Scanning Tunneling Microscope Topographic Images Research Articles

Nanoscale phase change on Ge2Sb2Te5 thin films induced by optical near fields with photoassisted scanning tunneling microscope

A scanning probe microscope coupled with either femtosecond laser pulses or terahertz pulses holds great promise not only for observing ultrafast phenomena but also for fabricating desirable structures at the nanoscale. In this study, we demonstrate that a few-nanometer-scale phase change can be non-thermally stored on the Ge2Sb2Te5 surface by a laser-driven scanning tunneling microscope (STM). An atomically flat Ge2Sb2Te5 surface was irradiated with the optical near-field generated by introducing femtosecond laser pulses to the STM tip-sample junction. The STM topographic images showed that few-nanometer-scale mounds appeared after irradiation. In addition, tunneling conductance spectra showed that the bandgap increased by 0.2 eV in the area of 5 × 5 nm2. These indicate that the nanoscale crystal-to-amorphous phase change was induced by the STM-tip-induced near field. Our approach presented here offers an unprecedented increase in the recording density of optical storage devices and is, therefore, expected to facilitate the development of next-generation information technology.

Mechanical properties of H2Pc self-assembled monolayers at the single molecule level by noncontact atomic force microscopy

The mechanical properties of molecular self-assembled monolayers (SAMs) play an important role in understanding the interactions between molecules in the self-assembly, the interactions between molecules and substrate, and thus the formation mechanism of SAMs. Using a high-resolution noncontact atomic force microscope (NC-AFM) combined with a scanning tunneling microscope (STM), we have successfully obtained the sub-molecular resolution of a H2Pc self-assembled monolayer grown on a Pb(111) surface. A 2 × 2 superstructure was observed in both AFM and STM topographic images. The lateral critical force of removing a H2Pcmolecule from its SAM and moving a single H2Pc molecule on Pb(111) were measured. An oscillation of the critical force along the edge of the H2Pc SAM with a period of two molecular sites was observed, which can be attributed to the 2 × 2 superstructure. The lateral critical force caused by intermolecular interaction was found to be 25 pN on average and is typically two times larger than the molecule–substrate interaction.

First-principles investigation forM(CO)n/Ag(110)(M=Fe,Co, Ni, Cu, Zn, and Ag;n=1,2) systems: Geometries, STM images, and vibrational frequencies

Fe, Cu atoms and CO molecules were manipulated with a scanning tunneling microscope (STM) on a Ag(110) surface, and one or two CO can transfer from the surface and bond with a metal atom through the STM tip [H. J. Lee and W. Ho, Science 286, 1719 (1999); Phys. Rev. B 61, R16347 (2000)]. We perform a density-functional cluster model investigation for the systems. The experimental geometries are validated and understood using the frontier orbital theory. The STM topographic images are reproduced. The vibrational frequencies of the adsorbate systems are obtained by diagonalizing the second-derivative matrices and are in excellent agreement with the experimental measurements. The geometries and C-O stretch frequencies are predicted for systems with the adsorbate metal atom being Co, Ni, Zn, and Ag. These systems can be divided to two classes, and each class exhibits a different set of properties.

Microcrystalline SWNT material

We report here on an optimised purification method for carbon nanotubes which yields microcrystalline single wall nanotube (SWNT) material. For this work the material is characterised using a scanning tunneling microscope (STM) with atomic resolution at room temperature. In the purified material, tubes with armchair structure prevail (diameter 1.7–2.5 nm), although tubes with other chiralities are also present. Based on STM topographic images the purity of SWNT material in the final step was estimated to be between 70 and 80%. In contrast to well-known ropes of SWNTs, the microcrystalline material is composed of parallel SWNTs which are not twisted, forming a crystalline structure in two dimensions. An interesting point is that tubes from one microcrystal appear from the STM images all to have the same chirality.

Visible Light Emission from Atomic Scale Patterns Fabricated by the Scanning Tunneling Microscope

Scanning tunneling microscope (STM) induced light emission from artificial atomic scale structures comprising silicon dangling bonds on hydrogen-terminated Si(001) surfaces has been mapped spatially and analyzed spectroscopically in the visible spectral range. The light emission is based on a novel mechanism involving optical transitions between a tip state and localized states on the sample surface. The wavelength of the photons can be changed by the bias voltage of the STM. The spatial resolution of the photon maps is as good as that of STM topographic images and the photons are emitted from a quasipoint source with a spatial extension similar to the size of a dangling bond.

Hydrogen-terminated Si Surfaces. STM Induced Light Emission from Nano Structures Comprising Si Dangling Bonds.

Scanning tunneling microscope (STM) induced light emission from nano structures comprising silicon dangling bonds on deuterium terminated Si(001) surfaces has been mapped spatially and analyzed spectroscopically in the visible spectral range. The light emission is based on a novel mechanism involving optical transitions between a tip state and localized states on the sample surface. The wavelength of the photons can be varied by the bias voltage of the STM. The spatial resolution of the photon maps is as accurate as that of STM topographic images and the photons are emitted from a quasi-point source with a spatial extension similar to the size of a dangling bond.

Fractal character of cold-deposited silver films determined by low-temperature scanning tunneling microscopy.

High-resolution scanning-tunneling-microscope (STM) topographic images of vacuum-deposited Ag films are reported. Films were formed and imaged at 100 and 300 K. Images of films deposited at 300 K, annealed to 560 K, and then returned to 300 K are also presented. The topographic surfaces of the low-temperature films are found to be self-affine fractals with a local Hausdorff-Besicovitch dimension D=2.5. The low-temperature films exhibit intense surface-enhanced Raman spectra (SERS). Films deposited at 300 K do not possess significant fractal character and are not SERS active. We show that the apparent local fractal dimension obtained by analyzing STM topographic images depends critically on the algorithm used. Three such methods (cube counting, triangulation, and power spectrum analysis) are assessed. A method is proposed for obtaining reliable fractal dimensions by analyzing the experimental STM topographic images using several algorithms and comparing the results to a calibration curve generated by applying the same algorithms to simulated fractal surfaces of known Hausdorff-Besicovitch dimension.

Atomic-scale view of AlGaAs/GaAs heterostructures with cross-sectional scanning tunneling microscopy

Atomically resolved cross-sectional scanning tunneling microscope topographic images of heterostructures that include a sequence of 1, 2, 5, and 10 nm AlGaAs and GaAs layers are presented. These layers clearly show alloy fluctuations and interface roughness on an atomic scale. In the thick AlGaAs layers a mottled structure with regions of higher Al content about 2 nm wide and elongated in [1̄12] or [11̄2] directions are observed. Similarly, the interfaces are rough on a 2 nm length scale. These results suggest that, for the conditions used for the epitaxial growth of the ternary layers, Al-rich regions nucleate and grow anisotropically.

Si(111) and Si(100) surfaces observed in air by scanning tunneling microscopy

Si(111) and Si(100) surfaces have been observed by an air-operating scanning tunneling microscope (STM). On the cleaved Si(111) surface STM topographic images show predominantly [2¯11]-oriented monatomic steps. The distribution of the width of the terraces is centered around 4 nm. Si(100) surfaces have been hydrogen-terminated by a treatment with fluoridic acid (HF). The surface appears in the topographic STM images to be quite stable and smooth with presumably no oxide.

Scanning tunneling microscopy of planar biomembranes

The scanning tunneling microscope (STM) is capable of imaging conductive surfaces at atomic resolution. When STMs are used to image biological samples, however, STM resolution is limited to nanometer levels whether samples are hydrated, air-dried, or metal-coated. Lateral resolution is poor due to the nature of biological macromolecules (large Image aspect ratios) as well as to STM tip effects (shape, multiple tips, and tip/sample Interactions). If samples are adsorbed to highly-oriented pyrolytic graphite (HOPG) surfaces and scanned in the topographic (constant current) mode, vertical resolution is also uncertain due to contamination-mediated surface deformation artifacts. Nevertheless, because the STM is capable of detecting sub-Ångstrom displacements in z (e.g. to 0.02 Å in UHV), we have examined the feasibility of using the STM to determine the thickness of planar membranes attached to glass and mica surfaces. Planar membrane monolayers also uniquely provide the opportunity to correlate biochemical and TEM information with STM topographic images.