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The work function (WF) of perovskite materials is essential for developing optoelectronic devices enabling efficient charge transfer at their interfaces. Perovskite's WF can be tuned by MXenes, a new class of two-dimensional (2D) early transition metal carbides, nitrides, and carbonitrides. Their variable surface terminations or the possibility of introducing elemental dopants could advance perovskites. However, the influence of doped-MXenes on perovskite materials is still not fully understood and elaborated. This study provides mechanistic insight into verifying the tunability of MAPbI3 WF by hybridizing with fluorine-terminated Ti3C2Tx (F-MXene) and nitrogen-doped Ti3C2Tx (N-MXene). We first reveal the interfacial interaction between MAPbI3 and MXenes via X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). UPS supported by density functional theory (DFT) calculations allowed the description of the influence of F and N on MXene's WF. Furthermore, we developed MAPbI3/MXene heterostructures using F- and N-MXenes. The F-MXenes extended the most WF of MAPbI3 from 4.50 eV up to 3.00 eV, compared to only a small shift for N-MXene. The underlying mechanism was charge transfer from low WF F-MXene to MAPbI3, as demonstrated by PL quenching in MAPbI3/F-MXene heterostructures. Altogether, this work showcases the potential of fluorine-doped MXenes over nitrogen-doped MXenes in advancing perovskite heterostructures, thus opening a door for efficient optoelectronic devices.
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We study the effect of boron and sulfur doping on graphene with atomically dispersed zirconium as an electrocatalyst for the oxygen reduction reaction (ORR) by using density functional theory (DFT). The use of Zr as a metal center offers a highly stable catalyst due to the high electronegativity difference between Zr and its ligand. The origin of the ORR activity improvement has been investigated thoroughly. Here, we proposed a novel geometric descriptor for an atomically dispersed zirconium on a nitrogen-doped graphene catalyst with an axial oxygen ligand, which is the fractional coordination number of the Zr atom. We found that the fractional coordination number can successfully describe the shift of the dz2 band center in the doped compound, which is related to the binding energy of the Zr to the O ligand. We also found that the oxygen ligand is mobile during the adsorption process of ORR intermediates, and hence it is imperative for the axial oxygen ligand to bind neither too strongly nor too weakly to the Zr atom. The coordination engineering strategy can successfully enhance the ORR activity, shifting the ORR overpotential from 0.75 V and 0.92 V to 0.33 V and 0.32 V. This study provides new insights into the origin of ORR activity by connecting the novel geometric descriptor to the electronic structure and finally it is connected to the ORR activity.
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We study the electronic, magnetic, and optical properties of (LaO1-xFx)MnAs (x = 0, 0.0625, 0.125, 0.25) systems, calculated using the generalized gradient approximation (GGA) corrected by Hubbard energy (U) = 1 eV. For x = 0, this system shows equal bandgap (Eg) values for spin-up and spin-down of 0.826 eV, with antiferromagnetic (AFM) properties and local magnetic moment in the Mn site of 3.86 µB per Mn. By doping F with x = 0.0625, the spin-up and spin-down Eg values decrease to 0.778 and 0.798 eV, respectively. This system, along with antiferromagnetic properties, also has a local magnetic moment in the Mn site of 3.83 µB per Mn. Increasing doping F to x = 0.125 induces increases of Eg to 0.827 and 0.839 eV for spin-up and spin-down. However, the AFM remains, where µMn slightly decreases to 3.81 µB per Mn. Furthermore, the excess electron from the F ion induces the Fermi level to move toward the conduction band and changes the bandgap type from indirect bandgap (Γ â M) to direct bandgap (Γ â Γ). Increasing x to 25% induces the decrease of spin-up and spin-down Eg to 0.488 and 0.465 eV, respectively. This system shows that the AFM changes to ferrimagnetism (FIM) for x = 25%, with a total magnetic moment of 0.78 µB per cell, which is mostly contributed by Mn 3d and As 4p local magnetic moments. The change from AFM to FIM behavior results from competition between superexchange AFM ordering and Stoner's exchange ferromagnetic ordering. Pristine (LaO)MnAs exhibits high excitonic binding energy (â¼146.5 meV) due to a flat band structure. Our study shows that doping F in the (LaO)MnAs system significantly modifies the electronic, magnetic, and optical properties for novel advanced device applications.
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We study the structural, electronic, and magnetic properties of the antiferromagnetic-layered oxyarsenide (LaO)MnAs system from the first-principle calculation. The increasing Hubbard energy (U) in the Mn 3d orbital induces the increasing local-symmetry distortions (LSDs) in MnAs4 and OLa4 tetrahedra. The LSD in MnAs4 tetrahedra is possibly promoted by the second-order Jahn-Teller effect in the Mn 3d orbital. Furthermore, the increasing U also escalates the bandgap (E g) and the magnetic moment of Mn (µMn). The value of U = 1 eV is the most appropriate by considering the structural properties. This value leads to E g and µMn of 0.834 eV and 4.31 µB, respectively. The calculated µMn is lower than the theoretical value for the high-spin state of Mn 3d (5 µB) due to the hybridization between Mn 3d and As 4p states. However, d xy states are localized and show the weakest hybridization with valence As 4p states. The Mott-insulating behavior in the system is characterized by the E g transition between the valence and conduction d zx /d zy states. This work shows new physical insights for advanced functional device applications, such as spintronics.
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Correction for 'The effect of crystallinity on the surface modification and optical properties of ZnO thin films' by Muhammad Abiyyu Kenichi Purbayanto et al., Phys. Chem. Chem. Phys., 2020, 22, 2010-2018, DOI: 10.1039/C9CP05464B.
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We have studied the effects of crystallinity on the emergence of porous morphology and strong green emission in ZnO thin films after H2 annealing treatment. The unique multiple-stacked porous structure is observed after performing H2 annealing treatment on the film with low crystallinity. However, the annealed high-crystallinity film exhibits surface morphology with a shallow porous structure, as revealed by SEM images. To study the effects of these unique porous structures on the optical properties, photoluminescence (PL) spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy (XPS) are conducted. The multiple-stacked porous structure produces strong green emission as compared to the shallow porous structure centered at 2.5 eV, as detected by PL. Here, the green emission originates from the electronic transition related to the oxygen vacancy (VO). XPS spectra show that the high density of VO located on the multiple-stacked porous surface is much higher as compared to that for the shallow porous structure due to a high surface-to-volume ratio. The results show that the multiple-stacked porous structure has the potential to enhance the functionality of ZnO for applications in light-emitting.
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We study the structural, electronic, and thermoelectric properties of p-type layered oxychalcogenides (La0.5Bi0.5O)CuCh (Ch = S, Se, Te) from first principles. Ch substitution from S to Te enhances the local-symmetry distortions (LSDs) in CuCh4 and OLa2Bi2 tetrahedra, where the LSD in OLa2Bi2 is more pronounced. The LSD in CuCh4 tetrahedra comes from the possible pseudo-Jahn-Teller effect, indicated by the degeneracy-lifted t2g and eg states of Cu 3d10 orbital. The Ch substitution decreases bandgap from 0.529, 0.256 (Γ â 0.4Δ), to 0.094 eV (Z â 0.4Δ), for Ch = S, Se, Te, respectively, implying the increasing carrier concentration and electrical conductivity. The split-off energy at Z and Γ points are also increased by the substitution. The valence band shows deep O 2p states in the electron-confining [LaBiO2]2+ layers, which is essential for thermoelectricity. (La0.5Bi0.5O)CuTe provides the largest thermoelectric power from the Seebeck coefficient and the carriers concentration, which mainly come from Te 5p x /p y , Cu 3d zx , and Cu 3d zy states. The valence band shows the partial hybridization of t2g and Chp states, implied by the presence of nonbonding valence t2g states. This study provides new insights, which predict experimental results and are essential for novel functional device applications.
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We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)Cu Ch ( Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperature-dependent electrical resistivity ρ( T). PE spectra are explained by calculating the electronic properties using the generalized-gradient approximation method. PE spectra and ρ( T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)Cu Ch ( Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.
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Spin transistors have attracted tremendous interest as new functional devices. However, few studies have investigated enhancements of the ON/OFF current ratio as a function of the electron spin behavior. Here, we found a significantly high spin-dependent current ratio-more than 10(2) at 1.5 V-when changing the relative direction of the magnetizations between FePt nanodots (NDs) and the CoPtCr-coated atomic force microscope (AFM) probe at room temperature. This means that ON and OFF states were achieved by switching the magnetization of the FePt NDs, which can be regarded as spin-diodes. The FePt magnetic NDs were fabricated by exposing a bi-layer metal stack to a remote H2 plasma (H2-RP) on ~1.7 nm SiO2/Si(100) substrates. The ultrathin bi-layers with a uniform surface coverage are changed drastically to NDs with an areal density as high as ~5 × 10(11) cm(-2). The FePt NDs exhibit a large perpendicular anisotropy with an out-of-plane coercivity of ~4.8 kOe, reflecting the magneto-crystalline anisotropy of (001) oriented L10 phase FePt. We also designed and fabricated double-stacked FePt-NDs with low and high coercivities sandwiched between an ultra-thin Si-oxide interlayer, and confirmed a high ON/OFF current ratio when switching the relative magnetization directions of the low and high coercivity FePt NDs.
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Searching for new plasmonic building blocks which offer tunability and design flexibility beyond noble metals is crucial for advancing the field of plasmonics. Herein, we report that solution-synthesized hexagonal Bi2Te3 nanoplates, in the absence of grating configurations, can exhibit multiple plasmon modes covering the entire visible range, as observed by transmission electron microscopy (TEM)-based electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Moreover, different plasmon modes are observed in the center and edge of the single Bi2Te3 nanoplate and a breathing mode is discovered for the first time in a non-noble metal. Theoretical calculations show that the plasmons observed in the visible range are mainly due to strong spin-orbit coupling induced metallic surface states of Bi2Te3. The versatility of shape- and size-engineered Bi2Te3 nanocrystals suggests exciting possibilities in plasmonics-enabled technology.