Charge Carrier Doping Research Articles

Tuneable electron\u2013magnon coupling of ferromagnetic surface states in PdCoO2

Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong electron–magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling is enhanced sevenfold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds light on electron–magnon interactions in solid-state materials, and the ways in which these can be controlled.

Structural, Electronic, and Physical Properties of a New Layered Cr-Based Oxyarsenide Sr2Cr2AsO3.

We report synthesis, crystal structure, and physical properties of SrCrAsO. The new compound crystallizes in a SrGaOCuS-type structure with two distinct Cr sites, Cr(1) in the perovskite-like block layers of “SrCrO” and Cr(2) in the ThCrSi-type layers of “SrCrAs”. An inter-block-layer charge transfer is explicitly evidenced, which dopes electrons in the CrO planes and simultaneously dopes holes into the CrAs layers. Measurements of electrical resistivity, magnetization, and specific heat, in combination with density-functional theoretical calculations, indicate that the title material is an antiferromagnetic metal. The Cr(2) magnetic moments in the CrAs layers order at 420 K, while the Cr(1) spins in the CrO planes show quasi-two-dimensional magnetism with long-range ordering below 80 K. Both Néel temperatures are significantly reduced, compared with those of the cousin material SrCrAsO, probably due to the intrinsic charge-carrier doping. Complex re-entrant magnetic transitions with a huge magnetic hysteresis were observed at low temperatures.

Recent advances in two-dimensional van der Waals magnets

Two-dimensional (2D) magnets have evoked tremendous interest within the research community due to their fascinating features and novel mechanisms, as well as their potential applications in magnetic nanodevices. In this review, state-of-the-art research into the exploration of 2D magnets from the perspective of their magnetic interaction and order mechanisms is discussed. The properties of these magnets can be effectively modulated by varying the external parameters, such as the charge carrier doping, thickness effect, pressure and strain. The potential applications of heterostructures of these 2D magnets in terms of the interlayer coupling strength are reviewed, and the challenges and outlook for this field are proposed.

Spin-orbit coupling locked robust thermoelectric performance of SrTe: A comparison with CaTe

We have investigated the pressure dependence of structural, electronic and transport properties of cubic CaTe and SrTe using density functional theory. By employing semi-classical Boltzmann-transport-formalism their transport properties are computed within constant scattering time approximation. Without spin–orbit-coupling (SOC), one finds multiple Dirac points along various directions around the M point in the Brillouin zone. SOC opens up gaps at all points except one Dirac point along the M-R direction. We have also explored the effect of isotropic compressive strain on thermoelectric properties. The effect of SOC is prominent for SrTe results into enhanced electrical conductivity by 20 times and power factor by almost 25 times. Doping of charge carriers results in improvedZT of CaTe. However, in SrTe only electron type doping is beneficial for improving ZT. We find thatSrTe is robust 3D Dirac material with promising thermoelectric performance. Specifically, SOC results in robust thermoelectric properties against strain and temperature changes.

Continuous Tuning of the Fermi Level in Disorder-Engineered Amorphous Films of Li-Doped ZnO for Thermoelectric Applications.

Amorphous metal-oxide semiconductors can be readily prepared by a solution process at low temperatures, and their energy band structures and carrier concentrations can be controlled based on the oxide composition or the addition of dopants in the design of thermoelectric (TE) materials. However, research on the correlation between the charge transport and TE performance of amorphous metal-oxide semiconductors is still in its infancy. Herein, we present the energy-dependent TE performance characteristics of Li-doped ZnO thin films with different doping levels and charge carrier concentrations. Thin films were prepared by the solution process, and the Li doping level was controlled by the Li precursor concentration added to a Zn precursor solution. Subsequently, a field-effect-modulated Seebeck coefficient measurement device was built to study the energy-dependent TE performance. Notably, the higher ratio of interstitial Li (Liinter) and oxygen vacancies (Ova) in the Li-ZnO device indicates an improved n-type TE performance. To investigate more thoroughly the charge transport phenomena, the localized density of states (DOS) was derived from the temperature-dependent transfer curve; the higher ratio of interstitial Li (Liinter) and oxygen vacancy (Ova) induces a reduction in the localized DOS and lowers the degree of disorder in their DOS. The determined energy-dependent TE characteristics can be used as guidance for the design of efficient TE devices with amorphous metal-oxide semiconductors.

Unveiling the Interplay among End Group, Molecular Packing, Doping Level, and Charge Transport in N‐Doped Small‐Molecule Organic Semiconductors

AbstractDoped small molecules with high electrical conductivity are desired because they typically show a larger Seebeck coefficient and lower thermal conductivity than their polymer counterparts. However, compared with conjugated polymers, only a few small molecules can show high electrical conductivities. In this study, three n‐type small‐molecule organic semiconductors with different end functional groups are synthesized to explore the reasons for the low electrical conductivity issue in n‐doped small‐molecule semiconductors. Charge carrier mobility and doping level are usually considered as two major parameters for achieving high electrical conductivity. TDPP‐ThIC with high electron mobility of 0.77 cm2 V−1 s−1 and high electron affinity, which can be easily n‐doped; however, it only displays an electrical conductivity ≈10−3 S cm−1. To explore the reasons, the single crystal structure of TDPP‐ThIC and the grazing incidence wide‐angle X‐ray scattering of its n‐doped films are carefully analyzed. TDPP‐ThIC with a 1D column packing is disclosed and easily distorted by the enthetic n‐dopants, which damages the charge transport pathways, and thereby results in low electrical conductivity. The results suggests that only high intrinsic charge carrier mobility and high doping level cannot guarantee high electrical conductivity, and keeping good charge transport pathways after doping is also critical.

How charge trapping affects the conductivity of electrochemically doped poly(3-hexylthiophene) films

Electrochemical doping is an elegant method of controlling the doping level and charge carrier densities of conjugated polymer films and enhancing their thermoelectric figure of merit. Applying this doping technique to films of poly(3-hexylthiophene) (P3HT) results in conductivities with values as high as 200 S/cm. The stability of the doped films in the solid state can be probed by UV-vis-NIR spectroscopy. We found that the choice of the conducting salt in the liquid electrolyte exerts a strong influence over the conductivity. Using TBAPF6 and LiClO4 provides highest conductivities for P3HT films, while LiTFSI and TBABF4 show overall lower performance. This effect is also reflected in cyclic voltammetry measurements coupled with in situ spectroscopy. Overall lower reversibility upon multiplex cycling in LiTFSI and TBABF4 electrolytes suggests strong charge trapping effects, which one might attribute to a considerable fraction of charges (holes/ions) remaining in the films after charge/discharge cycles. The salts with stronger charge irreversibility in the electrochemistry experiments show the poorer solid state conductivities. Our conclusion is that one should carefully choose the electrolyte to ensure good percolation pathways and delocalized charge transport throughout doped films.

(Invited) Probing Charge-Carrier Transport in Chemically Doped Polymeric Semiconductors

With organic semiconductors demonstrating potential for use in various electronic and optoelectronic applications, some attention has turned to an understanding of charge-carrier doping and transport processes in such material systems. Here we demonstrate a novel electrical transport measurement technique, based on microwave spectroscopy, to probe redox-based chemical p-type doping in conjugated semiconducting polymers. Through a comparison of these measurements with macroscopic charge-transport probes, we illustrate that charge injection from a prototypical dopant like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) initially results in charge-carriers that are bound to the reduced F4-TCNQ counterion, but that the coulomb attraction can be overcome by doping to high charge-carrier concentrations. In contrast, a series of novel charge-transfer dopants based on dodecaborane clusters appended with various organic moieties results in injected charge-carriers that are spatially separated from the counterion. This behavior results from the unique chemical and electronic structure of these dopants, which localizes the negative counter-charge on the center of the dodecaborane cluster, thereby reducing the coulomb attraction for all measurable charge-carrier concentrations and resulting in enhanced charge-carrier transport. These observations point to chemical control over the charge-injection and transport processes that can yield highly doped organic semiconductors with impressive electrical transport properties, in spite of the limitations that their low dielectric constants would suggest.

Supercontinuum generation in silicon Bragg grating waveguide

Supercontinuum generation is an extensively studied and arguably the most important and all-encompassing nonlinear phenomenon. Yet, we do not have a good control over all the signals generated in this process. Usually, a large part of an octave spanning spectrum has an orders of magnitude weaker signal than the peak to be useful for any application. In this work, we show strong signal generation within a supercontinuum generated in a complementary metal-oxide-semiconductor compatible silicon Bragg grating waveguide. We show up to 23 dB of signal enhancement over a 10 nm full-width-at-half-maximum bandwidth at the Bragg resonance in the telecom window. Additionally, the grating is made by depositing charge carriers periodically, thus avoiding any dimensional change in the waveguide, and it can allow other functionalities offered by the induced electric field, such as second harmonic generation and free carrier sweeping. We believe this work opens up an avenue for research in nonlinear integrated photonics and signal enhancement in the supercontinuum by the Bragg effect (whether created through grating formation with dimensional variation in insulators and/or periodic charge carrier doping in semiconductors).

AFeSe2 (A=Tl, K, Rb, or Cs) : Iron-based superconducting analog of the cuprates

It has long been a challenging task to find compounds with similar crystal and electronic structures as cuprate superconductors with low dimensionality and strong antiferromagnetic fluctuations. The parent compounds of cuprate superconductors are Mott insulators with strong in-plane antiferromagnetic exchange interactions between Cu moments. Here we show, based on first-principles density functional calculations, that AFeSe2 (A=Tl, K, Rb, or Cs) exhibit many of the physical properties common to the cuprate parent compounds: (1) the FeSe2 layer in AFeSe2 is similar in crystalline and electronic structures to the CuO2 plane in cuprates, although Se atoms are not coplanar to the square Fe-lattice; (2) they are antiferromagnetic insulators, but with relatively small charge excitation gaps; (3) their ground states are Neel antiferromagnetic ordered, similar as in cuprates; and (4) the antiferromagnetic exchange interactions between Fe moments are larger than in other iron-based superconducting materials, but comparable to those in cuprates. Like cuprates, these compounds may become high-Tc superconductors upon doping of charge carriers either by chemical substitution or intercalation or by liquid or solid gating.

(Invited) Carbon Nanotube Electronic Ratchets

Electronic ratchets are recently demonstrated thin-film semiconductor devices with the ability to harvest energy from high-frequency signals or electrical noise. The state-of-the-art electronic ratchets have been fabricated from doped conjugated polymers, but the device performance is limited by the low charge-carrier mobility and relatively poor electrical conductivity. Enriched semiconducting single-walled carbon nanotube (s-SWCNT) networks represent an attractive alternative to conjugated polymers, since they are easy to dope and exhibit good charge-carrier transport.While the electronic ratchet device is based on a simple field-effect transistor architecture, it requires the generation of asymmetry within the transistor channel. In this presentation, we will outline the approaches employed to control the charge-carrier doping profile to create this asymmetry. We will subsequently discuss the performance of these carbon nanotube electronic ratchets and make a comparison with their conjugated polymer counterparts.

Nature and origin of unusual properties in chemically exfoliated 2D MoS2

MoS2 in its two-dimensional (2D) form is known to exhibit many fundamentally interesting and technologically important properties. One of the most popular routes to form an extensive amount of such 2D samples is the chemical exfoliation route. However, the nature and origin of the specific polymorph of MoS2 primarily responsible for such spectacular properties have remained controversial with claims of both T and T′ phases and metallic and semiconducting natures. We show that a comprehensive scrutiny of the available literature data of Raman spectra from such samples allows little scope for such ambiguities, providing overwhelming evidence for the formation of the T′ phase as the dominant metastable state in all such samples. We also explain that this small bandgap T′ phase may attain substantial conductivity due to thermal and chemical doping of charge-carriers, explaining the contradictory claims of the metallic and semiconducting nature of such samples, thereby attaining a consistent view of all reports available so far.

Quantum valley Hall effect in wide-gap semiconductor SiC monolayer

We have investigated the valley Chern number and gapless edge states in wide-gap semiconductor SiC and BN monolayers by using the density functional theory calculations. We found that while SiC monolayer has a non-quantized valley Chern number due to a partial mixing of the Berry curvature peaks pertaining to the opposite valleys, there exist topologically protected gapless edge states within the bulk gap, leading to a quantum valley Hall effect. Doping of the opposite charge carriers causes a backscattering-free valley current flowing on the opposite edge, which can be used for experimental confirmation and application at room temperature. BN monolayer, on the other hand, was found to have gapped edge states due to the too large staggered AB-sublattice potentials.

Closely Packed Polypyrroles via Ionic Cross-Linking: Correlation of Molecular Structure-Morphology-Thermoelectric Properties.

A series of ionically interconnected polypyrrole (PPy) films are fabricated through two-monomer-connected-precursor polymerization by varying diacid linkers, thereby significantly influencing the crystalline morphology and electrical properties. The structure obtained using 1,5-napthalenedisulfonic acid (PPy-Nap) as a fused aromatic linker exhibits a higher electrical conductivity (∼78 S cm-1) than that (6.7 S cm-1) without a linker (PPy-ref). Cryogenic conductivity measurements reveal that the percolation carrier transport barrier of PPy-Nap is significantly smaller than that of PPy-ref, and the calculated carrier mobility of PPy-Nap is ∼5 times higher compared to PPy-ref. The carrier transport characteristics show a good agreement with morphological data by 2D grazing-incidence X-ray scattering. All PPys have similar doped charge carrier concentrations and, thus, similar Seebeck coefficients (5-8 μV K-1) but very different electrical conductivities. Consequently, PPy-Nap exhibits a higher power factor than that of PPy-ref (0.21 vs 0.043 μW m-1 K-2). The results show that the trade-off relationship between the Seebeck coefficient and electrical conductivity can be overcome by improving crystalline morphology and carrier transport. Thus, both the electrical conductivities and thermoelectric power factors can be improved with maintaining the Seebeck coefficients by enhancing the ordered conductive domains and carrier mobility while maintaining the doping level.

Enhanced thermoelectric properties of semiconducting carbon nanotube films by UV/ozone treatment

We examine the effect of UV/O3 oxidation on the thermoelectric properties of semiconducting carbon nanotube films. The oxidative UV/O3 treatment leads to the introduction of epoxy and carbonyl groups and a significant increase in the thermoelectric power factor up to 140 μW m−1 K−2. This power factor is three times larger than that of chemically-doped films because of the enhanced Seebeck coefficient. Characterization with Raman and mid-IR absorption/extinction spectroscopy reveals that the UV/ozone treatment results in simultaneous charge carrier doping and defect formation. This simple way of enhancing thermoelectric properties is suitable for the production of large-area, flexible thermoelectric devices based on semiconducting carbon nanotubes.

Enhanced Raman spectra of hydrogen-intercalated quasi-free-standing monolayer graphene on 4H-SiC(0001)

Abstract Epitaxial Chemical Vapor Deposition graphene on silicon carbide is a promising material for application in electronics. Its most developed form, i.e. quasi-free-standing monolayer that comes through hydrogen atom intercalation of carbon buffer layer grown on the Si-face of SiC, constitutes an excellent platform for sensor technology. Raman spectroscopy allows for a non-destructive analysis of graphene properties. The G and 2D bands widths and positions bring information on the number of graphene layers as well as charge carrier type and doping level. Their intensity is strong. However, the analysis of Si-H bonds remains a challenge. In this work we present how analytical methods can improve the visibility of hydrogen-related Raman bands and help monitor hydrogen presence in quasi-free-standing graphene.

Controlling Doping Profiles in Enriched Semiconducting Carbon Nanotube Networks for Novel Energy Harvesting Electronic Devices

Carbon nanotubes have drawn lots of interest due to their unique chemical structure, which confers excellent thermal, mechanical and electronic properties. In particular, semiconducting single-walled carbon nanotubes (s-SWCNTs) are being seen as the most promising candidate for future optical, electronic, and energy conversion applications, due to their structure-tunable electronic bandgap. The demand for high-performance, flexibility, and low weight miniaturized and portable electronic devices and power sources, suggest s-SWCNTs as a strong candidate for such applications. However, the ability to exert fine control over the optical and electronic performance of s-SWCNT components in these devices is critical to their widespread applicability. One strategy toward achieving this control is through manipulation of the Fermi level of the semiconducting single-walled carbon nanotubes through various doping approaches. In this presentation, we will first illustrate the strategies we have developed to create controlled charge-carrier doping profiles in highly pure s-SWCNT networks. And we characterize the spatial distribution of the charge-carrier doping profile using confocal resonant Raman microscopy and probe the resulting variation in the electronic transport properties using field effect transistor measurements. We will subsequently discuss the incorporation of these tailored spatial charge-carrier doping profiles into energy harvesting device architectures and discuss the various factors that determine the observed device performance.

TiO2 Self-Assembled, Thin-Walled Nanotube Arrays for Photonic Applications.

Two-dimensional arrays of hollow nanotubes made of TiO are a promising platform for sensing, spectroscopy and light harvesting applications. Their straightforward fabrication via electrochemical anodization, growing nanotube pillars of finite length from a Ti foil, allows precise tailoring of geometry and, thus, material properties. We theoretically investigate these photonic crystal structures with respect to reduction of front surface reflection, achievable field enhancement, and photonic bands. Employing the Rigorous Coupled Wave Analysis (RCWA), we study the optical response of photonic crystals made of thin-walled nanotubes relative to their bare Ti foil substrate, including under additional charge carrier doping which might occur during the growth process.

Graphene-Based Ammonia Sensors Functionalised with Sub-Monolayer V₂O₅: A Comparative Study of Chemical Vapour Deposited and Epitaxial Graphene †.

Graphene in its pristine form has demonstrated a gas detection ability in an inert carrier gas. For practical use in ambient atmosphere, its sensor properties should be enhanced with functionalisation by defects and dopants, or by decoration with nanophases of metals or/and metal oxides. Excellent sensor behaviour was found for two types of single layer graphenes: grown by chemical vapour deposition (CVD) and transferred onto oxidized silicon (Si/SiO2/CVDG), and the epitaxial graphene grown on SiC (SiC/EG). Both graphene samples were functionalised using a pulsed laser deposited (PLD) thin V2O5 layer of average thickness ≈ 0.6 nm. According to the Raman spectra, the SiC/EG has a remarkable resistance against structural damage under the laser deposition conditions. By contrast, the PLD process readily induces defects in CVD graphene. Both sensors showed remarkable and selective sensing of NH3 gas in terms of response amplitude and speed, as well as recovery rate. SiC/EG showed a response that was an order of magnitude larger as compared to similarly functionalised CVDG sensor (295% vs. 31% for 100 ppm NH3). The adsorption site properties are assigned to deposited V2O5 nanophase, being similar for both sensors, rather than (defect) graphene itself. The substantially larger response of SiC/EG sensor is probably the result of the smaller initial free charge carrier doping in EG.

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites.

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.