3 research outputs found
Ultrafast Photoluminescence in Quantum-Confined Silicon Nanocrystals Arises from an Amorphous Surface Layer
Here,
we examine ultrafast photoluminescence produced from plasma-grown,
colloidal silicon nanocrystals as a function of both particle size
and lattice crystallinity. In particular, we quantify the decay time
and spectral profiles of nominally few-picosecond direct-gap emission
previously attributed to phononless electronâhole recombination.
We find that the high-energy (400â600 nm, 2â3 eV) photoluminescence
component consists of two decay processes with distinct time scales.
The fastest photoluminescence exhibits an âź30 ps decay constant
largely independent of emission energy and particle size. Importantly,
nearly identical temporal components and blue spectral features appear
for amorphous particles. We thus associate high-energy, rapid emission
with an amorphous component in all measured samples, as supported
by Raman analysis and molecular dynamics simulation. Based on these
observations, we advise that the observed dynamics proceed too slowly
to originate from intraband carrier thermalization and instead suggest
a nonradiative origin associated with the amorphous component
Silicon Nanocrystals at Elevated Temperatures: Retention of Photoluminescence and Diamond Silicon to βâSilicon Carbide Phase Transition
We report the photoluminescence (PL) properties of colloidal Si nanocrystals (NCs) up to 800 K and observe PL retention on par with core/shell structures of other compositions. These alkane-terminated Si NCs even emit at temperatures well above previously reported melting points for oxide-embedded particles. Using selected area electron diffraction (SAED), powder X-ray diffraction (XRD), liquid drop theory, and molecular dynamics (MD) simulations, we show that melting does not play a role at the temperatures explored experimentally in PL, and we observe a phase change to β-SiC in the presence of an electron beam. Loss of diffraction peaks (melting) with recovery of diamond-phase silicon upon cooling is observed under inert atmosphere by XRD. We further show that surface passivation by covalently bound ligands endures the experimental temperatures. These findings point to covalently bound organic ligands as a route to the development of NCs for use in high temperature applications, including concentrated solar cells and electrical lighting
Broadband Absorbing ExcitonâPlasmon Metafluids with Narrow Transparency Windows
Optical
metafluids that consist of colloidal solutions of plasmonic and/or
excitonic nanomaterials may play important roles as functional working
fluids or as means for producing solid metamaterial coatings. The
concept of a metafluid employed here is based on the picture that
a single ballistic photon, propagating through the metafluid, interacts
with a large collection of specifically designed optically active
nanocrystals. We demonstrate water-based metafluids that act as broadband
electromagnetic absorbers in a spectral range of 200â3300 nm
and feature a tunable narrow (âź100 nm) transparency window
in the visible-to-near-infrared region. To define this transparency
window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped
silicon nanocrystals as opaque optical absorbers (âoptical
wallâ) in the UV and blue-green range of the spectrum. Water
itself acts as an opaque âwallâ in the near-infrared
to infrared. We explore the limits of the concept of a âsimpleâ
metafluid by computationally testing and validating the effective
medium approach based on the BeerâLambert law. According to
our simulations and experiments, particle aggregation and the associated
decay of the window effect are one example of the failure of the simple
metafluid concept due to strong interparticle interactions