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1.
Phys Chem Chem Phys ; 22(5): 2775-2782, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-31951228

RESUMO

van der Waals (vdW) interaction-based heterostructures are known for enhanced photon absorption. However, the origin of these phenomena is not yet completely understood. In this work, using first-principles calculations, we provide a comprehensive study to show the effect of vdW interactions on the optical and electrical characteristics of the device and its origin. Herein, MoS2/2D (where 2D varies as graphene, black and blue phosphorene, and InSe) vdW heterojunctions are considered as model structures. The change in the band gap of the heterostructures is because of hybridisation and the non-linearity of the exchange-correlation functional. Hybridisation is correlated with strain and the difference in interstitial potential between layers of the heterostructure and the vacuum level. Significantly, the estimated values of energy conversion efficiency are high in the case of MoS2/InSe and MoS2/BlackP vdW heterostructures as compared to MoS2/GR and MoS2/BlueP, suggesting their potential application in efficient and atomically thick excitonic solar cell devices.

2.
Sci Rep ; 9(1): 15983, 2019 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-31690735

RESUMO

The earth abundant and non-toxic solar absorber material kesterite Cu2ZnSn(S/Se)4 has been studied to achieve high power conversion efficiency beyond various limitations, such as secondary phases, antisite defects, band gap adjustment and microstructure. To alleviate these hurdles, we employed screening based approach to find suitable cationic dopant that can promote the current density and the theoretical maximum upper limit of the energy conversion efficiency (P(%)) of CZTS/Se solar devices. For this task, the hybrid functional (Heyd, Scuseria and Ernzerhof, HSE06) were used to study the electronic and optical properties of cation (Al, Sb, Ga, Ba) doped CZTS/Se. Our in-depth investigation reveals that the Sb atom is suitable dopant of CZTS/CZTSe and also it has comparable bulk modulus as of pure material. The optical absorption coefficient of Sb doped CZTS/Se is considerably larger than the pure materials because of easy formation of visible range exciton due to the presence of defect state below the Fermi level, which leads to an increase in the current density and P(%). Our results demonstrate that the lower formation energy, preferable energy gap and excellent optical absorption of the Sb doped CZTS/Se make it potential component for relatively high efficient solar cells.

3.
Soft Matter ; 15(39): 7787-7794, 2019 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-31515547

RESUMO

A generalized method for sorting nanoparticles based on their cores does not exist; it is an immediate necessity, and an approach incorporating cost-effectiveness and biocompatibility is in demand. Therefore, an efficient method for the separation of various mixed core-compositions or dissimilar metallic nanoparticles to their pure forms at the nano-bio interface was developed. Various simple core-combinations of monodispersed nanoparticles with dual cores, including silver plus gold, iron oxide plus gold and platinum plus gold, to the complex three-set core-combinations of platinum plus gold plus silver and platinum plus iron plus gold were sorted using step-gradient centrifugation in a sucrose suspension. Viscosity mediated differential terminal velocities of the nanoparticles permitted diversified dragging at different gradients allowing separation. Stability, purity and properties of the nanoparticles during separation were evaluated based on visual confirmation and by employing advanced instrumentations. Moreover, theoretical studies validated our experimental observations, revealing the roles of various parameters, such as the viscosity of sucrose, the density of the particles and the velocity and duration of centrifugation, involved during the separation process. This remarkably rapid, cost-efficient and sustainable strategy can be adapted to separate other cores of nanoparticles for various biomedical research purposes, primarily to understand nanoparticle induced toxicity and particle fate and transformations in natural biotic environments.

4.
Nanoscale ; 10(35): 16822-16829, 2018 Sep 13.
Artigo em Inglês | MEDLINE | ID: mdl-30167606

RESUMO

Graphene-based van der Waals (vdW) heterostructures can facilitate exciting charge transfer dynamics in between structural layers with the emission of excitonic quasi-particles. However, the chemical formation of such heterostructures has been elusive thus far. In this work, a simple chemical approach is described to form such van der Waals (vdW) heterostructures using few layer MoS2 sheet embedded quantum dots (QDs) and amine-functionalized graphene quantum dots (GQDs) to probe the energy transfer mechanism for tunable photoluminescence (PL). Our findings reveal an interesting non-radiative Förster-type energy transfer with the quenching of functional GQD PL intensity after GQD/MoS2 composite formation, which validates the existing charge transfer dynamics analogous to 0D and 2D systems. The non-radiative type of energy transfer characteristic from GQD into the MoS2 layer through vdW interactions has been confirmed by photoluminescence, time decay analyses and ab initio calculations with the shifting of the Fermi level in the density of states towards the conduction band in the stacked configuration. These results are encouraging for the fundamental exploration of optical properties in other chemically prepared QD/2D based heterostructures to understand the charge transfer mechanism and fingerprint luminescence quenching for future optoelectronic device and optical sensing applications.

5.
Nanotechnology ; 29(34): 345203, 2018 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-29862988

RESUMO

To realize a graphene based spintronic device, the prime challenge is to control the electronic structure of edges. In this work we find the origin of the spin filtering property in edge boron doped zigzag graphene nanoribbons (ZGNRs) and provide a guide to preparing a graphene based next-generation spin filter based device. Here, we unveil the role of orbitals (p-electron) to tune the electronic, magnetic and transport properties of edge B doped ZGNRs. When all the edge carbon atoms at one of the edges of ZGNRs are replaced by B (100% edge B doping), the system undergoes a semiconductor to metal transition. The role of passivation of the edge with single/double atomic hydrogen on the electronic properties and its relation with the p-electron is correlated in-depth. 50% edge B doped ZGNRs (50% of the edge C atoms at one of the edges are replaced by B) also show half-metallicity when the doped edge is left unpassivated. The half-metallic systems show 100% spin filtering efficiency for a wide range of bias voltages. Zero-bias transmission function of the other configurations shows asymmetric behavior for the up and down spin channels, thereby indicating their possible application potential in nano-spintronics.

6.
Small ; 14(10)2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29282859

RESUMO

Characteristic features of the d-band in electronic structure of transition metals are quite effective as descriptors of their catalytic activity toward oxygen reduction reaction (ORR). With the promise of graphene-based materials to replace precious metal catalysts, descriptors of their chemical activity are much needed. Here, a site-specific electronic descriptor is proposed based on the pz (π) orbital occupancy and its contribution to electronic states at the Fermi level. Simple structural descriptors are identified, and a linear predictive model is developed to precisely estimate adsorption free energies of OH (ΔGOH ) at various sites of doped graphene, and it is demonstrated through prediction of the most optimal site for catalysis of ORR. These structural descriptors, essentially the number of ortho, meta, and para sites of N/B-doped graphene sheet, can be extended to other doped sp2 hybridized systems, and greatly reduce the computational effort in estimating ΔGOH and site-specific catalytic activity.

7.
Sci Rep ; 7(1): 15550, 2017 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-29138441

RESUMO

First-principles based calculations are performed to investigate the dehydrogenation kinetics considering doping at various layers of MgH2 (110) surface. Doping at first and second layer of MgH2 (110) has a significant role in lowering the H2 desorption (from surface) barrier energy, whereas the doping at third layer has no impact on the barrier energy. Molecular dynamics calculations are also performed to check the bonding strength, clusterization, and system stability. We study in details about the influence of doping on dehydrogenation, considering the screening factors such as formation enthalpy, bulk modulus, and gravimetric density. Screening based approach assist in finding Al and Sc as the best possible dopant in lowering of desorption temperature, while preserving similar gravimetric density and Bulk modulus as of pure MgH2 system. The electron localization function plot and population analysis illustrate that the bond between Dopant-Hydrogen is mainly covalent, which weaken the Mg-Hydrogen bonds. Overall we observed that Al as dopant is suitable and surface doping can help in lowering the desorption temperature. So layer dependent doping studies can help to find the best possible reversible hydride based hydrogen storage materials.

8.
Nanotechnology ; 28(41): 415602, 2017 Oct 13.
Artigo em Inglês | MEDLINE | ID: mdl-28749376

RESUMO

We report a simple single step growth of α-MoO3 structures and energetically suitable site specific Ag nanoparticle (NP) decorated α-MoO3 structures on varied substrates, having almost similar morphologies and oxygen vacancies. We elucidate possible growth mechanisms in light of experimental findings and density functional theory (DFT) calculations. We experimentally establish and verified by DFT calculations that the MoO3(010) surface is a weakly interacting and stable surface compared to other orientations. From DFT study, the binding energy is found to be higher for (100) and (001) surfaces (∼-0.98 eV), compared to the (010) surface (∼-0.15 eV) and thus it is likely that Ag NP formation is not favorable on the MoO3(010) surface. The Ag decorated MoO3 (Ag-MoO3) nanostructured sample shows enhanced field emission properties with an approimately 2.1 times lower turn-on voltage of 1.67 V µm-1 and one order higher field enhancement factor (ß) of 8.6 × 104 compared to the MoO3 sample without Ag incorporation. From Kelvin probe force microscopy measurements, the average local work function (Φ) is found to be approximately 0.47 eV smaller for the Ag-MoO3 sample (∼5.70 ± 0.05 eV) compared to the MoO3 sample (∼6.17 ± 0.05 eV) and the reduction in Φ can be attributed to the shifting Fermi level of MoO3 toward vacuum via electron injection from Ag NPs to MoO3. The presence of oxygen vacancies together with Ag NPs lead to the highest ß and lowest turn-on field among the reported values under the MoO3 emitter category.

9.
Sci Rep ; 6: 37822, 2016 11 28.
Artigo em Inglês | MEDLINE | ID: mdl-27892532

RESUMO

First-principles calculations are performed to identify the pristine and Si doped 3D metallic T6 carbon structure (having both sp2 and sp3 type hybridization) as a new carbon based anode material. The π electron of C2 atoms (sp2 bonded) forms an out of plane network that helps to capture the Li atom. The highest Li storage capacity of Si doped T6 structure with conformation Li1.7Si1C5 produces theoretical specific capacity of 632 mAh/g which substantially exceeding than graphite. Also, open-circuit voltage (OCV) with respect to Li metal shows large negative when compared to the pristine T6 structure. This indicates modifications in terms of chemical properties are required in anode materials for practical application. Among various doped (Si, Ge, Sn, B, N) configuration, Si doped T6 structure provides a stable positive OCV for high Li concentrations. Likewise, volume expansion study also shows Si doped T6 structure is more stable with less pulverization and substantial capacity losses in comparison with graphite and silicon as an anode materials. Overall, mixed hybridized (sp2 + sp3) Si doped T6 structure can become a superior anode material than present sp2 hybridized graphite and sp3 hybridized Si structure for modern Lithium ion batteries.

10.
Nanoscale ; 8(15): 8245-54, 2016 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-27031679

RESUMO

In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.

11.
ACS Appl Mater Interfaces ; 8(8): 5290-9, 2016 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-26866799

RESUMO

Using the first-principles approach, we investigated the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer and thin film structures and substantiated their catalytic activity toward CO oxidation. 2H-SiC monolayer, being planar, is quite stable and has moderate binding with O2, while CO interacts physically; thus, the Eley-Rideal (ER) mechanism prevails over the Langmuir-Hinshelwood (LH) mechanism with an easily cleared activation barrier. Contrarily, 2H-SiC thin film, which exhibits a nonplanar structure, allows moderate binding of both CO and O2 on its surface, thus favoring the LH mechanism over the ER one. Comprehending these results leads to a better understanding of the reaction mechanisms involving structural contrast. Weak overlapping between the 2p(z)(C) and 3p(z)(Si) orbitals of the SiC monolayer system has been found to be the primary reason to revert the active site toward sp(3) hybridization, during interaction with the molecules. In addition, the influences of graphite and Ag(111) substrates on the CO oxidation mechanism were also studied, and it is observed that the ER mechanism is preserved on SiC/G system, while CO oxidation on the SiC/Ag(111) system follows the LH mechanism. The calculated Sabatier activities of the SiC catalysts show that the catalysts are very efficient in catalyzing CO oxidation.

12.
Nanotechnology ; 27(12): 125701, 2016 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-26883495

RESUMO

The lowering of the work function (Φ) can lead to a better field emission (FE) behavior at lower threshold fields. We report on enhanced FE from randomly oriented and faceted Au-capped ZnO hetero-nanostructures (HNs) having more oxygen defects. Large-area arrays of non-aligned, faceted Au-capped ZnO HNs, such as nanowires (NWs) and triangular nanoflakes (TNFs) are grown using the chemical vapor deposition (CVD) method. Enhanced FE properties from the TNF sample resulted in a turn-on field as low as 0.52 V µm(-1) at a current density of 0.1 mA cm(-2) and a field enhancement factor (ß) as high as ≈5.16 × 10(5). Under similar experimental conditions, drawing the same current density from an NW specimen needs a higher turn-on field (0.86 V µm(-1)) and to exhibit nearly four times less field enhancement factor compared to the TNFs samples. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements confirm the presence of more oxygen defects in the TNF samples compared to the NW samples. Kelvin probe force microscopy (KPFM) measurements show the average local work function to be 4.70 ± 0.1 eV for the TNF sample, which is ≈ 0.34 eV lower than the NW sample. Using density functional theory (DFT) calculations, the estimated Φ values are found to be 4.98 eV for ZnO(0001), 4.17 eV for Au(001)/ZnO(0001) and 3.91 eV for Au(001)/Ovac-ZnO(0001) surfaces. The DFT results are qualitatively in agreement with our experimental results. The presence of Au nanostructures on top of O-deficient and sharp-tipped TNFs results in enhanced FE performance following their reduced tunneling barrier via pinning of effective Φ.

13.
Sci Rep ; 5: 17460, 2015 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-26626147

RESUMO

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir-Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.

14.
ACS Appl Mater Interfaces ; 7(46): 25898-905, 2015 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-26524473

RESUMO

A scalable and controllable nanoscale perforation method for graphene is developed on the basis of the two-step thermal activation of a graphene aerogel. Different resistance to the thermal oxidation between graphitic and defective domains in the weakly reduced graphene oxide is exploited for the self-limiting nanoscale perforation in the graphene basal plane via selective thermal degradation of the defective domains. The resultant nanoporous graphene with a narrow pore-size distribution addresses the long-standing challenge for the high-level doping of graphene with lattice-mismatched large-size heteroatoms (S and P). Noticeably, this novel heteroatom doping strategy is demonstrated to be highly effective for oxygen reduction reaction (ORR) catalysis. Not only the higher level of heteroatom doping but also favorable spin and charge redistribution around the pore edges leads to a strong ORR activity as supported by density functional theory calculations.

15.
Dalton Trans ; 43(40): 15038-47, 2014 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-25090961

RESUMO

Single and multiple As adatom adsorptions on double vacancy (DV) defect graphene sheets are extensively analyzed using dispersive force corrected density functional theory (DFT). Defect pentagonal and heptagonal bridge sites and the immediate neighborhood of the defect center are found to be most favorable for this purpose. Quantitative analysis of electronic structures revealed the As-C bonding to be mostly ionic in nature with some covalency arising from the overlap of As p states and C p states. For multiple As adatoms adsorption in close vicinity, ionicity of the As-C bonds are found to decrease to support As-As cohesion; the net result of which is manifested as better binding of dioxygen molecule with them and additional weakening of the O-O bond in the adsorbed state. Free energy profile of oxygen reduction reaction (ORR) cycle using multiple As atom adsorbed DV graphene as electrocatalyst predicts high affinity towards four electron process and forbids the formation of H2O2via two electron process. Other traits, such as no intermediate O-O-H formation and high stability of the catalytic system throughout the reaction process indicate As adatoms adsorbed on DV graphene system to be efficient and highly stable as an alternate Pt free ORR electrocatalyst.

16.
Chemphyschem ; 15(12): 2542-9, 2014 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-24910355

RESUMO

Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by B-N bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene presents a prevalent doping site for nitrogen-rich BN clusters and they are also highly suitable for modulating (0.2-0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the O-O bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction.

17.
Nanoscale ; 6(6): 3384-91, 2014 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-24531861

RESUMO

Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.

18.
Nat Commun ; 4: 2076, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23797710

RESUMO

Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells.


Assuntos
Compostos Ferrosos/química , Indóis/química , Nanotubos de Carbono/química , Oxigênio/química , Catálise , Técnicas Eletroquímicas , Oxirredução , Platina/química , Piridinas/química
19.
J Phys Chem Lett ; 3(20): 3065-9, 2012 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-26292250

RESUMO

We investigated the microscopic mechanism of ion transport in iodide ionic liquid, using first-principles calculations. We show that the desorption barrier of polyiodides (I3(-) or I5(-)) from the cation is in a similar energy range as or higher than the barrier for the bond dissociation and ensued desorption of neutral iodine (I2). This suggests that, instead of the physical diffusion of such a negatively charged multiatomic species, the exchange of neutral iodine (I2) between the polyiodides can be an easier channel for the movement of polyiodide. For the transport of the monoiodide anion (I(-)), we suggest the contribution of the Grotthuss-type ion exchange through the intermediately formed even-member anion (I2n(-)), in addition to drift and diffusion. As a result, we suggest that, instead of the commonly cited diffusion of the triiodide/iodide (I3(-)/I(-)) redox couple, the exchange of neutral iodine (I2) and the Grotthuss-type transport (I(-)) constitute the dominant ion transport mechanism.

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