The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material

Pb6Ba3Si2S8I10: a new thiohalide with a quasi-two-dimensional structure and wide band gap

Pb-based chalcogenides display abundant structural diversity and distinguished properties. Based on a mixed anion and dimensional reduction combined strategy, a wide band gap Pb-based thiohalide, Pb6Ba3Si2S8I10, has been rationally designed and synthesized experimentally by the flux method. The compound crystallizes in the R3̄c space group with cell parameters a = 9.7925(2) Å, b = 9.7925(2) Å, and c = 70.628(3) Å and is composed of [SiS4] tetrahedra and unprecedented [PbI5S2] polyhedral units, resulting in a unique quasi-two-dimensional structure, which enriches the chemical and structural diversity of Pb-based thiohalides. The experimental band gap of Pb6Ba3Si2S8I10 was determined to be 2.80 eV. Based on statistical analyses and to the best of our knowledge, it is the largest experimental optical band gap among the known Pb-based thiohalides. The results demonstrate the feasibility of using highly electropositive Ba atoms to regulate the dimensions of the structural framework of thiohalides and give new insights into the structure and property modifications of thiohalides by the mixed anion and dimensional reduction combined strategy.

Structure-Prediction-Oriented Synthesis of Thiophosphates as Promising Infrared Nonlinear Optical Materials

Oriented synthesis of functional materials is a focus of attention in material science. As one of the most important function materials, infrared nonlinear optical materials with large second harmonic generation effects and broad optical band gap are in urgent need. In this work, directed by the theoretical structure prediction, the first series of non-centrosymmetric (NCS) alkali-alkaline earth metal [PS4]-based thiophosphates LiCaPS4 (Ama2), NaCaPS4 (P21), KCaPS4 (Pna21), RbCaPS4 (Pna21), CsCaPS4 (Pna21) were successfully synthesized. Comprehensive characterizations reveal that ACaPS4 could be regarded as promising IR NLO materials, exhibiting wide band gap (3.77-3.86 eV), moderate birefringence (0.027-0.064 at 1064 nm), high laser-induced damage threshold (LIDT, ~10×AGS), and suitable phase-matching second harmonic generation responses (0.4-0.6×AGS). Structure-properties analyses illustrate that the Ca-S bonds show non-ignorable covalent feature, and [PS4] together with [CaSn] units play dominant roles to determine the band gap and SHG response. This work indicates that Li-, Na- and K- analogs may be promising infrared nonlinear optical material candidates, and this is the first successful case of "prediction to synthesis" involving infrared (IR) nonlinear optical (NLO) crystals in the thiophosphate system and may provide a new avenue to the design and oriented synthesis of high-performance function materials in the future.

β-CsHg2I5, a compound with rare [Hg2I5] dimers and large optical anisotropy

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHg2I5 was reported. The high-temperature phase β-CsHg2I5 with rare [Hg2I5] dimers was synthesized by the flux method at 573 K, and it shows a reversible phase transition at a low temperature of ∼100 K to form the low-temperature phase α-CsHg2I5. The two phases crystallize in the same P21/c space group, with different crystal structures. β-CsHg2I5 is composed of rare [Hg2I5] dimers and [CsI11] polyhedral units, while α-CsHg2I5 is composed of [Hg4I11] and [CsI10] units. The experimental band gap of β-CsHg2I5 was found to be 2.58 eV. Owing to the presence of [Hg2I5]∞ pseudo-layers, β-CsHg2I5 exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β-CsHg2I5 is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [Hg2I5] dimer is a nonlinear active unit, which can be used as a new fundamental building block for the design of advanced nonlinear optical materials. Moreover, a CsI-HgI2 pseudo-binary diagram was drawn. The results enrich the structural diversity of Hg-based halides and give some insights into the development of new functional materials based on rare [Hg2I5] dimers.

Enhanced near-infrared optical transmission in zinc germanium phosphide crystals via precise magnesium doping

A zinc germanium phosphorus (ZnGeP2) crystal with a chalcopyrite structure is an efficient frequency converter in the mid-infrared region. However, point defect-induced optical absorption at the pumping wavelength (near infrared region) blocked the further application of ZnGeP2. To alleviate the absorption losses caused by point defects, in situ magnesium doping compensation was presented during the ZnGeP2 bulk crystal growth process via the vertical Bridgman method. Combined with theoretical calculations, the structural distortion of the magnesium-doped ZnGeP2 crystals in different orientations was illustrated. The thermodynamic and kinetic stability of the magnesium-doped ZnGeP2 structure were demonstrated. The transmission results indicated the improvement of transmittance within a wavelength range of 1.8-2.4 μm when doped with magnesium, which revealed the powerful ability of the appropriate dopant in optimizing near-infrared optical properties. Thus, the introduction of magnesium is a practical approach to improve the transmittance performance and extend the pumping source wavelengths of ZnGeP2 crystals.

Structural strategy for advancing nonlinear optical effects in 1D-[MX2]∞ chains: internal distortion and atomic types

This work aimed to alleviate the limitations of existing mid-infrared nonlinear-optics (MIR-NLO) crystals by conducting theoretical research on 1D-[MX2]∞ (1D = one-dimensional; M = metallic element; X = anionic element) structures in relation to NLO. An analysis was conducted on the electronic structure and optical properties of six selenides (BaZnGeSe4, KxBa1-x/2Ga2Se4, KxBa1-x/2GayIn2-ySe4, KxBa1-x/2In2Se4, BaZnSiSe4, SrZnSiSe4) and three arsenides (Cs2SnAs2, Rb2SnAs2, K2SnAs2) using first principles. Afterwards, the inherent characteristics of these 1D-[MX2]∞ chains were further examined. The calculation results suggest that the presence of internal distortion (non-centrosymmetric) and high electronegativity in M elements can greatly enhance the NLO capability of 1D-[MX2]∞ structures. Furthermore, the first-ever prediction of K2SnAs2's NLO ability has been made. Ultimately, the establishment of a theoretical structure (K2BaSn2As4) provides guidance for the subsequent creation of high-performance MIR-NLO crystals.

Wide band gap selenide infrared nonlinear optical materials AIIMg6Ga6Se16 with strong SHG responses and high laser-induced damage thresholds

Infrared (IR) nonlinear optical (NLO) materials with strong NLO response, wide band gap and high laser-induced damage threshold (LIDT) are highly expected in current laser technologies. Herein, by introducing double alkaline-earth metal (AEM) atoms, three wide band gap selenide IR NLO materials AIIMg6Ga6Se16 (AII = Ca, Sr, Ba) with excellent linear and NLO optical properties have been rationally designed and fabricated. AIIMg6Ga6Se16 (AII = Ca, Sr, Ba) are composed of unique [AIISe6] triangular prisms, [MgSe6] octahedra and [GaSe4] tetrahedra. The introduction of double AEMs effectively broadens the band gaps of selenide-based IR NLO materials. Among them, CaMg6Ga6Se16, achieving the best balance between the second-harmonic generation response (∼1.5 × AgGaS2), wide band gap (2.71 eV), high LIDT (∼9 × AgGaS2), and moderate birefringence of 0.052 @ 1064 nm, is a promising NLO candidate for high power IR laser. Theoretical calculations indicate that the NLO responses and band gaps among the three compounds are mainly determined by the NLO-active [GaSe4] units. The results enrich the chemical diversity of chalcogenides, and give some insight into the design of new functional materials based on the rare [AIISe6] prismatic units.

LaMg6Ga6S16: a chemical stable divalent lanthanide chalcogenide

Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical properties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tetrahedra single/double chains to construct an octahedral crystal field, we synthesized the stable crystalline La(II)-chalcogenide, LaMg6Ga6S16. The nature of the divalent La2+ cations can be identified by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and electron paramagnetic resonance, while the stability is confirmed by the differential thermal scanning, in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La2+(5d1), LaMg6Ga6S16 displays an ultrabroad-band green emission at 500 nm, which is the inaugural instance of La(II)-based compounds demonstrating luminescent properties. Furthermore, as LaMg6Ga6S16 crystallizes in the non-centrosymmetric space group, P-6, it is the second-harmonic generation (SHG) active, possessing a comparable SHG response with classical AgGaS2. In consideration of its wider band gap (Eg = 3.0 eV) and higher laser-induced damage threshold (5×AgGaS2), LaMg6Ga6S16 is also a promising nonlinear optical material.

AIB3 IIC2 IIIQ6 VIXVII: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS3] and Polycations

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS3] planar units and halide-centered polycations, six new metal thioborate halides ABa3B2S6X (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBamRbn/Csn] polycations and [BS3] units, belonging to a newA I B 3 II C 2 III Q 6 VI X VII ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}_{3}^{\mathrm{II}}{\mathrm{C}}_{2}^{\mathrm{III}}{\mathrm{Q}}_{6}^{\mathrm{VI}}{\mathrm{X}}^{\mathrm{VII}}$ family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABa3B2S6Cl) to Cmc21 (ABa3B2S6Br and ABa3B2S6I) driven by the clamping effect of polycationic frameworks. ABa3B2S6Br and ABa3B2S6I are the first series metal thioborate halide IR NLO materials, and the introduction of [BS3] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABa3B2S6Br and ABa3B2S6I, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS2), and strong SHG effects (0.5-0.6 × AgGaS2) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.

Na6Mg3P4S16 and RbMg2PS4Cl2: two Mg-based thiophosphates with ultrawide bandgaps resulting from [MgS6] and [MgSxCl6-x] octahedra

Designing wide-bandgap chalcogenides is one of the most important ways of obtaining high-performance infrared (IR) functional materials. In this work, two Mg-based metal thiophosphates, namely Na6Mg3P4S16 (NMPS) and RbMg2PS4Cl2 (RMPSC), were successfully obtained by introducing [MgS6] and [MgSxCl6-x] octahedra into thiophosphates. In addition, their crystal structures were determined, a first for Mg-containing [PS4]-based thiophosphates to the best of our knowledge. Their bandgaps were investigated in theoretical ways and verified by taking experimental measurements, and determined to be 3.80 eV for NMPS and 3.93 eV for RMPSC, values greater than those of the other investigated thiophosphate halides. The wide bandgaps of NMPS and RMPSC were attributed, based on theoretical calculations, to the [MgSxCl6-x] (x = 0-6) octahedron.

NaREP2Se6 (RE = Y, Sm, Gd-Dy): Quaternary Rare-Earth Selenophosphates with Unique 3D {[REP2Se6]-}∞ Framework Built by RESe8 and P2Se6 Motifs and Multiple Properties

Rare-earth (RE) chalcophosphates have been widely studied because of their abundant structures. Here, five new RE selenophosphates, NaREP2Se6 (RE = Y, Sm, Gd-Dy), were synthesized by a facile RE oxide-boron-selenium solid-state route. They crystallize in the triclinic P1̅ space group, featuring three-dimensional (3D) structures constructed by RESe8 and P2Se6 motifs, different from common 2D RE chalcophosphates A-RE-P2-Q6 (A = alkali metal; Q = S, Se) system. Their structural chemistry and relationship with related phases are analyzed. Both the size of A and the coordination geometry of RE have important influences on the system's structures. Their optical band gaps are tunable from 1.79 to 2.50 eV, and they exhibit diverse magnetic behaviors, including Van-Vleck-type paramagnetism, antiferromagnetism, and ferromagnetism. Their photocurrent responses and thermal stabilities are analyzed as well. Calculation results suggest that the RESe8 and P2Se6 units make a great contribution to the optical properties. This work enriches the chemistry and multifunctional properties of RE chalcophosphates.

Anisotropic structure building unit involving diverse chemical bonds: a new opportunity for high-performance second-order NLO materials

We define the anisotropic structure building unit that encompasses diverse chemical bonds (ABUCB). The ABUCB is highly likely to cause anisotropy in both crystallographic structure and spatial electron distribution, ultimately resulting in enhanced macroscopic optical anisotropy. Accordingly, the (PO3F)2- or (SO3F)- tetrahedron involving the unique P-F or S-F bond serves as such an ABUCB. The distinct chemical bond effectively alters the microscopic nature of the structure building unit, such as polarizability anisotropy, hyperpolarizability, and geometry distortion; this consequently changes the macroscopic second-order nonlinear optical (2nd-NLO) properties of the materials. In this review, we summarize both typical and newly emerged compounds containing ABUCBs. These compounds encompass approximately 90 examples representing six distinct categories, including phosphates, borates, sulfates, silicates, chalcogenides and oxyhalides. Furthermore, we demonstrate that the presence of ABUCBs in DUV/UV NLO compounds contributes to an increase in birefringence and retention of a large band gap, facilitating phase matching in high-energy short-wavelength spectral ranges. On the other hand, the inclusion of ABUCBs in IR NLO compounds offers a feasible method for increasing the band gap and consequently enhancing the larger laser-induced damage threshold. This review consolidates various trial-and-error explorations and presents a novel strategy for designing 2nd-NLO compounds, potentially offering an opportunity for the development of high-performance 2nd-NLO materials.

Synthesis, structure and characterization of Cd2TeO3Cl2 with unprecedented [Cd2O6Cl4] octahedral dimers

A new mixed anionic compound Cd2TeO3Cl2 with unprecedented [Cd2O6Cl4] octahedral dimers has been synthesized, and millimeter-scale single crystals of Cd2TeO3Cl2 have been grown by the vertical Bridgman method with CdCl2 as the flux. Cd2TeO3Cl2 crystallizes in the centrosymmetric P1̄ (no. 2) space group, and shows a mixed cationic layer structure constituted by distorted [TeO3] motifs, mixed anionic [Cd2O6Cl4] chains, and [Cd2O6Cl4] octahedral dimers. Experimental and theoretical results show that Cd2TeO3Cl2 is a direct band gap compound with an experimental band gap of ∼4.25 eV. Meanwhile, the compound has good optical transmittance in the 3-5 μm atmospheric window. The results indicate that Cd2TeO3Cl2 could be used as a promising mid-IR window material, and could enrich the chemical and structural diversity of oxides.

Ba3(BS3)(PS4): the first alkaline-earth metal thioborate-thiophosphate with strong optical anisotropy originating from planar [BS3] units

The first alkaline-earth metal thioborate-thiophosphate Ba3(BS3)(PS4) was designed from Ba3(BO3)(PO4) by S-O substitution and fabricated experimentally. The [BS3] pseudo-layers formed in the structure contribute to the strong optical anisotropy and a large birefringence of ∼0.11 at 1064 nm. The results enrich the structural and chemical diversity of chalcogenides.

Zn2 HgP2 S8 : A Wide Bandgap Hg-Based Infrared Nonlinear Optical Material with Large Second-Harmonic Generation Response

Hg-based chalcogenides, as good candidates for the exploration of high-performance infrared (IR) nonlinear optical (NLO) materials, usually exhibit strong NLO effects, but narrow bandgaps. Herein, an unprecedented wide bandgap Hg-based IR NLO material Zn2 HgP2 S8 (ZHPS) with diamond-like structure is rationally designed and fabricated by a tetrahedron re-organization strategy with the aid of structure and property predictions. ZHPS exhibits a wide bandgap of 3.37 eV, which is the largest one among the reported Hg-based chalcogenide IR NLO materials and first breaks the 3.0 eV bandgap "wall" in this system, resulting in a high laser-induced damage threshold (LIDT) of ≈2.2 × AgGaS2 (AGS). Meanwhile, it shows a large NLO response (1.1 × AGS), achieving a good balance between bandgap (≥3.0 eV) and NLO effect (≥1 × AGS) for an excellent IR NLO material. DFT calculations uncover that, compared to normal [HgS4 ]n , highly distorted [HgS4 ]d tetrahedral units are conducive to generating wide bandgap, and the wide bandgap in ZHPS can be attributed to the strong s-p hybridization between Hg─S bonding in distorted [HgS4 ]d , which gives some insights into the design of Hg-based chalcogenides with excellent properties based on distorted [HgS4 ]d tetrahedra.

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

[Ba4 (S2 )][ZnGa4 S10 ]: Design of an Unprecedented Infrared Nonlinear Salt-Inclusion Chalcogenide with Disulfide-Bonds

Salt-inclusion chalcogenides (SICs) have been receiving widespread attention due to their large second harmonic generation (SHG) responses and wide bandgaps, however most of them suffer from small birefringence limiting their technical application. Herein, by introducing the π-conjugated (S2 )2- units in the ionic guest of salt-inclusion structure, the first disulfide-bond-containing SIC, [Ba4 (S2 )][ZnGa4 S10 ] has been synthesized. It exhibits the widest bandgap up to 3.39 eV among polychalcogenides and strong SHG response as large as that of AgGaS2 (AGS). Importantly, its birefringence reaches a max value of 0.053@1064 nm among AGS-like SICs, indicating it is a promising IR nonlinear optical (NLO) material. Theoretical calculations reveal that the π-conjugated (S2 )2- units and covalent GaS layers favor the enhanced birefringence and large SHG response. This work provides not only a new type of SIC for the first time, but also new lights on the design of IR NLO materials.

MB3P2S10 (M = Rb, Cs): two new alkali metal thioboratephosphates with [B6P4S20] T3-supertetrahedra

Two new alkaline metal thioboratephosphates, RbB3P2S10 and CsB3P2S10, have been designed and fabricated by the flux method. The two compounds are composed of alkali metal polyhedral and [B6P4S20] T3-supertetrahedral units, and crystallize in I41/a and R3̄c space groups, respectively. The results enrich the chemical diversity of chalcogenides, and give insights for the exploration of new functional materials in thioboratephosphates.

Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit

Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra-decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K2 BaGeS4 and K2 BaGeS5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS5 ] group has much larger polarization anisotropy than [GeS4 ], further demonstrated that the linear [S2 ] led to the sharp birefringence enlargement of K2 BaGeS5 (0.19 vs 0.03 of K2 BaGeS4 ). This work provides a new guiding thought to improve the birefringence performance.

BaMo3O10 Polymorphs with Tunable Symmetries and Properties

Polymorphism is a well-known but important phenomenon in the field of solid-state chemistry. Crystalline materials can form various polymorphs and present drastically varied physical and chemical properties. Herein, systematic exploration of the BaO-MoO₃ binary system leads to the discovery of a new barium molybdate, α-BaMo₃O₁₀. The temperature-dependent phase transition from α-BaMo₃O₁₀ to β-BaMo₃O₁₀ is confirmed. The tunable linear and nonlinear optical properties induced by the phase transition are confirmed by both experimental and theoretical approaches. Also, β-BaMo₃O₁₀ is identified as a nonlinear-optical crystal for the first time. The origin of linear- and nonlinear-optical properties of BaMo₃O₁₀ polymorphs is confirmed by the additional theoretical means. This work indicates that a small change in the structure can induce tunable symmetries and thereby widely divergent optical properties.

Oxychalcogenides: A Promising Class of Materials for Nonlinear Optical Crystals with Mixed-Anion Groups

Infrared nonlinear optical (IR NLO) materials are of great significance in the development of IR laser technology. But rationally designing high-performance IR NLO materials remains a huge challenge due to the conflict between the necessary properties required for NLO materials. Notably, oxychalcogenides with mixed-anion groups have drawn extensive interest as a family of important IR NLO candidates because they integrate the property advantages of oxides and chalcogenides by chemical substitution engineering. In this review, we provide a survey of reported oxychalcogenides and aim to present the development of NLO oxychalcogenides from the perspective of rational design of their structural chemistry. Furthermore, we focus on the relationships between partial substitution and structural symmetry as well as optical properties. These provide some helpful guidance for the further exploration and design of novel oxychalcogenide materials with excellent NLO performance in the future.

Rb2CdSi4S10: novel [Si4S10] T2-supertetrahedra-contained infrared nonlinear optical material with large band gap

Infrared nonlinear optical (IR-NLO) materials with wide band gaps are important for generating high-power laser light for modern laser technologies. Herein, a wide band gap IR-NLO material, Rb₂CdSi₄S₁₀, was rationally designed and fabricated by introducing a NLO-active [Si₄S₁₀] T2-supertetrahedron (ST) into the quaternary sulfide system. The Rb₂CdSi₄S₁₀ shows the largest band gap (4.23 eV) among the quaternary chalcogenide IR-NLO materials reported, which results in a high laser-induced damage threshold (LIDT) of ∼5 × AgGaS₂ (AGS) at 1064 nm. At the same time, it has a moderate second-harmonic generation (SHG) response (0.6 × AGS). Based on statistical analyses, the Rb₂CdSi₄S₁₀ is the first compound to be discovered in the AI2B^IICIV4QVI10 family, and also the first Si-rich sulfide IR-NLO material with a [Si₄S₁₀] T2-supertetrahedra. The results indicate that Rb₂CdSi₄S₁₀ is a promising new IR-NLO material, and the NLO-active [Si₄S₁₀] T2-ST unit could be used for the exploration of IR-NLO material with excellent performances.

Emerging Chalcohalide Materials for Energy Applications

Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as "chalcohalides". As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists. This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic-inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials. We then proceed to discuss their optoelectronic properties, band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Heteroepitaxy Growth and Characterization of High-Quality AlN Films for Far-Ultraviolet Photodetection

The ultra-wide bandgap (~6.2 eV), thermal stability and radiation tolerance of AlN make it an ideal choice for preparation of high-performance far-ultraviolet photodetectors (FUV PDs). However, the challenge of epitaxial crack-free AlN single-crystalline films (SCFs) on GaN templates with low defect density has limited its practical applications in vertical devices. Here, a novel preparation strategy of high-quality AlN films was proposed via the metal organic chemical vapor deposition (MOCVD) technique. Cross-sectional transmission electron microscopy (TEM) studies clearly indicate that sharp, crack-free AlN films in single-crystal configurations were achieved. We also constructed a p-graphene/i-AlN/n-GaN photovoltaic FUV PD with excellent spectral selectivity for the FUV/UV-C rejection ratio of >103, a sharp cutoff edge at 206 nm and a high responsivity of 25 mA/W. This work provides an important reference for device design of AlN materials for high-performance FUV PDs.

Ag4SnGe2S7: A Noncentrosymmetric Chalcogenide in I4-II-IV2-VI7 System with Non-Diamond-Like Structure Featuring 1D ∞[SnGe2S8]6- Infinite Chain

The I₄-II-IV₂-VI₇ metal chalcogenide system has become an attractive research system because of its excellent physical and chemical properties. Here, we report the discovery of a new Sn^II-based quaternary chalcogenide in the I₄-II-IV₂-VI₇ system, Ag₄SnGe₂S₇, with a non-diamond-like structure and crystallizing in the Cc space group. The compound is characterized by isolated pyramid-like [SnS₃] units and one-dimensional _∞[SnGe₂S₈]^6- infinite chains with two orientations formed by the corner-sharing connected [SnGe₂S₈]^6- units. It has a band gap of 2.40 eV and is insensitive to air and moisture.

NH4IO2F2 and (NH4)3(IO2F2)3·H2O: A Series of Ammonium-Containing Fluoroiodates with Wide Band Gaps

A series of ammonium-containing fluoroiodates, NH₄IO₂F₂ and (NH₄)₃(IO₂F₂)₃·H₂O, with isolated [IO₂F₂] units have been fabricated by a fluorine-oxygen substitution strategy from NH₄IO₃. The two compounds crystallize in the orthorhombic system, but in different space groups, noncentrosymmetric Pca2₁ for NH₄IO₂F₂ and centrosymmetric Pnma for (NH₄)₃(IO₂F₂)₃·H₂O, and show wide band gaps of 4.53 eV for (NH₄IO₂F₂) and 4.55 eV for ((NH₄)₃(IO₂F₂)₃·H₂O). In addition, NH₄IO₂F₂ exhibits a 1.2 × KDP second harmonic generation response, a short ultraviolet cutoff edge in iodates, and a good crystal growth habit. The crystal of NH₄IO₂F₂ with a size of 11 × 5 × 2 mm³ was obtained by the aqueous solution method. The results enrich the structural diversity of iodate and supply a greater understanding of the design of new functional materials based on the fluoroiodates.

The Growth of High-Quality Hexagonal GaTe Nanosheets Induced by ZnO Nanocrystals

The monoclinic and hexagonal gallium tellurides (m-GaTe and h-GaTe) show different applications in optoelectronic devices. Compared to the m-GaTe, the h-GaTe is a metastable phase, which generally exists in ultrathin samples and is difficult to obtain by direct chemical reaction. Herein, a hexagonal ZnO-induced crystal growth strategy was used for the design and fabrication of h-GaTe. The high-quality h-GaTe nanosheets were successfully grown on the (001) surface of hexagonal ZnO by the chemical vapor deposition method under ambient pressure. The SEM, XPS, XRD, and HRTEM characterizations uncovered a flower-like nanosheet morphology and a hexagonal crystal structure for the obtained GaTe samples. Meanwhile, the conductive atomic force microscope measurement indicates that the obtained h-GaTe nanosheet is a p-type semiconductor. Based on the electron localization function simulation, the lattice-induced crystal growth of h-GaTe was demonstrated. The results give an insight into the synthesis of metastable phase crystal and open an avenue for fabricating new two-dimensional devices by p-type h-GaTe.