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Innovative Manufacturing of Nanoscale Calibration Spheres


Nanoscale calibration spheres serve an essential role in dimensional metrology, basically as a means of in-situ size calibration for industrial, scientific and medical applications. Modern usage has increased with the importance of detecting and characterizing nanoparticle materials in the environment. NIST has long been a source of particle standard reference materials such as polystyrene latex (PSL) spheres [Refs. 1- 4] but the quality of its presently available items has not kept pace with current applications and instrumentation. The unmet need is that commercial production methods for PSLs are based on 1950’s-era R&D, sufficient for industrial processes but not optimized, or appropriate, for current application requirements as nanoscale size transfer standards. The consequence is that the average size, size distribution, shape, and material properties vary considerably and rapidly degrade as particle size decreases, especially below 100 nm diameter.

There are many commercial vendors of calibration spheres and they rely on NIST reference materials for traceability. But bad reference materials result in poor metrological quality, i.e., traceability and uncertainty, since precision and accuracy of the measurement method are influenced by sample inhomogeneity.

In the interest of improving the practice of dimensional metrology, NIST recognizes that reviving R&D efforts in manufacturing of calibration spheres is essential for fulfilling NIST’s mission of disseminating the unit of length.

This subtopic focuses on significantly improving manufacturing methods for PSL spheres by the technique of emulsion polymerization. This industrially useful process has proven to be quite robust; nevertheless, it suffers from numerous secondary reactions which lead to highly polydisperse size distributions particularly for small average particle sizes. That the reaction conditions can be optimized to produce a very narrow, monodisperse size distribution was demonstrated in the production of the discontinued 100 nm diameter NIST SRM 1963 [Ref. 2]. This excellent size distribution was created by accident and has not been duplicated, either by the original manufacturer, JSR (Japan), or any other commercial vendor. However, it does offer a proof of principle and the historical literature offers numerous hints at alternative process conditions and choices of components which could result in the understanding and control necessary to synthesize PSL particles in the range of 20 nm to 100 nm diameter with a size distribution as good as or better than NIST SRM 1963. The project goal is the commercial realization of such high quality PSL calibration spheres for use as transfer standards by NIST and other national measurement institutes and for more general uses in industry and technology fields.

The expected outcome of Phase I is twofold: first, exploration of process control parameters and chemical constituents of the emulsion polymerization system will be undertaken. Rational understanding and control of the PSL particle size distribution parameters will be demonstrated. Multiple sizes below 100 nm will be synthesized and shown to possess a size distribution close to that of the NIST SRM 1963. Second, metrics to allow statistically significant comparison of the particle size distribution to SRM 1963 will be developed and validated. The expected outcome of Phase II will be the production of prototype amounts of PSL samples over the range of 20 nm to 100 nm diameter for which the particle size distribution is as good or better than that of the NIST SRM 1963. In addition, the stability and purity of the sample will be assessed for suitability as a viable commercial product.

NIST may be available to work with the awardee principally on choices of measurement and validation methods, such as atomic force microscopy, scanning electron microscopy, and dynamic light scattering.

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