Selective scanning tunneling microscope light emission from rutile phase of VO₂

Abstract

We observed scanning tunneling microscope light emission (STM-LE) induced by a tunneling current at the gap between an Ag tip and a VO2 thin film, in parallel to scanning tunneling spectroscopy (STS) profiles. The 34 nm thick VO2 film grown on a rutile TiO2 (0 0 1) substrate consisted of both rutile (R)- and monoclinic (M)-structure phases of a few 10 nm-sized domains at room temperature. We found that STM-LE with a certain photon energy of 2.0 eV occurs selectively from R-phase domains of VO2, while no STM-LE was observed from M-phase. The mechanism of STM-LE from R-phase VO2 was determined to be an interband transition process rather than inverse photoemission or inelastic tunneling processes.

Pump–probe STM light emission spectroscopy for detection of photo-induced semiconductor–metal phase transition of VO2

“…Through a number of measurements of conventional STM-LE spectroscopy at the points at which the metallic phase was confirmed by STS, we have observed LE peaks at around 2.0 and/or 2.7 eV, as shown in figure 4(a). Signals at 2.7 eV, as well as those at 2.0 eV, have been seen already (although not mentioned) in our previous studies (figures 3(b) and 3(d) of [15]). It should be noted that the real part of the dielectric function of R phase domains is positive in the observed spectral range from 1.5 to 3.0 eV [25], although this phase is electrically metallic (see figure 2(c)).…”

“…Therefore, STM-LE spectr oscopy characterizes solid materials at atomic-level spatial resolution and millielectronvolt energetic resolution. In our previous STM-LE study on a VO 2 film, in which semiconducting (M) and metallic (R) phase domains several tens of nanometers in size coexisted at room temperature (RT), we found that only the R phase causes STM-LE; the M phase does not [15]. Since the photon energy of STM-LE showed no bias voltage dependence, we concluded that the STM-LE is due to interband transition.…”

“…As in our previous study [15], we use the same VO 2 thin-film sample grown on a single-crystal rutile TiO 2 (0 0 1) substrate by pulsed laser deposition. XRD (see figure 1(a) of [15]) has revealed that the VO 2 film was oriented along the c-axis (in the R phase) and accompanied by compressive strain along the c-axis (c = 2.842 Å). Interference fringes in the XRD 2θ − ω profile indicate a film thickness of 34 nm and excellent crystallinity.…”

“…Interference fringes in the XRD 2θ − ω profile indicate a film thickness of 34 nm and excellent crystallinity. Partial lattice relaxation occurred at the first R-M transition while measuring resistivity versus temperature, resulting in the coexistence of domains of both M and R phases at RT (see figure 1(b) of [15]). After being placed in an ultra-high vacuum (UHV) chamber, the sample surface was cleaned by Ar-ion bombardment with an acceleration voltage of 600 V and an ion current of ~1 µA for 1 min.…”

Stress-Induced In Situ Modification of Transition Temperature in VO2 Films Capped by Chalcogenide

“…A bias power of 5 W was applied to the sample stage during the sputtering process. Other deposition conditions can be found in previous reports [42,43].…”