Uniform Dopant Distribution Research Articles

Control of the interfacial charge transfer resistance to improve the performance of quantum dot sensitized solar cells with highly electrocatalytic Cu-doped SnS counter electrodes

High charge transfer resistance at the interface between counter electrodes (CEs) and sulfide/polysulfide based electrolytes is responsible for the low photoelectrochemical performance of quantum dot sensitized solar cells (QDSSCs). Here, for the first time, we fabricated highly electrocatalytic and stable Cu-doped SnS CEs by inexpensive chemical bath deposition (CBD) method, to control the interfacial charge transfer resistance in QDSSCs and increase their photovoltaic performance. TiO2/CdS/CdSe/ZnS photoanodes and an aqueous polysulfide electrolyte were used to fabricate the QDSSCs with CEs. The QDSSCs with a 10% Cu-doped SnS CE achieved an outstanding conversion efficiency of 4.07% under standard simulated AM 1.5 illumination, exceeding that of a cell with an undoped SnS CE (2.80%). The surface morphologies of SnS CEs with different Cu contents were significantly different. The voids between SnS nanoparticles deposited on fluorine-doped tin oxide (FTO) substrate were reduced upon the uniform distribution of Cu dopants. The Cu-doped SnS CE had significantly improved electrocatalytic activity for catalysis during the reduction of electrolyte, low charge transfer resistance at the CE/electrolyte interface, and fast electron transport from SnS surface to the electrolyte to regenerate the redox couple. These findings may inspire the design of efficient CEs for the large-scale production of next-generation QDSSCs.

Dopant Segregation Boosting High-Voltage Cyclability of Layered Cathode for Sodium Ion Batteries.

As a widely used approach to modify a material's bulk properties, doping can effectively improve electrochemical properties and structural stability of various cathodes for rechargeable batteries, which usually empirically favors a uniform distribution of dopants. It is reported that dopant aggregation effectively boosts the cyclability of a Mg-doped P2-type layered cathode (Na0.67 Ni0.33 Mn0.67 O2 ). Experimental characterization and calculation consistently reveal that randomly distributed Mg dopants tend to segregate into the Na-layer during high-voltage cycling, leading to the formation of high-density precipitates. Intriguingly, such Mg-enriched precipitates, acting as 3D network pillars, can further enhance a material's mechanical strength, suppress cracking, and consequently benefit cyclability. This work not only deepens the understanding on dopant evolution but also offers a conceptually new approach by utilizing precipitation strengthening design to counter cracking related degradation and improve high-voltage cyclability of layered cathodes.

Axial segregation in crystal growth from a thin melt layer

During a crystal growth from a thin melt layer, the transport of dopant occurs mostly by diffusion. One can describe the dopant distribution by simplified analytical equations, as, for example, those previously obtained by Tiller.We performed numerical 2D axi-symmetric computations of heat and mass transfer in a thin melt layer (0.05<h/d<0.2), in order to find the limits of the validity of such approaches. Sb doped Ge, in a cylindrical volume with a diameter of 200 mm, was used as a model material. Compared with the heat transfer, the mass transfer was found to be much more sensitive to residual convection. Some restrictions in the application of the analytical equations for the description of the experimental data are revealed and necessary recommendations to improve the results are made. In order to provide the uniform axial dopant distribution in the very last portion of the ingot, too, it is necessary to vary the rate of solidification by changing the cooling rate with time.

Synthesis of Nickel‐Doped Ceria Catalysts for Selective Acetylene Hydrogenation

AbstractMetallic nickel is known to be an active, but not a selective hydrogenation catalyst for conversion of alkynes to alkenes. On the other hand, nickel oxide is not active. Recently, we have demonstrated that nickel doped into ceria provides an inexpensive catalyst for selective hydrogenation of acetylene in the presence of ethylene. Here, we evaluate various synthesis methods to achieve optimal selective hydrogenation performance. We examined incipient wetness impregnation, coprecipitation, solution combustion, and sol‐gel synthesis to study how the method of preparation affects catalytic structure and behavior. Sol‐gel synthesis, coprecipitation, and solution combustion synthesis methods favor nickel incorporation into the ceria lattice, while incipient wetness impregnation creates segregated nickel species on the ceria surface. For hydrogenation of acetylene, these nickel surface species lead to poor ethylene selectivity due to ethane and oligomer formation. However, when nickel is incorporated into the ceria lattice, ethane formation is prevented even while achieving 100 % conversion of acetylene. Coke formation is also significantly reduced on these catalysts compared to conventional nanoparticle counterparts. We conclude that sol‐gel synthesis provides the optimal method for creating a uniform dopant distribution within the high surface area ceria.

Development of the Growth Technology of Highly Homogeneous Semiconductor Crystals

Different technological solutions for growing GaSb(Te) and Ge(Ga) single crystals, making it possible to optimize the growth of highly homogeneous (at the microlevel) semiconductor crystals by the Bridgman method, are described. The possibility of implementing steady-state growth conditions (providing uniform dopant distribution over the crystal) has been experimentally confirmed.

Enriching the hydrogen storage capacity of carbon nanotube doped with polylithiated molecules

In a quest to find optimum materials for efficient storage of clean energy, we have performed first principles calculations to study the structural and energy storage properties of one-dimensional carbon nanotubes (CNTs) functionalized with polylithiated molecules (PLMs). Van der Waals corrected calculations disclosed that various PLMs like CLi, CLi2, CLi3, OLi, OLi2, OLi3, bind strongly to CNTs even at high doping concentrations ensuring a uniform distribution of dopants without forming clusters. Bader charge analysis reveals that each Li in all the PLMs attains a partial positive charge and transform into Li+ cations. This situation allows multiple H2 molecules adsorbed with each Li+ through the polarization of incident H2 molecules via electrostatic and van der Waals type of interaction. With a maximum doping concentration, that is 3CLi2/3CLi3 and 3OLi2/3OLi3 a maximum of 36 H2 molecules could be adsorbed that corresponds to a reasonably high H2 storage capacity with the adsorption energies in the range of −0.33 to −0.15 eV/H2. This suits the ambient condition applications.

Sulfur-doped porous graphene frameworks as an efficient metal-free electrocatalyst for oxygen reduction reaction

We developed sulfur-doped porous graphene frameworks as a highly efficient metal-free electrocatalyst for oxygen reduction reaction (ORR) which was prepared through MgO template directed approach by in situ sulfur doping based on the pyrolysis of p-methylbenzenesulfonic acid. The as-obtained electrocatalyst features distinctive porous graphene frameworks and uniform distribution of sulfur dopants with highly active sites. When employed for ORR, such catalyst shows pronounced catalytic activity, improved stability and better resistance toward methanol crossover than those of the commercial Pt/C, and a nearly four-electron transfer number in alkaline media.

Electronic and magnetic properties of Co doped MoS2 monolayer.

First principle calculations are employed to calculate the electronic and magnetic properties of Co doped MoS2 by considering a variety of defects including all the possible defect complexes. The results indicate that pristine MoS2 is nonmagnetic. The materials with the existence of S vacancy or Mo vacancy alone are non-magnetic either. Further calculation demonstrates that Co substitution at Mo site leads to spin polarized state. Two substitutional CoMo defects tend to cluster and result in the non-magnetic behaviour. However, the existence of Mo vacancies leads to uniform distribution of Co dopants and it is energy favourable with ferromagnetic coupling, resulting in an intrinsic diluted magnetic semiconductor.

Strong and Broad Visible Emission of Bismuth Doped Nano-Phase Separated Yttria-alumina-silica Optical Fibers

Background: Bismuth doped optical fibers (BDFs) have potential application in the area of broadband VIS and NIR luminescence in the range from 350-700 nm and 1100 to 1600 nm without presence of any rare-earth elements. Such fibers are studied intensively and used successfully in fiber amplification and lasing particularly in NIR region where no emission from any rare-earths are obtained. They have found that proper modeling for spectral behavior of Bi doped glasses and fibers, is still not available due to unpredictable oxidation states of Bi. Therefore there is scope for detail study for designing and fabricating BDFs with well organized spectral characteristics due to its long interaction length. Methods: Bi doped nano-phase separated yttria-alumino-silica (YAS) glass optical fibers are fabricated using modified chemical vapour deposition (MCVD) associated with Sloution doping (SD) technique. The micro-structures of the core of perform and fiber samples are analyzed by TEM along with electron diffraction (ED) using thin film sampling technique. The concentration distributions of the dopants in core are analysed by EPMA. Core refractive index (RI) profile is generated using preform analyzer model: PKL 2600 of Photon Kinetics. Using the measured RI of the core numerical aperture (NA) of preforms is calculated. The absorption spectra of BDFs are measured by cutback method at room temperature to avoid external interference. Photoluminescence spectra (PL) of BDFs in VIS region are obtained by FLS920 spectrometer (Edinburg Instrument) at room temperature using ~30 cm long BDFs at 532 nm excitation of 184 mW. Results: TEM images and ED patterns suggest that the core glasses are composed of amorphous nano-phase separated particles of size around 2-3 nm which are the Bi rich zones in the core. EPMA data show a uniform distribution of dopants within core. Absorption bands for bismuth active centres (BACs) appear mainly at ~515 nm, ~690 nm along with weak bands at ~800 nm, ~960nm and ~1200 nm and observe an overlapping zone within 400-1150 nm regime. Using Gaussian multi-peak fit technique we deconvolute them. The deconvoluted absorption bands are ~480, ~525, ~565, ~675, ~810, ~970 and ~1200 nm. The presence of multiple absorbance suggest the presence of multiple oxidation state of bismuth basically Bi0, Bi+, Bi2+ in the doped core glass. A schematic energy diagram is proposed based on the deconvoluted absorption bands. Using this diagram, the emission band, within 520 to 840 nm under 532 nm excitation, is explained. Conclusion: Fabrication process of nano phase separated Bi-YAS optical fibers through MCVD assisted with SD technique is discussed. Phase separation of core glass is analyzed by TEM. Almost uniform dopant distribution in the core obtained through EPMA analysis. Different absorption bands of Bi active centres at VIS to NIR have been identified using suitable Gaussian multi-peaks fitting technique. The analyses confirm the presence of multiple oxidation states of Bi into the fabricated BDFs. Intense broad PL from 520 nm to 840 nm are observed under 532 nm pumping. This unique feature would be helpful for fabricating wavelength tunable broadband fiber laser source from VIS to NIR region. Keywords: Absorption spectra, Bi doped fibers, nanophase separated core glass, visible fluorescence.

Thermally-Induced Dopant Segregation Effects on the Space Charge Layer and Ionic Conductivity of Nanocrystalline Gadolinia-Doped Ceria

Grain boundaries are known to block ionic conduction across grain boundaries in oxide ion conductors due to adjacent space charge layers. Since the positively charged grain boundary core is intensified with a high local concentration of defects such as oxygen vacancies, uniform distribution of a dopant may mitigate the formation of space charge layers and enhance the ionic conductivity. To investigate the dopant segregation effect on the space charge layer and ionic conductivity, we provided thermal energy to nanocrystalline gadolinia-doped ceria (GDC) thin film by post-annealing at different temperatures of 700°C, 900°C, and 1100°C. STEM-EELS analysis demonstrates strong dopant segregation and a higher Ce3+ content near the grain boundary than in the bulk after post-annealing. The concurrent segregation of dopants and Ce3+ ions implies that once thermal treatment is applied to nanocrystalline GDC thin films, complete space charge layers are formed while the non-thermally treated GDC film with a uniform distribution of dopants has less of a space charge effect and exhibits superior ionic conductivity.

Controlling the surface chemistry of cerium oxide nanoparticles for biological applications.

The catalytic activity of cerium oxide nanoparticles (CNPs) depends on the surface Ce3+/Ce4+ oxidation state. CNPs with a higher Ce3+ to Ce4+ ratio, oxygen vacancies and higher superoxide dismutase (SOD) mimetic activity are more effective against diseases associated with oxidative stress or inflammation. CNPs with a lower Ce3+/Ce4+ ratio show higher catalase mimetic activity and possess anticancer/antibacterial activity. However, different synthesis methods of CNPs and capping agents/surface coatings result in various Ce3+/Ce4+ oxidation states, thus limiting the use of particular CNPs for specific biological applications. In this study, we have shown that by selecting an appropriate doping method we can control the surface Ce3+/Ce4+ oxidation state to tune the catalytic activity and biological response. Importantly, superior SOD mimetic activity and efficient reactive oxygen species scavenging capability of one-step synthesized CNPs are linked to a uniform distribution of dopants in the CNP lattice and changes in the surface Ce3+/Ce4+ oxidation state.

Ion Transport and Surface Kinetics in Oxygen Ion Conducting Nanocrystalline Oxide Thin Films

Nanocrystalline oxide thin films, e.g., doped oxides of zirconia and ceria films, exhibit extraordinary electrical and electrochemical properties that are of great interest for applications such as solid oxide fuel cells (SOFCs). In nanocrystalline doped oxide thin films fabricated by atomic layer deposition (ALD) or pulsed laser deposition (PLD), interfacial properties are often found to dominate the electrochemical properties. For example, grain boundary plays a crucial role in ionic transport and surface exchange reactions. Ionic transport across grain-boundaries(GBs) is known to be hindered due to an electrical potential barrier built up by defect segregation, while exchange reaction is enhanced near the GBs due to the higher concentration of oxide ion vacancies. Such exotic electrochemical properties of oxide thin films are closely related to the distribution of defects near GBs. Along these lines, we investigated the defect structure near GBs in doped oxide thin films as well as the surface exchange and the ionic transport inside the films associated with grain size and the grain boundary density. We report on the atomic scale study of the defect structure near a single GB of yttria-stabilized zirconia (YSZ) [1]. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width of 0.6 nm, which corroborates well with electron energy loss spectroscopy (EELS) measurements. Additionally, we recently conducted near-atomic-scale EELS study on gadolinia-doped ceria (GDC) thin films [2]: Homogeneous distribution of dopants was observed in as-deposited films, while strong segregation of dopant was shown in the annealed (1100°C) film a width of 1.2 nm to 1.5 nm and an enhancement factor of 1.9. It has been reported that grain boundaries at the electrode(cathode)/electrolyte interface are of great importance in enhancing the oxygen reduction kinetics in SOFCs. Oxide-ion incorporation is greatly facilitated by these surface grain boundaries [3]. Thin-film electrolytes with high surface grain-boundary density consistently show higher exchange current density, and therefore, smaller activation loss. These experimental observations are supported by secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectroscopy (EIS) studies on doped zirconia and doped ceria thin films [3,4]. Polycrystalline YSZ and YDC were annealed in an 18O2 environment so that the exchange reaction between 16O2 inside YSZ and YDC and 18O in the environment can take place. Surface mapping using NanoSIMS clearly indicates the preferential enrichment of 18O along grain boundaries. The activation resistance at the cathode interface is significantly reduced by an electrolyte surface featuring a high density of grain boundaries that facilitates high surface exchange rates. In parallel, surfaces modified by compositionally graded cation doping layers of ~1 nm facilitate enhanced oxide-ion vacancy concentration and improve the power density of the thin-film SOFC by lowering the activation loss [5]. It is known that grain boundaries consisting of grain boundary core and the adjacent space charge layer pose a blocking effect on cross-boundary transport of ions. We show that thermal annealing, which in turn facilitates defect segregation and redistribution, plays an important role on the ionic conductivity of oxide thin films [2,6]. Nanocrystalline thin films of YDC and GDC were fabricated and then post-annealed for grain growth. Interestingly, as the grain size increases, ionic transport is indeed hindered, showing higher activation energy and lower ionic conductivity than the as-deposited films, which have the smallest grains. We speculate that the uniform distribution of dopants may attenuate the space charge layer barrier height and maintain high ionic conductivity, while the annealed films have fully developed space charge layer, which impedes ionic transport. In summary, the grain size and the grain boundary effects have a huge impact on surface exchange and ionic transport. We show that fundamental understanding and surface engineering of grain structures in oxide thin films may have significant implications in improving the electrochemical performance of thin film SOFCs, and therefore, widening their applications. References J. An, J. S. Park, A. L. Koh, H. B. Lee, H. J. Jung, J. Schoonman, R. Sinclair, T. M. Gür, F. B. Prinz, Sci. Rep., 3, 2680 (2013)J. Bae et al., in-preparation J. H. Shim, J. S. Park, T. P. Holme, K. Crabb, W. Lee, Y. B. Kim, X. Tian, T. M. Gür, F. B. Prinz, Acta Mater., 60, 1-7 (2012)Y. B. Kim, J. S. Park, T. M. Gür, F. B. Prinz, J. Power Sources, 196, 10550-10555 (2011)C. C. Chao, Y. B. Kim, F. B. Prinz, Nano Lett., 9, 3626-3628 (2009)J. An, J. Bae, S. Hong, B. Koo, Y. B. Kim, T. M. Gür, F. B. Prinz, Scr. Mater., 104, 45-48 (2015)

Preparation of Y-Doped BaZrO&lt;sub&gt;3&lt;/sub&gt; Proton Conducting Solid Electrolyte via Modified Low Temperature Pechini Method

One of the promising material for proton-conducting solid electrolyte operating at intermediate temperature range (400-600 °C) is the Yttrium-doped BaZrO3 (BZY) due to its high conductivity and chemical stability. In this study, a modified citrate-nitrate combustion method (Pechini method) has been employed for BZY powder preparation. A stoichiometric amounts of starting nitrates and oxide raw materials with nitric acid, citric acid and ethylene glycol for the synthesis of 20 mol% Y-doped BaZrO3 (BZY20) were prepared, then calcined and sintered at 1000 °C for two heat treatment durations of 24 hours and 48 hours. The obtained BZY20 powder samples have been fully characterized for its structure, morphology, and thermal properties. From the X-ray diffraction (XRD) results, the sample sintered for 48 hours showed a cubic phase of BZY20 which can be indexed to a Pm3m cubic structure which is also supported by Raman analysis. The calculated lattice parameter is 4.2067 Å which is higher than the reported lattice parameter of a pure BaZrO3 (BZ) of 4.1930 Å which indicates a successful doping due to higher ionic radius of Y3+ dopant as compared to Zr4+ in the B-site ABO3 perovskite sub-lattice. In addition, SEM-EDX analyses of the sintered pellet revealed a uniform distribution of Yttrium dopant in the BZY20 prepared solid electrolyte.

Enhancement of energy storage capacity of Mg functionalized silicene and silicane under external strain

The electronic structure, stability, and hydrogen storage capacity of strain induced Mg functionalized silicene (SiMg) and silicane (SiHMg) monolayers have been studied by means of van der Waals induced first principles calculations. A drastic increase in the binding energy of Mg adatoms on both the monolayers under the biaxial symmetric strain of 10% ensures the uniform distribution of dopants over the substrates. A significant positive charge on each Mg accumulates a maximum of six H2 molecules with H2 storage capacity of 8.10% and 7.95% in case of SiMg and SiHMg, respectively. The average adsorption energy for H2 molecules has been found ideal for practical H2 storage materials.

High power laser fiber fabricated through vapor phase doping of Ytterbium

We present the characteristics of an ytterbium-doped alumino-silicate fiber, fabricated through vapor phase doping of aluminum and ytterbium in the core, along with silica and in conjunction with the modified chemical vapor deposition process. The vapor phase doping of rare-earths provides the opportunity to fabricate large core active fibers with a uniform distribution of dopants. The fibers fabricated exhibited low OH− content, negligible center dip and good optical properties. Lasing performance was tested up to output power of 105 W, with a slope efficiency of 77% with respect to launched pump power. The linear variation of the laser power with a pump shows its potentiality for further power scaling.

Sulfur- and Nitrogen-Doped, Ferrocene-Derived Mesoporous Carbons with Efficient Electrochemical Reduction of Oxygen

Development of inexpensive and sustainable cathode catalysts that can efficiently catalyze the oxygen reduction reaction (ORR) is of significance in practical application of fuel cells. Herein we report the synthesis of sulfur and nitrogen dual-doped, ordered mesoporous carbon (SN-OMCs), which shows outstanding ORR electrocatalytic properties. The material was synthesized from a surface-templating process of ferrocene within the channel walls of SBA-15 mesoporous silica by carbonization, followed by in situ heteroatomic doping with sulfur- and nitrogen-containing vapors. After etching away the metal and silica template, the resulting material features distinctive bimodal mesoporous carbon frameworks with high nitrogen Brunauer-Emmett-Teller specific surface area (of up to ∼1100 m(2)/g) and uniform distribution of sulfur and nitrogen dopants. When employed as a noble-metal-free electrocatalyst for the ORR, such SN-OMC shows a remarkable electrocatalytic activity; improved durability and better resistance toward methanol crossover in oxygen reduction can be observed. More importantly, it performs a low onset voltage and an efficient nearly complete four-electron ORR process very similar to the observations in commercial 20 wt % Pt/C catalyst. In addition, we also found that the textural mesostructure of the catalyst has superseded the chemically bonded dopants to be the key factor in controlling the ORR performance.

Numerical Simulation on the Heat Transfer Performance of Multicrystalline Silicon in Directional Solidification Process

The temperature distribution has important influence on the position and shape of solid-liquid interface during directional solidification process. So the calculation of temperature field is fairly significant for both structural analysis and temperature control. In this paper, the finite element method is applied to establish the 2D axisymmetric model for modeling the temperature distribution and the solid-liquid interface shape of multicrystalline silicon in semi-industrial directional solidification furnace. The numerical results show that the temperature field and solid-liquid interface shape can be controlled by adjusting the pulling rate in directional solidification process, and an optimized pulling rate of this system was obtained for large diameter silicon crystals with low defect density and uniform dopant distribution.

Numerical Simulation on the Heat Transfer Performance of Multicrystalline Silicon in Directional Solidification Process

The temperature distribution has important influence on the position and shape of solid-liquid interface during directional solidification process. So the calculation of temperature field is fairly significant for both structural analysis and temperature control. In this paper, the finite element method is applied to establish the 2D axisymmetric model for modeling the temperature distribution and the solid-liquid interface shape of multicrystalline silicon in semi-industrial directional solidification furnace. The numerical results show that the temperature field and solid-liquid interface shape can be controlled by adjusting the pulling rate in directional solidification process, and an optimized pulling rate of this system was obtained for large diameter silicon crystals with low defect density and uniform dopant distribution.

Fluorescent polystyrene photonic crystals self-assembled with water-soluble conjugated polyrotaxanes

We demonstrate control of the photoluminescence spectra and decay rates of water-soluble green-emitting conjugated polyrotaxanes by incorporating them in polystyrene opals with a stop-band spectrally tuned on the rotaxane emission (405–650 nm). We observe a suppression of the luminescence within the photonic stop-band and a corresponding enhancement of the high-energy edge (405–447 nm). Time-resolved measurements reveal a wavelength-dependent modification of the emission lifetime, which is shortened at the high-energy edge (by ∼11%, in the range 405–447 nm), but elongated within the stop-band (by ∼13%, in the range 448–482 nm). We assign both effects to the modification of the density of photonic states induced by the photonic crystal band structure. We propose the growth of fluorescent composite photonic crystals from blends of “solvent-compatible” non-covalently bonded nanosphere-polymer systems as a general method for achieving a uniform distribution of polymeric dopants in three-dimensional self-assembling photonic structures.

Growth of Oxide Laser Crystals by Czochralski Method

The growth of series of actual laser crystals belonging to di erent structural types by the Czochralski method is presented. The primary attention is given to single crystalline compounds and their solid solutions with garnet structure (scandium-containing rare-earth garnets with general formula {LnSc}3[ScMe]2Me3O12, Me = Al, Ga) as well as with olivine (forsterite Mg2SiO4) and scheelite structure (double tungstates and molybdates with formula NaLn(TO4)2, where T = Mo or W). The broad bands of luminescence and absorption produced by activator ions in these laser hosts that appear either as a result of structural disorder (garnets, scheelites) or due to the doping by transition metal ions (chromium in forsterite) join together these representatives of di erent structural classes and determine their application in photonics. The paper considers the problems of growing of large crystals with uniform distribution of components and dopants, determination of congruently melting composition, especially in compounds with complex isomorphism. The mutual in uence of components and dopants, redistribution of ions along crystallographically non-equivalent sites in the structure, alteration of valency of transition metal ions accompanied with the change of their segregation coe cients under variation of redox conditions is discussed.