Outstanding NLO response of thermodynamically stable single and multiple alkaline earth metals doped C₂₀ fullerene

Highlights

• The nonlinear optical response of single and multi-doped alkaline earth metals on C₂₀ fullerene is studied theoretically.

• Results show that multi-doping is a selective strategy for enhancing the NLO properties of C₂₀ fullerene.

• Ca₅@C₂₀ complex has highest β_o value of 6.50 × 10⁷ au at CAM-B3LYP.

Abstract

In the current study, the NLO response of single and multi-doped M@C₂₀ (M = Be, Mg and Ca) fullerenes is investigated. Electronic and NLO properties of C₂₀ fullerene are enhanced with increasing number of doped metal atoms. Doping of single and multi-atoms on/in C₂₀ fullerene significantly reduces the HOMO-LUMO gap of C₂₀ fullerene. Density of state (DOS) analysis illustrates generation of new states which are responsible for decrease in H-L gap. Ca₅@C₂₀ complex has the lowest HOMO → LUMO of 1.40 eV and strong interaction energy of −99.02 kcal mol⁻¹. In all newly designed complexes, charge is transferred from metal to C₂₀ fullerene. The largest first hyperpolarizability of 6.5 × 10⁷ au is observed for Ca₅@C₂₀ complex. The hyperpolarizability values are also rationalized through two-level model analysis. This study will promote research to design nanomaterials with enhanced electronic and optical properties.

Introduction

In 1985 Kroto et al., discovered a new form of carbon in which atoms are present in close shell. This new allotropic form of carbon has structural similarity with truncated icosahedrons (resembling soccer ball) and therefore, known as Buckminster fullerene after the name of German architect Buckminster Fuller who introduced this structure in 1960 [1]. Besides diamond and graphite, fullerenes are considered as the third allotropic form of carbon [2]. Among all carbon fullerenes, C₆₀ has gained significant attention due to its high abundance among all carbon fullerenes. It consist of 12 pentagons and 20 hexagons to form a closed cage of carbon atoms [2,3]. Apart from C₆₀, carbon fullerenes exist in various sizes such as C₂₀, C₂₄, C₂₈, C₃₀, C₅₀, C₇₀, C₇₂, C₇₆, C₈₄ and C₅₄₀ etc. [3]. Carbon fullerene are widely used in biology and medicinal chemistry, due to their interesting properties. For example, polyhydroxy fullerenes are used for the treatment of neuro-degenerative disorders i.e. Parkinson's and Alzheimer's diseases [4]. Carbon fullerenes have electrophilic properties and the most reactive site is mostly CC bridging pentagons or double bond present at the junction of two hexagonal rings [5]. In 2003, Dolgonos observed that C₂₀ consists of only pentagons which are equally distributed on its sphere with bond length of 1.45 Å. Experimentally C₂₀ is synthesized by Prinzbach et al., by using debromination method in gas phase. In solid phase, C₂₀ is synthesized by two independent research groups [6]. Wang et al., synthesized closed packed hexagonal structure with ion beam irradiation [7], whereas Iqbal et al., synthesized C₂₀ fullerene by using laser ablation technique [8]. C₂₀ is highly symmetrical and its diameter is half to that of C₆₀ [9]. C₂₀ is used for adsorption of gases (H₂S and SO₂) as well [10].

Since, last two decades, extensive work has been carried out on the nonlinear optical properties (NLO) of different materials. NLO materials have applications in photonics, optical signal processing devices, dynamic imaging, data storage and computer devices [11]. The systematic study of NLO properties became possible after the invention of laser by Franken et al., in 1961 [12]. The NLO response of materials is directly linked with first hyperpolarizability (β_o) value. For the enhancement of first hyperpolarizability (β_o), various methods are known including: electron push-pull mechanism (particularly for organic systems in which electron donor and acceptor groups are linked through π conjugation) [13], metal ligand frameworks [14], bond-length alteration [15], diradical character and excess electrons [[16], [17], [18], [19], [20]]. Excess electrons are generated by doping metals on organic or inorganic nanocages [21]. Rad et al., theoretically studied the exohedrally transition metals (Ti, Cr and Ni) doped C₂₀ fullerenes. They found that Ti@C₂₀ has high first hyperpolarizability (2.5 × 10³ au) than Cr and Ni doped C₂₀ fullerenes [22]. Further, the NLO effect of 2nd row transition metal substitution on C₂₀ fullerenes is also examined by Rad et al., They observed that the polarizability (α_o) of substituted C₂₀ is higher than pure C₂₀ nanocage while the trend of first hyperpolarizability (β_o) is irregular [17]. In literature, extensive work has been reported on the electronic and NLO properties of alkali metal-doped organic and inorganic fullerenes through DFT (density functional theory) calculations [[23], [24], [25]]. Ayub studied NLO response of the alkali metals encapsulated X₁₂Y₁₂ nanocages (X = B, Al and Y = N, P). In the comparative study, it was revealed that the NLO response of phosphide nanocages is higher than nitride nanocages [19]. Maria et al., designed theoretically alkali metal substitutionally doped boron nitride nanocages (MB₁₂N₁₁, MB₁₁N₁₂) for enhancing electronic and NLO properties. The first hyperpolarizability of KB₁₂N₁₁ is the highest (1.3 × 10⁴ au) in their study [21].

Besides alkali metals, scientists have considered alkaline earth metals for various applications due to the presence of two valence electrons, although the ionization potentials of alkaline earth metals are relatively high. Wang et al., observed that hyperpolarizability of alkaline earth metal complexes [Be(NH₃)_nM (M = Be, Ca)] (β_o ~ 10⁵ au) is remarkably higher than the analogous alkali metal complexes [Li(NH₃)_nNa(n = 1–3)] [26]. Similarly, Nandi and co-workers demonstrated that doping of alkaline earth metals on NLi₃ enhances the first hyperpolarizability value. The 2p_z orbital of NLi₃ analogue of ammonia is empty due to the conversion of sp³ hybridization to sp² which can easily accept electrons from the alkaline earth metals [27]. In comparison to alkali metals, three alkaline-earth-doped compounds, i.e., Be-doped N-(2-(1H-imidazol-1-yl)ethyl)-N-(aminomethyl)-methanediamine (Be IAD), N-(aminomethyl)-N-((pyrimidin-5 yl)methyl)methanediamine (Be-PAD), and 2-(2-(1H-imidazol-1-yl)ethyl)-2 (aminomethyl)propane-1,3-diamine (Be-IPD) are analyzed by Wang et al., and they concluded that alkaline earth atom doping is an effective way to enhance the NLO response [28]. Kosar et al., examined alkaline earth metal (Be, Mg, and Ca) decorated beryllium and magnesium oxides nanocages for their NLO response [20]. Ayub and co-workers studied the first hyperpolarizability of alkaline earth metal (Be, Mg, and Ca) doped phosphide nanoclusters. They observed highest β_o (7.8 × 10⁴ au) for Ca@P_top-A₁₂P₁₂ complex [29].

The literature is rich in single atom doping on organic and inorganic fullerenes. However, the research is recently diverted toward multiple doping. In this regard, Shamouli and co-workers examined the multiple doping of Li_n (n = 1–6) on C₂₀ fullerene. In which Li₅@C₂₀ is quite interesting system, due to its higher first hyperpolarizability value (4.0 × 10⁶ au) [30]. Multiple alkali metal atoms doping enhance the NLO response of C₂₀ fullerene. Besides multiple alkali metal atoms@C₂₀, multiple alkaline earth doped C₂₀ complexes can also be used as novel candidates for high-performance NLO materials. However, literature is silent about the theoretical studies on the single and multiple alkaline earth metal doping. Keeping this idea in mind, we are committed in the current project to investigate the structural and NLO properties of single and multiple alkaline earth metal doped C₂₀ fullerene through DFT methods.