Organic Ligand Engineering for Tailoring Electron-Phonon Coupling in 2D Hybrid Perovskites.

In the emerging two-dimensional organic-inorganic hybrid perovskites, the electronic structures and carrier behaviors are strongly impacted by intrinsic electron-phonon interactions, which have received inadequate attention. In this study, we report an intriguing phenomenon of negative carrier diffusion induced by electron-phonon coupling in (2T)2PbI4. Theoretical calculations reveal that the electron-phonon coupling drives the band alignment in (2T)2PbI4 to alternate between type I and type II heterostructures. As a consequence, photoexcited holes undergo transitions between the organic ligands and inorganic layers, resulting in abnormal carrier transport behavior compared to other two-dimensional hybrid perovskites. These findings provide valuable insights into the role of electron-phonon coupling in shaping the band alignments and carrier behaviors in two-dimensional hybrid perovskites. They also open up exciting avenues for designing and fabricating functional semiconductor heterostructures with tailored properties.

Strain regulates the photovoltaic performance of thick-film perovskites.

Perovskite photovoltaics, typically based on a solution-processed perovskite layer with a film thickness of a few hundred nanometres, have emerged as a leading thin-film photovoltaic technology. Nevertheless, many critical issues pose challenges to its commercialization progress, including industrial compatibility, stability, scalability and reliability. A thicker perovskite film on a scale of micrometres could mitigate these issues. However, the efficiencies of thick-film perovskite cells lag behind those with nanometre film thickness. With the mechanism remaining elusive, the community has long been under the impression that the limiting factor lies in the short carrier lifetime as a result of defects. Here, by constructing a perovskite system with extraordinarily long carrier lifetime, we rule out the restrictions of carrier lifetime on the device performance. Through this, we unveil the critical role of the ignored lattice strain in thick films. Our results provide insights into the factors limiting the performance of thick-film perovskite devices.

Adaptive nanotube networks enabling omnidirectionally deformable electro-driven liquid crystal elastomers towards artificial muscles.

Artificial muscles that can convert electrical energy into mechanical energy promise broad scientific and technological applications. However, existing electro-driven artificial muscles have been plagued with problems that hinder their practical applications: large electro-mechanical attenuation during deformation, high-driving voltages, small actuation strain, and low power density. Here, we design and create novel electro-thermal-driven artificial muscles rationally composited by hierarchically structured carbon nanotube (HS-CNT) networks and liquid crystal elastomers (LCEs), which possess adaptive sandwiched nanotube networks with angulated-scissor-like microstructures, thus effectively addressing above problems. These HS-CNT/LCE artificial muscles demonstrate not only large strain (>40%), but also remarkable conductive robustness (R/R0 < 1.03 under actuation), excellent Joule heating efficiency (≈ 233 °C at 4 V), and high load-bearing capacity (R/R0 < 1.15 at 4000 times its weight loaded). In addition, our artificial muscles exhibit real-muscle-like morphing intelligence that enables preventing mechanical damage in response to excessively heavyweight loading. These high-performance artificial muscles uniquely combining omnidirectional stretchability, robust electrothermal actuation, low driving voltage, and powerful mechanical output would exert significant technological impacts on engineering applications such as soft robotics and wearable flexible electronics.

Patterned growth of two-dimensional atomic layer semiconductors.

Transition metal dichalcogenides (TMDCs), which are representative of two-dimensional (2D) semiconductors, have attracted tremendous attention over the last two decades. TMDCs are regarded as potential candidates in modern nano- and optoelectronic applications due to their unique crystal structures and outstanding electronic and optoelectronic properties. For practical use, 2D semiconductors need to be fabricated with diverse morphologies for integration into electronic devices and to perform different functionalities. Controlled patterning synthesis with programmable geometries is therefore highly desired. We review state-of-the-art strategies for the patterned growth of atomic layer TMDCs and their heterostructures, including additive manufacturing and subtractive manufacturing for patterning single TMDC materials and the introduction of other low-dimensional nanomaterials as growth templates or hetero-atoms for element conversion in patterning TMDC heterostructures. The optoelectronic and electronic applications of the as-grown monolayer TMDC patterns are introduced. Future challenges and the prospects for the patterned growth of 2D semiconductors are discussed based on present achievements.

Giant room-temperature nonlinearities in a monolayer Janus topological semiconductor.

Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials-the Janus transition metal dichalcogenides in the 1 T' phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T' MoSSe (e.g., > 50 times higher than 2H MoS2 for 18th order harmonic generation; > 20 times higher than 2H MoS2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.

Thickness control of organic semiconductor-incorporated perovskites.

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent. The length of the conjugated semiconducting organic cations and the dimensionality (n value) of the inorganic layers can be manipulated at the same time. The energy band offsets and exciton dynamics at the organic-inorganic interfaces can therefore be precisely controlled. Furthermore, we show that longer and more planar π-conjugated organic cations induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and better optoelectronic properties as compared to conventional two-dimensional perovskites. As a demonstration, optically driven lasing behaviour with substantially lower lasing thresholds was realized.

Phase-pure 2D tin halide perovskite thin flakes for stable lasing.

Ruddlesden-Popper tin halide perovskites are a class of two-dimensional (2D) semiconductors with exceptional optoelectronic properties, high carrier mobility, and low toxicity. However, the synthesis of phase-pure 2D tin perovskites is still challenging, and the fundamental understanding of their optoelectronic properties is deficient compared to their lead counterparts. Here, we report the synthesis of a series of 2D tin perovskite bulk crystals with high phase purity via a mixed-solvent strategy. By engineering the quantum-well thickness (related to n value) and organic ligands, the optoelectronic properties, including photoluminescence emission, exciton-phonon coupling strength, and exciton binding energy, exhibit a wide tunability. In addition, these 2D tin perovskites exhibited excellent lasing performance. Both high-n value tin perovskite (n > 1) and n = 1 tin perovskite thin flakes were successfully optically pumped to lase. Furthermore, the lasing from 2D tin perovskites could be maintained up to room temperature. Our findings highlight the tremendous potential of 2D tin perovskites as promising candidates for high-performance lasers.

Synthesis of Ultralong Carbon Nanotubes with Ultrahigh Yields.

Ultralong carbon nanotubes (CNTs) are in huge demand in many cutting-edge fields due to their macroscale lengths, perfect structures, and extraordinary properties, while their practical application is limited by the difficulties in their mass production. Herein, we report the synthesis of ultralong CNTs with a dramatically increased yield by a simple but efficient substrate interception and direction strategy (SIDS), which couples the advantages of floating-catalyst chemical vapor deposition with the flying-kite-like growth mechanism of ultralong CNTs. The SIDS-assisted approach prominently improves the catalyst utilization and significantly increases the yield. The areal density of the ultralong CNT arrays with length of over 1 cm reached a record-breaking value of ∼6700 CNTs mm-1, which is 2-3 orders of magnitude higher than the previously reported values obtained by traditional methods. The SIDS provides a solution for synthesizing high-quality ultralong CNTs with high yields, laying the foundation for their mass production.

Reconfiguring band-edge states and charge distribution of organic semiconductor-incorporated 2D perovskites via pressure gating.

Two-dimensional (2D) semiconductor heterostructures are key building blocks for many electronic and optoelectronic devices. Reconfiguring the band-edge states and modulating their interplay with charge carriers at the interface in a continuous manner have long been sought yet are challenging. Here, using organic semiconductor-incorporated 2D halide perovskites as the model system, we realize the manipulation of band-edge states and charge distribution via mechanical-rather than chemical or thermal-regulation. Compression induces band-alignment switching and charge redistribution due to the different pressure responses of organic and inorganic building blocks, giving controllable emission properties of 2D perovskites. We propose and demonstrate a "pressure gating" strategy that enables the control of multiple emission states within a single material. We also reveal that band-alignment transition at the organic-inorganic interface is intrinsically not well resolved at room temperature owing to the thermally activated transfer and shuffling of band-edge carriers. This work provides important fundamental insights into the energetics and carrier dynamics of hybrid semiconductor heterostructures.

Graphene protection improves the stability of two-dimensional halide perovskites under the electron irradiation.

The crystal structure of two-dimensional (2D) organic-inorganic halide perovskites undergoes fast structural collapse under the electron beam irradiation, hindering high-resolution transmission electron microscopy imaging. Graphene protection is an effective solution to mitigate the damage of electron-beam irradiation and has been applied in 2D materials such as MoS2 . However, the effectivity of graphene protection has not been demonstrated in 2D halide perovskites yet, as traditional wet-transfer of graphene with aqueous solution would cause serious degradation for moisture-sensitive halide perovskites. Here, we verified that graphene protection plays a protection role and developed a method using nonpolar solvent to transfer the graphene layer atop the perovskite nanosheets. With this method, the perovskite nanosheets might be well protected by graphene encapsulation. HIGHLIGHTS: Transfer method of graphene on moisture-sensitive 2D halide perovskites using nonpolar solvents was developed. Graphene substrate is proven to be able to mitigate electron-beam damage to 2D halide perovskites. Encapsulation structure of graphene/halide perovskite/graphene was demonstrated.

Soft-lock drawing of super-aligned carbon nanotube bundles for nanometre electrical contacts.

The assembly of single-walled carbon nanotubes (CNTs) into high-density horizontal arrays is strongly desired for practical applications, but challenges remain despite myriads of research efforts. Herein, we developed a non-destructive soft-lock drawing method to achieve ultraclean single-walled CNT arrays with a very high degree of alignment (angle standard deviation of ~0.03°). These arrays contained a large portion of nanometre-sized CNT bundles, yielding a high packing density (~400 µm-1) and high current carrying capacity (∼1.8 × 108 A cm-2). This alignment strategy can be generally extended to diverse substrates or sources of raw single-walled CNTs. Significantly, the assembled CNT bundles were used as nanometre electrical contacts of high-density monolayer molybdenum disulfide (MoS2) transistors, exhibiting high current density (~38 µA µm-1), low contact resistance (~1.6 kΩ µm), excellent device-to-device uniformity and highly reduced device areas (0.06 µm2 per device), demonstrating their potential for future electronic devices and advanced integration technologies.

Designing artificial two-dimensional landscapes via atomic-layer substitution.

Technology advancements in history have often been propelled by material innovations. In recent years, two-dimensional (2D) materials have attracted substantial interest as an ideal platform to construct atomic-level material architectures. In this work, we design a reaction pathway steered in a very different energy landscape, in contrast to typical thermal chemical vapor deposition method in high temperature, to enable room-temperature atomic-layer substitution (RT-ALS). First-principle calculations elucidate how the RT-ALS process is overall exothermic in energy and only has a small reaction barrier, facilitating the reaction to occur at room temperature. As a result, a variety of Janus monolayer transition metal dichalcogenides with vertical dipole could be universally realized. In particular, the RT-ALS strategy can be combined with lithography and flip-transfer to enable programmable in-plane multiheterostructures with different out-of-plane crystal symmetry and electric polarization. Various characterizations have confirmed the fidelity of the precise single atomic layer conversion. Our approach for designing an artificial 2D landscape at selective locations of a single layer of atoms can lead to unique electronic, photonic, and mechanical properties previously not found in nature. This opens a new paradigm for future material design, enabling structures and properties for unexplored territories.

Layer-by-layer anionic diffusion in two-dimensional halide perovskite vertical heterostructures.

Anionic diffusion in a soft crystal lattice of hybrid halide perovskites affects their stability, optoelectronic properties and the resulting device performance. The use of two-dimensional (2D) halide perovskites improves the chemical stability of perovskites and suppresses the intrinsic anionic diffusion in solid-state devices. Based on this strategy, devices with an enhanced stability and reduced hysteresis have been achieved. However, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Here we demonstrate the first quantitative investigation of the anionic interdiffusion across atomically flat 2D vertical heterojunctions. Interestingly, the halide diffusion does not follow the classical diffusion process. Instead, a 'quantized' layer-by-layer diffusion model is proposed to describe the behaviour of the anionic migration in 2D halide perovskites. Our results provide important insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform with an enhanced stability for heterostructure integration.

Two-dimensional halide perovskite lateral epitaxial heterostructures.

Epitaxial heterostructures based on oxide perovskites and III-V, II-VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics1-7. Halide perovskites-an emerging family of tunable semiconductors with desirable properties-are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers8-15. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration16,17. Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility, which leads to interdiffusion and large junction widths18-21, and owing to their poor chemical stability, which leads to decomposition of prior layers during the fabrication of subsequent layers. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance22-26. Here we report an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. We demonstrate highly stable and tunable lateral epitaxial heterostructures, multiheterostructures and superlattices. Near-atomically sharp interfaces and epitaxial growth are revealed by low-dose aberration-corrected high-resolution transmission electron microscopy. Molecular dynamics simulations confirm the reduced heterostructure disorder and larger vacancy formation energies of the two-dimensional perovskites in the presence of conjugated ligands. These findings provide insights into the immobilization and stabilization of halide perovskite semiconductors and demonstrate a materials platform for complex and molecularly thin superlattices, devices and integrated circuits.

Large-Scale Plasmonic Hybrid Framework with Built-In Nanohole Array as Multifunctional Optical Sensing Platforms.

Light coupling with patterned subwavelength hole arrays induces enhanced transmission supported by the strong surface plasmon mode. In this work, a nanostructured plasmonic framework with vertically built-in nanohole arrays at deep-subwavelength scale (6 nm) is demonstrated using a two-step fabrication method. The nanohole arrays are formed first by the growth of a high-quality two-phase (i.e., Au-TiN) vertically aligned nanocomposite template, followed by selective wet-etching of the metal (Au). Such a plasmonic nanohole film owns high epitaxial quality with large surface coverage and the structure can be tailored as either fully etched or half-way etched nanoholes via careful control of the etching process. The chemically inert and plasmonic TiN plays a role in maintaining sharp hole boundary and preventing lattice distortion. Optical properties such as enhanced transmittance and anisotropic dielectric function in the visible regime are demonstrated. Numerical simulation suggests an extended surface plasmon mode and strong field enhancement at the hole edges. Two demonstrations, including the enhanced and modulated photoluminescence by surface coupling with 2D perovskite nanoplates and the refractive index sensing by infiltrating immersion liquids, suggest the great potential of such plasmonic nanohole array for reusable surface plasmon-enhanced sensing applications.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites.

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm2 s-1, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals. These measurements demonstrate the charge transfer time around 10 ps and long-lived charge-separated state over the nanosecond time scale in two-dimensional ligand-perovskite heterostructures. The efficient charge transfer processes coupled with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in two-dimensional perovskite solar cells.

Molecular engineering of organic-inorganic hybrid perovskites quantum wells.

Semiconductor quantum-well structures and superlattices are key building blocks in modern optoelectronics, but it is difficult to simultaneously realize defect-free epitaxial growth while fine tuning the chemical composition, layer thickness and band structure of each layer to achieve the desired performance. Here we demonstrate the modulation of the electronic structure-and consequently the optical properties-of organic semiconducting building blocks that are incorporated between the layers of perovskites through a facile solution processing step. Self-aggregation of the conjugated organic molecules is suppressed by functionalization with sterically demanding groups and single crystalline organic-perovskite hybrid quantum wells (down to one-unit-cell thick) are obtained. The energy and charge transfers between adjacent organic and inorganic layers are shown to be fast and efficient, owing to the atomically flat interface and ultrasmall interlayer distance of the perovskite materials. The resulting two-dimensional hybrid perovskites are very stable due to protection given by the bulky hydrophobic organic groups.