Fermi-Level Pinning in ErAs Nanoparticles Embedded in III-V Semiconductors.

Embedding rare-earth monopnictide nanoparticles into III-V semiconductors enables unique optical, electrical, and thermal properties for THz photoconductive switches, tunnel junctions, and thermoelectric devices. Despite the high structural quality and control over growth, particle size (<3 nm), and density, the underlying electronic structure of these nanocomposite materials has only been hypothesized. Structural and electronic properties of ErAs nanoparticles with different shapes and sizes (cubic to spherical, 1.14, 1.71, and 2.28 nm) in AlAs, GaAs, InAs, and their alloys are investigated using first-principles calculations, revealing that spherical nanoparticles have lower formation energies. For the lowest-energy nanoparticles, the Fermi level is pinned near midgap in GaAs and AlAs but resonant in the conduction band in InAs. The Fermi level is shifted down as the particle size increases and is pinned on an absolute energy scale considering the band alignment at AlAs/GaAs/InAs interfaces, offering insights into the rational design of these nanomaterials.

Mott-Insulator State of FeSe as a Van der Waals 2D Material Is Unveiled.

We undertook a comprehensive investigation of the electronic structure of FeSe, known as a Hund metal, and found that it is not uniquely defined. Through accounting for all two-particle irreducible diagrams constructed from electron Green's function G and screened Coulomb interaction W in a self-consistent manner, a Mott-insulator phase of 2D-FeSe is unveiled. The metal-insulator transition is driven by the strong on-site Coulomb interaction in its paramagnetic phase, accompanied by the weakening of both local and nonlocal screening effects on the Fe-3d orbitals. Our results suggest that Mott physics may play a pivotal role in shaping the electronic, optical, and superconducting properties of monolayer or nanostructured FeSe.

Cause of Extremely Long-Lasting Room-Temperature Persistent Photoconductivity in SrTiO_{3} and Related Materials.

It has been recently revealed that strontium titanate (SrTiO_{3}) displays persistent photoconductivity with unique characteristics: it occurs at room temperature and lasts over a very long period of time. Illumination of SrTiO_{3} crystals at room temperature with sub-band-gap light reduces the electrical resistance by three orders of magnitude and persists for weeks or longer [Tarun et al., Phys. Rev. Lett. 111, 187403 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.187403]. Experiments indicate that oxygen vacancy and hydrogen play important roles, yet the microscopic mechanism responsible for this remarkable effect has remained unidentified. Using hybrid density functional theory calculations we show that an instability associated with substitutional hydrogen H_{O}^{+} under illumination, which becomes doubly ionized and leaves the oxygen site, can explain the experimental observations. H_{O} then turns into an interstitial hydrogen and an oxygen vacancy, leading to excess carriers in the conduction band. This phenomenon is not exclusive to SrTiO_{3}, but it is also predicted to occur in other oxides. Interestingly, this phenomenon represents an elegant way of proving the existence of hydrogen substituting on an oxygen site (H_{O}), forming an interesting, and rarely observed, type of three-center, two-electron bond.

Self-Catalyzed Sensitization of CuO Nanowires via a Solvent-free Click Reaction.

Recent advances in organic surface sensitization of metal oxide nanomaterials focused on two-step approaches with the first step providing a convenient functionalized chemical "hook", such as an alkyne functionality connected to a carboxylic group in prop-2-ynoic acid. The second step then took advantage of copper-catalyzed click chemistry to deliver the desired structure (such as benzyl or perylene) attached to an azide to react with the surface-bound alkyne. The use of this approach on CuO not only resulted in a successful morphology preserving chemical modification but also has demonstrated that surface Cu(I) can be obtained during the process and promote a surface-catalyzed click reaction without additional copper catalyst. Here, it is demonstrated that this surface-catalyzed chemistry can be performed on a surface of the CuO nanomaterial without a solvent, as a "dry click" reaction, as confirmed with spectroscopic and microscopic investigations with X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, solid-state nuclear magnetic resonance, and scanning electron microscopy. Computational studies provided instructive information on the interaction between the surface prop-2-yonate and azide functional group to better understand the mechanism of this surface-catalyzed click reaction.

Carrier-Density-Induced Ferromagnetism in EuTiO_{3} Bulk and Heterostructures.

EuTiO_{3} is an antiferromagnetic (AFM) material showing strong spin-lattice interactions, large magnetoelectric response, and quantum paraelectric behavior at low temperatures. Using electronic-structure calculations, we show that adding electrons to the conduction band leads to ferromagnetism. The transition from antiferromagnetism to ferromagnetism is predicted to occur at ∼0.08 electrons/Eu (∼1.4×10^{21} cm^{-3}). This effect is also predicted to occur in heterostructures such as LaAlO_{3}/EuTiO_{3}, where ferromagnetism is triggered by the formation of a high-density two-dimensional electron gas in the EuTiO_{3}. Our analysis indicates that the coupling between Ti 3d and Eu 5d plays a crucial role in lowering the Ti 3d conduction band in the ferromagnetic (FM) phase, leading to an almost linear dependence of the energy difference between the FM and AFM ordering on the carrier concentration. These findings open up possibilities in designing field-effect transistors using EuTiO_{3}-based heterointerfaces to probe fundamental interactions between highly localized spins and itinerant, polarized charge carriers.

Unique Surface Enhanced Raman Scattering Substrate for the Study of Arsenic Speciation and Detection.

In this study, a three-dimensional surface enhanced Raman scattering (SERS) substrate comprised of silver coated gold nanorods (Ag/AuNRs) decorated on electrospun polycaprolactone (PCL) fibers has been applied,  for the first time, to quantitative analytical measurements on various arsenic species: p-arsanilic acid ( pAsA), roxarsone (Rox), and arsenate (AsV), with a demonstrated sensitivity below 5 ppb. AsV detection in a solution of common salt ions has been demonstrated, showing the tolerance of the substrate to more complex environments. pAsA adsorption behavior on the substrate surface has been investigated in detail using these unique SERS substrates. Calculations based on density functional theory (DFT) support the spectral observation for pAsA. This substrate also has been shown to serve as a platform for in situ studies of arsenic desorption and reduction. This SERS substrate is potentially an excellent environmental sensor for both fundamental studies and practical applications.

Phase transformations upon doping in WO3.

High levels of doping in WO3 have been experimentally observed to lead to structural transformation towards higher symmetry phases. We explore the structural phase diagram with charge doping through first-principles methods based on hybrid density functional theory, as a function of doping the room-temperature monoclinic phase transitions to the orthorhombic, tetragonal, and finally cubic phase. Based on a decomposition of energies into electronic and strain contributions, we attribute the transformation to a gain in energy resulting from a lowering of the conduction band on an absolute energy scale.

Direct view at excess electrons in TiO2 rutile and anatase.

A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.

The role of native defects in the transport of charge and mass and the decomposition of Li4BN3H10.

Li4BN3H10 is of great interest for hydrogen storage and for lithium-ion battery solid electrolytes because of its high hydrogen content and high lithium-ion conductivity, respectively. The practical hydrogen storage application of this complex hydride is, however, limited due to irreversibility and cogeneration of ammonia (NH3) during the decomposition. We report a first-principles density-functional theory study of native point defects and defect complexes in Li4BN3H10, and propose an atomistic mechanism for the material's decomposition that involves mass transport mediated by native defects. In light of this specific mechanism, we argue that the release of NH3 is associated with the formation and migration of negatively charged hydrogen vacancies inside the material, and it can be manipulated by the incorporation of suitable electrically active impurities. We also find that Li4BN3H10 is prone to Frenkel disorder on the Li sublattice; lithium vacancies and interstitials are highly mobile and play an important role in mass transport and ionic conduction.

Tuning Excitonic Properties of Monochalcogenides via Design of Janus Structures.

Two-dimensional (2D) Janus structures offer a unique range of properties as a result of their symmetry breaking, resulting from the distinct chemical composition on each side of the monolayers. Here, we report a theoretical investigation of 2D Janus Q'A'AQ P3m1 monochalcogenides from group IV (A and A' = Ge and Sn; Q, Q' = S and Se) and 2D non-Janus QAAQ P3̅m1 counterparts. Our theoretical framework is based on density functional theory calculations combined with maximally localized Wannier functions and tight-binding parametrization to evaluate the excitonic properties. The phonon band structures exhibit exclusively real (nonimaginary) branches for all materials. Particularly, SeGeSnS has greater energetic stability than its non-Janus counterparts, representing an outstanding energetic stability among the investigated materials. However, SGeSnS and SGeSnSe have higher formation energies than the already synthesized MoSSe, making them more challenging to grow than the other investigated structures. The electronic structure analysis demonstrates that materials with Janus structures exhibit band gaps wider than those of their non-Janus counterparts, with the absolute value of the band gap predominantly determined by the core rather than the surface composition. Moreover, exciton binding energies range from 0.20 to 0.37 eV, reducing band gap values in the range of 21% to 32%. Thus, excitonic effects influence the optoelectronic properties more than the point-inversion symmetry breaking inherent in the Janus structures; however, both features are necessary to enhance the interaction between the materials and sunlight. We also found anisotropic behavior of the absorption coefficient, which was attributed to the inherent structural asymmetry of the Janus materials.

Ap-type dopable ultrawide-bandgap oxide.

A major shortcoming of ultrawide-bandgap (UWBG) semiconductors is unipolar doping, in which eithern-type orp-type conductivity is typically possible, but not both within the same material. For UWBG oxides, the issue is usually thep-type conductivity, which is inhibited by a strong tendency to form self-trapped holes (small polarons) in the material. Recently, rutile germanium oxide (r-GeO2), with a band gap near 4.7 eV, was identified as a material that might break this paradigm. However, the predicted acceptor ionization energies are still relatively high (∼0.4 eV), limitingp-type conductivity. To assess whether r-GeO2is an outlier due to its crystal structure, the properties of a set of rutile oxides are calculated and compared. Hybrid density functional calculations indicate that rutile TiO2and SnO2strongly trap holes at acceptor impurities, consistent with previous work. Self-trapped holes are found to be unstable in r-SiO2, a metastable polymorph that has a band gap near 8.5 eV. Group-III acceptor ionization energies are also found to be lowest among the rutile oxides and approach those of GaN. Acceptor impurities have sufficiently low formation energies to not be compensated by donors such as oxygen vacancies, at least under O-rich limit conditions. Based on the results, it appears that r-SiO2has the potential to exhibit the most efficientp-type conductivity when compared to other UWBG oxides.

Doping Limits of Phosphorus, Arsenic, and Antimony in CdTe.

Low p-type doping is a limiting factor to increase CdTe thin-film solar-cell efficiency toward the theoretical Shockley-Queisser limit of 33%. Previous calculations predict relatively high ionization energies for group-V acceptors (P, As, and Sb), and they are plagued by self-compensation, forming AX centers, severely limiting hole concentration. However, recent experiments on CdTe single crystals indicate a much more favorable scenario, where P, As, and Sb behave as shallow acceptors. Using hybrid functional calculations, we solve this puzzle by showing that the ionization energies significantly decrease with the supercell size. When including the effects of spin-orbit coupling and extrapolating the results to the dilute limit, we find these impurities behave as hydrogenic-like shallow acceptors, and AX centers are unstable and do not limit p-type doping. We address the differences between our results and previous theoretical predictions and show that our ionization energies predict hole concentrations that agree with recent temperature-dependent Hall measurements.

Emerging Nontrivial Topology in Ultrathin Films of Rare-Earth Pnictides.

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion. Spin-orbit coupling (SOC) opens a sizable nontrivial gap in the band structure of ultrathin films. Such effect is anticipated to be general in rare-earth monopnictides and may lead to interesting phenomena when coupled with the 4f magnetic moments present in other members of this family of materials.

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO3.

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO3 has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO3 using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO3 films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO3 with free electron concentrations, n3D, from 3.3 × 1019 cm-3 to 1.6 × 1020 cm-3. The films exhibit a maximum room-temperature mobility of 42 cm2 V-1 s-1 at n3D = 3.3 × 1019 cm-3. Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO3-based field-effect transistors. The transistors show promise for CaSnO3's high-voltage capabilities by exhibiting low off-state leakage below 2 × 10-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 106. This work serves as a starting point for future studies on the semiconducting properties of CaSnO3 and many devices that could benefit from CaSnO3's exceptionally wide band gap.

Engineering metal oxidation using epitaxial strain.

The oxides of platinum group metals are promising for future electronics and spintronics due to the delicate interplay of spin-orbit coupling and electron correlation energies. However, their synthesis as thin films remains challenging due to their low vapour pressures and low oxidation potentials. Here we show how epitaxial strain can be used as a control knob to enhance metal oxidation. Using Ir as an example, we demonstrate the use of epitaxial strain in engineering its oxidation chemistry, enabling phase-pure Ir or IrO2 films despite using identical growth conditions. The observations are explained using a density-functional-theory-based modified formation enthalpy framework, which highlights the important role of metal-substrate epitaxial strain in governing the oxide formation enthalpy. We also validate the generality of this principle by demonstrating epitaxial strain effect on Ru oxidation. The IrO2 films studied in our work further revealed quantum oscillations, attesting to the excellent film quality. The epitaxial strain approach we present could enable growth of oxide films of hard-to-oxidize elements using strain engineering.

Shallow versus deep nature of Mg acceptors in nitride semiconductors.

We investigate the properties of Mg acceptors in nitride semiconductors with hybrid functional calculations. We find that although the thermodynamic transition level is relatively close to the valence band in GaN (260 meV), Mg(Ga) exhibits key features of a deep acceptor: the hole is localized on a N atom neighboring the Mg impurity, inducing a large local lattice distortion and giving rise to broad blue luminescence. We show that the ultraviolet photoluminescence peak attributed to Mg acceptors in GaN is likely related to Mg-H complexes, explaining the results of photoluminescence and electron paramagnetic resonance experiments. Predictions for Mg acceptors in AlN and InN are also presented.

First-principles optical spectra for F centers in MgO.

The study of the oxygen vacancy (F center) in MgO has been aggravated by the fact that the positively charged and the neutral vacancy (F+ and F0, respectively) absorb at practically identical energies. Here we apply many-body perturbation theory in the G0W0 approximation and the Bethe-Salpeter approach to calculate the optical absorption and emission spectrum of the oxygen vacancy in all three charge states. We observe unprecedented agreement between the calculated and the experimental optical absorption spectra for the F0 and F+ center. Our calculations reveal that not only the absorption but also the emission spectra of different charge states peak at nearly the same energy, which leads to a reinterpretation of the F center's optical properties.

Decomposition mechanism and the effects of metal additives on the kinetics of lithium alanate.

First-principles density functional theory studies have been carried out for native defects and transition-metal (Ti and Ni) impurities in lithium alanate (LiAlH(4)), a potential material for hydrogen storage. On the basis of our detailed analysis of the structure, energetics, and migration of lithium-, aluminium-, and hydrogen-related defects, we propose a specific atomistic mechanism for the decomposition and dehydrogenation of LiAlH(4) that involves mass transport mediated by native point defects. We also discuss how Ti and Ni impurities alter the Fermi-level position with respect to that in the undoped material, thus changing the concentration of charged defects that are responsible for mass transport. This mechanism provides an explanation for the experimentally observed lowering of the temperature for the onset of decomposition and of the activation energy for hydrogen desorption from LiAlH(4).

In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials.

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In2Se3 has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys, and initial experiments on the growth and characterization of γ-In2Se3 thin films. The predicted band gap of 1.84 eV for γ-In2Se3 is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In2(Se1-xTex)3 alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In2Se3/γ-In2(Se1-xTex)3 bilayer could be employed in tandem solar-cell architectures absorbing at Eg ≈ 1.8 eV and at Eg ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys to the available toolbox of materials for solar cells and other optoelectronic applications.

Bi2Se3 Growth on (001) GaAs Substrates for Terahertz Integrated Systems.

Terahertz (THz) technologies have been of interest for many years due to the variety of applications including gas sensing, nonionizing imaging of biological systems, security and defense, and so forth. To date, scientists have used different classes of materials to perform different THz functions. However, to assemble an on-chip THz integrated system, we must understand how to integrate these different materials. Here, we explore the growth of Bi2Se3, a topological insulator material that could serve as a plasmonic waveguide in THz integrated devices, on technologically important GaAs(001) substrates. We explore surface treatments and find that an atomically smooth GaAs surface is critical to achieving high-quality Bi2Se3 films despite the relatively weak film/substrate interaction. Calculations indicate that the Bi2Se3/GaAs interface is likely selenium-terminated and shows no evidence of chemical bonding between the Bi2Se3 and the substrate. These results are a guide for integrating van der Waals materials with conventional semiconductor substrates and serve as the first steps toward achieving an on-chip THz integrated system.