Optical Second Harmonic Generation from ZnO Nanofluids—A Tight Binding Approach in Determining Bulk χ⁽²⁾

Abstract

ZnO nanomaterials exhibit attractive optical properties that are important in the realm of catalysis and nanotechnology involving the developments of solar cells, chemisensors and other optoelectronic devices. Herein, we provide experimental evidence of resonantly enhanced optical second harmonic generation (SHG) from a ZnO nanofluid. Moreover, we develop a simple theoretical model, based on tight binding theory, to calculate the bulk static second order susceptibility components, χ ijk(2) , of ZnO nanoparticles (NPs). We show that a tight binding approach, which is sensitive to local structure, can be used to determine χ ijk(2) of polar semiconducting crystals. The ratio, χ zzz (2) /χ zyy (2) = 3.07, obtained based on this approach is in excellent agreement with results for bulk ZnO crystals previously reported. However, our finding shows that there is a discrepancy between the theoretical prediction and experimental observation for the ZnO nanofluid. By modifying the ZnO nanoparticle surface via adsorption of an organic dye we further demonstrate that this discrepancy is due to a surface contribution to the overall SHG signal. Lastly, while the applicability of the tight binding method to calculate bulk χ ijk(2) for ZnO is demonstrated here, the potential advantage of this method to characterize surface second order susceptibility is also discussed. ■ INTRODUCTIONMetal oxide nanoparticles (NPs) 1,2 have become popular due to their wide range of applications in optoelectronics, 3−6 catalysis, 7,8 and environmental remediation. 9,10 Nanoparticles, in general, are of fundamental scientific interest because they represent an intermediate state of matter bridging the electronic and optical properties between those of the bulk materials, and those of molecules and small clusters. 11,12 Nanoparticles exhibit a large surface area to volume ratio, and therefore, surface chemistry plays a key role in various applications of NPs. Most common applications of metal oxide NPs are in the realm of nanotechnology and pollution remediation. At the heart of nanotechnology is the development of dye sensitized solar cells (DSSCs) 3−5 and sensors 13−15 using semiconducting NPs as the building blocks. An important example of metal oxides for this purpose is ZnO NPs. Zinc oxide exhibits numerous optoelectronic properties that are attractive for device performances. 2 These include a direct and wide band gap (3.37 eV at room temperature), large exciton binding energy (∼60 meV), large piezoelectric constants, and of particular interest, which provides the basis of this scientific exploration, a large nonlinear optical (NLO) response.While an extensive amount of research with respect to synthesis and design 9,16,17 of ZnO nanomaterials are on the rise, demand 1,2 for obtaining fundamental knowledge related to the optoelectronic properties of nanomaterials is at its apex. An interesting and useful aspect of ZnO NPs is that they are noncentrosymmetric and thus, exhibit strong NLO properties. This entails frequency doubling; i.e., a second ...

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