Controlled nanofabrication of metal-free SERS substrate on few layered black phosphorus by low power focused laser irradiation

Abstract

Black Phosphorous (BP) has intrinsic in-plane ferroelectric properties that may have the inherent capability of SERS response and can be considered as a replacement of metal nanoparticle-based SERS substrates. A simple one-step process has been demonstrated for the controlled nano-structuring and rapid prototyping on a BP flake to develop a metal-free SERS substrate by low power focused laser irradiation. The effect of focused laser irradiation on the surface morphology of the pristine BP flakes has been thoroughly investigated by real time Raman spectroscopy measurements and corresponding AFM height profiling, which confirms that the proposed laser irradiation technique has more advantages over the conventional lithography and is free from undesired contamination. For a 532 nm laser line, the minimum laser power needed to create a nano-void on the BP flake is 25 mW (Power density = ∼15.62 × 10⁵ W cm⁻²) with 5 s exposure time, where the etching rate is controlled by the laser power and exposure time. By analyzing the geometrical shape of the nano-void created due to laser irradiation, it is possible to identify the armchair and zigzag directions of the BP flake. The experimental results revealed that by controlling the exposure time and laser power, it was possible to perform layer by layer thinning of BP flakes. The proposed thinning process of the BP flake did not alter the pristine quality and no signature of oxidation was found in the Raman spectra, which signified the reliability of this low power laser irradiation technique towards the future nano-fabrication of BP-based devices. The controlled formation of the nano-void array on a few layered BP flake enhanced the local electric field (hot spots) in the vicinity of the nano-voids, resulting in ∼30% Raman intensity enhancement. Such nano-void induced hotspots on the BP flake open up a new species of metal-free SERS substrate, demonstrating pronounced enhancement in the Raman signal of Rhodamine B as high as ∼10⁶ and a limit of detection (LOD) up to ∼10 nM.