COMPUTATIONAL MODELING OF NANOSTRUCTURED MATERIALS

Research


Main Research Lines


Magnetic Nanostructures

  • Magnetic films
  • Ferrofluids (Magnetic Nanoparticles dispersed in a carrier liquid)
  • Diluted magnetic Semiconductor Nanostructures

Novel Perovskites for energy harvesting


Magnetic Nanostructures

Magnetic films

   Exchange bias effects in nanoparticle systems

Bi-magnetic structures that consist of two different magnetic constituents have  an extra unidirectional anisotropy along the interface of the two materials. This results to enhanced coercive field and asymmetric hysteresis loops, the so called exchange bias effects. Bi-magnetic structures find a variety of technological applications as sensors, biosensors, microwave devices, magnetic recording devices.

Our studies on bi-magnetic structures include:

Co nanoparticles in antiferromagnetic Mn matrix

Our numerical studies have demonstrated for first time that the exchange bias mechanism is responsible for the enhanced thermal stability in these structures. (Nanotechnology 28 (2017) 035701)

Iron oxide/manganese oxide systems

Unique robust antiferromagnetic coupling in iron oxide/manganese oxide systems is for first time demonstrated, giving strong exchange bias effects in these bi-magnetic systems (Nat. Comm. 4 (2013) 2960)

Films of Co/CoO  nanoparticles

The exchange bias mechanism of dense assemblies of 2D and 3D films of Co/CoO  nanoparticles is for first time investigated with the development of a multiscale model based in a mesoscopic approach to simulate large arrays of core/shell nanoparticles. It has been demonstrated that the combination of dipolar and exchange interparticle interactions enhances dramatically coercivity, exchange bias and blocking temperature at high particle densities in the Co/CoO nanoparticle assemblies. (Advanced Materials 24 (2012) 4331)

Magnetite nanoparticles characterized with electron tomography

Magnetite nanoparticles embedded in mesoporous silicon: using input from electron tomography which is a useful technique as it provides nanoscale three dimensional information of real materials. (Nanoscale 5 (2013) 11944)

Fe nanoparticles films and multilayers

Our simulations on Fe nanoparticles films produced  by femtosecond pulsed laser ablation: have demonstrated that the strong competition between the anisotropy and exchange energies in nonuniform dense assemblies results in a frustration of the nanoparticles moments coupling and creates plateaus and abrupt steps, which indicate a sudden, collective spin reversal, for low and intermediate dipolar strengths.  (Phys. Rev. B 86 (2012) 214425)

Monte-Carlo simulations on granular multilayers of   (0.4nm Fe / 5nm Ag)n  system with number of layers n=1-20  showed that the interplay of the dipole-dipole interactions with the out-of-plain magnetic anisotropy results to an enhancement of  the blocking temperature with the increase of n. The increase in the thermal stability with the increase of the number of layers n makes the granular multi-layered systems promising for magnetic recording applications. (J. Magn. Magn. Matt. 401 (2016) 386-390)

Ferrofluids

Ferrofluids consist of ferromagnetic or ferrimagnetic nanoparticles suspended in a carrier fluid (an organic solvent or water). Usually the particles are coated with a surfactant to prevent agglomeration. The application of a moderate magnetic field allows the control of the liquid flow and alters its physical characteristics (such as optical properties and viscosity) and  its magnetic properties leading to numerous applications  as microelectronic devices, hyperthermia agents,  contrast agents and more recently as thermoelectric materials, optical filters, microfluidic devices and microelectromechanical (MEMs) systems and magnetic biosensing systems.

Our numerical studies on ferrofluids include:

Structural characteristics of ferrofluids

The aggregation mechanism and the resulting fractal morphology depend on internal characteristics (anisotropy, magnetic moment) and external parameters (temperature, magnetic field).

Physical properties such as magnetization, moment-moment correlation function and magneto-optical properties (refraction index) can be characterized by scaling exponents that are strongly dependent on the growth conditions of the aggregate.

Magnetic Behavior of frozen ferrofluids

Effect of coating on the colloidal stability of ferrofluids

The physical parameters that influence the colloidal stability of nanoparticles in ferrofluids are the magnetic anisotropies, the magnetic moments and the nanoparticle charge. In view of this, we have also calculated the electrostatic field, the electric field and the ζ-potential.

Our study includes :

  • DFT calculations and mesoscopic modeling of the magnetic properties of nanoparticles covered with an organic surfactant

A) Diethylene glycol (DEG) and oleic acid (OA) bonded at the surface of small (∼5 nm in size) CoFe2O4 particles. Our calculations show that the 1.3 times larger atomic magnetic moments and the 1.5 times lower magnetocrystalline anisotropy of the DEG sample comparing to the corresponding values for the OA sample result to hysteresis loops with higher saturation magnetization and lower coercive field in the case of DEG sample. These effects are attributed to the different cationic distributions of the two samples. (Nanoscale 10 (2018) 21244)

B) Ultra-small MnFe2O4 nanoparticles covered by bovine serum albumin: The albumin coating on Mn ferrite nanoparticles serves as a protective layer providing stability, water solubility, and biocompatibility under physiological conditions. Our study on the effect of albumin on the interparticle interactions magnetic behavior of clusters of ultra-small MnFe2O4 nanoparticles covered by bovine serum albumin provides evidence that the strength of the dipolar interactions is not affected by the presence of the albumin (Delta M plots). DFT calculations show that the albumin coating reduces the surface anisotropy and the saturation magnetization in the nanoparticles leading to lower values of the coercive field in agreement with the experimental findings.  (Nanotechnology 31 (2020) 025707)

  • DFT calculations on the electrostatic behavior of  :

 1. Nanoparticles coated with an organic surfactant

A) Diethylene glycol (DEG) and oleic acid (OA) bonded at the surface of small (∼5 nm in size) CoFe2O4 particles. The coating influences more the charge of the Co atoms and less the Fe atoms charge while the influence is very small in the O atoms.The coating effect is more pronounced in the charge of the DEG CoFerrite particles.

The average electrostatic potential and electric field gives higher ζ-potential in DEG coated nanoparticles. (J. Al. Comp. 796 (2019) 9)

B) Bovine serum albumin bonded at the surface of small (∼2 nm in size) MnFe2O4 particles. The albumin coating increases the average electrostatic potential and the electric field giving higher ζ-potential. An increase of the interaction range is observed also resulting to finite electrostatic field  at larger distances.

  2. Maghemite nanoparticles with water molecules on their  surface  

Charge distributions on the γ-Fe2O3 nanoparticles’ interface with H2O  show that the adsorption energy depends on the shape of the surface.

The ionic state of Fe atoms increases with the addition of water while the magnetic moments of the structure do not show any significant change. The mean displacement of the charge with temperature is significant. The average electrostatic potential decreases with the addition of water and shows an oscillatory behavior near the surface. (J.Mat.Mat. 484 (2019) 74)

  • Binary assemblies of soft/hard nanoparticles

A) Nanoscale interparticle interaction strength in binary nanoparticle composites is a crucial parameter to enable the tuning of the properties beyond that of the simple superposition of the two constituents. A new insight into the subtle interplay between dipolar interactions, local anisotropy and sample heterogeneity is given to determine the behavior of dense assemblies of magnetic nanoparticles. (Chem. Mater. (2020) In press)

B) Despite the low particle volume fraction, small particle clusters were imaged and the existence of CoFe2O4 and MnFe2O4 neighboring particles was evidenced for the first time in the vitrified phase of a binary ferrofluid. Monte Carlo results and experimental findings provide the conclusive evidence that the modification of magnetic properties upon binary mixing of soft and hard ferrimagnetic particles is directly inherited from the short-range magnetic dipole interactions occurring between neighboring CoFe2O4 and MnFe2O4 nanoparticles. The intensity of dipole-dipole interactions is tuned by the value of the saturation magnetization, instead of the particle volume in size binary ferrofluids, opening the way for the engineering of novel magnetic responsive ferrofluids with optimum efficiency for biomedical  or magneto-optical applications. (submitted)

Heating performance  of magnetic Nanoparticles in ferrofluids

Hyperthermia applications

  • Optimizing the heating performance of magnetic nanoparticles

A) Magnetic heating due to susceptibility losses is demonstrated to give higher SAR values in the ferromagnetic (FM) core/ ferrimagnetic (FiM) shell nanoparticles compared to the commercially used Feoxide  ones. (Nanoscale 7 (2015) 7753)

B) Structural defects in the Fe oxide nanoparticles enhance their magnetic anisotropy allowing a remarkable ten-fold rise of the nanomaterial’s thermoresponsive performance (SAR), as compared to that obtained by defect-free nanoparticles .(Phys. Rev. X  9 (2019) 041044)

C) A promising way is opened to engineer further the technological potential in diagnosis and therapy of Fe oxide nanoparticles by creating assemblies of clusters of nanoparticles. The clusters’ optimized magnetic anisotropy (including microscopic surface spin disorder) and weak ferrimagnetism at room temperature, while they do not undermine colloidal stability, endow them a profound advantage as efficient MRI contrast agents and hyperthermic mediators with important biomedical potential, good solubility in aqueous media, low cytotoxicity, high r2/r1 ratio and SAR values. (Progress in Biomedical Optics and Imaging – Proceedings of SPIE, 8955 (2014)   895517 Colloidal Nanoparticles for Biomedical Applications IX).

Monte Carlo simulations corroborate the role of the inter-particle dipolar interactions and that of the constituent nanoparticles’ surface spin disorder in the emerging spin glass dynamics of frozen-liquid dispersion of nanoclusters of 50.2 and 85.6 nm with a polymeric coating composed of nanoparticles of 12.7nm size. (Nanoscale 6 (2014) 3764)

Magneto-thermo-electric properties of ionic liquid based ferrofluids

Our group participates in the H2020 FET Proactive project MAGENTA (MAGnetic nanoparticle based liquid ENergy materials for Thermoelectric device Applications) aiming the development of novel thermoelectric materials for waste-heat recovery applications based on ionic ferrofluids; i.e., colloidal dispersions of magnetic in ionic liquids, that are versatile, cost-effective and non-toxic. The originality of the project is based on the newly discovered thermal-to-electric energy conversion capacity of ionic-liquids and ferrofluids.

Significant enhancement of the Seebeck coefficient for magnetic nanoparticle systems with high anisotropy. This finding opens new perspectives in the study of the magneto-thermal effects of novel thermoelectric materials based on ferrofluids, for waste-heat recovery applications. (App. Mat. Today 19 (2020) 100587)

Diluted magnetic Semiconductor Nanostructures

(random ferromagnets Ga1-xMnxN)

The presence of ferromagnetic interactions without band carriers, together with a sizable spin splitting of the excitonic states makes the Ga1−xMnxN system suitable for magneto-optical devices, such as optical isolators, circumventing the destructive effect of antiferromagnetic interactions specific to II-VI Mn-based dilute magnetic semiconductors.

In the framework of tight-binding theory and Monte Carlo simulations, ferromagnetic superexchange is demonstrated to be the microscopic mechanism accounting for the ferromagnetic interaction between localized spins in Ga1-xMnxN. Because of its short-range character, this coupling leads to rather low TC(x) values. (Phys. Rev. B 85 2012  205204).

Ga1−xMnxN films with Mn content x reaching 10% exhibit high saturation magnetization comparing to other diluted magnetic semiconductors due to the high cation density and the absence of competing antiferromagnetic interactions. Therefore, Ga1−xMnxN emerges as a model system, making it possible to explore properties and functionalities specific to dilute ferromagnetic insulators. Our study on how disorder influences the critical behavior of continuous phase transitions, shows that the critical behavior of this dilute magnetic insulator shows strong deviations from the magnetically clean case (x = 1). (Phys. Rev. B 88 (R) (2013) 081201)


Novel perovskites for energy harvesting (Mixed-halide Cs2SnI3Br3, defect Cs2SnX6)

Defect perovskites Cs2SnX6 (X = Cl, Br, I) and mixed-ion perovskite Cs2SnI3Br3  are studied as a non-toxic alternative to hybrid organic−inorganic CH3NH3PbI3 perovskite compound which yields efficiencies close to 20% in solar cells.

DFT calculations show that the halogen atom has a marked effect both on the optical properties of these compounds as well as their chemical affinity toward organic solvents and titania substrates.The Cs2SnI6 compound demonstrates the higher performance of the studied materials attributed to efficient charge transport in the bulk material and hole extraction at the perovskite-Pt interface. (J. Phys. Chem. C 120 (2016) 11777-11785)

The Cs2SnI3Br3 perovskite shows significantly lower charge-transport resistance comparing to conventional hole-transporting materials incorporated in solid-state dye-sensitized solar cells (Electrochimica Acta  184 (2015) 466-474)