Correlation between thermal annealing temperature and Joule-heating based insulator-metal transition in VO2 nanobeams

Rapid thermal annealing of VO2 nanobeams in an ambient argon environment has been carried out at various temperatures after device fabrication. Our analysis revealed that increasing the annealing temperature from 200??C to 400??C results in the reduction of both ohmic and nanobeam resistances with an appreciable decrease in joule-heating based transition voltage and transition temperature, while samples annealed at 500??C exhibited a conducting rutile-phase like characteristics at room temperature. In addition, these variation trends were explored using a physical model and the results were found to be in agreement with the observed results, thus verifying the model.open2

Electric-field induced abrupt and multi-step insulator-metal transitions in vanadium dioxide nanobeams

Both abrupt and multi-step Insulator-metal transitions triggered by applied voltage (electric field) were observed in VO2 nanobeams. The multi-step transition occurs due to nucleation and subsequent expansion of multiple insulating and metallic domains across the nanobeam whereas the abrupt transition results from the propagation of a parallel metallic domain across the nanobeam. A simple power dissipation based model was also used to study the local temperature evolution during these transitions in the nanobeam. Both types of transitions are suitable for prospective application in resistive memory and switches.clos

Quantum size effect of surface-channeled charge carrier transport in Au nanoparticles-VO2 nanowire assembly

Confined charge carriers in nano-scaled structures have revealed great impacts on scientific and technological advances during the last few decades. Here we present quantum size effect of surface-channeled Au nanoparticles-VO2 nanowire assembly fabricated via ac dielectrophoresis. Carrier injection is manipulated through Au nanoparticles decorated on the surface of single crystal VO2 nanowire. Plateau structures are seen in the I-V characteristic of the assembly at 150 K, and the corresponding oscillation in channel conductance is analyzed in terms of quantum confinement induced two-dimensional layer of the carriers in a nanobelt formed around the insulating core of the VO2 nanowire. (C) 2015 Elsevier B.V. All rights reservedclose

Programmed dielectrophoretic assembly of Pd nanoparticles for conductance control in VO2 nanowires

Recently, hybrid nanodevices consisting of more than one type of nanomaterial, have been an intense research topic as far as science and advanced functionalities of such systems are concerned. However, lack of controlled, scalable and directed assembly techniques for these hybrid systems, particularly the accurate assembly of nanoparticles (NPs) on nanowires (NWs), has resulted in their limited applications. In the present work, a development has been reported using dielectrophoresis (DEP) technique through which a controlled assembly of palladium (Pd) NPs on VO2 NWs has been deposited. Furthermore, modulation in conductivity of VO2 NWs as a function of the NPs density is electrically measured and interpreted through work-function dependent surface doping effect of the Pd NPs on VO2 NWs. This well-controlled and scalable approach to functionalize VO2 NWs by assembling Pd NPs is significant in the view of a huge potential of Pd NPs like sensor applications thus adding functionalities to VO2 NWs. Our approach can be generalized for the large-scale assembly of a variety of NP-NW combination to fabricate advanced functional hybrid devices.clos

Unravelling the switching mechanisms in electric field induced insulator-metal transitions in VO2 nanobeams

We studied insulator-metal transitions in VO2 nanobeams for both abrupt and gradual changes in applied electric fields. Based on the observations, the Poole-Frenkel effect explained the abrupt transition, while the gradual case is found to be dominated by the Joule heating phenomenon. We also carried out power model and finite element method based simulations which supported the Joule heating phenomena for gradual transition. An in-principle demonstration of the Poole-Frenkel effect, performed using a square voltage pulse of 1 ?? s duration, further confirms the proposed insulator-metal transition mechanism with a switching time in the order of 100 ns. Finally, conductivity variations introduced via rapid thermal annealing at various temperatures validate the roles of both Joule heating and Poole-Frenkel mechanisms in the transitions.close0

Postfabrication Annealing Effects on Insulator–Metal Transitions in VO₂ Thin-Film Devices

In order to investigate the metal–insulator transition characteristics of VO₂ devices annealed in reducing atmosphere after device fabrication at various temperature, electrical, chemical, and thermal characteristics are measured and analyzed. It is found that the sheet resistance and the insulator–metal transition point, induced by both voltage and thermal, decrease when the devices are annealed from 200 to 500 °C. The V 2p_3/2 peak variation in X-ray photoelectron spectroscopy (XPS) characterization verifies the reduction of thin-films. A decrease of the transition temperature from voltage hysteresis measurements further endorse the reducing effects of the annealing on VO₂ thin-film

Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS₂/WSe₂ van der Waals Heterostructure

Two-dimensional (2D) material-based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto-electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS₂ and few layered WSe₂ has been demonstrated in which an atomically thin MoS₂ layer has been employed as a tunneling layer to the underlying WSe₂ layer. In this way, simultaneous contacts to both MoS₂ and WSe₂ 2D layers have been established by forming a direct metal–semiconductor to MoS₂ and a tunneling-based metal–insulator–semiconductor contacts to WSe₂, respectively. The use of MoS₂ as a dielectric tunneling layer results in an improved contact resistance (80 kΩ μm) for WSe₂ contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature-dependent measurement. Furthermore, this unique contact engineering and type-II band alignment between MoS₂ and WSe₂ enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 10⁴ for both electron (MoS₂) and hole (WSe₂) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS₂ and the heterostructure device, which agrees with the carrier recombination-induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high-performance opto-electrical devices and applications

Gate Tunable Self-Biased Diode Based on Few Layered MoS₂ and WSe₂

The operation of a self-biased diode (SBD) based on MoS₂ has been demonstrated by using an asymmetric top gate comprising metal-hexagonal boron nitride (h-BN)-MoS₂ structure. The rectification is achieved by asymmetric modulation of effective Schottky barrier and carrier density in the channel during forward and reverse bias, and a rectification factor of 1.3 × 10⁵ is achieved in <i>I–V</i> characteristics. The modulation of effective Schottky barrier is verified by temperature dependent measurement in a range of 173 to 373 K, and a difference of 300 meV is observed in effective Schottky barrier height for forward and reverse bias. The electrical characteristics of SBD exhibit close resemblance with an ideal thermionic emission model with an ideality factor of 1.3. SBD also exhibits strong photoelectrical response with a specific detectivity of 150 A/W and responsivity of 2.1 × 10¹⁰ Jones under 450 nm laser light illumination. In the end, to demonstrate the diversity of the proposed scheme, SBD based on WSe₂ has also been fabricated and the results have been discussed. These results suggest a new route toward the SBD based numerous electronics and optoelectronics applications and can in principle be implemented using other two-dimensional materials as well

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS₂ Device

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS₂) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on–off ratio drops by 3 orders of magnitude due to the increase in “off” current for both single and few layer MoS₂ FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 10³ and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS₂ interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices

Gate-Tunable Hole and Electron Carrier Transport in Atomically Thin Dual-Channel WSe2/MoS2 Heterostructure for Ambipolar Field-Effect Transistors

An ambipolar dual-channel field-effect transistor (FET) with a WSe2/MoS2 heterostructure formed by separately controlled individual channel layers is demonstrated. The FET shows a switchable ambipolar behavior with independent carrier transport of electrons and holes in the individual layers of MoS2 and WSe2, respectively. Moreover, the photo-response is studied at the heterointerface of the WSe2/MoS2 dual-channel FET © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim8