Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3)

Exploring Material Properties and Device Output Performance of a Miniaturized Flexible Thermoelectric Generator Using Scalable Synthesis of Bi2Se3 Nanoflakes

Environmental heat-to-electric energy conversion presents a promising solution for powering sensors in wearable and portable devices. However, the availability of near-room temperature thermoelectric (TE) materials is highly limited, posing a significant challenge in this field. Bi₂Se₃, as a room-temperature TE material, has attracted much attention. Here, we demonstrate a large-scale synthesis of Bi₂Se₃ nanoflakes used for the microflexible TE generator. A high-performance micro-TE generator module, utilizing a flexible printed circuit, has been designed and fabricated through the process of screen printing. The TE generator configuration comprises five pairs of PN TE legs. The p-type TE leg utilizes commercially available Sb₂Te₃ powder, while the n-type TE leg employs Bi₂Se₃ nanoflakes synthesized in this study. For comparative purposes, we also incorporate commercially available Bi₂Se₃ powder as an alternative n-type TE leg. The optimal performance of the single-layer microflexible TE generator, employing Bi₂Se₃ nanoflakes as the active material, is achieved when operating at a temperature differential of 109.5 K, the open-circuit voltage (V_OC) is 0.11 V, the short circuit current (I_SC) is 0.34 mA, and the maximum output power (P_MAX) is 9.5 μW, much higher than the generator consisting of commercial Bi₂Se₃ powder, which is expected to provide an energy supply for flexible electronic devices.

High thermoelectric performance enabled by convergence of nested conduction bands in Pb7Bi4Se13 with low thermal conductivity

Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450-800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

Lone-Electron-Pair Micelles Strengthen Bond Anharmonicity in MnPb16Sb14S38 Complex Sulfosalt Leading to Ultralow Thermal Conductivity

Designing crystalline solids in which intrinsically and extremely low lattice thermal conductivity mainly arises from their unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ∼0.34 W m^-1 K^-1 at 300 K in the new complex sulfosalt MnPb₁₆Sb₁₄S₃₈. We attribute the ultralow lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a C_p value of 0.206 J g^-1 K^-1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g^-1 K^-1). Further analysis of the C_p data and sound velocity (ν = 1834 m s^-1) measurement yielded Debye temperature values of 161 and 187 K, respectively. The resulting Grüneisen parameter, γ = 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.

Enhancing the Thermoelectric Properties of Misfit Layered Sulfides (MS)1.2+q (NbS2) n (M = Gd and Dy) through Structural Evolution and Compositional Tuning

The misfit monolayered sulfides, (GdS)_1.20NbS₂, (DyS)_1.22NbS₂, (Gd_0.1Dy_0.9S)_1.21NbS₂, (Gd_0.2Dy_0.8S)_1.21NbS₂, and (Gd_0.5Dy_0.5S)_1.21NbS₂ and the misfit bilayered sulfide (GdS)_0.60NbS₂ were synthesized via sulfurization under flowing CS₂/H₂S gas and consolidated by pressure-assisted sintering. The thermoelectric properties of the monolayered and bilayered sulfides perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction were investigated over a temperature range of 300-873 K. The crystal grains in all the sintered samples were preferentially oriented perpendicular to the pressing direction, which resulted in highly anisotropic electrical and thermal transport properties. All the sintered samples exhibited degenerate n-type semiconductor-like behavior, leading to a large thermoelectric power factor. The misfit layered structure yielded low lattice thermal conductivity. The evolution of the monolayered structures into bilayered structures affected their thermoelectric properties. The thermoelectric figure of merit (ZT) of monolayered (GdS)_1.20NbS₂ was higher than that of bilayered (GdS)_0.60NbS₂ due to the larger power factor and lower lattice thermal conductivity of (GdS)_1.20NbS₂. The lattice thermal conductivity of the monolayered sulfide was lower in (Gd _x Dy_1-x S)_1.2+q NbS₂ solid solutions. The large power factor and low lattice thermal conductivity allowed a ZT value of 0.13 at 873 K in (Gd_0.5Dy_0.5S)_1.21NbS₂ perpendicular to the pressing direction.

Nanomosaic of Topological Dirac States on the Surface of Pb5Bi24Se41 Observed by Nano-ARPES

We have performed scanning angle-resolved photoemission spectroscopy with a nanometer-sized beam spot (nano-ARPES) on the cleaved surface of Pb₅Bi₂₄Se₄₁, which is a member of the (PbSe)₅(Bi₂Se₃)_{3 m} homologous series (PSBS) with m = 4 consisting of alternate stacking of the topologically trivial insulator PbSe bilayer and four quintuple layers (QLs) of the topological insulator Bi₂Se₃. This allows us to visualize a mosaic of topological Dirac states at a nanometer scale coming from the variable thickness of the Bi₂Se₃ nanoislands (1-3 QLs) that remain on top of the PbSe layer after cleaving the PSBS crystal, because the local band structure of topological origin changes drastically with the thickness of the Bi₂Se₃ nanoislands. A comparison of the local band structure with that in ultrathin Bi₂Se₃ films on Si(111) gives us further insights into the nature of the observed topological states. This result demonstrates that nano-ARPES is a very useful tool for characterizing topological heterostructures.

Inelastic neutron scattering study of the lattice dynamics of the homologous compounds (PbSe)5(Bi2Se3)3m (m = 1, 2 and 3)

We report on the inelastic response of the homologous compounds (PbSe)5(Bi2Se3)3m for m = 1, 2 and 3 followed in a broad temperature range (50-500 K) using high-resolution powder inelastic neutron scattering experiments. These results are complemented by low-temperature measurements of the specific heat (2-300 K). The evolution of the anisotropic crystal structure of these compounds with varying m, built from alternate Pb-Se and mBi-Se layers, only weakly influences the generalized phonon density of states. In all the three compounds, intense inelastic signals, likely mainly associated with the dynamics of the Pb atoms, are observed in the 4.5-6 meV low-energy range. The response of these low-energy modes to temperature variations indicates a conventional quasi-harmonic behavior over the whole temperature range investigated. The modes located above 8 meV show a minor temperature effect regardless of the value of m. The low-energy excess of vibrational modes manifests itself in the low-temperature specific heat as a pronounced peak in the Cp(T)/T3 data near 10 K. The lack of significant anharmonicity beyond that associated with the thermal expansion of the lattice suggests that the inherent disorder in the monoclinic unit cell and scattering at interlayer interfaces are the most important ingredients that limit the heat transport in this series of compounds.

Synthesis and transport properties of the Te-substituted homologous compounds Pb5Bi6Se14-xTex (0 ≤ x ≤ 1.0)

The crystal structure and transport properties (2-723 K) of the homologous compound Pb5Bi6Se14 with partial substitution of Te for Se are studied by means of powder X-ray diffraction, scanning electron microscopy, electrical resistivity, thermopower, thermal conductivity and Hall effect measurements. Polycrystalline samples of Pb5Bi6Se14-xTex (0 ≤ x ≤ 1.0) were prepared by a two-step synthesis method based on the pseudo-binary PbSe-Bi2Se3 phase diagram combined with Te substitution in the PbSe precursor. The successful insertion of Te into the crystal structure of Pb5Bi6Se14 was confirmed by powder X-ray diffraction and scanning electron microscopy. Transport property measurements indicate an increase in the heavily doped character of the transport with increasing the Te concentration. The extremely low lattice thermal conductivity values (0.3-0.4 W m-1 K-1 at 723 K) that approach the glassy limit at high temperatures are nearly independent of the chemical composition suggesting no influence on point-defect scattering mechanisms in the substituted compounds. Despite the inherent complexity of this system, the evolution of the electronic properties with x is well described by a simple single-parabolic band model. Because the increase in the power factor with increasing x is compensated by the concomitant increase in the electronic thermal conductivity, this substitution does not yield enhanced ZT values with respect to the pristine compound with a similar peak ZT value of 0.5 achieved at 723 K. Nevertheless, the simple synthetic method used in this study to insert a doping element opens new avenues for controlling the transport properties of the homologous series (PbSe)5(Bi2Se3)3m (m = 1, 2 and 3).