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The Eley-Rideal abstraction involving [F.sup.+] projectiles and fluorinated Ag and Si surfaces occurring at high impact energies that result in hyperthermal [F.sub.2.sup.-] ions leaving the surface at energies of tens of electron volts is described. The results from energy analysis of the [F.sub.2.sup.-] products as well as bombardment of externally fluorinated surfaces with [super 22][Ne.sup.+] rather than [F.sup.+] showed that [F.sup.+] projectiles are required for fast-[F.sub.2.sup.-] formation.

Two-dimensional (2D) superconductors have attracted great attention in recent years due to the possibility of new phenomena in lower dimensions. With many bulk transition metal carbides being well-known conventional superconductors, here we perform first-principles calculations to evaluate the possible superconductivity in a 2D monolayer Mo2C. Three candidate structures (monolayer alpha-Mo2C, 1T MXene-Mo2C, and 2H MXene-Mo2C) are considered and the most stable form is found to be 2H MXene-Mo2C. Electronic structure calculations indicate that both unpassivated and passivated 2H forms exhibit metallic properties. We obtain phonon frequencies and electron–phonon couplings using density-functional perturbation theory, and based on the BCS theory and the McMillan equation, estimate the critical temperatures to be in the ∼0–13 K range, depending on the species of surface termination (O, H and OH). The optimal termination group is H, which can increase the electron–phonon coupling and bring the critical temperature to 13 K. This shows a rather high critical temperature, tunable by surface termination, making this 2D carbide an interesting test bed for low-dimensional superconductivity. [ABSTRACT FROM AUTHOR]

Recently synthesized graphitic honeycomb structures, consisting of sp 2 -bonded graphene nanoribbons connected by sp 3 -bonded “hinges” are investigated theoretically. Honeycombs of different “wall-chiralities” (armchair and zigzag) and sizes are studied. Simulation of the reconstruction of the hinges shows that zigzag honeycombs spontaneously rearrange, resulting in a new structure. Elastic mechanical simulations show that the Young's modulus of the structures is determined solely by the density of the hinges, regardless of the structural orientation or regularity. Compression tests display a distinct behavior of self-localized deformation, similar to that of macroscopic honeycombs. Interestingly, the failure strain of the honeycomb structure is affected significantly by its lattice size and geometrical regularity. Electronic band structures of different types of honeycombs are calculated, showing that the conductivity of armchair honeycombs follows the well-known “3 n ”-dependency, while zigzag honeycombs are always metallic. [ABSTRACT FROM AUTHOR]

With the lateral coplanar heterojunctions of two-dimensional monolayer materials turning into reality, the quantitative understanding of their electronic, electrostatic, doping, and scaling properties becomes imperative. In contrast to traditional bulk 3D junctions where carrier equilibrium is reached through local charge redistribution, a highly nonlocalized charge transfer (trailing off as 1/x away from the interface) is present in lateral 2D junctions, increasing the junction size considerably. The depletion width scales as p-1, while the differential capacitance varies very little with the doping level p. The properties of lateral 2D junctions are further quantified through numerical analysis of realistic materials, with graphene, MoS2, and their hybrid serving as examples. Careful analysis of the built-in potential profile shows strong reduction of Fermi level pinning, suggesting better control of the barrier in 2D metal-semiconductor junctions. [ABSTRACT FROM AUTHOR]

Buckling nanopatterns of monoatomic layer 2D materials on metal substrates attract significant attention due to their rich interface morphology affecting electronic applications. An experimental-theoretical study of a 2D boron-nitrogen-carbon (B x /2N x /2C1− x) alloy on a Ru(0001) surface is conducted and a profound relation between the composition x and the degree of buckling is discovered. Experimentally, real carbon-boron-nitrogen alloys on the Ru(0001) surface are demonstrated and various morphologies of pure and mixed compounds are shown. Density functional theory calculations are further carried out using the supercells of graphene, hexagonal boron nitride (h-BN), and random BNC on Ru(0001), as well as Monte Carlo simulations for elucidating the kinetics of their growth. The results show that unlike pure compounds (h-BN or C), the carbon-boron-nitrogen mix on Ru(0001) mostly exists in an uncorrugated form, thus greatly improving the interface contact. The likely cause of the diminished corrugation is a softening of bond angular interactions in the alloy relative to the pure phases. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides are promising materials for photoelectronic devices. Among them, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are some of the best candidates due to their favorable band gap values and band edge alignments. Here, various perturbative corrections to the DFT electronic structure, e.g. GW, spin-orbit coupling, as well as many-body excitonic and trionic effects are considered, and accurate band gaps as a function of homogeneous biaxial strain in these materials are calculated. All of these corrections are shown to be of comparable magnitudes and need to be included in order to obtain an accurate electronic structure. The strain at which the direct-to-indirect gap transition occurs is calculated. After considering all contributions, the direct to indirect gap transition strain is predicted to be at 2.7% in MoS2 and 3.9% in WS2. These values are generally higher than the previously reported theoretical values. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides are promising materials for photoelectronic devices. Among them, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are some of the best candidates due to their favorable band gap values and band edge alignments. Here, various perturbative corrections to the DFT electronic structure, e.g. GW, spin-orbit coupling, as well as many-body excitonic and trionic effects are considered, and accurate band gaps as a function of homogeneous biaxial strain in these materials are calculated. All of these corrections are shown to be of comparable magnitudes and need to be included in order to obtain an accurate electronic structure. The strain at which the direct-to-indirect gap transition occurs is calculated. After considering all contributions, the direct to indirect gap transition strain is predicted to be at 2.7% in MoS2 and 3.9% in WS2. These values are generally higher than the previously reported theoretical values. [ABSTRACT FROM AUTHOR]

We explore theoretically the possibility of growing GaN films in a nonpolar orientation on planar high-index Si(hhk) substrates. Candidate substrates were identified by requiring that they are well lattice matched, on a length scale of several unit cells, to GaN in the nonpolar m-plane orientation. These candidate orientations were then used to construct atomistic models of the GaN/Si(hhk) interface. Using density functional theory, we then computed the formation energies of these nonpolar interfaces and compared them to those of competing polar interfaces. We find that Si(112) and Si(113) offer potentially favorable substrates for the growth of nonpolar m-plane GaN. [ABSTRACT FROM AUTHOR]

Recent experiments have shown a dramatic increase in the conductivity of PbSe nanocrystalline films after hydrazine treatment, suggesting that hydrazine may create free carriers in PbSe nanocrystals. Here, we study the effect of hydrazine adsorption on the electronic structure of PbSe nanocrystals using density functional theory. The physisorption of the intact hydrazine molecule and its dissociative chemisorption on different surfaces of PbSe are considered. Despite experimental indications of the n-type doping by hydrazine in PbSe, no theoretical evidence of the effect is found. Instead, PbSe is predicted to remain an intrinsic semiconductor after surface doping by hydrazine, and become a p-type semiconductor after doping by the hydrazine fragments. We attribute the discrepancy between experiment and theory to indirect effects, such as self-doping by excess surface atoms due to selective surface etching. [ABSTRACT FROM AUTHOR]

Encapsulation by hexagonal boron nitride (hBN) has been widely used to address intrinsic properties of two-dimensional (2D) materials. The hBN encapsulation, however, can alter properties of 2D materials through interlayer orbital hybridization. In this paper, we present measurements of temperature dependence of photoluminescence intensity from monolayer MoS2 encapsulated by hBN flakes. The obtained temperature dependence shows an opposite trend to that of previously observed in a monolayer MoS2 on a SiO2 substrate. This is caused by the existence of stable momentum-forbidden dark excitons in the hBN-encapsulated MoS2. Ab-initio band-structure calculations have shown that orbital hybridization between MoS2 and hBN leads to upward shift of Γ-valley of MoS2, which results in lowering of energy of the momentum-forbidden dark excitons. This work shows an important implication that the hBN-encapsulated structures used to address intrinsic properties of two-dimensional crystals can alter basic properties of encapsulated materials. [ABSTRACT FROM AUTHOR]

Graphite, with many industrial applications, is one of the widely sought-after allotropes of carbon. Although the geometric design of the graphite in many of the applications could dictate its precision performance, conventional synthesis methods for formulating complex geometric graphite shapes are limited due to the intrinsic brittleness and difficulties of high-temperature processing. Here, we report the development of colloidal ink from commercial graphite powders that allows the fabrication of any complex architectures with tunable geometry and directionality via 3D printing at room temperature. The method is enabled via using very small amounts of clay as an additive, allowing the proper design of the graphite-based ink and subsequent binding of graphite platelets during printing. Sheared layers of clay are easily able to flow, adapt, and interface with graphite layers forming strong binding between the layers that make the larger structures. The direct ink writing of complex graphitic architectures without further heat treatments could lead to easy shape engineering and related applications of graphite at various length scales. The printed structures exhibit excellent thermal, electrical, and mechanical properties, and the clay additive does not seem to alter these properties due to the excellent inter-layer dispersion and mixing within the graphite material. [Display omitted] [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS 2 ), have aroused substantial research interest in recent years, motivating the quest for new synthetic strategies. Recently, halide salts have been reported to promote the chemical vapor deposition (CVD) growth of a wide range of TMD. Nevertheless, the underlying promoting mechanisms and reactions are largely unknown. Here, we employ first-principles calculations and ab initio molecular dynamics (AIMD) simulations in order to investigate the detailed molecular mechanisms during the salt-assisted CVD growth of MoS 2 monolayers. The sulfurization of molybdenum oxyhalides MoO 2 X 2 (X = F, Cl, Br, and I)─the form of Mo-feedstock dominating in salt-assisted synthesis─has been explored and displays much lower activation barriers than that of molybdenum oxide present during conventional "saltless" growth of MoS 2 . Furthermore, the rate-limiting barriers appear to depend linearly on the electronegativity of the halogen element, with oxyiodide having the lowest barrier. Our study reveals the promoting mechanisms of halides and allows growth parameter optimization to achieve even faster growth of MoS 2 monolayers in the CVD synthesis.

Electron optics is the systematic use of electromagnetic (EM) fields to control electron motions. In graphene, strain induces pseudo-electromagnetic fields to guide electron motion. Here we demonstrate the use of substrate topography to impart desirable strain on graphene to induce static pseudo-EM fields. We derive the quasi-classical equation of motion for Dirac Fermions in a pseudo-EM field in graphene and establish the correspondence between the quasi-classical and quantum mechanical snake states. Based on the trajectory analysis, we design sculpted substrates to realize various "optical devices" such as a converging lens or a collimator, and further propose a setup to achieve valley Hall effect solely through substrate patterning, without any external fields, to be used in valleytronics applications. Finally, we discuss how the predicted strain/pseudo-EM field patterns can be experimentally sustained by typical substrates and generalized to other 2D materials.

There has been growing interest in searching for new low-dimensional (low-D) materials for nanoelectronics and energy applications. Most materials have their structural units extended in three dimensions and connected with chemical bonds. When the dimension is reduced, these bonds will be broken, decreasing the stability and making their experimental realization difficult. Here, we show that stable low-D materials can be made from naturally existing planar structural units. This is demonstrated by first-principles study of boron chalcogenides (B-X), which can have various low-D structures with attractive properties. For example, B 2 O 3 can be the thinnest proton-exchange membrane for fuel cells. B-X are wide-gap semiconductors that can complement the narrow-gap 2D metal dichalcogenides for (opto)electronics. Our work sheds light on the stability of low-D materials and suggests guidelines for rational design of new materials.

Electronic transport through a metal|semiconductor (M|S) heterojunction is largely determined by its Schottky barrier. In 3D M|S junctions, the barrier height determines the turn-on voltage and is often pinned by the interface states, causing Fermi level pinning (FLP). The pinning strength in 3D depends on the ratio C i / C M between the interface quantum capacitance C i and the metal surface capacitance C M . In 2D, the interface dipole does not influence the band alignment, but still affects the Schottky barrier and transport. In light of the general interest in building 2D electronics, in this work we discover the relevant material parameters which dictate the behavior and strength of FLP in 2D M|S contacts. Using a multiscale model combining first-principles, continuum electrostatics, and transport calculations, we study a realistic Gr|MoS 2 interface as an example with high interface state density ( C i / C M ≫ 1). Transport calculations show partial pinning with a strength P ∼ 0.6, while a 3D junction with similar heterogeneity gives full pinning with P = 1 as expected. We further show that in 2D M|S contacts the turn-on voltage and pinning strength are affected by a physical parameter l /λ D , the ratio between the interface width l , and the thermal de Broglie wavelength λ D . Pinning is absent for ideal line-contacts ( l /λ D = 0), but increases for realistic l /λ D values.

The effect of flexoelectric voltage on the electronic and optical properties of single- and double-wall carbon nanotubes is evaluated by the first-principles calculations. The voltage between the inner channel of curved sp 2 carbon nanostructures and their surroundings scales linearly with nanotube wall curvature and can be boosted/reversed by appropriate outer wall functionalization. We predict and verify computationally that in double-wall nanotubes, flexoelectricity causes a straddling to staggered band gap transition. Accurate band structure calculations taking into account quasiparticle corrections and excitonic effects lead to an estimated critical diameter of ∼24 Å for this transition. Double-wall nanotubes above this diameter have staggered band alignment and could be potentially used for charge separation in photovoltaic devices.

Despite being only a few atoms thick, single-layer two-dimensional (2D) materials display strong electron-photon interactions that could be utilized in efficient light modulators on extreme subwavelength scales. In various applications involving light modulation and manipulation, materials with strong optical response at different wavelengths are required. Using qualitative analytical modeling and first-principles calculations, we determine the theoretical limit of the maximum optical response such as absorbance ( A) and reflectance ( R) in 2D materials and also conduct a computational survey to seek out those with best A and R in various frequency ranges, from mid-infrared to deep-ultraviolet. We find that 2D boron has broadband reflectance R > 99% for >100 layers, surpassing conventional thin films of bulk metals such as silver. Moreover, we identify 2D monolayer semiconductors with maximum response, for which we obtain quantitative estimates by calculating quasiparticle energies and accounting for excitonic effects by solving the Bethe-Salpeter equation. We found several monolayer semiconductors with absorbances ≳30% in different optical ranges, which are more than half of the maximum possible value, A lim = 1/2, for a freestanding 2D material. Our study predicts 2D materials which can potentially be used in ultrathin reflectors and absorbers for optoelectronic application in various frequency ranges.

Two-dimensional single-layer boron (borophene) has emerged as a new material with several intriguing properties. Recently, the β 12 polymorph of borophene was grown on Ag(111), and observed to host Dirac fermions. Similar to graphene, β 12 borophene can be described as atom-vacancy pseudoalloy on a closed-packed triangular lattice; however, unlike graphene, the origin of its Dirac fermions  is yet unclear. Here, using first-principles calculations, we probe the origin of Dirac fermions in freestanding and Ag(111)-supported β 12 borophene. The freestanding β 12 sheet hosts two Dirac cones and a topologically nontrivial Dirac nodal line with interesting Dirac-like edge states. On Ag(111), the Dirac cones develop a gap, whereas the topologically protected nodal line remains intact, and its position in the Brillouin zone matches that of the Dirac-like electronic states seen in the experiment. The presence of nontrivial topological states near the Fermi level in borophene makes its electronic properties important for both fundamental and applied research.

A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS 2 /MoS 2 , deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS 2 or MoS 2 monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G 0 W 0 , we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron-phonon coupling, which in turn can enable superconducting behavior. Here we perform first-principles analysis of electronic structure, phonon spectra, and electron-phonon coupling of selected 2D boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated superconductivity, with estimated critical temperature in the range of Tc ≈ 10-20 K.

Intrinsic semimetallicity of graphene and silicene largely limits their applications in functional devices. Mixing carbon and silicon atoms to form two-dimensional (2D) silicon carbide (SixC1-x) sheets is promising to overcome this issue. Using first-principles calculations combined with the cluster expansion method, we perform a comprehensive study on the thermodynamic stability and electronic properties of 2D SixC1-x monolayers with 0 ≤ x ≤ 1. Upon varying the silicon concentration, the 2D SixC1-x presents two distinct structural phases, a homogeneous phase with well dispersed Si (or C) atoms and an in-plane hybrid phase rich in SiC domains. While the in-plane hybrid structure shows uniform semiconducting properties with widely tunable band gap from 0 to 2.87 eV due to quantum confinement effect imposed by the SiC domains, the homogeneous structures can be semiconducting or remain semimetallic depending on a superlattice vector which dictates whether the sublattice symmetry is topologically broken. Moreover, we reveal a universal rule for describing the electronic properties of the homogeneous SixC1-x structures. These findings suggest that the 2D SixC1-x monolayers may present a new "family" of 2D materials, with a rich variety of properties for applications in electronics and optoelectronics.

Doped, substituted, or alloyed graphene is an attractive candidate for use as a tunable element of future nanomechanical and optoelectronic devices. Here we use the density functional theory, density functional tight binding, cluster expansion, and molecular dynamics to investigate the thermal stability and electronic properties of a binary 2D alloy of graphitic carbon and nitrogen (C(1-x)N(x)). The stability range naturally begins from graphene and must end before x = 1, where pure nitrogen rather forms molecular gas. This poses a compelling question of what highest x < 1 still permits stable 2D hexagonal lattice. Such upper limit on the nitrogen concentration that is achievable in a stable alloy can be found based on the phonon and molecular dynamics calculations. The stability switchover is predicted to between x = 1/3 (33.3%) and x = 3/8 (37.5%), and no stable hexagonal lattice two-dimensional CN alloys can exist at the N concentration of x = 3/8 (37.5%) and higher.

Binary alloys present a promising venue for band gap engineering and tuning of other mechanical and electronic properties of materials. Here we use the density-functional theory and cluster expansion to investigate the thermodynamic stability and electronic properties of 2D transition metal dichalcogenide (TMD) binary alloys. We find that mixing electron-accepting or electron-donating transition metals with 2D TMD semiconductors leads to degenerate p- or n-doping, respectively, effectively rendering them metallic. We then proceed to investigate the electronic properties of semiconductor-semiconductor alloys. The exploration of the configurational space of the 2D molybdenum-tungsten disulfide (Mo1-xWxS2) alloy beyond the mean field approximation yields insights into anisotropy of the electron and hole effective masses in this material. The effective hole mass in the 2D Mo1-xWxS2 is nearly isotropic and is predicted to change almost linearly with the tungsten concentration x. In contrast, the effective electron mass shows significant spatial anisotropy. The values of the band gap in 2D Mo1-xWxS2 and MoSe2(1-x)S2x are found to be configuration-dependent, exposing the limitations of the mean field approach to band gap analysis in alloys.

We study the Li clustering process on graphene and obtain the geometry, nucleation barrier, and electronic structure of the clusters using first-principles calculations. We estimate the concentration-dependent nucleation barrier for Li on graphene. While the nucleation occurs more readily with increasing Li concentration, possibly leading to the dendrite formation and failure of the Li-ion battery, the existence of the barrier delays nucleation and may allow Li storage on graphene. Our electronic structure and charge transfer analyses reveal how the fully ionized Li adatoms transform to metallic Li during the cluster growth on graphene.