Showing papers on "Doping published in 2010"
TL;DR: The results suggest that the doping-induced structural and size transition, demonstrated here in NaYF4 upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
Abstract: Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
2,835 citations
TL;DR: A new type of transistor in which there are no junctions and no doping concentration gradients is proposed and demonstrated, which has near-ideal subthreshold slope, extremely low leakage currents, and less degradation of mobility with gate voltage and temperature than classical transistors.
Abstract: All existing transistors are based on the use of semiconductor junctions formed by introducing dopant atoms into the semiconductor material. As the distance between junctions in modern devices drops below 10 nm, extraordinarily high doping concentration gradients become necessary. Because of the laws of diffusion and the statistical nature of the distribution of the doping atoms, such junctions represent an increasingly difficult challenge for the semiconductor industry. Here, we propose and demonstrate a new type of transistor in which there are no junctions and no doping concentration gradients. These devices have full CMOS functionality and are made using silicon nanowires. They have near-ideal subthreshold slope, extremely low leakage currents, and less degradation of mobility with gate voltage and temperature than classical transistors.
2,090 citations
TL;DR: A "structural doping" strategy was reported, in which phosphorus heteroatoms were doped into g-C(3)N(4) via carbon sites by polycondensation of the mixture of the carbon nitride precursors and phosphorus source (specifically from 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid).
Abstract: As a new kind of polymeric semiconductors, graphitic carbon nitride (g-C3N4) and its incompletely condensed precursors are stable up to 550 °C in air and have shown promising photovoltaic applications. However, for practical applications, their efficiency, limited e.g. by band gap absorption, needs further improvement. Here we report a “structural doping” strategy, in which phosphorus heteroatoms were doped into g-C3N4 via carbon sites by polycondensation of the mixture of the carbon nitride precursors and phosphorus source (specifically from 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid). Most of the structural features of g-C3N4 were well retained after doping, but electronic features had been seriously altered, which provided not only a much better electrical (dark) conductivity up to 4 orders of magnitude but also an improvement in photocurrent generation by a factor of up to 5. In addition to being active layers in solar cells, such phosphorus-containing scaffolds and materials are als...
1,109 citations
TL;DR: What is believed to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device is reported.
Abstract: Monolithic lasers on Si are ideal for high-volume and large-scale electronic-photonic integration. Ge is an interesting candidate owing to its pseudodirect gap properties and compatibility with Si complementary metal oxide semiconductor technology. Recently we have demonstrated room-temperature photoluminescence, electroluminescence, and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. Here we report what we believe to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device. The emission exhibited a gain spectrum of 1590-1610 nm, line narrowing and polarization evolution from a mixed TE/TM to predominantly TE with increasing gain, and a clear threshold behavior.
902 citations
TL;DR: It is proposed that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface and become stronger and more irreversible in the presence of moisture and over long periods of time.
Abstract: Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO2 and activates the ability of diatomic oxygen to accept charge from graphene. Gas flow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface.
745 citations
TL;DR: Titanium dioxide (M-TiO2) was synthesized by standard sol−gel methods and characterized by X-ray diffraction, BET surface area measurement, SEM, and UV−vis diffuse reflectance spectroscopy as mentioned in this paper.
Abstract: Titanium dioxide (M-TiO2), which was doped with 13 different metal ions (ie, silver (Ag+), rubidium (Rb+), nickel (Ni2+), cobalt (Co2+), copper (Cu2+), vanadium (V3+), ruthenium (Ru3+), iron (Fe3+), osmium (Os3+), yttrium (Y3+), lanthanum (La3+), platinum (Pt4+, Pt2+), and chromium (Cr3+, Cr6+)) at doping levels ranging from 01 to 10 at %, was synthesized by standard sol−gel methods and characterized by X-ray diffraction, BET surface area measurement, SEM, and UV−vis diffuse reflectance spectroscopy Doping with Pt(IV/II), Cr(III), V(III), and Fe(III) resulted in a lower anatase to rutile phase transformation (A−R phase transformation) temperature for the resultant TiO2 particles, while doping with Ru(III) inhibited the A−R phase transformation Metal-ion doping also resulted in a red shift of the photophysical response of TiO2 that was reflected in an extended absorption in the visible region between 400 and 700 nm In contrast, doping with Ag(I), Rb(I), Y(III), and La(III) did not result in a red s
709 citations
TL;DR: Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide–band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide– band-gap bipolar electronic devices of the future.
Abstract: Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.
661 citations
TL;DR: A critical review on the advancements in sol-gel method of doping TiO2 photocatalysts is provided in this paper, where various solgel and related systems of doping were considered, ranging from co-doping, transition metal ions doping, rare earth metal ion doping to other metals and non-metals ions doping of TiO 2.
Abstract: A critical review on the advancements in sol–gel method of doping TiO2 photocatalysts is provided Various sol–gel and related systems of doping were considered, ranging from co-doping, transition metal ions doping, rare earth metal ions doping to other metals and non-metals ions doping of TiO2 The results available showed that doping TiO2 with transition metal ions usually resulted in a hampered efficiency of the TiO2 photocatalyst, though in some few cases, enhancements of the photocatalytic activity of TiO2 were recorded by doping it with some transition metal ions In most cases, co-doping of TiO2 increases the efficiency of its photocatalytic activity The review reveals that there are some elemental ions that cannot be used to dope TiO2 because of their negative effects on the photocatalytic activity of the catalyst, while others must be used with caution as their doping will create minimal or no impacts on the TiO2 photocatalytic efficiency
495 citations
TL;DR: It is demonstrated that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio, and this ability to tune crystal structure between twin-free zinc blende and stacking-fault-free wurtzite will enhance the performance of nanowire devices.
Abstract: Controlling the crystallographic phase purity of III-V nanowires is notoriously difficult, yet this is essential for future nanowire devices. Reported methods for controlling nanowire phase require dopant addition, or a restricted choice of nanowire diameter, and only rarely yield a pure phase. Here we demonstrate that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio. Phase purity is achieved without sacrificing important specifications of diameter and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temperature coupled with a high V/III ratio. Conversely, a high growth temperature coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. We present a comprehensive nucleation model to explain the formation of these markedly different crystal phases under these growth conditions. Critical to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temperature and V/III ratio. This ability to tune crystal structure between twin-free zinc blende and stacking-fault-free wurtzite not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices, without restrictions on nanowire diameters or doping.
481 citations
TL;DR: In this paper, the authors show that gas-phase doping by means of NH3 plasma exposure is a highly flexible and manufacturable process for graphene electronics, and that the amount of charge transfer can be fine tuned by controlling the exposure time and monitored by the systematic shift in the Raman G mode and the Gds−Vg curves.
Abstract: Here we show that gas-phase doping by means of NH3 plasma exposure is a highly flexible and manufacturable process for graphene electronics. The nitrogen-containing radicals can readily form covalent bonds with the carbon lattice and keep stable in the postannealing for damage restoration. The amount of charge transfer can be fine tuned by controlling the exposure time and monitored by the systematic shift in the Raman G mode and the Gds−Vg curves in transport measurements. The maximum doping level can reach 1.5×1013 cm−2.
463 citations
TL;DR: In this paper, the authors demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) charge transfer complex.
Abstract: Epitaxial graphene on SiC(0001) suffers from strong intrinsic $n$-type doping We demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) Charge neutrality can be reached in monolayer graphene as shown in electron-dispersion spectra from angular-resolved photoemission spectroscopy In bilayer graphene the band-gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band-gap The reduction in the charge-carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy The structural and electronic characteristics of the graphene/F4-TCNQ charge-transfer complex are investigated by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy The doping effect on graphene is preserved in air and is temperature resistant up to $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry
TL;DR: It is demonstrated, both theoretically and experimentally, that cation vacancy can be the origin of ferromagnetism in intrinsic dilute magnetic semiconductors, and its presence reduces the formation energy of Zn vacancy, and thereby stabilizes the zinc vacancy.
Abstract: We demonstrate, both theoretically and experimentally, that cation vacancy can be the origin of ferromagnetism in intrinsic dilute magnetic semiconductors. The vacancies can be controlled to tune the ferromagnetism. Using Li-doped ZnO as an example, we found that while Li itself is nonmagnetic, it generates holes in ZnO, and its presence reduces the formation energy of Zn vacancy, and thereby stabilizes the zinc vacancy. Room temperature ferromagnetism with p type conduction was observed in pulsed laser deposited ZnO:Li films with certain doping concentration and oxygen partial pressure.
TL;DR: The chemical doping of monolayer and bilayer graphene with aluminium, silicon, phosphorus and sulfur was investigated in this paper, where the formation of interlayer bonds in bilayer GAs was investigated.
TL;DR: Control doping of epitaxial SrTiO(3) layers grown by MBE is demonstrated, which allows for low defect densities and precise control over doping concentrations and location and electron mobilities in these films exceed those of single crystals.
Abstract: The study of quantum phenomena in semiconductors requires epitaxial structures with exceptionally high charge-carrier mobilities. Furthermore, low-temperature mobilities are highly sensitive probes of the quality of epitaxial layers, because they are limited by impurity and defect scattering. Unlike many other complex oxides, electron-doped SrTiO(3) single crystals show high (approximately 10(4) cm(2) V(-1) s(-1)) electron mobilities at low temperatures. High-mobility, epitaxial heterostructures with SrTiO(3) have recently attracted attention for thermoelectric applications, field-induced superconductivity and two-dimensional (2D) interface conductivity. Epitaxial SrTiO(3) thin films are often deposited by energetic techniques, such as pulsed laser deposition. Electron mobilities in such films are lower than those of single crystals. In semiconductor physics, molecular beam epitaxy (MBE) is widely established as the deposition method that produces the highest mobility structures. It is a low-energetic, high-purity technique that allows for low defect densities and precise control over doping concentrations and location. Here, we demonstrate controlled doping of epitaxial SrTiO(3) layers grown by MBE. Electron mobilities in these films exceed those of single crystals. At low temperatures, the films show Shubnikov-de Haas oscillations. These high-mobility SrTiO(3) films allow for the study of the intrinsic physics of SrTiO(3) and can serve as building blocks for high-mobility oxide heterostructures.
TL;DR: The experimental results demonstrated that a well-known n-channel semiconductor, [6,6]-phenyl C(61) butyric acid methyl ester, can be effectively doped with N-DMBI by solution processing; the film conductivity is significantly increased by n-type doping.
Abstract: We present here the development of a new solution-processable n-type dopant, N-DMBI. Our experimental results demonstrated that a well-known n-channel semiconductor, [6,6]-phenyl C(61) butyric acid methyl ester (PCBM), can be effectively doped with N-DMBI by solution processing; the film conductivity is significantly increased by n-type doping. We utilized this n-type doping for the first time to improve the air-stability of n-channel organic thin-film transistors, in which the doping can compensate for the electron traps. Our successful demonstration of n-type doping using N-DMBI opens up new opportunities for the development of air-stable n-channel semiconductors. It is also potentially useful for application on solution-processed organic light-emitting diodes and organic photovoltaics.
TL;DR: Al-doped ZnO (AZO) films of ∼100nm thickness with various Al doping were prepared at 150°C by atomic layer deposition on quartz substrates as mentioned in this paper.
Abstract: Al-doped ZnO (AZO) films of ∼100 nm thickness with various Al doping were prepared at 150 °C by atomic layer deposition on quartz substrates. At low Al doping, the films were strongly textured along the [100] direction, while at higher Al doping the films remained amorphous. Atomic force microscopy results showed that Al–O cycles when inserted in a ZnO film, corresponding to a few atomic percent Al, could remarkably reduce the surface roughness of the films. Hall measurements revealed a maximum mobility of 17.7 cm2/V s. Film resistivity reached a minima of 4.4×10−3 Ω cm whereas the carrier concentration reached a maxima of 1.7×1020 cm−3, at 3 at. % Al. The band gap of AZO films varied from 3.23 eV for undoped ZnO films to 3.73 eV for AZO films with 24.6 at. % Al. Optical transmittance over 80% was obtained in the visible region. The detrimental impact of increased Al resulting in decreased conductivity due to doping past 3.0 at. % is evident in the x-ray diffraction data, as an abrupt increase in the opti...
TL;DR: A combination of CdTe QDs and ZnO nanowires for splitting water photoelectrochemically is examined, finding that one-dimensional nanostructures offer the additional potential advantage of improved charge transport over zero- dimensional nanostructureures such as nanocrystals.
Abstract: Increasing demand for clean energy has motivated considerable effort to exploit the properties of various materials in photovoltaics and related solar-harvesting devices. Splitting of water by sunlight to generate hydrogen is one of the forms of energy production with the most potential. Metal oxides such as TiO2, ZnO, and WO3 with various morphologies have been investigated for use in splitting water. However, most of these metal oxides have large band gaps, which limit light absorption in the visible region and overall efficiency. To reduce the band gaps of nanostructured metal oxides, doping and utilization of transition metals, carbon, or nitrogen have been investigated. One possibility is the use of semiconductor nanocrystals, known as quantum dots (QDs), as an alternative to photosensitive dyes. Quantum dots generally offer various significant advantages over dyes. It was recently established that QDs generate multiple electron– hole pairs per photon, improving device efficiency. Quantum dot sensitized nanostructures are widely studied for use in solar cells. However, little work has been done on metal oxide and semiconductor QD-based composite structures for use in water-splitting nanodevices. To elucidate this fundamental issue, we examined a combination of CdTe QDs and ZnO nanowires for splitting water photoelectrochemically (Scheme 1). One-dimensional nanostructures offer the additional potential advantage of improved charge transport over zero-dimensional nanostructures such as nanocrystals. Additionally, the typical electron mobility in ZnO is 10–100 times higher than that in TiO2, so the electrical resistance is lower and the electron-transfer efficiency higher. However, since the overall water-splitting reaction is tough, sacrificial reagents are commonly adopted to evaluate the photocatalytic activity for water splitting. When the photocatalytic reaction is carried out in an aqueous solution that contains a reductant, electron donors, or hole scavengers such as sulfide ions or selenium ions, photogenerated holes irreversibly oxidize the reductant rather than the water. Employment of CdTe QDs in water splitting system has major advantages. CdTe with a more favorable conduction band energy (ECB= 1.0 V vs. NHE) can inject electrons into ZnO faster than CdSe (ECB= 0.6 V vs. NHE). In addition, monolayer deposition of CdTe QDs on the surface of ZnO nanowires would further improve the stability in electrochemical reaction, by avoiding anodic decomposition/corrosion of CdTe and thus enhancing the overall watersplitting performance. During the photoirradiation of CdTe, two reactions can be expected to dominate after initial charge separation [Eqs. (1) and (2)].
TL;DR: Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission and this unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.
Abstract: Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission. Photoexcitation of these nanocrystals gives rise to strongly temperature dependent luminescence involving two distinct but interconnected emissive excited states of the same doped nanocrystals. The ratio of the two intensities is independent of nonradiative effects. The temperature window over which pronounced dual emission is observed can be tuned by changing the nanocrystal energy gap during growth. This unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.
TL;DR: The striking feature is an increase of the Raman intensity of the G mode at a high value of the positive electrode potential, found to be a function of laser excitation energy.
Abstract: Electrochemical charging has been applied to study the influence of doping on the intensity of the various Raman features observed in chemical vapor-deposition-grown graphene. Three different laser excitation energies have been used to probe the influence of the excitation energy on the behavior of both the G and G′ modes regarding their dependence on doping. The intensities of both the G and G′ modes exhibit a significant but different dependence on doping. While the intensity of the G′ band monotonically decreases with increasing magnitude of the electrode potential (positive or negative), for the G band a more complex behavior has been found. The striking feature is an increase of the Raman intensity of the G mode at a high value of the positive electrode potential. Furthermore, the observed increase of the Raman intensity of the G mode is found to be a function of laser excitation energy.
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08 Feb 2010
TL;DR: In this paper, the authors studied the effect of quantum mechanics on semiconductor nanostructures and showed that interference effects in two-dimensional quantum point contacts can affect the performance of the transmission phase.
Abstract: 1. Introduction 2. Semiconductor Crystals 3. Band Structure 4. Envelope function and effective mass approximation 5. Material aspects of heterostructures, doping, surfaces, and gating 6. Fabrication of semiconductor nanostructures 7. Electrostatics of Semiconductor nanostructures 8. Quantum mechanics of semiconductor nanostructures 9. Two-dimensional electron gases in heterostructures 10. Diffusive classical transport in two-dimensional electron gases 11. Ballistic electron transport in quantum point contacts 12. Tunneling transport through potential barriers 13. Multiterminal systems 14. Interference effects in nanostructures 15. Diffusive quantum transport 16. Magnetotransport in two-dimensional systems 17. Interaction effects in diffusive two-dimensional systems 18. Quantum dots 19. Coupled quantum dots 20. Electronic noise in semiconductor nanostructures 21. The Fano effect 22. Measurements of the transmission phase 23. Controlled dephasing experiments 24. Quantum information processing
TL;DR: In this paper, a spin-split defect band populated by charge transfer from a proximate charge reservoir was developed for defect-related ferromagnetism, in which a mixture of Fe2+ and Fe3+ ions in the oxide lattice was considered.
Abstract: The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1–5% of iron cations and find that the room temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism, which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir—in the present case a mixture of Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d–d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as non-magnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an anhysteretic magnetization curve, which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis, which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1–2% of the volume of the films is magnetically ordered.
TL;DR: It is shown that two competing models of doping in LECs are essentially limits of one master model, separated by different rates of carrier injection, and an analytical analysis yields an upper limit for the ratio of drift and diffusion currents.
Abstract: The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p-n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device.
TL;DR: In this article, the effect of substitutional doping on the crystal lattice of α-Fe2O3 films was investigated for photoelectrochemical hydrogen production, and the results indicated that the incident photon conversion efficiency (IPCE) for ∼ 0.45 at-% Al substitution increases by 2- to 3-fold over undoped samples.
Abstract: Substitutional doping can improve the electronic properties of α-Fe2O3 for the solar photoelectrochemical PEC) applications. Generally speaking, nonisovalent substitutional doping helps to enhance the electronic conductivity of α-Fe2O3. However, we found that the introduction of strain in the lattice, which is achieved by isovalent substitutional doping of an Al, can also improve the electronic properties. α-Fe2O3 films with the Al dopant atomic concentration varying from 0 to 10% were prepared by electrodeposition, and their performance for photoelectrochemical hydrogen production was characterized. Results indicate that the incident photon conversion efficiency (IPCE) for ∼0.45 at-% Al substitution increases by 2- to 3-fold over undoped samples. Density-functional theory (DFT) was utilized to interpret the experimental findings. It was shown that although no substantial change to the electronic structure, a contraction of the crystal lattice due to the isovalent replacement Of Fe3+ by an Al3+ benefits the small polaron migration, resulting in an improvement in conductivity compared to the undoped samples.
TL;DR: It is demonstrated that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process, and opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.
Abstract: Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)13 clusters with Mn2+ ions governs the Mn2+ incorporation during the nucleation stage. This highly efficient Mn2+ doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of ∼600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s–d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications. Synthesizing magnetic nanostructures, which could potentially be used in spintronic applications, is quite challenging owing to the difficulty in incorporating magnetic impurities in a non-magnetic matrix. It is now shown that up to 10% Mn can be incorporated in CdSe nanoribbons by nucleation-controlled doping, giving rise to very strong magnetic effects.
TL;DR: First-principles calculations are performed to study the geometry, electronic structure and magnetic properties of light non-metallic atom-doped graphene (B, N, O and F).
Abstract: First-principles calculations are performed to study the geometry, electronic structure and magnetic properties of light non-metallic atom-doped graphene (B, N, O and F). The planar structure and the quasi-linear energy dispersion near the Dirac point remain through doping with B and N atoms, by which p-type doping and n-type doping graphene are respectively induced. A bandgap of about 0.5 eV is generated through O doping, and geometrically the O atom is also in the graphene plane. No magnetic moment is detected in B- , N- and O-doped graphene. For F doping, the F atom bonds with one of the carbon atoms close to the vacancy, with the other two carbon atoms undergoing a Jahn-Teller distortion. A weak polarized magnetic moment of 0.71 µ(B) is detected through F doping.
TL;DR: In this paper, the intensity of the G band increases with increased graphene layers, and the shape of 2D band evolves into four peaks of bilayer graphene from a single peak of monolayer graphene.
Abstract: The recent progress using Raman spectroscopy and imaging of graphene is reviewed. The intensity of the G band increases with increased graphene layers, and the shape of 2D band evolves into four peaks of bilayer graphene from a single peak of monolayer graphene. The G band will blue shift and become narrow with both electron and hole doping, whereas the 2D band will blue shift with hole doping and red shift with electron doping. Frequencies of the G and 2D band will downshift with increasing temperature. Under compressed strain, the upshift of the G and 2D bands can be found. Moreover, the strong Raman signal of monolayer graphene is explained by interference enhancement effect. As for epitaxial graphene, Raman spectroscopy can be used to identify the superior and inferior carrier mobility. The edge chirality of graphene can be determined by using polarized Raman spectroscopy. All results mentioned here are closely relevant to the basic theory of graphene and application in nanodevices.
TL;DR: The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.
Abstract: Catalyst-free p-n homojunction ZnO nanowire (NW) arrays in which the phosphorus (P) and zinc (Zn) served as p- and n-type dopants, respectively, have been synthesized for the first time by a controlled in situ doping process for fabricating efficient ultraviolet light-emitting devices. The doping transition region defined as the width for P atoms gradually occupying Zn sites along the growth direction can be narrowed down to sub-50 nm. The cathodoluminescence emission peak at 340 nm emitted from n-type ZnO:Zn NW arrays is likely due to the Burstein-Moss effect in the high electron carrier concentration regime. Further, the electroluminescence spectra from the p-n ZnO NW arrays distinctively exhibit the short-wavelength emission at 342 nm and the blue shift from 342 to 325 nm is observed as the operating voltage further increasing. The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.
TL;DR: In this paper, the electrical conductivity and Seebeck coefficient of a series of heavily doped regioregular poly(3-hexylthiophene) films are measured between 220 and 370 K.
Abstract: Conducting polymers have recently been suggested as thermoelectric materials for use in large-area thermogenerators. To help assessing the feasibility of this the electrical conductivity and Seebeck coefficient of a series of heavily doped regioregular poly(3-hexylthiophene) films are measured between 220 and 370 K. $p$-type chemical doping of up to 34% is accompanied by the introduction of negatively charged counterions, ${\text{PF}}_{6}^{\ensuremath{-}}$. The counterions produce a disordered environment within which the $p$-type electronic carriers move. This disorder diminishes with increasing doping as the effect of the counterions is smoothed out. Concomitantly the thermally activated electrical conductivity rises strongly while its activation energy decreases. On the other hand, the Seebeck coefficient is found to be weakly dependent on temperature and it decreases with increasing doping. When combined, these results indicate that the thermoelectric power factor reaches a broad maximum between 20% and 31% doping. These results are discussed in terms of the thermally activated hopping-type mobility of bipolarons, deduced from the absence of electron spin resonance signal in the heavily doped materials.
TL;DR: In this paper, the authors demonstrate that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalysis activity by utilizing BiOI as the example.
Abstract: Foreign nonmetal or metal element doping has been widely used to tailor the electronic and band structures of wide band gap binary oxide semiconductor photocatalysts, extending their absorption edges into the visible light range for better utilization of solar light. Besides doping with foreign elements, self-doping can also tune the electronic and band structures of semiconductor photocatalysts but only limited to binary metal oxides, such as oxygen-deficient TiOx (x < 2). In this study, we demonstrate that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalytic activities by utilizing BiOI as the example. Density functional theory calculations revealed that iodine self-doping could effectively tune the electronic structures of BiOI. Motivated by the calculations, iodine self-doped bismuth oxyiodide photocatalysts were synthesized with a soft chemical method to illustrate this band structure tailoring appro...
TL;DR: It is suggested that high visible light photocatalytic efficiency can be achieved by combining conduction band control and surface ion modification, which provides a new approach for rational design and development of high-performance photocatallysts.
Abstract: Band-gap narrowing is generally considered to be a primary method in the design of visible-light-active photocatalysts because it can decrease the photo threshold to lower energies. However, controlling the valence band by up-shifting the top of the band or inducing localized levels above the band results in quantum efficiencies under visible light much lower than those under UV irradiation (such as those reported for N-doped TiO(2): Science 2001, 293, 269. J. Phys. Chem. B 2003, 107, 5483). Herein, we report a systematic study on a novel, visible-light-driven photocatalyst based on conduction band control and surface ion modification. Cu(II)-(Sr(1-y)Na(y))(Ti(1-x)Mo(x))O(3) photocatalysts were prepared by a soft chemical method in combination with an impregnation technique. It is found that Mo(6+) as well as Na(+) doping in the SrTiO(3) can lower the bottom of the conduction band and effectively extend the absorption edge to the visible light region. The Cu(II) clusters grafted on the surface act as a co-catalyst to efficiently reduce the oxygen molecules, thus consuming the excited electrons. Consequently, photocatalytic decomposition of gaseous 2-propanol into CO(2) is achieved, that is, CH(3)CHOHCH(3) + (9)/(2)O(2) → 3CO(2) + H(2)O. For Cu(II)-(Sr(1-y)Na(y))(Ti(1-x)Mo(x))O(3) at x = 2.0% under visible light irradiation, the maximum CO(2) generation rate can reach 0.148 μmol/h; the quantum efficiency under visible light is calculated to be 14.5%, while it is 10% under UV light irradiation. Our results suggest that high visible light photocatalytic efficiency can be achieved by combining conduction band control and surface ion modification, which provides a new approach for rational design and development of high-performance photocatalysts.