NMR tool allows analysis of nanometre-sized samples

The development, claimed to be a world first, is expected to reveal nanostructures at unprecedented levels of detail, opening up new areas of research and development, and device fabrication processes.

‘It is a unique technique,’ said Dr Evgeny Chekhovich from Sheffield University’s Department of Physics and Astronomy. ‘We demonstrate its capacity by measuring NMR spectra of single-strained semiconductor structures — quantum dots — something that was impossible prior to our work.’

Chekhovich explained via email that strained quantum dots have been widely investigated in the past two decades for concepts such as quantum computation and communication, quantum light sources, and solar cells.

Testing techniques

‘Naturally, structural testing techniques of strained semiconductor structures, especially non-destructive techniques such as NMR, are of great interest,’ he said.

Chekhovich explained that NMR probing involves exciting a sample with an oscillating magnetic field generated by a coil.

‘The effect of this excitation on the sample is then detected,’ he said. ‘The frequency of the radio field is then changed and its effect is measured again. This way the response of the sample as a function of frequency — namely the NMR spectrum — is measured.

‘However, this approach meets significant difficulties when applied to a broad range of objects containing quadrupole nuclei and subject to mechanical strain, as often happens in novel semiconductor nanostructures.’

To overcome this problem the Sheffield group, led by Dr Alexander Tartakovskii, modified the spectral pattern of the radio-field excitation.

‘Instead of a radio field at a given frequency we excite the sample with a radio signal that contains in its spectrum all possible frequencies except for a small number of spectral components,’ said Chekhovich.

‘In other words, we apply an oscillating magnetic field that has a spectrum similar to white noise. The only difference from the white noise is the absence of spectral components at a certain frequency that we call a “gap”. In our technique we measure NMR spectrum by scanning the frequency of this “gap”.’

This in turn enhances the amplitude of the NMR signal by a factor of ~100, allowing spectroscopy on nanoscale objects that were previously inaccessible with NMR.

‘In our case we probed semiconductor quantum dots embedded in a matrix of a different semiconductor material,’ he said. ‘Our NMR technique can provide information on structural properties such as chemical composition — we can measure the proportions of different chemical elements — physical dimensions, magnitude and spatial distribution of elastic strain within the nanometre-sized volume. All of these properties are very important for the performance of the final semiconductor device.’

Scientific set-up

Chekhovich added that the experimental set-up at Sheffield is based on a standard cryogenic optical microscope equipped with a superconducting magnet.

Bespoke items, designed and built by Chekhovich, included the sample holder and the minicoil that induces a radio field.

He added: ‘Our experiments were done at 5.3T, corresponding to ~220MHz proton frequency. Such magnetic fields are easily achieved by standard commercial superconducting magnets.

‘Overall we use very standard industrial/scientific equipment that significantly reduces the price of the set-up, and makes it easily reproducible.

‘The key innovation of our technique is something that I would describe as a “soft” approach, in contrast to “brute force” approaches that require extreme technical characteristics, huge cost, and very long time to assemble: a 1000MHz NMR machine, or Large Hadron Collider are good examples.’

A paper describing the work has been published in Nature Nanotechnology.