Improving the thermoelectric Seebeck coefficient, while simultaneously reducing thermal conductivity, is required in order to boost thermoelectric (TE) figure of merit (ZT). A common approach to improve the Seebeck coefficient is electron filtering where ‘cold’ (low energy) electrons are restricted from participating in transport by an energy barrier (Kim and Lundstrom 2011 J. Appl. Phys. 110 034511, Zide et al 2010 J. Appl. Phys. 108 123702). However, the impact of electron tunneling through thin barriers and resonant states on TE properties has been given less attention, despite the widespread use of quantum wells and superlattices (SLs) in TE applications. In our work, we develop a comprehensive transport model using the Wigner–Rode formalism. We include the full electronic bandstructure and all the relevant scattering mechanisms, allowing us to simulate both energy relaxation and quantum effects from periodic potential barriers. We study the impact of barrier shape on TE performance and find that tall, sharp barriers with small period lengths lead to the largest increase in both Seebeck coefficient and conductivity, thus boosting power factor and TE efficiency. Our findings are robust against additional elastic scattering such as atomic-scale roughness at side-walls of SL nanowires.

This article discusses design, prototype development and an experimental study of facade-integrated thermoelectric (TE) materials. TEs are smart materials that have the ability to produce a temperature gradient w hen electricity is applied, exploiting the Peltier effect, or to generate a voltage w hen exposed to a temperature gradient, utilizing the Seebeck effect. TEs can be used for heating, cooling, or pow er generation. In this research, heating and cooling potentials of these novel systems w ere explored. Initially, tw o low fidelity prototypes w ere designed and constructed, w here one prototype w as used to study integration of TE modules (TEM) as stand-alone elements in the facade, and one prototype w as used to explore integration of TEMs and heat sinks in facade assemblies. Both prototypes w ere tested, in ambient conditions and w ithin a thermal chamber. The thermal chamber w as used to represent four different exterior environmental conditions (0°F, 30°F, 60°F and 90°F), w hile the interior conditions w ere kept constant at room temperature. The supplied voltage to facade-integrated TEMs varied from 1 to 8 V. Temperature outputs of TEMs for all investigated thermal conditions w ere measured using thermal imaging, w hich are discussed in detail in this article. The results indicate that w hile stand-alone facade-integrated TEMs are not stable, addition of heat sinks improves their performance drastically. Facade-integrated TEMs w ith heatsinks show ed that they w ould operate w ell in heating and cooling modes under varying exterior environmental conditions.

This article discusses the design, prototype development and an experimental study of facade-integrated thermoelectric (TE) smart materials. TEs are semiconductors that have the ability to produce a temperature gradient when electricity is applied, exploiting the Peltier effect, or to generate a voltage when exposed to a temperature gradient, utilizing the Seebeck effect. TEs can be used for heating, cooling, or power generation. In this research, heating and cooling applications of these novel systems were explored. Initially, we designed and constructed two prototypes, where one prototype was used to study integration of TE modules (TEM) as stand-alone elements in the facade, and one prototype was used to explore integration of TEMs and heat sinks in facade assemblies. We tested both prototypes, where a thermal chamber was used to represent four different exterior environmental conditions (0°F, 30°F, 60°F and 90°F). The interior ambient conditions were kept constant at room temperature. The supplied voltage to facade-integrated TEMs varied from 1 to 8 V. We measured temperature outputs of TEMs for all investigated thermal conditions using thermal imaging, which are discussed in detail in this article. The results indicate that while stand-alone facade-integrated TEMs are not stable, addition of heat sinks improves their performance drastically. Facade-integrated TEMs with heatsinks showed that they would operate well in heating and cooling modes under varying exterior environmental

In many device architectures based on 2D materials, a major part of the heat generated in hot‐spots dissipates in the through‐plane direction where the interfacial thermal resistances can significantly restrain the heat removal capability of the device. Despite its importance, there is an enormous (1–2 orders of magnitude) disagreement in the literature on the interfacial thermal transport characteristics of MoS2 and other transition metal dichalcogenides (TMDs) (0.1–14 MW m−2 K−1). In this report, the thermal boundary conductance (TBC) across MoS2 and graphene monolayers with SiO2/Si and sapphire substrates is systematically investigated using a custom‐made electrical thermometry platform followed by 3D finite element analyses. Through comparative experiments, the TBC at 295 K across MoS2 is found to be 20.3–33.5 MW m−2 K−1 on SiO2/Si, and 19–37.5 MW m−2 K−1 on c‐sapphire, respectively, but far larger than the previous Raman‐based measurements on TMDs with optical heating (0.1–2 MW m−2 K−1). This study also investigates the effects of processing quality and potential interface contaminants, substrate properties, and encapsulation on TBC across MoS2 and graphene monolayers. Our results reveal that the emergence of Rayleigh wave modes dramatically contributes to the interfacial conductance across encapsulated 2D monolayers. This finding opens up an additional pathway to improve heat dissipation in 2D‐based devices through engineering of an encapsulating layer.

The steady-state behavior of thermal transport in bulk and nanostructured semiconductors has been widely studied, both theoretically [4] and experimentally [1], with an intense focus on 2-dimensional materials such as graphene and graphene nanoribbons (GNRs) in recent years. The effect of ribbon size (width and length) and temperature on steady-state thermal conductivity is now well understood. On the other hand, fast transients and frequency response of thermal conduction, sometimes called dynamical thermal conductivity has been given less attention. The response of thermal conductivity to rapidly varying heat sources may become more crucial in the future, especially with the constant growth in the clock frequencies in microprocessors and increase in giga- and terahertz applications of semiconductor devices. It has been theoretically predicted in 3-D materials that thermal conductivity in response to a time-varying temperature gradient starts decaying when the frequency of the applied heat source (Ω) exceeds a certain cut-off frequency üc, which was found to be related to the inverse of the average phonon relaxation time TC. The phonon relaxation time in bulk semiconductors such as silicon is short, on the order of 2–10 ps, leading to thermal conductivity that is independent of frequency up to very high iic exceeding 10 GHz. In contrast, 2-D materials like graphene have much longer phonon relaxation times, especially below room temperature. Therefore, in suspended graphene and wide graphene ribbons, Ω c can be expected to be much lower than that of silicon. Moreover, the presence of strong momentum-conserving normal phonon-phonon processes, overshadowing the momentum-destroying umklapp processes in graphene results in hydrodynamic transport [2] where heat does not diffuse but rather propagates in a wavelike fashion, giving rise to the second sound phenomenon[7].

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all-2D electronic circuitry. This study employs seed-free consecutive chemical vapor deposition processes to synthesize high-quality lateral MoS2 -graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS2 -graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS2 -metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS2 -graphene in-plane heterostructures are promising materials for the scale-up of all-2D circuitry with superlative electrical performance.

The ability to grow heterostructures with high-quality interfaces brings great flexibility to the design and development of modern electronic and optoelectronic devices. While nearly perfect from an electronic standpoint, these interfaces are exceedingly disruptive to thermal transport and are a major contributor to anisotropic heat conduction and localized heating. Doping, alloying, and strain — all commonly employed when tailoring the electronic and optical properties of heterostructures — are also highly detrimental to the transport of phonons, the dominant carriers of heat in semiconductors. From the theoretical standpoint of phonon dynamics in disordered systems, we discuss the present understanding of nanoscale thermal transport and its profound sensitivity to any deviation from single-crystallinity. The roles that boundaries, interfaces, point defects, and strain play in thermal transport and localized heating are illustrated on several examples of semiconductor nanostructures, such as nanowires, thin films, superlattices, and quantum cascade lasers.

Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant Ka, but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.

As dimensions of nanoelectronic devices become smaller, reaching a few nanometers in modern processors, CPU hot spots become increasingly more difficult to manage. Applying mechanical strain in nanostructures provides an additional tuning mechanism for both electronic band structures and phonon dispersions that is independent of other methods such as alloying and dimensional confinement. By breaking crystal symmetry, strain increases anisotropy. We present thermal conductivity calculations, performed in thin Si and Ge strained films, using first principles calculations of vibrational frequencies under biaxial strain, along with a phonon Boltzmann transport equation within the relaxation time approximation. We find that, while in-plane transport is not strongly dependent on strain, the cross-plane component of the thermal conductivity tensor shows a clear strain dependence, with up to 20% increase (decrease) at 4% compressive (tensile) strain in both Si and Ge. We also uncover that strain emphasizes the an...