Browsing by Author "Ou, Yi-Ching"

Now showing 1 - 4 of 4

Interplay of Spinodal and Diffusion-Limited Aggregation in Formation of Nanocrystal Assembled 2D Islands
(2010-10-21)
Ou, Yi-Ching
;
Zhuravelv, Konstantin S.
;
Fang, Jiye
;
Jian, Wen-Bin
;
電子物理學系
Although self-organization of semiconductor nanocrystals (or quantum dots) into 3D superlattices and exploration of their collective optical, magnetic, and transport properties have been (demonstrated. little is known about the underlying physics of self-assembly, growth mechanisms, and interdot-coupling-induced collective properties. Here we report a facile way of preparing nanocrystal-assembled 2D islands by dropcasting nanocrystal suspension on a hot substrate. Growth mechanisms such as scaling function, spinodal decomposition phase separation, and diffusion-limited aggregation are investigated based on the observation of quasi-monolayer coverage. After a curve fitting to several theoretical growth models, the pair bond (interaction) energy, critical nucleus size, and the phase of growth patterns were determined. Moreover, by heating the substrate and controlling the concentration of nanocrystal suspension, the spinodal decomposition and diffusion-limited aggregation can be tuned to modulate growth patterns of 2D nanocrystal islands. The interplay of these two mechanisms results in a variation or wavelength in spinodal growth patterns and of fractal pattern dimensions. By using this experimental approach. various sizes and shapes of nanocrystal-assembled 2D islands can be deposited on a flat surface of either graphite or gold.
Probing Capacitive Coupling and Collective Transport in PbSe Quantum-Dot Arrays Using Scanning Tunneling Spectroscopy
(2009-05-07)
Ou, Yi-Ching
;
Wu, Jiun-Ji
;
Fang, Jiye
;
Jian, Wen-Bin
;
電子物理學系
Semiconductor quantum dots with diameters of several to tens of nanometers have been largely synthesized through colloidal techniques for nanoscience exploration of quantum confinement, Coulomb staircase, and artificial-atom states in individual quantum dots and self-assembling growth behavior. Although charge transport of three-dimensional quantum dot arrays has been attempted for study on the micron scale, the electrical properties of a nanoscale array, self-assembled from a single quantum dot through a bottom-up procedure, have not been explored yet. We control growth parameters to self-assemble different sizes of PbSe quantum-dot arrays on flat gold surface for scanning tunneling spectroscopy measurements. The current-voltage curves of the arrays are analyzed using a double-barrier tunnel junction model to acquire the shunt capacitance between the array and the gold substrate. The increment of this capacitance is small as the particle number increases extremely from 1 to 40. Thus the array cannot be taken as a simple semiconductor island. We apply collective transport theory to exploration of electron tunneling and capacitive coupling between PbSe quantum dots in the array.
Size dependent magnetization and high-vacuum annealing enhanced ferromagnetism in Zn(1-x)Co(x)O nanowires
(2008-01-01)
Jian, Wen-Bin
;
Chen, I-Jan
;
Liao, Tai-Ching
;
Ou, Yi-Ching
;
Nien, Cheng-Hsun
Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn(1-x)Co(x)O nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn(1-x)Co(x)O nanowires displayed weak ferromagnetism while the high-vacuum annealed nanowires exhibited strong ferromagnetic ordering at room temperature. Size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. Field cooled and zero-field cooled magnetization and coercivity measurements were performed between 2 and 300 K to study the evolution of magnetism from the weak to the strong ferromagnetic states. In particular, superparamagnetic features were observed and shown to be intrinsic characteristics of the DMS Zn(1-x)Co(x)O nanowires. The room-temperature spontaneous magnetization of individual Zn(1-x)Co(x)O nanowires was also established by using magnetic force microscope measurements.
Size effects on thermal treatments and room-temperature ferromagnetism in high-vacuum annealed ZnCoO nanowires
(2008-06-26)
Chen, I-Jan
;
Ou, Yi-Ching
;
Wu, Zhong-Yi
;
Chen, Fu-Rong
;
Kai, Ji-Jung
Zn0.92Co0.08O nanowires with two different average diameters, 19 and 38 nm, have been made by the vapor transport and Co ion implantation method. The as-implanted nanowires were thermally annealed through multiple steps in a high vacuum. The morphology and crystal structure of the nanowires were inspected by use of scanning and transmission electron microscopes. Electron microscopy analysis was used to ensure the absence of Co nanocrystals in the annealed nanowires. Measurements of temperature-dependent hysteresis were carried out to demonstrate a strong magnetic state in the high-vacuum annealed samples. The coercive field obeying a square-root temperature dependence and the room-temperature ferromagnetism have been discovered. Moreover, by comparison of magnetic properties between different-diameter nanowires, field-dependent magnetizations reveal considerably stronger ferromagnetism in the smaller diameter (19 nm) Zn0.92Co0.08O nanowires than that in the larger diameter (38 nm) nanowires. We argue that the generation of point defects by thermal annealing is the origin for the enhanced ferromagnetism in our high-vacuum annealed nanowires.