Yon-Rui Toh

Efficient electron transfer in carbon nanodot–graphene oxide nanocomposites

Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer many promising applications in photovoltaics and photocatalysis. In this paper we investigate electron transfer in nanocomposites of CNDs–graphene oxide (GO), –multi-walled carbon nanotubes (MWNTs) and –TiO2 nanoparticles without linker molecules, using steady state and time-resolved spectroscopy. Significant fluorescence quenching was observed in the CND–GO system, and it is attributed to the ultrafast electron transfer from CNDs to GO with a time constant of 400 fs. In comparison, carbon nanotubes result in static quenching of fluorescence in CNDs. No charge transfer was observed in both CND–MWNT and CND–TiO2 nanocomposites. This finding suggests that the CND–GO nanocomposite can be an excellent candidate for hot carrier solar cells due to the effective carrier extraction, broad spectral absorption, weak electron–phonon scattering, and thus a slow cooling rate for hot carriers.

Ultrafast electron transfer in the nanocomposite of the graphene oxide–Au nanocluster with graphene oxide as a donor

Graphene oxide has been extensively investigated as an electron acceptor due to its exceptional electronic and optical properties. Here we report an unusual ultrafast electron transfer occurring in the nanocomposites of Au nanocluster (Au NC)–graphene oxide (GO) in which GO acts as an electron donor. An ultrafast electron transfer is corroborated from the excited states of graphene oxide into the highest occupied molecular orbital (HOMO) of Au NCs. It is found that the electron transfer rate is significantly higher in Au10–GO nanocomposites (4.17 × 1012 s−1) than that in Au25–GO (0.49 × 1012 s−1) due to a larger energy difference and smaller sized ligands. This finding suggests that graphene oxide–Au nanocluster nanocomposites can be very useful to construct novel nanostructures with enhanced visible light photovoltaic, photonic and photo-catalytic activities.

Near-infrared enhanced carbon nanodots by thermally assisted growth

The near-infrared emission, matching the biological window, is conducive to biological applications. To date, most of the reported carbon nanodots emit the blue to green fluorescence and few of carbon nanodots emit the near-infrared with relatively lower efficiency. Here, we report an approach to generate or to enhance the near-infrared luminescence from the green luminescent carbon nanodots. Experiments reveal that the near-infrared emission is significantly enhanced by thermally assisted growth in vacuum, which is attributed to the formation of the larger nano-domains from the small carbon clusters at elevated temperatures.

Induced pH-dependent shift by local surface plasmon resonance in functionalized gold nanorods

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful tool for chemical and biological sensing experiments. In this study, we observed LSPR shifts of 11-mercaptoundecanoic acid modified gold nanorods (GNR-MUA) for the pH range of 6.41 to 8.88. We proposed a mechanism involving changes of the dipole moment after protonation/deprotonation carboxylic groups of 11-mercaptoundecanoic acid (MUA) which plays an important role by modulating LSPR around the functionalized GNR. Such a stable and easily prepared GNR-MUA has potential to become one of the most efficient and promising pH nanosensors to study intra- or extra-cellular pH in a wide range of chemical or biological systems.

The enhancement of electron–phonon coupling in glutathione-protected Au25 clusters

Glutathione-protected Au25 clusters (Au25@GSH) are prospective for biological applications due to their biocompatibility and near infrared fluorescence. The weak electron-phonon coupling, however, restricts their applications in bioanalysis and therapeutics. Here we modify the properties of Au25@GSH by changing their ligands. The temperature dependent fluorescence shows that conjugation with different ligands results in modified temperature behavior. In particular, Au25@GSH-MPA evidently exhibits enhanced phonon coupling, therefore, resulting in a decrease in the emission energy and an increase in bandwidth upon increasing temperatures. The enhanced phonon coupling in modified Au25@GSH sheds new light on the future application of nanoclusters from early diagnosis towards therapeutics.

Dynamic study on the transformation process of gold nanoclusters

In this paper, the transformation process from Au8 to Au25 nanoclusters (NCs) is investigated with steady state fluorescence spectroscopy and time-resolved fluorescence spectroscopy at various reaction temperatures and solvent diffusivities. Results demonstrate that Au8 NCs, protected by bovine serum albumin, transform into Au25 NCs under controlled pH values through an endothermic reaction with the activation energy of 74 kJ mol(-1). Meanwhile, the characteristic s-shaped curves describing the formation of Au25 NCs suggest this process involves a diffusion controlled growth mechanism.

Studies of the photostability of CdSe/CdS dot-in-rod nanoparticles

Fluorescent CdSe/CdS dot-in-rod nanoparticles have attracted considerable interest for fundamental research and potential applications in bioscience and physical science. In this work, we investigated photoinduced phenomena in CdSe/CdS dot-in-rods using time-resolved photoluminescence (PL). Our experimental results show that photopassivation, photooxidation, and photoinduced defect/surface states occur in CdSe/CdS dot-in-rods, and these processes depend on the irradiation amount. The thickness of the CdS shell plays an important role for photostability. Photopassivation was found to initially result in an increase of PL intensity when fluence is low. Photooxidation was found to cause a spectral blue shift due to shrinkage of the core. We also found that a thick shell of CdS can effectively suppress photooxidation and hinder the diffusion of oxygen into the core. Irradiation could generate defect/surface states predominantly in the shell and also on the interface between the core and the shell. A weak PL shoulder in the blue side was observed in heavily irradiated samples as a result of photoinduced rupture of encapsulation.Graphical Abstract

Optical properties and electron dynamics in carbon nanodots

Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer promising applications in photonics, photovoltaics and photocatalysis. Herein we study the optical properties and electron dynamics in CNDs using steady state and time-resolved spectroscopy. The photoluminescence (PL) is determined to originate from both core and surface. The massive surface fluorophores result in a broad spectral fluorescence. In addition to various synthesis techniques, it is demonstrated that the PL of CNDs can be extended from the blue to the near infrared by thermal assisted growth. Directional electron transfer was observed as fast as femtosecond in CND-graphene oxide nanocomposites from CND into graphene oxide. These results suggest CNDs can be promising in many applications.