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The electromagnetic enhancement effect in scanning tunneling microscope (STM) light emission from GaAs has been investigated by the finite difference time domain method. We have calculated the intensity of light emitted by the recombination of minority carriers injected from the tip and majority carriers in the sample. The results depend not only on the material and the shape of the tip but also on light polarization. When the tip is tungsten whose dielectric function has a positive real part at 1.5 eV, the p-polarized light intensity depends strongly on the size of the tip and the location of recombination. However, the s-polarized light emission depends only weakly on these parameters. If the tip is a perfect metal, the p-polarized light intensity becomes a few times stronger than that for the W tip of the same shape. On the other hand, the s-polarized light intensity becomes weaker than that for the W tip due to the electromagnetic screening effect of a perfect metal. We conclude that the combination of the W tip and s-polarized light detection is suitable for precise measurement of the tip-position dependence of STM light emission properties. A tip with a negative value of the real part of dielectric function is preferable to the W tip, if one does not analyze the tip-position dependence of STM light emission, because STM light emission is enhanced relative to the case of the W tip. [ABSTRACT FROM AUTHOR]

We have measured the light emission (LE) spectra of the ordered C60 monolayer-island on the Au(1 1 1) surface and the bare Au surface, induced by the scanning tunneling microscope (STM). The optical measurements were performed at 78 K using electrochemically etched W-tips at a constant tunneling current of 2.0 nA for a sample bias voltage with respect to the tip of 2.3 V. For the bare Au surface, we observed an asymmetric peak at ∼2.01 eV, which is the typical STM-LE spectrum for the Au sample–W tip system. For the C60 monolayer-island, we observed a broad peak centered at ∼1.7 eV, which consists of the red-shifted emission from the underlying Au substrate (∼1.93 eV) and a new emission peak around 1.65 eV. The energy range of the new emission is consistent with that of the photoluminescence of the isolated molecules and a 100 nm-thick film of C60. Thus we conclude that the new emission peak is the fluorescence from the C60 molecules in the monolayer-island. [Copyright &y& Elsevier]

The honeycomb lattice is a fundamental two-dimensional (2D) network that gives rise to surprisingly rich electronic properties. While its expansion to 2D supramolecular assembly is conceptually appealing, its realization is not straightforward because of weak intermolecular coupling and the strong influence of a supporting substrate. Here, we show that the application of a triptycene derivative with phenazine moieties, Trip-Phz, solves this problem due to its strong intermolecular π-π pancake bonding and nonplanar geometry. Our scanning tunneling microscopy (STM) measurements demonstrate that Trip-Phz molecules self-assemble on a Ag(111) surface to form chiral and commensurate honeycomb lattices. Electronically, the network can be viewed as a hybrid of honeycomb and kagome lattices. The Dirac and flat bands predicted by a simple tight-binding model are reproduced by total density functional theory (DFT) calculations, highlighting the protection of the molecular bands from the Ag(111) substrate. The present work offers a rational route for creating chiral 2D supramolecules that can simultaneously accommodate pristine Dirac and flat bands.

We demonstrate the novel growth of silicene grown on Ag(111) using STM and reveal the mechanism with KMC simulation. Our STM study shows that after the complete formation of the first layer of silicene, it is transformed into bulk Si with the reappearance of the bare Ag surface. This dewetting (DW) during the epitaxial growth is an exception in the conventional growth behavior. Our KMC simulation reproduces DW by taking into account the differences in the activation energies of Si atoms on Ag, silicene, and bulk Si. The growth modes change depending on the activation energy of the diffusion, temperature, and deposition rate, highlighting the importance of kinetics in growing metastable 2D materials.

Monolayer transition metal dichalcogenides offer an appropriate platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS 2 and PtTe 2 , with the tunneling current as a control parameter. The bandgap of monolayer MoS 2 decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe 2 , by contrast, exhibits a much stronger gap reduction, and a reversible semiconductor-to-metal transition occurs at a moderate tunneling current. This unusual switching behavior of monolayer PtTe 2 , not seen in bulk semimetallic PtTe 2 , can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations.

We have explored the chemical reaction of the photoisomerization and thermal reaction of the photochromic spiropyran (SP) 1',3'-Dihydro-1',3',3' trimethyl-6-nitrospiro[2H-1 benzopyran-2,2'-(2H)-indole] molecule deposited on the atomic thin channel of a MoS 2 field-effect transistor (FET) through the analysis of the FET property. With four monolayers of SP molecules on the channel, we observed a clear shift of the threshold voltage in the drain-current vs gate-voltage plot with UV-light injection on the molecule, which was due to the change of the SP molecule to merocyanine (MC). A complete reset from MC to SP molecule was achieved by thermal annealing, while the injection of green light could revert the FET property to the original condition. In the process of change from MC to SP, two types of decay rates were confirmed. The quick- and slow-decay components corresponded to the molecules attached directly to the substrate and those in the upper layer, respectively. The activation energies for the conversion of MC to SP molecules were estimated as 71 kJ/mol and 90 kJ/mol for the former and latter, respectively. Combined with DFT calculations, we concluded that the I d - V g shift with photoisomerization from SP to MC is due to the upper layer molecules and the dipole moment in the surface normal direction. Based on the estimated activation energy of 90 kJ/mol for the reset process, we calculated the conversion rate in a controllable temperature range. From these values, we consider that the chemical state of MC can be maintained and switched in a designated time period, which demonstrates the possibility of this system in logical operation applications.

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS 2 channel when the channel part is exposed to Cl 2 gas. The gas exposure and the electrical measurement of the MoS 2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS 2 FET can provide and understand the surface properties. With the Cl 2 gas exposure to the channel, the plot of the drain current versus the gate voltage ( I d - V g curve) shifts monotonically toward the positive direction of V g , suggesting that the adsorbate acts as an electron acceptor. The I d - V g shifts are numerically estimated by measuring the onset of I d (threshold voltage, V th ) and the mobility as a function of the dosing amounts of the Cl 2 gas. The behaviors of both the V th shift and the mobility with the Cl 2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS 2 surface up to the monolayer coverage. The charge transfer makes the V th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS 2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl 2 molecule. The monotonic shift of V th with the coverage suggests that the MoS 2 device sensor has a good sensitivity to detect 10 -3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)

Topological materials have become promising materials for next-generation devices by utilizing their exotic electronic states. Their exotic states caused by spin-orbital coupling usually locate on the surfaces or at the edges. Scanning tunneling spectroscopy (STS) is a powerful tool to reveal the local electronic structures of condensed matters. Therefore, STS provides us with an almost perfect method to access the exotic states of topological materials. In this topical review, we report the current investigations by several methods based on the STS technique for layered topological material from transition metal dichalcogenide Weyl semimetals (WTe 2 and MoTe 2 ) to two dimensional topological insulators (layered bismuth and silicene). The electronic characteristics of these layered topological materials are experimentally identified.

Magnetic molecules are of interest for application in spintronic and quantum-information processing devices. Therein, control of the interaction between the spins of neighboring molecules is the critical issue. Substitution of outer moieties of the molecule can tune the molecule-molecule interaction. Here we show a novel spin behavior for a magnetic molecule of vanadyl tetrakis (thiadiazole) porphyrazine (abbreviated as VOTTDPz) adsorbed on Au(111), which is modified from vanadyl phthalocyanine (VOPc) by replacing the inert phthalocyanine ligand with a reactive thiadiazole moiety. The magnetic properties of the molecules are examined by observing the Kondo resonance caused by the screening of an isolated spin by conduction electrons using scanning tunneling spectroscopy. The Kondo features are detected at the molecule whose shape and intensity show site-dependent variation, revealing complex spin-spin interactions due to the enhanced interaction between molecules, originating from the functionalization of the ligand with a more reactive moiety., (© 2020. The Author(s).)

The electronic structure of type-II Weyl semimetal molybdenum ditelluride (MoTe 2 ) is studied by using scanning tunneling microscopy and density functional theory calculations. Through measuring energy-dependent quasiparticle interference (QPI) patterns with a cryogenic scanning tunneling microscope, several characteristic features are found in the QPI patterns. Two of them arise from the Weyl semimetal nature; one is the topological Fermi arc surface state and the other can be assigned to be a Weyl point. The remaining structures are derived from the scatterings relevant to the bulk electronic states. The findings lead to further understanding of the topological electronic structure of type-II Weyl semimetal MoTe 2 .

Weyl semimetals (WSMs) are classified into two types, type I and II, according to the topology of the Weyl point, where the electron and hole pockets touch each other. Tungsten ditelluride (WTe 2 ) has garnered a great deal of attention as a strong candidate to be a type-II WSM. However, the Weyl points for WTe 2 are located above the Fermi level, which has prevented us from identifying the locations and the connection to the Fermi arc surface states by using angle-resolved photoemission spectroscopy. Here, we present experimental proof that WTe 2 is a type-II WSM. We measured energy-dependent quasiparticle interference patterns with a cryogenic scanning tunneling microscope, revealing the position of the Weyl point and its connection with the Fermi arc surface states, in agreement with prior theoretical predictions. Our results provide an answer to this crucial question and stimulate further exploration of the characteristics of WSMs.

We present the transport characteristics of individual silicene nanoribbons (SiNRs) grown on Ag(110). By lifting up a single SiNR with a low-temperature scanning tunneling microscope tip, a nanojunction consisting of tip, SiNR and Ag is fabricated. In the differential conductance spectra of the nanojunctions fabricated by this methodology, a peak appears at the Fermi level which is not observed in the spectra measured either for the SiNRs before being lifted up or the clean Ag substrate. We discuss the origin of the peak as it relates to the SiNR.

A novel type of action spectrum representing multiple overtone excitations of the v(M-C) mode was observed for lateral hopping of a CO molecule on Ag(110) induced by inelastically tunneled electrons from the tip of a scanning tunneling microscope. The yield of CO hopping shows sharp increases at 261±4  mV, corresponding to the C-O internal stretching mode, and at 61±2, 90±2, and 148±7  mV, even in the absence of corresponding fundamental vibrational modes. The mechanism of lateral CO hopping on Ag(110) was explained by the multistep excitation of overtone modes of v(M-C) based on the numerical fitting of the action spectra, the nonlinear dependence of the hopping rate on the tunneling current, and the hopping barrier obtained from thermal diffusion experiments.

We present a novel reaction mechanism for a single adsorbed molecule that proceeds via simultaneous excitation of two different vibrational modes excited by inelastic tunneling electrons from a scanning tunneling microscope. Specifically, we analyze the dissociation of a single dimethyl disulfide (DMDS, (CH3S)2) molecule on Cu(111) by using a versatile theoretical method, which permits us to simulate reaction rates as a function of sample bias voltage. The reaction is induced by the excitation of C-H stretch and S-S stretch modes by a two-electron process at low positive bias voltages. However, at increased voltages, the dissociation becomes a single-electron process that excites a combination mode of these stretches, where excitation of the C-H stretch is the energy source and excitation of the S-S stretch mode enhances the anharmonic coupling rate. A much smaller dissociation yield (few orders of magnitude) at negative bias voltages is understood in terms of the projected density of states of a single DMDS on Cu(111), which reflects resonant excitation through the molecular orbitals.

We demonstrate that silicene, a 2D honeycomb lattice consisting of Si atoms, loses its Dirac fermion characteristics due to substrate-induced symmetry breaking when synthesized on the Ag(111) surface. No Landau level sequences appear in the tunneling spectra under a magnetic field, and density functional theory calculations show that the band structure is drastically modified by the hybridization between the Si and Ag atoms. This is the first direct example demonstrating the lack of Dirac fermions in a single layer honeycomb lattice due to significant symmetry breaking.

The adsorption site and vibrational energies of CO on a clean Ag(001) surface were determined using scanning tunneling microscopy, inelastic electron tunneling spectroscopy with a scanning tunneling microscope, and high-resolution electron energy loss spectroscopy. The CO molecules were found to adsorb on the atop site of the Ag(001) surface, which was similar to their adsorption on the Cu(001) surface. The vibrational energy of the CO internal stretching mode was found to be 263 meV, which is only 3 meV less than that of CO in the gas phase. This result indicates that the CO molecules chemisorb very weakly on the Ag(001) surface.

By measuring the very low energy photoemission spectra of the CO/Cu(001) surface with a high resolution, we have found the energy loss components due to inelastic scattering of electrons near the Fermi level by the CO vibrational modes. The main energy loss structure appears as a step at 254 meV below the Fermi edge for 12C16O. An isotope shift of the step to 240 meV was observed when 13C18O was adsorbed. This observation confirms that this step arises from the energy loss of photoelectrons near the Fermi level through the excitation of the C-O stretching mode.

By measuring the photoelectron spectra of the Cu(001) and Cu(110) surfaces excited by tunable-laser photons of very low energy (4.50-4.95 eV), we have found that the photoelectron can lose energy through interaction with its image charge. This energy loss occurs just outside the solid surface and appears as a spike structure at the vacuum edge in the photoemission spectra. The requirement for observing this energy loss structure is the absence of unoccupied states at the vacuum level at the Gamma point to which zero kinetic energy electrons can return.