Skip to main content

Nanoscale Vibrational Spectroscopy of Sensitive 2D Molecular Materials

Periodic Reporting for period 2 - 2DNanoSpec (Nanoscale Vibrational Spectroscopy of Sensitive 2D Molecular Materials)

Reporting period: 2019-03-01 to 2020-08-31

Two-dimensional (2D) materials are the materials that are only a few, or sometimes one atom thin, i.e. the thinnest one could possibly imagine. Generally, they exhibit different properties than their bulk counterparts. The discovery of graphene in 2004 triggered intense research in this field and, thanks to the tremendous development in the last decades, we are now able to synthesize a large variety of such materials. Some of them, like graphene, have extraordinary electric and thermal conductivity. Some others, like transition metal dichalcogenides, are semiconductors and promise to deliver huge improvements in transistor technology, reaching the boundaries of electronics miniaturization to previously unforeseeable levels. There is no doubt that 2D materials will shape the world of tomorrow.

While the production technology has dramatically improved with time, the currently available analytical methods tend to fail in characterizing their nanometer-size structure; obtaining information from such a small quantity of matter is indeed a very hard goal to reach. Tip-enhanced Raman spectroscopy (TERS) offers a solution to this problem. TERS consists in a combination of a scanning probe microscope, able to provide topographical characterization with lateral resolutions below 1 nm, and a Raman spectrometer, giving details about the chemical nature of the sample with a similar resolution. Unfortunately, while undergoing TERS imaging, the samples are exposed to very intense electromagnetic fields that could damage them, which represents one of the main limitations that hamper the widespread application of TERS to this area. In this project, our main goal is then to understand and control the sample degradation mechanism resulting from the exposure to the intense electromagnetic field and expand the application of TERS to “delicate” samples, such as 2D organic polymers or biological samples. Despite being very challenging, the perspective of applying TERS to biological membranes represents a pioneering investigation that we hope could lead to a major breakthrough in our understanding of how cell membranes are organized.
Initially, much work went into set-up of equipment and data processing pipelines. The stability of the TERS probe is also of key importance and determines the quality of the experimental results. We developed AFM-TERS tips coated with a thin layer of plasmonically active metal. We purchased, using own funds, and deployed a thermal evaporator inside of a glove box, for manufacturing ultra-clean and stable AFM-TERS tips. A method for recycling tips was also found. To maximize the chemical sensitivity of silver-coated TERS probes, a FDTD simulation model has been set up, for rapid testing of different silicon oxidation and silver coating parameters. Finally, our work has also led to a patent (submitted, application number EP19206592.8 not yet granted) on novel nanowire AFM tips that can also be used for TERS.

For providing a better understanding of TER spectra, we have simulated Raman spectra of molecular and periodic systems using various state-of-the-art computational methods. This allows us to compare our experimental results with what we expect from theory. This now allows the assignment of the complex TERS and Raman spectra from 2D-polymers, which are periodic in two directions.

In terms of sample preparation, much work was done to mitigate contaminants in the lipid membrane studies. Artificial membranes formed on a Langmuir-Blodgett trough and transfer of lipid monolayers onto template stripped gold, for STM-TERS measurement has been optimized. We have performed isotherm studies of the cationic antimicrobial peptide indolicidin and its interaction with DOPC. We have measured confocal Raman spectra of the peptide and identified the Raman bands of interest. Deposition of biomimetic and bacterial cell membranes (bilayers) on gold via vesicle fusion was optimized. We have achieved STM-TERS imaging of a DPPC self-assembled monolayer on gold, and are working on high-resolution chemical imaging of biphasic lipid systems consisting of DPPC and deuterated (d31) SM.

We have also built and tested an electrochemical TERS setup which helps performing experiments in liquid. One of the many applications for that technique is in catalysis, where we are studying the water oxidation reaction using an interfacial iridium compound as the active catalyst. We succeeded in recording a plethora of oxidation-reduction cycles with cyclovoltammetry. Finally, for the preparation of very air-sensitive samples, we have designed, built and brought to operation a continuous-flow inert gas purifier, which reduces the oxygen content in commercial boil-off nitrogen from roughly 60 ppm (parts per million) down to below the detection limit of our analyzer (≈1 ppm) and rigorously removes any water vapor.
Progress: TERS is a very powerful nano-spectroscopic technique. Not only it allows imaging of thin materials at ~10 nm spatial resolution, but it also yields chemical information. This makes it very useful in studying 2D materials which are very promising and useful in fields such as electronics, computer science, catalysis, biology, etc. We have studied reversible photoisomerization of molecular switches using TERS which are very promising materials for molecular electronics and high-density data storage. We have also elucidated the mechanism of sample degradation under the TERS tip which is very useful in optimizing the future TERS experiments of fragile samples, such as peptides, proteins, biological membranes, etc.

Expected Results: We expect to obtain a better theoretical understanding of the tip-enhanced Raman spectra of complex biological model systems using chemometrics and data analysis, as well as to better understand the TER response of 2D materials such as 2D polymers. Experimentally, we expect to obtain the TERS imaging of model biological systems such as lipid monolayer and bilayer using STM and AFM-TERS respectively. We expect to obtain a stable TERS map of a biphasic lipid membrane system to be able to differentiate between the different lipid components of the system. Furthermore, we expect to see the interaction of the peptide (such as indolicidin) and pore forming molecule (such as Amphotericin B) in the lipid bilayer, and chemically map their surrounding lipid environment to gain better understanding into their mechanism of action. Our ultimate aim is to do TERS imaging of living cells, and we are confident that we are moving in the right direction towards achieving it.