With support from the Chemical Measurement and Imaging Program in the Division of Chemistry and the NanoBiosensing Program in the Division of Chemical, Bioengineering, Environmental, and Transport Systems, Professors Ziegler and Reinhard at Boston University merge two very active research areas that have independently already impacted many aspects of our lives; nanomaterials and laser technology. The goal of the project is to exploit the special capabilities afforded by the combination of these two disciplines to advance a technical solution to some important societal needs. More specifically, these researchers explore how ultrafast pulsed laser light interacting with metal nano-materials can be used to enhance and control chemistry on metal surfaces for applications such as chemical catalysis, pollution mitigation, energy conversion, chemical imaging and sensing applications. The on-going experiments develop a detailed description of how molecules interact with nanostructured metal surfaces. Well-established methodologies are already in place that allow the design of nanoscale surfaces with exquisite control. The results of these experiments reveal molecular level details of how these materials interact with molecules and thus provide information on optimizing nanostructures design strategies for the applications cited above. The collaborative nature of this research effort provides participating students with unique experience at the interface of materials and ultrafast science, and thus promote cross-disciplinary activities in science and technology at Boston University. In synergy with the research, this grant supports a substantial education and outreach program to include participation of local Community College students and faculty, inner city and greater Boston area High School students, and High School teachers in conjunction with a newly awarded NSF research experience for undergraduates (REU) site program, as well as other BU based outreach programs.

The merger of ultrafast spectroscopy and nanotechnology is being used to study the dynamics and interactions of molecules on plasmonically active surfaces via three ultrafast laser techniques. Plasmonically enhanced (PE) optical heterodyne detected Raman spectroscopy (PE-OHD-RIKES) offers sensitivity advantages for viewing Raman responses on plasmonic surfaces, especially for low frequency modes resulting from molecular-surface physi-adsorption, and a phase sensitive methodology for understanding vibrational and plasmon contributions to nonlinear responses. Analysis of PE three-pulse photon echo peak shift measurements (PE 3PEPS) yields a dynamical description of optical dephasing, or equivalently solvation, of molecules on plasmonic surfaces (inhomogeneous energy distributions, spectral diffusion and fluctuation timescales). Finally, the successful implementation of PE femtosecond stimulated Raman spectroscopy (PE-FSRS), could have enormous impact as a new probe of surface chemistry allowing vibrationally-specific labels to follow the evolution of short-lived intermediates and rapid conformation changes of excited molecules on plasmonic surfaces. Determined dynamical and structural properties of analytes on plasmonic substrates is being contrasted with those of liquid solutions and correlated with observed plasmonic based phenomenon such as SERS enhancement factors. Substrates are being fabricated by a template-guided self-assembly procedure which results in electromagnetically strongly coupled nanoparticle cluster arrays where optical fields are enhanced by both near field coupling between nanoparticles, and diffractive coupling between clusters. Detailed molecular level information about how molecules interact with engineered plasmonic surfaces is providing rational design strategies for maximizing plasmon enhancement of optical responses and chemical outcomes. The implementation of this methodology is impactful upon optical imaging capabilities in terms of improved sensitivity and faster acquisition times, real time monitoring of photoinduced surface chemical reactivity, enhanced chemical and biological sensing capabilities, and improved strategies for subsequent spontaneous SERS and other plasmonic based techniques.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Lin He
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Boston University
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