Synthesis of Semiconductor Heterostructures by Surface Modification of Nanocrystals

Abstract

Nanocrystals (NCs) have significant portions of atoms on surfaces, and the surface atoms are passivated with surface ligand molecules. Especially, stoichiometry of NC surface atoms can heavily govern their optical, electronic, and catalytic properties. Understanding the interface chemistry between a NC and its surface ligand molecules can provide a systematic method to tailor NC properties. This thesis mainly describes the modulation of the NC surface stoichiometry for controlled optical properties of NC and heterostructure growth. In Chapter 2, surface atom stoichiometry was flexibly controlled for Ag2S NCs to yield dominantly cation-rich-surfaced Ag2S NCs (Ag-Ag2S), anion-rich-surfaced Ag2S NCs (S-Ag2S), and perfectly stoichiometric Ag2S NCs (st-Ag2S). Ion-pair ligand assisted NC surface reactions are performed for such stoichiometry control using combinations of cation and anion pairs. Depending on the NC surface stoichiometry and hardness/softness of ion-pair ligands, NCs undergo disparate surface reactions of (i) exchange by the anion X-type ligand expelling native X- and L-type ligands, (ii) binding of the whole ion-pair ligand, and (iii) one-to-one exchange by the positive cation ligand. When compared to non-stoichiometric NCs, stoichiometric Ag2S NCs show the fluorescence intensity increased by more than 10 times and more than three times enhanced ZnS growths when used as seeds for catalytic growths. In Chapter 3, the surface reaction study between off-stoichiometric Ag2S NCs and thiol was explained. Dodecanethiol (DDT), a representative thiol, is utilized to induce the surface reaction and investigated NC/ligand interface change. To reveal the effect of surface compositions in Ag2S NCs, both S-Ag2S and Ag-Ag2S was synthesized, and reacted with DDT ligands. During the surface reaction with DDT, S-Ag2S lost their S-rich surface and was modified into a stoichiometric surface with 190 times enhanced PL property, but Ag-Ag2S maintained Ag-rich surface. The surface reaction by DDT also caused the digestive ripening of both Ag2S NCs and thus their size distributions were narrowed. For better colloidal stability of DDT bound stoichiometric Ag2S NC, ion-pair ligands were used to intensify the NC/ligand binding. Labile DDT ligands were successfully exchanged by CTAB and exhibited stronger binding character. DDT bound stoichiometric Ag2S NCs were further used as a multifunctional seed for ZnS nanorod growth and cation exchange reaction with In3+, Bi3+, and Au3+. DDT bound stoichiometric Ag2S NC showed three times enhanced ZnS growth than off-stoichiometric Ag2S and produced alloy AgInS2, AgBiS2, and Ag3AuS2 NCs.