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Wang X, Hu Y, Kim SY, Addou R, Cho K, Wallace RM. Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides. ACS Nano 2023; 17:20353-20365. [PMID: 37788682 DOI: 10.1021/acsnano.3c06494] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/05/2023]
Abstract
Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance.
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Affiliation(s)
- Xinglu Wang
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Yaoqiao Hu
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Seong Yeoul Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States
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Rahman T, Martin NP, Jenkins JK, Elzein R, Fast DB, Addou R, Herman GS, Nyman M. Nb 2O 5, LiNbO 3, and (Na, K)NbO 3 Thin Films from High-Concentration Aqueous Nb-Polyoxometalates. Inorg Chem 2022; 61:3586-3597. [PMID: 35148102 DOI: 10.1021/acs.inorgchem.1c03638] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of Nb2O5, LiNbO3, and (Na, K)NbO3 (KNN) from high-concentration solutions, up to 2.5 M Nb for Nb2O5 and ∼1 M Nb for LiNbO3 and KNN. Deposition of KNN from 1 M Nb concentration represents a potentially important advancment in lead-free piezoelectrics, an application that requires thick films. Solution characterization via small-angle X-ray scattering and Raman spectroscopy described the speciation for all precursor solutions as the [HxNb24O72](x-24) POM, as did total pair distribution function analyses of X-ray scattering of amorphous gels prior to conversion to oxides. The tendency of the Nb24-POM to form extended networks without crystallization leads to conformal and well-adhered films. The films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, ellipsometry, and X-ray photoelectron spectroscopy. As a strategy to convert aqueous deposition solutions from {Nb10}-POMs to {Nb24}-POMs, we devised a general procedure to produce doped Nb2O5 thin films including Ca, Ag, and Cu doping.
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Affiliation(s)
- Tasnim Rahman
- Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331, United States
| | - Nicolas P Martin
- Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331, United States
| | - Jessica K Jenkins
- School of Chemical, Biological, and Environmental Engineering, 116 Johnson Hall, 105 SW 26th St. Corvallis, Oregon 97331, United States
| | - Radwan Elzein
- School of Chemical, Biological, and Environmental Engineering, 116 Johnson Hall, 105 SW 26th St. Corvallis, Oregon 97331, United States
| | - Dylan B Fast
- Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331, United States
| | - Rafik Addou
- School of Chemical, Biological, and Environmental Engineering, 116 Johnson Hall, 105 SW 26th St. Corvallis, Oregon 97331, United States
| | - Gregory S Herman
- School of Chemical, Biological, and Environmental Engineering, 116 Johnson Hall, 105 SW 26th St. Corvallis, Oregon 97331, United States
| | - May Nyman
- Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331, United States
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Lee J, Adiga P, Lee SA, Nam SH, Ju HA, Jung MH, Jeong HY, Kim YM, Wong C, Elzein R, Addou R, Stoerzinger KA, Choi WS. Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La 0.7 Sr 0.3 MnO 3 Heterostructures. Small 2021; 17:e2103632. [PMID: 34677915 DOI: 10.1002/smll.202103632] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/21/2021] [Revised: 09/06/2021] [Indexed: 06/13/2023]
Abstract
Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La0.7 Sr0.3 MnO3 (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO3 heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.
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Affiliation(s)
- Jegon Lee
- Department of Physics, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Prajwal Adiga
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, 97331, USA
| | - Sang A Lee
- Department of Physics, Sungkyunkwan University, Suwon, 16419, Republic of Korea
- Department of Physics, Pukyong National University, Busan, 48513, Republic of Korea
| | - Seung Hyun Nam
- Department of Physics, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Hyeon-Ah Ju
- Department of Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Min-Hyoung Jung
- Department of Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Hu Young Jeong
- Graduate School of Semiconductor Materials and Devices Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Republic of Korea
| | - Young-Min Kim
- Department of Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Cindy Wong
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, 97331, USA
| | - Radwan Elzein
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, 97331, USA
| | - Rafik Addou
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, 97331, USA
| | - Kelsey A Stoerzinger
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, 97331, USA
- Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99254, USA
| | - Woo Seok Choi
- Department of Physics, Sungkyunkwan University, Suwon, 16419, Republic of Korea
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Kampouri S, Ebrahim FM, Fumanal M, Nord M, Schouwink PA, Elzein R, Addou R, Herman GS, Smit B, Ireland CP, Stylianou KC. Enhanced Visible-Light-Driven Hydrogen Production through MOF/MOF Heterojunctions. ACS Appl Mater Interfaces 2021; 13:14239-14247. [PMID: 33749235 DOI: 10.1021/acsami.0c23163] [Citation(s) in RCA: 28] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
A strategy for enhancing the photocatalytic performance of MOF-based systems (MOF: metal-organic framework) is developed through the construction of MOF/MOF heterojunctions. The combination of MIL-167 with MIL-125-NH2 leads to the formation of MIL-167/MIL-125-NH2 heterojunctions with improved optoelectronic properties and efficient charge separation. MIL-167/MIL-125-NH2 outperforms its single components MIL-167 and MIL-125-NH2, in terms of photocatalytic H2 production (455 versus 0.8 and 51.2 μmol h-1 g-1, respectively), under visible-light irradiation, without the use of any cocatalysts. This is attributed to the appropriate band alignment of these MOFs, the enhanced visible-light absorption, and long charge separation within MIL-167/MIL-125-NH2. Our findings contribute to the discovery of novel MOF-based photocatalytic systems that can harvest solar energy and exhibit high catalytic activities in the absence of cocatalysts.
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Affiliation(s)
- Stavroula Kampouri
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Fatmah M Ebrahim
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Maria Fumanal
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Makenzie Nord
- Department of Chemistry, Oregon State University, Gilbert Hall 153, Corvallis, Oregon 97331-4003, United States
| | - Pascal A Schouwink
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Radwan Elzein
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States
| | - Rafik Addou
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States
| | - Gregory S Herman
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States
| | - Berend Smit
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Christopher P Ireland
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
| | - Kyriakos C Stylianou
- Laboratory for Molecular Simulations, Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l'Industrie 17, Sion 1951, Switzerland
- Department of Chemistry, Oregon State University, Gilbert Hall 153, Corvallis, Oregon 97331-4003, United States
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Diulus JT, Elzein R, Addou R, Herman GS. Surface chemistry of 2-propanol and O 2 mixtures on SnO 2(110) studied with ambient-pressure x-ray photoelectron spectroscopy. J Chem Phys 2020; 152:054713. [PMID: 32035445 DOI: 10.1063/1.5138923] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Tin dioxide (SnO2) has various applications due to its unique surface and electronic properties. These properties are strongly influenced by Sn oxidation states and associated defect chemistries. Recently, the oxidation of volatile organic compounds (VOCs) into less harmful molecules has been demonstrated using SnO2 catalysts. A common VOC, 2-propanol (isopropyl alcohol, IPA), has been used as a model compound to better understand SnO2 reaction kinetics. We have used ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to characterize the surface chemistry of IPA and O2 mixtures on stoichiometric, unreconstructed SnO2(110)-(1 × 1) surfaces. AP-XPS experiments were performed for IPA pressures ≤3 mbar, various IPA/O2 ratios, and several reaction temperatures. These measurements allowed us to determine the chemical states of adsorbed species on SnO2(110)-(1 × 1) under numerous experimental conditions. We found that both the IPA/O2 ratio and sample temperature strongly influence reaction chemistries. AP-XPS valence-band spectra indicate that the surface was partially reduced from Sn4+ to Sn2+ during reactions with IPA. In situ mass spectrometry and gas-phase AP-XPS results indicate that the main reaction product was acetone under these conditions. For O2 and IPA mixtures, the reaction kinetics substantially increased and the surface remained solely Sn4+. We believe that O2 replenished surface oxygen vacancies and that SnO2 bridging and in-plane oxygen are likely the active oxygen species. Moreover, addition of O2 to the reaction results in a reduction in formation of acetone and an increase in formation of CO2 and H2O. Based on these studies, we have developed a reaction model that describes the catalytic oxidation of IPA on stoichiometric SnO2(110)-(1 × 1) surfaces.
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Affiliation(s)
- J Trey Diulus
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, USA
| | - Radwan Elzein
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, USA
| | - Rafik Addou
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, USA
| | - Gregory S Herman
- School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, USA
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Zhu H, Addou R, Wang Q, Nie Y, Cho K, Kim MJ, Wallace RM. Surface and interfacial study of atomic layer deposited Al 2O 3 on MoTe 2 and WTe 2. Nanotechnology 2020; 31:055704. [PMID: 31618710 DOI: 10.1088/1361-6528/ab4e44] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The atomic layer deposition (ALD) of high-k dielectrics could build an efficient barrier against moisture and O2 adsorption. Such a barrier is highly needed for MoTe2 and WTe2 transition metal dichalcogenides because of the poor structural stability and the fast oxidization in ambient air. In situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy and scanning transmission electron microscopy were employed to report a comparative study between the growth of Al2O3 on MoTe2 and WTe2 by means of traditional thermal ALD and plasma-enhanced ALD (PEALD). Similar to what has been observed on other 2D materials such as MoS2 and Graphene, the thermal ALD results in an islanding growth of Al2O3 on MoTe2 due to the dearth of dangling bonds, whereas, a uniform coverage of Al2O3 on WTe2 is observed and likely contributed to the high concentration of intrinsic structural defects. The PEALD behavior is consistent between MoTe2 and WTe2 providing a conformal and linear growth rate (∼0.08 nm/cycle), which correlates with the creation of Te-O and metal-O nucleation sites. However, a thin layer of interfacial Mo or W oxides gradually forms, resulting from the plasma-induced damage in the topmost (1-2) layers. Attempts to enhance the Al2O3/MoTe2 interfacial quality by physically evaporating an Al2O3 seed layer are investigated as well. However, the evaporated Al2O3 process causes thermal damage on MoTe2, necessitating a more 'gentle' ALD technique for the surface passivation.
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Affiliation(s)
- H Zhu
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080, United States of America
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Barton AT, Walsh LA, Smyth CM, Qin X, Addou R, Cormier C, Hurley PK, Wallace RM, Hinkle CL. Impact of Etch Processes on the Chemistry and Surface States of the Topological Insulator Bi 2Se 3. ACS Appl Mater Interfaces 2019; 11:32144-32150. [PMID: 31416305 DOI: 10.1021/acsami.9b10625] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The unique properties of topological insulators such as Bi2Se3 are intriguing for their potential implementation in novel device architectures for low power and defect-tolerant logic and memory devices. Recent improvements in the synthesis of Bi2Se3 have positioned researchers to fabricate new devices to probe the limits of these materials. The fabrication of such devices, of course, requires etching of the topological insulator, in addition to other materials including gate oxides and contacts which may impact the topologically protected surface states. In this paper, we study the impact of He+ sputtering and inductively coupled plasma Cl2 and SF6 reactive etch chemistries on the physical, chemical, and electronic properties of Bi2Se3. Chemical analysis by X-ray photoelectron spectroscopy tracks changes in the surface chemistry and Fermi level, showing preferential removal of Se that results in vacancy-induced n-type doping. Chlorine-based chemistry successfully etches Bi2Se3 but with residual Se-Se bonding and interstitial Cl species remaining after the etch. The Se vacancies and residuals can be removed with postetch anneals in a Se environment, repairing Bi2Se3 nearly to the as-grown condition. Critically, in each of these cases, angle-resolved photoemission spectroscopy (ARPES) reveals that the topologically protected surface states remain even after inducing significant surface disorder and chemical changes, demonstrating that topological insulators are quite promising for defect-tolerant electronics. Changes to the ARPES intensity and momentum broadening of the surface states are discussed. Fluorine-based etching aggressively reacts with the film resulting in a relatively thick insulating film of thermodynamically favored BiF3 on the surface, prohibiting the use of SF6-based etching in Bi2Se3 processing.
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Affiliation(s)
- Adam T Barton
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Lee A Walsh
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
- Tyndall National Institute , University College Cork , Lee Maltings Complex , Cork T12R5CP , Ireland
| | - Christopher M Smyth
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Xiaoye Qin
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Rafik Addou
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Christopher Cormier
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Paul K Hurley
- Tyndall National Institute , University College Cork , Lee Maltings Complex , Cork T12R5CP , Ireland
| | - Robert M Wallace
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Christopher L Hinkle
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
- Department of Electrical Engineering , University of Notre Dame , Notre Dame , Indiana 46556 , United States
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Lo CL, Catalano M, Khosravi A, Ge W, Ji Y, Zemlyanov DY, Wang L, Addou R, Liu Y, Wallace RM, Kim MJ, Chen Z. Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature. Adv Mater 2019; 31:e1902397. [PMID: 31183907 DOI: 10.1002/adma.201902397] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/14/2019] [Revised: 05/02/2019] [Indexed: 06/09/2023]
Abstract
The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSx ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSx compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node.
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Affiliation(s)
- Chun-Li Lo
- School of Electrical and Computer Engineering, and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907, USA
| | - Massimo Catalano
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
- Institute for Microelectronics and Microsystems, National Council for Research (IMM-CNR), Via Monteroni, ed. A3, 73100, Lecce, Italy
| | - Ava Khosravi
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Wanying Ge
- Texas Materials Institute and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA
| | - Yujin Ji
- Texas Materials Institute and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA
| | - Dmitry Y Zemlyanov
- Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907, USA
| | - Luhua Wang
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Rafik Addou
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
- School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, OR, 93771, USA
| | - Yuanyue Liu
- Texas Materials Institute and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA
| | - Robert M Wallace
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Moon J Kim
- Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Zhihong Chen
- School of Electrical and Computer Engineering, and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907, USA
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Khosravi A, Addou R, Catalano M, Kim J, Wallace RM. High-κ Dielectric on ReS₂: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al₂O₃. Materials (Basel) 2019; 12:ma12071056. [PMID: 30935054 PMCID: PMC6479988 DOI: 10.3390/ma12071056] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/07/2019] [Revised: 03/26/2019] [Accepted: 03/27/2019] [Indexed: 11/16/2022]
Abstract
We report an excellent growth behavior of a high-κ dielectric on ReS₂, a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al₂O₃ thin film on the UV-Ozone pretreated surface of ReS₂ yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al₂O₃ thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al₂O₃ was achieved using a UV-Ozone pretreatment. The ReS₂ substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S⁻O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials.
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Affiliation(s)
- Ava Khosravi
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
- School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, OR 93771, USA.
| | - Massimo Catalano
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
- Institute for Microelectronics and Microsystems, National Council for Research (IMM-CNR), Via Monteroni, ed. A3, 73100 Lecce, Italy.
| | - Jiyoung Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
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Nie Y, Barton AT, Addou R, Zheng Y, Walsh LA, Eichfeld SM, Yue R, Cormier CR, Zhang C, Wang Q, Liang C, Robinson JA, Kim M, Vandenberghe W, Colombo L, Cha PR, Wallace RM, Hinkle CL, Cho K. Dislocation driven spiral and non-spiral growth in layered chalcogenides. Nanoscale 2018; 10:15023-15034. [PMID: 30052245 DOI: 10.1039/c8nr02280a] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.
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Affiliation(s)
- Yifan Nie
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA.
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11
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Zhou G, Addou R, Wang Q, Honari S, Cormier CR, Cheng L, Yue R, Smyth CM, Laturia A, Kim J, Vandenberghe WG, Kim MJ, Wallace RM, Hinkle CL. High-Mobility Helical Tellurium Field-Effect Transistors Enabled by Transfer-Free, Low-Temperature Direct Growth. Adv Mater 2018; 30:e1803109. [PMID: 30022534 DOI: 10.1002/adma.201803109] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/15/2018] [Revised: 06/18/2018] [Indexed: 05/07/2023]
Abstract
The transfer-free direct growth of high-performance materials and devices can enable transformative new technologies. Here, room-temperature field-effect hole mobilities as high as 707 cm2 V-1 s-1 are reported, achieved using transfer-free, low-temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO2 /Si. The Te nanostructures exhibit significantly higher device performance than other low-temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high-performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO2 /Si and HfO2 . The morphology of the Te films can be tailored by the growth temperature, and different carrier scattering mechanisms are identified for films with different morphologies. The transfer-free direct growth of high-mobility Te devices can enable major technological breakthroughs, as the low-temperature growth and fabrication is compatible with the severe thermal budget constraints of emerging applications. For example, vertical integration of novel devices atop a silicon complementary metal oxide semiconductor platform (thermal budget <450 °C) has been theoretically shown to provide a 10× systems level performance improvement, while flexible and wearable electronics (thermal budget <200 °C) can revolutionize defense and medical applications.
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Affiliation(s)
- Guanyu Zhou
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Rafik Addou
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Qingxiao Wang
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Shahin Honari
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Christopher R Cormier
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Lanxia Cheng
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Ruoyu Yue
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Christopher M Smyth
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Akash Laturia
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Jiyoung Kim
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - William G Vandenberghe
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Moon J Kim
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Robert M Wallace
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
| | - Christopher L Hinkle
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080, USA
- Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN, 46556, USA
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12
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Walsh LA, Green AJ, Addou R, Nolting W, Cormier CR, Barton AT, Mowll TR, Yue R, Lu N, Kim J, Kim MJ, LaBella VP, Ventrice CA, McDonnell S, Vandenberghe WG, Wallace RM, Diebold A, Hinkle CL. Fermi Level Manipulation through Native Doping in the Topological Insulator Bi 2Se 3. ACS Nano 2018; 12:6310-6318. [PMID: 29874037 DOI: 10.1021/acsnano.8b03414] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in Bi2Se3 and achieving Fermi levels ( EF) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the Bi2Se3 studies in the literature report strongly n-type materials with EF in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic Bi2Se3 with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune EF from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for EF control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of Bi2Se3.
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Affiliation(s)
- Lee A Walsh
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
- Tyndall National Institute, University College Cork , Lee Maltings Complex , Cork T12R5CP , Ireland
| | - Avery J Green
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Rafik Addou
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Westly Nolting
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Christopher R Cormier
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Adam T Barton
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Tyler R Mowll
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Ruoyu Yue
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Ning Lu
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Jiyoung Kim
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Moon J Kim
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Vincent P LaBella
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Carl A Ventrice
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Stephen McDonnell
- Department of Materials Science and Engineering , University of Virginia , Charlottesville , Virginia 22904 , United States
| | - William G Vandenberghe
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Robert M Wallace
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
| | - Alain Diebold
- Colleges of Nanoscale Science and Engineering , SUNY Polytechnic Institute , Albany , New York 12203 , United States
| | - Christopher L Hinkle
- Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States
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13
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Lin YC, Jariwala B, Bersch BM, Xu K, Nie Y, Wang B, Eichfeld SM, Zhang X, Choudhury TH, Pan Y, Addou R, Smyth CM, Li J, Zhang K, Haque MA, Fölsch S, Feenstra RM, Wallace RM, Cho K, Fullerton-Shirey SK, Redwing JM, Robinson JA. Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors. ACS Nano 2018; 12:965-975. [PMID: 29360349 DOI: 10.1021/acsnano.7b07059] [Citation(s) in RCA: 79] [Impact Index Per Article: 13.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 μA·μm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.
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Affiliation(s)
- Yu-Chuan Lin
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Bhakti Jariwala
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Brian M Bersch
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Ke Xu
- Department of Chemical and Petroleum Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15213, United States
| | - Yifan Nie
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Baoming Wang
- Department of Mechanical Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Sarah M Eichfeld
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Xiaotian Zhang
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Tanushree H Choudhury
- Two-Dimensional Crystal Consortium (2DCC), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Yi Pan
- Paul-Drude-Institut für Festkörperelektronik , Hausvogteiplatz 5-7, Berlin 10117, Germany
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Christopher M Smyth
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Jun Li
- Department of Physics, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States
| | - Kehao Zhang
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - M Aman Haque
- Department of Mechanical Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Stefan Fölsch
- Paul-Drude-Institut für Festkörperelektronik , Hausvogteiplatz 5-7, Berlin 10117, Germany
| | - Randall M Feenstra
- Department of Physics, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Susan K Fullerton-Shirey
- Department of Chemical and Petroleum Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15213, United States
- Department of Electrical and Computer Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15213, United States
| | - Joan M Redwing
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
- Two-Dimensional Crystal Consortium (2DCC), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Joshua A Robinson
- Department of Materials Science and Engineering, Materials Research Institute, and Center for 2D and Layered Materials (2DLM), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
- Two-Dimensional Crystal Consortium (2DCC), The Pennsylvania State University , University Park, Pennsylvania 16802, United States
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14
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Zhu H, Wang Q, Cheng L, Addou R, Kim J, Kim MJ, Wallace RM. Defects and Surface Structural Stability of MoTe 2 Under Vacuum Annealing. ACS Nano 2017; 11:11005-11014. [PMID: 29116754 DOI: 10.1021/acsnano.7b04984] [Citation(s) in RCA: 57] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/17/2023]
Abstract
Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe2 with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe2. Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature. Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a "wagon wheel" morphology after 400 °C annealing for 15 min. Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain centers. To prevent extensive Te loss at higher temperatures, where Mo6Te6 nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe2 is tested. With this passivation strategy, the structural stability of MoTe2 is greatly enhanced up to 500 °C without apparent structural defects.
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Affiliation(s)
- Hui Zhu
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Qingxiao Wang
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Lanxia Cheng
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Jiyoung Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Moon J Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
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15
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Dong H, Gong C, Addou R, McDonnell S, Azcatl A, Qin X, Wang W, Wang W, Hinkle CL, Wallace RM. Schottky Barrier Height of Pd/MoS 2 Contact by Large Area Photoemission Spectroscopy. ACS Appl Mater Interfaces 2017; 9:38977-38983. [PMID: 29035026 DOI: 10.1021/acsami.7b10974] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
MoS2, as a model transition metal dichalcogenide, is viewed as a potential channel material in future nanoelectronic and optoelectronic devices. Minimizing the contact resistance of the metal/MoS2 junction is critical to realizing the potential of MoS2-based devices. In this work, the Schottky barrier height (SBH) and the band structure of high work function Pd metal on MoS2 have been studied by in situ X-ray photoelectron spectroscopy (XPS). The analytical spot diameter of the XPS spectrometer is about 400 μm, and the XPS signal is proportional to the detection area, so the influence of defect-mediated parallel conduction paths on the SBH does not affect the measurement. The charge redistribution by Pd on MoS2 is detected by XPS characterization, which gives insight into metal contact physics to MoS2 and suggests that interface engineering is necessary to lower the contact resistance for the future generation electronic applications.
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Affiliation(s)
- Hong Dong
- Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, Nankai University , Tianjin 300071, China
| | - Cheng Gong
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Stephen McDonnell
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Angelica Azcatl
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Xiaoye Qin
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Weichao Wang
- Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, Nankai University , Tianjin 300071, China
| | - Weihua Wang
- Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, Nankai University , Tianjin 300071, China
| | - Christopher L Hinkle
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
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16
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Zhu H, Wang Q, Zhang C, Addou R, Cho K, Wallace RM, Kim MJ. New Mo 6 Te 6 Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe 2. Adv Mater 2017; 29:1606264. [PMID: 28295727 DOI: 10.1002/adma.201606264] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/18/2016] [Revised: 01/18/2017] [Indexed: 06/06/2023]
Abstract
A novel phase transition, from multilayered 2H-MoTe2 to a parallel bundle of sub-nanometer-diameter metallic Mo6 Te6 nanowires (NWs) driven by catalyzer-free thermal-activation (400-500 °C) under vacuum, is demonstrated. The NWs form along the 〈11-20〉 2H-MoTe2 crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe2 due to favorable band-alignment. In particular, an atomically sharp MoTe2 /Mo6 Te6 interface and van der Waals gap with the 2H layers are preserved. The work highlights an alternative pathway for forming a new transition metal dichalcogenide phase and will enable future exploration of its intrinsic transportation properties.
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Affiliation(s)
- Hui Zhu
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Qingxiao Wang
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Chenxi Zhang
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
| | - Moon J Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX, 75080, USA
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17
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Abstract
Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe2 is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe2 characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry. The STM images reveal the presence of atomic-size imperfections and a variation in the electronic structure caused by the presence of defects and impurities below the detection limit of XPS. Both STS and photoemission reveal a spatial variation in the Fermi level position. The analysis of the core levels indicates the presence of different doping levels. The presence of a large concentration of defects and impurities has a strong impact on the electronic properties of the WSe2 surface. Our findings demonstrate that the growth of controllable and high quality two-dimensional materials at nanometer scale is one of the most challenging tasks that requires further attention.
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Affiliation(s)
- Rafik Addou
- Department of Materials Science, Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Robert M Wallace
- Department of Materials Science, Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
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18
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K. C. S, Longo RC, Addou R, Wallace RM, Cho K. Electronic properties of MoS 2/MoO x interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts. Sci Rep 2016; 6:33562. [PMID: 27666523 PMCID: PMC5035990 DOI: 10.1038/srep33562] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2016] [Accepted: 08/30/2016] [Indexed: 01/14/2023] Open
Abstract
In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.
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Affiliation(s)
- Santosh K. C.
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA
- Materials Science and Technology Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, Tennessee 37831, USA
| | - Roberto C. Longo
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA
| | - Rafik Addou
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA
| | - Robert M. Wallace
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA
- Department of Physics, The University of Texas at Dallas, Richardson, TX 75080 USA
| | - Kyeongjae Cho
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA
- Department of Physics, The University of Texas at Dallas, Richardson, TX 75080 USA
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19
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Azcatl A, Qin X, Prakash A, Zhang C, Cheng L, Wang Q, Lu N, Kim MJ, Kim J, Cho K, Addou R, Hinkle CL, Appenzeller J, Wallace RM. Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure. Nano Lett 2016; 16:5437-43. [PMID: 27494551 DOI: 10.1021/acs.nanolett.6b01853] [Citation(s) in RCA: 137] [Impact Index Per Article: 17.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.
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Affiliation(s)
- Angelica Azcatl
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Xiaoye Qin
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Abhijith Prakash
- Department of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University , West Lafayette 47907, Indiana United States
| | - Chenxi Zhang
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Lanxia Cheng
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Qingxiao Wang
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Ning Lu
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Moon J Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Jiyoung Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Christopher L Hinkle
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Joerg Appenzeller
- Department of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University , West Lafayette 47907, Indiana United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
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20
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Lin YC, Li J, de la Barrera SC, Eichfeld SM, Nie Y, Addou R, Mende PC, Wallace RM, Cho K, Feenstra RM, Robinson JA. Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures. Nanoscale 2016; 8:8947-8954. [PMID: 27073972 DOI: 10.1039/c6nr01902a] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EG(FH)) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EG(FH) displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EG(PH) forms a Schottky barrier junction.
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Affiliation(s)
- Yu-Chuan Lin
- Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
| | - Jun Li
- Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
| | | | - Sarah M Eichfeld
- Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
| | - Yifan Nie
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
| | - Patrick C Mende
- Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
| | - Randall M Feenstra
- Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
| | - Joshua A Robinson
- Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
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21
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Amani M, Taheri P, Addou R, Ahn GH, Kiriya D, Lien DH, Ager JW, Wallace RM, Javey A. Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides. Nano Lett 2016; 16:2786-2791. [PMID: 26978038 DOI: 10.1021/acs.nanolett.6b00536] [Citation(s) in RCA: 117] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
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Affiliation(s)
- Matin Amani
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Peyman Taheri
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, University of Texas, Dallas , Richardson, Texas 75080, United States
| | - Geun Ho Ahn
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Daisuke Kiriya
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Der-Hsien Lien
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Joel W Ager
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, University of Texas, Dallas , Richardson, Texas 75080, United States
| | - Ali Javey
- Electrical Engineering and Computer Sciences, University of California , Berkeley, California 94720, United States
- Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
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22
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Zhang K, Feng S, Wang J, Azcatl A, Lu N, Addou R, Wang N, Zhou C, Lerach J, Bojan V, Kim MJ, Chen LQ, Wallace RM, Terrones M, Zhu J, Robinson JA. Correction to Manganese Doping of Monolayer MoS2: The Substrate Is Critical. Nano Lett 2016; 16:2125. [PMID: 26905941 DOI: 10.1021/acs.nanolett.6b00760] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
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23
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Cheng L, Jandhyala S, Mordi G, Lucero AT, Huang J, Azcatl A, Addou R, Wallace RM, Colombo L, Kim J. Partially Fluorinated Graphene: Structural and Electrical Characterization. ACS Appl Mater Interfaces 2016; 8:5002-5008. [PMID: 26820099 DOI: 10.1021/acsami.5b11701] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C-F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C-F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C-F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C.
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Affiliation(s)
- Lanxia Cheng
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Srikar Jandhyala
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Greg Mordi
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Antonio T Lucero
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Jie Huang
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Angelica Azcatl
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Luigi Colombo
- Texas Instruments , Dallas, Texas 75243, United States
| | - Jiyoung Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States
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24
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Amani M, Lien DH, Kiriya D, Xiao J, Azcatl A, Noh J, Madhvapathy SR, Addou R, KC S, Dubey M, Cho K, Wallace RM, Lee SC, He JH, Ager JW, Zhang X, Yablonovitch E, Javey A. Near-unity photoluminescence quantum yield in MoS2. Science 2015; 350:1065-8. [PMID: 26612948 DOI: 10.1126/science.aad2114] [Citation(s) in RCA: 477] [Impact Index Per Article: 53.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Affiliation(s)
- Matin Amani
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Der-Hsien Lien
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Computer, Electrical and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. Department of Electrical Engineering, Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China
| | - Daisuke Kiriya
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Jun Xiao
- National Science Foundation Nanoscale Science and Engineering Center, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Angelica Azcatl
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Jiyoung Noh
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Surabhi R Madhvapathy
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Rafik Addou
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Santosh KC
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Madan Dubey
- Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, MD 20723, USA
| | - Kyeongjae Cho
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Robert M Wallace
- Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA
| | - Si-Chen Lee
- Department of Electrical Engineering, Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China
| | - Jr-Hau He
- Computer, Electrical and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
| | - Joel W Ager
- Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Xiang Zhang
- National Science Foundation Nanoscale Science and Engineering Center, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Department of Physics, King Abdulaziz University, Jeddah 21589, Saudi Arabia
| | - Eli Yablonovitch
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
| | - Ali Javey
- Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
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25
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Zhang K, Feng S, Wang J, Azcatl A, Lu N, Addou R, Wang N, Zhou C, Lerach J, Bojan V, Kim MJ, Chen LQ, Wallace RM, Terrones M, Zhu J, Robinson JA. Manganese Doping of Monolayer MoS2: The Substrate Is Critical. Nano Lett 2015; 15:6586-6591. [PMID: 26349430 DOI: 10.1021/acs.nanolett.5b02315] [Citation(s) in RCA: 157] [Impact Index Per Article: 17.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e., SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.
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Affiliation(s)
| | | | | | - Angelica Azcatl
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Ning Lu
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | - Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | | | | | - Jordan Lerach
- Materials Characterization Laboratory, The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Vincent Bojan
- Materials Characterization Laboratory, The Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Moon J Kim
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
| | | | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
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26
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Addou R, McDonnell S, Barrera D, Guo Z, Azcatl A, Wang J, Zhu H, Hinkle CL, Quevedo-Lopez M, Alshareef HN, Colombo L, Hsu JWP, Wallace RM. Impurities and Electronic Property Variations of Natural MoS2 Crystal Surfaces. ACS Nano 2015; 9:9124-33. [PMID: 26301428 DOI: 10.1021/acsnano.5b03309] [Citation(s) in RCA: 91] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS2 crystals. Our findings reveal that the semiconductor 2H-MoS2 exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.3 eV. The presence of impurities in parts-per-million (ppm) and a surface defect density of up to 8% of the total area could explain the variation of the Fermi level position. High resolution RBS data also show a large variation in the MoSx composition (1.8 < x < 2.05) at the surface. Thus, the variation in the conductivity, the work function, and stoichiometry across small areas of MoS2 will have to be controlled during crystal growth in order to provide high quality uniform materials for future device fabrication.
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Affiliation(s)
- Rafik Addou
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Stephen McDonnell
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Diego Barrera
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
- Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), Unidad Monterrey, Alianza Norte 202, 66600 Apodaca, Nuevo León México
| | | | - Angelica Azcatl
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Jian Wang
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Hui Zhu
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Christopher L Hinkle
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Manuel Quevedo-Lopez
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | | | - Luigi Colombo
- Texas Instruments Incorporated, 13121 TI Boulevard, MS-365, Dallas, Texas 75243, Unites States
| | - Julia W P Hsu
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
| | - Robert M Wallace
- Department of Materials Science and Engineering, The University of Texas at Dallas , 800 Campbell Road, Richardson, Texas 75080, Unites States
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27
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Zhu H, McDonnell S, Qin X, Azcatl A, Cheng L, Addou R, Kim J, Ye PD, Wallace RM. Al2O3 on Black Phosphorus by Atomic Layer Deposition: An in Situ Interface Study. ACS Appl Mater Interfaces 2015; 7:13038-43. [PMID: 26016806 DOI: 10.1021/acsami.5b03192] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
In situ "half cycle" atomic layer deposition (ALD) of Al2O3 was carried out on black phosphorus ("black-P") surfaces with modified phosphorus oxide concentrations. X-ray photoelectron spectroscopy is employed to investigate the interfacial chemistry and the nucleation of the Al2O3 on black-P surfaces. This work suggests that exposing a sample that is initially free of phosphorus oxide to the ALD precursors does not result in detectable oxidation. However, when the phosphorus oxide is formed on the surface prior to deposition, the black-P can react with both the surface adventitious oxygen contamination and the H2O precursor at a deposition temperature of 200 °C. As a result, the concentration of the phosphorus oxide increases after both annealing and the atomic layer deposition process. The nucleation rate of Al2O3 on black-P is correlated with the amount of oxygen on samples prior to the deposition. The growth of Al2O3 follows a "substrate inhibited growth" behavior where an incubation period is required. Ex situ atomic force microscopy is also used to investigate the deposited Al2O3 morphologies on black-P where the Al2O3 tends to form islands on the exfoliated black-P samples. Therefore, surface functionalization may be needed to get a conformal coverage of Al2O3 on the phosphorus oxide free samples.
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Affiliation(s)
- Hui Zhu
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Stephen McDonnell
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Xiaoye Qin
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Angelica Azcatl
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Lanxia Cheng
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Rafik Addou
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Jiyoung Kim
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Peide D Ye
- ‡School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States
| | - Robert M Wallace
- †Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
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Abstract
Transition metal dichalcogenides (TMDs) are being considered for a variety of electronic and optoelectronic devices such as beyond complementary metal-oxide-semiconductor (CMOS) switches, light-emitting diodes, solar cells, as well as sensors, among others. Molybdenum disulfide (MoS2) is the most studied of the TMDs in part because of its availability in the natural or geological form. The performance of most devices is strongly affected by the intrinsic defects in geological MoS2. Indeed, most sources of current transition metal dichalcogenides have defects, including many impurities. The variability in the electrical properties of MoS2 across the surface of the same crystal has been shown to be correlated with local variations in stoichiometry as well as metallic-like and structural defects. The presence of impurities has also been suggested to play a role in determining the Fermi level in MoS2. The main focus of this work is to highlight a number of intrinsic defects detected on natural, exfoliated MoS2 crystals from two different sources that have been often used in previous reports for device fabrication. We employed room temperature scanning tunneling microscopy (STM) and spectroscopy (STS), inductively coupled plasma mass spectrometry (ICPMS), as well as X-ray photoelectron spectroscopy (XPS) to study the pristine surface of MoS2(0001) immediately after exfoliation. ICPMS used to measure the concentration of impurity elements can in part explain the local contrast behavior observed in STM images. This work highlights that the high concentration of surface defects and impurity atoms may explain the variability observed in the electrical and physical characteristics of MoS2.
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Affiliation(s)
- Rafik Addou
- †Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States
| | - Luigi Colombo
- ‡Texas Instruments Incorporated, 13121 TI Boulevard, MS-365, Dallas, Texas 75243, United States
| | - Robert M Wallace
- †Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States
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29
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Diaz HC, Avila J, Chen C, Addou R, Asensio MC, Batzill M. Direct observation of interlayer hybridization and Dirac relativistic carriers in graphene/MoS₂ van der Waals heterostructures. Nano Lett 2015; 15:1135-1140. [PMID: 25629211 DOI: 10.1021/nl504167y] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Artificial heterostructures assembled from van der Waals materials promise to combine materials without the traditional restrictions in heterostructure-growth such as lattice matching conditions and atom interdiffusion. Simple stacking of van der Waals materials with diverse properties would thus enable the fabrication of novel materials or device structures with atomically precise interfaces. Because covalent bonding in these layered materials is limited to molecular planes and the interaction between planes are very weak, only small changes in the electronic structure are expected by stacking these materials on top of each other. Here we prepare interfaces between CVD-grown graphene and MoS2 and report the direct measurement of the electronic structure of such a van der Waals heterostructure by angle-resolved photoemission spectroscopy. While the Dirac cone of graphene remains intact and no significant charge transfer doping is detected, we observe formation of band gaps in the π-band of graphene, away from the Fermi-level, due to hybridization with states from the MoS2 substrate.
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Affiliation(s)
- Horacio Coy Diaz
- Department of Physics, University of South Florida , Tampa, Florida 33620, United States
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30
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Dahal A, Addou R, Azcatl A, Coy-Diaz H, Lu N, Peng X, de Dios F, Kim J, Kim MJ, Wallace RM, Batzill M. Seeding atomic layer deposition of alumina on graphene with yttria. ACS Appl Mater Interfaces 2015; 7:2082-2087. [PMID: 25556522 DOI: 10.1021/am508154n] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Integrating graphene into nanoelectronic device structure requires interfacing graphene with high-κ dielectric materials. However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating a pinhole-free, uniform, and conformal graphene/dielectric interface challenging. Here, we demonstrate that an ultrathin layer of high-κ dielectric material Y2O3 acts as an effective seeding layer for atomic layer deposition of Al2O3 on graphene. Whereas identical Al2O3 depositions lead to discontinuous film on bare graphene, the Y2O3 seeding layer yields uniform and conformal films. The morphology of the Al2O3 film is characterized by atomic force microscopy and transmission electron microscopy. C-1s X-ray photoemission spectroscopy indicates that the underlying graphene remains intact following Y2O3 seed and Al2O3 deposition. Finally, photoemission measurements of the graphene/SiO2/Si, Y2O3/graphene/SiO2, and Al2O3/Y2O3/graphene/SiO2 interfaces indicate n-type doping of graphene with different doping levels due to charge transfer at the interfaces.
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Affiliation(s)
- Arjun Dahal
- Department of Physics, University of South Florida , Tampa, Florida 33620, United States
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31
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Yue R, Barton AT, Zhu H, Azcatl A, Pena LF, Wang J, Peng X, Lu N, Cheng L, Addou R, McDonnell S, Colombo L, Hsu JWP, Kim J, Kim MJ, Wallace RM, Hinkle CL. HfSe2 thin films: 2D transition metal dichalcogenides grown by molecular beam epitaxy. ACS Nano 2015; 9:474-80. [PMID: 25496648 DOI: 10.1021/nn5056496] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
In this work, we demonstrate the growth of HfSe2 thin films using molecular beam epitaxy. The relaxed growth criteria have allowed us to demonstrate layered, crystalline growth without misfit dislocations on other 2D substrates such as highly ordered pyrolytic graphite and MoS2. The HfSe2 thin films exhibit an atomically sharp interface with the substrates used, followed by flat, 2D layers with octahedral (1T) coordination. The resulting HfSe2 is slightly n-type with an indirect band gap of ∼ 1.1 eV and a measured energy band alignment significantly different from recent DFT calculations. These results demonstrate the feasibility and significant potential of fabricating 2D material based heterostructures with tunable band alignments for a variety of nanoelectronic and optoelectronic applications.
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Affiliation(s)
- Ruoyu Yue
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
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Lin YC, Chang CYS, Ghosh RK, Li J, Zhu H, Addou R, Diaconescu B, Ohta T, Peng X, Lu N, Kim MJ, Robinson JT, Wallace RM, Mayer TS, Datta S, Li LJ, Robinson JA. Atomically thin heterostructures based on single-layer tungsten diselenide and graphene. Nano Lett 2014; 14:6936-6941. [PMID: 25383798 DOI: 10.1021/nl503144a] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Green's function (NEGF).
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Affiliation(s)
- Yu-Chuan Lin
- Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University , University Park, Pennsylvania 16802, United States
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Abstract
Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.
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Affiliation(s)
- Santosh K C
- Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
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McDonnell S, Azcatl A, Addou R, Gong C, Battaglia C, Chuang S, Cho K, Javey A, Wallace RM. Hole contacts on transition metal dichalcogenides: interface chemistry and band alignments. ACS Nano 2014; 8:6265-6272. [PMID: 24797712 DOI: 10.1021/nn501728w] [Citation(s) in RCA: 78] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
MoOx shows promising potential as an efficient hole injection layer for p-FETs based on transition metal dichalcogenides. A combination of experiment and theory is used to study the surface and interfacial chemistry, as well as the band alignments for MoOx/MoS2 and MoOx/WSe2 heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. A Mo(5+) rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoOx contacts on MoS2 and WSe2.
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Affiliation(s)
- Stephen McDonnell
- Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States
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Addou R, Senftle TP, O'Connor N, Janik MJ, van Duin ACT, Batzill M. Influence of hydroxyls on Pd atom mobility and clustering on rutile TiO(2)(011)-2 × 1. ACS Nano 2014; 8:6321-33. [PMID: 24806092 DOI: 10.1021/nn501817w] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
Understanding agglomeration of late transition metal atoms, such as Pd, on metal oxide supports, such as TiO2, is critical for designing heterogeneous catalysts as well as for controlling metal/oxide interfaces in general. One approach for reducing particle sintering is to modify the metal oxide surface with hydroxyls that decrease adatom mobility. We study by scanning tunneling microscopy experiments, density functional theory (DFT) calculations, and Monte Carlo (MC) computer simulations the atomistic processes of Pd sintering on a hydroxyl-modified TiO2(011)-2 × 1 surface. The formation of small 1-3 atom clusters that are stable at room temperature is achieved on the hydroxylated surface, while much larger clusters are formed under the same conditions on a hydroxyl-free surface. DFT shows that this is a consequence of stronger binding of Pd atoms adjacent to hydroxyls and increased surface diffusion barriers for Pd atoms on the hydroxylated surface. DFT, kinetic MC, and ReaxFF-based NVT-MC simulations show that Pd clusters larger than single Pd monomers can adsorb the hydrogen from the oxide surface and form Pd hydrides. This depletes the surface hydroxyl coverage, thus allowing Pd to more freely diffuse and agglomerate at room temperature. Experimentally, this causes a bimodal cluster size distribution with 1-3 atom clusters prevalent at low Pd coverage, while significantly larger clusters become dominant at higher Pd concentrations. This study demonstrates that hydroxylated oxide surfaces can significantly reduce Pd cluster sizes, thus enabling the preparation of surfaces populated with metal clusters composed of single to few atoms.
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Affiliation(s)
- Rafik Addou
- Department of Physics, University of South Florida , Tampa, Florida 33620, United States
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36
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Abstract
Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.
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Affiliation(s)
- Stephen McDonnell
- Department of Materials Science and Engineering, University of Texas at Dallas , Richardson, Texas 75080, United States
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37
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Abstract
Heterostructures of dissimilar 2D materials are potential building blocks for novel materials and may enable the formation of new (photo)electronic device architectures. Previous work mainly focused on supporting graphene on insulating wide-band gap materials, such as hex-BN and mica. Here we investigate the interface between zero-band gap semiconductor graphene and band-gap semiconductor MoS2 as a potential building block for entirely 2D-material based semiconducting devices. We show that solution transfer results in water trapping at the interface which may be removed by annealing to ~300 °C in a vacuum. After removal of the water, by high temperature annealing, ultraflat graphene is obtained on MoS2 with only a very weak moiré pattern observable in scanning tunneling microscopy images due to lattice mismatch and random alignment of graphene with respect to the MoS2 substrate. Photoemission spectroscopy indicates interface dipole formation, p-type doping of graphene by ~0.09 eV downward shift of the Fermi-level below the Dirac point, and a negative space charge region in bulk MoS2. Interestingly, valence band spectra of the graphene covered MoS2 surface indicate a band gap narrowing of the MoS2 surface by ~0.1 eV. This band gap reduction at the surface is further evidence that interlayer van der Waals interactions critically influence the band structure of 2D-layered dichalcogenides and suggest that interfacing with dissimilar van der Waals materials allows tuning of their electronic properties.
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Affiliation(s)
- Horacio Coy Diaz
- Department of Physics, University of South Florida, Tampa, FL 33620, USA
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Addou R, Batzill M. Defects and domain boundaries in self-assembled terephthalic acid (TPA) monolayers on CVD-grown graphene on Pt(111). Langmuir 2013; 29:6354-6360. [PMID: 23627863 DOI: 10.1021/la400972k] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Self-assembly of terephthalic acid (TPA), vacuum deposited on Pt(111) supported graphene, has been investigated by scanning tunneling microscopy (STM). TPA organizes in an ordered 3 × 4 superstructure with respect to the graphene lattice. This structure is a consequence of hydrogen-bonded TPA chains that arrange in a commensurate overlayer on graphene. Due to the polycrystalline nature of graphene on Pt(111), the TPA layer exhibits various grain boundaries and dislocations. Molecular resolved STM imaging has been used to characterize these defect structures in the TPA monolayer.
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Affiliation(s)
- Rafik Addou
- Department of Physics, University of South Florida, Tampa, Florida 33620, USA
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Addou R, Dahal A, Batzill M. Growth of a two-dimensional dielectric monolayer on quasi-freestanding graphene. Nat Nanotechnol 2013; 8:41-45. [PMID: 23263724 DOI: 10.1038/nnano.2012.217] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/16/2012] [Accepted: 11/12/2012] [Indexed: 06/01/2023]
Abstract
Integrating graphene into device architectures requires interfacing graphene with dielectric materials. However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating continuous graphene/dielectric interfaces challenging. Here, we show that yttria (Y(2)O(3))--a high-κ dielectric--can form a complete monolayer on platinum-supported graphene. The monolayer interacts weakly with graphene, but is stable to high temperatures. Scanning tunnelling microscopy reveals that the yttria layer exhibits a two-dimensional hexagonal lattice rotated by 30° relative to the hexagonal graphene lattice. X-ray photoemission spectroscopy measurements indicate a shift of the Fermi level in graphene on yttria deposition, which suggests that dielectric layers could be used for charge doping of metal-supported graphene.
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Affiliation(s)
- Rafik Addou
- Department of Physics, University of South Florida, Tampa, Florida 33620, USA
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40
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Addou R, Shukla AK, de Weerd MC, Gille P, Widmer R, Gröning O, Fournée V, Dubois JM, Ledieu J. Pseudomorphy, surface alloys and the role of elementary clusters on the domain orientations in the Cu/Al13Co4(100) system. J Phys Condens Matter 2011; 23:435009. [PMID: 21983255 DOI: 10.1088/0953-8984/23/43/435009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We have used the pseudo-tenfold surface of the orthorhombic Al(13)Co(4) crystal as a template for the adsorption of Cu thin films of various thicknesses deposited at different temperatures. This study has been carried out by means of low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). From 300 to 573 K, Cu adatoms grow pseudomorphically up to one monolayer. At 300 K, the β-Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the β-Al(Cu, Co) phase further transforms into the γ-Al(4)Cu(9) phase. Both β and γ phases grow as two (110) domains rotated by 72° ± 1° from each other. Instead of following the substrate symmetry, it is the orientations of the bipentagonal motifs present on the clean Al(13)Co(4)(100) surface that dictate the growth orientation of these domains. The initial bulk composition and structural complexity of the substrate have a minor role in the formation of the γ-Al(4)Cu(9) phase as long as the amount of Al and the Cu film thickness reach a critical stoichiometry.
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Affiliation(s)
- R Addou
- Institut Jean Lamour (UMR 7198 CNRS-Nancy-Université-UPV-Metz), Ecole des Mines, Nancy, France
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