Two-dimensional penta-Sn3H2 monolayer for nanoelectronics and photocatalytic water splitting: a first-principles study

Exploring two-dimensional materials with novel properties is becoming particularly important due to their potential applications in future electronics and optoelectronics. In the current work, the electronic and optical properties of penta-Sn₃H₂ are investigated by density-functional theory. By assessing the phonon spectrum, we find that penta-Sn₃H₂ monolayer is energetically more favorable compared with pristine penta-stanene due to hydrogenation transforming the sp²–sp³ hybrid orbitals into sp³ hybridization. Our calculations revealed that penta-Sn₃H₂ is a semiconductor with indirect band gaps of 1.48 eV according to the GGA functional (2.44 eV according to the HSE06 functional). Moreover, the electronic structures of penta-Sn₃H₂ can be effectively modulated by biaxial tensile strain. Meanwhile, our calculations reveal that the indirect to direct band gap transition can be achieved in this monolayer sheet by >4% biaxial strain. On the other hand, the well-located band edge and visible light absorption make penta-Sn₃H₂ a potentially promising optoelectronic material for photocatalytic water splitting.

Exploring two-dimensional materials with novel properties is becoming particularly important due to their potential applications in future electronics and optoelectronics.     

Compressive behavior and electronic properties of ammonia ice: a first-principles study

Understanding the compressive behavior of ammonia ice is an enduring topic due to its salient implications in planetology and the origin of life as well as its applications in agriculture and industry. Currently, the most stable crystal structures of ammonia ice with increasing pressure have been determined to be P2₁3, P2₁2₁2₁, Pma2, Pca2₁, P2₁/m and Pnma, respectively. Taking these six crystal structures for consideration, the pressure-induced structural and electronic behavior of ammonia ice was systematically investigated using density functional theory calculations. According to our calculations, the transition from molecular phase P2₁2₁2₁ to ionic phase Pma2 can be ascribed to the bonds between H atoms and N atoms on adjacent NH₃ molecules. Analysis of the Mulliken population and electron density of states implies decreased charge transfer between the N and H atoms and enhanced bonds with increasing pressure. In addition, charge overlap between NH₃ molecules was found at high pressure in the molecular phases of ammonia ice, which is also observed between NH₂⁻ and NH₄⁺ groups in ionic phases. With increasing pressure, the band gap of ammonia ice increases rapidly and then decreases gradually, which is a consequence of the subtle competition between the strong coupling in the H 1s and N 2p states and the charge overlap. These simulations help us understand the characteristics of ammonia ice under high pressure and further provide valuable insights into the evolution of planets.

We performed systematic ab initio calculations to explore the structures and electronic properties of ammonia ice by hydrostatic compression.     

The mechanical,electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si,Ge, Sn) monolayers: a first-principles calculations

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure. The structural, mechanical, electronic, thermoelectric efficiency, and optical properties of XSb monolayers are studied. Ab initio molecular dynamic simulations and phonon dispersion calculations suggests their good thermal and dynamical stabilities. The mechanical properties of XSb monolayers shows that the monolayers are considerably softer than graphene, and their in-plane stiffness decreases from SiSb to SnSb. Our results shows that the single layers of SiSb, GeSb and SnSb are semiconductor with band gap of 1.48, 0.77 and 0.73 eV, respectively. The optical analysis illustrate that the first absorption peaks of the SiSb, GeSb and SnSb monolayers along the in-plane polarization are located in visible range of light which may serve as a promising candidate to design advanced optoelectronic devices. Thermoelectric properties of the XSb monolayers, including Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor and figure of merit are calculated as a function of doping level at temperatures of 300 K and 800 K. Between the studied two-dimensional materials (2DM), SiSb single layer may be the most promising candidate for application in the thermoelectric generators.

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure.     

A first-principles study of electronic and optical properties of the tetragonal phase of monolayer ZnS modulated by biaxial strain

Modulation of the electronic and optical properties of two-dimensional (2D) materials is of great significance for their practical applications. Here, by using first-principles calculations, we study a tetragonal phase of monolayer ZnS, and explore its associated electronic and optical properties under biaxial strain. The results from phonon dispersion and molecular dynamics simulation demonstrate that the tetragonal phase of monolayer ZnS possesses a very high stability. The monolayer ZnS has a direct band gap of 4.20 eV. It changes to an indirect band gap under both compression and tension, exhibiting a decrease in band gap. However, the band gap decreases more slowly under compression compared to the tension process such that the direct band gap remains within −8%, demonstrating excellent endurance under pressure. Fortunately, tetragonal ZnS exhibits a good absorption ability in the ultraviolet (UV) range regardless of strain. Our research results enrich the understanding of monolayer ZnS, which is helpful for the design and application of optoelectronic devices using the material.

The evolution of electronic property for monolayer tetragonal ZnS under biaxial strain.     

Correction: The mechanical,electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si,Ge, Sn) monolayers: a first-principles calculations

Correction for ‘The mechanical, electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si, Ge, Sn) monolayers: a first-principles calculations’ by Asadollah Bafekry et al., RSC Adv., 2020, 10, 30398–30405, DOI: 10.1039/D0RA05587E.

The authors regret that the name of one of the authors (Fazel Shojaei) was shown incorrectly in the original article. The corrected author list is as shown above.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

A first-principles study of strain tuned optical properties in monolayer tellurium

First-principles calculations are employed to study the optical properties of monolayer Te tuned by biaxial strain. Our results demonstrate that monolayer Te has strong absorption in the visible and ultraviolet regions, and that a structural transition occurs between the α-phase and the β-phase under certain strain. In addition, there is significant optical anisotropy in α- and β-Te, while γ-Te shows isotropic characteristics due to their different structural properties. Furthermore, strain has a significant impact on the optical properties. With increasing strain, the real and imaginary parts of the dielectric function exhibit redshift. In addition, the absorption spectrum is more likely to be excited under compressive strain rather than tensile strain in α- and β-Te, while only slight differences are induced in γ-Te. These findings can not only enhance the understanding of two-dimensional tellurium, but also provide an effective way to tune the optical properties for potential application in optoelectronic devices.

We have theoretically investigated the tunable optical properties of monolayer tellurium under biaxial strain and materials with better performance can be obtained with the application of strain.     

Anisotropic optical properties induced by uniaxial strain of monolayer C3N: a first-principles study

The optical properties, structural properties and electronic properties of a new two-dimensional (2D) monolayer C₃N under different strains are studied in this paper by using first-principles calculations. The applied strain includes in-layer biaxial strain and uniaxial strain. The monolayer C₃N is composed of a number of hexagonal C rings with N atoms connecting them. It is a stable indirect band gap 2D semiconductor when the strain is 0%. It could maintain indirect semiconductive character under different biaxial and uniaxial strains from ε = −10% to ε = 10%. As for its optical properties, when the uniaxial strain is applied, the absorption and reflectivity along the armchair and zigzag directions exhibit an anisotropic property. However, an isotropic property is presented when the biaxial strain is applied. Most importantly, both uniaxial tensile strain and biaxial tensile strain could cause the high absorption coefficient of monolayer C₃N to be in the deep ultraviolet region. This study implies that strain engineering is an effective approach to alter the electronic and optical properties of monolayer C₃N. We suggest that monolayer C₃N could be suitable for applications in optoelectronics and nanoelectronics.

The optical properties, structural properties and electronic properties of a new two-dimensional (2D) monolayer C₃N under different strains are studied in this paper by using first-principles calculations.     

Sc/C codoping effect on the electronic and optical properties of the TiO2 (101) surface: a first-principles study

Extension of the light absorption range and a reduction of the possibility of the photo-generated electron–hole pair recombination are the main tasks to break the bottleneck of the photocatalytic application of TiO₂. In this paper, we systematically investigate the electronic and optical properties of Sc-doped, C-doped, and Sc/C-codoped TiO₂ (101) surfaces using spin-polarized DFT+U calculations. The absorption coefficient of the Sc/C-codoped TiO₂ (101) surfaces were enhanced the most compared with the other two doped systems in the high energy region of visible light, which can be attributed to the shallow impurity states. Furthermore, we studied the optical absorption properties with the change of the impurity concentration. The Sc/C-codoped TiO₂ (101) surface with 5.56% impurity concentration exhibited optimal photocatalytic performance in the visible region. These results may be helpful for designing the high-performance of the photocatalysts by doping.

The Sc/C-codoped TiO₂ (101) surface with 5.56% impurity concentration exhibited optimal photocatalytic performance in the visible region, which may be helpful for designing the high-performance of photocatalysts by doping.     

Adsorption of gas molecules on buckled GaAs monolayer: a first-principles study

The design of sensitive and selective gas sensors can be significantly simplified if materials that are intrinsically selective to target gas molecules can be identified. In recent years, monolayers consisting of group III–V elements have been identified as promising gas sensing materials. In this article, we investigate gas adsorption properties of buckled GaAs monolayer using first-principles calculations within the framework of density functional theory. We examine the adsorption energy, adsorption distance, charge transfer, and electron density difference to study the strength and nature of adsorption. We calculate the change in band structure, work function, conductivity, density of states, and optical reflectivity for analyzing its prospect as work function-based, chemiresistive, optical, and magnetic gas sensor applications. In this regard, we considered the adsorption of ten gas molecules, namely NH₃, NO₂, NO, CH₄, H₂, CO, SO₂, HCN, H₂S, and CO₂, and noticed that GaAs monolayer is responsive to NO, NO₂, NH₃, and SO₂ only. Specifically, NH₃, SO₂ and NO₂ chemisorb on the GaAs monolayer and change the work function by more than 5%. While both NO and NO₂ are found to be responsive in the far-infrared (FIR) range, NO shows better spin-splitting property and a significant change in conductivity. Moreover, the recovery time at room temperature for NO is observed to be in the sub-millisecond range suggesting selective and sensitive NO response in GaAs monolayer.

NH₃, NO₂, and SO₂ chemisorb on the GaAs monolayer. NO adsorption induces a magnetic moment (1.02 μ_B per cell), and significantly changes the conductivity and reflectivity.     

High stability and visible-light photocatalysis in novel two-dimensional monolayer silicon and germanium mononitride semiconductors: first-principles study

Recently, two-dimensional semiconductor materials with moderate band gaps and significant light absorption have been highly sought for application in photocatalysis and nanoelectronics. In this study, novel monolayer SiN and GeN have been predicted by using first-principles calculations. They have excellent thermal and dynamic stabilities and present indirect band gaps of 2.58 eV and 2.21 eV with anisotropic carrier mobility, respectively. Suitable band gaps and band edges of SiN and GeN indicate that they can simultaneously produce both hydrogen and oxygen in the pH range of 6 to 14 and 0 to 10, respectively. Theoretical studies on strain engineering show that their band gaps could be effectively tuned by both biaxial tensile and compressive strain. Our work enriches the family of two-dimensional semiconductor materials and shows that monolayer SiN and GeN are promising candidates for electronic devices and photocatalysis.

The two-dimensional SiN and GeN semiconductors are expected to become novel photocatalysts for water-splitting.     

NO reduction over an Al-embedded MoS2 monolayer: a first-principles study

Converting toxic air pollutants such as nitric oxide (NO) and carbon monoxide (CO) into less harmful gases remains a critical challenge for many industrial technologies. Here, by performing first-principles calculations, we introduce a cheap, stable and novel catalyst for the conversion of NO and CO molecules into N₂O and CO₂ using Al-doped MoS₂ (Al–MoS₂). According to our results, dissociation of NO molecules on Al–MoS₂ has a large energy barrier (3.62 eV), suggesting that it is impossible at ambient temperature. In contrast, the coadsorption of NO molecules to form (NO)₂ moieties is characterized as the first step of the NO reduction process. The formed (NO)₂ is unstable on Al–MoS₂, and hence it is easily decomposed into N₂O molecules, and an oxygen atom is adsorbed onto the Al atom (O_ads). This reaction step is exothermic and needs an activation energy of 0.37 eV to be overcome. Next, the O_ads moiety is removed from the Al atom by a CO molecule, and thereby the Al–MoS₂ catalyst is recovered for the next round of reaction. The side reaction producing NO₂via the reaction of NO with the O_ads moiety cannot proceed on Al–MoS₂ due to its large activation energy.

By performing first-principles calculations, we introduce a stable and novel catalyst for the conversion of NO and CO molecules into N₂O and CO₂ using Al-doped MoS₂.     

Electronic and optical properties of silicene on GaAs(111) with hydrogen intercalation: a first-principles study

In this paper, we investigated the electronic and optical properties of silicene on GaAs(111) substrates (silicene/HGaAs) on the basis of first-principles density functional theory. The hydrogen intercalation introduced substantially weakened the interaction between silicene and the GaAs(111) substrate and induced considerable bandgaps in silicene/HGaAs heterostructures. The effects of the interlayer spacing (L) between silicene and the substrate, silicene buckling height (h), biaxial strain (ε), and external electric field (F) on the electronic properties were also considered. Our results showed that the electronic properties of silicene/HGaAs heterostructures could be controlled by adjusting L and h and applying ε and an external F. Silicene/HGaAs heterostructures possessed the typical optical absorption properties of freestanding silicene and had high absorption coefficients. Besides, some strong peaks of absorption spectra and energy loss spectra existed in the ultraviolet light region, which showed that silicene/HGaAs heterostructures had evident enhancement in the ultraviolet light region. Results laid a theoretical foundation for the study of the electronic and optical properties and applications of silicene on semiconductor substrate devices.

In this paper, we investigated the electronic and optical properties of silicene on GaAs(111) substrates (silicene/HGaAs) on the basis of first-principles density functional theory.     

Chemical functionalization of the ZnO monolayer: structural and electronic properties

Two-dimensional zinc oxide (ZnO) materials have been extensively investigated both experimentally and theoretically due to their novel properties and promising applications in optoelectronic and spintronic devices; however, how to tune the electronic property of the ZnO monolayer is still a challenge. Herein, employing the first-principles calculations, we explored the effect of chemical functionalization on the structural and electronic properties of the ZnO monolayer. The results demonstrated that the hydrogenated-, fluorinated- or Janus-functionalized ZnO monolayers were thermodynamically and mechanically stable except for the fully hydrogenated ZnO monolayer. The band gap of the ZnO monolayer could be effectively modulated by hydrogenation or fluorination, which varied from 0 to 2.948 eV, as obtained by the PBE functional, and from 0 to 5.114 eV, as obtained by the HSE06 functional. In addition, a nonmagnetic metal → nonmagnetic semiconductor transition was achieved after hydrogenation, whereas a transition from a magnetic half-metal to nonmagnetic semiconductor occurred after fluorination of the ZnO monolayer. These results demonstrate that tunability of the electronic properties of the ZnO monolayer can be realized by chemical functionalization for future nanoelectronic device applications.

After hydrogenation or fluorination, the band gap of the ZnO monolayer can be effectively modulated, and a nonmagnetic metal or magnetic half-metal → non-magnetic semiconductor transition can be achieved.     

The effect of oxidation on the electronic properties of penta-graphene: first-principles calculation

Herein, using first-principles calculations, we systematically studied the effect of oxidation on the structural and electronic properties of penta-graphene. We have found that the oxygen atom prefers to adsorb at the center of the C C bond, and the interaction between the oxygen atom and penta-graphene is a strong chemical bond. When the oxygen coverage increases, the band gap of penta-graphene gradually widens due to the rigid up-shift of the conduction band. More importantly, we found that the oxygen molecule on the penta-graphene surface could self-decompose into oxygen atoms without any metal catalyst. Our calculated results show that penta-graphene would be chemically unstable when it is exposed to air. Therefore, from the application point of view, penta-graphene-based devices must be encapsulated or functionalized before exposure to air. Oxidized penta-graphene exhibits a large band gap, which can facilitate its application as dielectric layers in electronic devices.

Herein, using first-principles calculations, we systematically studied the effect of oxidation on the structural and electronic properties of penta-graphene.     

Large bandgap quantum spin Hall insulator in methyl decorated plumbene monolayer: a first-principles study

Topologically protected edge states of 2D quantum spin Hall (QSH) insulators have paved the way for dissipationless transport. In this regard, one of the key challenges is to find suitable QSH insulators with large bandgaps. Group IV analogues of graphene such as silicene, germanene, stanene, plumbene etc. are promising materials for QSH insulators. This is because their high spin–orbit coupling (SOC) and large bandgap opening may be possible by chemically decorating these group IV graphene analogues. However, finding suitable chemical groups for such decoration has always been a demanding task. In this work, we investigate the performance of a plumbene monolayer with –CX₃ (X = H, F, Cl) chemical decoration and report very large bandgaps in the range of 0.8414 eV to 0.9818 eV with spin–orbit coupling, which is much higher compared to most other topological insulators and realizable at room temperature. The topological invariants of the samples are calculated to confirm their topologically nontrivial properties. The existence of edge states and topological nontrivial property are illustrated by investigating PbCX₃ nanoribbons with zigzag edges. Lastly, the structural and electronic stability of the topological materials against strain are demonstrated to extend the scope of using these materials. Our findings suggest that these derivatives are promising materials for spintronic and future high performance nanoelectronic devices.

Formulating methyl and trihalogenomethyl decorated plumbene monolayers as quantum spin Hall insulators for application in spintronic and dissipationless transport.     

Outstanding elastic,electronic, transport and optical properties of a novel layered material C4F2: first-principles study

Motivated by very recent successful experimental transformation of AB-stacking bilayer graphene into fluorinated single-layer diamond (namely fluorinated diamane C₄F₂) [P. V. Bakharev, M. Huang, M. Saxena, S. W. Lee, S. H. Joo, S. O. Park, J. Dong, D. C. Camacho-Mojica, S. Jin, Y. Kwon, M. Biswal, F. Ding, S. K. Kwak, Z. Lee and R. S. Ruoff, Nat. Nanotechnol., 2020, 15, 59–66], we systematically investigate the structural, elastic, electronic, transport, and optical properties of fluorinated diamane C₄F₂ by using density functional theory. Our obtained results demonstrate that at the ground state, the lattice constant of C₄F₂ is 2.56 Å with chemical bonding between the C–C interlayer and intralayer bond lengths of about 1.5 Å which are close to the C–C bonding in the bulk diamond. Based on calculations for the phonon spectrum and ab initio molecular dynamics simulations, the structure of C₄F₂ is confirmed to be dynamically and thermally stable. C₄F₂ exhibits superior mechanical properties with a very high Young''s modulus of 493.19 N m⁻¹. Upon fluorination, the formation of C–C bonding between graphene layers has resulted in a comprehensive alteration of electronic properties of C₄F₂. C₄F₂ is a direct semiconductor with a large band gap and phase transitions are found when a biaxial strain or external electric field is applied. Interestingly, C₄F₂ has very high electron mobility, up to 3.03 × 10³ cm² V⁻¹ s⁻¹, much higher than other semiconductor compounds. Our findings not only provide a comprehensive insight into the physical properties of C₄F₂ but also open up its applicability in nanoelectromechanical and optoelectronic devices.

Motivated by transformation of AB-stacking bilayer graphene into fluorinated single-layer diamond (fluorinated diamane C₄F₂), we investigate the structural, elastic, electronic, transport, and optical properties of fluorinated diamane C₄F₂ using density functional theory.     

The geometrical structure and electronic properties of trivalent Ho3+ doped Y2O3 crystals: a first-principles study

Trivalent rare-earth holmium ion (Ho³⁺) doped yttrium oxide (Y₂O₃) has attracted great research interest owing to its unique optoelectronic properties and excellent performances in many new-type laser devices. But the crystal structures of the Ho³⁺-doped Y₂O₃ system (Y₂O₃ : Ho) are still unclear. Here, we have carried out a first-principle study on the structural evolution of the trivalent Ho³⁺ doped Y₂O₃ by using the CALYPSO structure search method. The results indicate that the lowest-energy structure of Ho³⁺-doped Y₂O₃ possesses a standardized monoclinic P2 phase. It is found that the doped Ho³⁺ ion are likely to occupy the sites of Y³⁺ in the host crystal lattice, forming the [HoO₆]⁹⁻ local structure with C₂ site symmetry. Electronic structure calculations reveal that the band gap value of Ho³⁺-doped Y₂O₃ is approximately 4.27 eV, suggesting the insulating character of Y₂O₃ : Ho system. These findings could provide fundamental insights to understand the atomic interactions in crystals as well as the information of electronic properties for other rare-earth-doped materials.

Our study successfully identified the ground-state structure of Ho³⁺-doped Y₂O₃ crystal for the first time.     

Sequential BN-doping induced tuning of electronic properties in zigzag-edged graphene nanoribbons: a computational approach

We employed first-principles methods to elaborate doping induced electronic and magnetic perturbations in one-dimensional zigzag graphene nanoribbon (ZGNR) superlattices. Consequently, the incorporation of alternate boron and nitrogen (hole–electron) centers into the hexagonal network instituted substantial modulations to electronic and magnetic properties of ZGNR. Our theoretical analysis manifested some controlled changes to electronic and magnetic properties of the ZGNR by tuning the positions (array) of impurity centers in the carbon network. Subsequent DFT based calculations also suggested that the site-specific alternate electron–hole (B/N) doping could regulate the band-gaps of the superlattices within a broad range of energy. The consequence of variation in the width of ZGNR in the electronic environment of the system was also tested. The systematic analysis of various parameters such as the structural orientations, spin-arrangements, the density of states (DOS), band structures, and local density of states envisioned a basis for the band-gap engineering in ZGNR and attributed to its feasible applications in next generation electronic device fabrication.

Incorporation of an alternate impurity array in the ZGNR don''t break the spin-degeneracy, providing the freedom to tune the electronic behaviors without affecting the spin-dependent properties.     

Structural,electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation

Hybrid organic–inorganic perovskites have been one of the most active areas of research into photovoltaic materials. Despite the extremely fast progress in this field, the electronic properties of formamidinium lead iodide perovskite (FAPbI₃) that are key to its photovoltaic performance are relatively poorly understood when compared to those of methylammonium lead iodide (MAPbI₃). In this study, first-principles total energy calculations based on density functional theory were used to investigate the favored orientation of FA. Different theoretical methods, with or without incorporation of spin-orbit coupling (SOC) effects, were used to study the structure, electronic properties, and charge-carrier effective mass. Also the SOC-induced Rashba k-dependent band splitting, density of states and optical properties are presented and discussed. These results are useful for understanding organic–inorganic lead trihalide perovskites and can inform the search for new materials and design rules.

Different theoretical methods, including SOC effects, were used to study the detailed structure, electronic properties, charge-carrier mobility, and SOC-induced Rashba k-dependent band splitting in FAPbI₃.     

Insight into the electronic and thermodynamic properties of NbSi2 from first-principles calculations

The electronic and thermodynamic properties of NbSi₂ with four structures (C40, C11_b, C54 and C49) were studied in terms of first-principle calculations. The band structure and density of states of four NbSi₂ are calculated. Those disilicides show electronic properties because the band gap between the conduction band and the valence band is near the Fermi level. Their metallic character is mainly due to the strong metallic interaction between the Nb-4d state and Si-3p state. The formed Si–Si covalent bond is mainly concentrated in the valence band. The valence electron configuration of Nb–Si and Si–Si bonds is also explored. Besides, we study the thermodynamic properties of NbSi₂ as a function of temperature. The results indicate that the C54 structure has the best thermal stability in the obtained phases. In particular, the Debye temperature and heat capacity of the C54 structure are 547.8 K and 142.7 J mol⁻¹ K⁻¹, respectively. The calculated phonon DOS provides the explanation that the nature of the thermodynamic properties is mainly derived from the vibration of Si atoms.

The electronic and thermodynamic properties of NbSi₂ with four structures (C40, C11_b, C54 and C49) were studied in terms of first-principle calculations.