Haidong Ju

Study of electronic and spectroscopic properties on a newly synthesized red fluorescent material

The ground state (S(0)) and the lowest singlet excited state (S(1)) of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-Hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5,5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated. The S(0) and S(1) geometries are optimized at the ab initio Hartree-Fock and the singles configuration interaction (CIS) levels of theory, respectively. The CIS and semiempirical Zerners Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S(0)-->S(1)) and emission (S(1)-->S(0)) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are obtained. The absorption spectra in various solvents are calculated using the time-dependent density-functional theory method in combination with the polarized continuum model, which are in very good agreement with our experimental measurements. The solvent effects are discussed.

Experimental and theoretical studies of the structure and optical properties of nickel phthalocyanine nanowires

By using organic vapor phase deposition method, nickel phthalocyanines (NiPc) nanowires were successfully prepared, and the effects of heating temperature on the structural and optical properties of NiPc nanowires were investigated. Both the crystal structures of NiPc powders and nanowires are studied by x-ray diffraction patterns (XRD) and Fourier transform infrared spectra (FTIR). The lattice constants of NiPc nanowires from the fitting of XRD patterns are a = 13.04 A, b = 3.75 A, c = 24.32 A, β = 94.10°, belonging to the space group of monoclinic (P21/c). X-ray photo-electron spectroscopy (XPS) suggests main peaks at 284.82 eV and 286.18 eV for C-C and C-N bonds and two peaks at 397.8 eV and 398.8 eV for N-C and the N–Ni bonds, respectively, in NiPc nanowires. FTIR spectra show a structural transition from powder to nanowire. The optical properties of NiPc nanowires have been investigated via a comparison between theoretical and experimental approaches. The most significant absorption peaks of NiPc nanowires in the visible region are located at 626 nm and 672 nm, showing a blue shift comparing with the β-NiPc. We have also theoretically revealed the excitation energies in NiPc single molecule and dimer in the form of α and β phases using time-dependent density-functional theory. These theoretical results are in qualitative agreement with the measurements of optical properties. Moreover, no noticeable change in crystalline form is shown when tuning the heating temperature from 420 °C to 450 °C, suggesting a wide synthesis temperature window. The reduced Ni-Ni distance indicated a clear advantage over powder in terms of potential applications in organic electronics.

Crystal structure tuning in organic nanomaterials for fast response and high sensitivity visible-NIR photo-detector

A new polymorph of copper hexadecafluorophthalocyanine, η-F16CuPc, was fabricated via organic vapor phase deposition. The crystal structure, morphology and optical properties of η-F16CuPc were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transformed infrared spectroscopy and UV-Vis spectroscopy. Moreover, the phase transitions of F16CuPc from the η- to the β-phase were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry. In addition, the visible-NIR photodetectors based on F16CuPc nanowires were fabricated and their photo-response features were systematically studied. The absorption spectra of η-F16CuPc exhibited a broad band in the visible and near-infrared (NIR) regions, extending to a wavelength of ∼900 nm. The η-F16CuPc photodetector exhibited a fast response even at a low bias voltage of 0.5 V under a solar simulator with a power of 100 mW cm−2. The photocurrent response rise/fall time of the photodetector was ∼448 ms under various irradiation wavelengths. This organic-based photodetector also performed well for near-infrared light with wavelengths up to 940 nm.