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Materials physics

Doping control for nanotubes

Adding guest atoms to inorganic nanotubes, known as ‘doping’, influences their room-temperature magnetic properties — properties that could be exploited in ‘spintronic’ devices and computer memory.

Much of the technology underlying our computers and electronic devices is based on the transport of electronic charge across sub-micrometre structures made from perfectly crystalline silicon. The dimensions of silicon-based devices, transistors in particular, are constantly shrinking (90-nm transistor gates are now in production) and their integration densities are increasing — in line with Moore's law, which predicts at least a doubling of the transistor density on a chip every two years. Although this trend has been followed for almost 40 years, the rate of increase is expected to level off in the near future, when transistor size reaches a few tens of nanometres.

This intrinsic limitation of silicon devices has provoked much speculation about the ‘next big thing’ in information processing — might it be ‘spintronics’, which uses an electron's direction of magnetization, or spin, instead of its charge as the information carrier? Each spin could carry a bit of information — in effect, a single-spin transistor — which could lead to faster computers that consume less electricity, and alleviating the problem of heat dissipation. In designing spintronic devices, we can explore the exciting opportunities offered by the chemistry of nanotubes: Krusin-Elbaum et al.1 lead the way, on page 672 of this issue.

Spintronics requires the fabrication of ferromagnetic nanostructures that, at room temperature, can transport spin-polarized carriers, and which can be assembled into addressable hierarchies on a macroscopic chip. Most efforts have been directed towards the mixing of transition-metal atoms (such as Ni, Fe and Mn, which have permanent magnetic moments) into semiconductor devices based on compounds from groups II–VI (such as CdS) or III–V (GaAs) of the periodic table. Superstructures consisting of alternating ferromagnetic/diamagnetic, metallic/oxide thin films have also received attention; like spin valves, spin-polarized currents can be injected into them and transported.

Shortly after the discovery of carbon nanotubes, it was suggested that the layered structures of some inorganic compounds (such as WS2, MoS2 and BN; refs 24, respectively) should also fold in on themselves and close up into fullerene-like nanostructures and nanotubes. Various strategies have been developed to synthesize such nanotubes. These can be grossly divided into high-temperature processes and ‘chemie douce’ (soft chemistry), a generic name used for wet synthetic processes that are limited to temperatures of 300 °C and below. Among the early examples of chemie-douce synthesis were titanate5,6, niobate7 and vanadium-oxide (VOx) nanotubes8 — the last of these being the type of nanotube used by Krusin-Elbaum et al.1 in their study (Figs 1, 2).

Figure 1: Vanadium oxide (VOx) nanotubes, seen in false-colour by scanning electron microscope.


Krusin-Elbaum et al.1 show how the magnetic properties of such nanotubes can be engineered through doping with guest atoms.

Figure 2: Nano-scrolls.


These images from a transmission electron microscope show: a, a VOx nanotube in cross-section; b, with energy filtering.

To create nanostructures exhibiting room-temperature ferromagnetism, the authors1 synthesized VOx–alkylamine nanotubes through a well-established procedure (a combination of sol–gel and hydrothermal synthesis8). They then ‘doped’ the nanotubes, introducing foreign atoms into the gaps between layers: lithium atoms, which act as electron donors to the vanadium lattice; and iodine atoms, which act as electron acceptors, creating positive charge carriers in the lattice called ‘holes’. In contrast to the transition-metal devices mentioned above (in which a 3d transition-metal atom is added into a non-magnetic host), here both guest atoms are non-magnetic but the host is magnetized by the elimination or addition of one extra charge (positive or negative).

Careful analysis of the data has led Krusin-Elbaum et al.1 to conclude that the undoped nanotube is a frustrated spin liquid, meaning that its electron spins have random orientations. Although they have not clarified the exact location of the dopant atoms in the nanotube lattice, the authors found that when one charge is added to or eliminated from the highest-energy vanadium atom, the frustration is removed. There is then antiferromagnetic alignment of the vanadium spins in the lattice of the nanotube — that is, neighbouring spins have opposite orientations. If a small magnetic field is applied, the spins flip such that they all have the same orientation; this is ferromagnetic ordering. Furthermore, the undoped nanotube is an insulator (a Mott insulator), but the doping transforms it into a good conductor, by shifting the Fermi level (the surface defined by the highest occupied electron energy states) into the conduction or valence bands (for lithium or iodine atoms, respectively). So the doping process turns the VOx nanotubes into room-temperature magnets that are good charge carriers, accomplishing two of the main requirements for spintronics technology.

The room-temperature magnetization is, however, only modest. But this novel strategy1 paves the way for creating nanostructures with larger magnetic effects. Furthermore, in analogy to the ‘ballistic’ transport of electrons in single-wall carbon nanotubes, higher carrier mobilities can be expected if the crystalline order of the nanotubes is improved and if the doping atoms are more carefully controlled. The bipolar nature of the nanotubes — the possibility of making them conductors of either electrons or holes through appropriate doping — is important for the fabrication of transistor structures and junctions (p–n junctions).

Driven by scientific curiosity as well as by the possibilities of application, the burgeoning field of nanotube research benefits from the diversity of inorganic compounds from which the tubes can be made and the fact that their properties can be modified and controlled through judicious chemical transformation, as Krusin-Elbaum et al.1 have demonstrated. Potential applications are numerous — nanotubes as solid lubricants (based on fullerene-like WS2 and MoS2 nanoparticles9) are already heading for large-scale commercialization. Spintronics may well be the ‘next big thing’.


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