arXiv subjects

Jianxin Zhong

Publications and source records attributed to Jianxin Zhong.

53 records · Page 3Linked to original sources

arXiv2016

On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features

Although it is generally accepted that the so-called "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, the exact nature of their carriers remains unknown: whether they are free-flying, predominantly aromatic gas-phase molecules, or amorphous solids with a mixed aromatic/aliphatic composition are being debated. Recently, the 3.3 and 3.4 micrometer features which are commonly respectively attributed to aromatic and aliphatic C-H stretches have been used to place an upper limit of ~2\% on the aliphatic fraction of the UIE carriers (i.e. the number of C atoms in aliphatic chains to that in aromatic rings). Here we further explore the aliphatic versus aromatic content of the UIE carriers by examining the ratio of the observed intensity of the 6.2 micrometer aromatic C-C feature (I6.2) to that of the 6.85 micrometer aliphatic C-H deformation feature (I6.85). To derive the intrinsic oscillator strengths of the 6.2 micrometer stretch (A6.2) and the 6.85 micrometer deformation (A6.85), we employ density functional theory to compute the vibrational spectra of seven methylated polycyclic aromatic hydrocarbon molecules and their cations. By comparing I6.85/I6.2 with A6.85/A6.2, we derive the fraction of C atoms in methyl(ene) aliphatic form to be at most ~10\%, confirming the earlier finding that the UIE emitters are predominantly aromatic. We have also computed the intrinsic strength of the 7.25 micrometer feature (A7.25), another aliphatic C-H deformation band. We find that A6.85 appreciably exceeds A7.25. This explains why the 6.85 micrometer feature is more frequently detected in space than the 7.25 micrometer feature.

astro-ph.GA↗

arXiv2016

The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups

The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are ubiquitously seen in a wide variety of astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, e.g., polycyclic aromatic hydrocarbon (PAH) molecules. The 3.3 micrometer aromatic C--H stretching feature is often accompanied by a weaker feature at 3.4 micrometer. The latter is often thought to result from the C--H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3 micrometer aromatic C--H feature to that of the 3.4 micrometer aliphatic C--H feature allows one to estimate the aliphatic fraction of the UIE carriers, provided that the intrinsic oscillator strengths of the 3.3 micrometer aromatic C--H stretch (A3.3) and the 3.4 micrometer aliphatic C--H stretch (A3.4) are known. While previous studies on the aliphatic fraction of the UIE carriers were mostly based on the A3.4/A3.3 ratios derived from the mono-methyl derivatives of small PAH molecules, in this work we employ density functional theory to compute the infrared vibrational spectra of several PAH molecules with a wide range of sidegroups including ethyl, propyl, butyl, and several unsaturated alkyl chains, as well as all the isomers of dimethyl-substituted pyrene. We find that, except PAHs with unsaturated alkyl chains, the corresponding A3.4/A3.3 ratios are close to that of mono-methyl PAHs. This confirms the predominantly-aromatic nature of the UIE carriers previously inferred from the A3.4/A3.3 ratio derived from mono-methyl PAHs.

astro-ph.GA↗

arXiv2016

New Boys and Girls in Phosphorene Family from Gene Recombination: Different from Parents, Excellent than Parents

Based on the crystal structures of the previously proposed low energy phosphorene allotropes η-P and θ-P (Nano. Lett. 2015, 15, 3557), we propose five new structural stable phosphorene boys (XX-XY or XY-XY) and girls (XX-XX) through gene (XY from η-P and XX from θ-P) recombination methods. All of these five new phosphorene allotropes are obviously different from their parents, showing very different and fascinating two-dimensional patterns between each other. The dynamical stabilities of these phosphorene allotropes are confirmed positive and some of them are confirmed energetically more favorable than their parents (η-P and θ-P). Especially, the XX-XX type girl G1-P is confirmed energetically more favorable than all the previously proposed phosphorene allotropes, including black phosphorene (α-P, ACS Nano, 2014, 8, 4033) and blue phosphorene (β-P, Phys. Rev. Lett. 2014, 112, 176802), which is expected to be synthesized in future experiment through vapor deposition. Our results show that such a new promising phosphorene allotrope G1-P is an excellent candidate for potential applications in nano-electronics according to its middle band gap about 1.58 eV from DFT-HSE06 calculation.

cond-mat.mtrl-sci↗

arXiv2015

Two dimensional topological insulators with tunable band gaps: HgTe and HgSe monolayers

Employing ab initio electronic calculations, we propose a new type of two-dimensional (2D) topological insulator (TI), monolayer (ML) low buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gaps. Monolayer LB HgTe undergoes a transition to a topological nontrivial phase under the appropriate in-plane tensile strain (ε > 2.6%) due to the combination effects of strain and spin orbital coupling (SOC). Under the 2.6%< ε <4.2% tensile strain, the band inversion and topological nontrivial gap are induced by the SOC. For ε >4.2%, the band inversion is already realized by strain but the topological gap is induced by SOC. The band gap of monolayer LB HgTe TI phase can be tuned over a wide range from 0 eV to 0.20 eV as the tensile strain increases from 2.6% to 7.4%. Similarly, the topological phase transition of monolayer LB HgSe is induced by strain and SOC as the strain ε >3.1%. The topological band gap can be 0.05 eV as the strain increases to about 4.6%. The large band gap of 2D LB HgTe and HgSe monolayers make this type of material suitable for practical applications at room-temperature.

cond-mat.mes-hall↗

arXiv2015

Direct and quasi-direct band gap silicon allotropes with remarkable stability

In our present work, five previously proposed sp$^3$ carbon crystals were suggested as silicon allotropes and their stabilities, electronic and optical properties were investigated by first-principles method. We find that these allotropes with direct or quasi-direct band gaps in range of 1.2-1.6 eV are very suitable for applications in thin-film solar cells. They display strong adsorption coefficients in the visible range of the sunlight in comparison with diamond silicon. These five silicon allotropes are confirmed possessing positive dynamical stability and remarkable themodynamical stability close to that of diamond silicon. Especially, the direct band gap M585-silicon possessing energy higher than diamond silicon only 25 meV per atom is expected to be experimentally produced for thin-film solar cells.

cond-mat.mtrl-sci↗

arXiv2015

Nano-Stitching of Graphene Bilayers: A First-Principles Study

A nano-stitching method is proposed and investigated to modify graphene bilayers. Based this method, four types of low energy carbon allotropes, "wormhole graphene" allotropoes, are obtained and their structures, stabilities and electronic properties are investigated using first principles methods. We find that all of these wormhole graphene allotropoes are more favorable than graphdiyne and dynamically stable. Similar to carbon nanotubes and fullerences, these graphene allotropes are expected to act as two-dimensional periodic nano-capsules for encapsulating magnetic atoms or functional clusters for a variety of applications.

cond-mat.mtrl-sci↗

arXiv2014

Simple Cubic Carbon Phase C21-sc: A Promising Superhard Carbon Conductor

Traditionally, all superhard carbon phases including diamond are electric insulators and all conductive carbon phases including graphite are mechanically soft. Based on first-principles calculation results, we report a superhard but conductive carbon phase C21-sc which can be obtained through increasing the sp3 bonds in the previously proposed soft and conductive phase C20-sc (Phys. Rev. B 74, 172101 2006). We also show that further increase of sp3 bonds in C21-sc results in a superhard and insulating phase C22-sc with sp3 bonds only. With C20-sc, C21-sc, C22-sc and graphite, the X-ray diffraction peaks from the unidentified carbon material synthesized by compressing the mixture of tetracyanoethylene and carbon black (Carbon, 41, 1309, 2003) can be understood. In view of its positive stability, superhard and conductive features, and the strong possibility of existence in previous experiments, C21-sc is a promising multi-functional material with potential applications in extreme conditions.

cond-mat.mtrl-sci↗

arXiv2013

Pseudo-crystals of the group 14 elements with both 5-fold central rotation symmetry and divisional translation symmetry

Crystals are the materials which can be described by uniform periodic lattices. Traditionally, only the 1-, 2-, 3-, 4- and 6-fold rotation symmetries are allowed in crystals because other n-fold rotation symmetries are forbidden by the fully periodic translation symmetry. Materials containing rotation symmetries forbidden in crystals can be optionally described as quasicrystals with aperiodicity. The theoretical predictions and experimental discoveries of quasicrystals have enriched the knowledge of crystallography and topology, and expanded the capability of growing novel materials. Mathematically, one can fill the whole space with quasi-uniform lattices through divisional translations and arbitrary n-fold central rotations, including the 5-fold symmetry. However, it is unknown if such pseudo-crystals with 5-fold symmetry exist in reality. Here we propose a generalized crystal-prediction-method which can be used to search for potential pseudo-crystals possessing both arbitrary n-fold rotation symmetry and divisional translation symmetry. Successful examples with 5-fold central rotation symmetry based on the group 14 elements are discussed. We expect timely experimental explorations on such peculiar structures, especially on the very promising clathrate phases of the group 14 elements with only slightly larger binding energies compared with the diamond phases.

cond-mat.mtrl-sci↗

arXiv2013

Structures, stability, mechanical and electronic properties of a-boron and its twined brother a*-boron

The structures, stability, mechanical and electronic properties of a-boron and its twined brother a*-boron have been studied by first-principles calculations. Both a-boron and a*-boron consist of equivalent icosahedra B12 clusters in different connecting configurations of "3S-6D-3S" and "2S-6D-4S", respectively. The total energy calculations show that a*-boron is less stable than a-boron but more favorable than beta-boron and Gamma-boron at zero pressure. Both a-boron and a*-boron are confirmed dynamically and mechanically stable. The mechanical and electronic properties of a-boron and a*-boron indicate that they are potential superhard semiconducting phases of element boron.

cond-mat.mtrl-sci↗

arXiv2012

First-principles study of a novel superhard boron nitride phase

A superhard boron nitride phase dubbed as Z-BN is proposed as possible intermediate phase between h-BN and zinc blende BN (c-BN), and investigated using first-principles calculations within the framework of the density functional theory. Although the structure of Z-BN is similar to that of bct-BN containing four-eight BN rings, it is more energy favorable than bct-BN. Our study reveals that Z-BN, with a considerable structural stability and high density comparable to c-BN, is a transparent insulator with an indirect band gap about 5.27 eV. Amazingly, its Vickers hardness is 55.88 Gpa which is comparable to that of c-BN. This new BN phase may be produced in experiments through cold compressing AB stacking h-BN due to its low transition pressure point of 3.3 GPa.

cond-mat.mtrl-sci↗

arXiv2012

New Superhard Carbon Phases Between Graphite and Diamond

Two new carbon allotropes (H-carbon and S-carbon) are proposed, as possible candidates for the intermediate superhard phases between graphite and diamond obtained in the process of cold compressing graphite, based on the results of first-principles calculations. Both H-carbon and S-carbon are more stable than previously proposed M-carbon and W-carbon and their bulk modulus are comparable to that of diamond. H-carbon is an indirect-band-gap semiconductor with a gap of 4.459 eV and S-carbon is a direct-band-gap semiconductor with a gap of 4.343 eV. The transition pressure from cold compressing graphite is 10.08 GPa and 5.93 Gpa for H-carbon and S-carbon, respectively, which is in consistent with the recent experimental report.

cond-mat.mtrl-sci↗

arXiv2012

Structure, stability and electronic properties of tricycle type graphane

We propose a new allotrope of graphane, named as tricycle graphane,with a 4up/2down UUUDUD hydrogenation in each hexagonal carbon ring,which is different from previously proposed allotropes with UUDUUD(boat-1) and UUUUDD (boat-2) types of hydrogenation. Its stability and electronic structures are systematically studied using first-principles method. We find that the tricycle graphane is a stable phase in between the previously proposed chair and stirrup allotropes. Its electronic properties are very similar to those of chair, stirrup, boat-1, boat-2, and twist-boat allotropes. The negative Gibbs free energy of tricycle graphane is -91 meV/atom, which very close to that of the most stable chair one (-103 meV/atom). Thus, this new two-dimensional hydrocarbon may be produced in the process of graphene hydrogenation with a relative high probability compared to other conformers.

cond-mat.mtrl-sci↗

arXiv2012

Four superhard carbon allotropes: First-principle study

Using a generalized genetic algorithm, we propose four new sp3 carbon allotropes with 5-6-7 (5-6-7-type Z-ACA and Z-CACB) or 4-6-8(4-6-8-type Z4-A3B1 and A4-A2B2) carbon rings. Their stability, mechanical and electronic properties are systematically studied using first-principles method. We find that all these four carbon allotropes show amazing stability in comparison with recently proposed carbon phases. Both ZACA and Z-CACB are direct-band-gap semiconductors with band gaps of 2.261 eV and 4.196 eV, respectively. Whereas Z4-A3B1 and A4-A2B2 are indirect-band-gap semiconductors with band gaps of 3.105 eV and 3.271 eV, respectively. Their mechanical properties reveal that all these four carbon allotropes are superhard materials comparable to diamond.

cond-mat.mtrl-sci↗

arXiv2012

Enhanced thermoelectric properties in hybrid graphene-boron nitride nanoribbons

The thermoelectric properties of hybrid graphene-boron nitride nanoribbons (BCNNRs) are investigated using the non-equilibrium Green's function (NEGF) approach. We find that the thermoelectric figure of merit (ZT) can be remarkably enhanced by periodically embedding hexagonal BN (h-BN) into graphene nanoribbons (GNRs). Compared to pristine GNRs, the ZT for armchair-edged BCNNRs with width index 3p+2 is enhanced up to 10~20 times while the ZT of nanoribbons with other widths is enhanced just by 1.5~3 times. As for zigzag-edge nanoribbons, the ZT is enhanced up to 2~3 times. This improvement comes from the combined increase in the Seebeck coefficient and the reduction in the thermal conductivity outweighing the decrease in the electrical conductance. In addition, the effect of component ratio of h-BN on the thermoelectric transport properties is discussed. These results qualify BCNNRs as a promising candidate for building outstanding thermoelectric devices.

cond-mat.mes-hall↗

arXiv2005

Application of Random Matrix Theory to Biological Networks

We show that spectral fluctuation of interaction matrices of yeast a core protein interaction network and a metabolic network follows the description of the Gaussian orthogonal ensemble (GOE) of random matrix theory (RMT). Furthermore, we demonstrate that while the global biological networks evaluated belong to GOE, removal of interactions between constituents transitions the networks to systems of isolated modules described by the Poisson statistics of RMT. Our results indicate that although biological networks are very different from other complex systems at the molecular level, they display the same statistical properties at large scale. The transition point provides a new objective approach for the identification of functional modules.

q-bio.MN↗

arXiv2000

Dynamical Scaling Properties of Electrons in Quantum Systems with Multifractal Eigenstates

We study the intricate relationships between the dynamical scaling properties of electron wave packets and the multifractality of the eigenstates in quantum systems. Numerical simulations for the Harper model and the Fibonacci chain indicate that the root mean square displacement displays the scaling behavior $r(t)\sim t^β$ with $β=D_2^ψ$, where $D_2^ψ$ is the correlation dimension of the multifractal eigenstates. The equality can be generalized to $d$-dimensional systems as $β=D_2^ψ/d$, as long as the electron motion is ballistic in the effective $D_2^ψ$-dimensional space. This equality should be replaced by $β<D_2^ψ/d$ if the motion is non-ballistic, as supported by all known results.

cond-mat.dis-nn↗

arXiv2000

Morphological Symmetry Breaking During Epitaxial Growth at Grazing Incidence

It is shown that, in submonolayer growth at off-normal incidence, even much less than one percent of transfer from the condensation energy of the deposited atoms into adatom motion is sufficient to induce a net adatom current from the illuminated edge of a two-dimensional island to the other edges, thereby breaking the island symmetry. Such a symmetry breaking phenomenon is most pronounced for deposition at grazing incidence. Comparison between our theoretical predictions and existing experimental results confirms the general validity of the model.

cond-mat.mtrl-sci↗