21 research outputs found
Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study
Due to the advantage of tunability via size, shape, doping and relatively low
level of loss and high extent of spatial confinement, graphene quantum dots
(GQDs) are emerging as an effective way to control light by molecular
engineering. The collective excitation in GQDs shows both high energy plasmon
frequency along with frequencies in the terahertz (THz) region making these
systems powerful materials for photonic technologies. Here, we report a
systematic study of the linear and nonlinear optical properties of large
varieties of GQDs (400 systems) in size and topology utilizing the strengths of
both semiempirical and first-principles methods. Our detailed study shows how
the spectral shift and trends in the optical nonlinearity of GQDs depends on
their structure, size and shape. Among the circular, triangular, stripe, and
random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always
possess lower HOMO-LUMO gap, broadband absorption and high nonlinear optical
coefficients. Also, we find that for majority of the GQDs with interesting
linear and nonlinear optical properties have zigzag edges, although reverse is
not always true. We strongly believe that our findings can act as guidelines to
design GQDs in optical parametric oscillators, higher harmonic generators and
optical modulators.Comment: 21 pages, 11 figures, 4 table
Quantum scattering cross sections of O() + N collisions for planetary aeronomy
"Hot atoms", which are atoms in their excited states, transfer their energy
to the surrounding atmosphere through collisions. This process of energy
transfer is known as thermalization, and it plays a crucial role in various
astrophysical and atmospheric processes. Thermalization of hot atoms is mainly
governed by the amount of species present in the surrounding atmosphere and the
collision cross-section between the hot atoms and surrounding species. In this
work, we investigated the elastic and inelastic collisions between hot oxygen
atoms and neutral N molecules, relevant to oxygen gas escape from the
martian atmosphere and for characterizing the chemical reactions in hypersonic
flows. We conducted a series of quantum scattering calculations between various
isotopes of O() atoms and N molecules across a range of collision
energies (0.3 to 4 eV), and computed both their differential and collision
cross-sections using quantum timeindependent coupled-channel approach. Our
differential cross-section results indicate a strong preference for forward
scattering over sideways or backward scattering, and this anisotropy in
scattering is further pronounced at higher collision energies. By comparing the
cross-sections of three oxygen isotopes, we find that the heavier isotopes
consistently have larger collision cross-sections than the lighter isotopes
over the entire collision energy range examined. However, for all the isotopes,
the variation of collision cross-section with respect to collision energy is
the same. As a whole, the present study contributes to a better understanding
of the energy distribution and thermalization processes of hot atoms within
atmospheric environments. Specifically, the crosssectional data presented in
this work is directly useful in improving the accuracy of energy relaxation
modeling of O and N collisions over Mars and Venus atmospheres.Comment: 7 pages, 5 figures, 4 tables, submitted to MNRA
A first principles investigation of cubic BaRuO: A Hund's metal
A first-principles investigation of cubic-BaRuO, by combining density
functional theory with dynamical mean-field theory and a hybridization
expansion continuous time quantum Monte-Carlo solver, has been carried out.
Non-magnetic calculations with appropriately chosen on-site Coulomb repulsion,
and Hund's exchange, , for single-particle dynamics and static
susceptibility show that cubic-BaRuO is in a spin-frozen state at
temperatures above the ferromagnetic transition point. A strong red shift with
increasing of the peak in the real frequency dynamical susceptibility
indicates a dramatic suppression of the Fermi liquid coherence scale as
compared to the bare parameters in cubic-BaRuO. The self-energy also shows
clear deviation from Fermi liquid behaviour that manifests in the
single-particle spectrum. Such a clean separation of energy scales in this
system provides scope for an incoherent spin-frozen (SF) phase, that extends
over a wide temperature range, to manifest in non-Fermi liquid behaviour and to
be the precursor for the magnetically ordered ground state.Comment: 10 pages, 12 figures, 1 tabl
Resource-Efficient Quantum Circuits for Molecular Simulations: A Case Study of Umbrella Inversion in Ammonia
We conducted a thorough evaluation of various state-of-the-art strategies to
prepare the ground state wavefunction of a system on a quantum computer,
specifically within the framework of variational quantum eigensolver (VQE).
Despite the advantages of VQE and its variants, the current quantum
computational chemistry calculations often provide inaccurate results for
larger molecules, mainly due to the polynomial growth in the depth of quantum
circuits and the number of two-qubit gates, such as CNOT gates. To alleviate
this problem, we aim to design efficient quantum circuits that would outperform
the existing ones on the current noisy quantum devices. In this study, we
designed a novel quantum circuit that reduces the required circuit depth and
number of two-qubit entangling gates by about 60%, while retaining the accuracy
of the ground state energies close to the chemical accuracy. Moreover, even in
the presence of device noise, these novel shallower circuits yielded
substantially low error rates than the existing approaches for predicting the
ground state energies of molecules. By considering the umbrella inversion
process in ammonia molecule as an example, we demonstrated the advantages of
this new approach and estimated the energy barrier for the inversion process.Comment: 7 pages, 8 figure
Computing accurate bond dissociation energies of emerging per- and polyfluoroalkyl substances: Achieving chemical accuracy using connectivity-based hierarchy schemes
Understanding the bond dissociation energies (BDEs) of per- and polyfluoroalkyl substances (PFAS) bonds helps in devising their efficient degradation pathways. However, there is only limited experimental data on the PFAS BDEs, and there are uncertainties associated with the BDEs computed using density functional theory. Although quantum chemical methods like the G4 composite method can provide highly accurate BDEs (< 1 kcal mol-1), they are limited to small system sizes. To address DFT\u27s accuracy limitations and G4\u27s system size constraints, we examined the connectivity-based hierarchy (CBH) scheme and found that it can provide BDEs that are reasonably close to the G4 accuracy while retaining the computational efficiency of DFT. To further improve the accuracy, we modified the CBH scheme and demonstrated that BDEs calculated using it have a mean-absolute deviation of 0.7 kcal mol-1 from G4 BDEs. To validate the reliability of this new scheme, we computed the ground state free energies of seven PFAS compounds and BDEs for 44 C–C and C–F bonds at the G4 level of theory. Our results suggest that the modified CBH scheme can accurately compute the BDEs of both small and large PFAS at near G4 level accuracy, offering promise for more effective PFAS degradation strategies
Cobalt anti-MXenes as Promising Anode Materials for Sodium-ion Batteries
The current electric vehicle market is entirely dominated by lithium-ion batteries (LIBs). However, due to the limited and unequal distribution of LIB raw materials on earth, there is a continuous effort to design alternate storage devices. Among the alternatives to LIBs, sodium-ion batteries (NIBs) are at the forefront because sodium resources are ubiquitous worldwide and virtually inexhaustible. However, one of the major drawbacks of the NIBs is their low specific charge capacity. Since the specific charge capacity of a cell can be improved by increasing the specific charge capacity of the anode material, there is a constant effort to find suitable anode materials. Recent studies suggested that cobalt-boride (CoB) anti-MXene material (a newly discovered two-dimensional material) can yield superior specific charge capacities for LIBs than traditional graphite-based anodes. Inspired by these findings, in this work, we considered six cobalt-based anti-MXene materials (Co-anti-MXenes), namely, CoAs, CoB, CoP, CoS, CoSe, and CoSi, and examined their competency as anode materials for NIBs. Our findings suggest that Co-anti-MXenes possess superior specific charge capacities (~ 390–590 mAh/g) than many well-studied anode materials like MoS2 (146 mAh/g), Cr2C (276 mAh/g), expanded graphite (284 mAh/g), etc. Moreover, their greater affinity (-0.55 to -1.16 eV) to Na atoms, along with reasonably small diffusion energy barriers (0.32 to 0.59 eV) and low average sodiation voltages (0.2 to 0.64 V), suggest that these Co-anti-MXenes can serve as excellent anode materials for NIBs
Deep Neural Network Assisted Quantum Chemistry Calculations on Quantum Computers
The variational quantum eigensolver (VQE) is a widely employed method to solve electronic structure problems on the current noisy intermediate-scale quantum (NISQ) devices. However, due to inherent noise in the NISQ devices, VQE results on NISQ devices often deviate significantly from the results obtained on noiseless statevector simulators or traditional classical computers. The iterative nature of the VQE further amplifies the errors in each loop. Recent works have explored ways to integrate deep neural networks (DNN) with VQE to mitigate the iterative errors, albeit, primarily limited to the noiseless statevector simulators. In this work, we trained DNN models across various quantum circuits and examined the potential of two DNN-VQE approaches, DNN1 and DNNF, for predicting the ground state energies of small molecules in the presence of device noise. We carefully examined the accuracy of the DNN1, DNNF, and VQE methods on both noisy simulators and real quantum devices by considering different ansatzes of varying qubit counts and circuit depths. Our results illustrate the advantages and limitations of both VQE and DNN-VQE approaches. Notably, both DNN1 and DNNF methods consistently outperform the standard VQE method in providing more accurate ground-state energies in noisy environments. However, despite being more accurate than VQE, the energies predicted using these methods on real quantum hardware remain meaningful only at reasonable circuit depths (depth = 15, gates = 21). At higher depths (depth = 83, gates = 112), they deviate significantly from the exact results. Additionally, we find that DNNF does not offer any notable advantage over VQE in terms of speed. Consequently, our study recommends DNN1 as the preferred method for obtaining quick and accurate ground state energies of molecules on the current quantum hardware, particularly for quantum circuits with lower depth and fewer qubits
Promising anode materials for alkali metal ion batteries: A case study on cobalt anti-MXenes
There is a continuous demand for energy storage devices with high energy density in consumer electronics, electric vehicles, and the grid energy market. Although commercial lithium-ion batteries (LIBs) satisfy the current needs, the limited availability of raw materials and moderate specific charge capacities (SCC) of LIBS, motivated scientists to search for alternate anode materials for LIBs and also to find technologies beyond LIBs. In this work, we studied the potential of six Cobalt anti-MXenes (CoAs, CoB, CoP, CoS, CoSe, and CoSi), a class of newly discovered 2D materials, as anode materials for lithium, sodium, and potassium ion batteries (LIBs, NIBs, and KIBs). We found that these materials are good electrical conductors and have high adsorption stability for the alkali metal ions, which helps to prevent the formation of dendrites and increase the cycle life of the battery. They also show moderate to low migration energy barriers (MEBs), indicating the potential for faster charge-discharge kinetics. We also explained the slightly counter-intuitive result of observing low MEBs along with high adsorption stability. Furthermore, Co-anti-MXenes can adsorb multiple alkali atoms per formula unit, resulting in high specific charge capacities and low average anodic voltages. For example, as anode materials for lithium-ion batteries, CoP and CoSi have SCC values of 1075.4 mAh g-1 and 934 mAh g-1, and anodic voltages as low as 0.28 V and 0.43 V, respectively. Moreover, even the maximally metallated Co-anti-MXenes did not show agglomeration tendency at room temperature. Also, the volume expansion of these materials is minimum for both Li and Na adsorption. As a whole, we find that Co-anti-MXenes can act as promising anode materials for alkali metal ion batteries
AB-stacked bilayer β12–borophene as a promising anode material for alkali metal-ion batteries
As the lightest 2D- material, monolayer borophene exhibits a specific charge capacity of 1860 mAh g-1 for Li-ion batteries, which is four times higher than that of graphite and is one of the highest specific charge capacities ever reported for 2D anode materials. Additionally, it showed high mechanical strength and a low diffusion barrier. However, monolayer borophene suffers from stability issues in its free-standing form, which restricts its real-life applications. Inspired by the recent experimental investigations, which proved the higher stability of bilayer borophene polymorphs (BBPs) over their monolayer counterparts, in this work, we investigated the dynamical and thermodynamical stabilities of both AA and AB–stacked BBPs in their β12 phase using first-principles calculations. Between the two stacking patterns, we found that only the AB–stacked β12–BBP is both energetically and dynamically stable, and we further investigated its potential as a high-performance anode material for alkali metal ion batteries. Our investigations show that AB–stacked β12–BBP exhibits good electrical conductivity before and after metal atom (Li/Na/K) adsorption onto it. Further, AB–stacked β12–BBP adsorbs the metal atoms strongly with adsorption energies ranging between -0.89 to -1.44 eV, indicating that there is a lesser possibility of forming dendrites on this anode. Similarly, it has a low diffusion energy barrier (~ 0.13–0.49 eV) for metal atoms, meeting the fast charge/discharge rate requirements. Moreover, it exhibits a reasonably low average metal-insertion voltage (0.43 to 0.65 V) and a specific charge capacity of 330–413 mAh g-1 that is comparable to graphite. All the above findings suggest that the AB–stacked bilayer β12– borophene can be a potentially favorable anode material