Mujeeb Ahmad

Efficient thermoelectric (TE) conversion of waste heat to usable energy is a holy grail promising to address major societal issues related to energy crisis and global heat management. For these to be economical, synthesis of a solid‐state material with a high figure‐of‐merit (ZT) values is the key, with characterization methods quantifying TE and heat transport properties being indispensable for guiding the development of such materials. In the present study, a large enhancement of the TE power factor in Sb₂Te₃/MoS₂ multilayer structures is reported. A new approach is used to simultaneously experimentally determine the values of in‐plane (k_xy) and out‐of‐pane (k_z) thermal conductivities for multilayer samples with characteristic layer thickness of few nanometres, essential for the quantification of the ZT, the key parameter for the TE material. Combining simultaneous enhancement in the value of in‐plane power factor (to (4.9 ± 0.4) × mWm⁻¹ K⁻²) and reduction of the in‐plane value of the thermal conductivity (to 0.7 ± 0.1 Wm⁻¹ K⁻¹) for Sb₂Te₃/MoS₂ multilayer sample led to high values of ZT of 2.08 ± 0.37 at room temperature. The present study, therefore, sets the foundation for a new methodology of exploiting the properties of 2D/3D interfaces for the development of novel fully viable thermoelectric materials.

The present study reports a strong thickness-dependence and anomalously large enhancement in the values of the Seebeck coefficient and electrical conductivity in Bi₂Te₃ films at ultralow thickness. An opposite sign of the Hall coefficient (negative) and Seebeck coefficient (positive) is observed in an ultrathin Bi₂Te₃ film (65 nm) as compared to the normally observed identical sign in the case of Bi₂Te₃ thin films (520 nm). A simultaneous enhancement in the values of electrical conductivity and the Seebeck coefficient results in a giant enhancement in the value of power factor from 1.86 mW/m K² to 18.0 mW/m K² at 416 K, with a reduction in thickness. X-ray photoelectron spectroscopy investigation reveals the absence of any significant change in stoichiometry and chemical bonding upon reduction of thickness. Magnetoresistance vs magnetic field data show a sharp dip at the lower magnetic field values, indicating a weak antilocalization effect in the case of the ultrathin film sample suggesting the role of strong spin–orbit coupling toward the carrier filtering effect resulting in enhancement of thermoelectric properties. Observation of the large Seebeck coefficient and the power factor at lower thickness values and its relationship with spin–orbit coupling is an important result, both for practical applications and for better understanding of the thermoelectric properties.

In the present study, 2D-3D MoS₂/Sb₂Te₃ (n-p) and MoS₂/Bi₂Te₃ (n-n) heterojunctions with varying MoS₂ thicknesses have been investigated using the Kelvin Probe Force Microscopy technique. Nanoscale maps of interface measurements based on the difference of surface potential (SP) maps in surface charge and back natural modes have been carried out. The 2D-3D heterojunctions with lower MoS₂ thickness show a large difference in SP values in the two modes, which is observed to increase with a decrease in the MoS₂ thickness. In comparison, samples with larger (bulk-like) MoS₂ thickness show negligible SP differences, indicating complete Fermi level alignment, as expected in a normal bulk junction. The difference in the SP value in two modes represents large surface charge accumulation in the 2D layer due to a relatively high value of the depletion width required for achieving equilibrium in comparison to the atomic scale thickness of 2D MoS₂. In limited earlier reports, the current-voltage behavior of metal—2D MoS₂ junctions is explained on the basis of the Fermi level pinning effect, which is a very generic explanation given for bulk p-n heterojunctions and may not be applicable in 2D materials. The present study shows that surface charge accumulation has a large influence on the I-V characteristic of 2D junctions, and this may be a key factor influencing the physics of the 2D interface and their potential applications.

Thin film mesocarbon microbeads (MCMB) composite electrodes consisting of hybrid nano-fillers of carbon black (CB) and carbon nanotubes (CNT) have been developed and characterized by evaluating their electrical and electrochemical properties. The composite electrodes have been fabricated with various weight fractions of CB and CNT. It is shown that by partial replacement of CB by CNT in electrodes, a synergistic improvement in electrical conductivity, initial capacity and stability in cycling behavior at low (0.25C) as well as at high rates (4C) is achievable. This is attributed to the combined effect of (i) hybrid of CB-CNT providing short- and long-range conducting pathways by effectively filling the gaps between MCMB-MCMB, CNT-CNT and MCMB-CNT clusters, and (ii) formation of a compact and stable solid-electrolyte film resulting in a compact three-dimensional conducting scaffold. The electrodes with CB-CNT hybrid additives exhibit best performance in terms of capacity retention (342.8 mAhg⁻¹ after 50 cycles with almost 98% capacity retention at 0.25C and 280.0 mAhg⁻¹ after 100 cycles at 4C) and rate capability. This kind of approach to create a hybrid of geometrically distinct shape, aspect ratio and dispersion characteristics conducting fillers manifests a promising strategy to develop improved performance composite anodes for future lithium-ion batteries.

In this study, the effect of incorporation of 2D nanoflakes on an n type and a p type thermoelectric matrixes, Bi₂Te₃ and Sb₂Te₃, respectively, has been studied. MoS₂ has been used to prepare nanocomposite bulk samples having n-n or n-p 2D interfaces. Kelvin probe force microscopy based measurements were used to characterize nanocomposite samples which revealed a difference in potentials barrier at the 2D interface for Bi₂Te₃:MoS₂ and Sb₂Te₃:MoS₂ samples, respectively. The electrical conductivity of Bi₂Te₃:MoS₂ was observed to be lower as compared to the pristine Bi₂Te₃ due to increased electron scattering at 2D interfaces, whereas in the case of Sb₂Te₃:MoS₂, the incorporation of MoS₂ led to the increase in the value of electrical conductivity due to higher carrier mobility. In Bi₂Te₃:MoS₂, a large decrease in thermal conductivity due to reduced electronic contribution is observed in contrast to no change in the case of the Sb₂Te₃:MoS₂ nanocomposite sample. The Seebeck coefficient is observed to increase in both the types of nanocomposite samples but owing to different mechanisms. The presence of potential barrier for electrons restricts the flow of majority carriers in the Bi₂Te₃:MoS₂ nanocomposite, whereas in the case of Sb₂Te₃:MoS₂ nanocomposite samples, the increased potential barrier helps in assisting the flow of holes, thereby increasing the mobility of carriers in the case of Sb₂Te₃:MoS₂.