- Email: [email protected]
Growth of large fullerene C60 crystals and highly oriented thin films by physical vapor transport
j. . . . . . . . C R Y S T A L GIIROWTH
Journal of Crystal Growth 174 (1997) 821 827
ELSEVIER
Growth of large fullerene C6o crystals and highly oriented thin films by physical vapor transport Rong-Fu Xiao* Department of Physics. The Hong Kong University of Science and TechnoloL~', Clear ~ t e r Bay, Kowloon. Hong Kong
Abstract
We have grown C60 single crystals and thin films using physical vapor transport techniques. It took about 3M. weeks to grow C6o crystals with a dimension of up to about 8 mm in a sealed vacuum quartz tube. We have found that the growth proceeded by either the two-dimensional nucleation or step flow mode depending on the orientation of crystal surfaces. In contrast, C6o films grew much faster in an open quartz tube under vacuum. X-ray diffraction has revealed that these C6o films are highly oriented in either (1 1 l)/(cover-glass substrate) or (2 20)/(fused silica substrate) orientations. Our explanation for this phenomenon is that the orientation of C6o crystals in their early growth stage can be adjusted by the rotation of the individual C6o molecules at elevated temperatures. PA('S: 81.10.Bk: 68.55. - a K~wvords. Growth of C6o single crystal and thin film
1. I n t r o d u c t i o n
The discovery of fullerene C60 [1] has sparked intense interest directed towards both an understanding of the basic characteristics of the material and the possibility of new technological applications [2]. In addition to the studies of the properties of the molecule itself, efforts have been made to study condensed C6o for its characteristics as a van der Waals solid and a molecular crystal [3], as well as for its special properties such as superconductiv-
* Fax: +852 2358 1652; e-mail: [email protected].
ity [4], photoconductivity [5], and optical nonlinearity [6]. To uncover new chemical and physical properties of C6o fullerenes, high-quality single crystals are indispensable, Although C6o single crystals were first obtained from benzene solution [7], currently, the most widely used techniques to grow C~,o single crystals are physical v a p o r transport (PVT) [8] and deposition [9]. Thin films are a form of single crystalline sample that have a large surface area, and hence increase the effective sampling volume. Furthermore, thin films are necessary for studying the effect of finite size, low dimensionality, substrate/film interaction, and artificial layered structures. To
0022-0248/97/$17.00 Copyright :i' 1997 Elsevier Science B.V. All rights reserved Pll S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 5 1 - 1
822
R.-F. Xiao / Journal of Crystal Growth 174 (1997) 821-827
achieve heteroepitaxial growth of C6o thin films, various crystalline substrates have been used [10]. Although much effort has been made, most of the fullerene-based thin films currently available are polycrystalline over a large deposition area. In this article, we report our effort in the growth of large C6o bulk crystals and highly oriented thin films using hot-wall PVT techniques. The experimental setup for the growth of C6o crystals and thin films is described in Section 2 and the results are discussed in detail in Section 3.
Closed quartz vacuum tube TC1 \ Tea
(a)
C~ raw powder
vacuum
Quartz vacuum tube TCl~ ~
The experimental setup, which is schematically shown in Fig. 1, used a gold mirror furnace (from Trans Temp, USA). The furnace is transparent at high temperature ( > 600°C) or under a quartz fiber illumination. This is particularly useful for in situ viewing of the growing crystals or thin film. There are three independent temperature zones in the furnace, each controlled by a separate temperature controller (Eurotherm, model-818P). In order to grow large single C6o crystals without wasting too much source material, the growth chamber was sealed under vacuum (5 x 10 -v Torr) after the source material (C6o powder with 99.5% purity as specified by supplier) was inserted. The sealed tube was then put in the furnace with the C6o powder in the high-temperature zone-1 (Fig. la). To obtain crystals with highest quality, the raw C60 powder was first purified by condensing it to zone-2 using a symmetric temperature profile: T1 = T3 = 600°C a n d T 2 --- 550*C. Under these conditions, 70% of the C60 powder originally located at zone-1 was sublimed within 72 h. In our experiments, we had difficulty in evaporating all of the raw C6o powder. Throughout our growth experiments, we observed changes in the deposition rate in each growth process. The crystals grew very rapidly at the beginning and then growth slowed down gradually. Although there was still some remaining source material, it simply did not evaporate even at a higher sublimation temperature (TI = T3 = 620°C) over a prolonged period. Such unusual growth behavior has been observed previously [11]. We believe it is due to surface oxidation and/or
Sublimated C~
T~,
Growing crystals
Gold mirror furnace ~ ~oolingair
. r,
(b)
2. Experimental setup
Gold mirror furnace Ins-l-tin / TC~ - ;2 g
Insulating fibers 2 ,~
c60 powder Growing film
TCa
TC3
Removable joint
Fig. 1. Schematics of the hot-wall physical vapor deposition systems. TCj, TCz, TC3, and TC~ refer, respectively, to the thermocouple at zone-1, zone-2 (middle zone), zone-3, and at the back of the substrate: (a) sealed vacuum quartz tube for the growth of large C60 crystals; (b) setup for the growth of C60 films.
polymerization [12] of C6o powder at high temperatures after being exposed to the air. We found that once the raw powder had been purified in vacuum, the C6o could be completely evaporated in the next sublimation process in the same vaccum tube as long as the material was not exposed to the air again. The crystals were finally grown at zone-3. In order to obtain a small population of large crystals, the temperature profile was first set as T1 = 610°C, T 2 = 600°C and T3 = 580°C. Once a few small C6o seeds were formed (which took about 3~4 days), the T3 was raised to 590°C to reduce the supersaturation between zone-2 and zone-3 to prevent crystal morphological beakdown. It took about 3-4 weeks to obtain C6o crystals with a dimension of 4-8 mm. We found that the slower the growth rate, the higher the quality of the C6o crystals. C6o thin films were grown in a different experimental setup. As shown in Fig. 1b, the thin film was grown in a quartz chamber with a small opening at one end connected to a vacuum pump and a removable ground joint at the other end to facilitate the loading of source material and harvesting of the as-grown C6o films. The valve between the vacuum pump and the growth chamber was opened only occasionally to establish a sufficient vacuum to
R.-F. Xiao / Journal of C~stal Growth 174 (1997) 821-827
help material transport, and otherwise kept closed to prevent excessive loss of the material. The temperature at zone-1 and zone-3 was kept at a higher value than that at zone-2 to prevent the C6o molecules from condensing on the walls of zone-1 and zone-3. This temperature distribution was necessary to ensure that the thin films grew only in zone-2. A substrate was silver-pasted onto the flat tip of the inner tube which was located at the zone-2 of the furnace (see Fig. lb). To further ensure that the thin film only grew on the substrate and not on the wall of zone-2, air cooling was provided at the back of the substrate via a small glass tube which was heat-insulated from the inner tube of the vacuum chamber. By adjusting the temperatures between the outer wall of the quartz chamber and the substrate (via the adjustment of the flow rate of the cooling air), we were able to grow C~,o thin films with a thickness of 1 gm in 2-3 h.
3. Results and discussion
The pictures shown in Fig. 2 are typical C 6 o crystals grown in the sealed vacuum quartz tube (Fig. la). They are either single crystals (Fig. 2a) or crystals with some defects (Fig. 2b), with dimensions in the range of 4 8 mm. At the begining, all small crystals were single with shiny facets. The
823
defects were formed at the late stage (two weeks later) when crystals grew big. More detailed surface morphologies of the grown C60 crystals are shown in Fig. 3. The black dots seen on the crystal surfaces are C6o particles stuck on the crystal surfaces. They stemed from dropping the crystals, C~,o, into the powder when the quartz tube was broken to harvest the crystals. As one can see from this figure, both two-dimensional nucleation (2DN) growth (Fig. 3a) and step flow growth (Fig. 3b Fig. 3d) are present on the surfaces of these C6o crystals. It was quite unexpected to see 2DN growth iFig. 3at in this experiment since this mode usually occurs under conditions of high supersaturation. The presence of different growth modes indicates that, although the growth experiment was conducted very slowly, the growth of C6o crystals could proceed by different growth mechanisms for different surface orientations. To obtain C6o films with a high deposition rate, a higher supersaturation was used. The C6o thin film shown in Fig. 4a was grown with a temperature setting of T 1 = T 3 = 600c)C, T 2 = 550"C and Ts = 480°C. To see how the film was formed at the beginning, we have also taken a picture at a location 1 cm away from the middle of the film on the same substrate (fused silica). The resull in Fig. 4b can be viewed as an early growth stage of the film shown in Fig. 4a. One can see from this figure that the C6o film did not grow in a layer-by-layer
Fig. 2. C~,o crystals g r o w n in a v a c u u m - s e a l e d q u a r t z tube at T1 = T3 = 600'C. T 2 = 590'C: (a) single crystal: Ib) crystal with some defects.
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R.-F. Xiao / Journal of C~vstal Growth 174 (1997) 821-827
Fig. 3. Optical micrographs of the surfaces of C6o crystals grown under the conditions described in Fig. 2: (a) surface showing two-dimensional islands; (b) surface with steps; (c) surface with differently oriented steps; (d) surface with growth striations.
fashion. Instead, it grew in a multiple-nucleationand-spreading fashion as previosly shown by a computer simulation [13]. The nuclei formed earlier grew bigger while many other new nuclei were still forming. The continuous film was formed when these small crystals grew bigger and met. Although the Coo film shown in Fig. 4 does not exhibit a very smooth surface morphology, its high crystallinity is evident from the result of X-ray diffraction shown in Fig. 5a (note that a scale of square-root is used for diffraction intensity). The result in Fig. 5a matches all of the diffraction peaks of C6o thin films in their face-centered-cubic phase as observed in earlier experiments [14]. The strong diffraction peak (intensity ratio of I(2 2 0)/I(1 1 1) is about 16) in Fig. 5a indicates that the film is mostly aligned in its (2 2 0) orientation. Films from previous experiments, though grown on a crystalline substrate, usually showed three strong peaks, i.e., (1 1 1), (2 2 0), and (3 1 1), implying a random
oriented polycrystalline behavior. To check the growth behavior that we had found on a fused silica substrate, we repeated our growth experiment of C6o films on a regular microscope cover-glass under the same growth conditions used for the fused silica substrate shown in Fig. 4. In this case, a similar surface morphology was observed. Again, the C6o film exhibited a very high crystallinity as confirmed by X-ray diffraction (Fig. 5b). Instead of the preferred (2 2 0) orientation, this film was mainly aligned in the (1 1 1) direction (intensity ratio of I(1 1 1)/I(2 2 0) is about 24). We repeated our experiment several times, and similar results were obtained in each experiment, i.e., the C6o film was mainly aligned either in the (1 1 1) or (2 2 0) direction. The results in Figs. 4 and 5 imply that the choice of substrate is not as critical for the formation of crystalline C6o films as it is in other material systems. The reason for this may be that the
R.-F. Xiao / Journal ~?/ C~'stal Growth 174 (1997) 821 827
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10000"--
(a) 6400"
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,222 T~ 20
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Fig. 5. X-ray diffraction data of C~,o films: (al for the film shown in Fig. 4a, i.e., C6o/fused silica substrate; (b) for the fihn of C6o/cover-glass.
Fig. 4. C~, film grown on a fused silica substrate at T~ - T ~ - 6 0 0 'C. T 2 = 5 5 0 C , T ~ = 4 8 0 : C under vacuum. (a) A continuous film in the central area of the substrate: (bJ Discontinuous crystalline islands photographed at a location 1 cm away from the area shown in (a).
interaction between C60 molecules and the substrate is quite weak and, hence, the influence of the crystal structure of the substrate on the growing film is small. We have found that the as-grown C60 film can be easily separated from the substrate with a light tap (all crystals fell off when we broke the quartz tube to take them out). In contrast to the
substrate structure, substrate temperature has a significant effect on the crystallinity of the growing C60 film. According to our experience and that of others [10], crystalline C6o thin films can be obtained only at elevated substrate temperatures. At low growth temperatures, the C~,o films are amorphous no matter how well the lattice match between the substrate and the growing thin film. It is known that C60 molecules interact primarily through van der Waals forces. Owing to the quasispherical shape of individual C6o molecules, the C60 crystal is orientationally disordered [3], i.e., each molecule within the crystal can rotate freely around its equilibrium position, From the above results, we think that the orientation of an individual C6o crystal can be adjusted at an early stage in its growth by the freely rotating molecules in the crystal. Although the individual C~o nuclei are
826
R.-F. Xiao / Journal of C~stal Growth 174 (1997) 821 827
randomly oriented on an amorphous substrate at the beginning, they can adjust (relax) their orientations and grow in a preferred orientation if sufficient kinetic energy is provided by an elevated temperature at the substrate. When crystals grow bigger and meet each other, the orientation of each crystal will be affected by its neighboring crystals since the interaction between nucleated crystal and substrate is much weaker than that between the crystals themselves. As a result, the individual C6o crystals in the thin film will tend to align themselves in a preferred orientation so that they can reside in an energetically more stable position (state). Since the {1 1 1} planes in an fcc C6o crystal are closed-packed and the {2 2 0} planes are nearly closed-packed, the actual orientation that a growing thin film can assume will be possible in either of these two orientations, as shown in Fig. 5. In comparison to C6o thin films, we have found that isolated C6o single crystals were usually grown with {1 1 1}, {2 0 0} and {2 2 0} facets, and occasionally with {3 1 1} facets parallel to the substrate, even in the same experiment. We think the self-orientation-adjustment mechanism involved in the growth of C6o thin films is generally true for molecular crystals that possess orientational disorder, when the mutual interaction between the growing thin film and the substrate is weaker than that of the crystals themselves. Orientational disorder can provide large molecular mobility, and large mobility will help a crystal relax to its equilibrium configuration in a short time. In fact, large molecular mobility has been found in various molecular crystals [15]. For example, CBr4 can retain its equilibrium form in a nonequilibrium growth process due to the presence of orientational disorder [-16]. In conclusion, we have grown large C6o crystals and highly oriented C6o films by physical vapor deposition techniques. It took about 3~, weeks to grow C6o crystals of up to 8 mm long in a sealed vacuum quartz tube. Both two-dimensional nucleation growth and step flow growth were present during the growth process at different surfaces. In contrast, C6o films were grown much faster in an open quartz tube under vacuum. X-ray diffraction has revealed that these C6o films are highly oriented in either the (1 1 1)/(cover-glass substrate)
or (2 2 0)/(fused silica substrate) orientations. Our explanation for this phenomenon is that the orientation of C6o crystals at their early growth stage can be adjusted due to the free rotation of the individual C6o molecules at elevated temperatures. Hence, a single crystalline, lattice-matched substrate may not be necessary to grow a crystalline film of C6o fullerene.
Acknowledgements Financial support from Hong Kong Government RGC Grant (HKUST640/94P) is acknowledged.
References [1] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl and R.E. Smalley, Nature 318 (1985) 162. I-2] See, for example, D. Koruga, S. Hameroff, J. Withers, R. Loutfy and M. Sundareshan, Eds., Fullerene C6o History, Physics, Nanobiology, Nanotechnology (Elsevier, North-Holland, 1993). I-3] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglass and A.M. Mujsce, J. Phys. Chem. 95 (1991) 518; P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denestein, Jr., J.P. McCauley III and A.B. Smith, Phys. Rev. Lett. 66 (1991) 2991; W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare and K. Prassides, Europhys. Lett. 18 (1992) 219. I-4] See, for example: V. Kumar, T.P. Martin and E. Tosatti, Eds., Clusters and Fullerenes, Proc. Adriatico Research Conf. (World Scientific, Singapore, 1993) pp. 383445; J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini and M.-H. Whangbo, Organic Superconductors (including fullerenes): Synthesis, Structure, Properties, and theory (Prentice-Hall, Englewood Cliffs, NJ, 1992). 1-5] Y. Wang, Nature 356 (1992) 585. I-6] See, for example: D. Wilk, D. Johannsmann, C. Stanners and Y.R. Shen, Phys. Rev. B 51 (1995) 10057, and references therein. [7] W. Kratschmer, UD. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347 (1990) 354. 1-8] R.L. Meng, D. Ramirez, X. Jiang, P.C. Chow, C. Diaz, K. Matsuishi, S.C. Moss, P.H. Hor and C.W. Chu, Appl. Phys. Lett. 59 (1991) 3402; M.A. Verheijen, W.J.P. van Enckevort and G. Meijer, Chem. Phys. Lett. 216 (1993) 72. [9] J.Z. Liu, J.W. Dykes, M.D. Lan, P. Klavins, R.N. Shelton and M.M. Olmstead, Appl. Phys. Lett. 62 (1993) 531. 1-10] K. Tanigaki, S. Kuroshima and T.W. Ebbesen, Thin Solid Films 257 (1995) 154, and references therein.
R.-F. Xiao / Journal of Cr3,stal Growth 174 (1997) 821 827 [11] M.A. Verheijen, H. Meekes, G. Meier, E. Raas and P. Bennema, Chem. Phys. Lett. 191 (1992) 339; M. Haluska, H. Kuzmany, M. Vybornov, P. Rogl and P. Fejdi, Appl. Phys. A 56 (1993) 161. [12] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P.C. Wklund, D.S. Cornett, M.A. Duncan and I.J. Amster, Science 259 (1993) 955; Y.B. Zhao, D.M. Poirier, A.J. Pechman and J.H. Weaver, AppL Plays. Left. 64 (1994) 577.
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[13] R.-F. Xiao, J.I.D. Alexander and F. Rosenberger, Phys. Rev. A 43 (1991) 2977. [14] See, for example: A.F. Hebard, R.C. Haddon. R.M. Fleming and A.R. Kortan, Appl. Phys. Lett. 59 (1991) 2109: J.A. Dura, P.M. Pipperner, N.J. Halas, X.Z. Xiong, P.C. Chow and S.C. Moss, Appl. Phys. Lett. 63 (1993) 3443. [15] J. Timmermans, J. Phys. Chem. Solids 18 11961i I. [16] R.-F. Xiao and F. Rosenberger, J. Crystal Growth 114 (19911 536.
Journal of Crystal Growth 174 (1997) 821 827
ELSEVIER
Growth of large fullerene C6o crystals and highly oriented thin films by physical vapor transport Rong-Fu Xiao* Department of Physics. The Hong Kong University of Science and TechnoloL~', Clear ~ t e r Bay, Kowloon. Hong Kong
Abstract
We have grown C60 single crystals and thin films using physical vapor transport techniques. It took about 3M. weeks to grow C6o crystals with a dimension of up to about 8 mm in a sealed vacuum quartz tube. We have found that the growth proceeded by either the two-dimensional nucleation or step flow mode depending on the orientation of crystal surfaces. In contrast, C6o films grew much faster in an open quartz tube under vacuum. X-ray diffraction has revealed that these C6o films are highly oriented in either (1 1 l)/(cover-glass substrate) or (2 20)/(fused silica substrate) orientations. Our explanation for this phenomenon is that the orientation of C6o crystals in their early growth stage can be adjusted by the rotation of the individual C6o molecules at elevated temperatures. PA('S: 81.10.Bk: 68.55. - a K~wvords. Growth of C6o single crystal and thin film
1. I n t r o d u c t i o n
The discovery of fullerene C60 [1] has sparked intense interest directed towards both an understanding of the basic characteristics of the material and the possibility of new technological applications [2]. In addition to the studies of the properties of the molecule itself, efforts have been made to study condensed C6o for its characteristics as a van der Waals solid and a molecular crystal [3], as well as for its special properties such as superconductiv-
* Fax: +852 2358 1652; e-mail: [email protected].
ity [4], photoconductivity [5], and optical nonlinearity [6]. To uncover new chemical and physical properties of C6o fullerenes, high-quality single crystals are indispensable, Although C6o single crystals were first obtained from benzene solution [7], currently, the most widely used techniques to grow C~,o single crystals are physical v a p o r transport (PVT) [8] and deposition [9]. Thin films are a form of single crystalline sample that have a large surface area, and hence increase the effective sampling volume. Furthermore, thin films are necessary for studying the effect of finite size, low dimensionality, substrate/film interaction, and artificial layered structures. To
0022-0248/97/$17.00 Copyright :i' 1997 Elsevier Science B.V. All rights reserved Pll S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 5 1 - 1
822
R.-F. Xiao / Journal of Crystal Growth 174 (1997) 821-827
achieve heteroepitaxial growth of C6o thin films, various crystalline substrates have been used [10]. Although much effort has been made, most of the fullerene-based thin films currently available are polycrystalline over a large deposition area. In this article, we report our effort in the growth of large C6o bulk crystals and highly oriented thin films using hot-wall PVT techniques. The experimental setup for the growth of C6o crystals and thin films is described in Section 2 and the results are discussed in detail in Section 3.
Closed quartz vacuum tube TC1 \ Tea
(a)
C~ raw powder
vacuum
Quartz vacuum tube TCl~ ~
The experimental setup, which is schematically shown in Fig. 1, used a gold mirror furnace (from Trans Temp, USA). The furnace is transparent at high temperature ( > 600°C) or under a quartz fiber illumination. This is particularly useful for in situ viewing of the growing crystals or thin film. There are three independent temperature zones in the furnace, each controlled by a separate temperature controller (Eurotherm, model-818P). In order to grow large single C6o crystals without wasting too much source material, the growth chamber was sealed under vacuum (5 x 10 -v Torr) after the source material (C6o powder with 99.5% purity as specified by supplier) was inserted. The sealed tube was then put in the furnace with the C6o powder in the high-temperature zone-1 (Fig. la). To obtain crystals with highest quality, the raw C60 powder was first purified by condensing it to zone-2 using a symmetric temperature profile: T1 = T3 = 600°C a n d T 2 --- 550*C. Under these conditions, 70% of the C60 powder originally located at zone-1 was sublimed within 72 h. In our experiments, we had difficulty in evaporating all of the raw C6o powder. Throughout our growth experiments, we observed changes in the deposition rate in each growth process. The crystals grew very rapidly at the beginning and then growth slowed down gradually. Although there was still some remaining source material, it simply did not evaporate even at a higher sublimation temperature (TI = T3 = 620°C) over a prolonged period. Such unusual growth behavior has been observed previously [11]. We believe it is due to surface oxidation and/or
Sublimated C~
T~,
Growing crystals
Gold mirror furnace ~ ~oolingair
. r,
(b)
2. Experimental setup
Gold mirror furnace Ins-l-tin / TC~ - ;2 g
Insulating fibers 2 ,~
c60 powder Growing film
TCa
TC3
Removable joint
Fig. 1. Schematics of the hot-wall physical vapor deposition systems. TCj, TCz, TC3, and TC~ refer, respectively, to the thermocouple at zone-1, zone-2 (middle zone), zone-3, and at the back of the substrate: (a) sealed vacuum quartz tube for the growth of large C60 crystals; (b) setup for the growth of C60 films.
polymerization [12] of C6o powder at high temperatures after being exposed to the air. We found that once the raw powder had been purified in vacuum, the C6o could be completely evaporated in the next sublimation process in the same vaccum tube as long as the material was not exposed to the air again. The crystals were finally grown at zone-3. In order to obtain a small population of large crystals, the temperature profile was first set as T1 = 610°C, T 2 = 600°C and T3 = 580°C. Once a few small C6o seeds were formed (which took about 3~4 days), the T3 was raised to 590°C to reduce the supersaturation between zone-2 and zone-3 to prevent crystal morphological beakdown. It took about 3-4 weeks to obtain C6o crystals with a dimension of 4-8 mm. We found that the slower the growth rate, the higher the quality of the C6o crystals. C6o thin films were grown in a different experimental setup. As shown in Fig. 1b, the thin film was grown in a quartz chamber with a small opening at one end connected to a vacuum pump and a removable ground joint at the other end to facilitate the loading of source material and harvesting of the as-grown C6o films. The valve between the vacuum pump and the growth chamber was opened only occasionally to establish a sufficient vacuum to
R.-F. Xiao / Journal of C~stal Growth 174 (1997) 821-827
help material transport, and otherwise kept closed to prevent excessive loss of the material. The temperature at zone-1 and zone-3 was kept at a higher value than that at zone-2 to prevent the C6o molecules from condensing on the walls of zone-1 and zone-3. This temperature distribution was necessary to ensure that the thin films grew only in zone-2. A substrate was silver-pasted onto the flat tip of the inner tube which was located at the zone-2 of the furnace (see Fig. lb). To further ensure that the thin film only grew on the substrate and not on the wall of zone-2, air cooling was provided at the back of the substrate via a small glass tube which was heat-insulated from the inner tube of the vacuum chamber. By adjusting the temperatures between the outer wall of the quartz chamber and the substrate (via the adjustment of the flow rate of the cooling air), we were able to grow C~,o thin films with a thickness of 1 gm in 2-3 h.
3. Results and discussion
The pictures shown in Fig. 2 are typical C 6 o crystals grown in the sealed vacuum quartz tube (Fig. la). They are either single crystals (Fig. 2a) or crystals with some defects (Fig. 2b), with dimensions in the range of 4 8 mm. At the begining, all small crystals were single with shiny facets. The
823
defects were formed at the late stage (two weeks later) when crystals grew big. More detailed surface morphologies of the grown C60 crystals are shown in Fig. 3. The black dots seen on the crystal surfaces are C6o particles stuck on the crystal surfaces. They stemed from dropping the crystals, C~,o, into the powder when the quartz tube was broken to harvest the crystals. As one can see from this figure, both two-dimensional nucleation (2DN) growth (Fig. 3a) and step flow growth (Fig. 3b Fig. 3d) are present on the surfaces of these C6o crystals. It was quite unexpected to see 2DN growth iFig. 3at in this experiment since this mode usually occurs under conditions of high supersaturation. The presence of different growth modes indicates that, although the growth experiment was conducted very slowly, the growth of C6o crystals could proceed by different growth mechanisms for different surface orientations. To obtain C6o films with a high deposition rate, a higher supersaturation was used. The C6o thin film shown in Fig. 4a was grown with a temperature setting of T 1 = T 3 = 600c)C, T 2 = 550"C and Ts = 480°C. To see how the film was formed at the beginning, we have also taken a picture at a location 1 cm away from the middle of the film on the same substrate (fused silica). The resull in Fig. 4b can be viewed as an early growth stage of the film shown in Fig. 4a. One can see from this figure that the C6o film did not grow in a layer-by-layer
Fig. 2. C~,o crystals g r o w n in a v a c u u m - s e a l e d q u a r t z tube at T1 = T3 = 600'C. T 2 = 590'C: (a) single crystal: Ib) crystal with some defects.
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R.-F. Xiao / Journal of C~vstal Growth 174 (1997) 821-827
Fig. 3. Optical micrographs of the surfaces of C6o crystals grown under the conditions described in Fig. 2: (a) surface showing two-dimensional islands; (b) surface with steps; (c) surface with differently oriented steps; (d) surface with growth striations.
fashion. Instead, it grew in a multiple-nucleationand-spreading fashion as previosly shown by a computer simulation [13]. The nuclei formed earlier grew bigger while many other new nuclei were still forming. The continuous film was formed when these small crystals grew bigger and met. Although the Coo film shown in Fig. 4 does not exhibit a very smooth surface morphology, its high crystallinity is evident from the result of X-ray diffraction shown in Fig. 5a (note that a scale of square-root is used for diffraction intensity). The result in Fig. 5a matches all of the diffraction peaks of C6o thin films in their face-centered-cubic phase as observed in earlier experiments [14]. The strong diffraction peak (intensity ratio of I(2 2 0)/I(1 1 1) is about 16) in Fig. 5a indicates that the film is mostly aligned in its (2 2 0) orientation. Films from previous experiments, though grown on a crystalline substrate, usually showed three strong peaks, i.e., (1 1 1), (2 2 0), and (3 1 1), implying a random
oriented polycrystalline behavior. To check the growth behavior that we had found on a fused silica substrate, we repeated our growth experiment of C6o films on a regular microscope cover-glass under the same growth conditions used for the fused silica substrate shown in Fig. 4. In this case, a similar surface morphology was observed. Again, the C6o film exhibited a very high crystallinity as confirmed by X-ray diffraction (Fig. 5b). Instead of the preferred (2 2 0) orientation, this film was mainly aligned in the (1 1 1) direction (intensity ratio of I(1 1 1)/I(2 2 0) is about 24). We repeated our experiment several times, and similar results were obtained in each experiment, i.e., the C6o film was mainly aligned either in the (1 1 1) or (2 2 0) direction. The results in Figs. 4 and 5 imply that the choice of substrate is not as critical for the formation of crystalline C6o films as it is in other material systems. The reason for this may be that the
R.-F. Xiao / Journal ~?/ C~'stal Growth 174 (1997) 821 827
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Fig. 5. X-ray diffraction data of C~,o films: (al for the film shown in Fig. 4a, i.e., C6o/fused silica substrate; (b) for the fihn of C6o/cover-glass.
Fig. 4. C~, film grown on a fused silica substrate at T~ - T ~ - 6 0 0 'C. T 2 = 5 5 0 C , T ~ = 4 8 0 : C under vacuum. (a) A continuous film in the central area of the substrate: (bJ Discontinuous crystalline islands photographed at a location 1 cm away from the area shown in (a).
interaction between C60 molecules and the substrate is quite weak and, hence, the influence of the crystal structure of the substrate on the growing film is small. We have found that the as-grown C60 film can be easily separated from the substrate with a light tap (all crystals fell off when we broke the quartz tube to take them out). In contrast to the
substrate structure, substrate temperature has a significant effect on the crystallinity of the growing C60 film. According to our experience and that of others [10], crystalline C6o thin films can be obtained only at elevated substrate temperatures. At low growth temperatures, the C~,o films are amorphous no matter how well the lattice match between the substrate and the growing thin film. It is known that C60 molecules interact primarily through van der Waals forces. Owing to the quasispherical shape of individual C6o molecules, the C60 crystal is orientationally disordered [3], i.e., each molecule within the crystal can rotate freely around its equilibrium position, From the above results, we think that the orientation of an individual C6o crystal can be adjusted at an early stage in its growth by the freely rotating molecules in the crystal. Although the individual C~o nuclei are
826
R.-F. Xiao / Journal of C~stal Growth 174 (1997) 821 827
randomly oriented on an amorphous substrate at the beginning, they can adjust (relax) their orientations and grow in a preferred orientation if sufficient kinetic energy is provided by an elevated temperature at the substrate. When crystals grow bigger and meet each other, the orientation of each crystal will be affected by its neighboring crystals since the interaction between nucleated crystal and substrate is much weaker than that between the crystals themselves. As a result, the individual C6o crystals in the thin film will tend to align themselves in a preferred orientation so that they can reside in an energetically more stable position (state). Since the {1 1 1} planes in an fcc C6o crystal are closed-packed and the {2 2 0} planes are nearly closed-packed, the actual orientation that a growing thin film can assume will be possible in either of these two orientations, as shown in Fig. 5. In comparison to C6o thin films, we have found that isolated C6o single crystals were usually grown with {1 1 1}, {2 0 0} and {2 2 0} facets, and occasionally with {3 1 1} facets parallel to the substrate, even in the same experiment. We think the self-orientation-adjustment mechanism involved in the growth of C6o thin films is generally true for molecular crystals that possess orientational disorder, when the mutual interaction between the growing thin film and the substrate is weaker than that of the crystals themselves. Orientational disorder can provide large molecular mobility, and large mobility will help a crystal relax to its equilibrium configuration in a short time. In fact, large molecular mobility has been found in various molecular crystals [15]. For example, CBr4 can retain its equilibrium form in a nonequilibrium growth process due to the presence of orientational disorder [-16]. In conclusion, we have grown large C6o crystals and highly oriented C6o films by physical vapor deposition techniques. It took about 3~, weeks to grow C6o crystals of up to 8 mm long in a sealed vacuum quartz tube. Both two-dimensional nucleation growth and step flow growth were present during the growth process at different surfaces. In contrast, C6o films were grown much faster in an open quartz tube under vacuum. X-ray diffraction has revealed that these C6o films are highly oriented in either the (1 1 1)/(cover-glass substrate)
or (2 2 0)/(fused silica substrate) orientations. Our explanation for this phenomenon is that the orientation of C6o crystals at their early growth stage can be adjusted due to the free rotation of the individual C6o molecules at elevated temperatures. Hence, a single crystalline, lattice-matched substrate may not be necessary to grow a crystalline film of C6o fullerene.
Acknowledgements Financial support from Hong Kong Government RGC Grant (HKUST640/94P) is acknowledged.
References [1] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl and R.E. Smalley, Nature 318 (1985) 162. I-2] See, for example, D. Koruga, S. Hameroff, J. Withers, R. Loutfy and M. Sundareshan, Eds., Fullerene C6o History, Physics, Nanobiology, Nanotechnology (Elsevier, North-Holland, 1993). I-3] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglass and A.M. Mujsce, J. Phys. Chem. 95 (1991) 518; P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denestein, Jr., J.P. McCauley III and A.B. Smith, Phys. Rev. Lett. 66 (1991) 2991; W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare and K. Prassides, Europhys. Lett. 18 (1992) 219. I-4] See, for example: V. Kumar, T.P. Martin and E. Tosatti, Eds., Clusters and Fullerenes, Proc. Adriatico Research Conf. (World Scientific, Singapore, 1993) pp. 383445; J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini and M.-H. Whangbo, Organic Superconductors (including fullerenes): Synthesis, Structure, Properties, and theory (Prentice-Hall, Englewood Cliffs, NJ, 1992). 1-5] Y. Wang, Nature 356 (1992) 585. I-6] See, for example: D. Wilk, D. Johannsmann, C. Stanners and Y.R. Shen, Phys. Rev. B 51 (1995) 10057, and references therein. [7] W. Kratschmer, UD. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347 (1990) 354. 1-8] R.L. Meng, D. Ramirez, X. Jiang, P.C. Chow, C. Diaz, K. Matsuishi, S.C. Moss, P.H. Hor and C.W. Chu, Appl. Phys. Lett. 59 (1991) 3402; M.A. Verheijen, W.J.P. van Enckevort and G. Meijer, Chem. Phys. Lett. 216 (1993) 72. [9] J.Z. Liu, J.W. Dykes, M.D. Lan, P. Klavins, R.N. Shelton and M.M. Olmstead, Appl. Phys. Lett. 62 (1993) 531. 1-10] K. Tanigaki, S. Kuroshima and T.W. Ebbesen, Thin Solid Films 257 (1995) 154, and references therein.
R.-F. Xiao / Journal of Cr3,stal Growth 174 (1997) 821 827 [11] M.A. Verheijen, H. Meekes, G. Meier, E. Raas and P. Bennema, Chem. Phys. Lett. 191 (1992) 339; M. Haluska, H. Kuzmany, M. Vybornov, P. Rogl and P. Fejdi, Appl. Phys. A 56 (1993) 161. [12] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P.C. Wklund, D.S. Cornett, M.A. Duncan and I.J. Amster, Science 259 (1993) 955; Y.B. Zhao, D.M. Poirier, A.J. Pechman and J.H. Weaver, AppL Plays. Left. 64 (1994) 577.
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[13] R.-F. Xiao, J.I.D. Alexander and F. Rosenberger, Phys. Rev. A 43 (1991) 2977. [14] See, for example: A.F. Hebard, R.C. Haddon. R.M. Fleming and A.R. Kortan, Appl. Phys. Lett. 59 (1991) 2109: J.A. Dura, P.M. Pipperner, N.J. Halas, X.Z. Xiong, P.C. Chow and S.C. Moss, Appl. Phys. Lett. 63 (1993) 3443. [15] J. Timmermans, J. Phys. Chem. Solids 18 11961i I. [16] R.-F. Xiao and F. Rosenberger, J. Crystal Growth 114 (19911 536.