Chemical Vapor Deposition of Quaternary 2D BiCuSeO p-Type Semiconductor with Intrinsic Degeneracy

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10¹⁹ to 10²¹ cm^-3 , and the Hall mobility of BiCuSeO is ≈191 cm² V^-1 s^-1 (at 2 K). Furthermore, taking advantage of the p-type degeneracy of BiCuSeO, a prototypical BiCuSeO/MoS₂ TFET is fabricated. The emergence of the negative differential resistance trend and multifunctional diodes by modulating the gate voltage and temperature reveal the great practical implementation potential of BiCuSeO nanosheets. These results pave way for the CVD synthesis of multinary 2D materials and rational design of high-performance tunnel devices.

Active and conductive layer stacked superlattices for highly selective CO2 electroreduction

Metal oxides are archetypal CO₂ reduction reaction electrocatalysts, yet inevitable self-reduction will enhance competitive hydrogen evolution and lower the CO₂ electroreduction selectivity. Herein, we propose a tangible superlattice model of alternating metal oxides and selenide sublayers in which electrons are rapidly exported through the conductive metal selenide layer to protect the active oxide layer from self-reduction. Taking BiCuSeO superlattices as a proof-of-concept, a comprehensive characterization reveals that the active [Bi₂O₂]²⁺ sublayers retain oxidation states rather than their self-reduced Bi metal during CO₂ electroreduction because of the rapid electron transfer through the conductive [Cu₂Se₂]^2- sublayer. Theoretical calculations uncover the high activity over [Bi₂O₂]²⁺ sublayers due to the overlaps between the Bi p orbitals and O p orbitals in the OCHO* intermediate, thus achieving over 90% formate selectivity in a wide potential range from −0.4 to −1.1 V. This work broadens the studying and improving of the CO₂ electroreduction properties of metal oxide systems.

It is important yet challenging to improve the interface contact between commonly used carbon conductive layer and metal oxide catalysts. Here the authors propose BiCuSeO with alternant active [Bi₂O₂]²⁺ sublayers and conductive [Cu₂Se₂]^2- sublayer within its superlattice as efficient catalyst for CO₂ electroreduction to formate.

Intrinsically ultralow thermal conductive inorganic solids for high thermoelectric performance

Thermoelectric materials which can convert heat energy to electricity rely on crystalline inorganic solid state compounds exhibiting low phonon transport (i.e. low thermal conductivity) without much inhibiting the electrical transport. Suppression of phonons traditionally has been carried out via extrinsic pathways, involving formation of point defects, foreign nanostructures, and meso-scale grains, but the incorporation of extrinsic substituents also influences the electrical properties. Crystalline materials with intrinsically low lattice thermal conductivity (κlat) provide an attractive paradigm as it helps in simplifying the complex interrelated thermoelectric parameters and allows us to focus largely on improving the electronic properties. In this feature article, we have discussed the chemical bonding and structural aspects in determining phonon transport through a crystalline material. We have outlined how the inherent material properties like lone pair, bonding anharmonicity, presence of intrinsic rattlers, ferroelectric instability, weak and rigid substructures, etc. influence in effectively suppressing the heat transport. The strategies summarized in this feature article should serve as a general guide to rationally design and predict materials with low κlat for potential thermoelectric applications.

Ultralow cross-plane lattice thermal conductivity caused by Bi–O/Bi–O interfaces in natural superlattice-like single crystals

Revealing the impact of Bi–O/Bi–O interfaces with van der Waals interactions on the formation of ultralow cross-plane lattice thermal conductivity.

Two-Dimensional Oxides: Recent Progress in Nanosheets

Two-dimensional (2D) materials have been widely investigated for the last few years, introducing nanosheets and ultrathin films. The often superior electrical, optical and mechanical properties in contrast to their three-dimensional (3D) bulk counterparts offer a promising field of opportunities. Especially new research fields for already existing and novel applications are opened by downsizing and improving the materials at the same time. Some of the most promising application fields are namely supercapacitors, electrochromic devices, (bio-) chemical sensors, photovoltaic devices, thermoelectrics, (photo-) catalysts and membranes. The role of oxides in this field of materials deserves a closer look due to their availability, durability and further advantages. Here, recent progress in oxidic nanosheets is highlighted and the benefit of 2D oxides for applications discussed in-depth. Therefore, different synthesis techniques and microstructures are compared more closely.

BiCuSeO Thermoelectrics: An Update on Recent Progress and Perspective

A BiCuSeO system has been reported as a promising thermoelectric material and has attracted great attention in the thermoelectric community since 2010. Recently, several remarkable studies have been reported and the ZT of BiCuSeO was pushed to a higher level. It motivates us to systematically summarize the recent reports on the BiCuSeO system. In this short review, we start with several attempts to optimize thermoelectric properties of BiCuSeO. Then, we introduce several opinions to explore the origins of low thermal conductivity for BiCuSeO. Several approaches to enhance thermoelectric performance are also summarized, including modulation doping, introducing dual-vacancies, and dual-doping, etc. At last, we propose some possible strategies for enhancing thermoelectric performance of BiCuSeO in future research.