Raman Detection of Bond Breaking and Making of a Chemisorbed Up-Standing Single Molecule at Single-Bond Level

Abstract

Probing bond breaking and making as well as related structural changes at the single-molecule level is of paramount importance for understanding the mechanism of chemical reactions. In this work, we report in situ tracking of bond breaking and making of an up-standing melamine molecule chemisorbed on Cu(100) by subnanometer resolved tip-enhanced Raman spectroscopy (TERS). We demonstrate a vertical detection depth of about 4 Å with spectral sensitivity at the single chemical-bond level, which allows us not only to justify the up-standing configuration involving a dehydrogenation process at the bottom upon chemisorption, but also to specify the breaking of top N–H bonds and the transformation to its tautomer during photon-induced hydrogen transfer reactions. Our results indicate the chemical and structural sensitivity of TERS for single-molecule recognition beyond flat-lying planar molecules, providing new opportunities for probing the microscopic mechanism of molecular adsorption and surface reactions at the chemical-bond level.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

“…Moreover, of particular note is that during 2019–2021, a number of studies focused on chemical imaging with a resolution of a few angstroms (the chemical imaging of a single molecule or a single bond) were reported. 16,17,39,54–56 They are significantly different from TERS imaging studies before 2018, where the resolution was only at the nanometer level. Angstrom-resolved TERS microscopy, especially with the new scanning Raman picoscopy (SRP) methodology, can be used for visually constructing the chemical structure in real space and the determination of structural and chemical heterogeneities of surface species at the single-bond limit.…”

“…16 Wang et al reported the Raman detection of bond breaking and making of a chemisorbed up-standing single molecule at the single-bond level. 39 In situ nanospectroscopic imaging of plasmon-induced two-dimensional [4 + 4]-cycloaddition polymerization on Au(111) was reported by Shao et al 17 These three studies will be described in some detail later.…”

“…Chemical imaging using TERS rapidly moved from a nanometer resolution to an angstrom resolution. 16,39,56 Recently, we can even observe the bond breaking or the bond formation in the reactions. 17,39,56 We can acquire not only the morphology-related chemical information at the nanoscale but also the molecular fragment-related chemical information at the sub-nanoscale.…”

“…13–15 UHV-TERS enables angstrom-resolved TERS, which will be described in some detail in Section 4. 16,39 In addition, the Van Duyne group performed experiments to assess the feasibility of integrating pulsed laser excitation with TERS. 40 They used 1 ps pulses from an optical parametric oscillator (OPO) with an 80 MHz repetition rate to examine the TERS spectra of R6G and malachite green isothiocyanate.…”

Scanning probe microscopy in probing low-dimensional carbon-based nanostructures and nanomaterials

“…The combination of complementary spectroscopic information and topographic one provided by STM/AFM has shown its feasibility in this aspect (cf upper left panel of figure 9). For instance, application of tip-enhanced Raman spectroscopy (TERS) [166,167] and STM/AFM has been demonstrated to be capable of determining structural and chemical heterogeneities in the dehydrogenation processes at the single-bond limit [168] and identifying π-skeletons in coupling reactions [155], where the characteristic vibrational motions originated from specific bonds are the key to such chemical sensitivity. Moreover, TERS measurements have been shown to provide chemical information both laterally and vertically [167,168], which offer access to nonplanar stereo structures [167] and even more general 3D nanostructures.…”