Fabrication of single-electron transistors using field-emission-induced electromigration

We report a novel technique for the fabrication of planar-type Ni-based single-electron transistors (SETs) using electromigration method induced by field emission current. The method is so-called "activation" and is demonstrated using arrow-shaped Ni nanogap electrodes with initial gap separations of 21-68 nm. Using the activation method, we are easily able to obtain the SETs by Fowler-Nordheim (F-N) field emission current passing through the nanogap electrodes. The F-N field emission current plays an important role in triggering the migration of Ni atoms. The nanogap is narrowed because of the transfer of Ni atoms from source to drain electrode. In the activation procedure, we defined the magnitude of a preset current Is and monitored the current I between the nanogap electrodes by applying voltage V. When the current I reached a preset current Is, we stopped the voltage V. As a result, the tunnel resistance of the nanogaps was decreased from the order of 100 T(omega) to 100 k(omega) with increasing the preset current Is from 1 nA to 150 microA. Especially, the devices formed by the activation with the preset current from 100 nA to 1.5 microA exhibited Coulomb blockade phenomena at room temperature. Coulomb blockade voltage of the devices was clearly modulated by the gate voltage quasi-periodically, resulting in the formation of multiple tunnel junctions of the SETs at room temperature. By increasing the preset current from 100 nA to 1.5 microA in the activation scheme, the charging energy of the SETs at room temperature was decreased, ranging from 1030 meV to 320 meV. Therefore, it is found that the charging energy and the number of islands of the SETs are controllable by the preset current during the activation. These results clearly imply that the activation procedure allows us to easily and simply fabricate planar-type Ni-based SETs operating at room temperature.     

Integration of single-electron transistors using field-emission-induced electromigration

A novel technique for the integration of planar-type single-electron transistors (SETs) composed of nanogaps is presented. This technique is based on the electromigration procedure, which is caused by a field emission current. The technique is called "activation." By applying the activation to the nanogaps, SETs can be easily obtained. Furthermore, the charging energy of the SETs can be controlled by adjusting the magnitude of the applied current during the activation process. The integration of two SETs was achieved by passing a field emission current through two series-connected initial nanogaps. The current-voltage (I(D)-V(D)) curves of the simultaneously activated devices exhibited clear electrical-current suppression at a low-bias voltage at 16 K, which is known as the Coulomb blockade. The Coulomb blockade voltage of each device was also obviously modulated by the gate voltage. In addition, the two SETs, which were integrated by the activation procedure, exhibited similar electrical properties, and their charging energy decreased uniformly with increasing the preset current during the activation. These results indicate that the activation procedure allows the simple and easy integration of planar-type SETs.     

Optimal Configuration of Hydrogen-Embrittlement-Fabricated Nanogaps for Surface-Conduction Electron-Emitter Display

Application of nanogaps for electron sources is fascinating in surface-conduction electron-emitter display. In contrast to rather complicated fabrication processes of the focused ion beam technique for the extremely narrow fissure, nanogaps fabricated by hydrogen embrittlement (HE) have thus been proposed as novel surface-conduction electron emitters due to their low turn-on voltage, high emission current, high focus capability, and high emission efficiency. In this paper, we theoretically investigate effects of the separation width and the tilted angle of the nanogaps fabricated by HE method on the field emission efficiency using a 3-D finite-difference time-domain particle-in-cell simulation technique. The structure with a large tilted angle may result in a high emitted current, but the collected current on the anode is suppressed due to the strong local field around the tip. A small structure prevents the emitted electrons from spreading out, and thus, no current could be collected by the anode. Also, the structure with a wide (or a narrow) separation of gap weakens (or enhances) the field around the tip and reduces the collected electrons. For better emission efficiency and focus capability, the separation width and the tilted angle of the examined structure could vary from 57 to 117 nm and 30 $^{circ}$ to 60$^{circ}$ , respectively.      

Magnetoresistance Properties of Planar-Type Tunnel Junctions With Ferromagnetic Nanogap System Fabricated by Electromigration Method

We report electromigration techniques for the fabrication of planar-type tunnel junctions with ferromagnetic nanogap system. In these techniques, by monitoring the current passing through the devices, we are easily able to obtain the planar-type Ni-Vacuum-Ni tunnel junctions. In this paper, magnetoresistance (MR) properties of the planar-type Ni-based tunnel junctions formed by stepwise feedback-controlled electromigration (SFCE) and field-emission-induced electromigration (activation) are studied. We performed the SFCE method for Ni nanoconstrictions connecting asymmetrical butterfly-shape electrodes. Furthermore, the activation technique was applied to Ni nanogaps with separations of 15-45 nm. MR ratio of the devices formed by the SFCE exhibited approximately 4% at 16 K . On the other hand, the devices fabricated by the activation showed MR ratio of above 300% at 16 K. These results suggest that it is possible to fabricate planar-type ferromagnetic tunnel junctions with vacuum barriers by electromigration techniques.     

Effect of Process Variation on Field Emission Characteristics in Surface-Conduction Electron Emitters

In this paper, we explore the effect of process variation on field emission characteristics in surface-conduction electron emitters. The structure of Pd thin-film emitter is fabricated on the substrate and the nanometer scale gap is formed by the focused ion beam technique. Different shapes of nanogaps due to the process variations are investigated by the experiment and three-dimensional Maxwell particle-in-cell simulation. Four deformation structures are examined, and it is found that the type 1 exhibits high emission efficiency due to a stronger electric field around the apex and larger emission current among structures. The electron emission current is dependent upon the angle of inclination of surface. Hydrogen plasma treatment is also used to increase the edge roughness of the nanogap and thereby dramatically improve the field emission characteristics. For the nanogap with a separation of 90 nm, the turn-on voltage significantly reduces from 60 to 20 V after the hydrogen plasma treatment.      

Fabrication of In-doped SnO2 nanowire arrays and its field emission investigations

The field emission of In-doped SnO₂ wire array has been performed in parallel plate diode configuration. A maximum current density of 60?µA/cm² is drawn from the emitter at an applied field of 4?V/µm. The nonlinearity in the Fowler–Nordheim plot, characteristics of semiconductor emitter has been observed and explained on the basis of electron emission from both the conduction and the valence bands. The current stability recorded at a preset value of 1?µA is observed to be good. The high emission current density, good current stability and mechanically robust nature of the wires offer unprecedented advantages as promising cold cathodes for many potential applications based on field emission.     

Field emission and photo-catalytic investigations on hierarchical nanostructures of copper doped CdS synthesized by kitchen-chemistry approach

We herein report an economical and facile technique for the synthesis of hierarchical nanostructures of Cu doped CdS nanostructures by microwave assisted solvothermal technique using a household microwave oven. We attempted to establish the effect of variation of solvents ratio on the morphological and optical properties of the obtained nanoscale structures. The field emission characteristics of the copper doped CdS nanoarchitecture have been studied and the turn on field is found to be 2.8 V/microm for an emission current density of approximately 0.1 microA/cm2. Emission current stability is determined at the preset current of approximately 1 microA and approximately 10 microA for the stable duration of approximately 4 hrs. The observed field emission results envisage the possibility of using the present emitter in the field emission sources. We believe that this is a unique report on the synthesis as well as field emission studies of copper doped CdS nanostructures. Photocatalytic dye degradation ability of the Cu doped CdS nanostructures is observed to be less than the undoped CdS counterparts.     

Track technology for creating the arrays of nickel microtips,micronozzles, and microtubes

A method for obtaining surface arrays of nickel objects (microtips, micronozzles, and microtubes) with preset geometric parameters by means of a track technology has been developed using the data of scanning electron microscopy. It is shown that Ni microtips with a radius as small as 10–25 nm can be obtained, which allows these microobject arrays to be used as multiple-tip field emission cathodes capable of providing high emission current densities.     

Real time atomic force microscopy imaging during nanogap formation by electromigration

We present real time atomic force microscopy imaging during nanogap fabrication by feedback controlled electromigration of a gold nanowire. The correlated measurements of electrical resistance and atomic force microscopy reveal that the major structural changes appear at the early stage of the process. Moreover, despite important morphological changes, the resistance of the nanowire shows a weak increase of just a few ohms. The detailed analysis of the atomic force microscopy images clearly shows that the electromigration process is strongly influenced by the initial microstructure of the nanowire.     

Field emission studies of Te nanorods grown on Si (111) substrate

Tellurium nanorods were grown on silicon (111) substrates by thermal evaporation. The synthesized Te nanorods were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), prior to the field emission investigations. The TEM image revealed that the nanorods are needle-like having diameter less than 20 nm and length in the range of 200-400 nm. The selected area electron diffraction (SAED) pattern and high resolution TEM micrographs clearly reveal the crystalline nature of the Te nanorods. The field emission studies were carried out in a planar diode (close proximity) configuration at background pressure of ∼1 × 10⁻⁹ mbar. An emission current density of ∼8.5 μA/cm² has been drawn at an applied field of ∼3.2 V/μm. The Folwer-Nodhiem plot, showed a non-linear behaviour. The high value of field enhancement factor (β ∼ 1 × 10⁴), estimated from the slope of the F-N plot, suggests that the emission is indeed from the nanometric tips of the Te nanorods. The emission current stability studied at the preset value ∼3.5 μA over duration of more than 3 h is found to be very good, suggesting the use of Te nanorods as promising electron source for field emission based micro/nano-electronic devices.     

Field electron emission from hydrogen plasma treated nano-ZnO thin films

A nano-Zno films are deposited on the Mo film/ceramic substrates by using the electron beam vapor deposition technique. Then a hydrogen plasma treated method is used to improve the characteristics of ZnO thin films by microwave plasma chemical vapor deposition system. Effects of process parameters on morphologies and structures of the ZnO thin films are detected and analysed by field emission scanning electron microscopy, X-ray diffraction spectrum and energy dispersive spectrum. The experimental result indicates that the hydrogen plasma treated techniques can essentially reduce the surface resistance and improve the field emission current density of the nano-ZnO thin films. For the hydrogen plasma treated sample, its field emission current density can increased more than three times at 2.2 V/microm electric field condition.     

Ultra-Low-Powered Aqueous Shear Stress Sensors Based on Bulk EG-CNTs Integrated in Microfluidic Systems

Novel aqueous shear stress sensors based on bulk carbon nanotubes (CNTs) were developed by utilizing microelectricalmechanical system (MEMS) compatible fabrication technology. The sensors were fabricated on glass substrates by batch assembling electronics-grade CNTs (EG-CNTs) as sensing elements between microelectrode pairs using dielectrophoretic technique. Then, the CNT sensors were permanently integrated in glass–polydimethylsiloxane (PDMS) microfluidic channels by using standard glass–PDMS bonding process. Upon exposure to deionized (DI) water flow in the microchannel, the characteristics of the CNT sensors were investigated at room temperature under constant current (CC) mode. The specific electrical responses of the CNT sensors at different currents have been measured. It was found that the electrical resistance of the CNT sensors increased noticeably in response to the introduction of fluid shear stress when low activation current (≪1 mA) was used, and unexpectedly decreased when the current exceeded 5 mA. We have shown that the sensor could be activated using input currents as low as 100 $mu$A to measure the flow shear stress. The experimental results showed that the output resistance change could be plotted as a linear function of the shear stress to the one-third power. This result proved that the EG-CNT sensors can be operated as conventional thermal flow sensors but only require ultra-low activation power ($sim 1$ $mu$W), which is $sim 1000$ times lower than the conventional MEMS thermal flow sensors.      

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

Ramp-rate limitation experiment using induced current method. Part 2: analysis

This paper describes an analysis of ramp-rate limitation experiments performed by a background magnet only without a power supply for the tested cable. Three-strand U-shape cable-in-conduit conductor samples show distinctive ramp-rate limitation phenomena with sensitive transition between no-quench and quench results. Quench experimental results at various ramp rates are explained by a representative induced loop current model. Since the sample length is less than 1 m and the loop resistance through the joint is an order of 1 , the dominant loop current is induced mostly through the joint. Multiple quench-recovery processes during continuous magnetic field ramp are also explained due to fast recovery of the sample after quench. It is experimentally observed that the strand heat flux condition can be very influential to determine quench-recovery process near the critical heat flux regime of liquid helium.     

Stable field emission from arrays of vertically aligned free-standing metallic nanowires

We present a fully elaborated process to grow arrays of metallic nanowires with controlled geometry and density, based on electrochemical filling of nanopores in track-etched templates. Nanowire growth is performed at room temperature, atmospheric pressure and is compatible with low cost fabrication and large surfaces. This technique offers an excellent control of the orientation, shape and nanowires density. It is applied to fabricate field emission arrays with a good control of the emission site density. We have prepared Co, Ni, Cu and Rh nanowires with a height of 3?μm, a diameter of 80?nm and a density of ～10(7)?cm(-2). The electron field emission measurements and total energy distributions show that the as-grown nanowires exhibit a complex behaviour, first with emission activation under high field, followed by unstable emission. A model taking into account the effect of an oxide layer covering the nanowire surface is developed to explain this particular field emission behaviour. Finally, we present an in situ cleaning procedure by ion bombardment that collectively removes this oxide layer, leading to a stable and reproducible emission behaviour. After treatment, the emission current density is ～1?mA?cm(-2) for a 30?V?μm(-1) applied electric field.     

Scaled fabrication of single-nanotube-tipped ends from carbon nanotube micro-yarns and their field emission applications

Joule-heating-induced electrical breakdown was applied to break suspended carbon nanotube (CNT) micro-yarns. The yarn ends at the breaking points were well-shaped sharp tips and mostly terminated by a single nanotube. The uppermost CNT was approximately 5?nm in diameter and was firmly compacted with the CNTs below it, yielding better mechanical, electrical and thermal contacts. An individual end could provide an emission current of approximately 25?μA, with potential application as a point electron source. In addition, we developed a pixel structure for a field emission display using oppositely aligned ends as the cathode and gate, respectively.     

A DC-RF Substitution Error in Duel-Element Bolometer Mounts

Most of the coaxial type bolometer mounts in current use employ a pair of bolometer elements which are connected in series for the dc or audio-frequency bias powers, and in parallel at radio or microwave frequencies. In the frequency range where these techniques are most often employed, the dc-RF substitution error has been generally believed to be negligible. It is quite possible, however, for this to be true of the elements individually, and yet fail to be true of a pair of these elements as used in a typical coaxial mount. If only the sum of the resistances of the two elements is maintained at a constant value, and if the resistance division between the two elements changes with the application of RF power, an error is introduced which is given by the equation: e = [(1/?b)-(1/?a)]?r, where ?a and ?b are the "ohms per milliwatt" coefficients of the two bolometer elements, and ?r is the shift in resistance division. An experimental study indicates that, in the existing state-of-the-art, this error may be ignored in many applications but is large enough to be important in others, particularly at the higher power levels.     

Field emission from a selected multiwall carbon nanotube

The electron field emission characteristics of individual multiwalled carbon nanotubes were investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental set-up ensures a precise evaluation of the geometric parameters (multiwalled carbon nanotube length and diameter and anode-cathode separation) of the field emission system. For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour was obtained, with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of ～30?V?μm(-1) and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1?μm were evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities was extensively investigated.     

AZ31B镁合金无氰脉冲镀锌层的结构与性能

镁合金耐腐蚀性差,氰化镀和化学镀虽能提高其耐蚀性,但分别存在许多问题,如污染环境、工艺复杂、与镀层结合力差、镀液寿命短等.为此,采用直流(DC)、脉冲电流(PC)方法在A231B镁合金表面电沉积锌层,以扫描电子显微镜(SEM)、X射线衍射(XRD)、极化曲线(Tafel)和电化学阻抗谱(EIS)等方法研究了沉积方式对镀层表面形貌、结构和耐蚀性的影响.结果表明:随时间的增加,镀层从初期疏松、多孔,逐渐趋于致密、平整;相同时间下,脉冲镀层更致密、孔隙更少;脉冲方式下晶面衍射峰强度较直流方式有所降低,半峰宽有变大趋势,晶粒有所细化,晶面取向由(101)择优取向变为随机取向;经直流和脉冲电镀锌后,镁合金表面腐蚀电势分别正向移动了0.42 V和0.48 V,腐蚀电流为80.4μA/cm2和34.57μA/cm2,脉冲条件下制备的镀层耐蚀性较直流制备的镀层有所提高.