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Wang D, Telford EJ, Benyamini A, Jesudasan J, Raychaudhuri P, Watanabe K, Taniguchi T, Hone J, Dean CR, Pasupathy AN. Andreev Reflections in NbN/Graphene Junctions under Large Magnetic Fields. NANO LETTERS 2021; 21:8229-8235. [PMID: 34569787 DOI: 10.1021/acs.nanolett.1c02020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Hybrid superconductor/graphene (SC/g) junctions are excellent candidates for investigating correlations between Cooper pairs and quantum Hall (QH) edge modes. Experimental studies are challenging as Andreev reflections are extremely sensitive to junction disorder, and high magnetic fields are required to form QH edge states. We fabricated low-resistance SC/g interfaces, composed of graphene edge contacted with NbN with a barrier strength of Z ≈ 0.4, that remain superconducting under magnetic fields larger than 18 T. We establish the role of graphene's Dirac band structure on zero-field Andreev reflections and demonstrate dynamic tunability of the Andreev reflection spectrum by moving the boundary between specular and retro Andreev reflections with parallel magnetic fields. Through the application of perpendicular magnetic fields, we observe an oscillatory suppression of the 2-probe conductance in the ν = 4 Landau level attributed to the reduced efficiency of Andreev processes at the NbN/g interface, consistent with theoretical predictions.
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Moore SL, Ciccarino CJ, Halbertal D, McGilly LJ, Finney NR, Yao K, Shao Y, Ni G, Sternbach A, Telford EJ, Kim BS, Rossi SE, Watanabe K, Taniguchi T, Pasupathy AN, Dean CR, Hone J, Schuck PJ, Narang P, Basov DN. Nanoscale lattice dynamics in hexagonal boron nitride moiré superlattices. Nat Commun 2021; 12:5741. [PMID: 34593793 PMCID: PMC8484559 DOI: 10.1038/s41467-021-26072-7] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2021] [Accepted: 09/02/2021] [Indexed: 11/12/2022] Open
Abstract
Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moiré superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lattice response experiments only in their infancy. Here, we therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moiré superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moiré domains of twisted hBN. Here, the authors investigate the lattice dynamics of twisted hexagonal boron nitride layers via nano-infrared spectroscopy, showing local and stacking-dependent variations of the optical phonon frequencies associated to the interaction with the graphite substrate.
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Rhodes DA, Jindal A, Yuan NFQ, Jung Y, Antony A, Wang H, Kim B, Chiu YC, Taniguchi T, Watanabe K, Barmak K, Balicas L, Dean CR, Qian X, Fu L, Pasupathy AN, Hone J. Enhanced Superconductivity in Monolayer Td-MoTe 2. NANO LETTERS 2021; 21:2505-2511. [PMID: 33689385 DOI: 10.1021/acs.nanolett.0c04935] [Citation(s) in RCA: 23] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic Td-MoTe2, with critical temperature (Tc) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk Tc. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.
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Kerelsky A, Rubio-Verdú C, Xian L, Kennes DM, Halbertal D, Finney N, Song L, Turkel S, Wang L, Watanabe K, Taniguchi T, Hone J, Dean C, Basov DN, Rubio A, Pasupathy AN. Moiréless correlations in ABCA graphene. Proc Natl Acad Sci U S A 2021; 118:e2017366118. [PMID: 33468646 PMCID: PMC7848726 DOI: 10.1073/pnas.2017366118] [Citation(s) in RCA: 35] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022] Open
Abstract
Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene--a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3-5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.
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Benyamini A, Kennes DM, Telford EJ, Watanabe K, Taniguchi T, Millis AJ, Hone J, Dean CR, Pasupathy AN. Nonmonotonic Temperature-Dependent Dissipation at Nonequilibrium in Atomically Thin Clean-Limit Superconductors. NANO LETTERS 2021; 21:583-589. [PMID: 33372802 DOI: 10.1021/acs.nanolett.0c04024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Resistance in superconductors arises from the motion of vortices driven by flowing supercurrents or external electromagnetic fields and may be strongly affected by thermal or quantum fluctuations. The common expectation is that as the temperature is lowered, vortex motion is suppressed, leading to a decreased resistance. We show experimentally that in clean-limit atomically thin 2H-NbSe2 the resistance below the superconducting transition temperature may be nonmonotonic, passing through a minimum before increasing again as the temperature is decreased further. The effect is most pronounced in monolayer devices and cannot be understood in terms of known mechanisms. We propose a qualitative two-fluid vortex model in which thermal fluctuations of pinned vortices control the mobility of the free vortices. The findings provide a new perspective on fundamental questions of vortex mobility and dissipation in superconductors.
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Halbertal D, Finney NR, Sunku SS, Kerelsky A, Rubio-Verdú C, Shabani S, Xian L, Carr S, Chen S, Zhang C, Wang L, Gonzalez-Acevedo D, McLeod AS, Rhodes D, Watanabe K, Taniguchi T, Kaxiras E, Dean CR, Hone JC, Pasupathy AN, Kennes DM, Rubio A, Basov DN. Moiré metrology of energy landscapes in van der Waals heterostructures. Nat Commun 2021; 12:242. [PMID: 33431846 PMCID: PMC7801382 DOI: 10.1038/s41467-020-20428-1] [Citation(s) in RCA: 38] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2020] [Accepted: 12/02/2020] [Indexed: 01/29/2023] Open
Abstract
The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, represents a promising route for the design of quantum materials, as the twist-angle-induced superlattices offer means to control topology and strong correlations. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. Here we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers.
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Bai Y, Zhou L, Wang J, Wu W, McGilly LJ, Halbertal D, Lo CFB, Liu F, Ardelean J, Rivera P, Finney NR, Yang XC, Basov DN, Yao W, Xu X, Hone J, Pasupathy AN, Zhu XY. Author Correction: Excitons in strain-induced one-dimensional moiré potentials at transition metal dichalcogenide heterojunctions. NATURE MATERIALS 2020; 19:1124. [PMID: 32690914 DOI: 10.1038/s41563-020-0773-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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Bai Y, Zhou L, Wang J, Wu W, McGilly LJ, Halbertal D, Lo CFB, Liu F, Ardelean J, Rivera P, Finney NR, Yang XC, Basov DN, Yao W, Xu X, Hone J, Pasupathy AN, Zhu XY. Excitons in strain-induced one-dimensional moiré potentials at transition metal dichalcogenide heterojunctions. NATURE MATERIALS 2020; 19:1068-1073. [PMID: 32661380 DOI: 10.1038/s41563-020-0730-8] [Citation(s) in RCA: 91] [Impact Index Per Article: 22.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/25/2020] [Accepted: 06/10/2020] [Indexed: 06/11/2023]
Abstract
The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2 heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.
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Darlington TP, Carmesin C, Florian M, Yanev E, Ajayi O, Ardelean J, Rhodes DA, Ghiotto A, Krayev A, Watanabe K, Taniguchi T, Kysar JW, Pasupathy AN, Hone JC, Jahnke F, Borys NJ, Schuck PJ. Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe 2 at room temperature. NATURE NANOTECHNOLOGY 2020; 15:854-860. [PMID: 32661371 DOI: 10.1038/s41565-020-0730-5] [Citation(s) in RCA: 68] [Impact Index Per Article: 17.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/13/2020] [Accepted: 06/03/2020] [Indexed: 05/23/2023]
Abstract
In monolayer transition-metal dichalcogenides, localized strain can be used to design nanoarrays of single photon sources. Despite strong empirical correlation, the nanoscale interplay between excitons and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopic analysis of excitons in nanobubbles of monolayer WSe2 with atomistic models to study how strain induces nanoscale confinement potentials and localized exciton states. The imaging of nanobubbles in monolayers with low defect concentrations reveals localized excitons on length scales of around 10 nm at multiple sites around the periphery of individual nanobubbles, in stark contrast to predictions of continuum models of strain. These results agree with theoretical confinement potentials atomistically derived from the measured topographies of nanobubbles. Our results provide experimental and theoretical insights into strain-induced exciton localization on length scales commensurate with exciton size, realizing key nanoscale structure-property information on quantum emitters in monolayer WSe2.
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Telford EJ, Dismukes AH, Lee K, Cheng M, Wieteska A, Bartholomew AK, Chen YS, Xu X, Pasupathy AN, Zhu X, Dean CR, Roy X. Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e2003240. [PMID: 32776373 DOI: 10.1002/adma.202003240] [Citation(s) in RCA: 61] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2020] [Revised: 06/19/2020] [Indexed: 06/11/2023]
Abstract
The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, TN = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is ΔE = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.
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36
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Wang L, Shih EM, Ghiotto A, Xian L, Rhodes DA, Tan C, Claassen M, Kennes DM, Bai Y, Kim B, Watanabe K, Taniguchi T, Zhu X, Hone J, Rubio A, Pasupathy AN, Dean CR. Correlated electronic phases in twisted bilayer transition metal dichalcogenides. NATURE MATERIALS 2020; 19:861-866. [PMID: 32572205 DOI: 10.1038/s41563-020-0708-6] [Citation(s) in RCA: 234] [Impact Index Per Article: 58.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/22/2019] [Accepted: 05/11/2020] [Indexed: 05/06/2023]
Abstract
In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. Such flat bands in twisted graphene-based systems result in correlated insulator, superconducting and topological states. Here we report evidence of low-energy flat bands in twisted bilayer WSe2, with signatures of collective phases observed over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. At a 5.1° twist, zero-resistance pockets were observed on doping away from half filling at temperatures below 3 K, which indicates a possible transition to a superconducting state. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.
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McGilly LJ, Kerelsky A, Finney NR, Shapovalov K, Shih EM, Ghiotto A, Zeng Y, Moore SL, Wu W, Bai Y, Watanabe K, Taniguchi T, Stengel M, Zhou L, Hone J, Zhu X, Basov DN, Dean C, Dreyer CE, Pasupathy AN. Visualization of moiré superlattices. NATURE NANOTECHNOLOGY 2020; 15:580-584. [PMID: 32572229 DOI: 10.1038/s41565-020-0708-3] [Citation(s) in RCA: 92] [Impact Index Per Article: 23.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2020] [Accepted: 05/05/2020] [Indexed: 05/27/2023]
Abstract
Moiré superlattices in van der Waals heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. Indeed, electrons in these structures have been recently found to exhibit a number of emergent properties that the individual layers themselves do not exhibit. This includes superconductivity1,2, magnetism3, topological edge states4,5, exciton trapping6 and correlated insulator phases7. However, the lack of a straightforward technique to characterize the local structure of moiré superlattices has thus far impeded progress in the field. In this work we describe a simple, room-temperature, ambient method to visualize real-space moiré superlattices with sub-5-nm spatial resolution in a variety of twisted van der Waals heterostructures including, but not limited to, conducting graphene, insulating boron nitride and semiconducting transition metal dichalcogenides. Our method uses piezoresponse force microscopy, an atomic force microscope modality that locally measures electromechanical surface deformation. We find that all moiré superlattices, regardless of whether the constituent layers have inversion symmetry, exhibit a mechanical response to out-of-plane electric fields. This response is closely tied to flexoelectricity wherein electric polarization and electromechanical response is induced through strain gradients present within moiré superlattices. Therefore, moiré superlattices of two-dimensional materials manifest themselves as an interlinked network of polarized domain walls in a non-polar background matrix.
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Fu S, Kang K, Shayan K, Yoshimura A, Dadras S, Wang X, Zhang L, Chen S, Liu N, Jindal A, Li X, Pasupathy AN, Vamivakas AN, Meunier V, Strauf S, Yang EH. Enabling room temperature ferromagnetism in monolayer MoS 2 via in situ iron-doping. Nat Commun 2020; 11:2034. [PMID: 32341412 PMCID: PMC7184740 DOI: 10.1038/s41467-020-15877-7] [Citation(s) in RCA: 47] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2019] [Accepted: 03/24/2020] [Indexed: 12/15/2022] Open
Abstract
Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.
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Cheung SC, Shin JY, Lau Y, Chen Z, Sun J, Zhang Y, Müller MA, Eremin IM, Wright JN, Pasupathy AN. Dictionary learning in Fourier-transform scanning tunneling spectroscopy. Nat Commun 2020; 11:1081. [PMID: 32102995 PMCID: PMC7044214 DOI: 10.1038/s41467-020-14633-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2017] [Accepted: 01/17/2020] [Indexed: 11/15/2022] Open
Abstract
Modern high-resolution microscopes are commonly used to study specimens that have dense and aperiodic spatial structure. Extracting meaningful information from images obtained from such microscopes remains a formidable challenge. Fourier analysis is commonly used to analyze the structure of such images. However, the Fourier transform fundamentally suffers from severe phase noise when applied to aperiodic images. Here, we report the development of an algorithm based on nonconvex optimization that directly uncovers the fundamental motifs present in a real-space image. Apart from being quantitatively superior to traditional Fourier analysis, we show that this algorithm also uncovers phase sensitive information about the underlying motif structure. We demonstrate its usefulness by studying scanning tunneling microscopy images of a Co-doped iron arsenide superconductor and prove that the application of the algorithm allows for the complete recovery of quasiparticle interference in this material. Aperiodic structure imaging suffers limitations when utilizing Fourier analysis. The authors report an algorithm that quantitatively overcomes these limitations based on nonconvex optimization, demonstrated by studying aperiodic structures via the phase sensitive interference in STM images.
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Edelberg D, Rhodes D, Kerelsky A, Kim B, Wang J, Zangiabadi A, Kim C, Abhinandan A, Ardelean J, Scully M, Scullion D, Embon L, Zu R, Santos EJG, Balicas L, Marianetti C, Barmak K, Zhu X, Hone J, Pasupathy AN. Approaching the Intrinsic Limit in Transition Metal Diselenides via Point Defect Control. NANO LETTERS 2019; 19:4371-4379. [PMID: 31180688 DOI: 10.1021/acs.nanolett.9b00985] [Citation(s) in RCA: 77] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods, chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above 1013/cm2 to below 1011/cm2. Because these point defects act as centers for nonradiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.
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Zhou X, Kerelsky A, Elahi MM, Wang D, Habib KMM, Sajjad RN, Agnihotri P, Lee JU, Ghosh AW, Ross FM, Pasupathy AN. Atomic-Scale Characterization of Graphene p-n Junctions for Electron-Optical Applications. ACS NANO 2019; 13:2558-2566. [PMID: 30689949 DOI: 10.1021/acsnano.8b09575] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Graphene p-n junctions offer a potentially powerful approach toward controlling electron trajectories via collimation and focusing in ballistic solid-state devices. The ability of p-n junctions to control electron trajectories depends crucially on the doping profile and roughness of the junction. Here, we use four-probe scanning tunneling microscopy and spectroscopy (STM/STS) to characterize two state-of-the-art graphene p-n junction geometries at the atomic scale, one with CMOS polySi gates and another with naturally cleaved graphite gates. Using spectroscopic imaging, we characterize the local doping profile across and along the p-n junctions. We find that realistic junctions exhibit non-ideality both in their geometry as well as in the doping profile across the junction. We show that the geometry of the junction can be improved by using the cleaved edge of van der Waals metals such as graphite to define the junction. We quantify the geometric roughness and doping profiles of junctions experimentally and use these parameters in non-equilibrium Green's function-based simulations of focusing and collimation in these realistic junctions. We find that for realizing Veselago focusing, it is crucial to minimize lateral interface roughness which only natural graphite gates achieve and to reduce junction width, in which both devices under investigation underperform. We also find that carrier collimation is currently limited by the non-linearity of the doping profile across the junction. Our work provides benchmarks of the current graphene p-n junction quality and provides guidance for future improvements.
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Telford EJ, Benyamini A, Rhodes D, Wang D, Jung Y, Zangiabadi A, Watanabe K, Taniguchi T, Jia S, Barmak K, Pasupathy AN, Dean CR, Hone J. Via Method for Lithography Free Contact and Preservation of 2D Materials. NANO LETTERS 2018; 18:1416-1420. [PMID: 29385346 DOI: 10.1021/acs.nanolett.7b05161] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Atomically thin 2D materials span the common components of electronic circuits as metals, semiconductors, and insulators, and can manifest correlated phases such as superconductivity, charge density waves, and magnetism. An ongoing challenge in the field is to incorporate these 2D materials into multilayer heterostructures with robust electrical contacts while preventing disorder and degradation. In particular, preserving and studying air-sensitive 2D materials has presented a significant challenge since they readily oxidize under atmospheric conditions. We report a new technique for contacting 2D materials, in which metal via contacts are integrated into flakes of insulating hexagonal boron nitride, and then placed onto the desired conducting 2D layer, avoiding direct lithographic patterning onto the 2D conductor. The metal contacts are planar with the bottom surface of the boron nitride and form robust contacts to multiple 2D materials. These structures protect air-sensitive 2D materials for months with no degradation in performance. This via contact technique will provide the capability to produce "atomic printed circuit boards" that can form the basis of more complex multilayer heterostructures.
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Rhodes D, Chenet DA, Janicek BE, Nyby C, Lin Y, Jin W, Edelberg D, Mannebach E, Finney N, Antony A, Schiros T, Klarr T, Mazzoni A, Chin M, Chiu YC, Zheng W, Zhang QR, Ernst F, Dadap JI, Tong X, Ma J, Lou R, Wang S, Qian T, Ding H, Osgood RM, Paley DW, Lindenberg AM, Huang PY, Pasupathy AN, Dubey M, Hone J, Balicas L. Engineering the Structural and Electronic Phases of MoTe 2 through W Substitution. NANO LETTERS 2017; 17:1616-1622. [PMID: 28145719 DOI: 10.1021/acs.nanolett.6b04814] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/17/2023]
Abstract
MoTe2 is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T'- or β-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ∼1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1-xWxTe2 solid solution, which displays a semiconducting to semimetallic transition as a function of x. We find that a small critical W concentration xc ∼ 8% stabilizes the γ-phase at room temperature. This suggests that crystals with x close to xc might be particularly susceptible to phase transformations induced by an external perturbation, for example, an electric field. Photoemission spectroscopy, indicates that the γ-phase possesses a Fermi surface akin to that of WTe2.
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Zhou X, Kang K, Xie S, Dadgar A, Monahan NR, Zhu XY, Park J, Pasupathy AN. Atomic-Scale Spectroscopy of Gated Monolayer MoS2. NANO LETTERS 2016; 16:3148-3154. [PMID: 27064662 DOI: 10.1021/acs.nanolett.6b00473] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have strong electronic signatures in STM imaging.
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Adak O, Rosenthal E, Meisner J, Andrade EF, Pasupathy AN, Nuckolls C, Hybertsen MS, Venkataraman L. Flicker Noise as a Probe of Electronic Interaction at Metal-Single Molecule Interfaces. NANO LETTERS 2015; 15:4143-9. [PMID: 25942441 DOI: 10.1021/acs.nanolett.5b01270] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
Charge transport properties of metal-molecule interfaces depend strongly on the character of molecule-electrode interactions. Although through-bond coupled systems have attracted the most attention, through-space coupling is important in molecular systems when, for example, through-bond coupling is suppressed due to quantum interference effects. To date, a probe that clearly distinguishes these two types of coupling has not yet been demonstrated. Here, we investigate the origin of flicker noise in single molecule junctions and demonstrate how the character of the molecule-electrode coupling influences the flicker noise behavior of single molecule junctions. Importantly, we find that flicker noise shows a power law dependence on conductance in all junctions studied with an exponent that can distinguish through-space and through-bond coupling. Our results provide a new and powerful tool for probing and understanding coupling at the metal-molecule interface.
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Zhao L, He R, Zabet-Khosousi A, Kim KS, Schiros T, Roth M, Kim P, Flynn GW, Pinczuk A, Pasupathy AN. Dopant segregation in polycrystalline monolayer graphene. NANO LETTERS 2015; 15:1428-1436. [PMID: 25625227 DOI: 10.1021/nl504875x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Heterogeneity in dopant concentration has long been important to the electronic properties in chemically doped materials. In this work, we experimentally demonstrate that during the chemical vapor deposition process, in contrast to three-dimensional polycrystals, the substitutional nitrogen atoms avoid crystal grain boundaries and edges over micron length scales while distributing uniformly in the interior of each grain. This phenomenon is universally observed independent of the details of the growth procedure such as temperature, pressure, substrate, and growth precursor.
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Arguello CJ, Rosenthal EP, Andrade EF, Jin W, Yeh PC, Zaki N, Jia S, Cava RJ, Fernandes RM, Millis AJ, Valla T, Osgood RM, Pasupathy AN. Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H-NbSe2. PHYSICAL REVIEW LETTERS 2015; 114:037001. [PMID: 25659014 DOI: 10.1103/physrevlett.114.037001] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/19/2014] [Indexed: 06/04/2023]
Abstract
We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe2, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.
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Okamoto JI, Arguello CJ, Rosenthal EP, Pasupathy AN, Millis AJ. Experimental evidence for a Bragg glass density wave phase in a transition-metal dichalcogenide. PHYSICAL REVIEW LETTERS 2015; 114:026802. [PMID: 25635556 DOI: 10.1103/physrevlett.114.026802] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/22/2014] [Indexed: 06/04/2023]
Abstract
Analysis of the spatial dependence of current-voltage characteristics obtained from scanning tunneling microscopy experiments indicates that the charge density wave (CDW) occurring in NbSe_{2} is subject to locally strong pinning by a non-negligible density of defects, but that on the length scales accessible in this experiment the material is in a "Bragg glass" phase where dislocations and antidislocations occur in bound pairs and free dislocations are not observed. An analysis based on a Landau theory is presented showing how a strong local modulation may produce only a weak long range effect on the CDW phase.
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Zabet-Khosousi A, Zhao L, Pálová L, Hybertsen MS, Reichman DR, Pasupathy AN, Flynn GW. Segregation of Sublattice Domains in Nitrogen-Doped Graphene. J Am Chem Soc 2014; 136:1391-7. [DOI: 10.1021/ja408463g] [Citation(s) in RCA: 78] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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Zhao L, Levendorf M, Goncher S, Schiros T, Pálová L, Zabet-Khosousi A, Rim KT, Gutiérrez C, Nordlund D, Jaye C, Hybertsen M, Reichman D, Flynn GW, Park J, Pasupathy AN. Local atomic and electronic structure of boron chemical doping in monolayer graphene. NANO LETTERS 2013; 13:4659-65. [PMID: 24032458 DOI: 10.1021/nl401781d] [Citation(s) in RCA: 78] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
We use scanning tunneling microscopy and X-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film.
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