Junphil Hwang

Right sizes of nano- and microstructures for high- performance and rigid bulk thermoelectrics

Significance PbTe is known to be a promising thermoelectric material for waste heat recovery, so it has been the subject of extensive research involving new approaches. It is important to note that the performances of these developed materials can depend on the material synthesis conditions. We investigated three different routes of synthesizing 2% Na-doped PbTe and found that its thermoelectric figure of merit, zT, can be enhanced to ∼2.0 at 773 K. Also, the mechanical hardness of the sample synthesized by this condition was nearly double than that of the other samples. Our study shows that the size of nano- and microstructures can vary significantly by the choice of synthesis methods, which can explain the variation in zTs and mechanical hardness. In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to ∼2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2∼6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to ∼0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano- and microstructures of the material.

Large enhancement in the thermoelectric properties of Pb0.98Na0.02Te by optimizing the synthesis conditions

PbTe is known as a good thermoelectric material for waste heat recovery in the temperature range of 500 to 900 K. While various approaches such as nanostructuring for thermal conductivity reduction, resonant impurities, and band convergence by alloying for power factor enhancement have been proposed recently for enhancing the thermoelectric properties of PbTe, a systematic study on optimizing the synthesis conditions is also crucial to find a better base material, upon which those new approaches can be applied to further improve the material. In this paper, we systematically investigate the effect of various hot-press conditions on the thermoelectric properties of p-type 2% Na-doped PbTe, by varying the hot-press pressure from 70 to 130 MPa and the sintering time from 0.5 to 2 h. It is shown that the micro- and nano-scale structures in the hot-pressed material can be controlled by changing the sintering time and pressure. We demonstrate that by optimizing the hot-press conditions, the thermoelectric figure of merit of p-type 2% Na-doped PbTe can be enhanced up to zT = 1.74 at 774 K, which is about a 24% enhancement compared to the value of 1.4 presented by Pei et al. for the same material composition. Our electron transport modeling on bulk PbTe shows that this enhancement is due to the thermal conductivity reduction in both the electronic and lattice contributions. We believe that our findings can be accompanied with other recently-proposed techniques to further enhance the zT of this important thermoelectric material.

Enhancement of the thermoelectric performance of bulk SnTe alloys via the synergistic effect of band structure modification and chemical bond softening

SnTe alloys, which have the same crystal structure as PbTe, have attracted increasing attention. Here, we demonstrate that the synergistic effect of band structure modification and chemical bond softening can be realized simultaneously in In & Mn doped SnTe bulk alloys. The Seebeck coefficient and power factor are synergistically improved by co-doping of In and Mn. In doping is known to introduce a resonance level. Mn doping reduces the separation of light- and heavy-valence bands. The combination of these effects significantly enhances the Seebeck coefficient at room temperature owing to around a factor of five times increase in the band effective mass. The reduction of thermal conductivity is from the decrease of both the electronic and phononic parts. The electronic thermal conductivity is decreased by the increase in defect scattering, as can be confirmed by the carrier mobility. The force constant of the bonds around the Te site is decreased due to the co-doping of In & Mn, which indicates that the chemical bonds are softened, which leads to low sound velocity and lower lattice thermal conductivity. As a result, the peak thermoelectric figure of merit, zT = 1.03 has been achieved for Sn0.89In0.01Mn0.1Te at 923 K. This strategy of using the synergistic effect of band structure modification and chemical bond softening could be applicable to other thermoelectric materials.

High-quality CdTe(100)/GaAs(100) grown by hot-wall epitaxy using gold tube radiation shield

Abstract High-quality CdTe(100) layers grown on GaAs(100) substrates by hot-wall epitaxy using a gold tube radiation shield are reported for the first time. From the investigation of thermal properties, we find that the gold tube radiation shield is more effective in heat confinement and temperature stability than a stainless steel tube radiation shield. The CdTe lattice parameters perpendicular to the interface decrease as the layer thickness increases by strain relaxation. We obtain 89 arc sec full width at half maximum of the X-ray double-crystal rocking curve for a 15 μm thick CdTe layer which is the smallest value reported to date. Exciton emission and donor-acceptor pair emission along with longitudinal optical (LO) phonon replicas are obtained from PL measurements, confirming the good quality of the crystal.

The band offset measurement at the In0.5Ga0.5P/GaAs heterojunction by using properties of transition metal☆

Abstract It is well known that the energy level of a transition metal (TM) such as manganese is tied to the vacuum level rather than to a particular band extreme in semiconductors. Therefore, TM can be used as a common reference energy level in all the semiconductors. We have investigated for the first time the photoluminescence spectrum of Mn-doped In0.5Ga0.5P/GaAs heterojunction and by using the Mn acceptor level in photoluminescence (PL) spectra, the fractional band offsets Qc and Qv for the In0.5Ga0.5P/GaAs heterojunction are determined as 0.63 and 0.37, respectively.

A Light guide plate based flexible optical cuff for optogenetic stimulation of motor units

This paper reports a light guide plate based flexible optical cuff for optogenetic stimulation of motor units. We propose the optical cuff based on a single-sheet PDMS light guide plate (LGP). It has good flexibility, and can be improved the efficiency of the light source by distributing the light globally through the LGP. Moreover, the possibility of cell necrosis due to heat can help to reduce by preventing direct contact between the μ-LED and neurons.

Intersubband transitions of CdTe/Cd0.94Zn0.06Te strained single quantum wells grown by hot-wall epitaxy

Abstract Several CdTe/Cd 0.94 Zn 0.06 Te strained single quantum well structures with CdTe well widths ranging from 15 to 240 A were grown on GaAs (1 0 0) substrates by the hot-wall epitaxy method. Photoluminescence measurements on the strained single quantum well structures showed that the sharp e 1 h 1 excitonic transition peaks in the range 1.597–1.624 eV were shifted to higher energy with decreasing well width. The dependence of the e 1 h 1 excitonic transition energies on the CdTe well widths were calculated by taking into account the strain effects, and these theoretical values are in good qualitative agreement with the results from the photoluminescence measurements. These results indicate that the CdTe/Cd 0.94 Zn 0.06 Te strained single quantum wells have a good enough heterointerface with the necessary abruptness for the investigation of basic physics including optical properties.

Study on Metalizing 2% Na-PbTe for Thermoelectric Device

Heat emission from the laser diode used in the optical disc drive and the defects from the increased temperature at the system have attracted attentions from the field of the information storage device. Thermoelectric refrigerator is one of the fine solutions to solve these thermal problems. The refrigeration performance of thermoelectric device is dependent on the thermoelectric material’s figure-of-merit. Meanwhile, high electrical contact resistivity between metal electrode and p- and n-type thermoelectric materials in the device would lead increased total electrical resistance resulting in the degeneracy in performance. This paper represents the manufacturing process of the PbTebased material which has one of the highest figure-of-merit at medium-high-temperature, ~ 600K to 900 K, and the nickel contact layer for reduced electrical contact resistance at once, and the results showing the decent contact structure and figure-of-merit even after the long-term operation environment.

Growth of CdTe/CdZnTe strained-layer single quantum wells by modified hot-wall epitaxy method and their properties

Several CdTe/CdZnTe strained-layer single quantum well (SLSQW) structures, with CdTe well width ranging from 15 to 240 A, were grown on GaAs(100) substrates, by modified hot-wall epitaxy (HWE) method for the first time. Our HWE system is equiped with a gold tube radiation shield which is more effective in heat confinement and temperature stability than conventional metal tubes. Photoluminescence (PL) measurements of SLSQW showed that the sharp e 1 h 1 excitonic transition peaks in the range of 1.597 to 1.624 eV shifted to the higher energy with decreasing well width. From the PL excitation (PLE) measurements of SLSQW, we obtained the n=1, 2, 3 and n=1, 2 excitonic transition peaks for the 240 and 120 A CdTe well width SLSQW structures, respectively

Intersubband transition studies of Cd0.936Zn0.064Te/CdTe superlattices grown by double-well temperature-gradient vapor transport deposition

Abstract Cd0.936Zn0.064Te/CsTe strained superlattices (SLS) were grown by a simple method of the double-well temperature-gradient vapor deposition on GaAs (1 0 0) orientation substrates, with each well thickness ranging between 100 and 300 A. X-ray double crystal diffractometry measurements were performed to investigate the structural properties of the superlattices. Photoluminescence measurements showed that the sharp e 1h1 intersubband transition peaks were clearly observed on the Cd0.936Zn0.064Te/CdTe SLS and that the excitonic transition energies were shifted to higher energies with decreasing well width. The lowest intersubband transition energies in Cd0.936Zn0.064Te/CdTe SLS with well widths of 200 and 300 A were determined theoretically by taking into account the strain effects, and the values were in good qualitative agreement with those obtained from the experimental measurements. These results indicate that Cd0.936Zn0.064Te/CdTe SLS have good enough heterointerfaces with the necessary abruptness for the investigation of fundamental physics including optical properties.