9 research outputs found
Orientationally ordered silicon nanocrystal cuboctahedra in superlattices
This is an open access article published under an ACS AuthorChoice License. See Standard ACS AuthorChoice/Editors' Choice Usage Agreement - https://pubs.acs.org/page/policy/authorchoice_termsofuse.htmlUniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices
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Directing and characterizing silicon nanocrystal self-assembly
Silicon nanocrystals or quantum dots are non-toxic and exhibit unique size tunable opto-electronic properties. Their bright photoluminescence as well as their ability to generate more than one electron per photon absorbed make them good candidates for both bioimaging and photovoltaics applications.
Their surface can be functionalized with different ligands that protect them against oxidation and allow them to be dispersed and stable in a variety of solvents. Dodecane capped silicon nanocrystals can be dispersed in non-polar solvents such as hexane, chloroform or toluene for years without aggregating nor losing their optical properties.
When deposited on a substrate under certain conditions, dodecane capped silicon nanocrystals can self-assemble into 2D or 3D periodic arrays of quantum dots called superlattices. These ordered structures of nanocrystals can potentially enhance the conductivity of the nanocrystal thin film which would be helpful for photovoltaic applications. In order to perform charge transport measurements through superlattices, large uniform ordered nanocrystal films must be achieved. Two deposition processes are studied and optimized to lead to the formation of several microns large both 2D and 3D silicon nanocrystals superlattices. A model is developed to understand the superlattice growth mechanism and several parameters are found to influence the superlattice morphology.
Silicon nanocrystals can also be functionalized with chromophores to enhance their optical absorption. This can improve efficiencies of self-assembled quantum dots solar devices. Silicon nanocrystals functionalized with pyrene units are studied using transient absorption spectroscopy. They exhibit enhanced optical absorption and efficient carrier multiplication via energy transfer from the pyrene unit to the nanocrystal core.
Finally, carboxylate terminated silicon nanocrystals are stable in water over a wide range of pH which makes them suitable for self-assembly in aqueous media. Incorporating biological receptor-substrate sites to the nanocrystal surface can permit recognition-driven self-assembly. Carbodiimide activation is commonly used to biofunctionalize nanoparticles. This chemistry is tested to attach an amine terminated PEG (polyethylene glycol) molecule to silicon nanocrystals. PEG functionalization is found to improve silicon nanocrystal photoluminescence stability.Chemical Engineerin
Bubble Assemblies of Nanocrystals: Superlattices without a Substrate
A method was developed
to create free-standing nanocrystal films
in the form of solidified bubbles. Bubbles of octadecanethiol-capped
gold nanocrystals were studied by in situ grazing incidence small-angle
X-ray scattering (GISAXS) to determine how the absence of an underlying
substrate influences a disorder–order transition of a nanocrystal
superlattice. We find that the presence of the substrate does not
significantly change the nature of the disorder–order transition
but does lead to reduced interparticle separation and reduced thermal
expansion. Bubble assemblies of silicon and copper selenide nanocrystals
are also demonstrated
Structure–Properties Correlation in Si Nanoparticles by Total Scattering and Computer Simulations
High-energy synchrotron X-ray diffraction
coupled to atomic pair
distribution function analysis and computer simulations is used to
determine the atomic-scale structure of silicon (Si) nanoparticles
obtained by two different synthetic routes. Results show that Si nanoparticles
may have significant structural differences depending on the synthesis
route and surface chemistry. In this case, one method produced Si
nanoparticles that are highly crystalline but surface oxidized, whereas
a different method yields organic ligand-passivated nanoparticles
without surface oxide but that are structurally distorted at the atomic
scale. Particular structural features of the oxide-free Si nanoparticles
such as average first coordination numbers, length of structural coherence,
and degree of local distortions are compared to their optical properties
such as photoluminescence emission energy, quantum yield, and Raman
spectra. A clear structure–properties correlation is observed
indicating that the former may need to be taken into account when
considering the latter
Orientationally ordered silicon nanocrystal cuboctahedra in superlattices
This is an open access article published under an ACS AuthorChoice License. See Standard ACS AuthorChoice/Editors' Choice Usage Agreement - https://pubs.acs.org/page/policy/authorchoice_termsofuse.htmlUniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices
Cooling Dodecanethiol-Capped 2 nm Diameter Gold Nanocrystal Superlattices below Room Temperature Induces a Reversible Order–Disorder Structure Transition
We recently observed
that a disordered assembly of octadecanethiol-capped
gold (Au) nanocrystals can order when heated from room temperature
to 60 °C [Yu, Y.; Jain, A.; Guillaussier, A.; Voggu, V. R.; Truskett,
T. M.; Smilgies, D.-M.; Korgel, B. A. <i>Faraday Discuss.</i> <b>2015</b>, <i>181</i>, 181–192]. This “inverse
melting” structural transition was reversible and occurred
near the melting-solidification temperature of the capping ligands.
To determine the generality of this phenomenon, we studied by in situ
grazing incidence small-angle X-ray scattering (GISAXS) the structure
of assemblies of Au nanocrystals with shorter C<sub>12</sub> and C<sub>5</sub> alkanethiol capping ligands that form ordered superlattices
at room temperature and have a ligand melting-solidification temperature
below room temperature. Superlattices of dodecanethiol-capped Au nanocrystals
disorder when cooled below 260 K, which is the melting-solidification
temperature for dodecanethiol. Au nanocrystals capped with even shorter
pentanethiol ligands that have a melting transition below 100 K (the
lowest experimentally accessible temperature) do not undergo the disorder
transition
Silicon Nanocrystal Superlattice Nucleation and Growth
Colloidal
dodecene-passivated silicon (Si) nanocrystals were dispersed
in hexane or chloroform and deposited onto substrates as face-centered
cubic superlattices by slowly evaporating the solvent. The uniformity
of the nanocrystals enables extended order; however, the solvent and
the evaporation protocol significantly influence the self-assembly
process, determining the morphology of the films, the extent of order,
and the superlattice orientation on the substrate. Chloroform yielded
superlattices with step-flow growth morphologies and (111)<sub>SL</sub>, (100)<sub>SL</sub>, and (110)<sub>SL</sub> orientations. Hexane
led to mostly island morphologies when evaporated at room temperature
with exclusively (111)<sub>SL</sub> orientations. Higher evaporation
temperatures led to more extensive step-flow deposition. A model for
the surface diffusion of nanocrystals adsorbed on the superlattice
surface is developed
Orientationally Ordered Silicon Nanocrystal Cuboctahedra in Superlattices
Uniform silicon nanocrystals
were synthesized with cuboctahedral shape and passivated with 1-dodecene
capping ligands. Transmission electron microscopy, electron diffraction,
and grazing incidence wide-angle and small-angle X-ray scattering
show that these soft cuboctahedra assemble into face-centered cubic
superlattices with orientational order. The preferred nanocrystal
orientation was found to depend on the orientation of the superlattices
on the substrate, indicating that the interactions with the substrate
and assembly kinetics can influence the orientation of faceted nanocrystals
in superlattices