Duncan Stacey, Sales & Marketing Director, Linkam Scientific Instruments
Understanding the micro- and thermo- mechanical properties of materials is increasingly important. Existing materials are being deployed in new environments and exotic new materials are being developed to meet ever higher demands. Hot stage microscopy (HSM) or Temperature-controlled Microscopy covers a wide range of analysis techniques from basic study of a material at a different temperature, to full thermal analysis techniques that combine traditional thermo-analytical techniques, such as differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA), with microscopy.1 By merging these techniques, users can bring temperature control into their analysis as a testing parameter, allowing them to visualise and analyse the effects of temperature on samples in a variety of research fields, such as geology, pharmaceuticals, space science, and materials research.
The importance of temperature in understanding the behaviour and potential uses of materials being analysed is gaining more and more attention. Here, we discuss a selection of key applications of HSM, and the benefits this analytical technique can offer.
HSM and Other Analytical Methods
A useful technique both on its own and in conjunction with other characterisation techniques, HSM can be used with a variety of methods, such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, polarised light microscopy, and even X-ray and SAX/WAX. The introduction of computer-controlled programmable HSM offers further advantages when coupled with data analysis software. Such software can allow researchers to run multiple heating and cooling cycles during the same thermal experiment, enabling clearer results that offer a more complete picture during in-situ experiments.1
Ellipsometry is another technique that can benefit from temperature-controlled microscopy. Ellipsometry measures the changes in polarisation state of a light beam reflected or transmitted from a material structure,2 and is used mainly to characterise thin film and bulk material properties. Thin films are extremely thin layers of materials that can be used on a surface to modify its physical and optical characteristics. Properties such as electrical conductivity, hardness, and lubricity can also be adjusted through using thin films, to protect the surface from corrosion. To better understand the optical properties of thin films, it is important to recognise the role played by temperature and how this can affect the material. Temperature-controlled ellipsometry enables researchers to characterise a range of properties, such as thermal expansion coefficients (changes in thickness relating to temperature), surface-surface interfaces and temperature-dependent optical properties (a necessary insight for many applications), silicon photovoltaic panels, where quantum efficiency is temperature dependent, and satellites, which must be able to function across a wide range of temperatures.
The Heat is on in Pharma and Biopharma
HSM has many uses in pharmaceutical and biopharmaceutical research, such as obtaining data on the morphology of pharmaceutically relevant compounds such as active pharmaceutical ingredients (APIs) and interactions with the excipients. HSM allows researchers to observe the crystallisation process, melting/boiling points, polymorphisms, desolvation, and glass transitions, amongst other transitions that may take place within a sample.1
HSM has been used by researchers at the University of Glasgow in the study of phase transitions and crystallisation process of mixed liquid systems. Temperature control allows researchers to examine the phase transitions between liquids at high temperature. Controlling crystallisation is important when developing medications such as therapeutic proteins, as small tweaks to solvents can lead to issues such as the formation of different polymorphs, or failure of the protein to crystallise altogether. As such, results from experiments such as this are of interest to biopharmaceutical developers. By bringing in temperature as a testing parameter, the team was able to observe the small changes within a sample to predict how the sample would respond, meaning that developers may, in future, be able to retain some element of control over these factors.3
From Solar Power to Space Science
HSM is also widely used in semiconductor applications. Perovskite materials have received significant interest over the last few years, particularly as a material for photovoltaics or solar cells. Sample preparation of the perovskite has been found to be key to ensuring the performance of the material with changes in the temperature and other environmental conditions affecting the formation of different crystalline structures, which gives the material the desirable magnetic, electric, and superconducting properties for these applications.
To learn more about this material and its lifetime efficiency in solar cells, researchers at the University of Swansea recently used Raman spectroscopy to probe the degradation of perovskite thin film photovoltaic devices.4 Relative humidity (RH) and HSM were used to conduct in-situ Raman spectroscopy, to better understand the effects of temperature and humidity on degradation kinetics. Using a Linkam THMS600-H temperature stage allowed researchers to see how the material degraded between temperatures of -193°C and 26°C, which provided evidence for novel transitions within the perovskite that disappear at ambient temperatures. These transitions were also found to be magnetically sensitive, and these magnetic interactions could potentially be tuned and optimised for other optical applications.4
HSM is not just limited to terrestrial applications. The Perseverance 2020 mission, which landed on Mars in February 2021 as part of the ExoMars programme, has been in the headlines recently – and temperature-controlled microscopy is set to play a role. Geochemical activity on Earth and Mars are believed to have been similar in the past, so a key element of the Mars mission is the analysis of sulfate content from Martian samples. These samples are expected to contain evidence of other mixed sulfates, which could contain remnants of past microbial life and organic matter, similar to those found on Earth.
Researchers at the University of the Basque Country in Leioa, Spain, coupled temperature control to Raman spectroscopy, to examine gypsum, syngenite and görgeyite – the three mineral phases expected to be present on the red planet. These expected mineral phases potentially contain crystallised water, which is particularly sensitive to temperature changes. The team at the University of the Basque Country used HSM to control the temperature in these experiments, after which the sulfates were characterised with Raman and Near Infrared Spectroscopy to build a database of sulfates. With this database, researchers will be able to quickly interpret the analysis of sulfate-containing samples from Mars.
How Can Thermal Analysis Help Me?
Adding temperature control to your microscopy analysis can provide many benefits. Temperature is a key factor in the behaviour of materials and samples, and heating or cooling can provide key insights into how the sample behaves, degrades, crystallises, or melts/boils. This information is vital in sample characterisation, especially in areas such as drug development, solar power, and geology.
Thermal testing also opens the door for humidity testing, which allows researchers to measure the effects of humidity on samples, and how this may impact behaviour and degradation of materials. By controlling heat and humidity within a sample, researchers can create an environment in which to monitor these changes. Analysing and visualising how samples and materials behave under certain conditions offers a wealth of additional information and a better understanding of how the materials perform in real-world environments. Temperature and environment control can be additionally combined with mechanical testing such as tensile, compression and shear testing.
In pharmaceuticals, this insight allows researchers to understand details including how to maintain drug efficacy, such as ensuring APIs and finished products are stored in the right conditions before going to market. In photovoltaics, it allows researchers to evaluate the lifetime of certain materials within solar cells, and how these can be maximised and measured to ensure longevity. In space science and geology, it allows researchers to recreate the conditions of other planets and environments. The list of possible applications for thermal microscopy is extensive, and researchers from a variety of fields and industries can benefit from temperature control and HSM.
A range of temperature-controlled stages for thermal analysis are commercially available from manufacturers such as Linkam, so researchers can learn more about how samples behave across temperatures from -195°C and 1500°C.
References
- Kumar, A., Singh, P., Nanda, A. Hot stage microscopy and its applications in pharmaceutical characterization. Appl Microsc. 2020; 50(1):12. doi:10.1186/s42649-020-00032-9, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7818341/
- ‘What is Ellipsometry?’ J.A. Woollam, https://www.jawoollam.com/resources/ellipsometry-tutorial/what-is-ellipsometry
- Walton, F., Wynne, K. Control over phase separation and nucleation using a laser-tweezing potential. Nature Chem10, 506–510 (2018). https://doi.org/10.1038/s41557-018-0009-8
- K. E. A. Hooper, H. K. H. Lee, M. J. Newman, S. Meroni, J. Baker, T. M. Watson and W. C. Tsoi Probing the degradation and homogeneity of embedded perovskite semiconducting layers in photovoltaic devices by Raman spectroscopy, https://doi.org/10.1039/C6CP05123E
About the Author: Duncan Stacey, Sales and Marketing Director at Linkam Scientific Instruments, joined Linkam in 2014, bringing with him a wide ranging knowledge of scientific digital imaging and spectroscopy markets, technologies and applications. Before joining Linkam, Stacey worked in sales, marketing and product development for some of the leading photonics companies in imaging, spectroscopy and microscopy. He earned his PhD in optics and spectroscopy from the University of Liverpool.