In recent years water-repelling materials have gotten better and better at their job of fearing water. But even the best hydrophobic surfaces still take their time when repelling water. This becomes an issue when the surfaces you want to keep water-free operate in freezing conditions. If water is not repelled quickly, it can freeze and end up stuck there. Now Kripa Varanasi, of the Massachusetts Institute of Technology, has come up with a way of speeding up the process of repelling water. (Featuring an incredible GIF)
The ultimate dream of nanotechnology is to be able to manipulate matter atom by atom. To do that, we first need to know what they look like. 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.
The laws of nature limit what can be seen with the help of light alone. Only objects separated by more than half the wavelength of the light that illuminates it can be observed as separate objects. To overcome this limit, in 1928, Edward Hutchinson Synge came up with an idea of imaging things too small for the naked eye. The idea was to shine light on a small particle and study the scattering when reflected back, making the wavelength of incident light irrelevant.
The realisation of Synge’s idea had to wait till the 1980s, when Heinrich Rohrer, the father of nanotechnology, developed scanning tunnelling microscopy (STM). This method uses a special property of electric current called quantum tunnelling to achieve this.
Since the development of STM, techniques for imaging smaller and smaller objects have been improving incrementally. Today it is possible to identify the shapes of molecules and where the atoms reside. But none of these techniques can identify the atoms those 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 some form of energy, like heat or light, hits molecules it makes them vibrate and rotate, even in solid structures. This process is called “excitation”. The movement emits some of the energy back, which is called “emmission”. Raman spectroscopy works by detecting this tiny amount of emitted energy, which tells us things about the molecule that’s doing the emitting.
One of the many uses for Raman spectroscopy is analysing old ruined paintings. It can detect the presence of certain elements at very specific locations. The salts of these elements have specific colours and thus they can reveal what a particular part of the painting might have looked like originally.
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 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 the journal 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 which contains the molecule to be analysed. 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 to the left 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 analysed 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 require weeks to months of work just to be able to analyse a single molecule, and their paper reports work done with only one ring-shaped molecule.
Prabhat Verma at Osaka University has worked on using Raman spectroscopy to analyse 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.” Aizpurua and Hou have some theories about it, but are yet to nail down an explanation why they are able to get such high resolution.
Biology and nanotechnology are moving ever closer together. I recently wrote about the use of nanoparticles to aid delivery of stem cells in cardiac therapy. Now, Swiss researchers have developed nanoparticles that can detect, and one day could combat, viruses.
How high can you get? Resolutionwise, that is. In 2010, when launching the Apple iPhone 4, Steve Jobs claimed that the 326 dots per inch (dpi) resolution of that machine’s display would make it impossible to pick the pixels apart. His reason was that this density of dots is at the limit of the resolving power of the human eye when something is held at reading distance from it. This limit is not, however, the theoretical maximum resolution of an image. That is about 100,000 dpi, a figure imposed by the laws of physics. Place any more dots in an inch and the light waves coming from them start to interfere with each other, leading to a loss of clarity.
Printing at 100,000 dpi using either the inkjet technique (in which droplets of liquid ink are laid down side by side) or the laserjet technique (in which static electricity is used to direct bits of powdered ink onto paper, where a laser melts them) is impossible. Neither can manage more than about 10,000 dpi. But Karthik Kumar, a material scientist at Singapore’s Agency for Science, Technology and Research, thinks he can do better. As he and his colleagues report in Nature Nanotechnology, they have a prototype that can manage the full 100,000. The catch is that it uses “ink” made out of silver and gold.
Actually, that is not the only catch. For the image has to be created using an electron beam, rather than a laser or an inkjet, and such beams are rather hard to handle. But as a proof of principle it is interesting, and it might lead to cheaper and faster methods.
Dr Kumar and his team start with a plate of silicon. The electron beam carves bits of this away, leaving a pattern of cylindrical posts each about 140 nanometres (billionths of a metre) across and 50 nanometres apart. That “about” is important, though. The exact diameters of the posts and the distances between them are crucial. Varying them changes the colour that forms between the posts.
To create this colour, the plate is coated with a layer of silver and another of gold. The outer electrons of the atoms of these heavy metals often come loose, to form a cloud akin to an electronic gas. When light falls on this gas, it absorbs all frequencies bar one, which is reflected. Exactly which frequency is reflected depends on the resonant frequency at which the electron gas vibrates. And that, in turn, depends on how far apart the silicon posts (which constrain the gas’s movements) are.
A coloured image can thus be made by varying the size and spacing of the posts. This, the team did. Specifically, they recreated a widely used test image: that of Lenna, a pin-up girl from the 1970s whose picture is reckoned (ahem) a challenge to reproduce because of its wide range of tones. Dr Kumar’s version of Lenna was only 100 microns (about the thickness of a human hair) across, but matched the original with reasonable fidelity.
Carving images on silicon using electron beams, and then coating the result with precious metals, is unlikely ever to be a viable technology for the mass printing of images. It might, though, be a good way of storing data permanently—better, in terms of density, at least, than existing optical techniques such as CDs, DVDs and Blu-ray discs. It is also strangely reminiscent of the Daguerreotype, an early form of photography that formed images of silver on a copper plate. Bearing in mind the multi-billion dollar industry that Louis Daguerre’s idea eventually turned into, perhaps Dr Kumar’s version is not so strange, after all.