Be Aware of Single-Molecule Emitters
If you put fluorescence emitters into a polymer matrix and measure their fluorescence, you should expect that the signal you receive is that from your fluorescence emitter. However, any fluorescent impurity could give a signal.
Frustrated by frequent contamination, Alexander Högele, Ludwig Maximilian University of Munich and Munich Center for Quantum Science and Technology (MCQST), Germany, and colleagues put different substrates and common polymer films to the acid test. They found widespread contamination by emitting organic molecules in all materials, including the popular poly(methylene methacrylate) (PMMA) films.
The Riddle of Quantum Contamination
When the researchers first performed low-temperature fluorescence spectroscopy with colloidal quantum dots, they were part of a research community that was ready to ascribe the rich fine structure in the spectra to exciting new interactions among quantum dot excitons and vibrational modes of the ligands.
Colloidal quantum dots are nanometer-sized structures, composed of a semiconductor core and embedded in a shell of ligands, typically a hydrocarbon compound. Due to their near-atomic size, quantum dots behave like intermediate systems between the quantum and the macroscopic world. With their distinct electronic transitions and sharp fluorescence signals at near-zero temperatures, they can electronically interact with surrounding molecules and structures, and, therefore, are expected to be useful materials for quantum information processing.
However, they also exhibit “dark states” where photoemission is extinguished. And if particles are dark, the odds are especially high that any contaminating fluorescent signal in the matrix would taint the measurement. This explains what Högele and his team encountered when they studied the fine structure of presumably photostable quantum dots: it was just contamination in PMMA.
“Of course, we could have stopped at this point,” says Högele, “but then we faced the riddle.” And the physicists were surprised to find that confusion of sample and contaminant signatures was actually widespread and frequent and had already led to numerous misinterpretations. “In comparison to dark emitters, the contaminants that eventually turned out to be single fluorophores are actually not so dark,” explains Högele.
Scrub Everything Clean?
To investigate sources for contamination in more detail, the scientists cleaned silica substrates with various procedures, starting with the standard protocol of sonication in acetone and isopropyl alcohol. On all substrates except one they detected a broad distribution of fluorescing “hot spots”, all in sub-micrometer size. The only non-emitting substrate was treated with oxygen plasma. Then they coated the plasma-treated substrate with a PMMA film. The fluorescing hot spots turned up again, and, being in the matrix itself, they could not be removed.
So, what was the origin of the fluorescence? The signatures measured at cryogenic temperatures pointed to hydrocarbon molecules, and especially the vibration spectra resembled those of anthracene-related molecules. But this result opens only a small window into what is possible as contaminants. Högele points out that similar spectra of a “molecule X” had been identified in other matrices and associated with a non-identified impurity before . As fluorescing organic impurities were found in all different kinds of matrices and on various surfaces, they likely stem from the preparation process of the chemicals, the team concluded.
Lessons to Learn
This research shows several things. First, not every exciting signal coming from a very small dot on a surface must be that of your sample. Secondly, fluorescence spectroscopy at cryogenic temperatures can indeed very well detect fluorescing single organic molecules. Third, inorganic surfaces can be completely freed from those contaminants by plasma bleaching, but, fourth, organic films cannot. Here, the contaminant is present in the bulk owing to the preparation process.
Apart from these lessons to learn, a new application may emerge: as the contaminating molecules are in fact single-photon emitters, they could be useful in applications that rely on quantum emitters, for example, quantum cryptography. “If quantum efficiency is not the most important factor so that, for example, efficiencies of five percent at ambient conditions or thirty percent at low temperatures can be tolerated, all you need is a thin film of PMMA,” suggests Högele.
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