Surface Effect On The Nonlinear Optical Properties Of Transition Metal-Oxode Microcrystallites

The size dependent modifications of the optical and electronic properties of microcrystallites have attracted considerable attention recently[1-4]. As the diameter of the microcrystallite approaches its corresponding exciton Bohr diameter, its electronic and optical properties start to change because of the quantum confinement effect, dielectric effect and the effect of the surface[5]. For microcrystallites in such a small size regime, a large percentage of the atomes is on or near the surfaces. The existence of this vast interface between the microcrystallite and the surrounding medium can have a profound effect on the nonlinear optical properties of the microcrystallites. For the first time, we studied the nonlinear optical properties of translation metal-oxide microcrystallites by coating the surface with a layer of organic polar molecule(DBS etc.), and found that the change of the surface environment could alter the optical properties greatly. For Fe2O3 as example, (1) the absorption incresed toward the high energy side, (2) the laser induced luminescence intensity decreased by 2 orders in magnitude, and on the contrary, the Raman signal of the surface was enhanced greatly, (3) the saturable absorption phenomenon disappeared, (4) larger third order susceptibility and faster excited state relaxation were obtained compared with uncoated Fe2O3 microcrystallite. These phenomena are the results of the change of the electronic structure caused by the quantum confinement effect and the effect of the surface, unlike semiconductor microcrystallites in which the delocalized Wannier excitons can be influenced greatly by the quantum confinement effect (such as PbS microcrystallite). Transition metal oxide microcrystallite has more complicated electronic structure in which localized d electrons influence its electronic and optical properties greatly[6], and the small diameter Frenkel exciton in such material was effected little by the quantum confinement effect, therefore, the exciton structure could not be abserved in the absorption spectrum. But the size of the transition metal oxide microcrystallites influence their electronic structure strongly. For Fe2O3 as example, the energy structure can be quantitatively shown as the Figure (at the end of the paper), in which a is d-d transition, b represents charge transfer, c is orbital promotion and d is interband transitions. As the size of the microcrystallite decreases, the 3d and 4sp state couples increasingly, and the 3d-4sp (orbital promotion) state contribution increases correspondingly. To some extend, the d electrons and the Frenkel exciton will be delocalized, and the excited electron-hole pair can be ionized and scattered to the surface rapidly. In particular, when the surface was coated with a layer of organic polar molecule, the 3d-4sp state interaction was enhanced greatly under the strong polar interaction of the surface, and some 3d-4sp hydride state will exist, thus the d electrons and the Frenkel exciton will became more delocalization, and the laser induced electron-hole pairs interect and scatter to the surface very fast, so the surface delocalization state generate, accumulate and relax very rapidly and the electron-electron coherence effect[7] is enhanced greatly. Such changes not only increased the nonlinear response, but also resulted in shorter lifetime and stronger nonraditive process.