The STM learning curve and where it may take us*

The scanning tunnelling microscope (STM) has proved to be an extraordinary method to investigate surfaces in vacuum, air and liquid environments. Several issues regarding the use of the STM for atomic resolution studies are discussed. These include electronic contributions to STM images, the role of the tip in resolution and spectroscopy, as well as the need for complementary information about chemical composition or sub-surface structure.     

Scanning probe microscopy of biological samples and other surfaces

Scanning probe microscopes derived from the scanning tunnelling microscope (STM) offer new ways to examine surfaces of biological samples and technologically important materials. The surfaces of conductive and semiconductive samples can readily be imaged with the STM. Unfortunately, most surfaces are not conductive. Three alternative approaches were used in our laboratory to image such surfaces. 1. Crystals of an amino acid were imaged with the atomic force microscope (AFM) to molecular resolution with a force of order 10^?8 N. However, it appears that for most biological systems to be imaged, the atomic force microscope should be able to operate at forces at least one and perhaps several orders of magnitude smaller. The substitution of optical detection of the cantilever bending for the measurement by electron tunnelling improved the reliability of the instrument considerably. 2. Conductive replicas of non-conductive surfaces enabled the imaging of biological surfaces with an STM with a lateral resolution comparable to that of the transmission electron microscope. Unlike the transmission electron microscope, the STM also measures the heights of the features. 3. The scanning ion conductance microscope scans a micropipette with an opening diameter of 0·04-0·1 μm at constant ionic conductance over a surface covered with a conducting solution (e.g., the surface of plant leaves in saline solution).     

A cryogenic Quadraprobe scanning tunneling microscope system with fabrication capability for nanotransport research

We describe the development and the capabilities of an advanced system for nanoscale electrical transport studies. This system consists of a low temperature four-probe scanning tunneling microscope (STM) and a high-resolution scanning electron microscope coupled to a molecular-beam epitaxy sample preparation chamber. The four STM probes can be manipulated independently with subnanometer precision, enabling atomic resolution STM imaging and four-point electrical transport study of surface electronic systems and nanostructured materials at temperatures down to 10 K. Additionally, an integrated energy analyzer allows for scanning Auger microscopy to probe chemical species of nanostructures. Some testing results are presented.     

Surface investigations with a combined scanning electron–scanning tunneling microscope

We have combined a scanning tunneling microscope (STM) with a scanning electron microscope (SEM) for surface investigations of atomically flat surfaces, ultrathin adsorbate films, and material surfaces. The mechanical stability of the hybrid instrument allows high-resolution SEM of samples mounted on the STM stage and atomic resolution with the STM. Experimental results of combined SEM/STM investigations on textured material surfaces, submicron structures, and atomically flat conducting surfaces are presented. An example is given for surface machining with the STM under SEM control.     

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.     

Donor graphite intercalation compounds studied with a high stability STM

We have designed a small, rigid and easy operable scanning tunnelling microscope (STM) which shows low thermal drift rates due to its compact construction and the chosen materials. Using this instrument we have studied stage 1 donor graphite intercalation compounds (C₆Li and C₈K) with atomic resolution either in the current imaging, constant current or local tunnelling barrier height mode of operation. Ordered superlattices commensurate as well as incommensurate with the graphite lattice have been observed on the C₆Li surface. STM images on C₈K revealed a graphitic surface structure.     

Combining scanning tunneling microscopy and synchrotron radiation for high-resolution imaging and spectroscopy with chemical, electronic, and magnetic contrast

The combination of high-brilliance synchrotron radiation with scanning tunneling microscopy opens the path to high-resolution imaging with chemical, electronic, and magnetic contrast. Here, the design and experimental results of an in-situ synchrotron enhanced x-ray scanning tunneling microscope (SXSTM) system are presented. The system is designed to allow monochromatic synchrotron radiation to enter the chamber, illuminating the sample with x-ray radiation, while an insulator-coated tip (metallic tip apex open for tunneling, electron collection) is scanned over the surface. A unique feature of the SXSTM is the STM mount assembly, designed with a two free-flex pivot, providing an angular degree of freedom for the alignment of the tip and sample with respect to the incoming x-ray beam. The system designed successfully demonstrates the ability to resolve atomic-scale corrugations. In addition, experiments with synchrotron x-ray radiation validate the SXSTM system as an accurate analysis technique for the study of local magnetic and chemical properties on sample surfaces. The SXSTM system's capabilities have the potential to broaden and deepen the general understanding of surface phenomena by adding elemental contrast to the high-resolution of STM.     

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.     

Imaging an optical disc by the combined use of scanning tunnelling microscopy and scanning electron microscopy

We present the data obtained by scanning tunnelling microscopy combined with scanning electron microscopy of the digitally encoded structure on a stamper used to fabricate optical discs. The combination allows us to focus the STM tip on a preselected spot with a precision of ?0·3 μm. The data show the superiority of STM for a more detailed characterization of shape, width, length, height and fine structure appearing on the sample. We also show the influence of tip shape on STM resolution. Simultaneous use of both microscopes is possible but high electron doses produce an insulating layer of contaminants thick enough to make STM operation impossible.     

Hybrid scanning transmission electron/scanning tunnelling microscope system for the preparation and investigation of biomolecules

A hybrid scanning transmission electron/scanning tunnelling microscope vacuum system is introduced, which allows freeze drying and metal coating of biological samples and their simultaneous observation by scanning transmission electron microscopy and scanning tunnelling microscopy (STM). Different metal coatings and STM tips were analysed to obtain the highest possible resolution for such a system. Bovine liver catalase was used as a test sample and the STM results are compared to a molecular scale model.     

Scanning tunneling microscope combined with scanning electron microscope

A scanning tunneling microscope (STM) has been installed in a usual scanning electron microscope (SEM) with a vacuum of 10⁻⁶ Torr. The STM image is displayed on the cathode ray tube of the SEM, 512 × 512 pixels, with a scanning rate of 80 s/picture. The spatial resolution of the STM is about 1 Å, while that of the SEM is several tens of ångströms. The combined scanning microscope covers a wide magnification range from 10 to 10⁷, where STM covers the high magnification region from 10⁵ to 10⁷.     

Real-time scanning tunnelling microscopy of surfaces under active electrochemical control

A scanning tunnelling microscope has been designed which allows tunnelling microscopy to be performed in the presence of an externally applied electrochemical current. Separate, isolated electrodes were used for electrochemical control, and up to 1 mA was passed during real-time, video-rate, in situ STM observation of the surfaces, without interfering with the operation of the STM. The noise level of these STM images is only slightly higher than images taken with the electrochemical circuitry disconnected. Surfaces were observed during the formation of surface films in aqueous electrolytes.     

10-kV diffractive imaging using newly developed electron diffraction microscope

A new electron diffraction microscope based on a conventional scanning electron microscope (SEM), for obtaining atomic-level resolution images without causing serious damage to the specimen, has been developed. This microscope in the relatively low-voltage region makes it possible to observe specimens at suitable resolution and record diffraction patterns. Using the microscope we accomplished 10-kV diffractive imaging with the iterative phase retrieval and reconstructed the structure of a multi-wall carbon nanotube with its finest feature corresponding to 0.34-nm carbon wall spacing. These results demonstrate the possibility of seamless connection between observing specimens by SEM and obtaining their images at high resolution by diffractive imaging.     

In-situ STM studies of electrochemical underpotential deposition of Pb on Au(111)

A modified scanning tunnelling microscope (STM) has been used to observe in-situ deposition and stripping of an electrochemical film. With STM tip and sample immersed in an acid electrolyte, single atomic steps on Au(111) have been imaged during the deposition and stripping of a monolayer-thick, underpotential deposit (UPD) of Pb. Integration of the electrochemical current passed during the film deposition and evidence from the STM images themselves confirm monolayer coverage. Our images show enhanced film growth at step edges and defect sites. Observations of single plating and stripping cycles indicate that the Au substrate returns unaltered. Except for atomic resolution images of Au(111), which we have not yet achieved in an electrolyte, all types of Au surface features seen in air are reproduced under the electrolytic solution. The modifications made to our STM in order to perform in-situ electrochemical experiments are described.     

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.     

A compact sensor head for simultaneous scanning force and near-field optical microscopy

A compact sensor head based on scanning force microscopy (SFM) using cantilever probes has been developed. The idea is to replace the microscope objective of a conventional optical microscope by this compact module and turn the optical microscope into a scanning force and near-field optical microscope with subwavelength resolution. We describe our concept and present initial results showing images of the object’s optical properties and surface topography recorded simultaneously.     

A cooling system for samples for studying their surfaces in ultrahigh vacuum

The device described is designed to cool samples with liquid nitrogen to T=?165°C during low-energy electron diffraction (LEED) studies at an ultra-high-vacuum MALTIPROBE Compact system (Omicron) and versions thereof equipped with a scanning tunneling microscope and an LEED system. The efficiency of the system is demonstrated using the example of a low-temperature 2×1 ? c(4×2) phase transition on a (100)-oriented silicon surface observed using the LEED technique.     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

STM study of the structure of the sulphur (1×2) overlayer on molybdenum (001) in air: ordered domains,phase boundaries and defects

A (001) surface of molybdenum, covered by one monolayer of sulphur was prepared in UHV and characterized by LEED, Auger and XPS. This surface was found to be stable in air for periods of several days. STM images of the surface, obtained in air in the topographic and local barrier height modes, revealed the atomic arrangement of sulphur atoms in domains with 1×2 and 2×1 periodicities. Boundaries between domains, adsorbate and substrate defect structures and crystallites formed during the initial oxidation of the Mo substrate were observed.     

石英音叉扫描探针显微镜

石英音叉是一种谐振频率稳定、品质因数高的时基器件,其音叉臂的谐振参数(谐振振幅和谐振频率)对微力极其敏感。利用石英音叉对外力的敏感性,与钨探针结合,构成一种新型的表面形貌扫描测头。该测头与xyz压电工作台结合,利用测头音叉臂谐振频率对扫描微力的敏感性,研制基于相位反馈控制的扫描探针显微镜。首先介绍石英音叉测头的构成、工作原理和特性测试,以及由该测头构建的扫描探针显微镜的结构和测试、分析。通过对测头和系统的测试结果分析,系统达到1.2 nm的垂直分辨率,并通过对一维栅的测量,给出扫描获得的试样表面微观形貌图以及相位图,证明系统的有效性。另外,由于采用大长径比的钨探针,该系统具有测量大深宽比微器件表面轮廓的能力。