Tuning the electronic structure of rhombohedral and cubic GeTe for thermoelectric application: Influence of molybdenum doping

GeTe Thermoelectrics

Slight Symmetry Reduction in Thermoelectrics

The effect of spin-orbital coupling on the lattice parameters, volume and band gap of rhombohedral (R3m) GeTe has been reported using density functional theory. The effect of applied pressure on rhombohedral GeTe has been studied. This structure has two different bond lengths for Ge-Te bond i.e. 2.86 Å and 3.24 Å. As the applied pressure increases, the both bond lengths decreases and the band gap become narrow. At a pressure of 6.47 GPa, the band gap becomes equal to zero and GeTe has semimetal phase. With pressure applied under a constraint, the larger bond length starts to decrease and the smaller bond increases. As a result, GeTe shows transition to semimetal phase at a relatively lower pressure of 2.87 GPa. The effect of pressure on band structures has been discussed in terms of atomic positions and bond lengths in the real space.

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Rhombohedral GeTe can be approximated as the directional distortion of the cubic GeTe along [111]. Such a symmetry-breaking of the crystal structure results in an opposite arrangement in energy of the L and Σ valence bands, and a split of them into 3L+1Z and 6Σ+6η, respectively. This enables a manipulation of the overall band degeneracy for thermoelectric enhancements through a precise control of the degree of crystal structure deviating from a cubic structure for the alignment of the split bands. Here, we show the effect of AgBiSe2-alloying on the crystal structure as well as thermoelectric transport properties of rhombohedral GeTe. AgBiSe2-alloying is found to not only finely manipulate the crystal structure for band convergence and thereby an increased band degeneracy, but also flatten the valence band for an increased band effective mass. Both of them result in an increased density of state effective mass and therefore an enhanced Seebeck coefficient along with a decreased mobility. Moreover, a remarkably reduced lattice thermal conductivity of ∼0.4 W/m-K is obtained due to the introduced additional point defect phonon scattering and bond softening by the alloying. With the help of Bi-doping at the Ge site for further optimizing the carrier concentration, thermoelectric figure of merit, zT, of ∼1.7 and average zTave of ∼0.9 are achieved in 5% AgBiSe2-alloyed rhombohedral GeTe, which demonstrates this material as a promising candidate for low-temperature thermoelectric applications.

The local bonding structures of GexTe1-x (x = 0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te precursors were investigated using Ge K-edge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all of the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge-Te and Ge-Ge bonds but without any signature of Ge-GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like -Te-Ge-Ge-Te- in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 °C show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, whereas the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the nonsegregated Ge-rich phases and the low growth temperature of the ALD enables the achievement of the structurally metastable phases.

Electronic structure of transparent conducting Mo-doped indium oxide films grown by polymer assisted solution process

Investigation of mechanical, lattice dynamical, electronic and thermoelectric properties of half Heusler chalcogenides: A DFT study

Understanding ferromagnetism and thermoelectric behavior are crucial in spintronics and thermoelectric device applications. Using density functional theory-based WIEN2k code, we have examined the physical properties of vanadium-based MgV2S/Se4 spinels. The calculated negative formation energies and positive phonon frequency indicate the stability of the studied system. The lowest energy ground state has been predicted to be a ferromagnetic phase. The calculated electronic band structure and density of states show that these materials are half-metallic ferromagnetic. The existence of the ferromagnetic phase is described using the pd hybridization, double exchange interaction model by computing the exchange energy and constants. In addition, the quantum coupling of electrons is caused by the shift of the magnetic moment from the V site to non-magnetic sites (S/Se, Mg). Finally, electronic transport parameters like the Seebeck coefficient, electric and thermal conductivity, and power factor are also determined.

Investigation of GaBi1-xSbx based highly mismatched alloys: Potential thermoelectric materials for renewable energy devices and applications

Bulk electronic structure studied by hard X-ray photoelectron spectroscopy of the valence band: The case of intermetallic compounds

Amelioration of nickel-cobalt layered double hydroxides (NiCo-LDH) with a high specific theoretical capacitance is of great desire for high-power supercapacitors. Herein, a molybdenum (Mo) doping strategy is proposed to improve the charge-storage performance of NiCo-LDH nanosheets growing on carbon cloth (CC) via a rapid microwave process. The regulation of the electronic structure and oxygen vacancy of the LDH is consolidated by the density functional theory (DFT) calculation, which demonstrates that Mo doping narrows the band gap, reduces the formation energy of hydroxyl vacancies, and promotes ionic and charge transfer as well as electrolyte adsorption on the electrode surface. The optimal Mo-doped NiCo-LDH electrode (MoNiCo-LDH-0.05/CC) has an amazing specific capacity of 471.1mA h g-1 at 1 A g-1 , and excellent capacity retention of 84.8% at 32 A g-1 , far superior to NiCo-LDH/CC (258.3mA h g-1 and 76.4%). The constructed hybrid supercapacitor delivers an energy density of 103.3W h kg-1 at a power density of 750W kg-1 and retains the cycle retention of 85.2% after 5000 cycles. Two assembled devices in series can drive thirty LED lamps, revealing a potential application prospect of the rationally synthesized MoNiCo-LDH/CC as an energy-storage electrode material.

The electronic structure of tunnel-like VO2 is tailored by Mo doping (VO2-Mo). At 0.1 A g−1, VO2-Mo exhibits a specific capacity of 370 mA h g−1, surpassing that of VO2 (about 232 mA h g−1) and V-based materials reported for NH4+ storage.

Approach of fermi level and electron-trap level in cadmium sulfide nanorods via molybdenum doping with enhanced carrier separation for boosted photocatalytic hydrogen production

Boosting charge transfer via molybdenum doping and electric-field effect in bismuth tungstate: Density function theory calculation and potential applications