Nanostructuring of layered material surfaces

In recent years, formation of nanostructures on surfaces such as quantum dots, wires and clusters has been achieved giving new insights into an understanding of properties of structures with reduced dimensions and their potential technological applications. The most advanced tools for producing nanostructured surfaces are scanning probe methods for atomic manipulation [1] or electron beam lithography [2] where, in principal, any pattern can be formed. However, since these techniques are serial processes their application may be limited with regard to speed to produce extended nanostructures. This problem can be overcome by diffusion controlled aggregation [3] or nanocluster deposition on surfaces [4] and growth on pre-structured [5] or reconstructed [6] surfaces. Their application, however, may be limited to building one-dimensional chains and two dimensional island aggregates.

Figure 1: STM picture of a Rb nanowire network on TiTe2 (picture from R. Adelung, Ph.D. thesis)

We have found that deposition of atoms or molecules on extremely flat and highly unreactive surfaces of layered crystals such as transition metal dichalcogenides leads to self organized formation of nanowire networks associated with subtle strain effects on almost perfect single crystals. Their structure is mainly controlled by adjusting growth parameters during and after chemical vapor transport (CVT) growth of the substrates. Network dimensions are only limited by the size of the substrates (up to cm2).

Layered transition metal dichalcogenides have been studied extensively both because of interest in fundamental physical properties such as phase transitions, charge density waves and quasi two-dimensional (2D) electronic structures [7] and because of potential technological applications in solar cells [8] and batteries [9]. Depending on chemical composition they form semiconductors with varying band gaps or metals. Their structure is built by sandwiches consisting of chalcogen-metal-chalcogen planes forming layers held together by van der Waals interaction. Consequently, they exhibit a pronounced anisotropic two-dimensional character (e.g. concerning optical and electronic properties). Proper surfaces of these materials can be easily prepared in ultra high vacuum by cleavage between the layers. Surfaces created this way do not show any steps or defects over many microns. Compared to other semiconductors like silicon or simple metals this causes very large diffusion lengths, because almost no imperfection stops movement on the surface. In addition, surfaces of layered materials are highly unreactive and contain no dangling bonds. Depending on their size alkali atoms can either intercalate between the layers or be adsorbed [10].

Growth of nanostructures on layered material surfaces initially occurs along lines between weekly strained domains which form during the cooling process after CVT growth. Fig. 1 shows examples of the formation of metallic (Rb adsorbed at room temperature on TiTe2) nanowires. These growing nanowires finally form networks with typical mesh sizes in the micron range.

Figure 2: STM image showing the thickness of a Rb nanowire on TiTe2 (vertical image dimension 1 Ám, picture from R. Adelung, Ph.D. thesis).


Examples of the interesting physical properties of these nanostructures have been published in ref. [11]. Nanostructuring of layered surfaces can also be used to tune the dimensionality of electronic states between 3D and 2D (ref. [12]. This procedure opens new perspectives for designing a variety of physical phenomena such as phase transitions, charge density waves or metal-insulator transitions [13].

First results on adsorption of Au and Cu atoms on layered materials show different adsorption behaviour. While Au atoms form clusters adsorption of Cu atoms also leads to formation of nanostructures (see Fig. 2). Work on Formation of magnetic Nanostructures is in progress.

Figure 3: STM image of Cu nanowires on TiTe2. Horizontal image dimension is 500 nm (picture from R. Adelung, Ph.D. thesis).



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