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(3P^3P) + N2_2 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$_2$ 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($^3P$) atoms and N$_2$ molecules across a range of collision energies (0.3 to 4 eV), and computed both their differential and collision cross-sections using quantum time$-$independent 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 cross$-$sectional data presented in this work is directly useful in improving the accuracy of energy relaxation modeling of O and N$_2$ collisions over Mars and Venus atmospheres.Comment: 7 pages, 5 figures, 4 tables, submitted to MNRA

A first principles investigation of cubic BaRuO3_3: A Hund's metal

A first-principles investigation of cubic-BaRuO$_3$, 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, $U$ and Hund's exchange, $J$, for single-particle dynamics and static susceptibility show that cubic-BaRuO$_3$ is in a spin-frozen state at temperatures above the ferromagnetic transition point. A strong red shift with increasing $J$ 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$_3$. 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

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

The interaction of halogen molecules with SWNTs and graphene

The interaction of halogen molecules of varying electron affinity, such as iodine monochloride (ICl), bromine (Br<SUB>2</SUB>), iodine monobromide (IBr) and iodine (I<SUB>2</SUB>) with single-walled carbon nanotubes (SWNTs) and graphene has been investigated in detail. Halogen doping of the two nanocarbons has been examined using Raman spectroscopy in conjunction with electronic absorption spectroscopy and extensive theoretical calculations. The halogen molecules, being electron withdrawing in nature, induce distinct changes in the electronic states of both the SWNTs and graphene, which manifests with a change in the spectroscopic signatures. Stiffening of the Raman G-bands of the nanocarbons upon treatment with the different halogen molecules and the emergence of new bands in the electronic absorption spectra, both point to the fact that the halogen molecules are involved in molecular charge-transfer with the nanocarbons. The experimental findings have been explained through density functional theory (DFT) calculations, which suggest that the extent of charge-transfer depends on the electron affinities of the different halogens, which determines the overall spectroscopic properties. The magnitude of the molecular charge-transfer between the halogens and the nanocarbons generally varies in the order ICl &gt; Br<SUB>2</SUB> &gt; IBr &gt; I<SUB>2</SUB>, which is consistent with the expected order of electron affinities

A hexanuclear Cu(I) cluster supported by cuprophilic interaction: effects of aromatics on luminescence properties

A hexanuclear Cu(I) cluster {Cu<SUB>3</SUB>(L)<SUB>2</SUB>}<SUB>2</SUB> (1) based on a novel tripodal linker (L) has been synthesized. 1 shows intense emission (&#x03BB;<SUB>max</SUB> = 560 nm) with lifetime 〈&#x03C4;〉 = 224 &#x03BC;s and quantum yield = 27.6%. The emission is highly sensitive towards different electron rich and electron deficient aromatics. DFT calculations were performed to understand the origin of emission and sensing properties