engineering

Photothermal microspectroscopy

Photothermal microspectroscopy is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photothermal microspectroscopy rather than just read about it. In short: Photothermal microspectroscopy (PTMS), alternatively known as photothermal temperature fluctuation (PTTF), is derived from two parent instrumental techniques: infrared spectroscopy and atomic force microscopy (AFM). In one particular type of AFM, known as scanning thermal microscopy (SThM), the imaging probe is a sub-miniature temperature sensor, which may be a thermocouple or a resistance thermometer.

Key takeaways

Photothermal microspectroscopy belongs to engineering; place it in that map before memorising details.
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Reference excerpt

Photothermal microspectroscopy (PTMS), alternatively known as photothermal temperature fluctuation (PTTF), is derived from two parent instrumental techniques: infrared spectroscopy and atomic force microscopy (AFM). In one particular type of AFM, known as scanning thermal microscopy (SThM), the imaging probe is a sub-miniature temperature sensor, which may be a thermocouple or a resistance thermometer. This same type of detector is employed in a PTMS instrument, enabling it to provide AFM/SThM images: However, the chief additional use of PTMS is to yield infrared spectra from sample regions below a micrometer, as outlined below.

Technique The AFM is interfaced with an infrared spectrometer. For work using Fourier transform infrared spectroscopy (FTIR), the spectrometer is equipped with a conventional black body infrared source. A particular region of the sample may first be chosen on the basis of the image obtained using the AFM imaging mode of operation. Then, when material at this location absorbs the electromagnetic radiation, heat is generated, which diffuses, giving rise to a decaying temperature profile. The thermal probe then detects the photothermal response of this region of the sample. The resultant measured temperature fluctuations provide an interferogram that replaces the interferogram obtained by a conventional FTIR setup, e.g., by direct detection of the radiation transmitted by a sample. The temperature profile can be made sharp by modulating the excitation beam. This results in the generation of thermal waves whose diffusion length is inversely proportional to the root of the modulation frequency. An important advantage of the thermal approach is that it permits to obtain depth-sensitive subsurface information from surface measurement, thanks to the dependence of thermal diffusion length on modulation frequency.

Applications The two particular features of PTMS that have determined its applications so far are 1) spectroscopic mapping may be performed at a spatial resolution well below the diffraction limit of IR radiation, ultimately at a scale of 20-30 nm. In principle, this opens the way to sub-wavelength IR microscopy (see scanning probe microscopy) where the image contrast is to be determined by the thermal response of individual sample regions to particular spectral wavelengths and 2) in general, no special preparation technique is required when solid samples are to be studied. For most standard FTIR methods, this is not the case.

Related technique This spectroscopic technique complements another recently developed method of chemical characterisation or fingerprinting, namely micro-thermal analysis (micro-TA). This also uses an “active” SThM probe, which acts as a heater as well as a thermometer, so as to inject evanescent temperature waves into a sample and to allow sub-surface imaging of polymers and other materials. The sub-surface detail detected corresponds to variations in heat capacity or thermal conductivity. Ramping the temperature of the probe, and thus the temperature of the small sample region in contact with it, allows localized thermal analysis and/or thermomechanometry to be performed.

References

Further reading F L Martin & H M Pollock (2010). "Microspectroscopy as a tool to discriminate nano-molecular cellular alterations in biomedical research". In A V Narlikar & Y Y Fu (eds.). Oxford Handbook of Nanoscience and Technology vol. 2. pp. 285–336. Hammiche, A; German, MJ; Hewitt, R; Pollock, HM; et al. (2005). "Monitoring Cell Cycle Distributions in MCF-7 Cells Using Near-Field Photothermal Microspectroscopy". Biophysical Journal. 88 (5): 3699–706. Bibcode:2005BpJ....88.3699H. doi:10.1529/biophysj.104.053926. PMC 1305516. PMID 15722424. Grude, Olaug; Hammiche, Azzedine; Pollock, Hubert; Bentley, Adam J.; et al. (2007). "Near-field photothermal microspectroscopy for adult stem-cell identification and characterization". Journal of Microscopy. 228 (Pt 3): 366–72. doi:10.1111/j.1365-2818.2007.01853.x. PMID 18045331. S2CID 23356282. M. J. Walsh; et al. (2008). "FTIR micro-spectroscopy identifies symmetric PO2- modifications as a marker of the putative stem cell region of human intestinal crypts". Stem Cells. 26 (1): 108–118. doi:10.1634/stemcells.2007-0196. PMID 17901405. Grude, O; Nakamura, T; Hammiche, A; Bentley, A; et al. (2009). "Discrimination of human stem cells by photothermal microspectroscopy" (PDF). Vibrational Spectroscopy. 49: 22–27. doi:10.1016/j.vibspec.2008.04.008. A Hammiche; et al. (2004). "Mid-infrared micro-spectroscopy of difficult samples using near-field PhotoThermal Micro-Spectroscopy (PTMS)" (PDF). Spectroscopy. 19: 20–42. Archived from the original (PDF) on 2011-07-11. Retrieved 2009-10-27., erratum in 19(5), 14 (2004) J. G. Kelly; et al. (2011). "Biospectroscopy to metabolically profile biomolecular structure: a multi-stage approach linking computational analysis with biomarkers". J Proteome Res. 10 (4): 1437–48. doi:10.1021/pr101067u. PMID 21210632. H.M. Pollock (2011). "Towards chemical mapping at sub-micron resolution: near-field spectroscopic delineation of interphase boundaries" (PDF). Materials Science Forum. 662: 1–11. doi:10.4028/www.scientific.net/msf.662.1. S2CID 43540112. H M Pollock; S G Kazarian (2014). "Microspectroscopy in the Mid-Infrared". In R A Meyers (ed.). Encyclopedia of Analytical Chemistry. pp. 1–26. L Bozec; et al. (2001). "Localized photothermal infrared spectroscopy using a proximal probe". Journal of Applied Physics. 90 (10): 5159–65. Bibcode:2001JAP....90.5159B. doi:10.1063/1.1403671. M J German; et al. (2006). "IR spectroscopy with multivariate analysis potentially facilitates the segregation of different types of prostate cell". Biophysical Journal. 90 (10): 3783–3795. Bibcode:2006BpJ....90.3783G. doi:10.1529/biophysj.105.077255. PMC 1440759. PMID 16500983. A M Katzenmeyer; et al. (2015). "Mid-infrared spectroscopy beyond the diffraction limit via direct measurement of the photothermal effect†". Nanoscale. 7 (42): 17637–17641. Bibcode:2015Nanos...717637K. doi:10.1039/c5nr04854k. PMID 26458223. P M Donaldson; et al. (2016). "Broadband near-field infrared spectromicroscopy using photothermal probes and synchrotron radiation". Optics Express. 24 (3): 1852–1864. Bibcode:2016OExpr..24.1852D. doi:10.1364/oe.24.001852. PMID 26906764.

Worked examples

Example 1 — a first encounter with Photothermal microspectroscopy

Start with the simplest possible case. Write down what Photothermal microspectroscopy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photothermal microspectroscopy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photothermal microspectroscopy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photothermal microspectroscopy

In research: Photothermal microspectroscopy appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photothermal microspectroscopy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photothermal microspectroscopy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Scanning probe microscopy, Spectroscopy, so understanding it makes those chapters shorter.
In everyday life: Look for Photothermal microspectroscopy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Photothermal microspectroscopy in 20 minutes

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Frequently asked questions

What is Photothermal microspectroscopy in simple terms?

Why does Photothermal microspectroscopy matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photothermal microspectroscopy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photothermal microspectroscopy.