Nanoscale Sensing Vitrification of 3D Confined Glassy Polymers Through Refractory Thermoplasmonics

Abstract

Advances in plasmonics have been fundamentally rooted in minimizing ohmic losses in metallic nanostructures. However, the losses at resonance can play a positive role; for instance, in optical heating, there are two sides to every story. Under laser illumination, plasmonic nanostructures serve not only as near-field enhancers but heat generators. The emerging field of thermoplasmonics opens up unprecedented possibilities to probe temperature-dependent phase transitions locally. In this paper, we develop a new approach behind plasmon-assisted optical heating for spectroscopically recognizing the glass transition temperature (T g) of spatially confined poly­(methyl methacrylate) (PMMA) polymers deposited on a square-shaped titanium nitride (TiN) pad. A local photoheating is controlled through Raman thermometry of a c-Si (100) substrate that functions as a temperature-sensing Raman reporter. The reliability of temperature measurements is corroborated by using both the anti-Stokes/Stokes ratio and the Raman peak shift. We show that optical heating can be adjusted by extruding a c-Si substrate, for example, the temperature increase is achieved by making c-Si pillars beneath the TiN pads longer. This peculiarity gives the possibility to probe the T g in a broad temperature range for the diversity of glassy polymers. We believe that the developed method will pave the way for 2D mapping structural glass transitions of heterogeneous glassy polymers, polymeric blends, and eventually, 3D confined polymers.

Heat and Temperature Localization via Fabry–Pérot Resonances at the Tip of a Nanofocusing Cone

“…The combined effect of nanofocusing and FP-like resonances provides a double degree of freedom when designing conical nanofocusing structures to take advantage of heat and temperature localization. This feature is an added benefit for in situ nanofabrication, [48,49] nanochemistry, [50] Raman nano-thermometry, [51,52] thermally-enhanced electron emission spectroscopy, [22] or photon-enhanced thermionic emission. [53]…”

Nanoscale Melting of 3D Confined Azopolymers through Tunable Thermoplasmonics

“…This means that the optical heating can be directly tuned by the height of Si pillars rather than the pump power. 13 In the context of a 2D plasmonic sensor, the height controls an operational temperature range, whereas the pumping intensity does on-demand temperature. At the T g event, the defrosting of vibrational and rotational modes results in the enhancement of Raman scattering that simultaneously plays a role of a glass-to-rubber transition indicator and a remote thermometer.…”

“…The polymer size (the film thickness) is of importance because the temperature drops according to a fractional power law. 13 Temperature gradients inside the shaded region of 50 nm are equal to 1 K/nm for the heights of 100 nm (green) and 250 nm (red). This indicates that not only the chemical nature of a polymer but also its size plays an important role for choosing the height of Si pillars.…”

“…Below we will use a CW 632.8 nm laser light which enables the 3D TiN:Si voxel to be heated. In our previous work, 13 a poly[(methyl methacrylate) (PMMA) microstructure was chosen to develop a method for probing the glass transition temperature based on the temperature-dependent intensity of symmetric vibrational modes at 810 cm –1 (C–O–CH 3 ) and 1460 cm –1 (O–CH 3 ), 26 which are marked with an asterisk in Figure 2 . The azochromophore covalently attached to the PMMA backbone provides a broader set of spectral probes.…”

“…The temperature increment at a given pump power is controlled the height of Si pillars, so both the curves provide similar results: T g = T 0 + Δ T = 112 ± 5 °C, where T 0 is the room temperature (red circles) and T g = 115 ± 5 °C (blue squares). The quantification of the glass transition temperature was performed by using a cumulated Pearson’s correlation method developed in ref ( 13 ). In contrast to resistive heating, the rate of optical heating is almost instantaneous, and it is less than 1 μs.…”