Electron Beam Induced Modification of N-Heterocyclic Carbenes – Carbon Nanomembrane Formation

Electron irradiation of self-assembled monolayers (SAMs) is a versatile tool for a variety of lithographic methods and formation of new 2D materials such as carbon nanomembranes (CNMs) with potential application in ultrafiltration. While the interaction between the electron beam and standard aromatic or aliphatic thiolate SAMs on gold has been well studied, the behavior of more complex systems such as N-heterocyclic carbenes (NHCs), which are recently attracting growing attention due to their ultra-high chemical and thermal stability, remains completely unknown. In the current work, we analyze the low-energy electron irradiation of SAMs on gold based on the series of NHC molecules featuring different number of benzene moieties (0, 1 and 2), and different size of the nitrogen side groups (methyl, isopropyl) to modify their packing density. The changes in monolayer thickness and composition are carefully analyzed by the X-ray photoelectron spectroscopy (XPS) as a function of electron dose. Our results provide design rules to optimize NHC SAMs structure for their effective modification by the electron irradiation. Such optimization becomes particularly interesting considering that the analyzed NHC monolayers exhibit a much higher stability of their bonding with the metal substrate towards electron irradiation compared to standard thiols or recently applied carboxylic acids. Thus, the NHC SAMs offer an interesting alternative for chemical lithography where structural modification of SAMs by electron or photon beams should be limited mainly to the functional group. Moreover, delamination and transferring of electron irradiated NHC monolayers on holey TEM grid reveals formation of carbon nanomembranes (CNMs). As we show this process is also very sensitive to the structure of NHC SAMs and for properly designed system enables formation of continuous, freestanding CNMs, which are sulfur free and thus much more suitable for some ultrafiltration applications compared to standard CNMs fabricated from thiols and therefore contaminated with reactive sulfur.