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ABSTRACT: The band alignment in ZnO-GaN and related heterostructures is crucial for uses in solar harvesting technology. Here, we report our density functional calculations of the band alignment and optical properties of ZnO-GaN and ZnO-(Ga(1-x)Zn(x))(N(1-x)O(x))-GaN heterostructures using a Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. We found that the conventional GGA functionals underestimate not only the band gap but also the band offset of these heterostructures. Using the hybrid functional calculations, we show that the (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution has a direct band gap of about 2.608 eV, in good agreement with the experimental data. More importantly, this solid solution forms type-II band alignment with the host materials. A GaN-(Ga(1-x)Zn(x))(N(1-x)O(x))-ZnO core-shell solar cell model is presented to improve the visible light absorption ability and carrier collection efficiency.
Physical Chemistry Chemical Physics 10/2012; · 3.57 Impact Factor
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ABSTRACT: The band alignment in ZnO/GaN and related heterostructures are crucial for
the uses in solar harvesting technology. Here, we report our density functional
calculations of the band alignment and optical properties of ZnO/GaN and
ZnO/(Ga1-xZnx)(N1-xOx)/GaN heterostructures using a Heyd-Scuseria-Ernzerhof
(HSE) hybrid functional. We found that the conventional GGA functionals
underestimate not only the band gap but also the band offset of these
heterostructures. Using the hybrid functional calculations, we show that the
(Ga1-xZnx)(N1-xOx) solid solution has a direct band gap of about 2.608 eV, in
good agreement with the experimental data. More importantly, this solid
solution forms type-II band alignment with the host materials. A
GaN/(Ga1-xZnx)(N1-xOx)/ZnO core-shell solar cell model is presented to improve
the visible light adsorption ability and carrier collection efficiency.
06/2012;
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ABSTRACT: Graphdiyne, consisting of sp- and sp(2)-hybridized carbon atoms, is a new member of carbon allotropes which has a natural band gap ~1.0 eV. Here, we report our first-principles calculations on the stable configurations and electronic structures of graphdiyne doped with boron-nitrogen (BN) units. We show that BN unit prefers to replace the sp-hybridized carbon atoms in the chain at a low doping rate, forming linear BN atomic chains between carbon hexagons. At a high doping rate, BN units replace first the carbon atoms in the hexagons and then those in the chains. A comparison study indicates that these substitution reactions may be easier to occur than those on graphene which composes purely of sp(2)-hybridized carbon atoms. With the increase of BN component, the band gap increases first gradually and then abruptly, corresponding to the transition between the two substitution motifs. The direct-band gap feature is intact in these BN-doped graphdiyne regardless the doping rate. A simple tight-binding model is proposed to interpret the origin of the band gap opening behaviors. Such wide-range band gap modification in graphdiyne may find applications in nanoscaled electronic devices and solar cells.
The Journal of Physical Chemistry A 03/2012; 116(15):3934-9. · 2.95 Impact Factor
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ABSTRACT: Graphene quantum dots (QDs) hold great promises in spintronics. Here, we report our predictions of honeycomb-patterned QDs beyond graphene, on the basis of first-principles calculations and an extended Hubbard model. Our calculations showed that the electronic structures and spin-polarization of boron nitride (BN) and silicon carbide (SiC) QDs can be well tuned by controlling the shape and size of the QDs. Edge hydrogenation can not only greatly improve the stability but also diminish the spin-polarization of BN-QDs. Triangular SiC-QDs have spin-polarized ground states, and the magnetic moments increase with the increase of QD size. Hexagonal SiC-QDs, however, possess spin-unpolarized ground states whose energy gaps decrease with the increase of QD size. To understand the origins of the composition- and shape-dependent spin-polarization of these honeycomb-patterned QDs, we extended the single-orbital Hubbard model of graphene QDs by taking into account the onsite energy differences of the two sublattices. Our extended Hubbard model reproduces well the results of first-principles calculations and offers a simple model to predict the electronic structures of honeycomb-patterned QDs.
08/2011;
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ABSTRACT: We have carried out first-principles calculations to explore the energetics and dynamics of Li in graphyne, a novel carbon allotrope consisting of sp–sp2 hybridized carbon atoms, relevant for anode lithium intercalation in rechargeable Li-ion batteries. In contrast to graphite where Li diffusion is confined in the interlayer space (in-plane diffusion), the unique atomic arrangement and electronic structures enable both in-plane and out-plane diffusion of Li ions in graphyne with moderate barriers, 0.53–0.57 eV. The highest Li intercalation density in graphyne can be LiC4, exceeding the up limit of LiC6 in the commonly used graphite. The high lithium mobility and high storage capacity make graphyne a promising candidate for the anode material in battery applications.
04/2011;
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ABSTRACT: Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of graphene/boron nitride heterobilayers (C/BN HBLs) can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers (GBLs), and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.
Applied Physics Letters 02/2011; 98(8):083103-083103-3. · 3.84 Impact Factor
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ABSTRACT: We perform first-principles calculations to investigate the band offsets of (9,0) and (10,0) BN/C heterostructured nanotubes with different interfaces. We show that the built-in electric field induced by charge redistribution modulates the band offsets of these nanotubes in different ways. Remarkably enhanced field-emission properties of the heterostructures are also predicted.
Journal of Physics D Applied Physics 02/2011; 44(9):095405. · 2.54 Impact Factor
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ABSTRACT: We performed density-functional calculations to investigate the electronic structure of ZnO/GaN-core/shell heterostructured nanowires (NWs) orientating along <0001> direction. The built-in electric field arising from the charge redistribution at the {1 1 00} interfaces and the band offsets were revealed. ZnO-core/GaN-shell NWs rather than GaN-core/ZnO-shell ones were predicted to exhibit natural charge spatial separation behaviors, which are understandable in terms of an effective mass model. The effects of quantum confinement on the band gaps and band offsets were also discussed.
Journal of Applied Physics 01/2011; · 2.17 Impact Factor
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ABSTRACT: We carried out first-principles calculations to explore the oxidative longitudinal unzipping of single-walled carbon nanotubes (SWCNTs) of different diameters and chiralities. We found that the initial attack leading to nanotube unzipping prefers to occur in the middle region for armchair tubes, and at the tube ends for zigzag tubes. Once the initial attack has taken place, by overcoming an energy barrier whose value decreases with increasing tube diameter, the subsequent breakage of C-C bonds parallel to the ones broken in the former process is barrierless. The energetically preferred unzipping path is parallel to the tube axis for armchair tubes, resulting in straight zigzag-edged graphene nanoribbons. For zigzag tubes, there are two energetically equivalent unzipping directions corresponding to the opening of two types of C-C bonds tilted towards the tube axis, giving rise to helical unzipping paths. This is disadvantageous for the production of straight graphene ribbons. A local curvature modulation procedure is proposed to efficiently control the location of the initial attack and thus the shape of the produced graphene nanoribbons.
Physical Chemistry Chemical Physics 11/2010; 12(41):13674-80. · 3.57 Impact Factor
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ABSTRACT: We perform first-principles calculations to investigate the geometric and electronic properties of (10,0) and (5,5) BN/C nanotube heterostructures. We show that both of them have smooth interfaces which are free from bond mismatch and vacancy defect. Interface states appear in the band gaps, due to the discontinuity of π-π bonding of carbon nanotube segments, and exhibit asymmetric distribution in the two segments. The charge redistribution in the region near the interfaces gives rise to a build-in electric field and modulates the static electric potential profiles in the heterostructures. The band scheme diagrams of these heterostructures are also presented.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: The geometric, electronic, and magnetic properties of titania nanoribbons (TiO2NRs) are investigated with use of first-principles calculations within density-functional theory. The TiO2NRs formed by cutting ultrathin TiO2 nanosheet along armchair and zigzag axes have high energetic stability. Zigzag TiO2NRs are more preferable than armchair ones. The electronic structures of TiO2NRs highly depend on the growth orientation and the ribbon width. Introducing oxygen vacancy defects into the edges of zigzag TiO2NRs under poor oxygen conditions can reduce the band gap and trigger the spin-polarization of edge states. These TiO2NRs with well-defined atomic structures, high stability, and tunable electronic properties are expected to have potential applications in solar cells, spintronic devices, and sensors.
05/2010;
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ABSTRACT: We propose stable layered structures and ultrathin tubular configurations of titanium oxide (TiO2) nanomaterials on the basis of first-principles calculations within density functional theory. The thinnest TiO2 nanosheet is characterized by a reconstructed (001) bilayer of rutile TiO2, while ultrathin TiO2 nanotubes can be built by rolling up a TiO2NS. These nanotubes are predicted to have high stability, large Young’s modulus, and tunable electronic properties. A possible synthetic route toward these nanostructures is also presented.
06/2009;
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ABSTRACT: The energetic stability and electronic properties of hydrogenated silicon carbide nanowires (SiCNWs) with zinc blende (3C) and wurtzite (2H) structures are investigated using first-principles calculations within density functional theory and generalized gradient approximation. The [111]-orientated 3C-SiCNWs are energetically more stable than other kinds of NWs with similar size. All the NWs have direct band gaps except the 3C-SiCNWs orientating along [112] direction. The band gaps of these NWs decrease with the increase of wire size, due to the quantum-confinement effects. The direct-band-gap features can be kept for the 3C-SiCNWs orientating along [111] direction with diameters up to 2.8 nm. The superior stability and electronic structures of the [111]-orientated 3C-SiCNWs are in good agreement with the experimental results.
05/2009;
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ABSTRACT: The energetics and atomic and electronic structures of silicon carbide (SiC) nanowires (NWs) and nanotubes (NTs) with radii ranging from 0.45 to 2.9 nm are investigated using density functional theory in conjunction with an atomistic band-order potential. It is found that the formation energy (Eform) of the NWs decreases with the increase of wire radius, and that of the NTs decreases with the increase of wall thickness, irrespective of the tube radius. NTs with faceted single-crystalline walls are energetically more favorable than the cylindrical single- or multiwalled SiC NTs. Due to the surface states, the faceted NWs and NTs possess indirect band gaps, which are narrower than that of bulk SiC crystal. The highest valence band and the lowest conduction band mainly arise from the undercoordinated C and Si atoms on the facets. The surface states can be passivated by surface hydrogenation, and the hydrogenated SiC NWs and NTs become direct-band gap semiconductors with wider band gaps than that of bulk SiC crystal.
12/2008;
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ABSTRACT: We performed first-principles calculations to study the energetics, geometric and electronic properties of zinc sulfide (ZnS) nanostructures. ZnS nanowires (ZnSNWs), nanotubes (ZnSNTs) and nanosheets (ZnSNSs) were considered. Both ZnSNWs and ZnSNTs modeled using hexagonal prisms with the atomic arrangement displaying the characters of wurtzite crystal are more stable than the single-walled ZnS nanotubes presented in previous literature. The energy evolution of ZnSNWs and ZnSNTs as a function of tube diameter and wall thickness was calculated and explained using a simple model. The comparison between the energetics and electronic structures of these ZnS nanostructures was also addressed.
Nanotechnology 06/2008; 19(30):305708. · 3.98 Impact Factor
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Zexiao Xi,
Mingwen Zhao,
Ruiqin Zhang,
Shishen Yan,
Tao He,
Weifeng Li,
Xuejuan Zhang,
Xiaohang Lin, Zhenhai Wang,
Xiangdong Liu,
Yueyuan Xia
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ABSTRACT: We performed first-principles calculations on silica nanorings (NRs) designed via the assembly of two- (2MR), three- (3MR), four- (4MR), and six-membered rings (6MR) in a number of different ways. The stable configurations, energetics, and electronic structures of these NRs are presented. The most stable configurations were found to be size-dependent and to possess different structural features at different size ranges. For small-size silica NRs (SiO2)n with n < 12, the configurations with 2MR?3MR hybrid structures (2?3MR-NRs) were energetically most stable. For 12 < n < 22, the NRs formed from linked 2MRs (2MR-NRs) became most favorable. For n > 22, the configurations composed of uniformly hybrid 2MRs and 4MRs (2?4MR-NRs) were the most stable structures. The 2?4MR-NRs had the narrowest HOMO?LUMO gaps, which decreased with decreasing n.
The Journal of Physical Chemistry C. 01/2008; 112(44):17071-17075.
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ABSTRACT: We performed first-principles calculations to study the energetics, IR spectra, and electronic structures of silica nanorings (NR) consisting of two- and four-membered ring (2-4MR) units. A comparison study of other silica clusters, such as nanochains (NC) and nanorings formed by two-membered rings (2MRs) was made. The results show that for small-size (SiO2)n clusters with n<24, the nanochains composed of 2-4MRs (2-4MR-NCs) are more stable than other kinds of NRs and NCs. When n>24 the 2-4MR-NRs structures become energetically favorable. 2-4MR-NRs have the narrowest HOMO–LUMO gaps which increase with increasing cluster size, distinctive IR spectra characterized by several peaks at the 1000–1150 cm−1 region.
Physics Letters A. 373(47):4376-4380.
