Department of Materials Science and Engineering

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Structure of the InAlAs/InP interface by atomically resolved energy dispersive spectroscopy
(American Institute of Physics, 2011) Klenov, D. O.; Zide, J. M. O.; Klenov, D. O., Zide, J. M. O.; Zide, J. M. O. (orcid.org/0000-0002-6378-7221)
The structure of epitaxially grown InAlAs/InP interfaces was studied using atomically resolved x-ray energy dispersive spectroscopy in scanning transmission electron microscopy. As and P sublattices show sharp termination on the interface. The In sublattice is continuous across the interface. The study has shown the depletion of the Al concentration at the interface; at the last atomic columns of the InAlAs, In occupancy is close to 100%, while Al occupancy is almost zero. A monolayer of InAs at the interface is consistent with substitution of As for P at the surface preceding growth.
Contactless electroreflectance study of E0 and E0+ΔSO transitions in In0.53Ga0.47BiXAs1-X alloys
(American Institute of Physics, 2011) Kudrawiec, R.; Kopaczek, J.; Misiewicz, J.; Petropoulos, J. P.; Zhong, Y.; Zide, J. M. O.; R. Kudrawiec, J. Kopaczek, J. Misiewicz, J. P. Petropoulos, Y. Zhong, and J. M. O. Zide; Petropoulos, J. P.; Zhong, Y.; Zide, J. M. O. (orcid.org/0000-0002-6378-7221)
Energies of E0 and E0 + ΔSO transitions in In0.53Ga0.47BixAs1−x alloys with 0 < x ≤ 0.036 have been studied by contactless electroreflectance spectroscopy at room temperature. It has been clearly observed that the E0 transition shifts to longer wavelengths (∼50 meV/% of Bi), while the E0 + ΔSO transition is approximately unchanged with changes in Bi concentration. These changes in the energies of optical transitions are discussed in the context of the valence band anticrossing model as well as the common anion rule applied to III-V semiconductors.
Optical and electrical characterization of InGaBiAs for use as a new mid-infrared optoelectronic material
(American Institute of Physics, 2011) Petropoulos, J. P.; Zhong, Y.; Zide, J. M. O.; Petropoulos, J. P., Zhong, Y., Zide, J. M. O.; Petropoulos, J. P.; Zhong, Y.; Zide, J. M. O. (orcid.org/0000-0002-6378-7221)
In0.53Ga0.47BixAs1 x films were grown on InP:Fe substrates by molecular beam epitaxy, with Bi concentrations up to x¼3.60%. Bi content in the epilayers was determined by Rutherford backscattering spectroscopy, and channeling measurements show Bi incorporating substitutionally. Unlike previous work, electrical and optical data are obtained for all samples. A redshift in peak wavelength of about 56 meV/%Bi was observed using spectrophotometry. The valence band anti-crossing model is applied, showing InyGa1 yBixAs1 x lattice-matched to InP is possible by varying the composition, with a theoretical cutoff wavelength of about 6 lm.
Effects of molecular beam epitaxial growth conditions on composition and optical properties of InGaBiAs
(American Institute of Physics, 2012) Zhong, Y.; Dongmo, P. B.; Petropoulos, J. P.; Zide, J. M. O.; Y. Zhong, P. B. Dongmo, J. P. Petropoulos, and J. M. O. Zide; Zhong, Y.; Dongmo, P. B.; Petropoulos, J. P.; Zide, J. M. O.(orcid.org/0000-0002-6378-7221)
We describe the growth conditions of InxGa1 xBiyAs1 y (lattice-mismatched and matched) on InP substrates by molecular beam epitaxy and the resulting properties. Due to their anomalously narrow bandgaps and the presence of bismuth, these materials are promising for optoelectronics and thermoelectrics. Low growth temperature and moderate As/Bi beam equivalent pressure ratios are beneficial for Bi incorporation, in good qualitative agreement with GaBiyAs1 y on GaAs. Up to 6.75% bismuth is incorporated. High resolution x-ray diffraction and reciprocal space mapping show that InxGa1 xBiyAs1 y samples exhibit good crystalline quality and zero relaxation. The band gap is reduced in agreement with theoretical predictions. Lattice-matched samples have been produced with lattice mismatch 0.21%.
Understanding the role of buried interface charges in a metal-oxide-semiconductor stack of Ti/Al2O3/Si using hard x-ray photoelectron spectroscopy
(AIP Publishing, 2015) Opila, Robert Leon, 1953-; Weiland, C.; Church, J. R.; Church, J. R., Weiland C., Opila, R.L.; Church, J. R.
Hard X-ray photoelectron spectroscopy (HAXPES) analyses were carried out on metal-oxidesemiconductor (MOS) samples consisting of Si, thick and thin Al2O3, and a Ti metal cap. Using Si 1s and C 1s core levels for an energy reference, the Al 1s and Si 1s spectra were analyzed to reveal information about the location and roles of charges throughout the MOS layers. With different oxide thicknesses (2 nm and 23 nm), the depth sensitivity of HAXPES is exploited to probe different regions in the MOS structure. Post Ti deposition results indicated unexpected band alignment values between the thin and thick films, which are explained by the behavior of mobile charge within the Al2O3 layer. VC
Synthesis and characterization of bicontinuous cubic poly(3,4-ethylene dioxythiophene) gyroid (PEDOT GYR) gels
(Royal Society of Chemsitry, 2015-01-12) Cho, Whirang; Wu, Jinghang; Shim, Bong Sup; Kuan, Wei-Fan; Mastroianni, Sarah E.; Young, Wen-Shiue; Kuo, Chin-Chen; Epps, Thomas H. III; Martin, David C.; Whirang Cho, Jinghang Wu, Bong Sup Shim, Wei-Fan Kuan, Sarah E. Mastroianni, Wen-Shiue Young, Chin-Chen Kuo, Thomas H. Epps, III and David C. Martin; Cho, Whirang; Wu, Jinghang; Shim, Bong Sup; Kuan, Wei-Fan; Mastroianni, Sarah E.; Young, Wen-Shiue; Kuo, Chin-Chen; Epps, Thomas H. III; Martin, David C.
We describe the synthesis and characterization of bicontinuous cubic poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer gels prepared within lyotropic cubic poly(oxyethylene)10 nonylphenol ether (NP-10) templates with Ia[3 with combining macron]d (gyroid, GYR) symmetry. The chemical polymerization of EDOT monomer in the hydrophobic channels of the NP-10 GYR phase was initiated by AgNO3, a mild oxidant that is activated when exposed to ultraviolet (UV) radiation. The morphology and physical properties of the resulting PEDOT gels were examined as a function of temperature and frequency using optical and electron microscopy, small-angle X-ray scattering (SAXS), dynamic mechanical spectroscopy, and electrochemical impedance spectroscopy (EIS). Microscopy and SAXS results showed that the PEDOT gels remained ordered and stable after the UV-initiated chemical polymerization, confirming the successful templated-synthesis of PEDOT in bicontinuous GYR nanostructures. In comparison to unpolymerized 3,4-ethylenedioxythiophene (EDOT) gel phases, the PEDOT structures had a higher storage modulus, presumably due to the formation of semi-rigid PEDOT-rich nanochannels. Additionally, the storage modulus (G′) for PEDOT gels decreased only modestly with increasing temperature, from ∼1.2 × 105 Pa (10 °C) to ∼7 × 104 Pa (40 °C), whereas G′ for the NP-10 and EDOT gels decreased dramatically, from ∼5.0 × 104 Pa (10 °C) to ∼1.5 × 102 Pa (40 °C). EIS revealed that the impedance of the PEDOT gels was smaller than the impedance of EDOT gels at both high frequencies (PEDOT ∼102 Ω and EDOT 2–3 × 104 Ω at 105 Hz) and low frequencies (PEDOT 103–105 Ω and EDOT ∼5 × 105 Ω at 10−1 Hz). These results indicated that PEDOT gels were highly ordered, mechanically stable and electrically conductive, and thus should be of interest for applications for which such properties are important, including low impedance and compliant coatings for biomedical electrodes.
Controlled ionic conductivity via tapered block polymer electrolytes
(Royal Society of Chemistry, 2015-01-23) Kuan, Wei-Fan; Remy, Roddel; Mackay, Michael E.; Epps, Thomas H. III; Wei-Fan Kuan, Roddel Remy, Michael E. Mackay and Thomas H. Epps, III; Kuan, Wei-Fan; Remy, Roddel; Mackay, Michael E.; Epps, Thomas H. III
We present the design of novel solid electrolytes using tapered block polymers (TBPs). In this work, we synthesize a series of TBPs via atom transfer radical polymerization (ATRP) consisting of rigid polystyrene and ion-conducting poly(oligo-oxyethylene methacrylate) segments and explore the role of tapered interfaces on ion transport. Previous studies on TBPs have shown that manipulating the taper composition in block polymers can reduce the unfavorable polymer–polymer interactions between blocks, enabling the design for highly-processable (lower order–disorder transition temperature) polymer electrolytes. Herein, we demonstrate that the taper profile and taper volume fraction significantly impact the glass transition temperatures (Tgs) in block polymer electrolytes, thus affecting the ionic conductivity. Additionally, we find that the normal-tapered materials with z60 vol% tapering exhibit remarkable improvements in ionic conductivity (increase z190% at 20 C and increase z90% at 80 C) in comparison to their non-tapered counterparts. Overall, our TBPs, with controllable interfacial interactions, present an exciting opportunity for the fabrication of cost-effective, highly-efficient, and stable energy storage membranes.
Poly[3,4-ethylene dioxythiophene (EDOT)-co-1,3,5-tri[2-(3,4-ethylene dioxythienyl)]-benzene (EPh)] copolymers (PEDOT-co-EPh): optical, electrochemical and mechanical properties
(Royal Society of Chemistry., 2015-02-09) Ouyang, Liangqi; Kuo, Chin-Chen; Farrell, Brendan; Pathak, Sheevangi; Wei, Bin; Qu, Jing; Martin, David C.; Liangqi Ouyang, Chin-chen Kuo, Brendan Farrell, Sheevangi Pathak, Bin Wei, Jing Qu and David C. Martin; Ouyang, Liangqi; Kuo, Chin-chen; Farrell, Brendan; Wei, Bin; Qu, Jing; Martin, David C.
PEDOT-co-EPh copolymers with systematic variations in composition were prepared by electrochemical polymerization from mixed monomer solutions in acetonitrile. The EPh monomer is a trifunctional crosslinking agent with three EDOTs around a central benzene ring. With increasing EPh content, the color of the copolymers changed from blue to yellow to red due to decreased absorption in the near infrared (IR) spectrum and increased absorption in the visible spectrum. The surface morphology changed from rough and nanofibrillar to more smooth with rounded bumps. The electrical transport properties dramatically decreased with increasing EPh content, resulting in coatings that either substantially lowered the impedance of the electrode (at the lowest EPh content), leave the impedance nearly unchanged (near 1% EPh), or significantly increase the impedance (at 1% and above). The mechanical properties of the films were substantially improved with EPh content, with the 0.5% EPh films showing an estimated 5× improvement in modulus measured by AFM nanoindentation. The PEDOT-co-EPh copolymer films were all shown to be non-cytotoxic toward and promote the neurite outgrowth of PC12 cells. Given these results, we expect that the films of most interest for neural interface applications will be those with improved mechanical properties that maintain the improved charge transport performance (with 1% EPh and below).
Post-polymerization functionalization of poly(3,4-propylenedioxythiophene) (PProDOT) via thiol–ene “click” chemistry
(Royal Society of Chemistry., 2015-02-25) Wei, Bin; Ouyang, Liangqi; Liu, Jinglin; Martin, David C.; Bin Wei, Liangqi Ouyang, Jinglin Liua and David C. Martin; Wei, Bin; Ouyang, Liangqi; Liu, Jinglin; Martin, David C.
The surface functionalization of conjugated polymers such as the poly(alkoxythiophenes) poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT) provides a potential means for systematically tailoring their physical properties. We previously reported the synthesis of an alkene-functionalized 3,4-propylenedioxy-thiophene (ProDOT) derivative that could be readily modified through thiol–ene “click” chemistry. Here, we investigated the post-polymerization modification of PProDOT surfaces by using a dialkene functionalized variant (ProDOT-diene). The chemical structure of the ProDOT-diene monomer was confirmed by Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared spectroscopy (FTIR). The ProDOT-diene monomer was either chemically or electrochemically polymerized into the PProDOT-diene polymer, and then subsequently modified with alkyl, PEG, or ferrocene moieties via radical-based thiol–ene chemistry. We found that the normally insoluble PProDOT-diene could be converted into a soluble derivative by grafting alkyl groups onto the polymer chains after chemical polymerization. When electrochemically deposited on indium-tin oxide (ITO) glass electrodes, the conductivity, electroactivity and contact angles of the modified PProDOT-diene films could be tuned over a broad range. Scanning Electron Microscopy (SEM) revealed that post-polymerization modification did not significantly alter the surface morphology of the PProDOT-diene films. Overall, this method allows for efficient, facile tuning of the surface chemistry of poly(alkylthiophene) films, making it possible to tailor properties such as conductivity and wettability for different applications.
A Precise, Reduced-Parameter Model of Thin Film Electrolyte Impedance
(The Electrochemical Society, 2015-03-07) McNealy, Benjamin E.; Jiang, Jun; Hertz, Joshua L.; Benjamin E. McNealy, Jun Jiang, and Joshua L. Hertz; McNealy, Benjamin E.; Jiang, Jun; Hertz, Joshua L.
he extreme shape factors inherent in characterizing thin film electrolytes can present a challenge to quantitative interpretation f impedance spectra. Here, the impedance of a thin film ceramic electrolyte with surface microelectrodes is modeled via direct umerical solution of current conservation. Faradaic and non-faradaic currents at the electrode-electrolyte interface are modeled phe- omenologically using a formulation based on the Butler-Volmer equation. The model is able to reproduce complex, experimentally btained impedance spectra for Pt/YSZ and Pt/GDC cells using only four adjustable, physically intuitive parameters: electrolyte onductivity, permittivity, exchange current density, and double layer capacitance. Equivalent circuit models typically used to fit hese spectra instead require six or more adjustable parameters with ambiguous physical meaning. Notably, the model described here s able to capture a heretofore unexplained intermediate frequency arc seen in the experimental results. A parametric study enables he mechanism of the intermediate frequency feature to be identified as a spreading resistance in the electrolyte that vanishes at high requencies due to low-impedance dielectric transport of current across the electrode-electrolyte interface. The fitting results are validated by comparison of the parameter values with literature reports.
Peptide hydrogels – versatile matrices for 3D cell culture in cancer medicine
(Frontiers Media S.A., 2015-04-20) Worthington, Peter; Pochan, Darrin J.; Langhans, Sigrid A.; PeterWorthington, Darrin J. Pochan and Sigrid A. Langhans; Worthington, Peter; Pochan, Darrin J.
Traditional two-dimensional (2D) cell culture systems have contributed tremendously to our understanding of cancer biology but have significant limitations in mimicking in vivo conditions such as the tumor microenvironment. In vitro, three-dimensional (3D) cell culture models represent a more accurate, intermediate platform between simplified 2D culture models and complex and expensive in vivo models. 3D in vitro models can overcome 2D in vitro limitations caused by the oversupply of nutrients, and unphysiological cell–cell and cell–material interactions, and allow for dynamic interactions between cells, stroma, and extracellular matrix. In addition, 3D cultures allowfor the development of concentration gradients, including oxygen, metabolites, and growth factors, with chemical gradients playing an integral role in many cellular functions ranging from development to signaling in normal epithelia and cancer environments in vivo. Currently, the most common matrices used for 3D culture are biologically derived materials such as matrigel and collagen. However, in recent years, more defined, synthetic materials have become available as scaffolds for 3D culture with the advantage of forming well-defined, designed, tunable materials to control matrix charge, stiffness, porosity, nanostructure, degradability, and adhesion properties, in addition to other material and biological properties. One important area of synthetic materials currently available for 3D cell culture is short sequence, self-assembling peptide hydrogels. In addition to the review of recent work toward the control of material, structure, and mechanical properties, we will also discuss the biochemical functionalization of peptide hydrogels and how this functionalization, coupled with desired hydrogel material characteristics, affects tumor cell behavior in 3D culture.
Impact of Different Surface Ligands on the Optical Properties of PbS Quantum Dot Solids
(MDPI (Multidisciplinary Digital Publishing Institute), 2015-04-21) Xu, Fan; Gerlein, Luis Felipe; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.; Fan Xu, Luis Felipe Gerlein, Xin Ma, Chelsea R. Haughn, Matthew F. Doty and Sylvain G. Cloutier; Xu, Fan; Ma, Xin; Doty, Matthew F.; Cloutier, Sylvain G.
The engineering of quantum dot solids with low defect concentrations and efficient carrier transport through a ligand strategy is crucial to achieve efficient quantum dot (QD) optoelectronic devices. Here, we study the consequences of various surface ligand treatments on the light emission properties of PbS quantum dot films using 1,3-benzenedithiol (1,3-BDT), 1,2-ethanedithiol (EDT), mercaptocarboxylic acids (MPA) and ammonium sulfide ((NH4)2S). We first investigate the influence of different ligand treatments on the inter-dot separation, which mainly determines the conductivity of the QD films. Then, through a combination of photoluminescence and transient photoluminescence characterization, we demonstrate that the radiative and non-radiative recombination mechanisms in the quantum dot films depend critically on the length and chemical structure of the surface ligands.
Impact of Different Surface Ligands on the Optical Properties of PbS Quantum Dot Solids
(MDPI AG, 2015-04-21) Xu, Fan; Gerlein, Luis Felipe; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.; Fan Xu, Luis Felipe Gerlein, Xin Ma, Chelsea R. Haughn, Matthew F. Doty and Sylvain G. Cloutier; Xu, Fan; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.
The engineering of quantum dot solids with low defect concentrations and efficient carrier transport through a ligand strategy is crucial to achieve efficient quantum dot (QD) optoelectronic devices. Here, we study the consequences of various surface ligand treatments on the light emission properties of PbS quantum dot films using 1,3-benzenedithiol (1,3-BDT), 1,2-ethanedithiol (EDT), mercaptocarboxylic acids (MPA) and ammonium sulfide ((NH4)2S). We first investigate the influence of different ligand treatments on the inter-dot separation, which mainly determines the conductivity of the QD films. Then, through a combination of photoluminescence and transient photoluminescence characterization, we demonstrate that the radiative and non-radiative recombination mechanisms in the quantum dot films depend critically on the length and chemical structure of the surface ligands.
RAFT polymerization and associated reactivity ratios of methacrylate-functionalized mixed bio-oil constituents
(Royal Society of Chemsitry, 2015-04-22) Holmberg, Angela L.; Karavolias, Michael G.; Epps, Thomas H. III; Angela L. Holmberg, Michael G. Karavolias and Thomas H. Epps, III; Holmberg, Angela L.; Karavolias, Michael G.; Epps, Thomas H. III
This work features a new suite of correlations for estimating kinetic parameters from multicomponent reversible addition–fragmentation chain-transfer (RAFT) polymerizations and an improved methodology for determining reactivity ratios in the pursuit of cost-effective and renewable plastics prepared from moderately processed bio-oils. Select monomers representing possible derivatives of compounds found in renewable bio-oils, such as pyrolyzed Kraft lignin and vegetable oils, were polymerized to investigate the consequences of structural diversity on the kinetics of RAFT polymerization. To facilitate predictions of heteropolymer dispersities and molecular weights, apparent chain-transfer coefficients (Capptr's) and propagation rate constants (kappp's) from homopolymerizations were correlated to kinetic parameters associated with the polymerization of bio-oil mixtures. Capptr depended on the reactivity ratios of the bio-oil components and the composition of the bio-oil feed, whereas kappp was related to only the composition of the bio-oil feed. A modified approach for analyzing Mayo–Lewis plots resulted in more accurate reactivity ratios and with greater precision in comparison to conventional nonlinear fitting procedures and traditional linearization fitting methods, respectively. The measured compositional data readily mapped onto the predicted monomer distribution profiles in multicomponent polymers, confirming the validity of the improved method described herein to determine reactivity ratios. Altogether, this manuscript offers a strategy for improving the viability of biobased polymers, addressing two key factors: minimizing separations costs by polymerizing bio-oil mixtures and preventing batch-to-batch inconsistencies in polymer properties by applying a priori knowledge about the bio-oil constituents’ individual kinetic parameters.
Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules
(American Physical Society, 2015-05-19) Zhou, Xinran; Royo, Miquel; Liu, Weiwen; Lee, Jihoon H.; Salamo, Gregory J.; Climente, Juan I.; Doty, Matthew F.; Xinran Zhou, Miquel Royo, Weiwen Liu, Jihoon H. Lee, Gregory J. Salamo, Juan I. Climente, and Matthew F. Doty; Doty, Matthew F.
We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.
Defects as qubits in 3C- and 4H-SiC
(American Physical Society, 2015-07-20) Gordon, L.; Janotti, Anderson; Van de Walle, Chris G.; L. Gordon, A. Janotti, and C. G. Van de Walle; Janotti, Anderson
We employ hybrid density functional calculations to search for defects in different polytypes of SiC that may serve as qubits for quantum computing. We explore the divacancy in 4H- and 3C-SiC, consisting of a carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in 3C-SiC, in which the substitutional NC sits next to a Si vacancy (NC-VSi). The calculated excitation and emission energies of the divacancy in 4H-SiC are in excellent agreement with experimental data, and aid in identifying the four unique configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For 3C-SiC, we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room temperature as the spin-1 neutral divacancy. Finally, we show that the (NC-VSi)− center in 3C-SiC is highly promising for quantum information science, and we provide guidance for identifying this defect.
A Single-Step Monomeric Photo-Polymerization and Crosslinking via Thiol-Ene Reaction for Hydroxide Exchange Membrane Fabrication
(The Electrochemical Society, 2015-07-31) Tibbits, Andrew C.; Mumper, Laura E.; Kloxin, Christopher J.; Yan, Yushan; Andrew C. Tibbits, Laura E. Mumper, Christopher J. Kloxin, and Yushan S. Yan; Tibbits, Andrew C.; Mumper, Laura E.; Kloxin, Christopher J.; Yan, Yushan
A single step monomeric photo-polymerization and crosslinking via thiol-ene reaction is developed for the preparation of hydroxide exchange membranes (HEMs) in a ternary system with a triallyl triazine, a quaternary ammonium diallyl, and a dithiol. This facile method enables reproducible tuning of the ion exchange capacity and crosslink density. These HEMs demonstrate reasonable hydroxide conductivity, limited alkaline stability, and good thermal stability and have lower water uptakes than other photo-crosslinked HEMs produced with much longer reaction times. Furthermore, this new fabrication method allows the incorporation of catalyst nanoparticles in the hydroxide exchange materials to form thin catalyst layers that are resistant to dissolution in methanol which suggests these polymers can be used in direct alcohol fuel cells (DAFCs).
Determination of the Mott-Hubbard gap in GdTiO3
(American Physical Society, 2015-08-06) Bjaalie, L.; Verma, A.; Himmetoglu, B.; Janotti, Anderson; Raghavan, S.; Protasenko, V.; Steenbergen, E. H.; Jena, D.; Stemmer, S.; Van de Walle, Chris G.; L. Bjaalie, A. Verma, B. Himmetoglu, A. Janotti, S. Raghavan, V. Protasenko, E. H. Steenbergen, D. Jena, S. Stemmer, and C. G. Van de Walle; Janotti, Anderson
The band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.
Structural and electronic properties of SrZrO3 and Sr(Ti,Zr)O3 alloys
(American Physical Society, 2015-08-11) Weston, L.; Janotti, Anderson; Cui, X. Y.; Himmetoglu, B.; Stampfl, C.; Van de Walle, Chris G.; L. Weston, A. Janotti, X. Y. Cui, B. Himmetoglu, C. Stampfl, and C. G. Van de Walle; Janotti, Anderson
Using hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTix Zr1−x O3 structures with x = 0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard’s law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x = 0.5, we find that arranging the Ti and Zr atoms into a 1 × 1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3.
First-principles study of surface charging in LaAlO3/SrTiO3 heterostructures
(American Physical Society, 2015-08-19) Krishnaswamy, K.; Dreyer, C. E.; Janotti, Anderson; Van de Walle, Chris G.; K. Krishnaswamy, C. E. Dreyer, A. Janotti, and C. G. Van de Walle; Janotti, Anderson
The two-dimensional electron gas (2DEG) observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) is known to be very sensitive to the proximity of the LaAlO3 surface and the conditions to which the surface is exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of A. Y. Lozovoi et al. [J. Chem. Phys. 115, 1661 (2001)]. From the calculated electronic structure of the unreconstructed (but relaxed) AlO2 and LaO surface terminations of LAO, we find surface states having excess holes (AlO2 termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of 2DEG formation, and is consistent with a 2DEG of density 3.3 × 1014 cm−2 being intrinsic to the LaO-TiO2 interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy, and H-adatom surface reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of the surface significantly stabilizes the AlO2 termination versus the LaO termination. Overall, our methodology has the advantage of decoupling first-principles calculations for the interface from those for the charged surface, and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities in the bulk of a material. DOI: 10.1103/PhysRevB.

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