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1.
J Am Chem Soc ; 141(40): 16160-16168, 2019 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-31510737

RESUMO

Highly emissive isotropic CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals are typically observed in a six-faceted cube shape. When a unique approach is adopted and the reaction medium is enriched with halides, arm growth on all six facets was carried out and reported. Analysis suggested that these armed nanostructures were obtained from intermediate polyhedron shaped structures having 26 facets, and these were formed under halide-deficient conditions. Surface energy calculations further supported the possible existence of all facets for both of these structures under different halide composition environments. The entire study was first explored for CsPbBr3 and then extended to CsPbCl3; however, for CsPbI3 nanocrystals, Sr(II) dopant was used for obtaining stable emission. Arm lengths could also be tuned with a function of reaction temperature for CsPbBr3. Formation of stable facets in polyhedron shaped nanostructures and their transformation to respective hexapods under halide-deficient and halide-rich conditions add new fundamental concepts for these nanostructures and their shape evolutions.

2.
J Phys Chem Lett ; 7(16): 3270-7, 2016 Aug 18.
Artigo em Inglês | MEDLINE | ID: mdl-27494515

RESUMO

A few approaches have been employed to tune the band gap of colloidal organic-inorganic trihalide perovskites (OTPs) nanocrystals by changing the halide anion. However, to date, there is no report of electronic structure tuning of perovskite NCs upon changing the organic cation. We report here, for the first time, the room temperature colloidal synthesis of (EA)x(MA)1-xPbBr3 nanocrystals (NCs) (where, x varies between 0 and 1) to tune the band gap of hybrid organic-inorganic lead perovskite NCs from 2.38 to 2.94 eV by varying the ratio of ethylammonium (EA) and methylammonium (MA) cations. The tuning of band gap is confirmed by electronic structure calculations within density functional theory, which explains the increase in the band gap upon going toward larger "A" site cations in APbBr3 NCs. The photoluminescence quantum yield (PLQY) of these NCs lies between 5% to 85% and the average lifetime falls in the range 1.4 to 215 ns. A mixture of MA cations and its higher analog EA cations provide a versatile tool to tune the structural as well as optoelectronic properties of perovskite NCs.

3.
Nat Commun ; 6: 8627, 2015 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-26482121

RESUMO

Represented as graphs, real networks are intricate combinations of order and disorder. Fixing some of the structural properties of network models to their values observed in real networks, many other properties appear as statistical consequences of these fixed observables, plus randomness in other respects. Here we employ the dk-series, a complete set of basic characteristics of the network structure, to study the statistical dependencies between different network properties. We consider six real networks--the Internet, US airport network, human protein interactions, technosocial web of trust, English word network, and an fMRI map of the human brain--and find that many important local and global structural properties of these networks are closely reproduced by dk-random graphs whose degree distributions, degree correlations and clustering are as in the corresponding real network. We discuss important conceptual, methodological, and practical implications of this evaluation of network randomness, and release software to generate dk-random graphs.

4.
Sci Rep ; 4: 5636, 2014 Jul 09.
Artigo em Inglês | MEDLINE | ID: mdl-25005869

RESUMO

Observation of ferroelectricity among non-d(0) systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN = ) 32 K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d(0)-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

5.
Phys Rev Lett ; 111(7): 077601, 2013 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-23992082

RESUMO

Using density functional theory calculations, ultrathin films of SrVO3(d1) and SrCrO3(d2) on SrTiO3 substrates have been studied as possible multiferroics. Although both are metallic in the bulk limit, they are found to be insulating as a result of orbital ordering driven by lattice distortions at the ultrathin limit. While the distortions in SrVO3 have a first-order Jahn-Teller origin, those in SrCrO3 are ferroelectric in nature. This route to ferroelectricity results in polarizations comparable with conventional ferroelectrics.

6.
Phys Rev Lett ; 110(26): 267401, 2013 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-23848921

RESUMO

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width ≤150 meV in the orange-red region and a surprisingly large spectral width (≥180 meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (∼370 meV) covering the deep green--deep red region and (ii) exhibit widths substantially lower (∼60-75 meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

7.
J Phys Chem Lett ; 4(6): 1023-7, 2013 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-26291371

RESUMO

Mn doping in ZnS nanoplatelets has been shown to induce a structural transition from the wurtzite to the zinc blende phase. We trace the origin of this transition to quantum confinement effects, which shift the valence band maximum of the wurtzite and zinc blende polymorphs of ZnS at different rates as a function of the nanocrystal size, arising from different effective hole masses in the two structures. This modifies the covalency associated with Mn incorporation and is reflected in the size-dependent binding energy difference for the two structures.

8.
Phys Rev Lett ; 107(8): 085508, 2011 Aug 19.
Artigo em Inglês | MEDLINE | ID: mdl-21929179

RESUMO

Recently a novel phase of ZnO has been synthesized which is analogous to α-boron nitride, although more three dimensional, and consists of planar hexagonal sheets of ZnO. Examining the dynamic stability of the structure, we find unstable phonon modes over a considerable part of the Brillouin zone. Local-density approximation (LDA) and generalized gradient approximation level calculations have usually been able to predict the structural stability of s-p bonded systems. The failure in the present case is a surprise and is traced to the self-interaction error which incorrectly locates the localized Zn d states in the valence band of ZnO. Correcting for this with a Hubbard-like U on the Zn d states, the optimized structure is predicted to be stable. This highlights the fact that the large bond length contraction that one finds in going from sp(3)- to sp(2)-type bonding results in an increased necessity to correct for self-interaction errors.

9.
Phys Rev Lett ; 104(25): 256401, 2010 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-20867401

RESUMO

Usually metallicity accompanies ferromagnetism. K2Cr8O16 is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state. Analyzing the electronic and magnetic properties within first principles electronic structure calculations, we find that the doped electrons due to K induce a charge-ordered and insulating ground state and interestingly also introduce a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering gives rise to a rare example of a charge-order driven ferromagnetic insulator.

10.
Phys Rev Lett ; 105(5): 056403, 2010 Jul 30.
Artigo em Inglês | MEDLINE | ID: mdl-20867940

RESUMO

KO2 is a molecular solid consisting of oxygen dimers. K present in the lattice donates an electron which goes on to occupy the O p levels. As the basic electronic structure is similar to that of an oxygen molecule, except for broadening due to solid state effects, KO2 represents the realization of the doping of oxygen molecules arranged in a lattice. These considerations alone result in magnetism with high ordering temperatures as our calculations reveal. However, we find that the high temperature structure is unstable to an orbital ordering (OO) transition. The microscopic considerations driving the OO transition, however, are electrostatic interactions instead of the often encountered superexchange driven ordering within the Kugel-Khomskii model often used to describe the OO. This OO transition is also found to preclude any possibility of high magnetic ordering temperatures, which otherwise seemed possible.

11.
J Nanosci Nanotechnol ; 9(9): 5561-3, 2009 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-19928264

RESUMO

There has been a lot of effort to make Silicon optically active. In this work we examine two methods of generating nanocrystals of Silicon from bulk fragments. This approach of ours allows us to play with the shape of the nanocrystals and therefore the degeneracy of the conduction band minimum. We go on to examine whether similar sized particles with different shapes have the same physical properties, and finally whether Silicon may be rendered optically active by this route. While we do find that similar sized particles with different shapes may have different band gaps, this route of modifying the degeneracy of the conduction band minimum makes nano Si slightly optically active.

12.
J Nanosci Nanotechnol ; 9(9): 5564-6, 2009 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-19928265

RESUMO

We have examined the role of different terminating geometries in modifying the equilibrium lattice constant of semiconductor nanocrystals. In the case of binary semiconductors, the different criteria adopted for terminating the nanocluster lead to different surface stoichiometry. As a consequence similar sized nanocrystals are found to have different equilibrium lattice constants.

13.
J Nanosci Nanotechnol ; 9(9): 5673-5, 2009 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-19928286

RESUMO

A scheme has been devised to determine the shifts of the conduction band minimum of semiconductors as a function of size from ab-initio calculations. The method has been applied to CdSe as well as GaAs and the shifts determined. A monotonic decrease is found as a function of particle size, as expected. However, the most unusual aspect of our result is that the conduction band bottom approaches bulk-like values for the largest CdSe particles that we have studied here in contrast to expectations from other theories.

14.
J Nanosci Nanotechnol ; 7(6): 1779-82, 2007 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-17654938

RESUMO

Energetically the single sheet of graphite (graphene) is more stable than the nanotube. The energy difference between the two systems can be directly related to the strain energy involved in rolling up the graphene sheet to form the nanotube. We have carried out first-principle electronic structure calculations and evaluated the strain energy as a function of the nanotube radius. The dependence of the strain energy on the diameter of the nanotube has been found by several groups to be well-described by a continuum elasticity model. We attempt to examine why this is the case and show where atomistics enter the description.


Assuntos
Cristalização/métodos , Modelos Químicos , Modelos Moleculares , Nanotecnologia/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/ultraestrutura , Simulação por Computador , Elasticidade , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Estresse Mecânico , Propriedades de Superfície
15.
Phys Rev Lett ; 96(14): 147603, 2006 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-16712121

RESUMO

The electronic structure of the insulating sodium tungsten bronze, Na(0.025)WO(3), is investigated by high-resolution angle-resolved photoemission spectroscopy. We find that near-E(F) states are localized due to the strong disorder arising from random distribution of Na+ ions in the WO(3) lattice, which makes the system insulating. The temperature dependence of photoemission spectra provides direct evidence for polaron formation. The remnant Fermi surface of the insulator is found to be the replica of the real Fermi surface in the metallic system.

16.
Phys Rev Lett ; 93(17): 177201, 2004 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-15525118

RESUMO

Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor (DMS), has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy for Mn pairs, a dependence that cannot be explained within RKKY. In the limit of a hostlike hole engineered here where the RKKY model is applicable, the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for 110-oriented Mn pairs and minimal for 100-oriented Mn pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.

17.
Phys Rev Lett ; 88(4): 047205, 2002 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-11801165

RESUMO

The chalcopyrite CdGeP2 doped with Mn have been recently found to exhibit room-temperature ferromagnetism. Isovalent substitution of the Cd site is expected, however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn (d5) case. However, chalcopyrite semiconductors exhibit low-energy intrinsic defects. We show theoretically how ferromagnetism results from the interaction of Mn with hole-producing intrinsic defects.

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