Magnitude Change In Resistance Research Articles

Phase Transition Engineering of Vanadium Dioxide Induced by Oxygen Vacancies

A reconfigurable optical functional system is a pioneering and comprehensive approach for portable future flexible electronics, irrespective of traditional silicon-based technology. VO2 shows tremendous temperature-dependent structural and electrical phase transitions as a complex correlated metal oxide, presenting it as a potential candidate for bolometers, thermal camouflage, and optoelectronics applications. However, the structural stability and high transition temperature restrict room-temperature applications. The present study reports the oxygen vacancies-assisted phase transition engineering in the VO2/muscovite heterostructure. The ion bombardment alters the oxygen stoichiometry of VO2/muscovite thin film at room temperature. A significant decrease in lattice constant has been observed during the transition of the insulating M1 (monoclinic) phase into the metallic R (tetragonal) phase, which was confirmed through high-resolution XRD. The change in oxygen vacancy concentration determined using X-ray photoelectron spectroscopy (XPS) actively suppressed and stabilized the epitaxial thin film of VO2 near room temperature. We investigated whether applying increasing power-mediated ion bombardment for a few minutes affects the oxygen vacancy formation on the VO2/muscovite film’s surface. Consequently, a change in KPFM and 4 orders of magnitude change in resistance of VO2 thin film have been observed. This report reveals the fundamental understanding of controlling the MIT in epitaxial VO2 thin film via facile and clean phase transition engineering techniques for advanced flexible electronic devices.

Atomic Layer Deposited Ti2 O3 Thin Films.

Ti2 O3 thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti2 O3 thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3-4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.

Temperature dependent structural, electrical and electronic investigation of VO2 (B) thin film

Vanadium dioxide can exist in several polymorphs and among these the layered polymorph VO2 (B) with monoclinic symmetry has numerous applications. In this work, VO2 (B) phase thin film was prepared on quartz substrate via sputtering technique and its temperature dependent structural, electrical and electronic properties were investigated. We have witnessed a broader structural phase transition around 220 K; which encounter significant changes in the lattice constants yet the monoclinic symmetry is retained over a temperature range from 100 K to 380 K. Temperature dependent resistance measurement also exhibited a semi-metal to insulator like transition near 220 K displaying over 2 order of magnitude change in resistance across the transition. Small changes in the oxygen K-edge x-ray absorption spectrum were seen with change in temperature. At low temperature, an additional peak (d|| band) has emerged in the XAS spectra at energy higher than the σ* peak. The appearance of d|| band density of states is associated to the enhanced electron correlation effects driven by the localization of V–V pair's interactions at low temperature.

A Structural and ac Conductivity Study on Li Doped ZnO

In the present work, the incorporation of Li in a wurtzite ZnO host synthesized through a chemical route has been studied experimentally. Starting with Yoshio’s model, [K. Yoshio, A. Onodera, H. Satoh, N. Sakagami and H. Yamashita, Ferroelectrics, 264, 133 (2001)] structural refinement of the powder diffraction data suggests that Li has a mixed occupancy at both the substitutional 2b (≈1.2%) and interstitial 2a (≈12.6%) Wykoff sites. In comparison with a pure ZnO sample, the net result appears to be an overall expansion of the unit cell by ∼0.07% along both a and c -axes. Temperature dependant ac conductivity measurements carried out using a two electrode assembly and the influence of superimposed dc bias suggest that the conduction may be governed by both the presence of interstitial Li and polar defect complexes also. The grains seem to respond more to an applied dc bias (an order of magnitude change in resistivity) than grain boundaries.

Rheological conductor from liquid metal-polymer composites

<h2>Summary</h2> Advanced conductors are indispensable circuit elements in modern electronics with unique responses to environmental variations. This work successfully introduces the intriguing polymer relaxation into conductive materials and creates a special rheological conductor (RhC) from liquid metal (LM)-polymer composites. The electrical transition temperatures of RhC are found to be highly consistent with polymer rheological transition temperatures (T_g and T_f). Similar to the time-temperature superposition of polymers, a master curve can be constructed to effectively predict the conductivity of RhC. Further polymer rheological experiments and theoretical analysis successfully demonstrate the relationship between conductance and polymer chain relaxation. The RhC could realize several orders of magnitude change in resistance via temperature/time regulation. The polymer chain dynamics provide multiple resistive responses of RhC to temperature, time, and voltage that are remarkably different from traditional conductors. This work pioneers a new route to re-consider flexible conductors and provides diverse applications in high-tech devices.

Optical switching performance of thermally oxidized vanadium dioxide with an integrated thin film heater

Optical switching performance of vanadium dioxide produced by thermal oxidation of vanadium is presented in this paper. A 100nm thick vanadium was oxidized under controlled conditions in a quartz tube furnace to produce approximately 200nm thick VO2. The substrate was appropriately coated on the front and back side to reduce reflection in the cold state, and an integrated thin film heater was fabricated to allow in-situ thermal cycling. Electrical measurements show a greater than three orders of magnitude change in resistivity during the phase transition. Optical measurements exhibit 70% transparency at 1500nm and about 15dB extinction across a wide spectral band between 1000nm and 3000nm. These results are used to show a huge optical bistability effect in VO2-based devices.

Publisher’s Note

Abstract Vanadium dioxide can exist in several polymorphs and among these the layered polymorph VO2 (B) with monoclinic symmetry has numerous applications. In this work, VO2 (B) phase thin film was prepared on quartz substrate via sputtering technique and its temperature dependent structural, electrical, and electronic properties were investigated. We have witnessed a broader structural phase transition around 220 K; which encounter significant changes in the lattice constants yet the monoclinic symmetry is retained over a temperature range from 100 K to 380 K. Temperature dependent resistance measurement also exhibited a semi-metal to insulator like transition near 220K displaying over 2 order of magnitude change in resistance across the transition. Small changes in the oxygen K-edge x-ray absorption spectrum were seen with change in temperature. At low temperature, an additional peak (d|| band) has emerged in the XAS spectra at energy higher than the σ* peak. The appearance of d|| band density of states is associated with the enhanced electron correlation effects driven by the localization of V–V pair's interactions at low temperature.

B-Site Stoichiometry Control of the Magnetotransport Properties of Epitaxial Sr2FeMoO6 Thin Film

The high degree of spin-polarization observed in ordered double perovskite oxide Sr2FeMoO6 at room temperature has attracted considerable interest from both fundamental and practical points of view. In this paper, we have shown a significant change in spin-polarization with a small B-site off-stoichiometry. We have demonstrated a direct correlation between the growth process, Fe:Mo (B-site) stoichiometry, and the electrical properties of half metallic Sr2FeMoO6 thin film. We have shown that varying the Fe:Mo (B-site) stoichiometry by a small atomic percent results in an order of magnitude change in resistivity and at least four times change in magnetoresistance. Theoretical calculation suggests a strong correlation between electronic structure, electronic polarization, and B-site non-stoichiometry.

Scandium: An efficient dopant to modulate the optical spectrum of vanadium dioxide (VO2)

Transparency of VO2 thin films in the visible region is an important aspect of study for its use in smart window applications. In this regard, we experimentally validate the theoretical prediction regarding the influence of scandium (Sc) doping on modulating the optical spectrum of VO2. Sc-doped nano-crystalline VO2, bulk and thin films were synthesised using a two-step process involving a rapid non-equilibrium solution combustion process and ultrasonic nebulised spray pyrolysis of aqueous combustion mixture respectively; followed by a reduction step. The presence of Sc was determined by X-ray photoelectron spectroscopy where a peak observed at 530.06 eV is attributed to Sc–O interaction. Raman spectra showed a shift in 611 cm−1 peak position to a lower wavenumber due to tensile strain arising from Sc doping. Sc-doped VO2 thin films showed semiconductor to metal transition of four orders of magnitude change in resistance. Sc-doping increased the band gap from 1.76 eV for undoped to 2.02 eV with 2.0 at. % of doping thereby making the films more transparent in the visible region of the optical spectrum. This shift in the band gap of VO2 was consistent with the theoretical reports. Thus, Sc is a potential dopant for modulating the optical spectrum of VO2 for its application in smart windows.

Low-Voltage Domain-Wall LiNbO3 Memristors.

Application of conducting ferroelectric domain walls (DWs) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DW density using supercoercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of subcoercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multilevel memories and neuromorphic computing applications.

Fabrication of Phase Change Microstring Resonators via Top Down Lithographic Techniques: Incorporation of VO2/TiO2 Into Conventional Processes

The electronic, optical, and mechanical properties of vanadium dioxide (VO2) can be significantly altered by external stimuli such as heat, electric field, and so on, due to its metal-insulator transition (MIT). While the properties and potential applications of this material have been widely studied, minimal efforts were pursued to integrate VO2 into existing microfabrication processes. Here, in this paper, we present a process for fabricating VO2/TiO2-coated microstring resonators, where the stability of both films was monitored during the entire process. RIE, vapour HF, ablation, and ICP etching were used throughout the process and their effects on the VO2/TiO2 films, as well as the structural and masking layers was studied. Certain processes required the use of a metal hard mask rather than photo resist. VO2 shows excellent etch selectivity over the metal mask layer with RIE. Of all metals examined, Cr was found to be the most stable as a hard mask. Once fabricated, the mechanical and electrical characterizations of the microstring were carried out by measuring resonance frequency (fr) and electrical resistance which show over 6 % alteration in fr and two orders of magnitude change in resistance during the MIT, with a reversible transition and minimal hysteresis. This report provides a foundation for future processing of VO2 based MEMS/MOEMS devices.

Electro-Optical Gas Sensor Consisting of Nanostructured Paper Coating and an Ultrathin Sensing Element

This work describes the use of a paper substrate for electro-optical detection of toxic hydrogen sulfide (H2S) gas. For electrical detection, a chemiresistive type of gas sensor was developed. Ultrathin gold film electrodes (UTGFE) were produced by physical vapor deposition of gold on nanostructured latex-coated paper substrate. The gas-sensing film was deposited on the electrodes by inkjet printing. The sensing films were characterized by atomic force microscopy, X-ray photoelectron spectroscopy and conductometry. The sensing films showed more than seven orders of magnitude change in resistance when exposed to as low as 1 part per million (ppm) H2S gas at room temperature. Besides resistive response, the change in color of the sensing films was studied on a paper substrate, both as a function of print density of the sensing material and H2S concentration. For quantification of the analyte the red, green and blue color deconvolution was performed on the pictures of the paper strip indicator using an open source software. A clear response was obtained from the blue channel. The inexpensive disposable color strips produced on the paper substrate can be used for qualitative and quantitative detection (as low as 1.5 ppm) of H2S gas.

Inkjet-printed thin films on flexible substrate for electro-optical detection of methylamine gas

Methylamine (MA) is highly toxic even at low concentrations. The current U.S.A. Occupational Safety and Health Administration Permissible Exposure Limit (OSHA PEL) is 10 ppm in the gas phase. In this study we have developed low-cost, robust and stable copper sulphide film on flexible plastic and paper substrate. The sensitivity of the film towards MA as a function of print density is studied at room temperature. The electrical and colorimetric response of the sensing films is optimized with respect to print density. The performance parameters of the sensing films including, sensitivity, a lower limit of detection (LOD), selectivity and onset time of detection are determined. X-ray photoelectron spectroscopy is employed for chemical characterization of the sensing film. It is demonstrated that copper sulphide films register either change in resistance or colour when exposed to methylamine in a range of 1 to 100 ppm. A LOD of 1 ppm is achieved which is lower than the current OSHA PEL. The optimized sensing film has shown over three and six orders of magnitude change in resistance when exposed to 10 ppm and 500 ppm of MA respectively. The low cost, room temperature operation, electrical and colorimetric response of the developed sensing films make them promising candidates to be used for medical diagnostics as well as industrial use for example in chemical industry and environmental applications.

Effect of nAg concentration &amp; reaction time on the performance of nAg/0.1%PVP composite based humidity sensor

nAg/PVP composite is reported for the first time as humidity sensing material offering five orders of magnitude change in resistance. Other parameters are sensitivity of 0.066 (1/%RH), linear response (R2 = 0.99) over a wide humidity range [5–95 %relative humidity (%RH)] and fast response (8 s) and recovery (10 s) time. Three different proportions of silver salt (AgNO3) and reducing agent (Na3C6H5O7), keeping their ratio (0.05) same in 0.1% PVP (samples S1, S2 and S3) are used to study role of molar concentration on reaction time and metal nanocomposite formation. UV–Vis data reveals decrease in optical band gap and reaction time, and increase in full width at half maximum (FWHM) as a function of concentration. Increase in FWHM indicates polydispersity. Spherical nAg/PVP particles (5–20 nm for S1, 5–25 nm for S2) and spherical, triangular, hexagonal (10–35 nm for S3) are demonstrated by TEM. Coordination of nAg with C=O and tertiary nitrogen (i.e. [ ] Group) is shown by FTIR through decrease in intensities and shift in positions of corresponding peaks. When tested as resistive humidity sensors, S2 sample demonstrated best of the three. Humidity sensing mechanism is explained on the basis of complex impedance spectra at various values of RH and FTIR spectra in dry environment and high humidity environment.

Nanoscale Control of Oxygen Defects and Metal–Insulator Transition in Epitaxial Vanadium Dioxides

Strongly correlated vanadium dioxide (VO2) is one of the most promising materials that exhibits a temperature-driven, metal-insulator transition (MIT) near room temperature. The ability to manipulate the MIT at nanoscale offers both insight into understanding the energetics of phase transition and a promising potential for nanoelectronic devices. In this work, we study nanoscale electrochemical modifications of the MIT in epitaxial VO2 thin films using a combined approach with scanning probe microscopy (SPM) and theoretical calculations. We find that applying electric voltages of different polarity through an SPM tip locally changes the contact potential difference and conductivity on the surface of VO2 by modulating the oxygen stoichiometry. We observed nearly 2 orders of magnitude change in resistance between positive and negative biased-tip written areas of the film, demonstrating the electric field modulated MIT behavior at the nanoscale. Density functional theory calculations, benchmarked against more accurate many-body quantum Monte Carlo calculations, provide information on the formation energetics of oxygen defects that can be further manipulated by strain. This study highlights the crucial role of oxygen vacancies in controlling the MIT in epitaxial VO2 thin films, useful for developing advanced electronic and iontronic devices.

Resistance Switching and Memristive Hysteresis in Visible-Light-Activated Adsorbed ZnO Thin Films

The discovery of resistance switching memristors marks a paradigm shift in the search for alternative non-volatile memory components in the semiconductor industry. Normally a dielectric in these bistable memory cells changes its resistance with an applied electric field or current, albeit retaining the resistive state based on the history of the applied field. Despite showing immense potential, sustainable growth of this new memory technology is bogged down by several factors including cost, intricacies of design, lack of efficient tunability, and issues with scalability and eco-friendliness. Here, we demonstrate a simple arrangement wherein an ethanol-adsorbed ZnO thin film exhibits orders of magnitude change in resistance when activated by visible light. We show that there exists two stable ohmic states, one in the dark and the other in the illuminated regime, as well as a significant delay in the transition between these saturated states. We also demonstrate that visible light acts as a non-invasive tuning parameter for the bistable resistive states. Furthermore, a pinched hysteresis I-V response observed in these devices indicate what seems to be a new type of memristive behaviour.

Electric field induced modulation of transport characteristics in multiferroic BZT–BCT/FeCo thin films

We report the fabrication of 60Ba(Zr0.2Ti0.8)O3–40(Ba0.7Ca0.3TiO3)/Fe65Co35 (60BZT–40BCT/Fe65Co35) films on Pt/Ti/SiO2/Si substrates. The 60BZT–40BCT films exhibit well defined piezoelectric properties and the magnetic properties of Fe65Co35 films were characterized, which suggests the multiferroic nature of the films. The transport characteristics of Fe65Co35 films could be modified when the samples are subjected to a bias voltage along the thickness direction for 60BZT–40BCT/Fe65Co35 films, which manifests itself in the variation of current–voltage (I–V) curves of Fe65Co35 films under the influence of applied bias voltage. This produces more than an order of magnitude change in resistance. The manipulation could be attributed to the strain-mediated coupling and the charge accumulation and depletion at the interface.

Comparative study of TiN and TiN/Ti as bottom electrodes for layered type devices with phase transition VO2 films

DC-sputtered TiN and TiN/Ti conductive layers grown on Si substrates were investigated as bottom electrodes with the aim of realizing phase transition VO2-based layered type devices. The VO2 films prepared at a substrate temperature of 250°C on both layers were revealed to show more than two orders of magnitude change in resistance in out-of-plane direction. However, the oxidation of TiN layer was found to degrade the out-of-plane semiconductor-metal transition (SMT) of VO2 film in the case of VO2/TiN/Si structure at 400°C. On the other hand, VO2 film deposited on TiN/Ti/Si substrate at same temperature exhibits an abrupt out-of-plane SMT, in which oxidation of TiN layer was not observed. It was found through the X-ray photoelectron spectroscopy (XPS) depth profiles that the Ti layer played an indispensable role for avoiding oxidation of TiN, due to its high gettering effect for oxygen. Present results show advantage for introducing TiN/Ti layer as bottom electrode with high anti-oxidation ability.

Phase stabilization of VO2 thin films in high vacuum

A new growth approach to stabilize VO2 on Al2O3 in high vacuum is reported by reducing vanadium oxytriisopropoxide (VTIP) with vanadium metal. Phase stabilization and surface wetting behavior were studied as a function of growth parameters. The flux balance of VTIP to V in combination with growth temperature was identified to be critical for the growth of high quality VO2 thin films. High V fluxes were required to suppress the island formation and to ensure a coalesced film, while too high V fluxes ultimately favored the formation of the undesired, epitaxially stabilized V2O3 phase. Careful optimization of growth temperature, VTIP to V ratio, and growth rate led to high quality single phase VO2 thin films with &amp;gt;3.5 orders of magnitude change in resistivity across the metal-to-insulator transition. This approach opens up another synthesis avenue to stabilize oxide thin films into desired phases.

Synthesis of water-soluble dopamine–melanin for ultrasensitive and ultrafast humidity sensor

Natural melanin plays a vital role in living organisms, and is completely insoluble in almost all solvents. This article describes a simple method to synthesize water-soluble dopamine–melanin based on a new finding and the pH-dependent solubility of the dopamine–melanin. Use of the water-soluble dopamine–melanin in the humidity sensor system allows for the convenient and time-saving fabrication of dense and uniform films at the nanoscale, and features high device performance. The sensitivity of the thin-film humidity sensor made from such dopamine–melanin was evaluated as a more than five orders of magnitude change in resistance with relative humidity (RH) ranging from 100% to 0%. The dopamine–melanin sensor exhibited excellent response (0.42–0.47s) and recovery (0.43–0.49s) times under dynamic test, which are about one to two orders of magnitude faster than those of devices based on conventional inorganic materials.