2D Inorganic Bimolecular Crystals with Strong In-Plane Anisotropy for Second-Order Nonlinear Optics

Quantifying Structural Polarization by Continuous Regulation of Lone-Pair Electron Expression in Molecular Crystals

Rational design of structural polarization is vital for modern technologies, as it allows the physical properties of functional materials to be tailored. An effective approach for governing polarization involves the utilization of stereochemical lone-pair electrons (LPEs). However, despite the recognized significance of LPEs in controlling structural polarization, there remains a lack of understanding regarding the quantitative relationship between their expression and the extent of structural polarization. Here, by using pressure to continuously tune the LPE expression, we achieve the precise control and quantification of structural polarization, which brings enhanced second harmonic generation (SHG) of the molecular crystal SbI3·3S8. We introduce the I-Sb-I angle (α̅) that describes the degree of LPE expression and establishes a quantitative relationship between α̅ and structural polarization. That is, decreasing α̅ shapes LPE expression from delocalization to localization, which repels the bonding pairs of electrons and thus enhances the structural polarization. In addition, we extend this quantified relationship to a series of molecular crystals and demonstrate its applicability to the design of structural polarization by tailoring LPE expression.

Emerging van der Waals Dielectrics of Inorganic Molecular Crystals for 2D Electronics

In the landscape of continuous downscaling metal-oxide-semiconductor field-effect transistors, two-dimensional (2D) semiconductors with atomic thinness emerge as promising channel materials for ultimate scaled devices. However, integrating compatible dielectrics with 2D semiconductors, particularly in a scalable way, remains a critical challenge that hinders the development of 2D devices. Recently, 2D inorganic molecular crystals (IMCs), which are free of dangling bonds and possess excellent dielectric properties and simplicity for scalable fabrication, have emerged as alternatives for gate dielectric integration in 2D devices. In this Perspective, we start with the introduction of structure and synthesis methods of IMCs and then discuss the explorations of using IMCs as the dielectrics, as well as some remaining relevant issues to be unraveled. Moreover, we look at the future opportunities of IMC dielectrics in 2D devices both for practical applications and fundamental research.

SbI3·3S8: A Novel Promising Inorganic Adducts Crystal for Second Harmonic Generation

In the past twenty years, the basic investigation of innovative Non-Linear Optical (NLO) crystals has received significant attention, which has built the crucial heritage for the use of NLO materials. Fundamental research is essential given the scarcity of materials for NLO compounds, especially in the deep ultraviolet (DUV) and middle- and far-infrared (MFIR) regions. In the present work, we synthesized high-quality MFIR SbI₃·3S₈ NLO crystals having a length in the range of 1-5 mm through rapid facile liquid phase ultrasonic reaction followed by the assistance of instantaneous natural evaporation phenomenon of the solvent at room temperature. X-ray diffraction (XRD) results ratify the hexagonal R3m structure of SbI₃·3S₈ crystal, and energy-dispersive X-ray spectroscopy (EDX) demonstrates that the elemental composition of SbI₃·3S₈ crystal is similar to that of its theoretical composition. The direct and indirect forbidden energy gaps of SbI₃·3S₈ were measured from the optical transmittance spectra and they were shown to be 2.893 eV and 1.986 eV, respectively. The green sparkling signal has been observed from the crystal during the second harmonic generation (SHG) experiment. Therefore, as inorganic adducts are often explored as NLO crystals, this work on the MFIR SbI₃·3S₈ NLO crystal can bring about additional investigations on this hot topic in the near future.

Strong Anisotropic Two-Dimensional In2Se3 for Light Intensity and Polarization Dual-Mode High-Performance Detection

Detecting the light from different freedom is of great significance to gain more information. Two-dimensional (2D) materials with low intrinsic carrier concentration and highly tunable electronic structure have been considered as the promising candidate for future room-temperature multi-functional photodetectors. However, current investigations mainly focus on intensity-sensitive detection; the multi-dimensional photodetection such as polarization-sensitive photodetection is still in its early stage. Herein, the intensity- and polarization-sensitive photodetection based on α-In₂Se₃ is studied. By using angle-resolved polarized Raman spectroscopy, it is demonstrated that α-In₂Se₃ shows an anisotropic phonon vibration property indicating its asymmetric structure. The α-In₂Se₃-based photodetector has a photoelectric performance with a responsivity of 1936 A/W and a specific detectivity of 2.1 × 10¹³ Jones under 0.2 mW/cm² power density at 400 nm. Moreover, by studying the polarized angle-resolved photoelectrical effect, it is found that the ratio of maximum and minimum photocurrent (dichroic ratio) reaches 1.47 at 650 nm suggesting good polarization-sensitive detection. After post-annealing, α-In₂Se₃ in situ converts to β-In₂Se₃ which has similar in-plane anisotropic crystallinity and exhibits a dichroic ratio of 1.41. It is found that the responsivity of β-In₂Se₃ is 6 A/W, much lower than that of α-In₂Se₃. The high-performance light intensity- and polarization-detection of α-In₂Se₃ enlarges the 2D anisotropic materials family and provides new opportunities for future dual-mode photodetection.

Chiral Lead-Free Double Perovskite Single-Crystalline Microwire Arrays for Anisotropic Second-Harmonic Generation

Halide double perovskites present a new branch for versatile optoelectronic devices because of their huge structural compatibility and environmental friendliness, whereas nonlinear optics (NLO) devices remain blank for this fascinating family. Simultaneously, the precise patterning of single-crystalline perovskite microwire arrays remains a challenge for the integration of NLO devices. Herein, we designed lead-free chiral 2D double perovskites with the nonsymmetrical structure presenting second-harmonic generation (SHG). Furthermore, perovskite single-crystalline arrays with regulated geometry, pure orientation, and high crystallinity are prepared using the capillary-bridge confined assembly technique. The efficient SHG originates from the asymmetric crystal structure and high crystallinity of the microwire arrays. Compared with their polycrystalline thin-film counterparts, linearly polarized SHG and a higher SHG conversion efficiency are demonstrated based on microwire arrays. The results not only expand the applications of lead-free double perovskites in the NLO-integrated fields but also provide a viable way for lead-free optoelectronic devices.

Type-I Heterostructure Based on WS2/PtS2 for High-Performance Photodetectors

van der Waals (vdW) heterodiodes composed of two-dimensional (2D) layered materials led to a new prospect in photoelectron diodes and photovoltaic devices. Existing studies have shown that Type-I heterostructures have great potential to be used as photodetectors; however, the tunneling phenomena in Type-I heterostructures have not been fully revealed. Herein, a highly efficient nn⁺ WS₂/PtS₂ Type-I vdW heterostructure photodiode is constructed. The device shows an ultrahigh reverse rectification ratio of 10⁵ owing to the transmission barrier-induced low reverse current. A unilateral depletion region is formed on WS₂, which inhibits the recombination of carriers at the interface and makes the external quantum efficiency (EQE) of the device reach 67%. Due to the tunneling mechanism of the device, which allows the co-existence of a large photocurrent and a low dark current, this device achieves a light on/off ratio of over 10⁵. In addition, this band design allows the device to maintain a high detectivity of 4.53 × 10¹⁰ Jones. Our work provides some new ideas for exploring new high-efficiency photodiodes.

Strong In-Plane Optical and Electrical Anisotropies of Multilayered γ-InSe for High-Responsivity Polarization-Sensitive Photodetectors

Recently, identifying promising new two-dimensional (2D) materials with low-symmetry structures has aroused great interest for developing monolithic polarization-sensitive photodetectors with small volume. Here, after comprehensive research of the in-plane anisotropic structure and electronic and optoelectronic properties of layered γ-InSe, a superior responsivity polarization-sensitive photodetector based on multilayer γ-InSe is constructed by a facile method. Notably, the conductance and carrier mobility of the device along the armchair direction are 11.8 and 2.35 times larger than those along the zigzag direction, respectively. Benefitting from the high efficiency of light absorption and excellent carrier mobility (221 cm² V^-1 s^-1) of our multilayered γ-InSe along the armchair direction, the device exhibits a superior responsivity of 127 A/W and an external quantum efficiency (EQE) of 10⁴%. Especially, the highest responsivity along the armchair direction of our γ-InSe polarization-sensitive photodetectors can reach as high as 78.5 A/W under polarized light. This value is much higher than those of other devices even under unpolarized light. This work not only provides an insight into the in-plane anisotropic properties of 2D layered γ-InSe but also proposes a stable and environmentally friendly candidate for anisotropic optoelectronic applications.

Analyzing Anisotropy in 2D Rhenium Disulfide Using Dichromatic Polarized Reflectance

In-plane anisotropy in 2D rhenium disulfide (ReS₂ ) offers intriguing opportunities for designing future electronic and optical devices, and toward such applications, it is crucial to identify the crystal orientation in such 2D anisotropic materials. Existing spectroscopy or electron microscopy methods for determining the crystalline orientation often require complicated sample preparing procedures and specialized equipment, which could sometimes limit their application. In this work, a dichromatic polarized reflectance method is demonstrated, which can quickly and accurately resolve the crystal orientation (Re-Re chain) in 2D ReS₂ crystals with different thicknesses. Furthermore, it can be readily extended to multi-chromatic schemes to achieve greater measurement capability and can be easily tailored to work for different 2D materials. The method offers a simple and effective approach for studying anisotropy in 2D materials.

Emerging Two-Dimensional Inorganic Molecular Crystals: The Concept and Beyond

The concept of two-dimensional (2D) inorganic molecular crystals (IMCs) was first introduced by Zhai and coauthors in 2019. In contrast to the layered structures of graphene-like 2D materials, 2D IMCs consist of tiny inorganic molecules bonded together through all-around van der Waals (vdW) interactions. Their structural peculiarities lead to some special behaviors and appealing properties in their synthesis and applications. In this Perspective, we first introduce the concept of 2D IMCs and present the very first synthesis of 2D IMCs using a surface-passivated growth approach. The special intermolecular effects between the inorganic molecules are also summarized. In addition, because of its molecular structure, a vdW film of IMCs can be facilely fabricated, which exhibits appealing potential in integrated 2D devices. More importantly, we give a general outlook for the further development of 2D IMCs with the goal of attracting more attention to this emerging research frontier.

Scalable Van der Waals Encapsulation by Inorganic Molecular Crystals

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂ O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

A Universal Atomic Substitution Conversion Strategy Towards Synthesis of Large-Size Ultrathin Nonlayered Two-Dimensional Materials

Nonlayered two-dimensional (2D) materials have attracted increasing attention, due to novel physical properties, unique surface structure, and high compatibility with microfabrication technique. However, owing to the inherent strong covalent bonds, the direct synthesis of 2D planar structure from nonlayered materials, especially for the realization of large-size ultrathin 2D nonlayered materials, is still a huge challenge. Here, a general atomic substitution conversion strategy is proposed to synthesize large-size, ultrathin nonlayered 2D materials. Taking nonlayered CdS as a typical example, large-size ultrathin nonlayered CdS single-crystalline flakes are successfully achieved via a facile low-temperature chemical sulfurization method, where pre-grown layered CdI₂ flakes are employed as the precursor via a simple hot plate assisted vertical vapor deposition method. The size and thickness of CdS flakes can be controlled by the CdI₂ precursor. The growth mechanism is ascribed to the chemical substitution reaction from I to S atoms between CdI₂ and CdS, which has been evidenced by experiments and theoretical calculations. The atomic substitution conversion strategy demonstrates that the existing 2D layered materials can serve as the precursor for difficult-to-synthesize nonlayered 2D materials, providing a bridge between layered and nonlayered materials, meanwhile realizing the fabrication of large-size ultrathin nonlayered 2D materials.

Broken-Gap PtS2/WSe2 van der Waals Heterojunction with Ultrahigh Reverse Rectification and Fast Photoresponse

Broken-gap van der Waals (vdW) heterojunctions based on 2D materials are promising structures to fabricate high-speed switching and low-power multifunctional devices thanks to its charge transport versus quantum tunneling mechanism. However, the tunneling current is usually generated under both positive and negative bias voltage, resulting in small rectification and photocurrent on/off ratio. In this paper, we report a broken-gap vdW heterojunction PtS₂/WSe₂ with a bilateral accumulation region design and a big band offset by utilizing thick PtS₂ as an effective carrier-selective contact, which exhibits an ultrahigh reverser rectification ratio approaching 10⁸ and on/off ratio over 10⁸ at room temperature. We also find excellent photodetection properties in such a heterodiode with a large photocurrent on/off ratio over 10⁵ due to its ultralow forward current and a comparable photodetectivity of 3.8 × 10¹⁰ Jones. In addition, the response time of such a photodetector reaches 8 μs owing to the photoinduced tunneling mechanism and reduced interface trapping effect. The proposed heterojunction not only demonstrates the high-performance broken-gap heterodiode but also provides in-depth understanding of the tunneling mechanism in the development of future electronic and optoelectronic applications.

2D Rare Earth Material (EuOCl) with Ultra-Narrow Photoluminescence at Room Temperature

High color purity and color rendition of 2D luminescent materials have long been pursued for applications in low-dimensional lighting, display, biolabeling, and laser. However, the reported photoluminescence (PL) linewidth of most 2D luminescent materials is about dozens of meV. Herein, a brand-new luminescent system of 2D rare earth (RE) material EuOCl (1.1 nm) with ultra-narrow linewidth (1.2 meV) at room temperature is successfully synthesized via chemical vapor deposition (CVD). The linewidth of EuOCl flakes at room temperature is even narrower than most 2D luminescent materials and heterostructures detected at below 10 K. Impressively, the as-synthesized EuOCl flakes show abnormal temperature-dependent photoluminescent properties, which is absolutely different from the relatively stable 4f-4f transitions in RE owing to shielding from outer shell electrons. J-mixing effect has been successfully applied for this phenomenon. Undoubtedly, luminescent 2D EuOCl flakes will open new territory for the applications of 2D RE materials in the 2D luminescent areas, especially for the applications at room temperature.

Water Triggered Synthesis of Highly Stable and Biocompatible 1D Nanowire, 2D Nanoplatelet, and 3D Nanocube CsPbBr3 Perovskites for Multicolor Two-Photon Cell Imaging

Two-photon imaging in the near-infrared window holds huge promise for real life biological imaging due to the increased penetration depth. All-inorganic CsPbX3 nanocrystals with bright luminescence and broad spectral tunability are excellent smart probes for two-photon bioimaging. But, the poor stability in water is a well-documented issue for limiting their practical use. Herein, we present the development of specific antibody attached water-resistant one-dimensional (1D) CsPbBr3 nanowires, two-dimensional (2D) CsPbBr3 nanoplatelets, and three-dimensional (3D) CsPbBr3 nanocubes which can be used for selective and simultaneous two-photon imaging of heterogeneous breast cancer cells in the near IR biological window. The current manuscript reports the design of excellent photoluminescence quantum yield (PLQY), biocompatible and photostable 1D CsPbBr3 nanowires, 2D CsPbBr3 nanoplatelets, and 3D CsPbBr3 nanocubes through an interfacial conversion from zero-dimensional (0D) Cs4PbBr6 nanocrystals via a water triggered strategy. Reported data show that just by varying the amount of water, one can control the dimension of CsPbBr3 perovskite crystals. Time-dependent transition electron microscopy and emission spectra have been reported to find the possible pathway for the formation of 1D, 2D, and 3D CsPbBr3 nanocrystals from 0D Cs4PbBr6 nanocrystals. Biocompatible 1D, 2D, and 3D CsPbBr3 nanocrystals were developed by coating with amine-poly(ethylene glycol)-propionic acid. Experimental data show the water-driven design of 1D, 2D, and 3D CsPbBr3 nanocrystals exhibits strong single-photon PLQY of ∼66-88% as well as excellent two-photon absorption properties (σ2) of ∼8.3 × 105-7.1 × 104 GM. Furthermore, reported data show more than 86% of PL intensity remains for 1D, 2D, and 3D CsPbBr3 nanocrystals after 35 days under water, and they exhibit excellent photostability of keeping 99% PL intensity after 3 h under UV light. The current report demonstrates for the first time that antibody attached 1D and 2D perovskites have capability for simultaneous two-photon imaging of triple negative breast cancer cells and human epidermal growth factor receptor 2 positive breast cancer cells. CsPbBr3 nanocrystals exhibit very high two-photon absorption cross-section and good photostability in water, which are superior to those of commonly used organic probes (σ2 = 11 GM for fluorescein), and therefore, they have capability to be a better probe for bioimaging applications.

2D van der Waals Molecular Crystal β-HgI2 : Economical, Rapid, and Substrate-Free Liquid-Phase Synthesis and Strong In-Plane Optical Anisotropy

2D materials have a great potential for wide-range applications due to their adjustable bandgap characteristics and special crystal structures. β-HgI₂ is a new 2D van der Waals inorganic molecular crystal material with a wide bandgap of 4.03 eV, on whose preparation and properties there are few relevant reports due to the feature of instability of molecular crystals. Here, an economical method to control the synthesis of large-size 2D β-HgI₂ single crystal by using a mineralizer-assisted solution is reported. According to angle-resolved polarization Raman spectroscopy and first-principles optical absorption calculation, 2D β-HgI₂ flake has a strong in-plane anisotropic light scattering characteristic and high optical absorption dichroism (a_z /a_y  = 3.4), which is due to a low in-plane symmetry of the orthorhombic structure of β-HgI₂ . More importantly, due to the molecular crystal structure of β-HgI₂ , its sensitivity to temperature is less than that of 2D materials such as MoS₂ , which has been confirmed by temperature-dependent Raman spectroscopy. In the work, more 2D inorganic molecular crystals are studied in the aspect of growth, which provides a theoretical basis for 2D molecular crystal optoelectronic devices' potential applications.