Geunhee Lee

Maximum Li storage in Si nanowires for the high capacity three-dimensional Li-ion battery

Nanowires can serve as three-dimensional platforms at the nanometer scale for highly efficient chemical energy storage and conversion vehicles, such as fuel cells and secondary batteries. Here we report a coin-type Si nanowire (NW) half-cell Li-ion battery showing the Li capacity of approximately 4000 mAh/g, which nearly approaches the theoretical limit of 4200 mAh/g, with very high Coulombic efficiency of up to 98%. Concomitantly, we provide direct evidence of reversible phase transitions in the Si NW anodes at the full electrochemical cycles, varying from pure Si to Li22Si5 phase, which has been known empirically inaccessible in the bulk limit.

Heteroepitaxial ferroelectric ZnSnO3 thin film.

We investigated the ferroelectric characteristics of an epitaxial perovskite ZnSnO(3) thin film on a (111) SrRuO(3)/(111) SrTiO(3) substrate fabricated by pulsed laser deposition. We confirmed that the ZnSnO(3) thin film was epitaxially grown on the substrate, forming large terraces on the surface of the ZnSnO(3) thin film. The ZnSnO(3) thin film exhibited a high ferroelectric polarization of approximately 47 microC/cm(2), which was further supported by first-principles calculations.

Vertically aligned Si intrananowire p-n diodes by large-area epitaxial growth

We demonstrate fabrication of vertically aligned, intrananowire p-n diodes by large-area epitaxial growth of Si nanowires (NWs). The axially modulated doping profile of p-n junctions is achieved by in situ doping with alternating addition of dopants in the axial sequence during Au-assisted chemical vapor deposition. We provide direct evidence of the intra-NW p-n junctions using scanning local probes in both individual NWs and vertically aligned NWs at large areas. Our study suggests implication for integrated electronics and optoelectronics based on bottom-up Si NWs.

Directionally integrated VLS nanowire growth in a local temperature gradient.

Integrated nanowire (NW) ensembles can be used in various applications in electronic circuits, biological probes, and energy conversion systems. The self-organization of nanowires requires spontaneous ordering over a large anisotropic energy barrier set at the different length scales in the axial and radial directions. Herein, we report a simple and robust growth mechanism that coherently directs the nanowire growth directions by introducing a local temperature gradient as the local kinetic variable during the conventional vapor– liquid–solid (VLS) growth. This NW growth, which is the earliest and prevailing synthetic route for semiconductor NWs, typically occurs in spatially uniform heating zones that surround the growing crystals on substrates; thus, all the reactions for NW growth at the VLS phase boundaries are isothermal. The differences in the chemical potentials of the growth species are the thermodynamic driving force for the VLS growth, which occurs uniformly along the VLS interfaces, through which the growth species diffuse. The crystallographic orientation of a NW is thermodynamically determined at the LS interface within the eutectic liquid droplet of given size and geometry during the initial nucleation. 11] Nevertheless, the embryonic NWs nucleate in an isotropically random manner at the edges of the hemispheric droplets, 13] thus leading to an unpredictable growth direction, unless external constraints such as directional epitaxy 15] and guiding templates are imposed. Consequently, the systematic integration of VLS NWs usually requires supplementary processes after the NW growth. In principle, however, any local variation in the interfacial thermodynamics, that is, local temperature variations at the interfaces, can influence the elemental growth behavior. In our growth scheme, we imposed a temperature gradient normal to the substrate plane during the VLS Si NW growth, and observed that the NW growth parallel to the local temperature gradient is spontaneous and directional, with a significantly increased growth rate compared to the isothermal growth. We also provide a phenomenological model for the directional NW growth within the framework of the interfacial thermodynamic stability. In particular, we discuss the role of the temperature gradient on the redistribution of a local kinetic variable, that is, local interfacial supersaturation, on the thermodynamic stability at the fluctuating VL and LS interfaces. Our growth scheme provides practical implication for the growth of integrated NW ensembles. The design of our VLS chemical vapor reactor (Figure 1a), is much the same as the conventional hot-wall tube furnace, 23] except that the susceptor underneath the growth substrates can be cooled by air circulation (Figure S1 in the Supporting information). One can thus expect that the VLS growth is not isothermal, and instead that a stable temperature gradient is established perpendicular to the substrate. For example, when the reactor wall is heated at 650 8C by the furnace, the substrate temperature is maintained at 490 8C under air cooling. Figure 1b shows the simulated temperature distribution within the reactor. We wish to emphasize two aspects of our main observations. Firstly, the Si NW growth directions are vertical over the large areas (region A in Figure 1c). This observation is consistent with the direction of the temperature gradient. Secondly, the axial growth rate is unprecedentedly enhanced compared to the isothermal growth (see also Figure 3). The tendency to vertically aligned NW growth is found regardless of the types of substrates or the catalyst density, as similar behavior is observed on quartz, indium tin oxide, and alumina substrates (Figure S2 in the Supporting Information). Apparently, the growth direction is not dictated by possible epitaxial relations with crystalline substrates. Moreover, when Si NWs are grown on the edge of the substrate, over which the temperature gradient is radially distributed, (Figure 1c, region B), the NW growth direction precisely follows the temperature gradients at all different positions along the edge. The length of the NW growth is proportional to the magnitude of the temperature gradients, which increase with the proximity of lateral position to the edge (Figure 1d). These observations unequivocally demonstrate that the NW growth velocity, that is, its direction and magnitude, follows the local temperature gradient. We found two additional features by examining the individual NWs along the entire length from the bottom to top, as in the time-lapse SEM images (Figure 2a–e). In the very early growth stage (Figure 2 a,b), the individual NWs [*] Dr. G. Lee, Dr. Y. S. Woo, J.-E. Yang, D. Lee, C.-J. Kim, Prof. M.-H. Jo Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) San 31, Hyoja-Dong, Nam Gu, Pohang, Gyungbuk 790-784 (Korea) Fax: (+ 82)54-279-2399 E-mail: mhjo@postech.ac.kr Homepage: http://www.postech.ac.kr/lab/mse/ndmpl/

Surface charge dynamics on ferroelectric PbZr0.48Ti0.52O3 films responding to the switching bias of electric force microscope

We investigated the role of surface charges in writing and reading ferroelectric bits on an epitaxial PbZr0.48Ti0.52O3 thin film by electric force microscopy (EFM). The sign of EFM surface potential was reversed within several hundred microseconds for 10 V. For a negative bias voltage of −10 V, EFM surface potential was reversed in several milliseconds. The different time scales of the EFM surface potential reversals originate from the screening of the ferroelectric polarization charges by the surface charges which pass over two different Schottky barriers depending on the applied bias polarity.

Formation of self-assembled polyelectrolyte multilayer nanodots by scanning probe microscopy.

We demonstrate that patterned nanodots can be obtained from alternatively self-assembled polyelectrolytes which consist of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) by using atomic force microscopy. The surface potentials are easily reversible as positive or negative depending on the kind of the top polyelectrolyte layer. With the layer by layer growth, the nanodots are formed exactly on the charged area and their thicknesses proportionally increase to the total number of monolayers with a uniform thickness of about 0.5 nm.

Writing nanotriboelectric charge bits on insulator surface

We demonstrated nanotriboelectric charge bits on the surface of insulating SiO2 thin film, prepared by thermo-oxidation of Si substrate with about 100 nm in thickness, by Kelvin probe force microscope. We wrote and read nanotriboelectric charge bits with the various experimental parameters such as contact force, rubbing speed, and repeat number. We could write the nanotriboelectric charge bits with the minimum line width of about 17 nm.