In industries where the goal is to build smaller and more efficient materials and devices, nanotechnology is the logical outcome. With nanotechnology, humanity could fully exploit and control the fundamental building blocks of nature.
“I see nano as a new age of material sciences…one huge part of it, not the total, but a huge, new age of material science which found its roots in metallurgy many centuries ago,” says Professor Doug Perovic, chair of the department of materials science and engineering and also of the nanoengineering program, which last year saw its first graduating class.
Since the bronze age, humans have used a wasteful top down approach to manufacturing where large amounts of matter are chiseled away to get down to the desired size.
We can now manufacture very small materials down to the nanoscale, meaning the size of ten hydrogen atoms. This has a very significant impact on the way these things behave. There are for instance very interesting new magnetic and mechanical behaviours, and new potentials such as the ability to store hydrogen in the spaces between the nanoparticles.
The excitement lies in the fact that these materials could build themselves from the bottom up, through a process called self-assembly. The scientist applies the correct conditions, such as temperature, and the molecules assemble themselves into the desired structure.
A great deal of insight comes from a technique known as biomimetics, which models man-made devices after natural systems. “We need to do more with the space that we have, and figure out better ways to manufacture without creating the waste and the pollution,” explains Perovic. “And nature knows how to do that…it knows how to build and self-assemble. A cell is just unbelievable…a single cell can grow to produce one of us, the whole system just builds itself.”
Computing with light
Photonics is the technology of harnessing light for use in computers and other devices. Photonic components are currently made through a process in which one crystal is grown on another. But for this to work both crystals must have the same spacing between their constituent atoms.
“We make good devices that way, but [it was very difficult] to make different kinds of devices on one piece of material,” says professor Ted Sargent of the department of electrical and computer engineering.
“So with quantum dots in polymers we’ve lifted that constraint.” A polymer is a long chain of identical subunits. Quantum dots are a joint invention from the fields of physical and synthetic chemistry along with materials science, and are used in both biomedical and electronic applications.
Each semiconductor quantum dot is independent from the others, ultimately meaning that flexibility is enhanced and the need to match crystals is removed. Any polymer can now be placed onto a soft, thin substrate.
This advantage is also being used by professor Zheng-Hong Lu of the department of materials science and engineering towards research into flexible screens, using soft plastic-like substrates with organics grown on top (see Feb. 23 issue of The Varsity, “Flexible Screens: A Revolution Underway?”). An organic material contains carbon, an inorganic one does not.
Quantum dots can take advantage of something known as the quantum size effect, where the size of the quantum dot determines the wavelengths of the light that it will interact with and the colour of light it can produce. The light-emitting nanocrystal quantum dots can be placed on a piece of silicon interface electronic components with photonic ones. “I think that electronics and photonics do and will forever coexist, so to me the name of the game is actually harmonizing the two of them,” says Sargent.
Not only can photonic and electronic interact, but light can interact with light, in a field known as non-linear optics and involving the famous buckyball. Non-linear optics occur when one beam of light passing through a material influences the way other beams of light will pass through the same material.
Buckyballs, soccer-ball shaped molecules made up of 60 carbon atoms, are critical because they contain a lot of usable electrons-crucial for allowing light to act in new and unique ways. “We’ve made materials using buckyballs that allow light to control light ten to 100 times more efficiently than published before.” With this new ability to control how light influences light, the ability to set up a dynamic fibre-optic network for communicating vast amounts of information with unprecedented speed is now possible.
Materials that build themselves
Professor Geoffrey Ozin is the U of T Canada Research Chair in Materials Chemistry. He works in a field known as materials self-assembly which merges a “top-down” approach with a “bottom-up” molecular-based approach to create new materials of all sizes.
Benjamin Hatton, a grad student in materials chemistry, actually bridges departmental boundaries, working in collaborative projects with both Professors Perovic and Ozin. His research involves groups of materials known as periodic mesoporous silicas and organosilicas.
One application of conventional silica (glass) is as an insulating material in electronics systems. However, mesoporous silica (‘meso’ designates a length scale in the tens of nanometers) is made using a self-assembled organic template to direct the overlying inorganic silica structure. The organic template can then be burned away to leave a porous network.
Keeping some organics around has its advantages. Silicon itself is not interesting chemically, but adding organic molecules into the silica network can give the material extra capabilites.
The possible applications of these organic/inorganic hybrid materials are staggering. The new materials have insulating features that are currently being tested for a new generation of computer chips. In medicine, structures with small channels on the nanometer scale can potentially store drugs and release them in a controlled way. In chemistry, because the surface area to volume ratio is so much greater in the nano scale, there is a greater area on which reactions can take place. The universe of the very small seems to reach out very far indeed.
Increasing the size from several nanometers to a micron (1000nm), we run into the colloidal crystal and the domain of Dr. Vladimir Kitaev, a research associate working with professor Ozin and studying materials, polymers and colloid chemistry. Colloid spans the length scales between the meso and the macro
In particular Dr. Kitaev works with close-packed arrays of silica spheres, known as opals, which can form intricate patterns for manipulating light and the possible transmission of light encoded messages.
Since around 20 per cent of the opal is porous, one exciting innovation is to use a bigger opal as a template and fill it in with smaller materials. Enter Dr. Marc Mamak, also a research associate with Professor Ozin, who is currently studying the properties of these materials for use in fuel cells and lithium ion batteries.
He is also studying displays based on mesoporous nanocrystals that can change colour under electrical stimulation. Using these kinds of materials as relays between the circuitry and the dye molecules, it is possible to design display technologies that give unprecedented speed and colour contrast. This research is being sponsored by Xerox for their electronic paper technologies.
More than meets the eye
Professor Eugenia Kumacheva, professor of chemistry, works with two different sizes of particles: the nanoparticle and the colloidal crystal. If nanoparticles are placed inside colloidal crystals, the nanoparticle will emit light under certain conditions-but the wavelength of this light will be tuned by the size of the nanoparticle and the properties of the colloidal crystal.
These materials can be flipped between a transparent and an opaque state simply by shining light of the right frequency and intensity on them. In this way, they can be used as switches to build circuits based on light rather than electricity.
Professor Kumacheva also works with polymer beads with a fluorescent dye located in the core. The dye’s flurorescent colour can be turned on and off by shining light of a certain wavelength on it, allowing the beads to function as pixels in two possible states. These can serve as the building blocks for materials used for memory storage and security documents.
By putting different dyes in the core and the surrounding shell of the beads, documents could be made with several layers of information. This may have applications in data encryption for security documents. “Under visible light you see your picture but if you’re looking at different wavelengths you can see, for example, fingerprints or a signature.”
Reprogramming cells
A cell’s molecular network is the set of relationships between the molecules that make it up. Once this network is well understood, a malfunctioning cell can be easily identified. Targeting unhealthy cells is also possible through a method currently under exploration by professor Warren Chan of the Institute of Biomaterials and Biomedical Engineering (IBBME). “The cells that are healthy and unhealthy have a different set of molecules on the surface… once we figure out those molecules we can use that as a targeting scheme,” says Chan.
Once in the cell, the drug can inhibit the manufacturing of certain molecules or block undesirable biological pathways. However, to enter the cell and travel through the body the drug molecules have to be very small. Enter nanotechnology.
The mechanism of the drug delivery system involves producing a composite structure made up of different molecules that each serve a different purpose. “You produce one molecule to get into the right tissue, and another molecule that takes it into the cell and another that allows it to escape certain compartments within the cell,” says Chan.
This mechanism is modeled on the behaviour of a virus, proving once again the power of biomemetics. “We design it from the bottom up but modeled on the existing virus…so that’s where it becomes important to look at nature as a model scheme.”
Labeling using quantum dots is another powerful tool. These luminous nanocrystals were used in a study of mice where they were sent into a tumor and glowed different colours in each area. “Within the tumor the red decorated the blood vessels of the tumour, while the green the lymphatic vessels, etc.”
Chan explains the mechanism whereby quantum dots can be designed for cancer detection. “We want to utilize the different colours of the quantum dots as a way to molecularly code the disease…if you have cancer it just shines light.”
One of a kind
U of T’s nanoengineering program, capturing all these great new developments, is the first such undergraduate program in North America and is one of the options open to engineering science students during their final two years. “The [key] to our success is that we have this great university,” says Perovic proudly. “It has all the ingredients we need to get the right people together. No one single department could pull that off.”
The interdisciplinary engineering science program includes courses from seven departments in the faculties of arts and science and engineering, including chemistry and physics, material science engineering, electrical and computer engineering, chemical and mechanical engineering and the Institute of Biomaterials and Biomedical Engineering (IBBME). The innovations lie at the interface between these diverse disciplines.
