Enhanced Photoresponse in Phosphotungstic Acid-Functionalized Black Phosphorus Transistors—Insights from Scanning Photocurrent Microscopy

Black phosphorus (BP) is regarded as a promising 2D van der Waals material for optoelectronic applications due to its unique properties, such as a direct bandgap and high carrier mobility. However, BP rapidly oxidizes upon exposure to air, degrading its properties and reducing device performance. To prevent such oxidation-induced structural deformations, passivation layers like alumina have been applied, but they limit the photoresponse properties of BP. To address this issue, we aimed to improve the photoresponse properties of alumina-passivated BP transistors by functionalizing them with phosphotungstic acid (PTA).<br/><br/>Phosphotungstic acid (H₃PW₁₂O₄₀) belongs to the class of heteropoly acids with a Keggin-type structure, consisting of a central phosphate (PO₃^3-) ion surrounded by twelve tungsten atoms (W) coordinated with oxygen atoms, forming the complex anion [PW₁₂O₄₀]^3-. Due to its strong acidity and versatile solubility in water and many organic solvents, PTA is used in various applications, including as a staining agent in biology, in catalysis for organic synthesis, in the dye industry, and in corrosion inhibition.<br/><br/>By utilizing the high electron affinity of PTA, we confirmed the successful attachment of PTA to the BP surface using atomic force microscopy. Comparing the transfer characteristics of PTA-functionalized and non-functionalized BP transistors revealed that while both devices exhibited similar transfer characteristics, the functionalized devices demonstrated significantly enhanced photoresponse properties. Using scanning photocurrent microscopy (SPCM) under various bias conditions, we quantitatively analyzed the impact of PTA functionalization on the photoresponse of alumina-passivated BP transistors.<br/><br/>SPCM analysis provided a clear understanding of how PTA modifies the spatial photoresponse of BP transistors. Notably, the maximum photocurrent point was observed at the center of the BP channel when a 100mV drain-source voltage (<i>V</i>_ds) was applied. In contrast, the maximum photocurrent point in non-functionalized BP transistors occurred at the drain (or source) contact edge, indicating that the Schottky barrier-induced energy band bending is the dominant factor generating electron-hole pairs under light illumination. Additionally, PTA-functionalized BP transistors exhibited significantly higher photoresponsivity compared to non-functionalized devices, with the photocurrent increasing from 27nA to 285nA under a -100mV V_ds, representing an increase of over 10 times. This is attributed to PTA's high electron affinity, which facilitates efficient electron transfer from BP to PTA, leaving holes in the BP and suppressing electron-hole recombination. This mechanism results in higher photocurrent generation and a more uniform photocurrent distribution.<br/><br/>The combined effect of Al₂O₃ passivation, which preserves the intrinsic properties of BP and maintains high carrier mobility, and PTA functionalization significantly enhances the photoresponse of BP transistors. This study demonstrates that chemical functionalization with compounds like PTA can effectively improve the photoresponse of 2D van der Waals materials. These findings provide important insights for optimizing the performance of optoelectronic devices based on 2D van der Waals materials.<br/><br/><i>(1) Na, J.; Park, K.; Kim, J. T.; Choi, W. K.; Song, Y.-W., Air-Stable Few-Layer Black Phosphorus Phototransistor for near-Infrared Detection. Nanotechnology <b>2017,</b> 28, 085201.</i>