Enhancement in the Stability of Porous Silicon Using Electrochemical Grafting of Polymer 2,6 Dihydroxy Naphthalene
Authors: Dutta Susomon , Johnny Mejo A. , Suresh Sriram , Choudhury Tanushree Holme , Padmanabhan Revathy 
Published: August 2024

While there have been many advancements related to porous silicon, the reactivity of silicon with the ambient (particularly in environments with oxygen and humidity) has limited its use and integration in a wide variety of applications. Therefore, it is important to passivate the surface of nanostructured/porous silicon. To this end, there have been many reports in literature where passivation of bare (or unpassivated) porous silicon (UPSi) surface has been performed using gas phase techniques (thermal carbonization) and solution-based methods. In this work, we have studied the influence of the electrochemical deposition of polymer 2,6 dihydroxynaphthalene (2,6 DHN), in enhancing the stability of porous silicon surfaces.

Porous silicon was synthesized by the electrochemical etching of pre-cleaned p-type silicon substrates with resistivity of about 0.001 Ω-cm to 0.005 Ω-cm, in a PTFE cell that we have fabricated, with 48% HF and ethanol mixture (7:3) as the electrolyte solution, at a current density of about 1.08 A/cm². The polymer solution used for electrodeposition was prepared as a solution of 1 mM 2,6-dihydroxy naphthalene (2,6-DHN) in 100 mL of 1x phosphate buffer solution, which was vacuum infiltrated in the synthesized porous silicon and electrodeposited on the same with the standard three electrodes chronopotentiometry method, at a constant current density of about 2 mA/cm² for 120 seconds. Electrodeposited porous silicon was kept on the hot plate for 10 min at 220℃, followed by a thermal treatment at 600℃, for 180 min, in the presence of N2 flow (1 L/min).

The surface morphology of the unpassivated porous silicon (UPSi) and passivated porous silicon (PPSi) are shown in the insets of Fig. 1(a) and (b), respectively. This suggests the presence of an incredibly thin coating of the polymer on the pore walls. The pores in, both, unpassivated and passivated porous silicon samples (UPSi and PPSi) remained open and unobstructed, offering a uniform distribution of pores with sizes ranging between 5 nm and 20 nm.

To perform a corrosion test of the UPSi and PPSi, we look at these layers after they have been exposed to 1 M NaOH aqueous solution, for different time durations. We have seen the formation of a spontaneous bubble on the surface of the UPSi sample during the test. To this end, we have observed visible and morphological changes of both UPSi and PPSi samples after the corrosion stability test for 5 seconds and 120 minutes, respectively, as seen in Fig. 1(a) to (h). Due to the very high internal area and the hydrogen-terminated surface, in an aqueous solution, UPSi is oxidized by water at room temperature and degrades. In comparison, the PPSi sample is stable for more than 120 minutes.

Electrochemical characterisation of UPSi and PPSi was performed in a 10 mM NaCl aqueous electrolyte with a three-electrode cyclic voltammetry (C-V) setup. C-V measurements of the UPSi sample show strong redox reaction in the potential window of 0 V to 1 V, whereas, the PPSi sample shows a nearly constant charging current response in the potential window of -1 V to 1 V, as shown in Fig. 1(i). These results emphasize that the passivation of porous silicon using 2,6 DHN polymer makes it more stable and less reactive in an aqueous medium.

Figure 1