Ultrafast Vibrational Dephasing Times of Modified Graphene

Abstract

We have measured the ultrafast dephasing times of vibrational quanta in functional groups covalently bound to graphene. It has been previously shown that electronic relaxation in graphene occurs on subpicosecond timescales, but this is the first venture into the vibrational dephasing times in functionalized graphene. We have employed time-resolved sum-frequency generation spectroscopy to observe the decay of the coherent signal arising due to vibrational excitation by delaying the upconversion pulse with respect to the infrared (IR) laser pulse. We observe decay times of 0.8 and 1.4 ps for vibrational stretches in phenyl groups attached to graphene or gold, respectively, but for the vibrations of the bond covalently connecting H atoms to graphene in hydrogenated graphene, we measure much faster decay times of 0.3 ps. This is likely due to coupling between the C−H stretch and either the two-dimensional (2D) mode of graphene or nonadiabatic coupling to electrons photoexcited by the upconversion laser pulse.

Infrared Free Induction Decay (IR-FID) of Non-Interfacial Origin Observed in the Interfacial Sum-Frequency Generation Vibrational Spectroscopy (SFG-VS)

“…Time-scanning the visible pulse against the initial IR pulse generates the SFG-VS signal in the time domain, obtaining the time evolution of the vibrational polarization through the so-called SFG-FID process. So far, there have been SFG-FID studies on the dephasing dynamics of many different interfacial vibrations, such as CO adsorbates at metal interfaces, − −CN and −CH stretches of the Langmuir–Blodgett monolayers at air/water interface, air/solid interfaces, , −CH and −CN stretch vibrations at metal–liquid interface, −CH stretches of self-assembling monolayers (SAMs), − modified graphene samples, −CH and −CN stretch vibrations at liquid/solid interface, -OD and −OH stretches at the liquid/solid interface, − and −CH, CC– stretching vibrations at the air/liquid interface. , From these examples, one can see that SFG-FID has been used for systems with relatively simple vibrational spectra, since the time-resolved data for systems with complicated and congested vibrational bands usually generate less information on the vibrational coherences than that with spectral analysis of high-resolution BB-SFG-VS …”

Dephasing Dynamics across Different Local Vibrational Modes and Crystalline Environments

“…Rather than ignoring or filtering away PFID as a "coherent artifact" of pump-probe measurements [28], PFID may serve as a powerful tool in optics and ultrafast spectroscopy by providing subtle features that, when modeled accurately, yield accurate dephasing rates. These advances will be of interest in (i) chemical physics, where PFID as a spectroscopic technique is widely applicable to study the ultrafast vibrational dynamics of solidstate, liquid-phase, and gas-phase molecular systems [20][21][22][23][24][25][26][27]; (ii) molecular chemistry, where using PFID to accurately measure T 2 across different vibrational modes would advance the molecular design of popular systems such as modified graphene [40] and dye molecules [41], and could also uncover key molecule-environment interactions [42]; and (iii) semiconductor and quantum physics, where PFID may be used to probe defect-lattice interactions within tailored defect environments in diamond, as in the present work, or indeed in other semiconductor materials with important functional defects, such as GaN [43], silicon [44], or metal oxides [45]. The FTIR spectrum of S1 could not be resolved for N 0 s and A=B centers.…”