The first principles methods applied to enhance a performance of new photoconductive materials in X-ray imaging

应用第一原理方法增强新型光电导材料在 X 射线成像中的性能

• 批准号: RGPIN-2014-06490
• 负责人: Berashevich, Julia
• 金额: $ 1.38万
• 依托单位: Lakehead University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2017
• 资助国家: 加拿大
• 起止时间: 2017-01-01 至 2018-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=632999
• 关键词: first; principles; methods; applied; enhance

The goal of this program is to investigate the capabilities of the novel photoconductors for application in direct conversion X-ray imaging technology. The diagnostic imaging market in Canada is currently dominated by technology which utilizes a two-step approach: scintillators are used to convert the X-ray signal to light and then it is converted to the electrical signal by photoconductor. Withdrawing scintillators from this scheme would allow for a reduction in the signal loss and improvement of the spatial resolution being impaired by light scattering in scintillators. The success of this new direct conversion approach relies on finding novel photoconductive materials that have both the capacity to provide the high sensitivity to X-ray and compatibility with flat panel display technology. The compatibility is the primary component that limits our choice to amorphous and polycrystalline materials as these are the only available materials that provide uniform deposition over a large area of imaging matrixes at the required low temperature. To cover the whole range of the X-ray imaging applications, two potential candidates are considered: amorphous Se (a-Se) for mammographic energy range (20 keV) and high atomic number polycrystalline PbO for diagnostic energy range (60-120 keV). The amorphous and polycrystalline materials are disordered media, which are known to suffer from poor transport properties due to low charge mobilities and charge trapping on defects. While for a-Se the effect of trapping on photoconductivity is well discussed, the transport properties of polycrystalline PbO are unknown but the low charge mobility is believed to be caused by trapping. To suppress the trapping some material science and engineering solutions have to be applied, but this requires an understanding of fundamental properties, nature of defects and peculiarities of the charge transport. Another big issue of disordered materials is their structural instability: PbO compound degrades upon exposure to air, while a-Se is metastable with respect to crystalline Se and as such it experiences structural modifications upon exposure to light or X-ray. Any structural transformations are highly undesired as they result in permanent degradation of material properties, thus shortening utilization lifetime. Through the application of the first-principles methods we can gain the fundamental insight into the atomistic processes that drive behaviour of materials and to determine what affects their performance in different applications and suggest ways how to improve their technology. The computing technology has advanced simulations to the point where materials can be "grown virtually" with their properties predicted/manipulated theoretically before being created in a lab. Therefore, in this research program the first-principles methods are applied to model polycrystalline PbO and a-Se with focus on understanding of the mechanisms of carrier trapping and structural degradations of photoconductors. Such understanding is essential to provide a feedback for technology optimization and to develop the material science solutions directed on suppression of the parasitic effects degrading the photoconductor performance. It is expected to facilitate promotion of the Direct-conversion Flat-panel X-ray detector technology into the healthcare industry in Canada: our ultimate goal is to achieve a reduction of the radiation exposure of medical personals by enhancing performance of photoconductors. The collaboration with the experimental Advanced Imaging group at Thunder Bay Regional Research Institute (TBRRI) working on fabrication and commercialization of X-ray detectors, provides the unique opportunity to combine theory and experiment.

该计划的目标是研究新型光电导体在直接转换X射线成像技术中的应用能力。加拿大的诊断成像市场目前主要由采用两步方法的技术主导：使用闪烁器将X射线信号转换为光，然后通过光电导体将其转换为电信号。从该方案中撤回散射器将允许减少信号损失并改善被散射器中的光散射损害的空间分辨率。这种新的直接转换方法的成功依赖于找到新的光电导材料，既有能力提供对X射线的高灵敏度，又与平板显示技术兼容。兼容性是限制我们选择非晶和多晶材料的主要因素，因为这些材料是唯一可用的材料，可以在所需的低温下在大面积成像基质上提供均匀沉积。为了覆盖X射线成像应用的整个范围，考虑了两个潜在的候选者：用于乳房摄影能量范围（20 keV）的非晶Se（a-Se）和用于诊断能量范围（60-120 keV）的高原子序数多晶PbO。非晶和多晶材料是无序介质，已知其由于低电荷迁移率和缺陷上的电荷捕获而具有差的传输性质。虽然对于a-Se的捕获对光电导的影响进行了很好的讨论，多晶PbO的输运性质是未知的，但低电荷迁移率被认为是由捕获引起的。为了抑制陷阱，必须应用一些材料科学和工程解决方案，但这需要了解基本性质，缺陷的性质和电荷传输的特性。无序材料的另一个大问题是它们的结构不稳定性：PbO化合物在暴露于空气时降解，而a-Se相对于晶体Se是亚稳定的，因此它在暴露于光或X射线时经历结构改变。任何结构转变都是非常不希望的，因为它们导致材料性能的永久性劣化，从而缩短使用寿命。通过第一性原理方法的应用，我们可以获得对驱动材料行为的原子过程的基本洞察，并确定在不同应用中影响其性能的因素，并提出如何改进其技术的方法。计算技术已经将模拟推进到材料可以“虚拟生长”的程度，在实验室中创建之前，理论上可以预测/操纵它们的属性。因此，在本研究计划中，第一性原理方法被应用到模拟多晶PbO和a-Se，重点是了解载流子捕获和光电导体结构退化的机制。这样的理解是必不可少的，以提供反馈的技术优化，并开发针对抑制寄生效应降低光电导体性能的材料科学解决方案。预计将促进直接转换平板X射线探测器技术在加拿大医疗保健行业的推广：我们的最终目标是通过提高光电导体的性能来减少医疗人员的辐射暴露。与雷霆湾区域研究所（TBRRI）的实验先进成像小组合作，致力于X射线探测器的制造和商业化，为联合收割机理论和实验的结合提供了独特的机会。