Satya N. Guin

High thermoelectric performance in tellurium free p-type AgSbSe2

Enhanced electrical transport and ultra low thermal conductivity resulted in a high thermoelectric figure of merit, ZT, of ∼1 and ∼1.15 at ∼680 K in 4 mol% Pb and 2 mol% Bi doped AgSbSe2, which are 150 and 190% higher compared to that of the pristine sample, respectively. With this excellent thermoelectric performance, p-type AgSbSe2, constituting earth abundant Se, offers promise to replace traditional metal tellurides containing expensive and scarce Te for mid temperature (350–700 K) thermoelectric applications.

Temperature dependent reversible p-n-p type conduction switching with colossal change in thermopower of semiconducting AgCuS.

Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulfide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (ΔS of ~1757 μV K(-1)) at the superionic phase transition (T of ~364 K). In addition, its thermal conductivity is ultralow in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy, and positron annihilation spectroscopy measurements. Using first-principles calculations, we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulfur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.

Promising thermoelectric performance in n-type AgBiSe2: effect of aliovalent anion doping

Thermoelectric materials can convert untapped heat to electrical energy, and thus, it will have a significant role in future energy management. Recent industrial applications demand efficient thermoelectric materials which are made of non-toxic and inexpensive materials. Here, we report promising thermoelectric performance in halogen (Cl/Br/I) doped n-type bulk AgBiSe2, which is a Pb-free material and consists of earth abundant elements. Aliovalent halide ion doping (2–4 mol%) in the Se2− sublattice of AgBiSe2 significantly increases the n-type carrier concentration in AgBiSe2, thus improving the temperature dependent electronic transport properties. Temperature dependent cation order–disorder transition tailors the electronic transport properties in AgBiSe1.98X0.02 (X = Cl, Br and I) samples. Bond anharmonicity and disordered cation sublattice effectively scatter heat carrying phonon in the high temperature cubic phase of AgBiSe1.98X0.02 (X = Cl, Br and I), which limits the lattice thermal conductivity to a low value of ∼0.27 W m−1 K−1 at 810 K. The highest thermoelectric figure of merit, ZT, value of ∼0.9 at ∼810 K has been achieved for the AgBiSe1.98Cl0.02 sample, which is promising among the n-type metal selenide based thermoelectric materials.

Nanostructuring, carrier engineering and bond anharmonicity synergistically boost the thermoelectric performance of p-type AgSbSe2–ZnSe

Thermoelectric “waste heat-to-electrical energy” generation is an efficient and attractive option for robust and environmentally friendly renewable energy production. Simultaneous tailoring of interdependent thermoelectric parameters, i.e. electrical conductivity, thermopower and thermal conductivity, to improve the thermoelectric figure of merit is the utmost challenge in this field. Another important aspect is to develop high performance materials based on cheap and earth abundant materials. We have chosen AgSbSe2, a homologue of AgSbTe2 containing earth abundant selenium, as a model system for thermoelectric investigation due to its low thermal conductivity and favourable valence band structure. Herein, we show that by integrating different but synergistic concepts: (a) carrier engineering, (b) second phase endotaxial nanostructuring and (c) bond anharmonicity, we can achieve a maximum ZT of ∼1.1 at 635 K in AgSbSe2–ZnSe (2 mol%), which is significantly higher than that of pristine AgSbSe2. The above system therefore offers promise to replace traditional metal tellurides for mid-temperature power generation. We demonstrate a design strategy which provides simultaneous enhancement of electrical transport through optimized doping, superior thermopower by the convergence of degenerate valence bands, and glass-like thermal conductivity due to the effective scattering of phonons by nanostructuring, bond anharmonicity and a disordered cation sublattice.

Enhanced thermoelectric performance in p-type AgSbSe2 by Cd-doping

Thermoelectric materials can directly convert waste heat into electrical energy and will play a significant role in future energy management. Herein, we have achieved improved thermoelectric performance in p-type Te-free AgSb1−xCdxSe2 (x = 0.02–0.06) system. Simple doping of Cd2+ in the Sb3+ sublattice increases the carrier concentration, resulting in enhanced electrical conductivity in AgSb1−xCdxSe2 compared to the pristine AgSbSe2. Improved electrical transport and ultra low thermal conductivity give rise to a high thermoelectric figure of merit, ZT, of ∼1 at ∼640 K in AgSb0.98Cd0.02Se2, which is similar to the traditional market based expensive and scarce metal tellurides.

Origin of the Order–Disorder Transition and the Associated Anomalous Change of Thermopower in AgBiS2 Nanocrystals: A Combined Experimental and Theoretical Study

Bulk AgBiS2 crystallizes in a trigonal crystal structure (space group, P3̅m1) at room temperature, which transforms to a cation disordered rock salt structure (space group, Fm3̅m) at ∼473 K. Surprisingly, at room temperature, a solution-grown nanocrystal of AgBiS2 crystallizes in a metastable Ag/Bi ordered cubic structure, which transforms to a thermodynamically stable disorded cubic structure at 610 K. Moreover, the order-disorder transition in nanocrystalline AgBiS2 is associated with an unusual change in thermopower. Here, we shed light on the origin of a order-disorder phase transition and the associated anomalous change of thermopower in AgBiS2 nanocrystals by using a combined experimental, density functional theory based first-principles calculation and ab initio molecular dynamics simulations. Positron-annilation spectroscopy indicates the presence of higher numbers of Ag vacancies in the nanocrystal compared to that of the bulk cubic counterpart at room temperature. Furthermore, temperature-dependent two-detector coincidence Doppler broadening spectroscopy and Doppler broadening of the annihilation radiation (S parameter) indicate that the Ag vacancy concentration increases abruptly during the order-disorder transition in nanocrystalline AgBiS2. At high temperature, a Ag atom shuttles between the vacancy and interstitial sites to form a locally disordered cation sublattice in the nanocrystal, which is facilitated by the formation of more Ag vacancies during the phase transition. This process increases the entropy of the system at higher vacancy concentration, which, in turn, results in the unusual rise in thermopower.

Pressure induced structural, electronic topological, and semiconductor to metal transition in AgBiSe2

We report the effect of strong spin orbit coupling inducing electronic topological and semiconductor to metal transitions on the thermoelectric material AgBiSe2 at high pressures. The synchrotron X-ray diffraction and the Raman scattering measurement provide evidence for a pressure induced structural transition from hexagonal (α-AgBiSe2) to rhombohedral (β-AgBiSe2) at a relatively very low pressure of around 0.7 GPa. The sudden drop in the electrical resistivity and clear anomalous changes in the Raman line width of the A1g and Eg(1) modes around 2.8 GPa was observed suggesting a pressure induced electronic topological transition. On further increasing the pressure, anomalous pressure dependence of phonon (A1g and Eg(1)) frequencies and line widths along with the observed temperature dependent electrical resistivity show a pressure induced semiconductor to metal transition above 7.0 GPa in β-AgBiSe2. First principles theoretical calculations reveal that the metallic character of β-AgBiSe2 is induced mainl...

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe2 has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe2 crystalizes in cubic rock-salt structure with centro-symmetric space group (Fm–3m), and therefore, no ferroelect...

Low frequency noise and photo-enhanced field emission from ultrathin PbBi2Se4 nanosheets

Atomically thin two-dimensional layered materials have gained wide interest owing to their novel properties and potential for applications in nanoelectronic and optoelectronic devices. Here, we present the spectral analysis and photo-enhanced field emission studies of a layered intergrowth PbBi2Se4 nanosheet emitter, performed at the base pressure of ∼1 × 10−8 mbar. The emitter shows a turn-on field value of ∼4.80 V μm−1, corresponding to an emission current density of ∼1 μA cm−2. Interestingly, when the cathode was illuminated with visible light, it exhibited a lower turn-on field of ∼3.90 V μm−1, and a maximum emission current density of ∼893 μA cm−2 has been drawn at an applied electric field of ∼8.40 V μm−1. Furthermore, the photo-enhanced emission current showed reproducible, step-like switching behavior in synchronous with ON–OFF switching of the illumination source. The emission current–time plots reveal excellent stability over a duration of ∼6 h. Low-frequency noise is a significant limitation for the performance of nanoscale electronic devices. The spectral analysis performed on a Fast Fourier Transform (FFT) analyzer revealed that the observed noise is of 1/fα type, with the value of α ∼0.99. The low frequency noise, photo-enhanced field emission, and reproducible switching behavior characterized with very fast rise and fall times propose the layered PbBi2Se4 nanosheet emitter as a new promising candidate for novel vacuum nano-optoelectronic devices.

Sb deficiencies control hole transport and boost the thermoelectric performance of p-type AgSbSe2

Silver antimony selenide, AgSbSe2, a Te free analogue of AgSbTe2, has been known to show a promising thermoelectric performance when it is doped with monovalent (M+) and divalent (M2+) cations in the Sb sublattice. Here, we report a significant enhancement of the thermoelectric performance of p-type nonstoichiometric AgSbSe2 through Sb deficiencies. Sb deficiencies markedly increase the carrier concentration in AgSbSe2 without the addition of any foreign dopant, which in turn enhances electrical conductivity in the 300–610 K temperature range. Enhancement in the electrical transport results in a remarkable improvement in the power factor (σS2) values up to ∼6.94 μW cm−1 K−2 at 610 K in AgSb1−xSe2. Notably, we have achieved a nearly constant σS2 value of ∼6 μW cm−1 K−2 in the 400–610 K temperature range in Sb deficient samples. Additionally, AgSbSe2 exhibits ultra-low thermal conductivity due to phonon scattering because of bond anharmonicity and a disordered cation sub-lattice. With superior electronic transport and ultra-low thermal conductivity, a peak ZT value of ∼1 at 610 K was achieved for the AgSb0.9925Se2 and AgSb0.99Se2 samples. A maximum thermoelectric conversion efficiency (ηmax) of ∼8% was calculated by considering a virtual thermoelectric module consisting of the present p-type AgSb1−xSe2 and previously reported n-type AgBiSe2−xClx, by maintaining a temperature difference of ΔT = 400 K.