Narrow size-distributed silicon cluster beam generated using aspatiotemporal 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, Japanb Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japanc Department of Advanced Material Sciences, Graduate School of Frontier Sciences, The University of Tokyo,

7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japand 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 byan ArF excimer laser for mass analysis without dissociation, show narrow size dispersion with characteristically higher

abundance of stable Si23Hx (46%), Si19Hx (14%), and Si21Hx (12%) clusters. � 2002 Published by Elsevier Science B.V.

1. Introduction

As a fundamental common technology requiredto build up fine structure systems in a scale down tonanometer in the new material science, differentways have been developed: optical and electron

focused lithography, atomically controlled epitaxy,chemical methods for semiconductor nanocrystals,atomic manipulation using light, and biocatalyticsynthesis [1,2]. Vacuum synthesis of nanostructuresby low energy cluster deposition has such an ad-vantage over the preceding technologies that onecan carefully control the building block in na-nometer scale (cluster deposition rate) efficientlyand characterize the growth mechanisms [3].Depositing clusters on a substrate surface at suchlow impact energy as a few ten meV/atom, onewould like to expect spontaneous ordering with no

24 May 2002

Chemical Physics Letters 358 (2002) 36–42

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +81-298-61-5754.

E-mail address: y.iwata@aist.go.jp (Y. Iwata).1 Present address: Department of Materials Science, Nanjing

University, Nanjing 210093, China.

0009-2614/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0009-2614 (02 )00556-0

fragmentation. The deposited clusters diffuse atsignificantly high speed comparable with atomicdiffusion, and juxtapose with neighbor clusters toform stable fine structures in nanometer scale. Inan analogy of atomic ordering of adsorbate on acrystal surface [4], induced dipoles of clusters witha charge transferred from the substrate surfacecause long range interaction between clusters in apotential of PiPj=r3ij, where Pi is induced dipolemoment and rij is the distance between clusters. Ifthe magnitude of the induced dipole moment that isproportional to the number of cluster constituentatoms (cluster size) becomes uniform, equivalentperiodic potentials work on the clusters so thatlong range ordering of clusters is formed on thesurface. Further more, matching in the crystal lat-tice of clusters is also important in morphologydevelopment of a nanocrystalline regime. Attach-ment of anatase nanoparticles with oriented latticehas been observed in crystalline growth [5]. Ac-cordingly, well-defined stable clusters, which haveuniformly distributed cluster sizes, electronic statesand crystallographic structures, should be devel-oped for forming self-assembly an ordered stablestructure in nanometer scale.The cluster growth mechanism in the expanding

nozzle flow has been discussed to describe theproduction process of molecular clusters in su-personic beams [6]. While the expanding nozzleflow 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 thecollision rate of particles including atoms andclusters defined by their mean free path and localmean velocity. In the Knudsen effusion process ofparticles passing through a conical nozzle, the lo-cal density of particles and the temperature ofvapor are rapidly reduced. Metal vapor exiting thecluster source has a thermal velocity distributionand large angular divergence, which induce higherinternal excitation of the clusters. Kappes et al. [8]introduced the seeded beam technology improvedfrom the expanding nozzle flow method to reducethe internal excitation of clusters in the producingprocesses of the metal clusters with a lower meltingpoint such as sodium clusters. In helium-seededexpansion the faster mean flow velocity suppressescluster–cluster collisions in the gas phase. The

lower accommodation coefficient of a helium gasleads to production of smaller sodium clusters withreduced internal excitation.Otherwise the laser ablation type cluster source

is suited to generate clusters by condensation ofmaterials in the gas phase with a higher meltingpoint such as semiconductors and transition met-als. The most benefit advantage of the laser abla-tion methods is that one can obtain the samecomposition in the gas phase of alloy materials asin the solid phase. Then the laser ablation methodhas been widely used in vacuum synthesis of thecluster materials. It is generally difficult, however,to define the density and temperature of the ab-lated vapor following the pathway of clustergrowth in the cluster source, even though thethermo-dynamical conditions have great influenceon cluster growth. Then the internal states of thegenerated clusters cannot be uniformly defined,and the resultant size dispersion is commonly lar-ger. Copper clusters (CuN , N ¼ 1–500) and ironclusters (FeN , N ¼ 1–200) generated by laser ab-lation in a cell filled with a pulsed helium gas flowform a pulsed beam traveling at velocity of 600–700 m/s [9]. Before extracted into vacuum, parti-cles including clusters are confined in the cell for atime from 0.3 to 1.2 ms. Depending on the con-finement time, the size dispersion defined as a ratioDN=N of the distribution width DN to the meancluster size N has commonly large values from 0.2to 0.5 [10]. These results indicate that the thermo-dynamical conditions in the gas phase obviouslyhave influence on cluster growth in the cell.For the purpose to generate well-defined stable

clusters, we have developed a new laser ablationtype cluster source named ‘spatiotemporal confinedcluster source’ (SCCS), which gives well-definedthermodynamic conditions of density and temper-ature in the gas phase by locally confining the clustergrowth area in space and time. In this Letter, wedescribe the preliminary results of narrow size-dis-tributed silicon clusters generated using the SCCS.

2. Experimental

The cluster beam system equipped with theSCCS is shown in Fig. 1. The wall of the cluster

Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42 37

source has an ellipsoidal shape with a couple offocal points separated 20 mm far from each other.On the center axis of the ellipsoidal wall, a smallhole of 0.5 mm in diameter is opened at 3 mm farnear the focal point for introduction of the laserbeam and extraction of the cluster beam. A silicontarget is mounted on the top of a spherical-shapedholder so that the ablation point on the targetsurface is located at the focal point. The target hasa cylindrical shape with an edge surface cutslantwise at the angle of 4�, and the cylindrical axisleans at the same angle from the center axis of thecluster source. The setup enables the silicon sur-face to always make a right angle with the centeraxis while the target turns on its axis. Then thelaser-ablated vapor ejects from the target surfacesymmetrically with the center axis. We carefullypoured a helium gas continuously into the clustersource with no turbulent flow. Helium gas intro-duced through the laying pipe is spread all over thespherical surface of the target holder and makes astatic flow symmetric with the center axis. Thetypical helium gas pressure in the cluster source isPHe S ¼ 130 Pa, which is estimated from the inlet

pressure of PHe I ¼ 1300 Pa measured at the layingpipe.A pulsed Nd:YAG laser focused at the entrance

of the ellipsoidal wall (wave length 532 nm, pulseduration 10 ns, energy 50–300 mJ/pulse) is intro-duced along the center axis, and irradiates thetarget surface at a beam spot of 0.8 mm in dia-meter. The laser-ablated dense vapor induces ashock wave in the symmetric helium gas flow afterforming a Knudsen layer. The shock front prop-agates symmetrically to the ellipsoidal wall, andthen locally concentrates on the alternate focalpoint after reflection at the wall. The vapor fronttraveling at a slower velocity than the shock frontis stopped at the focal point by the dense heliumgas. At the contact front of the both gas phases, anarrow mixed gas layer of vapor and helium isformed in dense, where clusters grow up. Sufficientatomic collisions in the locally confined clustergrowth area possibly complete the uniform thermo-dynamical conditions. Excited particles in thevapor and helium gas emit lights in the initial stageof confinement. Emission is observed through asapphire window of 4:5 mm� 17:5 mm opened on

Fig. 1. Cluster beam system equipped with the SCCS. Nd:YAG laser is introduced through a hole opened at the center on the el-

lipsoidal-shaped wall to irradiate a silicon target set on the top of a spherical-shaped holder. A helium gas carefully poured contin-

uously 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).

38 Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42

the ellipsoidal wall. We use a fast timing CCDcamera (ICCD, Andor Tec., detectable wavelength from 180 to 850 nm) in timing operationfaster than 50 ns gated by the photo signals fromthe Nd:YAG laser. The cluster source system isevacuated by a turbo molecular pump (300l/s) toachieve the pressure at 1� 10�7 Pa as no loadingof the helium gas. Generated silicon clusters in theSCCS are extracted into vacuum following thehelium gas flow through a skimmer with an aper-ture of 3.0 mm in diameter set at 20 mm far fromthe cluster source. The ion components are sup-pressed to pass through the skimmer, to which anelectrostatic potential of 300 V is applied. Gener-ated neutral clusters are detected by the two-step-acceleration type time of flight mass spectrometer(TOFMS), the center of which is located at 253mm far from the cluster source. Silicon clusters areionized by irradiation of a pulsed ArF excimerlaser (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 particlesobserved through the sapphire window of theSCCS in the initial stage of the cluster growthprocess just after irradiation of the Nd:YAG laser.The laser-ablated vapor traveling in a helium gasflow is compressed in higher density at the contactfront with the helium gas, and the particles excitedby atomic collisions in the dense gas phase emitintense lights. The time development of the vaporin the helium gas flow at PHe I ¼ 670 Pa andPHe I ¼ 1300 Pa are displayed in Figs. 2a and b,respectively. In both helium pressure, the vaporfront travels at a velocity of 8:0� 103 m/s initiallyfor a time of 1 ls, and then approaches the focalpoint of the SCCS in 2.3 ls with deceleration. Theinduced shock near the target surface propagatesin the helium gas flow, and stops the travelingvapor front at the focal point in 2.3 ls after re-flection on the ellipsoidal wall. The mixed gas layerproduced at the boundary of the vapor and heliumgas is confined in a local space smaller than 1.0mm, which is estimated from the half-reductionlength of the emission intensity at the contact

front. If the pressure of the helium gas flow is re-duced lower than PHe I ¼ 150 Pa, such a sharpshape of the vapor front is not observed, and themixed gas layer disappears.A typical mass spectrum of silicon clusters

generated in the SCCS is shown in Fig. 3. In thehigher abundance region of smaller silicon clustersSiN with atomic size up to N ¼ 4, silicon mono-mers and dimers are dominantly observed. In thespectrochemical analysis of emission in the SCCS,several excited atomic lines of the neutral siliconmonomers and dimers are observed consistentlywith the results of mass analysis, even though theSCCS is not filled with a helium gas [10]. Thesemonomers and dimers directly emerge from thesilicon target by irradiation of the Nd:YAG laser.Silicon molecular products such as SiNHx ðN ¼1–4; x ¼ 0–4Þ, SiNCHx ðN ¼ 1–4; x ¼ 0–6Þ also get

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.

Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42 39

mixed in the spectrum. In the region of larger sil-icon clusters of N > 4, stable Si23 clusters includingSi23H3 and Si23H6 clusters show characteristicallyhigher abundance. Produced silicon clusters areconcentrated around the atomic size of N ¼ 23with narrow size dispersion. Separated from thecarbide products of SiNCHx that are generated inthe SCCS, histogram of the fractional siliconclusters of SiNHx in the size distribution fromN¼ 9 to 34 is shown in Fig. 3. Si23Hx;Si19Hx; Si21Hx clusters distribute in the highestabundance of 46%, 14%, 12%, respectively.The intensity of helium line in the mass spectra

shown in Fig. 3 is proportional to the density of thehelium gas flow, which carries the cluster productsto the TOFMS system. Sifting the delayed time ofionization at the TOFMS stage by trigger adjust-ment of the ArF excimer laser, we measured thedensity of the helium gas flow as shown in Fig. 4.The density of the helium gas flow without irradi-ation of the Nd:YAG laser is used as a standarddensity level of the constant helium gas flow withno shock. The excited particles locally confined inthe dense gas layers in the cluster source emitphotons as shown in Fig. 2, which cause a back-ground level of the MCP detector in the TOFMSsystem. The background level was measured underintroduction of the Nd:YAG laser but without ir-radiation of the ArF excimer laser. In 70 ls afterNd:YAG laser irradiation, the background levelwas reduced down to the lowest values enough to

identify the density of the helium gas in the inducedshock flow. The density of the helium gas flowrapidly increases at 90 ls, and reduces down to thelevel of the constant helium gas flow in 250 ls.Accordingly, the dense helium gas layer is pro-duced in the cluster source for a time of 160 ls, inwhich time clusters have possibility to grow up toseveral hundreds atomic size sufficiently.

4. Discussion

The fluence of 1:4–1:7 mJ=cm2 of the ArF ex-cimer laser used for ionization of neutral siliconclusters is much lower than the threshold inevaporation of copper clusters with lower cohesiveenergy, which have been studied previously [11]. Inthis fluence of the ArF excimer laser silicon clus-ters are actually stable [12]. Then there was nophoto-fragmentation of the generated siliconclusters in measurement of the TOFMS. In de-tection of the helium lines observed in Fig. 3,multi-photon ionization of helium atoms certainlyoccurs under the low fluence of the ArF excimerlaser. It can be speculated that the helium densityat the TOFM stage might be so high roughlyabove 1022–1023 m�3 in a moment.A shock front produced by moving a piston at a

constant velocity up in a one-dimensional tube

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.

Fig. 3. The TOF mass spectrum of silicon clusters generated

using the SCCS. The histogram of the fractional silicon clusters

of SiNHx (N ¼ 9–34) is shown in the inset.

40 Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42

filled with a polytropic gas with an adiabatic ex-ponent c propagates at a constant velocity U,which is described by

U ¼ 1

2

up1� l2

þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffic20 þ

1

4

up1� l2

� �2s

;

l2 ¼ c � 1

c þ 1;

ð1Þ

where c0 is the sound velocity in the initial gas zonebefore the shock front reaches [13]. In the case ofan induced shock in the cluster source, the siliconvapor front traveling at a velocity uv ¼ 8:0� 103

m/s plays the role of a piston to produce a shockwave. Applying the model to an induced shock inthe cluster source, where up ¼ uv, c of helium gas is5/3, and c0 ¼ 1:02� 103 m/s at the temperature of300 K, one can estimate the velocity of the shockfront propagating forward to the wall,Uþ ¼ 10:8�103 m/s. The reflected shock front on the wall is alsoestimated, U� ¼ 2:12� 104 m/s. On the otherhands, the experimental results in Fig. 2 show thatthe reflected shock front collides with the vaporfront at 2.3 ls after Nd:YAG laser irradiation.Accordingly, fixing Uþ ¼ 10:8� 103 m/s, thereflected shock velocity is experimentally deter-mined, U�;exp ¼ 2:0� 104 m/s. The good agreementon the reflected shock velocity indicates that shockbehavior in the cluster source is well described bythe model of a shock in a polytropic gas. Densityjump at the shock front traveling forward can beestimated, qþ=q0 ¼ 4.Condensation of a helium gas in the cluster

source is observed in Fig. 4 as rapid increment onthe density of a helium gas flow extracted intovacuum. The helium gas flow detected at theTOFMS stage is conserved in a higher density levelthan the constant helium gas flow while 160 ls.Since the dense helium gas front flies at a constantvelocity of 2.8 km/s in vacuum to the TOFMSstage located at 253 mm far in 90 ls, then it isconcluded that the helium gas phase inside thecluster source is densely conserved due to the in-duced shock just for the same time of 160 ls. Andit is also probable that a dense mixed gas layercreated locally in a fine space of sub millimeter atthe contact front of the both gas phase is keptunder confinement in a parallel time. Self-diffusion

of the mixed gas layer in the time is much small ina sub millimeter scale. Although clusters couldpossibly grow up to several hundred in atomic sizein a period of 160 ls, cluster growth is suppressedwhen the density of the mixed gas layer reduces.Change of the cluster growth rate is obviouslydisplayed in Fig. 5 by showing time-dependence ofthe TOF mass spectra. Cluster growth is stoppedconsistently with a clear cut of the helium densityin Fig. 4 at 250 ls. The cluster growth rate is muchrapidly reduced compared to the pulsed clusterbeam of a duration time in millisecond producedby normal laser ablation using a pulsed gas valve[9]. Accordingly, the silicon clusters generated inthe SCCS are resultantly distributed with a narrowsize 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.

Y. Iwata et al. / Chemical Physics Letters 358 (2002) 36–42 41

studies on the nanosurface chemistry. In the re-action rate analysis of size-selected silicon clusterions, Si13, Si19 and Si23 clusters are particularlyunreactive with oxygen, water and ethylene[14,15]. These stable clusters can be modeled bythe icosahedral growth sequence [16]. Siliconclusters generated using the SCCS show also nar-row size-dispersive mass spectra with a character-istically higher abundance of Si23Hx, Si19Hx, andSi21Hx clusters, although the spectra are contami-nated with the carbide products. The carbon ele-ments came from the spherical target holder notfrom the silicon target itself. The spectra are,however, completely different from the previouslyreported mass spectra obtained by laser ablation[17–20]. The SCCS can control the induced shockto produce a definite area in space and time at thecontact front of the helium and vapor gas phase.The area possibly gives well-defined thermo-dy-namical conditions suited for cluster growth with aso narrow dispersion in cluster size that the gen-erated silicon clusters show characteristicallyhigher abundance of Si23Hx, Si19Hx, and Si21Hx

clusters.

5. Conclusions

We have developed a new laser ablation typecluster source named ‘spatiotemporal confinedcluster source’ (SCCS), which gives well-definedthermo-dynamical conditions for cluster growthwith a narrow size dispersion suited for vacuumsynthesis of nanostructures by low energy clusterdeposition. The new structure of the SCCS en-ables one to make a continuous static helium gasflow with no turbulence symmetrically on thecenter axis of the ellipsoidal-shaped cluster cell,the flow which successfully controls propagationof the induced shock to produce a definite mixedgas layer of the atomic vapor and helium gas. Theformed mixed gas layer is confined in a fine spaceof sub millimeter at the contact front of the bothgas phase. The confined layer is conserved denselyin the cluster source for a time of 160 ls, and thenthe density is reduced rapidly. Cluster growth iswell-controlled in concert with the mixed gasdensity. Accordingly, the cluster growth area lo-

cally confined in space and time possibly makeswell-defined thermo-dynamical conditions forcluster growth with well-defined internal states.Resultantly, the new SCCS generates a siliconcluster beam of narrow size dispersion withcharacteristically higher abundance of stableSi23Hx (46%), Si19Hx (14%), and Si21Hx (12%)clusters.In the future works, we have to investigate the

effects of the thermal adiabatic compression due tothe induced shock on the internal states of growingclusters in the SCCS.

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