Scanning tunneling microscopy and spectroscopy of graphene layers on graphite

We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate.     

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 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.     

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     

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 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.     

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 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.     

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.     

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 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 microscopy of fluorinated fullerene molecules on silicon surfaces

Scanning tunneling microscopy (STM) under ultra-high vacuum conditions is used to study the initial stages of adsorption of C₆₀F₁₈ and C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 x 7 and Si(100)-2 x 1 surfaces. Spatially resolved STM images of individual molecules and ab initio calculations show that the fluorofullerene molecules interact with an Si surface, with the F atoms oriented toward the surface. The large electric dipole moment of the molecules induces strong polarization on the surface, but the charge transfer is weak. The presence of C₆₀F₃₆ isomers with different symmetry—T, C ₃, and C ₁—is revealed in STM images for the first time.     

Scanning tunneling microscopy and potentiometry at GaAs/AlGaAs heterojunctions

In certain problems of electrical transport through condensed matter, it is important to know the potential distribution with nanometer resolution, e.g., at interfaces (Schottky barriers) or pn junctions. Scanning tunneling potentiometry, a new application of scanning tunneling microscopy, is capable of providing this information. The tunnel current is used for simultaneously sensing probe-to-sample distance and local potential. We have used this technique for the localization of semiconductor junctions in cleaved MBE-grown AlGaAs-GaAs laser structures. Among other applications, the electron injection into the active GaAs layer of a heterostructure laser diode was readily observed.     

有机半导体外延生长的扫描隧道显微镜研究

有机半导体薄膜的光、电等性质取决于有机分子的取向以及长程有序性.研究有机半导体的生长机理以及内在驱动力是一个重要环节.文章通过两个典型生长体系,perylene 在Ru(0001)表面上的生长和tetracene在Ag(110)表面上生长过程的介绍,给出了形成有机半导体晶化薄膜的可能性以及决定其有序生长的内在驱动力.对于perylene 在Ru(0001)表面上的生长,决定其过程的主要驱动力是分子间的相互排斥作用,在单分子层时,由于这种相互作用导致形成Ru(0001)- 12×12-8 perylene有序超结构.而对于tetracene/Ag(110)体系,决定生长的驱动力主要表现为相互吸引作用,因此,在小于单分子层时,tetracene呈有序的岛状生长;而当tetracene膜的厚度大于单分子层时,呈逐层生长模式,并形成具有正交晶系结构的晶化薄膜.     

扫描隧道显微术中的微分谱学及其应用

文章介绍了扫描隧道显微术中微分谱学的原理及其在实验中的诸多应用。微分谱（dI／dV谱）和dI／dV成像可用来研究电子局域态密度在能量和空间的分布，即微分谱固定空间一点，反映电子态密度以能量为变量的分布；而dI／dV图像则反映某给定能量的电子局域态密度以空间为变量的分布，二次微分谱（d^2I／dV^2谱）和二次微分成像可以用来反映分子的非弹性隧穿过程，从而研究分子的振动态。