Li Doping Concentration Research Articles

Theoretical Study of the Reactive Mechanisms of Li-Doped Ni-Based Oxygen Carrier during Chemical Looping Combustion.

Ni-based oxygen carriers (OCs) are considered promising materials in the chemical looping combustion (CLC) process. However, the reactivity of Ni-based OCs still offers the potential for further enhancement. In this work, the Li doping method has been employed for the modification of Ni-based OCs. The reactivity and microreaction mechanisms of different concentrations of Li-doped Ni-based OCs with CO in CLC are clarified using density functional theory (DFT) simulation. The structures, energy, and density of states are obtained through computational investigation of the reaction path in elementary reactions. The results show that (1) the adsorption energies of CO molecules on NiO surfaces with 4, 8, and 12% Li doping concentrations are -0.53, -0.48, and -0.54 eV, respectively, demonstrating an enhanced reactivity compared to that of pure NiO (-0.41 eV); (2) the calculation of the transition state indicates that the most favorable pathway for CO oxidation takes place on the surface of NiO with an 8% Li doping concentration, exhibiting the lowest energy barrier of 0.51 eV; and (3) the oxygen vacancy formation energies on the surface of NiO are 3.05, 2.30, and 2.10 eV for 4, 8, and 12% doping concentrations, respectively. Additionally, the decrease in oxygen vacancy formation energies exhibits a gradual decline with an increasing Li doping concentration. By comprehensive analysis, 8% is considered to be the optimal doping concentration of NiO for chemical looping combustion.

Investigation of lithium (Li) doping on the resistive switching property of p-Li:NiO/n-β-Ga2O3 thin-film based heterojunction devices

Li-doped NiO/β-Ga2O3 polycrystalline bilayer thin-film pn-heterojunctions with different Li-doping concentrations are grown on Si-substrates using the pulsed laser deposition technique. Resistive switching property of these devices has been investigated in detail. This study shows that the Li-doping concentration in NiO layer significantly influences the performance of these devices. For an optimum Li-doping of 1.5%, a stable memory window of ∼102 with endurance of more than 100 cycles and long retention time can be achieved. The coefficient of variation (Cv) of SET and RESET voltages also found to ∼ 20% and ∼ 40%, respectively, satisfying the acceptability benchmark. A transition from complementary resistive switching (CRS) to bipolar resistive switching (BRS) after multiple sweeping operations has been observed in devices with intermediate Li-doping concentrations. Observation of CRS has been explained in terms of the formation of Li-rich metallic layer at the NiO/Ga2O3 interface as a result of out-diffusion of Li. Redistribution of the Li-ions from the Li-rich interfacial zone to whole of the NiO layer after first few sweeping cycles must be the reason for CRS-to-BRS transition. Results further suggest that return to high resistive state via Poole–Frenkel (PF) pathway during the RESET process might be the key to achieve high performance in p–n junction based resistive switching devices. This study, thus, presents Li-doping as a possible route to modulate the resistive switching property of bilayer Li:NiO/Ga2O3 based memory devices.

Improvement of Photoelectrochemical Properties of CuO Photoelectrode by Li Doping

We fabricated a Li doped CuO photoelectrode by doping CuO with Li to improve the photoelectrochemical properties of the CuO photoelectrode. The fabricated Li doped CuO photoelectrode was optimized by experimentally investigating Li doping concentration, annealing temperature, and spin coating deposition cycle. It was confirmed that Li doped CuO had increased light absorption, decreased energy band gap, and improved crystallinity. The Li-doped CuO photoelectrode had a porous surface, unlike the bare CuO photoelectrode, and had a low charge transfer resistance as well as a high flat band potential. The Li doping concentration experiment demonstrated that the 2 at% Li doped CuO photoelectrode had a superior photocurrent density value compared with a bare CuO photoelectrode. In the annealing temperature optimization experiment with a 2 at% Li doped CuO photoelectrode, it was found to have the best photocurrent density value at 500 oC. In experiments with various spin coating deposition cycles of the Li-doped CuO photoelectrode, the light absorption, energy bandgap, crystallinity, and electrical properties were affected by changes in the film thickness of the photoelectrode. In particular, we confirmed that a sample deposited with 4 spin coating cycles had the lowest interfacial resistance between the photoelectrode and the electrolyte, and the highest flat-band potential value. Consequently, we were able to obtain an improved photocurrent density of -1.68 mA/cm<sup>2</sup> compared to the bare CuO photoelectrode using the Li-doped CuO photoelectrode under the optimized conditions of Li 2 at%, an annealing temperature of 500 oC, and 4 cycles of spin coating depositions.

Characterization of Some Physical and Photocatalytic Properties of CuO Nanofilms Synthesized by a Gentle Chemical Technique

Pure and Li-doped CuO nanofilms were synthesized on heated glass substrates using the spray-pyrolysis technique. The deposited pure CuO nanofilms were achieved at a precursor molarity of 0.2 M using a solution prepared from copper nitrate trihydrate (Cu(NO3)2·3H2O). Doped Li–CuO nanofilms were obtained using several doping concentrations (3, 6, 9, 12 and 15%) by adding a solution prepared from lithium nitrate (LiNO3). The pure and Li–CuO samples were investigated by different techniques. XRD revealed three dominant peaks (-111), (111) and (211), which are the properties of monoclinic CuO. The increase in Li-doping concentration showed the appearance of other peaks of low intensities detected at 2θ ranging from 49 to 68°. AFM images showed a textured and inhomogeneous surface composed of spherical grains whose size decreased with increasing Li doping. UV–visible spectroscopy showed that the CuO samples were of low transparency; the transmittance was less than 50%. The band-gap energy determined from Tauc’s equation plot increased from 2.157 to 3.728 eV with the increase in Li doping. These values correspond well to the band gap of semiconducting CuO. The photocatalytic properties were accelerated by Li doping, as revealed by the discoloration of aqueous methylene-blue (MB) solution under ultraviolet irradiation.

Deposition and characterization of lithium doped direct current magnetron sputtered [formula omitted] films

Lithium doped cuprous oxide (Cu2O:Li) films were deposited on quartz substrates by direct current magnetron reactive co-sputtering of copper and Cu:Li targets. X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), Rutherford backscattering spectrometry, UV-VIS transmittance, and room temperature Hall measurements have been conducted to characterize the deposited films. SIMS revealed Li concentrations in the range 2×1018−5×1020cm−3 in the doped films. XRD confirms phase pure Cu2O for all doping concentrations. The doping concentration correlates with an increased free carrier density found from Hall effect measurements. The highest Li doping concentration results in low resistivity (4Ωcm) p-type Cu2O with acceptor concentrations up to 2×1017cm−3.

Li-Doping Effect on Characteristics of ZnO Thin Films Resistive Random Access Memory.

In this study, a Pt/Ag/LZO/Pt resistive random access memory (RRAM), doped by different Li-doping concentrations was designed and fabricated by using a magnetron sputtering method. To determine how the Li-doping concentration affects the crystal lattice structure in the composite ZnO thin films, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) tests were carried out. The resistive switching behaviors of the resulting Pt/Ag/LZO/Pt devices, with different Li-doping contents, were studied under direct current (DC) and pulse voltages. The experimental results showed that compared with the devices doped with Li-8% and -10%, the ZnO based RRAM device doped by 5% Li-doping presented stable bipolar resistive switching behaviors with DC voltage, including a low switching voltage (<1.0 V), a high endurance (>103 cycles), long retention time (>104 s), and a large resistive switching window. In addition, quick switching between a high-resistance state (HRS) and a low-resistance state (LRS) was achieved at a pulse voltage. To investigate the resistive switching mechanism of the device, a conduction model was installed based on Ag conducting filament transmission. The study of the resulting Pt/Ag/LZO/Pt devices makes it possible to further improve the performance of RRAM devices.

Low-frequency noise in high performance and stability of Li-doped ZnO thin-film transistors

In this work, thin film transistors (TFTs) based on the ZnO-doped with different concentrations of Li were fabricated by radio frequency sputtering. Using the optimal Li doping concentration of 1% and rationally designed passivation layer (PVL), a TFT with high mobility of 28.5 cm2/Vs, low SS of 0.28 V decade−1, high Ion/Ioff of 107 and small Vth of 0.8 V were obtained. Moreover, the 1% Li-doped ZnO TFT with Al2O3 PVL exhibits the best stability with the small Vth shifts of 1.1 (− 1.2) V and 1.6 (− 1.8) V under gate bias and light illumination test. Low-frequency noise and x-ray photo-electron spectroscopy band structure analysis suggests that the enhanced properties and stability are mostly due to the Li doping concentration. Because the Li doping not only increased the carrier concentration, but also reduced the oxygen vacancy defects and interface trap density. In addition, the high quality Al2O3 PVL reduced the photo-adsorption/desorption behavior, and protected the channel from environmental influences. Overall, the reliable and high performance Li-doped ZnO TFTs have a great potential in flat-panel display application.

Enhancing the mechanoluminescence of traditional ZnS:Mn phosphors via Li+ Co-doping

Mechanoluminescent materials that could convert mechanical energy into light energy have attracted increasing attention in recent years for their wide potential applications in stress sensing and imaging, structural damage monitoring, wearable illuminating devices, self-powered display, mechanical energy collection and conversion. However, their relatively low mechanoluminescence intensity greatly hinders the practical application. Here, we present a simple and effective strategy to enhance the mechanoluminescence performance of ZnS:Mn phosphor by Li ion co-doping. ZnS:Mn,Li phosphors of the hexagonal wurtzite structure have been prepared via high temperature solid phase sintering. The PL and ML intensity have been enhanced by 338% and 610% for samples obtained when 4 and 5 at. % Li is introduced in the precursor materials, respectively, in comparison with those without Li dopant. The photoluminescence lifetime of ZnS:Mn,Li decreases gradually, indicating the enhanced non-radiative energy transfer as the Li concentration increases. The electron paramagnetic resonance spectroscopy is carried out to investigate the changing tendency of the sulfur vacancy of samples with various Li-doping concentration. The mechanism for mechanoluminescence enhancement is supposed to be correlated with the increased sulfur vacancies by Li co-doping, which may facilitates the non-radiative energy transfer and thereby improve the luminescence properties of the samples. These findings provide a feasible strategy to enhance the mechanoluminescence performance of traditional ZnS derived phosphors, and may contribute to developing novel mechanoluminescence materials for practical applications.

Theoretical and experimental investigation of structural and optical properties of lithium doped cadmium oxide thin films

In this work, undoped CdO and Lithium doped CdO (Li:CdO) with 2% and 6% concentrations thin films were deposited using spray pyrolysis technique. Structural, morphological and optical properties of the deposited films were studied by x-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and UV–vis spectroscopy. The results and the density functional theory (DFT) showed that CdO have cubic structure. The exchange-correlation functions were indicated with generalized gradient approximation (GGA) by DFT. Experimental optical results showed that transmittance was decreased from 79% (undoped CdO) to 30% (CdO doped with 6% Li); and the theoretical results showed that it decreased from 81% (undoped CdO) to 31.55% (CdO doped with 6% Li). Similarly, energy gap (Eg) was decreased from 2.574 eV to 2.367 eV; while for the theoretical results it decreased from 2.611 eV to 2.390 eV with increasing Li doping concentration. Theoretical and experimental data were closely compatible.

Doping effect investigation of Li-doped CdS thin films

ABSTRACTLi-doped CdS (CdS:Li) thin films with 0, 1, 2 and 3 wt-% Li concentrations were prepared by the spray technique using a perfume atomiser. The effect of Li doping concentration on the structural, optical, electrical and magnetic properties was systematically investigated and the results are reported in this paper. Analysis of the microstructure indicates that all the films consist of a hexagonal CdS phase with a strong (0 0 2) preferential growth texture. Nanoneedles were evinced from the SEM images of the doped films. Optical studies reveal that the transparency and band gap were dependent on Li-doping content. Hall effect measurements reveal that all the doped films exhibit p-type conductivity and the resistivity values strongly depend on the occupancy of Li atoms on the substitutional and interstitial sites of the CdS lattice. Room temperature ferromagnetism was observed for the doped films.

Characterization and performance analysis of Li-doped ZnO nanowire as a nano-sensor and nano-energy harvesting element

A complete investigation of the effect of Li doping on the physical, material, electromechanical and piezoelectric properties of ZnO nanowires (NWs) is presented. Low temperature hydrothermal growth technique is used to grow vertically aligned crystalline ZnO NWs doped with different concentrations of Li. Characterization techniques reveal considerable physical, material and electromechanical property modifications of the ZnO NWs due to the incorporation of Li dopants. Atomic Force Microscope is utilized to apply controlled amount of force on the fabricated NWs to assess their piezoelectric response. More than twenty two-fold improvement is observed in sensitivity due to the combined effect of modifications in NW geometry and piezoelectric properties with the addition of Li. Finite element method simulations were performed to decouple the individual effect of Li doping on the NW size and on the piezoelectric coefficient and to see how much each effect plays a role in the sensitivity improvement. It is estimated that the changes in the material and electromechanical properties alone are responsible for more than seven-fold improvement in the sensitivity. The impact of ‘kick-out’ diffusion mechanism of Li in ZnO is one of the major factors responsible behind this sensitivity improvement. It is also observed that there is an optimum level of Li doping concentration which can lead to the best piezoelectric performance by the ZnO NWs. The novelty of this work lies on the detailed analyses to illustrate the physics and impact of Li dopants in ZnO NW structures from a piezoelectric point of view towards improving their application as a nano-sensing and nano-energy harvesting element.

Fabrication of Fast-response Ultraviolet Light Sensors with LZO Thin Films using Sol-gel Spin-coating Method

The Li-doped ZnO (LZO) thin films with various Li concentrations (0, 1, 3, 5, 7, and 9 at.%) were deposited on quartz substrates by sol-gel spin-coating method. The effects of Li doping on the structural and optical properties of the thin films were investigated. Undoped ZnO thin film and LZO thin films had three different diffraction peaks at 31.5 °, 34.2 °, and 36.0 °corresponding to the ZnO (100), (002), and (101) planes, respectively. The c-axis orientation of the LZO thin films increased with increasing Li doping concentration, owing to the high diffusivity of the Li dopant. A broad red emission was observed by photoluminescence of the undoped ZnO thin film, whereas LZO thin films had broad green emissions. The transmittance and optical band gap blue-shifted with increasing Li doping concentration. In terms of photoresponse, all LZO thin films had greater rise and decay rates than the undoped ZnO thin film and the photosensitivity increased with increasing Li doping concentration.

Effects of Li doping on the structural and electrical properties of solution-processed ZnO films for high-performance thin-film transistors

We report the structural and electrical properties of Li-doped ZnO (LZO) films, which are spin-coated via a simple solution process using Zn and Li precursor solutions. The Li doping improves the crystallinity of the ZnO films, and the LZO films exhibit a relatively dense and smooth surface morphology compared to that of the undoped ZnO films. Back-gated LZO thin-film transistors (TFTs) are fabricated onto a heavily p-doped Si substrate with a SiO2 gate dielectric, and operated in enhancement mode. In the case of the LZO TFT with a Li doping concentration of 0.50 mol%, the highest field-effect mobility of 5.18 cm2 V−1 s−1 is obtained; the on/off current ratio and subthreshold swing are 107 and 1.2 V dec−1, respectively. These results demonstrate that Li doping can effectively modulate the structural and electrical properties of ZnO-based films, facilitating the fabrication of LZO TFTs with enhanced performance.

Effect of Li-doping on low temperature solution-processed indium–zinc oxide thin film transistors

Lithium (Li)-doped indium zinc oxide (IZO) thin film transistors (TFTs) were fabricated on solution-processed zirconium oxide gate dielectrics using a low temperature all solution process. Li-doping in IZO thin films led to higher crystallinity, even at process temperature lower than 300°C, and to the formation of favorable oxidation states of metal ions. The results were confirmed by electrical property analysis of the Li-doped IZO TFTs. For Li content varied from 0 to 16.6 at.%, the highest field-effect mobility, on/off current ratio, subthreshold slope and stress bias stability were obtained for Li-doping concentration of 9.0mol%.

Doping effect investigation of Li-doped nanostructured ZnO thin films prepared by sol–gel process

In this work, we report the effects of Li doping concentration (5, 10 and 15 at.%) on structural, morphological, electrical and optical properties of Li doped ZnO films. We used X-ray diffraction (XRD), scanning electronic microscopy (SEM), Hall-Effect measurements, UV–Vis–NIR and photoluminescence (PL) spectroscopies. XRD patterns showed that the deposited films had a hexagonal-wurtzite crystal structure with preferred orientation along the c-axis. SEM micrographs indicated that grain dispersion in the films were homogeneous. Hall-effect measurements revealed that ZnO thin films doped with 5, 10 and 15 at.% of Li have p-type conduction. The UV–Vis–NIR spectroscopy analyses demonstrated that all thin films were highly transparent in the visible region and the optical transmittance was dependent on the Li doping content. Room temperature PL spectra put into evidence that all Li-doped ZnO thin films emitted a UV-blue light and the emission intensities were found to be affected by Li concentration.

Microstructural, optical and ethanol sensing properties of sprayed Li-doped Mn3O4 thin films

Lithium-doped manganese oxide thin films have been deposited onto glass substrate by the spray pyrolysis technique. The effect on structural, morphological, optical and ethanol sensitivity properties of the films with different Li concentrations was investigated. The X-ray diffraction (XRD) studies of Li doped Mn3O4 thin films revealed polycrystalline tetragonal structure with lattice parameters: a=5.73Å and c=9.46Å. The most intense band at 653.26cm−1 observed in room temperature Raman spectrum of all films corresponds to the tetragonal crystal structure of Mn3O4. Optical band gap energy was found to vary in the range of 2.23–2.82eV by an increase in Li-doping concentration up to 15%. The resistance based gas sensing measurements showed a clear response already at a low ethanol concentration of 0.1%. The films showed promising ethanol sensing behavior; especially the sample doped with 3% of lithium exhibits the most excellent sensing performance.

Lithium-doped PEO—a prospective solid electrolyte with high ionic conductivity, developed using n-Butyllithium in hexane as dopant

The present work is an effort to study the effects of Li doping on the structural and transport properties of the solid polymer electrolyte, poly-ethelene oxide (PEO) (molecular weight, 200,000). Li-doped PEO was synthesized by treating PEO with n-Butyllithium in hexane for different doping concentrations. It is seen that the crystallinity of the doped PEO decreases on increasing the Li doping concentration and XRD and FTIR studies support this observation. FESEM images give better details of surface morphology of doped PEO samples. The TGA curves of PEO and Li-doped PEO samples reveal the weight loss region, and it is observed that the weight loss process of the solid polymer electrolyte is gradual rather than abrupt, contrary to the case of liquid electrolytes. The purity and the electrochemical stability of the samples were established by cyclic voltammetry studies. Impedance measurements were carried out to estimate the ionic conductivity of Li-doped PEO samples. The present value of ionic conductivity observed at room temperature in Li-doped PEO is about five orders higher than that of pure PEO and is quite close to that of liquid electrolytes. It is inferred that, ionic conductivity of the sample is increasing on increasing the Li doping concentration due to enhanced charge carrier density and flexibility of the doped sample structure. The ionic mobility and ionic transport are significantly improved by the less crystallinity and higher flexibility of the Li-doped PEO samples which in turn are responsible for the enhanced ionic conductivity observed.

Ferromagnetism in alkali-metal-doped AlP: An ab initio study

By using first-principle calculations within the generalized gradient approximation, we investigated the magnetic and electronic properties of Group IA alkali-metal (Li, Na, K)-doped AlP. The main purpose of this study is to determine the effect that different types of alkali dopants have on the magnetic behavior of AlP. The magnetic moment induced by the Li dopant is negligible in (2×2×2) Li-doped AlP supercell. However, it is also found that the magnetism is affected by the Li doping concentration. The Na and K dopants in AlP induce a net magnetic moment of 2.0μB. Half-metallic characteristics are achieved in K-doped AlP. The calculated density of states and the spin density distribution of Na- and K-doped AlPs show that the magnetism derives from the p–p interaction between the dopants and the neighboring anions. However, the spontaneous polarization cannot occur in Na-doped AlP. The K-doped AlP has the AFM ground state which is the unstable at room temperature. The results reveal that the Na- and K-doped AlPs are unlikely to be potential DMSs for the development of new spintronic devices.

Enhancing blue luminescence from Ce-doped ZnO nanophosphor by Li doping.

Undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO nanophosphors were prepared by a sol-gel process. The effects of the additional doping with Li ions on the crystal structure, particle morphology, and luminescence properties of Ce-doped ZnO were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and photoluminescence spectroscopy. The results indicate that the obtained samples are single phase, and a nanorod shaped morphology is observed for (Li, Ce)-codoping. Under excitation with 325 nm light, Ce-doped ZnO phosphors show an ultraviolet emission, a green emission, and a blue emission caused by Zn interstitials. The spectrum of the sample codoped with a proper Li concentration features two additional emissions that can be attributed to the Ce3+ ions. With the increase of the Li doping concentration, the Ce3+ blue luminescence of (Li, Ce)-codoped ZnO is obviously enhanced, which results not only from the increase of the Ce3+ ion concentration itself but also from the energy transfer from the ZnO host material to the Ce3+ ions. This enhancement reaches a maximum at a Li content of 0.02, and then decreases sharply due to the concentration quench. These nanophosphors may promise for application to the visible-light-emitting devices.PACS78.55.Et; 81.07.Wx; 81.20.Fw

Electric and magnetic behaviors observed in NiO-based thin films under light-irradiation

We report the room-temperature ferromagnetic properties that can be tuned by light irradiation in the Li and Mn co-doped NiO films (LMNO) grown by the spinning coating. The optical tunable magnetic behavior is enhanced by the increase of the Li doping concentration. First-principle calculations reveal that the Li doping plays key roles in the optical tuned magnetic behavior, which brings a 3d-like impurity state to enhance a significant hybridization between the Mn3+ 3d state and the impurity band, thus strengthening the ferromagnetic coupling effects. Additionally, it can tune the band gap of the LMNO films and produce more holes under the light irradiation, enhancing the optical tuned magnetic behavior.