Unusual electronic transport in (1 - x)Cu2Se-(x)CuInSe2 hierarchical composites.

The ability to control the relative density of electronic point defects as well as their energy distribution in semiconductors could afford a systematic modulation of their electronic, optical, and optoelectronic properties. Using a model binary hybrid system Cu2Se-CuInSe2, we have investigated the correlation between phase composition, microstructure, and electronic transport behavior in the synthesized composites. We found that both Cu2Se and CuInSe2 phases coexist at multiple length scales, ranging from sub-ten nanometer to several micrometers, leading to the formation of a hybrid hierarchical microstructure. Astonishingly, the electronic phase diagram of the (1 - x)Cu2Se-(x)CuInSe2 (15% ≤ x ≤ 100%) hierarchical composites remarkably deviates from the trend normally expected for composites between a heavily doped semiconductor (Cu2Se) and a poorly conducting phase (CuInSe2). A sudden 3-fold increase in the electrical conductivity and carrier concentration along with a marginal increase in the carrier mobility is observed for composites at the vicinity of equimolar composition (48% ≤ x ≤ 52%). The carrier concentration increases from ∼1.5 × 1020 cm-3 for the composites with x ≤ 45% to 5.0 × 1020 cm-3 for x = 50%, and remains constant at 4.5 × 1020 cm-3 with x value in the range of 52% < x ≤ 90%, then quickly drops to 8 × 1018 cm-3 for pristine CuInSe2 phase (x = 100%). The atypical electronic behavior was rationalized in the light of the formation of an inter-band (IB) within the band gap, which arises from the hybridization of native electronic point defects from both Cu2Se and CuInSe2 phases in the resulting hierarchical composites. The result points to a new strategy to modulate the electronic structure of semiconductor composites to maximize interaction and coupling between two fundamentally contrasting properties enabling access to electronic hybrid systems with potential applications as interactive and stimuli-responsive multifunctional materials.

CuAlSe2 Inclusions Trigger Dynamic Cu+ Ion Depletion from the Cu2Se Matrix Enabling High Thermoelectric Performance.

Atomic-scale incorporation of CuAlSe2 inclusions within the Cu2Se matrix, achieved through a solid-state transformation of CuSe2 template precursor using elemental Cu and Al, enables a unique temperature-dependent dynamic doping of the Cu2Se matrix. The CuAlSe2 inclusions, due to their ability to accommodate a large fraction of excess metal atoms within their crystal lattice, serve as a "reservoir" for Cu ions diffusing away from the Cu2Se matrix. Such unidirectional diffusion of Cu ions from the Cu2Se matrix to the CuAlSe2 inclusion leads to the formation, near the CuAlSe2/Cu2Se interface, of a high density of Cu-deficient ß-Cu2-δSe nanoparticles within the α-Cu2Se matrix and the formation of Cu-rich Cu1+yAlSe2 nanoparticles with the CuAlSe2 inclusions. This gives rise to a large enhancement in carrier concentration and electrical conductivity at elevated temperatures. Furthermore, the nanostructuring near the CuAlSe2/Cu2Se interface, as well as the extensive atomic disorder in the Cu2Se and CuAlSe2 phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity. Consequently, a significant improvement in ZT is observed for selected Cu2Se/CuAlSe2 composites. This work demonstrates the use of in situ-formed interactive secondary phases in a semiconducting matrix as an elegant alternative approach for further improvement of the performance of leading thermoelectric materials.

Lone-Electron-Pair Micelles Strengthen Bond Anharmonicity in MnPb16Sb14S38 Complex Sulfosalt Leading to Ultralow Thermal Conductivity.

Designing crystalline solids in which intrinsically and extremely low lattice thermal conductivity mainly arises from their unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ∼0.34 W m-1 K-1 at 300 K in the new complex sulfosalt MnPb16Sb14S38. We attribute the ultralow lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a Cp value of 0.206 J g-1 K-1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g-1 K-1). Further analysis of the Cp data and sound velocity (ν = 1834 m s-1) measurement yielded Debye temperature values of 161 and 187 K, respectively. The resulting Grüneisen parameter, γ = 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.

Nanoscale Engineering of Polymorphism in Cu2Se-Based Composites.

Crystal polymorphism selection during synthesis is extremely challenging. However, promoting the formation of a specific metastable polymorph enables modulation of the functional properties of phase-change materials through alteration of the relative abundance of various polymorphs. Here, we demonstrate the stabilization of the superionic ß-Cu2Se phase under ambient conditions and the direct control over the relative ratio between the α-Cu2Se and ß-Cu2Se polymorphs in (x)CuGaSe2/(1-x)Cu2Se composites using CuGaSe2 nanoseeds. We found that the small lattice mismatch between ß-Cu2Se (cubic) and the ab plane of tetragonal CuGaSe2 nanoseeds promotes the formation of low-energy coherent CuGaSe2/ß-Cu2Se interfaces, leading to preferential stabilization of ß-Cu2Se. Astonishingly, the hierarchical microstructure of the resulting composites enables a remarkable decoupling of charge and heat transport, which is manifested by a breakdown of the Wiedemann-Franz law.

Charge Disproportionation Triggers Bipolar Doping in FeSb2- xSn xSe4 Ferromagnetic Semiconductors, Enabling a Temperature-Induced Lifshitz Transition.

Ferromagnetic semiconductors (FMSs) featuring a high Curie transition temperature ( Tc) and a strong correlation between itinerant carriers and localized magnetic moments are of tremendous importance for the development of practical spintronic devices. The realization of such materials hinges on the ability to generate and manipulate a high density of itinerant spin-polarized carriers and the understanding of their responses to external stimuli. In this study, we demonstrate the ability to tune magnetic ordering in the p-type FMS FeSb2- xSn xSe4 (0 ≤ x ≤ 0.20) through carrier density engineering. We found that the substitution of Sb by Sn FeSb2- xSn xSe4 increases the ordering of metal atoms within the selenium crystal lattice, leading to a large separation between magnetic centers. This results in a decrease in the Tc from 450 K for samples with x ≤ 0.05 to 325 K for samples with 0.05 < x ≤ 0.2. In addition, charge disproportionation arising from the substitution of Sb3+ by Sn2+ triggers the partial oxidation of Sb3+ to Sb5+, which is accompanied by the generation of both electrons and holes. This leads to a drastic decrease in the electrical resistivity and thermopower simultaneously with a large increase in the magnetic susceptibility and saturation magnetization upon increasing Sn content. The observed bipolar doping induces a very interesting temperature-induced quantum electronic transition (Lifshitz transition), which is manifested by the presence of an anomalous peak in the resistivity curve simultaneously with a reversal of the sign of a majority of the charge carriers from hole-like to electron-like at the temperature of maximum resistivity. This study suggests that while there is a strong correlation between the overall magnetic moment and free carrier spin in FeSb2- xSn xSe4 FMSs, the magnitude of the Curie temperature strongly depends on the spatial separation between localized magnetic centers rather than the concentration of magnetic atoms or the density of itinerant carriers.

Metal nanoparticle decorated n-type Bi2Te3-based materials with enhanced thermoelectric performances.

In this study, n-type Cu and Zn metal nanoparticle decorated Bi2(Te0.9Se0.1)3 ingots were prepared by a large-scale zone melting technique, with the concept of 'nanoparticle-in-alloy' to separately tune the electrical and thermal transport properties. Cu and Zn additions play multiple but different roles in the materials, whereas both of them form metal nanoinclusions embedded in van der Waals gaps or grain boundaries, exerting influences on thermoelectric properties. Cu addition, accommodated in the tetrahedral vacancies formed by four Te(1) atoms, effectively adjusts the electron concentration by donating its valence electron, and appreciably optimizes the power factor. Coupled with the significant frustration of heat-carrying phonons by Cu nanoinclusions, a highest ZT of 1.15 can be achieved for the 1 at.% Cu sample, which is an ∼20% improvement compared with that of commercial halogen-doped ingots. Zn addition, however, acting as weak donor, noticeably increases the density of state effective mass and Seebeck coefficient, and gives rise to a high ZT of 1.1. In particular, the kilogram-grade production technique coupled with the high ZT makes metal nanoparticle decorated n-type materials very promising for commercial applications.