Jong-Soo Rhyee

Peierls distortion as a route to high thermoelectric performance in In4Se3―δ crystals

Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

Enhancement of the Thermoelectric Figure-of-Merit in a Wide Temperature Range in In4Se3–xCl0.03 Bulk Crystals

IO N Because of the increasing awareness of renewable energy issues, much attention has been devoted to thermoelectric energy harvesting technology. The dimensionless thermoelectric fi gureof-merit is defi ned by ZT = S 2 σ T/ κ , where S , σ , T , and κ are the Seebeck coeffi cient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Regarding a high ZT , it has long been sought both by lowering the thermal conductivity by exploiting the phonon-glass and electron-crystal concept [ 1–5 ] and by enhancing the power factor S 2 σ by utilizing the quantum confi nement effect [ 6 , 7 ] in low-dimensional nanostructured materials. Recently, we proposed that the charge density wave represents a new direction for high thermoelectric performance in bulk crystalline materials. [ 8 , 9 ] Low-dimensional electronic transport with strong electron–phonon coupling breaks the translational symmetry of the lattice because of the energy instability in a high-symmetry crystalline lattice. [ 10 ] The lattice dimerization along the electronic transport plane lowers the lattice thermal conductivity as a result of lowering of the phonon energy. Through quasi one-dimensional lattice distortion (Peierls distortion) in In 4 Se 3– x bulk single crystals, we achieved a high ZT of 1.48 at 705 K. [ 9 ] However, two challenges remain for practical applications. Firstly, the reported ZT could be increased further if we could increase the carrier concentration of the In 4 Se 3– x crystals because it is far from the carrier concentration of a heavily doped semiconductor (on the order of 10 19 cm − 3 ) that is generally considered to be optimal for thermoelectric materials. Secondly, ZT decreases signifi cantly as the temperature decreases, which limits the operational temperature range to within 350 –430 ° C. [ 9 ] Here, we report a signifi cant increase in ZT (maximum ZT ( ZT max ) = 1.53) over a wide temperature range in chlorine-doped In 4 Se 3– x Cl 0.03

Thermoelectric properties and anisotropic electronic band structure on the In4Se3−x compounds

We report the high thermoelectric figure-of-merit (ZT) on the Se-deficient polycrystalline compounds of In4Se3−x (0.02≤x≤0.5) and the anisotropic electronic band structure. The Se-deficiency (x) has the effect of decreasing the semiconducting band gap and increasing the power factor. The band structure calculation for In4Se3−x (x=0.25) exhibits localized hole bands at the Γ-point and Y-S symmetry line, whereas the significant electronic band dispersion is observed along the c-axis. Here, we propose that the high ZT values on those compounds are originated from the anisotropic electronic band structure as well as Peierls distortion.

Formation of Cu nanoparticles in layered Bi2Te3 and their effect on ZT enhancement

Compounds of CuxBi2Te3 (0 ≤ x ≤ 0.1) were studied to measure the effect of Cu intercalation on their thermoelectric properties. HR-TEM images of freshly fractured surfaces of Cu0.07Bi2Te3 showed that Cu nanoparticles formed in the van der Waals gaps between Te layers in Bi2Te3. Such nanoparticles acted as electron donors, changing the native p-type character of Bi2Te3 to n-type. They also acted as phonon scatters, reducing thermal conductivity. Cu0.07Bi2Te3 had high electrical conductivity of 681 S cm−1 and Seebeck coefficient of −236 μV K−1 with low thermal conductivity of 1.0 W m−1 K−1, thus enhancing the figure of merit, ZT, to 1.15 at approximately 300 K. The intercalation of metal nanoparticles between Te layers in Bi2Te3 is a promising strategy for the preparation of highly efficient n-type thermoelectric materials.

Colors of graphene and graphene-oxide multilayers on various substrates

We investigated the colors of graphene and graphene-oxide multilayers that were deposited on various dielectric layers. In particular, the effects of the material thickness, the types of dielectric layers, and the existence of a back silicon substrate were analyzed. The colors of graphene-oxide layers on a SiO2/Si substrate were found to periodically change as the material thickness increased. However, the colors of graphene layers on the same substrate became saturated without a similar periodic change. The calculated colors corresponding to the material thicknesses were verified by optical microscopy and profilometry. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of graphene and graphene-oxide multilayers.

High‐Mobility Transistors Based on Large‐Area and Highly Crystalline CVD‐Grown MoSe2 Films on Insulating Substrates

Large-area and highly crystalline CVD-grown multilayer MoSe2 films exhibit a well-defined crystal structure (2H phase) and large grains reaching several hundred micrometers. Multilayer MoSe2 transistors exhibit high mobility up to 121 cm(2) V(-1) s(-1) and excellent mechanical stability. These results suggest that high mobility materials will be indispensable for various future applications such as high-resolution displays and human-centric soft electronics.

Thermoelectric properties of bipolar diffusion effect on In4Se3−xTex compounds

We present thermoelectric properties and electronic structure of the series compounds of In4Se3−xTex (0.0≤x≤3.0). Even if the Te-doping is an isoelectronic substitution, we found that the electron dominated carrier transport in Se-rich region (x≤0.2) evolves into the electron-hole bipolar transport properties in Te-rich region (x≥2.5) from the temperature-dependent thermal conductivity κ(T), Seebeck coefficient S(T), and Hall coefficient RH(T) measurements. The electronic band structures of In4Se3−xTex (x=0.0, 2.75, and 3.0) are not changed significantly with respect to Te-substitution concentrations. From the Boltzmann transport calculation, the electron-hole bipolar effect on thermoelectric transport properties in Te-rich region can be understood by lowering the chemical potential to the valence band maximum in the Te-rich compounds.

Superconducting Properties of a Stoichiometric FeSe Compound and Two Anomalous Features in the Normal State

This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a T zero c and T c at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 (±0.02). The Seebeck coefficient which was 12.3 μV/K at room temperature, changed to a negative value near 200 K, indicating it to be a two carriers material. Above Tc, the ρ(T ) curve revealed an ’S’ shape. Hence dρ(T )/dT , and dρ(T )/dT 2 showed pseudogap-like behavior at T ∗=110 K according to the resistivity curvature mapping (RCM) method for high Tc cuprates. Moreover, the magnetoresistance ρH(T )/ρH=0 under a magnetic field and the Seebeck coefficient S(T ) revealed revealed pseudogap-like behavior near T ∗. Interestingly, at the same temperature, 30 K, the sign of S(T ) and all signs of dρ(T )/dT 2 changed from negative to positive above Tc. PACS numbers: 74.70.-b, 74.62.Bf, 74.25.fg, 74.70.Ad

Effect of cationic substitution on the thermoelectric properties of In4−xMxSe2.95 compounds (M = Na, Ca, Zn, Ga, Sn, Pb; x = 0.1)

We report an electrical and thermal transport study for polycrystalline samples of In4Se3−x substituted with various metals. Our experimental and theoretical investigation revealed that the Na, Ca, and Pb substitutions at the In4 site in the 4:3 indium selenide lattice are more effective for increase in electron concentration than the Zn, Ga, and Sn substitutions at other sites, and the Peierls distortion in cationic substituted compounds may not be as strong as In4Se3−x based on higher lattice thermal conductivities of the former than the latter. It is found that the cationic substitutions may weaken the effect of Peierls distortion on thermal properties for In4Se3−x while they maintain the same trend for electrical properties of In4Se3−x with different electron concentrations.

Synthesis, anisotropy, and superconducting properties of LiFeAs single crystal

A LiFeAs single crystal with Tconset∼19.7 K was grown in a sealed tungsten crucible using the Bridgeman method. The electrical resistivity experiments revealed a ratio of room temperature to residual resistivity of approximately 46 and 18 for the in-plane and out-of plane directions, respectively. The estimated anisotropic resistivity, γρ=ρc/ρab, was approximately 3.3 at Tconset. The upper critical fields had large Hc2∥ab and Hc2∥c values of 83.4 T and 72.5 T, respectively, and an anisotropy ratio is γH=Hc2∥ab/Hc2∥c∼1.15. The high upper critical field value and small anisotropy highlight the potential use of LiFeAs in a variety of applications. The calculated critical current density (Jc) from the M-H loop is approximately 103 A/cm2