Self-assembled hollow nanosphere arrays used as low Q whispering gallery mode resonators on thin film solar cells for light trapping.

Optical micro/nano-spherical cavities can be designed to confine light by generating resonances in whispering gallery modes (WGM) and then couple them into the substrate through leaky modes, which can be potentially used in thin film solar cells for absorption enhancements. In this work, the transparent ZnO electrodes in a hollow nanosphere (HNS) structure were proposed as WGM resonators to trap sunlight for the absorption enhancement in silicon thin film solar cells. A low cost and high throughput template based technology was developed to fabricate the ZnO HNS arrays on the Si substrates. Significant simulated absorption enhancement has been demonstrated on the ZnO HNS arrays decorated thin film solar cell with an active layer down to 250 nm in thickness. A 9.3% enhancement in the short circuit current density can be achieved theoretically by comparing the ZnO HNS array modified amorphous Si thin film solar cell with an anti-reflection layer configured cell.

Engineered tunneling layer with enhanced impact ionization for detection improvement in graphene/silicon heterojunction photodetectors.

Here, an engineered tunneling layer enhanced photocurrent multiplication through the impact ionization effect was proposed and experimentally demonstrated on the graphene/silicon heterojunction photodetectors. With considering the suitable band structure of the insulation material and their special defect states, an atomic layer deposition (ALD) prepared wide-bandgap insulating (WBI) layer of AlN was introduced into the interface of graphene/silicon heterojunction. The promoted tunneling process from this designed structure demonstrated that can effectively help the impact ionization with photogain not only for the regular minority carriers from silicon, but also for the novel hot carries from graphene. As a result, significantly enhanced photocurrent as well as simultaneously decreased dark current about one order were accomplished in this graphene/insulation/silicon (GIS) heterojunction devices with the optimized AlN thickness of ~15 nm compared to the conventional graphene/silicon (GS) devices. Specifically, at the reverse bias of -10 V, a 3.96-A W-1 responsivity with the photogain of ~5.8 for the peak response under 850-nm light illumination, and a 1.03-A W-1 responsivity with ∼3.5 photogain under the 365 nm ultraviolet (UV) illumination were realized, which are even remarkably higher than those in GIS devices with either Al2O3 or the commonly employed SiO2 insulation layers. This work demonstrates a universal strategy to fabricate broadband, low-cost and high-performance photo-detecting devices towards the graphene-silicon optoelectronic integration.

Multiple coupling in plasmonic metal/dielectric hollow nanocavity arrays for highly sensitive detection.

Recently, the plasmonic coupled optical cavity has gained much attention due to its attractive properties in light manipulation, e.g. high Q optical resonance, local field enhancements and extraordinary transmission. The strongly enhanced local field originated from the plasmonic resonance hybridizing with the optical cavity mode presents great potential for application to chemical and biological sensing. Here, the multiple coupling effect between plasmonic mode and optical cavity mode has been demonstrated in self-assembled metal/dielectric hollow-nanosphere (HNS) arrays and the strongly enhanced local field originated from the inter-coupling of the plasmonic cavities was further employed for highly sensitive recyclable SERS sensing.

Multipole plasmon resonances in self-assembled metal hollow-nanospheres.

Recently, multipole plasmonic mode resonances in metal hollow structures, such as dipole, quadrupole, and octupole modes, have been widely investigated by researchers with the aim for potential applications in bio-sensing, fluorescence, nanolasers or nonlinear nano-photonics. Here, in this work, the multipole plasmon resonances in self-assembled metal hollow-nanospheres (HNSs) are theoretically and experimentally demonstrated and the hot spots originating from the higher order mode plasmonic resonance and interparticle coupling effect are proposed to be used for Raman scattering enhancements. Dipole, quadrupole, octupole and hexadecapole mode plasmonic resonances were clearly resolved in the extinction spectra of these Ag HNS arrays showing good agreement with the theoretical simulation results. Strong regular hot spots were obtained around the surface and in the gaps of the Ag HNSs through the higher order mode plasmonic resonances and corresponding interparticle coupling effect between the HNSs. Maximum local field intensity was accomplished by optimizing the size of as well as the coupling distance between the HNSs and then it was applied to SERS sensing. Raman mapping also demonstrated these self-assembled plasmonic cavity arrays to be a stable and uniform SERS-active substrate.

Effect of the surface-plasmon-exciton coupling and charge transfer process on the photoluminescence of metal-semiconductor nanostructures.

The effect of direct metal coating on the photoluminescence (PL) properties of ZnO nanorods (NRs) has been investigated in detail in this work. The direct coating of Ag nanoparticles (NPs) induces remarkable enhancement of the surface exciton (SX) emissions from the ZnO NRs. Meanwhile, the charge transfer process between ZnO and Ag also leads to notable increment of blue and violet emissions from Zn interstitial defects. A thin SiO2 blocking layer inserted between the ZnO and Ag has been demonstrated to be able to efficiently suppress the defect emission enhancement caused by the direct contact of metal-semiconductor, without weakening the surface-plasmon-exciton coupling effect. A theoretical model considering the type of contacts formed between metals, ZnO and blocking layer is proposed to interpret the change of the PL spectra.

Band edge emission enhancement by quadrupole surface plasmon-exciton coupling using direct-contact Ag/ZnO nanospheres.

Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices.