Interplanar Interactions Research Articles

Wafer‐Scale Synthesis of Mixed‐Dimensional Heterostructures via Manipulating Platinization Conditions

Abstract2D van der Waals (vdW) hetero integration, which features exotic interplanar interactions derived from mixed‐dimensional heterostructures, is an emergent platform for implementing high‐performance electronics and broadband/wavelength‐tunable photodetectors. However, the production of large‐area 2D spatially homogeneous transition‐metal dichalcogenides (TMDs) and elucidation of the electrostatic dynamics governing the interlayer interactions are two paramount prerequisites for realizing practical 2D‐TMD‐heterostructure‐based photodetectors. Here, a wafer‐scale synthesis of mixed‐dimensional Pt–MoS2‐based vdW heterostructures is unprecedentedly demonstrated by manipulating the platinization conditions. The rationally designed platinization yields dimensionality‐tailored Pt, including Pt nanofilm, Pt nanoparticles, and Pt atoms, with MoS2 as host platform. From density functional theory calculations, this study insights that Mo vacancy sites on the MoS2 surface are thermo‐dynamically favorable sites for Pt with an adsorption energy of −2.25 eV, then Pt clusters are sequentially formed neighboring the specific Pt‐substituted position with a formation energy of 1.30 eV. Intensive microscopic and spectroscopic analyses reveal the structural, chemical, and electrical features, validating the proposed dynamics‐related mechanism. The dimensionality‐tailored vdW heterostructures exhibit outstanding optoelectrical properties with excellent photoresponsivity (2.04 mA W−1) and highly sensitive detectivity (9.82 × 106 cm Hz1/2 W−1).

Magnetic Interactions of the Centrosymmetric Skyrmion Material Gd_{2}PdSi_{3}.

The experimental realization of magnetic skyrmion crystals in centrosymmetric materials has been driven by theoretical understanding of how a delicate balance of anisotropy and frustration can stabilize topological spin structures in applied magnetic fields. Recently, the centrosymmetric material Gd_{2}PdSi_{3} was shown to host a field-induced skyrmion crystal, but the skyrmion stabilization mechanism remains unclear. Here, we employ neutron-scattering measurements on an isotopically enriched polycrystalline Gd_{2}PdSi_{3} sample to quantify the interactions that drive skyrmion formation. Our analysis reveals spatially extended interactions in triangular planes, and large ferromagnetic interplanar magnetic interactions that are modulated by the Pd/Si superstructure. The skyrmion crystal emerges from a zero-field helical magnetic order with magnetic moments perpendicular to the magnetic propagation vector, indicating that the magnetic dipolar interaction plays a significant role. Our experimental results establish an interaction space that can promote skyrmion formation, facilitating identification and design of centrosymmetric skyrmion materials.

Prediction of the Mechanical Deformation Properties of Organic Crystals Based upon their Crystallographic Structures: Case Studies of Pentaerythritol and Pentaerythritol Tetranitrate.

Development of a quantitative model and associated workflow for predicting the mechanical deformation properties (plastic deformation or cleavage fracture) of organic single crystals from their crystallographic structures using molecular and crystallographic modelling. Intermolecular synthons, hydrogen bonding, crystal morphology and surface chemistry are modelled using empirical force fields with the data integrated into the analysis of lattice deformation as computed using a statistical approach. The approach developed comprises three main components. Firstly, the identification of the likely direction of deformation based on lattice unit cell geometry; secondly, the identification of likely lattice planes for deformation through the calculation of the strength and stereochemistry of interplanar intermolecular interactions, surface plane rugosity and surface energy; thirdly, identification of potential crystal planes for cleavage fracture by assessing intermolecular bonding anisotropy. Pentaerythritol is predicted to fracture by brittle cleavage on the {001} lattice planes by strong in-plane hydrogen-bond interactions in the <110>, whereas pentaerythritol tetranitrate is predicted to deform by plastic deformation through the slip system {110} < 001>, with both predictions being in excellent agreement with known experimental data. A crystallographic framework and associated workflow for predicting the mechanical deformation of molecular crystals is developed through quantitative assessment of lattice energetics, crystal surface chemistry and crystal defects. The potential for the de novo prediction of the mechanical deformation of pharmaceutical materials using this approach is highlighted for its potential importance in the design of formulated drug products process as needed for manufacture by direct compression.

Single-Phase Silicate Phosphors (Ba1.3Ca0.7−xSiO4:xDy3+) Doped with Dysprosium for White Solid-State Lighting

Single-phase phosphors have potential advantages such as simple processability, competitive cost, and other optical and optoelectronic properties. Hence, in this study, the silicate phosphors (Ba1.3Ca0.7−xSiO4:xDy3+) doped with Dy3+ ions (x = 0.01–0.05) were synthesized and characterized in detail. X-ray diffraction patterns showed that all the silicate phosphors have a τ-phase hexagonal unit cell independent of doping. However, d-spacing was reduced for the doped samples, indicating that the interplanar interactions were enhanced. Resultantly, the doped phosphors exhibited relatively larger domains with connectivity than the host, although there were sometimes microscale pores. Photoluminescence spectra stipulated that the optimized doping concentration is x = 0.03 for the silicate Ba1.3Ca0.7-xSiO4:xDy3+ phosphors. Finally, the CIE coordinates (0.30, 0.33) confirm that the Dy3+-doped silicate phosphors are well applicable to the white light-emitting diodes for solid-state lighting and display devices.

Electronic and magnetic properties of the Jahn-Teller active fluoride NaCrF3 from first-principles calculations

In perovskite-type compounds, the interplay of cooperative Jahn-Teller effect, electronic correlations and orbital degree of freedom leads to intriguing properties. $\mathrm{NaCrF_{3}}$ is a newly synthesized Jahn-Teller active fluoroperovskite where the $\mathrm{CrF_{6}^{4-}}$ octahedrons are considerably distorted. Based on the first-principles calculation, we analyze its electronic structure and magnetic properties. Our numerical results show that the $\mathrm{Cr^{2+}}$ ions adopt the high-spin $t_{2g\uparrow}^{3}e_{g\uparrow}^{1}$ configuration with $G$-type orbital ordering. We also estimate the magnetic exchange couplings and find that the in-plane and interplanar nearest-neighbor interactions are ferromagnetic and antiferromagnetic, respectively. The ground state of this material is $A$-type antiferromagnetic, in agreement with the experiments. Reasonable Curie-Weiss and $\mathrm{N\acute{e}el}$ temperatures compared to the experiments are given by mean-field approximation theory. Our results give a complete explanation of its electronic structure, magnetic and orbital order, and help to further comprehend the behaviors of Jahn-Teller active perovskite-type fluoride.

A comparative study of dirac 2D materials, TMDCs and 2D insulators with regard to their structures and photocatalytic/sonophotocatalytic behavior

A new era in the development of advanced functional materials was partly sparked by the discovery of 2D materials. In this respect, graphene is believed to have marked the origin of 2D materials. The ability to fabricate a vast majority of such advanced nanomaterials hinges upon the strength of interplanar interactions realized in their respective bulk counterparts. The present study undertakes the comparative analysis of oft-explored 2D materials such as Dirac 2D materials (GO, rGO), TMDCs (MoS2) and 2D insulators (BN) in the context of their structural, optical, thermal and morphological parameters. Despite implementing several methodologies including a combination of physical, chemical and biological techniques, aquatic and microbial pollution remains a challenge to this day. More recently, nanomaterials have attracted considerable attention as these are believed to hold an extraordinary prospective for utilization toward environmental remediation. Among several probable candidates, 2D materials hold immense appeal due to its useful properties including high absorptivity and large surface area, which enable them to be employed for multifaceted applications. In the present study, a wide range of experimental results extracted from numerous characterization techniques (i.e., XRD, UV–Vis, FTIR, HR-TEM, XPS, DSC-TGA, and Raman) is included. For instance, optical data obtained from these materials point toward a narrow bandgap, while HR-TEM images show large surface area, which suggests that these materials hold promising prospect for use in applications that require strong catalytic activity. Further experimental results indicate that photocatalytic and sonophotocatalytic potential is significantly enhanced by 2D materials. In this respect, rGO showed 60% degradation of synthetic pollutant in 100 min and MoS2 realized 55% degradation during the same duration. The present study suggests that rGO may be used as a superior photocatalyst in wastewater treatment and related environmental applications. Moreover, the sonophotocatalytic behavior exhibited by these materials showed consistently higher efficiency compared to the respective individual processes which is attributed to the formation of larger amounts of electron–hole pairs.

Synthetic investigation of competing magnetic interactions in 2D metal-chloranilate radical frameworks.

The discovery of emergent materials lies at the intersection of chemistry and condensed matter physics. Synthetic chemistry offers a pathway to create materials with the desired physical and electronic structures that support fundamentally new properties. Metal–organic frameworks are a promising platform for bottom-up chemical design of new materials, owing to their inherent chemical predictability and tunability relative to traditional solid-state materials. Herein, we describe the synthesis and magnetic characterization of a new 2,5-dihydroxy-1,4-benzoquinone based material, (NMe2H2)3.5Ga2(C6O4Cl2)3 (1), which features radical-based electronic spins on the sites of a kagomé lattice, a geometric lattice known to engender exotic electronic properties. Vibrational and electronic spectroscopies, in combination with magnetic susceptibility measurements, revealed 1 exhibits mixed valency between the radical-bearing trianionic and diamagnetic tetraanionic oxidation states of the ligand. This unpaired electron density on the ligand forms a partially occupied kagomé lattice where approximately 85% of the lattice sites are occupied with an S = ½ spin. We found that gallium mediates ferromagnetic coupling between ligand spins, creating a ferromagnetic kagomé lattice. By modulation of the interlayer spacing via post-synthetic cation metathesis of 1 to (NMe4)3.5Ga2(C6O4Cl2)3 (2) and (NEt4)2(NMe4)1.5Ga2(C6O4Cl2)3 (3), we determined the nature of the magnetic coupling between neighboring planes is antiferromagnetic. Additionally, we determined the role of the metal in mediating this magnetic coupling by comparison of 2 with the In3+ analogue, (NMe4)3.5In2(C6O4Cl2)3 (4), and we found that Ga3+ supports stronger superexchange coupling between ligand-based spins than In3+. The combination of intraplanar ferromagnetic coupling and interplanar antiferromagnetic coupling exchange interactions suggests these are promising materials to host topological phenomena.

Size Effects on the Mechanical Properties of Nanoporous Graphene Networks

AbstractIt is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy.

Effect of Copper Substitution on Magnetic Properties of NiFe2O4 Ferrite

The many-body Green’s function theory has been used to study the magnetic properties of CuxNi1−xFe2O4 spinels. We present a new method to calculate the expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green’s functions. Magnetization and magnetic susceptibility are given when external magnetic field is applied in (x–z) plane. The mean field theory has been used to calculate nearest-neighbor exchange and next-nearest-neighbor superexchange interactions. The intraplanar and the interplanar interactions are deduced. The magnetic phase diagram is deduced using high-temperature series expansions. The ferrimagnetic and paramagnetic phases are determined. The critical exponents associated with the magnetic susceptibility \(\left( \gamma \right)\) and the correlation lengths \(\left( \nu \right)\) have been deduced. These results are comparable with those obtained by magnetic measurements and are comparable with those of 3D Heisenberg model.

Equation of state of 1,3,5-triamino-2,4,6-trinitrobenzene up to 66 GPa

The equation of state and the unit cell parameters of triamino-trinitrobenzene (TATB) have been measured up to 66 GPa by x-ray diffraction on multi-grain samples. A custom indexing of the diffraction pattern was implemented to enable an unambiguous assignment of the diffraction peaks. The structural data reveal a more isotropic response to compression above 10 GPa, indicating that the van der Waals forces are no more the dominant interplanar interactions. The unit cell parameters are compared with density functional theory calculations, including the empirical D2 correction for the van der Waals interactions. An excellent agreement is obtained up to 20 GPa and an increasing deviation above. The present determination of the TATB isothermal equation of state is reliable for detonation physics applications.

Σ3(111)grain boundary of body-centered cubic Ti-Mo and Ti-V alloys: First-principles and model calculations

The energetics and atomic structures of $\mathrm{\ensuremath{\Sigma}}3[1\overline{1}0](111)$ grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-$\ensuremath{\omega}$ phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce the chemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

(Invited) Mixed Dimensional Nanoelectronic Heterostructures

Layered two-dimensional (2D) nanomaterials interact primarily via van der Waals bonding, which has created new opportunities for nanoelectronic heterostructures that are not constrained by epitaxial growth [1]. However, it is important to acknowledge that van der Waals interactions are not limited to interplanar interactions in 2D materials. In principle, any passivated, dangling bond-free surface interacts with another via non-covalent forces. Consequently, the emerging layered 2D nanomaterials can be integrated with a diverse range of other materials [2-4], including those of different dimensionality, to form van der Waals heterostructures. This talk will explore mixed dimensional combinations of 2D + n-D (n = 0, 1 and 3) materials, thus significantly expanding the van der Waals heterostructure concept. In order to efficiently explore the vast phase space for mixed dimensional heterostructures, our laboratory employs solution-based additive assembly. In particular, constituent nanomaterials (e.g., carbon nanotubes, graphene, transition metal dichalcogenides, black phosphorus, and boron nitride) are isolated in solution [5-9], and then deposited into thin films with scalable additive manufacturing methods (e.g., inkjet [10], gravure [11], and screen printing [12]). By achieving high levels of nanomaterial monodispersity and printing fidelity, large-area device arrays can realize complex electronic signal conditioning such as frequency and phase shift keying [4]. Furthermore, by integrating multiple nanomaterial inks into heterostructures, unprecedented device function has been demonstrated including anti-ambipolar p-n heterojunctions [2-4] and gate-tunable memristors [13]. In addition to technological implications for nanoelectronics and optoelectronics, this work allows the exploration of several fundamental issues including band alignment, doping, trap states, and charge/energy transfer across previously unexplored mixed dimensional heterointerfaces. [1] D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides,” ACS Nano, 8, 1102 (2014). [2] D. Jariwala, S. L. Howell, K.-S. Chen, J. Kang, V. K. Sangwan, S. A. Filippone, R. Turrisi, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Hybrid, gate-tunable, van der Waals p-n heterojunctions from pentacene and MoS2,” Nano Lett., DOI: 10.1021/acs.nanolett.5b04141 (2015). [3] D. Jariwala, V. K. Sangwan, C.-C. Wu, P. L. Prabhumirashi, M. L. Geier, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Gate-tunable carbon nanotube-MoS2 heterojunction p-n diode,” Proc. Nat. Acad. Sci. USA, 110, 18076 (2013). [4] D. Jariwala, V. K. Sangwan, J.-W. T. Seo, W. Xu, J. Smith, C. H. Kim, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Large-area, low-voltage, antiambipolar heterojunctions from solution-processed semiconductors,” Nano Lett., 15, 416 (2015). [5] N. D. Mansukhani, L. M. Guiney, P. J. Kim, Y. Zhao, D. Alducin, A. Ponce, E. Larios, M. J. Yacaman, and M. C. Hersam, “High-concentration aqueous dispersions of nanoscale two-dimensional materials using nonionic, biocompatible block copolymers,” Small, DOI: 10.1002/smll.201503082 (2015). [6] J. Zhu, X. Liu, M. L. Geier, J. J. McMorrow, D. Jariwala, M. E. Beck, W. Huang, T. J. Marks, and M. C. Hersam, “Layer-by-layer assembled two-dimensional montmorillonite dielectrics for solution-processed electronics,” Adv. Mater., DOI: 10.1002/adma.201504501 (2015). [7] J. Zhu, J. Kang, J. Kang, D. Jariwala, J. D. Wood, J.-W. T. Seo, K.-S. Chen, T. J. Marks, and M. C. Hersam, “Solution-processed dielectrics based on thickness-sorted two-dimensional hexagonal boron nitride nanosheets,” Nano Lett., 15, 7029 (2015). [8] J. Kang, J. D. Wood, S. A. Wells, J.-H. Lee, X. Liu, K.-S. Chen, and M. C. Hersam, “Solvent exfoliation of electronic-grade, two-dimensional black phosphorus,” ACS Nano, 9, 3596 (2015). [9] J. Kang, J.-W. T. Seo, D. Alducin, A. Ponce, M. J. Yacaman, and M. C. Hersam, “Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted density gradient ultracentrifugation,” Nature Communications, 5, 5478 (2014). [10] E. B. Secor, P. L. Prabhumirashi, K. Puntambekar, M. L. Geier, and M. C. Hersam, “Inkjet printing of high conductivity, flexible graphene patterns,” J. Phys. Chem. Lett., 4, 1347 (2013). [11] E. B. Secor, S. Lim, H. Zhang, C. D. Frisbie, L. F. Francis, and M. C. Hersam, “Gravure printing of graphene for large-area flexible electronics,” Adv. Mater., 26, 4533 (2014). [12] W. J. Hyun, E. B. Secor, M. C. Hersam, C. D. Frisbie, and L. F. Francis, “High-resolution patterning of graphene by screen printing with a silicon stencil for highly flexible printed electronics,” Adv. Mater., 27, 109 (2015). [13] V. K. Sangwan, D. Jariwala, I. S. Kim, K.-S. Chen, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS2,” Nature Nanotechnology, 10, 403 (2015).

Synthesis and characterization of Cu-doped polymeric carbon nitride

ABSTRACTPolymeric carbon nitride doped with copper through a solid-state reaction was characterized by several techniques, among them are UV-visible spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, etc. The material is a semiconductor with a wide band gap of 2.74 eV. Sites of both Cu(I) and Cu(II) were detected, apparently only coordinated by the polymer. The material comprises crumpled nanosheets, and is substantially an amorphous layered material with a prevalent 2D structure with low inter-planar interactions, as shown by X-ray diffractometry and TeraHertz spectroscopy. Photo-oxidation of benzyl alcohol was used to probe the active sites of the material, comparing them with the non-doped material. The higher activity and selectivity toward salicylic alcohol of the non-doped material can be due to both a more localized electron transfer and a longer lifetime of the hole–electron pair. Cu-CN favored the oxidation of hydroxymethyl group. Therefore, the presence of copper can favor different reaction pathways with respect to the non-doped material.

Multiphonon Raman spectroscopy properties and Raman mapping of 2D van der Waals solids: graphene and beyond

Strong in‐plane bonding (covalent) and weak van der Waals (vdW) interplanar interactions characterize a number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic two‐dimensional (2D) layers, isolated from mineral graphite via micromechanical exfoliation enabled the ability to pick, place and stack of arbitrary compositions. Moreover, this discovery implicated an access to other 2D vdW solids beyond graphene and artificially stacking atomic layers forming heterostructures/superlattices. Raman spectroscopy (RS) is a fast reliable non‐destructive analytical tool and an integral part for lattice dynamical structural characterization of crystalline solids at nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes and specifically used to determine the number of graphene layers and of other 2D vdW solids. We present Raman spectroscopy in first‐, second‐ and higher‐order vibrational modes involving 3 and 4 phonons (overtones and combination) and mapping of graphene (mono‐, bi‐, tri‐ and few‐) layers, semiconducting transition metal dichalcogenides (TMDs) [molybdenum disulfide (MoS2) and tungsten disulfide (WS2)] and wide bandgap hexagonal boron nitride (h‐BN) dispersed monolayers, revealing various molecular vibrations and structural quality/disorder. First‐ and higher‐order phonon modes are observed and analyzed in terms of Raman intensity (spatial inhomogeneity or thickness variation), band position (intrinsic mechanical strain) and intensity ratio (structural disorder) as a function of graphene layer (n). An empirical relation for G band position with n is corroborated. All of the higher order modes are observed to upshift almost linearly with n, betraying the underlying interlayer vdW interactions. These findings exemplify the evolution of structural parameters in layered materials in changing from 3‐ to 2‐ or low‐dimensional regime. The results are presented in view of applications of graphene by itself and in combination that help better understanding of physical and electronic properties for nano‐/optoelectronics. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Moiré patterns as a probe of interplanar interactions for graphene on h-BN.

By atomistic modeling of moiré patterns of graphene on a substrate with a small lattice mismatch, we find qualitatively different strain distributions for small and large misorientation angles, corresponding to the commensurate-incommensurate transition recently observed in graphene on hexagonal BN. We find that the ratio of C-N and C-B interactions is the main parameter determining the different bond lengths in the center and edges of the moiré pattern. Agreement with experimental data is obtained only by assuming that the C-B interactions are at least twice weaker than the C-N interactions. The correspondence between the strain distribution in the nanoscale moiré pattern and the potential energy surface at the atomic scale found in our calculations makes the moiré pattern a tool to study details of dispersive forces in van der Waals heterostructures.

2D to 3D transition of polymeric carbon nitride nanosheets

The transition from a prevalent turbostratic arrangement with low planar interactions (2D) to an array of polymeric carbon nitride nanosheets with stronger interplanar interactions (3D), occurring for samples treated above 650°C, was detected by terahertz-time domain spectroscopy (THz-TDS). The simulated 3D material made of stacks of shifted quasi planar sheets composed of zigzagged polymer ribbons, delivered a XRD simulated pattern in relatively good agreement with the experimental one. The 2D to 3D transition was also supported by the simulation of THz-TDS spectra obtained from quantum chemistry calculations, in which the same broad bands around 2THz and 1.5THz were found for 2D and 3D arrays, respectively. This transition was also in accordance with the tightening of the interplanar distance probably due to an interplanar π bond contribution, as evidenced also by a broad absorption around 2.6eV in the UV–vis spectrum, which appeared in the sample treated at 650°C, and increased in the sample treated at 700°C. The band gap was calculated for 1D and 2D cases. The value of 3.374eV for the 2D case is, within the model accuracy and precision, in a relative good agreement with the value of 3.055eV obtained from the experimental results.

Magnetic Properties of Mg x Cu1−x Cr2O4 Spinels are Studied by Different Theoretical Methods

The external magnetic field induced the reorientation of magnetization of a ferromagnetic (or antiferromagnetic) treated within the framework of many-body Green’s function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green’s functions. Magnetization and magnetic susceptibility are investigated when an external magnetic field is applied in (x-z)-plane. The mean field theory is applied to calculate the nearest neighbour and the next-neighbour super-exchange J 1(Cr−Cr) and J 2(Cr−(Mg(Cu)−O)−Cr), respectively, for the Mg x Cu1−x Cr2O4 in the range of 0 ≤ x ≤ 1. The intra-planar and the inter-planar interactions are deduced. The high-temperature series expansions (HTSEs) are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random-phase approximation (RPA). The magnetic-phase diagram is deduced. A spin-glass phase is predicted for intermediate range of concentration. The spin glass is obtained. The obtained results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The obtained values are comparable to those of 3D Heisenberg model.

Magnetic properties of ferromagnetic diluted Zn x Cd1–x Cr2Se4 spinels are studied by Green's functions, mean field theory and high temperature series expansions theories

The field induced reorientation of the magnetization of ferromagnetic (or antiferromagnetic) structure is treated within the framework of many-body Green's function theory by considering all components of the magnetization. The mean field theory is used to calculate the nearest neighbour and the next-neighbour super-exchange J1(Cr–Cr) and J2(Cr–(Zn(Cd)–Se)–Cr), respectively, for the Zn1–x Cd x Cr2Se4 in the range 0 < x < 1. The intraplanar and the interplanar interactions are deduced. The high temperature series expansions (HTSEs) are derived for the magnetic susceptibility and the two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation (RPA). The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The obtained results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The obtained values are comparable to those of 3D Heisenberg model.

Designing nanoscaled hybrids from atomic layered boron nitride with silver nanoparticle deposition

We have developed a microwave assisted one-pot approach to fabricate a novel hybrid nano-composite composed of two-dimensional chemically exfoliated layered hexagonal boron nitride (h-BN) and embedded silver nanoparticles (SNP). Atomic layered h-BN exfoliated using chemical liquid showed strong in-plane bonding and weak van der Waals interplanar interactions, which is utilized for chemically interfacing SNP, indicating their ability to act as excellent nano-scaffolds. The SNP/h-BN optical response, in particular band gap, is strongly dependent on the concentration of the metallic particles. In order to gain further insight into this behavior we have also carried out ab initio density functional theory (DFT) calculations on modeled structures, demonstrating that the bandgap value of SNP/h-BN hybrids could be significantly altered by a small percentage of OH− groups located at dangling B and N atoms. Our results showed that these novel SNP/h-BN nanohybrid structures exhibited excellent thermal stability and they are expected to be applied as devices for thermal oxidation-resistant surface enhanced Raman spectroscopy (SERS). The SNP/h-BN membrane showed remarkable antibacterial activity, suggesting their potential use in water disinfection and food packaging.

Artificially Stacked Atomic Layers: Toward New van der Waals Solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.