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47099-AC5
Hydrogenation Reactions on Palladium Involving Subsurface Hydride

Paul S. Weiss, Pennsylvania State University

            We have been utilizing low-temperature, ultrahigh vacuum scanning tunneling microscopy (STM) and spectroscopy (STS) to examine the interactions of sulfur-containing heterocycles with a single crystal palladium (Pd) surface.  We have observed thiophene at low to moderate coverages on Pd{111} to ascertain its adsorption orientation, low-temperature mobility, and electronic structure .  We are developing the methods necessary to induce the reactions of individual molecules with pre-defined subsurface hydride features.  These experiments will help us elucidate the role of subsurface hydride in critical hydrogenation processes reactions.

            We have observed that both the surface preparation and the temperature at which thiophene is deposited affects adsorption.  Figure 1 shows STM images representative of two separate dosing conditions, 4 K and ca. 20–40 K.  Dosing directly onto the surface at 4 K (Figure 1A) results in adsorption with random placement of molecules on the surface.  However, depositing immediately after the crystal is transferred to the low-temperature chamber, we find that thiophene adsorbs preferentially along the tops and bottoms of step edges, as seen in Figure 1B.  In this case, thiophene remains mobile while the surface cools, finding favorable binding sites along step edges.

            We acquired action spectroscopy over individual thiophene molecules to address their mobility at 4 K.  Figure 2 shows A) a representative action spectrum along with a sequence of STM images acquired B) before, C) during, and D) after acquisition.  The molecule over which we chose to perform action spectroscopy is denoted by the arrow in Figure 2A.  Because we acquire topography before and after the spectroscopic point, we see behavior typical of a molecule that becomes mobile beneath the probe (Figure 2B).  That is, since the thiophene moves out of the tunneling junction during spectroscopy and before topographic imaging is complete, we image only “half” of the molecule.  Subsequent topographic imaging shows that the thiophene indeed moved as a result of the tip perturbation (Figure 2C, arrow).  From these experiments, we are developing a technique that will allow us to manipulate single molecules into proximity of subsurface hydride features patterned in the substrate by STM manipulation.

            We have recorded initial differential conductance spectroscopy (dI/dV) over both the Pd{111} surface (Figure 3A) and single thiophene molecules (Figure 3B) in order to characterize the electronic structure of the hydrogenation system prior to reaction.  Although we observe effective quenching of the Pd surface state by thiophene adsorption, we will continue optimizing spectroscopic parameters in order to obtain reproducible and interpretable single-molecule vibrational spectroscopy results.

 FIGURES Figures

Figure 1. A) STM image (143 Å × 143 Å, V_s = 0.1 V, I_t = 150 pA) of molecular thiophene adsorbed on a Pd{111} surface at 4 K.  Dosed after the Pd surface was allowed to reach 4 K, thiophene appears randomly distributed on the surface.  The two regions seen in the image are atomic terraces separated by a single Pd atomic step edge.  B) STM image (286 Å × 286 Å, V_s = -0.3 V, I_t = 100 pA) of molecular thiophene aligned preferentially along the tops and bottoms of step edges on a Pd{111} surface at 4 K.  Thiophene was dosed at ca. 20 ‑ 40 K and thus had some surface mobility on adsorption.

Figure 2. A) STM image (57 Å × 38 Å, V_s = -0.2 V, I_t = 100 pA) of molecular thiophene adsorbed near a Pd{111} step edge at 4 K.  The arrow denotes a single molecule over which action spectroscopy is performed.  B) STM image (14 Å × 14 Å, V_s = -0.2 V, I_t = 100 pA) acquired before and after action spectroscopy.  The thiophene moved out of the frame of imaging during spectroscopy, resulting in what appears to be a partially imaged molecule.  C) STM image (57 Å × 38 Å, Vs =-0.2 V, I_t = 100 pA) after action spectroscopy.  This STM image shows the new location of the molecule from A. 
D) I vs. t spectrum (V_gap = -0.35 V, I_gap = 100 pA) acquired over 60 seconds exhibits motion of the molecule under the STM tip before finally moving out of the frame in B.

Figure 3. STM image (143 Å × 143 Å, V_s = -0.45 V, I_t = 40 pA) of molecular thiophene adsorbed on Pd{111} at 4 K and accompanying spectra acquired over A) Pd and B) thiophene (V_gap = -0.01 V, I_gap = 40 pA).  The broken red traces are the result of mathematically differentiating the dI/dV spectra (blue).  The I/V response and subsequently the dI/dV response of atomic Pd and molecular thiophene are markedly different.  A notable feature is the apparent quenching of the surface state about the Fermi energy by the adsorbate.  Once parameters are optimized for IETS, single-molecule vibrational spectra will be acquired simultaneously, enabling the assignment of vibrational modes.

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