Nanoscale Chemical Imaging of Coadsorbed Thiolate Self-assembled Monolayers on Au(111) by Tip-Enhanced Raman Spectroscopy

Self-assembled monolayers (SAMs) of thiolates on metal surfaces are of key importance for engineering surfaces with tunable properties. However, it remains challenging to understand binary thiolate SAMs on metals at the nanoscale under ambient conditions. Here we employ tip-enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations to investigate local information of binary SAMs on Au(111) coadsorbed from an equimolar mixture of p-cyanobenzenethiol (pCTP) and p-aminothiophenol (pATP), including chemical composition, coadsorption behavior, phase segregation, plasmon-induced photocatalysis, and solvation effects. We found that upon competitive adsorption of pCTP and pATP on Au(111) from a methanolic solution, the coadsorption initially occurs randomly and homogeneously; eventually, pATP is replaced by pCTP through gradual growth of pCTP nanodomains. TERS imaging also allows for visualization of the plasmon-induced coupling of pATP to p,p’-dimercaptoazobenzene (DMAB) and the solvation-induced phase segregation of the binary SAMs into nanodomains, with a spatial resolution of ~9 nm under ambient conditions. According to DFT calculations, these aromatic thiolates differing only in their functional groups, -CN versus –NH2, show different adsorption energy on Au(111) in vacuum and methanol, and thus the solvation effect on adsorption energy of these thiolates in methanol can determine the dispersion state and replacement order of the binary thiolates on Au(111).

Conformational heterogeneity of molecules physisorbed on a gold surface at room temperature

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the hyperspectral TERS imaging of single or a few brilliant cresyl blue (BCB) molecules at room temperature, along with quantitative spectral analyses. Robust chemical imaging is enabled by the freeze-frame approach using a thin Al2O3 capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contaminations in air. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm-1 is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies.

Synthesis and nanoscale characterization of hierarchically assembled molecular nanosheets

Chemical functionalization of molecular two-dimensional (2D) materials towards the assembly of hierarchical functional nanostructures is of great importance for nanotechnology including areas like artificial photocatalytic systems, nanobiosensors or ultrafiltration. To achieve the desired functionality of 2D materials, these need to be characterized down to the nanoscale. However, obtaining the respective chemical information is challenging and generally requires the application of complementary experimental techniques. Here, we demonstrate the synthesis and chemical characterization of hierarchically assembled molecular nanosheets based on about 1 nm thin, molecular carbon nanomembrane (CNM) and covalently grafted, single-molecule layer cobalt(III) catalysts for the light-driven hydrogen evolution reaction (HER). We employ X-ray photoelectron spectroscopy (XPS) and tip-enhanced Raman spectroscopy (TERS) to access both the transversal and lateral chemical information of the synthesized nanosheets with nanometer resolution. TERS and XPS data provide detailed information on the average and local surface distribution of the catalyst as well as mechanistic details of the grafting reaction. The proposed approach represents a general route towards a nanoscale structural analysis for a variety of molecular 2D materials - a rapidly growing materials class with broad prospects for fundamental science and applications.

Fabrication of Au and Ag – Coated AFM Probes for Tip-Enhanced Raman Spectroscopy

In this work, approaches for fabrication metal coated probes for Tip Enhanced Raman Spectroscopy (TERS) are considered. It was proposed to use optical characterization of probes to achieve the effective TERS of semiconductor nanoobjects. The shape and size of the metal cluster at the tip apex determines the position of the localized surface plasmon resonance, the electromagnetic field enhancement and, thus, TERS performance. The possibility of optimizing the characteristics of the probes for TERS studies of nanoobjects has been investigated.

Revealing DNA Structure at Liquid/Solid Interfaces by AFM-Based High-Resolution Imaging and Molecular Spectroscopy

DNA covers the genetic information in all living organisms. Numerous intrinsic and extrinsic factors may influence the local structure of the DNA molecule or compromise its integrity. Detailed understanding of structural modifications of DNA resulting from interactions with other molecules and surrounding environment is of central importance for the future development of medicine and pharmacology. In this paper, we review the recent achievements in research on DNA structure at nanoscale. In particular, we focused on the molecular structure of DNA revealed by high-resolution AFM (Atomic Force Microscopy) imaging at liquid/solid interfaces. Such detailed structural studies were driven by the technical developments made in SPM (Scanning Probe Microscopy) techniques. Therefore, we describe here the working principles of AFM modes allowing high-resolution visualization of DNA structure under native (liquid) environment. While AFM provides well-resolved structure of molecules at nanoscale, it does not reveal the chemical structure and composition of studied samples. The simultaneous information combining the structural and chemical details of studied analyte allows achieve a comprehensive picture of investigated phenomenon. Therefore, we also summarize recent molecular spectroscopy studies, including Tip-Enhanced Raman Spectroscopy (TERS), on the DNA structure and its structural rearrangements.