Extra Charge Carriers Research Articles

Delicate manipulation of cobalt oxide nanodot clusterization on binder-free TiO2-nanorod photoanodes for efficient photoelectrochemical catalysis

In this work, to probe the crucial role of photo nanosensitizer clusterization played in determining the performance of the photoelectrocatalytic process, an illustrative nano-regional heterostructure was constructed via novel in-situ loading ultra-small cobalt oxide nanodots (<3 nm) onto the surface of titanium dioxide nanorod array grown on FTO substrate. Photoelectrochemical studies showed that photocurrents of TiO2 nanorod anodes could be remarkably improved through loading with CoOx nanosensitizers, benefiting from the broadened light-responsive range and alleviated charge carrier recombination. For the first time, Förster-resonance energy transfer (FRET) assisted nanosensitizer re-excitation effect was observed. Beneficially, the lost energy through radiative-emission from TiO2 could be efficiently reutilized by adjacent CoOx, generating extra charge carriers on nanosensitizers for further improvement of photoelectrocatalytic performance. Importantly, we directly observed the strong clusterization tendency of CoOx and morphology evolution from sub-3 nm nanodots to clusters larger than 20 nm when their loading amount is slightly increased. Furthermore, photoelectrochemical (PEC) performance of photoanodes based on TiO2@CoOx hybrid structures underwent significant enhancement initially and sharp decline eventually. We reason it that the formation of large sensitizer clusters would severely block the charge carrier transfer from nanosensitizers to TiO2 nanorods and lead to exacerbated charge carrier recombination and photoelectrocatalytic performance.

Effect of excess charge carriers and fluid medium on the magnitude and sign of the Casimir-Lifshitz torque

Last year, we reported a perturbative theory of the Casimir-Lifshitz torque between planar biaxially anisotropic materials in the retarded limit [Phys. Rev. Lett. {\bf 120}, 131601 (2018)], which is applied here to study the change of sign and magnitude of the torque with separation distance in biaxial black phosphorus having excess charge carriers. The study is carried out both in vacuum as well as in a background fluid medium. The presence of extra charge carriers and that of an intervening fluid medium are both found to promote enhancement of the magnitude of the torque between identical slabs. The degree of enhancement of the magnitude of torque increases not only with an increased carrier concentration but also with separation distance. In the non-identical case when different planes of anisotropic black phosphorus face each other, owing to the non-monotonic characteristic of the sign-reversal effect of the torque, the enhancement by carrier addition and intervening medium also becomes non-monotonic with distance. In the presence of a background medium, the non-monotonic degree of enhancement of the torque with distance is observed even between identical slabs.

Redox-governed charge doping dictated by interfacial diffusion in two-dimensional materials

Controlling extra charge carriers is pivotal in manipulating electronic, optical, and magnetic properties of various two-dimensional materials. Nonetheless, the ubiquitous hole doping of two-dimensional materials in the air and acids has been controversial in its mechanistic details. Here we show their common origin is an electrochemical reaction driven by redox couples of oxygen and water molecules. Using real-time photoluminescence imaging of WS2 and Raman spectroscopy of graphene, we capture molecular diffusion through the two-dimensional nanoscopic space between two-dimensional materials and hydrophilic substrates, and show that the latter accommodate water molecules also serving as a hydrating solvent. We also demonstrate that HCl-induced doping is governed by dissolved O2 and pH in accordance with the Nernst equation. The nanoscopic electrochemistry anatomized in this work sets an ambient limit to material properties, which is universal to not only 2D but also other forms of materials.

Ferromagnetism and spin-valley liquid states in moiré correlated insulators

Motivated by the recent observation of evidences of ferromagnetism in correlated insulating states in systems with Moir\'{e} superlattices, we study a two-orbital quantum antiferromagnetic model on the triangular lattice, where the two orbitals physically correspond to the two valleys of the original graphene sheet. For simplicity this model has a SU(2)$^s$$\otimes$SU(2)$^v$ symmetry, where the two SU(2) symmetries correspond to the rotation within the spin and valley space respectively. Through analytical argument, Schwinger boson analysis and also DMRG simulation, we find that even though all the couplings in the Hamiltonian are antiferromagnetic, there is still a region in the phase diagram with fully polarized ferromagnetic order. We argue that a Zeeman field can drive a metal-insulator transition in our picture, as was observed experimentally. We also construct spin liquids and topological ordered phases at various limits of this model. Then after doping this model with extra charge carriers, the system most likely becomes spin-triplet/valley-singlet $d+id$ topological superconductor as was predicted previously.

Toward a Metallic Silicon Anode for Practical Lithium-Ion Battery Applications

The current state-of-the-art batteries comprising graphite-based anodes are facing its performance limit for the use in automotive applications, thus urging to develop electroactive materials with high capacity and low operating potentials [1]. Si has shown great promise as a next-generation anode, meeting the above requirements in natural abundance. However, unavoidable large volume change upon lithium insertion/extraction causes particle fracture and disintegration of the electrode, resulting in fast degradation of battery along with poor electric conductivity being the main hurdle for fast-chargeable battery design [2]. Recently, interesting breakthroughs have been proposed principally in nanostructuring of Si, composite formation with conductive carbons and mechanically stable binders, which achieved long-term cycle stability and rate capability [3]. Yet, these approaches lead to lowering the electrode density, initial Coulombic efficiency and energy density of battery and thus utilizing the Si microparticles can be rather feasible toward a practical system while stress-driven particle degradation becomes more serious. The existing technologies have focused on coalescing the fractured particles, not preventing itself, and have not considered ionic diffusion of Li-ion across the microparticles as well as the electronic conduction. Herein, we propose the material design concept of Si microparticles by integrating the chalcogen component into the bulk structure through a low temperature doping strategy. The low electric conductivity of intrinsic Si (~10-4 S m-1) with a near-insulator property undergoes a transition into the metallic state via this new method unlike other dopants of boron or phosphorous that only gives extra charge carriers without insulator-to-metal transition. In addition to electronic conduction, the chalcogen chains restrict the saturation of Si dangling bonding during recrystallization process of low temperature doping and create the internal channels inside the crystalline Si lattice. The incorporation of Li-ion channels further increases Li-ion diffusivity without any barriers in case of undoped Si microparticles due to the interfaces of lithium silicide and amorphous Si. Interestingly, chalcogen chains can sustain its internal structure with high flexibility and robustness which is directly corroborated by microscopy analysis, and lithium-induced intermediate can maintain the metallic nature at the interfaces that enhance the Li-ion diffusion over the cycles. Further, the porous but minimized surface area of Si structures increases the initial reversibility, extends the cycle life of battery up to hundreds of cycles with a high structural stability and facilitates fast-charging ability. [1] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nat. Mater. 2005, 4, 366. [2] H. Wu, Y. Cui, Nano Today 2012, 7, 414. [3] J. Ryu, T. Chen, T. Bok, G. Song, J. Ma, C. Hwang, L. Luo, H.-K. Song, J. Cho, C. Wang, S. Zhang, S. Park, Nat. Commun. 2018, 9, 2924

ZnO nanowire sensitization with Ru polypyridyl complexes: charge-transfer probed by spectral and relaxation photocurrent measurements

Dye-sensitized ZnO nanowire (NW) electrodes were fabricated using Ru polypyridyl complexes that use nitrile instead of carboxylic group as anchoring unit to the NW surfaces. The complexes formula is [Ru(bpy)3−x(Mebpy-CN)x]2+ (x = 1−3, bpy = 2,2′-bipyridine, Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile). The ZnO NWs were grown by a vapor transport method on insulating SiO2/Si substrates. The sensitized ZnO NW electrodes were studied by electron microscopy, Raman and PL spectroscopies, and spectral and relaxation photocurrent measurements. The Raman spectra confirm that the complexes were effectively anchored to the ZnO NWs through one of the pendant nitrile groups of the bipyridyl ligands. The nanostructured morphology of the NW electrodes was maintained so that their light trapping characteristics were preserved. The Ru complexes were found to be excellent sensitizers of the ZnO NWs, improving by orders of magnitude their photocurrent in the visible region. The Fe-based complex of formula [Fe(Mebpy-CN)3](PF6)2 was also tested; however it did not show any sensitizing effect. An order of magnitude shortening of the persistent photocurrent relaxation times (after the illumination is interrupted) was found to occur upon successful sensitization of the ZnO NWs with the Ru complexes. This effect is interpreted in terms of hole traps at ∼1 eV above the ZnO valence band edge, which are lowered by ∼50–60 meV in the soaked samples due to screening of the trap centers provided by the extra photoexcited charge carriers transferred from the sensitizing complex to the NWs.

Excessively Doped PbTe with Ge-Induced Nanostructures Enables High-Efficiency Thermoelectric Modules

Summary The application of thermoelectrics in waste heat recovery requires high conversion efficiency, which is best achieved through the combination of progressive performance-enhancing strategies. In this study, we combined engineered doping, nanostructuring, and module fabrication in PbTe-based thermoelectrics to generate high-ZT materials along with high-efficiency modules. The use of excess Na (4%) as a p-type dopant greatly enhanced the thermoelectric power factor by generating extra charge carriers at high temperature. The addition of minute amounts of Ge (≤1%) generated nanoprecipitates, which greatly reduced the lattice thermal conductivity. An optimal ZT of ∼1.9 at ∼805 K was achieved for Pb0.953Na0.040Ge0.007Te as a p-type leg, which was combined with PbTe0.9964I0.0036 as an n-type leg to fabricate thermoelectric modules. An exceptionally high efficiency of ∼12% for a temperature difference of 590 K was obtained in a cascade Bi2Te3/nanostructured PbTe module with eight p-n pairs.

Ionic liquid effect for efficiency improvement in poly(methyl acrylate)/poly(vinyl acetate)-based dye-sensitized solar cells

Polymer electrolytes based on the blends of poly(methyl acrylate) (PMA) and poly(vinyl acetate) (PVAc) were prepared using tetrapropylammonium iodide (TPAI) salt and 1-butyl-3-methylimidazolium iodide (BMII) ionic liquid. Single transition between the glass transition temperatures of the constituents indicates the miscibility of PMA and PVAc over the entire composition range. The presence of TPAI and BMII does not affect the miscibility of PMA and PVAc. The complexation in the PMA/PVAc-TPAI-BMII is supported by the differential scanning calorimetry and Fourier transform infrared spectroscopic studies. Conductivity increases by two order of magnitudes with the incorporation of 5 wt% of BMII. Conductivity increment is due to the extra charge carriers provided by BMII. Electrolyte films were sandwiched between titanium dioxide photoanode and platinum counter electrode for dye-sensitized solar cells (DSSCs) assembly. The addition of BMII increases the efficiency of DSSC from 2.79% to 4.62%.

Self-doped iron-based superconductors with intergrowth structures

The key structural unit of iron-based superconductors (FeSCs) is the Fe2X2 (X refers to a pnictogen or a chalcogen element) layer which stacks alternately along the crystallographic c axis with other spacer layers. This structural feature makes it possible to find FeSCs via rational material design. In this paper, we first review the crystal structure of FeSCs along with the relevant progress. Then we summarize several rules for designing the intergrowth structures. The rules include the following points. 1) Lattice match between the intergrowth layers should be good enough. Quantitatively, the lattice mismatch, defined as =2(aA-aB)/(aA + aB), where aA and aB are respectively the lattice parameters of the two constituent compounds, should be no larger than~2%. 2) The charge transfer between the intergrowth layers is mostly essential, which acts as the glue that combines the constituent layers together. Such a charge transfer also induces the extra charge carriers in the superconducting key layer to give rise to superconductivity without extrinsic doping (so-called self doping). 3) For the structure with similar yet crystallographically distinct sites, one needs to avoid forming solid solutions. 4) Each intergrowth layer is preferably thermodynamically stable. 5) The designed structure can be preliminary evaluated with the hard and soft acids and bases conception and ab initio calculations. Following these empirical rules, we introduce and analyze five examples, namely, (Li0.8Fe0.2OH)FeSe, Ba2Ti2Fe4As4O, 42214-type Ln4Fe2As2Te1-xO4 (Ln=Pr, Sm, Gd), 1144-type AkAeFe4As4 (Ak=K, Rb, Cs; Ae=Ca, Sr, Eu), and 12442-type AkCa2Fe4As4F2 and AkLn2Fe4As4O2 (Ak=K, Rb, Cs; Ln=Nd-Ho). For the last 12442-type compounds, we also discuss the unusual relation between superconducting transition temperature and crystallographic parameters. We conclude that the structural-design approach may serve as an effective route, not only for discovering new FeSCs but also for exploring other relevant functional materials with similar crystal structures.

Ambipolar thermoelectric power of chemically-exfoliated RuO2 nanosheets

The electrical conductivity and Seebeck coefficient of RuO2 nanosheets are enhanced by metal nanoparticle doping using Ag-acetate solutions. In this study, RuO2 monolayer and bilayer nanosheets exfoliated from layered alkali metal ruthenates are transferred to Si substrates for device fabrication, and the temperature dependence of their conductivity and Seebeck coefficients is investigated. For pristine RuO2 nanosheets, the sign of the Seebeck coefficient changes with temperature from 350–450 K. This indicates that the dominant type of charge carrier is dependent on the temperature, and the RuO2 nanosheets show ambipolar carrier transport behavior. By contrast, the sign of the Seebeck coefficient for Ag nanoparticle-doped RuO2 nanosheets does not change with temperature, indicating that the extra charge carriers from metal nanoparticles promote n-type semiconductor behavior.

Controlled electrical doping of organic semiconductors: a combined intra- and intermolecular perspective from first principles.

The process of introducing extra charge carriers into organic semiconductors, or simply molecular doping, takes place via intermolecular charge transfer from the donor to the acceptor molecule. Using density functional theory calculations on diverse donor-acceptor pairs, we show that there are two modes of charge transfer; in one, charge transfer is controlled by the sign and in the other, by the magnitude of the donor HOMO-acceptor LUMO level offset. Despite doping being an intermolecular process, the identification of the transfer modes requires a full account of intramolecular geometric changes during charge transfer. We further show that the degree of charge transfer can be represented entirely by the reorganization energy, a common measure of geometric changes, of either the donor or the acceptor.

Finding order in exotic superconductors

Superconductivity Physicists can coax some copper-oxide compounds into becoming superconducting by chemically adding extra charge carriers: holes or electrons. Concentrating on hole-doped materials, researchers have found a host of different phases in the neighborhood of or co-existing with

Recombination in liquid filled ionisation chambers with multiple charge carrier species: Theoretical and numerical results

Liquid-filled ionisation chambers (LICs) are used in radiotherapy for dosimetry and quality assurance. Volume recombination can be quite important in LICs for moderate dose rates, causing non-linearities in the dose rate response of these detectors, and needs to be corrected for. This effect is usually described with Greening and Boag models for continuous and pulsed radiation respectively. Such models assume that the charge is carried by two different species, positive and negative ions, each of those species with a given mobility. However, LICs operating in non-ultrapure mode can contain different types of electronegative impurities with different mobilities, thus increasing the number of different charge carriers. If this is the case, Greening and Boag models can be no longer valid and need to be reformulated.In this work we present a theoretical and numerical study of volume recombination in parallel-plate LICs with multiple charge carrier species, extending Boag and Greening models. Results from a recent publication that reported three different mobilities in an isooctane-filled LIC have been used to study the effect of extra carrier species on recombination. We have found that in pulsed beams the inclusion of extra mobilities does not affect volume recombination much, a behaviour that was expected because Boag formula for charge collection efficiency does not depend on the mobilities of the charge carriers if the Debye relationship between mobilities and recombination constant holds. This is not the case in continuous radiation, where the presence of extra charge carrier species significantly affects the amount of volume recombination.

ARPES study of the effect of Cu substitution on the electronic structure of NaFeAs

Using high-resolution angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe${}_{1\ensuremath{-}x}$Cu${}_{x}$As ($x=0.019,0.045,0.14$). With increasing the doping concentration, we found that the Cu dopant introduces extra charge carriers. The overall band dispersions barely change with doping, suggesting that the Cu substitution does not affect local correlations. Similar to the case of NaFe${}_{1\ensuremath{-}x}$Co${}_{x}$As, one electron pocket emerges at the Brillouin zone center at high doping levels. Moreover, the near-E${}_{F}$ spectral weight decreases with increasing the Cu dopant, which explains why the NaFe${}_{1\ensuremath{-}x}$Cu${}_{x}$As shows a poor electronical conductivity at high doping levels.

Fringe Field Effect on Electrical Characteristics of Pentacene Thin-Film Transistors

This paper reports the effects of the fringe field on the operating characteristics of geometrically-modified pentacene thin-film transistors (TFTs) with a polymeric gate insulator. When the dimensions of the pentacene film structurally exceeded the channel region, the output current at a given voltage was overestimated by the extra charge carriers, which were affected by the fringe field in the peripheral territory away from the channel region. By examining the range of operating frequencies, the fringe field was found to have a much more serious effect at low frequencies due to the slow movement of charge carriers stored around the periphery of the patterned source and drain electrodes. This suggests that minimizing the fringe field is important for improving the dynamic characteristics of the organic transistor-based circuits operating at low frequencies.

X-Ray Spectroscopic Study of the Conduction Band of K3:Anthracene and K3:Phenanthrene

We study anthracene and phenanthrene doped with potassium using X-ray absorption spectroscopy and electronic structure calculations. In addition, a comparison of molecular orbital calculations and solid state density functional theory calculations are presented. We find that potassium-doping partially populates the LUMO level of anthrancene and phenanthrene and that both the measured and calculated electronic structures of the doped systems are quite different from that of the pristine molecular systems. This suggests that the extra charge carriers in the doped system are responsible for the increased conductivity and greater intermolecular interaction. Finally, our calculations suggest that both K3:phenanthrene and K3:anthracene have reduced or nonexistent band gaps as compared to their pure counterparts, further supporting the conclusion that doping is responsible for increased conductivity.

Optical Probing of the Electronic Interaction between Graphene and Hexagonal Boron Nitride

Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few percent decrease in the Fermi velocity (v(F)) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in v(F) and mechanical strain but not by charge doping, unlike graphene supported on SiO₂ substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping.

Preparation of large-area dye-sensitized solar cells based on hydrothermally synthesized nitrogen-doped TiO2 powders

Low-cost, yellowish, nanocrystalline nitrogen-doped titanium dioxide (N-doped TiO2) powder was synthesized by a hydrothermal method. The as-prepared N-doped TiO2 powder was characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–Vis absorption spectra, X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller analysis techniques. The grain size of the prepared powder was around 13 nm as estimated by both Scherrer’s method and TEM images. The effect of the ratio of N-doped TiO2 particles to Degussa P25 on the photovoltaic performance of large-area dye-sensitized solar cells (DSSCs) was also investigated. The N-doped TiO2 electrode showed higher photovoltaic performance compared with that of pure P25 at constant irradiation of 100 mW cm−2, which is attributed to the large pore size and high surface area of N-doped TiO2 resulting in the introduction of extra charge carrier pathways that could be beneficial for overall charge transportation. Energy conversion efficiency of 5.12 % was achieved in a DSSC device with active area of 51.19 cm2.

Evolution of the band structure of superconducting NaFeAs from optimally doped to heavily overdoped Co substitution using angle-resolved photoemission spectroscopy

Using angle-resolved photoemission spectroscopy, we studied the evolution of electronic structure of NaFe${}_{1\ensuremath{-}x}$Co${}_{x}$As from an optimally doped superconducting compound ($x=0.028$) to a heavily overdoped nonsuperconducting one ($x=0.109$). As in ``122''-type iron pnictides, our data suggest that the Co dopant in NaFe${}_{1\ensuremath{-}x}$Co${}_{x}$As supplies extra charge carriers and shifts the Fermi level accordingly. The overall band renormalization remains basically the same throughout the doping range we studied, suggesting that the local magnetic and electronic correlations are not affected by carrier doping. In the $x=0.109$ compound, the holelike bands around the zone center $\ensuremath{\Gamma}$ move to deeper binding energies and an electron pocket appears instead, resulting in a Fermi surface topology similar to that of $A$${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ ($A=\mathrm{K}$, Cs, Rb, Tl). Our data suggest that a balance between itinerant properties of mobile carriers and local interactions plays an important role for the superconductivity in these materials.

Effect of Zn doping in hole-type 1111 phase (Pr, Sr)FeAsO

There is an anomalous broad hump in the normal state resistivity in hole-type 1111 phase FeAs-based superconductors and its origin is an open issue. We study the effect of Zn doping on this anomaly in order to determine whether it is associated with the residual structural/antiferromagnetic (AFM) phase transition as in the parent compounds. A series of Zn doped Pr0.8Sr0.2FeAsO samples are prepared and their resistivity, magnetoresistance, Hall effect and specific heat are measured. Zn doping should not introduce extra charge carriers, and instead it can suppress the structural/AFM transition efficiently in the parent LaFeAsO system. The hump in resistivity remains unchanged with 6% Zn doping in Pr0.8Sr0.2FeAsO. The measurements of magnetoresistance reveal that the magnetoresistance is negligible in the Zn doped Pr0.8Sr0.2FeAsO samples, in contrast to the large positive magnetoresistance below the temperature of structure/AFM phase transition in the parent compound PrFeAsO. The results indicate that the anomalous broad hump in resistivity does not originate from the structural/AFM transition. The Hall effect and specific heat data are also consistent with this conclusion.