In recent years, 2D layered organic–inorganic lead halide perovskites have attracted considerable attention due to the distinctive quantum confinement effects as well as prominent excitonic luminescence. Herein, we show that the recombination dynamics and photoluminescence (PL) of the 2D layered perovskites can be tuned by the organic cation length. 2D lead iodide perovskite crystals with increased length of the organic chains reveal blue-shifted PL as well as enhanced relative internal quantum efficiency. Furthermore, we provide experimental evidence that the formation of face-sharing [PbI6]4– octahedron in perovskites with long alkyls induces additional confinement for the excitons, leading to 1D-like recombination. As a result, the PL spectra show enhanced inhomogeneous broadening at low temperature. Our work provides physical understanding of the role of organic cation in the optical properties of 2D layered perovskites, and would benefit the improvement of luminescence efficiency of such materials.

Fundamental to understanding and predicting the optoelectronic properties of semiconductors is the basic parameters of excitons such as oscillator strength and exciton binding energy. However, such knowledge of CsPbBr3 perovskite, a promising optoelectronic material, is still unexplored. Here we demonstrate that quasi-two-dimensional (quasi-2D) CsPbBr3 nanoplatelets (NPLs) with 2D exciton behaviors serve as an ideal system for the determination of these parameters. It is found that the oscillator strength of CsPbBr3 NPLs is up to 1.18 × 104, higher than that of colloidal II–VI NPLs and epitaxial quantum wells. Furthermore, the exciton binding energy is determined to be of ∼120 meV from either the optical absorption or the photoluminescence analysis, comparable to that reported in colloidal II–VI quantum wells. Our work provides physical understanding of the observed excellent optical properties of CsPbBr3 nanocrystals and would benefit the prediction of high-performance excitonic devices based on such materials.

Organo-lead halide perovskite has emerged as a promising optical gain media. However, continuous efforts are needed to improve the amplified spontaneous emission (ASE) even lasing properties to evade the poor photostability and thermal instability of the perovskites. Herein, we report that simply through the coating of polymer layer, the CH3NH3PbBr3 polycrystalline films prepared by a modified sequential deposition process show remarkably enhanced photoluminescence and prolonged decay lifetime. As a result, under nanosecond pulse pumping, the ASE threshold of the perovskite films is significantly reduced from 303 to 140 μJ/cm2. Furthermore, the light exposure stability is improved greatly after the polymer coating. We confirmed that the polymer layer plays the roles of both surface passivation and symmetric waveguides. Our results may shed light upon the stable and sustained output of laser from perovskite materials.Keywords: amplified spontaneous emission; perovskite; surface passivation; thin films; waveguide;

Monolayer ZnO represents a class of new two-dimensional (2D) materials that are expected to exhibit unique optoelectronic properties and applications. Here we report a novel strategy to synthesize free-standing atomically thin ZnO layers via the oxidation of hydrothermally grown ultrathin zinc chalcogenide nanosheets. With micrometer-scaled lateral size, the obtained ultrathin ZnO layer has a thickness of ∼2 nm, and the layered structure still maintained well after high temperature oxidation. The thermal treatment strongly improves the crystal quality as well without inducing cracks or pinholes in the ultrathin layers. The atomically thin ZnO layers are highly luminescent with dominant green emission. High quality white light is obtained from the mixed phosphors containing the ZnO layers, exhibiting their potential as compelling ultraviolet-excited phosphors.Keywords: free-standing; light emission; two-dimensional materials; ZnO;

The photophysics of organolead trihalide perovskites are still not fully understood. Here we report that the photoluminescence (PL) lineshape, decay features, and intensity of solution-processed CH3NH3PbI3−xClx films are highly sensitive to the ambience. The radiative recombination actually consists of two decay channels corresponding to recombination of extended and localized states, respectively. The PL decay can be well described by a model considering the carrier localization, surface band bending, and trapping–detrapping of carriers.

Organic–inorganic hybrid perovskites have been widely recognized as highly luminescent materials for efficient light-emitting devices. Herein, we report a simple perovskite-based metal–insulator–semiconductor (MIS) device structure with green light emission. Electron tunnelling and subsequent recombination in the semiconductor–insulator interface region is confirmed as the working mechanism of the perovskite light-emitting devices.

Photoluminescence studies are carried out on porous silicon nanowires (Si NWs) undergoing rapid thermal oxidation. Despite maintaining porosity, the oxidized Si NWs fail to hold the efficient orange photoluminescence even under mild oxidation conditions. However, their PL stability in both air and ethanol achieves significant improvement after proper thermal oxidation.

We report a room-temperature colloidal synthesis of few-unit-cell-thick CsPbBr3 QWs with lengths over a hundred nanometers. The surfactant-directed oriented attachment growth mechanism was proposed to explain the formation of such CsPbBr3 QWs. Owing to the strong quantum confinement effect, the photoluminescence (PL) emission peak of few-unit-cell-thick CsPbBr3 QWs blue-shifted to 430 nm. The ensemble PL quantum yield (PLQY) of the few-unit-cell-thick CsPbBr3 QWs increased to 21.13% through a simple heat-treatment process. The improvement of PLQY was ascribed to the reduction of the density of surface trap states and defect states induced by the heat-treatment process. Notably, the dependence of the bandgap on the diameter with different numbers of unit cells was presented for the first time in 1-D CsPbBr3 QWs on the basis of the produced few-unit-cell-thick CsPbBr3 QWs.

AbstractThe powder form and low photoluminescence quantum yield (PLQY) of fluorescent metal–organic frameworks (MOFs) present a serious obstacle to fabricating high-efficiency film-like lighting devices. Here, we present a facile way to produce thin films of CdSexS1−x/ZnS quantum dots (QDs)@ZIF-8 with high PLQY by encapsulating red, green, and blue CdSexS1−x/ZnS QDs in ZIF-8 through a one-pot solid-confinement conversion process. The QDs@ZIF-8 thin film emits warm white light with good color quality and presents good thermal stability and long-term durability.

We report the formation of high-quality Cs0.4MA0.6PbBr3 thin films with nearly full surface coverage and good emission properties upon the introduction of Cs+ into perovskite crystals. The Cs0.4MA0.6PbBr3 thin films were applied as emissive layers in light-emitting diodes. A maximum external quantum efficiency of ~2.0% was achieved for these green-emitting devices.

Long-term ambiguity in knowledge has made individuals confused about the uncertain origin of magnetism and some defect-related issues in transition metal ions (TMs)-doped ZnO systems. In this paper, a facile colloidal chemistry procedure is employed to prepare amine-capped ZnO:Cu nanocrystals (NCs) with an optimized Cu–N defect configuration. The doping mechanism, dopant spatial distribution, valence state and defect structure are revealed in detail via electron microscopy, electron spin resonance (ESR) and photoluminescence (PL) techniques assisted by in situ chemical tracking. N-capping annealing-induced acceptor defects can activate high-temperature ferromagnetism in spin-coated ZnO:Cu nanocrystalline films, whereas O-capping ones cannot. Although the maximum magnetic moment of surface acceptor defect-mediated ferromagnetic films (1.58 μB/Cu) is comparable with that of a bulk donor defect-mediated case (1.41 μB/Cu), the corresponding physical nature is totally distinct and the former is considered more efficient. A comparative demonstration of the spin-exchange process in terms of acceptor and donor defects strongly indicates a diverse role of defects in mediating ferromagnetic ordering, like the case of carrier type-determined differences in ferromagnetism. The proposal of physical (annealing) or chemical (capping) means for magnetism control in the ZnO:Cu system expands the methodology of applications, which utilize spin and defects as controllable states.

A unique all-wurtzite ZnO/ZnSe hetero-nanohelix is formed via growing wurtzite ZnSe nanoteeth on ZnO nanobelts through a one step thermal evaporation method. The microstructure and growth mechanism of the hetero-nanohelix are investigated in detail. The formation of metastable wurtzite ZnSe is attributed to the wurtzite ZnO template. Mechanical forces, thermal expansion and polar plane in hexagonal crystals are suggested to contribute to the bending of the nanohelix. A boomerang-like structural block is proposed to assemble the zigzag ZnO nanobelts. The incorporation of Se into ZnO results in a strong orange emission. The heterostructure of the ZnO/ZnSe nanohelix is confirmed by elemental mapping and luminescence imaging. The fabrication of such a hetero-nanohelix may provide insights into the growth mechanism of the rich family of ZnO-based nanostructures.

We have demonstrated that photoluminescence (PL) is a non-damaging and powerful tool for the characterization of heavily-doped semiconductor nanostructures such as n-ZnO nanowires. The PL shows a redshift and a Gaussian-shaped low-energy wing, indicating a broadening mechanism governed by the impurity band. The electron concentration can be estimated from the PL linewidth.

We carried out comprehensive steady-state and time-resolved photoluminescence (PL) studies to elucidate the origin of luminescence from a porous Si nanowire array prepared by metal-assisted chemical etching. We provide evidence for the defect nature of the PL, and ascribe it to donor–acceptor pair recombination. A schematic energy-level diagram is proposed to interpret the overall PL features.

•Annealing at 1223 K removes H and greatly modifies the luminescence of ZnO nanorods.•VZn–H complex exists in the hydrothermally grown ZnO materials.•VZn and VZn–H complex are responsible for the different deep level emission.We carry out a comprehensive photoluminescence (PL) study on hydrothermally grown ZnO nanorods to provide insights into the role of hydrogen impurity in ZnO materials. Annealing at 1223 K greatly modifies the PL properties of the ZnO nanorods, both for the excitonic and deep level emissions. The suppression of excitons bound to H donors as well as enhancement of deep level emission after annealing suggests the removal of H from ZnO lattice. The temperature-dependent behavior of the deep level emissions suggests the formation of VZn–Hn complexes.

A systematic study was carried out to investigate the morphology and photoluminescence (PL) of porous silicon nanowire (Si NW) arrays fabricated by metal-assisted chemical etching with diverse etching conditions and starting Si wafers. The morphology of the Si NWs, including the orientation and porosity, could be well controlled. For Si NWs prepared from p-Si-(111) wafers, the orientation switched from the 〈100〉 to the 〈111〉 direction as the hydrogen peroxide (H2O2) concentration was increased. In addition, the NW porosity could be adjusted by changing the H2O2 concentration and the substrate doping elements. The PL intensity of the NWs correlated directly with their porosity. The PL intensity was strongly dependent on the oxidizer concentration, substrate orientation, and doping element of the starting Si wafer. The maximum PL enhancement ratio reached 14.5 when the concentration of H2O2 was doubled. The photoluminescence of p-Si-(100) was 1.15, 1.39 and 1.45 times stronger than that of p-Si-(111) with 0.25, 0.5, and 0.75 M H2O2, respectively. The morphology-controllable fabrication and tunable optical properties of such porous Si NWs will promote research on the application of semiconductor optical nanodevices.

Core–shell structured silicon nanowires (Si NWs) were obtained by coating Si NWs with an HfO2 layer. Enhanced photoluminescence (PL) and a slightly decreased PL lifetime are achieved by HfO2 coating. Furthermore, the sensing stability is strongly improved. The improvement of PL properties is interpreted in terms of surface passivation and the Purcell effect.

We carry out a comprehensive temperature-dependent photoluminescence (PL) study on chemically derived graphene oxide (GO) sheets. According to the unusual temperature dependence, we introduce a trap state ∼114 meV beneath the LUMO, which implies an additional carrier decay process.

Many factors affect the photoluminescence (PL) of graphene oxide (GO). The influence of Mn2+ on the PL in GO was systematically investigated when the universal heteroion was introduced in the common preparation based on the Hummers method, and for the first time we propose that a Mn2+ mediated energy transfer process may be a versatile origin in GO.

Polycrystalline ZnO:Cu-based film photodetectors with extended detection waveband (UV and visible light) were fabricated using facile colloidal chemistry and a post-annealing process. The obtained detectors are highly sensitive to visible light and can realize the response switch between UV and visible light. A native and extrinsic trap cooperatively controlled space charge limited (SCL) transport mechanism is proposed to understand this complex photoconduction behaviour.

The paper reports robust ferromagnetic Cu-doped ZnO micron-scale polycrystalline films via spin-coating using high-quality doped nanocrystals. A reliable magnetic response is observed in the 900 °C vacuum annealed film without any ferromagnetic contribution from other sources. Post-annealing treatment in terms of atmosphere and temperature can control the proportion of oxygen vacancies (VO) and zinc interstitials (Zni) defects and further help to precisely regulate defect-related ferromagnetic behavior. Complex charge transfer processes derived from dual-donor (Zni and VO) to Cu acceptor are revealed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra. Based on the above, specific charge transfer (CT)-type Stoner splitting and indirect double-exchange mechanisms are proposed to understand the ferromagnetic origin. The improvable FM performance and annealing-specific modulation further indicate that a thermal driven process can delicately tailor the magnetic property of the transition metal ion-doped ZnO system.

This work presents positive experimental evidence for the formation of a carbon–nitrogen complex in ZnO, which was theoretically predicted previously. A very high nitrogen content up to ∼8 at% can be doped into ZnO nanostructures via the formation of a carbon–nitrogen complex, which in turn suppresses the formation of a nitrogen acceptor.

This work presents positive experimental evidence for the hole traps in ZnO nanorods, which take part in recombination and change the thermal quenching of Cu-related green emission. The evolution of Cu impurity upon annealing, as well as the formation of Cu-related shallow donors with an energy level of ∼0.11 eV are also indicated by temperature-dependent photoluminescence.

Nanostructured ZnO is considered to be a promising building block in the design of nanoscale optoelectronic devices. It usually shows dominant donor-bound exciton (DX) emission at low temperatures. In this study, ZnO nanorods with high crystallinity and optical quality were grown by metal–organic chemical vapor deposition on a-plane sapphire (110) substrates. Dominant free exciton (FX) emission at a low temperature (14 K) was observed by photoluminescence spectroscopy. It was attributed to both the enhancement of the FX emission induced by the high crystalline quality of the nanorods and the suppression of the DX emission induced by hydrogen out-diffusion. The latter reason is believed to be more important from the analysis of the hydrogen distribution in the nanorods through photoluminescence spectroscopy and secondary ion mass spectrometry. A slow cooling process during the deposition is suggested to result in a better optical quality. These results can promote our understanding of the optical properties of ZnO nanostructures.

We investigate the negative thermal quenching behavior of the 3.338 eV emission in ZnO nanorods. A correlation between the 3.338 eV and the 3.368 eV (surface exciton) emissions is determined from temperature-dependent photoluminescence. The activation energies of the 3.338 eV emission, obtained using an approximated multi-level model, indicate an trap state between the two surface exciton emissions. The present study demonstrates a nondestructive and easy method to understand the surface effects on the optical properties of semiconductor nanostructures.Highlights► Negative thermal quenching of the 3.338 eV emission in ZnO is investigated. ► The activation energies of the 3.338 eV emission are exacted by a multi-level model. ► We attribute this emission as a near-surface structural defect-bound exciton. ► Trap states may exist between different surface exciton centers.

In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range.

Negative thermal quenching of both the excitonic and green emissions is observed in ZnO nanosheets, from which the energy level of surface traps can be extracted based on a model of multi-level transitions. The present study demonstrates a non-destructive and easy method to determine the trap levels in semiconductor nanostructures.

Three-dimensional SiOx nanostructures with various shapes have been prepared by chemical vapor deposition. The sunflower-like SiOx nanostructures provide a chance to probe the growth process. A typical Ga-assisted vapor–liquid–solid (VLS) model is responsible for the incipient SiOx nanostructures, while the growth of their branches is governed by a SiOx self-catalysis mechanism. Elemental analysis unveils the initial stage and evolution of SiOx nanostructure formation. The optical properties of SiOx nanostructures are also investigated.

We report the growth of uncommon layer-structured ZnO nanowire arrays via metal–organic chemical vapor deposition (MOCVD). The morphology, microstructure, and photoluminescence (PL) of the nanowires are investigated. The nanowires grow along the [0001] direction, with periodic zig-zag edges formed by the {101̄1}-type surfaces. The nanowires exhibit unique PL features. The PL spectra at low temperature are dominated by the surface exciton recombination at 3.366 eV and the controversial 3.32 eV emission. For the 3.32 eV emission, transformation from donor–acceptor pair recombination to free electron-to-acceptor transition is observed with increasing temperature. The stacking faults formed in the interface region between the layers are likely responsible for the strong emission around 3.32 eV.

One-dimensional semiconductor nanostructures are expected to show significant surface effects, which results in remarkable modification of the optical properties. In this work, an experimental study of surface effects on the photoluminescence (PL) of ZnO nanorods with different sizes is reported. A thin shell layer of Al2O3 is used to passivate the nanorods’ surface, which allows one to compare the PL spectra before and after dielectric coating. It is found that strong surface exciton recombination is present in ZnO nanorods with average diameter as large as ∼500 nm. Coating the nanorods by Al2O3 significantly reduces the surface state-related emissions, indicating that surface passivation rather than surface band-bending mechanism dominates. We also provide evidence that the long controversial 3.31 eV emission in ZnO is not related to surface states but a free-to-bound transition involving an unknown acceptor level of ∼125 meV. In the visible spectral region, an orange emission around 2.1 eV together with the normal green emission is observed in the thick nanorods. The little change in intensity after Al2O3 coating allows us to conclude that the visible emissions are unlikely from the surface. Based on a DAP-like transition model, we are able to interpret the blue shift of the orange emission with increasing temperature and attribute the emission to zinc vacancy defects.

We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at ∼3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be ∼0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D+A−eh complexes.

ZnO nanowires grown along non-polar direction were synthesized by a vapor–solid process using zinc and MgB2 powders as the source materials. Two kinds of nanowires with clearly different diameters and Mg distribution were discriminated. The thin nanowire exhibits <10–10> growth direction and zig-zag facets on the edge. The Mg concentration in the thin nanowires is uniform and can be as high as ∼ 30 at.%. It was found that both growth temperature and Mg source material are essential to obtain non-polar ZnO nanowires.

Temperature-dependent photoluminescence (PL) from p-type ZnO film codoped with Al and N has been investigated. In the whole temperature range of 10–300 K, the PL was dominated by a broad emission centered at 3.05 eV. The dependencies of its peak energy on temperature and compensation indicate that this emission is due to recombination of localized carriers. We suggest that the localization is due to potential fluctuations caused by strong compensation and local compositional fluctuations of the impurities. We obtain an activation energy of ∼69 meV from thermal quenching of the luminescence intensity and ascribe it to thermal ionization of shallow donors.

•Implanting Zn+ ions into ZnO single crystals produces VZn-Zni complex.•The complex is “invisible” in photoluminescence under low-level excitation.•The recombination at VZn-Zni takes place only when oxygen vacancies are saturated.Zinc vacancy (VZn) plays key roles in the optical and electrical properties of ZnO, but its behaviors are not fully understood. Here we report the formation and abnormal photoluminescence (PL) of VZn-related complex in Zn+-implanted ZnO single crystals. With increasing excitation density, we observed a new and gradually increased broad emission around 550 nm. The 550 nm emission is unexpectedly invisible under low power level excitation, indicating the lower priority of recombination of the related defect centers. Electron paramagnetic resonance (EPR) results suggest the formation of VZn-Zni complex after implantation, which is responsible for the abnormal PL properties.

We have demonstrated that photoluminescence (PL) is a non-damaging and powerful tool for the characterization of heavily-doped semiconductor nanostructures such as n-ZnO nanowires. The PL shows a redshift and a Gaussian-shaped low-energy wing, indicating a broadening mechanism governed by the impurity band. The electron concentration can be estimated from the PL linewidth.

This work presents positive experimental evidence for the hole traps in ZnO nanorods, which take part in recombination and change the thermal quenching of Cu-related green emission. The evolution of Cu impurity upon annealing, as well as the formation of Cu-related shallow donors with an energy level of ∼0.11 eV are also indicated by temperature-dependent photoluminescence.

The photophysics of organolead trihalide perovskites are still not fully understood. Here we report that the photoluminescence (PL) lineshape, decay features, and intensity of solution-processed CH3NH3PbI3−xClx films are highly sensitive to the ambience. The radiative recombination actually consists of two decay channels corresponding to recombination of extended and localized states, respectively. The PL decay can be well described by a model considering the carrier localization, surface band bending, and trapping–detrapping of carriers.

Long-term ambiguity in knowledge has made individuals confused about the uncertain origin of magnetism and some defect-related issues in transition metal ions (TMs)-doped ZnO systems. In this paper, a facile colloidal chemistry procedure is employed to prepare amine-capped ZnO:Cu nanocrystals (NCs) with an optimized Cu–N defect configuration. The doping mechanism, dopant spatial distribution, valence state and defect structure are revealed in detail via electron microscopy, electron spin resonance (ESR) and photoluminescence (PL) techniques assisted by in situ chemical tracking. N-capping annealing-induced acceptor defects can activate high-temperature ferromagnetism in spin-coated ZnO:Cu nanocrystalline films, whereas O-capping ones cannot. Although the maximum magnetic moment of surface acceptor defect-mediated ferromagnetic films (1.58 μB/Cu) is comparable with that of a bulk donor defect-mediated case (1.41 μB/Cu), the corresponding physical nature is totally distinct and the former is considered more efficient. A comparative demonstration of the spin-exchange process in terms of acceptor and donor defects strongly indicates a diverse role of defects in mediating ferromagnetic ordering, like the case of carrier type-determined differences in ferromagnetism. The proposal of physical (annealing) or chemical (capping) means for magnetism control in the ZnO:Cu system expands the methodology of applications, which utilize spin and defects as controllable states.

A systematic study was carried out to investigate the morphology and photoluminescence (PL) of porous silicon nanowire (Si NW) arrays fabricated by metal-assisted chemical etching with diverse etching conditions and starting Si wafers. The morphology of the Si NWs, including the orientation and porosity, could be well controlled. For Si NWs prepared from p-Si-(111) wafers, the orientation switched from the 〈100〉 to the 〈111〉 direction as the hydrogen peroxide (H2O2) concentration was increased. In addition, the NW porosity could be adjusted by changing the H2O2 concentration and the substrate doping elements. The PL intensity of the NWs correlated directly with their porosity. The PL intensity was strongly dependent on the oxidizer concentration, substrate orientation, and doping element of the starting Si wafer. The maximum PL enhancement ratio reached 14.5 when the concentration of H2O2 was doubled. The photoluminescence of p-Si-(100) was 1.15, 1.39 and 1.45 times stronger than that of p-Si-(111) with 0.25, 0.5, and 0.75 M H2O2, respectively. The morphology-controllable fabrication and tunable optical properties of such porous Si NWs will promote research on the application of semiconductor optical nanodevices.

Negative thermal quenching of both the excitonic and green emissions is observed in ZnO nanosheets, from which the energy level of surface traps can be extracted based on a model of multi-level transitions. The present study demonstrates a non-destructive and easy method to determine the trap levels in semiconductor nanostructures.

Core–shell structured silicon nanowires (Si NWs) were obtained by coating Si NWs with an HfO2 layer. Enhanced photoluminescence (PL) and a slightly decreased PL lifetime are achieved by HfO2 coating. Furthermore, the sensing stability is strongly improved. The improvement of PL properties is interpreted in terms of surface passivation and the Purcell effect.

This work presents positive experimental evidence for the formation of a carbon–nitrogen complex in ZnO, which was theoretically predicted previously. A very high nitrogen content up to ∼8 at% can be doped into ZnO nanostructures via the formation of a carbon–nitrogen complex, which in turn suppresses the formation of a nitrogen acceptor.