Controlled Nanofabrication of Metal Free SERS Substrate on Few Layer Black Phosphorus by Low Power Focused Laser Irradiation
Black Phosphorous (BP), having intrinsic in-plane ferroelectric properties may have inherent capability of SERS response and could be considered a replacement of metal nanoparticles based SERS substrate. A simple one-step process has been demonstrated for controlled nano-structuring and rapid prototyping on BP flake to develop metal free SERS substrate by low power focused laser irradiation. The effect of focused laser irradiation on the surface morphology of pristine BP flakes has been thoroughly investigated by real time Raman spectroscopic measurements and corresponding AFM height profiling, which confirms that the proposed laser irradiation technique has more advantages over conventional lithography process and free from undesired contamination. For 532 nm laser line, the minimum laser power needed to create a nano-void on BP flake is 25 mW (Power density ~ 15.62 x 105 W/cm2) with 5 sec exposure time where the etching rate is controllable 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 direction of BP flake. Experimental results reveal that by controlling the exposure time and laser power it is possible to perform layer by layer thinning of BP flakes. The proposed thinning process of the BP flake does not alter the pristine quality and no signature of oxidation is found in the Raman spectra, which signifies the reliability of this low power laser irradiation technique towards the future of nano-fabrication of BP based devices. Controlled formation of nano-voids array on few layer BP flake induces enhanced local electric field (hot spots) at the vicinity of the nano-voids, resulting in ~ 30% Raman intensity enhancement. Such nano-voids induced hotspots on BP flake open up a new species of metal free SERS substrate, demonstrating pronounced enhancement in Raman signal of Rhodamine B as high as of the order of ~106 and with a limit of detection (LOD) upto ~10 nM.