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Transformation of cluster structure at initial stage
Applied Surface Science 256 (2009) 1128–1131
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Transformation of cluster structure at initial stage T.Y. Fu a,*, Y.H. Wang a, T.T. Tsong b a b
Department of Physics, National Taiwan Normal University, 88, sec.4 Ting Chou Rd., Taipei 116, Taiwan, ROC Institute of Physics, Academia Sinica, Taipei, 115, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 30 May 2009
The initial stages of cluster nucleation for Pd or Ir adatoms on a W (1 1 0) surface and in the vicinity of surface steps are directly imaged by a field ion microscope (FIM). Three types of structures are observed. One is a one-dimensional linear chain, which is parallel to the nearest neighbor-stacking directions of the substrate. Another is a two-dimensional compact island, which is a pseudomorphic structure like the substrate. The other is a three-dimensional cluster, which shows a structural transition from bcc (1 1 0) to fcc (1 1 1). Factors affecting the structural transformation include coverage of atoms or atom chains, temperature of heat treatments and boundary of the substrate terrace. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Field ion microscopy Nucleation Surface thermodynamics Structural transformation
1. Introduction In the nano era, many efforts have been made to investigate the self-assembly of nanometer-scale clusters [1–3]. In order to control the growth structure, it is necessary to understand growth properties. A basic understanding of metal films on metal substrates is of interest not only in fundamental research, but also in technologically bimetallic applicable areas [4]. Transition metals of the platinum family adsorbed on the W (1 1 0) plane show interesting properties. For example, the adsorbed atoms prefer to form linear chains at the initial growth stage. The growth of these material systems has been investigated with various surface analysis techniques and has attracted continuing study for many years [5–11]. However, few studies have investigated the structural transformation between different dimensional clusters at initial growth. Studying cluster surface structures using field ion microscopy (FIM) is an ideal way. First, it is possible to observe individual atoms and monitor their diffusion directly. Second, the symmetry of the problem can be reduced to two dimensions and the entropy of the system is negligibly small for the small ensemble of atoms [12–14]. In addition, the size limitation of the FIM samples causes nucleation in the vicinity of surface steps and the growth effect of the terrace boundary to be more obvious. In the present paper, we report some FIM observations of structural transformation of Pd and Ir clusters on the W (1 1 0) surface. The transformation of one-dimensional (1D) linear chain to two-dimensional (2D) compact island with three-dimensional (3D) growth affected by surface steps, and a commensurate bcc
(1 1 0) to incommensurate fcc (1 1 1) structural phase transition in the Pd overlayer are shown. The important factors triggering structural transformation are discussed. 2. Experiment All investigations were made with a home-built atomic resolution ultra-high vacuum-field ion microscopy (UHV-FIM), which has already been described in detail elsewhere [15–17]. A tungsten tip was prepared from a poly-crystal wire of 0.1 mm in diameter by electrochemical etching in aqueous KOH (concentration 2 M) and was cleaned in a UHV environment by a combination of thermal degassing, neon cathode sputtering, and field evaporation. Pd or Ir atoms were deposited from well-outgassed evaporation sources with high purity (99.995%) coil spot-welded with two potential leads. In this experiment, helium gas of 1 10 5 torr was admitted as the image gas. The number of atoms on the terrace was controlled by alternating with vapor deposition and field evaporation. Vapor deposition can increase the number of atoms on the sample terrace and field evaporation can reduce the number of atoms. Heating was done by electronic-controlled current power supply which can heat up the tip mounting loop in less than 0.5 s. The temperature was determined by a resistance measurement of the loop. The resistance was double calibrated with the thermocouple in the cooling and warming processes and checked with room temperature. 3. Results and discussions 3.1. Cluster structures at initial stage
* Corresponding author. Tel.: +886 2 29346620 185; fax: +886 2 29326408. E-mail address: [email protected] (T.Y. Fu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.064
At initial growth stage, a small cluster with more than three atoms can form a 1D linear chain, and various types of 2D or 3D
T.Y. Fu et al. / Applied Surface Science 256 (2009) 1128–1131
structures. For palladium, previous observations reported that clusters containing as many as eight atoms formed chains [18], chains of nine Pd atoms were metastable [8], and structural rearrangements from 1D linear to 2D compact shapes were reversible for Pd8 [19]. Our results are consistent with that of previous research. Fig. 1 shows the FIM images of Pdx on W (1 1 0) surfaces. Fig. 1(a) shows the linear structure with seven Pd atoms. Fig. 1(b) shows the 2D compact shapes with eight Pd atoms. The most stable configurations for Pd8 on W (1 1 0) surfaces transform from the 1D linear chain to 2D compact islands. As the number of Pd atoms increases, the growth goes to the upper layer forming a 3D structure, as shown in Fig. 1(c). Some characteristics for the initial growth clusters are observed. First, linear chains are always oriented in a close-packed <1 1 1> direction on bcc (1 1 0) surfaces. Second, 2D compact islands usually follow the substrate structure to form a pseudomorphic layer. Last, the structure of 3D islands usually transform from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) overlayer for fcc adsorbates at higher coverages. Fig. 2(a) and (c) display FIM images which show the structure of the first layer of Pd and Ir compact islands on W (1 1 0) surfaces individually. Fig. 2(b) shows the schematic of possible atomic arrangement for clusters in (a) and (c). Because the Pd–W and Ir–W bonds are rather strong and the lattice mismatch between W (1 1 0) and Pd or Ir (1 1 1) is rather small, r/rs is only 0.47% and 0.80% respectively. Both Pd and Ir atoms follow the substrate structure to form a pseudomorphic layer. This result is consistent with that of the LEED study on obtaining (1 1) refraction pattern at submonolayer coverages [20]. Fig. 3 provides an evidence of structural phase transition from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) overlayer at higher coverages. In this case, the transition occurs at monolayer coverage. This transition can be judged by the variety of the intersecting angles. The directions of atom arrangements along [ 1, 1, 1] and [ 1, 1, 1] of bcc (1 1 0) intersect at angle 70.58; while the directions of atom arrangements along [ 1, 1, 0] and [ 1, 0, 1] of fcc (1 1 1) intersect at angle 608. The transition can be indicated by the reduction in intersecting angle. A LEED investigation of Pd on Ta (1 1 0) indicates that the Pd overlayer is commensurate at submonolayer coverages, yet undergoes a
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Fig. 3. (a) Schematic drawing shows the intersecting angle of bcc (1 1 0) structure is 70.58, (b) fcc (1 1 1) structure is 608, (c) FIM image shows the first layer of Pd cluster, the solid lines indicate the intersecting angle of the bcc pseudomorphic layer, and (d) the second layer of the Pd cluster, whose intersecting angle is smaller indicated by dashed line.
structural phase transition to become an incommensurate overlayer at approximately monolayer coverage [21]. In view of the interaction of adsorbate and adsorbate or adsorbate and substrate, the above-mentioned characteristics may exist in general cases. In other words, they are common characteristics. The oscillatory behavior has been proved in the interaction energy versus adsorbate separation curve [22]. For different adsorbate separations, the interactions may be attractive or repulsive. The curve which varies with the Fermi wave vector is different in different systems. In other words, the chemical nature of different elements plays a pronounced role. If the adsorbate prefers to form a linear chain, the interaction must be attractive in the nearest sites and repulsive or much less attractive in the next nearest sites. Therefore, the linear chain
Fig. 1. FIM images show (a) 1D, (b) 2D, and (c) 3D Pd clusters on W(1 1 0) surfaces.
Fig. 2. FIM images with the schematic drawing showing the pseudomorphic structure. (a) The pseudomorphic Pd cluster on the W(1 1 0) surface, (b) schematic drawing showing the pseudomorphic cluster on the bcc (1 1 0) surface, and (c) the pseudomorphic Ir cluster on the W(1 1 0) surface.
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Fig. 4. The schematic drawing shows the procedure of 3D Pd cluster growth on the W (1 1 0) surface. White blocks indicate the substrate W(1 1 0), black blocks indicate the Pd clusters, and the arrows indicate more deposition of Pd atoms.
always grows along the direction with the nearest neighbor sites. If the interaction is not so strong in the nearest sites, the cluster tends to form 2D structures. When the 2D structures are formed, the interaction between adsorbate and substrate is rather strong. Therefore, the adsorbate usually sits in the substrate sites and forms a pseudomorphic structure. If the interaction between adsorbate and substrate is not strong enough, the adsorbate would grow upward and form 3D structures. Because the interaction with substrate is weak, the structure of higher layers may resume the adsorbate properties. 3.2. Structural transformation and structural control The transformation conditions vary with the systems. In the case of Pdx on W (1 1 0) surfaces as shown in Fig. 1, the structure transforms with increase in number of atoms. However, the number of atoms required for forming a stable chain or 2D structure differs in different systems. The condition for structural transformation is not only the number of atoms. For instance, for Irx on W (1 1 0) surfaces, chain structures seem to be more stable. Previous research predicted that at least 16 atoms were required for structural change [7]. LEED study even claimed that the most stable structure is the linear chain of up to 40 atoms [23]. However, a 2D island of Ir13 can be found in the extremely small terrace on the W (1 1 0) surface. According to our observations, the terrace area also plays an important role as the critical number atoms required for 1D to 2D structural transformation. The terrace boundaries also affect the transition of 2D to 3D islands. With increase in deposited Pd atoms, the first pseudomorphic layer expands to become greater and greater until the whole W (1 1 0) terrace is covered. The 3D island growth begins after the substrate terrace is totally covered. It is very interesting that the terrace boundaries of Pd overlayers are very near the original W terrace boundaries. The terrace boundaries limit apparently the growth of the 2D Pd clusters. The procedure is described by the animation in Fig. 4. The phase transition of the Pd second layer may be the reason of step limitation of 2D Pd clusters. The coexistence of pseudomorphic bcc (1 1 0) and fcc(1 1 1)
structure of Pd clusters on the same layer makes it unstable and thus triggers 3D growth of Pd clusters on W (1 1 0) surfaces. Moreover, the stability of the Pd and Ir pseudomorphic layer is rather different. The Pd16 layer of Fig. 2(a) is the most stable structure, which can always be obtained by annealing at 330 K. However, the Ir16 layer of Fig. 2(c) will change its configuration upon heating to 500 K. This compact 2D structure of small Ir clusters can seldom be formed by annealing. Usually, it must be obtained by field evaporation from a bigger 2D Ir island. The island nucleus may form at chain intersection and chain step intersection. Because the Ir chains are too stable, they are hardly attached to the W (1 1 0) steps. Ir chains are distributed over the vicinity of surface steps. When the monolayer is covered, it is very difficult to determine. A special nucleation procedure of Ir islands on the W (1 1 0) surface is shown in Fig. 5. By controlling the atom chain density and annealing temperature, the Ir 2D compact cluster forms. At the beginning of deposition, the Ir adatom coverage is low. Three linear chains are formed after annealing at 440 K as shown in Fig. 5(a). After the deposition and annealing processes are repeated several times, many linear chains are formed and intersections occur as shown in Fig. 5(b). After being annealed at 650 K or higher temperatures, the Ir compact island can be formed as shown in Fig. 5(c). Notably, annealing the configurations such as those in Fig. 5(a) to 650 K, the chain density is too low to form compact islands. By keeping the annealing temperature at 440 K, which is not high enough, configurations like those in Fig. 5(b) cannot form the compact islands. The formation of compact island must satisfy both conditions, neither chain density nor annealing temperature can be neglected. In addition, we emphasize that the key role in structural change is chain density, not atom coverage. Because individual adatoms usually jump down the terrace boundary and disappear after annealing at 650 K, the Ir compact island cannot be formed just by high-temperature annealing after depositing large amount of atoms. Our diffusion experiments indicate that the Ir clusters with more than three atoms start diffusion at temperature higher than 450 K. Therefore, depositing Ir on W (1 1 0) surfaces and controlling the annealing temperature at 440 K results in the
Fig. 5. FIM images show the 1D to 2D structural transformation of Ir clusters. (a)Three Ir linear chains are formed when the Ir adatom density is low, (b) some 2D structures are formed around the chain intersections with increase in chain density, and (c) 2D compact structure is formed after 670 K annealing.
T.Y. Fu et al. / Applied Surface Science 256 (2009) 1128–1131
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Fig. 6. FIM images show the 1D structural pattern. (a) As the linear chain meets the terrace boundary, it grows in the other directions and forms a ‘Z’ shape. (b) With increase in Ir coverage, the long chains are arranged in a (1 3) structure. (c) The 1D structural pattern replaced the 2D compact structure form, when the annealing temperature is below 500 K.
growth of chain clusters. After the density of chain clusters becomes high enough to form chain–chain intersections, the 2D compact island can be formed by heating at 650 K. Fig. 6 shows the other case with the formation of 1D structure pattern being controlled. Under 440 K heat treatment, the Ir adatoms prefer to form clusters with a chain structure at the initial growth stage. As the linear chain meets the terrace boundary, it grows in the other directions and forms a ‘Z’ shape, as shown in Fig. 6(a). With increase in Ir coverage, the long chains arrange like a (1 3) structure, which is consistent with that of the previous LEED study [23]. Fig. 6(b) shows the distribution of chain structures. When the chain density is increasing and the annealing temperature is below 500 K, the 1D structural pattern is formed, as shown in Fig. 6(c). The desired 1D structural pattern or the 2D compact structure can be formed by controlling the chain cluster density and the annealing temperature. 4. Conclusion The atomic resolved field ion images of the chain and 2D island structures show conclusively that Pd and Ir overlayers on W (1 1 0) have pseudomorphic bcc (1 1 0) structures. Both Pd and Ir chains are always oriented in a close-packed <1 1 1> direction on W (1 1 0) surfaces. The structure transforms from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) at higher adsorbate layers. When the growth structure meets the terrace boundary, the chain cluster may turn the growth direction and the 2D cluster may transform to 3D growth. With the proper density of Ir chain clusters, the heat treatment below 500 K may grow 1D chain
pattern and treatment with higher temperatures may form 2D compact islands. Acknowledgement The research is supported by National Science Council of ROC under grant no. NSC 95-2112-M-003-022-MY3. References [1] Z. Gai, B. Wu, J.P. Pierce, G.A. Farnan, D. Shu, M. Wang, Z. Zhang, J. Shen, Phys. Rev. Lett. 89 (2002) 235502. [2] W.J. Ong, E.S. Tok, Phys. Chem. Chem. Phys. 9 (2007) 991. [3] J. Wang, M. Li, E.I. Altman, Surf. Sci. 596 (2005) 126. [4] J.H. Sinfelt, Bimetallic Catalysts, Wiley, New York, 1983. [5] D.W. Bassett, Surf. Sci. 21 (1970) 181. [6] V.R. Dhanak, D.W. Bassett, Surf. Sci. 238 (1990) 289. [7] H.W. Fink, G. Ehrlich, Surf. Sci. 110 (1981) L611. [8] P.R. Schwoebel, G.L. Kellogg, Phys. Rev. B38 (1988) 5326. [9] T.T. Tsong, Surf. Sci. 231 (1990) 81. [10] J. Kolaczkiewecz, E. Bauer, Surf. Sci. 366 (1996) 71. [11] R. Szukiewicz, J. Kolaczkiewicz, i.N. Yakovkin, Surf. Sci. 602 (2008) 2610. [12] T.T. Tsong, Rep. Prog. Phys. 51 (1988) 759. [13] G. Ehrlich, Surf. Sci. 299/300 (1994) 6281. [14] R. Gormer, Rep. Prog. Phys. 53 (1990) 917. [15] T.T. Tsong, Atom-Probe Field Ion Microscopy, Cambridge University Press, Cambridge, UK, 1990. [16] T.Y. Fu, W.J. Weng, T.T. Tsong, Appl. Surf. Sci. 254 (2008) 7831. [17] T.Y. Fu, L.C. Cheng, C.H. Nien, T.T. Tsong, Phys. Rev. B 64 (2001) 113401. [18] D.W. Bassett, Thin Solid Films 48 (1978) 237. [19] T.Y. Fu, Y.J. Hwang, T.T. Tsong, Appl. Surf. Sci. 219 (2003) 143. [20] W. Schlenk, E. Bauer, Surf. Sci. 93 (1980) 9. [21] M.W. Ruckman, V. Murgai, M. Strongin, Phys. Rev. B43 (1986) 6759. [22] T.Y. Fu, T.Y. Wu, T.T. Tsong, Chin. J. Phys. 43 (2005) 124. [23] J. Kolaczkiewecz, E. Bauer, Phys. Rev. B44 (1991) 5779.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Transformation of cluster structure at initial stage T.Y. Fu a,*, Y.H. Wang a, T.T. Tsong b a b
Department of Physics, National Taiwan Normal University, 88, sec.4 Ting Chou Rd., Taipei 116, Taiwan, ROC Institute of Physics, Academia Sinica, Taipei, 115, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 30 May 2009
The initial stages of cluster nucleation for Pd or Ir adatoms on a W (1 1 0) surface and in the vicinity of surface steps are directly imaged by a field ion microscope (FIM). Three types of structures are observed. One is a one-dimensional linear chain, which is parallel to the nearest neighbor-stacking directions of the substrate. Another is a two-dimensional compact island, which is a pseudomorphic structure like the substrate. The other is a three-dimensional cluster, which shows a structural transition from bcc (1 1 0) to fcc (1 1 1). Factors affecting the structural transformation include coverage of atoms or atom chains, temperature of heat treatments and boundary of the substrate terrace. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Field ion microscopy Nucleation Surface thermodynamics Structural transformation
1. Introduction In the nano era, many efforts have been made to investigate the self-assembly of nanometer-scale clusters [1–3]. In order to control the growth structure, it is necessary to understand growth properties. A basic understanding of metal films on metal substrates is of interest not only in fundamental research, but also in technologically bimetallic applicable areas [4]. Transition metals of the platinum family adsorbed on the W (1 1 0) plane show interesting properties. For example, the adsorbed atoms prefer to form linear chains at the initial growth stage. The growth of these material systems has been investigated with various surface analysis techniques and has attracted continuing study for many years [5–11]. However, few studies have investigated the structural transformation between different dimensional clusters at initial growth. Studying cluster surface structures using field ion microscopy (FIM) is an ideal way. First, it is possible to observe individual atoms and monitor their diffusion directly. Second, the symmetry of the problem can be reduced to two dimensions and the entropy of the system is negligibly small for the small ensemble of atoms [12–14]. In addition, the size limitation of the FIM samples causes nucleation in the vicinity of surface steps and the growth effect of the terrace boundary to be more obvious. In the present paper, we report some FIM observations of structural transformation of Pd and Ir clusters on the W (1 1 0) surface. The transformation of one-dimensional (1D) linear chain to two-dimensional (2D) compact island with three-dimensional (3D) growth affected by surface steps, and a commensurate bcc
(1 1 0) to incommensurate fcc (1 1 1) structural phase transition in the Pd overlayer are shown. The important factors triggering structural transformation are discussed. 2. Experiment All investigations were made with a home-built atomic resolution ultra-high vacuum-field ion microscopy (UHV-FIM), which has already been described in detail elsewhere [15–17]. A tungsten tip was prepared from a poly-crystal wire of 0.1 mm in diameter by electrochemical etching in aqueous KOH (concentration 2 M) and was cleaned in a UHV environment by a combination of thermal degassing, neon cathode sputtering, and field evaporation. Pd or Ir atoms were deposited from well-outgassed evaporation sources with high purity (99.995%) coil spot-welded with two potential leads. In this experiment, helium gas of 1 10 5 torr was admitted as the image gas. The number of atoms on the terrace was controlled by alternating with vapor deposition and field evaporation. Vapor deposition can increase the number of atoms on the sample terrace and field evaporation can reduce the number of atoms. Heating was done by electronic-controlled current power supply which can heat up the tip mounting loop in less than 0.5 s. The temperature was determined by a resistance measurement of the loop. The resistance was double calibrated with the thermocouple in the cooling and warming processes and checked with room temperature. 3. Results and discussions 3.1. Cluster structures at initial stage
* Corresponding author. Tel.: +886 2 29346620 185; fax: +886 2 29326408. E-mail address: [email protected] (T.Y. Fu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.064
At initial growth stage, a small cluster with more than three atoms can form a 1D linear chain, and various types of 2D or 3D
T.Y. Fu et al. / Applied Surface Science 256 (2009) 1128–1131
structures. For palladium, previous observations reported that clusters containing as many as eight atoms formed chains [18], chains of nine Pd atoms were metastable [8], and structural rearrangements from 1D linear to 2D compact shapes were reversible for Pd8 [19]. Our results are consistent with that of previous research. Fig. 1 shows the FIM images of Pdx on W (1 1 0) surfaces. Fig. 1(a) shows the linear structure with seven Pd atoms. Fig. 1(b) shows the 2D compact shapes with eight Pd atoms. The most stable configurations for Pd8 on W (1 1 0) surfaces transform from the 1D linear chain to 2D compact islands. As the number of Pd atoms increases, the growth goes to the upper layer forming a 3D structure, as shown in Fig. 1(c). Some characteristics for the initial growth clusters are observed. First, linear chains are always oriented in a close-packed <1 1 1> direction on bcc (1 1 0) surfaces. Second, 2D compact islands usually follow the substrate structure to form a pseudomorphic layer. Last, the structure of 3D islands usually transform from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) overlayer for fcc adsorbates at higher coverages. Fig. 2(a) and (c) display FIM images which show the structure of the first layer of Pd and Ir compact islands on W (1 1 0) surfaces individually. Fig. 2(b) shows the schematic of possible atomic arrangement for clusters in (a) and (c). Because the Pd–W and Ir–W bonds are rather strong and the lattice mismatch between W (1 1 0) and Pd or Ir (1 1 1) is rather small, r/rs is only 0.47% and 0.80% respectively. Both Pd and Ir atoms follow the substrate structure to form a pseudomorphic layer. This result is consistent with that of the LEED study on obtaining (1 1) refraction pattern at submonolayer coverages [20]. Fig. 3 provides an evidence of structural phase transition from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) overlayer at higher coverages. In this case, the transition occurs at monolayer coverage. This transition can be judged by the variety of the intersecting angles. The directions of atom arrangements along [ 1, 1, 1] and [ 1, 1, 1] of bcc (1 1 0) intersect at angle 70.58; while the directions of atom arrangements along [ 1, 1, 0] and [ 1, 0, 1] of fcc (1 1 1) intersect at angle 608. The transition can be indicated by the reduction in intersecting angle. A LEED investigation of Pd on Ta (1 1 0) indicates that the Pd overlayer is commensurate at submonolayer coverages, yet undergoes a
1129
Fig. 3. (a) Schematic drawing shows the intersecting angle of bcc (1 1 0) structure is 70.58, (b) fcc (1 1 1) structure is 608, (c) FIM image shows the first layer of Pd cluster, the solid lines indicate the intersecting angle of the bcc pseudomorphic layer, and (d) the second layer of the Pd cluster, whose intersecting angle is smaller indicated by dashed line.
structural phase transition to become an incommensurate overlayer at approximately monolayer coverage [21]. In view of the interaction of adsorbate and adsorbate or adsorbate and substrate, the above-mentioned characteristics may exist in general cases. In other words, they are common characteristics. The oscillatory behavior has been proved in the interaction energy versus adsorbate separation curve [22]. For different adsorbate separations, the interactions may be attractive or repulsive. The curve which varies with the Fermi wave vector is different in different systems. In other words, the chemical nature of different elements plays a pronounced role. If the adsorbate prefers to form a linear chain, the interaction must be attractive in the nearest sites and repulsive or much less attractive in the next nearest sites. Therefore, the linear chain
Fig. 1. FIM images show (a) 1D, (b) 2D, and (c) 3D Pd clusters on W(1 1 0) surfaces.
Fig. 2. FIM images with the schematic drawing showing the pseudomorphic structure. (a) The pseudomorphic Pd cluster on the W(1 1 0) surface, (b) schematic drawing showing the pseudomorphic cluster on the bcc (1 1 0) surface, and (c) the pseudomorphic Ir cluster on the W(1 1 0) surface.
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Fig. 4. The schematic drawing shows the procedure of 3D Pd cluster growth on the W (1 1 0) surface. White blocks indicate the substrate W(1 1 0), black blocks indicate the Pd clusters, and the arrows indicate more deposition of Pd atoms.
always grows along the direction with the nearest neighbor sites. If the interaction is not so strong in the nearest sites, the cluster tends to form 2D structures. When the 2D structures are formed, the interaction between adsorbate and substrate is rather strong. Therefore, the adsorbate usually sits in the substrate sites and forms a pseudomorphic structure. If the interaction between adsorbate and substrate is not strong enough, the adsorbate would grow upward and form 3D structures. Because the interaction with substrate is weak, the structure of higher layers may resume the adsorbate properties. 3.2. Structural transformation and structural control The transformation conditions vary with the systems. In the case of Pdx on W (1 1 0) surfaces as shown in Fig. 1, the structure transforms with increase in number of atoms. However, the number of atoms required for forming a stable chain or 2D structure differs in different systems. The condition for structural transformation is not only the number of atoms. For instance, for Irx on W (1 1 0) surfaces, chain structures seem to be more stable. Previous research predicted that at least 16 atoms were required for structural change [7]. LEED study even claimed that the most stable structure is the linear chain of up to 40 atoms [23]. However, a 2D island of Ir13 can be found in the extremely small terrace on the W (1 1 0) surface. According to our observations, the terrace area also plays an important role as the critical number atoms required for 1D to 2D structural transformation. The terrace boundaries also affect the transition of 2D to 3D islands. With increase in deposited Pd atoms, the first pseudomorphic layer expands to become greater and greater until the whole W (1 1 0) terrace is covered. The 3D island growth begins after the substrate terrace is totally covered. It is very interesting that the terrace boundaries of Pd overlayers are very near the original W terrace boundaries. The terrace boundaries limit apparently the growth of the 2D Pd clusters. The procedure is described by the animation in Fig. 4. The phase transition of the Pd second layer may be the reason of step limitation of 2D Pd clusters. The coexistence of pseudomorphic bcc (1 1 0) and fcc(1 1 1)
structure of Pd clusters on the same layer makes it unstable and thus triggers 3D growth of Pd clusters on W (1 1 0) surfaces. Moreover, the stability of the Pd and Ir pseudomorphic layer is rather different. The Pd16 layer of Fig. 2(a) is the most stable structure, which can always be obtained by annealing at 330 K. However, the Ir16 layer of Fig. 2(c) will change its configuration upon heating to 500 K. This compact 2D structure of small Ir clusters can seldom be formed by annealing. Usually, it must be obtained by field evaporation from a bigger 2D Ir island. The island nucleus may form at chain intersection and chain step intersection. Because the Ir chains are too stable, they are hardly attached to the W (1 1 0) steps. Ir chains are distributed over the vicinity of surface steps. When the monolayer is covered, it is very difficult to determine. A special nucleation procedure of Ir islands on the W (1 1 0) surface is shown in Fig. 5. By controlling the atom chain density and annealing temperature, the Ir 2D compact cluster forms. At the beginning of deposition, the Ir adatom coverage is low. Three linear chains are formed after annealing at 440 K as shown in Fig. 5(a). After the deposition and annealing processes are repeated several times, many linear chains are formed and intersections occur as shown in Fig. 5(b). After being annealed at 650 K or higher temperatures, the Ir compact island can be formed as shown in Fig. 5(c). Notably, annealing the configurations such as those in Fig. 5(a) to 650 K, the chain density is too low to form compact islands. By keeping the annealing temperature at 440 K, which is not high enough, configurations like those in Fig. 5(b) cannot form the compact islands. The formation of compact island must satisfy both conditions, neither chain density nor annealing temperature can be neglected. In addition, we emphasize that the key role in structural change is chain density, not atom coverage. Because individual adatoms usually jump down the terrace boundary and disappear after annealing at 650 K, the Ir compact island cannot be formed just by high-temperature annealing after depositing large amount of atoms. Our diffusion experiments indicate that the Ir clusters with more than three atoms start diffusion at temperature higher than 450 K. Therefore, depositing Ir on W (1 1 0) surfaces and controlling the annealing temperature at 440 K results in the
Fig. 5. FIM images show the 1D to 2D structural transformation of Ir clusters. (a)Three Ir linear chains are formed when the Ir adatom density is low, (b) some 2D structures are formed around the chain intersections with increase in chain density, and (c) 2D compact structure is formed after 670 K annealing.
T.Y. Fu et al. / Applied Surface Science 256 (2009) 1128–1131
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Fig. 6. FIM images show the 1D structural pattern. (a) As the linear chain meets the terrace boundary, it grows in the other directions and forms a ‘Z’ shape. (b) With increase in Ir coverage, the long chains are arranged in a (1 3) structure. (c) The 1D structural pattern replaced the 2D compact structure form, when the annealing temperature is below 500 K.
growth of chain clusters. After the density of chain clusters becomes high enough to form chain–chain intersections, the 2D compact island can be formed by heating at 650 K. Fig. 6 shows the other case with the formation of 1D structure pattern being controlled. Under 440 K heat treatment, the Ir adatoms prefer to form clusters with a chain structure at the initial growth stage. As the linear chain meets the terrace boundary, it grows in the other directions and forms a ‘Z’ shape, as shown in Fig. 6(a). With increase in Ir coverage, the long chains arrange like a (1 3) structure, which is consistent with that of the previous LEED study [23]. Fig. 6(b) shows the distribution of chain structures. When the chain density is increasing and the annealing temperature is below 500 K, the 1D structural pattern is formed, as shown in Fig. 6(c). The desired 1D structural pattern or the 2D compact structure can be formed by controlling the chain cluster density and the annealing temperature. 4. Conclusion The atomic resolved field ion images of the chain and 2D island structures show conclusively that Pd and Ir overlayers on W (1 1 0) have pseudomorphic bcc (1 1 0) structures. Both Pd and Ir chains are always oriented in a close-packed <1 1 1> direction on W (1 1 0) surfaces. The structure transforms from pseudomorphic bcc (1 1 0) to an incommensurate fcc (1 1 1) at higher adsorbate layers. When the growth structure meets the terrace boundary, the chain cluster may turn the growth direction and the 2D cluster may transform to 3D growth. With the proper density of Ir chain clusters, the heat treatment below 500 K may grow 1D chain
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