Scanning Tunneling Microscopy/scanning Tunneling Spectroscopy Research Articles

Checkerboard ordered state in a superconducting FeSe/SrTiO3(001) monolayer

Ordered electronic states have been extensively explored in cuprates and iron-based unconventional superconductors, but seldom observed in the epitaxial $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ monolayer (FeSe/STO) with an enhanced superconducting transition temperature $({T}_{c})$. Here, by using scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), we reveal a checkerboard charge order in the epitaxial FeSe/STO monolayer, with a period of four times the inter-iron-atom distance along two perpendicular directions of the iron lattice. This ordered state is uniquely present in the superconducting FeSe/STO monolayer, even at liquid nitrogen temperature, but is absent in the nonsuperconducting FeSe monolayer or bilayer. Quasi-particle interference measurements further confirm it as a static order without an energy-dependent dispersion and is gapped out within the superconductivity gap. The intensity of the charge order shows an enhancement near the superconducting transition temperature, thus implying a correlation with the $\text{high-}{T}_{c}$ superconductivity in the FeSe/STO monolayer. This study provides a new basis for exploring the ordered electronic states and their interplay with $\text{high-}{T}_{c}$ superconductivity in the FeSe monolayer.

Creation and Annihilation of Charge Traps in Silicon Nanocrystals: Experimental Visualization and Spectroscopy

Recent studies have shown the presence of an amorphous surface layer in nominally crystalline silicon nanocrystals (SiNCs) produced by some of the most common synthetic techniques. The amorphous surface layer can serve as a source of deep charge traps, which can dramatically affect the electronic and photophysical properties of SiNCs. We present results of a scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) study of individual intragap states observed on the surfaces of hydrogen-passivated SiNCs deposited on the Au(111) surface. STS measurements show that intragap states can be formed reversibly when appropriate voltage-current pulses are applied to individual SiNCs. Analysis of STS spectra suggests that the observed intragap states are formed via self-trapping of charge carriers injected into SiNCs from the STM tip. Our results provide a direct visualization of the charge trap formation in individual SiNCs, a level of detail which until now had been achieved only in theoretical studies.

Scanning tunneling microscopy/spectroscopy study of atomic and electronic structures of In2O on InAs and In0.53Ga0.47As(001)-(4×2) surfaces

Interfacial bonding geometry and electronic structures of In(2)O on InAs and In(0.53)Ga(0.47)As(001)-(4×2) have been investigated by scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS). STM images show that the In(2)O forms an ordered monolayer on both InAs and InGaAs surfaces. In(2)O deposition on the InAs(001)-(4×2) surface does not displace any surface atoms during both room temperature deposition and postdeposition annealing. Oxygen atoms from In(2)O molecules bond with trough In/Ga atoms on the surface to form a new layer of O-In/Ga bonds, which restore many of the strained trough In/Ga atoms into more bulklike tetrahedral sp(3) bonding environments. STS reveals that for both p-type and n-type clean In(0.53)Ga(0.47)As(001)-(4×2) surfaces, the Fermi level resides near the valence band maximum (VBM); however, after In(2)O deposition and postdeposition annealings, the Fermi level position is close to the VBM for p-type samples and close to the conduction band minimum for n-type samples. This result indicates that In(2)O bonding eliminates surface states within the bandgap and forms an unpinned interface when bonding with In(0.53)Ga(0.47)As/InP(001)-(4×2). Density function theory is used to confirm the experimental finding.

Identifying atomic geometry and electronic structure of (2×3)-Sr/Si(100) surface and its initial oxidation

We present a joint experimental and theoretical study on the geometric and electronic states and the initial oxidation of the (2x3)-Sr/Si(100) surface. With scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) measurements combined with ab initio calculations, the atomic geometry and the electronic states of the (2x3)-Sr/Si(100) surface are identified. The dimerization of the Si atoms in the single atom row based on a (1x3) Si substrate model plays a critical role in stabilization of the surface structure and in determining the electronic properties. At the very initial oxidation of the surface, four features corresponding to the primary adsorption and oxidation sites are determined. Three of them are corresponding to the most favored oxidation sites with single oxygen molecules, whose local density of states gives semiconducting behavior. One is corresponding to the oxidation site with two oxygen molecules, whose local density of states gives metallic behavior. These features all exhibit dark spots with different shapes in the occupied state images but display either dark spots or bright protrusions depending on the different oxidation sites in the empty state images. Compared with the theoretical calculations, the plausible adsorption and oxidation models are proposed.

Evaluation of Device Configurations through Cross-Sectional Planes along Gates of 0.1 µm Metal–Oxide–Semiconductor Field-Effect Transistors by Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy

Scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) has been used to evaluate device configurations along the gate through longitudinal cross sections of 0.1 µm metal–oxide–semiconductor field-effect transistors (MOSFETs). It is found that the STM images taken from the longitudinal cross sections of MOSFETs are classified into four groups according to the device configuration expected on the longitudinal cross sections, i.e., cross-sectional planes prepared by cutting off the samples at the source/drain, the channel, and the two spacers surrounding both sides of the gate. Among them, STM/STS observation of the longitudinal cross sections of MOSFETs cut off at a spacer visualizes actual structures of the sides of the gate and the spacer, in particular, fluctuations in device structure along the gate, at a resolution suitable for nanoscale devices.

STM/STS Study of Electron Density of States at the Bases Sites in the DNA Alternating Copolymers

Abstract Local electron density of state spectra at the bases sites in the two types (A–T pairs only and G–C pairs only) of double-stranded DNA molecules is measured with the use of STM/STS (scanning tunneling microscopy/scanning tunneling spectroscopy) technique. The difference of electronic structure between A–T pairs and G–C pairs is discussed in relation to the charge-transfer mechanisms in these molecules.

Visualization of the depleted layer in nanoscaled pn junctions on Si(001) surfaces with the use of scanning tunneling microscopy

We demonstrate that the microscopic variation of the bulk electronic properties of nanoscaled pn junctions formed on Si(001) surface is clearly visualized with the use of scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS). The STM measurement reveals that it is possible to distinguish among the n-type the p-type and the depleted p-type regions. It is shown that the contrasts between the regions strongly depend on the bias voltage for the STM measurement. Especially, for the negative sample bias voltage, it is observed that the width of the depletion layer gets wider with decreasing the sample bias voltage. We will discuss the origin of the bias voltage dependence of the STM image in terms of a metal–insulator–semiconductor structure model. It is also shown that the current imaging tunneling spectroscopy image can also visualize three regions of the nanoscaled pn junctions.

Atomic force microscopy and scanning tunneling microscopy/spectroscopy investigations of molybdenum ditellurides

The van der Waals surfaces of the layered material α-MoTe2 and the high temperature phase β-MoTe2 were investigated by means of atomic force microscopy (AFM) and scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS). On both samples atomically resolved images of the surface structures were obtained. The combined STS/STM measurements of β-MoTe2 provide unambiguous evidence that the contribution from the subsurface layer dominates the tunneling current. Furthermore, from the combined AFM and STM data we arrive at a model for a new structure modification of this surface. This modification represents an interesting kinetically controlled intermediate state of the transition from β- to α-MoTe2.