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