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Atomic-resolution images of radiation damage in KBr
Surface Science 474 (2001) L197±L202
www.elsevier.nl/locate/susc
Surface Science Letters
Atomic-resolution images of radiation damage in KBr R. Bennewitz a,*, S. Sch ar a, V. Barwich a, O. Pfeier a, E. Meyer a, F. Krok b, B. Such c, J. Kolodzej c, M. Szymonski c a b
Department of Physics and Astronomy, Institute of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland Regional Laboratory of Physicochemical Analyses and Structural Research, Jagellonian University, Ingardena 3, 30-060 Krakow, Poland c Institute of Physics, Jagellonian University, Reymonta 4, 30-059 Krakow, Poland Received 24 July 2000; accepted for publication 11 November 2000
Abstract The ®rst steps of electron irradiation induced modi®cation of a KBr(1 0 0) surface have been studied by dynamic force microscopy with atomic resolution. Rectangular pits of monatomic depth with not more than one kink site per pit have been found. The atomic structure of KBr(1 0 0) is preserved at the bottom of the pits. Possible deexcitation and desorption mechanisms are discussed based on these results. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Electron stimulated desorption (ESD); Alkali halides; Surface defects
The surface topography of alkali halide crystals that have been exposed to ionizing irradiation can exhibit structures, which re¯ect the high symmetry of the lattice structure [1±5]. The development of such structures may help to understand the underlying deexcitation and desorption processes, as it for example turns out that there is a close connection between surface topography and desorption yield [6]. Furthermore, the radiation-induced structures form a high density of well-de®ned steps at the surface, which play an important role for the growth of overlayers. Well-de®ned defect structures at surfaces of insulators on a nanometer scale
*
Corresponding author. Tel.: +41-61-267-3725; fax: +41-61267-3784. E-mail address: [email protected] (R. Bennewitz).
would allow a controlled nucleation of metal clusters. With the development of dynamic force microscopy (DFM), the study of surfaces of insulators became feasible even in atomic resolution [7,8]. In this paper, we report on a DFM study of electron-irradiated KBr(1 0 0) surfaces, where images of the ®rst steps of radiation damage in atomic resolution could be obtained. After a short introduction into the experimental procedures we present the results together with a discussion of the peculiarities of imaging small structures by DFM. In the ®nal discussion we compare the results to models of the desorption process. Single crystals of pure KBr were purchased from Kelpin, Neuhausen (Germany). Two springlike tungsten wires, which are pulled through two pits in the crystal, clamp it to a sample holder plate. Crystals cleaved in air were transferred to the vacuum chamber within 30 s after cleavage, and
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the chamber was immediately evacuated. Subsequently, the crystals were heated to 430 K for 30 min in order to desorb water layers. Crystals cleaved in ultra-high vacuum (UHV) were heated to 450 K for 30 min to remove charges built up in the cleavage process. The crystals were irradiated with the electron beam of a standard LEED electron source at an energy of 1 keV and with a beam diameter of 2 mm2 . A blue luminescence excited by the electron beam helps to adjust the electron beam. The current density exhibits a strong variation over the beam pro®le. For the results presented in this study, surfaces were irradiated for 5 s at a sample temperature of 430 K. The crystal surfaces were studied by means of DFM in UHV. The method and the home-built instrument have been described in detail previously with emphasis on dierent types of acting forces [9], on imaging at steps [10], and on imaging of NaCl thin ®lms together with a theoretical description of the imaging process [11]. In summary, a tip mounted to the end of a microfabricated cantilever is oscillated with constant amplitude A above the surface at its mechanical eigenfrequency. Forces between tip and surface cause a shift of this eigenfrequency, which is used to regulate the tipsample distance. Recording images at constant frequency shift produces maps of constant mean force between tip and sample. For homogeneous surfaces, such maps represent the surface topography in good approximation. The cantilever used in the present study had a spring constant of k 24 N/m, an eigenfrequency of f0 164 740 Hz, and a quality factor of Q 33 000. All images were recorded at room temperature with an oscillation amplitude of A 12 nm, the respective frequency shifts are given in the ®gure captions. The characteristic topography of the KBr(1 0 0) surface after irradiation with electrons is depicted in Fig. 1 on a 100-nm scale. The upper frame shows the DFM image of a surface, which had been cleaved in air. The electron irradiation removed approximately 20% of the uppermost monolayer in form of irregularly distributed rectangular pits. By comparing this image with images in Ref. [4] we can estimate the electron dose to be about 10 lC/cm2 , or about 6000 electrons on the imaged area. Atomically straight steps along
Fig. 1. Images of KBr(1 0 0) surfaces cleaved in (a) air and (b) UHV after irradiation with electrons (1 keV, 5 s, current density see text). Side length of the frames is 100 nm, the frequency shifts Df 9 Hz and Df 8 Hz, respectively. Atomic kink sites at steps are encircled.
h1 0 0i-type directions terminate nearly all pits with some rare single-atom kink sites, some of which are encircled in Fig. 1. Strikingly, only one if any kink site is observed in by far most of the pits. The shallow irregular elevations in the upper part of
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the frame are probably caused by an unidenti®ed irradiation damage below the surface, for example clustered defects. Apart from these irradiationproduced features the surface appears atomically ¯at and free of any irregularities, although it was cleaved in air. This is in contrast to ®ndings for the (1 1 1) surface of CaF2 where irreversible damage on nanometer-scale has been detected after cleavage in air [12]. Fig. 1b shows a DFM image of a similarly irradiated surface, however, cleaved in UHV. The occurrence of cleavage steps in this image re¯ects the signi®cantly higher step density compared to air-cleaved surfaces. The characteristic rectangular pits produced by the irradiation are the same as on the air-cleaved surface. Additional to the observations made on air-cleaved surfaces, the UHV-cleaved surfaces exhibit rectangular islands of monatomic height, which we believe are islands produced in the cleavage process and straightened by the eects of irradiation. In Fig. 2 we present DFM images showing the atomic periodicity of the KBr(1 0 0) surface. The surface imaged in Fig. 2a has been cleaved in air and heated in UHV as described above, while the surface in Fig. 2b was cleaved in UHV and electron irradiated. On both surfaces atomic periodicity was obtained on arbitrarily chosen spots, proving that indeed both ways of preparation produce a nearly perfect surface structure. In the surface shown in Fig 2b, the irradiation produced a rectangular pit of 10 missing KBr molecules. In Fig. 2c, cross-sections along the lines indicated in the images are plotted to quantify the atomic corrugation obtained in these experiments. The corrugation height of about 0.1 nm found in Fig. 2a is a value quite typical for imaging surfaces of ionic crystals [13]. Increasing the negative frequency shift would provoke instability and result in irreversible changes of tip and surface. The image in Fig. 2b, recorded with signi®cantly less negative frequency shift, exhibits a smaller atomic corrugation height of about 0.05 nm. However, a corrugation height comparable to Fig. 2a is found for the atoms forming the step edge of the pit. The enhanced corrugation at steps can be explained by both a stronger force and an easier deformability of these sites of lower crystallographic
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Fig. 2. High-resolution images of KBr(1 0 0) surfaces. The periodicity in these images is close to that of equally charged ions on the KBr(1 0 0) surface, i.e. 0.66 nm distance in the h0 0 1i direction. Deviations from the perfect symmetry are due to piezo creep. The surface in (a) was cleaved in air (side length 5 nm), the one in (b) was cleaved in UHV and irradiated with low-energy electrons (side length 4.86 nm). The cross in (c) sections reveal the atomic corrugation at Df 52 Hz in (a), Df 10 Hz in (b). Further increase of the frequency shift results in oscillation instabilities, in the case of frame (b) at the edge atoms of the pit.
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Fig. 3. Series of high-resolution images of small irradiationproduced pits in a KBr(1 0 0) surface. The images have been slightly ®ltered in order of suppress long-term vertical drift and pixel noise. The eect of such data processing can be estimated by comparing image f with the original data presented in Fig. 2b. The side lengths of the images and the number of KBr molecules removed by the irradiation are: (a) 7.1 nm, 34 molecules; (b) 7.2 nm, 54 molecules; (c) 6.0 nm, two molecules; (d) 4.7 nm, 12 molecules; (e) 8.1 nm, 10 molecules; (f) 4.5 nm, 10 molecules; (g) 8.6 nm, 24 molecules; (h) 7 nm, 10 molecules.
coordination [11]. Imaging at larger negative frequency shifts would yield a better atomic contrast on the terrace but would result in instabilities at the steps. The pronounced appearance of the step atoms allows one to easily analyze of the size of pits. Fig. 3 is a compilation of eight images recorded around irradiation-produced pits with atomic resolution. In the center of the ®gure, the atomic structure of the pits is schematically depicted as it was found in a close inspection of the step atoms. A ®nding already discussed for Fig. 1 is con®rmed for these small pits: the steps are straight along the h1 0 0i-type directions and not more than one kink site in steps is found per pit. Furthermore, the kink sites are always close to the corner of the pits. Although the statistic signi®cance is low based on eight images, we would like to point at the fact that all pits are formed by an even number of missing molecules. The atomic
Fig. 4. High-resolution image of an irradiation-induced pit in the KBr(1 0 0) surface, the frame size is 10 6:4 nm2 , Df 13 Hz. The depth of the pit is one atomic layer. The lower frame is a high-pass ®ltered image of the upper one, revealing the same atomic contrast on the terrace and in the pit. The white bar indicates the phase shift between rows of protrusions in adjacent (1 0 0) layers.
periodicity at the bottom of a pit is shown in Fig. 4. The contrast on the terrace and on the bottom of the pit appears the same. The white line drawn along the h1 1 0i direction runs on top of protrusions on the terrace, and between protrusions on the bottom of the pit. This shift of the atomic p lattice between adjacent atomic layers by 1= 2 lattice constants perpendicular to the h1 1 0i direction is inherent to the (1 0 0) surface of the rocksalt structure. We conclude that the bottom of the pits is formed by the KBr(1 0 0) surface perfectly restored with respect to structure and stoichiometry. Some uncertainty remains about the structure in the pit very close to the step edge, where the ®nite sharpness of the tip makes atomic resolution impossible. As a further experimental result, a bridge between two pits of only four-atom width is
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Fig. 5. High-resolution image of a small bridge between two pits of monatomic depth, the side length of the frame is 11.7 nm, Df 8 Hz. The insert depicts the structure of this fouratom wide atomic stripe.
shown in Fig. 5. With this example, we would like to demonstrate the stability of the radiationproduced structures reported here. While metal surfaces at room temperatures exhibit serious step ¯uctuation on the scale of a few atoms, for this KBr surface we observe structures of atomic dimensions, which are stable at least over several days and under repeated imaging by the DFM. The increase of atomic contrast on the bridge compared to the surrounding terraces is caused by the reduced contribution of long-range forces to the total force sensed by the tip [10]: due to the larger distance of the mesoscopic tip to the bottom of the pits left and right of the bridge, the total interaction between tip and surface is reduced and the tip comes closer to the surface in order to maintain the constant frequency shift. Thus, the contrast produced by the short-range interaction between the atoms at the tip apex and single atoms of the bridge is enhanced. In the following, we discuss the results with respect to desorption models. The damage of the KBr(1 0 0) surface by short electron irradiation as reported above is limited to rectangular pits of
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monatomic depth. The preservation of stoichiometry can be deduced from our DFM experiments with atomic resolution, which detect the characteristic contrast of the pure KBr(1 0 0) surface even at the bottom of pits. This result can be understood in terms of irradiation-induced defect processes, which have been recently described by Such et al. [6] based on well established models for electronic excitation in alkali halides, and a detailed theoretical study by Puchin et al. [14]. According to this description, the energy of crystal electronic excitations can be converted into pairs of F- and H-centers. When an H-center, essentially a halogen atom on an interstitial lattice site, diuses to the surface, the halogen atom is desorbed leaving behind the perfect surface. In contrast, F-centers (cation vacancies ®lled with an electron) hitting the surface do not necessarily initiate a desorption process. Only in their excited state (F -centers), they can cause desorption of an anion atom, resulting in a molecular vacancy at the surface. Therefore, both desorption processes preserve the stoichiometry of the surface even on the atomic scale. The aggregation of F-centers at the surface could not be observed on the surface under study here. Puchin et al. [14] found that the energy of an F-center at the surface is higher than in the bulk and, therefore, F-centers may be re¯ected from the surface. Aggregates of F-centers below the surface could explain the blurred topographic elevations observed in Fig. 1a. The observation of single F-centers by DFM has not been demonstrated yet in spite of its obvious ability of atomic resolution, possibly due to a high mobility of surface F-centers compared to the slow scanning speed of DFM, but possibly also due to a weak contrast in DFM. Such et al. found a correlation between the step density of rectangular pits and the desorption yield and concluded that the desorption process is predominantly eective at steps. This conclusion is supported by the ®nding that the desorption of metal atoms by an F -centers at the surface is hindered by a large potential barrier, which vanishes for anions at corner or kink sites [14]. However, there is no study of the desorption process from normal step sites. The idea of a preferential desorption from step sites compared to terrace sites is supported by
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the results of the present study: although pits consisting of only two molecules are found to be stable, mostly much larger pits are observed, as consequence of the eective desorption from steps of existing pits. The edges of the pits are oriented along the h1 0 0i-type directions, with not more than one kink site per pit. In order to explain this shape one has to assume a diusion of molecular vacancies along the steps and even around the inner corners of the pit, before the resulting shape with maximal one kink site is obtained. As an easier explanation for the characteristic shape of pits we suggest to assume a predominant desorption from kink sites, which could directly account for the shape of the pits. Furthermore, this hypothesis could explain why by far most pits do not exhibit any kink site at all in their steps: this con®guration would be a stable one, before the desorption from a step site would start a new round of desorption from kink sites. The preferential desorption from kink sites could be supported by the transfer of electronic excitation towards sites with lowest coordination, as it has recently been described by Shluger et al. [14,15]. So far we have no conclusive explanation for the preliminary observation of even numbers of missing molecules in each pit. An atomistic desorption mechanism with a yield of two molecules cannot be conceived. Possibly, the symmetry of desorption sites plays a role in the explanation of this observation. In summary, radiation damage at the cleavage face of an alkali halide crystal has been observed with atomic resolution for the ®rst time. Desorption induced by electron excitation results in the formation of rectangular pits of monatomic depth with steps running straight along h1 0 0i-type directions. Comparing our results with previously discussed deexcitation processes, we suggest that predominant desorption from kink sites accounts for the highly symmetric shape of the pits. The electron-irradiated KBr(1 0 0) surface shows a high density of well-de®ned nanometer-scale structures which could ®nd an application as nucleation centers for a controlled growth of clusters or overlayers.
Acknowledgements We would like to acknowledge valuable discussions with A. Shluger and the continued support of H.-J. G untherodt for our work. This work was ®nancially supported by the Swiss National Science Foundation and the ``Kommission zur F orderung von Technologie und Innovation''. The Krakow group acknowledges ®nancial support by Polish Committee for Scienti®c Research under the project number 2P03B 13717.
References [1] H. H oche, J. Toennies, R. Vollmer, Phys. Rev. B 50 (1994) 679. [2] R. Wilson, R. Williams, Nucl. Instr. Meth. Phys. B 101 (1995) 122. [3] R. Bennewitz, D. Smith, M. Reichling, Phys. Rev. B 59 (1999) 8237. [4] B. Such, P. Czuba, P. Piatkowski, M. Szymonski, Surf. Sci. 451 (2000) 203. [5] J. Engelhardt, H. Dabringhaus, K. Wandelt, Surf. Sci. 448 (2000) 187. [6] B. Such, J. Kolodziej, P. Czuba, P. Piatkowski, P. Struski, F. Krok, M. Szymonski, Phys. Rev. Lett. 85 (2000) 2621. [7] M. Bammerlin, R. L uthi, E. Meyer, A. Barato, J. L u, M. Guggisberg, C. Gerber, L. Howald, H.-J. G untherodt, Probe Microsc. 1 (1997) 3. [8] M. Reichling, C. Barth, Phys. Rev. Lett. 83 (1999) 768. [9] M. Guggisberg, M. Bammerlin, C. Loppacher, O. Pfeier, A. Abdurixit, V. Barwich, R. Bennewitz, A. Barato, E. Meyer, H.-J. G untherodt, Phys. Rev. B 61 (2000) 11151. [10] M. Guggisberg, M. Bammerlin, A. Barato, R. L uthi, C. Loppacher, F. Battiston, J. L u, R. Bennewitz, E. Meyer, H.-J. G untherodt, Surf. Sci. 461 (2000) 255. [11] R. Bennewitz, A. Foster, L. Kantorovich, M. Bammerlin, C. Loppacher, S. Schar, M. Guggisberg, E. Meyer, H.-J. G untherodt, A. Shluger, Phys. Rev. B 62 (2000) 2074. [12] M. Reichling, M. Huisinga, S. Gogoll, C. Barth, Surf. Sci. 439 (1999) 181. [13] M. Bammerlin, R. L uthi, E. Meyer, A. Barato, J. L u, M. Guggisberg, C. Loppacher, C. Gerber, H.-J. G untherodt, Appl. Phys. A66 (1998) 293. [14] V. Puchin, A. Shluger, Y. Nakai, N. Itoh, Phys. Rev. B 49 (1994) 11364. [15] A. Shluger, P. Sushko, L. Kantorovich, Phys. Rev. B 59 (1999) 2417.
www.elsevier.nl/locate/susc
Surface Science Letters
Atomic-resolution images of radiation damage in KBr R. Bennewitz a,*, S. Sch ar a, V. Barwich a, O. Pfeier a, E. Meyer a, F. Krok b, B. Such c, J. Kolodzej c, M. Szymonski c a b
Department of Physics and Astronomy, Institute of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland Regional Laboratory of Physicochemical Analyses and Structural Research, Jagellonian University, Ingardena 3, 30-060 Krakow, Poland c Institute of Physics, Jagellonian University, Reymonta 4, 30-059 Krakow, Poland Received 24 July 2000; accepted for publication 11 November 2000
Abstract The ®rst steps of electron irradiation induced modi®cation of a KBr(1 0 0) surface have been studied by dynamic force microscopy with atomic resolution. Rectangular pits of monatomic depth with not more than one kink site per pit have been found. The atomic structure of KBr(1 0 0) is preserved at the bottom of the pits. Possible deexcitation and desorption mechanisms are discussed based on these results. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Electron stimulated desorption (ESD); Alkali halides; Surface defects
The surface topography of alkali halide crystals that have been exposed to ionizing irradiation can exhibit structures, which re¯ect the high symmetry of the lattice structure [1±5]. The development of such structures may help to understand the underlying deexcitation and desorption processes, as it for example turns out that there is a close connection between surface topography and desorption yield [6]. Furthermore, the radiation-induced structures form a high density of well-de®ned steps at the surface, which play an important role for the growth of overlayers. Well-de®ned defect structures at surfaces of insulators on a nanometer scale
*
Corresponding author. Tel.: +41-61-267-3725; fax: +41-61267-3784. E-mail address: [email protected] (R. Bennewitz).
would allow a controlled nucleation of metal clusters. With the development of dynamic force microscopy (DFM), the study of surfaces of insulators became feasible even in atomic resolution [7,8]. In this paper, we report on a DFM study of electron-irradiated KBr(1 0 0) surfaces, where images of the ®rst steps of radiation damage in atomic resolution could be obtained. After a short introduction into the experimental procedures we present the results together with a discussion of the peculiarities of imaging small structures by DFM. In the ®nal discussion we compare the results to models of the desorption process. Single crystals of pure KBr were purchased from Kelpin, Neuhausen (Germany). Two springlike tungsten wires, which are pulled through two pits in the crystal, clamp it to a sample holder plate. Crystals cleaved in air were transferred to the vacuum chamber within 30 s after cleavage, and
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the chamber was immediately evacuated. Subsequently, the crystals were heated to 430 K for 30 min in order to desorb water layers. Crystals cleaved in ultra-high vacuum (UHV) were heated to 450 K for 30 min to remove charges built up in the cleavage process. The crystals were irradiated with the electron beam of a standard LEED electron source at an energy of 1 keV and with a beam diameter of 2 mm2 . A blue luminescence excited by the electron beam helps to adjust the electron beam. The current density exhibits a strong variation over the beam pro®le. For the results presented in this study, surfaces were irradiated for 5 s at a sample temperature of 430 K. The crystal surfaces were studied by means of DFM in UHV. The method and the home-built instrument have been described in detail previously with emphasis on dierent types of acting forces [9], on imaging at steps [10], and on imaging of NaCl thin ®lms together with a theoretical description of the imaging process [11]. In summary, a tip mounted to the end of a microfabricated cantilever is oscillated with constant amplitude A above the surface at its mechanical eigenfrequency. Forces between tip and surface cause a shift of this eigenfrequency, which is used to regulate the tipsample distance. Recording images at constant frequency shift produces maps of constant mean force between tip and sample. For homogeneous surfaces, such maps represent the surface topography in good approximation. The cantilever used in the present study had a spring constant of k 24 N/m, an eigenfrequency of f0 164 740 Hz, and a quality factor of Q 33 000. All images were recorded at room temperature with an oscillation amplitude of A 12 nm, the respective frequency shifts are given in the ®gure captions. The characteristic topography of the KBr(1 0 0) surface after irradiation with electrons is depicted in Fig. 1 on a 100-nm scale. The upper frame shows the DFM image of a surface, which had been cleaved in air. The electron irradiation removed approximately 20% of the uppermost monolayer in form of irregularly distributed rectangular pits. By comparing this image with images in Ref. [4] we can estimate the electron dose to be about 10 lC/cm2 , or about 6000 electrons on the imaged area. Atomically straight steps along
Fig. 1. Images of KBr(1 0 0) surfaces cleaved in (a) air and (b) UHV after irradiation with electrons (1 keV, 5 s, current density see text). Side length of the frames is 100 nm, the frequency shifts Df 9 Hz and Df 8 Hz, respectively. Atomic kink sites at steps are encircled.
h1 0 0i-type directions terminate nearly all pits with some rare single-atom kink sites, some of which are encircled in Fig. 1. Strikingly, only one if any kink site is observed in by far most of the pits. The shallow irregular elevations in the upper part of
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the frame are probably caused by an unidenti®ed irradiation damage below the surface, for example clustered defects. Apart from these irradiationproduced features the surface appears atomically ¯at and free of any irregularities, although it was cleaved in air. This is in contrast to ®ndings for the (1 1 1) surface of CaF2 where irreversible damage on nanometer-scale has been detected after cleavage in air [12]. Fig. 1b shows a DFM image of a similarly irradiated surface, however, cleaved in UHV. The occurrence of cleavage steps in this image re¯ects the signi®cantly higher step density compared to air-cleaved surfaces. The characteristic rectangular pits produced by the irradiation are the same as on the air-cleaved surface. Additional to the observations made on air-cleaved surfaces, the UHV-cleaved surfaces exhibit rectangular islands of monatomic height, which we believe are islands produced in the cleavage process and straightened by the eects of irradiation. In Fig. 2 we present DFM images showing the atomic periodicity of the KBr(1 0 0) surface. The surface imaged in Fig. 2a has been cleaved in air and heated in UHV as described above, while the surface in Fig. 2b was cleaved in UHV and electron irradiated. On both surfaces atomic periodicity was obtained on arbitrarily chosen spots, proving that indeed both ways of preparation produce a nearly perfect surface structure. In the surface shown in Fig 2b, the irradiation produced a rectangular pit of 10 missing KBr molecules. In Fig. 2c, cross-sections along the lines indicated in the images are plotted to quantify the atomic corrugation obtained in these experiments. The corrugation height of about 0.1 nm found in Fig. 2a is a value quite typical for imaging surfaces of ionic crystals [13]. Increasing the negative frequency shift would provoke instability and result in irreversible changes of tip and surface. The image in Fig. 2b, recorded with signi®cantly less negative frequency shift, exhibits a smaller atomic corrugation height of about 0.05 nm. However, a corrugation height comparable to Fig. 2a is found for the atoms forming the step edge of the pit. The enhanced corrugation at steps can be explained by both a stronger force and an easier deformability of these sites of lower crystallographic
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Fig. 2. High-resolution images of KBr(1 0 0) surfaces. The periodicity in these images is close to that of equally charged ions on the KBr(1 0 0) surface, i.e. 0.66 nm distance in the h0 0 1i direction. Deviations from the perfect symmetry are due to piezo creep. The surface in (a) was cleaved in air (side length 5 nm), the one in (b) was cleaved in UHV and irradiated with low-energy electrons (side length 4.86 nm). The cross in (c) sections reveal the atomic corrugation at Df 52 Hz in (a), Df 10 Hz in (b). Further increase of the frequency shift results in oscillation instabilities, in the case of frame (b) at the edge atoms of the pit.
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Fig. 3. Series of high-resolution images of small irradiationproduced pits in a KBr(1 0 0) surface. The images have been slightly ®ltered in order of suppress long-term vertical drift and pixel noise. The eect of such data processing can be estimated by comparing image f with the original data presented in Fig. 2b. The side lengths of the images and the number of KBr molecules removed by the irradiation are: (a) 7.1 nm, 34 molecules; (b) 7.2 nm, 54 molecules; (c) 6.0 nm, two molecules; (d) 4.7 nm, 12 molecules; (e) 8.1 nm, 10 molecules; (f) 4.5 nm, 10 molecules; (g) 8.6 nm, 24 molecules; (h) 7 nm, 10 molecules.
coordination [11]. Imaging at larger negative frequency shifts would yield a better atomic contrast on the terrace but would result in instabilities at the steps. The pronounced appearance of the step atoms allows one to easily analyze of the size of pits. Fig. 3 is a compilation of eight images recorded around irradiation-produced pits with atomic resolution. In the center of the ®gure, the atomic structure of the pits is schematically depicted as it was found in a close inspection of the step atoms. A ®nding already discussed for Fig. 1 is con®rmed for these small pits: the steps are straight along the h1 0 0i-type directions and not more than one kink site in steps is found per pit. Furthermore, the kink sites are always close to the corner of the pits. Although the statistic signi®cance is low based on eight images, we would like to point at the fact that all pits are formed by an even number of missing molecules. The atomic
Fig. 4. High-resolution image of an irradiation-induced pit in the KBr(1 0 0) surface, the frame size is 10 6:4 nm2 , Df 13 Hz. The depth of the pit is one atomic layer. The lower frame is a high-pass ®ltered image of the upper one, revealing the same atomic contrast on the terrace and in the pit. The white bar indicates the phase shift between rows of protrusions in adjacent (1 0 0) layers.
periodicity at the bottom of a pit is shown in Fig. 4. The contrast on the terrace and on the bottom of the pit appears the same. The white line drawn along the h1 1 0i direction runs on top of protrusions on the terrace, and between protrusions on the bottom of the pit. This shift of the atomic p lattice between adjacent atomic layers by 1= 2 lattice constants perpendicular to the h1 1 0i direction is inherent to the (1 0 0) surface of the rocksalt structure. We conclude that the bottom of the pits is formed by the KBr(1 0 0) surface perfectly restored with respect to structure and stoichiometry. Some uncertainty remains about the structure in the pit very close to the step edge, where the ®nite sharpness of the tip makes atomic resolution impossible. As a further experimental result, a bridge between two pits of only four-atom width is
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Fig. 5. High-resolution image of a small bridge between two pits of monatomic depth, the side length of the frame is 11.7 nm, Df 8 Hz. The insert depicts the structure of this fouratom wide atomic stripe.
shown in Fig. 5. With this example, we would like to demonstrate the stability of the radiationproduced structures reported here. While metal surfaces at room temperatures exhibit serious step ¯uctuation on the scale of a few atoms, for this KBr surface we observe structures of atomic dimensions, which are stable at least over several days and under repeated imaging by the DFM. The increase of atomic contrast on the bridge compared to the surrounding terraces is caused by the reduced contribution of long-range forces to the total force sensed by the tip [10]: due to the larger distance of the mesoscopic tip to the bottom of the pits left and right of the bridge, the total interaction between tip and surface is reduced and the tip comes closer to the surface in order to maintain the constant frequency shift. Thus, the contrast produced by the short-range interaction between the atoms at the tip apex and single atoms of the bridge is enhanced. In the following, we discuss the results with respect to desorption models. The damage of the KBr(1 0 0) surface by short electron irradiation as reported above is limited to rectangular pits of
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monatomic depth. The preservation of stoichiometry can be deduced from our DFM experiments with atomic resolution, which detect the characteristic contrast of the pure KBr(1 0 0) surface even at the bottom of pits. This result can be understood in terms of irradiation-induced defect processes, which have been recently described by Such et al. [6] based on well established models for electronic excitation in alkali halides, and a detailed theoretical study by Puchin et al. [14]. According to this description, the energy of crystal electronic excitations can be converted into pairs of F- and H-centers. When an H-center, essentially a halogen atom on an interstitial lattice site, diuses to the surface, the halogen atom is desorbed leaving behind the perfect surface. In contrast, F-centers (cation vacancies ®lled with an electron) hitting the surface do not necessarily initiate a desorption process. Only in their excited state (F -centers), they can cause desorption of an anion atom, resulting in a molecular vacancy at the surface. Therefore, both desorption processes preserve the stoichiometry of the surface even on the atomic scale. The aggregation of F-centers at the surface could not be observed on the surface under study here. Puchin et al. [14] found that the energy of an F-center at the surface is higher than in the bulk and, therefore, F-centers may be re¯ected from the surface. Aggregates of F-centers below the surface could explain the blurred topographic elevations observed in Fig. 1a. The observation of single F-centers by DFM has not been demonstrated yet in spite of its obvious ability of atomic resolution, possibly due to a high mobility of surface F-centers compared to the slow scanning speed of DFM, but possibly also due to a weak contrast in DFM. Such et al. found a correlation between the step density of rectangular pits and the desorption yield and concluded that the desorption process is predominantly eective at steps. This conclusion is supported by the ®nding that the desorption of metal atoms by an F -centers at the surface is hindered by a large potential barrier, which vanishes for anions at corner or kink sites [14]. However, there is no study of the desorption process from normal step sites. The idea of a preferential desorption from step sites compared to terrace sites is supported by
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the results of the present study: although pits consisting of only two molecules are found to be stable, mostly much larger pits are observed, as consequence of the eective desorption from steps of existing pits. The edges of the pits are oriented along the h1 0 0i-type directions, with not more than one kink site per pit. In order to explain this shape one has to assume a diusion of molecular vacancies along the steps and even around the inner corners of the pit, before the resulting shape with maximal one kink site is obtained. As an easier explanation for the characteristic shape of pits we suggest to assume a predominant desorption from kink sites, which could directly account for the shape of the pits. Furthermore, this hypothesis could explain why by far most pits do not exhibit any kink site at all in their steps: this con®guration would be a stable one, before the desorption from a step site would start a new round of desorption from kink sites. The preferential desorption from kink sites could be supported by the transfer of electronic excitation towards sites with lowest coordination, as it has recently been described by Shluger et al. [14,15]. So far we have no conclusive explanation for the preliminary observation of even numbers of missing molecules in each pit. An atomistic desorption mechanism with a yield of two molecules cannot be conceived. Possibly, the symmetry of desorption sites plays a role in the explanation of this observation. In summary, radiation damage at the cleavage face of an alkali halide crystal has been observed with atomic resolution for the ®rst time. Desorption induced by electron excitation results in the formation of rectangular pits of monatomic depth with steps running straight along h1 0 0i-type directions. Comparing our results with previously discussed deexcitation processes, we suggest that predominant desorption from kink sites accounts for the highly symmetric shape of the pits. The electron-irradiated KBr(1 0 0) surface shows a high density of well-de®ned nanometer-scale structures which could ®nd an application as nucleation centers for a controlled growth of clusters or overlayers.
Acknowledgements We would like to acknowledge valuable discussions with A. Shluger and the continued support of H.-J. G untherodt for our work. This work was ®nancially supported by the Swiss National Science Foundation and the ``Kommission zur F orderung von Technologie und Innovation''. The Krakow group acknowledges ®nancial support by Polish Committee for Scienti®c Research under the project number 2P03B 13717.
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