Enhanced patterned fluorescence from polystyrene through focused electron beam irradiation under various gases

We report on a novel method for tuning and enhancing fluorescence from irradiated polystyrene through focused electron-beam exposure in gaseous environments. We describe the effect of electron dose and ambient gas on the photoluminescence (PL) spectra and yield of irradiated polystyrene films on insulating and conductive substrates. Polystyrene films were exposed in an environmental scanning electron microscope using a 20 keV electron-beam, ambient gas pressures from<10^-5 mbar (high vacuum) to 3 mbar, and electron doses from 1.8 to 45 mC cm^-2. Irradiated polystyrene films were characterized using confocal microscopy, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and Fourier transform infrared (FTIR) spectroscopy. From emission spectra collected using confocal microscopy we found that the emission wavelength and photon yield of the irradiated film can be tuned by both dose and gas pressure. The emission wavelength blue-shifts with increasing pressure and red-shifts with increasing dose enabling an overall tuning range of 451-544 nm. Significant enhancement in the PL intensity, up to 18 times on sapphire substrates under helium when compared to high-vacuum, are observed. Overall, the highest PL yield is observed on soda lime glass substrates under argon. Also, the photon-yield on conductive substrates is significantly smaller than that yield from insulating substrates. TEM images revealed electron-beam irradiated polystyrene is amorphous in nature and elemental mapping EDS revealed no signs of film oxidation. FTIR spectroscopy revealed that under gaseous environments the decay of aromatic and aliphatic C-H stretches is reduced compared to the high vacuum exposure; in all cases, features associated with the phenyl rings are preserved. Localized electron-beam synthesis of fluorophores in polystyrene can be controlled by both dose and by ambient gas pressure. This technique could enable new approaches to photonics where fluorophores with tunable emission properties can be locally introduced by electron-beam patterning.