Assessing the thermal conductivity of amorphous SiN by approach-to-equilibrium molecular dynamics.

First-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics methodology is employed to calculate the thermal conductivity of non-stoichiometric amorphous SiN. This is achieved by implementing thermal transients in five distinct models of different sizes along the direction of the heat transport. Such models have identical structural features and are representative of the same material, thereby allowing for a reliable analysis of thermal conductivity trends as a function of the relevant cell dimension. In line with the known physical law of heat propagation at short scale, the thermal conductivity increases in size with the direction of heat transport. The observed behavior is rationalized accounting for previous results on crystalline and amorphous materials, thus providing a unified description holding for a large class of materials and spanning a wide range of heat propagation lengths.

Impact of the local atomic structure on the thermal conductivity of amorphous Ge2Sb2Te5.

Thermal properties are expected to be sensitive to the network topology, and however, no clearcut information is available on how the thermal conductivity of amorphous systems is affected by details of the atomic structure. To address this issue, we use as a target system a phase-change amorphous material (i.e., Ge2Sb2Te5) simulated by first-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics technique to access the thermal conductivity. Within the density-functional theory, we employed two models sharing the same exchange-correlation functional but differing in the pseudopotential (PP) implementation [namely, Trouiller-Martins (TM) and Goedecker, Teter, and Hutter (GTH)]. They are both compatible with experimental data, and however, the TM PP construction results in a Ge tetrahedral environment largely predominant over the octahedral one, although the proportion of tetrahedra is considerably smaller when the GTH PP is used. We show that the difference in the local structure between TM and GTH models impacts the vibrational density of states while the thermal conductivity does not feature any appreciable sensitivity to such details. This behavior is rationalized in terms of extended vibrational modes.

Impact of Dispersion Force Schemes on Liquid Systems: Comparing Efficiency and Drawbacks for Well-Targeted Test Cases.

First-principles molecular dynamics (FPMD) calculations were performed on liquid GeSe4 with the aim of inferring the impact of dispersion (van der Waals, vdW) forces on the structural properties. Different expressions for the dispersion forces were employed, allowing us to draw conclusions on their performances in a comparative fashion. These results supersede previous FPMD calculations obtained in smaller systems and shorter time trajectories by providing data of unprecedented accuracy. We obtained a substantial agreement with experiments for the structure factor regardless of the vdW scheme employed. This objective was achieved by using (in addition to FPMD with no dispersion forces) a selection of vdW schemes available within density functional theory. The first two are due to Grimme, D2 and D3, and the third one is devised within the so-called maximally localized Wannier functions approach (MLWF). D3 results feature a sizeable disagreement in real space with D2 and MLWF in terms of the partial and total pair correlation functions as well as the coordination numbers. More strikingly, total and partial structure factors calculated with D3 exhibit an unexpected sharp increase at low k. This peculiarity goes along with large void regions within the network, standing for a phase separation of indecipherable physical meaning. In view of these findings, further evidence of unconventional structural properties found by employing D3 is presented by relying on results obtained for a complex ionic liquid supported on a solid surface. The novelty of our study is multifold: new, reliable FPMD data for a prototypical disordered network system, convincing agreement with experimental data and assessment of the impact of dispersion forces, with emphasis on the intriguing behavior of one specific recipe and the discovery of common structural features shared by drastically dissimilar physical systems when the D3 vdW scheme is employed.

Reversible assembly of nanoparticles: theory, strategies and computational simulations.

The significant advances in synthesis and functionalization have enabled the preparation of high-quality nanoparticles that have found a plethora of successful applications. The unique physicochemical properties of nanoparticles can be manipulated through the control of size, shape, composition, and surface chemistry, but their technological application possibilities can be further expanded by exploiting the properties that emerge from their assembly. The ability to control the assembly of nanoparticles not only is required for many real technological applications, but allows the combination of the intrinsic properties of nanoparticles and opens the way to the exploitation of their complex interplay, giving access to collective properties. Significant advances and knowledge gained over the past few decades on nanoparticle assembly have made it possible to implement a growing number of strategies for reversible assembly of nanoparticles. In addition to being of interest for basic studies, such advances further broaden the range of applications and the possibility of developing innovative devices using nanoparticles. This review focuses on the reversible assembly of nanoparticles and includes the theoretical aspects related to the concept of reversibility, an up-to-date assessment of the experimental approaches applied to this field and the advanced computational schemes that offer key insights into the assembly mechanisms. We aim to provide readers with a comprehensive guide to address the challenges in assembling reversible nanoparticles and promote their applications.

Structural, dynamical, and electronic properties of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

We provide a microscopic insight, both structural and electronic, into the multifold interactions occurring in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] currently targeted for applications in next-generation low-power electronics and optoelectronic devices. To date, practical applications have remained hampered by the lack of fundamental understanding of the interactions occurring both inside the IL and at the interface with the substrate. Our first principles dynamical simulations provide accurate insights into the nature of bonding and non-bonding interactions, dynamical conformational changes and induced dipole moments, along with their statistical distributions, of this ionic liquid, that have so far not been completely unraveled. The mobilities of the two ionic species are obtained by long-lasting dynamical simulations at finite temperature, allowing simultaneous monitoring and quantification of the isomerization occurring in the IL. Moreover, a thorough analysis of the electronic structure and partial charge distributions characterizing the two components, the cation and anion, allow rationalization of the nature of the electrostatic interactions, hydrogen bonding properties of the two ionic counterparts, and the infra-red and dielectric response of the system, especially in the low frequency range, for the full characterization of the IL.

First-principles thermal transport in amorphous Ge2Sb2Te5 at the nanoscale.

Achieving a precise understanding of nanoscale thermal transport in phase change materials (PCMs), such as Ge2Sb2Te5 (GST), is the key of thermal management in nanoelectronics, photonic and neuromorphic applications using non-volatile memories. By resorting to a first-principles approach to calculate the thermal conductivity of amorphous GST, we found that size effects and heat transport via propagative modes persist well beyond extended range order distances typical of disordered network-forming materials. Values obtained are in quantitative agreement with the experimental data, by revealing a strong size dependence of the thermal conductivity down to the 1.7-10 nm range, fully covering the scale of current PCMs-based devices. In particular, a reduction of thermal conductivity as large as 75% occurs for dimensions lying below 2 nm. These results provide a quantitative description of the thermal properties of amorphous GST at the nanoscale and are expected to underpin the development of PCM-based device applications.

Atomic Structure of Glassy GeTe4 as a Playground to Assess the Performances of Density Functional Schemes Accounting for Dispersion Forces.

The atomic structure of glassy GeTe4 is obtained in the framework of first-principles molecular dynamics (FPMD) by considering five different approaches for the description of the electronic structure within density functional theory (DFT). Among these schemes, one is not corrected by accounting for the dispersion forces and it is based on the BLYP exchange-correlation (XC) functional, while all of the others consider the dispersion forces according to different theoretical strategies. In particular, by maintaining the BLYP expression for the XC functional, two of them (BLYP-D2 and BLYP-D3) exploit the Grimme expressions for the dispersion forces, while the fourth scheme is based on the maximally localized Wannier functions (MLWFs). Finally, we also considered the rVV10 functional constructed to include seamlessly the dispersion part. Our results point out the better performances of BLYP-D3 and MLWF in terms of comparison with experimental data for the total pair correlation functions, with BLYP-D2 and rVV10 being closer to the uncorrected BLYP data. The implications of such findings are discussed by considering the overall limited impact of dispersion forces on the atomic structure of glassy GeTe4.

Thermal resistance of an interfacial molecular layer by first-principles molecular dynamics.

The approach-to-equilibrium molecular dynamics (AEMD) methodology is applied in combination with first-principles molecular dynamics to investigate the thermal transfer between two silicon blocks connected by a molecular layer. Our configuration consists of alkanes molecules strongly coupled to the silicon surfaces via covalent bonds. In phase 1 of AEMD, the two Si blocks are thermalized at high and low temperatures to form the hot and cold reservoirs. During phase 2 of AEMD, a transfer between reservoirs occurs until thermal equilibrium is reached. The transfer across the interface dominates the transient over heat conduction within the reservoirs. The value of the thermal interface conductance is in agreement with the experimental data obtained for analogous bonding cases between molecules and reservoirs. The dependence on the length of the thermal interface resistance features two contributions. One is constant (the resistance at the silicon/molecule interface), while the other varies linearly with the length of the molecular chains (diffusive transport). The corresponding value of the thermal conductivity agrees well with experiments.

First-Principles Study of Dissociation Processes for the Synthesis of Fe and Co Oxide Nanoparticles.

Thermal decomposition is a practical and reliable tool to synthesize nanoparticles with monodisperse size distribution and reproducible accuracy. The nature of the precursor molecules and their interaction with the environment during the synthesis process have a direct impact on the resulting nanoparticles. Our study focuses on widely used transition-metal (Co, Fe) stearates precursors and their thermal decomposition reaction pathway. We show how the nature of the metal and the presence or absence of water molecules, directly related to the humidity conditions during the synthesis process, affect the decomposition mechanism and the resulting transition-metal oxide building blocks. This, in turn, has a direct effect on the physical and chemical properties of the produced nanoparticles and deeply influences their composition and morphology.

Impact of dispersion forces on the atomic structure of a prototypical network-forming disordered system: The case of liquid GeSe2.

A set of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van der Waals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localized Wannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes.

The role of 2D/3D spin-polarization interactions in hybrid copper hydroxide acetate: new insights from first-principles molecular dynamics.

The magnetic properties response of the layered hybrid material copper hydroxide acetate Cu2(OH)3CH3COO·H2O is studied as a function of the applied pressure within first-principles molecular dynamics. We are able to elucidate the interplay between the structural properties of this material and its magnetic character, both at the local (atomic) level and at the bulk level. We performed a detailed analysis of the intralayer spin configurations occurring for each value of the imposed projection along the z-axis for the total spin and of the applied pressure. The transition from an antiferromagnetic to a ferromagnetic state at high pressure (above 3 GPa) goes along with a vanishing difference between the spin polarizations pertaining to each layer. Therefore, at high pressure, copper hydroxide acetate is a ferromagnet with no changes of spin polarization in the direction perpendicular to the inorganic layers.

Thermal conductivity of glassy GeTe4 by first-principles molecular dynamics.

A transient thermal regime is achieved in glassy GeTe4 by first-principles molecular dynamics following the recently proposed "approach-to-equilibrium" methodology. The temporal and spatial evolution of the temperature do comply with the time-dependent solution of the heat equation. We demonstrate that the time scales required to create the hot and the cold parts of the system and observe the resulting approach to equilibrium are accessible to first-principles molecular dynamics. Such a strategy provides the thermal conductivity from the characteristic decay time. We rationalize in detail the impact on the thermal conductivity of the initial temperature difference, the equilibration duration, and the main simulation features.

Nanoporous chalcogenides for adsorption and gas separation.

The adsorption and gas separation properties of amorphous porous chalcogenides such as GeS2 are investigated using statistical mechanics molecular simulation. Using a realistic molecular model of such amorphous adsorbents, we show that they can be used efficiently to separate different gases relevant to environmental and energy applications (H2, CO2, CH4, N2). In addition to shedding light on the microscopic adsorption mechanisms, we show that coadsorption in this novel class of porous materials can be described using the ideal adsorbed solution theory (IAST). Such a simple thermodynamic model, which allows avoiding complex coadsorption measurements, describes the adsorption of mixture from pure component adsorption isotherms. Our results, which are found to be in good agreement with available experimental data, paves the way for the design of gas separation membranes using the large family of porous chalcogenides.

Origin of structural analogies and differences between the atomic structures of GeSe4 and GeS4 glasses: A first principles study.

First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe4 and GeS4. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge-Se-Se connections are more frequent than the corresponding Ge-S-S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS4 is rationalized in terms of a higher number of large size rings, accounting for extended Ge-Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge-S bonds when compared to Ge-Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.

Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides.

Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric-isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the surface-to-volume ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic-inorganic solids consisting of ionic liquids confined in porous chalcogenides.

Immobilization of monolayer protected lipophilic gold nanorods on a glass surface.

We present a novel process of immobilization of gold nanorods (GNRs) on a glass surface. We demonstrate that by exploiting monolayer protection of the GNRs, their unusual optical properties can be completely preserved. UV-visible spectroscopy and atomic force microscopy analysis are used to reveal the optical and morphological properties of monolayer protected immobilized lipophilic GNRs, and molecular dynamics simulations are used to elucidate their surface molecule arrangements.

Double phase transfer of gold nanorods for surface functionalization and entrapment into PEG-based nanocarriers.

A novel double phase transfer process was achieved to develop PEG-based targetable nanostructures containing gold nanorods: in the first phase transfer, lipophilic free-CTAB gold nanorods have been obtained, by a straightforward one-step ligand exchange reaction in hydro-alcoholic mixture with thiols, then a second phase transfer was performed to entrap them into PEG-based polymeric nanoparticles.