Increase In Carrier Concentration Research Articles

Synergistic Optimization of Electrical-Thermal-Mechanical Properties of the In-Filled CoSb3 Material by Introducing Bi0.5Sb1.5Te3 Nanoparticles.

How to realize the synergistic optimization of electrical-thermal-mechanical properties of thermoelectric materials is a key challenge. Using the Bi0.5Sb1.5Te3 nanoparticle as a mixed agent provides an effective way to address this challenge. Here, Bi0.5Sb1.5Te3/In0.25Co4Sb12 nanocomposites with different contents of Bi0.5Sb1.5Te3 nanoparticles were successfully prepared by ultrasonic dispersion combined with spark plasma sintering. Phase and microstructure characterization presented that Te nanoparticles were precipitated from Bi0.5Sb1.5Te3 during the SPS sintering process. Transport measurement results showed that the electrical conductivity was increased due to the increased carrier concentration induced by the charge transfer between Te nanoparticles and the matrix. The Seebeck coefficient was also increased due to the selected electron scattering and increased scattering factor. The lattice thermal conductivity was dramatically suppressed because of the enhanced phonon scattering induced by the Bi0.5Sb1.5Te3 nanoparticles and in situ-precipitated Te nanoparticles and increased dislocations. As a result, a higher average ZT value of 1 was obtained in the range of 300-700 K by the decoupling of the electrical and thermal transport properties for the nanocomposite with 0.1 wt % of Bi0.5Sb1.5Te3 nanometer suspension. Furthermore, the flexural strength, fracture toughness, and hardness of the nanocomposites were also improved significantly. This work demonstrates that using the Bi0.5Sb1.5Te3 nanoparticle as a mixed agent can realize the synergistic optimization of electrical-thermal-mechanical properties of the In-filled CoSb3 thermoelectric material.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Impact of doping on the morphology, structure and optical properties of c-oriented GaN nanorods

We report here the influence of Mg and Si-doping during growth, on the morphology, structure, and optical properties of single-crystalline GaN nanorods (NRs) grown on Si substrates using plasma-assisted molecular beam epitaxy. Mg-doping is shown to enhance the lateral growth of the NRs, leading to a higher degree of coalescence. Si-doping during the nucleation stage of the growth enhances the mutual misorientation of the NRs. Strain profile measurements along the length of individual NR by transmission electron microscopy shows that the top regions are relaxed. Evaluation of carrier concentration by Raman spectroscopy reveals that Si-doping leads to an increase of carrier concentration from 1016 to 1017 cm−3, and the optimal Mg incorporation for the realisation of p-doping is confirmed by photoluminescence spectroscopy. These results will significantly help in understanding and tuning the structural and optical properties of GaN NRs through doping in the fabrication of NR based optoelectronic devices.

Highly effective gating of graphene on GaN

By using four-layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm−1 corresponds to an increase in carrier concentration of about 1.2·1013 cm−2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron–phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.

Achievement of extra-high thermoelectric performance in doped copper (I) sulfide

The development of nontoxic and inexpensive thermoelectric copper (I) sulfide (Cu2S)-based materials could enable an increase in the application of thermoelectric materials in the domain of energy conversion. By using melt quenching, mechanical alloying and spark plasma sintering, we successfully prepared doped and undoped Cu2S with excellent thermoelectric properties. Owing to the increase of carrier concentration, the electrical conductivity of the dopants enhanced from 2.0 × 104 S/m to 1.0 × 105 S/m, with the highest value acquired for Mn doped Cu2S of about 1.0 × 105 S/m at 350 K. Besides, Cu1.8Sn0.1Mn0.1S, Cu1.9Mn0.1S, and Cu1.9Sn0.1S followed an increasing variation in Seebeck coefficient with final values of 325 μV K−1, 322 μV K−1, and 294 μV K−1 at 800 K, respectively. The maximum power factor of 3.24 mW m−1 K−2 is obtained for Cu1.9Mn0.1S at 650 K, which is about 40 times greater than undoped Cu2S (0.08 mW m−1 K−2). At the same time, a peak ZT of 1.63 is attained at 700 K for Cu1.9Mn0.1S, which is about 18 times enhanced over undoped Cu2S (0.09).

Strain-Induced Tunable Band Offsets in Blue Phosphorus and WSe2 van der Waals Heterostructure

The electronic structure and band offsets of blue phosphorus/WSe2 van der Waals (vdW) heterostructure are investigated via performing first-principles calculations. Blue phosphorus/WSe2 vdW heterostructure exhibits modulation of bandgaps by the applied vertical compressive strain, and a large compressive strain of more than 23% leads to a semiconductor-to-metal transition. Blue phosphorus/WSe2 vdW heterostructure is demonstrated to have a type-II band alignment, which promotes the spontaneous spatial separation of photo-excited electrons and holes. Furthermore, electrons concentrating in BlueP and holes in WSe2 can be enhanced by applied compressive strain, resulting in an increase of carrier concentration. Therefore, these properties make blue phosphorus/WSe2 vdW heterostructure a good candidate for future applications in photodetection.

Synthesis and characterization of ZnO/ZnAl2O4/Zn2TiO4 composite films by Ar–O2 mixture hollow cathode glow discharge

In this study, ZnO/ZnAl 2 O 4 /Zn 2 TiO 4 composite nanostructured thin films are deposited assisted with hollow-cathode glow discharge (HCD). The films are deposited using various argon-oxygen gases mixture (0–50% O 2 ), and its effect on film quality, optical and electrical properties is examined. Remarkably, instead of metallic targets, ZnO, TiO 2, and Al 2 O 3 powders filled in hollow-cathode are used for deposition, and thus no specific metallic targets are required in this technique. The deposited films consist of hexagonal wurtzite ZnO structure, ZnAl 2 O 4, and Zn 2 TiO 4 phases, and the crystalline size of films increases for higher oxygen contents. The thickness of films reduced by increasing oxygen contents, and all deposited films exhibit transmittance higher than 82%. The carrier concentration and mobility increase, whereas resistivity decreases when a higher amount of oxygen is added. This study shows that HCD can effectively synthesize the composite film with good optical and electrical properties. Additionally, the film's properties and elemental composition can be tuned by changing the gas composition, and thus, no separate target metallic materials with the specific composition are required.

Defect species in Ga-doped ZnO films characterized by photoluminescence

Photoluminescence (PL) spectra of Ga-doped ZnO (GZO) films were investigated with reference to those of undoped ZnO films to elucidate the effect of high-level Ga3+ doping. A transition from ZnO-like to GZO-like spectra occurred at a Ga content around 2 at. %. The room temperature PL spectra of sufficiently oxidized GZO films exhibited band edge and violet components, while emissions at wavelengths longer than 480 nm were sharply cut off. The close resemblance of the spectral shapes of the GZO and Zn-rich ZnO films indicated disordering of the ZnO lattice by excess Ga dopants. Deposition under a reducing atmosphere at 100 and 200 °C produced oxygen-deficient GZO films with additional emission signals corresponding to oxygen vacancy (VO) defects between 480 and 600 nm. For GZO films with Ga content larger than 4 at. %, increasing the deposition temperature above 400 °C or postannealing at 500 °C smeared out deep-level emission signals, suppressed the near-band edge emission, and deactivated the donor role of Ga3+. These changes can be ascribed to outdiffusion of Ga3+ from the cation sites and rearrangement of the ZnO crystal network. Argon plasma treatment of GZO films generated a VO-related emission band through preferential sputtering of oxygen atoms. Hydrogen donors trapped at the resulting VOs would be the origin of a slight increase in carrier concentration, by 1 × 1020 cm−3.

Investigation on many-body effects in micro-LEDs under ultra-high injection levels.

Micro-LEDs can work under an extremely high injection level and are widely used in high-brightness micro-displays and visible light communication. With the increase of carrier concentration, many-body effects gradually become important factors affecting devices' characteristics. Considering the effects of carrier scattering, bandgap renormalization, and Coulomb enhancement (CE), changes in the electroluminescence spectra of micro-LEDs are analyzed as the current density increases from 49.2 to 358.2 kA/cm2, the latter representing an ultra-high injection level. Affected by plasma screening, CE decreases below about 150 kA/cm2. After that, polarization screening dominates and effectively alleviates the spatial separation of electrons and holes, which results in CE increases to the maximum injection level of 358.2 kA/cm2. It is established that CE promotes radiative recombination processes. Different from the traditional phenomenon of "efficiency droop", the enhanced attraction between carriers leads to an abnormal increase of external quantum efficiency at high current density.

High mobility transparent and conducting oxide films of La-doped SrSnO$$_3$$

The synthesis and characterization of high mobility thin films of La-doped SrSnO $$_3$$ are reported. The mobility for the $$7\%$$ La-doped sample was found to be 228 cm $$^2$$ V $$^{-1}$$ s $$^{-1}$$ . The observed high mobility was associated with the reduced carrier effective mass and scattering centers of various scattering mechanisms. The enhancement in mobility and the increase in carrier concentration after doping reduced the resistivities of the thin films by five orders of magnitude. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy revealed that La-dopant and oxygen vacancies donated the electrons in the films. Films were highly transparent $$(> 90\%)$$ in the visible region. These materials have great potential to be used in optoelectronic and heterostructure devices

Effects of Charge Compensation on the Thermoelectric Properties of (La1-zCez)0.8Fe4-xCoxSb12 Skutterudites

La/Ce-partially double-filled and Co-charge-compensated (La1−zCez)0.8Fe4−xCoxSb12 skutterudites were synthesized, and their thermoelectric properties were studied by varying the filling ratio and charge compensation. X-ray diffraction analysis revealed that the matrix phase was skutterudite and a secondary phase was determined to the marcasite FeSb2. However, the formation of marcasite could be inhibited by increasing the Co content. Rare-earth antimonides, including LaSb2 and CeSb2, which were formed in fully filled La1−zCezFe4−xCoxSb12, were not found after La/Ce partial filling. La/Ce filling and Co substitution were confirmed by the decrease in lattice constants, from 0.9137 to 0.9099 nm, with increasing Ce and Co contents. Electrical conductivity showed negative temperature dependence, indicating metallic or degenerate semiconductor characteristics. Intrinsic conduction resulted in the maximum Seebeck coefficient at temperatures between 723 and 823 K. As the Co-substitution and Ce-filling contents increased, the Seebeck coefficient increased, while electrical and thermal conductivities decreased. This was considered to be due to difference in the valences of La<sup>3+</sup> and Ce<sup>3+/4+</sup> and the increase in carrier concentration caused by Co charge compensation. However, because they had similar atomic masses and ionic radii, the effects of the La/Ce filling ratio were not significant. Instead, Co charge compensation had the dominant effect on thermoelectric properties. The maximum Seebeck coefficient of 165.4 µVK<sup>-1</sup> was obtained for (La0.25Ce0.75)0.8Fe3CoSb12 at 823 K, and the highest electrical conductivity of 2.27 × 10<sup>5</sup> S m<sup>-1</sup> was achieved for (La0.75Ce0.25)0.8Fe4Sb12. (La0.25Ce0.75)0.8Fe3CoSb12 exhibited the lowest thermal conductivity of 2.15 W m<sup>-1</sup>K<sup>-1</sup> at 523 K and (La0.75Ce0.25)0.8Fe3.5Co0.5Sb12 showed the highest power factor of 2.53 mW m<sup>-1</sup> K<sup>-2</sup> at 723 K. The maximum dimensionless figure of merit, ZTmax = 0.71, was achieved at 723 K for (La0.75Ce0.25)0.8Fe3CoSb12.

Influence of the Physical Properties on the Antibacterial and Photocatalytic Behavior of Ag-Doped Indium Sulfide Film Deposited by Spray Pyrolysis

Spray pyrolysis was used to deposit indium sulfide (In2S3) films, with or without silver doping. The films are polycrystalline, and the inclusion of Ag in the In2S3 structure leads to the formation of a solid solution, with the crystallite size of the order of tens of nanometers. In2S3 films exhibit a semiconductive behavior, and the incorporation of Ag leads to an increase of the charge carrier concentration, enhancing the electrical conductivity of the films. The small polaron hopping mechanism, deduced by the fittings according to the double Jonscher variation, explains the evolution of the direct current (dc) conductivity at high temperature of the Ag-doped indium sulfide. From impedance spectroscopy, it was found that the doped film presents dielectric relaxation, and Nyquist diagrams indicate the importance of the grain and the grain boundaries’ contributions to the transport phenomena. The physical characteristics of the films have an influence on the photocatalytic performance, achieving photodegradation efficiency above 80% (85.5% in the case of Ag doping), and on the antibacterial activity. The obtained results indicate that indium sulfide films are good candidates for environmental and biological applications, confirming a multifunctional nature.

Scattering Mechanisms and Suppression of Bipolar Diffusion Effect in Bi2Te2.85Se0.15Ix Compounds.

We investigated the anisotropic thermoelectric properties of the Bi2Te2.85Se0.15Ix (x = 0.0, 0.1, 0.3, 0.5 mol.%) compounds, synthesized by ball-milling and hot-press sintering. The electrical conductivities of the Bi2Te2.85Se0.15Ix were significantly improved by the increase of carrier concentration. The dominant electronic scattering mechanism was changed from the mixed (T ≤ 400 K) and ionization scattering (T ≥ 420 K) for pristine compound (x = 0.0) to the acoustic phonon scattering by the iodine doping. The Hall mobility was also enhanced with the increasing carrier concentration. The enhancement of Hall mobility was caused by the increase of the mean free path of the carrier from 10.8 to 17.7 nm by iodine doping, which was attributed to the reduction of point defects without the meaningful change of bandgap energy. From the electron diffraction patterns, a lattice distortion was observed in the iodine doped compounds. The modulation vector due to lattice distortion increased with increasing iodine concentration, indicating the shorter range lattice distortion in real space for the higher iodine concentration. The bipolar thermal conductivity was suppressed, and the effective masses were increased by iodine doping. It suggests that the iodine doping minimizes the ionization scattering giving rise to the suppression of the bipolar diffusion effect, due to the prohibition of the BiTe1 antisite defect, and induces the lattice distortion which decreases lattice thermal conductivity, resulting in the enhancement of thermoelectric performance.

Y3Ba5Cu8Ox Superconductor Under Hydrostatic Pressure

Superconducting Y3Ba5Cu8O18 samples were prepared by standard solid-state reaction method in comparison with Y1Ba2Cu3O7 sample. Resistance vs. temperature of both samples was measured under hydrostatic pressures up to 0.93 GPa using a piston–cylinder pressure cell. Applying the pressure resulted in a decrease in resistance in the normal state and an increase of the transition temperature Tc for both polycrystalline samples. This phenomenon is then explained in terms of the increase in carrier concentrations. Our results show a positive value of $${\mathrm{dT}}_{\mathrm{c}}/\mathrm{dP}$$ =1.1 K/GPa and 1.4 K/GPa for Y123 and Y358, respectively.

Improved Dissolution Rate and Intestinal Absorption of Fexofenadine Hydrochloride by the Preparation of Solid Dispersions: In Vitro and In Situ Evaluation.

The objective of this study was to enhance dissolution and permeation of a low soluble, absorbable fexofenadine hydrochloride (FFH) by preparing solid dispersions using polyethylene glycol 20,000 (PEG 20,000) and poloxamer 188 as carriers. The phase solubility measurement for the supplied FFH revealed a linear increase in the solubility of fexofenadine with increasing carrier concentration in water (1.45 mg/mL to 11.78 mg/mL with 0% w/v to 30% w/v PEG 20,000; 1.45 mg/mL to 12.27 mg/mL with 0% w/v to 30% w/v poloxamer 188). To select the appropriate drug carrier concentration, a series of solid dispersions were prepared in the drug carrier weight ratios of 1:1, 1:2 and 1:4 by fusion method. The solid dispersions composed of drug carrier at 1:4 weight ratio showed highest dissolution with the time required for the release of 50% of the drug <15 min compared to the supplied FFH (>120 min). The intestinal absorption study presented a significant improvement in the absorption of drug from the solid dispersions composed of poloxamer 188 than PEG 20,000. In summary, the solid dispersions of FFH prepared using PEG 20,000 and poloxamer 188 demonstrated improved dissolution and absorption than supplied FFH and could be used to improve the oral bioavailability of fexofenadine.

Possibility of interstitial Na as electron donor in Yb14MgSb11

Here, we investigate Na-doping Yb14MgSb11 as a potential route to increase carrier concentration based on an improved multiband model. Experimental transport data were collected on Yb14-xNaxMgSb11 samples prepared by ball milling and hot pressing. We show that Na increases the Seebeck coefficient and resistivity, suggesting that it behaves not as a substituent but as an interstitial electron donor under this synthesis and processing conditions, decreasing hole carrier concentration. Density functional theory (DFT) calculations of equilibrium phases, defect formation enthalpies, and band diagrams shed light on the defect-modified carrier concentrations. Depending on the equilibrium phase, Na can behave as a substitutional or interstitial defect.

Synthesis, Bottom up Assembly and Thermoelectric Properties of Sb-Doped PbS Nanocrystal Building Blocks.

The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS.

Effects of NbCl5-doping on the thermoelectric properties of polycrystalline Bi2S3

Bi2S3 is considered a potential thermoelectric material owing to its non-toxic and earth-abundant components and intrinsically low thermal conductivity. However, owing to its low electrical conductivity and the coupling relationship between the electric transport properties and thermal transport properties, achieving a high ZT value for polycrystalline Bi2S3 is a major challenge. In this work, a high-valence halogen compound, NbCl5, was employed to enhance the thermoelectric properties of polycrystalline Bi2S3 fabricated by the melting method combined with the spark plasma sintering technique. The enhanced electrical conductivity of polycrystalline Bi2S3 is derived from the increased carrier concentration, which is caused by the substitution of Nb and Cl ions into Bi and S sites, respectively. A peak power factor of 363 μWm−1K‒2 was obtained for Bi2S3 with 1.0 ​wt% NbCl5 doping at 623 ​K. Finally, a maximum ZT of ~0.28, which is approximately four times higher than that of the pristine Bi2S3 sample, was obtained. This work proves that NbCl5 is an effective n-type dopant for the Bi2S3 system, and indicates that high thermoelectric performance can be realized for Bi2S3 with environmentally friendly and low-cost elements.

Effect of Additives on the Electrical, Structural and Mechanical Property Modification of PEO-NH4HF2 based Polymer Electrolytes

Objective: Polyethylene oxide (PEO) composed of 10 wt% ammonium bifluoride (NH4HF2) exhibited higher conductivity of 5.96x10-6 S/cm as compared to other concentrations of salt at room temperature. The effect of additives, i.e., nano-sized fumed silica concentration as nanofiller and propylene carbonate (PC) concentration as plasticizer on electrical, structural, as well as mechanical property studies of polymer electrolytes has been studied. Methods: The ionic conductivity, as well as dielectric studies of polymer electrolytes consisting of polyethylene oxide and different (x wt%) ammonium bifluoride (x=1, 2.5, 5, 10 and 15 wt %), have been measured using complex impedance spectroscopic technique. X-ray diffraction (XRD) and differential scanning calorimetry/thermogravimetric analysis (DSC/ TGA) studies have been conducted to observe the effect of additives on crystalline phase, crystallite size, melting temperature and weight loss of different polymer electrolytes. The effect of additives on the mechanical properties (tensile strength, modulus of elasticity and % elongation at break) of different polymer electrolytes has also been studied by Universal Testing Machine (UTM). Results: The maximum conductivity achieved was 1.55× 10-4 S/cm in case of plasticized nanocomposite polymer electrolytes with the simultaneous presence of 3 wt% fumed silica and 0.3 ml propylene carbonate. The variation of ionic conductivity at different temperatures and activation energy values of different polymer electrolytes were also measured and observed in good correlation. Conclusion: The observed enhancement in the ionic conductivity of polymer electrolytes with additives is due to an increase in carrier concentration, amorphous content, chain flexibility, as well as the formation of more conducting pathways. Hence, this new approach led to the development of plasticized nanocomposite polymer electrolytes with high ionic conductivity and improved structural and mechanical properties.

Effects of magnetic impurity concentration and hole carriers on ferromagnetic stability of Li(ZnCr)As and Li(ZnCrCu)As

First-principles calculations were performed to study electronic structure and magnetic properties of Li(ZnCr)As and Li(ZnCrCu)As. The ground state of Li(ZnCr)As was ferromagnetism. To enhance the ferromagnetic stability of the doping system, two methods were proposed and the exchange mechanism was analyzed. More Cr impurities increased the energy gain and decreased p-d hybridization, contributing to the double-exchange interaction and thus promoting ferromagnetic stability of Li(ZnCr)As. By substituting Zn(3d104s2) with Cu(3d104s1), additional hole carriers were introduced and the ferromagnetism was more stable than that in the single Cr doping case. The ferromagnetism of Li(ZnCrCu)As system was further stabilized with the increase of hole carrier concentration. The Li32(Zn22Cr8Cu2)As32 configuration showed the strongest ferromagnetic stability. Therefore, it was possible to fabricate a new type of dilute magnetic semiconductor material with high ferromagnetic stability by adjusting the concentration of Cr impurity and additional hole carriers.