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ABSTRACT: We show by first-principles calculations two types of magnetic magic Mn clusters on the Si(111)-(7 × 7) surface. The first is a small triangular Mn7 cluster stabilized by the solid-centered Mn-Si3 bonds on the top layer, and the second is a large hexagonal Mn13 cluster favored by the confining potential wells of the faulted half unit cells on the Si(111) surface. These two structural models are distinct from that of the planar group-III clusters on Si(111) and produce simulated scanning tunneling microscopy images in reasonable agreement with recent experimental observations. These results offer key insights for understanding the complex energetic landscape on the Si(111)-(7 × 7) surface, which is critical to precisely controlled growth of Mn nanocluster arrays with specific size, magnetic moment, and good uniformity.
The Journal of chemical physics 04/2013; 138(16):164705. · 3.09 Impact Factor
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ABSTRACT: We identify by ab initio calculations a simple monoclinic carbon in P2/m (C(2h)(1)) symmetry with an alternating zigzag and armchair buckling of the carbon sheets in AA stacking, which is formed via a distinct one-layer by one-layer conversion mechanism along the [210] direction assisted by a large lattice distortion. It is dynamically stable and energetically favorable as well as recently identified orthorhombic W- and monoclinic M-carbon. Moreover, this new phase has a wider band gap than diamond's, and is compatible with the experimental x-ray diffraction data. These results broaden our understanding of the direct graphite-to-diamond phase transition.
The Journal of chemical physics 07/2012; 137(2):024502. · 3.09 Impact Factor
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ABSTRACT: We identify by ab initio calculations an orthorhombic carbon (O-carbon) in Pbam (D2h9) symmetry for compressed graphite in AA stacking, which is formed via a distinct one-layer by one-layer slip and buckling mechanism along the [210] direction. It is dynamically stable and energetically more favorable than other known compressed graphite phases, albeit its slightly higher kinetic barrier. The O-carbon is comparable to diamond in ultralow compressibility, has a band gap wider than that of diamond, and is compatible with experimental x-ray diffraction data. The present results offer insights for understanding the complex structural landscape of compressed graphite and the versatile nature of carbon in forming a rich variety of structures under pressure.
Phys. Rev. B. 01/2012; 85(3).
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ABSTRACT: Geometry-based adhesion arising from hierarchical surface structure enables microspheres to adhere to cells strongly, which is essential for inorganic microcapsules that function as drug delivery or diagnostic imaging agents. However, constructing a hierarchical structure on the outer shell of the products via the current microcapsule synthesis method is difficult. This work presents a novel approach to fabricating hollow microspheres with a hierarchical shell structure through the vapor-liquid-solid (VLS) process in which liquid indium droplets act as both templates for the formation of silica capsules and catalysts for the growth of hierarchical shell structure. This hierarchical shell structure offers the hollow microsphere an enhanced geometry-based adhesion. The results provide a facile method for fabricating hollow spheres and enriching their function through tailoring the geometry of their outer shells.
Langmuir 06/2011; 27(13):7996-9. · 4.19 Impact Factor
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ABSTRACT: We identify by ab initio calculations an orthorhombic carbon polymorph in Pnma symmetry that has the lowest enthalpy among proposed cold-compressed graphite phases. This new phase contains alternating zigzag and armchair buckled carbon sheets transformed via a one-layer by three-layer slip mechanism. It has a wide indirect band gap and a large bulk modulus that are comparable to those of diamond. Its simulated x-ray diffraction pattern best matches the experimental data. Pressure plays a key role in lowering the kinetic barrier during the phase conversion process. These results provide a comprehensive understanding and an excellent account for experimental findings.
Physical Review Letters 02/2011; 106(7):075501. · 7.37 Impact Factor
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ABSTRACT: Using first-principles calculations, we unveil an atomistic nucleation process of monoatomic Mn nanowire self-assembly on Si(001). We find that Mn adatoms form magic 3n+1 atom dense trimerlike linear chains (DTLCs) driven by pedestal site adsorption. Moreover, the length of DTLCs is limited by a competing process to form three-dimensional D3h-Mn5 clusters in early stages. These results establish a new structural model distinct from the dimerlike linear chains for III and IV group elements on Si(001) and provide a natural explanation for recent experimental observations.
Physical Review Letters 09/2010; 105(11):116102. · 7.37 Impact Factor
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ABSTRACT: To explore the role of intraoperative indocyanine green angiography (ICGA) in the bypass surgery for moyamoya disease (MMD).
Thirty-seven MMD patients were surgically treated and prospectively recruited. Intraoperative ICGA was performed with a surgical microscope with integrated ICGA technology. ICGA was performed 60 times in 37 patients. In each patient, upon the completion of anastomosis, a near-infrared light emitted by laser diodes was used to illuminate the operating field and the intravascular fluorescence recorded by an optical filter-equipped video camera. Assessment of vascular patency and occlusion were also investigated. The findings of ICGA were compared with those of postoperative DSA (digital subtraction angiography).
In all cases, excellent visualization of cerebral arteries was achieved. The information provided by intraoperative ICGA significantly changed the surgical procedures. ICGA was used to identify 2 nonfunctioning STA-MCA (superficial temporal artery-middle cerebral artery) bypasses and both could be revised successfully. In all cases the final findings of ICGA could be positively validated by DSA during the postoperative course.
As one kind of intraoperative angiography, ICGA provides a reliable and rapid intraoperative assessment of bypass patency. ICGA has a high spatial resolution and it facilitates the identification of stenoses and obstruction at the anastomotic site or vascular obstructions. It is a helpful tool for the intraoperative evaluation of bypass patency.
Zhonghua yi xue za zhi 06/2010; 90(23):1628-30.
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ABSTRACT: Group-IV element clathrates have attracted considerable interest in recent years. Here, we report an ab initio study on the structural stability of carbon clathrates at high pressure and identify fcc- C <sub>136</sub> clathrate as the third most stable carbon phase after cubic diamond and hexagonal graphite. A pressure-induced phase transition is predicted to occur around 17 GPa from hexagonal graphite to fcc- C <sub>136</sub> , which is more stable than other carbon clathrates such as hex- C <sub>40</sub> and sc- C <sub>46</sub> , and the recently predicted metastable M -carbon up to 26 GPa. Phonon dispersion calculations confirm the dynamic stability of fcc- C <sub>136</sub> as well as diamond.
Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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ABSTRACT: We demonstrate an atomistic nucleation and growth mechanism for single-wall carbon nanotubes (SWNTs) on catalytic nanoparticle surfaces based on a core–shell model. We show by ab initio calculations that strain relief between the metal core and carbon shell plays a crucial role in facilitating the hexagonal tubular growth. The incipient nucleation begins with the formation of a hemispherical fullerene cap by a size-selected core–shell bonding process which is followed by a repeated phase-separating growth mode with increasing energetic stability via periodic pulsatile strain relief along the tubular growth pathway. These results provide an excellent account for experimental observations and shed new light on the origin and underlying dynamics of SWNT growth.
Nanotechnology 02/2010; 21(11):115602. · 3.98 Impact Factor
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ABSTRACT: Based on ab initio total energy calculations, Li, Na and Ag interstitials are found to be stable with at least a 1.56 eV energy barrier to transform to a zinc substitutional site in ZnO, whereas K interstitial has a relatively small energy barrier at 0.79 eV. The isolated dopant substitutional defects (Li(Zn), Na(Zn), K(Zn) and Ag(Zn)) are found to be rather stable, with at least a 3.4 eV energy barrier to transform to an interstitial site. All of the dopant interstitials (Li(i), Na(i), K(i) and Ag(i)) are fast diffusers. The diffusion of Li interstitial is isotropic, whereas the diffusion of Na, K and Ag interstitials is highly anisotropic. Fundamental processes of the vacancy-assisted mechanisms are systematically investigated and specific values of the energy barriers are obtained.
Journal of Physics Condensed Matter 08/2009; 21(34):345802. · 2.55 Impact Factor
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ABSTRACT: A comprehensive investigation of oxygen vacancy and interstitial diffusion in ZnO has been performed using ab initio total energy calculations with both the local density approximation (LDA) and the generalized gradient approximation (GGA). Based on our calculation results, oxygen octahedral interstitials are fast diffusers, contributing to annealing processes, as well as being responsible for the self-diffusion of oxygen for n-type ZnO, and oxygen vacancies are responsible for the self-diffusion of oxygen for p-type ZnO.
Journal of Physics Condensed Matter 05/2009; 21(19):195403. · 2.55 Impact Factor
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ABSTRACT: The geometry, stability, and electronic properties of B @ Co <sub>12</sub>@ B <sub>n</sub> and Co <sub>13</sub>@ B <sub>n</sub> clusters with a wide range n=55 up to 92 are studied by ab initio calculations. We find that B @ Co <sub>12</sub>@ B <sub>80</sub> and Co <sub>13</sub>@ B <sub>80</sub> with closed B <sub>80</sub> shell are two stable magic clusters with nearly perfect icosahedral symmetry, and B @ Co <sub>12</sub>@ B <sub>80</sub> is more stable than Co <sub>13</sub>@ B <sub>80</sub> energetically. The strong core-shell bonding yields a very large energy gain of ∼30 eV . This high stability is attributed to the favorable closed-shell atomic and electronic structures. The B @ Co <sub>12</sub>@ B <sub>80</sub> exhibits a large highest occupied and lowest unoccupied energy gap (0.96 eV) that is close to the value for isolated B <sub>80</sub> fullerene.
Applied Physics Letters 04/2009; · 3.84 Impact Factor
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ABSTRACT: Using first-principles total-energy calculations, we have systematically investigated the adsorption and diffusion of Ga dimers on Si(001) to form dimer linear chains (DLC) perpendicular to the surface dimer rows. We find that Ga dimers prefer forming DLC on Si(001) both energetically and kinetically with an anisotropy diffusing pathway, and the energy barriers for Ga dimers diffusing across or along the surface dimer rows along certain paths can be significantly reduced by the interaction of Ga dimers. As a result, Ga dimers can easily diffuse into the most stable sites to form one-dimensional DLC. These results reveal a natural explanation for the relevant experimental observation.
Journal of Physics Condensed Matter 09/2008; 20(44):445002. · 2.55 Impact Factor
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ABSTRACT: We report ab initio identification of initial dissociation pathways for Sb4 and Bi4 tetramer precursors on Si(001). We reveal a two-stage double piecewise rotation mechanism for the tetramer to ad-dimer conversion involving two distinct pathways: one along the surface dimer row via a rhombus intermediate state and the other across the surface dimer row via a rotated rhombus intermediate state. These two-stage double piecewise rotation processes play a key role in lowering the kinetic barrier by establishing and maintaining energetically favorable bonding between adatoms and substrate atoms. These results provide an excellent account for experimental observations and elucidate their underlying atomistic origin that may offer useful insights for other surface reaction processes.
Physical Review Letters 08/2006; 97(4):046103. · 7.37 Impact Factor
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ABSTRACT: In the digital television terrestrial broadcasting, the Single Frequency Network (SFN) has much advantage. SFN can serve an arbitrary large area with the same information broadcasted at the same frequency, resulting in the potential diversity gain. To improve the transmission performance of the Time Domain Synchronous-Orthogonal Frequency Division Multiplexing (TDS-OFDM) system in SFN, a general SFN structure with transmit diversity is introduced, theoretically analyzed and computer simulated. The proposed SFN structure is flexible to set up. Simulations show that this method can greatly improve the reliability of the signal transmission over the frequency selective fading channels and is suitable for the TDS-OFDM system in SFN.
IEEE Transactions on Broadcasting 07/2006; · 1.70 Impact Factor
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ABSTRACT: Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Néel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration.
Applied Physics Letters 03/2006; 88(13):132513-132513-3. · 3.84 Impact Factor
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ABSTRACT: Based on ab initio total energy calculation, we show that a dynamic ad-dimer twisting assisted (DATA) process plays a crucial role in facilitating a novel structural reconstruction involving surface and subsurface atoms on Si(001). It leads to self-assembly of long nanowires of group-V elements (Bi, Sb) in the trenches of surface dimer vacancy lines (DVLs) with a characteristic double-dimer configuration. The key to this is the lowering of the kinetic barrier by the DATA process in conjunction with a favorable interaction between ad-dimers and step edges in DVLs. The present results provide an excellent account for experimental observations and reveal the atomistic origin and the dynamic transformation path for nanowire self-assembly on Si(001).
Physical Review Letters 07/2005; 94(22):226103. · 7.37 Impact Factor
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ABSTRACT: Blueshifts in the ultraviolet absorption spectra have been observed in cerium oxide nanocrystallites. The mechanism of the absorption is a charge-transfer optical transition. The relationships among the blueshift, valence state of cerium ions, and the particle size are formulated. The blueshifts are well explained for diameters down to less than a few nanometers by the change in the electronic band structure. © 2003 American Institute of Physics.
Journal of Applied Physics 10/2003; · 2.17 Impact Factor
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ABSTRACT: First-principles total energy calculations on tetragonal MnAu superlattices are performed by means of the self-consistent full-potential linearized augmented-plane-wave method under the generalized gradient approximation. It is shown that the stability of the spin configuration strongly depends on the lattice distortion due to its competitive nature. Using Monte Carlo simulations with exchange parameters deduced from ab initio results, finite-temperature magnetism is also studied and a magnetic phase diagram connected with lattice distortion is obtained. © 2001 American Institute of Physics.
Applied Physics Letters 09/2001; 79(10):1507-1509. · 3.84 Impact Factor
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ABSTRACT: Ab initio calculations for layered Fe/Au(001) superlattices with a set of collinear spin configurations are performed by means of the self-consistent full-potential linearized augmented-plane-wave method under the generalized gradient approximation. To study the finite-temperature magnetism of such superlattices, Monte Carlo (MC) simulations are carried out based on a Heisenberg model with the exchange parameters extracted from the ab initio total energies and a phenomenological anisotropy constant. It is argued that the Curie temperature is rather insensitive to the anisotropy and is essentially determined by the ab initio exchange parameters. Due to the reduced coordination number of the magnetic atoms at interfaces, the Curie temperature obtained by this ab initio MC scheme decreases as decreasing of Fe layer thickness governed essentially by Weiss’ law. These results are discussed in connection with recent experimental and theoretical studies.
Phys. Rev. B. 07/2000; 62(5).
