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.     

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.     

A scanning tunnelling microscope for operation in air

In this paper a scanning tunnelling microscope for operation in air is described with emphasis on the coarse and fine sample positioning mechanism, piezoelectric scanner and vibration isolation. Line scan (amplitude modulation), top view (brightness modulation) and combined shaded topographical images of the surface of highly orientated pyrolitic graphite are shown.     

Photon emission experiments with the scanning tunnelling microscope

We have detected the light emitted from an STM at both ultraviolet (9·5 eV) and optical (1–4 eV) energies. We show that this light contains spectroscopic information on the sample surface comparable to conventional inverse photoelectric spectroscopy, but with nearly atomic spatial resolution. At optical energies we found sufficiently high intensities to allow spatial imaging of the emission probability. We propose that the high quantum efficiency is due to resonance phenomena, and present fluorescence spectra of the emitted light that support this view. We believe that atomic resolution inverse photoemission microscopy and spectroscopy will provide an important new dimension to surface studies.     

A small scanning tunnelling microscope with large scan range for biological studies

We have developed a scanning tunnelling microscope specially designed for biological applications presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5×20 μm with a vertical range of 20 μm and the total size of the system does not exceed 10×40 mm. The piezo-sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d₁₃ of ?1·71 Å/V at low voltages (under a few volts) going up to ?2 Å/V for higher voltages. Large-scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.     

An entirely enclosed scanning tunnelling microscope capable of being fully immersed in liquid helium

We present an ultrahigh‐vacuum (UHV)‐sealed high‐stability scanning tunnelling microscope (STM) that can be entirely immersed in liquid helium and readily used in a commercial Dewar or superconducting magnet. The STM head features a horizontal microscanner that can become standalone and ultrastable when the coarse approach inertial motor retracts. Low voltage is enough to operate the STM even at low temperature owing to the powerful motor. It is housed in a tubular chamber of 49 mm outer diameter, which can be pumped via a detachable valve (DV), UHV‐sealed and remain sealed after the DV is removed. The entire so‐sealed chamber can then be inserted into liquid helium, where in situ sample cleavage is done via vacuum bellow. This allows sample cleavage and STM measurements to take place in better UHV with higher cooling power. Quality atomic resolution images of graphite and charge density wave on 1T‐TiSe₂ taken in ambient and 14 K conditions, respectively, are presented.     

Imaging of granular high-Tc thin films using a scanning tunnelling microscope with large scan range

STM images are presented in the micron scale, taken in UHV with a single-tube scanner STM with SEM control of the tip position. For calibration, a carbon grid was used from which the coupling of x and y scan axes has been determined as well as the piezo-sensitivity factors. Images taken of a YBa₂Cu₃O_7-δ, film display the granular structure of the crystallites formed during the post-evaporation annealing. A direct comparison of STM scans with SEM micrographs was made which demonstrates the complementary information obtained by the two methods. The obtained resolution is compared to the tip shape which becomes crucial for the imaging of corrugated surfaces on the micron scale. With a simple geometric model, an attempt has been made to reconstruct the surface topography from STM scans based on the knowledge of the tip shape.     

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.     

Lateral resolution of the scanning tunnelling microscope

A theoretical study of the current distribution in a model which extracts the prominent characteristics of a tip-surface geometry in a scanning tunnelling microscope is presented. The sample is a Sommerfeld metal with a planar surface while the tip, also made of a Sommerfeld metal with a planar surface, presents a hemispherical protrusion. Schrödinger's equation is solved by using a form of the finite element method suitable to treat the three-dimensional tunnelling problem. We have derived the current distribution for several values of the protrusion radius and for various gaps between the electrodes. From these results, we investigate the ideal resolution of the scanning tunnelling microscope for realistic values of these geometrical parameters. Though applied to a rather simple model, at this stage, our method appears to be a suitable scheme for further, more refined computations, accounting for the detailed atomic structure of the electrodes.     

Automatic bias-reduction controller for a scanning tunneling microscope

In order to protect the sample and the tip against current transients in a scanning tunneling microscope, which in most cases damages the scanned surface and the tip, when using a bias higher than 1V, we have designed a simple and low-cost circuit that limits the tunneling current. During the evolution of the current transient, when the current exceeds a pre-determined value, a fast feedback control mechanism immediately reduces the bias and prevents the current transient from developing. In addition, we designed a fast pre-amplifier that works with this controller. We have shown that this mechanism provides a better scanning image compared to a system without such a mechanism.     

Secondary electron imaging in the variable pressure scanning electron microscope

Secondary electron imaging is not possible in the variable pressure scanning electron microscope because the mean free path of the secondaries in the gas is too short to permit them to reach the detector. This paper therefore investigates an alternative strategy for producing an image containing significant amounts of secondary electron contrast. This involves modifying the microscope by the addition of a biased electrode above the sample and then collecting a specimen current signal. This system, originally described by Farley and Shah (1988), is found to produce true secondary electron detail over a wide range of conditions.     

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.     

Optical-fibre scanning near-field optical microscope for cryogenic operation

We have developed a new type of scanning near-field optical microscope (SNOM) utilizing optical fibres. The probe tip is controlled by shear force feedback with a fibre interferometer and signal light is collected directly by a multimode fibre. These features make the SNOM head more compact and less sensitive to vibration. Further advantages of this new type of SNOM are that it obviates the need for optical windows in the cryostat and offers easy optical alignment.     

A graphite study with a new air operating scanning tunnelling microscope

A scanning tunnelling microscope working in air with good atomic resolution is described. Atomic corrugation of graphite samples cleaved in air has been observed with a lateral resolution considerably less than 1 Å. The height of the potential barrier is measured as a function of the time the system tip-sample is exposed to air. An explanation for the observed giant corrugation is given.     

In situ fabrication and regeneration of microtips for scanning tunnelling microscopy

Progress in STM, in particular for the interpretation of the tunnel signals of corrugated surfaces, is related to the possibility of sharpening and regenerating in situ reproducible tips for use as the probe. We proposed two techniques for the production of tips with controlled geometry at the atomic level. These two techniques are based on the production of tips with equilibrium profiles obtained under heat treatments in vacuum and in the presence of an electric field. Based on the pseudo-stationary profile principle, the microtips can be sharpened in situ and in a reproducible manner, either from initial cylindrical (111)-W wire, or from breakdown tips; in both cases without the need for heavy control devices (like FIM for example).     

Imaging of individual atoms on an Al(111) surface by scanning tunnelling microscopy

Due to the delocalized character of metal valence electrons the atomic corrugation of metal surfaces observed in Scanning Tunnelling Microscopy (STM) is found to be much smaller than in the case of semiconductor surfaces. In fact, there is only a single study in the literature which reports the resolution of the individual atoms on a metal surface (Hallmark et al., 1987). The present paper demonstrates the resolution of individual atoms on a close packed surface of a nearly free electron metal, Al(111), and presents systematic experiments on the physical origin of this phenomenon. A more detailed discussion will be given elsewhere (Wintterlin et al., 1988).     

扫描隧道显微镜系统的优化研究

对扫描隧道显微镜(STM)系统进行了优化研究,实现了对环境振动的隔绝和电噪声的屏蔽,并在此基础上,设计并完成CCD显微监测系统,实现了对样品—探针逼近过程的实时临控。对整个系统进行了联合调试,成功获得高定向热解石墨和金表面的结构图像。     

Modification of the sample holder for a variable temperature scanning tunneling microscope (Omicron)

The design of a sample holder for a variable temperature scanning tunneling microscope (VT STM (Omicron)) with a variable sample temperature is described. This design considerably extends the range of investigated materials whose surface structure is sensitive to low concentrations of contaminations. The device is manufactured on the basis of the components of a standard holder with the possibility of heat-treating samples in a temperature range of 100–1500 K. The working capacity of the modified sample holder was demonstrated for an example of obtaining a Si(100)?2 × 1 surface with an ultimately low concentration of structural defects.     

Surface structures of YBa2Cu3O7-x,BiCa1·7Sr0·7Cu2Ox and TlCaBaCuO4·5±x investigated by scanning tunnelling microscopy

Surface structures of the high-T_c superconductors YBa₂Cu₃O_7-x, BiCa_1·7Sr_0·7Cu₂O_x and TlCaBaCuO_4·5±x have been investigated with scanning tunnelling microscopy. The observed features include well-organized stripe corrugations in Y-Ba-Cu-O as well as orientated flake-like structures and steps with various heights in the Bi- and Tl-compounds. These observations suggest strong local variations of the elastic and electronic properties of the investigated materials.     

An experimental model of beam broadening in the variable pressure scanning electron microscope

In the variable pressure scanning electron microscope (VP-SEM) the incident electrons pass through a gaseous environment and the beam is scattered by these interactions. We show here that the experimental intensity profile of the scattered beam can be described as Gaussian in form to a high level of accuracy. This provides a rapid means of accounting for the effects of beam scatter in imaging and microanalysis because the standard deviation of the Gaussian is a simple function of parameters such as working distance, beam energy, gas type and pressure.