The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. ...Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our “embedded-ZnO nanowire structure” having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.
Organic material-based thermal switch is drawing much attention as one of the key thermal management devices in organic electronic devices. This study aims at tuning the switching temperature (T S) ...of thermal conductivity by using liquid crystalline block copolymers (BCs) with different order–order transition temperature (T tr) related to the types of mesogens in the side chain. The BC films with low T tr of 363 K and high T tr of 395 K exhibit reversible thermal conductivity switching behaviors at T S of ∼360 K and ∼390 K, respectively. The BC films also exhibit thermal conductivity variation originating from the anisotropy of the internal structures: poly(ethylene oxide) domains and liquid crystals. These results demonstrate that the switching behavior is attributed to an order–order transition between BC films with vertically arranged cylinder domains and the ones with ordered sphere domains. This highlights that BCs become a promising thermal conductivity switching material with tailored T S.
We investigated thermal conductivity of epitaxial germanane films: stacked structure of hydrogenated germanenes. It was confirmed that single crystalline germanane films were epitaxially grown on ...Ge(111). The films exhibited low out-of-plane thermal conductivity of 1.1 0.3 W m−1 K−1 which is lower than other layered materials composed of heavy atoms. This came from weak van der Waals interlayer interaction related to weak polarization in germanane composed of smaller atoms. This demonstrates that choice of small constituent atoms for weakening van der Waals interlayer interaction is a promising thermal conductivity reduction outline for developing ecofriendly high performance thermoelectric layered materials.
A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to ...strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained, not to give strain to the above Si layers. Therein, a drastic thermal conductivity reduction occurs by larger phonon scattering at the interfaces with the large atomic size difference between Si layers and Ge segregation layers, while unstrained Si layers preserve a high conduction band degeneracy leading to a high Seebeck coefficient. As a result, the n-type Si
Ge
/Si SL with controlled interfaces achieves a higher power factor of ∼25 μW cm
K
in the in-plane direction at room temperature, which is superior to ever reported SiGe-based films: SiGe-based SLs and SiGe films. The Si
Ge
/Si SL with controlled interfaces also exhibits a low thermal conductivity of ∼2.5 W m
K
in the cross-plane direction, which is ∼5 times lower than the reported value in a conventional Si
Ge
/Si SL. These results demonstrate that strain and atomic differences controlled by ultrathin layers can bring a breakthrough for realizing high-performance light-element-based thermoelectric films.
Toward drastic enhancement of thermoelectric power factor, quantum confinement effect proposed by Hicks and Dresselhaus has intrigued a lot of researchers. There has been much effort to increase ...power factor using step-like density-of-states in two-dimensional electron gas (2DEG) system. Here, we pay attention to another effect caused by confining electrons spatially along one-dimensional direction: multiplied 2DEG effect, where multiple discrete subbands contribute to electrical conduction, resulting in high Seebeck coefficient. The power factor of multiple 2DEG in GaAs reaches the ultrahigh value of ~100 μWcm
K
at 300 K. We evaluate the enhancement rate defined as power factor of 2DEG divided by that of three-dimensional bulk. The experimental enhancement rate relative to the theoretical one of conventional 2DEG reaches anomalously high (~4) in multiple 2DEG compared with those in various conventional 2DEG systems (~1). This proposed methodology for power factor enhancement opens the next era of thermoelectric research.
ZnO, a wide bandgap (3.3 eV) semiconductor has been expected to be a transparent thermoelectric material for the purpose of energy harvesting application because it is a low-cost and ubiquitous ...element material with a high optical transmittance and a high power factor. Bulk Ga-doped ZnO (GZO) is expected to have higher electrical conductivity and lower thermal conductivity than bulk Al-doped ZnO. However, because reports on their thermoelectric properties of GZO films have been scarce up to now, it has been unclear what film characters affect the thermoelectric properties effectively. In this work, GZO thin films with different characters (c-axis orientation, crystal domains and the domain interfaces, and carrier activation rate) were fabricated by two different methods, sol-gel method and pulsed laser deposition. All samples have optical transmittance over 80% in visible range. The highly-oriented GZO films exhibit the highest power factors up to 2.8 μWcm−1 K−2 in the reported GZO materials and low thermal conductivities of 8.4 Wm−1 K−1 (1/4 as high as that of bulk GZO). This enhanced thermoelectric performance is attributed to the high carrier activation rate, and the interfaces of highly-oriented crystal domains with the small carrier scattering effect.
•We studied transparent thermoelectric Ga-doped ZnO films by PLD and sol-gel method.•Both GZO films had highly (0001)-oriented nanometric crystal domains.•They have higher power factors and lower thermal conductivities than GZO bulks.
For realization of new informative systems, the memristor working like synapse has drawn much attention. We developed isolated high-density Fe
3
O
4
nanocrystals on Ge nuclei/Si with uniform and high ...resistive switching performance using low-temperature growth. The Fe
3
O
4
nanocrystals on Ge nuclei had a well-controlled interface (Fe
3
O
4
/GeO
x
/Ge) composed of high-crystallinity Fe
3
O
4
and high-quality GeO
x
layers. The nanocrystals showed uniform resistive switching characteristics (high switching probability of ~90%) and relatively high Off/On resistance ratio (~58). The high-quality interface enables electric field application to Fe
3
O
4
and GeO
x
near the interface, which leads to effective positively charged oxygen vacancy movement, resulting in high-performance resistive switching. Furthermore, we successfully observed memory effect in nanocrystals with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall nanocrystals is significant for realizing non-volatile nanocrystal memory leading to neuromorphic devices.
Layered silicene with deformed buckled structure attracts great interest as a next generation 2D Dirac thermoelectric material beyond conventional layered materials. However, the difficulty of ...modulating atomic positions in silicene prevents its realization. This study proposes a method to deform buckled structure in layered silicene by controlling the intercalated atoms, which can dramatically enhance its thermoelectric properties. Silicene buckled structure is deformed in epitaxial CaSi2 thin films, Ca‐intercalated layered silicenes, on Si(111) substrates, which is related to the composition of intercalated Ca. Therein, buckling height of silicene is changed. This CaSi2 film with deformed silicene exhibits not only metal‐like electrical conductivity but also three times larger Seebeck coefficient than the theoretically predicted value, resulting in ≈3000 times larger power factor (≈40 μW cm−1 K−2) than that of the reported CaSi2 film at room temperature. This result experimentally demonstrates that power factor can be greatly enhanced by deforming the silicene buckled structure.
Calcium‐intercalated layered silicene, CaSi2, is epitaxially grown on Si substrate. The silicene buckled structure is deformed through the control of the compositional ratio of Ca and Si. This deformation of silicene buckled structure can cause band convergence, enhancing Seebeck coefficient. As a result, thermoelectric power factor becomes ≈3000 times larger than that of the reported polycrystalline CaSi2 film.
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