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Many-body States Description of Single-molecule Electroluminescence Driven by Scanning Tunneling Microscope
摘要： Electron transport and optical properties of a single molecule in contact with conductive materials have attracted considerable attention owing to their scientific importance and potential applications. With recent progresses of experimental techniques, especially by the virtue of scanning tunneling microscope (STM)-induced light emission, where the tunneling current of the STM is used as an atomic-scale source for induction of light emission from a single molecule, it becomes possible to investigate single-molecule properties at sub-nanometer spacial resolution. Despite extensive experimental studies, the microscopic mechanism of electronic excitation of a single molecule in STM-induced light emission is yet to be clarified. Here we present a formulation of single-molecule electroluminescence driven by electron transfer between a molecule and metal electrodes based on a many-body state representation of the molecule. The effects of intra-molecular Coulomb interaction on conductance and luminescence spectra are investigated using the nonequilibrium Hubbard Green's function technique combined with first-principles calculations. We compare simulation results with experimental data and find that the intra-molecular Coulomb interaction is crucial for reproducing recent experiments for a single phthalocyanine molecule. The developed theory provides a unified description of both electron-transport and optical properties of a single molecule in contact with metal electrodes driven out of equilibrium, and thereby it contributes to a microscopic understanding of optoelectronic conversion in single molecules on solid surfaces and in nanometer-scale junctions.
关键词： Single molecule luminescence,exciton formation,nonequilibrium Hubbard Green's function technique,time-dependent density functional theory (TDDFT),scanning tunneling microscope-induced light emission,Vibronic interaction
更新于2025-09-19 17:15:36
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Imaging doubled shot noise in a Josephson scanning tunneling microscope
摘要： We have imaged the current noise with atomic resolution in a Josephson scanning tunneling microscope with a Pb-Pb junction. By measuring the current noise as a function of applied bias, we reveal the change from single-electron tunneling above the superconducting gap energy to double-electron charge transfer below the gap energy when Andreev processes become dominant. Our spatially resolved noise maps show that this doubling occurs homogeneously on the surface, and also on impurity locations, demonstrating that indeed the charge pairing is not influenced by disruptions in the superconductor smaller than the superconducting coherence length.
关键词： Josephson scanning tunneling microscope,Andreev reflections,current noise,atomic resolution,superconducting gap
更新于2025-09-19 17:13:59
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Lowering the power of OLEDs
摘要： Lower power OLED displays could emerge from the latest work from an international collaboration between researchers at RIKEN, the University of California San Diego, the University of Tokyo, and the Institute for Molecular Science. Organic light emitting diodes (OLEDs) have been with us for many years now. They offer many advantages over other types of display based on inorganic LEDs and liquid crystal displays. They can be ?exible, thin, and preclude the need for a power-draining backlight. Writing in the journal Nature, the team explains how they have found a new way to manipulate the “excitons”-the electron-hole pairs that are key for charge transport within an OLED. The team points out that a current passing through an OLED device creates exciton pairs and when these drop down to a lower energy level, they emit visible light in a quantum process. OLED excitons arise in one of two patterns - the spins are either in the same direction or opposing. The former, so-called triplet excitons, are three times more common than the latter, the singlet excitons. Singlet excitons are formed only at higher energy and they can ultimately convert into triplets, but it would cut energy costs in a device if singlets were not formed at all. The team has now demonstrated that lowering the voltage so that only triplets are formed is possible where it was always assumed this would not work. They used electroluminescence measurements with a scanning tunneling microscope (STM) and an optical detection system to observe the formation of excitons. A model system with a single, isolated molecule of the organic semiconductor 3, 4, 9, 10-perylenetetracarboxylic-dianhydride (PTCDA) on metal-supported ultrathin insulating ?lm was key to their understanding. By imparting a negative charge to this supported molecule, they could use the STM tip to induce luminescence in the molecule, and monitored record the emission spectrum of the resulting excitons. Critically, at low voltage, only triplets form. Theoretical calculations support the suggested mechanism.
关键词： OLEDs,scanning tunneling microscope,PTCDA,singlet excitons,electroluminescence,triplet excitons,excitons
更新于2025-09-12 10:27:22
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Isotropic charge screening of anisotropic black phosphorus revealed by potassium adatoms
摘要： Black phosphorus has attracted great research interest due to its numerous applications in electronic devices, optoelectronic devices, energy storage, and so on. Compared with the majority of two-dimensional materials, black phosphorus possesses a unique property—its strong in-plane anisotropy. All the properties reported so far, including its effective mass, electron mobility, light absorption, thermal conductivity, and so on, have shown great anisotropy in the basal plane. This property renders black phosphorus applications not achievable with other two-dimensional materials. In this work, however, we discover a remarkable isotropic behavior in the strongly anisotropic black phosphorus—its electrostatic screening of point charges. We use the tip-induced band bending of a scanning tunneling microscope to map out the Coulomb ?eld of ionized potassium adatoms on black phosphorus and reveal its isotropic charge screening. This discovery is important for understanding electron scattering and transport in black phosphorus.
关键词： electron scattering,black phosphorus,isotropic charge screening,potassium adatoms,scanning tunneling microscope
更新于2025-09-11 14:15:04
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Scanning frequency comb microscopy—A new method in scanning probe microscopy
摘要： A method for superimposing a microwave frequency comb (MFC) on the DC tunneling current in a scanning tunneling microscope (STM) is described in which a mode-locked laser is focused on the tunneling junction. The MFC is caused by optical rectification of the regular sequence of laser pulses due to the nonlinear current-voltage characteristics of the tunneling junction. Hundreds of harmonics, at integer multiples of the laser pulse-repetition frequency, are generated with a metal tip and sample. However, the harmonics have less power with a resistive sample due to the loss in its spreading resistance. The microwave power is greatest at a tip-sample distance that is unique for each sample resistivity. This distance may be set by using different pairs of the applied DC bias and the set-point for the DC tunneling current. However, the laser, and not the applied DC bias or the DC tunneling current, is the source of energy for the MFC so they are not required. Feedback control of the tip-sample distance may be based on maximizing the attowatt-level microwave power of the harmonics, which have a signal-to-noise ratio of 20 dB. This method shows promise for nondestructive carrier profiling of semiconductors with true sub-nanometer resolution which is essential in the continued progress below the 40-nm technology node. It may enable carrier profiling with 2-D materials such as graphene, and it also shows promise for finer resolution in images of biological materials or other resistive samples.
关键词： optical rectification,microwave frequency comb,resistive sample,feedback control,scanning tunneling microscope
更新于2025-09-04 15:30:14