Nano Lett. 11, 5553–5557 (2011)

Doping semiconductors with impurities has long been used to control their electronic properties, with materials scientists tuning them by using the appropriate dopant in the correct amount. In bulk materials, knowing precisely how many dopant atoms are present is not so critical, but this is certainly not the case when the materials to be doped are nanocrystals and the number of dopant atoms could be as low as one. The position of the dopant within the nanocrystal can also affect its properties, therefore being able to visualize dopant atoms within such nanomaterials is critical to a better understanding and control of their behaviour.

Credit: © 2011 ACS

Now a team led by David Norris from ETH in Zurich and Andre Mkhoyan from the University of Minnesota have shown that a combination of electron energy-loss spectroscopy (EELS) and annular dark-field scanning transmission electron microscopy (ADF-STEM) can be used to image manganese dopant atoms buried within zinc selenide nanocrystals. First, using an ADF-STEM image (pictured centre), Norris, Mkhoyan and colleagues selected a nanocrystal for further study. They then simultaneously measured the EELS map and the STEM image of the chosen nanocrystal.

In EELS, a sample is bombarded with electrons of known energy and the energy lost as a result of their interaction is measured. Different elements display distinct EELS spectra, therefore Norris, Mkhoyan and colleagues scanned across the nanocrystal, measuring the EELS spectrum at equally spaced points along the way. Areas with EELS features that matched a reference spectrum of a known manganese material therefore identified the position of manganese dopant atoms (pictured left and right).