triphenylamine-containing polymer (BTPA-F) organic redox system ( Figure 1 a). The conjugated copolymer of BTPA-F with a closed ring and steric crowded triphenylamine (TPA) group was used in the present study for its rich electrochemical redox behavior, [ 31,32 ] while the ethyl viologen diperchlorate (EV(ClO 4 ) 2 ) acted as the counter-electrode material for BTPA-F oxidation and a source of the mobile perchlorate ions to stabilize the charged form of the polymer. [ 20,[33][34][35] Sandwiched between two metal electrodes, the EV(ClO 4 ) 2 /BTPA-F bilayer structure exhibits history-dependent memristive behaviors, which meet the fundamental requirements for mimicking the potentiation and depression processes of a biological synapse. Consequently, a series of synaptic behaviors, including the spike-ratedependent and spike-timing-dependent plasticity (SRDP and STDP) characteristics, the transition from short-term memory (STM) to long-term memory (LTM), as well as the "learningforgetting-relearning" process, are successfully emulated in the present organic redox system. These demonstrations show the possibility of using organic materials for the construction of neuromorphic information storage and processing systems. Results and DiscussionPrepared via Suzuki coupling polymerization reaction (see Scheme S1 and the Supporting Information for details), the successful synthesis of the conjugated copolymer of BTPA-F was verifi ed through 1 H NMR, UV-vis absorption spectroscopy and electrochemistry analyses (Figures S1-S3, Supporting Information). When sandwiched between metal electrodes, the 450 nm EV(ClO 4 ) 2 /90 nm BTPA-F bilayer structure (Figure 1 b and Figure S4, Supporting Information) exhibits a distinctive history-dependent asymmetric resistive switching behavior at room temperature, as plotted in the current-voltage ( I -V ) characteristics of Figure 1 c. Initially, the Ta/EV(ClO 4 ) 2 /BTPA-F/Pt memristor shows a small conductivity of ≈ 0.03 S m −1 (read at 0.2 V). Upon being subjected to four consecutive positive voltage sweeps of 0 V → 1 V → 0 V, the device conductivity increases incrementally to 0.09 S m −1 . Afterward, fi ve consecutive negative voltage sweeps of 0 V → −1 V → 0 V have been applied onto the bilayer structure, while the device conductivity decreases continuously from 0.25 to 0.13 S m −1 (read at −0.2 V). The small rectifying effect may be ascribed to the difference in the molecular orbital energy levels of the BTPA-F polymer and the EV(ClO 4 ) 2 counter-electrode material, which in turn infl uences the charge transport across the EV(ClO 4 ) 2 /BTPA-F junction under electric fi elds of different polarities. Nevertheless, such a rectifying effect is useful for the single-direction

High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem. new particle formation | nucleation | ultrafine particles | growth | organics

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