Controlled growth of single-crystalline erbium chloride silicate with long-lived fluorescence.

Single-crystalline erbium chloride silicates have attracted extensive attention due to their high gain compatibility and silicon compatible properties. Long-lived near-infrared fluorescence is critical for reducing a pump density threshold when erbium containing materials are used as active devices. Here we developed a single-source chemical vapor deposition (CVD) method to grow high-quality single-crystalline erbium chloride silicate nanostructures. The growth mechanism is found composing of two steps, where silicon source comes from the minor evaporation of silicon substrate. The prepared single-crystalline erbium chloride silicate nanowires own diameter of about 200 nm with few lattice defects, and the fluorescence lifetime reaches up to 7.4 ms. A nanoscale thermometer based on their visible band fluorescence is realized.

Erbium chloride silicate-based vertical cavity surface-emitting laser at the near-infrared communication band.

Silicon-based integrated optoelectronics has become a hotspot in the field of computers and information processing systems. An integrated coherent light source on-chip with a small footprint and high efficiency is one of the most important unresolved devices. Here, we realize a silicon-based vertical cavity surface-emitting laser in the near-infrared communication band by making efforts in both controlled preparation of high-gain erbium silicate materials and novel design of high optical feedback microcavity. Single-crystal erbium/ytterbium silicate microplates with erbium concentration as high as 5 × 1021 cm-3 are controlled prepared by a chemical vapor deposition method. They can produce strong luminescence with quite a long lifetime (2.3 ms) at the wavelength of 1.5 µm. By embedding the erbium silicate microplates between two dielectric Bragg reflectors, we construct a vertical cavity surface-emitting laser at 1.5 µm, with a lasing threshold as low as 20 µJ/cm2 and Q factor of nearly 2000. Our study provides a new pathway to achieve a sub-micrometer coherent light source for optical communication.

Localized state effect and exciton dynamics for monolayer WS2.

The two-dimensional transition metal dichalcogenides (TMDCs) have been considered as promising candidates for developing a new generation of optoelectronic devices. Accordingly, investigations of exciton dynamics are of great importance for understanding the physics and the performance of devices based on TMDCs. Herein, after exposure to ambient environment for six months, monolayer tungsten disulfide (WS2) shows formation of localized states. Photoluminescence (PL) and time-resolved PL (TRPL) spectra demonstrate that these localized states have significant impacts on the exciton dynamics, including energy states filling, thermal activation and redistribution, and the decay behavior of excitons. These observations not only enrich the understanding for localized states and correlated exciton dynamics of aged monolayer WS2, but also reveal a possible approach to modulate the optical properties of TMDCs via the aging process.

Wavelength-Tunable Interlayer Exciton Emission at the Near-Infrared Region in van der Waals Semiconductor Heterostructures.

The wavelength-tunable interlayer exciton (IE) from layered semiconductor materials has not been achieved. van der Waals heterobilayers constructed using single-layer transition metal dichalcogenides can produce continuously changed interlayer band gaps, which is a feasible approach to achieve tunable IEs. In this work, we design a series of van der Waals heterostructures composed of a WSe2 layer with a fixed band gap and another WS2(1-x)Se2x alloy layer with continuously changed band gaps. The existence of IEs and tunable interlayer band gaps in these heterobilayers is verified by steady-state photoluminescence experiments. By tuning the composition of the WS2(1-x)Se2x alloy layers, we realized a very wide tunable band gap range of 1.97-1.40 eV with a wavelength-tunable IE emission range of 1.52-1.40 eV from the heterobilayers. The time-resolved photoluminescence experiments show the IE emission lifetimes over nanoseconds.

Probing and Manipulating Carrier Interlayer Diffusion in van der Waals Multilayer by Constructing Type-I Heterostructure.

van der Waals multilayer heterostructures have drawn increasing attention due to the potential for achieving high-performance photonic and optoelectronic devices. However, the carrier interlayer transportation behavior in multilayer structures, which is essential for determining the device performance, remains unrevealed. Here, we report a general strategy for studying and manipulating the carrier interlayer transportation in van der Waals multilayers by constructing type-I heterostructures, with a desired narrower bandgap monolayer acting as a carrier extraction layer. For heterostructures comprised of multilayer PbI2 and monolayer WS2, we find similar interlayer diffusion coefficients of ∼0.039 and ∼0.032 cm2 s-1 for electrons and holes in the PbI2 multilayer by fitting the time-resolved carrier dynamics based on the diffusion model. Because of the balanced carrier interlayer diffusion and the injection process at the heterointerface, the photoluminescence emission of the bottom WS2 monolayer is greatly enhanced by up to 106-fold at an optimized PbI2 thickness of the heterostructure. Our results provide valuable information on carrier interlayer transportation in van der Waals multilayer structures and pave the way for utilizing carrier behaviors to improve device performances.

WO3-WS2 Vertical Bilayer Heterostructures with High Photoluminescence Quantum Yield.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive for applications in a wide range of optoelectronic devices, due to their tremendous interesting physical properties. However, the photoluminescence quantum yield (PLQY) of TMDCs has been found to be too low, due to abundant defects and strong many-body effect. Here, we present a direct physical vapor growth of WO3-WS2 bilayer heterostructures, with WO3 monolayer domains attached on the surface of large-size WS2 monolayers. Optical characterizations revealed that the PLQY of the as-grown WO3-WS2 heterostructures can reach up to 11.6%, which is 2 orders of magnitude higher than that of WS2 monolayers by the physical vapor deposition growth method (PVD-WS2) and about 13-times higher than that of mechanical exfoliated WS2 (ME-WS2) monolayers, representing the highest PLQY reported for direct growth TMDCs materials so far. The PL enhancement mechanism has been well investigated by time-resolved optical measurements. The fabrication of WO3-WS2 heterostructures with ultrahigh PLQY provides an efficient approach for the development of highly efficient 2D integrated photonic applications.

Second-harmonic generation in single CdSe nanowires by focused cylindrical vector beams.

Cylindrical vector beams with radial or azimuthal polarization have created great interest due to their unique focusing characteristics and focal components. In this Letter, we investigate second-harmonic general (SHG) of single CdSe nanowires (NWs) excited by tightly focused cylindrical vector beams of 150 fs pulses at 800 nm. With the specific polarizations in the focal region, we demonstrate a three-dimensional interaction between the focal electric field components and the NWs. The excitation anisotropy of the SHG can directly be derived from the imaging patterns with the cylindrical vector beams. The highest SHG excitation efficiency is observed when the polarization is parallel to the long axis of the NW, which is confirmed by the conventional linear polarization approach. Our work with cylindrical vector beams provides a new approach to study the nonlinear phenomenon of single semiconductor NWs in three dimensions and it could be applied to many other nanoscale systems.

Second harmonic generation and waveguide properties in perovskite Na0.5Bi0.5TiO3 nanowires.

Nanowires with nonlinear optical properties such as second harmonic generation (SHG) are essential elements for an all-optical integrated photonic circuit. However, the existing materials face challenges for applications in a wide wavelength range. To cope with the challenges, ferroelectric nanowires are considered promising candidates, especially for SHG applications. In this Letter, we study SHG and waveguide properties in perovskite Na0.5Bi0.5TiO3 (NBT) nanowires. Strong SHG is observed in NBT nanowires illuminated by a 1064 nm laser radiation. For the waveguide studies, these NBT nanowires show a waveguide propagation loss as low as 0.01 dB/µm at 532 nm. This work suggests potential applications in future integrated optics with NBT nanowires.

Synthesis and optoelectronic properties of quaternary GaInAsSb alloy nanosheets.

Quasi-one-dimensional (1D) nanostructures have been extensively explored for electronic and optoelectronic devices on account of their unique morphologies and versatile physical properties. Here, we report the successful synthesis of GaInAsSb alloy nanosheets by a simple chemical vapor deposition method. The grown GaInAsSb alloy nanosheets are pure zinc-blende single crystals, which show nanosize-induced extraordinary optoelectronic properties as compared with bulk materials. µ-Raman spectra exhibit a multi-mode phonon vibration behavior with clear frequency shifts under varied laser power. Photoluminescence measurements reveal a strong light emission in the near-infrared region (1985 nm), and the obtained Varshni thermal coefficients α and ß are smaller than those of the bulk counterparts due to the size confinement effect. In addition, photodetectors (PDs) based on these single-alloy nanosheets were constructed for the first time. The PDs show a strong response in the near-infrared region with the external quantum efficiency of 8.05 × 104%, and the responsivity of 0.675 × 103 A W-1. These novel nanostructures would make contributions to the study of fundamental physical phenomena in quasi-1D nanomaterial systems and can be potential building blocks for optoelectronic and quantum devices.

Lateral Growth of Composition Graded Atomic Layer MoS(2(1-x))Se(2x) Nanosheets.

Band gap engineering of transition-metal dichalcogenides is an important task for their applications in photonics, optoelectronics, and nanoelectronics. We report for the first time the continuous lateral growth of composition graded bilayer MoS(2(1-x))Se(2x) alloys along single triangular nanosheets by an improved chemical vapor deposition approach. From the center to the edge of the nanosheet, the composition can be gradually tuned from x = 0 (pure MoS2) to x = 0.68, leading to the corresponding bandgap being continuously modulated from 1.82 eV (680 nm) to 1.64 eV (755 nm). Local photoluminescence scanning from the center to the edge gives single band edge emission peaks, indicating high crystalline quality for the achieved alloy nanosheets, which was further demonstrated by the microstructure characterizations. These novel 2D structures offer an interesting system for probing the physical properties of layered materials and exploring new applications in functional nanoelectronic and optoelectronic devices.

High Gain Submicrometer Optical Amplifier at Near-Infrared Communication Band.

Nanoscale near-infrared optical amplification is important but remains a challenge to achieve. Here we report a unique design of silicon and erbium silicate core-shell nanowires for high gain submicrometer optical amplification in the near-infrared communication band. The high refraction index silicon core is used to tightly confine the optical field within the submicron structures, and the single crystalline erbium-ytterbium silicates shell is used as the highly efficient gain medium. Both theoretical and experimental results show that, by systematically tuning the core diameter and shell thickness, a large portion of the optical power can be selectively confined to the erbium silicate shell gain medium to enable a low loss waveguide and high gain optical amplifier. Experimental results further demonstrate that an optimized core-shell nanowire can exhibit an excellent net gain up to 31 dB mm(-1), which is more than 20 times larger than the previously reported best results on the micron-scale optical amplifiers.

Room-temperature near-infrared photodetectors based on single heterojunction nanowires.

Nanoscale near-infrared photodetectors are attractive for their potential applications in integrated optoelectronic devices. Here we report the synthesis of GaSb/GaInSb p-n heterojunction semiconductor nanowires for the first time through a controllable chemical vapor deposition (CVD) route. Based on these nanowires, room-temperature, high-performance, near-infrared photodetectors were constructed. The fabricated devices show excellent light response in the infrared optical communication region (1.55 µm), with an external quantum efficiency of 10(4), a responsivity of 10(3) A/W, and a short response time of 2 ms, which shows promising potential applications in integrated photonics and optoelectronics devices or systems.

Growth of alloy MoS(2x)Se2(1-x) nanosheets with fully tunable chemical compositions and optical properties.

Band gap engineering of atomically thin two-dimensional layered materials is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, a simple one-step chemical vapor deposition approach for the simultaneous growth of alloy MoS2xSe2(1-x) triangular nanosheets with complete composition tunability. Both the Raman and the photoluminescence studies show tunable optical properties consistent with composition of the alloy nanosheets. Importantly, all samples show a single bandedge emission peak, with the spectral peak position shifting from 668 nm (for pure MoS2) to 795 nm (for pure MoSe2), indicating the high quality for these complete composition alloy nanosheets. These band gap engineered 2D structures could open up an exciting opportunity for probing their fundamental physical properties in 2D and may find diverse applications in functional electronic/optoelectronic devices.

The electric dipole moment of cobalt monoxide, CoO.

A number of low-rotational lines of the E(4)Δ7/2 ← X(4)Δ7/2 (1,0) band system of cobalt monoxide, CoO, were recorded field free and in the presence of a static electric field. The magnetic hyperfine parameter, h7/2, and the electron quadrupole parameter, eQq0, for the E(4)Δ7/2(υ = 1) state were optimized from the analysis of the field-free spectrum. The permanent electric dipole moment, µ(→)(el), for the X(4)Δ7/2 (υ = 0) and E(4)Δ7/2 (υ = 1) states were determined to be 4.18 ± 0.05 D and 3.28 ± 0.05 D, respectively, from the analysis of the observed Stark spectra of F' = 7 ← F″ = 6 branch feature in the Q(7/2) line and the F' = 8 ← F″ = 7 branch feature in the R(7/2) line. The measured dipole moments of CoO are compared to those from theoretical predictions and the trend across the 3d-metal monoxide series discussed.

Low-threshold nanowire laser based on composition-symmetric semiconductor nanowires.

Low-threshold nanoscale lasers are attractive for their promising applications in highly integrated photonic devices and systems. Here we report the controllable growth of composition-symmetric CdS(x)Se(1-x) nanowires by using a multistep thermal evaporation route with moving sources. Microstructure analyses reveal the obtained wires are high-quality single crystals with the composition gradually changed from the center toward their both ends. Under laser illumination, these wires exhibit symmetrical color distribution along the length direction, with red at the center and green at the both ends. Optically pumped lasing is realized at room temperature using these composition-symmetric nanowires, with the threshold several times lower than that of composition-homogeneous wires. This new nanowire structure will have potential applications as low-threshold nanoscale lasers in integrated nanophotonics.

570 Wh kg⁻1-Grade Lithium Metal Pouch Cell with 4.9V Highly Li+ Conductive Armor-Like Cathode Electrolyte Interphase via Partially Fluorinated Electrolyte Engineering.

Lithium-rich manganese-based layered oxides (LRMOs) are promisingly used in high-energy lithium metal pouch cells due to high specific capacity/working voltage. However, the interfacial stability of LRMOs remains challenging. To address this question, a novel armor-like cathode electrolyte interphase (CEI) model is proposed for stabilizing LRMO cathode at 4.9 V by exploring partially fluorinated electrolyte formulation. The fluoroethylene carbonate (FEC) and tris (trimethylsilyl) borate (TMSB) in formulated electrolyte largely contribute to the formation of 4.9 V armor-like CEI with LiBxOy and LixPOyFz outer layer and LiF- and Li3PO4-rich inner part. Such CEI effectively inhibits lattice oxygen loss and facilitates the Li+ migration smoothly for guaranteeing LRMO cathode to deliver superior cycling and rate performance. As expected, Li||LRMO batteries with such electrolyte achieve capacity retention of 85.7% with high average Coulomb efficiency (CE) of 99.64% after 300 cycles at 4.8 V/0.5 C, and even obtain capacity retention of 87.4% after 100 cycles at higher cut-off voltage of 4.9 V. Meanwhile, the 9 Ah-class Li||LRMO pouch cells with formulated electrolyte show over thirty-eight stable cycling life with high energy density of 576 Wh kg-1 at 4.8 V.

Facet-selective growth of halide perovskite/2D semiconductor van der Waals heterostructures for improved optical gain and lasing.

The tunable properties of halide perovskite/two dimensional (2D) semiconductor mixed-dimensional van der Waals heterostructures offer high flexibility for innovating optoelectronic and photonic devices. However, the general and robust growth of high-quality monocrystalline halide perovskite/2D semiconductor heterostructures with attractive optical properties has remained challenging. Here, we demonstrate a universal van der Waals heteroepitaxy strategy to synthesize a library of facet-specific single-crystalline halide perovskite/2D semiconductor (multi)heterostructures. The obtained heterostructures can be broadly tailored by selecting the coupling layer of interest, and can include perovskites varying from all-inorganic to organic-inorganic hybrid counterparts, individual transition metal dichalcogenides or 2D heterojunctions. The CsPbI2Br/WSe2 heterostructures demonstrate ultrahigh optical gain coefficient, reduced gain threshold and prolonged gain lifetime, which are attributed to the reduced energetic disorder. Accordingly, the self-organized halide perovskite/2D semiconductor heterostructure lasers show highly reproducible single-mode lasing with largely reduced lasing threshold and improved stability. Our findings provide a high-quality and versatile material platform for probing unique optoelectronic and photonic physics and developing further electrically driven on-chip lasers, nanophotonic devices and electronic-photonic integrated systems.

Modulated exciton-plasmon interactions in Au-SiO2-CdTe composite nanoparticles.

Well-defined Au-SiO(2)-CdTe composite nanoparticles were synthesized via a multistep chemical approach in water solution to gain insight into the interaction between metal and semiconductor nanostructures. Photoluminescence measurement reveals that the fluorescence of CdTe quantum dots (QDs) in this composite with optimized SiO(2) thickness (4 nm) has over ten times enhancement compared with that of bare CdTe QDs. The considerable fluorescence enhancement of CdTe QDs is attributed to the surface plasmon resonance, which is further confirmed by the lifetime measurement. The enhanced fluorescence can be used to improve the performance of CdTe QDs as fluorescence probe and may find potential applications in biolabeling.

Bandgap broadly tunable GaZnSeAs alloy nanowires.

Composition-tunable semiconductor alloy nanowires are emerging as an important class of materials for the realization of high-performance laterally-arranged multiple bandgap (LAMB) solar cells. Here we report the first growth of GaZnSeAs quaternary alloy nanowires with composed elements between two different groups using a temperature/space-selective CVD route. Under laser excitation, these special quaternary alloy nanowires exhibit composition-related characteristic emissions, with peak wavelengths gradually tunable from 470 nm (2.64 eV) to 832 nm (1.49 eV), covering almost the entire visible spectrum. Surface photovoltage measurements further reveal that these alloy nanowires have tunable bandgaps along the length of the substrate, making them promising candidates for developing high-efficiency LAMB solar cells. These quaternary alloy nanowires represent a new advancement in material synthesis and would have potential applications in a variety of function-tunable and broadband-response optoelectronic devices.

Wavelength-converted/selective waveguiding based on composition-graded semiconductor nanowires.

Compact wavelength-sensitive optical components are desirable for optical information processing and communication in photonic integrated system. In this work, optical waveguiding along single composition-graded CdS(x)Se(1-x) nanowires were systematically investigated. Under a focused laser excitation, the excited light can be guided passively along the bandgap-increased direction of the nanowire, keeping the photonic energy of the guided light almost unchanged during the whole propagation. In comparison, the excited light is guided actively through incessantly repeated band-to-band reabsorption and re-emitting processes along the bandgap-decreased direction, resulting in a gradual wavelength conversion during propagation. On the basis of this wavelength-converted waveguiding, a concept of nanoscale wavelength splitter is demonstrated by assembling a graded nanowire with several composition-uniform nanowires into branched nanowire structure. Our study indicates that composition-graded semiconductor nanowires would open new exciting opportunities in developing new wavelength-sensitive optical components for integrated nanophotonic devices.