Carrier Tuning Research Articles

Band Flattening and Strain Field Assists an Excellent Thermoelectric Performance of n‐type Bi2Se3 for Room to Mid‐Temperature Application

AbstractBismuth selenide, (Bi2Se3) is a typical n‐type V‐VI chalcogenide‐based thermoelectric (TE) material with high lattice thermal conductivity, which limits its TE performance. Single element doping is generally adopted to enhance the TE performance, since it is not affect significantly. Herein, it is revealed that the impact of the co‐doping strategy boost the TE performance of Bi2Se3 through tuning the band structure and phonon transport properties. The synergy of isovalent Ga (gallium) and aliovalent Ge (germanium) co‐doping produces a noteworthy improvement in carrier tuning from −1.1 × 1018 to −3.56 × 1018 cm−3. Bi1.95Ga0.05Ge0.033Se3 sample reached the enhanced power factor of 1403 µWm−1K−2 obtained at 303 K. The reduced low lattice thermal conductivity of 0.24 Wm−1K−1 is occurred by the multi‐scale phonon scattering mechanisms through the dissemination of strain field raised from various defects. The combined strategy of flat band structure and low lattice thermal conductivity significantly optimized the thermoelectric zT of 0.7 is achieved at 513 K and the highest zTaverage of 0.68 is obtained. The maximum zT values leads to attain a theoretical efficiency of 5.9% in the temperature range between 333–573 K. This work shows a practical approach to improve the zT of n‐type Bi2Se3 via co‐doping.

Stabilization and high thermoelectric performance of high-entropy-type cubic AgBi(S, Se, Te)2

As thermoelectric generators can convert waste heat into electricity, they play an important role in energy harvesting. The metal chalcogenide AgBiSe2 is one of the high-performance thermoelectric materials with low lattice thermal conductivity (κlat), but it exhibits temperature-dependent crystal structural transitions from hexagonal to rhombohedral, and finally a cubic phase as the temperature rises. The high figure-of-merit ZT is obtained only for the high-temperature cubic phase. In this study, we utilized the high-entropy-alloy (HEA) concept for AgBiSe2 to stabilize the cubic phase throughout the entire temperature range with enhanced thermoelectric performance. We synthesized high-entropy-type AgBiSe2−2xSxTex bulk polycrystals and realized the stabilization of the cubic phase from room temperature to 800 K for x ≥ 0.6. The ultra-low κlat of 0.30 Wm−1K−1 and the high peak ZT ∼ 0.9 at around 750 K were realized for cubic AgBiSe2−2xSxTex without carrier tuning. In addition, the average ZT value of x = 0.6 and 0.7 for the temperature range of 360–750 K increased to 0.38 and 0.40, respectively, which are comparable to the highest previously reported values.

Enhanced thermoelectric performance of iso-valent Al substituted Bi2S3via carrier tuning and multiscale phonon scattering

Bismuth sulphide is one of the promising thermoelectric material due to its low thermal conductivity, less toxicity, and abundance in nature. This work analyzed an Al-substituted Bi2S3 compounds for mid-temperature thermoelectric applications prepared by hydrothermal route. The combination of stacking faults and point defects significantly improved the phonon scattering, resulting in decreased thermal conductivity in (x = 0.10) Al substituted sample with the value of 0.498 Wm−1K−1 at 623 K. Also, it simultaneously contributes to the enhancement in carrier concentration of −2.15 × 1019 cm−3, resulting the increased electrical conductivity of 10,142 Sm-1 at 563 K. Further, x = 0.025 Al substitution in Bi2-xAlxS3 leads to a high power factor of 338 Wm−1K-2 and a low phonon thermal conductivity of 0.578 μWm−1K−1, leading to a maximum zT of 0.30 at 623 K.

Silane or Siloxane‐Side‐Chain Engineering of Photovoltaic Materials for Organic Solar Cells†

Comprehensive SummaryWith the tactful material design, skillful device engineering, and in‐depth understanding of morphology optimization, organic solar cells (OSCs) have achieved considerable success. Therefore, OSCs have reached high power conversion efficiencies (PCEs) exceeding 19%. Especially, continuously emerging new materials have been considered as one of the key factors to improve the PCEs of OSCs. Among molecular design strategies, side‐chain engineering is an easy and commonly‐used means which can optimize the solubility, alter intermolecular stacking arrangement, fine‐tune the open circuit voltage (VOC), thus ultimately improve the performance. As hybrid side chains, silane and siloxane side chains have considerable effects, not only in increasing the carrier mobility and tuning the energy level, but also in affecting the crystallinity and molecular orientation. In this review, the latest developments in photovoltaic materials based on silane and siloxane side chains are presented to illustrate the structure‐property relationships. The review comprehensively includes silane‐side based polymer/small molecule donors; siloxane‐side based polymer/small molecule donors, and polymer/small molecule acceptors. Then the similarities and differences between these two side chains are demonstrated. Finally, the possible applications and future prospects of silane and siloxane side chains are presented.

Ultrafast THz spectroscopy of carbon nanotube-graphene composites

Mixed nanomaterial composites can combine the excellent properties of well-known low-dimensional nanomaterials. Here we highlight the potential of one-dimensional single-walled carbon nanotubes interfaced with two-dimensional graphene by exploring the composite’s ac conductivity and photoconductivity, and the influence of HAuCl4 doping. In the composite, the equilibrium terahertz conductivity from free carrier motion was boosted, while the localised plasmon peak shifted towards higher frequencies, which we attribute to shorter conductivity pathways in the composite. A negative terahertz photoconductivity was observed for all samples under 410 nm optical excitation and was reproduced by a simple model, where the Drude spectral weight and the momentum scattering rate were both lowered under photoexcitation. The composite had an enhanced modulation depth in comparison to reference carbon nanotube films, while retaining their characteristically fast (picosecond) response time. The results show that carbon nanotube-graphene composites offer new opportunities in devices by controlling charge carrier transport and tuning their optoelectronic properties.

Frontier nanoarchitectonics of graphitic carbon nitride based plasmonic photocatalysts and photoelectrocatalysts for energy, environment and organic reactions

Plasmonic photocatalysis for effective charge carrier separation and tuning of optical response is very much desired. In this review, the recent advances in plasmon mediated graphitic carbon nitride materials for photocatalysis and photoelectrocatalysis are summarized.

Carrier tuning of 2D electron gas in field-effect devices based on Al2O3/ZnO heterostructures.

Two-dimensional electron gas (2DEG) formed at oxide heterointerfaces via atomic layer deposition (ALD) has attracted considerable interest toward fascinating electron-related physics and electronic device applications. The employment of oxide-based 2DEG in a confined channel in field-effect transistors (FETs) holds great promise for advanced electronic devices due to its high mobility, spatial confinement, and tunable conductivity. In this work, a 2DEG FET based on the Al2O3/ZnO heterostructure is fabricated with an optimized channel carrier density and oxide thickness. The carrier transport in the bulk and the oxide interface dominantly governed by percolation conduction, optical phonon scattering, and grain boundary scattering is comparatively studied through oxygen annealing and thickness engineering. A tunable carrier density from 4 × 1011 cm-2 to 2 × 1014 cm-2 is achieved with a maximum Hall mobility of ∼62 cm2 V-1 s-1. The electron distribution associated with the annealing process of the ZnO underlayer and the interface reaction during Al2O3 deposition are found to have an impact on the electrical characteristics of the devices. The fabricated Al2O3/ZnO-based 2DEG FET exhibits an on/off ratio over 108, a subthreshold swing of 224 mV dec-1, and a field-effect mobility of 5.7 cm2 V-1 s-1 which can be promising for advanced oxide thin film-containing device and system applications.

Fermiology of a topological line-nodal compound CaSb2 and its implication to superconductivity: Angle-resolved photoemission study

We performed angle-resolved photoemission spectroscopy with micro-focused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc~1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the G point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Ts's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.

Carrier and microstructure tuning for improving the thermoelectric properties of Ag8SnSe6 via introducing SnBr2

The argyrodite compounds (A(12−n)m/m+Bn+X62− (Am+ = Li+, Cu+, and Ag+; Bn+ = Ga3+, Si4+, Ge4+, Sn4+, P5+, and As5+; and X2− = S2−, Se2−, or Te2−)) have attracted great attention as excellent thermoelectric (TE) materials due to their extremely low lattice thermal conductivity Among them, Ag8SnSe6-based TE materials have high potential for TE applications. However, the pristine Ag8SnSe6 materials have low carrier concentration (< 1017 cm−3), resulting in low power factors. In this study, a hydrothermal method was used to synthesize Ag8SnSe6 with high purity, and the introduction of SnBr2 into the pristine Ag8SnSe6 powders has been used to simultaneously increase the power factor and decrease the thermal conductivity (κ). On the one hand, a portion of the Br− ions acted as electrons to increase the carrier concentration, increasing the power factor to a value of ∼698 µW·m−1·K−2 at 736 K. On the other hand, some of the dislocations and nanoprecipitates (SnBr2) were generated, resulting in a decrease of κ1 (−0.13 W·m−1·K−1) at 578 K. As a result, the zT value reaches ∼1.42 at 735 K for the sample Ag8Sn1.03Se5.94Br0.06, nearly 30% enhancement in contrast with that of the pristine sample (−1.09). The strategy of synergistic manipulation of carrier concentration and microstructure by introducing halogen compounds could be applied to the argyrodite compounds to improve the TE properties.

Superior carrier tuning in ultrathin superconducting materials by electric-field gating

Superior carrier tuning in ultrathin superconducting materials by electric-field gating

A comprehensive review on Bi2Te3‐based thin films: Thermoelectrics and beyond

AbstractBi2Te3‐based materials are not only the most important and widely used room temperature thermoelectric (TE) materials but are also canonical examples of topological insulators in which the topological surface states are protected by the time‐reversal symmetry. High‐performance thin films based on Bi2Te3 have attracted worldwide attention during the past two decades due primarily to their outstanding TE performance as highly efficient TE coolers and as miniature and flexible TE power generators for a variety of electronic devices. Moreover, intriguing topological phenomena, such as the quantum anomalous Hall effect and topological superconductivity discovered in Bi2Te3‐based thin films and heterostructures, have shaped research directions in the field of condensed matter physics. In Bi2Te3‐based films and heterostructures, delicate control of the carrier transport, film composition, and microstructure are prerequisites for successful device operations as well as for experimental verification of exotic topological phenomena. This review summarizes the recent progress made in atomic defect engineering, carrier tuning, and band engineering down to a nanoscale regime and how it relates to the growth and fabrication of high‐quality Bi2Te3‐based films. The review also briefly discusses the physical insight into the exciting field of topological phenomena that were so dramatically realized in Bi2Te3‐ and Bi2Se3‐based structures. It is expected that Bi2Te3‐based thin films and heterostructures will play an ever more prominent role as flexible TE devices collecting and converting low‐level (body) heat into electricity for numerous electronic applications. It is also likely that such films will continue to be a remarkable platform for the realization of novel topological phenomena.

Three-dimensional bulk Fermi surfaces and Weyl crossings of Co2MnGa thin films underneath a protection layer

Angle-resolved photoelectron spectroscopy utilizing soft x-ray synchrotron radiation was applied to Heusler-type ${\mathrm{Co}}_{2}\mathrm{MnGa}$ thin films that have a 1-nm Al capping layer. The bulk Fermi surfaces and band structures varied along the out-of-plane momentum, stemming from the three-dimensional crystal structure, in the absence of any in situ surface treatment. In addition, there were characteristic intersecting bands (Weyl cones), with crossing points near the Fermi level, which were consistent with computed results. The Weyl cones are of bulk origin and are responsible for the high anomalous Nernst and the anomalous Hall coefficients. A close comparison of the experimental band structures in ${\mathrm{Co}}_{2}\mathrm{MnGe}$ and ${\mathrm{Co}}_{2}\mathrm{MnGa}$ indicated that the rigid band picture is valid in both alloys and that fine carrier tuning is possible by replacing Ga with Ge to improve the anomalous conductivity.

Structure-property relationship and nascent applications of thermoelectric PEDOT:PSS/carbon composites: A review

The success of the future concept of green society relies on accelerated progress in reversing human impacts on the ecosystem. Thus, the pursuit of meeting the increasing energy needs of humanity through alternative benign means has stimulated a rapid technological evolution, which requires harnessing the dissipated waste heat. Therefore, this review presents an overview of the thermoelectric (TE) properties and applications of poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/carbon composites. The carbon allotropes considered herein vary across different length scales. Therefore, this review discusses the TE behavior of the composites of PEDOT:PSS and zero-, one-, two-, and three-dimensional carbon materials. The fabrication techniques for promoting dual electronic and ionic transport in PEDOT and the ion-exchange effect for carrier tuning have also been discussed. In summary, we have discussed the TE applications of PEDOT:PSS/carbon composites and their future perspectives for TE devices.

Embedded Metal Oxide Plasmonics Using Local Plasma Oxidation of AZO for Planar Metasurfaces.

New methods for achieving high-quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon-near-zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al-doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi-layer devices. To demonstrate the performance of the carrier-modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible-range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross-talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide-based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology.

Tuning the carrier density in SrTiO3/LaTiO3/SrTiO3 quantum wells

We discuss methods of built-in carrier density control in SrTiO3/LaTiO3/SrTiO3 heterostructures that exhibit quasi-two-dimensional carrier confinement in an interfacial quantum well. Unlike the electronically similar LaAlO3/SrTiO3 heterostructures, where the polar discontinuity at the interface defines the accumulated carrier density, the LaTiO3 heterostructures offer two means of carrier density control—changing the La doping level and utilizing the effect of surface depletion through the change in the SrTiO3 capping layer thickness. Dynamic carrier tuning over a limited range is possible by the application of a back-gate bias, which primarily affects the depth distribution of carriers. We find that small changes in the pre-annealing conditions of a SrTiO3 substrate can have a dramatic effect on the low-temperature sheet resistance of the heterostructures.

High-performance p-type elemental Te thermoelectric materials enabled by the synergy of carrier tuning and phonon engineering

An outstanding figure-of-merit zT ≈ 1.06 at 600 K for p-type elemental Te thermoelectrics is realized by synergistically tuning their carrier and phonon transport behaviors via a multicomponent alloying strategy.

Significantly enhanced thermoelectric performance in SWCNT films via carrier tuning for high power generation

Carbon nanotubes (CNTs) are promising flexible thermoelectric materials in the field of large-scale low-grade thermal energy utilization. However, the relatively poor thermoelectric performance of CNTs near ambient temperature is a big issue for their application as flexible power generator. Here, we improved the thermoelectric properties of SWCNT films in a wide temperature range from room temperature (RT) to 200 °C via facile thermal annealing process. The effects of heat treatment at different temperatures and atmospheres on the thermoelectric properties of SWCNT films were studied. A maximum power factor (PF) of 113.7 μW/m∙K2 at RT is obtained with average PF 111.7 μW/m∙K2 in the RT to 200 °C range, which is more than threefold the value of as-prepared SWCNT film. Flexible and compact power generator based on annealed p-type SWCNT films and n-type polyethylenimine-doped SWCNT films with 8 couples of p-n legs was assembled. An outstanding output power of 1.16 μW with power density of 0.48 W/m2 at temperature difference 49.5 K was achieved. The device showed good air stability within one month and excellent mechanical flexibility. This study thus provides a feasible and low-cost strategy for high power energy generation.

Carrier Tuning in ZnSnN2 by Forming Amorphous and Microcrystalline Phases.

ZnSnN2 (ZTN), an earth-abundant element semiconductor, is a potential candidate for photovoltaic applications. However, the excessively high n-type carrier concentration caused by intrinsic defects hinders its progress. In this work, a series of Zn1± xSnN2 thin films are fabricated by RF-magnetron sputtering deposition. The zinc-rich composition is found to promote the crystallization of ZTN. As a main source of n-type carriers in the zinc-rich thin films, the interstitial Zn dominates the change of carrier concentration with an increase in the Zn/Sn ratio. Near the stoichiometric ratio, amorphous ZTN (a-ZTN) thin films are fabricated, and the n-type carrier concentration is suppressed to 1016 cm-3. With an increase in the Zn/Sn ratio from 0.9 to 1.3, the n-type carrier concentration can be tuned in the range 1016-1019 cm-3, accompanied by the phase-transition from a-ZTN to microcrystalline ZTN (μc-ZTN). For the a-ZTN thin film, the carrier mobility reaches up to 7 cm2 V-1 s-1, and the photoresponse covers almost the whole visible band. The above properties demonstrate that a-ZTN and μc-ZTN are potential candidates for photovoltaic applications.

Tuning behaviour of slotted vernier widely tunable lasers

A detailed thermo-optic model combining the 1D transfer matrix method and a 3D finite element method is developed and used to simulate a widely tunable vernier laser based on surface etched slots. The model is used to investigate the experimentally observed tuning patterns. At low injection currents, carrier tuning dominates, while at high currents, thermal tuning is the dominant mechanism. These lasers are very simple to fabricate and have a wide tuning over 50 nm, but SMSR and linewidth performance is not yet optimised. Simulations give an insight into the observed tuning efficiency and linewidth performance of the lasers, with high carrier densities in the grating regions being identified as a key area, which is presently limiting these parameters.

Cu-incorporation by melt-spinning in n-type Bi2Te2.7Se0.3 alloys for low-temperature power generation

Herein, we report the enhanced thermoelectric performance of Cu incorporated Bi2Te2.7Se0.3 prepared by melt spinning. The electronic transport properties were significantly improved by Cu incorporation due to the synergetic effect of carrier tuning, band modification, and electron-phonon transport control. The steady dimensionless thermoelectric figure of merit higher than 0.90 was achieved in the wide range of temperature range of 350–450 K for Cu0.008Bi2Te2.7Se0.3, which is ~30% enhancement in comparison with pristine Bi2Te2.7Se0.3. This result can lead to high thermoelectric power generation efficiency 4.2% at ΔT = 180 K.