Superior Contact for Single-Molecule Conductance: Electronic Coupling of Thiolate and Isothiocyanate on Pt, Pd, and Au
Date Issued
2009
Date
2009
Author(s)
Ko, Chih-Hung
Abstract
One of the critical issues for molecular electronics is the development of optimal molecule-electrode contacts which have long been expected to substantially influence the measured single-molecule conductance. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal barrier have not been reported. This thesis work scrutinizes the effect of electrode materials on single-molecule conductance by examining alkanedithiols anchored onto Au and Pt electrodes as well as alkanediisothiocyanates on Au, Pd, and Pt. STM-BJ (scanning tunneling microscopy break junction) was employed to create thousands of molecular junctions and to obtain single-molecule conductance. The results show that all molecule-electrode combination exhibits high- and low-conductance datasets (HC and LC). Compared to the contact resistance measured using Au electrodes, alkanediisothiocyanates are about 20% ~ 60% less resistive on Pd and Pt and alkanedithiols are about 50% less resistive on Pt. The difference is ascribed to their Fermi energies and the pi characters of the atoms at the contact. The dependence of single-molecule conductance on the electrode materials is also true for a linear trimetal complex, [Ni3(dpa)4(NCS)2] (dpa = 2,2''-dipyridylamide), suggesting the generality of the findings for both saturated and highly conductive molecular wires. For alkanediisothiocyanates, the probability of acquiring staircase-like traces among all i-s traces increases from 27% on Au to 37-42% on Pd or Pt electrodes. Using density functional theory (DFT), adsorption energies on three-fold hollow site and ontop site are in agreement with probabilities of HC and LC. Calculations of transmission function and comformation of headgroup-metal interface are also carried out to facilitate the rationalization using HOMO-LUMO gap and electronic coupling at the contact.
Subjects
molecular electronics
STM, break junction
quantum point contact
single molecular conductance
Type
thesis
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