Electronic Resonance and Symmetry in Single-Molecule Inelastic Electron Tunneling

Hahn, Jong Hoon; Lee, H. J.; Ho, W.

doi:10.1103/physrevlett.85.1914

Cited by 158 publications

(190 citation statements)

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“…There is quantitative agreement with experiments [4,19]. (c) Onresonance STM configuration with an O 2 molecule on Ag(110) surface displaying a decrease in the conductance upon phonon emission [26]. The STM tip is modeled by a single Ag atom on a Ag(110) lead laterally displaced by 1.6Å corresponding to the experimental situation [26], T = 13 K, V rms = 7 meV.…”

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confidence: 74%

Unified Description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions

et al. 2008

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We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmeties in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states -namely those belonging to the most transmitting eigenchannels -need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.Electronic transport through atomic-size junctions is of immense scientific and technological interest. The importance of inelastic effects in electronic currents have been revealed in several ground-breaking experiments leading to the detection and identification of single molecules [1], chemical reactions [2,3], the detection of vibrations in atomic wires [4], the detection of inelastic effects by fluorescence [5], the modification of electron transport in nanotubes [6], the molecular motion induced by electronic currents [7], and the hydrogen detection in atomic wires [8], just to cite a few examples. Of particular importance due to its spreading use is the case of vibrational spectroscopy where the conductance changes due to phonon emission is measured [9,10,11]. This is often referred to as point contact spectroscopy or inelastic electron tunneling spectroscopy (IETS) [1], However, experiments alone are not able to give direct insight into the fundamental question on how the detailed atomic structure correlate with the electrical transport properties. There is experimental evidence of approximate selection rules (propensity rules [12]) such that only a small number out of the many possible vibrational modes give an inelastic signal. These propensity rules yield clues to the geometric and electronic structure of the junctions. It is therefore of fundamental interest to compare the experimental results with first-principles calculations.Existing calculations of inelastic effects in electron transport have been developed either for particular cases [12,13,14] or for simplified (one-level) models [15,16]. First-principles methods capable of treating both weak and strong coupling to the electrodes has also been developed [17,18,19]. However, the results of such detailed calculations involve many electronic states and vibrational modes. An advanced analysis is therefore needed in order to provide insight into the propensity rules.In this paper we propose a method for analysis of the inelastic transport based on just a few selected electronic scattering states, namely those belonging to the most transmitting eigenchannels at the Fermi energy (ε f ) [20]. These scattering states typically have the largest amplitude inside the junction and thus account for the majority of the electron-phonon (e-ph) scattering. To illustrate our method of analysis and to develop an understanding of the propensity rules we consid...

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confidence: 74%

Unified Description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions

et al. 2008

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“…Density-functional calculations performed by Gravil et al 8 shed light on this controversy showing the existence of two distinct chemisorption states with essentially equal energies. This theoretical finding was subsequently corroborated by EELS and TDS studies, 13 though the final confirmation came with the STM-IETS investigations by Hahn et al [14][15][16] Compared with other spectroscopies, the great advantage of IETS operated with STM is that it provides simultaneous topographical and spectroscopical images with atomic resolution. This permits an almost direct identification of the molecular state.…”

Section: Introductionmentioning

confidence: 79%

“…Figure 8͑b͒ shows in full line the peaks after convolution 47 using a modulation voltage of 7 mV rms as in the experiment. 14 …”

Section: Bias Dependence Of Iets: a Self-consistent Born Approximamentioning

confidence: 99%

“…The recorded inelastic signal is a decrease in conductance for one of the modes ͑the O-O stretch͒ and it can be a decrease or an increase in conductance depending on the position of the STM tip over the molecule for the other detected mode ͑the antisymmetric O 2 -Ag stretch͒. 14 The reasons behind this rich and complex IETS structure have been a matter of controversy that have recently been solved. 17 In IETS, the changes in conductance are recorded as a function of the tip-substrate voltage.…”

Section: Introductionmentioning

confidence: 99%

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Role of molecular electronic structure in inelastic electron tunneling spectroscopy:O2on Ag(110)

2010

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Density-functional theory ͑DFT͒ simulations corrected by the intramolecular Coulomb repulsion U are performed to evaluate the vibrational inelastic electron tunneling spectroscopy ͑IETS͒ of O 2 on Ag͑110͒. In contrast to DFT calculations that predict a spinless adsorbed molecule, the inclusion of the U correction leads to the polarization of the molecule by shifting a spin-polarized molecular orbital toward the Fermi level. Hence, DFT+ U characterizes O 2 on Ag͑110͒ as a mixed-valent system. This has an important implication in IETS because a molecular resonance at the Fermi level can imply a decrease in conductance while in the off-resonance case, an increase in conductance is the expected IETS signal. We use the lowest-order expansion on the electron-vibration coupling in order to evaluate the magnitude and spatial distribution of the inelastic signal. The final IET spectra are evaluated with the help of the self-consistent Born approximation and the effect of temperature and modulation-voltage broadening are explored. Our simulations reproduce the experimental data of O 2 on Ag͑110͒ ͓J. R. Hahn, H. J. Lee, and W. Ho, Phys. Rev. Lett. 85, 1914 ͑2000͔͒ and give extra insight of the electronic and vibrational symmetries at play. This ensemble of results reveals that the IETS of O 2 is more complicated that a simple decrease in conductance and cannot be ascribed to the effect of a single molecular-orbital resonance.

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“…24 This effect has been measured recently as dips in the dI 2 /dV 2 curve of O 2 on a silver surface. 26 We believe that the dip at 111 meV is due to a similar elastic contribution at the onset of vDOS singularity corresponding to the ZO(⌫) phonon branch. In the light of the above discussion of phonon-assisted tunneling on graphite, it is also not surprising that this effect is observed for phonons at ⌫.…”

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confidence: 81%

Phonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite

et al. 2004

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The inelastic electron tunneling spectrum ͑IETS͒ of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy ͑STS͒ at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles. We discuss the enhancement of certain phonon modes by phonon-assisted tunneling in STS based on the restrictions imposed by the electronic structure of graphite. We also demonstrate the local excitation of surface plasmons in IETS, which are detected at an energy of 40 meV. DOI: 10.1103/PhysRevB.69.121414 PACS number͑s͒: 68.35.Ja, 63.20.Ϫe, 68.37.Ef Initially applied to measure inelastic tunneling currents across metal-insulator-adsorbate-metal junctions, 1 inelastic electron tunneling spectroscopy ͑IETS͒ has in the recent past been revived and its concept extended to scanning tunneling microscopy ͑STM-IETS͒. 2 Superimposed on the elastic tunneling current, the inelastic tunneling process opens up alternative tunneling channels increasing the total conductance at the onset of its excitation. Very recently, elegant experiments ͑e.g., Refs. 2-5͒ have been performed in detecting local inelastic electron energy-loss spectra of individual molecule adsorbed on metallic substrates. The observed features in the tunneling spectra are found to correspond to the vibrational modes of the adsorbed species. Vibrations of single molecules as large as C 60 have have been detected. 6 Despite the increasing number of publications of STM-IETS experiments on single molecules adsorbed on metals, to our knowledge, only two studies were reported on collective vibrational excitations of surfaces. 7,8 In both these studies graphite was used as a substrate and several spectral features remained unexplained or were attributed to vibrational excitation in the tungsten tip. However, these tip effects have not been detected in many other STM-IETS experiments. In order to resolve the pure inelastic response of a graphite surface, we have performed STM-IETS experiments on a highly oriented pyrolitic graphite crystal ͑HOPG͒ at cryogenic temperature. In our experiments the whole vibrational spectrum of HOPG from the rigid layer shear mode at Ϸ5 meV to the optical modes at Ϸ200 meV is clearly observed without being perturbed by inelastic effects from the tip. The phonon modes of graphite can be clearly identified by the direct comparison of experimental data with first-principles density-function theory ͑DFT͒ calculations. We report also the observation of surface-plasmon losses with STM-IETS.Freshly cleaved HOPG has been introduced into the vacuum system and shortly annealed at 500 K. The experiments were performed at 6 K with a home built UHV-STM using a tungsten tip. The inelastic tunneling spectrum of HOPG was measured over a topographically clean and flat terrace with a lock-in technique. In this technique a sinusoidal signal with amplitude ⌬V is superimposed to the applied sample bias. In IETS the second-harmonic component of the modulated sig...

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Electronic Resonance and Symmetry in Single-Molecule Inelastic Electron Tunneling

Cited by 158 publications

References 17 publications

Unified Description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions

Unified Description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions

Role of molecular electronic structure in inelastic electron tunneling spectroscopy:O2on Ag(110)

Phonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite

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