Abstract
The quest for materials showing large thermoelectric power has long been one of the important subjects in material science and technology. Such materials have great potential for thermoelectric cooling and also high figure of merit ZT thermoelectric applications. We have fabricated bilayer graphene devices with ionic-liquid gating in order to tune its band gap via application of a perpendicular electric field on a bilayer graphene. By keeping the Fermi level at charge neutral point during the cool-down, we found that the charge puddles effect can be greatly reduced and thus largely improve the transport properties at low T in graphene-based devices using ionic liquid gating. At (Vig, Vbg) = (−1 V, +23 V), a band gap of about 36.6 ± 3 meV forms, and a nearly 40% enhancement of thermoelectric power at T = 120 K is clearly observed. Our works demonstrate the feasibility of band gap tuning in a bilayer graphene using ionic liquid gating. We also remark on the significant influence of the charge puddles effect in ionic-liquid-based devices.
Highlights
According to the Mott relation[10], TEP of a material is proportional to the density of states (DOS) at Fermi level
Since the ions are frozen below 180 K, one would need to warm up the ionic liquid to temperature higher than the freezing point to change the charge doping level
The progressive shift of the charge neutral point (CNP) to higher Vbg values as Vig goes from + 1.5 V to − 1.5 V is an indication of effective charge doping in the BLG
Summary
According to the Mott relation[10], TEP of a material is proportional to the density of states (DOS) at Fermi level. The progressive shift of the charge neutral point (CNP) to higher Vbg values as Vig goes from + 1.5 V to − 1.5 V is an indication of effective charge doping in the BLG. The black curve is the data at T = 220 K above the DEME-TFSI freezing point, which indicates that Fermi level can be shifted to the CNP by applying a Vbg = + 43 V.
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