The unique physical properties and ease of processing of nanoscale
materials has resulted in a focus on harnessing the core-dependent
properties of nanoscale materials, while the surface passivation
layer has been largely ignored. Our efforts in characterization
of nano-material surfaces have illustrated the importance of
molecular level interactions in these materials. The thermodynamics
of the interface between the quanutm dot surface and the surface
passivating organic capping moiety is dominated by inter-chain
interactions rather than head-group-surface interactions, producing
a 3-dimensional self-assembled monolayer. This packing motif
is reminiscent of self-assembled monolayer structures on single
crystals and liquid crystal structures, in which molecular level
interactions dominate the packing assembly thermodynamics. We
have probed the the surface packing of the passivant by correlating
phase transition behavior of the organic layer using attenuated
total reflection infrared spectroscopy (ATR-IR) coupled with
differential scanning calorimetry (DSC). The thermodynamic melting
dynamics in the passivant layer supports the description of
self-assembled monolayer like interactions, with an initial
constrained chain melting followed by alteration of the surface
packing. Correlation of chain length with particle diameter
and head group suggests in systems in which the radius of the
particle is smaller than the passivant layer thickness, chain-chain
interactions dominate thermodynamic stabilites, with only a
minimal contribution from head-group stabilization of the passivating
layer. This suggests quantum dot thermodynamic stability is
typically governed by chain-chain interactions in lyothermally
prepared materials. Our studies on the correlation of calorimetry,
Raman, IR, and XRD melting behavior of surface passivants is
leading to a new model for preparation of thermodynamically
stable nanoscale materials.
Interested in learning more?
Contact Artjay
Javier
or read the published articles:
"Effects of Alkylamine Chain Length on the Thermal Behavior
of CdSe Quantum Dot Glassy Films." Meulenberg, R.W.;
Bryan, S.; Yun, C.S.; Strouse, G.F. J. Phys. Chem B, 106,
7774-7780 (2002). [ view article -
PDF ]
"Chain Packing Analysis of the
Passivating Layer on Nanocrystalline Quantum Dot Surfaces."
Meulenberg, R.W.; Strouse, G.F. J. Phys. Chem. B 105, 7438-7445
(2001). [ view article
- PDF ]
Colloidal gold nanoparticles can be isolated with a variety
of surface-stabilizing ligands that allow them to be tailored
for specific reactions in both organic and aqueous media with
relatively good monodispersity and stability. The stability
of Au nanomaterials, as well as the frequency and bandwidth
of the surface plasmon resonance (SPR) band, is modulated by
the magnitude of the Lewis basicity, the bonding symmetry, and
the nature of the bonding to the metal particle surface. In
a recent report we have analyzed a series of Au nanomaterials
that have been isolated from excess citrate buffer by the addition
of oxyanions. This allows the nanomaterials to be isolated as
freely soluble powders that can be readily redissolved in water
or buffer solution for other applications, such as bio-labeling.
Analysis by UV-Vis and FTIR spectroscopy allows direct analysis
of the oxyanion interaction at the surface of the Au nanomaterial.
(Figure 3) These studies indicate the thermodynamic stability
and nature of binding is dictated by the availability of d-orbitals
for back bonding, while the SPR band frequency is largely modulated
by the electrostatics of the ligand. Comparison of the extinction
coefficient of the SPR band to Mie theory predictions further
illustrates the affect of ligand interactions on the properties
of the Au nanomaterials. For a series of oxyanions, the order
of thermodynamic stability is CO32- <
H2PO41- < SO42-,
while the shift in the SPR band follows the trend H2PO41-
> CO32- > SO42-.
Overall, this method produces gold colloidal nanoparticles with
labile surface-stabilizing groups that maintain the integrity
of the shape and size distribution of the original colloidal
gold.
Interested in learning
more?
Read the published articles:
"Analysis of the Nature of Oxyanion
Adsorption on Gold Nanomaterial Surfaces." Cumberland,
S.L.; Strouse, G.F. Langmuir, 18, 269-276 (2002).
[ view article - PDF
]
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