Most contemporary X‐ray detectors adopt device structures with non/low‐gain energy conversion, such that a fairly thick X‐ray photoconductor or scintillator is required to generate sufficient X‐ray‐induced charges, and thus numerous merits for thin devices, such as mechanical flexibility and high spatial resolution, have to be compromised. This dilemma is overcome by adopting a new high‐gain device concept of a heterojunction X‐ray phototransistor. In contrast to conventional detectors, X‐ray phototransistors allow both electrical gating and photodoping for effective carrier‐density modulation, leading to high photoconductive gain and low noise. As a result, ultrahigh sensitivities of over 105 μC Gyair−1 cm−2 with low detection limit are achieved by just using an ≈50 nm thin photoconductor. The employment of ultrathin photoconductors also endows the detectors with superior flexibility and high imaging resolution. This concept offers great promise in realizing well‐balanced detection performance, mechanical flexibility, integration, and cost for next‐generation X‐ray detectors.

Black phosphorus (BP) promises high‐performance humidity sensing, due to its unique electrical, mechanical, and surface properties. Different from traditional transistor‐based BP humidity sensors, herein, an all‐fiber sensing strategy is proposed by integrating BP nanosheets with etched single‐mode fiber (ESMF). The experiments show that transmitted light in the ESMF can be effectively modulated by the coated BP nanosheets as relative humidity varies from 35% to 80%. Due to the combination of unique sensing characteristics of BP materials with a high sensitivity of optical modulation strategy, highly responsive humidity sensing is achieved with the response time as fast as 7 ms. This work demonstrates a BP‐based all‐optical sensing platform not only for high‐performance humidity sensing, but also for opening up new opportunities for many potential vapor/gas sensing applications in the fields of biomedicine and chemistry.