Manipulating Peierls Distortion in van der Waals NbOX2 Maximizes Second-Harmonic Generation.

Two-dimensional (2D) van der Waals (vdW) materials, featuring relaxed phase-matching conditions and highly tunable optical nonlinearity, endow them with potential applications in nanoscale nonlinear optical (NLO) devices. Despite significant progress, fundamental questions in 2D NLO materials remain, such as how structural distortion affects second-order NLO properties, which call for advanced regulation and in situ diagnostic tools. Here, by applying pressure to continuously tune the displacement of Nb atoms in 2D vdW NbOI2, we effectively modulate the polarization and achieve a 3-fold boost of the second-harmonic generation (SHG) at 2.5 GPa. By introducing a Peierls distortion parameter, λ, we establish a quantitative relationship between λ and SHG intensity. Importantly, we further demonstrate that the SHG enhancement can be achieved under ambient conditions by anionic substitution to tune the distortion in NbO(I1-xBrx)2 (x = 0-1) compounds, where the chemical tailoring simulates the pressure effects on the structural optimization. Consequently, NbO(I0.60Br0.40)2 with λ = 0.17 exhibits a giant SHG of over 2 orders of magnitude higher than that in monolayer WSe2, reaching the record-high value among reported 2D vdW NLO materials. This work unambiguously demonstrates the correlation between Peierls distortion and SHG property and, more broadly, opens new paths for the development of advanced NLO materials by manipulating the structure distortions.

Building intercalation structure for high ionic conductivity via aliovalent substitution.

Two-dimensional (2D) materials, which possess robust nanochannels, high flux and allow scalable fabrication, provide new platforms for nanofluids. Highly efficient ionic conductivity can facilitate the application of nanofluidic devices for modern energy conversion and ionic sieving. Herein, we propose a novel strategy of building an intercalation crystal structure with negative surface charge and mobile interlamellar ions via aliovalent substitution to boost ionic conductivity. The Li2xM1-xPS3 (M = Cd, Ni, Fe) crystals obtained by the solid-state reaction exhibit distinct capability of water absorption and apparant variation of interlayer spacing (from 0.67 to 1.20 nm). The assembled membranes show the ultrahigh ionic conductivity of 1.20 S/cm for Li0.5Cd0.75PS3 and 1.01 S/cm for Li0.6Ni0.7PS3. This facile strategy may inspire the research in other 2D materials with higher ionic transport performance for nanofluids.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering.

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb2 O3 molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb2 O3 flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals.

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P4Se3 nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P4Se3 molecules, arising from an orderly and close packing of P4Se3 molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P4Se3 nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm2 V-1 s-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

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

2D inorganic bimolecular crystals, consisting of two different inorganic molecules, are expected to possess novel physical and chemical properties due to the synergistic effect of the individual components. However, 2D inorganic bimolecular crystals remain unexploited because of the difficulties in preparation arising from non-typical layered structures and intricate intermolecular interactions. Here, the synthesis of 2D inorganic bimolecular crystal SbI3 ·3S8 nanobelts via a facile vertical microspacing sublimation strategy is reported. The as-synthesized SbI3 ·3S8 nanobelts exhibit strong in-plane anisotropy of phonon vibrations and intramolecular vibrations as well as show anisotropic light absorption with a high dichroism ratio of 3.9. Furthermore, it is revealed that the second harmonic generation intensity of SbI3 ·3S8 nanobelts is highly dependent on the excitation wavelength and crystallographic orientation. This work can inspire the growth of more 2D inorganic bimolecular crystals and excite potential applications for bimolecular optoelectronic devices.

Hexagonal In2Se3: A Defect Wurtzite-Type Infrared Nonlinear Optical Material with Moderate Birefringence Contributed by Unique InSe5 Unit.

The balance between second harmonic generation (SHG) intensity and laser-induced damage threshold (LIDT), together with phase-matchable behavior, is the key point for exploration of novel nonlinear optical (NLO) materials. In this work, the NLO property of defect wurtzite-type hexagonal-In2Se3 (γ) is extensively explored first. It exhibits a strong SHG intensity of 2.6 × AgGaS2 (AGS) at 2.1 µm, and a high powder LIDT of 7.3 × AGS. From wurtzite to γ-In2Se3, the birefringence changes from 0.003 to 0.075, resulting in the phase-matchable phenomenon of γ-In2Se3. This is well ascribed to the contribution of the unique InSe5 unit in γ-In2Se3 from the result of birefringence calculation and analysis.

Partial substitution induced centrosymmetric to noncentrosymmetric structure transformation and promising second-order nonlinear optical properties of (K0.38Ba0.81)Ga2Se4.

Partial substitution of Ba by K in the Ba sites of BaGa2Se4 creates a new selenide (K0.38Ba0.81)Ga2Se4 (1), which is crystallized in the noncentrosymmetric space group I4cm, different from the centric structures of all the ternary MI-MIII2-Q4 (MII = divalent Sr, Ba, Pb, Sn, Eu; MIII = trivalent Ga, In; Q = S and Se) chalcogenides. Its 1D structure features {[GaSe2]-}∞ chains, and K/Ba occupying the interchain cavities as the counter cations. Its powder sample demonstrates a second-harmonic generation intensity which is around 0.9 times that of AgGaS2 at 2.1 µm, and a laser-induced damage threshold which is 13.4 times that of AgGaS2. DFT calculations on its electronic structure and optical properties were also performed.

Sn2Ga2S5: A Type of IR Nonlinear-Optical Material.

The deficiency of nonlinear-optical (NLO) materials in the IR region inspires strong research interest in this field. Here, Sn2Ga2S5 (1), crystallizing in the orthorhombic Pna21 space group, demonstrates obvious NLO activity, around a maximum of 1.6 times that of AgGaS2 and a strong laser-induced damage threshold of 9.7 times that of AGS. 1 represents the first NLO-active compound in the MII2MIII2Q5 (MII = divalent Ca, Sr, Ba, Pb, Sn, and Eu; MIII = B, Al, Ga, and In; Q = S and Se) family. The NLO performances of 1 are systematically studied experimentally and theoretically.

Large Second Harmonic Generation (SHG) Effect and High Laser-Induced Damage Threshold (LIDT) Observed Coexisting in Gallium Selenide.

A big challenge for nonlinear optical (NLO) materials is the application in high power lasers, which needs the simultaneous occurrence of large second harmonic generation (SHG) and high laser induced damage threshold (LIDT). Herein we report the preparation of a new Ga2 Se3 phase, which shows the SHG intensities of around 2.3 times and the LIDT of around 16.7 times those of AgGaS2 (AGS), respectively. In addition, its IR transparent window ca. 0.59-25 µm is also significantly wider than that of AGS (ca. 0.48-≈11.4 µm). The occurrence of the strong SHG responses and good phase-matching indicate that the structure of the new Ga2 Se3 phase can only be non-centrosymmetric and have a lower symmetry than the cubic γ-phase. The observed excellent SHG and phase-matching properties are consistent with our diffraction experiments and can be well explained by using the orthorhombic models obtained through our high throughput simulations.

Balanced Second-Order Nonlinear Optical Properties of Adducts CHI3·(S8)3 and AsI3·(S8)3: A Systematic Survey.

Two isostructural adducts CHI3·(S8)3 (1) and AsI3·(S8)3 (2) are synthesized by a simple solution method. Both of them crystallize in noncentrosymmetric R3 m, featuring van der Waals interaction linked CHI3 tetrahedra or SbI3 trigonal pyramids with crown-like S8 molecules. Both 1 and 2 show strong SHG responses and phase matchability under either 1.064 or 2.1 µm. The results of DFT calculations indicate that the electron transfer happens from S-3p to I-5p orbitals, and both of them demonstrate indirect band gaps. The NLO effects of 1 and 2 are almost fully contributed from the CHI3 or AsI3 units. Compared with AgGaS2, their powder laser damage thresholds are ca. 15 and 51 times higher.

Adduct-Type IR Nonlinear-Optical Crystal SbI3·(S8)3 with a Large Second-Harmonic Generation and a High Laser-Induced Damage Threshold.

A noncentrosymmetric adduct, SbI3·(S8)3, is obtained by a facile solution method. It crystallizes in the trigonal space group R3 m, and its structure features van der Waals force connected, isolated trigonal-pyramidal SbI3 and chairlike S8 units. SbI3·(S8)3 exhibits strong second-harmonic-generation responses around 4 times that of KH2PO4 (1.064 µm) and 1 times that of AgGaS2 (AGS; 2.1 µm) at 210-250 µm, respectively. Under both laser-radiation wavelengths, it exhibits type I phase-matching behavior. Its laser-induced damage threshold is measured to be around 26 times that of AGS under 1.064 µm. To better understand these optical data, theoretical calculations are performed to address its electronic structure and optical properties.