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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 � � 143 �, 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 � � 286 �, 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 � � 38 �, 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 � � 14 �, 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 � � 38 �, 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 � � 143 �, 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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