Amr M. S. Mohammed

Enhanced Thermoelectric Properties in Bulk Nanowire Heterostructure-Based Nanocomposites through Minority Carrier Blocking

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Composition Modulation of Ag2Te Nanowires for Tunable Electrical and Thermal Properties

In this article, we demonstrated that composition modulation of Ag2Te nanowires can be achieved during the self-templated transformation of Te nanowires into Ag2Te nanowires during solution phase synthesis, which provides a mean to tune the carrier density of the Ag2Te nanowires. Both nearly stoichiometric and Ag-rich nanowires have been synthesized, which give rise to p-type and n-type Ag2Te nanocomposites after hot press, respectively. The electrical and thermal properties of the two kinds of samples have been measured. Theoretical modeling based on the near-equilibrium Boltzmann transport equations has been used to understand the experimental results. We found that ZT of the heavily doped n-type sample reaches 0.55 at 400 K, which is the highest ZT value reported for Ag2Te at the same temperature mainly due to the reduced thermal conductivity by the nanostructures. Theoretical analysis on the carrier transport shows that the power factor is also very well optimized in the doped Ag2Te sample considering the reduced carrier mobility by the nanostructures.

Superdiffusive heat conduction in semiconductor alloys. II. Truncated Lévy formalism for experimental analysis

evy dynamics with fractal dimension α< 2. Here, we present a framework that enables full three-dimensional experimental analysis by retaining all essential physics of the quasiballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure L´ evy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any “effective” thermal parameters and without any ap rioriknowledge of microscopic phonon scattering mechanisms. Identified α values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal L´ evy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces.

Thermoelectric properties of solution-synthesized n-type Bi2Te3 nanocomposites modulated by Se: An experimental and theoretical study

We report the investigation of the thermoelectric properties of large-scale solution-synthesized Bi2Te3 nanocomposites prepared from nanowires hotpressed into bulk pellets. A third element, Se, is introduced to tune the carrier concentration of the nanocomposites. Due to the Se doping, the thermoelectric figure of merit (ZT) of the nanocomposites is significantly enhanced due to the increased power factor and reduced thermal conductivity. We also find that thermal transport in our hot-pressed pellets is anisotropic, which results in different thermal conductivities along the in-plane and cross-plane directions. Theoretical calculations for both electronic and thermal transport are carried out to establish fundamental understanding of the material system and provide directions for further ZT optimization with adjustments to carrier concentration and mobility.

Super-Joule heating in graphene and silver nanowire network

Transistors, sensors, and transparent conductors based on randomly assembled nanowire networks rely on multi-component percolation for unique and distinctive applications in flexible electronics, biochemical sensing, and solar cells. While conduction models for 1-D and 1-D/2-D networks have been developed, typically assuming linear electronic transport and self-heating, the model has not been validated by direct high-resolution characterization of coupled electronic pathways and thermal response. In this letter, we show the occurrence of nonlinear “super-Joule” self-heating at the transport bottlenecks in networks of silver nanowires and silver nanowire/single layer graphene hybrid using high resolution thermoreflectance (TR) imaging. TR images at the microscopic self-heating hotspots within nanowire network and nanowire/graphene hybrid network devices with submicron spatial resolution are used to infer electrical current pathways. The results encourage a fundamental reevaluation of transport models for network-based percolating conductors.

Fractal Lévy Heat Transport in Nanoparticle Embedded Semiconductor Alloys

Materials with embedded nanoparticles are of considerable interest for thermoelectric applications. Here, we experimentally characterize the effect of nanoparticles on the recently discovered Lévy phonon transport in semiconductor alloys. The fractal space dimension α ≈ 1.55 of quasiballistic (superdiffusive) heat conduction in (ErAs)x:InGaAlAs is virtually independent of the Er content 0.001 < x < 0.1 but instead controlled by alloy scattering of the host matrix. The increased nanoparticle concentration does reduce the diffusive recovery length by an order of magnitude. The bulk conductivity drops by 3-fold, in close agreement with a Callaway model. Our results may provide helpful hints toward engineering superdiffusive heat transport similar to what has been achieved with light in Lévy glasses.

Quasi-ballistic thermal transport in Al0.1Ga0.9N thin film semiconductors

We investigate thermal transport in high-quality Al0.1Ga0.9N thin films grown using plasma-assisted molecular beam epitaxy by time-domain thermoreflectance (TDTR) in the 100 K–500 K temperature range. The apparent thermal conductivity at 300 K and 500 K drops by 30% when the laser modulation frequency is increased from 0.8 MHz to 10 MHz. Tempered Levy analysis of the quasi-ballistic heat conduction reveals superdiffusion exponents α ≈ 1.70 ± 0.06 at room temperature and α ≈ 1.83 ± 0.16 at 500 K. We describe limitations in concurrent extraction of other model parameters and also discuss the impact of boundary scattering in the 100 K–200 K temperature range.

Evidence of Universal Temperature Scaling in Self-Heated Percolating Networks

During routine operation, electrically percolating nanocomposites are subjected to high voltages, leading to spatially heterogeneous current distribution. The heterogeneity implies localized self-heating that may (self-consistently) reroute the percolation pathways and even irreversibly damage the material. In the absence of experiments that can spatially resolve the current distribution and a nonlinear percolation model suitable to interpret them, one relies on empirical rules and safety factors to engineer these materials. In this paper, we use ultrahigh resolution thermo-reflectance imaging, coupled with a new imaging processing technique, to map the spatial distribution ΔT(x, y; I) and histogram f(ΔT) of temperature rise due to self-heating in two types of 2D networks (percolating and copercolating). Remarkably, we find that the self-heating can be described by a simple two-parameter Weibull distribution, even under voltages high enough to reconfigure the percolation pathways. Given the generality of the phenomenological argument supporting the distribution, other percolating networks are likely to show similar stress distribution in response to sufficiently large stimuli. Furthermore, the spatial evolution of the self-heating of network was investigated by analyzing the spatial distribution and spatial correlation, respectively. An estimation of degree of hotspot clustering reveals a mechanism analogous to crystallization physics. The results should encourage nonlinear generalization of percolation models necessary for predictive engineering of nanocomposite materials.

High temporal & spatial resolution imaging of catastrophic & soft breakdown in self-assembled nanodielectrics (SANDs) films

Breakdown processes in polymer dielectrics pose a limitation to energy storage and high-power operation of many electronic devices. These processes have been studied for many decades, and a great deal of progress has been made in developing phenomenological models [1]. Nonetheless, many open questions remain, particularly regarding the development of a single, unified theory to describe all stages of the breakdown process, from initiation, to sub-critical filament advance, to catastrophic breakdown and/or “soft breakdown” [2]. To develop and validate such a detailed understanding, it is essential to first develop high-speed, spatially-resolved in situ characterization techniques. Here we demonstrate that thermoreflectance imaging [3-5] can provide such a capability for voltage stress-induced defects in self-assembled polymer nanodielectrics (SANDs) films [6]. Temporal evolution of self-heating and material blister formation for soft and catastrophic failure sites in SANDs films are imaged at submicron spatial resolution. This result shows that thermoreflectance characterization techniques provide insight into the breakdown processes determining polymer dielectric performance and reliability.