Atomic resolution ADF-STEM imaging of organic molecular crystal of halogenated copper phthalocyanine

ARTICLE IN PRESS

Ultramicroscopy 108 (2008) 545–551 www.elsevier.com/locate/ultramic

Atomic resolution ADF-STEM imaging of organic molecular crystal of halogenated copper phthalocyanine Mitsutaka Haruta, Kaname Yoshida, Hiroki Kurata, Seiji Isoda Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received 23 June 2007; received in revised form 22 August 2007; accepted 28 August 2007

Abstract Annular dark-field (ADF) scanning transmission electron microscopy (STEM) measurements are demonstrated for the first time to be applicable for acquiring Z-contrast images of organic molecules at atomic resolution. High-angle ADF imaging by STEM is a new technique that provides incoherent high-resolution Z-contrast images for organic molecules. In the present study, low-angle ADF-STEM is successfully employed to image the molecular crystal structure of hexadecachloro-Cu-phthalocyanine (Cl16-CuPc), an organic molecule. The structures of CuPc derivatives (polyhalogenated CuPc with Br and Cl) are determined quantitatively using the same technique to determine the occupancy of halogens at each chemical site. By comparing the image contrasts of atomic columns, the occupancy of Br is found to be ca. 56% at the inner position, slightly higher than that for random substitution and in good agreement with previous TEM results. r 2007 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 61.66.Hq Keywords: ADF-STEM; LAADF; High resolution; Organic crystal

1. Introduction Annular dark-field (ADF) scanning transmission electron microscopy (STEM) is a powerful technique for acquiring high-resolution images of materials, as demonstrated by the pioneering works of Crewe and co-workers [1,2]. The high-angle annular detector proposed by Howie [3] makes it possible to suppress diffraction contrast effects in ADF images. Such high-angle annular dark-field (HAADF) STEM images are considered to be incoherent, showing chemical image contrast (Z-contrast) [4–6], due to the dominated effect of thermal diffuse scattering (TDS) in the detected signal [7,8]. ADF-STEM has been employed to obtain many excellent results in the field of inorganic crystals, including defects [9,10], interfaces [11–13], and surfaces [14]. To date, high-resolution HAADF STEM imaging has been performed invariably for inorganic crystals. Corresponding author. Tel.: +81 77 4383052; fax: +81 77 4383055.

E-mail address: [email protected] (M. Haruta). 0304-3991/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2007.08.011

Since Uyeda et al. [15] reported the first successful observation of the structural image of organic molecular crystals at atomic resolution by TEM, many high-resolution TEM studies have been performed for organic materials [16]. The use of ADF-STEM to observe the structures of molecular crystals is then an emerging challenge, with the potential to allow structures to be analyzed quantitatively. However, molecular crystals are usually very sensitive to electron irradiation requiring careful control of observation conditions under the strong convergent electron beam employed in STEM. In the present study, the structural image of a molecular crystal of hexadecachloro-Cu-phthalocyanine (Cl16-CuPc; Fig. 1(a)) is acquired at atomic resolution by ADF-STEM as a demonstration of the technique, and images obtained for various Cu-phthalocyanine (CuPc) derivatives (polyhalogenated CuPc with Br and Cl) are treated quantitatively so as to measure the occupancy of halogens at each chemical site. It has been reported that Cl atoms preferentially attach to the outer sites such as the 2 and 3 positions of CuPc (Fig. 1(b)) during direct chlorination due

ARTICLE IN PRESS M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

546

Cl Cl Cl Cl

Cl

C C

C

C C C

Cl

1X

Cl Cl

CN NC N C C C C C N Cu N C C C C C C N C C NC CN C

C

C

Cl

Cl

Cl

C C C

Cl

C C C

Cl

Cl

Cl

X

Cl

X

2

3

X

X

C

C

C C C C

X

X4 X

CN NC N C C C C C N Cu N C C C C C C N C C NC CN C

C

C

X

X

X

C C C

C C C

X

X

X X

X

X = Br or Cl

Fig. 1. Molecular structures of (a) Cl16-CuPc and (b) polyhalogenated CuPc with Br and Cl at positions 1–4 (x ¼ Br, Cl). 0 µm

2. Methods Specimens of Cl16-CuPc and Br8Cl8-CuPc were prepared as STEM samples on a (0 0 1)-cleaved surface of singlecrystal KCl by vacuum deposition consistent with previous studies [19,20]. During deposition, the temperature of the substrate was maintained at 320 1C, and the thickness of the deposited layer was controlled to 30–40 nm by monitoring the deposition rate using a quartz microbalance. The thickness of the formed films was measured by atomic force microscopy (AFM). A typical AFM image of a specimen of Cl16-CuPc on the substrate is shown in Fig. 2. The height profile of the AFM image indicates that the film is composed of flat crystallites of relatively uniform thickness (30–40 nm). In the case of Br8Cl8-CuPc, the averaged ratio of Br to Cl was determined by elemental analysis of the halogens species (Br and Cl atoms) using an energydispersive X-ray (EDX) spectroscope (EDAX-DX4, Philips) associated with a scanning electron microscope (SEM; ABT-150B, Topcon). The EDX measurements indicate an

90 nm

to the size effect [17]. In octabromooctachloro-Cu-phthalocyanine (Br8Cl8-CuPc, Fig. 1(b)), for which CuPc is halogenated simultaneously by both Br and Cl, a Br atom is expected to substitute preferentially at the 2 and 3 positions due to its larger size compared to Cl. Pioneering work on this subject by TEM [18] has revealed unexpectedly high (52%) occupancy of larger Br atoms at the inner positions compared to the random cases. In these high-resolution TEM measurements, a minute difference between the inner and outer contrasts of halogenated sites was detected. However, coherent TEM image contrast is strongly dependent on experimental conditions such as defocus, specimen thickness, and other factors. HAADF-STEM images, on the other hand, can be acquired in such a way that the intensity is approximately proportional to Zn, where Z is the atomic number of the scattering atom and n is a fixed number, allowing quantitative and straightforward interpretation. That is, the incoherent characteristics of HAADF image render the technique substantially less sensitive to imaging conditions compared to conventional TEM.

3.5 µm

0

-90 0.0

1.00

2.00 µm

3.00

4.00

Fig. 2. AFM image of Cl16-CuPc vacuum deposited on KCl at 320 1C and corresponding surface profile (inset).

atomic ratio of Br to Cl atoms of almost 1:1 based on the Cl-K and Br-L lines, showing that the compound is composed of equal amounts of both halogens on average. For high-resolution imaging, samples of halogenatedCuPc crystallites were prepared by formation on KCl and covering with a thin amorphous carbon film support. The KCl substrate was then removed on a surface of distilled water and the crystals covered with carbon film were fixed on a microgrid. ADF-STEM observations were performed at room temperature using a 200 kV TEM/STEM (JEM-2200FS, JEOL) instrument equipped with a Schottky emission gun. The annular detection angle was set at 24–64 mrad by selecting a camera length of 12 cm, corresponding to lowangle annular dark-field (LAADF) imaging rather than HAADF-STEM (typical detection angle: 50–170 mrad). The characteristics of the LAADF image were clarified by multislice simulation using the Win HREM v. 2.5 software package [21].

3. Results and discussion The crystal structure of Cl16-CuPc is illustrated schematically in Fig. 3. The molecules are packed in a basecentered monoclinic structure isomorphic to Br8Cl8-CuPc, with unit-cell dimensions of a ¼ 1.962 nm, b ¼ 2.604 nm, c ¼ 0.376 nm, and b ¼ 116.51 [22]. The a–b plane of the monoclinic structure (i.e., molecular planes) is parallel to the substrate surface as expected for vacuum deposition. The a*–b* net pattern (c-axis projection) was obtained by

ARTICLE IN PRESS M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

Cl Cl Cl Cl

C C C

Cl

Cl

Cl

Cl

C C C

Cl

C C C

b Cl Cl

Cl

C C C

Cl

Cl

C

Cl

C

Cl Cl

Cl

Cl

C C C

Cl Cl

Cl

C

Cl

C C C

Cl

Cl

Cl

a

C

Cl

Cl

C C C

Electron beam

26.5˚

Cl

C C

C

C C C

Cl Cl

C

Cl

Cl

c

Cl

Cl

CN NC N C C C C C N Cu N C C C C C C C N C NC CN C

Cl

Cl

Cl

C

Cl

Cl

Cl

Cl

Cl

Cl

Cl Cl

C C

Cl Cl

C C

Cl

Cl

C C C

Cl

Cl

C C

C

Cl

Cl

C C

Cl

C C C

CN NC C N C C C C C C C N Cu N C C C C C C N C C NC CN

C

C

Cl

Cl

Cl

C C

C

Cl

CN NC N C C C C C N Cu N C C C C C C N C C NC CN C

CN NC N C C C C C N Cu N C C C C C C N C C NC CN C

Cl

Cl

C

Cl

C C

C

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

CN NC C N C C C C C C C N Cu N C C C C C C N C C NC CN

Cl

Cl

Cl

Cl

C C

C

547

Cl

C C C

Cl

C C C

Cl

Substrate

a

Cl

Cl

Cl

Fig. 3. Schematic illustrations of (a) molecular arrangement in the crystal and (b) incident electron direction at the monoclinic Cl16-CuPc crystallite. Molecular images are obtained with the beam transmitting along the c-axis.

transmitting electrons along a direction inclined 26.51 from the normal to the basal plane of the crystallites (Fig. 3) [15]. High-resolution images were observed along the c-axis with the specimen inclined with respect to the incident electron beam. As the materials are radiation sensitive, crystal orientation was required to be adjusted as quickly as possible. The radiation damage to the specimen is estimated to be 30 C cm2 as a total end-point dose (TEPD) based on measurements of the decrease in diffraction intensity, similar to the TEPF for Br8Cl8-CuPc. To avoid excessive radiation damage, the alignment of crystal orientation was carried out by observing microdiffraction patterns with a convergence semi-angle of 3.4 mrad (aperture size: 10 mm) as shown in Fig. 4. The high-resolution ADF-STEM experiments were performed using an electron probe with a convergence semi-angle of 13.2 mrad (aperture size: 40 mm). As the TDS intensity is expected to be low in these specimens, high-resolution HAADF-STEM observations would have been very difficult. Therefore, observations under the conditions of LAADF-STEM were attempted in the present study in order to utilize the stronger electron intensity of TDS. A typical experimental LAADF-STEM image of the Cl16-CuPc molecular crystal is shown in Fig. 5(a). The image is strongly affected by noise, probably due to the weakness of the detected signals, electron radiation damage, and noise from the supporting carbon film. However, periodic bright spots can be observed in the image, corresponding to the atomic Cu columns at the center of each molecule (Z-contrast). Fast Fourier transformation (FFT) of this image (Fig. 5(b)) confirms that the spatial resolution of the raw image is 0.2 nm. Noise filtering improves the signal-to-noise ratio, as shown in Fig. 5(c). The central Cu columns and peripheral Cl columns in the

Fig. 4. Microdiffraction patterns of c-axis projection. Convergence semiangle of the electron probe is 3.4 mrad. Inner detection angle is indicated by a white circle.

molecules can be observed clearly in the noise-filtered image, and positions of columns correspond well with those determined by TEM of the same sample [15]. It should be emphasized that the image contrasts as shown in Fig. 5(c) are quantitative. The contrasts of the Cu columns appear consistently brighter than those of the Cl columns due to the higher atomic number of Cu compared to Cl. The details of quantitative contrast analysis will be discussed later. Lighter elements, the C and N columns, are invisible in this image, presumably due to the narrow dynamic range of the detector employed in these measurements and the limited spatial resolution of the present experiment.

ARTICLE IN PRESS M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

548

Normalized intensity (arb. unit)

Cu

Cu

1.0 0.8

120

0.6

Cl

Cl

0.4

100

0.2

80

0.0 0.0

0.5

1.0 1.5 2.0 Distance (nm)

2.5

0

0.2

0.4

0.6

nm

Fig. 5. Observed LAADF-STEM image of Cl16-CuPc projected along the c-axis at 4M-fold magnification: (a) experimental raw image, (b) FFT pattern, (c) noise-filtered image, (d) intensity profile along the line in (c) (solid line, observed; broken line, calculated), and (e) intensity profile of halogenated sites.

Using these high-resolution ADF-STEM images as a basis, the 12 cm camera length (detection angle: 24–64 mrad) corresponding to LAADF imaging was then applied. This detection angle range was found to provide incoherent Z-contrast images as in the usual HAADFSTEM case, which can be verified from the multislice image simulation as described below. Fig. 6 shows the results of LAADF-STEM image simulation. The image contrast due to elastic electrons (Fig. 6(a)) changes with specimen thickness as in the TEM case. Contrary to this, the image contrast of TDS (Fig. 6(b)) does not change with specimen thickness as expected. However, as the actual LAADFSTEM image contrast consists of both elastic and TDS components, both contributions should be considered, as shown in Fig. 6(c). The LAADF-STEM images still exhibit almost no change with specimen thickness, suggesting that the major contribution in the images originates from TDS, even under the present LAADF conditions. Fig. 7(a) shows the maximum intensities (from Fig. 6) of the elastic and TDS components as a function of specimen thickness based on multislice simulation, and Fig. 7(b) plots the ratio of these contributions. For the present specimen thickness of 30–40 nm, the TDS intensity is 60–80 times higher than the elastic intensity, ensuring incoherent imaging. As shown in Fig. 5(d), the experimental and simulated intensity profiles along a line crossing Cu and Cl columns are in good agreement. The intensity ratio between Cu and Cl columns is 2.5, corresponding to a ca. Z1.7 dependence of LAADF-STEM image contrast for Cl16-CuPc [23,24]. The reason for the good correspondence between the LAADF-STEM image of this molecule and the Z-contrast image can be understood as follows. As the lattice constants of Cl16-CuPc are comparatively large, elastically

Specimen thickness 0.376 nm

11.28 nm

20.68 nm

30.08 nm

37.60 nm

Fig. 6. Simulated through-thickness images of Cl16-CuPc, showing contributions from (a) elastically scattered electrons, (b) TDS and (c) LAADF-STEM images combining elastic and TDS components. Simulation conditions: incident energy, E0 ¼ 200 keV; spherical aberration, Cs ¼ 1 mm; defocus, Df ¼ 50 nm; convergence semi-angle, a ¼ 13.2 mrad; detection angle, b ¼ 24–64 mrad; slice thickness, 0.376 nm; 100 slice thickness, 37.6 nm.

diffracted beams are scattered mainly at lower angles (Fig. 8). This indicates that the elastic component could be low under the present experimental conditions. Moreover, the Debye–Waller factor for organic crystals is generally large compared to that of inorganic crystals, which means that the elastic component can be dumped more rapidly

ARTICLE IN PRESS

Maximum Intensity (TDS and elastic)

M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

Detection angle: 24–64 mrad

0.15

TDS

0.10 0.05

Elastic 0.00 0

10 20 30 Thickness (nm)

Intensity ratio Inelastic / elastic

100 80 60 40 20 0 10

20 30 Thickness (nm)

Fig. 7. (a) Intensity change of elastic and TDS contributions as a function of thickness and (b) corresponding ratio of elastic to TDS contributions.

Fig. 8. Selected-area diffraction pattern of c-axis projection for Cl16-CuPc crystal. In LAADF-STEM with 24–64 mrad detection angle, elastically diffracted beams pass almost entirely through the inside of the ADF detector.

and the TDS signals become dominant even at lower scattering angles. There is also the possibility that the highangle diffraction intensity is weakened by electron irradiation damage in organic molecular crystals [25]. Therefore,

549

the present detection angle ensures that incoherent imaging is dominant even in LAADF-STEM for organic crystals such as Cl16-CuPc with large lattice dimensions. LAADFSTEM of organic crystals is thus considered to be effective for Z-contrast imaging as in the case of HAADF imaging with an inner angle of 50 mrad or more for inorganic crystals. The present atomically resolved Cl16-CuPc molecular image obtained by ADF-STEM thus represents the first high-resolution Z-contrast imaging of organic molecular crystals. Atomic-resolution LAADF-STEM imaging of Br8Cl8CuPc was then attempted. In the raw experimental image (Fig. 9(a)), the brightest atomic columns are Cu columns (atomic number: 29), whereas the element with the largest atomic number is Br (atomic number: 35). This result indicates that the Br atom is not localized on one of the halogenated sites (sites 1–4, Fig. 1(b)), instead mixing with Cl atoms (atomic number: 17) along the column axis. In this paper, by considering the symmetry of the molecule, the halogenated sites are classified into inner sites (sites 1 and 4, Fig. 1(b)) and outer sites (sites 2 and 3, Fig. 1(b)). The noise-filtered images are superimposed by rotating by 1801 (Fig. 9(b)). Fig. 9(c) shows the intensity profile along sites 4 and 3 (from inner to outer), revealing that the contrast is clearly brighter at inner sites. If the substitution was random, the image contrast should be the same for all substituted positions, as in Cl16-CuPc (Fig. 5(e)). According to the quantitative estimation of LAADF-STEM contrast proportional to Z1.7 in Cl16-CuPc, the occupancy of Br atoms at inner positions (sites 1 and 4) is concluded to be approximately 56%, which is slightly higher than for random substitution and in good agreement with previous TEM results [18]. The reason for this kind of inhomogeneity in substitution remains unclear, as the synthesis route is considerably complex. The present results indicate that the size effect is not an effective mechanism in this synthesis and therefore an alternative mechanism should be considered. In TEM, the image contrast is strongly dependent on the experimental conditions, and quantitative determination of occupancy requires extensive processing. ADF-STEM is thus more quantitative than straightforward compared to TEM analysis, and can be expected to be useful as a general tool for studying both inorganic and organic crystals. The clearest images in the present study were observed at 6M-fold magnification, even though the maximum magnification available for atomic-resolution imaging was 8Mfold magnification. At magnification higher than 6M, the molecular crystal was completely destroyed by electron bombardment, and the lower part of the molecular image became completely obscured (Fig. 9(a)), as happened often in the present experiments. This may originate from the accumulation of small distortions of the crystal lattice during beam scanning. The ADF image in Fig. 9 was recorded with a step size of 0.05 nm and a dwell time of 60 ms per pixel. The probe current was set at approximately 5 pA, corresponding to an average electron dose of ca.

ARTICLE IN PRESS M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

550

10000

3

8000

4

6000 0

0.2

0.4 nm

0.6

Fig. 9. Observed LAADF-STEM image of c-axis projection of Br8Cl8-CuPc at 6M-fold magnification: (a) experimental raw image, (b) noise-filtered image and (c) intensity profile of halogenated sites.

camera. The full-width at half-maximum (FWHM) of the electron probe is 0.18 nm, and the full-width at tenthmaximum is roughly 0.35 nm. One pixel corresponds to dimensions of 0.05  0.05 nm2 at 6M-fold magnification, the probe size is larger than the pixel size, and unnecessary irradiation will occur before the probe arrives at an objective position (Fig. 10(b)). This is a serious problem inherent in STEM observation of radiation-sensitive materials. In order to achieve ADF-STEM at higher resolution for organic crystals, unnecessary irradiation should be minimized, requiring a finer electron probe. Such a system would promise lower-dose observation for organic crystals and allow imaging at higher spatial resolution.

0.18 nm ~ 0.35 nm

Probe position

FWHM

Scan 0.35 nm

0.05 nm

Pixels irradiated before probe arrives

Fig. 10. (a) STEM probe measured by CCD camera. (b) Probe size on scan area at 6M-fold magnification.

12 C cm2, smaller than the end-point dose for the molecular crystal. However, the effective electron dose at each scanning point will vary due to the finite size of the probe. Fig. 10(a) shows the intensity profile of the STEM probe under the above conditions measured by CCD

4. Conclusion High-resolution Z-contrast images of organic molecular crystals at atomic resolution were obtained for the first time by ADF-STEM. In molecular crystals with comparatively large lattice constants, such as the Cl16-CuPc sample examined in this study, LAADF-STEM observation with a detection angle of 24–64 mrad was found to be advantageous for acquiring incoherent Z-contrast images similar to the case of conventional HAADF-STEM, as verified by multislice image simulation. From the contrast differences in the halogenated sites in Br8Cl8-CuPc, LAADF-STEM image provides information on the occupancy of Br and Cl at each chemical site in the molecule based on a Z1.7 contrast scheme. Quantitative estimation in this manner indicates Br occupancy of 56% at the inner position (sites 1 and 4, Fig. 1(b)), slightly higher than that for random substitution and in agreement with previous TEM results [18].

ARTICLE IN PRESS M. Haruta et al. / Ultramicroscopy 108 (2008) 545–551

References [1] J. Wall, J. Langmore, M. Isaacson, A.V. Crewe, Proc. Natl. Acad. USA 71 (1974) 1. [2] A.V. Crewe, J.P. Langmore, M.S. Isaacson, in: B.M. Siegel, D.R. Beaman (Eds.), Physical Aspects of Electron Microscopy and Microbeam Analysis, Wiley, New York, 1975, pp. 47–62. [3] A. Howie, J. Microsc. 117 (1979) 11. [4] S.J. Pennycook, L.A. Boatner, Nature 336 (1988) 565. [5] D.H. Shin, E.J. Kirkland, J. Silcox, Appl. Phys. Lett. 55 (1989) 2456. [6] S.J. Pennycook, Ultramicroscopy 30 (1989) 58. [7] R.F. Loane, P. Xu, J. Silcox, Acta Crystallogr. A 47 (1991) 267. [8] D.E. Jesson, S.J. Pennycook, Proc. R. Soc. Lond. A 449 (1995) 273. [9] A.J. McGibbon, S.J. Pennycook, J.E. Angelo, Science 269 (1995) 519. [10] P.E. Batson, Phys. Rev. B 61 (2000) 16633. [11] M.M. McGibbon, N.D. Browning, M.F. Chisholm, A.J. McGibbon, S.J. Pennycook, V. Ravikumar, V.P. Dravid, Science 266 (1994) 102. [12] N.D. Browning, S.J. Pennycook, J. Phys. D 29 (1996) 1779. [13] Y. Xin, S.J. Pennycook, N.D. Browning, P.D. Nellist, S. Sivananthan, F. Omne`s, B. Beaumont, J.P. Faurie, P. Gibart, Appl. Phys. Lett. 72 (1998) 2680.

551

[14] C.M. Wang, Y. Zhang, V. Shutthanandan, D.R. Baer, W.J. Weber, L.E. Thomas, S. Thevuthasan, G. Duscher, Phys. Rev. B 72 (2005) 245421. [15] N. Uyeda, T. Kobayashi, K. Ishizuka, Y. Fujiyoshi, Chem. Scr. 14 (1979) 47. [16] T. Kobayashi, in: N. Karl (Ed.), Crystal, Growth, Properties and Application, vol. 13, Springer, Berlin, Heidelberg, 1991, p. 2. [17] S. Masahiro, Kogyo-Kagaku-Zassi 62 (1959) 110 (in Japanese). [18] T. Kobayashi, T. Ogawa, S. Moriguchi, T. Suga, K. Yoshida, H. Kurata, S. Isoda, J. Electron Microsc. 52 (2003) 85. [19] T. Kobayashi, Y. Fujiyoshi, K. Ishizuka, N. Uyeda, Proc. Intern. Congr. Electron Microsc. (1980) 158. [20] S. Moriguchi, S. Isoda, T. Kobayashi, J. Electron Microsc. 51 (2002) 303. [21] K. Ishizuka, Ultramicroscopy 90 (2002) 71. [22] N. Uyeda, T. Kobayashi, E. Suito, Y. Harada, M. Watanabe, J. Appl. Phys. 43 (1972) 5181. [23] E.J. Kirkland, R.F. Loane, J. Silcox, Ultramicroscopy 23 (1987) 77. [24] P.M. Voyles, J.L. Grazul, D.A. Mullar, Ultramicroscopy 96 (2003) 251. [25] W.R.K. Clark, J.N. Chapman, A.M. Macleod, R.P. Feffier, Ultramicroscopy 5 (1980) 195.