Reliable Contacts Research Articles

Patterning Superconductivity in a Topological Insulator.

Topologically protected states in combination with superconductivity hold great promise for quantum computing applications, but the progress on electrical transport measurements in such systems has been impeded by the difficulty of fabricating devices with reliable electrical contacts. We find that superconductivity can be patterned directly into Bi2Se3 nanostructures by local doping with palladium. Superconducting regions are defined by depositing palladium on top of the nanostructures using electron beam lithography followed by in situ annealing. Electrical transport measurements at low temperatures show either partial or full superconducting transition, depending on the doping conditions. Structural characterization techniques indicate that palladium remains localized in the targeted areas, making it possible to pattern superconducting circuits of arbitrary shapes in this topological material.

(Invited) Electronic and Photophysical Properties of Surface-Anchored Metal-Organic Frameworks (SURMOFs)

The huge versatility and unparalleled level of control in the design of metal-organic frameworks (MOFs) has recently been shown to also carry a huge potential for applications based on the electrical, electronic and photophysical properties of this rich class of materials. Recent examples are MOF-based electrochemical [1,2], photo-electrochemical [3] and photovoltaic devices [4,5], as well as photon-upconversion [6]. Since at present methods to provide reliable and reproducible contacts to MOF-materials are scarce, we have developed a method to produce well-defined, highly oriented and monolithic MOF thin films supported on different types of surfaces, also referred to as SURMOFs [7]. This procedure in particular allows the fabrication of hetero-multilayers (see Fig. 1). In this talk, we will describe the fabrication of such well-defined model systems as well as the results of detailed, multi-technique investigations of electrical [8] and photophysical properties exhibited by empty and loaded MOFs, including cyclic voltammetry, atomic force microscopy, quartz crystal microbalance and UV-Vis spectroscopy. In this talk, we will demonstrate that the huge size of the chemical space spanned by MOFs can be further drastically increased by loading MOFs with nanoparticles or quantum dots. In this context, SURMOFs offer particular advantages since the loading can be carried out in a layer-by-layer fashion, offering the potential to fabricate hetero-mulitlayers.

Electrochemical deposition of iridium and iridium-nickel-alloys

To develop durable and reliable electronic contacts, precious metals are still very important for finish plating of contact surfaces. The lesser-known iridium might be an interesting alternative to substitute gold alloys, platinum or rhodium for applications with highest demands to wear and corrosion resistance such as sliding and plug contacts. As matters stand there is no commercial electrolyte for iridium plating. Initial investigation screened the parameter range for different iridium compounds when an iridium layer occurred on the substrates. This approach showed that the oxidation state of iridium is crucial to reach contenting deposits. Best results came from Ir(IV) electrolyte with high bromine concentration coming from the starting compound, while electrolytes made from Ir(III) compounds gave very poor deposits. In subsequent experiments different organic compounds were added to the electrolytes to improve plating efficiency and stability of the solutions. So found electrolytes gave crack-free deposits up to two microns with a micro-hardness of 600 HV. To reduce the iridium content in the layer, iridium-nickel-alloys were investigated, finding that a nickel-content of 10 wt% raised the layer hardness to more than 900 HV.

A mobile microcell for measuring the electrical characteristics of individual nanowires and nanotubes

A mobile measuring cell was proposed and created using a focused ion beam as a nanoknife for measuring the electrical characteristics of individual nanowires and nanotubes by the four-wire method. The cell consists of two elements: the basic one, on whose surface a nanoconductor is deposited, and the auxiliary element that provides the electrical connection of the primary element to the wires of the external measuring circuit. Micro- and nanoscale wires in both elements of the cell were formed from gold films deposited on a silicon oxide surface. This provides reliable contacts between a nanoobject and the wires. In this paper, the application of the proposed method was demonstrated using examples of a multiwalled carbon nanotube and a thin platinum nanowire. The electrical resistance of the nanotube was measured using the two-wire method, whereas the current–voltage characteristic for the platinum nanowire was measured via the fourwire method.

Mo/Ni and Ni/Ta–W–N/Ni thin-film contact layers for (Bi,Sb)2Te3-based intermediate-temperature thermoelectric elements

We have examined the possibility of utilizing thin-film contact layers for producing reliable Ohmic contacts to proposed intermediate-temperature (Bi,Sb)2Te3-based thermoelectric materials with improved thermoelectric properties, which allow the working temperature range to be extended to 600 K. Three contact configurations have been produced by ion-plasma magnetron sputtering: a single Ni layer, Mo/Ni bilayer, and Ni/Ta–W–N/Ni three-layer system. It has been shown that reliable contacts can be produced using Mo/Ni and Ni/Ta–W–N/Ni layers, which prevent interdiffusion between the materials to be joined and ensure good adhesion to the thermoelectric element.

In situ reduced graphene oxide interlayer for improving electrode performance in ZnO nanorods

The effect of reduced graphene oxide (RGO) thin film on the transport characteristics of vertically aligned zinc oxide nanorods (ZnO NRs) grown on ITO substrate was studied. GO was uniformly drop casted on ZnO NRs as a passivation layer and then converted into RGO by heating it at 60 °C prior to metal electrode deposition. This low temperature reduction is facilitated by the thermally excited electrons from ZnI interstitial sites (~30 meV). Successful reduction of GO was ascertained from the increased disorder band (D) intensity in the Raman spectra. Temperature (298 K–10 K) dependent transport measurements of RGO–ZnO NRs indicate that the RGO layer not only acts as a short circuiting inhibitor but also reduces the height of the potential barrier for electron tunneling. This is confirmed from the temperature dependent electrical characteristics which revealed a transition of carrier transport from thermionic emission at high temperature (T > 100 K) to tunneling at low temperature (T < 100 K) across the interface. Our technique is the most promising approach for making reliable electrical contacts on vertically aligned ZnO NRs and improving the reproducibility of device characteristics.

Film Quality and Electronic Properties of a Surface‐Anchored Metal‐Organic Framework Revealed by using a Multi‐technique Approach

AbstractThe virtually unlimited versatility and unparalleled level of control in the design of metal‐organic frameworks (MOFs) has recently been shown to also entail a potential for applications based on the electrical and electronic properties of this rich class of materials. At present, methods to provide reliable and reproducible contacts to MOF materials are scarce; therefore, we have carried out a detailed, multi‐technique investigation of an empty and loaded prototype MOF, HKUST‐1. Epitaxial thin films of this material grown on a substrate by using liquid‐phase epitaxy have been studied by cyclic voltammetry, atomic force microscopy, and quartz crystal microbalance and their quality assessed. By using an ionic liquid as the electrolyte, it is shown that redox‐active molecules like ferrocene can be embedded in the pores, enabling a change in the overall conductivity of the framework and the study of the redox chemistry of guest molecules inside the MOF.

Piezomodulation of Connection Conductance TlInTe&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

This paper discusses the development method of strain sensors based on acicular single crystal compound TlInTe2 is grown by Bridgman-Stockbarger technique. Creating mechanically reliable ohmic contacts on said obtained single crystals was carried out directly by spot welding wires corresponding to the ends of the capacitor discharge heated in a stream of inert gas from the single crystal TlInTe2. This method of creating contacts proved effective and reliable. Sensors made by this manner stuck to the calibrated beams of steel thickness of 1 mm, a length of 30 mm and a corresponding optimal regime got sensors with a maximum piezo-sensitivity. It was found that if enshrined at one end of a thin steel sheet with glued crystals TlInTe2 initiate periodic mechanical vibrations to the same frequency, occurs conductivity modulation. The effect of piezo-modulation conductance depending on the degree of deformation, illumination in interval of deformation and lux illumination in room temperature were studied. The study showed that the more mechanical deformation and illumination are, the more modulation amplitude is. We studied the modulation of the conductivity of crystals during mechanical deformation values of 8, 14, 19, 23 and 27 × 10-5and illuminance 1000, 2750, 4750 and 6500 suites. Investigations the level piezo-signal depending on the amplitude of mechanical vibrations at frequency of 85 Hz, it is found that with increasing magnitude of the mechanical deformation of 7 × 10-5 and 26 × 10-5 amplitude increases six times. It is shown that the piezoresistive effect is clearly manifested in dynamic mode. Additional conductivity occurs during mechanical deformation. When the illuminated samples piezoconductivity magnitude increases linearly.

Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm(2)/(V s) in MoS2 at low temperature.

High-performance field-effect transistor-type glucose biosensor based on nanohybrids of carboxylated polypyrrole nanotube wrapped graphene sheet transducer

We report a rapid-response and high-sensitivity field-effect transistor (FET)-based sensor with specificity towards glucose, based on reduced graphene oxide (rGO)-carboxylated polypyrrole (C-PPy) nanotube (NT) hybrids as the conductive channel. The rGO, C-PPy NTs, and rGO/C-PPy NT hybrids were characterized using Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results indicate that a well-organized structure was successfully prepared, based on specific interactions between the C-PPy NTs and the graphene sheets. Reliable electrical contacts were developed between the rGO/C-PPy NTs and the patterned-microelectrodes, which remained stable when exposed to the liquid-phase electrolyte. Liquid-ion-gated FETs composed of these rGO/C-PPy NT hybrids exhibited hole-transport behavior with higher conductivity than those of graphene sheets or C-PPy NTs because the C-PPy NTs formed a bridge between the graphene layers, resulting in effective electron transport. Glucose oxidase (GOx) was used as the capture probe, and was tightly combined with C-PPy NTs via a chemical coupling reaction, and glucose detection was performed via GOx, which catalyzes the oxidation of glucose in the rGO/C-PPy NTs hybrid FET biosensor. The FET biosensor provided a rapid response (< 1s) with high sensitivity toward glucose with a limit of detection of 1nM. This result is ca. 2–3 orders more sensitive than previous reported glucose sensor. The FET-type biosensor was highly reproducible and stable in air over a period of one month. Furthermore, the liquid-gated FET-type biosensor displayed specificity toward glucose in a mixed solution containing compounds found in biological fluids.

Transferred metal electrode films for large-area electronic devices

The evaporation of metal-film gate electrodes for top-gate organic field-effect transistors (OFETs) limits the minimum thickness of the polymer gate dielectric to typically more than 300 nm due to deep hot metal atom penetration and damage of the dielectric. We show here that the self-release layer transfer method recently developed for high-quality graphene transfer is also capable of giving high-quality metal thin-film transfers to produce high-performance capacitors and OFETs with superior dielectric breakdown strength even for ultrathin polymer dielectric films. Dielectric breakdown strengths up to 5–6 MV cm−1 have been obtained for 50-nm thin films of polystyrene and a cyclic olefin copolymer TOPAS® (Zeon). High-quality OFETs with sub-10 V operational voltages have been obtained this way using conventional polymer dielectrics and a high-mobility polymer semiconductor poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene-2,5-diyl]. The transferred metal films can make reliable contacts without damaging ultrathin polymer films, self-assembled monolayers and graphene, which is not otherwise possible from evaporated or sputtered metal films.

Field-effect-induced two-dimensional electron gas utilizing modulation-doped ohmic contacts

Modulation-doped AlGaAs/GaAs heterostructures are utilized extensively in the study of quantum transport in nanostructures, but charge fluctuations associated with remote ionized dopants often produce deleterious effects. Electric field-induced carrier systems offer an attractive alternative if certain challenges can be overcome. We demonstrate a field-effect transistor in which the active channel is locally devoid of modulation-doping, but silicon dopant atoms are retained in the ohmic contact region to facilitate reliable low-resistance contacts. A high quality two-dimensional electron gas is induced by a field-effect and is tunable over a wide range of density. Device design, fabrication, and low temperature (T=0.3K) transport data are reported.

Gastroenterology in the former GDR (1975-1990) and the changes after German reunification

This short overview sketches the state of Gastroenterology in the GDR (1975 - 1990) from the point of view of an East-German contemporarywitness. The "Society for Gastroenterology/GDR" (GfG) has played a decisive role for the development of the Gastroenterology in the GDR. The society promoted medical education and constitutions of gastroenterological centers, fostered gastroenterological research and controlled the standards for the recognition of Gastroenterology as a state-accepted medical sub-discipline. An extensive program of scientific and educative events included two-annual meetings of scientific congresses, the "Berka-Talks", endoscopic workshops" and featured special symposia such as for Hepatology, Pancreatology and gastro-intestinal Microbiology. Temporary working groups developed technical and professional legal advice. Although the GfG was a full member of the respective international organizations (OMGE, ASNEMGE, ESGE), it was almost impossible building up reliable international contacts in a mutual interest. Especially, contacts with colleagues representing the "German Society of Digestion and Metabolic Diseases" (DGVS) were impeded. With the political changes of 1989/1990, an association of the two German Societies for Gastroenterology seemed within reach. At a meeting in Halle (Saale) (March, 22nd, 1990), representatives of DGVS and GfG quickly agreed on modalities to merge the two societies. After the 45th meeting of the DGVS (October 3rd-6th, Essen) more than 600 GDR physicians could join the BRD society under accommodating conditions. The GfG had fulfilled its historical function as a "bridge" during the division of Germany with dignity and was suspended (November, 24nd,1990).

Field Effect Transistor with Electrodeposited ZnO Nanowire Channel

ZnO nanowires were prepared by electrochemical deposition in polycarbonate ion track templates. After the deposition process the polymer templates were dissolved in dichloromethane and the nanowires were harvested by ultrasonication in isopropyl. A droplet of nanowire suspension was placed on a Si/SiO2 substrate patterned with interdigitated electrodes. By means of electron beam lithography single nanowires were selected and provided with electrical contacts. We found that in order to obtain reliable electrical contacts and typical field effect characteristics the electrode deposition process needs to be adapted to the 3 D shape of the wires and that annealing and passivation treatments are necessary.

Field Effect Transistor with Electrodeposited Nanowire Channels

Zinc oxide is one of the most intense studied materials these days and the applications sought range from energy production and photocathalysis to light emitting devices, sensors or logic circuits. A wide band gap (3.3 eV) semiconductor, with an excitonic binding energy of 60 meV zinc oxide is attractive besides other properties to its high occurrence/low cost and non-toxicity. The material is also remarkable from the point of view of polymorphism of its nanostructures, from belts and rods to platelets and flowers, tunable as a function of preparation method and specific conditions. Different electronic devices were fabricated using zinc oxide, among them of utmost importance being field effect transistors. ZnO nanowires were prepared by electrochemical deposition in polycarbonate ion track templates. After the deposition process the polymer templates were dissolved in dichloromethane and the nanowires were harvested by ultrasonication in isopropyl. A droplet of nanowire suspension was placed on a Si/SiO2substrate patterned with interdigitated electrodes. By means of electron beam lithography single nanowires were selected and provided with electrical contacts. We found that in order to obtain reliable electrical contacts and typical field effect characteristics the electrode deposition process needs to be adapted to the 3 D shape of the wires and that annealing and passivation treatments are necessary. Our results show an excellent behaviour of the nanowire based devices, including here typical field effect transistor behaviour, with saturated source – drain characteristics and Ion to Ioff ratios of about 104. The transport properties show carrier mobility in excess of 100 cm2/Vs.

Formation of reliable electrical and thermal contacts between graphene and metal electrodes by laser annealing

A new approach for electrical and thermal improvement of contacts between carbon nanostructures (multi-wall carbon nanotubes – MWCNTs and multi-layer graphene – MLG) and metal electrodes by localized laser heating is presented. The nanostructures were deposited over electrodes using the dielectrophoresis (DEP) technique. A focused laser beam was used for direct heating the samples in ambient atmosphere. The Raman spectroscopy was used to determine the process temperature by observations of the graphitic G-line downshift. In the laser annealing experiments, the G-line position was found first to downshift linearly with laser power indicating gradual heating of the sample proportional to the absorbed power. However, with increasing power the shift was found to saturate at levels that depend on the metal and the contact area. This saturation was attributed to gradual increase of the contact area and improvement of the thermal contacts between the nanostructures and metal electrode that can occur during sample heating. The maximum sample temperature in the beginning of the annealing process was always higher for MLG samples, due to smaller area of contact established between rigid multi-layer graphene and initially rough metal surface. The final result is the increased heat losses to the electrodes and, subsequently, the reduction of the samples temperature.The main advantage of this method, when compared with traditional and rapid thermal annealing, is that the thermal treatment is localized in a small pre-determined region, allowing individually controlled annealing process.

Graphene diffusion barrier for forming ohmic contact on N-polar n-type GaN for high-power vertical-geometry light-emitting diodes

Graphene sheets were employed as a diffusion barrier to form highly reliable ohmic contacts to N-polar n-GaN for high-power vertical-configuration LEDs (VLEDs). Before annealing, both Ti/Al and Ti/graphene/Al contacts exhibit ohmic behavior. After annealing at 250 °C, unlike the Ti/Al contact, the Ti/graphene/Al contacts remain ohmic with a contact resistivity of 7.5 × 10−4 Ω cm2. Secondary ion mass spectroscopy (SIMS) results show that the graphene sheets serve as a barrier to the outdiffusion of Ga atoms into the electrode. On the basis of SIMS and X-ray photoemission spectroscopy results, the ohmic and degradation behaviors of the Ti-based contacts are described and discussed.

Using ultra-high molecular weight hydrophilic polymer as cathode interlayer for inverted polymer solar cells: Enhanced efficiency and excellent air-stability

New cathode interlayers based on a water soluble cationic polyacrylamide (C-PAM) with a viscosity-averaged molecular weight of 3 million were introduced as cathode interlayers in inverted solar cells. The neat C-PAM and 5% CsF doped C-PAM thin layers on ITO substrate could decrease the work-function of ITO and hence lead to suitable energy level alignments for efficient electron collection. Obvious enhancements of energy conversion efficiency as well as air-stability were found, in comparison to well-known ZnO interlayer based device. The possible factors involving deteriorations at two interfaces of ITO/interlayer and interlayer/active layer during more than 1 year storage under an ambient condition were analyzed. Our results suggest that the interlayer polymer with giant molecular size and numerous side groups of hydrophilic amide and quaternary ammonium would facilitate the formations of large and intimate single-molecular binding area with the ITO substrate and the upper active layer, and might afford more reliable interfacial contacts to resist possible moisture- and/or stress-induced delamination. Therefore, hydrophilic polymer with an ultra-high molecular weight would be an ideal cathode interlayer for efficient and air-stable inverted solar cells.

Nanofabrication techniques of highly organized monolayers sandwiched between two electrodes for molecular electronics

AbstractIt is expected that molecular electronics, i.e., the use of molecules as critical functional elements in electronic devices, will lead in the near future to an industrial exploitable novel technology, which will open new routes to high value-added electronic products. However, despite the enormous advances in this field several scientific and technological challenges should be surmounted before molecular electronics can be implemented in the market. Among these challenges are the fabrication of reliable, robust and uniform contacts between molecules and electrodes, the deposition of the second (top) contact electrode, and development of assembly strategies for precise placement of molecular materials within device structures. This review covers advances in nanofabrication techniques used for the assembly of monomolecular films onto conducting or semiconducting substrates as well as recent methods developed for the deposition of the top contact electrode highlighting the advantages and limitations of the several approaches used in the literature. This contribution also aims to define areas of outstanding challenges in the nanofabrication of monomolecular layers sandwiched between two electrodes and opportunities for future research and applications.

Polypyrrole Nanotube Embedded Reduced Graphene Oxide Transducer for Field-Effect Transistor-Type H2O2 Biosensor

We report a rapid-response and high-sensitivity sensor with specificity toward H2O2 based on a liquid-ion-gated field-effect transistor (FET) using graphene-polypyrrole (PPy) nanotube (NT) composites as the conductive channel. The rGO, PPy, NTs, and nanocomposite materials were characterized using Raman spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). On the basis of these results, a well-organized structure is successfully prepared owing to the specific interactions between the PPy NTs and the rGO sheet. Reliable electrical contacts were developed between the rGO/PPy NTs and the microelectrodes, which remained stable when exposed to the liquid-phase analyte. Liquid-ion-gated FETs composed of these graphene nanocomposites exhibited hole-transport behavior with conductivities higher than those of rGO sheets or PPy NTs. This implies an interaction between the PPy NTs and the rGO layers, which is explained in terms of the PPy NTs forming a bridge between the rGO layers. The FET sensor provided a rapid response in real time and high sensitivity toward H2O2 with a limit of detection of 100 pM. The FET-type biosensing geometry was also highly reproducible and stable in air. Furthermore, the liquid-gated FET-type sensor exhibited specificity toward H2O2 in a mixed solution containing compounds found in biological fluids.