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Two-dimensional materials and their combined heterostructures have paved the way for numerous next-generation electronic and optoelectronic applications. Herein, we performed first principles calculations to computationally design the MoSe 2 /WS 2 heterostructure and consider its geometric structure, electronic properties and contact behavior, as well as the effects of the electric fields and strain. Our results show that the MoSe 2 /WS 2 heterostructure is energetically, thermodynamically and mechanically stable. Depending on the stacking configurations, the MoSe 2 /WS 2 heterostructure could form type-I or type-II band alignment. The versatility in contact behavior makes the MoSe 2 /WS 2 heterostructure attractive in electronics and optoelectronics. The combination of the MoSe 2 /WS 2 heterostructure also leads to an enhancement in the adsorption efficiency and the carrier mobility compared with the constituent components. More interestingly, our findings demonstrate that the electric field can induce the transition between type-I and type-II band alignments, as evident by the experimental measurement [J. Kistner-Morris, A. Shi, E. Liu, T. Arp, F. Farahmand, T. Taniguchi, K. Watanabe, V. Aji, C. H. Lui and N. Gabor, Nat. Commun. , 2024, 15 , 4075]. Additionally, we also find that strain can also induce the transition between type-I and type-II band alignments and lead to the transition from semiconductor to metal in the MoSe 2 /WS 2 heterostructure. Our findings prove that the MoSe 2 /WS 2 heterostructure holds significant potential for developing next-generation electronics.

In the present work, we propose GaGeX 2 (X = N, P, As) monolayers and explore their structural, vibrational, piezoelectric, electronic, and transport characteristics for multifunctional applications based on first-principles simulations. Our analyses of cohesive energy, phonon dispersion spectra, and ab initio molecular dynamics simulations indicate that the three proposed structures have good energetic, dynamic, and thermodynamic stabilities. The GaGeX 2 are found as piezoelectric materials with high piezoelectric coefficient d of -1.23 pm V 11 of -1.23 pm V -1 for the GaGeAs 2 monolayer. Furthermore, the results from electronic band structures show that the GaGeX 2 have semiconductor behaviours with moderate bandgap energies. At the Heyd-Scuseria-Ernzerhof level, the GaGeP 2 and GaGeAs 2 exhibit optimal bandgaps for photovoltaic applications of 1.75 and 1.15 eV, respectively. Moreover, to examine the transport features of the GaGeX 2 monolayers, we calculate their carrier mobility. All three investigated GaGeX 2 systems have anisotropic carrier mobility in the two in-plane directions for both electrons and holes. Among them, the GaGeAs 2 V 2 V -1 s -1 in the x and y directions, respectively. With high electron mobility, large piezoelectric coefficient, and moderate bandgap energy, the GaGeAs 2 material holds potential applicability for electronic, optoelectronic, piezoelectric, and photovoltaic applications. Thus, our findings not only predict stable GaGeX 2 structures but also provide promising materials to apply for multifunctional devices., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Recently, searching for a metal-semiconductor junction (MSJ) that exhibits low-contact resistance has received tremendous consideration, as they are essential components in next-generation field-effect transistors. In this work, we design a MSJ by integrating two-dimensional (2D) graphene as the metallic electrode and 2D Janus γ-Ge 2 SSe as the semiconducting channel using first-principles simulations. All the graphene/γ-Ge 2 SSe MSJs are predicted to be energetically, mechanically, and thermodynamically stable, characterized by the weak van der Waals (vdW) interactions. The graphene/γ-SGe 2 Se MSJ-vdWH form the n -type Schottky contact (SC), while the graphene/γ-SeGe 2 S MSJ-vdWH form the p -type one, suggesting that the switching between p -type and n -type SC in the graphene/γ-Ge 2 SSe MSJ-vdWHs can occur spontaneously by simply altering the stacking patterns, without requiring any external conditions. Notably, the contact features, including contact types and barriers of the graphene/γ-Ge 2 SSe MSJs are significant in versatility and can be altered by applying electric gating and adjusting interlayer spacing. Both the applied electric gating and strain engineering induce switchability between p - and n -type and SC to OC in the graphene/γ-Ge 2 SSE MSJs. This versatility underscores the potential of the graphene/γ-Ge 2 SSe MSJ for next-generation applications that require low-contact resistance and high performance.

First principles calculation was performed to study the Sn 2 Te 2 X 4 (X = P, As) monolayers. Structural investigation confirms the stability of the two monolayers with Young's modulus in the range of 30.34-33.65 N m -1 and a Poisson's ratio of 0.18-0.21. The two monolayers are semiconductors with a direct band gap of 1.52-1.66 eV. The light absorption rate of the two monolayers is rather high 10 4 -10 5 cm -1 . Both monolayers have high charge carrier mobility and suitable VBM and CBM positions for the redox reaction. The η STH efficiency of both materials (15.76-17.12%) is close to the theoretical limit of 18%. Moreover, moderate strains can improve the light absorption rate, while the suitable VBM and CBM positions are preserved. These characteristics suggest that Sn 2 Te 2 X 4 (X = P, As) monolayers are good candidates for being applied in flexible devices and for the conversion of solar energy to other types of energy., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

In this article, an attempt is made to explore new materials for applications in piezoelectric and electronic devices. Based on density functional theory calculation, we construct three Janus ZrGeZ 3 H (Z = N, P, and As) monolayers and study their stability, piezoelectricity, Raman response, and carrier mobility. The results from phonon dispersion spectra, ab initio molecular dynamics simulation, and elastic coefficients confirm the structural, thermal, and mechanical stability of these proposed structures. The ZrGeZ 3 H monolayers are indirect band gap semiconductors with favourable band gap energy of 1.15 and 1.00 eV for the ZrGeP 3 H and ZrGeAs 3 H, respectively, from Heyd-Scuseria-Ernzerhof functional method. It is found that the Janus ZrGeZ 3 H monolayers possess both in-plane and out-of-plane piezoelectric coefficients, revealing that they are potential piezoelectric candidates. In addition, the carrier mobilities of electrons and holes along transport directions are anisotropic. Notably, the ZrGeP 3 H and ZrGeAs 3 H monolayers have high electron mobility of 3639.20 and 3408.37 cm 2 V -1 s -1 , respectively. Our findings suggest the potential application of the Janus ZrGeZ 3 H monolayers in the piezoelectric and electronic fields., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Breaking structural symmetry in two-dimensional layered Janus materials can result in enhanced new phenomena and create additional degrees of piezoelectric responses. In this study, we theoretically design a series of Janus monolayers HfGeZ 3 H (Z = N, P, As) and investigate their structural characteristics, crystal stability, piezoelectric responses, electronic features, and carrier mobility using first-principles calculations. Phonon dispersion analysis confirms that HfGeZ 3 H monolayers are dynamically stable and their mechanical stability is also confirmed through the Born-Huang criteria. It is demonstrated that while HfGeN 3 H is a semiconductor with a large bandgap of 3.50 eV, HfGeP 3 H and HfGeAs 3 H monolayers have narrower bandgaps being 1.07 and 0.92 eV, respectively. When the spin-orbit coupling is included, large spin-splitting energy is found in the electronic bands of HfGeZ 3 H. Janus HfGeZ 3 H monolayers can be treated as piezoelectric semiconductors with the coexistence of both in-plane and out-of-plane piezoelectric responses. In particular, HfGeZ 3 H monolayers exhibit ultra-high electron mobilities up to 6.40 × 10 3 cm 2 V -1 s -1 (HfGeAs 3 H), indicating that they have potential for various applications in nanoelectronics., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Currently, tremendous efforts have been made to explore efficient glucose oxidation electrocatalysts for enzymeless glucose sensors to meet the urgent demands for accurate and fast detection of glucose in the fields of health care and environmental monitoring. In this work, an advanced nanostructured material based on the well-aligned CuO/Cu 2 S heteronanorods incorporated with P atoms is successfully synthesized on a copper substrate. The as-synthesized material shows high catalytic behavior accompanied by outstanding electrical conductivity. This, combined with the unique morphology of unstacked nanorod arrays, which endow the entire material with a greater number of exposed active sites, make the proposed material act as a highly efficient electrocatalyst for the glucose oxidation reaction. Density functional theory calculations demonstrate that P doping endows P-doped CuO/Cu 2 S with excellent electrical conductivity and glucose adsorption capability, significantly improving its catalytic performance. As a result, a non-enzymatic glucose sensor fabricated based on our proposed material exhibits a broad linear detection range (0.02-8.2 mM) and a low detection limit (0.95 μM) with a high sensitivity of 2.68 mA mM -1 cm -2 and excellent selectivity.

Tl 2 HgGeSe 4 crystal was successfully, for the first time, synthesized by the Bridgman-Stockbarger technology, and its electronic structure and peculiarities of optical constants were investigated using both experimental and theoretical techniques. The present X-ray photoelectron spectroscopy measurements show that the Tl 2 HgGeSe 4 crystal reveals small moisture sensitivity at ambient conditions and that the essential covalent constituent of the chemical bonding characterizes it. The latter suggestion was supported theoretically by ab initio calculations. The present experiments feature that the Tl 2 HgGeSe 4 crystal is a high-resistance semiconductor with a specific electrical conductivity of σ ∼ 10 -8 Ω -1 cm -1 (at 300 K). The crystal is characterized by p-type electroconductivity with an indirect energy band gap of 1.28 eV at room temperature. It was established that a good agreement with the experiments could be obtained when performing first-principles calculations using the modified Becke-Johnson functional as refined by Tran-Blaha with additional involvement in the calculating procedure of the Hubbard amendment parameter U and the impact of spin-orbit coupling (TB-mBJ + U + SO model). Under such a theoretical model, we have determined that the energy band gap of the Tl 2 HgGeSe 4 crystal is equal to 1.114 eV, and this band gap is indirect in nature. The optical constants of Tl 2 HgGeSe 4 are calculated based on the TB-mBJ + U + SO model.

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

The electronic and optical properties of an AgGaGeS 4 crystal were studied by first-principles calculations, where the full-potential augmented plane-wave plus local orbital (APW+lo) method was used together with exchange-correlation pseudopotential described by PBE, PBE+ U , and TB-mBJ+ U approaches. To verify the correctness of the present theoretical calculations, we have measured for the AgGaGeS 4 crystal the XPS valence-band spectrum and the X-ray emission bands representing the energy distribution of the electronic states with the biggest contributions in the valence-band region and compared them on a general energy scale with the theoretical results. Such a comparison indicates that, the calculations within the TB-mBJ+ U approach reproduce the electron-band structure peculiarities (density of states - DOS) of the AgGaGeS 4 crystal which are in fairly good agreement with the experimental data based on measurements of XPS and appropriate X-ray emission spectra. In particular, the DOS of the AgGaGeS 4 crystal is characterized by the existence of well-separated peaks/features in the vicinity of -18.6 eV (Ga-d states) and around -12.5 eV and -7.5 eV, which are mainly composed by hybridized Ge(Ga)-s/p and S-p state. We gained good agreement between the experimental and theoretical data with respect to the main peculiarities of the energy distribution of the electronic S 3p, Ag 4d, Ga 4p and Ge 4p states, the main contributors to the valence band of AgGaGeS 4 . The bottom of the conduction band is mostly donated by unoccupied Ge-s states, with smaller contributions of unoccupied Ga-s, Ag-s and S-p states, too. The AgGaGeS 4 crystal is almost transparent for visible light, but it strongly absorbs ultra-violet light where the significant polarization also occurs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The MXene SnSiGeN 4 monolayer as a new member of the MoSi 2 N 4 family was proposed for the first time, and its structural and electronic properties were explored by applying first-principles calculations with both PBE and hybrid HSE06 approaches. The layered hexagonal honeycomb structure of SnSiGeN 4 was determined to be stable under dynamical effects or at room temperature of 300 K, with a rather high cohesive energy of 7.0 eV. The layered SnSiGeN 4 has a Young's modulus of 365.699 N m -1 and a Poisson's ratio of 0.295. The HSE06 approach predicted an indirect band gap of around 2.4 eV for the layered SnSiGeN 4 . While the major donation from the N-p orbitals to the band structure makes SnSiGeN 4 's band gap close to those of similar 2D MXenes, the smaller distributions from the other orbitals of Sn, Si, and Ge slightly vary this band gap. The work functions of the GeN and SiN surfaces are 6.367 eV and 5.903 eV, respectively. The band gap of the layered SnSiGeN 4 can be easily tuned by strain and an external electric field. A semiconductor-metal transition can occur at certain values of strain, and with an electric field higher than 5 V nm -1 . The electron mobility of the layered SnSiGeN 4 can reach up to 677.4 cm 2 V -1 s -1 , which is much higher than the hole mobility of about 52 cm 2 V -1 s -1 . The mentioned characteristics make the layered SnSiGeN 4 a very promising material for use in electronic and photoelectronic devices, and for solar energy conversion., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Exploring Heusler based materials for different practical applications has drawn more and more attention. In this work, the structural, electronic, magnetic, and mechanical properties of NaTMGe (TM = all 3d transition metals) half-Heusler compounds have been systematically investigated using first-principles calculations. The TM modification plays a determinant role in the fundamental properties. Except NaNiGe and NaCuGe, the studied materials exhibit good dynamical stability. Calculations reveal the non-magnetic semiconductor of NaScGe with a direct energy gap of 1.21 eV. Prospective spintronic applications of NaVGe and NaCrGe-NaMnGe are also suggested by their magnetic semiconductor and half-metallic behavior, respectively, where their magnetic properties follow the Slater-Pauling rule. Nevertheless, the remaining materials are either magnetic or non-magnetic metallic. For the magnetic systems, the magnetism is induced mainly by the TM constituents with either spin-up (V, Cr, Mn, and Fe) or spin-down (Co) 3d states. Calculated elastic constants indicate that all compounds are mechanically stable. Furthermore, they exhibit significant elastic anisotropy, where NaScGe and NaZnGe are the least and most anisotropic materials, respectively. Also, modifying the TM elements influences the materials' ductile and brittle behaviors. Our work unravels clearly the effects of TM modification on the fundamental properties of NaTMGe compounds. NaTMGe materials show excellent versatility with promising properties for optoelectronic and spintronic applications., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (This journal is © The Royal Society of Chemistry.)

Road traffic constitutes a major source of air pollutants in urban Beijing, which are responsible for substantial premature mortality. A series of policies and regulations has led to appreciable traffic emission reductions in recent decades. To shed light on long-term (2014-2020) roadside air pollution and assess the efficacy of traffic control measures and their effects on public health, this study quantitatively evaluated changes in the concentrations of six key air pollutants (PM 2.5 , PM 10 , NO 2 , SO 2 , CO and O 3 ) measured at 5 roadside and 12 urban background monitoring stations in Beijing. We found that the annual mean concentrations of these air pollutants were remarkably reduced by 47%-71% from 2014 to 2020, while the concurrent ozone concentration increased by 17.4%. In addition, we observed reductions in the roadside increments in PM 2.5 , NO 2 , SO 2 and CO of 54.8%, 29.8%, 20.6%, and 59.1%, respectively, indicating the high effectiveness of new vehicle standard (China V and VI) implementation in Beijing. The premature deaths due to traffic emissions were estimated to be 8379 and 1908 cases in 2014 and 2020, respectively. The impact of NO 2 from road traffic relative to PM 2.5 on premature mortality was comparable to that of traffic-related PM 2.5 emissions. The public health effect of SO 2 originating from traffic was markedly lower than that of PM 2.5 . The results indicated that a reduction in traffic-related NO 2 could likely yield the greatest benefits for public health., (Copyright © 2022 Elsevier Inc. All rights reserved.)

We discuss and examine the stability, electronic properties, and transport characteristics of asymmetric monolayers XWGeN 2 (X = O, S, Se, Te) using ab initio density functional theory. All four monolayers of quintuple-layer atomic Janus XWGeN 2 are predicted to be stable and they are all indirect semiconductors in the ground state. When the spin-orbit coupling (SOC) is included, a large spin splitting at the K point is found in XWGeN 2 monolayers, particularly, a giant Rashba-type spin splitting is observed around the Γ point in three structures SWGeN 2 , SeWGeN 2 , and TeWGeN 2 . The Rashba parameters in these structures are directionally isotropic along the high-symmetry directions Γ - K and Γ - M and the Rashba constant α R increases as the X element moves from S to Te. TeWGeN 2 has the largest Rashba energy up to 37.4 meV (36.6 meV) in the Γ - K ( Γ - M ) direction. Via the deformation potential method, we calculate the carrier mobility of all four XWGeN 2 monolayers. It is found that the electron mobilities of OWGeN 2 and SWGeN 2 V 2 V -1 s -1 , which are suitable for applications in nanoelectronic devices.

In this work, we systematically examine the electronic features and contact types of van der Waals heterostructures (vdWHs) combining single-layer boron phosphide (BP) and Janus Ga 2 SSe using first-principles calculations. Owing to the out-of-plane symmetry being broken, the BP/Ga 2 SSe vdWHs are divided into two different stacking patterns, which are BP/SGa 2 Se and BP/SeGa 2 S. Our results demonstrate that these stacking patterns are structurally and mechanically stable. The combination of single-layer BP and Janus Ga 2 SSe gives rise to an enhancement in the Young's modulus compared to the constituent monolayers. Furthermore, at the ground state, the BP/Ga 2 SSe vdWHs possess a type-I (straddling) band alignment, which is desired for next-generation optoelectronic applications. The interlayer separation and electric field are effectively used to tune the electronic features of the BP/Ga 2 SSe vdWH from the type-I to type-II band alignment, and from semiconductor to metal. Our findings show that the BP/Ga 2 SSe vdWH would be appropriate for next-generation multifunctional optoelectronic and photovoltaic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this work, the effects of transition metal (TM = V and Cr) adsorption on AlN monolayer electronic and magnetic properties are investigated using first-principles density functional theory (DFT) calculations. TMs prefer to be adsorbed on-top of a bridge position as indicated by the calculated adsorption energy. V adatoms induce half-metallicity, while Cr adatoms metallize the monolayer. The magnetic properties are produced mainly by the V and Cr adatoms with magnetic moments of 3.72 and 4.53 μ , respectively. Further investigation indicates that antiferromagnetic (AFM) ordering is energetically more favorable than ferromagnetic (FM) ordering. In both cases, the AFM state is stabilized upon increasing adatom coverage. The AlN monolayer becomes an AFM semiconductor with 0.5 ML of V adatom, and metallic nature is induced with 1.0 ML. Meanwhile, the degree of metallicity increases with increasing Cr adatoms. Results reported herein may provide a feasible new approach to functionalize AlN monolayers for spintronic applications.B , respectively. Further investigation indicates that antiferromagnetic (AFM) ordering is energetically more favorable than ferromagnetic (FM) ordering. In both cases, the AFM state is stabilized upon increasing adatom coverage. The AlN monolayer becomes an AFM semiconductor with 0.5 ML of V adatom, and metallic nature is induced with 1.0 ML. Meanwhile, the degree of metallicity increases with increasing Cr adatoms. Results reported herein may provide a feasible new approach to functionalize AlN monolayers for spintronic applications., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (This journal is © The Royal Society of Chemistry.)

Due to the broken vertical symmetry, the Janus material possesses many extraordinary physico-chemical and mechanical properties that cannot be found in original symmetric materials. In this paper, we study in detail the structural, electronic, and transport properties of 1T Janus PdXO monolayers (X = S, Se, Te) by means of density functional theory. PdXO monolayers are observed to be stable based on the analysis of the vibrational characteristics and molecular dynamics simulations. All three PdXO structures exhibit semiconducting characteristics with indirect bandgap based on evaluations with hybrid functional Heyd-Scuseria-Ernzerhof (HSE06). The influences of the spin-orbit coupling (SOC) on the band diagram of PdXO are strong. Particularly, when the SOC is included, PdTeO is calculated to be metallic by the HSE06+SOC approach. With high electron mobility, Janus PdXO structures have good potential for applications in future nanodevices., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

First-principles calculations were performed to study a novel layered SnGe 2 N 4 compound, which was found to be dynamically and thermally stable in the 2H phase, with the space group P 6̄ m 2 and lattice constant a = 3.143 Å. Due to its hexagonal structure, SnGe 2 N 4 exhibits isotropic mechanical properties on the x-y plane, where the Young's modulus is 335.49 N m -1 and the Poisson's ratio is 0.862. The layered 2H SnGe 2 N 4 is a semiconductor with a direct band gap of 1.832 eV, allowing the absorption of infrared and visible light at a rate of about 10 4 cm -1 . The DOS is characterized by multiple high peaks in the valence and conduction bands, making it possible for this semiconductor to absorb light in the ultraviolet region with an even higher rate of 10 5 cm -1 . The band structure, with a strongly concave downward conduction band and rather flat valence band, leads to a high electron mobility of 1061.66 cm 2 V -1 s -1 , which is substantially greater than the hole mobility of 28.35 cm 2 V -1 s -1 . This difference in mobility is favorable for electron-hole separation. These advantages make layered 2H SnGe 2 N 4 a very promising photoelectric material. Furthermore, the electronic structure of 2H SnGe 2 N 4 responds well to strain and an external electric field due to the specificity of the p-d hybridization, which predominantly constructs the valence bands. As a result, strain and external electric fields can efficiently tune the band gap value of 2H SnGe 2 N 4 , where compressive strain widens the band gap, meanwhile tensile strain and external electric fields cause band gap reduction. In particular, the band gap is decreased by about 0.25 eV when the electric field strength increases by 0.1 V Å -1 , making a semiconductor-metal transition possible for the layered SnGe 2 N 4 ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this paper, the structural, electronic, and transport properties of Janus GaInX 3 (X = S, Se, Te) single-layers are investigated by a first-principles calculations. All three structures of GaInX 3 are examined to be stable based on the analysis of their phonon dispersions, cohesive energy, and Born's criteria for mechanical stability. At the ground state, The Janus GaInX 3 is a semiconductor in which its bandgap decreases as the chalcogen element X moves from S to Te. Due to the vertical asymmetric structure, a difference in the vacuum level between the two surfaces of GaInX 3 is found, leading to work functions on the two sides being different. The Janus GaInX 3 exhibit high directional isotropic transport characteristics. Particularly, GaInX 3 single-layers have high electron mobility, which could make them potential materials for applications in electronic nanodevices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Hypothesis: Janus particles (JPs) and surfactants express different behaviors at the oil-water interface under compression. When both are present at the interface, their synergies result in a different collapse mechanism than when present individually depending on the concentration of the JPs and surfactants., Experiments: Coarse-grained modeling methods were used to probe the synergies between Janus nanoparticles and nonionic surfactants on the stability of an oil-water interface under compression. When both JPs and surfactants were present, the interface was covered at 0-55% area by JPs and contained surfactants at 0-40% of the interfacial surfactant concentration corresponding to the critical micelle concentration (CMC)., Findings: Compression of the interface with only surfactants resulted in the expulsion of surfactant molecules to the water phase once the interfacial concentration of surfactant molecules reached the CMC value. Compression of a Janus particle-laden interface past the closed-packing point led to a buckled interface, so that the total interfacial area remained constant upon further compression. When both surfactants and JPs were present on the interface, JPs still caused buckling, which helped retain the surfactant molecules on the interface. The interface exhibited a higher level of deformation in presence of surfactants. When the surfactant concentration was high, under compression, the surfactants partitioned into the water phase, but the buckling of the interface persisted., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Coarse-grained modeling methods allow simulations at larger scales than molecular dynamics, making it feasible to simulate multifluid systems. It is, however, critical to use model parameters that represent the fluid properties with fidelity under both equilibrium and dynamic conditions. In this work, dissipative particle dynamics (DPD) methods were used to simulate the flow of oil and water in a narrow slit under Poiseuille and Couette flow conditions. Large surfactant molecules were also included in the computations. A systematic methodology is presented to determine the DPD parameters necessary for ensuring that the boundary conditions were obeyed, that the oil and water viscosities were represented correctly, and that the velocity profile for the multifluid system agreed with the theoretical expectations. Surfactant molecules were introduced at the oil-water interface (sodium dodecylsulfate and octaethylene glycol monododecyl ether) to determine the effects of surface-active molecules on the two-phase flow. A critical shear rate was found for Poiseuille flow, beyond which the surfactants desorbed to form the interface forming micelles and destabilize the interface, and the surfactant-covered interface remained stable under Couette flow even at high shear rates.

Motivated by the recent successful synthesis of 2D quintuple-layer atomic materials, for the first time, we design and investigate the electronic and transport properties of Janus Al 2 XY 2 (X/Y = S, Se, Te; X ≠ Y) monolayers by using the density functional theory. Our calculations demonstrate that most of the models of Al 2 XY 2 (except for Al 2 STe 2 monolayer) are dynamically and mechanically stable. By using the hybrid functional, all models of Al 2 XY 2 are semiconductors with an indirect bandgap. Meanwhile, Al 2 TeS 2 monolayer is found to be metal at the Perdew-Burke-Ernzerhof level. Due to the vertical asymmetry structure, an intrinsic built-in electric field exists in the Al 2 XY 2 and leads to a difference in the vacuum levels between the two sides of the monolayers. Carrier mobilities of Al 2 XY 2 monolayers are high directional anisotropic due to the anisotropy of their deformation potential constant. Al 2 XY 2 monolayers exhibit high electron mobility, particularly, the electron mobility of Al 2 SeS 2 exceeds 1 × 10 4 cm 2 V -1  s -1 , suggesting that they are suitable for applications in nanometer-sized electronic devices., (© 2021 IOP Publishing Ltd.)

Two-dimensional Janus monolayers have outstanding electronic and transport properties due to their asymmetric atomic structures. In the present work, we systematically study the structural, electronic, and transport properties of the Janus GaIn X 2 ( X = S, Se, Te) monolayers by using the first-principles calculations. The stability of the investigated monolayers is confirmed via the analysis of vibrational spectrum and molecular dynamics simulations. Our calculations demonstrate that while GaInS 2 and GaInSe 2 monolayers are direct semiconductors, GaInTe 2 monolayer exhibits the characteristics of an indirect semiconductor. The band gap of GaIn X 2 decreases when the chalcogen element X varies from S to Te. Obtained results reveal that small spin-orbit splitting energy in the valence band is found around the Γ point of the Brillouin zone when the spin-orbit coupling is included. Interestingly, GaInS 2 and GaInSe 2 have high and directional isotropic electron mobility meanwhile the directional anisotropy of the electron mobility is found in the Janus GaInTe 2 monolayer. Our findings not only present superior physical properties of GaIn X 2 monolayers but also show promising potential applications of these materials in nanoelectronic devices., (© 2021 IOP Publishing Ltd.)

This work aims to test the effectiveness of newly developed DFT-1/2 functional in calculating the electronic and optical properties of inorganic lead halide perovskites CsPbBr 3 . Herein, from DFT-1/2 we have obtained the direct band gap of 2.36 eV and 3.82 eV for orthorhombic bulk and 001-surface, respectively. The calculated energy band gap is in qualitative agreement with the experimental findings. The bandgap of ultra-thin film of CsPbBr 3 is found to be 3.82 eV, which is more than the expected range 1.23-3.10 eV. However, we have found that the bandgap can be reduced by increasing the surface thickness. Thus, the system under investigation looks promising for optoelectronic and photocatalysis applications, due to the bandgap matching and high optical absorption in UV-Vis (Ultra violet and visible spectrum) range of electro-magnetic(em) radiation., (© 2021. The Author(s).)

Motivated by very recent successful experimental transformation of AB-stacking bilayer graphene into fluorinated single-layer diamond (namely fluorinated diamane C 4 F 2 ) [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 C 4 F 2 by using density functional theory. Our obtained results demonstrate that at the ground state, the lattice constant of C 4 F 2 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 C 4 F 2 is confirmed to be dynamically and thermally stable. C 4 F 2 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 C 4 F 2 . C 4 F 2 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, C 4 F 2 has very high electron mobility, up to 3.03 × 10 3 cm 2 V -1 s -1 , much higher than other semiconductor compounds. Our findings not only provide a comprehensive insight into the physical properties of C 4 F 2 but also open up its applicability in nanoelectromechanical and optoelectronic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In 2 XY ( X / Y = S, Se, Te with X ≠ Y ) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In 2 XY are dynamically and thermally stable at room temperature. At equilibrium, both In 2 STe and In 2 SeTe are direct semiconductors while In 2 SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In 2 XY and their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In 2 XY is negligible. Meanwhile, the optical absorbance intensity of the Janus In 2 XY monolayers is remarkably increased by compressive strain. Also, In 2 XY monolayers exhibit very low lattice thermal conductivities resulting in a high figure of merit ZT , which makes them potential candidates for room-temperature thermoelectric materials., (© 2021 IOP Publishing Ltd.)

Two-dimensional MoSi 2 N 4 is an emerging class of 2D MA 2 N 4 family, which has recently been synthesized in experiment. Herein, we construct ultrathin van der Waals heterostructures between graphene and a new 2D Janus MoGeSiN 4 material and investigate their interfacial electronic properties and tunable Schottky barriers and contact types using first-principles calculations. The GR/MoGeSiN 4 vdWHs are expected to be energetically favorable and stable. The high carrier mobility in graphene/MoGeSiN 4 vdWHs makes them suitable for high-speed nanoelectronic devices. Furthermore, depending on the stacking patterns, either an n-type or a p-type Schottky contact is formed at the GR/MoGeSiN 4 interface. The strain engineering and electric field can lead to the transformation from an n-type to a p-type Schottky contact or from Schottky to Ohmic contact in graphene/MoGeSiN 4 heterostructure. These findings provide useful guidance for designing controllable Schottky nanodevices based on graphene/MoGeSiN 4 heterostructures with high-performance.

Epidemiological studies have consistently linked exposure to PM 2.5 with adverse health effects. The oxidative potential (OP) of aerosol particles has been widely suggested as a measure of their potential toxicity. Several acellular chemical assays are now readily employed to measure OP; however, uncertainty remains regarding the atmospheric conditions and specific chemical components of PM 2.5 that drive OP. A limited number of studies have simultaneously utilised multiple OP assays with a wide range of concurrent measurements and investigated the seasonality of PM 2.5 OP. In this work, filter samples were collected in winter 2016 and summer 2017 during the atmospheric pollution and human health in a Chinese megacity campaign (APHH-Beijing), and PM 2.5 OP was analysed using four acellular methods: ascorbic acid (AA), dithiothreitol (DTT), 2,7-dichlorofluorescin/hydrogen peroxidase (DCFH) and electron paramagnetic resonance spectroscopy (EPR). Each assay reflects different oxidising properties of PM 2.5 , including particle-bound reactive oxygen species (DCFH), superoxide radical production (EPR) and catalytic redox chemistry (DTT/AA), and a combination of these four assays provided a detailed overall picture of the oxidising properties of PM 2.5 at a central site in Beijing. Positive correlations of OP (normalised per volume of air) of all four assays with overall PM 2.5 mass were observed, with stronger correlations in winter compared to summer. In contrast, when OP assay values were normalised for particle mass, days with higher PM 2.5 mass concentrations (μgm -3 ) were found to have lower mass-normalised OP values as measured by AA and DTT. This finding supports that total PM 2.5 mass concentrations alone may not always be the best indicator for particle toxicity. Univariate analysis of OP values and an extensive range of additional measurements, 107 in total, including PM 2.5 composition, gas-phase composition and meteorological data, provided detailed insight into the chemical components and atmospheric processes that determine PM 2.5 OP variability. Multivariate statistical analyses highlighted associations of OP assay responses with varying chemical components in PM 2.5 for both mass- and volume-normalised data. AA and DTT assays were well predicted by a small set of measurements in multiple linear regression (MLR) models and indicated fossil fuel combustion, vehicle emissions and biogenic secondary organic aerosol (SOA) as influential particle sources in the assay response. Mass MLR models of OP associated with compositional source profiles predicted OP almost as well as volume MLR models, illustrating the influence of mass composition on both particle-level OP and total volume OP. Univariate and multivariate analysis showed that different assays cover different chemical spaces, and through comparison of mass- and volume-normalised data we demonstrate that mass-normalised OP provides a more nuanced picture of compositional drivers and sources of OP compared to volume-normalised analysis. This study constitutes one of the most extensive and comprehensive composition datasets currently available and provides a unique opportunity to explore chemical variations in PM 2.5 and how they affect both PM 2.5 OP and the concentrations of particle-bound reactive oxygen species., Competing Interests: Competing interests. The authors declare that they have no conflict of interest.

This study aims to critically evaluate the source apportionment of fine particles by multiple receptor modelling approaches, including carbon mass balance modelling of filter-based radiocarbon ( 14 C) data, Chemical Mass Balance (CMB) and Positive Matrix Factorization (PMF) analysis on filter-based chemical speciation data, and PMF analysis on Aerosol Mass Spectrometer (AMS-PMF) or Aerosol Chemical Speciation Monitor (ACSM-PMF) data. These data were collected as part of the APHH-Beijing (Atmospheric Pollution and Human Health in a Chinese Megacity) field observation campaigns from 10 th November to 12 th December in winter 2016 and from 22 nd May to 24 th June in summer 2017. 14 C analysis revealed the predominant contribution of fossil fuel combustion to carbonaceous aerosols in winter compared with non-fossil fuel sources, which is supported by the results from other methods. An extended Gelencsér (EG) method incorporating 14 C data, as well as the CMB and AMS/ACSM-PMF methods, generated a consistent source apportionment for fossil fuel related primary organic carbon. Coal combustion, traffic and biomass burning POC were comparable for CMB and AMS/ACSM-PMF. There are uncertainties in the EG method when estimating biomass burning and cooking OC. The POC from cooking estimated by different methods was poorly correlated, suggesting a large uncertainty when differentiating this source type. The PM 2.5 source apportionment results varied between different methods. Through a comparison and correlation analysis of CMB, PMF and AMS/ACSM-PMF, the CMB method appears to give the most complete and representative source apportionment of Beijing aerosols. Based upon the CMB results, fine aerosols in Beijing were mainly secondary inorganic ion formation, secondary organic aerosol formation, primary coal combustion and from biomass burning emissions.

Surface functionalization is one of the useful techniques for modulating the mechanical and electronic properties of two-dimensional systems. In the present study, we investigate the structural, elastic, and electronic properties of hexagonal boron phosphide monolayer functionalized by Br and Cl atoms using first-principles predictions. Once surface-functionalized with Br/Cl atoms, the planar structure of BP monolayer is transformed to the low-buckled lattice with the bucking constant of about 0.6 Å for all four configurations of functionalized boron phosphide, i.e. , Cl-BP-Cl, Cl-BP-Br, Br-BP-Cl, and Br-BP-Br. The stability of functionalized BP monolayers is confirmed via their phonon spectra analysis and ab initio molecular dynamics simulations. Our calculations indicate that the functionalized BP monolayers possess a fully isotropic elastic characteristic with the perfect circular shape of the angle-dependent Young's modulus and Poisson's ratio due to the hexagonal symmetry. The Cl-BP-Cl is the most stiff with the Young's modulus C 2D = 43.234 N m -1 . All four configurations of the functionalized boron phosphide are direct semiconductors with a larger band gap than that of a pure BP monolayer. The outstanding stability, isotropic elastic properties, and moderate band gap make functionalized boron phosphide a very intriguing candidate for next-generation nanoelectromechanical devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this paper, we investigate the electronic, optical, and thermoelectric properties of Ga 2 SSe monolayer by using density functional theory. Via analysis of the phonon spectrum and ab initio molecular dynamics simulations, Ga 2 SSe is confirmed to be stable at room temperature. Our calculations demonstrate that Ga 2 SSe exhibits indirect semiconductor characteristics and the spin-orbit coupling (SOC) effect has slightly reduced its band gap. Besides, the band gap of Ga 2 SSe depends tightly on the biaxial strain. When the SOC effect is included, small spin-orbit splitting energy of 90 meV has been found in the valence band. However, the spin-orbit splitting energy dramatically changes in the presence of biaxial strain. Ga 2 SSe exhibits high optical absorption intensity in the near-ultraviolet region, up to 8.444 × 10 4 cm -1 , which is needed for applications in optoelectronic devices. By using the Boltzmann transport equations, the electronic transport coefficients of Ga 2 SSe are comprehensively investigated. Our calculations reveal that Ga 2 SSe exhibits a very low lattice thermal conductivity and high figure of merit ZT and we can enhance its ZT by temperature. Our findings provide further insight into the physical properties of Ga 2 SSe as well as point to prospects for its application in next-generation high-performance devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Transparent conducting oxides (TCOs) of semiconductor family gained significant attention due to increasing trends in the optoelectronic and thermo-physical applications. In current work, we reported electronic, optical, transport and thermodynamical properties of spinel oxides ZnGaO 2 , [ZnGaO 2 ]:Mn 3+ and [ZnGaO 2 ]:Rh 3+ compounds. Based on DFT, we employed first-principles calculations implemented in Wien 2k using the modified-Becke-Johnson (mBJ) on parent spinel and generalized-gradient-approximation plus Hubbard potential U (GGA + U) on doped materials, respectively. The calculated band structure shows insulating nature of parent compound, while doped material observed semiconducting nature contains direct band gap for both spin channels with band gaps of [ZnGaO 2 ]:Mn 3+ (0.59 up, 2.4 eV dn) and [ZnGaO 2 ]:Rh 3+ (2.1 eV up/dn) respectively. The electronic and optical results reveal that hybridization occurred mainly due to O-p/Zn, Mn-d, Rh-d and Ga-s orbitals. It is analyzed that Mn-doped material shows good absorption in the visible region while other are good in UV region. The effective masses of spinel oxides are also computed at high symmetry directions hence varied nonlinearly with the doping. The stability of materials is checked by calculating formation energies which indicate Mn-doped spinel oxide is most stable as that of others. The thermoelectric properties of spinel oxides were carried out by Post-DFT (Boltztrap) calculations. Large values of Seebeck coefficient and power factor of Mn-doped spinel oxide indicate that this material can be used for thermoelectric devices. The thermodynamical properties are calculated by quasi-harmonic Debye model implemented in GIBBS 2 code. Moreover, the pressure and temperature dependence of all (TD) parameters of investigated spinel oxides are analyzed using quasi-harmonic Debye model., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

In this paper, we present a detailed investigation of the structural, electronic, and optical properties of pristine, nitrogenated, and fluorinated MgO monolayers using ab initio calculations. The two dimensional (2D) material stability is confirmed by the phonon dispersion curves and binding energies. Full functionalization causes notable changes in the monolayer structure and slightly reduces the chemical stability. The simulations predict that the MgO single layer is an indirect semiconductor with an energy gap of 3.481 (4.693) eV as determined by the GGA-PBE (HSE06) functional. The electronic structure of the MgO monolayer exhibits high sensitivity to chemical functionalization. Specifically, nitrogenation induces metallization of the MgO monolayer, while an indirect-direct band gap transition and band gap reduction of 81.34 (59.96)% are achieved by means of fluorination. Consequently, the functionalized single layers display strong optical absorption in the infrared and visible regimes. The results suggest that full nitrogenation and fluorination may be a quite effective approach to enhance the optoelectronic properties of the MgO monolayer for application in nano-devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

This study was designed to investigate the seasonal characteristics and apportion the sources of organic carbon during non-haze days (<75 μg m -3 ) and haze (≥75 μg m -3 ) events at Pinggu, a rural Beijing site. Time-resolved concentrations of carbonaceous aerosols and organic molecular tracers were measured during the winter of 2016 and summer 2017, and a Chemical Mass Balance (CMB) model was applied to estimate the average source contributions. The concentration of OC in winter is comparable with previous studies, but relatively low during the summer. The CMB model apportioned seven separate primary sources, which explained on average 73.8% on haze days and 81.2% on non-haze days of the organic carbon in winter, including vegetative detritus, biomass burning, gasoline vehicles, diesel vehicles, industrial coal combustion, residential coal combustion and cooking. A slightly lower percentage of OC was apportioned in the summer campaign with 64.5% and 78.7% accounted for. The other unapportioned OC is considered to consist of secondary organic carbon (SOC). During haze episodes in winter, coal combustion and SOC were the dominant sources of organic carbon with 23.3% and 26.2%, respectively, followed by biomass burning emissions (20%), whereas in summer, industrial coal combustion and SOC were important contributors. Diurnal contribution cycles for coal combustion and biomass burning OC showed a peak at 6-9 pm, suggesting domestic heating and cooking were the main sources of organic aerosols in this rural area. Backward trajectory analysis showed that high OC concentrations were measured when the air mass was from the south, suggesting that the organic aerosols in Pinggu were affected by both local emissions and regional transport from central Beijing and Hebei province during haze episodes. The source apportionment by CMB is compared with the results of a Positive Matrix Factorization (PMF) analysis of ACSM data for non-refractory PM 1 , showing generally good agreement., Competing Interests: Declaration of competing interest The authors have no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three years of beginning the submitted work that could inappropriately influence, or be perceived to influence, their work., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Nonmetal doping is an effective approach to modify the electronic band structure and enhance the photocatalytic performance of bismuth oxyhalides. Using density functional theory, we systematically examine the fundamental properties of single-layer BiOBr doped with boron (B) and phosphorus (P) atoms. The stability of the doped models is investigated based on the formation energies, where the substitutional doping is found to be energetically more stable under O-rich conditions than under Bi-rich ones. The results showed that substitutional doping of P atoms reduced the bandgap of pristine BiOBr to a greater extent than that of boron substitution. The calculation of the effective masses reveals that B doping can render the electrons and holes of pristine BiOBr lighter and heavier, respectively, resulting in a slower recombination rate of photoexcited electron-hole pairs. Based on the results of HOMO-LUMO calculations, the introduction of B atoms tends to increase the number of photocatalytically active sites. The top of the valence band and the conduction band bottom of the B doped BiOBr monolayer match well with the water redox potentials in an acidic environment. The absorption spectra propose that B(P) doping causes a red-shift. Overall, the results predict that nonmetal-doped BiOBr monolayers have a reduced bandgap, a slow recombination rate, more catalytically active sites, enhanced optical absorption edges, and reduced work functions, which will contribute to superior photocatalytic performance.

We report the relation between the optical properties and electronic structure of lithium thiogallate (LiGaS 2 ) by performing XPS and XES measurements and theoretical calculations. According to the XPS measurements, the LiGaS 2 crystals grown by the Bridgman-Stockbarger method possess promising optical qualities, low hygroscopicity and high stability upon middle-energy Ar + -ion irradiation. The difference in the LiGaS 2 band gaps obtained by theoretical calculations and experimental measurements was, for the first time, reduced down to 0.27 eV by applying the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential where the Coulomb repulsion was considered by introducing Hubbard parameter, U . The TB-mBJ+ U method also reproduces the XPS spectrum well. The TB-mBJ+ U band-structure calculations of LiGaS 2 are found to be in good agreement with the XPS and XES experimental data. The accurate electronic structure of LiGaS 2 allows further investigation of the optical properties. The relation between the photoluminescence of LiGaS 2 and its electronic structure was revealed. Moreover, the theoretical results show the possibility of emissions at higher energy levels in LiGaS 2 , that has not been measured in experiments yet. Good phase-matching of LiGaS 2 was expected to occur at energy levels of 5, 6, 6.2, 7, 7.2, and 8 eV., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

This paper focuses on the electronic and thermoelectric properties of monolayer MoS 2 . Here, we have examined the structure of MoS 2 , in which the hole in the center of the hexagonal cage is considered as a void atom, termed 1H-MoS 2 . Density functional theory (DFT) employing the generalized gradient approximation (GGA) and spin-orbit coupling (SOC) has been used for all calculations. Incorporation of SOC resulted in a significant change in the profile of the band energy, specifically the splitting of the valence band maximum (VBM) into two sub-bands. The "split-off" energy is found to be ∼20.6 meV. The reduction of the band gap with SOC is a prominent feature at the K-K location in the Brillouin zone. The band gap calculated with the GGA is ∼1.75 eV. However, on implementation of SOC, the GGA band gap was reduced to ∼1.68 eV. The frequency-dependent phonon dispersion curve was obtained to analyse the thermodynamical stability. 1H-MoS 2 is found to be thermodynamically stable with no imaginary frequency. We report a low value of lattice thermal conductivity ( κ l ) and low electron effective masses, which are desirable for potential applications in thermoelectric devices., Competing Interests: All authors declare that there are no conflicts of interest., (This journal is © The Royal Society of Chemistry.)

Constructing vertical heterostructures by placing graphene (Gr) on two-dimensional materials has recently emerged as an effective way to enhance the performance of nanoelectronic and optoelectronic devices. In this work, first principles calculations are employed to explore the structural and electronic properties of Gr/GeC and Gr/functionalized-GeC by H/F/Cl surface functionalization. Our results imply that the electronic properties of the Gr, GeC and all functionalized-GeC monolayers are well preserved in Gr/GeC and Gr/functionalized-GeC heterostructures, and the Gr/GeC heterostructure forms a p-type Schottky contact. Interestingly, we find that the p-type Schottky contact in Gr/GeC can be converted into the n-type one and into an n-type ohmic contact by H/F/Cl surface functionalization to form Gr/functionalized-GeC heterostructures. Furthermore, we find that electric fields and strain engineering can change both the Schottky barrier heights and the contact types of the Gr/functionalized-GeC vdWHs. These findings suggest that Gr/functionalized-GeC heterostructures can be considered as a promising candidate for designing high-performance optoelectronic and nanoelectronic devices.

The unique physical and chemical properties of β 12 -borophene stem from the coexistence of the Dirac and triplet fermions. The metallic phase of β 12 -borophene transitions to the semiconducting one when it is subjected to a perpendicular electric field or bias voltage. In this work, with the aid of a five-band tight-binding Hamiltonian, the Green's function approach and the Kubo-Greenwood formalism, the electronic thermal conductivity (ETC) of the semiconducting phase of β 12 -borophene is studied. Two homogeneous (H) and inversion symmetric (IS) models are considered depending on the interaction of the substrate and boron atoms. In addition, due to the anisotropic structure of β 12 -borophene, the swapping effect of bias poles is addressed. First of all, we find the pristine ETC IS < ETC H independent of the temperature. Furthermore, a decrease of 74.45% (80.62%) is observed for ETC H (ETC IS ) when strong positive bias voltages are applied, while this is 25.2% (47.48%) when applying strong negative bias voltages. Moreover, the shoulder temperature of both models increases (fluctuates) with the positive (negative) bias voltage. Our numerical results pave the way for setting up future experimental thermoelectric devices in order to achieve the highest performance.

The data on the electronic structure and optical properties of bromide K 0.5 Rb 0.5 Pb 2 Br 5 achieved by first-principle calculations and verified by X-ray spectroscopy measurements are reported. The kinetic energy, the Coulomb potential induced by the exchange hole, spin-orbital effects, and Coulomb repulsion were taken into account by applying the Tran and Blaha modified Becke-Johnson function (TB-mBJ), Hubbard U parameter, and spin-orbital coupling effect (SOC) in the TB-mBJ + U + SOC technique. The band gap was for the first time defined to be 3.23 eV. The partial density of state (PDOS) curves of K 0.5 Rb 0.5 Pb 2 Br 5 agree well with XES K Ll and Br Kβ 2 , and XPS spectra. The valence band (VB) is characterized by the Pb-5d 3/2 and Pb-5d 5/2 sub-states locating in the vicinities of -20 eV and -18 eV, respectively. The VB middle part is mainly formed by K-3p, Rb-4p and Br-4s states, in which the separation of Rb-4p 3/2 and Rb-4p 1/2 was also observed. The strong hybridization of Br-p and Pb-s/p states near -6.5 eV reveals a major covalent part in the Br-Pb bonding. With a large band gap of 3.23 eV, and the remarkably high possibility of inter-band transition in energy ranges of 4-7 eV, and 10-12 eV, the bromide K 0.5 Rb 0.5 Pb 2 Br 5 is expected to be a very promising active host material for core valence luminescence and mid-infrared rare-earth doped laser materials. The anisotropy of optical properties in K 0.5 Rb 0.5 Pb 2 Br 5 is not significant, and it occurs at the extrema in the optical spectra. The absorption coefficient α ( ω ) is in the order of magnitude of 10 6 cm -1 for an energy range of 5-25 eV., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this work, we consider the electronic and optical properties of chemically functionalized InN monolayers with F and Cl atoms ( i.e. , F-InN-F, F-InN-Cl, Cl-InN-F, Cl-InN-Cl monolayers) using first-principles calculations. The adsorption of the F and Cl atoms on the InN monolayer is determined to be chemically stable and the F-InN-F monolayer is most likely to occur. Our calculations show that the chemical functionalization with Cl and F atoms not only breaks the planar structure of InN monolayer but also increases its band gap. By using both Perdew, Burke, and Ernzerhof (PBE) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functionals, all four models of chemically functionalized InN monolayers are found to be semiconductors with direct energy gaps and these gaps depend on the constituent species. When the spin-orbit coupling (SOC) was included, the energy gap of these monolayers was reduced and an energy splitting was found at the Γ-point in the valence band. Chemically functionalized InN monolayers can absorb light in a wide region, especially the F-InN-F and Cl-InN-F monolayers have a strong ability to absorb the visible light. Our findings reveal that the chemically functionalized InN monolayers have potential applications in next-generation optoelectronic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The formation of van der Waals heterostructures (vdWHs) have recently emerged as promising structures to make a variety of novel nanoelectronic and optoelectronic devices. Here, in this work, we investigate the structural, electronic and optical features of ZnO/ZrSSe vdWHs for different stacking patterns of ZnO/SeZrS and ZnO/SZrSe by employing first-principles calculations. Binding energy and ab initio molecular dynamics calculations are also employed to confirm the structural and thermal stability of the ZnO/ZrSSe vdWHs for both models. We find that in both stacking models, the ZnO and ZrSSe layers are bonded via weak vdW forces, leading to easy exfoliation of the layers. More interestingly, both the ZnO/SeZrS and ZnO/SZrSe vdWHs posses type-II band alignment, making them promising candidates for the use of photovoltaic devices because the photogenerated electrons-holes are separated at the interface. The ZnO/ZrSSe vdWHs for both models possess high performance absorption in the visible and near-infrared regions, revealing their use for acquiring efficient photocatalysts. Moreover, the band gap values and band alignments of the ZnO/ZrSSe for both models can be adjusted by an electric field as well as vertical strains. There is a transformation from semiconductor to metal under a negative electric field and tensile vertical strain. These findings demonstrate that ZnO/ZrSSe vdWHs are a promising option for optoelectronic and nanoelectronic applications., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices. Here, using first principles calculations, we propose a GeC/C 2 N van der Waals heterostructure and investigate its electronic and optical properties. We demonstrate that the intrinsic electronic properties of both GeC and C 2 N monolayers are quite preserved in GeC/C 2 N HTS owing to the weak forces. At the equilibrium configuration, GeC/C 2 N HTS forms the type-II band alignment with an indirect band gap of 0.42 eV, which can be considered to improve the effective separation of electrons and holes. Besides, GeC/C 2 N vdW-HTS exhibits strong absorption in both visible and near ultra-violet regions with an intensity of 10 5 cm -1 . The electronic properties of GeC/C 2 N HTS can be tuned by applying an electric field and vertical strains. The semiconductor to metal transition can be achieved in GeC/C 2 N HTS in the case when the positive electric field of +0.3 V Å -1 or the tensile vertical strain of -0.9 Å is applied. These findings demonstrate that GeC/C 2 N HTS can be used to design future high-performance multifunctional devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In the present work, we investigate systematically the electronic and optical properties of Janus ZrSSe using first-principles calculations. Our calculations demonstrate that the Janus ZrSSe monolayer is an indirect semiconductor at equilibrium. The band gap of the Janus ZrSSe is 1.341 eV using the Heyd-Scuseria-Ernzerhof hybrid functional, larger than the band gap of ZrSe 2 monolayer and smaller than that of ZrS 2 monolayer. Based on the analysis of the band edge alignment, we confirm that the Janus ZrSSe monolayer possesses photocatalytic activities that can be used in water splitting applications. While strain engineering plays an important role in modulating the electronic properties and optical characteristics of the Janus ZrSSe monolayer, the influence of the external electric field on these properties is negligible. The biaxial strain, ε b , has significantly changed the band of the Janus ZrSSe monolayer, and particularly, the semiconductor-metal phase transition which occurred at ε b = 7%. The Janus ZrSSe monolayer can absorb light in both visible and ultraviolet regions. Also, the biaxial strain has shifted the first optical gap of the Janus ZrSSe monolayer. Our findings provide additional information for the prospect of applying the Janus ZrSSe monolayer in nanoelectronic devices, especially in water splitting technology., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

van der Waals heterostructures can be effectively used to enhance the electronic and optical properties and extend the application range of two-dimensional materials. Here, we construct for the first time MoSeTe/X(OH)2 (X = Ca, Mg) heterostructures and investigate their electronic and optical properties as well as the relative orientation of these layers with respect to each other and the effects of an electric field. Our results show that in the MoSeTe/X(OH)2 heterostructures, the Janus MoSeTe monolayer is bonded to the X(OH)2 layer via weak van der Waals forces. Owing to different kinds of chalcogen Se and Te atoms in both sides of Janus MoSeTe, there exist two main stacking types of the MoSeTe/X(OH)2 heterostructures, that are MoSeTe-Se/X(OH)2 and MoSeTe-Te/X(OH)2 heterostructures. Interestingly, the Se- and Te-interface can induce straddling type-II and type-I band alignments. The MoSeTe-Se/X(OH)2 heterostructure exhibits a type-II band alignment, thus endowing it with a potential ability to separate photogenerated electrons and holes. Whereas, the MoSeTe-Te/Ca(OH)2 heterostructure displays a type-I band alignment, which may result in an ultrafast recombination between electrons and holes, making the MoSeTe-Te/Ca(OH)2 heterostructure a suitable material for optoelectronic applications. The MoSeTe/X(OH)2 heterostructures show an isotropic behavior in the low energy region while an anisotropic behaviour in the high photon energy region. The dielectric function of the MoSeTe-Te/Ca(OH)2 heterostructure is high at low photon energy relative to other heterostructures verifying it to have a good optical absorption. Furthermore, the band gap values and band alignment of the MoSeTe/X(OH)2 heterostructures can be modulated by applying an electric field, which induces semiconductor-to-metal and type-I(II) to type-II(I) band alignment. These results demonstrate that the MoSeTe/X(OH)2 heterostructures are promising candidates for optoelectronic and photovoltaic nanodevices.

van der Waals heterostructures (vdWHs), obtained by vertically stacking different two-dimensional (2D) layered materials are being considered intensively as potential materials for nanoelectronic and optoelectronic devices because they can show the most potential advantages of individual 2D materials. Here, we construct the SnSe2/MoS2 vdWH and investigate its electronic and optical properties using first-principles calculations. We find that the band structures of both MoS2 and SnSe2 monolayers are well kept in the SnSe2/MoS2 vdWH because of their weakly interacting features via vdW interaction. The SnSe2/MoS2 vdWH forms a type-I band alignment and exhibits an indirect semiconductor band gap of 0.45 eV. The type-I band alignment makes the SnSe2/MoS2 vdWH a promising material for optoelectronic nanodevices, such as light emitting diodes because of ultra-fast recombination of electrons and holes. Moreover, the band gap and band alignment of the SnSe2/MoS2 vdWH can be tailored by the electric field and the insertion of a graphene sheet. After applying an electric field, type-I to type-II and semiconductor to metal transitions can be achieved in the SnSe2/MoS2 vdWH. Besides, when a graphene sheet is inserted into the SnSe2/MoS2 vdWH to form three stacking types of G/SnSe2/MoS2, SnSe2/G/MoS2 and SnSe2/MoS2/G, the p-type semiconductor of the SnSe2/MoS2 vdWH is converted to an n-type Ohmic contact. These findings provide theoretical guidance for designing future nanoelectronic and optoelectronic devices based on the SnSe2/MoS2 vdWH.

Hypothesis: Nanoparticles (NPs) can reduce the interfacial tension (IFT) of the oil-water system containing surfactants by reducing the interfacial area available to surfactants. The ability to reduce the IFT when surfactants are present in addition to NPs depends on the localization of the NPs on the interface, which is related to the nature of the NPs and the interaction between NPs and surfactant molecules., Experiments: Systems of NPs and surfactants on the oil-water interface were studied using dissipative particle dynamics (DPD). Heterogeneous NPs with different properties and interface coverage were placed on the interface with various surfactant concentrations. The IFT and the surfactant density profiles across the interface were analyzed., Findings: At constant surfactant concentration, adding NPs reduced the IFT; while with the absence of surfactant, NPs expressed no effect on the IFT. Among different types of heterogeneous NPs, the most effective were those that maximized their footprint on the interface, reducing thus the interfacial area available to surfactants. The interactions of the NPs with the surfactant molecules determined exactly which pattern of heterogeneity was most favorable. Based on these results, suggestions for designing NPs for maximum synergistic effects with surfactants were formulated., (Copyright © 2019 Elsevier Inc. All rights reserved.)

In order to investigate the interfacial region between oil and water with the presence of surfactants using coarse-grained computations, both the interaction between different components of the system and the number of surfactant molecules present at the interface play an important role. However, in many prior studies, the amount of surfactants used was chosen rather arbitrarily. In this work, a systematic approach to develop coarse-grained models for anionic surfactants (such as sodium dodecyl sulfate) and nonionic surfactants (such as octaethylene glycol monododecyl ether) in oil-water interfaces is presented. The key is to place the theoretically calculated number of surfactant molecules on the interface at the critical micelle concentration. Based on this approach, the molecular description of surfactants and the effects of various interaction parameters on the interfacial tension are investigated. The results indicate that the interfacial tension is affected mostly by the head-water and tail-oil interaction. Even though the procedure presented herein is used with dissipative particle dynamics models, it can be applied for other coarse-grained methods to obtain the appropriate set of parameters (or force fields) to describe the surfactant behavior on the oil-water interface.

The physical properties of indoor particles were measured with an Scanning Mobility Particle Sizer (SMPS) system (14.6-850 nm), an Aerodynamic Particle Sizer (APS, 0.54-18 μm) and an Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) in an apartment located in an urban background site in Prague (Czech Republic) from 15 August to 8 September, 2014. The total particle maximum number concentration was 9.38 × 10 4 , 1.46 × 10 5 , 2.89 × 10 4 , 2.25 × 10 5 and 1.57 × 10 6 particles cm -3 for particles released from vacuum cleaning, soap/W5 cleaning spray, smoking, incense burning and cooking (frying) activities, respectively. Particles emitted from cleaning activities showed unimodal number size distributions, with the majority of particles (>98.2 %) in the ultrafine size range (Dp <100 nm) and modes at a diameter of 19.8 nm for vacuum cleaning and 30.6 nm for soap/W5 cleaning. Smoking and incense burning predominantly generated particles in the accumulation mode with a count median diameter around 90-150 nm while cooking emissions showed a bimodal structure with a main mode at 47.8 nm. Particles from vacuum cleaning, incense burning, smoking and cooking emissions were found to be "nearly hydrophobic" with an average growth factor (G f ) around 1.01-1.10, while particles emitted from desk cleaning using organic compounds were found to be "less-hygroscopic" (G f ∼1.12-1.16). Based on an adjusted MPPD model with a consideration of the hygroscopic properties of particles, the total lung deposition fractions of these particles by number when they penetrate into the human lung were 0.73 ± 0.02, 0.62 ± 0.03, 0.37 ± 0.03, 0.32 ± 0.03 and 0.49 ± 0.02 for vacuum cleaning, desk cleaning, smoking, incense burning and cooking, respectively.

Density functional theory (DFT) calculations within the concept of the MBJ+U+SO (modified Becke-Johnson potential + U + spin orbit) approach were performed for a Tl 4 HgBr 6 single crystal for the first time assuming weak noncentrosymmetry (space group P4nc). Excellent agreement was achieved between the calculated and experimental band-gap-energy magnitudes as well as the density of electronic states measured by the X-ray photoelectron spectroscopy method. It is a very principal result because usually the DFT calculations underestimate the energy-gap values. In the present study, we carry out calculations of the optical properties (absorption coefficient, real and imaginary parts of the dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient, and optical reflectivity dispersions). It has been established that the principal origin of the observed weak acentricity is determined by delocalized band states at the top of the valence band originating from the p states of the Br atoms.