High-performance Nonlinear Optical Materials Research Articles

Doping superalkali on Zn12O12 nanocage constitutes a superior approach to fabricate stable and high-performance nonlinear optical materials

Fabrication of stable nonlinear optical materials is highly demanding due to their widespread applications in optical and optoelectronic devices. Novel inorganic complexes are theoretically designed and investigated by doping superalkalis on Zn12O12 cluster. The results reveal that doping causes shifting of excess electrons from superalkalis towards Zn12O12 cluster and enhances the NLO properties of these isomers. Moreover, the effect of doping different superalkalis (Li3O, Na3O and K3O) over Zn12O12 nanocages is also studied. All the isomers have shown excellent NLO response and are transparent under ultraviolet region of workable laser conditions. Comparative analysis reveals that hyperpolarizability shows monotonic dependence on atomic number of metal atom in the superalkalis. K3O@Zn12O12 exhibits the highest first hyperpolarizability (βo) value of 394,217 au. The band gap is reduced up to 70% as compared to pristine nanocage, making them n-type semiconductors. This study provides a pathway for designing thermally stable inorganic complexes as potential precursors for high-performance NLO materials.

A Theoretical Investigation on Second-Order NLO Properties of Organic Substitutions of [Mo6O19]<sup>2-</sup> and [MoW5O19]<sup>2-</sup>

In this study, density functional theory (DFT) was used to calculate second-order polarizabilities and second-order polarizabilities densities of a series of organic substitution for Lindqvist-type polyoxometalates (POMs), and the nonlinear optical (NLO) properties was also analyzed. We found that βzzz has the main contribution to β value. The expansion of molecular structure on z-axis greatly increased second-order polarizabilities. Both the size of the organic segments and metal hybridization exert an influence on β value. The analysis on the second-order polarizabilities density is used to explain the NLO phenomenon. In the present investigation, metal hybridization and π-conjugation changed the contribution of βzzz value from different parts. The results of this work will contribute to the potential applications in high-performance NLO materials.

First principle characterization of structural, electronic, mechanical, thermodynamic, linear and nonlinear optical properties of zinc blende InAs, InSb and their InAsxSb1-x ternary alloys

The structural and electronic properties of InAsxSb1-x ternary alloys are studied using plane-wave pseudopotential method based on the density functional theory. The Born effective charge, elastic constants, phonon dispersion curves, linear and nonlinear optical properties of these compounds are calculated using density functional perturbation theory. Based on the calculated phonon frequency, the specific heat at constant volume and entropy are obtained within the harmonic approximation. Moreover, due to the density functional theory band gap problem, the band gap correction is performed within the one-shot GW approximations. The distinguished role of In 4d states on the electronic energy band gap and nonlinear properties of these alloys are extensively explored by the comparison between the calculated results of Hartwigsen-Goedecker-Hutter and Trouiller-Martins-type pseudopotentials. The calculated results show that InAsxSb1-x alloys are mechanically stable and can be a good candidate for high-performance nonlinear optical material. The calculated results of InAs and InSb compounds are in acceptable agreement with available theoretical and experimental results, so the calculated results of InAsxSb1-x alloys will be helpful for future experimental and theoretical investigations.

Theoretical exploration of second-order nonlinear optical properties of mono- and bimetallic Pt(II) dithienylcyclopentene complexes: Ligands and photoisomerization effect

On the basis of great diverse applications of nonlinear optical (NLO) materials, organometallic complexes have attracted considerable attention. In this paper, we present a detailed investigation on a series of Pt(II) dithienylcyclopentene(DTE)-based complexes via density functional theory method with the aim of evaluating their structures, electronic absorption spectra and first hyperpolarizabilities. The calculations demonstrate that the first hyperpolarizabilities can be enhanced by the introduction of quinolone into the complexes because of the enlarged spatial separation of electron density. However, Pt(II) complexes containing perfluorocyclopentene ring exhibit decreasing NLO response attributed to lower amount of charge transferred and short effective CT distance. The static first hyperpolarizabilities (βtot) of monometallic Pt(II) complexes are larger than those of bimetallic Pt(II) complexes due to correlative mixed charge-transfer patterns. More importantly, the closed-ring (1c) complex comprising DTE unit exhibits the largest βtot value approaching 144 × 10−30 esu, with the contrast over five times compared to corresponding open-ring due to the better π-conjugated delocalization and smaller HOMO and LUMO energy gap. In general, we envision our work will be beneficial for further rational design of DTE-containing Pt(II) complexes as high performance NLO materials.

Effect of (super)alkali doping on the electronic and second-order nonlinear optical properties of graphitic C3N4

A systematic analysis of electronic and nonlinear optical properties of a novel class of (super)alkali-doped complexes M/M3O@g-C3N4 (MLi, Na, K) is presented. These compounds exhibit considerable stability with large interaction energies, and M3O@g-C3N4 has higher stability than the corresponding M@g-C3N4. It is found that doping the (super)alkali can significantly narrow the wide energy gap of the pure g-C3N4 in the range of 2.04–2.84 eV, and these doped g-C3N4 show enhanced visible-light absorption. Due to the large charge transfer from (super)alkali to g-C3N4 moiety, these doped compounds exhibited significantly large first hyperpolarizabilities up to 2165–63761 au. The ratio between the vibrational and electronic contribution on hyperpolarizability indicates that the vibrational contributions play an important role in determining the static hyperpolarizabilities of certain systems. Therefore, (super)alkali-doped g-C3N4 are expected to be potential candidates for visible-light catalysis and high-performance nonlinear optical materials. We expect this work could provide a new idea for designing functional materials using superalkali clusters.

Effects of Li doping and B3N3 substitution on the static first hyperpolarizabilities of biphenylene nanosheets: A computational and comparative study

Abstract Based on the density functional theory calculations, the BP-(B3N3)n and Li@BP-(B3N3)n (BP = biphenylene, n = 1–4) systems, were designed for the first time and explored the effects of Li doping and B3N3 substitution on their nonlinear optical (NLO) properties. We find that, when the carbon rings are replaced by B3N3 units in the biphenylene, systems of enhanced static hyperpolarizabilities (β0) are delivered. And the position and the number of B3N3 unit have an important effect on the β0 value. Moreover, when one Li atom is adsorbed on the surface of B3N3-substituted BP, the β0 values of these systems are further dramatically enhanced to 2.24 × 104–1.42 × 106 au. The further investigations show that small transition energy is the decisive factor for their large β0 values. Specially, these Li-doped compounds exhibits high stability and excellent infrared (IR) transparency. These findings might provide valuable insights into the design of novel high-performance NLO materials based on BP flake.

Alkaline-earthide: A new class of excess electron compounds Li-C6H6F6-M (M = Be, Mg and Ca) with extremely large nonlinear optical responses

Based on all-cis-1,2,3,4,5,6-hexafluor-ocyclohexane C6H6F6 (1), a new class of excess electron compounds, alkaline-earthides, Li-1-M (M = Be, Mg and Ca) with extremely large nonlinear optical (NLO) responses has been designed. Alkaline-earth M plays an important role in obtaining pretty small transition energies (0.520–0.621 eV) between the ground state and the crucial excited state. Consequently, the alkaline-earthides have large first hyperpolarizabilities (β0) up to 3.51 × 106 au. Our results indicate that the alkaline-earthides may be a novel idea for designing for high-performance NLO materials.

Removal of A-Site Alkali and Alkaline Earth Metal Cations in KBe2BO3F2-Type Layered Structures To Enhance the Deep-Ultraviolet Nonlinear Optical Capability.

In order to generate deep-ultraviolet (DUV, λ < 200 nm) coherent light, many DUV-transparent nonlinear optical (NLO) compounds have been synthesized experimentally over the past few decades, most of which contain alkali or/and alkaline earth metal cations. However, to date, practical DUV NLO materials beyond KBe2BO3F2 (KBBF) are still very scarce. In this work, based on analysis of the DUV NLO effect induced by the A-site alkali and alkaline earth metal cations, we attempt to expand the options for DUV NLO compounds from a molecular engineering point of view. Accordingly, a useful strategy is proposed to design densely stacked layered structures without A-site cations as new and promising DUV NLO materials. Along with the available experimental and first-principles calculation results, it is demonstrated that layered structures in which the A-site cations are removed, such as Be2BO3F, Be2CO3F2, AlCO3F, SiCO3F2, AlNO3F2, PBO3F2, and PB3O6F2, whether real or hypothetical, exhibit excellent DUV NLO performances. Based on the findings, our strategy may open up new opportunities for the design and exploration of high-performance DUV NLO materials beyond KBBF.

Improving the NLO response of bis-cyclometalated iridium(Ⅲ) complexes by modifying ligands: A DFT study

Density functional theory (DFT) have been carried out to investigate the nine recently reported iridium (Ⅲ) complexes which substituted various cyclometalated ligands (CˆN) and aryl isocyanide ancillary ligands (CNAr). The aim of this paper is to study the substituents effect on geometrical structures, electronic and second-order nonlinear optical (NLO) properties, UV-vis absorption spectra. It is found that the first hyperpolarizabilities can be easily modulated by the introduction of different electron-withdrawing/donating ligands on CˆN and CNAr. Complexes with 2-benzothienylpyidine (btp) CˆN ligands or the substituents on CNAr are electron-withdrawing groups -NO2 possess larger first hyperpolarizabilities values compared to remaining complexes. Significantly, the changes of the first hyperpolarizability can be qualitatively explained by means of the charge transfer pattern. It is anticipative that the theoretical investigation of these iridium complexes will be helpful for designing high-performance and versatile NLO materials.

Pb2 BO3 I: A Borate Iodide with the Largest Second-Harmonic Generation (SHG) Response in the KBe2 BO3 F2 (KBBF) Family of Nonlinear Optical (NLO) Materials.

Borate halides are an ideal materials class from which to design high-performance nonlinear optical (NLO) materials. Currently, borate fluorides, chlorides, and bromides are extensively investigated while borate iodide materials discovery remains rare because of the perceived synthetic challenges. We report a new borate iodide, Pb2 BO3 I, synthesized by a straightforward hydrothermal method. The Pb2 BO3 I chemical formula conceals that the compound exhibits a structure similar to the well-established KBe2 BO3 F2 (KBBF), which we show supports the highest second-harmonic generation (SHG) at 1064 nm in the KBBF family, 10 × KH2 PO4 (KDP), arising from the inclusion of Pb2+ and I- and the crystal chemistry. Our work shows that KBBF-related compounds can be synthesized incorporating iodide and exhibit superior NLO responses.

A comparative theoretical study on the electrical and nonlinear optical properties of Li atom adsorbed on AlN and BN single-walled nanotubes.

The geometrical structures, electrical properties, and nonlinear optical (NLO) properties of AlNNT-Li and BNNT-Li nanotube systems were investigated by means of the density functional theory (DFT) method. Frontier molecular orbitals and density of states analyses show that adsorption of the Li atom can significantly narrow the wide HOMO-LUMO gaps of pure AlNNT and BNNT. The results reveal that AlNNT-Li and BNNT-Li systems containing diffuse excess electrons can be regarded as inorganic electrides. The formation of diffuse excess electrons leads to a decrease in transition energies, thereby increasing the first hyperpolarizabilities (β 0) of AlNNT-Li and BNNT-Li. This work may contribute to the development of potential high-performance NLO materials. Graphical abstract The structural characteristics and nonlinear optical properties of the AlNNT-Li and BNNT-Li systems were studied by means of density functional theory. Introduction of Li atoms greatly enhances the static first hyperpolarizabilities of AlNNT-Li and BNNT-Li.

Bonding the superalkali M3O (M = Li and K): An effective strategy to improve the electronic and nonlinear optical properties of the inorganic B40 nanocage

Inspired by the fascinating finding of all-boron fullerene B40 (Nat Chem, 2014, 6, 727), we propose a new and effective strategy to construct a series of typical Donor-Acceptor (D-A) frameworks via linking the superalkali M3O (M = Li and K) unit with the low ionization potential to the B40 nanocage with large electron affinity. By means of the density functional theory computations, we have systematically investigated the structures, electronic properties, the first and second hyperpolarizabilities of these modified B40 nanocage systems. Owing to the formation of a B–O chemical bond, these composite systems (M3O)n-B40 (M = Li and K, n = 1 and 2) can possess the considerably large binding energy ranging from 57.0 to 99.8kcal/mol, indicating their high structure stabilities. Compared with the pristine B40 nanocage, linking the superalkali M3O can effectively narrow the wide energy gap from the original 2.86eV to 0.61–1.11eV, and significantly increase the first and second hyperpolarizabilities to as large as 5.00 × 104–2.46 × 105 au and 1.48 × 107–4.85 × 108 au, respectively, owing to the occurrence of evident electron transfer process in this kind of typical D-A framework. These fascinating findings will be advantageous for promoting the potential applications of the inorganic boron-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical materials.

Boron/nitrogen substituted the staggered hetero-dimers: Fascinating intermolecular charge-transfer and large NLO responses

Recently, the structure-property relationships of an attractive isomeric radical 1 (Phys. Chem. Chem. Phys., 2016, 18, 29041–29044) with famous phenalenyl (PLY) was investigated. Can PLY and 1 form stable π-dimers? The single point energy scan result shows that the staggered π-dimer (2) has the lowest energy. In order to explore intermolecular charge-transfer and nonlinear optical property, two π-dimers (2a and 2b by boron and nitrogen atoms substituting the central carbon atoms of 2) are designed. 2 has smaller intermolecular charge-transfer (0.020) and the first hyperpolarizability (βtot, 3.18 × 102 au). However, boron and nitrogen substitution effect can greatly enhance intermolecular charge-transfer (0.215 for 2a and 0.181 for 2b) and the βtot values (6.32 × 103 for 2a and 6.80 × 103 for 2b). Interestingly, the βtot values of 2a and 2b are close to each other, but their βz components and the NBO charge analysis show that 2a and 2b have opposite direction of charge transfer, which can be well explained by the difference of the dipole moment component (Δμz) between the ground and the excited states. Further, the first hyperpolarizability density study clearly substantiates the difference in their βzzz values: negative contribution for 2a is much larger than positive contribution, while the first hyperpolarizability density for 2b shows much larger positive contribution. Thus, the primary component of βzzz is negative value for 2a and βzzz is positive value for 2b. Combining with the frontier molecular orbital (FMO) and the electron density difference map (EDDM), intermolecular charge-transfer transition from B substituted fragment to N substituted fragment for 2a and 2b is concluded. The significant structure-property relationships in present π-dimers are beneficial for further theoretical and experimental investigations of high-performance nonlinear optical materials.

A theoretical investigation on doping superalkali for triggering considerable nonlinear optical properties of Si12 C12 nanostructure.

In this work, we designed a series of superalkali-doped Si12 C12 nanocage M3 O@Si12 C12 (M = Li, Na, K) with donor-acceptor framework. Density functional theory calculations demonstrated that the HOMO-LUMO gap of the complexes conspicuously narrowed with increase of atomic number of the alkali metal, the value decreased from 5.452 eV of pure Si12 C12 nanocage to 3.750, 2.984, and 2.634 eV of Li3 O@Si12 C12 , Na3 O@Si12 C12 , and K3 O@Si12 C12 , respectively. This finding shows that the pristine Si12 C12 cluster could be transformed to n-type semiconductor by introduction of the superalkali M3 O. We also showed that the superalkali doping remarkably enhanced the first hyperpolarizability of Si12 C12 . Among the studied systems, K3 O@Si12 C12 not only has the narrowest gap but also has the strongest nonlinear optical (NLO) properties, its first hyperpolarizability reached as high as 21695 a.u. The striking results presented in this work will be beneficial for potential applications of the Si12 C12 -based nanostructure in the electronic nanodevices and high-performance NLO materials. © 2017 Wiley Periodicals, Inc.

Introducing the triangular BN nanodot or its cooperation with the edge-modification via the electron-donating/withdrawing group to achieve the large first hyperpolarizability in a carbon nanotube system.

Based on the ab initio calculations, we first investigated the nonlinear optical (NLO) properties of the doped carbon nanotube (CNT) systems with a triangular BN nanodot with the central B or N atom. Our computed results reveal that introducing the triangular BN nanodot can be considered as a new strategy to effectively enhance the first hyperpolarizability β0 value of a CNT system, and the type and size of the triangular BN domain have an important effect on the β0 value of the hybrid CNT system. In addition, the n-type doping by introducing the triangular BN nanodot with more N atoms, viewed as injecting electrons into the CNT system, can more effectively increase the β0 value of the hybrid CNT system when the doped domains have the same size. Moreover, the β0 value of the hybrid CNT system can be further enhanced by employing the electron-donating or -withdrawing group (NH2/NO2) to modify the appropriate nanotube-edged site to build the typical donor-π-acceptor framework, in view of the occurrence of a more effective charge transfer process. Evidently, this type of cooperation of introducing the triangular BN nanodot and forming the donor-π-acceptor framework can be considered as another new strategy to significantly enhance the first hyperpolarizability of the CNT system. These appealing findings can provide valuable insights into the design of novel high-performance NLO materials based on carbon nanotubes.

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room-Temperature-Stable Organic Electride.

Excess-electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β0 ). A room-temperature-stable, excess-electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005, 127, 12416). In this work, the β0 of this electride was first evaluated to be 1.13×106 au, which revealed its potential as a high-performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β0 of Na@(TriPip222) could be further increased to 8.30×106 au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro-group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro-substituted Na@(TriPip222) compounds.

Theoretical insight into the substituent effects on linear and nonlinear optical properties of azobenzene-based chromophores

The linear and nonlinear optical properties of 4 kinds of experimental synthesized azobenzene-based chromophores were investigated by different density functional theories (DFTs) upon the electronic structures. The structure-property relationship was studied on each single molecule either in the gas phase or in diethylether and tetrahydrofuran (THF) solutions. The substituent effect on optical properties was revealed by checking the positions of substituent groups, and the influence of dynamic perturbation to the optical nonlinearity was investigated by simulating the experimental excitation. The results revealed that the substituent in the meta-position of the azobenzene group affects the optical properties more significantly than that in the ortho-position, which is in agreement with the experimental finding. The modulation of molecular hyperpolarizability of bridge-substituted azobenzene derived by dynamic perturbation is not recommended because of the reduced dynamic hyperpolarizability relative to the static one. The different functions of the DFT method hardly affect the calculated results, while solvent effects of diethylether and THF solutions are significant on the optical properties, especially for optical nonlinearity. The information derived from the single chromophore may be helpful in the design and preparation of high-performance nonlinear optical materials in further.

Enhanced electronic and non-linear optical properties of alkali metal (Li, Na, K) doped boron nitride nano-cages

Geometrical, electronic and nonlinear optical (NLO) properties of several alkali metal doped boron nitride nanocages (MB12N11 and MB11N12) are explored theoretically. NLO (Non Linear Optical) properties are obtained through first hyperpolarizability, and rationalized on the basis of dipole moment change during excitation to crucial transition state, and band gap. It is revealed that first hyperpolarizability (β0) is increased to a larger extent (1.3 × 104 au for KB12N11) in doped boron nitride nanocages as compared to pure B12N12 (0 au). MB12N11 nanocages show an increasing trend of β0 and βvec with increase in atomic number of alkali metal, in contrast, MB11N12 nanocages show unmonotonic dependency. Further, HOMO-LUMO gap is lowered significantly since a new level is introduced near Fermi level in all doped structures. All the doped structures show n-type semiconductor properties due to diffuse excess electrons in the system which give rise to new HOMO. The present idea offers potential application of these nanocages in new kind of electronic devices and is helpful in designing novel high-performance NLO materials.

Charge transfer and first hyperpolarizability: cage-like radicals C59X and lithium encapsulated Li@C59X (X=B, N).

Very recently, two new cage-like radicals (C59B and C59N) formed by a boron or nitrogen atom substituting one carbon atom of C60 were synthesized and characterized. In order to explore the structure-property relationships of combination the cage-like radical and alkali metal, the endohedral Li@C59B and Li@C59N are designed by lithium (Li) atom encapsulated into the cage-like radicals C59B and C59N. Further, the structures, natural bond orbital (NBO) charges, and nonlinear optical (NLO) responses of C59B, C59N, Li@C59B, and Li@C59N were investigated by quantum chemical method. Three density functional methods (BHandHLYP, CAM-B3LYP, and M05-2X) were employed to estimate their first hyperpolarizabilities (β tot) and obtained the same trend in the β tot value. The β tot values by BHandHLYP functional of the pure cage-like radicals C59B (1.30 × 10(3) au) and C59N (1.70 × 10(3) au) are close to each other. Interestingly, when one Li atom encapsulated into the electron-rich radical C59N, the β tot value of the Li@C59N increases to 2.46 × 10(3) au. However, when one Li atom encapsulated into the electron-deficient radical C59B, the β tot value of the Li@C59B sharply decreases to 1.54 × 10(2) au. The natural bond orbital analysis indicates that the encapsulated Li atom leads to an obvious charge transfer and valence electrons distribution plays a significant role in the β tot value. Further, frontier molecular orbital explains that the interesting charge transfer between the encapsulated Li atom and cage-like radicals (C59B and C59N) leads to differences in the β tot value. It is our expectation that this work will provide useful information for the design of high-performance NLO materials.

Investigation on Structures and Nonlinear Optical Properties of PPV and Its Derivatives Systems with Adsorbing Alkali Metal Atom

By means of the density functional theory(DFT) method,we investigated detailedly the structures and nonlinear optical(NLO) properties of the adsorbed phenylenevinylene(PPV) polymer with the alkali metal Li atom and its corresponding derivatives systems decorated with the donor/acceptor group.In these PPVbased systems,the Li atom can be adsorbed stably on the surfaces of PPV and its derivatives,as revealed by their considerable adsorption energies(62.3—78.2 k J/mol).It is revealed that when adsorbing the Li atom on the [PPV]n(n = 2—4),the evident charge transfer can occur between them,and this effect of lithium salt can effectively enhance the first hyperpolarizabilities β0values of PPV systems from original 249—756 a.u.to the range of 1.16 × 104—1.37 × 105 a.u.Comparatively,when adsorbing the alkali metal Li atom on the PPV derivatives modified with only donor(—NH2) or acceptor(—CN) group,the first hyperpolarizabilities of systems can be significantly improved further,where the β0values of Li@ [NH2-(PPV)4]and Li@ [(PPV)4-CN] can be as large as 1.61 × 105 and 2.85 × 105 a.u.,respectively.This can be attributed to the case that the cooperation effect of lithium salt and Donor-π-Acceptor can result in the more evident decrease of main transition energy of system.Further,this cooperation effect can be strengthened significantly in the Li-adsorbed PPV derivatives decorated with donor/acceptor(—NH2/ —CN) pairs,which can lead to the more effective improvement of β0value(up to 3.56 × 105 a.u.at n = 4).This work can provide some new valuable insights for designing the new type of high-performance NLO materials based on the excellent PPV systems.