Thermoelectric voltage modulation via backgate doping in graphene nanoconstrictions studied with STGM

Castanon, Eli and Kolosov, Oleg and Hamer, Matthew and Niblett, Andy and Gonzalez Munoz, Sergio and Zultak, Johanna and Gorbachev, Roman and Kazakova, Olga (2021) Thermoelectric voltage modulation via backgate doping in graphene nanoconstrictions studied with STGM. In: Microscience microscopy congress 2021, 2021-07-06 - 2021-07-09, Virtual event.

Abstract

The thermoelectric (TE) applications of graphene have generated great interest due to its extraordinary electronic and thermal properties, gate-controlled ambipolar behaviour, and competitive Seebeck coefficient (S). In recent studies, the effects of nanostructuring on the local variations of the Seebeck coefficient have been explored for bare graphene samples with patterned bow-tie constrictions[1], and mono- and bi-layer junctions[2]. These represent a new paradigm in the control of the TE properties on the nanoscale, enabling the development of single metal thermocouples[3] for temperature sensing and coolers for thermal load distribution and hot-spot removal with nanoscale dimensions. Here, we study the spatial distribution of the local Seebeck domains via the thermoelectric voltage (V_th )in encapsulated graphene devices with patterned constrictions. This novel approach explores two different strategies to improve the V_th signal and control of the S domains: (1) the enhancement of the Seebeck coefficient by encapsulation, gate carrier control, and temperature gradient control; and (2) the creation of local S domains by patterning constrictions of varying geometry and size as shown in figure 1(a). To study the response of the devices, maps of the thermovoltage with nanoscale resolution were created using scanning thermal gate microscopy (STGM), a novel SPM mode in which a hot tip is employed as the local heating source and scanned over the sample in open circuit configuration, thus creating thermovoltage signal that is proportional to the Seebeck coefficient. A schematic representation of the STGM is depicted in figure 1(b). The resulting thermovoltage maps were acquired for different gate voltages (V_BG ) and different temperature gradients between the tip and sample. In figure 1(c) and (d), V_th maps acquired for p- and n-doped graphene, respectively, are presented. Local TE junctions with an opposite gradient of Seebeck coefficient are formed across the device. Furthermore, there is a clear almost perfect inversion of the TE effect sign with the inversion of the charge carriers. One-dimensional profiles performed across the devices (see figure 1(e)) show that the highest Seebeck coefficient gradient occurs in the central rectangular constriction due to changes of the Seebeck coefficient in this area as well as in the half-bowtie bottom constriction. At these locations the electron mean free path (EMFP) would be reduced, leading to the observed sign change in S gradient. In this study, we have demonstrated the formation of the local Seebeck domains TE junctions in graphene devices with nanopatterned constrictions of varying geometries. Means to control the intensity and sign of the thermoelectric signal have also been shown. The combination of these solutions could lead to effective thermal management in electronic graphene devices, and the development of important applications such as single material thermocouples or coolers at the nanoscale. We also demonstrate the viability of STGM as a novel visualisation and characterisation tool able to provide much higher resolution than conventional optical methods for the characterisation of local TE properties.