Molecular image resolution in electron microscopyNatsu Uyeda, Takashi Kobayashi, Eiji Suito, Yoshiyasu Harada, and Masaru Watanabe Citation: Journal of Applied Physics 43, 5181 (1972); doi: 10.1063/1.1661094 View online: http://dx.doi.org/10.1063/1.1661094 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/43/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ballistic electron magnetic microscopy: Imaging magnetic domains with nanometer resolution Appl. Phys. Lett. 75, 1001 (1999); 10.1063/1.124578 Magnetization imaging at high spatial resolution using transmission electron microscopy J. Appl. Phys. 80, 3408 (1996); 10.1063/1.363207 Lattice and atomic structure imaging of semiconductors by high resolution transmission electron microscopy Appl. Phys. Lett. 47, 685 (1985); 10.1063/1.96058 Imaging of the silicon on sapphire interface by highresolution transmission electron microscopy Appl. Phys. Lett. 38, 439 (1981); 10.1063/1.92389 Imaging of thin intergranular phases by highresolution electron microscopy J. Appl. Phys. 50, 4223 (1979); 10.1063/1.326453

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

Molecular image resolution in electron microscopy·

Natsu Uyeda. Takashi Kobayashi. and Eiji Suito Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu, Japan

Yoshiyasu Harada and Masaru Watanabe JEOL Co. Ltd., Akishima, Tokyo, Japan

(Received 24 January 1972; in final form 26 June 1972)

In order to determine the ultimate molecular resolution attainable with a conventional electron microscope, the direct observation of hexadecachloro-Cu-phthalocyanine molecules was attempted. Since phthalocyanine derivatives are known to form crystalline films with columns of parallel stacks of planar molecules, the specimens were prepared by epitaxial growth on KCI cleavage face through vacuum evaporation so that the column axis was directed almost normal to the thin-film surface holding an orientation suitable for the observation. The molecular orientation was determined by Patterson synthesis based on the laser optical transform of the electron diffraction pattern obtained from the individual crystallites placed on the microgrid mesh. The direct observation was carried out with the lOO-kV electron beam incident on the specimen along the column axis. The crosslike images arrayed in a centered rectangular net were clearly resolved, well representing the molecular shape of phthalocyanine with the configuration like a four-leaf clover. The effect of the spherical aberration and the defocus value for the objective lens are also discussed in relation to the contrast inversion of the total images. It was also found that the chlorinated Cu-phthalocyanine is 40 times more resistant to the electron radiation damage than the ordinary phthalocyanines.

I. INTRODUCTION

Over the years, attempts have been made to roughly de­termine molecular structures of organic compounds with electron microscopes _ Particularly in the field of molecular biology and biochemistry, these attempts have proved considerably successful with respect to such large biopolymers as DNA, RNA, and enzyme proteins. However, since the specimen undergoes spe­cial treatment, such as staining and metal shadOwing, to enhance the image contrast, it is believed that the attainable resolution to determine the fine structure of the image could not be better than about 20 A. As to rigid inorganic crystals, however, electron opti­cal resolution as high as an atomic order was achieved by Iijima1 with respect to two-dimensional projections of titanium-thallium oxide crystals, where ordered and also disordered arrangements of metal-oxygen tetra­hedra were well resolved.

On the other hand, few or no attempts have been made to determine the structure of usual organic molecules of medium size (less than 20 A) through the direct ob­servation of the image, since it is obvious that many practical difficulties will result, although the problem of resolution as well as that of image contrast has been treated theoretically. 2, 3 One of the anticipated troubles is that since the component atoms are mostly light ele­ments, the amplitude of the electron waves which scat­ter from the atoms to form a molecular image is small, and the spherical aberration of the objective lens is too large to obtain a satisfactory phase contrast.

Another major trouble is that a high magnification which is essential for observing the molecular image requires a bright electron E\ource with increased current density. However, the molecules of the organic specimen are known to be so sensitive to radiation damage405 that they may become decomposed or deformed under the irradi­ation of an electron beam as dense as 1 A/cm2 at the object plane.

Though it is also essential to keep the molecules in a proper orientation for observation, it has been consid­ered almost unfeasible in view of the fact that the mole­cules make a fierce thermal vibration even at room temperature and also that the specimen-supporting

5181

membrane as well as the specimen molecule is com­posed of numerous similar atoms. We have sought a breakthrough in practical difficulties, in the belief that the solution of problems, mainly associated with the specimens, will be the prerequisite to the attainment of the molecular resolution. We used phthalocyanine com-

(a) H H H}{H

HMN_LN>--rJyH H~ I ~H

H N-QN H

H->-(H H H

(b) CI CI

CI~CI

~CI NVNXXCI CI I CI

N-Cu-N

CI I CI CI N~N CI

CI-\,-{CI

CI CI

FIG. 1. Molecular structure of phthalocyaniries. (a) Cu­phthalocyanine, (b) hexadecachloro-Cu-phthalocyanine.

J. Appl. Phys., Vol. 43, No. 12, December 1972

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

5182 UYEDA et al.

FIG. 2. Molecular column and its projection. The inter­molecular distance 15 is about 3.4 A.

pounds for the test specimen because they were known to be thermostable and more resistant to radiation damage than other aliphatic compounds, and also we have been familiar with how to control such compounds as the specimen through our previous studies of the polymorphic transformation6 and epitaxial growth, 7,8

since Menter9 succeeded in the observation of the lat­tice images of the compounds.

Supporting the extremely thin single crystallites formed by epitaxial growth upon the small holes of a microgrid was considered to be a most effective means to properly hold these planar molecules which might be suitable for the observation. In this case, the molecular image is expected to be the total projection of these planar mole­cules along the axis of a column which they usually form by parallel stacking.

II. CHARACTERISTICS OF THE SPECIMEN MOLECULE

Figure 1 (a) shows planar structure of the metal-phthalo­cyanine having a D 4" symmetry. Of all crystals of this compound, the Q1 and f3 polymorphs are the best known, as RobertsonlO and Brownll reported in detail. The structural characteristic of importance, common to these two polymorphs, is that the planar molecules are piled up in parallel with a constant intermolecular dis­tance of 3.4 A, forming a column in the direction of the b axis of the crystal as illustrated in Fig. 2. The molecular plane holds a given angle with the column

J. Appl. Phys., Vol. 43, No. 12, December 1972

axis, and its position is defined by periodic shifts along the b axis at regular intervals. As a result, all the molecules in the column are properly stacked so that their projection becomes identical with that of a single molecule, as viewed in the direction of the column axis. The projection of the total crystal in the coaxial direc­tion will appear as a two-dimensional lattice of these molecular images. This may be considered to be equiv­alent to a two-dimensional electron density map drawn on the basis of the inversed Fourier synthesis in terms of the structure factor F(hOZ) in x-ray structure analy­sis. The projection of molecules is almost sufficient to

FIG. 3. Epitaxial crystallites of hexadecachloro-Cu-phthalo­cyanine. (a) General view, (b) crystallites placed on a hole of unfilmed mocrogrid.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

RESOLUTION IN ELECTRON MICROSCOPY 5183

FIG. 4. Epitaxial crystallites of hexadecacbloro-Cu­phthalocyanine. (a) Slender type, (b) selected-area diffraction.

give the whole picture of their image, because the nor­mal of the molecular plane is inclined only about 30° with respect to the column axis. Since the molecules are superimposed along the b axis keeping the same orientation, the image contrast is expected to exceed that of a single molecule.

In order to use such crystals for observation, it was

pointed out by Heidenreich12 that the thickness should not exceed about 100 A, which is much less than the extinction distance for the dynamical interference of the electron waves which propagate in the crystal. However, the crystal lattice of a metal-phthalocyanine is formed through the overlapping of 1T-electron orbitals which produce a strong periodic bond chain, and long-needle crystals tend to grow along the column as pointed out by Hartman.l3 It has been considered difficult to grow thin crystalline films with such a special crystal habit as having the column axis almost normal to the thin-film surface. The problem can be solved by means of an

FIG. 5. Selected-area diffraction patterns obtained from the crystallites as shown in Fig. 3 with incident beam (a) normal to the habit face and (b) parallel to the c axis.

J. AppJ. Phys., Vol. 43, No. 12, December 1972

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

5184 UYEDA et al.

c

1

c*

/ -/ Refl. sphere II.

-a*

/

FIG. 6. Reciprocal lattice and reflection spheres viewed in the direction of the b axis.

epitaxial growth, using alkali-halide single crystals as the substrate. As previously reported,8 the molecules are adsorbed and oriented almost parallel to the sub­strate surface through an interaction between the 11

electrons of the molecules and the ions of the substrate crystal; and the molecules, maintaining those postures of oriented adsorption, become epitaxial nuclei, re­sulting in the growth of thin-film crystals holding an appropriate axial orientation. By controlling the amount of vapor deposit and the temperature of the substrate, it is possible to adjust the size as well as the thickness of the growing crystal.

III. ENHANCEMENT OF IMAGE CONTRAST AND RESISTIVITY OF RADIATION DAMAGE

The simple copper-phthalocyanine has the heavy-metal ion in its center as shown in Fig. l(a). In order to en­hance the image contrast by introducing atoms with a large scattering amplitude of electron waves, it was considered suitable to use copper-hexadecachloro­phthalocyanine [Fig. 1(b)] in which 16 hydrogens around the macromolecular ring are substitued with chlorines.

It has been found, through a preliminary examination with the electron microscope, that such a chlorine sub­stitution gives another advantage to the specimen. It is one of the drawbacks inherent in an electron microscope that the electron radiation is apt to damage the organic molecule. It has been discovered that Cl-substituted phthalocyanine is much less vulnerable to electron radi­ation. In the study of the effect of the accelerating volt­age upon radiation damage to polyethylene, Kobayashi et al. 4.5 assumed that the molecule is damaged when an electron diffraction pattern, obtainable from those crystals, was turned into halos. On the basis of the same criterion of fading time, it was found that the con­sumed electron dose upon the copper-phthalocyanine and the Cl-substituted one, which in thin films are nearly equal in mass thickness, were estimated to be 2.7 X 10-2 and 1.13 Amin/cm2 , respectively. This ex­plains that the resistance of the chlorinated molecules to damage is about 40 times stronger than that of the ordinary one.

1. Appl. Phys., Vol. 43, No. 12, December 1972

In order to prepare a crystalline thin film for observa­tion, a KCI single crystal 1 mm thick and 1 cm square having a fresh cleavage face, is placed in a vacuum of 10-7 Torr, its surface is baked out at 450°C, the tem­perature is decreased to 400°C, and powdered Cl-sub­stituted copper-phthalocyanine is sublimated on the crystal surface. In this case, the thickness of the evap­orated film is controlled to be within 100 A by means of a monitor microbalance equipped with a quartz crystal oscillator.

The film, together with the substrate crystal, is im­mersed in pure water so that the crystal thin film can be parted from the KCI crystal surface and left floating on the water surface. Then, the film is transferred upon a carbon microgrid with small holes 1 or 2 /J. in diameter. The crystallites thus prepared are hexagonal in shape as shown in Fig. 3 (a). Figure 3 (b) shows the same specimen held on a microgrid without the aid of supporting membrane.

On the other hand, if potassium chloride is cleaved in the air and Cl-substituted copper-phthalocyanine pow­der is evaporated on it, crystallites with a different habit appear, as shown in Fig. 4(a). Such a habit dif­ference, because of surface treatment, is also noted in a pure metal-phthalocyanine. 7.8

IV. MOLECULAR ORIENTATION IN CRYSTALLINE FILMS

Although the chlorination of copper-phthalocyanine has served to increase the image contrast and the resis­tance to radiation damage, its crystal structure has not been known as yet. Information on the crystal structure is essentially needed to identify whether planar mole­cules assume a column structure and whether the mole­cules are properly oriented with respect to the habit plane and, consequently, to the incident beam for the

.. . ".:-.. :.:: .. ~.: .. ..... . .... . .. .. . •....• .. :.: .... ~.: .. .. ... . .... . .. .. . . . .... .. ....

1

··· .. : :.. .. .. . ...... .0 • •••• • •••• •. : .. :.*!.:.: .• .. .:".. .. .. .. .. .. .. . ... . . .. - ... . ..... ::;:~ ... ;.;.: ::;'~"'~:;:: .... . .

a·sinj3

. ....• ..

FIG. 7. Presumed molecular arrangement in the unit cell.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

RESOLUTION IN ELECTRON MICROSCOPY 5185

FIG. 8. Comparison of Patterson syntheses and the mask for the optical transform.

actual observation. However, since an attempt to grow a single crystal with a suitable size turned out to be un­successful, the structure analysis by x-ray diffraction had to be abandoned. The only alternative was to derive an effective information from the selected-area diffrac­tion pattern of a single microcrystal.

Figure 5(a) shows an electron diffraction pattern ob­tained from one of the crystallites shown in Fig. 3, where the electron beam falls perpendicular to the habit plane of the crystal. The pattern is composed of a band structure which is parallel to the equator and a net which is symmetric with respect to the meridian, in­dicating that the main axis in the crystal thickness di­rection is tilted with respect to the incident electron beam. If the specimen is rotated about 30° around the crystal axis parallel to the equator with the aid of the gOniometer stage, a face-centered rectangular net of an accurate symmetry can be obtained, as shown in Fig. 5(b). This demonstrates that the crystal is symmetric with respect to the plane normal to the equator axis and the electron beam falls in the direction of the main axis which is included in that plane and tilted with respect to the habit plane. It is obvious from the above observation that the crystal belongs to the monoclinic system.

It is proper to deSignate the crystal axes parallel to the equator and the meridian as the b and a axes, respec­tively, and the axis in the thickness direction as the c axis. The selected-area diffraction pattern which ex-

plicitly represents the period of the c axis was obtained from the crystallite having a slender habit, as shown in Figs. 4(a) and 4(b). The unit cell lengths a, b, and c were accurately determined from these patterns, using a TICI evaporated film as the reference specimen. The weak satellite spots lined up parallel in each band in Fig. 5(a) are interpreted as the intersections of the in­terference regions of reciprocal lattice points extending in the direction of the c* axis and the reflection sphere (I) normal to the same axis, as shown in Fig. 6, which is the reciprocal lattice projected to the a*c* plane. On the other hand, the a*b* plane and the reflection sphere (II) coincide with each other in the case of Fig. 5(b). The axial angle f3 was determined from the ratio of the corresponding layer lines parallel to the equator in both patterns. From these relations, the unit-cell dimen­sions can be determined as follows:

a=19.62 A, b=26.04 A, c=3.76 A, f3=116.5°.

From Fig. 5(b), in which all reflections observed satis­fy the relation h + k = 2n, the space group may be con­side red to be C2/c or C2/m, the (hkO) projection of which has the same symmetry. Judging from the dif­fraction pattern and the following conditions, the molec­ular arrangement is assumably determined, as shown in Fig. 7.

(i) The structure and size of the planar molecule as well as the unit-cell dimensions are known.

(ii) From the extinction rule of reflection, it is evident that the crystal assumes a base-centered monoclinic lattice.

(iii) Planar molecules of polycyclic aromatic compounds having 7T electrons stack themselves in a crystal keeping an intermolecular distance of about 3.4 A.

FIG. 9. Vector synthesis of a point Patterson map for varying molecular inclination.

J. Appl. Phys., Vol. 43, No. 12, December 1972

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

5186 UYEDA et al.

1

.. 26A" --_. b

FIG. 10. Predicted molecular projections along the c axis.

(iv) From an empirical view point, 8 it is considered that some of the planar molecules are oriented parallel to the substrate and, in turn, to the basal plane of the crystal playing the part of the epitaxial nuclei.

To support this reasoning and to measure the tilting angle of the planar molecule more accurately, a struc­ture analysis was tried by the Patterson synthesis based on the optical transform14 with a laser beam as the light source. The typical diffraction patterns, shown in Fig. 5 (b) as an example, were used to deduce the intensities of diffraction spots neccessary for the analysis. The intensities of all reflections, which over the interplanar distances down to 1 A, were visually distinguished into ten grades. Each diffraction spot was transformed into a black dot, whose area represents the diffraction in­tensity proportional to the above-mentioned grade. The model diffraction pattern thus produced was reduced to a small mask of about 5 mm across, which was photo­graphically printed on a hard film as illustrated in Fig. 8(b).

When a parallel beam from a He-Ne gas laser with a wavelength of 6328 A was applied perpendicular to the mask, and the central beam was focused onto a photo­graphic film with a camera provided with a telephoto lens (j= 1200 mm), a dark-shaded pattern was obtained as shown in the background of Fig. 8 (a). This pattern represents a Patterson map resulted from an optical Fourier synthesis of the intensities of the (hkO) reflections.

On the other hand, with the molecular arrangement presumably determined as in Fig. 7, the atomic posi­tions of the central ion and the 16 chlorines were pro­jected to a plane normal to the c axis as in Fig. 9. The vector synthesis of a point Patterson map was carried out by the parallel displacement of the projection to determine the distribution of the expected peaks. In Fig. 7, only the strong peak points, each corresponding to the product of Cuxeu, CuxCI, or ClxCI, are shown. The white dots superimposed in Fig. 8(a) are the ex­pected positions of these peaks plotted by taking the symmetry elements of the projected unit cell into ac-

J. Appl. Phys., Vol. 43, No. 12, December 1972

count. In a series of the vector syntheses obtained by changing the tilting angle t (Fig. 9) at 5° intervals as 15°, 20°, 25°, and 30°, it was found that the peaks for t = 25° and the Patterson map obtained by the optical dif­fraction overlap most precisely. If t around 25° is varied finely to make the same comparison as above, a tilting angle, corresponding more closely to the Patter­son map, may be found furnishing the atomic param­eters for the two-dimensional Fourier analysis. How­ever, it has to be noted that the electron diffraction in­tensities are often subject to the effect of the dynamical interactions, particularly when many reflections are systematically excited. The effect is known to be sensi­tive to the crystal mass thickness and the wavelength of the scattering electrons, so that no thick crystallities can be used for the first-order scattering apprOximation to be applied to the structure determination in an ordin­ary way. In view of this, the crystal thickness in the present case has been restricted to about 100 A, which may be thin enough for 100-kV electrons when com­pared with the past example estimated by Turner and Cowley15 in regard to siver-thallium selenide crystal which consists of heavier atoms than those contained in the present organic material.

Even when the effect of many beam interactions was in­volved, it was pointed out by Cowley and Moodie16 that the position of the Patterson peak would not be changed for a proper thickness, though the peak height and peak area might differ in many ways from that expected in the kinematic case depending upon the vector combina­tions of various atomic numbers. Although more pre­cise determination of atomic parameters based on n-

1.0 M--870A

-1.0

10 M=O

>< 2 V> 0 0 (XxI0 2 rad. u

-1.0

1.0 M=870A

0 (Xc

-1.0

FIG. 11. Effect of defocus on the phase factor for the objective lens (100 kV, Cs=1.4 mml.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

RE SOL UTION IN E LE C TRON MIC ROSCOPY 5187

beam calculation is still in progress, the above result is considered to provide good enough information on the arrangement of the molecules of known configuration at present, as far as the practical observation of the molecular image is concerned.

From these results, it is presumed that the molecules are stacked in parallel with the c axis as the column axis. Then, if the crystal image is projected along this axis, the molecular arrangement will presumably be observed as illustrated in Fig. 10.

V. RESULTS OF DIRECT OBSERVATION

After the foregoing experiments, various problems con­cerning sample molecules have been solved. However, in the direct observation of the molecular image, one must consider some inherent limitations of the electron microscope in regard to the image formation. The principle of the image formation is based on the recom­bination of diffracted wave in the Gaussian plane of the objective lens. In order for a fine molecular structure to be resolved, it is essential that as large a number of effective reflected waves as possible must be utilized in accord with the Abbe prinCiple.

Even the most advanced objective lens of an electron microscope can never be free from spherical aberra­tion, the effect of which increases with the scattering angle of the electron waves, so that those with too large a scattering angle would not be admitted into the objec­tive lens.

In order to form an image through recombination of diffracted waves, it is necessary to hold all reflected waves in regular phase at the image plane. The spheri­cal aberration causes a phase shift between the scatter­ing and the paraxial waves. Since Scherzer17 and Hans­zen18 developed the theory of image formation based on

FIG. 12. Molecular images of hexa­decachloro-Cu-phthalocyanine in an epitaxial crystallite.

the phase contrast, it has been widely accepted that the image contrast is greatly influenced by a transfer ftmc­tion which modulates the components for the inverse Fourier transform of diffraction pattern into the image. The function, often referred to as the phase factor, sen­sitively changes its state depending upon the coefficient of spherical aberration Cs and defocus value t::..f of the objective lens. Using these variables, the phase factor is usually expressed as follows:

(1T 21T a 4

21T a 2) cosv:::::cos - - - C - + - t::...f-

A 2 A s4 A 2' (1)

where X is a phase difference between the deflected and the undeflected waves whose wavelength is given by A. Though it is desirable that the phase factor be + 1 or -1 through all the range of the scattering angle a so that the image might not be confused but maintain the highest contrast, it, in fact, oscillates between the two extrema more and more frequently as the angle a is increased. With a constant Cs given to define the electron micro­scope, the COs)(-vs-a curve changes its state only de­pending upon the defocus t::..f. In order to find the most suitable t::..fwith which the factor COS)( assumes almost constant value near either + 1 or - 1 over as wide range as possible, actual curves were obtained taking C,,=1.4 mm as the practical value for the electron microscope used in the present work. The computation was per­formed by varying t::..fbetween -900 A through +900 A with 50-A intervals according to the method reported by Eisenhandler and Siegel.

Shown in Fig. 11 are the characteristic curves selected from the obtained curves, each of which has a vibra­tional structure as seen especially in a larger scatter­ing angle. It has been clarified, however, that if the phase factor is assumed to be allowable up to :l:0. 7 in­stead of ± 1, the t::..f= 870 A is the most suitable defocus

J. Appl. Phys., Vol. 43, No. 12, December 1972

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.193.242.44

On: Tue, 02 Dec 2014 05:53:03

5188 UYEDA et al.

value to include as wide scattering angle as possible to meet the principle. The critical scattering angle O!c'

beyond which the phase factor oscillates rapidly, is approximately 1.08 x 10-2 rad and then the theoretical resolution corresponding to this maximum scattering angle becomes 3.4 A at an accelerating voltage of 100 kV. The reflected waves that scatter outside the o!c

were interrupted with an objective aperture. The JEM-100B electron microscope was used for observing and also photographing molecular images at an accelerating voltage of 100 kV and a direct magnification of 150 OOOX.

A magnified image was photographed as follows: First, the specimen was tilted by about 30", second, the elec­tron beam incident was made parallel to the c axis of the crystal, and finally, the specimen orientation was finely controlled so as to gain a selected-area diffrac­tion pattern of an accurate symmetry as shown in Fig. 5(b). The focus of the objective lens must be set in the Vicinity of the fixed optimum defocus value according to the foregoing consideration of the wave aberration. Generally, it is difficult to achieve the optimum defocus value. In the case of such a specimen as mentioned above, however, this condition was readily obtained, since the specimen surface was tilted by about 30° holding continuously changing distance from the princi­pal plane of the objective lens.

Figure 12 represents an image example obtained at 870 A of defocus value, i. e., on the optimum underfocus condition. Each of the crosslike forms, regularly ar­ranged as a centered rectagular net is a projection image of the phthalocyanine molecules. The molecular arrangement coincides precisely with that predicted in Fig. 10. Tho\lgh all individual heavy atoms have not yet been resolved, the observation of the outer form of molecules as a whole has been successfully achieved; for example, a clover-shaped leaf corresponds to a chlorinated isoindole ring.

J. Appl. Phys., Vol. 43, No. 12, December 1972

FIG. 13. Reversed contrast of molecular images due to a slight change in defocus value.

A similar two-dimensional array of the same molecular projection images is shown in Fig. 13, where the total contrast is inverted. As can be seen from Fig. 11, this may be ascribed to the fact that, as a whole, the phase factor is reversed by properly changing the defocus val­ue and setting it under a slight overfocus condition.

VI. CONCLUSION

In this study, the organic molecules with a medium size were observed through a conventional transmission electron microscope, and as a result, the molecular shape and its array state in a crystal were photographed for the first time. Many problems, however, must be solved, considering that the original purpose is to pur­sue the possibility of resolving individual atoms in the molecules. Although it may be almost impossible to ob­serve the molecular images in more detail, the follow­ing various conditions are requirements for the practi­cal experiment to improve the image resolution: (i) to make the crystallite as thin as possible, (ii) to elevate the accelerating voltage of the electron beam as high as possible while maintaining the same or better stability than that of the present 100 kV, (iii) to surpress the thermal vibrations of molecules by cooling a specimen with liquid helium, and (iv) to decrease the C.s' etc.

It is an intriguing item of discovery that the chlorina­tion makes the radiation damage surpression possible. This is considered to be useful in developing a chemi­cal means to protect the specimen radiation damage as far as the polycyclic aromatic compounds are concerned.

*Read at the Sept. Congr. Intern'l Micros. Electronique, Grenoble (1910). IS. lijirna, J. Appl. Phys. 42, 5891 (1911). 'c. B. Eisenhandler and B. M. Siegel, J. Appl. Phys. 31, 1613 (1966). 'B. M. Siegel, Ber. Bunsenges. Phys. Chern. 74, 1115 (1910). 'K. Kobayashi and K. Sakaoku, Bull. lnst. Chern. Res. Kyoto Univ. 42, 473

(1964).

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03

RESOLUTION IN ELECTRON MICROSCOPY 5189

'K. Kobayashi and M. Ohara, Proceedings of the Sixth International Congress on Electron Microscopy, Kyoto 1966, p. 579 (unpublished).

'E. Suito and N. Uyeda, Kolloid-Z. 193,97 (1963). 'N. Uyeda, M. Ashida, and E. Suito, J. App!. Phys. 36, 1453 (1965). 'N. Uyeda, M. Ashida, and E. Suito in Ref. 5, Vo!. 1, p. 485. 'J. W. Menter, Proc. R. Soc. A 236,119 (1956). 10M. Robertson, J. Chern. Soc. (Lond.) 1935, 615 (1935). lie. J. Brown, Acta Crystallogr. A 21, 137 (1966). 12R. D. Heidenreich, Fundamentals of Transmission Electron Microscopy

(Interscience, New York, 1964), Chaps. VIII and X. "P. Hartman, Acta Crystallogr. 8, 521 (1955). "C. A. Taylor and H. Lipson, Optical Transform (G. Bell & Sons, London,

1964). "P. S. Turner and J. M. Cowley, Acta Crystallogr. A 25,475 (1969). \6 1. M. Cowley and A. F. Moodie, Acta Crystallogr. 12, 360 (1959). 170. Scherzer, J. App!. Phys. 20, 20 (1949). 18K_J. Hanszen, Advance in Optical and Electron Microscopy (Academic

London, 1971), Vo!' 4.

Energy spectra transmitted through iron slabs of bremsstrahlung produced in iron and gold targets by O.5-1.44-MeV electrons

Takashi Nakamura, Morio Takemura*, Hideo Hirayama, and Tomonori Hyodo Depanment of Nuclear Engineering, Kyoto University, Kyoto, Japan

(Received 23 June 1972)

The bremsstrahlung spectra transmitted through iron slabs were measured as a function of incident electron energy and iron-slab thickness. In this experiment, the gold and the iron targets were bombarded by O.5-·1.44-MeV electron beams from the Van de Graaff accelerator. Bremsstrahlung produced in the target and transmitted through the iron slab, which emerged at the forward direction of the incident electron beam, was detected with a NaI(Tl) scintillator. The experimental results were in good agreement with those of the approximate numerical calculation of the forward photon spectra. From the slopes of attenuation curves of total photon number and intensity, the effective attenuation coefficients and the effective energies were obtained as a function of the incident electron energy.

I. INTRODUCTION

A wide range of studies concerning the bremsstrahlung resulting from the interaction of electrons with thin tar­gets has been reported and the review of these results has been published. 1 For several years after these re­sults were published, many theoretical2

-5 and experi­

mental6 - 9 studies on thick-target bremsstrahlung pro­duction have been performed. This rapid progress is based upon the fact that a radiation hazard to manned space vehicles from electrons in the radiation belt sur­rounding the earth is primarily in the form of penetrat­ing bremsstrahlung produced by the energy degradation of electrons in the space-vehicle wall.

From the viewpoint of the radiation-shielding research, many works concerning the transmission of photons through matter have been confined in the case of mono­energetic y rays. For the transmission of photons of the continuous energy spectrum, such as reactor y rays, the multigroup attenuation kernel method has been used,10 but there has been no systematic study for the bremsstrahlung.

In this paper, the bremsstrahlung spectra transmitted through iron slabs were measured as a function of inci­dent electron energy and iron-slab thickness. A numer­ical calculation of the transmitted bremsstrahlung spectra was made in order to compare the experimental results and the necessity for data over a wide electron energy and material range, based on a numerical method by Ferdinande et al. 5

II. EXPERIMENTAL PROCEDURE

A. Experimental arrangement

The experimental arrangement is shown schematically in Fig. 1. The experiments consist of three cases.

The first case, case 1, is the measurement of brems-

strahlung spectra produced in a gold target as a function of incident electron energy. The electron beam from the VE-20 electron Van de Graaff accelerator bombarded a gold target attached to the end of the beam accelerating tube. The gold target were 1 mm thick and were sur­rounded with a O. 5-mm-thick stainless-steel cover. The configuration of the target is shown in Fig. 1. Brems­strahlung produced in the target, which emerged at the forward direction of the incident electron beam, passed through a lead collimator and was detected by a scintil­lation detector with a lead shield. The detector axis was set to coincide with the electron-beam axis. The de­fining aperture of the collimator was a cylindrical hole 1. 0 cm in diameter and 16.5 cm in thickness, with its front edge at a fixed distance 39.5 cm from the target. Then the distance from the target to the detector was 56.0 cm.

The second experiment, case 2, is the measurement of the forward photon spectrum transmitted through an iron slab as a function of iron-slab thickness, when the bremsstrahlung produced in the gold target was incident normally to the slab surface. The iron scatterer 40 x40 cm 2 in area was placed on the front edge of the collima­tor, perpendicular to the detector axis.

In the case-3 experiment, the electron beam through a thin mica-plate cover of the accelerating tube directly bombarded the iron scatterer under the same geometri­cal arrangement as in the case-2 experiment. The dis­tance between the mica plate and the front edge of the collimator was 12.0 cm. The transmitted spectra of bremsstrahlung produced in the iron scatterer were measured as a function of iron-slab thickness. The values of incident electron energy Eo and scatterer thickness T used in these three experiments are listed in Table 1.

The detector head conSisted of a 3-in. -diam by 3-in. -long NaI(Tl) scintillator together with a photomultiplier

J. App!. Phys., Vo!. 43, No. 12, December 1972

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

131.193.242.44 On: Tue, 02 Dec 2014 05:53:03