Automatically assembling nanolenses
A team of researchers has created a scalable production method for a class of nanoscale lenses using the electrostatic forces between charged nanoparticles, enabling wider application of the technology.
These nanolenses use the gaps between triplets – or trimers – of different sized nanoparticles to focus light.
They are showing enormous promise in a range of optical and sensing applications due to their minuscule size and very high-yield enhancement. However, an expensive and laborious production process has limited their widespread uptake.
Dr Julian Lloyd, lead author of the research, and his colleagues’ method uses induced electrostatic forces to self-assemble the gold nanoparticle triplets. The process was developed at Monash University, Monash Centre for Electron Microscopy and the Melbourne Centre for Nanofabrication, ANFF VIC’s flagship facility and ANFF’s headquarters.
The method enables production of these trimers with a more than 60 percent yield, quantities that have never been achieved before, and removes the need for expensive “top-down” fabrication techniques traditionally used that inhibit the scalability of the fabrication process.
Julian’s trimers are short strings of nanoparticles 20nm, 30nm and 50nm in diameter. In his paper published in ACS Nano, Julian explains that by placing differing surface charges on the nanoparticles, the structures draw themselves into place.
Using a positively charged substrate as a base, the team add a number of negatively charged 30nm particles which neatly distribute themselves across the surface. Positively charged 20nm particles are added, each attracted to a 30nm particle, before a solution of positively charged 50nm particles is introduced. The particles align themselves because the 20 and 50nm particles repel each other, whilst both being drawn towards the centre 30nm particle.
“We can control the distance between individual trimers and also improve the trimer yield by tuning the surface charges,” Julian explained.
“As the whole assembly method relies on electrostatic interactions, it was vitally important to know the surface charges of the different components,” he continued. “The surface charge measurements were taken at the Melbourne Centre for Nanofabrication using the Centre’s Zeta sizer and a Zeta potential analyser.”
Ultrathin holograms open doors to 3D displays
A team led by Professor Min Gu from RMIT University and Beijing Institute of Technology has fabricated the world’s thinnest hologram – and it could revolutionise the way we interact with everyday technologies.
The team’s research, published in Nature Communications, takes a step closer to three-dimensional (3D) displays for smart devices by reducing physical dimensions of a hologram to the nanometer scale.
Holograms are the result of shining light on an interference pattern to recreate a seemingly 3D object within a film. Even the parallax effect is captured, meaning when the viewer moves, the image appears to move too.
An interference pattern is the result of light being split into two and travelling different paths of varying length. The additional time causes a phase difference in the light, resulting in peaks and troughs of intensity when the beams are recombined. Holograms were made conventionally made by splitting a laser beam – one direct, one detoured via an object – before hitting a film.
Since the ‘60s it’s been common practice to use computer-generated patterns – a process called computer-generated holography (CGH). CGH can in principle be applied to smart technologies but the physical size of holograms which currently ranges between micrometer to millimetres makes this currently impractical.
Holograms have to be thick enough to allow enough time for the phase difference to become noticeable.
The paper’s lead author, Dr Zengji Yue, explained that to reduce the depth the team began working with Antimony Telluride (Sb2Te3) that had been laser etched to feature the desired interference pattern.
The new film acts as a resonance cavity – light is bounced between the surfaces, amplifying the phase difference. Sb2Te3 is rare in the respect that its refractive index at the surface is far lower than within the body of the material which helps to retain the light.
The team worked closely with one of the Melbourne Centre for Nanofabrication’s process engineers, Dr Lachlan Hyde, to use the Centre’s atomic layer deposition (ALD) capabilities as the basis to fabricate the Sb2Te3 hologram film. The Centre’s ellipsometry equipment was used to compare the sample’s refractive index to theoretical models.
The result is a 60nm thick film that produces the holographic images. Considering most of the processes are scalable, the new holograms could be produced on a large scale.
The next step is to create smaller pixels to increase the resolution of the images, and to investigate dynamic displays.
Folding plasmene nanosheets
Formally introducing plasmene nanosheets to the world, he and his team have created self-folding sheets of nanoparticles that can change the behaviour of light.
The nanosheets consist of a superlattice of nanoparticles, organised in a hexagonal pattern much like the arrangement of carbon atoms in the wonder-material, graphene.
Wenlong’s team creates the materials by sparsely distributing nanoparticles covered with protruding polystyrene hairs, or ligands, across the surface of a water droplet. By evaporating the water the surface area shrinks, bringing the particles closer together. Attractive van der Waals forces between the nanoparticles draw them together while repulsive forces between the ligands manoeuvre the nanoparticles into the desired hexagonal configuration.
The nanoparticle-laden water droplet is suspended over an open cavity, in this case a hole in a copper sheet, which gives the nanosheet a place to form. Once all the water has been removed, a continuous nanosheet forms neatly across this cavity like the membrane on a drum, allowing Wenlong to begin to play with it.
Using ANFF-Vic’s focused ion beam (FIB) instrument at the Melbourne Centre for Nanofabrication, the team “scores” specific patterns into the sheets, deliberately weakening them at key points. The weakened regions are akin to the paper folding lines introduced in origami, where the layout and severity of the folds define the final three-dimensional structures.
Researchers have investigated the properties of this self-folding before, but the designs of the well-defined origami structures of this kind have not been realised until now.
Light interacts and reflects off differing origami shapes in a way that is unique to each contour. By changing the structure, Wenlong can exploit his plasmene sheets to perform some useful tricks.
One useful application for these tiny sculptures is in nanophotonics as plasmonic waveguides for directing and switching light at the nanoscale by circumventing diffraction limits.
The plasmene nanosheets produced can also be used as dual-coded encryption security labels as it’s almost impossible to fraudulently create the shapes without the initial blueprint. This poses enormous potential benefits to the banknote industry, which is always searching for more secure methods of deterring counterfeiters.
The plasmene sheet’s semitransparency, mechanical softness and uniform response to light suggest that it could be an ideal material in the production of secure identifying labels for chemicals, currency and commodities, among others.