— 70°C (1 Hz) in PE (auf S. 245 wird von -20 °C bei derselben Frequenz gesprochen) als Glasübergang der amorphen Bereiche des PE anzusehen ist, der (als Maximum des tan oder G\") deutlicher hervortritt, wenn man durch physikalisch-chemische Effekte, die den Kristallisationsgrad nicht beeinflussen (z.B.: Quellung in einem unpolaren, niedermolekularen Quellungsmittel), Ànderungen des Zustands der nichtkristallinen Phasen erreichen kann. Auf diese Weise kann z.B. durch Quellung in CClj oder C„H,„ die Relaxationsstarke der nichtkristallinen Phasen verstârkt werden. Die genannten Quellungsmittel zeigen keine eigenen Dispersionsgebiete im interessierenden Bereich, sofern sie molekular dispers vorliegen [ . H. Illers, Kolloid . 190 (1963) 16, 250 (1972) 426, 231 (1967) 622J.

Density Matrix Theory and Applications

The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

/pdf/thermoelectric-effects-in-graphene-nanostructures-1nv0pibydr.pdf

Thermoelectric effects in graphene nanostructures.

Graphene exhibits superior mechanical strength, high thermal conductivity, strong light-matter interactions, and, in particular, exceptional electronic properties. These merits make graphene an outstanding material for numerous potential applications. However, a graphene-based high-performance transistor, which is the most appealing application, has not yet been produced, which is mainly due to the absence of an intrinsic electronic bandgap in this material. Therefore, bandgap opening in graphene is urgently needed, and great efforts have been made regarding this topic over the past decade. In this review article, we summarise recent theoretical and experimental advances in interfacial engineering to achieve bandgap opening. These developments are divided into two categories: chemical engineering and physical engineering. Chemical engineering is usually destructive to the pristine graphene lattice via chemical functionalization, the introduction of defects, doping, chemical bonds with substrates, and quantum confinement; the latter largely maintains the atomic structure of graphene intact and includes the application of an external field, interactions with substrates, physical adsorption, strain, electron many-body effects and spin-orbit coupling. Although these pioneering works have not met all the requirements for electronic applications of graphene at once, they hold great promise in this direction and may eventually lead to future applications of graphene in semiconductor electronics and beyond.

Interfacial engineering in graphene bandgap

This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fourier's law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.

/pdf/thermal-transport-in-nanostructures-uga11xvi3w.pdf

Thermal transport in nanostructures

The typical conductive polymer of PEDOT:PSS has recently attracted intensive attention in thermoelectric conversion because of its low cost and low thermal conductivity as well as high electrical conductivity. However, compared to inorganic counterparts, the relatively poor thermoelectric performance of PEDOT:PSS has greatly limited its development and high-tech applications. Here, we report a dramatic enhancement in the thermoelectric performance of PEDOT:PSS by constructing unique composite films with graphene quantum dots (GQDs). At room temperature, the electrical conductivity and Seebeck coefficient of PEDOT:PSS/GQDs reached to 7172 S/m and 14.6 μV/K, respectively, which are 30.99% and 113.2% higher than those of pristine PEDOT:PSS. As a result, the power factor of the optimized PEDOT:PSS/GQDs composite is 550% higher than that of pristine PEDOT:PSS. These significant improvements are attributed to the ordered alignment of PEDOT chains on the surface of GQDs, originated from the strong interfacial interaction between PEDOT:PSS and GQDs and the separation of PEDOT and PSS phases. This study evidently provides a promising route for PEDOT:PSS applied in high-efficiency thermoelectric conversion.

/pdf/pedot-pss-graphene-quantum-dots-films-with-enhanced-28qvtlkh2o.pdf

PEDOT:PSS/graphene quantum dots films with enhanced thermoelectric properties via strong interfacial interaction and phase separation.

Thermoelectric properties of one-dimensional graphene antidot arrays

Within an one-dimensional tight-binding model, we investigate the inelastic scattering processes of oppositely charged polarons in conjugated polymers under the influence of an external electric field, by using a nonadiabatic evolution method. It is found that the polaron pair does not necessarily scatter into an entity(neutral exciton), but a mixed state composed of both polarons and excitons. The yield of the neutral exciton depends sensitively on the strength of applied fields. Additionally, effects of interchain coupling on the scattering processes are also discussed, which shows that the interchain coupling is of fundamental importance and facilitates the formation of the polaron-exciton.

/pdf/inelastic-scattering-of-oppositely-charged-polarons-in-4htdwszcp6.pdf

Inelastic scattering of oppositely charged polarons in conjugated polymers

Phonon effects on spin-charge separation in one dimension are investigated through the calculation of one-electron spectral functions in terms of the recently developed cluster perturbation theory together with an optimized phonon approach. It is found that the retardation effect due to the finiteness of phonon frequency suppresses the spin-charge separation and eventually makes it invisible in the spectral function. By comparing our results with experimental data of TTF-TCNQ, it is observed that the electron-phonon interaction must be taken into account when interpreting the angle-resolved photoemission spectroscopy data.

Phonon Effects on Spin-Charge Separation in One Dimension

The motion of polarons, which serve as charge carriers in conjugated polymers, is of fundamental importance for understanding transport properties of organic optoelectronic devices. We investigate the dynamics of a charged polaron in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field, which is modeled by the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model supplemented with a Hubbard on-site repulsion term. For this many-body dynamical evolution problem, we develop an adaptive time-dependent density matrix renormalization group ($t$-DMRG) method in combination with a Newtonian equation of motion for atomic displacements. Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interaction $U$. The polaron moves with a supersonic velocity, about four times the sound velocity at the small $U$ limit, and approaches the sound velocity at the large $U$ limit. Furthermore, the dependence of the polaron velocity and the polaron effective mass on the lattice structures are discussed.

/pdf/dynamics-of-polarons-in-conjugated-polymers-an-adaptive-time-1egpf6ec2x.pdf

Dynamics of polarons in conjugated polymers: An adaptive time-dependent density-matrix renormalization-group study

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-000455.pdf

Hui Zhao

Papers

Thermoelectric properties of one-dimensional graphene antidot arrays

Inelastic scattering of oppositely charged polarons in conjugated polymers

Phonon Effects on Spin-Charge Separation in One Dimension

Dynamics of polarons in conjugated polymers: An adaptive time-dependent density-matrix renormalization-group study

Phonon Effects on Spin-Charge Separation in One Dimension