Advanced Nanocluster Ion Source Based on High-Power Impulse

Article pubs.acs.org/JPCA

Advanced Nanocluster Ion Source Based on High-Power Impulse Magnetron Sputtering and Time-Resolved Measurements of Nanocluster Formation Chuhang Zhang,†,‡ Hironori Tsunoyama,†,‡ Hiroki Akatsuka,‡ Hiroki Sekiya,‡ Tomomi Nagase,‡ and Atsushi Nakajima*,†,‡ †

Nakajima Designer Nanocluster Assembly Project, JST-ERATO, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

‡

S Supporting Information *

ABSTRACT: We developed a new nanocluster (NC) ion source based on the high-power impulse magnetron sputtering (HiPIMS) technique coupled with a gas flow cell reactor. Silver NC anions (Agn−) with a maximum intensity of 5.5 nA (Ag11−) are generated with the size ranging from the atomic anion to the 70-mer, which is well-controlled by simply adjusting the peak power and repetition rate of the HiPIMS. By time-resolved density profiles of Agn−, we find that the ion beam generated by HiPIMS is characterized by individual 100 ms duration “bunches” below a repetition rate of 10 Hz, which is well-thermalized with a group velocity of 5 m/s. The high intensity of the NCs is attributable to the high ionization fraction by this HiPIMS ion source, while the underlying mechanism of the flexible size tuning of the ion source is understood by time-resolved mass spectrometry coupled with the sequential growth mechanism; the increment of the density of the target species in the bunches with the peak power and the overlapping of the bunches with the repetition rate cause the formation of large NCs.

1. INTRODUCTION Nanoclusters (NCs) composed of less than 100 atoms exhibit unique physical and chemical properties relevant to their functionality, found in neither bulk nor atomic species. Consequently, cluster science has become of increasing interest to researchers in both fundamental surface science and material science.1 Because their behavior depends on their size and composition, NCs are a promising candidate for applications such as NC-assembled functional materials.1 To realize such nanomaterial applications, selective NC fabrications must be coupled to mass spectrometric procedures for NC ions, which require an intense NC ion source. Laser vaporization is a versatile method, in which target atoms are vaporized by a pulsed laser and then condensed into NCs with He carrier gas.2 This method has proven quite successful in discovering magic number clusters, such as fullerenes,3 metal-encapsulated silicon cages,4,5 and Metcars,6 possibly because the high internal temperature causes the metastable NCs to fragment into more stable ones. However, © 2013 American Chemical Society

laser vaporization has been utilized very little in materials application, partly because of the limited input power and relatively poor scalability for mass production of NCs. In the past 2 decades, a direct current magnetron sputtering (DC-MSP) source with gas condensation has been developed for NC production.7,8 In the MSP method, the target materials are sputtered continuously under bombardment of working gas ions and cooled and condensed into NCs by collisions with the buffer gas in a condensation cell. Therefore, DC-MSP provides a temporally stable NC source with adjustable parameters (such as DC power and gas flow rates) for controlling the intensity of the NCs, as has been theoretically and experimentally demonstrated.9−13 However, few reports have focused on the NC formation mechanism by DC-MSP or radio frequency (RF)-MSP because both continuous DC- and RF-MSP hardly Received: July 2, 2013 Revised: September 9, 2013 Published: September 30, 2013 10211

dx.doi.org/10.1021/jp406521v | J. Phys. Chem. A 2013, 117, 10211−10217

The Journal of Physical Chemistry A

Article

Figure 1. Schematic of the experimental setup with a scheme for time-resolved measurements. After the trigger of HiPIMS sputtering, initiated at T0, the retarding voltage (−V) was grounded in the gate window time T1−T2.

yield sufficient information on time-resolved NC evolution. Furthermore, because the power density in DC-MSP is below 50 W/cm2,14 DC-MSP generates an ion beam with a low fraction of ionized target species below several percent,15,16 leading to a limited ion intensity of below 1 nA for NC production.9,10,17,18 A more advanced technique is pulsed MSP developed by high-power impulse magnetron sputtering (HiPIMS). 16 HiPIMS reveals the time-resolved evolution of NC formation and also enhances the degree of ionization of the target species by imparting a high-power pulse (maximum of 3 kW/cm2) to the target. Indeed, the fraction of the ionized target species enhances up to 50% regardless of the low duty cycle (under 10%).16 Furthermore, the duty cycle of HiPIMS has been recently increased by a modulated pulse power technique. This technique simultaneously achieves a high ionization fraction and high repetition rate by modulating the pulse shape, which is crucial to tailoring the thin film property and controlling the size of the nanoparticles.14,19−21 In this report, the HiPIMS technique is characterized to reveal time-resolved NC evolution using temporal gating mass spectrometry. The NC formation mechanism of a versatile NC ion source is investigated by finely controlling the NC size. In addition to higher ion flux, the HiPIMS NC source enables NC size control simply by tuning the peak power P and repetition rate f.

nitrogen, was introduced into the condensation cell from upstream of the MSP NC source. The typical flow rates of Ar and He were 100 and 600 sccm, respectively, and the pressure of the condensation cell was maintained at 20 Pa. The NCs were introduced to the next chamber (maintained at 10−1 Pa) and guided by an octopole ion guide (OPIG) to a quadrupole ion bender. NC ions of appropriate polarity were deflected to the quadrupole mass spectrometer (Q-MS; Extrel, MAX16000) in the third chamber (maintained at 10−3 Pa). The size and intensity of the NC ions were monitored by Q-MS with a Faraday cup ion detector, equipped with a picoammeter (Keithley, 6517B). For HiPIMS, a modulated power pulse was generated by a pulse power generator (Zpluser, AXIA III). The time course of the plasma advanced in three steps (see Figure 2). In the low

2. EXPERIMENTAL METHODS Figure 1 is a schematic of the experimental setup. The two main apparatuses are22 a MSP NC source and mass spectrometer. Briefly, a Ag target disk (diameter of 2 in.; purity of 99.99%) was mounted, and Ag NCs were produced by a HiPIMS placed in a condensation cell cooled with liquid nitrogen flowing in the surrounding jacket. The length of the condensation cell L can be varied from 190−290 mm. Argon (Ar) was introduced as the working gas nearby the target, while He, cooled by liquid

Figure 2. Typical HiPIMS pulse waveform used in this study.

ignition step (initial 240 μs; step 1), the voltage was programmed to increase gradually, that is, between −180 and −240 V, while maintaining the discharge current below 0.2 A. During the following 120 μs of step 2, the voltage was further increased to approximately −285 V, with resulting incremental increases in discharge current (1.0 A). Finally, the attained maximum voltage and current were retained for 1 ms in step 3, 10212



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generating a dense plasma of target species.23 Representative parameters of HiPIMS are the peak power P, the shape of the pulse waveform, and the repetition rate of pulses f. The pulse waveform was characterized by the duration and slope of the macropulse comprising the micropulses generated in steps 1 and 2, which was modulated by the on−off time ratio of the micropulses.23 Typical values of the peak voltage, discharge current, and peak power were between −280 and −400 V, 0.7 and 1.5 A, and 200 and 600 W, respectively. To obtain time-resolved measurements, an overall time profile of the total Ag NC ions was first measured at the entrance of the Q-MS, where stray electrons were deflected by an OPIG and ion bender. Furthermore, for time-resolved mass spectroscopy of the Ag NC ions, a retarding electrode made of nickel mesh (transmittance 80%) was mounted as an ion gate immediately beyond the condensation cell exit (see Figure 1). To ensure synchronization with the trigger of HiPIMS sputtering, initiated at T0, the retarding voltage +V1 (for cations) or −V1 (for anions) was grounded for a specified delay time T1 to allow the NC ions to pass through the gate. Following the gate window time T1−T2, the retarding voltage was again applied. The typical voltage V1 and gate time T1−T2 were 10 V and 0.5−4 ms, respectively.

Supporting Information). Note that the ion intensity obtained in HiPIMS is several to 10 times higher than that typically reported for Agn− (n > 10), namely, 10 pA.9,10 Figure 3b−d presents Agn− mass spectra under different peak powers at the lowest repetition rate (7 Hz). The total ion intensity, obtained as the integral of the mass spectrum, increases from 5.9 (Figure 3b) to 17.5 nA (Figure 3d) with P. This increase is ascribed to the higher densities of electrons and sputtered neutrals with increasing applied power.14,24 In addition, the average size increases with P, while the size distribution broadens. These results highlight the following three invaluable advantages of the HiPIMS NC source over conventional DC-MSP: (1) a marked increase in the intensity of size-selected NC ions for n = 5−70, (2) a relatively narrow size distribution,9,10 and (3) a flexible NC size tuning. To elucidate the peak power dependence on mass distributions, we focus on the growth mechanism. When the time profile of the total ion current was measured at a pulse width of 1.5 ms, the observed ion bunch exiting the condensation cell broadened to 100 ms, as shown in Figure 4. Because the discharge current decays within several milliseconds (see Figure 2), several to 10 times broadening of ion bunches originated from the translational velocity distribution of the sputtered species after collision cooling by the buffer gas. In fact, discrete ion bunches are clearly observed below the repetition rate of 7 Hz, whereas no ions are detected between neighboring bunches. The time-resolved mass spectra of each ion bunch were recorded by the above-mentioned pulsed gate method. Figure 5i shows time-resolved mass spectra of Agn−, where a time window of 4 ms is applied to a single ion bunch. The temporal density profile (Figure 5ii), obtained by integrating the ion current, almost identically reproduces that of the total ion current recorded by the Faraday cup prior to Q-MS analysis (Figure 5iii). This result indicates that no NC ions were sizespecifically dissipated through Q-MS, although the sizeindependent transmission efficiency in Q-MS seems to be approximately 12%. The time profile shows the temporal variation of the ion density; ions appear at 40 ms after discharge, increase up to 60 ms, and decay within approximately 150 ms (see Figure 5ii) at a condensation length of 290 mm. Figure 4 shows that the onset delay time and width of the ion bunches are altered by varying the condensation length; for L = 210 and 290 mm, the onset delay times are 25 and 40 ms (Figure 4 ii), respectively. From the relationship between the onset delay and condensation length, the group velocity of NC ions and the gas flow velocity inside of the cell are estimated. Figure 6a shows the dependence between the traveling time (ttra) of the ion bunch in the condensation cell and the length of the condensation cell, in which the group velocity of the ion bunch can be estimated to 5 m/s. In order to obtain the flow velocity of the buffer gas, the flow conductance is evaluated based on the sketch shown in Figure 6b. In the case of a simple orifice,25 the flow conductance is C = 157 × d2 = 2.3 × 10−2 m3/s, where d = 12 mm is the diameter of the exit nozzle of the condensation cell. Therefore, the average flow velocity of buffer gas inside of the condensation cell is given by C/π × (D/2)2 = 2.4 m/s, where D = 110 mm is the diameter of the condensation cell. Furthermore, if one assumes a simple parabolic velocity profile along the radial direction used for the gas flow as a viscous laminar flow,26 the maximum flow velocity along the center axis of the condensation cell is two times the

3. RESULTS AND DISCUSSION Figure 3a shows a mass spectrum of Ag NC anions (Agn−) generated by HiPIMS at a repetition rate of 60 Hz. All peaks

Figure 3. Representative mass spectra of Ag NC anions, Agn− (n = 1− 40), generated by the HiPIMS source. (a) Mass spectra at the optimum conditions for n = 11, where P = 210 W, f = 60 Hz, and L = 190 mm. The peak power dependence of the mass spectra when f = 7 Hz and L = 290 mm at (b) 210, (c) 320, and (d) 370 W. Other experimental parameters are indicated in the text.

are clearly assigned to Agn−, where n = 1−40. Under the optimized conditions, which occurred at n = 11, the maximum ion current intensity was 5.5 nA, while the maximum intensity achievable by DC-MSP was around 3.0 nA at Ag15− (see Figure 1S, Supporting Information). Furthermore, the intensity of the NC cations was also examined, and it was found that the maximum intensities available for DC-MSP and HiPIMS were 1.3 and 2.0 nA (see Figure 2S, Supporting Information), respectively. The above ion currents of silver NCs in our apparatus exceed previously reported results of silver NCs by more than 20 times9,10 and are also much higher than that in other metal NCs systems.18 This high intensity is probably due to (1) the relatively high ionization fraction of the target species in HiPIMS,14 (2) advanced size tuning, and (3) high ion transmission optics in the present setup.22 Indeed, optimized source conditions can generate Agn− ranging from 100 pA to several nanoamperes, depending on the NC size (see Figure 3S, 10213



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Figure 4. (i) Time profiles of the total ion current of ion bunches generated by the HiPIMS NC ion source, recorded in front of the Q-MS (without mass selection) at P = (a) 270, (b) 320, and (c) 380 W. Here, f = 7 Hz and L = 290 mm. Black lines indicate the discharge pulse of the HiPIMS. (ii) Time profile of the total ion current generated by the HiPIMS NC ion source, recorded in front of the Q-MS at varying lengths of the condensation cell: (a) L = 210, (b) L = 250, and (c) L = 290 mm. Here, P = 320 W, and f = 7 Hz.

Figure 5. (i) Time-resolved mass spectra of Ag NC anions in a single ion bunch, recorded by a pulsed ion gate method with a gate window of 4 ms. The times represent the delay times T1 from the discharge: (a) 40, (b) 60, (c) 90, and (d) 140 ms. (ii) Temporal density distribution of NC ions calculated from (i). (iii) Time profiles of the total ions prior to Q-MS; P = 320 W, f = 7 Hz, and L = 290 mm.

Figure 6. (a) Relationship between the traveling times of ion bunches ttra and the length of the condensation cell L. The solid line is the fitted curve. The group velocity of the ion bunch (5 m/s) is the reciprocal of the slope of the fitted curve. (b) Estimation of the flow velocity of the buffer gas in the condensation cell. d and D are the diameters of the exit nozzle and condensation cell, respectively.

decelerated by collisions with the buffer gas. Because the van der Waals radius of He (RHe) is 1.3 Å while the atomic radius of Ag (RAg) is 1.5 Å, the total number of collisions between He atoms and NC neutrals (Nn−a) during the traveling of NCs inside of the condensation cell is approximately given by the following equation:

average velocity, that is, 4.8 m/s. The group velocity of the ion bunch is quite close to the average and maximum flow velocity of the buffer gas and obviously different from the initial velocity of the sputtered ionic species (103−104 m/s),27 which indicates that the NC ions are well-thermalized by the collisions of buffer gas He. The dynamics of the neutral species inside of the condensation cell is also estimated because it is closely related to the growth of NC ions. In the target vicinity of HiPIMS, the velocity of the ionic species is known to exceed that of neutrals.27 In the cell, however, the ion velocity was effectively

Nn−a = π × (RAg + RHe)2 × ρgas × v × t tra

(1)

where ρgas is the number density of the gas atoms in the condensation cell. For the ideal gas model, ρgas = P/kT = 1.5 × 1022. v is the relative velocity between the NC neutrals and the 10214



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Figure 7. (Left) Two-dimensional colored diagrams representing the size distributions of Ag NC anions generated under different f and P = (a) 210, (b) 320, and (c) 370 W, plotted from mass spectrometric observations. The rows in each color map present the mass spectra at f = 7−150 Hz. The color bars show the actual ion current of the NC ions at L = 290 mm. In the right panels, (d−f) show how the average size of the NCs depends on the peak power with varying repetition rate. The error bars show the standard deviations.

Indeed, as shown in Figures 3 and 4i, the density of ions and neutrals increases with peak power. Further increasing the power again reduces the size of the formed NC ions (see Figure 4S, Supporting Information), probably because increasing P increases the ionization fraction and/or excessively heats the buffer gas. The target species density should also be increased if the ion bunches overlap. Therefore, to examine the larger NC formation at higher densities of target species, the relationship between the repetition rate and NC size distribution was investigated. The left panels of Figure 7 show Agn− mass spectra at f = 7, 40, 100, and 150 Hz and (from top to bottom) P = 210, 320, and 370 W. In these panels, the average size of the NC ions is seen to increase with repetition rate f, accompanying the broadening of size distributions. As mentioned above, according to the time profile (Figure 8), the ion bunches are characterized by a 100 ms width. Mass spectral changes become obvious above f = 40 Hz, although ion bunch overlap is

gas atoms. Because the group velocity of the NC is negligible compare to the average molecular velocity of the gas atom, v = (8kT/πM)1/2 = 480 m/s. Here, M = (6 × MHe + MAr)/7 is the effective mass of the He and Ar atoms, considering their flow rates. Figure 6a shows that ttra is 40 ms for a 290 mm length condensation cell, and thus, Nn−a = 7.6 × 104. The effective collision cross section between NC anions and He atoms might exceed that of the NC neutrals because of charge-induced dipoles on the gas atoms, as suggested by the Langevin− Gioumousis−Stevenson model.28 The thermal collision of NC anions (Ni−a) by gas atoms is thus given by Ni−a = π × σ × ρgas × v × t tra

(2)

where σ = π × (αq2/2πε0Ek)1/2 is the effective collision cross section. α = 0.205 × 10−30 m3, q = 1.6 × 10−19 C, and ε0 = 8.85 × 10−12 F/m are the polarizability of the He atom, the single electron charge, and the permittivity of vacuum, respectively. Therefore, the NC ions are anticipated to undergo approximately 2.2 × 105 collisions during passing through the condensation cell. On the basis of the above estimation, it may be reasonably considered that downstream of MSP, the group velocity of the anion bunch became close to that of neutrals by sufficient thermal collisions. In other words, the temporal profile of the neutrals approximates that of the ions, and the profiles highly overlapped during the passage through the cell. As shown in Figure 5, the size distribution of Agn− strongly correlates with the temporal density profile; larger NC anions appear at the peak of ion bunches, whereas smaller ones are temporally distributed throughout the ion bunches. The size distribution of Agn− can be discussed in terms of a sequential growth mechanism,12 in which NCs form by sequential attachment of neutral atoms to ionic nuclei; more specifically, (1) atomic neutrals and ions are initially generated by Ar+ sputtering, and (2) the neutral atoms adhere to ionic nuclei to form NC ions inside of the condensation cell. In this mechanism, larger NC ions should be formed at high densities of neutral atoms because these conditions will favor efficient formation of larger NC ions around the peak of ion bunches.

Figure 8. Time profile of the total ion current at different repetition rates: (a) 7, (b) 10, (c) 20, and (d) 100 Hz. The dashed lines in (d) show the schematic deconvolution by five traces shown in (a). Here, P = 320 W, and L = 290 mm. 10215


The Journal of Physical Chemistry A expected above f = 10 Hz. In fact, ion bunch overlap increases the local maximum current of NC ions to 100 (20 Hz) or 270 nA (150 Hz). As well as being affected by the peak power, NCs become larger as the repetition rate increases, as shown in Figure 7. If the NC growth were completed in the vicinity of the sputtering region, the size distributions would be insensitive to the repetition rate. Therefore, the observed change in the size distributions implies that NC growth primarily occurs widely in the cell. That is to say, incrementing the repetition rate enhances the quantities of ions and neutrals by increasing the overlap ratio of the bunches, in other words, by increasing their number density, as shown in Figure 8. From Figure 8, it should be noted that the ion intensity is relatively suppressed at higher repetition rates. This phenomenon arises mainly from the higher probability of electron detachments from the NC anions, induced by heating of buffer gas in the MSP vicinity.29 Furthermore, when large-size NCs are formed by ion bunch overlap at high repetition rates, their geometric structure might differ from those formed at the peak of the ion bunch. If ion bunches overlap, growth of larger NCs would occur by NC−NC collisions as well as NC−atom collisions, enhancing the NC assemblies at higher repetition rates.

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REFERENCES

(1) Johnson, R. L. Atomic and Molecular Clusters; Taylor & Francis: London, 2002. (2) Heiz, U.; Vanolli, F.; Trento, L.; Schneider, W.-D. Chemical Reactivity of Size-Selected Supported Clusters: An Experimental Setup. Rev. Sci. Instrum. 1997, 68, 1986−1994. (3) Kroto, H. W.; Health, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (4) Beck, S. M. Mixed Metal−Silicon Clusters Formed by Chemical Reaction in a Supersonic Molecular Beam: Implications for Reactions at the Metal/Silicon Interface. J. Chem. Phys. 1989, 90, 6306−6312. (5) Koyasu, K.; Akutsu, M.; Mitsui, M.; Nakajima, A. Selective Formation of MSi16 (M = Sc, Ti, and V). J. Am. Chem. Soc. 2005, 127, 4998−4999. (6) Wei, S.; Guo, B.; Deng, H.; Kerns, K.; Purnell, J.; Buzza, S.; Castleman, A. W. Formation of Met-Cars and Face-Centered Cubic Structures: Thermodynamically or Kinetically Controlled? J. Am. Chem. Soc. 1994, 116, 4475−4476. (7) Haberland, H.; Mall, M.; Moseler, M.; Qiang, Y.; Reiners, T.; Thurner, Y. Filling of Micron-Sized Contact Holes with Copper by Energetic Cluster Impact. J. Vac. Sci. Technol., A 1994, 12, 2925−2930. (8) Haberland, H.; Karrais, M.; Mall, M.; Thurner, Y. Thin Films from Energetic Cluster Impact: A Feasibility Study. J. Vac. Sci. Technol., A 1992, 10, 3266−3271. (9) Palmer, R. E.; Pratontep, S.; Boyen, H.-G. Nanostructured Surfaces from Size-Selected Clusters. Nat. Mater. 2003, 2, 443−448. (10) Pratontep, S.; Carroll, S. J.; Xirouchaki, C.; Streun, M.; Palmer, R. E. Size-Selected Cluster Beam Source Based on Radio Frequency Magnetron Plasma Sputtering and Gas Condensation. Rev. Sci. Instrum. 2005, 76, 045103/1−045103/9. (11) Shyjumon, I.; Gopinadhan, M.; Ivanova, O.; Quaas, M.; Wulff, H.; Helm, C. A.; Hippler, R. Structural Deformation, Melting Point and Lattice Parameter Studies of Size Selected Silver Clusters. Eur. Phys. J. D 2005, 37, 409−415. (12) Smirnov, B. M.; Shyjumon, I.; Hippler, R. Formation of Clusters through Generation of Free Atoms. Phys. Scr. 2006, 73, 288−295. (13) Kashtanov, P. V.; Smirnov, B. M.; Hippler, R. Efficiency of Cluster Generation in a Magnetron Discharge. Eur. Phys. Lett. 2010, 91, 63001/1−63001/6. (14) Hála, M.; Č apek, J.; Zabeida, O.; Klemberg-Sapieha, J. E.; Martinu, L. Pulse Management in High Power Pulsed Magnetron Sputtering of Niobium. Surf. Coat. Technol. 2012, 206, 4186−4193. (15) Petrov, I.; Myers, A.; Greene, J. E.; Abelson, J. R. Mass and Energy Resolved Detection of Ions and Neutral Sputtered Species Incident at the Substrate During Reactive Magnetron Sputtering of Ti in Mixed Ar+N2 Mixtures. J. Vac. Sci. Technol., A 1994, 12, 2846−2854. (16) Kouznetsov, V.; Macák, K.; Schneider, J. M.; Helmersson, U.; Petrov, I. A Novel Pulsed Magnetron Sputter Technique Utilizing Very High Target Power Densities. Surf. Coat. Technol. 1999, 122, 290−293. (17) Duffe, S.; Irawan, T.; Bieletzki, M.; Richter, T.; Sieben, B.; Yin, C.; von Issendorff, B.; Moseler, M.; Hövel, H. Softlanding and STM Imaging of Ag561 Clusters on a C60 Monolayer. Eur. Phys. J. D 2007, 45, 401−408. (18) Yasumatsu, H.; Hayakawa, T.; Koizumi, S.; Kondow, T. Unisized Two-Dimensional Platinum Clusters on Silicon(111)-7 × 7

ASSOCIATED CONTENT

S Supporting Information *

Mass spectrum of Agn− generated by DC-MSP under the most optimized condition (Figure 1S). Comparison of Agn + generated by DC-MSP and HiPIMS (Figure 2S). Representative Agn− mass spectra at varying peak powers and repetition rates (Figure 3S). Peak power dependence of Ag NC cations (Figure 4S). This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors acknowledge Professor J. M. Lisy (Univ. Illinois) for providing an electronic circuit of the radio frequency power supply for the OPIG. We also thank Professors H. Yasumatsu (Toyota Inst. Tech.) and A. Terasaki (Kyushu Univ.) for construction of the magnetron sputtering source and Mr. K. Tsukamoto (Ayabo Corp.) for the operation of HiPIMS. This work was partly supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009−2013.

4. CONCLUSIONS An intensive ion source for size-selected NCs, based on HiPIMS coupled with a gas flow cell reactor, was developed. The characteristics of the source were demonstrated for Agn−. Agn− NCs (n = 1−73) were successfully produced at ion currents from several hundred picoamperes to 5.5 nA. The NC ions were well-thermalized to a group velocity of 5 m/s, close to the average (2.4 m/s) and maximum gas flow velocity (4.8 m/s). Each ion bunch sputtered at a pulse of width of 1.5 ms was broadened to 100 ms by the time it exited the condensation cell. The size distribution of Agn− can be controlled by varying the peak power and repetition rate. In general, the average NC size increases with peak power and/or repetition rate. Timeresolved mass spectrometry reveals that NCs are formed by sequential attachment of neutral atoms to ionic nuclei,12 while size controllability is understood in terms of density modulations in the ion bunches. This research demonstrates the potential applicability of HiPIMS to selective synthesis of high-intensity NC ions that will facilitate the study of nanomaterials based on assembled NCs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-45-566-1697. Notes

The authors declare no competing financial interest. 10216



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Surface Observed with Scanning Tunneling Microscope. J. Chem. Phys. 2005, 123, 124709/1−124709/9. (19) Lin, J.; Moore, J. J.; Sproul, W. D.; Mishra, B.; Wu, Z. Modulated Pulse Power Sputtered Chromium Coatings. Thin Solid Films 2009, 518, 1566−1570. (20) Straňaḱ , V.; Block, S.; Drache, S.; Hubička, Z.; Helm, C.; Jastrabík, L.; Tichý, M.; Hippler, R. Size-Controlled Formation of Cu Nanoclusters in Pulsed Magnetron Sputtering System. Surf. Coat. Technol. 2011, 205, 2755−2762. (21) Pilch, I.; Söderström, D.; Brenning, N.; Helmersson, U. SizeControlled Growth of Nanoparticles in a Highly Ionized Pulsed Plasma. Appl. Phys. Lett. 2013, 102, 033108/1−033108/4. (22) Tsunoyama, H.; Zhang, C. H.; Akatsuka, H.; Sekiya, H.; Nagase, T.; Nakajima, A. Development of High Flux Ion Source for SizeSelected Nanoclusters Based on High Power Impulse Magnetron Sputtering. Chem. Lett. 2013, 42, 857−859. (23) Lin, J.; Moore, J. J.; Sproul, W. D.; Mishra, B.; Rees, J. A.; Wu, Z.; Chistyakov, R.; Abraham, B. Ion Energy and Mass Distributions of the Plasma during Modulated Pulse Power Magnetron Sputtering. Surf. Coat. Technol. 2009, 203, 3676−3685. (24) Poolcharuansin, P.; Bowes, M.; Petty, T. J.; Bradley, J. W. Ionized Metal Flux Fraction Measurements in HiPIMS Discharges. J. Phys. D: Appl. Phys. 2012, 45, 322001/1−322001/5. (25) Dushman, S. Scientific Foundation of Vaccum Technique; John Wiley & Sons: New York, 1949. (26) White, F. M. Fluid Mechanics, 4th ed.; McGraw-Hill: New York, 1998. (27) Tamura, T.; Matsusaki, H.; Koyasu, K.; Yamamoto, H.; Ishihara, D.; Tona, M.; Tsukamoto, K.; Misaizu, F. In 6th Annual Meeting of Japan Society for Molecular Science; Tokyo, 2012; p 35. (28) Armentrout, P. B. Fundamentals of Ion−Molecule Chemistry. J. Anal. At. Spectrom. 2004, 19, 571−580. (29) Meng, L.; Cloud, A. N.; Jung, S.; Ruzic, D. N. Study of Plasma Dynamics in a Modulated Pulsed Power Magnetron Discharge Using a Time-Resolved Langmuir Probe. J. Vac. Sci. Technol., A 2011, 29, 011024/1−011024/7.

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Advanced Nanocluster Ion Source Based on High-Power Impulse

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