The ultimate dream of nanotechnology is to be able to manipulate matter atom-by-atom. To do that, we first need to know where to find the atoms. In what could be a major step in that direction, researchers have developed a method that can determine the shape of a single molecule and identify its constituent atoms.

Nature limits what can be seen with the help of light alone. Only objects separated by more than half the wavelength of the light that illuminates them can be observed as separate objects. To overcome this limit, Edward Hutchinson Synge came up with an idea in 1928 of imaging things at an even finer scale. The idea was to shine light on a small particle and study the scattered light that reflected back, making the wavelength of incoming light irrelevant.

The realisation of Synge’s goal had to wait until the 1980s, when Heinrich Rohrer, the father of nanotechnology, developed a completely different technology: scanning tunnelling microscopy (STM). This method uses a special property of electric current called quantum tunnelling to achieve single-atom resolutions.

Since the development of STM, techniques for imaging smaller and smaller objects have been improving. Today it is possible to identify the shapes of molecules and where the atoms reside without needing to rely on crystallography. But none of these techniques can identify the atoms the molecules are made of.

Now, researchers from China, Spain, and Sweden have combined STM with another method called Raman spectroscopy to determine not just the shape, but also the constituent atoms of a single molecule.

When a form of energy like heat or light hits molecules, it makes them vibrate and rotate, even when they're part of solid structures. This process is called “excitation.” The molecule then emits some of the energy back, which is called “emission.” Raman spectroscopy works by detecting this tiny amount of energy, which tells us things about the molecule that's doing the emitting.

(One of the many uses for Raman spectroscopy is analyzing old ruined paintings. It can detect the presence of certain elements at very specific locations. The salts of these elements have specific colors and thus reveal what a particular part of the painting might have originally looked like.)

Analyzing trillions of molecules is easy because molecules of the same type will combine to produce a more intense signal since they all experience the same vibrations and rotations. Where things become tricky is when single molecules need to be excited and their weak energy emission is measured. Researchers led by Jianguo Hou at the University of Science and Technology of China have found a way to do that. The results of their work are published in Nature today.

They use a modified STM technique that produces just enough light to excite only a few atoms of a molecule at a time. A laser is focused in a metal cavity that contains the molecule to be analyzed. The laser’s energy creates an excited cloud of electrons called plasmons, which creates the local energy needed to excite different parts of a single molecule.

The image above is a pictorial representation of the process. Based on theoretical calculations by co-author Javier Aizpurua at the Center for Material Physics in Spain, the energy pattern received at the atomic level, called its Raman spectra, can be analyzed to reveal the chemical structure of the molecule.

The researchers claim that the method could be applied to any molecule. The trouble is that extreme conditions of high vacuum and low temperature (-200 deg C) are required to carry out this analysis. "At present this is very much a lab experiment," Aizpurua said. The setup also requires weeks to months of work just to be able to analyze a single molecule, and their paper reports on work done with only one ring-shaped molecule.

Prabhat Verma at Osaka University has worked on using Raman spectroscopy to analyze materials at the nanoscale. He was sceptical of the results. “This article claims a spatial resolution of better than 1 nanometer (billionths of a meter), without giving any explanation of how,” Verma said. Aizpurua and Hou have some ideas about it but are yet to nail down why they are able to get such a high resolution.

Nature, 2013. DOI: 10.1038/nature12151 (About DOIs)

This article was first published at The Conversation.