Broadband Green Luminescence and Phase Transition in Low-Dimensional Organic-Inorganic Hybrid Iodate.

Organic-inorganic hybrid iodide systems, which can form highly ordered chromophores and uniformly oriented transition dipole moments, serve as optimal host-guest systems for the fabrication of micrometer-scale optical devices. In particular, those with low-dimensional structures can exhibit strong quantum-limited and highly localized charges, enabling the generation of high exciton energies and stable excitation emission. In this study, we report a novel instance of an organic-inorganic hybrid iodate, (C13H11N2)(IO3), which was synthesized by incorporating the optically active organic compound, 9-aminoacridine. Upon crystallization in the monoclinic space group P21/c, this compound exhibits a direct optical band gap of 2.66 eV. The incorporation of discrete organic units within the low-dimensional structures induces pronounced local charges, culminating in broadband green luminescence with a peak at 540 nm under UV excitation. This corresponds to the CIE coordinates (0.37, 0.56). A potential phase transition was inferred through a comprehensive analysis of the variable temperature structure and emission spectra. Furthermore, first-principles calculations revealed the pivotal role of organic cations in facilitating broadband luminescence.

Molecularly Thin 2D Organic Single Crystals: A New Platform for High-Performance Polarization-Sensitive Phototransistors.

The inherent linear dichroism (LD), high absorption, and solution processability of organic semiconductors hold immense potential to revolutionize polarized light detection. However, the disordered molecular packing inherent to polycrystalline thin films obscures their intrinsic diattenuation, resulting in diminished polarization sensitivity. In this study, we develop filter-free organic polarization-sensitive phototransistors (PSPs) with both a high linear dichroic ratio (LDR) and exceptional photosensitivity utilizing molecularly thin dithieno[3,2-b:2',3'-d]thiophene derivatives (DTT-8) two-dimensional molecular crystals (2DMCs) as the active layer. The orderly molecular packing in 2DMCs amplifies the inherent LD, and their molecular-scale thickness enables complete channel depletion, significantly reducing the dark current. As a result, PSPs with an impressive LDR of 3.15 and a photosensitivity reaching 3.02 × 106 are obtained. These findings present a practical demonstration of using the polarization angle as an encryption key in optical communication, showcasing the potential of 2DMCs as a viable and promising category of semiconductors for filter-free, polarization-sensitive photodetectors.

Organic Single-Crystalline Microwire Arrays toward High-Performance Flexible Near-Infrared Phototransistors.

Flexible organic near-infrared (NIR) phototransistors hold promising prospects for potential applications such as noninvasive bioimaging, health monitoring, and biometric authentication. For integrated circuits of high-performance devices, organic single-crystalline micro-/nanostructures with precise positioning are prominently anticipated. However, the manufacturing of organic single-crystalline arrays remains a conundrum due to difficulties encountered in patterning arrays of dewetting processes at micron-scale confined space and modulating the dewetting dynamics. Herein, we utilize a capillary-bridge lithography strategy to fabricate organic 1D arrays with high quality, homogeneous size, and deterministic location toward high-performance flexible organic NIR phototransistors. Regular micro-liquid stripes and unidirectional dewetting are synchronously achieved by adapting micropillar templates with asymmetric wettability. As a result, high-throughput 1D arrays based organic field-effect transistors exhibit high electron mobility up to 9.82 cm2  V-1  s-1 . Impressively, flexible NIR phototransistors also show outstanding photoelectronic performances with a photosensitivity of 9.87 × 105 , a responsivity of 1.79 × 104  A W-1 , and a specific detectivity of 3.92 × 1014 Jones. This work paves a novel way to pattern high-throughput organic single-crystalline microarrays toward flexible NIR organic optoelectronics.

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.

High-energy, tunable, long-wave mid-infrared optical parametric oscillator based on BaGa4Se7 crystal.

A high-energy, tunable, long-wave mid-infrared optical parametric oscillator (OPO) based on the BaGa4Se7 crystal was demonstrated in this Letter with 1064 nm laser pumping. The mid-infrared OPO was designed as a double-pass single resonant oscillator (DP-SRO) to reduce the threshold and improve the outputs. Further optimization on the cavity length was theoretically and experimentally studied. With a short cavity length of 30 mm, the output energy of over 1 mJ/pulse at 11 µm was obtained with the pump energy of 39.5 mJ/pulse. In addition, a wide tuning range of 8-14 µm was experimentally achieved by rotating the BaGa4Se7 crystal.

Sr5Ga8O3S14: A Nonlinear Optical Oxysulfide with Melilite-Derived Structure and Wide Band Gap.

Sr5Ga8O3S14, which is a novel nonlinear optical oxysulfide, was discovered via solid-state reaction. The compound crystallizes in noncentrosymmetric space group P21212 (No. 18) of the orthorhombic system. The crystal structure is derived from the melilite structure and composed of anionic [Formula: see text] layers intercalated by Sr2+ cations. The anionic layers are constructed by [GaOS3]5- and [GaS4]5- tetrahedra via either corner sharing or edge sharing, while typical melilite only features edge-sharing mode. Because of the existence of electronegative O atoms, it possesses a large band gap (3.9 eV) with a wide transparent region of 0.3-13.4 µm. It exhibits second-harmonic-generation response, which is 0.8 times that of the reference AgGaS2 (AGS) in the particle size range of 20-41 µm under laser irradiation at a wavelength of 2.09 µm laser. Density functional theory calculations indicate that the band edges are mainly composed of the Ga-S bond orbitals, with little participation of O 2p near the valence band maximum.

Two rare-earth-based quaternary chalcogenides EuCdGeQ4 (Q = S, Se) with strong second-harmonic generation.

Two new rare-earth-based chalcogenides EuCdGeQ4 (Q = S, Se) have been designed and constructed by using Eu2+ and the classical NLO-active SBUs of [CdQ4] and [GeQ4]. They crystallize in a non-centrosymmetric Ama2 (no. 40) space group. Benefiting from the synergistic effects of [GeQ4] and highly distorted [CdQ4] tetrahedra, both compounds possess type-I phase-matching behaviour and large powder second harmonic generation (SHG) effects at 2.09 µm (2.6 and 3.8 × AgGaS2 for sulfide and selenide), as well as large direct band gaps (2.5 eV and 2.25 eV). Besides, they melt congruently at relatively low temperatures (997 °C for EuCdGeS4 and 882 °C for EuCdGSe4), which is suitable for bulk crystal growth by the Bridgman method. In addition, their electronic structures and some optical coefficients are calculated by first-principles.

AHgSnQ4 (A = Sr, Ba; Q = S, Se): A Series of Hg-Based Infrared Nonlinear-Optical Materials with Strong Second-Harmonic-Generation Response and Good Phase Matchability.

Four Hg-based IR nonlinear-optical materials, AHgSnQ4 (A = Sr, Ba; Q = S, Se), were discovered and investigated systematically. Their structures are built of two-dimensional [HgSnQ4]2- layers, which are assembled alternately by distorted (HgQ4 and SnQ4) tetrahedra and separated by eight-coordinated A2+ cations. The two sulfides AHgSnS4 (A = Ba, Sr) exhibit large second-harmonic-generation (SHG) responses (2.8 and 1.9 × AgGaS2 at 2.09 µm), as well as large band gaps (2.77 and 2.72 eV). The two selenides AHgSnSe4 (A = Ba, Sr) show even stronger SHG responses, about 5 times that of AgGaS2. Furthermore, all four compounds show phase-matching behavior, and the results of first-principles calculation elucidate the key role of the HgQ4 group in the enhanced SHG effect in ß-BaHgSnS4 and BaHgSnSe4.

High-conversion-efficiency tunable mid-infrared BaGa4Se7 optical parametric oscillator pumped by a 2.79-µm laser.

A mid-infrared BaGa4Se7 optical parametric oscillator with high conversion efficiency and beam quality is demonstrated, which is pumped by a 2.79-µm electro-optically Q-switched Cr, Er:YSGG laser. A pulse energy of 3.5 mJ with a pulse width of 21 ns at 10 Hz is obtained in the range of 3.94-9.55 µm, and the beam quality factors are measured to be Mx2=5.0 and My2=4.6. The optical-to-optical conversion efficiency is 18.9%, and the slope efficiency is 31.6%, which is a 59% improvement on the best of the previously reported slope efficiencies for BaGa4Se7-based OPOs.

High energy and tunable mid-infrared source based on BaGa4Se7 crystal by single-pass difference-frequency generation.

A high-energy and tunable mid-infrared source based on BaGa4Se7 crystal was demonstrated by single-pass difference-frequency generation (DFG). Orthogonally polarized wave at 1064 nm (λ1) and tunable idler wave (λ2) generated by KTP-OPO, which could be tuned in the wavelength range of 1360-1600 nm, were used as the DFG dual-wavelength pump. The pump parameters including total pump energy and energy ratio were studied. Maximum pulse energy of 5.72 mJ at 3.58 µm was obtained at the dual-wavelength pump energy of 58.4 mJ/pulse. The wavelength tuning range was 3.36-4.27 µm with a flat tunability. Moreover, a saturation phenomenon of DFG output was observed and experimentally inferred to be related to the input energy of λ2 in the BaGa4Se7 crystal.

Synthesis, Structure, and Characterization of Two Mixed-Cation Quaternary Chalcogenides K2BaSnQ4 (Q = S, Se).

Two new isostructural metal chalcogenides, K2BaSnS4 and K2BaSnSe4, have been isolated for the first time. They feature the intriguing ∞ 1[BaSnQ4]2- (Q = S, Se) tunnel structures which are separated by K+ cations. Experiments combined with theoretical calculations demonstrate that K2BaSnS4 possesses a large energy gap (3.09 eV), which is conducive for large laser-induced damage thresholds (LDTs), and maintains a relatively moderate SHG response (0.5 × AgGaS2) in the meanwhile. Moreover, K2BaSnS4 possesses good phase-matchability, which stems from its suitable birefringence (0.04@2.09 µm).

SnGa2GeSe6, a benign addition to the AMMIVQ6 family: synthesis, crystal structure and nonlinear optical performance.

A new selenide, SnGa2GeSe6, in the AMIII2MIVQ6 family was synthesized for the first time by a high-temperature solid-state reaction. It crystallized in the non-centrosymmetric space group Fdd2 with cell dimensions of a = 47.195(9) Å, b = 7.5213(15) Å, c = 12.183(2) Å, and Z = 16. SnGa2GeSe6's crystal structure is characterized by a crisscross network of two types of infinite chains (i.e. the 1∞[GaSe3] chain and the 1∞[M3Se7] chain, where M represents the two metal sites randomly occupied by Ga and Ge atoms in a 1 : 1 ratio), which is similar to SnGa2GeS6 and diverges strongly from its Ba analogue owing to the substitution of Ba with Sn atoms that contain stereochemically active lone pair electrons. Careful experimental research has revealed that SnGa2GeSe6 exhibits an optical band gap of 1.98 eV and incongruent melting behavior. Furthermore, the second harmonic generation (SHG) intensity of the SnGa2GeSe6 powder sample is about 1.7 × AgGaS2 at a particle size of 150-200 µm with a 2 µm laser as the fundamental light.

Sr6 Cd2 Sb6 O7 S10 : Strong SHG Response Activated by Highly Polarizable Sb/O/S Groups.

A new nonlinear optical (NLO) oxysulfide, Sr6 Cd2 Sb6 O7 S10 , which contains the functional groups [SbOx S5-x ]7- (x=0, 1) with a 5s2 electron configuration, is synthesized by a solid-state reaction. This compound displays a phase-matchable second harmonic generation (SHG) response four times stronger than AgGaS2 (AGS) under laser irradiation at 2.09 µm. Single-crystal-based optical measurements reveal a SHG intensity that can be tuned by temperature and novel photoluminescence properties. Theoretical analyses demonstrate that tetragonal [SbOS4 ]7- and [SbS5 ]7- pyramids make the predominant contribution to the enhanced SHG effect. Among those, the [SbOS4 ]7- units with mixed anions make a larger contribution. This work proposes that oxysulfide groups with an ns2 electron configuration can serve as new functional building units in NLO materials and opens a new avenue for the design of other optoelectronic materials.

Continuous-wave difference-frequency generation based on BaGa4Se7 crystal.

A continuous-wave mid-infrared radiation from difference frequency generation by mixing a continuous-wave Ti: sapphire laser and a continuous-wave YAG laser in a 15 mm long BaGa4Se7 crystal is demonstrated for the first time. The tunable range from 3.15 to 7.92 µm was achieved by rotating the crystal to fulfill the type I phase-matching condition. A maximum DFG power of 1.41 µW was obtained at 5 µm. Meanwhile the experimental DFG power conversion efficiency was 20.2 µW/W2, with a length-normalized slope efficiency of 15.5 µW/cmW2. The conversion efficiency decreases rapidly from 50 µW/cmW2 at 3.15 µm to 1 µW/cmW2 at 7.92 µm. The wavelength acceptance bandwidth and the angular acceptance bandwidth were measured to be 16.4 cm-1 and 44' for DFG at 5.1 µm, respectively.

Synthesis, Crystal Structure, and Optical Properties of Noncentrosymmetric Na2ZnSnS4.

A new chalcogenide Na2ZnSnS4 has been successfully synthesized by using Na2S2 as reactive flux. Na2ZnSnS4 crystallizes in the tetragonal system with space group of I4̅. Its cell parameters are a = 6.4835(6) Å and c = 9.134(1) Å. The structure is a derivative of AgGaS2, in which the Ag+ ions are replaced by Na+ ions and the Ga3+ ions are replaced by Zn2+ and Sn4+ ions. All three cations are in seriously distorted tetrahedral geometry with a distortion factor (η = c/ a) of 1.4. Optical measurements show that the Na2ZnSnS4 powder sample has a large transparent range from 0.8 to 25 µm and a wide band gap of 3.1 eV. It exhibits large second-harmonic generation intensity of 0.9 × AgGaS2 in the grain size range from 41 to 74 µm. First-principles calculation results reveal that the valence band maximum and conduction band minimum are mainly composed of S 3p, Zn 3d orbitals and Sn 5s, S 3p orbitals, respectively.

Zn3P2S8: A Promising Infrared Nonlinear-Optical Material with Excellent Overall Properties.

The thiophosphate Zn3P2S8 is reported for its potential application in infrared (IR) nonlinear optics. This nondeliquescent compound features a cubic closet packing of the (PS4)3- groups, with Zn2+ filling in three-quarters of the tetrahedral interspaces. The optical band gap of Zn3P2S8 is characterized as 3.12 eV, which is very beneficial to improving the laser damage threshold. Besides, Zn3P2S8 demonstrates good phase matchability (PM) with a strong second-harmonic-generation (SHG) response that is about 2.6 times that of AgGaS2, achieving a valuable balance among good phase matchability, strong SHG response, and large optical band gap. The origin of these attractive optical properties is investigated in detail via theoretical calculations. Moreover, the differential scanning calorimetry curve indicates a congruent-melting thermal behavior of Zn3P2S8, which is conducive to large single-crystal growth. The excellent comprehensive performance of Zn3P2S8 makes it a practically usable IR nonlinear-optical candidate.

K2ZnGe3S8: A Congruent-Melting Infrared Nonlinear-Optical Material with a Large Band Gap.

K2ZnGe3S8 belonging to the noncentrosymmetric space group P21 of the monoclinic system was discovered via a solid-state method. It possesses two-dimensional [ZnGe3S8]2- layers, with alkali-metal cations K+ located between the layers. On the basis of UV-vis-near-IR diffuse-reflectance spectrometry, the band gap of K2ZnGe3S8 is 3.36(2) eV. According to powder second-harmonic-generation (SHG) measurements, the SHG response of K2ZnGe3S8 is about 0.9 times that of AgGaS2 at the particle size range of 20-41 µm. Experimental results demonstrate that K2ZnGe3S8 keeps a good balance between a large band gap (3.36 eV) and a moderate SHG response. Moreover, according to the differential scanning calorimetry measurements, K2ZnGe3S8 melts congruently at around 1023 K and recrystallizes at about 963 K. Therefore, it is possible to obtain bulk single crystals via the Bridgman-Stockbarger method. The first-principles calculations indicate that the optical properties of K2ZnGe3S8 are dominantly determined by the [GeS4] tetrahedra as well as a small contribution from the [ZnS4] tetrahedra.

Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations.

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 ( d36 = 33 pm/V) at the particle size of 150-200 µm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.

Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 : Strong Nonlinear Optical Response in Quaternary Diamond-Like Selenide Networks.

Two new selenides with diamond-like structures, Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , were synthesized by using a conventional high-temperature solid-state reaction method. They crystallize in the space group Pna21 (no. 33) of the orthorhombic system. Their three-dimensional frameworks consist of corner-sharing LiSe4 , CdSe4 , and MSe4 (M=Ge, Sn) tetrahedra. These two compounds exhibit strong powder second-harmonic generation responses that are about 1.2 and 2.5 times that of the benchmark AgGaS2 at a laser wavelength of λ=2.09 µm, and also demonstrate type I phase-matchable behavior. The optical bandgaps were determined to be 2.18 and 1.95 eV for Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , respectively. Furthermore, these two materials exhibit congruent melting behavior at rather low temperatures of 985 and 1060 K, respectively, which makes bulk single crystal growth by using the Bridgman-Stockbarger method possible. Our study indicates that these two materials show advantages over the traditional IR NLO material CdSe and are promising for practical applications.

Pb0.65Mn2.85Ga3S8 and Pb0.72Mn2.84Ga2.95Se8: Two Quaternary Metal Chalcognides with Open-Tunnel-Framework Structures Displaying Intense Second Harmonic Generation Responses and Interesting Magnetic Properties.

By combining different nonlinear optical-active structural chromophores with transition metal Mn into a crystal structure, two novel quaternary metal chalcogenides Pb0.65Mn2.85Ga3S8 (1) and Pb0.72Mn2.84Ga2.95Se8 (2) were successfully synthesized. Compounds 1 and 2 are isostructural, and they represent a new structure type that crystallizes in the space group P6̅ (No. 174) in the hexagonal system. Their structures feature an interesting three-dimensional open-tunnel framework composed of bridged infinite chains with Pb2+ cations filling in the biggest tunnels. Interestingly, both 1 and 2 demonstrate intense second harmonic generation responses at 2.09 µm that is about 1.5 and 4.4 times, respectively, of that of the benchmark material AgGaS2. However, 1 and 2 possess different optical diffuse reflectance spectra: 1 displays an evident multiband absorption characteristic with two distinguishing absorption edges of 738 and 551 nm, corresponding to two band gaps of 1.68 and 2.25 eV, respectively, while 2 exhibits only one sharp edge, and the corresponding band gap was estimated to be 1.65 eV. Moreover, apart from the considerable structural similarity between 1 and 2, the dc temperature dependent susceptibility measurements indicate that compound 1 is paramagnetic, while compound 2 exhibits spin-glass-like behavior.