Intellectual Merit: Nanoscale metal tips behave like optical antenna to facilitate near-field amplification of scattered light and are being explored as high-resolution scanning probes for ultrasensitive vibrational spectroscopy. To obtain the highest signal gain from each probe, the metal tip must be designed with highly regulated size and shape. Current fabrication methods produce tips with a range of nanoscopic features that are difficult to characterize and suffer from both mechanical and thermal damage upon use. The research objective of this BRIGE project is to fabricate metal tips by assembling colloidal metal nanocrystals onto an atomic force microscope tip. This proposal will evaluate the optical response of nanocrystal-based tips as a function of nanocrystal shape, size, and assembly geometry. Nanocrystal assemblies will be engineered to achieve specific near-field electromagnetic properties, including tunable plasmon excitation wavelengths, electromagnetic coupling between multiple nanocrystals, and optimal evanescent field decay lengths. Finally, this project will seek to integrate these nanocrystal assemblies with scanning probe instrumentation and to demonstrate tip-enhanced Raman spectroscopy for applications such as chemical identification, detection, and mapping.
Broader Impacts: The proposed research will lead to the development of robust, tailored, and ultrasensitive nanoscale probe tips for tip-enhanced Raman spectroscopy and other near-field optical measurements. An optical technique that is able to pair quantitative chemical analysis with spatial resolution beyond the diffraction limit would have a tremendous impact in a broad range of fields that require surface characterization, including: chemical sensing and molecular identification, heterogeneous catalysis, nanostructure characterization, biomaterials development, and basic life sciences research. This research project will make broad impacts by integrating research with mentoring, education, and social outreach. The research outcomes of this proposal will be introduced to undergraduates within the NanoEngineering curriculum at UC San Diego as an example of how basic research in nanoengineering can drive technological innovation. Work will be conducted with community-based organizations to promote nanoscience education at the precollege level, with particular attention given toward broadening participation through the inclusion of underrepresented minorities and women. In addition, the proposed efforts include the development of online wikis for the dissemination of research results beyond the nanomaterials community. These different educational components will be designed to engage engineers at all levels of education, encouraging open lines of scientific discussion with their peers, mentors, and community.
The intellectual merit of the work performed in this project is the development of a new label-free method for the optical detection and characterization of surfaces and nanostructures. This award has led to the successful synthesis, characterization, and assembly of metal and semiconductor nanostructures that are capable of confining light to extreme subwavelength volumes. The probes developed by assembling these nanostructures onto atomic force microscopy tips are expected to provide unprecedented surface sensitivity in optical spectroscopy by focusing the maximum amount of incident light to molecules that lie within a few nanometers of the probe tip. This project has impacted nanotechnology research by contributing new methodologies for near-field optical measurements. The surface and interfacial properties of nanostructures are critical in determining the electronic, optical, magnetic, and chemical properties of nano-enabled devices. The optical probes developed by this project will be used to carry out scanning near-field and near-field Raman measurements to map the electromagnetic fields generated by self-nanostructures and nanostructured surfaces. These near-field probes will enable measurement via tip-enhanced Raman spectroscopy (TERS), which is a powerful scanning optical technique for chemical mapping that has the potential to enable the direct imaging and chemical characterization of complex surfaces and interfaces. These include materials such as carbon nanomaterials (e.g. nanotubes and graphene), inorganic nanostructures, membranes, and ultrathin films. Currently, no other optical technique possesses such capabilities. Our work over the past two years has resulted in five research papers published in peer-reviewed journals and 13 invited oral presentations for the PI. Research developments were coupled with extensive education and diversity activities. This project resulted in the mentoring of one female post-doctoral scientist, three graduate students, and four undergraduate students (two from underrepresented groups in STEM). Research findings regarding nanocrystal synthesis were integrated into the PIâ€™s undergraduate courses as an example of how basic research can drive technological innovation.