Manufacturing and Characterization of Large-scale Graphene and Metal Thin Film Membranes

Abstract

Membranes made from two-dimensional (2D) materials and thin films have emerged in recent years in the quest for ever-more permeable, highly selective membranes. Since the discovery of graphene, a single-layer sheet of carbon atoms, extensive research has been performed to manufacture and characterize such graphene membranes. Currently, one of the hurdles is the lack of controllable, uniform and large-scale perforation, as well as suitable mechanical supports and transfer techniques to create permeable membranes. As a consequence, few experimental demonstrations of mass transport and filtration using such membranes have been shown. The goal of this work was, therefore, the development, manufacturing and characterization of controllable and large-scale perforation methods of graphene and metal thin films. In the first part of this work, two methods to create perforated graphene membranes are demonstrated: one relying on block copolymer (BCP)-based nanolithographic etch masks, the other on the local inhibition of graphene formation during synthesis. Both methods enable the graphene perforation with pores < 100 nm, where the pore size is controlled via a respective process parameter. The pore density, an important figure of merit for porous membranes, is up to 2.1 · 1014 m−2, among the highest reported for graphene membranes of the respective pore size. The two methods are compared regarding their respective advantages and disadvantages. A graphene transfer technique is subsequently introduced, enabling the formation of permeable membranes consisting of patterned graphene on a polymer support as large as 5 cm × 5 cm. In addition, BCP-based nanolithography is demonstrated as a reliable and universal patterning method, enabling the perforation of other 2D materials and metal thin films. Subsequently, the mass permeation and filtration of graphene membranes, made from both methods, are modeled and characterized. The flow across these porous 2D membranes is modeled based on their unique transport regimes, where the resistance emerges, unlike in channels, solely from entrance effects, which minimizes the transport resistance. The theoretical and measured permeances coincide well for both, gases and liquids, finding a water permeance of up to 20’000 liters per square meter per hour per bar, among the highest measured for a pore size cut-off of ∼80 nm. These high flow rates, for gases and liquids likewise, are attained as the perforated graphene has a minimal mass transport resistance, such that graphene membranes can be used in e.g. high performance ultrafiltration.