Contact evaluation of the penta-PdPSe/graphene vdW heterojunction: tuning the Schottky barrier and optical properties.

In this work, we design a van der Waals heterojunction composed of semiconducting penta-PdPSe and semi-metallic graphene (G) monolayers based on state-of-the-art theoretical calculations. Our results show that both monolayers well preserve their intrinsic features and possess an n-type near Ohmic Schottky contact with a low Schottky barrier height of 0.085 eV for the electrons at the vertical interface. The electronic band alignment suggests a negative band bending of -1.47 eV at the lateral interface, implying electrons as the major transport carriers. Moreover, the transmission gap closely mirrors the heterojunction's band gap, indicating a subtle yet profound interaction between graphene and penta-PdPSe monolayers, which leads to enhanced optical absorption coefficient reaching 106 cm-1 and strong conductivity spanning the visible to ultraviolet region. In addition, our study demonstrates the ability to modify the penta-PdPSe/G heterojunction interface, switching between p-type as well as Ohmic contacts by applying external electric fields. These properties render the penta-PdPSe/G heterojunction promising for optoelectronic applications.

Three-dimensional porous borocarbonitride composed of pentagonal motifs as a high-performance pyroelectric material.

Pyroelectric materials have been attracting significant attention due to their intrinsic and permanent polarization, where the induced polarization is not associated with specific conditions, such as ferroelectric phase transition, strain gradient, dopants, and electric field. Thus, these materials have great potential for wide applications in energy conversion. Here, we propose a new 3D porous borocarbonitride termed PH-BCN, which is composed of pentagonal motifs with intrinsic polarization along the [0001] direction. Based on first-principles calculations, we show that PH-BCN possesses a record high longitudinal electromechanical coupling coefficient with the value of k33 = 97.99%, a remarkably strong SHG response (χ(2)xzx(0) = χ(2)yzy(0) = χ(2)zxx(0) = χ(2)zyy(0) = -6.23 pm V-1 and χ(2)zzz(0) = 21.21 pm V-1), and a record high shift current value of 908.58 µA V-2 due to the intrinsic vertical polarization. This study expands the family of pentagon-based materials, and may open a new frontier in the design of high-performance pyroelectric materials as well.

Immobilized triatomic CuB2 clusters on 2D carbon nitride: highly selective conversion of CO to ethanol at low potentials.

A promising pathway for carbon usage and energy storage is electrocatalytic reduction of CO to form high-value multi-carbon products. Herein, the d-p coupled triatomic catalyst CuB2@g-C3N4 with significant activity and selectivity for ethanol is presented for the first time. Density functional theory calculations elucidate that these spatially confined triatomic centers are capable of immobilizing multiple CO molecules, providing an exclusive reaction channel for direct C-C coupling. The CuB2@g-C3N4 catalyst can effectively reduce the energy barrier of CO dimerization to 0.46 eV. The limiting potential is only -0.19 V, which is much smaller than that of other Cu-based catalysts. Additionally, the CuB2@g-C3N4 catalyst can effectively inhibit the generation of competing C1 products and hydrogen evolution reactions. Excitingly, CuB2 loading makes g-C3N4 more optically active in visible and even infrared light. This work provides important ideas for the atomically precise design of novel d-p coupled catalysts for the direct conversion of CO2/CO into energetic fuels and high-value chemicals.

Imidazole-graphyne: a new 2D carbon nitride with a direct bandgap and strong IR refraction.

Six-membered rings are common building blocks of many carbon structures. Recent studies have shown that penta-graphene composed of five-membered carbon rings have properties very different from that of graphene. This has motivated the search for new carbon structures. Among this is cp-graphyne, composed of carbon pentagons and bridged by acetylenic linkers. However, the bandgap of cp-graphyne, like that of graphene, is zero, making it unsuitable for applications in electronics. Herein, we show that a new two-dimensional (2D) carbon nitride structure formed by assembling the five-membered imidazole molecules with acetylenic linker can overcome this limitation. Named ID-GY, this new material not only has a direct band gap of 1.10 eV, but it is dynamically and mechanically stable and can withstand temperatures up to 1200 K. In addition, due to its porous and anisotropic geometry, the Young's modulus of ID-GY along the diagonal direction is lower than that of most 2D materials reported previously. Equally important, ID-GY exhibits strong refraction near infrared (IR) and has potential for applications in nanoelectronics and optical devices. These results, based on density functional theory, can stimulate experimental studies.

Symmetry-breaking induced large piezoelectricity in Janus tellurene materials.

Structural symmetry-breaking can lead to novel electronic and piezoelectric properties in two-dimensional (2D) materials. In this paper, we propose a 2D Janus tellurene (Te2Se) monolayer with asymmetric Se/Te surfaces and its derived multilayer structures. The band structure calculations show that the 2D Janus Te2Se monolayer is an indirect gap semiconductor, and the intrinsic mirror asymmetry combined with the spin-orbit coupling induces the Rashba spin splitting and the out-of-plane spin polarization. Moreover, the absence of both the inversion symmetry and out-of-plane mirror symmetry, together with flexible mechanical properties, results in large in-plane and out-of-plane piezoelectric coefficients that are valuable in 2D piezoelectric materials. Furthermore, the out-of-plane piezoelectric effects can exist in multilayer structures under different stacking sequences while the in-plane piezoelectric effect can only exist in some specific stacking patterns. The piezoelectric coefficients of the Janus Te2Se monolayer and multilayers exceed those of many Janus transition metal dichalcogenides and other well-known piezoelectric materials (e.g., α-quartz and wurtzite-AlN). The combination of the SOC-induced spin splitting and large piezoelectricity endows the Janus Te2Se structures with potential for applications in spintronics, flexible electronics and piezoelectric devices.

Tuning the electronic and mechanical properties of penta-graphene via hydrogenation and fluorination.

Penta-graphene has recently been proposed as a new allotrope of carbon composed of pure pentagons, and displays many novel properties going beyond graphene [Zhang et al., Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 2372]. To further explore the property modulations, we have carried out a theoretical investigation of the hydrogenated and fluorinated penta-graphene sheets. Our first-principles calculations reveal that hydrogenation and fluorination can effectively tune the electronic and mechanical properties of penta-graphene: turning the sheet from semiconducting to insulating; changing the Poisson's ratio from negative to positive, and reducing the Young's modulus. Moreover, the band gaps of the hydrogenated and fluorinated penta-graphene sheets are larger than those of fully hydrogenated and fluorinated graphene by 0.37 and 0.04 eV, respectively. The phonon dispersions and ab initio molecular dynamics simulations confirm that the surface modified penta-graphene sheets are dynamically and thermally stable, and show that the hydrogenated penta-graphene has more Raman-active modes with higher frequencies as compared to the fluorinated penta-graphene.

Electronic and optical properties of silicon based porous sheets.

Si based sheets have attracted tremendous attention due to their compatibility with the well-developed Si-based semiconductor industry. On the basis of state-of-the-art theoretical calculations, we systematically study the stability, electronic and optical properties of Si based porous sheets including g-Si4N3, g-Si3N4, g-Si3N3 and g-Si3P3. We find that the g-Si3N3 and g-Si3P3 sheets are thermally stable, while the g-Si4N3 and g-Si3N4 are unstable. Different from the silicene-like sheets of SiN and Si3N which are nonplanar and metallic, both the porous g-Si3N3 and g-Si3P3 sheets are planar and nonmetallic, and the former is an indirect band gap semiconductor with a band gap of 3.50 eV, while the latter is a direct band gap semiconductor with a gap of 1.93 eV. Analysis of the optical absorption spectrum reveals that the g-Si3P3 sheet may have applications in solar absorbers owing to its narrow direct band gap and wide range optical absorption in the visible light spectrum.

Efficiency and optimization of government service resource allocation in a cloud computing environment.

According to the connotation and structure of government service resources, data of government service resources in L city from 2019 to 2021 are used to calculate the efficiency of government service resource allocation in each county and region in different periods, particularly by adding the government cloud platform and cloud computing resources to the government service resource data and applying the data envelopment analysis (DEA) method, which has practical significance for the development and innovation of government services. On this basis, patterns and evolutionary trends of government service resource allocation efficiency in each region during the study period are analyzed and discussed. Results are as follows. i) Overall efficiency level in the allocation of government service resources in L city is not high, showing an increasing annual trend among the high and low staggering. ii) Relative difference of allocation efficiency of government service resources is a common phenomenon of regional development, the existence and evolution of which are the direct or indirect influence and reflection of various aspects, such as economic strength and reform effort. iii) Data analysis for the specific points indicates that increased input does not necessarily lead to increased efficiency, some indicators have insufficient input or redundant output. Therefore, optimization of the physical, human, and financial resource allocation methods; and the intelligent online processing of government services achieved by the adoption of government cloud platform and cloud computing resources are the current objective choices to realize maximum efficiency in the allocation of government service resources.

First-Principles Study of the Structural, Electronic, and Enhanced Optical Properties of SnS/TaS2 Heterojunction.

Although the electronics and optoelectronics based on two-dimensional (2D) SnS have attracted great interest, their development is hindered by the large contact resistance at the interface of the metal-semiconductor junction. In this work, using first-principles calculations, we evaluate the contact performance in a van der Waals heterostructure composed of 2D SnS and TaS2. We demonstrate that holes can freely transfer from the electrode to the channel as a consequence of the Schottky-barrier-free interface as well as an upward band bending. Moreover, we show that the intrinsic properties of the SnS monolayer are well-preserved in the heterojunction, which is different from those of contact with metal surfaces. An enhanced optical response is also observed as compared with the freestanding sheet. Given the recent experimental synthesis of the SnS-TaS2 superlattice, this study enhances the understanding of the interface properties of SnS-based metal contact, which is essential for future device applications.

Downregulation of lncRNA-PVT1 participates in the development of progressive chronic kidney disease among patients with congestive heart failure.

BACKGROUND: Congestive heart failure (CHF) is a major cause of the development of progressive chronic kidney disease (CKD), while the mechanism is still unknown. LncRNA PVT1 contributes to kidney injury. This study aimed to explore the role of PVT1 in the development of CKD in CHF patients. METHODS: Expression of PVT1 in plasma samples of CHF patients with and without CKD was determined by RT-qPCR. The diagnostic value of plasma PVT1 for CKD was evaluated by ROC curve analysis. The predictive value of PVT1 for the development of CKD in CHF patients was analyzed by a 2-year follow-up study. Changes in PVT1 expression in CKD patients during treatment were analyzed by RT-qPCR and reflected by heatmaps. RESULTS: Plasma PVT1 was downregulated in CHF and further downregulated in CHF patients complicated with progressive CKD. ROC curve analysis showed that plasma PVT1 levels could be used to distinguish CHF patients complicated with CKD from CHF patients without CKD and healthy controls. During a 2-year follow-up, patients with high CHF levels had a low incidence of progressive CKD among CHF patients. Moreover, with the treatment of progressive CKD, plasma PVT1 was upregulated. CONCLUSIONS: LncRNA-PVT1 downregulation may participate in the development of progressive CKD among patients with CHF.

Penta-BCN: A New Ternary Pentagonal Monolayer with Intrinsic Piezoelectricity.

Going beyond conventional hexagonal sheets, pentagonal 2D structures are of current interest due to their novel properties and broad applications. Herein, for the first time, we study a ternary pentagonal BCN monolayer, penta-BCN, which exhibits intrinsic piezoelectric properties. Based on state-of-the-art theoretical calculations, we find that penta-BCN is stable mechanically, thermally, and dynamically and has a direct band gap of 2.81 eV. Due to its unique atomic configuration with noncentrosymmetric and semiconducting features, penta-BCN displays high spontaneous polarization of 3.17 × 10-10 C/m and a prominent piezoelectricity with d21 = 0.878 pm/V, d22 = -0.678 pm/V, and d16 = 1.72 pm/V, which are larger than those of an h-BN sheet and functionalized pentagraphene. Since B, C, and N are rich in resources, light in mass, and benign to the environment, the intrinsic polarization and piezoelectricity make the penta-BCN monolayer promising for technological applications. This study expands the family of 2D pentagonal structures with new features.

Piezoelectric Effects in Surface-Engineered Two-Dimensional Group III Nitrides.

Piezoelectric effects of two-dimensional (2D) group III-V compounds have received considered attention in recent years because of their wide applications in semiconductor devices. However, they face a problem that only metastable or unstable structures are noncentrosymmetric with piezoelectricity, thus leading to the difficulty in experimental observation. Motivated by the recent advances in the synthesis of 2D group III nitrides, in this paper, for the first time, we study the piezoelectric properties of the 2D group III nitrides (XN, X = Al, Ga, and In) with buckled hexagonal configurations by surface passivation, which are thermodynamically stable. Unlike the previously reported planar graphitic structure, we demonstrate that the hydrogenated 2D nitrides (H-XN-H, X = Al, Ga, and In) exhibit both the in-plane and out-of-plane piezoelectric effects in their monolayer and multilayer structures under an external strain in the basal plane. We further elucidate the underlying mechanism of the piezoelectricity by analyzing the correlations between the piezoelectric coefficients and their structural, electronic, and chemical properties. In addition, we show that H-F cofunctionalization not only enhances the stability, but also significantly improves the ionic polarization because of the charge redistribution, thus leading to large in-plane piezoelectric coefficients in F-XN-H. Our study advances the research in 2D piezoelectric materials and would stimulate more theoretical and experimental efforts in developing effective piezoelectric materials for device applications.

TiS3 sheet based van der Waals heterostructures with a tunable Schottky barrier.

Monolayer titanium trisulfide (TiS3), synthesized recently through exfoliation [Adv. Mater., 2015, 27, 2595], has emerged as a new 2D material with outstanding electronic and optical properties. Here, using first-principles calculations we show for the first time the great potential of the TiS3 monolayer as a channel material when in contact with graphene and other 2D metallic materials to form van der Waals (vdW) heterostructures, where the intrinsic properties of both the TiS3 monolayer and the 2D materials are preserved, different from the conventional 3D metal/TiS3 semiconductor heterojunction [Nanoscale, 2017, 9, 2068]. Moreover, the TiS3 monolayer forms an n-type Schottky barrier (Φe) when in contact with graphene, exhibiting a tunneling barrier and a negative band bending at the lateral interface; the Schottky barrier character can also be changed from n-type to p-type by doping graphene with boron atoms or replacing graphene with other high-work-function 2D metals, while a Schottky-barrier-free contact can be realized by doping graphene with nitrogen atoms, thus providing a solution to the contact-resistance problem in 2D electronics.

2D SnSe-based vdW heterojunctions: tuning the Schottky barrier by reducing Fermi level pinning.

Two-dimensional (2D) SnSe is a very promising material for semiconducting devices due to its novel properties. However, the contact behavior between a 2D SnSe sheet and a three-dimensional (3D) metal surface shows an un-tunable Schottky barrier because of the metallization of the SnSe sheet induced by strong Fermi level pinning at the contact interface. In this work, we use graphene rather than 3D metals as the metal electrode which comes into contact with a single-layer SnSe sheet to form a van der Waals (vdW) heterojunction. Based on state-of-the-art theoretical calculations, we find that the intrinsic properties of the SnSe sheet are preserved and the Fermi level pinning is weakened because of the vdW interaction between the SnSe sheet and graphene. We further demonstrate that an Ohmic contact can be realized by doping graphene with boron or nitrogen atoms or using other high-work-function 2D metals such as ZT-MoSe2, ZT-MoS2, or H-NbS2 sheet as the electrode to reduce the Fermi level pinning, leading to a spontaneous hole injection from the electrode to the channel material. This study sheds light on how to tune the Schottky barrier height for better device performance.

Physical Properties and Photovoltaic Application of Semiconducting Pd2Se3 Monolayer.

Palladium selenides have attracted considerable attention because of their intriguing properties and wide applications. Motivated by the successful synthesis of Pd2Se3 monolayer (Lin et al., Phys. Rev. Lett., 2017, 119, 016101), here we systematically study its physical properties and device applications using state-of-the-art first principles calculations. We demonstrate that the Pd2Se3 monolayer has a desirable quasi-direct band gap (1.39 eV) for light absorption, a high electron mobility (140.4 cm²V-1s-1) and strong optical absorption (~105 cm-1) in the visible solar spectrum, showing a great potential for absorber material in ultrathin photovoltaic devices. Furthermore, its bandgap can be tuned by applying biaxial strain, changing from indirect to direct. Equally important, replacing Se with S results in a stable Pd2S3 monolayer that can form a type-II heterostructure with the Pd2Se3 monolayer by vertically stacking them together. The power conversion efficiency (PCE) of the heterostructure-based solar cell reaches 20%, higher than that of MoS2/MoSe2 solar cell. Our study would motivate experimental efforts in achieving Pd2Se3 monolayer-based heterostructures for new efficient photovoltaic devices.

A C20 fullerene-based sheet with ultrahigh thermal conductivity.

A new two-dimensional (2D) carbon allotrope, Hexa-C20, composed of C20 fullerene is proposed. State-of-the-art first principles calculations combined with solving the linearized phonon Boltzmann transport equation confirm that the new carbon structure is not only dynamically and thermally stable, but also can withstand temperatures as high as 1500 K. Hexa-C20 possesses a quasi-direct band gap of 3.28 eV, close to that of bulk ZnO and GaN. The intrinsic lattice thermal conductivity κlat of Hexa-C20 is 1132 W m-1 K-1 at room temperature, which is much larger than those of most carbon materials such as graphyne (82.3 W m-1 K-1) and penta-graphene (533 W m-1 K-1). Further analysis of its phonons uncovers that the main contribution to κlat is from the three-phonon scattering, while the three acoustic branches are the main heat carriers, and strongly coupled with optical phonon branches via an absorption process. The ultrahigh lattice thermal conductivity and an intrinsic wide band gap make the Hexa-C20 sheet attractive for potential thermal management applications.

Phosphorus K4 Crystal: A New Stable Allotrope.

The intriguing properties of phosphorene motivate scientists to further explore the structures and properties of phosphorus materials. Here, we report a new allotrope named K4 phosphorus composed of three-coordinated phosphorus atoms in non-layered structure which is not only dynamically and mechanically stable, but also possesses thermal stability comparable to that of the orthorhombic black phosphorus (A17). Due to its unique configuration, K4 phosphorus exhibits exceptional properties: it possesses a band gap of 1.54 eV which is much larger than that of black phosphorus (0.30 eV), and it is stiffer than black phosphorus. The band gap of the newly predicted phase can be effectively tuned by appling hydrostastic pressure. In addition, K4 phosphorus exibits a good light absorption in visible and near ultraviolet region. These findings add additional features to the phosphorus family with new potential applications in nanoelectronics and nanomechanics.

TiC2: a new two-dimensional sheet beyond MXenes.

MXenes are attracting attention due to their rich chemistry and intriguing properties. Here a new type of metal-carbon-based sheet composed of transition metal centers and C2 dimers rather than individual C atom is designed. Taking the Ti system as a test case, density functional theory calculations combined with a thermodynamic analysis uncover the thermal and dynamic stability of the sheet, as well as a metallic band structure, anisotropic Young's modulus and Poisson's ratio, a high heat capacity, and a large Debye stiffness. Moreover, the TiC2 sheet has an excellent Li storage capacity with a small migration barrier, a lower mass density compared with standard MXenes, and better chemical stability as compared to the MXene Ti2C sheet. When Ti is replaced with other transition metal centers, diverse new MC2 sheets containing C=C dimers can be formed, the properties of which merit further investigation.

Thermal exfoliation of stoichiometric single-layer silica from the stishovite phase: insight from first-principles calculations.

Mechanical cleavage, chemical intercalation and chemical vapor deposition are the main methods that are currently used to synthesize nanosheets or monolayers. Here, we propose a new strategy, thermal exfoliation for the fabrication of silica monolayers. Using a variety of state-of-the-art theoretical calculations we show that a stoichiometric single-layer silica with a tetragonal lattice, T-silica, can be thermally exfoliated from the stishovite phase in a clean environment at room temperature. The resulting single-layer silica is dynamically, thermally, and mechanically stable with exceptional properties, including a large band gap of 7.2 eV, an unusual negative Poisson's ratio, a giant Stark effect, and a high breakdown voltage. Moreover, other analogous structures like single-layer GeO2 can also be obtained by thermal exfoliation of its bulk phase. Our findings are expected to motivate experimental efforts on developing new techniques for the synthesis of monolayer materials.

A New Silicon Phase with Direct Band Gap and Novel Optoelectronic Properties.

Due to the compatibility with the well-developed Si-based semiconductor industry, there is considerable interest in developing silicon structures with direct energy band gaps for effective sunlight harvesting. In this paper, using silicon triangles as the building block, we propose a new silicon allotrope with a direct band gap of 0.61 eV, which is dynamically, thermally and mechanically stable. Symmetry group analysis further suggests that dipole transition at the direct band gap is allowed. In addition, this new allotrope displays large carrier mobility (~10(4) cm/V · s) at room temperature and a low mass density (1.71 g/cm(3)), making it a promising material for optoelectronic applications.