The Primarily Undergraduate Nanomaterials Cooperative: A New Model for Supporting Collaborative Research at Small Institutions on a National Scale

The Primarily Undergraduate Nanomaterials Cooperative (PUNC) is an organization for research-active faculty studying nanomaterials at Primarily Undergraduate Institutions (PUIs), where undergraduate teaching and research go hand-in-hand. In this perspective, we outline the differences in maintaining an active research group at a PUI compared to an R1 institution. We also discuss the work of PUNC, which focuses on community building, instrument sharing, and facilitating new collaborations. Currently consisting of 37 members from across the United States, PUNC has created an online community consisting of its Web site (nanocooperative.org), a weekly online summer group meeting program for faculty and students, and a Discord server for informal conversations. Additionally, in-person symposia at ACS conferences and PUNC-specific conferences are planned for the future. It is our hope that in the years to come PUNC will be seen as a model organization for community building and research support at primarily undergraduate institutions

Tip-Enhanced Raman Spectroscopy with Picosecond Pulses

Tip-enhanced Raman spectroscopy (TERS) can probe chemistry occurring at surfaces with both nanometer spectroscopic and submolecular spatial resolution. Combining ultrafast spectroscopy with TERS allows for picosecond and, in principle, femtosecond temporal resolution. Here we couple an optical parametric oscillator (OPO) with a scanning tunneling microscopy (STM)-TERS microscope to excite the tip plasmon with a picosecond excitation source. The plasmonic tip was not damaged with OPO excitation, and TER spectra were observed for two resonant adsorbates. The TERS signal under ultrafast pulsed excitation decays on the time scale of 10 s of seconds; whereas with continuous-wave excitation no decay occurs. An analysis of possible decay mechanisms and their temporal characteristics is given

Tip-Enhanced Raman Spectroscopy with Picosecond Pulses

Tip-enhanced Raman spectroscopy (TERS) can probe chemistry occurring at surfaces with both nanometer spectroscopic and submolecular spatial resolution. Combining ultrafast spectroscopy with TERS allows for picosecond and, in principle, femtosecond temporal resolution. Here we couple an optical parametric oscillator (OPO) with a scanning tunneling microscopy (STM)-TERS microscope to excite the tip plasmon with a picosecond excitation source. The plasmonic tip was not damaged with OPO excitation, and TER spectra were observed for two resonant adsorbates. The TERS signal under ultrafast pulsed excitation decays on the time scale of 10 s of seconds; whereas with continuous-wave excitation no decay occurs. An analysis of possible decay mechanisms and their temporal characteristics is given

The Origin of Relative Intensity Fluctuations in Single-Molecule Tip-Enhanced Raman Spectroscopy

An explanation of the relative intensity fluctuations observed in single-molecule Raman experiments is described utilizing both single-molecule tip-enhanced Raman spectroscopy and time-dependent density functional theory calculations. No correlation is observed in mode to mode intensity fluctuations indicating that the changes in mode intensities are completely independent. Theoretical calculations provide convincing evidence that the fluctuations are not the result of diffusion, orientation, or local electromagnetic field gradients but rather are the result of subtle variations of the excited-state lifetime, energy, and geometry of the molecule. These variations in the excited-state properties will provide information on adsorbate–adsorbate and adsorbate–substrate interactions and may allow for inversion of experimental results to obtain these excited-state properties

Single-molecule Tip-enhanced Raman Spectroscopy

An existence proof for single-molecule tip-enhanced Raman spectroscopy (SMTERS) is given using the frequency domain approach involving the two isotopologues of Rhodamine 6G (R6G) that were previously employed for single-molecule surface-enhanced Raman spectroscopy (SMSERS). A combination of experimental and theoretical studies provides a detailed view of the isotopic response of R6G-d 0 and R6G-d 4 in the 600 - 800 cm -1 region. The single-molecule nature of the TERS experiment is confirmed through two lines of evidence. First, the vibrational signature of only one isotopologue at a time was observed from multiple TER spectra. Second, the spectral wandering of the 610 cm -1 mode of R6G-d 0 was less than ±4 cm -1, which in turn is less than the 10 cm -1 isotopic shift so that no confusion in assignment resulted. As a consequence, the total TERS enhancement factor can now be accurately established as EF TERS = 1.0 × 10 13 because only one molecule at a time is measured. Furthermore, EF TERS can be partitioned into an electromagnetic contribution of 10 6 and a molecule-localized resonance Raman contribution of 10 7. © 2011 American Chemical Society

Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy with Picosecond Excitation

Tip-enhanced Raman spectroscopy (TERS) provides chemical information about adsorbates with nanoscale spatial resolution, but developments are still required in order to incorporate ultrafast temporal resolution. In this Letter, we demonstrate that a reliable TER signal of rhodamine 6G (R6G) using picosecond (ps)-pulsed excitation can be obtained in ultrahigh vacuum (UHV). In contrast to our previous observation of irreversible signal loss in ambient TERS (Klingsporn, J. M.; Sonntag, M. D.; Seideman, T.; Van Duyne, R. P. J. Phys. Chem. Lett. 2014, 5, 106−110), we demonstrate that the UHV environment decreases irreversible signal degradation. As a complement to the TERS experiments, we examined the rate of surface-enhanced Raman (SER) signal decay under picosecond irradiation and found that it is also slowed in UHV compared to that in ambient. Signal decay kinetics suggest that the predominant mechanism responsible for signal loss in ps SERS of R6G is surface diffusion. Both diffusive and reactive phenomena can lead to pulsed excitation TER signal loss, and a UHV environment is advantageous in either scenario

Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy with Picosecond Excitation

Tip-enhanced Raman spectroscopy (TERS) provides chemical information about adsorbates with nanoscale spatial resolution, but developments are still required in order to incorporate ultrafast temporal resolution. In this Letter, we demonstrate that a reliable TER signal of rhodamine 6G (R6G) using picosecond (ps)-pulsed excitation can be obtained in ultrahigh vacuum (UHV). In contrast to our previous observation of irreversible signal loss in ambient TERS (Klingsporn, J. M.; Sonntag, M. D.; Seideman, T.; Van Duyne, R. P. J. Phys. Chem. Lett. 2014, 5, 106−110), we demonstrate that the UHV environment decreases irreversible signal degradation. As a complement to the TERS experiments, we examined the rate of surface-enhanced Raman (SER) signal decay under picosecond irradiation and found that it is also slowed in UHV compared to that in ambient. Signal decay kinetics suggest that the predominant mechanism responsible for signal loss in ps SERS of R6G is surface diffusion. Both diffusive and reactive phenomena can lead to pulsed excitation TER signal loss, and a UHV environment is advantageous in either scenario

Plasmon-Mediated Electron Transport in Tip-Enhanced Raman Spectroscopic Junctions

We combine experiment, theory, and first-principles-based calculations to study the light-induced plasmon-mediated electron transport characteristics of a molecular-scale junction. The experimental data show a nonlinear increase in electronic current perturbation when the focus of a chopped laser beam moves laterally toward the tip–sample junction. To understand this behavior and generalize it, we apply a combined theory of the electronic nonequilibrium formed upon decoherence of an optically triggered plasmon and first-principles transport calculations. Our model illustrates that the current via an adsorbed molecular monolayer increases nonlinearly as more energy is pumped into the junction due to the increasing availability of virtual molecular orbital channels for transport with higher injection energies. Our results thus illustrate light-triggered, plasmon-enhanced tunneling current in the presence of a molecular linker

Intramolecular Insight into Adsorbate–Substrate Interactions via Low-Temperature, Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) provides chemical information for adsorbates with nanoscale spatial resolution, single-molecule sensitivity, and, when combined with scanning tunneling microscopy (STM), Ångstrom-scale topographic resolution. Performing TERS under ultrahigh-vacuum conditions allows pristine and atomically smooth surfaces to be maintained, while liquid He cooling minimizes surface diffusion of adsorbates across the solid surface, allowing direct STM imaging. Low-temperature TER (LT-TER) spectra differ from room-temperature TER (RT-TER), RT surface-enhanced Raman (SER), and LT-SER spectra because the vibrational lines are narrowed and shifted, revealing additional chemical information about adsorbate–substrate interactions. As an example, we present LT-TER spectra for the rhodamine 6G (R6G)/Ag(111) system that exhibit such unique spectral shifts. The high spectral resolution of LT-TERS provides intramolecular insight in that the shifted modes are associated with the ethylamine moiety of R6G. LT-TERS is a promising approach for unraveling the intricacies of adsorbate–substrate interactions that are inaccessible by other means