Covalent Bonds versus van der Waals Forces: A Picture in Thermal Conduction of Organic Materials.

We present a direct comparison of the heat transport properties between the state in which the constituent molecules are assembled by intermolecular forces and the one in which they are covalently bonded, in a molecular system with identical constituent elements and masses, as well as a nearly identical structure and density. This comparison leading to an essential understanding of thermal conduction in organic materials is made possible by the unique compound found by Wudl et al., which exhibits a single-crystal-to-single-crystal topochemical polymerization with a yield of >99%, in combination with microtemperature wave analysis (µTWA), which allows accurate measurements of the thermal diffusivity of small single crystals. At room temperature, the thermal conductivity of monomer and polymer single crystals is not significantly different. For both crystals, the thermal conductivity increases monotonically with decreasing temperature. However, below the Debye temperature, the thermal conductivity of the polymer single crystal increases exponentially, giving much larger values than those of the monomer single crystal. Based on physical quantities related to the behavior of phonons, derived from the specific heat analysis, we discuss the differences in heat transport properties in the two states and provide guidelines for achieving high thermal conductivity in organic materials.

Polarimeters for the Detection of Anisotropy from Reflectance.

Polarimetry is used to determine the Stokes parameters of a laser beam. Once all four S0,1,2,3 parameters are determined, the state of polarisation is established. Upon reflection of a laser beam with the defined S polarisation state, the directly measured S parameters can be used to determine the optical properties of the surface, which modify the S-state upon reflection. Here, we use polarimetry for the determination of surface anisotropies related to the birefringence and dichroism of different materials, which have a common feature of linear patterns with different alignments and scales. It is shown that polarimetry in the back-reflected light is complementary to ellipsometry and four-polarisation camera imaging; experiments were carried out using a microscope.

A role of intermolecular interaction modulating thermal diffusivity in organosuperelastic and organoferroelastic cocrystals.

Although the finding of superelasticity and ferroelasticity in organic crystals has been serendipitous, an increasing number of organic crystals with such deformation properties have been witnessed. Understanding the structure-property relationship can aid in the rational selection of intermolecular interactions to design organic crystals with desired superelastic or ferroelastic properties. In this study, we investigated the mechanical deformation in two cocrystals, prepared with the parent compound, 1,4-diiodotetrafluorobenzene with two coformers, 1,2-bis(4-pyridyl)ethane and pyrene. The parent compound and coformers were chosen to introduce distinct weak interactions such as halogen bonds and C-H⋯F, and π⋯π interactions in the crystal structure. The two cocrystals exhibited different mechanical deformations, superelasticity, and ferroelasticity, respectively. The single-crystal X-ray diffraction and energy framework analysis of the crystal structure of the cocrystals revealed that both deformations were caused by mechanical twinning. Interestingly, a difference in the extent of deformation was observed, modulated by a combination of strong and weak intermolecular interactions in the superelastic cocrystal, and only weak interaction in the ferroelastic one. In this comparison, the superelastic cocrystal exhibited higher thermal diffusivity than the ferroelastic cocrystal, indicating the presence of symmetrical and relatively robust intermolecular interactions in the superelastic cocrystal.

Structural and Thermal Diffusivity Analysis of an Organoferroelastic Crystal Showing Scissor-Like Two-Directional Deformation Induced by Uniaxial Compression.

A two-directional ferroelastic deformation in organic crystals is unprecedented owing to its anisotropic crystal packing, in contrast to isotropic symmetrical packing in inorganic compounds and polymers. Thereby, finding and constructing multidirectional ferroelastic deformations in organic compounds is undoubtedly complex and at once calls for deep comprehension. Herein, we demonstrate the first example of a two-directional ferroelastic deformation with a unique scissor-like movement in single crystals of trans-3-hexenedioic acid by the application of uniaxial compression stress. A detailed structural investigation of the mechanical deformation at the macroscopic and microscopic levels by three distinct force measurement techniques (including shear and three-point bending test), single crystal X-ray diffraction techniques, and polarized synchrotron-FTIR microspectroscopy highlighted that mechanical twinning promoted the deformation. The presence of two crystallographically equivalent faces and the herringbone arrangement promoted the two-directional ferroelastic deformation. In addition, anisotropic heat transfer properties in the parent and the deformed domains were investigated by thermal diffusivity measurement on all three axes using microscale temperature-wave analysis (µ-TWA). A correlation between the anisotropic structural arrangement and the difference in thermal diffusivity and mechanical behavior in the two-directional organoferroelastic deformation could be established. The structural and molecular level information from this two-directional ferroelastic deformation would lead to a more profound understanding of the structure-property relationship in multidirectional deformation in organic crystals.

Lock-in photothermal method for in-plane thermal diffusivity measurements using arrayed temperature sensors on suspended SiNx membranes.

A device consisting of a line- or spiral-shaped temperature sensor array on a two-dimensional (2D) silicon nitride (SiNx) membrane of thickness 50 or 150 nm is developed for use in the lock-in photothermal method to determine the in-plane thermal diffusivity of SiNx membranes in air and in vacuum. The results of 2D heat diffusion are analyzed by the quadrupole method, and the system is approximated to the one-dimensional (1D) fin standing in a surrounding media (the fin approximation). The results show that 2D thermal diffusion on the membrane is affected not only by heat exchange with the surrounding environment but also by parallel thermal diffusion caused by heat conduction in the air along the membrane surface. The measurement using photothermal heating and contact detection of the temperature response enables the phenomenon to be detected consistently at a wide frequency range of temperature waves (50-1000 Hz). The measured thermal diffusivity values of the SiNx membrane are much smaller than those of bulk material, which can be reasonably considered an effect of the confined state of the phonon in the nanoscale geometry of the membrane.

Insights into Thermal Transport through Molecular π-Stacking.

π-Stacking, which is a ubiquitous structural motif in assemblies of aromatic compounds, is well-known to provide a transport pathway for charge carriers and excitons, while its contribution to thermal transport is still unclear. Herein, based on detailed experimental observations of the thermal diffusivity, thermal conductivity, and specific heat of a single-crystalline triphenylene featuring a one-dimensionally π-stacked structure, we describe the nature of thermal transport through the π-stacked columns. We reveal that acoustic phonons are responsible for thermal transport through the π-stacked columns, which exhibit crystal-like behavior. Importantly, the thermal energy stored as intramolecular vibrations can also be transported by coupling to the acoustic phonons. In contrast, in the direction perpendicular to the π-stacked columns, an amorphous-like thermal transport behavior dominates. The present finding offers deep insight into nanoscale thermal transport in organic materials, where the constituent molecules exist as discrete entities linked together by weak intermolecular interactions.

Composite formation of covalent organic framework crystals and sugar alcohols for exploring a new class of heat-storage materials.

We impregnated the sugar alcohols (SAs) erythritol and mannitol (Man) in >0.1 mm single crystals of a covalent organic framework, COF-300, to propose new solid-state heat storing materials. The fusion-freezing cycles of the Man-COF composite occur in a narrow range of 130-155 °C without large or random supercooling, which has been a crucial problem of SAs, indicating the significance of this materials concept. The Man-COF composite displayed two endothermic and two exothermic features in the heating/cooling cycles. We measured the heat transmission properties in detail, based on which we discuss and suggest a postulate on the phase change behaviour of Man in the pores of the COF.

Linearly Polarized Infrared Spectroscopy for the Analysis of Biological Materials.

The analysis of biological samples with polarized infrared spectroscopy (p-IR) has long been a widely practiced method for the determination of sample orientation and structural properties. In contrast to earlier works, which employed this method to investigate the fundamental chemistry of biological systems, recent interests are moving toward "real-world" applications for the evaluation and diagnosis of pathological states. This focal point review provides an up-to-date synopsis of the knowledge of biological materials garnered through linearly p-IR on biomolecules, cells, and tissues. An overview of the theory with special consideration to biological samples is provided. Different modalities which can be employed along with their capabilities and limitations are outlined. Furthermore, an in-depth discussion of factors regarding sample preparation, sample properties, and instrumentation, which can affect p-IR analysis is provided. Additionally, attention is drawn to the potential impacts of analysis of biological samples with inherently polarized light sources, such as synchrotron light and quantum cascade lasers. The vast applications of p-IR for the determination of the structure and orientation of biological samples are given. In conclusion, with considerations to emerging instrumentation, findings by other techniques, and the shift of focus toward clinical applications, we speculate on the future directions of this methodology.

Polarisation Control in Arrays of Microlenses and Gratings: Performance in Visible-IR Spectral Ranges.

Microlens arrays (MLAs) which are increasingly popular micro-optical elements in compact integrated optical systems were fabricated using a femtosecond direct laser write (fs-DLW) technique in the low-shrinkage SZ2080TM photoresist. High-fidelity definition of 3D surfaces on IR transparent CaF2 substrates allowed to achieve ∼50% transmittance in the chemical fingerprinting spectral region 2-5 µm wavelengths since MLAs were only ∼10 µm high corresponding to the numerical aperture of 0.3 (the lens height is comparable with the IR wavelength). To combine diffractive and refractive capabilities in miniaturised optical setup, a graphene oxide (GO) grating acting as a linear polariser was also fabricated by fs-DLW by ablation of a 1 µm-thick GO thin film. Such an ultra-thin GO polariser can be integrated with the fabricated MLA to add dispersion control at the focal plane. Pairs of MLAs and GO polarisers were characterised throughout the visible-IR spectral window and numerical modelling was used to simulate their performance. A good match between the experimental results of MLA focusing and simulations was achieved.

Structure and Optical Anisotropy of Spider Scales and Silk: The Use of Chromaticity and Azimuth Colors to Optically Characterize Complex Biological Structures.

Herein, we give an overview of several less explored structural and optical characterization techniques useful for biomaterials. New insights into the structure of natural fibers such as spider silk can be gained with minimal sample preparation. Electromagnetic radiation (EMR) over a broad range of wavelengths (from X-ray to THz) provides information of the structure of the material at correspondingly different length scales (nm-to-mm). When the sample features, such as the alignment of certain fibers, cannot be characterized optically, polarization analysis of the optical images can provide further information on feature alignment. The 3D complexity of biological samples necessitates that there be feature measurements and characterization over a large range of length scales. We discuss the issue of characterizing complex shapes by analysis of the link between the color and structure of spider scales and silk. For example, it is shown that the green-blue color of a spider scale is dominated by the chitin slab's Fabry-Pérot-type reflectivity rather than the surface nanostructure. The use of a chromaticity plot simplifies complex spectra and enables quantification of the apparent colors. All the experimental data presented herein are used to support the discussion on the structure-color link in the characterization of materials.

On the origin of elasticity and heat conduction anisotropy of liquid crystal elastomers at gigahertz frequencies.

Liquid crystal elastomers that offer exceptional load-deformation response at low frequencies often require consideration of the mechanical anisotropy only along the two symmetry directions. However, emerging applications operating at high frequencies require all five true elastic constants. Here, we utilize Brillouin light spectroscopy to obtain the engineering moduli and probe the strain dependence of the elasticity anisotropy at gigahertz frequencies. The Young's modulus anisotropy, E||/E⊥~2.6, is unexpectedly lower than that measured by tensile testing, suggesting disparity between the local mesogenic orientation and the larger scale orientation of the network strands. Unprecedented is the robustness of E||/E⊥ to uniaxial load that it does not comply with continuously transformable director orientation observed in the tensile testing. Likewise, the heat conductivity is directional, κ||/κ⊥~3.0 with κ⊥ = 0.16 Wm-1K-1. Conceptually, this work reveals the different length scales involved in the thermoelastic anisotropy and provides insights for programming liquid crystal elastomers on-demand for high-frequency applications.

THz Filters Made by Laser Ablation of Stainless Steel and Kapton Film.

THz band-pass filters were fabricated by femtosecond-laser ablation of 25-µm-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron's THz beamline. A 25-µm-thick Kapton film performed as a Fabry-Pérot etalon with a free spectral range (FSR) of 119 cm-1, high finesse Fc≈17, and was tuneable over ∼10µm (at ∼5 THz band) with ß=30∘ tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.

Analyses of chemiluminescence reactions of fluorophore-linked 1,2-dioxetane isomers in crystals heating at elevated temperature including a development of a simultaneous measurement method of thermal diffusivity and light emission for a single crystal.

It has been found that a fluorophore-linked adamantylideneadamantane 1,2-dioxetane derivative showed an isomeric difference in the reactivities of crystalline-state chemiluminescence (CL) under isothermal heating conditions, while there are still unsolved problems on the conditions for intracrystalline reactions. To confirm the suitable heating conditions for initiating crystalline-state CL reactions of the isomers, the CL reactions in their crystal samples heated at elevated temperature were investigated with monitoring of CL light emission and thermogravimetry-differential thermal analysis (TG-DTA) measurements together with a simultaneous measurement method of thermal diffusivity and light emission, to provide the information on suitable temperature range for the crystalline-state CL reactions and on the reactivities depending on open and closed conditions of the crystal samples. The newly developed simultaneous measurement method provides a methodology for a single crystal to analyze the relationship between a change of the thermophysical property of inside of the crystal and the progress of a crystalline-state CL reaction.

Probe-based microscale measurement setup for the thermal diffusivity of soft materials.

Based on the principle of the periodic heating method by using cantilever thermocouple nanoprobes, we developed a method and an apparatus to measure the thermal diffusivity of soft materials on a microscale. The contact position of the probe tip with the sample surface was defined by using the phenomenon that the DC component of the thermal electromotive force (EMF) of the probe changes significantly upon contact (i.e., the vertical temperature gradient near the sample surface changes significantly). This contact position was set as the surface reference position where the variation of the thermal contact conductance between the sample surface and the sensor probe is minimized. The phase shift from the micro-heater was measured by the AC component of the probe's thermal EMF and used to accurately determine the thermal diffusivity of micro-sized soft materials. The thermal diffusivity of the microstructured photoresist was determined with a deviation of ±3%.

Anisotropy of 3D Columnar Coatings in Mid-Infrared Spectral Range.

Polarisation analysis in the mid-infrared fingerprint region was carried out on thin (∼1 µm) Si and SiO2 films evaporated via glancing angle deposition (GLAD) method at 70∘ to the normal. Synchrotron-based infrared microspectroscopic measurements were carried out on the Infrared Microspectroscopy (IRM) beamline at Australian Synchrotron. Specific absorption bands, particularly Si-O-Si stretching vibration, was found to follow the angular dependence of ∼cos2θ, consistent with the absorption anisotropy. This unexpected anisotropy stems from the enhanced absorption in nano-crevices, which have orientation following the cos2θ angular dependence as revealed by Fourier transforming the image of the surface of 3D columnar films and numerical modeling of light field enhancement by sub-wavelength nano-crevices.

Photothermally Driven High-Speed Crystal Actuation and Its Simulation.

Mechanically responsive crystals have been increasingly explored, mainly based on photoisomerization. However, photoisomerization has some disadvantages for crystal actuation, such as a slow actuation speed, no actuation of thick crystals, and a narrow wavelength range. Here we report photothermally driven fast-bending actuation and simulation of a salicylideneaniline derivative crystal with an o-amino substituent in enol form. Under ultraviolet (UV) light irradiation, these thin (<20 µm) crystals bent but the thick (>40 µm) crystals did not due to photoisomerization; in contrast, thick crystals bent very quickly (in several milliseconds) due to the photothermal effect, even by visible light. Finally, 500 Hz high-frequency bending was achieved by pulsed UV laser irradiation. The generated photothermal energy was estimated based on the photodynamics using femtosecond transient absorption. Photothermal bending is caused by a nonsteady temperature gradient in the thickness direction due to the heat conduction of photothermal energy generated near the crystal surface. The temperature gradient was calculated based on the one-dimensional nonsteady heat conduction equation to simulate photothermally driven crystal bending successfully. Most crystals that absorb light have their own photothermal effects. It is expected that the creation and design of actuation of almost all crystals will be possible via the photothermal effect, which cannot be realized by photoisomerization, and the potential and versatility of crystals as actuation materials will expand in the near future.

Direct Measurement of Temperature Diffusivity of Nanocellulose-Doped Biodegradable Composite Films.

The thermal properties of novel nanomaterials play a significant role in determining the performance of the material in technological applications. Herein, direct measurement of the temperature diffusivity of nanocellulose-doped starch-polyurethane nanocomposite films was carried out by the micro-contact method. Polymer films containing up to 2 wt%. of nanocellulose were synthesised by a simple chemical process and are biodegradable. Films of a high optical transmittance T≈80% (for a 200 µm thick film), which were up to 44% crystalline, were characterised. Two different modalities of temperature diffusivity based on (1) a resistance change and (2) micro-thermocouple detected voltage modulation caused by the heat wave, were used for the polymer films with cross sections of ∼100 µm thickness. Twice different in-plane α‖ and out-of-plane α⟂ temperature diffusivities were directly determined with high fidelity: α‖=2.12×10-7 m2/s and α⟂=1.13×10-7 m2/s. This work provides an example of a direct contact measurement of thermal properties of nanocellulose composite biodegradable polymer films. The thermal diffusivity, which is usually high in strongly interconnected networks and crystals, was investigated for the first time in this polymer nanocomposite.

Tilted black-Si: ∼0.45 form-birefringence from sub-wavelength needles.

The self-organised conical needles produced by plasma etching of silicon (Si), known as black silicon (b-Si), create a form-birefringent surface texture when etching of Si orientated at angles of θi < 50 - 70° (angle between the Si surface and vertical plasma E-field). The height of the needles in the form-birefringent region following 15 min etching was d ∼ 200 nm and had a 100 µm width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si ∼λ/4 for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is proposed to directly measure the retardance Δn × d/λ ≈ 0.15 of the region with tilted b-Si needles. The quantified form birefringence of Δn = -0.45 over λ = 400 - 700 nm spectral window was obtained. Such high values of Δn at visible wavelengths can only be observed in the most birefringence calcite or barium borate as well as in liquid crystals. The replication of b-Si into Ni-shim with high fidelity was also demonstrated and can be used for imprinting of the b-Si nanopattern into other materials.

Laser-Inscribed Stress-Induced Birefringence of Sapphire.

Birefringence of 3 × 10 - 3 is demonstrated inside cross-sectional regions of 100 µ m, inscribed by axially stretched Bessel-beam-like fs-laser pulses along the c-axis inside sapphire. A high birefringence and retardance of λ / 4 at mid-visible spectral range (green) can be achieved using stretched beams with axial extension of 30-40 µ m. Chosen conditions of laser-writing ensure that there are no formations of self-organized nano-gratings. This method can be adopted for creation of polarization optical elements and fabrication of spatially varying birefringent patterns for optical vortex generation.

Temperature-Dependent Thermoelastic Anisotropy of the Phenyl Pyrimidine Liquid Crystal.

Controlling thermoelastic anisotropy of liquid crystals (LCs) is important for achieving reliable structural stability and efficient heat dissipation, especially for high-performance LC devices. A solid understanding of the thermoelastic anisotropy and its relation with the LC molecular structure is, however, still missing. Here, we studied the direction-dependent mechanical and thermal properties of 5-n-octyl-2-(4-n-octyloxy-phenyl)-pyrimidine (PYP8O8) in a wide temperature range, covering five phases (i.e., crystalline, smectic C, smectic A, nematic, and liquid), by Brillouin light spectroscopy and temperature wave analysis, respectively. We found that the mechanical anisotropy is much smaller than the thermal anisotropy at LC phases; both anisotropies show strong phase dependence, with the biggest change occurring at the crystalline to LC phase transition; and the anisotropy of the phonon mean-free path correlates with the structural anisotropy of the rigid core of the LC molecule. The analysis of the temperature-dependent thermoelastic anisotropy of LCs yields insights into structure-based phonon engineering.