Dopant Pairs Research Articles

Synthesis of Lanthanide-Ion-Doped NaYF4 RGB Up-Conversion Nanoparticles for Anti-Counterfeiting Application

Well-defined and mono-dispersed lanthanide-ion-doped NaYF4 up-conversion nanoparticles (UCNPs) were synthesized via thermal decomposition using lanthanide oleate as the precursor. By rational selecting the dopant pairs of the doped lanthanide ions (Y3+, Yb3+, Er3+ and Tm3+) with accurate molar ratios, three-primary-color (RGB) UCNPs which exhibited green (UCNPs-G), blue (UCNPs-B) and red (UCNPs-R) fluorescence, respectively, were prepared. The X-ray diffraction (XRD) patterns showed that the three UCNPs were purely hexagonal-phase NaYF4 crystals. Transmission electron microscopy (TEM) images revealed that the synthesized UCNPs exhibited well-defined nanosphere morphology with uniform size distribution. The average diameters were 23.95±3.35 nm for UCNPs-G, 20.63±2.59 nm for UCNPs-B, and 19.24±2.37 nm for UCNPs-R, respectively. After surface modification employing polyacrylic acid (PAA) as modifier, the obtained UCNPs were converted to be hydrophilic, which can be used as fillers to construct luminescent polymer films and luminescent ink in anti-counterfeiting application.

Two-dimensional semiconductors pave the way towards dopant-based quantum computing.

Since the proposal in 1998 to build a quantum computer using dopants in silicon as qubits, much progress has been made in the nanofabrication of semiconductors and the control of charge and spins in single dopants. However, an important problem remains unsolved, namely the control over exchange interactions and tunneling between two donors, which presents a peculiar oscillatory behavior as the dopants relative positions vary at the scale of the lattice parameter. Such behavior is due to the valley degeneracy in the conduction band of silicon, and does not occur when the conduction-band edge is at k = 0. We investigate the possibility of circumventing this problem by using two-dimensional (2D) materials as hosts. Dopants in 2D systems are more tightly bound and potentially easier to position and manipulate. Moreover, many of them present the conduction band minimum at k = 0, thus no exchange or tunnel coupling oscillations. Considering the properties of currently available 2D semiconductor materials, we access the feasibility of such a proposal in terms of quantum manipulability of isolated dopants (for single qubit operations) and dopant pairs (for two-qubit operations). Our results indicate that a wide variety of 2D materials may perform at least as well as, and possibly better, than the currently studied bulk host materials for donor qubits.

Multicolor Luminescent Anti-Counterfeiting Barcode Based on Transparent Lanthanide-Doped NaYF4/Poly(Vinyl Alcohol) Nanocomposite with Tunable Full-Color Upconversion Emission

In this paper, we report the synthesis of transparent lanthanide-doped NaYF4 phosphors/poly(vinyl alcohol) (UCPs/PVA) nanocomposites with full-color upconversion emission for construction of multicolor luminescent anti-counterfeiting barcode. By deliberate design of water-dispersible lanthanidedoped NaYF4 upconversion phosphors and precise control of the doped lanthanide ions (dopant pairs and molar ratio), three-primary RGB UCPs with fine red, green and blue upconversion emission were synthesized. The as-prepared three-primary RGB UCPs were incorporated into poly(vinyl alcohol) (PVA) matrix at a fixed concentration to balance the transparence and fluorescence intensity of the UCPs/PVA nanocomposite. The combination of emissions from different types of UCPs with selected ratios in PVA matrix was used to prepare UCPs/PVA nanocomposite with various upconversion emissions. It was demonstrated that the separation distance between UCPs was large enough to suppress inter-particle energy transfer, and thus the emissions of each UCPs were well preserved. This facilitated the fabrication of UCPs/PVA nanocomposite with tunable full-color upconversion emission based on the trichromatic additive color theory. The relevance of these multicolor UCPs/PVA nanocomposites to barcode materials and application in anti-counterfeiting field were also confirmed.

DFT Insights into the Role of Relative Positions of Fe and N Dopants on the Structure and Properties of TiO₂.

The location and nature of the doped elements strongly affect the structural, electronic and optical properties of TiO2. To tailor the band structure and modify the photoelectrochemical properties of TiO2, a pair of dopants is selected. Fe and N atoms are inserted in the TiO2 network at substitutional and interstitial sites with different relative distances. The main objective behind the different locations and sites of the doped elements is to banish the isolated unoccupied states from the forbidden region that normally annihilates the photogenerated carriers. Fe at the Ti site and N at the O site doped in the TiO2 network separated at a distance of 7.805 Å provided a suitable configuration of dopant atoms in terms of geometry and band structure. Moreover, the optical properties showed a notable shift to the visible regime. Individual dopants either introduced isolated unoccupied states in the band gap or disturbed the fermi level and structural properties. Furthermore, the other co-doped configurations showed no remarkable band shift, as well as exhibiting a suitable band structure. Resultantly, comparing the band structure and optical properties, it is argued that Fe (at Ti) and N (at O) doped at a distance of 7.805 Å would strongly improve the photoelectrochemical properties of TiO2.

Role of electron filling in the magnetic anisotropy of monolayer WSe2 doped with 5d transition metals

Exploring magnetic anisotropy (MA) in single-atom-doped two-dimensional materials provides a viable ground for realizing information storage and processing at ultimate length scales. Herein, the MA of $5d$ transition-metal doped monolayer ${\mathrm{WSe}}_{2}$ is investigated by first-principles calculations. Large MA energy (MAE) is achieved in several doping systems. The direction of MA is determined by the dopant in-plane $d$ states in the vicinity of the Fermi level in line with previous studies. An occupation rule that the parity of the occupation number of the in-plane $d$ orbital of the dopant determines the preference between in-plane and out-of-plane anisotropy is found in this $5d$-doped system. Furthermore, this rule is understood by second-order perturbation theory and proved by charge-doping analysis. Considering relatively little research on two-dimensional MA and not sufficiently large MAE, suitable contact medium dopant pairs with large MAE and tunable MA pave the way to novel data storage paradigms.

Efficient Strategy for Enhancement of Visible Light Photocatalytic Activity of NaTaO3 by a Significant Extent

Among different photocatalysts developed so far for the generation of hydrogen through water splitting, NaTaO3 has been at the forefront due to its excellent stability and tunable electronic and optical properties. However, to extend its applicability to the range of visible light, the band gap has to be reduced significantly. In this study, we propose an efficient way to improve its visible light photocatalytic activity by doping with carbon in the presence of Cr, or Mo, or W. Although, monodoping with either C or Cr/Mo is able to reduce the band gap, the presence of localized defect states limits their applicability for photocatalytic purpose. This can be avoided by using dopant pair leading to formation of codoped systems. In the present study, two different types of codoped systems for each pair [(Cr, C), (Mo, C) and (W, C)] have been considered by varying the relative proportions of the dopant elements (1:1 and 2:1). Unfortunately, electronic structure analysis of 1:1 type of codoped systems show lim...

Band gap engineering and visible light response for GaS monolayer by isovalent anion-cation codoping

Two-dimensional (2D) materials have been paid great attention due to their unique properties and promising applications in photochemistry and photoelectricity. In this theoretical study, a systematic investigation has been carried out to explore the effect of several isovalent ionic dopants (O, Se, Te, Al, In, Sc and Y) on the electronic structure of GaS monolayer using the HSE06 functionals. Although monodoping with O reduces the band gap of GaS monolayer, its conduction band minimum (CBM) shifts downward below the hydrogen reduction potential, while the band gap remains almost unchanged for monodoping with Se, Te, Al and In dopants. However, the scenario changes on monodoping with Sc and Y dopants, which can effectively narrow the band gap because of shifting up for valence band maximum (VBM) and shifting down for CBM, respectively. In addition, (Sc, Te) and (Y, Te)-codoped GaS monolayers are also investigated. The calculation reveals that band gap is further narrowed for both codoped cases. The extent of band gap narrowing in the case of codoping with (Y, Te) is quite significant (almost 1 eV) to effectively improve the visible light activity of GaS monolayer. Moreover, both codoped systems are more suitable for overall water splitting than that of monodoping systems, which has been confirmed though band edge alignment with respect to water redox levels. However, upon (Sc, Te) codoping system, the CBM mainly stems from 3d orbital of Sc, which is extremely detrimental for improving solar-to-hydrogen conversion efficiency. Fortunately, (Y, Te)-codoped GaS monolayer shows better for water splitting than that of (Sc, Te)-codoping. The calculated optical absorption spectra confirm that (Y, Te)-codoped system has significantly improved visible light absorption. According to the defect formation energy calculation, the formation energies are all negative values except monodoping with In and Te systems, indicating dopants are possible introduced in the experiments. Thus, these findings imply that (Y, Te) is a suitable dopant pair for the controlled band gap engineering of GaS monolayer and (Y, Te)-codoped GaS monolayer should be a promising visible light photocatalyst for water splitting.

Enhancing the photoactivity of TiO2 by codoping with silver and molybdenum: the effect of dopant concentration on the photoelectrochemical properties

The concentration and nature of foreign atoms in titanium dioxide (TiO2) play a vital role in stabilizing the system as well as generating its optimal photocatalytic properties. A new pair of dopants is introduced to simultaneously improve the visible light harvesting and increase the separation between photoexcited carriers. Silver–molybdenum-codoped TiO2 with different doping concentrations was synthesized using a single-pot hydrothermal method. The synthesized samples showed a pure anatase phase with good crystallinity. Codoping shifted the absorption range of TiO2 towards the visible regime, and agglomerated particles with nearly spherical morphology were found. The existence of doped elements in TiO2 was verified by x-ray photoelectron spectroscopy and peaks relating to silver and molybdenum were detected. Based on their binding energy value, the peaks were attributed to their respective charge state and doping configuration. Among the synthesized samples, the 4% Ag–Mo-codoped TiO2 sample demonstrated the best visible light photodegradation response, attributed to the optimal doping concentration.

Theoretical study on geometric, electronic and catalytic performances of Fe dopant pairs in graphene.

The formation geometries, electronic structures and catalytic properties of monovacancy and divacancy graphene sheets with two embedded Fe dopants (2Fe-MG and 2Fe-DG) have been systematically investigated using the first-principles calculations. It was found that the configuration of 2Fe-DG is slightly more stable than that of 2Fe-MG sheets and the two doped Fe atoms in graphene (2Fe-graphene) as active sites could regulate the stability of gas molecules. In addition, the adsorption of O2 and CO molecules could modulate the electronic and magnetic properties of 2Fe-graphene systems. Moreover, the adsorption behaviors of reactants could determine the reaction pathway and energy barrier of the catalytic oxidation of CO. On the 2Fe-graphene substrates, the adsorptive decomposition of O2 molecules (<0.20 eV) and the subsequent Eley-Rideal (ER) reaction (2Oads + 2CO → CO2) (<0.60 eV) have low energy barriers. In comparison, the CO3 complex is quite stable and its formation needs to overcome a higher energy barrier (>0.90 eV). Hence, the dissociation of O2 as an initial step is an energetically more favored process. These results provide valuable guidance for the design of functionalized graphene-based devices.

Improving visible light photocatalytic activity of KTaO3 using cation-anion dopant pair

Increasing photocatalytic application of KTaO3 motivated us to find efficient strategy to reduce its band gap, so that it can better utilize the solar spectrum. In this study, the effect of (N, Mo/W) pair on the electronic structure of KTaO3 has been investigated using density functional theory. The experimental band gap of KTaO3 (3.6eV) has been successfully reproduced (3.61eV) by employing hybrid functional proposed by Heyd, Scuseria, and Ernzerhof. The present study reveals that the dopant elements individually can reduce the band gap either by introducing localized unoccupied states or occupied states in between valence band and conduction band. Unfortunately, these localized states in combination with charge compensating vacancy defects may reduce the photoconversion efficiency. In case of codoping with (N, Mo/W) pair total electrical charge neutrality is maintained. Therefore, vacancy promoted electron-hole recombination can be avoided. Moreover, the feasibility of N doping into crystal structure of KTaO3 increases in presence of (Mo/W) due to reduction in formation energy. Interestingly, in presence of both cationic (Mo/W) and anionic dopant (N) a clean band structure is produced, which is favorable for good photoconversion efficiency. The reduction in band gap due to codoping is quite significant for enhancement of absorption of visible light, and it is more prominent in the case of (Mo, N) pair. The final conclusion has been drawn by checking the relative positions of the band edges of both the codoped systems with respect to water redox potential. Thus present study explores the role of (Mo/W, N) pair in improving the visible light photocatalytic activity of KTaO3.

HyperSolar reaches new milestones and reveals next steps for its renewable hydrogen technology

Two-dimensional (2D) materials have been paid great attention due to their unique properties and promising applications in photochemistry and photoelectricity. In this theoretical study, a systematic investigation has been carried out to explore the effect of several isovalent ionic dopants (O, Se, Te, Al, In, Sc and Y) on the electronic structure of GaS monolayer using the HSE06 functionals. Although monodoping with O reduces the band gap of GaS monolayer, its conduction band minimum (CBM) shifts downward below the hydrogen reduction potential, while the band gap remains almost unchanged for monodoping with Se, Te, Al and In dopants. However, the scenario changes on monodoping with Sc and Y dopants, which can effectively narrow the band gap because of shifting up for valence band maximum (VBM) and shifting down for CBM, respectively. In addition, (Sc, Te) and (Y, Te)-codoped GaS monolayers are also investigated. The calculation reveals that band gap is further narrowed for both codoped cases. The extent of band gap narrowing in the case of codoping with (Y, Te) is quite significant (almost 1 eV) to effectively improve the visible light activity of GaS monolayer. Moreover, both codoped systems are more suitable for overall water splitting than that of monodoping systems, which has been confirmed though band edge alignment with respect to water redox levels. However, upon (Sc, Te) codoping system, the CBM mainly stems from 3d orbital of Sc, which is extremely detrimental for improving solar-to-hydrogen conversion efficiency. Fortunately, (Y, Te)-codoped GaS monolayer shows better for water splitting than that of (Sc, Te)-codoping. The calculated optical absorption spectra confirm that (Y, Te)-codoped system has significantly improved visible light absorption. According to the defect formation energy calculation, the formation energies are all negative values except monodoping with In and Te systems, indicating dopants are possible introduced in the experiments. Thus, these findings imply that (Y, Te) is a suitable dopant pair for the controlled band gap engineering of GaS monolayer and (Y, Te)-codoped GaS monolayer should be a promising visible light photocatalyst for water splitting.

Geometric, electronic and optical properties of zinc/tin codoped In2O3 modulated by the bixbyite/corundum phase transition

As transparent conducting oxides (TCOs), In2O3 in the high pressure phase attracts extensive research interests. Because physical properties are determined by the geometric structures, we investigate the electronic and optical properties of Zn/Sn codoped In2O3 materials (IZTO) being modulated by the bixbyite/corundum phase transition via Density Functional Theory calculations. For IZTO in high pressure phase, i.e. corundum phase, Sn/Zn dopant pair tends to form face-sharing ZnO6 and SnO6 octahedrons. The radius differences between Zn2+/Sn4+ dopants and In3+ host cations make Jahn-Teller effect occur and IZTO transform from bixbyite to corundum phase under a slight higher pressure than that of pure In2O3. Although Zn/Sn cosubstitution of In ions may increase the free carrier effective mass m* near the band edge, when IZTO crystal transforms to corundum phase, the more dense packing structure results in stronger cation s-orbital overlaps than in bixbyite phase, which makes m* recover to a smaller value. In addition, corundum IZTO has a larger indirect band gap and a high dopant solubility. So these investigations may open a new way to search for TCOs materials with low indium content.

Improving KNbO3 photocatalytic activity under visible light

An efficient strategy for improving KNbO3 photocatalytic activity under visible light is presented using N and W as dopant pair, which reduces the band gap mainly by elevation of the valence band maxima.

Improving visible light photocatalytic activity of NaNbO3: a DFT based investigation

The unique characteristics of the (W, N) dopant pair is that it reduces the band gap of NaNbO3 mainly by elevation of valence band maxima, which is very much crucial for photocatalyst for hydrogen evolution through water splitting.

Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids.

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts. The procedure relies on vibrational frequencies and excitation energies of single-ion active centers, and on differences between ion-ligand distances of the donor and the acceptor, which are attainable empirically or through ab initio calculations. The configuration coordinate diagrams of the Yb(2+)/Yb(3+) mixed-valence pair in Yb-doped YAG and the Ce(3+)/Yb(3+) heteronuclear pair in Ce,Yb-codoped YAG, are obtained and described. They are drawn from empirical data of the single-ions and their usefulness is discussed. The first diagram suggests that IVCT states of Yb(2+)/Yb(3+) pairs may play an important role in the quenching of the Yb(3+) emission and it provides the details of the quenching mechanism. The second diagram supports the interpretation recently given for the energy transfer from Ce(3+) to Yb(3+) in Ce,Yb-codoped YAG via a MMCT Ce(4+)-Yb(2+) state and it provides the details. The analyses of the two diagrams suggest the formation of Yb(2+)/Yb(3+) pairs after the Ce(3+)-to-Yb(3+) MMCT, which is responsible for the temperature quenching of the Yb(3+) emission excited via Ce(3+) (4f → 5d) absorption in Ce,Yb-codoped YAG.

Band gap engineering of NaTaO3using density functional theory: a charge compensated codoping strategy

In this theoretical study, we employ a codoping strategy to reduce the band gap of NaTaO3 aimed at improving the photocatalytic activity under visible light. The systematic study includes the effects of metal (W) and nonmetal (N) codoping on the electronic structure of NaTaO3 in comparison to the effect of individual dopants. The feasibility of the introduction of N into the NaTaO3 crystal structure is found to be enhanced in the presence of W, as indicated by the calculated formation energy. This codoping leads to formation of a charge compensated system, beneficial for the minimization of vacancy related defect formation. The electronic structure calculations have been carried out using a hybrid density functional for an accurate description of the proposed system. The introduction of W in place of Ta leads to the appearance of donor states below the conduction band, while N doping in place of oxygen introduces isolated acceptor states above the valence band. The codoping of N and W also passivates undesirable discrete midgap states. This feature is not observed in the case of (Cr, N) codoped NaTaO3 in spite of its charge compensated nature. We have also studied charge non-compensated codoping using several dopant pairs, including anion-anion and cation-anion pairs. However, this non-compensated codoping introduces localized states in between the valence band and the conduction band, and hence may not be effective in enhancing the photocatalytic properties of NaTaO3. The optical spectrum shows that the absorption curve for the (W, N)-codoped NaTaO3 is extended to the visible region due to narrowing of the band gap to 2.67 eV. Moreover, its activity for the photo decomposition of water to produce both H2 and O2 remains intact. Hence, based on the present investigation we can propose (W, N) codoped NaTaO3 as a promising photocatalyst for visible light driven water splitting.

Atomic structure and dynamics of metal dopant pairs in graphene.

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

A combined DFT + U and Monte Carlo study on rare earth doped ceria

We investigate the dopant distribution and its influence on the oxygen ion conductivity of ceria doped with rare earth oxides by combining density functional theory and Monte Carlo simulations. We calculate the association energies of dopant pairs, oxygen vacancy pairs and between dopant ions and oxygen vacancies by means of DFT + U including finite size corrections. The cation coordination numbers from ensuing Metropolis Monte Carlo simulations show remarkable agreement with experimental data. Combining Metropolis and Kinetic Monte Carlo simulations we find a distinct dependence of the ionic conductivity on the dopant distribution and predict long term degradation of electrolytes based on doped ceria.

Microstructure and thermal properties of inflight rare-earth doped thermal barriers prepared by suspension plasma spray

Rare-earth doped yttria-stabilized zirconia (YSZ) coatings with lower thermal conductivity have been fabricated via suspension plasma spray by dissolving rare-earth nitrates into YSZ powder-ethanol suspensions prior to plasma spraying. The effect of dopant concentration and dopant type on properties of the coatings was determined by comparing two coatings containing different concentrations of the same dopant pair (Nd 2 O 3 /Yb 2 O 3 ), and three coatings having similar concentrations of different dopant pairs (Nd 2 O 3 /Yb 2 O 3 , Nd 2 O 3 /Gd 2 O 3 , and Gd 2 O 3 /Yb 2 O 3 ). The porosity content of the coating was found to increase with increased total rare-earth dopant concentration but did not significantly change with dopant pairs. The cross-sectional morphology of every coating displayed a cauliflower-like structure. However, the most heavily doped coating exhibited a larger surface roughness and feathery features in the columnar structures. The thermal conductivity measurement showed that the thermal conductivity decreased with increased Nd 2 O 3 /Yb 2 O 3 concentration. Among coatings containing different dopant pairs, the Gd 2 O 3 /Yb 2 O 3 doped coating exhibited lowest conductivity.

First-principles study of Pt-film stability on doped graphene sheets

The stability of doped graphene sheets and a catalytic platinum (Pt) film on the doped sheets was investigated for a variety of dopants, using the first-principles calculations in the density functional theory. It was shown that the doping (dopant-substitution) energy increases as the differences of atomic radius and valency increase between the dopant atom and the host carbon atom. On the other hand, the adsorption energy of the Pt film becomes larger on doped graphene sheets, compared to that on undoped graphene sheet. We found a strong correlation between the adsorption and doping energies: the adsorption energy increases as the doping energy increases. This correlation occurs because the Pt-film adsorption partially releases hetero-valency-induced doping-energy loss through electron redistribution and atom-position change. We showed that cation–anion dopant pairs in the graphene sheet are also effective to stabilize adsorbed Pt films.