Riccardo Rurali

Materials Science Institute of Barcelona · Theory and Simulation Department

2D materials provide a rapidly expanding platform for the observation of novel physical phenomena and for the realization of cutting-edge optoelectronic devices. In addition to their peculiar individual characteristics, 2D materials can be stacked into complex van der Waals heterostructures, greatly expanding their potential. Moreover, thanks to th...

Enabling on-demand control of heat flow is key for the development of next-generation electronic devices, solid-state heat pumps, and thermal logic. However, precise and agile tuning of the relevant microscopic material parameters for adjusting thermal conductivities remains elusive. Here, we study several single crystals of lanthanum aluminate (La...

The possibility to tune the functional properties of nanomaterials is key to their technological applications. Super7 lattices, i.e., periodic repetitions of two or more materials in different dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer...

Organic thermoelectrics (TE) has attracted a fair amount of attention in recent years due to remarkable advances achieved in terms of the figure of merit, ZT. However, much of the efforts up until now have just been devoted towards the improvement of the power factor, whereas thermal properties have often been overlooked. In order to fill this gap,...

Point defects can be used to tailor the properties of semiconductors, but can also have undesired effects on electronic and thermal transport, particularly in ultrascaled nanostructures, such as nanowires. Here we use all-atom molecular dynamics to study the effect that different concentrations and spatial distributions of vacancies have on the the...

Solid-state cooling applications based on electrocaloric (EC) effects are particularly promising from a technological point of view due to their downsize scalability and natural implementation in circuitry. However, EC effects typically involve materials that contain toxic substances and require relatively large electric fields (∼100-1000 kV cm-1)...

Perovskite oxides offer tremendous potential for applications in information storage and energy conversion, owing to a subtle interplay between their spin, charge, orbital and lattice degrees of freedom. Here, we further expand the possible range of perovskite oxides operation towards the fields of thermal management and thermal computing by exploi...

A density functional theory study of the electronic structure of nanostructures based on the hexagonal layers of LuI3 is reported. Both bulk and slabs with one to three layers exhibit large and indirect bandgaps. Different families of nanotubes can be generated from these layers. Semiconducting nanotubes of two different chiralities have been studi...

Organic materials for thermoelectric (TE) applications have attracted a fair amount of attention in recent years due to remarkable advances achieved in terms of their figure of merit, ZT: a value of 0.42 has been reported by for poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films treated with dimethyl sulfoxide, while 0.25 has...

Solid-state electrolytes (SSEs) with high ion conductivity are pivotal for the development and large-scale adoption of green-energy conversion and storage technologies such as fuel cells, electrocatalysts and solid-state batteries. Yet, SSEs are extremely complex materials for which general rational design principles remain indeterminate. Here, we...

Solid-state electrolytes (SSE) with high ion conductivity are pivotal for the development and large-scale adoption of green-energy conversion and storage technologies such as fuel cells, electrocatalysts and solid-state batteries. Yet, SSE are extremely complex materials for which general rational design principles remain indeterminate. Here, we un...

Crystal phase engineering gives access to new types of periodic nanostructures, such as the so-called twinning superlattices, where the motif of the superlattice is determined by a periodic rotation of the crystal. Here, by means of atomistic nonequilibrium molecular dynamics calculations, we study to what extent these periodic systems can be used...

Strategies for tuning the thermal conductivity of crystals by means of external fields are rare. Here, we predict the existence of large magnetophononic effects in materials that undergo antiferromagnetic (AFM) ↔ ferromagnetic (FM) phase transitions, which allow for the modulation of the lattice heat conductivity, κL, via the application of magneti...

Detecting hazardous and toxic gasses is important to avoid harmful effects on human health and two-dimensional nanostructures have emerged as candidate materials for sensing or scavenging gasses. The chemical interactions between NO, O2, and N2 gas molecules and Cu-, Ag-, and Au-decorated germanene were investigated by using density functional theo...

Crystal phase engineering gives access to new types of superlattices where, rather than different materials, different crystal phases of the same material are juxtaposed. Here, by means of atomistic nonequilibrium molecular dynamics calculations, we study to what extent these periodic systems can be used to alter phonon transport, similarly to what...

Interfacial thermal transport plays a prominent role in the thermal management of nanoscale objects and is of fundamental importance for basic research and nanodevices.[1] At metal/insulator interfaces, a configuration commonly found in electronic devices, heat transport strongly depends upon the effective energy transfer from thermalized electrons...

In this paper we give a general and basic introduction to the most important concepts related with phonon transport. We first revise Fourier’s equation and the diffusive transport regime, focusing on the main scattering mechanisms that yield a finite thermal conductivity in insulators: anharmonic phonon-phonon scattering, impurity scattering, and b...

We compute the thermal conductivity, κ, of five representative III-V ternary alloys - namely In x Ga1 - x As, GaAs1 - x P x , InAs1 - x Sb x , GaAs1 - x N x , and GaP1 - x N x - in the whole range of compositions, and in zincblende and wurtzite crystal phases, using a first-principles approach and solving the phonon Boltzmann transport equation bey...

Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experimental observation was previously restricted to a small number of materials, usually in rather narrow temperature windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying temperature fiel...

Many of graphene’s remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin–orbit coupling. Because of these...

We present here a new interatomic potential parameter set to predict the thermal conductivity of zirconium trisulfide monolayers. The generated Tersoff-type force field is parameterized using data collected with first-principles calculations. We use non-equilibrium molecular dynamics simulations to predict the thermal conductivity. The generated pa...

Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial...

Early detection of diabetes, a worldwide health issue, is key for its successful treatment. Acetone is a marker of diabetes, and efficient, non-invasive detection can be achieved with the use of nanotechnology. In this paper we investigate the effect of acetone adsorption on the electronic properties of silicon nanowires (SiNWs) by means of density...

Recent advances in nanowire synthesis have enabled the realization of crystal phases that in bulk are attainable only under extreme conditions, i.e. high temperature and/or high pressure. For group IV semiconductors this means access to hexagonal-phase SixGe1-x nanostructures (with a 2H type of symmetry), which are predicted to have a direct band g...

We review several fundamental and practical aspects of the theoretical treatment of neutral and charged point defects—i.e., vacancies, interstitials, substitutional impurities, and complexes—in bulk and nanowires (NWs). In particular, we show how a few of the issues preventing the straightforward application of bulk methods to NWs can be partially...

This book covers virtually all aspects of semiconductor nanowires, from growth to related applications, in detail. First, it addresses nanowires’ growth mechanism, one of the most important topics at the forefront of nanowire research. The focus then shifts to surface functionalization: nanowires have a high surface-to-volume ratio and thus are wel...

The formation energies of n- and p-type dopants in III–V arsenide and phosphide semiconductors (GaAs, GaP, and InP) are calculated within a first-principles total energy approach. Our findings indicate that—for all the considered systems—both the solubility and the shallowness of the dopant level depend on the crystal phase of the host material (wu...

At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transp...

The goal of this work is to investigate the influence of mechanical deformation on the electronic and thermoelectric properties of ZrS 3 monolayers. We employ density functional theory (DFT) calculations at the hybrid HSE06 level to evaluate the response of the electronic band gap and mobilities, as well as the thermopower, the electrical conductiv...

Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the f...

Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experimental observation was previously restricted to a small number of materials, usually in rather narrow temperature windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying temperature fiel...

Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density functional theory calculations. We provide the experimental proof of the f...

Recent advances in nanowire synthesis have enabled the realization of crystal phases that in bulk are attainable only under extreme conditions, i.e., high temperature and/or high pressure. For group IV semiconductors this means access to hexagonal-phase Si x Ge1-x nanostructures (with a 2H type of symmetry), which are predicted to have a direct ban...

The goal of this work is to evaluate the effect of mechanical strain on a number of electronic and thermoelectric properties of TiS3 monolayers. We have used density-functional theory (DFT) calculations at the hybrid HSE06 level to evaluate the response of the electronic band gap and mobilities, as well as the thermopower, the electrical conductivi...

We study the thermal conductivity of monolayer, bilayer, and bulk titanium trisulphide (TiS 3 ) by means of an iterative solution of the Boltzmann transport equation based on ab-initio force constants. Our results show that the thermal conductivity of these layers is anisotropic and highlight the importance of enforcing the fundamental symmetries i...

Among the most common few-layers transition metal dichalcogenides (TMDs), WSe2 is the most challenging material from the lattice dynamics point of view. Indeed, for a long time the main two phonon modes (A1g and ) have been wrongly assigned. In the last few years, these two modes have been properly interpreted, and their quasi-degeneracy in the mon...

We compute the thermal conductivity of the most common polymorphs of titanium dioxide (TiO2), rutile and anatase, within a first principles approach and solving the phonon Boltzmann Transport Equation beyond the Relaxation Time Approximation. We find that both polytypes are anisotropic, as expected from their crystal structure, but while κxx=κyy<κz...

We present a theoretical proposal for the design of a thermal switch based on the anisotropy of the thermal conductivity of PbTiO3 and the possibility to rotate the ferroelectric polarization with an external electric field. Our calculations are based on an iterative solution of the phonon Boltzmann Transport Equation and rely on interatomic force...

We demonstrate theoretically how, by imposing epitaxial strain in a ferroelectric perovskite, it is possible to achieve a dynamical control of phonon propagation by means of external electric fields, which yields a giant electrophononic response, i.e. the dependence of the lattice thermal conductivity on external electric fields. Specifically, we s...

We demonstrate theoretically how, by imposing epitaxial strain in a ferroelectric perovskite, it is possible to achieve a dynamical control of phonon propagation by means of external electric fields, which yields a giant electrophononic response, i.e., the dependence of the lattice thermal conductivity on external electric fields. Specifically, we...

Among the most common few-layers transition metal dichalcogenides (TMDs), WSe2 is the most challenging material from the lattice dynamics point of view. Indeed, for a long time the main two phonon modes (A1g and E12g) have been wrongly assigned. In the last few years, these two modes have been properly interpreted, and their quasi-degeneracy in the...

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results of ab-initio...

We present a theoretical proposal for the design of a thermal switch based on the anisotropy of the thermal conductivity of PbTiO3 and of the possibility to rotate the ferroelectric polarization with an external electric field. Our calculations are based on an iterative solution of the phonon Boltzmann Transport Equation and rely on interatomic for...

We calculate the lattice thermal conductivity (κ) for cubic (zinc-blende) and hexagonal (wurtzite) phases for eight semiconductors using ab initio calculations and solving the phonon Boltzmann transport equation, explaining the different behavior of the ratio κhex/κcub between the two phases. We show that this behavior depends on the relative impor...

We combine state-of-the-art Green's-function methods and nonequilibrium molecular dynamics calculations to study phonon transport across the unconventional interfaces that make up crystal-phase and twinning superlattices in nanowires. We focus on two of their most paradigmatic building blocks: cubic (diamond / zincblende) and hexagonal (lonsdaleite...

One of the current challenges in nanoscience is tailoring the phononic properties of a material. This has long been a rather elusive task because several phonons have wavelengths in the nanometer range. Thus, high quality nanostructuring at that length-scale, unavailable until recently, is necessary for engineering the phonon spectrum. Here we repo...

Using a combination of equilibrium classical molecular dynamics (within the Green-Kubo formalism) and the Boltzmann transport equation, we study the effect of strain on the ZnO thermal conductivity focusing in particular on the case of hydrostatic and uniaxial strain. The results show that in the case of hydrostatic strain up to ±4%, we can obtain...

The validity of Fourier’s law in telescopic nanowires is tested by means of molecular dynamics simulations. We observe that the radial dependence of the heat-flux profile, the temperature jump, and the appearance of vorticity obtained in molecular dynamics telescopic wires near the contact point acquire a hydrodynamic character, and we show that th...

We present a theoretical study of the lattice thermal conductivity of SrTiO3 in its antiferrodistortive ferroelastic phase and of its dependence on an applied external electric field, via electrophononic couplings. The calculations are done by using second-principles density-functional theory and the full solution of the Boltzmann transport equatio...

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX$_2$ (M = Mo, W; X = S, Se) from first-principles by solving the Boltzmann Transport Equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the Relaxation Time Approximation (RTA), using the same se...

We calculate the lattice thermal conductivity ($\kappa$) for cubic (zinc-blende) and hexagonal (wurtzite) phases for 8 semiconductors using {\it ab initio} calculations and iteratively solving the Phonon Boltzmann Transport Equation, explaining the different behavior of the ratio $\kappa_{\rm hex}/\kappa_{\rm cub}$ between the two phases. We show t...

We combine first-principles electronic structure calculated thermal conductivity data with a numerical solution of the one-dimensional heat equation to show that an asymmetric distribution of impurity scattering, if suitably designed, yields the conditions for a low-temperature thermal rectification. This happens as a result of the differences in t...

We studied the physics of common p- and n-type dopants in hexagonal-diamond Si –a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions– by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed...

We report on a series of controlled computational experiments based on nonequilibrium molecular dynamics and show that at the nanoscale, the thermal rectification is determined by the thermal boundary resistance, i.e., the thermal resistance of the interface, and cannot be explained without it. In the graphene-bilayer graphene system that we study,...

Efficiently controlling phonon propagation is important in many applications, ranging from the design of thermoelectric materials to thermal budget engineering. Phonon manipulation, nonetheless, has proved to be an elusive task. Here, we review some basic strategies to alter vibrational modes throughout the entire phonon spectrum by means of point...

According to our current understanding, thermal rectification in heterojunctions derives from the different temperature dependence of the thermal conductivity of the constituent materials and from the thermal boundary resistance (TBR). This view, however, neglects the importance of the relation between these two factors, namely, how in nanoscale sy...

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Semiconducting nanowires (NWs) offer the unprecedented opportunity to host different crystal phases in a nanostructure, which enables the formation of polytypic heterostructures where the material composition is unchanged. This characteristic boosts the potential of polytypic heterostructured NWs for optoelectronic and phononic applications. In thi...

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individu...

We critically readdress the definition of thermal boundary resistance at an interface between two semiconductors. By means of atomistic simulations we provide evidence that the widely used Kapitza formalism predicts thermal boundary resistance values in good agreement with the more rigorous Onsager non–equilibrium thermodynamics picture. The latter...

There is now both experimental and theoretical data relating to conductivity changes in porous silicon and other silicon nanostructures arising from the adsorption of specific molecules. The phenomenon is reviewed with emphasis on the potential mechanisms involved and its exploitation with regard sensing applications. © Springer International Publi...

We demonstrate a fully electric control of the heat flux, which can be continuously modulated by an externally applied electric field in PbTiO3, a prototypical ferroelectric perovskite, revealing the mechanisms by which experimentally accessible fields can be used to tune the thermal conductivity by as much as 50% at room temperature.

The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this work we evaluate the thermal boundary resistance (TBR) of 180$^{\circ}$ DWs in prototype ferroelectric perovskite PbTiO$_3$ within the numerical formalisms of nonequilibrium molecular dyna...

Modulating the polarization of a beam of quantum particles is a powerful method to tailor the macroscopic properties of the ensuing energy flux as it directly influences the way in which its quantum constituents interact with other particles, waves or continuum media. Practical polarizers, being well developed for electric and electromagnetic energ...

By means of first-principles calculations, we investigate the thermal properties of silica as it evolves, under hydrostatic compression, from a stishovite phase into a CaCl$_2$-type structure. We compute the thermal conductivity tensor by solving the linearized Boltzmann transport equation iteratively in a wide temperature range, using for this the...

We calculate the thermal conductivity, κ, of the recently synthesized hexagonal diamond (lonsdaleite) Si using first-principles calculations and solving the Boltzmann Transport Equation. We find values of κ which are around 40% lower than in the common cubic diamond polytype of Si. The trend is similar for [111] Si nanowires, with reductions of the...

Recent advances in the synthetic growth of nanowires have given access to crystal phases that in bulk are only observed under extreme pressure conditions. Here, we use first-principles methods based on density functional theory and many-body perturbation theory to show that a suitable mixing of hexagonal Si and hexagonal Ge yields a direct bandgap...

We theoretically study the electrical, thermal and thermoelectric transport properties of graphene nanoribbons under torsional deformations. The modelling follows a nonequilibrium Green's function approach in the ballistic transport regime, describing the electrical and phononic properties through \textit{ab-initio} density functional theory and em...

We present thermal and electron transport calculation in the ballistic regime of a graphene nanoribbon with a nanogap bridged by a stilbene molecular switch. We show that by commuting the stilbene molecule back and forth between its two stable isomers the thermal conductance can be reversibly switched from a low to a high conduction state, providin...

This review focuses on the emerging field of core–multishell (CMS) semiconductor nanowires (NWs). In these kinds of wires, a NW grown vertically on a substrate acts as a template for the coaxial growth of two or more layers wrapped around it. Thanks to the peculiar geometry, the strain is partially released along the radial direction, thus allowing...