Critical Doping Level Research Articles

Comprehensive performance analysis of perovskite solar cells based on different crystalline structures of MAPbI3

Perovskite solar cells (PSCs) have shown high optical absorption and consequently provide high conversion efficiency with stable performance. In our work, CH3NH3PbI3 (MAPbI3) as an absorber layer is analyzed for different crystalline structures. Cubic, tetragonal, and orthorhombic phases of perovskite material are investigated to check the impact of the crystalline structure on the solar cell performance. Both density of states and band structure are studied using Quantum-ESPRESSO package depending on density functional theory. Then, all relevant parameters were employed in SCAPS software and comprehensive study was done for examining the effect of the crystalline structure of perovskite layer on the solar cell performance. In-depth, analyses were conducted to evaluate key parameters, including open circuit voltage (Voc), short circuit current (Isc), fill factor (FF), and power conversion efficiency (PCE) considering the variations of perovskite layer thickness and bulk defect densities. The obtained results indicate that cells with cubic MAPbI3, which shows a notably higher bandgap of 1.7 eV and an enhanced optical absorption coefficient, especially in the higher wavelength range (around 105 cm−1), show better performance for almost all three scenarios. Cubic MAPbI3 cells achieve relatively higher peak efficiency of 26% when the absorber layer thickness is almost 900 nm. The investigation into absorber bulk defect densities reveals the critical role of defect levels in PSC performance. Adjusting defect levels from 1014 cm−3 to 1018 cm−3 results in deteriorating trends in Voc, Jsc, FF, and PCE. Jsc remains stable until a defect level of 1017 cm−3, highlighting a threshold where defects begin to impact charge carrier generation and separation. Doping effect has been studied, PCE remains stable until a critical doping level of 1016 cm−3 after which it drops significantly which indicates that doping is cautioned against due to its adverse effects on material and carrier transport. This finding holds significant promise for experimental solar cell fabrication, as it suggests that cubic MAPbI3’s superior bandgap and enhanced optical absorption could lead to more efficient and robust photovoltaic devices in real-world applications.

A quadruple transition metal dichalcogenide for variously synergetic electron behaviors during photocatalytic carbon dioxide reduction

A quadruple 2H transition metal dichalcogenide of platinum doped molybdenum selenide sulfide (MoSSe) has been achieved. The critical doping amount of Pt is estimated to be approximately 0.9 a.t.% to prevent the formation of a heterogeneous phase. It has been observed that the excess Pt beyond this critical doping level cannot be settled into the lattice of MoSSe but forms an independent metal phase of Pt nanoparticles, composing a heterostructure with MoSSe, in which the behavior of photogenerated electrons will be predominantly governed by metal Pt and transported along the pathways for gas production in the photocatalytic CO2 reduction. However, as long as Pt can be located in the lattice of MoSSe, the elemental variety of Pt doped MoSSe has provided various active sites during the photocatalytic CO2 reduction process, favoring the localized coupling of matching intermediates, consequently enhancing the production of high value-added liquid products. In essence, the successful integration of Pt into the 2H MoSSe lattice opens up diverse active sites, optimizing the photocatalytic performance on the selectivity of liquid production reduced from CO2.

Ag-Doped Free-Standing 2D TiO2 Sheets: Electronic, Optical, Magnetic, and Self-Healing Behaviour.

Beyond a critical doping level, Ag - 2D TiO2 sheets (ATO) are deemed to be a flexible transparent conductor, useful for visible-range functional photonic/optoelectronic devices/sensors, sunlight-sensitive catalysis, and light-activated resistive switching. Due to the lack of control of surface energy which often leads to the formation of structural defects and even dimensionality crossover (2D to 0D) of materials during doping reaction, it is challenging to obtain ATO with a controlled doping level. Gauging the urgency, therefore we report the surface energy-controlled synthesis of ATO employing liquid phase exfoliation of TiO2 and subsequent hydrothermal Ag-doping in the presence of Hexamethylenetetramine (HMTA). Electron microscopy and atomic force microscopy reveal ATO sheets with large lateral dimensions. 6-fold, 4-fold, and strain-mediated crystallographic phases of 2D ATO have been revealed by high-resolution electron imaging. Successful tuning of the band gap down to ~ 2 eV with Ag doping up to ~ 10% is obtained. Synthesized 2D ATO have been investigated for their electrical, optical, optoelectronic, photoluminescence, and ferromagnetic behaviour. Visible light-sensitive thermally/structurally robust semiconductor/conductor via tuneable doping will pave the way for their flexible as well as wearable device applications. Self-healing effect of AFM tip-generated mechanical stress has also been demonstrated.

Cu-Doping Induced Phase Transformation in CsPbI3 Nanocrystals with Enhanced Structural Stability and Photoluminescence Quantum Yield

The synthesis of air-stable and cubic phases of all-inorganic halide perovskite nanocrystals (HPNCs) by a hot-injection approach is still challenging due to their rapid in situ phase transformations. Therefore, understanding and preventing this phase conversion by doping of cations is the key to improve the structural stability, environmental durability, and photoluminescence quantum yield. Here, the doping of divalent Cu ions at the Pb-site of cesium lead iodide (CsPbI3) is reported to achieve cubic-phase (α-phase) HPNCs with superior quantum yield and enhanced stability compared to undoped CsPbI3. Rietveld refined X-ray diffraction patterns and atomic-resolution transmission electron microscopy analyses reveal structural transformation from a mixed (γ-orthorhombic and α-cubic) phase to a cubic (α-CsPbI3) one with a smaller lattice constant at an optimal (5.6%) Cu concentration. Computational density functional theory (DFT) analysis credits the resultant structural and environmental stability for the Cu-doped sample to the charge accumulation at the dopant site that leads to increased bond strength with consequent minimization of the energy of the system to give rise to the maximum stability of the HPNCs at this dopant ratio. With the increase of Cu-doping, a red shift is observed in the absorbance and photoluminescence spectra up to a critical doping level, making the system optically tuneable. The optimally doped (5.6% Cu) CsPbI3 HPNCs exhibit stronger light emission with higher carrier lifetime and higher quantum yield (>80%), which are absolutely essential requirements in air-stable optoelectronic devices.

Electronic and superconducting properties of hydrogenated graphene from first-principles calculations

We performed first-principles calculations on two hydrogenated graphene systems with different hydrogen coverages, C8H2 and C50H2, to analyze their electronic and superconducting properties. Our results show that their electronic properties are highly correlated to the hydrogenation positions. If the two hydrogen atoms are attached to the same sublattice, the final system will be ferromagnetic. Otherwise, it will maintain nonmagnetic rather than anti-ferromagnetic. Moreover, the distance between the doped hydrogens can trigger the movement of Dirac points, and even annihilate Dirac points when the distance is close to the maximum. We further studied their superconducting properties by applying hole doping and tensile strains. The results show that the superconducting transition temperature T c increases with more holes and reaches its maximum of about 20.2 K at the critical doping level (x c = 0.17 holes/cell). Our results show that the superconductivity mainly originates from the coupling between the out-of-plane lattice vibration modes and the electronic p z orbitals of carbon atoms. The increase of T c can be attributed to the stronger coupling between the electrons and the low-frequency phonon. However, the application of biaxial and uniaxial tensile strain will depress the superconductivity because of the modulation of the low-frequency phonon. It is worthy to note that weak anharmonicity exists in the hydrogenated graphene systems. This work provides a systematic study on tuning the superconductivity of hydrogenated graphene.

Large antiferromagnetic fluctuation enhancement of the thermopower at a critical doping in magnetic semimetal Cr1+δTe2

Cr1+dTe2 is a self-intercalated transition metal dichalcogenide that hosts tunable electronic filling and magnetism in its semimetallic band structure. Recent angle-resolved photoemission spectroscopy (ARPES) studies have unveiled a systematic shift in this semimetallic band structure relative to the chemical potential with increased Cr doping. This report presents the temperature and magnetic field dependence of the longitudinal thermopower Sxx for different Cr1+dTe2 compositions. We show that as doping increases, the sign of Sxx changes from positive to negative at the critical doping level of d ~ 0.5. This observed doping-dependent trend in the thermopower is consistent with the evolution of the semimetallic band structure from ARPES. Importantly, an anomalous enhancement of the thermoelectric response is also observed around d~0.5. Combining information from magnetometry and ARPES measurements, existence of the critical nature of the doping level dc (~0.5) is unveiled in magnetic semimetal Cr1+dTe2, where antiferromagnetic fluctuation and near-Fermi-energy pseudogap formation play a potential vital role in enhancing thermoelectric energy conversion.

Strange metal electrodynamics across the phase diagram of Bi2−xPbxSr2−yLayCuO6+δ cuprates

Unlocking the mystery of the strange metal state has become the focal point of high T$_{c}$ research, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed `quantum supreme matter'. Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low T$_{c}$ cuprate superconductor, Bi$_{2-x}$Pb$_{x}$Sr$_{2-y}$La$_{y}$CuO$_{6+\delta}$ allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are two-fold, (i): a low energy Drude response with Drude width on the order of temperature and (ii): a high energy conformal tail with doping dependent power-law exponent. While the Drude weight evolves monotonously throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed 'return to normalcy' is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width.

Revised phase diagram of the high- Tc cuprate superconductor Pb-doped Bi2Sr2CaCu2O8+δ revealed by anisotropic transport measurements

Although phase diagrams can be leveraged to investigate high transition temperature (high-${T}_{c}$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-${T}_{c}$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the ``weak'' pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the ``strong'' pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-${T}_{c}$ superconductivity.

Striping of orbital-order with charge-disorder in optimally doped manganites

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the {frac{3}{8}}^{th} doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn{}_{3}^{A^{prime} }Mn{}_{4}^{B}O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

Nonlinear Hall effect in two-dimensional class-AI metals

In a time-reversal invariant system, while the anomalous Hall effect identically vanishes in the linear response regime due to the constraint of time-reversal symmetry on the distribution of Berry curvature, a nonlinear Hall effect can emerge in the second-order response regime if the inversion symmetry is broken to allow a nonzero Berry curvature dipole (BCD) on the Fermi surface. In this work, we study the nonlinear Hall effect of the BCD origin in two-dimensional doped insulators and semimetals belonging to the symmetry class AI which has spinless time-reversal symmetry. Despite that the class AI does not host any strong topological insulator phase in two dimensions, we find that they can still be classified as topologically obstructed insulators and trivial insulators if putting certain constraint on the Hamiltonians. When the insulator gets closer to the phase boundary of the two distinct phases, we find that the BCDs will become more prominent if the doping level is located near the band edge. Moreover, when the insulator undergoes a phase transition between the two distinct phases, we find that the BCDs will dramatically change their signs. For the semimetals without inversion symmetry, we find that the BCDs will sharply reverse their signs when the doping level crosses the Dirac points. With the shift of the locations of Dirac points in energy, the critical doping level at which the BCDs sharply reverse their signs will accordingly change. Our study reveals that class AI materials can also have interesting geometrical and topological properties, and remarkable nonlinear Hall effect can also appear in this class of materials even though the spin-orbit coupling is negligible. Our findings broaden the scope of materials to study the nonlinear Hall effect and provide new perspectives for the application of this effect.

Multiple quantum phase transitions of different nature in the topological kagome magnet Co3Sn2\u2212xInxS2

The exploration of topological electronic phases that result from strong electronic correlations is a frontier in condensed matter physics. One class of systems that is currently emerging as a platform for such studies are so-called kagome magnets based on transition metals. Using muon spin-rotation, we explore magnetic correlations in the kagome magnet Co3Sn2−xInxS2 as a function of In-doping, providing putative evidence for an intriguing incommensurate helimagnetic (HM) state. Our results show that, while the undoped sample exhibits an out-of-plane ferromagnetic (FM) ground state, at 5% of In-doping the system enters a state in which FM and in-plane antiferromagnetic (AFM) phases coexist. At higher doping, a HM state emerges and becomes dominant at the critical doping level of only xcr,1 ≃ 0.3. This indicates a zero temperature first order quantum phase transition from the FM, through a mixed state, to a helical phase at xcr,1. In addition, at xcr,2 ≃ 1, a zero temperature second order phase transition from helical to paramagnetic phase is observed, evidencing a HM quantum critical point (QCP) in the phase diagram of the topological magnet Co3Sn2−xInxS2. The observed diversity of interactions in the magnetic kagome lattice drives non-monotonous variations of the topological Hall response of this system.

Dislocation generation mechanisms in heavily boron-doped diamond epilayers

Doping diamond layers for electronic applications has become straightforward during the last two decades. However, dislocation generation in diamond during the microwave plasma enhanced chemical vapor deposition growth process is still not fully understood. This is a truly relevant topic to avoid for an optimal performance of any device, but, usually, it is not considered when designing diamond structures for electronic devices. The incorporation of a dopant, here boron, into a lattice as close as that of diamond, can promote the appearance of dislocations in the epilayer. The present contribution analyzes the different processes that can take place in this epilayer and gives some rules to avoid the formation of dislocations, based on the comparison of the different dislocation generation mechanisms. Indeed, competitive mechanisms, such as doping atom proximity effect and lattice strain relaxation, are here quantified for heavily boron-doped diamond epilayers. The resulting growth condition windows for defect-free heavily doped diamond are here deduced, introducing the diamond parameters and its lattice expansion in several previously published critical thickness (hc) and critical doping level relationships for different doping levels and growth conditions. Experimental evidence supports the previously discussed thickness-doping-growth condition relationships. Layers with and without dislocations reveal that not only the thickness but also other key factors such as growth orientation and growth parameters are important, as dislocations are shown to be generated in epilayers with a thickness below the People and Bean critical thickness.

Hidden magnetism at the pseudogap critical point of a cuprate superconductor

The mysterious pseudogap phase of cuprate superconductors ends at a critical hole doping level p* but the nature of the ground state below p* is still debated. Here, we show that the genuine nature of the magnetic ground state in La2-xSrxCuO4 is hidden by competing effects from superconductivity: applying intense magnetic fields to quench superconductivity, we uncover the presence of glassy antiferromagnetic order up to the pseudogap boundary p* ~ 0.19, and not above. There is thus a quantum phase transition at p*, which is likely to underlie highfield observations of a fundamental change in electronic properties across p*. Furthermore, the continuous presence of quasi-static moments from the insulator up to p* suggests that the physics of the doped Mott insulator is relevant through the entire pseudogap regime and might be more fundamentally driving the transition at p* than just spin or charge ordering.

A new sight into the glass forming ability caused by doping on Ba- and Ti-site in BaTi2O5 glass

La- and Nb-doped BaTi2O5 (BT2) spherical glasses were prepared by a containerless aerodynamic levitation method and their glass-forming regions were established. It is found that La-doping on the Ba-site (network-modifier) and Nb-doping on the Ti-site (network-former) show distinct difference in the glass-forming region: less than 10 % La can replace Ba whereas 40 % Nb can incorporate into BT2 glass. The distinction in glass-forming ability induced by La- and Nb-doping is discussed mainly from the structural arrangement of the glass. Raman spectroscopy analysis shows that La-doping elongates the short Ti–O bonds in the distorted [TiO5] polyhedra and thus relaxes the network. Nb-doping introduces [NbO6] polyhedra into BT2 and there exists a critical doping level (20 %), below which incorporation of Nb into BT2 relaxes the [TiOn] polyhedra by shortening the long Ti–O bond and above which [NbO6] starts to participate in the network skeleton construction resulting in a dramatic change in the glass structure, which is supported by the dramatic change in the exothermic peak on the DTA curves. This work triggers the speculation that the network-modifiers in BT2 glass possess a very important role in the structure of network-former skeleton than those in glasses based on traditional network-former oxides such as SiO2, GeO2 and B2O3, which may provide a useful strategy for modifying the properties of these novel glasses by chemical doping.

Contrasted Sn substitution effects on Dirac line node semimetals SrIrO3 and CaIrO3

Perovskite-type iridates SrIrO3 and CaIrO3 are a Dirac line node semimetal protected by crystalline symmetry, providing an interesting playground to investigate electron correlation effects on topological semimetals. The effect of Sn doping was examined by growing SrIr1−xSnxO3 and CaIr1−xSnxO3 thin films epitaxially on the SrTiO3(001) substrate using pulsed laser deposition. Upon Sn doping, the semimetallic ground state switches into an insulator. As temperature is lowered, the resistivity, ρ(T), of SrIr1−xSnxO3 above a critical doping level (xc ∼ 0.1) shows a well-defined transition from the semimetal to a weakly ferromagnetic insulator at T = Tc. In contrast, ρ(T) of CaIr1−xSnxO3 with increasing x shows a rapid increase in magnitude but does not show a clear signature of metal-insulator transition in the temperature dependence. We argue that the contrasted behavior of the two closely related iridates reflects the interplay between the effects of electron correlation and disorder enhanced by Sn doping.

Impact of electrostatic doping level on the dissipative transport in graphene nanoribbons tunnel field-effect transistors

The impact of electrostatic doping level on the dissipative transport of Armchair GNR-TFET is studied using the Quantum Perturbation Theory (QPT) with the Extended Lowest Order Expansion (XLOE) implementation method. Results show that the doping level of the source and drain sides of the GNR-TFET has a significant impact on the phonon contribution to the carrier transport process. Unlike in other similar studies, where phonons are believed to have a constant detrimental influence on the ION/IOFF ratio and Subthreshold Swing (SS) of the TFET devices due to the phonon absorption-assisted tunneling, we show that by a proper engineering of the doping level in the source and drain, the phonon absorption assisted tunneling can be effectively inhibited. We also show that as temperature increase, the device switching property deteriorates in both the ballistic and dissipative transport regimes, and there exists a temperature-dependent critical doping level where the device has optimal switching behavior.

Effect of Substitution on the Hysteretic Phase Transition in a Bistable Phenalenyl-Based Neutral Radical Molecular Conductor.

The ability to tune the physical properties of bistable organic functional materials by means of chemistry can facilitate their development for molecular electronic switching components. The butylamine-containing biphenalenyl boron neutral radical, [Bu]2 B, crystalline compound has recently attracted significant attention by displaying a hysteretic phase transition accompanied by simultaneous bistability in magnetic, electrical, and optical properties close to room temperature. In this report, substitutional doping was applied to [Bu]2 B by crystallizing solid solutions of bistable [Bu]2 B and its non-radical-containing counterpart [Bu]2 Be. With increasing doping degree, the hysteretic phase transition is gradually suppressed in terms of reducing the height, but conserves the width of the hysteresis loop as observed through magnetic susceptibility and electrical conductivity measurements. At the critical doping level of about 6 %, the abrupt transformation of the crystal structure to that of the pure [Bu]2 Be crystal packing was observed, accompanied by a complete collapse of the hysteresis loop. Further study of the structure-properties relationships of bistable neutral radical conductors based on the [Bu]2 B host can be conducted utilizing a variety of biphenalenyl-based molecular conductors.

The effect of humidity on the dielectric properties of (In + Nb) co-doped SnO2 ceramics

(In + Nb) co-doped SnO2 ceramics, (In0.5Nb0.5)xSn1−xO2 (INSO) with x = 0, 0.0005, 0.001, 0.01, and 0.1, were prepared by sol-gel method. Dielectric properties and humidity sensitivity of these samples were systematically investigated. Our results revealed that there exists a critical doping level of xc ∼ 0.001 separating the dielectric behavior and humidity sensitivity of INSO into two different regions: In region I of x ≤ xc, the main relaxing species are InSn'-OHO• complex dipoles, which greatly enhances the dielectric constant. The humidity sensitivity was found to decrease with increasing doping level and have positive effect on the dielectric properties of the samples. In region II of x > xc, the main relaxing species are localized holes, which leads to polaronic relaxation and gradually weakens the dielectric constant enhancement. The humidity sensitivity was found to increase with increasing doping level and have negative effect on the dielectric properties of the samples.

Probing influence of rare earth ions (Er3+, Dy3+ and Gd3+) on structural, magnetic and optical properties of magnetite nanoparticles

Fe3−xRExO4 (RE = Er, Dy and Gd) nanoparticles with x varying from 0 to 0.1 were synthesized using co-precipitation method. The synthesized nanoparticles were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and UV-Vis spectroscopy techniques. TEM images reveal round shaped particles of ∼8–14 nm diameter in case of undoped magnetite (Fe3O4) nanoparticles whereas there is evolution of rod like structures by the doping of RE ions with aspect ratio in the range of 6–16. The room temperature saturation magnetization (Ms) values show gradual increase with doping till a critical doping level which is found to depend on the ionic radius of dopant ion (x = 0.01 for Er, 0.03 for Dy and 0.04 for Gd). There is a variation in the maximum value of saturation magnetization which is directly proportional to the number of unpaired 4f electrons in the dopant element. Low temperature magnetization study, carried out at 5 K and 120 K reveal an increase in the value of Ms as well as coercivity. The direct bandgaps calculated from UV-Visible data are found to decrease with increasing number of unpaired electrons in the dopant ions.

Charge-density-wave order takes over antiferromagnetism in Bi2Sr2\u2212xLaxCuO6 superconductors

Superconductivity appears in the cuprates when a spin order is destroyed, while the role of charge is less known. Recently, charge density wave (CDW) was found below the superconducting dome in YBa2Cu3Oy when a high magnetic field is applied perpendicular to the CuO2 plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped. Here by 63Cu-nuclear magnetic resonance, we report the discovery of CDW induced by an in-plane field, setting in above the dome in single-layered Bi2Sr2−xLaxCuO6. The onset temperature TCDW takes over the antiferromagnetic order temperature TN beyond a critical doping level at which superconductivity starts to emerge, and scales with the pseudogap temperature T*. These results provide important insights into the relationship between spin order, CDW and the pseudogap, and their connections to high-temperature superconductivity.