One-dimensional self-consistent solution of modulation doped nanocrystalline/crystalline Si heterojunctions

A kind of modulation doped structure of n-type nanocrystalline hydrogenated silicon (nc-Si:H) film with intrinsic nc-Si:H layer with p-type bulk Si 100 substrate was proposed. The numerical self-consistent solutions of one-dimensional Schrödinger and Poisson equations along the direction normal to the heterojunction were performed to calculate the distribution of electron density and profiles of conduction-band as a function of ionized donor concentration in doped film, thickness of intrinsic layer, and other device parameters in the junctions. The calculated results are shown to be in agreement with experimental data. The relation of mobility vs sheet density of two-dimensional electron gases under different scattering mechanisms was analyzed. The obtained consequences may be used to evaluate optimum design for the modulation doped nc-Si:H-based devices.     

Ratiometric fluorescent sensor based on inhibition of resonance for detection of cadmium in aqueous solution and living cells

Although cadmium has been recognized as a highly toxic heavy metal and poses many detrimental effects on human health, the Cd(2+)-uptake and nosogenesis mechanisms are still insufficiently understood, mainly because of the lack of facile analytical methods for monitoring changes in the environmental and intracellular Cd(2+) concentrations with high spatial and temporal reliability. To this end, we present the design, synthesis, and photophysical properties of a cadmium sensor, DQCd1 based on the fluorophore 4-isobutoxy-6-(dimethylamino)-8-methoxyquinaldine (model compound 1). Preliminary investigations indicate that 1 could be protonated under neutral media and yield a resonance process over the quinoline fluorophore. Upon excitation at 405 nm, 1 shows a strong fluorescence emission at 554 nm with a quantum yield of 0.17. Similarly, DQCd1 bears properties comparable to its precursor. It exhibits fluorescence emission at 558 nm (Φ(f) = 0.15) originating from the monocationic species under physiological conditions. Coordination with Cd(2+) causes quenching of the emission at 558 nm and simultaneously yields a significant hypsochromic shift of the emission maximum to 495 nm (Φ(f) = 0.11) due to inhibition of the resonance process. Thus, a single-excitation, dual-emission ratiometric measurement with a large blue shift in emission (Δλ = 63 nm) and remarkable changes in the ratio (F(495 nm)/F(558 nm)) of the emission intensity (R/R(0) up to 15-fold) is established. Moreover, the sensor DQCd1 exhibits very high sensitivity for Cd(2+) (K(d) = 41 pM) and excellent selectivity response for Cd(2+) over other heavy- and transition-metal ions and Na(+), K(+), Mg(2+), and Ca(2+) at the millimolar level. Therefore, DQCd1 can act as a ratiometric fluorescent sensor for Cd(2+) through inhibition of the resonance process. Confocal microscopy and cytotoxicity experiments indicate that DQCd1 is cell-permeable and noncytotoxic under our experimental conditions. It can indeed visualize the changes of intracellular Cd(2+) in living cells using dual-emission ratiometry.     

A theoretical study of the Y$\mathsf{_{3}}$O clusters

Hybrid density functional calculations are performed to study the structural and electronic properties of neutral, anionic and cationic Y₃O clusters. The most stable structures of these clusters are found to be triply bridging oxygen atom structures with C_S symmetry. The ground states of Y₃O, Y₃O^- and Y₃O ⁺ are doublet (²A), triplet (³A) and singlet (¹A), respectively. The calculated electron affinities and ionization potentials are in good agreement with the available experimental data. Time-dependent density functional theory is used to calculate the low-lying excited states. A theoretical assignment for the features in the experimental photoelectron spectra is given.Received: 5 November 2003, Published online: 20 January 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 31.15.Ew Density-functional theory - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.)     

Theoretical study of nitric oxide adsorption on Au clusters

The adsorption properties of NO molecule on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory with the generalized gradient approximation, and with the hybrid functional. For anionic and cationic clusters, the charge transfer between the Au clusters and NO molecule and the corresponding weakening and elongation of the N-O bond are essential factors of the adsorption. The neutral Au clusters have also remarkable adsorption ability to NO molecule. The adsorption energies of NO on the cationic clusters are generally greater than those on the neutral and anionic clusters.     

Halogen bonds and metal bonds involving superalkalies M2OCN/M2NCO (M = Li,Na) complexes

Structural Chemistry - A new type of halogen bond formed by supermetals or superalkalies with dihalogen molecules was analyzed by means of ab initio at the MP2/aug-cc-pVTZ level. The results reveal...     

N-(1-Naphthyl) Ethylenediamine Dinitrate: A New Matrix for Negative Ion MALDI-TOF MS Analysis of Small Molecules

An organic salt, N-(1-naphthyl) ethylenediamine dinitrate (NEDN), with rationally designed properties of a strong UV absorbing chromophore, hydrogen binding and nitrate anion donors, has been employed as a matrix to analyze small molecules (m/z?相似文献   

Facile synthesis of 1-bromo-7-alkoxyl perylene diimide dyes: toward unsymmetrical functionalizations at the 1,7-positions

In this Letter, we report a facile approach to synthesize unsymmetrical 1-bromo-7-alkoxyl perylene diimides by the nucleophilic substitution of one of the two 1,7-dibromo units with an alkyl alcohol using K₂CO₃ as the base. A further replacement of another bromo, for example, by using 4-hydroxylpyridine resulted in unsymmetrical functionalizations at the 1,7-positions. The optical properties of the unsymmetrical PDI derivative were reported and compared with those of the symmetrical derivative.     

Dependence of solar cell performance on Si:H nanostructure

Nanocrystalline Si:H (nc-Si:H) films were prepared to fabricate solar cells, of which the output properties were investigated. Nanocrystals are indicated in the films by electron microscopy. Raman experimental data of phosphorus doped nc-Si:H films were well simulated by Fano effect (FE) profiles due to Fano interference between discrete optical phonons and a continuum of electronic excitations in the nanocrystals. Raman measurement signals of the phosphorus doped and intrinsic nc-Si:H layers can be fitted well by a strain-calibrated phonon confinement effect (S-PCE) model owing to incorporated effects of phonon confinement and tensile strain. For the Raman signals from intrinsic films, the fit according to the S-PCE model is better than that based on FE. The output properties of P–I–N type nc-Si:H solar cells, depending on the material structures, including the mean size of the nanocrystals, crystalline volume fraction and disorder, were analyzed.     

Conduction mechanism of Aviram-Ratner rectifiers with single pyridine-sigma-C(60) oligomers

We present the electron transport of pyridyl aza[60]fulleroid oligomers, abbreviated as C(60)NPy, which is based on the donor-barrier-acceptor (D-sigma-A) architecture, at a single molecular scale using scanning tunneling microscopy. A rectifying effect is observed in the current-voltage characteristics. The theoretical calculation shows that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are well localized either on the Py moiety (donor) or on the C(60) moiety (acceptor), indicating the sigma-bridge decouples the LUMO and the HOMO of the donor and the acceptor, respectively. This structure accords well with the unimolecular rectifying model proposed by Aviram and Ratner [Chem. Phys. Lett. 1974, 29, 277]. The mechanism of the rectifying effect is understood by analyzing in detail the electron transport through energy levels of the donor and the acceptor of the C(60)NPy molecules. By directly comparing the experimental conductance peaks and the calculated density of states of the C(60)NPy, we find that the observed rectification is attributed to the asymmetric positioning of the LUMOs and the HOMOs of both sides of the acceptor and the donor of the C(60)NPy molecules with respect to the Fermi level of the electrodes. When a main voltage drop is over the molecule-electrode vacuum junction but a small fraction over the molecule itself, the shift of the energy levels between the donor and the acceptor will be small. This behavior deviates from the original proposal by Aviram and Ratner in which a large shift of the energy level is expected.