Nanomaterials represent an active area of research that isn’t entirely new to science. These materials scale between one and 100 nanometers (nm). For comparison, a single human hair is approximately 60,000 nm in thickness while the DNA double helix has a radius of 1 nm. Today, nanomaterials are everywhere. From sunscreens, textiles, and clothing to the electronics that drive phones, computers, and cars, nanomaterials have infiltrated crucial aspects of modern technology.
Tunable properties at the nanoscale
Materials at the nanoscale demonstrate drastically different properties compared to their macroscale counterparts. At the nanoscale, quantum effects dominate and influence changes in a material’s physical, mechanical, optical, and chemical properties, including chemical reactivity, electrical conductivity, magnetic permeability, melting point, fluorescence, etc. For example, color is defined by a material’s interaction with light. Unlike the familiar yellow of bulk gold, nanoscale gold particles exhibit a reddish or purple hue. This behavior is directly related to the motion confinement of the gold’s electrons due to the reduction in size, resulting in gold nanoparticles reacting differently with light compared to bulk gold.
A fascinating consequence of these quantum effects is the “tunability” of material properties at the nanoscale. By changing the size of a material, the material property of interest could be fine-tuned toward specific applications. Nanoscale gold particles can be used for laser tumor therapy, especially with cancers. Their ability to selectively accumulate in tumors enables precise imaging and targeted destruction of the tumors using a laser while avoiding undue harm to nearby healthy cells. The ability to alter the color of a nanoparticle has also been put to practical use with nanomaterials commonly utilized as fluorescence markers for sensing and imaging.
Another reason for the differing behavior of nanoscale materials is that a large percentage of their atoms are located on the surface of the material. The resultant high surface-to-volume ratio of nanomaterials promotes greater chemical reactivity than found in bulk materials. The improved reactivity of nanomaterials has been utilized to create better catalysts with diverse applications for nanoengineered batteries, fuel cells, and artificial photocatalysis for clean energy and storage. By further functionalizing nanomaterial surfaces, innovations have been discovered in nanoscale drug delivery, water treatment and desalination, and insulated clothing.
Nanomaterial manufacturing: top-down approach
Manufacturing nanomaterials breaks down into two classes of methods: top-down and bottom-up. Top-down approaches consider moving from the very large to the very small, adopting some form of force to break bulk materials into nanomaterials. A top-down synthesis method takes a bulk material and etches out nanostructures by removing crystal planes already present on the substrate. It is an approach where the building blocks of the bulk material are removed to form the nanostructure.
Ball milling, for example, utilizes mechanical force to grind or blend materials through impact and attrition to smaller sizes. The coarse approach of ball milling is distinct from other top-down methods such as optical lithography. The working principle of optical lithography is similar to how photographic prints are developed; it is a refined and popular printing technique using electromagnetic energy or light to pattern an exposed surface and deposit material atop the stencil to produce desired geometries.
Optical lithography’s limitations are dictated by the resolution of the process. As light passes through an aperture on the photomask (containing the desired pattern) it diffracts and smears out. There is an interplay between the aperture size and position, and the wavelength of light utilized to provide for the best resolution. By fixing the size of the aperture, a sharper pattern can be obtained using smaller wavelengths. This ignores the presence of a lens to focus the illumination, but describes the significant role the wavelength of light plays in the process.
Technologies such as extreme ultraviolet (EUV) and X-ray lithography have been developed to further improve the resolution threshold and enable smaller lithographic patterns. Replacing light with ions and electrons has also led to electron beam lithography, where resolutions on the order of 1-10 nm are achievable. Methods to break the diffraction limit of light using metamaterials are also being considered in the development of a superlens. In general, the efforts toward achieving better resolution and reducing feature size are characteristic of the top-down approach.
Nanomaterial manufacturing: bottom-up approach
Bottom-up approaches consider a reverse process flow exclusively focusing on chemical synthesis and physical assembly techniques to build nanomaterials atom-by-atom or molecule-by-molecule. The development of refined microscopic technologies including the scanning electron microscope (SEM), scanning tunneling microscope (STM), transmission electron microscope (TEM), atomic force microscope (AFM), and their corresponding spectroscopic techniques ramped up research interest and innovation in bottom-up approaches for nanotechnology. A bottom-up synthesis method creates nanostructures from the ground up by stacking atoms onto one another, giving rise to crystal planes that further stack upon one another, and resulting in the final nanostructure.
Chemical synthesis produces rough materials that can be used directly in their bulk disordered form or as building blocks for more advanced architectures. Colloidal synthesis is representative of a chemical synthesis method and involves dissolving a metal salt or metal-containing complex in a solvent. The metal ions are reduced, resulting in the formation of small metal nuclei in the growth solution. With appropriate control of the reduction technique, time, and capping materials, diverse shapes and sizes of nanoparticles can be generated, including spheres, rods, cubes, disks, wires, tubes, and prisms. Colloidal solutions of nanoparticles are now commercially available for industrial and research purposes. Beyond colloidal synthesis, other exotic methods, such as self-assembly methods, take advantage of natural chemical and physical interactions between atoms or molecules to organize them into an ordered nanostructure. Positional assembly, a variant of this, allows for atoms, molecules, or clusters to be positioned freely one-by-one.
Top-down and bottom-up methods accompany selective differences in quality, speed, and cost. Top-down methods provide sophisticated and flexible tools for the synthesis and fabrication of nanostructures but are limited to minimum feature sizes on the order of 1-10 nm, at least for now. These methods dominate the microelectronics and semiconductor industries where precision engineering is key, although the advantages are offset by high cost and maintenance coupled with low-throughput and defect-ridden samples. In contrast, bottom-up methods represent a radical technological shift in fabricating nanomaterials. Moving atoms individually is a time-consuming process and researchers continue to seek efficient methods of building nanoscopic architectures. Overall, bottom-up methods provide for nanostructures with fewer defects, better homogeneity in chemical composition, and efficient short- and long-range ordering of components, albeit being laborious and time-consuming.
The future of nanotechnology
Nanotechnology is an emerging technology. At its crux is the fabrication of nanomaterials and products of great potential with substantial influence in human life. The success of futuristic technologies depends on the ability to effectively manipulate and create nanomaterials for various applications in electronics (advanced computing, nano-sensors), energy storage (hydrogen fuel and solar cells), environmental remediation (artificial photocatalysis, clean energy), medicine (drug delivery and cancer therapeutics), etc. Top-down and bottom-up approaches encompass diverse possibilities for synthesizing nanomaterials. Depending on the application of concern and experimental objectives, a combination of the two approaches is often utilized by researchers to fabricate nanostructures. Although critical challenges continue to exist in the development and effective utilization of nanomaterials, there remains much room at the bottom for fresh innovation and solutions.