Fluorescence Spectroscopy: Solid Sampling & Cryogenic Applications

Fluorescence spectroscopy is considered a mature technique, but its sensitivity and selectivity could enable its use in new application areas. Participants in this Technology Forum are Jim Mattheis of Horiba Jobin Yvon, Rob Morris and Monde Qhobosheane of Ocean Optics, and Michael W. Allen of Thermo Fisher Scientific.

Although typically fluorescence is thought of as a technique for analyzing liquid samples, what new applications for solid sampling are accessible by fluorescence spectroscopy?

Mattheis: New applications of solid sample fluorescence include
• In the study of semiconductor function and production molecular fluorescence can provide significant information.
• The presence of oil and oil quality can be measured by fluorescence both macroscopically and microscopically in solids such as shale and other rock formations.
• Fluorescence can be used for measuring food quality.
• Gemstone quality and origin can be studied using room temperature and low temperature fluorescence.
• Fluorescence signatures of currency, ink and stamps are used for authentication and other security measures.
• The fluorescence lifetime of photovoltaic material can reveal defects that reduce the efficiency of the process.

Morris and Qhobosheane: This isn’t quite new, but there are folks who have taken advantage of clever fiber-optic probe designs to optimize signal collection and make fluorescence measurements of solids such as biological samples. One of the more surprising applications we’ve encountered involves measuring the fluorescence of minerals — spectral response that is used to help identify minerals at deposit sites.

Allen: While fluorescence spectrometers have a long history of analyzing solid samples such as gemstones and mineral samples, new applications are appearing daily. For samples such as solar cells, OLEDs, and polymer films, fluorescence is emerging as a popular analysis tool. As new sample types emerge, it is critical that instrumentation change to support these samples.

A solid sample will have much sharper fluorescence peaks due to the elimination of nonradiative relaxation pathways available in the liquid phase. This makes resolution capacity, achieved through a narrow spectral bandwidth, of the fluorescence spectrometer a key driver. Many applications in solid materials research require this resolution capability to get a complete picture of the sample’s characteristics.

Accessories for measuring solid samples are also important. For example, solar cell researchers need to adjust the position of the sample in the excitation beam to mimic the motion of the change in angle of incidence as the sun traverses the sky. Fiber-optic probes for measuring fluorescence are also advantageous as they eliminate the need to dissect the sample in order to fit it in the sample compartment of the spectrometer.

Fluorescence emission is temperature dependent — what information can be obtained from samples at cryogenic temperatures that is not accessible at room temperature?

Mattheis: Cryogenic applications of fluorescence include
• Enhanced sensitivity of fluorescence and phosphorescence.
• Enhanced spectral resolution for identification and quantitative studies.
• Characterize bandgap changes in semiconductors due to material treatment.
• The study of intrinsic anisotropy of fluorophores.• Mechanisms of photochemical reactions.

Morris and Qhobosheane: We’re not experts in this area by a long shot, but certainly where you have samples with discrete emission wavelengths such as quantum dots, the ability to discriminate them more precisely may be important. At room temperature, the same emission peaks may be broader, resulting in overlap.

Allen: Analyzing samples at cryogenic temperatures essentially turns the sample into a solid matrix as not many compounds are liquids at 77 K. Of particular interest are aromatic compounds that show very different spectroscopic characteristics at cryogenic temperatures. The solid matrix changes intra- and intermolecular forces allowing spectroscopists to observe transitions that are blurred in a liquid matrix. Again resolution capacity, achieved through a narrow spectral bandwidth, is critical to derive as much information as possible from these samples. Many distinctive spectral characteristics appear only under cryogenic conditions. Examples include identification of soil contaminants including uranium, detection of adulterations in gemstones, and polarization measurements to elucidate charge transfer mechanisms.