Jeung Hun Park

Control of Electron Beam-Induced Au Nanocrystal Growth Kinetics through Solution Chemistry.

Measurements of solution-phase crystal growth provide mechanistic information that is helpful in designing and synthesizing nanostructures. Here, we examine the model system of individual Au nanocrystal formation within a defined liquid geometry during electron beam irradiation of gold chloride solution, where radiolytically formed hydrated electrons reduce Au ions to solid Au. By selecting conditions that favor the growth of well-faceted Au nanoprisms, we measure growth rates of individual crystals. The volume of each crystal increases linearly with irradiation time at a rate unaffected by its shape or proximity to neighboring crystals, implying a growth process that is controlled by the arrival of atoms from solution. Furthermore, growth requires a threshold dose rate, suggesting competition between reduction and oxidation processes in the solution. Above this threshold, the growth rate follows a power law with dose rate. To explain the observed dose rate dependence, we demonstrate that a reaction-diffusion model is required that explicitly accounts for the species H(+) and Cl(-). The model highlights the necessity of considering all species present when interpreting kinetic data obtained from beam-induced processes, and suggest conditions under which growth rates can be controlled with higher precision.

E-beam deposited Ag-nanoparticles plasmonic organic solar cell and its absorption enhancement analysis using FDTD-based cylindrical nano-particle optical model

We report the plasmon-assisted photocurrent enhancement in Ag-nanoparticles (Ag-NPs) embedded PEDOT:PSS/P3HT:PCBM organic solar cells, and systematically investigate the causes of the improved optical absorption based on a cylindrical Ag-NPs optical model which is simulated with a 3-Dimensional finite difference time domain (FDTD) method. The proposed cylindrical Ag-NPs optical model is able to explain the optical absorption enhancement by the localized surface plasmon resonance (LSPR) modes, and to provide a further understanding of Ag-NPs shape parameters which play an important role to determine the broadband absorption phenomena in plasmonic organic solar cells. A significant increase in the power conversion efficiency (PCE) of the plasmonic solar cell was experimentally observed and compared with that of the solar cells without Ag-NPs. Finally, our conclusion was made after briefly discussing the electrical effects of the fabricated plasmonic organic solar cells.

Ion Beam Alignment of Liquid Crystal on Amorphous SiOx Film

Liquid crystal alignment on a-SiOx film surfaces through the ion beam exposure is studied. The pre-tilt angle of liquid crystals on a-SiOx film surfaces can be controlled from about 8° to about 89° by changing the ion beam incident angle from 25° to 80°. Vertical alignment of liquid crystal can be ascribed to high contact angles on ion-beam exposed inorganic film surfaces.

Visualization of Active and Passive Control of Morphology during Electrodeposition

Morphological instability, particularly dendrite formation, can cause potentially catastrophic failure in rechargeable batteries. Morphological instabilities can lower the quality of electroplated coatings, yet may also be useful in forming porous deposits. Thus, it is important to develop strategies to control these instabilities. Liquid cell electron microscopy allows us to image in real time and with nanoscale resolution the evolution of the solid-liquid interface during electrochemical deposition as a function of process conditions [1-3]. This allows us to obtain insights into the mechanisms leading to instabilities and to investigate strategies for controlling electrodeposited morphology.

Control of Liquid Crystal Alignment on SiOx Film Surfaces by Low-Energy Ion Beam

In this paper, liquid crystal alignment on SiOx film surfaces irradiated by a low-energy ion beam is systematically examined experimentally. As a consequence of low-energy ion beam exposure, LCs can be aligned vertically when a high rms of electric potential is present on SiOx film surfaces on the basis of electric force microscopy data, while they can be aligned homogeneously when a low rms of electric potential is present on those surfaces. It is also found by X-ray photoemission spectroscopy and the contact angle method that, for the vertical alignment, the absorption curve shifts to a lower binding energy and a higher contact angle, respectively. All experimental results consistently show that the Coulomb interaction between LC molecules and inorganic film surfaces has a dominant effect on LC alignment on films irradiated by a low-energy ion beam.

Electrochemical electron beam lithography: Write, read, and erase metallic nanocrystals on demand

We develop an electrochemistry- and radiolysis-based patterning technique for site-specific deposition and dissolution of metallic nanocrystals. We develop a solution-based nanoscale patterning technique for site-specific deposition and dissolution of metallic nanocrystals. Nanocrystals are grown at desired locations by electron beam–induced reduction of metal ions in solution, with the ions supplied by dissolution of a nearby electrode via an applied potential. The nanocrystals can be “erased” by choice of beam conditions and regrown repeatably. We demonstrate these processes via in situ transmission electron microscopy using Au as the model material and extend to other metals. We anticipate that this approach can be used to deposit multicomponent alloys and core-shell nanostructures with nanoscale spatial and compositional resolutions for a variety of possible applications.

Automated analysis of evolving interfaces during in situ electron microscopy

In situ electron microscopy allows one to monitor dynamical processes at high spatial and temporal resolution. This produces large quantities of data, and hence automated image processing algorithms are needed to extract useful quantitative measures of the observed phenomena. In this work, we outline an image processing workflow for the analysis of evolving interfaces imaged during liquid cell electron microscopy. As examples, we show metal electrodeposition at electrode surfaces; beam-induced nanocrystal formation and dissolution; and beam-induced bubble nucleation, growth, and migration. These experiments are used to demonstrate a fully automated workflow for the extraction of, among other things, interface position, roughness, lateral wavelength, local normal velocity, and the projected area of the evolving phase as functions of time. The relevant algorithms have been implemented in Mathematica and are available online.

In Situ LC-TEM Studies of Corrosion of Metal Thin Films in Aqueous Solutions

Jeung Hun Park, See Wee Chee, Suneel Kodambaka, and Frances M. Ross 1 Department of Materials Science and Engineering, University of California Los Angeles, 410 Westwood Plaza, Los Angeles, CA 90095, USA 2 IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA 3 Centre of Bioimaging Sciences, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117557

Control of Growth Front Evolution by Bi Additives during ZnAu Electrodeposition

The performance of many electrochemical energy storage systems can be compromised by the formation of metal dendrites during charging. Additives in the electrolyte represent a useful strategy to mitigate dendrite formation, but understanding the mechanisms involved requires knowledge of the nanoscale effects of additives during electrochemical deposition. Here we quantify the effects of an inorganic additive on the morphology of an evolving electrochemical growth front, using liquid cell electron microscopy to provide the necessary spatial and temporal resolution. We examine deposition of ZnAu on Au in the presence of Bi additive, and show that low concentrations of Bi delay but do not prevent the formation of growth front instabilities. We describe a model in which Bi segregates at the growth front and promotes the surface diffusion and relaxation of Zn, allowing better coverage of the initial Au electrode surface. A more precise knowledge of the mechanism of inorganic additive effects may help in designing electrolyte chemistry for battery and other applications where morphology control is essential.

Estimation of Nanoscale Current Density Distributions during Electrodeposition

The control of interfacial morphology in electrochemical processes is essential for various applications. Morphological instability, particularly dendrite formation, can cause potentially catastrophic failure in rechargeable batteries and can lower the quality of electroplated coatings, yet may also be useful in forming porous deposits. Thus, it is important to understand the temporal development of morphology and the nature of the forces that govern the geometry of the electrode-electrolyte interface. Liquid cell electron microscopy allows us to image, in real time and with nanoscale resolution, the evolution of the solid-liquid interface during electrochemical deposition as a function of process conditions [13]. Our nanoscale resolution allows us to infer the current density along the interface.