A combined scanning tunnelling and field ion microscope

The construction of a combined scanning tunnelling and field ion microscope allowing the in situ preparation and analysis of the tunnelling tip is described. The apparatus is based on the Besocke type STM and includes a fast, automated sample approach, a sample transfer manipulator and a tip-cooling mechanism for the FIM operation mode. The design is simple and needs a mechanical feedthrough only for sample transfer. A method has been developed to produce sharp tungsten tips with small clusters on the apex plane. Test-run results of FIM and STM operation modes are discussed.     

Tip to substrate distances in STM imaging of biomolecules

STM images of single biomolecules adsorbed on conductive substrates do not reproduce the expected physical height, which generally appears underestimated. This may cause the tip to interfere with the soft biological sample during the imaging scans. Therefore, a key requirement to avoid invasive STM imaging is the knowledge, and the control, of the initial tip to substrate distance. This is connected to the setting of the tunnelling current and applied voltage, which define a tunnelling resistance. The height of the STM tip was measured by calibrating the tunnelling resistance, as a function of its vertical displacement until establishing a mechanical contact. At a tunnelling resistance of 4 x 10(9)Omega, distances of about 3 and 6 nm are estimated when flat Au substrates are imaged in water and in air, respectively. On such a ground, the relevance of the starting tip-substrate distance in determining a non-invasive imaging has been investigated for a plastocyanin mutant chemisorbed on Au(111) electrodes. At tunnelling distances sufficient to overcome the physical height of the imaged biomolecules, their lateral dimensions are found to be consistent with the crystallography, whereas they are significantly broadened for smaller distances.     

Electrically conductive and optically transparent Sb-doped SnO2 STM-probe for local excitation of electroluminescence

We demonstrate that an optically transparent and electrically conductive antimon-doped tin-oxide tip that is prepared in a sol-gel process can be used as a probe for scanning tunnelling microscopy (STM), yielding atomic vertical and nanometre lateral resolution. Emission of visible light from the tunnelling junction between gold particles and the tip is observed for bias voltages above 7 V. In contrast to the metallic tips generally used in STM, this tip does not significantly perturb the local optical response. Therefore, the tunnelling induced light can be used to map the optical near-field of surface structures with the tunnel gap acting as highly localised light source for the investigation of near-field enhancement in complex metal structures.     

Scanning tunnelling and atomic force microscopy performed with the same probe in one unit

The technique demonstrated here provides features of both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). The metallic probe acts to record current variations and sense forces from the same sample area simultaneously. Thus, separate images may be recorded, in registry. The collected data allows real space correlations between some electrical properties and the geometric structure of a sample surface. The same tip is used since the geometry and condition of the tip can effect the data recordings. Platinum alloys, tungsten and graphite tips have been employed successfully. An AFM lever can respond to surface contact forces, within the elastic limits of the sample, while electric current is sensed by the tip of the lever. The usefulness of this experimental procedure is tested here by an application to semiconducting samples of Ag-doped CdTe in air and in paraffin oil media.     

Atomic force microscopy of a hydrated bacterial surface protein

The protein surface layer of the bacterium Deinococcus radiodurans (HPI layer) was examined with an atomic force microscope (AFM). The measurements on the air-dried, but still hydrated layer were performed in the attractive imaging mode in which the forces between tip and sample are much smaller than in AFM in the repulsive mode or in scanning tunnelling microscopy (STM). The results are compared with STM and transmission electron microscopy (TEM) data.     

Force sensing in scanning tunnelling microscopy: observation of adhesion forces on clean metal surfaces

Tip sample interaction forces were investigated during normal tunnelling operation of the STM using an Ir tip and a polycrystalline Ir sample. Metallic adhesion interaction was observed for tunnel conductivity ranging from 10^?6 to 19^{?9 Ω-1} implying that the actual gap width was of the order of 1–4 Å. Similar experiments performed on a polycrystalline Al sample exposed to 1 Langmuir O₂ showed that tip sample interaction changed from attractive to repulsive on well-defined areas extending over ～100 Ų which we identified with the oxidized Al surface.     

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.     

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.     

Preparation of tips coated with poly(dimethylsiloxane) for scanning tunneling microscopy in aqueous solutions

A simple and reliable method for preparing tips coated with poly(dimethylsiloxane) (PDMS) for scanning tunneling microscopy (STM) in aqueous solutions is reported. Au STM tips were dip-coated with PDMS and subsequently cured. The coated tips were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and voltammetry. We show that the PDMS layer smoothly covers the tip surface except for its very apex. These tips enable atomically resolved imaging of a sample surface and measurement of single-molecule conductance below 1.0 nS with STM in aqueous solutions. Because of the superior compatibility of PDMS with organic solvents, the PDMS-coated tips would be useful for preparing molecular tips via their chemical modification in such solvents, enabling chemically selective imaging of a single molecule in water.     

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.     

A fully automated, ‘thimble-size’ scanning tunnelling microscope

A novel, fully automated high-stability, high-eigenfrequency scanning tunnelling microscope (STM) has been developed. Its key design feature is the application of two piezoelectric ceramic tubes, one for the x-y-z motion of the tip and one for a linear motor (‘nano-worm’) used for the coarse positioning of the tip relative to the specimen. By means of the nano-worm, the tip can be advanced in steps between 16 and 0·2 nm. The walking distance is >2 mm, with a maximum speed of 2000 steps/s. The nano-worm positioning implies that this STM is fully controlled by electronic means, and that no mechanical coupling is needed, which makes operation of the STM extremely convenient. The axial-symmetry construction is rigid, small and temperature-compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. The sample is simply placed on a piece of invar which surrounds the scanner tube and the nano-worm and is held by gravity alone. This allows for easy sample mounting. The performance of the microscope has been tested in air by imaging a variety of surfaces, including graphite and biological samples.     

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。     

Reproducible tip fabrication and cleaning for UHV STM

Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatments of the scanning tunneling microscope (STM) tips have been implemented to improve a reliability of the tip preparation for high-resolution STM. Widely used electrochemical etching drop-off technique has been further refined to enable a reproducible fabrication of the tips with a radius 相似文献   

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.     

The importance of current density ‘focusing’ in STM image resolution

In a previous paper we incorporated Tersoff and Hamann's model of an STM tip into Lang's transfer Hamiltonian result for the tunnelling current density and applied the resulting simple expression to Tersoff's six-plane-wave model of a monolayer of graphite. In this paper the results for a more realistic model of a graphite surface and a more complete selection of tip positions are presented. They support the previous conclusions that the normal component of the current density takes on both positive and negative values in a complex flow pattern and that its lateral falloff away from the (projected) tip position is much slower than expected from the extraordinary lateral resolution (～1 Å) evident in the best STM images of graphite. This shows that sharp ‘focusing’ of the current density directly under the tip is not a necessary condition for high lateral resolution.     

Differential scanning tunnelling microscopy

We review the principle of differential imaging and its application to scanning tunnelling microscopy (STM). It is shown that placing a lateral dither on an STM tip at high frequency provides the means for transfering topographic information to a frequency range where noise is small. Differential STM imaging on graphite and gold is demonstrated. A simple relation between the differential image and the conventional topographic image is described.     

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.     

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.     

Fabrication of silver tips for scanning tunneling microscope induced luminescence

We describe a reliable fabrication procedure of silver tips for scanning tunneling microscope (STM) induced luminescence experiments. The tip was first etched electrochemically to yield a sharp cone shape using selected electrolyte solutions and then sputter cleaned in ultrahigh vacuum to remove surface oxidation. The tip status, in particular the tip induced plasmon mode and its emission intensity, can be further tuned through field emission and voltage pulse. The quality of silver tips thus fabricated not only offers atomically resolved STM imaging, but more importantly, also allows us to perform challenging "color" photon mapping with emission spectra taken at each pixel simultaneously during the STM scan under relatively small tunnel currents and relatively short exposure time.     

Single-electron effects observed with a low-temperature STM

We have used a low-temperature scanning tunnelling microscope to study the effect of the Coulomb charging energy on the tunnelling behaviour of low capacitance point-contact junctions. The tunnelling I-V characteristics between a tungsten tip and various materials, such as stainless steel, aluminium, carbon and YBa₂Cu₃O_7-δ, show a quadratic behaviour at low voltages and a displaced asymptotic behaviour at high voltages. The I-V characteristics can be quantitatively understood using the model of single-electron tunnelling induced by the Coulomb blockade. The capacitances of this type of point-contact tunnel junctions are in the 10^?18 F range, and are adjustable by varying the distance between tip and surface. These capacitances are at least two orders of magnitude lower than can presently be achieved by electron lithography. In a series configuration of two low-capacitance tunnel junctions, with an isolated small particle as a common electrode, we have observed the so-called Coulomb staircase, due to the quantization of the charge on a single small particle. The experimental results are in good agreement with semi-classical Monte-Carlo simulations. The low capacitance of an STM assembly can cause a serious complication in the interpretation of low temperature spectroscopic data. We will discuss the implications for the measurements on superconductors, and for the case of inelastic tunnelling spectroscopy.