We demonstrate that graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes can be chemically functionalized by diazonium salts. We show that functional groups form a thin layer on a GNR and modify its electrical properties. The kinetics of the functionalization can be monitored by probing the electrical properties of GNRs, either in vacuum after the grafting, or in situ in the solution. We derive a simple kinetics model that describes the change in the electrical properties of GNRs. The reaction of GNRs with 4-nitrobenzene diazonium tetrafluoroborate is reasonably fast, such that >60% of the maximum change in the electrical properties is observed after less than 5 min of grafting at room temperature.

III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.Material science plays a key role in quantum computing research. Long quantum state lifetimes -the fundamental prerequisite for realizing quantum computers -rely on the ability to produce materials with high purity and structural quality. Together with the requirements of scalability and reproducibility, these properties are what mainly defines the challenges of material science in quantum computing today. Proposals for topological quantum computing, 1-3 which are based on hybrid semiconductor-superconductor nanowire (NW) networks, are being pursued by numerous research groups and have ignited intense research efforts on hybrid epitaxy. 4-8 NW scalability is tightly related to the semiconductor growth approach. Top-down lithography has been used to define NWs in two-dimensional layers 5,9 and a variety of methods have been pursued for alignment and positioning of bottom-up vapor-liquid-solid (VLS) grown NWs, such as dielectrophoresis techniques, 10 nanoscale combing 11 and magnetic aligning of NWs. 12 Despite of these developments, large-scale synthesis of bottom-up grown high-mobility NW networks that are compatible with epitaxial interwire connections and semiconductor/superconductor epitaxy has still not been realized. To realize the epitaxial connections, a lot of effort has been put into the growth of branched NWs via the VLS method. 8,13-15 A scalable approach has been developed in Ref. [16,17] using template assisted growth of inplane NW networks. 18 Nonetheless, this approach is not yet compatible with superconductor epitaxy. An alternative scalable approach is to use lithographically defined openings in a mask on a crystalline substrate. This method is referred to as selective area growth (SAG) and until recently has mainly been used in conjunction with metal organic chemical vapour deposition 19,20 , metal organic vapour phase epitaxy 21,22 , chemical beam epitaxy and metal organic molecular beam epitaxy (chemical beam epitaxy). [23][24][25][26] In contrast to molecular beam epitaxy (MBE), the dissociation kinetics of the chemical precursors in these methods enhance the growth selectivity on masked substrates by expanding the growth parameter window, ...

Selective-area growth is a promising technique for enabling of the fabrication of the scalable III–V nanowire networks required to test proposals for Majorana-based quantum computing devices. However, the contours of the growth parameter window resulting in selective growth remain undefined. Herein, we present a set of experimental techniques that unambiguously establish the parameter space window resulting in selective III–V nanowire networks growth by molecular beam epitaxy. Selectivity maps are constructed for both GaAs and InAs compounds based on in situ characterization of growth kinetics on GaAs(001) substrates, where the difference in group III adatom desorption rates between the III–V surface and the amorphous mask area is identified as the primary mechanism governing selectivity. The broad applicability of this method is demonstrated by the successful realization of high-quality InAs and GaAs nanowire networks on GaAs, InP, and InAs substrates of both (001) and (111)B orientations as well as homoepitaxial InSb nanowire networks. Finally, phase coherence in Aharonov–Bohm ring experiments validates the potential of these crystals for nanoelectronics and quantum transport applications. This work should enable faster and better nanoscale crystal engineering over a range of compound semiconductors for improved device performance.

Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses to a candidate material for magnetoelectronic devices. In 1939, Verwey found that bulk magnetite undergoes a transition at TV approximately 120 K from a high-temperature 'bad metal' conducting phase to a low-temperature insulating phase. He suggested that high-temperature conduction is through the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering on cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial. Here, we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.

We report on the structural and electrical properties of graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes. GNRs were reduced by hydrazine at 95 °C and further annealed in Ar/H2 at 900 °C; monolayer ribbons were selected for the fabrication of electronic devices. GNR devices on Si/SiO2 substrates exhibit an ambipolar electric field effect typical for graphene. The conductivity of monolayer GNRs (∼35 S/cm) and mobility of charge carriers (0.5–3 cm2/V s) are less than the conductivity and mobility of pristine graphene, which could be explained by oxidative damage caused by the harsh H2SO4/KMnO4 used to make GNRs. The resistance of GNR devices increases by about three orders of magnitude upon cooling from 300 to 20 K. The resistance/temperature data is consistent with the variable range hopping mechanism, which, along with the microscopy data, suggests that the GNRs have a nonuniform structure.

The realization of hybrid superconductor–semiconductor quantum devices, in particular a topological qubit, calls for advanced techniques to readily and reproducibly engineer induced superconductivity in semiconductor nanowires. Here, we introduce an on-chip fabrication paradigm based on shadow walls that offers substantial advances in device quality and reproducibility. It allows for the implementation of hybrid quantum devices and ultimately topological qubits while eliminating fabrication steps such as lithography and etching. This is critical to preserve the integrity and homogeneity of the fragile hybrid interfaces. The approach simplifies the reproducible fabrication of devices with a hard induced superconducting gap and ballistic normal-/superconductor junctions. Large gate-tunable supercurrents and high-order multiple Andreev reflections manifest the exceptional coherence of the resulting nanowire Josephson junctions. Our approach enables the realization of 3-terminal devices, where zero-bias conductance peaks emerge in a magnetic field concurrently at both boundaries of the one-dimensional hybrids.

In many transition metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, Fe3O4, below the Verwey temperature, an electric field driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high-to low-resistance transition is driven by electric field, the hysteresis observed in I − V curves results from Joule heating in the low resistance state. The shape of the hysteresis loop depends on pulse parameters, and reduces to a hysteresis-free "jump" of the current provided thermal relaxation is rapid compared to the time between voltage pulses. A simple relaxation time thermal model is proposed that captures the essentials of the hysteresis mechanism.PACS numbers: 71.30.+h,72.20.Ht Dramatic changes in resistance induced by electric fields, so called resistive switching (RS), have recently attracted much attention due to this phenomenon's potential application in memory devices (resistive random access memory, ReRAM) [1,2]. RS from high-to lowresistance states is driven by application of high voltage, and corresponding up-and-down sweeps of currentvoltage (I-V ) characteristics often show hysteresis, i.e. in sweeps up and down in bias voltage, the current does not retrace itself. Systems exhibiting hysteretic RS include organic compounds [3] and transition-metal oxides such as widely-studied colossal resistance manganites [4], perovskites (e.g. SrTiO 3 [5]), 1D cuprates Sr 2 CuO 3 [6], NiO [7], TiO 2 [8] etc.For some RS systems, while sweeping out a hysteresis loop in I-V with a switch to a low resistance state at high bias, the low resistance state persists down to zero current as voltage approaches zero. This behavior is often the case for RS systems where the switching is based on metallic filament formation at a transition point [5]. However, for some RS systems the low-resistance state persists only in some voltage interval, and the system returns to the high-resistance state before voltage returns to zero. This is the case for some complex oxides [4,9,10] as well as for magnetite nanostructures, which were recently shown to exhibit RS at low temperatures [11,12].Magnetite, Fe 3 O 4 , is an example of strongly correlated material. In equilibrium, bulk magnetite undergoes a structural transition at the Verwey temperature, T V ∼120 K, accompanied by three-order-of-magnitude change in electrical conductivity, i.e. a metal-insulator transition (MIT) [13]. Recently we demonstrated that magnetite nanoparticles and thin films, once in the insulating state below T V , exhibit RS under a sufficiently large voltage bias [11]. By examining RS systematically in different device geometries, the switching was demon-strated to be driven by the applied in-plane electric field. This is in contrast to previously observed transitions in magnetite driven by Joule heating of the samples above T V...

demonstrated ten-fold improvement of charge carrier mobility in graphene-based field-effect transistors (FETs) when a conventional Si/SiO 2 substrate is replaced by an epitaxial lead zirconate titanate Pb(Zr,Ti)O 3 (PZT) film. Several other studies also showed that graphene-based FETs on ferroelectric substrates have nonvolatile memory properties. [3][4][5][6][7][8] While these studies demonstrate some practical characteristics of graphene-ferroelectric FETs (FeFETs), their electrical properties are not yet completely understood. In particular, these devices exhibit an unusual antihysteresis of electronic transport, which contradicts the hysteretic polarization dependence of PZT. [3][4][5][6] The electronic behavior of graphene FeFETs is schematically illustrated by Figure 1. Figure 1A shows the scheme of a typical graphene FeFET considered in prior studies, as well as in this work. It consists of a graphene channel bridging the source (S) and drain (D) electrodes on a PZT film covering a back gate (G) electrode. When sufficient positive voltage is applied to the gate electrode (V G ), the polarization of ferroelectric is pointing upward, and with sufficient negative V G the polarization is pointing downward. In these experiments, the drain-source current (I DS ) is measured as a function of gate voltage V G . When a certain V G is applied, it creates an electric field E, which affects the polarization of PZT, P. The resulting dielectric displacement D, which changes the Fermi level of graphene and thus I DS , can be expressed as D = ε o E + P. Figure 1B shows a general hysteretic P-V G dependence for PZT. The condition for the charge neutrality in graphene where its conductivity is the lowest is D = 0, which occurs when P = −ε o E. As shown schematically in Figure 1B, there are two gate voltages (shown as green dots) at which this condition is met. This means that in cyclic I DS -V G dependences of graphene FeFETs two points of minimum conductivity, or Dirac points (V DP ), should be observed-one when V G is scanned forward (V DP,F ), and another one when V G is scanned back (V DP,B ). The expected I DS -V G dependence for a graphene FeFET is schematically shown in Figure 1C. In order to describe the effect of a ferroelectric polarization on conductivity of graphene we will define the V DP shift as ΔV DP =V DP,B − V DP,F . For the expected, i.e., polarization-related I DS -V G hysteresis Ferroelectric field-effect transistors (FeFETs) employing graphene on inorganic perovskite substrates receive considerable attention due to their interesting electronic and memory properties. They are known to exhibit an unusual hysteresis of electronic transport that is not consistent with the ferroelectric polarization hysteresis and is previously shown to be associated with charge trapping at graphene-ferroelectric interface. Here, an electrical measurement scheme that minimizes the effect of charge traps and reveals the polarization-dependent hysteresis of electronic transport in graphene-Pb(Zr,Ti)O 3 FeFETs is demonstrated....