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Carbon Monosulfide Nanostructures: Chains Arrays, Monolayers, And

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Carbon Monosulfide Nanostructures: Chains Arrays, Monolayers, And July 22, 2018 Stable Carbon Monosulfide Nanostructures: Chains Arrays and Monolayers T. Alonso-Lanza,1 F. Aguilera-Granja,1;2 J. W. Gonz´alez1 A. Ayuela1 1Centro de F´ısica de Materiales CFM-MPC CSIC-UPV/EHU, Donostia International Physics Center (DIPC), Departamento de F´ısica de Materiales, Fac. de Qu´ımicas, UPV-EHU, 20018 San Sebasti´an,Spain 2Instituto de F´ısica, Universidad Aut´onomade San Luis de Potos´ı,78000 San Luis Potos´ıS.L.P., M´exico Theoretical predictions have lead to the experimental synthesis of new low dimensional layered structures. Herein we show for the very first time that compounds of carbon monosulfide exhibit a great variety of layered nanostructures, such as chains arrays, monolayers, and thin films. We find that the chains arrays are the most stable because they are mainly dimensionality-driven by the sp2 hybridization of sulfur and carbon orbitals. Furthermore, the chains arrays are direct gap semiconductors. In contrast to thin films, the monolayers are stable at room temperature with a semiconductor phase followed in energy by a metallic phase. Then, we achieve a semiconductor- to-metal phase transition in carbon sulfur monolayers, which can be driven by strain engineering controlling conductivity and carrier mobility. I. INTRODUCTION analyzed to obtain 2D materials with improved stability and new properties30; one of the hottest topic nowadays New phenomena in condensed matter physics are based focuses on phosphorene isoelectronics compounds. on two-dimensional materials. An example is a single There are two different approaches to obtain phos- layer of carbon atoms known as graphene1. Graphene phorene isoelectronic compounds, by taking elements of characterization shows outstanding structural and elec- the group V or by mixing elements of the groups IV-VI tronic properties such as high stiffness or flexibility, and also called group IV monochalcogenides31{36. The phos- massless Dirac fermions with low resistivity2{7, which al- phorene isoelectronic compounds of the group V are ni- low for many applications. The main drawback is the trogene, phosphorene, arsenene37{39 and antimonene39, absence of band gap in the graphene electronic structure and their binary compounds40,41. They are cur- and the small on=off ratio4,8, a band-gap may be opened rently undergoing characterization, for instance, by via different techniques, such as chemical tailoring, in- studying doping42, point defects42,43, and oxygen ducing strains, and geometric patterning9{13. Although contamination44. The group IV monochalcogenides in- graphene looks promising for the future, research is to- cludes a combination of light elements, such as in silicon day focusing on other carbon nanostructured materials monosulfide monolayers and thin films23{25, and a com- and other two-dimensional materials14. bination of heavy (Ge, Sn) and light (O, S) atoms45, such The possibility to exfoliate materials and the ex- as in silicon telluride46 and germanium monosulfide47 istence of accurate experimental techniques to study monolayers, which exhibit strain-tunable indirect band single-layer materials15{17 supposed a big boost to study gap. Nevertheless, none of the today proposed phospho- other layered materials. Layers with other elements in rene isoelectronic compounds is based on carbon. In fact, the carbon group have been studied to produce coun- the layered carbon compounds, such as carbon nitride48 terparts to graphene, such as silicene, germanene or and carbon phosphide49,50, are not isoelectronic to phos- stanene18{21. Graphene is flat because there is sp2 hy- phorene. Our aim is to design new layered nanostructures bridization, and these other compounds adopt a buckled isoelectronic to black phosphorus, but still composed of structure because they prefer sp3 hybridization. Other carbon. layered nanostructures isoelectronic to graphene are ob- We present here for the first time stable carbon mono- tained either combining III-V group elements, such as sulfide nanostructures covering chains, monolayers, and in hexagonal boron nitride22. 2D materials research is thin films. We have performed phonon and molecular recently widen to other new materials based on phos- dynamics calculations to ensure the stability of those arXiv:1703.02756v1 [cond-mat.mtrl-sci] 8 Mar 2017 phorus, whose isoelectronic counterparts are a central structures at room temperature. The carbon monosulfide issue nowadays23{25. Black phosphorus is formed by nanostructures vary from metallics in thin films to semi- single layers mediated by van der Waals interaction, so conductors in chains and most monolayers. The chains that analogously to graphene, few layers are isolated to and thin films are more stable than the monolayers be- form phosphorene26,27. Phosphorene has a two dimen- cause sulfur atoms have two bonds instead of three (or sional honeycomb puckered structure where each atom four) for the monolayers25. The large variety of nanos- is bonded to three neighbors. It displays a direct band tructures that we explored with either metallic or semi- gap, tunable attending to the number of layers and the conductor character makes this material an interesting stacking28,29. compound for several applications based on electronic More and more systems are being proposed and deeply transport within semiconductors devices. 2 II. COMPUTATIONAL DETAILS and electronic structure of the four most stable struc- tures, and finally consider the ground state in more de- We use the SIESTA (Spanish Initiative for Electronic tail. Figure 1 shows four monolayers of carbon mono- Simulations with Thousands of Atoms) package to carry sulfide after structural relaxation including their relative out density functional theory (DFT) calculations for car- energies. The most stable α monolayer presents a simi- bon monosulfide nanostructures. For the exchange and lar structure to that of a single layer of black phosphorus. correlation potentials we used the generalized gradient This structure has also been reported recently for group 23,31 approximation (GGA) in the form of Perdew-Burke- IV monochalcogenides , and group V semiconductors 40,41,63 Ernzenhof51. We fully relax atomic positions and unit such as phosphorous-nitrogen or arsenic-nitrogen . cells until forces are below 0.006 eV/A.˚ The used unit The results obtained with VASP method yield energy cells have large vectors about 24.5 A˚ in the perpendic- differences slightly larger, although the stability order is ular direction to the layers in order to avoid interaction the same. Second in energy we find the δ monolayer with images. An electronic temperature of 25 meV and that is a regular structure composed of squares looked a meshcutoff of 250 Ry are used. The sampling of the from above. A side view reveals two different heights for Brillouin zone has a fine grid of 20×20×1 k points. The sulfur atoms. Next there is a similar structure, the κ atomic cores are described by nonlocal norm-conserving structure, with all the carbon atoms shifted towards one Troullier-Martins52 pseudopotentials factorized in the side producing long and short distances with the sulfur Kleynman-Bylander form. Orbitals are developed in ba- atoms. When the distortion is maximized the α structure sis sets on each atom, and the basis size is double zeta is reached. The κ layer seems an intermediate structure plus polarization orbitals. Details on the used pseu- between the α and δ monolayers, so that the three mono- dopotentials and basis were described previously9,25,53. layer monosulfide-carbon structures seems related, to be Anyhow our main results are checked by repeating cal- discussed at length in next section III A 2. Phonon dis- culations with the VASP code using the projected aug- persions, shown in Supplemental Information, indicate mented wave method (PAW) and within the PBE formal- that the α and δ structures are really stable, and the κ ism for the exchange and correlation54,55, and the Quan- structure is unstable because there are negative frequen- tum ESPRESSO method with equivalent parameters56. cies. Furthermore, molecular dynamics simulations show We estimate the van der Waals contribution to the to- that the α and δ structures are stable even at room tem- tal energy for a two dimensional array with well sepa- perature. Last we find the β structure which from a top rated chains using the implementation within the VASP view it displays a hexagonal pattern, and in a side view it code57,58 of the vdW-DF59 functional. Last but not has a buckled structure, reported for other phosphorene 23 least, we even confirm the stability of such highly stable isoelectronic compounds . The β structure is similar to 64,65 obtained structures by carrying out molecular dynam- that of blue phosphorene . The phonon dispersion ics calculations within the SIESTA code using the Nose curves show that the carbon monosulfide β phase is un- thermostat at room temperature. We employed a Nose stable. mass60{62 of 10 Ry fs2, and a time step of 1 fs. We have To gain insight on the electronic structure of the car- chosen 3000 as final time step and the relaxation time to bon monosulfide monolayers we consider the band struc- reach target temperature was 2500 fs. Phonon dispersion tures shown in Fig. 2. On the one hand, three of the curves are obtained using the Quantum ESPRESSO monolayers, labeled as α, κ and β, are semiconductors code, with details included in the Supplemental Informa- with indirect band gaps of 0.97, 0.77 and 1.64 eV, re- tion. spectively. For the α and κ layers, the minimum of the conduction band is located at the gamma point. For the α case, the highest occupied valence band is well sepa- III. RESULTS AND DISCUSSION rated from lower valence bands with a pseudogap of 2 eV. The κ structure has still a pseudogap opening in the We study three different types of carbon monosulfide valence band, although the value is reduced to 0.5 eV. nanostructures, monolayers, thin films, and chains ar- On the other hand, the δ structure is metallic.
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