- Email: [email protected]
Spiral growth mechanisms of CMTD crystals
ARTICLE IN PRESS
Journal of Crystal Growth 261 (2004) 63–69
Spiral growth mechanisms of CMTD crystals Kunpeng Wang*, Daliang Sun, Jianxiu Zhang, Wentao Yu, Hong Liu, Xiaobo Hu, Shiyi Guo, Yanling Geng State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, 27 Shanda, Nanlu, Jinan 250100, PR China Received 23 February 2003; accepted 5 September 2003 Communicated by D.T.J. Hurle
Abstract We present the results of an in situ atomic force microscopy investigation of the growth mechanisms of CdHg(SCN)4(H6C2OS)2CMTD crystals. The results show that spiral growth at screw dislocations dominates at a supersaturation s ¼ B0:1%. On the (0 0 1) surface, alternate CMTD layers are related through a 21 screw axis and the development of a spiral protuberance was observed with alternate growth modes having two different characteristic layers. The growth of the first CMTD layer continued for 16 min before the next layer began to grow on top of it. However, it took only 2 min for the next layer to continue its growth. We felicitously refer to them as ACTIVE layers and LAZY layers in this paper. The LAZY layers separated into two different types of elementary-steps with step height h1 ¼ 0:69 nm and h2 ¼ 0:72 nm on top of the spiral protuberance on the {0 0 1} face of the CMTD crystals. Two different types of straight edge always in the same direction were periodically observed when the screw protuberance grows. r 2003 Elsevier B.V. All rights reserved. PACS: 81.10.h; 81.10.aj Keywords: A1. Atomic force microscopy; A1. Growth modes; A1. Spectral dislocation; A1. Straight step; B1. CdHg(SCN)4 (H6C2OS)2CMTD
1. Introduction Materials are unsuitable for in situ observation using AFM when they grow quickly. The reason is that the scanning of the microscope tip lags behind the motion of the step if the crystal grows too fast. Therefore, the collected image is not exactly true to *Corresponding author. Tel.: +86-531-683-2632; fax: +86531-836-4451. E-mail address: [email protected] (K. Wang).
the actual morphology. In this paper, we report success in observing a series of consecutive growth processes using in situ AFM. These observation include a novel periodic growth process of spiral protuberances, an interaction process between twodimensional nuclei and spiral dislocations and a reconstruction process involving three-dimensional nuclei and amorphous aggregates on the {0 0 1} surface of CMTD crystals, which are semi organic coordination compounds with a relatively high kinetic growth coefficient under normal conditions.
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.09.011
ARTICLE IN PRESS 64
K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
CMTD is a highly efficient nonlinear optical (NLO) material for generating blue-violet light by laser frequency doubling. It belongs to the orthorhombic system, space group P212121 with lattice parameters of a ¼ 0:85188ð6Þ nm, b ¼ 0:85398ð7Þ nm, c ¼ 2:8224ð6Þ nm, v ¼ 2:0533ð5Þ nm3, and Z ¼ 4 [1–3].
instrument (Digital Instruments). A 90 mm 90 mm scanner with standard 200 mm SiN cantilevers with a small force constant of 0.12 N/m integrated with NP (Nanoprobe) tips was used. Images were collected at a scan rate of 2 Hz with a scan raster of 256 256. During imaging, care was taken to continually adjust the set point voltage to the lowest possible value.
2. Experimental procedure 3. Results and discussion Crystals of CMTD were prepared by using dimethy1 sulphoxide as a ligand to react with cadmium mercury thiocyanate (CMTC) in a mixture of dimethy1 sulphoxide and de-ionized water. The chemical reaction is CdHgðSCNÞ4 þ 2ðCH3 Þ2 SO -CdHgðSCNÞ4 2ðCH3 Þ2 OS: Samples were crystallized by spontaneous nucleation in a saturated solution made of CMTC as the raw material, and dissolved in a mixture of analytical grade reagent dimethy1 sulphoxide and de-ionized water (volume ratio 3:1). When the crystal samples reach the required size, a single crystal without macro defects is quickly removed from the culture dish and fixed by double adhesive tape in an AFM sample holder. Care was taken to keep the {0 0 1} crystal faces parallel to the surface of the holder and to keep the surface fresh all the while. Before making observations, the surface was daubed several times using a strip of damp filter paper that was immersed into the equilibrium solution for a short time. After the tip is engaged, solute molecules, that have not been completely attached to the steps can possibly escape from the surface due to the energy increase resulting from the tip’s scanning motion, and they can subsequently enter the solution on the surface. On the other hand, the solubility of CMTD will decrease if nonionized water is added to the solution and will increase upon injecting dimethy1 sulphoxide. The 70% relative humidity of the laboratory made the crystal surface absorb more water and therefore decreased the solubility of CMTD on the surface. Images were collected in situ in contact mode using a Nano-scope, IIIa dimension 3100 AFM
3.1. The formation of a partial dislocation with burgers vector h1+h2=0.69+0.72 nm=1.41 nm In the AFM experiment, the development from the initiation through the evolution of a screw dislocation with Burgers vector b ¼ h1 þ h2 ¼ 0:69 þ 0:72 nm ¼ 1:41 nm (Figs. 1a and 3i), was observed and its velocity was measured to be about 0.36 nm/s. The enlarged stereoscopic map, containing the location labeled ‘‘o’’ where the screw dislocation has formed, is shown on the right upper frame in Fig. 1a. Normally the formation of a screw dislocation is considered to be result of stress. From the image, we can assume that impurity particles have probably dropped on to the common boundary between the bunched steps and the elementary steps (Fig. 1a(o)). A spiral dislocation may be produced when a step encounters an impurity particle. Sketches are shown in Fig. 1c. 3.2. The alternating growth of the ACTIVE layers and LAZY layers of the spiral protuberance and the formation of two different types of elementary step To make the images clearer, we adjusted the scan angle several times but kept the other parameters unchanged from beginning to end of the experiment. As a result we were able to collect a new image every 2 min. A spiral protuberance was observed to evolve in an alternating growth mode consisting of two different characteristic layers. The growth of the first CMTD layer continued for 16 min before the next layer began to grow on top of it. However, the next layer continued to grow for only 2 min. As
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
65
Fig. 1. (a–b) The formation process of a screw dislocation along with a stereoscopic enlarged map, showing the location where the screw dislocation formed is shown on the upper right. (c) Diagrammatic sketch of the formation causes due to impurity particles. Scan areas are 10 10 mm2 for all AFM images.
a matter of convenience, we will felicitously refer to them as ACTIVE layers and LAZY layers, respectively, with the labels ‘‘a’’ and ‘‘L’’, respectively in Fig. 3a. On the (0 0 1) surface, alternate CMTD layers are related by a 21 screw axis, and so the anisotropy of the alternating layers should be reversed. This prediction is reflected by the alternating growth of the LAZY and ACTIVE layers in our experiment. The growth velocity of the ACTIVE layers is measured to be about 8 times that of the LAZY layers. A total of five entire periodic growth processes have been observed in our experiment. Two types of elementary-steps are measured on the top of the spiral dislocation and they are measured to be 0.69 and 0.72 nm, respectively (Figs. 3j and k). The height of an elementary-step should be consistent with the dimensionality of the growth motif. From the Raman spectrum of the CMTC solution [4–7], we can assume that the tetrahedral HgS4 and octahedral Cd(CN)4(OS)2
growth motifs can probably be associated with the kinks on the CMTD crystal surfaces. From the structure of the CMTD unit cell (Fig 2a), we can see that there are four molecular layers along the c direction. The theoretical height of the first layer within the unit-cell is calculated to be 0.69 nm and the second layer is calculated to be 0.72 nm along the c-axis (Fig. 2a). The third and fourth layers are simply related to the first and second layers by the 21 axis. The first and second layers are usually incorporated into bunched steps (h1 þ h2 ¼ 0:69+0:72 nm¼ 1:41 nm). We designate the bunched steps as the ACTIVE layers. The third and fourth layers often appear separated into two elementary-steps (corresponding to one layer of molecules), and we designate them as the LAZY layers. Figs. 3(b–e) show the consecutive growth process of the LAZY layers. The LAZY layers are difficult to grow along the (a b) plane for some unknown reason and are therefore separated
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K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
Fig. 2. (a) The structure of the CMTD unit cell along the c-axis. (b) The structure of the {0 0 1} face.
into two types of elementary-steps with step heights h1 ¼ 0:69 nm and h2 ¼ 0:72 nm, respectively. It is surprising that the upper step (0.72 nm) stops moving around the screw dislocation and forms a perfectly straight step along (a þ b) by lengthening itself before the straight step disappears. The lower step (0.69 nm) keeps moving slowly around the dislocation but a growth motif cannot be attached to the straight step. Therefore, the lower step (0.69 nm) piles up on the formerly straight step (0.72 nm). Thus a novel straight step made up of two different sections is formed (Figs. 3c and h): One section is a bunched step with step height h1 þ h2 ¼ 0:69+0:72 nm=1:41 nm close to the dislocation core and the other section is a unitstep with step height h2 ¼ 0:72 nm linked to the bunched section. The straight step consists of both bunched and unit steps that have stopped moving forward for about 10 min as the former upper step (0.72 nm) breaks out from the region where its growth is inhibited. Once the straight step is formed it will naturally have difficulty in growing because the kink density of a straight step is lower than that of an arched
step except when impurity poisoning is present. We can clearly see that the arched steps were expanding from the screw dislocation at all times. After remaining motionless for 12 min the unitstep (h2 ¼ 0:72 nm) broke through the inhibition and kept on moving around the dislocation. The former bunched step turned into a unit-layer step with a step height h1 ¼ 0:69 nm. Once the inhibition was broken through, it grew quickly and finished one cycle around the screw dislocation in 3 min. Eventually the LAZY layer finished its cycle. At the same time the growth along the layers turned slowly. The layer growth rate we measured is about 0.17 nm/s Once the unit-step has moved around the screw dislocation through one whole cycle (0.69 nm), can we say that the LAZY layer has finished one period of growth in 16 min. Subsequently, the ACTIVE layer with step height h1 þ h2 ¼ 0:69+0:72 nm¼ 1:41 nm began to grow. The ACTIVE layer grew quickly and finished one period within 2 min (Fig. 3f). The layered growth rate decreased quickly, especially in the (a b) direction because the growth of the
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
ACTIVE layer is limited by the underlying LAZY layer. The ACTIVE layer did not separate into unit-steps probably because the growth velocity is too great. A total ‘‘step’’ height of 1.41 nm corresponds to a layer thickness of (0 0 2), in accordance with the space group (P212121). Therefore, (0 0 2) steps are
67
expected for step flow. In this case, however, the steps stem from a screw dislocation which can only have a Burgers vector which is a multiple of the caxis (2.82 nm). A Burgers vector of 1=2c must be a partial dislocation, which needs another partial dislocation which is connected to the first one by a stacking fault. In the images this stacking fault is
Fig. 3. (a) The alternating growth of the ACTIVE layers and the LAZY layers of the spiral protuberance. (b–e) The growth process of the LAZY layer. (b–c, g–h) The upper step (0.72 nm) stops moving around the screw dislocation and forms a perfectly straight step along (a+b) but the lower step (0.69 nm) keeps moving. (d–e) The former upper step (0.72 nm) breaks through the inhibition and thus finishes one periodic growth of the LAZY layer). (f) The growth process of the ACTIVE layer. (i–k) The survey of step heights. The intervals are 4, 8, 2, 6, 2, 4, 8 and 2 min between images (a–i), respectively. Scan angles are 0 for (a–d) and 90 for (e–k).
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K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
Fig. 3 (continued).
not observed. We assume that the straight step that is observed for one of the sublayers is an indication for this stacking fault. In this case the second partial dislocation is not observed and, moreover, the straight step disappears after some time though it appears periodically again in exactly the same direction. Anisotropic growth of the screw dislocation was also observed in our experiment (Figs. 1b and 3). From the structure drawing (Fig. 2b), we can see
that the {0 0 1} surface is a three-dimensional network-like structure formed by tetrahedral HgS4 and octahedral Cd(CN)4(OS)2. The coordination force between {0 0 1} faces is relatively weak. The Periodic Bond Chains (PBC) along (a þ b) and (a b) are approximately the same. They are constructed by the tetrahedral HgS4 and octahedral Cd(CN)4 linked by a –SQC–N–bridge. Following the BFDH law, there should be about the same growth rate for the (a þ b) and (a b)
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
orientations, but in fact the growth rates were measured to be 7 and 3 nm/s, respectively. This fact explains why the spiral growth protuberance we visualized was not a circle but an ellipse. Through further observation, we can clearly see that the step rows that expand from the dislocation core intersect each other (Fig. 3). This phenomenon can be explained by the structure on the (0 0 1) surface. The LAZY layer grew slowly along the (a b) direction, whereas the next layer, namely the ACTIVE layer, grew rapidly in all directions but rapid growth along (a b) is prevented by the slow growth of the underlying LAZY layer. This alternation of LAZY and ACTIVE layers limited the growth along (a b) and therefore made the growth velocity along (a þ b) and (a b) different. This result was also observed by Pina and Becker in their research of the growth mechanisms of barite [8].
4. Conclusions Using in situ AFM we have succeeded in observing consecutive dynamic growth processes in CMTD crystals with a relatively high kinetic growth coefficient. Two different types of elemen-
69
tary-steps with step height h1 ¼ 0:69 nm and h2 ¼ 0:72 nm, respectively, were observed for the first time on the same surface of a crystal. Two differential types of straight edges formed on the top of the screw dislocation that were observed to be periodic. The growth velocity of the ACTIVE layers is about 8 times that of the LAZY layers and it is difficult to explain by means of existing crystal growth theory.
References [1] Shiyi Guo, Duorong Yuan, Dong Xu, et al., Progr. Crystal Growth Charact. 40(2000) 75. [2] Shiyi Guo, Dong Xu, Mengkai Lv, et al., Progr. Crystal Growth Charact. Mater. 2000, 111. [3] Kunpeng Wang, Daliang Sun, J. Funct. Mater. 34 (2003) 215. [4] X.N. Jiang, D. Xu, D.R. Yuan, et al., Crystal Res. Technol. 37 (2002) 67. [5] X.N. Jiang, D. Xu, D.L. Sun, et al., J. Crystal Growth 233 (2001) 196. [6] X.N. Jiang, D. Xu, D.L. Sun, et al., J. Crystal Growth 233 (2001) 318. [7] X.N. Jiang, D. Xu, D.R. Yuan, et al., J. Crystal Growth 236 (2001) 267. [8] C.M. Pina, U. Becker, P. Risthaus, D. Bosbach, A. Putnis, Nature 395 (1998) 483.
Journal of Crystal Growth 261 (2004) 63–69
Spiral growth mechanisms of CMTD crystals Kunpeng Wang*, Daliang Sun, Jianxiu Zhang, Wentao Yu, Hong Liu, Xiaobo Hu, Shiyi Guo, Yanling Geng State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, 27 Shanda, Nanlu, Jinan 250100, PR China Received 23 February 2003; accepted 5 September 2003 Communicated by D.T.J. Hurle
Abstract We present the results of an in situ atomic force microscopy investigation of the growth mechanisms of CdHg(SCN)4(H6C2OS)2CMTD crystals. The results show that spiral growth at screw dislocations dominates at a supersaturation s ¼ B0:1%. On the (0 0 1) surface, alternate CMTD layers are related through a 21 screw axis and the development of a spiral protuberance was observed with alternate growth modes having two different characteristic layers. The growth of the first CMTD layer continued for 16 min before the next layer began to grow on top of it. However, it took only 2 min for the next layer to continue its growth. We felicitously refer to them as ACTIVE layers and LAZY layers in this paper. The LAZY layers separated into two different types of elementary-steps with step height h1 ¼ 0:69 nm and h2 ¼ 0:72 nm on top of the spiral protuberance on the {0 0 1} face of the CMTD crystals. Two different types of straight edge always in the same direction were periodically observed when the screw protuberance grows. r 2003 Elsevier B.V. All rights reserved. PACS: 81.10.h; 81.10.aj Keywords: A1. Atomic force microscopy; A1. Growth modes; A1. Spectral dislocation; A1. Straight step; B1. CdHg(SCN)4 (H6C2OS)2CMTD
1. Introduction Materials are unsuitable for in situ observation using AFM when they grow quickly. The reason is that the scanning of the microscope tip lags behind the motion of the step if the crystal grows too fast. Therefore, the collected image is not exactly true to *Corresponding author. Tel.: +86-531-683-2632; fax: +86531-836-4451. E-mail address: [email protected] (K. Wang).
the actual morphology. In this paper, we report success in observing a series of consecutive growth processes using in situ AFM. These observation include a novel periodic growth process of spiral protuberances, an interaction process between twodimensional nuclei and spiral dislocations and a reconstruction process involving three-dimensional nuclei and amorphous aggregates on the {0 0 1} surface of CMTD crystals, which are semi organic coordination compounds with a relatively high kinetic growth coefficient under normal conditions.
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.09.011
ARTICLE IN PRESS 64
K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
CMTD is a highly efficient nonlinear optical (NLO) material for generating blue-violet light by laser frequency doubling. It belongs to the orthorhombic system, space group P212121 with lattice parameters of a ¼ 0:85188ð6Þ nm, b ¼ 0:85398ð7Þ nm, c ¼ 2:8224ð6Þ nm, v ¼ 2:0533ð5Þ nm3, and Z ¼ 4 [1–3].
instrument (Digital Instruments). A 90 mm 90 mm scanner with standard 200 mm SiN cantilevers with a small force constant of 0.12 N/m integrated with NP (Nanoprobe) tips was used. Images were collected at a scan rate of 2 Hz with a scan raster of 256 256. During imaging, care was taken to continually adjust the set point voltage to the lowest possible value.
2. Experimental procedure 3. Results and discussion Crystals of CMTD were prepared by using dimethy1 sulphoxide as a ligand to react with cadmium mercury thiocyanate (CMTC) in a mixture of dimethy1 sulphoxide and de-ionized water. The chemical reaction is CdHgðSCNÞ4 þ 2ðCH3 Þ2 SO -CdHgðSCNÞ4 2ðCH3 Þ2 OS: Samples were crystallized by spontaneous nucleation in a saturated solution made of CMTC as the raw material, and dissolved in a mixture of analytical grade reagent dimethy1 sulphoxide and de-ionized water (volume ratio 3:1). When the crystal samples reach the required size, a single crystal without macro defects is quickly removed from the culture dish and fixed by double adhesive tape in an AFM sample holder. Care was taken to keep the {0 0 1} crystal faces parallel to the surface of the holder and to keep the surface fresh all the while. Before making observations, the surface was daubed several times using a strip of damp filter paper that was immersed into the equilibrium solution for a short time. After the tip is engaged, solute molecules, that have not been completely attached to the steps can possibly escape from the surface due to the energy increase resulting from the tip’s scanning motion, and they can subsequently enter the solution on the surface. On the other hand, the solubility of CMTD will decrease if nonionized water is added to the solution and will increase upon injecting dimethy1 sulphoxide. The 70% relative humidity of the laboratory made the crystal surface absorb more water and therefore decreased the solubility of CMTD on the surface. Images were collected in situ in contact mode using a Nano-scope, IIIa dimension 3100 AFM
3.1. The formation of a partial dislocation with burgers vector h1+h2=0.69+0.72 nm=1.41 nm In the AFM experiment, the development from the initiation through the evolution of a screw dislocation with Burgers vector b ¼ h1 þ h2 ¼ 0:69 þ 0:72 nm ¼ 1:41 nm (Figs. 1a and 3i), was observed and its velocity was measured to be about 0.36 nm/s. The enlarged stereoscopic map, containing the location labeled ‘‘o’’ where the screw dislocation has formed, is shown on the right upper frame in Fig. 1a. Normally the formation of a screw dislocation is considered to be result of stress. From the image, we can assume that impurity particles have probably dropped on to the common boundary between the bunched steps and the elementary steps (Fig. 1a(o)). A spiral dislocation may be produced when a step encounters an impurity particle. Sketches are shown in Fig. 1c. 3.2. The alternating growth of the ACTIVE layers and LAZY layers of the spiral protuberance and the formation of two different types of elementary step To make the images clearer, we adjusted the scan angle several times but kept the other parameters unchanged from beginning to end of the experiment. As a result we were able to collect a new image every 2 min. A spiral protuberance was observed to evolve in an alternating growth mode consisting of two different characteristic layers. The growth of the first CMTD layer continued for 16 min before the next layer began to grow on top of it. However, the next layer continued to grow for only 2 min. As
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
65
Fig. 1. (a–b) The formation process of a screw dislocation along with a stereoscopic enlarged map, showing the location where the screw dislocation formed is shown on the upper right. (c) Diagrammatic sketch of the formation causes due to impurity particles. Scan areas are 10 10 mm2 for all AFM images.
a matter of convenience, we will felicitously refer to them as ACTIVE layers and LAZY layers, respectively, with the labels ‘‘a’’ and ‘‘L’’, respectively in Fig. 3a. On the (0 0 1) surface, alternate CMTD layers are related by a 21 screw axis, and so the anisotropy of the alternating layers should be reversed. This prediction is reflected by the alternating growth of the LAZY and ACTIVE layers in our experiment. The growth velocity of the ACTIVE layers is measured to be about 8 times that of the LAZY layers. A total of five entire periodic growth processes have been observed in our experiment. Two types of elementary-steps are measured on the top of the spiral dislocation and they are measured to be 0.69 and 0.72 nm, respectively (Figs. 3j and k). The height of an elementary-step should be consistent with the dimensionality of the growth motif. From the Raman spectrum of the CMTC solution [4–7], we can assume that the tetrahedral HgS4 and octahedral Cd(CN)4(OS)2
growth motifs can probably be associated with the kinks on the CMTD crystal surfaces. From the structure of the CMTD unit cell (Fig 2a), we can see that there are four molecular layers along the c direction. The theoretical height of the first layer within the unit-cell is calculated to be 0.69 nm and the second layer is calculated to be 0.72 nm along the c-axis (Fig. 2a). The third and fourth layers are simply related to the first and second layers by the 21 axis. The first and second layers are usually incorporated into bunched steps (h1 þ h2 ¼ 0:69+0:72 nm¼ 1:41 nm). We designate the bunched steps as the ACTIVE layers. The third and fourth layers often appear separated into two elementary-steps (corresponding to one layer of molecules), and we designate them as the LAZY layers. Figs. 3(b–e) show the consecutive growth process of the LAZY layers. The LAZY layers are difficult to grow along the (a b) plane for some unknown reason and are therefore separated
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Fig. 2. (a) The structure of the CMTD unit cell along the c-axis. (b) The structure of the {0 0 1} face.
into two types of elementary-steps with step heights h1 ¼ 0:69 nm and h2 ¼ 0:72 nm, respectively. It is surprising that the upper step (0.72 nm) stops moving around the screw dislocation and forms a perfectly straight step along (a þ b) by lengthening itself before the straight step disappears. The lower step (0.69 nm) keeps moving slowly around the dislocation but a growth motif cannot be attached to the straight step. Therefore, the lower step (0.69 nm) piles up on the formerly straight step (0.72 nm). Thus a novel straight step made up of two different sections is formed (Figs. 3c and h): One section is a bunched step with step height h1 þ h2 ¼ 0:69+0:72 nm=1:41 nm close to the dislocation core and the other section is a unitstep with step height h2 ¼ 0:72 nm linked to the bunched section. The straight step consists of both bunched and unit steps that have stopped moving forward for about 10 min as the former upper step (0.72 nm) breaks out from the region where its growth is inhibited. Once the straight step is formed it will naturally have difficulty in growing because the kink density of a straight step is lower than that of an arched
step except when impurity poisoning is present. We can clearly see that the arched steps were expanding from the screw dislocation at all times. After remaining motionless for 12 min the unitstep (h2 ¼ 0:72 nm) broke through the inhibition and kept on moving around the dislocation. The former bunched step turned into a unit-layer step with a step height h1 ¼ 0:69 nm. Once the inhibition was broken through, it grew quickly and finished one cycle around the screw dislocation in 3 min. Eventually the LAZY layer finished its cycle. At the same time the growth along the layers turned slowly. The layer growth rate we measured is about 0.17 nm/s Once the unit-step has moved around the screw dislocation through one whole cycle (0.69 nm), can we say that the LAZY layer has finished one period of growth in 16 min. Subsequently, the ACTIVE layer with step height h1 þ h2 ¼ 0:69+0:72 nm¼ 1:41 nm began to grow. The ACTIVE layer grew quickly and finished one period within 2 min (Fig. 3f). The layered growth rate decreased quickly, especially in the (a b) direction because the growth of the
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
ACTIVE layer is limited by the underlying LAZY layer. The ACTIVE layer did not separate into unit-steps probably because the growth velocity is too great. A total ‘‘step’’ height of 1.41 nm corresponds to a layer thickness of (0 0 2), in accordance with the space group (P212121). Therefore, (0 0 2) steps are
67
expected for step flow. In this case, however, the steps stem from a screw dislocation which can only have a Burgers vector which is a multiple of the caxis (2.82 nm). A Burgers vector of 1=2c must be a partial dislocation, which needs another partial dislocation which is connected to the first one by a stacking fault. In the images this stacking fault is
Fig. 3. (a) The alternating growth of the ACTIVE layers and the LAZY layers of the spiral protuberance. (b–e) The growth process of the LAZY layer. (b–c, g–h) The upper step (0.72 nm) stops moving around the screw dislocation and forms a perfectly straight step along (a+b) but the lower step (0.69 nm) keeps moving. (d–e) The former upper step (0.72 nm) breaks through the inhibition and thus finishes one periodic growth of the LAZY layer). (f) The growth process of the ACTIVE layer. (i–k) The survey of step heights. The intervals are 4, 8, 2, 6, 2, 4, 8 and 2 min between images (a–i), respectively. Scan angles are 0 for (a–d) and 90 for (e–k).
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K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
Fig. 3 (continued).
not observed. We assume that the straight step that is observed for one of the sublayers is an indication for this stacking fault. In this case the second partial dislocation is not observed and, moreover, the straight step disappears after some time though it appears periodically again in exactly the same direction. Anisotropic growth of the screw dislocation was also observed in our experiment (Figs. 1b and 3). From the structure drawing (Fig. 2b), we can see
that the {0 0 1} surface is a three-dimensional network-like structure formed by tetrahedral HgS4 and octahedral Cd(CN)4(OS)2. The coordination force between {0 0 1} faces is relatively weak. The Periodic Bond Chains (PBC) along (a þ b) and (a b) are approximately the same. They are constructed by the tetrahedral HgS4 and octahedral Cd(CN)4 linked by a –SQC–N–bridge. Following the BFDH law, there should be about the same growth rate for the (a þ b) and (a b)
ARTICLE IN PRESS K. Wang et al. / Journal of Crystal Growth 261 (2004) 63–69
orientations, but in fact the growth rates were measured to be 7 and 3 nm/s, respectively. This fact explains why the spiral growth protuberance we visualized was not a circle but an ellipse. Through further observation, we can clearly see that the step rows that expand from the dislocation core intersect each other (Fig. 3). This phenomenon can be explained by the structure on the (0 0 1) surface. The LAZY layer grew slowly along the (a b) direction, whereas the next layer, namely the ACTIVE layer, grew rapidly in all directions but rapid growth along (a b) is prevented by the slow growth of the underlying LAZY layer. This alternation of LAZY and ACTIVE layers limited the growth along (a b) and therefore made the growth velocity along (a þ b) and (a b) different. This result was also observed by Pina and Becker in their research of the growth mechanisms of barite [8].
4. Conclusions Using in situ AFM we have succeeded in observing consecutive dynamic growth processes in CMTD crystals with a relatively high kinetic growth coefficient. Two different types of elemen-
69
tary-steps with step height h1 ¼ 0:69 nm and h2 ¼ 0:72 nm, respectively, were observed for the first time on the same surface of a crystal. Two differential types of straight edges formed on the top of the screw dislocation that were observed to be periodic. The growth velocity of the ACTIVE layers is about 8 times that of the LAZY layers and it is difficult to explain by means of existing crystal growth theory.
References [1] Shiyi Guo, Duorong Yuan, Dong Xu, et al., Progr. Crystal Growth Charact. 40(2000) 75. [2] Shiyi Guo, Dong Xu, Mengkai Lv, et al., Progr. Crystal Growth Charact. Mater. 2000, 111. [3] Kunpeng Wang, Daliang Sun, J. Funct. Mater. 34 (2003) 215. [4] X.N. Jiang, D. Xu, D.R. Yuan, et al., Crystal Res. Technol. 37 (2002) 67. [5] X.N. Jiang, D. Xu, D.L. Sun, et al., J. Crystal Growth 233 (2001) 196. [6] X.N. Jiang, D. Xu, D.L. Sun, et al., J. Crystal Growth 233 (2001) 318. [7] X.N. Jiang, D. Xu, D.R. Yuan, et al., J. Crystal Growth 236 (2001) 267. [8] C.M. Pina, U. Becker, P. Risthaus, D. Bosbach, A. Putnis, Nature 395 (1998) 483.