The Theoretical Investigation On Nonlinear Optical Properties Of Graphene

Due to the important role that nonlinear optical （NLO） materials have played inthe domain of photoelectronics and photonics, the development of nonlinear optical（NLO） materials has always been in the spotlight of scientific interest. The organicNLO material has been in the focus of current research owing to the large and fastnonlinearities as well as tailorable structure characteristics. The discovery of graphenehas taken us into the new material revolution. Carbon is the chemical genius and itcan create structures （such as graphite, fullerene, carbon nanotube, carbon nanoconeand so forth） with entirely different properties by bonding in different ways. Owing toits unique structure and properties, graphene could be utilized for potentialapplications in many fields. On the basis of graphene, the conversion between carbonnanotube and carbon nanocone is not only fascinating but also characteristic, which isbeneficial in the research of NLO materials.The NLO properties of graphene have been investigated by using quantumchemical calculations. The relationship between molecular structures and NLOproperties has been explored. The research results suggest:（1） How do the number and location of lithium atoms affect the firsthyperpolarizability （βtot） of graphene? In this paper, based on pentacene, a series ofgraphene （multi）lithium salts Lin@pentacene （n=1,2,3,4,5and6） have beendesigned to answer this question. The result shows that the βtotincreases obviouslystepwise with increasing the number of the lithium atom:1369¹843forLi@pentacence <3510⁴081for Li2@pentacence <6933⁷934for Li3@pentacence<11188¹2145for Li4@pentacence <14904au for Li5@pentacence, which are greatlylarger than pentacence. It suggests that the lithium salt effect on the firsthyperpolarizability is very large. Unexpectedly, when an additional lithium atombeing doped into graphene multilithium salt Li5@pentacence, which leads toLi6@pentacence, the βtotvalue dramatically mounts into a value of4501764au with aremarkable increase of302-fold by contrast with Li5@pentacence. On the other hand,when the number of lithium atoms is equal, the location of lithium also affects the βtotvalue: the lithium cluster is closer, the βtotis larger: for Li3@pentacence,6933ofsystem10<7401of system9<7934au of system8. Furthermore, their transition energies （E） are also obtained. The result shows that the E decreases stepwise withincreasing the number of the lithium atoms and the E of Li6@pentacence is sharplydecreased into0.299eV, which may explain the huge βtotvalue. This study mayarouse the possibility to explore a new type of graphene NLO materials based onalkali metal for NLO application.（2） Trumpet-shaped carbon nanocone （CNC） is used as π-conjugated bridge todesign high-performance nonlinear optical （NLO） material. Owing to the attractivetrumpet-shaped structure, O₂N–CNC–NH₂exhibits nonlinear optical behavior that isboth surprisingly and qualitatively distinct from conventional π-conjugated organicspecies. It has been explored that the electron-acceptor nitryl （–NO2） at the apex ofthe trumpet and the electron-donor amino （–NH₂） at the bottom edge of the trumpet toform O₂N–CNC–NH₂is beneficial to improve the NLO response of CNC.Significantly, the first hyperpolarizabilitiies （βtot） of O₂N–CNC–NH₂show anarc-shaped change as–NH₂substituting different H atoms along the arc-shapedbottom edge of CNC. Interestingly, the βtotvalues of H–CNC–NH₂also show anarc-shaped change. By comparing the βtotvalues of O₂N–CNC–NH₂andH–CNC–NH₂, it has been found that no matter which bottom H of H–CNC–H issubstituted by NH₂, the βtotvalue of H–CNC–NH₂will be increased about1.6timesafter substituting the top H with–NO2into the corresponding molecule. This studymay stimulate the search for new types of NLO materials based on CNC forapplication.