Multiple Conduction Bands Research Articles

Multiple conduction bands with strong valley anisotropy yielding ultrahigh thermoelectric power factors in n-type elemental Ge

Materials with high power factors are of great importance for thermoelectric power generation. However, currently most state-of-the-art thermoelectric compounds with excellent ZT values mainly benefit from thermal conductivity reduction. Herein, we report an ultrahigh power factor of 64.8 μWcm−1K−2 at 673 K in phosphorus-doped n-type elemental Ge. This is achieved because of the presence of multiple conduction bands with strong valley anisotropy that ensures both large band degeneracy and high electron mobility. By contrast, for the gallium-doped p-type Ge whose electrical properties are governed by one valence band, the optimized power factor is only 26.2 μWcm−1K−2, far less than that of Ge1-xPx. Consequently, a decent ZT of ∼0.5 at 873 K is achieved for n-type elemental Ge. Our work highlights the importance of multi-valley electronic structure and band anisotropy in optimizing thermoelectric performance.

BiSbSe3: A promising Te-free thermoelectric material

BiSbSe3 is a Te-free thermoelectric material with a chain-like complex crystal structure and multiple band configuration. This work overviews recent advancements in BiSbSe3, clearly demonstrating its excellent thermal and electrical characteristics. The strategies to optimize the thermoelectric performance of BiSbSe3 mainly focus on two aspects: manipulating carrier concentration and effective mass based on multiple conduction bands, and elevating carrier mobility based on maneuvering electrical parameters and microstructures. Additionally, strategies for boosting the carrier mobility are outlined as the prospects for the future development of BiSbSe3, including sharpening bands, improving orientation, and designing composites.

Pressure-induced insulator-to-metal transition in the van der Waals compound CoPS3

We have studied the insulator-to-metal transition and crystal structure evolution under high pressure in the van der Waals compound ${\mathrm{CoPS}}_{3}$ through in situ electrical resistance, Hall resistance, magnetoresistance, x-ray diffraction, and Raman scattering measurements. ${\mathrm{CoPS}}_{3}$ exhibits a $C2/m\ensuremath{\rightarrow}P\overline{3}$ structural transformation at 7 GPa accompanied by a $2.9%$ reduction in the volume per formula unit. Concomitantly, the electrical resistance decreases significantly and ${\mathrm{CoPS}}_{3}$ becomes metallic. This metallic ${\mathrm{CoPS}}_{3}$ is a hole-dominant conductor with multiple conduction bands. The linear magnetoresistance and the small volume collapse at the metallization suggest the incomplete high-spin $\ensuremath{\rightarrow}$ low-spin transition in the metallic phase. Thus, the metallic ${\mathrm{CoPS}}_{3}$ possibly possesses an inhomogeneous magnetic moment distribution and short-range magnetic ordering. This paper summarizes the comprehensive phase diagram of $M{\mathrm{PS}}_{3}$ ($M\phantom{\rule{4pt}{0ex}}=\phantom{\rule{4pt}{0ex}}\mathrm{V}$, Mn, Fe, Co, Ni, and Cd) that metalize under pressures.

Enhanced thermoelectric properties in dual cation doped CuCrO2 nanocrystals mediated by magnon-carrier drag

The open-layered CuCrO2 delafossite has received great attention in medium-range temperature thermoelectric applications. The addition of Ni with Mg upsurged the carrier concentration and effective mass by 10-20 times owing to the multiple conduction bands in CuCrO2 hence increased mobility of 38.21 cm2/Vs was achieved. The power factor obtained is 350 μW/mK2 for the CuCr(MgNi)O2 (Mg=0.5, Ni=0.2 wt%) at 700 °C, which is above two times the Mg alone doped CuCrO2 nanocrystals. The lowest thermal conductivity of 3.43 W/Km and the highest ZT of 0.099 were also obtained for the same sample. The augmented Seebeck coefficient of 423 μV/K due to magnon-carriers interaction in the Cr3+ site, the enhanced conductivity of 2083 S/m, and reduced thermal conductivity caused by decreased carrier-phonon interactions are the reasons for the enhanced power factor and improved thermoelectric figure of merit.

Pressure induced bands convergence and strength enhancement in thermoelectric semiconductor β-InSe

The semiconductor InSe possesses an exceptional ductility at room temperature, but its weak electronic transport properties leads to the low thermoelectric properties, limiting its applications in flexible thermoelectric devices. In this work, we applied first principles to find that the hydrostatic pressure can remarkably improve the thermoelectric and mechanical properties of β-InSe. The multiple conduction bands convergence is observed at the hydrostatic pressure of 5 GPa, resulting in a maximum zT value of 0.235 at 300 K, which is 170 % higher than the intrinsic zT value of 0.087. The point defect calculations suggest that In0.96Pb0.04Se or InSe0.96Cl0.04 could obtain the optimal carrier concentration of 7.65 × 1019 cm−3 with the highest zT value. In addition, we find that the (001)/<110>slip system of InSe exhibits the lowest ideal shear strength of 0.68 GPa, due to the weak interlayered van der Waals SeSe bonds. While the hydrostatic pressure can reduce the bond lengths and strengthen the van der Waals SeSe bonds. At 5 GPa, a maximum stretching force constant of van der Waals SeSe bond is obtained, giving rise to a maximum shear ideal strength of 1.45 GPa, which is 113 % higher than that without pressure. This hydrostatic pressure induced strain engineering strategy provides a theoretical guidance to synthesize high-performance thermoelectric β-InSe with robust mechanical properties.

Direct observation of multiple conduction-band minima in high-performance thermoelectric SnSe

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within ±20 fs of the pump pulse arrival. Around 30–50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized.

Realizing high-ranged thermoelectric performance in PbSnS2 crystals

Great progress has been achieved in p-type SnS thermoelectric compound recently, while the stagnation of the n-type counterpart hinders the construction of thermoelectric devices. Herein, n-type sulfide PbSnS2 with isostructural to SnS is obtained through Pb alloying and achieves a maximum ZT of ~1.2 and an average ZT of ~0.75 within 300–773 K, which originates from enhanced power factor and intrinsically ultralow thermal conductivity. Combining the optimized carrier concentration by Cl doping and enlarged Seebeck coefficient through activating multiple conduction bands evolutions with temperature, favorable power factors are maintained. Besides, the electron doping stabilizes the phase of PbSnS2 and the complex-crystal-structure induced strong anharmonicity results in ultralow lattice thermal conductivity. Moreover, a maximum power generation efficiency of ~2.7% can be acquired in a single-leg device. Our study develops a n-type sulfide PbSnS2 with high performance, which is a potential candidate to match the excellent p-type SnS.

Signatures of Multiband Effects in High-Harmonic Generation in Monolayer MoS_{2}.

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS_{2}, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multiband recombination dipoles.

Remarkable anisotropy in rhombohedral Ge2Sb2Te5 compound: a promising thermoelectric material with multiple conduction bands and acoustic-optical branches coupling

First-principles calculations and Boltzmann transport theory are used to evaluate the electronic structure and thermoelectric properties of the high-temperature rhombohedral Ge2Sb2Te5 compound. In this work, we have investigated the phonon spectrum, band structure and density of states with K-H configuration, respectively. In the phonon spectrum, the acoustic and optical branches are coupled, and their combined effect significantly reduces the thermal lattice conductivity. The conduction band contributed by the Ge, Sb, and Te has the characteristics of more multiple valleys and degenerate features. When the electron-doping level is 4.3 × 1019 cm−3, a large number of conduction bands are activated, thereby enhancing the conductivity and seebeck coefficient. The layered Ge2Sb2Te5 has significant anisotropy, and its thermoelectric performance along the z-axis is excellent. The thermoelectric transport coefficients are calculated using Boltzmann transport theory combined with relaxation time approximation. For p type, the largest ZT value of 1.509 could be obtained with the carrier density of 1.3 × 1020 cm−3 at 900 K, while for n type, the considerable ZT value of 3.124 could be obtained with the carrier density of 4.5 × 1019 cm−3 at 700 K.

Crystal, local atomic, and local electronic structures of YbFe2Zn20−xCdx(0≤x≤1.4) : A multiband system with possible coexistence of light and heavy fermions

The partial (up to 7 %) substitution of Cd for Zn in the Yb-based heavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$ %) reduction of the Sommerfeld specific heat coefficient $\gamma$ and a huge (up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient $A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio $A/\gamma ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy shows that the Yb valence state is close to $3+$ for all $x$, whereas X-ray diffraction reveals that Cd replace the Zn ions only at the $16c$ site of the $Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn occupation. Ab-initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with substantial contribution of Cd($16c$) $5p$ levels that have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermions coexist with heavy fermions originated from other conduction bands with larger participation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupled with the Yb$^{3+}$ local moments.

Enhanced thermoelectric performance in Cl-doped BiSbSe3 with optimal carrier concentration and effective mass

Possessing inherently low thermal conductivity, BiSbSe3 is a promising thermoelectric material for medium temperature. Therefore, to substantially optimize the thermoelectric performance of BiSbSe3, researchers mainly focus on the strategies to improve its electrical transport properties. Among these strongly coupled thermoelectric parameters, carrier concentration and effective mass are two intrinsic variables to decisively affect the electrical transport properties. In this work, Cl as a donor dopant is effective to provide extra electrons in n-type BiSbSe3, and the carrier concentration and effective mass can be well optimized simultaneously with increasing Cl content owing to the multiple conduction bands in BiSbSe3. What’s more, maximum weighted mobility ∼53 cm2 V−1 s−1 is obtained in Cl-doped BiSbSe3, which contributes to a largely enhanced power factor ∼4.8 μW cm−1 K-2 at room temperature and outperforms other halogen-doped BiSbSe3 samples. Finally, combining the significantly enhanced power factor and maintained low thermal conductivity, a maximum ZT ∼1.0 is achieved in Cl-doped BiSbSe3 at 800 K.

Crystal-momentum-resolved contributions to multiple plateaus of high-order harmonic generation from band-gap materials

We study the crystal-momentum-resolved contributions to the high-order harmonic generation (HHG) in band-gap materials, and identify the relevant initial crystal momenta for the first and higher plateaus of the HHG spectra. We do so by using a time-dependent density-functional theory model of one-dimensional linear chains. We introduce a self-consistent periodic treatment for the infinitely extended limit of the linear chain model, which provides a convenient way to simulate and discuss the HHG from a perfect crystal beyond the single-active-electron approximation. The multi-plateau spectral feature is elucidated by a semiclassical k-space trajectory analysis with multiple conduction bands taken into account. In the considered laser-interaction regime, the multiple plateaus beyond the first cutoff are found to stem mainly from electrons with initial crystal momenta away from the Gamma point (k = 0), while electrons with initial crystal momenta located around the Gamma point are responsible for the harmonics in the first plateau. We also show that similar findings can be obtained from calculations using a sufficiently large finite model, which proves to mimic the corresponding infinite periodic limit in terms of the band structures and the HHG spectra.

Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K2.5Bi8.5Se14

We studied the narrow band-gap (0.55 eV) semiconductor K2.5Bi8.5Se14 as a potential thermoelectric material for power generation. Samples of polycrystalline K2.5Bi8.5Se14 prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κlat) of 0.57–0.33 W m–1 K–1 in the temperature range of 300–873 K. The physical origin of such low κlat in K2.5Bi8.5Se14 is related to the strong anharmonicity and low phonon velocity caused by its complex low symmetry, large unit cell crystal structure, and mixed occupancy of Bi and K atoms in the lattice. High-resolution scanning transmission electron microscopy studies and microanalysis indicate that the K2.5Bi8.5Se14 sample is a single phase without intergrowth of the structurally related K2Bi8Se13 phase. The undoped material exhibits an n-type character and a figure-of-merit (ZT) value of 0.67 at 873 K. Electronic band structure calculations indicate that K2.5Bi8.5Se14 is an indirect band-gap semiconductor with multiple conduction bands clo...

Intrinsically Low Thermal Conductivity in BiSbSe3: A Promising Thermoelectric Material with Multiple Conduction Bands

AbstractBi2Se3, as a Te‐free alternative of room‐temperature state‐of‐the‐art thermoelectric (TE) Bi2Te3, has attracted little attention due to its poor electrical transport properties and high thermal conductivity. Interestingly, BiSbSe3, a product of alloying 50% Sb on Bi sites, shows outstanding electron and phonon transports. BiSbSe3 possesses orthorhombic structure and exhibits multiple conduction bands, which can be activated when the carrier density is increased as high as ≈3.7 × 1020 cm−3 through heavily Br doping, resulting in simultaneously enhancing the electrical conductivities and Seebeck coefficients. Meanwhile, an extremely low thermal conductivity (≈0.6–0.4 W m−1 K−1 at 300–800 K) is found in BiSbSe3. Both first‐principles calculations and elastic properties measurements show the strong anharmonicity and support the ultra‐low thermal conductivity of BiSbSe3. Finally, a maximum dimensionless figure of merit ZT ∼ 1.4 at 800 K is achieved in BiSb(Se0.94Br0.06)3, which is comparable to the most n‐type Te‐free TE materials. The present results indicate that BiSbSe3 is a new and a robust candidate for TE power generation in medium‐temperature range.

Extraordinary thermoelectric performance in n-type manganese doped Mg3Sb2 Zintl: High band degeneracy, tuned carrier scattering mechanism and hierarchical microstructure

Zintl phases are ideal candidates for thermoelectric applications due to their rich chemistry and structural complexity. However, the persistent p-type conduction due to intrinsic defects strongly restricts their practical applications. Recently, several typical n-type Zintl materials have been designed, where Te-doped Mg3Sb1.5Bi0.5 as the most promising. To enhance its overall thermoelectric performance, we introduce Mn to synergistically optimize the electrical and thermal transport properties. Both experimental and computational results demonstrate that multiple conduction bands with high band degeneracy are responsible for the enhanced Seebeck coefficient. Mn doping on Mg sites changes the low-temperature carrier scattering mechanism from ionized impurity scattering to mixed scattering with acoustic phonons and ionized impurities, resulting in a significant enhancement of carrier mobility and therefore power factor. Simultaneously, the total thermal conductivity is observably reduced after Mn doping. We employed aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) to thoroughly investigate its hierarchical microstructure, including sub-micron grains, nanoscale Bi precipitates segregated at grain boundaries, nanoscale endotaxial Bi-rich precipitates within the Mg3Sb2 based matrix, as well as the resulting strain fields around these defects. The synergistic optimization of electrical and thermal transport contributes to extraordinary performance, namely a peak ZT ~ 1.85 at 723 K and an average ZT ~ 1.25 (from 300 K to 723 K), which are the highest ever reported in any n-type thermoelectric material.

Negative Photoconductance in Heavily Doped Si Nanowire Field-Effect Transistors.

We report the first observation of negative photoconductance (NPC) in n- and p-doped Si nanowire field-effect transistors (FETs) and demonstrate the strong influence of doping concentrations on the nonconventional optical switching of the devices. Furthermore, we show that the NPC of Si nanowire FETs is dependent on the wavelength of visible light due to the phonon-assisted excitation to multiple conduction bands with different band gap energies that would be a distinct optoelectronic property of indirect band gap semiconductor. We attribute the main driving force of NPC in Si nanowire FETs to the photogenerated hot electrons trapping by dopants ions and interfacial states. Finally, comparing back- and top-gate modulation, we derive the mechanisms of the transition between negative and positive photoconductance regimes in nanowire devices. The transition is decided by the competition between the light-induced interfacial trapping and the recombination of mobile carriers, which is dependent on the light intensity and the doping concentration.

Multiple Converged Conduction Bands in K2Bi8Se13: A Promising Thermoelectric Material with Extremely Low Thermal Conductivity.

We report that K2Bi8Se13 exhibits multiple conduction bands that lie close in energy and can be activated through doping, leading to a highly enhanced Seebeck coefficient and a high power factor with elevated temperature. Meanwhile, the large unit cell, complex low symmetry crystal structure, and nondirectional bonding lead to the very low lattice thermal conductivity of K2Bi8Se13, ranging between 0.42 and 0.20 W m-1 K-1 in the temperature interval 300-873 K. Experimentally, we further support the low thermal conductivity of K2Bi8Se13 using phonon velocity measurements; the results show a low average phonon velocity (1605 ms-1), small Young's modulus (37.1 GPa), large Grüneisen parameter (1.71), and low Debye temperature (154 K). A detailed investigation of the microstructure and defects was carried out using electron diffraction and transmission microscopy which reveal the presence of a K2.5Bi8.5Se14 minor phase intergrown along the side of the K2Bi8Se13 phase. The combination of enhanced power factor and low thermal conductivity results in a high ZT value of ∼1.3 at 873 K in electron doped K2Bi8Se13 material.

Berry phase and band structure analysis of the Weyl semimetal NbP.

Weyl semimetals are often considered the 3D-analogon of graphene or topological insulators. The evaluation of quantum oscillations in these systems remains challenging because there are often multiple conduction bands. We observe de Haas-van Alphen oscillations with several frequencies in a single crystal of the Weyl semimetal niobium phosphide. For each fundamental crystal axis, we can fit the raw data to a superposition of sinusoidal functions, which enables us to calculate the characteristic parameters of all individual bulk conduction bands using Fourier transform with an analysis of the temperature and magnetic field-dependent oscillation amplitude decay. Our experimental results indicate that the band structure consists of Dirac bands with low cyclotron mass, a non-trivial Berry phase and parabolic bands with a higher effective mass and trivial Berry phase.

Effect of multiple conduction bands on high-harmonic emission from dielectrics

We find that, for sufficiently strong mid-IR fields, transitions between different conduction bands play an important role in the generation of high-order harmonics in a dielectric. The transitions make a significant contribution to the harmonic signal, and they can create a single effective band for the motion of an electron wave packet. We show how high harmonic spectra produced during the interaction of ultrashort laser pulses with periodic solids provide a spectroscopic tool for understanding the effective band structure that controls electron dynamics in these media.

Disorder-induced valley-orbit hybrid states in Si quantum dots

Quantum dots in silicon are promising candidates for implementation of solid-state quantum information processing. It is important to understand the effects of the multiple conduction band valleys of silicon on the properties of these devices. Here we introduce a novel, systematic effective mass theory of valley-orbit coupling in disordered silicon systems. This theory reveals valley-orbit hybridization effects that are detrimental for storing quantum information in the valley degree of freedom, including non-vanishing dipole matrix elements between valley states and altered intervalley tunneling.