Low Doping Levels Research Articles

Molecular dynamics simulation of friction in DLC films with different Cr doping levels

This study systematically investigates the impact of varying Cr doping levels on the friction properties of DLC films using molecular dynamics simulations. The results demonstrate that under identical friction velocities and load conditions, the friction force of DLC films significantly decreases when the Cr doping content reaches 16.5 %. High Cr doping levels lead to the formation of Cr atomic clusters at the interface, hindering contact and reducing the number of interface bonds. These clusters act as lubricants, filling the interface gaps and thereby reducing the friction force. Internal stress calculations indicate that Cr doping effectively reduces the average internal stress of DLC films. For DLC films with lower Cr doping levels, the friction force exhibits similar trends under different friction velocities, while films with higher Cr doping levels show a more pronounced response, with friction force increasing at higher velocities. Increasing the load significantly raises the friction force, especially for DLC films with higher Cr doping levels. Furthermore, high Cr doping levels under high loads lead to significant structural damage and adhesive wear. This study provides theoretical and technical references for optimizing the friction performance of DLC films.

Phonon-assisted Auger decay of excitons in doped transition metal dichalcogenide monolayers.

The competition between the radiative and nonradiative lifetimes determines the optical quantum yield and plays a crucial role in the potential optoelectronic applications of transition metal dichalcogenides (TMDCs). Here, we show that, in the presence of free carriers, an additional nonradiative decay channel opens for excitons in TMDC monolayers. Although the usual Auger decay channel is suppressed at low doping levels by the simultaneous momentum and energy conservation laws, exciton-phonon coupling relaxes this suppression. By solving a Bethe-Salpeter equation, we calculate the phonon-assisted Auger decay rates in four typical TMDCs as a function of doping, temperature, and dielectric environment. We find that even for a relatively low doping of 1012 cm-2, the nonradiative lifetime ranges from 16 to 165ps in different TMDCs, offering competition to the radiative decay channel.

The Impact of 1030 nm fs-Pulsed Laser on Enhanced Rayleigh Scattering in Optical Fibers.

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.

An effective strategy to enhance phosphoric acid retention and proton conductivity stability: Construction of proton transfer channels with starch rather than H3PO4

To enhance the phosphoric acid (PA) retention as well as maintain high proton conductivity of phosphoric acid doped proton exchange membranes at high temperature, we successfully design a series of phosphoric acid doped biobased composite membranes by incorporation of starch and graphene oxide (GO) into poly arylene ether ketones (PAEK). Proton transfer channels should be mainly built through dense hydrogen bonds formed from massive oxygen-containing groups of starch mainchain, which is confirmed by Molecular dynamics (MD) simulation, FT-IR and XRD analysis. The dense hydrogen-bond structure could construct fast proton transfer channels with extreme low doping level (0.00484 molH3PO4). The excellent PA retention properties with almost unchanged proton conductivity at high temperature (200 °C) for 600 min indicates that PA molecules are firmly fixed into membranes. Thus, in this study, we suggest a novel strategy for stablizing proton conductivity at high temperature and improving PA retention properties of PA doped membranes, which is building dense hydrogen-bond structure with low PA doping level.Based on the results in this study and the Grotthuss proton transfer mechanism, dense hydrogen-bonds from oxygen-containing groups in polymer backbones should be more stable than hydrogen-bonds from massive H3PO4 molecules with high acid doping levels to promote proton conduction.

(Invited) Bulk Traps in Layered 2D Gate Dielectrics

Ultra-thin ferromagnets, when coupled with magnetoelectric or multiferroic materials, could potentially enable highly energy-efficient electric field control of the magnet for use in nanoelectronic memories. Substitutional doping of magnetic impurities in monolayer transition metal dichalcogenides (TMDs) may be a promising way to create 2D ferromagnets but, according to theoretical calculations, require high doping levels (10-20 %) to achieve above room temperature (RT) Curie temperature. Room-temperature ferromagnetism has been reported for very low doping levels (0.1-1%), in conflict with the theoretical calculations, and always in the presence of high concentrations of defects, making it unclear if the dopants alone are responsible for this ferromagnetism. In this work, we use a combination of molecular beam epitaxy (MBE), plan-view TEM, XRD, XPS, Raman, and magnetometry to study iron and vanadium doping in monolayer WSe2. We show that optimally grown monolayers with up to 35% doping and low defect density show no ferromagnetism. Interestingly, ferromagnetism is observed when these monolayers contain a significant amount of selenium vacancies (Sevac), intentionally created via a post-growth heat treatment process, and magnetism is seen to scale with heating time/vacancy concentration. Moreover, even undoped WSe2 shows similar ferromagnetism for Sevac > 1014 cm-2. This ferromagnetism can later be quenched by filling these vacancies via Se annealing. For films with low Se vacancy concentration, TEM analysis reveals significant clustering of the V and Fe dopants which likely suppresses the ferromagnetism predicted by theory - a problem that has previously plagued III-V based dilute magnetic semiconductors as well. We go so far as to claim that all of the above room-temperature ferromagnetism reports in the literature thus far are due to Se vacancies and not magnetic doping.

Wettability of anatase TiO2 surface: Effect of niobium doping

The effect of niobium doping on the hydrophilic behavior of anatase TiO2 surface was demonstrated on a series of x-Nb-TiO2 nanocoatings with dopant concentrations in the range of 0.0 - 1.0 at.% Nb. Thin films prepared on glass substrates from sols by dip-coating were characterized by tensiometric, work function and isoelectric point measurements along with FT-IR spectroscopy in situ method. Using macroscopic and microscopic techniques, it was found that, at low doping level, it is the functionalization of the anatase surface with niobium ions that leads to a drastic change in its surface acid-base as well as electronic properties and, as a consequence, to a sharp increase in the initial surface hydrophilicity. The rearrangement in the hydroxyl‑hydrated layer with the formation of additional hydroxyl groups as a result of surface functionalization has an increased effect on wettability only at low levels of niobium inclusion, whereas with increasing dopant content the electronic factor plays a significant role on the properties of titanium dioxide, including its wettability. The kinetic dependencies of the photoinduced superhydrophilic conversion for all Nb-doped titanium dioxide surfaces were obtained and discussed in terms of proposed mechanism. Both the initial hydrophilic state and the initial rate of photoinduced hydrophilic conversion depend on the Nb concentration in titanium dioxide, while the final UV-induced superhydrophilic state is achieved for all Nb-doped TiO2 regardless of the dopant content.

Effects of high valency and polarizability ion (W6+) on the phase evolution of CaZr1–2xLn2xTi2–xWxO7 zirconolite-based solid solution (Ln = Nd, Sm, Gd, Ho, Yb)

Zirconolite-based structures have served as robust matrices for the incorporation of minor actinide-rich high-level waste (HLW). The co-doping effects are critical for exploring the phase evolution of zirconolite-based structures, thereby providing a guide for HLW immobilization. Nevertheless, few studies have studied the synergistic effect of high valent ion W6+ and Lanthanides (Ln, surrogates to minor actinides) co-doping. This study proposed a systematical study on the Ln-W (Ln = Nd, Sm, Gd, Ho, Yb) co-doping in zirconolite-derived structure using X-ray diffraction (XRD) and scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). The results showed that near-single-phase zirconolite-2M was recorded for samples with low level doping of Ln-W (Nd–W, Sm–W and Gd–W with x = 0.05; Ho–W and Yb–W with x = 0.05–0.1), and its structure underwent a moderate expansion along the c-axis, as revealed by a Pawley refinement method. One key finding is that near-single-phase zirconolite-2M was recorded for samples with low level doping of Ln-W (Nd–W, Sm–W and Gd–W with x = 0.05; Ho–W and Yb–W with x = 0.05–0.1), and its structure underwent a moderate expansion along the c-axis, as revealed by a Pawley refinement method. Notably, in addition to a transformation from zirconolite-2M to pyrochlore, a pseudo-yellow phase CaWO4 was also noted with increasing co-doping contents. This work demonstrates that the high charge compensator W6+ and lanthanides can be simultaneously incorporated into zirconolite structure, and a different phase transformation was revealed compared to that in low valent element doping system, which gives a new insight into the optimization of HLW immobilization route.

Interface-Suppressed Nematicity and Enhanced Superconducting Pairing Strength of FeSe/NdFeO3 in the Low-Doping Regime.

The discovery of interfacial superconductivity in monolayer FeSe/oxides has spurred intensive research interest. Here we not only extend the FeSe/FeOx superconducting interface to FeSe/NdFeO3 but also establish robust interface-enhanced superconductivity at a very low doping level. Specifically, well-annealed FeSe/NdFeO3 exhibits a low doping level of 0.038-0.046 e-/Fe with a larger superconducting pairing gap without a nematic gap, indicating an enhancement of the enhanced superconducting pairing strength and suppression of nematicity by the FeSe/FeOx interface compared with those of thick FeSe films. These results improve our understanding of the roles of the oxide interface in the low-electron-doped regime.

Er3+/ Yb3+ co-doped multi-functional silica phosphate glass films for integrated photonic applications

The multi-component silicate glasses co-doped with Er3+/Yb3+ were fabricated as thin films by the spin coating technique, which was analyzed for their optoelectronic response. Their monolith counterparts synthesized by the sol-gel route were used for the electrical studies. The films evidenced nanocristobalite phases at low levels of doping. The film micrographs explicated gas-trapped circular dimensions on the surface, due to the spreading of the sol on the substrate during the spin coating process. The glasses show a decrease in the number of polarizable non-bridging oxygen units with doping, reducing the polarizability and optical basicity. The films exhibit prominent green lasing in the downshifting process. The energy transfer from the Yb3+ ions endows the glasses with significant red lasing, suppressing the green emissions by the upconversion mechanism, which is also validated by the colorimetric chromaticity analysis. The gain cross-section of the glass doped with 1.5 mol% of Yb3+ is 23.3 × 10−20 cm2 in the S-band telecommunication domain. The glass film evidences its potential for NIR lasers, beyond a population inversion of 40 %, and a decreasing metastable lifetime of 6.6 μs. Beyond 1.5 mol% of the co-dopant Yb3+, energy transfer dominates, due to the dipole-quadrupole interactions, which has been corroborated by the Inokuti-Hirayama model. The conductivity in the glasses shows a decreasing trend with doping. The Randle's equivalent circuitry of the Nyquist impedance plots, predicts Warburg diffusion impedance that deters the space charge polarization. The consequential decrease in the dielectric constant and capacitance makes the glasses suggestive of multi-layer dielectric substrates in the integrated microelectronic technology for soldering of the printed circuit boards (PCB) that demand low signal losses.

Comparative analysis of radiation-induced effects on the performance of p-type PERC and TOPCon solar cells for space applications

This work presents a comprehensive numerical evaluation of PERC and TOPCon technologies, focusing on the impact of radiation-induced defects. This assessment is conducted for p-type silicon solar cells as they are intrinsically more resistant to radiation defects. By rigorously calibrating recombination parameters, radiation-induced defect profiles, and other pertinent details, a robust basis is established for an in-depth comparison of the performance characteristics displayed by both architectures under space conditions. The investigation reveals that when utilizing substrates with high doping levels, both PERC and TOPCon cells exhibit nearly identical beginning-of-life (BOL) and end-of-life (EOL) performance. However, with lower substrate doping concentrations, both technologies show improved BOL efficiency. Notably, this enhanced BOL efficiency does not translate into superior EOL efficiency. This distinction in EOL efficiency can be attributed to two primary factors triggered by radiation exposure. Firstly, the emergence of defects leads to a reduction in open-circuit voltage. Secondly, dopant compensation contributes to an increase in series resistance. Specifically, for PERC cells, the challenge of elevated series resistance is further exacerbated by the requirement for majority carriers to traverse both vertically and laterally to reach the rear metal contact. When a robust defect recovery mechanism or resilient cover glass is absent, substrates characterized by lower doping levels display increased susceptibility to the adverse effects of radiation-induced defects and the subsequent dopant compensation. Under these circumstances, the TOPCon technology demonstrates a significant advantage over PERC, particularly for high electron fluence due to its full area contacts for both minority and majority charge carriers.

Effect of Low-level Doping With Strontium On Structure, Magnetism, and Electron States in SmMnO3 Manganite

Effect of Low-level Doping With Strontium On Structure, Magnetism, and Electron States in SmMnO3 Manganite

Regulation of electrons between Ru and VN for high performance HER of low-level Ru doping in vanadium-nitrogen doped carbon catalysts

Transition metal nitrides (TMNs) have garnered significant attention for their catalytic activity similar to noble metals, attributed to their distinctive electronic structures. However, the sluggish reaction kinetics have hindered the progress in TMN development. In this study, an alloying approach was employed to create Ru@VN/C catalysts by uniformly dispersing low levels of ruthenium (Ru) in vanadium-nitrogen doped carbon (VN/C) on carbon spheres. Through alloying, the electronic structures of ruthenium and vanadium atoms were further adjusted, leading to exceptional catalytic activity resulting from unique electron transfer between the atoms. The Ru@VN/C catalyst demonstrated a low overpotential of 5 mV and a Tafel slope of 27 mV·dec−1 at 10 mA·cm−2 in alkaline seawater solution, as well as superior performance in alkaline and simulated seawater solutions. The interatomic synergistic interactions, driven by charge redistribution as confirmed by density functional theory calculations and experimental data, play a crucial role in expanding the applications of TMNs in green hydrogen production.

Thermoelectric performance enhancement of Pb-doped α -MgAgSb near room temperature

α-MgAgSb is taken as the p-type leg material for recently focused Mg-based thermoelectric devices because of the high thermoelectric performance near room temperature. However, the thermoelectric performance of α-MgAgSb is inhibited by the existence of the Ag-rich second phase. The ordinary methods like carrier concentration optimization and minimizing lattice thermal conductivity were nearly invalid because of the extremely low doping level for heteroatoms and intrinsically low lattice thermal conductivity. The crystal structure of α-MgAgSb can be viewed as Ag atom filled in half distorted hexahedron in the distorted rock salt skeleton formed by the Mg–Sb sublattice. In this work, by replacing the smaller Mg in the sublattice with Pb, the volume of the distorted hexahedron is effectively expanded to accommodate Ag atoms and then lead to the re-dissolution of Ag-rich second phase in the matrix. In addition, as Ag is the main source of low-frequency phonons, the enhanced lattice anharmonicity by Pb doping leads to stronger scattering of phonons in the distorted hexahedron and results in 20% reduction of lattice thermal conductivity in the temperature range of 300–500 K. Finally, the figure of merit zT is enhanced by ∼40% in the whole temperature range, demonstrating that lattice management is a promising method for the optimization of α-MgAgSb materials.

Exploration of an atomic‐scale boron additive in SiAlON ceramics

AbstractSiAlON ceramics are of interest for high‐risk applications such as biomedical implants and combustion engine turbines. This work is part of a larger study aimed at leveraging atomic‐ or molecular‐scale additives to control the densification, microstructures, and ultimate structural properties of SiAlONs. Here, we investigate the effects of boron in silicon nitride‐based ceramics. The present work demonstrates a possible chemical method for controlling the microstructural development of SiAlONs by incorporating boric acid (H3BO3) into the starting powder blend. Raman spectroscopy and 11B solid‐state magic angle spinning nuclear magnetic resonance cooperatively indicate that after sintering, boron exists in threefold coordination with nitrogen in the turbostratic boron nitride (t‐BN) structure. The results of this work indicate that the incorporation of boron and generation of t‐BN bonding in the SiAlON system result in a narrower grain size distribution, a suppression of second phases such as yttrium aluminosilicates, and ultimately, increased flexure strength. A separate fractographic study showed that SiAlONs fabricated with 3 wt% boric acid exhibited fracture origins such as subtle surface flaws or cracks, while lower dopant levels and undoped SiAlONs typically failed from flaws such as inclusions or large grains. It is argued that the modification of the intergranular glass chemistry and resulting generation of t‐BN reduces atomic diffusion through the grain boundary phase and inhibits the crystallization of second phases as well as exaggerated grain growth that often characterizes the development of β′‐SiAlON.

Enhancing Electrical Conductivity and Power Factor in Poly‐Glycol‐Bithienylthienothiophene with Oligoethylene Glycol Side Chains Through Tris (pentafluorophenyl) Borane Doping

AbstractDoping of organic semiconductors has served as an effective method to achieve high electrical conductivity and large thermoelectric power factor. This is of importance to the development of flexible/wearable electronics and green energy‐harvesting technologies. The doping impact of the Lewis acid tris (pentafluorophenyl) borane (BCF) on the thermoelectric performance of poly(2‐(4,4′‐bis(2‐methoxyethoxy)‐5′‐methyl‐[2,2′‐bithiophen]‐5‐yl)‐5‐methylthieno[3,2‐b]thiophene (pgBTTT), a thiophene‐based polymer featuring oligoethylene glycol side chains is investigated. Tetrafluorotetracyanoquinodimethane (F4TCNQ), a well‐established dopant, is utilized as a comparison; however, its inability to co‐dissolve with pgBTTT in less polar solvents hinders the attainment of higher doping levels. Consequently, a comparative study is performed on the thermoelectric behavior of pgBTTT doped with BCF and F4TCNQ at a very low doping level. Subsequent investigation is carried out with BCF at higher doping levels. Remarkably, at 50 wt% BCF doping level, the highest power factor of 223 ± 4 µW m−1 K2 is achieved with an electrical conductivity of 2180 ± 360 S cm−1 and a Seebeck coefficient of 32 ± 1.3 µV K−1. This findings not only contribute valuable insights to the dopant interactions with oxygenated side chain polymers but also open up new avenues for high conductivity thermoelectric polymers in flexible electronic applications.

A New Approach to Fuel Cell Electrodes: Lanthanum Aluminate Yielding Fine Pt Nanoparticle Exsolution for Oxygen Reduction Reaction

AbstractDesigning an electrocatalyst with low Pt content is an immediate need for essential reactions in low temperature fuel cell systems. In the present work, La0.9925Ba0.0075Al0.995Pt0.005O3 is aimed at using with low (only 0.5%) Pt doping as an electrocatalyst for oxygen reduction reaction (ORR). The low doping level renders exsolution of 1–2 nm nanoparticles with uniform dispersion upon reduction in H2/N2 at low temperatures. Pt exsolved perovskite oxides deliver significantly enhanced catalytic activity for ORR and improved stability in alkaline media. This study demonstrates that LaAlO3 with low noble metal content holds immense potential as an electrocatalyst in real fuel cell systems.

MXene-Integrated Solid-Solid Phase Change Composites for Accelerating Solar-Thermal Energy Storage and Electric Conversion.

The high thermal storage density of phase change materials (PCMs) has attracted considerable attention in solar energy applications. However, the practicality of PCMs is often limited by the problems of leakage, poor solar-thermal conversion capability, and low thermal conductivity, resulting in low-efficiency solar energy storage. In this work, a new system of MXene-integrated solid-solid PCMs is presented as a promising solution for a solar-thermal energy storage and electric conversion system with high efficiency and energy density. The composite system's performance is enhanced by the intrinsic photo-thermal behavior of MXene and the heterogeneous phase transformation properties of PCM molecular chains. The optimal composites system has an impressive solar thermal energy storage efficiency of up to 94.5%, with an improved energy storage capacity of 149.5Jg-1 , even at a low MXene doping level of 5wt.%. Additionally, the composite structure shows improved thermal conductivity and high thermal cycling stability. Furthermore, a proof-of-concept solar-thermal-electric conversion device is designed based on the optimized M-SSPCMs and commercial thermoelectric generators, which exhibit excellent energy conversion efficiency. The results of this study highlight the potential of the developed PCM composites in high-efficiency solar energy utilization for advanced photo-thermal systems.

Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses.

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35W) limited the cw powers we could achieve experimentally to 2.25W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10W level.

Impurity reduction in lightly doped n-type gallium nitride layer grown via halogen-free vapor-phase epitaxy

The development of gallium nitride (GaN) vertical-type metal-oxide-semiconductor field-effect transistors and p–i–n diode devices has gathered increasing attention. These devices require an n-type drift layer with a low doping level of 1016 cm−3 or less, minimized point defects inhibiting electron conduction, and a layer approximately 10 μm thick. Therefore, a practical method with a growth rate of at least several tens of μm/h and impurity concentrations of less than 1015 cm−3, except for that of dopants, is necessary. Halogen-free vapor-phase epitaxy (HF-VPE) has a high growth rate suitable for fabricating thick drift layers and utilizes a simple reaction between Ga vapor and ammonia gas (without a corrosive halogen gas), resulting in lower impurity levels. Herein, we eliminated the quartz content from the high-temperature zone to reduce the excess unintentional Si doping and identified that the nitrile gloves used for the growth preparation are other impurity contamination sources. We obtained a lightly n-type ([Si]=∼1016 cm−3) GaN layer, in which C, O, B, Fe, Mg, Al, Ca, Cr, Zn, Ni, Mn, and Ti impurity contents were below the detection limits of secondary ion mass spectrometry. Deep-level transient spectroscopy revealed that electron traps at EC − 0.26 and at EC − 0.59 eV were 2.7 × 1013 and 5.2 × 1014 cm−3, respectively. Moreover, the Hall effect analysis showed the acceptor-type defect-compensating donor content as approximately 2.7 × 1015 cm−3, resulting in a high electron mobility of HF-VPE GaN in the 30–710 K temperature range. Furthermore, we identified the Ca impurity as a deep acceptor, another killer defect leading to mobility collapse.

Fabrication of mechanically robust nanoporous ZrSiO4 ceramics at low temperature with a low doping level of Mn dopant

AbstractZircon (ZrSiO4) ceramics have been widely used in many fields due to their excellent physical and chemical properties. However, ZrSiO4 ceramics typically possess moderately low mechanical properties, which hinders their wider application. Meanwhile, elevated temperatures (∼1500°C) are required to obtain high‐purity synthetic ZrSiO4 ceramics, which is time‐ and energy‐consuming. In the present study, we prepared mechanically robust ZrSiO4 ceramics at low temperature (∼1170°C) with a low doping level of Mn dopant (<2 mol%). The ZrSiO4 ceramic processed by hot isostatic pressing with .5 mol% Mn dopant achieved the highest flexural strength (512 MPa), elastic modulus (341 GPa), and nanohardness (20.8 GPa). These values are significantly higher than conventional ZrSiO4 ceramics. The strengthening mechanisms of the prepared ZrSiO4 ceramics were attributed to the formation of homogeneously‐distributed nanopores due to incomplete densification and submicron ZrSiO4 grains (∼300 nm). The nanopores avoided stress concentration and deflected microcracks during loading, and the submicron ZrSiO4 grains endowed the ZrSiO4 ceramics with grain refinement strengthening. The results reported in this study would offer guidance to fabricate mechanically robust ZrSiO4 ceramics at low temperatures with a low doping level of dopant.