β-Carotene on Au(111)ALI-1000 Deposition System
The biomolecule β-Carotene, pictured below, is a hydrocarbon pigment abundant in plants and fruits, and the precursor to vitamin A.
The ALI deposition system was tested with this molecule: a solution of β-Carotene in Tetrahydrofuran (THF) was pulse injected in UHV onto a Au(111) substrate. The sample was subsequently characterized by XPS and STM, showing that deposition was successful. The STM image on the left shows a layer of β-Carotene, showing order in small patches with different configurations.
The XPS spectrum of the sample before (black) and after (red) injection with ALI is displayed below. The O1s, C1s and Au4f core levels are shown.
In the spectrum after injection, an intense C1s signal is observed. This of course is associated with the β-Carotene (a hydrocarbon) that has arrived at the sample. Interestingly, an O1s signal, which would indicate the presence of the solvent THF on the sample, is completely absent, showing that the solvent THF does not adsorb to the Au(111) surface (and that the C1s signal is associated only with the solute, β-Carotene).
In the STM images (above) of β-Carotene/Au(111) patches of molecular order can be seen, in which the molecules appear to lie in different configurations.
It is worth noting that β-Carotene is very susceptible to oxidation, and is therefore stored in a Nitrogen atmosphere at -4ºC. In powder form, the molecule is quickly oxidized when it comes into contact with air and/or exposed to light, changing its color from dark brown to orange. In solution the process is slowed down, and so by injecting from solution the molecule can be deposited in its unoxidized state. Even so, if exposed to light for a prolonged period, the solution will turn from dark red to yellow, indicating oxidation.
Courtesy of F. Himpsel and C. Rogero (NanoPhysics Lab, Centro de Física de Materiales CSIC-UPV/EHU, San Sebastián, Spain)
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