Search

Clathrate-I-based materials are promising for waste-heat recovering applications via thermoelectric (TE) effects. However, the lack of highly efficient p-type materials hampers the development of clathrate-based TE devices. In this work, the synthesis of the p-type semiconductor Ba7.8Au5.33Ge40.67 with clathrate-I structure is up-scaled by steel-quenching and spark plasma sintering treatment at 1073 K. A thermoelectric figure of merit ZT ≈ 0.9 at 670 K is reproducibly obtained, and 40 chemically homogeneous module legs of 5 × 5 × 7 mm3 are fabricated. By using a carbon layer as a diffusion barrier, electrical contacts are sustainable at elevated application temperatures. Eight couples with the clathrate-I compounds Ba7.8Au5.33Ge40.67 as p-type and Ba8Ga16Ge30 as n-type materials are integrated into a TE module with an output power of 0.2 W achieved under a temperature difference ΔT = 380 K (T1 = 673 K and T2 = 293 K). The thermoelectric performance of Ba7.8Au5.33Ge40.67 demonstrates the potential of type-I clathrates for waste heat recycling.

The p-type properties of carbon nanotubes (CNTs) in organic thermoelectric devices need urgent improvement for large-scale, low-grade thermal energy applications. Here, we present a suitable approach to significantly enhance the power factor (PF) by increasing the electrical conductivity through the low-temperature calcination-induced pyrolysis of the insulating γ-cyclodextrin polymer (PγCyD), which is used as a solubilizer of film-like CNTs. The low-temperature calcination method, which can be used to realize good electrical contact between CNT bundles, shows enhancement behavior as a universal phenomenon for not only PγCyD but also other commonly used polymers for CNT films. To moderate the calcination temperature, the Pd catalyst was added, and the optimal temperature was reduced from 340 °C to 250 °C. Consequently, the PF value of the CNT film was 570 μW m−1 K−2, which was found to be more than twice that of the original CNT film. In addition, we demonstrated the energy harvesting capability of a thermoelectric generator based on this p-type CNT film; a thermoelectric generator with 10 p-type thermoelectric elements showed a maximum power output of 10.3 μW with a temperature difference of 75 °C, which is comparable to the maximum power output of some of the best single-component organic thermoelectric devices demonstrated to date. This outstanding output power shows that easy-to-handle CNT films with low-temperature heat treatment can open new avenues for the development of thermoelectric generators.

While thermally cycling monolayer, bilayer, and trilayer graphene between 5 K and 300 K, Raman spectroscopy has shown that cooling to 5 K induces a strain in these graphene flakes of − 0.081 ± 0.003 %. This strain was used to measure the graphene thermal expansion coefficient (TEC), which was found to be ( − 3.2 ± 0.2 ) × 10 − 6 K − 1 for monolayers, ( − 3.4 ± 0.4 ) × 10 − 6 K − 1 for bilayers, and ( − 3.8 ± 0.6 ) × 10 − 6 K − 1 for trilayers at room temperature. The TEC showed a similar temperature dependence across all graphene thicknesses and was found to be in good agreement with theoretical predictions. This study, thus, represents the first measurement of the TEC of bilayer and trilayer graphene. Modification of graphene flakes of all thicknesses with various electrical contact designs was found to have no significant impact on the resulting strain, and thus the TEC, compared to the pristine graphene.

Homoepitaxial film and semi-insulating bulk β-Ga2O3 with (001) orientation were studied using terahertz time-domain spectroscopy (THz-TDS) in the frequency region from 0.2 to 3.0 THz parallel to the [100] and [010] directions. The static permittivity of the bulk was determined to be 10.0 and 10.4 along the a-axis and b-axis, respectively, and the refractive index values at 0.2 THz are 3.17 and 3.23 for each axis. The electrical resistivity of the epilayer was extracted with good accuracy by employing the Drude–Lorentz model and without the use of electrical contacts. This noninvasive and contact-free material evaluation through THz-TDS proves to be a powerful tool for probing and obtaining various types of information about β-Ga2O3 materials such as bulk and thin films for the development of β-Ga2O3-based device applications.

Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostructures composed of MoSi$_2$N$_4$ contacted by graphene and NbS$_2$ monolayers using first-principle density functional theory calculations. We show that the MoSi$_2$N$_4$/NbS$_2$ contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For MoSi$_2$N$_4$/graphene contact, the SBH can be modulated via interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights on the physics of 2D electrical contact to MoSi$_2$N$_4$, and shall offer a critical first step towards the design of high-performance electrical contacts to MoSi$_2$N$_4$-based 2D nanodevices., Comment: 5 pages, 5 figures (accepted version, to appear in Applied Physics Letters)

This study demonstrates a low-temperature Ohmic contact to WSe2 using a van der Waals (vdW) junction between highly p-doped MoS2 (p+-MoS2) and WSe2. p+-MoS2 exhibits a large work function comparable to that of a well-known metal such as Pt. Owing to its layered crystal structure, p+-MoS2 can easily be exfoliated to obtain atomically flat, freshly cleaved surfaces. Moreover, it is stable in air; therefore, this material can be used as an efficient hole-injection contact to a transition metal dichalcogenide semiconductor like WSe2. An h-BN encapsulated WSe2 field effect transistor (FET) was fabricated, having electrical contacts in the form of two flakes of exfoliated p+-MoS2. The fabricated FET demonstrated Ohmic contact behavior under hole doping between room temperature (295 K) and liquid helium temperature (4.2 K). Further, owing to the low contact resistance of the p+-MoS2/p-WSe2 junction, metal-to-insulator transition of WSe2 was observed upon hole doping, as well as quantum oscillation under the application of a magnetic field. On the basis of the Arrhenius plot, a potential barrier height of ∼41 meV at the p+-MoS2/p-WSe2 junction was determined; we infer that this value is limited by the carrier depletion region of p+-MoS2 at the junction. Overall, this appears to indicate potential high performance of the p+-MoS2/WSe2 vdW Ohmic contact.

Intrinsic Si(111) surfaces passivated with atomic hydrogen are an ideal platform to host two-dimensional electron systems. Traditional methods to probe these surfaces, however, typically involve the placement of dopants and metals directly onto the surface and subsequent high temperature processing, which can be harsh and invasive and lead to surface degradation. Here, we detail a non-invasive gating approach for probing two-dimensional electron systems on intrinsic H-Si(111) surfaces using a silicon-on-insulator (SOI) gating assembly. In this architecture, all harsh device fabrication is performed on a single SOI chip, ensuring that the H-Si(111) surface remains in pristine condition, or as close to the original manufactured intrinsic-Si wafer as possible. To achieve this, we intentionally keep our H-Si(111) surfaces free of any dopants or metals, which are instead placed on the adjacent SOI chip. All electrical components, including Ohmic contacts and accumulation and depletion gates, are housed in the SOI piece. The Ohmic contacts on the SOI piece are brought into physical and electrical contact with the pristine H-Si(111) piece after being van der Waals bonded at room temperature, while all gates on the SOI piece are separated from the H-Si(111) surface by vacuum. Architecture details, baseline operation tests, and 77 K device characterization measurements will be discussed, as well as the implications of going beyond H-Si(111) surfaces and using our device architecture to facilitate transport measurements on halogen-terminated Si surfaces.

A high-sensitivity 4H–SiC temperature sensor and an alpha detector have been fabricated using additively printed metal contacts. The surface morphology and electrical conductivity of the printed electrodes were established prior to Schottky diode development. 4H–SiC Schottky diodes with direct-write printed silver contacts on the 5 μm-thick epilayer on 4H–SiC were characterized electrically in terms of the forward and reverse current–voltage and high-frequency capacitance–voltage characteristics. The turn-on voltage of the Schottky diodes, as established from the forward current–voltage characteristics measured up to a temperature of 400 °C, showed a linear temperature dependence. Schottky diodes with direct-write printed Ag electrodes were able to measure alpha particles emitted from Americium-241. The high temperature and radiation response of the Schottky diodes show their suitability for multi-modal sensor fusion on the 4H–SiC platform for harsh environment applications.

Memristor devices have history-dependent charge transport properties that are ideal for neuromorphic computing applications. We reveal a memristor material and mechanism in the layered Mott insulator α-RuCl3. The pinched hysteresis loops and S-shaped negative differential resistance in bulk crystals verify memristor behavior and are attributed to a nonlinear coupling between charge injection over a Schottky barrier at the electrical contacts and concurrent Joule heating. Direct simulations of this coupling can reproduce the device characteristics.

An approach to measuring electrical contact resistance as a direct function of the true contact size at the nanoscale is presented. The approach involves conductive atomic force microscopy (C-AFM) measurements performed on a sample system comprising atomically flat interfaces (up to several hundreds of nanometers in lateral size) formed between gold islands and a highly oriented pyrolytic graphite (HOPG) substrate. The method overcomes issues associated with traditional C-AFM such that conduction can be correlated with a measurable true, conductive contact area. Proof-of-principle experiments performed on gold islands of varying size point toward an increasing contribution of the island-HOPG junction to the measured total resistance with decreasing island size. Atomistic simulations complement and elucidate experimental results, revealing the maximum island size below which the electrical contact resistance at the island-HOPG junction can be feasibly extracted from the measured total resistance.

MoS2 is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of monolayer (ML) MoS2 devices. Significant p-type (hole-transport) behavior was only observed for chemically doped MoS2 devices with high work function palladium (Pd) contacts, while MoS2 devices with low work function metal contacts made from titanium showed ambipolar behavior with electron transport favored even after prolonged p-doping treatment. ML MoS2 transistors with Pd contacts exhibit effective hole mobilities of (2.3 ± 0.7) cm2 V−1 S−1 and an on/off ratio exceeding 106. We also show that p-doping can help to improve electrical contacts in p-type field-effect transistors: relatively low contact resistances of (482 ± 40) kΩ μm and a Schottky barrier height of ≈156 meV were obtained for ML MoS2 transistors. To demonstrate the potential application of 2D-based complementary electronic devices, a MoS2 inverter based on pristine (n-type) and p-doped monolayer MoS2 was fabricated. This work presents a simple and effective route for contact engineering, which enables the exploration and development of high-efficiency 2D-based semiconductor devices.

In this paper, we present the proof-of-concept of a device for rectifying ac-magnetic-field effects by mixing thermal spin and charge currents. The device consists of a NiO/Pt bilayer in which a pure spin current is generated perpendicularly to the sample plane by means of the spin Seebeck effect. The pure spin current, created in the NiO layer, is injected into the Pt layer and is converted into a charge current by means of the inverse spin Hall effect, which is measured by electrical contacts. Due to the combination of two effects, the spin Seebeck and the planar Nernst effect, the overall response of the device as a function of a magnetic field is similar to the one of a rectifying diode. The resulting charge current mainly flows in one direction, approaching zero for negative values of the magnetic field when the spin Seebeck voltage has an opposite sign and magnitude close to the planar Nernst effect contribution. This remarkable effect is well explained by analyzing the response function as well as taking into account the magnon diffusion theory for the spin Seebeck effect. We believe that this proof of concept is of interest to the development of spintronic devices.

For electronic and photoelectronic devices based on GeSe, an emergent two dimensional monochalcogenide with many exciting properties predicted, good electrical contacts are of great importance for achieving high device performances and exploring the intrinsic physics of GeSe. In this article, we use temperature-dependent transport measurements and thermionic emission theory to systematic investigate the contact-barrier heights between GeSe and six common electrode metals, Al, Ag, Ti, Au, Pt and Pd. These metals cover a wide range of work functions (from ~ 3.6 eV to ~ 5.7 eV). Our study indicates that Au forms the best contact to the valence band of GeSe, even though Au does not possess the highest work function among the metals studied. This behavior clearly deviates from the expectation of Schottky-Mott theory and indicates the importance of the details at the interfaces between metals and GeSe., 10 pages, 4 figures

We have demonstrated pressure-induced transition in a c-axis oriented vanadium sesquioxide (V2O3) thin film from a strongly correlated metal to a Mott insulator in a submicrometric region by inducing a local stress using contact atomic force microscopy. To have an access to a pressure range of sub-gigapascal, a tip with a large radius of 335 nm was prepared by chemical vapour deposition of platinum onto a commercial tip with a focused ion beam (FIB). The FIB-modified tip gives a good electrical contact at low working pressures (0.25–0.4 GPa) allowing unambiguously to evidence reversible metal-insulator transition in a pulsed laser-deposited V2O3 thin film by means of local investigations of current-voltage characteristics. A finite element method has confirmed that the diminution of the c/a ratio under this tip pressure explains the observed phase transition of the electron density of states in the film.We have demonstrated pressure-induced transition in a c-axis oriented vanadium sesquioxide (V2O3) thin film from a strongly correlated metal to a Mott insulator in a submicrometric region by inducing a local stress using contact atomic force microscopy. To have an access to a pressure range of sub-gigapascal, a tip with a large radius of 335 nm was prepared by chemical vapour deposition of platinum onto a commercial tip with a focused ion beam (FIB). The FIB-modified tip gives a good electrical contact at low working pressures (0.25–0.4 GPa) allowing unambiguously to evidence reversible metal-insulator transition in a pulsed laser-deposited V2O3 thin film by means of local investigations of current-voltage characteristics. A finite element method has confirmed that the diminution of the c/a ratio under this tip pressure explains the observed phase transition of the electron density of states in the film.

We measured thermal conductivity and electrical conductivity in organic/inorganic composites with Ag nanowires (NWs) embedded in a poly(vinylidene fluoride) matrix. High thermal and electrical conductivities of 8.43 W / ( mK ) and 1.02 × 10 6 S / m are achieved, respectively, when the volume fraction of Ag NWs reaches 28.34 %. Both measured electrical and thermal conductivities obey the universal power law commonly described in the percolation theory. The percolation behaviors of thermal and electrical conductivities are clearly observed when the volume fraction of Ag NWs is above the critical volume fraction (2.25%), due to the formation of a percolation spanning cluster. Further calculations on the Lorenz number as a function of Ag NW volume fraction also confirm the percolation behaviors. The power-law exponent for the thermal percolation is slightly smaller than that for the electrical percolation, which is likely due to the “dead-end” structures that do not contribute to electrical percolation. To understand the effect of contact resistance between Ag NWs, we modeled the electron contribution to the electrical and thermal resistance at the contact. The non-ideal contact will cause the interfacial thermal resistance increase much more than the electrical contact resistance. The interfacial Lorenz number will decrease from the Sommerfeld value to a much lower value if the contact is non-ideal. Our work can shed some light on the thermal percolation in composite materials.

Interface Engineering is an effective route to tune structural and electrical properties in semiconductor heterostructures. Two kinds of typical van der Waals (vdW)-type electrical contacts, i.e., the electrical contacts of the Pb-I interface and Cs-I interface with a black phosphorus (BP) monolayer, respectively, in CsPbI3-BP heterostructures are studied by first-principles calculations. The electronic band structures of both CsPbI3 slabs and the BP monolayer are preserved in the combined vdW CsPbI3-BP heterostructures. The heterostructure of the Pb-I interface contacting with BP demonstrates the type-I band alignment, and the Cs-I interface contacting with the BP heterostructure demonstrates the type-II band alignment. The reason for the energy level shift is the work function difference of CsPbI3 slabs relative to the BP monolayer, which drives electrons and holes to move spontaneously. In addition, the CsPbI3-BP heterostructures show much better optical properties than CsPbI3 slabs. The light absorptions are enhanced in the CsPbI3-BP heterostructures, especially in the infrared region, which would improve the use of infrared light in CsPbI3 perovskite solar cells. This work suggests that such inorganic perovskite-BP heterostructures have significant potential for future optoelectronic applications and can enable broad possibilities with compositional tunability in inorganic perovskites.

We describe an all-optical lithography process that can make electrical contact to nanometer-precision donor devices fabricated in silicon using scanning tunneling microscopy (STM). This is accomplished by implementing a cleaning procedure in the STM that allows the integration of metal alignment marks and ion-implanted contacts at the wafer level. Low-temperature transport measurements of a patterned device establish the viability of the process.

We demonstrate few-layer molybdenum diselenide (MoSe2) dual-gated field effect transistors (FETs) with few-layer hexagonal boron nitride (hBN) flakes as encapsulation and multilayer graphene as electrical contacts. A high current on/off ratio of up to ∼108, a two-terminal electron mobility of 38.5 cm2/V·s at room temperature, and negligible hysteresis are achieved in hBN encapsulated MoSe2 FETs. Our results also indicate that the flicker (1/f) current noise in hBN encapsulated MoSe2 transistors is governed by Hooge's carrier mobility fluctuation and the normalized current noise in the dual-gated configuration can be dramatically reduced by applying a positive bias on the bottom gate. All these suggest that dual-gated MoSe2 FETs are very promising candidates for sensing applications.

We present an efficient broadband single-photon source which is fabricated by a flip-chip gold-bonding technique and in-situ electron beam lithography. The device comprises a single InGaAs quantum dot that is centered at the bottom of a monolithic mesa structure and located above a gold mirror for enhanced photon-extraction efficiency. We show a photon-extraction efficiency of ηext=(18±2) % into a numerical aperture of 0.4 and a high suppression of multi-photon events from this source with g(2)(0)=0.015±0.009. Our deterministic device with a backside gold mirror can be combined with electrical contacts and piezo-tuning capabilities in future refinements, which represents an important step towards a spectrally tunable plug-and-play quantum-light source with broadband enhancement for photonic quantum networks.

Tubular germanium (Ge) resistors are demonstrated by rolling-up thin Ge nanomembranes (NMs, 50 nm in thickness) with electrical contacts. The strain distribution of rolled-up Ge microtubes along the radial direction is investigated and predicted by utilizing micro-Raman scattering spectroscopy with two different excitation lasers. Electrical properties are characterized for both unreleased GeNMs and released/rolled-up Ge microtubes. The conductivities of GeNMs significantly decrease after rolling-up into tubular structures, which can be attributed to surface charging states on the conductance, band bending, and piezo-resistance effect. When illuminated with a light source, facilitated by the suppressed dark current of rolled-up Ge tubes, the corresponding signal-to-noise ratio can be dramatically enhanced compared with that of planar GeNMs.

Techniques for the preparation of highly integrated arrays of IV–VI semiconductors are presented. PbSnTeSe films were grown by molecular-beam epitaxy and array structures are fabricated by photolithography, electrical contact formation, and etching. The rationale for forming electrical contacts with low specific contact resistivity is provided. Characterization of the specific contact resistivity was performed using a transmission line technique. Using a procedure discussed in this letter, specific contact resistivity measurements as low as 7×10−8Ωcm2 have been obtained and demonstrate the feasibility of a class of highly integrated IV–VI device technology.

Bilayers of aluminum and an alkali fluoride are well-known top cathode contacts for organic light-emitting devices but have never been successfully applied as bottom contacts. We describe a bilayer bottom cathode contact for organic electronic devices based on reversing the well-known top cathode structure such that the aluminum, rather than the alkali fluoride, contacts the organic material. Electron-only devices were fabricated showing enhanced electron injection from this bottom contact. Kelvin probe, x-ray photoelectron spectroscopy experiments, and thermodynamic calculations suggest that the enhancement results from n doping of the organic material by dissociated alkali metals.

A straightforward method for removing thin insulating films from contact interfaces of conductive surfaces that enables changing the contact behavior from nonohmic to ohmic was derived in this study. The efficacy of this method is demonstrated by experiments performed with a special microdevice consisting of polysilicon. It is shown that the native oxide film can be removed from asperity nanocontacts at the contact interface by electromechanical means without damaging the microdevice. Measurements of electrical contact resistance versus applied current are used to characterize the contact interface. The present approach for removing oxide surface films that are responsible for the nonohmic behavior at microdevice contact interfaces is straightforward and can easily be applied to other types of miniaturized devices to enhance their reliability and performance. An additional benefit of this method is the release of adhered microdevices by nondestructive means.

We demonstrate the fabrication of organic light-emitting devices based on a ruthenium complex with indium tin oxide anodes and laminated Au cathodes. Light emission was uniform over the whole device area, indicating a high-quality mechanical and electrical contact. The devices showed no rectification, indicating that the laminated contact was ohmic and caused no damage to the ruthenium complex. Comparison with devices using evaporated Au cathodes confirmed the quality of the lamination process.

Electrical transport through high-density arrays of carbon nanotubes grown within vertical pores of anodized alumina was measured. Individual nanotubes were studied using conductive tip atomic force microscopy, with bias applied between the tip and platinum back electrode. Multiwalled nanotubes of diameter about 50 nm, with 5 nm thick walls were found to have a resistivity lower than 1.4×10−5 Ω m. A potential barrier was found to exist between the sensing tip and nanotube, resulting in nonlinear current–voltage characteristics. Low-resistance contact was formed by breaking down this barrier, once the circuit was stressed beyond 1.5 V.

We demonstrate a spectroscopic technique based on scanning tunneling microscopy that provides high-resolution maps of local carrier transport across contacts to polycrystalline thin-film solar cells. Using this technique, preferential transport channels across a p+-ZnTe/p-CdTe back contact of a p-CdTe/n-CdS solar cell are imaged with 20 nm spatial resolution. Transport across this contact is highly nonuniform. Large areas of high resistance coexist with nanoscale low-resistance regions that are strongly correlated with grain boundaries in the CdTe absorber. These results suggest an important role of grain boundaries as near-contact conducting channels.

A scheme that allows on-chip growth of multiwalled carbon nanotubes at designed locations is demonstrated. The nanotubes were grown by thermal chemical vapor deposition and were contacted to nanoscaled Cr electrodes fabricated by standard e-beam lithography techniques. The contacts were found to be Ohmic with resistance values on the order of 103 Ω at room temperature. Remarkably, the contacts showed weak temperature dependence down to 40 mK and were insensitive to the magnetic field up to 5 T.

Controlling electronic transport across metal-thermoelectric interfaces is important for realizing high-efficiency solid-state refrigeration and waste-heat harvesting devices. We report up to 34-fold increase in electrical contact conductivity Σc across Cu-alkanedithiol-Bi2Te3 interfaces. Longer chain dithiols are more effective in curtailing Cu diffusion, telluride formation, and reducing interfacial oxides of Bi and Te, leading to higher Σc. In contrast, Σc is insensitive to the alkanedithiol chain length at Ni-alkanedithiol-Bi2Te3 interfaces due to weak Ni-S bonding. These results indicate that interfacial bonding and phase formation are primary determinants of Σc rather than charge transport through the alkanedithiol molecules. Our findings provide insights for tuning electronic transport across metal-thermoelectric interfaces using an interfacial nanolayer comprising molecules with suitably chosen chemical termini and molecular length.

Amorphous alloys containing Ni-Si-Nb-C were formed on 4H-SiC creating a low resistance Ohmic contact electrode. In a conventional nickel silicide (NiSi) electrode on SiC, a carbon agglomeration at the silicide/SiC interface occurs, and contact resistance between NiSi and SiC substrate becomes larger. For carbon agglomeration suppression, nanosecond non-equilibrium laser annealing was introduced, and to form metal carbides, carbon-interstitial type metals Nb and Mo were introduced. Ni, Nb, Mo, Nb/Ni, Mo/Ni multilayer contacts, and NbNi mixed contact were formed on the C-face side of n-type 4H-SiC wafers. The electrical contact properties were investigated after a 45 ns pulse laser annealing in N2 ambient. As a result, with NbNi film, an amorphous alloy with Ni-Si-Nb-C was formed, and a low specific contact resistance of 5.3 × 10−4 Ω cm2 was realized.

In classical contact theory, the temperature T of contact spots in an electrical interface passing a DC current is determined by the voltage drop V across the contact, i.e., the V-T relation. This paper reports on evaluations of the temperature distribution in a single circular contact spot in a copper-copper contact heated by an AC current. The steady-state maximum temperature was computed numerically for an AC electrical current of fixed amplitude and a frequency ranging from 100 Hz to 100 MHz. The computed temperature was compared with the predictions of the V-T relation.

The current-induced domain wall dynamics along curved ferromagnetic strips is studied by coupling the magnetization dynamics to the heat transport. Permalloy strips with uniform and non-uniform cross section are evaluated, taking into account the influence of the electrical contacts used to inject the current pulses and the substrate on top of which the ferromagnetic strip is sited. Micromagnetic simulations indicate that the geometry and the non-ferromagnetic materials in the system play a significant role in the current-induced domain wall dynamics. Due to the natural pinning, domain walls are hardly affected by the spin-transfer torques when placed in uniform cross section strips under current pulses with reduced magnitude. On the contrary, the current-induced domain wall displacement is significantly different in strips with non-uniform cross section, where thermal gradients emerge as due to the Joule heating. It is found that these thermal gradients can assist or act against the pure spin-transfer torques, in agreement with the recent experimental observations.

We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide (PAn/PEO) blend with sub-30 nm diameter. Fiber diameters near 5 nm are obtained for optimized process parameters. Scanning conductance microscopy (SCM) shows that fibers with diameter below 15 nm are electrically insulating; the small diameter may allow complete dedoping in air or be smaller than phase-separated grains of PAn and PEO. Electrical contacts to nanofibers are made by shadow mask evaporation with no chemical or thermal damage to the fibers. Single fiber I–V characteristics show that thin fibers conduct more poorly than thick ones, in agreement with SCM data. I–Vs of asymmetric fibers are rectifying, consistent with formation of Schottky barriers at the nanofiber-metal contacts.

Findings are given from the experimental observation of the vacuum breakdown of carbon-nanotube (CNT) field emitters on a Si tip. The CNTs were grown on the apex of a Si microtip by microwave plasma-enhanced chemical vapor deposition. The electrical contact of the CNT-Si junction was shown to be of ohmic type. A fine tungsten microprobe in combination with a scanning electron microscopy (SEM) system was employed for both the field emission and the contact conductivity measurements. This arrangement allows to precisely measure the characteristics of individual CNT and to in situ inspect the morphology of the CNT emitters on Si tips before and after vacuum breakdown events. An upper limit in emission current density of ∼103 A/m2 from the CNT emitters was recorded before a vacuum breakdown event is initiated. Clear evidence was found to show that the vacuum breakdown of the CNTs results in melting of the Si tip. These findings enhance the understanding of the failure mechanism of CNT emitters. It also has im...

The contact behavior of various metals on n-type nitrogen-doped ultrananocrystalline diamond (UNCD) thin films has been investigated. The influences of the following parameters on the current-voltage characteristics of the contacts are presented: (1) electronegativity and work function of various metals, (2) an oxidizing acid surface cleaning step, and (3) oxide formation at the film/contact interface. Near-ideal ohmic contacts are formed in every case, while Schottky barrier contacts prove more elusive. These results counter most work discussed to date on thin diamond films, and are discussed in the context of the unique grain-boundary conductivity mechanism of the nitrogen-doped UNCD.

The dielectric properties of alkyl monolayers with various chain lengths [CH3(CH2)n–1- (n = 12, 14, 16, and 18)] covalently bonded to a hydrogen terminated n-type silicon (111) surface in a nanoscale region were evaluated using current sensing atomic force microscopy (AFM). A reliable electrical contact between the alkyl monolayers and the metal-coated AFM tip was achieved under slight stress. At a force less than 2 nN, current sharply increased as the bias was scanned over a critical value, showing that breakdown took place. The breakdown voltage linearly depended on the chain length of the alkyl monolayers and the dielectric strength of 2.0 GV/m was derived from the slope of this relation.

We have developed a suture analogue, suitable for use in laparoscopic surgery, in the form of a staple constructed from NiTi shape memory alloy (SMA). Closure of the staple is effected by resistive heating via a (50–100 ms) pulse of electrical current (≈5 A). In order to optimize deployment protocols and minimize thermal collateral damage to tissue, the heat sink effect of the electrical contact rails, as well as the presence of contact hotspots must be considered. Here, we have employed high-resolution thermal imaging to observe the dynamic temperature distributions in SMA staples as a function of the pulse parameters. This has facilitated process optimization and also provided data from which to validate computational finite-element models of the heat transport phenomena.

Soft contact lamination and metal-coated elastomeric stamps provide the basis for a convenient and noninvasive approach to establishing high resolution electrical contacts to electroactive organic materials. The features of relief on the stamps define, with nanometer resolution, the geometry and separation of electrically independent electrodes that are formed by uniform, blanket evaporation of a thin metal film onto the stamp. Placing this coated stamp on a flat substrate leads to “wetting” and atomic scale contact that establishes efficient electrical connections. When the substrate supports an organic semiconductor, a gate dielectric and a gate, this soft lamination process yields high performance top contact transistors with source/drain electrodes on the stamp. We use this approach to investigate charge transport through pentacene in transistor structures with channel lengths that span more than three decades: from 250 μm to ∼150 nm. We also report some preliminary measurements on charge transport through organic monolayers using the same laminated transistor structures.

We describe a method for contact printing metal patterns with nanometer features over large areas. This nanotransfer printing (nTP) technique relies on tailored surface chemistries to transfer metal films from the raised regions of a stamp to a substrate when these two elements are brought into intimate physical contact. The printing is purely additive, fast (

We present anisotropic magnetoresistance measurements of magnetic switching processes in narrow ferromagnetic permalloy rings fabricated with six nonmagnetic electrical contacts. We demonstrate that measuring the resistance between different contacts allows the determination of the domain wall positions. Furthermore, these measurements also yield the possibility of determining the local switching fields of different parts of the ring. This provides a very useful tool to explore the complete switching process of a single ring. We show that, by using notches of suitable size and by applying fields along appropriate directions, it is possible to select the circulation direction of the vortex state using a uniform field only.

The surfaces of electronic and optical materials such as nitrides, carbides, oxides, sulfides, fluorides, selenides, diamond, silicon, and GaAs are known to be very difficult to bond with low melting point solders (

For many experiments and device applications, electrical contacts to a two-dimensional conduction layer must remain reliable under repeated temperature cycling between 300 and 77 K or lower. This work introduces the use of a silicon-doped InAs contact to the AlGaAs/GaAs two-dimensional electron gas which demonstrates exceptional reliability under such temperature cycling. The noise spectrum of AlGaAs/GaAs contacted with silicon-doped InAs shows almost no dependence on bias current; this fact can be used to improve the performance of device applications such as Hall sensors. In addition, this work introduces an alternative two-dimensional conduction structure, highly mismatched InAs/GaP. InAs/GaP contacted with Ti/Au shows reliability equal to AlGaAs/GaAs contacted with silicon-doped InAs. The InAs/GaP material may be more desirable for some applications because of the lower temperature dependence of its electronic properties and potentially easier integration with silicon-based microelectronics.

Atomic force microscopy tips are used to apply point-like local gates to manipulate the electrical properties of individual single-walled carbon nanotubes (SWNT) contacted by Ti electrodes. Depleting a semiconducting SWNT at a local point along its length leads to orders of magnitude decrease of the nanotube conductance, whereas local gating to metallic SWNTs causes no change in the conductance of the system. These results shed light into gating effects on metal-tube contacts. Electrical properties of SWNT crosses are also investigated. Scanning-probe gating is used to identify the metallic or semiconducting nature of the nanotube components in the crosses.

Transport measurements were carried out on 15–35 nm diameter silicon nanowires grown using SiH4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440°C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 10^4. Three terminal devices indicate that the doping of the wires is p type.

Electrical contacts to a buried chemical-vapor-deposited (CVD) diamond layer can be accomplished using a process in which Pt columns automatically grow from the buried layer to the diamond surface by diamond overgrown on a thin Pt film. Hall effect measurements were employed in a temperature region from room temperature to 450 °C to investigate and compare transport properties of a buried B-doped homoepitaxial diamond film with such Pt column electrodes before and after the overgrowth of the upper layer. A useful ohmic contact to the B-doped CVD diamond interlayer was formed by this type of buried electrodes.

We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.

Electrical contacts to carbon nanotubes typically exhibit high resistance, posing a serious obstacle to their application in electronic devices. One important factor may be their unique electronic structure, which gives weak electronic coupling at the Fermi surface. This suggests some possible ways to reduce contact resistance.

We report on a nanomachined electromechanical resonator applied as a mechanically flexible tunneling contact. The resonator was machined out of a single-crystal silicon-on-insulator substrate and operates at room temperature with frequencies up to some 73 MHz, transferring electrons by mechanical motion.

We report a versatile computation method to quantitatively determine the quantum capacitance of graphene when it is in contact with metal. Our results bridge the longstanding gap between the theoretically predicted and experimentally measured quantum capacitance of graphene. Contrary to popular assumptions, the presence of charged impurities or structural distortions of graphene are not the only sources of the asymmetric capacitance with respect to the polarity of the bias potential and the higher-than-expected capacitance at the Dirac point. They also originate from the field-induced electronic interactions between graphene and metal. We also provide an improved model representation of a metal–graphene junction.

We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses a novel, on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the number of wires available in cryostat measurement systems. Two device innovations are introduced. Firstly, a multiplexer design which utilises split gates to allow the multiplexer to divide three or more ways at each branch. Secondly we describe a device architecture that utilises a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total number exceeds that of the available electrical contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate devices utilising these innovations on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approximately one Coulomb oscillation per hour., 5 pages, 7 figures

High aspect ratio CuO nanowires are synthesized by a simple and scalable method, thermal oxidation in air. The structural, morphological, optical, and electrical properties of the semiconducting nanowires were studied. Au-Ti/CuO nanowire and Pt/CuO nanowire electrical contacts were investigated. A dominant Schottky mechanism was evidenced in the Au-Ti/CuO nanowire junction and an ohmic behavior was observed for the Pt/CuO nanowire junction. The Pt/CuO nanowire/Pt structure allows the measurements of the intrinsic transport properties of the single CuO nanowires. It was found that an activation mechanism describes the behavior at higher temperatures, while a nearest neighbor hopping transport mechanism is characteristic at low temperatures. This was also confirmed by four-probe resistivity measurements on the single CuO nanowires. By changing the metal/semiconductor interface, devices such as Schottky diodes and field effect transistors based on single CuO p-type nanowire semiconductor channel are obtained. These devices are suitable for being used in various electronic circuits where their size related properties can be exploited.