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.
Using single-molecule confocal imaging techniques combined with time-correlated
single-photon counting we investigated the electron transfer (ET) rates to the single-walled
carbon nanotubes from various types of semiconductor hetero-nanocrystals of
type-I or type-II band alignment. We observed significantly larger ET rate for type-II
ZnSe/CdS dot-in-rod nanostructures as compared to type-I spherical CdSe/ZnS
core/shell quantum-dots, and to CdSe/CdS dot-in-rod structures. We demonstrated
that such rapid ET dynamics can compete with both Auger and radiative
recombination processes, leading to potentially more effective photovoltaic
operation. In another work, we used aligned single-walled carbon nanotubes as
saturable absorbers for ps laser pulse generation. Using the vertical evaporation
technique we fabricated saturable absorbers by transferring the water-soluble single
wall carbon nanotubes onto a hydrophilic quartz substrate. The fast recovery times of
the absorber were measured to be 136 fs and 790 fs. The modulation depth of the
absorber was about 1.5%. Passive mode-locked Nd: GdVO4 laser using such an
absorber was demonstrated. The continuous wave mode-locked pulses with the pulse
duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum
average output power of the mode-locked laser is 2.4 W at the pump power of 13 W.
Such a kind of absorbers has potential to be put into practical use.
Fluorescence of gold nanocubes is investigated down to the single-particle level using time-correlated single photon (SP) counting techniques with a combined confocal and atomic force microscopy system. In comparison with most fluorophores, gold nanocubes exhibit nonblinking and nonbleaching behavior at ambient environment with much faster fluorescence decay. Also, the photon statistics of single gold nanocubes follows a Poissonian distribution, unlike the SP emission characteristics of single semiconducting quantum dots. Therefore, from the single-particle perspective, the fluorescence of gold nanocubes is attributed to the radiative emission from excited localized surface plasmons.
Efficient diode-pumped Yb:LuY2SiO5 laser mode locked by single walled carbon nanotube saturable absorber is reported for the first time. Under the absorbed pump power of 12.83 W, continuous wave mode-locked pulses were generated with the maximum average output power of 1.50 W. We obtained pulses as short as 5.2 ps around a center wavelength of 1058.6 nm. The peak power and the single pulse energy of the mode-locked laser were up to 2.9 kW and 15.0 nJ, respectively.
In this report we explore how the surrounding environments around a single semiconducting nancrystal affect the
photoinduced electron transfer, charge trapping fluorescence lifetime and fluorescence blinking. Using the
time-correlated single photon counting techniques combined with confocal microscopy, we investigated
photoluminescence of single CdSe/ZnS quantum dots embedded in agarose gel and on conductive substrates as examples.
Understanding of the underlying mechanisms would allow us to better control of the photoluminescence properties of
nanoparticles and to improve their performance in biophotonics and optoelectronics applications such as fluorescence
markers, single-photon sources, photovoltaics and quantum dot lasers.
We report here our study on suppression of fluorescence blinking of colloidal CdSe/ZnS quantum
dots (QDs) for potential applications as high performance single-photon sources. Blinking is an
interesting property for single QDs, but it is undesirable for their applications. We have
demonstrated that by coupling these QDs to adjacent silver nanoprisms, we could not only
completely suppress the blinking but also enhance the fluorescence quantum yield, and also
increase the fluorescence decay rates. These single QDs also exhibit anti-bunching behavior which
is a signature for a single-photon emitter. In addition, we have also achieved blinking suppression
by embedded the QDs in agarose gel. The electrostatic environment around QDs due to negatively
charged fibers of gel might strongly affect the extent of blinking suppression. The mechanisms of
blinking suppression will be discussed in the frame work of diffusion-controlled electron transfer
model.
We will report our study of photoinduced structural dynamics of nanomaterials of various shapes and sizes by a femtosecond laser heating pulse as detected by time-resolved electron diffraction or transient optical absorption. This work improves the understanding of nanoscale heat transfer and the ultrafast structural dynamics in nanomaterials such as thin films, spheres, prisms, discs, rods, pyramids and cubes. This work allows us to elucidate the roles of dynamic expansion/contraction and the more well-known static linear expansion. Both mechanisms play an important but different role in the observed structural changes at a different time scale. We will present comparative analysis of the experimental measurements for various samples as obtained by our own group and those reported by others.
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