Individual Dopants Research Articles

Exploring the True Nature of Doping Effects: The Origin of the Discrepancy in Doping Effects on Ni-Rich Cathode Materials

Co-free, Ni-rich batteries have gained significant traction due to their importance and popularity in the field. Despite numerous doping studies aimed at mitigating the drawbacks associated with Co-free batteries, the outcomes have been inconsistent. This research endeavors to elucidate the true essence of doping effects by employing dopants (Nb, Ti, Zr, Hf, Ta, W, Mo) commonly utilized in Li(Ni0.95Mn0.05)O2 cathode. The findings reveal the advantageous impact of various dopants on doping, albeit to varying extents contingent upon temperature. Notably, each dopant exhibits distinct effects on both the promotion and inhibition of grain size, contingent upon temperature variations. These findings underscore the significance of optimizing temperature conditions to maximize the doping effect, aligning with the unique characteristics of each dopant. Consequently, comprehending temperature modulation in accordance with the growth and suppression traits of individual dopants holds paramount importance for the advancement of high-performance Co-free, Ni-rich cathode.

Nb impurity-bound excitons as quantum emitters in monolayer WS2

Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb.

Pushing the Energy-Lifetime Frontier of Li-Ion Batteries: Study of Ni-Rich, Co-Free NMAW Cathode Material

Abstract Dopants and coatings have been widely used to improve the performance of Ni-rich positive electrode active materials. Previous studies have aimed to elucidate the mechanism by which Al and W improve lithium metal oxides, providing valuable insight on the design of enhanced electrode materials for Li-ion batteries. In this work, Al and W are compared as individual dopants as well as co-dopants in order to design an optimal Ni-rich, Co-free material. This involved studying the effect of synthesis temperature in the presence of Al and/or W as well as the effect that these metals have on the morphology of the resultant polycrystalline materials. In addition, structural analysis by X-ray diffraction, electrochemical analysis, and characterization of the mechanical strength of the materials were also conducted. The change in performance with the addition of Al and W depends greatly on particle size and chemical composition. Small sized Ni-rich polycrystalline particles (Ni content of 94%) with low contents of Al (3%) and W (1%) showed the greatest enhancement in energy density with good cycle life.

Overlooked Ionic Contribution of a Chiral Dopant in Cholesteric Liquid Crystals.

This study focuses on the ionic contribution by a chiral dopant added into a nematic host for preparing cholesteric liquid crystals (CLCs). Chiral structures were designated by individually incorporating two enantiomers, R5011 and S5011, into the nematic E44 to construct right- and left-handed CLCs, respectively. Characterized by the space-charge polarization, the dielectric spectra of the CLCs were investigated in the low-frequency regime, where f ≤ 1 kHz. The role of the individual chiral dopant, R5011 or S5011, at concentrations of 0-4.0 wt.% in altering the ionic properties of the CLC material was analyzed by deducing the electrical conductivity, ion density, and ion diffusivity. Regardless of the cell structure to be antiparallel or twisted by 90°, a significant ionic response was observed in the right-handed CLCs in comparison with the left-handed counterparts, suggesting that excess ions originating from our R5011 were introduced into the mesogenic mixtures. This work alarms the potential contribution of notorious impurity ions by a chiral dopant, which is often ignored in fabricating CLCs for electro-optical applications.

Challenges to extracting spatial information about double P dopants in Si from STM images

The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images.

Sequential Cascade Doping of Conjugated-Polymer-Wrapped Carbon Nanotubes for Highly Electrically Conductive Platforms.

To explore a highly conductive flexible platform, this study develops PIDF-BT@SWCNT by wrapping single-walled carbon nanotubes (SWCNTs) with a conjugated polymer, PIDF-BT, known for its effective doping properties. By evaluating the doping behaviors of various dopants on PIDF-BT, appropriate dopant combinations for cascade doping are selected to improve the doping efficiency of PIDF-BT@SWCNT. Specifically, using F4TCNQ or F6TCNNQ as the first dopant, followed by AuCl3 as the second dopant, demonstrates remarkable doping efficiency, surpassing that of the individual dopants and yielding an exceptional electrical conductivity exceeding 6000 S/cm. Characterization using X-ray photoelectron spectroscopy and Raman spectroscopy elucidates the doping mechanism, revealing an increase in the proportion of electron-donating atoms and the ratio of quinoid structures upon F4TCNQ/AuCl3 cascade doping. These findings offer insights into optimizing dopant combinations for cascade doping, showcasing its advantages in enhancing doping efficiency and resulting electrical conductivity compared with single dopant processes.

Single-Atom Control of Arsenic Incorporation in Silicon for High-Yield Artificial Lattice Fabrication.

Artificial lattices constructed from individual dopant atoms within a semiconductor crystal hold promise to provide novel materials with tailored electronic, magnetic, and optical properties. These custom-engineered lattices are anticipated to enable new, fundamental discoveries in condensed matter physics and lead to the creation of new semiconductor technologies including analog quantum simulators and universal solid-state quantum computers. This work reports precise and repeatable, substitutional incorporation of single arsenic atoms into a silicon lattice. A combination of scanning tunneling microscopy hydrogen resist lithography and a detailed statistical exploration of the chemistry of arsine on the hydrogen-terminated silicon (001) surface are employed to show that single arsenic dopants can be deterministically placed within four silicon lattice sites and incorporated with 97 ± 2% yield. These findings bring closer to the ultimate frontier in semiconductor technology: the deterministic assembly of atomically precise dopant and qubit arrays at arbitrarily large scales.

Trends in high-temperature H2 production on CeO2 Co-doped with trivalent cations in solid oxide electrolysis cells

CeO2-based catalysts incorporated with cooperative dopants show excellent potential for promoting H2 production via the water-splitting reaction (WSR) in solid oxide electrolysis cells (SOECs). Here, we report that CeO2 co-doped with Gd + Sb, Pr + Sb, and Bi + Sb trivalent cations have superior performance for WSR with the reaction rate 2–5 orders of magnitude larger than that of individual trivalent dopants including Ga, Sb, Lu, Gd, Sm, Pr, Bi, and La, owing to improved stability of hydridic H-species and accordingly decreased barrier for their decompositions into H2 by co-doping. The energy of hydridic H-species and the turn-over frequency (TOF) toward H2 production is shown to correlate well with the average ionic radius of trivalent cations, where the TOF increases with decreasing ionic radius and Gd + Sb, Pr + Sb, and Bi + Sb with smaller ionic radius show better WSR activity than other dopants. Our studies suggest incorporating ceria with proper co-dopants with an average radius close to Gd + Sb, Pr + Sb, or Bi + Sb pairs as an effective strategy for enhancing WSR performance on CeO2 in SOECs.

Chemical Speed Dating: The Impact of 52 Dopants in Na–Mn–O Cathodes

Na–Mn–O cathodes are very promising for sodium-ion batteries but suffer major setbacks related to long-term cycling and stability in air. With our high-throughput approach, a systematic investigation of 52 different dopants of Na0.66MnO2 from across the periodic table was performed. The chemical composition of Na0.66Mn0.9M0.1O2+δ (M = dopant) is utilized to unravel the impact of dopants on the layered structure and investigate how different dopants influence the battery performance and air and moisture stability. A broad range of doping was possible, with 20 different dopants fully integrating into the Na–Mn–O structures, including several previously unstudied dopants (Si, Sc, Ga, Rb, Rh, Cs, Re, and Tl). This yields high-interest novel cathodes, including a Rb-doped sample with a high specific capacity of 200 mA h g–1, as well as Mo- and Nb-doped samples with excellent capacity retentions of 98% and 100%, respectively, after 10 cycles compared to 92% in undoped Na0.66MnO2. The air and moisture stability of the cathode material is studied systematically, and a number of compositions show ultrahigh stability in air. This systematic approach provides a rapid overview of the benefits of individual dopants and also provides an excellent opportunity to elucidate trends across the periodic table. Significantly, we find that the presence of reversible anionic redox (absent in the undoped samples) correlates remarkably well to the bond valence sum of the dopants, implying that dopants can be used to tune the polarity of M–O bonds and encourage anionic redox behavior. Such “speed dating” reveals fundamental chemical insights and guides further design.

Individual dopant profiles in high energy multiple implantation under channeling conditions

Individual dopant profiles in high energy multiple implantation under channeling conditions

Single-dopant band bending fluctuations in MoSe2 measured with electrostatic force microscopy

In this paper, we experimentally demonstrate two-state fluctuations in a metal-insulator-semiconductor device formed out of a metallic atomic force microscopy tip, vacuum gap, and multilayer ${\mathrm{MoSe}}_{2}$ sample. We show that noise in this device is intrinsically bias dependent due to the bias-dependent surface potential and does not require that the frequency or magnitude of individual dopant fluctuations are themselves bias dependent. Finally, we measure spatial nonhomogeneities in band bending (charge reorganization) timescales.

Spin excitations of individual magnetic dopants in an ionic thin film

Individual magnetic transition metal dopants in a solid host usually exhibit relatively small spin excitation energies of a few meV. Using scanning tunneling microscopy and inelastic electron tunneling spectroscopy (IETS) techniques, we have observed a high spin excitation energy around 36 meV for an individual Co substitutional dopant in ultrathin NaCl films. In contrast, the Cr dopant in the NaCl film shows much lower spin excitation energy around 2.5 meV. Electronic multiplet calculations combined with first-principles calculations confirm the spin excitation induced IETS, and quantitatively reveal the out-of-plane magnetic anisotropies for both Co and Cr. They also allow reproducing the experimentally observed redshift in the spin excitations of Co dimers and ascribe it to a charge and geometry redistribution.

Atomic-scale 3D imaging of individual dopant atoms in an oxide semiconductor

The physical properties of semiconductors are controlled by chemical doping. In oxide semiconductors, small variations in the density of dopant atoms can completely change the local electric and magnetic responses caused by their strongly correlated electrons. In lightly doped systems, however, such variations are difficult to determine as quantitative 3D imaging of individual dopant atoms is a major challenge. We apply atom probe tomography to resolve the atomic sites that donors occupy in the small band gap semiconductor Er(Mn,Ti)O3 with a nominal Ti concentration of 0.04 at. %, map their 3D lattice positions, and quantify spatial variations. Our work enables atomic-level 3D studies of structure-property relations in lightly doped complex oxides, which is crucial to understand and control emergent dopant-driven quantum phenomena.

Quantification of Ion-Implanted Single-Atom Dopants in Monolayer MoS2 via HAADF STEM Using the TEMUL Toolkit.

In recent years, atomic resolution imaging of two-dimensional (2D) materials using scanning transmission electron microscopy (STEM) has become routine. Individual dopant atoms in 2D materials can be located and identified using their contrast in annular dark-field (ADF) STEM. However, in order to understand the effect of these dopant atoms on the host material, there is now the need to locate and quantify them on a larger scale. In this work, we analyze STEM images of MoS2 monolayers that have been ion-implanted with chromium at ultra-low energies. We use functions from the open-source TEMUL Toolkit to create and refine an atomic model of an experimental image based on the positions and intensities of the atomic columns in the image. We then use the refined model to determine the likely composition of each atomic site. Surface contamination stemming from the sample preparation of 2D materials can prevent accurate quantitative identification of individual atoms. We disregard atomic sites from regions of the image with hydrocarbon surface contamination to demonstrate that images acquired using contaminated samples can give significant atom statistics from their clean regions, and can be used to calculate the retention rate of the implanted ions within the host lattice. We find that some of the implanted chromium ions have been successfully integrated into the MoS2 lattice, with 4.1% of molybdenum atoms in the transition metal sublattice replaced with chromium.

Study of variability induced by random dopant fluctuation in Fe DS-SBTFET

This paper explores the affectability of random dopant fluctuation (RDF) on the performance of Ferroelectric Dopant Segregated Schottky Barrier (Fe DS-SB) TFET by utilizing 3-D TCAD simulation. The simulation technique involves the device bifurcation into rectangular coordinates where the doping charges are placed randomly as individual and discrete dopants. Threshold voltage varies significantly due to variation in source-channel dopant concentration. The dependency of threshold voltage standard deviation on various source doping concentration has been studied. Doping variability within the channel and drain regions mainly leads to alteration in ON state current. Moreover, the impact of temperature (300 K–500 K) on switching characteristics of the proposed device has been investigated.

Ab Initio Study on Dopant Relaxation Mechanism in Ti and Ce Cationically Substituted in Wurtzite Gallium Nitride.

The changes in properties of materials upon introduction of impurities is well documented but less is known about the location of foreign atoms in different hosts. This study is carried out with the motivation to explore dopant location in hexagonal GaN using density functional theory based calculations. The dopant site location of the individual dopants Ti, Ce, and Ti-Ce codoped wurtzite GaN was investigated by placing the dopants at cationic lattice sites as well as off-cationic sites along the c-axis. The geometry optimization relaxed individual dopants on cationic Ga sites but in the case of codoping Ce settled at site 7.8% away along and Ti adjusted itself at site 14% away along from regular cationic sites. The analysis of the results indicates that optimized geometry is sensitive to the starting position of the dopants. The magnetic exchange interactions between Ti and Ce ions are responsible for their structural relaxation in the matrix.

Aberration-corrected scanning transmission electron microscopy: the potential for nano- and interface science

Abstract The sub-Ångström probe of an aberration-corrected scanning transmission electron microscope will enable imaging and analysis of nanostructures and interfaces with unprecedented resolution and sensitivity. In conjunction with first-principles theory, new insights are anticipated into the atomistic processes of growth and the subtle link between structure and functionality. We present initial results from the aberration-corrected microscopes at Oak Ridge National Laboratory that indicate the kinds of studies that will become feasible in the near future. Examples include (1) the three-dimensional location and identification of individual dopant and impurity atoms in semiconductor interfaces, and their effect on local electronic structure; (2) the accurate reconstruction of surface atomic and electronic structure on nanocrystals, and the effect on optical properties; and (3) the ability to distinguish which configurations of catalyst atoms are active, and why.

Visualisation of individual dopants in a conjugated polymer: sub-nanometre 3D spatial distribution and correlation with electrical properties.

While molecular doping is ubiquitous in all branches of organic electronics, little is known about the spatial distribution of dopants, especially at molecular length scales. Moreover, a homogeneous distribution is often assumed when simulating transport properties of these materials, even though the distribution is expected to be inhomogeneous. In this study, electron tomography is used to determine the position of individual molybdenum dithiolene complexes and their three-dimensional distribution in a semiconducting polymer at the sub-nanometre scale. A heterogeneous distribution is observed, the characteristics of which depend on the dopant concentration. At 5 mol% of the molybdenum dithiolene complex, the majority of the dopant species are present as isolated molecules or small clusters up to five molecules. At 20 mol% dopant concentration and higher, the dopant species form larger nanoclusters with elongated shapes. Even in case of these larger clusters, each individual dopant species is still in contact with the surrounding polymer. The electrical conductivity first strongly increases with dopant concentration and then slightly decreases for the most highly doped samples, even though no large aggregates can be observed. The decreased conductivity is instead attributed to the increased energetic disorder and lower probability of electron transfer that originates from the increased size and size variation in dopant clusters. This study highlights the importance of detailed information concerning the dopant spatial distribution at the sub-nanometre scale in three dimensions within the organic semiconductor host. The information acquired using electron tomography may facilitate more accurate simulations of charge transport in doped organic semiconductors.

Synergistic Effects of Surface Coating and Bulk Doping in Ni-Rich Lithium Nickel Cobalt Manganese Oxide Cathode Materials for High-Energy Lithium Ion Batteries.

Ni‐rich layered oxide cathodes are promising candidates to satisfy the increasing energy demand of lithium‐ion batteries for automotive applications. Thermal and cycling stability issues originating from increasing Ni contents are addressed by mitigation strategies such as elemental bulk substitution (“doping”) and surface coating. Although both approaches separately benefit the cycling stability, there are only few reports investigating the combination of two of such approaches. Herein, the combination of Zr as common dopant in commercial materials with effective Li2WO4 and WO3 coatings was investigated with special focus on the impact of different material processing conditions on structural parameters and electrochemical performance in nickel‐cobalt‐manganese (NCM) || graphite cells. Results indicated that the Zr4+ dopant diffusing to the surface during annealing improved the electrochemical performance compared to samples without additional coatings. This work emphasizes the importance to not only investigate the effect of individual dopants or coatings but also the influences between both.

Rotational Resonances and Regge-like Trajectories in Lightly Doped Antiferromagnets.

Understanding the nature of charge carriers in doped Mott insulators holds the key to unravelling puzzling properties of strongly correlated electron systems, including cuprate superconductors. Several theoretical models suggested that dopants can be understood as bound states of partons, the analogues of quarks in high-energy physics. However, direct signatures of spinon-chargon bound states are lacking, both in experiment and theory. Here we propose a rotational variant of angle-resolved photo-emission spectroscopy (ARPES) and calculate rotational spectra numerically using the density-matrix renormalization group. We identify long-lived rotational resonances for an individual dopant, which we interpret as a direct indicator of the microscopic structure of spinon-chargon bound states. Similar to Regge trajectories reflecting the quark structure of mesons, we establish a linear dependence of the rotational energy on the superexchange coupling. The rotational peaks we find are strongly suppressed in standard ARPES spectra, but we suggest a multiphoton extension of ARPES which allows us to access rotational spectra. Our findings suggest that multiphoton spectroscopy experiments should provide new insights into emergent universal features of strongly correlated electron systems.