Scanning tunnelling microscopy and tunnelling spectroscopy of titanium before and afterin vitro immersion

In an effort to understand thein vivo interactions of titanium and its alloys with a biological environment, surface science methods have been used on specimens retrieved fromin vitro andin vivo experiments. A relatively new technique that has the potential to further our knowledge of the oxide-solution interface is scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (TS). This work documents the use of STM/TS in the study of titanium thin films before and after immersion in anin vitro solution. Titanium thin films were fabricated using a procedure which produced an oxide that had minimal contaminants. Half of the thin films were immersed in an electrolyte. STM/TS was performed immediately after the immersion period. Constant current images were obtained. Current-voltage characteristics were recorded at regions of interest. The topography of the nonimmersed films revealed that the surface was qualitatively the same as other sputter deposited metal films. I–V curves showed little spatial variation. The topography of the immersed film showed little change from the nonimmersed ones. However, significant spatial variation of the local electronic structure was noted. This indicates that titanium surface-fluid interactions do not occur uniformly on the film.     

Current tunnelling through MOS devices

In this study, we have calculated the tunnelling current through ultra thin gate oxides for MOS structure. In the aim to reduce the large gate leakage while scaling SiO₂ down oxide thickness, it has become necessary to use high-k gate dielectrics. We have used HfO₂/SiO₂ dual layer as gate oxide. According to the importance of these alternative gate dielectrics, it becomes essential to take into account the existence of electron trap at the HfO₂/SiO₂ interface. The gate current of n poly-Si/HfO₂/trap/SiO₂/p Si substrate capacitors is underestimated for low voltage if the effect of traps is not taken into account. The influence of trap parameters like width, depth and material masse on gate current has been examined.     

Hydrogen and deuterium tunnelling in scandium

A detailed study of the relaxation processes occurring below room temperature has been carried out by charging polycrystalline Sc with different amount of hydrogen and deuterium. For all the concentrations, we have observed relaxation effects ascribed to tunnelling of H (D) between the two equivalent first neighbour tetrahedral sites in the c direction, which strongly depend on the H (D) content. In diluted Sc–H (up to few hundreds ppm H), due to the presence of interstitial O, the main anelastic peak is related to the tunnelling of H trapped by O. Increasing the H (D) concentration the traps are saturated and new peaks appear at lower temperature. A marked isotope effect is observed; the peaks shift in temperature and are at least four times smaller with H than with D, when measured at the same frequency. In deuterated sample the main peak has been analysed in terms of the two-level-systems (TLS). A good fit for the case of 1 at.% D could be obtained with a relaxation time τ∝T⁻⁷ and a small distribution of the asymmetries. This indicates that, quite surprisingly, the rate in this temperature range is dominated by multiple phonons processes.     

Scanning tunnelling microscopy of carbon nanotubes

This paper briefly reviews how scanning tunnelling microscopy (STM) and spectroscopy (STS) are used to analyse the atomic structure and the electronic properties of individual single-wall carbon nanotubes. In this area, the progress accomplished over the past several years has been spectacular. As this paper demonstrates, all the effects predicted by theory have been verified experimentally. Geometrical and electronic effects specific to carbon nanotubes are illustrated by analysing a series of STM images and STS spectra computed using a tight-binding theory. The simulations include a catalogue of images of 27 single-wall nanotubes, Stone-Wales defects in semiconducting nanotubes, and a symmetric Y-junction.     

Confinement effects and tunnelling through quantum dots

Several recent theoretical advances concerning semiconductor quantum dots are reviewed. First of all, the effect of the quantum confinement on the energy gap is revisited on the basis of GW and Bethe-Salpeter calculations, showing that the excitonic gap is practically equal to the ordinary eigenvalue gap of single-particle approximations. The second part demonstrates that it is now possible to calculate the conductance peaks for the tunnelling current through a nanostructure. Finally, we discuss in some detail the concept of a macroscopic dielectric constant for nanostructures, showing that, except for a thin surface layer, the local dielectric constant still keeps its bulk value down to pretty small nanostructures.     

Long-range electron tunnelling in oligo-porphyrin molecular wires

Short chains of porphyrin molecules can mediate electron transport over distances as long as 5-10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction.     

Channel selective tunnelling through a nanographene assembly

We report selective tunnelling through a nanographene intermolecular tunnel junction achieved via scanning tunnelling microscope tip functionalization with hexa-peri-hexabenzocoronene (HBC) molecules. This leads to an offset in the alignment between the energy levels of the tip and the molecular assembly, resulting in the imaging of a variety of distinct charge density patterns in the HBC assembly, not attainable using a bare metallic tip. Different tunnelling channels can be selected by the application of an electric field in the tunnelling junction, which changes the condition of the HBC on the tip. Density functional theory-based calculations relate the imaged HBC patterns to the calculated molecular orbitals at certain energy levels. These patterns bear a close resemblance to the π-orbital states of the HBC molecule calculated at the relevant energy levels, mainly below the Fermi energy of HBC. This correlation demonstrates the ability of an HBC functionalized tip as regards accessing an energy range that is restricted to the usual operating bias range around the Fermi energy with a normal metallic tip at room temperature. Apart from relating to molecular orbitals, some patterns could also be described in association with the Clar aromatic sextet formula. Our observations may help pave the way towards the possibility of controlling charge transport between organic interfaces.     

The dynamical conductance of graphene tunnelling structures

The dynamical conductances of graphene tunnelling structures were numerically calculated using the scattering matrix method with the interaction effect included in a phenomenological approach. The overall single-barrier dynamical conductance is capacitative. Transmission resonances in the single-barrier structure lead to dips in the capacitative imaginary part of the response. This is different from the ac responses of typical semiconductor nanostructures, where transmission resonances usually lead to inductive peaks. The features of the dips depend on the Fermi energy. When the Fermi energy is below half of the barrier height, the dips are sharper. When the Fermi energy is higher than half of the barrier height, the dips are broader. Inductive behaviours can be observed in a double-barrier structure due to the resonances formed by reflection between the two barriers.     

磁性隧道结中自旋相关的隧穿输运

全面回顾和总结了磁性隧道结中自旋相关的隧穿这一研究领域的理论和实验方面的最新研究进展。讨论了影响磁性隧道结的自旋极化和隧穿磁电阻的各种因素及反映铁磁层和铁磁／绝缘层界面电子结构在隧穿中重要作用的理论模型和近期实验，同时也讨论了绝缘势垒和铁磁／绝缘层界面中的无序性在隧穿过程中对自旋极化与磁电阻效应的影响。     

Ultralow voltage resonant tunnelling diode electroabsorption modulator

Embedding a double barrier resonant tunnelling diode (RTD) in a unipolar InGaAlAs optical waveguide gives rise to a very low driving voltage electroabsorption modulator (EAM) at optical wavelengths around 1550 nm. The presence of the RTD within the waveguide core introduces high non-linearity and negative differential resistance in the current-voltage (I-V) characteristic of the waveguide. This makes the electric field distribution across the waveguide core strongly dependent on the bias voltage: when the current decreases from the peak to the valley, there is an increase of the electric field across the depleted core. The electric field enhancement in the core-depleted layer causes the Franz-Keldysh absorption band-edge to red shift, which is responsible for the electroabsorption effect. High-frequency ac signals as low as 100 mV can induce electric field high-speed switching, producing substantial light modulation (up to 15 dB) at photon energies slightly lower than the waveguide core band-gap energy. The key difference between this device and conventional p-i-n EAMs is that the tunnelling characteristics of the RTD are employed to switch the electric field across the core-depleted region; the RTD-EAM has in essence an integrated electronic amplifier and, therefore, requires considerably less switching power.     

Elastic and inelastic tunnelling in single-layer Langmuir films

The electron tunnel effect in single-layer Langmuir films sandwiched between metal electrodes (MIM devices) has been investigated in the present work by a study of both the elastic and inelastic tunnelling components. Results of inelastic tunnelling experiments in single-layer barium stearate Langmuir films are presented and it is shown that this offers convincing evidence that tunnelling is the predominant electron transport mechanism in these films over a more extensive voltage regime than previously studied. Data obtained from d.c. investigations of such MIM devices is shown to fit Stratton's model for tunnelling in symmetrical barriers for applied voltage V ? 0.6 V. A value of 1.8 eV is obtained for the aluminium-barium stearate barrier height. It is also shown that the same d.c. data can alternatively be fitted to a log I versus $\text{V}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ relation. The voltage-current characteristics of such MIM devices were studied at low temperature and low voltage. Structure related to the superconducting energy gap is clearly visible when one electrode is made superconducting.The modified fabrication technique employed is described in detail and the close agreement with results of other workers gives confidence that the method is adequate.     

Scanning tunnelling microscopy study of activated carbon fibres

Scanning tunnelling microscopy (STM) has been used to study isotropic pitch-based carbon fibres before and after steam activation. The results show that the present carbon fibre precursor exhibits a particulate surface which is very favourable for the formation of activated carbon fibre. After activation, the carbon fibre surface becomes much more porous and rougher, and the mesopores are evidently present on the surface. Because the scale is down to atomic resolution, the STM observations offer direct evidence for the existence of micropores on the surface of the activated carbon fibres. In addition, the surface textures of both fibres are presented and discussed.     

The elastic tunnelling current in asymmetrical aluminum-insulator-metal diodes

We examined the conductivities of the two limiting cases of diodes: (a) nearly symmetrical aluminum-insulator-aluminum junctions and (b) strongly asymmetrical aluminum-insulator-gold junctions. The fit of our experimental data to a power series for the current up to a bias of 0.5 V yielded excellent confirmation of our recently introduced phenomenological theory of the elastic tunnelling current. In addition we were able to describe even extremely asymmetrical Al-I-Au diodes within the framework of our theory. With a step potential instead of the common trapezoidal barrier we obtained physically reasonable results for our barrier parameters.