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Chemical reactions on solid surfaces: Atomistic observations by scanning tunneling microscopy
applied
surfacescience ELSEVIER
Applied Surface Science 113/l 14 (1997) 539-545
Chemical reactions on solid surfaces: Atomistic observations by scanning tunneling microscopy R.J. Madix
*,
W.W. Crew, X.-C. Guo
Depurtments of Chemical Engineering und Chemistry, Stanford University, Stanford, CA 94305, USA
Abstract A scanning tunneling microscope has been developed which allows manipulation and control of the surface as well as scanning tunneling microscopy without transfer of the crystal off the manipulator to the microscope. The microscope is of the ‘Johnnie Walker’ type and is lowered onto the crystal, which is itself housed in a versatile transfer carriage. In this fashion surfaces can be ion bombarded, chemically treated, etc. and examined by the microscope at temperatures ranging from 120-400 K with atomic resolution without change in surface temperature. With this microscope we have studied the reaction of carbon monoxide with oxygen on Cu(ll0). At an ambient pressure of 10mJ Torr carbon monoxide reacts with the p(2 X 1) islands of oxygen at a measurable rate only at 400 K or above. Reaction is approximately 1000 times faster along the (001) direction of the oxide islands than along the (110) direction. Reaction does not occur internal to the island unless there are defects to initiate the reaction. At 400 K the oxide islands do not maintain an equilibrium shape or distribution when reacting with ambient carbon monoxide, though in the presence of a low pressure of oxygen they appear to do so. At 150 K the p(2 X 1) oxide islands are inert to reaction with carbon monoxide and, additionally, short Cu-0 chains are also unreactive. Reaction between pre-adsorbed carbon monoxide and ambient oxygen is observed, implying that mobile oxygen atoms which have not yet formed surface-bound pseudomolecules, can react with adsorbed carbon monoxide.
1. Introduction Since its invention in 1981, the scanning tunneling microscope (STM) has had a profound impact on surface science and is playing an ever-increasing role in surface science. The number of papers in the literature using STM has increased very rapidly in the last 10 years [I]. The principle advantage of STM is its capacity for real space imaging of local phenomena at the atomic level, in contrast to conventional surface science techniques, which measure spatially averaged properties of the substrate and
1 Corresponding author.
adsorbates.
For example,
Auger
spectroscopy
or pho-
toemission spectroscopies monitor properties integrated over the entire surface; information regarding the effects of local inhomogeneities in coverage or defects on surface phenomenon are not readily obtainable with this technique. While there are many STM studies in the literature that report surface reconstructions and adatom adsorption on metal surfaces [2,3], there are few investigations of chemical reactions on surfaces [471. Because one can atomically resolve surface reactants with a scanning tunneling microscope [4], it is in principle possible to investigate the effects of spatial organization of reactants on surface reactions at the atomic level. However, monitoring reactions
0169-4332/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PIf SO169-4332(96)00962-2
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R.J. Madix et al. /Applied Surface Science I I3/ I14 (19971539-545
can be experimentally difficult because it requires temperature cycling of the sample by balancing simultaneous heating and cooling to achieve the desired steady state temperature. Sample temperature drift and temperature differences between sample and microscope make it difficult to image a selected section of the surface for an extended period without losing registry. Therefore, most microscopes operate at one temperature. We have developed a variable temperature UHV STM capable of monitoring reactions on surfaces between 120 and 450 K [8]. The apparatus is equipped with STM, Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed reaction spectroscopy (TPRS), Ar’ bombardment and sample dosing. This system is based on a design concept by Thomas Michely [6], but is itself unique because it allows imaging with atomic resolution while cooling continuously with liquid nitrogen and it is quite compact, incorporating ‘airlegs’ for vibrational isolation rather than a large, spring suspension system.
STM Sample
2. UHV system To minimize vibration transmission of building vibrations to the sample, the chamber is mounted to an I-beam frame supported by four laminar flow isolation legs, which are essential in order to filter vibrational noise (Fig. 1). The chamber is bolted to the frame by four stainless steel struts that are welded to the frame and tilted outward from the vertical position by 20” to prevent chamber torsion vibration modes. This is the only contact between the chamber and the frame; the pump-well hangs unsupported. In order that the vibration isolation legs work properly, the system weight was fixed at 800 lb and to minimize low frequency motions of the apparatus, a low center of gravity was maintained. The large pump-well beneath the top half of the chamber lowers the center of mass below the platform level of the Newport isolation legs. This makes the system less susceptible to hobby-horse oscillations because the system behaves like a pendulum at rest.
AES, LEED, MS Differentially pumped
Fig. 1. A three dimensional cross section of the UHV chamber showing the chamber, the rectangular beam frame and the air legs. The chamber is divided into two halves, top and bottom. The top half contains the surface science techniques and sample manipulator. The bottom half contains the ion and titanium sublimation pumps.
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R.J. Madix et al. /Applied Surface Science 113 / I14 (IYY7) 539-545
The chamber is pumped to ultra-high vacuum conditions by a 220 L/s ion pump and a titanium sublimation pump. A 60 L/s turbomolecular pump evacuates the chamber after argon ion bombardment and after the chamber has been vented to atmosphere, as well as the gas handling lines. A mechanical pump, which sits on the floor, backs the turbomolecular pump. The turbomolecular and mechanical pumps must be turned off and the hose between the mechanical and turbomolecular pumps removed during microscope operation; a gate valve isolates the turbomolecular pump from the chamber. When the turbomolecular pump stops, a safety valve vents the pump to atmosphere; a gate valve between the turbomolecular pump and chamber must be closed when the pump is turned off to prevent the entire system from venting to atmosphere. All other connections to the chamber except the electrical connections necessary to operate the STM and the ion pump are also disconnected. The chamber is divided into a top and bottom section. The top section houses all of the surface science techniques and the bottom section contains the ion and titanium sublimation pumps. A poppet valve separates the two halves of the chamber. The top-half of the chamber is designed for a combination of physical and chemical measurements. Provision is made for scanning tunneling microscopy @TM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), argon ion bombardment, temperature programmed reaction spectroscopy (TPRS) and gas dosing. The design is sufficiently general that other techniques could be incorporated. The existing techniques are distributed between two stations along the horizontal axis of the chamber (Fig. 1); the first station is for the microscope, the second is for all other techniques. The microscope is a ‘Johnnie Walker’ type STM originally designed by Frohn et al. [9]. This microscope is particularly well suited for monitoring reactions on surfaces because all of the piezoelectric tubes are aligned in one direction so that thermal drift is reduced in the direction perpendicular to the sample. The microscope is lowered onto the sample using a conventional manipulator with X, Y, Z and 0 degrees of freedom. The rearview LEED screen, which also serves as a retarding field analyzer for AES, ion bombardment gun, dosers and quadrupole mass spec-
trometer are distributed
radially in a semicircle
at the
second station. To position the sample for the various techniques, the sample carriage can be moved along the length of the chamber and rotated up to 180”. The sample carriage is mechanically linked to a rotary, differentially pumped flange mounted on a motorized bellows. The sample is linearly translated by the motorized bellows and rotated by turning the differentially pumped flange.
3. Sample transfer system The sample carriage is a stainless steel tube weighing 24 lb. The tube lies along the horizontal axis of the chamber (Fig. 1). The weight of the tube and thickness of its walls ensure that any vibrational noise conducted to the sample carriage is not amplified. The outside of the tube is finely polished with 1 km diamond paste so that it slides smoothly on Teflon buttons placed at each end of the chamber. Many factors were incorporated into the carriage and sample holder design to minimize vibration transmission from the environment to the sample. By using Teflon as a sliding surface, we minimized the number of moving parts for sample motion and vibrational coupling between the chamber and the sample. To increase vibration isolation, a loose mechanical linkage between the sample manipulator and the rotary, differentially pumped flange isolate the bellows and rotary flange from the sample manipulator. Liquid nitrogen tubes which run down the center of the manipulator are rigidly clamped and are isolated from the original feedthroughs and the feedthrough nipple by metal bellows. Sample heating leads and thermocouple wires are connected directly to the manipulator tube so that any vibration conducted through the wires must excite the vibrational modes of the tube to continue to the sample. In the center of the sample carriage is a compartment for the sample, the second vibration isolation stage, a heater and a liquid nitrogen reservoir. The sample holder consists of the ramp on which the STM rests, the sample, a sapphire insulator for electrical isolation and a copper ring holder. The STM ramp and sample are biased for STM through the thermocouple wires attached to the crystal.
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et al. /Applied
Surface Science I13 / I14
The sample is attached to, but thermally and electrically isolated from, a large copper block that serves as the second UHV low frequency vibration isolation stage. Three glass balls placed between the copper block and the horseshoe-shaped piece, which itself is fastened to the large copper block by two screws, thermally isolate the sample holder and copper ring from the large copper block while providing rigid mounting. The weight of the block (6 lb) and the elasticity of the viton comprises a vibration isolation stage with damping whose resonance frequency is between 5 and 10 Hz. Sample temperatures ranging from 120 to 1000 K can be achieved. The sample is radiatively heated by a filament directly underneath it. To prevent vibra-
(1997)
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tion transmission from the heating wires to the sample, the filament does not touch the sample holder. The sample is cooled via conduction through a copper braid that is connected to a liquid nitrogen reservoir. The braid is flexible and U-shaped to minimize vibration transmission between the sample and cooling reservoir. The liquid nitrogen reservoir is connected to the outside liquid nitrogen supply by two l/4” diameter tubes that run straight down the middle of the sample manipulator tube. To minimize vibration transmission bellows are placed between the terminal feedthrough and the sample manipulator tube and the liquid nitrogen tubes are clamped to the sample manipulator tube at the reservoir and the end of the
Fig. 2. (a), (b) and (c) show sequential STh4 images of p(2 X 1) 0 reduced by CO. The time between each frame is I.50 s. In (b) arrow ‘A’ marks a row that fragmented and shifted one row to the left, arrow ‘B’ marks a row at an island edge that fragmented and arrow ‘C’ marks a transient row not observed in either (a) or (c). Image (d) shows a topographic image of the clean surface after the oxygen is completely reduced.
R.J. Madix et al. /Applied Surface Science 113/114
tube. In addition, the tubes are as straight as possible to minimize vibration generation from liquid nitrogen boiling in the tubes. The liquid nitrogen tubes are thermally insulated from the carriage by Teflon and ceramic spacers. Provided the tubes and reservoir were free of traces of water, liquid nitrogen flowing through the reservoir did nor induce sample vibrations.
4. Microscope performance With this instrument we have achieved atomically resolved images of the p(2 X 1) oxide rows of Cu-0 on Cu(l10) at 120, 150, 293 and 400 K. In addition we have studied the oxidation of CO at 400 and 150 K on oxygen-precovered Cu( 1 IO) without experienc-
(1997) 539-545
543
ing excessive thermal drift [lo,1 11, we have imaged phenoxy molecular fragments, an intermediate in the oxidation of phenol, on Cu(l10) at room temperature [ 121 and we have successfully followed the oxidation of ammonia on Cu(l10) between room temperature and 400 K [ 131. In this paper we focus on the oxidation of CO on this surface. Oxygen forms a very well ordered p(2 X 11 pattern on Cu(l10) up to oxygen coverage of 0.5 ML [ 14-191 consisting of Cu-0-Cu chains in an ‘added-row’ type structure, where oxygen atoms bind to copper on top of the (170) copper rows in the short bridge position. When the oxygen-covered surface is annealed at or above 400 K, the oxygen rows aggregate into well-ordered bands providing a supergrating of regularly spaced oxygen islands 8-14 rows thick with very few defects.
‘4
Fig. 3. (a-f) show sequential STM topographic images of isosteric experiments where that the rate of oxygen adsorption equals the rate of oxygen consumption by reaction Multi-colored tick marks highlight evolving features. The elapsed time for each image (a-f), respectively. Images are 356 X 359 A* and were recorded with a tunneling current not shown with this color contrast, individual atoms are resolved in the oxide structure.
cl
both CO and O? are present with CO. The CO pressure is 6:47, 14:45, 29:OO. 43:45, of 1.5 nA and a sample bias
in the gas phase such was 1.7 X 10-j Torr. 54:30 and 1:16:30 for of - 17 mV. Although
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R.J. Madix et al. /Applied Surface Science I I3 / I14 (19971539-545
The catalytic oxidation of CO to CO, over oxygen covered Cu(1 10) proceeds via the reaction between adsorbed oxygen and CO [20-221. CO reacts with oxygen primarily along the oxygen rows, or (001) direction, leaving a clean surface (Fig. 2) shows a topographic image of a typical oxygen covered surface before the experiment. As shown in Fig. 2a and b the oxygen rows on the island perimeter react away first in a sequential fashion, leaving two individual rows shown in Fig. 2c. Although not shown in Fig. 2, the STM image taken after Fig. 2c showed no oxygen on the surface. Fig. 2d is a close-up image of the resulting clean surface obtained at 400 K showing individual Cu atoms, verifying that the surface is clean after the reaction. If the defect density internal to the p(2 X 1) islands is low, the reaction appears to be initiated by penetration into an oxygen (2 X 1) island along the (liO> direction, followed by rapid reaction along the oxygen rows in the (001) direction. This effect is already
evident in Fig. 2b, which clearly shows fragments of a single oxide chain formed by reaction along the (001) direction (arrow B) without significant penetration into the island in the (l‘i0) direction. Defects in the oxygen overlayer can also play a critical role in the oxidation of CO on Cu(l10) [lo]. In the kinetic steady state, when the global concentration of adsorbed oxygen remains constant, the island structure fluctuates. Islands shrink and grow and the kink structure on the island perimeter changes (Fig. 3). The continual fluctuation in the structure of these kink sites suggest that CO reacts with oxygen and that oxygen subsequently adds to the island at defect kink-sites. Indeed, oxygen addition to defect sites in the p(2 X 1) overlayer was established in previous studies. In addition, mobile copper adatoms are always present on the terrace; copper atoms are continuously released from oxygen reaction with CO and reincorporated into the p(2 X 1) island structure by reaction with adsorbed oxygen. This fluctuating
Fig. 4. Topographic images showing oxygen on Cu(l10). (a) p(2 X I)0 on clean Cu(l10) at 150 K (169 X 172A*, - 1.44 V, 1.9 nA). (b) the surface in (a) dosed with 10 L of oxygen at 150 K. -Cu-O- pseudomolecules form in the interstitial regions between existing oxygen islands and are immobile (136 x 147 2, - 1.44mV, 1.9nA). The closely spaced hash marks in the line scan denote oxide rows and the last hash mark on the right highlights a pseudomolecule. The ‘A’ on the topograph corresponds to the ‘A’ in the cross-section plot. (c) shows them surface heated to 300 K. The pseudomolecules become mobile at this temperature and aggregate into islands. The one atomic layer deep pits in the terrace are characteristic of p(2 X 1)oxide formation (241 X 247 A2, 187 mV, 1.2 nA).
R.J. Madix et al. /Applied Surface Science 113 / 114 C19971539-54s
structure is apparently typical of the surface during the steady state reaction. We conclude that oxygen from the gas phase adsorbs and replaces the chemisorbed oxygen consumed in the reaction. The copper originally incorporated into the 0 p(2 X 1) ‘added-row’ structure appears to migrate to terrace edges while oxygen is removed by reaction with CO. In all of the images recorded for this type of experiment, we never observed Cu atom aggregation on a terrace to form a Cu island or surface roughening. Oxygen dosed onto Cu(l10) at 150 K forms a somewhat disordered structure, comprised of short -Cu-O- chains aligned in the (001) direction (Fig. 4). Carbon monoxide also does not react with either the p(2 X 110 islands or the Cu-0 pseudomolecules at 150 K. Long CO strands along the (001) direction coadsorb with the oxygen, terminating at either the boundaries of the (2 X 1) islands or at the pseudomolecules. The fact that CO strands terminated at pseudomolecules proves that CO in contact with pseudomolecules does not react at this temperature. The fact that pseudomolecules are also unreactive to CO indicates that the state of binding of oxygen in the pseudomolecules is very similar to that in the larger p(2 X 1)-O islands. When the surface is predosed with carbon monoxide and exposed to oxygen at 145 K, oxygen reacts with carbon monoxide to form carbon dioxide. The carbon monoxide stripes spanning (2 X 110 islands are stable for at least 30 min unless oxygen is introduced to the system. Upon exposure to ambient oxygen, however, carbon monoxide disappears and is replaced by oxide pseudomolecules in the interstitial regions. Mass spectrometric experiments confirm that CO, is formed concomitantly [IO].
5. Summary A novel STM capable of imaging surface reactions over a range of surface temperatures was described, using as an illustration its use to study the
545
oxidation of carbon monoxide on Cu(l10). The design of this instrument is compatible with incorporation of a wide range of surface experiments with STM.
References [l] H.-J. Guntherodt and R. Wiesendanger teds.), Scanning Tunneling Microscopy I: General Principles and Applications to Clean and Adsorbate-Covered Surfaces (Springer-Verlag. New York, 1992). [2] J.L. Solomon. R.J. Madix and J. Stiihr, Surf. Sci. 255 (1991) 12. [3] E.M. Stuve and R.J. Madix, J. Phys. Chem. 89 (1985) 3183. [4] L. Ruan, F. Besenbacher, I. Stensgaard and E. Lzgsgaard. Phys. Rev. Lett. 69 (1992) 3523. [5] F.M. Leibsle, S.M. Francis, R. Davis, N. Xiang, S. Haq and M. Bowker, Phys. Rev. Len. 72 (1994) 2569. [6] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger and G. Comsa, J. Chem. Phys. 97 (1992) 6774. [7] F.M. Leiblse, S.M. Francis, S. Haq and M. Bowker, Surf. Sci. 318 (1994) 46. [S] W.W. Crew and R.J. Madix, Rev. Sci. Instrum. 66 (1995) 4552. [9] J. Frohm, J.F. Wolf, K. Besocke and M. Teske. Rev. Sci. Instrum. 60 (1989) 1200. [lo] W.W. Crew and R.J. Madix, Surf. Sci. 349 (1996) 275 [II] W.W. Crew and R.J. Madix, Surf. Sci., 356 (1996) 1. [12] X.-C. Guo and R.J. Madix, Surf. Sci. 341 (1995) L1065. 1131 X.-C. Guo and R.J. Madix, to be published. [ 141 D. Coulman, J. Wintterlin, R.J. Behm and G. Ertl. Phys. Rev. Lett. 64 (1990) 1761. [15] D. Dorenbos, M. Breeman and D.O. Boerma, Phys. Rev. B 47 (1993) 1580. [16] U. Dobler. K. Baberschke, J. Haase and A. Puschmann, Phys. Rev. Lett. 52 (1984) 1437. [17] H. Niehus and G. Comsa, Surf. Sci. 140 (1984) 18. [IL?] S.R. Parkin, H.C. Zheng, M.Y. Zhou and A.R. Mitchell, Phys. Rev. B 41 (1990) 5432. [19] R. Freidenhans’l. F. Grey, R.L. Johnson. S.G.J. Mochrie, J. Bohr and M. Nielsen, Phys. Rev. B 41 (1990) 5420. 1201 F.H.P.M. Habraken and G.A. Bootsma, Surf. Sci. 87 (1979) 333. 1211 F.H.P.M. Habraken, G.A. Bootsma, P. Hofmann, S. Hachicha and A.M. Bradshaw, Surf. Sci. 88 (1979) 285. 1221 O.P. van Pruissen. M.M.M. Dings and O.L.J. Gijzeman, Surf. Sci. 179 (1987) 377.
surfacescience ELSEVIER
Applied Surface Science 113/l 14 (1997) 539-545
Chemical reactions on solid surfaces: Atomistic observations by scanning tunneling microscopy R.J. Madix
*,
W.W. Crew, X.-C. Guo
Depurtments of Chemical Engineering und Chemistry, Stanford University, Stanford, CA 94305, USA
Abstract A scanning tunneling microscope has been developed which allows manipulation and control of the surface as well as scanning tunneling microscopy without transfer of the crystal off the manipulator to the microscope. The microscope is of the ‘Johnnie Walker’ type and is lowered onto the crystal, which is itself housed in a versatile transfer carriage. In this fashion surfaces can be ion bombarded, chemically treated, etc. and examined by the microscope at temperatures ranging from 120-400 K with atomic resolution without change in surface temperature. With this microscope we have studied the reaction of carbon monoxide with oxygen on Cu(ll0). At an ambient pressure of 10mJ Torr carbon monoxide reacts with the p(2 X 1) islands of oxygen at a measurable rate only at 400 K or above. Reaction is approximately 1000 times faster along the (001) direction of the oxide islands than along the (110) direction. Reaction does not occur internal to the island unless there are defects to initiate the reaction. At 400 K the oxide islands do not maintain an equilibrium shape or distribution when reacting with ambient carbon monoxide, though in the presence of a low pressure of oxygen they appear to do so. At 150 K the p(2 X 1) oxide islands are inert to reaction with carbon monoxide and, additionally, short Cu-0 chains are also unreactive. Reaction between pre-adsorbed carbon monoxide and ambient oxygen is observed, implying that mobile oxygen atoms which have not yet formed surface-bound pseudomolecules, can react with adsorbed carbon monoxide.
1. Introduction Since its invention in 1981, the scanning tunneling microscope (STM) has had a profound impact on surface science and is playing an ever-increasing role in surface science. The number of papers in the literature using STM has increased very rapidly in the last 10 years [I]. The principle advantage of STM is its capacity for real space imaging of local phenomena at the atomic level, in contrast to conventional surface science techniques, which measure spatially averaged properties of the substrate and
1 Corresponding author.
adsorbates.
For example,
Auger
spectroscopy
or pho-
toemission spectroscopies monitor properties integrated over the entire surface; information regarding the effects of local inhomogeneities in coverage or defects on surface phenomenon are not readily obtainable with this technique. While there are many STM studies in the literature that report surface reconstructions and adatom adsorption on metal surfaces [2,3], there are few investigations of chemical reactions on surfaces [471. Because one can atomically resolve surface reactants with a scanning tunneling microscope [4], it is in principle possible to investigate the effects of spatial organization of reactants on surface reactions at the atomic level. However, monitoring reactions
0169-4332/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PIf SO169-4332(96)00962-2
540
R.J. Madix et al. /Applied Surface Science I I3/ I14 (19971539-545
can be experimentally difficult because it requires temperature cycling of the sample by balancing simultaneous heating and cooling to achieve the desired steady state temperature. Sample temperature drift and temperature differences between sample and microscope make it difficult to image a selected section of the surface for an extended period without losing registry. Therefore, most microscopes operate at one temperature. We have developed a variable temperature UHV STM capable of monitoring reactions on surfaces between 120 and 450 K [8]. The apparatus is equipped with STM, Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed reaction spectroscopy (TPRS), Ar’ bombardment and sample dosing. This system is based on a design concept by Thomas Michely [6], but is itself unique because it allows imaging with atomic resolution while cooling continuously with liquid nitrogen and it is quite compact, incorporating ‘airlegs’ for vibrational isolation rather than a large, spring suspension system.
STM Sample
2. UHV system To minimize vibration transmission of building vibrations to the sample, the chamber is mounted to an I-beam frame supported by four laminar flow isolation legs, which are essential in order to filter vibrational noise (Fig. 1). The chamber is bolted to the frame by four stainless steel struts that are welded to the frame and tilted outward from the vertical position by 20” to prevent chamber torsion vibration modes. This is the only contact between the chamber and the frame; the pump-well hangs unsupported. In order that the vibration isolation legs work properly, the system weight was fixed at 800 lb and to minimize low frequency motions of the apparatus, a low center of gravity was maintained. The large pump-well beneath the top half of the chamber lowers the center of mass below the platform level of the Newport isolation legs. This makes the system less susceptible to hobby-horse oscillations because the system behaves like a pendulum at rest.
AES, LEED, MS Differentially pumped
Fig. 1. A three dimensional cross section of the UHV chamber showing the chamber, the rectangular beam frame and the air legs. The chamber is divided into two halves, top and bottom. The top half contains the surface science techniques and sample manipulator. The bottom half contains the ion and titanium sublimation pumps.
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R.J. Madix et al. /Applied Surface Science 113 / I14 (IYY7) 539-545
The chamber is pumped to ultra-high vacuum conditions by a 220 L/s ion pump and a titanium sublimation pump. A 60 L/s turbomolecular pump evacuates the chamber after argon ion bombardment and after the chamber has been vented to atmosphere, as well as the gas handling lines. A mechanical pump, which sits on the floor, backs the turbomolecular pump. The turbomolecular and mechanical pumps must be turned off and the hose between the mechanical and turbomolecular pumps removed during microscope operation; a gate valve isolates the turbomolecular pump from the chamber. When the turbomolecular pump stops, a safety valve vents the pump to atmosphere; a gate valve between the turbomolecular pump and chamber must be closed when the pump is turned off to prevent the entire system from venting to atmosphere. All other connections to the chamber except the electrical connections necessary to operate the STM and the ion pump are also disconnected. The chamber is divided into a top and bottom section. The top section houses all of the surface science techniques and the bottom section contains the ion and titanium sublimation pumps. A poppet valve separates the two halves of the chamber. The top-half of the chamber is designed for a combination of physical and chemical measurements. Provision is made for scanning tunneling microscopy @TM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), argon ion bombardment, temperature programmed reaction spectroscopy (TPRS) and gas dosing. The design is sufficiently general that other techniques could be incorporated. The existing techniques are distributed between two stations along the horizontal axis of the chamber (Fig. 1); the first station is for the microscope, the second is for all other techniques. The microscope is a ‘Johnnie Walker’ type STM originally designed by Frohn et al. [9]. This microscope is particularly well suited for monitoring reactions on surfaces because all of the piezoelectric tubes are aligned in one direction so that thermal drift is reduced in the direction perpendicular to the sample. The microscope is lowered onto the sample using a conventional manipulator with X, Y, Z and 0 degrees of freedom. The rearview LEED screen, which also serves as a retarding field analyzer for AES, ion bombardment gun, dosers and quadrupole mass spec-
trometer are distributed
radially in a semicircle
at the
second station. To position the sample for the various techniques, the sample carriage can be moved along the length of the chamber and rotated up to 180”. The sample carriage is mechanically linked to a rotary, differentially pumped flange mounted on a motorized bellows. The sample is linearly translated by the motorized bellows and rotated by turning the differentially pumped flange.
3. Sample transfer system The sample carriage is a stainless steel tube weighing 24 lb. The tube lies along the horizontal axis of the chamber (Fig. 1). The weight of the tube and thickness of its walls ensure that any vibrational noise conducted to the sample carriage is not amplified. The outside of the tube is finely polished with 1 km diamond paste so that it slides smoothly on Teflon buttons placed at each end of the chamber. Many factors were incorporated into the carriage and sample holder design to minimize vibration transmission from the environment to the sample. By using Teflon as a sliding surface, we minimized the number of moving parts for sample motion and vibrational coupling between the chamber and the sample. To increase vibration isolation, a loose mechanical linkage between the sample manipulator and the rotary, differentially pumped flange isolate the bellows and rotary flange from the sample manipulator. Liquid nitrogen tubes which run down the center of the manipulator are rigidly clamped and are isolated from the original feedthroughs and the feedthrough nipple by metal bellows. Sample heating leads and thermocouple wires are connected directly to the manipulator tube so that any vibration conducted through the wires must excite the vibrational modes of the tube to continue to the sample. In the center of the sample carriage is a compartment for the sample, the second vibration isolation stage, a heater and a liquid nitrogen reservoir. The sample holder consists of the ramp on which the STM rests, the sample, a sapphire insulator for electrical isolation and a copper ring holder. The STM ramp and sample are biased for STM through the thermocouple wires attached to the crystal.
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et al. /Applied
Surface Science I13 / I14
The sample is attached to, but thermally and electrically isolated from, a large copper block that serves as the second UHV low frequency vibration isolation stage. Three glass balls placed between the copper block and the horseshoe-shaped piece, which itself is fastened to the large copper block by two screws, thermally isolate the sample holder and copper ring from the large copper block while providing rigid mounting. The weight of the block (6 lb) and the elasticity of the viton comprises a vibration isolation stage with damping whose resonance frequency is between 5 and 10 Hz. Sample temperatures ranging from 120 to 1000 K can be achieved. The sample is radiatively heated by a filament directly underneath it. To prevent vibra-
(1997)
539-545
tion transmission from the heating wires to the sample, the filament does not touch the sample holder. The sample is cooled via conduction through a copper braid that is connected to a liquid nitrogen reservoir. The braid is flexible and U-shaped to minimize vibration transmission between the sample and cooling reservoir. The liquid nitrogen reservoir is connected to the outside liquid nitrogen supply by two l/4” diameter tubes that run straight down the middle of the sample manipulator tube. To minimize vibration transmission bellows are placed between the terminal feedthrough and the sample manipulator tube and the liquid nitrogen tubes are clamped to the sample manipulator tube at the reservoir and the end of the
Fig. 2. (a), (b) and (c) show sequential STh4 images of p(2 X 1) 0 reduced by CO. The time between each frame is I.50 s. In (b) arrow ‘A’ marks a row that fragmented and shifted one row to the left, arrow ‘B’ marks a row at an island edge that fragmented and arrow ‘C’ marks a transient row not observed in either (a) or (c). Image (d) shows a topographic image of the clean surface after the oxygen is completely reduced.
R.J. Madix et al. /Applied Surface Science 113/114
tube. In addition, the tubes are as straight as possible to minimize vibration generation from liquid nitrogen boiling in the tubes. The liquid nitrogen tubes are thermally insulated from the carriage by Teflon and ceramic spacers. Provided the tubes and reservoir were free of traces of water, liquid nitrogen flowing through the reservoir did nor induce sample vibrations.
4. Microscope performance With this instrument we have achieved atomically resolved images of the p(2 X 1) oxide rows of Cu-0 on Cu(l10) at 120, 150, 293 and 400 K. In addition we have studied the oxidation of CO at 400 and 150 K on oxygen-precovered Cu( 1 IO) without experienc-
(1997) 539-545
543
ing excessive thermal drift [lo,1 11, we have imaged phenoxy molecular fragments, an intermediate in the oxidation of phenol, on Cu(l10) at room temperature [ 121 and we have successfully followed the oxidation of ammonia on Cu(l10) between room temperature and 400 K [ 131. In this paper we focus on the oxidation of CO on this surface. Oxygen forms a very well ordered p(2 X 11 pattern on Cu(l10) up to oxygen coverage of 0.5 ML [ 14-191 consisting of Cu-0-Cu chains in an ‘added-row’ type structure, where oxygen atoms bind to copper on top of the (170) copper rows in the short bridge position. When the oxygen-covered surface is annealed at or above 400 K, the oxygen rows aggregate into well-ordered bands providing a supergrating of regularly spaced oxygen islands 8-14 rows thick with very few defects.
‘4
Fig. 3. (a-f) show sequential STM topographic images of isosteric experiments where that the rate of oxygen adsorption equals the rate of oxygen consumption by reaction Multi-colored tick marks highlight evolving features. The elapsed time for each image (a-f), respectively. Images are 356 X 359 A* and were recorded with a tunneling current not shown with this color contrast, individual atoms are resolved in the oxide structure.
cl
both CO and O? are present with CO. The CO pressure is 6:47, 14:45, 29:OO. 43:45, of 1.5 nA and a sample bias
in the gas phase such was 1.7 X 10-j Torr. 54:30 and 1:16:30 for of - 17 mV. Although
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The catalytic oxidation of CO to CO, over oxygen covered Cu(1 10) proceeds via the reaction between adsorbed oxygen and CO [20-221. CO reacts with oxygen primarily along the oxygen rows, or (001) direction, leaving a clean surface (Fig. 2) shows a topographic image of a typical oxygen covered surface before the experiment. As shown in Fig. 2a and b the oxygen rows on the island perimeter react away first in a sequential fashion, leaving two individual rows shown in Fig. 2c. Although not shown in Fig. 2, the STM image taken after Fig. 2c showed no oxygen on the surface. Fig. 2d is a close-up image of the resulting clean surface obtained at 400 K showing individual Cu atoms, verifying that the surface is clean after the reaction. If the defect density internal to the p(2 X 1) islands is low, the reaction appears to be initiated by penetration into an oxygen (2 X 1) island along the (liO> direction, followed by rapid reaction along the oxygen rows in the (001) direction. This effect is already
evident in Fig. 2b, which clearly shows fragments of a single oxide chain formed by reaction along the (001) direction (arrow B) without significant penetration into the island in the (l‘i0) direction. Defects in the oxygen overlayer can also play a critical role in the oxidation of CO on Cu(l10) [lo]. In the kinetic steady state, when the global concentration of adsorbed oxygen remains constant, the island structure fluctuates. Islands shrink and grow and the kink structure on the island perimeter changes (Fig. 3). The continual fluctuation in the structure of these kink sites suggest that CO reacts with oxygen and that oxygen subsequently adds to the island at defect kink-sites. Indeed, oxygen addition to defect sites in the p(2 X 1) overlayer was established in previous studies. In addition, mobile copper adatoms are always present on the terrace; copper atoms are continuously released from oxygen reaction with CO and reincorporated into the p(2 X 1) island structure by reaction with adsorbed oxygen. This fluctuating
Fig. 4. Topographic images showing oxygen on Cu(l10). (a) p(2 X I)0 on clean Cu(l10) at 150 K (169 X 172A*, - 1.44 V, 1.9 nA). (b) the surface in (a) dosed with 10 L of oxygen at 150 K. -Cu-O- pseudomolecules form in the interstitial regions between existing oxygen islands and are immobile (136 x 147 2, - 1.44mV, 1.9nA). The closely spaced hash marks in the line scan denote oxide rows and the last hash mark on the right highlights a pseudomolecule. The ‘A’ on the topograph corresponds to the ‘A’ in the cross-section plot. (c) shows them surface heated to 300 K. The pseudomolecules become mobile at this temperature and aggregate into islands. The one atomic layer deep pits in the terrace are characteristic of p(2 X 1)oxide formation (241 X 247 A2, 187 mV, 1.2 nA).
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structure is apparently typical of the surface during the steady state reaction. We conclude that oxygen from the gas phase adsorbs and replaces the chemisorbed oxygen consumed in the reaction. The copper originally incorporated into the 0 p(2 X 1) ‘added-row’ structure appears to migrate to terrace edges while oxygen is removed by reaction with CO. In all of the images recorded for this type of experiment, we never observed Cu atom aggregation on a terrace to form a Cu island or surface roughening. Oxygen dosed onto Cu(l10) at 150 K forms a somewhat disordered structure, comprised of short -Cu-O- chains aligned in the (001) direction (Fig. 4). Carbon monoxide also does not react with either the p(2 X 110 islands or the Cu-0 pseudomolecules at 150 K. Long CO strands along the (001) direction coadsorb with the oxygen, terminating at either the boundaries of the (2 X 1) islands or at the pseudomolecules. The fact that CO strands terminated at pseudomolecules proves that CO in contact with pseudomolecules does not react at this temperature. The fact that pseudomolecules are also unreactive to CO indicates that the state of binding of oxygen in the pseudomolecules is very similar to that in the larger p(2 X 1)-O islands. When the surface is predosed with carbon monoxide and exposed to oxygen at 145 K, oxygen reacts with carbon monoxide to form carbon dioxide. The carbon monoxide stripes spanning (2 X 110 islands are stable for at least 30 min unless oxygen is introduced to the system. Upon exposure to ambient oxygen, however, carbon monoxide disappears and is replaced by oxide pseudomolecules in the interstitial regions. Mass spectrometric experiments confirm that CO, is formed concomitantly [IO].
5. Summary A novel STM capable of imaging surface reactions over a range of surface temperatures was described, using as an illustration its use to study the
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oxidation of carbon monoxide on Cu(l10). The design of this instrument is compatible with incorporation of a wide range of surface experiments with STM.
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