Chain-like Materials Research Articles

Anisotropic Klemens model for the thermal conductivity tensor and its size effect

With the constraint from Onsager reciprocity relations, here we generalize the Klemens model for intrinsic phonon-phonon scattering from isotropic to anisotropic. Combined with the anisotropic Debye dispersion, this anisotropic Klemens model leads to analytical expressions for heat transfer along both ab-plane (κab) and c-axis (κc), suitable for both layered and chainlike materials with any anisotropy ratio of the dispersion and the scattering. This combination of dispersion and scattering is further applied to the equation of phonon radiative transfer to model heat conduction across anisotropic thin films. The model is compared to experimental κab and κc of bulk graphite at high temperatures (T≥0.1×max(θD,ab,θD,c), where θD,ab and θD,c are the two Debye temperatures), leading to best-fit scattering parameters that capture the harmonic and anharmonic nature of the intra- and inter-layer bonding of graphite. With no further adjusting of these parameters, the anisotropic scattering model performs at least 9 times better than the isotropic one at explaining the experimental κc of graphite thin films. This size effect is further justified by comparing the accumulation function of κc.

Direct observation of one-dimensional disordered diffusion channel in a chain-like thermoelectric with ultralow thermal conductivity

Structural disorder, highly effective in reducing thermal conductivity, is important in technological applications such as thermal barrier coatings and thermoelectrics. In particular, interstitial, disordered, diffusive atoms are common in complex crystal structures with ultralow thermal conductivity, but are rarely found in simple crystalline solids. Combining single-crystal synchrotron X-ray diffraction, the maximum entropy method, diffuse scattering, and theoretical calculations, here we report the direct observation of one-dimensional disordered In1+ chains in a simple chain-like thermoelectric InTe, which contains a significant In1+ vacancy along with interstitial indium sites. Intriguingly, the disordered In1+ chains undergo a static-dynamic transition with increasing temperature to form a one-dimensional diffusion channel, which is attributed to a low In1+-ion migration energy barrier along the c direction, a general feature in many other TlSe-type compounds. Our work provides a basis towards understanding ultralow thermal conductivity with weak temperature dependence in TlSe-type chain-like materials.

Photocarrier relaxation pathways in selenium quantum dots and their application in UV-Vis photodetection.

Recently, chain-like materials have attracted significant attention due to their unique structure and outstanding electro-optical properties. However, the photocarrier dynamics and pathways in these materials that determine the electro-optical performances of the prepared devices have barely been touched. Herein, selenium quantum dots (Se QDs), one typical chain-like material, were prepared via a facile liquid phase exfoliation approach. The photocarrier dynamics in selenium quantum dots were systematically investigated by ultrafast transient absorption spectroscopy in the ultraviolet-visible regime. Four photocarrier decay pathways with different lifetimes were firstly detected, and they assist in the elucidation and reconstruction of the energy schematic diagram of Se QDs. Owing to the broadband photo-response and fast recovery time of Se QDs, a photoelectrochemical (PEC)-type photodetector was proposed for the first time to our knowledge, demonstrating its excellent photodetection properties. Considering the feasible fabrication method and clear photocarrier pathways, the excellent photocurrent density and stability of this photodetector undoubtedly guarantee that selenium is a promising candidate for advanced photonic devices.

An anisotropic model for the minimum thermal conductivity

The Cahill-Pohl/Einstein model of minimum phonon thermal conductivity (κmin), which assumes isotropic material properties, is widely successful as the lower limit for fully dense amorphous and disordered materials. However, measurements of disordered highly anisotropic layered WSe2 [Chiritescu et al., Science 315, 351 (2007)] fall below the isotropic κmin limit by at least a factor of four. Here, we generalize the isotropic κmin to be anisotropic, suitable for both layered and chain-like materials with any anisotropy ratio. We obtain compact algebraic expressions in limiting temperature regimes for heat transfer along both c-axis (κmin−c) and ab-plane (κmin−ab). Applying this framework to the disordered layered WSe2 with no free parameters brings the theoretical κmin−c back in line with the experimental results. The anisotropic corrections result from both a phonon focusing effect and a first Brillouin zone truncation effect.

Anisotropic Debye model for the thermal boundary conductance

Most standard models for the thermal boundary conductance (TBC) assume isotropic properties and thus are inappropriate for layered and chainlike materials such as graphite, Bi${}_{2}$Te${}_{3}$, and high-density polyethylene (HDPE). To model such anisotropic materials, here a framework is introduced whereby the first Brillouin zone and the isoenergy surfaces of the Debye dispersion relation are both generalized from spherical to ellipsoidal. This model is checked by comparison with the experimental specific heat capacity of graphite and HDPE, as well as the phonon irradiation of graphite calculated from lattice dynamics. The anisotropic TBC model performs at least six times better than the standard isotropic diffuse mismatch model at explaining the measured TBC between graphite and various metals reported by Schmidt et al. [J. Appl. Phys. 107, 104907 (2010)]. The model further reveals an unexpected guideline to engineer the TBC: due to phonon focusing effects, in many cases the TBC across an interface can be increased by reducing a phonon velocity component parallel to the plane of the interface.

Spin Transition Regime in New One-Dimensional Polymeric Iron(II) Compounds. Importance of the Water Content for the Thermal and Optical Properties

A new series of polymeric chainlike materials exhibiting the spin crossover phenomenon has been synthesized in aqueous conditions using different iron(II) salts and 4-(formylamino)-1,2,4-triazole (fatrz). The magnetic and thermogravimetric behaviors of the compounds of general formula [Fe(fatrz) 3 ](A) 2 xH 2 O are presented for 1 (A = trifluoromethanesulfonate and x = 3), 2 (A = tetrafluoroborate and x = 2), 3 (A = p-tolylsulfonate and x = 3), and 4 (A = nitrate and x = 1). In 1, the high (S = 2) ↔ low (S = 0) spin transition of the iron(II) ions occurs sharply in the close vicinity of 0 °C. Reflectivity measurements show that this transition can be cycled and used as a temperature sensor. However, 1 loses water when warmed above 320 K, whereby its transitions become more gradual and shift to lower temperatures. Calorimetric parameters for the spin transition of 1 have been obtained and indicate a strongly cooperative phenomenon, with ΔH = 17.19 kJ mol -1 and ΔS = 64.26 J K -1 mol -1 in the warming mode. 2 has a more gradual transition in the range 265-285 K. 3 and 4 are low spin at room temperature and possess large apparent hysteresis due to a loss of water upon the first L - H spin change. The reabsorption of water in 3 makes it a possible reusable temperature threshold sensor. The prominent role of water molecules in the whole family of chainlike spin crossover materials is discussed.

The tellurium halide glasses

The reaction of tellurium with the halogens Cl, Br and I leads to a new family of halide glasses which are structurally related to Se. These chain-like materials are characterized by a broad optical window lying from 1 to 13 μm or 1 to 20 μm, depending on the weight of the halogen. When combined with S or specially Se, many of these new IR glasses exhibit a strong resistance towards devitrification and water corrosion. Infrared optical fibers have been prepared from these so-called TeX glasses. The position and the dependence of the multiphonon edge as well as the bandgap have been investigated by measuring the attenuation spectra. It has been shown that these glasses exhibit low optical losses in the 8–12 μm atmospheric window. Thin layers of TeX glasses have been deposited by sputtering on silica or fluoride glass substrates, and antireflection coatings deposited on bulk TeX glasses allow the reduction of optical losses by reflection.