Polycrystalline VO2 Research Articles

Vanadium oxide thin films growth by a chemical solution deposition method and its pH sensor toward bio-sensing devices

Abstract We report on the evaluation of the structure of VOx thin films under different fabrication conditions of the chemical solution deposition method and the pH response of an extended gate field effect transistor (EGFET)-type pH sensor. Polycrystalline V2O5 thin films of 30 and 60 nm thick and amorphous VOx thin films of about 30 nm thick were successfully prepared under different precursor solution conditions. Using polycrystalline V2O5 thin films and amorphous VOx thin films as an extended gate (EG) electrode, protypes of EGFET-type pH sensors were fabricated and investigated their pH response characteristics. The average pH responsibility of 64 mV/pH, which exceeded the theoretical Nernst limit, was observed over a wide pH range from 2.4 to 8.0 in the amorphous VOx extended gate sensor. It was also found that stable pH response measurements could be performed on amorphous VOx film samples.

The influence of structural defects on the electrical and infrared emission properties of VO2 thin films

Defect engineering is widely used in the modification of transition metal oxides. In this study, defects are introduced into the polycrystalline VO2 film by the energetic Ar ion irradiation, and the influence of structural defects on the electrical and infrared emission properties of VO2 thin films have been investigated. The decrease in electrical switching ratio with the increase of defect ratio (displacement per atom (dpa)) can be described as R10°C/R90°C=2.032×(dpa+0.005)−1.8069, while the infrared modulation performance is Δε=−0.119×ln(dpa+0.061). The dpa thresholds of the electrical switching ratio and the infrared modulation performance are ηdpaRR = 0.005/atom and ηdpaΔε = 0.0426/atom, respectively, indicating that the electrical switching ratio of VO2 films is more responsive to defects than the infrared modulation performance. The relationship between transition temperature and defect ratio has be determined. Concerning electrical properties, the heating phase transition temperature can be described as TMITHeating=25.88−10.93×ln(dpa+0.01); while for infrared emission modulation, the heating phase transition temperature is given by Tε−MITHeating=31.65−8.18×ln(dpa+0.01). While reducing the phase transition temperature, the VO2 thin films still exhibit an electrical switching ratio of more than two orders of magnitude and obvious infrared modulation performance. It not only offers a viable solution for broadening the application scope of VO2 but also contributes to understanding the role of defects in VO2 thin films for further research.

Polarized Raman mapping and phase-transition by CW excitation for fast purely optical characterization of VO2 thin films

Vanadium dioxide has attracted much interest due to the drastic change of the electrical and optical properties it exhibits during the transition from the semiconductor state to the metallic state, which takes place at a critical temperature of about 68 °C. Much study has been especially devoted to developing advanced fabrication methodologies to improve the performance of VO2 thin films for phase-change applications in optical devices. Films structural and morphological characterisation is normally performed with expensive and time consuming equipment, as x-ray diffractometers, electron microscopes and atomic force microscopes. Here we propose a purely optical approach which combines Polarized Raman Mapping and Phase-Transition by Continuous Wave Optical Excitation (PTCWE) to acquire through two simple measurements structural, morphological and thermal behaviour information on polycrystalline VO2 thin films. The combination of the two techniques allows to reconstruct a complete picture of the properties of the films in a fast and effective manner, and also to unveil an interesting stepped appearance of the hysteresis cycles probably induced by the progressive stabilization of rutile metallic domains embedded in the semiconducting monoclinic matrix.

Tracking optical properties of VOx films to optimize polycrystalline VO2 fabrication

In-situ real-time spectroscopic ellipsometry (RTSE) measurements are performed on a growing amorphous vanadium oxide (a-VOx) film to determine the complex dielectric function (ε = ε1 + iε2) spectra, structure, and oxygen content (x) with depth during deposition. VOx with x ∼2 is annealed to produce polycrystalline VO2 and characterized by near-infrared to ultraviolet (0.75–5.9 eV) spectroscopic ellipsometry during heating and cooling from room temperature (RT) to 343 K to RT to track hysteresis effects in the semiconducting-to-metal transition (SMT). Spectra in ε are measured for as-deposited and annealed films prepared with varying deposition parameters to determine the as-deposited film x and verify the SMT. Temperature-dependent IR extended spectroscopic ellipsometry is performed from 0.06 to 0.74 eV to determine the IR ε spectra of polycrystalline VO2 across the SMT. As-deposited amorphous VOx films with x from 1.89 ≤ x ≤ 2.14 transition to polycrystalline VO2 after annealing and exhibit the SMT when heated from RT to 343 K which are verified by substantially increased ε2 magnitude at ≥343 K. By identifying x of a-VOx from ε spectra, a shortened path of qualifying a-VOx films which will crystallize to phase change VO2 is developed.

Формирование наноструктур из поликристаллических пленок диоксида ванадия с помощью сканирующей зондовой литографии

Vanadium dioxide (VO2) undergoes a reversible first-order metal-insulator phase transition near room temperature, accompanied by a structural phase transition. This causes a significant change in its electrical and optical properties, making it useful for practical applications. VO2-based nanostructures exhibit notable mechanical resistance to structural transition caused by their small size and demonstrate vivid properties during the phase transition. Obtaining VO2 nanostructures is an extremely demanded objective. This research study presents the use of scanning probe lithography techniques to fabricate nanostructures on polycrystalline VO2 films. The present study focuses on the modification of VO2 films when a positive bias is applied to the sample. The effect of the value and duration of the applied voltage, relative humidity on the quality of the formed nanolithographic pattern was analyzed. The oxidation mechanism was determined. It was found that as a result of local anodic oxidation the formed oxide structures consisting of vanadium penta-oxide (V2O5) completely dissolve in water. This process leads to the separation of the continuous polycrystalline VO2 film into individual nanostructures with precise dimensions. The presented method of forming nanostructures from crystalline VO2 films is promising for nanophotonics and nanoelectronics.

Near room temperature tunability of thermochromic VO2 thin films prepared via thermal oxidation method, influence of CO2:N2 gas flux ratios

In this study, we have investigated the thermochromic characterizations of VO2 thin films synthesized by the thermal oxidation method. The oxidation process of a DC sputtered metallic vanadium layer on glass substrates, at 450 °C for 1 h, was carried out in the presence of CO2:N2 gases with different flux ratios of 30:70, 40:60, 50:50, 60:40, and 70:30, respectively. Using CO2, as the oxidizing gas, provides an easy control route for obtaining the VO2 phase among various vanadium oxide phases. The layers were characterized by FESEM, XRD & Raman spectra, sheet resistance vs. temperature (20–120 °C), and UV–Vis–NIR spectra at 25 and 90 °C. The XRD and Raman spectra confirmed all prepared layers have a VO2 polycrystalline structure in monoclinic phase. We found among the studied samples CN30-70 and CN50-50 having desirable optical characteristics of peak visible transmittance of about 55%, with low transition temperatures (Tcr) of ∼42 and 31 °C, and also relatively high amounts of ΔT1700nm, ΔTsol and Tlum,av are good candidates for thermochromic smart windows, both from optical properties and economical fabrication method points of view.

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. We explore the novel combination of two methods of VO2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO2. Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

Insights into Electrode Architectures and Lithium-Ion Transport in Polycrystalline V2 O5 Cathodes of Solid-State Batteries.

Polymer-based solid-state batteries (SSBs) have received increasing attentions due to the absence of interfacial problems in sulfide/oxide-type SSBs, but the lower oxidation potential of polymer-based electrolytes greatly limits the application of conventional high-voltage cathode such as LiNix Coy Mnz O2 (NCM) and lithium-rich NCM. Herein, this study reports on a lithium-free V2 O5 cathode that enables the applications of polymer-based solid-state electrolyte (SSE) with high energy density due to the microstructured transport channels and suitable operational voltage. Using a synergistic combination of structural inspection and non-destructive X-ray computed tomography (X-CT), it interprets the chemo-mechanical behavior that determines the electrochemical performance of the V2 O5 cathode. Through detailed kinetic analyses such as differential capacity and galvanostatic intermittent titration technique (GITT), it is elucidated that the hierarchical V2 O5 constructed through microstructural engineering exhibits smaller electrochemical polarization and faster Li-ion diffusion rates in polymer-based SSBs than those in the liquid lithium batteries (LLBs). By the hierarchical ion transport channels created by the nanoparticles against each other, superior cycling stability (≈91.7% capacity retention after 100 cycles at 1C) is achieved at 60 °C in polyoxyethylene (PEO)-based SSBs. The results highlight the crucial role of microstructure engineering in designing Li-free cathodes for polymer-based SSBs.

Annealing Enhanced Phase Transition in VO2 Thin Films Deposited on Glass Substrates via Chemical Vapor Deposition

In this work, VO2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO2 in simple monoclinic crystal structure (P21/c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies.

Hydrothermal synthesis and thermochromism effects in Eu-doped VO2 polycrystalline materials

Hydrothermal synthesis and thermochromism effects in Eu-doped VO2 polycrystalline materials

Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.

Strain-Induced Modulation of Resistive Switching Temperature in Epitaxial VO2 Thin Films on Flexible Synthetic Mica.

The resistive switching temperature associated with the metal-insulator transition (MIT) of epitaxial VO2 thin films grown on flexible synthetic mica was modulated by bending stress. The resistive switching temperature of polycrystalline VO2 and V2O5 thin films, initially grown on synthetic mica without a buffer layer, was observed not to shift with bending stress. By inserting a SnO2 buffer layer, epitaxial growth of the VO2 (010) thin film was achieved, and the MIT temperature was found to vary with the bending stress. Thus, it was revealed that the bending response of the VO2 thin film depends on the presence or absence of the SnO2 buffer layer. The bending stress applied a maximum in-plane tensile strain of 0.077%, resulting in a high-temperature shift of 2.3 °C during heating and 1.8 °C during cooling. After 104 bending cycles at a radius of curvature R = 10 mm, it was demonstrated that the epitaxial VO2 thin film exhibits resistive switching temperature associated with MIT.

Strategic texturation of VO2 thin films for tuning mechanical, structural, and electronic couplings during metal-insulator transitions

Owing to its pronounced metal-insulator transition at ∼340 K, vanadium dioxide (VO2) has emerged as a promising candidate material to emulate neuronal logic and memory functions for neuromorphic computing applications. For viable implementation into practical devices, it is critical to understand the fundamental mechanical behavior of VO2 during this phase transformation. Herein, we implemented sputter deposition under various conditions to strategically texture VO2 thin films, thereby enabling us to examine the influence of crystal orientation on mechanical, structural, and electrical properties across its characteristic metal-insulator transition. Notably, polycrystalline VO2 and epitaxial VO2/sapphire (0001) films developed tension, whereas epitaxial VO2/TiO2 (001) developed compression in heating through the phase transformation. Through structural analysis, we attribute this tension/compression disparity to highly anisotropic deformation that occurs during the phase transformation. Corresponding analyses from linear elastic fracture mechanics enable the prediction of a critical film thickness, below which polycrystalline VO2 films will not fracture, which has implications for the design of resilient neuromorphic architectures. Similarly, by analyzing the stress evolution in epitaxial VO2/TiO2 (001) films, we find that fracture occurs during sputter deposition itself. Finally, we conduct simultaneous measurements of mechanical stress and electrical conductance of polycrystalline and epitaxial VO2 thin films during thermal cycling. Surprisingly, we unveiled that the orientation of the film can even alter the temperature-sequence of the macroscopic electrical response and overall stress response during the phase transformation, which we attribute to spatial heterogeneities in the transformation.

Modulations in electrical properties of sputter deposited vanadium oxide thin films: Implication for electronic device applications

• Optimized growth of polycrystalline VO 2 films on SiO 2 /Si substrates by Sputtering. • Appreciable insulator to metal transition (IMT) observed down to 23 nm film thickness. • Film thickness induce systematic modulation in IMT order and IMT temperature. • Low resistivity (∼10 −2 Ω-cm) achieved for insulator state in 23 and 38 nm films. Engineering electronic properties of VO 2 across its insulator to metal transition enable to design wide range of electronic devices operating with low voltage such as memristor, field effect transistor, and in neuromorphic device as an active circuit element. We investigated the electrical and structural properties of sputter deposited VO 2 thin films grown on SiO 2 /Si substrates. In polycrystalline VO 2 films, we observed appreciable insulator to metal transition (IMT) down to film thickness of 23 nm. Further, thickness dependence study reveals a systematic variation in the IMT order between 30-3000, accompanied by modulation in transition temperature and hysteresis. The genesis of the modulations is attributed to morphological and structural growth dynamics. Significantly, we have attained low values for insulating states (∼10 −2 Ω-cm) in low dimensional thin films (23 and 38 nm), which are desirable for Joule heating driven IMT based VO 2 devices with low voltage switching, as well as for sensing applications.

The structural, optical properties, surface and electrical properties of vanadium pentoxide (V2O5) films produced at different substrate temperatures

Vanadium pentaoxide (V2O5) coatings generated by ultrasonic spray pyrolysis on glass substrates at various temperatures have been investigated. The XRD analysis revealed polycrystalline V2O5 films with an orthorhombic structure. The optimal orientation of V2O5 films is the peak direction (101) for substrate temperatures 300 °C, 350 °C, 450 °C, and (110) for substrate temperatures 400 °C, according to structural analyses. The optical characteristics were investigated using a UV–VIS spectrofotometer. As the temperature of the substrate increased, the transmittance of the films increased by up to 42 %. Using the optical absorption method, the optical band gaps of the films were estimated. It can be shown that when the substrate temperature rises, the optical band gap values decrease. The increase in substrate temperature caused porous structures to emerge, as well as the formation of region borders, as shown in SEM pictures. The electrical properties of films were searched by four probe technique It was seen that the resistivity values of the V2O5 films produced by the ultrasonic spray pyrolysis method decreased as the substrate temperature increased, thus the conductivity values increased.

Electrical and dielectric properties of polycrystalline VO2 discriminating between bulk and grain boundary conduction

Electrical and dielectric properties were measured on polycrystalline VO2 samples in the neighborhood of the metal → insulator transition (MIT) temperature of ∼340 K, prepared with the ceramic sintering technique. It is shown that in certain temperature ranges in the monoclinic semiconducting M1-phase below the MIT, bulk and grain boundary (GB) effects can be distinguished applying the method of impedance spectroscopy. An enormous literature has accumulated, dealing with electrical properties of VO2 small films and nanowires, with the aim to prepare various kinds of micro-devices for technology. Evidently, investigations to separate bulk from GB effects are largely missing, although many small films and nanowires are polycrystalline. The structure of their GBs doubtless differs from that in macroscopic samples, nevertheless valuable information might be supplied concerning electrical conduction across and along GBs, compared with conduction in the bulk. GB processes in our VO2 samples could be measured between room temperature and the MIT, bulk properties along with GB conduction could only be studied at low temperatures.

Surface Morphology and Optical Mutation Properties of V2O5 Films Prepared with Different Oxygen Partial Pressures

vanadium pentoxide films(V2O5) were deposited on sapphire substrates under four different oxygen partial pressures by magnetron sputtering. Then the films’ crystallinity, surface morphology and optical mutation properties were researched. The results show that all the prepared films are polycrystalline V2O5, and the crystallization performance gets better with the oxygen pressure increases. In addition, the Root-Mean-Square(RMS) roughness decrease with the increase of oxygen pressure. Furthermore, with the oxygen pressure increases, the optical turn-off time and turn-on time are 2.2 and 36 ms, respectively down to 1.7 and 30 ms, and before/after phase-change transmittance, meanwhile, decreases from 75%/20% to 57%/11%, showing a great improvement in optical mutation properties.

In situ Raman-scattering mapping and resistance study of the VO2 metal-insulator phase transition

In this study, a large-area single-phase VO2 film was successfully prepared on a Si (100) substrate by reactive magnetron sputtering deposition. The thickness of the VO2 film is approximately 220 nm. The phase transition temperature is about 67 °C, with more than three orders of magnitude of the resistance change. X-ray diffraction patterns obtained at different temperatures indicate the coexistence of two phases during the VO2 phase transition process, where a portion of the grains first transform from a monoclinic structure to a rutile structure, and then phase transition expands throughout the film. In situ Raman-scattering mapping scans further prove that the monoclinic-to-rutile phase transition occurs gradually in polycrystalline VO2 films, in which monoclinic and rutile phases coexist from 55 °C to 80 °C. Electrical resistance of the film will change dramatically when the volume fraction of monoclinic phase decreases to 43.5%, or the volume fraction of rutile phase increases to 55.5%. The phase transition temperature differs among the grains in the polycrystalline VO2 film. This study provides an important scientific approach for elucidating the mechanism of the metal-insulator phase transition.

Effect of the bottom layer thickness on the structural and optical phase transition properties of V2O5/V/V2O5 thin films

Vanadium dioxide (VO2 (M)) is an intelligence material that undergoes a phase transformation that can be tuned. This material has the potential to be used as an energy-efficient coating. Scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and optical transmittance measurements were used to characterise the multi-layered thin films with a varying bottom layer thickness. Surface morphology reveals irregular shapes, boundary layers, and porous at the nanoscale, and the presence of tiny cracks. For all samples, the root means square (RMS) roughness reveals a smooth film surface. The formation of polycrystalline VO2 (M) thin films is confirmed by XRD patterns. Switching thermochromic properties are demonstrated by temperature-dependent of optical transmittance measurements. Transmittance increases significantly in the infrared region when compared to the visible region. The transmittance in the UV (ultra-violent) ranges between 15 and 25%, while the transmittance in the NIR (near infrared region) ranges between 5 and 40%, depending on the thickness of the bottom layer. We also reduced the phase transition temperature from 48 to 50 °C when compared to pure VO2 (∼68 °C), this reduction in phase transition is attributed to the distinction of nanostructured in VO2. This research could help to advance the use of smart window coatings in the real world.

Photoinduced Multistable Resonance Frequency Switching of Phase Change Microstring at Room Temperature

AbstractVanadium dioxide (VO2), a promising phase change material, exhibits insulator to metal transition at 68 °C, manifests a drastic change in multiple physical properties, such as electrical resistance, mechanical modulus, lattice parameters, etc. From technological perspective, the transition temperature can be reduced by precise strain engineering. Here a noncontact, all‐optical, and highly energy efficient platform is demonstrated to study macroscopic dynamics related to the localized structural rearrangements at room temperature. A thin layer (≈25 nm) of polycrystalline VO2 deposited on a platinum coated silicon nitride microstring resonator shows a fast controlled mechanical resonance frequency response upon variations in optical power and wavelength. It is shown multiple stable frequencies of the resonator, designated as different equilibrium states, can be activated at different optical powers (≈200 µW) and wavelength, i.e., 450, 520, and 635 nm. The observed multiple resonance states of the microstring are explained because of the generation of stress due to the interplay between thermal expansion and the temperature‐induced phase change of VO2. It is believed this change in frequency states under the controlled external optical excitation can have potential applications in ultrafast optical switching, intelligent temperature sensors, and neuromorphic devices operated at room temperature.