Dopant Chemistry Research Articles

First-principles evaluation of dopant impact on structural deformability and processability of Li7La3Zr2O12.

Li7La3Zr2O12 (LLZO) and related ceramic solid electrolytes feature excellent stability and reasonable ionic conductivity, but processing remains challenging. High-temperature co-sintering is required for successful integration with the electrode, which is energetically costly and can lead to unacceptable cathode degradation. The introduction of dopants can promote lower-temperature processing by improving deformability and disrupting lattice integrity; however, an unbiased, systematic study correlating these properties to the dopant chemistry and composition is lacking. Here, we rely on a set of static and dynamic metrics derived from first-principles simulations to estimate the impact of doping on LLZO processability by quantifying LLZO structural deformability. We considered three distinct dopants (Al, Ba, and Ta) as representatives of substitutional incorporation on Li, La, and Zr sites. Our descriptors indicate that doping in general positively impacts lattice deformability, although significant sensitivities to dopant identity and concentration are observed. Amongst the tested dopants, Al doping (on the Li site) appears to have the greatest impact, as signaled across nearly the entire set of computed features. We suggest that these proxy descriptors, once properly calibrated against well-controlled experiments, could enable the use of first-principles simulations to computationally screen new ceramic electrolyte compositions with improved processability.

Copper shape-templated N-doped carbons: exercising selective surface area control for lithium-ion batteries & beyond

An N-doped carbon synthesis platform is developed which allows for carbon morphology to be changed without altering nitrogen dopant chemistry, allowing chemical and physical properties to be decoupled for study in lithium-ion batteries.

Thermal stability of MxOy‐doped zirconia aerogels (M = Y, Yb, Gd, Ce, Ca) studied through 1200°C

AbstractThe high porosities and low densities of ceramic aerogels offer outstanding insulative performance in applications where weight is a critical factor. The high surface area‐to‐volume ratios and specific surface areas provide extremely low thermal conductivity, but also contribute to rapid densification of the pore structure at elevated temperatures. This densification diminishes their favorable properties and inhibits use of aerogels in high‐temperature applications. This work contributes to a design framework for thermally stable aerogels via the study of dopant chemistry (Y, Yb, Gd, Ca, Ce) in zirconia aerogels. The structural evolution was studied through 1200°C using nitrogen physisorption, scanning electron microscopy, and X‐ray diffraction. The role of dopant identity and concentration in thermal stability was elucidated. In the context of the design framework, dopant chemistry is an aggregate for many closely related material properties, each of which may contribute to aerogel structural evolution. To develop a truly predictive design framework for ceramic‐based aerogels, systematic and comprehensive evaluation of thermodynamic and kinetic properties must be performed in conjunction with studies on structural evolution.

Atomistic Mechanisms Underlying the Maximum in Diffusivity in Doped Li7La3Zr2O12

Doped lithium lanthanum zirconium oxide (LLZO) is a promising class of solid electrolytes for lithium-ion batteries due to their good electrochemical stability and compatibility with Li metal anodes. Ionic diffusivity in these ceramics is known to occur via correlated, vacancy-mediated jumps of Li+ between alternating tetrahedral and octahedral sites. Aliovalent doping at the Zr site increases the concentration of vacancies in the Li+ sublattice and cation diffusivity, but such an increase is universally followed by a decrease for Li+ concentration lower than 6.3–6.5 Li molar content. Molecular dynamics simulations based on density functional theory show that the maximum in diffusivity originates from competing effects between the increased vacancy concentration and the increasing occupancy of the low-energy tetrahedral sites by Li+, which increases the overall activation energy associated with diffusion. For the relatively high temperatures of our simulations, Li+ concentration plays a dominant role in transport as compared to dopant chemistry.

Doped 2D VX2 (X = S, Se, Te) monolayers as electrocatalysts for ammonia production: A DFT based study

Electrocatalytic nitrogen fixation under ambient conditions on vanadium dichalcogenides (VX2) with non-metal dopants has been explored herein. Understanding the interface chemistry, inherent electronic and acute synergistic nature of non-metal dopants on two unique phases of VX2 has been meticulously explored through a scrutiny of several non-metal atoms as catalytic centers. The efficacity of N2 chemisorption and NN bond activation has been implemented as crucial parameters to realize boron and carbon doped VX2 monolayers to be electrocatalytically active for nitrogen reduction reaction (NRR). Detailed investigation on the NRR mechanism brings out the pivotal role of thermodynamic favourability for product formation obtained from Gibbs free energy differences. The charge transfer on N and π-π* orbital hybridization and electron “donor–acceptor” mechanism between the non-metal and N2 has been found to modulate the electrocatalytic barrier for NRR on VX2 monolayers. This study proposes boron doped VS2 as an efficient chemically feasible, earth abundant sustainable electrocatalyst for NRR with an overpotential as low as 0.06 eV.

Hazard profiling of a combinatorial library of zinc oxide nanoparticles: Ameliorating light and dark toxicity through surface passivation

Zinc oxide (ZnO) is one of the high-volume production nanoparticles (NPs) currently used in a wide range of consumer and industrial goods. The inevitable seepage into environmental matrices and the photoactive nature of ZnO NPs warrants hazard profiling under environmentally related conditions. In this paper, the influence of simulated solar light (SSL) on dissolution behaviour and phototoxicity of ZnO NPs was studied using a combinatorial library of ZnO NPs with different sizes, surface coatings, dopant chemistry, and aspect ratios in a fish cell line (BF2) and zebrafish embryos. Generally, the cytotoxicity and embryo mortality increased when exposed concomitantly to SSL and ZnO NPs. The increase in toxic potential of ZnO NPs during SSL exposure concurred with release of Zn ions and ROS generation. Surface modification of NPs with poly(methacrylic acid) (PMAA), silica or serum coating decreased toxicity and ZnO with serum coating was the only NP that had no significant effect on any of the cytotoxicity parameters when tested under both dark and SSL conditions. Results from our study show that exposure to light could increase the toxic potential of ZnO NPs to environmental lifeforms and mitigation of ZnO NP toxicity is possible through modifying the surface chemistry.

Stoichiometric Irreversibility of Aged Garnet Electrolytes

Solid-state lithium batteries (SSLBs) have been regarded as one of the next-generation energy storage systems. With the adoption of solid-state electrolytes (SSEs) and lithium metal anodes, SSLBs enable higher energy density and more reliable safety than the state-of-the-art lithium-ion batteries. Among potential SSEs, the cation-doped Li7La3Zr2O12 (LLZO) is promising for its high ionic conductivity (~10-3 S cm-2) at room temperature and high stability with Li metal anode. However, the storage of doped LLZO under ambient condition suffers the severe aging effects, including the structural transition (i.e. low-temperature cubic form) and the stoichiometric changes (i.e. Li2CO3). These changes are detrimental for LLZO ionic conductivity and interfacial stability in SSLBs. To this end, we are motivated to investigate the structural and stoichiometric reversibility of aged LLZO during thermal treatment. With the help of in-situ synchrotron-based high-energy X-ray diffraction technique, our experiments revealed that the high temperature cubic form can be restored after a thermal treatment of the aged LLZO powder. However, the restoration of the stoichiometry remained a challenge, and the degree of the stoichiometric restoration showed a strong dependence on the dopant chemistry. This study provides a key understanding on the restoration of aged LLZO and aims to enhance the development of LLZO in solid-state lithium batteries. Figure 1

Material properties of IR-to-IR down-converting Er and Nd-Doped ZrO2 nanopowders

Specially tuned luminescent lanthanide-doped IR-to-IR down-converting nanopowders are utilized as taggants to tag and track explosives and valuables. Zirconia (ZrO 2 )-based oxides doped with an Er or Nd dopant offer a cheaper alternative to fluoride taggants. Maximum IR-to-IR down-conversion intensity was achieved with low concentrations (1 mol%) of Er or Nd dopant. Er-doped ZrO 2 powders possessed strong emission bands from 1444 nm to 1600 nm. Nd-doped ZrO 2 powders possessed strong emission bands from 1055 nm to 1151 nm as well as 1323 nm–1473 nm. No visible light emission was detected for any of the doped compositions. For all doped compositions, increasing the phase fraction of monoclinic structure substantially increased the luminescent intensities and is hypothesized to be a result of decreased lattice symmetry and oxygen vacancy concentration decreasing the number of non-radiative pathways for energy transfer. Our results suggest that Er and Nd-doped ZrO 2 represents an excellent candidate for an IR-IR taggant system because it possesses a wide array of unique emission spectra which may be tailored by controlling dopant chemistry, dopant concentration, and annealing temperature. •Rare-earth doped zirconia nanopowders were fabricated using aqueous precursors •Lower symmetry crystal systems had the highest tested luminescence response •The lowest dopant concentrations resulted in the highest luminescence intensity •Each nanopowder tested displayed a unique emission spectrum implying a high tunability

Chemistry and physics of dopants embedded in helium droplets.

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Effect of Applying a Carbon Coating on the Crystal Structure and De-/Lithiation Mechanism of Mn-Doped ZnO Lithium-Ion Anodes

The introduction of transition metal dopants such as Fe and Co in zinc oxide enables substantially enhanced reversible capacities and greater reversibility of the de-/lithiation reactions occurring. Herein, we report a comprehensive analysis of the electrochemical processes taking place in Mn-doped ZnO (Zn0.9Mn0.1O) and carbon-coated Zn0.9Mn0.1O upon de-/lithiation. The results shed light on the impact of the dopant chemistry and, especially, its coordination in the crystal structure. When manganese does not replace zinc in the wurtzite structure, only a moderate improvement in electrochemical performance is observed. However, when applying the carbonaceous coating, a partial reduction of manganese and its reallocation in the crystal structure occur, leading to a substantial improvement in the material’s specific capacity. These results provide important insights into the impact of the lattice position of transition metal dopants—a field that has received very little, essentially no attention, so far.

Stoichiometric irreversibility of aged garnet electrolytes

Solid-state lithium batteries (SSLBs) have been regarded as one of the next-generation energy storage systems. With the adoption of solid-state electrolytes (SSEs) and lithium metal anodes, SSLBs enable higher energy density and more reliable safety than the state-of-the-art lithium-ion batteries. Among potential SSEs, the cation-doped Li7La3Zr2O12 (LLZO) is promising for its high ionic conductivity (~10−3 S cm−2) at room temperature and high stability with Li metal anode. However, the storage of doped LLZO in the ambient condition suffers the aging effect, including the structural transition (i.e. low-temperature cubic form) and the stoichiometric changes (i.e. Li2CO3). These changes are detrimental to LLZO ionic conductivity and interfacial stability in SSLBs. To this end, we are motivated to investigate the structural and stoichiometric reversibility of aged LLZO during thermal treatment. With the help of an in-situ synchrotron-based high-energy X-ray diffraction technique, our experiments revealed that the LLZO powders became a low-temperature cubic phase when exposed to the ambient condition for an extended period of time. A high temperature cubic form can be restored after a thermal treatment of the aged LLZO powder, regardless of the type of dopant. However, the restoration of the stoichiometry remained a challenge, and the degree of the restoration showed a clear dependence on the dopant chemistry.

Preparation and X-ray photoelectron spectroscopic characterization of Sn-doped C12A7:e− electride nanoparticles

Understanding the chemistry of dopants in electride nanoparticles is essential and challenging owing to the active nature of the electride and the low atomic concentration of the dopants. In this work, we developed conductive Sn-doped C12A7:e− electride nanoparticles by a modified sol-gel method, and performed a comprehensive X-ray photoelectron spectroscopic characterization on the surface chemistry of the doped electride nanoparticles. The resulting electride Ca12Al(14−x)Snx:e− (x = 1) exhibited an electron concentration of 1.65 × 1021 cm−3 and a specific area of 53.9 m2/g. XPS results showed that Sn was in the form of Sn4+ at the surface of the doped C12A7:e− nanoparticles. The depth distribution of elements was investigated by XPS Ar+ ion and Ar cluster ion sputtering. Results showed that Sn was enriched at the surface and sputtering partially reduced Sn4+ to metallic Sn, and Ar cluster ion sputtering showed much weaker effect on the reduction of Sn4+ than the Ar+ ion sputtering. The chemical states and distribution of elements Ca, Al, O did not show obvious change after sputtering. The present study provides novel insights on the chemical states analysis of doped-C12A7:e− nanoparticles.

Influence of annealing conditions on the photoelectrocatalytic performance of WO3 nanostructures

Nanostructured WO3 photoanodes have been synthesized by electrochemical anodization under controlled hydrodynamic conditions in acidic media in the presence of 0.05 M H2O2. Subsequently, samples have been subjected to a thermal treatment (annealing) at different temperatures (400 °C, 500 °C and 600 °C) and under different gaseous atmospheres (air, N2, Ar). The influence of these annealing conditions on the morphology, crystallinity, photoelectrochemical behavior and dopant chemistry of the different photoanodes has been investigated through Electronic Microscopy, Raman Spectroscopy, Photoelectrochemical Impedance Spectroscopy and Mott-Schottky analysis. In general, higher annealing temperatures resulted in samples with higher degrees of crystallinity, which in turn favored the transport of electron-hole pairs through the semiconducting photoanodes. Besides, an increase in annealing temperature implied higher densities of donor species within the samples structure, which can explain the observed enhancement in charge transfer. Annealing temperature was observed to have a more marked impact on the photoelectrocatalytic performance of WO3 nanostructures than the gaseous atmosphere.

Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% of EuI2. Two types of precipitate structures were found. First, needle-shaped EuI2 precipitates with a layered structure are likely responsible for scattering the scintillation light. In transparent crystals with good scintillation properties, precipitates with a cubic crystal structure and a size below 4 nm were found. A surprisingly low concentration of point defects was detected in all of the investigated samples. Upon annealing, Eu segregates towards the surface, which results in the formation of an ordered hexagonal overlayer with the EuI2 composition and a pronounced, unidirectional moire pattern.

Discriminative potential of ion mobility spectrometry for the detection of fentanyl and fentanyl analogues relative to confounding environmental interferents.

The opioid crisis and emergence of fentanyl, fentanyl analogues, and other synthetic opioids has highlighted the need for sensitive and robust detection for interdiction at screening points, notably vehicles at border crossings and packages at postal facilities. This work investigates the discriminative potential, sensitivity and specificity, of ion mobility spectrometry (IMS) for the detection of fentanyl and fifteen (15) fentanyl-related compounds (analogues, other opioids, and metabolites) relative to confounding environmental interferents. The environmental background interferent levels, frequency and intensity, were derived from over 10 000 screening samples collected from delivery vehicles entering a federal site. A receiver operating characteristic (ROC) curve methodology was employed to quantify the relationship between sensitivity and specificity for these target compounds on two instruments/configurations. These instrument configurations differed in desorption and drift tube temperatures, reactant ion dopant chemistry, and analysis time. This work identified reduced mobility areas of high interference that resulted in increased false positive rates (FPR), effectively reducing sensitivity (true positive rate: TPR) in those regions. Except for a few target compounds on either of the instruments that exhibited elevated FPRs, detection of fentanyl and fentanyl-related species was achieved at single to tens of nanograms with ≥90% TPR and ≤2% FPR. This work established the importance of systematic environmental background characterization at each specific screening setting in evaluating a platform's true performance.

Influence of Chemistry and Misfit Dislocation Structure on Dopant Segregation at Complex Oxide Heterointerfaces

AbstractComplex oxide heterostructures and thin films have emerged as promising candidates for diverse applications. Lattice mismatch between the two oxides lead to the formation of misfit dislocations, which influence vital material features. Trivalent dopant segregation to misfit dislocations at semi‐coherent oxide heterointerfaces, while not well understood, is anticipated to impact interface‐governed properties. Here, atomistic simulations elucidating the influence of misfit dislocations on dopant segregation at SrTiO3/MgO heterointerfaces are reported. SrO‐ and TiO2‐terminated interfaces that have differing misfit dislocation structure were considered for trivalent dopants segregation. At SrO‐terminated interface, dopants tend to segregate toward but not precisely to the heterointerface, whereas at TiO2‐terminated interface, dopants exhibit a thermodynamic preference to accumulate at the heterointerface. Most favorable segregation sites at SrO‐terminated interface are located within the coherent terrace, whereas those at TiO2‐terminated interface are at misfit dislocation intersections. Atomic layer chemistry and the resulting misfit dislocation structure at the heterointerface, along with concomitant strain at the heterointerface due to mismatched dopants, play a critical role in influencing the observed trends for dopant segregation. Overall, the present results offer a fundamental atomic scale perspective of dopant behavior at semi‐coherent complex oxide heterointerfaces and the interplay between dopant chemistry, interface chemistry, and misfit dislocation structure.

Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations.

We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.

The influence of dopants and complexion transitions on grain boundary fracture in alumina

The fracture of micro-cantilever Al2O3 specimens containing single grain boundaries is quantified via in situ bending experiments performed in the transmission electron microscope. The type of grain boundary complexion plays a dominant role in determining the fracture response, while the dopant chemistry has a secondary effect for the same complexion type. More ‘ordered’ complexions, those containing sub-monolayer adsorbates, are tougher than the undoped material. The behaviors of SiO2 and Y2O3 doped Al2O3 with different complexions are compared and contrasted. In general, grain boundaries exhibiting more ‘disordered’ complexions types, those with multilayer adsorbates, are embrittled relative to undoped Al2O3.

Defect-Controlled Electronic Structure and Phase Stability in Thermoelectric Skutterudite CoSb3

Controlling extrinsic defects to tune the carrier concentration of electrons or holes is a crucial point with regard to the engineering application of thermoelectric semiconductors. To understand the defect-controlled electronic structure in thermoelectric materials, we apply density functional theory (DFT) to investigate the defect chemistry of dopants M (M = O, S, Se, or Te) in CoSb3. DFT predicts that the breakage of Sb4 rings induced by these dopants produces the unexpected (n- or p-type) conductivity behavior in CoSb3. For example, energetically dominant O interstitials (Oi) chemically break Sb4 rings and form O–4Sb five-membered rings, leading to the charge neutral behavior of Oi. While S interstitials (Si) collapse Te–3Sb four-membered rings within Te doped CoSb3 leading to p-type conduction behavior, Se substitution on Sb (SeSb) breaks the Se–Te–2Sb four-membered ring, resulting in a charge neutral behavior of the SeSb+TeSb complex defect. Furthermore, the solubility limits of M dopants (M = S, Se...

Cationic Redistribution at Epitaxial Interfaces in Superconducting Two-Dimensionally Doped Lanthanum Cuprate Films.

The exploration of interface effects in complex oxide heterostructures has led to the discovery of novel intriguing phenomena in recent years and has opened the path toward the precise tuning of material properties at the nanoscale. One recent example is space-charge superconductivity. Among the complex range of effects which may arise from phase interaction, a crucial role is played by cationic intermixing, which defines the final chemical composition of the interface. In this work, we performed a systematic study on the local cationic redistribution of two-dimensionally doped lanthanum cuprate films grown by oxide molecular beam epitaxy, in which single LaO layers in the epitaxial crystal structure were substituted by layers of differently sized and charged dopants (Ca, Sr, Ba, and Dy). In such a model system, in which the dopant undergoes an asymmetric redistribution across the interface, the evolution of the cationic concentration profile can be effectively tracked by means of atomically resolved imaging and spectroscopic methods. This allowed for the investigation of the impact of the dopant chemistry (ionic size and charge) and of the growth conditions (temperature) on the final superconducting and structural properties. A qualitative model for interface cationic intermixing, based on thermodynamic considerations, is proposed. This work highlights the key role which cationic redistribution may have in the definition of the final interface properties and represents a further step forward the realization of heterostructures with improved quality.