Jong Wook Roh

Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

Squeezing out efficient thermoelectrics Thermoelectric materials hold the promise of converting waste heat into electricity. The challenge is to develop high-efficiency materials that are not too expensive. Kim et al. suggest a pathway for developing inexpensive thermoelectrics. They show a dramatic improvement of efficiency in bismuth telluride samples by quickly squeezing out excess liquid during compaction. This method introduces grain boundary dislocations in a way that avoids degrading electrical conductivity, which makes a better thermoelectric material. With the potential for scale-up and application to cheaper materials, this discovery presents an attractive path forward for thermoelectrics. Science, this issue p. 109 Pressure-assisted liquid-phase compaction allows synthesis of high–conversion efficiency thermoelectric materials. The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi0.5Sb1.5Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

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 ]

Observation of Anisotropy in Thermal Conductivity of Individual Single- Crystalline Bismuth Nanowires

The thermal conductivity of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON-OFF) has been investigated. We observed that the thermal conductivity of single-crystalline Bi nanowires is highly anisotropic. Thermal conductivity of nanowires (diameter ∼100 nm) in the off-axis [102] and [110] directions exhibits a difference of ∼7.0 W/m·K. The thermal conductivity in both growth directions is diameter-dependent, which indicates that thermal transport through the individual Bi nanowires is limited by boundary scattering of both electrons and phonons. This huge anisotropy in thermal conductivities of Bi nanowires suggests the importance of direction-dependent characterization of charge, thermal transport, and thermoelectric properties of Bi nanowires.

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.

Synthesis of Multishell Nanoplates by Consecutive Epitaxial Growth of Bi2Se3 and Bi2Te3 Nanoplates and Enhanced Thermoelectric Properties

We herein demonstrate the successive epitaxial growth of Bi2Te3 and Bi2Se3 on seed nanoplates for the scalable synthesis of heterostructured nanoplates (Bi2Se3@Bi2Te3) and multishell nanoplates (Bi2Se3@Bi2Te3@Bi2Se3, Bi2Se3@Bi2Te3@Bi2Se3@Bi2Te3). The relative dimensions of the constituting layers are controllable via the molar ratios of the precursors added to the seed nanoplate solution. Reduction of the precursors produces nanoparticles that attach preferentially to the sides of the seed nanoplates. Once attached, the nanoparticles reorganize epitaxially on the seed crystal lattices to form single-crystalline core-shell nanoplates. The nanoplates, initially 100 nm wide, grew laterally to 620 nm in the multishell structure, while their thickness increased more moderately, from 5 to 20 nm. The nanoplates were pelletized into bulk samples by spark plasma sintering and their thermoelectric properties are compared. A peak thermoelectric figure of merit (ZT) ∼0.71 was obtained at 450 K for the bulk of Bi2Se3@Bi2Te3 nanoplates by simultaneous modulation of electronic and thermal transport in the presence of highly dense grain and phase boundaries.

Thermoelectric properties of individual single-crystalline PbTe nanowires grown by a vapor transport method

We present the thermoelectric properties of individual PbTe nanowires grown by a vapor transport method. Temperature-dependent thermopower and electrical conductivity in PbTe nanowires were investigated. A PbTe nanowire {d = 136 nm) was found to have a Seebeck coefficient of −72 µV/K and electrical conductivity of 45 S/cm at 300 K. Thermal conductivity of a PbTe nanowire {d = 290 nm) was also found to be 1.36 W/mK at 300 K. Our results demonstrate the enhanced thermoelectric properties of individual single-crystalline PbTe nanowires, which are higher than that in PbTe bulk.

Transport properties of single-crystalline n-type semiconducting PbTe nanowires

Single-crystalline PbTe nanowires were synthesized using the chemical vapor transport method. They consisted of rock-salt structure PbTe nanocrystals uniformly grown in the [100] direction. We fabricated field-effect transistors using a single PbTe nanowire, providing evidence for its intrinsic n-type semiconductor characteristics. The values of the carrier mobility and concentration were estimated to be 0.83 cm(2) V(-1) s(-1) and 8.8 x 10(17) cm(-3), respectively. The Seebeck coefficients (-72 muV K(-1)) of individual nanowires were measured to show their n-type carrier-dominated thermoelectric transport properties.

Enhancing the Thermoelectric Properties of p-Type Bulk Bi-Sb-Te Nanocomposites via Solution-Based Metal Nanoparticle Decoration

Embedding nanosized particles in bulk thermoelectric materials is expected to lower the lattice thermal conductivity by enhancing the degree of interface phonon scattering, thus improving their thermoelectric figure of merit ZT. We have developed a wet chemical process to fabricate Bi0.5Sb1.5Te3-based thermoelectric nanocomposites which include nanometer-sized metal particles. By simple solution mixing of metal acetate precursors and Bi0.5Sb1.5Te3 powders in ethyl acetate as a medium for homogeneous incorporation, it is possible to apply various types of metal nanoparticles onto the surfaces of the thermoelectric powders. Next, bulk Bi0.5Sb1.5Te3 nanocomposites with homogeneously dispersed metal nanoparticles were fabricated using a spark plasma sintering technique. The lattice thermal conductivities were reduced by increasing the long-wavelength phonon scattering in the presence of metal nanoparticles, while the Seebeck coefficients increased for a few selected metal-decorated nanocomposites, possibly due to the carrier-energy-filtering effect. Finally, the figure of merit ZT was enhanced to 1.4 near room temperature. This approach highlights the feasibility of incorporating various types of nanoparticles into an alloy matrix starting by wet chemical routes, which is an effective means of improving the thermoelectric performance of Bi-Te-based alloys.

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.