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Rothhardt D, Kimouche A, Klamroth T, Hoffmann-Vogel R. Local work function on graphene nanoribbons. BEILSTEIN JOURNAL OF NANOTECHNOLOGY 2024; 15:1125-1131. [PMID: 39224533 PMCID: PMC11368052 DOI: 10.3762/bjnano.15.91] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/09/2024] [Accepted: 07/30/2024] [Indexed: 09/04/2024]
Abstract
Graphene nanoribbons show exciting electronic properties related to the exotic nature of the charge carriers and to local confinement as well as atomic-scale structural details. The local work function provides evidence for such structural, electronic, and chemical variations at surfaces. Kelvin prove force microscopy can be used to measure the local contact potential difference (LCPD) between a probe tip and a surface, related to the work function. Here we use this technique to map the LCPD of graphene nanoribbons grown on a Au(111) substrate. The LCPD data shows charge transfer between the graphene nanoribbons and the gold substrate. Our results are corroborated with density functional theory calculations, which verify that the maps reflect the doping of the nanoribbons. Our results help to understand the relation between atomic structure and electronic properties both in high-resolution images and in the distance dependence of the LCPD.
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Affiliation(s)
- Daniel Rothhardt
- Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm, Germany
- current address: Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland
- current address: Department of Physics, University of Basel, CH-4056 Basel, Switzerland
| | - Amina Kimouche
- Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm, Germany
| | - Tillmann Klamroth
- Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm, Germany
| | - Regina Hoffmann-Vogel
- Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm, Germany
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2
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Schumacher Z, Rejali R, Cowie M, Spielhofer A, Miyahara Y, Grutter P. Charge Carrier Inversion in a Doped Thin Film Organic Semiconductor Island. ACS NANO 2021; 15:10377-10383. [PMID: 34048210 DOI: 10.1021/acsnano.1c02600] [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
Inducing an inversion layer in organic semiconductors is a highly nontrivial, but critical, achievement for producing organic field-effect transistor (OFET) devices, which rely on the generation of inversion, accumulation, and depletion regimes for successful operation. Here, we develop a pulsed bias technique to characterize the dopant type of any organic material system, without prior knowledge or characterization of the material in question. We use this technique on a pentacene/PTCDI heterostructure and thus deduce that pentacene is exhibiting n-doped like response. The source of the additional charges in the pentacene island can be identified by charging rings in the dissipation channel of the noncontact atomic force microscopy (AFM) signal, a typical signature for localized charge transfer from the AFM tip to the sample. Additionally, through tip-induced band-bending, we generate inversion, depletion, and accumulation regimes over a 20 nm radius, three monolayer thick n-doped pentacene island. Our findings demonstrate that nanometer-scale lateral extent and thickness are sufficient for an OFET device to operate in the inversion regime.
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Affiliation(s)
- Zeno Schumacher
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
| | - Rasa Rejali
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
| | - Megan Cowie
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
| | - Andreas Spielhofer
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
| | - Yoichi Miyahara
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
| | - Peter Grutter
- Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada
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Wagner P, Foster A, Yi I, Abe M, Sugimoto Y, Hoffmann-Vogel R. Role of tip apices in scanning force spectroscopy on alkali halides at room temperature-chemical nature of the tip apex and atomic-scale deformations. NANOTECHNOLOGY 2021; 32:035706. [PMID: 33052141 DOI: 10.1088/1361-6528/abbea8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We have revealed processes of the tip apex distortion in the measurements of non-contact scanning force microscopy. High-spatial-resolution two-dimensional force mapping on KCl(100) surfaces for a large number of tips, seven tips, enabled us to see the complex behavior of the tip apex distortion. The tips are from Si without additional coating, but are altered by the tip-sample interaction and show the behavior of different atomic species. On the KCl(001) surfaces, the tip apex, consisting of K and Cl atoms or of Si, distorted several times while changing the distance even in a weak attractive region. There are variations in rigidity of the tip apex, but all tips distorted in the small attractive region. This complex behavior was categorized in patterns by our analyses. We compare the experimental force-distance data to atomistic simulations using rigid KCl-terminated tips and KCl-terminated tips with an additional KCl-pair designed to perform atomic jumps. We also compare the experimental force-distance data to first principles simulations using Si tips. We mainly find K-terminated tips and Si-terminated tips. We find that Si tips show only one force minimum whereas KCl-terminated tips show two force minima in line with the stronger rigidity of Si compared to KCl. At room temperature, the tip apex atoms can perform atomic jumps that change the atomic configuration of the tip apex.
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Affiliation(s)
- Philipp Wagner
- Physikalisches Institut, Karlsruher Institut für Technologie, D-76128 Karlsruhe, Germany
| | - Adam Foster
- Department of Applied Physics, Aalto University School of Science, PO Box 11100, FI-00076 Aalto, Finland
- WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
- Graduate School Materials Science in Mainz, Staudinger Weg 9, D-55128, Germany
| | - Insook Yi
- Graduate School of Engineering, Osaka University, Japan
| | - Masayuki Abe
- Graduate School of Engineering, Osaka University, Japan
| | | | - Regina Hoffmann-Vogel
- Department of Physics, University of Konstanz, Universitätsstrasse 10, D-78464 Konstanz, Germany
- Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht Strasse 24-25, D-14476 Potsdam-Golm, Germany
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Günder D, Watanabe K, Taniguchi T, Witte G. Van der Waals Bound Organic/2D Insulator Hybrid Structures: Epitaxial Growth of Acene Films on hBN(001) and the Influence of Surface Defects. ACS APPLIED MATERIALS & INTERFACES 2020; 12:38757-38767. [PMID: 32846485 DOI: 10.1021/acsami.0c09527] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Combining 2D materials with functional molecular films enables the fabrication of van der Waals bound organic/inorganic hybrids that are of interest for future device architectures. Recently, the 2D dielectric hexagonal boron nitride (hBN) has received particular attention since exfoliation allows the preparation of crystalline layers which have been utilized as ultrathin dielectrics in electronic devices. Here, we have studied the formation and structure of molecular films of the prototypical organic semiconductors pentacene (PEN) and perfluoropentacene (PFP) on hBN. Special attention was paid to the influence of substrate surface defects on the film formation by comparing molecular films that were grown on hBN substrates of various quality, including single crystals (representing the most ideal surface), briefly ion bombarded substrates, and exfoliated flakes. While X-ray diffraction (XRD) yields precise information about the crystalline structure of films grown on (large) single crystals, it is hardly applicable to analyze the films formed on exfoliated flakes because of their small size. Here, we demonstrate that in the case of flakes detailed structural analyses of the molecular films are possible by combining atomic force microscopy (AFM) with microspot UV/vis spectroscopy and optical polarization microscopy. On well-ordered hBN single crystal surfaces both acenes form very smooth and epitaxial crystalline films where molecules adopt a recumbent orientation (even in 100 nm thick films). By contrast, both materials adopt an upright molecular orientation and different polymorphs on defective hBN surfaces and reveal distinctly different film morphologies. On exfoliated flakes, PFP shows a film structure similar to that on the hBN single crystals, while PEN films exhibit a structure as on defective hBN substrates. In addition, a pronounced decoration of defect steps, which are probably created by the exfoliation process, was observed for PEN leading to the formation of tall and extended fibers where molecules adopt a recumbent orientation. The present study reveals different robustness in film growth on exfoliated hBN flakes for various molecules, which has to be considered in their device integration, especially with regard to their optoelectronic properties such as light absorption or charge transport, which depend critically on the molecular orientation and crystalline order.
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Affiliation(s)
- Darius Günder
- Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg, Germany
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan
| | - Gregor Witte
- Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg, Germany
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Hoffmann-Vogel R. Imaging prototypical aromatic molecules on insulating surfaces: a review. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2018; 81:016501. [PMID: 28958993 DOI: 10.1088/1361-6633/aa8fda] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF2 are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, [Formula: see text] and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. [Formula: see text] nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of [Formula: see text] islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules 'climb' to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.
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Affiliation(s)
- R Hoffmann-Vogel
- Physikalisches Institut, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany. Institut für Angewandte Physik, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany
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Späth T, Popp M, Pérez León C, Marz M, Hoffmann-Vogel R. Near-equilibrium measurement of quantum size effects using Kelvin probe force microscopy. NANOSCALE 2017; 9:7868-7874. [PMID: 28555693 DOI: 10.1039/c7nr01874f] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
In nano-structures such as thin films electron confinement results in the quantization of energy levels in the direction perpendicular to the film. The discretization of the energy levels leads to the oscillatory dependence of many properties on the film thickness due to quantum size effects. Pb on Si(111) is a specially interesting system because a particular relationship between the Pb atomic layer thickness and its Fermi wavelength leads to a periodicity of the oscillation of two atomic layers. Here, we demonstrate how the combination of scanning force microscopy (SFM) and Kelvin probe force microscopy (KPFM) provides a reliable method to monitor the quantum oscillations in the work function of Pb ultra-thin film nano-structures on Si(111). Unlike other techniques, with SFM/KPFM we directly address single Pb islands, determine their height while suppressing the influence of electrostatic forces, and, in addition, simultaneously evaluate their local work function by measurements close to equilibrium, without current-dependent and non-equilibrium effects. Our results evidence even-odd oscillations in the work function as a function of the film thickness that decay linearly with the film thickness, proving that this method provides direct and precise information on the quantum states.
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Affiliation(s)
- Thomas Späth
- Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Str. 1, 76128 Karlsruhe, Germany.
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Yu L, Niazi MR, Ngongang Ndjawa GO, Li R, Kirmani AR, Munir R, Balawi AH, Laquai F, Amassian A. Programmable and coherent crystallization of semiconductors. SCIENCE ADVANCES 2017; 3:e1602462. [PMID: 28275737 PMCID: PMC5336352 DOI: 10.1126/sciadv.1602462] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/07/2016] [Accepted: 01/26/2017] [Indexed: 05/28/2023]
Abstract
The functional properties and technological utility of polycrystalline materials are largely determined by the structure, geometry, and spatial distribution of their multitude of crystals. However, crystallization is seeded through stochastic and incoherent nucleation events, limiting the ability to control or pattern the microstructure, texture, and functional properties of polycrystalline materials. We present a universal approach that can program the microstructure of materials through the coherent seeding of otherwise stochastic homogeneous nucleation events. The method relies on creating topographic variations to seed nucleation and growth at designated locations while delaying nucleation elsewhere. Each seed can thus produce a coherent growth front of crystallization with a geometry designated by the shape and arrangement of seeds. Periodic and aperiodic crystalline arrays of functional materials, such as semiconductors, can thus be created on demand and with unprecedented sophistication and ease by patterning the location and shape of the seeds. This approach is used to demonstrate printed arrays of organic thin-film transistors with remarkable performance and reproducibility owing to their demonstrated spatial control over the microstructure of organic and inorganic polycrystalline semiconductors.
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Affiliation(s)
- Liyang Yu
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Muhammad R. Niazi
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Guy O. Ngongang Ndjawa
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Ruipeng Li
- Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14850, USA
| | - Ahmad R. Kirmani
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Rahim Munir
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Ahmed H. Balawi
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Frédéric Laquai
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
| | - Aram Amassian
- King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), and Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia
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Park SH, Kwon S. Dynamics of Molecular Orientation Observed Using Angle Resolved Photoemission Spectroscopy during Deposition of Pentacene on Graphite. Anal Chem 2016; 88:4565-70. [PMID: 26999332 DOI: 10.1021/acs.analchem.6b00986] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A real-time method to observe both the structural and the electronic configuration of an organic molecule during deposition is reported for the model system of pentacene on graphite. Structural phase transition of the thin films as a function of coverage is monitored by using in situ angle resolved photoemission spectroscopy (ARPES) results to observe the change of the electronic configuration at the same time. A photoemission theory that uses independent atomic center approximations is introduced to identify the molecular orientation from the ARPES technique. This study provides a practical insight into interpreting ARPES data regarding dynamic changes of molecular orientation during initial growth of molecules on a well-defined surface.
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Affiliation(s)
- Sang Han Park
- Beamline Division Group of PAL-XFEL Project Headquarters, Pohang University of Science and Technology , 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, Korea 790-784
| | - Soonnam Kwon
- Beamline Division Group of PAL-XFEL Project Headquarters, Pohang University of Science and Technology , 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, Korea 790-784
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Casalegno M, Moret M, Resel R, Raos G. Surface Reconstructions in Organic Crystals: Simulations of the Effect of Temperature and Defectivity on Bulk and (001) Surfaces of 2,2':6',2″-Ternaphthalene. CRYSTAL GROWTH & DESIGN 2016; 16:412-422. [PMID: 26834509 PMCID: PMC4720359 DOI: 10.1021/acs.cgd.5b01405] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 10/01/2015] [Revised: 11/17/2015] [Indexed: 06/05/2023]
Abstract
2,2':6',2″-Ternaphthalene (NNN) is a novel, blue-emitting material, suitable for preparation of organic light-emitting diodes. Its crystal structure has been solved recently, but its thermal behavior and surface properties have not yet been explored, partly due to the difficulty in obtaining high quality crystals. In the present study we use classical molecular dynamics to investigate the thermal behavior of bulk and (001) surfaces of NNN. Our bulk simulations indicate the occurrence of a phase transition at about 600 K. The transition is facilitated by the presence of a free (001) surface, since a reconstruction leading to a very similar structure occurs around 550 K in our surface models. This holds for both ideal and defective surface models, containing a small number of vacancies (one or two missing molecules in the outermost layer). In all cases, the process is triggered by thermal motion and involves the reorientation of the molecules with respect to the (001) plane. Both the bulk and surface phases share the monoclinic space group P21/a with a herringbone disposition of molecules. These findings and their implications for the use of NNN in organic electronics are discussed.
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Affiliation(s)
- Mosè Casalegno
- Dipartimento di Chimica, Materiali e Ingegneria Chimica "G. Natta", Politecnico di Milano , Via L. Mancinelli 7, 20131 Milano, Italy
| | - Massimo Moret
- Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca , Via R. Cozzi 55, 20125 Milano, Italy
| | - Roland Resel
- Institut für Festkörperphysik, Technische Universität Graz , 60101 Graz, Austria
| | - Guido Raos
- Dipartimento di Chimica, Materiali e Ingegneria Chimica "G. Natta", Politecnico di Milano , Via L. Mancinelli 7, 20131 Milano, Italy
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