Low Charge Carrier Density Research Articles

Ultra-high sensitivity D-type photonic crystal fiber sensor deposited with MoO2 nanofilm

Fiber optic sensing technology has extensive applications in many fields and demands high sensitivity and cost-effectiveness for sensors. In this study, we propose an ultra-high sensitivity D-type photonic crystal fiber sensor based on MoO2 nano-film coating to meet this requirement. Firstly, MoO2 is selected as the plasmonic excitation material. Compared to expensive metals such as Au and Ag, MoO2 has the characteristics of low cost and charge carrier density, which is suitable for use in optical fiber sensing modules. The sensitivity of the sensor to the external environment can be increased by coating the surface of the optical fiber with MoO2 nano-film. Then, we use the finite element method to evaluate the performance of the sensor. Numerical simulation results show that the average wavelength sensitivity of the sensor in the x-polarization direction is as high as 24000 nm/RIU when analyzing substances with refractive indices ranging from 1.38 to 1.43. This means that the sensor has a very sensitive response to refractive index changes and can be widely applied to sensing needs in different fields. Overall, the D-type photonic crystal fiber sensor based on MoO2 nanofilm coating has enormous potential for applications. This sensor can achieve high-sensitivity detection within a specified range of refractive indices, providing new ideas and methods for research and applications in the field of fiber sensing.

Temperature-assisted electromagnetic surface modes in graphene-based temperature sensitive metafilms

Graphene has the potential to manipulate surface modes in frequency bands from THz to mid-IR regions. Typically, due to single-atom thickness and low charge-carrier density, the thermal response of graphene is ineffective. Temperature-sensitive materials (TSMs) can play an active role in enhancing the thermal response of graphene-based devices. In the present work, graphene-based temperature-sensitive metafilms have been proposed for thermally tunable propagation of electromagnetic surface modes. A detailed analytical and numerical solution for temperature-dependent electromagnetic surface (even and odd) modes supported by the graphene-based temperature-sensitive metafilm has been studied. The Kubo’s formulation has been used to model optical conductivity (σ g ) while the hybrid Drude’s model is implemented to realize the indium antimonide (InSb) as temperature-sensitive material. To simulate the metafilm, the waveguide modal analysis approach was implemented, while the realization of the graphene sheets was achieved by the use of impedance boundary conditions (IBCs). The propagation characteristics for even/odd surface modes were analyzed under different values of temperature (T), chemical potential (µ c ), and thickness of metafilm (d). Further, the numerical results for even and odd surface modes under two phases of InSb [Insulator phase (T = 200 K) and metallic phase (T = 300 K)] were compared under different values of chemical potential (µ c ) and TSM film thickness (d). It is concluded that the propagation characteristics of surface modes are sensitive to the external temperature and can be tailored by tuning the temperature, chemical potential (µ c ), and TSM film thickness (d). Moreover, the degeneracy of the even and odd modes can be controlled by varying the temperature and TSM film thickness. The work is suitable for designing temperature-assisted dual channel waveguides, THz optical switches, THz optical logic designs, and flexible thermal-optical sensors.

Impact of doping on the mechanical properties of conjugated polymers.

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Next-Generation Polymeric Organic Semiconductors for Electrochemical Transistors with Applications in Bioelectronics

Over the last decade, significant progress has been made in developing efficient mixed-organic-ionic-electronic conductors (OMIECs) for electrochemical transistors. Improvements in device performance for both transconductance and operational stability paved the way for applications of electrochemical transistors for health monitoring, neuromorphic computing, or electrochemical sensors. While the tuning of the energy levels (via backbone engineering) and the local environment of the polymers (via side-chain engineering) is reported to be a successful strategy for improving the electronic and ionic charge transport properties, little is known about how the short- and long-range order of polymers impacts the device performance.In my talk, I will discuss the importance of microstructure and dynamic microstructural rearrangements of OMIECs which greatly influence the electronic charge transport properties in electrochemical transistors [1]. By combining electrochemical and spectroelectrochemical measurements alongside electrochemical X-ray diffraction measurements, we observe that holes and anions are placed in ordered aggregates and crystallites at relatively low charge carrier densities (< 2x1020 cm-3) for selected OMIECs, explaining the large increase in transconductance at low operational voltages. Moreover, we find that the electrochemical charging of p-type polymeric OMIECs induces structural order along the polymer backbones. We interpret this structural ordering as the key factor for achieving high transconductance since percolating networks can form more easily when polymer chains are planarizing. Surprisingly, these dynamic microstructural rearrangements are highly reversible, however only when high charge carrier densities are avoided where bipolarons are forming. Based on these findings, I will discuss chemical design strategies for next-generation OMIECs with improved transconductance and operation stability in electrochemical transistors.[1] Garrett LeCroy, Camila Cendra, Tyler J. Quill, et.al., Role of aggregates and microstructure of mixed-ionicelectronic-conductors on charge transport in electrochemical transistors, Mater. Horiz., 2023, accepted.

Thermally tunable electromagnetic surface waves supported by graphene loaded indium antimonide (InSb) interface

The thermal agitation plays a vital role in tunability of optoelectronic, structural and chemical characteristics of the temperature sensitive materials. Graphene enables the THz optics, due to its unprecedent controlling characteristics over the traditional materials. The influence of temperature on the monolayer graphene is very negligible due to its low free charge carrier density, to enhance the thermal sensitivity of graphene, the graphene loaded temperature sensitive material interface has been proposed. A theoretical analysis has been carried out on temperature dependent propagation characteristics of electromagnetic surface waves supported by the graphene loaded semi-infinite indium antimonide (InSb). The InSb has been taken as temperature sensitive material. The Drude model has been used for the modeling of InSb in the THz region while the modeling of the graphene has been done by random phase approximation-based Kubo’s formulism. To realize the graphene loaded indium antimonide interface, the impedance boundary conditions (IBCs) have been employed. The numerical analysis has been conducted to analyze the influence of temperature on the characteristics of electromagnetic surface waves i.e., dispersion curve, effective mode index (Neff), penetration depth (δ), propagation length (Lp), phase speed (Vp) and field profile, propagating along the graphene loaded InSb. In all the numerical results, the temperature variation has been considered from 200 to 350 K. It has been concluded that the graphene–InSb interface provides more temperature assisted tunability to the interfacial surface modes, commonly known as surface waves, as compared to monolayer graphene. Further, the graphene parameters can play a vital role in the dynamical tuning of electromagnetic surface waves in THz to IR frequency range. The numerically computed results have potential applications in designing of thermo-optical waveguides, temperature assisted communication devices, thermo-optical sensors and near field thermal imaging platforms.

Cl−1 doped Sb2Se3 polycrystals and films for optoelectric application

Antimony selenide (Sb2Se3) has been considered potential candidate material for the high-performance photovoltaic and photo-detecting application due to its significant optical absorption coefficient (∼105), suitable direct band gap (1.0–1.2 eV), low toxicity, and natural abundance in the crust. To overcome its low electrical conductivity and low-charge carrier density deficiencies, we propose a one-step melting quenching method strategy to prepare Cl− doped Sb2Se3 semiconductors. The photocurrent density of Sb2(Se0.9983Cl0.0017)3 crystal was greatly improved, with fast response speed and photo-response stability. The Sb2(Se0.9983Cl0.0017)3 films were then fabricated with rapid thermal evaporation. The beneficial effect of Sb2(Se0.9983Cl0.0017)3 films on photoelectric performance enhancement is confirmed by researching aspects such as heat treatment-dependent morphology, growth orientation and photoconductivity.

Chemical Defects and Energetic Disorder Impact the Energy‐Level Alignment of Functionalized Hexaazatriphenylene Thin Films

The energy‐level alignment (ELA) at interfaces ubiquitous in organic optoelectronic applications is decisive for the device performance. Due to the notoriously low density of free charge carriers in organic thin films, the ELA at organic––inorganic interfaces is determined by gap states and/or tailing states of the frontier molecular orbitals, that is, the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO). Informed by modeling defect energies on the density‐functional theory level, it is deduced from ultraviolet and X‐ray photoelectron spectroscopy data that chemical‐defect induced gap states lead to the substrate‐independent pinning of the Fermi level (EF) to the LUMO for 1,4,5,8,9,12‐hexaazatriphenylene‐2,3,6,7,10,11‐hexacarbonitrile thin films. For 5,6,11,12,17,18‐hexaazatrinaphthylene thin films, the ELA is instead governed by tailing states due to energetic disorder, which put the EF closer to midgap position. It is highlighted in the study that the susceptibility of conjugated organic material to forming chemical and structural defects is key for the ELA at interfaces and, therefore, must be considered in the synthesis of novel materials and their processing into functional structures.

Terahertz Electromodulation Spectroscopy for Characterizing Electronic Transport in Organic Semiconductor Thin Films

Terahertz (THz) spectroscopy is a well-established tool for measuring the high-frequency conductance of inorganic semiconductors. Its application to organic semiconductors, however, is challenging, because of the low carrier mobilities in organic materials, which rarely exceed 10cm2/Vs. Furthermore, low charge carrier densities in organic field-effect devices lead to sheet conductivities that are often far-below the detection limits of conventional THz techniques. In this contribution, we present the application of THz electromodulation spectroscopy for characterizing charge transport in organic semiconductors. Pulses of THz radiation are transmitted through organic field-effect devices and are time-resolved by electro-optic sampling. A differential transmission signal is obtained by modulating the gate voltage of the devices. This controls charge injection into the semiconductors, where the charge carriers reduce the THz transmission by their Drude response. Advantageous is that a nearly noise-free differential transmission can be obtained. Furthermore, electromodulation allows to sense specifically either injected electrons or holes. Because the method exclusively probes transport of mobile carriers, it provides access to fundamental transport properties, which are difficult to access with conventional characterization methods, such as conductance measurements of organic field-effect transistors. The outstanding property that a relative differential signal is measured allows to obtain charge carrier mobilities with high reliability. Mobilities as small as 1cm2/Vs can be probed, which makes THz electromodulation spectroscopy an attractive tool for studying charge transport in most technologically relevant organic semiconductors.

Massively parallel FDTD full-band Monte Carlo simulations of electromagnetic THz pulses in p-doped silicon at cryogenic temperatures

In semiconductor samples with a low density of free charge carriers the current response to an intense electromagnetic pulse is weak and can be treated as a perturbation in the finite-difference-time-domain calculation of the electromagnetic field. Up to first order the electric field that drives the particles does not depend on their density and the particles can be simulated independently. This opens the door for massively parallel Monte Carlo simulations that can be processed efficiently on a computer cluster and a large number of particles can be simulated resulting in a very low noise level. By this full-band Monte Carlo finite-difference-time-domain approach the generation of higher harmonics in p-doped silicon at cryogenic temperatures is investigated. It is found that the higher harmonics are mostly due to the warped valence bands, which are discretized on adaptive unstructured tetrahedral grids in the k-space to avoid numerical artifacts.

Efficient p‐Type Doping of Tin Halide Perovskite via Sequential Diffusion for Thermoelectrics

Metal halide perovskites (MHPs) hold great potential in thermoelectric (TE) applications, thanks to their regular and soft lattice in nature. However, the poor electrical conductivity caused by low charge carrier density (&lt;1014 cm−3 for lead‐based MHPs) strongly impedes its TE development. In this scenario, tin halide perovskites (THPs) emerge as promising TE candidates owing to their high background hole densities (&gt;1019 cm−3). However, further electrical doping remains challenging, originating from the limited capability of accommodating heterogeneous dopants and the heavy compensation in THPs. Herein, a novel diffusion‐mediated doping approach is demonstrated to prominently increase the p‐type doping level of THPs by a sequence of air exposure and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) surface treatments. In paradigm photovoltaic THP materials—CH(NH2)2SnI3 (namely FASnI3), the electrical conductivity is dramatically increased by 300× from 0.06 to 18 S cm−1 in thin films, leading to a remarkable enhancement of power factor by 25× up to 53 μW m−1 K−2. In contrast, only a slight variation of thermal conductivity is observed after F4TCNQ deposition, which is in accordance with the increase in electrical conductivity, indicating that the lattice structures of FASnI3 remain intact after doping. This study paves an illuminating way to ameliorate TE properties in halide perovskites.

Polythiophene hybrid film with zirconium–porphyrin metal–organic framework for improved charge carrier transport and NO2 gas sensing

Noxious gas sensors based on polymer thin films are attracting increasing attention because of their unique advantages. However, polymeric gas sensors usually exhibit low sensitivity and slow response rates for target gas molecules. These drawbacks are attributed to the poor charge transport in polymer active layers and to a structural limit resulting from the use of thick semiconductor films because of low charge carrier density. In this study, we examined the electrical and physical properties of Fe porous coordination network 222 (PCN-222) and explored its potential as an analyte interacting material inserted into a polymer active layer. The morphologies, molecular structures, and electrical characteristics of PCN-222/polythiophene blend films were systematically examined. The superior electrical characteristic of the PCN-222 enhanced the field-effect mobility in the polythiophene transistors. In addition, transistor-based sensors with the PCN-222/polythiophene blend films also showed significantly enhanced nitrogen dioxide sensing characteristics, including an enhanced responsivity, response rate. The improved sensing performance was due to the porous structures and large specific surface area of the PCN-222. These results provide useful guidelines for the application of transistor-type gas sensor based on metal–organic framework/polymer semiconductor blend films.

A New Microscopic Approach To Deal with the Temperature- and Applied Magnetic Field–Dependent Critical Current Densities of Superconductors

Based on the formalism of the Bethe-Salpeter equation, we present here a new approach to deal with the temperature (T)- and the applied magnetic field (H)-dependent critical current densities (jcs) of superconductors (SCs). Saliently, it employs (i) the premise that the chemical potential μ (T, H) subsumes most of the factors to which the widely varying empirical values of jc (T, H) of an SC are conventionally attributed, (ii) the basic equation that defines jc as a product of the electronic charge, the number density of superconducting electrons (ns) and their critical velocity (vc), (iii) a recently derived equation for ns (T, H), and (iv) the empirical values of jc (T, H)—which it is meant to explain—as inputs into one of its equations which, paradoxical though it may seem, is shown to have the virtue of shedding light on several parameters related to jc (T, H), such as μ, ns, vc, m*, nL, and λm, where m* is the effective mass of an electron, nL the number of occupied Landau levels, and λm the magnetic interaction parameter. Remarkably, the approach is found to lead to two branches of μ-dependent solutions with distinctly different values of the parameters corresponding to the same value of jc (T, H). This is reminiscent of the behavior of the chemical potential in SCs with a low density of charge carriers that was reported by Marel [Physica C 165, 35 (1990)] and described as anomalous. Finally, we show that our approach suggests the exciting possibility of fabricating an SC that has a bespoke value of jc at suitably chosen values of T and H. For the sake of concreteness, we employ here some of the available empirical data on the jc (T, H) values of Bi-2212.

Novel, low-cost device to simultaneously measure the electrical conductivity and the Hall coefficient from room temperature up to 600 °C

Abstract. A novel measurement device for simultaneous high temperature measurements of the electrical conductivity and the Hall coefficient has been developed. Simulations were used to design a suitable screen-printed planar platinum heating structure that generates temperatures of up to 600 ∘C by Joule heating. Simulations of the temperature distribution have been validated using thermal imaging. With the hardware setup of two permanent magnetic yoke systems with a magnetic flux density of ±760 mT, the electrical conductivity and the charge carrier densities of a silicon wafer and a gold film were measured, as examples of a typical semiconductor with low charge carrier densities but high mobility and a metal representing materials with very high charge carrier densities but moderate mobilities, respectively. Measurements were compared with data from the literature to validate the functionality of the novel, low-cost measurement device.

Effect of binary additives in mixed 2D/3D Sn-based perovskite solar cells

Additive doping improves the morphology and stability of tin (Sn)-based perovskite films under ambient conditions. However, the role of additives in the perovskite layer is not clearly understood, especially when two or more additives are incorporated. In this study, the effect of binary additives composed of 2-phenylethylammonium iodide (PEAI) and ethylenediammonium diiodide (EDAI2) on formamidinium tin iodide (FASnI3) perovskite solar cells (PSCs) is studied. PEAI plays an important role in reducing the dimensionality of the perovskite crystals from 3D to mixed 2D/3D, whereas EDAI2 resides on the grain periphery as a linker to improve the interconnection between neighboring grains. Overall, the optimum blending of the two additives, 8 and 1 mol % of PEAI and EDAI2, produces pinhole-free, compact perovskite film with superior crystallinity and preferential orientation, exhibiting very low background charge carrier density, reduced trap states and suppressed recombination losses. The reduced dimensionality and favored perovskite layer stacking lead to a better interlayer coupling that improves charge transfer at the interfaces. Consequently, the best PSC exhibits a power conversion efficiency (PCE) of 8.47% with an open-circuit voltage (VOC) of 0.634 V and the device stability is enhanced up to 5 days in ambient conditions without any additional encapsulation.

Exploring the Effect of Double Additives in Mixed 2D/3D Sn-Based Perovskite Solar Cells

Additive doping improves the tin (Sn)-based perovskite film morphology and ambient stability, however the function of additive in the perovskite film is limited, especially when two or more additives are used. In this study, the effect of double additives such as 2-phenylethylammonium iodide (PEAI) and ethylenediammonium diiodide (EDAI2) on the formamidinium tin iodide (FASnI3) films were explored. The perovskite film with only PEAI has showed several pinholes which is not suitable for ideal large area solar cell. While mixing of the two additives for an optimum concentration of PEAI (8 mol%) and EDAI2 (1 mol%) produced pinhole-free compact perovskite film with superior crystallinity, preferential orientation, low background charge carrier density, reduced trap states, and suppressed non-geminated recombination losses. The reduced dimensionality and favored perovskite layer stacking led to better interlayer coupling and increase charge transfer at interfaces. Consequently, a high-power conversion efficiency (PCE) of 8.47% and an open circuit voltage (V OC) of 0.634 V were achieved for the perovskite solar cell (PSC) with mixed additive, and the fabricated solar cells exhibits excellent stability for 5 days in ambient conditions.

Complete removal of phenolic contaminants from bismuth-modified TiO2 single-crystal photocatalysts

Exploring low-cost and highly active photocatalysts is very urgent to accomplish complete removal of phenolic contaminants and overcome the limitations of the existing photocatalysts. In this study, we designed and synthesized noble metal-free TiO2 photocatalysts by introducing bismuth nanoparticles as modifiers of a TiO2 single crystal (Bi-SCTiO2). The Bi-SCTiO2 can make full use of the synergistic effect of a small band overlap and low charge carrier density (Bi) with a high conductivity (single crystal), significantly boosting the separation and migration of the photogenerated charge pairs. Therefore, the Bi-SCTiO2 photocatalyst exhibits a significantly enhanced degradation rate (12 times faster) of 4-nitrophenol than a TiO2 single crystal under simulated sunlight irradiation. Notably, the complete removal of phenolic contaminants is achieved in various water matrices, which not only successfully overcomes the incomplete degradation in many reported photocatalytic systems, but also manifests a significant practical potential for sewage disposal. Therefore, this work presents a new insight in designing and constructing noble metal-free decorated semiconductor single-crystal photocatalysts with excellent activity and cyclability.

Structure, Morphology, and Photoelectric Performances of Te-Sb2Se3 Thin Film Prepared via Magnetron Sputtering.

Antimony selenide (Sb2Se3) has been widely investigated as a promising absorber material for photovoltaic devices. However, low open-circuit voltage (Voc) limits the power conversion efficiency (PCE) of Sb2Se3-based cells, largely due to the low-charge carrier density. Herein, high-quality n-type (Tellurium) Te-doped Sb2Se3 thin films were successfully prepared using a homemade target via magnetron sputtering. The Te atoms were expected to be inserted in the spacing of (Sb4Se6)n ribbons based on increased lattice parameters in this study. Moreover, the thin film was found to possess a narrow and direct band gap of approximately 1.27 eV, appropriate for harvesting the solar energy. It was found that the photoelectric performance is related to not only the quality of films but also the preferred growth orientation. The Te-Sb2Se3 film annealed at 325 °C showed a maximum photocurrent density of 1.91 mA/cm2 with a light intensity of 10.5 mW/cm2 at a bias of 1.4 V. The fast response and recovery speed confirms the great potential of these films as excellent photodetectors.

Electrical and Thermal Transport Properties of Layered Superconducting Ca10(Pt₄As8)((Fe0.86Pt0.14)2As₂)₅ Single Crystal.

We have synthesized single crystals of iron-based superconducting Ca10(Pt4As8)((Fe0.86Pt0.14)2As2)5 and performed extensive measurements on their transport properties. A remarkable difference in the behavior and a large anisotropy between in-plane and out-of-plane resistivity was observed. Disorder could explain the in-plane square-root temperature dependence resistivity, and interlayer incoherent scattering may contribute to the out-of-plane transport property. Along the ab plane, the estimated value of the coherence length is 15.5 Å. From measurements of the upper critical magnetic field Hc2 (T ≥ 20 K), we estimate Hc2(0) = 313 T. Thermal conductivity for Ca10(Pt4As8)((Fe0.86Pt0.14)2As2)5 is relatively small, which can be accounted for by the disorder in the crystal and the low-charge carrier density as verified by the Hall effect.

Quantum Oscillations of Magnetization in Antiferromagnetic Semimetals on a Triangular Lattice

The specific features of magnetization in antiferromagnetic semimetals with a low charge carrier density on a triangular lattice in a high magnetic field are studied. It is demonstrated that the well-known plateau in the magnetic field dependence of the magnetization manifesting itself in the subsystem of localized S = 1/2 spins is actually not strictly horizontal but has a slight positive slope. It is found that an abrupt change in the frequency of quantum oscillations of the magnetization in the itinerant subsystem should be observed at the magnetic field values corresponding to the edges of this plateau owing to the strong s–d(f) exchange coupling.

Topological surface Fermi arcs in the magnetic Weyl semimetal Co3Sn2S2

Very recently, the half-metallic compound Co$_3$Sn$_2$S$_2$ was predicted to be a magnetic WSM with Weyl points only 60 meV above the Fermi level ($E_F$). Owing to the low charge carrier density and large Berry curvature induced,Co$_3$Sn$_2$S$_2$ possesses both a large anomalous Hall conductivity (AHC) and a large anomalous Hall angle (AHA), which provide strong evidence for the existence of Weyl points in Co$_3$Sn$_2$S$_2$. In this work, we theoretically studied the surface topological feature of Co$_3$Sn$_2$S$_2$ and its counterpart Co$_3$Sn$_2$Se$_2$. By cleaving the sample at the weak Sn--S/Se bonds, one can achieve two different surfaces terminated with Sn and S/Se atoms, respectively. The resulting Fermi arc related states can range from the energy of the Weyl points to $E_F$--0.1 eV in the Sn-terminated surface. Therefore, it should be possible to observe the Fermi arcs in angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, in order to simulate quasiparticle interference (QPI) in scanning tunneling microscopy (STM) measurements, we also calculated the joint density of states (JDOS) for both terminals. This work would be helpful for a comprehensive understanding of the topological properties of these two magnetic WSMs and further ARPES and STM measurements.