Probing the size dependence on the optical modes of anatase nanoplatelets using STEM-EELS

Abstract

†Electronic supplementary information (ESI) available: Histograms of length and thickness of the platelets; perpendicular component of dielectric function; thickness dependence of perpendicular component of loss function; VEEL spectra of edge-on platelet along c-axis; VEEL spectra of face-on and edge platelets stacks. 2 AbstractAnatase titania nanoplatelets predominantly exposing {001} facets have been reported to have enhanced catalytically properties in comparison to bulk anatase. To understand their unusual behaviour, it is essential to fully characterize their electronic and optical properties at the nanometer scale. One way of accessing these fundamental properties is to study the dielectric function. Valence electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscope (STEM) is the only analytical method that can probe the complex dielectric function with both high energy (< 100 meV) and high spatial (< 1 nm) resolution. By correlating experimental STEM-EELS data with simulations based on semiclassical dielectric theory, the dielectric response of thin (< 5 nm) anatase nanoplatelets was found to be largely dominated by characteristic (optical) surface modes, which are linked to surface plasmon modes of anatase. For platelets less than 10 nm thick, the frequency of these optical modes varies according to their thickness. This unique optical behaviour prompts the enhancement of light absorption in the ultraviolet regime. Finally, the effect of finite size on the dielectric signal is gradually lost by stacking consistently two or more platelets in a specific crystal orientation, and eventually suppressed for large stacks of platelets.

Present Perspectives of Advanced Characterization Techniques in TiO2-Based Photocatalysts

“…96 EELS measurements can provide the evidence for charge transfer from the TiO 2 surface to the O 2 molecule based on disappearance of the vacancy loss feature. 97 Liberti et al combined valence EELS with scanning TEM to obtain the dielectric response of anatase nanoplatelets, 98 and they were able to characterize the complex dielectric properties with superior spatial (<1 nm) and energy (<100 meV) resolution. The authors found that the energies of the symmetric mode and antisymmetric mode shifted toward lower and higher energy-losses, respectively, with decrease in the thickness of the anatase nanoplatelet.…”

Mapping the Origins of Luminescence in ZnO Nanowires by STEM-CL

“…The S signal is strongest at the edges, where the entire primary electron beam trajectory intersects with the surface region; in the center of the NW, the beam only intersects with the upper and lower surfaces. The significant S intensity when the electron beam is aloof (nearby but not intersecting with the nanowire) is likely due to the excitation of surface plasmons, , which decay to excitons and subsequently recombine via surface traps in the ZnO, but may also be caused by stray electrons that directly excite excitons preferentially at the nanowire surface. The peak of S intensity extends into the nanowire due to excitons excited in the nanowire diffusing to the surface, where they subsequently decay.…”