Abstract
This work investigates the surface dynamics of vanadium dioxide with nitric oxide in dependence of its metal-to-insulator transition (MIT) at 68 °C. At lower temperatures VO2 has a monoclinic lattice structure which has low electrical conductance. VO2(M) is therefore considered an insulator. At higher temperature it is found in its rutile phase, which shows several orders of magnitude more electrical conductance than the monoclinic phase. VO2(R) is therefore considered a metal. In previous works it was found that when vibrationally excited NO scatters from metal surfaces, such as Au(111) and Ag(111), vibrational energy is transferred via an electron abstraction mechanism to the surface degrees of freedom. It is found that the vibrational relaxation probability of NO scattered from metals is dependent on the difference between the molecule’s vertical binding energy at the outer turning point of the vibration and the work function of the surface. This mechanism is not present for scattering from insulator materials such as LiF because they have no electron density at energies similar to the vibrational energy of NO. As VO2 is a material which can switch between metal and insulator phase it is investigated how it fits into the previously seen picture of vibrational relaxation of scattered NO. For this samples of VO2 films are prepared with chemical vapor deposition (CVD) and radio frequency magnetron sputtering (RFMS). The former results in films with grainy surface structure. The latter yields VO2 films with a structure close to single crystals and relatively flat surfaces. Molecular beams of vibrationally excited NO are scattered from various VO2 films and analyzed for their translational, rotational and vibrational excitation after scattering. This is done with time-of-flight spectroscopy and REMPI spectroscopy of the y-band system of NO. It is shown that NO(v = 2, J = 0.5) and NO(v = 11, J = 0.5) scattered from CVD prepared films are dominantly scattered via the direct scattering channel. Scattered NO molecules retain mostly their initial vibrational state during scattering and have a hyperthermal rotational excitation after scattering. Furthermore, the translational energy after scattering and the rotational excitation increase both with the translational energy of the incident molecular beam. During the experiments no trapping desorption is observed but a certain amount of NO is seen with lower rotational and vibrational excitation and lower translational energy than the directly scattered molecules. It is assumed that they penetrate into the gaps between the VO2 grains of the CVD prepared surface and bounce multiple times before they leave the surface again. For NO scattered from CVD prepared VO2 no clear sign of a increased vibrational relaxation probability of NO scattered from VO2(R) compared to VO2(M) could be observed. This is attributed to the fact that the CVD prepared samples are cleaned with Ar+- ion sputtering which is found to eliminate oxygen from the VO2 film. This cleaning technique is yet not able to remove all carbon contaminations present on the surface of CVD prepared VO2. The former results in a changed chemical composition of the VO2 film, which causes a reduced contrast of the MIT. The latter results in surfaces partially covered in carbon. In consequence, it can not be outruled that a certain amount of NO scatters from carbon instead of VO2. To improve the surface cleaning procedure this work also investigates the application of O2+-ion sputtering. It is shown that O2+-ion sputtering is significantly more efficient in removing carbon contaminations from the surface. Furthermore, it negates the effect of oxygen elimination from the VO2 film by inserting oxygen back into the surface. This retains the chemical composition of VO2 during sputtering and in consequence also the contrast of the MIT. The O2+-ion sputtering technique is applied to the later RFMS prepared VO2 thin films instead of Ar+-ion sputtering. NO(v = 3, J = 0.5) is prepared in a molecular beam with a translational energy of Einc,trans = 0.99 eV and scattered from RFMS prepared VO2 thin films. The directly scattered molecules relax with a probability of 2% to the (v = 2) state when scattered from VO2(M). The vibrational relaxation probability increases by 0.3% when the thin film changes to VO2(R) at 68 C. No population is found for NO(v = 1) for both phases of VO2. NO(v = 11, J = 0.5) is prepared in molecular beams with translational energies of Einc,trans = 0.92 eV, Einc,trans = 0.73 eV and Einc,trans = 0.47 eV. For direct scattering from RFMS prepared VO2(M) vibrational relaxation probabilities of about 20% are found which increase with the translational energy of the incident molecular beam. For VO2(R) the relaxation probability increases by 3% compared to scattering from VO2(M). A detailed comparison with the vibrational relaxation probabilities found for NO scattered from Ag(111) and Au(111) is done to understand why relaxation of NO from VO2 is significantly less likely than from other metals. For this the image charge stabilization, work function and the charge carrier density of VO2(M), VO2(R), Au(111) and Ag(111) are compared. This work also shows the comparison of NO(v = 11) scattered from SiO2 and VO2. It is shown that for scattering from SiO2 NO is only found in the vibrational states v = 11 and v = 10. For scattering from VO2 additionally a small amount of population is observed in the vibrational states 4 < v < 10. Rotational state distributions of scattered NO(v = 3) and NO(v = 2) from the scattering experiments of NO(v = 3, J = 0.5) from RFMS prepared VO2 thin films are analyzed. It is shown that the rotational energy of scattered NO(v = 3) is significantly higher that the rotational excitation of thermal NO(v = 3) at the surface temperature. The rotational excitation of the vibrationally relaxed NO(v = 2) is more than 50% higher than the rotational excitation of NO(v = 3). The same effect for the rotational excitation of NO(v = 11) and NO(v = 10) is also shown in the scattering experiments of NO(v = 11, J = 0.5) scattered from RFMS prepared VO2 thin films and for NO(v = 11, J = 0.5) scattered from SiO2. This suggests that a mechanism is present during the scattering which transfers vibrational energy to the rotational quanta which has yet not been reported for NO scattering from metal surfaces.
