92 research outputs found
Memristors with diffusive dynamics as synaptic emulators for neuromorphic computing
The accumulation and extrusion of Ca2+ in the pre- and postsynaptic compartments
play a critical role in initiating plastic changes in biological synapses. To emulate this fundamental process in electronic devices, we developed diffusive Ag-in-oxide
memristors with a temporal response during and after stimulation similar to that of the
synaptic Ca2+ dynamics. In situ high-resolution transmission electron microscopy and nanoparticle dynamics simulations both demonstrate that Ag atoms disperse under electrical bias and regroup spontaneously under zero bias because of interfacial energy minimization, closely resembling synaptic influx and extrusion of Ca2+, respectively. The diffusive memristor and its dynamics enable a direct emulation of both short- and long-term plasticity of biological synapses and represent a major advancement in hardware implementation of neuromorphic functionalities
Anatomy of Ag/Hafnia‐Based Selectors with 1010 Nonlinearity
Sneak path current is a significant remaining obstacle to the utilization of large crossbar arrays for non-volatile memories and other applications of memristors. A two-terminal selector device with
an extremely large current-voltage nonlinearity and low leakage current could solve this problem.
We present here a Ag/oxide-based threshold switching (TS) device with attractive features such
as high current-voltage nonlinearity (~1010
), steep turn-on slope (less than 1 mV/dec), low OFF-state leakage current (~10-14 A), fast turn ON/OFF speeds (108
cycles). The feasibility of using this selector with a typical memristor has been demonstrated by
physically integrating them into a multilayered 1S1R cell. Structural analysis of the nanoscale
crosspoint device suggests that elongation of a Ag nanoparticle under voltage bias followed by
spontaneous reformation of a more spherical shape after power off is responsible for the observed
threshold switching of the device. Such mechanism has been quantitatively verified by the Ag nanoparticle dynamics simulation based on thermal diffusion assisted by bipolar electrode effect and interfacial energy minimization
Revealing Brown Carbon Chromophores Produced in Reactions of Methylglyoxal with Ammonium Sulfate
Atmospheric
brown carbon (BrC) is an important contributor to light
absorption and climate forcing by aerosols. Reactions between small
water-soluble carbonyls and ammonia or amines have been identified
as one of the potential pathways of BrC formation. However, detailed
chemical characterization of BrC chromophores has been challenging
and their formation mechanisms are still poorly understood. Understanding
BrC formation is impeded by the lack of suitable methods which can
unravel the variability and complexity of BrC mixtures. This study
applies high performance liquid chromatography (HPLC) coupled to photodiode
array (PDA) detector and high resolution mass spectrometry (HRMS)
to investigate optical properties and chemical composition of individual
BrC components produced through reactions of methylglyoxal (MG) and
ammonium sulfate (AS), both of which are abundant in the atmospheric
environment. A direct relationship between optical properties and
chemical composition of 30 major BrC chromophores is established.
Nearly all of these chromophores are nitrogen-containing compounds
that account for >70% of the overall light absorption by the MG+AS
system in the 300–500 nm range. These results suggest that
reduced-nitrogen organic compounds formed in reactions between atmospheric
carbonyls and ammonia/amines are important BrC chromophores. It is
also demonstrated that improved separation of BrC chromophores by
HPLC will significantly advance understanding of BrC chemistry
Organosulfates in Humic-like Substance Fraction Isolated from Aerosols at Seven Locations in East Asia: A Study by Ultra-High-Resolution Mass Spectrometry
Humic-like substances (HULIS) in ambient aerosols collected
at
seven locations in East Asia were analyzed using electrospray ionization
(ESI) coupled with an ultra-high-resolution mass spectrometer (UHRMS).
Locations included a 3 km high mountaintop site in Taiwan, rural,
suburban, and urban locations in the Pearl River Delta (PRD), South
China, and in Taiwan. Organosulfates (OS) in the HULIS fraction were
tentatively identified through accurate mass measurements and MS/MS
spectra interpretation. In the two mountaintop samples collected in
regional background atmosphere, little OS were detected, while a few
hundred OS formulas were identified in the six samples taken in Taiwan
and PRD. Many of the OS ions were among the most intense peaks in
the negative ESI–UHRMS spectra, and their elemental formulas
were identical to OS derived from biogenic volatile organic compounds
(BVOCs) (e.g., monoterpenes) that have been identified in chamber
studies. With OS having less than 6 carbon atoms too hydrophilic to
be effectively retained in the HULIS fraction, OS containing 10 carbon
atoms were the most abundant, indicating monoterpenes as important
precursors of OS in the HULIS fraction. Clear spatial variation in
abundance of OS was found among different atmospheric environments,
with enhanced coupling of BVOCs with anthropogenic acidic aerosols
observed in the PRD samples over the Taiwan samples. The double bond
equivalent (DBE) values indicate the majority of OS (>90%) in the
HULIS fraction are aliphatic. The elemental compositions of OS compounds
containing N atoms (defined as CHONS) indicate that they are probably
nitrooxy OS. Some insights into OS formation mechanisms are also gained
through examining the presence/absence of perceived reactant–product
formula pairs in the mass spectra. The results suggest the dominant
epoxide intermediate pathway for formation of OS compounds without
N atoms (defined as CHOS) and confirm the more readily hydrolyzed
characteristics of the −ONO<sub>2</sub> group than the −OSO<sub>3</sub> group. There is a lack of evidence for the epoxide pathway
to account for the formation of OS in the CHONS subgroup
Al-TiO<sub>2</sub> Composite-Modified Single-Layer Graphene as an Efficient Transparent Cathode for Organic Solar Cells
While there are challenges in tuning the properties of graphene (surface wettability, work function alignment, and carrier transport) for realizing an efficient graphene cathode in organic solar cells (OSCs), we propose and demonstrate using an Al-TiO<sub>2</sub> composite to modify single-layer graphene as an efficient cathode for OSCs. To unveil the contributions of the composite in addressing the aforementioned challenges, the evaporated aluminum nanoclusters in the composite benefit the graphene cathode by simultaneously achieving two roles of improving its surface wettability for subsequent TiO<sub>2</sub> deposition and reducing its work function to offer better energy alignment. To address challenges related to charge transport, solution-processed TiO<sub>2</sub> with excellent electron transport can offer charge extraction enhancement to the graphene cathode, which is essential to efficient devices. However, it is a well-known issue for methods such as spin-coating to produce uniform films on the initially hydrophobic graphene, even with improved wettability. The undesirable morphology of TiO<sub>2</sub> by such methods considerably inhibits its effectiveness in enhancing charge extraction. We propose a self-assembly method to deposit the solution-processed TiO<sub>2</sub> on the Al-covered graphene for forming the Al-TiO<sub>2</sub> composite. Compared with spin-coating, the self-assembly method is found to achieve more uniform coating on the graphene surface, with highly controllable thickness. Consequently, the graphene cathode modified with the Al-TiO<sub>2</sub> composite in inverted OSCs gives rise to enhanced power conversion efficiency of 2.58%, which is 2-fold of the previously best reported efficiency (1.27%) for graphene cathode OSCs, reaching ∼75% performance of control devices using indium tin oxide
Elemental Composition of HULIS in the Pearl River Delta Region, China: Results Inferred from Positive and Negative Electrospray High Resolution Mass Spectrometric Data
The HUmic-LIke Substances (HULIS) fraction isolated from
aerosol
samples collected at a rural location of the Pearl River Delta Region
(PRD), China, during the harvest season was analyzed by both positive
and negative mode electrospray ionization (ESI) coupled with an ultrahigh
resolution mass spectrometer (UHRMS). With the remarkable resolving
power and mass accuracy of ESI-UHRMS, thousands of elemental formulas
were identified. Formulas detected in the positive (ESI+) and the
negative (ESI-) mode complement each other due to differences in the
ionization mechanism, and the use of both provides a more complete
characterization of HULIS. Compounds composed of C, H, and O atoms
were preferentially detected in ESI- by deprotonation, implying their
acidic properties. Tandem MS and Kendrick Mass Defect analysis implies
that carboxyl groups are abundant in the CHO compounds. This feature
is similar to those of natural fulvic acids, but relatively smaller
molecular weights are observed in the HULIS samples. A greater number
of reduced nitrogen organic compounds were observed in the ESI+ compared
to ESI-. Compounds with biomass burning origin including alkaloids,
amino acids, and their derivatives are their probable constituents.
Sulfur-containing species were dominantly detected in ESI-. The presence
of sulfate fragments in the MS/MS spectra of these species and their
high O/S ratios implies that they are mainly organosulfates. Organosulfates
and nitrooxy-organosulfates were often the most intensive peaks in
the ESI- spectra. They are believed to be products of reactive uptake
of photooxidation products of reactive volatile organic compounds
by acidic sulfate particles. The elemental compositions deduced from
the UHRMS analysis confirm the conclusion from our previous study
that biomass burning and SOA formation are both important sources
of HULIS in the PRD region
Elemental Composition of HULIS in the Pearl River Delta Region, China: Results Inferred from Positive and Negative Electrospray High Resolution Mass Spectrometric Data
The HUmic-LIke Substances (HULIS) fraction isolated from
aerosol
samples collected at a rural location of the Pearl River Delta Region
(PRD), China, during the harvest season was analyzed by both positive
and negative mode electrospray ionization (ESI) coupled with an ultrahigh
resolution mass spectrometer (UHRMS). With the remarkable resolving
power and mass accuracy of ESI-UHRMS, thousands of elemental formulas
were identified. Formulas detected in the positive (ESI+) and the
negative (ESI-) mode complement each other due to differences in the
ionization mechanism, and the use of both provides a more complete
characterization of HULIS. Compounds composed of C, H, and O atoms
were preferentially detected in ESI- by deprotonation, implying their
acidic properties. Tandem MS and Kendrick Mass Defect analysis implies
that carboxyl groups are abundant in the CHO compounds. This feature
is similar to those of natural fulvic acids, but relatively smaller
molecular weights are observed in the HULIS samples. A greater number
of reduced nitrogen organic compounds were observed in the ESI+ compared
to ESI-. Compounds with biomass burning origin including alkaloids,
amino acids, and their derivatives are their probable constituents.
Sulfur-containing species were dominantly detected in ESI-. The presence
of sulfate fragments in the MS/MS spectra of these species and their
high O/S ratios implies that they are mainly organosulfates. Organosulfates
and nitrooxy-organosulfates were often the most intensive peaks in
the ESI- spectra. They are believed to be products of reactive uptake
of photooxidation products of reactive volatile organic compounds
by acidic sulfate particles. The elemental compositions deduced from
the UHRMS analysis confirm the conclusion from our previous study
that biomass burning and SOA formation are both important sources
of HULIS in the PRD region
Reactive Oxygen Species Production Mediated by Humic-like Substances in Atmospheric Aerosols: Enhancement Effects by Pyridine, Imidazole, and Their Derivatives
Ambient particulate matter (PM) can
cause adverse health effects
via their ability to produce reactive oxygen species (ROS). Humic-like
substances (HULIS), a complex mixture of amphiphilic organic compounds,
have been demonstrated to contain the majority of redox activity in
the water-extractable organic fraction of PM. Reduced organic nitrogen
compounds, such as alkaloids resulting from biomass burning emissions,
are among HULIS constituents. In this study, we examined the redox
activities of pyridine, imidazole and their alkyl derivatives using
a cell-free dithiothreitol (DTT) assay under simulated physiological
conditions (37 °C, pH = 7.40). These compounds were found to
have little redox activity on their own as measured by the DTT assay,
but they enhanced ROS generation catalyzed by 1,4-naphthoquinone (as
a model quinone compound) and HULIS isolated from multiple aerosol
samples. The enhancement effect by the individual nitrogen-containing
bases was determined to be proportional to their amount in the assay
solutions. It is postulated that the underlying mechanism involves
the unprotonated N atom acting as a H-bonding acceptor to facilitate
hydrogen-atom transfer in the ROS generation cycle. The enhancement
capability was found to increase with their basicity (i.e., p<i>K</i><sub>a</sub> of their conjugated acids, BH<sup>+</sup>),
consistent with the proposed mechanism for enhancement. Among the
imidazole homologues, a linear relationship was observed between the
enhancement factors (in log scale) of the unprotonated form of the
imidazole compounds (B) and the p<i>K</i><sub>a</sub> of
their conjugated acids (BH<sup>+</sup>). This relationship predicts
that the range of alkylimidazole homologues (C<sub>6</sub>–C<sub>13</sub>) observed in atmospheric HULIS would be 1.5–4.4 times
more effective than imidazole in facilitating HULIS-mediated ROS generation.
Our work reveals that the ability of atmospheric PM organics to catalyze
generation of ROS in cells could be affected by coexisting redox inactive
organic constituents and suggests further work deploying multiple
assays be conducted to quantify redox capabilities and enhancement
effects of the HULIS components
Major behavioral and serologic variables by Hukou among drug users from the detoxification center and community.
<p>IDU: injection drug use.</p>*<p>Variable represents characteristic of injection drug users.</p
Social demographic characteristics among drug users from the detoxification center and community.
<p>IDU injection drug use. <i>CI</i> confidence interval.</p>*<p>Denotes significance at alpha value of 0.05.</p
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