Novel three-dimensional TiO2 structure with a unique quasi-direct band gap for photocatalysts.

Motivated by fundamental interests and practical applications, three-dimensional (3D) photocatalysts are a fascinating area of research in clean energy. Based on first-principles calculations, we predicted three new 3D polymorphs of TiO2: δ-, ε-, and ζ-TiO2. Our results indicate that the band gaps of TiO2 decrease almost linearly with an increase in the coordination number of Ti. Moreover, δ-TiO2 and ζ-TiO2 are semiconductors, whereas ε-TiO2 is a metal, and the lowest energy of ζ-TiO2 is a quasi-direct band gap semiconductor with a distinctive band gap of 2.69 eV, calculated by the HSE06 level. In addition, the calculated imaginary part of the dielectric function indicates that the optical absorption edge is located in the visible light region, suggesting that the proposed ζ-TiO2 may be a good photocatalyst candidate. Importantly, ζ-TiO2 with the lowest energy is dynamically stable, and phase diagrams based on total energies at a specific pressure indicate that ζ-TiO2 can be synthesized from rutile TiO2 at high-pressure conditions.

High mobility and excellent thermoelectric performance monolayer ZnX2Z4 (X = In, Al, Ga; Z = S, Se, Te) materials.

Recently, two-dimensional (2D) layered polarized ZnIn2S4 nanosheets have been successfully synthesized in experiments. However, the polarized monolayers are unstable in air, which hinders their practical applications. Therefore, in this work, we proposed a new family of nonpolarized monolayers (ß2-phase) ZnX2Z4 (X = In, Al, and Ga; Z = S, Se, and Te) by first-principles. It is confirmed that the energies of ß2-phase ZnX2Z4 are lower than those of the polarized and ß-phase ZnX2Z4 monolayers. Moreover, these ZnX2Z4 monolayers have not only desirable indirect band gaps but also high electron mobility (up to 103 cm2 V-1 s-1), revealing a fascinating visible light absorption range. Furthermore, ß2-phase ZnX2Te4 (X = In, Al, and Ga) has ultra-low lattice thermal conductivity and high ZT value (up to 0.89), suggesting that these monolayers can be good candidates for thermoelectric materials. These new 2D ternary monolayers not only effectively broaden the family of 2D materials but also provide promising candidates for optoelectronic and thermoelectric materials.

Atomic mechanism of lithium intercalation induced phase transition in layered MoS2.

The phase transition in layered MoS2 has attracted wide attention but the detailed phase transition process is still unclear. Here, the H → T' phase transition mechanism of single- and bilayer MoS2 induced by lithium intercalation has been systematically studied using first principles. The results indicated that the lithium intercalation can effectively reduce the sliding barrier of the S atom layer. Moreover, we demonstrated that the phase transition process in bilayer MoS2 is induced by S atom transition one by one instead of the collective behavior of the S atoms. Importantly, we found that the phase transition process in bilayer MoS2 consists of the formation, diffusion and recombination of S vacancies, and the phase transition originates from interlayer lithium defects. In addition, the lithium defects cannot induce phase transition in monolayer MoS2 due to the larger sliding barrier of the S atom.

Dual Defect-Passivation Using Phthalocyanine for Enhanced Efficiency and Stability of Perovskite Solar Cells.

Semiconducting molecules have been employed to passivate traps extant in the perovskite film for enhancement of perovskite solar cells (PSCs) efficiency and stability. A molecular design strategy to passivate the defects both on the surface and interior of the CH3 NH3 PbI3 perovskite layer, using two phthalocyanine (Pc) molecules (NP-SC6 -ZnPc and NP-SC6 -TiOPc) is demonstrated. The presence of lone electron pairs on S, N, and O atoms of the Pc molecular structures provides the opportunity for Lewis acid-base interactions with under-coordinated Pb2+ sites, leading to efficient defect passivation of the perovskite layer. The tendency of both NP-SC6 -ZnPc and NP-SC6 -TiOPc to relax on the PbI2 terminated surface of the perovskite layer is also studied using density functional theory (DFT) calculations. The morphology of the perovskite layer is improved due to employing the Pc passivation strategy, resulting in high-quality thin films with a dense and compact structure and lower surface roughness. Using NP-SC6 -ZnPc and NP-SC6 -TiOPc as passivating agents, it is observed considerably enhanced power conversion efficiencies (PCEs), from 17.67% for the PSCs based on the pristine perovskite film to 19.39% for NP-SC6 -TiOPc passivated devices. Moreover, PSCs fabricated based on the Pc passivation method present a remarkable stability under conditions of high moisture and temperature levels.

Two-Dimensional Semiconducting Boron Monolayers.

The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations. Unexpectedly, we have first found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+px,y and pz orbitals, with which various semiconducting boron monolayers are designed to realize the band gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.

Tuning charge transport by manipulating concentration dependent single-molecule absorption configurations.

Understanding and tuning charge transport in molecular junctions is pivotal for crafting molecular devices with tailored functionalities. Here, we report a novel approach to manipulate the absorption configuration within a 4,4'-bipyridine (4,4'-BPY) molecular junction, utilizing the scanning tunneling microscope break junction technique in a concentration-dependent manner. Single-molecule conductance measurements demonstrate that the molecular junctions exhibit a significant concentration dependence, with a transition from high conductance (HC) to low conductance (LC) states as the concentration decreases. Moreover, we identified an additional conductance state in the molecular junctions besides already known HC and LC states. Flicker noise analysis and theoretical calculations provided valuable insights into the underlying charge transport mechanisms and single-molecule absorption configurations concerning varying concentrations. These findings contribute to a fundamental comprehension of charge transport in concentration-dependent molecular junctions. Furthermore, they offer promising prospects for controlling single-molecule adsorption configurations, thereby paving the way for future molecular devices.

Optimization of the Mixed Gas Detection Method Based on Neural Network Algorithm.

Real-time mixed gas detection has attracted significant interest for being a key factor for applications of the electronic nose (E-nose). However, mixed gas detection still faces the challenge of long detection time and a large amount of training data. Therefore, in this work, we propose a feasible way to realize low-cost fast detection of mixed gases, which uses only the part response data of the adsorption process as the training set. Our results indicated that the proposed method significantly reduced the number of training sets and the prediction time of mixed gas. Moreover, it can achieve new concentration prediction of mixed gas using only the response data of the first 10 s, and the training set proportion can reduce to 60%. In addition, the convolutional neural network model can realize both the smaller training set but also the higher accuracy of mixed gas. Our findings provide an effective way to improve the detection efficiency and accuracy of E-noses for the experimental measurement.

Ultralow diffusion barrier induced by intercalation in layered N-based cathode materials for sodium-ion batteries.

Sodium-ion batteries (SIBs) have attracted huge attention due to not only the similar electrochemical properties to Lithium-ion batteries (LIBs) but also the abundant natural reserves of sodium. However, the high diffusion barrier has hindered its application. In this work, we have theoretically studied the relationship between the strain and the diffusion barrier/path of sodium ions in layered CrN2 by first-principles calculation. Our results show that the strain can not only effectively decrease the diffusion barrier but also change the sodium diffusion path, which can be realized by alkali metal intercalation. Moreover, the diffusion barrier is as low as 0.04 eV with the Cs atoms embedding in layered CrN2 (Cs1/16CrN2), suggesting an excellent candidate cathode for SIBs. In addition, the decrease of the barrier mainly originated from the fact that interlayer electronic coupling weakened with the increase of interlayer spacing. Our findings provide an effective way to enhance sodium diffusion performance, which is beneficial for the design of SIB electrode materials.

Wafer-Scale Field-Effect Transistor-Type Sensor Using a Carbon Nanotube Film as a Channel for Ppb-Level Hydrogen Sulfide Detection.

Sulfur hexafluoride is widely used in power equipment because of its excellent insulation and arc extinguishing properties. However, severe damage to power equipment may be caused and a large-scale collapse of the power grid may occur when SF6 is decomposed into H2S, SOF2, and SO2F2. It is difficult to detect the SF6 concentration as it is a kind of inert gas. Generally, the trace gas decomposed in the early stage of SF6 is detected to achieve the function of early warning. Consequently, it is of great significance to realize the real-time detection of trace gases decomposed from SF6 for the early fault diagnosis of power equipment. In this work, a wafer-scale gate-sensing carbon-based FET gas sensor is fabricated on a four-inch carbon wafer for the detection of H2S, a decomposition product of SF6. The carbon nanotubes with semiconductor properties and the noble metal Pt are respectively used as a channel and a sensing gate of the FET-type gas sensor, and the channel transmission layer and the sensing gate layer each play an independent role and do not interfere with each other by introducing the gate dielectric layer Y2O3, giving full play to their respective advantages to forming an integrated sensor of gas detection and signal amplification. The detection limit of the as-prepared gate-sensing carbon-based FET gas sensor can reach 20 ppb, and its response deviation is not more than 3% for the different batches of gas sensors. This work provides a potentially useful solution for the industrial production of miniaturized and integrated gas sensors.

High Anisotropic Optoelectronics in Monolayer Binary M8X12 (M = Mo, W; X = S, Se, Te).

Exploring high performance and excellent ambient stability in two-dimensional (2D) monolayer photoelectric materials is motivated by not only practical applications but also scientific interest. Here, a new 2D monolayer W8Se12 structure is synthesized via in situ electron-beam irradiation on 2D WSe2. Moreover, we systematically studied the photoelectric properties of the class of monolayer M8X12 (M = Mo, W; X = S, Se, and Te) materials by first principles. The results indicated that Mo8S12, Mo8Se12, W8S12, and W8Se12 monolayers possess desirable direct band gaps and remarkable anisotropic optical absorption in visible light, while Mo8Te12 and W8Te12 monolayers are metals. Impressively, the monolayer W8Se12 can result in a direct-indirect-metal transition under uniaxial strain. In addition, they show high anisotropic carrier mobilities (up to 104 cm2 V-1 s-1), significantly over those of transition-metal dichalcogenides. These new binary monolayer M8X12 structures can effectively broaden the 2D material family and may provide four potential candidates in photoelectric applications.

New family of layered N-based cathode materials for sodium-ion batteries.

The cathode materials of sodium-ion batteries (SIBs) have received considerable attention not only because of their abundant natural reserves and chemical properties similar to those of lithium-ion batteries but also their great potential in energy storage and conversion technologies. However, their low capacity and high diffusion barrier remain unsolved problems. In this work, we systematically studied the theoretical capacity and sodium ion diffusion barrier in a new family of layered transition metal compounds, named MX2 (M = Ti, V, Cr, Mn, and Fe; X = C, N, and O), as the cathode materials of SIBs. The results indicate that all 2H-phase MX2 materials possess a high theoretical capacity of over 300 mA h g-1. Moreover, it is found that the 2H-phase CrN2 exhibits a desirable sodium ion diffusion barrier, indicating high mobility of sodium ions. In addition, the layered CrN2 has a remarkable voltage window (3.1-3.8 V) and outstanding electrochemical performance arising from the charge transfer between Na and N atoms, which is induced by the large electronegativity of nitrogen. Our research provides a promising candidate for application in SIB cathode materials in the future.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy.

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 µW m K-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, εmax, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher εmax values, which significantly improved the processability of the solution-derived films.

High Anisotropic Optoelectronics in Two Dimensional Layered PbSnX2 (X = S/Se).

We systematically study the giant anisotropic optoelectronics in layered PbSnX2 (X = S/Se). The highly anisotropic optoelectronics mainly originates from the asymmetric sublattices SnX, resulting in the anisotropy of photoelectronic properties with fascinating visible light absorption range in single-layer and bilayer PbSnX2. We employ uniaxial strain in both the x and y directions and find an indirect-to-direct band gap transition, while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer PbSnX2. We also demonstrate ultrahigh anisotropic mobilities of electrons (µy > µx) and holes (µx > µy) in both single-layer and bilayer PbSnX2 (X = S/Se), and spin-orbit coupling effects and the increase of layer number significantly reduce exciton binding energies and band gaps. Finally, the strong layer dependence of the band structure is clearly seen when the film thickness is less than 4 layers. Our results provide a fundamental understanding of highly anisotropic PbSnX2 (X = S/Se) and show two potential candidates in photoelectric applications.

The Dirac cone in two-dimensional tetragonal silicon carbides: a ring coupling mechanism.

The exploration of novel two-dimensional semimetallic materials is always an attractive topic. We propose a series of two-dimensional silicon carbides with a tetragonal lattice. The band structure of silicon carbides with tetragonal carbon rings and silicon rings exhibits Dirac cones. Interestingly, the Dirac cone of tetragonal SiC originates from a "ring coupling" mechanism. This mechanism refers to the mutual coupling between the four carbon atoms in the tetragonal C ring, and the same coupling in the tetragonal Si ring. Additionally, the "ring coupling" mechanism is applicable to other group IV binary compounds such as monolayer GeC and SnC. This work provides reliable evidence for the argument that two-dimensional tetragonal materials can produce Dirac cones.

Surface Adsorption and Vacancy in Tuning the Properties of Tellurene.

The emerging two-dimensional tellurene has been demonstrated to be a promising candidate for photoelectronic devices. However, there is a lack of comprehensive insight into the effects of vacancies and common adsorbates (i.e., O2 and H2O) in ambient conditions, which play a crucial role in semiconducting devices. In this work, with the aid of first-principles calculations, we demonstrate that H2O and O2 molecules behave qualitatively differently on tellurene, while water adsorption can be remarkably promoted by adjacent preadsorbed O2. Upon the formation of Te vacancies, the adsorption of both O2 and H2O molecules is enhanced. More importantly, the existence of H2O and Te vacancies can dramatically facilitate the dissociation of O2, suggesting that tellurene may be readily oxidized in humid conditions. In addition, it is found that the electronic properties of tellurene are well preserved upon either H2O or O2 adsorption on the surface. In sharp contrast, vacancies enable significant modification on the band structure. Specifically, an indirect-to-direct band gap transition is found at a vacancy concentration of 5.3%.

Two-Dimensional Li-Based Ternary Chalcogenides for Photocatalysis.

Motivated by fundamental interest and practical applications, the investigations of two-dimensional photocatalysts are fascinating subjects in clean energy. Herein, we propose that two-dimensional Li-based ternary chalcogenides LiXY2 (X = Al, Ga, In; Y = S, Se, Te) have intrinsic polarization and direct band gaps. Our results show that LiXY2 materials possess optical absorption spectra covering the visible and ultraviolet range. We show that these materials possess extremely high electron mobility (∼103 cm2 V-1 s-1), providing great potential in overall water splitting. Furthermore, LiAlS2 and LiGaS2 can facilitate overall water splitting regardless of their energy gaps because of the large differences of surface electronic potentials of LiXY2. Importantly, it is feasible to exfoliate the layered LiAlTe2 from its bulk counterpart in experiments. Our findings open an exotic pathway to realizing promising photocatalytic applications in two-dimensional ternary materials.

New Family of Two-Dimensional Ternary Photoelectric Materials.

Screening unique two-dimensional (2D) materials with high mobility and applicable band gaps is motivated by not only the interest in basic science but also the practical applications for photoelectric materials. In this work, we have systematically studied a new family of 2D ternary quintuple layers (QLs), named ABC (A = Na, K, and Rb; B = Cu, Ag, and Au; C = S, Se, and Te). Our results indicate that the QLs of KCuTe, KAgS, KAgSe, KAuTe, RbCuTe, RbAgSe, and RbAgTe host direct band gaps. Moreover, KCuTe, RbCuTe, and RbAgTe QLs show extremely high mobilities of ∼104 cm2 V-1 s-1. Interestingly, the linear scaling between exciton binding energy and quasiparticle band gap for ABC QLs exhibits an unexpected deviation with the 1/4 law. In addition, KAgSe, KAgS, RbAgSe, and RbAgTe show outstanding power energy conversion efficiencies of up to 21.5%, suggesting that they are good potential donor materials. Our results provide many potential candidates for applications in photoelectric materials, which may be realized in experiments due to the possible exfoliation from their parent compounds.

NiSe2 as Co-Catalyst with CdS: Nanocomposites for High-Performance Photodriven Hydrogen Evolution under Visible-Light Irradiation.

One-dimensional NiSe2 /CdS nanocomposites (NCs), with NiSe2 nanoparticles (NPs) integrated onto the surface of CdS nanowires (NWs) were designed and fabricated through a two-step in situ solvothermal method. Density functional theory (DFT) calculations showed that NiSe2 plays an important role in enhancing photocatalystic hydrogen (H2 ) production when coupled with CdS catalyst, which arises from 1) the contribution of Ni 3d and Se 4p orbitals to the electronic structures and 2) the charge redistribution at the interface region of the heterostructure results in efficient promotion of the charge transfer and separation of the photogenerated electron-hole pairs. Under visible-light illumination (λ≥400 nm), the NCs showed improved photocatalytic H2 -generation activity in the presence of lactic acid. The peak value of 7610.1 µmol h-1 g-1 for H2 production was achieved with an apparent quantum efficiency (AQE) of 41.68 % when the Ni/Cd ratio was set to 10 %, which was approximately 48 times higher than that of bare CdS NWs. Photoelectrochemical measurements also confirmed the pivotal role of NiSe2 in the improvement of the H2 -evolution performance of NiSe2 /CdS NCs, which is consistent with the DFT investigations.

Insights into the unusual semiconducting behavior in low-dimensional boron.

Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of the boron nanostructures have been found to be metallic, despite their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importantly, the low-dimensional semiconducting boron allotropes are predicted to be an excellent solar-cell material with a power conversion efficiency of up to 22%, paving the way for their promising optoelectronic application.

Oxidation-Induced Topological Phase Transition in Monolayer 1T'-WTe2.

Monolayer (ML) tungsten ditelluride (WTe2) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe2 in ambient conditions. To reveal the changes of topological properties of WTe2 arising from oxidation, we systematically study the surface oxidation reaction of ML 1T'-WTe2 using first-principles calculations. The calculated results indicate that the fast oxidation of WTe2 originates from the existence of H2O in air, which significantly promotes the oxidation of ML 1T'-WTe2. More importantly, this low-coverage oxidized WTe2 loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe2 that can still possess the QSH insulator states. The topological phase transition induced by oxidation provides exotic insight into understanding the topological features of layered transition-metal dichalcogenide materials.