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Particle MLD

The molecular layer deposition (MLD) method,1,2 which is similar to atomic layer deposition (ALD) and is also based on sequential, self-limiting surface reactions, can be utilized to deposit polymer films. In this process, molecules are stacked on substrates one by one in order of preference. The MLD technique offers the same advantages for polymer film deposition as ALD does for ceramic films. In addition, MLD can deposit hybrid polymer films using suitable precursors, such as trimethylaluminum (TMA) and ethylene glycol (EG) for aluminum alkoxide (alucone) hybrid polymer.3 Examples of alucone MLD on particles are shown in Figures 1 and 2. This vapor-phase method, which operates under vacuum and does not require solvents or catalysts, is a useful and promising technique for the fabrication of functional ultra-thin polymeric layers.

Many challenges are encountered during MLD on large quantities of nanoparticles. First, the native cohesive properties of the nanoparticles will form agglomerates that are several times larger than the primary particles. Therefore, the particles need to be fluidized or agitated to perform the MLD surface reactions in reasonable times and to prevent the particles from being aggregated by the MLD polymer film. Second, it is difficult to deliver the low vapor pressure, bulky organic monomer precursor to the reaction chamber and to subsequently remove the relatively sticky bifunctional monomers from the primary nanoparticle surfaces. This low vapor pressure requires the heating of the precursors to obtain sufficient reactant flux. Sufficient reactant flux can be obtained by preheating the organic precursors and using bubbler to dilute the sticky precursor vapors. Third, polymer films fabricated by MLD have more of a tendency to stick than those ceramic films fabricated by ALD, which may increase particle aggregation during the MLD coating process.

The MLD particle coating process can be carried out in a fluidized bed reactor (FBR), which can be used to efficiently deliver gas phase reactive precursors to bulk quantities of particles. Fluidized bed reactors (FBRs) have been successfully applied for ALD particle coating. This type of unit operation is commonly used for various steps throughout powder manufacturing processes. For example, pharmaceutical particles can be manufactured in a fine powder form and dried in an FBR. Integration of the particle MLD process into an existing unit operation would allow for precision polymer coatings on surfaces without significant capital equipment costs.

References

1. Yoshimura T, Tatsuura S, Sotoyama W. Polymer-films formed with monolayer growth steps by molecular layer deposition. Applied Physics Letters 1991; 59: 482-484.
2. Yoshimura T, Tatsuura S, Sotoyama W, Matsuura A, Hayano T. Quantum wire and dot formation by chemical vapor-deposition and molecular layer deposition of one-dimensional conjugated polymer. Applied Physics Letters 1992; 60: 268-270.

3. Dameron AA, Seghete D, Burton BB, Davidson SD, Cavanagh AS, Bertrand JA, George SM. Molecular layer deposition of alucone polymer films using trimethylaluminum and ethylene glycol. Chemistry of Materials 2008; 20: 3315-3326.

 

Figure 1. Scanning transmission electron microscopy (STEM) image of 50 cycles alucone MLD coated silica nanoparticles at 100 °C.

 

 

Figure 2. Transmission electron microscopy (TEM) image of 20 cycles alucone MLD coated titania nanoparticles at 160 °C.

 

 

 

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© 2009. Team Weimer


University of Colorado at Boulder University of Colorado at Boulder