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

Abstract

Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary by up to a factor of 3 for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front-atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single-atom tips, while the intensity drops sharply for multiatom tips. Adsorption of the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that the molecule is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multiatom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111). DOI: 10.1103/PhysRevB.93.165415 Inelastic electron tunneling spectroscopy (IETS) with scanning tunneling microscopy (STM) is an effective method to analyze the vibrational modes of a single adsorbed molecule with subnanometer lateral resolution [1,2]. The vibrational energy of a molecule on a substrate strongly depends on the surrounding environment, such as the substrate structure and composition [3]. 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 discuss γ inel on the basis of the intensity of IETS, we have to consider that IETS intensity is described by the multiplication of two factors: (1) the ratio of the tunneling current passing through a molecule to the total tunneling current (I molecule /I total ) and (2) the efficiency of the inelastic process (γ inel ). These factors should in principle be affected by the geometrical structure of the substrate and of the tip.The geometrical structure of a metal tip apex can be determined by using carbon monoxide (CO) front-atom identification (COFI) provided by AFM [23,24], where the tip apex of a force sensor is probed by a CO molecule that stands upright on a metal surface [inset of Fig. 1(e)]. The metallic tip apex atom has a dipole moment induced by the Smoluchowski effect [25], whose direction is the same as that of the CO molecule adsorbed on the surface [26]. Thus in the distance regime where the electrostatic int...

Vibrations of a molecule in an external force field

“…To measure the conductance (dI/dV) and IET signal (d 2 I/dV 2 ), a modulation voltage (2,338.7 Hz and 1-mV rms amplitude) 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). In the text, the IETS signal is normalized to the conductance (6,21), i.e., (d 2 I/dV 2 )/(dI/dV) has been plotted. The complete set of the IETS are shown in SI Appendix, Figs.…”

“…(L) Tip-height-dependent vibrational energy of the FT and FR modes at positive bias (solid lines), negative bias (dotted lines), and average values (squares).semiclassical approach similar to the calculations of phonons in crystals(22) with the Lagrange formalism (23), we solve the equation of motion for small oscillations of the double pendulum. From the vibrational energies E FT = 4.2 meV and E FR = 35.4 meV(8,9,21,24) and the bonding lengths l 1 = 187 pm and l 2 = 115 pm(25), the two angular force constants are calculated to be D 1 = 135 zNm and D 2 = 216 zNm. Quantization must be taken into account to estimate the zero-point amplitude of the vibrations: The FT mode has an energy spectrum given by e(n FT ) = (1/2 + n FT ) × Zω FT , where Z is Planck's constant and ω is the angular frequency.…”

Role of valence states of adsorbates in inelastic electron tunneling spectroscopy: A study of nitric oxide on Cu(110) and Cu(001)

“…9(d)] shows an almost round distribution. We assume that the experimental distribution is broadened by the measurement with a relatively blunt tip apex [29], and that a burred intensity distribution around the dimer in the simulated maps is also reflected by the experimental map. For (NO) 2 /Cu(001), the 41 meV signal is assigned to FR mode is active.…”

“…Nonetheless, recent theoretical studies have demonstrated valid models for elastic and inelastic tunneling processes, and have successfully reproduced experimental STM-IET spectra [11][12][13][14][15][16][17][18][19][20]. IETS for CO on metal surfaces have been thoroughly investigated as a typical model of a simple * shiotari@k.u-tokyo.ac.jp molecule-metal substrate system [12,15,[21][22][23][24][25][26][27][28][29][30]. O 2 /Ag(110) is also an interesting subject of study [15][16][17] because on-resonant IETS signals, i.e., dips (peaks) in d 2 I /dV 2 at positive (negative) sample biases, are detected [31].…”

STM contrast of a CO dimer on a Cu(1 1 1) surface: a wave-function analysis

“…With atomic force microscoscopy (AFM), the interaction between tip and sample is characterized, which allows for examination of the tip-apex structure [5][6][7][8]. The tip-apex structure may strongly influence the tunneling process, both when making a quantitative comparison between experimental and theoretical STM images [9], and when investigating the inelastic tunneling signal from an adsorbate species [10].…”