High Charge Carrier Density Research Articles

Electrically active boron doping in the core of Si nanocrystals by planar inductively coupled plasma CVD

An improvement in the doping efficiency in p-type nc-Si:H, a two-phase structure consisting of Si-nanocrystallites embedded in an amorphous matrix, has been pursued via low-temperature, spontaneous, plasma processing of low-pressure and high-density SiH4 plasma with added B2H6, diluted in H2, in inductively coupled plasma CVD. With increased dopant incorporation, the gradually reduced overall crystallinity systematically dominates via an ultrananocrystalline component in the thermodynamically stable ⟨220⟩ orientation. The microstructure factor decreases continuously, and the bonded-H content of the network increases arbitrarily, with a significant fraction being associated with Si in an Si-H-Si plateletlike configuration. Effective doping by B atoms, mostly in fourfold coordination with Si in its nanocrystalline core, contributes to the rapid increase in conductivity of the doped p-nc-Si:H network, up to a moderate B2H6 flow rate. However, gradually enhanced doping by threefold coordinated electrically inactive B-atoms, mostly at the nanocrystalline grain boundary at a higher B2H6 flow rate, induces gross structural deviation in the degree of nanocrystallinity and reduces dark conductivity substantially. Further, the logarithmic magnitude of the conductivity prefactor (σ0) demonstrates the negative characteristic energy (EMN) in the Meyer–Neldel (MN) relation. The high density of charge carriers in the heavily doped configuration of the nc-Si network forms a deep and continuous band tail near the valence-band edge and induces subsequent narrowing of the bandgap as well as a shift of the Fermi level into the valence band. Degenerate-semiconductor type behavior has been accomplished in a heavily doped p-nc-Si:H network, demonstrating reverse MN characteristics in electrical transport.

Impact of Intrinsic Capacitances on the Dynamic Performance of Printed Electrolyte-Gated Inorganic Field Effect Transistors

Electrolyte-gated, printed field-effect transistors exhibit high charge carrier densities in the channel and thus high on-currents at low operating voltages, allowing for the low-power operation of such devices. This behavior is due to the high area-specific capacitance of the device, in which the electrolyte takes the role of the dielectric layer of classical architectures. In this paper, we investigate intrinsic double-layer capacitances of ink-jet printed electrolyte-gated inorganic field-effect transistors in both in-plane and top-gate architectures by means of voltage-dependent impedance spectroscopy. By comparison with deembedding structures, we separate the intrinsic properties of the double-layer capacitance at the transistor channel from parasitic effects and deduce accurate estimates for the double-layer capacitance based on an equivalent circuit fitting. Based on these results, we have performed simulations of the electrolyte cutoff frequency as a function of electrolyte and gate resistances, showing that the top-gate architecture has the potential to reach the kilohertz regime with proper optimization of materials and printing process. Our findings additionally enable accurate modeling of the frequency-dependent capacitance of electrolyte/ion gel-gated devices as required in the small-signal analysis in the circuit simulation.

CdS supported on electrochemically reduced rGO for photo reduction of water to hydrogen

The paper reports synthesis of CdS supported rGO where reduction of GO to rGO was carried out electrochemically in situ. This catalyst had a better activity for photo-electrochemical dissociation of water to hydrogen. Catalysts were characterized by XRD, FTIR, DRS, XPS, EIS and Mott-Schottky analysis. It is reported that this catalyst showed formation of heterojunction at the interface of CdS and rGO by chemical interaction between the two. This resulted in a greater band bending, a higher charge carrier density, low resistance for electronic mobility and low recombination rate. A better activity of the catalyst is due to the above mentioned attributes.

Optimizations of r GO supported CdS photo-electrocatalyst for dissociation of water

The paper reports fabrication of electrode containing rGO supported CdS catalyst for photoelectrochemical reduction of water to hydrogen. The reduction of GO to r GO was carried out electrochemically in situ. The catalyst which contained 10 wt % GO with CdS showed the best activity. Catalysts were characterized by XRD, FTIR, DRS, SEM, TEM, SED, XPS, PL, EIS and Mott-Schottky analysis. It is reported that incorporation of r GO to CdS resulted into formation of heterojunction at the interface of CdS and rGO by a chemical interaction between the two. Consequently, a greater band bending, a higher charge carrier density, a low resistance for electronic mobility and low recombination rate result in. The high activity of the catalyst is due to the above mention attributes. These effects become prominent with the increase in the loading of GO, however, any loading in excess of 10 wt % resulted into its shielding of CdS which adversely affected the activity.

Zn2+ Ion Surface Enrichment in Doped Iron Oxide Nanoparticles Leads to Charge Carrier Density Enhancement.

Here, we report the development of monodisperse Zn-doped iron oxide nanoparticles (NPs) with different amounts of Zn (ZnxFe3–xO4, 0 < x < 0.43) by thermal decomposition of a mixture of zinc and iron oleates. The as-synthesized NPs show a considerable fraction of wüstite (FeO) which is transformed to spinel upon 2 h oxidation of the NP reaction solutions. At any Zn doping amounts, we observed the enrichment of the NP surface with Zn2+ ions, which is enhanced at higher Zn loadings. Such a distribution of Zn2+ ions is attributed to the different thermal decomposition profiles of Zn and Fe oleates, with Fe oleate decomposing at much lower temperature than that of Zn oleate. The decomposition of Zn oleate is, in turn, catalyzed by a forming iron oxide phase. The magnetic properties were found to be strongly dependent on the Zn doping amounts, showing the saturation magnetization to decrease by 9 and 20% for x = 0.05 and 0.1, respectively. On the other hand, X-ray photoelectron spectroscopy near the Fermi level demonstrates that the Zn0.05Fe2.95O4 sample displays a more metallic character (a higher charge carrier density) than undoped iron oxide NPs, supporting its use as a spintronic material.

Effects of Fe doping on the photoelectrochemical properties of CuO photoelectrodes

In this study, CuO photoelectrodes doped with various iron concentrations were grown by using a modified chemical bath deposition method. We investigated the effects of the iron doping concentration on the morphological, structural, optical, electrical and photoelectrochemical properties of the CuO photoelectrode by using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), UV-visible spectrophotometry (UV-Vis), electrochemical impedance spectroscopy (EIS) and potentiostatic/galvanostatic analysis, respectively. In this study, the smallest dislocation density, highest charge carrier density and lowest charge transfer resistance values were obtained from the sample with an Fe doping concentration of 2 at%. As a result, the maximum photocurrent density value of −1.44 mA/cm2 was obtained at −0.55 V (vs. SCE) from the photoelectrode with an Fe doping concentration of 2 at%, which was 53% higher than that of the undoped CuO photoelectrode.

Quantum spin fluctuations and evolution of electronic structure in cuprates

Correlation effects in CuO2 layers give rise to a complicated landscape of collective excitations in high-Tc cuprates. Their description requires an accurate account for electronic fluctuations at a very broad energy range and remains a challenge for the theory. Particularly, there is no conventional explanation of the experimentally observed “resonant” antiferromagnetic mode, which is often considered to be a mediator of superconductivity. Here we model spin excitations of the hole-doped cuprates in the paramagnetic regime and show that this antiferromagnetic mode is associated with electronic transitions between anti-nodal X and Y points of the quasiparticle band that is pinned to the Fermi level. We observe that upon doping of 7–12%, the electronic spectral weight redistribution leads to the formation of a very stable quasiparticle dispersion due to strong correlation effects. The reconstruction of the Fermi surface results in a flattening of the quasiparticle band at the vicinity of the nodal MΓ/2 point, accompanied by a high density of charge carriers. Collective excitations of electrons between the nodal MΓ/2 and XM/2 points form the additional magnetic holes state in magnetic spectrum, which protects the antiferromagnetic fluctuation. Further investigation of the evolution of spin fluctuations with the temperature and doping allowed us to observe the incipience of the antiferromagnetic ordering already in the paramagnetic regime above the transition temperature. Additionally, apart from the most intensive low-energy magnetic excitations, the magnetic spectrum reveals less intensive collective spin fluctuations that correspond to electronic processes between peaks of the single-particle spectral function.

Characteristics of TiO2 Compact Layer prepared for DSSC application

Dye-sensitized solar cells (DSSCs) offer an economically reliable and suitable alternative in moderating the challenges presented by the existing convectional photovoltaic cells. However, the efficiency of dye-sensitized solar cells has remained relatively low. For this reason, this research was aimed at studying the characteristics of TiO 2 compact layer that can be applied in DSSCs as a way of improving efficiency. To achieve this, TiO 2 compact layer was deposited on a conductive glass substrate by using Holmarc’s Spray Pyrolysis system, using Ultrasonic Spray Head and spraying in the vertical geometry. X-ray Diffraction studies revealed that TiO 2 compact layer was of anatase phase and had tetragonal crystalline structure. Raman spectroscopy showed that the most intense peak appeared at 142 cm -1 due to the external vibration of the anatase structure. Hall Effect studies revealed that TiO 2 compact layer has a high density of charge carriers’ value of 1.25 × 1019 cm -3 hence it can be used in DSSC applications.

Brightness Enhancement in Pulsed-Operated Perovskite Light-Emitting Transistors.

Perovskite light-emitting field-effect transistors (PeLEFETs) provide a versatile device architecture to control transport and electroluminescence properties of hybrid perovskites, enabling injection of high charge carrier density and spatial control of the radiative recombination zone. Ionic screening and organic cation polarization effects typical of metal-halide perovskites, however, critically affect PeLEFET efficiency and reliability. In this work, we demonstrate a new device operation mode based on high-frequency modulation of the applied voltages, which allows significant reduction of ionic drift/screening in methylammonium lead iodide light-emitting transistors. In optimized top contact PeLEFETs, AC operation results in brighter and more uniform electroluminescence compared to DC-driven devices, whereas high-frequency modulation enables electroluminescence emission up to room temperature.

Low-Voltage, Flexible IGZO Transistors Gated by PSSNa Electrolyte

A carving, cutting, and flip-chip bonding process is proposed for the fabrication of flexible electric double layer transistors (EDLTs) with low cost. Solution processed poly(styrenesulfonic acid sodium salt) is used as a gate dielectric. The large EDL-specific capacitance ( $4.5~\mu \text{F}$ /cm2 at 20 Hz) can induce very high charge carrier density in the InGaZnO (IGZO) channel layer, enabling the EDLTs to operate at a single-battery-drivable low voltage of 1.0 V with a high on-current of 10−4A. The effect of IGZO layer thickness on the performance of EDLTs was investigated. The flexible EDLT with optimized IGZO thickness of 100 nm has achieved a high on/off ratio of $1.4 \times 10^{7}$ , a low threshold voltage of 0.51 V, a saturated field-effect mobility of 1.14 cm2/Vs, and high positive gate bias stress stability. Furthermore, the achieved subthreshold swing, 76 mV/dec, is very close to the theoretical ideal minimum value.

Co-catalyst-free photocatalytic hydrogen evolution on TiO2: Synthesis of optimized photocatalyst through statistical material science

An active TiO2 for co-catalyst-free photocatalytic hydrogen evolution was successfully synthesized employing a simple Evaporation-Induced Self-Assembly (EISA) method that was developed and optimized with the help of Design of Experiment (DoE) coupled with Full Factorial Design (FFD) methodology. Coupling DoE with FFD provides a statistical tool for optimizing the synthesis process while carrying out the smallest number of experiments. This tool builds a statistical framework to determine the significance of the studied factors, i.e., titanium-precursor type, surfactant type and surfactant quantity, along with their potential interactions, as well as with their optimum levels. The choice of the titanium-precursor type is found to be the predominant factor affecting the efficiency of TiO2 for hydrogen gas evolution. The interaction between precursor type and surfactant type is also statistically significant. The statistically optimized study identifies that combining F-108 amphiphilic block copolymers with titanium(III) chloride solution leads to TiO2 exhibiting the highest photocatalytic efficiency for the generation of molecular hydrogen. The thus prepared TiO2 shows relatively high photocatalytic hydrogen evolution rates (1.22 mmol h−1 g−1) compared to the commercially available TiO2 photocatalysts which are not active for hydrogen generation in the absence of a co-catalyst. Significant photocatalytic reforming of ethanol is achieved over the synthesized bare TiO2 with the formation of acetaldehyde as the main by-product in the gas phase. This unexpected photocatalytic performance is mainly attributed to the shift of flat band potential towards more negative potentials as revealed from the characterization results in addition to the high density of charge carriers exhibiting longer lifetime shown by laser transient reflectance measurements. The latter showed the presence of a high number of trapped states, which are beneficial for the photocatalytic properties.

Equilibrium and non-equilibrium free carrier dynamics in 2D Ti3C2Tx MXenes: THz spectroscopy study

MXenes is an emerging class of 2D transition metal carbides, nitrides and carbonitrides which exhibit large conductivity, ultrahigh volumetric capacitance, high threshold for light-induced damage and nonlinear optical transmittance, making them attractive candidates for a variety of optoelectronic and electrochemical applications. Here, we report on equilibrium and non-equilibrium free carrier dynamics of Ti3C2Tx gleaned from THz spectroscopic studies for the first time. Ti3C2Tx showed high (~2 × 1021 cm−3) intrinsic charge carrier density and relatively high (~34 cm2 V−1 s−1) mobility of carriers with an exceptionally large, ~46 000 cm−1 absorption in the THz range, which suggests that Ti3C2Tx is well suited for THz detection. We also demonstrate that Ti3C2Tx conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared, 800 nm pulses is found to cause transient suppression of the conductivity that recovers over hundreds of picoseconds. The possibility of control over THz transmission and conductivity by photoexcitation suggests the promise for application of Ti3C2Tx Mxenes in THz modulation devices and variable electromagnetic shielding.

Periodically Ordered Nanoporous Perovskite Photoelectrode for Efficient Photoelectrochemical Water Splitting.

Nonmetallic materials with localized surface plasmon resonance (LSPR) have a great potential for solar energy harvesting applications. Exploring nonmetallic plasmonic materials is desirable yet challenging. Herein, an efficient nonmetallic plasmonic perovskite photoelectrode, namely, SrTiO3, with a periodically ordered nanoporous structure showing an intense LSPR in the visible light region is reported. The crystalline-core@amorphous-shell structure of the SrTiO3 photoelectrode enables a strong LSPR due to the high charge carrier density induced by oxygen vacancies in the amorphous shell. The reversible tunability in LSPR of the SrTiO3 photoelectrode was observed by oxidation/reduction treatment and incident angle adjusting. Such a nonmetallic plasmonic SrTiO3 photoelectrode displays a dramatic plasmon-enhanced photoelectrochemical water splitting performance with a photocurrent density of 170.0 μA cm-2 under visible light illumination and a maximum incident photon-to-current-conversion efficiency of 4.0% in the visible light region, which are comparable to the state-of-the-art plasmonic noble metal sensitized photoelectrodes.

Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production on CdS/Cu7S4/g-C3N4 Ternary Heterostructures.

Hydrogen production through photocatalytic water splitting has attracted much attention because of its potential to solve the issues of environmental pollution and energy shortage. In this work, CdS/Cu7S4/g-C3N4 ternary heterostructures are fabricated by ion exchange between CdS and Cu+ and subsequent ultrasonication-assisted self-assembly of CdS/Cu7S4 and g-C3N4, which provide excellent visible-light photocatalytic activity for hydrogen evolution without any noble metal cocatalyst. With the presence of p-n junction, tuned band gap alignments, and higher charge carrier density in the CdS/Cu7S4/g-C3N4 ternary heterostructures that can effectively promote the spatial separation and prolong the lifetime of photogenerated electrons, a high hydrogen evolution rate of 3570 μmol g-1 h-1, an apparent quantum yield of 4.4% at 420 nm, and remarkable recycling stability are achieved. We believe that the as-synthesized CdS/Cu7S4/g-C3N4 ternary heterostructures can be promising noble metal-free catalysts for enhanced hydrogen production from photocatalytic water splitting.

Electrochemical synthesis of photoactive carbon-carbide structure on silicon in molten salt

Carbon-carbide coatings on silicon with a distinctively developed surface were electrochemically synthesized in molten CaCl2 using Si-SiO2 precursor and a graphite anode. Energy-dispersive X-ray spectroscopy showed approximately one quarter of the structure (in at.%) was composed of carbon. X-ray diffraction identified crystalline phases of graphite, silicon carbide and carbonate. X-ray photoelectron spectroscopy identified the carbide/carbon concentration ratio varied from 0.2 in the external region of the structure to 0.7 at the silicon substrate boundary. Mott-Schottky analysis showed p-type semiconductivity of the structure with high density of charge carriers N > 1016 cm−3. The surfaces absorbed over 90% of white light in a broad region of wavelengths from 400 nm to 1100 nm. The obtained composite is promising for application as an antireflection and protective coating or a photoelectrode.

Fabrication of Nickel–Cobalt Bimetal Phosphide Nanocages for Enhanced Oxygen Evolution Catalysis

AbstractReplacement of precious metals with earth‐abundant electrocatalysts for oxygen evolution reaction (OER) holds great promise for realizing practically viable water‐splitting systems. It still remains a great challenge to develop low‐cost, highly efficient, and durable OER catalysts. Here, the composition and morphology of Ni–Co bimetal phosphide nanocages are engineered for a highly efficient and durable OER electrocatalyst. The nanocage structure enlarges the effective specific area and facilitates the contact between catalyst and electrolyte. The as‐prepared Ni–Co bimetal phosphide nanocages show superior OER performance compared with Ni2P and CoP nanocages. By controlling the molar ratio of Ni/Co atoms in Ni–Co bimetal hydroxides, the Ni0.6Co1.4P nanocages derived from Ni0.6Co1.4(OH)2 nanocages exhibit remarkable OER catalytic activity (η = 300 mV at 10 mA cm−2) and long‐term stability (10 h for continuous test). The density‐functional‐theory calculations suggest that the appropriate Co doping concentration increases density of states at the Fermi level and makes the d‐states more close to Fermi level, giving rise to high charge carrier density and low intermedia adsorption energy than those of Ni2P and CoP. This work also provides a general approach to optimize the catalysis performance of bimetal compounds.

Influence of Co2+ on electrical and optical behavior of Mn2+-doped ZnS quantum dots

Co2+-doped Zn0.98Mn0.02S quantum dots with various concentrations of Co2+ from 0% to 4% have been successfully synthesized by a simple co-precipitation method. X-ray diffraction (XRD) pattern confirmed the acquirement of cubic structure and phase purity in all the samples. The average crystallite size of the particles was ∼3 nm observed from XRD result. Surface morphology of the samples was studied using scanning electron microscope (SEM). TEM study was also taken to know the structural parameters of the samples. Fourier transform infrared (FTIR) spectra proved the presence of Co2+ and Mn2+ in ZnS host lattice. Energy dispersive X-ray (EDX) analysis confirmed the elemental composition with their normal stoichiometric ratio. In the dielectric study, dielectric dispersion and dielectric loss were increased with Co2+ composition due to the increase of carrier concentration. From the AC conductivity measurement, the maximum conductivity was observed for Co2+ = 2% due to their higher charge carrier density and it was decreased for Co2+ = 4% due to the scattering of charge carriers. Because of the low dielectric constant at higher frequency, these materials can be used for high-frequency applications. The variation of peak intensity and wavelength shifting in UV-vis absorption and transmittance were discussed on the basis of formation of secondary phase and variation of charge carrier density. The continuous red shift of energy gap by Co2+-doping is attributed to the direct energy transfer between excited states and 3d levels of Co2+ ions. Photoluminescence spectra showed the strong and broad blue emission bands between 468 nm and 483 nm. Since higher transmittance was observed for Co2+ = 2% addition, this material can be selected for optimum applications of optoelectronic devices.

Comparative study of α-Fe2O3 films prepared by electrodeposition and spray pyrolysis methods as photoanode

Photoelectrochemical (PEC) water splitting using solar energy offers a clean and sustainable approach for the production of renewable hydrogen with less carbon footprint. Hematite is one of the promising materials, which is being considered as photoanode for this application. Accordingly, in this study, α-Fe2O3 films deposited by the electrodeposition and spray pyrolysis methods were investigated as photoanodes for PEC water splitting. X-ray diffraction, scanning electron microscopy and UV–Vis spectroscopy techniques were carried out to study the structural, morphological and optical characteristics of the hematite films, respectively. Moreover, the PEC performance and electrical properties of the films were examined by the linear sweep voltammetry under chopped illumination and electrochemical impedance spectroscopy, respectively. The results indicated that the photocurrent density of the electrodeposited film increased by twofold over the spray pyrolysed counterpart. The higher performance of the electrodeposited film was caused by a combination of the higher surface area, more visible light absorption, less charge transfer resistance and higher charge carrier density.

Selective charge transfer to dioxygen on KPF6-modified carbon nitride for photocatalytic synthesis of H2O2 under visible light

Photochemical production of H2O2 through O2 reduction has been proposed as an alternative method of solar energy storage. A carbon nitride (CN) photocatalyst was selected for this purpose. The incorporation of KPF6 into the CN structure greatly enhanced the apparent quantum yield (AQY) of H2O2 production in the UV and visible light region. The AQY of KPF6-modified CN was measured to be 35.9% and 24.3% under monochromatic irradiation at 370 and 420 nm, respectively, which are 8.3 and 26.1 times higher than for bare CN. The KPF6-enhanced activity is ascribed to several factors including (i) enhanced absorption of UV and visible light, (ii) higher charge carrier density, (iii) retarded radiative recombination of charge pairs, (iv) highly selective two-electron transfer to O2, and (v) hindered photodecomposition of in-situ generated H2O2. The markedly high selectivity of KPF6-modified CN toward the two-electron reduction of O2 (leading to H2O2) was demonstrated in comparision with other photoreductive conversions such as the reduction of polyoxometalate (POM → POM−), hexavalent chromium (CrVI → CrIII), CCl4 (dechlorination), and protons (H2 production). This study developed a simple method of efficient production of H2O2 using visible light, which could be utilized for a variety of applications that employ H2O2 as a solar fuel or a green oxidant.

Ultrafast non-radiative dynamics of atomically thin MoSe2

Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.