STM investigation of cobalt silicide nanostructures’ growth on Si(111)-(√19 × √19) substrate

Continuing miniaturization of electronic devices necessarily requires assembly of several different objects or devices in a small space. Therefore, besides thin films growth, the possibility of fabricating wires and dots [1, 2] at the nanometre scale composed of metal silicides is of the top interest. This report is about the STM/STS investigation of cobalt silicides’ nanostructures created on Si(111)-(√19 × √19) substrates via Co evaporation and post deposition annealing. This (√19 × √19) reconstruction was induced by Ni doping. Less than 1ML of Co on surface was obtained. Surface reconstruction induced growth of agglomerates of clusters rather than an uniform layer. The post deposition annealing of a crystal sample (up to 670 K, 770 K, 870 K, 970 K, 1070 K and 1170 K) led to creation of silicides’ nanostructures. Measurements showed that coalescence of Co nanoislands begun around 970 K. Annealing above 1070 K led to alloying of a Co, Ni and Si. As a consequence the Si(111)-(7×7) reconstruction occurred at the cost of Si(111)-(√19 × √19).      

STM study of di-indenoperylene molecules on Cu(1 0 0) surfaces: Mobility, stability and epitaxy

We report a detailed study of the self-assembly process of di-indenoperylene (DIP) on Cu(1 0 0) surfaces, investigated by variable temperature scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. During the initial growth stages a preferred nucleation at the step edges is revealed. Subsequently, large ordered 2D islands grow from the step edges, while smaller 2D islands form on the terrace sites. The equilibrium fluctuation of these 2D clusters has been monitored in real-time, thereby obtaining a direct insight into the temperature dependence of the molecular surface diffusion. The substrate–adsorbate interactions determine the azimuthal molecular orientation, finally lead to the formation of highly ordered chiral domains which are commensurate with the substrate.     

Morphology and electronic structure of bcc Co(1 1 0) and fcc/hcp Co(1 1 1) on Fe(1 1 0) investigated by STM and STS

We report on the growth of ultrathin epitaxial Co films on Fe(1 1 0) examined by scanning tunneling microscopy and spectroscopy (STM and STS). At room temperature Co forms pseudomorphic, ideally ordered body-centered cubic (bcc) layers for the first two monolayers as confirmed by atomically resolved STM images. This is in contrast to the related case of Co/Cr(1 1 0) where a superstructure occurs in the second layer. The third monolayer forms a close-packed structure and causes a transformation of the buried second monolayer into a close-packed structure. The Fe(1 1 0) substrate strongly influences the electronic structure of the first Co monolayer as concluded from the dI/dU spectra. This influence is less important for the second monolayer. The measured local density-of-states function for the bcc Co double layer is in agreement with theoretical predictions for bcc Co.     

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5-10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon-oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25-30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO₂) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.     

表面科学中的单分子研究

简要介绍了表面科学中单分子研究的主要技术手段、研究现状及其发展趋势,特别介绍了扫描隧道显微术（scanning tunneling microscopy,STM)在此研究领域的重要作用以及所获成果等,内容涉及STM技术的特点,材料的表面结构、表面吸附分子的观察、基体对吸附分子成像的影响以及分子操纵和分子器件的构筑等。     

取代烷基侧链长度对二维纳米结构的影响

合成了一系列烷基取代的间苯三酚衍生物,并在大气条件下用扫描隧道显微镜研究了它们在高定向裂解石墨表面的吸附和组装行为.实验结果表明,这些自组装分子具有条状结构特征.在链长较短的分子图像中,两条平行的烷氧基链肩并肩地排列在苯环的一侧,另一条烷氧基链则排列在苯环的另一侧,链与链之间彼此相互交错排列形成均一的烷基条带.当链长增加时,这种高稳定性和密排结构遭到破坏,出现单个分子和分子对共存的组装结构.这是由于烷基链与烷基链之间以及烷基链与基底之间的作用力共同决定的.通过调控分子烷基链的长度可以得到不同的表面二维纳米结构.     

Collective and plastic vortex motion probed by STM

The motion of flux line lattices (VL) in various single crystals of NbSe₂ has been observed using fast scanning tunneling microscopy. In crystals with intrinsic weak point disorder the VL creeps coherently as a large bundle along one of the principal axes of the VL. However, we observe striking and for the creep regime unanticipated velocity modulations in the direction of motion, related to the VL periodicity, i.e. with the “washboard” frequency. In the transverse direction the fluctuations of the center-of-mass displacement are random. In crystals with strong line disorder (correlated pinning) caused by columnar defects due to swift ion irradiation collective pinning is observed in fields up to almost double the dose-equivalent field Bφ while interesting plastic flow patterns are detected at 2Bφ.     

直立在金膜表面的单壁碳纳米管的研究

用水溶胶体将单壁碳纳米管（ＳＷＣＮＴｓ）组装在晶态金膜表面,用扫描隧道显微镜（ＳＴＭ）研究了表面单壁碳纳米管直立于金膜表面,单根碳管的长度约在１０－１５ｎｍ之间。     

Three-dimensional Wentzel–Kramers–Brillouin approach for the simulation of scanning tunneling microscopy and spectroscopy

We review the recently developed three-dimensional (3D) atom-superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS) based on ab initio electronic structure data. In the method, contributions from individual electron tunneling transitions between the tip apex atom and each of the sample surface atoms are summed up assuming the one-dimensional (1D) Wentzel–Kramers–Brillouin (WKB) approximation in all these transitions. This 3D WKB tunneling model is extremely suitable to simulate spin-polarized STM and STS on surfaces exhibiting a complex noncollinear magnetic structure, i.e., without a global spin quantization axis, at very low computational cost. The tip electronic structure from first principles can also be incorporated into the model, that is often assumed to be constant in energy in the vast majority of the related literature, which could lead to a misinterpretation of experimental findings. Using this approach, we highlight some of the electron tunneling features on a prototype frustrated hexagonal antiferromagnetic Cr monolayer on Ag(111) surface. We obtain useful theoretical insights into the simulated quantities that is expected to help the correct evaluation of experimental results. By extending the method to incorporate a simple orbital dependent electron tunneling transmission, we reinvestigate the bias voltage- and tip-dependent contrast inversion effect on theW(110) surface. STM images calculated using this orbital dependent model agree reasonably well with Tersoff-Hamann and Bardeen results. The computational efficiency of the model is remarkable as the k-point samplings of the surface and tip Brillouin zones do not affect the computational time, in contrast to the Bardeen method. In a certain case we obtain a relative computational time gain of 8500 compared to the Bardeen calculation, without the loss of quality. We discuss the advantages and limitations of the 3D WKB method, and show further ways to improve and extend it.