No abstract

Luminescent copper nanoclusters (Cu NCs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Here, we demonstrate the synthesis of L-cysteinecapped Cu NCs in aqueous medium having a bright cyan emission (489 nm) with a quantum yield of 6.2%. The structure of the Cu NCs (Cu 7 L 3 ) is investigated by using density functional theory (DFT) calculation and mass spectrometric study. Further, time-dependent density functional theory (TD-DFT) calculations provide the insights of electronic transitions, and it is correlated with experimental data. With near-HOMO−LUMO gap excitation, Cu NCs are excited to the S 4 state and subsequently relaxed to the S 1 state through an internal conversion process with a time scale in the ultrafast region (326.8 ± 6.5 fs). Furthermore, the structural relaxation in S 1 takes place at a picosecond time scale, and the radiative relaxation occurs from S 1 to S 0 . Finally, Cu NCs are attached with imidazole-functionalized partially reduced graphene oxide (ImRGO) via electrostatic attraction. A dramatic photoluminescence (PL) quenching and shortening of the decay time of the Cu cluster in the presence of ImRGO indicate the photoinduced electron transfer process, which is confirmed from ultrafast spectroscopic study. The photoinduced electron transfer in a Cu NC−ImRGO nanocomposite should pave the way for potential applications in light harvesting.

Gold nanoclusters (Au NCs) are new class of fluorescent nanomaterials with widespread applications in energy, water and healthcare. Here, we report a green synthesis of Au NCs with tunable emission wavelength from 590 to 510 nm in aqueous medium by core etching and ligand exchange method. Investigation reveals that the number of Au atoms present in the core of nanoclusters controls the emission wavelength. The quantum yield (QY) of nanoclusters increases from 0.57 to 3.15% with changing core from Au 12 to Au 6 . Time resolved spectroscopic study reveals that the emission with higher lifetime (>100 ns) originates from ligand to metal charge transfer (LMCT; S to gold core of NCs). It is demonstrated that the highly green emitting NCs (Au-510) are more sensitive than orange emitting NCs (Au-590) toward Pb 2+ . The detection limit of Pb 2+ is found to be 10 nM which is much lower than allowed concentration of Pb 2+ in drinking water. Thus, Au NCs based optical sensor is promising for the selective detection of Pb 2+ in drinking water.

The development of graphene oxide (GO)/semiconductor quantum dots (QDs) hybrid composite remains a frontier area of research to design optoelectronic, photovoltaic, and light harvesting devices based on an electron transfer process. Therefore, the examination of the electron transfer process from QDs to GO as a function of the number of sites of QD and the mean fractional surface coverage of QD by GO sheet with changing the size of QD and concentration of GO is an important issue to manipulate the performance of devices. Here, we have assembled graphene oxide-CdTe QD composite by the attachment of positively charged cysteamine capped CdTe QDs with negatively charged GO. The structural changes due to electronic interaction of graphene oxide with QDs have been evaluated using Raman spectroscopy. The shifting of G-band and increase of I D /I G intensity ratio reveal the electron transfer from excited QDs to GO. The fluorescence dynamics of QD has been investigated by time-resolved fluorescence spectroscopy, and the electron transfer rate (2.24 × 10 8 to 1.18 × 10 8 s −1 ) is found to be decreased with increasing the size of QDs. We analyze the decays of fluorescence by assuming a binomial distribution of number of available sites of QD and the mean fractional surface coverage of QD by GO sheet which control the quenching process. Analysis suggests that the average number of available sites (152 to 396) increases, the mean fractional surface coverage and the total quenching rate (1.3 × 10 8 to 0.18 × 10 8 s −1 ) are decreased with increasing the size of QD. It is noteworthy that an ∼6 fold increase in the photocurrent is found in this composite device under light illumination. Such graphene oxide-QD functional materials open up new possibilities in solar energy conversion, photovoltaic, and various potential applications.

Top down and bottom up approaches for the synthesis of alloy metal clusters.