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Nanoscale Electron Beam Damage Studied by Atomic Force Microscopy Sam M. Stevens,†,‡ Pablo Cubillas,† Kjell Jansson,‡ Osamu Terasaki,‡,§ and Michael W. Anderson*,† Centre for Nanoporous Materials, School of Chemistry, The UniVersity of Manchester, Chemistry Building, Oxford Road, Manchester M13 9PL, United Kingdom, Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, S-106 91, Stockholm, Sweden, and Graduate School of EEWS, WCU Energy Science & Engineering, KAIST, 335 Gwahangno, Yuseong-Gu, Daejeon 305-701, Republic of Korea ReceiVed: July 29, 2009; ReVised Manuscript ReceiVed: September 4, 2009
High-resolution scanning electron microscopy (HRSEM) has recently been added to the arsenal of characterization tools for material scientists to observe nanoscale surface features on both conducting and insulating materials. It is now therefore crucial to understand whether the intense electron beam will damage the features of interest. We have been able, for the first time, to measure and quantify this damage using a combination of HRSEM and atomic force microscopy (AFM), and as a consequence, we demonstrate that the bulk of the damage, expressed as a depression on the crystal surface, is confined primarily to a subsurface volume. Simulations demonstrate that the depth of the depression is proportional to the interaction volume of impact electrons below the crystal surface. More importantly, the nanometer surface features are conserved, and there is negligible associated loss of the critical information in nanoscopic surface topography. These results confirm the usefulness of HRSEM as a tool for surface analysis not only for scientists investigating crystal growth but also for materials scientists analyzing any surface at the nanoscale. High-resolution scanning electron microscopy (HRSEM) has recently been added to the arsenal of characterization tools for material scientists to observe nanoscale surface features on both conducting and insulating materials.1 However, as a consequence, it is crucial to understand whether the intense electron beam will damage the features of interest. We have been able, for the first time, to measure and quantify this damage using a combination of HRSEM and atomic force microscopy (AFM), and as a consequence, we demonstrate that the bulk of the damage is confined primarily to a subsurface volume. Beam damage occurs by a variety of mechanisms, all of which result in a loss of crystallinity of the specimen, and the collective phenomenon is therefore referred to as irradiationinduced crystalline-amorphous (C-A) transitions.2 With respect to beam damage of porous materials, considerable work has been done at transmission electron microscopy (TEM) accelerating voltages (100 kV or greater), including the observation of in situ C-A transitions in zeolites.3 However, of the very few literature examples of damage at SEM accelerating voltages, (30 kV or less), there are no reports on the condition of a sample surface topography after electron irradiation. To further assess and understand the nature of the surface damage on nanoporous materials, two zeotype crystals were chosen due to their importance in providing answers to understanding crystal growth through their surface features and submitted to a range of electron doses in a JEOL JSM-7401F * To whom correspondence should be addressed. E-mail: m.anderson@ manchester.ac.uk. † University of Manchester. ‡ Stockholm University. § KAIST.
10.1021/jp907245z CCC: $40.75
scanning electron microscope and a landing energy of 1 kV. The systems studied were (1) STA-7, an aluminophosphate zeotype with framework code SAV,4,5 exhibiting ∼30 µm single crystals with tetragonal morphology and molar, chemical composition 12Al2O3/7.2SiO2/8.4P2O5/xH2O, and (2) aluminosilicate zeolite A, exhibiting ∼10 µm cubic crystals and molar chemical composition Na2O/Al2O3/2SiO2/xH2O with frameowork code LTA.6 The crystal surfaces were oriented orthogonally with respect to the optic axis of the microscope and irradiated with an electron probe. This produced rectangular areas of contrast, as can been seen in the SEM image of a (100) surface of STA-7, where the irradiated areas appear as rectangles of brighter contrast, corresponding to the scanned area of interest, when viewed from a lower magnification (Figure 1b) that were absent before irradiation (figure 1a). The rectangular areas were analyzed ibidem (in the same place) by atomic force microscopy, and the height of the surface depressions were analyzed for a number of beam conditions using a similar method to a procedure in a previous communication.7 It is noteworthy that areas with very low electron exposure possess a rectangular contrast in the SEM but exhibit a zero height depression as measured by AFM. This establishes that contrast from contamination, another contributor to loss of information in a SEM image, is still present even at nondestructive electron doses. (Contamination is a commonly experienced but scarcely documented phenomenon where hydrocarbon contaminants, either from within the sample itself or from the ambient environment, are ionized by the electron beam and then precipitate on the scanned area. The result is a rectangle of contrast, usually darker than the surrounding area because of a 2009 American Chemical Society
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Letters when the beam is in stop mode, where the electron beam scans an extra amount of time while imaging is temporarily frozen. When viewed in the SEM, the contrast change of the damaged area and thick borders (caused by surface contamination and beam stop, respectively) imply that the surface undergoes chemical changes. The lack of distortion of the crystal surface, when measured by AFM, is therefore surprising. Despite a surface depression of up to 20 nm, there is no observable change in the nanometer step heights. This implies that the area of the crystal experiencing C-A transitions is sufficiently below the surface to leave it geometrically unaltered. This, along with the difference in the maximum depression depth between zeolite A and STA-7, can be explained from simulations of impact electrons and their interaction with the sample beneath the sample surface using the CASINO software.8 Scattering cross sections of both zeolite A and STA-7 are shown in Figure 2g and h, respectively. The red lines represent trajectories from back-scattered electrons, high-angle coulomb (Rutherford) scattering between impact electrons and the nuclei of the atoms in the material. As this is essentially an elastic collision, such events are not responsible for beam damage on the same scale as those produced by inelastically scattered electrons known as secondary electrons. It is the inelastic collisions occurring close to the surface that contribute to topographic contrast in the SEM as their length of attinuity (the depth from the surface of the crystal at which an electron is successfully able to escape) to the surface is no more than 10 nm. The majority of impact electrons pass much deeper into the sample before undergoing inelastic collisions. The result of these inelastic collisions is a loss of energy, and this is represented by the change of color in the electron’s trajectory from yellow (high energy) to blue (less energy). Again, it is these inelastic collisions which are responsible for the C-A transitions, and because they occur mainly at sufficient depth, the surface remains chemically and structurally intact.
Figure 1. HRSEM image of STA-7 before (a) and after (b) electron exposure at high magnification. Graphs depicting the change in surface depression as a function of (c) time of exposure for STA-7 and zeolite A (constant electron beam current of 60 pA) and (d) electron beam current for STA-7 (constant time of exposure of 60 s).
change in composition, and therefore a different electron yield relative to that of the sample.) At destructive doses, there is a correlation between the depth of the depression (difference in height between the damaged and undamaged surface) created by the beam damage with both time of exposure and probe current (Figures 1c and d, respectively). The depth increased with increasing time of electron exposure and increasing probe current to an eventual maximum depression of just under 20 and 15 nm for STA-7 and zeolite A, respectively, with the former crystals exhibiting a more rapid collapse. Figure 2 shows AFM images of irradiated samples of both zeolite A and STA-7. In both materials, the nanometer surface terraces are preserved with no discernible distortion in the irradiated area (Figure 2a-d). Figure 2f illustrates how the AFM cross section in Figure 2d is the result of translation of the surface in a direction orthogonal to the crystal face. The measured step heights within the damaged area differ by less than 5% (i.e., within the precision limit of the AFM) when compared to the step heights outside of the damaged area. The peaks and troughs at the edges of scanned areas (represented by yellow arrows) are formed from the scanning electron probe
The depth at which these interactions occurs is often depicted as a bell or tear shape, depending on conditions, the locus of which is known as the interaction volume.9 The interaction volume is therefore a volume in which beam damage occurs. As shown in Figures 2g and h, the interaction volume is calculated to be 33% larger in STA-7 than that in zeolite A, which should result in 33% more densification of the former material. This is corroborated by AFM measurements between STA-7 and zeolite A under analogous beam damage conditions; see Figure 1c. This uniform subsidence of the crystal surface and conservation of the nanometer features thereon confirms that while HRSEM is a destructive technique, there is negligible associated loss of the critical information in the surface topography. This result confirms the usefulness of HRSEM as a form of surface analysis not only for scientists investigating crystal growth but also for any materials scientist wishing to analyze any surface at the nanoscale. Acknowledgment. The authors thank The Knut and Alice Wallenberg Foundation, EC Marie-Curie Research Training Networks “INDENS” (MRTRN-CT-2002-005503), EPSRC, and ExxonMobil Research and Engineering for financial support, L. Itzel Meza for the zeolite A sample, and Marı´a Castro and Paul A. Wright at the University of St. Andrews for the STA-7 sample.
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Figure 2. AFM images and associated cross sections (represented by the green line) for both zeolite A (a and c) and STA-7 (b and d). Panels (e) and (f) refer to the cross section of STA-7 during scanning by the electron beam and a depiction of the resulting surface translation down into the crystal, respectively. Panels (g) and (h) are CASINO electron simulations of 4000 electron trajectories for both zeolite A and STA-7, respectively, at the beam conditions of the microscope (1 kV landing energy, 2 nm spot size). Red indicates back-scattered electrons, blue low energy, and yellow high energy electron trajectories.
References and Notes (1) (a) Che, S.; Lund, K.; Tatsumi, T.; Iijima, S.; Joo, S. H.; Ryoo, R.; Terasaki, O. Angew. Chem., Int. Ed. 2003, 42, 2182. (b) Loiola, A. R.; da Silva, L. R. D.; Cubillas, P.; Anderson, M. W. J. Mater. Chem. 2008, 18, 4985. (2) (a) Egerton, R. F.; Li, P.; Malac, M. Micron 2004, 35, 399. (b) Eby, R. K.; Ewing, R. C. J. Mater. Res. 1992, 7 (11), 3080. (3) Bursill, L. A.; Thomas, J. M.; Rao, K. J. Nature 1981, 289, 157. (4) (a) Wright, P. A.; Maple, M. J.; Slawin, A. M. Z.; Parinec, V.; Aitken, R. A.; Welsch, S.; Cox, P. A. J. Chem. Soc., Dalton Trans. 2000, 1243. (b) Wright, P. A. In Atlas of Zeolite Framework Types 5th ed.; Baerlocher, Ch., Meier, W. M., Olson, D. H., Eds.; Elsevier: Amsterdam, The Netherlands, 2001.
(5) Castro, M.; Garcia, R.; Warrender, S. J.; Slawin, A. M. Z.; Wright, P. A.; Cox, P. A.; Fecant, A.; Mellot-Draznieks, C.; Bats, N. Chem. Commun. 2007, 33, 3470. (6) Charnell, J. F. J. Cryst. Growth 1971, 8, 291. (7) Stevens, S. M.; Cubillas, P.; Jansson, K.; Terasaki, O.; Anderson, M. W.; Wright, P. A.; Castro, M. Chem. Commun. 2008, 3894. (8) Drouin, D.; Couture, A. R.; Joly, D.; Tastet, X.; Aimez, V.; Gauvin, R. Scanning 2007, 29 (3), 92. (9) Reimer, L. Image Formation in Low-Voltage Scanning Electron Microscopy; SPIE: Bellinham, WA, 1993.
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