EXPERIMENTAL CHARACTERIZATION AND MODELING OF ASPECT-RATIO-DEPENDENT DIFFUSION OF NANOCRYSTALS IN NANOCHANNELS
The investigation of aspect-ratio-dependent diffusion and equilibrium partitioning of semiconductor nanocrystals in rectangular nanochannels has been undertaken. The nanocrystals are terminated with hydrocarbon ligands and suspended in toluene, considered an aprotic solvent. Time-dependent concentration profiles are measured as a function of distance from the channel entrance by the calibrated fluorescence intensity of the nanocrystals. Both the forward diffusion of the nanocrystals and their reverse diffusion are characterized. Nanocrystals are first allowed to diffuse forward from a reservoir into the nanochannels over a long period (\u2265 3 days) until the inlet concentration reaches a steady state. Subsequently, these nanocrystals are allowed to diffuse out, in reverse, from the nanochannels back into the reservoir that is filled with neat toluene. The experimentally observed concentration profiles during forward diffusion are compared to the profiles resulting from a continuum transport model accounting for diffusion and wall-adsorption of nanocrystals. The transport model provides estimates on the diffusion constant, equilibrium coefficients, and the adsorption/desorption rate constants. For the aspect ratios ranging from 1 to 6, the observed concentration within the nanochannels is significantly higher (\u2265 x4) than the initial bulk concentration within the reservoir, indicating wall adsorption of nanocrystals. Despite the significant level of fluorescence intensity originating from the wall adsorbed NCs, the fractional surface coverage of nanocrystals remains below 1 monolayer. Using a two-site adsorption model, the diffusion constants from the forward diffusion profiles range from 10^-8 to 10^-9 cm^2 s^-1, while the adsorption rate coefficients range from 0.36 to 1.60 (x10^-15 cm^2 s^-1). In general, we observe an attractive van der Waals force between the NCs and the channel walls that allows for a reversible wall adsorption that strongly affects the apparent diffusion of NCs within nanochannels.
nanochannel, nanofluidic, CdSe, nanocrystal, diffusion, adsorption
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