Epitaxy: Programmable Atom Equivalents Versus Atoms

Abstract

The programmability of DNA makes it an attractive structure-directing ligand for the assembly of nanoparticle (NP) superlattices in a manner that mimics many aspects of atomic crystallization. However, the synthesis of multilayer single crystals of defined size remains a challenge. Though previous studies considered lattice mismatch as the major limiting factor for multilayer assembly, thin film growth depends on many interlinked variables. Here, a more comprehensive approach is taken to study fundamental elements, such as the growth temperature and the thermodynamics of interfacial energetics, to achieve epitaxial growth of NP thin films. Both surface morphology and internal thin film structure are examined to provide an understanding of particle attachment and reorganization during growth. Under equilibrium conditions, single crystalline, multilayer thin films can be synthesized over 500 × 500 μm 2 areas on lithographically patterned templates, whereas deposition under kinetic conditions leads to the rapid growth of glassy films. Importantly, these superlattices follow the same patterns of crystal growth demonstrated in atomic thin film deposition, allowing these processes to be understood in the context of well-studied atomic epitaxy and enabling a nanoscale model to study fundamental crystallization processes. Through understanding the role of epitaxy as a driving force for NP assembly, we are able to realize 3D architectures of arbitrary domain geometry and size.

Position‐ and Orientation‐Controlled Growth of Wulff‐Shaped Colloidal Crystals Engineered with DNA

“…[ 15 ] Although there have been crude early examples of colloidal crystals engineered on substrates, the ability to deliberately control the orientation, shape, and location of each individual crystal has not been realized. [ 15,16 ] Without such capabilities, it is difficult to envision how one could exploit such materials in device fabrication. Therefore, a synthetic approach needs to be developed that allows for such structural control.…”

Programmable Atom Equivalents: Atomic Crystallization as a Framework for Synthesizing Nanoparticle Superlattices

“…By tailoring the structure of the lithographically defined patterning, monolayers were epitaxially grown to match the {100}, {110}, and {111} crystal planes of a bcc superlattice . Later work demonstrated that subsequent rounds of deposition enabled the synthesis of a multilayer, millimeter‐sized single‐crystal (Figure B) . This proxy platform was then used to investigate the alleviation of strain due to lattice mismatch in the PAE thin films by patterning nanodots with periodicities inconsistent with the equilibrium, bulk PAE lattice parameter .…”

“…This work also highlights a conceptual advance in controlling PAE microscale structure through combining top‐down lithographic templating and bottom‐up PAE self‐assembly. With such a template, the overall shape of the crystallite is dictated by a set of arbitrary boundary conditions, and its size is theoretically unlimited beyond the conditions that limit the size of the pattern that can be lithographically fabricated …”

“…Recently, preliminary work has been done to extend spatial control over PAEs at a surface by templating the deposition of PAE films using top‐down lithography methods (Section .). Strategies include using site‐specific e‐beam irradiation to damage a DNA‐functionalized interface prior to deposition of a PAE monolayer resulting in NP size segregation, depositing PAEs onto lithographically defined gold “landing pads” or into PMMA wells, and epitaxial deposition on arbitrarily shaped arrays of gold nanodots . Future work may be done to investigate postprocessing techniques to manipulate the PAE material after assembly.…”

Light‐Responsive Colloidal Crystals Engineered with DNA

“…To integrate colloidal crystals into device structures, a variety of different lithography techniques have been developed, including nanoimprinting, direct‐write 3D printing, and e‐beam lithography (EBL) . Although quite useful for many purposes, nanoimprinting often causes structural deformation, direct write 3D printing is limited with respect to the size and composition of the crystal building blocks and the types of symmetries that can be accessed, and EBL is time‐consuming, labor‐intensive, and difficult to implement over large scales.…”