Eutectic gallium-indium based thermoelectrics in tunnelling junctions comprising self-assembled monolayers

In his thesis, Lungani Mthembu presents a new EGaIn-based thermoelectric set-up with high flexibility and extreme capabilities. Self - assembled monolayers have been measured for many years using numerous different measurement platforms to determine transport properties of various molecules. Charge transport in SAMs can happen through tunnelling and hopping. In tunnelling, an electron is transferred between energy levels across energy barriers in the scattering zone while hopping transport is a stepwise process mediated by the redox levels of the linker molecules. Tunneling junctions of SAMs have been extensively studied using EGaIn as a top electrode contact and it is therefore interesting to use EGaIn as a top electrode to study thermoelectrics and compute the seebeck coefficient. 

In one study, Mthembu studied the length dependence of even numbered alkanethiols from C2 to C18SH by first reproducing the literature reported length dependence trend at ambient conditions. Using a recently developed EGaIn based thermoelectric measurement platform, the bottom electrode (AuTS) temperature was systematically lowered creating various ΔTs which affects transport. The temperature gradient is reversed from positive to negative which significantly affects transport resulting in a flattened curve. This study raises important questions about metal induced gap states and the rate of change of the Seebeck with molecular length. Mthembu observed the disappearance of the previously reported two length dependent regimes with lowering temperatures. The electrode work function, substrate, SAM, EGaIn were possible sources of the phenomenon observed by lowering electrode temperatures and observing the complete elimination of two length dependent regimes in alkanethiols. The direction of charge carriers was changed from +ΔT to -ΔT which affected molecular behavior.

In another study, the measurement of alkanethiols C12 - C16SH at ambient conditions confirmed the well known length dependence. Seebeck measurements were undertaken on a +ΔT and −ΔT gradient. When the temperature of AuTS is lower than EGaIn ΔT is negative and when AuTS is heated above EGaIn, ΔT is positive. In both ΔTs, fullerene based SAMs and SABs PTEG-1weremeasured and the sign of the Seebeck coefficient in the same molecular system changed from + on SAM to − on SAB. Transport was confirmed to be non-resonent tunnellig and a molecular orbital shift from HOMO to LUMO is responsible for the change in the sign of the Seebeck coefficient.

Finally, Mthembu describes a comparative study of a single-level model (SLM) to describe tunneling charge-transport between alkanethiols (CnSH; = 7 - 16) and oligophenylene dithiol (TnC4; = 1 - 4) chemisorbed on ultra-flat Au substrates. Using Vtrans values extracted from plots of current versus voltage, Mthembu applies the SLM to data obtained using EGaIn which he compares to ultraviolet photoelectron spectroscopy measurements. Mthembu concludes that, across the two measurement platforms in both molecular systems, the substrate (bottom electrode) and molecular-length strongly influence coupling for both CnSH and TnC4, but the energy gap between the frontier molecular orbitals and Fermi level are systematically larger for EGaIn than UPS. This observation holds true for both alkanethiols and aromatic/-conjugated molecular systems and suggests that the molecule-electrode interface influences the level-alignment in accordance with what is predicted by work functions. These results demonstrate the reproducibility of the SLM in both measurement platforms, expanding its utility and shedding light on the discrepancies often observed when comparing junctions measured by CP-AFM platform as reported in literature.

Dissertation: https://hdl.handle.net/11370/c824ae8d-e5c0-456b-a0dd-5c8dfff00e86