Hydrothermally-grown ZnO nanowire tips for scanning tunnelling microscopy

The probe tip is pivotal in determining the resolution and nature of features observed in the Scanning Tunnelling Microscope (STM). We have augmented a conventional Pt/Ir metallic tip with a hydrothermally grown ZnO nanowire (NW). Atomic resolution imaging of graphite is attained. Current-voltage (IV) characteristics demonstrate an asymmetry stemming from the unintentional n-type doping of the ZnO NW, whereas the expected Schottky barrier at the ZnO-Pt/Ir interface is shown to have negligible effect. Moreover the photoconductivity of the system is investigated, paving the way towards a photodetector capable of atomic resolution.     

Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography

The practical realization of nanoscale electronics faces two major challenges: the precise engineering of the building blocks and their assembly into functional circuits. In spite of the exceptional electronic properties of carbon nanotubes, only basic demonstration devices have been realized that require time-consuming processes. This is mainly due to a lack of selective growth and reliable assembly processes for nanotubes. However, graphene offers an attractive alternative. Here we report the patterning of graphene nanoribbons and bent junctions with nanometre-precision, well-defined widths and predetermined crystallographic orientations, allowing us to fully engineer their electronic structure using scanning tunnelling microscope lithography. The atomic structure and electronic properties of the ribbons have been investigated by scanning tunnelling microscopy and tunnelling spectroscopy measurements. Opening of confinement gaps up to 0.5 eV, enabling room-temperature operation of graphene nanoribbon-based devices, is reported. This method avoids the difficulties of assembling nanoscale components and may prove useful in the realization of complete integrated circuits, operating as room-temperature ballistic electronic devices.     

Direct atomic scale imaging of III-V nanowire surfaces

We have succeeded in direct atomic scale imaging of the exterior surfaces of III-V nanowires by scanning tunneling microscopy (STM). By using atomic hydrogen, we expose the crystalline surfaces of InAs nanowires with regular InP segments in vacuum while retaining the wire morphology. We show images with atomic resolution of the two major types of InAs wurtzite nanowire surface facets and scanning tunneling spectroscopy (STS) data. Ab initio calculations of the lowest energy surface structures and simulated STM images, agree very well with experiments.     

The imaging of streptavidin and avidin using scanning tunnelling microscopy

Streptavidin and avidin were adsorbed onto graphite surfaces and images of their structures were obtained using a scanning tunnelling microscope. Individual protein molecules and their subunits were observed, and their sizes were found to agree closely with measurements from a subunit model based upon the crystal structure of streptavidin. The process of adsorption of streptavidin-FITC and avidin succinylfluorescein isothiocyanate onto the graphite surface was also investigated using fluorescence microscopy.     

The imaging of streptavidin and avidin using scanning tunnelling microscopy

Streptavidin and avidin were adsorbed onto graphite surfaces and images of their structures were obtained using a scanning tunnelling microscope. Individual protein molecules and their subunits were observed, and their sizes were found to agree closely with measurements from a subunit model based upon the crystal structure of streptavidin. The process of adsorption of streptavidin-FITC and avidin succinylfluorescein isothiocyanate onto the graphite surface was also investigated using fluorescence microscopy.     

Morphological structure study of pitch fibre surfaces by scanning tunnelling microscopy: influence of the mesophase content

The surface morphology of pitch fibres with different mesophase contents was studied by scanning tunnelling microscopy (STM) and the results were related to their mechanical properties. The tensile strength and modulus increase with the mesophase content and correspondingly the surface structure becomes more anisotropic. Inversely, the STM method can also be used to evaluate the precursor mesophase content of a pitch-based carbon fibre.     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy

We directly image the interior of GaAs/AlGaAs axial and radial nanowire heterostructures with atomic-scale resolution using scanning tunneling microscopy. We show that formation of monolayer sharp and smooth axial interfaces are possible even by vapor-phase epitaxy. However, we also find that instability of the ternary alloys formed in the Au seed fundamentally limits axial heterostructure control, inducing large segment asymmetries. We study radial core-shell nanowires, imaging even ultrathin submonolayer shells. We demonstrate how large twinning-induced morphological defects at the wire surfaces can be removed, ensuring the formation of wires with atomically flat sides.     

Study on the surface topology of vacuum-fired stainless steel by scanning tunnelling microscopy

The surface of 304L stainless steel, after normal bakeout and vacuum firing, has been imaged in the scanning tunnelling microscope (STM). Probe tips with sharp apexes and well-known reproducible geometry which were previously characterized in the field ion microscope (FIM) were used. After bakeout the STM surface profiles show on chemical polished surfaces an average roughness of 3-5 nm and over 300-500 nm variations in height of 30-50 nm. After vacuum firing a significant change of the surface structure and topology is observed. Crystallites with almost (1 1 1) orientation show wide (1 1 1) terraces with monoatomic steps. On tilted crystallites again wide (1 1 1) terraces, intersected with bunched steps and facets forming nearly regularly pattern and corresponding in orientation almost to the (1 0 0) and (1 1 0) planes can be observed. From the general appearance of the surface after vacuum firing it can be concluded that the surface became less active with respect to gas surface interaction which supports the present understanding of outgassing of this material.     

Application of scanning tunnelling microscopy to fatigue and fracture

The scanning tunnelling microscope (STM) is applied to the characterization of fine-scale details associated with the fatigue fracture surface of a steel alloy, and with fibre-coating microfracture in a fibre-reinforced metal matrix composite. Results are compared with those obtained by conventional scanning electron microscopy, and discussed in terms of implications for exploiting the STM in the study of deformation and fracture.     

A new elastic-wave-based imaging method for scanning the defects inside the structure

In this paper, a new nondestructive testing method using elastic waves for imaging possible voids or defects in concrete structures is proposed. This method integrates the point-source/point receiver scheme with the synthetic aperture focusing technique (SAFT) process to achieve the effect like scanning with a phase array system. This method also is equipped with large functioning depth because of the high-energy feature that elastic waves usually possess over traditional ultrasound. Both numerical simulations and experimental tests were carried out to explore the capabilities of this method in revealing single or multiple defects implied in a matrix material. The results from numerical simulations indicate that this method can clearly reveal the number of the voids or defects, their locations, and front-end profiles. The influence of the accuracy of the wave velocity determination on the resultant image also was evaluated in this study. Furthermore, the effects of the types of the responses to be recorded and the wavelength of the introduced waves also were evaluated so that very good resultant images may be obtained. Both the results from the numerical simulations and the experimental tests indicate that this elastic-wave-based method exhibits high potential in inspecting the defects of in-situ concrete structures by imaging.     

Studying multilayer langmuir film structure by confocal laser scanning and atomic force microscopy techniques

Features of extracting information on the surface structure of a multilayer organic film from data on its optical properties and surface relief are considered. The object of investigation was a multilayer Langmuir-Blodgett film based on a prepolymer (polyamic acid salt). It is suggested that the formation of the film volume is influenced by inhomogeneities in the structure of layers.     

Shifting atomic patterns: on the origin of the different atomic-scale patterns of graphite as observed using scanning tunnelling microscopy

We present an in-depth study of the myriad atomically resolved patterns observed on graphite using the scanning tunnelling microscope (STM) over the past three decades. Through the use of highly resolved atomic resolution images, we demonstrate how the interactions between the different graphene layers comprising graphite affect the local surface atomic charge density and its resulting symmetry orientation, with particular emphasis on interactions that are thermodynamically unstable. Moreover, the interlayer graphene coupling is controlled experimentally by varying the tip-surface interaction, leading to associated changes in the atomic patterns. The images are corroborated by first-principles calculations, further validating our claim that surface graphene displacement, coming both from lateral and vertical displacement of the top graphene layer, forms the basis of the rich variety of atomic patterns observed in STM experiments on graphite.     

Two-dimensional imaging without scanning by multifocal multiphoton microscopy

We describe multifocal multiphoton microscopy giving images without laser scanning. A multitude of 8 x 8 laser beams is focused into a sample yielding two-photon excitation in a plane. The focal spots are arranged in a rectangular array with close spacing between individual points (approximately 0.5 microm). The fluorescence emission from the sample is recorded with a CCD camera, but, owing to the close distance between the beams, they can no longer be regarded as individual points but rather as an illumination of the plane that is covered by the array of focal points. The axial sectioning capability is comparable with an ordinary single-beam two-photon microscope. Interference between the beams that could compromise the axial sectioning capability does not occur in our setup owing to small temporal delays between the individual beams. The axial sectioning capability of the setup is discussed in detail by means of the step response in which the foci are scanned axially into a uniformly fluorescent medium.     

Surface morphology of high-temperature superconductor thin films using scanning tunnelling microscopy

The surface morphology of laser-deposited superconducting YBa₂Cu₃O_7?x thin films has been studied using scanning tunnelling microscopy (STM). Very high resolution of the surface morphology has been obtained showing well-textured surfaces with very distinct growth patterns. The thin films produced byin situ laser deposition are highly oriented with typicalJ _c values > 1× 10⁶A cm^?2 at 77 K. The morphology of these surfaces as revealed by STM is a terraced, layer-like structure with a discrete rise of each terrace level approximately equal to thec-axis lattice spacing. These terraces are stacked upon each other in an ordered fashion. These terraces can also be viewed as a series of chip-like morphologies which increase in size at the lower levels. The surface structure (amount of texturing and size of terraces near the surface) shows a small but observable dependence upon the deposition conditions, with lower deposition rates producing larger grain sizes. We have also studied the chemical etching of these surfaces and the resultant morphologies have been followed using atomic force microscopy. Upon chemical etching the individual grains are revealed and due to the epitaxial growth of these films, most likely represent mutually aligned but separate grains. These etching studies provide valuable information about substructures. A model for film growth based upon nucleation and growth will be described.     

Spiral scanning method for atomic force microscopy

A spiral scanning method is proposed for atomic force microscopy with thoroughgoing analysis and implementation. Comparing with the traditional line-by-line scanning method, the spiral scanning method demonstrates higher imaging speed, minor image distortion, and lower acceleration, which can damage the piezoelectric scanner. Employing the spiral scanning method to replace the line-by-line scanning method, the experiment shows that the time to complete an imaging cycle can be reduced from 800 s to 314 s without sacrificing the image resolution.