A stiff scanning tunneling microscopy head for measurement at low temperatures and in high magnetic fields

We have developed a measurement head for scanning tunneling microscopy (STM) and specifically for spectroscopic imaging STM which is optimized for high mechanical stiffness and good thermal conductivity by choice of material. The main components of the microscope head are made of sapphire. Sapphire has been chosen from several competing possibilities based on finite element modeling of the fundamental vibrational modes of the body. We demonstrate operation of the STM head in topographic imaging and tunneling spectroscopy at temperatures down to below 2 K.     

Supported metal model catalyst surfaces examined by scanning tunnelling microscopy

Topographic and/or barrier-height images of ultrafine Pt and Au metal particles supported on a vacuum-deposited carbon film or titanium oxide thin films grown on titanium metal sheets were obtained. The topographic images of colloidal Au particles (5-nm diameter) adsorbed on a titanium oxide thin film showed a structure elongated in the direction normal to the x scan, indicating their weak interaction with the support surface. The topographic images of Pt vacuum-deposited on a carbon film showed c. 4-nm diameter particles, larger than those observed in electron microscopy. The problems inherent to the STM observation of such dispersed metal systems are identified. In the case of Pt particles vacuum-deposited on titanium oxide film, its barrier-height image gave better indication of different phases on the surface than its topographic image. The significance of obtaining barrier-height images along with topographic images for such sample systems is demonstrated.     

新型纳米阻抗显微镜前置放大器的设计

基于扫描隧道显微镜的新型纳米阻抗显微镜（STM-NIM）能够测量材料表面的纳米微区阻抗性质并具有分辨率高的突出优点，但传统STM前置放大器的频率带宽不能满足STM-NIM测量模式的要求，因而需要设计新型前置放大器。STM-NIM前置放大器必须在噪声和频率带宽两方面同时具有非常优良的性能。本文利用STM及NIM信号的特点，通过将信号一分为二并对电路进行优化，研制出了满足STM-NIM测量要求的新型前置放大器。测试表明，该前置放大器能同时用于NIM阻抗测量和STM形貌扫描；其NIM信号的频率带宽达到300kHz；其STM信号的噪声约为0．8mV，能满足原子级分辨成像的要求。     

Observation of removal of an Fmoc protecting group by scanning tunneling microscopy

We demonstrate here by imaging successive surface reactions in self-assembled monolayers (SAMs) on Au(1 1 1) at molecular scale with a scanning tunneling microscope (STM): (i) SAM matrices formation with 1-octanethiol on Au(1 1 1) in ethanol, (ii) insertion of N-Fmoc-aminooctanethiol into the SAM matrices in ethanol, and (iii) removal of the Fmoc protecting group with tris(2-aminoethyl)amine (TAEA). The total reaction is formation of SAMs containing a small amount of NH₂ terminated molecules in the CH₃ terminated SAM matrices. After the reaction of the protecting group with TAEA, STM imaging revealed the decrease in heights of the inserted molecules on average. We attributed this observation to removal of the protecting group by taking account of a convolution of electronic and topographic contributions to observed STM heights. Apparent areas of the terminal groups, however, became larger on removal. The increase in the areas was attributed to water adsorption to the NH₂ terminal group under air.     

Antenna-based ultrahigh vacuum microwave frequency scanning tunneling microscopy system

The instrumental synthesis of high resolution scanning tunneling microscopy (STM) with the ability to measure differential capacitance with atomic scale resolution is highly desirable for fundamental metrology and for the study of novel physical characteristics. Microwave frequency radiation directed at the tip-sample junction in an STM system allows for such high-resolution differential capacitance information. This ability is particularly critical in ultrahigh vacuum environments, where the additional parameter space afforded by including a capacitance measurement would prove powerful. Here we describe the modifications made to a commercial scanning tunneling microscope to allow for broad microwave frequency alternating current scanning tunneling microscopy (ACSTM) in ultrahigh vacuum conditions using a relatively simple loop antenna and microwave difference frequency detection. The advantages of our system are twofold. First, the use of a removable antenna on a commercial STM prevents interference with other UHV processes while providing a simple method to retrofit any commercial UHV-STM with UHV-ACSTM capability. Second, mounting the microwave antenna on a translator allows for specific tuning of the system to replicate experimental conditions between samples, which is particularly critical in sensitive systems like organic thin films or single molecules where small changes in incident power can affect the results. Our innovation therefore provides a valuable approach to give nearly any commercial STM, be it an ambient or UHV system, the capability to measure atomic-scale microwave studies such as differential capacitance or even single molecule microwave response, and it ensures that experimental ACSTM conditions can be held constant between different samples.     

STM imaging of molecular collagen and phospholipid membranes

The application of STM to biological materiais has been limited by poor conductivity, sample geometry and stability of biological materials. In this paper we describe an STM study of the monomeric helical forms of collagen, a stable, conductive and widely prevalent structural protein. We have also used STM to image artificial Langmuir DPE (dipalmitoyl phosphatidyl ethanolamine) phospholipid membranes. Both molecular collagen and the phospholipid membranes were dried in air on highly oriented pyrolytic graphite (HOPG). Our STM images of collagen dried on HOPG reveal strands 15Å in diameter with a periodicity of about 30Å which correlates with that known to occur in collagen. Spikes which periodically protrude from strands in our STM images of collagen appear to represent pyrrolidine ring structures in the amino acids proline and hydroxyproline. Thus, we report the first STM imaging of native biomolecules revealing intramolecular details and what appear to be specific amino acids. STM imaging of phospholipid membranes show a lattice pattern with densities spaced ～4–5Å apart. These are thought to represent individual phospholipid molecules in an artificial membrane formed on the HOPG. We believe STM and its related technologies will have great future utility in biomolecular studies.     

STM imaging of noble metal electrodes in solution under potentiostatic control

We have combined a three-electrode cell arrangement with a scanning tunnelling microscope (STM) to image the surfaces of polycrystalline noble metal electrodes under potentiostatic conditions. STM imaging was made in the potential range where a tip and the electrodes are ideally polarized. We have observed on a submicron scale that the electrochemically roughened Ag electrode surface relaxed over times of 40 min at ?0·16 V versus SCE in 0·1 m KCl. We have also imaged a dynamic process (formation and dissociation) of the submicron-scale reconstruction of the Au electrode subjected to electrochemical oxidation-reduction cycles. The results can be explained by the absorption of Cl^? ions and the surface diffusion of adatoms or clusters. STM imaging under potentiostatic control leads to the discovery of phenomena occurring at solid metal/electrolyte interfaces.     

High-pressure scanning tunneling microscopy: tip reactions

Scanning tunneling microscopy (STM) is an ideal tool to image conducting and semiconducting surfaces with atomic resolution. The technique provides high-resolution images in vacuum or even high-pressure environments. Since STM can be operated at elevated pressures and temperatures, images can be collected in situ under catalytic conditions. In this work, we demonstrate that artifacts can be observed when imaging in situ since reactions can occur on the tip, and care should be taken when analyzing the data obtained.     

STM/SEM correlative study of facetted gold

We have integrated an STM unit with a conventional scanning electron microscope in order to perform STM–SEM correlative microscopy. The method is applied to an electrochemically facetted gold sample, which provides a surface structure suitable for this study. We discuss the factors which are relevant in order to obtain a quantitative resolution of the topographic surface structure, by taking advantage of the performances of both techniques. In particular we suggest the use of the STM height distribution as the best parameter for STM/SEM correlation. Finally, from the STM data we deduce that the main process during electrochemical etching is the formation of (111) faces.     

Potential-induced resonant tunneling through a redox metalloprotein investigated by electrochemical scanning probe microscopy.

The redox metalloprotein azurin self-chemisorbed onto Au(1 1 1) substrates has been investigated by electrochemically controlled scanning tunneling (STM) and scanning force/lateral force microscopy (SFM/LFM) and cyclic voltammetry (CV) in aqueous solution. The combined use of STM and SFM/LFM under electrochemical control in the negative side of the azurin redox midpoint (+116 mV vs. SCE) has delivered unique information on the nature of the STM images. While in STM the bright spots, believed to be associated with azurin molecules, are visible only for potential values higher than -125 mV, the concurrent electrochemical SFM results show adsorbed proteins over the whole potential range investigated (from -225 to +75 mV). Stepping the potential back and forth (between -25 and -125 mV) in STM imaging, it has been possible to make bright spots appearing and disappearing repeatedly, indicating that STM image formation arises possibly through resonant tunneling via the redox levels of azurin. These results represent the first clear evidence of potential-dependent tunneling in proteins adsorbed onto a conductive substrate.     

Versatile optical access to the tunnel gap in a low-temperature scanning tunneling microscope

We developed a setup that provides three independent optical access paths to the tunnel junction of an ultrahigh vacuum low temperature (4.2 K) scanning tunneling microscope (STM). Each path can be individually chosen to couple light in or out, or to image the tunnel junction. The design comprises in situ adjustable aspheric lenses to allow tip exchange. The heat input into the STM is negligible. We present in detail the beam geometry and the realization of lens adjustment. Measurements demonstrate the characterization of a typical light source exemplified by emission from tip-induced plasmons. We suggest employing the Fourier transforming properties of imaging lenses and polarization analysis to obtain additional information on the light emission process. Performance and future potential of the instrument are discussed.     

Dependence of tunneling current through a single molecule of phenylene oligomers on the molecular length

The electrical properties of single phenylene oligomers were studied in terms of the dependence of the tunneling current on the length of the oligomers using self-assembling techniques and scanning tunneling microscopy (STM). It is important to isolate single molecules in an insulating matrix for the measurement of the conductivity of the single molecule. We demonstrate here a novel self-assembled monolayer (SAM) matrix appropriate for isolation of the single molecules. A bicyclo[2.2.2]octane derivative was used for a SAM matrix, in which the single molecules were inserted at molecular lattice defects. The isolated single molecules of phenylene oligomers inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights of the molecular protrusions using STM and estimated the decay constant, beta, of the tunneling current through the single phenylene oligomers using a bilayer tunnel junction model.     

STM探针电化学腐蚀装置的设计及实验研究

本文根据扫描隧道显微镜(STM)探针电化学腐蚀方法的特点,在总结其它STM探针电化学腐蚀装置的基础上,自制了STM探针电化学腐蚀装置。并用此装置进行探针制备的实验研究,摸索出了此装置制备高质量钨探针的较佳工艺条件,同时给出了所制探针的STM成像结果。     

近视防控离焦镜片像质评价方法的探讨

近视防控离焦镜片的光学性能评价的过程中,通常以镜片的屈光度地形图作为分析依据,对成像质量的影响目前研究较少.从像质评价入手,搭建了一套镜片?相机测试系统用于测量离焦镜片的光学传递函数(MTF).整个系统是由光源、成像模块、镜片夹持工具、棋盘标定板和MTF分析软件系统组成的.实验过程中对相机镜头、国内明月离焦镜片、国外Z...     

High-resolution scanning tunneling microscopy for molecules

Scanning tunneling microscopy (STM) can detect individual molecular configuration with its high spatial resolution ability, but some intrinsical and extrinsic factors result in the complexities of STM imaging of single molecules. By combining STM experimental work and theoretical simulation with the local density approximation based on Bardeen perturbation method, we have explored the atomic-scale configuration of the following molecular systems: C(60) molecules adsorbed on Si(111)-(7x7); alkanethiol self-assembly monolayers on Au(111); C(60) molecule imaged by STM tip adsorbed with another C(60) molecule; O(2) molecule adsorbed on Ag(110) and CO molecule adsorbed on Cu(111) imaged by CO chemically modified STM tip. Some related problems including: molecule-substrate interactions, STM imaging mechanism, chemically modified STM tip, etc., are discussed.     

Imaging of metal-coated biological samples by scanning tunneling microscopy

A method for imaging biological samples by scanning tunneling microscopy (STM) is presented. There are two main difficulties in imaging biological samples by STM: (1) the low conductivity of biological material and (2) finding a method of reliably depositing the sample on a flat conducting surface. The first of these difficulties was solved by coating the samples with a thin film of platinum-carbon. The deposition problem was solved by a method similar to a procedure used to deposit biological molecules onto field ion microscope (FIM) tips. STM images of bacteriophage T7 and filamentous phage fd are shown. The substrate on which the samples were absorbed was atomically flat gold. The images do not show molecular detail due to the metal coating, but the gross dimensions and morphology are correct for each type of virus. Also, the surface density of virus particles increases and decreases in the way expected when the conditions of deposition are changed. These methods allow reliable and reproducible STM imaging of biological samples.     

Imaging colloidal gold labels in LVSEM

On biological samples, the topographic imaging capabilities of the new generation of scanning electron microscopes (SEM) (those having both field-emission guns and low aberration lenses) rival those of the replica techniques. In addition, they permit the localization of specific molecules on the sample surface using one of several labeling techniques utilizing heavy metal colloids. Normally, colloidal gold can be detected in the SEM both by the secondary electron signal (shape) and by the backscattered electron signal (BSE, Z-contrast). The new instruments seem to produce their best topographic images using low-beam voltage (1–5 kV) where topographic contrast is higher and the required thickness of the metal coating is less (Haggis and Pawley 1988, Ris and Pawley 1988). Although the detection of backscattered electrons is more difficult at low-beam voltage, we are able to show here that the secondary electron (SE) signal produced with a 2–5-kV beam permits the unambiguous detection of gold particles as small as 5 nm on carbon-coated specimens while a 1-kV beam produces a high-quality topographic image of the same sample.     

Imaging biological macromolecules by STM: quantitative interpretation of topographs

Methods are discussed which permit the calibration of x-, y-, z-sensitivities, non-linearities and frequency responses of the scanning device of a scanning tunneling microscope (STM) either by interferometry or directly from STM topographs. A technique is presented to measure the frequency response of the complete STM feedback unit and to derive a maximum speed in z direction which allows one to estimate the maximum scanning speed still permitting one to track surface corrugations. The signal transfer characteristics of a STM are evaluated in a direct comparison with high resolution transmission electron microscopy on an identical specimen area. The various effects of contaminants between tip and specimen and the finite tip radius receive special attention.     

基于图像差分的精密畸变校正研究

图像畸变是影响视觉检测精度的关键因素.通过图像差分建立畸变场微分方程,提出了摄像机光学成像系统畸变场微分方程参数的理论计算方法,实现了基于图像差分的精密视觉检测.首先给出差分成像方式及图像畸变场微分方程建立方法,然后高精度标定摄像机确定畸变场微分方程理想平移量参数,改进Harris角点提取法使其达到亚像素精度,再采用SAD角点特征匹配法确定畸变场微分方程局部位移量参数,最后利用Nelder-Mead-Simplex优化策略实现畸变场微分方程求解,获得图像畸变完整精确的描述.在石油管螺纹梳刀上的实验表明,该方法检测精度高,且仅需要2个平移自由度,极大地简化了图像检测机械装置.     

The FAST module: an add-on unit for driving commercial scanning probe microscopes at video rate and beyond

We present the design and the performance of the FAST (Fast Acquisition of SPM Timeseries) module, an add-on instrument that can drive commercial scanning probe microscopes (SPM) at and beyond video rate image frequencies. In the design of this module, we adopted and integrated several technical solutions previously proposed by different groups in order to overcome the problems encountered when driving SPMs at high scanning frequencies. The fast probe motion control and signal acquisition are implemented in a way that is totally transparent to the existing control electronics, allowing the user to switch immediately and seamlessly to the fast scanning mode when imaging in the conventional slow mode. The unit provides a completely non-invasive, fast scanning upgrade to common SPM instruments that are not specifically designed for high speed scanning. To test its performance, we used this module to drive a commercial scanning tunneling microscope (STM) system in a quasi-constant height mode to frame rates of 100 Hz and above, demonstrating extremely stable and high resolution imaging capabilities. The module is extremely versatile and its application is not limited to STM setups but can, in principle, be generalized to any scanning probe instrument.