X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (1 mm–1 cm), resulting in rigid structures, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broadband, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy −1 cm −3 for “soft” X-rays and 30 and 58 µC mGy −1 cm −3 under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (−10 V) and low power, enabling detector operation powered by coin cell batteries. X-rays are widely used in homeland security, therapeutic and diagnostic healthcare and industrial process control (e.g. Pharmaceuticals) with each application necessitating specific detector requirements. For example, direct conversion detectors based on materials such as amorphous selenium are currently used in mammography, but are limited by their low X-ray attenuation for energies higher than 50 keV.
Detectors based on p-type silicon with its high radiation-damage resistance are used in radiotherapy for dose measurement or beam imaging. However, their propensity to damage from accumulated dose and drift due to environmental effects makes these less useful for beam calibration. High-quality single crystal Cd(Zn)Te is used for homeland security screening, but suffers from being limited to small dimensions, high cost, charge carrier trapping and high-voltage operation (500 V). Similarly, X-ray detection in the non-destructive evaluation sector is currently dominated by CsI (Tl) scintillator screens coupled to a-Si which, despite their high stopping power and spatial resolution, are limited to sizes less than 60 × 60 cm 2.
Some detectors are operated as simple counting systems by using a single-channel. The radiation-sensitive region of the detector, produces excitation or ionization effects that are not directly observable. The ideal radiation detector achieves a high absolute detection. Diagnostic nuclear medicine.
Therefore, there is a demand for broadband, high sensitivity, low-cost radiation detectors, which current inorganic detectors fail to fulfil.Organic semiconductors can be fabricated over large areas in a flexible format, enabling conformability to complex structures at low cost and are now commercialized for photovoltaics, displays etc. There is increasing attention given to organic photodetectors for X-ray detection. This often involves the coupling of scintillator screens with organic photodiodes, insertion of high-atomic number ( Z) nanoparticles (NPs), quantum dots or scintillator particles into organic diodes, or the use of thin film organic semiconductors or crystals.
Of these, the use of X-ray scintillators is often preferred as this enables the already mature organic photodetector technologies to be adapted for X-ray detection. However, the absorption of light by the organic semiconductor forms bound electron–hole pairs (excitons), which need to be dissociated resulting in significant losses, limiting detector sensitivity as opposed to a direct conversion process.Here, we introduce a broadband, direct, X-ray detector concept based on a thin film, hybrid semiconductor diode consisting of an organic bulk heterojunction (BHJ)—bismuth oxide (Bi 2O 3) NP composite. These direct X-ray detectors demonstrate high sensitivities of 1712 µC mGy −1 cm −3 under 50 kV soft X-rays and 30 and 58 µC mGy −1 cm −3 under 6 and 15 MV hard X-rays. Furthermore, we also demonstrate a flexible detector based on the same device concept which offer a high sensitivity of 280 µC mGy −1 cm −3.
More importantly, these sensitivities are achieved at −10 V. X-ray response of BHJ-NP detectorsThe device compromises of a diode architecture where the BHJ-NP composite is sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes (Fig. ). Here, the introduction of the Bi 2O 3 ( Z = 83 for Bi) is utilised to increase the X-ray attenuation. We have chosen Bi 2O 3 from the many metal oxides available based on its direct conversion of X-rays and lower environmental impact and health risks when compared to, for example, high Z Pb-based semiconductors.
Given its existing use as a non-toxic dental material such as in the case of hydraulic silicate cements with an opacity to X-rays makes it an ideal candidate for our application. Regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and 6,6-Phenyl C 71 butyric acid methyl ester (PC 70BM) were selected as the BHJ system. The formation of nanoscale diodes throughout the volume of the BHJ, in close proximity to the NPs leads to an in-built depletion region, with local electric fields as high as 200 V µm −1, which has been experimentally quantified with Fourier-transform IR-absorption spectroscopy for the P3HT:PCBM system. This is further enhanced by dielectric inhomogeneities in the material. The above factors, in combination with the high crystallinity of P3HT:PC 70BM enables efficient electron and hole extraction from the entirety of the depleted active layer under low reverse-bias voltages (. X-ray detector overview. A Device schematic structure.
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B Performance comparison of current solid state X-ray detectors—(1), (2), (3), (4), (5), (7), (9), (11), (14), (15), (16), (17), (18), (19), (20), and (24) are direct detectors, (6), (8), (21), and (22) are inorganic detectors, and (10), (12), (13) and (23) are indirect detectors—with the technology developed in this work—(25) Bi 2O 3-40, (26) Bi 2O 3-80, (27) Bi 2O 3-40 and (28) Bi 2O 3-40. The operating voltage is given adjacent to each data point. The total attenuation coefficient values of carbon, selenium, methylammonium lead iodide (MAPbI 3) and Bi 2O 3 are given as shaded areas showing the previous limits to detector technology based only on bulk attenuation processes. C An X-ray imager based on the hybrid X-ray detector and 70 kV X-ray image of a bolt taken using the X-ray imager. For optimisation of the detector, the NP loading within the device active volume was increased in order to increase the X-ray attenuation by varying the Bi 2O 3 content in the parent solution (0, 10, 20, 30, 40, 50, 60, 70 and 80 mg ml −1: X mg ml −1 is noted as Bi 2O 3-X). It is worth to note that the highest NP-loaded device which could be fabricated using the given procedure is Bi 2O 3-80, due to the formation of cracks during the annealing process at higher loadings beyond Bi 2O 3-80.
However, with the appropriate selection of the organic bulk heterojunction and with tuning of the solvent used, much higher Bi 2O 3-loaded device fabrication maybe possible. Assuming a periodic structure for the NPs within the BHJ, this enables the unit cell dimensions to be reduced from 72 nm for Bi 2O 3-40 to 64 nm for Bi 2O 3-80 (Supplementary Figure and Supplementary Note ). The detectors with thicknesses of 10–30 μm (Supplementary Figure ) demonstrate dark current densities in the range of 10 −4 (Bi 2O 3-80) to 10 −6 (Bi 2O 3-0) A cm −2 at −10 V, and 1 and 40 nA cm −2 under 0 and −1 V, respectively (Supplementary Figure ). We note recent work in the literature where the dark current can be tuned to meet industrial requirements through appropriate BHJ selection as a promising route for further improvements.Visible light photocurrent measurements are a useful tool in determining whether NP incorporation disrupts the BHJ phase separation thereby impeding charge transport.
The lack of significant variation in the visible light photocurrent response for different Bi 2O 3 loadings indicates that the phase separation within the BHJ remains undisturbed (Supplementary Figure ). The X-ray photocurrent response of the detectors tested under a 50 kV X-ray source at −10 V bias, demonstrates a linear increase with increasing NP loading from Bi 2O 3-0 to Bi 2O 3-40 (Fig. ), followed by a non-linear increase for Bi 2O 3-60 and upwards. The X-ray sensitivity ( S) depends on the amount of X-rays stopped, which depends on both the device cross section and its thickness, and hence, the sensitivity of the detector is calculated.
Graphene's benefits are opening possibilities in high-performance IR imaging and spectroscopy. Researchers from the Graphene Flagship, working at the University of Cambridge (UK), Emberion Ltd. (UK), the Institute of Photonic Sciences (ICFO; Spain), Nokia UK, and the University of Ioannina (Greece) have developed a graphene-based pyroelectric bolometer that detects infrared (IR) radiation by measuring tiny temperature changes with an ultra-high level of accuracy. The work, published in Nature Communications, demonstrates the highest reported temperature sensitivity for graphene-based uncooled thermal detectors, capable of resolving temperature changes down to a few tens of µK. Only a few nano-Watts of IR radiation power are required to produce such a small temperature variation in isolated devices, about 1000 times smaller than the IR power delivered to the detector by a human hand in close proximity.The high sensitivity of the detector is of great use for spectroscopic applications beyond thermal imaging. With a high-performance graphene-based IR detector that gives a strong signal with less incident radiation, it is possible to isolate different parts of the IR spectrum.
This is of key importance in security applications, where different materials – such as explosives – can be distinguished by their characteristic IR absorption or transmission spectra.Dr Alan Colli, Principal Engineer at Emberion and co-leader of the research, said: 'With a higher sensitivity detector, one can restrict the large thermal band and still form an image using photons in a very narrow spectral range and do multi-spectral IR imaging. For security screening, there are specific signatures that materials emit or absorb in narrow bands.
So, you want a detector that is trained in that narrow band. This can be useful while looking for explosives, hazardous substances, or anything of the sort.' Typical IR photodetectors operate either via the pyroelectric effect, or as bolometers, which measure changes in resistance due to heating.
The graphene-based pyroelectric bolometer combines both approaches with the excellent electrical properties of graphene, for maximum performance. Graphene acts as a built-in amplifier for the signal, removing the need for external transistors – meaning no losses from parasitic capacitance, and remarkably low noise. The high conductivity of graphene also offers a convenient impedance matching with the external readout integrated circuit (ROIC) used to interface with the detector pixels and the recording device. With the continuous improvement in the quality of graphene (e.g., higher mobility), robust devices with an extended dynamic range (temperature range over which the device will operate reliably) can be fabricated while maintaining the same excellent temperature responsivity. Andrea Ferrari, Director of the Cambridge Graphene Centre and co-author of the work said 'This work is another example of the steady march of graphene on the roadmap towards applications. Emberion is a new company created to produce graphene photonics and electronics for infrared photodetectors and thermal sensors, and this work exemplifies how basic science and technology can lead to swift commercialisation.'
Ferrari is the Science and Technology Officer of the Graphene Flagship, and Chair of the Flagship Management Panel.Prof. Frank Koppens, co-author of the work, is leader of the Quantum Nano-Optoelectronics at ICFO, and leads the Photonics and Optoelectronics work package of the Graphene Flagship. 'One of the most promising applications of graphene is broadband photodetection and imaging. Combining visible and infrared detection in one material system is not possible with any other existing technology. The Graphene Flagship program will further build on this work to develop hyperspectral imaging systems, and exploiting the directions where graphene is unique,' he said.Dr Daniel Neumaier (AMO, Germany) is the leader of the Graphene Flagship Electronics and Photonics Integration Division and was not directly involved in the work.
He said 'The market size of IR detectors has increased dramatically in the last couple of years and these devices are entering more and more application areas. In particular, spectroscopic security screening is becoming more important. This requires under room temperature operation.
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The present is a huge step forward in meeting these requirements in graphene-based IR detectors.'