Strategy of nanocluster and nanostructure synthesis by conventional pulsed laser ablation
Introduction
In this paper, we describe the basic principles of nanocluster synthesis by conventional pulsed laser ablation (PLA). Clusters are aggregates of atoms, or molecules, with a typical size from some tens up to a few thousands of atoms.1 The small dimensions of these particles determine their properties strongly characterized by quantum effects. The properties of large clusters of 10–20 nm approach those of the corresponding bulk materials.
Nanometer-sized semiconductor particles proved to be a very promising material, since quantum confinement in such structures modifies the bulk Si band structure and results in radiative transitions. The elemental semiconductor nanocrystals Si and Ge at room temperature exhibit visible light emission at energies greater than the band gap of the bulk semiconductors. An efficient photoluminescence from the blue to the infrared has been reported from Si, oxidized Si and Ge nanoparticles.
In most of the previous laboratory experiments, nanoclusters were produced using a PLA supersonic expansion source of the Smalley type [2] or one of its modifications. For this source, collisions between monoatomic particles and the expanding high-pressure gas (generally He at pressure of about 1–10 atm), cool the particles and allow a heterogeneous nucleation and the growth of the clusters. However, this technique produces a relatively broad cluster size distribution.
An alternative method of nanocluster synthesis is the conventional PLA. In this case, ablation is performed at relatively low gas pressure, 0.2–10 Torr, of inert or reactive gas. During expansion and cooling, condensation starts within the ablated vapor and the condensed particles undergo multiple collisions with ambient gas molecules, leading to the stabilization of the nanoclusters before they arrive to the substrate surface. The size of the nanoclusters can be controlled by the laser parameters: fluence, wavelength, pulse duration and by the ambient gas conditions: pressure, nature and flow parameters.
Condensation of nanoparticles within the laser-induced vapor plume is a common phenomenon in PLA and was observed in numerous pulsed laser deposition (PLD) experiments. For more details, see, for example, the review by Chen [3] and references therein. The first synthesis of Si-based nanocluster films by conventional PLA has been reported by Movtchan et al. [4] and further development has been described in [5], [6]. Motivated by the importance of the Si nanocluster synthesis, a number of groups reproduced these results [7], [8], and the first fabrication of electroluminescent diodes of Si nanoclusters by PLD has been reported by Yoshida and Yamada [9]. Geohegan et al. have recently published new results on the dynamics of Si nanocluster formation [10] by PLA and on the luminescence of free, gas phase Si nanoclusters [11].
This new, rapidly growing, area of application of conventional PLA and PLD started with the synthesis of Si nanoclusters and is expanding towards the synthesis of clusters of compound materials [12], [13], of oxides [14] and metals [15], [16]. In this paper, we describe the basic principle of nanocluster condensation and present recent experimental results on the Si nanocluster synthesis.
Section snippets
Condensation of vapor and nanocluster formation
An idealized scheme of the cluster film synthesis by PLA can be separated into four independent steps: cluster nucleation, growth, cluster cooling and deposition.
(i) The nucleation is determined by the thermodynamic parameters of the material and by the initial conditions, like temperature and density of the vapor ejected after PLA. The nucleation process is characterized by the condensation temperature which can be found from general thermodynamic considerations.
(ii) The cluster growth will
Experimental results
A survey of the published experimental results shows that clusters formed from the vapor phase, typically in the nanometer range, are much smaller than the well-known droplets (0.1 to 5–10 μm) produced by melt ejection from a target [3]. The size of nanoclusters can be controlled by the ambient gas pressure, whereas the largest particles are essentially the same in vacuum or in a gaseous ambient.
At present, the best documented material is Si. Due to its important technological applications,
Conclusion
In this paper, we introduced the basic principles of nanocluster synthesis by conventional PLA. We showed that, for clusters moving together with the vapor, one can distinguish three different waves propagating through the plume: (1) the wave of saturation (where the vapor becomes saturated), (2) the supercooling wave where the highest supercooling is reached, and (3) the quenching wave where the growth stops. After the condensation and the growth stages, the clusters follow a long
References (40)
- et al.
Thin Solid Films
(1995) - et al.
Appl. Surf. Sci.
(1996) - et al.
Appl. Surf. Sci.
(1998) - et al.
Appl. Surf. Sci.
(1998) - et al.
Appl. Surf. Sci.
(1998) - et al.
Appl. Surf. Sci.
(1996) - et al.
J. Chem. Phys.
(1985) - et al.