Poster Abstracts

Combining density functional theory and experimental investigations we elucidate a unique tetragonal ground state of MnTi₂O₄ [1]. On lowering the temperature, cubic (Fd-3m) MnTi₂O₄ enters into a chiral polar tetragonal (P41) structure accompanied with an unusual higher order tetramer orbital ordering among Ti³⁺ (3d₁) orbitals along equivalent <111> directions involving all three t_2g orbitals. Magnetic superexchange interactions among Mn and Ti spins, and minimization of strain energy associated with co-operative Jahn-Teller distortions stabilize the unique orbital ordered ground state which further gives rise to lattice chirality through short and long Ti-Ti bond-length modulations.

Reference

[1] A. Rahaman, M. Chakraborty, T. Paramanik, R. K. Maurya, S. Mahana, R. Bindu,

D. Topwal, P. Mahadevan, and D. Choudhury , Phys. Rev. B 100, 115162 (2019).

The carrier-density distribution near a conducting interface and related band structure is an important topic of condensed matter physics. We propose a scheme combining Photoluminescence (PL) spectroscopy, time-correlated photon counting (TCSPC) with electrical measurements to reveal the distribution of the carriers, the shape of the quantum well, energy subbands, and Fermi surfaces of the conducting interface of LaVO₃ and SrTiO₃ (LVO/STO). Our optical measurement shows the quantum well depth at the interface is 10nm and the carrier density is 2.21x1014 cm^-2 which is very close to the carrier density predicted from polar catastrophe model (3x1014 cm^-2). We have also shown two channel conductivity from the Hall measurement at low temperature (3K). Electronic properties such as Carrier density, mobility estimated from the electrical measurements are in excellent agreement with that estimated from optical spectroscopy-based methods through theoretical modeling. The proper knowledge of band structure can help us to understand the fascinating physics of “Rashba band splitting” due to high spin orbit coupling which give rise to planar hall effect, non-trivial berry phase.

Bifurcation amplifiers, such as the Josephson bifurcation amplifier, are highly sensitive to external stimuli. However, their operation is restricted to mK temperatures. We demonstrate a nanomechanical bifurcation amplifier using parametrically excited MoS₂ nanoresonator that can detect charge fluctuations on the order of 10 electrons in real time and can also store information of short-lived signals like a set-reset charge flip-flop.

In this work, Si doped n⁺-InGaN epilayer has been grown on a 100 nm thick AlN template on an n-type Si (111) substrate to form semiconductor-insulator-semiconductor (SIS) heterostructure by plasma-assisted molecular beam epitaxy (PAMBE). The n⁺-InGaN/AlN/n-Si (111) device shows excellent self-powered and broad band photo response under UV-Visible (300-800 nm) light illumination and maximum response is observed at 580 nm for low intensity irradiance (0.1 mW/cm2), owing to the intermediate energy states present in InGaN lattice due to nitrogen vacancies. At zero bias, the device exhibits a high responsivity of 9.64 A/W with ultrafast rise and fall times of 19.9 and 21.4 μs, respectively. This is the highest reported responsivity for the InGaN based photodetectors at zero bias to best of our knowledge. Introduction of AlN buffer layer and doping enhance the photoelectrical properties of the device compared to other conventional detectors. This work opens up a new avenue for SIS heterojunction photo detectors with much improved performance as self-powered and broadband detectors over the previously reported values on InGaN. It can be used as a promising device for nanoscale electronics, optoelectronics, integrated circuits and light wave communications.

MoS₂ monolayer, a prototypical transition metal dichalcogenide (TMD), with high carrier mobility and apt optical properties have often been used in photocatalysis. Janus (MoSSe) too follows the same. However, their usage is limited by the carrier (e-h) recombination, making it imperative to examine their bilayer van der Waals heterostructure (vdW HTS), where the spatial separation of e-h on two layers reduce recombination. The design of bilayer vdW HTSs here, consists of TMDs (HfS₂, ZrS₂, TiS₂, WS₂) and transition metal oxides (TMOs) (HfO₂, T-SnO₂, T-PtO₂,). The present work displays curiosity in their Z-scheme photocatalytic capability, where faster interlayer e-h recombination is required for efficient charge separation. We have performed first-principles based calculations un-der the framework of (hybrid) density functional theory (DFT) and many-body perturba-tion theory (GW approximation), to obtain the band edge levels and optical spectra, re-spectively. The comparative study of vdW HTSs and the constituent monolayers, have been undertaken by analyzing their electrostatic potential, work function and e-h recombination. Finally, with the promising optical response, we find MoSSe/HfS2, MoSSe/TiS2, MoS2/T-SnO2, MoS2/ZrS2 and MoSSe/ZrS2 as probable Z-scheme photo-catalysts.

We have addressed the unsolved issues on the local and long-range crystal structure of perovskite type oxides, Na_0.5Bi_0.5-xTi_1-yMg_yO_3-1.5x-y (for x=0.0 and y=0.0, x=0.01 and y=0.02, x=0.01 and y=0.04). Na_0.5Bi_0.5TiO₃ ceramic is a poor conductor, whereas Na_0.5Bi_0.49Ti_0.98Mg_0.02O2.965 and Na_0.5Bi_0.49Ti_0.96Mg_0.04O2 ceramics are excellent oxide-ion conductor, verified by AC impedance spectroscopy investigations. Based on the powder XRD and 23Na MAS NMR results, we see a clear evidence of the coexistence of monoclinic-Cc and rhombohedral-R3c symmetries in all three ceramics at room temperature. EXAFS investigations have revealed the ordering of Bi³⁺ and Na⁺, displacements of the cations, oxygen-vacancy generation and their migration pathways. Our EXAFS results demonstrates Bi^-, and Na^- rich planes formation due to short-range ordering of Bi³⁺/Na⁺ in the perovskite units. Oxygen-vacancies are found to be located in the Bi-rich planes. Reasons for the different conducting behavior of Na_0.5Bi_0.5-xTi_1-yMg_yO_3-1.5x-y ceramics will be discussed in the context of the local, long-range and micro structure.

GeSe, a structural analogue of layered orthorhombic SnSe, has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2, but experimentally it is still elusive. However, rhombohedral GeSe, a structural analogue of ferroelectric GeTe, is promising due to the higher earth abundance of Se than Te. Here, we demonstrate high thermoelectric performance in the rhombohedral GeSe crystal, stabilized by 10 mol% AgBiSe₂ alloying and the crystal grown using Bridgman method. The lattice thermal conductivity (kL) is found to be ultra-low 0.74-0.47 W/mK in the 300-723 K range with high zT ~1.25 at 723 K. First-principles density functional theoretical (DFT) analysis reveals its vicinity to a ferroelectric instability with large anomalous Born effective charges, and strong coupling of low energy polar optical phonons with acoustic phonons in rhombohedral (GeSe)_0.9(AgBiSe₂)_0.1, which results in suppressed kL and high thermoelectric performance. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)_0.9(AgBiSe₂)_0.1 are directly evident in the low temperature heat capacity (Cp) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively.

Bioinspired superhydrophobic material is well known for both its extreme water repellency and super oil-affinity underwater (underwater superoleophilicity (UWSOPHI)) which make such material suitable for selective separation of oil/water mixtures. However, the UWSOPHI of such materials is compromised upon continuous exposure to aqueous phase for less than 2 days, as well as exposure to elevated temperature (>50°C), due to the complete replacement of meta-stable trapped air by water. In this report, a comparison between superhydrophobic and moderately hydrophobic (water contact angle ≤130°) multilayers was accomplished to investigate the superior activity of the multilayers in terms of super oil affinity underwater. Moderately hydrophobic multilayer which allows co-existence of discontinuous trapped air and aqueous phase, displayed UWSOPHI, like a superhydrophobic one. Due to the presence of discontinuous metastable trapped air, the moderately hydrophobic multilayer was capable of displaying UWSOPHI for consecutive 7 days and also at elevated temperature (~90°C). Thus, the superior performance of hydrophobicity over superhydrophobicity provides a potential avenue to exploit the hydrophobic multilayers UWSOPHI for energy-efficient separation of oil-in-water emulsion.

Since the discovery of perovskites, extensive research has been carried out to decipher the underlying mechanisms that play an imperative role in deciding the ultimate device performance. But with time, scientific community has been further fascinated by new avenues of research that have opened up by the heterojunction of in-organic perovskites and other semiconductor materials. Among the various probable combinations, the CsPbBr₃ - PbS hetero-structure system has proved itself as a strong contender for usage in phototransistors. This system has already demonstrated its dominance over the constituting systems in terms of superior photoresponsivity and wider response-width. Here we have used femtosecond transient absorption spectroscopy employing energetically distinct pump excitations to understand the interplay between native carrier relaxation and carrier transfer mechanisms in this CsPbBr₃/ PbS Type -1 architecture. It was observed that the hot carrier transfer process tends to be bidirectional (from CsPbBr₃ to PbS and vice versa) when the excitation takes place in the hot states (300 nm). On the contrary, it was found that for the instance of close to band edge excitation (480 nm), the transfer of carriers occurs merely in one direction, i.e. from CsPbBr₃ to PbS states.

The major limitation of the adoption of renewables and electric automobiles is the high cost of batteries and the inability to produce them worldwide. This is due to the usage of expensive minerals primarily cobalt as raw material for electrode fabrication. Alongside batteries, the global market growth of supercapacitors also increasing due to its potential applications in automobiles, grid balancing, and electronics. As bimetallic compound electrode shows superior characteristics than monometallic electrodes, we have developed nickel-copper selenide (NiCuSe) and compared its charge storage characteristics with nickel-cobalt selenide (NiCoSe) for supercapacitors. Porous bimetallic selenide electrodes were synthesized by adapting M – benzene tricarboxylic acid (M-Co, Cu) metal-organic framework as a template to enhance the surface area. NiCuSe delivered a specific capacity of 450 mAh/g while NiCoSe delivered a specific capacity of 638.44 mAh/g in 3M KOH electrolyte. Charge storage kinetics, Coulombic efficiency, and rate capability of the electrodes were also analyzed.

We have studied the effect of uniaxial strain on the electronic and magnetic properties of LaCrS₃ using first principle density functional theory + Hubbard U (DFT+U) approach. At ambient condition, Cr ions form an antiferromagnetic zigzag spin chain along the crystallographic b-axis with dominant nearest neighbour exchange (J1) and a smaller second nearest neighbour exchange interaction (J2) which induce magnetic frustration. However, with the application of compressive strain along b-axis, J₁ and J₂ become almost equal, resulting a significant enhancement of magnetic frustration in the triangular Cr-nework. Such compressive strain also modifies the electronic structure and the system shows an interesting insulator-metal transition (IMT). A similar IMT is also observed when strain is applied along the a-axis. However, a strain along c-axis changes the nature of J₁ and J₂ from antiferromagnetic to ferromagnetic type and a subsequent first order IMT is observed. Thus, our results demonstrate that the application of uniaxial strain along various crystallographic axis will lead to different kind of electronic and magnetic transition in LaCrS₃.

We have investigated structural, electronic, magnetic and thermodynamic properties of poly-crystal Sr₃MRhO₆ (M=Na, Li) as a function of temperature T and applied magnetic field H. Both the systems crystalize in rhombohedral R3 ̅c space group. No anomaly was observed in dc magnetic susceptibility (χ) vs Temperature(T) measurements down to 2 K for Na compound, while a broad hump like feature is observed for the Li compound. A reasonable anti-site disorder between Rh and M = Na, Li cation was found from x-ray diffraction analysis which is consistent with the local structure investigation, done by EXAFS. Temperature variation of dc resistivity (ρ) and valence band x-ray spectroscopy confirms that the system is insulating at room temperature and follows a 1-D Mott-variable range hopping (VRH) mechanism. No λ-like anomaly was found in the heat capacity (Cp vs T) measurement at least down to 1.8 K, however Schottky effect was observed at low temperature.

Two-dimensional (2D) layered Ruddlesden–Popper metal halide perovskites (MHPs) show enhanced stability compared to three-dimensional (3D) MHPs. The general formula of 2D layered perovskite is L₂A_n−1M_nX_3n+1, where L is the large organic spacer, and ‘n’ is the number of metal octahedra. Such 2D layered perovskites generally demand non-conducting bulky organic spacer for the syntheses and they hamper charge carrier transport of optoelectronic devices. We have sonochemically synthesized 2D layered of type (MA)_n+1Pb_nI_3n+1 perovskite without the bulky organic spacer. With the help of absorption and emission spectroscopy, we have traced the reaction mechanism and shown that the dimensionality, ‘n’, can be controlled by both reaction time and temperature. At both lower temperature and early stage of the reaction, 2D layered perovskites with lower dimensionality forms and eventually covert to higher dimensional layered perovskite before transforming to 3D perovskite. The dissimilarity in the solubility of the precursors (PbI₂ and MAI) is responsible for such transformations. We show that these mixed (2D layered and 3D MAPI) perovskites can be used to fabricate a white light-emitting diode.

Recently discovered 2M phase of bulk WS₂ was observed to exhibit superconductivity with a critical temperature of 8.8 K, the highest reported among superconducting transition metal dichalcogenides [1]. Predicted and experimentally confirmed to support robust surface states, this novel phase could be a potential topological superconductor hosting Majorana modes [2]. The topological surface states manifests as a single Dirac cone at the brillouin zone centre. In this study, we give support for the non-trivial band nature of the bulk 2M WS₂, by the calculation of Z₂ invariant and surface states. Also, deriving a low dimensional analogue of this novel phase of the material, we predict a bilayer 2M WS₂ to display similar topologically protected surface states. Analysing the stability of such a bilayer and identifying band inversions present, we compute the electronic band structure and nature of edge states in the system. We also propose that the broken inversion symmetry in the bilayer leads to presence of Berry curvature dipoles and the resulting non-linear responses, which can serve as its signature in experiments [3].

[1] Fang et al., Adv. Mater. 30, 1901942 (2019)

[2] Yuan et al., Nature Physics 15, 1046 (2019)

[3] N. B. Joseph and A. Narayan, arXiv:2009.00849 (submitted)

Low thermal conductive crystalline solids are important to thermoelectric, thermal barrier coating, and photovoltaics. Two-dimensional layered halide perovskites have recently attracted in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κL) in the single crystal of Ruddlesden−Popper perovskite, Cs₂PbI₂Cl₂. We have observed an ultralow κL value of ∼0.37−0.28 W/mK in the temperature range of 295−523 K. First-principles density functional theory analysis of the phonon spectrum uncovers the presence of soft optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm−1 that originates from anharmonic vibration of Cl atoms induced by a 3s² lone pair. We provide experimental evidence for such low energy optical phonon modes. The coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons. The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic−optical phonon coupling results in ultralow κL value in the Cs₂PbI₂Cl₂.

Copper doping in II-VI semiconductor nanocrystals (NCs) has sparked enormous debate regarding the oxidation state of Cu ions and their hugely differing consequences in optoelectronic applications. The identity of a magnetically active Cu²⁺ ion or a magnetically inactive d¹⁰ Cu⁺ ion has generally been probed using optical techniques, and confusion arises from the spatial clutter that is part of the technique. One major probe that could declutter the data obtained from ensemble emission is single-particle fluorescence spectroscopy. In this work, using this very technique along with X-ray absorption spectroscopy probing the local environment of dopant ions, we study Cu-doped II-VI semiconductor NCs to find conclusive evidence on the oxidation state of Cu dopants and hence the mechanism of their emission. Detailed analysis of blinking properties has been used to study the single particle nature of the NCs.

Reference: Mondal, P.; Chakraborty, S.; Grandhi, G. K.; Viswanatha, R. J. Phys. Chem. Lett. 2020, 11, 13, 5367–5372.

Solution processing of nanomaterials is a promising technique for use in various applications owing to its simplicity and scalability. However, the studies on liquid phase exfoliation (LPE) of WO₃ are limited, unlike others, by a lack of commercial availability of bulk WO₃ with layered structures. Herein, a one-step topochemical synthesis approach to obtain bulk layered WO₃ from commercially available layered WS₂ by optimizing various parameters like reaction time and temperature is reported. Further, LPE was carried out on topochemically converted layered WO₃ in 22 different solvents; among the solvents studied, the propan-2-ol/water (1: 1) cosolvent system appeared to be the best. This indicates that the possible values of surface tension and Hansen solubility parameters of WO₃ could be close to that of the co-solvent system. The obtained WO₃ dispersions in a low boiling point solvent enable to fabricate thin films with varying thickness using spray coating method. The obtained thin films were used as active materials in supercapacitors without any conductive additives/binders and exhibited an areal capacitance of 31.7 mF/cm2 at 5 mV/s. Photoelectrochemical measurements revealed that these thin films can also be used as photoanodes for photoelectrochemical water oxidation.

In this review, the various methods of preparation of Cu(OH)₂PO₄ by many researchers are mentioned. The XRD, SEM of this material was discussed. The photocatalytic application is also discussed.

We present the detailed structural and magnetic properties of Li₂Mn₃O₇ with the help of various characterization techniques. Rietveld refinement of XRD data reveals that this compound has a rhombhohedral structure composed of a layered triangular lattice. Onset of spin-glass transition was confirmed by dc magnetization and ac susceptibility measurements. Dynamic scaling laws were used to analyze and classify the glassy behavior. Magnetic field dependence of irreversible temperature follows the Almeida-Thouless line, which is characteristic for an Ising spin-glass system. Fitting of the frequency-dependent freezing temperature with a power law indicates cluster-glass behavior. Further evidence of cluster-glass behavior comes from the frequency dependence of the freezing temperature fitted with the Vogel-Fulcher law, which considers interaction among bigger magnetic entities. The presence of magnetic relaxation below freezing temperature and the magnetic memory effect confirms the nonequilibrium dynamics of the system through a number of metastable states. Moreover, observation of the exchange bias effect reflects the presence of intrinsic phase inhomogeneity. In conclusion, triangular lattice causes a degenerate magnetic ground state as a result of competing exchange interactions.

Graphene coating on arbitrary substrates finds numerous applications. Coating on metal acts as a corrosion barrier in harsh environments, while graphene on metal mesh produces hybrid transparent conducting electrodes (HTCE) for optoelectronic applications. This work features a novel process of growing graphene using bio-polymer (Shellac) over a larger area (15 cm x 15 cm) under rotary pressure (0.01 mbar) and at moderate temperature ~ 800 °C on any arbitrary substrate. The graphene coating on copper showed better corrosion protection (~0.02 mm/year) than bare copper (~0.2 mm/year). In HTCE fabrication, the metal mesh was fabricated on a quartz substrate by the crackle lithography process, followed by graphene growth. The as-obtained HTCE exhibited a transmittance of ~80% and sheet resistance of ~10 Ω /□ , thus acting as an efficient transparent heater. The fabrication of HTCE by the direct growth of graphene on the metal mesh is reported for the first time to the best of our knowledge. Interestingly, the graphene growth can be extended to substrates of any arbitrary shape.

Members of A₂(MO₄)₃ structure type have been identified as negative thermal expansion (NTE) ceramics. Specifically, tungstates (Sc₂(WO₄)₃)and molybdates (Al₂(MoO₄)₃) with orthorhombic structure exhibits NTE due to the transverse thermal vibration of A-O-M linkage. Langbeinite is a similar type structure consist of three dimensional open framework with MO₆ octahedral and PO₄ tetrahedral interconnection. In the present work, we synthesized orthorhombic phosphomolydates of the chemical formula K_2-xZr₂P_2-xMo_xSiO₁₂ (x = 0, 0.1, 0.2) to develop new NTE ceramics with langbeinite structure. The compounds were synthesized by solution method and characterized by powder XRD, FT-IR, SEM and TG-DTA. Rietveld refinement study revealed that the synthesized compounds were crystallized in orthorhombic structure with P212121 space group. The characteristic stretching and bending vibrational bands of P-O, Si-O and Mo-O were observed from FT-IR spectra. The compounds were found to be stable up to 1000°C and the effect of Mo substitution was observed from the micrographs with the particle size variation. In contrary to A₂(MO₄)₃ structure type, the average thermal expansion coefficient of K_1.9Zr₂Mo_0.1P_1.9SiO₁₂ and K_1.8Zr₂Mo_0.2P_1.8SiO₁₂ was found to be 8.19 and 6.89 x 10-6/°C, respectively.

Ultrafast internal conversion between excited states of molecular aggregates like photosynthetic proteins and organic photovoltaic thin films has been of particular interest in terms of the possibility of strong non-adiabatic mixing between vibrational and electronic degrees of freedom in the presence of resonance between exciton energy gap and vibrational quanta, termed as vibronic resonance in recent literature. One pertinent question to ask in this context is what kinds of approximations can allow us to treat such couplings exactly without oversimplifying excitonic dynamics while still reducing the computational cost. I will show that one-particle approximation, a commonly used approximation in vibronic exciton models, completely fails to describe vibronic resonance. Further, I will present an effective-mode approach where a N-dimensional energy transfer problem can be analyzed in terms of separable one-dimensional Hamiltonians. This allows us to identify vibrational motions which most strongly couple promote vibronic mixing, against those which are mere spectators. By identifying promoter versus spectator vibrational motions, we exemplify our approach on a trimer toy model to demonstrate a novel design concept of trap-mediated energy transport through a vibronic resonance.

Ferroelectric (FE) materials have shown great potential as the promising alternative for green energy harvesting, especially in photovoltaic (PV) and photodetector applications. Anomalous open circuit voltage with photocurrent response is the main advantages of FE- PV effect than the conventional p-n junction solar cell. In this regard, the PV response in organic FE materials upsurge attention over the inorganic FE due to their recently observed high spontaneous polarization and better stability factor. Among that, diisopropylammonium bromide (DIPAB) is a promising material with superior FE response. However, making a continuous film of DIPAB restricts its application potential in optoelectronic devices. In the present work, DIPAB continuous film is successfully fabricated on Si(100) substrate by the thermal evaporation method. The electrical current measurements under UV-Vis light illumination on the DIPAB film displayed a remarkable photocurrent response. Importantly, it is demonstrated that the DIPAB film exhibits notable self-powered photodetector characteristics with responsivity of 0.66 mA/W and detectivity of 2.2 x 109 Jones at 11.45 mW cm^-2 light intensity. The fabricated DIPAB film reported in this work can widen its application potential in other optoelectronic devices.

We have studied the interactions producing particle aggregations in colloid polymer mixtures for phase stability control, an important concern for industrial applications. Phase contrast microscopy (PCM) and SEM were employed to study solution and film state of PEG-bentonite solution and starch films with bentonite, CMC and Ag nanoparticles. Cracks detected in SEM images of starch films get enhanced on addition of bentonite, with crack fractal dimension of starch-bentonite film increase to dF=1.64 from dF=1.57(starch). Ag nanoparticles were uniformly dispersed in starch but at the cracks, SEM images reveal aggregation of Ag nanoparticles with fractal structuring (dF=1.45). Film response to tensile forces show that fractal nature of nanoparticle clusters and cracks can store information about film mechanical properties. By proper choice of polymer and nanoparticle concentration, aggregation could be tuned in PEG-bentonite solution. PCM images reveal enhancement of clustering of bentonite nanoparticles with cluster size increase from 25μm(0% PEG) to 70μm(20% PEG). Storage modulii decreased with increasing polymer concentration, due to aggregation of bentonite in PEG matrix, mainly a result of entropic forces called depletion interactions.

The adsorption of negatively charged alcohol dehydrogenase (ADH) and positively charged lysozyme (L_yz) on positively charged (CTAB capped) gold nanoparticles (GNPs) have been studied by second harmonic light scattering (SHLS) and dynamic light scattering (DLS) in solution at physiological pH (7.4). The binding constants for both the proteins with gold nanoparticles were extracted from the SHLS data by using a modified Langmuir adsorption model. If electrostatics dominate the interaction of the protein with the GNP surface, ADH being negatively charged at pH 7.4 is expected to have a higher binding constant with positively charged GNPs compared to that of L_yz which is positively charged. But it is observed that both the proteins bind equally strongly with the GNP surface. The binding constants were rechecked by DLS. The agreement between the binding constants obtained from SHLS and DLS is very good asserting that electrostatic is not the dominating interaction in the adsorption of the two proteins on the GNP surface. The mode and mechanism of binding, the number of protein molecules bound to the surface of the GNPs, etc. will be presented.

Multi-phase co-existence ferroelectric systems are found to exhibit anomalous photovoltaic (PV) responses. However, the phenomenological theory of bulk photovoltaic effect (BPVE) is mostly studied on single-phase ferroelectric systems. In this work, we have carried out a detailed investigation of BPVE on Ba_1−x(Bi_0.5Li_0.5)_xTiO₃ having co-existence of tetragonal and orthorhombic phases. The linear and sinusoidal photocurrent response as a function of light intensity and polarization-direction, respectively, elucidate the experimental evidence for the linear BPVE. Notably, the temperature-dependent PV studies demonstrates 2-fold enhancement in photovoltage near the ferroelectric phase transition temperature (TC). The observed features in photovoltage follow inverse of the photoconductive effect. The linear relationship between the calculated bulk-photovoltaic tensor element and the photocurrent density validates the proposed phenomenological model. This work provides an insight into engineering the ferroelectric system for better PV characteristics suitable for device applications.

Cadmium sulfide (CdS), an II-VI group semiconductor material, is one of the most investigated semiconductors in thin film form. We synthesized CdS thin films with improved film morphology in the presence of ethylene diamine (EA) as the complexing agent by chemical bath deposition (CD) at lower pH. The resultant morphology was significantly influenced by the composition of the growth solution and showed prospective application as a humidity sensor with a high sensor response of 2.61 corresponding to 80% relative humidity. The synthesis process can be further exploited for fabrication of CdS/p-Si Nanowire heterojunction and their potential application for electricity generation from atmospheric moisture. The CdS nanostructures are widely exploited for the fabrication of humidity sensors because of its hygroscopic nature and 1D SiNWs array provide an excellent template for the crystal growth and assembly of semiconductors and quantum dots. Such heterojunctional architectures shows promising potential with enhanced performance owing to their large interfacial area and facile charge transport. A single CdS-Si NWs hetero-junction based HEC device exhibits a saturated maximum output voltage of 250 mV and a saturation current of 2 µA in presence of humid conditions.

HfO₂ first garnered interest as a possible alternative for SiO₂ for gate dielectric applications. In recent years, discovery of unconventional ferroelectricity in HfO₂ has fueled intense research because its CMOS compatibility can help in preparing lowest power devices like FeRAMs and FeFETs. The property of ferroelectricity is expressed only in its thin film morphology as a result of transformation from monoclinic to non-centrosymmetric orthorhombic structure. This is achieved by doping and here we use La as the dopant. Synthesis through various routes have been carried out and microwave assisted route remains unexplored. We present an original synthesis recipe for doped and undoped HfO₂ through microwave route. The films were synthesized on Si/HfO₂-seeded and Si/SiO₂/TiO₂/Pt substrates of approximate dimension 5 mm x 15 mm. Post-synthesis annealing was done to achieve crystallinity. The films were smooth and had surface roughness ranging from 1.7 - 3.7 nm and a grain size of 20 nm. Cross-sectional SEM imaging revealed that the films had thickness of 260 nm. Post-synthesis of films, photolithography was performed to fabricate interdigitated and circular devices for in plane and out of plane ferroelectric and electrical measurements which we intend to perform in the future.

After the observation of topological Fermi arcs on the surface, cubic B20 systems; CoSi, and RhSi have attracted enormous research interests. Another isostructural system, FeSi, has been predicted to show bulk chiral fermions, but whether FeSi possesses the topological surface Fermi arcs associated with the exotic chiral fermions in vicinity of the Fermi level, is yet to be confirmed both theoretically, and experimentally. Here, we present the low-energy electronic band structure of FeSi using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). Both theoretical, and experimental studies confirm that, FeSi has no topological Fermi arcs near the Fermi level. Further, we observed that the ARPES data show spin-orbit coupling (SOC) band splitting of 40 meV, which is in good agreement with bulk band-structure calculations. We also noticed an anomalous temperature-dependent resistivity in FeSi which can be understood through the electron-phonon interactions as we find a Debye energy of 80 meV from the ARPES data.

Magnetoelectric materials exhibit induction of magnetization(polarization) by an electric field(magnetic field), which are promising for various applications. Recently, magnetoelectric effect has been observed in the A₄B₂O₉ (A = Mn, Fe, Co and B = Nb, Ta) class of compounds where the magnetic point group break both space-inversion and time-reversal symmetries. Therefore, materials with linear ME can be predicted on the basis of symmetry aspects. Here we report a comprehensive investigation of the structural, magnetic, and electrical properties in a quasi-two-dimensional planar antiferromagnet BaNi₂(PO₄)₂, which crystallizes in the rhombohedral structure (space group R3 ̅) consisting of honeycomb layers of Ni²⁺ ions. Magnetic susceptibility and heat capacity data reveal a long-range antiferromagnetic ordering of Ni²⁺ ions at TN = 24 K. Interestingly, an applied magnetic field induces a dielectric anomaly and an electric polarization at TN with the polarization proportional to the applied magnetic fields, demonstrating the linear magnetoelectric effect in BaNi₂(PO₄)₂ with a coupling coefficient of 1.67 ps/m. It is interesting to note that the magnetic symmetry associated with the magnetoelectric effect is 1 ̅', which allows all tensor elements of the magnetoelectric susceptibility.

The optoelectronic response of any material has a profound reliance on the efficient absorption of electromagnetic radiation and subsequent relaxation of the photogenerated hot carriers in the system. In layered transition metal dichalcogenides (TMDCs), the excitonic features are spread over a broad absorption range (A, B, C and D in energetic order), where the high energy (C and D) excitons possess extraordinary influence over the optical properties of these systems. Hence, comprehensive understanding of C, D (along with A, B) is very important toward the advancement 2D optoelectronics field. Here, we employed transient absorption spectroscopy to monitor the underlying photophysical processes involved with the different excitonic features in few layer WS₂. We observed a strong intervalley coupling across the momentum space, where C and D exciton dynamics were significantly slower as compared to canonical A and B excitons, as a consequence of the indirect Λ−Γ relaxation in C and D and direct K−K combination in A and B. Most importantly, all four excitons emerge in the system and influence each other irrespective of the incident photon energy, which would be extremely impactful in fabricating wide range photonic devices.

Cesium antimony iodide (Cs₃Sb₂I₉) is a lead-free perovskite vouched as a promising alternative to its lead-based counterparts. Cs₃Sb₂I₉ exists in two polymorphs – the dimer form (indirect bandgap of 2.3 eV) and the layered form (direct band gap of 2.05 eV). However, halide perovskites are also known for their poor stability in the air, which makes their commercialization a challenge. A detailed study on the stability of Cs₃Sb₂I₉ was essential for its efficient use in photovoltaic modules. This work explains the degradation of the two polymorphs of Cs₃Sb₂I₉ in water, light, and elevated temperature – the well-known factors causing degradation in perovskites, using X-ray diffraction and thermogravimetric analysis. We observed that all these factors contribute to the degradation of Cs₃Sb₂I₉ individually as well as combined. We propose a possible mechanism for degradation caused by the diffusion of iodine from the system. Also, the reactivity of antimony iodide (SbI₃) with oxygen accelerates the degradation process. The degradation of both the polymorphs, however, is observed to be reversible to the dimer form in the presence of hydroiodic acid. Hence, use of this material for device applications in the ambient atmosphere would need proper encapsulation or necessary measures.

Here, we present hyperfluorescence organic light emitting diodes by using different TADF materials as an assistant dopants in the emissive layers which permits efficient energy transfer of all the excitons generated to fluorescent and phosphorescent emitters. The resulting OLEDs achieved high external quantum efficiency above 20% accompanied with good color stability. These findings demonstrate the effectiveness of TADF materials as an assistant dopant to fabricate high efficiency hyperfluorescence OLEDs.

Tetrahedrite (Cu₁₂Sb₄S₁₃) and tennantite (Cu₁₂As₄S₁₃) are isomorphous cubic structures (I-43m) at room temperature and exhibit phase transitions at 85 K and 124 K, respectively. We have investigated the phase transitions using single crystal synchrotron X-ray diffraction data. The low-temperature structure of Cu₁₂Sb₄S₁₃ is described by 2a × 2a × 2c supercell of the cubic phase and successfully refined in the I-42m space group [1]. The distortion of S(2)Cu(2)6 octahedra is found to be responsible for the structural phase transformation. In contrast, the structure of Cu₁₂As₄S₁₃ preserves its cubic symmetry and periodicity below the transition temperature. The low-temperature structure is characterized by positional disorder of S(1) and As(1) atoms along with a significant change in the occupancy of disordered sites of the Cu(2) atom. Besides distinct low temperature structures for Cu₁₂Sb₄S₁₃ and Cu₁₂As₄S₁₃, the structural phase transition is further accompanied by negative thermal expansion of lattices (per formula unit) in both compounds.

Reference:

1. Hathwar, V. R. et al., Cryst. Growth Des. 2019, 19, 3979−3988.

Bistable molecules that exhibit on/off switching of their magnetization with the application of various external perturbations e.g. temperature, pressure, light, electric-field are the best candidates for information storage, switches, and sensor devices at the molecular level.^[1] In this aspect after the discovery of photomagnetic effect in [FeCo] Prussian Blue Analogues (PBAs),^[2] various molecular model complexes of [FeCo] PBAs e.g. molecular cubes, squares, and dinuclear units have been reported by different research groups.^[1] Among all {Fe₂Co₂} molecular square complexes, none of them exhibits thermo-induced two-stepped MMET accompanied with thermal hysteresis, ON/OFF photo-switching and LITH effect in a single system. Herein, I will present a [Fe₂Co₂] molecular square complex, {[Fe(Tp)(CN)₃]₂[Co(L)₂]₂}(BF₄)₂∙2CH₃CN∙6H₂O (Tp = hydrotris(pyrazol-1-yl)borate, L = bis(1-ethyl-1H-imidazol-2-yl)ketone) which exhibits reversible switching between diamagnetic state, {Fe^II_LS-CN-Co^III_LS}, and paramagnetic state, {Fe^III_LS-CN-Co^II_HS} under the application of temperature and light.^[3]

References:

[1] Chem. Soc. Rev. 2016, 45, 203-224.

[2] Science 1996, 272, 704-705.

[3] Inorg. Chem. 2020, 59, 11879-11888.