Jin-Seo Noh

Reduction of Lattice Thermal Conductivity in Single Bi‐Te Core/Shell Nanowires with Rough Interface

Reducing the thermal conductivity of nanometer-scale materials is of signifi cant interest for a broad range of applications in the dissipation of heat from electronics and optoelectronics, and in thermoelectric energy conversion. When the relevant length scale of a nanostructure is comparable to the mean free path of the heat carriers, the heat transport can be effectively controlled, which often results in the reduction of the thermal conductivity of the nanostructure compared to its bulk counterpart. The reduced thermal conductivity provides an effective strategy for optimizing thermoelectric energy conversion as well as for managing heat generated in electronic and photonic devices. Considerable effort has been invested in developing methods to reduce thermal conductivity, largely because the reduction of thermal conductivity helps increase the thermoelectric fi gureof-merit ( ZT ) (defi ned as ZT = S 2 σ T/ κ , where S , σ , κ , and T are the Seebeck coeffi cient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). [ 1 ] In this respect, rational synthetic routes for κ reduction include the insertion of nanometer scale inclusions in bulk materials, [ 2 ] the epitaxial growth of superlattice thin fi lms, [ 3 ] one-dimensional heterostructures, [ 4,5 ] and the use of photonic nanomesh structures. [ 6,7 ] A particularly versatile technique is to introduce a rough surface on silicon nanowires, [ 8 ] which provides effi cient scattering across the broad phonon spectrum, and thus reduces κ as much as two orders of magnitude relative to bulk crystalline silicon. In a core/shell structure, which is low-dimensional heterostructures, has the signifi cant advantage in the enhancement of ZT owing to low thermal conductivity by interface phonon scattering. [ 4,5 ]

Size-dependent thermal conductivity of individual single-crystalline PbTe nanowires

We investigated the thermal conductivity of individual single-crystalline PbTe nanowires grown by a chemical vapor transport method. Thermal conductivities of PbTe nanowires 182–436 nm in diameter were measured using suspended microdevices. The thermal conductivity of a PbTe nanowire appeared to decrease with decreasing nanowire diameter and was measured to be 1.29 W/mK for a 182 nm nanowire at 300 K, which is about half of that of bulk PbTe. Our results indicate that phonon transport through a PbTe nanowire is effectively suppressed by the enhanced phonon boundary scattering due to size effects.

High-performance vertical hydrogen sensors using Pd-coated rough Si nanowires

We fabricated Pd-coated rough Si nanowires, using a combination of Si electroless etching and Pd sputtering. The semi-densely distributed, vertical-standing rough Si nanowires, which were clustered locally with inter-cluster distances of several nm to several μm, were selected as a basal platform. Pd was coated only on the upper part of the Si nanowires in the semi-dense configuration, and their surface profile replicated the surface morphology of Si nanowires. The Pd-coated rough Si nanowires showed good reversibility and excellent hydrogen-sensing performance in terms of sensitivity (>300%), response time (<3 s), and detection limit (∼5 ppm). These attributes are discussed in terms of the vertical-standing nanowire structures and the rough surface effect of the nanowires. Interestingly, the variation in sensitivity of the Pd-coated Si nanowire sensors was divided into two regimes, which appeared in the low and high hydrogen concentration ranges, respectively. A simple model is also provided to account for this unusual finding.

Direct observation of the semimetal-to-semiconductor transition of individual single-crystal bismuth nanowires grown by on-film formation of nanowires

We have systematically investigated the semimetal-to-semiconductor transition of individual single-crystalline Bi nanowires. For this work, we developed a technique to reduce the diameter of Bi nanowires grown by our unique on-film formation of nanowires (OFF-ON) method. Cooling down the substrate temperature during Bi film deposition by use of liquid nitrogen, film structures with small-sized grains were obtained. Through thermal annealing of these fine-granular Bi films, single-crystalline Bi nanowires can be produced with minimum diameter of approximately 20 nm. Elaborative nanofabrication techniques were employed to shape state-of-the-art four-probe devices based on the individual small diameter Bi nanowires. Diameter-dependent transport measurements on the individual Bi nanowires revealed that the semimetal-to-semiconductor transition really occurred at about d(w) = 63 nm. Moreover, band structure calculations supported this occurrence of the semimetal-to-semiconductor transition.

Highly conductive and stretchable poly(dimethylsiloxane):poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) blends for organic interconnects

Naturally immiscible PEDOT:PSS and PDMS, which are a typical conducting polymer and an transparent elastomer, respectively, were blended by the support of PDMS-b-PEO. A block copolymer, PDMS-b-PEO, consisting of hydrophobic PDMS backbones and hydrophilic PEO side chains, significantly improved the miscibility of PEDOT:PSS and PDMS. At an optimal PDMS-b-PEO concentration of 30%, a cured PEDOT:PSS:PDMS film was found to be comprised of three-dimensional PDMS networks and a PEDOT:PSS phase filling in between the networks. The optimal blend film exhibited a conductivity comparable to a pure PEDOT:PSS film and a maximum strain to rupture of about 75%. It was also demonstrated that interconnects made of this blend film functioned well irrespective of the substrate and the pattern size, and could reproducibly operate under strains up to 50%. These results indicate that the PEDOT:PSS:PDMS blends could be a practical choice for organic interconnects for future stretchable electronics.

Novel surfactant-free multi-branched gold stars characterized by inverse photocurrent

Multi-branched gold stars were spontaneously formed on a semiconductor (Ge) substrate in high yield via a surfactant-free galvanic displacement method at room temperature using a DMF–water (9/1) mixed solvent. The average length of the branches was estimated to be 561 nm, and the size and shape of the multi-branched gold stars can be controlled by varying the reaction time of the Ge wafer and gold precursor. A high volume ratio of DMF was found to be crucial for the formation of these multi-branched gold stars. Interestingly, the photocurrent of the prepared gold stars decreased by 10% upon irradiation with a 532 nm visible laser. The photocurrent was switched on and off >10 times without significant degradation, indicating high reproducibility and reliability of the inverse photoresponse of the gold stars under visible light.

Simple two-step fabrication method of Bi2Te3 nanowires

Bismuth telluride (Bi2Te3) is an attractive material for both thermoelectric and topological insulator applications. Its performance is expected to be greatly improved when the material takes nanowire structures. However, it is very difficult to grow high-quality Bi2Te3 nanowires. In this study, a simple and reliable method for the growth of Bi2Te3 nanowires is reported, which uses post-sputtering and annealing in combination with the conventional method involving on-film formation of nanowires. Transmission electron microscopy study shows that Bi2Te3 nanowires grown by our technique are highly single-crystalline and oriented along [110] direction.

Perpendicular Magnetic Anisotropy in FePt Patterned Media Employing a CrV Seed Layer

A thin FePt film was deposited onto a CrV seed layer at 400°C and showed a high coercivity (~3,400 Oe) and high magnetization (900–1,000 emu/cm3) characteristic of L 10 phase. However, the magnetic properties of patterned media fabricated from the film stack were degraded due to the Ar-ion bombardment. We employed a deposition-last process, in which FePt film deposited at room temperature underwent lift-off and post-annealing processes, to avoid the exposure of FePt to Ar plasma. A patterned medium with 100-nm nano-columns showed an out-of-plane coercivity fivefold larger than its in-plane counterpart and a remanent magnetization comparable to saturation magnetization in the out-of-plane direction, indicating a high perpendicular anisotropy. These results demonstrate the high perpendicular anisotropy in FePt patterned media using a Cr-based compound seed layer for the first time and suggest that ultra-high-density magnetic recording media can be achieved using this optimized top-down approach.

Cracked titanium film on an elastomeric substrate for highly flexible, transparent, and low-power strain sensors

Strain-dependent cracking behaviors in thin titanium (Ti) films on polydimethylsiloxane (PDMS) substrates were systematically investigated for their application to sensitive, flexible, transparent, and portable strain sensors. When uniaxially elongated, vertical cracks were developed in the low-strain range, and beyond a critical strain, tilted cracks appeared to intersect the vertical cracks. The cracking behaviors were also dependent on Ti film thickness. The varying strain-dependent crack patterns produced a significant resistance change in response to the applied strain, particularly, in the high- and broad-strain range. For a 180-nm-thick Ti film on PDMS substrate, a gauge factor of 2 was achieved in the range of 30% to 50% strain. The operation power was extremely low. All the Ti films on PDMS substrates were transparent, highly flexible, and very easy to fabricate. These results suggest that cracked Ti films on PDMS substrates could be a viable candidate for realizing a low-cost, flexible, transparent, and portable strain sensor.

Ag nanowire/ZnO nanobush hybrid structures for improved photocatalytic activity

Reverse-engineered Ag nanowire/ZnO nanobush hybrid structures have been synthesized for the first time by a combination of polyol method and low-temperature solution method. Through the elaborate control of Ag-to-ZnO weight ratio, vertically aligned ZnO nanobushes grew on the surface of well-faceted Ag nanowires. The Ag/ZnO hybrid nanostructures showed the crystal features of both Ag nanowires and wurtzite ZnO nanostructures. They exhibited strong UV absorption, whereas their photoluminescence spectra were much weaker than pure ZnO nanostructures due to the inhibited carrier recombination. The photocatalytic activity of Ag/ZnO hybrid nanostructures was greatly improved compared to pure ZnO nanostructures. Furthermore, they showed good cyclic performance and easy recovery from the test solution, demonstrating the possibility of their practical use.