A Burlaka

A Peigney

A. V. Strungar

B Aufray

B Lalmi

DC Rapaport

I. V. Кomarov

J Tersoff

O. V. Mykhailenko

OV Mykhailenko

R Quhe

S Dorfman

SM Aghaei

TM Radchenko

VF Korolovych

Yu I. Prylutskyy

Nanoscale Research Letters

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NANO EXPRESS Open AccessStructure and Thermal Stability of Co- andFe - Intercalated Double Silicene LayersO. V. Mykhailenko1*, Yu I. Prylutskyy1, I. V. Кomarov1 and A. V. Strungar2AbstractThe arrangement of Fe and Co atoms between two silicene planes was theoretically investigated. The research hasshown that below 500 K Co atoms form stable lattices—“hexagonal” (with the lattice parameters 0.635 nm of ABconfiguration) and cubic (with the lattice parameter of 0.244 nm), whereas Fe atoms form cubic lattices only (withthe lattice parameter of 0.281 nm). The system intercalated with Co atoms is stable enough at high temperaturesup to ~625 K, while the Fe-silicene system is stable only at ~770 K. The silicene UV-spectrum depending on theintercalate concentration and association constant of the “silicene-intercalate” system was calculated.Keywords: Double-layer silicene, Intercalated Fe and Co atoms, Thermal stability, PM3 and Monte-Carlo simulationsBackgroundSilicene is a newly discovered material that is 1-au layerthin [1–3]. It is a two-dimensional (2D) nanomaterialthat is classified as a nanosheet, which has large lateraldimensions up to micrometres, but thickness of severalnanometres or less. The unique properties and morph-ology of such materials make them ideal for a variety ofapplications, including electronic devices, batteries andsensors. 2D nanosheet of silicene can be considered asan analogue of graphene [3].Opening a sizable band gap without degrading its highcarrier mobility is as vital for silicene to its applicationas a high-performance field-effect transistor [4].Besides, the specific surface area of nanomaterials canplay one of the key roles on their application. So, thecarbon nanotubes (CNTs) distinguish themselves amongother nanocarbon structures by a very large specificsurface area, the value of which is in the range of 50–1315 m2/g [5]. The developed surface area of CNTsreflects on the properties of liquid systems with addingCNTs [6] and by changing the thermodynamic parame-ters. There is a possibility of enhancing the bioactivity ofCNT by carboxyl group functionalization [7]. Theseexamples motivate the use of double-layer silicene asfiller in combination with Fe and Co atoms.Inspired by recent successes in the development of 2Dgas-based sensors capable of single gas molecule detec-tion, the adsorption of gas molecules (N2, NO, NO2, NH3,CO, CO2, CH4, SO2 and H2S) on silicene nanoribbons(SiNRs) were investigated using the density functional the-ory (DFT) and nonequilibrium Green’s function (NEGF)methods [8]. The most stable adsorption configurations,adsorption sites, adsorption energies, charge transfer,quantum conductance modulation and electronic proper-ties of gas molecules on SiNRs have been studied. Theresults indicate that NO, NO2 and SO2 are chemisorbedon SiNRs via strong covalent bonds, suggesting its poten-tial application for disposable gas sensors.As the identification of structure-to-property relations isan important task of 2D chemistry and material physics[9, 10], the aims of this work were to study the structureof intercalated Fe and Co atoms double-layer silicene sys-tems under heating by the MM+, PM3 and Monte-Carlo,to calculate the UV-spectrum of silicene layers dependingon intercalate concentration as well as to determine theassociation constant of the “silicene-intercalate” system.MethodsOur main object of interest, a double-layer silicene, wasmodelled with 32 silicon atoms in each plane. Thedistance between the planes was chosen to be 0.3855 nm(we selected the AB… order of silicon atoms between theplanes). Double-layer silicene intercalation is produced viaintroduction of the intercalate atoms into the interlayer* Correspondence: alexm-@ukr.net1Taras Shevchenko National University of Kyiv, Volodymyrska Str., 64, 01601Kyiv, UkraineFull list of author information is available at the end of the article© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.Mykhailenko et al. Nanoscale Research Letters  (2017) 12:110 DOI 10.1186/s11671-017-1874-6space. As stated above, our goal was to study the systemof two silicene planes, with Co or Fe atoms intercalatedbetween them. Of particular interest are the positions ofthe metal atoms that are relative to each other and thesilicon atoms’ mutual positioning of the silicene planes inthe presence of the metal intercalates (with or withoutkeeping the AB… order of silicon atoms between theplanes), as well as the stability of the system upon thermaltreatment [11, 12].In the model considered, the interaction potential(Leonard-Jones potential) between metal atoms (see Eq. 1)directly mated the pair potential of high energy of atomicexcitation [13] and it was described by the Born-Mayerequation within 0–0.68 nm of effective interaction radius(see Eq. 2)U rð Þ ¼ 4ε σr 12−σr 6 ð1Þwhere r is distance between particle centres, ε is depth ofpotential pit, and σ is distance at which the interactionenergy is equal to zero (parameters ε and σ characteriseatoms of corresponding substances);U rð Þ ¼ A exp −r=bð Þ ð2Þwhere A, b is constants, for every pair of colliding particles,and r is distance between interacting particles.To describe the atom interaction at a distance smallerthan 0.2 nm, we have used the Tersoff-Brenner potentialof interatomic interaction [14]. Total potential energy ofsystem U is expressed as a sum of bonding energies forall pairs of atoms forming this systemU ¼XiXj>iV R rij −BijV R rij h i ð3Þwhere rij is distance between i and j atoms, VR(r) is theexponential function included into the Morse potentialtype which corresponds to the energies of attraction andrepulsion between the atoms, and Bij* is the functionexpressing the dependence of binding energy of the iand j atoms from the angles θijk between the bond i–jand close bonds i–k and j–k.To describe the atom interaction at a distance greaterthan 0.21 nm, we have employed the Tersoff-Brennerpotential of interatomic interaction [14] along with theZiegler-Biersack-Litmark pair potential [13]. The lengthof Si–Si bonds in a silicene layer was 0.220 nm, and theinteraction of the Co and Fe atoms with silicene wasdescribed by the Lennard-Jones pairwise potential withthe energy of 0.112 and 0.110 eV for the Co-silicene andFe-silicene systems, respectively. The modelled period ofone excitation cascade was 2 ps, and the energy conserva-tion law in every calculation cycle was correlated within0.15%. The initial coordinates of intercalate were selectedin conformity with the law of random numbers.To do the task above, the following numerical modellingscheme was used:(1)the first calculation stage was based on the MM+molecular mechanics method;(2)the second stage was based on the semi-empiricalPM3 method. It should be noted that the maindistinction of this method from the others is theirdifferent parameterisations. In our case, PM3method was parameterised to the best match ofcalculated and experimental molecule formationheats;(3)the third stage was based on the Monte-Carlomethod.To calculate the association constant of the “silicene-intercalate” complex formed, the modified Benes-Hilderbrand method [15] that accounts the data onmaximum silicene absorption values at various intercal-ate concentrations in the UV-spectra was employed.Results and DiscussionIt was found that at 273 K, Co atoms, when placed be-tween two silicene planes, form a stable quasi-hexagonallattice with the following lattice parameters 0.635 nm(Fig. 1). At 625 K, however, the hexagonal Co lattice (orFig. 1 Calculated optimal geometric model of the Co-silicene system: (a, b) orthogonal projections and (c) side viewMykhailenko et al. Nanoscale Research Letters  (2017) 12:110 Page 2 of 5the α-structure) is transformed into β-structure—Coatoms forming a cubic lattice with the lattice parameter of0.244 nm. The silicene planes do not move relatively eachother, and the order of the silicon atoms between theplanes remains the same–AB… (akin to the single-crystalsilicene). However, when heated above 770 K, the distancebetween the silicene planes grows to 0.645 nm (ABconfiguration).At 273 K, Fe atoms placed between the two siliceneplanes form a stable cubic lattice with the lattice parameterof 0.281 nm. The silicene planes do not move relativelyeach other and the order of the silicon atoms between theplanes remains the same–AB configuration. The Fe-silicene system turned out to be sufficiently heat resistantin a wide temperature range (up to 770 K) compared tothe Co-silicene system (up to only 625 K). Deformationaloscillations of the silicene lattice did not exceed 0.014 nm,and those of the Co and Fe lattices, 0.020 and 0.021 nm re-spectively, suitable for configurational and conformationalstability of the system.Apparently, the Co cubic lattice is more sensitive tothe temperature changes compared to the Fe lattice.Energy dependence of the Fe- and Co-silicene systemsversus temperature is shown in Fig. 2.Fig. 2 Energy dependence of the Fe-silicene (1) and Co-silicene (2) systems versus temperatureFig. 3 Dependence of silicene absorption with intercalate added in Benes-Hilderbrand coordinates: Fe-silicene system (1) and Co-silicene system (2)Mykhailenko et al. Nanoscale Research Letters  (2017) 12:110 Page 3 of 5One can see that in the range of 273–625 K, theenergy of Co-silicene system (Fig. 2, curve 2) graduallydecreases, but just above 625 K, it increases sharply.This behaviour is consistent with the transformation ofthe α-structure into the β-structure due to the abruptenergy absorption by the system. Further temperatureincrease causes only gradual disintegration of the cubicCo lattice.At the same time, the energy of Fe-silicene system(Fig. 2, curve 1) slightly decreases when heated from 273to ~625 K, and then, with further temperature increase,it reaches a plateau indicating its high resistance totemperature up to ~770 K. When being heated from 273to 625 K for Co and 273 to 575 K for Fe, the metallattice becomes somewhat deformed, opposite (relativeto the symmetry centre) sites move apart. This trans-formation is approximately equivalent to the shift of oneplane (two atoms) along the main axis of the fourthorder by 0.006 nm for Co and by 0.005 nm for Fe.Increasing the temperature further by ~50–75 K, onecan observe greater configurational changes of the Co-silicene system, which correspond to allotrope transitionsin Co lattice. At ~625–725 K, a gradual disintegration ofthe Co lattice is observed. Yet, as the lattice structurebreaks down, the interplane distance between the silicenelayers remains the same (within the proper range of latticeoscillations). The behaviour of the Fe-silicene systemshows certain distinct features.Finally, theoretical calculations of UV-absorption spec-tra of the Fe- and Co-silicene systems depending on theintercalate concentration (Fe and Co) in terms of themodified Benes-Hilderbrand method show that the asso-ciation constant is 6.745 l · mol-1 for Fe-silicene systemand 3.26 l · mol-1 for Co-silicene system (Fig. 3).ConclusionsCalculations elucidating arrangement of Co and Featoms between two silicene planes have been made. Inparticular, we demonstrated that at 273 K, Co atomsform a hexagonal lattice, heat resistance up to 625 K.Above this temperature interval, the lattice transformsinto a cubic one due to allotropic changes. In the Fe-silicene system, Fe atoms form a cubic lattice, thermallyresistant up to 770 K.Above 725 K for the Co-silicene system and above770 K for the Fe-silicene system, there is a gradualbreakdown of the metal lattices consistent with theorder-disorder phase transition. This, in all likelihood,may produce significant changes in the physical andchemical properties of both the intercalated metal and thesystem as a whole. Concurrently, the distance between thesilicene planes was practically unchanged for both Co-and Fe-intercalated systems.Differences in the behaviour of Co- and Fe-silicenesystems stem from the decrease of the atomic radius inthe Fe–Co row of the periodic table, which is caused byd-compression, i.e., by the increasing attraction of thens-electrons to the nucleus with the increasing charge ofthe latter. This, in turn, yields a decrease in chemicalactivity of the metals in the Fe–Co row.Silicene planes can be considered as inorganic li-gands for sandwich structures (similar to ferrocenes).d-compression of the electron shells of Co, as comparedto Fe, is responsible for the difference in properties ofthose metals, in particular, the decreasing affinity of Co toaromatic and quasi-aromatic structures (such as silicene).Thus, Co interaction with silicon is weaker than that ofFe. As a result, while intercalated with Co and Fe, theorder of silicon atoms between the silicene planes remainsthe same, i.e., AB configuration.Abbreviations2D: Two-dimensional; CNT: Carbon nanotube; DFT: Density functional theory;NEGF: Nonequilibrium Green’s function; SiNRs: Silicon nanoribbons; UV-spectrum: Ultraviolet-visible spectrum; MM+: Molecular mechanics method;PM3: Parameterised model number 3—semi-empirical methodAuthors’ ContributionsAll authors read and approved the final manuscript.Competing InterestsThe authors declare that they have no competing interests.Author details1Taras Shevchenko National University of Kyiv, Volodymyrska Str., 64, 01601Kyiv, Ukraine. 2Vernadsky National Library of Ukraine, Holosiivskyi ave., 3,03039 Kyiv, Ukraine.Received: 15 November 2016 Accepted: 27 January 2017References1. Aufray B, Kara A, Vizzini S, Oughaddou H, Landri C, Ealet B, Le Lay G (2010)Graphene-like silicon nanoribbons on Ag(110): a possible formation ofsilicone. 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Mat Sci Eng 15:191–201Submit your manuscript to a journal and beneﬁ t from:7 Convenient online submission7 Rigorous peer review7 Immediate publication on acceptance7 Open access: articles freely available online7 High visibility within the ﬁ eld7 Retaining the copyright to your article    Submit your next manuscript at 7 springeropen.comMykhailenko et al. Nanoscale Research Letters  (2017) 12:110 Page 5 of 5

Structure and Thermal Stability of Co- and Fe - Intercalated Double Silicene Layers

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Structure and Thermal Stability of Co- and Fe - Intercalated Double Silicene Layers

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