Transport In Nanostructured Materials Research Articles

Temperature dependences of thermal conductivity of solid heterogeneous crystalline and amorphous materials: An empirical approach to the description in the high-temperature region

This paper presents a detailed analysis of the thermal conductivity behaviors exhibited by a diverse array of nanostructured materials, ranging from multilayer graphene nanocomposites to semiconductor-based nanostructures such as Bi0.5Sb1.5Te3 and In0.53Ga0.47As composites. The investigation extends to superlattices, nanowires, and hybrid nanostructures, encompassing materials like hexagonal boron nitride flakes, iron oxide nanoporous films, and organic-inorganic hybrid materials. The thermal conductivity of these materials is characterized by distinct trends, with some showcasing crystal-like behavior and others demonstrating glass-like characteristics. The analysis employs empirical expressions to discern the contributions of phonons and diffusons in crystal-like materials and incorporates Peierls contributions and Arrhenius-type terms for glass-like behavior. Noteworthy observations include deviations in fitting certain materials at lower temperatures and the identification of negative diffuson contributions in specific cases. These findings contribute to a nuanced understanding of thermal transport in nanostructured materials and have implications for applications in advanced thermal management systems and thermoelectric devices. The extracted parameters provide valuable insights for researchers exploring the thermal conductivity of diverse nanostructured materials.

Features of phonon scattering by a spherical pore: Molecular dynamics insight

There is still a gap in understanding phonon scattering by geometrical defects at the nanoscale, and it remains a significant challenge for heat transfer management in nanoscale devices and systems. In this study, we aim to explore the characteristics of phonon scattering by a single pore to gain insights into thermal transport in nanostructures. The paper outlines a methodology for assessing the spatial distribution of the magnitude of the radial, azimuthal, and polar components of the velocity of scattered phonons by a spherical pore. We demonstrated that the size parameter, commonly employed in electromagnetic wave scattering theory, is vital in determining the scattering regime. Specifically, we show that the calculated scattering efficiency has the same pattern as that commonly obtained in classical wave scattering theory. However, we found that crystallographic directions are pivotal in shaping the scattering patterns, especially in the regions where scattering patterns are defined by the Mie resonances. This observation holds significance in understanding the influence of phonon coherence on thermal transport in nanostructured materials.

Computationally efficient Monte Carlo electron transport algorithm for nanostructured thermoelectric material configurations

Monte Carlo statistical ray-tracing methods are commonly employed to simulate carrier transport in nanostructured materials. In the case of a large degree of nanostructuring and under linear response (small driving fields), these simulations tend to be computationally overly expensive due to the difficulty in gathering the required flux statistics. Here, we present a novel Monte Carlo ray-tracing algorithm with computational efficiency of at least an order of magnitude compared to existing algorithms. Our new method, which is a hybrid of the analytical Boltzmann transport equation and Monte Carlo used a reduced number of ray-tracing particles, avoids current statistical challenges, such as the subtraction of two opposite going fluxes, the application of a driving force altogether, and the large simulation time required for low-energy carriers. We demonstrate the algorithm’s efficiency and power in accurate simulations in large domain nanostructures with multiple defects. We believe that the new method we present is indeed more robust and user friendly compared to common methods and can enable the efficient study of transport in nanostructured materials under low-field steady-state conditions.

Effects of Different Factors on the Heat Conduction Properties of Carbon Films and Fibers

The increasing popularity of carbon nanotubes has created a demand for greater scientific understanding of the characteristics of thermal transport in nanostructured materials. However, the effects of impurities, misalignments, and structure factors on the thermal conductivity of carbon nanotube films and fibers are still poorly understood. Carbon nanotube films and fibers were produced, and the parallel thermal conductance technique was employed to determine the thermal conductivity. The effects of carbon nanotube structure, purity, and alignment on the thermal conductivity of carbon films and fibers were investigated to understand the characteristics of thermal transport in the nanostructured material. The importance of bulk density and cross-sectional area was determined experimentally. The results indicated that the prepared carbon nanotube films and fibers are very efficient at conducting heat. The structure, purity, and alignment of carbon nanotubes play a fundamentally important role in determining the heat conduction properties of carbon films and fibers. Single-walled carbon nanotube films and fibers generally have high thermal conductivity. The presence of non-carbonaceous impurities degrades the thermal performance due to the low degree of bundle contact. The thermal conductivity may present power law dependence with temperature. The specific thermal conductivity decreases with increasing bulk density. At room temperature, a maximum specific thermal conductivity is obtained but Umklapp scattering occurs. The specific thermal conductivity of carbon nanotube fibers is significantly higher than that of carbon nanotube films due to the increased degree of bundle alignment.

Karbon nanotüplerin ve fiberlerin ısı iletim özellikleri üzerine yapı, saflık ve hizalamanın etkileri

The increasing popularity of carbon nanotubes has created a demand for greater scientific understanding of the characteristics of thermal transport in nanostructured materials. However, the effects of impurities, misalignments, and structure factors on the thermal conductivity of carbon nanotube films and fibers are still poorly understood. Carbon nanotube films and fibers were produced, and the parallel thermal conductance technique was employed to determine the thermal conductivity. The effects of carbon nanotube structure, purity, and alignment on the thermal conductivity of carbon films and fibers were investigated to understand the characteristics of thermal transport in the nanostructured material. The importance of bulk density and cross-sectional area was determined experimentally. The results indicated that the prepared carbon nanotube films and fibers are very efficient at conducting heat. The structure, purity, and alignment of carbon nanotubes play a fundamentally important role in determining the heat conduction properties of carbon films and fibers. Single-walled carbon nanotube films and fibers generally have high thermal conductivity. The presence of non-carbonaceous impurities degrades the thermal performance due to the low degree of bundle contact. The thermal conductivity may present power law dependence with temperature. The specific thermal conductivity decreases with increasing bulk density. At room temperature, a maximum specific thermal conductivity is obtained but Umklapp scattering occurs. The specific thermal conductivity of carbon nanotube fibers is significantly higher than that of carbon nanotube films due to the increased degree of bundle alignment.

Effects of Carbon Nanotube Structure, Purity, and Alignment on the Heat Conduction Properties of Carbon Films and Fibers

The increasing popularity of carbon nanotubes has created a demand for greater scientific understanding of the characteristics of thermal transport in nanostructured materials. However, the effects of impurities, misalignments, and structure factors on the thermal conductivity of carbon nanotube films and fibers are still poorly understood. Carbon nanotube films and fibers were produced, and the parallel thermal conductance technique was employed to determine the thermal conductivity. The effects of carbon nanotube structure, purity, and alignment on the thermal conductivity of carbon films and fibers were investigated to understand the characteristics of thermal transport in the nanostructured material. The importance of bulk density and cross-sectional area was determined experimentally. The results indicated that the prepared carbon nanotube films and fibers are very efficient at conducting heat. The structure, purity, and alignment of carbon nanotubes play a fundamentally important role in determining the heat conduction properties of carbon films and fibers. Single-walled carbon nanotube films and fibers generally have high thermal conductivity. The presence of non-carbonaceous impurities degrades the thermal performance due to the low degree of bundle contact. The thermal conductivity may present power law dependence with temperature. The specific thermal conductivity decreases with increasing bulk density. At room temperature, a maximum specific thermal conductivity is obtained but Umklapp scattering occurs. The specific thermal conductivity of carbon nanotube fibers is significantly higher than that of carbon nanotube films due to the increased degree of bundle alignment.

Characterizing carrier transport in nanostructured materials by force-resolved microprobing

The advent of novel nanostructured materials has enabled wearable and 3D electronics. Unfortunately, their characterization represents new challenges that are not encountered in conventional electronic materials, such as limited mechanical strength, complex morphology and variability of properties. We here demonstrate that force-resolved measurements can overcome these issues and open up routes for new applications. First, the contact resistance to 2D materials was found to be sensitively depending on the contact force and, by optimizing this parameter, reliable contacts could be repeatably formed without damage to the fragile material. Moreover, resistance of three-dimensional surfaces could be investigated with high accuracy in spatial position and signal through a force-feedback scheme. This force-feedback approach furthermore permitted large-scale statistical characterization of mobility and doping of 2D materials in a desktop-sized automatic probing system that fits into glove boxes and vacuum enclosures using easily available and low-cost components. Finally, force-sensitive measurements enable characterization of complex electronic properties with high lateral resolution. To illustrate this ability, the spatial variation of a surface’s electrochemical response was investigated by scanning a single electrolyte drop across the sample.

Suppressed-to-enhanced thermal transport in a Fermi-Pasta-Ulam superlattice: Mediation roles of solitons and phonons

Managing thermal transport in nanostructured materials possesses both theoretical and application value in thermoelectric and microelectronics design. Though a suppressed thermal conductivity could be easily achieved through disorder-induced phonon scattering in a superlattice, it is challenging to enhance thermal transport in a periodically designed lattice. In this paper, we show the possibility of mediating thermal conductivity from a suppressed to an enhanced value in a Fermi-Pasta-Ulam β superlattice with periodic cells of arithmetically increased nonlinearity. When the cell length is increased, thermal conductivity in the superlattice crosses over a suppressed region into an enhanced one and it is even higher than in a homogeneous lattice with the same nonlinearity strength. The mediation originates from the long-lived nonlinear wave packets as solitons across the disorder-induced interface between cells of the superlattice, while at the same time the normal vibrational modes as phonons are suppressed. Our result shows a promising strategy to manipulate thermal transport over a wide range in a superlattice with strong nonlinearity.

Effect of wave versus particle phonon nature in thermal transport through nanostructures

Comprehensive understanding of thermal transport in nanostructured materials needs large scale simulations bridging length scales dictated by different physics related to the wave versus particle nature of phonons. Yet, available computational approaches implicitly treat phonons as either just waves or as particles. In this work, using a full wave-based Non-Equilibrium Green's Function (NEGF) method, and a particle-based ray-tracing Monte Carlo (MC) approach, we investigate the qualitative differences in the wave and particle-based phonon transport at the vicinity of nanoscale features. For the simple example of a nanoporous geometry, we show that phonon transmission agrees very well for both methods with an error margin of ±15%, across phonon wavelengths even for features with sizes down to 3–4 nm. For cases where phonons need to squeeze in smaller regions to propagate, we find that MC underestimates the transmission of long wavelength phonons whereas wave treatment within NEGF indicates that those long wavelength phonons can propagate more easily. We also find that particle-based simulation methods are somewhat more sensitive to structural variations compared to the wave-based NEGF method. The insight extracted from comparing wave and particle methods can be used to provide a better and more complete understanding of phonon transport in nanomaterials.

Quantum Phonon Transport in Nanomaterials: Combining Atomistic with Non-Equilibrium Green's Function Techniques.

A crucial goal for increasing thermal energy harvesting will be to progress towards atomistic design strategies for smart nanodevices and nanomaterials. This requires the combination of computationally efficient atomistic methodologies with quantum transport based approaches. Here, we review our recent work on this problem, by presenting selected applications of the PHONON tool to the description of phonon transport in nanostructured materials. The PHONON tool is a module developed as part of the Density-Functional Tight-Binding (DFTB) software platform. We discuss the anisotropic phonon band structure of selected puckered two-dimensional materials, helical and horizontal doping effects in the phonon thermal conductivity of boron nitride-carbon heteronanotubes, phonon filtering in molecular junctions, and a novel computational methodology to investigate time-dependent phonon transport at the atomistic level. These examples illustrate the versatility of our implementation of phonon transport in combination with density functional-based methods to address specific nanoscale functionalities, thus potentially allowing for designing novel thermal devices.

Understanding the mechanism of diffuse phonon scattering at disordered surfaces by atomistic wave-packet investigation

Phonon surface scattering is of great importance for understanding thermal transport in nanostructured materials and has been widely utilized to tailor thermal properties. However, the current understanding of phonon surface scattering is largely based on the Ziman's formula which was derived at the continuum limit of the scalar wave equation and, thus, ignoring the atomistic information of the surface. In this work, we applied the atomistic phonon wave-packet simulations to study the impact of surface amorphization and surface roughness on phonon reflection. We found that for both types of surfaces, the energies of the specularly reflected components decrease with increasing wavelength. However, the underlying reflection mechanisms are different. For the amorphous surface, the energy of the incident wave packet will first enter the amorphous layer before being partially and diffusely scattered in this layer. The local density and elastic moduli fluctuations as well as the localized vibrational modes are reasons for the diffuse scattering in the amorphous region, which affects the short wavelength phonons more severely. For the rough surface, the diffuse reflection is the combined effect from surface irregularity, surface Rayleigh waves, and spatially localized modes induced by the surface roughness. The extent of diffuse reflection at the rough surface can be tuned by engineering the surface boundary condition.

Charge Transport in Nanostructured Materials: Implementation and Verification of Constrained Density Functional Theory.

The in silico design of novel complex materials for energy conversion requires accurate, ab initio simulation of charge transport. In this work, we present an implementation of constrained density functional theory (CDFT) for the calculation of parameters for charge transport in the hopping regime. We verify our implementation against literature results for molecular systems, and we discuss the dependence of results on numerical parameters and the choice of localization potentials. In addition, we compare CDFT results with those of other commonly used methods for simulating charge transport between nanoscale building blocks. We show that some of these methods give unphysical results for thermally disordered configurations, while CDFT proves to be a viable and robust approach.

Thermoelectric properties of Ge/Si heterostructures: A combined theoretical and experimental study

We present a combined experimental and theoretical investigation of the thermoelectric properties of p‐doped Ge/Si superlattices grown on Si(001) substrates by molecular beam epitaxy. Electrical conductivity is measured both in the direction parallel and perpendicular to the interfaces by means of a modified transfer length method. Electronic transport is strongly anisotropic, with the cross‐plane conductivity being about five times lower than in plane. This result is in very good agreement with the theoretical predictions based on the tight‐binding method combined with the Boltzmann equation applied to the experimentally investigated structure. The cross‐plane thermal conductivity of doped superlattices is measured with the differential 3 method and compared with that of undoped superlattices and alloys with similar average Ge content. The comparison reveals that superlattices have strongly reduced thermal conduction compared to alloys, and that doping increases their thermal conductivity by about 50%. Considering the used doping level, this increase appears surprising. The Seebeck coefficient of the structures is addressed theoretically and displays a less pronounced anisotropy compared to the electric conductivity. Combined with the knowledge of the other thermoelectric parameters, we conclude that, while p‐doped Si/Ge superlattices may be used as model systems for the investigation of thermoelectric transport in nanostructured materials, their relevance for application is limited.

Heat Conduction in Nanostructured Materials Predicted by Phonon Bulk Mean Free Path Distribution

We develop a computational framework, based on the Boltzmann transport equation (BTE), with the ability to compute thermal transport in nanostructured materials of any geometry using, as the only input, the bulk cumulative thermal conductivity. The main advantage of our method is twofold. First, while the scattering times and dispersion curves are unknown for most materials, the phonon mean free path (MFP) distribution can be directly obtained by experiments. As a consequence, a wider range of materials can be simulated than with the frequency-dependent (FD) approach. Second, when the MFP distribution is available from theoretical models, our approach allows one to include easily the material dispersion in the calculations without discretizing the phonon frequencies for all polarizations thereby reducing considerably computational effort. Furthermore, after deriving the ballistic and diffusive limits of our model, we develop a multiscale method that couples phonon transport across different scales, enabling efficient simulations of materials with wide phonon MFP distributions length. After validating our model against the FD approach, we apply the method to porous silicon membranes and find good agreement with experiments on mesoscale pores. By enabling the investigation of thermal transport in unexplored nanostructured materials, our method has the potential to advance high-efficiency thermoelectric devices.

Phonon Dynamics at Surfaces and Interfaces and Its Implications in Energy Transport in Nanostructured Materials—An opinion Paper

Nanoscale thermal transport has attracted considerable attention because of both fundamental scientific interest and important engineering applications. For semiconductors and insulators, energy transport is dominated by phonons, whose behavior at surfaces and interfaces plays a significant role in energy transport processes. In this article, we present opinions on phonon dynamics at surfaces and interfaces and the implications on nanoscale thermal transport. The effects of roughness, bonding strength, coherence, and nanoscale constrictions are discussed. The existence of two specularity parameters at an interface (separate values for transmitted and reflected phonons) and the implications of phonon reflection at free surfaces to the thermal conductance of nearby interfaces are two concepts that have not been previously discussed in the literature. We provide some outlook and potential topics for future studies.

Toward phonon-boundary engineering in nanoporous materials

Tuning thermal transport in nanostructured materials is a powerful approach to develop high-efficiency thermoelectric materials. Using a recently developed approach based on the phonon mean free path dependent Boltzmann transport equation, we compute the effective thermal conductivity of nanoporous materials with pores of various shapes and arrangements. We assess the importance of pore-pore distance in suppressing thermal transport, and identify the pore arrangement that minimizes the thermal conductivity, composed of a periodic arrangement of two misaligned rows of triangular pores. Such a configuration yields a reduction in the thermal conductivity of more than 60% with respect the simple circular aligned case with the same porosity.

Enhancing phonon transmission across a Si/Ge interface by atomic roughness: First-principles study with the Green's function method

Knowledge on phonon transmittance as a function of phonon frequency and incidence angle at interfaces is vital for multiscale modeling of heat transport in nanostructured materials. Although thermal conductivity reduction in nanostructured materials can usually be described by phonon scattering due to interface roughness, we show how a Green's function method in conjunction with the Landauer formalism suggests that interface roughness induced by atomic mixing can increase phonon transmission and interfacial thermal conductance. This is an attempt to incorporate first-principles force constants derived from ab initio density-functional theory (DFT) into Green's function calculation for infinitely large three-dimensional crystal structure. We also demonstrate the importance of accurate force constants by comparing the phonon transmission and thermal conductance using force constants obtained from semiempirical Stillinger-Weber potential and first-principles DFT calculations.

Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces

Understanding thermal transport in nanostructured materials is important for the development of energy conversion applications and the thermal management of microelectronic and optoelectronic devices. Most nanostructures interact through van der Waals interactions, and these interactions typically lead to a reduction in thermal transport. Here, we show that the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon. Moreover, the thermal conductivity of the bundle can be switched between the enhanced values and that of a single nanoribbon by wetting the van der Waals interface between the nanoribbons with various solutions.

Molecular dynamics simulations of strain engineering and thermal transport in nanostructured materials

Molecular dynamics simulations of strain engineering and thermal transport in nanostructured materials

Charge transport in nanostructured materials for solar energy conversion studied by time-resolved terahertz spectroscopy

Spectra of far-infrared conductivity contain useful information on charge transport at nanoscopic length scales. However, decrypting the mechanisms and parameters of charge transport from the measured spectra is a complex task in nanostructured systems: in particular, the conductivity is strongly influenced by charge carrier interaction with surfaces or interfaces between constituents of the composite material as well as by local field effects. Here we review our work on transient far-infrared conductivity in polymer:fullerene bulk heterojunctions and in bare and dye-sensitized semiconductor nanoparticles. Measurements performed by time-resolved terahertz spectroscopy are complemented by Monte-Carlo calculations which clearly link the charge transport properties and the terahertz conductivity spectra.