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Electron-positron annihilation is the process in which a positron collides with an electron resulting in the annihilation of both particles. Electrons (or β- particles) and positrons (or β+ particles) are of equal mass but opposite charge. Positrons are the antimatter equivalent of an electron, produced from B+ decay
What emerges on the other side of the annihilation process is a new pair of particles. They can have the same identities as before; they can transform into a brand new particle-antiparticle pair; or they can convert entirely into photons, particles that carry electromagnetic energy.
Astrophysicists have posited that annihilation could help us to indirectly detect particles of dark matter, which have made their presence known only via the pull of gravity. If dark-matter particles and antiparticles annihilate, they could convert their energy into a more easily detectable form.
The population extinction pulse we describe here shows, from a quantitative viewpoint, that Earth's sixth mass extinction is more severe than perceived when looking exclusively at species extinctions. Therefore, humanity needs to address anthropogenic population extirpation and decimation immediately. That conclusion is based on analyses of the numbers and degrees of range contraction (indicative of population shrinkage and/or population extinctions according to the International Union for Conservation of Nature) using a sample of 27,600 vertebrate species, and on a more detailed analysis documenting the population extinctions between 1900 and 2015 in 177 mammal species. We find that the rate of population loss in terrestrial vertebrates is extremely high-even in \"species of low concern.\" In our sample, comprising nearly half of known vertebrate species, 32% (8,851/27,600) are decreasing; that is, they have decreased in population size and range. In the 177 mammals for which we have detailed data, all have lost 30% or more of their geographic ranges and more than 40% of the species have experienced severe population declines (>80% range shrinkage). Our data indicate that beyond global species extinctions Earth is experiencing a huge episode of population declines and extirpations, which will have negative cascading consequences on ecosystem functioning and services vital to sustaining civilization. We describe this as a \"biological annihilation\" to highlight the current magnitude of Earth's ongoing sixth major extinction event.
Despite the impressive progress made in the optoelectronic devices based on 2D perovskites11, the understanding of exciton transport is extremely limited and currently there is no direct measurement on exciton diffusion. Because of the requirement of two excitons in proximity, exciton diffusion typically proceeds annihilation in 1D or 2D materials. EEA can occur either in the diffusion-limited or the reaction-limited regime27. To differentiate these two regimes, measurements of exciton population dynamics in both spatial and temporal domains are required. However, the majority of studies on exciton annihilation so far are based on time-resolved photoluminescence (PL) or transient absorption (TA) spectroscopy that offers no spatial resolution, leading to annihilation rates convoluted with exciton diffusion constants28,29,30, making it difficult to elucidate factors that control these two processes independently.
To address this challenge, here we employ transient absorption microscopy (TAM) as a direct means to image exciton population in space and in time to disentangle exciton diffusion and annihilation in 2D perovskites. We have demonstrated in a previous work the capability of TAM in imaging free carrier diffusion and extracting Auger recombination constants in bulk 3D perovskites31. Distinct from the results in 3D perovskites, the measurements on 2D perovskites elucidated the critical role of electron-hole interactions in controlling exciton dynamics and transport. These results showcase the unique ability of 2D perovskites in achieving a combination of large exciton binding energy, long-range exciton transport, and slow annihilation, suggesting their large potential in optoelectronic applications.
a Illustration of the crystal structures and excitons of (BA)2(MA)n-1PbnI3n+1 with n varying from 1 to 5. b Scheme of exciton diffusion and annihilation. c Linear extinction coefficient α and d PL spectra of (BA)2(MA)n-1PbnI3n+1 with n varying from 1 to 5.
a Exciton diffusion constant D and diffusion length L0 for n ranging from 1 to 5. D and L0 increase as n increases. The error bars are estimated from the sensitivity of model to the parameters. b Total annihilation rate as a function of n for (BA)2(MA)n-1PbnI3n+1, showing enhanced annihilation at decreasing n. As a comparison, the exciton recombination rates k1 are denoted by the circles.
A major challenge in realizing optoelectronic applications for low-dimensional nanostructures is to achieve long-range transport and suppressed annihilation while maintaining large exciton binding energy. As unraveled by the spatially- and temporally resolved measurements reported here, 2D perovskites present unique opportunities in addressing this challenge. The exciton binding energies of 2D perovskites are comparable to those found in TMDCs but their annihilation rates are orders of magnitude lower. These properties make 2D perovskites excellent candidates for light emitting applications. Further, 2D perovskites are also promising as solar cell materials, with long-range exciton diffusion over hundreds of nanometers. Finally, their structures are uniquely programmable, which allows for further enhancement of exciton transport and suppression of annihilation through composition engineering. 59ce067264