Organic-inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic-inorganic Ruddelsden-Popper perovskites (RPPs) of (BA)(MA) PbI ( n = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic-inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media.

This work presents an ultrahigh gain InSebased photodetector by using a novel approach called the surface oxidation doping (SOD) technique. The carrier concentration of multilayered two-dimensional (2D) InSe semiconductor surface has been modulated by controlling the formation of a surface oxide layer. The SOD through surface charge transfer at the interface of the oxide/2D InSe semiconductor heterostructure can lead to the creation of a vertical built-in potential and band bending as a result of the carrier concentration distribution gradient. The internal electric field caused by the formation of a carrier concentration gradient in InSe layers can facilitate charge separation of photogenerated electron−hole pairs under light illumination. Consequently, the record high photoresponsivities of InSe-based photodetector with ∼5 × 10 6 A/W at the excitation wavelength of 365 nm and 5 × 10 5 A/W at the wavelength of 530 nm can be obtained, outperforming the majority of photodetectors based on other 2D materials, such as graphene, MoS 2 , and even highly sensitive multilayer GaTe and In 2 Se 3 flakes. The approach based on SOD induced efficient photogenerated charge separation can be also applied to other 2D layered semiconductors.

binding energies of electron-hole pairs, [1] and long carrier diffusion lengths. [2] Most state-of-the-art perovskite solar cells typically consist of either mesoporous [3] or planar heterojunction device architectures. [4] The mesoporous perovskite solar cells consist of a device structure that is based on a mesoporous (or mesosuperstructured) metal oxide scaffold (for example, TiO 2 or Al 2 O 3 ) and the organometal halide perovskites are then infiltrated into the mesoporous metal oxides as light harvesting materials. [3,5] In parallel to the mesoporous architecture, the planar heterojunction perovskite solar cells have a simple device structure with the perovskite films sandwiched between electron and hole transporting layers. [5b] For high-performance perovskite solar cells consisting of either mesoporous or planar heterojunction device architectures, electron transporting layers (ETLs) and hole transporting layers (HTLs) are usually used to extract both photogenerated electrons and holes in the perovskite light absorber layers toward two opposite electrodes. In addition, the appropriate design of the interfacial ETLs and HTLs with well-matched energy levels between electrodes and perovskite light absorbers is known to be responsible to maximize the built-in potential and the open circuit voltage of devices. [6] A novel atomic stacking transporting layer (ASTL) based on 2D atomic sheets of titania (Ti 1−δ O 2 ) is demonstrated in organic-inorganic lead halide perovskite solar cells. The atomically thin ASTL of 2D titania, which is fabricated using a solution-processed self-assembly atomic layer-by-layer deposition technique, exhibits the unique features of high UV transparency and negligible (or very low) oxygen vacancies, making it a promising electron transporting material in the development of stable and high-performance perovskite solar cells. In particular, the solution-processable atomically thin ASTL of 2D titania atomic sheets shows superior inhibition of UV degradation of perovskite solar cell devices, compared to the conventional high-temperature sintered TiO 2 counterpart, which usually causes the notorious instability of devices under UV irradiation. The discovery opens up a new dimension to utilize the 2D layered materials with a great variety of homostructrual or heterostructural atomic stacking architectures to be integrated with the fabrication of large-area photovoltaic or optoelectronic devices based on the solution processes.

of more than 20% in the last few years. [1,2] Hybrid lead halide perov skites typically have a 3D ABX 3 (where, A = CH 3 NH 3 + (MA), B = Pb, and X = Cl, Br, and I) crystal structure con sisting of [PbI 6 ] and MA + units in their lattice framework and their band gaps can be tuned by halide engineering. [3][4][5] However, the key challenge in commer cializing 3D organic-inorganic hybrid perovskite photovoltaics is related to well known issues with longterm instability of devices under ambient conditions. [6,7] Recently, another class of 2D organicinorganic hybrid perovskite counterparts have attracted attention owing to their superior ambient stability and prom ising optoelectronic properties. [8][9][10][11][12][13][14] There is now considerable interest in 2D hybrid perovskite compounds, typi cally represented by the generic for mula (Aʹ) 2 (MA) n−1 M n X 3n+1 , where Aʹ is a longchain organic spacer, MA is small organic cation, M is a divalent metal, and n is the number of perovskite layers per unit cell. [15,16] These 2D hybrid halide perovskites exhibit a characteristic quantum well (QW)like structure owing to the selfassembled periodic array of perovskite [PbI 6 ] This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA) 2 (MA) n−1 Pb n I 3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10 −3 , 8.3 × 10 −2 , and 1.25 cm 2 V −1 s −1 at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals. Field Effect Transistors

The development of optical memory with attractive features such as long-lasting, nonvolatile, high-speed, and low-energy consumption is vitally important in the information age. Owing to these advantages, optical memory has been popular for more 10 years. Recently, flexibility has become desirable for the application of wearable devices and smart artificial intelligence; for conventional optical memory, this is still difficult to achieve. To combine optical memory with soft materials, this study presents a flexible and photoelectronic switchable multilevel memory device with long-lasting nonvolatile properties. On the basis of the integration of nanoscale (graphene nanoflakes) and macroscale graphene heterojunctions, a device achieves switchable memory states up to 196 distinct levels under the illumination of lasers with different wavelengths. The photoelectronic memory device can be written optically and erased by both optical and electric methods. Additionally, the device possesses several unique features including a low working bias of 0.5 V, nonvolatility for over 10 000 s, and mechanical stability for more than 10 000 bending cycles. Notably, in previous studies, polymers with poor mobility were used as a conducting channel, which can greatly limit the amplitude of the light-induced switching ratio and electrical performance. In stark contrast, in our device, the graphene layer with the mobility exceeding several orders of magnitude was used to serve as a conducting channel, enabling one to overcome the existing shortcoming. Our approach therefore not only provides an alternative paradigm for the development of photoelectronic memory but also holds great promise for practical applications due to its compatibility with current technologies.

A large enhancement of color-conversion efficiency of colloidal quantum dots in light-emitting diodes (LEDs) with novel structures of nanorods embedded in microholes has been demonstrated. Via the integration of nano-imprint and photolithography technologies, nanorods structures can be fabricated at specific locations, generating functional nanostructured LEDs for high-efficiency performance. With the novel structured LED, the color-conversion efficiency of the existing quantum dots can be enhanced by up to 32.4%. The underlying mechanisms can be attributed to the enhanced light extraction and non-radiative energy transfer, characterized by conducting a series of electroluminescence and time-resolved photoluminescence measurements. This hybrid nanostructured device therefore exhibits a great potential for the application of multi-color lighting sources.

Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boosting light extraction efficiency of LEDs with an enhancement of 78% but also collimating the view angle of LEDs from 131.5゜to 114.0゜. This could be because the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs points the way towards a promising avenue of solid-state lighting.

This work demonstrates the direct visualization of atomically resolved quantum-confined electronic structures at organic−inorganic heterointerfaces of twodimensional (2D) organic−inorganic hybrid Ruddlesden−Popper perovskites (RPPs); this is accomplished with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) by using solvent engineering to prepare perpendicularly oriented 2D RPPs. Atomically resolved band mapping images across the organic−inorganic interfaces of 2D RPPs yield typical quantum-well-like type-I heterojunction band alignment with band gaps depending on the thicknesses or n values of the inorganic perovskite slabs. The presence of edge states within the band gap due to organic cation vacancies is also observed. In addition, real-space visualization of atomic-scale structural phase transition behavior and changes in local electronic band structures are obtained simultaneously. Our results provide an unequivocal observation and explanation of the quantumconfined electronic structures formed at organic−inorganic interfaces of 2D RPPs.