Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4 TiSe4

Liquid-Metal-Assisted Synthesis of Patterned GaN Thin Films for High-Performance UV Photodetectors Array

GaN's outstanding physical characteristics allow for a wide range of applications in numerous industries. Although individual GaN-based ultraviolet (UV) photodetectors are the subject of in-depth research in recent decades, the demand for photodetectors array is rising as a result of advances in optoelectronic integration technology. However, as a prerequisite for constructing GaN-based photodetectors array, large-area, patterned synthesis of GaN thin films remains a certain challenge. This work presents a facile technique for pattern growing high-quality GaN thin films for the assembly of an array of high-performance UV photodetectors. This technique uses UV lithography, which is not only very compatible with common semiconductor manufacturing techniques, but also enables precise patterning modification. A typical detector has impressive photo-response performance under 365 nm irradiation, with an extremely low dark current of 40 pA, a high Ilight /Idark ratio over 105 , a high responsivity of 4.23 AW-1 , and a decent specific detectivity of 1.76 × 1012 Jones. Additional optoelectronic studies demonstrate the strong homogeneity and repeatability of the photodetectors array, enabling it to serve as a reliable UV image sensor with enough spatial resolution. These outcomes highlight the proposed patterning technique's enormous potential.

Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu₄TiS₄ and Cu₄TiSe₄ were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu₄TiSe₄ compared to that in Cu₄TiS₄, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu₄TiSe₄. In addition, the strong lattice anharmonicity in Cu₄TiSe₄ enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ_L) of 0.11 W m^-1 K^-1 at room temperature in Cu₄TiSe₄ compared with that of 0.58 W m^-1 K^-1 in Cu₄TiS₄. Owing to the suitable band gaps of Cu₄TiS₄ and Cu₄TiSe₄, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu₄TiSe₄ are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu₄TiS₄, due to its low κ_L, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu₄TiSe₄ demonstrates its great potential for applications in thermoelectric conversion.

Cu4TiTe4: Synthesis, Crystal Structure, and Chemical Bonding

A new compound Cu₄TiTe₄ in the Cu-Ti-Te ternary system is prepared using high-temperature solid-state synthesis and characterized by single-crystal X-ray diffraction and energy-dispersive X-ray spectroscopy. The average structure of Cu₄TiTe₄ crystallizes in the cubic space group P4̅3m (cP9; a = 5.9484(1) Å) and adopts the Cu₄TiSe₄ structure type. Like Cu₄TiSe₄, it shows positional disorder in one of the two Cu sites. The three-dimensional structure of Cu₄TiTe₄ is viewed as a cubic close-packed (ccp) array of Te, where half of the tetrahedral holes are orderly occupied by three Cu and one Ti and the disordered Cu atoms effectively occupied 1/4 of the octahedral holes. The calculated density of states (DOS) discerns that the compound is a narrow-bandgap semiconductor, and the crystal orbital Hamilton population (COHP) analysis shows that though the individual Cu-Te short contact is relatively weak compared to the Ti-Te contact, Cu-Te bonds largely contribute toward the overall stability. Due to the unique atomic arrangements, some Te atoms in the unit cell have unsaturated coordination, which presents 5s² lone pairs on the Te atoms. This has been confirmed by the density of states (DOS) and electron localization function (ELF) calculations.

Self-Modulation-Guided Growth of 2D Tellurides with Ultralow Thermal Conductivity

Ultralow thermal conductivity materials have triggered much interest due to diverse applications in thermal insulation, thermal barrier coating, and especially thermoelectrics. Two dimensional (2D) indium tellurides with ultralow thermal conductivity provide a versatile platform for tailoring the heat transfer, exploring new candidates for thermoelectrics, and achieving miniature, lightweight, and highly integrated devices. Unfortunately, their nanostructure and structure-related heat transfer properties at a 2D scale are much less studied due to difficulties in material fabrication. The ionic character between interlayers and strong covalent bonds in 3D directions impede the anisotropic growth of indium telluride flakes; meanwhile, the low environmental stability and chemical reactivity of tellurium also limit the fabrication of high-quality tellurides, thus hindering the exploration of thermal transport properties. Here, a self-modulation-guided growth strategy to synthesize high-quality 2D In₄ Te₃ single crystals with ultralow thermal conductivity (0.47 W m^-1  K^-1 ) is developed. This strategy can also be extended to synthesize a series of highly crystallized metal tellurides, providing excellent candidates for further application in thermoelectrics.

Phase-engineered high-entropy metastable FCC Cu2-y Ag y (In x Sn1-x )Se2S nanomaterials with high thermoelectric performance

Crystal-phase engineering to create metastable polymorphs is an effective and powerful way to modulate the physicochemical properties and functions of semiconductor materials, but it has been rarely explored in thermoelectrics due to concerns over thermal stability. Herein, we develop a combined colloidal synthesis and sintering route to prepare nanostructured solids through ligand retention. Nano-scale control over the unconventional cubic-phase is realized in a high-entropy Cu2-y Ag y (In x Sn1-x )Se2S (x = 0-0.25, y = 0, 0.07, 0.13) system by surface-ligand protection and size-driven phase stabilization. Different from the common monoclinic phase, the unconventional cubic-phase samples can optimize electrical and thermal properties through phase and entropy design. A high power factor (0.44 mW m-1 K-2), an ultralow thermal conductivity (0.25 W m-1 K-1) and a ZT value of 1.52 are achieved at 873 K for the cubic Cu1.87Ag0.13(In0.06Sn0.94)Se2S nanostructured sample. This study highlights a new method for the synthesis of metastable phase high-entropy materials and gives insights into stabilizing the metastable phase through ligand retention in other research communities.

Cubic Stuffed-Diamond Semiconductors LiCu3TiQ4 (Q = S, Se, and Te)

Lithium chalcogenides have been understudied, owing to the difficulty in managing the chemical reactivity of lithium. These materials are of interest as potential ion conductors and thermal neutron detectors. In this study, we describe three new cubic lithium copper chalcotitanates that crystallize in the P4̅3m space group. LiCu₃TiS₄, a = 5.5064(6) Å, and LiCu₃TiSe₄, a = 5.7122(7) Å, represent two members of a new stuffed diamond-type crystal structure, while LiCu₃TiTe₄, a = 5.9830(7) Å crystallized into a similar structure exhibiting lithium and copper mixed occupancy. These structures can be understood as hybrids of the zinc-blende and sulvanite structure types. In situ powder X-ray diffraction was utilized to construct a "panoramic" reaction map for the preparation of LiCu₃TiTe₄, facilitating the design of a rational synthesis and uncovering three new transient phases. LiCu₃TiS₄ and LiCu₃TiSe₄ are thermally stable up to 1000 °C under vacuum, while LiCu₃TiTe₄ partially decomposes when slowly cooled to 400 °C. Density functional theory calculations suggest that these compounds are indirect band gap semiconductors. The measured work functions are 4.77(5), 4.56(5), and 4.69(5) eV, and the measured band gaps are 2.23(5), 1.86(5), and 1.34(5) eV for the S, Se, and Te analogues, respectively.

Ultra-low lattice thermal conductivity and anisotropic thermoelectric transport properties in Zintl compound β-K2Te2

A good thermoelectric (TE) performance is usually the result of the coexistence of an ultralow thermal conductivity and a high TE power factor in the same material. In this paper, we investigate the thermal transport and TE properties of the Zintl compound β-K₂Te₂ based on a combination of first-principles calculations and the Boltzmann transport equation. Remarkably, the calculated lattice thermal conductivity κ_L in hexagonal β-K₂Te₂ is ultralow with a value of 0.19 (0.30) W m^-1 K^-1 along the c (a and b) axis at 300 K due to the small phonon group velocity and phonon lifetime, which is comparable to the κ_L for wood and promises possible good TE performance. By taking the fully anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering into account, the rational electron scattering and transport properties are captured, which indicates a power factor exceeding 2.0 mW m^-1 K^-2. As a result, the anomalously high n-type ZT of 2.62 and p-type ZT of 3.82 at 650 K along the c axis are obtained in the hexagonal β-K₂Te₂, breaking the long-term record of ZT < 3.5 in the majority of the reported TE materials until now. These findings support that hexagonal β-K₂Te₂ is a potential candidate for high-efficiency TE applications.

Self-Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-Type Half-Heuslers

Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence electron count, namely, ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising thermoelectric properties. Exemplarily, it has been experimentally observed that a slight change in the content of Y site atoms (by ∼3-12.5% i.e., m = 0.03 and 0.125 in ZrY_1+mZ) leads to a drastic decrease in lattice thermal conductivity κ_L by more than 65-80% in many of these compounds. The present work aims at exploring the possibility of maximizing the electronic transport scenario after achieving the low κ_L limit in these compounds. By taking into account the full anharmonicity of the lattice dynamics, Boltzmann transport calculations are performed under the framework of density functional theory. Our results show that these excess atoms present in the vacant lattice site induce scattering either by acting as a rattling mode or by hybridizing with the acoustic modes of the host depending upon their mass and bonding chemistry, respectively. Furthermore, the introduction of these scattering centers may lead either to the formation of a defect midgap state in the electronic band structure (detrimental for electronic transport) or to light doping of the host compound. The latter is found to be particularly conducive for attaining synergy in both thermal and electronic transport.