Purely organic 4HCB single crystals exhibiting high hole mobility for direct detection of ultralow-dose X-radiation

Precise detection of low-dose X-radiation using purely organic direct detectors is vital for tissue-equivalent dosimeters and safety control in medical radiation treatment, but it still remains a challenge. Here, we report a promising organic radiation detector based on 4-hydroxycyanobenzene (4HCB, C₇H₅NO) single crystals. Plate-like 4HCB single crystals up to 18 × 15 × 1.2 mm³ in size are obtained by an optimized solvent evaporation method, thanks to the clarification of the two-dimensional nucleation growth mechanism. After post surface treatment, the leakage current of the 4HCB detector is no larger than 0.1 pA under an electric field of 600 V cm^-1. The fabricated detectors show a capability of detecting ²⁴¹Am 5.49 MeV α particles with a well resolved full energy peak. The calculated hole mobility (μ_h) and hole mobility lifetime product (μτ)_h are 3.40 cm² V^-1 s^-1 and 8.50 × 10^-5 cm² V^-1, respectively. Simultaneously, under a 50 kVp X-ray beam, a detection limit as low as 0.29 μGy_air s^-1 with a high sensitivity of 10 μC Gy_air^-1 cm^-2 is achieved in the bias range of 40-100 V, contributing to a superior X-ray imaging capability with a spatial resolution of 0.9 lp mm^-1 at a low-dose rate (below 150 μGy_air s^-1) of exposure.