The surface-to-volume ratio: a key parameter in the thermoelectric transport of topological insulator Bi2Se3 nanowires

Topological insulators for thermoelectrics: A perspective from beneath the surface

Thermoelectric properties of topological insulators have traditionally been examined in the context of their metallic surface states. However, recent studies have begun to unveil intriguing thermoelectric effects emerging from the bulk electronic states of topological insulators, which have largely been overlooked in the past. Charge transport phenomena through the bulk are especially important under typical operating conditions of thermoelectric devices, necessitating a comprehensive review of both surface and bulk transport in topological insulators. Here, we review thermoelectric properties that are uniquely observed in topological insulators, placing special emphasis on unconventional phenomena emerging from bulk states. We demonstrate that unusual thermoelectric effects arising from bulk states, such as band inversion-driven warping, can be discerned in experiments through a simple analysis of the weighted mobility. We believe that there is still plenty to uncover within the bulk of topological insulators, yet our current understanding can already inspire new strategies for designing and discovering new materials for next-generation thermoelectrics.

Localization and Directionality of Surface Transport in Bi2Te3 Ordered 3D Nanonetworks

The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami-Larkin-Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.

Advancing Thermoelectric Materials: A Comprehensive Review Exploring the Significance of One-Dimensional Nano Structuring

Amidst the global challenges posed by pollution, escalating energy expenses, and the imminent threat of global warming, the pursuit of sustainable energy solutions has become increasingly imperative. Thermoelectricity, a promising form of green energy, can harness waste heat and directly convert it into electricity. This technology has captivated attention for centuries due to its environmentally friendly characteristics, mechanical stability, versatility in size and substrate, and absence of moving components. Its applications span diverse domains, encompassing heat recovery, cooling, sensing, and operating at low and high temperatures. However, developing thermoelectric materials with high-performance efficiency faces obstacles such as high cost, toxicity, and reliance on rare-earth elements. To address these challenges, this comprehensive review encompasses pivotal aspects of thermoelectricity, including its historical context, fundamental operating principles, cutting-edge materials, and innovative strategies. In particular, the potential of one-dimensional nanostructuring is explored as a promising avenue for advancing thermoelectric technology. The concept of one-dimensional nanostructuring is extensively examined, encompassing various configurations and their impact on the thermoelectric properties of materials. The profound influence of one-dimensional nanostructuring on thermoelectric parameters is also thoroughly discussed. The review also provides a comprehensive overview of large-scale synthesis methods for one-dimensional thermoelectric materials, delving into the measurement of thermoelectric properties specific to such materials. Finally, the review concludes by outlining prospects and identifying potential directions for further advancements in the field.

Experimental evidence of a size-dependent sign change of the Seebeck coefficient of Bi nanowire arrays

The electrical transport in bismuth nanowires is strongly influenced by both sample geometry and crystallinity. Compared to bulk bismuth, the electrical transport in nanowires is dominated by size effects and influenced by surface states, which gain increasing relevance with increasing surface-to-volume ratios, i.e. with decreasing wire diameter. Bismuth nanowires with tailored diameter and crystallinity constitute, therefore, excellent model systems, allowing to study the interplay of the different transport phenomena. Here, we present temperature-dependent Seebeck coefficient and relative electrical resistance measurements of parallel bismuth nanowire arrays with diameters between 40 and 400 nm synthesized by pulsed electroplating in polymer templates. Both electrical resistance and Seebeck coefficient exhibit a non-monotonic temperature dependence, with the sign of the Seebeck coefficient changing from negative to positive with decreasing temperature. The observed behavior is size-dependent and is attributed to limitations of the mean free path of the charge carriers within the nanowires. The observed size-dependent Seebeck coefficient and in particular the size-dependent sign change opens a promising avenue for single-material thermocouples with p- and n-legs made from nanowires with different diameters.

Interface-Dominated Topological Transport in Nanograined Bulk Bi2 Te3

3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi₂ Te₃ nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system. As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami-Larkin-Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 10³ cm² V^-1 s^-1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi₂ Te₃ features surface carrier properties that are of importance for technical applications.

High-Yield Growth and Tunable Morphology of Bi2Se3 Nanoribbons Synthesized on Thermally Dewetted Au

The yield and morphology (length, width, thickness) of stoichiometric Bi₂Se₃ nanoribbons grown by physical vapor deposition is studied as a function of the diameters and areal number density of the Au catalyst nanoparticles of mean diameters 8–150 nm formed by dewetting Au layers of thicknesses 1.5–16 nm. The highest yield of the Bi₂Se₃ nanoribbons is reached when synthesized on dewetted 3 nm thick Au layer (mean diameter of Au nanoparticles ~10 nm) and exceeds the nanoribbon yield obtained in catalyst-free synthesis by almost 50 times. The mean lengths and thicknesses of the Bi₂Se₃ nanoribbons are directly proportional to the mean diameters of Au catalyst nanoparticles. In contrast, the mean widths of the Bi₂Se₃ nanoribbons do not show a direct correlation with the Au nanoparticle size as they depend on the contribution ratio of two main growth mechanisms—catalyst-free and vapor–liquid–solid deposition. The Bi₂Se₃ nanoribbon growth mechanisms in relation to the Au catalyst nanoparticle size and areal number density are discussed. Determined charge transport characteristics confirm the high quality of the synthesized Bi₂Se₃ nanoribbons, which, together with the high yield and tunable morphology, makes these suitable for application in a variety of nanoscale devices.

A New Possibility for Fermentation Monitoring by Electrical Driven Sensing of Ultraviolet Light and Glucose

Industrial fermentation generates products through microbial growth associated with the consumption of substrates. The efficiency of industrial production of high commercial value microbial products such as ethanol from glucose (GLU) is dependent on bacterial contamination. Controlling the sugar conversion into products as well as the sterility of the fermentation process are objectives to be considered here by studying GLU and ultraviolet light (UV) sensors. In this work, we present two different approaches of SnO₂ nanowires grown by the Vapor–Liquid–Solid (VLS) method. In the GLU sensor, we use SnO₂ nanowires as active electrodes, while for the UV sensor, a nanowire film was built for detection. The results showed a wide range of GLU sensing and as well as a significant influence of UV in the electrical signal. The effect of a wide range of GLU concentrations on the responsiveness of the sensor through current–voltage based on SnO₂ nanowire films under different concentration conditions ranging was verified from 1 to 1000 mmol. UV sensors show a typical amperometric response of SnO₂ nanowires under the excitation of UV and GLU in ten cycles of 300 s with 1.0 V observing a stable and reliable amperometric response. GLU and UV sensors proved to have a promising potential for detection and to control the conversion of a substrate into a product by GLU control and decontamination by UV control in industrial fermentation systems.

High-Density Sb2 Te3 Nanopillars Arrays by Templated, Bottom-Up MOCVD Growth

Sb₂ Te₃ exhibits several technologically relevant properties, such as high thermoelectric efficiency, topological insulator character, and phase change memory behavior. Improved performances are observed and novel effects are predicted for this and other chalcogenide alloys when synthetized in the form of high-aspect-ratio nanostructures. The ability to grow chalcogenide nanowires and nanopillars (NPs) with high crystal quality in a controlled fashion, in terms of their size and position, can boost the realization of novel thermoelectric, spintronic, and memory devices. Here, it is shown that highly dense arrays of ultrascaled Sb₂ Te₃ NPs can be grown by metal organic chemical vapor deposition (MOCVD) on patterned substrates. In particular, crystalline Sb₂ Te₃ NPs with a diameter of 20 nm and a height of 200 nm are obtained in Au-functionalized, anodized aluminum oxide (AAO) templates with a pore density of ≈5 × 10¹⁰ cm^-2 . Also, MOCVD growth of Sb₂ Te₃ can be followed either by mechanical polishing and chemical etching to produce Sb₂ Te₃ NPs arrays with planar surfaces or by chemical dissolution of the AAO templates to obtain freestanding Sb₂ Te₃ NPs forests. The illustrated growth method can be further scaled to smaller pore sizes and employed for other MOCVD-grown chalcogenide alloys and patterned substrates.

Surface structure promoted high-yield growth and magnetotransport properties of Bi2Se3 nanoribbons

In the present work, a catalyst-free physical vapour deposition method is used to synthesize high yield of Bi2Se3 nanoribbons. By replacing standard glass or quartz substrates with aluminium covered with ultrathin porous anodized aluminium oxide (AAO), the number of synthesized nanoribbons per unit area can be increased by 20-100 times. The mechanisms of formation and yield of the nanoribbons synthesized on AAO substrates having different arrangement and size of pores are analysed and discussed. It is shown that the yield and average length of the nanoribbons can base tuned by adjustment of the synthesis parameters. Analysis of magnetotransport measurements for the individual Bi2Se3 nanoribbons transferred on a Si/SiO2 substrate show the presence of three different populations of charge carriers, originating from the Dirac surface states, bulk carriers and carriers from a trivial 2DEG from an accumulation layer at the Bi2Se3 nanoribbon interface with the substrate.

Thermoelectrics of Nanowires

The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to enhance it, including earlier theoretical and experimental work on quantum confinement effects and semimetal-to-semiconductor transition, surface engineering and complex heterostructures to enhance the carrier mobility and power factor, and the recent emergence of topological insulator NWs. For phonon transport, we broadly categorize the work on thermal conductivity of NWs into five different effects: classic size effect, acoustic softening, surface roughness, complex NW morphology, and dimensional crossover. Finally, we discuss the integration of NWs for device applications for thermoelectric power generation and cooling. We conclude our review with some outlooks for future research.

Growth and quantum transport properties of vertical Bi2Se3 nanoplate films on Si substrates

Controlling the growth direction (planar versus vertical) and surface-to-bulk ratio can lead to lots of unique properties for two-dimensional layered materials. We report a simple method to fabricate continuous films of vertical Bi₂Se₃ nanoplates on Si substrate and investigate the quantum transport properties of such films. In contrast to (001) oriented planar Bi₂Se₃ nanoplate film, vertical Bi₂Se₃ nanoplate films are enclosed by (015) facets, which possess high surface-to-bulk ratio that can enhance the quantum transport property of topological surface states. And by controlling the compactness of vertical Bi₂Se₃ nanoplates, we realized an effective tuning of the weak antilocalization effect from topological surface states in Bi₂Se₃ films. Our work paves a way for exploring the unique transport properties of this unconventional structure topological insulator film.

Transmission Electron Microscope Nanosculpting of Topological Insulator Bismuth Selenide

We present a process for sculpting Bi₂Se₃ nanoflakes into application-relevant geometries using a high-resolution transmission electron microscope. This process takes several minutes to sculpt small areas and can be used to cut the Bi₂Se₃ into wires and rings, to thin areas of the Bi₂Se₃, and to drill circular holes and lines. We determined that this method allows for sub 10 nm features and results in clean edges along the drilled regions. Using in situ high-resolution imaging, selected area diffraction, and atomic force microscopy, we found that this lithography process preserves the crystal structure of Bi₂Se₃. TEM sculpting is more precise and potentially results in cleaner edges than does ion-beam modification; therefore, the promise of this method for thermoelectric and topological devices calls for further study into the transport properties of such structures.

Materials analysis and focused ion beam nanofabrication of topological insulator Bi2Se3

Focused ion beam milling allows manipulation of the shape and size of nanostructures to create geometries potentially useful for opto-electronics, thermoelectrics, and quantum computing. We focus on using the ion beam to control the thickness of Bi₂Se₃ and to create nanowires from larger structures. Changes in the material structure of Bi₂Se₃ nanomaterials that have been milled using a focused ion beam are presented. In order to characterize the effects of ion beam processing on the samples, we use a variety of techniques including analytical transmission electron microscopy and atomic force microscopy. The results show that while part of the material remains intact after shaping, amorphous regions form where the beam has been used to thin the sample. For wires created by thinning the material down to the substrate, the sidewalls of the wires appear intact based on diffraction images from samples cut at an angle, but thin crystalline regions remain at the wire edges. Even with the resulting defects and limitations when thinning, focused ion beam milling can be used to fabricate custom geometries of Bi₂Se₃ nanostructures.

Simultaneous observation of surface- and edge-states of a 2D topological insulator through scanning tunneling spectroscopy and differential conductance imaging

Gapless edge-states with a Dirac point below the Fermi energy and band-edges at the interior observed in 2D topological insulators.