Chemistry at the Space-Time Limit (CaSTL) Center at UCI is dedicated to interrogating molecules at their natural length and time scales, with the purpose to unravel the details of such dynamic events as electron transfer, configurational motions and, ultimately, a chemical reaction. To make this possible, CaSTL develops and employs new research tools, both on the experimental as well as on the theoretical end. One such example is a femtosecond scanning tunneling microscope, which will produce a brand new view of the dynamic molecular world. The Centers research scope is vast. Below a summary of the current major thrust areas. (See Figure 1)
Fast and sharp: a femtosecond scanning tunneling microscope
An instrument central to the CaSTL Center is an optical scanning tunneling microscope (STM) coupled to a multi-color fs laser system. The goal of combining femtosecond lasers with STM is to merge the unique capabilities of two instruments to obtain the highest spatial and temporal resolution possible for probing chemistry. The overall setup of the time-resolved STM (t-STM) is shown in Fig. 2, and it features two independently tunable noncollinear parametric amplifiers, STM tunnel junction optimized for optical access and a sensitive CCD camera with single photon sensitivity.
Theory of single molecule optical STM
CaSTL puts great effort into developing the theoretical framework necessary for understanding the ultrafast-STM measurements on single molecules. Under the guidance of Shaul Mukamel theoretical effort focused on the development of the non-equilibrium super-operator Green’s function theory (NESGFT) for describing the response of a single molecule to an external electric bias/current. For instance, this formalism was applied to calculate the tunneling induced florescence (TIF) spectra of single molecules in an STM junction and compared to measurements. Kieron Burke will lead current efforts that employ a density functional (DFT) approach to zoom in on the details of the molecular response. (See Figure 3)
Time resolved plasmon dynamics in single molecules
The coherent optical response from single molecules is weak, but plasmonic enhancement of the electric field at metallic interfaces can come to the rescue. CaSTL is studying the physics of plasmonic enhancement in nonlinear optical experiments of single molecules. CaSTL studies of silver nanorod systems have already revealed important clues as to the nonlinear optical response of plasmonic systems. In a new collaborative study with Guy Bazan from UCSB, a novel plasmonic system – two metallic nanoparticles connected via a distyrylbenzene (DSB) linker, as shown in Fig. 4, is investigated. Time-resolved CARS measurements of the linker vibrations, as a function of the separation between the nano-particles is being conducted to fully characterize the enhancement of nonlinear optical mixing in a geometry that is closely related to that of the STM junction.
Single molecule nonlinear imaging
CaSTL has pioneered the nonlinear coherent imaging of carbon nanotubes, very large single molecules. Fig. 5 shows the first CARS image of a single carbon nanotube attached to a titanium electrode. The single walled nanotubes were visualized by tuning the lasers to the defect induced D-band at 1315 cm-1. These studies indicate that, under the condition of highly polarizable electron densities, CARS signals from single molecular structures can be observed. In yet another collaborative study, the G mode at ~1,600 cm-1 is observed by FTIR and investigated by using nonlinear time-resolved IR studies. For these studies, the Center grows dilute, isolated SWNTs on different IR-transparent substrates, such as CaF2 and sapphire.
Single molecule characterization by electronic circuitry
In collaboration with G. Weiss (Chemistry, UCI), the CaSTL has pursued an electronic circuit architecture based on SWNT conductors. By covalently linking a molecule to the sidewall of an SWNT, they have demonstrated circuits that electronically transduce the dynamics of single attached molecules. Various protein attachments have been robustly achieved and the resulting circuits can be measured in liquids versus temperature and pH to study chemically-relevant processes. Of course, for this area of investigation the relevant space-time limit is the 10nm-1μs range, which is well-suited to an electronic approach.(See Fig 6)