TUESDAY, 14 MARCH 2017Nanotechnology, the science of the small, emerged as a hot-button topic around the 1990s. It now has far-reaching impacts in multiple fields of technology, ranging from medicine to transportation, and has been particularly influential in electronics. It is an area of engineering focusing on the design of new materials and devices at the nanometre (nm) scale, one billionth of a metre, which is a size range difficult to conceptualise. One nanometre is the length of just seven carbon atoms in a row; 100,000 nm span the thickness of a human hair.
The first musing about nanotechnology as a field is said to come from Nobel Laureate Dr. Richard Feynman’s famous 1959 talk “There’s Plenty of Room at the Bottom.” Here, he forecasted the invention of “infinitesimal machines” with wires and gears invisible to the human eye, an idea which seemed visionary at the time. Nanotechnology has just started to blossom in the last few decades because scientists have only recently acquired the tools to manipulate, and indeed visualise, matter at the atomic scale. The scanning tunnelling microscope (STM), invented in 1981, can be used to visualise individual atoms on a material’s surface, while the molecular beam epitaxy (MBE) deposition system, invented in the late 1960’s, builds up new materials, which may have never before existed on earth, one atomic layer at a time.
"At this infinitesimal scale, classical physical laws which govern the macroscopic world start to fall apart, and we enter the weird and wonderful world of the quantum"
The first evidence of nanoscale engineering, however, not only predated the invention of these research tools, but also came before many influential scientists including Feynman, Faraday, and Newton. In fact, the oldest recorded use of nanotechnology dates to a civilization known for famous ‘firsts’: the Ancient Romans. Thousands of years ago, they used metal nanoparticles to create eye-catching colours and other optical effects in stained glass artefacts.
The definition of a nanoparticle can be somewhat nebulous, but it is typically considered a cluster of atoms in which at least one dimension is less than 100 nm. At this infinitesimal scale, classical physical laws which govern the macroscopic world start to fall apart, and we enter the weird and wonderful world of the quantum. For instance, nanoparticles interact with light in unusual and remarkable ways, as the dimensions of the particle are on the same scale as the wavelength of visible light (200-800nm). This phenomenon is something the Ancient Romans used, perhaps unknowingly, in their artwork.
"So, did the Romans discover nanoparticles and master nanotechnology several millennia ago, only to have it lost and rediscovered by modern-day scientists?"
The Lycurgus Cup is so named because its outer carvings depict the legend of King Lycurgus from the 6th book of Homer’s Iliad. The chalice currently sits in the British Museum and dates back to the 4th century AD. What is remarkable about the cup is that it exhibits a dual-colour optical effect known as dichroism: it is a jade colour when light is reflected from the surface (front-lit), but a ruby red colour when light is transmitted through the cup (back-lit). The optical effect began attracting scholarly attention in the 1950s, when a sample of the cup was sent to the General Electric Company in Wembley, yet the science underlying this effect was not pinned down until the 1990s. Scientists Barber and Freestone, from the University of Essex and the British Museum Research Laboratory respectively, studied a sample of the cup under a transmission electron microscope (TEM) and discovered the presence of metal nanoparticles with diameters ranging from 50-100 nm. Further X-ray analysis revealed that the composition of these nanoparticles was approximately 70% gold and 30% silver.
So, did the Romans discover nanoparticles and master nanotechnology several millennia ago, only to have it lost and rediscovered by modern-day scientists? This is unlikely. The use of nanoparticles to colour Roman ceramics seems to have been short-lived: many pieces of recovered pottery appear to be failed attempts to recreate the dichroism, suggesting the Romans just got astonishingly lucky.
The metal nanoparticles in the Lycurgus Cup interact with light in unusual ways because of a phenomenon known as surface plasmon resonance. Light propagates as an electromagnetic wave, composed of orthogonal electric and magnetic fields. Metals, being conductors, have a large density of freely moving electrons on their surface. When light hits the surface of a metal, like a chrome mirror or a piece of aluminium foil, its oscillating electric field causes electrons on the metal’s surface to oscillate at the same frequency. These oscillations are known as surface plasmons, and they block the light from entering the metal, causing it to reflect off of the surface instead. This is what causes metals to be shiny and reflective.
As a result of the minuscule dimensions of metal nanoparticles, the electrons on the surface of the particle oscillate at specific frequencies, known as resonant frequencies. Only specific frequencies of light, those at the resonance frequency, are reflected, while light of other frequencies travels through and so is transmitted. By changing the size, shape, and chemical composition of the metal nanoparticle, you can change the resonant frequencies and hence tune which wavelengths of light get reflected and which get transmitted. In the case of the Lycurgus cup, the gold-silver nanoparticles reflect blue-green light but transmit red light, leading to the dichroism.
"It is always exciting to think about what the future holds in terms of technology development, but what happens when ‘the next big thing’ has really been around for centuries?"
Over 1,600 years later, the same physical phenomenon at work in the Lycurgus Cup is making a comeback in the research labs of Cambridge’s Department of Engineering. In a study published in the journal PNAS in 2014, a Cambridge research team, led by then PhD student Yunuen Montelongo, produced multi-colour holograms by taking advantage of the plasmonic resonance effect in silver nanoparticles. The group fabricated arrays of nanoparticles with two different shapes (rods and spheres), spaced at distances less than the wavelength of visible light (<200 nm). The two types of nanoparticles scatter different frequencies of light. The scattered light can be reconstructed into a multicolour image, which is then projected. In the future, this effect could therefore be used for 3D display technology.
Moreover, each of the nanoparticles in the array essentially stores its own independent information in the form of the wavelength and polarization of the light it scatters. Utilising this knowledge, new technologies could be developed that dramatically improve the capacity of optical data storage devices like DVDs or Blu-ray discs.
Of course, it will take some time for this technology to become commercially viable. In a Cambridge Research press release, Montelongo told interviewers, “The potential of this technology will be realised when they are mass-produced and integrated into the next generation of ultra-thin consumer electronics.”
It is always exciting to think about what the future holds in terms of technology development, but what happens when ‘the next big thing’ has really been around for centuries? Perhaps the story of the Lycurgus Cup is a gentle reminder that sometimes it is necessary to reach back 1,600 years in order to take the next step forward. It is both baffling and extremely humbling that the Romans were manufacturing using nanoparticles before there was even a word to describe them.
Ramya Gurunathan is an MPhil student at the Centre for Scientific Computing