Diameter dependent thermoelectric properties of individual SnTe nanowires

Pentagonal nanowires from topological crystalline insulators: a platform for intrinsic core-shell nanowires and higher-order topology

We report on the experimental realization of Pb1-xSnx Te pentagonal nanowires (NWs) with [110] orientation using molecular beam epitaxy techniques. Using first-principles calculations, we investigate the structural stability of NWs of SnTe and PbTe in three different structural phases: cubic, pentagonal with [001] orientation and pentagonal with [110] orientation. Within a semiclassical approach, we show that the interplay between ionic and covalent bonds favors the formation of pentagonal NWs. Additionally, we find that this pentagonal structure is more likely to occur in tellurides than in selenides. The disclination and twin boundary cause the electronic states originating from the NW core region to generate a conducting band connecting the valence and conduction bands, creating a symmetry-enforced metallic phase. The metallic core band has opposite slopes in the cases of Sn and Te twin boundaries, while the bands from the shell are insulating. We finally study the electronic and topological properties of pentagonal NWs unveiling their potential as a new platform for higher-order topology and fractional charge. These pentagonal NWs represent a unique case of intrinsic core-shell one-dimensional nanostructures with distinct structural, electronic and topological properties between the core and the shell region.

Density Functional Theory Study of the Spin-Orbit Insulating Phase in SnTe Cubic Nanowires: Implications for Topological Electronics

We investigate the electronic, structural, and topological properties of the SnTe and PbTe cubic nanowires using ab initio calculations. Using standard and linear-scale density functional theory, we go from the ultrathin limit up to the nanowire thicknesses observed experimentally. Finite-size effects in the ultrathin limit produce an electric quadrupole and associated structural distortions; these distortions increase the band gap, but they get reduced with the size of the nanowires and become less and less relevant. Ultrathin SnTe cubic nanowires are trivial band gap insulators; we demonstrate that by increasing the thickness, there is an electronic transition to a spin-orbit insulating phase due to trivial surface states in the regime of thin nanowires. These trivial surface states with a spin-orbit gap of a few meV appear at the same k-point of the topological surface states. Going to the limit of thick nanowires, we should observe the transition to the topological crystalline insulator phase with the presence of two massive surface Dirac fermions hybridized with the persistent trivial surface states. Therefore, we have the copresence of massive Dirac surface states and trivial surface states close to the Fermi level in the same region of the k-space. According to our estimation, the cubic SnTe nanowires are trivial insulators below the critical thickness tc1 = 10 nm, and they become spin-orbit insulators between tc1 = 10 nm and tc2 = 17 nm, while they transit to the topological phase above the critical thickness of tc2 = 17 nm. These critical thickness values are in the range of typical experimental thicknesses, making the thickness a relevant parameter for the synthesis of topological cubic nanowires. Pb1-xSnxTe nanowires would have both these critical thicknesses tc1 and tc2 at larger values depending on the doping concentration. We discuss the limitations of density functional theory in the context of topological nanowires and the consequences of our results on topological electronics.

Two-Dimensional Weak Antilocalization Signatures Due to Quantum Coherent Transport in Nanocrystalline SnTe

Nanostructured topological crystalline insulators (TCIs) in the presence of exotic surface states with spin momentum locking reported in individual nanostructures are predicted to hold a great promise for spintronics and quantum computing applications. However, practical application demands a strategy with large-scale production and integration for device applications. In this work, we demonstrate through prominent signatures of weak antilocalization (WAL), arising predominantly from destructive quantum interference on robust surface states, that a correlated TCI phase is possible in the nanobulk assembly of carefully nanostructured quasi-two-dimensional SnTe (edge-to-edge length ∼ 382 nm) synthesized by a simple, rapid, and scalable microwave-assisted solvothermal method. Hikami-Larkin-Nagaoka analysis (T^-0.71), as well as the temperature dependence of resistivity, illustrates an interplay of both conductions from 2D channels and 3D EEI effects as the precursor for the observed WAL at low temperatures (2-6 K). Interestingly, the enhanced thermoelectric power of the sample of ∼45 μV/K, with a p-type carrier concentration of ∼10¹⁸/cm³ at 300 K, makes this SnTe nanocrystalline assembly more attractive as a multifunctional material for large-scale technological applications.

Development of topological insulator and topological crystalline insulator nanostructures

Topological insulators (TIs), a class of quantum materials with time reversal symmetry protected gapless Dirac-surface states, have attracted intensive research interests due to their exotic electronic properties. Topological crystalline insulators (TCIs), whose gapless surface states are protected by the crystal symmetry, have recently been proposed and experimentally verified as a new class of TIs. With high surface-to-volume ratio, nanoscale TI and TCI materials such as nanowires and nanoribbons can have significantly enhanced contribution from surface states in carrier transport and are thus ideally suited for the fundamental studies of topologically protected surface state transport and nanodevice fabrication. This article will review the synthesis and transport device measurements of TIs and TCIs nanostructures.

Multimodality imaging of naturally active melanin nanoparticles targeting somatostatin receptor subtype 2 in human small-cell lung cancer

Somatostatin receptor subtype 2 (SSTR2) is highly expressed in pulmonary neuroendocrine tumors, which account for approximately 25% of all lung cancers including small-cell lung cancer (SCLC). It is possible to establish SCLC-specific imaging agents for multimodal imaging to obtain tumor integrity information. Herein, we constructed novel multifunctional organic melanin nanoparticles (MNPs) as a carrier and surface-loaded somatostatin analog octreotide to produce a human small-cell lung cancer-targeted nanoprobe OCT-PEG-MNPs. MNPs have an excellent photoacoustic imaging (PAI) function and can be directly chelated with the magnetic resonance contrast agent Mn2+, and N-bromo succinimide (NBS) can be used as an oxidant to label the nanoparticles with the long half-life radionuclide 124I by an electrophilic substitution reaction. Therefore, (124I, Mn) OCT-PEG-MNPs can not only be used for PAI but also be used for positron emission tomography (PET) and magnetic resonance imaging (MRI). The NCI-H69 SCLC tumor xenograft model with high SSTR2 expression was constructed to evaluate the multimodal imaging ability of (124I, Mn) OCT-PEG-MNPs. This nanoprobe showed good imaging abilities in PAI, MRI and PET. The PA images showed that the photoacoustic signal in the NCI-H69 tumor site gradually increased with time, and the NCI-H69 xenograft showed a clear increase in the T1-weighted signal intensity after injection of Mn-OCT-PEG-MNPs at 24 h compared to that in the prescan. MicroPET and biodistribution studies showed that the uptake of NCI-H69 tumors (8.03 ± 0.37% ID g-1) was significantly higher than that in the control A549 model (3.35 ± 0.54% ID g-1) after injection of (124I, Mn) OCT-PEG-MNPs at 24 h. The (124I, Mn) OCT-PEG-MNPs were successfully applied to multimodal imaging in a small-cell lung cancer model with high SSTR2 expression. This nanoprobe may be considered for clinical trials since it combines the numerous advantages of organic nanoparticles.

Microscopic study of thermoelectric In-doped SnTe

SnTe is a p-type thermoelectric material that is isostructural with PbTe, for which it is a potential environmentally friendly replacement. By doping the SnTe lattice with In, the thermal conductivity of SnTe can be significantly reduced and the thermoelectric conversion efficiency improved. A large number of precipitates were present in the In-doped SnTe samples; based on atomic-resolution high-angle annular dark-field images and electron energy loss spectra, these precipitates were identified as the zinc-blende phase of In₂Te₃. Through geometry phase analysis, a new phonon scattering mechanism is discussed.

Realizing zT of 2.3 in Ge1-x-y Sbx Iny Te via Reducing the Phase-Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral-to-cubic phase transition is a promising lead-free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon-phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1-x Sb_x Te with optimized carrier concentration, a record-high figure-of-merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase-transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high-density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high-performance Pb-free thermoelectric materials.

Surface oxidation and thermoelectric properties of indium-doped tin telluride nanowires

The recent discovery of excellent thermoelectric properties and topological surface states in SnTe-based compounds has attracted extensive attention in various research areas. Indium doped SnTe is of particular interest because, depending on the doping level, it can either generate resonant states in the bulk valence band leading to enhanced thermoelectric properties, or induce superconductivity that coexists with topological states. Here we report on the vapor deposition of In-doped SnTe nanowires and the study of their surface oxidation and thermoelectric properties. The nanowire growth is assisted by Au catalysts, and their morphologies vary as a function of substrate position and temperature. Transmission electron microscopy characterization reveals the formation of an amorphous surface in single crystalline nanowires. X-ray photoelectron spectroscopy studies suggest that the nanowire surface is composed of In₂O₃, SnO₂, Te and TeO₂ which can be readily removed by argon ion sputtering. Exposure of the cleaned nanowires to atmosphere leads to rapid oxidation of the surface within only one minute. Characterization of electrical conductivity σ, thermopower S, and thermal conductivity κ was performed on the same In-doped nanowire which shows suppressed σ and κ but enhanced S yielding an improved thermoelectric figure of merit ZT compared to the undoped SnTe.

Fabrication of Bi2Te3-x Se x nanowires with tunable chemical compositions and enhanced thermoelectric properties

Uniform Bi₂Te_3-x Se _x nanowires (NWs) with tunable components are synthesized by a modified solution method free of any template, and inter-diffusion mechanism is proposed for the growth and transformation of ternary nanowires. Spark plasma sintering is adopted to fabricate the pellets of Bi₂Te_3-x Se _x NWs and thermoelectric transport properties are measured. As compared to Bi₂Te₃ pellets, Se doping results in lowered electrical conductivity because of the reduced carrier concentration, both the Seebeck coefficient and the power factor are enhanced substantially. The Bi₂Te_2.7Se_0.3 pellet exhibits the highest power factor at room temperature as a result of optimized carrier concentration (4.37 × 10¹⁹ cm^-3) and mobility (60.22 cm² V^-1 s^-1). As compared to Bi₂Te₃, the thermal conductivity of Bi₂Te_3-x Se _x is lowered owing to the enhanced phonon scattering by dopants and grain boundaries. As a result, the ZT value at 300 K is substantially improved from 0.045 of Bi₂Te₃ to 0.42 of Bi₂Te_2.7Se_0.3. It is suggested that Se doping is an effective way to enhance the thermoelectric performance of Bi₂Te₃ based materials.

Benchmark characterization of the thermoelectric properties of individual single-crystalline CdS nanowires by a H-type sensor

A precision H-type sensor method has been developed to measure the thermoelectric performance of individual single-crystalline CdS nanowires for the first time.

Ultrahigh power factor and enhanced thermoelectric performance of individual Te/TiS2 nanocables

Here, we present the successful fabrication of Te/TiS2 heterostructure nanocables with enhanced thermoelectric (TE) performance by a two-step route (a facile solvothermal approach for Te nanowires and then the Te nanowires are used as templates for the controllable growth of the Te/TiS2 nanocables), which is scalable for practical nanodevice applications. The heterostructure nanocables of different sizes can be prepared by varying the synthetic composition. Measurements of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) are carried out on the same nanowires over a temperature range of 2-350 K. The heterostructure nanocables show an ultrahigh power factor (S(2) σ) with a maximum value of 0.58 Wm(-1) K(-2), which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient. The figure of merit (ZT) can reach 1.91 at room temperature from a single nanocable with a diameter of 60 nm, which is thought to contribute to the formation of the hetero-phase core/shell structure. These results are expected to open up new application possibilities in nanoscale TE devices based on individual Te/TiS2 heterostructure nanocables.

Template-based syntheses for shape controlled nanostructures

A variety of nanostructured materials are produced through template-based synthesis methods, including zero-dimensional, one-dimensional, and two-dimensional structures. These span different forms such as nanoparticles, nanowires, nanotubes, nanoflakes, and nanosheets. Many physical characteristics of these materials such as the shape and size can be finely controlled through template selection and as a result, their properties as well. Reviewed here are several examples of these nanomaterials, with emphasis specifically on the templates and synthesis routes used to produce the final nanostructures. In the first section, the templates have been discussed while in the second section, their corresponding synthesis methods have been briefly reviewed, and lastly in the third section, applications of the materials themselves are highlighted. Some examples of the templates frequently encountered are organic structure directing agents, surfactants, polymers, carbon frameworks, colloidal sol-gels, inorganic frameworks, and nanoporous membranes. Synthesis methods that adopt these templates include emulsion-based routes and template-filling approaches, such as self-assembly, electrodeposition, electroless deposition, vapor deposition, and other methods including layer-by-layer and lithography. Template-based synthesized nanomaterials are frequently encountered in select fields such as solar energy, thermoelectric materials, catalysis, biomedical applications, and magnetowetting of surfaces.

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

We systematically investigated the role of topological surface states on thermoelectric transport by varying the surface-to-volume ratio (s/v) of Bi2Se3 nanowires. The thermoelectric coefficients of Bi2Se3 nanowires were significantly influenced by the topological surface states with increasing the s/v. The Seebeck coefficient of Bi2Se3 nanowires decreased with increasing the s/v, while the electrical conductivity increased with increasing the s/v. This implies that the influence of metallic surface states become dominant in thermoelectric transport in thin nanowires, and the s/v is a key parameter which determines the total thermoelectric properties. Our measurements were corroborated by using a two-channel Boltzmann transport model.

Measuring methods for thermoelectric properties of one-dimensional nanostructural materials

Measuring methods for the Seebeck coefficient and thermal conductivity of 1D nanostructural materials have been reviewed and structures, principles, merits and shortcomings, as well as examples of each method are discussed in detail.

Low-Dimensional Topological Crystalline Insulators

Topological crystalline insulators (TCIs) are recently discovered topological phase with robust surface states residing on high-symmetry crystal surfaces. Different from conventional topological insulators (TIs), protection of surface states on TCIs comes from point-group symmetry instead of time-reversal symmetry in TIs. The distinct properties of TCIs make them promising candidates for the use in novel spintronics, low-dissipation quantum computation, tunable pressure sensor, mid-infrared detector, and thermoelectric conversion. However, similar to the situation in TIs, the surface states are always suppressed by bulk carriers, impeding the exploitation of topology-induced quantum phenomenon. One effective way to solve this problem is to grow low-dimensional TCIs which possess large surface-to-volume ratio, and thus profoundly increase the carrier contribution from topological surface states. Indeed, through persistent effort, researchers have obtained unique quantum transport phenomenon, originating from topological surface states, based on controllable growth of low-dimensional TCIs. This article gives a comprehensive review on the recent progress of controllable synthesis and topological surface transport of low-dimensional TCIs. The possible future direction about low-dimensional TCIs is also briefly discussed at the end of this paper.

Horizontal growth of MoS2 nanowires by chemical vapour deposition

We describe a single step route for the synthesis of MoS₂ wires using a chemical vapour deposition (CVD) method.