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First principles study of oxide semiconductors for solar energy applications
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , viii, 96 p.
Keyword [en]
photocatalysis; water splitting; oxynitrides; dielectric function; first-principles calculation; density functional theory; electronic structures; Casimir interaction
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-165070ISBN: 978-91-7595-451-6 (print)OAI: oai:DiVA.org:kth-165070DiVA: diva2:807069
Public defence
2015-05-22, D3, Lindstedtsvägen 5, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20150423

Available from: 2015-04-23 Created: 2015-04-22 Last updated: 2015-04-23Bibliographically approved
List of papers
1. Band gap reduction and dielectric function of Ga1-xZnxN1-xOx and In1-xZnxN1-xOx alloys
Open this publication in new window or tab >>Band gap reduction and dielectric function of Ga1-xZnxN1-xOx and In1-xZnxN1-xOx alloys
2012 (English)In: Physica Status Solidi. A: Applications and Materials Science (Print), ISSN 1862-6300, Vol. 209, no 1, 75-78 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
absorption, band gap, dielectric function, GaN, InN, semiconductor alloys, ZnO
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-95277 (URN)10.1002/pssa.201100148 (DOI)000303380700015 ()2-s2.0-84055218396 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120522Available from: 2012-05-22 Created: 2012-05-21 Last updated: 2015-04-23Bibliographically approved
2. Free exciton absorption in Ga1-xZnxN1-xOx alloys
Open this publication in new window or tab >>Free exciton absorption in Ga1-xZnxN1-xOx alloys
2012 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 350, no 1, 17-20 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
GaN, ZnO, Semiconductor alloys, Dielectric function, Exciton
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-97990 (URN)10.1016/j.jcrysgro.2011.12.015 (DOI)000304457800005 ()2-s2.0-84861644813 (Scopus ID)
Conference
7th International Workshop on Bulk Nitride Semiconductors (IWBNS), Mars 15-20, 2011, Japan
Note

QC 20120619

Available from: 2012-06-19 Created: 2012-06-18 Last updated: 2017-12-07Bibliographically approved
3. ZnO-InN nanostructures with tailored photocatalytic properties for overall water-splitting
Open this publication in new window or tab >>ZnO-InN nanostructures with tailored photocatalytic properties for overall water-splitting
2013 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 38, no 36, 16727-16732 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
ZnO, InN, Hydrogen production, Alloys, Band gap, Dielectric function
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-139276 (URN)10.1016/j.ijhydene.2013.02.071 (DOI)000327904500098 ()2-s2.0-84888001850 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research Council
Note

QC 20140109

Available from: 2014-01-09 Created: 2014-01-08 Last updated: 2017-12-06Bibliographically approved
4. Analysis of the Semi local States in ZnO-InN Compounds
Open this publication in new window or tab >>Analysis of the Semi local States in ZnO-InN Compounds
2014 (English)In: Crystal Growth & Design, ISSN 1528-7483, E-ISSN 1528-7505, Vol. 14, no 10, 4937-4943 p.Article in journal (Refereed) Published
Abstract [en]

ZnO alloys are extensively explored for the developments of optoelectronics. In this work we analyze the rather unconventional type of ZnO-based compound ZnOX (ZnO)(1y)Xy with X = InN. The compound forms alloy with ZnO and/or assembles cluster structures in the ZnO host. Importantly, this type of alloy benefits from being isovalent which implies a more stable crystalline structure, and at the same time it benefits from the oxynitride anion-alloying that alters the optoelectronic properties. Theoretical studies reveal that incorporating InN in ZnO strongly narrows the fundamental band gap energy Eg. For example, the (ZnO)(0.875)(InN)(0.125) alloy has the gap energy E-g = 2.20 eV = E-g(ZnO) 1.14 eV. The origin of this effect is a hybridization of the anion N 2p-like and O 2p-like orbitals. Intriguingly, the presence of InN nanoclusters enhances this effect and narrows the gap further, and moreover, the nanostructured configurations show more disperse energy distribution of the hybridized anion states compared with the random alloy. Nanoclustering affects the ZnO host more compared to structures with more random distribution of the InN dimers. On the basis of the different characters of the alloys and the nanostructures, we conclude that fine-tuned synthesizing of the (ZnO)(1-y)(InN)(y) alloys can be beneficial for a variety of novel nanosystems for optoelectronic and photoelectrochemical applications.

National Category
Other Chemistry Topics Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-155467 (URN)10.1021/cg500279q (DOI)000342609300012 ()2-s2.0-84907494769 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20141112

Available from: 2014-11-12 Created: 2014-11-06 Last updated: 2017-12-05Bibliographically approved
5. Comparative study of rutile and anatase SnO2 and TiO2: Band-edge structures, dielectric functions, and polaron effects
Open this publication in new window or tab >>Comparative study of rutile and anatase SnO2 and TiO2: Band-edge structures, dielectric functions, and polaron effects
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 8, 083703- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Born effective charge, Chemical compositions, Electron phonon couplings, Electronic band gaps, Exchange-correlations, First-principles approaches, Optical and electronic properties, Static dielectric constants
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-121488 (URN)10.1063/1.4793273 (DOI)000315667500038 ()2-s2.0-84874863000 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
6. Adjusting the Electronic and Optical Properties of Mesoporous MCM-41 Materials by Ti Doping
Open this publication in new window or tab >>Adjusting the Electronic and Optical Properties of Mesoporous MCM-41 Materials by Ti Doping
2013 (English)In: Sensor Letters, ISSN 1546-198X, Vol. 11, no 8, 1530-1534 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Mesoporous Materials, Chemical Synthesis, First-Principles Calculation, Hybrid Functional, Density Functional, Optical Absorption
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-141739 (URN)10.1166/sl.2013.2834 (DOI)000330418100023 ()2-s2.0-84899889328 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research Council
Note

QC 20140224

Available from: 2014-02-24 Created: 2014-02-21 Last updated: 2015-04-23Bibliographically approved
7. Casimir quantum levitation tuned by means of material properties and geometries
Open this publication in new window or tab >>Casimir quantum levitation tuned by means of material properties and geometries
Show others...
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 20, 201407- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-147426 (URN)10.1103/PhysRevB.89.201407 (DOI)000336754300002 ()2-s2.0-84901441427 (Scopus ID)
Funder
Swedish Research Council, C0485101
Note

QC 20140630

Available from: 2014-06-30 Created: 2014-06-27 Last updated: 2017-12-05Bibliographically approved
8. Increased porosity turns desorption to adsorption for gas bubbles near water-SiO2 interface
Open this publication in new window or tab >>Increased porosity turns desorption to adsorption for gas bubbles near water-SiO2 interface
Show others...
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 7, 075403Article in journal (Refereed) Published
Abstract [en]

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).

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-161107 (URN)10.1103/PhysRevB.91.075403 (DOI)000348873500003 ()2-s2.0-84961288284 (Scopus ID)
Funder
Swedish Research Council, C0485101
Note

QC 20150323

Available from: 2015-03-23 Created: 2015-03-09 Last updated: 2017-12-04Bibliographically approved
9. Ultrathin nanosheet induced repulsive Casimir force with two transition points
Open this publication in new window or tab >>Ultrathin nanosheet induced repulsive Casimir force with two transition points
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-165088 (URN)
Funder
Swedish Research Council
Note

QS 2015

Available from: 2015-04-23 Created: 2015-04-23 Last updated: 2015-04-23Bibliographically approved

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