Theoretical study of the charge transport through C60-based single-molecule junctions

Abstract

We present a theoretical study of the conductance and thermopower of single-molecule junctions based on C 60 and C 60 -terminated molecules. We first analyze the transport properties of gold-C 60 -gold junctions and show that these junctions can be highly conductive (with conductances above 0.1G 0 , where G 0 = 2e 2 /h is the quantum of conductance). Moreover, we find that the thermopower in these junctions is negative due to the fact that the lowest unoccupied molecular orbital dominates the charge transport, and its magnitude can reach several tens of microvolts per kelvin, depending on the contact geometry. On the other hand, we study the suitability of C 60 as an anchoring group in single-molecule junctions. For this purpose, we analyze the transport through several dumbbell derivatives using C 60 as anchors, and we compare the results with those obtained with thiol and amine groups. Our results show that the conductance of C 60 -terminated molecules is rather sensitive to the binding geometry. Moreover, the conductance of the molecules is typically reduced by the presence of the C 60 anchors, which in turn makes the junctions more sensitive to the functionalization of the molecular core with appropriate side groups.

Nanoelectrical analysis of single molecules and atomic-scale materials at the solid/liquid interface

“…Two distinct molecular resonance peaks in an energy span of ±1.5 V are visible in the dI /dV spectra centred around −0.5 and +0.7 eV, which correspond to the HOMO and LUMO peak positions, respectively. The DFT-calculated HOMO and LUMO eigenstates in the gas-phase molecule (indicated by the green arrow) show the HOMO localized at the interface between the C 60 anchor and the functional group whereas the LUMO is localized on the C 60 lobe, comparable to previous DFT calculations on C 60 -terminated molecular dumbbells 39 , in which the LUMO remains unperturbed on molecular functionalization. Also the LUMO-derived peak position for the functionalized C 60 molecule with respect to the Fermi edge is equivalent to the LUMO position of pristine C 60 but there is a significant shift in the HOMO peak position closer to the Fermi edge resulting in a decreased conductance gap for the functionalized C 60 molecule.…”

Thermal conductance and thermoelectric figure of merit of C60 -based single-molecule junctions: Electrons, phonons, and photons

“…Notice that in both cases the electronic transmission at the Fermi energy is determined by the LUMO of the molecules, as it has been reported in numerous studies, see for instance Ref. 31 and references therein.…”

“…The fullerene C 60 is a test-bed molecule for molecular electronics and its electrical transport properties have been extensively investigated both experimentally [14][15][16][17][18][19][20][21][22] and theoretically [18,[20][21][22][23][24][25][26][27][28][29][30][31][32]. Also in the context of thermoelectricity, C 60 -based single-molecule junctions have been analyzed and several groups have reported room-temperature thermopower measurements [33][34][35].…”

Voltage tuning of vibrational mode energies in single-molecule junctions

“…However, in a junction environment, where C 60 is contacted on both sides with rough surfaces, E 0 will be closer to E F (37). Depending on the specifics of the Au-C 60 contact within a particular junction, E 0 may vary somewhat.…”

“…4 shows a simple model for the energy level alignment of the junction. At zero applied bias, the triply degenerate LUMO resonance will be positioned near the Fermi energy (37,43), and broadened by its coupling to the source and drain electrodes. Assuming that electrons tunneling from the source to drain and drain to source, respectively; that they are noninteracting and therefore are occupied according to their original source or drain quasi-Fermi levels (44); and that the resonance line shape is Lorentzian with a width Γ = Γ S + Γ D , the change in steady-state occupation, δρ, of a single triply degenerate level at energy E 0 above the equilibrium Fermi level E F , at bias V, may be expressed as (1,27) …”