Mildred Dresselhaus: The wonderful world of the tiny
Nanoscience, the science of tiny things, is leading to some extraordinary developments and applications. Many people believe that nanotechnology is the next big thing. But its business application is only just beginning. Mildred Dresselhaus explains what it is and where it is heading –
Nanotechnology deals with objects on the nanometre size scale, typically more than 1,000 times smaller than a human hair (one nanometre is one-billionth of a metre).
Despite their small size, nano-objects have been sparking huge excitement among scientists in the past few years. That excitement is starting to spread – to school children, to the business community and to government planners.
Since the nano-field is at such an early stage of development and is moving so rapidly, there are many views on how it started, why it is happening, and where it is all going.
It is difficult to make cast-iron predictions about practical outcomes for this infant technology, or on how these outcomes might affect our lives. But one thing is certain: it will trigger major changes in the next decade.
Nanotechnology is the application of nanoscience, which deals with objects and physical phenomena on such a minuscule scale that new physical principles apply – those governed largely by quantum mechanical phenomena.
The era of nanoscience and nanotechnology emerged in the past few years because of three factors: the push of industrial progress; the availability of scientific know-how to fabricate, measure, model and manipulate objects of nanometre size; and the almost daily discovery of new phenomena on the nanoscale, which fuel new discoveries and enable new levels of control of molecular self-assembly.
Many people imagine nanotechnology as a remote field, but it is already here and all around us.
Consider, for example, the magnetic information storage system in your computer. Here, magnetic heads move at great speed just eight nanometres (nm) above the disk as they read and write information at a density approaching 100 gigabits per square inch, and where the magnetically active film thickness is only about 25nm.
Even more amazing is the low cost of this huge functionality and the speed at which manufacturers are compelled to develop the next generation system (less than one year), or face obsolescence.
Nanotechnology was ushered in by what is called Moore’s law in the semiconductor electronics, magnetic recording and optoelectronics industries. This says that the functionality of chips will increase exponentially over time, while their size and cost will decrease exponentially.
The size of some elements of devices in these industries have already reached nanoscale because technology has been able to produce objects of this size in a controllable way.
Techniques such as scanning-probe microscopes and high-resolution electron microscopes have been developed to measure and manipulate objects down to molecular and atomic levels (1nm and below in scale).
This “top-down” approach continues to dominate industrial progress at the same relentless pace, pushing down the size of devices year by year, increasing their functionality and lowering their cost.
But this is not where the real excitement lies. What people are drawn to increasingly is the “bottom-up” approach. This means starting from new systems that can be controlled at the nanoscale then seeing what can be built up from these objects and what they can be used for.
This approach does not set out to look for a replacement for the integrated circuit as such, or for a new type of magnetic information storage bit, but rather looks for new properties and functions not previously envisaged.
One of the most celebrated examples of the new “building blocks” resulting from the bottom-up method is the single-wall carbon nanotube. This is a single layer of a graphite crystal rolled up into a seamless cylinder one atom in thickness, typically about one nanometre in diameter, and with length to diameter ratios that can be a thousand or more.
The prediction that these objects would be semiconducting or metallic depending on their precise geometry – and the detailed experimental verification of this prediction – was an early triumph for nanoscience.
The modelling capability and ability to carry out quantitative measurements has led to the carbon nanotube becoming a model system for understanding new phenomena at the nanoscale. Carbon nanotubes were shown not only to have a variety of remarkable scientific properties, but also to possess attributes of practical interest.
Besides being extremely tiny the tubes are also excellent electron emitters, with high potential for flat panel display applications – which is already under commercial development in Korea.
In the electronics field, early demonstration of a field-effect transistor based on a single carbon nanotube, one nanometre in diameter, created much excitement and enthusiasm.
So did the early illustration of the full range of basic logic functions that nanotubes could provide.
The ability to grow a single nanotube at a desired location and in a desired direction showed great promise for control at the nanoscale.
The solubility of nanotubes in aqueous solutions and their ability to be used as individual sensors and actuators have also attracted attention, with regard to miniaturization as well as high spatial and temporal sensitivity.
A great challenge in the development of nanotubes for electronics applications has been finding a way to control the growth process.
It is necessary to produce nanotubes that have the same diameter and chirality (left or right handedness), so that they have reproducible semiconducting or metallic properties.
Other areas of application, such as providing structural reinforcement to processible polymeric composites are not hampered as much by the need for controlled nanotubes.
Another remarkable building block that offers more control for electronic applications has been the nanowire. Using the vapour-liquid-solid approach, a nanometre-sized molten gold catalyst particle provides the active region to which semiconductor constituents are supplied in the vapor phase.
This produces a supersaturated active region from which single crystal wires are precipitated under highly controlled conditions.
The development of nanowire building blocks over the past five years has proceeded at a phenomenal pace. This is because developers’ ability to control the growth process to produce nanowires of controlled diameter, down to 2nm, has allowed them to generalize the growth technique to produce nanowires covering a large variety of semiconducting materials, including group IV, III-V and II-VI semiconducting wires, and to dope the nanowire over a wide range of n- and p-type carrier concentrations.
Nanowire properties have provided incredible possibilities for use in electronics, opto-electronics, sensors and actuators, both at the individual nanowire level or in concert with one another. For example, there have been demonstrations of: transistor action from individual nanowires; superior p-type carrier mobility in single nanowire devices; nanowires with superlattices of constituents along the length and with the possibility of different chemical species in radial shells; the controlled preparation of p-n junctions; and light emission at p-n junctions and of single nanowire.
In addition to nanotubes and nanowires there are many more building blocks under development. There is, for example, great promise in nanosystems that bridge the organic and inorganic worlds. This offers the control that is possible with semiconducting nanowires plus the processibility and low costs that can be provided with organic constituents.
Nanostructures could also provide the necessary short transport lengths for the low mobility carriers typically associated with organic constituents.
Nanosystems bridging inorganic nanowires with nanoscale biological constituents with high binding specificity likewise offer great promise for using the direct electronic readout possibilities of the nanowires and the multifunctional sensor arrays that can be assembled by this route.
Already small companies are growing up around many universities and research institutes to develop the commercial potential of these recent developments. These activities are likely to lead to miniature and portable devices and instruments with high sensitivity and speed; new categories of sensors, actuators and detectors; greater coupling between the inorganic and organic worlds, as well as between the inorganic and bio-worlds; and more extensive use of simulation and design to optimize nanosystem performance.
Many conferences are being organized by the venture capital community and by government research funding organizations to look at what has been achieved in the laboratory.
Since the initial major advances in nanoscience, research funding to explore new ideas has become increasingly available worldwide for individual and team activities at universities or small commercial organizations.
Some of the nanotechnology claims are hype, but the demonstration of so many real achievements in the laboratory and the excitement of young people working in the field suggest that there is substance beyond the puff.
Mildred dresselhaus
Mildred Dresselhaus is professor of physics and electrical engineering at the Massachusets Institute of Technology.
