Apoorva Chaturvedi

Preparation of Single-Layer MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 Nanosheets with High-Concentration Metallic 1T Phase.

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS(2x)Se2(1- x) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

Preparation of High‐Percentage 1T‐Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth-abundant electrocatalysts to potentially replace precious platinum-based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low-density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high-yield, large-scale production of water-dispersed, ultrasmall-sized, high-percentage 1T-phase, single-layer TMD nanodots with high-density active edge sites and clean surface, including MoS2 , WS2 , MoSe2 , Mo0.5 W0.5 S2 , and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of -140 mV at current density of 10 mA cm-2 , a Tafel slope of 40 mV dec-1 , and excellent long-term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high-density active edge sites, high-percentage metallic 1T phase, alloying effect and basal-plane Se-vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging.

Unveiling two-dimensional TiS2 as an insertion host for the construction of high energy Li-ion capacitors

Herein, we report, for the first time, the possibility of using TiS2 as an insertion host for the fabrication of high energy and high power Li-ion capacitors with commercial activated carbon (AC). The Li-insertion and PF6− adsorption/desorption properties are determined using Li in TiS2 and AC electrodes, respectively, which is very useful to balance the mass loading between the electrodes to realize maximum energy. Under balanced loading, the AC/TiS2 based LIC delivers a maximum energy density of ∼49 W h kg−1. Upon long-term cycling, the AC/TiS2 based LIC experiences a marginal fade in density; specifically ∼76% of its initial value is retained after 2000 cycles. Prior to this, a nearly 100% crystalline TiS2 insertion host was prepared via chemical vapor transport with high yield.

Fe–Ni–Mn Nanoparticles for Magnetic Cooling Near Room Temperature

We have studied the magnetocaloric effect in high-energy ball-milled (Fe₇₀Ni₃₀)₉₅Mn₅ alloy nanoparticles. The partial substitution of Fe and Ni by Mn decreases the Curie temperature (T_C) of the alloy to 338 K from 443 K. The change in entropy (Δ S_M) occurs over a broad range of temperatures, which results in high relative cooling power (RCP). RCP increases from 26 to 470 J · kg^-1 for a field change of 0.5 T and 5 T, respectively; these values are comparable to the benchmark magnetocaloric material, gadolinium. The RCP is proportional to field H to the power 1+1/δ, with a critical exponent δ of 4.34.

High energy Li-ion capacitors using two-dimensional TiSe0.6S1.4 as insertion host

A remarkable improvement in energy density was found using Se-substituted two-dimensional TiS2 (TiSe0.6S1.4) as the insertion matrix in a Li-ion capacitor (LIC) assembly with activated carbon (AC) as the counter electrode. Chemical vapor transport was successfully used to synthesize high quality TiSe0.6S1.4 with near-100% crystallinity and subsequently employed as a battery component in LICs. Prior to LIC fabrication, the Li-insertion properties were assessed in single-electrode configuration with metallic Li. Single-electrode performance is crucial for adjusting the mass loading of AC and TiSe0.6S1.4 to realize the maximum energy density. Accordingly, an AC/TiSe0.6S1.4-based LIC delivered an energy density of ∼50 W h kg−1 with a good cyclability of 5000 cycles. Few undesirable side reactions with the electrolyte counterpart were observed for TiSe0.6S1.4, as supported by the impedance measurements.

From Linear to Angular Isomers: Achieving Tunable Charge Transport in Single‐Crystal Indolocarbazoles Through Delicate Synergetic CH/NH⋅⋅⋅π Interactions

Weak intermolecular interaction in organic semiconducting molecular crystals plays an important role in molecular packing and electronic properties. Here, four five-ring-fused isomers were rationally designed and synthesized to investigate the isomeric influence of linear and angular shapes in affecting their molecular packing and resultant electronic properties. Single-crystal field-effect transistors showed mobility order of 5,7-ICZ (3.61 cm2  V-1  s-1 ) >5,11-ICZ (0.55 cm2  V-1  s-1 ) >11,12-ICZ (ca. 10-5  cm2  V-1  s-1 ) and 5,12-ICZ (ca. 10-6  cm2  V-1  s-1 ). Theoretical calculations based on density functional theory (DFT) and polaron transport model revealed that 5,7-ICZ can reach higher mobilities than the others thanks to relatively higher hole transfer integral that links to stronger intermolecular interaction due to the presence of multiple NH⋅⋅⋅π and CH⋅⋅⋅π(py) interactions with energy close to common NH⋅⋅⋅N hydrogen bonds, as well as overall lower hole-vibrational coupling owing to the absence of coupling of holes to low frequency modes due to better π conjugation.

Light-Tunable 1T-TaS2 Charge-Density-Wave Oscillators

External stimuli-controlled phase transitions are essential for fundamental physics and design of functional devices. Charge density wave (CDW) is a metastable collective electronic phase featured by the periodic lattice distortion. Much attention has been attracted to study the external control of CDW phases. Although much work has been done in the electric-field-induced CDW transition, the study of the role of Joule heating in the phase transition is insufficient. Here, using the Raman spectroscopy, the electric-field-driven phase transition is in situ observed in the ultrathin 1T-TaS2. By quantitative evaluation of the Joule heating effect in the electric-field-induced CDW transition, it is shown that Joule heating plays a secondary role in the nearly commensurate (NC) to incommensurate (IC) CDW transition, while it dominants the IC-NC CDW transition, providing a better understanding of the electric field-induced phase transition. More importantly, at room temperature, light illumination can modulate the CDW phase and thus tune the frequency of the ultrathin 1T-TaS2 oscillators. This light tunability of the CDW phase transition is promising for multifunctional device applications.

Magnetocaloric behavior of Fe-Ni-Mn nanoparticies

We have studied the magnetocaloric effect in high energy ball milled (Fe₇₀Ni₃₀)₉₅Mn₅ alloy nanoparticles. The partial substitution of Fe and Ni by Mn decreases the Curie temperature of the alloy to near room temperature (338 K). Relative cooling power (RCP) increases from 26 to 470 J-kg^-1 for a field change of 0.5 and 5 T, respectively, these values are comparable to the benchmark magnetocaloric material, gadolinium.