Phase transitions, hysteresis and mesoscopics in phases of adsorbates on suspended graphene

We study the states of adsorbed layers of simple gases using electrical conductance measurements of suspended graphene flakes. Conductance isotherms as a function of the three-dimensional pressure reveal multiple phase transitions in the chemical potential-temperature phase diagram of the adsorbed films. The technique is exquisitely sensitive both to the density of adsorbates and to their ordering. We compare the behavior seen using monolayer, bilayer, and trilayer graphene and graphene on hBN, for Ar, Ne, N₂, CO, O₂, and Kr. In addition to probing the classic 2D phase diagrams of gas, liquid and solid phases of these adsorbates, in some cases much more precisely that possible with traditional adsorption isotherms on bulk materials, we observe modifications due to the weaker binding and two-sided adsorption. Additionally, we observe reproducible mesoscopic fluctuations and hysteresis at the phase transitions, phenomena that we believe have not been oserved in adsorption before which are revealed because of the tiny area and unique form of the sample.