Boron surface phase on Si(111): atomic structure and Si overgrowth studied by scanning tunneling microscopy and work function measurement

Surface reconstructions of boron (B) on silicon (Si) have been well known for several years. One reconstruction of special interest to us is the so called R30° boron surface phase (BSP) on Si(111). This reconstruction can occur in two different forms, one with B located on T₄ lattice sites (B-T₄), the second one with B residing in S₅ sites (B-S₅) directly underneath a Si adatom in a T₄ site. The two forms of the R30° reconstruction are expected to exhibit completely different properties due to their different chemical binding. In this paper we present work function measurements of these surface phases which clearly show their distinctively different behaviour and allow the determination of the temperature at which the boron atoms migrate from the T₄ sites to the S₅ sites. Furthermore, STM results concerning the overgrowth of BSPs with Si films of variable thicknesses and its effect on the BSP itself are shown.     

Lyocell基炭纤维表面微结构的STM研究

采用SEM揭示出Lyocell基炭纤维(LCF)表面比粘胶基炭纤维(RCF)表面光滑,而后者粗糙的表面上还存在裂纹和沟槽,这也是导致RCF强度偏低的一个原因。尝试利用扫描隧道显微镜(STM)的原子级分辨率来研究LCF的表面微结构。在500nm×500nm观测范围内,发现了由25nm宽,150nm长的条状微结构组成的约150nm2大小的沿纤维轴呈近乎45°角排列的块状结构;并测得LCF表面碳原子间距离为0.331nm,结果表明Lyocell基炭纤维表面尚未形成完整的碳网六元环结构。     

Si(111)表面亚稳态重构的STM研究

利用扫描隧道显微镜 (STM) ,研究了Si(111)表面的 2× 2、c2× 4、9× 9和 11× 11等各种亚稳态结构。与已经发表的研究结果相比 :2× 2和c2× 4重构区域规则且面积更大 ,原子分辨的图像清晰稳定 ;在不同亚稳态之间的畴界处 ,发现存在2种不同的环形结构。从原子密度和能量两方面 ,对各种亚稳态重构的形成机制进行了讨论     

Sub-molecular features of single proteins in solution resolved with scanning tunneling microscopy

Scanning tunneling microscopy (STM) can be used to image individual biological molecules, such as proteins, in vacuum or air. This requires sample dehydration and thus may not reflect the native state of the molecule. Extensive efforts have been made to image single proteins in solution using STM; however, the images have revealed only round or oval shapes with no sub-molecular details. Here, we present the sub-molecular features of streptavidin proteins under physiological conditions using a homebuilt low-leakage-current and highstability liquid phase STM. The N-lobe, C-lobe, and C-terminal tail of the epidermal growth factor receptor kinase domains were also resolved in solution. Our results demonstrate that the structure, morphology, and dynamics of a protein molecule can be examined under physiological conditions by the STM.

Open image in new window

     

Assembling of redox proteins on Au(111) surfaces: A scanning probe microscopy investigation for application in bio-nanodevices

The morphology and conductive properties of azurin molecules, chemically attached to sulfhydryl terminated alkanethiol monolayer assembled on Au(111) surface, are mapped at single molecule level and compared with those observed for the same molecule immobilised on bare Au(111). High-resolution Tapping Mode Atomic Force Microscopy shows that the protein molecules immobilised on modified gold, better reproduces the crystallographic height of the protein, than that immobilised on bare gold. Such a height recovering is also found in the Scanning Tunnelling Microscopy images. Consistently, a good tunnelling conduction of azurins on the modified gold electrode is demonstrated by Tunnelling Spectroscopy. Cyclic voltammetry measurements show, in addition, that the redox activity of azurin molecules covalently immobilised on sulfhydryl functionalised Au(111) surface is retained. These results are discussed in connection with possible use of this linker in the assembling of nano-hybrid systems.     

Growth of copper and vanadium on a thin Al2O3-film on Ni3Al(111)

The growth of different metals on thin Al₂O₃-films on Ni₃Al(111) was investigated using scanning tunneling microscopy (STM). These thin alumna films are well ordered showing two superstructures, which appear in the STM images at different bias voltages. These superstructures, with periodicities of 2.6 and 4.5 nm, respectively, are shown here to govern the nucleation of the deposited metals. Copper clusters grow on these nucleation centers only at room temperature. Higher temperatures lead to an increase of the cluster size and the loss of order. In turn, vanadium forms ordered cluster arrays at room and higher temperature. Due to the stronger metal–oxide interaction compared to copper vanadium forms smaller clusters at low and high coverages, which do not show any ripening after annealing. Based on these observations, Al₂O₃-films on Ni₃Al(111) prove to be an interesting template for the fabrication of periodic cluster arrays.     

Under-surface observation of thin-film alumina on NiAl(100) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to investigate the oxide structures underlying the surface of alumina thin-film grown on NiAl(100). At a bias voltage (on the sample) below 2.0 V, STM topography images of the alumina layer beneath the surface were obtained. A probe with depth of 2-8 Å was readily attained. The under-surface observation shows that the film consists of stacked layers of alumina whereas the layered alumina unnecessarily comprises entire θ-Al₂O₃ unit cells. The lattice orientation of the upper alumina layer differs from that of the lower one by 90° — the newly grown oxide structurally matching the horizontal oxide rather than the lower oxide. The results indicate a growth process competing with the typical mode of epitaxial growth for the growth of alumina film.     

Developments and perspectives of scanning probe microscopy (SPM) on organic materials systems

In this paper recent developments in scanning probe microscopy ( SPM ) on organic materials are reviewed with selected examples. Presented subjects include instrumentation and particularities in connection with SPM imaging on soft organic materials. Exemplary cases of structure-properties investigations with SPM in organic materials science including amorphous polymers, polymer crystal growth effects, interface structure and stability, spinodal decomposition effects, copolymer and liquid crystal (LC) nanophase separation, LC phase transitions on a molecular scale, molecular manipulation as well as structure and properties of other organic materials are presented. Naturally, this paper cannot review all papers published about SPM on organic materials. Rather, main principles and problems as well as the strategy of probe microscopy on organic materials is elucidated with selected examples. SPM is shown to be an effective and forceful organic materials analytic tool for the materials scientist.     

Growth, structure and electronic properties of ultrathin cerium oxide films grown on Pt(111)

Ultrathin cerium oxide films have been prepared by oxidizing Ce-Pt/Pt(111) surface alloys with 65 L O₂ at 900 K. According to low electron energy diffraction data, phonon spectra recorded by high resolution electron energy loss spectroscopy and scanning tunneling microscopy measurements the films are of fluorite-type CeO₂(111) structure. They grow three dimensionally as islands, which are flat and up to several hundred square nanometers large. The film thickness varies between one and five layers. The band gap of the oxide films has been probed by scanning tunneling spectroscopy and turned out to be significantly smaller than for bulk CeO_2.     

Two-dimensional vacancy islands induced by the growth of Cr on Cu(111)

Surface structures induced by the growth of Cr on Cu(111) at room temperature are studied with scanning tunneling microscopy. At sub-monolayer Cr coverage, islands and vacancy islands with lateral sizes of about 10 nm in length are observed. The islands show one- or two-monolayer heights with irregular shapes, while vacancy islands have one-monolayer depth and hexagonal shapes with openings. Atomic structures inside and near the vacancy islands are the same as those of bare Cu(111), implying that Cu atoms detach from the vacancy islands and attach to other step edges and islands. Possible origins and atomic processes for the observed structures are discussed.     

Thermal evolution of the morphology of Ni/Ag/Si(111)-√3 × √3 surface

The temperature-driven changes in morphology of the interface formed by room temperature (RT) deposition of Ni atoms onto an Ag/Si(111)-√3 × √3 surface were investigated by scanning tunneling microscopy. Roughly 70% of Ni deposition diffused into bulk substrate within the temperature range between RT and 573 K. The images as obtained after annealing up to 670 K correspond to the formation of nano-sized islands of nickel silicides. Two types of islands, large triangular islands typical of the whole range of applied coverage, and smaller islands of different shapes, coexist at Ni coverage higher than 1 monolayer. Annealing above 870 K led to the formation of a 7 × 7 phase in coexistence with small 5 × 5 domains at the expense of a complete disappearance of the √3 × √3 phase. Also, formation of Ni,Si alloy was observed at the temperature, along with segregation of bulk-dissolved Ni species onto the surface.     

Si(337)表面的STM研究

利用超高真空扫描隧道显微镜 (UHVSTM) ,研究了Si(337)高指数面的形貌和原子结构 ,发现Si(337)表面平台的生长与退火速度有关 ,相对较慢的退火速度有利于生长尺寸更大、结构更完整的平台。高分辨STM结果显示Si(337)表面平台原子结构为 (5 5 1 2 )的周期性结构 ,表明Si(337)表面退火过程中稳定性较差 ,易重构成临近的更稳定的高指数面     

A comparison of different carbon filaments on the nanometer and atomic scales by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed as a tool to elucidate the structural differences between carbon nanotubes (CNTs) and carbon nanofibers (CNFs) at the nanometer and atomic scales. As opposed to the seamless, virtually defect-free structure of CNTs that was visualized by STM at both scales, CNFs displayed a fragmented graphitic conformation (nanographites), where the nanographitic domains were below 2-3 nm in lateral size. Thus, STM is proposed as a suitable technique to discriminate these two types of carbon nanostructures.     

Structure of ultrathin MgO films on Mo(001)

We have studied the structure of ultrathin MgO films grown on a single crystal Mo(001) surface. Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) were used to investigate the effect of substrate temperature and oxygen partial pressure on the growth and morphology of these films. LEED indicates the growth of (100) films with MgO 〈110〉 directions oriented along 〈100〉 directions of the substrate. Despite the insulating nature of bulk MgO, films up to 25-Å thick are sufficiently conducting to perform STM measurements. STM reveals Mg deposition in an oxygen ambient at substrate temperatures from 300 to 900 K produces uniform films. Films as thick as eight atomic layers typically have only three layers exposed. These films consist of small domains between 20 and 60 Å in diameter. The domain shapes are random and the perimeters show no preferred orientation. In contrast, films grown at temperatures in excess of 1000 K exhibit larger three-dimensional MgO islands (Volmer-Weber growth). Steps on these high temperature films orient preferentially along thermodynamically favored MgO 〈100〉 directions. STM images of films deposited at high temperature exhibit a checkerboard pattern. The dimensions and symmetry of this pattern are consistent with the coincidence arising from the mismatch of the MgO(100) and Mo(001) lattice. Annealing room temperature deposited films results in island coalescence and produces uniform films with domains in excess of 100 Å. The perimeters of these domains are oriented along MgO 〈100〉 directions.     

In situ high-temperature scanning tunneling microscopy study of bilayer graphene growth on 6H-SiC(0001)

Using in situ high-temperature (1395 K), ultra-high vacuum, scanning tunneling microscopy (STM), we investigated the growth of bilayer graphene on 6H-SiC(0001). From the STM images, we measured areal coverages of SiC and graphene as a function of annealing time and found that graphene grows at the expense of SiC. Graphene domains were observed to grow, at comparable rates, at (I) graphene-free SiC step edges, (II) graphene-SiC interfaces, and (III) the existing graphene domain edges. Based upon our results, we suggest that the rate-limiting step controlling bilayer graphene growth is the desorption of Si from the substrate.     

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

Abstract

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Manifestations of strain-relaxation in the structure of nano-sized Co-2 × 2 islands grown on Ag/Ge(111)-√3 × √3 surface

We have examined strain-relaxation of Co-2 × 2 islands grown on the Ag/Ge(111)-√3 × √3 surface by analyzing scanning tunneling microscopy images. We have found that the Co-2 × 2 islands commonly adopt a more compact arrangement as compared to that of the Ge(111) substrate, however they differ in a degree of an atomic compactness. We have not found a distinct relation between strain-relaxation and the island height. Three groups of islands have been identified upon analyzing a correspondence between strain-relaxation and the island size: (i) small islands (not bigger than 80 nm²) with a high atomic compactness, displaying fixed inter-row distances, (ii) small islands with unfixed distances between atomic rows, and (iii) big islands (bigger than 80 nm²) with fixed inter-row distances, but with a less compact atomic arrangement compared to that of the first two groups. We propose a model to account for the relation between the relaxation and the island size.     

Structure of Pt3Co(1 1 0)c(2×4)-O surface studied by scanning tunneling microscopy

A Pt₃Co(1 1 0)c(2×4)-O surface has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. At a very initial oxidation stage exposed at 500°C, creation of missing and/or added row structures running to the [0 0 1] direction on clean Pt₃Co(1 1 0)2×1 surface was imaged from the steps. The surface is fully covered by a well-ordered c(2×4) structure at 2 L oxygen exposure. Atomic-resolution STM image shows the added row-type anti-phase Co-O zigzag chains along the [0 0 1] direction. Based on the images, the structure model for the c(2×4) surface was suggested as a first oxidized layer, which comes from the chemical reaction forming stoichiometric Co monoxide. Further oxygen exposure above 5 L, Co-O clusters imaged to large dots were formed on the surface with the size of about 5-7 Å.