(166f) Searching for Ideal Structure-Directing Agents in Colloidal Copper Nanocrystal Synthesis

Cu nanocrystals are usually prepared using solution-phase synthesis, where structure-directing agents (SDAs) play an important role in facilitating shape-selective nanocrystal growth as additive molecules on Cu surfaces. There has been great interest to find promising SDAs, among which candidates with amine functionalities are demonstrated experimentally effective. We have investigated the adsorption of a series of alkylamines on Cu(100) and Cu(111) using first-principles density functional theory with dispersion correction. DFT results on the effective SDA ethylenediamine show that it binds stronger to Cu(100) than Cu(111) and forms a more densely-packed monolayer on Cu(100), agreeing with the experimental trend to produce {100}-faceted nanowires. Three groups of candidates with different symmetries were studied including linear-, cyclic-, and branched-alkylamines. The branched amines with rectangular structures show stronger preference on Cu(100) while cyclic amines with threefold symmetry selectively bind to Cu(111).

The projected density of states has been calculated upon the adsorption of these candidates, as well as the charge distribution and work function, to provide further insights into the binding process. Alkylamines are chemisorbed on top of the Cu surface atoms via lone-pair electrons of N atoms with charge depleted from Cu substrates to adsorbates, which is consistent with experimental NMR study. The bond strength is generally higher on Cu(100) due to stronger ability to donate electrons. Our results indicate that a high density of amine groups and good geometric matching with a specific Cu facet can promote nanocrystals that tend to express that facet.