Insights into Photogenerated Carrier Dynamics and Overall Water Splitting of the CrS3/GeSe Heterostructure.

Based on the nonadiabatic molecular dynamics (NAMD) simulations and the first-principles calculations, we explore the overall water-splitting schemes and the photogenerated carrier dynamics for two configurations (CG and CyG) of the CrS3/GeSe van der Waals heterostructures. The photocatalytic direct Z-schemes and carrier migration pathways for hydrogen and oxygen evolution reactions (HER/OER) are constructed based on the electronic properties. The solar-to-hydrogen efficiency (η'STH values) of the schemes can reach 10.60% and 10.17% and further rise under tensile strain. The NAMD results demonstrate similar transfer times of the electron/hole for HER/OER and more rapid electron-hole recombination in CG enables it to be superior to CyG in photocatalytic performance. Moreover, the Gibbs free energy indicates that both the HERs and OERs turn to spontaneously proceed with CG and CyG at pH = 0-12.37 and pH = 2.55-11.01, respectively. These facts reveal that the CrS3/GeSe heterostructure is promising in photocatalytic overall water splitting.

The effect of the CPP size on the nonlinear optical properties of the new necklace-type molecules formed by carborane and [n]Cycloparaphenylenes(n = 8-11).

The new necklace-type molecules were formed by [8-13]CPP and carborane, which further manipulated the size of the macroring, revealing the effect of size on its luminescence behavior. In this work, the effects of ring size on the absorption spectrum, electron excitation and nonlinear optical properties of the compounds were investigated in detail, aiming to reveal an effective way to improve the optical properties of these necklace-type compounds. The absorption spectra of the compounds showed that the size of the CPP ring had little effect on the spectral shape and position, but the electron transition information showed that there were the significant charge transfer within the CPP ring and a gradual enhancement of interfragment charge transfer from the CPP ring to carborane. The increasing order of polarizability, first and second hyperpolarizability values of these compounds with the increase of CPP size indicated that increasing the size of the CPP ring was an effective way to increase the nonlinear optical properties of necklace-type molecules. Among the frequency dependent hyperpolarizability values, the γ(-ω;ω,0,0) value increased by a factor of 4 from complex 1 to 6 with the increase of CPP ring size, which indicated that increasing the size of the CPP ring was an effective way to increase the optical Kerr effect of necklace-type molecules. Therefore, these the new necklace-type nolecules formed by carborane and [n]Cycloparaphenylenes would be excellent nonlinear optical materials in the field of the all-optical switch.

Electron-phonon coupling, bipolar effects, and thermoelectric performance of the CuSbS2 monolayer.

The thermoelectric performance of the CuSbS2 monolayer is determined using the relaxation times obtained from electron-phonon coupling calculations and the transport properties of phonons and electrons. Based on the fully relaxed structure, the lattice thermal conductivity and the electronic transport coefficients are evaluated by solving the Boltzmann transport equation for phonons and electrons under relaxation time approximation, respectively. The tendencies of the transport coefficients depending on the carrier concentrations and temperatures are studied to understand the thermoelectric performance. Based on the bipolar effect, the transport coefficients and intrinsic carrier concentrations, we determined the dimensionless figure of merit ZT in the 300-800 K range. The results demonstrate that the CuSbS2 monolayer should be an p-type semiconductor, and the maximum ZT of 1.36 is obtained, indicating that the monolayer is a good candidate for high-temperature thermoelectric devices. Substantial bipolar effects are observed, and the ones in the x-direction are stronger in comparison to those in the y-direction, which is responsible for the smaller ZT in the x-direction.

Effect of intermolecular interaction of the charge-transfer complex between molecular "tweezers" and C60/C70 on second-order nonlinear optical properties.

To enhance understanding of the correlation between the intermolecular interaction and second-order nonlinear optical (NLO) properties, we studied a "molecular tweezer" with two corannulene substituents linked by a tetrahydro[5]helicene imide, which enabled highly sensitive and selective complexation of C60/C70 through convex-concave π-π interactions. The geometric structure, molecular orbitals, intermolecular interactions, electron absorption spectra and second-order NLO properties of the charge-transfer (CT) complexes formed by molecular tweezers and C60/C70 were studied by density functional theory. Larger fullerenes helped to increase the intermolecular interaction and CT, thereby increasing the first hyperpolarizabilities of CT complexes. Embedding of lithium ions helped to enhance the electron-absorption ability of fullerenes, thereby increasing the intermolecular interaction and intermolecular CT and, thus, enhancing their first hyperpolarizability significantly. Our data indicated that, through structure adjustment (including increasing the volume of fullerene and embedding alkali metal ions), we could enhance intermolecular interactions and improve intermolecular CT significantly. These actions could improve the second-order NLO properties of CT complexes.

Efficient photocatalytic hydrogen evolution and CO2 reduction by HfSe2/GaAs3 and ZrSe2/GaAs3 heterostructures with direct Z-schemes.

The elaborate configuration of the heterostructure is crucial and challenging to achieve high solar-to-hydrogen efficiency or CO2 reduction efficiency . Here, we predict two heterostructures composed of HfSe2, ZrSe2, and GaAs3 monolayers. The maximum of 42.71%/35.12% with the heterostructures can be reached with the perfect match between the bandgap and band edges. The configurations of the heterostructures are discovered from 12 possible stacking types of the three monolayers. The formation energy, potentials of band edges, carrier mobilities, and optical absorption were used to identify the feasibility of the CO2 reduction reaction (CO2RR), the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). The and based on overpotentials and bandgaps and the Gibbs free energies (ΔGs) are evaluated to quantificationally access the photocatalytic performance of the constructed heterostructures. The results demonstrate that high can be obtained for the solar photocatalytic Z-schemes with the HfSe2/GaAs3 and ZrSe2/GaAs3 heterostructures, and these values can be further enhanced through strain engineering. Moreover, small changes in ΔGs were observed for HER, OER, and CO2RR. Therefore, the two heterostructures have excellent performance in photocatalytic hydrogen evolution and CO2 reduction. The results of the electronic properties revealed that the delicate matching of the projected band edges of the monolayers in the heterostructures is responsible for the high photocatalytic performance.

Effects of Atypical Hydrogen Bonds and π-π Interactions on Nonlinear Optical Properties: Dimers of Triangular Structures Based on Perylene, Naphthalene, and Pyromellitic Diimides.

Nonlinear optical (NLO) materials have become important materials in the field of high-speed optical devices due to the changes in light absorption and refraction caused by the photoelectric field. Compounds tend to exist as aggregates rather than single molecules, so intermolecular interactions are crucial to the nature of aggregates. Therefore, to study the effects of intermolecular interactions on nonlinear optical properties, we use a dimer simplified model and adopt the methods of controlling variables, which are the different intermolecular interactions resulting from the different stacking patterns of dimers based on the same monomer structures (2PMDI-1NDI and 2NDI-1PDI). It is found that compared with dimers involving π-π interactions, dimers involving C-H···O interactions have shorter intermolecular distances, larger intermolecular interaction energies, and smaller highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. Moreover, the C-H···O interactions are more conducive to the intermolecular charge transfers and more beneficial for increasing the nonlinear optical response values of aggregates with respect to π-π interactions. This work provides an important basis for the influence of intermolecular interactions on nonlinear optical properties.

The organic co-crystals formed using naphthalenediimide-based triangular macrocycles and coronene: intermolecular charge transfers and nonlinear optical properties.

Formation of organic co-crystals is an effective strategy to synthesize near infrared emission and nonlinear optical (NLO) materials, which often show "1 + 1 > 2" performance. Moreover, the crystallization process can be effectively regulated through supramolecular interactions; thus the properties of co-crystal materials can also be flexibly regulated. Here, in order to further understand the nature and formation mechanism of co-crystals from the perspective of theoretical research, we studied the structures, intermolecular interactions, absorption spectra, charge transfer (CT) characteristics and nonlinear optical (NLO) properties of the newly synthesized organic co-crystals formed between naphthalenediimide based triangles (NDI, acceptor) and coronene (COR, donor). According to the analysis of decomposition of intermolecular interaction energy, dispersion energy played a major role, so the co-crystal properties can be regulated by regulating the intermolecular dispersion energy. More importantly, the formation of co-crystals NDI-COR and NDI-2COR reduced the Egap values with respect to those of their components. And there was significant intermolecular CT from COR to NDI and the degree of CT in NDI-COR was larger than that in NDI-2COR, so that the αtot and γtot values of NDI-COR and NDI-2COR were significantly greater than those of their components. Thus, the NLO properties of organic co-crystals can be further improved by enhancing the electron-donating ability of the donor and the electron-withdrawing ability of the acceptor to enhance the degree of intermolecular interaction energy and CT.

Assembling of Perylene, Naphthalene, and Pyromellitic Diimide-Based Materials and Their Third-Order Nonlinear Optical Properties.

π-conjugated aromatic diimides with chemical stability, heat resistance, and redox activity have attracted more attention due to their excellent fluorescence quantum yield in solution. The planar perylene diimide (PDI) derivatives generally have aggregation-induced emission quenching in the solid state, while the cyclic trimers based on pyromellitic diimides (PMDIs), naphthalene diimides (NDIs), and PDIs can increase the fluorescence quantum yield in the solid state and have large two-photon absorption cross section, which can be used as excellent nonlinear optical (NLO) materials. Therefore, this paper will study the effects of multiple assembly modes of the three monomers on the NLO responses of materials. It was found that the assembly modes of 2PMDI-1NDI and 2NDI-1PDI exhibit larger third-order NLO response (γ) values, which was due to the larger conjugate surface of PDI effectively reducing the energy gap between the HOMO and LUMO. Compared with other assembly methods, 2PMDI-1NDI and 2NDI-1PDI were conducive to causing redshifts (150 nm) in the absorption spectrum. Therefore, the larger conjugate surface of PDI and the assembly mode of the isosceles triangle were more favorable for intramolecular charge transfer, thus improving its NLO properties.

2D XBiSe3(X = Ga, In, Tl) monolayers with high carrier mobility and enhanced visible-light absorption.

The geometrical configurations of the XBiSe3 (X = Ga, In, Tl) monolayers are identified by employing the first-principles density functional theory calculations, and the stabilities are confirmed by phonon dispersion, formation energy, and ab initio molecular dynamics simulation, respectively. The bandgap and band edges, the density of states, the optical absorption, mobility, and effect of strain engineering are evaluated to understand the photoelectronic properties of the monolayers. The results show that the XBiSe3 monolayers have the indirect bandgaps of 1.14-1.69 (1.20-1.84) eV by HSE06(GW), leading to the enhanced optical absorption from the visible to near-ultraviolet region. The large mobility of the electron and hole are also observed, which is helpful for the separation and transfer of the photogenerated carrier pair. The band edges and bandgaps, as well as the optical absorptions, can effectively be tuned by strain engineering. It should be noted that the band edges of the InBiSe3 monolayer could satisfy the condition of redox potential for the hydrogen evolution reaction under the compressive strain heavier than -3%, implicating this monolayer can also be used for photocatalytic water splitting to produce hydrogen. Therefore, these monolayers have potential applications in photocatalytic materials or photoelectronic devices such as energy harvesters and visible-light sensors.

Two-dimensional SiMI4(M = Ge, Sn) monolayers as visible-light-driven photocatalyst of hydrogen production.

Two-dimensional (2D) materials of SiMI4(M = Ge, Sn) monolayers are identified as promising visible-light-driven photocatalyst for hydrogen evolution reaction by DFT calculations. The dynamical and thermal stabilities of the two monolayers are confirmed by the phonon dispersion calculations and ab initiomolecular dynamics (AIMD) simulations, respectively.The results show that the two 2D materials have indirect bandgaps of 2.45 and 2.43 eV, and the band edges can match the hydrogen evolution reaction conditions. Absorption spectra show that the monolayers respond tovisible light and can be tuned by different strains.Besides, the hole and electron mobilitiesare different, which is beneficial for photoelectronic performance. The mechanisms of the hydrogen evolution reaction and the direct water splitting process are also explored. The calculational results support the promising applications of SiMI4(M = Ge, Sn) monolayers asvisible-light-driven photocatalyst of hydrogen production.

The high power conversion efficiency of a two-dimensional GeSe/AsP van der Waals heterostructure for solar energy cells.

Constructing a van der Waals heterostructure is a practical way to promote the conversion efficiency of solar energy. Here, we demonstrate the efficient performance of a GeSe/AsP heterostructure in solar energy cells based on the first-principles calculations. The electronic properties, optical absorption, and optoelectronic properties are calculated to evaluate the efficiency of the newly designed heterostructure. The results indicate that the GeSe/AsP heterostructure possesses a type-II band alignment with an indirect bandgap of 1.10 eV, which greatly promotes the effective separation of photogenerated carriers. Besides, an intrinsic electric field is formed in the direction from the AsP to GeSe monolayer, which is beneficial to prevent the recombination of the photogenerated electron-hole pair. Simultaneously, a strong optical absorption is observed in the visible light range. The predicted power conversion efficiency (PCE) of the GeSe/AsP heterostructure is 16.0% and can be promoted to 17.3% by applying 1% biaxial compression strain. The present results indicate that the GeSe/AsP heterostructure is a promising candidate material for high-performance solar cells.

Direct laser cooling schemes for the triatomic SOH and SeOH molecules based on ab initio electronic properties.

Direct laser cooling is a very promising method to obtain cold molecules for various applications. However, a molecule with satisfactory electronic and optical properties for the optical scheme is difficult to identify. By suggesting criteria for the qualified molecules, we develop a method to identify the suitable polyatomic molecules for direct laser cooling. The new criteria from the equilibrium geometrical structures and fundamental frequencies of the ground and low-lying excited states are used to replace the past ones based on Franck-Condon factors. The new method can rapidly identify the preferable one among many candidate polyatomic molecules based on ab initio calculations because the new criteria are free from the construction of potential energy surfaces. The method is testified by using triatomic molecules containing OH. All the reported and two new molecules suitable for direct laser cooling are identified by comparing 168 electronic states of 28 molecules with the new criteria. The newly found molecules have been confirmed using the Franck-Condon factors from the construction of potential energy surfaces. Finally, the optical schemes for the direct laser cooling of the SOH and SeOH molecules are established.

Direct laser cooling the NH molecule with the pseudo-closed loop triplet-triplet transition including intervening electronic states.

Direct laser cooling molecule is useful way to obtain the accurate molecular spectroscopy. However, most of the reported direct laser cooling schemes are only involved the molecules with a singlet or doublet ground state because the one with a triplet ground state is more complex, especially when the first-excited state is not suitable for the pseudo-closed loop transition. Using NH as the prototype of the simplest heteronuclear molecule with a triplet ground state, we focus on constructing the direct laser cooling scheme with a pseudo-closed loop triplet-triplet transition including intervening electronic states. The potential energy curves and transition dipole moments are calculated for the X3Σ-, a1Δ, b1Σ+, and A3Π states by using the multireference configuration interaction including spin-orbit coupling with the aug-cc-pV5Z basis sets. The rotational and vibrational energy levels of each electronic state are obtained by solving the Schrödinger equation of nuclear motion with the obtained potential energy curves. A two-color laser cooling scheme is established based on the 3Π1 â†’ X3Σ- transition because the highly diagonal Franck-Condon factors make the transition suitable for constructing the pseudo-closed loop transition. The radiative lifetimes, the Doppler temperature, and the recoil temperature are calculated to access the cooling effect of the optical scheme. The results demonstrate that the 3Π1 â†’ X3Σ- transition is much superior to the other transitions and the intervening a1Δ and b1Σ+ will not significantly impact the pseudo-closed loop transition of the laser cooling scheme. The accumulate FCF reach 0.99996 implies that about 25,000 scattering photons are available before leaking, which can cool the NH molecule to the Doppler temperature of 20.2 µK.

First-principles investigation on the thermoelectric performance of half-Heusler compound CuLiX(X = Se, Te).

The remarkable thermoelectric performance is predicted for half-Heusler (HH) compounds of CuLiX (X = Se, Te) based on the first-principles calculation, the deformation potential (DP) theory, and semi-classical Boltzmann theory. The Slack model is employed to evaluate the lattice thermal conductivity and the result is in good agreement with the previously reported data. The results of mechanical properties demonstrate that CuLiSe is ductile but CuLiTe is brittle. The relaxation time and the carrier mobility are calculated with DP theory. The electrical and thermal conductivities are obtained by using the semi-classical Boltzmann theory based on the relaxation approximation. The Seebeck coefficient and power factor are obtained and their characters are analyzed. The dimensionless figure of merits (ZT) is obtained for the p- and n-type CuLiX. The maximum ZT of 2.65 can be achieved for n-type CuLiTe at the carrier concentration of 3.19 × 1019 cm-3 and 900 K, which indicates that this compound is a very promising candidate for the highly efficient thermoelectric materials.

The shape selectivity of corannulene dimers based on concave-convex and convex-convex shape complementarity as hosts for C60 and C70.

In the formation of noncovalent complexes, the stacking arrangements of corannulene and fullerene are diverse, most of which are combinations of multiple corannulenes and fullerene. Here, a composition ratio of 2 : 1 was selected for the complex between corannulene and fullerene (C60 and C70) to investigate the effects of different superposition modes, including concave-convex and convex-convex interactions, on the stability and third-order nonlinear optical (NLO) properties of the composite materials. It was found that the concave-convex interaction was stronger and it was reported to stabilize the charge-transfer (CT) complex more effectively than the convex-convex interaction. The dispersion range of the concave-convex interaction was larger than that of the convex-convex interaction, which is consistent with the interaction energy results. The packing design with the double convex-convex interactions exhibited the largest linear optical response and third-order NLO response, which showed that the convex-convex interaction was more likely to be excited and cause intermolecular CT as compared to the concave-convex interaction. This work confirmed that the packing arrangement significantly affected the NLO response and will advance the development of NLO crystals.

Photocatalytic water splitting for hydrogen generation driven by tetragonal, trigonal, hexagonal and cubic LiCoO2 and visible light.

The photocatalytic properties of LiCoO2 are not explored up to date although its cubic and trigonal structures are explored experimentally. Here, we investigate the feasibility of photocatalytic hydrogen production from water splitting driven by the tetragonal, trigonal, hexagonal and cubic LiCoO2 with the irradiation of the visible light. The band structure, density of state, optical absorption and mobility are calculated by the first-principles density functional theory. The results show that the band edges of all the four structures of LiCoO2 match to the conditions of the redox potentials of water splitting reaction and the enhanced optical absorption in the visible light range is observed. The obvious difference between the mobilities of the hole and electron are identified, especially for the cubic LiCoO2. All the obtained results suggest that the considered structures of LiCoO2 are promising candidates for the photocatalytic water splitting to produce hydrogen with the irradiation of the visible light.

High thermoelectric figure of merit and thermopower of HfTe5 at room temperature.

We predict a high thermoelectric efficiency of HfTe5, based on the first-principles calculations of the electronic structure and thermal conductivity, and the transport coefficients obtained by using the semi-classical Boltzmann transport theory in a wide temperature and carrier concentration range. The lattice thermal conductivity is calculated based on the Slack model and the result is in good agreement with the experimental value. The results of all the thermoelectric transport coefficients demonstrate anisotropic characteristics with the obvious small values along with the b direction. The figure of merit ZT computed with a temperature-dependent relaxation time can reach 2.68 along with the c direction of the n-type HfTe5 at 300 K and an optimal carrier concentration of 5.80 × 1019 cm-3. The Seebeck thermopower coefficients are between 100 and 300 µV K-1 at the optimal carrier concentration, but can reach nearly 1000 µV K-1 at low concentration. Therefore, HfTe5 could achieve high thermoelectric performance at room temperature by controlling the transport direction and carrier concentration.

ZnCdO2 monolayer - A complex 2D structure of ZnO and CdO monolayers for photocatalytic water splitting driven by visible-light.

ZnO monolayer possesses band structure matching the conditions of water splitting for hydrogen generation but cannot well response to the visible light, while CdO one, contrariwise, have obvious optical absorption in the visible light range but no satisfactory band edges for the water splitting to produce hydrogen. Here, we predict a two-dimensional ZnCdO2 structure comprising of ZnO and CdO ones to achieve their strengths. The band structures, optical properties, carrier mobility, and the strain engineering for ZnCdO2, ZnO and CdO monolayers are investigated by using the first-principles hybridization functional calculations. The results demonstrate that the two-dimensional ZnCdO2 structure is a promising candidate for water splitting to produce hydrogen. All the structures show a direct band energy gap and the character remains unchanged under the considered biaxial strains. All the conduction band minimums are suitable for water splitting reaction even under the -4% to +4% strain. Moreover, the valence band maximum of ZnCdO2 monolayer matches the conditions of the water-splitting reaction under the -2% to +4% strain. Interestingly, the unsatisfactory valence band maximum of CdO monolayer can be overcome by strain larger than +2%. As expected, the enhanced optical absorption in the visible light range is observed for the ZnCdO2 monolayer. Additionally, the mobilities of the hole and the electron are significantly different for the three monolayers, implying that the low recombination ratio of the photogenerated carrier pairs is available, which is also beneficial for the photocatalytic performance. Therefore, ZnCdO2 monolayer and CdO monolayer (with tensile strain larger than 2%) is a promising candidate for the water splitting to produce hydrogen under the irradiation of the solar light.

Remarkably High Thermoelectric Efficiencies of the Half-Heusler Compounds BXGa (X = Be, Mg, and Ca).

The thermoelectric materials with high values of the dimensionless figure of merit (ZT) are among the most important new energy resources. Too much attention has been paid to the search of high-ZT thermoelectric materials, and the one with ZT = 5 has been reported recently. Here, a remarkably high ZT = 7.38 is predicted for the n-type half-Heusler compound of BCaGa at 700 K. To understand the high-ZT behavior, we studied electronic properties of BXGa (X = Be, Mg, and Ca) with first-principles calculations based on the density functional theory. The stabilities of the structures of BXGa (X = Be, Mg, and Ca) are confirmed by phonon dispersion. The transport properties are determined by the semiclassical Boltzmann transport theory. We evaluate the relaxation time by using the deformation potential theory and the lattice thermal conductivity based on the elastic coefficients. The results demonstrate that such a high efficiency of BCaGa arises from the intrinsic coordination of the ultralow lattice and electronic thermal conductivity and the larger power factor at certain carrier concentration and temperature. The high n-type power factor originates from the large relaxation time, which results in a light, twofold degenerate conduction-band pocket at the Γ point. In contrast, the power factors of BBeGa and BMgGa are smaller because of their flat-and-dispersive valence band. It is expected that the remarkable results for BXGa could encourage more experimental and theoretical investigations to develop efficient thermoelectric materials with BXGa.

2D AlP3 with high carrier mobility and tunable band structure.

The exploration of new monolayer materials always attracts much attention due to the extraordinary properties and promising applications. Here we predict two monolayered aluminum triphosphides (AlP3) with C2/m and P3m1 space groups with a tunable bandgap under strain as the new members of the 2D XP3 family by using the first principles calculations. The stabilities of the predicted structures are confirmed with the phonon dispersion curves and molecular dynamics. Unlike the narrow bandgaps of the reported XP3 monolayers, the larger bandgaps of 1.78 (HSE06) or 1.91 eV (G0W0) for C2/m and 1.42 (HSE06) or 2.14 eV (G0W0) for P3m1 AlP3 monolayers are observed. The high mobility of 1.01 × 105 and 1.62 × 104 cm2 V-1 s-1 are observed for the electron of P3m1 and the hole of C2/m. The optical absorptions of the AlP3 monolayers, in particular, the one with C2/m, are obviously strong in the visible light range. These results imply that the monolayers are promising in the optoelectronic application. Unfortunately, the undesirable band edges make them not suitable for water splitting in spite of the strong optical absorption coefficient in the visible light range. However, an obvious effect of strain engineering is demonstrated for the monolayers. Under -2% and -3% biaxial strain, the band edges of P3m1 AlP3 can straddle the redox potential of water and meet the requirement of photocatalytic water splitting. Therefore, the P3m1 AlP3 monolayer can also be a promising candidate for the photocatalytic water splitting to produce hydrogen driven by the visible light.