Heavy-to-light electron transition enabling real-time spectra detection of charged particles by a biocompatible semiconductor

Wearable and implantable dosimeters are needed for real-time, position-sensitive radiation monitoring in contexts such as medical imaging and radiotherapy, where ionizing radiation is effective but poses risks to healthy tissues. Current in-vitro dose estimation methods are limited and cannot capture random or systematic errors during treatment. There is a lack of suitable detector materials that combine high performance, biocompatibility, and low cost. State-of-the-art inorganic semiconductors (e.g., CdZnTe, Si, CsPbBr3) either contain toxic elements or are expensive to fabricate. Solution-grown organic single crystalline semiconductors (OSCSs) have shown promise, but their spectral detection performance is constrained by slow charge transport, leading to long acquisition times. Reported organic detectors have not achieved real-time spectroscopic detection for charged particles due to elongated pulse rising times (tens of microseconds), unlike inorganic detectors. Clarifying the origin of the two-stage rising pulses and improving charge transport are essential. Here, the OSCS 4-hydroxyphenylacetic acid (4HPA), a biocompatible molecule, is introduced as a radiation detector. The work investigates its anisotropic structure and transport along in-plane (a axis) versus out-of-plane (c axis) directions, uses DFT and Monte Carlo simulations to predict a radiation-stimulated heavy-to-light electron transition, validates transport via TOF measurements, elucidates exciton dissociation mechanisms with photocurrent and AFM, and demonstrates real-time charged-particle spectra as well as X-ray and neutron detection.

Inorganic detectors like CdZnTe, Si, and halide perovskites can deliver single-photon sensitivity but face biocompatibility or cost issues. OSCSs such as 4-hydroxycyanobenzene (4HCB) and rubrene have directly detected X-rays, charged particles, and neutrons. However, typical OSCS operation relies on photocurrent mode rather than spectroscopic mode due to slow transport; for example, 4HCB needed ~30 minutes per spectrum, limiting real-time use. Organic detectors usually exhibit two-stage rising pulses with significant slow components, unlike inorganic detectors where rise times are often under 1 µs. The mechanisms behind these slow components, including exciton diffusion/dissociation and potential trapping at interfaces or structural defects, are recognized but not fully resolved. This work positions 4HPA within this context, aiming to overcome transport limits and achieve real-time, energy-resolved detection with a biocompatible material.

- Materials and crystal growth: 4HPA (99%) was purified and single crystals were grown via controlled solvent evaporation from ethylic ether (1–8 mg mL^−1) at 5–10 °C, followed by washing. Growth was performed in an ultra-clean environment. - Structural and thermal characterization: Powder and single-crystal XRD (Bruker D8, Cu Kα) determined orthorhombic P2_12_12_1 symmetry with a=0.5 nm, b=0.9 nm, c=1.5 nm and dominant (001) facet; DSC-TG assessed thermal stability (melting point 146 °C). - Optical properties: UV-Vis transmission measured the optical bandgap (Tauc plot): experimental Eg ≈ 3.97 eV (DFT indirect gap 3.75 eV). - Device fabrication and electrical measurements: Gold electrodes (~100 nm) were thermally evaporated in sandwich and coplanar configurations to probe c- and a-axis transport, respectively. I–V and photocurrent measurements were done with a Keithley 6517 Picoammeter. - Time-of-flight (TOF) with alpha particles: Using a 241Am source, electron-only and hole-only transients were measured by orienting irradiation on cathode or anode and applying bias (ORTEC 556/142PC/572A chain). Pulses were averaged (n=200) to extract drift times (10–90% rise) and mobility; spectra were recorded digitized from amplified signals. - DFT calculations: VASP with PAW, PBE-GGA, 520 eV cutoff; structures relaxed to forces <0.01 eV Å^−1; effective masses derived from band curvature; anisotropic effective masses for HOMO/LUMO and higher sub-bands (H1/L1) along a and c assessed; electron cloud density visualized. - Monte Carlo (Geant4): Simulated 5.49 MeV α interaction in 4HPA (C8H8O3, ρ=1.253 g cm^−3) to obtain penetration depth (~35 µm) and energy distributions of ionized electrons (0.8–150 eV). - Photocurrent/X-ray imaging: Amptek Mini-X (40 kVp, 5–60 µA) with Cu attenuator; dose rate 0.35–2.34 µGy_air s^−1 calibrated by Fluke RaySafe X2; dose-response, sensitivity S (µC Gy_air^−1 cm^−2) via linear J–D fits; LoD based on SNR≥3 (IUPAC). High-dose stability: Spellman X4060 up to 150 kVp, 1.2 mA; 10 h continuous irradiation (total ~690 Gy_air). Imaging performed on a simple object (metal wire in capsule). - AFM: Characterized quasi-2D surface layer edges (~1.5 nm step height) to infer donor/acceptor-like edge states aiding exciton dissociation. - Biocompatibility: HUVEC cell viability by CCK-8 after 24 h exposure to 4HPA dispersions (up to 2 mg mL^−1).

- Biocompatible OSCS detector: Solution-grown 4HPA single crystals show >90% HUVEC viability at 2 mg mL^−1 and improved thermal stability (Tm=146 °C) over 4HCB (Tm=123 °C). - Anisotropic structure and transport: Orthorhombic P2_12_12_1 with strong π–π overlap along a axis and hydrogen bonds along c axis. DFT reveals wide bandgap (Eg ~3.75–3.97 eV) and anisotropic effective masses; high-energy L1 level along a has much lighter effective mass (1/m* up to 4.98) than LUMO and H1/HOMO. - Heavy-to-light electron transition under radiation: Monte Carlo shows ionized electrons (0.8–150 eV) that, under continuous radiation and high fields, populate L1 leading to fast-rising pulse components; slow components arise from exciton diffusion/dissociation. Correcting for exciton dissociation probability f(p), experimental fast/slow mobility ratios align with theory (f(p)≈0.73 along a; 0.09 along c). - Surface-edge-assisted exciton dissociation: AFM identifies dense 1.5 nm surface layer edges acting as donor/acceptor-like states, enhancing exciton dissociation along a (LAED + EAED) at lower fields (~2.8 kV cm^−1) versus predominantly EAED along c requiring ~11 kV cm^−1. X-ray photocurrent along a can decrease at low light due to hole de-trapping/recombination, confirming edge-state trapping. - Transport metrics: Electron drift velocity up to 5×10^5 cm s^−1; field-dependent mobility along a: 0.84±0.01 cm^2 V^−1 s^−1 below ~2.8 kV cm^−1, rising to 4.17±0.04 cm^2 V^−1 s^−1 above; single-pulse rise time as low as 0.52 µs at 2.8 kV cm^−1. High dark resistivity (1.28±0.002)×10^12 Ω cm with dark current ~12 pA at 100 V cm^−1. - Charged-particle spectroscopy: Alpha (5.49 MeV) electron-only spectra along a show best energy resolution ~36% (FWHM) among organic detectors. Mobility–lifetime product (µτ)e along a: (4.91±0.07)×10^−5 cm^2 V^−1; along c: (4.62±0.08)×10^−5 cm^2 V^−1. Real-time spectra acquisition down to 3 ms with resolvable peak; linear counts–dose relation; superior pulse-mode operation at low bias (e.g., 200 V). - X-ray detection (integration mode): LoD as low as 20 nGy_air s^−1 at 40 kVp, well below 5.5 µGy_air s^−1 clinical requirement; sensitivity S ≈ 330 µC Gy_air^−1 cm^−2 at 150 V, equivalent to 16,612 µC Gy_abs^−1 cm^−3 considering absorption; S/J_dark ≈ 1.5×10^4 µGy_air^−1 s^−1. Stable response with negligible hysteresis; long-term stability after 10 h continuous 150 kVp irradiation (total dose ~690 Gy_air) and sustained operation. - Additional: Response to fast neutrons demonstrated; overall performance compares favorably with advanced halide perovskites in drift velocity while maintaining biocompatibility.

The study addresses the key challenge of realizing real-time, energy-resolved charged-particle detection in a biocompatible, solution-processable semiconductor. 4HPA’s quasi-2D packing yields strong in-plane π–π overlap, enabling efficient electron transport. Under radiation and high field, electrons undergo a heavy-to-light transition by occupying the higher L1 conduction sub-band with lower effective mass, producing fast-rising pulse components and high drift velocities. The slow pulse components in organics are attributed to exciton diffusion/dissociation; here, surface layer-edge states act as donor/acceptor interfaces that, together with the electric field, significantly enhance exciton dissociation along the a axis, reducing the effective drift time and enabling pulse-mode spectroscopy at practical fields. This anisotropic mechanism explains why a-axis-oriented devices show superior spectral resolution, µτ, and counting capability at low bias and high rates, culminating in real-time spectra acquisition within milliseconds. In integration mode, despite limited X-ray absorption, high photoconductive gain/CCE—likely aided by edge-state-mediated charge injection—yields very low LoD and high sensitivity with excellent stability, making 4HPA competitive with leading perovskite detectors. Overall, elucidating the heavy-to-light transition and layer-edge-assisted exciton dissociation provides a mechanistic foundation to guide material and device engineering for improved organic radiation detectors.

This work demonstrates that biocompatible, solution-grown 4HPA single crystals enable real-time, single-particle spectroscopic detection of charged particles, achieving 36% FWHM energy resolution for 5.49 MeV α particles, (µτ)e ≈ 4.9×10^−5 cm^2 V^−1 (a axis), and spectra acquisition down to 3 ms. Under radiation, a heavy-to-light electron transition to the L1 sub-band and surface layer-edge-assisted exciton dissociation along the a axis jointly produce high drift velocities (~5×10^5 cm s^−1) and efficient charge collection while maintaining ultralow dark current and high resistivity. In integration mode, 4HPA exhibits a low X-ray detection limit (20 nGy_air s^−1), high sensitivity (16,612 µC Gy_abs^−1 cm^−3), and robust long-term stability after ~690 Gy_air irradiation. These findings position 4HPA as a compact, tissue-equivalent, and low-cost platform for wearable/implantable dosimeters and radiation imagers for X-rays and fast neutrons. Future work can pursue molecular engineering, purity/crystallinity improvements, device architecture optimization, and electronics refinement to further enhance energy resolution and overall performance.

- The energy resolution (~36% FWHM for α particles) remains inferior to state-of-the-art inorganic detectors (e.g., CdZnTe, Si), indicating room for improvement via material purification, molecular engineering, and device/electronics optimization. - Anisotropic performance requires favorable crystallographic orientation (a axis) to realize best transport and exciton dissociation; c-axis operation demands higher fields for comparable exciton dissociation. - X-ray absorption in 4HPA is relatively weak; high sensitivity relies on photoconductive gain/CCE mechanisms, which may be device-architecture dependent.