Electrical readouts of single and few molecule systems in metal-molecule-metal device structures

Electrical conduction through molecular junctions are measured in different local environments through two test beds that are ideal for single/few molecule and molecular monolayer systems. A technique has been developed to realize Au films with approximately 1.5 A surface roughness comparable to the best available techniques and suitable for formation of patterned device structures. The technique utilizes room temperature e-beam evaporated Au films over oxidized Si substrates silanized with (3-mercaptopropyl)trimethoxysilane (MPTMS). The lateral (single/few molecule) and vertical (many molecules) device structures are both enabled by the process for realizing ultraflat Au layer. Lateral metal-molecule-metal (M-M-M) device structures are fabricated by forming pairs of Au electrodes with nanometer separation (nano-gap) through an electromigration-induced break-junction (EIBJ) technique at room temperature and conductivity measurements are carried out for dithiol functionalized single molecules. We have used the flat Au layer (using the current technique) as the bottom contact in vertical M-M-M device structures. Here, molecular self-assembly are formed on the Au surface, and patterned (20 x 20 microm2) top Au contacts were successfully transferred on to the device using a stamping technique (where the Au is deposited on a polydimethylsiloxane (PDMS) pad and following a physical contact on the thiolated Au layer). The single molecular property of XYL, a highly conductive molecule and many molecular property of HS-C9-SH, an insulating molecule in its molecular monolayer form are measured. Observation of enhanced conduction following molecular deposition, and comparison of conductance-voltage characteristics to those predicted theoretically, confirms the success of trapping single/few molecules in the nano-gap. The observed approximately 10(2) less conductance through the molecular monolayer of HS-C9-SH compared to the estimation of a linear sum of single molecule conductances over large area indicate that either all the molecules are not in physical contact with the top stamping electrode or electrode-molecule coupling has a less broadening in presence of it own environment or both.     

Formation of electrically conducting mesoscale wires through self-assembly of atomic clusters

Bi and Sb clusters deposited from an inert gas aggregation source have been used to form cluster-assembled wires on unpassivated, and SiO/sub 2/ passivated, V-grooved Si substrates. V-grooves (4-7 /spl mu/m in width, 6 /spl mu/m-1 mm in length) were prepared using optical lithography and anisotropic etching in KOH solution. The effectiveness of the surface templating technique was demonstrated by scanning electron microscope analysis carried out after deposition. When Sb clusters were deposited onto SiO/sub 2/ passivated substrates, the surface coverage was seen to vary from <20% on the unpatterned (normal-to-beam) surface (which is required to be nonconducting) to >100% at the apexes of the V-grooves used to promote growth of the wire. Sb wires produced with this technique currently have minimum widths of /spl sim/400 nm and lengths of /spl sim/1 mm. Electrical contacts can be positioned within the V-grooves prior to cluster deposition, thus enabling the initial onset of conduction and subsequent I(V) characteristic of a wire to be monitored in vacuum.     

Fabrication of resonant-tunneling diodes by Sb surfactant modified growth of Si films on CaF/sub 2//Si

Molecular beam epitaxy growth of Si thin films on CaF/sub 2//Si(111) substrates has been studied. A surfactant-modified solid-phase epitaxy method, where the room temperature Si deposition was followed by annealing under Sb flux, resulted in a continuous, smooth epitaxial crystalline Si film with a sharp (/spl radic/3/spl times//spl radic/3)R30/spl deg/ reconstruction and a surface roughness of 0.15-nm rms for a 2.8-nm Si thin film. This growth technique was used to fabricate CaF/sub 2//Si/CaF/sub 2/ double-barrier resonant tunneling diodes in SiO/sub 2/ windows patterned on Si(111) substrates. A negative differential resistance (NDR) peak was found at /spl sim/0.35 V at 77 K, and the current density at the NDR peak was estimated to be 3-4 orders of magnitude higher than in earlier reports.     

Localized growth of suspended SWCNTs by means of an "all-laser" process and their direct integration into nanoelectronic devices

We have successfully developed an "all-laser" processing for the localized growth of suspended single-wall carbon nanotubes (SWCNTs) on prepatterned SiO/sub 2//Si substrates. Our "all-laser" process stands out by its exclusive use of the same KrF excimer laser, first, to deposit the embedded-catalyst electrodes with a controllable architecture and, second, to grow SWCNTs through the pulsed laser ablation of a pure graphite target. Under the optimal growth conditions, the suspended SWCNTs are shown to bridge laterally adjacent electrodes separated by a gap of /spl sim/2 /spl mu/m. These SWCNTs (having diameters in the 1.25-1.64-nm range) generally tend to auto-assemble into bundles of /spl sim/5--15 nm in diameter. The "all-laser" process here developed offers the advantage of a direct integration of the SWCNTs into field-effect-transistor-like devices with no postprocessing, thereby permitting the investigation of their electrical transport properties. Thus, the suspended SWCNT bundles are shown to behave collectively as an ambipolar transistor with ON/OFF switching ratios as high as /spl sim/10/sup 4/.     

Nanometer-scale analysis of current limited stresses impact on SiO/sub 2/ gate oxide reliability using C-AFM

A conductive atomic force microscope (C-AFM) has been used to analyze at a nanometer scale the impact of the current limitation on the breakdown (BD) of thin (<6 nm) SiO/sub 2/ gate oxides of metal-oxide-semiconductor (MOS) structures. The high-lateral resolution of the technique (/spl sim/10 nm) allows to get more insight in the BD phenomenology and to study, independently, the effect of the current limit on different post-BD oxide properties such as the oxide conductivity at the primary location where the event is triggered (S/sub 0/) and the size of the broken-down region (S/sub BD/). The results show that the conductivity at S/sub 0/, the total area affected by the BD and the structural damage of the oxide increase when a current limitation is not imposed during the electrical stress, leading to harder BD events. The results demonstrate that the C-AFM is a very suitable tool to perform a complete analysis of the BD phenomenology at such reduced scale.     

Spontaneous Assembly of Extremely Long,Horizontally‐Aligned,Conductive Gold Micro‐Wires in a Langmuir Monolayer Template

“Bottom‐up” technologies are based upon the premise that organized systems – from the nano‐scale up to the macro‐scale – can be assembled spontaneously from basic building blocks in solution. We demonstrate a simple strategy for the generation of extremely long (up to several centi­meters), horizontally‐aligned gold micro‐wires, produced through a surfactant monolayer template deposited from gold thiocyanate [Au(SCN)₄⁻] aqueous solution. Specifically, we show that the surfactant, octyl‐maleimide (OM), spontaneously forms oriented micro‐wires at the air/water interface, which constitute a template for deposition of metallic gold through binding and crystallization of the soluble gold complex. The Au micro‐wires can be subsequently transferred onto solid substrates, and following plasma treatment and gold enhancement exhibit excellent conductivity even at electrode spacings of several centimeters. Importantly, the micro‐wire alignment determines the direction of electrical current, demonstrating that long‐range ordering of the micro‐wires can be accomplished, significantly affecting the physical properties of the system. The new approach is simple, robust, and can be readily exploited for bottom‐up fabrication of micro‐wire assemblies and transparent conductive electrodes.     

Au/锗/氧化硅纳米多层膜/p-Si结构的电致发光机制研究

用射频磁控溅射法制备了锗/氧化硅纳米多层膜,在室温下测量了Au/锗/氧化硅纳米多层膜/pSi结构的电致发光.利用位形坐标模型分析了锗/氧化硅纳米多层膜的发光中心,并用量子限制-发光中心模型对该纳米结构的电致发光过程作了研究,研究表明锗/氧化硅纳米多层膜的电致发光主要来自SiO2层的发光中心.     

Gamma radiation-induced changes in the electrical and optical properties of tellurium dioxide thin films

Thin films of tellurium dioxide (TeO/sub 2/) were investigated for /spl gamma/-radiation dosimetry purposes. Samples were fabricated using thermal evaporation technique. Thin films of TeO/sub 2/ were exposed to a /sup 60/Co /spl gamma/-radiation source at a dose rate of 6 Gy/min at room temperature. Absorption spectra for TeO/sub 2/ films were recorded and the values of the optical band gap and energies of the localized states for as-deposited and /spl gamma/-irradiated samples were calculated. It was found that the optical band gap values were decreased as the radiation dose was increased. Samples with electrical contacts having a planar structure showed a monotonic increase in the values of current with the increase in radiation dose up to a certain dose level. The observed changes in both the optical and the electrical properties suggest that TeO/sub 2/ thin film can be considered as an effective material for room temperature real time /spl gamma/-radiation dosimetry.     

Controlled nucleation and growth of surface-confined gold nanoparticles on a (3-aminopropyl)trimethoxysilane-modified glass slide: a strategy for SPR substrates

The thickness of the gold film and its morphology, including the surface roughness, are very important for getting a good, reproducible response in the SPR technique. Here, we report a novel alternative approach for preparing SPR-active substrates that is completely solution-based. Our strategy is based on self-assembly of the gold colloid monolayer on a (3-aminopropyl)trimethoxysilane-modified glass slide, followed by electroless gold plating. Using this method, the thickness of films can be easily controlled at the nanometer scale by setting the plating time in the same conditions. Surface roughness and morphology of gold films can be modified by both tuning the size of gold nanoparticles and agitation during the plating. Surface evolution of the Au film was followed in real time by UV-vis spectroscopy and in situ SPRS. To assess the surface roughness and electrochemical stability of the Au films, atomic force microscopy and cyclic voltammetry were used. In addition, the stability of the gold adhesion is demonstrated by three methods. The as-prepared Au films on substrates are reproducible and stable, which allows them to be used as electrodes for electrochemical experiments and as platforms for studying SAMs.     

Heat conduction across monolayer and few-layer graphenes

We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.     

Stochastic assembly of sublithographic nanoscale interfaces

We describe a technique for addressing individual nanoscale wires with microscale control wires without using lithographic-scale processing to define nanoscale dimensions. Such a scheme is necessary to exploit sublithographic nanoscale storage and computational devices. Our technique uses modulation doping to address individual nanowires and self-assembly to organize them into nanoscale-pitch decoder arrays. We show that if coded nanowires are chosen at random from a sufficiently large population, we can ensure that a large fraction of the selected nanowires have unique addresses. For example, we show that N lines can be uniquely addressed over 99% of the time using no more than /spl lceil/2.2log/sub 2/(N)/spl rceil/+11 address wires. We further show a hybrid decoder scheme that only needs to address N=O(W/sub litho-pitch//W/sub nano-pitch/) wires at a time through this stochastic scheme; as a result, the number of unique codes required for the nanowires does not grow with decoder size. We give an O(N/sup 2/) procedure to discover the addresses which are present. We also demonstrate schemes that tolerate the misalignment of nanowires which can occur during the self-assembly process.     

CO-sensing properties of In/sub 2/O/sub 3/-doped SnO/sub 2/ thick-film sensors: effect of doping concentration and grain size

In/sub 2/O/sub 3/-doped SnO/sub 2/ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl/sub 4/ 5H/sub 2/O) and indium nitrate. The doping concentration was varied from 7.718/spl times/10/sup -5/ to 3.859/spl times/10/sup -4/ moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300/spl deg/C-900/spl deg/C. Thick-film samples with a thickness of /spl sim/15 /spl mu/m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220/spl deg/C-240/spl deg/C. A blue shift in the sensing temperature and increase in sensitivity factor (S/sub f/) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of /spl sim/5 is found for the highest doping concentration (3.859/spl times/10/sup -4/ moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.     

Recent developments in high curie temperature perovskite single crystals

The temperature behavior of various relaxor-PT piezoelectric single crystals was investigated. Owing to a strongly-curved morphotropic phase boundary, the usage temperature of these perovskite single crystals is limited by T/sub R-T/- the rhombohedral to tetragonal phase transformation temperature - which occurs at the significantly lower temperatures than the Curie temperature T/sub c/. Attempts to modify the temperature usage range of Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZNT) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMNT) rhombohedral crystals (T/sub c/ /spl sim/ 150-170/spl deg/C, T/sub R-T/ /spl sim/ 60-120/spl deg/C) using minor dopant modifications were limited, with little success. Of significant potential are crystals near the morphotropic phase boundary in the Pb(Yb/sub 1/2/Nb/sub 1/2/)O/sub 3/-PbTiO/sub 3/ (PYNT) system, with a T/sub c/ > 330/spl deg/C, even though T/sub R-T/ was found to be only half the value at /spl sim/160/spl deg/C. Single crystals in the novel BiScO/sub 3/-PbTiO/sub 3/ system offer significantly higher T/sub c/s > 400/spl deg/C, while exhibiting electromechanical coupling coefficients k/sub 33/ > 90% being nearly constant till the T/sub R-T/ temperature around 350/spl deg/C, which greatly increases the temperature range for transducer applications.     

Demagnetizing factors for rectangular prisms

For rectangular prisms of dimensions 2a/spl times/2b/spl times/2c with constant material susceptibility /spl chi/, we have calculated and tabulated the fluxmetric and magnetometric demagnetizing factors N/sub f/ and N/sub m/, defined along the 2c dimension as functions of c/(ab)/sup 1/2/(=1/spl sim/500), a/b(=1/spl sim/256), and /spl chi/(=0/spl sim/10/sup 9/). We introduce an interpolation technique for obtaining N/sub f,m/ with arbitrary values of c/(ab)/sup 1/2/, a/b, and /spl chi/.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Amperometric determination of total cholesterol at gold electrodes covalently modified with cholesterol oxidase and cholesterol esterase with use of thionin as an electron mediator

Immobilization of cholesterol oxidase (EC 1.1.3.6) (ChOx) on a gold electrode was attempted by cross-linking using glutaraldehyde between ChOx molecules and a self-assembled monolayer of 2-aminoethanethiolate. The resulting electrode (ChOx/Au) exhibits an amperometric response to free cholesterol in the presence of thionin as an electron mediator, and a steady-state response is obtained approximately 60 s after injection of cholesterol into the electrolyte solution. Coimmobilization of cholesterol esterase (EC 3.1.1.13) (ChE) and ChOx (ChE/ChOx/Au) allows the amperometric determination of both esterified cholesterol and free cholesterol. Cyclic voltammetry of the ChE/ChOx/Au and the dependence of the amperometric response to cholesterol on the concentration of thionin suggest that thionin is encapsulated in the enzyme film on the electrode surface. Apparent Michaelis constants of the ChOx/Au and the ChE/ChOx/Au electrodes suggest that the amperometric response was controlled by penetration of the reaction substrate into the films of the enzyme(s). The concentration of total (free and esterified) cholesterol in human serum samples, determined by using the techniques developed in the present study, is in good agreement with that determined by the well-established technique using colorimetry.     

Gold-nanoparticle-functionalized In₂O₃ nanowires as CO gas sensors with a significant enhancement in response

We present the room-temperature sensing of gold nanoparticle (AuNP)-functionalized In(2)O(3) nanowire field-effect transistor (NW-FET) for low-concentration CO gas. AuNPs were functionalized onto In(2)O(3) nanowires via a self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS-SAM). The nanowires were mounted onto the Au electrodes with both ends in Schottky contacts. High sensor response toward low concentration of CO gas (200 ppb-5 ppm) at room temperature is achieved. The presence of AuNPs on the surface of In(2)O(3) nanowire serves to enhance the CO oxidation due to a higher oxygen ion-chemisorption on the conductive AuNP surfaces. Detailed studies showed that the sensing capabilities were greatly enhanced in comparison to those of bare nanowires or low coverage of Au NP-decorated nanowires. When the sensor is exposed to CO, the CO molecules interact with the preadsorbed oxygen ions on the AuNP surface. The CO oxidation on the AuNPs leads to the transfer of electrons into the semiconducting In(2)O(3) nanowires and this is reflected as the change in conductance of the NW-FET sensor. This work provides a promising approach for fabricating nanowire devices with excellent sensing capabilities at room temperature.     

Dielectrophoretic batch fabrication of bundled carbon nanotube thermal sensors

We present a feasible technology for batch assembly of carbon nanotube (CNT) devices by utilizing ac electrophoretic technique to manipulate multiwalled bundles on an Si/SiO/sub 2/ substrate. Based on this technique, CNTs were successfully and repeatably manipulated between microfabricated electrodes. By using this parallel assembly process, we have investigated the possibility of batch fabricating functional CNT devices when an ac electrical field is applied to an array of microelectrodes that are electrically connected together. Preliminary experimental results showed that over 70% of CNT functional devices can be assembled successfully using our technique, which is considered to be a good yield for nanodevices manufacturing. Besides, the devices were demonstrated to potentially serve as novel thermal sensors with low power consumption (/spl sim/microwatts) with electronic circuit response of approximately 100 kHz in constant current mode operation. In this paper, we will present the fabrication process of this feasible batch manufacturable method for functional CNT-based thermal sensors, which will dramatically reduce production costs and production time of nanosensing devices and potentially enable fully automated assembly of CNT-based devices. Experimental results from the thermal sensors fabricated by this batch process will also be discussed.     

Thickness-dependent leakage current of (polyvinylidene fluoride/lead titanate) pyroelectric detectors

The novel pyroelectric IR detectors have been fabricated using the Polyvinylidene Fluoride (PVDF)/Lead Titanate (PT) pyroelectric bilayer thin films, which were deposited onto Pt(111)/SiO/sub 2//Si(100) substrates by a sol-gel process. The ceramic/polymer structure was constructed of the randomly oriented polycrystalline PT film (/spl sim/1 /spl mu/m) heated at 700/spl deg/C for 1 h and the /spl beta/-phase PVDF film crystallized at 65/spl deg/C for 2 h. The effects of PVDF thin film thickness (100 /spl sim/ 580 nm) on the pyroelectric response of IR detectors were studied. The results show that the depositions of PVDF thin films onto the PT films will cause the leakage current (J) of the detectors decrease from 6.37/spl times/10/sup -7/ A/cm/sup 2/ to 3.86/spl times/10/sup -7/ A/cm/sup 2/. The specific detectivity (D*) measured at 100 Hz decreased from 2.72/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W for detector without PVDF to 1.71/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W for detector with PVDF thickness of 580 nm. By optimizing the ratio of the specific detectivity (D*) to leakage current, D*/J, the detector with PVDF thickness of 295 nm exhibits the best performance.     

Ultrahigh-sensitive tin-oxide microsensors for H/sub 2/S detection

Ultrahigh-sensitivity SnO/sub 2/-CuO sensors were fabricated on Si(100) substrates for detection of low concentrations of hydrogen sulfide. The sensing material was spin coated over platinum electrodes with a thickness of 300 nm applying a sol-gel process. The SnO/sub 2/-based sensors doped with copper oxide were prepared by adding various amounts of Cu(NO/sub 3/)/sub 2/.3H/sub 2/O to a sol suspension. Conductivity measurements of the sensors annealed at different temperatures have been carried out in dry air and in the presence of 100 ppb to 10-ppm H/sub 2/S. The nanocrystalline SnO/sub 2/-CuO thin films showed excellent sensing characteristics upon exposure to low concentrations of H/sub 2/S below 1 ppm. The 5% CuO-doped sensor having an average grain size of 20 nm exhibits a high sensitivity of 2.15/spl times/10/sup 6/ (R/sub a//R/sub g/) for 10-ppm H/sub 2/S at a temperature of 85/spl deg/C. By raising the operating temperature to 170/spl deg/C, a high sensitivity of /spl sim/10/sup 5/ is measured and response and recovery times drop to less than 2 min and 15 s, respectively. Selectivity of the sensing material was studied toward various concentrations of CO, CH/sub 4/, H/sub 2/, and ethanol. SEM, XRD, and TEM analyses were used to investigate surface morphology and crystallinity of SnO/sub 2/ films.