Ever since the discovery of two-dimensional (2D) material graphene, there has been huge interest in the exploration of low-dimensional materials that can be exfoliated from their three-dimensional counterpart with enriched properties due to quantum confinement. Two members of the Sn-S family, Pnma-SnS and P3Im1-SnS 2 that possess a layered structure with 2D nanosheets stacked via weak van der Waals (vdW) interactions, have widely been studied in this regard. The other member, Pnma-Sn 2 S 3 , comprising one-dimensional (1D) nanochains bound via vdW interactions, has never been investigated in the view of exfoliated 1D analogue. In this work, we therefore comprehensively studied 1D-Sn 2 X 3 (X = S and Se) nanochains and demonstrated them to be stable and exfoliable from their bulk counterpart. Further, it is also shown that the exfoliated 1D nanochains can easily be identified from their bulk counterpart using Raman, infrared, and X-ray spectroscopies. Our calculations predict a direct band gap of 2.35 eV (1.67 eV) for 1D-Sn 2 S 3 (1D-Sn 2 Se 3 ) nanochains under the Heyd, Scuseria, and Ernzerhof functional, with a broad absorption region lying between 2 and 8 eV, lower reflection, high charge-carrier mobility with ambipolar characteristics, as well as a larger value of the Seebeck coefficient and a smaller value of the thermal conductivity, resulting in a better thermoelectric figure of merit. These interesting electronic, optical, transport, and thermoelectric properties make 1D-Sn 2 X 3 nanochains a potential candidate for the application in future optoelectronic and thermoelectronic devices, in fact, better than three-dimensional (3D)-Sn 2 X 3 for few of the applications. Moreover, 3D-Sn 2 Se 3 is also investigated in detail in this work, which to the best of our knowledge has not been done before. © 2019 American Chemical Society.

Priya Sudhir Johari

Physics

(SoNS) School of Natural Sciences

Sen R.

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Shiv Nadar University

ACS Applied Materials and Interfaces

An annealing temperature-driven appearance of a sharp dual-band edge in the ultraviolet-visible absorption spectrum of electrochemically anodized TiO2 nanotubes (TNTs) and the corresponding impact on the photodegradation of organic dyes is reported based on the evolution of an anatase (A)/rutile (R) phase junction. Systematic X-ray diffraction followed by a micro-Raman analysis illustrates the impact of annealing between 500 and 900 °C in promoting the anatase-to-rutile phase transformation with a higher degree of crystallinity through an evolution of an intermediate mixed phase. X-ray photoelectron spectroscopy (XPS) further reveals a clear transition from the sub-stoichiometric to stoichiometric TNTs with increasing annealing temperature. Moreover, the valence band (VB) XPS along with the ultraviolet photoelectron spectroscopy (UPS) analysis suggests the possible band alignment at the A/R interface, whereas the X-ray absorption spectroscopy (XAS) at the Ti L- and O K-edges confirms a systematic change from an anatase to a rutile phase owing to the modification in local symmetry. To validate our experimental results, especially the formation of a dual-band edge at the A/R phase junction in TNTs, theoretical calculations using density functional theory (DFT) were also performed. Here, a one-to-one analysis of our theoretical and experimental results not only shows a good agreement to each other but also illustrates the microscopic origin behind the formation of a dual-band edge in mixed phase TNTs as well as the observed variation in Raman and XPS spectra with annealing temperature. Finally, an improved photocatalytic degradation of methylene blue aqueous solution is demonstrated in the presence of a mixed phase and explained in the framework of an efficient transfer of photogenerated electrons from the conduction band (CB) of R to the CB of A and holes from the VB of A to the VB of R across the phase junction in tube walls. © 2021 American Chemical Society.

Journal of Physical Chemistry C

Unveiling Temperature-Mediated Dual-Band Edge in TiO2Nanotubes with Enhanced Photocatalytic Effect

Despite having the fascinating physical, electronic, and optical properties of two-dimensional (2D) crystals of MoS2, h-BN, and InSe, none of them solely meet all the desired criteria required for high efficiency optoelectronic devices, such as a suitable band gap with very high carrier mobility, a moderate excitonic lifetime, a desirable bending modulus, environmental stability against air and water, etc. Herein, we demonstrate that these fundamental limitations can easily be overcome by building a van der Waals heterostructure (vdW-HS) of monolayer InSe either with single-layer MoS2 or h-BN. Our first-principles calculations suggest that compared to individual monolayers, the examined InSe/MoS2 and InSe/h-BN vdW-HSs are not only thermodynamically and mechanically more robust but also possess improved electronic and optical properties, which can be particularly useful for solar harvesting devices. Importantly, through a systematic study, we elucidate that the band gap and its nature can largely be modulated (∼0.1-1.6 eV, indirect direct, type I type II) for both the examined heterobilayers by applying mechanical strain and transverse electric field. Even more interestingly, we further show that with such bilayer heterostructures it is possible to get electron and hole mobility almost in the same order of magnitude (103-104cm2V-1s-1), either naturally or by applying small biaxial strain. © 2020 American Physical Society.

Physical Review B

Modulation of electronic and transport properties of bilayer heterostructures: InSe/MoS2 and InSe/h -BN as the prototype

Low-energy linear absorption spectrum of a series of 48 donor-acceptor-donor (D-A-D) scheme based thiophone-benzo(bis-)X-diazole molecules with X = O, S, Se, or Te are calculated using time dependent density functional theory in order to propose strategical design of molecules that can efficiently absorb light in the infrared and visible region of the solar spectrum. Our study establishes that optical properties of the D-A-D based organic molecules significantly depend on the donor-to-acceptor (D/A) ratio and the strength of the acceptor moiety. Thus, by choice of a suitable D/A ratio and type of the acceptor moiety, the linear absorption spectrum can be largely shifted, in general, while the optical gap can be engineered over a wide energy range of ∼0.2-2.3 eV, in particular. It is also noticed that the increase in acceptor units (i.e., when D/A ≤ 1) leads to increase in steric hindrance in between them. This, in turn, disrupts the effective conjugation length and increases the optical gap. However, this effect is found to dominate strongly in the bis-configurations of the molecules as compared to the nonbis compositions. In order to reduce this effect for rational designing of effective D-A-D type chromophores with less steric hindrance, the role of π-conjugated ethylene (-CH=CH-) linkage/spacer between the A-A units is explored further. Here, it is found that introduction of such linkage substantially decreases the steric hindrance and, thereby, the optical gap as well. Besides this, our study also highlights and explains the impact of the acceptor moiety in improving the absorption capabilities of these molecules in the low-energy region. © 2017 American Chemical Society.

Journal of Physical Chemistry A

Strategical Designing of Donor-Acceptor-Donor Based Organic Molecules for Tuning Their Linear Optical Properties

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li-Sn batteries. It can, however, be overcome with the help of void space engineering by using a LixSn phase as the prelithiated anode, where an optimal value for x is desired. Currently, Li4.25Sn is known as the most lithiated Li-Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (x &gt; 4.25) Li-Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li-Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li-Sn system, in general, but also in improving our understanding of the reaction mechanism in Li-Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li-Sn compounds, our study reveals the existence of five unreported stoichiometries (Li8Sn3, Li3Sn1, Li4Sn1, Li5Sn1, and Li7Sn1) and their associated crystal structures at ambient and high pressure. Although Li8Sn3 has been identified as one of the most stable Li-Sn compound in the entire pressure range (1 atm-20 GPa) with R3m symmetry, the Li-rich compounds like Li3Sn1-P2/m, Li4Sn1-R3m, Li5Sn1-C2/m, and Li7Sn1-C2/m are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li5Sn1 and Li7Sn1 opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li17Sn4. © 2017 American Chemical Society.

Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li-Sn Compounds at Ambient and Moderately High Pressure