Probing the Role of Environmental and Sample Characteristics in Gap Mode Tip-Enhanced Raman Spectroscopy

Abstract

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical tool for nondestructive and label-free molecular characterization at the nanoscale. However, the influence of environmental factors and sample characteristics on the occurrence of spurious signals, enhancement of TERS signals, and longevity of TERS probes is not well understood yet. Herein, we present a detailed investigation of the influence of oxygen, humidity, and atmospheric carbon contaminants on scanning tunneling microscopy-TERS (STM-TERS) measurements of self-assembled monolayer systems in ambient and inert environments. Our results reveal a consistent increase of TERS signals, significant reduction of spurious signals, and drastically improved longevity of TERS probes in the inert environment. Additionally, sample characteristics such as molecular packing, chemisorption behavior, and hydrophilicity are found to have a direct impact on signal enhancement in the TERS measurements of molecular self-assembled monolayers (SAMs). The novel insights gained in this study are expected to pave the way for a more robust data analysis and improved experimental design in the future gap mode STM- and atomic force microscopy-TERS (AFM-TERS) studies.

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip‐Enhanced Raman Spectroscopy

“…A key question that arises from the TERS measurements of 4‐ATP adlayers after O 2 exposure (Figures 2 and 3) is: How can the 4‐ATP molecules oxidize to 4‐NTP considering the direct activation of O 2 on the Au(111) surface is a very difficult process (activation barrier of 2.44 eV [1] )? It is known that under ambient conditions, an interfacial water layer is present on Au(111) surface due to the relative humidity [46–48] . Theoretical studies have proposed that water molecules present on an atomically flat bulk Au(111) surface can interact with molecular oxygen to form a H 2 O−O 2 complex, which can heterolytically cleave to produce oxidative hydroperoxyl, atomic oxygen, and hydroxyl species as schematically illustrated in Figure 5a [18] .…”

Nanoscale Chemical Analysis of Thin Film Solar Cell Interfaces Using Tip-Enhanced Raman Spectroscopy

“…Conversely, atom probe tomography can also furnish nanoscale information but is destructive and necessitates time-consuming sample preparation. 14 Over the past two decades, tip-enhanced Raman spectroscopy (TERS) has emerged as a promising analytical tool 15,16 for nanoscale molecular characterization across various materials and processes such as two-dimensional (2D) materials, 17−19 catalysts, 20,21 polymers, 22,23 biomaterials, 24−26 organic photovoltaic devices, 27 self-assembled monolayers, 28,29 surface chemical reactions, etc. 30 TERS employs a metallic scanning probe microscopy tip positioned at the focal point of an excitation laser, which generates an enhanced and confined electromagnetic field (also called near-field) at the tip apex through a combination of the localized surface plasmon resonance and lightning rod effect.…”

“…Over the past two decades, tip-enhanced Raman spectroscopy (TERS) has emerged as a promising analytical tool , for nanoscale molecular characterization across various materials and processes such as two-dimensional (2D) materials, − catalysts, , polymers, , biomaterials, − organic photovoltaic devices, self-assembled monolayers, , surface chemical reactions, etc . TERS employs a metallic scanning probe microscopy tip positioned at the focal point of an excitation laser, which generates an enhanced and confined electromagnetic field (also called near-field) at the tip apex through a combination of the localized surface plasmon resonance and lightning rod effect. − This intense and localized near-field at the TERS tip enables the circumvention of conventional Raman microscopy’s diffraction-limited spatial resolution and enhances sensitivity by several orders of magnitude.…”

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip‐Enhanced Raman Spectroscopy

“…It is known that under ambient conditions, an interfacial water layer is present on Au(111) surface due to the relative humidity. [46][47][48] Theoretical studies have proposed that water molecules present on an atomically flat bulk Au(111) surface can interact with molecular oxygen to form a H 2 OÀ O 2 complex, which can heterolytically cleave to produce oxidative hydroperoxyl, atomic oxygen, and hydroxyl species as schematically illustrated in Figure 5a. [18] These oxidative species can oxidize 4-ATP molecules to 4-NTP.…”