Can interaction with informal urban green space reduce depression levels? An analysis of potted street gardens in Tangier, Morocco.

OBJECTIVES: The purpose of this study was to investigate the association between ownership of a potted street garden (PSG) and depression levels in a densely populated, disadvantaged Moroccan neighbourhood. STUDY DESIGN: The study design used was a cross-sectional study. METHODS: Data were collected through a face-to-face survey conducted in January 2019. In total, there were 388 participants, in three densely populated neighbourhoods of the Beni-Makada district of Tangier, Morocco. We measured depression levels using the Patient Health Questionnaire-9 and data were analysed using weighted moderated ordinary least squared regression analysis. RESULTS: PSG ownership was associated with a .74-point increase in depression score (b = .74, 95% confidence interval [CI] = .38, 1.10, ß = .22, Variance Inflation Factor (VIF) = 1.15; P < .001). PSG ownership also moderated the negative association between depression levels and neighbourhood life satisfaction (F [3,336] = 5.058, P < .001, R2 change = .039). A one-level increase in PSG being perceived as a public amenity by their owners was associated with a .36-point decrease in depression score (b = -.36, 95% CI = -.71, -.01, ß = -.14, VIF = 1.08; P < .05), whereas a 1-min increase in PSG daily care duration was associated with .04-point increase in depression score (b = .04, 95% CI = .01, .06, ß = .24, VIF = 1.68; P < .01). CONCLUSIONS: Our findings suggest that PSG ownership might have a negative impact on mental health in densely populated, disadvantaged neighbourhoods. This negative association might be due to the fact of PSGs being deemed as private property present in an unsafe and uncontrolled environment.

Non-regular hexagonal 2D carbon, an allotrope of graphene: a first-principles computational study.

In a first principle computational study, using density functional theory, we have identified four types of 2D carbon sheets, similar to graphene, made entirely of non-regular hexagons. In one case, we get a structure where the non-regular hexagons have four sides of length d1 = 1.416 Å and two sides of length d2 = 1.68 Å. Next case, in the non-regular hexagons the side d1 (two times) and d2 (four times) are exchanged. In two other cases, the non-regular hexagons have three pairs (opposite sides) of different lengths (d1 = 1.529 Å, d2 = 1.567 Å, and d3 = 1.612 Å; d1 = 1.387 Å, d2 = 1.348 Å, and d3 = 1.387 Å). By propper choice of the non-regular hexagon sides, one could arrive at a 2D carbon system like graphene, but with a tunable band gap. The structure is more stable when the system has more number of regular C-C bonds than the longer C-C bonds. Due to its non-regular hexagons, special atomic configuration, this system may have, like graphene, unusual properties. It is semiconducting, and there is no need to functionalize it for opening the band gap as is the case with graphene.

Penta-BCN: A New Ternary Pentagonal Monolayer with Intrinsic Piezoelectricity.

Going beyond conventional hexagonal sheets, pentagonal 2D structures are of current interest due to their novel properties and broad applications. Herein, for the first time, we study a ternary pentagonal BCN monolayer, penta-BCN, which exhibits intrinsic piezoelectric properties. Based on state-of-the-art theoretical calculations, we find that penta-BCN is stable mechanically, thermally, and dynamically and has a direct band gap of 2.81 eV. Due to its unique atomic configuration with noncentrosymmetric and semiconducting features, penta-BCN displays high spontaneous polarization of 3.17 × 10-10 C/m and a prominent piezoelectricity with d21 = 0.878 pm/V, d22 = -0.678 pm/V, and d16 = 1.72 pm/V, which are larger than those of an h-BN sheet and functionalized pentagraphene. Since B, C, and N are rich in resources, light in mass, and benign to the environment, the intrinsic polarization and piezoelectricity make the penta-BCN monolayer promising for technological applications. This study expands the family of 2D pentagonal structures with new features.

Low thermal conductivity of peanut-shaped carbon nanotube and its insensitive response to uniaxial strain.

Motivated by the experimental synthesis of peanut-shaped carbon nanotubes (PSNTs) that combine the novel features of fullerene and carbon nanotubes (CNTs), we study the thermal conductivity of a PSNT (1dp08) and its response to different strains by using non-equilibrium molecular dynamics simulations and lattice dynamics together with density functional theory. We find that the thermal conductivity of the PSNT is reduced by more than 90% as compared to that of CNTs, and remains almost the same when different strains applied, exhibiting very different behaviors from that of CNTs, where the thermal conductivity decreases monotonically with the increase of strain. Through phonon mode calculations, we show that the reduced phonon group velocity, phonon lifetime and the vibrational mismatch are responsible for the low thermal conductivity of the PSNT, and the insensitive response of thermal conductivity to strain is due to the insensitivity of its phonon density of states and group velocity to strain. These features endow the PSNT with the potential applications in thermal devices, and add new features to one-dimensional carbon nanomaterials going beyond conventional CNTs.

A theoretical exploration of the intermolecular interactions between resveratrol and water: a DFT and AIM analysis.

The polyphenolic compound resveratrol, classified under stilbenes, offers a broad range of health advantages, including neuroprotection and playing a role in autophagy in the nervous system. However, resveratrol has poor water solubility and is soluble in the gel phase in liposomal membranes. The main aim of this work was to understand the nature of the interactions between resveratrol and water molecules. In the present study, we used the dispersion corrected density functional theory (DFT) method to study hydrogen bonding interactions. Eight different geometries of resveratrol-water complexes were identified by optimizing the geometries by placing water at various locations. We observed the two lowest energy structures to be isoenergetic. In most complexes, water interaction occurs with phenolic hydrogen as all the phenolic hydroxyl groups have identical Vs,max values. Energy decomposition analysis shows that the dispersion contribution was minimal in these complexes, while electrostatic and orbital contributions were larger. Complex formation between water and the resveratrol molecule results in a blue shift in the vibrational frequency, along with an increase in intensity due to the transfer of electron density. The hydrogen bonds in the resveratrol-water complexes have closed-shell interactions with a medium-to-strong bonding nature. Noncovalent index analysis of the complexes shows that, in addition to hydrogen bonding, electrostatic and van der Waal's interactions play a key role in stabilizing the complexes. Graphical abstract Noncovalent index analysis showing that, in addition to hydrogen bonding, electrostatic and van der Waal's interactions play a major role in stabilizing resveratrol-water complexes.

Extensive first-principles molecular dynamics study on Li encapsulation into C60 and its experimental confirmation.

The aim of increasing the production ratio of endohedral C60 by impinging foreign atoms against C60 is a crucial matter of the science and technology employed towards industrialization of these functional building block materials. Among these endohedral fullerenes, Li+@C60 exhibits a wide variety of physical and chemical phenomena and has the potential to be applicable in areas spanning the medical field to photovoltaics. However, currently, Li+@C60 can be experimentally produced with only ∼1% ratio using the plasma shower method with a 30 eV kinetic energy provided to the impinging Li+ ion. From extensive first-principles molecular dynamics simulations, it is found that the maximum production ratio of Li+@C60 per hit is increased to about 5.1% (5.3%) when a Li+ ion impinges vertically on a six-membered ring of C60 with 30 eV (40 eV) kinetic energy, although many C60 molecules are damaged during this collision. On the contrary, when it impinges vertically on a six-membered ring with 10 eV kinetic energy, the production ratio remains at 1.3%, but the C60 molecules are not damaged at all. On the other hand, when the C60 is randomly oriented, the production ratio reduces to about 3.7 ± 0.5%, 3.3 ± 0.5%, and 0.2 ± 0.03% for 30 eV, 40 eV, and 10 eV kinetic energy, respectively. Based on these observations we demonstrate the possibility of increasing the production ratio by fixing six-membered rings atop C60 using the Cu(111) substrate or UV light irradiation. In order to assess the ideal experimental production ratio, the 7Li solid NMR spectroscopy measurement is also performed for the multilayer randomly oriented C60 sample irradiated by Li+ using the plasma shower method combined with inductively coupled plasma atomic emission spectroscopy (ICP-AES). Time-of-flight mass spectroscopy measurements are also performed to cross check whether Li+@C60 molecules are produced in the sample. The resulting experimental estimate, 4% for 30 eV incident kinetic energy, fully agrees with our simulation results mentioned above, suggesting the consistency and accuracy of our simulations and experiments.

CO Oxidation Prefers the Eley-Rideal or Langmuir-Hinshelwood Pathway: Monolayer vs Thin Film of SiC.

Using the first-principles approach, we investigated the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer and thin film structures and substantiated their catalytic activity toward CO oxidation. 2H-SiC monolayer, being planar, is quite stable and has moderate binding with O2, while CO interacts physically; thus, the Eley-Rideal (ER) mechanism prevails over the Langmuir-Hinshelwood (LH) mechanism with an easily cleared activation barrier. Contrarily, 2H-SiC thin film, which exhibits a nonplanar structure, allows moderate binding of both CO and O2 on its surface, thus favoring the LH mechanism over the ER one. Comprehending these results leads to a better understanding of the reaction mechanisms involving structural contrast. Weak overlapping between the 2p(z)(C) and 3p(z)(Si) orbitals of the SiC monolayer system has been found to be the primary reason to revert the active site toward sp(3) hybridization, during interaction with the molecules. In addition, the influences of graphite and Ag(111) substrates on the CO oxidation mechanism were also studied, and it is observed that the ER mechanism is preserved on SiC/G system, while CO oxidation on the SiC/Ag(111) system follows the LH mechanism. The calculated Sabatier activities of the SiC catalysts show that the catalysts are very efficient in catalyzing CO oxidation.

Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study.

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir-Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.

Role of Interlayer Coupling on the Evolution of Band Edges in Few-Layer Phosphorene.

Using first-principles calculations, we have investigated the evolution of band edges in few-layer phosphorene as a function of the number of P layers. Our results predict that monolayer phosphorene is an indirect band gap semiconductor and its valence band edge is extremely sensitive to strain. Its band gap could undergo an indirect-to-direct transition under a lattice expansion as small as 1% along the zigzag direction. A semiempirical interlayer coupling model is proposed, which can reproduce the evolution of valence band edges obtained by first-principles calculations well. We conclude that the interlayer coupling plays a dominant role in the evolution of the band edges via decreasing both band gap and carrier effective masses with the increase of phosphorene thickness. Scrutiny of the orbital-decomposed band structure provides a better understanding of the upward shift of the valence band maximum, surpassing that of the conduction band minimum.

Selective gas adsorption in microporous metal-organic frameworks incorporating urotropine basic sites: an experimental and theoretical study.

The sorption of CO, CO2 and C2H2 by two urotropine-containing porous metal-organic framework materials [Zn4(dmf)(ur)2(ndc)4] (H2ndc = 2,6-naphthalenedicarboxylic acid; ur = urotropine; dmf = dimethylformamide) and [Zn11(H2O)2(ur)4(bpdc)11] (H4bpdc = 4,4'-biphenyldicarboxylic acid) incorporating free N-donors has been investigated. These materials show pronounced affinity for CO2 and C2H2, and these observations are supported by interaction energy and ab initio DFT calculations.

Stability and properties of 2D porous nanosheets based on tetraoxa[8]circulene analogues.

Two-dimensional porous nanosheets based on tetraoxa[8]circulene and its analogue are theoretically studied using first principles calculations focusing on their thermal stability and mechanical, electronic, optical and thermoelectric properties. It is found that the nanosheets composed of tetraoxa[8]circulene (TO8C) and tetraaza[8]circulene (TA8C) are thermodynamically and kinetically stable. Both sheets show anisotropic Young's moduli corresponding to tetragonal symmetry. However, due to their porosity, the Young's moduli of both sheets are much smaller than that of graphene. Electronic structure calculations indicate that both TO8C and TA8C nanosheets are direct semiconductors with a band gap of 1.92 eV and 1.83 eV respectively, and they can adsorb strongly visible light and exhibit a huge Seebeck coefficient. Thus they can be promising candidates for solar and thermoelectric applications.

The Intrinsic Ferromagnetism in a MnO2 Monolayer.

The Mn atom, because of its special electronic configuration of 3d(5)4s(2), has been widely used as a dopant in various two-dimensional (2D) monolayers such as graphene, BN, silicene and transition metal dichalcogenides (TMDs). The distributions of doped Mn atoms in these systems are highly sensitive to the synthesis process and conditions, thus suffering from problems of low solubility and surface clustering. Here we show for the first time that the MnO2 monolayer, synthetized 10 years ago, where Mn ions are individually held at specific sites, exhibits intrinsic ferromagnetism with a Curie temperature of 140 K, comparable to the highest TC value achieved experimentally for Mn-doped GaAs. The well-defined atomic configuration and the intrinsic ferromagnetism of the MnO2 monolayer suggest that it is superior to other magnetic monolayer materials.

Insight into the vertical detachment energy oscillation of Na(n)C60(-) clusters.

We have performed a detailed density functional theory study on the structural and electronic properties of Na(n)C(60)(-) (n = 1-12) clusters. The calculated vertical detachment energies show good agreement with the experimental data, which confirms the 3p (n = 3p) oscillation rule. The oscillation can be attributed to the combination of the charge depletion distribution induced by removing electrons and the number of the sodium atoms in direct contact with the fullerene. Based on the structural and electronic properties, the Na atoms can be categorized into two groups, one is for the metal atoms directly bonded to the fullerene surface, and the other one is for those without bonding to the fullerene. The Na atoms in group one would donate electrons to both the fullerene and the Na atoms in group two. As the total number of the sodium atoms increases, the number of Na atoms in group one would continue increasing till the size n = 3p - 1 to meet a shoulder from n = 3p - 1 to n = 3p, which accounts for the maximum vertical detachment energy at the size of n = 3p as drawn from the detailed electronic property studies.

Coinage metal (4, 4) nanotubes, simulated by first-principles calculations.

The structural stability of coinage metal nanotubes with a square cross-section has been investigated by the first-principles numerical simulations. In addition to the reported (4, 4) silver tube, it is found that the hollow (4, 4) copper and gold nanotubes can also be formed by applying an appropriate stress to an 8(A)/8(B) fcc wire. The stability of these coinage metal (4, 4) nanotubes, formed by tip-stretching the wires, has been explained by a local minimum in the string tension variation with their tube lengths. Interestingly, we have explained why a low-stress stretching is needed to obtain the (4, 4) Cu tube in contrast to a higher one for both the (4, 4) Ag and Au tubes due to the larger stiffness coefficient of copper than those of silver and gold, which could be proved by future experiments.

Gate-controlled current and inelastic electron tunneling spectrum of benzene: a self-consistent study.

We use density functional theory based nonequilibrium Green's function to self-consistently study the current through the 1,4-benzenedithiol (BDT). The elastic and inelastic tunneling properties through this Au-BDT-Au molecular junction are simulated, respectively. For the elastic tunneling case, it is found that the current through the tilted molecule can be modulated effectively by the external gate field, which is perpendicular to the phenyl ring. The gate voltage amplification comes from the modulation of the interaction between the electrodes and the molecules in the junctions. For the inelastic case, the electron tunneling scattered by the molecular vibrational modes is considered within the self-consistent Born approximation scheme, and the inelastic electron tunneling spectrum is calculated.

Tuning magnetic properties of Mn(4) cluster with gold coating.

The magnetic properties of transition metal clusters are a unique function of their size and differ from their bulk behavior due to quantum confinement. Here we show that surface modification provides another channel to tune their magnetic properties. This is demonstrated by taking Mn(4) as an example. Although Mn(4) carries a giant magnetic moment of 20 micro(B), the magnetic coupling can be tuned from ferromagnetic to ferrimagnetic by changing the number of gold atoms coated on its surface. We found that 26 gold atoms are needed to fully cover a Mn(4) cluster. When partially coated, the system exhibits ferromagnetic coupling with a total magnetic moment of 18 micro(B) and it becomes ferrimagnetic with a moment of 8 micro(B) when fully coated. This magnetic cross-over is caused by the shrinking of the Mn-Mn bond length, suggesting that the magnetic properties of a Mn(4) cluster can be tuned by controlling the surface coverage.

Polarization-induced switching effect in graphene nanoribbon edge-defect junction.

With nonequilibrium Green's function approach combined with density functional theory, we perform an ab initio calculation to investigate transport properties of graphene nanoribbon (GNR) junctions self-consistently. Tight-binding approximation is applied to model the zigzag (ZGNR) electrodes, and its validity is confirmed in comparison to the GAUSSIAN03 periodic boundary condition calculation result of the same system. The origin of abnormal jump points usually appearing in the transmission spectrum is explained with the detailed tight-binding ZGNR band structure. Transport property of an edge-defect ZGNR junction is investigated, and the tunable tunneling current can be sensitively controlled by transverse electric fields.

Ferromagnetism in semihydrogenated graphene sheet.

Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized pi bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.

Stabilization of square planar silicon: a new building block for conjugated Si-containing systems.

The feasibility of square planar silicon as a building block for conjugated systems was investigated by ab initio calculations. A five-membered ring model system was used to map electronic and steric substituent effects that might help in the stabilization of the planar structure. A pi push-pull arrangement around the silicon was found to prefer planarity. Aromaticity was proven to play an important role in stabilization as well. With the help of steric constraints, a new structure was proposed as a synthetic target containing square planar silicon. The kinetic stability of this structure was also investigated.

Interaction of gas molecules with Ti-benzene complexes.

Using first-principles calculations based on gradient corrected density functional theory, we have studied the interaction of NH(3), H(2), and O(2) with Ti-benzene complexes [Ti(Bz)(2) and Ti(2)(Bz)(2)]. The energy barriers as the gas molecules approach the Ti-benzene complexes as well as the geometries of the ground state of these interacting complexes were obtained by starting with several initial configurations. While NH(3) and H(2) were found to physisorb on the Ti(Bz)(2) complex, the O(2) reacts with it strongly leading to dissociative chemisorption of the oxygen molecule. In contrast all the gas molecules react with the Ti(2)(Bz)(2) complex. These studies indicate that the reaction of certain, but not all, gas molecules can be used to probe the equilibrium geometries of organometallic complexes. Under special conditions, such as high pressure, the Ti atom intercalated between benzene molecules in Ti(Bz)(2) and the Ti(2)(Bz)(2) complexes could store hydrogen in chemisorbed states. The results are compared to available experimental data.