锁定放大技术用于扫描隧道显微镜

介绍了锁定放大技术应用于扫描隧道显微镜（STM）中调制隧道间隙而获得物质表面原子的局域势垒分布的原理,并给出了高定向石墨的观测结果,与恒电流模式下STM图像相比较,具有分辨率高、针尖的影响小、抗干扰性能强的特点。另外,根据局域势垒分布还可区分不同的原子,给解释STM原子图像提供了一种手段。     

基于小波变换的STM图像处理

根据扫描隧道显微镜(STM)的特点，阐述了将小波变换应用于扫描隧道显微镜的降噪、增强及融合的方法，对于STM扫描获得的石墨原子图像，选用不同的小波基函数和分解层数进行分解和重构，结果表明，利用小波变换对扫描隧道显微镜图像进行处理是有效的、可行的，图像质量得到了明显提高。     

利用STM制作有机LB膜超高密度存储器

利用有机材料制作高密度存储器是当前纳米电子学研究的一个热点.有机LB膜的有序分子排列和能精确控制厚度的膜层被认为是高密度存储器理想的基体.具有原子分辨率及纳米量级局域作用微区的扫描隧道显微镜(STM)的出现则提供了制作超高密度存储器一种有力的高技术手段.本文介绍利用STM在有机LB膜上制作超高密度存储器.以硬脂酸制备多层LB膜,其形貌由STM成像.施加于STM针尖上的脉冲偏压,在LB膜表面的局域区域产生足够的强电场,使该微区转换为低阻导电状态,以高、低阻态分别表示两种逻辑状态,就完成了一次"写入"操作.LB膜材料的高、低阻态由STM的伏安特性(I-V)和扫描隧道谱(dI/dV-V)加以表征.用幅值较小的脉冲实现"读出"操作.该材料LB膜上存储的信息能保持很长时间,显示了潜在的应用前景.     

石墨中碳原子的扫描隧道显微镜观察及分形研究

用扫描隧道显微镜观察了石墨表面碳原子形貌,并与其晶体结构的理论模型进行了比较,获得了一致的结果.同时,计算了石墨原子STM图像的分形维数,得到了合理的结果,表明了计算方法是可行的,从而为扫描隧道显微图像的分形研究做出了初步的尝试.     

扫描隧道显微镜的一种新模式

本文提出了扫描隧道显微镜(STM))在高隧道电压区(Fowler-Nordheim区)工作的一种新模式——恒流和极大电导率(CCMC)模式。分析和计算表明,由此得到的CCMC图象能给出不依赖于表面功函数变化及表面局域态密度结构的表面形貌图,同时还能给出表面局域功函数分布图。最后对CCMC图象的分辨率和适用范围作了讨论。     

分子束外延制备金单原子台阶样品的工艺参数改进研究

利用常规分子束外延开展单晶金原子台阶标准样品的制备,制备的样品通过扫描隧道显微镜(STM)测试发现,原子台阶数量较少且台阶高度均匀性较差。本课题针对分子束外延制备系统的基底与蒸发源距离、温度、时间、真空度等关键工艺参数进行了改进研究,并利用扫描隧道显微镜(STM)对改进参数后所生长金单原子台阶的质量和高度进行了分析测试,结果表明通过调节基底与蒸发源距离、温度、时间和真空度等工艺过程参数能够实现云母基底金原子的可控生长,为单原子台阶标准物质的制备提供了实践经验。     

原子力显微镜AFM

一、AFM的发展 世界最新型原子力显微镜AFM(Atomic Force Micro-scope)是1986年由电子扫描隧道显微镜STM(ScamningTunneling Microscope)的发明者Binning试制成功的,1990年完成了实用化装置并开始投放市场,目前AFM在世界上的普及速度大大超过了STM。 其理由是STM不能测定绝缘材料,而AFM不但具有与STM同等高的分辨能力,还能直接测定包括绝缘材料在内的各种物质。由于STM能够逐个地识别原子,所以作为显微镜来说是划时代的,但是不能直接测定绝缘物质是它的最大缺点。使用STM测定绝缘物质时,必须     

STM研究的新进展

STM是80年代初期发明的一种用于表面结构研究的新技术(注:扫描隧道显微技术和扫描隧道显微镜的英文缩写都是STM),它能观察物质表面原子的几何排列和表面形貌,配合扫描隧道谱学(scanning tunneling spectroscopy)原理,还能够获取表面价键、能隙等电子结构信     

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     

场离子-扫描隧道显微镜

由IBM的Binnig和Rohrer发明的扫描隧道显微镜STM是基于与原先的各种表面研究手段完全不同的原理,能直接观察一个个表面原子并能测出每个原子位置上的电子状态的划时代的表面研究装置。STM之所以引起表面科学工作者的注目主要原因是由于它能在极高的空间分辨率(dX/Y～0.1nm,dz～0.01nm)下直接观察表面的原子结构。而且,它不一定需要超高真空条件,而可以在大气或溶液等环境中操作。这是以往任何一种表面研究手段不可能做到的。然而,即使如此,STM也暴露出它的弱点,如操作特性的不稳定性以及操作过程中原子分辨能力的不可控性。很显然主要原因是扫描探针的尖端曲率半径及其稳定性。为了在STM实验中随时监测其扫描探针尖端的原子排列状况,Kuk及Sakurai等人先后设计研制了一种将场离子显微镜FIM和STM组合在一起的复合型场离子-扫描隧道显微境FI-STM。使用这种仪器可以确保在极高的原子分辨的情况下从事STM研究。下面将就FIM的原理、STM的基础、FI-STM的技术细节以及它的应用实例作简单的介绍。     

Studying local surface electric conductivity of a ZnSe/CdSe/ZnSe heterostructure by scanning tunneling microscopy in the field electron emission regime

The surface morphology and local electric conductivity of a ZnSe/CdSe/ZnSe nanoheterostructure have been studied by scanning tunneling microscopy (STM) in the field electron emission regime. The homogeneity of the local conductivity distribution in a near-surface layer has been evaluated. The main parameters of a potential barrier in the local field contact are determined. It is shown that the STM probe can be used for creating local regions with nonequilibrium carrier concentration in a semiconductor.     

Dirac electrons of a split-gate Hall bar

In this work we study several unusual properties of Klein tunneling through the abrupt and flat barriers of a split-gate Hall bar system of graphene. We show that Klein tunneling of Dirac electrons can be rather strong in such a system, and that a significant electron density can be present under the barrier. It can be shown that the probability wavefunctions for large angular momenta are identical to the probability wavefunctions of the same angular momenta in the absence of the potential barrier, i.e., it is as if the barrier does not exist and the Klein tunneling is complete. This is a unique effect in a magnetic field. We propose that STM measurements may be used to detect the presence of such a density. We have also investigated drift velocity of electrons as the center of probability wavefunction varies from outside to inside of the flat potential barrier, and find a significant deviation from the semiclassical result.     

Studies of 2D Cryocrystals by STM Techniques

Scanning tunneling microscopy (STM) studies of two-dimensional (2D) cryocrystals ⁴ He, Kr, Xe) physisorbed on graphite surfaces are presented. Individual helium atoms, usually thought to be invisible with STM, were recently observed on graphite surfaces at a density corresponding to the commensurate solid. Here we show that a local elastic deformation seems to be the principal mechanism responsible to render the atoms visible. Recent tight-binding calculations of the local density of states (LDOS) of graphite which predict the appearance of an energy gap support this picture. I-z curve measurements for the case of ⁴ He show a sharp drop (increase) of the tunneling current I at a certain tip-surface distance z during retraction (approach) of the tip. This drop (increase) may be associated with the tunneling of a single He adatom, opening new possibilities to study the quantum tunneling of atoms via STM.     

Resonant electron tunneling through azurin in air and liquid by scanning tunneling microscopy

Electron transfer through the redox metalloprotein azurin immobilized on Au (111) by its disulphide bridge is studied by scanning tunneling microscopy (STM) in buffer solution and (for the first time) in air. STM analysis gives evidences of a stable and robust binding of the molecules in both cases. Bright spots, associated with azurin molecules, are clearly visible in STM images. The image contrast of adsorbed azurin is highly affected by the bias potential with proteins visible only for some well-defined voltage values. This experimental demonstration of the possibility to induce tunneling through azurin in air could disclose very interesting perspectives for the development of protein-based hybrid nanodevices operating in nonliquid environments.     

STM tunneling spectroscopy on high Tc superconductors

STM tunneling spectroscopy has been performed on the bulk single crystals of BiSrCaCuO (BSCCO) and the epitaxial thin films of YBaCuO (YBCO) at cryogenic temperatures. The STM images and tunneling spectra observed on the (001) surfaces can be classified into three cases; 1) Atomic image is visible. However, the tunneling spectrum shows semiconducting or smeared superconducting gap structures, depending on the tip-sample distance. 2) Clear atomic image can not be obtained. But, the tunneling spectrum shows flat bottom region with quite low zero bias conductance. 3) Tunneling spectra demonstrate gapless behavior, independent of the tip-sample separation. These observations support the quasi-2D electronic picture in whichs-wave like 2D superconducting layers are coupled with each other through the Josephson effect.     

Probing quantized image-potential states at supported carbon nanotubes

Discrete image-potential states (ISs) are revealed at double-walled carbon nanotubes by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in the distance-voltage z(V) spectroscopy mode. The nanotubes are supported by flat Au(111) substrates. Due to the high sensitivity of the hot IS electrons to local variations of the surface potential, they can be considered as a sensitive probe to investigate interactions with the supporting substrate and impurities or defects at the nanotube surface. ISs provide information on the local electronic structure as well as on the electron dynamics at supported nanotubes.     

Features of Transport in Ultrathin Gold Nanowire Structures

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation‐induced tunneling conduction through the nanowire‐metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.     

Cryogenic STM/STS observation on oxide superconductors

The topographic and electronic properties of the surfaces of (001) and (110) oriented YBa₂Cu₃O_y, epitaxial films have been probed by atomic resolution STM/STS at 4.2 K. The STM image on the (001) surface clearly revealed the atomic corrugation of the tetragonal lattice with an average spacing of 0.4 nm. while on the (110) surface the orthorhombic atomic lattice, corresponding to the Cu atoms of both CuO₂ and CuO chain planes, was observed. The STS result on the (001) surface indicated the semiconducting nature of the terminating layer. As the tunneling tip came closer to the surface, however, the shape of the tunneling spectrum became more metallic and showed a superconducting energy gap, which seems to arise from the underlying superconducting layer. On the other hand, the tunneling spectra on the (110) surface indicated superconducting gap structures, independent of the tip-sample distance.     

Direct observation of X-ray induced atomic motion using scanning tunneling microscope combined with synchrotron radiation

X-ray induced atomic motion on a Ge(111)-c(2 x 8) clean surface at room temperature was directly observed with atomic resolution using a synchrotron radiation (SR)-based scanning tunneling microscope (STM) system under ultra high vacuum condition. The atomic motion was visualized as a tracking image by developing a method to merge the STM images before and after X-ray irradiation. Using the tracking image, the atomic mobility was found to be strongly affected by defects on the surface, but was not dependent on the incident X-ray energy, although it was clearly dependent on the photon density. The atomic motion can be attributed to surface diffusion, which might not be due to core-excitation accompanied with electronic transition, but a thermal effect by X-ray irradiation. The crystal surface structure was possible to break even at a lower photon density than the conventionally known barrier. These results can alert X-ray studies in the near future about sample damage during measurements, while suggesting the possibility of new applications. Also the obtained results show a new availability of the in-situ SR-STM system.     

NaCl multi-layer islands grown on Au(111)-([Formula: see text]) probed by scanning tunneling microscopy

The growth of multi-layer NaCl islands on Au(111)-([Formula: see text]) surfaces was investigated using scanning tunneling microscopy (STM). We observed that the aspect of the NaCl islands drastically differs depending on the tunneling conditions. It is therefore possible to observe the layers forming an NaCl island or to image the gold reconstruction below the first NaCl layer. Atomically resolved STM images obtained on the first NaCl layer demonstrate that NaCl grows as an epitaxial crystalline film on Au(111)-([Formula: see text]). STM images also suggest that some NaCl layers can be non-crystalline.