Vibrationally Induced Two-Level Systems in Single-Molecule Junctions

Abstract

Single-molecule junctions are found to show anomalous spikes in dI/dV spectra. The position in energy of the spikes are related to local vibration mode energies. A model of vibrationally induced two-level systems reproduces the data very well. This mechanism is expected to be quite general for single-molecule junctions. It acts as an intrinsic amplification mechanism for local vibration mode features and may be exploited as a new spectroscopic tool.PACS numbers: 81.07. Nb, 73.63.Rt, 85.65.+h, 63.22.+m A single atom or molecule with an almost transparent single conductance channel leading to a conductance near the conductance quantum 2e 2 /h (= 1 G 0 ) can be contacted to leads. Conduction electrons can pass through such junction ballistically for low bias voltages since the mean free path of the electrons is much larger than the size of the contact. However the contact is not entirely ballistic in the sense that once the excess energy of the conduction electrons becomes equal or larger than the energy of a local mode of the contact, the electrons can scatter inelastically by exciting a local mode. This results in the case of a perfectly transmitting single channel contact to a small decrease in the conductance, since the forward travelling electrons are backscattered due to the energy loss in the inelastic scattering process. Differential conductance (dI/dV) measurements have identified vibration modes of single molecules in an atomic contact [1]. This technique, also called Point Contact Spectroscopy (PCS) is analogous to inelastic electron tunnelling spectroscopy (IETS) for single molecules [2,3], with the difference that the conductance in the latter case increases due to the opening of an additional conductance channel.In this letter we present the observation of anomalous spikes, rather than steps, in dI/dV measurements on various single molecule contacts. We present a model that involves two-level systems, which describes our data very well. It may be used as a new spectroscopic tool for identifying molecular vibration modes in single molecule junctions.We create atomic contacts using a mechanically controlled break junction (MCBJ) setup in cryogenic vacuum at 4.2 K (see ref.[4] for a detailed description). Break junctions for the metals Au, Ag, Pt and Ni have been investigated with the molecules H 2 , D 2 , O 2 , C 2 H 2 , CO, H 2 O and benzene. In most of these cases regular vibration mode spectra displaying a step down in conductance have been observed, but for all systems anomalous spectral features as displayed in Fig. 1 were also found.In order to admit these molecules to the metal atomic contacts at 4.2 K, the insert is equipped with a capillary that has a heating wire running all along its interior to prevent premature condensation of the gasses. The amount of gas admitted is of order 10 µmol. Previous measurements have clearly demonstrated that it is possible to capture a single molecule in the atomic junction and measure its vibration modes [1,5].Figure 1(a) shows a dI/dV spectrum of a single ...

Vibrational Nonequilibrium Effects in the Conductance of Single Molecules with Multiple Electronic States

“…Due to the small size of molecules, the charging of the molecular bridge is often accompanied by significant changes of the nuclear geometry that indicate strong coupling between electronic and nuclear (in particular vibrational) degrees of freedom [8]. Electronic-vibrational (vibronic) coupling manifests itself in vibrational structures in the conductance, which have been observed for a variety of different systems [5,6,9,10,11], and may result in current-induced vibrational excitation that destabilizes the junction and causes local heating [12,13]. Furthermore, conformational changes of the geometry of the conducting molecule are possible mechanisms for switching behavior and negative differential resistance [14,15].…”

Conductance of single 1,4-disubstituted benzene molecules anchored to Pt electrodes

“…First, the conductance trace shows a conductance fluctuation, which would originate from the structural transformation of the molecular junction. 13 Second, the trace shows a plateau in which the conductance is nearly constant ͑arrow in Fig. 1͒, and the conductance value of the plateau is an integer multiple of ϳ3 ϫ 10 −2 G 0 ͑=2e 2 / h͒.…”

Current-induced conformational switching in single-molecule junctions

“…An essential requirement for electric circuits of nanoscale dimensions is a molecular device that can be switched between two distinct conductive states. Because of intrinsic bistabilities, many single-molecule junctions reveal switching behavior, e.g., involving cis and trans isomers of a molecule [15][16][17][18][19][20][21][22][23][24][25].…”