Self- & Directed Nano/Microwire Assembly

Despite the tremendous promise of nanomaterials that offer desirable electronic, optical, and mechanical properties, it remains quite challenging to combine functional nano- and microparticles into predetermined architectures to produce working devices.   The Keating group is active in particle synthesis, surface functionalization, and assembly to meet these challenges.  In addition to more traditional chemical synthesis and particle functionalization, we also take advantage of lithographic microfabrication and applied electric fields in our assemblies. A fundamental question in this work is, “how can the construction of complex, multifunctional architectures be controlled on the nano- and microscale?” Long-term target structures include electronic and optical devices, with an emphasis on multicomponent assemblies and reconfigurability.  Our efforts in particle assembly are supported in part by IRG4 of the Penn State Center for Nanoscale Science, an NSF-funded Materials Research Science and Engineering Center (MRSEC). 

Patterned van der Waals’ attractions. Understanding the fundamental forces that act on anisotropic nanoparticles is important in bringing about improved “bottom-up” assembly methods, i.e., the rational design of functional structures assembled from particle building blocks. For example, we are investigating assembly mechanisms that take advantage of anisotropic van der Waals attractions along the length of microns-long nanocylinders to generate different types of assemblies depending on the particle pattern. This work is done in collaboration with the Fichthorn lab in Chemical Engineering at PSU, who perform simulations valuable in understanding and designing our experiments.

Assembly in applied electric fields. Spatiotemporally-controlled electric fields can be used to orient and position individual particles or particle assemblies at desired locations on a patterned substrate. We have worked with the Mayer lab in Electrical Engineering to design electrode geometries and fabrication strategies that enable control over particle assembly by dielectrophoretic assembly in applied AC fields. With this approach, we have prepared arrays of nanoresonators and placed different populations of DNA-functionalized nanowires at predetermined locations on an integrated circuit chip. We are also interested in using changes in applied field to reconfigure assemblies in real-time in order to change their optical properties.

DNA-functionalized nanowire resonators

TOC Figure crop

Nanowire lattice assemblies are reconfigurable by changing the frequency or turning the field on and off. These particle assemblies can also be re-oriented using perpendicular electrode sets to form reconfigurable polarizers for infrared light. The polarizer work is a collaboration with the Werner lab in Electrical Engineering.

Re-orientation of titania nanowires in an applied AC electric field can be used to turn on and off the emission from a random laser formed by multiple scattering in a dye solution of the nanowires. This work is a collaboration with the Liu lab in Electrical Engineering.

Vertical nanocylinder assemblies. Of particular interest are particle types that form arrays of standing composite particles, oriented with their long axes perpendicular to the underlying substrate. Self-assembly of partially etched nanowires (PENs) occurs spontaneously during sedimentation from suspension, without drying or applied fields. PENs, which have segments that are either gold or “empty” (solvent-filled) surrounded by a silica shell, are produced from striped metal nanowires by first coating with silica and then removing sacrificial segments by acid etching. The aspect ratio and relative center of mass (COM) of the PENs are important for determining whether the PEN long axes align vertically or horizontally in an assembly. Arrays with predominantly vertically aligned particles are observed for PENs with a large offset in COM relative to the geometric center, while other types of PENs produce horizontal arrays.  The assembly mechanism is scalable and quite versatile. It should be applicable to a wide range of particle, solution, and substrate types. Columnar nanowire and composite particle arrays such as these could find application in solar energy conversion.

Binary and multinary particle assemblies. Structures that combine multiple micro- or nanoparticles could incorporate their distinct properties to enable functions not possible from single-population assemblies. However, in mixed particle populations, the assembly forces may differ between the particle types, which will in turn influence the final assembled structures. The image below shows binary mixtures of partially-etched nanowires in which most of the particles are vertically oriented and are consequently seen as bright points in these reflectance optical microscopy images, which are taken from below the assembly through a glass coverslip (some particles can be seen laying horizontally; these appear as rods). Differences in the wavelength-dependent reflectivity of the different materials makes it possible to distinguish the two particle types in these images.

Representative publications: 

Formation and frequency response of two-dimensional nanowire lattices in an applied electric field. Boehm, S. J.; Lin,L.; Guzmán Betancourt, K.; Robyn Emery, R.; Mayer, J. S.; Mayer, T. S. Keating, C. D. Langmuir 201531, 5779-5786.

Self-assembled binary mixtures of partially etched nanowires.  Smith, B. D.; Kirby, D. J.; Boehm, S. J.; Keating, C. D.  Part. Part. Syst. Char. 2014 (DOI: 10.1002/ppsc.201400139). 

Microwell directed self-assembly of vertical nanowire arrays. Kirby, D. J.; Smith, B. D.; Keating, C. D., Particle and Particle Systems Characterization 210431, 492-499.

Self-assembly of segmented anisotropic particles: Tuning compositional anisotropy to form vertical or horizontal arrays. Smith, B. D.; Kirby, D. J.; Ortiz Rivera, I.; Keating, C. D. ACS Nano 20137, 825-833.

Deterministic assembly of functional nanostructures using nonuniform electric fields. Smith, B. D.; Mayer, J. S.; Keating, C. D. Annu. Rev. Phys. Chem201263, 12.1-12.23.

Vertical arrays of anisotropic particles by gravity-driven self-assembly. Smith, B. D.; Kirby, D. J.; Keating, C. D. Small 2011, 7, 781-787.

Assembly of gold nanowires by sedimentation from suspension: Experiments and simulation. Triplett, D. A.; Dillenback, L. M.; Smith, B. D.; Hernadez Rodriguez, D.; St. Angelo, S.; Gonzalez, P.; Keating, C. D.; Fichthorn, K. A. J. Phys. Chem. C 2010,114, 7346-7355.

Programmed assembly of DNA-coated nanowire arrays. Morrow, T. J.; Li, M.; Kim, J.; Mayer, T. S.; Keating, C. D. Science 2009323, 352.

Thanks to our Sponsors:


Kaufmann Foundation

National Science Foundation (IRG4 of the Penn State MRSEC)

               Spri© Chris Keating 2013