Rolled-up semiconductor nanotubes

A very elegant method of producing nanotubes has been developed by scientists at the Max Planck Institute for Solid State Research in Stuttgart, Germany. The new technique makes it possible to prepare tubes from very different substances, e.g. silicon, as well as to vary their dimensions and to deposit the nano objects very exactly (nature, March 8, 2001).

Fig.1: Schematic illustration of the roll-up procedure. Once a strained bilayer is detached from its own substrate by selective etching, the epitaxially defined layer rolls up into a nanotube.

Illustration: Max Planck Institute for Solid State Research

When a strained semiconductor sheet springs free of the crystalline substrate that holds it flat, the sheet curls up into a nanotube. This is a rather laxly formulated description of what happens when a strained semiconductor bilayer is deposited on a sacrificial buffer layer on a semiconductor substrate and subsequently detached from the substrate by a selective etching procedure (see Fig.1). The bilayer consists of two thin layers - the lower one having a larger lattice constant, the upper one having a smaller lattice constant. As these two layers are freed from their substrate, each layer tries to obtain its inherent lattice constant, resulting in a force perpendicular to the layer plane. The force bends the layers upwards - eventually forming a nanotube after one complete rotation.

An example of such a nanotube is shown in Fig.2. The nanotube consists of a rolled-up SiGe layer which has formed along the [010] edge of a Si (001) substrate. One fascinating point of this new technology is that you can position the nanotubes in almost any place on a substrate surface. In the case reported in nature, the position was precisely defined by the sample edge and the time of selective etching. Instead of the sample edge, one could easily etch a nanoslit into the sample surface at a certain deliberate position, which would then be the starting point of the nanotube roll-up.

Fig. 2: Silicon-germanium nanotube that has rolled up along the edge of a Si substrate.

Photo: Max Planck Institute for Solid State Research

The nanotube in Fig. 2 is still very large, having a diameter of 530 nanometers and a length of 20 micrometers. Another fascinating point of this new nanotechnology is, however, that the free-standing nanotubes are scalable over a huge size scale. The built-in strain in the original SiGe bilayer (from which the nanotube in figure 2 was formed) was only 1.5%. If one builds in 4% strain (this is the lattice mismatch between pure Si and pure Ge) into a bilayer of only 2 atomic layers in thickness, the radius of such a nanotube would shrink to only a few nanometers.

Deposition techniques are capable of combining materials of almost unlimited diversity, including semiconductors, insulators, metals, polymers, etc. This richness will create new nano-objects of unknown diversity, which will find their fortune in the wide and interdisciplinary fields of micro- and nano-electromechanical systems.

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