Methods for Achieving Ultracleanness to Enable Freestanding Nanno-Membrane Research

Even sub-monolayer, low-coverage contaminants, on <i>both</i> surfaces, obscure intrinsic properties of a freestanding atomically thin membrane or 2D material, and their ultimate thinness precludes the effective conventional cleaning methods that are almost always sacrificial. In this regard, suspended single-layer graphene and carbonaceous contaminants represent the most salient challenge.<br/>We have achieved ultraclean suspended graphene, attested by electron energy loss (EEL) spectra, for the first time, as clearly resolved as the best data of <i>graphite</i>, with fine-structure features corresponding to the bonding characters and band structure of <i>graphene</i>. The notorious electron beam induced carbon deposition in electron microscopy has been eradicated, as demonstrated for the first time by dwelling an intense focused electron beam for one minute and continuously raster scanning the beam for a full work day at intensities ~ 6 × 10⁸ e^–/(Ų s) without inducing carbon deposition in a transmission electron microscope. It is this level of cleanness that allowed for unprecedentedly high accumulated dose (10¹² e^– in a 1 nm² scanned area) and long acquisition time (5 min) to achieve the high-quality spectra mentioned above.<br/>The methods, while involving <i>ex situ</i> transfer and pre-cleaning and <i>in situ</i> cleaning, are readily adaptable to other experimental settings and other materials, to enable previously inhibited undertakings that aim at revealing or exploiting intrinsic properties of 2D materials or ultrathin membranes, especially those using graphene as an encapsulator for other membranes or as a supporting or buffer layer (e.g. in remote epitaxy). Importantly, the surprisingly simple and robust methods are easily implementable with common lab equipment.