科研成果

Spiking Neural Networks (SNNs) have been attached great importance due to their biological plausibility and high energy-efficiency on neuromorphic chips. As these chips are usually resource-constrained, the compression of SNNs is thus crucial along the road of practical use of SNNs. Most existing methods directly apply pruning approaches in artificial neural networks (ANNs) to SNNs, which ignore the difference between ANNs and SNNs, thus limiting the performance of the pruned SNNs. Besides, these methods are only suitable for shallow SNNs. In this paper, inspired by synaptogenesis and synapse elimination in the neural system, we propose gradient rewiring (Grad R), a joint learning algorithm of connectivity and weight for SNNs, that enables us to seamlessly optimize network structure without retraining. Our key innovation is to redefine the gradient to a new synaptic parameter, allowing better exploration of network structures by taking full advantage of the competition between pruning and regrowth of connections. The experimental results show that the proposed method achieves minimal loss of SNNs' performance on MNIST and CIFAR-10 datasets so far. Moreover, it reaches a ~3.5% accuracy loss under unprecedented 0.73% connectivity, which reveals remarkable structure refining capability in SNNs. Our work suggests that there exists extremely high redundancy in deep SNNs. Our codes are available at https://github.com/Yanqi-Chen/Gradient-Rewiring.

Both naphthalene and perylene derivatives bearing six-membered-ring dicarboximide groups are proven valuable modules for preparing organic electron-transporting semiconductors. However, obtaining their analogue with pyrene appeared challenging, as all previous endeavors resulted in 5- instead of 6-membered dicarboximide rings. Here, we report the first successful synthesis of pyrene-1,5,6,10-tetracarboxyl diimide (PyDI), a centro-symmetric structure comprising two 6-membered dicarboximide groups. Moreover, this molecule undergoes regioselective dihalogenation to afford a 3,8-dibromo-substituted derivative, which may conveniently be incorporated into conjugated polymers via cross-coupling with suitable comonomers. Two representative conjugated polymers, PyDI-T and PyDI-TT, are thus prepared, composed of alternating PyDI and thiophene or bithiophene units. OFETs are then fabricated using these polymers under solution-processing conditions. An optimal electron mobility of 0.335 cm(2) V(−1) s(−1) is achieved by PyDI-TT, which clearly demonstrates the potential of 1,5,6,10-PyDI as a potent building block for developing new high-performance electronic-transporting materials.

Stabilized Criegee intermediates (SCIs) have the potential to oxidize trace species and to produce secondary organic aerosols (SOA), making them important factors in tropospheric chemistry. This study quantitatively investigates the performance of SCIs in SOA formation at different relative humidity (RH), and the first- and second-generation oxidations of endo- and exo-cyclic double bonds ozonated in limonene ozonolysis are studied separately. Through regulating SCIs scavengers, the yields and rate constants of SCIs in reaction system were derived, and the amounts of SCIs were calculated. The amount of SOA decreased by more than 20% under low-humidity conditions (10–50% RH), compared to that under dry conditions due to the reactions of SCIs with water, while the inhibitory effect of water on SOA formation was not observed under high-humidity conditions (60–90% RH). When using excessive SCIs scavengers to exclude SCIs reactions, it was found that the effect of water on SOA formation with the presence of SCIs was different from that without the presence of SCIs, suggesting that SCIs reactions were relevant to the non-monotonic impact of water. The fractions of SCIs contribution to SOA were similar between dry and high-humidity conditions, where the SCIs reactions accounted for ~ 63% and ~ 73% in SOA formation in the first- and second-generation oxidation, however, marked differences in SOA formation mechanisms were observed. SOA formation showed a positive correlation with the amount of SCIs, and the SOA formation potential of SCIs under high-humidity conditions was more significant than that under dry and low-humidity conditions. It was estimated that 20–30% of SCIs could convert into SOA under high-humidity conditions, while this value decreased nearly by half under dry and low-humidity conditions. The typical contribution of limonene-derived SCIs to SOA formation is calculated to be (8.21 ± 0.15) × 10–2 μg m–3 h–1 in forest, (4.26 ± 0.46) × 10–2 μg m–3 h–1 in urban area, and (2.52 ± 0.28) × 10–1 μg m–3 h–1 in indoor area. Water is an uncertainty on the role of SCIs playing in SOA formation, and the contribution of SCIs to SOA formation needs consideration even under high RH in the atmosphere.

In recent decade the ambient fine particle (PM2.5) levels have shown a trend of distinct dropping in China, while ground-level ozone concentrations have been increasing in Beijing and many other Chinese mega-cities. The variation pattern in Los Angeles was markedly different, with PM2.5 and ozone decreasing together over past decades. In this study, we utilize observation-based methods to establish the parametric relationship between PM2.5 concentration and key aerosol physical properties (including aerosol optical depth and aerosol surface concentration), and an observation-based 1-D photochemical model to quantify the response of PM2.5 decline in enhancing ground-level ozone pollution over a large PM2.5 concentration range (10–120 μg m−3). We find that the significance of ozone enhancement due to PM2.5 dropping depends on both the PM2.5 levels and optical properties of particles. Ozone formation increased by 37% in 2006–2016 due to PM2.5 dropping in Beijing, while it becomes less important (7%) as PM2.5 reaches below 40 μg/m3, similar to Los Angeles since 1980s. Therefore, the two cities show the convergence of air pollutant characteristics. Hence a control strategy prioritizing reactive volatile organic compound abatement is projected to yield simultaneous ozone and PM2.5 reductions in Beijing, as experienced in Los Angeles.

We quantify the macroeconomic and welfare effects of a social health insurance reform that occurred in China in 2016 using a two-sector model with endogenous rural-to-urban migrant workers. The calibrated model mimics the rural-to-urban migration and rural-urban wage gap from 2007 through 2016. We find that the health insurance reform depresses rural-to-urban migration and leads to reallocation of labor and capital in both the rural and urban sectors. As the result, we find that the consolidation of premium and reimbursement expands the rural-urban wage gap by approximately 6.8% but universal health insurance coveragenarrows the rural-urban wage gap by approximately 0.9%. Keeping the government deficit unchanged, the welfare results favor universal health insurance relative to pure consolidation.

With the successful implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer, the atmospheric abundance of ozone-depleting substances continues to decrease slowly and the Antarctic ozone hole is showing signs of recovery. However, growing emissions of unregulated short-lived anthropogenic chlorocarbons are offsetting some of these gains. Here, we report an increase in emissions from China of the industrially produced chlorocarbon, dichloromethane (CH2Cl2). The emissions grew from 231 (213–245) Gg yr−1 in 2011 to 628 (599–658) Gg yr−1 in 2019, with an average annual increase of 13 (12–15) %, primarily from eastern China. The overall increase in CH2Cl2 emissions from China has the same magnitude as the global emission rise of 354 (281−427) Gg yr−1 over the same period. If global CH2Cl2 emissions remain at 2019 levels, they could lead to a delay in Antarctic ozone recovery of around 5 years compared to a scenario with no CH2Cl2 emissions.

The cardiovascular risk equations for diabetes patients from New Zealand and Chinese electronic health records (CREDENCE) study is a unique prospectively designed investigation of cardiovascular risk in two large contemporary cohorts of people with type 2 diabetes from New Zealand (NZ) and China. The study was designed to derive equivalent cardiovascular risk prediction equations in a developed and a developing country, using the same epidemiological and statistical methodology. Two similar cohorts of people with type 2 diabetes were identified from large general population studies in China and New Zealand, which had been generated from longitudinal electronic health record systems. The CREDENCE study aims to determine whether cardiovascular risk prediction equations derived in patients with type 2 diabetes in a developed country are applicable in a developing country, and vice versa, by deriving and validating equivalent diabetes-specific cardiovascular risk prediction models from the two countries. Baseline data in CREDENCE was collected from October 2004 in New Zealand and from January 2010 in China. In the first stage of CREDENCE, a total of 93,207 patients (46,649 from NZ and 46,558 from China) were followed until December 31st 2018. Median follow-up was 7.0 years (New Zealand) and 5.7 years (China). There were 5926 (7.7% fatal) CVD events in the New Zealand cohort and 3650 (8.8% fatal) in the Chinese cohort. The research results have implications for policy makers, clinicians and the public and will facilitate personalised management of cardiovascular risk in people with type 2 diabetes worldwide.

CO2 sequestration and CO2 enhanced oil recovery (CO2-EOR) are two major processes that can expose the rock to CO2. The behavior of a rock when saturated with CO2 changes over time, affecting both the mechanical and chemical properties of the host rock. CO2 operation involves the injection and pressurization of reservoirs that usually results in changes to the state of in situ stresses that may initiate fractures. This can lead to slippage along pre-existing fracture and fault systems. CO2 storage in tight formations, either for EOR or sequestration purposes, is imperative to contribute to the current energy transition and mitigate climate change. Thus, injection of CO2 may alter the mineralogy, pore structure, mechanics, and other properties and behavior of tight reservoirs, and sometimes may be susceptible to leakage through induced fractures or reactivated faults. Here, we aim to evaluate and reassess studies on CO2 sequestration in tight reservoirs and associated formations. This report focuses on the changes in properties and behavior of tight rocks (shale and tight carbonate rocks) due to CO2 exposure through CO2 sequestration or CO2-EOR. We highlight the most important findings from available studies to date, and we recommend promising areas of research that can advance the knowledge and development of CO2 sequestration in tight formations.

Photocatalytic nitrogen fixation represents an effective technology for the artificial production of ammonia from atmospheric nitrogen, a critical step toward a sustainable economy. Bismuth oxyhalides (BiOX, X = Cl, Br, and I) have emerged as viable catalysts for photocatalytic reduction of nitrogen into ammonia, due to their unique electronic structures and optical properties. Herein, the recent progress of BiOX-based photocatalysts for nitrogen fixation, with a focus on the reaction mechanism and pathways, materials preparation, and strategies of structural engineering for enhanced performance, is summarized. The article is concluded with a perspective where the promises and challenges of bismuth-based photocatalysts for nitrogen reduction to ammonia are highlighted, along with possible future research directions.