Changes In Density Of States Research Articles

The Density Functional Tight Binding (DFTB) Approach for Investigating Vacancy and Doping in Graphene as Hydrogen Storage

A study on graphene defects for hydrogen storage has been successfully conducted using the Density Functional Tight Binding (DFTB) approach. The research aimed to modify solid materials for hydrogen storage. A 4 × 4 × 1 unit cell was used as the basis, while the supercell used for sampling was enlarged to 40 × 40 × 1. The analyzed data included changes in Density of States (DOS), Fermi level shifts, electronic band structures, formation energy, adsorption energy, and isosurfaces for each graphene orientation. It has been observed that modifying the surface structure of graphene can alter the electronic properties of graphene. This is indicated by shifts in DOS intensity, characterized by increased electronic intensity around the Fermi level total density charge different. The interaction energy between graphene and hydrogen gas has been determined to be -0.0155 eV for H-epoxy graphene, -0.4941 eV for H-monovacancy graphene, and -0.0424 eV for HN-monovacancy graphene. The presence of the vacancy increase the potential to adsorp hydrogen.

Understanding into atomic structures and electronic properties from the analysis of intramolecular junctions in carbon nanotubes

Octuplet arrangements of intra-molecular junctions (IJs) are selected and studied in order to forecast each one's atomic structure, quantum density of states of CNTs with imperfections, and electronic transmittals. Each of these can be divided into groups of triplets, specifically those that exhibit comparable characteristics to one another, such as Semiconductor_Semiconductor IJs (SC_SC IJs) (in arrangements of first, second, fifth and eight), Metal_Metal IJs (in arrangements of third and seventh) and Semiconductor_Metal IJs (in arrangements of fourth and sixth). The quantum density of states of SC_SC IJs results in semiconductors with the same order as before, but with a reduced band gap as compared to the primary standards. Additionally, a resized image of quantum density of states is provided to demonstrate the specifics of the divergence near the Fermi limit. quantum density of states for CNTs-IJs (fifth, fourth) – (fifth, fourth) (SC_SC) estimated across a handful of rings at the intersection indicated by the blue lines is shown through the illustration. The hexagonal-assembly of CNTs contains pentagons & heptagon-paired imperfections, which can alter CNTs’ helicity and their basic electronic framework. The red lines indicate the quantum density of states of Perfect-CNTs with indices (fifth, fourth). Despite being semiconductors by nature, Perfect-CNTs (fifth, fourth) exhibit a band gap around 1.22 eV. The semiconductor Perfect-CNTs with indices (seventh, fourth) have a quantum density of states that is mirrored around the Fermi limit (E = 0). Additionally, at the Fermi limit, it has a value that does not equal zero, along with a difference in particle energy. The picture shows that the behavior of CNTs-IJs quantum density of states changes from being purely semiconductor-like to becoming less like a semiconductor when mixed with metallic CNTs at the intersection. The combination of metal and semiconductor is tangible. It changes from one system to another, beginning as a semiconductor and changing into a semiconductor with a considerable number of switching operations. The observation shows that the semiconductor with the indices (seventh, third) and the metal carbon nanotubes with the indices (seventh, fourth) both have zero QDS at the Fermi energy level. Despite being diluted within a few atomic rings, the quantum density of states in the case of CNTs-IJs (seventh, third) – (seventh, fourth) remains at 0.0 at the Fermi energy level due to being diluted within a few atomic rings.

Effect of electronic alteration on hydrogen storage and optical response in NaMgF3 using DFT approach

Due to the depletion of non-renewable fossil fuels, energy production and storage is a matter of concern for scientific community. Hydrogen being environmental friendly was presented by the scientists a fruitful replacement of fossil fuel, but the storage of hydrogen is still a matter of concern. Safety concerns are associated with hydrogen as an alternative fuel because it is difficult to store and transport. Perovskite materials have attracted researchers as new materials for solid hydrogen storage. In this present work, Density Functional Theory (DFT) calculations have been performed for perovskite NaMg F3−xHx with the GGA-PBE formalism as implemented in the CASTEP code. The aim of the work is to investigate the structural, electronic and optical properties of NaMg F3−xHx perovskite hydride material for hydrogen storage applications with varying substituent concentrations (x = 0, 0.3, 0.6, 0.9, 1.2). The formation enthalpy of the studied material for each concentration reveals that these compounds are stable and synthesizable experimentally. The hydrogen inclusion in pristine material affects the electronic states significantly which is elaborated using band structure and density of states plots. After Hydrogen inclusion, change in density of states has been observed and the band gap is decreased from 5.74eV to 3.311eV. The alteration of electronic band gap directly influences the optical behavior of the material, thus optical response such as dielectric function, absorption and refractive index was also calculated. For hydrogen storage application, gravimetric storage capacity (Cwt%) was calculated for all concentrations of hydrogen inclusion. The value varied from 0.30% to 1.87%, indicating that the presented material is an efficient material not only for opto-electronic applications but also for the hydrogen storage applications.

Importance of self-consistency in first-principles Eliashberg calculation for superconducting transition temperature

We have performed first-principles Eliashberg calculations for several conventional superconductors. Using our recently developed scheme of solving the anisotropic Eliashberg gap equation considering the retardation effect and mass renormalization effect (Wang et al. 2020), we investigate the importance of the self-consistency in the calculation. We compare the estimated Tc given by the self-consistent calculation and the one-shot calculation for the self-energy of electrons. We show that the difference of the one-shot results and self-consistent results will become larger for systems for which the density of states changes rapidly around the Fermi level, such as the presence of the van Hove singularities in pressurized hydrides.

Enhanced electrochemical properties of graphene-based screen-printed carbon electrode by PPy modification: Experimental and DFT investigations

Here, a novel method to enhance the electrochemical properties of graphene-modified screen-printed carbon electrode (G/SPCE) with polypyrrole (PPy) nanosheets is proposed. The mechanism of improving the electrochemical sensitivity of G/SPCE was investigated at the molecular level. G/SPCE was modified with PPy nanosheets synthesized by oxidative polymerization to improve its electrochemical properties. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) response of PPy-modified G/SPCE (PPy/G/SPCE) to K3[Fe(CN)6] (PF) were tested experimentally. The results show that the oxidation current sensitivity of G/SPCE to PF after PPy nanosheets modification increased from 11.12 μA·mM−1 to 12.91 μA·mM−1 in CV response, and from 8.44 μA·mM−1 to 10.72 μA·mM−1 in DPV response. The effective reaction area of PPy/G/SPCE was found to be increased by scanning electron microscopy. And the functional group CO/CO in PPy/G/SPCE, acting as a catalyst for the redox reaction of PF, was also found to be improved by X-ray photoelectron spectroscopy. Furthermore, the adsorption energy, electron transfer and density of states changes of PF molecule on pure and PPy-modified graphene were analyzed based on density functional theory calculation. The calculation results show that the adsorption energy and electron transfer of PPy-modified graphene to PF molecule are significantly increased. The response sensitivity of graphene to PF molecule is greatly enhanced by the synergistic effect generated of PPy and graphene.

Carbonyl sulfide and dimethyl sulfide adsorption studies on novel square-octagon antimonene sheets – A first-principles study

Using the first-principles study, we studied the structural and electronic attributes of square-octagon antimonene nanosheet (so-SbNS). Firstly, the structural and thermal stability of so-SbNS is ascertained. The energy band gap of so-SbNS is recorded as 0.814 eV. Furthermore, the so-SbNS is deployed as base material to adsorb carbonyl sulfide (COS), and dimethyl sulfide (DMS) molecules. The physical adsorption of COS and DMS molecules is noticed on so-SbNS. The so-SbNS behaves as the acceptor of electrons observed from charge transfer and electron density difference results. The band structure and projected density of states changes are noticed due to COS and DMS adsorption on so-SbNS. The results convey that so-SbNS is new material towards the detection of COS and DMS molecules emitted from solid waste.

Gate-tunable charge carrier electrocaloric effect in trilayer graphene

The electrocaloric (EC) effect is the change in temperature and entropy of a material driven by the application of an electric field. Our tight-binding calculations linked to Fermi statistics, show that the EC effect can be produced in trilayer graphene (TLG) structures connected to a heat source, triggered by changes in the electronic density of states (DOS) at the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate that entropy changes are sensitive to the stacking arrangement in TLG systems. The AAA-stacked TLG presents an inverse EC response (cooling) regardless of the temperature value and gate field potential strength, whereas the EC effect in ABC-stacked TLG remains direct (heating) above room temperature. We reveal otherwise the TLG with Bernal-ABA stacking generates both the direct and inverse EC response within the same sample, associated with gate-dependent electronic transitions of thermally excited charge carriers from the valence band to the conduction band in the band structure. The novel charge carrier electrocaloric effect we propose in quantum layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the thermal response in nanodevices.

First-Principles Studies of the Surface and Interface between/Among Metal Oxides Used in SOFCs

On the surface and interface, all materials change the property such as stability and electronic states from them on bulk. Human being continue to apply the phenomena to the key technologies from early times. For example, in solid oxide fuel cells (SOFCs), important phenomena such as fuel decomposition, charge transfer etc., occur at surface or triple phase boundary (TPB) that consists of catalyst, electrolyte, and gas phase. To solve remaining issues of SOFCs such as degradation or efficiency improvement, important is to know the detail of property changes of materials used in SOFCs on the surface and interface. Theoretical study is efficient approach to investigate material property, especially electronic states. Therefore, in this study, we have focused on surface and interface of metal oxides used as electrolytes in SOFCs and investigated theoretically their property using two calculation models with first-principles calculation. In concrete, one is density functional theory (DFT) calculations with the TPB model that consists of Ni catalyst cluster, YSZ electrolyte, and gas phase. In this case, we have tried to decide the reliable structure to discuss properties on TPB and evaluated adsorption reactivity etc. The other is DFT calculations with the hetero-structure interface model that consists of zirconia (ZrO2) and ceria (CeO2). In this case, we have tried to evaluated electronic states on that interface and discussed in terms of conductivity control. Here, only the results of optimized YSZ (111) surface model and density of state (DOS) change in ZrO2/CeO2 hetero-structure are shown as examples for word limitation. The CASTEP software was used for all calculations.Geometry optimization of YSZ (111) surface model have been performed with some arrangements of yttrium (Y) atoms and oxygen vacancy in order to identify the most stable structure. Our model consists of (111) three layers includes 2×2 or 2×4 ZrO2 unit cells, 15 Å vacuum layer, and 9.1 % substituted Y atoms. In the most stable YSZ 2×2 model, one Y atom is on the top Zr atoms layer and another Y atom is on the middle Zr atoms layer nearest to the top Y atom, and oxygen vacancy is on the 2nd O atoms layer nearest to the top Y atom. We have found the position of oxygen vacancy against Y atoms and surface is sensitive to the YSZ surface stability. Oxygen vacancy does not prefer to stick around two Y atoms. It indicates that improvement for the geometry unstability of cubic ZrO2 is more important than keeping local electroneutrality even near on the surface. By applying this finding, we have performed geometry optimization of YSZ 2×4 unit cell model with changing Y atom arrangements in the same layer and keeping the stable structure in 2×2 unit cell model. The most stable YSZ 2×4 surface model finally-obtained is shown in attached figure (A).Our 2nd topic is for the electronic states change on the interface of metal oxides. We have calculated DOS with pure ZrO2, pure CeO2, and several ZrO2/CeO2 models hetero-connected with (111) surfaces. DOS changes by hetero-connection are shown in attached figure (B). A new small peak not appeared in the cases of pure ZrO2 and pure CeO2 is observed in 01 eV near Fermi level. It is considered that the peak is caused by the interaction between 4f orbital shown in 13 eV of Ce atoms and 2p orbital shown in -50 eV of O atoms in ZrO2 on the interface. In addition, the peak height is influenced only by the thickness of CeO2 layers, not that of ZrO2 layers. Our results suggest that we can control electronic conductivity of these metal oxides with hetero-structuration in atomic level especially at low temperature. Other results and detailed discussion will be reported at our meeting publications (ECS Transactions) and presentation in this conference. Figure 1

Raman measurement of the effect of lattice defects on the two‐phonon density of states in ZnO

AbstractThe resonance of the mode of the zinc oxide (ZnO) lattice occurs near a ridge‐like structure of the two‐phonon density of states (DOS). The renormalization of the harmonic frequency of the mode using perturbation‐theory with first‐principles calculations of the two‐phonon DOS is usually used to describe the Raman spectrum of this mode. In this contribution, we tackle the inverse problem; that is, we derive the two‐phonon DOS in ZnO using numerical analysis of Raman measurements of the mode. We vary the temperature to displace the resonance of the mode along the ridge and detect a wide spectrum of the two‐phonon DOS in ZnO. We first derive the two‐phonon DOS by increasing the temperature from 300 K to 1000 K with a 20 K step. Next, we perform the same study but decrease the temperature from 1000 K to 300 K to observe how the two‐phonon DOS changes due to thermal annealing. The results show that lattice defects significantly enhance the two‐phonon DOS, which highlights the need for an effective thermal management strategy when ZnO lattice defects are harnessed for technology applications.

Theoretical study of the adsorption and sensing properties of pure and metal doped C24N24 fullerene for its potential application as high-performance gas sensor

Density functional theory (DFT) calculation was used to study the potential applications of pure and metal-doped C24N24 fullerene as highly sensitive and selective gas sensors. It is found that gases commonly existing in the air, including NO2, CO, NO, NH3, N2 and CO2, are weakly adsorbed on pure C24N24. The adsorption strength is increased when they are adsorbed on the metal-doped (Co-, Cr-, Cu-, K-, Li-, Fe-, Ni-, Pd-, Ti-, Mn-, Na-, Zn-, V-) C24N24. The pure and metal-doped C24N24 have a specific response to certain gas according to adsorption energy, charge transfer, band gap and the density of states analyses. Among various metal-doped C24N24, the adsorption energies of NO2 on K- and Na-doped C24N24 are apparently lager than those of other gas molecules, which fall between the suitable range of strong physical adsorption and weak chemical adsorption. Upon NO2 adsorption, the band gap changes of K- and Na-doped C24N24 which corelate with its conductivity reach −12.75% and −26.67%, respectively. In accordance with the band gap changes, the density of states changes and charge transfer for NO2 adsorption on K- and Na-doped C24N24 are most obvious. Sensor performance analysis further shows K- and Na-doped C24N24 are highly selective in NO2 detection. The recycle time of Na–C24N24 under ambient condition is also in the suitable range (1.80 s). This study concludes the potential application of the Na-doped C24N24 as high-performance NO2 sensor.

A New Approach to Stable Cationic and Anionic Redox Activity in O3‐Layered Cathode for Sodium‐Ion Batteries

AbstractHerein, stable cationic and anionic redox in an O3‐type layered Na[Ni2/3Ru1/3]O2 cathode for sodium‐ion batteries (SIBs) is revealed. Density functional theory (DFT) calculation shows that the electron density features change in density of state with mixing of delocalized valence states as well as localized deeper energy states of O(p), Ni(d), and Ru(d) for the highly desodiated Na1−x[Ni2/3Ru1/3]O2 electrode, revealing the covalent characteristic of the transition metal (TM)O and TMTM bonds in the charged system. These properties lead to cycling stability for 200 cycles, with ≈79% of the capacity retained at a rate of 1C (210 mA g−1). Operando X‐ray diffraction, X‐ray absorption spectroscopy, and DFT calculations reveal the reversible electrochemical activity of the Ni2+/Ni3+ and O2−/O1− redox reactions, which are sustainable throughout the cycles. In addition, no loss of oxygen from the crystal structure of Na[Ni2/3Ru1/3]O2 occurs according to differential electrochemical mass spectrometry. The findings provide additional insight into the complex mechanism of the oxygen redox activity of high‐capacity O3‐type cathode materials for SIBs, encouraging further studies on their development.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Changes in the phonon density of states of Fe induced by external strain

Nuclear inelastic scattering of synchrotron radiation is used to study the changes induced by external tensile strain on the phonon density of states (pDOS) of polycrystalline Fe samples. The data are interpreted with the help of dedicated atomistic simulations. The longitudinal phonon peak at around 37 meV and also the second transverse peak at 27 meV are decreased under strain. This is caused by the production of defects under strain. Also the thermodynamic properties of the pDOS demonstrate a weakening of the force constants and of the mean phonon energy under strain. Remaining differences between experiment and simulation are discussed.

Band structure of twisted bilayer graphene on hexagonal boron nitride

The effect of an hexagonal boron nitride (hBN) layer close aligned with twisted bilayer graphene (TBG) is studied. At sufficiently low angles between twisted bilayer graphene and hBN, $\theta_{hBN} \lesssim 2^\circ$, the graphene electronic structure is strongly disturbed. The width of the low energy peak in the density of states changes from $W \sim 5 - 10$ meV for a decoupled system to $\sim 20 - 30$ meV. Spikes in the density of states due to van Hove singularities are smoothed out. We find that for a realistic combination of the twist angle in the TBG and the twist angle between the hBN and the graphene layer the system can be described using a single moir\'e unit cell.

The influence of phonon softening on the superconducting critical temperature of Sn nanostructures

The increase in superconducting transition temperature (TC) of Sn nanostructures in comparison to bulk, was studied. Changes in the phonon density of states (PDOS) of the weakly coupled superconductor Sn were analyzed and correlated with the increase in TC measured by magnetometry. The PDOS of all nanostructured samples shows a slightly increased number of low-energy phonon modes and a strong decrease in the number of high-energy phonon modes in comparison to the bulk Sn PDOS. The phonon densities of states, which were determined previously using nuclear resonant inelastic X-ray scattering, were used to calculate the superconducting transition temperature using the Allen-Dynes-McMillan (ADMM) formalism. Both the calculated as well as the experimentally determined values of TC show an increase compared to the bulk superconducting transition temperature. The good agreement between these values indicates that phonon softening has a major influence on the superconducting transition temperature of Sn nanostructures. The influence of electron confinement effects appears to be minor in these systems.

Analytical Approach to Study Sensing Properties of Graphene Based Gas Sensor.

Over the past years, carbon-based materials and especially graphene, have always been known as one of the most famous and popular materials for sensing applications. Graphene poses outstanding electrical and physical properties that make it favorable to be used as a transducer in the gas sensors structure. Graphene experiences remarkable changes in its physical and electrical properties when exposed to various gas molecules. Therefore, in this study, a set of new analytical models are developed to investigate energy band structure, the density of states (DOS), the velocity of charged carriers and I-V characteristics of the graphene after molecular (CO, NO2, H2O) adsorption. The results show that gas adsorption modulates the energy band structure of the graphene that leads to the variation of the energy bandgap, thus the DOS changes. Consequently, graphene converts to semiconducting material, which affects the graphene conductivity and together with the DOS variation, modulate velocity and I-V characteristics of the graphene. These parameters are important factors that can be implemented as sensing parameters and can be used to analyze and develop new sensors based on graphene material.

Graphene-Based Ultrasensitive Strain Sensors

We performed density function theory calculations to study the effect of strain on electronic properties of pristine and doped graphene. A dramatic change in density of states (DOS) and band struct...

Simulations of Graphene Nanoribbon Field Effect Transistor for the Detection of Propane and Butane Gases: A First Principles Study.

During the last few years graphene has emerged as a potential candidate for electronics and optoelectronics applications due to its several salient features. Graphene is a smart material that responds to any physical change in its surrounding environment. Graphene has a very low intrinsic electronic noise and it can detect even a single gas molecule in its proximity. This property of graphene makes is a suitable and promising candidate to detect a large variety of organic/inorganic chemicals and gases. Typical solid state gas sensors usually requires high operating temperature and they cannot detect very low concentrations of gases efficiently due to intrinsic noise caused by thermal motion of charge carriers at high temperatures. They also have low resolution and stability issues of their constituent materials (such as electrolytes, electrodes, and sensing material itself) in harsh environments. It accelerates the need of development of robust, highly sensitive and efficient gas sensor with low operating temperature. Graphene and its derivatives could be a prospective replacement of these solid-state sensors due to their better electronic attributes for moderate temperature applications. The presence of extremely low intrinsic noise in graphene makes it highly suitable to detect a very low concentration of organic/inorganic compounds (even a single molecule ca be detected with graphene). In this article, we simulated a novel graphene nanoribbon based field effect transistor (FET) and used it to detect propane and butane gases. These are flammable household/industrial gases that must be detected to avoid serious accidents. The effects of atmospheric oxygen and humidity have also been studied by mixing oxygen and water molecules with desired target gases (propane and butane). The change in source-to-drain current of FET in the proximity of the target gases has been used as a detection signal. Our simulated FET device showed a noticeable change in density of states and IV-characteristics in the presence of target gas molecules. Nanoscale simulations of FET based gas sensor have been done in Quantumwise Atomistix Toolkit (ATK). ATK is a commercially available nanoscale semiconductor device simulator that is used to model a large variety of nanoscale devices. Our proposed device can be converted into a physical device to get a low cost and small sized integrated gas sensor.

Haeckelite boron nitride as nano sensor for the detection of hazardous methyl mercury

A new two-dimensional (2D) member in the boron nitride family, haeckelite BN (haeck-BN) is proposed as a nanosensor for the detection of environmental hazardous material, methyl mercury (CH3Hg). Using first principles-based dispersion corrected density functional calculations, we have studied interaction of CH3Hg with haeck-BN to understand its role as toxic gas sensor. The interaction of CH3Hg with haeck-BN is validated by calculating the adsorption energy and electronic density of states (DOS). The change in DOS and work function (WF) upon adsorption of CH3Hg molecules further confirms the sensing ability of haeck-BN. In the pristine form, haeck-BN shows weak interaction towards CH3Hg with adsorption energy of −0.45 eV. To tune the sensing and electronic properties of haeck-BN, we doped haeck-BN with Al, S, Si, and P atoms. The substitutions were found to alter the electronic properties of haeck-BN, turning it from semiconducting to metallic. However, with the substitution of heteroatom, a significant increment in the adsorption energy is observed in the range of −1.9 eV to −3.8 eV. Our finding suggests that haeck-BN can be utilized as a sensor for the detection of hazardous toxins.

Change in Density of States of 2D Ising Model when Next-Neighbor Interaction Is Introduced

In the present paper we analyzed a change in the density of states of a two-dimensional Ising model when a next-next-neighbor interaction is introduced. In other words, we examined two-dimensional lattices with diagonal connections. The same as in a three-dimensional model in this case each spin has 6 connections. Since the model is planar, we can calculate the free energy and other characteristics of the system using a polynomial algorithm. We performed computer simulations using the Kasteleyn–Fisher algorithm, which allowed us to study changes of critical values and the density of states when a long-range interaction is taken into account. From the obtained results it follows that the interactions of the type analyzed here result in quantitative changes of the system’s characteristics, but they did not change them qualitatively. In particular, we again obtained a logarithmic divergence of the heat capacity in the critical point.