Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1-unit cell (UC) limit. The ferroelectric transition temperature T(c) of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

We propose new two-dimensional (2D) topological insulators (TIs) in functionalized germanenes (GeX, X =H, F, Cl, Br or I) using first-principles calculations. We find GeI is a 2D TI with a bulk gap of about 0.3 eV, while GeH, GeF, GeCl and GeBr can be transformed into TIs with sizeable gaps under achievable tensile strains. A unique mechanism is revealed to be responsible for large topologically-nontrivial gap obtained: owing to the functionalization, the σ orbitals with stronger spin-orbit coupling (SOC) dominate the states around the Fermi level, instead of original π orbitals with weaker SOC; thereinto, the coupling of the pxy orbitals of Ge and heavy halogens in forming the σ orbitals also plays a key role in the further enlargement of the gaps in halogenated germanenes. Our results suggest a realistic possibility for the utilization of topological effects at room temperature.

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the power conversion efficiencies (PCEs) of OSCs show rapid increase and the values have increased to over 18%. [16][17][18][19][20][21][22][23][24] However, owing to the brittle nature of small molecules, [25] the mechanical properties of polymer:NF-SMA blends are generally insufficient and can hardly meet the requirement of stretchable electronics. [26,27] The mechanical imperceptibility of OSCs requires low stiffness and high extensibility for wearable and portable applications. The human skin exhibits a ductility of about 30%, which is the benchmark for skin-wearable devices. [28] Studies have been carried out to determine the mechanical properties of nonfullerene OSCs [29][30][31] and drive the development of stretchable OSCs. [32][33][34][35] For instance, the fracture strain of the well-known PTB7-Th:(3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC) blend films decreased dramatically with the increase of ITIC, and the blend films became significantly stiffer as a result of increased elastic modulus. [26,36] Therefore, methods should be adopted to improve the stretchability and reduce the stiffness of OSCs based on polymer:NF-SMA blends.As the mechanical performance of polymer:NF-SMA blends are often poor, a representative high-efficiency Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of highefficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

absorption coefficients, which can be used to capture the far-infrared of solar radiation. [13,14] This outperforms silicon-based and other solution-based semiconductor materials. Additionally, lead chalcogenide CQDSCs have the potential to break through the Shockley-Quesser limit, via multiple exciton generation. [15][16][17][18] In the past decade, lead chalcogenide CQDSCs have attracted abundant attention and their power conversion efficiencies (PCEs) have increased from ≈3% to ≈14%. [19][20][21] Moreover, the facile solution-processability (synthesis and ligand exchange) allows the production of solar cells by spraying, scraping, and roll-to-roll process, enabling great commercial potential. [22][23][24][25] Recently, some reports discussed the commercial viability of lead chalcogenide CQDSCs. [26,27] The results demonstrated that the cost of flexible lead chalcogenide CQDSCs can be as low as ≈0.94 $ per W, indicating the strong competitiveness of these solution-processability solar cells. To reach this low production cost, the PCEs of the CQDSCs are assumed to be 19% and the devices are assumed to be manufactured via a roll-toroll process. [27] Thus, it is still a great challenge for the commercialization of lead chalcogenide CQDSCs, and the low PCE is still the main obstacle for its market competitiveness.We summarize the main developments of lead chalcogenide CQDSCs (see Figure 1). In the early stage, the Schottky structure was used and it achieved the highest PCE of ≈5.2% in 2013. [28] But after 2014, there exist few reports of lead chalcogenide CQDSCs with the Schottky structure, due to the mismatch between optical absorption and charge extraction. [29] Subsequently, the n-p structure and depleted bulk heterojunction (DBH) structure have greatly improved the PCEs of lead chalcogenide CQDSCs. [30,31] N-p structure uses p-n junction to improve carrier extraction efficiency and reduce recombination, thus increasing the PCE of lead chalcogenide CQDSCs from ≈3% to above 9% in 2015. [19,32] Additionally, the absorption coefficient of lead chalcogenide CQDSCs reaches ≈10 6 cm −3 , which means that it is essential to use ≈1 μm solar absorber to fully absorb solar radiation. The DBH structure uses a bulk electron transport layer (ETL) to address the challenge of insufficient carrier diffusion length for the n-p structure, thus increasing the absorber thickness and greatly improving the short-circuit current (J sc ). [33][34][35] Through continued efforts, the PCE of the DBH structure has reached ≈10.8%. [36] To date, more research Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have received considerable attention due to their broad and tunable absorption and high stability. Presently, lead chalcogenide CQDSC has achieved a power conversion efficiency of ≈14%. However, the state-of-the-art lead chalcogenide CQDSC still has an open-circuit voltage (V oc ) loss of ≈0.45 V, which is significantly higher than those of c-Si and perovskite solar cells. Such high V oc loss severely limits the performance im...

The hexagonal boron nitride (hBN) encapsulation has been widely used in the electronics applications of 2D materials to improve device performance by protecting 2D materials against contamination and degradation. It is often assumed that hBN layers as a dielectric would not affect the electronic structure of encapsulated 2D materials.Here we studied few-layer MoS 2 encapsulated in hBN flakes by using a combination of theoretical and experimental Raman spectroscopy. We found that after the encapsulation the out-of-plane A 1g mode is upshifted, while the in-plane E 1 2g mode is downshifted.The measured downshift of the E 1 2g mode does not decrease with increasing the thickness of MoS 2 , which can be attributed to tensile strains in bilayer and trilayer MoS 2 caused by the typical experimental process of the hBN encapsulation. We estimated the strain magnitude and found that the induced strain may cause the K-Q crossover in the conduction band of few-layer MoS 2 , so greatly modifies its electronic properties as an n-type semiconductor. Our study suggests that the hBN encapsulation should be 1 arXiv:1905.05493v2 [cond-mat.mtrl-sci] 16 May 2019 used with caution, as it may affect the electronic properties of encapsulated few-layer 2D materials.

Radiative cooling (RC) dissipates terrestrial heat to outer space through the atmospheric window, without external energy input and production of environmental pollutants. More and more efforts have been devoted to this clean promising cooling technology; thus diverse radiative coolers have emerged. However, the performance, cost, and effectiveness of various radiative coolers are not exactly the same. In addition, the large-scale application of RC technology is impeded by the low energy density, uncontrollable cooling power, and limited sky-facing area. Here, we critically review the recent progress of RC technology, evaluate the cooling performance of various radiative coolers, and discuss the challenges and feasible solutions to commercialize RC technology. Furthermore, valuable insights are provided to make new breakthroughs in this field.

CO 2 in 2050. [5] Due to the second law of thermodynamics, cooling is generally more challenging than heating. Conventional vapor-compression cooling not only results in excessive energy consumption, accelerating the depletion of fossil fuels, but also degrades the atmospheric ozone, leading to notorious environmental issues. Therefore, developing novel and pollutionfree cooling technology has become an urgent demand in the following decades.Radiative sky cooling (RSC) can harvest the coldness of the universe via the 8-13 µm atmospheric window without any pollution and energy consumption, which has witnessed great progress in the last few years. [6][7][8] Since the first demonstration of daytime radiative cooling via multilayer photonic structure by Fan and co-workers in 2014, this facile cooling technology has drawn worldwide attention. [9,10] With the joint efforts, RSC technology has achieved striking advances in building cooling, [4,11] photovoltaic cooling, [12][13][14] cryogenic cooling, [15,16] RSC driving thermoelectricity, [17][18][19] atmospheric water harvesting, [20,21] personal thermal management [3,22,23] and wearable devices. [24][25][26] For building cooling, the pioneering reports by Goldstein et al. and Zhao et al. have demonstrated that building-integrated RSC modules can provide continuous day-and-night cooling and can potentially save 32%-45% of the electricity consumption for cooling in summer. [4,11] For photovoltaic cooling, selective photonic cooler can lower the temperature of solar panels by over 5.7 K and afford the greater cooling