Optofluidic crystallithography for directed growth of single-crystalline halide perovskites.

Crystallization is a fundamental phenomenon which describes how the atomic building blocks such as atoms and molecules are arranged into ordered or quasi-ordered structure and form solid-state materials. While numerous studies have focused on the nucleation behavior, the precise and spatiotemporal control of growth kinetics, which dictates the defect density, the micromorphology, as well as the properties of the grown materials, remains elusive so far. Herein, we propose an optical strategy, termed optofluidic crystallithography (OCL), to solve this fundamental problem. Taking halide perovskites as an example, we use a laser beam to manipulate the molecular motion in the native precursor environment and create inhomogeneous spatial distribution of the molecular species. Harnessing the coordinated effect of laser-controlled local supersaturation and interfacial energy, we precisely steer the ionic reaction at the growth interface and directly print arbitrary single crystals of halide perovskites of high surface quality, crystallinity, and uniformity at a high printing speed of 102 µm s-1. The OCL technique can be potentially extended to the fabrication of single-crystal structures beyond halide perovskites, once crystallization can be triggered under the laser-directed local supersaturation.

Super-Resolution Exciton Imaging of Nanobubbles in 2D Semiconductors with Near-Field Nanophotoluminescence Microscopy.

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides, have emerged as important candidate materials for next-generation chip-scale optoelectronic devices with the development of large-scale production techniques, such as chemical vapor deposition (CVD). However, 2D materials need to be transferred to other target substrates after growth, during which various micro- and nanoscale defects, such as nanobubbles, are inevitably generated. These nanodefects not only influence the uniformity of 2D semiconductors but also may significantly alter the local optoelectronic properties of the composed devices. Hence, super-resolution discrimination and characterization of nanodefects are highly demanded. Here, we report a near-field nanophotoluminescence (nano-PL) microscope that can quickly screen nanobubbles and investigate their impact on local excitonic properties of 2D semiconductors by directly visualize the PL emission distribution with a very high spatial resolution of ∼10 nm, far below the optical diffraction limit, and a high speed of 10 ms/point under ambient conditions. By using nano-PL microscopy to map the exciton and trion emission intensity distributions in transferred CVD-grown monolayer tungsten disulfide (1L-WS2) flakes, it is found that the PL intensity decreases by 13.4% as the height of the nanobubble increases by every nanometer, which is mainly caused by the suppression of trion emission due to the strong doping effect from the substrate. In addition to the nanobubbles, other types of nanodefects, such as cracks, stacks, and grain boundaries, can also be characterized. The nano-PL method is proven to be a powerful tool for the nondestructive quality inspection of nanodefects as well as the super-resolution exploration of local optoelectronic properties of 2D materials.

Quantum Emitters with Narrow Band and High Debye-Waller Factor in Aluminum Nitride Written by Femtosecond Laser.

Solid-state quantum emitters (QEs) are central components for photonic-based quantum information processing. Recently, bright QEs in III-nitride semiconductors, such as aluminum nitride (AlN), have attracted increasing interest because of the mature commercial application of the nitrides. However, the reported QEs in AlN suffer from broad phonon side bands (PSBs) and low Debye-Waller factors. Meanwhile, there is also a need for more reliable fabrication methods of AlN QEs for integrated quantum photonics. Here, we demonstrate that laser-induced QEs in AlN exhibit robust emission with a strong zero phonon line, narrow line width, and weak PSB. The creation yield of a single QE could be more than 50%. More importantly, they have a high Debye-Waller factor (>65%) at room temperature, which is the highest result among reported AlN QEs. Our results illustrate the potential of laser writing to create high-quality QEs for quantum technologies and provide further insight into laser writing defects in relevant materials.

Module-Level Polaritonic Thermophotovoltaic Emitters via Hierarchical Sequential Learning.

Thermophotovoltaic (TPV) generators provide continuous and high-efficiency power output by utilizing local thermal emitters to convert energy from various sources to thermal radiation matching the bandgaps of photovoltaic cells. Lack of effective guidelines for thermal emission control at high temperatures, poor thermal stability, and limited fabrication scalability are the three key challenges for the practical deployment of TPV devices. Here we develop a hierarchical sequential-learning optimization framework and experimentally realize a 6″ module-scale polaritonic thermal emitter with bandwidth-controlled thermal emission as well as excellent thermal stability at 1473 K. The 300 nm bandwidth thermal emission is realized by a complex photon polariton based on the superposition of Tamm plasmon polariton and surface plasmon polariton. We experimentally achieve a spectral efficiency of 65.6% (wavelength range of 0.4-8 µm) with statistical deviation less than 4% over the 6″ emitter, demonstrating industrial-level reliability for module-scale TPV applications.

All-Optical Reconfigurable Excitonic Charge States in Monolayer MoS2.

Excitons are quasi-particles composed of electron-hole pairs through Coulomb interaction. Due to the atomic-thin thickness, they are tightly bound in monolayer transition metal dichalcogenides (TMDs) and dominate their optical properties. The capability to manipulate the excitonic behavior can significantly influence the photon emission or carrier transport performance of TMD-based devices. However, on-demand and region-selective manipulation of the excitonic states in a reversible manner remains challenging so far. Herein, harnessing the coordinated effect of femtosecond-laser-driven atomic defect generation, interfacial electron transfer, and surface molecular desorption/adsorption, we develop an all-optical approach to manipulate the charge states of excitons in monolayer molybdenum disulfide (MoS2). Through steering the laser beam, we demonstrate reconfigurable optical encoding of the excitonic charge states (between neutral and negative states) on a single MoS2 flake. Our technique can be extended to other TMDs materials, which will guide the design of all-optical and reconfigurable TMD-based optoelectronic and nanophotonic devices.

Optical Visualization of Photoexcitation Diffusion in All-Inorganic Perovskite at High Temperature.

All-inorganic halide perovskites are promising candidates for optoelectronic and photovoltaic devices because of their good thermal stability and remarkable optoelectronic properties. Among those properties, carrier transport properties are critical as they inherently dominate the device performance. The transport properties of perovskites have been widely studied at room and lower temperatures, but their high-temperature (i.e., tens of degrees above room temperature) characteristics are not fully understood. Here, the photoexcitation diffusion is optically visualized by transient photoluminescence microscopy (TPLM), through which the temperature-dependent transport characteristics from room temperature to 80 °C are studied in all-inorganic CsPbBr3 single-crystalline microplates. We reveal the decreasing trend of diffusion coefficient and the almost unchanged trend of diffusion length when heating the sample to high temperature. The phonon scattering in combination with the variation of effective mass is proposed for the explanation of the temperature-dependent diffusion behavior.