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Narrow size-distributed silicon cluster beam generated using a spatiotemporal confined cluster source
24 May 2002
Chemical Physics Letters 358 (2002) 36–42 www.elsevier.com/locate/cplett
Narrow size-distributed silicon cluster beam generated using a spatiotemporal confined cluster source Yasushi Iwata a,*, Masaaki Kishida a, Makiko Muto b, Shengwen Yu a,1, Tsuguo Sawada c, Akira Fukuda a, Toshio Takiya d, Akio Komura d, Koichiro Nakajima e a
Cluster Advanced Nanoprocesses CRT, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, Tsukuba 305-8568, Japan b Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan c Department of Advanced Material Sciences, Graduate School of Frontier Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan d Technical Research Institute, Hitachi Zosen Corporation, 2-11 Funamachi 2, Taisho, Osaka 551-0022, Japan e Kohshien Kinzoku Corporation, 8-2 Tsuto-Ootuka, Nishinomiya, 663-8241, Japan Received 19 February 2002; in final form 15 March 2002
Abstract We have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermo-dynamical conditions for cluster growth with narrow size dispersion. A laser-induced shock is controlled to produce a definite mixed gas layer of the silicon vapor and helium gas, which is locally confined in a sub millimeter space, and conserved densely for a time of 160 ls. The generated silicon clusters, which are ionized by an ArF excimer laser for mass analysis without dissociation, show narrow size dispersion with characteristically higher abundance of stable Si23 Hx (46%), Si19 Hx (14%), and Si21 Hx (12%) clusters. Ó 2002 Published by Elsevier Science B.V.
1. Introduction As a fundamental common technology required to build up fine structure systems in a scale down to nanometer in the new material science, different ways have been developed: optical and electron
*
Corresponding author. Fax: +81-298-61-5754. E-mail address: [email protected] (Y. Iwata). 1 Present address: Department of Materials Science, Nanjing University, Nanjing 210093, China.
focused lithography, atomically controlled epitaxy, chemical methods for semiconductor nanocrystals, atomic manipulation using light, and biocatalytic synthesis [1,2]. Vacuum synthesis of nanostructures by low energy cluster deposition has such an advantage over the preceding technologies that one can carefully control the building block in nanometer scale (cluster deposition rate) efficiently and characterize the growth mechanisms [3]. Depositing clusters on a substrate surface at such low impact energy as a few ten meV/atom, one would like to expect spontaneous ordering with no
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 5 6 - 0
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
fragmentation. The deposited clusters diffuse at significantly high speed comparable with atomic diffusion, and juxtapose with neighbor clusters to form stable fine structures in nanometer scale. In an analogy of atomic ordering of adsorbate on a crystal surface [4], induced dipoles of clusters with a charge transferred from the substrate surface cause long range interaction between clusters in a potential of Pi Pj =rij3 , where Pi is induced dipole moment and rij is the distance between clusters. If the magnitude of the induced dipole moment that is proportional to the number of cluster constituent atoms (cluster size) becomes uniform, equivalent periodic potentials work on the clusters so that long range ordering of clusters is formed on the surface. Further more, matching in the crystal lattice of clusters is also important in morphology development of a nanocrystalline regime. Attachment of anatase nanoparticles with oriented lattice has been observed in crystalline growth [5]. Accordingly, well-defined stable clusters, which have uniformly distributed cluster sizes, electronic states and crystallographic structures, should be developed for forming self-assembly an ordered stable structure in nanometer scale. The cluster growth mechanism in the expanding nozzle flow has been discussed to describe the production process of molecular clusters in supersonic beams [6]. While the expanding nozzle flow method is also used to produce metal clusters, the intensity of larger clusters is actually very small [7]. The cluster growth rate is proportional to the collision rate of particles including atoms and clusters defined by their mean free path and local mean velocity. In the Knudsen effusion process of particles passing through a conical nozzle, the local density of particles and the temperature of vapor are rapidly reduced. Metal vapor exiting the cluster source has a thermal velocity distribution and large angular divergence, which induce higher internal excitation of the clusters. Kappes et al. [8] introduced the seeded beam technology improved from the expanding nozzle flow method to reduce the internal excitation of clusters in the producing processes of the metal clusters with a lower melting point such as sodium clusters. In helium-seeded expansion the faster mean flow velocity suppresses cluster–cluster collisions in the gas phase. The
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lower accommodation coefficient of a helium gas leads to production of smaller sodium clusters with reduced internal excitation. Otherwise the laser ablation type cluster source is suited to generate clusters by condensation of materials in the gas phase with a higher melting point such as semiconductors and transition metals. The most benefit advantage of the laser ablation methods is that one can obtain the same composition in the gas phase of alloy materials as in the solid phase. Then the laser ablation method has been widely used in vacuum synthesis of the cluster materials. It is generally difficult, however, to define the density and temperature of the ablated vapor following the pathway of cluster growth in the cluster source, even though the thermo-dynamical conditions have great influence on cluster growth. Then the internal states of the generated clusters cannot be uniformly defined, and the resultant size dispersion is commonly larger. Copper clusters (CuN , N ¼ 1–500) and iron clusters (FeN , N ¼ 1–200) generated by laser ablation in a cell filled with a pulsed helium gas flow form a pulsed beam traveling at velocity of 600– 700 m/s [9]. Before extracted into vacuum, particles including clusters are confined in the cell for a time from 0.3 to 1.2 ms. Depending on the confinement time, the size dispersion defined as a ratio DN =N of the distribution width DN to the mean cluster size N has commonly large values from 0.2 to 0.5 [10]. These results indicate that the thermodynamical conditions in the gas phase obviously have influence on cluster growth in the cell. For the purpose to generate well-defined stable clusters, we have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermodynamic conditions of density and temperature in the gas phase by locally confining the cluster growth area in space and time. In this Letter, we describe the preliminary results of narrow size-distributed silicon clusters generated using the SCCS.
2. Experimental The cluster beam system equipped with the SCCS is shown in Fig. 1. The wall of the cluster
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Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
Fig. 1. Cluster beam system equipped with the SCCS. Nd:YAG laser is introduced through a hole opened at the center on the ellipsoidal-shaped wall to irradiate a silicon target set on the top of a spherical-shaped holder. A helium gas carefully poured continuously with no turbulence is spread all over the spherical surface of the target holder to get a symmetric static flow. The front path of the induced shock propagating in the ellipsoidal-shaped cluster cell is drown as white lines. The system is evacuated by a turbo molecular pump (300l/s) and a diffusion pump (4000l/s).
source has an ellipsoidal shape with a couple of focal points separated 20 mm far from each other. On the center axis of the ellipsoidal wall, a small hole of 0.5 mm in diameter is opened at 3 mm far near the focal point for introduction of the laser beam and extraction of the cluster beam. A silicon target is mounted on the top of a spherical-shaped holder so that the ablation point on the target surface is located at the focal point. The target has a cylindrical shape with an edge surface cut slantwise at the angle of 4°, and the cylindrical axis leans at the same angle from the center axis of the cluster source. The setup enables the silicon surface to always make a right angle with the center axis while the target turns on its axis. Then the laser-ablated vapor ejects from the target surface symmetrically with the center axis. We carefully poured a helium gas continuously into the cluster source with no turbulent flow. Helium gas introduced through the laying pipe is spread all over the spherical surface of the target holder and makes a static flow symmetric with the center axis. The typical helium gas pressure in the cluster source is PHe S ¼ 130 Pa, which is estimated from the inlet
pressure of PHe I ¼ 1300 Pa measured at the laying pipe. A pulsed Nd:YAG laser focused at the entrance of the ellipsoidal wall (wave length 532 nm, pulse duration 10 ns, energy 50–300 mJ/pulse) is introduced along the center axis, and irradiates the target surface at a beam spot of 0.8 mm in diameter. The laser-ablated dense vapor induces a shock wave in the symmetric helium gas flow after forming a Knudsen layer. The shock front propagates symmetrically to the ellipsoidal wall, and then locally concentrates on the alternate focal point after reflection at the wall. The vapor front traveling at a slower velocity than the shock front is stopped at the focal point by the dense helium gas. At the contact front of the both gas phases, a narrow mixed gas layer of vapor and helium is formed in dense, where clusters grow up. Sufficient atomic collisions in the locally confined cluster growth area possibly complete the uniform thermodynamical conditions. Excited particles in the vapor and helium gas emit lights in the initial stage of confinement. Emission is observed through a sapphire window of 4:5 mm 17:5 mm opened on
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
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the ellipsoidal wall. We use a fast timing CCD camera (ICCD, Andor Tec., detectable wave length from 180 to 850 nm) in timing operation faster than 50 ns gated by the photo signals from the Nd:YAG laser. The cluster source system is evacuated by a turbo molecular pump (300l/s) to achieve the pressure at 1 107 Pa as no loading of the helium gas. Generated silicon clusters in the SCCS are extracted into vacuum following the helium gas flow through a skimmer with an aperture of 3.0 mm in diameter set at 20 mm far from the cluster source. The ion components are suppressed to pass through the skimmer, to which an electrostatic potential of 300 V is applied. Generated neutral clusters are detected by the two-stepacceleration type time of flight mass spectrometer (TOFMS), the center of which is located at 253 mm far from the cluster source. Silicon clusters are ionized by irradiation of a pulsed ArF excimer laser (PSX-100, MBP Technology, 3 ns, 6.41 eV, 1:4–1:7 mJ=cm2 pulse).
3. Results Fig. 2 shows light emission from the particles observed through the sapphire window of the SCCS in the initial stage of the cluster growth process just after irradiation of the Nd:YAG laser. The laser-ablated vapor traveling in a helium gas flow is compressed in higher density at the contact front with the helium gas, and the particles excited by atomic collisions in the dense gas phase emit intense lights. The time development of the vapor in the helium gas flow at PHe I ¼ 670 Pa and PHe I ¼ 1300 Pa are displayed in Figs. 2a and b, respectively. In both helium pressure, the vapor front travels at a velocity of 8:0 103 m/s initially for a time of 1 ls, and then approaches the focal point of the SCCS in 2.3 ls with deceleration. The induced shock near the target surface propagates in the helium gas flow, and stops the traveling vapor front at the focal point in 2.3 ls after reflection on the ellipsoidal wall. The mixed gas layer produced at the boundary of the vapor and helium gas is confined in a local space smaller than 1.0 mm, which is estimated from the half-reduction length of the emission intensity at the contact
Fig. 2. Transient emission of lights in the SCCS taken by a fast timing CCD camera. The time sequence of the vapor front in a helium gas flow at (a) PHe I ¼ 670 Pa, and (b) PHe I ¼ 1300 Pa.
front. If the pressure of the helium gas flow is reduced lower than PHe I ¼ 150 Pa, such a sharp shape of the vapor front is not observed, and the mixed gas layer disappears. A typical mass spectrum of silicon clusters generated in the SCCS is shown in Fig. 3. In the higher abundance region of smaller silicon clusters SiN with atomic size up to N ¼ 4, silicon monomers and dimers are dominantly observed. In the spectrochemical analysis of emission in the SCCS, several excited atomic lines of the neutral silicon monomers and dimers are observed consistently with the results of mass analysis, even though the SCCS is not filled with a helium gas [10]. These monomers and dimers directly emerge from the silicon target by irradiation of the Nd:YAG laser. Silicon molecular products such as SiN Hx ðN ¼ 1–4; x ¼ 0–4Þ, SiN CHx ðN ¼ 1–4; x ¼ 0–6Þ also get
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Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
Fig. 3. The TOF mass spectrum of silicon clusters generated using the SCCS. The histogram of the fractional silicon clusters of SiN Hx (N ¼ 9–34) is shown in the inset.
mixed in the spectrum. In the region of larger silicon clusters of N > 4, stable Si23 clusters including Si23 H3 and Si23 H6 clusters show characteristically higher abundance. Produced silicon clusters are concentrated around the atomic size of N ¼ 23 with narrow size dispersion. Separated from the carbide products of SiN CHx that are generated in the SCCS, histogram of the fractional silicon clusters of SiN Hx in the size distribution from N ¼ 9 to 34 is shown in Fig. 3. Si23 Hx ; Si19 Hx ; Si21 Hx clusters distribute in the highest abundance of 46%, 14%, 12%, respectively. The intensity of helium line in the mass spectra shown in Fig. 3 is proportional to the density of the helium gas flow, which carries the cluster products to the TOFMS system. Sifting the delayed time of ionization at the TOFMS stage by trigger adjustment of the ArF excimer laser, we measured the density of the helium gas flow as shown in Fig. 4. The density of the helium gas flow without irradiation of the Nd:YAG laser is used as a standard density level of the constant helium gas flow with no shock. The excited particles locally confined in the dense gas layers in the cluster source emit photons as shown in Fig. 2, which cause a background level of the MCP detector in the TOFMS system. The background level was measured under introduction of the Nd:YAG laser but without irradiation of the ArF excimer laser. In 70 ls after Nd:YAG laser irradiation, the background level was reduced down to the lowest values enough to
Fig. 4. Time-dependence of the density of the helium gas flow in vacuum after Nd:YAG irradiation. The induced shock flow () and the standard density level of the static constant flow () and the background level caused by photoemission from the dense gas layer in the cluster source are summarized.
identify the density of the helium gas in the induced shock flow. The density of the helium gas flow rapidly increases at 90 ls, and reduces down to the level of the constant helium gas flow in 250 ls. Accordingly, the dense helium gas layer is produced in the cluster source for a time of 160 ls, in which time clusters have possibility to grow up to several hundreds atomic size sufficiently.
4. Discussion The fluence of 1:4–1:7 mJ=cm2 of the ArF excimer laser used for ionization of neutral silicon clusters is much lower than the threshold in evaporation of copper clusters with lower cohesive energy, which have been studied previously [11]. In this fluence of the ArF excimer laser silicon clusters are actually stable [12]. Then there was no photo-fragmentation of the generated silicon clusters in measurement of the TOFMS. In detection of the helium lines observed in Fig. 3, multi-photon ionization of helium atoms certainly occurs under the low fluence of the ArF excimer laser. It can be speculated that the helium density at the TOFM stage might be so high roughly above 1022 –1023 m3 in a moment. A shock front produced by moving a piston at a constant velocity up in a one-dimensional tube
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
filled with a polytropic gas with an adiabatic exponent c propagates at a constant velocity U, which is described by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi 1 up 1 u p ; U¼ þ c20 þ 2 1 l2 4 1 l2 ð1Þ c1 2 l ¼ ; cþ1 where c0 is the sound velocity in the initial gas zone before the shock front reaches [13]. In the case of an induced shock in the cluster source, the silicon vapor front traveling at a velocity uv ¼ 8:0 103 m/s plays the role of a piston to produce a shock wave. Applying the model to an induced shock in the cluster source, where up ¼ uv , c of helium gas is 5/3, and c0 ¼ 1:02 103 m/s at the temperature of 300 K, one can estimate the velocity of the shock front propagating forward to the wall, Uþ ¼ 10:8 103 m/s. The reflected shock front on the wall is also estimated, U ¼ 2:12 104 m/s. On the other hands, the experimental results in Fig. 2 show that the reflected shock front collides with the vapor front at 2.3 ls after Nd:YAG laser irradiation. Accordingly, fixing Uþ ¼ 10:8 103 m/s, the reflected shock velocity is experimentally determined, U;exp ¼ 2:0 104 m/s. The good agreement on the reflected shock velocity indicates that shock behavior in the cluster source is well described by the model of a shock in a polytropic gas. Density jump at the shock front traveling forward can be estimated, qþ =q0 ¼ 4. Condensation of a helium gas in the cluster source is observed in Fig. 4 as rapid increment on the density of a helium gas flow extracted into vacuum. The helium gas flow detected at the TOFMS stage is conserved in a higher density level than the constant helium gas flow while 160 ls. Since the dense helium gas front flies at a constant velocity of 2.8 km/s in vacuum to the TOFMS stage located at 253 mm far in 90 ls, then it is concluded that the helium gas phase inside the cluster source is densely conserved due to the induced shock just for the same time of 160 ls. And it is also probable that a dense mixed gas layer created locally in a fine space of sub millimeter at the contact front of the both gas phase is kept under confinement in a parallel time. Self-diffusion
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of the mixed gas layer in the time is much small in a sub millimeter scale. Although clusters could possibly grow up to several hundred in atomic size in a period of 160 ls, cluster growth is suppressed when the density of the mixed gas layer reduces. Change of the cluster growth rate is obviously displayed in Fig. 5 by showing time-dependence of the TOF mass spectra. Cluster growth is stopped consistently with a clear cut of the helium density in Fig. 4 at 250 ls. The cluster growth rate is much rapidly reduced compared to the pulsed cluster beam of a duration time in millisecond produced by normal laser ablation using a pulsed gas valve [9]. Accordingly, the silicon clusters generated in the SCCS are resultantly distributed with a narrow size dispersion around the atomic size of N ¼ 23. Jarrold et al. have discussed stability of silicon clusters of atomic size up to 70 in the drift tube
Fig. 5. Time-dependence of the generated silicon mass spectra measured at 100, 150, 300 ls after Nd:YAG irradiation. The cluster growth rate decreases following reduction of the mixed gas density in the cluster source.
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studies on the nanosurface chemistry. In the reaction rate analysis of size-selected silicon cluster ions, Si13 , Si19 and Si23 clusters are particularly unreactive with oxygen, water and ethylene [14,15]. These stable clusters can be modeled by the icosahedral growth sequence [16]. Silicon clusters generated using the SCCS show also narrow size-dispersive mass spectra with a characteristically higher abundance of Si23 Hx , Si19 Hx , and Si21 Hx clusters, although the spectra are contaminated with the carbide products. The carbon elements came from the spherical target holder not from the silicon target itself. The spectra are, however, completely different from the previously reported mass spectra obtained by laser ablation [17–20]. The SCCS can control the induced shock to produce a definite area in space and time at the contact front of the helium and vapor gas phase. The area possibly gives well-defined thermo-dynamical conditions suited for cluster growth with a so narrow dispersion in cluster size that the generated silicon clusters show characteristically higher abundance of Si23 Hx , Si19 Hx , and Si21 Hx clusters.
5. Conclusions We have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermo-dynamical conditions for cluster growth with a narrow size dispersion suited for vacuum synthesis of nanostructures by low energy cluster deposition. The new structure of the SCCS enables one to make a continuous static helium gas flow with no turbulence symmetrically on the center axis of the ellipsoidal-shaped cluster cell, the flow which successfully controls propagation of the induced shock to produce a definite mixed gas layer of the atomic vapor and helium gas. The formed mixed gas layer is confined in a fine space of sub millimeter at the contact front of the both gas phase. The confined layer is conserved densely in the cluster source for a time of 160 ls, and then the density is reduced rapidly. Cluster growth is well-controlled in concert with the mixed gas density. Accordingly, the cluster growth area lo-
cally confined in space and time possibly makes well-defined thermo-dynamical conditions for cluster growth with well-defined internal states. Resultantly, the new SCCS generates a silicon cluster beam of narrow size dispersion with characteristically higher abundance of stable Si23 Hx (46%), Si19 Hx (14%), and Si21 Hx (12%) clusters. In the future works, we have to investigate the effects of the thermal adiabatic compression due to the induced shock on the internal states of growing clusters in the SCCS.
References [1] J.H. Davies, A.R. Long, Physics of Nanostructures, The Scottish Universities Summer School in Physics Pub., 1992. [2] G. Timp, Nanotechnology, Springer, New York, 1999. [3] P. Jensen, Rev. Mod. Phys. 71 (1999) 1695. [4] P.J. Kundrotas, S. Lapinskas, A. Rosengren, Phys. Rev. 56 (1997) 6486. [5] R. Lee Penn, J.F. Banfield, Geochim. Cosmochim. Acta 63 (1999) 1549. [6] E.W. Becker, K. Bier, Z. Phys. 146 (1956) 333. [7] A. Herrmann, S. Leutwyler, E. Schumacher, L. Woste, Helv. Chim. Acta 61 (1978) 453. [8] M.M. Kappes, R.W. Kunz, E. Schumacher, Chem. Phys. Lett. 91 (1982) 413. [9] N. Saito, Y. Iwata, K. Koyama, T. Kitamura, M. Tanimoto, Surf. Rev. Lett. 3 (1996) 161. [10] Y. Iwata, M. Muto, M. Han, T. Sawada, A. Fukuda, S. Kiyama, A. Komura, T. Takiya, in preparation. [11] M. Muto, Y. Iwata, M. Han, T. Sawada, in: Proceedings of the International Seminar Photothermal Phenomena Applications Tomakomai, 1998, p. 201. [12] Y. Liu, Q.-L. Zhang, F.K. Tittel, R.F. Curl, R.E. Smalley, J. Chem. Phys. 85 (1986) 7434. [13] R. Courant, K.O. Friedrichs, Supersonic Flow and Shock Waves, Springer, Berlin, 1976. [14] M.F. Jarrold, U. Ray, K.M. Creegan, J. Chem. Phys. 93 (1990) 224. [15] U. Ray, M.F. Jarrold, J. Chem. Phys. 94 (1991) 2631. [16] J.R. Chelikowsky, J.C. Phillips, Phys. Rev. B 41 (1990) 5735. [17] J.R. Heath, Y. Liu, S.C. O‘Brien, Q. Zhang, R.F. Curl, F.K. Tittel, R.E. Smalley, J. Chem. Phys. 83 (1985) 5520. [18] L.A. Bloomfield, M.E. Geusic, R.R. Freeman, W.L. Brown, Chem. Phys. Lett. 121 (1985) 33. [19] Y. Guyot, B. Champagnon, M. Boudeullue, P. Melinon, B. Prevel, V. Dupuis, A. Perez, Thin Solid Films 297 (1997) 188. [20] S. Messerli, S. Schintke, K. Morgenstern, A. Sanchez, U. Heiz, W. Schneider, Surf. Sci. 465 (2000) 331.
Chemical Physics Letters 358 (2002) 36–42 www.elsevier.com/locate/cplett
Narrow size-distributed silicon cluster beam generated using a spatiotemporal confined cluster source Yasushi Iwata a,*, Masaaki Kishida a, Makiko Muto b, Shengwen Yu a,1, Tsuguo Sawada c, Akira Fukuda a, Toshio Takiya d, Akio Komura d, Koichiro Nakajima e a
Cluster Advanced Nanoprocesses CRT, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, Tsukuba 305-8568, Japan b Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan c Department of Advanced Material Sciences, Graduate School of Frontier Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan d Technical Research Institute, Hitachi Zosen Corporation, 2-11 Funamachi 2, Taisho, Osaka 551-0022, Japan e Kohshien Kinzoku Corporation, 8-2 Tsuto-Ootuka, Nishinomiya, 663-8241, Japan Received 19 February 2002; in final form 15 March 2002
Abstract We have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermo-dynamical conditions for cluster growth with narrow size dispersion. A laser-induced shock is controlled to produce a definite mixed gas layer of the silicon vapor and helium gas, which is locally confined in a sub millimeter space, and conserved densely for a time of 160 ls. The generated silicon clusters, which are ionized by an ArF excimer laser for mass analysis without dissociation, show narrow size dispersion with characteristically higher abundance of stable Si23 Hx (46%), Si19 Hx (14%), and Si21 Hx (12%) clusters. Ó 2002 Published by Elsevier Science B.V.
1. Introduction As a fundamental common technology required to build up fine structure systems in a scale down to nanometer in the new material science, different ways have been developed: optical and electron
*
Corresponding author. Fax: +81-298-61-5754. E-mail address: [email protected] (Y. Iwata). 1 Present address: Department of Materials Science, Nanjing University, Nanjing 210093, China.
focused lithography, atomically controlled epitaxy, chemical methods for semiconductor nanocrystals, atomic manipulation using light, and biocatalytic synthesis [1,2]. Vacuum synthesis of nanostructures by low energy cluster deposition has such an advantage over the preceding technologies that one can carefully control the building block in nanometer scale (cluster deposition rate) efficiently and characterize the growth mechanisms [3]. Depositing clusters on a substrate surface at such low impact energy as a few ten meV/atom, one would like to expect spontaneous ordering with no
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 5 6 - 0
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
fragmentation. The deposited clusters diffuse at significantly high speed comparable with atomic diffusion, and juxtapose with neighbor clusters to form stable fine structures in nanometer scale. In an analogy of atomic ordering of adsorbate on a crystal surface [4], induced dipoles of clusters with a charge transferred from the substrate surface cause long range interaction between clusters in a potential of Pi Pj =rij3 , where Pi is induced dipole moment and rij is the distance between clusters. If the magnitude of the induced dipole moment that is proportional to the number of cluster constituent atoms (cluster size) becomes uniform, equivalent periodic potentials work on the clusters so that long range ordering of clusters is formed on the surface. Further more, matching in the crystal lattice of clusters is also important in morphology development of a nanocrystalline regime. Attachment of anatase nanoparticles with oriented lattice has been observed in crystalline growth [5]. Accordingly, well-defined stable clusters, which have uniformly distributed cluster sizes, electronic states and crystallographic structures, should be developed for forming self-assembly an ordered stable structure in nanometer scale. The cluster growth mechanism in the expanding nozzle flow has been discussed to describe the production process of molecular clusters in supersonic beams [6]. While the expanding nozzle flow method is also used to produce metal clusters, the intensity of larger clusters is actually very small [7]. The cluster growth rate is proportional to the collision rate of particles including atoms and clusters defined by their mean free path and local mean velocity. In the Knudsen effusion process of particles passing through a conical nozzle, the local density of particles and the temperature of vapor are rapidly reduced. Metal vapor exiting the cluster source has a thermal velocity distribution and large angular divergence, which induce higher internal excitation of the clusters. Kappes et al. [8] introduced the seeded beam technology improved from the expanding nozzle flow method to reduce the internal excitation of clusters in the producing processes of the metal clusters with a lower melting point such as sodium clusters. In helium-seeded expansion the faster mean flow velocity suppresses cluster–cluster collisions in the gas phase. The
37
lower accommodation coefficient of a helium gas leads to production of smaller sodium clusters with reduced internal excitation. Otherwise the laser ablation type cluster source is suited to generate clusters by condensation of materials in the gas phase with a higher melting point such as semiconductors and transition metals. The most benefit advantage of the laser ablation methods is that one can obtain the same composition in the gas phase of alloy materials as in the solid phase. Then the laser ablation method has been widely used in vacuum synthesis of the cluster materials. It is generally difficult, however, to define the density and temperature of the ablated vapor following the pathway of cluster growth in the cluster source, even though the thermo-dynamical conditions have great influence on cluster growth. Then the internal states of the generated clusters cannot be uniformly defined, and the resultant size dispersion is commonly larger. Copper clusters (CuN , N ¼ 1–500) and iron clusters (FeN , N ¼ 1–200) generated by laser ablation in a cell filled with a pulsed helium gas flow form a pulsed beam traveling at velocity of 600– 700 m/s [9]. Before extracted into vacuum, particles including clusters are confined in the cell for a time from 0.3 to 1.2 ms. Depending on the confinement time, the size dispersion defined as a ratio DN =N of the distribution width DN to the mean cluster size N has commonly large values from 0.2 to 0.5 [10]. These results indicate that the thermodynamical conditions in the gas phase obviously have influence on cluster growth in the cell. For the purpose to generate well-defined stable clusters, we have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermodynamic conditions of density and temperature in the gas phase by locally confining the cluster growth area in space and time. In this Letter, we describe the preliminary results of narrow size-distributed silicon clusters generated using the SCCS.
2. Experimental The cluster beam system equipped with the SCCS is shown in Fig. 1. The wall of the cluster
38
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
Fig. 1. Cluster beam system equipped with the SCCS. Nd:YAG laser is introduced through a hole opened at the center on the ellipsoidal-shaped wall to irradiate a silicon target set on the top of a spherical-shaped holder. A helium gas carefully poured continuously with no turbulence is spread all over the spherical surface of the target holder to get a symmetric static flow. The front path of the induced shock propagating in the ellipsoidal-shaped cluster cell is drown as white lines. The system is evacuated by a turbo molecular pump (300l/s) and a diffusion pump (4000l/s).
source has an ellipsoidal shape with a couple of focal points separated 20 mm far from each other. On the center axis of the ellipsoidal wall, a small hole of 0.5 mm in diameter is opened at 3 mm far near the focal point for introduction of the laser beam and extraction of the cluster beam. A silicon target is mounted on the top of a spherical-shaped holder so that the ablation point on the target surface is located at the focal point. The target has a cylindrical shape with an edge surface cut slantwise at the angle of 4°, and the cylindrical axis leans at the same angle from the center axis of the cluster source. The setup enables the silicon surface to always make a right angle with the center axis while the target turns on its axis. Then the laser-ablated vapor ejects from the target surface symmetrically with the center axis. We carefully poured a helium gas continuously into the cluster source with no turbulent flow. Helium gas introduced through the laying pipe is spread all over the spherical surface of the target holder and makes a static flow symmetric with the center axis. The typical helium gas pressure in the cluster source is PHe S ¼ 130 Pa, which is estimated from the inlet
pressure of PHe I ¼ 1300 Pa measured at the laying pipe. A pulsed Nd:YAG laser focused at the entrance of the ellipsoidal wall (wave length 532 nm, pulse duration 10 ns, energy 50–300 mJ/pulse) is introduced along the center axis, and irradiates the target surface at a beam spot of 0.8 mm in diameter. The laser-ablated dense vapor induces a shock wave in the symmetric helium gas flow after forming a Knudsen layer. The shock front propagates symmetrically to the ellipsoidal wall, and then locally concentrates on the alternate focal point after reflection at the wall. The vapor front traveling at a slower velocity than the shock front is stopped at the focal point by the dense helium gas. At the contact front of the both gas phases, a narrow mixed gas layer of vapor and helium is formed in dense, where clusters grow up. Sufficient atomic collisions in the locally confined cluster growth area possibly complete the uniform thermodynamical conditions. Excited particles in the vapor and helium gas emit lights in the initial stage of confinement. Emission is observed through a sapphire window of 4:5 mm 17:5 mm opened on
Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42
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the ellipsoidal wall. We use a fast timing CCD camera (ICCD, Andor Tec., detectable wave length from 180 to 850 nm) in timing operation faster than 50 ns gated by the photo signals from the Nd:YAG laser. The cluster source system is evacuated by a turbo molecular pump (300l/s) to achieve the pressure at 1 107 Pa as no loading of the helium gas. Generated silicon clusters in the SCCS are extracted into vacuum following the helium gas flow through a skimmer with an aperture of 3.0 mm in diameter set at 20 mm far from the cluster source. The ion components are suppressed to pass through the skimmer, to which an electrostatic potential of 300 V is applied. Generated neutral clusters are detected by the two-stepacceleration type time of flight mass spectrometer (TOFMS), the center of which is located at 253 mm far from the cluster source. Silicon clusters are ionized by irradiation of a pulsed ArF excimer laser (PSX-100, MBP Technology, 3 ns, 6.41 eV, 1:4–1:7 mJ=cm2 pulse).
3. Results Fig. 2 shows light emission from the particles observed through the sapphire window of the SCCS in the initial stage of the cluster growth process just after irradiation of the Nd:YAG laser. The laser-ablated vapor traveling in a helium gas flow is compressed in higher density at the contact front with the helium gas, and the particles excited by atomic collisions in the dense gas phase emit intense lights. The time development of the vapor in the helium gas flow at PHe I ¼ 670 Pa and PHe I ¼ 1300 Pa are displayed in Figs. 2a and b, respectively. In both helium pressure, the vapor front travels at a velocity of 8:0 103 m/s initially for a time of 1 ls, and then approaches the focal point of the SCCS in 2.3 ls with deceleration. The induced shock near the target surface propagates in the helium gas flow, and stops the traveling vapor front at the focal point in 2.3 ls after reflection on the ellipsoidal wall. The mixed gas layer produced at the boundary of the vapor and helium gas is confined in a local space smaller than 1.0 mm, which is estimated from the half-reduction length of the emission intensity at the contact
Fig. 2. Transient emission of lights in the SCCS taken by a fast timing CCD camera. The time sequence of the vapor front in a helium gas flow at (a) PHe I ¼ 670 Pa, and (b) PHe I ¼ 1300 Pa.
front. If the pressure of the helium gas flow is reduced lower than PHe I ¼ 150 Pa, such a sharp shape of the vapor front is not observed, and the mixed gas layer disappears. A typical mass spectrum of silicon clusters generated in the SCCS is shown in Fig. 3. In the higher abundance region of smaller silicon clusters SiN with atomic size up to N ¼ 4, silicon monomers and dimers are dominantly observed. In the spectrochemical analysis of emission in the SCCS, several excited atomic lines of the neutral silicon monomers and dimers are observed consistently with the results of mass analysis, even though the SCCS is not filled with a helium gas [10]. These monomers and dimers directly emerge from the silicon target by irradiation of the Nd:YAG laser. Silicon molecular products such as SiN Hx ðN ¼ 1–4; x ¼ 0–4Þ, SiN CHx ðN ¼ 1–4; x ¼ 0–6Þ also get
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Fig. 3. The TOF mass spectrum of silicon clusters generated using the SCCS. The histogram of the fractional silicon clusters of SiN Hx (N ¼ 9–34) is shown in the inset.
mixed in the spectrum. In the region of larger silicon clusters of N > 4, stable Si23 clusters including Si23 H3 and Si23 H6 clusters show characteristically higher abundance. Produced silicon clusters are concentrated around the atomic size of N ¼ 23 with narrow size dispersion. Separated from the carbide products of SiN CHx that are generated in the SCCS, histogram of the fractional silicon clusters of SiN Hx in the size distribution from N ¼ 9 to 34 is shown in Fig. 3. Si23 Hx ; Si19 Hx ; Si21 Hx clusters distribute in the highest abundance of 46%, 14%, 12%, respectively. The intensity of helium line in the mass spectra shown in Fig. 3 is proportional to the density of the helium gas flow, which carries the cluster products to the TOFMS system. Sifting the delayed time of ionization at the TOFMS stage by trigger adjustment of the ArF excimer laser, we measured the density of the helium gas flow as shown in Fig. 4. The density of the helium gas flow without irradiation of the Nd:YAG laser is used as a standard density level of the constant helium gas flow with no shock. The excited particles locally confined in the dense gas layers in the cluster source emit photons as shown in Fig. 2, which cause a background level of the MCP detector in the TOFMS system. The background level was measured under introduction of the Nd:YAG laser but without irradiation of the ArF excimer laser. In 70 ls after Nd:YAG laser irradiation, the background level was reduced down to the lowest values enough to
Fig. 4. Time-dependence of the density of the helium gas flow in vacuum after Nd:YAG irradiation. The induced shock flow () and the standard density level of the static constant flow () and the background level caused by photoemission from the dense gas layer in the cluster source are summarized.
identify the density of the helium gas in the induced shock flow. The density of the helium gas flow rapidly increases at 90 ls, and reduces down to the level of the constant helium gas flow in 250 ls. Accordingly, the dense helium gas layer is produced in the cluster source for a time of 160 ls, in which time clusters have possibility to grow up to several hundreds atomic size sufficiently.
4. Discussion The fluence of 1:4–1:7 mJ=cm2 of the ArF excimer laser used for ionization of neutral silicon clusters is much lower than the threshold in evaporation of copper clusters with lower cohesive energy, which have been studied previously [11]. In this fluence of the ArF excimer laser silicon clusters are actually stable [12]. Then there was no photo-fragmentation of the generated silicon clusters in measurement of the TOFMS. In detection of the helium lines observed in Fig. 3, multi-photon ionization of helium atoms certainly occurs under the low fluence of the ArF excimer laser. It can be speculated that the helium density at the TOFM stage might be so high roughly above 1022 –1023 m3 in a moment. A shock front produced by moving a piston at a constant velocity up in a one-dimensional tube
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filled with a polytropic gas with an adiabatic exponent c propagates at a constant velocity U, which is described by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi 1 up 1 u p ; U¼ þ c20 þ 2 1 l2 4 1 l2 ð1Þ c1 2 l ¼ ; cþ1 where c0 is the sound velocity in the initial gas zone before the shock front reaches [13]. In the case of an induced shock in the cluster source, the silicon vapor front traveling at a velocity uv ¼ 8:0 103 m/s plays the role of a piston to produce a shock wave. Applying the model to an induced shock in the cluster source, where up ¼ uv , c of helium gas is 5/3, and c0 ¼ 1:02 103 m/s at the temperature of 300 K, one can estimate the velocity of the shock front propagating forward to the wall, Uþ ¼ 10:8 103 m/s. The reflected shock front on the wall is also estimated, U ¼ 2:12 104 m/s. On the other hands, the experimental results in Fig. 2 show that the reflected shock front collides with the vapor front at 2.3 ls after Nd:YAG laser irradiation. Accordingly, fixing Uþ ¼ 10:8 103 m/s, the reflected shock velocity is experimentally determined, U;exp ¼ 2:0 104 m/s. The good agreement on the reflected shock velocity indicates that shock behavior in the cluster source is well described by the model of a shock in a polytropic gas. Density jump at the shock front traveling forward can be estimated, qþ =q0 ¼ 4. Condensation of a helium gas in the cluster source is observed in Fig. 4 as rapid increment on the density of a helium gas flow extracted into vacuum. The helium gas flow detected at the TOFMS stage is conserved in a higher density level than the constant helium gas flow while 160 ls. Since the dense helium gas front flies at a constant velocity of 2.8 km/s in vacuum to the TOFMS stage located at 253 mm far in 90 ls, then it is concluded that the helium gas phase inside the cluster source is densely conserved due to the induced shock just for the same time of 160 ls. And it is also probable that a dense mixed gas layer created locally in a fine space of sub millimeter at the contact front of the both gas phase is kept under confinement in a parallel time. Self-diffusion
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of the mixed gas layer in the time is much small in a sub millimeter scale. Although clusters could possibly grow up to several hundred in atomic size in a period of 160 ls, cluster growth is suppressed when the density of the mixed gas layer reduces. Change of the cluster growth rate is obviously displayed in Fig. 5 by showing time-dependence of the TOF mass spectra. Cluster growth is stopped consistently with a clear cut of the helium density in Fig. 4 at 250 ls. The cluster growth rate is much rapidly reduced compared to the pulsed cluster beam of a duration time in millisecond produced by normal laser ablation using a pulsed gas valve [9]. Accordingly, the silicon clusters generated in the SCCS are resultantly distributed with a narrow size dispersion around the atomic size of N ¼ 23. Jarrold et al. have discussed stability of silicon clusters of atomic size up to 70 in the drift tube
Fig. 5. Time-dependence of the generated silicon mass spectra measured at 100, 150, 300 ls after Nd:YAG irradiation. The cluster growth rate decreases following reduction of the mixed gas density in the cluster source.
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studies on the nanosurface chemistry. In the reaction rate analysis of size-selected silicon cluster ions, Si13 , Si19 and Si23 clusters are particularly unreactive with oxygen, water and ethylene [14,15]. These stable clusters can be modeled by the icosahedral growth sequence [16]. Silicon clusters generated using the SCCS show also narrow size-dispersive mass spectra with a characteristically higher abundance of Si23 Hx , Si19 Hx , and Si21 Hx clusters, although the spectra are contaminated with the carbide products. The carbon elements came from the spherical target holder not from the silicon target itself. The spectra are, however, completely different from the previously reported mass spectra obtained by laser ablation [17–20]. The SCCS can control the induced shock to produce a definite area in space and time at the contact front of the helium and vapor gas phase. The area possibly gives well-defined thermo-dynamical conditions suited for cluster growth with a so narrow dispersion in cluster size that the generated silicon clusters show characteristically higher abundance of Si23 Hx , Si19 Hx , and Si21 Hx clusters.
5. Conclusions We have developed a new laser ablation type cluster source named ‘spatiotemporal confined cluster source’ (SCCS), which gives well-defined thermo-dynamical conditions for cluster growth with a narrow size dispersion suited for vacuum synthesis of nanostructures by low energy cluster deposition. The new structure of the SCCS enables one to make a continuous static helium gas flow with no turbulence symmetrically on the center axis of the ellipsoidal-shaped cluster cell, the flow which successfully controls propagation of the induced shock to produce a definite mixed gas layer of the atomic vapor and helium gas. The formed mixed gas layer is confined in a fine space of sub millimeter at the contact front of the both gas phase. The confined layer is conserved densely in the cluster source for a time of 160 ls, and then the density is reduced rapidly. Cluster growth is well-controlled in concert with the mixed gas density. Accordingly, the cluster growth area lo-
cally confined in space and time possibly makes well-defined thermo-dynamical conditions for cluster growth with well-defined internal states. Resultantly, the new SCCS generates a silicon cluster beam of narrow size dispersion with characteristically higher abundance of stable Si23 Hx (46%), Si19 Hx (14%), and Si21 Hx (12%) clusters. In the future works, we have to investigate the effects of the thermal adiabatic compression due to the induced shock on the internal states of growing clusters in the SCCS.
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