摘要:

We have performedabinitioquantum mechanical calculations to describe scanning tunneling microscopy (STM) images of MoS2and MoTe2. These results indicate that the interpretation of the STM images of these and related materials depends sensitively on experimental conditions. For example, determining whether the maximum tunneling current correlates to the top atom (S or Te) or to the second‐layer atom (Mo) requires information on the tip‐sample separation. Based on these results we discuss some STM experimental procedures which would allow assignment of the chemical identity of STM spots with greater certainty.

ISSN:0734-211X    

DOI:10.1116/1.585591

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Tunneling current and conductance in scanning tunneling microscopy/spectroscopy (STM/STS) are calculated for various kind of tips using first‐principle local density approximation (LDA) method with the tunneling‐Hamiltonian formalism. As models of tip, tungsten cluster, platinum cluster, carbon adsorbed tungsten cluster, and titanium carbide cluster are used. Graphite surface is used for the simulation. STM image and qualitative feature of STS spectrum mainly depend on the arrangement of atoms at the tip apex and not so much affected by the atomic compositions of the tip because the most of tunneling current comes from a single or a few atom at the tip apex.

ISSN:0734-211X    

DOI:10.1116/1.585592

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We present a first‐principles study of elastic surface deformations, limits of atomic resolution, and of atomic‐scale friction in atomic force microscopy (AFM). In the case of a Pd AFM tip interacting with a graphite surface, we find that atomic resolution can be achieved in a narrow load range near ≊10−8N (per Pd tip atom). For these loads, we determine the microscopic friction coefficient to be μ≊10−2.

ISSN:0734-211X    

DOI:10.1116/1.585550

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The tunneling current using Bardeen formalism is calculated in a scanning tunneling microscope (STM). We take into account that the process is nonspecular tunneling, so the no conservation of the transverse wave number is considered. The tip is a three‐dimensional box with free electrons inside and with the transversal dimensions in the atomic range. Note that with this description the allowed states inside the tip are quantified and that it is necessary for the electrons to have at least a minimum value for the energy in the parallel direction. The wave functions inside the sample are described as extended plane waves. Using a perturbative potential, the wave functions of the electrons inside and outside the tip are calculated. The matrix element to apply Bardeen formalism is calculated using the second Green’s identity which makes unnecessary the tip’s wave function outside it. Thus, instead of doing the integration on a tunnel surface, we have done the integration over the tip volume. Finally a discussion of the lateral resolution of the STM in terms of the transverse wave number is made.

ISSN:0734-211X    

DOI:10.1116/1.585551

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We present an analytical method for treating the tunneling current between a probe tip and a sample in scanning tunneling microscopy (STM). The choice of a highly idealized tip permits a direct calculation of the tunnel current without resorting to perturbation theory (Bardeen approximation). In our model, the tip is a semi‐infinite chain of spherical potential wells oriented orthogonal to the sample surface. The sample surface, however, is arbitrary. The wavefunction for the entire system is obtained by a matching procedure, from which the total current is determined. For small bias voltage the result is comparable in simplicity with that of Tersoff and Hamann [Phys. Rev. B31, 805 (1985)]. We find that the tunnel conductance is proportional to the local density of states (LDOS) of the surface, but renormalized to include multiple reflections to all orders: σ∝ρL(r0,EF)/D, whereDdepends on both the tip and sample electronic structure, and the tip positionr0. As the tip‐surface separation increasesDapproaches unity, and we recover the result of TH. We find thatDcan be smaller or greater than unity, leading to a relative reduction or enhancement of the current. This effect which can be significant, both in the interpretation of STM images and in related spectroscopies, depends on the particular surface electronic structure.

ISSN:0734-211X    

DOI:10.1116/1.585552

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Mechanism of the negative differential resistance (NDR) observed in the scanning tunneling spectroscopy of the Si(111)√3×√3‐B surface is discussed based on the simulation with the first‐principles electronic states calculation. The NDR is reproduced by the W10[111] tip model, but not by the W14[110] and Pt10[111] models. The presence of the localized dangling bond state nearEFfor the surface in conjunction with a single tunnel active orbital at the tip apex causes the NDR.

ISSN:0734-211X    

DOI:10.1116/1.585553

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have used scanning tunneling microscopy (STM) to investigate the fractal character of gold deposits grown by electroreduction of hydrous oxide layers. We have been able to precisely measure the fractal dimension (D’) of several samples grown under different conditions by processing three‐dimensional STM images. Lake patterns were generated by computer simulating the filling with ‘‘water’’ up to a given level of the surface topography. Lake perimeter (L) versus area (A) relationships could be established leading to a fractal description (L∝AD’/2) of the sample surface.

ISSN:0734-211X    

DOI:10.1116/1.585554

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Scanning tunneling microscopy experiments on specific semiconducting samples show negative resistance characteristics. It is predicted that similar behavior can be achieved on metallic samples. The requirement for those samples is the existence of a bulk band gap for electron momentum perpendicular to sample surface. Calculations for a tip‐vacuum‐Ni(100) junction have been performed. In this system, negative resistance characteristics arise due to the presence of localized surface barrier states. Another property of those systems is that negative resistance disappears with increasing temperature.

ISSN:0734-211X    

DOI:10.1116/1.585555

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Experimental measurements of the apparent barrier height as a function of tip‐sample separation using a scanning tunneling microscope (with cleanWtips and clean Si surfaces in ultra high‐vacuum) show that the barrier height starts at 3.5 eV at large separations, increases to 4.8 eV at about 1.5 Å before the mechanical contact, and then drops to below 0.3 eV within a fraction of an ångström. At the distances encountered in scanning tunneling microscopy, forces between sample and tip can be significant. Using a simple model of this system including tip‐sample forces leads to a calculated apparent barrier height which quantitatively reproduces the observed variation in apparent barrier height over the entire range of tip‐sample separations.

ISSN:0734-211X    

DOI:10.1116/1.585556

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale. Some of them are correlated to the surface topography.

ISSN:0734-211X    

DOI:10.1116/1.585557

出版商:American Vacuum Society    

年代:1991

数据来源: AIP