UC Davis

The focus of my thesis is the development of an advanced methodology to create 3D nanostructures by design, and to demonstrate the control over geometry and chemical functionalities of the nanostructures produced. The driving motive behind this are the pressing need for 3D nanostructures in biomaterials development, modern nanodevices and biomedical applications. My approach is scanning probe microscopy-based nanolithography in combination with advanced surface chemistry. This thesis clearly demonstrated the concept and feasibility of the approach. While the power of 3D printing has proven to be a powerful tool in additive manufacture, extending the spatial precision to nanometer scale would lay the foundation for the next science and technology revolutions. Specific applications impacted by this work include surface science, catalysis, modern sensors, nanophotonics, and nanoelectronics.

Three significant goalposts are reported in this thesis followed by future prospective research. First focuses on mechanically sensitive molecules known as mechanophores, which have recently attracted much interest due to the need for mechanoresponsive materials. Maleimide−anthracene mechanophores located at the interface between poly- (glycidyl methacrylate) (PGMA) polymer brushes and Si wafer surfaces were activated locally using atomic force microscopy (AFM) probes to deliver mechanical stimulation. Each individual maleimide−anthracene mechanophore exhibits binary behavior: undergoing a retro-[4 + 2] cycloaddition reaction under high load to form a surface-bound anthracene moiety and free PGMA or remaining unchanged if the load falls below the activation threshold. In the context of nanolithography, this behavior allows the high spatial selectivity required for the design and production of complex and hierarchical patterns with nanometer precision. The high spatial precision and control reported in this work brings us closer to molecular level programming of surface chemistry, with promising applications such as 3D nanoprinting, production of coatings, and composite materials that require nanopatterning or texture control as well as nanodevices and sensors for measuring mechanical stress and damage in situ.

Following our success with creating structures by design using nanolithography and mechanophore chemistry, we set out to fabricate organizational chirality on surfaces, which has been an interest in chemistry and materials science due to the need for enantioselective catalysis, separation, and reactions. Current methods for production of organizational chirality are primarily based upon self-assembly of molecules. While powerful, the chiral structures produced are restricted to those dictated by reaction thermodynamics. This work introduces a method to create organizational chirality by design. Using atomic force microscopy in conjunction with our chosen surface chemistry, various chiral structures were designed and produced with nanometer precision, from simple chiral spirals to arrays of chiral nanofeatures to hierarchical chiral structures. The size, geometry, and organizational chirality faithfully follow the designs with a high degree of spatial control. The concept and methodology reported here provides researchers a new means to carry out organizational chiral chemistry, with the intrinsic advantages of chiral structures by design. The results open new and promising applications including organizational chiral sensors, 3D nanoprinting of chiral structures, enantiomeric separation, and enantiomeric heterogeneous catalysis.

Building upon the theme of controlled fabrication of nanostructures, we utilized controlled assembly to create structures by design. Our prior work has demonstrated the concept of controlled assembly of macromolecules such as star polymers [molecular weight (Mw) ∼383 kDa,hydrodynamic radius R ∼ 13.8 nm] in droplets. This work extends this concept to smaller molecules, in this case, poly(ethylene glycol) bis-tetrazine (PEGbisTz, Mw 8.1 kDa, R ∼1.5 nm). The key to controlled molecular assembly is to first deliver ultrasmall volumes (sub-fL) of solution containing PEG-bisTz to a substrate. The solvent evaporates rapidly due to the minute volume, thus forcing the assembly of solute, whose overall size and dimension are dictated by the initial liquid geometry and size. Using prepatterned surfaces, this work revealed that the initial liquid shape can be further tuned, and we could control the final assembly of solute such as PEGbisTz molecules. The degree of control was demonstrated by varying the micropatterns and delivery conditions. This work demonstrated the validity of controlled assembly for PEG-bisTz and enables three-dimensional (3D) nanoprinting of functional materials. The technology has promising applications in nanophotonics, nanoelectronics, nanocomposite materials, and tissue engineering.

These investigations into fabrication of a variety of nanostructures demonstrated the success in creating, complex, hierarchical, and chiral structures with nanometer precision. This success lays the foundation for utilization of scanning probe lithography to create functional nanostructures out of new and exciting materials. Future investigations of this technology will focus on incorporating materials such as mithrene, a unique 3D material that possesses 2D properties, and anthraquinone modified cellulose nanocrystals. Combined with the methodology presented here, the development of these structures will be useful in future applications such as modern sensors, nanodevices, nanophotonics, and nanoelectronics.