Metal Oxide Heterostructure Array via Spatially Controlled-Growth within Block Copolymer Templates.

Nanofabrication is continuously searching for new methodologies to fabricate 3D nanostructures with 3D control over their chemical composition. A new approach for heterostructure nanorod array fabrication through spatially controlled-growth of multiple metal oxides within block copolymer (BCP) templates is presented. Selective growth of metal oxides within the cylindrical polymer domains of polystyrene-block-poly methyl methacrylate is performed using sequential infiltration synthesis (SIS). Tuning the diffusion of trimethyl aluminum and diethyl zinc organometallic precursors in the BCP film directs the growth of AlOx and ZnO to different locations within the cylindrical BCP domains, in a single SIS process. BCP removal yields an AlOx -ZnO heterostructure nanorods array, as corroborated by 3D characterization with scanning transmission electron microscopy (STEM) tomography and a combination of STEM and energy-dispersive X-ray spectroscopy tomography. The strategy presented here will open up new routes for complex 3D nanostructure fabrication.

The Development and Atomic Structure of Zinc Oxide Crystals Grown within Polymers from Vapor Phase Precursors.

Sequential infiltration synthesis (SIS), also known as vapor phase infiltration (VPI), is a quickly expanding technique that allows growth of inorganic materials within polymers from vapor phase precursors. With an increasing materials library, which encompasses numerous organometallic precursors and polymer chemistries, and an expanding application space, the importance of understanding the mechanisms that govern SIS growth is ever increasing. In this work, we studied the growth of polycrystalline ZnO clusters and particles in three representative polymers: poly(methyl methacrylate), SU-8, and polymethacrolein using vapor phase diethyl zinc and water. Utilizing two atomic resolution methods, high-resolution scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy, we probed the evolution of ZnO nanocrystals size and crystallinity level inside the polymers with advancing cycles─from early nucleation and growth after a single cycle, through the formation of nanometric particles within the films, and to the coalescence of the particles upon polymer removal and thermal treatment. Through in situ Fourier transform infrared spectroscopy and microgravimetry, we highlight the important role of water molecules throughout the process and the polymers' hygroscopic level that leads to the observed differences in growth patterns between the polymers, in terms of particle size, dispersity, and the evolution of crystalline order. These insights expand our understanding of crystalline materials growth within polymers and enable rational design of hybrid materials and polymer-templated inorganic nanostructures.

Revealing the 3D Structure of Block Copolymers with Electron Microscopy: Current Status and Future Directions.

Block copolymers (BCPs) are considered model systems for understanding and utilizing self-assembly in soft matter. Their tunable nanometric structure and composition enable comprehensive studies of self-assembly processes as well as make them relevant materials in diverse applications. A key step in developing and controlling BCP nanostructures is a full understanding of their three-dimensional (3D) structure and how this structure is affected by the BCP chemistry, confinement, boundary conditions, and the self-assembly evolution and dynamics. Electron microscopy (EM) is a leading method in BCP 3D characterization owing to its high resolution in imaging nanosized structures. Here we discuss the two main 3D EM methods: namely, transmission EM tomography and slice and view scanning EM tomography. We present each method's principles, examine their strengths and weaknesses, and discuss ways researchers have devised to overcome some of the challenges in BCP 3D characterization with EM- from specimen preparation to imaging radiation-sensitive materials. Importantly, we review current and new cutting-edge EM methods such as direct electron detectors, energy dispersive X-ray spectroscopy of soft matter, high temporal rate imaging, and single-particle analysis that have great potential for expanding the BCP understanding through EM in the future.

Polymer dewetting in solvent-non-solvent environment- new insights on dynamics and lithography-free patterning.

HYPOTHESIS: We show that one may employ polymer dewetting in solvent-non-solvent environment to obtain lithography-free fabrication of well-defined nano- to micro- scale polymer droplets arrays from pre-patterned polymer films. The polymer droplet pattern may be converted to a series of hybrid organic-inorganic and inorganic well-defined nano-patterns by using sequential infiltration synthesis (SIS). In particular, we scrutinize the physical parameters which govern the dewetting of flat and striped polymer thin films, which is the key to obtaining our objective of lithography-free ordered nano-patterns. EXPERIMENTS: We immerse polystyrene (PS) and polymethyl methacrylate (PMMA) thin films in water in the presence of chloroform vapors. We study the ensuing polymer dewetting dynamics and the pattern formation of nanospheres by employing in-situ light microscopy and scanning electron microscopy. We then investigate pattern formation by dewetting of polymer stripes, fabricated by directed solvent evaporation, and SIS of AlOx from vapor phase precursors, trimethyl aluminum (TMA) and H2O, within the nanosphere patterns. FINDINGS: We find that solvent- non-solvent environments render film dewetting rates, which are an order of magnitude faster than solvent vapor dewetting, and supports the formation of small solid polymer droplets, down to sub-100 nm droplet size, of large contact angles with the solid substrate. Pre-patterned polymer film stripes support the formation of highly ordered structures of polymer droplets, which are easily transformed to hybrid polymer-AlOx nanosphere patterns and templated AlOx nanosphere via SIS.

Tin oxide nanostructure fabrication via sequential infiltration synthesis in block copolymer thin films.

Tin oxide (SnO2) nanostructures are attractive for sensing, catalysis, and optoelectronic applications. Here we investigate the fabrication of SnOx nanostructures through sequential infiltration synthesis (SIS) in block copolymer (BCP) film templates. While the growth of metal and metal oxides within polymers and BCP films via SIS has been demonstrated until now using small precursors such as trimethyl aluminum and diethyl zinc, we hypothesize that SIS can be performed using larger precursors and demonstrate SnOx SIS with tetrakis(dimethylamino)tin (TDMASn) and hydrogen peroxide. Tuning the SIS reaction and BCP chemistry resulted in highly ordered, polystyrene-block-poly(2-vinyl pyridine) (P2VP)-templated porous SnOx - AlOx and SnOx nanostructures. Detailed investigation using in-situ microbalance, high resolution electron microscopy, elemental analysis and infra-red spectroscopy shows that SnOx can directly grow within P2VP homopolymer and BCP films. Simultaneously with the growth, SnOx SIS process also contributes to the polymer etch. Performing SnOx SIS with pretreatment of a single AlOx SIS cycle increases the SnOx growth and protects the BCP template from etching. This is the first report of SnOx SIS opening a pathway for additional tetrakis-based organometallic precursors to be utilized in growth processes within polymers.