Local Tunneling Barrier Height Research Articles

Revealing buried hydrogen bonds

Surface Science One way to stabilize self-assembly monolayers of alkane thiols on gold surfaces is to introduce amide groups into the chain near the thiol groups to form hydrogen-bonding networks. This subsurface location has made direct structural studies challenging, but Thomas et al. have used scanning tunneling microscopy and local tunneling barrier height studies of 3-mercapto- N -nonylpropionamide on Au{111} (gold films on mica) at cryogenic temperatures to map the spatial orientation of the hydrogen bond. These networks can cross the boundaries between the domains that form with different rotational orientations. ACS Nano 10.1021/acsnano.6b01717 (2016).

Nanoscale surface element identification and dopant homogeneity in the high-Tcsuperconductor PrxCa1−xFe2As2

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in Pr${}_{x}$Ca${}_{1\ensuremath{-}x}$Fe${}_{2}$As${}_{2}$, the highest-${T}_{c}$ member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-${T}_{c}$ superconducting phase is unlikely to originate from Pr inhomogeneity.

Spatially resolved electronic inhomogeneities of graphene due to subsurface charges

We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO 2 substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long-range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.

Topographic and electronic contrast of the graphene moiré on Ir(111) probed by scanning tunneling microscopy and noncontact atomic force microscopy

Epitaxial graphene grown on transition metal surfaces typically exhibits a moir\'e pattern due to the lattice mismatch between graphene and the underlying metal surface. We use both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) experiments to probe the electronic and topographic contrast of the graphene moir\'e on the Ir(111) surface. While STM topography is influenced by the local density of states close to the Fermi energy and the local tunneling barrier height, AFM is capable of yielding the 'true' surface topography once the background force arising from the van der Waals (vdW) interaction between the tip and the substrate is taken into account. We observe a moir\'e corrugation of 35$\pm$10 pm, where the graphene-Ir(111) distance is the smallest in the areas where the graphene honeycomb is atop the underlying iridium atoms and larger on the fcc or hcp threefold hollow sites.

Local Tunneling Barrier Height at and around Subsurface Dopant Sites on p-GaAs(110)

The local tunneling barrier height at and around subsurface Zn acceptor atoms on GaAs(110) has been investigated by scanning tunneling microscopy (STM) and local barrier height (LBH) imaging. The measured LBH showed the bias dependence, which was in good agreement with the theoretical simulation that takes into account the effect of tip-induced band bending (TIBB). Above the acceptor sites, the bias dependence of LBH shifted to the positive-bias side as compared with that of the area distant from acceptors. We explained the observed shift by the additional local band bending that is induced by ionized acceptors.

Scanning tunneling microscopy contrast in lateral Ge-Si nanostructures onSi(111)−3×3-Bi

We investigate the origin of scanning tunneling microscope (STM) contrast in lateral Ge-Si nanostructures prepared on the $\text{Si}(111)\text{\ensuremath{-}}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-Bi}$ surface [M. Kawamura, N. Paul, V. Cherepanov, and B. Voigtl\"ander, Phys. Rev. Lett. 91, 096102 (2003)]. At low sample bias, the voltage-dependent apparent height difference between Si- and Ge-terminated areas in STM images corresponds exceptionally well to the difference in voltage-integrated scanning tunneling spectroscopy (STS) curves measured in Si- and Ge-terminated areas. The STS curves and the STM contrast reflect both differences in local density of states and in tip-induced effects in Si- and Ge-terminated areas. At higher bias voltage, the tunneling into unoccupied states on Ge-terminated areas is strongly influenced by lowering of the local height of the tunneling barrier with respect to Si. The lowering of the local tunneling barrier height vanishes for the occupied states and can be traced back to different tip-induced band bending on Si- and Ge-terminated areas.

Nanometer-scale distribution of field emission current from the arc-prepared carbon thin film

In order to clarify the mechanism of low-macroscopic-field (LMF) electron emission from carbon related materials, atomistic properties including local tunneling barrier height (LBH) and field emission (FE) current distributions of the arc-prepared carbon thin film have been measured by using scanning tunneling microscopy. From the LBH images, it is found that the carbon thin film surface consists of nanometer-scale graphite grains whose crystallinities and electronic properties are different although the local work function of the film is rather homogeneous. The obtained FE images show that the FE currents are varied more than one order of magnitude, depending on the grains, and that the FE current is higher at the edge of the individual grains. The authors cannot find any specific emission sites that give extremely higher emission currents, indicating that the specific emission sites such as high-aspect-ratio nanoprotrusions or atomistic defects are not responsible to the LMF electron emission from carbon related materials.

Local Tunneling Barrier Height Observations on Ni3Al(111)

Local tunneling barrier height (LBH) distribution, regarded as the local work function distribution, of Ni3Al(111) is observed and compared with that of NiAl(110). Although element-specific and atomically abrupt LBH distribution has been observed on NiAl(110), such a definite distribution is not observed on Ni3Al(111). This indicates that the abrupt LBH distribution is not due to the elemental combination of Ni and Al but due to the electronic structure of the surface.

水素終端Si(100)面上の探針により誘起されるバンドベンディングと局所トンネル障壁高さの理論解析

水素終端Si(100)面上の探針により誘起されるバンドベンディングと局所トンネル障壁高さの理論解析

局所トンネル障壁高さの理論解析

局所トンネル障壁高さの理論解析

Localized and Delocalized Features of Microscopic Work Functions

Highly localized and widely delocalized features of the microscopic work function have been observed on the basis of the local tunneling barrier height by means of scanning tunneling microscopy. The effect of work function reduction by Cs adsorption on Pt(111) extends over an area wider than tens of nanometers. On the other hand, the individual atoms on NiAl(110) show element-specific local work functions. These results indicate that the work function distribution can not be explained within the framework of electric dipoles induced by surface atoms, and that detailed features of wave functions should be taken into account.

Local tunneling barrier height observations of NiAl(1 1 0)

Local work functions at individual atoms on a compound metal surface have been experimentally examined. The simultaneously obtained STM and local tunneling barrier height (LBH) images of NiAl(1 1 0) show that LBH at geometrically lower Ni sites is much higher than that at geometrically higher Al sites, indicating that each individual atom on the NiAl(1 1 0) surface has a specific potential barrier for relevant electrons.

Local tunneling barrier height imaging of a reconstructed Pt(1 0 0) surface

The effect of surface reconstruction on work function of a quasi-hexagonal reconstructed Pt(100) surface has been studied by local tunneling barrier height (LBH) imaging with scanning tunneling microscopy (STM). Relatively low LBH is observed at the areas providing the strongly-corrugated geometric structure, where the bright contrast is observed in the STM image. This reduction in the local work function has been interpreted in terms of the dipoles induced by the Smoluchowski smoothing effect. This suggests the reason why the macroscopic work function of the reconstructed Pt(100) surface is lower than that of the Pt(111) surface, despite the similar hexagonal surface structures.

Local Tunneling Barrier Height Studies of Thermally Treated CO- and O-covered Pt(100) Surfaces

CO- and O-covered Pt(100) surfaces after thermal treatment have been studied by scanning tunneling microscopy (STM) and local tunneling barrier height (LBH) imaging. The coexistence of the reconstructed (hex) and reconstruction-deconstructed (1×1) domains with similar well-defined shapes is observed for both surfaces. However, the LBH images show that the local work function of the (1×1) domains is lower than that of the hex domains in the case of the CO-covered surface, but higher in the case of the O-covered surface. This feature is considered due to the electric dipole moment induced by O2 adsorption being higher than that induced by CO adsorption.

Local Tunneling Barrier Height Measurement on Au(111)

The systematic local tunneling barrier height (LBH) measurement on a Au(111) surface has been carried out in order to clarify the origin of the previously-reported peculiar behaviors; extremely small LBH values and its strong gap-distance dependence. In our study where carefully chosen tunneling tips are used, the LBH of Au(111) shows no such anomalies, indicating that these peculiar behaviors are artifacts due to tip-sample interaction. Furthermore, the bias and distance dependence of the LBH of Au(111) obtained here can be explained well by the simplified one-dimensional rectangular potential model for tunneling, which is similar to the case of Pt(111).

Microscopic Study on the Work Function Reduction Induced by Cs-Adsorption

The effects of Cs adsorption on Pt(111) and Si(111)-(7×7) surfaces have been investigated using the local tunneling barrier height (LBH) measurement with scanning tunneling microscope (STM). LBH has been evaluated from the tunneling current versus tip-sample gap distance (I-z) characteristic measurement. The histograms of LBHs obtained from the surfaces with different Cs coverages imply that the Cs adsorption effect on reduction in microscopic work function extends over a long distance beyond the classical picture. We conclude that the long-range modification in electronic structure of the substrate due to the presence of Cs atoms may be responsible for this long-range effect.

Moiré-like Distribution of Local Tunneling Barrier Height of the Monolayer Graphite Adsorbed on Pt(111) Surface

The Moiré-like distribution of the local tunneling barrier height (LBH) in the surface of monolayer graphite (MG) adsorbed on Pt(111) is discussed on the basis of atom-resolved LBH imaging. The Moiré-like LBH distribution is explained by the local work function distribution induced by the lateral variations of inter-layer interaction between C and Pt atoms, which results from the lattice mismatch between MG and Pt(111). Further, the tip-sample distance dependence of the LBH image is discussed. When the tip-sample separation is improperly small, we find that the apparent LBH is influenced by the surface deformation induced by the interaction between the tip and the sample, resulting in the reversal of the Moiré-like contrast. This is also explained in the framework of the inhomogeneous inter-layer interaction.

Local Tunneling Barrier Height Studies of the Initial Stage of Cs Adsorption on a Si(111) 7×7 Surface

The initial stage of Cs adsorption on a Si(111)7×7 surface is studied by means of scanning tunneling microscope (STM) and local tunneling barrier height (LBH) imaging which provides the microscopic work function distribution. Utilizing the combination of STM and LBH imaging, the Cs adsorption features can be classified into three types; clustering, adsorption on an adatom site, and adsorption on a rest atom site. For all these adsorption types, LBH is reduced locally just at the Cs adsorption sites. On the other hand, the increase in the local density of states at the adatom next to the adsorbed Cs is observed. This is considered as a long-ranged effect of Cs adsorption, which has been pointed out in theoretical works.

Scanning Tunneling Microscopy (STM)/Local Tunneling Barrier Height (LBH) Studies on Cs Adsorption on a Pt(111) Surface

The initial stage of Cs adsorption and the formation process of a Cs monolayer on a Pt(111) surface, as well as the variations of the local work function, are investigated by scanning tunneling microscopy (STM) and local tunneling barrier height (LBH) imaging. The STM/LBH images of the surface with a small amount of Cs adsorbate indicate increases in local density of states (LDOS) and LBH near the Cs adsorption sites, which are considered to provide a long-range effect of the Cs adsorption. At the Cs coverage of about 0.1, small (2×2) patches are seen throughout the surface, but they are so unstable that the Cs atoms in the patches diffuse to form stabilized patches at room temperature. With increasing Cs coverage, the area covered by the stabilized (2×2) patches increases. The stabilized (2×2) structure gives low LBH values and large atomic corrugations. The charge density in this structure is considered to be strongly localized due to the bond formation between Cs and Pt. This induces a strong surface dipole, resulting in a large work function reduction.

分子線を用いた最近の研究 局所トンネル障壁高さ計測

The local tunneling barrier height (LBH) measurements of two cases, monolayer graphite (MG) on Pt (111) and Cs/ Si (111) 7 × 7, are discussed. In the LBH measurements on MG/Pt (111), the distribution of microscopic work function was observed, which is due to inhomogeneous inter-layer interactions. From the LBH imaging of Cs/Si (111) 7 × 7 Cs adsorption structure can be classified into three types. The LBH reduction upon Cs adsorption is found only at the Cs adsorption sites. The increase in the density of states (DOS) around the Cs adsorption sites are also observed. This is rather long-ranged effect of Cs adsorption.