Atomic resolution imaging

Researchers from three universities in the Netherlands and Belgium have achieved the following of a chemical reaction in real time and at atomic resolution. This became possible because the chemical reaction, taking place inside nanoparticles, was monitored inside a transmission electron microscope. This instrument uses electrons rather than light to look at matter, which enables the user to zoom in to the level of individual atoms. The chemical reaction was triggered by heating, and in the process a new growth mechanism of nanoparticles was discovered. This new growth method will bring about new types of nanoparticles which can be used for making brighter displays in computers and smartphones, and to make more efficient solar cells.

Research leader Dr Marijn van Huis from the Soft Condensed Matter group of Utrecht University explains how following chemical reactions at the atomic scale can bring about new insights: “In general, we investigate nanomaterials only before and after a chemical transition has taken place. That leaves us in the dark about what really happens to the material during the chemical transition. By a smart combination of technologies, we are now able to look with atomic resolution at particular chemical reactions while they take place, providing information about the chemical process that could not be acquired in any other way. We can then discover, for example, that the reaction consists of multiple stages, or that the process takes place only at one particular surface of the nanoparticle.”

The research team investigated nanocrystals that are shaped as dumbbells, with a length of approximately 20 nanometres and a width of approximately four nanometres. The dumbbells consist of a rod of the semiconductor cadmium selenide (CdSe), with tips at both ends that consist of the semiconductor lead selenide (PbSe). This type of particle is developed for use in solar cells, detectors and optical computers. The CdSe-PbSe nanodumbbells were synthesised at Utrecht University, while the experiments were performed at the Delft University of Technology and the University of Antwerp.

When heated, the PbSe material at the tip started to grow into the CdSe nanorod, whereby Cd atoms were replaced by Pb atoms during a so-called ‘cation exchange reaction’. Because of revolutionary improvements in the design of the heating holders in the electron microscope, these chemical reactions could be imaged for the first time with atomic resolution. By analysing the X-rays emitted from the sample at every pixel of the image, the distribution of chemical elements could be monitored, as well (so-called ‘chemical mapping’). In this way, it was discovered that cadmium disappears through evaporation, after which excess Pb atoms in the nanocrystal occupy the empty sites in the atomic lattice that were left behind by the Cd. Surprisingly, this process is exactly the opposite of what typically happens during chemically ‘wet’ conditions whereby cation exchange is performed. This new growth process was called solid-solid-vapour growth by the researchers. It is a chemical reaction of one solid to another solid whereby a vapour of Cd atoms is formed. This growth mechanism is completely different from previously known growth methods because the driving force for the reaction is the evaporation of Cd.

Crucial for this scientific achievement is the development of the heater holder which allows the insertion of the nanoparticles into the electron microscope while heating them at the same time. In conventional heating holders, the holders themselves also heat up, which affects the stability at the atomic scale and completely blurs the resolution of the microscope. In this revolutionary design, developed by the group of Professor Henny Zandbergen at the Delft University of Technology, the heating experiment is miniaturised using semiconductor clean room technology. In this manner, only the nanoparticles are heated, while the holder remains unaffected, thereby retaining the atomic resolution of the microscope.

To better understand the new growth process, the researchers also performed quantum mechanical calculations and force-field atomistic simulations. Surprisingly, it turned out that the presence of empty sites at the atomic lattice is required for the cation exchange reaction to take place, and it also became clear that the chemical reaction occurs only very close to the interface between the two materials.

The investigators expect that through this new growth process, a novel family of nanoparticles with new optical and electronic properties can be created. It is still unknown to how many materials this new growth process can be applied. The future outcome of these investigations is still wide open and offers opportunities for fabrication of more efficient solar cells, brighter displays, novel catalytic materials, and cost-effective lasers.

A movie of the growth process can be appreciated at:

http://pubs.acs.org/doi/suppl/10.1021/nl501441w/suppl_file/nl501441w_si_005.avi.

Reference:

• Yalcin, A O et al., ‘Atomic Resolution Monitoring of Cation Exchange in CdSe-PbSe Heteronanocrystals during Epitaxial Solid−Solid−Vapor Growth’, Nano Letters 14 (2014) 3661-3667.

Dr M A van Huis
Assistant Professor
Soft Condensed Matter Group
Debye Institute for Nanomaterials Science
Utrecht University
Princetonplein 5
3584CC Utrecht
tel. +31 30 2532409

www.uu.nl/staff/mavanhuis
www.colloid.nl