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We are investigating various properties of diverse materials including semconductors, metals, and nanostructured materials, and their potential applications, using theoretical and computational methods.
Following are brief summaries of research projects conducted by Computational NanoPhysics Lab.
A phase change material is a material exhibiting easy and fast transition between crystalline and amorphous phases. Due to this characteristic, phase change materials have been used for rewritable DVDs and Blueray disks, and are expected to be used for phase change random access memory. Ge-Sb-Te compounds are major phase change materials. We are performing molecular dynamics simulations to identify amorphous phases of various Ge-Sb-Te compounds. Our simulation data are analyzed to explore local atomic structures of amorphous phases, which are compared with those of their crystalline counterparts.
We investigate structural properties of varius amorphous phases and crystalline using molecular dynamics simulation. Also, based on a Radial Distribution Functions(RDFs), Angle Distribution Functions(ADFs) and Order Parameters(OPs) analyses, we study local structure characteristics of several phases. Our calculated OPs and RDFs are analyzed and compared to EXAFS data available. Finally, we estimate energy barriers not only between crystalline and one of amorphous phases, but also between different amorphous phases to explore their structural stability, using nudged elastic band method.
The phase transition of Ge2Sb2Te5 between its crystalline and amorphous phases was investigated by constructing its various amorphous phases.
Sb$_2$Te$_3$ is a very interesting material for rewritable optical media. We study of the switching mechanism between the amorphous and crystal states theoretically.
Phosphorene is a single layer of black phosphorous, recently synthesized. Its two-dimentional structure is similar to the carbon-based graphene, however, it is semiconductor compared to graphene with zero gap. So this material has attracted much attention due to its potential usage for electronic or optoelectronic devices. Because of a lone pair of electrons on each phosphorous atom, however, pristine phosphorene may be extremely reactive with air, especially with oxygen, and thus the oxidation of phosphorene would be unavoidable during synthesis and/or fabrication processes unless they were done in a vacuum environment.
Due to the existence of a lone pair of electrons on each P atom, phosphorene can be easily oxidized. At first, we have investigated the structural stability of POx with 0<x<1. For every given 0<x<1 of POx, diverse configurations are considered and optimized to find its equilibrium structure to know whether such a phosphorene oxide can be formed or not.
As x increases in POx, the electronic properties of phosphorene oxides changes. We found an intriguing phenomenon that at around 16% uniaxial tensile strain, PO (with x=1) becomes a metal with a modified Dirac cone at the gamma point in its Brillouin zone.
Transition metal dichalcogneides (MX$_2$) are another 2D layered materials similar to graphene. We investigate the physical properties of various MX$_2$, where M is a transition metal atom in group 4 (Ti, Zr, Hf), 5 (V, Nb, Ta), or 6 (Cr, Mo, W) elements; and X is a chalcogen atom, either S or Se.
We investigate the structural, electronic properties of MX$_2$. (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) (X=S, Se) and their phase transition. We calculate the relative stability of two phase structures (octahedral-T and trigonal-H) determined by considering relative positions of X. We find interesting phase existence for the each group in the periodic table. The stable phase structure of IV-group (Ti, Zr, Hf) is octahedral, while VI-group (Cr, Mo, W) is trigonal, and V-group (V, Nb, Ta) is possible two phase structures. Two phase structures of MX$_2$ have a same total energy at the same lattice constant near the each most stable lattice constant. From these results, we also study activation energy barriers at the various lattice constants such as from stable T phase to H phase, same total energy, and from stable H to T. Interestingly we find that the activation energy barrier is small at the small and large lattice constants for the stable lattice constants. It means that the phase transition occurs by compressive and tensile strains.
We investigate the effect of the interlayer interaction on the structural and electronic properties of layered MX2(M=Ti, Zr, Hf, Cr, Mo, W) structures with van der Waals interaction. We calculate the relative stability of various layer-layer stacking configurations determined by considering relative positions and orientations between neighboring layers. It is found that an intriguing coupling effect between stacking and electronic structure. Some stacking configuration with a lager inter-layer distance exhibit larger electronic band gap than the other structures.
We have investigated the thermal expansion behaviors of three different types (α, β, and γ) of graphyne which is a two-dimensional carbon allotrope composed of sp and sp2 bonds. We have found that all three types of graphyne exhibit negative in-plane thermal expansion. The observed in-plane thermal contraction can be attributed to the ripple effect, similarly in graphene. However, the ripple effect itself is not sufficient to explain anomalously larger thermal contraction found in graphyne than in graphene. Our deliberate analysis on the phonon modes observed in graphyne enables us to discover another source causing such amplified contraction behavior. We found that there are particular phonon modes with frequencies around a few hundreds of cm-1 existing exclusively in graphyne that may fill empty spaces resulting in area reduction. These modes are attributed to the libration of “rigid units” composed of triple bonds
We have found a strain-induced metal-insulator transition in graphyne blah blah
It is very important to understand the electronic structure of GaAs and GaSb for fabrication of electronic and optoelectronic devices. Recently, tenary compounds, GaAs1-xSbx began to attract much attention from researchers because they were synthesized and expected to be new materials for optoeletronic devices. For x = 1 and x = 0, there have been a lot of computational studies to investigate their structural, electronic and optical properties, but few studies for general cases. Moreover, it has been known that “regular” density functional calculations does not describe the correct electronic structures including underestimated band gap and incorrect band splitting. To resolve such issues, we consider spin-orbit coupling effects and GW corrections on GaAs1-xSbx with the composition of x = 0, 0.25, 0.5, 0.75, and 1. To calculate their band structures, we make use of maximally localized Wannier functions to interpolate GW quasiparticle band structures. Based on our GW calculations, we discuss the trend of band gaps at various high symmetric points in the Brillouin zone with the composition x.
We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).
We have studied the growth and characterization of patterned and uniformly distributed GaN microcrystals with well-defined facets and epitaxy. The microcrystals were grown on a mask pattertned by lithography, and formed by selective-area epitaxy using metal-organic chemical vapour deposition. The GaN microcrystals have similar sizes and shapes. Each microcrystal consists of an upper and lower part, which are rotated by 30 deg. Transmission electron microscopy shows that there is a rather clear interface between the two parts of the crystal, suggesting a sudden change in the growth direction. We performed ab initio calculations for the surface energies of hexagonal GaN, and the growth morphology is explained based on surface energy consideration.
Iron is one of the most important and widely-used materials in our lives. We have studied iron nanoclusters and iron-based alloys using ab initio and parametrized calculations.
We have examined the structural and mechanical properties of Fe-Al alloys at various Al concentrations with or without a small amount of Mn. For each disordered Fe-Al alloy, we considered sundry disordered configurations to determine the equilibrium structure. It was found that the disordered Fe-Al alloy with high Al content (~25 at%) becomes less stable and more brittle than its ordered counterpart DO3 second phase. To predict the ductility of Fe-Al alloys, we estimated Pugh's constant defined by the ratio between bulk and shear moduli and apply the Pugh's criterion. We found that an addition of a small amount of Mn may stabilize disordered solid solutions over the DO3 second phase. Moreover, the Mn addition also improves its mechanical properties making the disordered Fe-Al alloy more ductile than DO3.
Using tight-binding method implemented with Stoner model of itinerant magnetism, we have searched the magic numbers of iron nano clusters with up to N=30 iron atoms.
Schwarzite is a carbon allotrope with negative Gaussian curvature.
We have studied the stability, elastic properties and electronic structure of sp2 carbon minimal surfaces with negative Gaussian curvature, called schwarzites. We focused on two systems with cubic unit cells containing 152 and 200 carbon atoms, which are metallic and very rigid. The porous schwarzite structure allows for efficient and reversible doping by electrondonors and acceptors, amking it a promising candidate for the next generation of alkali ion batteries. We identified schwarzite structures that act as arrays of interconnected spin quantum dots or become magnetic when doped. We introduced two interpenetrating scwarzite structures that may find their use as the ultimate super-capacitor.
Using ab initio density functional theory and Boltzman transport theory, we have investigated the thermoelectric properties of schwarzite and estimate its ZT values. blah blah.
We have investigate various phenomena and changes in properties occurring at interfaces between metal and organic molecules using ab initio density functional theory
Using ab initio density functional theory, we have investigated the structural and electronic properties of self-assembled HATCN patterns on graphene.
Using ab initio density functional theory, we have investigated the self-assembled pattern of HATCN molecules on gold surface.
Using ab initio density functional theory, we have investigated the electronic structures of self-assembled chains comprising Ag atoms and organic molecules.
We have investigated various properties of divers gold nanostructures using ab initio density functional theory with spin-orbit coupling.
We have investigated the adsorption properties of chalcogen elements (oxygen and sulfur) and nitrogen on an anionic golden fullerene Au16- and its effects on the structural and electronic properties of the golden cage. In particular, we found that when a sulfur atom is encapsulated inside Au16-, its bonding character with Au atoms appears ionic due to electron transfer from sulfur to the gold nanocage. In contrast, the exohedrally adsorbed S atom tends to have strong orbital hybridization with the gold nanocage. For an oxygen adsorption case, electrons from the golden cage tend to be shared with the adsorbed O atom exhibiting strong orbital hybridization, regardless of its adsorption sites. To investigate the transition behaviors between the most stable exohedral and endohedral adsorption configurations, we calculate the activation and reaction energies in the transition. The oxygen atom experiences a lower energy barrier than the sulfur atom due to its smaller atomic radius. Finally, we explore the vibrational properties of S- or O-adsorbed Au16- 16 buckyballs by calculating their infrared spectra. When the N atom is adsorbed on the cage, electrons are transferred to nitrogen from Au 16. The nitrogen atom may move thermally from the exterior to the interior through a bridge site. In infrared spectra, exohedral doping causes greater intensities at higher frequencies than endohedral doping.
We have studied the dimerization of metal-encapsulated gold nanoclusters M@Au12 (M = W, Mo) with Ih or Oh symmetry, and their structural and electronic properties.To determine the most stable dimer structure in each case, various configurations are considered. We find that during dimerization, gold atoms near the interface tend to form inter-cluster triangular bonds, which stabilize two monomer clusters by about 3.3–3.5 eV. The dimerization along a specific axis selected as the z axis causes the symmetry reduction of each M@Au12 cluster resulting in the modification of electronic structures. It is found that all the stable dimers exhibit a much smaller HOMO–LUMO gap than those of their comprising monomers. Such a gap decrease is mainly attributed to the dz2 orbital splitting of the central atoms owing to dimerization. We also calculate the vibrational modes and the corresponding IRactive spectra, which are distinguishable for different dimer configurations. In addition, we find that the IR-active modes of the Oh-based dimer structures appear to be red-shifted in comparison to those of Ih-based ones. Thus, the IR spectra may be utilized experimentally to discriminate dimer configurations with different central metal atoms and/or dissimilar structural symmetries.
We have studied the 1D chain formation of metal-encapsulated gold nanoclusters M@Au12 (M = W, Mo) with Ih or Oh symmetry, and their structural and electronic properties.
We have investigated various nanowires.
Mo6S9-xIx nanowires exhibit interesting structural and electronic properties due to the flexible S linkages, which can be either short or long linke in an accordion. We have investigated the structrual and electronic properties of isolated and bundled Mo6S9-xIx nanowires. We find the optimum interwire distance depends on composition and to a smaller degree on the orientation of the wires. Structural order is expected in bundles with x=0 and x=6, since there is no disorder in the decoration of the Mo clusters. In bundles with other stoichiometries we expect structural disorder to occur. We find that the electronic structure of some nanowires can be switched from metallic to semiconducting by changing the interwire separation. The most intriguing changes occur in nanowires with x=4.5 and 6,which change from metal to semiconductor or undergo a magnetic transition upon axially stretching or compressing the nanowires or upon changing the inter-wire separation.
It is expected that the Moore's law would be broken in Si-based technology in the near future. To overcome this issue, we have performed research and development of various nanodevices based on nanostructured materials.
Recent progress in silicon photonics has dramatically advanced the possible realization of heterogeneous logic circuits. A variety of Boolean optoelectronic circuits have been proposed5. In this context, experimental investigation of logic operations with both optical and electrical inputs in chip-integrable devices is highly desirable. Here, we present a new kind of photodiode-based logic device using scalable heterojunctions of carbon nanotubes and silicon, the output currents of which can be manipulated completely by both optical and electrical inputs. This provides a novel platform for heterogeneous optoelectronic logic elements with voltage-switchable photocurrent responsivity of > 1 AW-1, photovoltage responsivity of >1×105 VW-1, electrical on/off ratios of >1×105 and optical on/off ratios of >1×104. To demonstrate their scalability, we fabricated a large array of photoactive elements on a centimetre-scalewafer. We also present bidirectional phototransistors and novel clock-triggerable logic elements such as a mixed optoelectronic AND gate, a 2-bit optoelectronic ADDER/OR gate and a 4-bit optoelectronic digital-to-analog converter.
We have reported the large-scale assembly of type-switchable field effect transistors (FETs) based on carbon nanotubes (CNTs) and nanoparticles (NPs). In this device, the charges stored in NPs adjacent to ambipolar CNT channels were adjusted to control the carrier type and density in the channels. We demonstrated the real-time reconfiguration of individual FET types and logic circuit functionality. Theoretical simulation of a model system was provided to explain this doping effect. This work takes advantage of the ambipolar properties of CNTs and opens up the possibility to build new types of devices with reconfigurable functionalities.
Based on the percolation theory, we have performed device simulations on network devices. We have found that it is possible to make semiconducting devices out of mixture of metallic and semiconducting nanotubes.
We investigate the internal dynamics of a related model system, consisting of a K@C+60 endohedral complex enclosed in a C480 nanocapsule. We show this to be a tunable two-level system, where transitions between the two states can be induced by applying an electric field between the C480 end caps, and discuss its potential application as a nonvolatile memory element.
Nanostructured materials, which ususally posses high surface/volume ratios, can be hosts of variouis adsorbates.
Using density functional theory, we investigate the adsorption properties of acetylsalicylic acid (aspirin) on the outer surfaces of a (10,0) carbon nanotube (CNT) and a (8,0) triazine-based graphitic carbon nitride nanotube (CNNT). The adsorption energies for the CNNT and CNT are 0.67 and 0.51 eV, respectively, and hence, the aspirin molecule binds more strongly to the CNNT. The stronger adsorption energy for the binding to the CNNT is ascribed to the high reactivity of its nitrogen atoms with high electron affinity. The CNNT exhibits local electric dipole moments that cause strong charge redistribution in the adsorbed aspirin molecule. The influence of an external electric field on the adsorption of aspirin on the nanotubes is explored by examining modifications in their electronic band structures, partial densities of states, and charge distributions. An electric field applied along a particular direction is found to induce molecular states of aspirin that lie within the in-gap region of the CNNT. This implies that the CNNT can be potentially utilized for the detection of aspirin.
Adsorption properties of Amino acids on graphitic carbon structures.
Here we report the highly effective detection of hydrogen sulfide (H2S) gas by redox reactions based on single-walled carbon nanotubes (SWCNTs) functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a catalyst and we also discuss the important role of water vapor in the electrical conductivity of SWCNTs during the sensing of H2S molecules. To explore the H2S sensing mechanism, we investigate the adsorption properties of H2S on carbon nanotubes (CNTs) and the effects of the TEMPO functionalization using first-principles density functional theory (DFT) and we summarize currentchanges of devices resulting from the redox reactions in the presence of H2S. The semiconducting-SWCNT (s-SWCNT) device functionalized with TEMPO shows a very high sensitivity of 420% at 60% humidity, which is 17 times higher than a bare s-SWCNT device under dry conditions. Our results offer promising prospects for personal safety and real-time monitoring of H2S gases with the highest sensitivity and low power consumption and potentially at a low cost.
Using first-principles calculations, we investigate the electronic structures and binding properties of nicotine and caffeine adsorbed on single-walled carbon nanotubes to determine whether CNTs are appropriate for filtering or sensing nicotine and caffeine molecules. We find that caffeine adsorbs more strongly than nicotine. The different binding characteristics are discussed by analyzing the modification of the electronic structure of the molecule-adsorbed CNTs. We also calculate the quantum conductance of the CNTs in the presence of nicotine or caffeine adsorbates and demonstrate that the influence of caffeine is stronger than nicotine on the conductance of the host CNT.
Glycines are spontaneously adsorbed to form into self-assembled nanoclutsers on single-walled carbon nanotubes (SWNTs). After formation of glycine nanoclusters on SWNTs, the field effect transistor (FET) devices show selective sensing ability to alcohols, such as isopropyl alcohol (IPA), methanol, and ethanol. Upon the adsorption of alcohol, the glycine-coated SWNT-FET devices exhibit pseudo-metallic transport behaviors, whereas the original and glycine-coated devices display conventional p-type transport characteristics. Computational studies support that the gate field screening effect induced by instantly formed glycine-alcohol pair layers seems to be responsible for the pseudo-metallic transport behavior.
A Li ion battery is one of rechargeable battery types, where Li ions move from the negative to the positive electrondes. blah blah
Using ab initio density functional theory, we have investigated the adsorption properties of Li on WS2.
Using ab initio density functional theory, we have investigated the adsorption properties of Li on schwarzites.
hydrogen storage is one of three main technologies to achieve so-called hydrogen economy in future. blah blah
Using ab initio density functional theory, we have investigated the adsorption properties of H2 on various organic systems
Using ab initio density functional theory, we have investigated the adsorption properties of H2 on materials with “pocket” structures.
Using ab initio density functional theory, we have investigated the adsorption properties of H2 on various carbon-based materials.
Using ab initio density functional theory and semi-empirical theories, we have studied various properties and applications of carbon nanotubes (CNTs), which are seamless cylinders wrapped from graphene.
There are variety of different computational results on the same CNTs with the same computational method. To resolve this issue, we swapped completely the configurational spaces of each CNT considering its structural symmetry determined by either as well as achiral (armchair and zigzag) and chiral CNTs. As a result, We were able to identify the most stable structure and the coupling effects between structural and electronic properties.
Using Monte Carlo simulations, we have investigated the possibility of an orientational melting transition within a “rope” of CNTs. hen twisting nanotubes bundle up during the synthesis, orientational dislocations or twistons arise from the competition between the anisotropic interbute interactions, which tend to align neighboring tubes, and the torsion rigidity that tends to keep individual tubes straight. We mapped the energetics of a rope containing twistons onto a alttice gas model and found that the onset of a free “diffusion” of twistons, conrresponding to orientational melting, occurs at TOM ≥ 160 K.
We explored the effect of intertube interaction in bundles of single-wall CNTs and multiwall CNTs. Because of small intertube interaction and lattice frustration, we predicted very soft libration modes to occur and to disappear above the orientational melting temperature which marks the onset of free tube rotations about their axis. We also predicted such weak intertube coupling causes an additional electronic band dispersion of < 0.2 eV and opens up a pseudogap of the same magnitude at the Fermi level in the rope of (10,10) SWCNTs. In a (5,5)@(10,10) double wall CNT, the weak interwall interaction and the symmetry reduction causes four pseudogaps to open and close periodically near the Fermi level during the soft librational motaion. This electron-libration coupling, absent in solids composed of fullerenes and SWCNTs, may yield superconductivity in MWCNTs.
We investigated the morphology and structural stability at the growing edge of multi-wall CNTs using ab initio and parametrized calculations. We found that the open-ended structures are stabilized against dome closure by strong covalent bonds connecting the exposed edgs of adjacent walls. Growth at the open edge involves rearrangement of these bonds, which are mediated by carbon atoms bridging the gap and change the tip morphology significantly. Presence of a strong ``lip-lip'' interaction can explain formation of carbon nanotubes under annealing conditions.
Following are other carbon materials we have investigated for their properties and applications.
We have explored the suitability of nanotube-based hooks for bonding. Our results indicate that a large force of 3.0 nN is required to disengage two hooks, which are formed by the insertion of pentagon-heptaon pairs in a (7,0) CNT. Nanohooks based on various nanotubes are resilient and keep their structrual integrity during the opeing process. Arrays of hooks, which are permanetly anchored in solid surfaces, are a nanoscale counterpart of velcro fasteners, forming tough bonds with a capability of self-repair.
This micro-fastening system was rigisterd as a US patent (US7,181,811).
Using molecular dynamics calculations, we have investigated the absorption of a C60 molecule in a (10, 10) nanotube either through the open and or a large defect in the tube wall as possible scenarios for the hierarchical self-assembly of (C60)n@(10,10 “nano-peapods”. We found the absorption through a defect to be significantly more efficient than the end-on absorption. This process occurs most likely within a narrow launch velocity range for the fullerene that agrees well with the observed optimum temperature window for peapod formation.
We have determined the stability, optimun geometry, and electronic properties of nanometer-sized capped graphitic cones, called “nanohorns”. Different nanohorn morphologies were considered, which differe in the relative location of the five terminating pentagons. Simulated scanning tunneling microscopy images of the various structures at different bias voltages reflect a net electron transfer towards the epntagon vertex sites. We found that the density of states at the tip, ovservable by scanning tunneling spectroscopy, can be used to discriminate between different tip structures.
We are also performing our research on various interesting problems.
Integration of semiconductor on 2D atomic layers has offered flexible optoelectronic devices. Among various 2D atomic layers BN has great potentials for diverse device applications due to its excellent thermal conductivity and chemical stability as a wide-bandgap electric insulator. We found ZnO and BN were interacted through van der waals interaction using aberration-corrected high resolution transmission electron microscopy (HR-TEM) and ab initio calculations based on density function theory (DFT). We used DFT-D2 method of Grimme to describe London dispersion interactions.
The nature of magnetic ordering in LaCoO3 epitaxial thin films has been the subject of considerable debate. We present direct observations of the spin-state modulation of Co ions in LaCoO3 epitaxial thin films on an atomic scale using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and ab initio calculations based on density functional theory (DFT) calculations. The results of an atomic-resolution STEM/EELS study indicate that the superstructure is not associated with oxygen vacancies; rather, it is associated with a higher spin state of Co3+ ions and their ordering. DFT calculations successfully reproduced the modulation of lattice spacing with the introduction of spin ordering. This result identifies the origins of intrinsic phenomena in strained LaCoO3 and provides fundamental clues for understanding ferromagnetism in Co-base oxides.
Here we report direct observations of spatial movements of nanodroplets of Pb metal trapped inside sealed carbon nanocontainers. We find drastic changes in the mobility of the liquid droplets as the particle size increases from a few to a few ten nanometers. In open containers the droplet becomes immobile and readily evaporates to the vacuum environment. The particle mobility strongly depends on confinement, particle size, and wetting on the enclosed surface. The collisions between droplets increase mobility but the tendency is reversed if collisions lead to droplet coalescence. The dynamics of confined nanodroplets could provide new insights into the activity of nanostructures in spatially constrained geometries.
The assessment of information transfer in the global economic network helps to understand the current environment and the outlook of an economy. Most approaches on global networks extract information transfer based mainly on a single variable. This paper establishes an entirely new bioinformatics-inspired approach to integrating information transfer derived from multiple variables and develops an international economic network accordingly. In the proposed methodology, we first construct the transfer entropies (TEs) between various intra- and inter-country pairs of economic time series variables, test their significances, and then use a weighted sum approach to aggregate information captured in each TE. Through a simulation study, the new method is shown to deliver better information integration compared to existing integration methods in that it can be applied even when intra-country variables are correlated. Empirical investigation with the real world data reveals that Western countries are more influential in the global economic network and that Japan has become less influential following the Asian currency crisis.