Geometry and Surface Structures of Supported Nanostructures

The strong interest in nanostructural research originates from the novel properties of nanoparticles that can be quite different from the bulk, and the sensitive dependence of these properties on particle size, shape, and surface atomic configuration [1-5]. The size- and shape-dependent properties are closely related to important applications of nanoparticles, including quantum dots (QDs) in electronics and optoelectronics [6,7], catalysts [8-11], and single-domain magnets [12,13]. Although some extrinsic factors such as a surface passivation layer may have a strong influence on nanoparticles' properties [14], most of their novel properties originate from their atomic configuration and geometry. Free-standing nanoparticles have been investigated extensively in past decades in order to extract their intrinsic properties [1,3,4,15-19]. But various experimental difficulties have limited the variety of completely free-standing nanoparticles that can be studied.

Taking advantage of the Volmer-Weber growth mode, nanoparticles in the form of cluster and crystallite can nucleate and grow on inert substrates such as graphite, oxide, and ceramic surfaces. To display the intrinsic properties of nanostructures, their interaction with the substrate should be significantly weaker than that in an epitaxial system. Nevertheless, the interaction is strong enough to maintain mechanical bonding required for analyses. In particular, graphite, oxide, and nitride thin films formed on conductive substrates provide us with the "inert" templates for growing nanostructures that can be characterized subsequently using popular analytical tools including photoelectron spectroscopy and scanning tunneling microscopy (STM) [9,10,20]. Such supported nanoparticles are relatively close to the state of their applications in catalysis and sensors [21]. They also make it relatively easy to extract geometric, electronic, chemical, and magnetic properties of a particular nanostructure and establish correlations among these properties [22,23].

In many applications, the surface atomic structure, shape, and crystal orientation with respect to the substrate are important to particular functions of nanoparticles. For example, the catalytic and sensor performances are closely related to the surface structures of nanoparticles. For single-domain nanomagnets in the patterned medium used for ultrahigh-density information storage, it is desirable to let all magnetic crystallites take an identical orientation and a unique shape so that full advantage may be taken of the shape and magnetocrystalline anisotropy to enhance the stability against superparamagnetism [12,13,24]. In addition, the nucle-ation and growth of nanoparticles are the initial stages of film growth on an inert substrate. The properties of nanoparticles, including interparticle mass transport kinetics, have intimate effects on the texture of films obtained at a later stage of growth [25-31].

Depending on the strength of the interaction with the supporting surface, the properties of nanoparticles grown on inert substrates vary between those formed in epitaxy and those of free-standing clusters without any support. The epitaxial film or island growth [32,33] on one extreme and free cluster formation [1-4] on the other have been quite well understood now. In epitaxy, the lattice parameters and orientation of islands nucleated initially are fixed largely by the requirement of registry with the substrate due to a strong interaction across the interface. For particles formed on an ideal inert substrate, their crystalline orientation could be completely random as a free-standing nanoparticle, and the shape and atomic configuration of clusters or crystallites are determined solely by minimizing the particle energy, which includes contributions from electronic and atomic configurations. For the intermediate systems we deal with here, all the factors important in the extreme cases must be considered. Additionally, growth kinetics [26,31-34], surfactant [35-37], buffer layer [38], and surface termination [39,40] can be used to adjust the geometric and other properties of nanoparticles as well as the texture of thin film derived from these nanoparticles.

The equilibrium shape and surface atomic configuration of nanoparticles often show sensitive size dependence. Small metal clusters (consisting of <103 atoms) are normally spherical (with a diameter within ~3 nm) to minimize quantum confinement energy of electrons [3,4,41]. In the intermediate size (~ 103-104 atoms) range, both quantum confinement and atomic configuration energies are important so that the particles are usually in near-spherical crystalline or quasicrystalline polyhedral forms (e.g., cuboctahedron, icosahedron, and decahedron) [1,34]. At even larger particle size, the interior atomic configuration should take a crystal form the same as the bulk. But the particle shape may differ remarkably from the macroscopic equilibrium crystal shape (ECS) [32]. Quantum confinement may remain an important factor. Furthermore, the surface energy anisotropy of a nanocrystallite can deviate significantly from that of a macroscopic crystal due to reduced surface area to accommodate surface reconstruction. Additionally, the contribution to surface energy from atoms located at edge and corner sites, which scales as is no longer negligible for nanoparticles. All these factors favor an isotropic ECS. On the other hand, the interface binding with the substrate generally reduces the profile of supported nanoparticles and favors a relative anisotropic ECS [8,42].

To reveal the interplay of various factors, we performed a systematic study, mainly using in situ ultrahigh vacuum (UHV) STM, combining with Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), on the nucleation and growth, shape, and surface atomic structures of nanostructures on highly oriented pyrolytic graphite (HOPG) [43,44] and silicon nitride (SiNx) thin films obtained by thermal nitridation of Si [45-47]. Much previous research work has been carried out with (mostly ex situ) scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM), and X-ray scattering [3-5,8,29,34,48]. Although each of these techniques offers certain strength, UHV STM is unique in providing atomic-resolution surface structural and other geometric information of nanostructures under a well-controlled environment. Of course, STM normally can only image the top surface of an object, so investigations using STM and other techniques should be complementary to each other.

HOPG has been used as a prototypical inert substrate in many nanostructural nucleation and growth studies [8,26,49-54]. It has been found that the bonding of many metal and semiconductor atoms (e.g., Au, Ag , Sb, and Si) with HOPG is extremely weak (the sticking coefficient of Si on HOPG is much less than unity [49]), so a variety of nearly free-standing nanostructures has been observed to form on HOPG. HOPG is also used as an ideal template for studying the kinetics and thermodynamics governing the migration and coalescence of nanoparticles. Here, we present mainly our investigations of Al, Sb, and Ge nanostructures formed on HOPG.

For comparison, we also show some results of Ge and Si nucleation and growth on SiNx films. SiNx has been used widely as a high-quality dielectric material in microelectronics [55], and as coating and buffer layers with excellent thermal, mechanical, and chemical stabilities [56]. Crystalline and amorphous SiNx thin films (thickness 1-3 nm) can be formed on Si(111) and Si(001) substrates, respectively, allowing us to examine processes relevant to different applications [45-47]. Semiconductor and magnetic nanoparticle growth on SiNx has been investigated by a few groups including ours [22,37,57-60]. In general, studies of formation of semiconductor QDs and magnetic nanoparticles on Si-based substrates are important for integration of optoelectronic and magnetic functions on Si-based integrated circuits [5,61]. Si and Ge low-dimensional structures are especially attractive because their luminescence efficiency could be much higher than that of the bulk [2,6,7,62,63]. Clusters made of up to a few tens of Si or Ge atoms (size ~1 nm) have been investigated extensively [15,16], and studies of free-standing particles of size beyond a few nm have been rather rare [17-19] because of experimental and calculation difficulties. For supported nanoparticles (besides the well-known epitaxial pyramidal and dome Ge (or SiGe) QDs on Si(001) [64,65]), experimental works of deposition on Si(111) [66], SiOx [67,68], CaF2 [69], HOPG [49], and Ag(111) [70] have been reported. It has been found that the electronic properties of Si nanoparticles on graphite depend sensitively on their size [71,72]. However, the evolution of shape and surface structures with the particle size, the effects of surfactants on nanoparticle growth, and the texture of continuous films obtained subsequently were not examined extensively. These issues are addressed in our study.

In the following, after a brief description of procedure and precautions in experiments and data analyses, results of nucleation, coarsening, morphology and surface atomic structures of Al, Sb and Ge nanostructures on HOPG are presented first, followed with the evolution from clusters via crystallites to continuous films of Si and Ge on SiNx. The effects of an Sb atomic layer on the nucleation and shape evolution of nanoparticles on SiNx will be briefly illustrated. Based on the comparison of nanostructures formed in different cases, a few general observations regarding the correlation between the nanostructural geometry and the thermodynamic and kinetic driving forces will be made. Particularly, we will explore the surface properties of nano-particles, their interaction with each other and with the substrates, and the consequences on the texture of more sophisticated nanostruc-tures and continuous films they form later.

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