Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.     

Real-space imaging of alternate localization and extension of quasi-two-dimensional electronic States at graphite surfaces in magnetic fields

We measured the local density of states (LDOS) of a quasi-two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.     

原子围栏中金属表面电子的态函数及能级

本文用薛定谔方程求解被束缚在原子围栏中金属表面电子的态函数及能级,同时用计算机绘制出围栏中电子概率分布图。     

Spontaneous-emission control by local density of states of photonic crystal cavity

The local density of states (LDOS) of two-dimensional square lattice photonic crystal (PhC) defect cavity is studied.The results show that the LDOS in the centre is greatly reduced,while the LDOS at the point off the centre (for example,at the point (0.3a,0.4a),where a is the lattice constant) is extremely enhanced.Further,the disordered radii are introduced to imitate the real devices fabricated in our experiment,and then we study the LDOS of PhC cavity with configurations of different disordered radii.The results show that in the disordered cavity,the LDOS in the centre is still greatly reduced,while the LDOS at the point (0.3a,0.4a) is still extremely enhanced.It shows that the LDOS analysis is useful.When a laser is designed on the basis of the square lattice PhC rod cavity,in order to enhance the spontaneous emission,the active materials should not be inserted in the centre of the cavity,but located at positions off the centre.So LDOS method gives a guide to design the positions of the active materials (quantum dots) in the lasers.     

A theoretical investigation on Pt3Sn(1 0 2) surface alloy and CO-Pt3Sn(1 0 2) system

The adsorption properties of CO on experimentally verified stepped Pt₃Sn(1 0 2) surface were investigated using quantum mechanical calculations. The two possible terminations of Pt₃Sn(1 0 2) were generated and on these terminations all types of possible adsorption sites were determined. The adsorption energies and geometries of the CO molecule for all those sites were calculated. The most favorable sites for adsorption were determined as the short bridge site on the terrace of pure-Pt row of the mixed-atom-ending termination, atop site at the step-edge of the pure row of pure-Pt-ending termination and atop site at the step-edge of the pure-Pt row of the mixed-atom-ending termination. The results were compared with those for similar sites on the flat Pt₃Sn(1 1 0) surface considering the fact that Pt₃Sn(1 0 2) has terraces with (1 1 0) orientation. The LDOS analysis of bare sites clearly shows that there are significant differences between the electronic properties of Pt atoms at stepped Pt₃Sn(1 0 2) surface and the electronic properties of Pt atoms at flat (1 1 0) surface, which leads to changes in the CO bonding energies of these Pt atoms. Adsorption on Pt₃Sn(1 0 2) surface is in general stronger compared to that on Pt₃Sn(1 1 0) surface. The difference in adsorption strength of similar sites on these two surface terminations is a result of stepped structure of Pt₃Sn(1 0 2). The local density of states (LDOS) of the adsorbent Pt and C of adsorbed CO was utilized. The LDOS of the surface metal atoms with CO-adsorbed atop and of their bare state were compared to see the effect of CO chemisorption on the electron density distribution of the corresponding Pt atom. The downward shift in energy peak in the LDOS curves as well as changes in the electron densities of the corresponding energy levels indicate the orbital mixing between CO molecular orbitals and metal d-states. The present study showed that the adsorption strength of the sites has a direct relation with their LDOS profiles.     

Fluctuations of the local density of states probe localized surface plasmons on disordered metal films

We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale.     

Spatial imaging of valence band electronic structures in a GaSb/InAs quantum well

We measure local density of states (LDOS) for GaSb/InAs heterostructures with quantum wells in the valence band by scanning tunneling spectroscopy (STS) on the cleaved surface. Clear standingwave patterns of LDOS corresponding to the holes confined in the quantum wells are observed.     

用LMTO方法对清洁Nb(100)面电子结构的研究

用LMTO-ASA方法计算了铌的体态密度和清洁Nb(100)表面的局域态密度以及有关的表面能。所计算的体态密度与用其它方法计算的态密度符合得很好,表面能则支持了低能电子衍射(LEED)动力学计算的结果,并且通过分析表面局域态密度,提出了清洁Nb(100)表面存在表面态,表面态位于费密能级的上方。 关键词：     

Ab initio electronic structure of the Ge(111)-(2 × 1) surface in the presence of surface vacancy. Application to STM data analysis

We present the results of ab initio modeling of Ge(111)-(2 × 1) surface in the presence of atomic vacancy in surface bilayer. We showed that simple crystal structure defect affects surface electronic structure to the extent comparable with the influence of doping atom. We demonstrated the strong difference of surface LDOS structure above surface defects of different kind. We have proved that spatial oscillations of LDOS exist around individual surface vacancy in the same tunneling bias range as in case of donor doping atom on Ge(111)-(2 × 1) surface.     

Defects of SiC nanowires studied by STM and STS

For the first time the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are employed to investigate the morphology and the surface electronic structure of the defective silicon carbide nanowires (SiCNWs). The SiCNWs produced via combustion synthesis route are studied. The STS measurements are performed in the current imaging tunneling spectroscopy mode (CITS) that allows us to determine the correlation between STM topography and the local density of electronic states (LDOS) around the bend of an isolated SiCNW. The measurements reveal fluctuations of LDOS in the vicinity of the defect. The local graphitisation and the inhomogeneous concentration of doping impurities (e.g. nitrogen, oxygen) are considered to explain these fluctuations of metallic-like LDOS in the vicinity of the SiCNW's deformation.     

β-NaGdF4纳米球中Tb3+-Er3+下转换对局域态密度的尺寸效应

以Span 80 为模板,采用水热法合成了不同尺寸（4.7~115.5 nm）的β-NaGdF₄：1%Tb³⁺,1%Er³⁺ 纳米晶（NCs）。在Rayleigh限（粒子尺寸小于跃迁波长）下,研究了纳米晶尺寸对局域态密度（Local density of states,LDOS）的影响以及镶嵌在β-NaGdF₄纳米球中的Tb³⁺-Er³⁺的辐射和无辐射特性,进一步揭示下转换过程的物理机制。基于Tb³⁺-Er³⁺ 处在 β-NaGdF₄纳米球中的模型,用Green函数方法计算了Tb³⁺-Er³⁺ 发射体的自发发射速率。在介电纳米球内,Tb³⁺-Er³⁺发射体的LDOS没有显著的变化。在小尺寸（R<<λ）介电纳米球外,按照Chew的理论,发现LDOS有一个类-Gauss分布。如果R>35 nm（在本实验条件下）,介电纳米球外则只能观测到LDOS 的下降边,LDOS与局域场强的平方E²成正比,因而LDOS的类高斯分布出现的原因应归于小尺寸发射体与局域场相互作用的增强。通过计算纳米晶尺寸与体材料自发辐射速率的比值可直接确定纳米材料中的填充因子。     

Ni adsorbate dynamics on a NiO(0 0 1) surface

We present results concerning the dynamical behavior of a Ni³⁺ adsorbate on a NiO(0 0 1) surface obtained by molecular dynamics simulations. In a first place, we examined at low temperature the position of the Ni³⁺ ion as an adatom on the surface and the corresponding modification of its local environment as reflected on the pair-wise radial distribution function. The calculation of the vibrational properties of the adatom by means of the phonon local density of states (LDOS) shows that there is an anisotropic behavior both in the two principal in-plane directions as well as in the direction normal to the surface in accordance with the structural results. We compare the phonon LDOS of the Ni³⁺ adatom with the corresponding results for the Ni²⁺ adatom and the Ni²⁺ surface cations.Static energetic calculations are indicative that the exchange of the Ni³⁺ ion with a surface Ni²⁺ ion could be favorable. Such a behavior is confirmed by results observed at temperatures higher than 700 K where the Ni³⁺ adsorbate is located on a substitutional position on the surface and not on adatom position. The exchange takes place through simple or double exchange mechanisms. The structural and dynamical behavior of the Ni³⁺ ion at the substitution position was investigated in the temperature range 700-2000 K through the calculation of the pair distribution function, the relaxed interlayer relative position (RIRP), mean-square displacements (MSDs) and phonon LDOS. Results show that in comparison with the Ni²⁺ surface ions the Ni³⁺ ion at substitution position is more tightly bound especially in the direction normal to the surface as is indicated by the local structure and the contraction it presents as well as its phonon LDOS. As temperature increases the binding of the Ni³⁺ ion becomes less important as reflected on the physical properties mentioned above.     

STM/STS measurements of two-dimensional electronic states trapped around surface defects in magnetic fields

The local density of states (LDOS) near point defects on a surface of highly oriented pyrolytic graphite (HOPG) was studied at very low temperatures in magnetic fields up to 6 T. We observed localized electronic states over a distance of the magnetic length around the defects in differential tunnel conductance images at the valley energies of the Landau levels (LLs) as well as relatively extended states at the peak ones of LLs. These states appear mainly at energies above the Fermi energy corresponding to the electron LL bands. The data suggest that the quantum Hall state is realized in the quasi two dimensional electron system in HOPG. At the peak energy associated with the n=0 (electron) and -1 (hole) LLs characteristic of the graphite structure, a reduced LDOS around the defects is observed. The spatial distribution is almost field independent, which indicates that it represents the potential shape produced by the defects.     

Dyadic Green's functions and electromagnetic local density of states

A formal proof to relate the concept of electromagnetic local density of states (LDOS) to the electric and magnetic dyadic Green's functions (DGF) is provided. The expression for LDOS is obtained by relating the electromagnetic energy density at any location in a medium at uniform temperature T to the electric and magnetic DGFs. The appropriate boundary conditions governing the DGFs are obtained and it is seen that the two types of DGFs are electromagnetic duals of each other. With this the concept of LDOS is also extended to material media. The LDOS is split into two terms—one that originates from the energy density in an infinite, homogeneous medium and the other that takes into account scattering from inhomogeneities. The second part can always be defined unambiguously, even in lossy materials. For lossy materials, the first part is finite only if spatial dispersion is taken into account.     

Direct comparison between potential landscape and local density of states in a disordered two-dimensional electron system

The local density of states (LDOS) of the adsorbate-induced two-dimensional electron system (2DES) on n-InAs(110) is studied by scanning tunneling spectroscopy. In contrast to a similar 3DES, the 2DES LDOS exhibits 20 times stronger corrugations and rather irregular structures. Both results are interpreted as consequences of weak localization. Fourier transforms of the LDOS reveal that the k values of the unperturbed 2DES still dominate the 2DES, but additional lower k values contribute. To clarify the origin of the LDOS patterns, we measure the potential landscape of the 2DES area. We use it to calculate the expected LDOS and find reasonable agreement between calculation and experiment.     

扫描隧道显微术中的微分谱学及其应用

文章介绍了扫描隧道显微术中微分谱学的原理及其在实验中的诸多应用。微分谱（dI／dV谱）和dI／dV成像可用来研究电子局域态密度在能量和空间的分布，即微分谱固定空间一点，反映电子态密度以能量为变量的分布；而dI／dV图像则反映某给定能量的电子局域态密度以空间为变量的分布，二次微分谱（d^2I／dV^2谱）和二次微分成像可以用来反映分子的非弹性隧穿过程，从而研究分子的振动态。     

Adsorption properties of CO on low-index Pt3Sn surfaces

The adsorption properties of CO on Pt₃Sn were investigated by utilizing quantum mechanical calculations. The (1 1 1), (1 1 0) and (0 0 1) surfaces of Pt₃Sn were generated with all possible bulk terminations, and on these terminations all types of active sites were determined. The adsorption energies and the geometries of the CO molecule at those sites were found. Those results were compared with the results obtained from the adsorption of CO on similar sites of Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) surfaces. The comparison reveals that adsorption of CO is stronger on Pt surfaces; this may be the reason why catalysts with Pt₃Sn phase do not suffer from CO posioning in experimental works. Aiming to understand the interactions between CO and the metal adsorption sites in detail, the local density of states (LDOS) profiles were produced for atop-Pt adsorption, both for the carbon end of CO for its adsorbed and free states, and for the Pt atom of the binding site. LDOS profiles of C of free and adsorbed CO and Pt for corresponding pure Pt surfaces, Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) were also obtained. The comparison of the LDOS profiles of Pt atoms of atop adsorption sites on the same faces of bare Pt₃Sn and Pt surfaces showed the effect of alloying with Sn on the electronic properties of Pt atoms. Comparison of LDOS profiles of the C end of CO in its free and atop adsorbed states on Pt₃Sn and LDOS of Pt on bare and CO adsorbed Pt₃Sn surface were used to clear out the electronic changes occurred on CO and Pt upon adsorption. The study showed that (i) inclusion of a Sn atom at the adsorption site structure causes dramatic decrease in stability which limits the number of possible CO adsorption sites on Pt₃Sn surface, (ii) the presence of Sn causes angles different from 180° for M-C-O orientation, (iii) the presence of Sn in the neighborhood of Pt on which CO is adsorbed causes superposition of the 5σ/1π derived-state peaks at the carbon end of CO and changes in adsorption energy of CO, (iv) Sn present beneath the adsorption site strengthens the CO adsorption, whereas neighboring Sn on the surface weakens it for all Pt₃Sn surfaces tested and (v) the most stable site for CO adsorption is the atop-Pt site of the mixed atom termination of Pt₃Sn(1 1 0).     

Frequency shift of sources embedded in finite two-dimensional photonic clusters

The frequency (Lamb) shift and local density of states (LDOS) in two-dimensional photonic crystals composed of a cluster of infinitely long circular cylinders is calculated classically using the radiation reaction mechanism. We investigate the frequency shift and LDOS as a function of the size of the cluster and show that, at the edges of the band gap, both quantities can be large and increase in magnitude with cluster size. We explain this in terms of poles of a scattering operator and also show that both the Lamb shift and LDOS are sensitive functions of the shape of the cluster.     

利用扫描隧道谱(STS)对石墨单晶表面局域电子态的研究

利用ＳＴＳ测量并结合扫描隧道显微镜（ＳＴＭ）扫描图象，给出一组沿石墨单晶表面原子分辨的ＳＴＭ图象上某一线段各点处的扫描隧道谱．ｄ（lｎI）／ｄ（lｎＶ）～ｅＶ由测量谱给出的样品表面Ｅ_Ｆ附近局域态密度分布与由体能带结构计算得到的结果在一定程度上相符合，将各条曲线中Ｅ_Ｆ附近的态密度峰能量对相应的空间位置作图，给出石墨表面E_F附近能态密度在测量区域内实空间的变化。通过对表面不等价Ａ，Ｂ类原子处局域电子结构的分析并利用简单模型进行计算，给出了与实验 关键词：     

Frequency shift of sources embedded in finite two-dimensional photonic clusters

The frequency (Lamb) shift and local density of states (LDOS) in two-dimensional photonic crystals composed of a cluster of infinitely long circular cylinders is calculated classically using the radiation reaction mechanism. We investigate the frequency shift and LDOS as a function of the size of the cluster and show that, at the edges of the band gap, both quantities can be large and increase in magnitude with cluster size. We explain this in terms of poles of a scattering operator and also show that both the Lamb shift and LDOS are sensitive functions of the shape of the cluster.