Browsing by Author "Avishan, N."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Open Access Laser photochemical nanostructuring of silicon for surface enhanced raman spectroscopy(Wiley-VCH Verlag GmbH & Co. KGaA, 2022-07-18) Akbıyık, A.; Avishan, N.; Demirtaş, Ö.; Demir, Ahmet Kemal; Yüce, E.; Bek, A.In this work, a novel method of fabricating large-area, low-cost surface-enhanced Raman spectroscopy (SERS) substrates is explained which yields nanostructured surface utilizing laser-induced chemical etching of crystalline silicon (Si) in an hydrofluoric acid solution. Nanostructuring of Si surface is followed by deposition of a thin noble metal layer to complete the fabrication procedure. A 50 nm thick silver (Ag) layer is shown to maximize the SERS performance. The SERS effect is attributed to the electromagnetic field enhancement originating from the nanoscale surface roughness of Si that can be controlled by the illumination power, etch duration, and the spot size of the laser beam. The SERS substrates are found to be capable of detecting a Raman analyte dye molecule down to 10−11 m. SERS performance of the Ag deposited substrates are compared to gold (Au) deposited substrates at 660 and 532 nm excitation. Nanostructured Si surface with Ag exhibits stronger SERS than with Au under 532 nm excitation exhibiting an enhancement factor as high as 109. Raman enhancement factor is calculated both by SERS spectra experimentally, and using finite-elements simulation of the electric field enhancement. The applicability of the fabricated substrates is examined by adsorbing different analytes.Item Open Access Ultra-narrow linewidth photo-emitters in polymorphic selenium nanoflakes(Wiley, 2022-07-12) Hussain, N.; Ahmed, S.; Tepe, H. U.; Huang, K.; Avishan, N.; He, S.; Rafique, M.; Farooq, U.; Kasirga, Talip Serkan; Bek, A.; Turan, R.; Shehzad, K.Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τc) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of −0.45% cm−1 in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics. © 2022 Wiley-VCH GmbH.