Surface-Functionalization-Dependent Optical Properties of II–VI Semiconductor Nanocrystals

Abstract

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

“…It is known that synthesis of QDs in the presence of anionic X-type ligands, e.g., the benzoate ion, results in surface metal cation rich nanocrystals to maintain the overall charge neutrality of ligand-coated QDs. [44][45][46][47][48][49] We believe that the existence of a similar atomic structure at the surface of our ultrasmall nanocrystals influenced the peak sharpness for 1S(e)-2S3/2(h) transition, as was recently demonstrated for CdSe QDs. 44 The ~8 nm red shift in the broad emission band of our nanocrystals also indicates that their surface was composed of different ligands, which influenced exciton delocalization as will be discussed later.…”

Surface Modification of CdSe and CdS Quantum Dots-Experimental and Density Function Theory Investigation

“…It is difficult to reproducibly apply chemical and biochemical procedures to these QDs because of their unstable nature. In addition to the use of thiol molecules to cap the surfaces of CdSe QDs, most researchers used various amines as ligands to modify the optical properties of CdSe QDs [40][41][42][43][44]. Talapin et al synthesized CdSe QDs in a three-component hexadecylamine (HDA)-trioctylphosphine oxide (TOPO)-trioctylphosphine (TOP) mixture [35].…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…They provide ''trap states'' for charge carriers, i.e., energy levels within the bandgap where the charge carrier is spatially localized. Indeed, the quantum efficiency of NC emission improves when the NC surface is covered with a protective shell of high-bandgap material [13,15,16,61,74,[243][244][245][246], or when ligands saturate chemical bonds on the surface [16,123,[247][248][249][250][251].…”

“…The simplest picture is that quenching can be suppressed by saturating chemical bonds of the surface atoms [15,16,61,[243][244][245][246][247][248][249]251]. This can be achieved by overcoating the NC either by a shell of another semiconductor or by a ligand layer.…”