Scanning Tunneling Luminescence of Individual CdSe Nanowires

The local luminescence properties of individual CdSe nanowires composed of segments of zinc blende and wurtzite crystal structures are investigated by low‐temperature scanning tunneling luminescence spectroscopy. Light emission from the wires is achieved by the direct injection of holes and electrons, without the need for coupling to tip‐induced plasmons in the underlying metal substrate. The photon energy is found to increase with decreasing wire diameter due to exciton confinement. The bulk bandgap extrapolated from the energy versus diameter dependence is consistent with photon emission from the zinc blende‐type CdSe sections.     

The preparation of high quality y-type polyimide Langmuir-Blodgett films

This paper describes a method which we have developed to prepare high quality polyimide Langmuir-Blodgett films. The key was to achieve good Y-type deposition of the polyamic acid Langmuir films from which the polyimide films are derived. It was found to be important to control two factors carefully: the time between preparation of the spreading solution and its use, and the pressure- annealing time of the Langmuir film. The characterization of the films, which involved liquid crystal alignment properties, Fourier transform IR, scanning electron microscopy, and scanning tunneling microscopy, showed that the structural integrity was much higher for the Y-type films. Also the degree of polymer chain alignment within each layer was again much higher for the Y-type films.     

CVD金刚石膜的场发射机制

利用热灯丝化学气相沉积方法在光滑的钼上沉积了金刚石膜,用扫描电子显微镜和Raman谱对金刚石膜进行了分析。结果表明金刚石膜是由许多金刚石晶粒组成,晶粒间界主要是石墨相,并且在膜内有许多缺陷。金刚石膜的场发射结果表明高浓度CH4形成的金刚石膜场发射阈位电场较低浓度CH4形成的金刚石为低。这意味着杂质（如石墨）和缺陷（悬挂键）极大地影响了膜的场发射性能。根据以上结果,提出了一种CVD金刚石膜的场发射机制即膜内的缺陷增强膜内的电场,石墨增大电子的隧穿系数以增强CVD金刚石膜的场发射。     

Studies of topological features of the HFCVD surface of a nanocrystalline diamond film using a scanning tunneling microscope with a diamond tip

Structural features of HFCVD nanocrystalline diamond films have been considered and a physicomathematical model of the atomic structure of its surface has been constructed. The film has been studied in air using a scanning tunneling microscope with a tip of a boron-doped semiconducting diamond single crystal. Fractal analysis of images of the film surface from scanning tunneling microscope has shown the presence of diamond single crystals of different orientations having a mean size of 31 nm with a fractal dimensionality of 2.36 and 2.73.     

Nanoimprint lithography for nanophotonics in silicon

A novel inverse imprinting procedure for nanolithography is presented which offers a transfer accuracy and feature definition that is comparable to state-of-the-art nanofabrication techniques. We illustrate the fabrication quality of a demanding nanophotonic structure: a photonic crystal waveguide. Local examination using photon scanning tunneling microscopy (PSTM) shows that the resulting nanophotonic structures have excellent guiding properties at wavelengths in the telecommunications range, which indicates a high quality of the local structure and the overall periodicity.     

Numerical simulations of global approach for photon scanning tunneling microscopy: coupling of finite-element and boundary integral methods

An accurate global model is proposed for a two-dimensional probe-sample system of photon scanning tunneling microscopy in near-field optics. A coupling of a finite-element method in the inhomogeneous sample and a boundary integral method on the artificial boundary of the truncated domain is developed. Numerical experiments are included to demonstrate the effectiveness of the proposed method and to show the features of wave propagation in photon scanning tunneling microscopy.     

Ice-crystal absorption: a comparison between theory and implications for remote sensing

The problem of the disagreement between cirrus crystal sizes determined remotely and by in situ measurements is shown to be due to inappropriate application of Mie theory. We retrieved the absorption optical depth at 8.3 and 11.1 mum from 11 tropical anvil cirrus clouds, using data from the High Resolution Infrared Radiation Sounder (HIRS). We related the absorption optical depth ratio between the two wavelengths to crystal size (the size was defined in terms of the crystal median mass dimension) by assuming Mie theory applied to ice spheres and anomalous diffraction theory (ADT) applied to hexagonal columns, hexagonal plates, bullet rosettes, and aggregates (polycrystals). The application of Mie theory to retrievals yielded crystal sizes approximately one third those obtained with ADT. The retrievals of crystal size by use of HIRS data are compared with measurements of habit and crystal size obtained from in situ measurements of tropical anvil cirrus particles. The results of the comparison show that ADT provides the more realistic retrieval. Moreover, we demonstrate that at infrared wavelengths retrieval of crystal size depends on assumed habit. The reason why Mie theory predicts smaller sizes than ADT is shown to result from particle geometry and enhanced absorption owing to the capture of photons from above the edge of the particle (tunneling). The contribution of particle geometry to absorption is three times greater than from tunneling, but this process enhances absorption by a further 35%. The complex angular momentum and T-matrix methods are used to show that the contribution to absorption by tunneling is diminished as the asphericity of spheroidal particles is increased. At an aspect ratio of 6 the contribution to the absorption that is due to tunneling is substantially reduced for oblate particles, whereas for prolate particles the tunneling contribution is reduced by 50% relative to the sphere.     

Free charges produced by carrier multiplication in strongly coupled PbSe quantum dot films

We show that in films of strongly coupled PbSe quantum dots multiple electron-hole pairs can be efficiently produced by absorption of a single photon (carrier multiplication). Moreover, in these films carrier multiplication leads to the generation of free, highly mobile charge carriers rather than excitons. Using the time-resolved microwave conductivity technique, we observed the production of more than three electron-hole pairs upon absorption of a single highly energetic photon (5.7E(g)). Free charge carriers produced via carrier multiplication are readily available for use in optoelectronic devices even without employing any complex donor/acceptor architecture or electric fields.     

真空蒸发沉积聚苯胺晶态薄膜的研究

利用傅里叶变换红外光谱、透射电子显微镜、高分辨扫描电子显微镜和扫描隧道显微镜等多种分析技术 ,对用真空蒸发沉积技术制备的不同聚苯胺 (PANI)薄膜试样进行了综合表征。研究发现 ,PANI薄膜是无定形态的绝缘膜 ;经HCl掺杂的PANI薄膜尽管其电导率有很大提高 ,但仍是非晶态的薄膜 ;而PANI TCNQ薄膜确是多晶薄膜 ,且薄膜的电导率较PANI薄膜提高几个数量级。研究发现PANI TCNQ薄膜中的PANI聚合链与TCNQ分子之间存在着电荷转移。与PANI薄膜和TCNQ薄膜相比 ,PANI TCNQ复合薄膜的结构更加完善 ,表面更加平整     

真空蒸发沉积聚苯胺—TCNQ复合薄膜的STM研究

利用扫描隧道显微镜对真空蒸发沉积的聚苯胺－ＴＣＮＱ（ＰＡＮＩ－ＴＣＮＱ）复合薄膜、纯聚苯胺薄膜及纯ＴＣＮＱ薄膜试样进行了对比分析。研究发现,纯ＰＡＮＩ薄膜和纯ＴＣＮＱ薄膜都是绝缘膜,而用ＴＣＮＱ挽杂获得具有较高导电特性的ＰＡＮＩ－ＴＣＮＱ复合薄膜。而且与表面粗糙不连续的ＰＡＮＩ薄膜和ＴＣＮＱ薄膜相比,ＰＡＮＩ－ＴＣＮＱ复合薄膜易形成较大面积的表面结构完善的连续膜。傅里叶变换红外光谱（ＦＴＩＲ）分析     

Surface structure of silver thin films on In2O3:Sn and Al2O 3 †

Surface structure of thin silver films (200 Å) on two technologically important films, indium tin oxide (ITO) and aluminium oxide, has been studied using scanning tunneling microscope. ITO films were prepared by reactive electron beam evaporation. Aluminium oxide films were prepared by oxidizing 2000 Å thick aluminium films evaporated on to H₂ terminated single crystal silicon substrates. The surface structure of silver on ITO and aluminium oxide appeared to be same and was characteristic of Stranski-Krastanov type. The observed asymmetry in the island shape was attributed to the anisotropic nature of the strain fields surrounding the nucleation centres.     

Initial stages of WO3 growth on silicon substrates

Ultrathin films (5 nm, 10 nm and 20 nm effective thickness) of WO₃ have been deposited in high vacuum (10^− 6 Torr) onto single crystal Si(100) substrates and studied with X-ray diffraction, atomic force microscopy, scanning tunneling microscopy and spectroscopy. The experiments have been carried out on “as-deposited” thin films or after 1 h post-deposition annealing at various temperatures (ranging from 300 °C to 500 °C). A size induced increase of the amorphous to crystalline (monoclinic) phase transition has been observed for the 5 nm and 10 nm films, with a critical crystallite size of 25 ± 5 nm and a critical temperature of 345 ± 5 °C. All the experimental evidences show that, upon annealing, there is a diffusion limited aggregation growth of WO₃ that forms large flat two-dimensional islands composed by aggregates of individual crystallites approximately uniform in size and shape. These islands are isolated in the 5 nm thin films, are connected in the 10 nm case and form a uniform patchwork in the 20 nm thin films. Scanning tunneling spectroscopy shows the opening of a large surface band gap (2.7 eV) in the 500 °C annealed films and the significant presence of in gap states for thin films prepared with a lower (below 400 °C) annealing temperature. These findings are discussed in view of the optimization of the best morphological, structural and electronic parameters to fabricate WO₃ gas sensing devices at the sub-micrometer length scale.     

Atomic force microscopy silicon tips as photon tunneling sensors: a resonant evanescent coupling experiment

Evanescent wave conversion by transparent dielectric nanoprobes has long been achieved in photon scanning tunneling microscopy experiments. Nevertheless, the exact mechanism (i.e., resolution limit) of this optical interaction is not satisfactorily explained theoretically nor evidenced experimentally. We study the ability of doped silicon atomic force microscopy tips to capture infrared near-field waves standing at the flat surface of a semiconductor (semi-insulating InP) material. It is shown that, unlike silicon nitride tips previously studied, the transmitted intensity of these silicon tips does not obey the classical frustrated total internal reflection model but a more complex dependence that involves a resonant tunneling transfer. An explanation is proposed that follows the theoretical predictions for the electromagnetic coupling between subwavelength objects.     

On-line characterization of YBCO coated conductors using Raman spectroscopy methods

The use of Raman spectroscopy for on-line monitoring of the production of superconducting YBa2Cu3O6+X (YBCO) thin films on long-length metal tapes coated with textured buffer layers is reported for the first time. A methodology is described for obtaining Raman spectra of YBCO on moving tape exiting a metal-organic-chemical-vapor-deposition (MOCVD) enclosure. After baseline correction, the spectra recorded in this way show the expected phonons of the specific YBCO crystal orientation required for high supercurrent transport, as well as phonons of non-superconducting second-phase impurities when present. It is also possible to distinguish YBCO films that are properly textured from films having domains of misoriented YBCO grains. An investigation of the need for focus control on moving tape indicated that focusing of the laser on the surface of the highly reflective YBCO films exiting the MOCVD enclosure tends to produce aberrant photon bursts that swamp the Raman spectrum. These photon bursts are very likely a consequence of optical speckle effects induced by a combination of surface roughness, crystallographic texture, and/or local strain within the small grain microstructure of the YBCO film. Maintaining a slightly out-of-focus condition provides the best signal-to-noise ratio in terms of the obtained Raman spectra. In addition to examining moving tape at the post-MOCVD stage, Raman spectra of the film surface can also be recorded after the oxygen anneal performed to bring the YBCO to the optimum superconducting state. Consideration is given to data processing methods that could be adapted to the on-line Raman spectra to allow the tagging of out-of-specification tape segments and, at a more advanced level, feedback control to the MOCVD process.     

NP1024神经网络处理器中的数据传输与图形显示

介绍在ＮＰ１０２４神经网络系统中如何将网络中有关数据按要求高速送到ＬＣ－ＳＡ屏上的接口及显示电路的软件硬件设计技巧。     

Morphology and current-voltage characteristics of nanostructured pentacene thin films probed by atomic force microscopy

Atomic force microscopy was used to study the growth modes (on SiO2, MoS2, and Au substrates) and the current-voltage (I-V) characteristics of organic semiconductor pentacene. Pentacene films grow on SiO2 substrate in a layer-by-layer manner with full coverage at an average thickness of 20 A and have the highest degree of molecular ordering with large dendritic grains among the pentacene films deposited on the three different substrates. Films grown on MoS2 substrate reveal two different growth modes, snowflake-like growth and granular growth, both of which seem to compete with each other. On the other hand, films deposited on Au substrate show granular structure for thinner coverages (no crystal structure) and dendritic growth for higher coverages (crystal structure). I-V measurements were performed with a platinum tip on a pentacene film deposited on a Au substrate. The I-V curves on pentacene film reveal symmetric tunneling type character. The field dependence of the current indicates that the main transport mechanism at high field intensities is hopping (Poole-Frenkel effect). From these measurements, we have estimated a field lowering coefficient of 9.77 x 10(-6) V-1/2 m1/2 and an ideality factor of 18 for pentacene.     

室温直流磁控溅射氮化钛薄膜研究

利用直流磁控溅射在室温下沉积出性能优良的氮化钛薄膜,研究了N2流量和偏压对氮化钛薄膜性能和结构的影响,并采用扫描隧道显微镜(STM)技术对其表面形貌进行了较为详细的研究.结果表明,随着N2流量的增加,薄膜的结构从四边型混合结构转变为面心立方 NaCl型结构,最后变为无定型结构,薄膜结构的变化也使薄膜的硬度随之发生变化;施加负偏压不仅能让薄膜中缺陷减少,使膜层变得更致密,而且还能优化氮化钛晶粒,从而获得性能优良的薄膜.从TiN薄膜的表面形貌图可知,薄膜表面平整,缺陷很少,晶粒排列非常致密,且空位及表面缺陷较少.     

Properties of CdS Films Prepared from Thiourea Complexes of Cadmium Salts

Cadmium sulfide films were prepared by spray pyrolysis of thiourea complexes, and their optical, electrical, luminescent, and structural properties were investigated. The results are used to elucidate the mechanism underlying the effect of the nature of the thiourea complex on the properties of the films. The crystal structure and perfection of the films are shown to correlate with the structure of the first coordination sphere of the thiourea complex.     

Nano and micro structural studies of thin films of ZnO

Zinc oxide thin films grown by sol–gel and RF sputtering methods have been characterized. The characterization techniques used involve ellipsometry, optical absorption, scanning tunneling microscopy, scanning and transmission electron microscopy. The films grown by sol–gel spin method which followed zinc acetate route exhibited a smoother texture than the films, which were deposited by using zinc nitrate route. The later type of films showed a dendritic character. Nano-structured fine grains of size ranging from 20 to 60 nm were observed with zinc nitrate precursor film. Individual grains show a sharp contrast with different facets and boundaries. Crystal planes and lattice parameters calculated by electron diffraction and X-ray diffraction are quite close and in agreement with the reported values in literature. Scanning tunneling microscopy has been used for measuring the average roughness of the surface and estimating the lattice constants. The STM studies of RF sputtered films, although showing a ZnO structure, exhibited a disturbed lattice. This was presumably due to the fact that after deposition the films were not annealed. Nanographs of 2D and 3D view of atomic positions of ZnO have been presented by using scanning tunneling microscopy.     

Electron-tunneling analysis of defect structure in near-surface layers of conducting materials

A new general approach to calculations of the tunneling characteristics of heterostructures involving nanodimensional disordered insulating films has been developed with allowance for both elastic and inelastic electron tunneling. It is shown that, by determining the value of the exponent in the dependence of the differential conductivity of these heterocontacts on the applied voltage, it is possible to characterize the structure of a transition layer between metal electrodes. The proposed approach is exemplified by analysis of the tunneling curves for silver contacts with conducting films of manganites and cuprates.