Dejian Dai

Red shift in the photoluminescence of colloidal carbon quantum dots induced by photon reabsorption

We synthesize the colloidal carbon/graphene quantum dots 1–9 nm in diameter and study their photoluminescence properties. Surprisingly, the luminescence properties of a fixed collection of colloidal carbon quantum dots can be systematically changed as the concentration varies. A model based on photon reabsorption is proposed which explains well the experiment. Infrared spectral study indicates that the surfaces of the carbon quantum dots are substantially terminated by oxygen atoms, which causes their ultra-high hydrophilicity. Our result clarifies the mystery of distinct emission colors in carbon quantum dots and indicates that photon reabsorption can strongly affect the luminescence properties of colloidal nanocrystals.

Excitation and recombination photodynamics in colloidal cubic SiC nanocrystals

We studied the photodynamics of the different-sized colloidal cubic SiC nanocrystals in distinct polar and nonpolar solvents. The UV-visible absorption spectral study indicates that the SiC nanocrystals with an average size of 4 nm retain an indirect energy gap; whereas the smaller quantum dots about 1 nm in size exhibit discrete and sharp absorption features indicating their discrete energy levels and the result agrees well with theoretical results. The colloidal SiC nanocrystals exhibit triple-exponential photoluminescence decay with nanosecond-order lifetimes which show slight size-dependence.

Identification of luminescent surface defect in SiC quantum dots

The surface defect that results in the usually observed blue luminescence in the SiC quantum dots (QDs) remains unclear. We experimentally identify that the surface defect C=O (in COO) is responsible for this constant blue luminescence. The HO···C=O [n(OH) → π*(CO)] interaction between the hydroxyl and carbonyl groups changes the energy levels of C=O and makes the light absorption/emission arise at around 326/438 nm. Another surface defect (Si–Si) is identified and its light absorption contributes to both C=O-related luminescence and quantum-confinement luminescence of the SiC QDs.

Highly bright tunable blue-violet photoluminescence in SiC nanocrystal–sodium dodecyl sulfonate crosslinked network

We report strong photoluminescence in an ultra-small surface oxidized SiC quantum dot-sodium dodecyl sulfonate crosslinked network. The peak emission wavelength is tunable spanning a wide blue-violet spectral region showing clear quantum confinement effects. The photoluminescence decay exhibits triple recombination dynamics with an average lifetime of 13.65 ns.

UV-blue photoluminescence from close-packed SiC nanocrystal film

We observed stable photoluminescence from close-packed cubic SiC nanocrystals that are self-assembled into thin solid film. The peak wavelength shifts from blue to near UV with increasing excitation energy and follows well the quantum-confinement effect. The photoluminescence excitation spectrum indicates a 3.47 eV bandgap corresponding to a particle size of 2.3 nm. The nanocrystal film shows triple-exponential photoluminescence decay with lifetimes of 2.3, 8.5, and 36.9 ns. The results open the possibility of the solid state UV-blue light emitting by use of the SiC nanocrystals in photonics and photonics/electronics integration.We observed stable photoluminescence from close-packed cubic SiC nanocrystals that are self-assembled into thin solid film. The peak wavelength shifts from blue to near UV with increasing excitation energy and follows well the quantum-confinement effect. The photoluminescence excitation spectrum indicates a 3.47 eV bandgap corresponding to a particle size of 2.3 nm. The nanocrystal film shows triple-exponential photoluminescence decay with lifetimes of 2.3, 8.5, and 36.9 ns. The results open the possibility of the solid state UV-blue light emitting by use of the SiC nanocrystals in photonics and photonics/electronics integration.

Experimental evidence of α → β phase transformation in SiC quantum dots and their size-dependent luminescence

Phase transformation can occur among different SiC polytypes under extreme conditions such as high pressure or temperature. It remains unknown whether phase transformation can occur under normal conditions. We demonstrate that the α → β phase transformation can occur at ambient temperature and pressure in nanoscale SiC. The microstructural characterization and light absorption and emission spectroscopy demonstrate the occurrence of this phase transformation. It is found that the quantum-confinement luminescence dominates in larger SiC quantum dots (QDs) and the surface-defect luminescence dominates in ultrasmall SiC QDs. The rare phenomenon of multiple-phonon-assisted light absorption is observed in the SiC QDs.

Photoluminescence and light reabsorption in SiC quantum dots embedded in binary-polyelectrolyte solid matrix

We report photoluminescence (PL) and light reabsorption in the SiC quantum dots (QDs) embedded in the binary poly(allylamine hydrochloride)-sodium poly(styrene sulfonate) polyelectrolytes solid matrix. The emission maximum wavelength can be tuned over a wide blue-violet spectral region indicating a PL origin of quantum confinement. The emission maximum exhibits a red shift with increasing concentration of the QDs in the polyelectrolytes matrix. The analysis shows that this shift stems from reabsorption of PL of smaller SiC QDs by larger ones in the matrix. We find that deposition of Ag nanoparticles on the surface of the solid matrix can dramatically reduce light reabsorption owing to surface plasmon-induced concentration of incident light. Additionally, the PL intensity was enhanced by a factor of 1.4. Our results open the possibility of the SiC QDs-based solid blue-UV light emitters for applications in full-solid-state lighting and display.