Kondo insulator like material FeSb2 was found to exhibit colossal Seebeck coefficient. It would have had huge potential in thermoelectric applications in cryogenic temperature range if it had not been for the large thermal conductivity. Here we studied the influence of Te doping at Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. Surprisingly, only 5% Te doping suppresses thermal conductivity by two orders of magnitude, which may be attributed to the substitution disorder. Te doping also results in transition from an semiconductor to a metal. Consequently thermoelectric figure of merit (ZT ? 0:05) in Fe(Sb0:9Te0:1)2 at ? 100K was enhanced by about one order of magnitude when compared to ZT < 0:005 in undoped FeSb2.
We study the effect of doping in the suppression of tunneling observed in multi-walled nanotubes, incorporating as well the influence of the finite dimensions of the system. A scaling approach allows us to encompass the different values of the critical exponent $α$ measured for the tunneling density of states in carbon nanotubes. We predict that further reduction of $α$ should be observed in multi-walled nanotubes with a sizeable amount of doping. In the case of nanotubes with a very large radius, we find a pronounced crossover between a high-energy regime with persistent quasiparticles and a …
In this work, the thermal transport properties of thorium dioxide (ThO$_2$, thoria) with low levels of substitutional uranium (U) doping are explored. We observe strong indications of resonant phonon scattering, an interaction between phonons and electronic degrees of freedom, induced by this doping in addition to common ``impurity'' scattering due to mass and interatomic force constant differences. Uranium doping levels of 6\%, 9\%, and 16\% were studied in a single hydrothermally synthesized U-doped thoria crystal with spatially-varying U doping levels. Within this crystal, isoconcentration …
(La1-xSrx)(Zn1-yMny)AsO is a two dimensional diluted ferromagnetic semiconductor that has the advantage of decoupled charge and spin doping. The substitution of Sr2+ for La3+ and Mn2+ for Zn2+ into the parent semiconductor LaZnAsO introduces hole carriers and spins, respectively. This advantage enables us to investigate the influence of carrier doping on the ferromagnetic ordered state through the control of Sr concentrations in (La1-xSrx)(Zn0.9Mn0.1)AsO. 10 % Sr doping results in a ferromagnetic ordering below TC ~ 30 K. Increasing Sr concentration up to 30 % heavily suppresses the Curie temp…
Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3 creates structural changes that inflect the electrical conductivity caused by small polaron hopping via exchange interactions and charge transfer. The substitution forms electron holes and a structural crossover from orthorhombic to rhombohedral symmetry, and then to cubic symmetry. The structural crossover is accompanied by a crossover from Fe3+-O2--Fe3+ superexchange interaction to Fe3+-O2--Fe4+ double exchange interaction, as evidenced by a considerable increase of conductivity. These interactions and charge trans…
We report the thermoelectric properties (Seebeck coefficient, thermal conductivity, and electrical resistivity) of lightly doped single crystals of (001)-oriented SrTiO3 (STO). Hall effect measurements show that electron doping around 10^-5 carriers per unit cell can be achieved by vacuum annealing of the crystals under carefully controlled conditions. The steep density of states near the Fermi energy of STO at this doping level (confirmed by ab initio calculations) retains an unusually large Seebeck coefficient, in spite of an increase in the electronic conductivity by several orders of magni…
"Secondary doping" in poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) is quite effective and a frequently used method for the conductivity enhancement. This simple approach, the addition of co-solvents to the PEDOT:PSS solution before film fabrication, is an essential way to derive the high electrical performance of PEDOT:PSS, but the mechanism still remains unclear. In this study, nanoscale structural changes synchronized with the conductivity enhancement via secondary doping in PEDOT:PSS films were investigated. During secondary doping with ethylene glycol near the criti…
We report the structural, magnetic and electrical transport properties of Y$_2$Ir$_{2-x}$Cr$_x$O$_7$ pyrochlore iridates. The chemical doping leads to order of magnitude enhancement of electrical conductivity. The introduction of Cr3+ at Ir4+ site tends to distort the Ir-O6 octahedra and suppresses antiferromagnetic correlation. The X-ray photoemission spectroscopy measurements suggest the coexistence of Ir4+ and Ir5+ valence states in the Y$_2$Ir$_{2-x}$Cr$_x$O$_7$ compounds. The concentration of Ir5+ is enhanced with Cr doping, leading to weak ferromagnetism and enhanced electrical conductiv…
Decreasing thermal conductivity of a thermoelectric material is always a prerequisite for its potential application. Using first-principle calculations, we examine the magnetism induced change in lattice thermal transport in bismuth telluride. The source of magnetic moment, Cr in the doped system, weakly magnetizes the coordinated Te atoms to make the latter's phonon softer than that in the pure compound. Though the transition metal dopants do not participate directly in the heat conduction process, the anharmonicity induced by them favor in reducing the lattice thermal conductivity. Large anh…
Thermal conductivity of graphene nanoribbons (GNR) with length 106~Å and width 4.92~Å after isotopic doping is investigated by molecular dynamics with quantum correction. Two interesting phenomena are found: (1) isotopic doping reduces thermal conductivity effectively in low doping region, and the reduction slows down in high doping region; (2) thermal conductivity increases with increasing temperature in both pure and doped GNR; but the increasing behavior is much more slowly in the doped GNR than that in pure ones. Further studies reveal that the physics of these two phenomena is related to …
In this work, we introduce models for deriving lower limits for the key parameters doping efficiency, charge carrier concentration, and charge carrier mobility from conductivity data of doped organic semiconductors. The models are applied to data of thin layers of Fullerene C60 n-doped by four different n-dopants. Combining these findings with thermoelectric Seebeck data, the energetic position of the transport level can be narrowed down and trends for the absolute values are derived.
Declining the lattice thermal conductivity in graphene is essential for its thermoelectric applications. In high electron density systems, scatterings of phonons by electrons are no less than the phonon scatterings by other phonons. With the aid of first-principle calculations we examine the lattice thermal conductivity in graphene by inducing electron-phonon scattering externally. With experimentally tunable charge carrier density we find $\sim 50\%$ reduction of the lattice thermal conductivity at 200 K. The present study opens up new avenues for potential thermoelectric applications of grap…
Rutile ($R$) phase VO$_2$ is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic phase below T$_{MIT} \sim 340K$. It has been experimentally shown that one can control this transition by doping VO$_2$. In particular, doping with oxygen vacancies ($V_O$) has been shown to completely suppress this MIT {\em without} any structural transition. We explain this suppression by elucidating the influence of oxygen-vacancies on the electronic-structure of the metallic $R$ ph…
Based on experimental observations in A{x}MO{2} (A = Na, Li; M = Co, Ni), a model for suppression of magnetic frustration by electron doping in a nearest-neighbour antiferromagnetic triangular lattice is presented. It is found that frustration can be quantified, as determined by geometry and bond-counting, and its magnitude is a non-monotonic function of $x$. A mean-field calculation provides temperature-dependent magnetization, spin-entropy and heat capacity. Low-doping (x = 0.25, 0.33) results in a highly frustrated regime. A{0.5}MO{2} has strongest order and no frustration, while high dopin…
Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity $κ$. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At $5\%$ of Te doping at Sb site thermal conductivity is suppressed from $\sim 250$ W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20$\%$…
We study effects of the Coulomb repulsion on the electron-phonon interaction (EPI) in a model of cuprates at zero and finite doping. We find that antiferromagnetic correlations strongly enhance EPI effects on the electron Green's function with respect to the paramagnetic correlated system, but the net effect of the Coulomb interaction is a moderate suppression of the EPI. Doping leads to additional suppression, due to reduced antiferromagnetic correlations. In contrast, the Coulomb interaction strongly suppresses EPI effects on phonons, but the suppression weakens with doping.
Simultaneously optimizing performances, processability and fabrication cost of organic electronic materials is the continual source of compromise hindering the development of disruptive applications. In this work, we identified a strategy to achieve record conductivity values of one of the most benchmarked semiconducting polymers by doping with an entirely solution-processed, water-free and cost-effective technique. High electrical conductivity for poly(3-hexylthiophene) up to 21 S/cm has been achieved, using a commercially available electron acceptor as both a Lewis acid and an oxidizing agen…
In this paper, the effect of B and N doping on the phonon induced thermal conductivity of graphene has been investigated. This study is important when one has to evaluate the usefulness of electronic properties of B and N doped graphene. We have performed the calculations by employing density functional perturbation theory(DFPT) to calculate the inter-atomic forces=force constants of pristine/doped graphene. Thermal conductivity calculations have been carried out by making use of linearized Boltzmann transport equations (LBTE) under single-mode relaxation time approximation (RTA). The thermal …
Near-interfacial oxide traps and chemical impurities on the graphene surface or at the graphene-dielectric interface can be a source of intentional or unintentional doping of graphene sheet. The efficiency of such chemical doping can vary in a wide range depending on parameters of graphene field effect devices. Mechanisms of such sensitivity of doping efficiency to the device characteristics need to be understood. The objective of this paper is to theoretically derive the analytical relations, adapted to the explicit calculation of graphene chemical doping.
The conductivity of the electron hole and polaron conductor La1-xSrxFe0.75Ni0.25O3-δ, a potential cathode material for intermediate temperature solid oxide fuel cells, was studied for 0 <x < 1 and for temperatures 300 K <T < 1250 K. In LaSrFe-oxide, an ABO3 type perovskite, A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we have in addition a B-site substitution by Ni. The compound for x = 0.5 is identified as the one with the highest conductivity (σ ~ 678 S/cm) and lowest activation energy for p…
Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high-efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unpre…
Very recently, a 3$d$ based honeycomb cobaltate Na$_2$Co$_2$TeO$_6$ has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its 4$d$/5$d$ counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on Na$_2$Co$_{2-x}$Zn$_x$TeO$_6$ by structural, magnetic, and thermodynamic measurements, and track the doping evolution of its magnetic ground states. Due to the extremely close radii of Zn$^{2+}$ and high-spin Co$^{2+}$ ions, the substitution can be easily achieved. X-ray diffractions reveal no structural transition but …
Cobalt and silver co-doping has been undertaken in ZnO thin films grown by pulsed laser deposition in order to investigate the ferromagnetic properties in ZnO-based diluted magnetic materials and to understand the eventual relation between ferromagnetism and charge carriers. Hall transport measurements reveal that Ag doping up to 5% leads to a progressive compensation of the native n-type carriers. The magnetization curves show ferromagnetic contributions for all samples at both 5 K and room temperature, decreasing with increasing the Ag concentration. First principles modeling of the possible…
We propose a new approach to theoretical description of doped cuprate like $La_{2-x}Sr_{x}CuO_4$ and $YBa_2Cu_3O_{6+x}$, assuming phase separation and treating it as inhomogeneous composite material, containing the dielectric and metallic stripe-like nanoparticles. The formalism of effective medium theory is then applied for calculation of dielectric permittivity, optical and EELS spectra of $La_{2-x}Sr_{x}CuO_4$ with $x$ varying in a wide range. Reasonable semi-quantitative agreement with experiment has been obtained even for the simplest version of the theory. The model was found able to rep…
Conductive polymers or "organic metals" are highly engineered nanostructured materials made from organic building blocks. They are candidates as molecular wires for nanotechnology applications in molecular electronics. The conduction in these polymers is due to the presence of delocalized molecular orbitals. In this work, we present the synthesis of chloride doped polyaniline by bulk oxidative chemical polymerization using a solid aniline salt as a monomer instead of liquid aniline to diminish toxic hazards. The FTIR and UV-visible spectra confirmed the expected structure of the polymer. The e…
Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free carriers increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to cause a portion of the lowest unoccupied band to merge into the valance band, leading to a decrease in conductivity. This behavior we dub as 'anti-doping' was seen in rare-earth nickel oxides SmNiO3, cobalt oxides SrCoO2.5, Li-ion battery materials and e…
The empty perovskite ScF3 exhibits negative thermal expansion up to 1100 K. We demonstrate that ab-initio calculations of temperature-dependent effective phonon spectra allow to quantitatively describe the behavior of this compound and the suppression of negative thermal expansion. Based on this result, we predict an anomalous temperature dependence of the thermal conductivity and interpret it as a general feature of the perovskite class. Finally, we comment on the fact that the suppression of negative thermal expansion at such a high temperature is beyond the reach of the quasi-harmonic appro…
Chemical modification, such as intercalation or doping of novel materials is of great importance for exploratory material science and applications in various fields of physics and chemistry. In the present work, we report the systematic intercalation of chemically exfoliated few-layer graphene with potassium while monitoring the sample resistance using microwave conductivity. We find that the conductivity of the samples increases by about an order of magnitude upon potassium exposure. The increased of number of charge carriers deduced from the ESR intensity also reflects this increment. The do…
Substrate engineering provides an opportunity to modulate the physical properties of quantum materials in thin film form. Here we report that TiSe$_2$ thin films grown on TiO$_2$ have unexpectedly large electron doping that suppresses the charge density wave (CDW) order. This is dramatically different from either bulk single crystal TiSe$_2$ or TiSe$_2$ thin films on graphene. The epitaxial TiSe$_2$ thin films can be prepared on TiO$_2$ via molecular beam epitaxy (MBE) in two ways: by conventional co-deposition using selenium and titanium sources, and by evaporating only selenium on reconstruc…
The promise enabled by BAs high thermal conductivity in power electronics cannot be assessed without taking into account the reduction incurred when doping the material. Using first principles calculations, we determine the thermal conductivity reduction induced by different group IV impurities in BAs as a function of concentration and charge state. We unveil a general trend, where neutral impurities scatter phonons more strongly than the charged ones. $\text{C}_{\text{B}}$ and $\text{Ge}_{\text{As}}$ impurities show by far the weakest phonon scattering and retain BAs $κ$ values of over $\sim$…
A key challenge in manipulating the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque is to identify materials that exhibit an efficient charge-to-spin current conversion. Ab initio electronic structure calculations reveal that the intrinsic spin Hall conductivity (SHC) for pristine $β$-W is about sixty percent larger than that of $α$-W. More importantly, we demonstrate that the SHC of $β$-W can be enhanced via Ta alloying. This is corroborated by spin Berry curvature calculations of W$_{1-x}$Ta$_x$ ($x$ $\sim$ 12.5%) alloys which show a giant enhancement of spin Ha…
We have performed a comparative study of the electronic properties of six different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc, NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by means of potassium intercalation. In spite of the complexity of these systems, we find that the nature of the underlying molecular orbitals produce observable effects in the doping dependence of the electrical conductivity of the materials. For all the MPc's in which the added electrons are expected to occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the dop…
The evolution of the density of states (DOS) and conductivity as function of well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene) [OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6 have been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains non-metallic, while the latter crosses the metal-insulator transition. In both systems a similar almost linear increase in DOS as function of charges per unit volume c* has been observed from the electrochemical gated transistor data. In PPy-PF_6, when compared to doped OC_1C_…
We investigate how field-effect doping affects the structural properties, the electronic structure and the Hall coefficient of few-layers transition metal dichalcogenides by using density-functional theory. We consider mono-, bi-, and trilayers of the H polytype of MoS2, MoSe2, MoTe2, WS2, and WSe2 and provide a full database of electronic structures and Hall coefficients for hole and electron doping. We find that, for both electron and hole doping, the electronic structure depends on the number of layers and cannot be described by a rigid band shift. Furthermore, it is important to relax the …
N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1-xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0E19 F/cm3.The dramat…
In semiconductors almost all heat is conducted by phonons (lattice vibrations), which is limited by their quasi-particle lifetimes. Phonon-phonon interactions represent scattering mechanisms that produce thermal resistance. In thermoelectric materials, this resistance due to anharmonicity should be maximised for optimal performance. We use a first-principles lattice-dynamics approach to explore the changes in lattice dynamics across an isostructural series where the average atomic mass is conserved: ZnS to CuGaS$_2$ to Cu$_2$ZnGeS$_4$. Our results demonstrate an enhancement of phonon interacti…
Conjugated polymers are increasingly used as organic mixed ionic-electronic conductors in electrochemical devices for neuromorphic computing, bioelectronics and energy harvesting. The design of efficient applications relies on high electrochemical doping levels, high electronic conductivity, fast doping/dedoping kinetics and high ionic uptake. In this work, we establish structure-property relations and demonstrate how these parameters can be modulated by the co-existence of order and disorder. We use in-situ time-resolved spectroelectrochemistry, resonant Raman and terahertz conductivity measu…
We report X-ray diffraction, magnetization and transport measurements for polycrystalline samples of the new layered superconductor Bi4-xAgxO4S3 (0<x<0.2). The superconducting transition temperature (TC) decreases gradually and finally suppressed for x>0.10. Accordingly, the resistivity changes from a metallic behavior for x<0.1 to a semiconductor-like behavior for x>0.1. The analysis of Seebeck coefficient shows there are two types of electron-like carriers dominate at different temperature regions, indicative of a multiband effect responsible for the transport properties. The suppression of …
Transparent conductors (TC's) combine the usually contraindicated properties of electrical conductivity with optical transparency and are generally made by starting with a transparent insulator and making it conductive via heavy doping, an approach that generally faces severe 'doping bottlenecks'. We propose a different idea for TC design-starting with a metallic conductor and designing transparency by control of intrinsic interband transitions and intraband plasmonic frequency. We identify the specific design principles for three such prototypical intrinsic TC classes and then search computat…
Ceria (CeO$_{2}$) co-doping has been suggested as a means to achieve ionic conductivities that are significantly higher than those in singly-doped systems. Rekindled interest in this topic over the last decade has given rise to claims of much improved performance. The present study makes use of computer simulations to investigate the bulk ionic conductivity of Rare Earth (RE) doped ceria, where RE = Sc, Gd, Sm, Nd and La. The results from the singly doped systems are compared to those from ceria co-doped with Nd/Sm and Sc/La. The pattern that emerges from the conductivity data is consistent wi…
This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect VNi (nickel vacancy) in the wide-gap p-type semiconductor nickel oxide (NiO). VNi can easily be introduced into NiO thin films by offering high oxygen partial pressures during film growth, rendering nonstoichiometric semiconducting structures. However, exposure to lower oxygen supply after growth, e.g. in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration equilibrates. In this study, we observe this process in sit…
We examine the effect of acoustic phonon scattering on the conductivity of two-dimensional Dirac electrons. The temperature ($T$) dependence of the conductivity ($σ$) is calculated using the electron Green's function with damping by both the impurity ($Γ_0$) and phonon ($Γ_{\rm ph}$).For zero or small doping, on which the present Rapid Communication focuses, $σ(T)$ increases and becomes almost constant due to the competition between the Dirac electrons and the phonon scattering. Such strange behavior of $σ(T)$ is ascribed to an exotic mechanism of phonon scattering, whose momentum space is str…
Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion co…
For simultaneously achieving the high-power factor and low lattice thermal conductivity of Si-Ge based thermoelectric materials, we employed, in this study, constructively modifying the electronic structure near the chemical potential and nano-structuring by low temperature and high-pressure sintering on nano-crystalline powders. Nickel was doped to create the impurity states near the edge of the valence band for enhancing the power factor with boron for tuning the carrier concentration. The nanostructured samples with the nominal composition of Si0.65-xGe0.32Ni0.03Bx (x = 0.01, 0.02, 0.03, an…
