- Email: [email protected]
Interaction of chlorine with nickel (110) studied by scanning tunnelling microscopy
surface science ELSEVIER
Surface Science 377-379 (1997) 629-633
Interaction of chlorine with nickel (110) studied by scanning tunnelling microscopy T.W. Fishlock
a~*, J.B. Pethica
a, F.H. Jones b, R.G. Egdell b, J.S. Foord b
aDepartment of Materials, Oxford University, Parks Road, Oxford OX1 3PH, UK b New Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QT, UK Received 1 August 1996; accepted for publication 9 September 1996
Abstract
The role of forces in STM image formation has been established from studies on copper and nickel, and the resulting potential for controlled atom movement has generated interest in STM imaging of low coverages of halogens on these surfaces. Here we report atom-resolved images of clean and chlorine-covered nickel (110) surfaces over a range of chlorine coverages. All the images show that chlorine dissociates on the (110) surface into chemisorbed atom pairs oriented along the [OOl] direction. The atom-atom separation is comparable to the bulk nickel lattice constant. For very low chlorine exposures, strings of chlorine pairs running in the [ iTO] direction are observed, propagating from [OOl] step edges onto the terraces. High exposures of chlorine produce a number of different co-existing reconstructions, which will be discussed. Keywords: Chemisorption; Chlorine; Low energy electron diffraction (LEED); Nickel; Scanning tunnelling microscopy; Scanning tunnelling spectroscopies; Surface relaxation and reconstruction
1. Introduction
The controlled and reproducible modification of surfaces at the atomic scale in STM is of interest not only for potential technological applications, but also for the investigation of physical and chemical processes on a very local scale [l]. In order to gain such precise control, information on the forces required to initiate atomic motion is essential. The role of such forces in STM imaging has been established for Cu and Ni surfaces. Analysis by Clarke et al. [2] of the variation of corrugation height with tunnel current has given insight into the imaging mechanism, and coupled * Corresponding author. e-mail: [email protected]
with MD simulations, provides a method of assessing the tip-surface forces just before they induce actual motion of atoms on the surface. The potential for controlled atom manipulation at room temperature has generated interest in STM imaging of low coverages of halogens on copper and nickel surfaces. Here we present a scanning tunnelling microscopy study of the interaction of chlorine with Ni( 110) at room temperature. Atomically resolved images were recorded after both low and high chlorine exposures. Low-exposure images coupled with atom-resolved scanning tunnelling spectroscopy (STS) data suggest that chlorine is imaged directly by the STM. In agreement with Shuxian et al. [3], we show that Cl, chemisorbs dissociatively on the Ni(ll0) surface, but we also show
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01476-8
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T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
that the Cl atoms retain short-range order or correlation. They form a basic structural unit imaged as a pair of STM greyscale maxima 3.5 A apart. The pairs initially form at [OOl] step edges and then grow further in the form of strings directed in the [ liO] direction. Strings are also seen to form directly on the Ni terraces, but appear only marginally stable at room temperature against migration to step edges. Images recorded after higher exposures of chlorine indicate a number of different reconstructions, within which the basic Cl atom-pair unit remains.
2. Experimental The experiments were performed with a commercial Omicron UHV-STM. The STM is mounted in an ion- and turbomolecular-pumped UHV chamber with a base pressure of 5 x lo-l1 mbar. The UHV chamber is equipped with facilities for X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and argon-ion sputtering. An existing solid-state electrochemical Ag/AgCl/Pt cell is used as a UHV-compatible molecular chlorine source [4]. The cell currents used are typically 0.1-0.2 mA. The Ni crystal is mounted on a tantalum sample-holder, and annealing is performed by radiative heating on the rear side of the mount. The Ni( 110) crystal, a semicircle 10 mm in diameter and 3 mm thick, was mechanically polished with diamond paste down to 0.25 ,um. The initial cleaning of the surface was achieved with repeated Ar sputtering (1.5 kV, 30 min) and annealing cycles (7OO”C,20 min) until good-quality LEED patterns indicated a well-ordered (1 x 1) surface, and no impurities could be detected by XPS. Large, flat terraces of the clean (1 x 1) lattice were easily imaged by STM.
3. Results and discussion The exposure of the clean Ni(ll0) surface to a low chlorine exposure results in the formation of an adsorbed species imaged as a discrete pair of maxima 3.5 A apart. As will be discussed below,
the pairs are interpreted as chemisorbed Cl atoms imaged directly by the STM. The pairs are oriented with their long axis along the [OOl] direction and appear to be the basic structural unit forming the chlorine overlayer. Fig. 1 shows an STM image of the Ni( 110) surface after a chlorine dose of 5 x lo-’ mbar*s and a subsequent anneal to 300°C. The discrete pairs are visible at the [OOl] step edge. The pairs are seen to form chains running in the close-packed [ liO] direction, indicated at points A and B in Fig. 1. The chains appear at a height of 0.3 A above the Ni substrate surface. Strings of chlorine pairs have been imaged before on metal surfaces. Schott et al. [ 51 studied chlorine adsorption on a Ag( 111) surface, and observed a similar parallel doublerow structure. In the present work, strings are seen to propagate from [OOl] step edges, but also form directly on the Ni terraces. In both cases the
Fig. 1. 200 A x 200 A image of Ni( 110) after a chlorine exposure of 5 x 10-9mbar*s (at 10m9mbar chlorine pressure) and subsequent anneal to 300°C (4nA tunnelling current and -0.01 V sample bias). Strings of chlorine pairs growing out from the [OOl] step edges are indicated at points A and B. Adjacent strings are one lattice spacing out of registry in the [OOl] direction, highlighted at points E and F. Points C and D and points G and H refer to the spectroscopy and corrugation profile shown in Fig. 2. Line G-H runs along the close-packed [ 1101 direction.
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T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
distance between the pairs in the [liO] direction is 5 A, or twice the lattice spacing. Note that these pairs are not due to a tip-structure artefact. We have obtained numerous such images, with different tips and at different times. The pair and its orientation is always a feature in the image. The stability of the strings on the terraces appears to be low. Chlorine strings seen on the terraces at room temperature after the initial exposure are absent when the surface is imaged again after the sample had been heated to 300°C. This suggests diffusion of the adsorbed atoms to the step edges. Indeed, after the sample had been heated, the terraces were almost completely clear of adsorbates, but the pairs were still present at the [OOl] step edges. The terrace strings also seemed susceptible to tip-induced motion at room temperature. Imaging above a threshold value of the tunnel current (> 5 nA at - 0.01 V sample bias), the position of the chlorine strings on the terrace varies from scan to scan, and sometimes the adatoms are not imaged at all. This seems to indicate a large tipadsorbate interaction resulting in the movement of the adsorbed chlorine. Fig. 1 also shows the initial stages of longerrange order between the Cl pair strings. Here interest focuses on the strings marked E and F in Fig. 1. Two types of longer-range order are evident. Firstly there is double spacing of the pairs along the [ 1701 direction. As mentioned earlier, the pairs in the strings are separated by 5 A, i.e. two lattice spacings in the [ liO] direction. Secondly, adjacent chlorine pairs are out of registry in the [OOl] direction. There is in effect a doubling of the periodicity in both the [OOl] and [ 1701 directions. This apparent reluctance for the pairs to reside in immediately adjacent sites would seem to suggest the presence of longer-range strain fields generated by the interaction of the adsorbed chlorine with the underlying Ni substrate. Assignment of the imaged pairs as chemisorbed chlorine is made tentatively by means of scanning tunnelling spectroscopy data. By interrupting the feedback loop, and keeping the tunnelling gap width fixed and applying a voltage ramp on the tunnelling junction, the tunnel current as a function of bias provides information about the sample density of states (DOS) [ 61. 1/I’ spectra from
-0.8-0.4 Sample
0.0
0.4
Bias
(V)
0.8 Oistance packed
along close direction
Fig. 2. (a) Current-voltage (I/V) curves for chlorine on Ni( 110). The dotted line corresponds to a step-edge pair (site-type C, Fig. l), and the solid line corresponds to bare Ni (site-type D, Fig. 1). The current is limited to 50 nA at high sample bias. (b) Corrugation pro6le along the close-packed [ 1701 direction taken across a [OOl] step edge (line G-H, Fig. 1). The maximum at about 20A corresponds to the chemisorbed chlorine atom two lattice spacings (5 A) removed from the terminal Ni atom.
single atom sites (C and D in Fig. 1) are shown in Fig. 2a. The plot shows that at positive bias (empty sample states) the relationship between tunnel current and sample bias for site-type C (step-edge pair) is the same as it is for site-type D (Ni substrate). This suggests that the empty DOS of the two species are similar. However, at negative sample biases (filled sample states) there is a distinct difference in the plot between the two species. For example, at the same sample bias of -0.2 V the tunnel current between the tip and an atom in one of the pairs is much greater than for the bare Ni substrate. Now, Lang [7] has shown that for chlorine chemisorbed on simple metals the broadened 3p level of the chemisorbed Cl atom lies approximately 2.5 eV below the Fermi energy of the metal. It is therefore totally occupied and can be considered as ionic adsorption. For negative biases, tunnelling will proceed mainly from the hlled portion of the tail of the broadened 3p Cl-ion level into empty states of the tip. Since the presence of the Cl 3p level only affects the filled sample states and not the empty sample states, it will cause an increase in the tunnel current at negative biases. We can therefore say that we are directly imaging chlorine atoms. This is an important result, since it will enable us to spectroscopically differentiate between substrate and adsorbate, thus making tracking of the Cl atoms across the Ni surface possible.
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T. W. Fishlock et al. 1 Surface Science 377-379
Fig. 2b corresponds to a corrugation profile taken along line G-H in Fig. 1. It is interesting to note that the onset of the double periodicity in the [ liO] direction occurs between the terminal Ni at the [OOl] step edge and the first adsorbed chlorine. We are not, however, limited to a topographic determination of this fact, because spectroscopic data also enables us to chemically resolve the step edge species. For higher chlorine doses we observed that the double strings of chlorine atoms extend outward from the [OOl] step edge, again each double string being one lattice spacing out of registry with the adjacent string. We have also investigated significantly higher coverages (fractions of a monolayer). Fig. 3 is an image recorded after a chlorine exposure of 10m7mbarss and a subsequent anneal to 300°C. The chlorine pairs are again evident, which gives credence to the fact that they are the basic unit for chlorine adsorption on Ni( 110). There are three points of interest to note. Firstly, the bright pairs appear to have identical geometry to the
(1997) 629633
pairs seen at the step edges in the low-dose images. They are again oriented with their long axes along the [OOl] direction, and are 3.5 A (one lattice spacing) apart. Secondly, the substrate underlying the brighter pairs is not a (1 x 1) reconstruction, and thirdly there is a different reconstruction in the top right-hand corner of Fig. 3. The LEED pattern associated with this high chlorine exposure had strong spots at (3, t), (3, 3) and ($, +), the intensity of the (3, 3) spot being the greatest. Vector analysis leads us to believe that the pattern corresponds to two different surface reconstructions. The (3, 3) spots are indicative of a c(2 x 2) reconstruction which correlates with the reconstruction at the top-right hand corner of Fig. 3. The (3, $), (f, 3) and (+, 3) spots, on the other hand, correspond to a c(2 x 4) reconstruction which, we believe from analysis of Fig. 3 and other images, is the reconstruction of the underlying substrate beneath the bright on-top pairs.
4. Conclusions
Fig. 3. 200 A x 200 A image of Ni( 110) after a chlorine exposure of 10m7 mbar*s (at lo-* mbai chlorine pressure) and subsequent anneal to 300°C. Discrete chlorine atom pairs with the same geometry as in Fig. 1 are seen. c(2 x 2) reconstruction is also visible at the top right-hand corner of the image. The crystal is in the same orientation as Fig. 1.
We have studied the interaction of Cl with the Ni( 110) surface at room temperature and after annealing to 300°C. It is found that even though chlorine appears to chemisorb dissociatively, there is still some apparent interaction between the Cl atoms causing formation of Cl “atom pairs”. Assignment of the imaged pairs as Cl was made by analysis of atomically resolved STS data. Comparison of the I/V plots of the Ni substrate and an adsorbed pair revealed the existence of a greater density of filled states for the pair. Theoretical work by Lang and Williams [7] has placed the Cl 3p state approximately 2.5 eV below the Fermi energy, creating a greater filled DOS. Therefore, the larger filled DOS for the adsorbed Cl tallies well with the observed increase in tunnelling current over an adsorbed pair at negative biases. For low chlorine exposures we have demonstrated the formation of strings of Cl pairs directed along the [ 1701 direction. The strings propagate from [OOl] step edges, but also form on the Ni terraces. The strings on the terraces are only
T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
partially stable at room temperature and diffuse completely to step edges at elevated temperatures. It would appear that at low enough gap resistance, the tip can also induce motion of the terrace strings, therefore strengthening the possibility of controlled atom manipulation of this system. For progressively higher chlorine exposures, elongation of the Cl strings is observed. At still higher exposures more complex structures are imaged, most still involving the basic Cl atom pairs. c( 2 x 2) and c( 2 x 4) chlorine reconstructions have been imaged and correlate well with the corresponding LEED patterns.
633
References [ 11 P. Avouris, Act. Chem. Res. 28 (1995) 95. [2] A.R.H. Clarke, J.B. Pethica, J.A. Nieminen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. Lett. 76 (1996) 1276. [3] Z. Shuxian, J. Minrong, W. Jianxin and K. Wandelt, Surf. Sci 2511252 (1991) 759. [4] N.D. Spencer, P.J. Goddard, P.W. Davies, M. Kitson and R.M. Lambert, J. Vat. Sci. Technol. A 1 (1983) 1554. [S] J.H. Schott and H.S. White, J. Phys. Chem. 98 (1994) 291. [6] R.M. Feenstra, J.A. Stroscio and A.P. Fein, Surf. Sci. 181 (1987) 295. [7] N.D. Lang and A.R. Williams, Phys. Rev. B 18 (1978) 616.
Surface Science 377-379 (1997) 629-633
Interaction of chlorine with nickel (110) studied by scanning tunnelling microscopy T.W. Fishlock
a~*, J.B. Pethica
a, F.H. Jones b, R.G. Egdell b, J.S. Foord b
aDepartment of Materials, Oxford University, Parks Road, Oxford OX1 3PH, UK b New Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QT, UK Received 1 August 1996; accepted for publication 9 September 1996
Abstract
The role of forces in STM image formation has been established from studies on copper and nickel, and the resulting potential for controlled atom movement has generated interest in STM imaging of low coverages of halogens on these surfaces. Here we report atom-resolved images of clean and chlorine-covered nickel (110) surfaces over a range of chlorine coverages. All the images show that chlorine dissociates on the (110) surface into chemisorbed atom pairs oriented along the [OOl] direction. The atom-atom separation is comparable to the bulk nickel lattice constant. For very low chlorine exposures, strings of chlorine pairs running in the [ iTO] direction are observed, propagating from [OOl] step edges onto the terraces. High exposures of chlorine produce a number of different co-existing reconstructions, which will be discussed. Keywords: Chemisorption; Chlorine; Low energy electron diffraction (LEED); Nickel; Scanning tunnelling microscopy; Scanning tunnelling spectroscopies; Surface relaxation and reconstruction
1. Introduction
The controlled and reproducible modification of surfaces at the atomic scale in STM is of interest not only for potential technological applications, but also for the investigation of physical and chemical processes on a very local scale [l]. In order to gain such precise control, information on the forces required to initiate atomic motion is essential. The role of such forces in STM imaging has been established for Cu and Ni surfaces. Analysis by Clarke et al. [2] of the variation of corrugation height with tunnel current has given insight into the imaging mechanism, and coupled * Corresponding author. e-mail: [email protected]
with MD simulations, provides a method of assessing the tip-surface forces just before they induce actual motion of atoms on the surface. The potential for controlled atom manipulation at room temperature has generated interest in STM imaging of low coverages of halogens on copper and nickel surfaces. Here we present a scanning tunnelling microscopy study of the interaction of chlorine with Ni( 110) at room temperature. Atomically resolved images were recorded after both low and high chlorine exposures. Low-exposure images coupled with atom-resolved scanning tunnelling spectroscopy (STS) data suggest that chlorine is imaged directly by the STM. In agreement with Shuxian et al. [3], we show that Cl, chemisorbs dissociatively on the Ni(ll0) surface, but we also show
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01476-8
630
T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
that the Cl atoms retain short-range order or correlation. They form a basic structural unit imaged as a pair of STM greyscale maxima 3.5 A apart. The pairs initially form at [OOl] step edges and then grow further in the form of strings directed in the [ liO] direction. Strings are also seen to form directly on the Ni terraces, but appear only marginally stable at room temperature against migration to step edges. Images recorded after higher exposures of chlorine indicate a number of different reconstructions, within which the basic Cl atom-pair unit remains.
2. Experimental The experiments were performed with a commercial Omicron UHV-STM. The STM is mounted in an ion- and turbomolecular-pumped UHV chamber with a base pressure of 5 x lo-l1 mbar. The UHV chamber is equipped with facilities for X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and argon-ion sputtering. An existing solid-state electrochemical Ag/AgCl/Pt cell is used as a UHV-compatible molecular chlorine source [4]. The cell currents used are typically 0.1-0.2 mA. The Ni crystal is mounted on a tantalum sample-holder, and annealing is performed by radiative heating on the rear side of the mount. The Ni( 110) crystal, a semicircle 10 mm in diameter and 3 mm thick, was mechanically polished with diamond paste down to 0.25 ,um. The initial cleaning of the surface was achieved with repeated Ar sputtering (1.5 kV, 30 min) and annealing cycles (7OO”C,20 min) until good-quality LEED patterns indicated a well-ordered (1 x 1) surface, and no impurities could be detected by XPS. Large, flat terraces of the clean (1 x 1) lattice were easily imaged by STM.
3. Results and discussion The exposure of the clean Ni(ll0) surface to a low chlorine exposure results in the formation of an adsorbed species imaged as a discrete pair of maxima 3.5 A apart. As will be discussed below,
the pairs are interpreted as chemisorbed Cl atoms imaged directly by the STM. The pairs are oriented with their long axis along the [OOl] direction and appear to be the basic structural unit forming the chlorine overlayer. Fig. 1 shows an STM image of the Ni( 110) surface after a chlorine dose of 5 x lo-’ mbar*s and a subsequent anneal to 300°C. The discrete pairs are visible at the [OOl] step edge. The pairs are seen to form chains running in the close-packed [ liO] direction, indicated at points A and B in Fig. 1. The chains appear at a height of 0.3 A above the Ni substrate surface. Strings of chlorine pairs have been imaged before on metal surfaces. Schott et al. [ 51 studied chlorine adsorption on a Ag( 111) surface, and observed a similar parallel doublerow structure. In the present work, strings are seen to propagate from [OOl] step edges, but also form directly on the Ni terraces. In both cases the
Fig. 1. 200 A x 200 A image of Ni( 110) after a chlorine exposure of 5 x 10-9mbar*s (at 10m9mbar chlorine pressure) and subsequent anneal to 300°C (4nA tunnelling current and -0.01 V sample bias). Strings of chlorine pairs growing out from the [OOl] step edges are indicated at points A and B. Adjacent strings are one lattice spacing out of registry in the [OOl] direction, highlighted at points E and F. Points C and D and points G and H refer to the spectroscopy and corrugation profile shown in Fig. 2. Line G-H runs along the close-packed [ 1101 direction.
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T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
distance between the pairs in the [liO] direction is 5 A, or twice the lattice spacing. Note that these pairs are not due to a tip-structure artefact. We have obtained numerous such images, with different tips and at different times. The pair and its orientation is always a feature in the image. The stability of the strings on the terraces appears to be low. Chlorine strings seen on the terraces at room temperature after the initial exposure are absent when the surface is imaged again after the sample had been heated to 300°C. This suggests diffusion of the adsorbed atoms to the step edges. Indeed, after the sample had been heated, the terraces were almost completely clear of adsorbates, but the pairs were still present at the [OOl] step edges. The terrace strings also seemed susceptible to tip-induced motion at room temperature. Imaging above a threshold value of the tunnel current (> 5 nA at - 0.01 V sample bias), the position of the chlorine strings on the terrace varies from scan to scan, and sometimes the adatoms are not imaged at all. This seems to indicate a large tipadsorbate interaction resulting in the movement of the adsorbed chlorine. Fig. 1 also shows the initial stages of longerrange order between the Cl pair strings. Here interest focuses on the strings marked E and F in Fig. 1. Two types of longer-range order are evident. Firstly there is double spacing of the pairs along the [ 1701 direction. As mentioned earlier, the pairs in the strings are separated by 5 A, i.e. two lattice spacings in the [ liO] direction. Secondly, adjacent chlorine pairs are out of registry in the [OOl] direction. There is in effect a doubling of the periodicity in both the [OOl] and [ 1701 directions. This apparent reluctance for the pairs to reside in immediately adjacent sites would seem to suggest the presence of longer-range strain fields generated by the interaction of the adsorbed chlorine with the underlying Ni substrate. Assignment of the imaged pairs as chemisorbed chlorine is made tentatively by means of scanning tunnelling spectroscopy data. By interrupting the feedback loop, and keeping the tunnelling gap width fixed and applying a voltage ramp on the tunnelling junction, the tunnel current as a function of bias provides information about the sample density of states (DOS) [ 61. 1/I’ spectra from
-0.8-0.4 Sample
0.0
0.4
Bias
(V)
0.8 Oistance packed
along close direction
Fig. 2. (a) Current-voltage (I/V) curves for chlorine on Ni( 110). The dotted line corresponds to a step-edge pair (site-type C, Fig. l), and the solid line corresponds to bare Ni (site-type D, Fig. 1). The current is limited to 50 nA at high sample bias. (b) Corrugation pro6le along the close-packed [ 1701 direction taken across a [OOl] step edge (line G-H, Fig. 1). The maximum at about 20A corresponds to the chemisorbed chlorine atom two lattice spacings (5 A) removed from the terminal Ni atom.
single atom sites (C and D in Fig. 1) are shown in Fig. 2a. The plot shows that at positive bias (empty sample states) the relationship between tunnel current and sample bias for site-type C (step-edge pair) is the same as it is for site-type D (Ni substrate). This suggests that the empty DOS of the two species are similar. However, at negative sample biases (filled sample states) there is a distinct difference in the plot between the two species. For example, at the same sample bias of -0.2 V the tunnel current between the tip and an atom in one of the pairs is much greater than for the bare Ni substrate. Now, Lang [7] has shown that for chlorine chemisorbed on simple metals the broadened 3p level of the chemisorbed Cl atom lies approximately 2.5 eV below the Fermi energy of the metal. It is therefore totally occupied and can be considered as ionic adsorption. For negative biases, tunnelling will proceed mainly from the hlled portion of the tail of the broadened 3p Cl-ion level into empty states of the tip. Since the presence of the Cl 3p level only affects the filled sample states and not the empty sample states, it will cause an increase in the tunnel current at negative biases. We can therefore say that we are directly imaging chlorine atoms. This is an important result, since it will enable us to spectroscopically differentiate between substrate and adsorbate, thus making tracking of the Cl atoms across the Ni surface possible.
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T. W. Fishlock et al. 1 Surface Science 377-379
Fig. 2b corresponds to a corrugation profile taken along line G-H in Fig. 1. It is interesting to note that the onset of the double periodicity in the [ liO] direction occurs between the terminal Ni at the [OOl] step edge and the first adsorbed chlorine. We are not, however, limited to a topographic determination of this fact, because spectroscopic data also enables us to chemically resolve the step edge species. For higher chlorine doses we observed that the double strings of chlorine atoms extend outward from the [OOl] step edge, again each double string being one lattice spacing out of registry with the adjacent string. We have also investigated significantly higher coverages (fractions of a monolayer). Fig. 3 is an image recorded after a chlorine exposure of 10m7mbarss and a subsequent anneal to 300°C. The chlorine pairs are again evident, which gives credence to the fact that they are the basic unit for chlorine adsorption on Ni( 110). There are three points of interest to note. Firstly, the bright pairs appear to have identical geometry to the
(1997) 629633
pairs seen at the step edges in the low-dose images. They are again oriented with their long axes along the [OOl] direction, and are 3.5 A (one lattice spacing) apart. Secondly, the substrate underlying the brighter pairs is not a (1 x 1) reconstruction, and thirdly there is a different reconstruction in the top right-hand corner of Fig. 3. The LEED pattern associated with this high chlorine exposure had strong spots at (3, t), (3, 3) and ($, +), the intensity of the (3, 3) spot being the greatest. Vector analysis leads us to believe that the pattern corresponds to two different surface reconstructions. The (3, 3) spots are indicative of a c(2 x 2) reconstruction which correlates with the reconstruction at the top-right hand corner of Fig. 3. The (3, $), (f, 3) and (+, 3) spots, on the other hand, correspond to a c(2 x 4) reconstruction which, we believe from analysis of Fig. 3 and other images, is the reconstruction of the underlying substrate beneath the bright on-top pairs.
4. Conclusions
Fig. 3. 200 A x 200 A image of Ni( 110) after a chlorine exposure of 10m7 mbar*s (at lo-* mbai chlorine pressure) and subsequent anneal to 300°C. Discrete chlorine atom pairs with the same geometry as in Fig. 1 are seen. c(2 x 2) reconstruction is also visible at the top right-hand corner of the image. The crystal is in the same orientation as Fig. 1.
We have studied the interaction of Cl with the Ni( 110) surface at room temperature and after annealing to 300°C. It is found that even though chlorine appears to chemisorb dissociatively, there is still some apparent interaction between the Cl atoms causing formation of Cl “atom pairs”. Assignment of the imaged pairs as Cl was made by analysis of atomically resolved STS data. Comparison of the I/V plots of the Ni substrate and an adsorbed pair revealed the existence of a greater density of filled states for the pair. Theoretical work by Lang and Williams [7] has placed the Cl 3p state approximately 2.5 eV below the Fermi energy, creating a greater filled DOS. Therefore, the larger filled DOS for the adsorbed Cl tallies well with the observed increase in tunnelling current over an adsorbed pair at negative biases. For low chlorine exposures we have demonstrated the formation of strings of Cl pairs directed along the [ 1701 direction. The strings propagate from [OOl] step edges, but also form on the Ni terraces. The strings on the terraces are only
T. W. Fishlock et al. / Surface Science 377-379 (1997) 629-633
partially stable at room temperature and diffuse completely to step edges at elevated temperatures. It would appear that at low enough gap resistance, the tip can also induce motion of the terrace strings, therefore strengthening the possibility of controlled atom manipulation of this system. For progressively higher chlorine exposures, elongation of the Cl strings is observed. At still higher exposures more complex structures are imaged, most still involving the basic Cl atom pairs. c( 2 x 2) and c( 2 x 4) chlorine reconstructions have been imaged and correlate well with the corresponding LEED patterns.
633
References [ 11 P. Avouris, Act. Chem. Res. 28 (1995) 95. [2] A.R.H. Clarke, J.B. Pethica, J.A. Nieminen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. Lett. 76 (1996) 1276. [3] Z. Shuxian, J. Minrong, W. Jianxin and K. Wandelt, Surf. Sci 2511252 (1991) 759. [4] N.D. Spencer, P.J. Goddard, P.W. Davies, M. Kitson and R.M. Lambert, J. Vat. Sci. Technol. A 1 (1983) 1554. [S] J.H. Schott and H.S. White, J. Phys. Chem. 98 (1994) 291. [6] R.M. Feenstra, J.A. Stroscio and A.P. Fein, Surf. Sci. 181 (1987) 295. [7] N.D. Lang and A.R. Williams, Phys. Rev. B 18 (1978) 616.