Density Functional Theory Study of Two-Dimensional Boron Nitride Films

Since graphene was isolated in 2004, the number of two-dimensional (2D) materials and their scientific relevance have grown exponentially. Besides graphene, one of the most important and technolocially promizing 2D materials that has emerged in recent years is hexagonal boron nitride, in its monolayer or multilayer form. In my thesis work, I used density functional theory (DFT) calculations to investigate the properties of boron nitride films. In particular, I first studied the properties (i.e. formation energy, defect states, and structure) of point charged defects in monolayer and bilayer hexagonal boron nitride, and subsequently, I focused on the linear and nonlinear mechanical properties of boron nitride films with sp2 and sp3 bonding structures. Experimental detection and characterization of defects in 2D materials are challenging tasks. My research work has been focused on points defects in monolayer (1L-) and bilayer hexagonal boron nitride (2L-hBN). I carried out technical developments and DFT calculations, and I have investigated the structural, formation energy, and electronic properties of both neutral and charged nitrogen and boron vacancy defects, as well as carbon substitutions for nitrogen and boron sites in 1L-hBN and 2L-hBN. These studies have shown that, due to an electrostatic polarization effect, the formation energies of charged defects in 2L-hBN are, in all cases, about 0.5 eV lower than in monolayer 1L-hBN. Moreover, I found that, under the assumption that vacancies and carbon substitutions defects are all present in a 2D h-BN film, there is at least one point defect species that is in a charged state, regardless the value of the Fermi energy. Carbon and boron nitride form a variety of solid allotropes, including layered materials such as graphite and h-BN, and sp3-bonded materials such as diamond and cubic BN. Similarly to the case of graphene, which can be considered a single layer of graphite, in recent years experimental investigations have shown that a diamond or sp3-rich phase can exist in the ultrathin down to the monolayer form. In particular, these experimental studies agree that under pressure and/or upon surface passivation (with hydrogen or hydroxyl groups), a few-layer graphene can transform into a sp3-rich C film, exhibiting a stiffness comparable to that one of diamond. As for BN, although high-purity polycrystalline h-BN is known to form a sp3-bonded phase upon compression, to the best of our knowledge only one very recent experimental study has shown that upon compression, nanosheets of h-BN transform into films rich in sp3 bonds. In this work, the sp3-bonded BN film was proposed to have a surface in contact with the substrate, and hydroxyl groups terminating the surface exposed to air. The aforementioned experimental works underline the importance of studying the mechanical properties of BN ultrathin membranes having both a sp2 and sp3 bonding structure. In my thesis work, I have used a DFT approach to calculate linear and nonlinear elastic constants of BN membranes. These elastic constants were used to estimate the ideal breaking strength under biaxial strain, and this theoretical parameter was then used to quantify the effects of film thickness, surface passivants, and structure on the mechanical strength of the membranes. In my work, I found that compression of a few-layer h-BN film can lead to the formation of various plausible conformations of an ultrathin BN film with a sp3 bonding structure. Most of these sp3-bonded 2D films are energetically stable upon passivation of at least one surface. Nonetheless, I found that three-layer BN films can form stable ultra- thin films with a sp3 bonding structure and clean surfaces. Although the BN sp3-bonded membranes exhibit longitudinal mechanical properties comparable to those of the layered BN film, the benefits of a sp3-bonded membrane include the enhanced mechanical strength in the transverse direction of the film, and potentially, the possibility to form conformations of the films with anisotropic and tunable mechanical and electrical properties

Monoamine oxidase (MAO) inhibitory effects of candidate MAO inhibitors found in cigarette smoke.

There is strong evidence that tobacco smoke inhibits both MAO A and MAO B isoforms in the body. However, which components of cigarette smoke are responsible for MAO inhibition is not clear yet. Our group has identified six previously unidentified candidate MAO inhibitors from the tobacco smoke. The MAO inhibitory effects of these candidate inhibitors were compared with that of nicotine and TPM (Tobacco Particulate Matter). An SH-SY5Y cell line was exposed to different regimens of ethanol (control), nicotine, TPM and the cocktail of candidate inhibitors. A final concentration 0.2 μM nicotine was used and the concentration of each candidate inhibitor was relative to that originally found in TPM. We found that nicotine did not have any significant MAO inhibitory effect compared to the control. TPM inhibited overall MAO activity by 39%, while the MAO inhibition by cocktail of candidate inhibitors was 47%. The results suggest that the candidate inhibitors identified by our group are the major contributors to the total MAO inhibitory activity depicted by cigarette smoke and potentially unlocks the mystery behind the components responsible for MAO inhibition by cigarette smoke in smokers

Geocentric Gestures as a Research Tool

This study1 is part of a large-scale cross-cultural research project on the development of spatial language and cognition, in India, Indonesia and Nepal, that focuses on a culturally particular way of organizing small-scale, table space, using a large-scale geocentric spatial orientation system (Dasen & Mishra, in preparation). One of the main questions is at what age this geocentric frame of reference starts to be effective. The study of language development does not provide a clear answer, because young children (ages 4 to 7) use ambiguous “deictic” descriptions, i.e., they just say “this way” accompanied by a gesture. Can these gestures be used to clarify the meaning of language? To answer this question, 234 video recordings of Nepalese children performing the “Perspectives” task (in which they have to describe the location of three objects placed on a table in front of them, under three different conditions) were analyzed separately for both language and gestures. The results show a good correspondence between language and gestures in 9 to 12 year olds. This allows us to interpret further the frame of reference used by the younger children. Out of 367 items on which young children (4 to 9 years) give an ambiguous deictic answer, only 17% are accompanied with an egocentric gesture, and 83% with a geocentric one (combining 48% large gestures linked to the use of cardinal directions, and 35% medium-large gestures linked to the use of situational local landmarks). This shows that a geocentric frame is at play as early as age 4, even when the child cannot express it clearly in the language

Signature of structural distortion in optical spectra of YFe2O4 thin film

We report structural, optical, and electro-optical properties of polycrystalline YFe2O4 thin films, deposited on (0001) sapphire substrates using the electron-beam deposition technique. The optical spectra of a 120 nm YFe2O4 show Fe d to d on-site and O 2p to Fe 3d, Y 4d, and Y 5s charge-transfer electronic excitations. Anomalies in the temperature dependence data of the charge-transfer excitations and the splitting of the 4.46 eV charge-transfer peak strongly suggest a structural distortion at 180 ± 10 K. Evidence of such a structural distortion is also manifested in the surface resistance versus temperature data. In addition, the YFe2O4 thin film at low temperatures shows strong electro-optical properties, as high as 9% in the energy range of 1 - 2.5 eV, for applied electric fields up to 500 V.cm−1

How Might Recharge Change Under Projected Climate Change in the Western U.S.?

Although groundwater is a major water resource in the western U.S., little research has been done on the impacts of climate change on groundwater storage and recharge in the West. Here we assess the impact of projected changes in climate on groundwater recharge in the near (2021–2050) and far (2071–2100) future across the western U.S. Variable Infiltration Capacity model was run with RCP 6.0 forcing from 11 global climate models and “subsurface runoff” output was considered as recharge. Recharge is expected to decrease in the West (−5.8 ± 14.3%) and Southwest (−4.0 ± 6.7%) regions in the near future and in the South region (−9.5 ± 24.3%) in the far future. The Northern Rockies region is expected to get more recharge in the near (+5.3 ± 9.2%) and far (+11.8 ± 12.3%) future. Overall, southern portions of the western U.S. are expected to get less recharge in the future and northern portions will get more. Climate change interacts with land surface properties to affect the amount of recharge that occurs in the future. Effects on recharge due to change in vegetation response from projected changes in climate and CO2 concentration, though important, are not considered in this study.Key PointsClimate change interacts with land surface properties to affect the amount of recharge that occurs in the futureSouthern portions of the western U.S. are expected to get less and northern portions more recharge in the futureThe large variability in projected recharge across the GCMs is associated with variability in projected precipitationPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139906/1/grl56569.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139906/2/grl56569_am.pd

Problematic attention processing and fear learning in adolescent anxiety: Testing a combined cognitive and learning processes model

Newton Fund (managed by the UK Medical Research Council) (HB, VK, SN, JL) and the India Department of Biotechnology (RP