Hole-rich Regions Research Articles

Construction of Cu-Fe bimetallic oxide/biochar/Ag3PO4 heterojunction for improving photocorrosion resistance and photocatalytic performance achieves efficient removal of phenol

Ag3PO4 has an excellent visible light response, but its serious photocorrosion has limited its application in environmental remediation. Here, we constructed a ternary heterojunction photocatalyst (CuFe2O4/Biochar/Ag3PO4) to inhibit its photocorrosion and enhance photocatalytic activity. Notably, the composite catalyst (0.5 g/L) thoroughly degraded phenol (20 mg/L) at 18 min, and its apparent kinetic constant is fivefold that of pure Ag3PO4. The analysis of XPS, XRD, and cyclic degradation experiments confirmed that the photocorrosion of the heterojunction photocatalyst was significantly suppressed. The enhanced photocorrosion resistance of Ag3PO4 can be ascribed to the rapid transfer and consumption of photoelectrons on the Ag3PO4 surface by biochar and CuFe2O4, thereby avoiding photocorrosion. Especially, Ag3PO4 as a hole-rich region can protect Ag+ from being reduced. DFT calculations further revealed the electron transfer and anti-photocorrosion mechanism of Ag3PO4. The photogenerated O2– and h+ played a major role in phenol degradation. A possible degradation pathway was proposed by DFT calculations and GC–MS analysis. This study provides a novel ternary heterojunction photocatalyst to improve the catalytic performance and photocorrosion resistance of Ag3PO4.

In-situ Raman investigation and application of MXene-stabilized polypyrrole composite for flexible aqueous batteries

Although polypyrrole (PPy) has been recognized as a promising organic electrode for rechargeable aqueous batteries (RABs), the practical application is still restricted by its rapid capacity fading after repeated cycles. Herein, a multilayer structural MXene-stabilized PPy (MXene@PPy) composite has been purposefully designed via a facile dip-coating approach, wherein the MXene coating effectively inhibits the structural degradation and irreversible redox reaction of PPy during the electrochemical process, demonstrated by electrochemical measurements combined with in-situ Raman investigation. Density functional theory (DFT) calculation further confirms the obvious electron transfer from MXene to PPy, leading to the formation of electron-rich region on PPy and hole-rich region on MXene, thereby promoting the redox reaction of PPy in the MXene@PPy composite with increased density of states (DOS). As an electrode, the MXene@PPy composite exhibits a large specific capacity of 124.9 mAh g−1 at 1.0 mA cm−2, high coulombic efficiency of ∼100%, superior charge-transfer capability, excellent rate performance and long-term cycling stability with high capacity retention of ∼80.3% over 2500 cycles in acidic aqueous electrolyte. For real application, a high-performance flexible RAB device constructed with such MXene@PPy composite electrode has been substantiated as the efficient power source, revealing its potential applications in high-safety portable/wearable electronics.

Two-component electronic phase separation in the doped Mott insulator Y1−xCaxTiO3

The Mott insulator-metal transition has been the subject of intense research in condensed matter physics. Here, the authors study this transition in the prototypical perovskite oxide Y(Ca)TiO${}_{3}$, using a combination of x-ray absorption spectroscopy measurements and density functional theory calculations. They uncover electronic phase separation into hole-poor Mott-insulating and hole-rich metallic regions, i.e., an inherent first-order and percolative nature of the transition that occurs at nonzero hole doping.

Distinctive Features of Metal-Insulator Transitions, Multiscale Phase Separation, and Related Effects in Hole-Doped Cuprates

We study the distinctive features of the metal-insulator transitions, multiscale phase separation, and evolution of coexisting insulating and metallic/superconducting phases in hole-doped cuprates. We show how these interrelated phenomena and related effects manifest themselves in a wide doping range from the lightly doped to optimally doped regime in these systems, where the localized and mobile hole carriers reside in hole-poor (insulating) and hole-rich (metallic or superconducting) regions. We argue that small hole-rich regions (i.e. narrow nanoscale metallic islands or stripes) can persist in the insulating phase of the lightly doped cuprates, while the competing insulating, metallic, and superconducting phases would coexist in the under-doped cuprates. When the doping level is increased further, the hole-poor regions (or insulating zones) gradually narrow from macroscale to nanoscale insulating stripes and disappear in the optimally doped cuprates. We demonstrate clearly that the metal-insulator transitions and the coexisting insulating and metallic/superconducting phases are manifested in the suppression of superconductivity in underdoped cuprates and in the different temperature-dependent behaviors of the magnetic susceptibility and c-axis resistivity of lightly to optimally doped cuprates.

Electronic structure and polar catastrophe at the surface of LixCoO2 studied by angle-resolved photoemission spectroscopy

We report an angle-resolved photoemission spectroscopy (ARPES) study of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ single crystals which have a hole-doped ${\mathrm{CoO}}_{2}$ triangular lattice. Similar to ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, the Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ band crosses the Fermi level with strongly renormalized band dispersion while the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}^{\ensuremath{'}}$ bands are fully occupied in ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ ($x=0.46$ and 0.71). At $x=0.46$, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at $x=0.71$, the Fermi surface area is larger than the expectation which can be associated with the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co $3d$ peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at $x=0.71$ can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ is very robust around $x=0.5$ even in the topmost ${\mathrm{CoO}}_{2}$ layer due to the absence of the polar catastrophe.

Investigation and modeling of photocurrent collection process in multiple quantum well solar cells

Solar cells employing quantum wells can enhance the light absorption but suffer from the difficulty in photocarrier extraction. Here, we analyzed the spectral response and the photocarrier collection mechanism of p-i-n multiple quantum well (MQW) solar cells using the effective-mobility model. Both the simulation and experimental results imply that the spatial profiles of electron and hole densities in MQWs play an important role in the carrier collection process. By considering the recombination increment under illumination, our findings suggest that the concept of the majority/minority carriers is important even in the intrinsic region: photogenerated electrons and holes only experience significant recombination when passing through the hole-rich and electron-rich regions, respectively. This can accurately explain the photocurrent behavior in cells with background doping, background illumination, and different MQW positions. Based on the experimental findings, we derived analytical formulae for carrier collection efficiency, which directly show the impact of each cell parameter and can be used for the systematic cell design.

Dislocations as a boundary between charge density wave and oxygen rich phases in a cuprate high temperature superconductor

Multiple functional ionic and electronic orders are observed in high temperature superconducting cuprates. The charge density wave order is one of them and it is spatially localized in different regions of the material. It is also known that the oxygen interstitials introduced by chemical intercalation self-organize in different oxygen rich regions corresponding with hole rich regions in the CuO2 layers left empty by the charge density wave order domains. However, what happens in between these two orders is not known, and neither there is a method to control this spatial separation. Here we demonstrate by using scanning nano x-ray diffraction, that dislocations or grain boundaries in the material can act as boundary between charge density wave and oxygen rich phases in a optimally doped high temperature superconductor. Dislocations can be used therefore to control the anti-correlation of the charge density wave order with the oxygen interstitials in specific portion of the material.

Relaxation of charge in monolayer graphene: Fast nonlinear diffusion versus Coulomb effects

Pristine monolayer graphene exhibits very poor screening because the density of states vanishes at the Dirac point. As a result, charge relaxation is controlled by the effects of zero-point motion (rather than by the Coulomb interaction) over a wide range of parameters. Combined with the fact that graphene possesses finite intrinsic conductivity, this leads to a regime of relaxation described by a nonlinear diffusion equation with a diffusion coefficient that diverges at zero charge density. Some consequences of this fast diffusion are self-similar superdiffusive regimes of relaxation, the development of a charge depleted region at the interface between electron- and hole-rich regions, and finite extinction times for periodic charge profiles.

Formation of exciton rings and localized bright spots in coupled semiconductor quantum wells

We consider indirect excitons generated at the ring-shaped boundaries between electron- and hole-rich regions in semiconductor quantum wells (QW's). We show theoretically that the in-plane translational motion of the excitons is confined in the radial direction. The confinement potential results from the electrostatic interaction of the exciton dipole moment with the in-plane electric field induced at the boundary by the macroscopic charge separation. Our results directly apply to the external ring and the localized bright spots (LBS) observed in the photoluminescense (PL) pattern of indirect excitons.

First-principles study of water adsorption on α-SiO2 [110] surface

We have investigated the structural and electronic properties of water molecule adsorbed silicon dioxide (α-SiO2) [110] surface and analyzed the influence of water molecule on its energetics, structure and elctronic propertes using density functional theory based first principles calculations. The inhomogeneous topology of the α-SiO2 clean surface promotes a total charge density displacement on the adsorbed water molecule and giving rise to electron-rich as well as hole-rich region. The electronic charge transfer from a α-SiO2 to the water molecule occurs upon the formation of a partially occupied level laying above conduction band level.

Self-powered graphene thermistor

Self-powered flexible graphene thermistor device is successfully fabricated by modifying the graphene layers with ferroelectric polarization. Chemical vapor deposited graphene monolayer was transferred to the surface of a ferroelectric polymer poly[(vinylidenefluoride-co-trifluoroethylene)] P(VDF-TrFE) thin film. By utilizing alternating polarization charge, the graphene surface contains polarization induced hole-rich, intrinsic and electron-rich regions (p-i-n regions). The potential difference created in the graphene layer leads to the drift of the extra carriers created in the graphene monolayer with increasing temperature and gives rise to an exemplary self-powered or autonomous thermistor.

MoS2 on an amorphous HfO2 surface: An ab initio investigation

The energetic stability, electronic and structural properties of MoS2 adsorbed on an amorphous a-HfO2 surface (MoS2/HfO2) are examined through ab initio theoretical investigations. Our total energy results indicate that the formation of MoS2/HfO2 is an exothermic process with an adsorption energy of 34 meV/Å2, which means that it is more stable than similar systems like graphene/HfO2 and MoS2/SiO2. There are no chemical bonds at the MoS2-HfO2 interface. Upon formation of MoS2/HfO2, the electronic charge distribution is mostly localized at the interface region with no net charge transfer between the adsorbed MoS2 sheet and –HfO2 surface. However, the MoS2 sheet becomes n-type doped when there are oxygen vacancies in the HfO2 surface. Further investigation of the electronic distribution reveals that there are no electron- and hole-rich regions (electron-hole puddles) on the MoS2 sheet, which makes this system promising for use in high-speed nanoelectronic devices.

Graphene on amorphous HfO2surface: Anab initioinvestigation

The energetic stability, electronic, and structural properties of graphene adsorbed on the amorphous HfO${}_{2}$ surface (G/HfO${}_{2}$) have been examined through ab initio theoretical investigations. We have considered the graphene adsorption on (i) defect-free (pristine) and defective HfO${}_{2}$ surfaces, (ii) oxygen vacancy, and (iii) interstitial oxygen atoms. We find that the formation of G/HfO${}_{2}$ is an exothermic process, ruled by van der Waals interactions. In (i) and (iii) there is no net charge transfer between the graphene and the HfO${}_{2}$ surface. In contrast, upon the presence of oxygen vacancy, the adsorbed graphene sheet becomes $n$-type doped, due to a donor level lying above the Dirac point of the graphene. The absence of G--HfO${}_{2}$ chemical bonds has been maintained, however, the graphene adsorption energy increases when compared with (i) and (iii). Finally, in (ii) we find that HfO${}_{2}$ surface potential becomes more inhomogeneous, strengthening the formation of electron- and hole-rich regions on the graphene sheet.

Resonant X-Ray Scattering Measurements of a Spatial Modulation of the Cu3dand O2pEnergies in Stripe-Ordered Cuprate Superconductors

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole-rich and hole-poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered La(1.475)Nd(0.4)Sr(0.125)CuO(4) at the Cu L and O K absorption edges that identify an additional feature of stripe order. Analysis of the energy dependence of the scattering intensity reveals that the dominant signature of the stripe order is a spatial modulation in the energies of Cu 3d and O 2p states rather than the large modulation of the charge density (valence) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure and may point to a valence-bond-solid interpretation of the stripe phase.

Evolution of the spin-state transition with doping in La1−xSrxCoO3

The thermally induced spin-state transition of Co${}^{3+}$ ions in the cobaltite LaCoO${}_{3}$, found at temperatures in the range 40 to 120 K, has been the subject of extensive experimental and theoretical investigation. Much less is known about what happens to the spin-state transition in hole-doped La${}_{1\ensuremath{-}x}$Sr${}_{x}$CoO${}_{3}$ (LSCO). The present ${}^{139}$La NMR experiments show that spin-state transitions persist in nanoscale hole-poor regions of the inhomogeneous doped material. In fact, thermally induced spin-state transitions remain important for doping levels close to the metal-insulator critical concentration of ${x}_{C}=0.17$. This finding suggests that the unusual glassy behavior seen in doped LSCO for $x<0.18$ results from the interplay of spin-state transitions in hole-poor regions and ferromagnetism in hole-rich regions.

Ab initio study of the influence of nanoscale doping inhomogeneities in the phase separated state of La1−xCaxMnO3

The chemical influence in the phase separation phenomenon that occurs in perovskite manganites is discussed by means of ab initio calculations. Supercells have been used to simulate a phase separated state, that occurs at Ca concentrations close to the localized itinerant crossover. We have first considered a model with two types of magnetic ordering coexisting within the same compound. This is not stable. However, a non-isotropic distribution of chemical dopants is found to be the ground state. This leads to regions in the system with different effective concentrations, that would always accompany the magnetic phase separation at the same nanometric scale, with hole-rich regions being more ferromagnetic in character and hole-poor regions being in the antiferromagnetic region of the phase diagram, as long as the system is close to a phase crossover.

Hybrid Graphene and Graphitic Carbon Nitride Nanocomposite: Gap Opening, Electron–Hole Puddle, Interfacial Charge Transfer, and Enhanced Visible Light Response

Opening up a band gap and finding a suitable substrate material are two big challenges for building graphene-based nanodevices. Using state-of-the-art hybrid density functional theory incorporating long-range dispersion corrections, we investigate the interface between optically active graphitic carbon nitride (g-C(3)N(4)) and electronically active graphene. We find an inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer. The composite displays significant charge transfer from graphene to the g-C(3)N(4) substrate, which alters the electronic properties of both components. In particular, the strong electronic coupling at the graphene/g-C(3)N(4) interface opens a 70 meV gap in g-C(3)N(4)-supported graphene, a feature that can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors. Additionally, the 2-D planar structure of g-C(3)N(4) is free of dangling bonds, providing an ideal substrate for graphene to sit on. Furthermore, when compared to a pure g-C(3)N(4) monolayer, the hybrid graphene/g-C(3)N(4) complex displays an enhanced optical absorption in the visible region, a promising feature for novel photovoltaic and photocatalytic applications.

Doping of graphene adsorbed on the a-SiO2 surface

We have performed an ab initio theoretical investigation of a graphene sheet adsorbed on amorphous SiO2 surface (G/a-SiO2). We find that graphene adsorbs on the a-SiO2 surface through van der Waals interactions. The inhomogeneous topology of the a-SiO2 clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as hole-rich regions on the graphene. Furthermore, the adsorbed graphene sheet exhibits a net total charge density gain. In this case, the graphene sheet becomes n-type doped, however, no chemical bonds form at the graphene–SiO2 interface. The electronic charge transfer from a-SiO2 to the graphene sheet occurs upon the formation of a partially occupied level lying above the Dirac point. We find that this partially occupied level comes from the three-fold coordinated oxygen atoms in the a-SiO2 substrate.

Coexistence of superconductivity and ferromagnetism in La 1.85Sr 0.15CuO 4–La 2/3Sr 1/3MnO 3 matrix composites

In order to clarify the interaction between superconductivity and magnetism, a series of (La 1.85Sr 0.15CuO 4) 1− x (La 2/3Sr 1/3MnO 3) x matrix composites ( x = 0–0.2, mole fraction) was successfully prepared by solid-state reaction method. Based on the electrical transport measurements, it is found that the superconductivity is gradually suppressed as increasing the content of La 2/3Sr 1/3MnO 3 (LSMO) manganites and that the superconductivity still exists in the composites even though plenty of LSMO is introduced into La 1.85Sr 0.15CuO 4 (LSCO) superconducting cuprate. At the same time, the results of the magnetic measurements also demonstrate the coexistence between superconductivity and ferromagnetism when the CuO 2 planes are intact as follows from the results of X-ray diffraction (XRD). In the whole, the present experiments show that the ferromagnetism in the microscale does not destroy superconductivity for LSCO cuprate in this kind of the matrix composites, and the intercalation of LSMO may lead to an electronic phase separation in LSCO with the hole rich and/or hole poor regions.

Microscopic inhomogeneity and superconducting properties of a two-dimensional Hubbard model for high-Tccuprates

Recent scanning tunneling microscopy measurements on cuprate superconductors have revealed remarkable spatial inhomogeneities in the single-particle energy gap. Using cellular dynamical mean-field theory, we study the zero temperature superconducting properties of a single-band Hubbard model with a spatial modulation of the electron density. We find that the inhomogeneity in the electronic structure results in a substantial spatial variation in the superconducting order parameter and single-particle energy gap, reminiscent of the experimental results. In particular, we find that the order parameter and gap amplitudes in the hole-rich regions are significantly enhanced over the corresponding quantities in a uniform system, if the hole-rich regions are embedded in regions with smaller hole density.