Templated self-assembly of non-close-packed colloidal crystals: Toward diamond cubic and novel heterostructures
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Deying Xia and Chee C. Wong School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798
W. Craig Carter and Yet-Ming Chianga) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 7 June 2010; accepted 13 August 2010)
We demonstrate the formation of non-close-packed binary colloidal structures through a novel layer-by-layer directed self-assembly methodology. In this approach we deposit colloidal suspensions of particle concentration and controlled electrostatic potential onto a planar template with a periodic array of features that is able to trap the particles, nucleating ordered domains with a template-defined symmetry and periodicity that permits subsequent, sequential deposition to produce an ordered heterostructure. Specifically, a silicon template with a hole pattern formed by interference lithography that corresponds to [100] symmetry of a cubic system has been used. At low particle concentrations, and using a Debye length that is on the order of the particle diameter, ordered domains in which polystyrene (PS) particles occupy every other site in the template are formed. The remaining sites on the 2D template are then filled by identically sized silica particles using vertical deposition. This process is repeated to produce a second layer of the same structure. Upon removing the PS particles, a two-layer non-close-packed structure that is a half-unit-cell precursor to the diamond cubic structure is obtained. To our knowledge this is the first demonstration of colloidal self-assembly to obtain a non-close-packed multilayer structure. Challenges that remain in applying the approach to create extended three-dimensional structures are discussed. I. INTRODUCTION
In the absence of a template, like-sized spherical particles spontaneously assemble into close-packed face centered cubic (fcc) or hexagonally close-packed (hcp) arrays, generally with (111) orientation against a planar deposition surface, if one is available. Taken in analogy with atomic crystal structures, these well-known structures are interesting but constitute a very small subset of the ordered structures that might be possible—they are the colloidal equivalent of elemental fcc and hcp metals among the much larger constellation of all known crystal structures. In particular, non-close-packed colloidal crystals are difficult to fabricate, although they are of interest for their optical and other properties. The diamond cubic structure is an exemplar of particular interest due to its complete photonic band gap.1,2 Researchers have also used spontaneous self-assembly of particles (through spin-coating,3,4 convective assembly,5,6 etc.) to create a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.8 J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
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dense-packed binary colloidal crystals. In this instance, a non-close-packed crystal of one particle type c
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