Logic Computing Field-Effect Transistors Based on a Monolayer WSe2 Homojunction for the Semi-adder and Decoder.

Two-dimensional reconfigurable field-effect transistors (FETs) are promising candidates for next-generation computing hardware. However, exploring the cascade design of FETs for logic computing remains challenging. Here, by using density functional theory combined with the nonequilibrium Green's function method, we design a 5 nm split-gate FET based on a monolayer WSe2 homojunction, which can implement dynamic polarity control in different gate configurations. The series array of two FETs shows a functional family of logic gates (NOR, AND, XOR, A̅B, and AB̅), and the semi-adder designed by the logic functions AND and XOR reduces the number of transistors by 66.7%. The parallel array of two FETs demonstrates reconfigurable logic gates with NAND/OR/A̅+B/A+B̅ quadruple functions, which can realize the decoding function of 00-11 in the decoder. The cascade design of the electrically tunable FETs helps to tackle the logic device downscaling and integration dilemmas.

Strong interlayer coupling in p-Te/n-CdSe van der Waals heterojunction for self-powered photodetectors with fast speed and high responsivity.

Self-driven photodetectors, which can detect optical signals without external voltage bias, are highly attractive in the field of low-power wearable electronics and internet of things. However, currently reported self-driven photodetectors based on van der Waals heterojunctions (vdWHs) are generally limited by low responsivity due to poor light absorption and insufficient photogain. Here, we report p-Te/n-CdSe vdWHs utilizing non-layered CdSe nanobelts as efficient light absorption layer and high mobility Te as ultrafast hole transporting layer. Benefiting from strong interlayer coupling, the Te/CdSe vdWHs exhibit stable and excellent self-powered characteristics, including ultrahigh responsivity of 0.94 A W-1, remarkable detectivity of 8.36 × 1012 Jones at optical power density of 1.18 mW cm-2 under illumination of 405 nm laser, fast response speed of 24 µs, large light on/off ratio exceeding 105, as well as broadband photoresponse (405-1064 nm), which surpass most of the reported vdWHs photodetectors. In addition, the devices display superior photovoltaic characteristics under 532 nm illumination, such as large Voc of 0.55 V, and ultrahigh Isc of 2.73 µA. These results demonstrate the construction of 2D/non-layered semiconductor vdWHs with strong interlayer coupling is a promising strategy for high-performance and low-power consumption devices.

Effects of p-d orbital mixings on magnetocrystalline anisotropy for D3d-symmetric ferromagnetic semiconducting monolayers.

Understanding magnetic anisotropy based on electronic properties is vital for theoretical and applied research on ferromagnetic semiconductors. Here, for several representative D3d-symmetric ferromagnetic semiconducting monolayers, we investigate the effects of mixings between d-orbitals of central magnetic atoms and p-orbitals of ligands on magnetocrystalline anisotropy energy (MAE). For high-spin materials, the weakening of p-d mixing increases the electron occupation of spin-up bonding d-orbitals at the expense of the electron occupation in the corresponding spin-down orbitals, In contrast, the weakening of p-d mixing decreases the electron occupation of the spin-up antibonding d-orbitals and enhances the electron occupation in the corresponding spin-down orbitals. The weakening mixings also result in an overall shift of the spin-down band toward a higher energy with respect to the spin-up band. These changes are just the opposite in a low-spin material. More interestingly, we find that the transition point between the bonding and the antibonding spin-up bands plays a significant role in tuning the MAE. Its shift with strain is almost linearly related to the p-d bond strength and significantly affects both the electron occupation of occupied spin-up antibonding d-bands and the band shift of unoccupied spin-up d-bands. Furthermore, the correlation of these mixing-related changes in electronic structures with the MAE is qualitatively and quantitatively analyzed. Our findings can deepen the understanding of the correlation between MAE and p-d orbital mixings and provide theoretical guidance for modulating the MAE.

Tunable Room-Temperature Ferromagnetism in Two-Dimensional Cr2Te3.

The manipulation of magnetism provides a unique opportunity for the development of data storage and spintronic applications. Until now, electrical control, pressure tuning, stacking structure dependence, and nanoscale engineering have been realized. However, as the dimensions are decreased, the decrease of the ferromagnetism phase transition temperature (Tc) is a universal trend in ferromagnets. Here, we make a breakthrough to realize the synthesis of 1 and 2 unit cell (UC) Cr2Te3 and discover a room-temperature ferromagnetism in two-dimensional Cr2Te3. The newly observed Tc increases strongly from 160 K in the thick flake (40.3 nm) to 280 K in 6 UC Cr2Te3 (7.1 nm). The magnetization and anomalous Hall effect measurements provided unambiguous evidence for the existence of spontaneous magnetization at room temperature. The theoretical model revealed that the reconstruction of Cr2Te3 could result in anomalous thickness-dependent Tc. This dimension tuning method opens up a new avenue for manipulation of ferromagnetism.

Self-assembly of an alkynylpyrene derivative for multi-responsive fluorescence behavior and photoswitching performance.

Highly emissive fluorophores based on polyaromatic hydrocarbons with tunable emission properties and aggregated structures play a very important role in relevant functional studies. In this study, a novel alkynylpyrene derivative 1 was synthesized, which exhibits unimolecular to excimer emission in methanol with an increasing concentration accompanied by the formation of nanovesicles via the π-π stacking, hydrogen bond and hydrophobic interaction. The self-assembly behavior as well as emission properties of 1 in aprotic polar solvents (ACN, acetone, DMF and DMSO) can also be adjusted by the volume fraction of the poor solvent H2O, which can induce 1 self-assembly to excimer state and could be applied in information transfer. Moreover, upon visible light irradiation, photoswitchable performance of nanovesicles of 1 was observed in which the emission markedly changes from yellow to blue; this is attributed to the cycloaddition reaction of alkynyl groups and singlet oxygen, which can be generated without the addition of external photosensitizers. The multi-responsive and fluorescence behavior of the alkynylpyrene derivative show that the self-assembly can be used to expand the development of this type of fluorophores, and the novel photoinduced tunability of the fluorescence emission provides an effective strategy to obtain high-performance transmitting and sensing materials.

Amphiphilicity Regulation of AgI Nanoclusters: Self-Assembly and Its Application as a Luminescent Probe.

Research on the self-assembly of various amphiphilic molecules is a relatively new research area and of great significance. However, new kinds of metal-nanocluster (NC)-based amphiphilic molecule have rarely been explored. Herein, hydrophobic cation 1-hexadecyl-3-methylimidazolium (C16 mim+ ) was chosen to modify hydrophilic (NH4 )6 [Ag6 (mna)6 ] (Ag6 -NCs, H2 mna=2-mercaptonicotinic acid) and Ag6 @C16 mim-NCs were obtained. Ag6 @C16 mim-NCs displayed thermotropic liquid crystal and thermofluorescent properties. Moreover, the Ag6 @C16 mim-NCs exhibits benign amphiphilicity, and it can self-assemble into ordered nanosheets and nanorods through aggregation in water/dimethyl sulfoxide (DMSO) binary solvent mixtures, whereas single Ag6 -NCs do not. Meanwhile, the Ag6 @C16 mim-NCs also displays aggregation-induced emission properties owing to the restriction of intramolecular vibrations of the capping ligands. Furthermore, the luminescent aggregates could detect arginine selectively with the detection limit at 28 µm. This study introduces a new kind of metal-NC-based amphiphilic molecule in a supramolecular self-assembly field, and they have potential to be used as optical materials in applied research.

Size-controlled excitonic effects on electronic and optical properties of Sb2S3 nanowires.

In this work, the electronic and optical properties of one-dimensional (1D) Sb2S3 nanowires (NWs) with different sizes are investigated using first-principles calculations. The indirect-direct band transition of Sb2S3 NWs can be tuned effectively by the NW size and various uniaxial strains. In the Sb2S3 NWs, the quantum confinement effects result in wider bandgaps while the significantly enhanced electron-hole interaction that is expected to produce excitonic bound states generates a bandgap narrowing. The exciton binding energies for the Sb2S3 NWs are predicted by the effective masses of electrons and holes to lie in the range of 0-1 eV, which are larger than that of bulk Sb2S3, suggesting that excitons in Sb2S3 NWs may bind possible defects to promote luminescence. The size-controlled absorption edge blueshift and redshift of Sb2S3 NWs suggest that Sb2S3 NWs may be promising in the applications of nanoscale light emitting devices.

Band structure engineering of SnS2/polyphenylene van der Waals heterostructure via interlayer distance and electric field.

Constructing a van der Waals heterostructure (vdWH) by stacking different two-dimensional (2D) materials has been considered to be an effective strategy to obtain the desired properties. Here, based on first-principle methods, we systematically explored the electronic structures of 2D SnS2/polyphenylene vdWH. The theoretical results predicted that vdWH exhibited type-II band alignment, which realized effective separation of carriers. The band gap values could be tuned effectively by interlayer coupling effects. Moreover, the vertical electric field not only modulated the band gap, but also transformed the type-II band alignment to type-I or type-III band alignment, realizing multi-functional device applications. Thus, these predicted results reveal the possibility of realizing next-generation electronic applications for 2D SnS2/polyphenylene-based materials.

Modulation of the electronic properties and spin polarization of 2H VS2 nanoribbons by tuning ribbon widths and edge decoration.

The band structures and spin-polarization characteristics of armchair and zigzag VS2 nanoribbons with different terminated edges are investigated based on density functional theory (DFT) calculations with a spin polarized meta-GGA. The results reveal that zigzag 2H VS2 nanoribbons exhibit metal, half-metal, or semiconductor electrical characteristics with different edge decorations or ribbon widths. And the spin polarized ratio can achieve 100% self-polarization for the zigzag VS2 nanoribbons with V atom edges. The Curie temperatures (TC) estimated by mean field approximation simulations for the zigzag 2H VS2 nanoribbons with terminated edges of V systems are 276 K. These preliminary findings offer an effective treatment option for controllable and adjustable spintronic devices.

Type-I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects.

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type-II band alignment and interlayer exciton. However, the studies of type-I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX2 (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer-dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type-I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX2 homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type-I homojunction based 2D semiconductor materials.

Fabrication of three-dimensionally ordered macroporous TiO2 film and its application in quantum dots-sensitized solar cells.

Engineering of TiO2 photoanode is an important strategy for increasing the photovoltaic conversion efficiency of quantum dots-sensitized solar cells (QDSSCs). In this work, three-dimensional ordered macroporous (3DOM) TiO2 films are fabricated by the controlled infiltrating-calcination method using the close-packed polystyrene spheres colloidal crystals as templates. The as-prepared macroporous TiO2 films are then applied as the photoanode in colloidal CdSe QDSSCs. This structure not only facilitates the penetration of thioglycolic acid capped CdSe QDs, and thus achieving a high coverage of the internal surface with QDs sensitizer, but also exhibits a photonic band gap with tunable positions, which could enhance the light absorption. As a result, the liquid-junction QDSSCs assembled with the CdSe sensitized 3DOM TiO2 yields a power conversion efficiency of 3.60% under solar illumination of 100 mW cm-2, and this value is much higher than that of the device using nanoporous TiO2 photoanode (1.82%). Our results indicate that the 3DOM TiO2 is a promising candidate for the construction of high-efficiency QDSSCs.

Coassembly of Mixed Weakley-Type Polyoxometalates to Novel Nanoflowers with Tunable Fluorescence for the Detection of Toluene.

In this work, three-dimensional nanoflowers with tunable fluorescent properties constructed with mixed Weakley-type polyoxometalates (POMs, Na9[LnW10O36]·32H2O, Ln = Eu, Tb, abbreviated to LnW10) and tetraethylenepentamine (TEPA) have been successfully prepared through a facile ionic self-assembly (ISA) method. The shape and petal size of the nanoflower as well as its fluorescent behaviors can be tuned through varying the ratio of EuW10/TbW10. The varied-temperature emission behaviors at 80-260 K show that the fluorescent intensity of both Tb3+ and Eu3+ decreased with the increase in temperature, which makes them potential luminescent ratiometric thermometers. Moreover, after being mixed with polydimethylsiloxane (PDMS), the as-formed hybrid films showed stable fluorescence along with good transparency. The robustness of the hybrid films was also demonstrated by corrosion resistance upon treatment with strong acid and alkali and thus can be used as a sensor to detect toluene circularly. Our results provide a new avenue to the facile construction of fluorescent composites and demonstrate that the POM complexes can be further used in supramolecular chemistry and nanomaterials.

PtSe2/graphene hetero-multilayer: gate-tunable Schottky barrier height and contact type.

Graphene-based two-dimensional hybrid materials are attracting significant attention because they can preserve novel characteristics of Dirac cone. Here, based on first-principles methods, we focus on the electronic characteristics of PtSe2/graphene hetero-multilayer. The negative binding energies indicate that the hybrid materials can be fabricated easily in practice. Also, the n-type Schottky contact is formed and its barrier height is robust to the number of graphene layer. Moreover, the gate-voltage can effectively induce the Schottky barrier transformation from n-type to p-type and contact type transformation from Schottky to Ohmic in the PtSe2/graphene hetero-multilayer. Thus, the work demonstrates that the graphene stacking configuration and gate-voltage will tune the electronic characteristics of PtSe2/graphene-based nanodevices.

Even-odd oscillation of bandgaps in GeP3 nanoribbons and a tunable 1D lateral homogenous heterojunction.

GeP3 is a new kind of IV-V two dimensional material that has been predicted very recently. Here, we have theoretically explored the electronic properties of GeP3 nanoribbons (NRs) by employing first-principles calculations within density functional theory. We find that, unlike other monolayer materials, the bandgaps of armchair GeP3 NRs exhibit a strong even-odd oscillation as a function of nanoribbon width and such oscillations can remain intensive even when the width reaches up to 9 nm. The underlying physics of such oscillation originates from both the parity-dependent geometric symmetry and quantum size effects. Furthermore, we also find that suitable chemical decoration at the nanoribbon edge, e.g., by passivating hydrogen atoms, effectively tunes the indirect band gap into a direct one, making these nanoribbons potentially important for photovoltaic applications. Based on the strong bandgap oscillating nature of GeP3 NRs, we have conceptually designed a lateral homogenous heterojunction, constructed by GeP3 nanoribbons with different widths, which has shown a type-II band alignment beneficial for photo-detector applications.

Hierarchical Nanostructures Self-Assembled by Polyoxometalate and Alkylamine for Photocatalytic Degradation of Dye.

A novel simple strategy for alkylamine-directed self-assembly of Weakley-type polyoxometalate (POM, Na9[EuW10O36]·32H2O, abbreviated to EuW10) to form three-dimensional nanoflowers has been successfully developed through the ionic self-assembly (ISA) method. For comparison, different molecular weights of alkylamines including diethylenetriamine, triethylenetetramine, and tetraethylenepentamine (TEPA) were selected to construct hierarchical nanostructures. Our results revealed that the morphologies and sizes of the nanostructures could be simply controlled by varying the molecular weights and concentrations of alkylamines. The fluorescent color of EuW10/TEPA nanoflowers changed compared with that of EuW10 owing to the varied symmetry degree of europium coordination in EuW10/TEPA nanoflowers. It is demonstrated that this effective self-assembly occurs mainly though the hydrogen bond and electrostatic interaction between EuW10 and TEPA. What's more, the EuW10/TEPA nanoflowers after calcining showed excellent decomposition efficiency toward methylene blue dyes. Our results further confirmed that ISA method between small molecules and POM can provide a unique "bottom-up" strategy to construct novel structures with functional properties.

Band engineering of the MoS2/stanene heterostructure: strain and electrostatic gating.

In a fast developing field, it has been found that van der Waals heterostructures can overcome the weakness of single two-dimensional layered materials and extend their electronic and optoelectronic applications. Through first-principles methods, the studied MoS2/stanene heterostructure preserves high-speed carrier characteristics and opens the direct band gap. Simultaneously, the band alignment shows that the electrons transfer from stanene to MoS2, which forms an internal electric field. As an effective strategy, the out-of-plane strain remarkably changes the band gaps of the heterostructure and enhances its carrier concentration. In addition, the combined effects of the internal and external electric fields can further open the band gaps and induce a direct-to-indirect gap transition in the heterostructure. More interestingly, when the external electric field is equal to the reverse internal one, the heterostructure regains a Dirac cone. Our results show that the MoS2/stanene heterostructure has potential applications in high-speed optoelectronic devices.

Band gap scaling laws in group IV nanotubes.

By using the first-principles calculations, the band gap properties of nanotubes formed by group IV elements have been investigated systemically. Our results reveal that for armchair nanotubes, the energy gaps at K points in the Brillouin zone decrease as 1/r scaling law with the radii (r) increasing, while they are scaled by -1/r 2 + C at Γ points, here, C is a constant. Further studies show that such scaling law of K points is independent of both the chiral vector and the type of elements. Therefore, the band gaps of nanotubes for a given radius can be determined by these scaling laws easily. Interestingly, we also predict the existence of indirect band gap for both germanium and tin nanotubes. Our new findings provide an efficient way to determine the band gaps of group IV element nanotubes by knowing the radii, as well as to facilitate the design of functional nanodevices.

A theoretical simulation of small-molecules sensing on an S-vacancy SnS2 monolayer.

Using first-principle atomistic simulations, we focused on the electronic structures of small gas molecules (CO, H2O, NH3, NO, and NO2) adsorbed on the S-vacancy SnS2 monolayer. The results show that H2O and CO molecules were physisorbed on the S-vacancy SnS2 monolayer, whereas NH3, NO, and NO2 molecules were chemisorbed on the S-vacancy SnS2 monolayer via strong covalent bonds. Moreover, our calculations show that H2O and NH3 act as charge donors, whereas CO, NO, and NO2 gas molecules act as acceptors. Different adsorption behaviors of common gas molecules on the S-vacancy SnS2 monolayer provide a feasible way to exploit chemical gas sensors and electrical devices. In particular, our results also show that under applied biaxial strains, the adsorption energy and charge transfer of gas molecules on the S-vacancy SnS2 monolayer dramatically changed, which indicates that external factors on the S-vacancy SnS2 monolayer are highly preferred.

Influences of the adsorption of different elements on the electronic structures of a tin sulfide monolayer.

Based on first-principles calculations, we investigated the adsorption energy, structural parameters, and electronic and magnetic properties for the adsorption of different atoms, including light metals, hydrogen, oxygen, and 3D transition metals (TM) adatoms, on a tin sulfide (SnS) monolayer. The results showed that Li- and Al-atom adsorption can effectively induce n-type carriers, whereas O atom adsorption can produce p-type doping in the SnS monolayer. In addition, except for Ni atoms, the other adatoms can induce magnetism in the SnS monolayer. Moreover, for Fe- and Co-atom adsorption, the occupied and unoccupied states belong to the same spin-channel. These results indicate that surface adsorption is an effective method to tune the electronic structures of the SnS monolayer.

Dimensionality and Valency Dependent Quantum Growth of Metallic Nanostructures: A Unified Perspective.

Quantum growth refers to the phenomena in which the quantum mechanically confined motion of electrons in metallic wires, islands, and films determines their overall structural stability as well as their physical and chemical properties. Yet to date, there has been a lack of a unified understanding of quantum growth with respect to the dimensionality of the nanostructures as well as the valency of the constituent atoms. Based on a first-principles approach, we investigate the stability of nanowires, nanoislands, and ultrathin films of prototypical metal elements. We reveal that the Friedel oscillations generated at the edges (or surfaces) of the nanostructures cause corresponding oscillatory behaviors in their stability, leading to the existence of highly preferred lengths (or thicknesses). Such magic lengths of the nanowires are further found to depend on both the number of valence electrons and the radial size, with the oscillation period monotonously increasing for alkali and group IB metals, and monotonously decreasing for transition and group IIIA-VA metals. When the radial size of the nanowires increases to reach ∼10 Å, the systems equivalently become nanosize islands, and the oscillation period saturates to that of the corresponding ultrathin films. These findings offer a generic perspective of quantum growth of different classes of metallic nanostructures.