J. L. Shen

Synthesis, Characterization, and Bioconjugation of Fluorescent Gold Nanoclusters toward Biological Labeling Applications

Synthesis of ultrasmall water-soluble fluorescent gold nanoclusters is reported. The clusters have a decent quantum yield, high colloidal stability, and can be readily conjugated with biological molecules. Specific staining of cells and nonspecific uptake by living cells is demonstrated.

Fluorescent Gold Nanoclusters as a Biocompatible Marker for In Vitro and In Vivo Tracking of Endothelial Cells

We have been investigating the fluorescent property and biocompatibility of novel fluorescent gold nanoclusters (FANC) in human aortic endothelial cells (HAEC) and endothelial progenitor cells (EPC). FANC (50-1000 nmol/L) was delivered into cells via the liposome complex. The fluorescence lasted for at least 28 days with a half-life of 9 days in vitro. Examination of 12 transcripts regulating the essential function of endothelial cells after a 72 h delivery showed that only the vascular cell adhesion molecule 1 and the vascular endothelial cadherin were down-regulated at high concentration (500 nmol/L). In addition, no activation of caspase 3 or proliferating cell nuclear antigens was detected. 3-[4,5-Dimethylthiazol-2-yl]-2,5- diphenyltetrazolium bromide (MTT) assay demonstrated that, unlike the markedly suppressed viability in cells treated with quantum dots, FANC had minimal effect on the viability, unless above 500 nmol/L, at which level a minor reduction of viability mainly caused by liposome was found. Tube formation assay showed no impaired angiogenesis in the EPC treated with FANC. In vivo study using hindlimb ischemic mice with an intramuscular injection of FANC-labeled human EPC showed that the cells preserved an angiogenic potential and exhibited traceable signals after 21 days. These findings demonstrated that FANC is a promising biocompatible fluorescent probe.

Improvement of conversion efficiency for multi-junction solar cells by incorporation of Au nanoclusters.

We studied the photoluminescence (PL) and photovoltaic current-voltage characteristics of the three-junction InGaP/InGaAs/Ge solar cells by depositing Au nanoclusters on the cell surface. The increases of the PL intensity and short-circuit current after incorporation of Au nanoclusters are evident. An increase of 15.3% in energy conversion efficiency (from 19.6 to 22.6%) is obtained for the three-junction solar cells in which Au nanoclusters have been incorporated. We suggest that the increased light trapping due to radiative scattering from Au nanoclusters is responsible for improving the performance of the three-junction solar cells.

Surface-micromachined micro-XYZ stages for free-space microoptical bench

Micro-XYZ stages have been monolithically integrated with microactuators and out-of-plane microoptical elements, all fabricated by the same surface-micromachined process, on Si free-space microoptical bench. Optical beam adjustment with three degrees of freedom has been realized without angular squinting. A positioning accuracy of 11 nm has been achieved by the integrated scratch drive actuators, with the travel distance larger than 30 /spl mu/m in each direction.

Concentration dependence of carrier localization in InN epilayers

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

Red-light emission in MCM-41 meso-porous nanotubes

Photoluminescence has been used to study the red-light emission from siliceous MCM-41 with rapid thermal annealing (RTA). A broad emission ranging from 580 to 670 nm was observed and interpreted as non-bridging oxygen hole center (NBOHC). Based on the evolution of NBOHC after RTA, the hydroxyl group and peroxy linkage were demonstrated to be the possible precursors of NBOHC in siliceous MCM-41. We also found the photoluminescence intensity of NBOHC in MCM-41 degrade with time during photoexcitation. The degradation rate of NBOHC increases with increasing RTA temperature and excitation power. The degradation mechanism of photoluminescence may be associated with the recombination of the optically induced hydrogen species and NBOHC.

Single fluorescent gold nanoclusters.

Both ensemble and single-molecule measurements were performed to explore the fluorescence properties of Au nanoclusters (NCs). Photoinduced fluorescence enhancement was observed for ensemble NCs in solution, but photobleaching was found at ambient environments. At the single-molecule level, fluorescence blinking and single-step photobleaching were observed. Furthermore, their time-resolved fluorescence shows a single exponential decay with a lifetime of approximately 7 ns and is insensitive to changes in fluorescence intensity. The lifetime distribution is more homogeneous within ensemble Au NCs as compared to CdSe QDs. Therefore, Au NCs have potential applications as nontoxic fluorescent labels for lifetime-based imaging microscopy. However, their low quantum yields and poor photostability are disadvantageous factors, which require further improvement.

A Facile and Low-Cost Method to Enhance the Internal Quantum Yield and External Light-Extraction Efficiency for Flexible Light-Emitting Carbon-Dot Films

Solution-processed, non-toxic carbon dots (CDs) have attracted much attention due to their unique photoluminescence (PL) properties. They are promising emissive layers for flexible light-emitting devices. To this end, the CDs in pristine aqueous solutions need to be transferred to form solid-state thin films without sacrificing their original PL characteristics. Unfortunately, solid-state PL quenching induced by extra non-radiative (NR) energy transfer among CDs would significantly hinder their practical applications in optoelectronics. Here, a facile, low-cost and effective method has been utilized to fabricate high-performance CD/polymer light-emitting flexible films with submicron-structured patterns. The patterned polymers can serve as a solid matrix to disperse and passivate CDs, thus achieving high internal quantum yields of 61%. In addition, they can act as an out-coupler to mitigate the waveguide-mode losses, approximately doubling the external light-extraction efficiency. Such CD/polymer composites also exhibit good photo-stability, and thus can be used as eco-friendly, low-cost phosphors for solid-state lighting.

Temperature dependence of time-resolved photoluminescence spectroscopy in InAs/GaAs quantum ring

We present detailed experimental results of the temperature dependence of continuous wave and time-resolved photoluminescence (PL) spectroscopy in self-assembled InAs/GaAs quantum dot and quantum ring nanostructures. A dramatic increase in PL decay time of the excited and ground states is observed in InAs quantum rings at high temperature. We speculate that the longer PL lifetime in quantum rings is due to the interplay among the dark states, ground states, and the reduced wave function overlapping between electrons and holes. A rate equation model is proposed to interpret the observed temperature dependence of the ground state exciton lifetime.

Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition

a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL.