Fractionation of metal and semiconductor nanoparticles and their deposition into wide area thin films and ordered arrays using CO2-expanded liquid solutions
Multi-scale and/or multi-disciplinary approach to process-product innovation
Nanotechnology & Nanomanufacturing (T3-1)
Keywords: nanoparticles, size-separation, CO2 gas-expanded liquid, thin films
The size dependent properties of metallic and semi-conductor nanoscale materials allow them to be engineered for specific applications such as in catalysis and quantum dots in optoelectronic devices. Solution based nanoparticle synthesis techniques are among the most simple, but, they often result in particles with a wide size range. To address this limitation, post synthesis processing is required to further refine the size distribution. This paper presents an environmentally friendly and efficient process for size selective fractionation of polydisperse metal and semiconductor nanoparticle dispersions into multiple narrow size populations (± 0.5 nm) using the pressure tunable physico-chemical properties of CO2 gas expanded liquid solutions. Our work has shown that ligand stabilized nanoparticles can be size selectively precipitated by controlling the addition of CO2 antisolvent (through pressurization) to an organic dispersion of nanoparticles. Compared to current liquid techniques, this CO2-expanded liquid approach provides for faster and more efficient size separation, a reduction in organic solvent usage, tunable size selection, and controllable deposition. In this CO2-expanded liquid nanoparticle precipitation technique, ligand capped particles are first dispersed in solution where the interaction between the solvent and the ligand tails provides enough repulsive force to overcome the inherent van der Waals attraction between the particles that would otherwise result in agglomeration and precipitation. Through the addition of CO2 antisolvent, the resultant poorer solvent mixture interacts less with the ligand tails than did the pure solvent, thereby reducing the ability of the solvent/antisolvent mixture to disperse the particles. Larger particles possess greater interparticle van der Waals attractions and therefore precipitate first upon worsening solvent conditions followed by subsequent precipitation of the smaller sized particles with further addition of antisolvent. To achieve these separations using CO2 as an antisolvent, a novel high pressure apparatus has been designed that allows the controlled nanoparticle precipitations to occur from a liquid situated at a specific location on a surface by simply tuning the CO2 pressure applied above the liquid dispersion. The efficiency of nanoparticle size fractionation was investigated on several types of metallic (Ag, Au, Pd, Pt) and semiconductor (CdSe/ZnS) nanoparticles to both illustrate the general applicability of the process and to provide fundamental information on the effects of several processing parameters.
In addition to demonstrating the benefits of this new size fractionation technique, we have extend the general phenomenon of nanoparticle precipitation with CO2 expanded liquids to an improved method for nanoparticle thin film deposition. Full exploitation of nanoparticles and their novel properties for application in areas such as catalysis, optical systems, electronic devices, and sensors requires the ability to effectively process and maneuver particles onto surfaces or support structures. This is often performed by simply evaporating a liquid solution containing ligand stabilized nanoparticles to leave behind dry nanoparticles coated on a surface. However, solvent dewetting and capillary forces at the liquid/vapor interfaces can lead to film defects such as nanoparticle islands, percolating networks, ring-like particle arrays, and uneven particle concentration. We have developed a novel nanoparticle deposition technique which utilizes CO2 as an anti-solvent for low defect, wide area metallic nanoparticle film formation employing monodisperse nanoparticles. Ligand stabilized metallic particles are precipitated from organic solvents by controllably expanding the solution with carbon dioxide. Subsequent addition of carbon dioxide as a dense supercritical fluid then provides for removal of the organic solvent while avoiding the dewetting effects common to evaporating solvents. These dewetting effects and interfacial phenomena can be very detrimental to nanoscale structures. Controllable expansion of the liquid solution via CO2 injection allows for control over the thermophysical properties that govern this deposition and assembly process. Ordered thin film arrays of metal nanoparticles were successfully produced using this CO2-expanded liquid technique provided that the original dispersion of nanoparticles was fairly monodisperse.
Presented Monday 17, 15:40 to 16:00, in session Nanotechnology & Nanomanufacturing (T3-1).
