研究目的
To investigate the linear and second-order nonlinear optical (NLO) properties of titanium-based MIL-125 metal-organic frameworks (MOFs) and their derivatives through ligand functionalization.
研究成果
The study reveals that the linear and second-order NLO properties of MIL-125 can be tuned by introducing functional groups into BDC linker or by extending the length of aromatic ring. The chemical modifications of BDC tend to increase the dielectric constants and the magnitude of birefringence of MIL-125 at the low energy region, especially for two aminated derivatives. The SHG response is sensitive to the type of functional group incorporated into BDC ligand, with the inclusion of amino group enhancing the SHG activity of MIL-125. MIL-126 and MIL-127 with longer aromatic linking unit are not suitable to act as NLO materials due to their poor phase matching performances, but they are promising candidates for the low dielectric constant materials.
研究不足
The study is theoretical and based on computational models, which may not fully capture all experimental conditions and variations. The PBE functional tends to underestimate the band gap of semiconductor with respect to the experimental value, and the hybrid HSE06 functional was used to determine the band gap of different derivatives based on MIL-125.
1：Experimental Design and Method Selection:
Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) and the projected augmented wave (PAW) method. The generalized gradient approximation Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed, and the kinetic cut-off energy for the plane-wave expansion was set to 500 eV. The influences of van der Waals interactions were taken into account by using the dispersion corrected vdW-DF2 functional.
2：Sample Selection and Data Sources:
The study focused on MIL-125 and its derivatives with modified BDC linkers by introducing different functional groups (-NH2, -OH, -NO2) or by extending BDC ligand to contain two (MIL-126) and three (MIL-127) benzene rings.
3：List of Experimental Equipment and Materials:
Computational tools and software including VASP and PAW method were used.
4：Experimental Procedures and Operational Workflow:
Structural optimizations were performed with only Γ point involved in the Brillouin zone integration, and the convergence thresholds of the energy change and the maximum force were set to 10-5 eV and
5：03 eV/ ?, respectively. Optical properties were evaluated after structural optimizations. Data Analysis Methods:
The linear optical response was related to the complex dielectric function, and the imaginary part of the dielectric function was calculated. The real part of the dielectric function was yielded from the imaginary part by a Kramer-Kronig transformation. The second-order nonlinear susceptibility was calculated using the length-gauge formalism.
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