Haifei Lu

Selective Growth and Integration of Silver Nanoparticles on Silver Nanowires at Room Conditions for Transparent Nano-Network Electrode

Recently, metal nanowires have received great research interests due to their potential as next-generation flexible transparent electrodes. While great efforts have been devoted to develop enabling nanowire electrodes, reduced contact resistance of the metal nanowires and improved electrical stability under continuous bias operation are key issues for practical applications. Here, we propose and demonstrate an approach through a low-cost, robust, room temperature and room atmosphere process to fabricate a conductive silver nano-network comprising silver nanowires and silver nanoparticles. To be more specific, silver nanoparticles are selectively grown and chemically integrated in situ at the junction where silver nanowires meet. The site-selective growth of silver nanoparticles is achieved by a plasmon-induced chemical reaction using a simple light source at very low optical power density. Compared to silver nanowire electrodes without chemical treatment, we observe tremendous conductivity improvement in our silver nano-networks, while the loss in optical transmission is negligible. Furthermore, the silver nano-networks exhibit superior electrical stability under continuous bias operation compared to silver nanowire electrodes formed by thermal annealing. Interestingly, our silver nano-network is readily peeled off in water, which can be easily transferred to other substrates and devices for versatile applications. We demonstrate the feasibly transferrable silver conductive nano-network as the top electrode in organic solar cells. Consequently, the transparent and conductive silver nano-networks formed by our approach would be an excellent candidate for various applications in optoelectronics and electronics.

Plasmonic optical trap having very large active volume realized with nano-ring structure.

The feasibility of using gold nano-rings as plasmonic nano-optical tweezers is investigated. We found that at a resonant wavelength of λ=785 nm, the nano-ring produces a maximum trapping potential of ~32k(B)T on gold nanoparticles. The existence of multiple potential wells results in a very large active volume of ~10(6) nm(3) for trapping the target particles. The report nano-ring design provides an effective approach for manipulating nano-objects in very low concentration into the high-field region and is well suited for integration with microfluidics for lab-on-a-chip applications.

Smooth CH3NH3PbI3 from controlled solid–gas reaction for photovoltaic applications

The merits of high power conversion efficiency (PCE) and easy preparation make organic–inorganic perovskite solar cells one of the most promising solar devices. However, PCE is greatly dependent on the morphology of perovskite thin film. Here, we report a solid–gas reaction method to fabricate very smooth CH3NH3PbI3 thin film with high coverage. Through controlling the reaction rate between CH3NH3I and PbI2 by tuning the PbI2 substrate temperature and the evaporation rate of CH3NH3I, we obtain a CH3NH3PbI3 layer with roughness of 7.37 nm. Besides, no post-treatment annealing is needed after film formation using our approach. With about 250 nm perovskite active layer, the solar cells exhibit a PCE of 10.0% with little hysteresis.

Plasmonic graded nano-disks as nano-optical conveyor belt

We propose a plasmonic system consisting of nano-disks (NDs) with graded diameters for the realization of nano-optical conveyor belt. The system contains a couple of NDs with individual elements coded with different resonant wavelengths. By sequentially switching the wavelength and polarization of the excitation source, optically trapped target nano-particle can be transferred from one ND to another. The feasibility of such function is verified based on the three-dimensional finite-difference time-domain technique and the Maxwell stress tensor method. Our design may provide an alternative way to construct nano-optical conveyor belt with which target molecules can be delivered between trapping sites, thus enabling many on-chip optofluidic applications.

Detection of Panton-Valentine Leukocidin DNA from methicillin-resistant Staphylococcus aureus by resistive pulse sensing and loop-mediated isothermal amplification with gold nanoparticles

This report describes a novel diagnostic assay for rapid detection of the Panton-Valentine Leukocidin (PVL) toxin of methicillin-resistant Staphylococcus aureus (MRSA) utilizing resistive pulse sensing (RPS), loop-mediated isothermal DNA amplification (LAMP) in combination with gold nanoparticles (AuNPs). The PVL DNA from MRSA was specifically amplified by LAMP using four primers at one temperature (65 °C). The DNA products with biotin were then conjugated to a first AuNP1 (55±2 nm) through biotin-avidin binding. A second AuNP2 (30±1.5 nm) coated with a specific DNA probe hybridized with the LAMP DNA products at the loop region to enhance assay sensitivity and specificity, to generate supra-AuNP1-DNA-AuNP2 assemblies. Scanning electron microscopy confirmed the presence of these supra-assemblies. Using RPS, detection and quantitation of the agglomerated AuNPs were performed by a tunable fluidic nanopore sensor. The results demonstrate that the LAMP-based RPS sensor is sensitive and rapid for detecting the PVL DNA. This technique could achieve a limit of detection (LOD) up to about 500 copies of genomic DNA from the bacteria MRSA MW2 and the detection can be completed within two hours with a straightforward signal-to-readout setup. It is anticipated that this LAMP-based AuNP RPS may become an effective tool for MRSA detection and a potential platform in clinical laboratory to report the presence or absence of other types of infectious agents.

Synthesis of size-controlled silver nanodecahedrons and their application for core–shell surface enhanced Raman scattering (SERS) tags

We report the synthesis of silver nanodecahedrons (Ag NDs) and their use as surface enhanced Raman scattering (SERS) nano-composites. The as-prepared Ag NDs possess strong localized surface plasmon resonance (LSPR) with widely tunable peaks between 420–660 nm, which was formerly not possible, thus greatly improving the prospect of using silver nanoparticles for SERS applications. The growth of large size Ag NDs (LSPR peak longer than 490 nm) results from a seed-mediated step involving the reduction of silver cations by photo-excitation (illumination wavelength at 500 nm). Ag ND-SERS composites formed from a layer-by-layer coating technique show strong Raman signal enhancement and good material stability because of passivation effects from the coating. The reported silica-coated Ag NDs may be used with other molecular species to take advantage of field enhancement for a variety of applications, including non-linear harmonics generation and fluorescence enhancement.

Diffraction resonance with strong optical-field enhancement from gain-assisted hybrid plasmonic structure

We propose and analyze the diffraction coupling of localized plasmon resonances (LPRs) through gain-assisted propagation surface plasmons (PSPs). The coupling process involves localization of incident light by LPR and LPR-PSP interaction. We demonstrate a significantly strong enhancement of electromagnetic power for LPRs in the event of diffraction resonance through incorporation of experimentally feasible optical gain to the PSP. Based on such phenomenon, we propose a hybrid plasmonic structure, which would potentially give rise to device realization of the nano-lasers. In addition, it is also a promising platform for applications such as surface enhanced Raman scattering, nonlinear optics, plasmonic trapping, etc.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Experimental and Theoretical Investigation of Macro-Periodic and Micro-Random Nanostructures with Simultaneously Spatial Translational Symmetry and Long-Range Order Breaking

Photonic and plasmonic quasicrystals, comprising well-designed and regularly-arranged patterns but lacking spatial translational symmetry, show sharp diffraction patterns resulting from their long-range order in spatial domain. Here we demonstrate that plasmonic structure, which is macroscopically arranged with spatial periodicity and microscopically constructed by random metal nanostructures, can also exhibit the diffraction effect experimentally, despite both of the translational symmetry and long-range order are broken in spatial domain simultaneously. With strategically pre-formed metal nano-seeds, the tunable macroscopically periodic (macro-periodic) pattern composed from microscopically random (micro-random) nanoplate-based silver structures are fabricated chemically through photon driven growth using simple light source with low photon energy and low optical power density. The geometry of the micro-structure can be further modified through simple thermal annealing. While the random metal nanostructures suppress high-order Floquet spectra of the spatial distribution of refractive indices, the maintained low-order Floquet spectra after the ensemble averaging are responsible for the observed diffraction effect. A theoretical approach has also been established to describe and understand the macro-periodic and micro-random structures with different micro-geometries. The easy fabrication and comprehensive understanding of this metal structure will be beneficial for its application in plasmonics, photonics and optoelectronics.