Single-doped System Research Articles

First-principles calculation of P-type Mg3Sb2 thermoelectric performance modification by Ge and Si doping

Mg3Sb2 has been considered a highly promising thermoelectric material for mid-temperature applications. Optimizing the properties of the material is crucial for accelerating its commercial use. In this work, first-principles molecular simulations of P-type Mg3Sb2 doped with the carbon group elements Ge and Si have been carried out. Results indicate that doping with Ge and Si enhances the thermodynamic stability and electrical conductivity of the material. This improvement is achieved by decreasing the bandgap, increasing the local and peak density of states, flattening the band structure, and elevating the relative mass of carriers. Additionally, doping with Ge and Si decreases the phonon velocity and Debye temperature, which weakens the thermal transport properties of the material. These findings suggest that Ge and Si doping is an effective method for improving the thermoelectric properties of the material. At the same doping concentration, the Si single-doped system possesses the smallest bandgap value with the highest peak density of states and forms an indirect bandgap, leading to the best electrical transport properties; the Ge single-doped system has the lowest phonon velocity and Debye temperature, which has the most significant effect in attenuating the thermal transport properties of the material; and the Ge–Si co-doped system has the highest relative mass of carriers, which is conducive to the enhancement of Seebeck coefficient. The results offer theoretical guidance for experimentally analyzing the effects of Ge and Si doping on the thermoelectric properties of Mg3Sb2.

Engineering a novel role for Yb3+ to overcome thermal quenching and improve photothermal conversion efficiency as well as temperature sensing performance

In this paper, two types of phosphors, Cs3GdGe3O9:Er3+ and Cs3GdGe3O9:Yb3+/Er3+, were synthesized using the high-temperature solid-state method, and a new role for Yb3+ was successfully achieved. The presence of low phonon energy in the matrix and the low energy level mismatch between the 2F5/2 and the 4I11/2 enables the construction of a phonon-assisted channel between Yb3+ and Er3+ through doping with Yb3+ ions. This channel overcomes thermal quenching and achieves thermal enhancement at high temperatures. Additionally, Yb3+ increases the chance of non-radiative transition within the matrix, effectively converting the 980 nm laser into heat energy. This leads to a significant change in the surface temperature of the sample, greatly improving the photothermal conversion efficiency. Moreover, Yb3+ greatly enhances the emission intensity of green light, resulting in a maximum relative sensitivity of 1.11 % K−1 for temperature measurements in the Er3+ single-doped system and 1.28 % K−1 in the Yb3+/Er3+ co-doped system, based on the Er3+ thermally coupled energy level. These results demonstrate that this paper provides a new idea for materials to overcome thermal quenching and improve the efficiency of photothermal conversion and temperature sensing performance.

Effect of 4d transition metals on the electronic and magnetic properties of twin graphene

To explore new uses for twin graphene (TG) in spintronic devices, we tuned the electronic properties and introduced a magnetic moment for TG by doping it with 4d transition metal (TM = Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, and Cd) atoms. Some single TM doped TG systems have spin-polarized states, and their magnetic moments can be divided into three categories. TG doped with single Y, Mo, and Ag exhibits dilute magnetic semiconductor (DMS) properties, and the doping of Tc introduces a half-metallic character. For dual-doped systems, the electronic properties of TG can be effectively regulated by the doping positions of various TM atoms. The α-2Mo/TG, α-2Tc/TG, β-2Y/TG, and γ-2Y/TG systems exhibit DMS behavior. Some dual-doped systems exhibit antiferromagnetic (AFM) and ferromagnetic (FM) behaviors. For β configurations, the electronic properties of TM atoms dual-doped on the same and different sides of TG are similar to those of the corresponding single-doped systems, except for the β/D-2Zr/TG and β/D-2Ag/TG systems. The dual-doping of Y and Rh atoms on the same and different sides of TG can realize the switch between AFM and FM. The results of this study might provide new inspiration for applying TG in spintronic and nanomagnet devices.

Enhanced linear and nonlinear optical response of superhalogen (Al7) doped graphitic carbon nitride (g-C3N4)

• Effect of Al 7 on g-C 3 N 4 is investigated for the second- and third-order NLO polarizabilities. • Doping reduces the band gap by about 50 % in the single doped system. Similarly, linear optical properties increase from 55 to 108 %. • The single doped system exhibited the first and second hyperpolarizability of about 39 and 5 times greater than the pure g-C 3 N 4 . In this study, pure and superhalogen doped g-C 3 N 4 clusters are computed with the CAM-B3LYP method of DFT to investigate the electronic and optical properties. Single Al 7 @g-C 3 N 4 and double doped 2Al 7 @g-C 3 N 4 systems are investigated. The doping of the superhalogen cluster on g-C 3 N 4 especially in a single doped system significantly improves the charge transfer properties by reducing the band gap energies from 6.42 eV to 3.23 eV. The average and total first hyperpolarizabilities ( β o and β total ) remarkably increase from 82 to 3204 au and 136–5340 au, whereas average second hyperpolarizability ( γ o ) increases from 14 × 10 4 to 64 × 10 4 au due to their lower transition energies. The selected systems were also compared with the external reference molecule p -NA which is a well-known reference molecule for NLO applications. TD-DFT simulations reveal that among pure, single, and double doped systems, the only single doped system shows significantly better outcomes including interaction energies, vertical ionization potential, HOMO-LUMO energies, and UV–vis absorption. The single doped system shows high absorption in the far-red region that use in lasers. The entitled systems especially single doped systems have the potential to use them as an efficient NLO material.

Effect of Cu F Co-doping on the Properties of AgSnO2 Contact.

The crystal structures, mechanical properties, and electrical properties of Cu doped SnO2, F doped SnO2, and Cu F co-doped SnO2 were studied by using the first-principles method. Meanwhile, AgSnO2, AgSnO2-F, AgSnO2-Cu, and AgSnO2-Cu-F contacts were prepared by using the sol-gel method for a series of experiments to verify the theoretical analysis. According to the XRD patterns, the doping does not change the structure of SnO2, but increases its lattice constant and volume. Compared with the single-doped system, the doping formation energy of Cu F co-doped system is the smallest and the structure is more stable. Among the three groups of doping systems, the Cu F co-doped system has the highest shear modulus, Young’s modulus, hardness, and Debye temperature, and its mechanical properties and wear resistance are relatively best, and the melting point is also the highest. Cu F co-doping can further narrow the band gap of SnO2, reduce the electron effective mass and donor ionization energy, increase the electron mobility, and further enhance the conductivity of SnO2. The wetting angle of SnO2-Cu-F sample with Ag liquid is 1.15°, which indicates that Cu and F co-doping can significantly improve the wettability of SnO2 and Ag liquid. AgSnO2-Cu-F contact has a hardness of 82.03 HV, an electrical conductivity of 31.20 mS⋅m−1, and a contact resistance of 1.048 mΩ. Cu F co-doping can improve the shortcomings of AgSnO2 contact properties.

Tunable emission with efficient energy transfer in Na2SrSi2O6:Ce3+, Tb3+ phosphor for near-UV LED

Two series of Na2SrSi2O6:Ce3+ and Na2SrSi2O6:Ce3+, Tb3+ phosphors were synthesized via high-temperature solid-state reaction. The obtained Na2SrSi2O6:Ce3+ phosphors exhibit an intense excitation peak at 284 and 360 nm, which was suited to the dominant emission band of a NUV light-emitting-diode (LED) chip. In the Ce3+ single doped system, the critical quench concentration of the phosphor was approximately 4 mol% and a dipole-dipole interaction was verified for the concentration quench mechanism. In addition, Tb3+ ions were introduced as a sensitizer, and the effects of introducing the Tb3+ ions are investigated by measuring the photoluminescence emission and excitation spectra, the PL decay curves, and the quantum efficiencies. Results confirm the occurrence of non-radiative energy transfer from Ce3+ to Tb3+ with an efficiency of over 60%, indicating that the developed phosphor can be used as a potential white-light-emitting phosphor for near-UV LED.

Electronic and optical properties of n-pr co-doped anatase TiO<sub>2</sub> from first-principles

ZnO is a wide bandgap semiconductor with the advantages of good stability, strong radiation resistance, and low cost. It has become a hot material in the field of photocatalysis, but it can only absorb purple light. Therefore, it is a valuable problem to study how to expand the response range of ZnO to visible light. Doping modification is a common method to solve this problem. In order to carry out the relevant research, the calculation in this paper are carried out by the CASTEP tool in Materials Studio software based on the first-principles of ultrasoft pseudopotential of density functional theory, the geometric structures of ZnO, Zn<sub>0.875</sub>Pr<sub>0.125</sub>O, ZnO<sub>0.875</sub>N<sub>0.125</sub>, Zn<sub>0.875</sub>Pr<sub>0.125</sub>O<sub>0.875</sub>N<sub>0.125</sub>, Zn<sub>0.75</sub>Pr<sub>0.25</sub>O<sub>0.875</sub>N<sub>0.125</sub>, Zn<sub>0.625</sub>Pr<sub>0.375</sub>O<sub>0.875</sub>N<sub>0.125</sub> are constructed. All the models are based on the optimization of the geometry structure. By using the method of generalized gradient approximation plus U, we calculate the band structure, density of states, population, absorption spectra and dielectric functions of the models. The results show Co-doped system is easier to form than single-doped system, and the stability of the co-doped system increases first and then decreases with the increase of Pr concentration. The population ratio of the shortest Zn-O bond to the longest Zn-O bond in the same system increases first and then decreases with the impurity concentration, which shows that the doping of impurities has a great influence on the lattice distortion of the system, and the distortion is benefit for the separation of photogenerated hole-electron pairs. Therefore, the photocatalytic activity of the materials can be improved. Hybridization of N-2p and Pr-4f states destroys the integrity of crystals and forms crystal fields around impurity atoms, which results in splitting of energy levels and narrowing of bandgap. Compared with intrinsic ZnO, the static dielectric constant of all doped systems increases, especially the constant of Pr-N co-doped systems increases with the increase of doped Pr concentration, which indicates that the polarization ability of the co-doped systems increases with the increase of doped Pr atomic concentration. The main peaks of the dielectric function imaginary part of the doping systems move to the low energy region, and the absorption spectrums are red-shifted. As the concentration of impurity Pr atom increases, in the visible region, the absorption capacity of each co-doped system increases, their response range is enlarged in turn, showing the co-doping of N and Pr is benefit for improving the photocatalytic activity of ZnO.

Electronic Structure and Optical Absorption Spectra of C–Cr Co‐Doped Anatase TiO2 Based on First Principles

The geometrical structure, defect formation energies, electrical and optical properties of C and/or Cr doped anatase TiO2 are calculated by GGA + U method under the framework of density functional theory. The relationship between band structure and light absorption is revealed. The results show that the lattice is obviously distorted after doping, and the stability of C and Cr co‐doping is higher than just Cr doping, second only to Cr doped TiO2. The contribution of C is forming impurity energy levels, and the contribution of Cr is to reduce the band gap in the single doped system. In the co‐doped system, Cr atoms are used as donor impurities and C atoms act as acceptor impurities. The impurity energy is composed of C 2p and Cr 3d orbital hybridization, which reduces the band gap and promotes the separation of the impurity levels. C and Cr co‐doping expands the light absorption of TiO2 to the entire visible region, thereby increasing the absorption coefficient and effectively enhancing the photocatalytic efficiency.

Enhanced infrared-to-visible up-conversion emission and temperature sensitivity in (Er3+,Yb3+, and W6+) tri-doped Bi4Ti3O12 ferroelectric oxide

Strong up conversion (UC) luminescence at 527, 550, and 662 nm is compared under an excitation of 980 nm in single doped (Er3+), co-doped (Er3+/Yb3+), and (Er3+/Yb3+/W6+) tri-doped bismuth titanate (Bi4Ti3O12). For the co-doped system, the frequency (UC) emission intensity due to Er3+ ions is enhanced significantly in the green bands due to the efficient energy transfer from Yb3+ to Er3+ ions. Further increase in the emission intensity is seen with non-luminescent W6+ ions in the tri-doped system due to the modification in the local crystal field around the Er3+ ions, and is evidenced through a gradual change in the crystal structure of the host lattice with increasing W6+ content. The observed changes in the fluorescence lifetime and the associated energy transfer mechanisms are discussed. A progressive reduction of the lifetime of the 4S3/2 levels of Er3+ ions from 72 to 58.7 μs with the introduction of Yb3+ and W6+ dopant increases the transition probability and enhances the UC emission intensity. The efficiency of the energy transfer process (η) in the co-doped and tri-doped systems is found to be 9.4% and 18.6%, respectively, in comparison to the single doped system. Temperature sensing based on the fluorescence intensity ratio (FR) technique shows high sensitivity (0.0123 K−1) in the high temperature range (293 to 523 K) for an optimum content of Er3+, Yb3+, and W6+ with x = 0.03, y = 0.18, and z = 0.06 at. % in the tri-doped Bi4−x−yErxYbyTi3−zWzO12 ferroelectric composition, and is found useful for potential applications in optical thermometry.

First-Principles Investigation of Energetics and Electronic Structures of Ni and Sc Co-Doped MgH<sub>2</sub>

First-principles calculations were used to study the energetics and electronic structures of Ni and Sc co-doped MgH2 system. The preferential positions for dopants were determined by the minimal total electronic energy. The results of formation enthalpy indicate that Ni and Sc co-doped MgH2 system is more stable than Ni single-doped system. The hydrogen desorption enthalpies of these two hydrides are investigated. Ni and Sc co-doping can improve the dehydrogenation properties of MgH2. The lowest hydrogen desorption enthalpy of 0.30 eV appears in co-doped system, which is significantly lower than that of Ni doping. The electronic structure analysis illustrates that the hybridization of dopants with Mg and H atom together weakens the Mg-H interaction. And the Mg-H bonds are more susceptible to dissociate by Ni and Sc co-doping because of the reduced magnitude of Mg-H hybridization peaks. These behaviors effectively improve the dehydrogenation properties of Ni and Sc co-doped cases.

New insight into the enhanced visible-light photocatalytic activities of B-, C- and B/C-doped anatase TiO2 by first-principles

The geometry structures, formation energies and electronic properties of the B-, C- and B/C-doped anatase TiO2 were investigated by the density functional theory (DFT) calculations of first-principles. The results indicated that the visible-light absorption and photocatalytic activities of the B-, C- and B/C-doped anatase TiO2 were not only influenced by the energy gaps (Eg) and the distributions of impurity states, but also affected by the locations of Fermi levels (EF) and the energies of the edges of band gaps (Ev for the top of valence bands and Ec for the bottom of conduction bands). However, the above four factors changed with the doped models of TiO2. The impurity states in the band gaps reduced the maximum energy gaps in the band gaps, which is responsible for the absorption of visible light. The Fermi levels at the bottom of conduction bands indicated the existence of Ti(3+) ions, which enhanced the separation rates of photogenerated electrons and holes. Further, the energies of the edges of band gaps, determining the dominant types of oxidants (O2(-), hole, ˙OH) in the photocatalytic degradation, were discussed. Moreover, the stability of the doped TiO2 depended on its growth conditions (O-rich or Ti-rich environment). The O-rich growth condition is beneficial to the substitutional B and C atoms to Ti atoms, while the Ti-rich growth condition is favorable to the other doped TiO2 including the most stable co-doped TiO2 with the interstitial B atom and the substitutional C atom to O atom. In addition, our results also showed that the B/C-doped TiO2 inherited the partial electronic properties of single-doped TiO2, but also exhibited many new electronic properties, implying that the electronic properties of co-doped systems are not a mechanical mixture of those of both single-doped systems.

Structural and superconducting properties of co-doped YBa2−xLaxCu3−xMxOz and La-free YBa2Cu3−xMxOz (M = Al, Zn) high-TC superconductors

Two co-doped high-Tc superconducting systems, YBa2−xLaxCu3−xAlxOz and YBa2−xLaxCu3−xZnxOz (0⩽x⩽0.3), both of which have not been reported up to the present, were synthesized. The structural and superconducting properties have been investigated by X-ray diffraction (XRD) and DC magnetization measurement. Comparing the properties of these co-doped systems with single-doped systems YBa2Cu3−xAlxOz and YBa2Cu3−xZnxOz, it shows that in the Al-single-doped YBCO system, the depression of the critical temperature (Tc) with doping is stronger than that in (La, Al)-co-doped system, however, in the Zn-single-doped system, the Tc descends slower than that in (La, Zn)-co-doped system. This is possibly due to the opposite change of the distance between the Ba site and the CuO2 plane induced by the La doping. Besides, the La doping has another effect of improving the solid solubility compared with the Al- or Zn-single-doped system.

Preparation and luminescence properties of Ca3(VO4)2: Eu3+, Sm3+ phosphor for light-emitting diodes

Rare-earth ions co-activated red phosphors Ca 3 (VO 4 ) 2 : Eu 3+ , Sm 3+ were synthesized by modified solid-state reactions. The samples were characterized by X-ray powder diffractometer (XRD), energy-dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM) and luminescence spectrometer (LS). The results showed that the Eu–Sm system exhibits higher emission intensity than those of the Eu single-doped system and Sm separate-doped system under blue light. Samarium (III) ions are effective in broadening and strengthening absorptions around 467 nm. Furthermore, they exhibit enhanced luminescence emission. Luminescent measurements showed that the phosphors can be efficiently excited by ultraviolet (UV) to visible region, emitting a red light with a peak wavelength of 616 nm. The material has potential application as a phosphor for light-emitting diodes (LEDs).

Luminescence properties of Eu 3+ and Sm 3+ coactivated Gd(III) tungstate phosphor for light-emitting diodes

Rare-earth ions coactivated red phosphors Gd 0.2RE 1.8(WO 4) 3 (RE=Eu 3+ and Sm 3+) were synthesized by conventional solid-state reaction using boric acid as a flux agent. The samples were characterized by X-ray diffractometer (XRD), energy-dispersive X-ray spectrometer (EDS) and luminescence spectrometer (LS). The results showed that the Eu–Sm system exhibits higher emission intensity than those of the Eu single-doped system and Sm separate-doped system under ultraviolet (UV) radiation. Samarium(III) ions are effective in broadening and strengthened absorptions around 400 nm. Furthermore, it exhibits enhanced luminescence emission. when the mole ratio of boric acid is about 0.16, the luminescence capability is optimum. Two strongest lines at ultraviolet (394 nm) and blue (465 nm) in excitation spectra of these phosphors match well with the output wavelengths of UV and blue GaN-based light-emitting diodes (LEDs) chips.

Numerical analysis of intrinsic bistability and chromatic switching in Tm3+ single-doped systems under photon avalanche pumping scheme

In this paper, we predict and numerically demonstrate the intrinsic intensity bistability, spectra bistability and chromatic switching of visible-infrared emission in Tm3+ single-doped systems that are pumped by the photon avalanche scheme at 648 nm. Based on the coupled rate equation theory, the evolutions of the populations at various Tm3+ energy levels, emission spectra and fluorescence intensity versus pump excitation are numerically investigated in detail. The results show that intrinsic optical bistability (IOB) associated with emission spectra and luminescence intensity takes place in the vicinity of the avalanche threshold (∼10 kW cm−2). When the pump excitation rises above the switching threshold (∼17.5 kW cm−2), the chromatic switching between the infrared (1716 nm) and the visible blue (452/469 nm) spectra can be performed. Moreover, the influences of system parameters on IOB and the origin of chromatic switching are discussed. These unique characteristics of Tm3+-doped systems would lead to the new possibility of the development of pump-controlled all-solid-state luminescence switches and optical bistability switches.

The suppression of charge ordering in [formula omitted] and [formula omitted] systems

Two series of samples, single-doped Pr 0.5 Ca 0.5 Mn 1 - x Ti x O 3 ( x = 0 , 0.05 , 0.1 , 0.2 ) and double-doped Pr 0.5 - 0.5 x Ca 0.5 + 0.5 x Mn 1 - x Ti x O 3 ( x = 0.05 , 0.1 , 0.2 ) with fixed Mn 3 + : Mn 4 + ratio as 1:1, are prepared and investigated experimentally. The comparison of the destroying process of the charge ordering/orbital ordering (CO/OO) in the two systems is studied by the magnetization and electron spin resonance measurement. The collapse of CO/OO with Ti doping is less prominent in the double-doped system than in the single-doped system. The results indicate that the destroying process of the CO/OO is relative to the variety degree of the Mn 3 + : Mn 4 + ratio.

Impurity resistivity of the double-donor system Si:P,Bi

The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. @S0163-1829~99!11747-8#

Energy Transfer in Anthracene-and Tetracene-Doped Naphthalene Crystals

Energy transfer in thin crystals of anthracene-doped naphthalene and tetracene-doped naphthalene was investigated by monitoring the time evolution of the fluorescence intensities. The observed time dependence of the energy transfer cannot be explained by the theory of exciton diffusion generally invoked to explain energy transfer in molecular crystals. The time dependence is consistent with the predictions of long-range resonance energy transfer, but an anomalously large interaction strength is needed to fit the data. The magnitude of this interaction strength is the same for both types of impurity molecules despite the fact that there is a significant difference in the overlap of their absorption spectra with the host emission spectrum. Similar measurements were made on energy transfer from anthracene molecules to tetracene molecules in a naphthalene host. In this case, the mechanism for energy transfer is expected to be long-range resonance interaction, and it is found that the data can be fitted with the predictions of this theory using the theoretically derived value for the strength of the interaction. Thus the anomalies observed in the single-doped systems are associated only with host-sensitized energy transfer.