Abstract: Strain engineering is an effective way to tune the intrinsic properties of a material. Here, we show by using first-principles calculations that both uniaxial and biaxial tensile strain applied to monolayer TiS3 are able to significantly modify its intrinsic mobility. From the elastic modulus and the phonon dispersion relation we determine the tensile strain range where structure dynamical stability of the monolayer is guaranteed. Within this region, we find more than one order of enhancement of the acoustic phonon limited mobility at 300 K (100 K), i.e. from 1.71 x 10(4) (5.13 x 10(4)) cm(2) V-1 s(-1) to 5.53 x 10(6) (1.66 x 10(6)) cm(2) V-1 s(-1). The degree of anisotropy in both mobility and effective mass can be tuned by using tensile strain. Furthermore, we can either increase or decrease the band gap of TiS3 monolayer by applying strain along different crystal directions. This property allows us to use TiS3 not only in electronic but also in optical applications.

Abstract: When confined between graphene layers, water behaves differently from the bulk and exhibits unusual properties such as fast water flow and ordering into a crystal. The hydrogen-bonded network is affected by the limited space and by the characteristics of the confining walls. The presence of an extraordinary number of hydronium and hydroxide ions in narrow channels has the following effects: (i) they affect water permeation through the channel, (ii) they may interact with functional groups on the graphene oxide surface and on the edges, and (iii) they change the thermochemistry of water, which are fundamentally important to understand, especially when confined water is subjected to an external electric field. Here we study the physical properties of water when confined between two graphene sheets and containing hydronium and hydroxide. We found that: (i) there is a disruption in the solvation structure of the ions, which is also affected by the layered structure of confined water, (ii) hydronium and hydroxide occupy specific regions inside the nanochannel, with a prevalence of hydronium (hydroxide) ions at the edges (interior), and (iii) ions recombine more slowly in confined systems than in bulk water, with the recombination process depending on the channel height and commensurability between the size of the molecules and the nanochannel height – a decay of 20% (40%) in the number of ions in 8 ps is observed for a channel height of h = 7 angstrom (bulk water). Our work reveals distinctive properties of water confined in a nanocapillary in the presence of additional hydronium and hydroxide ions.

Abstract: Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate the adsorption of Mg and Na adatoms on five M2CS2 monolayers (where M = Mo, Nb, Ti, V, and Zr) for battery applications. We assessed the stability of the adatom (i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo2CS2 cannot support a full layer of Na nor even a single Mg atom. Na and Mg exhibit the strongest binding on Zr2CS2, followed by Ti2CS2, Nb2CS2 and V2CS2. Using the nudged elastic band method (NEB), we computed promising diffusion barriers for both dilute and nearly full ion surface coverage cases. In the dilute ion adsorption case, a single Mg and Na atom on Ti2CS2 experience similar to 0.47 eV and similar to 0.10 eV diffusion barriers between the lowest energy sites, respectively. For a nearly full surface coverage, a Na ion moving on Ti2CS2 experiences a similar to 0.33 eV energy barrier, implying a concentration-dependent diffusion barrier. Our molecular dynamics results indicate that the three (one) layers (layer) of the Mg (Na) ion on both surfaces of Ti2CS2 remain stable at T = 300 K. While, according to voltage calculations, Zr2CS2 can store Na up to three atomic layers, our MD simulations predict that the outermost layers detach from the Zr2CS2 monolayer due to the weak interaction between Na ions and the monolayer. This suggests that MD simulations are essential to confirm the stability of an ion-electrode system – an insight that is mostly absent in previous studies.

Abstract: In this study, it is predicted by density functional theory calculations that graphene-like novel ultra-thin phases of manganese fluoride crystals, that have nonlayered structures in their bulk form, can be stabilized by fluorination of manganese dichalcogenide crystals. First, it is shown that substitution of fluorine atoms with chalcogens in the manganese dichalcogenide host lattice is favorable. Among possible crystal formations, three stable ultra-thin structures of manganese fluoride, 1H-MnF2, 1T-MnF2 and MnF3, are found to be stable by total energy optimization calculations. In addition, phonon calculations and Raman activity analysis reveal that predicted novel single-layers are dynamically stable crystal structures displaying distinctive characteristic peaks in their vibrational spectrum enabling experimental determination of the corresponding phases. Differing from 1H-MnF2 antiferromagnetic (AFM) large gap semiconductor, 1T-MnF2 and MnF3 single-layers are semiconductors with ferromagnetic (FM) ground state.

Abstract: The absence of a direct-to-indirect band gap transition in ReS2 when going from the monolayer to bulk makes it special among the other semiconducting transition metal dichalcogenides. The functionalization of this promising layered material emerges as a necessity for the next generation technological applications. Here, the structural, electronic, and magnetic properties of substitutionally doped ReS2 monolayers at either the S or Re site were systematically studied by using first principles density functional calculations. We found that substitutional doping of ReS2 depends sensitively on the growth conditions of ReS2. Among the large number of non-metallic atoms, namely H, B, C, Se, Te, F, Br, Cl, As, P. and N, we identified the most promising candidates for n-type and p-type doping of ReS2. While Cl is an ideal candidate for n-type doping, P appears to be the most promising candidate for p-type doping of the ReS2 monolayer. We also investigated the doping of ReS2 with metal atoms, namely Mo, W, Ti, V. Cr, Co, Fe, Mn, Ni, Cu, Nb, Zn, Ru, Os and Pt. Mo, Nb, Ti, and V atoms are found to be easily incorporated in a single layer of ReS2 as substitutional impurities at the Re site for all growth conditions considered in this work. Tuning chemical potentials of dopant atoms energetically makes it possible to dope ReS2 with Fe, Co, Cr, Mn, W, Ru, and Os at the Re site. We observe a robust trend for the magnetic moments when substituting a Re atom with metal atoms such that depending on the electronic configuration of dopant atoms, the net magnetic moment of the doped ReS2 becomes either 0 or 1 mu(B). Among the metallic dopants, Mo is the best candidate for p-type doping of ReS2 owing to its favorable energetics and promising electronic properties.

Abstract: Nitrogen-doped graphene was recently synthesized and was reported to be a catalyst for hydrogen dissociative adsorption under a perpendicular applied electric field (F). In this work, the diffusion of H atoms on N-doped graphene, in the presence and absence of an applied perpendicular electric field, is studied using density functional theory. We demonstrate that the applied field can significantly facilitate the binding of hydrogen molecules on N-doped graphene through dissociative adsorption and diffusion on the surface. By removing the applied field the absorbed H atoms can be released efficiently. Our theoretical calculation indicates that N-doped graphene is a promising hydrogen storage material with reversible hydrogen adsorption/desorption where the applied electric field can act as a switch for the uptake/release processes.

Abstract: Experiments are supplemented with ab initio density functional theory (DFT) calculations in order to investigate how the structural, electronic and optical properties of zinc oxide (ZnO) thin films are modified upon Cu doping. Changes in characteristic properties of doped thin films, that are deposited on a glass substrate by sol-gel dip coating technique, are monitored using X-ray diffraction (XRD) and UV measurements. Our ab initio calculations show that the electronic structure of ZnO can be well described by DFT+U/G(0)W(0) method and we find that Cu atom substitutional doping in ZnO is the most favourable case. Our XRD measurements reveal that the crystallite size of the films decrease with increasing Cu doping. Moreover, we determine the optical constants such as refractive index, extinction coefficient, optical dielectric function and optical energy band gap values of the films by means of UV-Vis transmittance spectra. The optical band gap of ZnO the thin film linearly decreases from 3.25 to 3.20 eV at 5% doping. In addition, our calculations reveal that the electronic defect states that stem from Cu atoms are not optically active and the optical band gap is determined by the ZnO band edges. Experimentally observed structural and optical results are in good agreement with our theoretical results.

Abstract: The ability of different materials to display self-limiting growth has recently attracted an enormous amount of attention because of the importance of nanoscale materials in applications for catalysis, energy conversion, (opto)-electronics, and so forth. Here, we show that the electrochemical deposition of palladium (Pd) between graphene oxide (GO) sheets result in the self-limiting growth of 5-nm-thick Pd nanosheets. The self-limiting growth is found to be a consequence of the strong interaction of Pd with the confining GO sheets, which results in the bulk growth of Pd being energetically unfavorable for larger thicknesses. Furthermore, we have successfully carried out liquid exfoliation of the resulting Pd-GO laminates to isolate Pd nanosheets and have demonstrated their high efficiency in continuous flow catalysis and electrocatalysis.

Abstract: A periodic spatial modulation, as created by a moire pattern, has been extensively studied with the view to engineer and tune the properties of graphene. Graphene encapsulated by hexagonal boron nitride (hBN) when slightly misaligned with the top and bottom hBN layers experiences two interfering moire patterns, resulting in a so-called supermoire (SM). This leads to a lattice and electronic spectrum reconstruction. A geometrical construction of the nonrelaxed SM patterns allows us to indicate qualitatively the induced changes in the electronic properties and to locate the SM features in the density of states and in the conductivity. To emphasize the effect of lattice relaxation, we report band gaps at all Dirac-like points in the hole doped part of the reconstructed spectrum, which are expected to be enhanced when including interaction effects. Our result is able to distinguish effects due to lattice relaxation and due to the interfering SM and provides a clear picture on the origin of recently experimentally observed effects in such trilayer heterostuctures.

Abstract: We report the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultralow energy implantation of noble gas ions (He, Ne, Ar) into graphene grown on a Pt(111) surface. We show that the universal scaling of the aspect ratio, which has previously been established for larger bubbles, breaks down when the bubble radius approaches 1 nm, resulting in much larger aspect ratios. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element. We interpret these dependencies in terms of the atomic compressibility of the noble gas as well as of the adhesion energies between graphene, the substrate, and trapped atoms.

Abstract: Recent experiments demonstrated that interfacial water dissociation (H2O ⇆ H+ + OH-) could be accelerated exponentially by an electric field applied to graphene electrodes, a phenomenon related to the Wien effect. Here we report an order-of-magnitude acceleration of the interfacial water dissociation reaction under visible-light illumination. This process is accompanied by spatial separation of protons and hydroxide ions across one-atom-thick graphene and enhanced by strong interfacial electric fields. The found photoeffect is attributed to the combination of graphene's perfect selectivity with respect to protons, which prevents proton-hydroxide recombination, and to proton transport acceleration by the Wien effect, which occurs in synchrony with the water dissociation reaction. Our findings provide fundamental insights into ion dynamics near atomically thin proton-selective interfaces and suggest that strong interfacial fields can enhance and tune very fast ionic processes, which is of relevance for applications in photocatalysis and designing reconfigurable materials.

Abstract: Recent experiments have revealed that the macroscopic Kelvin equation remains surprisingly accurate even for nanoscale capillaries. This phenomenon was so far explained by the oscillatory behavior of the solid−liquid interfacial free energy. We here demonstrate thermodynamic and capillarity inconsistencies with this explanation. After revising the Kelvin equation, we ascribe its validity at nanoscale confinement to the effect of disjoining pressure.
To substantiate our hypothesis, we employed molecular dynamics simulations to evaluate interfacial heat transfer and wetting properties. Our assessments unveil a breakdown in a previously established proportionality between the work of adhesion and the Kapitza conductance at capillary heights below 1.3 nm, where the dominance of the work of adhesion shifts primarily from energy to entropy. Alternatively, the peak density of the initial water layer can effectively probe the work of adhesion. Unlike under bulk conditions, high confinement renders the work of adhesion entropically unfavorable.

Abstract: We propose a new system where electron and hole states are electrostatically confined into a quantum ring in bilayer graphene. These structures can be created by tuning the gap of the graphene bilayer using nanostructured gates or by position-dependent doping. The energy levels have a magnetic field (B0) dependence that is strikingly distinct from that of usual semiconductor quantum rings. In particular, the eigenvalues are not invariant under a B0 ¨ −B0 transformation and, for a fixed total angular momentum index m, their field dependence is not parabolic, but displays two minima separated by a saddle point. The spectra also display several anticrossings, which arise due to the overlap of gate-confined and magnetically confined states.

Abstract: An array of quantum rings with local (ring by ring) modulation of the spin orbit interaction (SOI) can lead to novel effects in spin state transformation of electrons. It is shown that already small (3 x 3, 5 x 5) networks are remarkably versatile from this point of view: Working in a given network geometry, the input current can be directed to any of the output ports, simply by changing the SOI strengths by external gate voltages. Additionally, the same network with different SOI strengths can be completely analogous to the Stern-Gerlach device, exhibiting spatial-spin entanglement.