Molecular Oxygen Network as a Template for Adsorption of Ammonia

Aug 11, 2013 - At adsorption temperatures below 50 K, the chemisorbed O2 molecules form an ordered network at high coverages. The STM images reveal th...
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Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111) Zhu Liang,† Hyowon Kim,‡ Yousoo Kim,*,‡ and Michael Trenary*,† †

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan

‡

S Supporting Information *

ABSTRACT: Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH3−O2 overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O2 molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to ontop sites of the Pt lattice that are surrounded by two or three O2 molecules. Different hole−hole distances are observed with 0.73 nm most common. These holes in the O2 network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH3−O2 complex that serves as a precursor to ammonia oxydehydrogenation. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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examples of one adsorbed molecule influencing the reactivity of another. For example, Sasaki et al. found an enhanced dissociation of ammonia coadsorbed with CO on Ru(0001), which was attributed to an indirect substrate-mediated COammonia interaction.14,15 Because ammonia oxidation by Pt catalysts is an important industrial process, the reaction has been the subject of ultrahigh-vacuum (UHV) studies on stepped Pt(S)-12(111) × (111)16 and on Pt(111).17,18 A previous reflection absorption infrared spectroscopy (RAIRS) study of ammonia adsorbed on an O2 overlayer on Pt(111) indicated the possibility of forming an NH3−O2 complex,18 which had also been suggested to form on Ag(111),19 Cu(111),20 and Zn(0001).21 The complex is characterized by a strongly blue-shifted O−O stretch vibration indicating significant perturbation of the O−O bond. However, determination of the local geometry of such an NH3−O2 complex is not achievable with techniques such as RAIRS, temperature programmed desorption, X-ray photoelectron spectroscopy, or low-energy electron diffraction (LEED). In this study we present results of the surface morphology and local configuration of a molecular O2 network and a mixed NH3−O2 layer, using a low-temperature scanning tunneling microscope (LT-STM). Previous studies have revealed several forms of O2 on the Pt(111) surface, depending on the adsorption temperature. At temperatures below 40 K, only physisorbed O2 was observed.22 Chemisorbed molecular O2 on Pt(111) exists in the temperature range from 40 to 150 K. Two chemisorbed O2 species were identified in studies by electron energy loss spectroscopy

olecules of various sizes often form ordered twodimensional structures when adsorbed onto crystalline metal surfaces. In the case of larger molecules, two-dimensional (2D) molecular porous networks (MPNs) have been exploited extensively because of their applications in the development of molecule-based devices, such as molecular electronics,1 and gas sensors.2 Both single-3,4 and two-component5,6 molecular selfassemblies have been explored in the construction of MPNs. The size and shape of the nanopore can be controlled by tuning the building blocks and the molecule−molecule interactions.7−10 These 2D porous networks can be employed as host nanotemplates that offer patterned binding sites for guest molecules. For example, different types of ordered arrays of C60 molecules have been observed on MPNs by Wei et al.11 and by Stepanow et al.12 Madueno et al.13 created a bimolecular network on Au(111) with a tunable pore size that could accommodate other species including thiols that would then form regular arrays of patches of self-assembled monolayers. Organic molecules, especially aromatic hydrocarbons and derivatives, are commonly used as the building blocks for molecular networks. In contrast to these studies, the interaction of smaller molecules with transition metal surfaces are generally motivated by their reactive chemistry in relation to heterogeneous catalysis in which case the interaction of a guest molecule with an ordered template of host molecules is seldom studied. Here we show that a molecule as small as O2 can form a molecular network on a metal substrate that can serve as a template for the adsorption of a second small molecule, NH3. The adsorption of ammonia molecules in O2-network sites is associated with the formation of an NH3−O2 complex, which precedes the oxydehydrogenation reaction on Pt(111). Although the structure observed here is unique and is presumed to serve as a reaction precursor, there are many © 2013 American Chemical Society

Received: July 18, 2013 Accepted: August 10, 2013 Published: August 11, 2013 2900

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Figure 1. STM images (a−d, h) and models (e−g) of chemisorbed molecular O2 on Pt(111) at 50 K. (a) Low coverage (area = 15 nm × 15 nm, Vs = 80 mV, It = 2 nA). (b) High coverage (area = 20 nm × 20 nm, Vs = 50 mV, It = 1.0 nA). (c) Superoxo islands with individual O2 molecule resolved (area = 8 nm × 8 nm, Vs = 200 mV, It = 0.7 nA). (d) Four-leaf clover shape of superoxo (area = 5.0 nm × 5.0 nm, Vs = 2 mV, It = 4 nA). (e,f) (√7 × √7)R19.1°-3O2 structure. (g) Arrangement of superoxo in the square area of (c). (h) Zoomed-in image of the black square in (d), with superimposed drawing of three overlapping four-leaf clovers.

(EELS),23 in which O−O stretch vibrations were observed at 700 and 875 cm−1, and were assigned to peroxo (O22‑) and superoxo (O2−), respectively, with the latter species also observed with RAIRS.24 A scanning tunneling microscope (STM) study assigned the adsorption sites of peroxo and superoxo to top-bridge and bridge sites, respectively.25 Topbridge means one oxygen atom sits at an atop site, while the other one sits at a bridge site. The same study also found compact islands formed exclusively by bridge site molecules, which was attributed to the long lifetime of the precursor state.26 A later RAIRS study indicated that peroxo dominates at low coverage and converts to superoxo as coverage increases.27 Although an explanation was not offered, it may be that repulsion between the doubly charged peroxo species makes it less stable at higher coverages than the singly charged superoxo species. At temperatures above 150 K, chemisorbed molecular O2 dissociates to O atoms, which adsorb at face-centered cubic (fcc) sites.25 After low O2(g) exposures at 50 K, we observe both peroxo and superoxo in the image of a single Pt(111) terrace shown in Figure 1a, which is consistent with the STM observations by Stipe et al.26 and calculations by Eichler and Hafner28 and Bocquet et al.29 Based on the previous studies,23,25−29 we assign isolated molecules, which are often seen as pairs, and chains to peroxo, and compact islands to superoxo, indicated by solid and dashed arrows in Figure 1a, respectively. The peroxo molecules in chains and pairs are separated by two Pt lattice constants. When superoxo islands start to grow on the surface, peroxo molecules are also found to sit along the island perimeters (see Figure S1). As we gradually increase the coverage of molecular O2, most of the surface was covered with superoxo islands (Figure 1b). Moreover, with a specific tip state, the detailed structure within the superoxo islands (Figure 1c) can be resolved, which has not been reported previously. The change in tip state occurred randomly, but frequently, and more detailed structure, such as shown in Figure 1c, was observed reproducibly. Also under certain tip conditions, superoxo molecules appear in the shape of a four-leaf clover (Figure 1d). This shape was also observed by Stipe et al.,25 who suggested that its similarity to the shape of the π* orbital

implies that this orbital makes the dominant contribution to the local density of states at the Fermi level. A drawing of overlapping four-leaf clovers centered on three O2 molecules is shown in red in Figure 1h and is superimposed on the image. The fact that the peroxo and superoxo molecules appear so differently in the STM images is a clear manifestation of their different electronic structures, which is also reflected in their different adsorption sites (bridge for superoxo, top-bridge sites for peroxo), different formal charges (−2 for peroxo, −1 for superoxo), and their distinctly different O−O stretch frequencies. These differences are also readily apparent in simulated STM images based on electronic structure calculations.29 Figure 1c shows that the superoxo molecules within islands form an ordered network. The “holes” that appear in the O2 network are located at platinum atoms that are surrounded by, but not occupied by, O2 molecules. Another characteristic feature in the O2 network is the triple-hole structure, highlighted by the red circle in Figure 1c. A model of the arrangement of the O2 molecules inside the black square in Figure 1c, including both single- and triple-holes, is shown in Figure 1g. The single-hole seen in the STM image is surrounded by three bridge O2 molecules (red) with the molecular axis aligned with the Pt lattice, while the triple-hole is surrounded by five O2 molecules with four of them sitting at bridge sites (superoxo) and one molecule possibly sitting at a top-bridge site (peroxo). An image showing a close-up view of a triple-hole that supports the arrangement shown in Figure 1g is provided in the Supporting Information in Figure S2. The brighter appearance at the edge of the triple-hole in the STM image could be the effect of a conjugate π bond among the three O2 molecules (two superoxo and one peroxo). As superoxo islands grow in size, peroxo molecules are incorporated into their perimeters but not converted to superoxo. If we assume the surface was covered by a perfect network of superoxo islands, possible arrangements would be those shown in Figure 1e,f. There would be three equivalent directions for the molecular axes of O2 occupying bridge sites, which are the [1̅01], [01̅1], and [11̅0] directions of the Pt(111) surface. When three O2 molecules along these three directions 2901

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Figure 2. Schemes showing the possible hole−hole distances from 0.48 nm up to 1.54 nm on Pt(111). (a) Arrows corresponding to each possible value of the distance, from shortest to longest, are colored from blue to red. (b−k) Models of each hole−hole distance.

Figure 3. (a) STM image of the O2 network (area = 3.5 nm × 3.5 nm, Vs = 200 mV, It = 0.7 nA). Hole−hole distances of 0.73 nm (green arrows) and 0.48 nm (blue arrows) are shown. (b) Model showing the O2 molecular arrangements corresponding to the hole−hole distance of 0.73 and 0.48 nm. (c) Histogram of hole−hole distances of superoxo network. (d) STM image of mixed NH3−O2 overlayer on Pt(111) (area = 3.5 nm × 3.5 nm, Vs = 20 mV, It = 1 nA). (e) Model showing NH3 molecules sitting inside the holes (also on top of Pt atoms). Oxygen molecules are colored red, nitrogen atoms blue, and hydrogen atoms white. (f) Histogram of NH3−NH3 molecular distances.

LEED after the surface was exposed to ≥6 × 1015 molecules/ cm2 of oxygen at 100 K, while such an ordered overlayer could not be formed at a lower exposure (2.4 × 1014 molecules/cm2). The matrix notation for the (3/2 × 3/2)R15° superlattice, 1.67 0.45 −0.45 1.22 , has irrational values, which indicates that it is incommensurate with respect to the Pt lattice.30 Consequently, in contrast to the (√7 × √7)R19.1°-3O2 structure in Figure 1, a structure with (3/2 × 3/2)R15° periodicity in which all O2 molecules occupy identical sites of the Pt lattice cannot be drawn. The fact that this latter superlattice was not observed here is presumably because the coverage was not quite high

come close to each other, the smallest unit cell they could form are shown by the dashed rhombuses in Figure 1e,f. This is a (√7 × √7)R19.1°-3O2 structure, which has a maximum O2 coverage of 0.43 ML, i.e., three times 1/7 (the coverage of the (√7 × √7)R19.1° structure itself). Although the same unit cell describes the structures corresponding to the single-hole in Figures 1e,f, the STM images clearly favor the structure in panel e in which three superoxo O2 molecules located at bridge sites surround each hole in a symmetric way. The saturation coverage of O2 on Pt(111) was reported to be 0.44 ML by Steininger et al. in their study of O2 adsorption on Pt(111).23 They observed an ordered (3/2 × 3/2)R15° overlayer with

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(error range ±0.02 nm). The dominant NH3−NH3 distance is 0.74 nm, which matches the value of 0.73 nm of the hole−hole distance in the O2 network. Comparing the two histograms in Figure 3c,f, we conclude that the distance among the adjacent NH3 neighbors matches the hole−hole distance of the O2 overlayer. This suggests that the preferred adsorption sites for NH3 are the holes in the O2 overlayer, which is also the atop site of the Pt substrate. Based on the above results, we conclude that in the mixed NH3−O2 overlayer, the NH3 molecules are adsorbed on top of Pt atoms, each of which is surrounded by two or three O2 molecules that adsorb at bridge sites, as shown in Figure 3e, where NH3 molecules are presumed to sit inside the hole, and are oriented such that their three H atoms point toward the O2 molecules. The distance between the O2 and NH3 molecules in this proposed structure is 0.240 nm, which is less than the sum of the van der Waals radii of 0.305 nm,32 suggesting that the molecules are in contact, and that a favorable orientation, such as the one shown, must be adopted. The interaction between the NH3 and O2 molecules associated with this bonding arrangement presumably corresponds to the NH3−O2 complex identified previously with RAIRS.18 The complex is characterized by an O−O stretch of 1451 cm−1, compared to 1556 cm−1 for gas phase33 O2 and 878 cm−1 for the superoxo O2 on Pt(111).24,27 As the low O−O stretch frequency of the superoxo species is due to electron transfer into the π* orbital, which is antibonding with respect to the O−O bond, there is evidently little excess negative charge on the O2 molecules that are part of the complex. Whether this occurs due to direct transfer to the H atoms of the NH3 molecules or a reduction in charge transferred from the metal is unclear. As this complex precedes the dehydrogenation reaction, the results here show that the O2 molecular network provides a template for adsorption of ammonia molecules in a configuration that facilitates subsequent reactivity. Such prereaction structures may exist transiently under the actual reaction conditions employed in catalysis, conditions that would preclude direct imaging by the methods employed here.

enough. Based on a Pt−Pt distance of 0.277 nm, the calculated hole−hole distance in the O2 network represented by Figures 1e and 1f is 0.73 nm, which exactly matches the values measured from the line profile of the STM images. However, from Figures 1b and 1c, it is easily seen that there are many defects in the network with various arrangements of O2 molecules. The hole−hole distance varies corresponding to different arrangements of O2 molecules. For example, hole− hole distances of 0.48 nm (blue arrow) and 0.73 nm (green arrow) and the corresponding models are shown in Figures 2b and 2h. The hole−hole distance of 0.73 nm is expected to be most common as it achieves the highest density of holes that are surrounded by three equivalent O2 molecules. In order to understand the various structures and defects associated with the experimentally observed O2 network, we tabulated all hole−hole distances in the network from the smallest value of 0.48 nm up to 1.54 nm. Larger distances are also possible but to simplify our analysis we consider only the holes adjacent to a given hole. As shown in Figure 2a, colored arrows represent ten discrete values from 0.48 to 1.54 nm. The color for each value is chosen to go from blue, for the smallest distance, to red, for the largest. Figure 2b−k are the models corresponding to the 10 hole−hole distances. One possible arrangement of O2 molecules corresponding to each value is shown as an example. In Figure 2h−k, there are two possible sets of hole−hole distances at each value, labeled as dark and light brown. They are rotational alternatives for the same distance on a hexagonal surface. Figure 3c is a histogram of the number of occurrences of each of the 10 hole−hole distances corresponding to the 10 structures in Figure 2. The color of each column in the histogram is the same as that of the models in Figure 2 for each value of the hole−hole distance. As anticipated, the distance of 0.73 nm predominates. The 10 distances given in the histograms are the averages of the measured values clustered about the ideal values. A table with the average measured values and their standard deviations for both the hole−hole and the NH3−NH3 distances compared to their ideal values based on the structures of Figure 2 is given in the Supporting Information. Images obtained following ammonia adsorption on the saturated O2 overlayer on Pt(111) are shown in Figure 3d. Although the ammonia molecules are easily seen as bright protrusions, the underlying O2 network is not observable in this image, although it can be observed at slightly lower ammonia coverages as shown in Figure S2 of the Supporting Information. Nevertheless, analysis of the images indicates that the NH3 molecules are adsorbed on top of the Pt atoms in the holes in the O2 network structure. It is reasonable since the most stable adsorption site for ammonia on Pt(111) is the atop site.31 There is some variability in the apparent height of the ammonia molecules, which presumably reflects the number and configuration of surrounding O2 molecules. Although there is no long-range ordered structure of NH3 molecules, only certain NH3−NH3 distances are observed. For example, an NH3−NH3 distance of 0.74 nm (green arrow) and 0.47 nm (blue arrow) are shown in Figure 3d. Models of the corresponding arrangements of O2 and NH3 are shown in Figures 3b and 3e. A histogram of the measured NH3−NH3 distances is shown in Figure 3f. The number of these distances measured was 219, whereas 256 measured hole−hole distances were used for the histogram of Figure 3c. For ease of comparison, we use the same color for the column if the value of the molecule− molecule distance is the same as that of the hole−hole distance

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EXPERIMENTAL SECTION The experiments were carried out in linked ultrahigh-vacuum (UHV) STM and preparation chambers with base pressures of 1 × 10−11 Torr and 1 × 10−9 Torr, respectively. STM images were obtained at 4.7 K with an Omicron Low-Temperature Scanning Probe Microscope (LT-SPM) system. Images were acquired with Omicron SCALA PRO 4.0 software and data processing was carried out with the WSxM program provided by Nanotec.34 The Pt(111) crystal was cleaned in the preparation chamber. The sample was heated by electron bombardment to 1220 K and held at that temperature for 10 min, followed by argon ion sputtering at room temperature for 10 min (Ar pressure, 3 × 10−5 Torr; energy, 1000 eV). After 5 to 8 cycles of annealing and sputtering, the sample was treated by O2 (1 × 10−7 Torr) at 750 K for 10 min, followed by flash annealing to 1220 K and then was transferred to the STM chamber where the surface stayed clean for several days. Surface cleanness was verified by STM and occasionally by LEED. A chemisorbed molecular O2 overlayer on Pt(111) was prepared by repeated exposure to O2(g) at 50 K via a nozzle in the STM chamber. The pressure measured with an ion gauge was 2.5 × 10−10 Torr and the time was 20 s for each exposure. However, the actual partial pressure at the nozzle exit should be considerably higher than that measured at the ion gauge. 2903

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After each O2 exposure (2.5 × 10−10 Torr, 20 s) at 50 K, the crystal was cooled to 4.7 K for STM measurement in order to verify the coverage. After saturating the surface with molecular O2 (2.5 × 10−10 Torr for 20 s, repeated 3 times), NH3 gas was then exposed to the surface at 50 K via the nozzle in the STM chamber (6.0 × 10−11 Torr NH3 for 20 s, repeated 2 times). The NH3−O2 covered sample was then cooled down to 4.7 K for STM measurements. The temperature of 50 K for the O2 and NH3 exposures was based on the time it took to remove the sample from the STM sample holder, move it to the dosing position, and to complete the gas exposures. The relationship between time and temperature was established through a calibration procedure in which a thermocouple was directly attached to the sample. The accuracy of the dosing temperature is estimated to be ±5 K.

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(8) Xiao, W.; Feng, X.; Ruffieux, P.; Gröning, O.; Müllen, K.; Fasel, R. Self-Assembly of Chiral Molecular Honeycomb Networks on Au(111). J. Am. Chem. Soc. 2008, 130, 8910−8912. (9) Liang, H.; Sun, W.; Jin, X.; Li, H.; Li, J.; Hu, X.; Teo, B. K.; Wu, K. Two-Dimensional Molecular Porous Networks Formed by Trimesic Acid and 4,4′-Bis(4-pyridyl)biphenyl on Au(111) through Hierarchical Hydrogen Bonds: Structural Systematics and Control of Nanopore Size and Shape. Angew. Chem., Int. Ed. 2011, 50, 7562− 7566. (10) Bartels, L. Tailoring Molecular Layers at Metal Surfaces. Nat. Chem. 2010, 2, 87−95. (11) Wei, Y.; Robey, S. W.; Reutt-Robey, J. E. TiOPc Molecular Dislocation Networks as Nanotemplates for C60 Cluster Arrays. J. Am. Chem. Soc. 2009, 131, 12026−12027. (12) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Steering Molecular Organization and Host−Guest Interactions Using TwoDimensional Nanoporous Coordination Systems. Nat. Mater. 2004, 3, 229−233. (13) Madueno, R.; Raisanen, M. T.; Silien, C.; Buck, M. Functionalizing Hydrogen-Bonded Surface Networks with SelfAssembled Monolayers. Nature 2008, 454, 618−621. (14) Sasaki, T.; Aruga, T.; Kuroda, H.; Iwasawa, Y. Interaction between CO and NH3 Coadsorbed on Ru(001): Its Effects on the Ordering in Mixed Adlayers and the Ammonia Dissociation. Surf. Sci. 1990, 240, 223−244. (15) Sasaki, T.; Aruga, T.; Kuroda, H.; Iwasawa, Y. Coadsorption of NH3 and CO on Ru(001): The Ordering in Mixed Layers and the Effect of Intermolecular Interactions on NH3 Dissociation. Surf. Sci. 1989, 224, L969−L978. (16) Gland, J. L.; Korchak, V. N. Ammonia Oxidation on a Stepped Platinum Single-Crystal Surface. J. Catal. 1978, 53, 9−23. (17) Mieher, W. D.; Ho, W. Thermally Activated Oxidation of NH3 on Pt(111): Intermediate Species and Reaction Mechanisms. Surf. Sci. 1995, 322, 151−167. (18) Herceg, E.; Jones, J.; Mudiyanselage, K.; Trenary, M. Formation and Hydrogenation of p(2 × 2)-N on Pt(111). Surf. Sci. 2006, 600, 4563−4571. (19) Carley, A. F.; Davies, P. R.; Roberts, M. W.; Yan, S. Oxygen States Present at a Ag(111) Surface in the Presence of Ammonia: Evidence for a NH3−O2δ− Complex. Chem. Commun. 1998, 0, 35−36. (20) Davies, P. R.; Roberts, M. W.; Shukla, N.; Vincent, D. J. Oxygen States at a Cu(111) Surface: The Influence of Coadsorbed Ammonia. Surf. Sci. 1995, 325, 50−56. (21) Carley, A. F.; Roberts, M. W.; Yan, S. Evidence from Coadsorption Studies for a Molecular Precursor State in the Oxidation of Zn(0001). J. Chem. Soc., Chem. Commun. 1988, 0, 267−268. (22) Luntz, A. C.; Grimblot, J.; Fowler, D. E. Sequential Precursors in Dissociative Chemisorption: O2 on Pt(111). Phys. Rev. B 1989, 39, 12903−12906. (23) Steininger, H.; Lehwald, S.; Ibach, H. Adsorption of Oxygen on Pt(111). Surf. Sci. 1982, 123, 1−17. (24) Fan, J.; Trenary, M. Symmetry and the Surface Infrared Selection Rule for the Determination of the Structure of Molecules on Metal Surfaces. Langmuir 1994, 10, 3649−3657. (25) Stipe, B. C.; Rezaei, M. A.; Ho, W.; Gao, S.; Persson, M.; Lundqvist, B. I. Single-Molecule Dissociation by Tunneling Electrons. Phys. Rev. Lett. 1997, 78, 4410−4413. (26) Stipe, B. C.; Rezaei, M. A.; Ho, W. Atomistic Studies of O2 Dissociation on Pt(111) Induced by Photons, Electrons, and by Heating. J. Chem. Phys. 1997, 107, 6443−6447. (27) Gustafsson, K.; Andersson, S. Infrared Spectroscopy of Physisorbed and Chemisorbed O2 on Pt(111). J. Chem. Phys. 2004, 120, 7750−7754. (28) Eichler, A.; Hafner, J. Molecular Precursors in the Dissociative Adsorption of O2 on Pt(111). Phys. Rev. Lett. 1997, 79, 4481−4484. (29) Bocquet, M. L.; Cerdà, J.; Sautet, P. Transformation of Molecular Oxygen on a Platinum Surface: A Theoretical Calculation of STM Images. Phys. Rev. B 1999, 59, 15437−15445.

ASSOCIATED CONTENT

* Supporting Information S

Additional STM images showing peroxo O2 near islands of superoxo O2; close up views of the triple-hole structure in the O2 network; a mixed NH3−O2 covered surface showing areas free of NH3 where the O2 network structure is the same as on the clean surface. A table of measured hole−hole and NH3− NH3 distances is also provided. This material is free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel: 312 996-0777, Fax: 312 9960431 (M.T.). E-mail: [email protected], Tel: +81-48-467-4073, Fax: +81-48-467-1945 (Y.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation (CHE-1012201) and from the Surface and Interface Science Laboratory (SISL) of RIKEN, Japan.

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REFERENCES

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