The thermal and electrical properties of the promising semiconductor MXene Hf2CO2

In this work, we investigate the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes using first-principles calculations. Hf2CO2 is found to exhibit a thermal conductivity better than MoS2 and phosphorene. The room temperature thermal conductivity along the armchair direction is determined to be 86.25-131.2 Wm-1K-1 with a flake length of 5-100 um, and the corresponding value in the zigzag direction is approximately 42% of that in the armchair direction. Other important thermal properties of M2CO2 are also considered, including their specific heat and thermal expansion coefficients. The theoretical room temperature thermal expansion coefficient of Hf2CO2 is 6.094x10-6 K-1, which is lower than that of most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility. At room temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag direction) is determined to be as high as 13.5x103 cm2V-1s-1 (17.6x103 cm2V-1s-1), which is comparable to that of phosphorene. Broader utilization of Hf2CO2 as a material for nanoelectronics is likely because of its moderate band gap, satisfactory thermal conductivity, low thermal expansion coefficient, and excellent carrier mobility. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided here for comparison. Notably, Ti2CO2 presents relatively low thermal conductivity and much higher carrier mobility than Hf2CO2, which is an indication that Ti2CO2 may be used as an efficient thermoelectric material.Comment: 26 pages, 5 figures, 2 table

An Updated Search of Steady TeV γ−\gamma-Ray Point Sources in Northern Hemisphere Using the Tibet Air Shower Array

Using the data taken from Tibet II High Density (HD) Array (1997 February-1999 September) and Tibet-III array (1999 November-2005 November), our previous northern sky survey for TeV $\gamma-$ray point sources has now been updated by a factor of 2.8 improved statistics. From $0.0^{\circ}$ to $60.0^{\circ}$ in declination (Dec) range, no new TeV $\gamma-$ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV $\gamma-$ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.Comment: This paper has been accepted by hepn

Genetic Regulation of the Thymic Stromal Lymphopoietin (TSLP)/TSLP Receptor (TSLPR) Gene Expression and Influence of Epistatic Interactions Between IL-33 and the TSLP/TSLPR Axis on Risk of Coronary Artery Disease

The thymic stromal lymphopoietin (TSLP)/TSLP receptor (TSLPR) axis is involved in multiple inflammatory immune diseases, including coronary artery disease (CAD). To explore the causal relationship between this axis and CAD, we performed a three-stage case-control association analysis with 3,628 CAD cases and 3,776 controls using common variants in the genes TSLP, interleukin 7 receptor (IL7R), and TSLPR. Three common variants in the TSLP/TSLPR axis were significantly associated with CAD in a Chinese Han population [rs3806933T in TSLP, Padj = 4.35 × 10−5, odds ratio (OR) = 1.18; rs6897932T in IL7R, Padj = 1.13 × 10−7, OR = 1.31; g.19646A>GA in TSLPR, Padj = 2.04 × 10−6, OR = 1.20]. Reporter gene analysis demonstrated that rs3806933 and rs6897932 could influence TSLP and IL7R expression, respectively. Furthermore, the “T” allele of rs3806933 might increase plasma TSLP levels (R2 = 0.175, P < 0.01). In a stepwise procedure, the risk for CAD increased by nearly fivefold compared with the maximum effect of any single variant (Padj = 6.99 × 10−4, OR = 4.85). In addition, the epistatic interaction between TSLP and IL33 produced a nearly threefold increase in the risk of CAD in the combined model of rs3806933TT-rs7025417TT (Padj = 3.67 × 10−4, OR = 2.98). Our study illustrates that the TSLP/TSLPR axis might be involved in the pathogenesis of CAD through upregulation of mRNA or protein expression of the referenced genes and might have additive effects on the CAD risk when combined with IL-33 signaling

Surface potential-determined performance of Ti- and Zr-containing MXenes as anode materials of sodium ion batteries

Sodium ion batteries have attracted increasing attention due to their low cost and abundant reserves of sodium, but their ideal anode materials still need to be explored. MXenes could be candidate electrode materials due to their excellent electrical conductivity and large specific surface area. In this work, the theoretical performance of the Ti- and Zr-containing MXenes Ti3C2T2 (T=O, F, OH) and Zr3C2T2 (T=O, F, OH, S) as SIB anode materials is investigated. The influence of Hubbard U correction is discussed, and the behaviour at MXene surface with the partial occupation of sodium atoms is considered. Including the weight and volume of adsorbed sodium atoms, Ti3C2O2 presents the best performance among the seven MXenes studied. Its mass and volumetric capacities are 299 mA h g-1 and 993 mA h cm-3 respectively, and the migration barrier and open circuit voltage are 0.138 eV and 0.421 V. Both Zr3C2O2 and Zr3C2S2 can adsorb double layers of sodium atoms on both sides, and the former shows higher capacity because of its lower weight and smaller volume. The mass and volumetric capacities of Zr3C2O2 are 254 mA h g-1 and 913 mA h cm-3 respectively. More importantly, the surface potential is determined to be an effective descriptor for selecting electrode materials. The migration barrier is proportional to the fluctuation amplitude of surface potential. A low surface potential generally implies a high capacity. A large open circuit voltage is prone to appear in the structure with large fluctuation amplitude and a low average value in its surface potential

First-principles Study on Electronic and Magnetic Properties of Mn-doped Strontium Ferrite SrFe12O19

First-principles Study on Electronic and Magnetic Properties of Mn-doped Strontium Ferrite SrFe12O1

Theoretical investigations on helium trapping in the Zr/Ti2AlC interface

MAX phases have been suggested to be applicable in the next generation nuclear reactors for their advantages in thermal/mechanical properties at high temperatures and radiation damage resistance. In the present investigations, the stabilities and formation energies of helium defects in the Zr/Ti2AlC interface are studied by first principles calculations. The calculations are performed using four possible interface models, and the trap of helium atoms in interstitial or vacancy sites are taken into consideration. According to the predicted formation energies, a single He atom tends to reside between the Al and Ti planes. The results also imply that Al vacancies are better able to trap He atoms. The stepwise rise for the trapping energies with the increasing number of trapped He atoms are determined in an Al vacancy and the corresponding swelling effect is examined. These investigations may provide new insight into the underlying mechanisms of helium bubble nucleation and formation in the Zr/Ti2AlC interface. (C) 2017 Elsevier B.V. All rights reserved

Role of the surface effect on the structural, electronic and mechanical properties of the carbide MXenes

The two-dimensional material MXene has recently attracted interest for its excellent performance in diverse perspectives. Etched from the parental MAX phase with hydrofluoric acid, the synthesized MXene surface is normally functionalized by oxygen (-O), fluorine (-F) or hydroxyl (-OH) groups. Herein, using first-principles density functional calculations, we investigate the structural, mechanical and electronic properties of the carbide MXene M2CT2 (M=Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; T=-O, -F, -OH). Both the M atom and the surface group T have a significant effect on the MXenes properties. Generally, oxygen functionalized MXenes present smaller lattice parameters and stronger mechanical strength compared to those functionalized by fluorine and hydroxyl groups. Sc2CO2 exhibits the smallest interlayer thickness and W2CO2 shows the strongest mechanical strength. In regard to electronic properties, five oxygen functionalized members M2CO2 (M=Sc, Ti, Zr, Hf, W), two fluorine functionalized members M2CF2 (M=Sc, Mo), and hydroxyl functionalized Sc2C(OH)(2) present semiconducting characteristics, but only Sc2C(OH)(2) exhibits a direct band gap. Copyright (C) EPLA, 201

Structural, mechanical and electronic properties of two-dimensional chlorine-terminated transition metal carbides and nitrides

Two-dimensional transition metal carbides and nitrides (MXenes) have attracted intensive attention since 2011, and surface groups have been determined to show a key role in MXene properties. Recently, an emerging functional group of chlorine was realized in MXenes, such as in Ti3C2Cl2 and Ti2CCl2. In order to understand and apply MXenes terminated by this type functional group, the structural, mechanical and electronic properties of M2X2Cl2 and M3X2Cl2 (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; X = C, N) are investigated in this work from first-principles theory. For M2X2Cl2, the chlorine groups are all stabilized on the top-sites of the bottom M atoms on both sides. In M3X2Cl2, the surface groups of most configurations are functionalized on the top-sites of the middle M atoms. Regarding to the mechanical properties, the elastic constants vary significantly with the types of M and X elements. The highest Young modulus of 208.3 GPa is determined in Ta3C2Cl2. The mechanical, dynamical and thermodynamic stabilities are further tested. Thirteen members. ie. M2CCl2 (M = Sc, Ti, Zr, Nb, Hf), M2NCl2 (M = Sc, Zr), M3C2Cl2 (M = Ti, Zr, Hf) and M3N2Cl2 (M = Sc, Ti, Zr) are determined to be stable. In addition, Ta2CCl2, Hf2NCl2, M3C2Cl2 (M = Sc, Nb, Ta) and Hf3N2Cl2 could be metestable. For these stable and metastable configurations, the electronic structures and work functions are studied. Most configurations are metallic, except for Sc2CCl2, Zr3N2Cl2 and Hf3N2Cl2. These three members are indirect band gap semiconductors, with their band gap values of 1.65, 0.135 and 0.246 eV, respectively. All the structures show high work functions, and the smallest value is approximate to 3.97 eV determined in Ti2CCl2. Our work implies that the chlorine-functionalized MXenes could be utilized in semiconductors and metallic films