Ruoming Tian

A solution-processed TiS2/organic hybrid superlattice film towards flexible thermoelectric devices

Liquid-exfoliation has proven to be a scalable and versatile technique to produce high-yield two-dimensional nanosheets in graphene, BN, layered perovskites and transition metal dichalcogenides. This also provides new insights into the construction of novel nanoelectronics and nanophotonics through the assembly of nanosheets. Here we present a simple exfoliation-and-reassembly approach to produce a flexible n-type TiS2/organic hybrid film for low-temperature thermoelectric applications. The obtained film shows a superlattice structure with alternative layers of TiS2 and organic molecules. Charge transfer occurs when TiS2 and organic molecules form intercalation complexes, which gives rise to a high electrical conductivity but a low Seebeck coefficient. However, the power factor can be further enhanced by annealing the film under vacuum, and the value reaches 210 μW m−1 K−2 at room temperature in this study. Our flexible thermoelectric device can generate a high power density of 2.5 W m−2 at a temperature gradient of 70 K, which hits a new record among organic-based thermoelectric devices.

Eco-friendly p-type Cu2SnS3 thermoelectric material: crystal structure and transport properties

As a new eco-friendly thermoelectric material, copper tin sulfide (Cu2SnS3) ceramics were experimentally studied by Zn-doping. Excellent electrical transport properties were obtained by virtue of 3-dimensionally conductive network for holes, which are less affected by the coexistence of cubic and tetragonal phases that formed upon Zn subsitition for Sn; a highest power factors ~0.84 mW m−1 K−2 at 723 K was achieved in the 20% doped sample. Moreover, an ultralow lattice thermal conductivity close to theoretical minimum was observed in these samples, which could be related to the disordering of atoms in the coexisting cubic and tetragonal phases and the interfaces. Thanks to the phonon-glass-electron-crystal features, a maximum ZT ~ 0.58 was obtained at 723 K, which stands among the tops for sulfide thermoelectrics at the same temperature.

Ultrahigh thermoelectric power factor in flexible hybrid inorganic-organic superlattice

Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. However, the substantial challenge in optimizing carrier concentration without disrupting the superlattice structure prevents further improvement of the thermoelectric performance. Here we demonstrate a strategy for carrier optimization in a hybrid inorganic–organic superlattice of TiS2[tetrabutylammonium]x[hexylammonium]y, where the organic layers are composed of a random mixture of tetrabutylammonium and hexylammonium molecules. By vacuum heating the hybrid materials at an intermediate temperature, the hexylammonium molecules with a lower boiling point are selectively de-intercalated, which reduces the electron density due to the requirement of electroneutrality. The tetrabutylammonium molecules with a higher boiling point remain to support and stabilize the superlattice structure. The carrier concentration can thus be effectively reduced, resulting in a remarkably high power factor of 904 µW m−1 K−2 at 300 K for flexible thermoelectrics, approaching the values achieved in conventional inorganic semiconductors.Hybrid inorganic-organic superlattices show promise for flexible thermoelectric applications, yet they suffer from limited performance. Here, the authors devise a strategy for carrier optimization in a hybrid inorganic-organic superlattice of TiS2[tetrabutylammonium]x[hexylammonium]y, achieving an ultrahigh power factor of 904 μW m−1 K−2.

Cobalt-doping in Cu2SnS3: enhanced thermoelectric performance by synergy of phase transition and band structure modification

Mohite-type ternary sulfide Cu2SnS3, which has been intensively studied in the photovoltaic field, has recently attracted much attention as an outstanding p-type eco-friendly thermoelectric material. In the present work, significant synergistic effects of d-orbital-unfilled transition metal (Co) doping on the crystal structure and electrical/thermal properties of Cu2SnS3 are reported. Crystal structure evolution with Co doping, involving not only monoclinic to cubic and tetragonal transitions but also the formation of a hierarchical architecture (Cu–S nano-precipitates, metal and S vacancies, and even nano-scaled stacking faults), is related to bond softening and intensified phonon scattering. Thus, an ultralow lattice thermal conductivity of 0.90 W m−1 K−1 at 323 K to 0.33 W m−1 K−1 at 723 K was obtained. Moreover, an increased effective mass is derived from the contribution of the Co 3d states to the inherent Cu 3d and S 3p states in the valence band, leading to a remarkable power factor (0.94 mW m−1 K−2, x = 0.20 at 723 K) with optimal doping. As a result, the high ZT of ∼0.85 at 723 K elevates the modified Cu2SnS3 to the level of state-of-the-art mid-temperature eco-friendly sulfide thermoelectric materials.

Thermoelectric properties of sol–gel derived lanthanum titanate ceramics

In this work, the thermoelectric properties of lanthanum titanate ceramics with different La/Ti ratios were reported. Samples were prepared using a sol–gel method followed by a conventional sintering process. At 973 K, the electrical resistivity of La2/3TiO2.87 ceramic is ∼91 μΩ m, with a Seebeck coefficient of −192 μV K−1 and the thermal conductivity is 2 W m−1 K−1. The ZT value of La2/3TiO2.87 is 0.18 at 973 K, demonstrating its potential for high temperature thermoelectric applications.

Doubling the ZT record of TiS2-based thermoelectrics by incorporation of ionized impurity scattering

1T-type TiS2 has been recognized as a promising environmentally friendly thermoelectric material with a record ZT of around 0.4 so far. We hereby report the high thermoelectric performance of a series of TiS2–xAgSnSe2 composites with nano-AgSnSe2 as an ionized impurity for electron scattering. Electrical conductivities (σ) in the composites increased possibly due to intercalation; moreover, they showed an exceptional power law decay of σ ∝ Tm with m much increased in the temperature range of 400–580 K due to a significant contribution of ionized impurity scattering. Correspondingly, Seebeck coefficients (S) experience a much faster growth from ∼450 K to approach and even surpass that in pristine TiS2. The simultaneous enhancement of σ and S enabled a high power factor of ∼1.55 mW m−1 K−1 at 700 K. Besides, the lattice thermal conductivity was effectively reduced due to enhanced phonon scattering and intercalations, almost by half from 1.9 W m−1 K−1 to 1.0 W m−1 K−1 at 700 K. These synergistic effects renewed the record of ZT for TiS2-based thermoelectrics by almost doubling it to ∼0.8 at 700 K in the TiS2–4%AgSnSe2 composite.

Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity.

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.