Jae Yong Song

Size-dependent thermal instability and melting behavior of Sn nanowires

Thermal instability and melting behavior of tin nanowires were studied with a decrease of wire radius(rNW=7–30nm) via differential scanning calorimetry (DSC). Two sequential DSC measurements showed a 1∕rNW dependency of the melting temperature depression; the first melting temperature decreased from 502to486K with 1∕rNW whereas the second one was more depressed between 0.8 and 5K. The melting temperature difference between the first and second cycles increased linearly with 1∕rNW. This variation was attributed to fragmentation of nanowires due to Rayleigh instability. Here, fragmentation of long nanowires was suppressed by a template confinement, resulting in the formation of short nanorods.

Electrodeposition of Triangular Pd Rod Nanostructures and Their Electrocatalytic and SERS Activities

We report a simple one-step electrodeposition of triangular Pd rod nanostructures on clean Au substrates without additives. Scanning electron microscopy, transmission electron microscopy, and electrochemical techniques were utilized to characterize the structural features of the triangular Pd rod nanostructures. The regulation of the electrodeposition rate by optimizing the electrolyte concentration and applied potential was critical for the anisotropic growth of Pd in the vertical direction. The triangular Pd rod structures exhibited electrocatalytic activities for oxygen reduction and methanol oxidation reactions. These surfaces could be effectively utilized as reproducible surface-enhanced Raman scattering (SERS) active substrates to produce stable SERS signals under electrochemical systems. A simple preparation of well-defined triangular Pd rod structures would allow new opportunities in various areas utilizing Pd-based nanostructured surfaces.

Size dependence of lattice deformation induced by growth stress in Sn nanowires

We report on size-dependent lattice expansion of single crystalline Sn nanowires (NWs) with the wire radius (rNW=6.9–34.7 nm), where the NWs are deposited under confinement of a nanotemplate. The longitudinal lattice expansion in the NWs increases up to approximately 1.0% with the reciprocal radius (1/rNW), contrary to the general theoretical prediction that the surface relaxation causes lattice contraction of nanomaterials. The longitudinal dilatation of the NW lattice can be understood by the Poisson effect induced by the compressive growth stress in the radial direction, which increases with the reciprocal radius.

Galvanic synthesis of three-dimensional and hollow metallic nanostructures

We report a low-cost, facile, and template-free electrochemical method of synthesizing three-dimensional (3D) hollow metallic nanostructures. The 3D nanoporous gold (3D-NPG) nanostructures were synthesized by a galvanic replacement reaction (GRR) using the different reduction potentials of silver and gold; hemispherical silver nanoislands were electrochemically deposited on cathodic substrates by a reverse-pulse potentiodynamic method without templates and then nanoporous gold layer replicated the shape of silver islands during the GRR process in an ultra-dilute electrolyte of gold(III) chloride trihydrate. Finally, the wet etching process of remaining silver resulted in the formation of 3D-NPG. During the GRR process, the application of bias voltage to the cathode decreased the porosity of 3D-NPG in the voltage range of 0.2 to -0.62 V. And the GRR process of silver nanoislands was also applicable to fabrication of the 3D hollow nanostructures of platinum and palladium. The 3D-NPG nanostructures were found to effectively enhance the SERS sensitivity of rhodamine 6G (R6G) molecules with a concentration up to 10-8 M.

Sensor based on chemical vapour deposition-grown molybdenum disulphide for gas sensing application

Over the past few decades, sensors based on field-effect transistors have drawn much attention. Initially three dimensional materials were used for sensing, which were later replaced by two dimensional materials because of their ease of manufacturing and large specific areas. Amongst the transition metal dichalcogenides, MoS2 has been widely used for the fabrication of sensors owing to its ability to differentiate between a charge donor and an acceptor analyte. In this work, we fabricated sensors using chemical vapour deposition grown-MoS2. MoS2 was grown on a p-Si/SiO2 substrate using Mo(CO)6 as a precursor, the growth was carried out by the sublimation of the precursor under a flow of high purity H2S at high temperature. The aim of this work is to achieve a level of sensitivity that would enable the detection of individual gas analytes upon adsorption to the MoS2 surface. To efficiently detect individual gas analytes upon adsorption to the surface, we used interdigitated electrodes in the device architecture to increase the area of the channels for analyte adsorption. We used CO2 and O2 gases, which acted as charge donors. A trilayer MoS2 film was examined, and the detection sensitivity for O2 was higher in comparison to CO2. The fabricated device showed significant sensitivity up to parts per million detection level.

Interface formation between tris(8-hydroxyquinoline) aluminum and ZnO nanowires and film

The energy level alignments at the interface between tris(8-hydroxyquinoline) aluminum (Alq3) and ZnO nanowires and thin film were studied with in situ x-ray and ultraviolet photoemission spectroscopy. The changes of work functions, highest occupied molecular orbitals, and core levels were measured with step-by-step deposition of Alq3 on each ZnO substrate. Although both substrates show similar electronic structures, a larger interface dipole is induced at the interface between Alq3 and ZnO nanowires. This results in the reduction of the electron injection barrier at the interface of Alq3/ZnO nanowires. Thus, the ZnO nanowire substrate is expected to show better performance than that of ZnO film when used as a cathode. We discussed the different interface dipole formation at each interface.

Effects of doping and planar defects on the thermoelectric properties of InAs nanowires

Undoped InAs and Si-doped InAs nanowires with stacking faults and twins were synthesized by catalyst-free molecular beam epitaxy and their thermoelectric enhancements due to planar defects were experimentally and theoretically demonstrated. The Seebeck coefficients, electrical resistivities, and thermal conductivities of the Si-doped and undoped InAs nanowires were measured using a micro-fabricated thermoelectric measurement platform over the temperature range of 50 to 300 K. The Si-doping increased electrical conductivity from 1.0 × 10−4 to 7.8 × 10−4 S m−1, due to the increase in carrier concentration from 2 × 1017 to 8 × 1017 cm−3, and then decreased the thermopower from −216 to −81 μV K−1 at 300 K, in agreement with the two-band model based on the Boltzman transport theory. Phonon scattering, caused by planar defects such as surfaces, twins, and stacking-fault boundaries, suppressed the lattice thermal conductivity below 3 W m−1 K−1 following the Callaway model. The planar defect-induced phonon scattering as well as the optimization of carrier concentration is very effective at enhancing the thermoelectric properties of InAs nanowires and is expected to be utilized for improving the thermoelectric properties of other thermoelectric materials.

Phase-change-induced martensitic deformation and slip system in GeSbTe

Films with a newly observed monoclinic phase of Ge2Sb2Te5 (GST) were analyzed by high-resolution transmission electron microscopy (HRTEM) analysis and ab initio calculations. After an annealing treatment at 220 °C, the amorphous GST films were partly crystallized to an unknown monoclinic crystal structure. The transformation of the face-centered cubic (FCC) phase to monoclinic phases resulted from crystallization-induced stress caused by volume change during FCC formation. The crystallization-induced stress at the amorphous-FCC boundary was estimated to be 1.18 GPa. The volume per atom in the monoclinic phase was about 7.3% greater than that in the FCC phase. The stress value measured in situ was much smaller than the zx stress tensor (shear stress) calculated ab initio because the stress in the actual film was minimized by plastic deformation of the GST itself. Moreover, there is an activation stress barrier to deformation; this barrier corresponds to a deformation angle (γ) of approximately 78°. Slip of the (111) plane along the [110] direction also occurs in the FCC phase during annealing treatment. Based on the calculated total energy difference per atom in GST, the martensitic deformation as well as the slip system can occur at deformation angles as low as 70°.

Structural deformation and void formation driven by phase transformation in the Ge2Sb2Te5 film

The failure mechanism of reversibility in the Ge2Sb2Te5 (GST) film was analyzed microscopically based on transmission electron microscopy and ab initio density functional theory (DFT). In this study, the crystallization region was limited to the range of 500–600 °C under fast ramping rates, enabling the GST amorphous phase to approach a supercooled region above the glass transition temperature. The densification accompanying phase transformation under fast ramping rates induces two deformation behaviors: phase separation and void formation. In the disorder–order transition, the disordered domain of GST is dominated by low viscosity while approaching the supercooled region, which induces phase separation to compensate for the densification. However, coexisting cubic and hexagonal phases show void formation around the interfaces with some phase separation. The DFT calculation shows that the polymorphic transition, a fast, martensitic transformation between the cubic and hexagonal phases, induces a vacancy cluster at the interface. In the tensile stress state, void growth can be easily driven from the vacancy cluster as the nuclei for compensating for densification.

Lattice deformation of Sn nanowires for the application to nano-interconnection technology

Nano-interconnection technology is expected to replace some part of solder bump technology of electronic packaging in near future. Metallic nanowires (NWs) are one of the candidates for the electrical interconnection materials. In this study, as a well-known material for the interconnection in the electronic packaging, Sn was selected for the application to nano-interconnection technology. Since the physical properties of Sn NWs are important for the interconnection applications, we have already reported the size-dependency of melting behaviors and lattice parameters of single crystalline Sn NWs. In this study, the effects of the NW microstructure and the kinds of templates, which were used for the growth of Sn NWs, on the lattice parameter were investigated. Results showed that the single crystalline Sn NWs were elongated along the longitudinal direction up to 0.64 % depending upon their microstructures and kinds of the templates. The nanowire elongation was gradually reduced when the NW microstructure were single crystalline, granular, and bamboolike structures, in sequence.