The electrical properties of hydrogen-terminated silicon (H-Si) surfaces were investigated by the assembly and testing of reliable and reproducible mercury-silicon junctions. When a H-Si surface is exposed to air under ambient conditions, the current density-bias voltage curve of the thus formed mercury-silicon junction initially maintains ohmic characteristics for a period of 8-12 h and then evolves to diode behavior. The current density substantially decreases, but can be recovered to a certain extent upon sonication in organic solvents. In agreement with infrared spectroscopic results, organic contamination of H-Si is suggested to play an important role in the transition of the electrical properties.

The structures of several recently reported organometallic NLO chromophores, (thiophene) manganese tricarbonyl cations, were fully optimized at the DFT non-local (GGA) level. The calculated results show that the fragments 2-SC4H3CH=CHC6H4-R in these organometallic chromophores are not planar with dihedral angles of 42.2～59.8° between two aromatic rings, which are different from those of uncoordinated counterparts. Based on the DFT geometry optimization, the second-order nonlinear optical polarizabilities were calculated by using RPA method. The calculated results indicate that incorporation of Mn(CO)+ 3 unit with thiophene leads to a substantial increase in the second-order polarizability (β).

On the basis of the ZINDO program, we have designed a program to calculate the second-order nonlinear polarizabilitiesβlike, β0 andβµaccording to the sum-over-states (SOS) expression. A series of new 4-(dicyanomethylene)-2-methyl-6-(p-dithyla-mino-styryl)-4H-pyran (DCM) derivatives were designed and their electron spectra and nonlinear optical properties were studied. It is proposed that these compounds possess two important excited states close to each other in energy, both contri-buting to hyperpolarizability in an additive fashion; 4-(dicyanomethylene)-2,6-bis-(p-donor-styryl)-4H-pyran derivatives aremore nonlinear than 4-(dicyanomethylene)-2,6-bis-(p-donor-phenyl)-azo-4H-pyran derivatives. The high nonlinearities, good thermal stability and high transparency make them attractive candidates for second-order nonlinear applications such as electro-optic modulators and frequency doublers. © 2001 Elsevier Science B.V. All rights reserved.

Based on DCM structure moiety, we have designed a series of symmetric (dicyanomethylene)-pyran derivatives in which benzene rings were replaced by thiophene rings. Their two-photon absorption (TPA) properties were studied by using a combination of intermediate neglect of differential overlap Hamiltonian with the configuration interaction technique combined with sum-over-states expression. The calculated results show that the TPA cross-sections are sensitive to molecular structure. The electron excessivity of heterocycle and double-bridge effect on TPA cross-sections is discussed.© 2001 Elsevier Science B.V. All rights reserved.

AM1 semiempirical calculation method was used to study the structures and electronic properties of N-methyl-2-(21-thiophene)-pyrrolo[3,4]C60 (MTPC) and N-methyl-pyrrolo[3,4]C60 (MPC). Geometry ptimization results indicate that MTPC possesses eight stable isomers (four pairs of enantiomers), nd among which, MTPC-1A, MTPC-3A, MTPC-1B, MTPC-3B correspond to the lowest total energies and they have larger dipole moments. The frontier molecular orbitals of C60 moiety dictated that of MTPC and MPC. For MTPC and MPC, there exists stronger intramolecular electron transfer, with C60 moiety as the electron acceptor while thiophene and pyrrolidine part are electron donors. Compared with that of N-methyl-pyrroloβ,4]C60 (MPC), the C60 moiety of MTPC isomers possesses less net charge. Based on the AM1 geometry optimiza-tion, the electronic spectra and second-order nonlinear optical polarizabilities (β) of MTPC and MPC were calculated by using INDO/SCI method combined with sum-over-states (SOS) expression. The calculated results reveal that there exist weak absorption peaks above 400 nm for MTPC and MPC. The thiophene-containing molecule (MTPC) exhibits good optical nonlinearity with the β values slightly less than that of MPC. For MTPC, β values vary with different isomers. When electron donor such as amino connected with thiophene, β values increase by one factor.