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Shibuta M, Nakajima A. Imaging of ultrafast photoexcited electron dynamics in pentacene nanocrystals on a graphite substrate. NANOSCALE 2024. [PMID: 38832543 DOI: 10.1039/d4nr00720d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2024]
Abstract
Understanding molecular film growth on substrates and the ultrafast electron dynamics at their interface is crucial for advancing next-generation organic electronics. We have focused on studying the ultrafast photoexcited electron dynamics in nanoscale organic crystals of an aromatic molecule, pentacene, on a two-dimensional material of graphite substrate. Through the use of time-resolved two-photon photoelectron emission microscopy (2P-PEEM), we have visualized the ultrafast lateral evolution of photoexcited electrons. By resonantly tuning the incident photon to excite pentacene molecules, polarization-dependent 2P-PEEM has revealed that pentacene nanocrystals (sub- to several μm) on the substrate exhibit a preferential orientation, in which a molecular π-orbital contacts the substrate in a "lying flat" orientation, facilitating electron transfer to the substrate. The time-resolved 2P-PEEM captures the motion of excited electrons in a femto- to pico-second timescale, clearly imaging the ultrafast charge transfer and lateral expansion two-dimensionally on the graphite substrate. Moreover, we found that the lying-flat molecular orientation of pentacene nanocrystals is transformable into a "standing-up" one through gentle heating up to 50 °C. These experimental insights using time-resolved 2P-PEEM will be highly valuable in enhancing the photofunctionalities of organic electronic devices by controlled molecular deposition.
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Affiliation(s)
- Masahiro Shibuta
- Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
| | - Atsushi Nakajima
- Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
- Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
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Nakayama M, Kajimoto K, Misaka T, Mishima N, Yamada T, Ohoyama H, Matsumoto T. Probing Energy-Level Alignment in Molecular Multilayers by Frequency-Modulation Electrostatic Force Microscopy under Tapping-Mode-Combined Fowler-Nordheim Tunneling Spectroscopy. ACS APPLIED MATERIALS & INTERFACES 2023; 15:47704-47714. [PMID: 37751421 PMCID: PMC10573325 DOI: 10.1021/acsami.3c08553] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/13/2023] [Accepted: 08/17/2023] [Indexed: 09/28/2023]
Abstract
The alignment of molecular electronic levels in a molecular multilayer is of crucial importance to realize desired functions for molecular devices. Amplitude-modulation-feedback frequency-modulation electrostatic force microscopy combined with Fowler-Nordheim tunneling spectroscopy is utilized as a probe for the energy-level alignment in an organic multilayer. Bias-dependent electrostatic force spectra were examined for bilayers including a Ru complex as a benchmark multilayer system. Electrostatic properties in the low-bias region were captured well by a single-capacitor model, which indicates weak coupling at the bilayer interface between the Ru complex and self-assembled monolayer. In contrast, in the high-bias region, significant disagreement with the expected electrostatic force was recognized for the bilayers and evaluated as the loss of electrostatic energy through the Fowler-Nordheim tunneling process. Alignment of the lowest unoccupied molecular orbital (LUMO) level of the Ru complex was determined by Fowler-Nordheim emission through resonant tunneling. These results indicate an effective method to probe level alignment at interfaces inside multilayers and to provide the partition factor β that depicts a divided electric field.
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Affiliation(s)
- Masahiro Nakayama
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Kentaro Kajimoto
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Tomoki Misaka
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Naoya Mishima
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Takashi Yamada
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Hiroshi Ohoyama
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
| | - Takuya Matsumoto
- Department of Chemistry,
Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
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Yamagiwa K, Shibuta M, Nakajima A. Two-photon photoelectron emission microscopy for surface plasmon polaritons at the Au(111) surface decorated with alkanethiolate self-assembled monolayers. Phys Chem Chem Phys 2017; 19:13455-13461. [PMID: 28530270 DOI: 10.1039/c7cp01693j] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
In this study, we have employed dual-color photoelectron emission microscopy (2P-PEEM) to visualize surface plasmon polaritons (SPPs) propagating along a chemically modified organic/metal interface of alkanethiolate self-assembled monolayers (Cn-SAMs; n is the number of alkyl carbon atoms) formed on Au(111). In dual-color 2P-PEEM, near-infrared photons around 900 nm generate SPPs at the Cn-SAMs/Au(111) interface, which interfere with the remaining light field. The resulting surface polarization beats are imaged as local distributions of 2P-photoelectrons probed by ultraviolet photons. Through dual-color 2P-PEEM for various alkyl chain lengths of Cn-SAMs, it is revealed that SPP properties are largely modified by an interfacial electronic state, particularly formed by the chemical interaction between surface Au atoms and adsorbate thiol molecules, thereby allowing the quantification of their group velocity at ∼0.86 times the speed of light. Since the SPP properties are controllable in terms of their height as organic dielectric layers, a bottom-up tailored technique using SAMs exhibits designer capability in adjusting the dielectric properties toward applications in surface plasmonic devices.
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Affiliation(s)
- Kana Yamagiwa
- Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
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Shibuta M, Hirata N, Eguchi T, Nakajima A. Photoexcited State Confinement in Two-Dimensional Crystalline Anthracene Monolayer at Room Temperature. ACS NANO 2017; 11:4307-4314. [PMID: 28399361 DOI: 10.1021/acsnano.7b01506] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Organic thin film electronics place a high demand on bottom-up technology to form a two-dimensionally (2D) functional unit consisting of a single molecular crystalline layer bound to a layered structure. As the strong interaction between a substrate and molecules makes it difficult to evaluate the electronic properties of organic films, the nature of electronic excited states has not been elucidated. Here, we study a 2D crystalline anthracene monolayer electronically decoupled by alkanethiolates on a gold substrate using scanning tunneling microscopy and time-resolved two-photon photoemission spectroscopy and unravel the geometric/electronic structures and excited electron dynamics. Our data reveal that dispersive 2D excited electrons on the surface can be highly coupled with an annihilation of nondispersive excitons that facilitate electron emission with vibronic interaction. Our results provide a fundamental framework for understanding photoexcited anthracene monolayer and show how the coupling between dispersive and nondispersive excited states may assist charge separation in crystalline molecular layers.
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Affiliation(s)
- Masahiro Shibuta
- Keio Institute of Pure and Applied Science (KiPAS), Keio University , 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
| | - Naoyuki Hirata
- Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST) , 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
- Department of Chemistry, Faculty of Science and Technology, Keio University , 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
| | - Toyoaki Eguchi
- Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST) , 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
- Department of Chemistry, Faculty of Science and Technology, Keio University , 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
| | - Atsushi Nakajima
- Keio Institute of Pure and Applied Science (KiPAS), Keio University , 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
- Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST) , 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
- Department of Chemistry, Faculty of Science and Technology, Keio University , 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
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Ueba T, Yamada T, Munakata T. Electronic excitation and relaxation dynamics of the LUMO-derived level in rubrene thin films on graphite. J Chem Phys 2016; 145:214703. [PMID: 28799400 DOI: 10.1063/1.4968847] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- Takahiro Ueba
- Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
| | - Takashi Yamada
- Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
| | - Toshiaki Munakata
- Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
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Shibuta M, Hirata N, Eguchi T, Nakajima A. Probing of an adsorbate-specific excited state on an organic insulating surface by two-photon photoemission spectroscopy. J Am Chem Soc 2014; 136:1825-31. [PMID: 24451024 DOI: 10.1021/ja4088456] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
In this study, we investigate the photoexcited electronic states of ferrocene (Fc) molecules adsorbed on an organic insulating surface by two-photon photoemission spectroscopy. This insulating layer, composed of a decanethiolate self-assembled monolayer formed on an Au(111) substrate, enables us to probe the electronically excited states localized at the adsorbed Fc molecules. The adsorbate-specific state is resonantly excited by photons at 4.57 eV, which is 0.5 eV smaller than the energy of the first molecular Rydberg state of free Fc in the gas phase. This result indicates that the electrons are bound to both the excited hole formed in the adsorbate and the positive image charge induced in the substrate. The hybridized electronic characteristics of the adsorbate-specific state are responsible for the strong transition selectivity and short lifetime of the excited state.
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Affiliation(s)
- Masahiro Shibuta
- ERATO, Nakajima Designer Nanocluster Assembly Project, Japan Science and Technology Agency, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
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Excited electron dynamics at ferrocene-terminated self-assembled monolayers on Au(111): Lengthened lifetime of image potential state. Chem Phys Lett 2013. [DOI: 10.1016/j.cplett.2013.01.053] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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Nakaya M, Shikishima M, Shibuta M, Hirata N, Eguchi T, Nakajima A. Molecular-scale and wide-energy-range tunneling spectroscopy on self-assembled monolayers of alkanethiol molecules. ACS NANO 2012; 6:8728-8734. [PMID: 22958159 DOI: 10.1021/nn302405r] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The electronic properties of alkanethiol self-assembled monolayers (alkanethiolate SAMs) associated with their molecular-scale geometry are investigated using scanning tunneling microscopy and spectroscopy (STM/STS). We have selectively formed the three types of alkanethiolate SAMs with standing-up, lying-down, and lattice-gas phases by precise thermal annealing of the SAMs which are conventionally prepared by depositing alkanethiol molecules onto Au(111) surface in solution. The empty and filled states of each SAM are evaluated over a wide energy range covering 6 eV above/below the Fermi level (E(F)) using two types of STS on the basis of tunneling current-voltage and distance-voltage measurements. Electronic states originating from rigid covalent bonds between the thiol group and substrate surface are observed near E(F) in the standing-up and lying-down phases but not in the lattice-gas phase. These states contribute to electrical conduction in the tunneling junction at a low bias voltage. At a higher energy, a highly conductive state stemming from the alkyl chain and an image potential state (IPS) formed in a vacuum gap appear in all phases. The IPS shifts toward a higher energy through the change in the geometry of the SAM from the standing-up phase to the lattice-gas phase through the lying-down phase. This is explained by the increasing work function of alkanethiolate/Au(111) with decreasing density of surface molecules.
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Affiliation(s)
- Masato Nakaya
- Nakajima Designer Nanocluster Assembly Project, ERATO, JST, KSP, 3-2-1 Sakado, Kawasaki 213-0012, Japan
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