Showing papers on "Doping published in 2013"

Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping

Abstract: We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.

Fullerene Derivative‐Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low‐Bandgap Polymer (PTB7‐Th) for High Performance

Abstract: Modification of a ZnO cathode by doping it with a hydroxyl-containing derivative - giving a ZnO-C60 cathode - provides a fullerene-derivative-rich surface and enhanced electron conduction. Inverted polymer solar cells with the ZnO-C60 cathode display markedly improved power conversion efficiency compared to those with a pristine ZnO cathode, especially when the active layer includes the low-bandgap polymer PTB7-Th.

Degenerate n-doping of few-layer transition metal dichalcogenides by potassium.

Abstract: We report here the first degenerate n-doping of few-layer MoS2 and WSe2 semiconductors by surface charge transfer using potassium. High-electron sheet densities of ~1.0 × 10(13) cm(-2) and 2.5 × 10(12) cm(-2) for MoS2 and WSe2 are obtained, respectively. In addition, top-gated WSe2 and MoS2 n-FETs with selective K doping at the metal source/drain contacts are fabricated and shown to exhibit low contact resistances. Uniquely, WSe2 n-FETs are reported for the first time, exhibiting an electron mobility of ~110 cm(2)/V·s, which is comparable to the hole mobility of previously reported p-FETs using the same material. Ab initio simulations were performed to understand K doping of MoS2 and WSe2 in comparison with graphene. The results here demonstrate the need of degenerate doping of few-layer chalcogenides to improve the contact resistances and further realize high performance and complementary channel electronics.

Metal nanoparticles at mesoporous N-doped carbons and carbon nitrides: functional Mott-Schottky heterojunctions for catalysis.

Abstract: Porous carbons and porous carbon nitrides are well known support materials. Some of these materials are, however, not only a geometric construct for immobilization, enabling mass transport at the same time, but contribute due to their extended electronic structure to a potential catalytic event as such. When appropriate band schemes and electron reactivity are chosen, immobilized metal nanoparticles can exhibit a highly enhanced chemical reactivity. This is due to electronic interaction and electron transfer between the metal and semiconductor, as introduced by Mott and Schottky for planar metal–semiconductor interfaces. A rational choice of mesoporous semiconductor and metal particle allows to create a new generation of catalysts and catalytic schemes with unparalleled performances. This tutorial review highlights the latest development in the synthesis and applications of mesoporous N-doped carbon and carbon nitride supported metal nanoparticles, and concentrates on the catalytic effect of the charge transfer between the metal nanoparticles and semiconductive components.

Prediction of two-dimensional diluted magnetic semiconductors: Doped monolayer MoS 2 systems

Abstract: Using first-principles calculations, we propose a two-dimensional diluted magnetic semiconductor: monolayer MoS2 doped by transition metals. Doping of transition metal atoms from the IIIB to VIB groups results in nonmagnetic states, since the number of valence electrons is smaller or equal to that of Mo. Doping of atoms from the VIIB to IIB groups becomes energetically less and less favorable. Magnetism is observed for Mn, Fe, Co, Zn, Cd, and Hg doping, while for the other dopants from these groups it is suppressed by Jahn-Teller distortions. Analysis of the binding energies and magnetic properties indicates that (Mo,X)S2 (X = Mn, Fe, Co, and Zn) are promising systems to explore two-dimensional diluted magnetic semiconductors.

Possible doping strategies for MoS 2 monolayers: An ab initio study

Abstract: Density functional theory is used to systematically study the electronic properties of doped MoS2 monolayers, wherethedopantsareincorporatedbothviaS/Mosubstitutionorasadsorbates.Amongthepossiblesubstitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS2 conduction band, making the system n type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both n -a nd p-type conductivity, depending on the charge polarity of the adsorbed species.

Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide

Abstract: Bismuth vanadate (BiVO4) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ~3 was observed along the c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ~0.2 cm(2) V(-1) s(-1) for Mo and W:BiVO4 at 300 K. By application of the Gartner model, a hole diffusion length of ~100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott-Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03-0.08 V versus the reversible H2 electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode.

In Situ Fabrication of Porous Graphene Electrodes for High-Performance Energy Storage

Abstract: In the development of energy-storage devices, simultaneously achieving high power and large energy capacity at fast rate is still a great challenge. In this paper, the synergistic effect of structure and doping in the graphene is demonstrated for high-performance lithium storage with ulftrafast and long-cycling capabilities. By an in situ constructing strategy, hierarchically porous structure, highly conductive network, and heteroatom doping are ideally combined in one graphene electrode. Compared to pristine graphene, it is found that the degree of improvement with both structure and doping effects is much larger than the sum of that with only structure effect or doping effect. Benefitting from the synergistic effect of structure and doping, the novel electrodes can deliver a high-power density of 116 kW kg–1 while the energy density remains as high as 322 Wh kg–1 at 80 A g–1 (only 10 s to full charge), which provides an electrochemical storage level with the power density of a supercapacitor and the ene...

Designing band gap of graphene by B and N dopant atoms

Abstract: Ab initio calculations have been performed to study the geometry and electronic structure of boron (B) and nitrogen (N) doped graphene sheets. The effect of doping has been investigated by varying the concentrations of dopants from 2% (one atom of the dopant in 50 host atoms) to 12% (six dopant atoms in 50 atoms host atoms) and also by considering different doping sites for the same concentration of substitutional doping. All of the calculations have been performed using VASP (Vienna Ab initio Simulation Package) based on density functional theory. By B and N doping, p-type and n-type doping are induced, respectively, in the graphene sheet. While the planar structure of the graphene sheet remains unaffected on doping, the electronic properties change from semi-metal to semiconductor with increasing number of dopants. It has been observed that isomers formed by choosing different doping sites differ significantly in the stability, bond length and band gap introduced. The band gap is found to be at a maximum when dopants are placed at same sublattice points of graphene due to the combined effect of symmetry breaking of sublattices and the band gap is closed when dopants are placed at adjacent positions (alternate sublattice positions). These interesting results provide the possibility of tuning the band gap of graphene as required and its application in electronic devices, such as replacements to Pt-based catalysts in Polymer Electrolytic Fuel Cells (PEFCs).

Doping TiO2 with p-block elements: Effects on photocatalytic activity

Abstract: A critical overview is presented on the role that first row p-block elements boron, carbon, nitrogen and fluorine, employed as dopants of TiO2, have in improving the capability of this photocatalyst in harvesting solar light for photocatalytic applications. The peculiar physicochemical properties of doped TiO2 materials are described in terms of the results of both theoretical calculations and photocatalytic efficiency tests, in relation to their bulk and surface features. The limitations of doping titania with non metal elements are outlined and a few recent examples of very promising co-doping effects are discussed.

Enhancement of the Electrical Properties of Graphene Grown by Chemical Vapor Deposition via Controlling the Effects of Polymer Residue

Abstract: Residual polymer (here, poly(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO2/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the −NH2 functional group in formamide that is absorbed in the polymer r...

Sulfur-Doped Graphene via Thermal Exfoliation of Graphite Oxide in H2S, SO2, or CS2 Gas

Abstract: Doping of graphene with heteroatoms is an effective way to tailor its properties. Here we describe a simple and scalable method of doping graphene lattice with sulfur atoms during the thermal exfoliation process of graphite oxides. The graphite oxides were first prepared by Staudenmaier, Hofmann, and Hummers methods followed by treatments in hydrogen sulfide, sulfur dioxide, or carbon disulfide. The doped materials were characterized by scanning electron microscopy, high-resolution X-ray photoelectron spectroscopy, combustible elemental analysis, and Raman spectroscopy. The ζ-potential and conductivity of sulfur-doped graphenes were also investigated in this paper. It was found that the level of doping is more dramatically influenced by the type of graphite oxide used rather than the type of sulfur-containing gas used during exfoliation. Resulting sulfur-doped graphenes act as metal-free electrocatalysts for an oxygen reduction reaction.

Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles

Abstract: TiO2 doped with Cu 2+ initiates the formation of brookite phase along with anatase. Doping of Cu 2+ introduces structural defects into TiO2. The direct evidence is the low intense and broad diffraction peaks. Raman peaks of doped TiO2 are also broad and are blueshifted. Pure TiO2 exhibits an absorption in the UV region, the position of which is shifted towards the visible region on incorporation of Cu into it. The visible absorption peaks arise due to the d-d transition of Cu 2+ in the crystalline environment of TiO2. Incorporation of Cu 2+ distorts the local structure of TiO2, resulting in the loss of octahedral symmetry surrounding Cu 2+ . The Jahn-Teller distortion splits the 2 Eg and 2 T2g state of Cu 2+ into several d states. Interaction of light excites the electron from ground to several of the excited states and gives the visible absorption peaks in the framework of TiO2 .T hese Cu 2+ d states and oxygen defects create band states, thereby favoring electronic transition to these levels and resulting in lowering of band gap of TiO2. A direct confirmation is the increase in the magnitude of Urbach energy with the reduction in the band gap of doped TiO2.

First-Principles Investigations of Metal (Cu, Ag, Au, Pt, Rh, Pd, Fe, Co, and Ir) Doped Hexagonal Boron Nitride Nanosheets: Stability and Catalysis of CO Oxidation

Abstract: By means of first-principles computation, metal (Cu, Ag, Au, Pt, Rh, Pd, Fe, Co, and Ir) doped hexagonal boron nitride nanosheets (h-BNNSs) have been systematically investigated. The strong interaction between the metal atoms and defect sites in h-BNNS, such as the boron vacancy and nitrogen edge, suggests that metal doped h-BN nanosheets (M-BNNSs) should be stable under high temperatures. The catalytic activity of Co doped h-BNNS is also investigated by using CO oxidation as a probe, and the calculated low barrier suggests that the Co-BNNS is a viable catalyst for CO oxidation. Based on electronic structure analysis, the catalytic capacity of Co-BNNS is attributed to the strong mixing between the cobalt 3d orbitals and oxygen 2p orbitals, which activates the adsorbed molecular or atomic oxygen.

Metallic-like Stoichiometric Copper Sulfide Nanocrystals: Phase- and Shape-Selective Synthesis, Near-Infrared Surface Plasmon Resonance Properties, and Their Modeling

Abstract: In the realm of semiconductor nanomaterials, a crystal lattice heavily doped with cation/anion vacancies or ionized atomic impurities is considered to be a general prerequisite to accommodating excess free carriers that can support localized surface plasmon resonance (LSPR). Here, we demonstrate a surfactant-assisted nonaqueous route to anisotropic copper sulfide nanocrystals, selectively trapped in the covellite phase, which can exhibit intense, size-tunable LSPR at near-infrared wavelengths despite their stoichiometric, undoped structure. Experimental extinction spectra are satisfactorily reproduced by theoretical calculations performed by the discrete dipole approximation method within the framework of the Drude–Sommerfeld model. The LSPR response of the nanocrystals and its geometry dependence are interpreted as arising from the inherent metallic-like character of covellite, allowed by a significant density of lattice-constitutional valence-band free holes. As a consequence of the unique electronic pr...

Three-dimensional B,N-doped graphene foam as a metal-free catalyst for oxygen reduction reaction

Abstract: Using a modified chemical vapor deposition (CVD) method, we have prepared a class of new graphene foams (GFs) doped with nitrogen, boron or both. Nitrogen-doped graphene foams (N-GFs) with a nitrogen doping level of 3.1 atom% were prepared by CVD of CH4 in the presence of NH3 while boron-doped graphene foams (B-GFs) with a boron doping level of 2.1 atom% were produced by using toluene and triethyl borate as a carbon and a boron source. On the other hand, graphene foams co-doped with nitrogen (4.5 atom%) and boron (3 atom%) (BN-GFs) were prepared by CVD using melamine diborate as the precursor. In all cases, scanning electron microscope (SEM) images revealed well-defined foam-like microstructures, while electrochemical measurements showed much higher electrocatalytic activities toward oxygen reduction reaction for the doped graphene foams than their undoped counterparts.

Laser cooling of a semiconductor by 40 kelvin

Abstract: Net laser cooling from 290 kelvin to about 250 kelvin is achieved in semiconductor cadmium sulphide ‘nanobelts’ and attributed to strong coupling between excitons and longitudinal optical phonons. Laser cooling of solids, or optical refrigeration, is attractive as a route to compact, cryogen-free and vibration-free refrigeration devices. Laser cooling, based on removing heat due to blue-shifted emission, has been reported previously in rare-earth-metal-doped glasses and crystals. Now Jun Zhang et al. demonstrate a substantial net laser cooling of a semiconductor CdS nanobelt — by about 40 K from 290 K pumped by a 514-nm laser. This achievement opens up a route to optical refrigeration based on semiconductors, where the mechanisms involve excitonic rather than atomic resonances. Laser cooling media based on II–VI semiconductors are potentially highly efficient, capable of achieving extremely low temperatures and readily integrated into optoelectronic devices. Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling, or optical refrigeration, which was proposed by Pringsheim in 19291. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling2, and laser cooling of ultradense gas has been demonstrated by collisional redistribution of radiation3. In solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence. Since the first experimental demonstration in glasses doped with rare-earth metals4, considerable progress has been made, particularly in ytterbium-doped glasses or crystals: recently a record was set of cooling to about 110 kelvin from the ambient temperature, surpassing the thermoelectric Peltier cooler5,6. It would be interesting to realize laser cooling in semiconductors, in which excitonic resonances dominate7,8,9, rather than in systems doped with rare-earth metals, where atomic resonances dominate. However, so far no net cooling in semiconductors has been achieved despite much experimental10,11,12 and theoretical7,8,9,13,14 work, mainly on group-III–V gallium arsenide quantum wells. Here we report a net cooling by about 40 kelvin in a semiconductor using group-II–VI cadmium sulphide nanoribbons, or nanobelts, starting from 290 kelvin. We use a pump laser with a wavelength of 514 nanometres, and obtain an estimated cooling efficiency of about 1.3 per cent and an estimated cooling power of 180 microwatts. At 100 kelvin, 532-nm pumping leads to a net cooling of about 15 kelvin with a cooling efficiency of about 2.0 per cent. We attribute the net laser cooling in cadmium sulphide nanobelts to strong coupling between excitons and longitudinal optical phonons (LOPs), which allows the resonant annihilation of multiple LOPs in luminescence up-conversion processes, high external quantum efficiency and negligible background absorption. Our findings suggest that, alternatively, group-II–VI semiconductors with strong exciton–LOP coupling could be harnessed to achieve laser cooling and open the way to optical refrigeration based on semiconductors.

Energy-Level Matching of Fe(III) Ions Grafted at Surface and Doped in Bulk for Efficient Visible-Light Photocatalysts

Abstract: Photocatalytic reaction rate (R) is determined by the multiplication of light absorption capability (α) and quantum efficiency (QE); however, these two parameters generally have trade-off relations. Thus, increasing α without decreasing QE remains a challenging issue for developing efficient photocatalysts with high R. Herein, using Fe(III) ions grafted Fe(III) doped TiO2 as a model system, we present a novel method for developing visible-light photocatalysts with efficient R, utilizing the concept of energy level matching between surface-grafted Fe(III) ions as co-catalysts and bulk-doped Fe(III) ions as visible-light absorbers. Photogenerated electrons in the doped Fe(III) states under visible-light efficiently transfer to the surface grafted Fe(III) ions co-catalysts, as the doped Fe(III) ions in bulk produced energy levels below the conduction band of TiO2, which match well with the potential of Fe3+/Fe2+ redox couple in the surface grafted Fe(III) ions. Electrons in the surface grafted Fe(III) ions e...

Energy-level structure of nitrogen-doped graphene quantum dots

Abstract: The doping of carbon-based materials is of great importance due to its ability to modulate their optical, electrical and optoelectronic properties. Nitrogen-doped graphene quantum dots (N-GQDs) have received significant attention due to their superior electrocatalytic activity, optical properties and biocompatibility. The energy-level structure of N-GQDs remains unknown, which hinders the development of N-GQDs for various applications. Here, we report a one-pot synthesis method to prepare large-quantity N-GQDs at room temperature and atmospheric pressure under a prolonged reaction time. Using this approach, we can effectively dope N into the N-GQDs. As revealed by electron energy loss spectroscopy, N-doping introduces a new energy level into the electronic structure, which is responsible for tuning the optical properties of the N-GQDs.

Nitrogen-Doped Graphene Sheets Grown by Chemical Vapor Deposition: Synthesis and Influence of Nitrogen Impurities on Carrier Transport

Abstract: A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.

Effect of graphene oxide doping on superconducting properties of bulk MgB2

Abstract: In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB2. Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB2?+?x?wt% of GO (x?=?0, 1, 2, 3, 5, 7 and 10) by sintering at ?850?? C in a reducing atmosphere of Ar/H2 (9:1). The x-ray diffraction results confirm the formation of the MgB2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, Jc(H), has been observed in the entire magnetic field range (0?8?T) for samples x?=?1, 2 and 3 as compared to the undoped sample. In addition to Jc(H), marginal improvements in the upper critical field (Hc2) and the irreversibility field (Hirr) have been observed for the doped samples x?=?1, 2 and 3 with respect to pristine MgB2. Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (Tc) up to a doping level of 10?wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.

Formation of a passivating CH3NH3PbI3/PbI2 interface during moderate heating of CH3NH3PbI3 layers

Abstract: Layers of CH3NH3PbI3 are investigated by modulated surface photovoltage spectroscopy (SPV) during heating in vacuum. As prepared CH3NH3PbI3 layers behave as a p-type doped semiconductor in depletion with a band gap of 1.5 eV. After heating to 140 °C the sign of the SPV signals of CH3NH3PbI3 changed concomitant with the appearance of a second band gap at 2.36 eV ascribed to PbI2, and SPV signals related to charge separation from defect states were reduced after moderate heating.

New Member of BiS2-Based Superconductor NdO1-xFxBiS2

Abstract: We have successfully synthesized the new BiS2-based superconductor NdOBiS2 by F doping. This compound is composed of superconducting BiS2 layers and blocking NdO layers, which indicates that the BiS2 layer is analogous to the CuO2 layer in cuprates or to the Fe–As layer in Fe-based superconductors. We can obtain NdO1-xFxBiS2 with bulk superconductivity by a solid-state reaction. Therefore, NdO1-xFxBiS2 should be a suitable material for elucidating the mechanism of superconductivity in the BiS2 layer.