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LASER DIAGNOSTICS AND SPECTROSCOPY
SERS-spectroscopy on microcrakes of metal coating of track-etched
membranes
N. Kovalets1, E. Kozhina2, S. Bedin1,2,3, I. Razumovskaya1, A. Naumov1
, 2
1-Moscow State Pedagogical University, Moscow, Russia 2- Lebedev Physical Institute RAS, Moscow, Russia 3- Center of Crystallography and Photonics of RAS, Moscow, Russia
bserg5@gmail.com
There are various methods for synthesizing SERS-active metasurfaces that allow achieving high enhancement and detecting ultra-low concentrations of various substances. One of the options for metasurfaces are substrates with nano-structures of a given geometry made of plasmonic metals, adjusted to the required resonant spectral range of the excited radiation source [1]. Another option for substrates is porous templates, for example, polymer track membranes (TM) chemically or physically coated with a plasmonic metal. Interest in the latter is largely related to the possibility of their application as flow systems for SERS spectroscopy of low concentrations of substances under study.
TM are a film with a system of calibrated pores. The pore diameter can vary from several tens of nanometers to several micrometers. If the surface of such a membrane is coated with a thin layer of plasmonic metal by vacuum deposition, then due to the presence of calibrated holes on the surface, a noticeable increase in the Raman signal from the detected compounds can be observed.
In this work, it was found that during uniaxial tension of a metallized track membrane in a metallized layer, a system of small pores arises on the surface, propagating between the pores. At small deformations, the distance between the crack edges is a few nanometers, which corresponds to the conditions for the formation of a gap hot spot between sharp crack edges.
In this work, commercial polymeric membranes were used (Dubna, Russia) made of polyethylene terephthalate (PET) with a thickness of 10 |im with a system of intersecting pores directed at an angle of about 43° to the membrane surface was chosen to create SERS-active metasurfaces 39. A pore diameter was 70 nm, and its surface density was 1,2*109 cm-2. A layer of silver or gold with a thickness of about 50 and 25 nm, respectively, was applied to one of the TM surfaces by thermal sputtering in a vacuum. For the uniformity of sputtering, TMs were placed on a rotating holder at an angle of 45 degrees to the sputtering source.
Samples of metalized TMs with a working part of 5x40 mm were cut using a REY RAN (Great Britain) manual cutting press. For uniaxial tension of the metallized TM, a manual tensile machine was used, which makes it possible to observe the deformation process on an optical microscope.
It was shown that the intensity of the Raman scattering signal of the characteristic malachite green peaks increases with increasing strain up to 15%. (Fig. 1) This effect seems to be associated with an increase in the number of small cracks. More surprising is the increase in signal intensity upon removal of deformation and relaxation of the metallized track membrane. In this case, the deformed polymer tends to return to its original size and tightens the edges of the cracks, pinching the malachite green molecules in the area of hot spots.
Рис. 1. Image of silver-metallized TM at 15% deformation and SERS spectra of malachite green adsorbed on the surface of TM at different deformations.
[1] E. Kozhina S. Bedin A., N. Nechaeva, S. Podoynitsyn, V. Tarakanov, S. Andreev, Y. Grigoriev, A. Naumov. Ag-Nanowire Bundles with Gap Hot Spots Synthesized in Track-Etched Membranes as Effective SERS-Substrates, Applied Sciences. Vol.11, pp. 1375, (2021)
