Uniform Electrical Properties Research Articles

A percolation network model to predict the electrical property of flexible CNT/PDMS composite films fabricated by spin coating technique

Carbon nanotubes (CNTs) based flexible films have great potential on the flexible electronics due to their relatively uniform electrical properties in the film and ease of fabrication technology. In this work, CNT/polydimethylsiloxane (PDMS) composite films with various film thickness was fabricated successfully. By introducing a percolation network model, this work reveals theoretically that the observed rapidly drops of conductivity in thin films mainly depends on the change of CNT network morphology during spin coating. Junctions formed by structural distorted tubes are also observed at the cross section of film samples. By considering both CNT re-orientation and CNT structural distortion into percolation network model, the simulation results are in good agreement with the experimental data. Furthermore, a comprehensive numerical investigation reveals the relationship between film conductivity and some core parameters including CNT loadings, CNT intrinsic conductivity and CNT aspect ratio.

A Facile Method for the Growth of Organic Semiconductor Single Crystal Arrays on Polymer Dielectric toward Flexible Field‐Effect Transistors

AbstractPolymer dielectrics with intrinsic mechanical flexibility are considered as a key component for flexible organic field‐effect transistors (OFETs). However, it remains a challenge to fabricate highly aligned organic semiconductor single crystal (OSSC) arrays on the polymer dielectrics. Herein, for the first time, a facile and universal strategy, polar surface‐confined crystallization (PSCC), is proposed to grow highly aligned OSSC arrays on poly(4‐vinylphenol) (PVP) dielectric layer. The surface polarity of PVP is altered periodically with oxygen‐plasma treatment, enabling the preferential nucleation of organic crystals on the strong‐polarity regions. Moreover, a geometrical confinement effect of the patterned regions can also prevent multiple nucleation and misaligned molecular packing, enabling the highly aligned growth of OSSC arrays with uniform morphology and unitary crystallographic orientation. Using 2,7‐dioctyl[1]benzothieno[3,2‐b]benzothiophene (C8‐BTBT) as an example, highly aligned C8‐BTBT single crystal arrays with uniform molecular packing and crystal orientation are successfully fabricated on the PVP layer, which can guarantee their uniform electrical properties. OFETs made from the C8‐BTBT single crystal arrays on flexible substrates exhibit a mobility as high as 2.25 cm2 V−1 s−1, which has surpassed the C8‐BTBT polycrystalline film‐based flexible devices. This work paves the way toward the fabrication of highly aligned OSSCs on polymer dielectrics for high‐performance, flexible organic devices.

Channel-restricted meniscus self-assembly for uniformly aligned growth of single-crystal arrays of organic semiconductors

A channel-restricted meniscus self-assembly (CRMS) method is developed for wafer-scale, highly uniform, aligned growth of organic semiconductor single crystals (OSSCs). Field-effect transistors (FETs) made from the OSSC arrays exhibit a high average mobility of up to 30.3 cm 2 V −1 s −1 with good uniformity among devices. This method is general for the growth of various OSSC arrays, facilitating the applications of OSSCs in large-area, high-performance organic electronic devices. Organic semiconductor single-crystal (OSSC)-based field-effect transistors (FETs) with high mobility and small device-to-device variation enable OSSCs to be adapted for practical applications. Research attention has recently been focused on developing simple ways of fabricating large-area OSSC arrays by means of solution-coating techniques. However, the lack of control of the meniscus front, where the nucleation and growth of organic crystals occur, leads to inconsistent crystal alignment and consequently induces large variation in device performance. Here, we propose a universal strategy, termed the channel-restricted meniscus self-assembly (CRMS) method to fabricate ultrahigh-mobility, uniform OSSC arrays. The microscale photoresist channels used in this method produce a confinement effect to reduce the size of the meniscus, enabling the homogeneous nucleation of OSSCs at the meniscus front. Meanwhile, the dip-coating process ensures consistent molecular packing in the OSSCs and thus guarantees their highly uniform electrical properties. Using 2,6-diphenylanthracene as an example, wafer-scale (>2 inch) OSSC arrays with very small size variations (10%) are successfully prepared, which is very difficult to achieve by using the previously reported methods. As a result, field-effect transistors (FETs) based on the OSSC arrays show a high average hole mobility of up to 30.3 cm 2 V −1 s −1 with good uniformity among devices. This method is general for the growth of various OSSC arrays, facilitating the applications of OSSCs in large-area, high-performance organic electronic devices.

Electrochemical Mechanism and Structure Simulation of 2D Lithium‐Ion Battery

AbstractFor decades, lithium batteries have attracted much attention. Highly efficient, safe, and convenient batteries are worthy of being investigated and applied in modern life. However, most of the investigation is on the macroscopic device electrical properties such as current–voltage curves. Investigation on the microscopic electrical properties including potential distribution, electrolyte salt concentration distribution, and lithium ion concentration distribution within the batteries during the discharging is still needed. Here, a detailed study of the key electrical properties distributions inside the lithium battery in the discharging status is given. After comparing three widely used cathode materials (LiCoO2, LiMn2O4, and LiFePO4), LiFePO4 is found to show the most stable working performance which is consistent with previous reports. This is probably attributed to its most uniform key electrical properties distributions, its high electric conductivity, and crystal structure. A structure is also proposed which might give a hint to stress and temperature release for a longer and safer application of batteries. This simulation study on the key parameters distribution inside the battery may help to achieve a better understanding and hence control on the lithium battery performance and reliability.

Selective sorting of semiconducting single-walled carbon nanotubes using thienylenevinylene-based conjugated polymers with high alkyl side-chain density

The demand for high-purity semiconducting single-walled carbon nanotubes (sc-SWNTs) has increased considerably with the aim of utilizing their superior properties in a range of future applications. Among post-sorting methods, conjugated polymers have been regarded as one of the candidates to selectively isolate sc-SWNTs with uniform electrical properties. Herein, we demonstrate the ability to selectively sort two types of SWNTs by the two polymers PCTV18T and PC12TV18T, which have different alkyl side-chain densities. PC12TV18T, with a high alkyl chain density, shows great sorting ability for both high-pressure carbon monoxide and plasma-torch-grown SWNTs with a weight ratio of almost 1:1 in toluene solution. In addition, it is found that PC12TV18T selectively sorts sc-SWNTs with the high purity. The chirality and diameters of the enriched sc-SWNTs are further confirmed by Raman spectroscopy and photoluminescence excitation/emission mapping. Finally, we fabricate bottom gate/bottom contact thin-film transistors using the enriched sc-SWNTs as semiconductors to verify the electrical performance. Devices with well-percolated sc-SWNT networks displayed p-dominant properties with average charge-carrier mobilities of 2.05 cm2 V−1 S−1 and 9.87 cm2 V−1 S−1 and on/off current ratios of approximately 104 and 105 for high-pressure carbon monoxide and plasma-torch-grown SWNTs, respectively.

Dry and Direct Deposition of Aerosol-Synthesized Single-Walled Carbon Nanotubes by Thermophoresis.

Single-walled carbon nanotubes (SWCNTs) show great potential as an active material in electronic and photonic devices, but their applicability is currently limited by shortcomings in existing deposition methods. SWCNTs can be dispersed from liquid solutions; however, their poor solubility requires the use of surfactants and ultrasonication, causing defects and degradation in device performance. Likewise, the high temperatures required by their chemical vapor deposition growth limit substrates on which SWCNTs can be directly grown. Here, we present a systematic study of the direct deposition of pristine, aerosol-synthesized SWCNTs by thermophoresis. The density of the deposited nanotube film can be continuously adjusted from individual, separated nanotubes to multilayer thin films by changing the deposition time. Depending on the lateral flow inside the thermophoretic precipitator, the angular distribution of the deposited SWCNT film can be changed from uniform to nonuniform. Because the substrate is kept at nearly ambient temperature, deposition can be thus carried out on practically any flat substrate with high efficiencies close to unity. The thermophoretic terminal velocity of SWCNTs, determined by aerosol loss measurements, is found to be approximately one-third of the usual prediction in the free molecular regime and shows a weak dependence on the nanotube diameter. As a demonstration of the applicability of our technique, we have used thermophoretic deposition in the fabrication of carbon nanotube thin-film transistors with uniform electrical properties and a high, over 99.5%, yield.

Alleviation of parasitic reactions for III-nitride epitaxy in MOCVD with a spatial separated source delivery method by controlling the main reaction type

A spatial separated source delivery (SSSD) method has been proposed to alleviate parasitic reactions between group III precursors and ammonia (NH3) in gas phase using the MOCVD. AlGaN alloys and GaN films grown on sapphire has been prepared with this method. The influence of parasitic reactions on the qualities and properties of AlGaN and GaN films is investigated in detail. High-resolution X-ray diffraction (HRXRD) measurements show that the aluminum composition in materials rises up close to the ratio of trimethylaluminum (TMAl) supply when the supply ratio of TMAl is 0.2, 0.3 and 0.4 respectively, which demonstrates that the parasitic reactions are suppressed mostly. In-situ monitor reflectometer spectra for GaN show that the growth rate gets a 20% increase in comparison with materials grown without the SSSD method. Subsequently, the relationship between parasitic reactions and other reaction types are elucidated. The effect of the SSSD method on parasitic reactions is also systematically studied. Reactions are controlled to change from combination reactions to pyrolysis reactions and surface reactions with the SSSD method. A 2-inch GaN/AlGaN heterostructure is also prepared to evaluate the uniform electrical properties grown with this method. The result indicates that the SSSD method provides a better way to prepare high-quality III-nitride materials, especially for AlN and related alloys.

Arbitrary cross-section SEM-cathodoluminescence imaging of growth sectors and local carrier concentrations within micro-sampled semiconductor nanorods.

Future one-dimensional electronics require single-crystalline semiconductor free-standing nanorods grown with uniform electrical properties. However, this is currently unrealistic as each crystallographic plane of a nanorod grows at unique incorporation rates of environmental dopants, which forms axial and lateral growth sectors with different carrier concentrations. Here we propose a series of techniques that micro-sample a free-standing nanorod of interest, fabricate its arbitrary cross-sections by controlling focused ion beam incidence orientation, and visualize its internal carrier concentration map. ZnO nanorods are grown by selective area homoepitaxy in precursor aqueous solution, each of which has a (0001):+c top-plane and six {1–100}:m side-planes. Near-band-edge cathodoluminescence nanospectroscopy evaluates carrier concentration map within a nanorod at high spatial resolution (60 nm) and high sensitivity. It also visualizes +c and m growth sectors at arbitrary nanorod cross-section and history of local transient growth events within each growth sector. Our technique paves the way for well-defined bottom-up nanoelectronics.

Surface and Electrical Properties of 2 wt% Cr-doped Ni Ultrathin Film Electrode for MLCCs

In this study, 2 wt% Cr-doped Ni thin films were deposited using DC sputtering on a bare Si substrate using a 4 inch target at room temperature. In order to obtain ultrathin films from Cr-doped Ni thin films with high electrical properties and uniform surface, the micro-structure and electrical properties were investigated as a function of deposition time. For all deposition times, the Cr-doped Ni thin films had low average resistivity and small surface roughness. However, the resistivity of the Cr-doped Ni thin films at various ranges showed large differences for deposition times below 90 s. From the results, 120 s is considered as the appropriate deposition time for Cr-doped Ni thin films to obtain the lowest resistivity, a low surface roughness, and a small difference of resistivity. The Cr-doped Ni thin films are prospective materials for microdevices as ultrathin film electrodes.

Observatory geoelectric fields induced in a two-layer lithosphere during magnetic storms

We report on the development and validation of an algorithm for estimating geoelectric fields induced in the lithosphere beneath an observatory during a magnetic storm. To accommodate induction in three-dimensional lithospheric electrical conductivity, we analyze a simple nine-parameter model: two horizontal layers, each with uniform electrical conductivity properties given by independent distortion tensors. With Laplace transformation of the induction equations into the complex frequency domain, we obtain a transfer function describing induction of observatory geoelectric fields having frequency-dependent polarization. Upon inverse transformation back to the time domain, the convolution of the corresponding impulse-response function with a geomagnetic time series yields an estimated geoelectric time series. We obtain an optimized set of conductivity parameters using 1-s resolution geomagnetic and geoelectric field data collected at the Kakioka, Japan, observatory for five different intense magnetic storms, including the October 2003 Halloween storm; our estimated geoelectric field accounts for 93% of that measured during the Halloween storm. This work demonstrates the need for detailed modeling of the Earth’s lithospheric conductivity structure and the utility of co-located geomagnetic and geoelectric monitoring.

Large-area formation of self-aligned crystalline domains of organic semiconductors on transistor channels using CONNECT

The electronic properties of solution-processable small-molecule organic semiconductors (OSCs) have rapidly improved in recent years, rendering them highly promising for various low-cost large-area electronic applications. However, practical applications of organic electronics require patterned and precisely registered OSC films within the transistor channel region with uniform electrical properties over a large area, a task that remains a significant challenge. Here, we present a technique termed "controlled OSC nucleation and extension for circuits" (CONNECT), which uses differential surface energy and solution shearing to simultaneously generate patterned and precisely registered OSC thin films within the channel region and with aligned crystalline domains, resulting in low device-to-device variability. We have fabricated transistor density as high as 840 dpi, with a yield of 99%. We have successfully built various logic gates and a 2-bit half-adder circuit, demonstrating the practical applicability of our technique for large-scale circuit fabrication.

Wrinkle-free graphene with spatially uniform electrical properties grown on hot-pressed copper

The chemical vapor deposition (CVD) of graphene on Cu substrates enables the fabrication of large-area monolayer graphene on desired substrates. However, during the transfer of the synthesized graphene, topographic defects are unavoidably formed along the Cu grain boundaries, degrading the electrical properties of graphene and increasing the device-to-device variability. Here, we introduce a method of hot-pressing as a surface pre-treatment to improve the thermal stability of Cu thin film for the suppression of grain boundary grooving. The flattened Cu thin film maintains its smooth surface even after the subsequent high temperature CVD process necessary for graphene growth, and the formation of graphene without wrinkles is realized. Graphene field effect transistors (FETs) fabricated using the graphene synthesized on hot-pressed Cu thin film exhibit superior field effect mobility and significantly reduced device-to-device variation.

Time causal operational estimation of electric fields induced in the Earth's lithosphere during magnetic storms

In support of projects for monitoring geomagnetic hazards for electric power grids, we develop a simple mathematical formalism, consistent with the time causality of deterministic physics, for estimating electric fields that are induced in the Earth's lithosphere during magnetic storms. For an idealized model of the lithosphere, an infinite half-space having uniform electrical conductivity properties described by a galvanic tensor, we work in the Laplace-transformed frequency domain to obtain a transfer function which, when convolved with measured magnetic field time series, gives an estimated electric field time series. Using data collected at the Kakioka, Japan observatory, we optimize lithospheric conductivity parameters by minimizing the discrepancy between model-estimated electric field variation and that actually measured. With our simple model, we can estimate 87% of the variance in storm time Kakioka electric field data; a more complicated model of lithospheric conductivity would be required to estimate the remaining 13% of the variance. We discuss how our estimation formalism might be implemented for geographically coordinated real-time monitoring of geoelectric fields.

20am2-E2 酸化銅ナノ粒子還元を用いたフェムト秒レーザ直接描画プロセス

CuO nanoparticles were directly reduced to form Cu micropattems using femtosecond laser writing. CuO nanoparticles were dispersed into reductant agent and base polymers. When femtosecond laser pulses were focused and irradiated into the CuO nanoparticle based films on glass substrate, Cu micropattems were formed by reducing and sintering of CuO nanoparticles. X-ray diffraction spectra of the laser irradiated area shows Cu peaks although the Cup peaks were not observed in that of non-laser irradiated area. The width of the Cu micropattems was increased with decreasing laser pulse energy and scanning speed. Cu-rich patterns were obtained by laser irradiation with high laser scanning speed. The resistance of the line patterns were inversely proportional to the line width which indicated that the Cu micropattems have uniform electrical properties.

Uniform and Non-uniform Thermoelement Subject to Lateral Heat Convection

A general energy equation of quasi-one-dimensional heat flow in a longitudinal thermoelement (TE) of a curved side that is subjected to an electric field and convection heat transfer on the curved surface is developed. The energy equation is solved for the temperature distribution in two cases; uniform cross-section TE and non-uniform cross-section TE. Analytical solutions for a uniform cross-section TE with uniform electrical and thermophysical properties are obtained, whereas numerical solutions are provided for a non-uniform cross-section TE. Two parameters playing a vital role in the thermal performance of the TE are identified: the heat resistance ratio (HRR) and the energy growing ratio (EGR). The HRR represents the ratio of the longitudinal conduction maximum thermal resistance to the lateral convection maximum thermal resistance. The EGR represents the ratio of Joule’s electrical heating to Fourier’s heat conduction. The effects of varying these two parameters, as well as the TE geometry, have been thoroughly investigated.

Electromechanical behavior of single-walled carbon nanotube thin films

Carbon nanotubes have been intensively studied owing to their great potential in nanoelectronics and nanomechanical devices. Recently, experimental results have shown that single-walled carbon nanotubes (SWCNTs) change their electronic properties when subjected to strain. In this study, the electromechanical characteristics of SWCNT networks were investigated for the application of printable strain sensors on flexible substrates. SWCNT films were formed on plastic substrates of poly(ethylene terephthalate) using a spray process. In this manner, we were able to control the transparency and obtain uniform electrical properties of the films. The films are isotropic on account of the random orientation of bundles of SWCNTs. Experimental results showed a nearly linear change in the resistance across a film when it was subjected to tensile strain, even in the inelastic range of the flexible substrate. The results demonstrate the potential use of SWCNT films for highly sensitive printable strain sensors on a macroscale.

Anodic Alumina Films Prepared by Powerful Pulsed Discharge Oxidation

A novel powerful pulsed discharge anodization technique has been applied to prepare dense, homogeneous oxide films of 16–180 nm thick on aluminum surface. The Volta potential difference (VPD) of the obtained films was studied by scanning Kelvin probe force microscopy. The VPD value was measured as a function of the film thickness and was compared with the similar dependences for the anodic alumina films prepared by the conventional galvanostatic and potentiostatic methods. The anodic films present polarization because of the embedded charges localized near the film interfaces. It was shown that despite the apparent differences in the respective anodization processes, the pulsed discharge films demonstrate the magnitude of polarization which is entirely comparable with that of the conventional anodic alumina films of the same thickness. At the same time, the pulsed discharge films are characterized by more uniform surface structure and electrical properties. The characteristic features of the films such as...

Thickness Control of Graphene Overlayer via Layer-by-Layer Growth on Graphene Templates by Chemical Vapor Deposition

We examined graphene growth on graphene templates prepared by the mechanical exfoliation of graphite crystals using a sloped-temperature chemical vapor deposition (CVD) apparatus with ethanol. The structural characterization of the grown graphene layers in detail by atomic force microscopy and Raman spectroscopy reveals that the film thickening of graphene overlayers proceeds by the layer-by-layer growth mode, and that film thickness increases linearly as a function of growth time. This result indicates that a graphene growth technique using the CVD apparatus is a potential approach to precisely controlling the number of graphene layers, which is one of the important subjects for fabricating practical devices with uniform electrical properties using graphene as the channel material, such as field effect transistors.

Efficient Numerical Modal Solutions for RF Propagation in Lossy Circular Waveguides

The propagation of electromagnetic waves is reconsidered in the context of cylindrical tunnels through material of uniform electrical properties. In the absence of internal conductors, the wave solutions reduce to well-known analytic expressions, requiring the solution of a single transcendental equation in complex phase space. Any given tunnel configuration has a wide range of solutions in phase and frequency space. Identifying these solutions can be a significant numerical challenge. Both the Newton-Raphson method and the winding number technique have been described in previous publications, and each can be prone to missing solutions. This paper reviews these two solution techniques and develops a combination of the two to provide a fast and efficient procedure for locating the roots in well-defined regions of phase space.

Controlled synthesis of millimeter-long silicon nanowires with uniform electronic properties.

We report the nanocluster-catalyzed growth of ultralong and highly uniform single-crystalline silicon nanowires (SiNWs) with millimeter-scale lengths and aspect ratios up to approximately 100,000. The average SiNW growth rate using disilane (Si 2H 6) at 400 degrees C was 31 microm/min, while the growth rate determined for silane (SiH 4) reactant under similar growth conditions was 130 times lower. Transmission electron microscopy studies of millimeter-long SiNWs with diameters of 20-80 nm show that the nanowires grow preferentially along the 110 direction independent of diameter. In addition, ultralong SiNWs were used as building blocks to fabricate one-dimensional arrays of field-effect transistors (FETs) consisting of approximately 100 independent devices per nanowire. Significantly, electrical transport measurements demonstrated that the millimeter-long SiNWs had uniform electrical properties along the entire length of wires, and each device can behave as a reliable FET with an on-state current, threshold voltage, and transconductance values (average +/-1 standard deviation) of 1.8 +/- 0.3 microA, 6.0 +/- 1.1 V, 210 +/- 60 nS, respectively. Electronically uniform millimeter-long SiNWs were also functionalized with monoclonal antibody receptors and used to demonstrate multiplexed detection of cancer marker proteins with a single nanowire. The synthesis of structurally and electronically uniform ultralong SiNWs may open up new opportunities for integrated nanoelectronics and could serve as unique building blocks linking integrated structures from the nanometer through millimeter length scales.