Regulating the orientation of a single coordinate bond by the synergistic action of mechanical forces and electric field.

The molecular binding orientation with respect to the electrode plays a pivotal role in determining the performance of molecular devices. However, accomplishing in situ modulation of single-molecule binding orientation remains a great challenge due to the lack of suitable testing systems and characterization approaches. To this end, by employing a developed STM-BJ technique, we demonstrate that the conductance of pyridine-anchored molecular junctions decreases as the applied voltage increases, which is determined by the repeated formation of thousands of gold-molecule-gold dynamic break junctions. In contrast, the static fixed molecular junctions (the distance between two electrodes is fixed) with identical molecules exhibit a reverse tendency as the bias voltage increases. Supported by flicker noise measurements and theoretical calculations, we provide compelling evidence that the orientation of nitrogen-gold bonds (a universal coordinate bond) in the pyridine-anchored molecular junctions can be manipulated to align with the electric field by the synergistic action of the mechanical stretching force and the electric fields, whereas either stimulus alone cannot achieve the same effect. Our study provides a framework for characterizing and regulating the orientation of a single coordinate bond, offering an approach to control electron transport through single molecular junctions.

Nonlinear Optical Properties of CdSe Quantum Dots Having Different Sizes.

We synthesis different sizes of cadmium selenide quantum dots (CdSe QDs) and their linear and nonlinear optical properties. Size-controlled CdSe QDs are characterized by ultraviolet-visible absorption, photoluminescence, transmission electron microscope (TEM), and Z-scan techniques. Redshift was observed in the linear absorption and photoluminescence with an increase in the size of CdSe QDs. TEM images show the sizes of the QDs are 2.2 nm-4.9 nm. Bandgap tuning CdSe QDs of third-order nonlinear optical properties are investigated at 532 nm with an open aperture Z-scan technique using a picosecond laser. These CdSe QDs can be used as optical limiters.

Photoluminescence and Nonlinear Optical Properties of Transition Metal (Ag, Ni, Mn) Doped ZnO Nanoparticles.

ZnO nanoparticles doped with transition metal (Ag, Ni, Mn) were prepared by using co-precipitation method. Absorption spectra showed exciton peak for ZnO nanoparticles at 367 nm and for Ag, Ni, Mn doped ZnO nanoparticles at 328, 354 nm and 342 nm respectively. Photoluminescence dynamics showed shallow level-trap and deep-level emission where the intensity of deep-level trap increases with transition metal doping due to the oxygen deficiency. Nonlinear absorption measurements showed the two-photon absorption increasing with transition metal following the trend Ag > Ni > Mn when excited with 532 nm, 30 ps laser pulses. The transition metal doped and un-doped ZnO nanoparticles can be used as optical limiters.