Shot Noise of 1,4-Benzenedithiol Single-Molecule Junctions

Abstract

We report measurements of the shot noise on single-molecule Au-1,4-benzenedithiol-Au junctions, fabricated with the mechanically controllable break junction (MCBJ) technique at 4.2 K in a wide range of conductance values from 10(-2) to 0.24 conductance quanta. We introduce a simple measurement scheme using a current amplifier and a spectrum analyzer and that does not imply special requirements regarding the electrical leads. The experimental findings provide evidence that the current is carried by a single conduction channel throughout the whole conductance range. This observation suggests that the number of channels is limited by the Au-thiol bonds and that contributions due to direct tunneling from the Au to the π-system of the aromatic ring are negligible also for high conductance. The results are supported by quantum transport calculations using density functional theory.

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

“…In particular, we find common signatures of the =C-H in-plane and outof-plane deformations in the energy range 100-130 meV (890-1050 cm −1 ) [34,35,42,43,[46][47][48], where signatures of the N-H deformation or C-C stretching are also predicted [21,37,42,48,49]. Common signatures of the stretching of different carbon based bonds are also detectable, such as C-N, C=C, and C=O in the energy range 160-210 meV (1290-1690 cm −1 ) [21,35,42,43,[47][48][49]. This crossed analysis shows that IETS combined with the QI effect allows one to highlight modes equivalent to what was found by optical spectroscopy.…”

“…Furthermore, the peak around 10 meV (80 cm −1 ) can be attributed to AQ skeletal deformation [43,44]. Note, however, that resonances in the low energy range usually involve metal/metal bonds as in the case of tip-apex metal vibrational modes measured in STM [45] and MBJ [47] based experiments. More generally, we recognize signatures that can be related to typical CC, CN, CH, and CO bonds.…”

“…2, realized by comparison to existing vibrational spectroscopy studies in literature [21,34,35,[42][43][44][45][46][47][48][49][50][51][52][53]. Typical signatures of expected modes related to bonds of the AQ molecule are present in the whole energy range.…”

“…Note finally that some peaks can be also attributed to residual traces of organic solvents, such as a C-C stretching vibration at 130 meV (1050 cm −1 ) [34,35,42,47,48], a CH 2 stretching at 345-360 meV (2780-2900 cm −1 ). Hydroxyl O-H group bands are attributable to the peaks at 410 meV (3300 cm −1 ) and 437 meV (3525 cm −1 ) and can be also related to vibrations of water molecules [42].…”

“…Most evident vibrational bands are highlighted by black vertical lines. Their assignment is done following the literature [21,34,35,[42][43][44][45][46][47][48][49][50][51][52][53][54] and is summarized in Table II. While IETS is typically difficult to interpret since there are no strict theoretical determined selection rules and propensity rules have only been proposed [63,64], in IR spectroscopy selection rules exist, simplifying the assignment of the vibrational modes. By comparing Tables I and II common modes between IETS and IR spectra can be recognized in the energy range 100 meV < E < 200 meV.…”