Surface States Of Nanowires Research Articles

Fabry–Pérot interferometry with gate-tunable 3D topological insulator nanowires

Three-dimensional topological insulator (3D TI) nanowires display remarkable magnetotransport properties that can be attributed to their spin-momentum-locked surface states such as quasiballistic transport and Aharonov–Bohm oscillations. Here, we focus on the transport properties of a 3D TI nanowire with a gated section that forms an electronic Fabry–Pérot (FP) interferometer that can be tuned to act as a surface-state filter or energy barrier. By tuning the carrier density and length of the gated section of the wire, the interference pattern can be controlled and the nanowire can become fully transparent for certain topological surface-state input modes while completely filtering out others. We also consider the interplay of FP interference with an external magnetic field, with which Klein tunneling can be induced, and transverse asymmetry of the gated section, e.g. due to a top-gated structure, which displays an interesting analogy with Rashba nanowires. Due to its rich conductance phenomenology, we propose a 3D TI nanowire with gated section as an ideal setup for a detailed transport-based characterization of 3D TI nanowire surface states near the Dirac point, which could be useful towards realizing 3D TI nanowire-based topological superconductivity and Majorana bound states.

Optically-controlled resistive switching effects of CdS nanowire memtransistor* *Project supported by the National Natural Science Foundation of China (Grant No. 51702245) and the Fundamental Research Funds for the Central Universities, China (Grant No. WUT2020IB010).

Since it was proposed, memtransistors have been a leading candidate with powerful capabilities in the field of neural morphological networks. A memtransistor is an emerging structure combining the concepts of a memristor and a field-effect transistor with low-dimensional materials, so that both optical excitation and electrical stimuli can be used to modulate the memristive characteristics, which make it a promising multi-terminal hybrid device for synaptic structures. In this paper, a single CdS nanowire memtransistor has been constructed by the micromechanical exfoliation and alignment lithography methods. It is found that the CdS memtransistor has good non-volatile bipolar memristive characteristics, and the corresponding switching ratio is as high as 106 in the dark. While under illumination, the behavior of the CdS memtransistor is similar to that of a transistor or a memristor depending on the incident wavelengths, and the memristive switching ratio varies in the range of 10 to 105 with the increase of the incident wavelength in the visible light range. In addition, the optical power is also found to affect the memristive characteristics of the device. All of these can be attributed to the modulation of the potential barrier by abundant surface states of nanowires and the illumination influences on the carrier concentrations in nanowires.

(Invited) Axial Silicon-Germanium Nanowires: Properties and Device Applications

Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface. It is also shown that the nanowire surface states act as shunt resistors and considerably degrade the Si-Ge nanowire device characteristics.

Surface-States-Modulated High-Performance InAs Nanowire Phototransistor.

We report a high-performance InAs nanowire phototransistor with the photoresponse mechanism governed by the gate-controlled surface states. Detailed characterizations suggest that the high density of surface defect states of the InAs nanowire can capture electrons from the nanowire core to form negative surface charge centers. Before and after light illumination, nanowire surface states undergo processes of capturing and neutralizing the electrons, respectively. This leads to an increase in the concentration and mobility of electrons after light illumination, which endows the device with remarkable photoresponsivity. After modulating the surface states through gate voltage and surface passivation, significantly high responsivity of up to 4.4 × 103 A/W and gain of up to 2.7 × 103 under the illumination of light at the wavelength of 2000 nm are obtained, putting our devices among the high-performance short-wave infrared nanowire photodetectors. This work provides an important reference for understanding the surface effects of nanomaterials and enhancing the performance of nanophotodetectors by modulating the surface states.

The high-speed ultraviolet photodetector of ZnO nanowire Schottky barrier based on the triboelectric-nanogenerator-powered surface-ionic-gate

Nanowires have a high specific surface area and high surface activity, and the surface states are key factors controlling the nanowire Schottky barrier. Therefore, adjusting the surface states is an important strategy for developing high-performance electronic and optoelectronic devices based on nanowire Schottky barriers. Here, a surface-ionic-gate modulation technique was developed to modulate the surface states and Schottky barrier of nanowires, based on the gas discharge induced by triboelectric nanogenerators (TENGs). It is demonstrated that the Schottky barrier height and electrical transport characteristics of the Ag/ZnO nanowires can be modulated by effectively controlling the surface states. The experimental results from different atmospheres show that O2− ions and electrons generated by gas discharge can form local ions on nanowires surface, which is the source for modulating surface states. Reversible modulation of the surface states and barrier height could be achieved by combining the surface-ionic-gate modulation with ultraviolet (UV) light. In addition, the recovery time of the ZnO nanowire Schottky barrier UV photodetector was reduced from 87 s to 0.3 s using the surface-ionic-gate modulation technique, the recovery speed increased 290 times, making it a universal method for achieving fast UV detection in different environments, such as in air, in oxygen, and in nitrogen. The TENG-based surface-ionic-gate modulation technique proposed here can modulate the surface states of nanowires in real time and in situ. This provides a novel method to study the dynamic generation and relaxation process of surface states and to develop high-performance electronic and optoelectronic nanodevices with broad application prospects.

Tungsten oxide nanowire gas sensor preparation and P-type NO2 sensing properties at room temperature

Gas sensor has been widely used to monitor the air quality. Metal oxide semiconductor (MOS) is one of the most popular materials used for gas sensors due to its low-cost, easy preparation and good sensing properties. However, the working temperature of tungsten oxide gas sensor is still high, which restricts its applications in special environment. Researchers try to lower the working temperature of WO3 by doping or changing morphology. Tungsten oxide nanowire has great potential to be applied to the gas sensing field because of its high specific surface area. In this work, one-dimensional WO3 nanowire structure is synthesized by sputtering W and followed by the twostep thermally oxidation method. The first step of oxidation is carried out in vacuum tube furnace to obtain the WO2 nanowires and the second step of oxidation is an air annealing treatment in which we will control the temperatures (S0, without treatment; S1, 300℃; S2, 400℃) to study the morphologies and gas sensing properties. The obtained WO3 nanowires are investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM) techniques. The SEM results indicate that WO3 nanowires grow along different directions in space. Nanowires have an average length of 1 μm and a diameter of 40 nm. Besides, nanowires have better crystallinity after higher-temperature (400℃) annealing as indicated by the XRD results, which means less surface defects and surface states. The XPS spectrum indicates the existence of oxygen vacancy in nanowires after 300℃ annealing. The TEM results show that nanowires preferred growth direction is changed after different annealing treatments and the crystal lattice of nanowires after 400℃ has better order than that of nanowires after 300℃. The influences of annealing temperature in the second step on the sensing properties to variousconcentration NO2 gases are investigated at working temperature ranging from room temperature (RT) to 150℃. The results show that the WO3 nanowires after 300℃ annealing show better response than after 400℃ annealing and without annealing treatment. The best response of nanowires to 6 ppm NO2 is 2.5 at RT after 300℃ annealing treatment, and the lowest NO2 detection limit is 0.5 ppm. The room temperature enhancement in gas sensing property may be attributed to the large WO3 nanowire surface states caused by oxidation degree controlled twostep thermal oxidation method. Besides, p-type response to testing gas is found. This might be caused by the lattice defect and the adsorption of oxygen from atmosphere which leads to the formation of surface inversion layer. And the dominated carriers of nanowires will convert from electrons into holes. In conclusion, these results demonstrate that the WO3 nanowires have great potential applications in future NO2 gas detection with low consumption and good performance.

Aharonov–Bohm oscillations caused by non-topological surface states in Dirac nanowires

One intriguing fingerprint of surface states in topological insulators is the Aharonov-Bohm effect in magnetoconductivity of nanowires. We show that surface states in nanowires of Dirac materials (bismuth, bismuth antimony, and lead tin chalcogenides) being in non-topological phase, exhibit the same effect as amendment to magnetoconductivity of the bulk states. We consider a simple model of a cylindrical nanowire, which is described by the 3D Dirac equation with a general $T$-invariant boundary condition. The boundary condition is determined by a single phenomenological parameter whose sign defines topological-like and non-topological surface states. The non-topological surface states emerge outside the gap. In longitudinal magnetic field $B$ they lead to Aharonov-Bohm amendment for the density of states and correspondingly for conductivity of the nanowire. The phase of these magnetooscillations increases with $B$ from $\pi$ to $2\pi$.

Quantum Transport and Nano Angle-resolved Photoemission Spectroscopy on the Topological Surface States of Single Sb2Te3 Nanowires.

We report on low-temperature transport and electronic band structure of p-type Sb2Te3 nanowires, grown by chemical vapor deposition. Magnetoresistance measurements unravel quantum interference phenomena, which depend on the cross-sectional dimensions of the nanowires. The observation of periodic Aharonov-Bohm-type oscillations is attributed to transport in topologically protected surface states in the Sb2Te3 nanowires. The study of universal conductance fluctuations demonstrates coherent transport along the Aharonov-Bohm paths encircling the rectangular cross-section of the nanowires. We use nanoscale angle-resolved photoemission spectroscopy on single nanowires (nano-ARPES) to provide direct experimental evidence on the nontrivial topological character of those surface states. The compiled study of the bandstructure and the magnetotransport response unambiguosly points out the presence of topologically protected surface states in the nanowires and their substantial contribution to the quantum transport effects, as well as the hole doping and Fermi velocity among other key issues. The results are consistent with the theoretical description of quantum transport in intrinsically doped quasi-one-dimensional topological insulator nanowires.

Surface trap mediated electronic transport in biofunctionalized silicon nanowires

Silicon nanowires (SiNWs), fabricated via a top-down approach and then functionalized with biological probes, are used for electrically-based sensing of breast tumor markers. The SiNWs, featuring memristive-like behavior in bare conditions, show, in the presence of biomarkers, modified hysteresis and, more importantly, a voltage memory component, namely a voltage gap. The voltage gap is demonstrated to be a novel and powerful parameter of detection thanks to its high-resolution dependence on charges in proximity of the wire. This unique approach of sensing has never been studied and adopted before. Here, we propose a physical model of the surface electronic transport in Schottky barrier SiNW biosensors, aiming at reproducing and understanding the voltage gap based behavior. The implemented model describes well the experimental I–V characteristics of the device. It also links the modification of the voltage gap to the changing concentration of antigens by showing the decrease of this parameter in response to increasing concentrations of the molecules that are detected with femtomolar resolution in real human samples. Both experiments and simulations highlight the predominant role of the dynamic recombination of the nanowire surface states, with the incoming external charges from bio-species, in the appearance and modification of the voltage gap. Finally, thanks to its compactness, and strict correlation with the physics of the nanodevice, this model can be used to describe and predict the I–V characteristics in other nanostructured devices, for different than antibody-based sensing as well as electronic applications.

In-surface confinement of topological insulator nanowire surface states

The bandstructures of [110] and [001] Bi2Te3 nanowires are solved with the atomistic 20 band tight binding functionality of NEMO5. The theoretical results reveal: The popular assumption that all topological insulator (TI) wire surfaces are equivalent is inappropriate. The Fermi velocity of chemically distinct wire surfaces differs significantly which creates an effective in-surface confinement potential. As a result, topological insulator surface states prefer specific surfaces. Therefore, experiments have to be designed carefully not to probe surfaces unfavorable to the surface states (low density of states) and thereby be insensitive to the TI-effects.

Resolving ambiguities in nanowire field-effect transistor characterization.

We have modeled InAs nanowires using finite element methods considering the actual device geometry, the semiconducting nature of the channel and surface states, providing a comprehensive picture of charge distribution and gate action. The effective electrostatic gate width and screening effects are taken into account. A pivotal aspect is that the gate coupling to the nanowire is compromised by the concurrent coupling of the gate electrode to the surface/interface states, which provide the vast majority of carriers for undoped nanowires. In conjunction with field-effect transistor (FET) measurements using two gates with distinctly dissimilar couplings, the study reveals the density of surface states that gives rise to a shallow quantum well at the surface. Both gates yield identical results for the electron concentration and mobility only at the actual surface state density. Our method remedies the flaws of conventional FET analysis and provides a straightforward alternative to intricate Hall effect measurements on nanowires.

Semiconductor–metal–semiconductor transition in Bi and Bi–Ag nanowires

We report on the experimental observation of temperature dependent (195–475 K) semiconductor–metal–semiconductor transition in the electrical resistance of 50 nm Bi nanowires grown in polymer. The distorted lattice structure of Bi nanowires opens up a semiconducting band gap (162 meV) in contrast to semi-metallic bulk as supported by the electron transport and infrared absorption data. The electron transport plausibly arises out of both surface electronic and bulk-like states, where the former dominates at lower temperatures and the latter at higher temperatures. In the intermediate temperature range, electron transport is dominated by phonon interactions, giving rise to the metal-like transport of Bi nanowires. Attempts to dope Bi nanowires with silver results in a significant modification of Bi–Ag nanowire surface states without changing the crystal structure. The modification of surface states plays an important role in shifting the transition temperatures significantly without any appreciable change in the semiconducting gap of Bi nanowires.

Resolving the Dirac cone on the surface of Bi2Te3 topological insulator nanowires by field-effect measurements

We validate the linear dispersion relation (Dirac cone) on the surface of a single Bi2Te3 nanowire via a combination of field-effect and magnetoresistance measurements by which we unambiguously prove the topological insulator nature of the nanowire surface states. Moreover, we show that the experimentally determined carrier concentration, mobility, and cyclotron mass of the surface states are in excellent agreement with relativistic models. Our method provides a facile way to identify topological insulators that too small for angle-resolved photoemission spectroscopy.

Analysis of surface states in ZnO nanowire field effect transistors

Nanowires (NWs) have attracted considerable interests for scaled electronic and optoelectronic device applications. However, NW based semiconductor devices normally suffer from surface states due to the existence of dangling bonds or surface reconstruction. Because of their large surface-to-volume ratio, surface states in NWs can easily affect the metallic contacts to NWs and electron transport in NW. Here, we present ZnO NW surface analysis by performing current–voltage characterization on ZnO NW Schottky barrier field effect transistors with different metal contacts (Ti, Al, Au) at both room temperature and cryogenic temperature. Our results show that three metal contacts are all Schottky contacts to ZnO NWs due to surface states. Our further study reveals: (a) the surface states related Schottky barrier height (SBH) can be extracted from a back to back Schottky diodes model and the SBH values are in the range of 0.34–0.37eV for three metal contacts; (b) the trap activation energy determined from the Arrhenius plots of different Schottky metal contacts is in the range of 0.23–0.29eV, which is oxygen vacancies related; and (c) based on the space-charge-limited model, the surface state density of ZnO NW is in the range of 1.04×1010–1.24×1010/cm2.

II–VI nanowire radial heterostructures

There are many reports of ZnSe nanowire synthesis, but photoluminescence measurements on these nanowires indicate weak band-edge and high sub-bandgap defect emission. The two main contributors to the non-optimal photoluminescence are nanowire growth at high temperatures and unpassivated surface states. In this paper, the synthesis of II–VI core–shell nanowires by metal organic vapor phase epitaxy is reported. We demonstrate that larger bandgap shells that passivate the nanowire surface states can be deposited around the nanowires by increasing the partial pressures of the shell reactants without a large increase in growth temperature, allowing high quality material to be obtained. The deposition of nearly lattice-matched ZnMgSSe shells on the ZnSe nanowires increases the band-edge luminescent intensity of the ZnSe nanowires by more than four orders of magnitude and improves the band-edge to defect photoluminescence intensity ratio to 12 000:1. The corresponding full widths at half maximum of the band-edge exciton peaks of the core–shell nanowires can be as narrow as 2.8 nm. It is also shown that magnesium and chlorine can be incorporated into the ZnSe nanowire cores, which shortens the emission wavelength and is known to act as an n-type dopant, respectively.

Carrier transport processes in dye sensitized solar cells based on Zn2SnO4 nanostructures studied by intensity modulated photocurrent/photovoltage spectroscopy

In this work, the electron transport and recombination processes in dye sensitized solar cells (DSSCs) based on Zn2SnO4 nanowires and nanoparticles were studied by intensity modulated photocurrent spectroscopy (IMPS)/photovoltage spectroscopy. Two distinct transport time constants were obtained from the IMPS spectra of Zn2SnO4 nanowire-DSSCs, which were associated with a fast, direct transport through the single crystalline structure of the nanowires and a slow, percolation transport that was assisted by the nanowire surface states, respectively. The charge collection efficiency for the fast transport process was close to unity in all the measurements at different light intensities, indicating that the overall charge collection efficiency of nanowire-DSSCs could be enhanced if the slow transport process could be suppressed.

Direct measurement of surface states density and energy distribution in individual InAs nanowires

InAs nanowires are candidates for future high-speed electronic and optoelectronic applications due to their high electron mobility and large coherence length. However, InAs surfaces are known to possess a high concentration of donor-type surface states, which results in an electron accumulation layer and, consequently, Fermi level pinning. Since the surface to volume ratio in nanowires is very large, the effect of surface states is greatly enhanced. We present a method for directly determining the density and energy distribution of single nanowire surface states using Kelvin probe force microscopy measured on a nanowire field-effect transistor and interpreted by electrostatic modeling. Here, the method is applied to individual InAs nanowires, which similarly to bulk InAs exhibit a prominent accumulation layer consisting of a large concentration of donor-type surface states. Nevertheless, due to the small diameter of the nanowires, the electron accumulation and Fermi level pinning take place within the entire nanowire.

Fabrication of reliable semiconductor nanowires by controlling crystalline structure

One-dimensional SnO2 nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparentdisplays and high-performance nano-electronics. In this study, the crystallinity ofSnO2 nanowires was regulated by controlling their growth temperatures. Moreover, the correlation ofthe crystallinity of nanowires with optical and electrical characteristics was analyzed. WhenSnO2 nanowires were grown at temperatures below900 °C, they showed various growth directions and abnormal discontinuity intheir crystal structures. On the other hand, most nanowires grown at950 °C exhibited a regular growth trend in the direction of [100]. In addition, thelow temperature photoluminescence measurement revealed that the highergrowth temperatures of nanowires gradually decreased the 500 nm peak ratherthan the 620 nm peak. The former peak is derived from the surface defectrelated to the shallow energy level and affects nanowire surface states. Owing tocrystallinity and defects, the threshold voltage range (maximum–minimum) ofSnO2 nanowire transistorswas 1.5 V at 850 °C, 1.1 V at 900 °C,and 0.5 V at 950 °C, with dispersion characteristics dramatically decreased. This study successfullydemonstrated the effects of nanowire crystallinity on optical and electrical characteristics.It also suggested that the optical and electrical characteristics of nanowire transistors couldbe regulated by controlling their growth temperatures in the course of producingSnO2 nanowires.

Heterojunction Photovoltaics Using GaAs Nanowires and Conjugated Polymers

We demonstrate an organic/inorganic solar cell architecture based on a blend of poly(3-hexylthiophene) (P3HT) and narrow bandgap GaAs nanowires. The measured increase of device photocurrent with increased nanowire loading is correlated with structural ordering within the active layer that enhances charge transport. Coating the GaAs nanowires with TiO(x) shells passivates nanowire surface states and further improves the photovoltaic performance. We find that the P3HT/nanowire cells yield power conversion efficiencies of 2.36% under white LED illumination for devices containing 50 wt % of TiO(x)-coated GaAs nanowires. Our results constitute important progress for the use of nanowires in large area solution processed hybrid photovoltaic cells and provide insight into the role of structural ordering in the device performance.

Optical characteristics of GaInP/GaP double-heterostructure core-shell nanowires embedded in polydimethylsiloxane membranes

The authors report on the optical properties of GaInP/GaP double-heterostructure (DH) core-shell nanowires (NWs) embedded in polydimethylsiloxane (PDMS) membranes. Self-catalyzed NW structures are grown on Si (111) substrates by initiating with the formation of Ga droplets as a catalyst which is followed by the growth of GaP core and GaInP DH shells. Optical characteristics of GaInP/GaP DH core-shell NWs transferred from Si substrates into PDMS membranes show enhanced 77 K light emission at 630 nm. The signal at 775 nm from the surface states of NWs can be mitigated by embedding the NWs in a PDMS membrane that acts as a surface state passivant.