Pinned emission from ultrasmall cadmium selenide nanocrystals

Abstract

We report pinning of the emission spectrum in ultrasmall CdSe nanocrystals with a diameter of 1.7 nm and smaller. It was observed that the first emission feature ceased to blueshift once the band edge absorption reached 420 nm, though the band edge absorption continued to blueshift with decreasing nanocrystal diameter.

On ultrasmall nanocrystals

“…Ultrasmall CdSe nanocrystals synthesized with a surfactant mixture of dodecylphosphonic acid, hexadecylamine and tri-n-octylphosphine oxide have an additional emission feature residing between a strongly Stoke-shifted first emission peak (not believed to be band-edge) and the broad deep trap emission 35, 40,41 Figure 1. shows a typical absorption and emission spectrum of ultrasmall CdSe, which produces a balanced white-light emission with Commission internationale de l'éclairage (C.I.E.)…”

Pinned Luminescence Emission and Absorbance Band from Ultrasmall Ball-Milled Cd0.3Zn0.7Se Nanocrystals

“…Similarly, Puzder et al suggested that surface states are responsible for the pinning of the conduction band [21]. Recently, Dukes et al concluded that the pinned emission is due to a surface state introduced by a phosphoric acid ligand bound to the nanocrystal surface [18]. By contrast, Zanella et al merely associated the fixed peaks in the absorption spectra with the discontinuous growth of groups II-VI semiconductor nanocrystals [22].…”

“…Decrease in nanocrystal diameter results in the blueshifting of the band-edge emission and band-edge absorption through the quantum confinement effects [14][15][16][17][18]. However, optical band gap insensitivity occurs when the particle sizes are less than 2 nm.…”

“…In this method, the influence of the surface ligand, particularly that ascribed to the root cause of the surface state by Dukes et al [18], is completely eliminated.…”

“…The nanocrystals exhibit new features, such as a fixed absorption peak at the short wavelength of the absorption spectrum, when their sizes decrease further to values less 2 nm [17]. The first emission feature might also become pinned and appear as a broad emission [18].…”