Electronic transport experiments involving the topologically protected states found at the surface of Bi2Se3 and other topological insulators require fine control over carrier density, which is challenging with existing bulk-doped material. Here we report on electronic transport measurements on thin (<100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions. Moreover, the conductance dependence on top-gate voltage is ambipolar, consistent with tuning between electrons and hole carriers at the surface.
We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are suitable metal electrodes for BP channels to control the transition from a p-type unipolar property to amb…
Topological insulators (TIs) constitute a new class of materials with unique properties resulting from the relativistic-like character and topological protection of their surface states. Theory predicts these to exhibit a rich variety of physical phenomena such as anomalous magneto-electric coupling and Majorana excitations. Although TI surface states have been detected in Bi-based compounds by ARPES and STM techniques, electrical control over their density, required for most transport experiments, remains a challenge. Existing materials are heavily doped in the bulk, thus preventing electrica…
We have used a polarized microluminescence technique to investigate photocarrier charge and spin transport in n-type depleted GaAs nanowires ($ \approx 10^{17}$ cm$^{-3}$ doping level). At 6K, a long-distance tail appears in the luminescence spatial profile, indicative of charge and spin transport, only limited by the length of the NW. This tail is independent on excitation power and temperature. Using a self-consistent calculation based on the drift-diffusion and Poisson equations as well as on photocarrier statistics (Van Roosbroeck model), it is found that this tail is due to photocarrier d…
We study the electronic transport properties of a dual-gated bilayer graphene nanodevice via first principles calculations. We investigate the electric current as a function of gate length and temperature. Under the action of an external electrical field we show that even for gate lengths up 100 Ang., a non zero current is exhibited. The results can be explained by the presence of a tunneling regime due the remanescent states in the gap. We also discuss the conditions to reach the charge neutrality point in a system free of defects and extrinsic carrier doping.
One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe2 devices in …
The energy efficiency and power of a three-terminal thermoelectric nanodevice are studied by considering elastic tunneling through a single quantum dot. Facilitated by the three-terminal geometry, the nanodevice is able to generate simultaneously two electrical powers by utilizing only one temperature bias. These two electrical powers can add up constructively or destructively, depending on their signs. It is demonstrated that the constructive addition leads to the enhancement of both energy efficiency and output power for various system parameters. In fact, such enhancement, dubbed as thermoe…
Using local scanning electrical techniques we study edge effects in side-gated Hall nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ~60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. Howeve…
Transition metal dichalcogenides (TMDCs) have recently shown much promise as thin layer semiconductors in application to transistor technology. TMDCs provide a unique vantage point for studying the properties of phosphorene, a highly efficient and tunable super-material. Phosphorene-TMDC heterostructure nanodevices have remained largely unexplored due to the lack of air stability observed in phosphorene under ambient conditions. This study investigates a novel nanofabrication technique that effectively enhances the air stability and practical scalability of graphene-analogous phosphorene. A ph…
The fabrication of micro- and nano-scale silicon electronic devices requires precision lithography and controlled processing to ensure that the electronic properties of the device are optimized. Importantly, the Si-SiO2 interface plays a crucial role in defining these properties. While transmission electron microscopy (TEM) can be used to observe the device architecture, substrate / contact crystallinity and interfacial roughness, the preparation and isolation of the device active area is problematic. In this work, we describe the use of focussed ion beam technologies to isolate and trench-cut…
We propose a nanodevice for single-electron spin initialization. It is based on a gated planar semiconductor heterostructure with a quantum well and with potentials generated by voltages applied to local gates. Initially we insert an electron with arbitrary spin into the nanodevice. Next we perform a sequence of spin manipulations, after which the spin is set in a desired direction (e.g., the growth direction). The operations are done all-electrically, do not require any external fields and do not depend on the initial spin direction.
Two-dimensional (2D) magnetic materials are essential for the development of the next-generation spintronic technologies. Recently, layered van der Waals (vdW) compound MnBi2Te4 (MBT) has attracted great interest, and its 2D structure has been reported to host coexisting magnetism and topology. Here, we design several conceptual nanodevices based on MBT monolayer (MBT-ML) and reveal their spin-dependent transport properties by means of the first-principles calculations. The pn-junction diodes and sub-3-nm pin-junction field-effect transistors (FETs) show a strong rectifying effect and a spin f…
We demonstrate ambipolar graphene field effect transistors individually controlled by local metal side gates. The side gated field effect can have on/off ratio comparable with that of the global back gate, and can be tuned in a large range by the back gate and/or a second side gate. We also find that the side gated field effect is significantly stronger by electrically floating the back gate compared to grounding the back gate, consistent with the finding from electrostatic simulation.
We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $μ$(n),…
A few-layer palladium diselenide (PdSe2) field effect transistor is studied under external stimuli such as electrical and optical fields, electron irradiation and gas pressure. We observe ambipolar conduction and hysteresis in the transfer curves of the PdSe2 material unprotected and as-exfoliated. We tune the ambipolar conduction and its hysteretic behavior in the air and pure nitrogen environments. The prevailing p-type transport observed at room pressure is reversibly turned into dominant n-type conduction by reducing the pressure, which can simultaneously suppress the hysteresis. The press…
Achieving true bulk insulating behavior in Bi$_2$Se$_3$, the archetypal topological insulator with a simplistic one-band electronic structure and sizable band gap, has been prohibited by a well-known self-doping effect caused by selenium vacancies, whose extra electrons shift the chemical potential into the bulk conduction band. We report a new synthesis method for achieving stoichiometric Bi$_2$Se$_3$ crystals that exhibit nonmetallic behavior in electrical transport down to low temperatures. Hall effect measurements indicate the presence of both electron- and hole-like carriers, with the lat…
Feedback-controlled electric breakdown of graphene in air or vacuum is a well-established way of fabricating tunnel junctions, nanogaps, and quantum dots. We show that the method is equally applicable to encapsulated graphene constrictions fabricated using hydrogen silsesquioxane. The silica-like layer left by hydrogen silsesquioxane resist after electron-beam exposure remains intact after electric breakdown of the graphene. We explore the conductance switching behavior that is common in graphene nanostructures fabricated via feedback-controlled breakdown, and show that it can be attributed to…
Inefficient screening of electric fields in nanoconductors makes electric manipulation of electronic transport in nanodevices possible. Accordingly, electrostatic (charge) gating is routinely used to affect and control the Coulomb electrostatics and quantum interference in modern nanodevices. Besides their charge, another (quantum mechanical) property of electrons - their spin - is at the heart of modern spintronics, a term implying that a number of magnetic and electrical properties of small systems are simultaneously harvested for device applications. In this review the possibility to achiev…
We report on ambipolar gate-defined quantum dots in silicon on insulator (SOI) nanowires fabricated using a customised complementary metal-oxide-semiconductor (CMOS) process. The ambipolarity was achieved by extending a gate over an intrinsic silicon channel to both highly doped n-type and p-type terminals. We utilise the ability to supply ambipolar carrier reservoirs to the silicon channel to demonstrate an ability to reconfigurably define, with the same electrodes, double quantum dots with either holes or electrons. We use gate-based reflectometry to sense the inter-dot charge transition(IDT…
The spin-dependent localization of electrons in quadruple quantum dots (QD's) has been studied by the configuration interaction method. We have investigated two nanodevices that consist of laterally coupled quadruple QD's. We have shown that -- in both the nanodevices with suitably chosen parameters -- the exclusive OR (XOR) logic gate can be realized by all-electrical control with the readout of output via the spin-to-charge conversion. We have determined the nanodevice parameters that are optimal for the performance of the XOR logic gate.
In this paper, several nanodevices which realize basic single heavy hole qubit operations are proposed and supported by time dependent self consistent Poisson-Schrödinger calculations using a four band heavy hole-light hole model. In particular we propose a set of nanodevices which can act as Pauli X, Y, Z quantum gates and as a gate that acts similar as a Hadamard gate (i.e. it creates a balanced superposition of basis states but with an additional phase factor) on the heavy hole spin qubit. We also present the design and simulation of a gated semiconductor nanodevice which can realize an arb…
Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With differe…
In this paper, we examine the conditions under which the nonlinear transport theory is inescapable, when a correlated quantum dot is symmetrically coupled to two leads submitted to temperature and voltage biases. By detailed numerical comparisons between nonlinear and linear currents, we show that the claimed nonlinear behavior in a temperature gradient for the electric current is not so genuine, and the linear theory made at the operating temperature $\bar{T}= (T_H+T_C)/2$ is unexpectedly robust. This is demonstrated for the single impurity Anderson model, in different regimes: resonant tunne…
Classical and quantum dynamics are important limits for the understanding of the transport characteristics of interacting electrons in nanodevices. Here we apply an intermediate semiclassical approach to investigate the dynamics of two interacting electrons in a planar nanochannel as a function of Coulomb repulsion and electric field. We find that charge is mostly redistributed to the channel edges and that an electric field enhances the particle-like character of electrons. These results may have significant implications for the design and study of future nanodevices.
Polymer field-effect transistors with 2D graphene electrodes are devices that merge the best of two worlds: on the one hand, the low-cost and processability of organic materials and, on the other hand, the chemical robustness, extreme thinness and flexibility of graphene. Here, we demonstrate the tuning of the ambipolar nature of the semiconductor polymer N2200 from Polyera ActiveInk by incorporating graphene electrodes in a transistor geometry. Our devices show a balanced ambipolar behavior with high current ON-OFF ratio and charge carrier mobilities. These effects are caused by both the effe…
We have fabricated suspended few layer (1-3 layers) graphene nanoribbon field effect transistors from unzipped multiwall carbon nanotubes. Electrical transport measurements show that current-annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large band-gap and subsequent high on/off ratio (which can exceed 104). Such fabricated devices are thermally and mechanically stable: re…
Electric-field effect control of two-dimensional electron gases (2-DEG) has enabled the exploration of nanoscale electron quantum transport in semiconductors. Beyond these classical materials, transition metal-oxide-based structures have d-electronic states favoring the emergence of novel quantum orders absent in conventional semiconductors. In this context, the LaAlO3/SrTiO3 interface that combines gate-tunable superconductivity and sizeable spin-orbit coupling is emerging as a promising platform to realize topological superconductivity. However, the fabrication of nanodevices in which the el…
We measure gate-tuned thermoelectric power of mechanically exfoliated Bi2Se3 thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near charge neutrality point and at low temperatures, the gate dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states, but is larger than expected at high elect…
Electric Field-Dependent Charge-Carrier Velocity in Semiconducting Carbon Nanotubes
研究了高偏压下具有肖特基势垒接触的半导体单壁纳米管（SWNTs）中的电荷输运。我们观察到几乎对称的双极传输与电子和空穴电流显着超过25微安，在金属单壁碳纳米管由于光学声子发射报告的电流极限。在单极状态下，研究了四种简单的场相关速度模型（弹道、电流饱和、速度饱和和恒定迁移率）;饱和速度为2 x 10^7 cm/s的速度饱和模型可以最好地解释高偏压行为。
We report on transport measurement study of top-gated field effect transistors made out of InSb nanowires grown by chemical vapor deposition. The transistors exhibit ambipolar transport characteristics revealed by three distinguished gate-voltage regions: In the middle region where the fermi level resides within the bandgap, the electrical resistance shows an exponential dependence on temperature and gate voltage. With either more positive or negative gate voltages, the devices enter the electron and hole transport regimes, revealed by a resistance decreasing linearly with decreasing temperatu…
Monolayers (MLs) of transition metal dichalcogenides (TMDs) exhibit unusual electrical behavior under magnetic fields due to their intrinsic spin-orbit coupling and lack of inversion symmetry. While recent experiments have also identified the critical role of carrier interactions within these materials, a complete mapping of the ambipolar Landau level (LL) sequence has remained elusive. Here, we use single-electron transistors to perform LL spectroscopy in ML WSe$_2$, for the first time providing a comprehensive picture of the electronic structure of a ML TMD for both electrons and holes. We f…
We study coherent transport in density tunable micro-devices patterned from thin films of the topological insulator (TI) Bi2Se3. The devices exhibit pronounced electric field effect, including ambipolar modulation of the resistance with an on/off ratio of 500%. We show that the weak antilocalization (WAL) correction to conductance is sensitive to the number of coherently coupled channels, which in a TI includes the top and bottom surface and the bulk carriers. These are separated into coherently independent channels by the application of gate voltage and at elevated temperatures. Our results a…
Although it was demonstrated that discrete molecular levels determine the sign and magnitude of the thermoelectric effect in single-molecule junctions, full electrostatic control of these levels has not been achieved to date. Here, we show that graphene nanogaps combined with gold microheaters serve as a testbed for studying single-molecule thermoelectricity. Reduced screening of the gate electric field compared to conventional metal electrodes allows control of the position of the dominant transport orbital by hundreds of meV. We find that the power factor of graphene-fullerene junctions can …
Two-dimensional (2D) metal-diboride ZrB2 monolayers was predicted theoretically as a stable new electronic material [A. Lopez-Bezanilla, Phys. Rev. Mater., 2018, 2, 011002 (R)]. Here, we investigate its electronic transport properties along the zigzag (z-ZrB2) and armchair (a-ZrB2) directions, using the density functional theory and non-equilibrium Green's function methods. Under low biases, the 2D ZrB2 shows a similar electrical transport along zigzag and armchair directions as electric current propagates mostly via the metallic Zr-Zr bonds. However, it shows an electrical anistropy under hig…
The photovoltaic effect is one of the fundamental light-matter interactions in light energy harvesting. In conventional photovoltaic solar cells, the photogenerated charge carriers are extracted by the built-in electric field of a PN junction, typically defined by ionic dopants in a semiconductor. In atomically thin semiconductors, the doping level can be controlled by the field-effect without need of implanting dopants in the lattice, which makes 2D semiconductors prospective materials to implement electrically tunable PN junctions. However, most 2D semiconducting materials do not show ambipo…
We investigate the impact of mechanical strains and a perpendicular electric field on the electronic and magnetic ground-state properties of two-dimensional monolayer CrI$_3$ using density functional theory. We propose a minimal spin model Hamiltonian, consisting of symmetric isotropic exchange interactions, magnetic anisotropy energy, and Dzyaloshinskii-Moriya (DM) interactions, to capture most pertinent magnetic properties of the system. We compute the mechanical strain and electric field dependence of various spin-spin interactions. Our results show that both the amplitudes and signs of the…
Thermal Hall conductivity of semimetallic graphite dominated by ambipolar phonon drag
Wuhan National High Magnetic Field Center, School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院）；Laboratoire de Physique et d’Étude des Matériaux (ESPCI - CNRS - Sorbonne Université), PSL Research University（巴黎高等物理化工学院-法国国家科学研究中心-索邦大学物质物理与研究实验室）
The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Green's functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly con…
Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tuneable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfil this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this pro…
Tailoring spin coupling to electric fields is central to spintronics and spin-based quantum information processing. We present an optimal micromagnet design that produces appropriate stray magnetic fields to mediate fast electrical spin manipulations in nanodevices. We quantify the practical requirements for spatial field inhomogeneity and tolerance for misalignment with spins, and propose a design scheme to improve the spin-rotation frequency (to exceed 50MHz in GaAs nanostructures). We then validate our design by experiments in separate devices. Our results will open a route to rapidly contr…
Electron transport through nanodevices of atoms in a single-layer rectangular arrangement with free (open) boundary conditions parallel to the direction of the current flow is studied within the single-band tight binding model. The Landauer formula gives the electrical conductance to be a function of the electron transmission probability, ${\cal T}(E)$, as a function of the energy $E$ of the incoming electron. A quantum dragon nanodevice is one which has a perfectly conducting channel, namely ${\cal T}(E)=1$ for all energies which are transmitted by the external leads even though there may be …
We examine spin-dependent displacement of a single electron, resulting in separation and relocation of the electron wavefunction components, and thus charge parts, corresponding to opposite spins. This separation is induced by a pulse of an electric field which generates varying Rashba type spin-orbit coupling. This mechanism is next implemented in a nanodevice based on a gated quantum dot defined within a quantum nanowire. The electric field pulse is generated by ultrafast changes of voltages, of the order of several hundred mV, applied to nearby gates. The device is modeled realistically wit…
The aim of presented research is to design a nanodevice, based on a MoS$_2$ monolayer, performing operations on a well-defined valley qubit. We show how to confine an electron in a gate induced quantum dot within the monolayer, and to perform the NOT operation on its valley degree of freedom. The operations are carried out all electrically via modulation of the confinement potential by oscillating voltages applied to the local gates. Such quantum dot structure is modeled realistically. Through these simulations we investigate the possibility of realization of a valley qubit in analogy with a r…
Spin transistors (whose on-off operation is achieved by electric-field-controlled spin orientation 1), if realized, can revolutionize modern electronics through the implementation of a faster and a more energy-efficient performance as well as non-volatile data storage 2, 3. The original proposal by Datta and Das 1 that relies on electric-field-controlled spin precession in a semiconductor channel faces significant challenges including inefficient spin injection, spin relaxation and spread of the spin precession angle 4, 5. Recent demonstration of electric-field switching of magnetic order 6-8 …
Multiferroic van der Waals (vdW) heterostrucutres offers an exciting route towards novel nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructure composed of ferromagnetic FeCl$_2$ monolayer and ferroelectric Sc$_2$CO$_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that FeCl$_2$/Sc$_2$CO$_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of Sc…
We report measurements of the infrared optical response of thin black phosphorus under field-effect modulation. We interpret the observed spectral changes as a combination of an ambipolar Burstein-Moss (BM) shift of the absorption edge due to band-filling under gate control, and a quantum confined Franz-Keldysh (QCFK) effect, phenomena which have been proposed theoretically to occur for black phosphorus under an applied electric field. Distinct optical responses are observed depending on the flake thickness and starting carrier concentration. Transmission extinction modulation amplitudes of mo…
It is well known that the electronic thermal conductivity of clean compensated semimetals can be greatly enhanced over the electric conductivity by the availability of an ambipolar mechanism of conduction, whereby electrons and holes flow in the same direction experiencing negligible Coulomb scattering as well as negligible impurity scattering. This enhancement -- resulting in a breakdown of the Wiedemann-Franz law with an anomalously large Lorenz ratio -- has been recently observed in two-dimensional monolayer and bilayer graphene near the charge neutrality point. In contrast to this, three-d…
On-chip magnets can be used to implement relatively large local magnetic field gradients in na- noelectronic circuits. Such field gradients provide possibilities for all-electrical control of electron spin-qubits where important coupling constants depend crucially on the detailed field distribution. We present a double quantum dot (QD) hybrid device laterally defined in a GaAs / AlGaAs het- erostructure which incorporates two single domain nanomagnets. They have appreciably different coercive fields which allows us to realize four distinct configurations of the local inhomogeneous field distri…
