Bond-Selected Photodissociation of Single Molecules Adsorbed on Metal Surfaces

Shaowei Li (李绍巍), Gregory Czap, Hui Wang, Likun Wang, Siyu Chen (陈思宇), Arthur Yu, Ruqian Wu, and W. Ho

Phys. Rev. Lett. 122, 077401 – Published 22 February 2019

Abstract

We report the photoassisted activation of selected $C ─ H$ bonds in individual molecules adsorbed on metal surfaces within the junction of a scanning tunneling microscope. Photons can couple to the $C ─ H$ bond activation of specific hydrocarbons through a resonant photoassisted tunneling process. The molecule to be activated can be selected by positioning the tip with subangstrom resolution. Furthermore, structural tomography of the molecule and its dissociation products are imaged at different heights by the inelastic tunneling probe. The demonstration of single bond dissociation induced by resonant photoassisted tunneling electrons implies the attainment of atomic scale spatial resolution for bond-selected photochemistry.

Received 13 May 2018
Revised 2 December 2018
Corrected 25 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.077401

Physics Subject Headings (PhySH)

Chemical bonding Light-matter interaction Photodissociation Photoinduced effect Reactions at surfaces & chemisorption

Scanning tunneling microscopy

Interdisciplinary PhysicsCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Corrections

25 February 2019

Correction: The Chinese characters for the name of the fifth author were misplaced and have now been relocated properly.

Authors & Affiliations

Shaowei Li (李绍巍)¹, Gregory Czap¹, Hui Wang¹, Likun Wang¹, Siyu Chen (陈思宇)¹, Arthur Yu¹, Ruqian Wu^1,*, and W. Ho^1,2,†

¹Department of Physics and Astronomy, University of California, Irvine, California 92697-4575, USA
²Department of Chemistry, University of California, Irvine, California 92697-2025, USA

^*Corresponding author. wur@uci.edu
^†Corresponding author. wilsonho@uci.edu

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Issue

Vol. 122, Iss. 7 — 22 February 2019

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Images

Figure 1
(a)–(e) A series of STM topographic images taken before and after dissociating several nearby azulene molecules with laser illumination. The arrows indicate the positions over which the tip was parked. The molecules can be turned into a “crescent” shape as shown in (b)–(d). A crescent species indicated by the arrow in (d) further dissociated into a “peanut” shape as shown in (e). (f) Topographic image showing an intact azulene molecule and two dissociation products. The round depressions are the coadsorbed CO molecules. (g)–(i) Enlarged STM topographic images of the intact azulene molecule (g), the crescent product (h), and the peanut product (i). Images in (a)–(e) were taken with 0.5 V and 1 nA set points in constant current mode. (j)–(l) The semiconstant height itProbe images of azulene and the two products corresponding to the molecules shown in (g)–(i). (m)–(o) Top views of the adsorption structures given by DFT calculations of the intact azulene (m), after one $C ─ H$ bond scission (n), and after scission of two $C ─ H$ bonds (o).
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Figure 2
(a)–(c) Constant height itProbe images of (a) an intact azulene, (b) a crescent product, and (c) a peanut product, taken with feedback off at 100 mV, 0.1 nA, and over the center of the molecule, followed by advancing the tip 0.59 Å towards the molecule. (d)–(i) The constant height itProbe images taken with the tip advanced 0.72 Å (d)–(f) and 0.85 Å (g)–(i) towards the molecule. (j)–(l) Side views of the adsorption structures given by DFT calculations of azulene and its dissociation products.
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Figure 3
(a) The azulene dissociation rate as a function of bias with (red) or without (black) pulse laser illumination. The red and black lines are the exponential fits to the data. (b) The semilog plot of the data in (a). (c)–(e) Schematic diagrams of three possible mechanisms of electron-photon coupling in the STM junction.
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Figure 4
(a) $d I / d V$ spectra taken over the center of an azulene molecule (red) and Ag(110) background (black). The inset shows the $d I / d V$ image taken at 0.9 V. (b) Calculated density of states of an azulene adsorbed on Ag (110). The HOMO is found at 1.6 eV below the Fermi level. The LUMO and $LUMO + 1$ are found at 0.5 and 1.3 eV above the Fermi level, respectively. The insets show the spatial distributions of the local density of states of HOMO, LUMO, and $LUMO + 1$ . Red indicates occupied states and blue indicates the empty states. (c) DFT calculations of the activation barriers of dissociating an azulene into a crescent product (black) and dissociating a crescent product into a peanut product (red). To determine the energies of the transition states, we stretch out the C to H distance in the DFT model and allow the molecule to relax to a locally optimized structure. The open symbols denote the C to H distance before relaxation and the solid symbols after relaxation. (d),(e) Top views (top) and side views (bottom) of the calculated intermediate structures of the molecule during the first (d) and second (e) $C ─ H$ bond dissociation. The corresponding energies and positions along the C to H reaction coordinate are labeled in (c).
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