Electronic Properties of Suspended Few-Layer Graphene Membranes
Graphene, the two-dimensional (2D) honeycomb lattice of sp2-hybrized carbon atoms, has emerged as a “wonder” material with unique properties, such as its linear energy dispersion with massless Dirac fermions, so-called half-integer quantum Hall (QH) effect, unparalleled tensile strength, and high optical transparency and thermal conductivity. Its few-layer counterparts have similar mechanical but remarkably different electrical properties, including layer- and stacking-dependent band structures, massive charge carriers, and energy gaps that may arise from single particle effect as well as electronic interactions.
This dissertation reports my six year study of dual-gated suspended few-layer graphene (FLG) field effect transistor (FET) devices. In particular, we focus on their electronic transport properties at low temperature as a function of out-of-plane electric field E_⊥ and interlayer potential U_⊥, charge carrier density n, temperature T, and out-of-plane (B_⊥) and parallel (B_⫽) magnetic fields. A number of broken symmetry states in the absence and presence of external fields are observed in rhombohedral-stacked bilayer- (BLG), trilayer- (r-TLG), and tetralayer graphene (r-4LG). We also study the morphological deformation of suspended graphene membranes under electrostatic and thermal manipulation, which is relevant for analyzing low temperature transport data.
In particular, in BLG, r-TLG and r-4LG, we observe intrinsic insulating states in the absence of external fields, with energy gaps of 2, ~40, and ~80 meV, respectively. We attribute this increasing gap size with number of layers N to enhanced electronic-interactions near the charge neutrality point, due to the layer-dependent energy dispersions k^N in r-FLG, which give rise to increasingly diverging density of states and interaction strength with increasing N, at least up to four layers. Our observations of the spontaneous insulating state in r-FLG are consistent with a layer antiferromagnetic state with broken time reversal symmetry, where the top and bottom layers are oppositely spin polarized.