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A New Strategy of Bi-Alkali Metal Doping to Design Boron Phosphide Nanocages of High Nonlinear Optical Response with Better Thermodynamic Stability

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A New Strategy of Bi-Alkali Metal Doping to Design Boron Phosphide Nanocages of High Nonlinear Optical Response with Better Thermodynamic Stability A New Strategy of bi-Alkali Metal Doping to Design Boron Phosphide Nanocages of High Nonlinear Optical Response with Better Thermodynamic Stability Rimsha Baloach University of Education Khurshid Ayub University of Education Tariq Mahmood University of Education Anila Asif University of Education Sobia Tabassum University of Education Mazhar Amjad Gilani ( [email protected] ) University of Education Original Research Full Papers Keywords: Boron phosphide (B12P12), Bi-alkali metal doping, Nonlinear optical response (NLO), Density functional theory Posted Date: February 11th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-207373/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Version of Record: A version of this preprint was published at Journal of Inorganic and Organometallic Polymers and Materials on April 16th, 2021. See the published version at https://doi.org/10.1007/s10904-021-02000-6. A New Strategy of bi-Alkali Metal Doping to Design Boron Phosphide Nanocages of High Nonlinear Optical Response with Better Thermodynamic Stability ABSTRACT: Nonlinear optical materials possess high rank in fields of optics owing to their impacts, utilization and extended applications in industrial sector. Therefore, design of molecular systems with high nonlinear optical response along with high thermodynamic stability is a dire need of this era. Hence, the present study involves investigation of bi-alkali metal doped boron phosphide nanocages M2@B12P12 (M=Li, Na, K) in search of stable nonlinear optical materials. The investigation includes execution of geometrical and opto-electronic properties of complexes by means of density functional theory (DFT) computations. Bi-doped alkali metal atoms introduce excess of electrons in the host B12P12 nanocage. These electrons contribute towards the formation of new HOMO, thus reducing HOMO-LUMO gaps. The reduced HOMO-LUMO gap ranges from 0.63eV to 3.69eV. The diffused excess electrons also come up with increased hyperpolarizability values of complexes i.e. up to 4.0×104au. TD-DFT calculations have been performed to examine crucial transition states and for UV-VIS analysis. IR and DOS spectra have been plotted to support our obtained results. Non covalent interaction (NCI) calculations along with quantum theory of the atoms in molecules (QTAIM) analysis were carried out to understand the bonding interactions between alkali metal atoms and B12P12 nanocage. All obtained results suggest bi-alkali metal doped nanocages as exceptionally stable materials with improved NLO response and superb candidates for their vast applications in optics. Keywords: Boron phosphide (B12P12); Bi-alkali metal doping; Nonlinear optical response (NLO); Density functional theory 1. Introduction Nonlinear optical materials play important roles in this modern era of optics with their limitless utilization in different fields of science and technology[1]. They own numerous applications in the fields of modern computing, optical data storage, telecommunication, and photonics[2][3][4]. Furthermore, they also find their potential uses in adsorption[5], laser technology, waveguides, gyroscopes, and scanners as well[6][7][8][9]. At present, their use has been extended in designing of catalysts [10], pulse wave sensors[9], field-effect transistors[11], luminescent materials[12] and light emitting diodes[13]. In addition,, these materials are widely used in signal processing[14] logical technologies, and holographic imaging[15]. All these reported applications highlight the prime need to design new NLO materials for academic and industrial fields. At present, various promising materials have been reported possessing outstanding NLO properties. These materials include inorganic fullerenes[16], donor-π bridge-acceptor systems, chiral π conjugated molecules[17]. Most recently, inorganic NLO crystals[18][19], carbon dots[20], octupolar molecules[21], graphidyne nanosheets[22], metal organic frameworks[23] multi-decker sandwich clusters[24]. Hyperpolarizability (βtot) is the key feature to characterize the performance of NLO materials.[25] Numerous studies have been reported to explore inorganic fullerenes (XY)n as they possess unique properties and find extensive applications to design optoelectronic devices. To date, various fullerenes (XY)n from group Ⅲ-Ⅴ have been synthesized by different experimental methods[26]. Carbothermal reduction, chemical vapor deposition and direct nitridation[27] solution growth method[28] are some of these techniques. Molecular simulations show that (XY)n [X=Al, B; Y= N, P; n= 12] are the most stable structures. Fullerenes are semiconductors with wide HOMO-LUMO gap[29][30]. Usually, fullerenes are non-responsive towards NLO response and scientists have introduced numerous strategies to induce NLO response by both experimental and theoretical techniques. These strategies include doping[31], electron push– pull mechanism[32], extensive π conjugation[33], utilization of bond length alteration theory (BLAT)[34], substitution effect[35], introduction of electron acceptor/donor group[36] and consideration of diradical character of materials[37]. Recently, Li et al. reported an effective strategy where diffused excess electrons are introduced in materials to increase their NLO response[38][39]. Fullerenes hold centrosymmetric structure therefore; they have zero dipole moment and hyperpolarizability (βo). The introduction of diffused excess electrons causes remarkable increase in hyperpolarizability, efficiently narrows down HOMO-LUMO gap and results in increased NLO response[40]. Certainly, doping of foreign metals is an efficient strategy to induce excess electrons in materials. Alkali metals have low ionization potential, therefore valence electrons from these metal atoms are easily diffused out to act as excess electrons in materials. This technique is being frequently used to design high performance[41] NLO materials. Literature reveals that lithium doping on B10H14 framework leads to considerable increase in hyperpolarizability (2.31×104 au) because of diffused excess electrons of Li atom[42]. Huang et al. have studied alkali metal doped AlN nanocages and proved that hyperpolarizability of the resultant complex (Li@b66-Al12N12) is 5 increased to (8.89×10 au)[26]. A related study is performed for B12N12 which confirms that alkali metal doping has increased its hyperpolarizability to manifold[43]. In addition, alkali metal atom doped complexes, Li+( calix [4] pyrrole)M-; [M =Li, Na, and K], have shown remarkable increase in hyperpolarizability value of up to 7.3×103 au[44]. Likewise, alkali metal doped aromatic rings (indole, thiophene, and benzene) show enhanced electronic properties and exhibit large hyperpolarizability values from 6.7 × 103 to 9.3 × 103 au[45]. Muhammad et al. have reported that decoration of Si12C12 with alkali metals shows enhancement in first hyperpolarizability up to 5.7×103 au[46]. A remarkable nonlinear optical response of 5 7.9 × 10 au for K@Ptop-B12P12 has been studied by Maria et al. [47]. Literature reveals lot of reports on single and multi-atom doping but no study is found on bi-metal doping of nanocages. There is a need to explore bi-metal doping strategy to design novel NLO materials. B12P12 is a refractory semiconducting structure. Boron phosphide has been synthesized by molten nickel or nickel phosphide reactions[48], solid state metathesis reaction[49], sun fluxed assisted synthesis[50] and high temperature reactions of elements[51]. It holds extreme binding affinity for a number of surfaces, making it an excellent contestant for various practical applications[52]. To date, boron phosphide nanocages have been explored by using number of strategies for their applications as NLO materials[53][54], hydrogen storage media[55][56], chemical sensors specifically for SO2[57] and phenol[58], and as catalytic materials[59]. Doping single alkali metal atoms is a popular and the most used strategy to introduce excess electrons in doped systems. In this study, we are specifically interested to study the considerable impacts of bi-alkali metal atoms doping on B12P12 nanocage and subsequently its NLO response. All parameters including geometries, NBO analysis, dipole moment, polarizability and hyperpolarizability, NCI and QTAIM analysis are studied to get deep insight of NLO response of bi-alkali metal doped nanocage. Moreover, UV-Vis, IR spectra have been explained to examine the effects of bi-metal doping on the absorption properties of B12P12. This study will provide a new approach to design highly stable bi-metal doped B12P12 based NLO materials. 2. Computational Details All calculations in this study were performed using the Gaussian 16 software package[60] and structures have been visualized by using GaussView 5.0[61]. The reported structures in this work are optimized at (Becke’s three-parameter hybrid functional (B3) in relation to the gradient corrected correlation functional of Lee-Yang-Parr (LYP) using B3LYP with 6- 31G(d,p) computational level of theory. True minima for all the structures are characterized by frequency calculations at B3LYP/6-31G(d,p) level of theory. To confirm the extent of stability for designed structures, adsorption energies are analyzed by using given equation: (1) 푎푑푠 푐표푚푝푙푒푥 푐푎푔푒 푀2 In this equation Ecomplex is퐸 the =energy 퐸 of doped− (퐸 nanocage,+ 퐸 ) Ecage is the energy of bare BP nanocage while EM2 stands for the sum energy of doped bi-metal atoms only. These energies
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