S. Joon Kwon

Solvent-driven dewetting and rim instability

An experimental method suitable for reproducible results has been used to investigate dewetting behavior of thin films of solvent-laden polymer. This solvent-driven dewetting enables one to change spreading coefficient by an order of magnitude that is not readily realizable in thermal dewetting and to study polar interactions that have not been fully exploited experimentally. While the film instability is similar to that found in thermal dewetting, the rim instability is quite different. Two different types of the rim instability have been found. With a polar solvent, the rim instability changes from one type to another with increasing film thickness whereas the unstable rim becomes stable for an apolar solvent.

Microshaping metal surfaces by wave-directed self-organization

Self-organization in the shaping of a metal surface is dictated by an internal wave that selects the type of modes from an externally imposed periodic pattern. An elastomeric mold, when placed on a thin bilayer of metal on polymer and heated, provides periodic nodes that give rise to a periodic wave of harmonic series. The internal wave in the bilayer selects the type, number of harmonic modes, and the fractional magnitude that each allowed harmonic mode contributes to the overall surface shape, thereby permitting shape engineering of the metal surface.

Wave interactions in buckling: Self-organization of a metal surface on a structured polymer layer

We report on self-organized metal surface patterns that result when a thin bilayer of a metal on a periodically corrugated polymer layer is annealed. A theoretical basis is provided for the self-organization that is verified by experiment. The wave number of the surface pattern is determined by a square law relationship involving the corrugation wave number and the intrinsic wave number of the bilayer. The phase shift between the wave on the ridge and that on the groove is related to an uncertainty in the corrugation wave number. Free energy minimization dictates connectivity of the two waves and fidelity of the metal surface pattern.

Simultaneous enhancement of upconversion and downshifting luminescence via plasmonic structure.

We describe a metal nanodisk-insulator-metal (MIM) structure that enhances lanthanide-based upconversion (UC) and downshifting (DS) simultaneously. The structure was fabricated using a nanotransfer printing method that facilitates large-area applications of nanostructures for optoelectronic devices. The proposed MIM structure is a promising way to harness the entire solar spectrum by converting both ultraviolet and near-infrared to visible light concurrently through resonant-mode excitation. The overall photoluminescence enhancements of the UC and DS were 174- and 29-fold, respectively.

Patterned growth of ZnO nanorods by micromolding of sol-gel-derived seed layer

An effective method for fabricating micropatterned ZnO nanorods without a catalyst via hydrothermal synthesis using a micromolding technique is presented. Micromolding of a sol-gel-derived seed layer was flawlessly accomplished by conformal contact of elastic mold with microscale pattern. The patterned growth of the nanorods over a large area of the preconfined seed layer was observed with a pattern feasibility attaining 1μm. The orientation of the resulting crystalline nanorods was observed to be normal to the seed layer while characteristic sizes of the nanorods did not show any dependences on the diameter of the seed particles. The presented synthetic process can provide a simpler and more inexpensive way for the fabrication of an array of one-dimensional nanostructures using a wet-chemical method.

Theoretical analysis of the graphitization of a nanodiamond

We report on a theoretical analysis of the graphitization of a nanosize diamond (nanodiamond) in the metastable state. A nanodiamond annealed at a relatively lower temperature suffers morphological transition into a nanodiamond–graphite core–shell structure. Thermodynamic stability analysis of the nanodiamond showed that the phase diagram (relationship between the annealing temperature and radius) of the nanodiamond–graphite has three regimes: smaller nanodiamond, nanodiamond–graphite, and larger nanodiamond. These regimes of nanodiamond–graphite are due to an additional phase boundary from finding the maximum size of the nanodiamond which can be graphitized. In the theoretical analysis, the most probable and the maximum volume fractions of graphite in the nanodiamond were 0.76 and 0.84 respectively, which were independent of the annealing temperature and the initial radius of the nanodiamond. Therefore, the nanodiamond is not completely transformed into graphite by simple annealing at relatively lower process temperature and pressure. The highest graphitization probability decreased with increasing annealing temperature. Raman spectra for the F2g vibration mode of nanodiamond were also calculated, and we found that the variation in properties of the spectral line was strongly dependent on the graphitization temperature and the initial size of the nanodiamond.

Structural origin of the band gap anomaly of quaternary alloy Cd x Zn 1-x S y Se 1-y nanowires, nanobelts, and nanosheets in the visible spectrum

Single-crystalline alloy II-VI semiconductor nanostructures have been used as functional materials to propel photonic and optoelectronic device performance in a broad range of the visible spectrum. Their functionality depends on the stable modulation of the direct band gap (Eg), which can be finely tuned by controlling the properties of alloy composition, crystallinity, and morphology. We report on the structural correlation of the optical band gap anomaly of quaternary alloy CdxZn1-xSySe1-y single-crystalline nanostructures that exhibit different morphologies, such as nanowires (NWs), nanobelts (NBs), and nanosheets (NSs), and cover a wide range of the visible spectrum (Eg = 1.96-2.88 eV). Using pulsed laser deposition, the nanostructures evolve from NWs via NBs to NSs with decreasing growth temperature. The effects of the growth temperature are also reflected in the systematic variation of the composition. The alloy nanostructures firmly maintain single crystallinity of the hexagonal wurtzite and the nanoscale morphology, with no distortion of lattice parameters, satisfying the virtual crystal model. For the optical properties, however, we observed distinct structure-dependent band gap anomalies: the disappearance of bowing for NWs and maximum and slightly reduced bowing for NBs and NSs, respectively. We tried to uncover the underlying mechanism that bridges the structural properties and the optical anomaly using an empirical pseudopotential model calculation of electronic band structures. From the calculations, we found that the optical bowings in NBs and NSs were due to residual strain, by which they are also distinguishable from each other: large for NBs and small for NSs. To explain the origin of the residual strain, we suggest a semiempirical model that considers intrinsic atomic disorder, resulting from the bond length mismatch, combined with the strain relaxation factor as a function of the width-to-thickness ratio of the NBs or NSs. The model agreed well with the observed optical bowing of the alloy nanostructures in which a mechanism for the maximum bowing for NBs is explained. The present systematic study on the structural-optical properties correlation opens a new perspective to understand the morphology- and composition-dependent unique optical properties of II-VI alloy nanostructures as well as a comprehensive strategy to design a facile band gap modulation method of preparing photoconverting and photodetecting materials.

Morphological dynamics of swelling-induced surface patterns in metal-capped polymer bilayer

We report on the morphological dynamics of surface patterns induced by swelling of metal-capped polymer bilayer on a substrate. When the bilayer is subject to solvent vapor, the strain is generated in the polymer layer that is confined by the substrate and the metal capping layer. An increase in the strain induces the development of the stress in the bilayer to deform the lower polymer layer perpendicularly to the surface of the bilayer. Isotropic surface wave patterns results from the stress relaxation, the wave number of the patterns shows a characteristic temporal dependency on the swelling time, such that km(t) approximately t(-1/8). This temporal evolution accompanied by the morphological dynamics gives smaller value of the growth rate of the characteristic wavelength than that of the case of swelling of gel.

Germanium microflower-on-nanostem as a high-performance lithium ion battery electrode

We demonstrate a new design of Ge-based electrodes comprising three-dimensional (3-D) spherical microflowers containing crystalline nanorod networks on sturdy 1-D nanostems directly grown on a metallic current collector by facile thermal evaporation. The Ge nanorod networks were observed to self-replicate their tetrahedron structures and form a diamond cubic lattice-like inner network. After etching and subsequent carbon coating, the treated Ge nanostructures provide good electrical conductivity and are resistant to gradual deterioration, resulting in superior electrochemical performance as anode materials for LIBs, with a charge capacity retention of 96% after 100 cycles and a high specific capacity of 1360 mA h g−1 at 1 C and a high-rate capability with reversible capacities of 1080 and 850 mA h g−1 at the rates of 5 and 10 C, respectively. The improved electrochemical performance can be attributed to the fast electron transport and good strain accommodation of the carbon-filled Ge microflower-on-nanostem hybrid electrode.

Theoretical analysis of the radius of semiconductor nanowires grown by the catalytic vapour–liquid–solid mechanism

We present a theoretical analysis of the radius, rC*, of semiconductor nanowires (SNWs) grown by the catalytic vapour–liquid–solid (VLS) mechanism. Two types of the catalytic metal were examined, namely case I: thin film, and case II: colloids or cluster. In case I, the number density along with the inter-distance between two neighbouring nucleus, D, and the critical radius of the catalytic metal nucleus, RC*, were correlated and determined by either the thermodynamic relationship (determination of RC* followed by D) or structural instability (determination of D followed by RC*). In case II, the linearly scaling behaviour of rC* with the observed radius of the SNWs was explained by comparing with experimental data obtained from the literature. Commonly in both cases, it was shown that rC* is thermodynamically determined, assuming the kinetic effect due to the initial diffusion length of the gaseous semiconductor precursor in the early stages of the growth of the SNWs, and that rC* mirrors RC* only when rC* is greater than RC*. We also found that the theoretical analysis of rC* is matched well with experimental data in the literature.