Significant nonlinear optical response of alkaline earth metals doped beryllium and magnesium oxide nanocages

Journal Pre-proof Significant nonlinear optical response of alkaline earth metals doped beryllium and magnesium oxide nanocages Naveen Kosar, Saman Gul, Khurshid Ayub, Ali Bahader, Mazhar Amjad Gilani, Muhammad Arshad, Tariq Mahmood PII:

S0254-0584(19)31318-5

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122507

Reference:

MAC 122507

To appear in:

Materials Chemistry and Physics

Received Date: 22 December 2018 Revised Date:

28 July 2019

Accepted Date: 29 November 2019

Please cite this article as: N. Kosar, S. Gul, K. Ayub, A. Bahader, M.A. Gilani, M. Arshad, T. Mahmood, Significant nonlinear optical response of alkaline earth metals doped beryllium and magnesium oxide nanocages, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122507. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Significant nonlinear optical response of alkaline earth metals doped beryllium and magnesium oxide nanocages Naveen Kosar1, Saman Gul2, Khurshid Ayub1, Ali Bahader2, Mazhar Amjad Gilani3, Muhammad Arshad4, Tariq Mahmood*1 1) Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad-22060, Pakistan 2) Department of Chemistry, Hazara University Mansehra, Pakistan 3) Department of Chemistry, COMSATS University, Lahore Campus, Lahore, Pakistan. 4) Institute of Chemistry, University of the Punjab, Lahore-54590, Pakistan.

*To whom correspondence can be addressed: E-mail: [email protected] (T. M)

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Abstract The electronic and non-linear optical (NLO) properties of alkaline earth metals (Be, Mg, Ca) doped Be12O12 and Mg12O12 nanocages were investigated. For each nanocage, both endo- as well as exo-hedral doping of alkaline earth metals were studied. Interaction energies and geometric parameters were calculated to understand the structure and stability of Be12O12 and Mg12O12 nanocages. Exohedral doping was favored (exothermic) over the endohedral (endothermic) doping, as reflected by counterpoise corrected energy values. The HOMO-LUMO gaps were effectively lowered for all doped Be12O12 and Mg12O12 nanocages. The first hyperpolarizability was significantly enhanced (compared to pure nanocage) by alkaline earth metals doping on Be12O12 and Mg12O12 nanocages. The maximum value of hyperpolarizability (2.38×104 au) was observed for calcium encapsulated Mg12O12 (endo-Ca@Mg12O12) nanocage. Two-level model was used to rationalize the trend of first hyperpolarizability (βo) values. Βvec. of all the complexes is in good agreement with the βo.

Keywords: Oxide nanocage; Density functional theory; First hyperpolarizability; Two-level model

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1. Introduction The design and synthesis of new non-linear optical (NLO) materials is an area of immense scientific investigations [1–4]. These materials have applications in dynamic image processing, optical computing, optical data storage, optical communication, sensing, microfabrication and cancer therapy [5–9]. In opto-electronic devices, the applied electric field changes the refractive index of the NLO materials. The work on NLO materials flourished in early 1970s when dipolar compounds were the main focus of study. These molecules were quite important because they could be more polarized in one direction than the other. Recently, interest is diverted on designing novel and highly efficient organic and inorganic NLO materials. A number of strategies has been proposed to design high performance NLO materials. For organic systems, electron donor and acceptor species connected through π conjugation has successfully been applied to enhance NLO response [10–13]. A very well-known example reported in the literature involving push-pull mechanism, is 4-(N,Ndimethylamino)-4՛ -nitrostilbene (DANS) (Fig. 1). The electronic communication between electron donor and acceptor is responsible for increase in the first hyperpolarizability value, which is directly proportional to the NLO response of any compound [13] .

Figure 1: Structure of 4-(N,N-dimethylamino)-4-nitrostilbene Another strategy to increase the first hyperpolarizability of organic compounds is addition of transition metal where charge transfer from metal to organic compound increases the first 3

hyperpolarizability value of the system [14]. Recently, introduction of excess electrons has emerged as an excellent strategy to enhance the NLO response of organic and inorganic systems. These excess electrons have tendency to modify the electronic as well as geometric properties of a system, which ultimately increases the hyperpolarizability values. Doping of alkali and alkaline earth metals is reported in the literature to produce excess electrons in a system and improve the NLO response [15–20]. Electrides [21,22], alkalides [23,24] and alkaline-earthides [25], are the most common classes of excess electrons compounds with remarkable non-linear optical response. Recently, alkaline earth metals oxides such as BeO and MgO have emerged as wurtzite insulators due to iconicity of Be-O and Mg-O bond. Moreover, these oxides have large band gap, very high melting points and high thermal conductivity [26–32]. Ren et al. studies (BeO)n clusters of different sizes through density functional theory (DFT) by applying genetic algorithm and concluded that Be12O12 nanocage is the most stable structure [33]. Shakerzdeh et al. studied the endohedral and exohedral interactions of alkali metals with Be12O12 and Mg12O12 nanoclusters using DFT calculations. They observed that exohedrally doped systems were thermodynamically more stable with large first hyperpolarizability (βo) than the endohedral analogues [27]. Masoomeh et al. examined the scandium doping on the Be12O12, Mg12O12 and Ca12O12 nanoclusters. As a result of Sc doping, the energy gap (Eg) was decreased which resulted in large polarizability and first hyperpolarizability. They observed a monotonic behavior of increasing hyperpolarizability with increasing atomic number i.e. Be < Mg < Ca [34]. Shamlouei and his co-workers theoretically investigated the effects of BF3, BF4, BCl3, AlF3, AlCl3, AlBr3, BeF3, GaF3, GaCl3, GaBr3, NO3, BS2, BSO, BO2, F2, PF5, PCl5, and ASF5 molecules adsorption on the structural, electronic, and optical (linear, nonlinear) properties of Mg12O12 nanocage.

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Their results showed that BeF3@Mg12O12 nanocluster has the highest first hyperpolarizability value (βo ≈ 5775 au) [35]. DFT studies were carried out to investigate the effect of superalkalis M3O (M = Li, Na, K) doping on the electronic and optical properties of Be12O12 nanocage. The results illustrated that K3O@Be12O12 nanocluster showed the highest first hyperpolarizability (βo ≈ 214000 au). The TD-DFT calculations revealed that the high first hyperpolarizabilities was due to the lower transition energy [36]. Asghar et al; through density functional theory methods, investigated the effect of alkali metal oxides MnO (M = Li, Na, K; n = 2, 3, 4) doping on the geometric, electronic, linear and nonlinear optical properties of the Mg12O12 nanocage. Static first hyperpolarizability (βo) of the nanocage increased under the influence of the alkali metal oxides, and the highest first hyperpolarizability (βo ≈ 600,000 au) was observed for K3O@Mg12O12, orders of magnitude higher than the pure Mg12O12 nanocage [37]. Previously, we performed density functional theory calculations to explore the changes in the electronic properties of Mg12O12 nanocluster through exohedral doping with nickel. HOMO-LUMO gap was decreased with remarkable increases in the polarizability (αo) and first hyperpolarizabilities (βo) of MgO [38]. Wang et al. reported that hyperpolarizability of alkaline earth metal doped Be(NH3)nM (M = Be, Ca) complexes (β-105 au) is remarkably higher than the analogous alkali metal doped Li(NH3)nNa(n = 1–3) complexes [39]. Similarly, Nandi and his co-workers demonstrated that doping of alkaline earth metals on NLi3 complexes results in enhancement of their first hyperpolarizability. The reason is the conversion of sp3 hybridization of ammonia to sp2 hybridization and as a result the 2pz orbital of NLi3 molecule becomes electron acceptor from the alkaline earth metals.[40]. These reports reflect the role of excess electrons on alkaline earth metals in efficiently improving first hyperpolarizability of nanomaterials. Keeping in mind the efficiency of alkaline earth metals doping, we intend to study Be12O12 and Mg12O12 nanocages

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for NLO response. Be12O12 and Mg12O12 nanocages are excellent substitute for high performance NLO material, as revealed from alkali metal doping.

2. Computational methods

All calculations were performed by using GAUSSIAN 09 software [41]. Geometries were visualized by using GaussView 5.0 [42]. The Dell server (computing hardware) was used for all types of simulations. This computer was an Intel (R) Xenon (R) CPU X5690@ 3.47 GHz (2 processors). The installed memory (RAM) was 32.0 GB on a 64-bit operating system. The structures of bare and doped Be12O12 and Mg12O12 nanocages were optimized at B3LYP/631G(d) level of theory. The true optimization was confirmed through frequency analysis (no imaginary frequency is observed). All the considered geometries were optimized without symmetry constraints. B3LYP is a reliable functional for accurately predicting the geometries [43–48]. Interaction energies of stable complexes (endohedral and exohedral) with alkaline earth metals were calculated with the help of following equation (Eq. 1). Eint. = Ecomplex - (E(nanocage)+ EM)

……………….

Eq. (1)

Where, Ecomplex is the electronic energy of the complex between alkaline earth metal and nanocage. E(nanocage) and EM are the energies of free nanocage and the alkaline earth metal respectively. Additionally, the interaction energies were corrected for basis set superposition error (BSSE). The BSSE arises in the complexes where two species interact with each other through weak van der Waals forces. At shorter distance, the idle basis functions of one specie (dopant) interact with the basis function of other specie (nanocage) and results in BSSE. Boys 6

and Bernardi proposed CP method for the correction of such error. The counterpoise corrected energies were calculated by: Ecp = Eint. - EBSSE

………………. Eq. (2)

Ecp represented counterpoise corrected energy. Whereas, Eint. and EBSSE terms represent interaction energy and energy corrected for basis set superposition error, respectively. Literature reveals that the Eint. at B3LYP/6-31G(d) and Ecp at B3LYP-CP/6-31G(d) are almost comparable [49,50]. Molecular electrostatic potentials (MEP) were studied to explore the reactive sites (Nucleophilic and electrophilic sites) in the newly formed complexes. MEP was calculated by following mathematical equation: V r = ∑

ZA RA -r

-

ρ(r') r' -r

dr'

……………….

Eq. (3)

Σ represented the summation on the whole complex, ZA represents the atomic charge of complex A, RA is its distance and ρ(r') represents the electronic density function of the complex. TDOS (Total Density of States) graphics were generated by using Gauss sum software. The polarizability (αo) and first hyperpolarizability (βo) were calculated at CAM-B3LYP/6-31G(d) level of theory. CAM-B3LYP provides best agreement between efficiency and quality which leads to more accurate values of hyperpolarizability. It is known as best method for calculating NLO properties and its accuracy is comparable to the ab initio method (couple cluster) [51]. For analyzing the crucial excited states, TD-DFT analysis was also executed at long range corrected coulomb attenuating CAM-B3LYP method with 6-31G(d) basis set. Because the dominant part of the NLO response of doped systems is attributed to hyperpolarizability and to charge transfer which require proper treatment of intramolecular long rang excitation. Therefore, CAM-B3LYP

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functional provides better treatment of excited states and non-linear optical properties calculations [52]. The mathematical equation for polarizability is given as; αo = 1/3(αxx, αyy, αzz)

……………….

Eq. (4)

First hyperpolarizability, known as nonlinear optical response and wascalculated by the following equation; βo= [(βxxx + βxyy + βxzz)2 + (βyyy + βyzz + βyxx)2 + (βzzz + βzxx+ βzyy)2]1/2 ………………. Eq. (5) To gain deep insight into the variational trend of first hyperpolarizability in alkaline earth metal doped Be12O12 and Mg12O12 nanocages, two-level model was also implemented. According to the two-level model: βo ≈ ∆µ × fo / ∆E3

………………. Eq. (6)

∆µ = Difference of dipole moments between ground state and crucial excited state (energy state having the largest oscillator strength) fo = Largest oscillator strength ∆E = Crucial transition energy Eq. 6 clearly indicates that first hyperpolarizability (βo) is inversely proportional to the third power of excitation energy (∆E3). βvec (static first hyperpolarizability) is projection of βo on dipole moment vector µ and related to electric field induced second harmonic generation (EFISH). The βvec provides crucial information about the polarization mechanism. βvec was calculated for all complexes by using the following mathematically expression;

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βvec = ∑i

µi β i |µ|

i=x, y, z .................. Eq. (7)

where µi is dipole moment along the direction i of ground state, |µ| is the total dipole moment, and βi (polarizability along the direction i) can be defined as; βi = (1/3) ∑j (βijj +βjji +βjij )

i, j = {x, y, z}

.................. Eq. (8)

3. Results and discussion 3.1 Structural and optical properties of pure Be12O12 and Mg12O12 nanocages Both Be12O12 and Mg12O12 nanocages have been optimized without any symmetry constraints at B3LYP-6-31G(d) method (Fig. 2). The optimized geometries show that both Be12O12 and Mg12O12 consist of six four membered rings and eight six membered rings. The Be−O bond length in Be12O12 nanocage is 1.58 Å whereas Mg−O bond length in Mg12O12 nanocage is 1.99 Å. Furthermore, Th symmetry is observed in both Be12O12 and Mg12O12 nanocages in isolated form. The pictorial diagrams of pure Be12O12 and Mg12O12 along with their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been shown in Figure 2.

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Figure 2: The optimized geometries and HOMO-LUMO distributions of the pristine Be12O12 and Mg12O12 nanocages.

From HOMO–LUMO diagrams, it is obvious that in cases of Be12O12 and Mg12O12 nanocages, the HOMO orbitals are concentrated over the oxygen atoms of nanocage whereas the LUMO orbitals are spread over the beryllium and magnesium atoms of cages. The obtained energies of HOMO, LUMO and HOMO-LUMO gap (HLG) of both nanocages along with polarizability and hyperpolarizability values are given in Table 1. H-L gaps of pure Be12O12 and Mg12O12 are 8.24 and 4.86 eV, which are in accordance with the previous literature reports [27,53], which reflect the semiconductor (insulator like) behavior of both cages [54]. 10

Table 1: The bond length between two six membered rings (b66, in Å) and the bond length between six membered and four membered rings (b64, in Å), frontier molecular orbital energies (EH and EL, in eV), HOMO-LUMO energy gap (HLG, in eV), polarizability (αo, in au) and hyperpolarizability (βo, in au) values for the pure Be12O12 and Mg12O12 nanocages. Nanocages

b66

b64

EH

EL

HLG

αo

βo

Be12O12

1.52

1.58

-8.52

-0.27

8.24

122.43

0.00

Mg12O12

1.89

1.95

-6.57

-1.71

4.86

225.99

0.00

3.2 Structural and optical properties of Be, Mg and Ca doped Be12O12 and Mg12O12 nanocages Next, the interaction of the alkaline earth metals (Be, Mg and Ca) with Be12O12 and Mg12O12 at endohedral and exohedral sites are investigated in detail. First the encapsulation of alkaline earth metals (endohedral doping) is studied. The obtained local minima geometries of encapsulated metals in both nanocages belong to the C1 point group of symmetry (see 2A–F in Fig. 3). The optimized geometries show that the Ca is relaxed at the center of nanocage, whereas the Be and Mg reside close to surface because of their small atomic sizes. The encapsulation of alkaline earth metals in these nanocages leads to the slight increase in the bond length between two six membered hexagonal rings (b66) and the bond length between six and four membered rings (b64).

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Figure 3: The optimized geometries of endohedral doped Be12O12 complexes i.e. endoBe@Be12O12 (2A), endo-Mg@Be12O12 (2B) and endo-Ca@Be12O12 complexes as well as endohedral doped Mg12O12 complexes i.e. endo-Be@Mg12O12 (2D), endo-Mg@Mg12O12 (2E) and endo-Ca@Mg12O12 (2F) complexes.

The b66/b64 bond length in alkaline earth metal encapsulated Be12O12 nanocage (endoM@Be12O12, M = Be, Mg a 4nd Ca), changes from 1.52/1.58 Å to 1.63/1.67 Å (Be, 2A) and 1.57/1.65 Å (2B, Mg), respectively. In case of alkaline earth metal encapsulated Mg12O12 (endo-M@Mg12O12, M = Be, Mg and Ca), the same bond lengths (b66/b64) are changed to 1.90/1.95 Å for Be encapsulated nanocage (2D) and 1.93/2.06 Å for Mg encapsulated nanocage (2E), respectively. Ca atom is relaxed exactly in the center of both cages (2C and 2F), and therefore it does not have any pronounced effect on the b66/b64 bond lengths, whereas the Be and Mg encapsulated structures are deformed, and metals tend to go towards the surface. The interaction energies (∆E) of alkaline earth metal encapsulated structures of both nanocages are calculated and are listed in 12

Table 2. The encapsulation of alkaline earth metals in Be12O12 and Mg12O12 nanocages is endothermic and energetically non-favorable. The counterpoise corrected energy (Ecp) values for the Be12O12 are more as compared to Mg12O12 nanocage, which indicate that the encapsulation of alkaline earth metals in Be12O12 is more energy demanding (highly endothermic). The Ecp value for endo-Be@Be12O12 (2A), endo-Mg@Be12O12 (2B) and endo-Ca@Be12O12 (2C) are 66.81 kcal mol-1, 158.21 kcal mol-1 and 175.74 kcal mol-1, respectively. In case of endo-M@Mg12O12 (M = Be, Mg and Ca) nanocages (2D-2F), the Ecp values are 14.02 kcal mol-1, 28.23 kcal mol-1 and 21.91 kcal mol-1 for endo-Be@Mg12O12, endo-Mg@Mg12O12 and endo-Ca@Mg12O12, respectively. Table 2: The calculated interaction energy (Eint., in kcal mol-1), counterpoise corrected energy (Ecp, in kcal mol-1) and symmetry (Sym.) of exohedrally and endohedrally alkaline earth metals doped Be12O12 and Mg12O12 nanocages. Endohedral Codes

Complexes

Eint.

ECP

Sym.

2A

endo-Be@Be12O12

61.72

66.81

C1

2B

endo-Mg@Be12O12

153.31

158.21

C1

2C

endo-Ca@Be12O12

171.45

175.74

C1

2D

endo-Be@Mg12O12

11.19

14.02

C1

2E

endo-Mg@Mg12O12

23.85

28.23

C1

2F

endo-Ca@Mg12O12

18.06

21.91

C1

Exohedral 3A

exo-Be@Be12O12

1.25

2.72

C1

3B

exo-Mg@Be12O12

-0.33

0.18

C1

3C

exo-Ca@Be12O12

-3.53

-2.43

C1

3D

exo-Be@Mg12O12

-49.09

-42.23

C1

3E

exo-Mg@Mg12O12

-18.12

-16.64

C1

3F

exo-Ca@Mg12O12

-21.00

-19.83

C1

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From interaction energy results, it is clear that the considered alkaline earth metal encapsulated Be12O12 and Mg12O12 cages are energetically disfavored and more energy demanding. This reflects that alkaline earth metals (Be, Mg and Ca) would prefer to stay outside, at the exterior of the surface of both nanocages. By keeping in view this observation, the adsorption of the alkaline earth metals on the surfaces of the considered nanocages has also been investigated to see the trend of surface adsorption. For this, all possible adsorption sites on the surface i.e. on the top of a hexagonal or square ring, and on the top of b66 or b64 bonds, are considered. In case of exohedral doping of both nanocages, the only structures having the metal on the top of the oxygen (which is the preferred site in case of oxide nanocages) are optimized to local minima [27]. The obtained optimized geometries are displayed in Figure 4. All of the optimized exohedral structures have C1 point group of symmetry.

3.49

2.33

2.54

Be O 3A

3B 1.45

3D

3C

2.31

1.99

3E

Mg Ca

3F

Figure 4: The optimized geometries of exohedral doped Be12O12 complexes i.e. exoBe@Be12O12 (3A), exo-Mg@Be12O12 (3B) and exo-Ca@Be12O12 (3C) complexes as well as

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exohedral doped Mg12O12 complexes i.e. exo-Be@Mg12O12 (3D), exo-Mg@Mg12O12 (3E) and exo-Ca@Mg12O12 (3F) complexes. The vertical distances between the alkaline earth atom and the oxygen (the interaction bond distances) are shorter in Mg12O12 than Be12O12 one. This reflects that considered alkaline earth metals are more efficiently adsorbed on the exterior of Mg12O12 nanocage, which is evident form the interaction energy results as well. It is noted that the interaction energies of alkaline earth metals with Be12O12 are less as compared to the Mg12O12, and this may be due to the large interaction distance in case of Be12O12. Mg-O bond (where electronegativity difference is 2.3) is more polar than the Be-O (where electronegativity difference is 2.0) bond. The electropositive Mg atom increase the polarizability of the oxygen atom, and as a result, the interactions between oxygen and alkaline earth metals are stronger. Exo-M@Mg12O12 (M = Be, Mg and Ca) complexes are thermodynamically more stable than the exo-M@Be12O12 (M = Be, Mg and Ca) complexes. The counterpoise corrected energies (Ecp) of exohedral alkaline earth metal doped nanocages have been summarized in Table 2. The Ecp results reflect that in contrast to the encapsulation of these metals in the both nanocages, adsorption of alkaline earth metals on the surface is energetically favorable (exothermic) except exo-Be@Be12O12. The Ecp value for exoBe@Be12O12 (3A) is 2.72 kcal mol-1, for exo-Mg@Be12O12 (3B) is 0.18 kcal mol-1 and for exoCa@Be12O12 (3C) is -2.43 kcal mol-1, respectively. In case of Mg12O12, the Ecp values are -42.23 kcal mol-1, -16.64 kcal mol-1 and -19.83 kcal mol-1 for exo-Be@Mg12O12 (3D), exoMg@Mg12O12 (3E) and exo-Ca@Mg12O12 (3F), respectively. Form the Ecp results, it is observed that that the interaction of alkaline earth metals on the exterior of Mg12O12 is more favorable in comparison to the Be12O12. The values of Eint. and Ecp are almost similar (Table 2) as observed in previous literature (vide supra).

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Frontier molecular orbitals (FMO’s), HOMO-LUMO gap (HLG) and total density of sates (TDOS) spectra of pure and alkaline earth metal doped Be12O12 and Mg12O12 have been studied thoroughly to investigate the electronic properties. Energies of HOMOs (EH) and LUMOs (EL) along with HLG and % change in HLG of doped cages are given in Tables 3. After alkaline earth metal (Be, Mg and Ca) doping on both nanocages, remarkable decrease in the HLG is observed. One of the reasons for this decrease in HLG is the increase in the energy of HOMOs and decrease in the energy of LUMOs. EH(EL) of endo-Be@Be12O12 (2A), endo-Mg@Be12O12 (2B) and endo-Ca@Be12O12 (2C) are -3.78(-1.69) eV, -2.75(-1.15) eV and -2.41(-1.18) eV, respectively. The HLG of respective encapsulated nanocages (2A-2C) are 2.09 eV, 1.60 eV and 1.23 eV, respectively. Similarly, EH(EL) of endo-Be@Mg12O12 (2D), endo-Mg@Mg12O12 (2E) and endo-Ca@Mg12O12 (2F) are -4.79(-1.36) eV, -3.35(-1.92) eV and -2.80(-1.58) eV, respectively and their HLG values are 3.43 eV, 1.44 eV and 1.22 eV, respectively. A monotonic decrease in HLG is observed in case of alkaline earth metal encapsulated isomers (2A-2F). A similar decrease in HLG with the increase in atomic number has been reported in the literature for alkali metal doping [55]. On the other side, in exohedral alkaline earth metal doped nanocages, exo-M@Be12O12, M = Be, Mg, Ca (3A-3C) show non-monotonic decrease of HLG with respect to atomic number. The HLG first increases down the group and then decreases. This behavior is comparable to those of alkali metal doped aluminum nitride nanocages [17]. The increasing trend of HLG is exo-Mg@Be12O12 (4.23 eV) > exo-Be@Be12O12 (4.09 eV) > exoCa@Be12O12 (2.62 eV). The variation in trend is due to changes in energies of HOMOs (EH) and energies of LUMOs (EL) after exohedral doping on cages. EH(EL)-4.96(-0.87) eV, -5.11(-0.88) eV and -3.73(-1.11) eV for exo-Be@Be12O12 (3A), exo-Mg@Be12O12 (3B) and exo-Ca@Be12O12 (3C), respectively. In case of exo-M@Mg12O12 (M = Be, Mg and Ca), exo-Be@Mg12O12 (3D),

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exo-Mg@Mg12O12 (3E) and exo-Ca@Mg12O12 (3F) complexess, the EH(EL) are -4.08(-1.82) eV, -4.20(-1.83) eV and -3.82(-1.79) eV, respectively.

The trend of decreasing HLG is exo-

Ca@Mg12O12 (2.03 eV) < exo- Be@Mg12O12 (2.16 eV) < exo -Mg@Mg12O12 (2.37 eV). Conclusively, in all doped complexes, endo-Ca@Mg12O12 (2F) complex has high energy of HOMOs and low energy of LUMOs which results in the lowest HLG (1.22 eV). The % drop in HLG is about 70%. Total density of state (TDOS) spectra of pure and doped Be12O12 and Be12O12 nanocages are plotted for better understanding of electronic properties of these complexes (2A-3F). TDOS clearly indicates the formation of new HOMOs in these complexes after interaction of alkaline earth metals with both oxide nanocages. These new HOMOs are concentrated on the metal (M = Be, Mg and Ca) which is a strong indication of excess electrons (see SI1-4). After doping of alkaline earth metals on Be12O12 and Mg12O12 nanocages, the excess electrons may go towards the nanocages. The excess electrons occupy the new HOMOs which have higher energy than the old HOMOs (now becomes HOMO-1), and due to this reason, HLG is reduced in doped systems (as shown in Fig 5). The EH values of all doped complexes (2A-3F) increases, as compared to EH of pure nanocage. Besides EH, the EL of all complexes increases to some extent. Such an increase in the EH and EL values results in the lowering of HLG. Thus, TDOS explicitly interprets the formation of these new HOMOs, which are occupied by excess electrons of alkaline earth metals. Among all complexes, 2F has the lowest HLG due to lower value of EL (-1.57 eV) and higher value (-2.80 eV) of new HOMO level.

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Figure 5: Total density of states (TDOSs) of pristine Be12O12 (a), Mg12O12 (b), 2C (c) and 2F (d) nanocages.

3.3 TD-DFT studies of Be, Mg and Ca doped Be12O12 and Mg12O12 nanocages NLO materials having enhanced first hyperpolarizability (βo) are used in second harmonic generation for frequency doubling. Absorption spectroscopy at TD-DFT method provides information about the absorption maxima (wavelength) of these materials. For this purpose, TDDFT analysis of pure and alkaline earth metals (Be, Mg and Ca) doped Be12O12/Mg12O12 nanocages have been performed (Fig. 6). Absorption maxima (λmax) of all complexes increase with increasing atomic number of alkaline earth metals (dopant). For endohedral alkaline earth metal doped Be12O12, the λmax increases monotonically. The λmax for 2A, 2B and 2C are 505 nm, 781 nm and 1136 nm, respectively. The λmax of 2B and 2C lie in near infra-red region. The λmax of endo-M@ Mg12O12 are comparatively less red shifted. The λmax for 2D, 2E and 2F are 368 nm,

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788 nm and 993 nm, respectively. The λmax for exo-M@ Be12O12 complexes i.e. 3A, 3B and 3C are 255 nm, 283 nm and 442 nm, respectively. The λmax of 3A and 3B lie in UV region. The λmax for 3D, 3E and 3F are 437 nm, 435 nm and 534 nm, respectively. The λmax of all these complexes (3D-3F) lie in visible region. UV-VIS-NIR spectra show the highest λmax of 1136 nm for 2C complex. On the other side, the lowest λmax of 255 nm has been observed for 3A complex. An inverse relationship is observed between HLG and λmax. As the HLG decreases in a series, the λmax increases.

19

Figure 6: UV-VIS-NIR spectra of endohedral (2A-2F at top) and exohedral (3A-3F at bottom) alkaline earth metals doped Be12O12 and Mg12O12 nanocages.

3.4 Molecular electrostatic potentials (MEP) properties of Be, Mg and Ca doped Be12O12 and Mg12O12 nanocages Molecular electrostatic potential (MEP) is an informative property used to analyze and predict the reactive sites of a molecule. MEP is generated on the surfaces of pure and doped complexes which describes the electrophilic and nucleophilic sites. The potentials on the surfaces of complexes are represented by different colors and represented in increasing order of red < orange < yellow < green < blue. The red and blue colors show the nucleophilic and electrophilic sites while green color is used for neutral region of the complex [56]. The MEP surfaces of pure and alkaline earth meta doped nanocages Be12O12 and Mg12O12 have been given in Figure 7. The electrostatic potential mapped over the surfaces of pure and doped nanocages provide an insight of interaction between metal atoms and nanocages. Both nanocages in pure form do not 20

show any red region indicating the absence of nucleophilic sites [57]. This allows the adsorption/encapsulation of alkaline earth metals on/in the nanocages. No particular change in electrostatic potential is observed by comparing the MEP surfaces of pure nanocages and their corresponding endohedral nanocages. It shows a very insignificant charge transfer from metal to nonocage and therefore, MEP surfaces are unchanged. This supports the non-feasibility of endohedral doping as indicated by large positive interaction energy values. In case of exohedral doping, the electrophilic regions (blue or slight blue) are faded (when compared with corresponding pure nanocages) at the regions where alkaline earth metals are doped. This effect is more pronounced in structures 3B and 3C where high charge is transfer from metal to nanocage.

21

Mg12O12

Be12O12

-6.01E-2

2A

-5.58E-2

2B

5.58E-2 -2.18E0

3A

2.55E-2

4.21E-2 -2.55E-2

6.01E-2 -4.21E-2

2D

1.82E-2 -2.12E-2

1.56E-1 -1.82E-2

-1.56E-1

3C

3D

1.68E0

1.76E0 -1.68E0

1.73E0 -1.76E0

-1.73E0

2C

2.18E0

3B

1.69E0

-1.69E0

4.89E2

-4.89E2

2E

2F

2.12E-2 -1.85E-2

3E

1.85E-2

3F

Figure 7: The MEP surfaces of pure and doped Be12O12 and Mg12O12. 3.5 Non-linear optical (NLO) properties of Be, Mg and Ca doped Be12O12 and Mg12O12 nanocages The presence of excess electrons in any system enhances the first hyperpolarizability (βo) value of a system. As a result, this system must have remarkable NLO response to the external applied field [19]. The polarizability (αo) and first hyperpolarizability (βo) of all complexes have been evaluated (Table 3). The trend of first hyperpolarizability of all considered configurations has been shown in Figure 8. αo value of bare Be12O12 is 122 au, which increases after doping of 22

alkaline earth metals. The αo of alkaline earth metal encapsulated Be12O12 nanocages 2A, 2B and 2C are 185 au, 336 au and 852 au, respectively. The exohedrally doped Be12O12 nanocages 3A, 3B and 3C have αo of 161 au, 193 au and 303 au, respectively. The αo value of pure Mg12O12 is 226 au, and after interaction with alkaline earth metals, it further increases to 275 au, 562 au and 342 au for 2D, 2E and 2F, respectively. The αo of 3D, 3E and 3F (exohedrally doped Mg12O12 complexes) are 282 au, 321 au and 425 au, respectively. The highest αo is observed for endoCa@Be12O12 (852 au) complex and lowest αo is obtained for exo-Be@Be12O12 (161 au). Thus, a monotonic trend of increasing polarizability with increasing atomic number is observed in all exohedral and endohedral doped Be12O12 and Mg12O12 nanocages (except 2F complex). Table 3: The frontier molecular orbital energies (EH and EL, in eV), HOMO-LUMO gap (HLG, in eV, % drop in HOMO-LUMO gap (%HLG), Wavelength (λmax, in nm) polarizability (αo, in au) and first hyperpolarizability (βo, in au) values of alkaline earth metals doped Be12O12 and Mg12O12 nanocages. Nanocages

EH

EL

HLG

% HLG

λmax

µo

αo

βo

2A

-3.78

-1.69

2.09

54

505

2.48

185

2.14×103

2B

-2.75

-1.15

1.60

80

781

1.17

336

2.73×103

2C

-2.41

-1.18

1.23

85

1136

0.08

852

1.54×103

2D

-4.79

-1.36

3.43

29

368

0.71

275

2.09×102

2E

-3.35

-1.92

1.44

70

788

1.65

562

8.28×103

2F

-2.80

-1.58

1.22

74

993

1.14

342

4.72×104

3A

-4.96

-0.87

4.09

48

255

3.00

161

5.93×102

3B

-5.11

-0.88

4.23

48

283

0.81

193

2.42×102

3C

-3.73

-1.11

2.62

68

442

3.56

303

1.30×103

23

3D

-4.08

-1.82

2.16

49

437

5.19

282

6.52×103

3E

-4.20

-1.83

2.37

51

435

4.01

321

5.43×103

3F

-3.82

-1.79

2.03

58

534

2.38

425

5.79×103

In the case of alkaline earth metals encapsulated Be12O12 systems, the βo values of 2A, 2B and 2C are 2.14×103 au, 2.73×103 au and 1.54×103 au, respectively. Similarly, the βo values of encapsulated alkaline earth metals Mg12O12 systems (2D-2F) are 2.09×102 au, 8.28×103 au and 4.72×104 au for endo-Be@Mg12O12 (2D), endo-Mg@Mg12O12 (2E) and endo-Ca@Mg12O12 (2F), respectively. The βo values of endo-M@Be12O12 (M = Be, Mg and Ca) reflect the non-monotonic trend, whereas monotonic trend (by increasing the atomic number, βo value increases) is observed for 2D-2F. From first hyperpolarizability results, it is obvious that the encapsulation of the alkaline earth metals of both considered nanocages (Be12O12 and Mg12O12) has strong positive impact on their NLO response. The encapsulation of Ca atom inside the Mg12O12 (2F) leads to the highest first hyperpolarizability value among the considered encapsulated complexes (2A-2F). Such a high first hyperpolarizability value is anticipated due to low ionization potential of Ca, and lowest HLG of 2F. The first hyperpolarizability (βo) show inverse relationship with the HLG. The lower HLG, the higher is βo in 2A-2C and 2D-2F complexes. Next, βo of exohedrally doped Be12O12 and Mg12O12 nanocages has been calculated. The results (Table 3) reflect that the adsorption of alkaline earth metals (Be, Mg and Ca) on the surface of the considered nanocages has significant effect on βo value. The more pronounced increase in βo is observed when alkaline earth metals are doped exohedrally on Mg12O12 nanocage, as compared to Be12O12 nanocage. The pronounced effect may be due to the strong binding of alkaline earth metals on the surface of Mg12O12 (vide supra). In case of surface doped Be12O12 24

complexes (3A-3C), the βo values are 5.93×102 au, 2.42×102 au and 1.30×102 au for 3A, 3B and 3C, respectively. In contrast to 2A-2C complexes, a monotonic decreasing behavior is observed in the 3A-3C. Moreover, βo values of 3D, 3E and 3F are 6.52×103 au, 5.43×103 au and 5.79 ×103 au, respectively. Interestingly, in 3A-3F complexes, βo decreases with increase of atomic number which shows a monotonic decreasing trend. It is observed that remarkable NLO properties are observed in case of alkaline earth metals doped Mg12O12 nanocages. But, the encapsulation of alkaline earth metals in both nanocages can effectively enhance their NLO response in comparison to the surface adsorption on nanocages.

Figure 8: Comparison of hyperpolarizabilities of exohedral and endohedral doped alkaline earth metals on Be12O12 and Mg12O12 nanocages.

3.4 Two-level model calculations (2LM) In order to elucidate the controlling factors of first hyperpolarizability, Oudar and Chemla have proposed a simple link between the βo and a low-lying charge transfer transition through a twolevel model (2LM)) from the complex sum-over-states expression [58,59]. In current study, two 25

level model is implemented to explore these factors (∆µ, fo and ∆E3). In case of endoM@Be12O12 (M = Be, Mg and Ca), two level model results are comparable to the results of βo. βo (2LM) values for 2A, 2B and 2C are 1.32×102 au, 3.96×102 au and 6.60×101 au. For exoM@Be12O12 (M = Be, Mg and Ca) complexes, it is observed that the trend of βo (2LM) do not match with βo. Deep analysis reveal that change in dipole moment is a decisive factor in this series. ∆µ of 3A, 3B and 3C complexes are 0.01 au, 0.00 au and 0.16 au respectively. A direct relationship is observed in ∆µ and βo for 3A-3C complexes. However, for exo-M@Mg12O12 and endo-M@Mg12O12 (M = Be, Mg and Ca) nanocages, excitation energy (∆E3) has been found to be a decisive factor. The ∆E3 values of endo-M@Mg12O12 (M = Be, Mg, Ca) complexes are 1.90×10-3 au, 1.93×10-4 au and 9.65×10-5 au for 2D, 2E and 2F respectively. Further, ∆E3 values of exo-M@Mg12O12 (M = Be, Mg, Ca) complexes are 1.13×10-3 au, 1.15×10-3 au and 6.20×10-4 au for 3D, 3E and 3F, respectively. ∆E3 shows inverse relationship with the βo. In both of these cases (2A-2F and 3D-3F), the trend of (2LM) deviates from βo. βvec. is the projection of hyperpolarizability along the dipole moment vector. All the newly designed (2A-3F) complexes had M-X axes (M = Be, Mg and Ca where X = O) as the principle axes. The dipole moment lies on or close to the x axis in all these complexes. The βvec. values for 2A, 2B, 2C, 2D 2E and 2F are 2.14 × 103 au, 2.73×103 au, 1.01×103 au, 2.09 ×102 au, -9.98×102 au and 4.36×104 au, respectively. The βvec. values of surface doped complexes i.e. 3A, 3B, 3C, 3D, 3E and 3F are 5.93×102 au, -2.41×102 au, 1.30×103 au, 6.52×103 au, -5.42×103 au and 5.79×103 au respectively, which are quite similar to βo values (Table 4). The variational trend of βvec is almost comparable to the βo values in endohedral and exohedral doped M@Mg12O12 and M@Be12O12 (M = Be, Mg, Ca) nanocages. These results reflect that charge transfer is unidirectional and parallel to molecular dipole moment (Table 4).

26

Table 4: The first hyperpolarizability (βo, in au), hyperpolarizability where the vector components are along the dipole moment (βvec., in au), two-level model (βo (2LM ) au), difference in ground state and crucial excited state dipole moment (∆µ, in au) oscillating strength (fo, in au), and crucial excitation energy (∆E3, in au) of alkaline earth metals doped Be12O12 and Mg12O12 nanocages. βo

βvec.

βo (2LM)

∆µ

fo

∆E3

2A

2.14×103

-2.14×103

1.32×102

0.33

0.30

7.36×10-4

2B

2.73×103

2.73×103

3.96×102

0.33

0.24

1.98×10-4

2C

1.54×103

1.01×103

6.60×101

0.01

0.37

6.46×10-5

2D

2.09×102

2.09×102

9.96×101

0.01

0.29

1.90×10-3

2E

8.28×103

-9.98×102

4.69× 10-1

0.02

0.50

1.93×10-4

2F

4.72×104

4.36×104

1.53×103

0.26

0.56

9.65×10-5

3A

5.93×102

5.93×102

3.74×101

0.01

0.40

5.74×10-3

3B

2.42×102

-2.41×102

6.34×101

0.00

0.78

4.19×10-3

3C

1.30×103

1.30×103

1.05×102

0.16

0.72

1.10×10-3

3D

6.52×103

6.52×103

3.11×101

0.24

0.22

1.13×10-3

3E

5.43×103

-5.42×103

7.15×101

0.23

0.35

1.15×10-3

3F

5.79×103

5.79×103

2.91×102

0.32

0.56

6.20×10-4

S. No.

4. Conclusions DFT calculations have been executed on the alkaline earth metals (Be, Mg and Ca) exohedral and endohedral doped Be12O12 and Mg12O12 nanocages to analyze the stability, electronic, linear and nonlinear optical properties. Exohedrally doped nanocages contain alkaline earth metal on the oxygen of these clusters. Encapsulation of alkaline earth metals in both cages is endothermic whereas surface adsorption is exothermic (energetically favorable). The highest Ecp of -42.23 kcal mol-1 has been observed for exo-Be@Mg12O12 nanocage. Doping of alkaline earth metals 27

effectively reduces the HLG for all the considered exohedral and endohedral complexes of Be12O12 and Mg12O12 nanocages. In case of alkaline earth metal doped Be12O12 complexes, the HLG are observed in the range of 4.22-1.60 eV. For alkaline earth metal doped Mg12O12 complexes, the HLG lies in the range of 2.47-1.22 eV. The maximum reduction in HLG is observed for endo-Ca@Mg12O12 (2F). TDOS graphs are plotted which confirm the formation of new HOMO in doped Be12O12 and Mg12O12 nanocages. The first hyperpolarizability values of the exohedrally and endohedrally doped Be12O12 and Mg12O12 nanocages are calculated. The first hyperpolarizability values of M@Mg12O12 (M = Be, Mg and Ca) complexes (encapsulated as well as surface doped) are substantially increased after doping, as compared to M@Be12O12 (M = Be, Mg and Ca) complexes. The maximum βo value calculated for endo-Ca@Mg12O12 is 4.72×104 au.

Furthermore,

the

two-level

model

rationalized

the

results

of

first

hyperpolarizability. The βvec. of all nanocages are in good agreement with βo.

Acknowledgments

Authors acknowledge Higher Education Commission Pakistan (HEC) (Grant no. 3013 and 5309) and COMSATS University, Abbottabad campus for financial support.

Supplementary information

The HOMO-LUMO surfaces, TDOS spectra and cartesian coordinates of all alkaline earth metal doped Be12O12 and Mg12O12 complexes are provided as supplementary information data.

28

References [1]

D.F. EATON, Nonlinear Optical Materials, Science (80-. ). 253 (1991) 281–287.

[2]

S.R. Marder, Large Molecular Third-Order Optical Nonlinearities in Polarized Carotenoids, Science (80-. ). 276 (1997) 1233–1236.

[3]

T. Le Bouder, O. Maury, A. Bondon, K. Costuas, E. Amouyal, I. Ledoux, J. Zyss, H. Le Bozec, Synthesis, Photophysical and Nonlinear Optical Properties of Macromolecular Architectures Featuring Octupolar Tris(bipyridine) Ruthenium(II) Moieties: Evidence for a Supramolecular Self-Ordering in a Dentritic Structure, J. Am. Chem. Soc. 125 (2003) 12284–12299.

[4]

A. Avramopoulos, H. Reis, J. Li, M.G. Papadopoulos, The Dipole Moment, Polarizabilities, and First Hyperpolarizabilities of HArF. A Computational and Comparative Study, J. Am. Chem. Soc. 126 (2004) 6179–6184.

[5]

J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutartre, P. Geneste, P. Bernier, P.M. Ajayan, Application of Carbon Nanotubes as Supports in Heterogeneous Catalysis, J. Am. Chem. Soc. 116 (1994) 7935–7936.

[6]

S.S. Varghese, S. Lonkar, K.K. Singh, S. Swaminathan, A. Abdala, Recent advances in graphene based gas sensors, Sensors Actuators B Chem. 218 (2015) 160–183.

[7]

A.S. Rad, S.S. Shabestari, S. Mohseni, S.A. Aghouzi, Study on the adsorption properties of O 3 , SO 2 , and SO 3 on B-doped graphene using DFT calculations, J. Solid State Chem. 237 (2016) 204–210.

[8]

A.S. Rad, K. Ayub, Ni adsorption on Al 12 P 12 nano-cage: A DFT study, J. Alloys

29

Compd. 678 (2016) 317–324. [9]

Y. Qiang, J. Antony, A. Sharma, J. Nutting, D. Sikes, D. Meyer, Iron/iron oxide core-shell nanoclusters for biomedical applications, J. Nanoparticle Res. 8 (2006) 489–496.

[10]

S. Priyadarshy, M.J. Therien, D.N. Beratan, Acetylenyl-Linked, Porphyrin-Bridged, Donor−Acceptor Molecules: A Theoretical Analysis of the Molecular First Hyperpolarizability in Highly Conjugated Push−Pull Chromophore Structures, J. Am. Chem. Soc. 118 (1996) 1504–1510.

[11]

B. Kirtman, B. Champagne, D.M. Bishop, Electric Field Simulation of Substituents in Donor−Acceptor Polyenes: A Comparison with Ab Initio Predictions for Dipole Moments, Polarizabilities, and Hyperpolarizabilities, J. Am. Chem. Soc. 122 (2000) 8007–8012.

[12]

T.-G. Zhang, Y. Zhao, I. Asselberghs, A. Persoons, K. Clays, M.J. Therien, Design, Synthesis, Linear, and Nonlinear Optical Properties of Conjugated (Porphinato)zinc(II)Based Donor−Acceptor Chromophores Featuring Nitrothiophenyl and Nitrooligothiophenyl Electron-Accepting Moieties, J. Am. Chem. Soc. 127 (2005) 9710– 9720.

[13]

L.R. Dalton, A.W. Harper, B. Wu, R. Ghosn, J. Laquindanum, Z. Liang, A. Hubbel, C. Xu, Polymeric Electro-Optic Modulators: Matereials synthesis and processing, Adv. Mater. 7 (1995) 519–540.

[14]

W.-M. Sun, X.-H. Li, J. Wu, J.-M. Lan, C.-Y. Li, D. Wu, Y. Li, Z.-R. Li, Can Coinage Metal Atoms Be Capable of Serving as an Excess Electron Source of Alkalides with Considerable Nonlinear Optical Responses?, Inorg. Chem. 56 (2017) 4594–4600. 30

[15]

H.L. Tan, C.Y. Jia, C. Xiang, Y.X. Yang, Impact Electrical Property of Alkali Metal Doped ZnO, Adv. Mater. Res. 468–471 (2012) 1501–1507.

[16]

A.A. Peyghan, M. Noei, The alkali and alkaline earth metal doped ZnO nanotubes: DFT studies, Phys. B Condens. Matter. 432 (2014) 105–110.

[17]

M. Niu, G. Yu, G. Yang, W. Chen, X. Zhao, X. Huang, Doping the Alkali Atom: An Effective Strategy to Improve the Electronic and Nonlinear Optical Properties of the Inorganic Al 12 N 12 Nanocage, Inorg. Chem. 53 (2014) 349–358.

[18]

S. Munsif, Maria, S. Khan, A. Ali, M.A. Gilani, J. Iqbal, R. Ludwig, K. Ayub, Remarkable nonlinear optical response of alkali metal doped aluminum phosphide and boron phosphide nanoclusters, J. Mol. Liq. 271 (2018) 51–64.

[19]

G. Yu, X.-R. Huang, W. Chen, C.-C. Sun, Alkali metal atom-aromatic ring: A novel interaction mode realizes large first hyperpolarizabilities of M@AR (M = Li, Na, and K, AR = pyrrole, indole, thiophene, and benzene), J. Comput. Chem. 32 (2011) 2005–2011.

[20]

Maria, J. Iqbal, R. Ludwig, K. Ayub, Phosphides or nitrides for better NLO properties? A detailed comparative study of alkali metal doped nano-cages, Mater. Res. Bull. 92 (2017) 113–122.

[21]

F. Ullah, N. Kosar, K. Ayub, M.A. Gilani, T. Mahmood, Theoretical study on a boron phosphide nanocage doped with superalkalis: novel electrides having significant nonlinear optical response, New J. Chem. 43 (2019) 5727–5736.

[22]

F. Ullah, N. Kosar, K. Ayub, T. Mahmood, Superalkalis as a source of diffuse excess electrons in newly designed inorganic electrides with remarkable nonlinear response and

31

deep ultraviolet transparency: A DFT study, Appl. Surf. Sci. 483 (2019) 1118–1128. [23]

N. Uludağ, G. Serdaroğlu, A. Yinanc, A novel synthesis of octahydropyrido[3,2]carbazole framework of aspidospermidine alkaloids and a combined computational, FTIR, NMR, NBO, NLO, FMO, MEP study of the cis-4a-Ethyl-1-(2hydroxyethyl)2,3,4,4a,5,6,7,11c-octahydro-1H-pyrido[3,2-c]carbazole, J. Mol. Struct. 1161 (2018) 152– 168.

[24]

B.D. Joshi, NBO, chemical reactivity, thermodynamic properties and hyperpolarizability analysis of aristolochic acid II, BIBECHANA. 14 (2016) 86–97.

[25]

J. Hou, Linsheng, Zhu, D. Jiang, J. Qin, Q. Duan, Alkaline-earthide: A new class of excess electron compounds Li-C6H6F6-M (M = Be, Mg and Ca) with extremely large nonlinear optical responses, Chem. Phys. Lett. 711 (2018) 55–59.

[26]

G.F. Fine, L.M. Cavanagh, A. Afonja, R. Binions, Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring, Sensors. 10 (2010) 5469–5502.

[27]

E. Shakerzdeh, E. Tahmasebi, H.R. Shamlouei, The influence of alkali metals (Li, Na and K) interaction with Be12O12 and Mg12O12 nanoclusters on their structural, electronic and nonlinear optical properties: A theoretical study, Synth. Met. 204 (2015) 17–24.

[28]

J. Kakemam, A.A. Peyghan, Electronic, energetic, and structural properties of C- and Sidoped Mg12O12 nano-cages, Comput. Mater. Sci. 79 (2013) 352–355.

[29]

M.B. Javan, Magnetic properties of Mg 12 O 12 nanocage doped with transition metal atoms (Mn, Fe, Co and Ni): DFT study, J. Magn. Magn. Mater. 385 (2015) 138–144.

[30]

H.R. Shamlouei, A. Nouri, A. Mohammadi, A.D. Tehrani, Influence of transition metal

32

atoms doping on structural, electronic and nonlinear optical properties of Mg 12 O 12 nanoclusters: A DFT study, Phys. E Low-Dimensional Syst. Nanostructures. 77 (2016) 48–53. [31]

S. Duman, A. Sütlü, S. Bağcı, H.M. Tütüncü, G.P. Srivastava, Structural, elastic, electronic, and phonon properties of zinc-blende and wurtzite BeO, J. Appl. Phys. 105 (2009) 33719.

[32]

B. Baumeier, P. Krüger, J. Pollmann, Structural, elastic, and electronic properties of SiC, BN, and BeO nanotubes, Phys. Rev. B. 76 (2007) 85407.

[33]

L. Ren, L. Cheng, Y. Feng, X. Wang, Geometric and electronic structures of (BeO) N ( N = 2–12, 16, 20, and 24): Rings, double rings, and cages, J. Chem. Phys. 137 (2012) 14309.

[34]

M. Omidi, H.R. Shamlouei, M. Noormohammadbeigi, The influence of Sc doping on structural, electronic and optical properties of Be12O12, Mg12O12 and Ca12O12 nanocages: a DFT study, J. Mol. Model. 23 (2017) 82.

[35]

A.M. Hesari, H.R. Shamlouei, Influences of adsorptions of some inorganic molecules on electronic, optical, and thermodynamic properties of Mg 12 O 12 nanocage: A computational approach, Chinese Phys. B. 27 (2018) 84210.

[36]

A. Raoof Toosi, H. Reza Shamlouei, A. Mohammadi Hesari, Influence of alkali metal superoxides on structure, electronic, and optical properties of Be 12 O 12 nanocage: Density functional theory study, Chinese Phys. B. 25 (2016) 94220.

[37]

A. Mohammadi Hesari, H.R. Shamlouei, A. Raoof Toosi, Effects of the adsorption of alkali metal oxides on the electronic, optical, and thermodynamic properties of the

33

Mg12O12nanocage: a density functional theory study, J. Mol. Model. 22 (2016) 189. [38]

A.S. Rad, K. Ayub, Nonlinear optical, IR and orbital properties of Ni doped MgO nanoclusters: A DFT investigation, Comput. Theor. Chem. 1138 (2018) 39–47.

[39]

Y.-F. Wang, B. Li, J. Huang, J. Li, Z.-R. Li, Effect of alkaline earth metal atom on the large static first hyperpolarizabilities of alkaline earth-based alkalides Be(NH3)nM (M=Be and Ca) in comparison with alkalides Li(NH3)nNa (n=1–3), Comput. Theor. Chem. 1051 (2015) 10–16.

[40]

A. Mondal, K. Hatua, P.K. Nandi, Second hyperpolarizability of diffuse-electron compounds M-NH3, M-NLi3 and M-NLi3-M of alkaline earth metals: Effect of lithiation, J. Mol. Graph. Model. 88 (2019) 81–91.

[41] J.L. W.C. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.P.; Izmaylov, A.F.; Bloino, J.; Zheng, G.; Sonnenberg, Gaussian 09, Rev. C.01, Gaussian, Inc., Wallingford CT, 2010. [42] R. Dennington, T. Keith, J. Millam, GaussView 5.0, Semichem Inc., 2009. [43] M.A. Gilani, S. Tabassum, U. Gul, T. Mahmood, A.I. Alharthi, M.A. Alotaibi, M. Geesi, R. Sheikh, K. Ayub, Copper-doped Al12N12 nano-cages: potential candidates for nonlinear optical materials, Appl. Phys. A. 124 (2018) 14. [44]

M.N. Ahmed, B. Sadiq, N.A. Al-Masoudi, K.A. Yasin, S. Hameed, T. Mahmood, K. Ayub, M.N. Tahir, Synthesis, crystal structures, computational studies and antimicrobial activity of new designed bis((5-aryl-1,3,4-oxadiazol-2-yl)thio)alkanes, J. Mol. Struct. 1155 (2018) 403–413.

34

[45]

S. Abbas, H.H. Nasir, S. Zaib, S. Ali, T. Mahmood, K. Ayub, M.N. Tahir, J. Iqbal, Carbonic anhydrase inhibition of Schiff base derivative of imino-methyl-naphthalen-2-ol: Synthesis, structure elucidation, molecular docking, dynamic simulation and density functional theory calculations, J. Mol. Struct. 1156 (2018) 193–200.

[46]

M. Madni, M.N. Ahmed, S. Hameed, S.W. Ali Shah, U. Rashid, K. Ayub, M.N. Tahir, T. Mahmood, Synthesis, quantum chemical, in vitro acetyl cholinesterase inhibition and molecular docking studies of four new coumarin based pyrazolylthiazole nuclei, J. Mol. Struct. 1168 (2018) 175–186.

[47]

H. Sajid, T. Mahmood, K. Ayub, High sensitivity of polypyrrole sensor for uric acid over urea, acetamide and sulfonamide: A density functional theory study, Synth. Met. 235 (2018) 49–60.

[48]

T. Mahmood, N. Kosar, K. Ayub, DFT study of acceleration of electrocyclization in photochromes under radical cationic conditions: Comparison with recent experimental data, Tetrahedron. 73 (2017) 3521–3528.

[49]

A. Shokuhi Rad, M.R. Zardoost, E. Abedini, First-principles study of terpyrrole as a potential hydrogen cyanide sensor: DFT calculations, J. Mol. Model. 21 (2015) 273.

[50]

A. Shokuhi Rad, P. Valipour, Interaction of methanol with some aniline and pyrrole derivatives: DFT calculations, Synth. Met. 209 (2015) 502–511.

[51]

M. Torrent-Sucarrat, J.M. Anglada, J.M. Luis, Evaluation of the Nonlinear Optical Properties for Annulenes with Hückel and Möbius Topologies, J. Chem. Theory Comput. 7 (2011) 3935–3943.

35

[52]

T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP), Chem. Phys. Lett. 393 (2004) 51–57.

[53]

E. Shakerzadeh, A Theoretical Study on the Influence of Carbon and Silicon Doping on the Structural and Electronic Properties of (BeO)12 Nanocluster, J. Inorg. Organomet. Polym. Mater. 24 (2014) 694–705.

[54]

L. Chen, C. Xu, X.-F. Zhang, T. Zhou, Raman and infrared-active modes in MgO nanotubes, Phys. E Low-Dimensional Syst. Nanostructures. 41 (2009) 852–855.

[55]

W.-M. Sun, X.-H. Li, D. Wu, Y. Li, H.-M. He, Z.-R. Li, J.-H. Chen, C.-Y. Li, A theoretical study on superalkali-doped nanocages: unique inorganic electrides with high stability, deep-ultraviolet transparency, and a considerable nonlinear optical response, Dalt. Trans. 45 (2016) 7500–7509.

[56]

M.N. Arshad, A.M. Asiri, K.A. Alamry, T. Mahmood, M.A. Gilani, K. Ayub, A.S. Birinji, Synthesis, crystal structure, spectroscopic and density functional theory (DFT) study of N[3-anthracen-9-yl-1-(4-bromo-phenyl)-allylidene]-N-benzenesulfonohydrazine, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 142 (2015) 364–374.

[57]

N. Rasool, A. Kanwal, T. Rasheed, Q. Ain, T. Mahmood, K. Ayub, M. Zubair, K. Khan, M. Arshad, A. M. Asiri, M. Zia-Ul-Haq, H. Jaafar, One Pot Selective Arylation of 2Bromo-5-Chloro Thiophene; Molecular Structure Investigation via Density Functional Theory (DFT), X-ray Analysis, and Their Biological Activities, Int. J. Mol. Sci. 17 (2016) 912.

[58]

J.L. Oudar, D.S. Chemla, Hyperpolarizabilities of the nitroanilines and their relations to the excited state dipole moment, J. Chem. Phys. 66 (1977) 2664–2668. 36

[59]

J.L. Oudar, Optical nonlinearities of conjugated molecules. Stilbene derivatives and highly polar aromatic compounds, J. Chem. Phys. 67 (1977) 446–457.

37

Highlights •

Alkaline earth metals doped BeO and MgO nanocages are studied theoretically for the first time.

•

Exohedral and endohedral configurations are investigated for electronic and NLO properties.

•

HOMO-LUMO gap of doped systems is decreased significantly.

•

First Hyperpolarizability (βo), Two level model and βvec. values corroborated nicely.