1. This is standard 4-point probe configuration. You can source current through the left probes and measure the voltage through the right probes. OPTICAL AND ELECTRICAL PROPERTIES OF VANADIUM DIOXIDE (VO 2 ) THIN FILM DOPED WITH NIOBIUM (Nb) Tanviben Patel, Ama Agyapong, and Adetayo Adedeji Department of Natural Science, Elizabeth City State University, Elizabeth City, NC METHODSRESULTS CONCLUSION BACKGROUND ABSTRACT VO 2 is a thermochromic material with reversible structural transition at a transition temperature of 68°C. At this temperature, VO 2 structural changes from insulating monoclinic structure to metallic tetragonal rutile structure. The structural change allows the material to be spectrally selective[1-4]. Not only optical switches has been observed in this material, electrical and magnetic properties have been observed to switch as well at the same transition temperature. Application of VO 2 thermochromic property in energy conservation is hindered by its relatively high transition temperature. However, literature explains that doping VO 2 with impurities like heavy transition metals can decrease the transition temperature to suitable temperature[2]. Our goal is to systematically add transition metal impurities to VO2 to drive its transition temperature to about 35°C desired applications. According to previous research, vanadium dioxide (VO 2 ) is a spectrally selective thermochromic material that endures reversible structural changes at temperature of 68 °C. The objective of this research is to observe and enhance the thermochromic properties of VO 2 by adding small amount of niobium (Nb) on the samples. Literature shows that impurities added to VO 2 can drive down the transition temperatures. The samples were coated by DC magnetron sputtering of vanadium metal and RF co-sputtering of Nb metal. Samples deposited on quartz and diamond substrates were annealed at 500 °C in pure oxygen and nitrogen gas for 4 hours at the pressure of 800 mtorr to obtain VO 2 with added impurity. The optical and electrical data on the samples were acquired as a function of temperatures, from room temperature to about 90 °C and back to room temperature. Surface morphology of the samples has been obtained using SEM before and after oxidization. The images of surface morphology indicate the uniformity of the samples. Energy Dispersive Spectroscopy (EDS) gave the atomic composition of the oxidized coatings. The deposition and characterization steps were repeated to ensure reproducibility. Future work includes variation in the thickness of the coating. OBJECTIVE The objective of this experiment is to drive down the transition temperature of VO 2 from 68°C to less than 35°C for energy conservation application. Magnetron Sputtering Purpose: Deposit Thin Films of V and V-Nb on Quartz and CVD Diamond Substrates. Conditions: Samples were sputtered for 60 minutes at a pressure of 10 milli-torr (mtorr). Vanadium was sputtered using DC power of 100 watts (W). Niobium and Tantalum were co-sputtered with Vanadium using RF power of 30W. All the sputtering were done with pure Argon gas at room temperature. Sputtering CombinationDC Power (W)RF Power (W) V100- V-Nb10030 Thermal Oxidation The V-Nb and V-Ta samples were oxidized in nitrogen/oxygen gas mixture for 4 hours at 500°C in an annealing furnace with at a pressure of 800 millitorr. The flow rate of the nitrogen to oxygen gases used is 13 sccm to 0.03 sccm. The furnace was ramped to 500°C in one hour. Thin Film Optical Spectroscopy Using the Filmetric software, the reflectance and transmittance data of each sample was acquired at different sample temperatures. A transformer was utilized to control the samples heating process Van Der Pauw Method (Electrical data) The resistance and resistivity of an arbitrarily shaped sample can be determined using a four point probe method. The resistivity is given by: Ρ=πt/(ln(2))× ((R_12,34+ R_23,41))/2 x F where F is the correction factor, R is the resistance obtained by Ohm’s law. Scanning Electron Microscopy Images of the samples were taking using a scanning electron microscope at different magnifications (30K, 10K, and 300) to observe surface morphology changes. The images are secondary electron images at 20 kV accelerating voltage. SEM images of the samples before and after the thermal oxidization are shown below. Energy Dispersive Spectroscopy (EDS) Atomic compositions of the thin films were obtained with the in- built x-ray detector. Atomic percentages of V:O:Nb and V:O of the corresponding metals oxidized on CVD diamond were obtained from the x-ray peaks. SEM at 10,000X Magnification As-deposited Vanadium on CVD diamondAfter oxidation process: VO 2 on diamond As-deposited Vanadium-Niobium on CVD diamond After oxidation process: VO 2 :Nb diamond The reflectance and transmittance data of the samples showed a jump around 60°C. The transition temperature of VO 2 was driven down with addition of Nb as impurity. The thermal cycle for both optical and electrical data shows “hysteresis” curve. No significant changes in the samples’ surface morphology after the thermal oxidization. EDS may not be the most accurate way to obtain the atomic composition. However, based on the EDS, it is observed that all elements expected on the samples were present. Samples that were reproduced with same parameters have different characterization. REFERENCE & ACKNOWLEDGEMENTS [1]. Kanu, S., & Binions, R. (2009). Thin films for solar control applications. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 19-44. [2]. Mlyuka, N., Niklasson, G., & Granqvist, C. (n.d.). Thermochromic VO-based multilayer films with enhanced luminous transmittance and solar modulation. Physica Status Solidi (a), 2155-2160. [3]. Qureshi, U., Manning, T., Blackman, C., & Parkin, I. (n.d.). Composite thermochromic thin films: (TiO 2 )–(VO 2 ) prepared from titanium isopropoxide, VOCl 3 and water. Polyhedron, 334-338. [4]. Tazawa, M., Jin, P., Miki, T., Yoshimura, K., Igrashi, K., & Tanemura, S. (n.d.). IR properties of SiO deposited on V 1−x W x O 2 thermochromic films by vacuum evaporation. Thin Solid Films, 100-103. Acknowledgments: This research is partly funded by the National Science Foundation grant (grant #: 1337141) EDS oxidized samples We plan to use Rutherford Backscattering Spectroscopy (RBS) and X-ray Diffraction to obtain better atomic composition. Change the sputtering parameters such as the sputtering time, vary power, and oxidation conditions. FUTURE WORK Graph 1: Shows the reflectance of V/Nb and compares the hysteresis loop between two samples that were produced using same parameters Graph 2: Shows the transmittance of V/Nb and compares the hysteresis loop between two samples that were produced using same parameters Van Der Pauw Method (Electrical) Thin Film Optical Spectroscopy