Phase-Controlled Growth of CuInS2 Shells to Realize Colloidal CuInSe2/CuInS2 Core/Shell Nanostructures.

Synthetic routes to deposit CuInS2 (CIS) shells with either a cubic chalcopyrite (CP) or a hexagonal wurtzite (WZ) phase on trigonal pyramidal-shaped CuInSe2 (CISe) core nanocrystals (NCs) with a cubic CP crystal structure have been developed and governed by tuning the amount of the sulfur precursor tert-dodecanethiol. During the synthesis of CP-CIS/CP-CISe core/shell NCs, the CP-CIS shell initially starts to grow epitaxially in a uniform way, while the further addition of the CIS precursor induces islandlike growth, and finally a branched CIS shell is formed. In a stark contrast, when a WZ-CIS shell is deposited, it initially grows on a portion of each of the facets of the trigonal pyramidal-shaped CISe cores to form a monolayer, which then continues to increase in thickness and forms a multilayered WZ-CIS shell. Both CP-CISe/CP-CIS core/shell NCs and CP-CISe/WZ-CISe core/shell NCs exhibit rather low photoluminescence quantum yields (<10%), even with a smaller-sized CISe core, which calls for further refinements of the shell growth methods. Synthetic methods for the growth of CIS shells as described here allow for direct deposition of cadmium-free ternary compounds as shell materials and provide important insights into the different modes of growth of heterostructured NCs, ranging from epitaxial to island- and branched-like, as well to the facet-specific multilayer deposition.

Broad-Band Photodetectors Based on Copper Indium Diselenide Quantum Dots in a Methylammonium Lead Iodide Perovskite Matrix.

Low-temperature solution-processed methylammonium lead iodide (MAPbI3) crystalline films have shown outstanding performance in optoelectronic devices. However, their high dark current and high noise equivalent power prevent their application in broad-band photodetectors. Here, we applied a facile solution-based antisolvent strategy to fabricate a hybrid structure of CuInSe2 quantum dots (CISe QDs) embedded into a MAPbI3 matrix, which not only enhances the photodetector responsivity, showing a large on/off ratio of 104 at 2 V bias compared with the bare perovskite films, but also significantly (for over 7 days) improves the device stability, with hydrophobic ligands on the CuInSe2 QDs acting as a barrier against the uptake of environmental moisture. MAPbI3/CISe QD-based lateral photodetectors exhibit high responsivities of >0.5 A/W and 10.4 mA/W in the visible and near-infrared regions, respectively, partly because of the formation of a type II interface between the respective semiconductors but most significantly because of the efficient trap-state passivation of the perovskite grain surfaces, and the reduction in the twinning-induced trap density, which stems from both CISe QDs and their organic ligands. A large specific detectivity of 2.2 × 1012 Jones at 525 nm illumination (1 µW/cm2), a fast fall time of 236 µs, and an extremely low noise equivalent power of 45 fW/Hz1/2 have been achieved.

Lead Halide Perovskite Nanostructures for Dynamic Color Display.

Nanoprint-based color display using either extrinsic structural colors or intrinsic emission colors is a rapidly emerging research field for high-density information storage. Nevertheless, advanced applications, e. g., dynamic full-color display and secure information encryption, call for demanding requirements on in situ color change, nonvacuum operation, prompt response, and favorable reusability. By transplanting the concept of electrical/chemical doping in the semiconductor industry, we demonstrate an in situ reversible color nanoprinting paradigm via photon doping, triggered by the interplay of structural colors and photon emission of lead halide perovskite gratings. It solves the aforementioned challenges at one go. By controlling the pumping light, the synergy between interlaced mechanisms enables color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment. Our design presents a promising realization of in situ dynamic color nanoprinting and will empower the advances in structural color and classified nanoprinting.

Tailoring the Performances of Lead Halide Perovskite Devices with Electron-Beam Irradiation.

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10-20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro-LED with well-preserved optical properties and improves the photocurrent of photodetector.

Miscellaneous Lasing Actions in Organo-Lead Halide Perovskite Films.

Lasing actions in organo-lead halide perovskite films have been heavily studied in the past few years. However, due to the disordered nature of synthesized perovskite films, the lasing actions are usually understood as random lasers that are formed by multiple scattering. Herein, we demonstrate the miscellaneous lasing actions in organo-lead halide perovskite films. In addition to the random lasers, we show that a single or a few perovskite microparticles can generate laser emissions with their internal resonances instead of multiple scattering among them. We experimentally observed and numerically confirmed whispering gallery (WG)-like microlasers in polygon shaped and other deformed microparticles. Meanwhile, owing to the nature of total internal reflection and the novel shape of the nanoparticle, the size of the perovskite WG laser can be significantly decreased to a few hundred nanometers. Thus, wavelength-scale lead halide perovskite lasers were realized for the first time. All of these laser behaviors are complementary to typical random lasers in perovskite film and will help the understanding of lasing actions in complex lead halide perovskite systems.

All-Dielectric Full-Color Printing with TiO2 Metasurfaces.

Recently, color generation in resonant nanostructures have been intensively studied. Despite of their exciting progresses, the structural colors are usually generated by the plasmonic resonances of metallic nanoparticles. Due to the inherent plasmon damping, such plasmonic nanostructures are usually hard to create very distinct color impressions. Here we utilize the concept of metasurfaces to produce all-dielectric, low-loss, and high-resolution structural colors. We have fabricated TiO2 metasurfaces with electron-beam lithography and a very simple lift-off process. The optical characterizations showed that the TiO2 metasurfaces with different unit sizes could generate high reflection peaks at designed wavelengths. The maximal reflectance was as high as 64% with full width at half-maximum (fwhm) around 30 nm. Consequently, distinct colors have been observed in bright field and the generated colors covered the entire visible spectral range. The detailed numerical analysis shows that the distinct colors were generated by the electric resonance and magnetic resonances in TiO2 metasurfaces. Based on the unique properties of magnetic resonances, distinct colors have been observed in bright field when the metasurfaces were reduced to a 4 × 4 array, giving a spatial resolution around 16000 dpi. Considering the cost, stability, and CMOS-compatibility, this research will be important for the structural colors to reach real-world industrial applications.

Coupling the normal incident light into waveguide modes of DBR mirrors via a diffraction grating.

Here we numerically and experimentally demonstrate the conversion of normally incident light into the guiding modes of distributed Bragg reflector (DBRs) mirror. By fabricating a gold grating onto a 7.5 pairs TiO2/SiO2 DBR mirror, a series of asymmetrical resonances have been formed at the bandgap range of the DBR mirror. The detailed numerical calculations show that these Fano resonances are attributed to the coupling of incident waves into guiding modes of the DBR mirror. Compared with the other resonances, this coupling mechanism can be simply realized and it has also been revealed to be quite robust to the environmental changes, making the conversion between propagating waves and guiding waves to be practically interesting for many applications.