Local strain and tunneling current modulated excitonic luminescence in MoS2 monolayers

The excitonic luminescence of monolayer molybdenum disulfide (MoS2) on a gold substrate is studied by scanning tunneling microscopy (STM). STM-induced light emission (STM-LE) from MoS2 is assigned to the radiative decay of A and B excitons. The intensity ratio of A and B exciton emission can be modulated by the tunneling current, since the A exciton emission intensity saturates at high tunneling currents. Moreover, the corrugated gold substrate introduces local strain to the monolayer MoS2, resulting in significant changes of electronic bandgap and valence band splitting. The modulation rates of strain on A and B exciton energies are estimated as -72 meV/% and -57 meV/%, respectively. STM-LE provides a direct link between exciton energy and local strain in monolayer MoS2 with a spatial resolution <10 nm.

Leakage mechanism in AlxGa1-xN/GaN heterostructures with AlN interlayer

Leakage of AlxGa1-xN/GaN heterostructures was investigated by admittance–voltage profiling. Nominally undoped structures were grown by low-pressure metal-organic vapor-phase epitaxy (MOVPE). The investigated structures had an Al-content of 30 %. They are compared to structures with an additional 1 nm thick AlN interlayer placed before the Al0.3Ga0.7N layer growth, originally to improve device performance. Conductance of FET devices with AlN interlayer, measured from depletion of the two-dimensional electron gas (2DEG) to zero volt bias at frequencies ranging from 50 Hz to 10 kHz, could be described by free charge carriers using a Drude model. The voltage dependent conductance shows a behavior described by either Poole-Frenkel emission or Schottky emission. From the size of the conductance, as well as simulation of the tunneling current injected from the gate under off-state conditions by universal Schottky tunnelling, Schottky Emission is obvious. Evaluating the data by Schottky emission, we can locate the leakage path, of tens of nm in the range between gate and drain/source with contact to the 2DEG, originating from the AlN interlayer. The static dielectric constant in growth direction, necessary for the evaluation, is determined from various AlxGa1 xN/GaN heterostructures to ε||(0) = 10.7 +/- 0.1.

Analytical models for inter-layer tunneling in two-dimensional materials

Analytical formula of the transmission function of the inter-layer intra-band tunneling is derived for coupled narrow two-dimensional materials. Analytical models of the intra-band tunneling conductance G, the transmission function of the inter-layer band-to-band tunneling, and the maximum band-to-band tunneling current Imax, are also obtained. G and Imax are shown to exhibit different characteristics depending on the channel length.

Electronic Parameters of Diode Based Organometallic Semiconductor Dyes Centered Ruthenium Complexes with Active COOH Terminals

In this study, the ruthenium complexes, which is an organometallic N-3 and C-106 semiconductor material, was coated on indium tin oxide (ITO) by using the self-assembled technique and thus a diode containing an organometallic interface was produced. The effects of this interface on the electronic parameters of the diode were investigated. It is aimed to improve the heterogeneity problem of the inorganic/organic interface by chemically bonding these materials from COOH active parts to the ITO surface. In order to understand how the electronic parameters of the diode change with this modification, the Schottky diode electrical characterization approach has been used. The charge mobility of the diode was calculated using the current density-voltage curve (J–V) characteristic with Space Charge Limited Current (SCLC) technique. When the electrical field is applied to the diode, it can be said that the ruthenium complexes molecules create an electrical dipole and the tunneling current is transferred to the anode contact ITO through the ruthenium molecule through the charge carrier, thus contributing to the hole injection. The morphology of these interface modifications was examined by Atomic Force Microscope (AFM) and surface potential energy by KelvinProbe Force Microscope (KPFM). To investigate local conductivity of bare ITO and modified ITO surface, Scanning Spreading Resistance Microscopy (SSRM) that is a conductive AFM analyzing technique were performed by applying voltage to the conductive tip and to the sample. According to the results of this work the diode containing N-3 material shows the best performance in terms of charge injection to the ITO due to possess the lowest barrier height Φb as 0.43 eV.

Predicting Finite-Bias Tunneling Current Properties from Zero-Bias Features: The Frontier Orbital Bias Dependence at an Exemplar Case of DNA Nucleotides in a Nanogap

The electrical current properties of single-molecule sensing devices based on electronic (tunneling) transport strongly depend on molecule frontier orbital energy, spatial distribution, and position with respect to the electrodes. Here, we present an analysis of the bias dependence of molecule frontier orbital properties at an exemplar case of DNA nucleotides in the gap between H-terminated (3, 3) carbon nanotube (CNT) electrodes and its relation to transversal current rectification. The electronic transport properties of this simple single-molecule device, whose characteristic is the absence of covalent bonding between electrodes and a molecule between them, were obtained using density functional theory and non-equilibrium Green’s functions. As in our previous studies, we could observe two distinct bias dependences of frontier orbital energies: the so-called strong and the weak pinning regimes. We established a procedure, from zero-bias and empty-gap characteristics, to estimate finite-bias electronic tunneling transport properties, i.e., whether the molecular junction would operate in the weak or strong pinning regime. We also discuss the use of the zero-bias approximation to calculate electric current properties at finite bias. The results from this work could have an impact on the design of new single-molecule applications that use tunneling current or rectification applicable in high-sensitivity sensors, protein, or DNA sequencing.

Probing the electronic properties of the electrified silicon/water interface by combining simulations and experiments

Silicon (Si) is broadly used in electrochemical and photoelectrochemical devices, where the capacitive and Faradaic reactions at the Si/water interfaces are critical for signal transduction or noise generation. However, probing the electrified Si/water interface at the microscopic level remains a challenging task. Here we focus on hydrogenated Si surfaces in contact with water, relevant to transient electronics and photoelectrochemical modulation of biological cells and tissues. We show that by carrying out first-principles molecular dynamics simulations of the Si(100)/water interface in the presence of an electric field we can realistically correlate the computed flat-band potential and tunneling current images at the interface with experimentally measured capacitive and Faradaic currents. Specifically, we validate our simulations in the presence of bias by performing pulsed chronoamperometry measurements on Si wafers in solution. Consistent with prior experiments, our measurements and simulations indicate the presence of voltage-dependent capacitive currents at the interface. We also find that Faradaic currents are weakly dependent on the applied bias, which we relate to surface defects present in newly prepared samples.

Current—voltage characteristics of connecting tunnel diodes at temperature heating up to 80°C

Investigations of the temperature stability of the peak tunneling current density of connecting tunneling diodes, which are necessary for the creation on their basis of multijunction photoconverters of powerful optical radiation, have been carried out. The structures of n++-GaAs/i-GaAs/i-AlGaAs/p++-AlGaAs of connecting TD with an intermediate undoped layer thickness of 7.5 nm and a growth temperature of 500 °C (structure ”A”) and with a thickness of 10 nm and a temperature of 450 °C (structure ”B”) were investigated. When heated to 80 °C, an increase in the peak tunneling current density of the TD structure ”B” by 4% is observed. However, for structure ”A”, a decrease in the peak tunneling current density by 5% with heating is observed. The factors leading to the appearance of a negative or positive temperature coefficient of the peak tunneling current density are determined using mathematical modeling of tunneling diodes based on GaAs/AlGaAs materials. By reducing the epitaxial growth temperature of n++–GaAs/i-GaAs/i-AlGaAs/p++–AlGaAs tunnel diode structure to 450 °C and including an undoped i-layer 10 nm thick between the degenerate layers ensure the temperature stability of peak current density when heated to 80 °C.

ATOMIC STRUCTURE OF A SINGLE STEP AND DYNAMICS OF Sn ADATOMS ON THE Si(111)- v3 ? v3-Sn SURFACE

The atomic structure of single steps on the Si(111)- 3 3 ? -Sn surface and the dynamics of Sn adatoms in the vicinity of these steps were studied. The work was performed using scanning tunneling microscopy (STM) and ab initio calculations based on the density functional theory. The atomic structure model of the single steps consisting of Sn atomic chains along the steps was developed. This structure leads to the formation of potential double-wells near the steps acting as traps for Sn atoms and explains the fluctuating tunneling current recorded in these areas.