Theoretical Investigations On The Effects Of Functional Moieties On The Nonlinear Optical Properties Of Fullerene Onion And Graphene Carbon-based Nanomaterials

There is an urgent need to explore novel high-performance nonlinear optical(NLO)materials because NLO materials have crucial applications in national defense(such as laser protection),optoelectronic industry(particularly laser photolithography technology),medical(such as photodynamic therapy)fields,etc.Carbon-based nanomaterials(e.g.,fullerene,graphene,and so on)not only have bonding diversity,but also have good thermostability,chemical stability and mechanical strength.In addition,the?electron conjugation and special geometry lead to the distinct electronic properties of carbon-based nanomaterials,which make those nano-materials have potential applications in nonlinear optics.Due to the non-polarity(or weak polarity),pristine carbon-based materials could have good third-order NLO properties,but the second-order NLO properties are missing(or weak).Therefore,the structures of pristine carbon-based materials need to be further engineered.Although the fullerenes with embedded functional moieties(cluster or small molecule)have excellent NLO properties,the contribution of each functional moiety to the NLO responses of system and the influence mechanism of the interaction between functional moieties on NLO responses are not clear.Therefore,this thesis systematically studies the contribution and roles of functional moieties on the NLO properties of fullerene onions and azulene-defect graphene nanosheets(GNSs).This will help to clarify the structure-properties correlation in carbon-based nanomaterials,and provide practical strategies and useful information for future carbon-based NLO materials design.This thesis includes the following works:The third order static and dynamic NLO responses of I_h symmetry fullerenes(C₆₀,C₂₄₀,and C₅₄₀)and fullerene onions(C₆₀@C₂₄₀ and C₆₀@C₂₄₀@C₅₄₀)are predicted with the ZINDO method and the sum-over-states(SOS)model.The static second hyperpolarizability(<?₀>)of I_h symmetry fullerenes has an exponent increase[<?₀>(C₅₄₀)?<?₀>(C₆₀)×9^2.63]with fullerene size(from C₆₀ to C₅₄₀).The external fields of strong third order NLO responses of I_h symmetry fullerenes change from ultra-violet(C₆₀)to visible region(C₅₄₀)as the fullerene size gets bigger.The contribution of each component fullerene as a functional moiety to the third-order NLO properties of the fullerene onions is analyzed and the“Faraday cage effect”of the outer fullerene in the fullerene onion is clarified.Although outer fullerene contributes crucially to the third-order NLO properties of fullerenes onions,inter-shell charge-transfer excitations have conspicuous contributions to the third-order NLO properties of the fullerene onions.Two-dimensional two-photon absorption(TPA)spectra of C₆₀ and C₂₄₀ are simulated to provide a new means for characterizing the TPA cross section of nano-materials.The existing literatures have not been reported on the two-dimensional TPA spectra of materials.The azulene-defect GNSs with two different relative position and orientation are designed by introducing the polar azulene functional moieties into the pristine GNSs for applications in optoelectronics and nonlinear optics.The NLO properties of those GNSs depend on the relative orientation of two azulenes.The azulene-defect GNSs with two parallel azulenes have larger NLO responses than those with reverse and angular aligning azulenes.The all-carbon Paral-2 has good kinetic stability as well as a large static first hyperpolarizability(<?₀>)of 1032.28×10^-30 esu(18.43×10^-30 esu per heavy atom).The substitution of edge C atoms with N atoms enhances the kinetic stability of azulene-defect GNSs while essentially keeping the strong NLO responses.The Paral-0-4N,formed by replacing four edge C atoms with N atoms in the Paral-0,has the largest<?₀>(1247.68×10^-30 esu,22.28×10^-30 esu per heavy atom)due to the charge transfer based electron excitations and the polar structure.The two-dimensional second order NLO spectra of those azulene-defect GNSs provide vital information for further experimental exploration.Based on kinetically stable all-armchair-edged GNSs,common topological defects[i.e.,Stone-Wales defect(55-77)and Stone-Wales-like divacancy defects(555-777 and5555-6-7777)]constituted by 2,3 and 4 azulenes are introduced to regulate the second-order and third-order NLO properties of these GNSs under the condition that the edge shape of GNSs remains unchanged.The influence of the shape of the nanosheet on the NLO properties of systems is also considered.The results show that the parallelogram GNSs has a larger NLO response than the hexagonal and triangular GNSs.The number of defects plays a leading role in the second-order NLO responses of these GNSs,while in third-order NLO responses,the shape of the nanosheet has a greater effect.In all investigated all-carbon GNSs,AP-4a has the largest<?₀>(1990.37×10^-30 esu,22.12×10^-30 esu per heavy atom),and AP-2a has the largest<?₀>(-199.22×10^-34 esu,-2.16×10^-34 esu per heavy atom).With the number of azulenes increases,the dipole moment of azulene-defect GNSs increases.The fragment composed of azulene functional moieties becomes a functional center in GNSs,which not only triggers polarization of GNSs,but also plays a major role in electron transitions involved in the major electronic excitation processes.