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.     

Measurements of polyphosphoric acid on HOPG

As a sample for initial study on biological materials by using scanning tunnelling microscope (STM), we selected phosphoric acid consisting of phosphorous atoms which play biologically an important role. We measured polyphosphoric acid coated on HOPG by STM and scanning tunnelling spectroscopy (STS) in air. In order to identify molecules on STM images, local I-V curves were taken simultaneously. The averaged I-V curves and the normalized conductance spectra of graphite coated with the acid show characteristics different from those of clean graphite surface. Around a step of the substrate, we found domains where normalized conductance spectra were different from those of clean graphite surface and ascribable to that of the adsorbed molecules.     

Voltage dependence of the tunnelling current: an exact expression

An exact expression for the tunnelling current measured with a scanning tunnelling microscope (STM) is obtained. It clarifies the information deducible from the ‘spectroscopic mode’ of the STM and raises the question of the observability of surface states. The connection with the Transfer Hamiltonian approach is made, and the conditions of validity of the latter are analysed.     

Nanomanipulation and nanofabrication with multi-probe scanning tunneling microscope: from individual atoms to nanowires

The wide variety of nanoscale structures and devices demands novel tools for handling, assembly, and fabrication at nanoscopic positioning precision. The manipulation tools should allow for in situ characterization and testing of fundamental building blocks, such as nanotubes and nanowires, as they are built into functional devices. In this paper, a bottom-up technique for nanomanipulation and nanofabrication is reported by using a 4-probe scanning tunneling microscope (STM) combined with a scanning electron microscope (SEM). The applications of this technique are demonstrated in a variety of nanosystems, from manipulating individual atoms to bending, cutting, breaking carbon nanofibers, and constructing nanodevices for electrical characterizations. The combination of the wide field of view of SEM, the atomic position resolution of STM, and the flexibility of multiple scanning probes is expected to be a valuable tool for rapid prototyping in the nanoscience and nanotechnology.     

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.     

Study of the fractal character of surfaces by scanning tunnelling microscopy: Errors and limitations

The possibility of using the intrinsic three-dimensional imaging capability of scanning tunnelling microscopes to study the fractal character of surfaces by Mandelbrot's method of ‘filling’ with water up to a given height is discussed. By plotting on a log-log plot the area against the perimeter of the ‘lakes' that appear, the fractal dimension is obtained from the slope of the straight line fitting the data points. The possible errors and limitations of the method are discussed from results obtained from both simulated and real surfaces. The effect of noise and resolution in the scanning tunnelling microscope on the calculation of the fractal dimension is also discussed.     

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Anomalous voltage dependence of tunnelling microscopy in WSe2

We present scanning tunnelling microscopy (STM) investigations of the layered semiconductor WSe₂. The tunnelling experiments were performed in air and under silicone oil with markedly different results. In air, atomic resolution images of the hexagonally structured surface could be obtained for sample-to-tip voltages of both negative and positive polarities, from ?1·5 to ?0·3 V for negative sample and from +0·6 to +1·6 V for positive sample, respectively. Under silicone oil, however, good atomic images could be seen for negative sample biases down to at least ?14 V, while for positive sample biases no difference with respect to the tunnelling in air was found.     

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.     

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.     

Atomic resolution imaging using a novel,compact and stiff scanning tunnelling microscope in cryogen-free superconducting magnet

We present the design and performance of a novel scanning tunnelling microscope (STM) operating in a cryogen-free superconducting magnet. Our home-built STM head is compact (51.5 mm long and 20 mm in diameter) and has a single arm that provides complete openness in the scanning area between the tip and sample. The STM head consists of two piezoelectric tubes (PTs), a piezoelectric scanning tube (PST) mounted on a well-polished zirconia shaft, and a large PT housed in a sapphire tube called the motor tube. The main body of the STM head is made of tantalum. In this design, we fixed the sapphire tube to the frame with screws so that the tube's position can be changed quickly. To analyse the stiffness of the STM head unit, we identified the lowest eigenfrequencies with 3 and 4 kHz in the bending modes, 8 kHz in a torsional mode, and 9 kHz in a longitudinal mode by finite element analysis, and also measured the low drift rates in the X–Y plane and in the Z direction. The high performance of the home-built STM was demonstrated by images of the hexagonal graphite lattice at 300 K and in a sweeping magnetic field from 0 T to 9 T. Our results confirm the high stability, vibration resistance, insensitivity to high magnetic fields and the application potential of our newly developed STM for the investigation of low-frequency systems with high static support stiffness in physics, chemistry, material and biological sciences.     

Oriented immobilization of Pseudomonas putida putidaredoxin at a gold (111)–buffer interface: a real time scanning tunnelling microscopy study

A scanning tunnelling microscopy study of adsorption of wild‐type Pseudomonas putida putidaredoxin at a gold (111)–buffer interface has been made in real time. Reversible adsorption has been observed reflecting weak interaction of the wild‐type protein with a gold (111) electrode. A genetically engineered mutant, C73S‐D58C, which contains a surface thiol, has been used for ‘immobilization’ and ‘orientated adsorption’ on the gold surface. The implication of such orientated immobilization in development of a bio‐electrode surface has been predicted.     

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.     

STM and AFM investigations of high-Tc superconductors

We have studied the (001) surface of single crystal YBa₂Cu₃O_7-x high-T_c superconductors using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) at room temperature at ambient pressure. Both methods show flat terraces with steps which are multiples of the c-axis lattice constant (of 1·17 nm) high. Our results show that the bulk crystal structure extends to the surface and that the crystals were formed by island growth. Only occasionally tunnelling was possible with sample bias voltages below +1·0 V. We interpret the observed voltage dependence and the difficulty to get good STM images to be due to the presence of a less-conducting surface layer. Auger spectroscopy indicates that carbon is present at the surface, which is probably related to a contamination layer.     

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.     

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.     

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.     

Scanning tunnelling microscopy of 1-D and 2-D charge-density wave systems

We have used a new variable temperature scanning tunnelling microscope (STM) to study quasi-1D and 2-D charge-density wave (CDW) systems. The 1-D systems, typified by NbSe₃ and TaS₃, are of special interest since they exhibit unusual transport phenomena associated with moving CDW above a threshold electric field. In the case of NbSe₃, room temperature STM images show both major and subtle details of the lattice structure. At present, however, images taken below the Peierls transition temperature of T_P=144 K resolve major lattice details but are not sufficiently clear to resolve the CDW. On the other hand, for the fully gapped CDW system orthorhombic-TaS₃, the CDW modulation superimposed on the lattice structure and having the correct period of four times the S-S spacing of 3·3 Å, is observed below T_P=215 K. Above T_P, the main observable feature is the S-S spacing along the chains. STM measurements have also been performed on the 2-D CDW system 1T-TaS₂ in its incommensurate, nearly commensurate, fully commensurate and trigonal phases. For the nearly commensurate phase, STM images show uniform commensurability with a relatively low concentration of small, time-varying discommensurations in contrast to models pradicting a regular domain structure. In the trigonal phase, however, evidence is seen for the striped phase composed of long, nearly parallel discommensurations.     

The STM as an information storage device

The high spatial resolution of the scanning tunnelling microscope (STM) makes its use as an information storage device very attractive. However, even without specifying the means of information storage, several limits can be placed on the characteristics of these devices. One would expect that a bit of information could not be stored in a dimension smaller than an atom. This limits the storage density to 10¹⁵ bits/cm². Present-day STM have fields of view that range up to 2times10^?6cm² yielding a total potential capacity of 2times10⁹ bits. This capacity would be expected to grow as STM design becomes more refined. At 1 nA, shot noise will limit the read rate to 20 Mbits/s. This limit can only be overcome by using larger currents. The write rate may also be limited by this phenomenon if the writing mechanism demands that a certain charge threshold be reached to write a bit. For a particular device structure, operating conditions will be identified where the above limitations will become important. The above analysis leads to the conclusion that an STM storage device with reasonable read/write rates and a large storage capacity should be possible.     

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.