Research

What is Nanotechnology?

by Zhong L. Wang

    In the history of industrial engineering, technology characterized by length only occurred in microelectronics, but now we have nanotechnology. How small is one nanometer? The typical width of a human hair is 50 micrometers. One nanometer is 50,000th of a hair width.

    Nanotechnology is the construction and use of functional structures designed from atomic or molecular scale with at least one characteristic dimension measured in nanometers. Their size allows them to exhibit novel and significantly improved physical, chemical, and biological properties, phenomena, and processes because of their size. When characteristic structural features are intermediate between isolated atoms and bulk materials in the range of about one to 100 nanometers, the objects often display physical attributes substantially different from those displayed by either atoms or bulk materials.

    Phenomena at the nanometer scale are likely to be a completely new world. Properties of matter at nanoscale may not be as predictable as those observed at larger scales. Important changes in behavior are caused not only by continuous modification of characteristics with diminishing size, but also by the emergence of totally new phenomena such as quantum confinement, a typical example of which is that the color of light emitting from semiconductor nanoparticles depends on their sizes. Designed and controlled fabrication and integration of nanomaterials and nanodevices is likely to be revolutionary for science and technology.

    Nanotechnology can provide unprecedented understanding about materials and devices and is likely to impact many fields. By using structure at nanoscale as a tunable physical variable, we can greatly expand the range of performance of existing chemicals and materials. Alignment of linear molecules in an ordered array on a substrate surface (self-assembled monolayers) can function as a new generation of chemical and biological sensors. Switching devices and functional units at nanoscale can improve computer storage and operation capacity by a factor of a million. Entirely new biological sensors facilitate early diagnostics and disease prevention of cancers. Nanostructured ceramics and metals have greatly improved mechanical properties, both in ductility and strength.

    From the fundamental units of materials, all natural materials and systems establish their foundation at nanoscale; controlling matter at atomic or molecular levels means tailoring the fundamental properties, phenomena, and processes exactly at the scale where the basic properties are initiated. Nanotechnology could impact the production of virtually every human-made object from automobiles and electronics to advanced diagnostics, surgery, advanced medicines, and tissue and bone replacements. To build electronic devices using atom-by-atom engineering, for example, we have to understand the interaction among atoms and molecules, how to manipulate them, how to keep them stable, how to communicate signals among them, and how to face them with the real world. This goal requires new knowledge, new tools, and new approaches.

Much more than miniaturization

    To many people, nanotechnology may be understood as a process of ultra-miniaturization. Philosophically, changes in quantity result in changes in quality. Shrinkage in device size may lead to a change in operation principle due to quantum effect, which is the physics that governs the motion and interaction of electrons in atoms. In fact, the trend in product miniaturization will require new process measurement and control systems that can span across millimeter-, micrometer-, and nanometer-sized scales while accounting for the associated physics that govern the device and environment interaction at each specific size scale.

    To consider the interactions among atoms in the nanometer scale, we need to introduce quantum mechanics and each atom has to be treated as a unit. To face the atoms with the real world in the millionmeter scale, we need to consider the collective properties of millions and millions of atoms, so that the matter is considered to be a continuous medium, and we use classical mechanics. The bridging of the two length scales requires new standardized architecture definitions that support multiple physics-based models and new computational representations that allow seamless transition and traversing through these various models.

Manufacturing nanomaterials

    Nanomanufacturing technologies that will support tailor-made products having functionally critical nanometer-scale dimensions are produced using massively parallel systems or self-assembly. The current research mainly focuses on nanoscience for discovering new materials, novel phenomena, new characterization tools, and fabricating nanodevices. The future impact of nanotechnology to human civilization is manufacturing. The small feature size in nanotechnology that limits application of wellestablished optical lithography and manipulation techniques causes industrial nanomanufacturing to remain a serious challenge to our technological advances.

    Synthesis of nanomaterials is one of the most active fields in nanotechnology. There are numerous methods for synthesizing nanomaterials of various characteristics. An essential challenge in synthesis is controlling the structures at a high yield for industrial applications. Techniques are needed for atomic and molecular control of material building blocks, which can be assembled, used, and tailored for fabricating devices of multifunctionality in many applications.

    The oxide nanobelt discovered in my laboratory is an example. Ultra-long nanobelts have been successfully synthesized for a wide range of oxides by simply evaporating the desired commercial metal oxide powders at high temperatures. These materials are semiconductors with important applications in sensors and transducers. The as-synthesized oxide nanobelts are pure and structurally uniform; they have a rectangular-like cross-section. The semiconducting oxide nanobelts could be doped with different elements and be used for fabricating nanometer-sized sensors based on the characteristics of individual nanobelts, which could be potentially useful for in-situ, real-time, and remote detection of molecules, cancel cells, or proteins based on electronic signal. The nanobelts could also be used for fabrication of nanoscale electronic and optoelectronic devices because they are semiconductors.

Characterizing the performance and properties of nanostructures

    Property characterization of nanomaterials is challenging because of the difficulties in manipulating structures of such small size. New tools and approaches must be developed to meet new challenges. Due to the high size and structure selectivity of nanomaterials, their physical properties could be quite diverse, depending on their atomic-scale structure, size, and chemistry. A typical example is the carbon nanotube, which is made of concentrical cylindrical graphite sheets with a diameter range from one to 400 nanometers and length of a few micrometers. Characterizing the mechanical properties of individual nanotubes, for example, is a challenge to many testing and measuring techniques because of the following constraints. First, the size (diameter and length) is rather small, prohibiting the application of wellestablished testing techniques. Tensile and creep testing require that the size of the sample be sufficiently large to be clamped rigidly by the sample holder without sliding. This is impossible for one-dimensional nanomaterials using conventional means. Second, the small size of the nanostructure makes their manipulation rather difficult, and specialized techniques are needed for picking up and installing individual nanostructures. Therefore, new methods and methodologies must be developed to quantify the properties of individual nanostructures.

    In-situ transmission electron microscopy technique, or TEM, has been developed for measuring the modulus of individual carbon nanotubes. We have to see the object while its properties are being measured, thus, a microscope is required. To carry out the property measurement of a nanotube, a specimen holder for a TEM was built for applying a voltage across a nanotube and its counter electrode. To measure the bending modulus of a carbon nanotube, an oscillating voltage is applied on the nanotube that can tune the frequency of the applied voltage. By changing the frequency of the applied voltage onto the nanotube, mechanical resonance can be induced in carbon nanotubes at specific frequencies from which the bending modulus can be derived. This type of technique works well for small objects.

Large-scale manipulation and self-assembly

    Manipulation of nanostructures relies on scanning probe microscopy. Using a fine tip, atoms, nanoparticles, or nanowires can be manipulated for a variety of applications. This type of approach is outstanding for scientific research. For manufacturing, an array of scanning tips, if synchronized, may be used for achieving atom-by-atom engineering. But the building rate is rather slow. If a device has a feature size of five nanometers and a scanning tip can move atoms 10 atoms per second, it will take about six months to build 1012 devices on an eight-inch wafer.

    The ultimate solution is self-assembly. Like many biological systems, self-assembly is the most fundamental process for forming a functional and living structure. The genetic codes and sequence built in a biosystem guide and control the self-assembling process. Self-assembly is the organization and pattern formed naturally by the fundamental building blocks such as molecules and cells. Designed and controlled self-assembly is a possible solution for future manufacturing needs.

    Size- and shape-selected nanocrystals behave like molecular matter that can be used as fundamental building blocks for constructing nanocrystal-assembled superlattices. Self-assembled arrays involve selforganization into monolayers, thin films, and superlattices of size-selected nanocrystals encapsulated in a protective compact organic coating. Nanocrystals are the hard cores that preserve the ordering at the atomic scale; the organic molecules adsorbed on their surfaces serve as the interparticle molecular bonds and as protection for the particles in order to avoid direct core contact with a consequence of coalescing. The interparticle interaction can be changed via control over the length of the molecular chains, resulting in tunable electronic, optical, and transport properties.

Large-scale parallel device fabrication and system integration

    System integration involves at least an integration of numerous functional materials and components for achieving a complex, preprogrammed action. This involves patterned materials growth on a designed substrate; large-scale, parallel integration of nanowires, nanoparticles, and functional groups; interconnection among the components; and defect-tolerated path design following neuron networks. A wide range of novel approaches has been developed for fabrication of single nanodevices. Nanomanufacturing requires a simultaneous, parallel fabrication of a large amount of nanodevices under precisely controlled conditions and repeatability. This remains a major challenge to the development of nanotechnology, especially for nanoelectronics. A possible solution is to integrate patterns produced by lithographic technique with self-assembly process. Self-assembly of single-walled carbon nanotubes is one example.

    By functionalizing the substrate produced by lithographic technique so that individual carbon nanotubes selectively recognize the locations for self-assemble on the substrate following specific patterns, mass producing carbon nanotube-based circuit structures is possible. To achieve the process, the substrate was coated with patterns of organic molecules using techniques such as dip-pen nanolithography and microcontact stamping. Two surface regions have been produced: one patterned with polar chemical groups, and the second coated with non-polar groups. A suspension of single-walled carbon nanotubes solution was added, and the nanotubes were attracted to the polar regions and self-assembled to form predesigned structures. Millions of individual nanotubes have been patterned on stamp-generated microscale patterns, covering areas of about one square centimeter on gold.

Integrating nanometer-to-millimeter manufacturing technologies

    Over the next decade, major industrial and scientific trends that emerged during the 1990s will influence not only how manufacturing will be done, but also what is manufactured. The size of many manufactured goods continues to decrease, resulting in ultra-miniature electronic devices and new hybrid technologies. For example, micro-electromechanical system, or MEMS, devices integrate physical, chemical, and even biological processes in micro- and millimeter-scale technology packages. MEMS devices are used in many sectors: information technology, medicine and health, aerospace, automotive, environment, and energy, to name a few.

    The future relies on the integration of nanotechnology with existing technology. The challenge remains in integrating nanotechnology with microelectronics-based technology. The nanometer-scale components have to be connected with micrometer- and millimeter-scale components to communicate with the real world. This requires an integration of not only the technologies covering nanometer-to-millimeter multilength scales, but also the physics and chemistry covering the entire length scale. We are facing the merging of quantum mechanics and classical mechanics. Any ultra small components have to be connected with the real world. The goal should be on how to use nanotechnology to make microtechnology more efficient, multifunctional, and intelligent as well as faster, smaller, and achieving the impossible. Nanotechnology comes to life if we can achieve the integration of nanoscale building blocks with lithographically produced structures through self-assembly and genetically engineered growth.

Building nanomanufacturing standards

    Nanomanufacturing needs the measurements and standards required to achieve effective and validated nanoscale product and process performance. This challenge is mainly in the following three directions:

Atomic-scale manufacturing: Develop and assemble the technologies required to fabricate standards that are atomically precise. This will include work directed at solving artifact integrity, precision placement, dimensional metrology, and manufacturing issues.

Molecular-scale manipulation and assembly: Identify and address the fundamental measurement, control, and standards issues related to manipulation and assembly of microscale or nanoscale devices using optical, physical, or chemical methods. This entails building the manipulation technology and using it to understand and address the measurement issues that arise when assembling devices at the microscale or nanoscale level.

Micro-to-millimeter-scale manufacturing technologies: Develop the technologies required to position, manipulate, assemble, and manufacture across nanometer-to-millimeter multilength scales.

Not just an engineering process

    Traditionally, manufacturing is attributed to an engineering field. For nanomanufacturing, we must go beyond engineering. Once we approach the atomic-scale precision and control, fundamental physics and chemistry have to be applied. The nanoscale manufacturing is multidisciplinary -- involving but not limited to mechanics, electrical engineering, physics, chemistry, biology, and biomedical engineering. The future view of nanomanufacturing is the integration of engineering, science and biology. This complex task requires not only innovative research and development themes, but also a new education system for training future scientists and engineers.