Computational design of materials towards next generation energy conversion and quantum information science
Quantum defects
Nuclear Spin via Interactions with a Rare-Earth Ion in the Solid State
Erbium-doped yttrium orthosilicate (Y2SiO5) can be used in microwave-addressable quantum memory applications. Optically detected magnetic resonance of a single Er3+ ion in Y2SiO5 (performed by collaborators) revealed coupling to a proton nuclear spin; this proton, in turn, has a magnetic dipole coupling to other nuclear dark spins in the lattice.
We performed DFT calculations to reveal the origin of the dark spins: hydrogen is unintentionally introduced during growth or processing, and the CdY-HO or PO-Hi complexes lead to the coupling strength consistent to the experimental values, which explains the experimentally observed frequency shift.
Coherent control of a nuclear spin via interactions with a rare-earth ion in the solid-state M. T. Uysal, M. Raha, S. Chen, C. M. Phenicie, S. Ourari, M. Wang, C. G. Van de Walle, V. V. Dobrovitski, and J. D. Thompson, PRX Quantum, 4, 010323 (2023).
Doping and growth of semiconductors for (opto)electronics
Surface reconstructions on bare and hydrogenated β-Ga2O3 surfaces and implications for growth
Hydrogen is present during the growth of β-Ga2O3 using chemical vapor deposition techniques. A detailed understanding of hydrogen-related surface reconstructions is therefore essential for controlling the material properties. This work is focused on the adsorption of hydrogen, gallium, and oxygen adatoms on the Ga2O3 (010) and Ga2O3 (110) surfaces.
By creating a surface phase diagram, surface reconstructions are illustrated as a function of Ga and H chemical potentials. We find that the reconstructions on (110) and (010) surfaces are similar, due to the similarity in bonding. Under more H-rich conditions, multiple hydrogen-containing reconstructions are found, with H adsorption being more stable under O-rich conditions. We propose that such surface will be found under chemical vapor deposition growth conditions, enhancing the prospect that H would be incorporated, which will benefit material quality and contribute to passivating native defects and impurities.
Surface reconstructions on bare and hydrogenated β-Ga2O3 surfaces: Implications for growth M. Wang, S. Mu, and C. G. Van de Walle, Phys. Rev. Materials, 7, 064603 (2023).
Planar defects and the role in electrical properties in β-Ga2O3
Planar defects including stacking faults (SFs) or twin boundaries (TBs) may form during the growth of semiconductors and possibly affect the electrical properties. To understand and control the material performance, a thorough understanding of the structure and formation mechanism of the planar defects is required. This study explored the energetics and electronic structures of planar defects in monoclinic β-Ga2O3, including TBs and SFs. TBs on the (001)A, (001)B, (100)A, (100)B, and (−102) planes are examined.
We found that (100)A has a very low formation energy (0.01 Jm-2), consistent with its observation in a number of experiments. SFs on the (100) plane have much lower energy (0.03 Jm-2) than SFs formed on the (010) and (001) planes. I propose that growing β-Ga2O3 on a (100) surface is thus expected to result in more planar-defect formation, which is also consistent with experimental observations.
First-Principles Study of Twin Boundaries and Stacking Faults in β-Ga2O3 M. Wang∗, S. Mu, J. S. Speck, and C. G. Van de Walle, Adv. Mater. Interfaces, 2300318 (2023).
Incorporation of Al and impurities in gallium oxide
(AlxGa1−x)2O3 is an ideal candidate to form heterostructures with Ga2O3 for applications in high power electronics.
Al behaves differently on the surface from the bulk: thermodynamically Al prefer the octahedrally coordinated cation site; Al adsorbs on tetrahedral sites in most of the surface reconstructions and the high migration barrier of the Al adatom makes it hard to migrate to an octahedral site.
This work demonstrates that surface reconstructions and kinetic limitations can greatly affect the Al incorporation in alloys.
This work also inspires the research in Si and Sn incorporation, which substitutes Ga sites and behaves like a shallow donor. Under a wide range of Si/Sn and Ga chemical potentials, we also find that Si and Sn also prefer the thermodynamically unfavored sites due to the surface reconstruction.
Incorporation of Si and Sn donors in β-Ga2O3 through surface reconstructions M. Wang, S. Mu, and C. G. Van de Walle, J. Appl. Phys., 130, 185703 (2021). (Editor’s pick)
Adsorption and Diffusion of Aluminum on β-Ga2O3 (010) Surfaces M. Wang, S. Mu, and C. G. Van de Walle, ACS Appl. Mater. Interfaces, 13, 10650 (2021).
Atomic scale investigation of aluminum incorporation, defects, and phase stability in β-(AlxGa1−x)2O3 films J. M. Johnson, H.-L. Huang, M. Wang, S. Mu, J. B. Varley, A. F. M. A. U. Bhuiyan, Z. Feng, N. K. Kalarickal, S. Rajan, H. Zhao, C. G. Van de Walle, and J. Hwang, APL Mater. 9, 051103 (2021). (Featured article)
Role of carbon and hydrogen in limiting n-type doping of (AlxGa1−x)2O3
Modulation doping is required for high mobility in field-effect transistors, raising the issue of whether n-type doping of (AlxGa1−x)2O3 can be achieved. We find that Si is an effective donor up to 70% Al in (AlxGa1−x)2O3. However, C and H, which are commonly present during the metal-organic chemical vapor deposition (MOCVD) growth, are deep centers in (AlxGa1−x)2O3. H can also form defect complexes with Si and C that interfere with the Si doping of (AlxGa1−x)2O3. We find that CGa–H and CO–H complexes can explain experimental observations of carbon-related compensation grown by MOCVD.
Role of carbon and hydrogen in limiting n-type doping of monoclinic (AlxGa1−x)2O3 S. Mu, M. Wang, J. B. Varley, J. L. Lyons, D. Wickramaratne, and C. G. Van de Walle, arxiv.org/pdf/2111.07194.pdf
Indium catalyzed growth of gallium oxide
The molecular beam epitaxy (MBE) growth of Ga2O3 suffers from the desorption of suboxides under high growth temperature that reduces the growth rate. Ga2O3(010) surface reconstructions shows that the co-adsorption of In and Ga increases the stability of the metal-covered surface and the desorption of suboxides is suppressed by the presence of In.
In works like a catalyst, which improves the growth rate, but does not incorporate in the material, thus contributing to enhancement in the growth rate.
Role of Ga and In adatoms in the epitaxial growth of β-Ga2O3 M. Wang, S. Mu, and C. G. Van de Walle, Phys. Rev. B, 102, 035303 (2020).
Heterogeneous catalysis
n-butane monomolecular cracking and dehydrogenation in zeolites
We investigated the monomolecular cracking and dehydrogenation of n-butane molecules over the acidic hydroxyl groups in the nanocages and nanochannels of zeolite frameworks (CHA, TON, MEL, MEI, and VFI) and on the surface of the 2D CHA thin films. The intrinsic free energy of activation is the highest for dehydrogenation and lowest for central C–C bond cracking in bulk chabazite, which is discovered in all zeolite frameworks. The effects of the zeolite channel size on the n-butane adsorption and monomolecular cracking are studied using bulk zeolite frameworks with decreasing special confinement (TON, MEL, MEI, and VFI). As the confinement of channels decreases, n-butane adsorption becomes weaker, and the free energy of activation of terminal C–C cracking increases. The activation energy barriers (dehydrogenation and terminal and central C–C cracking) on the 2D bilayer film surface, which may be considered as zeolite cages at the infinite cage size limit, are close to that in VFI with the largest channel size.
First-Principles Study of n-Butane Monomolecular Cracking and Dehydrogenation on Two-Dimensional-Zeolite Model Systems: Reaction Mechanisms and Effects of Spatial Confinement M. Wang, J. A. Boscoboinik, and Deyu Lu, J. Phys. Chem. C, 127, 13593 (2023).
Nanoconfinement enables new chemical reaction pathway
The rate-limiting step in the direct hydrogenation pathway on the Ru(0001) surface is the first hydrogen addition step (H + O = OH).
Interestingly, the confinement at the silicate/Ru(0001) interface activates an alternative disproportionation reaction pathway (H2O + O = 2OH), as the presence of the silica increases the desorption barrier for water molecules at the interface.
The increased residence time allows trapped water molecules to react with chemisorbed oxygen to produce two OH per H2O, with an activation energy 0.25 eV lower than that of the first hydrogen addition step.
This work reveals the origin of the observed accelerated water formation reaction at the silica/Ru(0001) interface, and points to a route to engineer chemical reaction pathways by leveraging sub-nano-scale confined spaces at metal-oxide interfaces.
Mechanism of the accelerated water formation reaction under interfacial confinement M Wang, C Zhou, N Akter, WT Tysoe, JA Boscoboinik, D Lu, ACS Catal., 10, 6119, (2020).
Nano-confined space of the silica allows noble gas separation
The nanoscale confinement of noble gases at non-cryogenic temperatures is crucial for many applications including noble gas separations, nuclear waste remediation, and the removal of radon. Due to the nanoporous structure of the silicate bilayer and the space at the silicate/Ru interface, noble gas atoms (Ar, Kr, and Xe) can be trapped inside nano-cages of ultrathin silicate crystalline nanoporous frameworks.
DFT studies provides evidence of an initial ionization process that significantly reduces the apparent trapping barrier. The desorption barriers increase as atomic size increases, which leads to increasing desorption temperatures 348 K (Ar), 498 K (Kr) and 673 K (Xe). Rn is theoretically predicted to be trapped with an even higher desorption temperature of 775 K. This work highlights a new ionization-facilitated trapping mechanism resulting in an ultrathin porous material for single-atom trapping and separation applications.
Ionization-facilitated formation of two-dimensional (alumino)silicate– noble gas clathrate compounds J. Q. Zhong‡, M. Wang‡, N. Akter, J. Kestell, T. Niu, A. M. Boscoboinik, T. J. Kim, D. Stacchiola, Q. Wu, D. Lu, J. A. Boscoboinik, Adv. Funct. Mater, 29, 1806583, (2019).
Immobilization of single Ar atoms in two-dimensional zeolite nano-cages J. Q. Zhong, M. Wang, N. Akter, J. Kestell, I. Waluyo, A. M. Boscoboinik, T. Kim, D. Stacchiola, D. Lu, J. A. Boscoboinik, Nat. Commun., 8, 16118 (2017).
Exfoliating silica bilayers via intercalation
Novel applications of silica bilayer can be further explored as a constituent of van der Waals assembly of 2D materials. Key to these applications is an unmet technical challenge to exfoliate and transfer silica bilayer films in large area from one substrate to another without material damage.
I perform DFT studies and propose a new exfoliation mechanism based on gas molecule intercalation. I find that the intercalation of O atoms and CO molecules at the silica/TM interface weakens the silica – transition metal hybridization, which results in an exponential decrease of the exfoliation energy against the interface distance as the coverage of interfacial species increases.
This new intercalation mechanism opens up the opportunity for non-damaging exfoliation and transfer of large area silica bilayers.
Exfoliating silica bilayers via intercalation at the silica/transition metal interface M Wang, JA Boscoboinik, and D Lu, Nanotechnology, (2021).
First-principles study of interface structures and charge rearrangement at the aluminosilicate/Ru(0001) heterojunction M. Wang, J. Q. Zhong, D. Stacchiola, J. A. Boscoboinik, D. Lu, J. Phys. Chem. C, 123, 7731, (2018).
Energy level shifts at the silica/Ru(0001) heterojunction driven by surface and interface dipoles M. Wang, J. Q. Zhong, J. Kestell, I. Waluyo, D. Stacchiola, D. Lu, J. A. Boscoboinik, Top. Catal., 60, 481 (2017).
Nanoconfinement activates copper catalyst
The confined space at the silicate bilayer/Ru interface controls the oxidation state of copper. Copper is an important industrial catalyst. The ability to manipulate the oxidation state of copper clusters in a controlled way is critical to understanding structure-reactivity relations of copper catalysts at the molecular level. Here bilayer silica films grown on a metallic substrate are used to trap diluted Cu2+ while Cu and Cu+ are commonly observed on bare metal surfaces. Density functional theory computations revealed that the confined space created by the silicate film increases Cu diffusion barriers, which leads to more dispersed Cu clusters, enabling stronger charge transfer to form Cu2+.
Stabilization of Oxidized Copper Nanoclusters in Confined Spaces N. Akter, M. Wang, J. Q. Zhong, Z. Liu, T. Kim, D. Lu, J. A. Boscoboinik, D. Stacchiola, Top. Catal., 61, 419 (2018).