Surveying Molecular Vibrations during the Formation of Metal−Molecule Nanocontacts

Abstract

Molecular junctions have been characterized to determine the influence of the metal contact formation in the electron transport process through a single molecule. With inelastic electron tunneling spectroscopy and first-principles calculations, the vibration modes of a carbon monoxide molecule have been surveyed as a function of the distance from a copper electrode with unprecedented accuracy. We observe a continuous but nonlinear blue shift of the frustrated rotation mode in tunneling with decreasing distance followed by an abrupt softening upon contact formation. This indicates that the presence of the metal electrode sensibly alters the structural and conductive properties of the junction even without the formation of a strong chemical bond.KEYWORDS Inelastic electron tunneling spectroscopy, electron-vibration coupling, single molecule-metal contact, electron conductance, density functional theory, nonequilibrium Green's function, point contact spectroscopy C harge transport through metal-molecule systems is a major subject of study in a rapidly growing interdisciplinary research field. It deals with fundamental and applied aspects of science at the nanoscale aiming to control the electron conductance at the molecular level and the uprising of nanotechnology.1,2 One of the major results of this research is that the properties of an isolated molecule are not the only fundamental parameters determining the conductance in a metal-molecule junction. Indeed, similarly to the adsorption of molecules on surfaces, the atomistic arrangement at the junction and the coupling between the molecule and the metal electrodes can significantly alter the electronic and structural properties of the molecule.3-5 In particular, the stronger the metal-molecule interaction is, the less the measured conductance can be ascribed to molecular properties alone.Accessing the influence of the molecular contact to the metal in the transport properties is, however, experimentally challenging. Despite the continuous progress, the electron conductance measured using either a scanning tunneling microscope (STM) or a break-junction device 6-13 has so far not elucidated this question clearly. The strength of the metal-molecule coupling, and in particular its variation during the contact formation, can be characterized through the spectroscopic signal of the molecular vibrations as measured by inelastic electron tunneling spectroscopy (IETS). 6,9,[14][15][16][17] This can be recorded as a function of the tip-molecule distance allowing understanding to which extent the contact formation influences the molecular properties.A good molecular prototype to study the influence of the metal bond is carbon monoxide (CO). Indeed, this molecule is used as ligand to contact metal atoms in coordination chemistry and its electronic and vibration properties are very sensitive to the local adsorption environment. Shifts of the vibration frequencies are observed by coadsorption with other gases 18 and explained by subtle mechanisms of charge transfer. 19...

Lowest order in inelastic tunneling approximation: Efficient scheme for simulation of inelastic electron tunneling data

“…8(a)], we find a large signal for the low-frequency FTs and the FRs with hardly any contribution from the stretch modes. The resulting IETS curve is a well-defined double-peak structure which is in line with previous experimental 13,14 and theoretical 14,34,35 studies for the same system. However, our FT contribution seems to be overestimated with respect to these works, where a similar height was found for the FT and FR modes.…”

“…14,44 These studies showed that the contribution of the inelastic processes tends to increase the total current in the former regime, which is in accordance with our LOIT approach where expression (43) has the same sign as its elastic counterpart (42). However, in those studies the AO basis employed was rather localized and therefore, as shown in Sec.…”

“…13,14,34,35,40 The CO is known to adsorb at a top site, 35 despite that DFT does not always identify this site as the most stable one, with the C attached to the metal atom. Here, we model the surface via a (4 × 4) supercell.…”

“…Within the context of the STM, this technique is denoted as inelastic electron tunneling spectroscopy (STM-IETS) and is extensively employed to characterize the molecule's vibrational spectra. [9][10][11][12][13][14][15] From the theoretical side, a wealth of formalisms dealing with the inelastic transport have been developed in parallel to these delicate experiments. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] The earliest models for IETS employed a multiple scattering formalism (multichannels) with simplified semiempirical Hamiltonians 16 or a Tersoff-Hamman-(TH-) type approach.…”

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.…”