Probing Interaction between Individual Submonolayer Nanoislands and Bulk MoS₂ Using Ambient TERS

Abstract

Tip-enhanced Raman spectroscopy (TERS) has shown that detecting single molecules with a high spatial resolution is possible in ultrahigh vacuum (UHV) at low temperature with plasmonic metallic substrates. It is still challenging to probe interactions of molecules with semiconductors, which is important in biosensing, photovoltaics, and many other applications. Here we demonstrate that in ambient conditions it is possible to obtain Raman signals from submonolayer molecular islands on bulk MoS 2 using TERS. Analysis of relative Raman signal intensity ratio and Raman spectral peak position from spatial TERS mapping showed differences in the adsorbate− adsorbate and adsorbate−substrate interactions on Au and MoS 2 substrates. The Raman transition which involves the vibration of the metal center of the CuPc molecule experienced a change in the relative Raman signal intensity ratio due to the differences in the molecule−substrate charge transfer interaction. In comparison to the other vibrational modes, the vibrational modes of the surface charge transfer interacting moieties involving the metal center experienced the smallest shift in the Raman spectral peak position on both Au and MoS 2 substrates. Further, the distributions of the peak position and relative intensity were narrower at the center of the island with respect to the isolated molecules due to the adsorbate−adsorbate interaction. This study shows the contribution of charge transfer between molecules and MoS 2 in the TERS spectra.

Are charged tips driving TERS-resolution? A full quantum chemical approach

“…In particular, we investigate singly positively vs. singly negatively charged silver tips in comparison to the uncharged reference. Planar molecules, such as (metallo-)porphyrins 40,46,61,69,70 and phthalocyanines [71][72][73][74][75] have been the subject of several studies in the context of plasmon-enhanced spectroscopy like SERS and TERS. SnPc in particular features bright excited states that can respond to light at different wavelengths and allows for possible charge transfer to or from the tip, both in the visible spectrum.…”

The chemical effect goes resonant – a full quantum mechanical approach on TERS

“…As a realistic model system, we chose the surface-immobilized molecule tin(II) phthalocyanine (SnPc) which is three-dimensionally mapped by a single silver atom employing DFT and TDDFT calculations. This class of molecules, (metallo-)phthalocyanines [69][70][71][72] and (metallo-)porphyrins, 43,48,64,73,74 has been subject to numerous experimental and theoretical studies in the scope of plasmon-enhanced spectroscopy (SERS and TERS). In particular, SnPc was selected for the present computational investigation as pronounced resonance effects are anticipated for SnPc stemming from bright absorption features in the visible region.…”

Charge transfer and electromagnetic enhancement processes revealed in the SERS and TERS of a CoPc thin film

“…Here we follow the same considerations to evaluate the contributions of the charge-transfer process between CoPc and the Au substrate in our experiments. For the CoPc molecule, the vibrational mode involving the metal center appears at 754 cm −1 while the non-metal bond N-C stretch is located at 1540 cm −1 [52]. The results are shown in Figure 4C.…”

“…It has been reported that for copper phthalocyanine (CuPc) in the spectral region at about 650 nm, the symmetric Raman modes (e.g. B 1g ) borrow intensity from allowed molecular transitions (Q bands) by charge-transfer resonances [52]. The intensity ratio between the metal-centerrelated vibrational mode to that of the benzene ring vibrational mode has been used to evaluate the strength of the charge-transfer interaction between CuPc and Au foil or MoS 2 [52].…”

“…B 1g ) borrow intensity from allowed molecular transitions (Q bands) by charge-transfer resonances [52]. The intensity ratio between the metal-centerrelated vibrational mode to that of the benzene ring vibrational mode has been used to evaluate the strength of the charge-transfer interaction between CuPc and Au foil or MoS 2 [52]. For the CuPc molecule, the 749 cm −1 vibration is assigned to the in-plane N-Cu stretch, while the peak at 1530 cm −1 is the in-plane ring symmetric non-metal-related N-C stretch.…”