Inelastic Tunneling Spectroscopy of Alkanethiol Molecules: High-Resolution Spectroscopy and Theoretical Simulations

Abstract

We investigate inelastic electron tunneling spectroscopy (IETS) for alkanethiol self-assembled monolayers (SAM) with a scanning tunneling microscope and compare it to first-principles calculations. Using a combination of partial deuteration of the molecule and high-resolution measurements, we identify and differentiate between methyl (CH3) and methylene (CH2) groups and their symmetric and asymmetric C-H stretch modes. The calculations agree quantitatively with the measured IETS in producing the weight of the symmetric and asymmetric C-H stretch modes while the methylene stretch mode is largely underestimated. We further show that inelastic intermolecular scattering is important in the SAM by plotting the theoretical current densities.

Current-induced atomic dynamics, instabilities, and Raman signals: Quasiclassical Langevin equation approach

“…The equilibrium zerocurrent dynamical matrix is denoted by K. This Hamiltonian has been used to describe IETS in molecular contacts with parameters obtained, e.g., with density functional theory (DFT) for concrete systems. 20,49 We have previously discussed the adiabatic limit, where the perturbation is in terms of the velocitiesu k and where the full nonlinear effects of u k can be included. Here, we instead assume small displacements from equilibrium and expand the electronic Hamiltonian to first order in u k .…”

Inelastic electron tunneling process for alkanethiol self-assembled monolayers

“…In contrast, inelastic electron tunneling spectroscopy (IETS) can be adopted for even a single molecule. In this article, we review the recent progress that has been made on the use of IETS for alkanethiol SAMs, where IETS is compared to alternative, more conventional vibrational spectroscopy techniques [39][40][41][42][43]. IET spectra obtained for various junctions are also compared and the propensity rule of IETS is discussed using state-of-the-art theory based on DFT and NEGF [9,44,45].…”

Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory

“…By studying these subtle changes of the vibrational energy using STM-IETS with a molecularfunctionalized tip, it has been demonstrated that STM-IETS can provide information on the inner structure of a molecule [4,5] similarly to atomic force microscopy (AFM) [6]. These advantages of STM-IETS have accelerated research in related fields [7][8][9][10][11][12][13][14][15][16]. Owing to recent progress in the theoretical description of IETS [17][18][19][20][21][22], the qualitative understanding has been improved considerably: the symmetry of the wave functions of a tip and a molecule on a substrate and a vibrational mode of the molecule are predicted to influence the efficiency of the inelastic process (γ inel ) for the tunneling current involving the molecule.…”

“…In order to measure the conductance (dI /dV ) and IET signal (d 2 I/dV 2 ), a modulation voltage (2338.7 Hz, 1 mV rms ) is added to the sample bias and the first and second harmonics in the current are detected by a lock-in amplifier (HF2LI, Zurich Instruments, Zürich, Switzerland). Throughout the whole paper, the IET signal is normalized by the differential conductance, i.e., IETS = (d 2 I/dV 2 )/(dI/dV ) [10,14,18,20]. The tip is formed from an etched tungsten wire, cleaned by field evaporation and repeatedly poked into the Cu substrate to prepare various tip apexes [23,24,30].…”