Air-stable, non-encapsulated Bi2O2Se ultrathin films grown by chemical vapour deposition display high electron mobility and exceptional semiconducting transport properties, making the observation of quantum oscillations possible and suggesting potential applications in electronics.

High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi2O2Se.

Skutterudite materials are widely considered for thermoelectric waste heat recovery. While the skutterudite structure effectively scatters the high frequency phonons, grain-boundary engineering is needed to further reduce the thermal conductivity beyond simply decreasing grain size. Here, we show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene. Wrapping even micron sized grains with graphene leads to such a significant reduction in the thermal conductivity that a high thermoelectric figure of merit zT = 1.5 was realized in n-type YbyCo4Sb12, while a zT of 1.06 was achieved in p-type CeyFe3CoSb12. A 16 leg thermoelectric module was made by using n- and p-type skutterudite–graphene nanocomposites that exhibited conversion efficiency 24% higher than a module made without graphene. Engineering grain boundary complexions with 2-D materials introduces a new strategy for advanced thermoelectric materials.

Skutterudite with graphene-modified grain-boundary complexion enhances zT enabling high-efficiency thermoelectric device

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA–VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

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Recent Progress of Two-Dimensional Thermoelectric Materials

Two-dimensional layered materials (2DLMs) have attracted a tremendous amount of attention as photodetectors due to their fascinating features, including high potentials in new-generation electronic devices, wide coverage of bandgaps, ability to construct van der Waals heterostructures, extraordinary light–mass interaction, strong mechanical flexibility, and the capability of enabling synthesis of 2D nonlayered materials. Until now, most attention has been focused on the well-known graphene and transition metal dichalcogenides (TMDs). However, a growing number of functional materials (more than 5619) with novel optoelectronic and electronic properties are being re-discovered, thereby widening the horizon of 2D libraries. In addition to showing common features of 2DLMs, these new 2D members may bring new opportunities to their well-known analogues, like wider bandgap coverage, direct bandgaps independence with thickness, higher mechanical flexibility, and new photoresponse phenomena. The impressive results communicated so far testify that they have shown high potentials with photodetections covering THz, IR, visible, and UV ranges with comparable or even higher performances than well-known TMDs. Here, we give a comprehensive review on the state-of-the-art photodetections of two-dimensional materials beyond graphene and TMDs. The review is organized as follows: fundamentals of photoresponse first are discussed, followed by detailed photodetections of new 2D members including both layered and non-layered ones. After that, photodiodes and hybrid structures based on these new 2D materials are summarized. Then, the integration of these 2D materials with flexible substrates is reviewed. Finally, we conclude with the current research status of this area and offer our perspectives on future developments. We hope that, through reading this manuscript, readers will quickly have a comprehensive view on this research area.

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2D library beyond graphene and transition metal dichalcogenides: a focus on photodetection

Despite the many attempts to build ultrathin 2D-2D heterojunctions via wet chemical methods, the resulting composite materials reported to date suffer from poor interfacial bonding and/or complexity of the synthetic protocols. Encouraged by the structural compatibility of Bi2WO6 and Bi2O2S, both of which are 2D semiconductors sharing a rather similar structural unit of a [Bi2O2]2+ slice in their crystal structures, we have successfully fabricated an ultrathin nanosheet with a tightly bonded 2D-2D heterojunction between the two components by facilely joining the [Bi2O2]2+ and [S]2- slices using a simple two-step hydrothermal method. Such a Bi2WO6-Bi2O2S 2D-2D heterojunction has a five-alternating-layer (Bi2O2S-Bi2WO6-Bi2O2S-Bi2WO6-Bi2O2S) sandwich structure and a thickness down to ca. 5 nm and was obtained by simply growing the Bi2O2S layer in situ on the surface of monolayer Bi2WO6 nanosheets. The judicious combination of Bi2WO6 and Bi2O2S through a 2D-2D heterojunction not only extended light absorption in the visible range but also significantly enhanced photo(electro)chemical water splitting efficiencies in comparison to the bare Bi2WO6 nanosheets alone due to the close-bonding-promoted interfacial charge separation. Our findings provide a viable methodology to build a host of nanomaterials with closely bonded 2D nanosheets, which can be used as photocatalysts and electrocatalysts, among others.

From One to Two: In Situ Construction of an Ultrathin 2D-2D Closely Bonded Heterojunction from a Single-Phase Monolayer Nanosheet.

With the addition of oxygen into the chain-like bismuth sulfide of Bi2S3, there are two interesting functional compounds of Bi2O2S (photoelectric) and Bi4O4S3 (superconducting) containing the PbO-like [Bi2O2] layers. Nanoscale Bi2O2S crystals with an indirect band gap of 1.12 eV are synthesized via a facile hydrothermal method. This semiconductor shows excellent photoelectric response under the irradiation of visible light lamp at room temperature. Theoretical calculations and packing factor model both indicate that the loosely packed Bi2O2S is an excellent photoelectric material. When the Bi2O2S phase was annealed at 500 °C in an evacuated quartz tube, nanocrystals of Bi4O4S3 were obtained. The powder X-ray diffraction and electron microscope analyses (SEM, TEM, EDX) confirmed the thermal decomposition from orthorhombic Bi2O2S to tetragonal Bi4O4S3. The superconducting transition temperature of Bi4O4S3 was observed to be 4.6 K from the temperature-dependence measurements of electrical resistivity and mag...

Thermal Decomposition of Bismuth Oxysulfide from Photoelectric Bi2O2S to Superconducting Bi4O4S3

CuInTe2/graphene composites were fabricated by spark plasma sintering and their thermoelectric properties were investigated. Embedding graphene sheets into CuInTe2 led to decreased thermal conductivity, which can be ascribed to the enhanced phonon scattering by the interface between graphene and the matrix. An improved dimensionless figure of merit (ZT) of 0.40 at 700 K was achieved in a CuInTe2/graphene composite with a mass ratio of 80 : 1.

Thermoelectric properties of CuInTe2/graphene composites

[Bi2O2]-containing tetragonal compounds have received enormous attention due to unique functions including ferroelectricity, photocatalysis, and superconductivity. Here, a new layered compound Bi9O7.5S6 was synthesized via a facile hydrothermal route. The compound, belonging to a new structure type crystallizes in a rhombohedral system with space group R3m (a = 4.0685(1) A, c = 31.029(5) A, V = 444.8(1) A3, Z = 1). The overall crystal structure consists of alternatively packed unique [Bi2O2] and [BiS2] layers along [001] which are combined with each other by van der Waals interaction. The phase purity of the product is confirmed by powder X-ray diffraction. XPS analyses indicate +3 for Bi and −2 for S atoms. The temperature dependence of resistivity ρ(T) indicates that the semiconducting sample follows the mechanisms of variable range hopping (VRH) and adiabatic small polaron hopping (SPH). The direct-transition band gap, Eg = 1.27 eV derived from optical absorption spectrum, falls in the optimal region ...

Synthesis, Crystal Structure, and Photoelectric Properties of a New Layered Bismuth Oxysulfide.

Layered chalcogenides have attracted much attentions due to their unique physical properties. Herein, two novel layered quaternary chalcogenides K2FeCu3Q4 (Q = S and Se) with the ThCr2Si2-type structure have been synthesized via a facile hydrothermal approach. Their structures were confirmed by XRD, XPS and TEM. The magnetic studies revealed that K2FeCu3S4 nanoparticles show a superparamagnetic behaviour at low temperature while K2FeCu3Se4 microcrystals exhibit a weak ferromagnetic behaviour with a high magnetic transition temperature. The temperature dependence, ρ(T) is semiconducting for all the compounds and explained well by the variable range hopping (VRH) rather than the adiabatic small polaron hopping (SPH). The band gaps of K2FeCu3Q4 are in the range of 0.99–1.28 eV. Their photoelectric responses were also characterized, which demonstrated their potential for application in photovoltaic devices.

Facile synthesis, magnetic, electrical and photoelectric properties of layered quaternary chalcogenides K2FeCu3Q4 (Q = S and Se)

La1−xMgxO1−2xF2xBiS2 (x = 0.1−0.35) were synthesized, and their superconductive properties were investigated. The superconducting transition temperature (Tc) increased below the codoping level (x ≤ 0.25). La1−xMgxOBiS2 (x =0 −0.2) and La1−xMgxO0.6F0.4BiS2 (x = 0.1−0.3) were further prepared to explore the effect of Mg 2+ . We found that the introduction of Mg 2+ and F − leads to local structure distortion. Larger distortion is beneficial for superconductivity in LaOBiS2, which was further confirmed by the results in La1−xCaxO1−2xF2xBiS2 (x = 0.2, 0.3).

Ganghua Zhang

Papers

Thermal Decomposition of Bismuth Oxysulfide from Photoelectric Bi2O2S to Superconducting Bi4O4S3

Thermoelectric properties of CuInTe2/graphene composites

Synthesis, Crystal Structure, and Photoelectric Properties of a New Layered Bismuth Oxysulfide.

Facile synthesis, magnetic, electrical and photoelectric properties of layered quaternary chalcogenides K2FeCu3Q4 (Q = S and Se)

Effect of Local Structure Distortion on Superconductivity in Mg- and F-Codoped LaOBiS2