Majority Charge Carriers Research Articles

Quantum chemical calculations on molecules with donor-π-acceptor structures for efficient organic field effect transistor

Novel organic compounds for possible use in organic field effect transistors (OFETs) have been widely researched in recent times. Polythiophene [1] was the first organic compound to be used for development of OFET in 1986. OFETs with p-type semiconductor properties have been found to be most suitable for designing of devices, since stability of such compounds is higher as compared to n-type semiconducting compounds. In this paper, two molecular structures (naphthalene-pyrrole-propanoic acid and naphthalene-pyrrole-pyridine) have been designed based on the donor-π-acceptor architecture. Both structures were optimized using density functional theory (DFT) at B3LYP/6–31 + G(d,p) theory level. These structures were also checked to confirm whether they correspond to global minima on potential energy surface (PES) through frequency calculation in the Gaussian 16 W software package. Thorough analysis of calculated parameters such as hole reorganization energy (λh), electron reorganization energy (λe), ionization potential (vertical and adiabatic) and electron affinity (vertical and adiabatic), has been done to investigate charge transport properties. In both the designed systems, λh values were found to be smaller as compared to λe values. Hence, it may be inferred that our designed molecules have holes as majority charge carriers and can be utilized as p-type semiconductors. Behavior of both designed molecules under electric field was also checked to determine their feasibility for use as conducting channel in OFET.

An investigation of structural and magnetotransport features of half-Heusler ScPtBi thin films

Half-Heusler materials have grabbed enormous devotion from scientific community owing to their novel electrical, magnetic, and optical features. In this paper, we report on ScPtBi half-Heusler thin films grown at 200 and 300 °C on Ta/Si (100) substrates by magnetron co-sputtering. X-ray diffraction studies have proved a polycrystalline structure of the synthesized films. In addition, the film deposited at 300 °C has possessed a secondary PtBi phase. Cross-sectional microstructures and surface topographies of the films have been investigated via scanning transmission electron microscopy and scanning electron microscopy, respectively. The stoichiometric composition of the films has been investigated by X-ray photoelectron spectroscopy studies. Magnetotransport measurements, conducted by a conventional four-probe method in a temperature range of 5–300 K and at fields up to 9 T, have unveiled the existence of the weak antilocalization (WAL) effect in the examined samples. The phase coherence length, lφ, and the coefficient α have been extracted from the Hikami-Larkin-Nagaoka (HLN) relation. It has been shown that the extracted α values are between 800 and 1000, which highlights that the origin of the WAL effect results from the strong spin-orbit coupling (SOC) nature of the studied films rather than from topologically protected surface states. The Hall measurements performed at room temperature have shown that the major charge carriers are electrons and their mobility has been identified as 61 cm2.V−1.s−1 and 113 cm2.V−1.s−1 for the film deposited at 200 °C and 300 °C, respectively.

Cobalt doped SnO2: A new material for thermoelectric application

Due to the environmental crisis of energy, the depletion and increasing price of fossil fuel resources, current research focuses on sustainable and renewable energy sources for power generation. Meanwhile, electricity remains the most convenient form of energy in the near future, the conversion of heat into electricity acquires wide acceptance. Thermoelectric generation can be considered as an amble method to scavenge the heat energy for the generation of electricity. In this work we investigate the structural, chemical and thermoelectric property of SnO2 based samples with varying concentration of cobalt ion. The thermoelectric properties were measured in a temperature 100-700 0C and found that cobalt doped samples posses a remarkable effect on transport properties compared to pristine SnO2. Negative thermopower ensured that electrons are the majority charge carriers. Seebeck coefficient for 5 wt % of cobalt doping was found to be -364 µV/K, so that it can be considered as a potential candidate for thermoelectric application. The figure of merit (ZT) of the material gets enhanced on cobalt doping and found to increase with increment in temperature so that it can be used for high temperature thermoelectric application. Copyright © 2018 VBRI Press.

Decoupling of Implied and External VOC Due to Ionic Movement Explaining Transient VOC Overshoot in Perovskite Solar Cells

An overshoot of the open‐circuit voltage (V OC) after switching off the illumination is observed for perovskite solar cells, while the simultaneously measured photoluminescence (PL) intensity decreases continuously. Similarly, a dip in the photovoltage transient is detected at the beginning of a light pulse added to a continuous bias light, while the PL increases. This decoupling of external and implied V OC (as derived from the PL data) originates from a strong gradient of the majority charge carrier quasi‐Fermi level in the vicinity of a nonideal contact. This gradient reduces the external voltage much more than the implied voltage. The V OC overshoot is observed whenever the gradient decreases faster than when the separation of the quasi‐Fermi levels is reduced by charge carrier recombination. As shown in previous work, in perovskite solar cells, the magnitude of the gradient is strongly influenced by mobile ionic species and it decreases upon light soaking. This is why a fully light‐soaked device does not show this kind of V OC overshoot.

Investigation of spin gapless semiconducting behaviour in quaternary CoFeMnSi Heusler alloy thin films on Si (1 0 0)

Spin gapless semiconducting (SGS) behaviour of polycrystalline CoFeMnSi (CFMS) Heusler alloy thin films was investigated through structural, magnetic, magnetotransport and electron-transport characterization. In this study, we have tailored the structural ordering in CFMS thin films (∼60 nm) grown at different substrate temperatures (TS) in 150 °C to 550 °C range on Si(100) substrates using pulsed DC magnetron sputtering. All the grown films were polycrystalline in nature, showing an enhancement in structural ordering with TS. Among all the samples, the film grown at 550 °C showed an optimal structure with L21 ordering. The magnetic measurements revealed that saturation magnetization (Ms) of the sample decreased with the an increment in measurement temperature with a value ∼ 3.42 μB/f.u. at 5 K to 3.11 μB/f.u. at 300 K. The Curie temperature (TC) of the sample was estimated to be 778 ± 13 K. The longitudinal resistivity (ρXX) measurements clearly exhibited a negative temperature coefficient of resistivity, indicating the non-metallic behaviour of these CFMS films. The fitting of resistivity curve dig out contribution of electron–phonon scattering and defect scattering to resistivity. Fitting also suggest that holes are the majority charge carriers in optimized CFMS thin film which was further confirmed through positive slope of anomalous Hall resistivity curve. The temperature-dependent anomalous Hall effect (AHE) measurements have also been carried out to identify the different scattering mechanisms responsible for AHE. The results suggested that both the intrinsic as well as extrinsic factors contribute to the AHE of CFMS deposited film with non-zero contribution of phonon-related skew scattering. The carrier concentration (n) and mobility (μ) were found to be nearly temperature-independent, revealing typical SGS characteristics. At room temperature, the n and μ values were found to be 3.91 ± 0.04 × 1021 cm−3 and 88.83 ± 1.58 cm2/Vs, respectively. The value of anomalous Hall conductivity at 5 K is found to be 53.79 S/cm..

Thermally Controlled Charge-Carrier Transitions in Disordered PbSbTe Chalcogenides.

Binary and ternary chalcogenides have recently attracted much attention due to their wide range of applications including phase-change memory materials, topological insulators, photonic switches, and thermoelectrics. These applications require a precise control of the number and mobility of charge carriers. Here, an unexpected charge-carrier transition in ternary compounds from the PbTe-Sb2 Te3 pseudo-binary line is reported. Upon thermal annealing, sputtered thin films of PbSb2 Te4 and Pb2 Sb2 Te5 undergo a transition in the temperature coefficient of resistance and in the type of the majority charge carriers from n-type to p-type. These transitions are observed upon increasing structural order within one crystallographic phase. To account for this striking observation, it is proposed that the Fermi energy shifts from the tail of the conduction band to the valence band because different levels of overall structural disorder lead to different predominant types of native point defects. This view is confirmed by an extensive computational study, revealing a transition from excess cations and SbPb for high levels of disorder to PbSb prevailing for low disorder. The findings will help fine-tune transport properties in certain chalcogenides via proper thermal treatment, with potential benefits for memories, thermoelectric material optimization, and neuromorphic devices.

Pt‐Induced Defects Curing on BiVO4 Photoanodes for Near‐Threshold Charge Separation

AbstractPhotostability is one of the most essential properties for evaluating photoelectrochemical (PEC) water splitting performance on semiconductors. Herein, the oxygen‐deficiency conditions are applied to tune and activate BiVO4 photoanodes with a class of oxygen vacancies across the whole bulk material, and regulate the electronic occupancy of these states upon the charge carrier processes that determine PEC water oxidation activity. Through the experimental results and nonadiabatic molecular dynamics with time‐domain density functional theory calculations, the charge carrier lifetime can be influenced by the oxygen vacancies concentration on BiVO4, and the semiconductor can be flexibly photoactivated under oxygen‐sufficient and deficient atmospheres for enhancing the charge carrier density and photovoltage. The PEC performance of BiVO4 is further boosted by Pt doping, which exhibits a record photocurrent density of 5.45 mA cm–2 at 1.23 VRHE with solar conversion efficiency of 2.1% at 0.65 VRHE. The Pt can prevent the unnecessory charge recombination on the defected BiVO4, which also enhances the majority charge carrier density, resulting in one of the best charge separation efficiencies, close to 100%, among the steady‐state PEC performance for BiVO4. More importantly, the resulting Pt:BiVO4 presents long‐term stability over 50 h at 0.8 VRHE.

Exploring pressure induced thermoelectric properties of LiAeH3 (Ae = Ca, Sr, Ba) perovskite hydrides along with optoelectronic features

This report aims to demonstrate the plane wave pseudopotential approached ab initio computation based on density functional method integrated within semi-classical Boltzmann transport theory to investigate the pressure induced electronic, optical and thermoelectric properties of LiAeH3 (Ae = Ca, Sr, Ba) perovskite hydrides. The computed electronic structures of these compounds possess indirect band gaps with mixed covalent and ionic bonding in the whole range of studied pressure (0–50 GPa) in which the band gaps of LiCaH3 decrease while those of LiSrH3 and LiBaH3 increase as the pressure increases. The rational analyses of optical spectra devote these perovskites to optoelectronic applications. For instance, the investigated hydrides reveal plasmonic excitation owing to their unique loss spectra, which are significant in optical sensing devices. The estimated electronic and optical parameters are compared, whenever possible, to the available results and found a reasonable agreement among them. To further assess physical behavior, the pressure and temperature dependent thermoelectric properties in terms of Seebeck coefficients, thermal conductivities, electrical conductivities, power factors and figure of merits are carried out first time as implemented in the BoltzTraP code. The estimated positive Seebeck coefficients of the studied perovskites exhibit p-type semiconducting nature, and thus holes are the majority charge carriers for conduction rather than electrons. The large figure of merits and small thermal to electrical conductivity ratios disclose their suitability in thermoelectric device applications.

Low thermal conductivity and semimetallic behavior in some TiNiSi structure-type compounds

Motivated by recent advances in half-Heusler based thermoelectric materials, we investigated the phase stability and thermoelectric properties of compounds ZrNiSi, ZrNiGe, HfNiSi, NbCoSi, and ZrNiSb, some of which were recently reported in literature as promising half-Heuslers for thermoelectric applications using the first-principles density functional theory based calculations. Here, we show that all the named compounds actually crystallize with the orthorhombic TiNiSi structure type, which remains stable above room temperature up to at least 1100 K. In ZrNiSb, $5%$ excess Zr is required to obtain the pure orthorhombic phase. Our first-principles electronic band structure calculations reveal that they are semimetals. In ZrNiSi, ZrNiGe, and HfNiSi, the Fermi surface consists of small electron and hole pockets with electrons as the majority charge carriers. In NbCoSi and ZrNiSb, the majority carriers are holes. A pseudogaplike feature is observed in the electronic density of states with Fermi energy (${E}_{F}$) located either slightly below (ZrNiSi, ZrNiGe, and HfNiSi) or above the pseudogap (NbCoSi). In ZrNiSb no pseudogap is observed; however, the density of states at ${E}_{F}$ is still small. The electrical conductivity ($\ensuremath{\sigma}$) near room temperature is of the order of ${10}^{3}\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which is intermediate between that of the degenerate semiconductors and metallic alloys. Near room temperature the thermopower is negative for $\mathrm{ZrNi}X$ ($X$ = Si, Ge) and HfNiSi, and positive for NbCoSi and ZrNiSb as predicted theoretically. The average value of Seebeck coefficient is small, of the order of $10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Despite reasonably high electrical conductivity, the thermal conductivity ($\ensuremath{\kappa}$) of these compounds is found to be generally low ($<15\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ near $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$). In ${\mathrm{Zr}}_{1.05}\mathrm{Ni}\mathrm{Sb}$, which has the highest electrical conductivity ($\ensuremath{\approx}4000\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), $\ensuremath{\kappa}$ is as low as $\ensuremath{\approx}4\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$, of which almost 70% is estimated to be due to the electronic contribution resulting in a lattice contribution which is $<1\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. This uncommon combination of high electrical conductivity and low thermal conductivity is interesting and invites further attention.

Synthesis and Characterization of N-Type CuGaS2 Nanoparticles and Films for Purpose of Photoelectrocatalytic Water Splitting

In this work, n-type CuGaS2 (CGS) nanoparticles were synthesized using an original colloidal method in N-Methylimidazole solvent with GaCl3, Li2S and CuCl as Ga3+, S2− and Cu+ precursors, respectively. After annealing at 600 °C, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed an excess of gallium with a bulk Ga/Cu atomic ratio of 1.01 and a larger value at the surface. The CGS X-ray diffraction pattern is compatible with a chalcopyrite crystalline phase with crystallites size of about 17 nm. UV-visible spectroscopy measurements showed that CGS sample has a direct band gap energy of 2.41 eV. Capacitance measurements, carried out in aqueous 0.5 M KOH on CGS thin film, using electrochemical impedance spectroscopy, confirmed the material n-type conductivity. The Fermi level was found to be −3.93 eV and the majority charge carrier density is 1.93 × 1018 cm−3. The valence band higher energy level and the conduction band lower energy level are −6.31 eV and −3.90 eV, respectively. The energy level diagram of the n-CuGaS2/electrolyte junction suggests that photo-oxidation of OH− species to produce O2 gas would be favorable; however, a small external cathodic bias would be required to generate H2 gas at the platinum counter electrode.

Reduction in low-temperature thermal conductivity of Cu2Se via substitution of Se by Te atoms

We report the structural and thermoelectric properties of the Cu2Se compound with a systematic substitution of Se by Te atom. The bulk polycrystalline Cu2Se1-xTex samples for the Te concentrations of x = 0.00, 0.02, 0.04, 0.06, and 0.08 were prepared by the solid-state reaction method. The room-temperature XRD studies revealed that the studied samples possess a monoclinic crystal structure. The oxidation state and electronic structure are confirmed by employing X-ray photoemission spectroscopy (XPS). The hardness of the samples increased systematically with an increase in Te doping concentration. The electrical resistivity of the doped samples was found to be increased in comparison to the pure sample. The Seebeck coefficient was positive throughout the temperature range under investigation, indicating that holes are the majority charge carriers in the studied compounds. The thermal conductivity of the doped samples decreased, which is presumably attributed to the point defect scattering. The systematic evolution of thermoelectric properties via substitution of Se by Te over the range (0.00 ≤ x ≤ 0.08) concludes that the overall thermoelectric power factor (PF) and figure of merit (ZT) decrease with Te substitution compared to the pristine Cu2Se compound.

Determination of Partial Proton Conductivity in Fluorite-Type Oxides Using Electrochemical Technique

Fluorite-type oxides such as YSZ and GDC are typical oxide ion conductors which are used for electrolyte materials for SOFCs. Although major charge carriers in these materials are of course oxide ions, it has been suggested that under wet conditions dissolved hydrogen becomes minor charge carriers. Existence of the dissolved hydrogen was confirmed by measuring deuterium concentration using SIMS, but whether the dissolved hydrogen really becomes charge carrier or not is still unclear. It is possible to examine this by whether proton conduction is detected or not in electrochemical measurements. In this study, in order to detect partial proton conduction in YSZ and GDC, oxide ion blocking measurements using hydrogen permeable Pd-plate as the blocking electrode were performed.Pelletized 8YSZ and 10GDC powders by CIP at 390 MPa were sintered at 1300~1700 °C. The surfaces of the obtained dense pellets were mirror polished. As blocking electrodes, Pd-plates were fixed on upper surfaces of the pellets using Pd-pastes by firing them at 1000 °C and the boundaries between the pellets and Pd-plates were sealed using BaO-Al2O3-SiO2-glasses by firing them at 1250 °C. As reversible electrodes, Pd-pastes were painted on lower surfaces and fired at 1000 °C. Under the conditions with constant temperature and p O2 (1×10−4 atm), constant voltage (20 mV) was applied between two electrodes with the reversible electrodes being positive electrical potential and the current responses to humidification of atmosphere by H2O and D2O were recorded.In the case of 8YSZ sintered at 1300 °C, the current under dry atmosphere at 900 °C was 0.152 μA. When the atmosphere was humidified by 18°C H2O and D2O, the current increased to 0.226 and 0.206 μA, respectively. The current increment by humidification with H2O (ΔI H2O = 0.074 μA) was approximately 1.4 times as compared to D2O (ΔI D2O = 0.054 μA), which indicates that H+/D+ isotopic effect was reflected on the current increments. The current increments are suggested to be caused by conduction of dissolved hydrogen and deuterium because not oxygen but only hydrogen and electrons can pass through Pd-plates. This means that dissolved hydrogen becomes charge carriers, i.e., protons, in YSZ. Similar behavior was also observed for the case of 10GDC.

Study the electrical and magnetic properties of Ferrite Ni1-x ZnxFe2O4 films and their potential use in the magnetic core of transformers

Membranes (Ni1-x ZnxFe2O4) were prepared by chemical precipitation method and with different weight ratios of values (x = 0, 0.3, 0.5, 0.7, 0.9), the electrical properties of the prepared samples were studied with the presence of a magnetic field of magnitude (0.45T), the result (Hall Effect) The majority of charge carriers are electrons, while the results of magnetic tests showed that the prepared films of different weights are of a type (soft Ferrite) The membranes behaved as a ferromagnetic material. The results indicate that the membranes can be used in the fields of microwave frequencies and in the transformers inside the magnetic core due to the smallness of the hysterical loop.

Highly Efficient Experimental Approach to Evaluate Metal to 2D Semiconductor Interfaces in Vertical Diodes with Asymmetric Metal Contacts.

The energy band alignments and associated material properties at the contacts between metal and two-dimensional (2D) semiconducting transition metal dichalcogenide (SCTMD) films determine the important traits in 2D SCTMD-based electronic and optical device applications. In this work, we realize 2D vertical diodes with asymmetric metal-SCTMD contact areas where currents are dominated by the contact-limited charge flows in the transport regimes of Fowler-Nordheim tunneling and Schottky emission. With straightforward current-voltage characteristics, we can accurately evaluate the interface parameters such as Schottky barrier heights and the vertical effective masses of tunneling charges. In particular, the differing contact areas and resultant current rectifications allow us to address specific Schottky barrier locations with respect to the conduction and valence band edges of 2D semiconducting WSe2, WS2, MoSe2, and MoS2, thereby determining whether p-type holes or n-type electrons become the majority charge carriers in the SCTMD devices. We demonstrate that our experimental and analytical approaches can be utilized as a simple but powerful material metrology to qualitatively and quantitatively evaluate various metal-SCTMD contacts.

Inverted Ion Current Rectification-Based Chemical Delivery Probes for Stimulation of Neurons.

Ion current rectification (ICR), diodelike behavior in surface-charged nanopores, shows promise in the design of delivery probes for manipulation of neural networks as it can solve diffusive leakages that might be critical in clinical and research applications. However, it has not been achieved because ICR has restrictions in nanosized dimension and low electrolyte concentration, and rectification direction is inappropriate for delivery. Herein, we present a polyelectrolyte gel-filled (PGF) micropipette harnessing inverted ICR as a delivery probe, which quantitatively transports glutamate to stimulate primary cultured neurons with high efficiency while minimizing leakages. Since the gel works as an ensemble of numerous surface-charged nanopores, the current is rectified in the micro-opening and physiological environment. By extending the charge-selective region using the gel, inverted ICR is generated, which drives outward deliveries of major charge carriers. This study will help in exploring new aspects of ICR and broaden its applications for advanced chemical delivery.

From n- to p-Type Material: Effect of Metal Ion on Charge Transport in Metal-Organic Materials.

An intriguing new class of two-dimensional (2D) materials based on metal-organic frameworks (MOFs) has recently been developed that displays electrical conductivity, a rarity among these nanoporous materials. The emergence of conducting MOFs raises questions about their fundamental electronic properties, but few studies exist in this regard. Here, we present an integrated theory and experimental investigation to probe the effects of metal substitution on the charge transport properties of M-HITP, where M = Ni or Pt and HITP = 2,3,6,7,10,11-hexaiminotriphenylene. The results show that the identity of the M-HITP majority charge carrier can be changed without intentional introduction of electronically active dopants. We observe that the selection of the metal ion substantially affects charge transport. Using the known structure, Ni-HITP, we synthesized a new amorphous material, a-Pt-HITP, which although amorphous is nevertheless found to be porous upon desolvation. Importantly, this new material exhibits p-type charge transport behavior, unlike Ni-HITP, which displays n-type charge transport. These results demonstrate that both p- and n-type materials can be achieved within the same MOF topology through appropriate choice of the metal ion.

Galvanomagnetic and Optical Properties of Type-II Weyl Semimetal Candidate WTe₂

The galvanomagnetic properties (magnetoresistivity and Hall Effect) and resistivity at temperatures from 2 K to 300 K in magnetic fields of up to 9 T as well as optical characteristics (real and imaginary parts of the complex permittivity, optical conductivity, and reflectivity) at room temperature were studied. It was shown that the temperature dependence of the electrical resistivity has a “metallic” type and is quadratic at low temperatures (< 60 K) that can be caused by the “electron-phonon-surface” interference scattering mechanism in WTe2. Whereas an applied magnetic field induces a minimum in the temperature dependence of the resistivity at low temperatures, that shifts to higher temperature with increasing field, one of the possible explanation of which is the transition from high to weak effective magnetic fields. The magnetoresistivity increases with the magnetic field according to the quadratic law, reaching 1750% at 2 K, which is due to the compensation of charge carriers in WTe2. Hall Effect studies showed that the majority charge carriers are electrons with the concentration 1019 cm-3 and the mobility 7500 cm2/V•s at 2 K. At the same time optical investigations did not reveal the features characteristic of metals. The optical conductivity spectrum is a broad band centered at 3.4 eV, formed by interband transitions. The presence of peaks in the infrared region indicates the formation of low-energy gaps in the band spectrum of WTe2. The obtained data on the real and imaginary parts of the complex permittivity also evidence the absence of a contribution from free carriers up to 0.2 eV. The optical characteristics are shown to be in good agreement with the data on electronic transport properties. The revealed features are a manifestation of the topological nature of the material.

Experimental validation of bulk-graphene as a thermoelectric generator

Quest for alternate energy sources is the core of most of the research activities these days. No matter how small or large amount of energy can be produced by utilizing the non-conventional techniques and sources, every bit of innovation can reshape the future of energy. In this work, experimental analysis of the thermoelectric (TE) properties of bulk-graphene in the temperature range of (303 to 363) K is presented. Graphene powder was pressed to form a pellet which was used to fabricate the TE device. The effects of temperature on the Seebeck coefficient, electrical and thermal conductivities, and the dimensionless figure of merit (FOM) were measured. The increasing value of the Seebeck coefficient (thermopower) with temperature is indicant of the metallic behavior. Additionally, the observed thermopower (TEP) is positive, which shows that the majority charge carriers are holes and peaked to a value of 56 μV K−1 at 363 K. The thermopower of the pellet is four times larger than the previously reported values for single layer graphene (SLG) and few layer graphene (FLG). In addition to this, low values of the thermal conductivity were observed for the pellet which is one of the requirements of a good TE material. Besides this, an upward trend is observed with increasing temperature for FOM, which attains a peak value of 0.0016 at 363 K, which is almost ten times that of the previously reported values.

Electron transport properties of electrically doped guanine nano-sheet based bio-Zener diode: a first principle paradigm

The quantum ballistic transmission properties of an electrically-doped guanine-nanosheet-based bio-Zener diode are investigated using density functional theory and nonequilibrium Green’s function-based first-principles calculations. The bio-Zener diode is gate-bias modulated, and its various quantum-electronic properties, for example, the V–I characteristic, the transmission spectra, and the device density of states, depend on both the electrical doping concentration and on the applied gate bias voltage. The junctionless highly doped bio-Zener diode shows high levels of reverse-bias current which is dominated by majority charge carriers. It is also found that, due to the presence of a wide bandgap and the backscattering effect, the forward-bias current is highly suppressed. The quantum simulation results confirm a strong reverse gate-bias-modulated biased current–voltage response as well as a charge transport phenomenon through the effective device region. The bio-Zener diode exhibits a specific reverse breakdown that can be varied from −0.78 to −3.2 V without affecting the forward current–voltage characteristic. The current findings are obtained by including the coherent tunneling and incoherent hopping processes with a minimal Hamiltonian model approach.

Organic Doping at Ultralow Concentrations

AbstractOrganic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C60 metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C60 OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 107 are realized. The devices have low subthreshold swing in the range of 80 mV dec−1 and a large transconductance of up to 8 mS mm−1.