Directed exfoliating and ordered stacking of transition-metal-dichalcogenides.

Two-dimensional van der Waals crystals provide a limitless scope for designing novel combinations of physical properties by controlling the stacking order or twist angle of individual layers. Lattice orientation between stacked monolayers is significant not only for breaking the engineering symmetry but also for the study of many-body quantum phases and band topology. Thus far the state-of-the-art exfoliation approaches focus on the achievements of quality, size, yield, and scalability, while lacking sufficient information on lattice orientation. Consequently, interlayer alignment is usually determined by later experiments, such as the second harmonic generation spectroscopy, which increase the number of trials and errors for a designed artificial ordering and hampered the efficiency of systematic study. Herein, we report a lattice orientation distinguishable exfoliation method via gold favor epitaxy along the specific atomic step edges, meanwhile, fulfilling the requirements of high-quality, large-size, and high-yield monolayers. Hexagonal- and rhombohedral-stacking configurations of bilayer transition metal dichalcogenides are built directly at once as a result of foreseeing the lattice orientation. Optical spectroscopy, electron diffraction, and angle-resolved photoemission spectroscopy are used to study crystal quality, symmetric breaking, and band tuning, which support the exfoliating mechanism we proposed. This strategy shows the ability to facilitate the development of ordering stacking especially for multilayers assembling in the future.

Giant moiré trapping of excitons in twisted hBN.

Excitons in van der Waals (vdW) stacking interfaces can be trapped in ordered moiré potential arrays giving rise to the attractive phenomena of quantum optics and bosonic many-body effects. Compared to the prevalent transition metal dichalcogenides (TMDs) systems, due to the wide bandgap and low dielectric constant, excitons in twist-stacked hexagonal boron nitride (hBN) are anticipated trapped in deeper moiré potential, which enhances the strength of interactions. However, constrained by the common low detectivity of weak light-emitting in the deep-ultraviolet (DUV) bands, the moiré excitons in twist-hBN remain elusive. Here, we report that a remarkable DUV emitting band (peak located at ∼260 nm) only emerges at the twisted stacking area of hBN, which is performed by a high collection efficiency and spatially-resolved cathodoluminescence (CL) at room temperature. Significant peak red shifting contrast to defect-bound excitons of bulk hBN indicates the giant trapping effects of moiré potential for excitons. The observation of deeply trapped excitons motivates further studies of bosonic strongly correlation physics based on the twist-hBN system.

Electron-hole plasma Fabry-Perot lasing in a Ga-incorporated ZnO microbelt via Ag nanoparticle deposition.

In this work, individual ZnO via Ga-doped (ZnO:Ga) microbelts with excellent crystallinity and smooth facets can enable the realization of lateral microresonator Fabry-Perot (F-P) microlasers, and the F-P lasing action originates from excitonic state. Interestingly, introducing Ag nanoparticles (AgNPs) deposited on the microbelt can increase F-P lasing characteristics containing a lower threshold and enhanced lasing output. Especially for the large size AgNPs (the diameter d is approximately 200 nm), the lasing features also exhibit a significant redshift of each lasing peak and an observable broadening of the spectral line width with an increase of the excitation fluence. And the remarkable lasing characteristics are belonging to the electron-hole plasma (EHP) luminescence. The behavior and dynamics of the stimulated radiation in an AgNPs@ZnO:Ga microbelt are studied, suggesting the Mott-transition from the excitonic state to EHP state that is responsible for the F-P lasing. These features can be attributed to the working mechanism that the hot electrons created by the large size AgNPs through nonradiative decay can fill the conduction band of nearby ZnO:Ga, leading to a downward shift of the conduction band edge. This novel filling influence can facilitate bandgap renormalization and result in EHP emission. The results provide a comprehensive understanding of the transition between excitonic and EHP states in the stimulated emission process. More importantly, it also can provide new scheme to developing high efficiency and ultra-low threshold microlasing diodes.

The Deformation Behavior and Bending Emissions of ZnO Microwire Affected by Deformation-Induced Defects and Thermal Tunneling Effect.

The realization of electrically pumped emitters at micro and nanoscale, especially with flexibility or special shapes is still a goal for prospective fundamental research and application. Herein, zinc oxide (ZnO) microwires were produced to investigate the luminescent properties affected by stress. To exploit the initial stress, room temperature in situ elastic bending stress was applied on the microwires by squeezing between the two approaching electrodes. A novel unrecoverable deformation phenomenon was observed by applying a large enough voltage, resulting in the formation of additional defects at bent regions. The electrical characteristics of the microwire changed with the applied bending deformation due to the introduction of defects by stress. When the injection current exceeded certain values, bright emission was observed at bent regions, ZnO microwires showed illumination at the bent region priority to straight region. The bent emission can be attributed to the effect of thermal tunneling electroluminescence appeared primarily at bent regions. The physical mechanism of the observed thermoluminescence phenomena was analyzed using theoretical simulations. The realization of electrically induced deformation and the related bending emissions in single microwires shows the possibility to fabricate special-shaped light sources and offer a method to develop photoelectronic devices.

High-performance flexible UV photodetector based on self-supporting ZnO nano-networks fabricated by substrate-free chemical vapor deposition.

Self-supporting ZnO nano-networks have been demonstrated by a substrate-free chemical vapor deposition process for the application as flexible ultraviolet (UV) photodetector. The device shows a responsivity of ∼300 mA W-1over a wide wavelength range from 254 to 365 nm and a high UV/visible rejection ratio of more than 104. More interestingly, a short 90%-10% decay time of <0.12 s can be observed in the air atmosphere, and the current can fully recover to its original dark value within 1 s after switching off the light. The quick response speed should be associated with the wire-wire junction barriers and the adsorption/desorption process of oxygen molecules on the oxygen vacancies near the surface of the ZnO. In addition, the photocurrent, the dark current and the response speed of the ZnO nano-networks flexible UV photodetector nearly stay the same under different bending conditions, suggesting the excellent photoelectric stability and repeatability. Such a simple and cheap way for fabricating self-supporting ZnO-based devices has broad application prospects in the fields of flexible and wearable electronic devices.

Low-Temperature-Induced Controllable Transversal Shell Growth of NaLnF4 Nanocrystals.

Highly controllable anisotropic shell growth is essential for further engineering the function and properties of lanthanide-doped luminescence nanocrystals, especially in some of the advanced applications such as multi-mode bioimaging, security coding and three-dimensional (3D) display. However, the understanding of the transversal shell growth mechanism is still limited today, because the shell growth direction is impacted by multiple complex factors, such as the anisotropy of surface ligand-binding energy, anisotropic core-shell lattice mismatch, the size of cores and varied shell crystalline stability. Herein, we report a highly controlled transversal shell growth method for hexagonal sodium rare-earth tetrafluoride (ß-NaLnF4) nanocrystals. Exploiting the relationship between reaction temperature and shell growth direction, we found that the shell growth direction could be tuned from longitudinal to transversal by decreasing the reaction temperature from 310 °C to 280 °C. In addition to the reaction temperature, we also discussed the roles of other factors in the transversal shell growth of nanocrystals. A suitable core size and a relative lower shell precursor concentration could promote transversal shell growth, although different shell hosts played a minor role in changing the shell growth direction.

High-Performance Planar-Type Ultraviolet Photodetector Based on High-Quality CH3NH3PbCl3 Perovskite Single Crystals.

A hybrid perovskite MAPbCl3 (MA = CH3NH3+) single crystal is considered to be one of the most viable candidates for the development of photodetectors because of its outstanding optoelectronic properties. However, the relatively lower crystalline quality of the reported MAPbCl3 single crystals fabricated by the traditional one-step inverse temperature crystallization results in momentous degradation in the performance of their photodetectors. Here, we present a novel two-step temperature process to fabricate high-quality MAPbCl3 single crystals, namely, lower temperature nucleation and higher temperature crystallization. These MAPbCl3 single crystals present low defect density (∼7.9 × 109 cm-3) commensurate with the best-quality crystals of hybrid organic-inorganic lead halide perovskites reported so far. Moreover, a high-performance ultraviolet photodetector was demonstrated on MAPbCl3 single crystals. At 30 V, the peak responsivity at 415 nm of the photodetector is as high as 3.73 A W-1 (light intensity = 1 mW cm-2), ∼2-3 orders of magnitude higher than that of the previously reported MAPbCl3 photodetectors. Meanwhile, the device has an ultrafast response speed with a rise time of 130 ns, which is one of the shortest values of MAPbX3-based photodetectors. Our findings open a new way to obtain high-quality perovskite single crystals and their high-performance photodetectors.

Oleylamine-assisted and temperature-controlled synthesis of ZnO nanoparticles and their application in encryption.

A temperature-controlled synthesis process for ZnO nanoparticles (NPs) with the assist of oleylamine (OAm) has been demonstrated, and the ZnO NPs show bright fluorescence under ultraviolet illumination. In this process, zinc nitrate was firstly converted to zinc nitrate hydroxide (Zn5(OH)8(NO3)2) sheets with the assist of OAm, then the Zn5(OH)8(NO3)2 was decomposed into fluorescent ZnO NPs by increasing the ambient temperature. Furthermore, information encryption has been realized based on this process. For encryption, the encrypted information cannot be observed, while the encrypted information appears when they are proceeded in the temperature of 120 °C for about one minute. The results shown in this work provide a controllable way for the synthesis of ZnO NPs by adjusting the reaction temperature, and this may inspire wide applications of ZnO in information encryption.

Surface related intrinsic luminescence from carbon nanodots: solvent dependent piezochromism.

There has been a long standing debate on the luminescence origination from carbon nanodots (CDs). Herein, we report the solvent dependent piezochromism of CDs by diamond anvil cells experiment. Red- and blue-shift piezochromism were observed in CDs with N,N-dimethylformamide and water as pressure transmitting medium (PTM) with increasing pressure from atmospheric pressure to 25 GPa, which were related to increased π-π stacking and protic-solvent-induced surface chemical structural changes, respectively. Based on theoretical modeling and structural analysis of hydrothermally treated CDs (h-CDs), the reversible and irreversible piezochromism from green emission to blue emission with water as PTM was attributed to pressure induced enhanced intermolecular hydrogen bonding and addition reaction between water molecules and surface electron withdrawing groups on the CDs, respectively. The decreased electron withdrawing ability of the surface chemical structures of CDs further affects their intrinsic luminescence. This work provides a new understanding of the piezochromic luminescence origination from CDs, which is related to the surface related intrinsic luminescence.

In vivo theranostics with near-infrared-emitting carbon dots-highly efficient photothermal therapy based on passive targeting after intravenous administration.

Carbon dots that exhibit near-infrared fluorescence (NIR CDs) are considered emerging nanomaterials for advanced biomedical applications with low toxicity and superior photostability and targeting compared to currently used photoluminescence agents. Despite progress in the synthesis of NIR CDs, there remains a key obstacle to using them as an in vivo theranostic agent. This work demonstrates that the newly developed sulfur and nitrogen codoped NIR CDs are highly efficient in photothermal therapy (PTT) in mouse models (conversion efficiency of 59%) and can be readily visualized by photoluminescence and photoacoustic imaging. The real theranostic potential of NIR CDs is enhanced by their unique biodistribution and targeting. Contrary to all other nanomaterials that have been tested in biomedicine, they are excreted through the body's renal filtration system. Moreover, after intravenous injection, NIR CDs are accumulated in tumor tissue via passive targeting, without any active species such as antibodies. Due to their accumulation in tumor tissue without the need for intratumor injection, high photothermal conversion, excellent optical and photoacoustic imaging performance, and renal excretion, the developed CDs are suitable for transfer to clinical biomedical practice.

Controlled compensation via non-equilibrium electrons in ZnO.

Doping wide-band-gap semiconductor with impurities always accompanied spontaneous compensation of opposite charged intrinsic defects, which lead to invalid control of the type of free carriers. We demonstrate an effectual route to overcoming such detrimental defects formation during doping by suppressing Fermi level shifting using non-equilibrium carriers gathering on the polar epitaxial surfaces. Non-equilibrium carriers are generated by ultraviolet light excited interband transitions (photon energy greater than bandgap). Because the p-type dopants are compensated by non-equilibrium electrons at metal-polar surfaces, donor-type native defects are inhibited. This new doping strategy provides an attractive solution to self-compensation problems in wide-band-gap semiconductors with spontaneous polarization of the future.

Electrically pumped Fabry-Perot microlasers from single Ga-doped ZnO microbelt based heterostructure diodes.

Semiconducting micro/nanostructures possessing naturally optical waveguiding behaviors and Fabry-Perot (F-P) like resonances are emerging as versatile building blocks for the assembly of photonic and optoelectronic devices, such as photodetectors, light-emitting diodes, lasers and so on. Individual ZnO micro/nanowires with a rectangular cross-section, such as microwires and microbelts possessing naturally smooth facets along both sides for good optical feedback, can be employed as an underlying F-P mode microcavity whilst as the gain medium for light amplification. In this context, electrically pumped F-P mode microlasers comprising a single ZnO:Ga microbelt and p-GaN substrate have been realized. By treating as the precondition, electrically driven exciton-polariton light-emitting behavior was achieved from the heterojunction diodes, which could be ascribed to strong exciton-photon coupling and waveguided nature of the synthesized microbelts. Once the applied bias exceeded the threshold value, an electrically pumped F-P mode lasing behavior could be observed, the lasing peaks centered at 410.5 nm and 450.5 nm respectively, accompanied with a dramatic narrowing of the spectral line-width to be around 1.0 nm emerging on the waveguided emission spectrum. Therefore, the realization of electrically pumped F-P mode lasing using single microbelt based heterojunction diodes opens the door not only to the fabrication of coherent light sources and model systems for waveguided resonators, but also affords a competitive candidate to develop electrically pumped and ultralow threshold polariton lasers.

Multilevel Data Encryption Using Thermal-Treatment Controlled Room Temperature Phosphorescence of Carbon Dot/Polyvinylalcohol Composites.

Thermal-treatment controlled room temperature phosphorescence is realized by embedding either originally synthesized carbon dots (CDs) or 200 °C thermal-treated CDs into a polyvinylalcohol (PVA) matrix through post-synthetic thermal annealing at 200 or 150 °C. The thermal-treatment controlled phosphorescence is attributed to the transfer of photoexcitation from the excited singlet state to the triplet state through intersystem crossing, followed by radiative transition to the ground state, which is due to decrease of quenchers (oxygen) in the CDs and suppression of the vibrational dissipations through the chemical bonding of CDs in the PVA matrix. Multilevel fluorescence/phosphorescence data encryption is demonstrated based on the thermal-treatment controlled phosphorescence from CD@PVA composites.

Investigation of Interface Effect on the Performance of CH3NH3PbCl3/ZnO UV Photodetectors.

Recent investigations indicate that the performance of organic-inorganic perovskite optoelectronic devices can be improved by combining the perovskites and the inorganic materials. However, very few studies have focused on the investigation of perovskites/inorganic semiconductor hybrid UV photodetectors and their detailed performance-enhancement mechanism is still not very clear. In this work, a CH3NH3PbCl3/ZnO UV photodetector has been first demonstrated and investigated. Both the photoresponsivity and response speed of the hybrid device are higher than those of pure CH3NH3PbCl3 and ZnO devices. The photoluminescence and transient absorption spectra indicate that the photoinduced electron transfer between CH3NH3PbCl3 and ZnO should be responsible for the performance enhancement of the hybrid device. In addition, the high crystal quality of CH3NH3PbCl3 on ZnO film is another important reason for the excellent UV detection performance. Our findings in this work provide new insights into the intrinsic photophysics essential for perovskite optoelectronic devices.

Red carbon dots-based phosphors for white light-emitting diodes with color rendering index of 92.

Exploration of solid-state efficient red emissive carbon dots (CDs) phosphors is strongly desired for the development of high performance CDs-based white light-emitting diodes (WLEDs). In this work, enhanced red emissive CDs-based phosphors with photoluminescence quantum yields (PLQYs) of 25% were prepared by embedding red emissive CDs (PLQYs of 23%) into polyvinyl pyrrolidone (PVP). Because of the protection of PVP, the phosphors could preserve strong luminescence under long-term UV excitation or being mixed with conventional packaging materials. By applying the red emissive phosphors as the color conversion layer, WLEDs with high color rendering index of 92 and color coordinate of (0.33, 0.33) are fabricated.

Electrically driven lasers from van der Waals heterostructures.

Van der Waals heterostructures (vdWHs) have opened new avenues for fundamental scientific studies and design of novel devices. Although numerous reports have demonstrated vdWH optoelectronic devices, no report on vdWH lasers can be found to date. In this paper we demonstrated electrically driven vdWH lasers for the first time, and the lasers were realized from ZnO microwire/MgO/p-GaN structures. By coating Ag films on the top surfaces of the ZnO microwires, the current injection and lasing directionality of the vdWH lasers have been improved significantly, and this improvement can be attributed to the high conductivity and reflectivity of the Ag film. The output power of the device can reach 2.41 µW under 14 mA drive current, which is among the highest values ever reported for ZnO based lasers. Our results may provide a promising way to electrically pumped lasers based on micro/nano-structures.

n-type Rashba spin splitting in a bilayer inorganic halide perovskite with external electric field.

Here, we investigated the Rashba effect of the CsPbBr3 bilayers under the external electric field (EEF) with first-principles calculations. For the PbBr2 terminated bilayer, we found that only electrons experience the Rashba splitting under EEF, while holes do not. Such an n-type Rashba effect can be ascribed to the surface relaxation effect that reverses the positions of the top valence bands. The n-type Rashba parameter can be tuned monotonically to the maximum of 0.88 eV Å at EEF of 1.35 V nm-1 at which the sequence of top valence bands recover to the bulk style. During this process the p-type spins will not survive in this 2D CsPbBr3, that indeed reveals a new way for making advanced functional spintronic devices.

Carbon-ZnO alternating quantum dot chains: electrostatic adsorption assembly and white light-emitting device application.

Aggregation-induced quenching (ACQ) in carbon nanodots (CNDs) impede their applications in solid devices. Herein, the concept of alternating quantum dot (QD) chains was proposed to overcome the common issue of fluorescence quenching in CNDs; in this study, CNDs and ZnO QDs were interlinked to form carbon-ZnO alternating quantum dot chains (CZA-QDCs), which overcame the ACQ of CNDs and hence ensured efficient full-spectrum fluorescence for white light-emitting devices (WLEDs) without excessive blue emission. Under the excitation of 365 nm lines, white emission resulting from the combination of blue emission from the CNDs and yellow emission from the ZnO QDs has been achieved from these powders. The quantum efficiency of the CZA-QDC powders can reach 49% and remain stable for two months. By coating the powders onto an ultraviolet chip as phosphors, WLEDs with a luminous efficiency of 20.1 lm W-1, color coordinate of (0.30, 0.35), correlated color temperature of 5205 K, and a color rendering index of 84 have been fabricated. Due to the relatively high abundance and eco-friendly characteristics of both carbon and ZnO, the results reported herein may provide a promising alternative to fluorescent phosphors that are widely used in WLEDs.

Electrically excited hot-electron dominated fluorescent emitters using individual Ga-doped ZnO microwires via metal quasiparticle film decoration.

The generation of hot electrons from metal nanostructures through plasmon decay provided a direct interfacial charge transfer mechanism, which no longer suffers from the barrier height restrictions observed for metal/semiconductor interfaces. Metal plasmon-mediated energy conversion with higher efficiency has been proposed as a promising alternative to construct novel optoelectronic devices, such as photodetectors, photovoltaic and photocatalytic devices, etc. However, the realization of the electrically-driven generation of hot electrons, and the application in light-emitting devices remain big challenges. Here, hybrid architectures comprising individual Ga-doped ZnO (ZnO:Ga) microwires via metal quasiparticle film decoration were fabricated. The hottest spots could be formed towards the center of the wires, and the quasiparticle films were converted into physically isolated nanoparticles by applying a bias onto the wires. Thus, the hot electrons became spatially localized towards the hottest regions, leading to a release of energy in the form of emitting photons. By adjusting the sputtering times and appropriate alloys, such as Au and Ag, wavelength-tunable emissions could be achieved. To exploit the EL emission characteristics, metal plasmons could be used as active elements to mediate the generation of hot electrons from metal nanostructures, which are located in the light-emitting regions, followed by injection into ZnO:Ga microwire-channels; thus, the production of plasmon decay-induced hot-electrons could function as an efficient approach to dominate emission wavelengths. Therefore, by introducing metal nanostructure decoration, individual ZnO:Ga microwires can be used to construct wavelength-tunable fluorescent emitters. The hybrid architectures of metal-ZnO micro/nanostructures offer a fantastic candidate to broaden the potential applications of semiconducting optoelectronic devices, such as photovoltaic devices, photodetectors, optoelectronic sensors, etc.

Near-Infrared Excitation/Emission and Multiphoton-Induced Fluorescence of Carbon Dots.

Carbon dots (CDs) have significant potential for use in various fields including biomedicine, bioimaging, and optoelectronics. However, inefficient excitation and emission of CDs in both near-infrared (NIR-I and NIR-II) windows remains an issue. Solving this problem would yield significant improvement in the tissue-penetration depth for in vivo bioimaging with CDs. Here, an NIR absorption band and enhanced NIR fluorescence are both realized through the surface engineering of CDs, exploiting electron-acceptor groups, namely molecules or polymers rich in sulfoxide/carbonyl groups. These groups, which are bound to the outer layers and the edges of the CDs, influence the optical bandgap and promote electron transitions under NIR excitation. NIR-imaging information encryption and in vivo NIR fluorescence imaging of the stomach of a living mouse using CDs modified with poly(vinylpyrrolidone) in aqueous solution are demonstrated. In addition, excitation by a 1400 nm femtosecond laser yields simultaneous two-photon-induced NIR emission and three-photon-induced red emission of CDs in dimethyl sulfoxide. This study represents the realization of both NIR-I excitation and emission as well as two-photon- and three-photon-induced fluorescence of CDs excited in an NIR-II window, and provides a rational design approach for construction and clinical applications of CD-based NIR imaging agents.