The effectiveness of nanomaterials depends on a range of factors, which include critical dimensions and chemical, mechanical, optical, and thermal properties. With increasingly stringent requirements for product performance, accurate metrology and characterisation becomes critical. The Centre of Excellence in Metrology for Micro and Nano Technologies (CEMMNT) provides access to equipment and expertise to accelerate nanomaterials commercialisation. This article highlights nanomaterials application examples.
Carbon nanotubes can be grown by passing carbon based gases over heated metal catalyst surfaces. Single and multi-wall nanotubes grow on the surface of the catalyst particles. It is critical to control both the tube type and tube length. Transmission Electron Microscopy (TEM) can be used to visualise the nanotubes and determine tube length and morphology. TEM additionally provides key information relating to nanotube properties, including the number of walls (single, double or multi-wall), tube diameter, wall roughness (which relates to graphitic crystallinity in the case of multi-walled tubes) and location of co-located species such as fillers or catalyst particles. In the catalysed growth of carbon nanotubes, Energy Dispersive X-ray Analysis (EDX) is used to identify the elemental compositions of individual metal particles in conjunction with TEM imaging to test for inter-relationships between catalyst particle size, composition and nanotube form and diameter (Figure 1).
Figure 1. TEM-EDX elemental analysis of individual metal catalyst particle compositions from a carbon nanofibre material. An example EDX spectrum is shown inset. (Copyright QinetiQ)
Nanofibres are being used in high-strength lightweight composite materials, which have obvious applications in the automotive, aerospace, and defence industries, as well as having increasing potential in the biomedical industry with devices such as surgical implants. The nanoscale dimensional composition and physical and thermal properties of composite materials is key to determining bulk material performance, and can be directly characterised. The Atomic Force Microscope (AFM) is a versatile tool that generates three dimensional images and surface property information by scanning a micro fabricated stylus (typical tip diameter 5-10 nanometres) with Angstrom precision. Of particular relevance to composite materials, Force Modulation Mode (FMM) is used to detect variations in the mechanical properties of a surface, such as elasticity, adhesion, and friction, by ramping the applied force at each imaging pixel and monitoring the cantilever deflection as a function of applied load. FMM also measures local stiffness and gives information on boundary interface between fibre and matrix. Figure 2a shows a fibre/polymer composite generated by AFM. Figure 2b shows an FMM image measured simultaneously, which highlights the higher stiffness of the embedded fibres. Although the interface boundary appears sharp, when imaged at higher resolution (Figure 2c) there is potentially evidence of local reduction in modulus of the matrix around the perimeter of the fibre. Nanoindentation can be performed on an AFM to determine additional mechanical properties, including quantitative stiffness, storage, and loss modulus.
Figure 2a (right). Topographic contact AFM image of fibre/polymer composite (Copyright National Physical Laboratory)
Figure 2b (left). FMM image of fibre/polymer composite (Copyright National Physical Laboratory)
Figure 2c. High resolution FMM image of fibre/polymer composite shows variation in interphase properties (Copyright National Physical Laboratory)
Healthcare and Cosmetics
As the global population ages, the market for healthcare and anti-aging products is flourishing. With increasing demand from consumers for more effective products, manufacturers are engaged in research and development of formulations containing nanoparticles. European cosmetics regulations require manufacturers to provide documented proof or claims they make concerning their products. Therefore, skincare and healing products must be extensively tested prior to commercialisation. Nanoscale comparisons between skin samples before and after treatment with anti-aging products enable manufacturers to quantify the efficacy of a product, test new compositions, and monitor the production of existing formulas.
One key measurement tool is optical profilometry, which gives Angstrom level vertical resolution. The technique is completely non-contact and non-destructive. Measurements are achieved by imaging silicone imprints of an area of the skin before and after a product has been applied. A wide range of application specific parameters can be calculated such as mean depth of a wrinkle, wrinkle volume, and developed surface. It is possible to analyse individual wrinkles, overall texture direction, and substrate complexity. Before and after results are compared to provide a method of quantifying product effectiveness. Figure 3, taken using a Taylor Hobson non-contact profiler, shows the analysis of a skin furrow for depth and volume after application of an anti-aging cream and subsequent replication. In addition to anti-aging creams, this process is also vital in testing skin healing products, anti-cellulite treatments, and lip care products.
Figure 3. CCI Interferometer analysis of wrinkle (Copyright Taylor Hobson).
Recent technology advances permit the thickness of multi-layer transparent thin films to be determined non-destructively and are likely to expand the market for this technique for applications such as measurements of passivation layers in the semiconductor industry and implant coatings in the medical devices sector.
An extensive armoury of nanoscale measurement, characterisation, and analytical techniques can be harnessed to enable new product and process development across nearly every industrial sector. Careful choice of multiple complementary analysis techniques provides the most comprehensive and accurate understanding of the material or device being analysed. The cost benefits are substantial but rely on both state of the art equipment and skilled operators familiar with not only the technology but also the applications for which it is being utilised.