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There is an urgent need for alternative compact technologies that can derive and store energy from the sun, especially the large amount of solar heat that is not effectively used for power generation. Here, we report a combination of solution- and neat-film-based molecular solar thermal (MOST) systems, where solar energy can be stored as chemical energy and released as heat, with microfabricated thermoelectric generators to produce electricity when solar radiation is not available. The photophysical properties of two MOST couples are characterized both in liquid with a catalytical cycling setup and in a phase-interconvertible neat film. Their suitable photophysical properties let us combine them individually with a microelectromechanical ultrathin thermoelectric chip to use the stored solar energy for electrical power generation. The generator can produce, as a proof of concept, a power output of up to 0.1 nW (power output per unit volume up to 1.3 W m−3). Our results demonstrate that such a molecular thermal power generation system has a high potential to store and transfer solar power into electricity and is thus potentially independent of geographical restrictions., This work was supported by the K. & A. Wallenberg Foundation, the Swedish Foundation for Strategic Research, the Swedish Research Council Formas, the Swedish Energy Agency, the European Research Council (ERC) under grant agreement CoG, PHOTHERM - 101002131, the Catalan Institute of Advanced Studies (ICREA), and the European Union's Horizon 2020 Framework Programme under grant agreement no. 951801. The MEMS-TEG chip manufacture and experimentation were supported by the National Natural Science Foundation of China (grant 51776126). The authors would like to thank the Center for Advanced Electronic Materials and Devices (AEMD) and Instrumental Analysis Center of Shanghai Jiao Tong University (SJTU) and the startup fund of Shanghai Jiao Tong University. We thank Dr. Sarah Lerch and Prof. Ben Greatrex for reading and commenting on the manuscript. We acknowledge Neuroncollective.com and Daniel Spacek for the graphical abstract., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Bi2Te3, a typical thermoelectric (TE) material as well as topological insulator, exhibits anisotropy characteristics due to its layered structure. (00l)-oriented Bi2Te3 reported with good TE proper...

This paper demonstrated a novel energy utilization that thermoelectric generation (TEG) device can achieve self-powering by radiative cooling (RC) continuously. An ultrathin TEG with a multilayer thermal emitter was fabricated to convert the heat from the environment into electricity directly by using radiative cooling. The TEG device was consisted of more than 46,000 P-N modules in series, and each two TE modules were connected by air bridge. Thermal emitter had 80.8% emissivity in the atmospheric window (8–13 µm). Besides, maximum temperature drop of 4 K was achieved. The output voltage of the TEG-RC reached up to 0.5 mV, and the TEG-RC exhibited a continuous average 0.18 mV output for 24 h. Extreme temperature gradients were surprisingly formed in cross section of 1.5 µm thick for micro/nano film materials.

Multilayer structure is one of the research focuses of thermoelectric (TE) material in recent years. In this work, n-type 800 nm Bi2Te3/(Pt, Au) multilayers are designed with p-type Sb2Te3 legs to fabricate ultrathin microelectromechanical systems (MEMS) TE devices. The power factor of the annealed Bi2Te3/Pt multilayer reaches 46.5 μW cm−1 K−2 at 303 K, which corresponds to more than a 350% enhancement when compared to pristine Bi2Te3. The annealed Bi2Te3/Au multilayers have a lower power factor than pristine Bi2Te3. The power of the device with Sb2Te3 and Bi2Te3/Pt multilayers measures 20.9 nW at 463 K and the calculated maximum output power reaches 10.5 nW, which is 39.5% higher than the device based on Sb2Te3 and Bi2Te3, and 96.7% higher than the Sb2Te3 and Bi2Te3/Au multilayers one. This work can provide an opportunity to improve TE properties by using multilayer structures and novel ultrathin MEMS TE devices in a wide variety of applications.

Thermoelectricity, green technology which can convert huge free thermal energy to electricity without time and geography limitations, is vital for bright future energy to alleviate global warming. In recent decades, numerous efforts have been made in the development of thermoelectric (TE) materials and their devices for various applications. Here, the latest progress of TE materials and devices is summarized. Multiple strategies for improving the performance of TE materials via regulating carriers and phonons are discussed. Besides the common heat source from industrial, natural, radioisotope, human and solar energy harvesting in various approaches, the attractive radiative cooling technology can provide a cold source for TE devices to generate electricity. Furthermore, TE devices are utilized to develop self-powered temperature/optical/chemical/biological sensors as well as temperature controllers and desalination. Especially, it is proposed that thermoelectric devices can be used to detect chemical endothermic reactions and the heat released by cell activity. In addition, it is expected that an uninterrupted power generation could be realized by integrating radiative cooling emitters and photothermal materials with thermoelectric devices. The future tendency is to further enhance material performance, optimize device design and develop adaptive circuit units while looking for exclusive broad application scenarios. There is plenty of room for thermoelectricity.

Thermoelectric multilayer thin films used in nanoscale energy conversion have been receiving increasing attention in both academic research and industrial applications. Thermal transport across multilayer interface plays a key role in improving thermoelectric conversion efficiency. In this study, the cross-plane thermal conductivities of nano-constructed Sb2Te3/(Cu, Ag, Au, Pt) thermoelectric multilayer thin films have been measured using time-domain thermoreflectance method. The interface morphology features of multilayer thin film samples were characterized by using scanning and transmission electron microscopes. The effects of interface microstructure on the cross-plane thermal conductivities of the multilayer thin films have been extensively examined and the thermal transfer mechanism has been explored. The results indicated that electron–phonon coupling occurred at the semiconductor/metal interface that strongly affected the cross-plane thermal conductivity. By appropriately optimizing the period thickness of the metal layer, the cross-plane thermal conductivity can be effectively reduced, thereby improving the thermoelectric conversion efficiency. This work presents both experimental and theoretical understanding of the thermal transport properties of Sb2Te3/metal multilayer thin film junctions with important implications for exploring a novel approach to improving the thermoelectric conversion efficiency.

Generating sufficient current density for powering electronic devices remains as one of the critical challenges of mechanical energy harvesting techniques based on piezo and triboelectricity, mainly due to the high impedance of the insulating material systems. Here we report on producing sustainable tunneling current using an unbiased, triboelectrically charged metal-insulator-semiconductor (MIS) point contact system, consisting of p-type silicon, silicon oxide and a metal tip. The native thin oxide (~ 1.6 nm) on the silicon surface provides a natural pathway for quantum mechanical tunneling of the triboelectrically generated electrons into the silicon substrate. Lateral back and forth sliding motion of the tip, irrespective of the direction of motion, generates a constant direct current (d.c.) with very high current density. The measured current shows an exponential decay with the thickness of oxide layer deposited with atomic layer deposition (ALD), confirming the quantum mechanical tunneling mechanism. It is proposed that the contact potential difference enhanced by triboelectric charging provides potential difference between metal point contact and the substrate. With single metallic micro probe sliding on a moderately doped p-type silicon, an open circuit voltage (Voc) of 300–400 mV and a short-circuit direct current (Isc) of 3–5 μA (a corresponding high current density, J, in the order of 1–10 A/m2) have been observed. It is predicted from conductive-atomic force microscopy (C-AFM) experiment that the theoretical J can be as high as 104 A/m2. This new concept has the potential as a green energy harvesting technique where a broad range of material candidates and device configurations could be used.

The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting 1–3 . A conventional triboelectric nanogenerator converts frictional energy into electricity by producing alternating current (a.c.) triboelectricity. However, this approach is limited by low current density and the need for rectification 2 . Here, we show that continuous direct-current (d.c.) with a maximum density of 106 A m−2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2). Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (105−106 V m−2) at the conductive nanoscale tip 4 . We hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current−voltage spectra analysis indicates that the rectifying Schottky barrier at the tip–sample interface plays a critical role in efficient d.c. energy harvesting. This concept is scalable when combined with microfabricated or contact surface modified electrodes, which makes it promising for efficient d.c. triboelectricity generation. A large triboelectric direct current can be generated via the nanoscale sliding friction of a conductive-AFM tip on a MoS2 thin film.

A 20 nm flat fire pattern was prepared by magnetron sputtering (MTS) deposition using Pt/Al2O3 nanocatalytic structure. Atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM) were used to characterize the microstructure of nanocatalystic film. Uniform temperature distributions across the nanocatalytic surface as well as rapid temperature response were observed using infrared thermal microscope under various methanol/air mixed flows. We observed as high as 15 °C over 20 nm Pt/ Al2O3 layer (vertical thermal gradient > 750 °C/μm or 750,000 °C/mm); when the vertical temperature difference is about 5 °C the horizontal thermal gradient is larger than 1.33 °C/μm (or 1330 °C/mm). In addition, the nanoscale heat transfer mechanisms were explored based on the experimental data and theoretical model. The results indicate that this kind of patterned nanoscale fire has the advantages of rapid temperature response and uniform temperature distribution, as well as larger temperature gradient. The nanoscale thin catalytic fire exhibits excellent properties of ultra-low catalyst loading and high conversion efficiency which are suitable for energy conversion at nanoscale. To develop a controllable and localized ultrathin two-dimensional fire may provide a possibility of making chip-scale power MEMS in the near future.

Nanoscale heat conduction with ultra-low thermal conductivity across metal-nonmetal Si/Au multilayer films has been investigated. Si/Au multilayer films with different Au thickness were prepared by magetron sputtering, of which the multilayer structures were confirmed by grazing incidence small angle X-ray scattering (GISAXS) and field emission scanning electron microscopy (FESEM). Moreover, the cross-plane thermal conductivities of the films deposition were investigated by a differential 3ω method at room temperature. It is possible that we can control thermal transport across multilayer films by constructing ultrathin Au layers. The reduced thermal conductivity (∼0.6 Wm−1K−1) of multilayer films with Au thickness of 1 nm is ∼50% of that with Au thickness above 8 nm and 42% of amorphous Si film (1.44 Wm−1K−1). The result is attributed to the low contribution of phonons to the overall thermal conductivity in ultrathin Au layer (below 8 nm), leading to a relatively high film thermal resistances compare to thicker Au layer due to strong electron-phonon coupling at metal-nonmetal interfaces. Meanwhile, experimental results show excellent agreement with two temperature model over 8 nm but not below 8 nm. It can be found that conventional thermal conducitvity models fail to explain the observed thermal conducitvity tendency as a function of intercalating metal layer thickness. Accordingly, a revised two temperature model (TTM) has been proposed, which shows well agreement with experimental results. The results provide us with more insight about the thermal transport mechanism of the heterogeneous multilayer system, and would give more instruction for next-generation thermoelectric material development.

Combustion is a chemical reaction that emits heat and light. Nanofire is a kind of flameless combustion that occurs on the micro–nano scale. Pt/Al2O3 film with a thickness of 20 nm can be prepared as a catalyst by micro–nano processing. When the methanol-air mixture gas passes through the surface of the catalyst, a chemical reaction begins and a significant temperature rise occurs in the catalyst region. Compared to macroscopic combustion, Nanofire has many special properties, such as large temperature gradients, uniform temperature distribution, and fast temperature response. The large temperature gradient is the most important property of Nanofire, which can reach 1330 K/mm. Combined with thermoelectric materials, it can realize the efficient conversion of chemical energy to electric energy. Nanoscale thickness offers the possibility of establishing thermal gradient. On the other hand, large thermal gradient has an effect on the transport properties of phonons and electrons in film materials. From these we can get the scale effects of heat. This article will provide an overview of the preparation, properties and applications of Nanofire, and then a comprehensive introduction to the thermal scale and thermal scale effects. Electronic supplementary material The online version of this article (10.1186/s40580-019-0175-4) contains supplementary material, which is available to authorized users.

In this work, bismuth telluride (Bi2Te3) thin films have been fabricated on Bi2Te3/ITO substrates by constant potential electrochemical deposition at room temperature. Bi2Te3 seed layers with different thicknesses (2 nm, 4 nm and 6 nm) were deposited onto ITO substrates using molecular beam epitaxy (MBE) method. The SEM images show that the morphology of Bi2Te3 thin films can be controlled not only by the deposition potential, but also the thickness of seed layer. Moreover, the morphologies of Bi2Te3 thin films with different thickness of seed layers tend to be similar and contain two-layer structure along the vertical direction after prolonged deposition time. Due to the two layers structure, Bi2Te3 thin films have shown different electrical conductivity performances. At room temperature, Bi2Te3 thin films with 4 nm-thick seed layer possess the maximum electrical conductivity value of 617.9 s cm-1.

The manipulation of heat transport across multilayer thin films with metal-semicounductor interfaces is of great interest for thermoelectric material optimization. Here we fabricated Sb 2 Te 3 /Au multilayer films with different Au thickness by magnetron sputtering. We demonstrated that the thermal conductivity of the system can be facilely manipulated by simply changing the Au layer thickness, where an optimal thickness (5 nm) value exists with the lowest thermal conductivity (∼0.44 Wm −1 K −1 , 44% of the pure Sb 2 Te 3 thin film thermal conductivity). It has been proved that the decreased thermal conductivity was mainly attributed to the strong electron–phonon coupling in a metal-nonmetal multilayered system with Au layer thickness larger than 5 nm, where the Two Temperature Model (TTM) predicts the experimental data perfectly. It was also proposed that the grain boundary effect may dominiate the phonon scattering when the Au layer is in a discountinuous form (

The role of localized surface plasmon resonance (LSPR) in UV-Vis light irradiated Au/TiO2 photocatalysis systems has been investigated, and it is demonstrated experimentally for the first time that both pros and cons of LSPR exist simultaneously for this photocatalytic reaction. We have proved that when operating under mixed UV and green light irradiation, the LSPR injected hot electrons (from the Au nanoparticles to TiO2 under green light irradiation) may surmount the Schottky barrier (SB) formed between the Au nanoparticles and TiO2, and flow back into the TiO2. As a result, these electrons may compensate for and even surpass those transferred from TiO2 to the Au nanoparticles, thus accelerating the recombination of UV excited electron-hole pairs in TiO2. This is the negative effect of LSPR. On the other hand, more hot electrons existing on the surface of the Au nanoparticles due to LSPR would favor the photocatalytic reaction, which accompanied by the negative effect dominates the overall photocatalytic performance. The presented results reveal the multi-faceted essence of LSPR in Au/TiO2 structures, and is instructive for the application of metal-semiconductor composites in photocatalysis. Moreover, it is confirmed that the extent to which the above pros and cons of LSPR dominate the overall photocatalytic reaction depends on the intensity ratio of visible to UV light.

Antimony telluride (Sb2Te3) is one of the best thermoelectric materials at room temperature. Low dimension has the potential to improve the material thermoelectric properties. Herein, we demonstrate experimental evidence of the positive influence of tellurium (Te) nanoparticles on thermopower of Sb2Te3 thin film. Sb2Te3 films with Te nanoparticles are prepared by alternate growth of Sb2Te3 layers and Te layers. When the single Te layer thickness is 1 nm, Te nanoparticle diameter is about 5 nm and areal density is approximately 160 µm−2, the Seebeck coefficient increases by ~25 %, thermoelectric power factor by ~50 %, and thermal conductivity decreases by ~26 % compared with Te layer free Sb2Te3 film without Te nanoparticles. The enhancement of thermoelectric thermopower could be attributed to carrier energy filtering effect induced by Te–Sb2Te3 nanocrystal boundary, and the thermal conductivity reduction may be ascribed to enhance phonon scattering by Te nanoparticles. The results show the Te nanoparticles have the ability to improve Sb2Te3 material thermoelectric properties.

The cross-plane thermal conductivities of four X/Si (X = SiGe, Au, Cr, Ti) periodic multilayer films deposited by magnetron sputtering with period thicknesses of about 20 nm were investigated by a differential 3ω method at room temperature. Structure characterization and sharp interfaces of multilayer films were confirmed by grazing incidence small angle X-ray scattering and field emission scanning electron microscopy. The measured thermal conductivities were compared with the classic heat conduction model and the two temperature model (TTM). The interfacial thermal resistance is the sum of the phonon interfacial thermal resistance and electron–phonon thermal resistance. The experimental results show great agreement with the theoretical results calculated by the TTM. It indicates that in metal–semiconductor periodic multilayered system for a relatively low coupling factor G (1016–1017 Wm−3K−1), the thermal conductivity of the samples is significantly affected by the electron–phonon coupling, providing a more insightful understanding of the thermal transport mechanism of the thin-film system.

Bi0.5Sb1.5Te3.0 thin films were deposited on silicon substrates at room temperature by co-sputtering and the effects of annealing temperatures on structure and thermoelectric properties were investigated. The composition, crystallinity, and microstructure of these thin films were characterized by energy dispersive X-ray spectroscopy, X-ray diffraction, and scanning electron microscopy. The crystalline quality of the thin films was enhanced with a rising annealing temperature. When annealed at 573 K, the layered structure of the Bi0.5Sb1.5Te3.0 thin films with a preferred orientation along the (00l) plane was formed. However, excessive high annealing temperature caused the thin films to become porous due to the separation of substantial Sb-rich precipitates. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. By optimizing the annealing temperature and time to 573 K for 6 h, the thermoelectric power factor was enhanced to 22.54 μW/(cm K2) at its maximum with a moderate electrical conductivity of 6.21 × 102 S/cm and a maximum Seebeck coefficient of 190.6 μV/K.

A new combination of techniques named evaporation-induced self-assembly–plasma etching (EISA–PE) method for mesoporous titania thin films preparation has been developed. The main advantage of this method is that the organic templates can be removed by plasma etching method thus the inorganic frameworks shrinkage or collapse induced by calcination can be avoided. Titania films prepared by this method have been characterized by Fourier transform infrared spectroscopy, thermogravimetric analyses, small and wide-angle X-ray diffraction, and field emission scanning electron microscope. The results show that titania thin film with well-resolved mesostructure can be created by EISA–PE method. The results also prove that plasma parameters have a great influence on the mesoporous property of the film and it is a good kind of catalyst support for methanol catalytic combustion.

Au@TiO2 core–shell hollow nanoparticles were prepared by a simple hydrothermal method without surfactants or templates. The core–shell structure materials were characterized by transmission electron microscopy, X-ray powder diffraction, scanning electron microscopy, and specific surface area of the test (BET). The catalytic activity was tested in a stainless reactor with a fixed bed and connected with a gas chromatograph. The results show that the microstructure, crystallography, and morphology were correlated with the hydrothermal reaction time and temperature, and the properties of the solvent. The crystallinity degree of TiO2 and the particle size increased with the reaction time and temperature. Particles with different morphologies can be obtained when using different solvents. The size of microsphere can be controlled easily by changing the amount of TiF4. This material exhibited the complete CO conversion temperature to be about 130 °C and no deactivation was observed after 1,000 min reaction.

Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core–shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core–shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core–shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core–shell nanoparticles in this paper. The current research trends of these core–shell catalysts are also highlighted.