Perovskite Quantum Dots Lasing in Double-Heterostructure through Energy Transfer.

Planar double heterostructures were initially investigated and have been successfully applied in III-V semiconductor lasers due to their excellent roles in confining both the photons and carriers. Here, we design and fabricate a (PEA)2Csn-1PbnX3n+1 (quasi-2D)/CsPbBr3 QD/quasi-2D double-heterostructure sandwiched in a 3/2 λ DBR microcavity, and then demonstrate a single-mode pure-green lasing with a threshold of 53.7 µJ/cm2 under nanosecond-pulsed optical pumping. The thresholds of these heterostructure devices decrease statistically by about 50% compared to the control group with no energy donor layers, PMMA/QD/PMMA in an identical microcavity. We show that there is efficient energy transfer from the barrier regions of the quasi-2D phases to the QD layer by transient absorption and luminescence lifetime spectra and that such energy transfer leads to marked threshold reduction. This work indicates that the double-heterostructure configurations should play a significant role in the future perovskite electrically pumped laser.

Large-area large-grain CsPbCl3perovskite films by confined re-growth for violet photodetectors.

CsPbCl3perovskite is an attractive semiconductor material with characteristics such as a wide bandgap, high chemical stability, and excellent optoelectronic properties, which broaden its application prospects for ultraviolet (UV) and violet photodetectors (PDs). However, large-area CsPbCl3films with high coverage, large grains, and controllable thickness are still difficult to prepare by using the solution method due to the extremely low solubility of their precursors in conventional solvents. Herein, a water-assisted confined re-growth method is developed, and a CsPbCl3microcrystalline film with an area of 3 cm × 3 cm is grown, the thickness of which is controllable within a range of several microns. The as-prepared thin film exhibits a flat and smooth surface, large grains, and enhanced photoluminescence. Furthermore, the fabricated violet PDs based on the prepared CsPbCl3film show a high responsivity of 2.17 A W-1, external quantum efficiency of 664%, on/off ratio of 2.58 × 103, and good stability. This study provides a prospective solution for the growth of large-area, large-grain, and surface-smooth CsPbCl3films for high-performance UV and violet PDs.

Growth of hopper-shaped CsPbCl3 crystals and their exciton polariton emission.

CsPbCl3 is an attractive wide-bandgap perovskite semiconductor. Herein, we have grown hopper-shaped CsPbCl3 crystals in a solution droplet dripped on a heated substrate. During the growth, we have observed the impacts of the coffee ring effect and Marangoni flow, which may result in the hopper shape. Their photoluminescence spectra feature double peaks, which are located at 413.9 nm and 422.0 nm, respectively, and the latter increases faster in intensity than the former as the excitation power increases. We believe that the higher-energy peak originates from the excitonic emission and the lower-energy one is from the polaritons' emission, where the polaritons are generated in the exciton-exciton inelastic scattering process. Based on such an explanation, the exciton binding energy of CsPbCl3 is estimated to be 76.7 meV in our experiments, consistent with the previous reports.

Centimeter-scale 2D perovskite (PEA)2PbBr4 single crystal plates grown by a seeded solution method for photodetectors.

Large-sized single-crystal two-dimensional (2D) perovskites are highly desirable owing to their fundamental properties and intriguing ability to boost devices. Herein, 2-phenylethylammonium lead bromide [(PEA)2PbBr4] single crystals, which are a violet-light-emitting 2D perovskite material, with typical lateral sizes of about one centimeter were successfully grown using a seeded solution method. The single-crystal plates showed a well-defined shape (rectangle or hexagon), a natural thickness (300-500 µm) similar to that of conventional silicon and InP wafers, a large aspect ratio of ∼20, and a smooth surface (root mean square, ∼0.7 nm). We integrated these single crystal plates into an ultraviolet photodetector, achieving a low dark current of ∼10-13 A and an efficient photoresponse (on/off ratio, ∼103). This experiment could easily be extended to grow freestanding 2D perovskite single crystals on a wafer scale for practical integrated optoelectronics.

Low-threshold room-temperature continuous-wave optical lasing of single-crystalline perovskite in a distributed reflector microcavity.

Organic-inorganic halide perovskites have achieved remarkable success in various optoelectronic devices. A high-quality CH3NH3PbBr3 single-crystalline thin film has been directly grown in a micrometer gap between a pair of distributed reflectors with over 99.9% reflectivity, which naturally form a vertical cavity surface-emitting laser device with a single mode or several modes. The single-crystalline perovskite has an exciton lifetime of 426 ns and evidence of the exciton-photon coupling is observed. At room temperature and under continuous-wave optical pumping conditions, this device lases at a threshold of 34 mW cm-2 in the green gap. The extremely low lasing threshold suggests that polariton lasing may occur in the strongly confined optical cavity comprising the high-quality single-crystalline perovskite.

Acetone vapour-assisted growth of 2D single-crystalline organic lead halide perovskite microplates and their temperature-enhanced photoluminescence.

We adopt an acetone vapour-assisted method to grow high quality single-crystalline microplates of two-dimensional (2D) perovskite, 2-phenylethylammonium lead bromide [(C6H5C2H4NH3)2PbBr4]. The microplates, converted from the spin-coated films, are well-defined rectangles. Temperature dependent photoluminescence (PL) spectroscopy shows that the band gap PL is enhanced markedly with increasing temperature up to 218 K, accompanied by the quenching of the PL related to the trap states, which perhaps results from the exciton-phonon couplings. The optical phonon energy around 50 meV and the exciton binding energy around 120 meV are derived by fitting the band gap PL linewidths and intensities at different temperatures, respectively.

Epitaxial Single-Layer MoS2 on GaN with Enhanced Valley Helicity.

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12-time increased valley spitting in single-layer WSe2 through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band-to-band tunnel field-effect transistors with a subthreshold swing below 60 mV dec-1 at room temperature based on bilayer n-MoS2 and heavily doped p-germanium, etc. Here, it is demonstrated that epitaxially grown single-layer MoS2 on a lattice-matched GaN substrate, possessing a type-I band alignment, exhibits strong substrate-induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron-phonon interaction, resulting in a short exciton lifetime in the MoS2 /GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady-state and time-resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley-optoelectronic device applications.

Hybrid single-mode laser based on graphene Bragg gratings on silicon.

We exploit distributed optoelectronic properties enabled by graphene Bragg gratings (GBGs) to realize a hybrid single-mode laser on silicon. This hybrid laser achieves single-mode, continuous-wave operation at 1540 nm with a remarkable side-mode suppression ratio of 48 dB, benefitting from the coupling of the GBGs. These results suggest that graphene thin films can be used as an essential and cost-saving component for hybrid photonic integration on silicon.

Fully Transparent Quantum Dot Light-Emitting Diode with a Laminated Top Graphene Anode.

A new method to employ graphene as top electrode was introduced, and based on that, fully transparent quantum dot light-emitting diodes (T-QLEDs) were successfully fabricated through a lamination process. We adopted the widely used wet transfer method to transfer bilayer graphene (BG) on polydimethylsiloxane/polyethylene terephthalate (PDMS/PET) substrate. The sheet resistance of graphene reduced to ∼540 Ω/□ through transferring BG for 3 times on the PDMS/PET. The T-QLED has an inverted device structure of glass/indium tin oxide (ITO)/ZnO nanoparticles/(CdSSe/ZnS quantum dots (QDs))/1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC)/MoO3/graphene/PDMS/PET. The graphene anode on PDMS/PET substrate can be directly laminated on the MoO3/TAPC/(CdSSe/ZnS QDs)/ZnO nanoparticles/ITO/glass, which relied on the van der Waals interaction between the graphene/PDMS and the MoO3. The transmittance of the T-QLED is 79.4% at its main electroluminescence peak wavelength of 622 nm.

Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure.

Graphene and other layered materials, such as transition metal dichalcogenides, have rapidly established themselves as exceptional building blocks for optoelectronic applications because of their unique properties and atomically thin nature. The ability to stack them into van der Waals (vdWs) heterostructures with new functionality has opened a new platform for fundamental research and device applications. Nevertheless, near-infrared (NIR) photodetectors based on layered semiconductors are rarely realized. In this work, we fabricate a graphene-MoTe2-graphene vertical vdWs heterostructure on a SiO2/p+-Si substrate by a facile and reliable site-controllable transfer method and apply it for photodetection from the visible to NIR wavelength range. Compared to the layered semiconductor photodetectors reported thus far, the graphene-MoTe2-graphene photodetector has a superior performance, including high photoresponsivity (∼110 mA W-1 at 1064 nm and 205 mA W-1 at 473 nm), high external quantum efficiency (EQE; ∼12.9% at 1064 nm and ∼53.8% at 473 nm), rapid response and recovery processes (a rise time of 24 µs and a fall time of 46 µs under 1064 nm illumination), and free from an external source-drain power supply. We have employed scanning photocurrent microscopy to investigate the photocurrent generation in this heterostructure under various back-gate voltages and found that the two Schottky barriers between the graphenes and MoTe2 play an important role in the photocurrent generation. In addition, the vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back-gate (p+-Si) voltage. We compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling next-generation high-responsivity, high-speed, flexible, and transparent NIR devices.

Efficient small molecular organic light emitting diode with graphene cathode covered by a Sm layer with nano-hollows and n-doped by Bphen:Cs2CO3 in the hollows.

Graphene is a favorable candidate for electrodes of organic light emitting diodes (OLEDs). Graphene has quite a high work function of ∼4.5 eV, and has been extensively studied when used as anodes of OLEDs. In order to use graphene as a cathode, the electron injection barrier between the graphene cathode and the electron transport layer has to be low enough. Using 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs2CO3 to n-dope graphene is a very good method, but the electron injection barrier between the n-doped graphene and Bphen:Cs2CO3 is still too high to be ∼1.0 eV. In this work, in order to further reduce the electron injection barrier, a novel method is suggested. On the graphene cathode, a Sm layer with a lot of nano-hollows, and subsequently a layer of Bphen:Cs2CO3, are deposited. The Bphen:Cs2CO3 can n-dope graphene in the nano-hollows, and the Fermi level of the graphene rises. The nano Sm layer is very easily oxidized. Oxygen adsorbed on the surface of graphene may react with Sm to form an O--Sm+ dipole layer. On the areas of the Sm oxide dipole layer without nano-hollows, the electron injection barrier can be further lowered by the dipole layer. Electrons tend to mainly inject through the lower electron barrier where the dipole layer exists. Based on this idea, an effective inverted small molecular OLED with the structure of graphene/1 nm Sm layer with a lot of nano-hollows/Bphen:Cs2CO3/Alq3:C545T/NPB/MoO3/Al is presented. The maximum current efficiency and maximum power efficiency of the OLED with a 1 nm Sm layer are about two and three times of those of the reference OLED without any Sm layer, respectively.

4-λ InGaAsP-Si distributed feedback evanescent lasers with varying silicon waveguide width.

A four-wavelength silicon hybrid laser array operating at room temperature is realized by evanescently coupling the optical gain of InGaAsP multi-quantum wells to the silicon waveguides of varying widths and patterned with distributed feedback gratings based on selective-area metal bonding technology. The lasers have emission peaks between 1539.9 and 1546.1 nm with a wavelength spacing of about 2.0 nm. The single laser has a typical threshold current of 50 mA and side-mode suppression ratio of 20 dB. The silicon waveguides are fabricated simply by standard photolithography and holographic lithography which are CMOS compatible.

[Effects of Ag nanocrystals on electroluminescence in Si oxide films].

Ag nanocrystal-embedded silicon oxide (SiO2 : Ag) films with varying Ag fractions were prepared on p-Si substrate by magnetron co-sputtering and thermal annealing. Visible electroluminescence (EL) was observed from the structures of ITO/SiO2 : Ag/p-Si. The authors found that Ag nanocrystals in the SiO2 film can not only shift the EL peak evidently but also enhance the EL intensity markedly. The larger the Ag fractions in the EL structures, the longer the peak wavelengths. The electromagnetic interactions of the Ag nanocrystals with the emitters in the film via local surface plasmons are considered responsible for these experimental results.

[Electroluminescence from a Mn2+ activated SiO2 : Si film on N(+)-Si substrate].

Recently, a monolithic integration of optics and electronics in a single Si chip has attracted a great deal of attention due to its attractive application prospects: the potential for forming high speeded information processing and transmission, and inexpensive and low power silicon chip. Developing high-efficiency silicon-based light sources is the main task in silicon photonics. In the present paper the authors explore a potential way for silicon-based light-emitting application. A Mn(2+)-activated silicon-rich silicon oxide (SiO2 : Si : Mn2+) film was prepared on the n(+)-type silicon substrate using co-sputtering technique followed by doping and activation of Mn with a thermal diffusion method. High-resolution transmission electronic microscope study shows that the film is embedded with 3-5 nm silicon nanocrystals. Bright green photoluminescence (PL) from the film was observed under ultraviolet radiation and peaked at 524 nm (2.36 eV), the decay time of which is 0.8 ms. It is generally believed that the green radiation originates from 4T1 --> 6A1 transition in Mn2+. The PL excitation spectrum of the film, monitored at 524 nm, has a peak of 254 nm, similar to that of the Zn2 SiO4 : Mn film. It is believed that the strong 254 nm absorption is attributed to Mn2+ --> Mn3+ ionization or ds --> d4s transition. A very broad electroluminescence spectrum ranging from 400 to 800 nm, covering almost the whole visible band, was observed from the device made of the SiO2 : Si : Mn2+ film at low reverse biases. The threshold voltage of the device is as low as 5 V. Spectra of the device demonstrate that the electroluminescence is attributed to Mn2+ and luminescence centers in the Si-rich SiO2 film. The authors interpret that Mn2+ excitation is mainly due to direct impact excitation of hot electrons, silicon nanocrystals in the SiO2 film help electrons tunnel from a silicon nanocrystal to an adjacent one, and are advantageous for generating hot electrons to excite Mn2+.