Explore the vast range of titles available.

Thermoelectric materials convert heat into electricity and can provide solid-state cooling for spot-sized refrigeration. One important barrier for adopting these materials beyond niche applications is their low efficiency. He and Tritt review the mechanisms and strategies for improving thermoelectric efficiency. They discuss how to report material performance and highlight the most promising materials. With new materials and strategies for performance enhancement, thermoelectrics are poised to alter the renewable energy landscape. Science , this issue p. eaak9997 High-performance thermoelectric materials lie at the heart of thermoelectrics, the simplest technology applicable to direct thermal-to-electrical energy conversion. In its recent 60-year history, the field of thermoelectric materials research has stalled several times, but each time it was rejuvenated by new paradigms. This article reviews several potentially paradigm-changing mechanisms enabled by defects, size effects, critical phenomena, anharmonicity, and the spin degree of freedom. These mechanisms decouple the otherwise adversely interdependent physical quantities toward higher material performance. We also briefly discuss a number of promising materials, advanced material synthesis and preparation techniques, and new opportunities. The renewable energy landscape will be reshaped if the current trend in thermoelectric materials research is sustained into the foreseeable future.

The existence of electronic symmetry breaking in the underdoped cuprates and its disappearance with increased hole density p are now widely reported. However, the relation between this transition and the momentum-space ($\\overrightarrow{\\mathrm{k}}$-space) electronic structure underpinning the superconductivity has not yet been established. Here, we visualize the $\\overrightarrow{\\mathrm{Q}}=0$ = 0 (intra–unit-cell) and $\\overrightarrow{\\mathrm{Q}}\\ne 0$ (density-wave) broken-symmetry states, simultaneously with the coherent $\\overrightarrow{\\mathrm{k}}$-space topology, for Bi2Sr2CaCu2O8+δ samples spanning the phase diagram 0.06 ≤ p ≤ 0.23. We show that the electronic symmetry-breaking tendencies weaken with increasing p and disappear close to a critical doping pc = 0.19. Concomitantly, the coherent $\\overrightarrow{\\mathrm{k}}$-space topology undergoes an abrupt transition, from arcs to closed contours, at the same pc. These data reveal that the $\\overrightarrow{\\mathrm{k}}$-space topology transformation in cuprates is linked intimately with the disappearance of the electronic symmetry breaking at a concealed critical point.

Doping is required to modulate the electrical properties of semiconductors but introduces impurities that lead to Coulomb scattering, which hampers charge transport. Such scattering is a particular issue in two-dimensional semiconductors because charged impurities are in close proximity to the atomically thin channel. Here we report the remote modulation doping of a two-dimensional transistor that consists of a band-modulated tungsten diselenide/hexagonal boron nitride/molybdenum disulfide heterostructure. The underlying molybdenum disulfide channel is remotely doped via controlled charge transfer from dopants on the tungsten diselenide surface. The modulation-doped device exhibits two-dimensional-confined charge transport and the suppression of impurity scattering, shown by increasing mobility with decreasing temperature. Our molybdenum disulfide modulation-doped field-effect transistors exhibit a room-temperature mobility of 60 cm 2  V –1  s – 1 ; in comparison, transistors that have been directly doped exhibit a mobility of 35 cm 2  V –1  s – 1 . Carriers in a molybdenum disulfide transistor can be modulated without decreasing mobility by remote doping and charge transfer through a van der Waals heterostructure, which avoids dopant-induced impurity scattering in the channel.

Quantum anomalous Hall (QAH) systems are of great fundamental interest and potential application because of their dissipationless conduction without the need for an external magnetic field1–9. The QAH effect has been realized in magnetically doped topological insulator thin films10–14. However, full quantization requires extremely low temperature (T < 50 mK) in the earliest works, athough it has been significantly improved by modulation doping or co-doping of magnetic elements15,16. Improved ferromagnetism has been shown in these thin films, yet direct evidence of long-range ferromagnetic order is lacking. Herein, we present direct visualization of long-range ferromagnetic order in thin films of Cr and V co-doped (Bi,Sb)2Te3 using low-temperature magnetic force microscopy with in situ transport. The magnetization reversal process reveals typical ferromagnetic domain behaviour—that is, domain nucleation and possibly domain wall propagation—in contrast to much weaker magnetic signals observed in the endmembers, possibly due to superparamagnetic behaviour17–19. The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way towards high-temperature dissipationless conduction by exploring magnetic topological insulators.

Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal–insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La 1− x Sr x MnO 3 ( x = 0, 1/8, and 1/3) at the interface between disordered LaAlO 3 and crystalline SrTiO 3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov–de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density. The insertion of La 1− x Sr x MnO 3 in the interface between LaAlO 3 and SrTiO 3 enhances the electron mobility due to charge-transfer-induced modulation doping. Shubnikov–de Haas oscillations and fingerprints of the quantum Hall effect are observed.

With the rising popularity of organic thermoelectrics, the interest in doping strategies for organic semiconductors has increased strongly over the last decade. Here, we use aggregate data to discuss how far the approaches pursued till date have brought the community in terms of typical performance indicators for doped semiconductors in the context of thermoelectric applications. Surprisingly, despite the superlinear increase in the number of publications on the subject matter, the performance indicators show no clear upward trend in the same time range. In the second part, we discuss possible approaches to break this deadlock. A specifically promising approach, controlling the distribution of dopant atoms in the host material, is discussed in some quantitative detail by experiments and numerical simulations. We show that spontaneous modulation doping, that is, the spatial separation between static dopant ions and mobile charge carriers, leads to a dramatic conductivity increase at low dopant loading. Graphical abstract

Bismuth telluride‐based thermoelectric (TE) materials are historically recognized as the best p‐type (ZT = 1.8) TE materials at room temperature. However, the poor performance of n‐type (ZT≈1.0) counterparts seriously reduces the efficiency of the device. Such performance imbalance severely impedes its TE applications either in electrical generation or refrigeration. Here, a strategy to boost n‐type Bi2Te2.7Se0.3 crystals up to ZT = 1.42 near room temperature by a two‐stage process is reported, that is, step 1: stabilizing Seebeck coefficient by CuI doping; step 2: boosting power factor (PF) by synergistically optimizing phonon and carrier transport via thermal‐driven Cu intercalation in the van der Waals (vdW) gaps. Theoretical ab initio calculations disclose that these intercalated Cu atoms act as modulation doping and contribute conduction electrons of wavefunction spatially separated from the Cu atoms themselves, which simultaneously lead to large carrier concentration and high mobility. As a result, an ultra‐high PF ≈63.5 µW cm−1 K−2 at 300 K and a highest average ZT = 1.36 at 300–450 K are realized, which outperform all n‐type bismuth telluride materials ever reported. The work offers a new approach to improving n‐type layered TE materials. The intercalated Cu atoms in the Bi2Te2.7Se0.3 structure act as modulation doping and effectively improve carrier mobility while maintaining the carrier concentration similar to that in the uniformly doped sample. A record‐high ZT of 1.42 at 375 K is successfully realized in (CuI)0.002Bi2Te2.7Se0.3 + 0.2 % Cu.

A BiCuSeO system has been reported as a promising thermoelectric material and has attracted great attention in the thermoelectric community since 2010. Recently, several remarkable studies have been reported and the ZT of BiCuSeO was pushed to a higher level. It motivates us to systematically summarize the recent reports on the BiCuSeO system. In this short review, we start with several attempts to optimize thermoelectric properties of BiCuSeO. Then, we introduce several opinions to explore the origins of low thermal conductivity for BiCuSeO. Several approaches to enhance thermoelectric performance are also summarized, including modulation doping, introducing dual-vacancies, and dual-doping, etc. At last, we propose some possible strategies for enhancing thermoelectric performance of BiCuSeO in future research.

Thermoelectric materials, which directly convert thermal energy to electricity and vice versa, are considered a viable source of renewable energy. However, the enhancement of conversion efficiency in these materials is very challenging. Recently, multiphase thermoelectric materials have presented themselves as the most promising materials to achieve higher thermoelectric efficiencies than single-phase compounds. These materials provide higher degrees of freedom to design new compounds and adopt new approaches to enhance the electronic transport properties of thermoelectric materials. Here, we have summarised the current developments in multiphase thermoelectric materials, exploiting the beneficial effects of secondary phases, and reviewed the principal mechanisms explaining the enhanced conversion efficiency in these materials. This includes energy filtering, modulation doping, phonon scattering, and magnetic effects. This work assists researchers to design new high-performance thermoelectric materials by providing common concepts.

Doping silicon on the nanoscale by the intentional introduction of impurities into the intrinsic semiconductor suffers from effects such as dopant deactivation, random dopant fluctuations, out‐diffusion, and mobility degradation. This paper presents the first experimental proof that doping of silicon nanowires can also be achieved via the purposeful addition of aluminium‐induced acceptor states to the SiO2 shell around a silicon nanowire channel. It is shown that modulation doping lowers the overall resistance of silicon nanowires with nickel silicide Schottky contacts by up to six orders of magnitude. The effect is consistently observed for various channel geometries and systematically studied as a function of Al2O3 content during fabrication. The transfer length method is used to separate the effects on the channel conductivity from that on the barriers. A silicon resistivity is achieved as low as 0.04–0.06 Ω ·cm in the nominal undoped material. In addition, the specific contact resistivity is also strongly influenced by the modulation doping and reduced down to 3.5E‐7 Ω · cm2, which relates to lowering the effective Schottky barrier to 0.09 eV. This alternative doping method has the potential to overcome the issues associated with doping and contact formation on the nanoscale. Beyond conventional impurity doping: Modulation doping of Silicon nanowires via purposeful addition of Aluminium‐induced acceptor states to SiO2 dielectric shell surrounding the Silicon nanowire. Undoped Silicon channel with low resistivity and significantly reduced contact barrier height.