Ambient Molecules Research Articles

Theoretical insights into the dimerization mechanism of aluminum species at two different pH conditions

Dimerization mechanism of aluminum species is investigated in weakly acidic solutions and in strongly alkaline solutions using density functional theory, respectively. The results indicate the dimerization is controlled by dehydration of monomeric aluminum and the dissolution of dimer is controlled by the hydroxyl bridge cleavage in weakly acidic solutions. The two processes are enhanced by the rising pH. In addition, the main pathway generating the fluorine bridge is not the direct polymerization, but the terminal substitution of F− into polymeric aluminum followed by the transition of terminal F to bridging F. The formation of fluorine bridge promotes dissolution and di-fluorine-bridged dimer should be excluded. In strongly alkaline solutions, the formation processes of three dimers were speculated, and the existence of mono-hydroxyl-bridged dimer is denied due to the extraordinary instability and high formation energy barriers. It is possible that mono-oxygen-bridged and di-hydroxyl-bridged dimers coexist in highly alkaline solutions. However, the salts of mono-oxygen-bridged anion are much more stable than salts of di-hydroxyl-bridged anion due to the effect of counter cations and the removal of ambient water molecules.

Simultaneous Random Number Generation and Optical Tweezers Calibration Employing a Learning Algorithm Based on the Brownian Dynamics of a Trapped Colloidal Particle

True random number generators are in high demand for secure cryptographic algorithms. Unlike algorithmically generated pseudo-random numbers they are unclonable and non-deterministic. A particle following Brownian dynamics as a result of the stochastic Ornstein-Uhlenbeck process is a source of true randomness because the collisions with the ambient molecules are probabilistic in nature. In this paper, we trap colloidal particles in water using optical tweezers and record its confined Brownian motion in real-time. Using a segment of the initial incoming data we train our learning algorithm to measure the values of the trap stiffness and diffusion coefficient and later use those parameters to extract the “white” noise term in the Langevin equation. The random noise is temporally delta correlated, with a flat spectrum. We use these properties in an inverse problem of trap-calibration to extract trap stiffnesses, compare it with standard equipartition of energy technique, and show it to scale linearly with the power of the trapping laser. Interestingly, we get the best random number sequence for the best calibration. We test the random number sequence, which we have obtained, using standard tests of randomness and observe the randomness to improve with increasing sampling frequencies. This method can be extended to the trap-calibration for colloidal particles confined in complex fluids, or active particles in simple or complex environments so as to provide a new and accurate analytical methodology for studying Brownian motion dynamics using the newly-emerged but robust machine learning platform.

Surface and interface engineering of two-dimensional bismuth-based photocatalysts for ambient molecule activation

This review endeavors to reveal the inherent functionality of the surface and interface to the reactivity and selectivity of photocatalytic ambient molecule activation, with 2D Bi-based photocatalysts as the platform.

The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films

AbstractWe utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h‐BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h‐BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h‐BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal‐to‐noise ratio under ambient conditions, we propose a line‐scan method for the acquisition of UV Raman spectra in atomically thin h‐BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures.

Dynamics of respiratory droplets carrying SARS-CoV-2 virus in closed atmosphere

From the epidemiological point of view, the lifetime of cough and sneeze droplets in the ambient atmosphere plays a significant role in the transmission rate of Coronavirus. The lifetime of indoor respiratory droplets, per se, is a function of droplet size, ambient temperature, and humidity. In the attempt to explore the effective factors of droplet lifetime, sufficient knowledge of atomic-scale interactions and dynamics of the droplet with themselves, as well as the airflow molecules in the room space, is necessary. In this study, the vertical traveling of a wide range (100 nm–10 μm) of representative carrier droplets is studied in three ambient temperatures of 258, 298, and 318 K using all-atom molecular dynamics simulation. Our obtained results confirm that by increasing the room temperature, the suspending time of aerosol (suspended droplets carrying virus particles) increases due to the higher dynamics of air and evaporated water molecules in room space. In fact, by increasing the indoor temperature, the collision rate of aerosol and ambient atmosphere molecules increases significantly. Our result shows this higher rate of collision could have a dual effect on the lifetime of aerosol considering the fact of faster deposition of larger (heavier) droplet due to the gravitational force. On one hand, in higher temperatures, the higher collision can split the droplets to smaller ones with a semi-permanent suspension period. On the other hand, the higher dynamics of ambient molecules can lead to meet and coalesce of smaller cough/sneeze droplets making larger (heavier) droplets with faster sediment times. So, the role of indoor humidity to fuel the probability of coalescence phenomenon and lifetime of droplets becomes more determinant in the warmer spaces.

The formation of atomic oxygen and hydrogen in atmospheric pressure plasmas containing humidity: picosecond two-photon absorption laser induced fluorescence and numerical simulations

Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio.

Beyond lipid peroxidation: Distinct mechanisms observed for POPC and POPG oxidation initiated by UV-enhanced Fenton reactions at the air-water interface.

Fenton or Fenton-like reactions are ubiquitous in nature, and the hydroxyl radicals (·OH) generated in these reactions are accountable for a plethora of oxidation processes both in the environment and in vivo. Among these oxidation reactions, lipid oxidation initiated by ·OH radicals has long been oversimplified as a peroxidation mechanism, but in reality, it is a highly complicated process that can result in a large variety of products. Using the unique field-induced droplet ionization mass spectrometry (FIDI-MS) methodology that is capable of selective sampling of amphiphilic molecules that reside at the air-water interface, here, we show distinct mechanisms from the ultraviolet (UV)-enhanced Fenton oxidations of two phospholipids, POPC and POPG, even though these two lipids possess the same functional groups that are vulnerable to ·OH attack. We postulate that it is the different packing densities that determine the permeability of ambient NO molecules into the monolayers, resulting in highly distinct reaction pathways and products. We anticipate that this work will be a wake-up call that the lipid peroxidation mechanism is sometimes taken for granted and that lipid oxidation can be subtly affected by various factors that deserves deeper investigations.

Spatiotemporal regulation of GABA concentration in extracellular space by gliotransmission crucial for extrasynaptic receptor-mediated improvement of sensory tuning performance in schizophrenia.

In schizophrenic patients, sensory tuning performance tends to be deteriorated (i.e., flattened sensory tuning), for which impaired intracortical tonic inhibition arising from a reduction in GABA concentration in extracellular space might be responsible. The δ subunit-containing GABAA receptor, located on extrasynaptic sites, is known to be involved in mediating tonic inhibitory currents in cortical pyramidal cells and is considered to be one of the beneficial therapeutic targets for the treatment of schizophrenia. The transporter GAT-1 in glial (astrocytic) membrane controls concentration of GABA molecules by removing them from extracellular space. We speculated that the upregulation of extrasynaptic receptors might compensate for the impaired tonic inhibition and thus improve their sensory tuning performance, in which the astrocytic GABA transporter might play an important role. To test our hypothesis, we simulated a schizophrenic neural network model with a GABAergic gliotransmission (i.e., GABA transport by transporters embedded in astrocytic membranes) mechanism that modulates local ambient (extracellular) GABA levels in a neuronal activity-dependent manner. Upregulating extrasynaptic GABA receptors compensated the impaired tonic inhibition and sharpened the sensory tuning, provided that ambient GABA molecules around stimulus-sensitive pyramidal cells were actively removed during sensory stimulation. We suggest that the upregulation of extrasynaptic GABA receptors can improve the performance of sensory tuning in schizophrenic patients, for which spatiotemporal regulation of ambient GABA concentration by gliotransmission may be crucial.

Optical emission spectroscopy of non-equilibrium microwave plasma torch sustained by focused radiation of gyrotron at 24 GHz

The excitation and gas temperature of a non-equilibrium argon plasma torch sustained by a focused 24 GHz microwave beam in the external atmosphere of air was measured by optical emission spectroscopy. The excitation temperature of argon atoms was found about 5000 K, while the vibrational and rotational temperatures of ambient nitrogen molecules were respectively estimated at 3000 K and 2000 K. The effective mixing of the external gas atmosphere into the plasma torch is experimentally demonstrated. The vibrational and rotational temperatures of nitrogen molecules slightly increased with the distance from the nozzle exit. Experimental results demonstrate that microwave discharge at atmospheric pressure may be relevant for the decomposition of highly stable molecules, e.g. carbon dioxide, in non-equilibrium conditions.

A prolonged evaluation of air contamination level with CO2 in a college student’s laboratory using nanosized MgO

Three nanocrystalline eco-friendly MgO samples, as pure and modified with 1% (wt/wt) F− or K2O at 600 °C, with high surface area and mesoporous nature were prepared by a simple sonochemical process. The three samples were characterized by XRD, surface area, porosity and TEM techniques. These samples were kept in contact with the atmosphere in the middle of a student’s organic chemistry laboratory and were examined toward capturing the ambient CO2 molecules through continuous exposure for a period of 8 weeks. The capturing capacity of these samples was shown to be in the following order: MgO > MgO-1%F > MgO-1%K2O. After 8 weeks, the amount captured by MgO was 346.2 $$ {\text{mg}}_{{{\text{CO}}_{2} }} \,{\text{g}}^{ - 1} $$, while MgO-1%F captured 311.4 $$ {\text{mg}}_{{{\text{CO}}_{2} }} \,{\text{g}}^{ - 1} $$, and finally MgO-1%K2O had the lowest capturing capacity of 252.9 $$ {\text{mg}}_{{{\text{CO}}_{2} }} \,{\text{g}}^{ - 1} $$. The obtained results showed that our samples are characterized by prolonged activity and reactivity toward the capturing of ambient CO2 molecules. Another series of experiments were carried out by passing pure CO2 gas (instead of the atmospheric CO2) over such samples, inside a glass cubic chamber, for 1 week. The results showed a similar trend in the TG desorption and TPD curves of pure CO2 as those recorded in ambient atmosphere. Values of the molar enthalpy change (ΔH) for CO2 desorption, corresponding to the two steps recorded for each sample in all durations, were calculated and were in agreement with those published formerly. Deconvolution of the IR-broad band, between 1800 and 1200 cm−1, revealed the presence of different types of basic sites on the surface of these samples under investigation.

Residual Gas Adsorption and Desorption in the Field Emission of Titanium-Coated Carbon Nanotubes.

Titanium (Ti)-coated multiwall carbon nanotubes (CNTs) emitters based on the magnetron sputtering process are demonstrated, and the influences of modification to CNTs on the residual gas adsorption, gas desorption, and their field emission characteristic are discussed. Experimental results show that Ti nanoparticles are easily adsorbed on the surface of CNTs due to the “defects” produced by Ar+ irradiation pretreatment. X-ray photoelectron spectroscopy (XPS) characterization showed that Ti nanoparticles contribute to the adsorption of ambient molecules by changing the chemical bonding between C, Ti, and O. Field emission of CNTs coated with Ti nanoparticles agree well with the Fowler–Nordheim theory. The deviation of emission current under constant voltage is 6.3% and 8.6% for Ti-CNTs and pristine CNTs, respectively. The mass spectrometry analysis illustrated that Ti-coated CNTs have a better adsorption capacity at room temperature, as well as a lower outgassing effect than pristine CNTs after degassing in the process of field emission.

Destruction of dioxin and furan pollutants via electrophilic attack of singlet oxygen

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) remain of particular concern owing to their extensive toxicity towards health and accumulation in the environment. Atmospheric oxidation (by ambient oxygen molecules) of this class of persistent environmental pollutants has little to no kinetic feasibility due to very sizable activation energies in the entrance channel. The current control measures involve energy-intensive source incineration of contaminated materials at high temperatures as high as 850 °C. This study finds an alternative low-energy approach of destroying dioxin-like compounds, proposing that advanced oxidation by highly reactive singlet oxygen (O21Δg, originated from chemical, surface-mediated and photochemical processes) can initiate low-temperature remediation of these pollutants. This contribution completes the first milestone in mapping out the mechanisms of the electrophilic addition of singlet oxygen to unsubstituted and chlorinated dibenzo-p-dioxin (DBD) and dibenzofuran (DBF) structures, according to density functional theory DFT-B3LYP method in conjunction with the 6–311+g(d,p) basis set, as well as energy refinements based on the approximate spin-projection scheme. The [2+2]-cycloaddition mechanism appears dominant for singlet oxidation of dibenzo-p-dioxin with a fitted rate constant of k(T) = 5.01 × 10−14 exp(-98000/RT). On the other hand, the addition of singlet oxygen to the aromatic ring of dibenzofuran primarily transpires via [4+2]-cycloaddition channel with a fitted rate constant of k(T) = 2.16 × 10−13 exp(-119000/RT). The results suggest that application of singlet oxygen can reduce the energy cost of recycling halogenated and flame retarded materials.

Elastic rate coefficients forLi+H2collisions in the calibration of a cold-atom vacuum standard

Ongoing efforts at the National Institute of Standards and Technology in creating a cold-atom vacuum standard device have prompted theoretical investigations of atom-molecule collision processes that characterize its operation. Such a device will operate as a primary standard for the ultrahigh-vacuum and extreme-high-vacuum regimes. This device operates by relating loss of ultracold lithium atoms from a conservative trap by collisions with ambient atoms and molecules to the background density and thus pressure through the ideal gas law. The predominant background constituent in these environments is molecular hydrogen H2. We compute the relevant Li+H2 Born-Oppenheimer potential energy surface, paying special attention to its uncertainty. Coupled-channel calculations are then used to obtain total rate coefficients, which include momentum-changing elastic and inelastic processes. We find that inelastic rotational quenching of H2 is negligible near room temperature. For a (T = 300)-K gas of H2 and 1.0-μK gas of Li atoms prepared in a single hyperfine state, the total rate coefficients are 6.0(1) × 10-9 cm3/s for both 6Li and 7Li isotopes, where the number in parentheses corresponds to a one-standard-deviation combined statistical and systematic uncertainty. We find that a 10-K increase in the H2 temperature leads to a 1.9% increase in the rate coefficients for both isotopes. For Li temperatures up to 100 μK, changes are negligible. Finally, a semiclassical Born approximation significantly overestimates the rate coefficients. The difference is at least ten times the uncertainty of the coupled-channel result.

A review on the upper atmospheric sodium observations from India: Insights

This review article briefly brings out the historical development of atmospheric sodium (Na) measurements over India and the importance of coordinated measurements with multiple techniques to address physical processes in the Earth's upper atmosphere. These measurements were initiated in the early 1970s by observing Na airglow emission intensities with broad band airglow photometer from Mt. Abu, a low-latitude hill station in India. Considerable amount of night-to-night variations in nocturnal emission intensities of the Na airglow were observed. Later, investigations regarding the dependence with the magnetic activities from the equatorial and low latitudes were carried out and double-humped structures in the nocturnal variation of intensities were reported. With the advent of Na lidar at Gadanki around 2005, the measurements of atmospheric neutral Na atom concentration became possible and more frequent occurrences of sporadic Na layers over the magnetic low latitude station compared to other latitudes were detected indicating the role played by electrodynamics. Later, a possible relationship between E-region field aligned plasma irregularities and the concentration of neutral Na atoms was investigated using coordinated measurements of VHF radar and Na lidar. Further, simultaneous measurements with Na lidar and a narrow band airglow photometer with narrow field of view brought out the importance of coordinated observation wherein the characterization of gravity waves could be carried out and also revealed the importance of collisional quenching due to ambient molecules in the Na airglow emission processes. In addition, combining the ground based measurements of Na lidar and meteor wind radar along with satellite measurements made possible to hypothesize the over-turning Kelvin–Helmholtz billow in the Na layer manifests “C-type” or inverted lambda shape structures in the height-time-concentration map of neutral Na atoms. This review paper presents a synoptic view mostly based on the previously reported observations of Na airglow emission, Na lidar and coordinated Na airglow and Na lidar observations from the Indian sector and highlights the importance of simultaneous measurements of mesospheric Na and its emissions along with satellite-borne measurements to address interesting geophysical processes in the Earth's upper atmosphere.

An optimized resistive CNT-based gas sensor with a novel configuration by top electrical contact

The paper proposes a new fabrication method of a resistive gas sensor made of vertically aligned carbon nanotubes (CNT) by the perforated electrode. This new configuration can be also utilized with vertically aligned nanomaterial such as ZnO wires. This method contains two aspects, which are getting top electrical contact from MWCNTs and devising hexagonal pattern holes on the sensor surface to ease the gas flow through the sensor. Reaching ambient molecules to these dense CNT forests is always of the essence; also, the top electrical contact of the CNTs is a fabrication challenge. The high sensitivity of proposed CNT-based sensor is attributed to their collective large porous surface along with etched circular holes located at the apexes and center of a hexagonal pattern on sensor top contact. For optimal sensor performance, the simulation approach by ANSYS-FLUENT software has optimized circular holes diameters and distances. The fabrication process proposes an oxygenated plasma treated CNTs with top electrical contact devised by a certain simple method for the very first time. Top electrical contact has been fabricated possible by the procedure of immersing the CNTs in viscosity liquid and providing electrical contact using conductive paste. Well-aligned multi-walled carbon nanotubes (MWCNT) have been grown on glass substrate placed into DC-PECVD unit bearing approximately temperature of 350 °C. In this paper, the resistivity of carbon nanotubes at different temperatures for the ambient gas and linear sensitivity to oxygen gas has been investigated. The sensors with larger diameter nanotubes are more sensitive. Etching patterned holes with 200 μm diameter enhances the sensitivity up to 50%.

Frictional characteristics of nano-confined water mediated hole-doped single-layer graphene on silica surface

We have investigated the frictional properties of single-layer graphene (SLG) coated rough silica substrate under the influence of nano-confined hydration layer underneath SLG. Through the friction and surface potential measurements by atomic force microscopy (AFM), we found polygonal features in AFM images of SLG-protected silica surface that exhibit simultaneously larger friction and higher surface potential as compared to their surrounding areas due to water layers confined under SLG. Nano-confined water layers at the SLG-silica interface can induce the hole-doping effect in SLG, resulting in a more positively-charged and hydrophilic surface that favors adsorption of ambient water molecules. Therefore, during friction measurements, nanoscale capillary bridges can form within the interstices of AFM probe-SLG contact, leading to larger adhesion and friction. The friction forces were found to respectively have negative and positive dependence on the sliding velocity inside and outside the polygonal regions due to different surface wettability. Hence, it is possible to manipulate the frictional properties of SLG-coated silica by the amount of hydration layer confined underneath SLG. Our results may find applications in friction control for future nano-devices.

Titanium dioxide reveals basis of key surface property

Titanium dioxide has made a name for itself as a catalytic coating that renders glass self-cleaning. Despite the popularity, the molecular basis of some of TiO2’s useful properties puzzles scientists. For example, researchers know that shining ultraviolet light on TiO2 exposed to air and water makes the material’s surface hydrophilic by oxidizing ambient organic molecules, which produces hydroxylated species. It’s unclear though, why when left in the dark, the surface gradually switches to hydrophobic, and why it develops a so-called 2 x 1 overlayer, which suggests there are molecules binding to every other Ti atom. A team led by Jan Balajka and Ulrike Diebold of Technical University of Vienna and Melissa A. Hines of Cornell University has an answer. By controlling the purity of TiO2’s environment, the team shows that trace levels of acetic and formic acids in air bind to TiO2 and form hydrophobic layers that repel other air

Structure and Excitation Dynamics of β-Carotene Aggregates in Cetyltrimethylammonium Bromide Micelle

The β-carotene(β-Car) aggregate was prepared by self-assembly in cetyltrimethylammonium bromide (CTAB) micelle. The ground state absorption measurement showed that the aggregate has J-type characteristics and resonance Raman spectra gave the intrinsic explanation of molecular interaction in aggregate. Upon excitation at the optical allowed S2 state of aggregate, direct generation of triplet state via singlet fission(SF) mechanism was observed. Excitation dynamics was elucidated by fs-transient absorption spectroscopy and ns-flash photolysis, respectively. The triplet state life time of aggregate was found to be independent of the ambient oxygen molecules.

Environment-dependent and anion-vacancy-controlled reversible phase transition of MoS2 synthesized by chemical vapor deposition

The crucial role of anion vacancies on the phase stability of synthetic metal chalcogenides was elucidated by demonstrating the environment-dependent, reversible phase transition (2H ↔ 1T/1T′) of the MoS2 films synthesized by chemical vapor deposition. The origins of the sulfur (S)-vacancy-controlled phase transition of the synthetic MoS2 films were supported by various transmission electron microscopy (TEM) studies as well as density functional theory calculations. Under high-vacuum conditions, transition to the metastable 1T/1T′ phases was induced by weak electron irradiation during a TEM observation and explained by the detachment of molecules chemisorbed on the S-vacancy sites, and the subsequent electron-delocalization (charge-transfer) process. In addition, a spontaneous backward transition to a 2H phase could be triggered by exposing the sample to air, which induced electron localization by re-adsorption of ambient molecules on the S-vacancy sites.

A dispersion-corrected DFT investigation of [formula omitted] adsorption by silver-decorated monolayer graphene in the presence of ambient oxygen molecules

The main purpose of the present article is an investigation of electronic properties of Ag-decorated graphene (Ag-G), in the presence of ambient O2 molecules, when methane gas is adsorbed. To pursue this aim, we employ the dispersion-corrected density functional theory (DFT-D2) in which the Van der Waals interactions, as well as spin-polarization-mode, are explicitly activated. Our calculations, thereby, demonstrate the stable geometrical configuration, the corresponding parameters and the adsorption energy of CH4/(O2-Ag-G). For a comparison, moreover, we also report the geometrical properties and adsorption energies for CH4/(O2-G),CH4/(Ag-G) and O2/(Ag-G). We then proceed by calculating the charge transfer and charge redistribution, as a result of adsorption, for the aforementioned combinations. The manner of charge transfer and redistribution serves to justify the electronic spin-polarized band structures and densities of states, which we also report. Our results then demonstrate that the adsorption energy of CH4 onto Ag-decorated graphene, even in the presence of ambient O2, is by far larger than the undecorated one. In addition, considerations of the band structures, reveal an increase in the response of O2-Ag-G to the presence of CH4 in comparison with that of O2-G. The last point, in combination with the enhancement of adsorption energy, provide strong motivation for the use of Ag-decorated graphene as a low-cost and highly sensitive methane gas detector.