Min-Yi Zhang

Vacuum electron field emission from SnO2 nanowhiskers synthesized by thermal evaporation

Rod-shaped and wire-shaped SnO2 nanowhiskers were synthesized by thermal evaporating of tin powders at 900 °C. Three Raman peaks (474, 632, 774 cm−1) showed the typical feature of the rutile phase of as-synthesized SnO2 nanowhiskers, which was consistent with the result of x-ray diffraction. A relatively low turn-on field of 1.37 V µm−1 at a current density of 0.1 µA cm−2 was obtained. The dependence of emission current density on the electric field followed a Fowler–Nordheim relationship. Our results indicated that SnO2 nanowhiskers had an interesting FE property as a wide band gap semiconductor.

First-principles study of CN carbon nitride nanotubes

The structural, electronic and optical properties of carbon nitride (CN) nanotubes (CNNTs) built from triazine units were investigated by using density-functional theory (DFT) within the generalized gradient approximation. The total energies per CN unit (relative values) of armchair CNNTs are smaller than those of zigzag CNNTs for the same tube index. This leads to unsmooth tubular surfaces for nitrogen-nitrogen lone pair repulsions and porous structures. The armchair CNNTs appear as polygons while zigzag CNNTs are circles. Both the armchair and zigzag CNNTs are direct band gap semiconductors and their band gaps are dependent on tube size and chirality. The calculated dielectric functions of CNNTs are dependent on tube size and the directions of polarization. Moreover, they may be used as photocatalysts to split water to produce hydrogen.

First-principles study of one- and two-photon absorption of the H-bonding complexes from monomeric red fluorescent proteins with large Stokes shifts.

LSSmKate1 and LSSmKate2 are monomeric red fluorescent proteins (RFPs) with large Stokes shifts (LSS). The hydrogen-bonding (H-bonding) network within LSSmKate1 or LSSmKate2, which is composed of a chromophore and some surrounding amino acid residues, supports their special spectral properties. In this work, we propose H-bonding complex models to simulate the H-bonding network of LSSmKate1 and LSSmKate2 and employ the time-dependent density functional theory combining with the sum-overstates method to calculate their one- and two-photon absorption characters. We discuss the influence of the hydrogen bond on the one- and two-photon absorption properties of these H-bonding complexes through intermolecular hyperconjugation of the hydrogen-bond form.

Buried tungsten silicide layer in silicon on insulator substrate by Smart-Cut®

A single-crystalline Si/SiO2/poly-WSix/Sub-Si structure has been successfully fabricated by a new method incorporating a standard smart-cut® technique and a high temperature reaction between tungsten and silicon. Annealing at 800–1100 °C does not only strengthen the bonding of the wafers but also induces solid phase reaction of deposited tungsten and silicon. A poly-crystalline WSix (1 < x < 2) layer with a tetragonal structure is formed below the buried oxide layer. Cross section images of TEM show three steep interfaces of the four layers. It is found that increasing the annealing temperature is in favour of decreasing the sheet resistance of tungsten silicide and improving the crystal quality of the top silicon layer. However, a spreading resistance profile measurement shows that annealing under high temperature (≥1000 °C) will induce diffusion of tungsten into the Si substrate which is confirmed by the EDX results and the reason is presented.

Theoretical study of the proton transfer wires influence on the one- and two-photon absorption properties of green fluorescent protein chromophore

The excited-state proton transfer (ESPT) via proton transfer wires in green fluorescent protein (GFP) plays an important role on the spectroscopic of GFP. In this work, we use the proton transfer wires and the chromophore complex to simulate the tautomer structures of neutral state and the intermediate state in wt-GFP. And we employ the time-dependent density functional theory combined with the sum-over-states method to calculate the one- and two-photon absorption properties of these complexes in GFP. We obtain the large stokes shift from 383 nm to 500 nm in GFP when simulating the ESPT process by these isomerous H-bonding complexes. We find that the TPA spectrum of the H-bonding complex of the intermediate state agrees more with experimental measurement than that of the H-bonding complex of the neutral state. The TPA spectrum of GFP might be mainly dominated by the structure which is similar to the H-bonding complex of intermediate state. Further, we simulate another kind of complex which possess short-strong hydrogen bonds in proton transfer wires, and find that TPA properties of these complexes are much stronger than that of the complexes with the long distance proton wires from GFP.

THEORETICAL STUDY OF ONE- AND TWO-PHOTON ABSORPTION PROPERTIES FOR THREE SERIES OF DIPHENYLAMINE AND DIFLUORENYLAMINE SUBSTITUTED CONJUGATED COMPOUNDS

The one-photon (OPA) and two-photon (TPA) absorption properties of three series of symmetrically substituted quadrupolar compounds with structure of donor-π bridge-donor (D–π–D) were investigated by time-dependent density functional theory (TDDFT) based on the Hartree–Fock (HF)-optimized geometrical structures. These compounds were constructed with either phenyl or fluorenyl groups connected by vinylene unit as the central π-conjugated bridges and either diphenylamine or difluorenylamine groups as terminal electron donors. The calculated OPA spectra are dominated by two strong transitions which are attributed to the charge transfers from the donor groups to central conjugated chains. The OPA and TPA transition strength all increase with the extension of conjugated chain length in each series and the corresponding wavelength shifts red in general. The transition strength in either OPA or TPA process also increases from series one to series two or three by replacing the phenyl groups with fluorenyl groups. The intramolecular charge transfers make significant contributions to the TPA activity. According to the three-state model, the enhanced TPA activity comes from the enhancement of transition moment between states–states as conjugated chain increases.

A THEORETICAL STUDY ON SECOND HARMONIC GENERATION HYPERPOLARIZABILITIES OF PHENYLALANINE POLYPEPTIDES

The second harmonic generation (SHG) hyperpolarizabilities of phenylalanine and homopolypeptides are investigated by configuration interaction among singly excited configurations (CIS) technique combined with the sum-over-states (SOS) method. The geometries of peptides containing phenylalanine (Phe)n(n = 1–8) are optimized by B3LYP/6-31g(d) method, and they form the special structures like β-sheet (a common protein secondary structure). It is found that the energy gaps of various peptides are reduced and the hyperpolarizabilities are increased with the peptide chains lengthened. We discuss the origin of the second-order nonlinear optical response in phenylalanine homopolypeptides and confirm that the π → π* transitions in the aromatic residue of phenylalanine make the most important contributions to the second-order polarizability. Our results strongly suggest that the hyperpolarizabilities are dominated from the propagation direction of peptide chains.

ELECTRONIC STRUCTURES, EXCITED SPECTRA, AND NONLINEAR OPTICAL PROPERTIES OF N-METHYL-FULLEROPYRROLIDINE-OLIGOANILINE DYADS NMPC60-AN(n)(n = 1-5): QUANTUM CHEMISTRY STUDY

The geometrical and electronic structures of a series of dyads NMPC60–ANn (n = 1–5) were investigated at B3LYP/6-31G* level. Then, the excited states were calculated at TDB3LYP/3-21G* level to investigate the photophysical properties. The third-order polarizabilities of degenerate four-wave mixing (DFWM), electric-field-induced second-harmonic generation (EFISHG), and third-harmonic generation (THG) were calculated in combination with the sum-over-states (SOS) method. The obtained absorption spectra show a remarkable red shift. The average value of third-order polarizabilities 〈γ〉 away from the resonant region increases with n for NMPC60–ANn. The charge transfers from the oligoanilines to C60 cage make the main contribution to the large third-order polarizabilities of this series of compounds. It is found that 〈γ〉 can be expressed as a function of the chain length of linked oligoaniline and 11.74 × 10-34esu is the upper limit after n = 7 for NMPC60–ANn in the static state.

SECOND-HARMONIC GENERATION OF FLUORESCENT PROTEINS

We theoretically study the second-order nonlinear optical properties of six fluorescent proteins (FPs), such as green fluorescent protein (GFP), BFP, enhanced BFP (eBFP), CFP, YFP, and DsRed. To begin with, the geometries of all these FP chromophores are optimized at B3LYP/6-311++G** level in a water medium and the polarized continuum model (PCM in water) method is adopted. Using a time-dependent density functional theory (TDDFT) method, the electronic structures and excited-state properties of chromophores are determined. Here we employ TDDFT combining with the sum-over-states (SOS) method to calculate the first-order hyperpolarizability for second-harmonic generation (SHG) optical process. Moreover, we discuss the origin of the nonlinear optical response and determine what caused the variation of first-order hyperpolarizability. Our calculations show that the charge transfers of π → π* in the central conjugated structure and p → π* charge transfers from the side chain R1 to conjugated structure of chromophores markedly affect the first-order hyperpolarizability.