Theoretical study on novel superalkali doped graphdiyne complexes: Unique approach for the enhancement of electronic and nonlinear optical response
Graphical abstract
Introduction
From the past few decades, numerous reports have appeared on the design and synthesis of nonlinear optical (NLO) materials, due to their potential applications in electro optics, rechargeable batteries, sensors, detectors etc. [[1], [2], [3], [4]]. In order to design such high performance NLO materials, various strategies have been put forth for both organic and inorganic systems. For organic systems, NLO response is generally enhanced by increasing the degree of charge transfer [5]. For example, electron-push pull mechanism in organic NLO materials where appropriate electron donor and acceptor groups are linked through π conjugation [6]. Moreover, extended conjugation in π network is another approach to design better NLO materials [7]. Various NLO materials have been designed by using this strategy. NLO response can also be enhanced by metal ligand frameworks in organometallic compounds [8].
The most effective strategy for the enhancement of NLO response is the introduction of excess electrons in a system. This strategy was revealed after the study of solvated electrons systems in 2004 [9,10]. The reported hyperpolarizability of molecular cluster having excess electrons (H2O)3{e} is 106 times higher than that of equivalent molecular cluster lacking excess electrons [11]. Electrides and alkalides are the advanced types of compounds showing large NLO response due to the presence of excess electrons. Electrides are ionic compounds in which trapped electrons are present as ions on anionic sites. Li3O@Al12N12 is an example of electride [12]. Alkalides contain negative charges on alkali metals i.e. OLi3-M-Li3O (M = Li, Na, and K). These alkalides are designed without use of any ligand, and it is found that the OLi3-M-Li3O alkalide possesses significantly high hyperpolarizability value (1.93×104 au) [13]. Various methods are used to introduce excess electrons in any system but most effective mode is the doping of alkali metals or superalkalis [14]. Various NLO materials were designed in past few years as well e.g. barium borate crystal is synthesized commercially for optoelectronic devices [12,15]. Similarly, alkali-metal fluorooxoborates, AB4O6F (FK, Rb, Cs) are reported for deep-ultraviolet (DUV) applications [16]. All of these compounds show high NLO response but 2D carbon materials have caught specific attention in the past decades [17] due to their highly delocalized π conjugation structure [18] and high surface to volume ratio [19].
Graphdiyne, an allotropic form of carbon with atoms in the form of layers with highly delocalized π conjugation is studied [16]. In 2010 Li et al., experimentally synthesized graphdiyne (GDY) as a member of GYs series [17]. This new form of carbon has symmetrical resemblance with graphene but it is structurally different from graphene [18]. Normally, graphynes (GYs) consist of benzene rings bonded with acetylenic linkages [20]. However, number of acetylenic linkages present between carbon hexagones classify GYs as graphdiyne, graphtriyne and so on [21]. GDY has been manufactured in various forms like nanotube arrays, nanowires, nanosheets, nanowalls etc. GDY has high degree of sp and sp2 hybridization in which two acetylenic chains join two hexagonal rings [22]. The planar layers of the GDY have self-assembly of 18 hexagonal unit with 3 large hollow cavities [20]. Various models have been used for graphdiyne such as C30H12, C28H12 and C26H12 [23]. The 2D structure of GDY shows large delocalized π conjugation, which gives rise to optical and electrical properties. Various properties shown by GDY’s layered structures, such as conduction of electricity [24], nonlinear optical sensitivity [25], resistance to heat [26], extensive absorption in visible region of light [27] etc. are not seen in graphene. The GDY is widely used in electronic devices [28,29], solar cells, detectors, biomedicine, gas storage, water purification [30], and in cancer treatment [31].
Most interestingly, the doping of GDY with metals has attracted great attention, since the external dopant effectively modifies the electronic and magnetic properties of GDY [32,33]. Recently, Li and co-workers studied the effect of alkali metal doping in GDY through DFT methods, and it is found that doping of alkali metal atoms significantly enhanced both electronic and magnetic properties [20]. It is known that alkali metals, due to low ionization potentials, can easily generate excess electrons to nanosheets, nanotubes and nanocages [34,35]. These excess electrons directly influence the hyperpolarizability [36]. Since low ionization potential of alkali metals increases the hyperpolarizability value then, according to Gutsev and Boldyrev, superalkalis possessing ionization potential even lower than the alkali metals are expected to further increase βo values of graphdiyne [37]. In this regard, tetrahedral Li3NM molecules were doped on the highly delocalized π conjugated GDY surface and βo values are increased up to 2.88 × 105 au [38].
Similarly, the nonlinear optical response of superalkali doped Si12C12 is also higher (2.1 × 104 au), as compared to alkali doped Si12C12 nanostructures [39]. Sun et al., studied superalkalis (M3O, M = Li, Na and K) doped X12Y12 (X = B, Al; Y = N, P) nanocages at Møller−Plesset perturbation (MP2) method and observed that Li3O@Al12N12 has the highest hyperpolarizability value (1.86 × 107 au) [40]. Tu et al., theoretically studied superalkalis (M3O, M = Li, Na and K) doped boron-heterofullerene BC59 for electronic and NLO properties. The hyperpolarizability of Li3O–BC59 is 4921 au, which is higher than those of alkali doped K@6-BC59 (2621 au) and K@5-BC59 (3352 au) complexes [41]. In 2017, Srivastava et al., studied the interaction of superalkali doped C60 with superhalogen and noticed the dependence of NLO properties on size of superalkalis and superhalogen [42]. Subsequently, (M3O)+(e@C20F20)-(M = Na, K) were considered as a new class of superalkali-based electride molecules with large NLO response [39].
In this report, we aim to explore the enhancement the NLO response of GDY by doping different superalkalis such as M3O and M3S (M3 = Li, Na, K) on the surface of graphdiyne (GDY). DFT method is implemented to thoroughly study the geometric and electronic properties of the doped structures. We also compared the results of M3O@GDY doped structures with M3S@GDY to study how the nature of dopant can change the opto-electronic properties of GDY.
Section snippets
Computational methods
To study the of interactions between GDY and various superalkalis, different doped structures are optimized. Structural optimization of all doped complexes M3O@GDY and M3S@GDY (M = Li, Na, and K) is performed at ωB97XD in combination 6-311G(d,p) basis set by using Gaussian 09 program [43]. ωB97XD is the best reported method for the optimization of systems with alkali metals and the systems with noncovalent interactions [19,44]. In order to confirm the stationary points as true minima on
Geometric parameters
For stable equilibrium structures, the 18 membered carbon site of graphdiyne with highly delocalized π conjugated surface is selected for the doping of superalkalis. The selected site provides large cavity for external doping. In this study, we have selected M3O and M3S (M = Li, Na and K) dopants. The only difference between the two series is the presence of oxygen or sulphur atom. In both series, dopant is positioned horizontally over the GDY sheet. The optimized structures of different
Conclusions
In the current study, we have theoretically studied the geometric, electronic and nonlinear optical properties of M3O and M3S doped graphdiyne complexes. Our calculated results show that dopants are preferably adsorbed on hollow cavities of GDY surface, and the highest Eint of −67.9 kcal/mol and -69.2 kcal/mol are observed for lithium superalkali adsorption. Superalkalis effectively reduce the HOMO-LUMO gaps in doped structures. In overall comparison, the HOMO-LUMO gap reduction of M3O@GDY
Acknowledgements
The authors acknowledge the financial support from Higher Education Commission of Pakistan (Grant No. 5309) and COMSATS University Islamabad, Abbottabad Campus.
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