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Using non-contact AFM to study the local doping and damping through the transition in an ultrathin VO2 film
No full textBulk VO2 undergoes an insulator-to-metal transition (IMT) with up to five order of magnitude change its resistivity at 340 K. However, when VO2 is deposited as a film on a substrate, the strain from the substrate can alter the IMT temperature, resistivity ratio, and hysteresis. Here, we present single-phase VO2 ultrathin films (thick- ness less than 20 nm) grown using oxygen plasma molecular beam epitaxy (MBE) on TiO2(001) and Al2O3(0001) substrates. First, we modify existing recipes employing ozone MBE and reproduce the best reported films on TiO2(001); maintaining an almost three order of magnitude transition in a 12 nm thick film. We then extend our recipe to Al2O3(0001) substrates where we stabilize a 12 nm thin single-phase VO2 film and observe a two order of magnitude transition, expanding the possible growth methods for ultrathin VO2 films on Al2O3(0001).
In a separate, approximately 10 nm thick VO2 film on TiO2(001), we use non-contact AFM (nc-AFM) to track electronic properties across the expected temperature range of transition. We first observe a change in the work function of VO2 at the expected bulk transition temperature. We then measure the frequency shift and dissipation at varying bias and tip-sample separation at seven stable temperatures, spanning the insulating to the metallic state. Using the frequency shift data, we ex- tract the tip-sample capacitance, then calculate the induced carrier doping within the sample as a result of the proximity of the tip. The work function, carrier density, and damping results above and below the IMT suggest that we observe the IMT; with the damping in particular showing consistent behavior with a decrease in resistance starting as low at 270 K and continuing to 300 K. Furthermore, we observe a change in work function at fixed temperature at close tip-sample separation as low as 240 K, along with elevated damping, suggesting the electric field from the tip itself may be initiating the transition, at least in the surface of the film, at temperatures as low as 240 K in the close tip-sample separation regime
The importance of Joule heating on the voltage-triggered insulator-to-metal transition in VO₂
No full textThe large change in resistivity in the material VO₂ has attracted considerable attention since it was first discovered in 1959. Recently, the ability to trigger the insulator-to-metal transition (IMT) with a strong electric field has been observed, but there has been debate about whether the transition is due to field-effects. We apply a voltage bias across a VO₂ thin film via a conductive atomic force microscope (CAFM) tip and measure the resultant current. We observe the IMT as a jump in the measured current in the IV curves. We fit the IV curves to the Poole-Frenkel (PF) conduction mechanism in the insulating state, immediately preceding the IMT. The PF conduction mechanism describes the thermal excitation of electrons into the conduction band in insulators, facilitated by strong electric fields. The PF mechanism is temperature dependent, and we use the temperature dependence to calculate the local temperature of the film before the transition. We directly compare the local electric field and local temperature of the film immediately preceding the IMT. We determine that the transition is not solely due to the applied electric field, but rather that the tip has locally warmed the film close to its IMT temperature through Joule heating.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
Nanoscale thermal imaging of VO<sub>2</sub> via Poole–Frenkel conduction
No full textWe present a method for nanoscale thermal imaging of insulating thin films using atomic force microscopy (AFM), and we demonstrate its utility on VO2. We sweep the applied voltage V to a conducting AFM tip in contact mode and measure the local current I through the film. By fitting the IV curves to a Poole-Frenkel conduction model at low V, we calculate the local temperature with spatial resolution better than 50 nm using only fundamental constants and known film properties. Our thermometry technique enables local temperature measurement of any insulating film dominated by the Poole-Frenkel conduction mechanism and can be extended to insulators that display other conduction mechanisms. © 2022 Author(s).11Nsciescopu
Overview of Four Prescription Monitoring/Review Programs in Canada
No full textBACKGROUND: Prescription monitoring or review programs collect information about prescription and dispensing of controlled substances for the purposes of monitoring, analysis and education. In Canada, it is the responsibility of the provincial institutions to organize, maintain and run such programs.OBJECTIVE: To describe the characteristics of four provincial programs that have been in place for >6 years.METHODS: The managers of the prescription monitoring/review programs of four provinces (British Columbia, Alberta, Saskatchewan and Nova Scotia) were invited to present at a symposium at the Canadian Pain Society in May 2012. In preparation for the symposium, one author collected and summarized the information.RESULTS: Three provinces have a mix of review and monitoring programs; the program in British Columbia is purely for review and education. All programs include controlled substances (narcotics, barbiturates and psychostimulants); however, other substances are differentially included among the programs: anabolic steroids are included in Saskatchewan and Nova Scotia; and cannabinoids are included in British Columbia and Nova Scotia. Access to the database is available to pharmacists in all provinces. Physicians need consent from patients in British Columbia, and only professionals registered with the program can access the database in Alberta. The definition of inappropriate prescribing and dispensing is not uniform. Double doctoring, double pharmacy and high-volume dispensing are considered to be red flags in all programs.CONCLUSIONS: There is variability among Canadian provinces in managing prescription monitoring/review programs.Peer Reviewe
