Deposition of Small Organic Molecules by the Displacement of Two Immiscible Supercritical Phases

摘要

A new coating process is described(deposition from two immiscible supercritical phases,or DISP)in which a solution of supercritical carbon dioxide(scCO_2 with a desired solute is displaced by supercritical helium(scHe).After depressurization,the solute is deposited on substrates initially submerged in the coating solvent.Micron-sized particles and thin films of sucrose octaacetate(SOA)were formed on silicon wafer substrate coupons from DISP at relatively low temperatures and pressures(= 1.5 wt at 4500 psi).Particle sizes decreased with increasing displacement velocity and increasing pressure.Estimates of characteristic times for diffusion and nucleation indicate that DISP is a diffusion-limited process.Optical microscopy and atomic force microscopy(AFM)were used to characterize film morphology,including defect formations and film roughness.Highly uniform films with low root-mean-square(RMS)roughness(approx 10 A)were obtained at a low displacement velocity of 0.0035 cm/s,while ring-like defect structures were observed in films deposited at a higher displacement velocity of 0.035 cm/s.The film thickness and morphology of the films deposited from DISP were compared with films from normal dip coating with typical organic solvents(acetone and toluene).Films deposited from scCO_2 by DISP were much thicker,more uniform,and exhibited much fewer drying defects and lower RMS roughness compared with films from the organic solvents.