Stoner Ferromagnetism in Hole-Doped CuMIIIAO2 with MIIIA = Al, Ga, and In.
ABSTRACT: Using density functional theory calculations, we examine the effect of hole doping on the magnetic and electronic properties of CuMIIIAO2, with MIIIA = Al, Ga, and In. CuMIIIAO2 nonmagnetic semiconductors switch to ferromagnetic half-metals upon hole doping. For CuAlO2, the nonmagnetic-to-ferromagnetic transition occurs for hole densities of ∼7 × 1019/cm3. Ferromagnetism arises from an exchange splitting of the electronic states at the valence band edge, and it can be attributed to the high-lying Cu-d states. Hole doping induced by cation vacancies and substitutional divalent dopants is also investigated. Interestingly, both vacancies and nonmagnetic divalent dopants result in the emergence of ferromagnetism.
Project description:In recent experiments, superconductivity and correlated insulating states were observed in twisted bilayer graphene (TBG) with small magic angles, which highlights the importance of the flat bands near Fermi energy. However, the moiré pattern of TBG consists of more than ten thousand carbon atoms that is not easy to handle with conventional methods. By density functional theory calculations, we obtain a flat band at EF in a novel carbon monolayer coined as cyclicgraphdiyne with the unit cell of eighteen atoms. By doping holes into cyclicgraphdiyne to make the flat band partially occupied, we find that cyclicgraphdiyne with 1/8, 1/4, 3/8 and 1/2 hole doping concentration shows ferromagnetism (half-metal) while the case without doping is nonmagnetic, indicating a hole-induced nonmagnetic-ferromagnetic transition. The calculated conductivity of cyclicgraphdiyne with 1/8, 1/4 and 3/8 hole doping concentration is much higher than that without doping or with 1/2 hole doping. These results make cyclicgraphdiyne really attractive. By studying several carbon monolayers, we find that a perfect flat band may occur in the lattices with both separated or corner-connected triangular motifs with only including nearest-neighboring hopping of electrons, and the dispersion of flat band can be tuned by next-nearest-neighboring hopping. Our results shed insightful light on the formation of flat band in TBG. The present study also poses an alternative way to manipulate magnetism through doping flat band in carbon materials.
Project description:Searching for two-dimensional (2D) group V materials with ferromagnetism, elastic anisotropy, and carrier mobility and tunable band structure is one key to developing constantly developing nanodevices. The 2D monolayers Sn<sub>x</sub>P<sub>y</sub> with x/y (1/1, 1/2, 1/3, and so on) coordination number are studied based on the particle-swarm optimization technique combined with the density functional theory optimization. Its thermal stability can be confirmed by molecular dynamics at 70K and 300K, indicating that the novel 2D materials have a stable existence. The electronic band structures of four stable structures suggest that all the monolayers of Sn<sub>x</sub>P<sub>y</sub> are fully adjustable and flexible tunable band gaps semiconductors under the biaxial strain. The monolayer of P[Formula: see text]m-SnP<sub>2</sub> with unique valence band structure can go from nonmagnetic to ferromagnetic by the hole doping because of the "Stoner criterion," and Pmc2<sub>1</sub>-SnP<sub>2</sub> is a direct-like gap semiconductor with in-plane elastic anisotropy to possess a high electron mobility as high as 800?cm<sup>2</sup>V<sup>-1</sup>?s<sup>-1</sup> along the k<sub>b</sub> direction, which is much higher than that of MoS<sub>2</sub> (? 200?cm<sup>2</sup>V<sup>-1</sup>?s<sup>-1</sup>). The optical absorption peak of the material is in the ultraviolet region. These discoveries expand the potential applications of the emerging field of 2D Sn<sub>x</sub>P<sub>y</sub> structures in nanoelectronics.
Project description:The process of photocatalysis is appealing to huge interest motivated by the great promise of addressing current energy and environmental issues through converting solar light directly into chemical energy. However, an efficient solar energy harvesting for photocatalysis remains a critical challenge. Here, we reported a new full solar spectrum driven photocatalyst by co-doping of Gd<sup>3+</sup> and Sn<sup>4+</sup> into A and B-sites of BiFeO<sub>3</sub> simultaneously. The co-doping of Gd<sup>3+</sup> and Sn<sup>4+</sup> played a key role in hampering the recombination of electron-hole pairs and shifted the band-gap of BiFeO<sub>3</sub> from 2.10?eV to 2.03?eV. The Brunauer-Emmett-Teller (BET) measurement confirmed that the co-doping of Gd<sup>3+</sup> and Sn<sup>4+</sup> into BiFeO<sub>3</sub> increased the surface area and porosity, and thus the photocatalytic activity of the Bi<sub>0.90</sub>Gd<sub>0.10</sub>Fe<sub>0.95</sub>Sn<sub>0.05</sub>O<sub>3</sub> system was significantly improved. Our work proposed a new photocatalyst that could degrade various organic dyes like Congo red, Methylene blue, and Methyl violet under irradiation with different light wavelengths and gave guidance for designing more efficient photocatalysts.
Project description:<i>α</i>-CsPbI<sub>3</sub> nanocrystals (NCs) with poor stability prevent their wide applications in optoelectronic fields. Ca<sup>2+</sup> (1.00 Å) as a new B-site doping ion can successfully boost CsPbI<sub>3</sub> NC performance with both improved phase stability and optoelectronic properties. With a Ca<sup>2+</sup>/Pb<sup>2+</sup> ratio of 0.40%, both phase and photoluminescence (PL) stability could be greatly enhanced. Facilitated by increased tolerance factor, the cubic phase of its solid film could be maintained after 58 days in ambient condition or 4 h accelerated aging process at 120°C. The PL stability of its solution could be preserved to 83% after 147 days in ambient condition. Even using UV light to accelerate aging, the T<sub>50</sub> of PL could boost 1.8-folds as compared to CsPbI<sub>3</sub> NCs. Because Ca<sup>2+</sup> doping can dramatically decrease defect densities of films and reduce hole injection barriers, the red light-emitting diodes (LEDs) exhibited about triple enhancement for maximum the external quantum efficiency (EQE) up to 7.8% and 2.2 times enhancement for half-lifetime of LED up to 85 min. We believe it is promising to further explore high-quality CsPbI<sub>3</sub> NC LEDs via a Ca<sup>2+</sup>-doping strategy.
Project description:We report on the evolution of the average and depth-dependent magnetic order in thin-film samples of biaxially stressed and electron-doped EuTiO<sub>3</sub> for samples across a doping range < 0.1 to 7.8 × 10<sup>20</sup> cm<sup>-3</sup>. Under an applied in-plane magnetic field, the <i>G</i>-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated Eu<sup>2+</sup> moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.
Project description:Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron-hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO3 film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron-hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.
Project description:Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS<sub>2</sub> monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS<sub>2</sub> crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS<sub>2</sub> monolayers that appears at 2.28?eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS<sub>2</sub>. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS<sub>2</sub> monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.
Project description:Magnetic properties of Mott-Hubbard systems are generally dominated by strong antiferromagnetic interactions produced by the Coulomb repulsion of electrons. Although theoretical possibility of a ferromagnetic ground state has been suggested by Nagaoka and Penn as single-hole doping in a Mott insulator, experimental realization has not been reported more than half century. We report the first experimental possibility of such ferromagnetism in a molecular Mott insulator with an extremely light and homogeneous hole-doping in ?-electron layers induced by net polarization of counterions. A series of Ni(dmit)<sub>2</sub> anion radical salts with organic cations, where dmit is 1,3-dithiole-2-thione-4,5-dithiolate can form bi-layer structure with polarized cation layers. Heat capacity, magnetization, and ESR measurements substantiated the formation of a bulk ferromagnetic state around 1.0 K with quite soft magnetization versus magnetic field (M-H) characteristics in (Et-4BrT)[Ni(dmit)<sub>2</sub>]<sub>2</sub> where Et-4BrT is ethyl-4-bromothiazolium. The variation of the magnitude of net polarizations by using the difference of counter cations revealed the systematic change of the ground state from antiferromagnetic one to ferromagnetic one. We also report emergence of metallic states through further doping and applying external pressures for this doping induced ferromagnetic state. The realization of ferromagnetic state in Nagaoka-Penn mechanism can paves a way for designing new molecules-based ferromagnets in future.
Project description:It is widely reported during last decade on the observation of room temperature ferromagnetism (RTFM) in doped ZnO and other transition metal oxides. However, the origin of RTFM is not understood and highly debated. While investigating the origin of RTFM, magnetic ion doped oxides should be excluded because it is not yet settled whether RTFM is intrinsic or due to the magnetic ion cluster in ZnO. Hence, it is desirable to investigate the origin of RTFM in non-magnetic ion doped ZnO and Cu-doped ZnO will be most suitable for this purpose. The important features of ferromagnetism observed in doped ZnO are (i) observation of RTFM at a doping concentration much below than the percolation threshold of wurtzite ZnO, (ii) temperature independence of magnetization and (iii) almost anhysteretic magnetization curve. We show that all these features of ferromagnetism in ZnO are due to overlapping of bound magnetic polarons (BMPs) which are created by exchange interaction between the spin of Cu<sup>2+</sup> ion and spin of the localized hole due to zinc vacancy [Formula: see text]. Both the experimental and theoretical investigation show that the exchange interaction between Cu<sup>2+</sup>-Cu<sup>2+</sup> ions mediated by [Formula: see text] is responsible for RTFM in Cu-doped ZnO.
Project description:K<sup>+</sup>/Cl<sup>-</sup> and K<sup>+</sup>/F<sup>-</sup> co-doped LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> (LNMO) materials were successfully synthesized via a solid-state method. Structural characterization revealed that both K<sup>+</sup>/Cl<sup>-</sup> and K<sup>+</sup>/F<sup>-</sup> co-doping reduced the Li<i><sub>x</sub></i>Ni<sub>1</sub><sub>-</sub><i><sub>x</sub></i>O impurities and enlarged the lattice parameters compared to those of pure LNMO. Besides this, the K<sup>+</sup>/F<sup>-</sup> co-doping decreased the Mn<sup>3+</sup> ion content, which could inhibit the Jahn-Teller distortion and was beneficial to the cycling performance. Furthermore, both the K<sup>+</sup>/Cl<sup>-</sup> and the K<sup>+</sup>/F<sup>-</sup> co-doping reduced the particle size and made the particles more uniform. The K<sup>+</sup>/Cl<sup>-</sup> co-doped particles possessed a similar octahedral structure to that of pure LNMO. In contrast, as the K<sup>+</sup>/F<sup>-</sup> co-doping amount increased, the crystal structure became a truncated octahedral shape. The Li<sup>+</sup> diffusion coefficient calculated from the CV tests showed that both K<sup>+</sup>/Cl<sup>-</sup> and K<sup>+</sup>/F<sup>-</sup> co-doping facilitated Li<sup>+</sup> diffusion in the LNMO. The impedance tests showed that the charge transfer resistances were reduced by the co-doping. These results indicated that both the K<sup>+</sup>/Cl<sup>-</sup> and the K<sup>+</sup>/F<sup>-</sup> co-doping stabilized the crystal structures, facilitated Li<sup>+</sup> diffusion, modified the particle morphologies, and increased the electrochemical kinetics. Benefiting from the unique advantages of the co-doping, the K<sup>+</sup>/Cl<sup>-</sup> and K<sup>+</sup>/F<sup>-</sup> co-doped samples exhibited improved rate and cycling performances. The K<sup>+</sup>/Cl<sup>-</sup> co-doped Li<sub>0.97</sub>K<sub>0.03</sub>Ni<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>3.97</sub>Cl<sub>0.03</sub> (LNMO-KCl0.03) exhibited the best rate capability with discharge capacities of 116.1, 109.3, and 93.9 mAh g<sup>-1</sup> at high C-rates of 5C, 7C, and 10C, respectively. Moreover, the K<sup>+</sup>/F<sup>-</sup> co-doped Li<sub>0.98</sub>K<sub>0.02</sub>Ni<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>3.98</sub>F<sub>0.02</sub> (LNMO-KF0.02) delivered excellent cycling stability, maintaining 85.8% of its initial discharge capacity after circulation for 500 cycles at 5C. Therefore, the K<sup>+</sup>/Cl<sup>-</sup> or K<sup>+</sup>/F<sup>-</sup> co-doping strategy proposed herein will play a significant role in the further construction of other high-voltage cathodes for high-energy LIBs.