Electron Microscopy and Diffraction Techniques for the Study of Small

28 Electron Microscopy and Diffraction Techniques for the Study of Small Particles

Downloaded by TUFTS UNIV on November 26, 2015 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch028

J. M. Cowley Department of Physics, Arizona State University, Tempe, AZ 85287 Recent advances in electron microscopy with instru ments having a resolution of 2Å or better provide the possibility of atomic-scale imaging of small particles and, in favorable cases, atom positions can be deter mined with an accuracy approaching 0.1Å. The scanning transmission electron microscope provides complemen tary information through the use of special detector configurations and the possibilities for obtaining microdiffraction patterns and microanalysis signals from very small specimen regions, 10Å or less in diameter. Examples are given of the analysis of supported catalyst systems using electron beams of about 10Å in diameter to obtain diffraction patterns from individual metal particles of comparable diameter. Advances i n the design of transmission electron microscopes, combined with the use of accelerating voltages higher than the 100keV of the older high r e s o l u t i o n instruments, have provided the very important improvements of the r e s o l u t i o n l i m i t which allow the atom positions i n many inorganic s o l i d s to be distinguished. Point-to-point resolutions of 28 or better approached by some of the older m i l l i o n v o l t microscopes and achieved by some of the newer instruments should allow meaningful images of the atom configurations i n small regions of t h i n specimens to be interpreted q u a n t i t a t i v e l y and r e l i a b l y . Developments of the s p e c i a l detector configurations for scanning transmission electron microscopy (STEM) have made i t possible to perform s e l e c t i v e imaging making use of known c h a r a c t e r i s t i c s of the specimen, such as composition or c r y s t a l l i n i t y , to answer more s p e c i f i c questions. The techniques of m i c r o d i f f r a c t i o n have advanced to the stage that d i f f r a c t i o n patterns from regions 108 or l e s s diameter can be obtained r e a d i l y . This can provide information on the structures of i n d i v i d u a l small p a r t i c l e s or regions within small p a r t i c l e s , thus complementing i n an important way the information from the selected area electron d i f f r a c t i o n and X-ray d i f f r a c t i o n methods which refer to averages over very large numbers of individual particles. 0097^6156/85/0288-0329$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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In a d d i t i o n , use of the scanning p r i n c i p l e allows microanalysis of very small specimen regions to be performed by detection of e i t h e r the c h a r a c t e r i s t i c X-rays emitted or the c h a r a c t e r i s t i c energy l o s s peaks i n the energy spectrum of transmitted electrons. In t h i s review an attempt w i l l be made to assess the current c a p a b i l i t i e s of these techniques i n t h e i r a p p l i c a t i o n to the study of small metal and oxide p a r t i c l e s which are of i n t e r e s t i n r e l a t i o n to c a t a l y s i s . Some examples w i l l be given of recent a p p l i c a t i o n s and some suggestions w i l l be made concerning probable d i r e c t i o n s for future developments.


Transmission electron microscopy

(TEM)

For many years i t has been possible to detect s i n g l e heavy atoms as small black (or white) spots i n TEM images. Also i t has been possible to detect d e t a i l due to d i f f r a c t i o n e f f e c t s i n c r y s t a l s on a scale of 1A or better even with electron microscopes having a nominal "point-to-point" r e s o l u t i o n of 3-^8 Π,2). However with such microscopes the i n t e r p r e t a t i o n of image d e t a i l on t h i s scale i n terms of structure i s possible only for very s p e c i a l cases of extended, perfect t h i n c r y s t a l s of very simple structure and i s not possible for small c r y s t a l s or c r y s t a l s with defects. The p r a c t i c a l use of electron microscopes as a means for d e r i v i n g the atom arrangements i n small p a r t i c l e s or other t h i n specimens had to wait for the development of electron microscopes having a point-to-point resolution around or better since the interatomic distances i n projections of the structures of metals, semiconductors, oxides and other materials tend to be 1.5-28 for even the most favorable o r i e n t a t i o n s . The required r e s o l u t i o n has been attained by use of microscopes having higher accelerating voltages than the 100keV which has been conventional i n the past. Interpretation of the images i s s t i l l not straightforward even when there seems to be a simple one-to-one correspondence between black (or white) dots i n the image and atom p o s i t i o n s . E s p e c i a l l y when q u a n t i t a t i v e data on interatomic distances i s to be derived, detailed c a l c u l a t i o n s based on many-beam dynamical theory (3) must be applied to derive calculated images for comparison with experiment. For t h i s purpose the experimental parameters describing the imaging conditions and the specimen thickness and o r i e n t a t i o n must be known with high accuracy. A recent example of a successful analysis comes from the studies of small gold p a r t i c l e by Marks and Smith (4,5) using the 600keV Cambridge microscope, (see also t h e i r a r t i c l e i n t h i s volume). With the incident beam p a r a l l e l to the (110) face of a gold c r y s t a l , i n [100] d i r e c t i o n , the configuration of atom rows extending about 5θ8 i n the beam d i r e c t i o n could be seen c l e a r l y , showing the 2x1 surface reconstruction, which had previously been postulated from LEED r e s u l t s . Displacements of the gold surface atoms from the bulk l a t t i c e p o s i t i o n s could be determined with an accuracy of about 0.l8. These displacements, derived by comparison with calculated images

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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were d i s t i n c t l y d i f f e r e n t from those suggested by the positions of the black dots associated with the atom rows i n the images. An extreme case of the apparent d i s t o r t i o n of structures due to the complications of the imaging process i s shown i n figure 1, an image of the corner of a cubic c r y s t a l of MgO smoke viewed along the [ 0 0 1 ] d i r e c t i o n , p a r a l l e l to the edge of the cube. The image was obtained with JEM 2 0 0 C X microscope by Dr. T. Tanji i n our laboratory. The 2 A square g r i d corresponding to the 2 0 0 and 0 2 0 l a t t i c e p e r i o d i c i t i e s i s v i s i b l e i n the bulk of the c r y s t a l . In the small c r y s t a l projecting from the top of the large c r y s t a l and at the corner of the large c r y s t a l the l a t t i c e planes appear to be bent, curving away from the c r y s t a l face by 1 or 28. There may, of course, be some s l i g h t displacements of the atoms present, but these large apparent displacements are almost c e r t a i n l y the r e s u l t of an a r t i f a c t produced by dynamical d i f f r a c t i o n processes. Determinations of projected atom positions are much more d i f f i c u l t for atoms i n the i n t e r i o r of the p a r t i c l e i f the atoms are not conveniently aligned i n s t r a i g h t rows i n the d i r e c t i o n of the incident electron beam. For the immediate future only the most favorable cases w i l l be studied but with the a p p l i c a t i o n of a n t i c i pated improvements of r e s o l u t i o n to the 1.58 l e v e l or better and the means for more accurate and automated measurement of the necessary instrumental parameters, the d e t a i l e d study of configurations of atoms i n small p a r t i c l e s should become generally f e a s i b l e . In the meantime a great deal of more q u a l i t a t i v e but h i g h l y s i g n i f i c a n t information on small p a r t i c l e s should flow from the high resolution instruments now a v a i l a b l e . Scanning transmission electron microscopy (STEM) While STEM instruments have not shown the same r e s o l u t i o n or picture q u a l i t y as the fixed beam TEM instruments f o r b r i g h t - f i e l d imaging, they have important advantages derived from the f l e x i b i l i t y with which d i f f e r e n t detector systems may be arranged to obtain a v a r i e t y of image s i g n a l s . Also the fact that multiple images from d i f f e r e n t detectors can be produced as p a r a l l e l e l e c t r o n i c s i g n a l s i n s e r i a l form allows great f l e x i b i l i t y i n on-line image processing. Early work by Crewe and associates (6,7) established the benefits of STEM for d a r k - f i e l d imaging and for images using combinations of s i g n a l s from i n e l a s t i c and e l a s t i c s c a t t e r i n g . These, and other means involving s p e c i a l detector configurations, have increasingly been applied to the s p e c i a l problems of imaging small heavy-atom p a r t i c l e s supported on, or embedded i n , light-atom m a t e r i a l . The Ζ contrast method, i n v o l v i n g signals from i n e l a s t i c a l l y and e l a s t i c a l l y scattered electrons, has been shown to be e f f e c t i v e for the study of supported c a t a l y s t p a r t i c l e s ( 8 ) . Later, advantage was taken of the fact that heavy atoms scatter more strongly to higher angles than l i g h t atoms and i t was shown that heavy atom p a r t i c l e s could be revealed more r e a d i l y i f the images were obtained only with electrons scattered to high angles, of the order of 1 0 ~ radians ( 9 ) . unless the s c a t t e r i n g angle i s s u f f i c i e n t l y l a r g e , the remaining


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signal s t i l l shows some contrast due to c r y s t a l l i n i t y of the l i g h t atom m a t e r i a l , with maxima or minima of i n t e n s i t y from small regions giving strong d i f f r a c t i o n e f f e c t s ,


A further refinement of the method has been made i n which only those electrons were used which were scattered to higher angle regions of the d i f f r a c t i o n pattern where c r y s t a l l i n e r e f l e c t i o n s were weak or absent. Then a difference s i g n a l was obtained from electrons scattered to very high angles and those scattered to high angles (10). The s i g n a l l e v e l s from such a scheme are low but the discrimination between heavy and l i g h t atoms can be very good i f the specimen i s not too t h i c k . For most studies of c a t a l y s t p a r t i c l e s the electron microscopy i s done on very t h i n specimens and the use of high angle scattering for dark f i e l d imaging i s f e a s i b l e . When circumstances require the use of t h i c k specimens these techniques are not so e f f e c t i v e but even then the choice of s p e c i f i c detector configurations can enhance the contrast of small p a r t i c l e s . I t has been shown, for example, that the v i s i b i l i t y of small p a r t i c l e s on t h i c k supports can be improved considerably by using a detector displaced from the normal bright f i e l d imaging p o s i t i o n so that i t l i e s on the edge of the c e n t r a l spot (the d i r e c t l y transmitted beam) i n the detector plane ( 1 1 ) . Figure 2 shows two STEM images of small gold p a r t i c l e s on a c r y s t a l of MgO. For the image on the l e f t , the detector was c e n t r a l in the beam spot containing the d i r e c t l y transmitted electrons, as for normal bright f i e l d imaging. The other image was obtained with the detector displaced so that i t was j u s t at the edge of the c e n t r a l beam spot, giving an image produced p a r t l y by the d i r e c t l y transmitted electrons and p a r t l y by electrons deflected by e l a s t i c and i n e l a s t i c s c a t t e r i n g processes. In t h i s , the small gold p a r t i c l e s are seen much more c l e a r l y . Microanalysis When the fine electron beam of a STEM instrument passes through a specimen, i t generates secondary r a d i a t i o n through i n e l a s t i c scattering processes. When inner s h e l l electrons of the atoms are excited, the secondary r a d i a t i o n signals may be c h a r a c t e r i s t i c of the elements present and so provide a basis for the microanalysis of the small specimen regions which are i r r a d i a t e d . F i r s t l y , the energy losses of the incident electrons which produce the inner s h e l l e x c i t a t i o n s may be detected as peaks i n electron energy l o s s spectroscopy (EELS). The elecrons transmitted by the specimen are dispersed i n a magnetic f i e l d spectrometer and the peaks, due to K, L and other s h e l l e x c i t a t i o n s giving energy losses i n the range of 0-2000eV, may be detected and measured. Secondly, the c h a r a c t e r i s t i c X-rays, emitted as the electrons displaced from the inner s h e l l s of the atoms are replaced, can be detected by use of an energy-sensitive detector placed close to the specimen. An account of the a p p l i c a t i o n of both the energy dispersive spectroscopy (EDS) of the emitted X-rays and EELS to the



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Figure 1. High r e s o l u t i o n Electron Micrograph of a cubic MgO c r y s t a l viewed i n [100] d i r e c t i o n showing square net of 2A fringes and apparent bending of atom planes at edges. Courtesy of Dr. T. T a n j i .

Figure 2. (a) Bright f i e l d STEM image of small gold c r y s t a l s on a large MgO smoke c r y s t a l . Marker indicates 100A. (b) As f o r (a) but with a displaced detector.



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study of the composition of small p a r t i c l e s of i n t e r e s t i n c a t a l y s i s i s given by the a r t i c l e of Lyman i n t h i s volume. In each case the analysis may be done of very small specimen regions of diameter l e s s than 100A. The l i m i t a t i o n s on s i z e of p a r t i c l e which may be analysed, or on the percentage of a p a r t i c u l a r element present i n any sample area, are determined i n each case by the s i g n a l strength. The relevant parameters include the i n t e n s i t y of the incident beam, the scattering cross section for inner s h e l l e x c i t a t i o n s , the detection e f f i c i e n c y and the r a t i o of signal to background. In general the detection e f f i c i e n c y i s high for EELS but the background l e v e l s of the s i g n a l s are also high. For X-ray EDS the detection e f f i c i e n c y i s not so good but the signals show lower background l e v e l s . Each technique has i t s own p a r t i c u l a r areas of strength and weakness. A t h i r d s i g n a l , dependent on the nature of the elements present, i s given by the Auger electrons, emitted as an a l t e r n a t i v e to X-rays when the energy of an inner s h e l l e x c i t a t i o n i s dispersed. Because the emitted electrons are of r e l a t i v e l y low energy (0-2000eV) and so have only l i m i t e d penetration through s o l i d s , Auger electron spectroscopy (AES) and the corresponding scanning imaging technique (SAM: scanning Auger Microscopy) have been thought of as surface a n a l y s i s techniques to be applied to bulk samples. Currently, however, instruments are being b u i l t to combine AES with STEM imaging i n the transmission or r e f l e c t i o n mode. With a projected s p a t i a l r e s o l u t i o n of 508 or l e s s , AES and SAM may w e l l take t h e i r place as major t o o l s for the i n v e s t i g a t i o n of the composition and surface modifications of small p a r t i c l e s . P a r t i c u l a r l y for l i g h t elements, the cross sections for the production of Auger electrons may be greater than for X-rays and the c o l l e c t i o n e f f i c i e n c i e s may also be greater. REM electron microscopy

(REM)

In recent years the technique f o r imaging with d i f f r a c t e d beams, obtained i n the RHEED mode with an incident beam at grazing incidence to the f l a t surface, has been shown to be e f f e c t i v e as a means for studying surface structure and surface reactions 02,J3.). While i t i s desirable to use a microscope having an u l t r a - h i g h vacuum specimen environment i f surface reactions are to be studied, some valuable determinatons of structure can be made with a conventional i n s t r u ment. In each case s i n g l e atom steps on the surface give good contrast, d i s l o c a t i o n s i n t e r s e c t i n g the surface are c l e a r l y v i s i b l e and a number of other i n t e r e s t i n g surface features have been seen and explored. In the case of the regular arrays of steps seen on v i c i n a l surfaces of gold c r y s t a l s , a r e s o l u t i o n of better than 10A has been demonstrated (JJO. This technique has been applied most e f f e c t i v e l y for the study of extended surfaces of bulk samples and the i m p l i c a tions for the knowledge of surfaces of small p a r t i c l e s are, at best, i n d i r e c t . The equivalent type of imaging using the scanning mode, i s more d i r e c t l y relevant. Scanning r e f l e c t i o n electron microscopy (SREM) By use of a scanning transmission electron microscope, with the incident beam grazing the c r y s t a l surface, the s t r u c t u r a l features on surfaces have also been revealed with a r e s o l u t i o n of 10A or better


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(15). This technique has been applied e f f e c t i v e l y to examine d e t a i l s of surfaces of p a r t i c l e s 1ym or l e s s i n diameter. I t has been used, for example, t o detect the ordering i n l i n e a r arrays of small gold p a r t i c l e , 2θ8 i n diameter, on the surfaces of MgO c r y s t a l s (16). The extraordinary r e s u l t i s that the gold p a r t i c l e ( s ) apparently are aligned on surface steps which are i n c l i n e d to each other, and to the [100] c r y s t a l edge d i r e c t i o n s , at angles of only 2 or 3 degrees.


As i n the case of STEM, the main benefit a r i s i n g from the use of the scanning mode i s that the incident electron probe can be stopped or c o n t r o l l e d i n i t s motion and a v a r i e t y of detector types and configurations can be used to obtain p a r t i c u l a r s i g n a l s , g i v i n g information beyond that obtained i n the normal imaging modes. When the scan of the incident beam i s stopped at a chosen part of the image, a d i f f r a c t i o n pattern can be obtained from the selected region which may have a diameter as small as the image r e s o l u t i o n of 10A or l e s s . Also electron energy loss analysis of the scattered electrons may allow deductions concerning the energy states of very small surface regions. The most s t r i k i n g r e s u l t s obtained i n t h i s way come from experiments i n which an electron beam of 1θ8 diameter i s made to traverse the vacuum j u s t outside the face of a small c r y s t a l (17-19). In t h i s way the surface e x c i t a t i o n s can be examined with no complication from s c a t t e r i n g or e x c i t a t i o n s of the bulk c r y s t a l . The main features of the energy l o s s spectra have been shown to be i n e s s e n t i a l agreement with the deductions from the known d i e l e c t r i c constant of MgO, but there are i n d i c a t i o n s of e f f e c t s due to surface states appearing w i t h i n the band gap of the bulk c r y s t a l and to surface channelling phenomena (V7). Experiments have also been conducted to investigate the form of the p o t e n t i a l f i e l d extending from the c r y s t a l i n t o the surrounding vacuum by detection of the d e f l e c t i o n of electrons passing close to the c r y s t a l surface (20) .

M i c r o d i f f r a c t i o n i n a STEM instrument The d i f f r a c t i o n pattern obtained i n the detector plane when the beam scan i n a STEM instrument i s stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 38 i n diameter. In order to form a probe of t h i s diameter i t i s necessary to i l l u m i n a t e the specimen with a convergent beam. The pattern obtained i s then a convergent beam electron d i f f r a c t i o n (CBED) pattern i n which the c e n t r a l spot and a l l d i f f r a c t i o n spots from a t h i n c r y s t a l are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for p a r t i c u l a r postulated d i s t r i b u t i o n s of atoms. This has been attempted, as yet, for only a few cases such as on the d i f f r a c t i o n study of the planar, nitrogen-rich defects i n diamonds (21) . D i f f r a c t i o n patterns having r e l a t i v e l y well-defined sharp spots can be obtained from small u n i t - c e l l c r y s t a l s with an incident beam of diameter 10-158. Such patterns have been used i n the study of the structures of small metal p a r t i c l e s (22). For p a r t i c l e s 10-20A diameter the electron beam can i l l u m i n a t e the whole of the p a r t i c l e


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so that any s t r u c t u r a l features such as twin or f a u l t s can be revealed. For larger p a r t i c l e s , i n the 20-508 s i z e range, the d i f f r a c t i o n pattern may be seen to change as the beam i s moved across the p a r t i c l e .


For the smaller p a r t i c l e s which include only a few tens or hundreds of atoms, any twinning or f a u l t i n g reduces the range of ordering to the extent that the pattern can not be interpreted i n the same way as a pattern from an extended c r y s t a l . The i n d i v i d u a l s i n g l e - c r y s t a l regions may contain only two or three atomic planes. Interpretation can be made only by c a l c u l a t i o n of patterns from postulated models for the configurations of atoms (22). This technique has been applied, for example, to test the t h e o r e t i c a l p r e d i c t i o n that for small p a r t i c l e s of face-centered cubic metals the equilibrium state w i l l be a multiply-twinned form with preference for configurations i n which e i t h e r f i v e or twenty t e t r a h e d r a l l y shaped s i n g l e c r y s t a l regions are r e l a t e d by twining on (111) planes (23)· For p a r t i c l e s of gold i n a polyester f i l m , formed by co-sputtering (24), i t was shown that i n the s i z e range of 30-50A approximately h a l f were m u l t i p l y twinned but i n the size range of 15-20A a much smaller proportion of the p a r t i c l e s could be i d e n t i f i e d as such. Most were s i n g l e c r y s t a l s or had at most one or two twin planes. I t i s not necessarily to be concluded that, i n general, the proportion of small metal p a r t i c l e s having the m u l t i p l i c i t y twinned form decreases as the s i z e i s decreased. The evidence concerning p a r t i c l e s formed i n other ways shows a great deal of v a r i a b i l i t y . For example 20A gold p a r t i c l e s epitaxed on MgO (100) faces are almost i n v a r i a b l y s i n g l e c r y s t a l s when formed by i n d i r e c t evaporation on faces not exposed to the d i r e c t f l u x of incident gold atoms (16), although gold p a r t i c l e s formed on MgO (100) faces by d i r e c t deposit i o n from the source are i n random o r i e n t a t i o n , u s u a l l y with (111) planes p a r a l l e l to the surface, and are frequently twinned or multiply twinned. P a r t i c l e s of Pd on s i n g l e - c r y s t a l ot-Al-O- faces were sometimes twinned and sometimes not for d i f f e r e n t regions of the same specimen. In agglomerates of pure Pt p a r t i c l e s , i n d i v i d u a l p a r t i c l e s i n the 100A s i z e range showed a r e l a t i v e l y high incidence of twinning and multiple twinning but Pt p a r t i c l e s i n the s i z e range of 15-30A, supported on alumina or s i l i c a substrates gave mostly s i n g l e - c r y s t a l patterns. The extent to which small p a r t i c l e s of Pd and Pt show evidence of oxidation a f t e r exposure to a i r i s also highly v a r i a b l e . I t i s d i f f i c u l t to confirm the evidence of X-ray d i f f r a c t i o n and EXAFS (25) that most p a r t i c l e s i n the 15-20A s i z e range consist e n t i r e l y of oxide. We have found that such p a r t i c l e s usually give s i n g l e c r y s t a l patterns a t t r i b u t a b l e to the metals. There i s , however, considerable evidence t h a t , i n the case of Pt on alumina, the Pt c r y s t a l s have a well-defined e p i t a x i a l r e l a t i o n s h i p with the c r y s t a l l i t e s (20-50A diameter) of the nominally "amorphous" alumina substrate. Similar observations of e p i t a x i a l r e l a t i o n s h i p s have been observed for small c r y s t a l s of Ru and Au on MgO (26). Figure 3(a)


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for example, shows the pattern of the MgO c r y s t a l substrate i n [111] o r i e n t a t i o n , obtained with an incident beam of diameter approximately 10A. The a d d i t i o n a l hexagonal array of spots of figure 3(b) comes from a c r y s t a l of Ru about 158 i n diameter, aligned i n a p a r a l l e l o r i e n t a t i o n . Figure 3(c) shows the spots from an Au c r y s t a l , about 20A i n diameter, seen i n an approximate [110] d i r e c t i o n with one set of (111) planes almost p a r a l l e l to MgO (220) planes, with some i n d i c a t i o n that i t i s accompanied by a small Ru c r y s t a l aligned as for Figure 3(b). The STEM images obtained when the incident beam, used to obtain m i c r o d i f f r a c t i o n patterns such as i n Figure 3f i s scanned over the specimen w i l l have a r e s o l u t i o n no better than the beam diameter of about 10A, as i n Figure 4(a). This i s usually s u f f i c i e n t to allow the p a r t i c l e s i n question to be located and i d e n t i f i e d i n images subsequently obtained with better r e s o l u t i o n , using larger objective aperture, such as Figure 4(b). The r e s o l u t i o n of the STEM instrument i s currently l i m i t e d to about 38 but t h i s i s s u f f i c i e n t to provide considerable information on the p a r t i c l e morphology and to allow some c o r r e l a t i o n with more d e t a i l e d images now possible with the best TEM instruments. S t a t i s t i c a l information from s i n g l e c r y s t a l patterns The p o s s i b i l i t y of obtaining s i n g l e c r y s t a l d i f f r a c t i o n patterns from regions of very small diameter can obviously be an important addition to the means for i n v e s t i g a t i n g the structures of c a t a l y t i c materials. The d i f f i c u l t y a r i s e s that data on i n d i v i d u a l small p a r t i c l e s i s usually, at best, merely suggestive and at worst, completely meaningl e s s . What i s normally required i s s t a t i s t i c a l data on the r e l a t i v e frequencies of occurrence of the various s t r u c t u r a l features. For adequate s t a t i s t i c s , i t would be necessary to record and analyse very large numbers of d i f f r a c t i o n patterns. The powder patterns obtained by X-ray d i f f r a c t i o n and selected area electron d i f f r a c t i o n do represent averages over very large numbers of p a r t i c l e s but the averaging over s i z e , o r i e n t a t i o n and imperfection of c r y s t a l s removes much of the important information, e s p e c i a l l y that on the c o r r e l a t i o n s of properties,e.g. the o r i e n t a t i o n a l r e l a t i o n s h i p of adjacent c r y s t a l regions or the dependence of twinning on s i z e . In order to take advantage of the c a p a b i l i t i e s of the microd i f f r a c t i o n method i t thus seems necessary to find some a l t e r n a t i v e to the laborious compilation of vast numbers of analyses of i n d i v i d u a l r e s u l t s . One a l t e r n a t i v e which we have explored i s to use our automatic d i g i t a l data c o l l e c t i o n equipment (25) i n combination with a pattern recognition device (26). In our system the small electron probe of the STEM instrument i s scanned over a chosen area of a specimen and the m i c r o d i f f r a c t i o n patterns from each successive probe p o s i t i o n are viewed by a low l i g h t - l e v e l TV camera and d i s played on a video screen. A set o f detectors i s arranged such that when a d i f f r a c t i o n pattern which includes a p a r t i c u l a r array of spots appears on the screen, a s i g n a l i s generated to stop the scan and record the d i f f r a c t i o n pattern i n d i g i t a l form i n the computer



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Figure 3. M i c r o d i f f r a c t i o n patterns obtained with an electron beam of diameter about 1o8 from p a r t i c l e s of Ru and Au on a MgO support, (a) MgO c r y s t a l , (b) Ru c r y s t a l , 15& i n diameter, on MgO. (c) Au c r y s t a l , 20& i n diameter, on MgO.

Figure 4. STEM images of Au p a r t i c l e s on a MgO support, (a) Image taken with the small objective aperture used f o r m i c r o d i f f r a c t i o n ; (b) Image obtained with larger objective aperture showing better r e s o l u t i o n .


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memory. I t i s then possible to perform d i g i t a l c o r r e l a t i o n analysis of a l l such patterns recorded and derive answers to s p e c i f i c questions such as, f o r example, i f a small metal p a r t i c l e has a p a r t i c u l a r o r i e n t a t i o n , i s there evidence that neighboring parts of the metal p a r t i c l e or of the supporting material have a tendency to occur i n a s i m i l a r or related orientation? There are many variants of t h i s system which can be envisaged as means by which the current p o s s i b i l i t i e s for automation i n data c o l l e c t i o n can be applied for s p e c i f i c purposes. There are consider able dangers i n t h i s approach i n that i t may be a l l too easy to b u i l d i n r e s t r i c t i o n s which predetermine the r e s u l t s . These dangers, however, are not l i k e l y to be worse than those normally encountered i n electron microscopy or single c r y s t a l d i f f r a c t i o n where the one p a r t i c u l a r l y "good-looking" picture i s taken as being " t y p i c a l " of a sample. I t i s f e l t that the use of electron microbeam methods o f f e r s the basis for a revolutionary new approach to the study of c a t a l y s t p a r t i c l e s . Some r e s u l t s can be obtained immediately but to r e a l i s e the f u l l p o t e n t i a l of the method a considerable amount of further exploration of data c o l l e c t i o n and data analysis methods w i l l be needed. Acknowledgment The author wishes to thank Dr. J.B. Cohen for supplying samples of Pt and Pd on alumina and s i l i c a and Drs. J . Schwank and A.K. Dayte for samples of Ru and Au on magnesia and s i l i c a . This work was supported by the US Department of Energy under Contract DMR-76ER02995 and has make use of the resources of the ASU F a c i l t i t y for High Resolution Electron Microscopy, supported by NSF grant DMR 8306501. Literature Cited 1. Hashimoto, H.; Endoh, H.; Tanji, T.; Ono, Α.; Watanabe, E.; J. Phys. Soc. Japan 1977, 42, 1073. 2. Izui, K.; Furuno, S.; Ono, Α.; J. Electron Microscopy 1977, 26, 129. 3. Cowley, J.M.; Diffraction Physics, 2nd Edit. North Holland Publ. Co., 1981. 4. Marks, L.D.; Surface Sci. 1984, 139, 281-98. 5. Marks, L.D.; D.J. Smith; Ultramicroscopy (1984) In Press. 6. Crewe, A.V. in "Electron Microscopy in Material Science;" U. Valdre, Ed.; Academic Press, New York, 1971, p. 62. 7. Langmore, J.P.; Wall, J.; Isaacson M.; Optik 1973, 38, 335. 8. Brown, L.M. in Developments in Electron Microscopy and Analysis 1978; D.L. Misell Ed.; Institute of Physics, Bristol, England 1977 p. 14. 9. Treacy, M.M.J.; Howie, Α.; Pennycook, S.J. in Electron Microscopy and Analysis, 1979, (T. Mulvey, Ed.) Institute of Physics, Bristol, England 1980, p. 261. 10. Butler, J.H.; Turner, P.S.; Cowley, J.M. 1984 In preparation. 11. Cowley, J.M.; J. Electron Microscopy Techniques 1984, 1, 83.


340 12. 13. 14. 15.


16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

CATALYST CHARACTERIZATION SCIENCE Osakabe, N.; Tanishiro, Y . ; Yagi, K.; Honjo, G . ; Surface Sci. 1981, 109, 353. Hsu, Tung; Cowley, J.M.; Ultramicroscopy 1983, 11, 239. Hsu, Tung; Ultramicroscopy 1983, 11, 167. Cowley, J.M. in Microbeam Analysis 1980, D.B., Wittry, Ed., San Francisco Press, San Francisco 1980, p. 33. Cowley, J.M.; Neumann, K.D.; Surface Sci. 1984, In press. Cowley, J . M . , Surface Sci. 1982, 114, 587. Marks, L.D. in Electron Microscopy and Analysis, 1981, M.J. Goringe, Ed.; Institute of Physics, Bristol, England 1981, p. 259. Batson, P . E . ; Ultramicroscopy 1983, 11, 299. Tan, C.S.; Cowley, J.M.; Ultramicroscopy 1983-4, 12, 333-44. Cowley, J.M.; Osman, Μ.Α.; and Humble, P.; Ultramicroscopy 1984, In press. Monosmith, W.B.; Cowley, J.M.; Ultramicroscopy 1983-4, 12, 177. Allpress, J . A . ; Sanders, J . V . ; Surface S c i . , 1965, 7, 1. Roy, R.A.; Messier, R.; Cowley, J.M.; Thin Solid Films 1979, 79, 207. Cohen, J . B . , These proceedings. Datye, A.K.; Schwank, J . in Proceedings of 8th International congress on catalysis, Berlin, 1984. In press. Strahm, M.; Butler, J . H . ; Rev. Sci. Instr. 1981, 52 840. Monosmith, W.B.; Cowley, J.M.; Ultramicroscopy 1983, 12, 51.

RECEIVED March 11, 1985


Electron Microscopy and Diffraction Techniques for the Study of Small

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