Dense Perovskite Thick Film Enabled by Saturated Solution Filling for Sensitive X-ray Detection and Imaging.

Thick polycrystalline perovskite films synthesized by using solution processes show great potential in X-ray detection applications. However, due to the evaporation of the solvent, many pinholes and defects appear in the thick films, which deteriorate their optoelectronic properties and diminish their X-ray detection performance. Therefore, the preparation of large area and dense perovskite thick films is desired. Herein, we propose an effective strategy of filling the pores with a saturated precursor solution. By adding the saturated perovskite solution to the polycrystalline perovskite thick film, the original perovskite film will not be destroyed because of the solution-solute equilibrium relationship. Instead, it promotes in situ crystal growth within the thick film during the annealing process. The loosely packed grains in the original thick perovskite film are connected, and the pores and defects are partially filled and fixed. Finally, a much denser perovskite thick film with improved optoelectronic properties has been obtained. The optimized thick film exhibits an X-ray sensitivity of 1616.01 µC Gyair-1 cm-2 under an electric field of 44.44 V mm-1 and a low detection limit of 28.64 nGyair s-1 under an electric field of 22.22 V mm-1. These values exceed the 323.86 µC Gyair-1 cm-2 and 40.52 nGyair s-1 of the pristine perovskite thick film measured under the same conditions. The optimized thick film also shows promising working stability and X-ray imaging capability.

Spatiotemporal responses of habitat quality to land use changes in the source area of Pearl River, China.

Habitat quality assessment provides an effective interface for exploring the mechanisms linking land use change and biodiversity. We used InVEST model to assess the spatial and temporal variations of habitat quality in the Pearl River source area from 2000 to 2020, and to examine the impacts of land use change on habitat quality. The results showed that during the study period, construction land had the largest change, with an area increase of 321.48 km2, and the area of grassland decreased most significantly. Habitat quality in the Pearl River source area was generally at a high level, with a small downward trend. The low value areas were concentrated in the town centers and main agricultural production areas, while habitat quality was high in the mountainous areas in the south and north region. The cold spots of habitat quality change had the most significant aggregation effect in the Quzhan Basin along the Nanpan River. The number of hot spots was small and scattered. Among the various types of nature reserves, habitat quality of Jiache Nature Reserve in Huize County was the lowest, and that of Zhujiangyuan Nature Reserve had a slight decreasing trend, mainly due to the transformation of grassland to cropland. The contribution of habitat quality degradation driven by land function change was 5.6 times as that of improvement. The encroachment of urban and rural construction space into grassland ecological space and agricultural production space was the dominant factor driving the reduction of habitat quality in the Pearl River source area. The contribution of construction land expansion decreased from 69.9% (2000-2010) to 46.7% (2010-2020). The expansion of construction land was the main cause for the degradation of habitat quality. The degradation threat from arable land expansion overtook the traffic network construction as the secondary threat source from 2010 to 2020. It was necessary to increase control of nature reserves in the Pearl River source area and to implement land use regulation strictly to guarantee no net loss and integrity of the ecosystem.

Urban heat vulnerability: A dynamic assessment using multi-source data in coastal metropolis of Southeast China.

Extreme heat caused by global climate change has become a serious threat to the sustainable development of urban areas. Scientific assessment of the impacts of extreme heat on urban areas and in-depth knowledge of the cross-scale mechanisms of heat vulnerability forming in urban systems are expected to support policymakers and stakeholders in developing effective policies to mitigate the economic, social, and health risks. Based on the perspective of the human-environment system, this study constructed a conceptual framework and index system of "exposure-susceptibility-adaptive capacity" for urban heat vulnerability (UHV) and proposed its assessment methods. Taking Xiamen City, a coastal metropolis, as an example, spatial analysis and Geodetector were used to explore the spatial and temporal changes, spatial characteristics, and patterns of UHV under multiple external disturbances from natural to anthropological factors, and to reveal the main factors influencing UHV forming and spatial differentiation. Results showed that the exposure, susceptibility, adaptive capacity, and UHV in Xiamen City had a spatial structure of "coastal-offshore-inland". On the hot day, both the exposure and UHV showed a temporal pattern of "rising and then falling, peaking at 14:00" and a spatial pattern of "monsoonal-like" movement between coast and inland. Coastal zoning with favorable socioeconomic conditions had less magnitude of changes in UHV, where the stability of the urban system was more likely to be maintained. During the hot months, the high UHV areas were mainly distributed in the inland, while coastal areas showed low UHV levels. Further, coastal UHV was mainly dominated by "heat exposure", offshore by "comprehensive factors", and inland in the northern mountainous areas by "lack of adaptive capacity". Multi-scale urban adaptive capacity was confirmed to alter spatial distribution of exposure and reshape the spatial pattern of UHV. This study promotes the application of multi-scale vulnerability framework to disaster impact assessment, enriches the scientific knowledge of the urban system vulnerability, and provides scientific references for local targeted cooling policy development and extreme heat resilience building programs.

Bulk Heterostructure BA2PbI4/MAPbI3 Perovskites for Suppressed Ion Migration To Achieve Sensitive X-ray Detection Performance.

Three-dimensional (3D) lead-halide perovskites with outstanding mobility-lifetime products and large attenuation coefficients for X-ray photons have demonstrated highly sensitive X-ray detection. However, there exists severe ion migration, especially under electrical bias, that results in dark-current drift and poorer device stability. Theoretical analyses suggest that 3D perovskites with two-dimensional (2D) perovskites may mitigate ion migration and reduce the dark current to achieve a drastically lower detection limit, which is badly needed for X-ray diagnostics. A bulk 2D/3D perovskite heterostructure is therefore designed and prepared by hot-pressing a mixture of BA2PbI4 and MAPbI3 particles. Compared with the pure MAPbI3 pellet, the bulk 2D/3D heterostructure pellet shows much higher resistivity, hence, significantly reduced ion migration and a much smaller dark-current drift of 4.84 × 10-5 nA cm-1 s-1 V-1, which is much lower than that of the pristine MAPbI3 pellet, thus demonstrating its effectiveness for the suppression of ion migration. The bulk 2D/3D heterostructure pellet attains an X-ray sensitivity of 2.0 × 103 µC Gyair-1 cm-2 as well as a lower detection limit of 111.76 nGy s-1 under 10 V bias. This work provides a successful strategy to prepare X-ray detectors with suppressed ion migration and negligible dark current drift, which will further benefit the development of lead-halide perovskite X-ray detectors.