Deposition of Size-Selected Cu Nanoparticles by Inert Gas Condensation
© to the authors 2009
Received: 17 June 2009
Accepted: 4 October 2009
Published: 6 November 2009
Nanometer size-selected Cu clusters in the size range of 1–5 nm have been produced by a plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge sputtering with an inert gas condensation technique. With this method, by controlling the experimental conditions, it was possible to produce nanoparticles with a strict control in size. The structure and size of Cu nanoparticles were determined by mass spectroscopy and confirmed by atomic force microscopy (AFM) and scanning electron transmission microscopy (STEM) measurements. In order to preserve the structural and morphological properties, the energy of cluster impact was controlled; the energy of acceleration of the nanoparticles was in near values at 0.1 ev/atom for being in soft landing regime. From SEM measurements developed in STEM-HAADF mode, we found that nanoparticles are near sized to those values fixed experimentally also confirmed by AFM observations. The results are relevant, since it demonstrates that proper optimization of operation conditions can lead to desired cluster sizes as well as desired cluster size distributions. It was also demonstrated the efficiency of the method to obtain size-selected Cu clusters films, as a random stacking of nanometer-size crystallites assembly. The deposition of size-selected metal clusters represents a novel method of preparing Cu nanostructures, with high potential in optical and catalytic applications.
Owing to their unique catalytic, electronic, magnetic, and optical properties, different from their bulk species, nanoparticles continue to attract the attention of researchers . Metal nanoparticles (NP’s) have been synthetically produced by different techniques for many years [2–8]. The approach described in this paper is an inert gas condensation (IGC) technique used before for synthesis of metal NP’s [9–12]. The optical, electronic, and thermal properties of metal NP’s endow them with potential application in electrical and third-order nonlinear optical devices [13–18], solid dielectric materials , nano-biomaterials , and high thermal conductivity nanofluids . Recently, attention has been focused on Cu nanoparticles due to their optical and catalytic properties [22–25]. Particularly, the optical and catalytic behavior in Cu nanoparticles is not well understood. It is believed that absorption of light by metal nanoparticles is dominated by the surface plasmon (SP) resonance changing both the static and dynamic optical properties. These and other properties depend on their size, size distribution, shape, and structure in addition to the interaction between the surfaces of the nanoparticle. Control of these features is not trivial as Cu nanoparticles formation is extremely sensitive to reaction conditions [26, 27]. Although a variety of techniques for the production of Cu nanoparticles have been developed, the main problem is still the control of size that let us to explain the size effects on its properties. In the present work, we describe the synthesis of Cu nanoparticles using a grow-discharge sputtering with an inert gas condensation (IGC) technique. The resulting nanoparticles were analyzed by mass spectroscopy, scanning transmission electron microscopy, and atomic force microscopy. We present the results of the different experimental analysis of the Cu NP’s and these results are discussed and contrasted for different experimental conditions.
Cu Clusters Sputtered
The dc magnetron type discharge is used to generate clusters from the target, connected to the magnetron assembly. The magnetron-based source has an advantage over all other types of cluster sources in terms of the wide cluster size range, which varies from fraction of a nanometer to a few tens of nanometers.
DC plasma is ignited in a mixture of argon (Ar) and helium (He) gases and confined close to the Cu target by the magnetic field of the magnetron set-up. In the IGC process, a supersaturated vapor of Cu atoms is originated by sputtering a Cu target in an inert gas atmosphere of Ar and He. The Nanogen system was kept at low temperature by a coolant mixture, and before the nanoparticles deposition, the system pressure was set at 1 × 10−8 Torr. The cluster size can be adjusted by varying three main source parameters: the length over which the clusters aggregate (variable using a linear drive), the power to the magnetron, and the flow of the aggregation gases. In terms of the cluster size range, the magnetron-based source has the advantage over all other types of cluster source as it is the most flexible. For a large number of materials, the source is capable of producing clusters consisting of a few tens of atoms up to particles with diameters of around 20 nm. Due to the nature of the gas aggregation technique, narrow size distributions can be achieved.
Typically, sputtered clusters are swept through the aggregation region (10−1 Torr) by argon and helium gases, where these clusters nucleate to form a distribution of nanoclusters of various sizes as represented in Fig. 2. Energetic Cu atoms sputtered from the target are cooled by the He gas leading to the nucleation of clusters. The nucleation of these small cluster ‘seeds’ is followed by the growth of the seeds into larger clusters. The growth of clusters is heavily dependent of interatomic collisions; this states the importance of Ar and He. Once the Cu clusters grow in size to exceed a critical radius, large clusters grow from the cluster seeds at a faster rate than that at which new seeds are formed. Since He is primarily responsible for the cluster-condensation process, the He partial pressure is used to control the cluster size distribution. The residence time within the aggregation zone can be varied by varying the length of the aggregation region with the linear motion drive. By controlling the aggregation length, and the residence time, one can control the distribution of the nanocluster size within the aggregation region. The nucleation and growth of clusters ceases after expansion through a nozzle where clusters expands into the filtering zone, which is maintained at a much lower pressure (10−4 Torr). In our system, the experimental conditions restrict the size of the nanoparticles in the range of 1–20 nm governed by the thermodynamic stabilities related to the cluster binding energies and recombination processes.
Cu Clusters Filtering
Along with the nanocluster source, our system also consists of a quadrupole mass filter (MesoQ), intercepted between the nanocluster source and the main deposition chamber. A large percentage of the Cu clusters generated by the source are ionized, typically 40% for Cu clusters, which makes possible manipulate them electrostatically by the mass filter. MesoQ is used to filter nanoclusters of a particular size from the widely varied size distribution of nanoclusters present in the aggregation region. This mass filter has been specifically added for the purpose of high-resolution measurement and filtering of nanoclusters between 1 × 103 and 3 × 107 a.m.u. The MesoQ utilizes the high ionized content of clusters generated by the Nanogen 50 to achieve a high transmission and to acquire clear mass spectra. The electronics unit allows us to acquire a mass scan of the clusters from the source and filter the ionized clusters.
The focused cluster beam is then mass-selected by a mass spectrometer and then accelerated by a high-voltage pulse applied to a substrate in a high-vacuum chamber with a base pressure of 10−8 Torr. The system is capable of depositing at rates between <0.001 nm/s and >0.5 nm/s measured at a distance of 100 mm for Cu clusters. The deposition rate achieved depends on a number of parameters, which includes the material and the size of the clusters deposited. In our case, in the deposition we applied one bias voltage between the substrate (negative) and the chamber, and for this reason we only working with the positive ions. The size of the nanoparticles was controlled through the variation in (1) gas flow (Ar and He), (2) partial pressure, (3) magnetron power, and (4) zone condensation length. These parameters were varied to produce particles of different sizes onto Si substrates. The latter choice was taken for purposes of analysis in a SEM, Nova200 Nanosem. The average size of the nanoparticles was monitored in situ from the synthesis conditions by a linear quadrupole to measure the mass distribution and to act as a mass filter. AFM imaging and manipulation experiments were carried out using a Veeco Instrument Multimode scanning probe microscopy by hard tapping mode (low amplitude setpoint voltage).
Results and Discussion
Nanocluster Size Variation with Experimental Parameters
Ar and He Flow Rate
Magnetron Power and Aggregation Length
Monodispersed Cu Nanoclusters: Structural and Morphological Analysis
Cu Cluster Films
We have investigated the growth of Cu nanoparticles in a plasma enhanced sputtering gas aggregation type growth region. Various sputtering parameters were varied to observe the change in the size distributions of the nanoparticles. It was found that Ar flow rate favors the cluster growth but when this flow reach some critical value the mean cluster size decreases since Ar also sweep the cluster through the aggregation zone this reducing the time for particle growth. A continuous decreasing in mean cluster size was also observed with the increasing in He flow, which reveals its effective role to reduce the size by reducing the associated residence time in the aggregation zone. Cu nanoparticle size is also controlled with the sputtering power, a nearly linear relationship was found, however, the most determinant experimental parameter to control size seems to be the aggregation length. From STEM and AFM measurements, we found that nanoparticles were monodispersed with the selected size, this demonstrates the efficiency of IGC technique to obtain cluster with the desired size. Additionally, controlling experimental parameters by IGC technique leads to obtain size-selected Cu cluster films. The deposition of size-selected Cu clusters represents a novel method of preparing Cu nanostructures, with high potential in optical and catalytic applications.
M. A. Gracia-Pinilla thank to PROMEP for their support through grant PROMEP/103.5/09/3905. The authors thank J. Sainz, J. Flores, and J. Aguilar for their technical help.
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