Despite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip‐enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip–molecule point contact. A combined physico‐chemical mechanism is proposed to explain such an enhancement via the generation of a ground‐state charge‐transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.

Single-molecule tip-enhanced Raman spectroscopy (TERS) has emerged as an important technique for structural analysis at sub-molecular scale. Here in this work, we report a TERS study of an isolated free-base porphyrin molecule adsorbed on the Ag(100) surface at cryogenic temperature (∼7 K). Site-dependent TERS spectra reveal distinct local vibrational information for the chemical constituents within a single molecule. Moreover, distinct spatial features among different Raman peaks can be resolved from the TERS mapping images. These images are found to associate with related vibrational modes, enabling to resolve the mode associated with N−H bonds at the sub-nanometer level. This study will provide deep insights into the symmetry of adsorption configurations and local vibrational information within a single molecule.

[n]Cycloparaphenylene ([n]CPP) molecules have attracted broad interests due to their unique properties resulting from the distorted and strained aromatic hoop structures. In this work, we apply sub-nanometer resolved tip-enhanced Raman spectroscopy (TERS) to investigate the adsorption configurations and structural deformations of [12]CPP molecules on metal substrates with different crystallographic orientations. The TERS spectra for a [12]CPP molecule adsorbed on the isotropic Cu(100) surface are found to be essentially the same over the whole nanohoop, indicating an alternately twisted structure that is similar to the [12]CPP molecule in free space. However, when the [12]CPP molecules are adsorbed on the anisotropic Ag(110) surface, the molecular shape is found to be severely deformed into two types of adsorption configurations: one showing an interesting “Möbius-like” feature and the other showing a symmetric bending structure. Their TERS spectral features are found to be site-dependent over the hoop and even show peak splitting for the out-of-plane C–H bending vibrations. The deformed structural models gain strong support from the spatial distribution of “symmetric” TERS spectra at different positions on the hoop. Further TERS imaging, with a spatial resolution down to ∼2 Å, provides a panoramic view on the local structural deformations caused by different tilting of the benzene units in real space, which offers insights into the subtle changes in the aromatic properties over the deformed hoop owing to inhomogeneous molecule−substrate interactions. The ability of TERS to probe the molecular structure and local deformation at the sub-molecular level, as demonstrated here, is important for understanding surface science as well as molecular electronics and optoelectronics at the nanoscale.

The formation of well‐ordered supramolecular self‐assembly based on cytosine and 4,4′‐bipyridine on Ag(111) has been investigated with subnanometer‐resolved tip‐enhanced Raman spectroscopy (TERS) using a low‐temperature ultrahigh‐vacuum scanning tunneling microscope (STM). The co‐deposition of two molecules on the Ag(111) surface held at room temperature results in well‐ordered hydrogen‐bonded monolayer islands, which transform into well‐aligned double‐stranded chains after thermal annealing. Site‐dependent TERS spectra help to identify the packing structures of molecular assemblies. The changes of molecule‐specific vibrational fingerprints across the assemblies can be clearly monitored by combining high‐resolution TERS spectra with advanced multivariate analysis. Furthermore, the peak positions and relative intensities for characteristic vibrational modes of the two molecules are found to vary due to the changes of the local hydrogen bonding and interaction environment in different molecular samples. Our findings demonstrate that TERS is sensitive to the local molecular interactions, provides new potentials to monitor the functionality of molecular architectures on demand, and opens new opportunities to probe surface chemistry at the nanoscale.

We demonstrate the B-band electroluminescence from the high-lying S2 excited state of a single zinc porphyrin molecule with the scanning tunneling microscope-induced luminescence technique by using an aluminum tip. The nanocavity plasmon mode is found to be critical for the occurrence of S2 electroluminescence. When using a silver tip to excite the molecule electronically decoupled from the Ag(100) substrate by an ultrathin sodium chloride spacer, we only observe the Q-band electroluminescence originating from the radiative decay of the S1 first excited state, without any B-band emission due to the lack of effective plasmonic enhancement for the B-band. However, when the nanocavity plasmon resonance is tuned to a bluer range by using an aluminum tip, the S2 electroluminescence from a single zinc porphyrin shows up because the nanocavity plasmon mode can now spectrally overlap with the B-band emission to generate efficient plasmonic enhancement for the radiative decay directly from the S2 state. Interestingly, the excitation mechanisms for these two types of emission are found to be different. While the Q-band emission is attributed mainly to a carrier-injection mechanism, the B-band electroluminescence is found to be excited via an inelastic electron scattering process. Our results open a route to investigate the photophysical property and dynamic behavior of isolated molecules in their excited states.