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The trapping effect of Al-dopants on Li-ions is greatly reduced at high Al-dopants due to the delocalization of polarization interactions and the depopulation of oxygen atoms, which results in a smooth energy landscape and destabilization of Li-ions.
Thermochemical air separation via cyclic redox reactions of oxide-based oxygen sorbents has the potential to achieve high energy efficiency. Although a number of promising sorbents have been investigated, further improvements in sorbent performance through a fundamental understanding of the structure–performance relationships are highly desirable. In this study, we systematically investigated the effects of A and B site dopants on the oxygen uptake/release properties (i.e., vacancy formation energy, reduction enthalpy, oxygen release temperature, and oxygen capacity) of the SrFeO 3 family of perovskites as oxygen sorbents. A monotonic correlation between DFT calculated oxygen vacancy formation energy and oxygen release temperature demonstrates the effectiveness of DFT for guiding sorbent selection. Combining vacancy formation energy with stability analysis, dopants such as Ba and Mn have been identified for tuning the redox property of SrFeO 3 sorbents, and increasing the oxygen capacity for temperature and pressure swings when compared to undoped SrFeO 3 . The Mn doped sample proved to be highly stable, with less than a 3% decrease in capacity over 1000 cycles. Although the dynamic nature of the redox process makes it difficult to use a single vacancy formation energy as the descriptor, a systematic approach was developed to correlate the oxygen storage capacities with the sorbents’ compositional properties and vacancy formation energies. The combination of DFT calculations with experimental studies from this study provides a potentially effective strategy for developing improved sorbents for thermochemical air separation.
Molecular spin qubits with long spin coherence time as well as non-invasive operation methods on such qubits are in high demand. It was shown that both molecular electronic and nuclear spin levels can be used as qubits. In solid state systems with dopants, an electric field was shown to effectively change the spacing between the nuclear spin qubit levels when the electron spin density is high at the nucleus of the dopant. Inspired by such solid-state systems, we propose that divalent lanthanide (Ln) complexes with an unusual electronic configuration of Ln 2+ have a strong interaction between the Ln nuclear spin and the electronic degrees of freedom, which renders electrical tuning of the interaction. Furthermore, as an example, we study electronic structure and hyperfine interaction of the 159 Tb nucleus in a neutral Tb(II)(Cp iPr5 ) 2 single-molecule magnet (SMM), which exhibits unusually long magnetization relaxation time, using the complete active space self-consistent field (CASSCF) method with spin–orbit interaction included within the restricted active space state interaction (RASSI). Our calculations show that the low-energy states arise from 4f 8 (6s,5d z 2 ) 1 , 4f 8 (5d x 2 -y 2 ) 1 , and 4f 8 (5d xy ) 1 configurations. We compute the hyperfine interaction parameters and the electronic–nuclear spectrum within our multiconfigurational approach. We find that the hyperfine interaction is about one order of magnitude greater than that for Tb(III)Pc 2 SMMs. This stems from the strong Fermi contact interaction between the Tb nuclear spin and the electron spin density at the nucleus that originates from the occupation of the (6s,5d) orbitals. We also uncover that the response of the Fermi contact term to electric field results in electrical tuning of the electronic–nuclear level separations. This hyperfine Stark effect may be useful for applications of molecular nuclear spins for quantum computing.
Inorganic halide perovskites (HP's) of the CsPbX 3 (X = I, Br, Cl) type have reached prominence in photovoltaic solar cell efficiencies, leading to the expectation that they are a new class of semiconductors relative to the traditional ones. Peculiarly, they have shown an asymmetry in their ability to be doped by holes vs. electrons. Indeed, both structural defect-induced doping as well as extrinsic impurity-induced doping strangely often result in HP's in a unipolar doping (dominantly p-type) with low free carriers’ concentration. This raises the question whether such doping limitations presents just a temporary setback due to insufficient optimization of the doping process, or perhaps this represents an intrinsic, physically-mandated bottleneck. Here we study three fundamental Design Principles (DP's) for ideal doping, applying them via density functional doping theory to these HP's, thus identifying the violated DP that explains the doping limitations and asymmetry in these HP's. Here, the target DP are: (i) requires that the thermodynamic transition level between different charge states induced by the dopants must ideally be energetically shallow both for donors (n-type) or acceptors (p-type); DP-(ii) requires that the ‘Fermi level pinning energies’ for electrons E (n) pin and holes E (p) pin (being the limiting value of the Fermi level before a structural defect that compensate the doping forms spontaneously) should ideally be located inside the conduction band for n-type doping and inside the valence band for p-type doping. DP-(iii) requires that the doping-induced shift in equilibrium Fermi energy ΔE (n) F towards the conduction band for n-type doping (shift of ΔE (p) F towards the valence band, for p-type doping) to be sufficiently large. We find that, even though in HP's based on Br and Cl there are numerous shallow level dopants that satisfy DP-(i), in contrast DP-(ii) is satisfied only for holes and DP-(iii) fails for both holes and electrons, being the ultimate bottleneck for the n-type doping in Iodide HP's. This suggests an intrinsic mechanism for doping limitations in this class of semiconductors in terms of recognized physical mechanisms.
While p-type BiCuSeO is a well-known mid-temperature oxide thermoelectric (TE) material, computations predict that superior TE performance can be realized through n-type doping. In this study, we use first-principles defect calculations to show that Cu vacancies are responsible for the native p-type self doping; yet, we find that BiCuSeO is n-type dopable under Cu-rich growth conditions, where the formation of Cu vacancies is suppressed. We computationally survey a broad suite of 23 dopants and find that only Cl and Br are effective n-type dopants. Therefore, we recommend that future experimental doping efforts utilize phase boundary mapping to optimize the electron concentration and resolve the anomalous p–n–p transitions observed in halogen-doped BiCuSeO. The prospect of n-type doping, as revealed by our defect calculations, paves the path for rational design of BiCuSeO chemical analogues with similar doping behavior and even better TE performance.
Here, the effects of halogen (F, Cl, Br, I, and At) doping in the direct-band-gap β-Fe 2 O 3 semiconductor on its band structures and electron–hole recombination have been investigated by density functional theory. Doping Br, I, and At in β-Fe 2 O 3 leads to transformation from a direct-band-gap semiconductor to an indirect-band-gap semiconductor because their atomic radii are too large; however, F- and Cl-doped β-Fe 2 O 3 remain as direct-band-gap semiconductors. Due to the deep impurity states of the F dopant, this study focuses on the effects of the Cl dopant on the band structures of β-Fe 2 O 3 . Two impurity levels are introduced when Cl is doped into β-Fe 2 O 3 , which narrows the band gap by approximately 0.3 eV. After doping Cl, the light-absorption edge of β-Fe 2 O 3 redshifts from 650 to 776 nm, indicating that its theoretical solar to hydrogen efficiency for solar water splitting increases from 20.6% to 31.4%. In addition, the effective mass of the holes in halogen-doped β-Fe 2 O 3 becomes significantly larger than that in undoped β-Fe 2 O 3 , which may suppress electron–hole recombination.
Self-doping organic semiconductors provide a promising route to avoid instabilities and morphological issues associated with molecular n-type dopants. Structural characterization of a naphthalenetetracarboxylic diimide (NDI) semiconductor covalently bound to an ammonium hydroxide group is presented. The dopant precursor was found to be the product of an unexpected base catalyzed hydrolysis, which was reversible. The reversible hydrolysis had profound consequences on the chemical composition, morphology, and electronic performance of the doped films. In addition, we investigated the degradation mechanism of the quaternary ammonium group and the subsequent doping of NDI. These findings reveal that the products of more than one chemical reaction during processing of films must be considered when utilizing this promising class of water-soluble semiconductors.
The design of organic light emitting diode (OLED) materials with the potential for exhibiting thermally-activated delayed fluorescence (TADF) is reported. Using computational methods (DFT/TD-DFT) as a guiding tool, six materials with a benzobisoxazole (BBO) core and donor–acceptor–donor architectures were designed by changing the conjugation position of carbazole-substituted phenyl substituents and the type of BBO isomer (cis- vs. trans-). Experimental steady-state and transient absorption spectroscopic techniques were utilized to probe the TADF activity of these molecules. Each material was then used in host–guest OLED devices as either near-UV dopants or host with low singlet-triplet energy differences. The electroluminescent properties show that when used as dopants these materials provide near-UV emission (CIE y < 0.06 and CIE x = 0.16), whereas when used as hosts, these materials show reduced operating voltages and increased performance efficiencies when compared to commercial materials.
Delafossites, typically denoted by the formula ABO 2 , are a class of layered materials that exhibit a wide range of electronic and optical properties. Recently, the idea of modifying these delafossites into ordered kagome or honeycomb phases via strategic doping has emerged as a potential way to tailor these properties. In this study, we use high-throughput density functional theory calculations to explore many possible candidate kagome and honeycomb phases by considering dopants selected from the parent compounds of known ternary delafossite oxides from the inorganic crystal structure database. Here our results indicate that while A-site in existing delafossites can host a limited range of elemental specifies, and display a low propensity for mixing or ordering, the oxide sub-units in the BO 2 much more readily admit guest species. Our study identifies four candidate B-site kagome and fifteen candidate B-site honeycombs with a formation energy more than 50 meV f.u. −1 below other competing phases. The ability to predict and control the formation of these unique structures offers exciting opportunities in materials design, where innovative properties can be engineered through the selection of specific dopants. A number of these constitute novel correlated metals, which may be of interest for subsequent efforts in synthesis. These novel correlated metals may have significant implications for quantum computing, spintronics, and high-temperature superconductivity, thus inspiring future experimental synthesis and characterization of these proposed materials.
The spatial distribution of copper (Cu) dopant in CdSe quantum dots (QDs) significantly impact the photophysical properties including emission characteristics at ambient conditions. The dopant state fluctuates more widely for surface Cu doping.
Na-doped BiFeO 3 demonstrates an enhanced p-type behavior compared to p-type BiFeO 3 prepared without extrinsic dopants, and Na-doped BiFeO 3 can serve as a photocathode for solar O 2 reduction to H 2 O 2 when coupled with Ag nanoparticle catalysts.
Strontium zirconium sulfide (SrZrS 3 ) has garnered significant attention for photovoltaic (PV) applications due to its excellent optoelectronic properties, high chemical and moisture stability, and non-toxicity. However, the bandgaps of both the α- and β-phases lie outside the optimum ranges for both single-junction solar cells (SJSCs) and tandem solar cells (TSCs), thereby limiting their applications in PV technologies. In this study, we employed hybrid density functional theory to engineer the band gaps of α- and β-SrZrS 3 through substitutional doping. We considered three dopants (Hf, Sn, and Ti) at the Zr site of SrZrS 3 with varying doping concentrations and investigated their effects on the structural, electronic, and optical properties of the materials. We found that Sn and Ti doping effectively lowers the band gaps of both α- and β-SrZrS 3 , whereas Hf doping increases them. For x values up to 0.25, the band gaps of the α-SrZr 1−x Sn x S 3 and α-SrZr 1−x Ti x S 3 are within the optimum range for SJSCs, and those of β-SrZr 1−x Sn x S 3 and β-SrZr 1−x Ti x S 3 lie within the optimum range for Si/perovskite as well as perovskite/perovskite TSCs. The three dopants exhibited significant effects on the optical properties of both α- and β-SrZrS 3 , including the absorption coefficient, energy-loss functions, reflectivity, and refractivity spectra. Thermodynamic stability analysis revealed that for both phases, SrZr 1−x Hf x S 3 can be synthesized via exothermic processes, whereas the formation of SrZr 1−x Ti x S 3 and SrZr1−xSn x S 3 is endothermic and hence, not thermodynamically favorable. Further analysis showed that SrZr 1−x Ti x S 3 in both α- and β-phases are stable under thermodynamic equilibrium conditions, whereas SrZr 1−x Sn x S 3 is prone to dissociation into ternary phases (SrZrS 3 and SrSnS 3 ), especially at higher doping concentrations. These results show that Ti doping is effective in tuning the band gaps of α- and β-SrZrS 3 toward the optimal values for PV applications.
Carbon-based materials, such as graphite and its functionalized/doped derivatives, are promising lightweight layered materials for hydrogen activation and storage. Their propensity to control the thermodynamics of hydrogen binding and the kinetics of hydrogen mobility strongly depends on the speciation and the arrangement of dopants. In this study, we demonstrate precise control over dopant speciation and clustering in nitrogen-containing layered carbon materials during hydrothermal synthesis. Through extensive spectroscopic characterization and first principles simulations, we demonstrate that the formation of N-motifs can be controlled by the choice of precursor and synthesis temperature. The distinct three-dimensional architecture and porosity in graphene oxide and carbon nitride-derived materials furnish a synthetic pathway for precise control over the local and global structure of nitrogen-doped carbon materials and their activity toward the activation of molecular hydrogen.
Green hydrogen is of great interest as a replacement for traditional fossil fuels in a variety of energy applications. However, due to the poor kinetics present in the oxygen evolution reaction (OER) half-reaction, nanostructured catalysts are needed to reduce the reaction overpotential. Nickel oxide has previously been shown to be a promising alternative to expensive Pt-group based catalysts for the OER in alkaline media. Herein, facetted NiO nanosheets have been doped with Fe, Mn, or Co to reduce its catalytic overpotential for the OER. A supercritical synthesis process was used to promote the mass transport of the reactants while preserving catalytic surface area. Microscopy, diffraction, spectroscopy, and adsorption techniques were used to understand the morphological changes resulting from the inclusion of each dopant, as well as characterize the surface chemistry presented by the doped (111) facet. The pH was found to affect the properties of mixing due to difference in hydrolysis rates and catalysis of the hydrolysis/condensation. The dopants exhibited distinct effects on OER activity: Mn increased the overpotential to 742 mV vs. RHE, while Co and Fe reduced it to 502 mV and 457 mV, respectively. In summary, a straightforward and novel synthesis method is presented to prepare doped NiO(111) nanosheets, and their surface characteristics are explored to understand their varied electrochemical performances.
Modulation doping is a commonly adopted technique to create two-dimensional (2D) electrons or holes in semiconductor heterostructures. One constraint, however, is that the intentional dopants required for modulation doping are controlled and incorporated during the growth of heterostructures. Here, using undoped strained germanium quantum wells as the model material system, we show that modulation doping can be achieved post-growth of heterostructures by ion implantation and dopant-activation anneals. The carrier density is controlled ex situ by varying the ion fluence and implant energy, and an empirical calibration curve is obtained. While the mobility of the resulting 2D holes is lower than that in undoped heterostructure field-effect transistors built using the same material, the achievable carrier density is significantly higher. Potential applications of this modulation-doping technique are discussed.
Understanding the irradiation-induced defects in oxides is of interest for a wide range of applications. ZnO is an interesting oxide with mixed ionic and covalent bonding that contains a variety of point defect structures—making it an excellent model for studying irradiation-induced defects and their impact on properties. Here, we investigate the effects of neutron irradiation on the formation of defects and on the structural, optical, and electrical properties of ZnO single crystals. We observe the formation of vacancies and voids via positron annihilation spectroscopy. Neutron irradiation led to a significant deterioration of the ZnO structure and formed a high concentration of point defects, vacancy clusters, and voids with large disparities in their structure across variable irradiation times. It also led to significant changes in the optical properties and sample color. Irradiation for 444 h induced a high concentration of Cu acceptors as well as a high concentration of Ga donors. Additionally, temperature-dependent Hall effect measurements revealed the competing production of donors and acceptors and showed an increase in the slope of the carrier freeze-out curve with increasing irradiation dose. This work demonstrates the combined effects of neutron irradiation in producing a wide range of structural defects, impurities, and dopants in oxides and their enormous impact on modifying the oxide structure and both the optical and electronic properties. It particularly emphasizes the importance of considering the production of new impurities and dopants during the neutron irradiation of oxides.
Doped SrTiO 3 is a superconductor whose pairing mechanism is still not fully understood. The response of a superconductor to impurities has long been used to obtain insights into the nature of the superconducting state. In this work, we investigate the superconductivity of SrTiO 3 films that are doped or alloyed with different rare earth ions, which carry a magnetic moment.. It is shown that large concentrations (up to a few percent) of rare earth ions with unpaired f-electrons, such as Sm and Eu, do not reduce the superconducting critical temperature and critical fields. The finding is independent of whether the rare earth ion acts as a dopant or is an isovalent impurity. The interactions between the superconducting condensate and the magnetic dopants that could result in the observed insensitivity to magnetic impurities are discussed.
Heteroatom substitution can favorably alter electronic transport in conductive polymers to improve their thermoelectric performance. Here we report the spectroscopic, structural, and thermoelectric properties of poly(3–(3',7'-dimethyloctyl) chalcogenophenes) or P3RX doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), where the heteroatom [X = thiophene (T), selenophene (Se), tellurophene (Te)], the doping methodology, and extent of doping are systematically varied. Spectroscopic measurements reveal that while all P3RX polymers are appreciably doped, the doping mechanism is inherently different. Poly(3-hexylthiophene) (P3HT, used as a control) and poly(3–(3',7'-dimethyloctyl)tellurophene) (P3RTe) are doped primarily via integer charge transfer (ICT), whereas poly(3–(3',7'-dimethyloctyl)selenophene) (P3RSe) and poly(3–(3',7'-dimethyloctyl)thiophene) (P3RT) are doped via charge transfer complex (CTC) mechanisms. Despite these differences, all polymers saturate with roughly the same number of F4TCNQ counterions (1 dopant per 4 to 6 heterocycles), reinforcing the idea that the extent of charge transfer varies with the doping mechanism. Grazing incidence wide-angle x-ray scattering measurements provide insight into the structural driving forces behind different doping mechanisms—P3RT and P3RSe have similar microstructures in which F4TCNQ intercalates between the π-stacked backbones resulting in CTC doping (localized carriers), while P3HT and P3RTe have microstructures in which F4TCNQ intercalates in the alkyl side chain region, giving rise to ICT doping (delocalized carriers). These structural and spectroscopic observations shed light on why P3HT and P3RTe obtain electrical conductivities ca. 3 S/cm, while P3RT and P3RSe have conductivities <10–3 S/cm under the same thin film processing conditions. Ultimately, this work quantifies the effects of heteroatom, microstructural ordering, extent of doping, and doping mechanism, thereby providing rational guidance for designing future thermoelectric polymer-dopant systems.