topics

Nanoparticle functional materials

Recently a strong impetus is given to studies of nano-particle/nano-structured thin layers for two main reasons. The first stems from the desire to miniaturise electronic devices. Specifically, one would like to grow organised nanometer-size islands with specific electronic properties. The second subfield of nano-structured materials, is thin or thick films which show mechanical and magnetic properties different from their microcrystalline counterparts. The precise reasons for these effects is currently being investigated, but one can cite the presence of a significant fraction of atoms in configurations different from the bulk configuration, for example, in interfaces. Interestingly the particles are grown in extreme non-equilibrium conditions, which allow one to obtain metastable structures or alloys. Because one avoids the effects of nucleation and growth on a specific substrate with this method one may tune the properties of the films by choosing the appropriate preparation conditions [1]. In addition, nanoparticle films are also of strong interest to catalysis studies due to large surface to volume ratio owing to the porous structure of films build by soft landing nanoparticles [1-4].

Figure 1. Diagram of the nanoparticle deposition source

At any rate the formation of ordered aggregates of nanoparticles exhibit interesting magnetic, electronic and/or opto-electronic properties. At the present nanoparticle source (obtained from Oxford Applied Research) metal atoms are generated by magnetron discharge and particleed by the so called gas aggregation method [1]. For the proposed research we will employ the nano-particle source that is available in our group (Fig. 1). At present we have installed a NC200U nanoparticle deposition source (obtained from Oxford Applied Research) onto a vacuum system where metal atoms are generated by magnetron discharge and particleed by the so-called gas aggregation method developed by Haberland et al. [1]. Briefly metal atoms due to collisions with Ar atoms loose energy and combine to form particles, which are subsequently jet-propelled through a nozzle to form a particle beam. Interestingly the particles are grown in extreme non-equilibrium conditions, which allow the occurrence of metastable structures or alloys. An important point here is that the particle size distribution is very monodisperse. Because one avoids the effects of nucleation and growth on a specific substrate with this method one may tune the properties of the films by choosing the appropriate preparation conditions [1].

Furthermore, the morphology and properties of particle films depend strongly on the particle impact energy. Generally, there are three regimes that can be distinguished, i.e. low evergy (~ 0.1 eV/atom), medium energy (~ 1-10 eV/atom), and high evergy (>10 eV/atom), see Fig.2 by Haberland et al. [1]. The low and medium energy deposition produces films with particles that stay rather intact upon impact. However, these films are porous (preserving their high surface/volume ratio which is important for other applications like catalysis) and weakly adhering films. On the other hand high-energy deposition leads to particle disruption and damage on the substrate surface, which run several layers deep. Despite these anomalies, high impact energy deposition has been more than a shining success in producing exceptionally good (smooth and highly adhering) thin films [1]. This is because particle fragmentation is followed by diffusion and local rapid annealing of the constituent atoms removing thus hills and valleys and producing very smooth films. Moreover, the particle fragmentation enhances the formation of nucleation sites and adatom diffusion allowing the formation of epitaxial thin films at room temperature despite a 25 % lattice misfit (i.e., Al/Si(111)) [1].

Figure 2. Deposition with variable particle energy (per atom) (a) 0.1 eV, (b) 1 eV, (c) 10 eV

The particle size and composition can strongly influence particle properties as the particle transforms from an ordered to a disordered state. Former studies of the particle size effect on the order-disorder transition (by means of Mode-Carlo simulations) in Cu₃Au [5] have shown that i) the difference in the ground state energy between perfectly ordered and disordered states becomes less in nanoparticles than that in the bulk, ii) The critical temperature in where order-disorder transition occurs is depressed in nanoparticles (in comparison with the bulk) and decreases with decreasing particle size, iii) finally the cohesion energy is higher for the ordered state also for nanoparticles with cohesion energy difference in between ordered-disordered state decreasing as the particle size decreases. The size effects on order-disorder transitions have impact on the particle habit, particle-particle coalescence phenomena (and the associated surface diffusion mechanisms), as well as particle diffusion as a whole onto substrates. Note that the degree of slow or fast of coalescence processes has important consequences for the particle-assembled materials. Indeed, the type of morphology (compact or ramified) depends critically on the ratio of the coalescence time and the time it takes a new particle to join an existing group. If this ratio is larger than 1 then ramified objects will be formed. In the opposite case where compact object formation occurs, the material will loose memory of the initial building blocks from which it was formed.

Besides the particle based by itself, the presence of a substrate plays important role. The important role of the latter onto the particle aggregation dynamics is that during coalescence of supported particles it ensures their thermalization. Moreover, particles can burrow into substrate which a phenomenon that is driven by extremely large capillary forces on the particle particles. Indeed, burrowing could occur in all systems if the nanoparticles have a significantly higher surface energy than the substrate. If the particle had a smaller surface energy, it would simply wet the substrate [1]. Finally, if chemical reaction at the particle substrate interface can occur (i.e., Co and Ni particles onto atomically clean Si substrates leading to silicidation even at room temperature) would also affect the adhesion of deposited particle.

Our present studies include Cu, Co, Fe and Nb nanoparticle films [2, 3, 4]. Their study is performed by means of Atomic force microscopy (AFM), magnetic force microscopy (MFM), and high resolution transmmision electron microscopy (HRTEM).

Growth of Cu nanoparticle films [2]: Up to now, growth front aspects of Cu nanoparticle films deposited with low energy (soft landing films) onto silicon substrates at room temperature have been investigated by AFM [6]. Analyses of the height-difference correlation function yield a roughness exponent H=0.45± 0.05. The rms roughness amplitude w evolves with deposition time as a power law with exponent b =0.62± 0.07, leading also to a power law increase of the local surface slope r with an exponent c=0.73± 0.09. The measured scaling exponents, in combination with an asymmetrical height distribution, point at a complex nonlinear roughening mechanism dominated by the formation of pores.

Cu nanoparticle films (open and closed films). Rms amplitude w vs. deposition time

Nano-sized Fe particles investigated with in-situ transmission electron microscopy: Transmission electron microscopy was employed for investigating the structural stability of nano-sized iron particles as deposited and after in-situ annealing treatments under high vacuum conditions. The thin iron oxide shell that is formed around the iron particles (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of bcc iron. The oxide shell breaks down upon annealing at relatively low temperatures (~500 °C) leading to pure iron particles having a bcc crystal structure. Annealing of particles, which are in contact, leads to their fusion and formation of larger particles preserving their crystallographic structure and being free of any oxide shell. On the other hand isolated particles appear rather immobile (upon annealing). The truncated rhombic dodecahedron was found as the most probable shape of the particles which differs from former theoretical predictions based on calculations of stable structural forms

AFM images of Fe nanoparticles TEM images of Fe nanoparticles (left: oxide shell structure)

Magnetic versus structural properties of Co nanoparticle thin films: A magnetic force microscopy study: Magnetic force microscopy (MFM) has been employed to study thin films consisting of low-energy deposited cobalt nanoparticles. On continuous layers a clear magnetic stray field pattern can be observed, although measurements on individual particles are complicated by interference from topography. The magnetic correlation length determined from MFM images is substantially larger than the size of a single particle. This indicates that the particles are magnetically coupled to form stable domains associated with the formation of a correlated super-spin glass state [6].

Co particle (25 nm diameter) coalescence during in-situ annealing in TEM AFM topography of Co & MFM phase map

Niobium nanoparticles studied with in situ transmission electron microscopy: Structural aspects of deposited niobium nanoparticles approximately 10 nm in size have been explored by means of high-resolution transmission electron microscopy. The niobium particles have a bcc structure and a crystal habit of rhombic dodecahedron. In-situ heating up to ~ 800°C revealed a resistance to high temperatures, in the sense that the particle habit is preserved. However, the internal structural order of the particles is altered due to formation of niobium oxide domains within the particles. Coalescence does not occur even at the highest temperatures, which is attributed to the presence of facets and the occurrence of oxidation during heat treatment [4].

Nb nanoparticles do not show coalescence during relatively high temperature annealing

Mo nanoparticles: (a) TEM bright field picture of Mo nanoparticles. (b) HRTEM image. (c) Formation of a large cube.

Related references

[1] H. Haberland M. mosseler, Y. Qiang, O. Rattude, T. Reiners, and Y. Thunrner, Surf. Review and Lett. 3, 887 (1996); H. Haberland, M. Mall, M. Moseler, Y. Quiang, Th. Reiners, and Y. thurner, Nucl. Instrum. Methods Phys. Res. Sect. B 80/81, 1320 (1993); G. Fuchs, P. Melinon, F. Santos Aires, M. Treileux, B. Cabaud, and A. Hoareau, Phys. Rev. B 44, 3926 (1991); C. G. Zimmermann, M. Yeadon, K. Nordlund, J. M. Gibson, R. S. Averback, U. Herr, and K. Samwer, Phys. Rev. Lett. 83, 1163 (1999); For main reviews in the field see also : Nanomaterials (Synthesis, properties and Applications) ed. by A. S. Edelstein and R. c. Cammarata (Institute of Physics Publishing, london 1998), C. Binns, Surf. Sci. Rep. 44, 1 (2001); W. Eberhardt, Surf. Sci. 500, 242 (2002); P. Jensssen, Rev. Mod. Phys. 71, 1695 (1999); I. Yamada, H. Inokawa, and T. Takagi, J. Appl. Phys. 56, 2746 (1984).

[2] G. Palasantzas, S. A. Koch, J. Th. M. De Hosson, "Growth front roughening of room temperature deposited copper nanoparticle films" .Appl. Phys. Lett. 81, 1089 (2002)

[3] T. Vystavel, G. Palasantzas, S. A. Koch, J. Th. M. De Hosson, Nano-sized iron particles investigated with in-situ transmission electron microscopy, Appl. Phys. Lett. 82, 197 (2003)

[4] T. Vystavel, G. Palasantzas, S. A. Koch, J. Th. M. De Hosson, Nano-sized niobium particles investigated with in-situ transmission electron microscopy, Appl. Phys. Lett. 83, 3909 (2003)

[5] T. Tadaki, T. Kinoshita, Y. Nakata, T. Ohkubo, Y. Hirota, Z. Phys. D 40, 493 (1997).

[6] S. A. Koch, R. H. te Velde, G. Palasantzas, J. Th. M. De Hosson, Magnetic Force Microscopy of Co nanoparticle films, Appl. Surf. Sci. 226, 185 (2004); S. A. Koch, R. H. te Velde, G. Palasantzas, J. Th. M. De Hosson, Magnetic versus structural properties of Co nanoparticle thin films: A magnetic force microscopy study, Appl. Phys. Lett 84, 556 (2004); G. Palasantzas, S. Koch, T. Vystavel, J. Th. M. De Hosson, Nano-Sized Cobalt Particle Films: Structural Stability and functionality, To appear as Research News (invited paper) Advanced Engineering Materials (2004).