Nanotechnology is coordinated movement: a choreographed dance among atoms and molecules to achieve a desired effect. Nanotechnologists seek to harmonize with, and thereby exploit, the laws of physics and chemistry, coaxing nature to assemble matter into new and useful forms. The resulting materials may have the power to produce breakthroughs in medicine, computing, and energy.
But nanotechnology is not new. For four billion years, nature has organized atoms into simple molecules, molecules into complex proteins, proteins and sugars and fats into intricate societies of cells, and cells into the infinitely variegated life that surrounds us. Nature builds using an array of one hundred distinct atomic elements that are rigorously disciplined, and limited to a small set of simple, but powerful rules. With this modest set of elements, nature invents limitless variety, beauty, form, and purpose.
Scientists have been exploiting nature’s ready-made molecular assembly line for centuries. We have been linking molecules together to form long, perfect polymer chains with predictable properties. We have been introducing lumps of imperfect, impure material into a vacuum chamber, letting atoms evaporate, and growing from them strikingly perfect crystals of defined shape, size, and orientation. We have controlled how these designer materials produce light, conduct electricity, and respond to touch.
Today, nanotechnologists have at their disposal a vast new set of methods for the visualization and manipulation of matter. They also have a growing cultural advantage. Nanotechnology is an intersection, a point of confluence at the heart of contemporary science. At this junction, traditional scientific disciplines merge; mix engineering with medicine and produce chips, diagnoses, and therapies that no sequestered specialist could generate. This produces the convergent thinking that emerges when paradigms collide.
Nanotechnology Applied: Sensing the world around us, and turning the sun’s energy into electrical power
Our enhanced control over matter’s self-organization could transform many technologies of great importance to society. Examples include the light-sensing chips in digital cameras; and the sheets we use to capture the sun’s energy—photovoltaics.
These fields rely on semiconductor technology: a proven, powerful platform. Further major advances in semiconductors could come not only through further miniaturization, but also by building semiconductors on the grand scale such as painting new nanostructured semiconductor materials onto chips, and even onto flexible building materials.
Semiconductors are the physical foundation of the information age. Whether it be computers, the Internet, digital cameras or even military night-vision cameras, all rely on a semiconductor of some form.
The equipment used to develop conventional semiconductor crystals costs in the millions of dollars, and its operation, in view of the dangerous chemicals required, is also expensive. In the past few decades, researchers, and now companies, have instead begun painting a new class of semiconductors on top of chips. They’ve made transistors, sensors, solar cells, and even lasers. But can their performance compete with the conventional semiconductor crystals?
In short, yes. They can even outperform them. In July 2006, we reported in Nature about a paint-on sensor for light that outperformed its traditional semiconductor crystal counterpart by a factor of about 10. The figure illustrates the simplicity with which the device was built. What is perhaps most remarkable about this device is that it worked so well, yet was made from anything but a perfect, pure single crystal of semiconductor. Careful examination of what makes a good sensor for light, however, reveals that, in this application, atomic-scale crystal perfection is not a prerequisite. Instead, what’s needed is a strong signal—a very significant flow of electronic current in response to light—combined with a minimum of noise. Engineering materials at the nanoscale allowed us to maximize the flow of signal current while bringing noise down to almost the very limits that fundamental physical laws allow.
Next on the Horizon: Harnessing the sun’s power
The research on sensors is tantalizing not only because it could allow us to make low-cost cameras that can see in the dark. It also leads us to wonder if we could make use of solar power on a mass scale.
The solar energy opportunity is astounding. One thousand times more sun reaches the earth’s landmass each day than we consume across all our energy habits. If we could cover one percent of the earth with 10 percent power-conversion-efficiency solar cells, we could then harness this powerful, clean source of energy to considerable effect.
However, today’s polymer solar cells typically give only a few percent of the sun’s energy back as electricity. Of the challenges to overcome in making processible solar cells efficient, one is to make the best use of the sun’s rainbow of colours. Nearly half of the sun’s power is invisible to us, lying in the infrared wavelengths beyond the red that our eyes can see. Visible-absorbing polymer photovoltaics access only a fraction of the sun’s power. However, our group has recently shown that it is possible to build infrared solution-processed solar cells, and we have shown a rapid improvement in their performance. A year and a half after the first report, these devices are now a factor of 20-40 from tapping the sun’s infrared rays with sufficient efficiency to augment significantly the overall performance of polymer solar cells. Our group is working to bridge the remaining gap.
Nanotechnology is increasingly revealing to us not only its beauty, but also its power to ultimately change our lives for the better. Taking advantage of nanoparticles, suitably painted onto a chip, we can sense light with exquisite finesse and even infrared colours, lifting the obscurity of night.
Though we have yet to make it happen, nanotechnology holds promise to help us tap the greatest power source of all—the sun—an abundant source of clean, safe energy. With such exciting potential, nanotechnology is a dance worth waiting for.
Introduction to Nanotechnology:
Ted Sargent, The Dance of Molecules: How Nanotechnology is Changing Our Lives, Penguin, 2005.
Research in Nanotechnology in the Sargent Group at UofT:
Papers cited in this work:
Gerasimos Konstantatos, Ian Howard, Armin Fischer, Sjoerd Hoogland, Jason Clifford, Ethan Klem, Larissa Levina, Edward H. Sargent, “Ultrasensitive solution-cast quantum dot photodetectors,” Nature, vol. 442, pp. 180-183, 2006.
S. A. McDonald, G. Konstantatos, S. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, E. H. Sargent, “Solution-processed PbS quantum dot infrared photodetectors and photovoltaics,” Nature Materials, vol. 4, no. 2, pp. 138-142.
The views and ideas expressed in this column do not necessarily reflect those of the Canada Foundation for Innovation or its Board Directors and Members.