Current Imaging Tunneling Spectroscopy Measurements Research Articles

Engineering the Interface Characteristics of Ultrananocrystalline Diamond Films Grown on Au-Coated Si Substrates

Enhanced electron field emission (EFE) properties have been observed for ultrananocrystalline diamond (UNCD) films grown on Au-coated Si (UNCD/Au-Si) substrates. The EFE properties of UNCD/Au-Si could be turned on at a low field of 8.9 V/μm, attaining EFE current density of 4.5 mA/cm(2) at an applied field of 10.5 V/μm, which is superior to that of UNCD films grown on Si (UNCD/Si) substrates with the same chemical vapor deposition process. Moreover, a significant difference in current-voltage curves from scanning tunneling spectroscopic measurements at the grain and the grain boundary has been observed. From the variation of normalized conductance (dI/dV)/(I/V) versus V, bandgap of UNCD/Au-Si is measured to be 2.8 eV at the grain and nearly metallic at the grain boundary. Current imaging tunneling spectroscopy measurements show that the grain boundaries have higher electron field emission capacity than the grains. The diffusion of Au into the interface layer that results in the induction of graphite and converts the metal-to-Si interface from Schottky to Ohmic contact is believed to be the authentic factors, resulting in marvelous EFE properties of UNCD/Au-Si.

Synthesis, Magnetic Properties, and STM Spectroscopy of Cobalt(II) Cubanes [CoII4(Cl)4(HL)4

Reaction of cobalt(II) chloride hexahydrate with N-substituted diethanolamines H(2)L(2-4) (3) in the presence of LiH in anhydrous THF leads under anaerobic conditions to the formation of three isostructural tetranuclear cobalt(II) complexes [Co(II) (4)(Cl)(4)(HL(2-4))(4)] (4) with a [Co(4)(mu(3)-O)(4)](4+) cubane core. According to X-ray structural analyses, the complexes 4 a,c crystallize in the tetragonal space group I4(1)/a, whereas for complex 4 b the tetragonal space group P$\bar 4$ was found. In the solid state the orientation of the cubane cores and the formation of a 3D framework were controlled by the ligand substituents of the cobalt(II) cubanes 4. This also allowed detailed magnetic investigations on single crystals. The analysis of the SQUID magnetic susceptibility data for 4 a gave intramolecular ferromagnetic couplings of the cobalt(II) ions (J(1) approximately 20.4 K, J(2) approximately 7.6 K), resulting in an S=6 ground-state multiplet. The anisotropy was found to be of the easy-axis type (D=-1.55 K) with a resulting anisotropy barrier of Delta approximately 55.8 K. Two-dimensional electron-gas (2DEG) Hall magnetization measurements revealed that complex 4 a is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies below a blocking temperature of about 4.4 K. The hysteresis loops collapse at zero field owing to fast quantum tunneling of the magnetization (QTM). The structural and electronic properties of cobalt(II) cubane 4 a, deposited on a highly oriented pyrolytic graphite (HOPG) surface, were investigated by means of STM and current imaging tunneling spectroscopy (CITS) at RT and standard atmospheric pressure. In CITS measurements the rather large contrast found at the expected locations of the metal centers of the molecules indicated the presence of a strongly localized LUMO.

Cross-sectional scanning tunneling microscopy and spectroscopy of strain in buried heterostructure lasers

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase in both the density of filled valence band states and empty conduction band states as a result of the strain, with a particularly large increase at −3.1 V. Current imaging tunneling spectroscopy measurements show an increase in the tunneling current in to and out of the strained regions at both gap voltage polarities, consistent with the spectroscopy. Moving towards tensile strain, InP is known to maintain much the same bandgap, with the split-off level and lower lying states being drawn up towards the valence band edge, consistent with the data.

Current imaging tunneling spectroscopy characterization of YBa 2Cu 3O 7− x and Bi 2Sr 2CaCu 2O 8+ x single crystals

A combined scanning electron microscope–scanning tunneling microscope (SEM–STM) system has been used to investigate surface topographic and electronic properties of YBa 2Cu 3O 7− x (YBCO) and Bi 2Sr 2CaCu 2O 8+ x (Bi-2212) single crystals. Current imaging tunneling spectroscopy (CITS) measurements reveal the coexistence of areas with metallic and semiconducting characteristics in regions close to micron-sized growth steps imaged by SEM in the YBCO and Bi-2212 samples. On the contrary, a metallic behaviour is always found by CITS in extended areas of the crystals far from the growth steps. This inhomogeneous electronic behaviour is attributed to the defects related to the surface oxygen stoichiometry or distribution. Modifications induced by means of voltage pulses applied with the STM have also been investigated. We report on the threshold voltages necessary to modify the surface of YBCO and Bi-2212 single crystals. Furthermore, CITS measurements have been carried out in the modified areas in order to analyze the surface alterations caused in these materials.

Scanning tunneling microscopy studies of Ge/Si films on Si(111): From layer by layer to quantum dots

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on the Si(111) surface kept at 500 °C. For Ge thickness smaller than 0.45 monolayers (ML), STM images show large 7×7 flat regions without protrusions while at higher coverages flat, triangular 5×5 islands start nucleating. We have followed the evolution of this wetting layer up to its completion and investigated its surface composition at 3 ML by current imaging tunneling spectroscopy measurements. At larger coverages thick Ge islands (quantum dots) start to nucleate according to the Stranski-Krastanov mechanism. We analyze the evolution of the lattice strain both on the wetting layer and on the islands up to 15 ML coverage. A clear expansion of the lattice parameter as a function of the coverage is evidenced both on the islands’ top and on the wetting layer. The luminescence yield measured at 10 K on samples covered by 40 Å of Ge and capped with 10 Å of Si evidences a structure that could be assigned to Ge quantum dots.

Spatially varying energy gap in the CuO chains of YBa2Cu3O7-x detected by scanning tunneling spectroscopy.

Current-imaging tunneling spectroscopy (CITS) was performed on cold-cleaved single crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ at 20 K. CITS data include $I(V)$ curves taken simultaneously with a topographic scanning tunneling microscopic image. $I(V)$ curves taken on CuO chains show an energy gap of about 20 meV which disappears near oxygen vacancies. We explain several features of large-junction $I(V)$ measurements, photoemission spectroscopy, and single-point $I(V)$ spectroscopy in terms of local effects detected by our CITS measurements. Finally, we consider the possibilities that this energy gap is due to either a charge-density wave or proximity-coupled superconductivity from the ${\mathrm{CuO}}_{2}$ planes.