Oxygen sensitivity of atomically passivated CdS nanocrystal films

CdS nanocrystals (NCs) synthesized by colloidal chemistry methods have been intensively studied for various applications. However, little attention has been paid to the interaction between the oxygen molecules present in air and the NC surface, which has a strong influence on the electrical properties of NC films. Here, we discuss the effect of oxygen adsorption at the NC surface on the photoconductivity of CdS NC films that were treated by propyltrichlorosilane, which is known to replace the native ligands at the NC surface with chloride ions. The photocurrent-voltage (PIV) characteristics of NC@Cl films recorded under oxygen atmosphere reveal a significant reduction in the photocurrent, as compared to those recorded under argon or vacuum. We demonstrate that this reduction can be related to adsorbed oxygen ions that effectively passivate the NC surface. This passivation reduces the free electron concentration and thereby reduces the photocurrent. Furthermore, we have investigated the light intensity dependence of the photocurrent dynamics of our devices in argon and in oxygen. These measurements confirm that the adsorption of oxygen is a photo-assisted process. Eventually, the potential of using our devices as oxygen sensors is assessed. A remarkable sensitivity of 35 is obtained at room temperature for 10% (concentration) oxygen flow, which is at least one order of magnitude higher than the results reported in the literature. Our work clarifies the mechanism of the photoconductivity reduction in CdS NC films upon oxygen adsorption and opens up opportunities of exploring such devices for gas sensing applications.

Transfer from trap emission to band-edge one in water-soluble CdS nanocrystals

A simple and rapid route to water-soluble CdS nanocrystals stabilized by citrate was reported, and the transfer of citrate-stabilized CdS NCs from trap emission to band-edge one was studied systematically for the first time. It was found that heating in air, alkaline activation and illumination, all efficiently manipulated surface states of CdS NCs and controlled the emission states, leading to transferring CdS NCs from a broad trap emission (FWHM ~125 nm) to their strong, narrow band-gap emission (FWHM ~25 nm), comparable to that of CdS NCs synthesized by organic routes. Lifetime decay kinetic studies demonstrated that the average lifetimes for CdS NCs before and after transferred were 131.1 and 32.7 ns, respectively. The freshly-synthesized NCs were predominated by trap emission (~94%), while the transferred CdS NCs with well cubic structure dominated by band-edge emission (up to 91%). The tunable emissions of CdS NCs from violet to green could be achieved by controlling emission states of CdS NCs with different Cd/S molar ratios. The transfer mechanisms of CdS NCs from trap to band-edge emission were proposed to be epitaxial growth of a Cd(OH)(2) shell on CdS NCs core. The transition probability of energy states before and after transferred was further investigated.

Recovery of CdS nanocrystal defects through conjugation with proteins

The luminescence behavior of CdS nanocrystals in aqueous solution and in the presence of proteins has been deeply investigated. CdS nanocrystals have been prepared in water by thermal decomposition of a single organometallic precursor assisted by thioglycerol, which acts as capping agent. Different experimental conditions have been explored to gain insights into the parameters affecting the nanocrystal growth. The CdS samples were characterized in terms of absorption and emission spectra, luminescence quantum yields, and decay times. These data together with size distribution analysis gave information on the growth mechanism and on the nature of the trap states formed in different experimental conditions. The emission properties of the nanocrystals in the presence of bovine serum albumin (BSA) have been examined to test how the electrostatic bioconjugation can influence the optical properties of the nanocrystals. The spectral changes observed upon addition of BSA indicated a direct interaction of the protein with the nanocrystal surface able to recover (at least partially) the defects formed during the crystal growth, resulting in improved emission properties.