Compositional-spread and temperature-gradient approaches
High-throughput materials synthesis
Multi-sample approaches allow us to systematically investigate the effect of composition or growth temperature on simultaneously grown thin films. To this purpose, we have developed a continuous compositional-spread method based on pulsed laser deposition that is applicable to metastable materials and heterostructures, thereby going well beyond the limitations of previous combinatorial synthesis techniques. Similarly, a temperature-gradient method can be used to determine the optimal film-growth temperature in a single run (Review, 2005), (see below for more details).
Continuous compositional spread
Compositional-spread approaches, in which a sample of continuously varying chemical composition is obtained by the simultaneous deposition of multiple constituents, have been used for more than 35 years. However, the earliest methods were based on naturally-occurring spatial deposition-rate variations, and led to simultaneous spatial variations of film thickness and deposition energetics. Alternative approaches based on moving shutters yield well-controlled composition variations, but on length scales that are too small for standard characterization techniques. The present approach is based on a moving substrate—more difficult to implement but yielding samples that can be investigated with conventional measurement techniques (RSI, 2003).
(Film growth with Isao Ohkubo, now at Tokyo University; ellipsometry: G.E. Jellison, Jr.; piezoresponse force microscopy: S. Kalinin)
Optimization of the growth temperature is a first – and often time-consuming – step in the development of a new material. Depositing simultaneously onto multiple samples (held at temperature from 200°C to 800°C) significantly increases productivity. Furthermore, the growth temperature-dependence of optical properties, crystallinity, and electro-mechanical behavior, can systematically be investigated. In the growth of materials for which systematic information about crystallization temperatures is lacking, this method allows us to quickly screen a large number of candidate materials for a particular application.
Application to the catalysis of carbon nanotubes
(Nanotube synthesis: A.A. Puretzky and D.B. Geohegan, microscopy: A.A. Puretzky and H.Cui)
The characteristics of nanomaterials are known to depend strongly on the size and activity of the catalyst nanoparticles responsible for their growth. Here we use the compositional-spread approach to quickly and systematically study the properties of multiwalled carbon nanotubes as a function of the metal catalyst composition. Tube height is used as first and most accessible parameter, but other properties (such as thermal conductivity and Raman spectra) are also investigated (NanoLetters, 2004).