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Nourozi Bromand, Boochani Arash, Abdolmaleki Ahmad, Sartpi Elmira, Darabi Pezhman, Naderi Sirvan. Ti Impurity Effect on the Optical Coefficients in 2D Cu2Si: A DFT Study. Communications in Theoretical Physics, 2018, 69(1): 101  
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Ti Impurity Effect on the Optical Coefficients in 2D Cu2Si: A DFT Study
Nourozi Bromand1, †, Boochani Arash1, Abdolmaleki Ahmad2, Sartpi Elmira1, Darabi Pezhman1, Naderi Sirvan3
Department of Physics, Islamic Azad University, Kermanshah Branch, Kermanshah, Iran
Department of Physics, Lorestan University, Khoramabad, Iran
Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran

 

† Corresponding author. E-mail: bromanddalaho@yahoo.com

Abstract
Abstract

The electronic and optical properties of 2D Cu2Si and Cu2Si:Ti are investigated based on the density functional theory (DFT) using the FP-LAPW method and GGA approximation. The 2D Cu2Si has metallic and non magnetic properties, whereas adding Ti impurity to its structure changes the electronic behavior to the half-metallic with 3.256μB magnetic moment. The optical transition is not occurred in the infrared and visible area for the 2D Cu2Si in x-direction and by adding Ti atom, the real part of dielectric function in the x-direction, Re (ε(ω))x is reached to a Dirac peak at this energy range. Moreover, the absorption gap tends to zero in x-direction of the 2D Cu2Si:Ti.

PACS: ;73.90.+f;;75.70.Ak;;78.20.Ci;
Keyword:2D Cu2Si;Ti impurity;DFT;optical property
1 Introduction

The amazing electronic behavior of graphene leads to a high mobility concluding from the hexagonal sheet structure.[1] Considering the Dirac cone graphene materials, several electronic and spintronic behaviors[2–5] such as ballistic charge transport,[6] high mobility,[7] and quantum Hall effects[8] are appeared. According to some restriction in carbon graphenes, many 2D materials are synthesized and predicted such as Silicene, Germanene,[9] and Ti2B.[10] It is reported that 2D materials with the honeycomb structure have different physical behaviors than those in bulk phase.[11] Hence, it is necessary to find a new class of 2D materials with new electronic, magnetic and optical properties to use in some industrial applications such as optoelectronic, spintronic, and solar cell devices.

After discovery of graphene,[12] new 2D materials have been considered such as BN,[13, 14] tertialy B-C-N,[15] etc. These 2D materials have more unique physical properties than carbon graphene and are very attractive in the electronic and optoelectronic industries. Most of the 2D chemical and physical structures have tri-coordinated motifs, as their graphene structures are illustrated,[16–22] so only a few samples of these materials have been reported by quasiplanar tetra coordinated structure. Hoffman et al., were the first group reporting based on tetra coordinated carbon motifs. In addition, new planar hexa-coordinated structures, such as the 2D boron were predicted by computational studies.[23]

In recent years, making planar hyper coordinate 2D materials has been a big challenge as their compound elements do not mach. The planar hyper coordinate molecules were firstly predicted by Schleger et al.,[24] a class of materials, which are very interesting in scientific and industrial points of view, and also B2C monolayer in the quasi planar hexa-coordinate carbon shape has been recently investigated.[25]

Due to the transition metals applications in the electronic industry, they are even more attractive. The 2D materials based the Cu atom have the interesting electronic and optical treatment. Recently, 2D Cu2Si has been synthesized by laying down the Cu cluster on Si (111),[25] also, as well as ab-initio calculations indicated the good stability of this 2D material. The electronic calculations by Yang et al..[26] have shown the metallic behavior for the 2D Cu2Si. Furthermore, this 2D material has strong chemical bonding between atoms and high implant stiffness. The optical properties of the 2D Cu2Si have been less considered so far, and also according to the metallic behavior of the 2D Cu2Si, it is expected that by absorbing Ti to its structure, the electronic and the optical properties of the 2D Cu2Si:Ti have been improved for optoelectronic and solar cell applications.

Since Ti belongs to the transition metals, there is a good compatibility between its electronic structure and Cu atom, thus Ti is placed instead of Cu in the 2D Cu2Si. By doping Ti into the 2D Cu2Si, the electronic and the optical properties are changed significantly. Additionally, it is noticeable that by adding Ti, magnetic behaviors emerge in the 2D Cu2Si:Ti especially at the Fermi level. In Sec. 2 the computational details are explained and the thermodynamic stability, the electronic and the magnetic properties are calculated in Sec. 3. Finally, the optical properties are reported in Sec. 4.

2 Computational Methods

Calculations are based on the density functional theory (DFT) framework and the FP-LAPW method with the GGA approximations.[27–29] After optimization of the input parameter, the RKmax, Gmax, and KPoint are selected as 8.0, 13.0, 5000, and 8.0, 13.5, 500 for the pure and the impurity cases respectively. The lattice parameters of the 2D Cu2Si hexagonal super lattice are set to a = b = 23.374 (Bohr) and c = 18.897 (Bohr). The structures are relaxed by miniposition command with 1.0 a.u./dyne accuracy, and the optical calculations are performed by the RPA approximation.[30] For better comparison, the 2D Cu2Si and the 2D Cu2Si:Ti shapes are shown in Fig. 1.

Fig. 1 (Color online) (a) The 2D Cu2Si structure, (b) The 2D Cu2Si:Ti lattice by XCrysden code.
3 Results and Discussion
3.1 Electronic and Magnetic Properties

According to the other studies, the 2D Cu2Si has not good metallic treatment due to a few electron states at the Fermi level and lack of the magnetic property.[25–26] In the electronic discussions, the DOS diagram shows important information about metallic or non-metallic and magnetic or non-magnetic properties of matter. Figure 2(a) illustrates the 2D Cu2Si DOS at up and down spins indicating a completely isotropic behavior at the all energies below and above the Fermi level confirming the non-magnetic property of this compound, which is in good agreement with the other works.[25–26]

Fig. 2 (Color online) (a) Total 2D Cu2Si DOS versus energy, (b) Total 2D Cu2Si:Ti DOS versus energy, (c) Total DOS of 2D Cu2Si:Ti at Fermi area.

Considering Ar3 and Ar electronic structures of Ti and Cu atoms respectively, they are different in 4s and 3d orbitals so that Cu-3d is occupied while two electrons are located in Ti-3d orbital. It is clear that the Ti atoms produce the magnetic moment in the 2D Cu2Si:Ti compound, so the electronic calculations are based on spin polarization. The DOS diagram of the 2D Cu2Si:Ti is shown in Fig. 2(b) in the up and down spins; it is clear that they have different behaviors especially at the Fermi level. It is shown that the DOS in both mentioned spins at the range of −10 eV to −2.5 eV have a completely isotropic behavior, but at the Fermi area [–1 eV, 1 eV], it shows an anisotropic behavior (see Fig. 2(c)) claimed to the magnetic properties of 2D Cu2Si:Ti. So, the total and interstitial magnetic moments of the 2D Cu2Si:Ti are listed in Table 1. Moreover, the cohesive energy (EC) represents that the 2D Cu2Si:Ti has a good thermodynamic stability (Table 1) confirmed by Ti-Si bond length, which is less than the Cu-Si one.

Table 1

The bond length of 2D Cu2Si in pure and impurity cases (Å) with Cohesive energy (Ec), Atomic bond length (Å) and Total magnetic moment ().

.
3.2 Optical Properties

The optical properties are described by the response of matter to incident light leading to the electronic structure such as the energy band gap, the conductivity, the semiconductor behavior and the glassiness. The 2D Cu2Si and Cu2Si:Ti have two symmetric directions x and z, where x is on the plane of the structure and z is perpendicular to it. The real part of the dielectric function ) of the 2D Cu2Si is shown in Fig. 3 for the two mentioned directions of the incident photon. After 10 eV of the photon energy, the in x and the z directions are completely similar and tend to a constant value about 1, therefore, at this energy range, the 2D Cu2Si response to incident light has not been changed. However, in the infrared and visible regions, their behaviors are completely different so the static value of in the x and z directions are 2 and screaming value, respectively indicating the metallic behavior in the x direction. At (0–2) eV and (4–10.5) eV, the sign is negative while the in the vicinity of 9 eV is negative. Hence, no optical transitions occurred at these energy intervals. In addition, several roots are appeared at 3 eV and 9 eV for the and 8.5 eV and 9.5 eV for the , which by comparing the and the Eloss curve, the Plasmon frequencies have been confirmed at the mentioned energies.

Fig. 3 (Color online) Real part of dielectric function for x and z directions for 2D Cu2Si and 2D Cu2Si with Ti impurity versus photon energy.

The Real part of dielectric function of the 2D Cu2Si:Ti along x and z directions are shown in Fig. 3, representing the metallic and semiconductor behaviors in both directions, and the static values of the real part of the dielectric function are infinitive and 2.2 for z and x directions, respectively. By increasing photon energy, the is considerably increased and dramatically decrease at +1 eV (infrared region), respectively, so it has two roots in this region indicating the metallic treatment in the infrared region, and getting incident photons at low energies, the surface electrons are vibrated causing the Plasmon frequencies. The reaches to a constant and small value in the UV area indicating no response to the incident photon energy.

A quick look at the graph indicates lack of sensible fluctuations in the visible area, but a moderate loop in the is observed in Fig. 3. The reaches a plateau after the two roots (4.4 eV and 5.5 eV) in the UV edge showing low response of the 2D Cu2Si:Ti to incident light at the mentioned energy range. In contrast, the has a different behavior in UV area so that by increasing photon energy, a significant escalation is observed while there are two roots at the energy range of 9 eV to 10 eV.

It is noticeable that from 1 eV to 2.2 eV and from 9 eV to 10 eV have negative sign confirming the occurrence of no optical transition at the mentioned energy ranges for the 2D Cu2Si:Ti compound. The imaginary part of the dielectric function ) illustrates the electronic structure of matter. Each peak of the represents the electron transition from an occupied to an unoccupied level. Comparing the diagrams of Fig. 3, it is clear that adding Ti to the 2D Cu2Si causes no significant change in the z direction, while in x direction, owing to its high fluctuations in lower energy ((0–2) eV), the metallic behavior is observed confirming the intraband transition.

The for the coplanar and the perpendicular components (x and z) of the 2D Cu2Si are illustrated in Fig. 4. From zero to 10 eV, they are extremely anisotropic so that the static amount of tends to infinity. By increasing the photon energy in the visible area, the is considerably dropped confirming the root and the first Plasmon frequency in this area. Moreover, the high amount of the in the infrared region indicates the high metallic behavior and the intraband transitions.

Fig. 4 (Color online) The Imaginary part of dielectric function of 2D Cu2Si & 2D Cu2Si:Ti versus photon energy in the x & z directions.

Afterward, the second main peak of is appeared on the UV edge (4 eV), but no peak exists for the before 2 eV confirming the semiconductor behavior in the optical view. It has three peaks in the UV region, each peak indicates the electron transition from occupied to unoccupied levels (from Cu-p, Cu-d and Ti-d to Cu-d and Ti-d orbitals) causing the intraband or interband transitions.

The of the 2D Cu2Si:Ti graphene is shown in Fig. 4 for the x and the z directions. The optical anisotropy of this parameters for the mentioned directions is observed so that the reaches its highest peak at 0 eV to 4 eV, and it is decreased by increasing the photon energy. Another small peak is seen at 4.2 eV (UV edge). However, the has a completely different behavior so that it is started at 4.2 eV and reached its highest peak at 5.7 eV, then two smaller peaks occurred in the area of 8 eV to 10 eV.

It is noticeable that the all curves in both directions are leveled out to zero after fluctuations at 12 eV, and after this energy the incident light completely transmits in both directions. On the visible edge (2 eV), the tends to infinity indicating the high metallic behavior and intraband transition in this direction. The different response of the 2D Cu2Si:Ti to the incident photons in the x and z directions indicates a promising optical sensor for electronic and optoelectronic devices. It should be mentioned that by adding Ti to the 2D Cu2Si, the second peak of the is decreased and in contrast, the first peak is increased showing a greater tendency to the metallic properties in the 2D Cu2Si:Ti.

The energy loss function diagram (Eloss) involves important information about the optical properties such as absorption, refraction and Plasmon frequency. Figure 5 shows the 2D Cu2Si and the 2D Cu2Si:Ti energy loss function curves in the x and z directions. As it is clear from the diagram, the most electron transitions belong to the x direction, so the Eloss-x is occurred at lower energies than the Eloss-z. Each Eloss peak indicates the energy loss of the incident light lead to the Plasmon frequency or the light refraction. For example, the Eloss-x of the imaginary case has a peak at 4 eV equivalent to root and significant reduction of the reflectivity at this energy. It is concluded that the Plasmon frequency occurs at 4 eV photon energy, and also several peaks have been observed in from 6 eV to 8 eV. However, no loss of the incident light is occurred at the energy range of 0 eV to 5 eV, which by comparing with the , the lack of any electron transition at the mentioned energy range have been seen and the has a relatively constant behavior. At 6 eV photon energy, the first Eloss-z peak is occurred and the absorption and the reflection of the are decreased while conductivity is increased. Hence, this Eloss peak can not be a sign for the Plasmon frequency. The major Eloss-x and -z peaks are at 10 eV and 11 eV, respectively. By comparing the and the curves, it is concluded that the volume Plasmon frequency area is located at this energy range, which the optical conductivity diagram confirms the above points in Fig. 5. Comparing the pure and the impurity cases of 2D Cu2Si in Fig. 5 indicates that there is no peak until 2.5 eV unlike the 2D Cu2Si:Ti. The first Eloss-x peak in pure case is occurred at 2.5 eV that is smaller than the impurity case, and the main Eloss-x peak is located at 11 eV, which is greater than the impurity case. In the z direction, the maximum energy loss function is nearly half of the x direction in maximum case.

Fig. 5 (Color online) Eloss function of 2D Cu2Si and (:Ti) cases versus photon energy in x and z directions.

The optical conductivity is resulted from the electron transitions to the conduction bands caused by the incident radiation to matter including the important electronic and the optical information about the material. The optical conductivity in the x and z directions are in agreement with the above discussions. The optical conductivity of the 2D Cu2Si in the x and z directions are shown in Fig. 6 and show high conducting property at zero energy along the x direction with a gradually reduction at 2.5 eV. While the Plasmon frequency is occurred at this photon energy, the next peak occurs at 5 eV. Moreover, no conduction is observed until 3 eV in the z direction, and in the UV region (6 eV, 8 eV, and 9 eV) three peaks are seen. By increasing the photon energy after the main Plasmon frequency, the conductivity for both mentioned axes are decreased and tend to zero.

Fig. 6 (Color online) 2D Cu2Si, 2D Cu2Si:Ti conductivity in x and z directions versus photon energy.

Figure 6 indicates a sharp reduction in the optical conductivity at 1 eV area for the x direction confirming the Plasmon frequency at this energy (compatible with diagram). Furthermore, the optical conductivity reduces gradually on the visible edge and reaches a plateau in the UV area by increasing the photon energy. A brief like in optical conductivity on z direction shows that there is no conductivity before 2 eV confirming the semiconductor behavior and it experiences a steady improvement on the visible edge to reach a maximum at 5.5 eV, which is in agreement with the curve. By adding the Ti impurity to the 2D Cu2Si, the conductivity behavior in the z direction remains nearly the same as the pure case, but the static conductivity is finite and its maximum occurs in the infrared region. An important note is that the conductivity increases to a constant amount at higher energies (Fig. 6).

The absorption peaks of the 2D Cu2Si are presented in Fig. 7. In the both x and z directions, absorption gaps are about 2.5 eV and then it is started the mentioned directions, respectively. The maximum absorption is at 10 eV photon energy, and the anisotropy behavior is clear from 2.5 eV to 11 eV. After 25 eV, the absorption drops to zero, i.e. the light is completely transmitted through the matter.

Fig. 7 (Color online) Absorption function of 2D Cu2Si in pure and impurity cases versus photon energy in the in-plane and normal directions.

The 2D Cu2Si:Ti absorption diagram in the x and z directions are shown in Fig. 7. The absorption diagram in the x direction is increased immediately by photon energy, thus the 2D Cu2Si:Ti has a good metallic behavior. The absorption diagram experiences a moderate escalation and reaches two higher peaks in the visible and UV area, and then it is decreased by increasing the photon energy to reach a smaller constant value. Conversely, there is no absorption in infrared region for the z direction (an absorption gap). The absorption-z reaches a maximum at 5.5 eV, while the and the have minimum and maximum, respectively. Two peaks are also located at the energy range of 9 eV to 9.7 eV indicating the main Plasmon frequencies.

Reflection parameter is an important optical parameter for matter representing the percentage of the light transmission and reflection in the material surfaces; metals have high reflection (about 90%), for example shining matters and non-metals are opaque. The 2D Cu2Si has complete reflection at zero energy in the x direction, but it is nearly zero for the z direction indicating the different behaviors for the two mentioned directions so that the light is fully transmitted in the direction perpendicular to 2D Cu2Si surface, and completely reflected in the coplanar state. By increasing the photon energy, the reflectivity in the z direction is dropped significantly at 3.5 eV (the visible area) to reach its second maximum at 4.7 eV. Figure 8 shows that for all energy ranges, the reflection in the z direction is smaller than the x one confirming the high metallic behavior of the optical point of view.

Fig. 8 (Color online) Reflection index of 2D Cu2Si & 2D Cu2Si:Ti versus photon energy in x & z directions.

It is clear that the static amount of the reflection for the 2D Cu2Si:Ti in the x direction is 90% confirming its metallic behavior, whereas this compound shows the semiconductor behavior in the z direction due to a low static reflectivity (Fig. 8). By increasing the intensity of the incident light, where has roots, a gradual reduction in the reflection on the z direction is observed. The 2D Cu2Si:Ti reflection in the x direction reaches a plateau (zero) after 10 eV and the optical incident light is transmitted completely. Moreover, this phenomenon is similarly occurred in the z direction.

4 Conclusion

In summary, calculations of the Ti impurity effect on the 2D Cu2Si are performed based on the DFT framework by the GGA approximation. In the pure case, the DOS diagrams cut the Fermi level and do not have magnetic moment, but the 2D Cu2Si:Ti has half-metallic behavior with the magnetic moment of . Furthermore, the Ti effect causes changing in the optical parameters, especially in the infrared and the visible area.

In the pure case, no optical transition occurred in the x direction in the infrared and the visible areas, but in the impurity case, it has a Dirac peak in the infrared area. The major electron transitions are occurred from the Cu-d and the Ti-d to the S-p and the Cu-d orbitals. The absorption gap is reduced to zero in the x direction and is sensitive to the incident light, but it is relatively the same in the pure and the impurity cases in the z direction. According to the Ti atomic structure, the 2D Cu2Si:Ti has lower metallicity in zero energy, whereas its optical conductivity and reflectivity are smaller than the 2D Cu2Si case.

Owing to the semi-metallic behavior of Cu2Si:Ti at the Fermi level, it is expected that this compound will be used in the spintronics and giant magnetoresistance (GMR). In addition, by adding Ti to the 2D structure of Cu2Si, the light absorption gap along xx has been vanished, and an increase in the light absorption can be observed in the infrared and visible regions making this composition suitable for solar cells and optical devices.

Reference
[1] Wallace P. R. Phys. Rev. 71 1947 622
[2] Ma F. Jiao Y. Gao G. et al. Nano Lett. 16 2016 3022
[3] Wang A. J. Li H. Huang H. Qian Z. S. Feng Jiu-Ju J. Mater. Chem. C 4 2016 8146
[4] Mhamane D. et al. J. Mater. Chem. A 4 2016 5578
[5] Nosolov K. S. et al. Nature (London) 438 2005 197
[6] Bliokh Y. P. Freilikher V. Nori F. Phys. Rev. B 87 2013 245134
[7] Castro Neto A. H. et al. Rev. Mod. Phys. 81 2009 109
[8] Zhang Y. Tan Y. W. Stormer H. L. Kim P. Nature (London) 438 2005 201
[9] Cahangirov S. Topsakal M. Akturk E. et al. Phys. Rev. Lett. 102 2009 236804
[10] Zhou X. F. Dong X. Oganov A. R. et al. Phys. Rev. Lett. 112 2014 085502
[11] Li X. Yu S. Wu S. et al. J. Phys. Chem. C 117 2013 15347
[12] Noselov K. S. Geim A. K. Morozov S. V. et al. Science 306 2004 666
[13] Noselov K. S. Jiang D. Schedin F. et al. Proc. Natl. U.S.A. 102 2005 10451
[14] Jin C. Lin F. Suenaga K. Iijima S. Phys. Rev. Lett. 102 2009 195505
[15] Ci L. Song L. Jin C. et al. Nat. Mater 9 2010 430
[16] Xu M. Liang T. Shi M. Chen H. Chem. Rev. 113 2013 3766
[17] Zhang Sheng-Li Yan Zhong. Li Ya-Fei et al. Angew. Chem. Int. Ed. 54 2015 3112
[18] Khromova I. Navarro-Ca M. Brener I. et al. Appl. Phys. Lett. 107 2015 022102
[19] Zhang Sheng-Li Xie Mei-Qiu Li Feng-Yu et al. Angew. Chem. Int. Ed. 55 2016 1666
[20] Zhang Sheng-Li Xie Mei-Qiu Cai Bo et al. Phys. Rev. B 93 2016 245303
[21] Zhang Sheng-Li Guo Shi-Ying Huang Ya-Xin et al. 2D Materials 4 2016 015030
[22] Zhang Sheng-Li Zhou Wen-Han Ma Yan-Dong et al. Nano Lett. 17 2017 34343440
[23] Tang H. Ismail-Beigi S. Phys. Rev. Lett. 99 2007 115501
[24] Yang L. M. Ganz E. Chen Z. et al. Angew. Chem. Int. ed. 53 2014 7248
[25] Saranin A. A. Zotov A. V. Uta O. A. et al. Surf. Sci. 603 2009 2874
[26] Yang L. M. Basic V. Popov I. A. et al. JACS 137 2015 2757
[27] Blaha P. Schwarz K. Sorantin P. Tricky S. B. Comput. Phys. Commun. 59 1990 399
[28] Blaha P. Schwarz K. Madesen G. K. H. et al. WIEN2K an Augmented Plane Wave Local Orbitals Program for Calculating Crystal Properties Karlheinz Schwarz, Techn. Universitaetwien Wien, Austria 2001
[29] Perdew J. Chevary J. A. Vosko S. H. et al. Phys. Rev. B 46 1992 6671
[30] Kronig R. L. J. Opt. Soc. Am. 12 1926 547