Photon Reabsorption and Nonradiative Energy-Transfer-Induced Quenching of Blue Photoluminescence from Aggregated Graphene Quantum Dots

A deep understanding of the photoluminescence (PL) from aggregated graphene quantum dots (GQDs) is very important for their practical applications. Here the PL spectra from GQDs solutions at different concentrations are studied. We find that the intensity of the green emission (ca. 530–560 nm) linearly relies on the concentration of GQDs, whereas the blue PL (ca. 425 nm) intensity is below the linear relationship, indicating a concentration-induced partial quenching of blue PL. Confocal fluorescence images explicitly demonstrate the aggregation of GQDs at high concentration. The concentration-induced PL quenching is successfully interpreted by a model of photon reabsorption and nonradiative energy transfer, indicating that, at the aggregated states, the excited electrons of GQDs may nonradiatively relax to ground states through couplings with neighboring ones. Simulated fluorescence decay results show that the energy transfer between neighboring GQDs results in a prolonged dwell time of electron on high-energy state and thus increases the decay time of 425 nm emission, while 550 nm emission remains unaffected, which is consistent with the experimental results. This work will contribute to a deep understanding on PL of GQDs and is also of huge importance to extend GQDs’ applications

Photothermal Contribution to Enhanced Photocatalytic Performance of Graphene-Based Nanocomposites

Photocatalysts possessing high efficiency in degrading aquatic organic pollutants are highly desirable. Although graphene-based nanocomposites exhibit excellent photocatalytic properties, the role of graphene has been largely underestimated. Herein, the photothermal effect of graphene-based nanocomposites is demonstrated to play an important role in the enhanced photocatalytic performance, which has not been considered previously. In our study on degradation of organic pollutants (methylene blue), the contribution of the photothermal effect caused by a nanocomposite consisting of P25 and reduced graphene oxide can be as high as ∼38% in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal characteristic of graphene-based nanocomposite is vital to photocatalysis. It implies that designing graphene-based nanocomposites with the improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity

Quantifying Mie Scattering in Luminescent Solar Concentrators for Improved Performance

Luminescent solar concentrators (LSCs) are waveguiding devices that collect solar light to supplement photovoltaic devices. Scattering in waveguides is critical to the LSC performance, but it is challenging to quantify its contribution, severely hindering the development of LSCs. In this work, we developed an analytical approach to quantify the Mie scattering coefficient (αs) and anisotropy factor (g) in LSCs. By using the αs and g, we conduct theoretical calculation and Monte Carlo ray-tracing simulation to estimate the optical properties and optical efficiency of LSCs based on Gd1.5Y1.5Al5O12:Ce3+ phosphors. In all cases, both the calculated and simulated results agree well with the experimental observations, evidencing the feasibility of this approach. According to our model, scattering in the waveguide improves LSC performance by rerouting incident photons propagation to the edges but degrades LSC performance by redirecting radiated photons into the escape cone. We demonstrate that the enhancement of scattering increases both the scattering gain and loss as well as improves the absorption of LSCs. Finally, an external quantum efficiency (ηext) of 4.2% for a 20 × 20 cm2 LSC is achieved by optimizing the scattering. This work provides quantitative guidelines for estimating the scattering in LSCs, which will guide future research on LSC designs

Emission from Trions in Carbon Quantum Dots

The photoluminescence (PL) spectra acquired from 1 to 6 nm large carbon quantum dots (CQDs) prepared by refluxing activated carbon in HNO₃ show blue emission independent of the excitation wavelength as well as long-wavelength emission depending on the excitation wavelength. The dependence of the two emissions on pH is investigated, and the experimental results show that the peak position of the long-wavelength emission does not change with pH; however, the blue emission becomes more asymmetrical, and obvious shoulder peaks emerge as the pH increases. A model based on defect-bound trions in the CQDs is proposed to explain the shoulder peaks in the blue emission at high pH, and the calculated results agree well with experimental data concerning the integral intensity ratio of the trion to exciton emissions versus pH. Our experimental and theoretical results demonstrate for the first time emission from trions in CQDs

Graphene Metamaterial 3D Conformal Coating for Enhanced Light Harvesting

Silicon (Si) photovoltaic devices present possible avenues for overcoming global energy and environmental challenges. The high reflection and surface recombination losses caused by the Si interface and its nanofabrication process are the main hurdles for pursuing a high energy conversion efficiency. However, recent advances have demonstrated great success in improving device performance via proper Si interface modification with the optical and electrical features of two-dimensional (2D) materials. Firmly integrating large-area 2D materials with 3D Si nanostructures with no gap in between, which is essential for optimizing device performance, has rarely been achieved by any technique due to the complex 3D morphology of the nanostructures. Here we propose the concept of a 3D conformal coating of graphene metamaterials, in which the 2D graphene layers perfectly adapt to the 3D Si curvatures, leading to a universal 20% optical reflection decrease and a 60% surface passivation improvement. In a further application of this metamaterial 3D conformal coating methodology to standard Si solar cells, an overall 23% enhancement of the solar energy conversion efficiency is achieved. The 3D conformal coating strategy could be readily extended to various optoelectronic and semiconductor device systems with peculiar performance, offering a pathway for highly efficient energy-harvesting and storage solutions

Supplementary document for Modulating afterglow time of Mn2+ doped double perovskites by size tuning and its applications in dynamic information display - 6306062.pdf

PDF document with expanded result

Supplementary document for Modulating afterglow time of Mn2+ doped double perovskites by size tuning and its applications in dynamic information display - 6288358.pdf

PDF document with expanded descriptions and result

Ultrasensitive Detection of Matrix Metalloproteinase 2 Activity Using a Ratiometric Surface-Enhanced Raman Scattering Nanosensor with a Core–Satellite Structure

Matrix metalloproteinase 2 (MMP-2) has been considered a promising molecular biomarker for cancer diagnosis due to its related dysregulation. In this work, a core–satellite structure-powered ratiometric surface-enhanced Raman scattering (SERS) nanosensor with high sensitivity and specificity to MMP-2 was developed. The SERS nanosensor was composed of a magnetic bead encapsulated within a 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB)-labeled gold shell as the capture core and a 4-mercaptobenzonitrile (MBN)-encoded silver nanoparticle as the signal satellite, which were connected through a peptide substrate of MMP-2. MMP-2-triggered cleavage of peptides from the core surface resulted in a decrease of the SERS intensity of MBN. Since the SERS intensity of DTNB was used as an internal standard, the reliable and sensitive quantification of MMP-2 activity would be realized by the ratiometric SERS signal, with a limit of detection as low as 2.067 ng/mL and a dynamic range from 5 to 100 ng/mL. Importantly, the nanosensor enabled a precise determination of MMP-2 activity in tumor cell secretions, which may provide an avenue for early diagnosis and classification of malignant tumors