| 1. |
- Boström, Mathias, et al.
(författare)
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Casimir attractive-repulsive transition in MEMS
- 2012
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:11, s. 377-
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Tidskriftsartikel (refereegranskat)abstract
- Unwanted stiction in micro-and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable nonmonotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.
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| 2. |
- Boström, Mathias, et al.
(författare)
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Enlarged molecules from excited atoms in nanochannels
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 86:1, s. 014701-
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Tidskriftsartikel (refereegranskat)abstract
- The resonance interaction that takes place in planar nanochannels between pairs of excited-state atoms is explored. We consider interactions in channels of silica, zinc oxide, and gold. The nanosized channels induce a dramatically different interaction from that in free space. Illustrative calculations for two lithium and cesium atoms demonstrate that there is a short-range repulsion followed by long-range attraction. The binding energy is strongest near the surfaces. The size of the enlarged molecule is biggest at the center of the cavity and increases with channel width. Since the interaction is generic, we predict that enlarged molecules are formed in porous structures, and that the molecule size depends on the size of the nanochannels.
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| 3. |
- Boström, Mathias, et al.
(författare)
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Increased porosity turns desorption to adsorption for gas bubbles near water-SiO2 interface
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:7
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Tidskriftsartikel (refereegranskat)abstract
- We consider theoretically the retarded van der Waals interaction of a small gas bubble in water with a porous SiO2 surface. We predict a possible transition from repulsion to attraction as the surface is made more porous. It highlights that bubbles will interact differently with surface regions with different porosity (i.e., with different optical properties).
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| 4. |
- Dou, Maofeng, et al.
(författare)
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Adjusting the Electronic and Optical Properties of Mesoporous MCM-41 Materials by Ti Doping
- 2013
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Ingår i: Sensor Letters. - : American Scientific Publishers. - 1546-198X. ; 11:8, s. 1530-1534
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Tidskriftsartikel (refereegranskat)abstract
- Solid mesoporous materials with ordered pore structures can be used for catalysis, genome sequencing, and drug delivering etc. The common method of synthesizing Ti doped mesoporous silica is to use costly ethyl silicate and tetrabutyl titanate as inorganic sources. In this work, however, low cost water glass and TiCl4 were used as the inorganic sources. Pore structures were studied by small angle X-ray diffraction, transmission electron microscopy, and N-2 adsorption-desorption. This Ti doped mesoporous material has binary mesopore structures with preferable pore sizes of 2.60 and 3.85 nm, respectively. Results from the ultraviolet-visible spectroscopy as well as density functional calculation employing a screened hybrid functional indicate that the mespore matrix contains tetrahedral coordinated Ti dopants (Ti-Si) and oxygen vacancies (V-O). The electron transitions from occupied V-O to the unoccupied Ti-Si contribute to the red-shift of the optical absorption edge. The hybrid potential describes fairly accurately the electronic structure and optical absorption, and we find an overall good qualitative agreement with the experimental characterization.
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| 5. |
- Dou, Maofeng, et al.
(författare)
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Band gap reduction and dielectric function of Ga1-xZnxN1-xOx and In1-xZnxN1-xOx alloys
- 2012
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Ingår i: Physica Status Solidi. A: Applications and Materials Science (Print). - : Wiley. - 1862-6300. ; 209:1, s. 75-78
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Tidskriftsartikel (refereegranskat)abstract
- The band gap reductions, dielectric functions and absorption coefficients of the Ga1-xZnxN1-xOx and In1-xZnxN1-xOx (x=0.00, 0.25, 0.50, 0.75, and 1.00) alloys were calculated, employing the partial self-consistent GW approximation. As a comparison, the local density approximation (LDA) and the Heyd-Scueria-Ernzerhof (HSE) hybrid functional were also used to calculate the gap reduction. Both Ga1-xZnxN1-xOx and In1-xZnxN1-xOx alloys show strong band gap bowing. As a result, the band gap energy in Ga1-xZnxN1-xOx is reduced by E-g(GaN) E-g (Ga1-xZnxN1-xOx) - 1.61, 2.01 and 1.91 eV for x=0.25, 0.50, and 0.75, respectively. This allows optoelectronic devices based on GaN and ZnO with more efficient absorption or emission of light in the visible light range. The calculated dielectric functions and absorption spectra demonstrate that the band gap reduction enhances the optical absorption around the 2.5 eV region. Interestingly, the In1-xZnxN1-xOx alloy with x=0.25 has the large optical absorption coefficient in the energy region 0.69-6.0 eV, and the alloy has very good absorption at 2-3 eV.
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| 6. |
- Dou, Maofeng, et al.
(författare)
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Casimir quantum levitation tuned by means of material properties and geometries
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:20, s. 201407-
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Tidskriftsartikel (refereegranskat)abstract
- The Casimir force between two surfaces is attractive in most cases. Although stable suspension of nano-objects has been achieved, the sophisticated geometries make them difficult to be merged with well-established thin film processes. We find that by introducing thin film surface coating on porous substrates, a repulsive to attractive force transition is achieved when the separations are increased in planar geometries, resulting in a stable suspension of two surfaces near the force transition separation. Both the magnitude of the force and the transition distance can be flexibly tailored though modifying the properties of the considered materials, that is, thin film thickness, doping concentration, and porosity. This stable suspension can be used to design new nanodevices with ultralow friction. Moreover, it might be convenient to merge this thin film coating approach with micro- and nanofabrication processes in the future.
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| 7. |
- Dou, Maofeng, et al.
(författare)
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Comparative study of rutile and anatase SnO2 and TiO2 : Band-edge structures, dielectric functions, and polaron effects
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:8, s. 083703-
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Tidskriftsartikel (refereegranskat)abstract
- SnO2 and TiO2 polymorphs (rutile and anatase) are oxides with similar crystal structures, comparable bond lengths, and electronic band-gap energies, but different optical and electronic properties. In this work, we have studied the origin of these differences from the band-edge structures and electron-phonon coupling. The band-edge structures, dielectric functions, and effective masses were calculated by means of a first-principles approach with the exchange-correlation described by a hybrid functional. The phonon frequencies were calculated using a finite displacement method with non-analytic correction, and the phonon contribution to the dielectric functions was modeled using a multi-phonon Lorentz model. The calculated band-edge structures show that the bottommost conduction bands are highly dispersive for SnO2 polymorphs but flat dispersive for TiO2 polymorphs because of the strongly localized Ti-3d states. Consequently, SnO2 polymorphs present small effective electron masses and a weak optical absorption, whereas the TiO2 polymorphs present a strong optical absorption and larger effective electron masses. Due to the strong ionic bonds, TiO2 have larger Born effective charges than that of SnO2, result in stronger polaron effect and larger average static dielectric constant epsilon(0). For example, epsilon(0) = 115 for rutile TiO2 whereas epsilon(0) = 9.5 for rutile SnO2. Moreover, it is interesting to note that the epsilon(0) in rutile TiO2 is much larger than in anatase TiO2 (epsilon(0) = 28) although they have the same chemical compositions, which related to the local structure distortion of the phases.
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| 8. |
- Dou, Maofeng, 1982-
(författare)
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First principles study of oxide semiconductors for solar energy applications
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The objectives of this thesis are to understand the electronic structures of oxides and oxynitrides for photocatalytic water splitting, examine the Casimir interaction between oxides, and explore possible approach to bridge the Casimir force and material properties for advanced material research. The studies were performed in the framework of the density functional theory, many-body perturbation theory, i.e, the GW approximation and Bethe-Salpeter equation, as well as the Casimir-Lifshitz approach.The thesis consists of two sets of results. In the first part (papers I-VI), the electronic structures of oxynitrides, i.e., ZnO-GaN and ZnO-InN, with different compositions and local structures have been studied. The oxynitrides reduce the band-gap energies significantly compared to the binary counterparts, enabling the oxynitrides to act as visible light active photocatalysts. Formation of cluster--like structures further reduces the band-gap and delocalizes the valence bands, benefiting higher optical absorption. Furthermore, the energy levels between oxynitride and water were aligned using a surface model adapted from semiconductor heterostructure.In the second part (papers V-IX), the electronic structures of oxides as well as the Casimir interactions have been examined. In particular, we investigated the differences of optical and electronic properties between SnO2 and TiO2 polymorphs in terms of band-edge characters and electron-phonon coupling. In addition, we synthesized a mesoporous material possessing two types of pore structures (one is hexagonal ordered with pore diameter of 2.60 nm and the other is disordered with pore diameter of 3.85 nm). The pore framework contains four-coordinated titanium and oxygen vacancies, verified by both experimental measurements and density-functional theory calculations. Utilizing the predicted properties of the materials, we studied the Casimir interactions. A stable equilibrium of Casimir force is achieved in planar geometry containing a thin film and porous substrates. Both the force and equilibrium distance are tuned through modification of the material properties, for instance, optical properties and porosity. Furthermore, we adapted this concept to study the interactions between gas bubbles and porous SiO2 in water. A transition from repulsion to attraction is predicted, which highlights that the bubbles may interact differently at different surface regions.
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| 9. |
- Dou, Maofeng, et al.
(författare)
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Free exciton absorption in Ga1-xZnxN1-xOx alloys
- 2012
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 350:1, s. 17-20
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of the Ga1-xZnxN1-xOx alloy (x=0.0, 0.25, 0.50, 0.75, and 1.00) are studied by first-principles means, employing the GW method to describe single-particle excitations and the Bethe-Salpeter equation (BSE) to model the two-particle exciton interactions. Intriguingly, we find that the band gaps of the Ga1-xZnxN1-xOx alloy are reduced significantly compared with that of bulk ZnO and GaN. By including the electron-hole interactions within the BSE approach, the imaginary part epsilon(2)(omega) of the dielectric function shows an optical absorption enhancement in the low energy region with the exciton peak below the band gap energy. By comparing the energy difference between the exciton absorption peaks E-ex and the energy gaps E-g, we qualitatively estimate that the strength of excitonic coupling is weaker in the Ga1-xZnxN1-xOx alloy than in both GaN and ZnO. Interestingly, the exciton absorption intensity increases with respect to ZnO content.
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| 10. |
- Dou, Maofeng, et al.
(författare)
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Nanostructured ZnO-X alloys with tailored optoelectronic properties for solar-energy technologies
- 2013
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Ingår i: Materials Research Society Symposium Proceedings. - : Materials Research Society.
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Konferensbidrag (refereegranskat)abstract
- Alloying ZnO with isovalent compounds allows tailoring the material's optoelectronic properties. In this work, we theoretically analyze the ZnO-based alloys ZnO-X ≡ (ZnO)1-x(X)x where X = GaN and InN, employing a first-principles Green's function method GW0 based on the density functional approach. Since the alloy compounds are isovalent to ZnO, we find relatively small distortion of the crystalline structure, however, nanocluster structures are expected to be present in the alloy. ZnO-X reveal intriguing optoelectronic properties. Incorporating GaN or InN in ZnO strongly narrows the energy gap. The band gap energy is reduced from Eg = 3.34 eV in intrinsic ZnO to ∼2.17 and ∼1.89 eV in ZnO-X by alloying ZnO with 25% GaN and InN, respectively. Moreover, clustering enhances the impact on the electronic structure, and the gap energy in ZnO-InN is further reduced to 0.7-1.5 eV if the 25% compound contains nanoclusters. The dielectric function - 2(ω) varies weakly in ZnO-GaN with respect to alloy composition, while it varies rather strongly in ZnO-InN. Hence, by properly growing and designing ZnO-X, the alloy can be optimized for a variety of novel integrated optoelectronic nano-systems.
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| 11. |
- Dou, Maofeng, et al.
(författare)
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ZnO-InN nanostructures with tailored photocatalytic properties for overall water-splitting
- 2013
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Ingår i: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 38:36, s. 16727-16732
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Tidskriftsartikel (refereegranskat)abstract
- ZnO-based electrodes for one-step photocatalytic water splitting are designed by incorporating InN. The electronic and optical properties of (ZnO)(1-x)(InN)(x) alloys and ZnO with InN-like cluster formations ZnO:(InN)(x) are analyzed by means of first-principles approaches. We calculate the energy gaps E-g, the band-edge energies relative to the vacuum level, and the optical absorption, employing the GW(o) method to describe single-particle excitations and the Bethe-Salpeter equation to model the two-particle exciton interactions. For ZnO and InN, the valence-band maximum (VBM) is E-VBM approximate to -7.3 and -5.7 eV, and the energy gap is E-g approximate to 3.3 and 0.7 eV, respectively. Incorporating InN into ZnO, the random (ZnO)(1-x)(InN)(x) alloys up-shifts the VBM and down-shifts the conduction-band minimum (CBM). In addition, the presence of InN-like clusters enhances this effect and significantly narrows the band gap. For instance, the VBM and the energy gap for 12.5% InN are E-VBM approximate to -6.5 and -6.1 eV, and E-g approximate to 2.2 and 1.9 eV for the alloy and the cluster structure, respectively. This impact on the electronic structure favors thus visible light absorption. With proper nanoclusters, the band edges straddle the redox potential levels of H+/H-2 and O-2/H2O, suggesting that ZnO-InN nanostructures can enhance the photocatalytic activity for overall solar-driven water splitting.
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| 12. |
- Huang, Dan, et al.
(författare)
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Investigation on AgGaSe2 for water splitting from first-principles calculations
- 2014
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 105:3, s. 37007-
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of AgGaSe2 has been investigated as a photocatalyst candidate by first-principles calculation. Our results demonstrate that the band edge positions of bulk AgGaSe2 straddle the water redox potentials. From the band offset calculation, we find that Al-doping of AgGaSe2 shifts the conduction band minimum upwards, whereas Cu-doping of AgGaSe2 shifts the valence band maximum upwards. By (Ag, Cu)(Ga, Al)Se-2 alloying one can thereby tailor both the band edge positions and the band gap energy, and this effect provides an approach to optimize the band properties for overall water splitting. Moreover, AgGaSe2 forms a suitable junction with CuGaSe2 with a type-II band offset, which facilitates electron-hole separation. The AgGaSe2 and CuGaSe2 junction can be designed as a tandem photoelectrochemical device to improve the photocatalytic properties of the system.
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| 13. |
- Liang, Jie, et al.
(författare)
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A multi-dimensional quasi-zeolite with 12 x 10 x 7-ring channels demonstrates high thermal stability and good gas adsorption selectivity
- 2016
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 7:5, s. 3025-3030
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Tidskriftsartikel (refereegranskat)abstract
- A novel quasi-zeolite PKU-15, with a rare 3-dimensional structure containing interconnected large (12-ring), medium (10-ring) and small (7-ring) multi-pore channels, was hydrothermally synthesised and characterised. A unique tri-bridging O2- anion is found to be encapsulated in the cage-like (Ge,Si)(12)O-31 building unit and energetically stabilises the PKU-15 framework. The removal of this oxygen atom would convert PKU-15 into a hypothetical zeolite PKU-15H. Thus, PKU-15 can be considered as a unique 'quasi-zeolite', which bridges porous germanates and zeolites. Owing to the absence of terminal Ge-OH groups in its structure, PKU-15 shows a remarkably high thermal stability of up to 600 degrees C. PKU-15 is also the first microporous germanate that exhibits permanent porosity, with a BET area of 428 m(2) g(-1) and a good adsorption affinity toward CO2.
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| 14. |
- Wang, Jing, et al.
(författare)
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Visible light-driven g-C3N4/m-Ag2Mo2O7 composite photocatalysts : synthesis, enhanced activity and photocatalytic mechanism
- 2014
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 4:92, s. 51008-51015
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Tidskriftsartikel (refereegranskat)abstract
- The g-C3N4/m-Ag2Mo2O7 composite photocatalysts with well-aligned band structures are successfully fabricated by a simple two-step method with different mass contents of m-Ag2Mo2O7. The as-prepared samples are evaluated as photocatalysts toward rhodamine B (RhB) degradation in aqueous solution under visible light irradiation (lambda > 420 nm). The results demonstrate that the photocatalytic activities of the composites are strongly influenced by the weight ratio of g-C3N4 to m-Ag2Mo2O7. When it is 6 : 1, the composite exhibits the highest photocatalytic efficiency. More specifically, this value is as high as 3.4 and 6.1 times that of pure g-C3N4 and P25 respectively. In order to investigate the mechanism causing the enhanced photocatalytic degradation, the band structures are determined by UV-vis diffuse reflection spectroscopy and the Mott-Schottky technique. Moreover, the reactive radicals involved in the photocatalytic process are examined in detail via active species trapping (AST) experiments. The improved photocatalytic activities can be attributed to the efficient separation of the photo-induced charge carriers and the strong redox capacities benefit from the synergetic effect between g-C3N4 and m-Ag2Mo2O7.
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