How the Number and Location of Lithium Atoms Affect the First Hyperpolarizability of Graphene

Abstract

How do the number and location of lithium atoms affect the first hyperpolarizability (βtot) of graphene? In this paper, based on pentacene, a series of graphene (multi)lithium salts Lin@pentacene (n = 1, 2, 3, 4, 5, and 6) have been designed to answer this question. βtot obviously increases stepwise with an increase in the number of lithium atoms: 1369−1843 for Li@pentacence < 3510−4081 for Li2@pentacence < 6933−7934 for Li3@pentacence < 11 188−12 145 for Li4@pentacence < 14 904 au for Li5@pentacence, which are much larger than pentacence. This pattern suggests that the lithium salt effect on the first hyperpolarizability is very large. Unexpectedly, when an additional lithium atom is doped into the graphene multilithium salt Li5@pentacence, which leads to Li6@pentacence, the βtot value dramatically increases to a value of 4 501 764 au with a remarkable increase of 302-fold in contrast to Li5@pentacence. On the other hand, when the number of lithium atoms is equal, the location of lithium atoms also affects the βtot value: the closer the lithium atoms are clustered, the larger the βtot value: for Li3@pentacence, 6933 au of system 10 < 7401 au of system 9 < 7934 au of system 8. Furthermore, their transition energies (ΔE) are also obtained. The results show that ΔE decreases stepwise with an increase in the number of the lithium atoms, and ΔE of Li6@pentacence sharply decreases to 0.299 eV, which may explain the huge βtot value. This study may stimulate the search for new types of graphene NLO materials based on alkali metals for NLO application.

How the Number and Location of Lithium Atoms Affect the First Hyperpolarizability of Graphene

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