Donghun 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.

Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks

Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

Quantum Confinement Effects in Transferrable Silicon Nanomembranes and Their Applications on Unusual Substrates

Two dimensional (2D) semiconductors have attracted attention for a range of electronic applications, such as transparent, flexible field effect transistors and sensors owing to their good optical transparency and mechanical flexibility. Efforts to exploit 2D semiconductors in electronics are hampered, however, by the lack of efficient methods for their synthesis at levels of quality, uniformity, and reliability needed for practical applications. Here, as an alternative 2D semiconductor, we study single crystal Si nanomembranes (NMs), formed in large area sheets with precisely defined thicknesses ranging from 1.4 to 10 nm. These Si NMs exhibit electronic properties of two-dimensional quantum wells and offer exceptionally high optical transparency and low flexural rigidity. Deterministic assembly techniques allow integration of these materials into unusual device architectures, including field effect transistors with total thicknesses of less than 12 nm, for potential use in transparent, flexible, and stretchable forms of electronics.

Atomic Layer-by-Layer Thermoelectric Conversion in Topological Insulator Bismuth/Antimony Tellurides

Material design for direct heat-to-electricity conversion with substantial efficiency essentially requires cooperative control of electrical and thermal transport. Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3), displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin-orbit interaction in a-few-monolayers thickness regime, thus possibly providing another degree of freedom for electron and phonon transport at surfaces. Here, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi2Te3 (n-type) and Sb2Te3 (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the monolayer steps, we show that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi2Te3) and holes (Sb2Te3) through 2D subbands and topologically protected surface states in a geometrically deterministic manner. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit.

Tunable Catalytic Alloying Eliminates Stacking Faults in Compound Semiconductor Nanowires

Planar defects in compound (III-V and II-VI) semiconductor nanowires (NWs), such as twin and stacking faults, are universally formed during the catalytic NW growth, and they detrimentally act as static disorders against coherent electron transport and light emissions. Here we report a simple synthetic route for planar-defect free II-VI NWs by tunable alloying, i.e. Cd(1-x)Zn(x)Te NWs (0 ≤ x ≤ 1). It is discovered that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates interfacial instability during the catalytic growth by the surface energy minimization and forms homogeneous zinc blende crystals as opposed to unwanted zinc blende/wurtzite mixtures. As a direct consequence of the tunable alloying, we demonstrated that intrinsic energy band gap modulation in Cd(1-x)Zn(x)Te NWs can exploit the tunable spectral and temporal responses in light detection and emission in the full visible range.

Spatially Resolved Photodetection in Leaky Ferroelectric BiFeO3

Potential gradients due to the spontaneous polarization of BiFeO(3) yield asymmetric and nonlinear photocarrier dynamics. Photocurrent direction is determined by local ferroelectric domain orientation, whereas magnitude is spectrally centered around charged domain walls that are associated with oxygen vacancy migration. Photodetection can be electrically controlled by manipulating ferroelectric domain configurations.

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/

Large electroabsorption susceptibility mediated by internal photoconductive gain in Ge nanowires.

Large spectral modulation in the photon-to-electron conversion near the absorption band-edge of a semiconductor by an applied electrical field can be a basis for efficient electro-optical modulators. This electro-absorption effect in Group IV semiconductors is, however, inherently weak, and this poses the technological challenges for their electro-photonic integration. Here we report unprecedentedly large electro-absorption susceptibility at the direct band-edge of intrinsic Ge nanowire (NW) photodetectors, which is strongly diameter-dependent. We provide evidence that the large spectral shift at the 1.55 μm wavelength, enhanced up to 20 times larger than Ge bulk crystals, is attributed to the internal Franz-Keldysh effect across the NW surface field of ~10(5) V/cm, mediated by the strong photoconductive gain. This classical size-effect operating at the nanometer scale is universal, regardless of the choice of materials, and thus suggests general implications for the monolithic integration of Group IV photonic circuits.

Determination of the photocarrier diffusion length in intrinsic Ge nanowires.

We quantitatively determined the photocarrier diffusion length in intrinsic Ge nanowires (NWs) using scanning photocurrent microscopy. Specifically, the spatial mapping of one-dimensional decay in the photocurrent along the Ge NWs under the scanning laser beam (λ= 532 nm) was analyzed in a one-dimensional diffusion rate equation to extract the diffusion length of ~4-5 μm. We further attempt to determine the photocarrier lifetime under a finite bias across the Ge NWs, and discuss the role of surface scattering.