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Structure and Thermal Stability of Co and Fe  Intercalated Double Silicene Layers
Nanoscale Research Letters volume 12, Article number: 110 (2017)
Abstract
The arrangement of Fe and Co atoms between two silicene planes was theoretically investigated. The research has shown that below 500 K Co atoms form stable lattices—“hexagonal” (with the lattice parameters 0.635 nm of AB configuration) and cubic (with the lattice parameter of 0.244 nm), whereas Fe atoms form cubic lattices only (with the lattice parameter of 0.281 nm). The system intercalated with Co atoms is stable enough at high temperatures up to ~625 K, while the Fesilicene system is stable only at ~770 K. The silicene UVspectrum depending on the intercalate concentration and association constant of the “siliceneintercalate” system was calculated.
Background
Silicene is a newly discovered material that is 1au layer thin [1–3]. It is a twodimensional (2D) nanomaterial that is classified as a nanosheet, which has large lateral dimensions up to micrometres, but thickness of several nanometres or less. The unique properties and morphology of such materials make them ideal for a variety of applications, including electronic devices, batteries and sensors. 2D nanosheet of silicene can be considered as an analogue of graphene [3].
Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene to its application as a highperformance fieldeffect transistor [4].
Besides, the specific surface area of nanomaterials can play one of the key roles on their application. So, the carbon nanotubes (CNTs) distinguish themselves among other nanocarbon structures by a very large specific surface area, the value of which is in the range of 50–1315 m^{2}/g [5]. The developed surface area of CNTs reflects on the properties of liquid systems with adding CNTs [6] and by changing the thermodynamic parameters. There is a possibility of enhancing the bioactivity of CNT by carboxyl group functionalization [7]. These examples motivate the use of doublelayer silicene as filler in combination with Fe and Co atoms.
Inspired by recent successes in the development of 2D gasbased sensors capable of single gas molecule detection, the adsorption of gas molecules (N_{2}, NO, NO_{2}, NH_{3}, CO, CO_{2}, CH_{4}, SO_{2} and H_{2}S) on silicene nanoribbons (SiNRs) were investigated using the density functional theory (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 properties of gas molecules on SiNRs have been studied. The results indicate that NO, NO_{2} and SO_{2} are chemisorbed on SiNRs via strong covalent bonds, suggesting its potential application for disposable gas sensors.
As the identification of structuretoproperty relations is an important task of 2D chemistry and material physics [9, 10], the aims of this work were to study the structure of intercalated Fe and Co atoms doublelayer silicene systems under heating by the MM+, PM3 and MonteCarlo, to calculate the UVspectrum of silicene layers depending on intercalate concentration as well as to determine the association constant of the “siliceneintercalate” system.
Methods
Our main object of interest, a doublelayer silicene, was modelled with 32 silicon atoms in each plane. The distance between the planes was chosen to be 0.3855 nm (we selected the AB… order of silicon atoms between the planes). Doublelayer silicene intercalation is produced via introduction of the intercalate atoms into the interlayer space. As stated above, our goal was to study the system of two silicene planes, with Co or Fe atoms intercalated between them. Of particular interest are the positions of the metal atoms that are relative to each other and the silicon atoms’ mutual positioning of the silicene planes in the presence of the metal intercalates (with or without keeping the AB… order of silicon atoms between the planes), as well as the stability of the system upon thermal treatment [11, 12].
In the model considered, the interaction potential (LeonardJones potential) between metal atoms (see Eq. 1) directly mated the pair potential of high energy of atomic excitation [13] and it was described by the BornMayer equation within 0–0.68 nm of effective interaction radius (see Eq. 2)
where r is distance between particle centres, ε is depth of potential pit, and σ is distance at which the interaction energy is equal to zero (parameters ε and σ characterise atoms of corresponding substances);
where 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 smaller than 0.2 nm, we have used the TersoffBrenner potential of interatomic interaction [14]. Total potential energy of system U is expressed as a sum of bonding energies for all pairs of atoms forming this system
where r _{ ij } is distance between i and j atoms, V _{ R } (r) is the exponential function included into the Morse potential type which corresponds to the energies of attraction and repulsion between the atoms, and B _{ ij } * is the function expressing the dependence of binding energy of the i and j atoms from the angles θ _{ ijk } between the bond i–j and close bonds i–k and j–k.
To describe the atom interaction at a distance greater than 0.21 nm, we have employed the TersoffBrenner potential of interatomic interaction [14] along with the ZieglerBiersackLitmark pair potential [13]. The length of Si–Si bonds in a silicene layer was 0.220 nm, and the interaction of the Co and Fe atoms with silicene was described by the LennardJones pairwise potential with the energy of 0.112 and 0.110 eV for the Cosilicene and Fesilicene systems, respectively. The modelled period of one excitation cascade was 2 ps, and the energy conservation law in every calculation cycle was correlated within 0.15%. The initial coordinates of intercalate were selected in conformity with the law of random numbers.
To do the task above, the following numerical modelling scheme was used:

(1)
the first calculation stage was based on the MM+ molecular mechanics method;

(2)
the second stage was based on the semiempirical PM3 method. It should be noted that the main distinction of this method from the others is their different parameterisations. In our case, PM3 method was parameterised to the best match of calculated and experimental molecule formation heats;

(3)
the third stage was based on the MonteCarlo method.
To calculate the association constant of the “siliceneintercalate” complex formed, the modified BenesHilderbrand method [15] that accounts the data on maximum silicene absorption values at various intercalate concentrations in the UVspectra was employed.
Results and Discussion
It was found that at 273 K, Co atoms, when placed between two silicene planes, form a stable quasihexagonal lattice with the following lattice parameters 0.635 nm (Fig. 1). At 625 K, however, the hexagonal Co lattice (or the αstructure) is transformed into βstructure—Co atoms forming a cubic lattice with the lattice parameter of 0.244 nm. The silicene planes do not move relatively each other, and the order of the silicon atoms between the planes remains the same–AB… (akin to the singlecrystal silicene). However, when heated above 770 K, the distance between the silicene planes grows to 0.645 nm (AB configuration).
At 273 K, Fe atoms placed between the two silicene planes form a stable cubic lattice with the lattice parameter of 0.281 nm. The silicene planes do not move relatively each other and the order of the silicon atoms between the planes remains the same–AB configuration. The Fesilicene system turned out to be sufficiently heat resistant in a wide temperature range (up to 770 K) compared to the Cosilicene system (up to only 625 K). Deformational oscillations of the silicene lattice did not exceed 0.014 nm, and those of the Co and Fe lattices, 0.020 and 0.021 nm respectively, suitable for configurational and conformational stability of the system.
Apparently, the Co cubic lattice is more sensitive to the temperature changes compared to the Fe lattice. Energy dependence of the Fe and Cosilicene systems versus temperature is shown in Fig. 2.
One can see that in the range of 273–625 K, the energy of Cosilicene system (Fig. 2, curve 2) gradually decreases, but just above 625 K, it increases sharply. This behaviour is consistent with the transformation of the αstructure into the βstructure due to the abrupt energy absorption by the system. Further temperature increase causes only gradual disintegration of the cubic Co lattice.
At the same time, the energy of Fesilicene system (Fig. 2, curve 1) slightly decreases when heated from 273 to ~625 K, and then, with further temperature increase, it reaches a plateau indicating its high resistance to temperature up to ~770 K. When being heated from 273 to 625 K for Co and 273 to 575 K for Fe, the metal lattice becomes somewhat deformed, opposite (relative to the symmetry centre) sites move apart. This transformation is approximately equivalent to the shift of one plane (two atoms) along the main axis of the fourth order by 0.006 nm for Co and by 0.005 nm for Fe.
Increasing the temperature further by ~50–75 K, one can observe greater configurational changes of the Cosilicene system, which correspond to allotrope transitions in Co lattice. At ~625–725 K, a gradual disintegration of the Co lattice is observed. Yet, as the lattice structure breaks down, the interplane distance between the silicene layers remains the same (within the proper range of lattice oscillations). The behaviour of the Fesilicene system shows certain distinct features.
Finally, theoretical calculations of UVabsorption spectra of the Fe and Cosilicene systems depending on the intercalate concentration (Fe and Co) in terms of the modified BenesHilderbrand method show that the association constant is 6.745 l · mol^{1} for Fesilicene system and 3.26 l · mol^{1} for Cosilicene system (Fig. 3).
Conclusions
Calculations elucidating arrangement of Co and Fe atoms between two silicene planes have been made. In particular, we demonstrated that at 273 K, Co atoms form a hexagonal lattice, heat resistance up to 625 K. Above this temperature interval, the lattice transforms into a cubic one due to allotropic changes. In the Fesilicene system, Fe atoms form a cubic lattice, thermally resistant up to 770 K.
Above 725 K for the Cosilicene system and above 770 K for the Fesilicene system, there is a gradual breakdown of the metal lattices consistent with the orderdisorder phase transition. This, in all likelihood, may produce significant changes in the physical and chemical properties of both the intercalated metal and the system as a whole. Concurrently, the distance between the silicene planes was practically unchanged for both Co and Feintercalated systems.
Differences in the behaviour of Co and Fesilicene systems stem from the decrease of the atomic radius in the Fe–Co row of the periodic table, which is caused by dcompression, i.e., by the increasing attraction of the nselectrons to the nucleus with the increasing charge of the latter. This, in turn, yields a decrease in chemical activity of the metals in the Fe–Co row.
Silicene planes can be considered as inorganic ligands for sandwich structures (similar to ferrocenes). dcompression of the electron shells of Co, as compared to Fe, is responsible for the difference in properties of those metals, in particular, the decreasing affinity of Co to aromatic and quasiaromatic structures (such as silicene). Thus, Co interaction with silicon is weaker than that of Fe. As a result, while intercalated with Co and Fe, the order of silicon atoms between the silicene planes remains the same, i.e., AB configuration.
Abbreviations
 2D:

Twodimensional
 CNT:

Carbon nanotube
 DFT:

Density functional theory
 NEGF:

Nonequilibrium Green’s function
 SiNRs:

Silicon nanoribbons
 UVspectrum:

Ultravioletvisible spectrum
 MM+:

Molecular mechanics method
 PM3:

Parameterised model number 3—semiempirical method
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Mykhailenko, O.V., Prylutskyy, Y.I., Кomarov, I.V. et al. Structure and Thermal Stability of Co and Fe  Intercalated Double Silicene Layers. Nanoscale Res Lett 12, 110 (2017). https://doi.org/10.1186/s1167101718746
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DOI: https://doi.org/10.1186/s1167101718746
Keywords
 Doublelayer silicene
 Intercalated Fe and Co atoms
 Thermal stability
 PM3 and MonteCarlo simulations