First-principles analysis of a molecular piezoelectric meta-nitroaniline.

The piezoelectric and elastic properties of a molecular piezoelectric meta-nitroaniline (mNA) in its single-crystal form were investigated in the framework of first-principles density functional perturbation theory (DFPT). Results support the recent experimental findings those despite being soft and flexible, mNA's piezoelectric coefficients are an order of magnitude greater than that of ZnO and LiNbO3. A molecular-level insight into the piezoelectric properties of mNA is provided. These results are helpful not only for better understanding mNA, but also for developing new piezoelectric materials.

A terahertz study of taurine: Dispersion correction and mode couplings.

The low-frequency characteristics of polycrystalline taurine were studied experimentally by terahertz (THz) absorption spectroscopy and theoretically by ab initio density-functional simulations. Full optimizations with semi-empirical dispersion correction were performed in spectral computations and vibrational mode assignments. For comparison, partial optimizations with pure density functional theory were conducted in parallel. Results indicate that adding long-range dispersion correction to the standard DFT better reproduces the measured THz spectra than the popular partial optimizations. The main origins of the observed absorption features were also identified. Moreover, a coupled-oscillators model was proposed to explain the experimental observation of the unusual spectralblue-shift with the increase of temperature. Such coupled-oscillators model not only provides insights into the temperature dynamics of non-bonded interactions but also offers an opportunity to better understand the physical mechanisms behind the unusual THz spectral behaviors in taurine. Particularly, the simulation approach and novel coupled-oscillators model presented in this work are applicable to analyze the THz spectra of other molecular systems.

Experimental and Theoretical Study on Terahertz Spectra for Regenerated Cellulose.

In this work, regenerated cellulose films were prepared with an iced dissolution method, while the physical morphologies and crystal types of the products were systematically characterized with scanning electron microscope (SEM), Fourier transform infrared(FTIR), while X-Ray Diffraction (XRD). The results demonstrate that the as-prepared continuous and uniform films are indeed cellulose Ⅱ, whose morphology and crystal type are significantly different from those of the degreased cotton. Moreover, Terahertz time domain system (THz-TDS) and FTIR were employed to measure the THz spectra of the regenerated cellulose films. Accordingly, the THz characteristic peaks for the regenerated cellulose films are experimentally identified for the first time. In addition, the increase of the THz transmittance with the decrease of the wavenumber is attributed to the existence of amorphous components in the regenerated cellulose films. Although the shapes of Far-IR spectra in the range of 100～700 cm-1 are similar, the absorption peaks of the regenerated cellulose films move to lower wavenumbers (blue shift) compared with those of the degreased cotton. Based on this, we developed a new approach to distinguish the allomorphism of cellulose Ⅱ and cellulose Iß by Far-IR. Particularly, geometry optimization and IR calculation for the crystal structure of cellulose Ⅱ have been successfully processed by Density Functional Theory (DFT) using periodic boundary condition via CASTEP package. The calculated absorption peak positions are in good agreement with those experimentally measured. Consequently, the THz characteristic peaks of the regenerated cellulose films have been systematically and successfully assigned. Theoretical calculations reveal that the peaks at 42 and 54 cm-1 are assigned to the lattice vibration modes coupled with translational mode and rotational mode, respectively. Moreover, the absorption peaks in the range of 68～238 cm-1 are related with the torsion vibration of ­CH2OH group and deformation vibration of C­H bond and O­H bond, while those in the range of 351～583 cm-1 are assigned to the skeletal vibration of C­O­C bond and pyranoid ring, and those at 611 and 670 cm-1 are originated from the out-of-plane bending vibration of O­H bond. Each absorption peak is involved in more than single vibration mode. The THz spectra presented in this work, together with the theoretical simulations, indicate that the THz responses of regenerated cellulose are closely associated with both its chemical constituents and molecular structure. These results will be helpful not only for better understanding the relations between the molecular structure of the regenerated cellulose and its THz spectrum, but also for providing valuable information for future studies on the physical mechanisms of THz responses of other partially-crystalline polymers and organic biological macromolecules.