58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Our research has explored self-assembled structures of nanowires as a potential route to produce solar cell substrates. Solar cells made of vertically aligned arrays of anisotropic particles isolate absorption and charge separation along the long and short particle axes. This isolation enables the use of cheaper polycrystalline material, while assembly could provide fast production of large arrays. Our studies used nanowires (~300 nm in diameter by 2-12 μm in length) stabilized by an amorphous silica shell. These particles are attractive because they can be synthesized in high yield, imaged with optical microscopy, and are comparable in dimensions to feature size in top-down fabricated nanowire solar cells. When nanowires are allowed to sediment, they form arrays that maximize van der Waals interactions (vdW) between their metal cores. We have developed the ability to control the particle orientation by introducing asymmetry via segmentation and through surface patterning. Assembly mechanisms have been evaluated with respect to particle composition and assembly substrate while also examining the assembly of mixed particle populations.  A brief summary of our findings follows.

Single-component particles such as solid Au nanowires form smectic rows that balance vdW attractions and electrostatic repulsion. Small changes in vdW due to asymmetric core segmentation can impact ordering within these assemblies. For instance, changing the material, size, position, and/or number of segments causes particles to assemble into either horizontally or vertically oriented arrays. In the case of horizontally aligned arrays, segmented particles show alignment with respect to their nearest neighbors. By comparing experimental results with computational predictions performed by our collaborators, we find that even very small differences in the vdW interactions of the segments is sufficient to align like materials in nearest neighbor particles. By increasing the difference in the vdW attractions of the segment materials the orientational ordering of nearest neighbors was improved. If a particle has both a large vdW and density difference between end segments, we have observed the spontaneous formation of vertically oriented arrays. We can produce particles such as this through the selective removal of a segment. We term these segmented hybrid nanowire/nanotube particles partially etched nanowires (PENs). They are synthesized by selectively etching a specific metal segment from silica-coated, striped nanowires to leave partially hollow silica nanotubes with a partial metal core. The density and vdW differences allowed the “hollow” end to diffuse off the surface while the gold end stayed on the substrate. At high particle densities, vertical orientations are supported by increased nearest neighbor interactions. Arrays of PENs have displayed ~70% of the particles oriented vertically (standing) and we have shown the process to be scalable from 0.01 to greater than 10 cm². By using the proper segment dimensions, vertical assembly was even achieved at 70% standing for particles with an aspect ratio of ~40. These results represent significant steps in assembly over large areas and with high aspect ratio particles without using applied fields.

We have also explored surface patterning as a way to improve and guide the orientation of assembled nanowires. Microwells were fabricated using standard lithographic processing to be used as selective assembly sites that could passively direct particle alignment. Microwell dimensions affected the particle orientation and quality of resulting structures, whether they formed vertical or horizontal arrays. For PENs, the standing percentage was observed to increase as the well dimensions approached the particle length. Average standing percentages as high as 92 % were observed in the smallest lithographic features studied (4.3 × 4.3 µm and 3.2 µm deep). Many wells contained perfect assemblies (100 % standing). The increased standing is driven by photoresist walls, which supported vertical orientations, and the geometry of smaller microwells (i.e., the particles could not lay down). Assembly of solid nanowires on the same patterned surfaces formed smectic rows aligned with the walls of the microwell. Again, as the well dimensions approached the particle length, these nanowires were forced into columnar arrays. Deep microwells (4.5 μm) with edge dimensions smaller than the target nanowire lengths (6 μm) restricted assembly to the vertical orientation only and showed average standing percentages over 90 % and again many perfectly assembled wells.

Microwell-based assembly has presented a route to further processing including sample drying, electron microscopy, and potential integration with functional surface materials. Electron microscopy confirms that assembled nanowires retain their vertical orientation after drying. Additionally, dried assemblies have been covered with and cured into polydimethyl siloxane (PDMS). Once cured, the PDMS was peeled from the assembly substrate and nanowires were shown to transfer to the PDMS and retain their structure. This serves as a proof of concept for removal of the assembly from the original surface and shows the potential for nanowires to be used in flexible devices.

An advantage of bottom-up assembly is the potential for mixing multiple types of component particles together into a single assembly. Currently very little work has been done in multicomponent assembly for nanowires of this size. Using PENs, we have started exploring the assembly of multiple nanowire populations on both open and patterned substrates. Combined populations of PENs with different core materials or different segmentation (number, location, size) can be assembled. 

Financial support from the ACS-PRF has been extremely important in seeding this new direction in our research.  We were recently awarded a grant from the Kaufman Foundation to study assembly of semiconducting and photoactive nanowires, and are in the process of applying for funding from the National Science Foundation for collaborative work in which assemblies driven by external fields will be employed to generate optical and electrical devices. We hope to ultimately use the knowledge generated here to advance the production of a solar energy collection device based on particle assembly.