| 1. |
- Panda, Swarup K., et al.
(författare)
-
High photon energy spectroscopy of NiO : Experiment and theory
- 2016
-
Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 93:23
-
Tidskriftsartikel (refereegranskat)abstract
- We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.
|
|
| 2. |
- Mukherjee, Soham, et al.
(författare)
-
Structure and Electronic Effects from Mn and Nb Co-doping for Low Band Gap BaTiO3 Ferroelectrics
- 2021
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:27, s. 14910-14923
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated the doping-induced local structural and electronic effects in the recently developed low band gap room temperature ferroelectric Mn-Nb co-doped BaTiO3. Experimental and theoretical Raman spectroscopies are utilized to quantify the Ti off-centering, identified to be the intrinsic origin of ferroelectricity in these systems. Mn and Nb exhibit contrasting doping behaviors that have remarkable effects on BaTiO3 functionality. Jahn-Teller distorted Mn3+ is primarily associated with lowering of the bulk band gap, while charge-compensating Nb5+ off-centers within the O-6 octahedra, creating a polar mode that stabilizes the ferroelectric ground state. The charge neutral aliovalent Mn3+-Nb5+ pair effectively couples to the inherent ferroelectric instability of the BaTiO3 lattice, restoring some spontaneous polarization lost by doping Mn3+ (d(4)) ions at Ti4+ (d(0)) sites.
|
|
| 3. |
- Pa, Jaya, et al.
(författare)
-
Synthesis and Optical Properties of Colloidal M3Bi2I9 (M = Cs, Rb) Perovskite Nanocrystals
- 2018
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:19, s. 10643-10649
-
Tidskriftsartikel (refereegranskat)abstract
- Bulk Cs3Bi2I9 exhibits zero-dimensional (0-D) perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to three-dimensional perovskite structures. Here, 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs3Bi2I9 nanocrystals and elucidated the unique optical properties arising from their 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic band gap of Cs3Bi2I9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting in a very high excitonic binding energy, E-b(X) = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Because EbX (300 meV) >> effective phonon energy (36 meV), the phonon-mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two photoluminescence peaks related to both the bulk band edge and the excitonic transitions are observed. Furthermore, Rb3Bi2I9 nanocrystals have also been synthesized, but they exhibit two-dimensional layered structure, unlike the 0-D structure of Cs3Bi2I9.
|
|
| 4. |
- Phuyal, Dibya, et al.
(författare)
-
The origin of low bandgap and ferroelectricity of a co-doped BaTiO3
- 2018
-
Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 124:2
-
Tidskriftsartikel (refereegranskat)abstract
- We recently demonstrated the lowest bandgap bulk ferroelectric material, BaTi1-x(Mn1/2Nb1/2)xO3, a promising candidate material for visible light absorption in opto- electronic devices. Using a combination of x-ray spectroscopies and density functional theory (DFT) calculations, we here elucidate this compound’s electronic structure and the modifications induced by Mn doping. In particular, we are able to rationalize how this compound retains its ferroelectricity even through a significant reduction of the optical gap upon Mn doping. The local electronic structure and atomic coordination are investigated using x-ray absorption at the Ti K, Mn K, and O K edges, which suggests only small distortions to the parent tetragonal ferroelectric system, BaTiO3, thereby providing a clue to the substantial retention of ferroelectricity in spite of doping. Features at the Ti K edge, which are sensitive to local symmetry and an indication of Ti off-centering within the Ti-O6 octahedra, show modest changes with doping and strongly corroborates with our measured polarization values. Resonant photoelectron spectroscopy results suggest the origin of the reduction of the bandgap in terms of newly created Mn d bands that hybridize with O 2p states. X-ray absorption spectra at the O K-edge provide evidence for new states below the conduction band of the parent compound, illustrating additional contributions facilitating bandgap reduction.
|
|
| 5. |
- Sarkar, Debasish, et al.
(författare)
-
A Cost-Effective and High-Performance Core-Shell-Nanorod-Based ZnO/alpha-Fe2O3//ZnO/C Asymmetric Supercapacitor
- 2017
-
Ingår i: Journal of the Electrochemical Society. - : ELECTROCHEMICAL SOC INC. - 0013-4651 .- 1945-7111. ; 164:6, s. A987-A994
-
Tidskriftsartikel (refereegranskat)abstract
- A novel core-shell design for nano-structured electrode materials is introduced for realizing cost-effective and high-performance supercapacitors. In the proposed core-shell design, thin shell-layers of highly pseudo-capacitive materials provide the platform for surface or near-surface-based faradaic and non-faradaic reactions together with shortened ion-diffusion path facilitating fast-ion intercalation and deintercalation processes. The highly-conducting core serves as highway for fast electron transfer toward current collectors, improving both energy and power performance characteristics of the core-shell structure in relation to pristine component materials. Furthermore, use of carbon (C)-based materials as a shell layer in either electrode not only enhances capacitive performance through double-layer formation but also provides enough mechanical strength to sustain volume changes in the core material during long-cycling of the supercapacitor improving its cycle life. In order to enhance electrochemical performance in terms of specific capacitance and rate capability via core-shell architecture and nano-structuring, an asymmetric supercapacitor (ASC) is assembled using ZnO/alpha-Fe2O3 and ZnO/C core-shell nanorods as respective negative and positive electrodes. The ASC exhibits a specific capacitance of similar to 115 F/g at a scan rate of 10 mV/s in a potential window as large as 1.8 V with a response time as short as 39 ms and retains more than 80% of its initial capacitance after 4000 cycles. Interestingly, the ASC can deliver an energy density of similar to 41 Wh/kg and a power density of similar to 7 kW/kg that are significantly higher than those reported hitherto for iron-oxide-based ASCs.
|
|
