Gongxun Bai

Deep Ultraviolet to Near-Infrared Emission and Photoresponse in Layered N-Doped Graphene Quantum Dots

Material that can emit broad spectral wavelengths covering deep ultraviolet, visible, and near-infrared is highly desirable. It can lead to important applications such as broadband modulators, photodetectors, solar cells, bioimaging, and fiber communications. However, there is currently no material that meets such desirable requirement. Here, we report the layered structure of nitrogen-doped graphene quantum dots (N-GQDs) which possess broadband emission ranging from 300 to >1000 nm. The broadband emission is attributed to the layered structure of the N-GQDs that contains a large conjugated system and provides extensive delocalized π electrons. In addition, a broadband photodetector with responsivity as high as 325 V/W is demonstrated by coating N-GQDs onto interdigital gold electrodes. The unusual negative photocurrent is observed which is attributed to the trapping sites induced by the self-passivated surface states in the N-GQDs.

Thermal Assisted Oxygen Annealing for High Efficiency Planar CH3NH3PbI3 Perovskite Solar Cells

We report investigations on the influences of post-deposition treatments on the performance of solution-processed methylammonium lead triiodide (CH3NH3PbI3)-based planar solar cells. The prepared films were stored in pure N2 at room temperature or annealed in pure O2 at room temperature, 45°C, 65°C and 85°C for 12 hours prior to the deposition of the metal electrodes. It is found that annealing in O2 leads to substantial increase in the power conversion efficiencies (PCEs) of the devices. Furthermore, strong dependence on the annealing temperature for the PCEs of the devices suggests that a thermally activated process may underlie the observed phenomenon. It is believed that the annealing process may facilitate the diffusion of O2 into the spiro-MeOTAD for inducing p-doping of the hole transport material. Furthermore, the process can result in lowering the localized state density at the grain boundaries as well as the bulk of perovskite. Utilizing thermal assisted O2 annealing, high efficiency devices with good reproducibility were attained. A PCE of 15.4% with an open circuit voltage (VOC) 1.04 V, short circuit current density (JSC) 23 mA/cm2, and fill factor 0.64 had been achieved for our champion device.

Magnetic-Assisted Noncontact Triboelectric Nanogenerator Converting Mechanical Energy into Electricity and Light Emissions.

A magnetic-assisted noncontact triboelectric nanogenerator (TENG) is developed by combining a magnetic responsive layer with a TENG. The novel TENG device is applied to harvest mechanical energy which can be converted into electricity and light emissions. This work has potential for energy harvesting, magnetic sensors, self-powered electronics and optoelectronics applications.

2D Layered Materials of Rare‐Earth Er‐Doped MoS2 with NIR‐to‐NIR Down‐ and Up‐Conversion Photoluminescence

A 2D system of Er-doped MoS2 layered nanosheets is developed. Structural studies indicate that the Er atoms can be substitutionally introduced into MoS2 to form stable doping. Density functional theory calculation implies that the system remains stable. Both NIR-to-NIR up-conversion and down-conversion light-emissions are observed in 2D transition metal dichalcogenides, ascribed to the energy transition from Er(3+) dopants.

Mapping live cell viscosity with an aggregation-induced emission fluorogen by means of two-photon fluorescence lifetime imaging

Intracellular viscosity is a crucial parameter that indicates the functioning of cells. In this work, we demonstrate the utility of TPE-Cy, a cell-permeable dye with aggregation-induced emission (AIE) property, in mapping the viscosity inside live cells. Owing to the AIE characteristics, both the fluorescence intensity and lifetime of this dye are increased along with an increase in viscosity. Fluorescence lifetime imaging of live cells stained with TPE-Cy reveals that the lifetime in lipid droplets is much shorter than that from the general cytoplasmic region. The loose packing of the lipids in a lipid droplet results in low viscosity and thus shorter lifetime of TPE-Cy in this region. It demonstrates that the AIE dye could provide good resolution in intracellular viscosity sensing. This is also the first work in which AIE molecules are applied in fluorescence lifetime imaging and intracellular viscosity sensing.

Efficiency enhancement by defect engineering in perovskite photovoltaic cells prepared using evaporated PbI2/CH3NH3I multilayers

We report, for the first time, on the synthesis of perovskite films by thermal annealing of evaporated lead(II) iodide (PbI2)/methylammonium iodide (CH3NH3I) multilayers. Detailed characterization of the resulting films is presented. Our work demonstrates that compact, high quality and uniform perovskite films can be grown using this technique. Optimization of the device structure was achieved by careful design of the layer thickness and the number of PbI2/CH3NH3I pairs used in the formation of the absorber layer. Utilizing additional annealing steps in a controlled atmosphere was shown to result in significant improvement in the device performance. Our experimental data indicate that O2 treatments may result in substantial reduction in the trap density of the device and thereby significant improvement in the lifetimes of the carriers. A high power conversion efficiency (PCE) of 12.5% was recorded for the champion device.

A graphene oxide based fluorescence resonance energy transfer (FRET) biosensor for ultrasensitive detection of botulinum neurotoxin A (BoNT/A) enzymatic activity

Botulinum neurotoxins (BoNTs) are among the most potent toxic bacterial proteins for humans, which make them potential agents for bioterrorism. Therefore, an ultrasensitive detection of BoNTs and their active states is in great need as field-deployable systems for anti-terrorism applications. We report the construction of a novel graphene oxide (GO)-peptide based fluorescence resonance energy transfer (FRET) biosensor for ultrasensitive detection of the BoNT serotype A light chain (BoNT-LcA) protease activity. A green fluorescence protein (GFP) modified SNAP-25 peptide substrate (SNAP-25-GFP) was optimally designed and synthesized with the centralized recognition/cleavage sites. This FRET platform was constructed by covalent immobilization of peptide substrate on GO with BSA passivation which have advantages of low non-specific adsorption and high stability in protein abundant solution. BoNT-LcA can specifically cleave SNAP-25-GFP substrate covalently immobilized on GO to release the fragment with GFP. Based on fluorescence signal recovery measurement, the target BoNT-LcA was detected sensitively and selectively with the linear detection range from 1fg/mL to 1pg/mL. The limit of detection (LOD) for BoNT-LcA is around 1fg/mL.

Effects of dopant concentration on structural and near-infrared luminescence of Nd3+-doped beta-Ga2O3 thin films

We have investigated structural and near-infrared (NIR) luminescence of Nd3+-doped β-Ga2O3 thin films (Nd:Ga2O3) with different Nd3+ doping concentrations. With an increase of Nd3+ content, the crystal lattice of the films expands, while the energy band gap shrinks. Moreover, NIR luminescence is investigated as a function of Nd3+ doping concentration. The measured results are related to the structural change and energy transfer of cross relaxation process ascribed to 4F3/2—4I9/2, 4F3/2—4I11/2, and 4F3/2—4I13/2 of the phosphor films. This work implies that the enhanced NIR luminescence and blue-shift observation are associated with the lattice distortion and the variation in the crystal field of Nd: Ga2O3.

Tuning of near-infrared luminescence of SrTiO3:Ni2+ thin films grown on piezoelectric PMN-PT via strain engineering

We report the tunable near-infrared luminescence of Ni2+ doped SrTiO3 (STO:Ni) thin film grown on piezoelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrate via strain engineering differing from conventional chemical approach. Through controlling the thickness of STO:Ni film, the luminescent properties of the films including emission wavelength and bandwidth, as well as lifetime can be effectively tuned. The observed phenomena can be explained by the variation in the crystal field around Ni2+ ions caused by strain due to the lattice mismatch. Moreover, the modulation of strain can be controlled under an external electric field via converse piezoelectric effect of PMN-PT used in this work. Consequently, controllable emission of the STO:Ni thin film is demonstrated in a reversible and real-time way, arising from the biaxial strain produced by piezoelectric PMN-PT. Physical mechanism behind the observation is discussed. This work will open a door for not only investigating the luminescent properties of the phosphors via piezoelectric platform, but also potentially developing novel planar light sources.

Hierarchical donut-shaped LiMn2O4 as an advanced cathode material for lithium-ion batteries with excellent rate capability and long cycle life

Porous micrometer-sized architecture that consists of aggregated single-crystalline nanoparticles is critical for LiMn2O4 to achieve good rate capacity and cycling stability, since it can increase the contact area between the electrolyte/electrode and shorten the transport paths for electrons and lithium ions. In this paper, hierarchical porous donut-shaped LiMn2O4 comprising aggregated single-crystalline nanoparticles has been successfully fabricated with MnO2 nanosheet coated polystyrene spheres as a precursor and characterized in terms of structure and performance as the cathode for lithium ion batteries. The charge/discharge tests show that the as-obtained donut-shaped LiMn2O4 exhibits excellent rate capability and high-rate cyclic stability. Surprisingly, even at a high charge/discharge rate of 10 C, the battery yields a capacity retention of over 95% after 500 cycles. The superior performance of the synthesized product can be attributed to its intrinsic structure: porous donut-shaped LiMn2O4 consisting of well-connected single-crystalline nanoparticles. The interpenetrating nanoparticle reduces the path of Li ion diffusion and increases the number of reaction sites for lithium insertion/extraction; the pores provide void space to buffer the volume changes during high-rate charge/discharge.