Design and operation of a variable temperature scanning tunnelling microscope

A new variable temperature STM has been developed which utilizes two concentric piezoelectric tubes; an inner scanning tube, and an outer thermal compensation tube which also provides for inertial translation of the sample into tunnelling range. With this design, continuously variable temperature operation is demonstrated for the first time in an STM. Also, by eliminating all mechanical components such as springs, levers and gears, which normally couple directly to the tunnelling gap in other designs, atomic resolution operation is demonstrated in which no vibration isolation is necessary. During operation, the inside of the scanning tube is maintained at ground potential while the feedback signal is electronically summed to the scanning voltages applied to the outer quadrants. In addition to shielding the sensitive tunnelling circuit, this mode of operation enables one to electronically balance out mechanical imperfections of the scanning tube. To date, this new STM has been operated over the 77–400 K temperature range with the observed thermal drift as low as 1 Å/h and 10 Å/K. Another useful feature of this new design is the ability to reposition a sample to within 200 Å of the same location after it has been translated macroscopic distances (several mm) out of tunnelling range.     

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.     

Preparation of STM tips for in-situ characterization of electrode surfaces

The total current between the tip and the sample in a scanning tunnelling microscopy study of a solid/liquid interface can be dominated by Faradaic charge transfer currents. In such a situation, feedback control of the tunnelling gap, and imaging, is precluded. In this contribution we describe the preparation of glass and polymer coated STM tips that possess < 100 Ų of exposed metal. These tips effectively discriminate against Faradaic current and enable STM imaging in the presence of reversible electroactive solution species at appreciable tip/sample biases.     

SNOM/STM using a tetrahedral tip and a sensitive current-to-voltage converter

Scanning near-field optical microscopes (SNOM) using the tetrahedral-tip (T-tip) with scanning tunnelling microscopy (STM) distance control have been realized in transmission and reflection mode. Both set-ups used ordinary STM current-to-voltage converters allowing measurement of metallic samples. In the transmission mode, a resolution of 10 nm to 1 nm with regard to material contrast can be achieved on binary metal samples. Because of the great near-field optical potential of the T-tip with respect to the optical resolution, it is a challenging task to find out whether these results can be transferred to non-metallic sample systems as well. This paper reports on a newly designed SNOM/STM transmission mode set-up using the tetrahedral-tip. It implements a sensitive current-to-voltage converter to widen the field of measurable sample systems. Beyond this, mechanical and optical measuring conditions are substantially improved compared to previous set-ups. The new set-up provides a basis for the routine investigation of metal nanostructures and adsorbed organic monolayers at resolutions in the 10 nm range.     

Digital filters to restore information from fast scanning tunnelling microscopy images

A transfer function—similar to that used in optical cases to correct blurring effects due to the circular aperture of the system—is presented here to restore scanning tunnelling microscopy (STM) images. Due to the conical geometry of the tip-sample system, we have established an analogy between the process of image formation in STM and in optical systems. The transfer function utilized, similar to that calculated by Stokseth, allows us to differentiate between the blurring effects introduced along the x and y axes. These effects are different due, mainly, to the different velocities achieved along the x and y directions. Furthermore we have measured the β parameter that characterizes the classical 1/fβ noise present in STM data, demonstrating its independence from experimental conditions. A Wiener filter is utilized to restore the images using the previous assumptions given for the transfer function and noise effects.     

Low-temperature scanning tunnelling microscopy and spectroscopy on Pb and Au

We have designed and built a scanning tunnelling microscope (STM) in order to work at temperatures ranging from 1·2 to 300 K. Tunnelling spectroscopy has been performed in Au and Pb with this STM. Our results on Au show different conductance-voltage behaviours, their relation with the cleaning state of the tip being discussed. For Pb, the fitting of our I-V characteristics to a BCS density of states gives A = 1·25 meV for the superconducting energy gap at about 5 K.     

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.     

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.     

Spatially resolved ballistic electron spectroscopy of subsurface interfaces

Buried interfaces have traditionally been inaccessible to direct investigation by surface-sensitive techniques. A unique and novel electronic probe, which is sensitive to subsurface electronic structure, has been utilized to probe Schottky barrier interfaces. The method, ballistic electron emission microscopy (BEEM), is based on scanning tunnelling microscopy (STM) techniques. A theoretical treatment has been developed to describe the ballistic electron spectra obtained by BEEM and this treatment is applied to data obtained on the Au-Si Schottky barrier interface system. Excellent agreement between experiment and theory is obtained. In addition, the treatment pradicts nanometre spatial resolution for interface imaging by BEEM.     

Images of 16S ribosomal RNA by scanning tunnelling microscopy

We report the use of scanning tunnelling microscopy (STM) to study surface topographies of complex nucleic acid structures. From low-resolution STM images of uncoated 16S ribosomal RNA, we demonstrate the possibility of determining several objective parameters (molecular mass and radius of gyration) in order to characterize and identify the molecules observed. These parameters were compared with values obtained by other physical methods and the radius of gyration was found to be the most reliable. At high resolution, it was possible to measure the main dimensions of selected V-form particles more precisely than with electron microscopy. Images of the more compact form have been also obtained that show different domains in the macromolecular structure.     

钢中下贝氏体组织的扫描隧道显微镜研究

在大汽环境中利用扫描隧道显微镜（ＳＴＭ）观察钢铁表面的精细组织结构。对照光学显微镜（ＯＭ）、扫描电子显微镜（ＳＥＭ）、透射电子显微镜（ＴＥＭ）的观察结果,民得的钢铁的组织形貌进行分析讨论。初步建立ＳＴＭ下图像形貌和组织之间的对应关系。利用ＳＴＭ优良的表面垂直分辨能力。观察到贝氏体铁素体上人有比马氏体铁素体更高的耐蚀性。     

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.     

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.     

Biological structures imaged in a hybrid scanning transmission electron microscope and scanning tunneling microscope

A hybrid scanning transmission electron microscope (STEM) and scanning tunneling microscope (STM) is described which allows simultaneous imaging of biological structures adsorbed to electron-transparent specimen supports in both modes of scanning microscopy, as demonstrated on uncoated phage T4 polyheads. We further discuss the reproducibility and validity of height data obtained from STM topographs of biomacromolecules and present raw data from topographs of freeze-dried, metal-coated nuclear envelopes from Xenopus laevis oocytes.     

Electronic property investigations of single-walled carbon nanotube bundles in situ within a transmission electron microscope: an evaluation

We have evaluated a combined transmission electron microscopy (TEM)–scanning tunnelling microscopy (STM) (hence TEM-STM) sample holder for the investigation of the mechanical and electrical properties of individual bundles of single-walled carbon nanotubes (SWCNTs) together with their simultaneous observation, analysis and mechanical modification in the TEM. Current-voltage (I–V) measurements from bundles of SWCNTs were observed to change as the bundles were deformed both reversibly and irreversibly, although the observed behaviour was somewhat complex. Electron energy loss spectroscopy (EELS) analysis revealed measurable changes in the bonding of the carbon atoms within the graphene layers upon bundle deformation, with measurable changes in the π*/(π*+σ*) peak ratios observed at the carbon K-edge. Reversible deformation of the bundles was consistent with the sensitivity of σ bonding to deviations from nonplanarity, whereas irreversible deformation was consistent with the introduction of nonhexagonal defects into the graphene sheets.     

STM of metal embedded and coated DNA and DNA–protein complexes

Bare and Pt/Ir/C-coated DNA has been analysed using scanning tunnelling microscopy (STM). To achieve reproducible imaging of bare DNA on mica ethanol/air-dried molecules were embedded in Pt/C. By peeling the metal film off the mica, the previously mica-exposed side of the Pt/C-film with the embedded DNA molecules was accessible for STM analysis. By applying this replica/anchoring technique only hollow trenches in the metal film, and not the DNA itself, could be visualized. The gaps averaged 3.1 nm (± 0.9 nm) wide and 1 nm (± 0.5 nm) deep. Using scanning force microscopy it could be confirmed that the DNA remained in the Pt/C film during the peel-off procedure. For STM, DNA fragments were also coated with 0.7–1 nm Pt/Ir/C. Owing to the high Z-resolution the STM samples were coated at a high elevation angle (65°), thereby minimizing the problem of self-shadowing. Coating by Pt/Ir/C allowed routine imaging and quantitative analysis of both ethanol/air- and freeze-dried DNA under atmospheric conditions. After ethanol/air drying measured values for DNA width and height were 5.1 nm (± 1.8 nm) and 0.9 nm (± 0.2 nm), respectively. Freeze-dried DNA averaged 4.2 nm (± 1.3 nm) wide and 1.1 nm (± 0.1 nm) high. A Pt/Ir/C-coating was also applied to visualize DNA–protein interaction using STM.     

Study of 4Hb-TaS2 and graphite intercalation compound by STM/STS

Investigations of charge-density waves (CDW) in 4Hb-TaS₂ and the shifted Fermi level in a CoCl₂ graphite intercalation compound (GIC) were performed by scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) A discrimination of the surface electronic structure of the octahedral structure and of the trigonal-prismatic structure in 4Hb-TaS₂ was made successfully in terms of the electronic properties. The measured value of the charge-density-wave gap in 4Hb-TaS₂ at room temperature was about 85 meV. The shifted Fermi level in CoCl₂-GIC was measured to be about 400 meV.     

Fractal characterization by frequency analysis: III. Effect of noise

The effect of noise in the fractal characterization by frequency analysis of surface images obtained by scanning tunnelling microscopy (STM), atomic force microscopy (AFM) or profilometry has been studied. The origin of noise and its relationship to the signal is discussed. A procedure to simulate noisy images is presented. From the study it is concluded that the method usually used to characterize noise in STM is not valid and it is shown that fractal characterization of surfaces when noise is present by traditional frequency analysis methods is not possible. A new method to perform both the noise characterization and the fractal characterization of surfaces when noise is present is proposed.     

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.     

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.