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Asymmetric Surface Modification

We are developing methods which will enable us to site-specifically modify particle surfaces resulting in "patchy" particles. The simplest form of a surface-asymmetric particle is a so-called Janus particle. Janus particles are named after the Greek god Janus, who is usually displayed with two faces looking in opposite directions. In order to be a Janus particle, the particle has to have two halves with differing chemical composition or physical properties. Such a Janus particle is interesting, because it has:
• amphiphilic properties similar to that of a surfactant,
• a giant dipole moment, and
• directional reactivity.

There are two general approaches to surface-asymmetric particles. One uses a mask to protect part of the particle surface from modification and the other employs the shadowing effect. We have employed both approaches.
Surface-asymmetric particles via masking[1]

Electroless deposition in combination with PDMS masking is used to generate silver-capped surface-anisotropic particles. First, particles are assembled in a monolayer on a silicon surface. Next, the monolayer is pressed into a precured PDMS film, which is subsequently cured at 70°C. After curing, the PDMS film with the partially embedded particles is peeled off the silicon wafer. The particle/PDMS composite is then exposed to a silver acetate solution which upon addition of ammonia and formaldehyde leads to deposition of silver islands on the exposed particle surface. Next, the PDMS composite is stretched and the particles are released from the PDMS (Fig. 1)leading to surface-anisotropic particles in solution (Fig. 2).

Preliminary results indicate that the deposition mechanism is diffusion limited (see Poster). As the stirring rate is increased during electroless deposition, the number and size of the silver islands increases.

Asymmetric particles via metal deposition.

Recently, we acquired a metal evaporation system (see equipment page), which enables us to make surface-asymmetric particles via the second approach using the shadowing effect. Particles are assembled into a close-packed monolayer onto a silicon wafer. Next, silver or gold metal is evaporated perpendicular to the wafer surface. The side of the particles facing the metal source is covered with a thin film of metal (Fig. 3), while the back of the particle is shadowed and therefore not coated yielding surface-anisotropic particles (Fig. 4).

Currently, we are investigating the effect of the angle between the particle layer and the metal source[2] on the patch size created. This method is also know as glancing angle deposition - GLAD.

Current Graduate Students:
Current Undergraduate Students:
Former Contributors: J.-Q. Cui, A. B. Pawar, Romesh Collins, Saumya Banerjee, Maria Mukovoztchick, Minna Carlberg

1)“Synthesis of Asymmetric Silver-decorated Polystyrene Spheres by Electroless Deposition” J.-Q. Cui and I. Kretzschmar* MRS Spring 2006 Meeting, San Francisco, CA PDF

Relevant Literature:
[1]“Surface-anisotropic Polystyrene Spheres by Electroless Deposition” J.-Q. Cui and I. Kretzschmar* Langmuir, 2006, 22 8281-8284. PDF
[2]“Fabrication of Si Nanocolumns and Si Square Spirals on Self-Assembled Monolayer Colloid Substrates” Y.-P. Zhao, D.-X. Ye, P.-I. Wang, G.-C. Wang, T.-M. Lu International Journal of Nanoscience, 2002, 1, 87 - 97.

• CCNY and CUNY start-up funds
• This work was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-02-13574,
by the New York State Office of Science Technology and Academic Research (NYSTAR), and by the NSEC Program of the
National Science Foundation under Award Number CHE-01-17752.
• CUNY Research grant for doctoral students.
• Funds from PSC-CUNY under Award Number 676310036