Probing molecular junctions using surface plasmon resonance spectroscopy

The optical absorption spectra of nanometer-thick organic films and molecular monolayers sandwiched between two metal contacts have been measured successfully using surface plasmon resonance spectroscopy (SPRS). The electric field within metal-insulator (organic)-metal (MIM) cross-bar junctions created by surface plasmon-polaritons excited on the metal surface allows sensitive measurement of molecular optical properties. Specifically, this spectroscopic technique extracts the real and imaginary indices of the organic layer for each wavelength of interest. The SPRS sensitivity was calculated for several device architectures, metals, and layer thicknesses to optimize the organic film absorptivity measurements. Distinct optical absorption features were clearly observed for R6G layers as thin as a single molecular monolayer between two metal electrodes. This method also enables dynamic measurement of molecular conformation inside metallic junctions, as shown by following the optical switching of a thin spiropyran/polymer film upon exposure to UV light. Finally, optical and electrical measurements can be made simultaneously to study the effect of electrical bias and current on molecular conformation, which may have significant impact in areas such as molecular and organic electronics.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

The role of an amorphous carbon layer on a multi-wall carbon nanotube attached atomic force microscope tip in making good electrical contact to a gold electrode

Multi-wall carbon nanotube (MWNT) attached atomic force microscope (AFM) tips (MWNT tips) have good potential for use in AFM lithography. Good conducting MWNT tips are needed in such applications. However, characterizing the conductance of MWNT tips is nontrivial: making a good electrical contact between the MWNT and electrode is difficult. We observed that MWNT tips produced by hydrocarbon-deposition attachment usually do not make good electrical contacts to gold electrodes because of the thin and rough amorphous carbon layer on the MWNT that was unintentionally deposited during the attachment. We found that good contacts can be made if a more amorphous carbon layer is deposited to form a thick and smooth amorphous carbon layer on MWNTs. Good contact was made either by transformation of the amorphous carbon layer into a conducting or peel-off layer, exposing the bare MWNT surface. MWNT tips with an exposed MWNT surface showed the well-known high-current-flowing capacity and the stepped-cutting behavior of bare MWNTs. The peeling-off behavior of a thick amorphous carbon layer may be utilized in producing bare-surfaced MWNT tips that have good conductance and therefore are useful for applications.     

Deposition of conductive materials on textile and polymeric flexible substrates

This paper describes the study, analysis and selection of textile and similar materials to be used as flexible substrates for thin conductive film deposition, in the context of integrating electronics into textiles. Kapton^® polyimide was chosen as reference substrate material, was characterized regarding mechanical and electrical properties and was used as a basis for a comparison with several textile substrates. Samples were fabricated using physical vapour deposition (thermal evaporation) to deposit a thin layer of aluminium on top of Kapton and textile substrates. The measurement of electrical resistance of the thin aluminum films was carried out using the Kelvin method. To characterize the mechanical behaviour of the substrate and aluminum film, several mechanical tests were performed and results were compared between Kapton and these textile materials. The chemical composition of the textile substrates and aluminum films as well as the continuity of the films was characterized. This selection process identified the material that was closer to the behaviour of polyimide, a flexible, but non-elastic woven textile coated on both sides with PVC.     

Electrical contacts to one- and two-dimensional nanomaterials

Existing models of electrical contacts are often inapplicable at the nanoscale because there are significant differences between nanostructures and bulk materials arising from unique geometries and electrostatics. In this Review, we discuss the physics and materials science of electrical contacts to carbon nanotubes, semiconductor nanowires and graphene, and outline the main research and development challenges in the field. We also include a case study of gold contacts to germanium nanowires to illustrate these concepts.     

Rational design of amorphous indium zinc oxide/carbon nanotube hybrid film for unique performance transistors

Here we report unique performance transistors based on sol-gel processed indium zinc oxide/single-walled carbon nanotube (SWNT) composite thin films. In the composite, SWNTs provide fast tracks for carrier transport to significantly improve the apparent field effect mobility. Specifically, the composite thin film transistors with SWNT weight concentrations in the range of 0-2 wt % have been investigated with the field effect mobility reaching as high as 140 cm(2)/V·s at 1 wt % SWNTs while maintaining a high on/off ratio ～10(7). Furthermore, the introduction SWNTs into the composite thin film render excellent mechanical flexibility for flexible electronics. The dynamic loading test presents evidently superior mechanical stability with only 17% variation at a bending radius as small as 700 μm, and the repeated bending test shows only 8% normalized resistance variation after 300 cycles of folding and unfolding, demonstrating enormous improvement over the basic amorphous indium zinc oxide thin film. The results provide an important advance toward high-performance flexible electronics applications.     

STM"接触"式测量中的整流效应

在利用扫描隧道显微镜(STM)对Ag-TCNQ薄膜进行"接触"式的I-V特性测量中,发现由于接触势垒引起的整流效应.这为有机分子在电子学的应用方面,提供一种新的思路,从而设计新的有机电子器件.     

Recrystallization of amorphous silicon layers on sapphire

Electrical conductance measurements, X-ray diffraction, optical microscopy with transmitted light, megaelectronvolt ⁴He⁺ backscattering and channeling techniques were used to investigate the recrystallization characteristics of amorphous silicon layers on sapphire obtained by ion implantation or vacuum evaporation. It was found that the electrical conductance characteristics strongly depend on the mode of recrystallization which in turn depends on the ability of the amorphous silicon layer to form an interconnecting network of crystalline silicon before or after the crystallites have grown vertically through the entire film. The limitations of using electrical conductance measurements to deduce growth rates are discussed. It is also shown that optical microscopy can be useful in monitoring the nucleation and growth process. The temperature range for growth rate determination can be extended to 800 °C or more if a lateral motion method is used. The growth rates determined by this method were found to be in agreement with those determined by channeling and reflectivity measurements.     

还原氧化石墨烯基纳米银线透明导电薄膜研究进展

透明导电薄膜已广泛应用于印刷电子领域,传统的透明导电薄膜氧化铟锡（ITO）因其高脆性低柔韧性而不能满足高速发展的柔性电子行业;纳米银线（AgNWs）和石墨烯均具有良好光学性能、导电性能以及机械性能,使其能成为制备透明导电薄膜的理想材料。综述了近年来还原氧化石墨烯（rGO）基AgNWs透明导电薄膜的研究进展。介绍了柔性导电薄膜的关键参数及rGO/AgNWs透明导电薄膜的成膜工艺;归纳了影响rGO/AgNWs透明导电薄膜光电性能的主要因素和相关研究;阐述了rGO/AgNWs透明导电薄膜在印刷电子领域的应用现状,并展望了rGO/AgNWs透明导电薄膜的未来发展趋势。     

Evaluation of metal-nanowire electrical contacts by measuring contact end resistance

It is known, but often unappreciated, that the performance of nanowire (NW)-based electrical devices can be significantly affected by electrical contacts between electrodes and NWs, sometimes to the extent that it is really the contacts that determine the performance. To correctly understand and design NW device operation, it is thus important to carefully measure the contact resistance and evaluate the contact parameters, specific contact resistance and transfer length. A four-terminal pattern or a transmission line model (TLM) pattern has been widely used to measure contact resistance of NW devices and the TLM has been typically used to extract contact parameters of NW devices. However, the conventional method assumes that the electrical properties of semiconducting NW regions covered by a metal are not changed after electrode formation. In this study, we report that the conventional methods for contact evaluation can give rise to considerable errors because of an altered property of the NW under the electrodes. We demonstrate that more correct contact resistance can be measured from the TLM pattern rather than the four-terminal pattern and correct contact parameters including the effects of changed NW properties under electrodes can be evaluated by using the contact end resistance measurement method.     

Material Design of p‐Type Transparent Amorphous Semiconductor,Cu–Sn–I

Transparent amorphous semiconductors (TAS) that can be fabricated at low temperature are key materials in the practical application of transparent flexible electronics. Although various n‐type TAS materials with excellent performance, such as amorphous In‐Ga‐Zn‐O (a‐IGZO), are already known, no complementary p‐type TAS has been realized to date. Here, a material design concept for p‐type TAS materials is proposed utilizing the pseudo s‐orbital nature of spatially spreading iodine 5p orbitals and amorphous Sn‐containing CuI (a‐CuSnI) thin film is reported as an example. The resulting a‐CuSnI thin films fabricated by spin coating at low temperature (140 °C) have a smooth surface. The Hall mobility increases with the hole concentration and the largest mobility of ≈9 cm² V^?1 s^?1 is obtained, which is comparable with that of conventional n‐type TAS.     

Perspectives and challenges for organic thin film transistors: materials, devices, processes and applications

This paper reviews recent advancements in the field of organic electronics. Performance of p- and n-type conducting polymers and small molecule organic semiconductors is presented primarily in terms of mobility and current on/off ratio. Moreover, it presents a deep insight into different organic/inorganic materials used for the dielectric layer, electrodes and substrate for thin film transistors (TFTs). The electrical characteristics and performance parameters of single and dual gate structures are compared. In addition, performance dependence of organic TFT (OTFT) is discussed on the basis of contact resistance, channel length and thickness of the active layer. The paper thoroughly discusses several important applications of OTFTs including inverter, organic static random access memory, radio frequency identification tag and DNA sensors. It also includes several limitations and future prospects of organic electronics technology.     

Effects of the composition of sputtering target on the stability of InGaZnO thin film transistor

In this study, we investigated the electrical characteristics and the stability of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) from the viewpoint of active layer composition. Active layers of TFTs were deposited by r.f. sputtering. Two kinds of sputtering targets, which have different compositional ratios of In:Ga:Zn, were used to make variations in the active layer composition. All the fabricated IGZO TFTs showed more excellent characteristics than conventional amorphous silicon TFTs. However, in accordance with the Ga content, IGZO TFTs showed somewhat different electrical characteristics in values such as the threshold voltage and the field effect mobility. The device stability was also dependent on the Ga content, but had trade-off relation with the electrical characteristics.     

Thermal Conductivity Measurements of Thin Insulating Layers Deposited on High-Conducting Sheets

The thermal conductivity of thin insulating layers and coatings deposited on high-conducting sheets has been measured using the hot disk technique. The need for this type of measurements stems mainly from the electronics industry. In many situations, the materials supporting the thin layers or films are in the shape of thin sheets—often highly conducting ceramics, metals or anisotropic composites with a high-conducting component in the plane of the sheet. The present measurement setup has some interesting advantages with possibilities to design and optimize a system for performing convenient measurements on textiles. Although apparent properties are studied in the present investigation, the need to address thermal contact problems in general engineering constructions, including interfacial layers and thermal contact resistances, is discussed here. Experiences in this field indicate that, in order to perform correct thermal analysis and design, it is necessary to treat bulk material, thermal contact resistances, and interfaces separately. This is demonstrated by the fact that there is often a difference in interface conditions when performing a measurement as compared with the situation in which a manufactured component is being used.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Process-dependent Thermal-Mechanical Behaviors of an Advanced Thin-Flip-Chip-on-Flex Interconnect Technology with Anisotropic Conductive Film Joints

User experiences for electronic devices with high portability and flexibility, good intuitive human interfaces and low cost have driven the development of semiconductor technology toward flexible electronics and display. In this study proposes, an advanced flexible interconnect technology is proposed for flexible electronics, in which an ultra-thin IC chip having a great number of micro-bumps is bonded onto a very thin flex substrate using an epoxy-based anisotropic conductive film (ACF) to form fine-pitch and reliable interconnects or joints (herein termed ACF-typed thin-flip-chip-on-flex (TFCOF) technology). The electrical and thermal -mechanical performances of the micro-joints are the key to the feasibility and effectiveness of the technology. Thus, the main goal of the study is to assess the process-induced thermal-mechanical behaviors of the interconnect technology during the bonding process. To undertake the process modeling, a process-dependent simulation methodology that integrates both thermal and nonlinear thermal-mechanical finite element (FE) analyses together with ANSYS^® birth-death modeling technique is proposed. The validity of the process modeling is confirmed through various temperature and warpage measurements. Subsequently, the contact behaviors of the ACF joints under four-point bending and static bending tests are characterized through FE modeling. The simulated contact stresses are further correlated with the measured electrical resistance data using four-point probe method, by which the minimum threshold contact stress for achieving a reliable contact electrical performance is determined.     

Der Nachweis dünner Fremdschichten auf Kontaktoberflächen

The detection of thin contaminated layers on contact surfaces . Today electrical contacts must satisfy requirements which are growing steadily. The detection of very thin contaminated layers at contact surfaces therefore becomes very important. Clad gold contacts were contaminated with very small amounts of Zink. Contact resistance measurements and surface analysis measurements by AES and SIMS were shown to be adequate for the detection of this surface contamination.     

WSe2 Homojunction Devices: Electrostatically Configurable as Diodes,MOSFETs, and Tunnel FETs for Reconfigurable Computing

In this paper, electrostatically configurable 2D tungsten diselenide (WSe₂) electronic devices are demonstrated. Utilizing a novel triple‐gate design, a WSe₂ device is able to operate as a tunneling field‐effect transistor (TFET), a metal–oxide–semiconductor field‐effect transistor (MOSFET) as well as a diode, by electrostatically tuning the channel doping to the desired profile. The implementation of scaled gate dielectric and gate electrode spacing enables higher band‐to‐band tunneling transmission with the best observed subthreshold swing (SS) among all reported homojunction TFETs on 2D materials. Self‐consistent full‐band atomistic quantum transport simulations quantitatively agree with electrical measurements of both the MOSFET and TFET and suggest that scaling gate oxide below 3 nm is necessary to achieve sub‐60 mV dec^?1 SS, while further improvement can be obtained by optimizing the spacers. Diode operation is also demonstrated with the best ideality factor of 1.5, owing to the enhanced electrostatic control compared to previous reports. This research sheds light on the potential of utilizing electrostatic doping scheme for low‐power electronics and opens a path toward novel designs of field programmable mixed analog/digital circuitry for reconfigurable computing.     

Scaling down of organic thin film transistors: short channel effects and channel length-dependent field effect mobility

Organic thin film transistors with P3HT (poly-3-hexylthiophene) as active semiconducting layer, channel lengths from 0.3 to 20 μm, and gate oxide thicknesses from 15 to 170 nm have been successfully fabricated on Si substrates. The measurement results show that the channel length over oxide thickness ratio should be large enough (i.e., the vertical electric field should be at least 10 times higher than the lateral electric filed) in order to suppress the short channel effects of transistors. The field effect mobility of long channel devices (L ≥ 5 μm) is about an order of magnitude larger than small channel devices (L from 0.3 to 2.5 μm), which could be attributed to the more severe contact resistance effects between organic materials and metal contacts for devices with smaller dimensions.     

Zinc oxide thin films: Characterization and potential applications

Zinc oxide (ZnO) has attracted recent interest for a range of applications, including use as a transparent conductive oxide (TCO) and in gas sensor devices. This paper compares ZnO films grown using two methods designed for the production of thin films, namely sol-gel and aerosol assisted chemical vapour deposition (AACVD) for potential use in sensor and TCO applications. Materials produced by the sol-gel route were observed to be amorphous when annealed at 350 °C, but were crystalline when annealed at higher temperatures and had a relatively open grain structure when compared to the AACVD films. Electrical characterization showed that materials were highly resistive, but that their properties varied considerably when the measurements were performed in vacuum or in air. This behaviour was rapidly reversible and reproducible for room temperature measurement.In contrast materials grown by aerosol-assisted CVD were non-porous, polycrystalline and conductive. Measured electrical properties did not vary with changing measurement atmosphere. These differences are discussed in terms of the structural characterisation of the films and some comments are made regarding the suitability of both approaches for the growth of ZnO thin film sensor materials.     

Estimation of thermal properties of composite materials without instrumentation inside the samples

Recent contributions of parameter estimation in the measurement of thermal properties are of great importance. In comparison with other techniques such as steady state (hot guarded plate, etc.) or transient (line source method, flash method, etc.), the use of parameter estimation provides more information and, in most cases, produces faster results. With this technique the thermal conductivity and the volumetric specific heat are estimated simultaneously and as a function of time, temperature, or position. This method requires experimental data, such as transient temperature and heat flux measurements. Previously, the temperature measurements came from thermocouples embedded in the sample. These thermocouples are introduced in the sample either by drilling holes or by molding the material around a series of thermocouples. Both operations are time-consuming and costly and are needed for each sample. In this study, temperature measurements are made only on the two sides of the samples with thin resistance thermometers. Since the sensors are not inside the material, the effect of the thermal contact conductance between sensor and sample was first investigated. The value of this thermal contact conductance was estimated by using samples of high-conductivity material. Using these values, the estimated thermal properties obtained with surface temperature measurements are compared with values provided by other methods for several low-thermal conductivity materials; agreement has been very good.