Plasmonic nanohole array with strong mode coupling for hot carrier generation

Abstract

Surface plasmon resonance (SPR) effect has shown drastic enhancement the interaction of light with noble metal nanostructures and has been widely used for photosensing and solar energy conversion. However, most of the reported work utilizes discontinuous nanostructures like nanoparticles and nanoparticle arrays. This research turns to study the complementary counterpart - nanohole arrays. The continuous nature of the structure would enable surface plasmon plariton (SPP) and direct external connection of hot carriers, which are beneficial to photocurrent generation but not present in nanoparticle arrays. The originality contribution of this research lie in two aspects. One is that this is the first systematic study and comparison of the optical properties of a pair of complementary structures - AuNPA and AuNHA. The other major contribution of this work is that this is the first contribution of AuNHA into the MDM structure for drastically enhanced hot carrier generation, obtaining the photocurrent density of 170 μA·cm-2 under > 490 nm visible light and an enhancement factor of 680 as compared to the bare TiO₂ film. In one major part of this thesis, a pair of complementary structures made of gold nanohole array (AuNHA) and gold nanoparticle array (AuNPA) are analyzed, simulated and experimentally studied in terms of the mechanisms and the plasmonic properties. In experiments, the AuNHA and the AuNPA are individually fabricated by microsphere lithography on a TiO₂ layer and their hot carrier generation properties are studied using an electrochemical system. The results show that AuNHA generally performs superior to AuNPA in terms of photocatalytic performance upon visible light illumination (>420 nm). This study presents to be the first direct comparison of AuNHA and AuNPA. In the other part of this research, the AuNHA is incorporated into a metal-dielectric-metal (MDM) architecture consisting of an ultrathin AuNHA film, a TiO₂ layer and a gold reflector (from top to bottom) that enhance the overall absorption and the photocurrent. Both the FDTD and the experimental results have demonstrated the MDM structure has multiple absorption bands as compared to the single band of the above structures of AuNHA on TiO₂. By optimizing the thickness of the TiO₂ layer, strong coupling of modes associated with the absorption bands is obtained and shows superior performance in both IPCE and broadband photoresponse measurements as compared to the other structures. More specifically, the AuNHA in the MDM abosrber obtains the photocurrent density of 170 μA·cm-2, which is 680 times of that of the bare TiO₂ film. Such a large enhancement factor well demonstrates the benefit of AuNHA for plasmonic photocurrent. The superior performance is attributed to the strong field confinement in the TiO₂ layer that results from the hybridization of the cavity mode, the SPR mode of the AuNHA, and the gap surface plasmon polariton (GSPP) mode, as experimentally suggested by the enhanced Raman spectrum. In summary, this work proposes the AuNHA structures and studies the mechanisms of hot carrier generation under visible light. Simulation and experiments both show that the AuNHA structure has superior performance as compared to the complementary nanoparticles, which would enable their broad applications in plasmonic sensing, photocatalysis and solar energy conversion.