Dopant Distribution Research Articles

Nucleation and Growth of Bipyramidal Yb:LiYF4 Nanocrystals-Growing Up in a Hot Environment.

Lanthanide-doped LiYF4 (Ln:YLF) is commonly used for a broad variety of optical applications, such as lasing, photon upconversion and optical refrigeration. When synthesized as nanocrystals (NCs), this material is also of interest for biological applications and fundamental physical studies. Until now, it was unclear how Ln:YLF NCs grow from their ionic precursors into tetragonal NCs with a well-defined, bipyramidal shape and uniform dopant distribution. Here, we study the nucleation and growth of ytterbium-doped LiYF4 (Yb:YLF), as a template for general Ln:YLF NC syntheses. We show that the formation of bipyramidal Yb:YLF NCs is a multistep process starting with the formation of amorphous Yb:YLF spheres. Over time, these spheres grow via Ostwald ripening and crystallize, resulting in bipyramidal Yb:YLF NCs. We further show that prolonged heating of the NCs results in the degradation of the NCs, observed by the presence of large LiF cubes and small, irregular Yb:YLF NCs. Due to the similarity in chemical nature of all lanthanide ions our work sheds light on the formation stages of Ln:YLF NCs in general.

Synthesis and characterization of polyphenylene sulfide resin-derived S-doped porous carbons for efficient CO2 capture

Porous carbons have long been studied for their capacity to capture CO2 due to their enhanced textural characteristics and customizable surface decoration. However, developing porous carbons with significant CO2 uptake capacity and strong CO2/N2 selectivity is still a challenge. This study presents introducing sulfur (S) atoms into the porous carbon structure to increase CO2 uptake using a pre-carbonized polyphenylene sulfide resin (PPS) via chemical activation. By adjusting KOH activator amount and activation temperature, various textural features and S contents were obtained, resulting in a strong adsorption effect for CO2. The optimal sample displayed CO2 uptake of 3.64 and 5.13 mmol/g at 25 and 0 °C and a CO2/N2 selectivity of 19 due to the judicious heteroatom doping and optimal pore size distributions. The findings demonstrate the significant potential of PPS-derived porous carbon adsorbents for CO2 adsorption, and the novel synthesis method provides new possibilities for the scalable production of CO2 adsorbent.

Pt and Pd Nanoparticle Crystallization in the Sol-Gel-Derived Thin SiO2 Films

The crystallization and distribution the features of Pt, Pd and Pt/Pd nanoparticles in spin-on glass SiO2 films were studied within a wide range of the dopant concentrations in silica sol (from 10 to 80 mol.% Pt, Pd or Pt/Pd per 100 mol.% Si). The grazing incidence X-ray diffraction (GIXRD) characterization revealed that the formation of 4–8 nm sized crystalline Pt, Pd and Pt/Pd nanoparticles in SiO2 films began at the dopant concentrations of at least 10 mol.% Pt and/or Pd per 100 mol.% Si. The nanoparticles obtained from sols with the lower Pt, Pd or Pt/Pd concentrations were characterized by an amorphous structure. The dopants distribution over the film thickness (~21–47 nm) was studied using X-ray reflectometry. The effect of the dopant concentration, spin-coating modes and heat treatment temperature on the film thickness was characterized. When only one of the dopants (Pt or Pd) was introduced into the silica sol, the resulting nanoparticles were preferentially localized close to the film surface. When dopants were used together, the Pt/Pd nanoparticles were distributed more evenly.

Periodic Arrays of Dopants in Silicon by Ultralow Energy Implantation of Phosphorus Ions through a Block Copolymer Thin Film.

In this work, block copolymer lithography and ultralow energy ion implantation are combined to obtain nanovolumes with high concentrations of phosphorus atoms periodically disposed over a macroscopic area in a p-type silicon substrate. The high dose of implanted dopants grants a local amorphization of the silicon substrate. In this condition, phosphorus is activated by solid phase epitaxial regrowth (SPER) of the implanted region with a relatively low temperature thermal treatment preventing diffusion of phosphorus atoms and preserving their spatial localization. Surface morphology of the sample (AFM, SEM), crystallinity of the silicon substrate (UV Raman), and position of the phosphorus atoms (STEM- EDX, ToF-SIMS) are monitored during the process. Electrostatic potential (KPFM) and the conductivity (C-AFM) maps of the sample surface upon dopant activation are compatible with simulated I-V characteristics, suggesting the presence of an array of not ideal but working p-n nanojunctions. The proposed approach paves the way for further investigations on the possibility to modulate the dopant distribution within a silicon substrate at the nanoscale by changing the characteristic dimension of the self-assembled BCP film.

The Formation of NaYF4 : Er3+, Yb3+ Nanocrystals Studied by In Situ X‐ray Scattering: Phase Transition and Size Focusing

Abstractβ‐NaYF4 nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core‐multi‐shell geometries with controlled dopant distributions. Here, we follow the synthesis of β‐NaYF4 nanocrystals from α‐NaYF4 precursor particles using in situ small‐angle and wide‐angle X‐ray scattering and ex situ electron microscopy. We observe an evolution from a unimodal particle size distribution to bimodal, and eventually back to unimodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α‐to‐β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α‐NaYF4 precursor particles, potentially due to inter‐particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics.

The Formation of NaYF4:Er3+,Yb3+ Nanocrystals Studied by In Situ X-ray Scattering: Phase Transition and Size Focusing.

β-NaYF4 nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core-multi-shell geometries with controlled dopant distributions. Here, we follow the synthesis of β-NaYF4 nanocrystals from α-NaYF4 precursor particles using in situ small-angle and wide-angle X-ray scattering and ex-situ electron microscopy. We observe an evolution from a monomodal particle size distribution to bimodal, and eventually back to monomodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α-to-β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α-NaYF4 precursor particles, potentially due to inter-particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics.

Al Implantation in Sic; Where Will the Ions Come to Rest?

In all implantations into crystalline targets, quite a few ions find a path along a crystal channel or plane, so called channeling, and these ions travel deep into the crystal. This paper treats aluminum (Al) implantation in 4H-SiC and show how the crystal lattice will guide incoming ions deep into the target and modify the final dopant distribution. 4H-SiC samples have been implanted with 100 keV Al-ions, in a “random” direction using the wafer miscut angle of 4°, as well as with the impact beam aligned anti-parallel to the [0001] direction. Aluminium concentration versus depth profiles has been recorded by secondary ion mass spectrometry (SIMS). To track the most probable ion paths during stopping process, SIIMPL, a Monte Carlo simulation code based on the binary collision approximation (MC-BCA) has been used. In addition, the remaining ion energy has been extracted from SIIMPL at various depth along the ion path. Our results show that, independent of the used impact angle, some ions will be steered by crystal planes predominantly into the direction and also along the six directions. The energy loss is smaller along these low index axes. Therefore, at a depth of 1.2 μm, some Al ions along a path may still have kinetic energy, more than 40% of the original 100 keV, and continues to move deep into the SiC sample. The mean projected range of 100 keV ions in 4H-SiC is about 120 nm.

Direct observation of the distribution of impurity in phosphorous/boron co-doped Si nanocrystals

Doping in Si nanocrystals is an interesting topic and directly studying the distribution of dopants in phosphorous/boron co-doping is an important issue facing the scientific community. In this study, atom probe tomography is performed to study the structures and distribution of impurity in phosphorous/boron co-doped Si nanocrystals/SiO2 multilayers. Compared with phosphorous singly doped Si nanocrystals, it is interesting to find that the concentration of phosphorous in co-doped samples can be significantly improved. Theoretical simulation suggests that phosphorous–boron pairs are formed in co-doped Si nanocrystals with the lowest formation energy, which also reduces the formation energy of phosphorous in Si nanocrystals. The results indicate that co-doping can promote the entry of phosphorous impurities into the near-surface and inner sites of Si nanocrystals, which provides an interesting way to regulate the electronic and optical properties of Si nanocrystals such as the observed enhancement of conductivity and sub-band light emission.

DFT analysis of the sensitivity of graphene/MoS2 heterostructures toward H2CO

Indoor toxic gases detection is very important for human health, and the use of novel two-dimensional materials to detect or adsorb toxic gases is significant. With these prospects in mind, the potential application of graphene/MoS2 two-dimensional heterostructure as a gas sensor was explored. This study investigated the sucking stability and electronic and magnetic properties of H2CO adsorbed on perfect, vacancy-defected, transition metal (TM)-doped graphene/MoS2 heterostructures through density functional theory analysis. Perfect graphene/MoS2 (GMoS2) exhibited small adsorption energy and low charge transfer, and the TM-doped Mo-vacancy-defected heterostructures exhibited physical adsorption toward H2CO molecules with a shorter desorption time. However, the interaction between H2CO and the substrate changed to chemisorption when TMs (Sc, V, Cr, Mn) were doped with S-vacancy-defected graphene/MoS2. Meanwhile, the density of states of the S-defected heterostructure indicated the hybridization of the d orbitals of the dopants and the p orbital of the H2CO molecule that considerably enhanced the adsorption energy. Furthermore, the asymmetric electron spin distribution of dopants and Mo atoms induced a strong magnetic moment. In summary, this study established an effective approach for H2CO gas detection based on heterostructure.

Impact of high-temperature Mg-implantation on defects and dopants distribution in GaN

We have investigated the impact of high-temperature Mg-implantation in GaN layers on distribution of Mg-enriched defects using scanning transmission electron microscopy and atom probe tomography. For this, 1 × 1019 cm−3 Mg ions have been implanted in GaN layers at room temperature (RT) and 1000 °C, followed by annealing at 1300 °C. A smaller number of Mg-enriched defects were observed in the sample implanted at 1000 °C in comparison to the sample implanted at RT. The implantation of Mg ions at 1000 °C resulted in a higher amount of randomly distributed Mg in the GaN matrix, which, in turn, leads to more uniform and enhanced donor–acceptor pair emission, leading to higher Mg activation.

Direct atomic layer deposition of ultra-thin Al2O3 and HfO2 films on gold-supported monolayer MoS2

In this paper, the atomic layer deposition (ALD) of ultra-thin films (<4 nm) of Al2O3 and HfO2 on gold-supported monolayer (1L) MoS2 is investigated, providing an insight on the mechanisms ruling the nucleation in the early stages of the ALD process. A preliminary multiscale characterization of large area 1L-MoS2 exfoliated on sputter-grown Au/Ni films demonstrated: (i) a tensile strain (from 0.1 to 0.3%) and p-type doping (from 1 × 1012 to 4 × 1012 cm−2) distribution at micro-scale; (ii) an almost conformal MoS2 membrane to the Au grains topography, with some locally detached regions, indicating the occurrence of strain variations at the nanoscale; (iii) atomic scale variability (from ∼ 4.0 to ∼ 4.5 Å) in the Mo-Au atomic distances was detected, depending on the local configuration of Au nanograins. Ab initio DFT calculations of a free-standing MoS2 layer and a simplified MoS2/Au(111) interface model showed a significant influence of the Au substrate on the MoS2 energy band structure, whereas small differences were accounted for the adsorption of H2O, TMA (co-reactant, and Al-precursor, respectively) molecules, and a slight improved adsorption was predicted for TDMAHf (Hf-precursor). This suggests a crucial role of nanoscale morphological effects, such as the experimentally observed local curvature and strain of the MoS2 membrane, in the enhanced physisorption of the precursors. Afterwards the nucleation and growth of Al2O3 an HfO2 films onto 1L-MoS2/Au was investigated in detail, by monitoring the surface coverage as a function of the number (N) of ALD cycles, with N from 10 to 120. At low N values, a slower growth rate of the initially formed nuclei was observed for HfO2, probably due to the bulky nature of the TDMAHf precursor as compared to TMA. On the other hand, the formation of continuous films was obtained in both cases for N > 80 ALD cycles, corresponding to ∼ 3.6 nm Al2O3 and ∼ 3.1 nm HfO2. Current mapping on these ultra-thin films by conductive-AFM showed, for the same applied bias, a uniform insulating behavior of Al2O3 and the occurrence of few localized breakdown spots in the case of HfO2, associated to a less compact films regions. Finally, an increase of the 1L-MoS2 tensile strain was observed by Raman mapping after encapsulation with both high-κ films, accompanied by a reduction in the PL intensity, explained by the effects of strain and the higher effective dielectric constant of the surrounding environment.

Mitigating the High-Charge Detrimental Phase Transformation in LiNiO2 Using Doping Engineering

Cobalt-free layered LiNiO2 has gained increased interest due to the scarcity and high cost of cobalt. However, LiNiO2 suffers from poor cycling stability, which is mainly due to oxygen loss and structural instability, especially when operating at high voltages. Herein, we present a doping strategy to mitigate the detrimental O3-to-O1 phase transformation in LiNiO2 from first-principles calculations. Temperature–composition phase diagrams of pristine and doped Li1–xNiO2 are obtained using a cluster-expansion and Monte Carlo simulation approach. We investigate the effects of dopant oxidation states, sizes, and concentrations on the dopant distribution in LiNi1–yMyO2 (M = Sb, Ti, Si, Al, and Mg) as well as the phase transitions during delithiation. We find that introducing high-valence dopants with ionic radii similar to that of Ni3+ into LiNiO2 stabilizes the O3-phase cathode bulk structure at high charge. Our results provide a general guidance on using doping engineering to realize Ni-rich, Co-free cathodes for lithium-ion batteries.

Effect of Y, La, and Yb simultaneous doping on the thermal conductivity and thermoelectric performances of CaMnO3 ceramics

Triple doping of CaMnO3 in different stoichiometric proportions has been studied in bulk sintered materials. They were prepared through the classical ceramic route using planetary milling to decrease the precursors particle sizes. It has been found that particle sizes decrease with the amount of dopant, both in the precursors and in the sintered bodies. XRD patterns showed that all samples were nearly single phase, with small amounts of CaMn2O4 phase. SEM and TEM observations revealed a homogeneous distribution of dopants in the CaMnO3 phase, while EDX showed a composition close to the nominal one. However, HREM imaging has shown some dark regions in the crystallites, with different Ca(Y,La,Yb)/Mn ratios found through STEM image-chemical mapping, which points out to the coexistence of CaMnO3 and CaMn2O4–type domains in the crystallites. Electrical resistivity, absolute Seebeck coefficient and thermal conductivity have been drastically decreased with doping. The highest PF values at 800 ºC have been achieved in 0.02(Y,La,Yb) doped samples (∼ 0.37 mW/K2m), which is among the best reported values in literature. On the other hand, lattice thermal conductivity is dramatically decreased with doping due to the phonon scattering produced by the dopants, the decrease in the grain sizes, and the strains present inside the crystallites, reaching the minimum values at 800 ºC in 0.03(Y,La,Yb) doped samples (∼ 0.8 W/K m). Consequently, ZT reaches the maximum values (∼ 0.29) in 0.03(Y,La,Yb) doped samples due to their very low thermal conductivity, being higher than the best ZT reported values in the literature.

Ultrahigh-Voltage LiCoO2 at 4.7V by Interface Stabilization and Band Structure Modification.

Lithium cobalt oxide (LCO) is widely used in Li-ion batteries due to its high volumetric energy density, which is generally charged to 4.3 V. Lifting the cut-off voltage of LCO from 4.3 V to 4.7 V will increase the specific capacity from 150 to 230 mAh g-1 with a significant improvement of 53%. However, LCO suffers serious problems of H1-3/O1 phase transformation, unstable interface between cathode and electrolyte, and irreversible oxygen redox reaction at 4.7 V. Herein, interface stabilization and band structure modification are proposed to strengthen the crystal structure of LCO for stable cycling of LCO at an ultrahigh voltage of 4.7 V. Gradient distribution of magnesium and uniform doping of nickel in Li layers inhibit the harmful phase transitions of LCO, while uniform LiMgx Ni1- x PO4 coating stabilizes the LCO-electrolyte interface during cycles. Moreover, the modified band structure improves the oxygen redox reaction reversibility and electrochemical performance of the modified LCO. As a result, the modified LCO has a high capacity retention of 78% after 200 cycles at 4.7 V in the half cell and 63% after 500 cycles at 4.6 V in the full cell. This work makes the capacity of LCO one step closer to its theoretical specific capacity.

Concentration Phase Separation of Substitution-Doped Atoms in TMDCs Monolayer.

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.

Structural modification of nano titania by doping with Barium and Copper – Impact on photocatalysis: Applications in degradation of dye and pathogens

Ba/Cu co-doped TiO2 nanoparticles with visible light activity were fabricated by the sol-gel method with different weight percentages for the decontamination of toxic organic azo dye pollutants. The characterizations of fabricated samples were evaluated by using UV–Vis Diffuse Reflectance Spectroscopy, X-ray Diffraction Spectroscopy, Brunauer-Emmett-Teller, Transmission Electron Microscopy, Scanning Electron Microscopy-Energy Dispersive X-ray, X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, and X-ray Fluorescence techniques. The results of co-doping revealed a lower band gap of 2.59 ​eV (UV–Vis DRS), a high surface area of 138 ​m2/g (BET), and anatase phase formation (XRD), which are the major contributing factors for efficient photocatalytic activity. SEM-EDX and TEM analyses showed the rough morphology, uniform distribution of dopants with spherical shape. XPES and XRF confirmed the incorporation of Ba and Cu into TiO2 lattice. Owing to the results of Ba/Cu co-doped samples, BCT3 (Ba-0.25% and Cu-0.75%) has shown the best photocatalytic activity by degrading carcinogenic food/textile dye Amaranth, within 50 ​min with optimum reaction parameters of pH ​= ​3, catalyst dosage of 0.1 ​g/L, and dye concentration of 10 ​mg/L under visible light irradiation. BCT3 showed efficient antibacterial activity when tested for Bacillus subtilis (gram ​+ ​ve) and Klebsiella pneumoniae (gram -ve).

Synergistically Augmented ZnO via Cobalt and Copper Simultaneous Doping for Pollutant Reduction

AbstractContinuous electron‐hole transfer and the ability of nanoscale materials to absorb visible light are critical for pollutant catalytic reduction. Cobalt and copper co‐doped ZnO composites were shaped by the sol‐gel solution combustion (SG‐SC) approach. The x‐ray diffraction analysis confirmed the crystalline nature and interstitial doping of copper and cobalt in the zinc oxide lattice and the development of ZnO‐CuO local contact. The PL and DRS‐UV‐vis investigations showed the NCs’ superior charge transfer and considerable redshift (from 3.17 to 2.21 eV) properties over ZnO. TEM and SEM‐EDS analyses confirmed the foam‐type surface morphology, elemental composition, and dopant distribution. The NCs showed greater potency than ZnO NPs in the catalytic reduction reactions of 4‐nitrophenol and methylene blue. Thus, the SG‐SC approach has the potential for industrially scalable catalytic pollutant reduction applications.

Enhancing light extraction efficiency of GaN LED by combining complex‐period photonic crystals with doping

AbstractThe light extraction efficiency (LEE) of GaN‐based light‐emitting diodes (LEDs) was limited by intense total internal reflection and the photothermal effect. In order to solve this problem, a method to synergistically control the LEE of GaN‐based LEDs by ​​combining the complex‐period photonic crystals (PhCs) with M (M = Al, In) material doping is proposed. The forbidden band width of two‐dimensional (2D) PhC array, three‐dimensional (3D) LED model, M doping, and electromagnetic field distribution are investigated respectively. By doping the M, the LED emission wavelength range is regulated to achieve the dual‐band emission. Furthermore, a triangular complex‐period photonic structure is introduced to establish stacked or etched PhCs models. By combining the plane wave expansion and finite difference time domain algorithm, the structural parameters of PhCs and M concentration dependent LEE are investigated, and the electromagnetic field distribution is explored also. The results show that the optimal LEEs can be achieved are 19.08% and 13.96% for blue light and ultraviolet, respectively, which are larger than that of traditional flat‐panel LED (4%). This work provides theoretical results and technical support for the design of LEDs with high luminous efficiency.

11.2: The Effect of Doping Distribution on the Property of Green Phosphorescent Organic Light‐Emitting Diodes

In mass production OLED structures, doping ratio distribution across the thickness of emitting layer is not uniform owing to the linear evaporating configuration, where dopant and host materials are added in different sources. In this study, we firstly confirmed that different types of doping distribution among emitting layer of green phosphorescent OLED exhibited over 15% difference of current efficiency. By carefully designing and comparing emitting layers with a series of doping distribution modes, we found that dramatic variation of doping ratio near electron blocking layer was the critical factor. Then, trapping behavior of dopant molecules was confirmed to affect the carrier distribution among the emitting layer according to analysis of current density‐voltage (JV) and capacitance‐voltage (CV) curves from carrier only devices based on two types of doping distribution modes. Combining with the result of hole‐electron recombination zone (RZ), the area where possessed both high doping ratio and excitons recombination probability exhibited a high current efficiency and low driving voltage. Finally, PL and EL transient decay lifetime combined with triplet‐polaron quenching (TPQ) efficiency measurement demonstrated that TPQ was the main factor causing current efficiency difference between two types of doping distribution modes. Therefore, the doping distribution and trapping property of dopant material in emission layer should be carefully evaluated for OLED devices for mass production.

Orbital Angular Momentum Mode-Multiplexed Amplification and Transmission Based on a Ring-Core Erbium-Doped Fiber

Recently, orbital angular momentum (OAM) multiplexing has attracted much attention for providing an exponential growth in optical fiber capacity. As an irreplaceable component to promote long-haul OAM mode fiber transmission system, the on-line OAM mode-multiplexed fiber amplifier need to be developed. In this paper, we propose a Gaussian-like erbium doping distribution design method for ring-core erbium doped fiber (RC-EDF) to optimize the differential modal gain (DMG), which can reduce the fabrication complexity of the fiber preform. With a good match with original design, the fabricated RC-EDF was characterized by all the supporting four OAM modes ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\vert l \vert$</tex-math></inline-formula> = 1, 2, 3, 4) multiplexed amplification, which achieves an ultra-low DMG less than 0.3 dB at 1550 nm. The gain is above 15 dB and DMG is less than 1.2 dB for four OAM modes across the C-band under a 500 mW pump power. As a proof for transmission, 32 channels (8 wavelengths × 4 OAM modes) carrying 32-Gbaud quadrature phase-shift keying (QPSK) OAM modes multiplexed-WDM transmission over the OAM-EDFA is demonstrated. The proposed OAM-EDFA with the average OSNR penalties of 0.34 dB may promote the practical implementation for the long-haul OAM modes fiber transmission system.