Mechanics and Gas Transport of Ultrathin Membranes

Luda Wang, University of Colorado Boulder


This thesis focuses on the gas transport of porous graphene membranes. Moreover, it includes the mechanical properties of ultrathin films of atomic layer deposition (ALD) Al2O3.

The ability to control the quantity and location of a single file molecular flux to a precise location in space has important applications to nanoscale 3D printing, catalysis, and sensor design. Barrier materials containing pores with molecular dimensions have been used to control molecular compositions in the gas phase, but unlike their aqueous counterparts, none has enabled an ability to observe or control single pore transport. Herein, we demonstrate gas transport through atomically thin, monolayer graphene opened with a single molecularly-sized, sub-nm pore demonstrating the ability to detect and control the gas flux. This is accomplished using ~nm sized gold clusters formed on the surface of the graphene. Such clusters migrate and partially block the pore. We also observe stochastic switching of small magnitude in the gas flux indicative of modulation by a single pore even without gold clusters. The stochastic switching is fit to discrete and repeatable states. These nanopore molecular valves open possibilities for unique sensors, catalytic processes, and approaches to molecular synthesis based on the controllable switching of a molecular gas flux reminiscent of ion channels in biological cell membranes and solid state nanopores.

In this thesis, a method is also presented to create and characterize mechanically robust, free standing, ultrathin, oxide films with controlled, nanometer-scale thickness using ALD on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Young's modulus of 154 ± 13 GPa. This Young's modulus is comparable to much thicker alumina ALD films. This behavior indicates that these ultrathin two-dimensional films have excellent mechanical integrity. The films are also impermeable to standard gases suggesting they are pinhole-free. These defect-free, micron-dimensioned, 2-D ultrathin films are expected to enable new applications in fields such as thin film coatings, membranes and flexible electronics.