Interaction between O2 and ZnO films probed by time-dependent second-harmonic generation

Abstract

The interaction between O2 and ZnO thin films prepared by atomic layer deposition has been investigated by time-dependent second-harmonic generation, by probing the electric field induced by adsorbed oxygen molecules on the surface. The second-harmonic generated signal decays upon laser exposure due to two-photon assisted desorption of O2. Blocking and unblocking the laser beam for different time intervals reveals the adsorption rate of O2 onto ZnO. The results demonstrate that electric field induced second-harmonic generation provides a versatile non-contact probe of the adsorption kinetics of molecules on ZnO thin films.

Enhancement of the Sub-Band-Gap Photoconductivity in ZnO Nanowires through Surface Functionalization with Carbon Nanodots

“…Therefore, the fast injection of photoexcited electrons from the C-dots into the conduction band of the ZnO NWs does not only induce a quick increase of the device photoconductivity thanks to the enhanced electron density in the NWs, but causes indirectly also a reduction of the NW surface band bending, which leads to a further increase of the device photoconductivity. The latter contribution proceeds on a significantly longer temporal scale than the electron injection into the NWs, since it is driven by the slow desorption dynamics of the chemical species from the surface of ZnO [34,8,16], and accounts for the slow temporal dynamics of the photocurrent increase observed in the functionalized device (Figure 5 (a)).…”

“…The former determines a quick increase of the density of charge carriers in the ZnO NWs, while the latter induces a reduction of the surface band bending and therefore an enlargement of the cross-sectional area available for the charge transport in the core of the NWs. In particular, the latter process is expected to proceed on a significantly longer temporal scale than the electron injection into the NWs, due to the slow desorption dynamics of the oxygen-related species from the surface of ZnO, ,, and accounts likely for the slow temporal dynamics of the photocurrent increase observed in the functionalized device (Figure a).…”

Second harmonic generation from ZnO films and nanostructures

“…Thus, surface SHG represents an efficient tool to probe the dynamics of absorption (and desorption) of atomic and molecular species on ZnO thin films. As an example, time dependent SHG using a pump beam tuned at 740 nm was used to investigate the interaction between O2 and ZnO surface defect sites [48]. Adsorption of O2 on ZnO thin films, in fact, induce an electric field at the surface of ZnO leading to electric field induced SHG (EFISH).…”

Influence of the spatial extent of the space-charge region in c-Si on the electric-field-induced second-harmonic-generation effect

“…Second-harmonic generation (SHG) is a nonlinear optical process used to study (buried) interfaces of solids, a purpose for which SHG is particularly well suited due to its inherent interface and surface sensitivity [1,2]. With SHG it is possible to probe interface structure [3][4][5], electronic transitions at interfaces [6], local strain at interfaces [7], and adsorbates at surfaces [8,9], among other related properties. The sensitivity of the SHG process toward interface and surface properties stems from its second-order nonlinear nature: SHG is not allowed at a site with inversion symmetry (in the dipole approximation) [10,11].…”