Optimization of the Consolidation Parameters for Enhanced Thermoelectric Properties of Gr-Bi2Te2.55Se0.45 Nanocomposites.

Hot pressing represents a promising consolidation technique for ball-milled bismuth telluride alloys, yet deep investigations are needed to understand its effect on the thermoelectric properties. This paper studies the effect of hot-pressing parameters (temperature and pressure) on the thermoelectric properties of the n-type Gr-Bi2Te2.55Se0.45 nanocomposite. Ultra-high pressure, up to 1.5 GPa, is considered for the first time for consolidating Bi2(Te,Se)3 alloys. Results from this study show that increasing the temperature leads to changes in chemical composition and causes noticeable grain growth. On the contrary, increasing pressure mainly causes improvements in densification. Overall, increments in these two parameters improve the ZT values, with the temperature parameter having a higher influence. The highest ZT of 0.69 at 160 °C was obtained for the sample hot-pressed at 350 °C and 1 GPa for 5 min, which is indeed an excellent and competitive value when compared with results reported for this n-type Bi2Te2.55Se0.45 composition.

Democratizing the Assessment of Thermal Robustness of Metal-Organic Frameworks.

With the pressing need of having reliable materials for carbon dioxide capture, metal-organic frameworks (MOFs) have shown promising performance due to their flexible sign and tunable functionality by applying reticular chemistry principles. One of the main characteristics of practical MOFs is to design thermally robust candidates for sustainable functionality. Here, we introduce a comprehensive methodology for examining the thermal stability of MOFs by combining theoretical calculations and affordable experimental methods to fully describe their performance under thermal variations. We chose the prototypical MOF, HKUST-1, to assess the methodology by performing density functional theory and classical molecular dynamics simulations and validating with experiments such as in situ powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. HKUST-1 shows thermal robustness until a temperature of 240 °C at different atmospheric gases with a reversible breathing trend with temperature. This methodology is affordable as it uses minimal experimental testing and can be applied to any MOF materials to explore its suitability for practical applications.

Electronic transport descriptors for the rapid screening of thermoelectric materials.

The discovery of novel materials for thermoelectric energy conversion has potential to be accelerated by data-driven screening combined with high-throughput calculations. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. Using a data-driven screening, we selected 12 potential candidates in the trigonal ABX2 family, followed by charge transport property simulations from first principles. The results suggest that carrier scattering processes in these materials are dominated by ionised impurities and polar optical phonons, contrary to the oft-assumed acoustic-phonon-dominated scattering. Using these data, we further derive ground-state transport descriptors for the carrier mobility and the thermoelectric powerfactor. In addition to low carrier mass, high dielectric constant was found to be an important factor towards high carrier mobility. A quadratic correlation between dielectric constant and transport performance was established and further validated with literature. Looking ahead, dielectric constant can potentially be exploited as an independent criterion towards improved thermoelectric performance. Combined with calculations of thermal conductivity including Peierls and inter-branch coherent contributions, we conclude that the trigonal ABX2 family has potential as high performance thermoelectrics in the intermediate temperature range for low grade waste heat harvesting.

Direct measurement of the thermoelectric properties of electrochemically deposited Bi2Te3 thin films.

The best known thermoelectric material for near room temperature heat-to-electricity conversion is bismuth telluride. Amongst the possible fabrication techniques, electrodeposition has attracted attention due to its simplicity and low cost. However, the measurement of the thermoelectric properties of electrodeposited films is challenging because of the conducting seed layer underneath the film. Here, we develop a method to directly measure the thermoelectric properties of electrodeposited bismuth telluride thin films, grown on indium tin oxide. Using this technique, the temperature dependent thermoelectric properties (Seebeck coefficient and electrical conductivity) of electrodeposited thin films have been measured down to 100 K. A parallel resistor model is employed to discern the signal of the film from the signal of the seed layer and the data are carefully analysed and contextualized with literature. Our analysis demonstrates that the thermoelectric properties of electrodeposited films can be accurately evaluated without inflicting any damage to the films.

Correlating charge and thermoelectric transport to paracrystallinity in conducting polymers.

The conceptual understanding of charge transport in conducting polymers is still ambiguous due to a wide range of paracrystallinity (disorder). Here, we advance this understanding by presenting the relationship between transport, electronic density of states and scattering parameter in conducting polymers. We show that the tail of the density of states possesses a Gaussian form confirmed by two-dimensional tight-binding model supported by Density Functional Theory and Molecular Dynamics simulations. Furthermore, by using the Boltzmann Transport Equation, we find that transport can be understood by the scattering parameter and the effective density of states. Our model aligns well with the experimental transport properties of a variety of conducting polymers; the scattering parameter affects electrical conductivity, carrier mobility, and Seebeck coefficient, while the effective density of states only affects the electrical conductivity. We hope our results advance the fundamental understanding of charge transport in conducting polymers to further enhance their performance in electronic applications.

Employing a Bifunctional Molybdate Precursor To Grow the Highly Crystalline MoS2 for High-Performance Field-Effect Transistors.

Growth of the large-sized and high-quality MoS2 single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS2 contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS2 is yet far below the expected theoretical values, implying that the MoS2 crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS2 FETs with room-temperature electron mobility up to ∼90 cm2 V-1 s-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS2 crystals (edge length up to ∼260 µm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS2 single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS2 crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS2 sheds light on its potential in low-voltage and self-powered electronics.

Effects Of Structural Phase Transition On Thermoelectric Performance in Lithium-Intercalated Molybdenum Disulfide (Li xMoS2).

Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient ( S). Here, we study the thermoelectric properties of mixed-phase Li xMoS2 as a function of its phase composition tuned by in situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T' phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase Li xMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.

Integrating carbon nanotubes into silicon by means of vertical carbon nanotube field-effect transistors.

Single-walled carbon nanotubes have been integrated into silicon for use in vertical carbon nanotube field-effect transistors (CNTFETs). A unique feature of these devices is that a silicon substrate and a metal contact are used as the source and drain for the vertical transistors, respectively. These CNTFETs show very different characteristics from those fabricated with two metal contacts. Surprisingly, the transfer characteristics of the vertical CNTFETs can be either ambipolar or unipolar (p-type or n-type) depending on the sign of the drain voltage. Furthermore, the p-type/n-type character of the devices is defined by the doping type of the silicon substrate used in the fabrication process. A semiclassical model is used to simulate the performance of these CNTFETs by taking the conductance change of the Si contact under the gate voltage into consideration. The calculation results are consistent with the experimental observations.