Thickness-dependent exciton relaxation dynamics of few-layer rhenium diselenide

Rhenium diselenide(ReSe_(2))has gathered much attention due to its low symmetry of lattice structure,which makes it possess in-plane anisotropic optical,electrical as well as excitonic properties and further enables ReSe_(2)have an important application in optoelectronic devices.Here,we report the thickness-dependent exciton relaxation dynamics of mechanically exfoliated few-layer ReSe_(2)flakes by using time-resolved pump–probe transient transmission spectroscopies.The results reveal two thickness-dependent relaxation processes of the excitons.The fast one correlates with the exciton formation(i.e.,the conversion of hot carriers to excitons),while the slow one is attributed to the exciton recombination dominated by defect-assisted exciton trapping besides photon emission channel.The decrease of scattering probability caused by defects leads to the increase of fast lifetime with thickness,and the increase of slow lifetime with thickness is related to the trap-mediated exciton depopulation induced by surface defects.Polarization-dependent transient spectroscopy indicates the isotropic exciton dynamics in the two-dimensional(2D)plane.These results are insightful for better understanding of excitonic dynamics of ReSe_(2)materials and its application in future optoelectronic and electronic devices.

Epitaxial Growth and Characteristics of Nonpolar a-Plane InGaN Films with Blue-Green-Red Emission and Entire In Content Range

Nonpolar(1120)plane In_(x)Ga_(1-x)N epilayers comprising the entire In content(x)range were successfully grown on nanoscale Ga N islands by metal-organic chemical vapor deposition.The structural and optical properties were studied intensively.It was found that the surface morphology was gradually smoothed when x increased from 0.06 to 0.33,even though the crystalline quality was gradually declined,which was accompanied by the appearance of phase separation in the In_(x)Ga_(1-x)N layer.Photoluminescence wavelengths of 478 and 674 nm for blue and red light were achieved for x varied from 0.06 to 0.33.Furthermore,the corresponding average lifetime(τ_(1/e))of carriers for the nonpolar In Ga N film was decreased from 406 ps to 267 ps,indicating that a high-speed modulation bandwidth can be expected for nonpolar In Ga N-based light-emitting diodes.Moreover,the bowing coefficient(b)of the(1120)plane In Ga N was determined to be 1.91 e V for the bandgap energy as a function of x.

Doped microspheres for whispering gallery mode microlasing

Whispering gallery mode(WGM)optical microcavities,which are capable of light field confinement and manipulation in a relatively small volume,have gained considerable attention from fundamentals in enhanced light-matter interactions to laser applications[1].Undergoing decades of development,the improvement of lasing performance is always a significant issue,including low threshold and high quality(Q)factor[2].Increasing optical gain and reducing the losses are the key aspects towards the realization of a high-quality WGM laser.A perfect cavity structure and a smooth surface are the most crucial determinants to reduce optical losses and enhance light field confined abilities.

One-dimensional core/shell radial heterojunction with cascade type-II energyband alignment for enhanced broadband photodetection

Constructing a one-dimensional(1D)core/shell heterostructure is a rational and efficient way to integrate multiple functional materials into a single device,in which a distinct and reliable interface and suitable energy-band alignment play important roles in optoelectronic applications.Here,using a typical magnetron sputtering system,we constructed a 1D ZnO/CdS/CdTe core/shell nanorod arrays radial heterojunction with a well-designed cascade type-II energy band alignment and improved the broadband photodetector(PD)performance.The well-formed shell layers compensated for the defect states on the ZnO surface and extended the photoresponse range from ultraviolet to visible and near-infrared.Moreover,reliable and distinct heterointerfaces with a cascade type-II energy band alignment can guarantee more stable carrier migration and reduce energy loss,promoting effective photogenerated charge carrier separation and resulting in an enhanced photoresponse.The optimised 1D ZnO/CdS/CdTe core/shell heterojunction PD exhibited a fast photoresponse at 0 V bias with high responsivity and detectivity.These results provide an important reference for the rational design and controllable synthesis of multifunctional optoelectronic nanodevices.

High-performance self-powered ultraviolet to near-infrared photodetector based on WS_(2)/InSe van der Waals heterostructure

van der Waals heterostructures(vdWHs)based on two-dimensional(2D)materials without the crystal lattice matching constraint have great potential for high-performance optoelectronic devices.Herein,a WS_(2)/InSe vdWH photodiode is proposed and fabricated by precisely stacking InSe and WS_(2)flakes through an all-dry transfer method.The WS_(2)/InSe vdWH forms an n–n heterojunction with strong built-in electric field due to their intrinsic n-type semiconductor characteristics and energy-band alignments with a large Fermi level offset between WS_(2)and InSe.As a result,the device displays excellent photovoltaic behavior with a large open voltage of 0.47 V and a short-circuit current of 11.7 nA under 520 nm light illumination.Significantly,a fast rising/decay time of 63/76μs,a large light on/off ratio of 105,a responsivity of 61 mA/W,a high detectivity of 2.5×10^(11) Jones,and a broadband photoresponse ranging from ultraviolet to near-infrared(325–980 nm)are achieved at zero bias.This study provides a strategy for developing high-performance self-powered broadband photodetectors based on 2D materials.

All-optical generation,detection,and manipulation of picosecond acoustic pulses in 2D semiconductor/dielectric heterostructures

Launching,tracking,and controlling picosecond acoustic(PA)pulses are fundamentally important for the construction of ultrafast hypersonic wave sources,ultrafast manipulation of matter,and spatiotemporal imaging of interfaces.Here,we show that GHz PA pulses can be all-optically generated,detected,and manipulated in a 2D layered MoS_(2)∕glass heterostructure using femtosecond laser pump-probe.Based on an interferometric model,PA pulse signals in glass are successfully decoupled from the coexisting temperature and photocarrier relaxation and coherent acoustic phonon(CAP)oscillation signals of MoS_(2)lattice in both time and frequency domains.Under selective interface excitations,temperature-mediated interfacial phonon scatterings can compress PA pulse widths by about 50%.By increasing the pump fluences,anharmonic CAP oscillations of MoS_(2)lattice are initiated.As a result,the increased interatomic distance at the MoS_(2)∕glass interface that reduces interfacial energy couplings can markedly broaden the PA pulse widths by about 150%.Our results open new avenues to obtain controllable PA pulses in 2D semiconductor/dielectric heterostructures with femtosecond laser pump-probe,which will enable many investigations and applications.

Plasmon-enhanced ZnO whispering-gallery mode lasing

Collaborative enhancements from surface plasmons （SPs） and whispering-gallery modes （WGMs） can induce intense near-field effects with high spatial localization around the surface of a semiconducting material. One can construct a highly efficient hybrid microcavity using semiconducting materials through resonant coupling between SPs and WGMs. Hexagonal ZnO micro-/nanostructures, which have been employed as natural WGM microcavities for ultraviolet （UV） lasing, can be used as ideal platforms to construct such hybrid microcavities. Here, we comprehensively review the recent efforts for improving lasing performance by resonant coupling between SPs and WGMs. Traditional SPs originating from various metals as well as novel SPs originating from atomic layers such as graphene are considered. Moreover, we discuss the mechanism of light-matter interactions beyond the improvements in lasing performance.

Spatiotemporal sectioning of two-photon fluorescence ellipsoid with a CsPbBr_(3) nanosheet

Two-photon fluorescence (TPF) ellipsoid formed by a focused femtosecond laser into luminescent media serves as a fundamental pixel for TPF spatiotemporal imaging. Visualizing spatiotemporal evolution of the TPF ellipsoid itself in a selected luminescent medium is important for correctly reconstructing and interpreting spatiotemporal information of imaged targets. Here, we report a new spatiotemporal sectioning technique with a luminescent CsPbBr_(3) nanosheet and visualize the spatiotemporal evolution of TPF ellipsoid along the axial direction. Time-resolved axial lengths of TPF ellipsoids turn out to broaden nonlinearly with a turning point at about 600 ps. By comparison experiments, observed phenomena are attributed to photocarrier trapping and TPF photon recycling processes within CsPbBr_(3) nanosheets. The spatiotemporal sectioning technique is expected to be widely applicable, which will ignite a plethora of investigations and applications utilizing TPF ellipsoid.

SERS-encoded nanocomposites for dual pathogen bioassay

Surface-enhanced Raman spectroscopy(SERS)has been successfully applied to detect various biomolecules,but it is still in challenge to assay living cells or bacteria sensitively,selectively and quantitatively in complex environments.In this paper,4-ATP and DTNB are assembled on Ag nanoparticle(NP)-decorated poly(styrene-co-acrylic acid)(PSA)nanospheres and then sealed by silica shells to form sensitive SERS labels based on the localized surface plasmon resonance of Ag NPs and large light scattering cross-sections of PSA nanospheres.They are further developed as encoding tags for dual detection of S.aureus and E.coli after assembling corresponding aptamers,which demonstrate ultralow detection limits of 8 cell L-1 for S.aureus and 2 cell L-1 for E.coli.Such a bioassay indicates a point-of-care strategy of ultrasensitively biomedical detections by encoding specific SERS tags.