Scanning tunneling microscopy and spectroscopy study of charge inhomogeneities in bilayer graphene

We report a room-temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on a SiO₂/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contributions from regions of varying electron density, indicating significant charge inhomogeneity. Large-scale topographic images show ripple-like structures with a roughness of ∼1 nm, while the small-scale atomic resolution images show graphite-like triangular lattices. The local () tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation, indicating inhomogeneity in electron density of order 10¹¹ cm⁻². The minimum in spectrum at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field-induced carrier density.     

Scanning tunneling microscopy and spectroscopy on iron-pnictides

Tremendous excitement has followed the recent discovery of superconductivity up to T_c = 56 K in iron–arsenic based materials (pnictides). This discovery breaks the monopoly on high-T_c superconductivity held by copper-oxides (cuprates) for over two decades and renews hope that high-T_c superconductivity may finally be theoretically understood and widely applied.Since scanning tunneling microscopy (STM) and spectroscopy (STS) have been key tools in the investigation and understanding of both conventional and unconventional superconductivity, these techniques are also applied to the pnictides. While the field is still in its early stages, several important achievements by STM and STS have been reported on the pnictides. In this paper, we will review their contribution towards an understanding of superconductivity in this new class of materials.     

长链烷烃和醇在石墨表面吸附的扫描隧道显微镜研究

用扫描隧道显微镜（STM）研究了室温下十八醇（1-C18H37OH）和十四烷（C14H30）在石墨表面的吸附行为.十八醇在石墨表面自组装形成条状结构.实验发现了十八醇分子在石墨表面的两种不同的密排方式，它们形成并列的不同宽度的条状结构.还发现，由于溶剂（正十四烷C14H30）的完全蒸发，留下的十八醇分子在石墨表面吸附的自组装结构与固液界面中十八醇分子在石墨表面的吸附形成的结构在分子方向、表面分子晶体的晶格常数及条状结构宽度等方面有很大的不同.此外，实验中也发现溶剂十四烷在某种情况下也能在石墨表面吸附形成可供STM观察的有较长时间稳定性的条状周期结构.实验观察到的十四烷在石墨表面形成的晶体结构与固液界面中观察到的有序结构的晶体结构常数和分子排列方向也是不同的.根据STM图像，提出了十八醇和十四烷在石墨表面吸附的结构模型. 关键词： 十四烷 十八醇 石墨 扫描隧道显微镜     

Scanning tunneling microscopy and spectroscopy of the semi-insulating CdZnTe(110) surface

The morphology and electronic structure of the (110) surface of semi-insulating CdZnTe has been studied by scanning tunneling microscopy and spectroscopy. The surface shows a 1 × 1 reconstruction whilst the tunneling spectra are highly rectified implying that imaging could only be performed at negative sample bias. Theoretical computations of the tunnel current have been used to fit to experiment to reveal the origin of each tunneling component and explain the rectification observed. The implications of various surface defects and surface states are considered. A discussion on scanning tunneling microscopy and spectroscopy on semi-insulating materials in general is also given.     

A combined scanning tunneling microscopy and reflectance anisotropy spectroscopy investigation of tetraphenylporphyrin deposited on graphite

The ordering properties of tetraphenylporphyrin molecules sublimated on a highly oriented pyrolitic graphite surface have been investigated. The morphological information obtained by scanning tunneling microscopy has been linked with the optical spectra by reflectance anisotropy spectroscopy. Measurements were performed in situ as a function of the nominal coverage.     

Scanning probe microscopy of graphene

A material with novel fundamental properties that challenge our current understanding is always exciting for research. If the novel properties extend to the realm of device engineering and promise a revolution in applications, then the scope of its research knows no bounds. The story of graphene, the two dimensional form of carbon, has followed this path. Graphene has been the subject of numerous experimental and theoretical investigations since 2004 when an elegant and a simple technique to make monolayer graphene set the stage for extensive research. Many other techniques to make graphene were developed in parallel to this technique. As graphene is replete with unique structural and electronic properties scanning probe microscopy has proved to be an exciting and a rewarding venture. In this review we discuss the findings of scanning probe microscopy and how it has served as an indispensable tool to understand the properties of graphene and further graphene research.     

Scanning tunneling microscopy and spectroscopy on cracked surfaces of superconducting CeRu2

Scanning tunneling microscopy and scanning tunneling spectroscopy measurements have been performed on a single crystal of CeRu₂ down to 2.2 K under a magnetic field up to 2.0 T. The sample surfaces for the measurements are prepared by cutting or cracking the single crystal at 4.2 K. The vortex lattice has been imaged by mapping the quasiparticle density of states at the Fermi energy on the surface. We have observed that the surface has been covered with microstructures a few nm in diameter. These microstructures are characteristic of the surface when the sample is cut or cracked at low temperature.     

Scanning tunneling microscopy imaging of nanotubes

Samples of carbon paper containing multiwalled carbon nanotube films are produced by current annealing. A scanning tunneling microscope is used to examine the structure of the modified carbon paper. X-, Y-, and V-shaped nanotubes are found.     

Scanning tunneling microscopy of carbon nanotubes

This article reports on the application of scanning tunneling microscopy for the study of surface structures and electronic properties of carbon nanotubes. Geometric effects resulting from the cylindrical shape of the tubes as well as the particular band structure of the graphitic crystal lattice can lead to a variety of contrast patterns. On the atomic scale, it is sometimes possible to see the full honeycomb lattice structure but often different structures are observed. Besides distortions caused by tip–sample interactions, we find that a complex superstructure superimposed on the simple atomic contrast pattern arises from elastic scattering of the Fermi states at defects or impurities. From a careful analysis of high-resolution images it is possible to extract information about elastic strain of individual tubes. A new combination of scanning tunneling and scanning force microscopy enables near-atomic point resolution of the force signal the tubes can be identified without the need of a conducting substrate. This imaging mode is a crucial step for the characterization of electronic devices based on individual single-wall tubes. This mode can be further enhanced by the use of single-walled tubes as probe tips. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 4 August 1999     

Interlayer tunneling spectroscopy of graphite at high magnetic field oriented parallel to the layers

Interlayer tunneling in graphite mesa-type structures is studied at a strong in-plane magnetic field H up to 55 T and low temperature T = 1.4 K. The tunneling spectrum dI/dV vs. V has a pronounced peak at a finite voltage V ₀. The peak position V ₀ increases linearly with H. To explain the experiment, we develop a theoretical model of graphite in the crossed electric E and magnetic H fields. When the fields satisfy the resonant condition E = vH, where V is the velocity of the two-dimensional Dirac electrons in graphene, the wave functions delocalize and give rise to the peak in the tunneling spectrum observed in the experiment.     

Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO₂ tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO₂ tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.     

Scanning tunneling microscopy of RF oscillating surfaces

Amplitude and phase of high frequency surface acoustic wave (SAW) fields are investigated by a novel scanning tunneling microscopy technique. The gap voltage is modulated at a slightly detuned high frequency. Due to the nonlinearity of the tunneling process a frequency mixing appears. For scanned areas with dimensions much smaller than the wavelength of the SAW a remarkable local variation of amplitude and phase of the tunneling current at the difference frequency is observed. Depending on the local morphology different components of the particle displacement vector are detected. Model calculations of amplitude and phase images are presented for a real topography.     

Scanning tunneling microscopy of a luttinger liquid

Explicit predictions for scanning tunneling microscopy (STM) on interacting one-dimensional electron systems are made using the Luttinger liquid formalism. The STM current changes with the distance from an impurity or boundary in a characteristic way, which reveals the spin-charge separation and the interaction strength in the system. The current exhibits Friedel-like oscillations, but also carries additional modulated behavior as a function of voltage and distance, which shows the spin-charge separation in real space. Moreover, very close to the boundary the current is strongly reduced, which is an indication of the interaction strength in the system.     

Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.     

石墨单晶表面原子的扫描隧道显微象

扫描隧道显微镜(STM)是近几年发展起来的一种直接观察物质表面微观结构的仪器.利用量子隧道效应,将极细的金属针尖接近样品表面扫描,从而获得样品表面的三维图象,可以反映表面原子排列和原子形态.图1是我们设计制造的扫描隧道显微镜原理图.采用压电陶瓷管P作为x,y和z方向的三维扫描器件.管表面等分为相邻90°的四个电极,针尖T固定在其中的一个电极上.尖端曲率半径为100nm左右的金属针尖,可用化学腐蚀法制备.两对电极上施加扫描电压时,针尖便在垂直于管轴z的x-y方向扫描,而z方向的高低变化则由加在内管壁上的电     

Scanning tunneling microscopy investigations of silicon carbide nanowires

Scanning tunneling microscopy (STM) images have been obtained from the surfaces of silicon carbide nanowires produced in the thermolysis-induced carbonization of halocarbons (combustion synthesis). The morphology of the nanowires shows trench-like features perpendicular to the fibres’ axis, which is assigned to the existence of microfacets on their sidewalls. For the first time high-resolution STM images of the SiC nanowires are presented. The results are in agreement with the previous reports suggesting the presence of microfacets on the SiC whiskers’ surface.     

Scanning tunneling microscopy studies of diazo dye monolayers on HOPG

We report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-[4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C₂₀H₁₉N₅) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.     

Scanning tunneling spectroscopy on the novel superconductor CaC6

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.