Template-Guided Self-Assembly of Colloidal Quantum Dots Using Plasma Lithography

Quantum dots are building blocks in emerging applications such as diagnostics, imaging, nanoelectronics, nanophotonics, and energy. Selective placement of colloidal quantum dots into large-area micro–nanopatterns, however, remains unresolved. We describe a nonconventional technique wherein selective plasma modification of a substrate guides self-assembly of colloidal quantum dots with features as small as 100 nm. In addition, we demonstrate plasma lithography applicability to diverse nanoscale building blocks, including fluorescent nanoparticles, gold nanoparticles, salts, and proteins. Several methods currently exist for arranging colloidal quantum dots. These methods include molecular scaffold-based assembly, microbead-based assembly, and arranging free quantum dots from a liquid solution. These techniques can yield useful nanoscale structures, but long-range order and control over the pattern and the ability to produce deterministic shapes necessary for a useable device can be lacking due to the nature of the processes. On the other hand, template-guided self-assembly on a solid substrate is a promising strategy for developing functional quantum-dot devices that can be integrated with other top-down-fabricated systems. Despite the recent development of quantum-dot self-assembly techniques, complex and time-consuming steps are required to spatially arrange quantum dots on solid substrates. Few techniques are available for batch fabrication of quantum-dot-based structures that span the 100 nm to 1 cm scale and that are applicable to different substrate materials. Furthermore, the ability to modulate the density of the quantum dots over patterned areas, which has important implications in sub-wavelength nanophotonic integrated circuits and other coupled quantum-dot devices, are limited. One inherently rapid and effective method for surface patterning, which is compatible with a large variety of materials and top-down fabricated systems, is plasma surface modification. Using this patterning method, we have developed a technique, plasma lithography, that is suitable for guiding the self-assembly of nanoscale building blocks. The basis of using plasma lithography to guide the self-assembly of quantum dots is realizing a spatially selective plasma modification as a means to guide attachment of quantum dots to selective areas of a substrate. The fabrication method involves creating a nanoscale mold with 3D topography that when placed in contact with a substrate can shield selective areas of the substrate from the modification of a plasma, while allowing plasma surface treatment with other exposed areas. This results in a spatial pattern of functionalized templates that can guide the attachment of quantum dots. The plasma-patterning technique uses a deformable mold to produce conformal contact with a surface, allowing patterning of both planar and nonplanar surfaces. Existing plasma-patterning methods typically use resist-defined masks or open-contact (stencil)-type masks where plasma contacts the substrate through holes defined in the masks. A deformable mold with topographically defined channels can achieve high-resolution structures without the need to create high-aspect-ratio structures or very thin masks, because it is not necessary to cut completely through the mask. The plasma lithography approach we have developed is a rapid, batch method that uses a simple, reusable master and is generally applicable to patterning a variety of nanoparticles and molecules onto a surface. The plasma surface modification can also be directly applied tomodify a variety of substrate materials, and is one of the few techniques that can be specialized for functionalizing polymeric substrates that, due to their biocompatibility and tunable properties, are attractive for nanoscale and other applications. To create templates for self-assembly of quantum dots, 3D molds were placed on polystyrene substrates during plasma treatment in order to spatially functionalize the polystyrene surface (Fig. 1a–d). Weights amounting to a pressure of 5–10 kPa were placed on top of the mold in order to ensure conformal contact between the deformable mold and the surface. Themold cavities provided for plasma treatment of selective areas of the substrate, which then spatially functionalized the surface. We have demonstrated that the plasma lithography technique can be applied for different substrate materials including polystyrene as well as glass, polydimethylsiloxane (PDMS), and other polymers (data not shown). For instance, the surface of polystyrene, which was used for most of the experiments, has phenyl functional groups, and is hydrophobic (contact angle 818) in its untreated state. After plasma treatment with atmospheric gas (air), it becomes hydrophilic (contact angle 108), and presents oxygen-based functional groups (see also Supporting Information Fig. S5). This type of plasma modification is transient over long periods of time (hours to days), as the contact angle will tend to return to its unmodified state over time with exposure to air post-plasma modification. For