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## Publications

Publications (107)

D-loops, a new type of structural defect in carbon fibers, are presented, which have highly detrimental effect on their mechanical properties and can define a new fundamental upper limit to their strength. These defects form exclusively during polyacrylonitrile carbonization, act as stress concentrators in the graphitic basal plane, and cannot be r...

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX2) and underscored the need for a new 2D semiconductor without defect-induced deep gap states. In this work, we demonstrate that a 2D mono-elem...

In the formation of a carbon nanotube (CNT) nucleus, a hemispherical fullerene end-cap, a specific pattern of six pentagons encodes what unique (n,m) chirality a nascent CNT would inherit, with many possible pentagon patterns corresponding to a single chirality. This configurational variety and its potential role in the initial stages of CNT cataly...

Recent research progress in nanostructured carbon has built upon and yet advanced far from the studies of more conventional carbon forms such as diamond, graphite, and perhaps coals. To some extent, the great attention to nano-carbons has been ignited by the discovery of the structurally least obvious, counterintuitive, small strained fullerene cag...

Ziel der vorliegenden Arbeit ist es, Einblicke in die Oberflächendiffusion in der Heteroepitaxie von InAs auf GaAs(001) auf atomarer Skala zu liefern. Die Methode, die wir benutzt haben, um das Problem der Oberflächendiffusion von In anzugehen, basiert auf der Dichtefunktionaltheorie. Bei der Untersuchung des Wachstums von InAs/GaAs(001) haben wir...

Hydrocarbon conversion to advanced carbon nanomaterials with concurrent hydrogen production holds promise for clean energy technologies. This has been largely enabled by the floating catalyst chemical vapor deposition (FCCVD) growth of carbon nanotubes (CNTs), where commonly catalytic iron nanoparticles are formed from ferrocene decomposition. Howe...

Correlating mechanical performance with mesoscale structure is fundamental for the design and optimization of light and strong fibers (or any composites), most promising being those from carbon nanotubes. In all forms of nanotube fiber production strategies, due to tubes’ mutual affinity, some degree of bundling into liquid crystal-like domains can...

Carbon and hydrogen bonding constitute the backbone of life; in the form of graphene, possibly functionalized by DNA nucleobases, these hold promise for the programmable assembly of graphene-based nanoelectronic devices. It is still unknown how hydrogen-bonded junctions inherent in such devices will perform as electron transport media. Here, we des...

Metal corrosion from exposure to supercritical fluids is a long-standing challenge lacking atomistic understanding for effective corrosion prevention in diverse important applications. Here, reactions relevant to corrosion in supercritical CO2 (sCO2), with H2O and NO2 as impurities, are revealed using ab initio molecular dynamics and enhanced sampl...

Two-dimensional metals offer intriguing possibilities to explore the metallic and other related properties in systems with reduced dimensionality. Here, following recent experimental reports of synthesis of two-dimensional metallic gallium (gallenene) on insulating substrates, we conduct a computational search of gallenene structures using the Part...

Despite being one of the most consequential processes in the utilization of structural materials, fatigue at the nano- and mesoscale has been marginally explored or understood even for the most promising nanocarbon forms—nanotubes and graphene. By combining atomistic models with kinetic Monte Carlo simulations, we show that a pristine carbon nanotu...

Two-dimensional metals offer intriguing possibilities to explore metallicity and other related properties in systems with reduced dimensionality. Here, following recent experimental reports of synthesis of two-dimensional metallic gallium (gallenene) on insulating substrates, we conduct a computational search of gallenene structures using the Parti...

Predictive modeling of two-dimensional (2D) materials is at the crossroad of two current rapidly growing interests: 2D materials per se, massively sought after and explored in experimental laboratories, and materials theoretical-computational models in general, flourishing on a fertile mix of condensed-matter physics and chemistry with advancing co...

The extreme thinness of graphene combined with its tensile strength made it a material appealing for discussing and even making complex cut-kirigami or folded-only origami. In the case of origami, its stability is mainly defined by the positive energy of the single- or double-fold curvature deformation counterbalanced by the energy reduction due to...

Carbon nanotubes (CNTs) individually display exceptional mechanical properties, but the strength of their mesoscale assemblies such as bundles has a fundamental disconnect, with limited understanding of its scaling. Here we use coarse-grained implementation of a CNT interface with prescribed length distributions and parametrized cross-link density,...

The contact between a carbon nanotube (CNT) edge and a catalyst is a curvilinear interface of fundamental and practical importance. Here, the ﬁrst-principles evidence shows that on a rigid/solid catalyst the faceted CNT edge is signiﬁcantly lower in energy compared to the minimal-length circle, with the interface energy difference decreasing on mor...

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNT...

Solid Co-W catalysts have been shown to yield single-wall carbon nanotubes (CNT) with high selectivity, simplistically attributed to CNT-catalyst symmetry match for certain chiral indices (n,m). Here, based on large-scale first-principles calculations combined with kinetic Monte Carlo simulations, we show instead that such selectivity arises from a...

Correction for ‘Two-dimensional boron: structures, properties and applications’ by Zhuhua Zhang et al. , Chem. Soc. Rev. , 2017, DOI: 10.1039/c7cs00261k.

Nanocarbons have been catalytically grown since 1993. However, even today, the formation mechanisms of carbon nanotubes (CNTs) and graphene are not sufficiently understood. This sustained challenge has been an engine for the development in theory concepts and computational methods, tackling the problem of well-controlled production of these nanomat...

Situated between metals and non-metals in the periodic table, boron is one of the most chemically versatile elements, forming at least sixteen bulk polymorphs composed of interlinked boron polyhedra. In low-dimensionality, boron chemistry remains or becomes even more intriguing since boron clusters with several to tens of atoms favor planar or cage...

Helicity of a carbon nanotube is determined by the pentagon distribution in the nucleus-cap. In catalytic growth, the shape of the metal catalyst could possibly affect the pentagon arrangement---intuitively, by matching the disclinations-pentagons to the vertices of underlying solid metal. Here we explore this effect by atomistic simulations of all...

We show how a jellium model can represent a catalyst particle within the density-functional theory based approaches to the growth mechanism of carbon nanotubes(CNTs). The advantage of jellium is an abridged, less computationally taxing description of the multi-atom metal particle, while at the same time in avoiding the uncertainty of selecting a pa...

One can utilize the folding of paper to build fascinating 3D origami architectures with extraordinary mechanical properties and surface area. Inspired by the same, the morphology of 2D graphene can be tuned by addition of magnetite (Fe3O4) nanoparticles in the presence of magnetic field. The innovative 3D architecture with enhanced mechanical prope...

Recently synthesized graphitic honeycomb structures, consisting of sp²-bonded graphene nanoribbons connected by sp³-bonded “hinges” are investigated theoretically. Honeycombs of different “wall-chiralities” (armchair and zigzag) and sizes are studied. Simulation of the reconstruction of the hinges shows that zigzag honeycombs spontaneously rearrang...

We report a comprehensive first-principles study of the structural and chemical properties of the recently discovered B40 cage. It is found to be highly reactive and can exothermically dimerize, regardless of the orientation, by overcoming a small energy barrier ≃0.06 eV. The energy gap of the system varies widely with the aggregation of the increa...

On page 10317, B. I. Yakobson and co-workers discuss “D-loops”, which are new types of defect that form during the carbonization stages of polyacrylonitrile precursors. If the process is viewed as zipping together several polymeric nanoribbons, then faulty zipping can result in the formation of D-loops. These defects can account for the large discr...

Carbon nanotubes are one of the most promising reinforcement for polymer based composite. The interface engineering between nanotube and polymer is still challenging. Here, we are using interconnected carbon nanotubes (iCNT) with different geometries such as X, Y, and Λ as reinforcement. The unique geometry of the reinforcement helps in improvement...

We study the mechanical properties of two-dimensional (2D) boron, borophenes, by first-principles calculations. The recently synthesized borophene with 1/6 concentration of hollow hexagons (HH) is shown to have in-plane modulus C up to 210 N/m and bending stiffness as low as D = 0.39 eV. Thus, its Foppl-von Karman number per unit area, defined as C...

Graphene nanoribbons (GNR), can be prepared in bulk quantities for large-area applications by reducing the product from the lengthwise oxidative unzipping of multiwalled carbon nanotubes (MWNT). Recently, the biomaterials application of GNR has been explored, for example, in the pore to be used for DNA sequencing. Therefore, understanding the polym...

Early theories suggested the possibility of atomically thin boron layers, but electron-deficient boron favours multicentre bonds and assembles into various polymorphs, making the synthesis of such layers challenging. Now, in two independent experiments, the deposition of atomic boron has offered this long-sought material on a silver platter.

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron-phonon coupling, which in turn can enable superconducting behavior. Here we perform first-principles analysis of electronic structure, phonon spectra, and electron-phonon coupling of selected 2...

Here we report a unique method to locally determine the mechanical response of individual covalent junctions between carbon nanotubes (CNTs), in various configurations such as "X", "Y" and "Λ"-like. The setup is based on in-situ indentation using a picoindenter integrated within a scanning electron microscope. This allows for precise mapping betwee...

3D carbon nanotube (CNT)-based macrostructures are the subject of extensive attention because the outstanding properties of 1D and 2D nanostructures have not been fully translated into key engineering applications. Generation of 3D CNT architectures with covalent junctions could endow the new materials with extraordinary mechanical properties. In t...

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX2) and underscored the need for a new 2D semiconductor without defect-induced deep gap states. The talk will discuss why a 2D mono-elemental se...

Carbon nanotubes hold enormous technological promise. It can only be harnessed if one controls their chirality, the feature of the tubular carbon topology that governs all the properties of nanotubes—electronic, optical, mechanical. Experiments in catalytic growth over the last decade have repeatedly revealed a puzzling strong preference towards mi...

We will discuss the developments in theory of sp2-carbon growth [1], the similarities and contrasts between nanotubes and graphene, including the dynamics of defect healing [2]. Comprehensive DFT computations allow one to build a "nanoreactor" diagram of all important energy states, and then to predict the rate of carbon addition, the growth speed...

Binary alloys present a promising venue for band gap engineering and tuning of other mechanical and electronic properties of materials. Here we use the density-functional theory and cluster expansion to investigate the thermodynamic stability and electronic properties of 2D transition metal dichalcogenide (TMD) binary alloys. We find that mixing el...

We will discuss the developments in theory of sp ² -carbon growth [1], the similarities and contrasts between nanotubes and graphene, including the dynamics of defect healing [2]. Comprehensive DFT computations allow one to build a "nanoreactor" diagram of all important energy states, and then to predict the rate of carbon addition, the growth spee...

Carbon nanotubes (CNT) hold enormous technological promise. It can turn into reality only if one can control in a practical way the CNT chirality---the geometric feature of the tubular carbon topology that governs the CNT electronic properties. Experimental efforts over the last decade have consistently revealed a puzzling strong preference towards...

In the initial stages of carbon nanotube (CNT) growth, a fixed pattern of six pentagons encodes what unique (n,m) chirality a nascent CNT would inherit and can be viewed as its ``inorganic gene''. We shall present the results from a large-scale computational effort designed to establish a quantitative structure--property (intrinsic elastic energy)...

Boron synthesis, in theory: Although two-dimensional boron sheets have attracted considerable interest because of their theoretically predicted properties, synthesis of such sheets remains a challenge. The feasibility of different synthetic methods for two-dimensional boron sheets (red, see figure) was assessed using first-principles calculations,...

The structural stability and diversity of elemental boron layers are evaluated by treating them as pseudoalloy B(1-x)[hexagon](x), where [hexagon] is a vacancy in the close-packed triangular B lattice. This approach allows for an elegant use of the cluster expansion method in combination with first-principles density-functional theory calculations,...

We investigate the role of lipid chemical potential on the shape, thickness, and molecular orientation (lipid tilting relative to the monolayer surface normal) of lipid bilayers via a continuum-level model. We predict that decreasing the chemical potential at constant temperature, which is associated with an increase in surface tension via the Gibb...

In situ observation of the carbon nanotube nucleation process accompanied by dynamic reconstruction of the catalyst particle morphology is considered within a thermodynamic approach. It reveals the driving force for the detachment of the sp(2)-carbon cap, so-called lift-off-a crucial event in nanotube growth. A continuum model and detailed atomisti...

Drawing from the broad spectrum of phenomena, described in more than 100,000 articles on high-Tc superconductivity, in this book, the authors analyze those basic properties for which understanding can be achieved within the framework of traditional methods of theoretical physics. This is the case of the overdoped cuprates for which the "Bardeen pro...

Electronic, magnetic, and structural properties of graphene flakes depend sensitively upon the type of edge atoms. We present a simple software tool for determining the type of edge atoms in a honeycomb lattice. The algorithm is based on nearest neighbor counting. Whether an edge atom is of armchair or zigzag type is decided by the unique pattern o...

Band structure trends in hole-doped cuprates and correlations with T_c are
interpreted within the s--d exchange mechanism of high-T_c superconductivity.
The dependence of T_c on the position of the copper 4s level finds a natural
explanation in the generic Cu 3d, Cu 4s, O 2p_x and O 2p_y four-band model. The
Cu 3d--Cu 4s intra-atomic exchange inter...

Complementary electronic properties and a tendency to form sharp graphene-graphane interfaces open tantalizing possibilities for two-dimensional nanoelectronics. First-principles density functional and tight-binding calculations show that graphane can serve as natural host for graphene quantum dots, clusters of vacancies in the hydrogen sublattice....

In this article, we present a comprehensive characterization of three carbon nanomaterials of technological interest: graphene, graphane, and fluorinated graphene. By means of first principles and tight-binding calculations in combination with analytical methods, we carried out detailed comparative studies of their structural, mechanical, thermal,...

We present a driver program for performing replica-exchange molecular dynamics simulations with the Tinker package. Parallelization is based on the Message Passing Interface, with every replica assigned to a separate process. The algorithm is not communication intensive, which makes the program suitable for running even on loosely coupled cluster s...

The minima hopping algorithm (MHOP) to find global minima on potential energy surfaces is used for protein structure prediction. The energy surface of the protein is represented with an all-atom OPLS force field and an implicit free energy solvation term. The system we studied here is the small 10-residue beta-hairpin mini-protein, chignolin. Start...

We present a novel method for comparing the long-range part of force fields in the presence of a maximally cooperative network of nonbonded interactions. The method is based on mapping the potential energy surface of an infinite polypeptide chain in the gas phase by using cylindrical coordinates (the twist and pitch) as geometry descriptors. We app...

Empirical force fields for proteins come in a variety of flavors. Despite their ubiquitous use in biomolecular simulations, the differences and similarities between them remain poorly understood. In this work, we present a novel method for comparing the longrange part of force fields in the presence of a maximally cooperative network of hydrogen bo...

The surface structure of GaAs(0 0 1)-c(4 × 4) was investigated by
synchrotron surface X-ray diffraction which is sensitive to both
in-plane and out-of-plane structures. The atomic coordinates and Debye
Waller factors for up to the sixth layer from the surface are given. The
resultant atomic coordinates were compared with those given by a
first-prin...

The atomic structure of the c(4×4) reconstruction, formed on the GaAs(001) surface under high arsenic overpressure, has recently been attracting renewed interest. This has lead to a revision of the commonly accepted c(4×4) structural model but a definitive understanding of the driving force for the newly proposed structure was lacking. Targeting th...

An explicit expression for the temperature dependence of the specific heat of clean anisotropic-gap superconductors is derived within the weak-coupling BCS approximation. The specific heat is presented as a functional of the superconducting gap on the Fermi surface. The obtained formula interpolates between the correct low coupling jump at Tc and t...

Density-functional theory calculations are employed to obtain important information about the morphology of III–V semiconductor surfaces and kinetics of epitaxial growth. In this way, insight into the microscopic processes governing quantum dot formation in InAs/GaAs(001) heteroepitaxy is gained. First, we investigate theoretically the atomic struc...

The GaAs(001)-c(4x4) surface was studied using ab initio atomistic thermodynamics based on density-functional theory calculations. We demonstrate that in a range of stoichiometries, between those of the conventional three As-dimer and the new three Ga-As-dimer models, there exists a diversity of atomic structures featuring Ga-As heterodimers. These...

In the Stranski-Krastanow growth of strained pseudomorphic films, material transport by surface diffusion plays a crucial role for the development of the three-dimensional island morphology. In an attempt to elucidate the atomistic aspects of this growth mode, we study diffusion of a single indium adatom on (1×3)- and (2×3)-reconstructed subcritica...

Within the weak-coupling BCS scheme we derive a general form of the coefficients in the Ginzburg-Landau expansion of the free energy of a superconductor for the case of a Fermi level close to a van Hove singularity (VHS). A simple expression for the influence of the VHS on the specific heat jump is then obtained for the case where gaps for differen...

We demonstrate the use of density-functional theory (DFT) calculations for understanding molecular beam epitaxy (MBE) of GaAs and InGaAs films at the atomic level. For analyzing island growth in homoepitaxy of GaAs(001), the use of DFT in conjunction with kinetic Monte Carlo (kMC) simulations is discussed. This approach enables us to elucidate the...

It is analyzed what fundamental new information for the properties of the superconductors can be obtained by systematic investigation of the Bernoulli effect. It is shown that it is a tool to determine the effective mass of Cooper pairs, the volume density of charge carriers, the temperature dependence of the penetration depth and condensation ener...

An analytical result for the renormalization of the jump of the heat capacity ΔC/CN by the anisotropy of the order parameter is derived within the framework of the very recent model proposed by Posazhennikova, Dahm and Maki (Europhys. Lett., 60 (2002) 134), for both oblate and prolate anisotropy. The graph of ΔC/CNvs. the ratio of the gaps on the e...

High-temperature superconductors are nowadays found in great variety and hold technological promise. It is still an unsolved mystery that the critical temperature T_c of the basic cuprates is so high. The answer might well be hidden in a conventional corner of theoretical physics, overlooked in the recent hunt for exotic explanations of new effects...

The single-site two-electron exchange amplitude J_sd between the Cu 4s and Cu 3d_{x^2-y^2} states is found to be the pairing mechanism of high-T_c overdoped cuprates. The noninteracting part of the Hamiltonian spans the copper Cu 4s, Cu 3d_{x^2-y^2} and oxygen O 2p_x and O 2p_y states. Within the standard BCS treatment an explicit expression for th...

The Ginzburg-Landau coefficients, and the jump of the specific heat are calculated for a disordered two-band superconductor. We start with the analysis of a more general case arbitrary anisotropy. While the specific heat discontinuity at the critical temperature T_c decreases with increasing disorder, its ratio to the normal state specific heat at...

An analytical result for the renormalization of the jump of the heat capacity DeltaC/C-N by the anisotropy of the order parameter is derived in the framework of the model proposed by Haas and Maki [Phys. Rev. B 65, 020502(R) (2001)], for both prolate and oblate anisotropy. The graph of DeltaC/C-N versus the ratio of the gaps on the equator and the...

A detailed Linear Combination of Atomic Orbitals (LCAO) tight-binding model is developed for the layered High-Temperature Superconductor (HTSC) cuprates. The band structure of these materials is described using a σ-band Hamiltonian employing Cu4s, Cu3dx2 - y2, O2px and O2py atomic orbitals. The Fermi level and the shape of the resulting Fermi surfa...

Contents 1 The Murnaghan fit 1 1.1 Getting started: Murnaghan equation of state . . . . . . . . . . . 1 1.2 Implementation: the murn package . . . . . . . . . . . . . . . . . 2 1.2.1 Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.2 Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Command-line interface: mu...

The free energy, non-gradient terms of the Ginzburg-Landau (GL) expansion, and the jump of the specific heat of a multiband anisotropic-gap BCS superconductor are derived in the framework of a separable-kernel approximation. Results for a two-band superconductor, d-wave superconductor, and some recent models for MgB2 are worked out as special cases...

An analytical result for the renormalization of the jump of the heat capacity Delta C/C_N by the anisotropy of the order parameter is derived within the framework of the very recent model proposed by Posazhennikova, Dahm and Maki [cond-mat/0204272, submitted to Europhys. Lett.], for both oblate and prolate anisotropy. The graph of Delta C/C_N versu...

Recent low-temperature scanning tunneling experiments have questioned the generally accepted picture of buckled silicon dimers as the ground state reconstruction of the Si(100) surface, undermining the ability of density functional theory to accurately describe electronic correlations at surfaces. We present quantum Monte Carlo calculations on larg...