Local Tunneling Barrier Height Images Research Articles

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 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.

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.

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.

Microscopical analysis of structure and work function of Ba-covered Si(111)-(7×7) surface

Microscopic `work function' is evaluated by local tunneling barrier height (LBH) measurements of clean and Ba-deposited Si(111) surfaces in order to clarify the mechanism of work function reduction by Ba deposition. Atomically resolved LBH images are obtained corresponding to scanning tunneling microscopy (STM) images. The difference in the LBH measurement for clean Si(111)-(7×7) and Ba deposited Si(111)-(3×1) surfaces agrees with macroscopic work function reduction due to Ba adsorbate. The result suggests that the work function is strongly related to the atomic arrangement of the Si(111) surface induced by Ba deposition.

Local Tunneling Barrier Height Image of the Si(111)-(7×7) Surface

The local tunneling barrier height (LBH) image of Si(111)-(7×7) surface is compared with the corresponding scanning tunneling microscope (STM) image. At a negative sample bias voltage, while the faulted half adatoms in the 7×7 unit cell in the STM image are observed to be 0.24 Å higher than the unfaulted half adatoms, there exists no difference in the LBH image contrast between faulted and unfaulted halves. At a positive sample bias voltage, we observe no difference between the faulted and unfaulted halves both in the STM and LBH images. The results suggest that there exists an unexpected mechanism which determines the STM and LBH images.

Local Tunneling Barrier Height on Si(111) Reconstructed Surfaces

We observed local tunneling barrier height images on reconstructed Si(111) surfaces. Local tunneling barrier height on √3×√3 reconstructed structure was almost the same as that of 7×7 structure. In contrast, local tunneling barrier height on the c(4×2) reconstructed structure was lower than that on the 7×7 structure. Both reconstructed structures were induced by transition metal atom adsorption. This is influence of three-dimensional spatial distribution of surface wave function. We also found that local tunneling barrier height on the ring cluster structure was lower than that on 7×7 structure.

Local Tunneling Barrier Height Imaging on Si(111) Surface

We measured the spatial distribution of a local tunneling barrier height by means of the AC height modulation method. We performed this measurement on clean Si(111)7×7 surfaces and the surfaces exposed to O2 gas. On clean surfaces, 7×7 structures, which are the same as those in topographic images, were observed in the tunneling barrier height mode. It was found that the spatial distribution of local tunneling barrier height was modified along a step. It was also found that the topography and the spatial distribution of local tunneling barrier height were modified by exposure to O2 gas.

Hydrogenated amorphous silicon studied by scanning tunneling microscopy

We have used scanning tunneling microscopy under ultrahigh vacuum conditions in order to study the local electronic properties, as well as the topographical and chemical structure of hydrogenated amorphous silicon. Local characteristics were measured and images of the topography, as well as simultaneously recorded images of the local tunneling barrier height, were obtained with subnanometer resolution. The experimental results demonstrate that chemical inhomogeneities on a subnanometer scale, which may be caused by the presence of hydrogen in these samples, can be detected in the local tunneling barrier height images.

Local tunneling barrier height images obtained with the scanning tunneling microscope

Local tunneling barrier height images obtained with the scanning tunneling microscope

Local tunneling barrier height images obtained with the scanning tunneling microscope

We have routinely measured both topograhic and local tunneling barrier height data simultaneously with our UHV version of a scanning tunneling microscope. The experimental results obtained on surfaces of several single- and multi-component systems reveal new information about chemical inhomogeneities and crystallographic anisotropies with an unprecedented real space resolution in the nanometer range.