Kristie J. Koski

The negatively charged boron vacancy (VB−) in hexagonal boron nitride (hBN) is a promising quantum defect that can be used to sense pressure, temperature, and magnetic field with high spatial resolution. hBN enriched with the boron-10 and nitrogen-15 isotopes, denoted h10B15N, has good contrast and coherence for quantum sensing because nitrogen-15 has nuclear spin (1/2), reducing hyperfine interactions. Boron vacancies can be generated by neutron irradiation, which causes the transmutation of the boron-10 isotope to lithium-7. In this study, the ancillary structural, compositional, and mechanical properties of h10B15N crystals that have been subjected to neutron irradiation fluences from 1.4 × 1016 to 8.4 × 1017 n/cm2 were thoroughly characterized. Besides creating VB−, the process also induces other defects that generate strain in the crystal lattice. In turn, the mechanical properties of these crystals change drastically. Investigated here are the visual changes, lattice integrity, composition, crystal strain, and elastic constants (C33 and C66) to assess how these characteristics change as a function of neutron fluence.

Hydrogels are three-dimensional networks of hydrophilic polymers that retain large amounts of water and can be tailored for medicine, agriculture, electronics, and cosmetics. Their softness and tunable density complicate contact-based mechanical testing. Here, we explore the mechanics of a cellulose-based hydrogel using Brillouin light scattering (BLS) spectroscopy, a non-contact optical probe. We study micro-fibrillated cellulose hydrogels prepared via two drying routes—a dense, ambient-dried film and a foam-like, freeze-dried aerogel—pre-characterized by tensile testing. BLS is then used to extract longitudinal sound velocity and stiffness in three scattering geometries: reflective backscattering 180a and 90r and transmissive 90a. As expected, the structural arrangement imposed by drying (i.e., material density) dominates the stiffness, resulting in a stiffness of 3 GPa for the aerogel and 19 GPa for the hydrogel film. We further track moisture effects by changing the relative humidity (RH) level (40% RH and 75% RH), which leads to a decrease in frequency shift and a broadening of the Brillouin peaks with increasing RH, and a drop in stiffness by factor two. Time-resolved BLS tracks dehydration kinetics: fully wetting the hydrogel film and merely changing RH produce different Brillouin frequency shift dynamics. These results explore BLS as non-contact method for in situ measurement of mechanical properties during conditioning, with further potential applications during processing of technologically relevant soft and polymeric materials.

Brillouin scattering of bulk layered hexagonal gallium sulfide (GaS) and gallium selenide (GaSe) was measured both as pristine materials and intercalated with copper and silver. The sound velocities, refractive index, and four of the five independent elastic stiffnesses (cij) were determined. Values of the c11 elastic stiffness in thin crystals are higher than in previous measurements, bringing the elastic anisotropy (c11/c33) into a range typical of other layered materials. Copper intercalation showed a larger effect on acoustic phonons in GaS than in GaSe.

Germanium sulfide (GeS) is a 2D semiconductor with potential for high-speed optoelectronics and photovoltaics due to its near-infrared band gap and high mobility of optically excited charge carriers. Here, we use time-resolved THz spectroscopy to investigate the differences in ultrafast carrier dynamics in GeS following near-band gap photoexcitation (1.55 eV), which penetrates deep into the multilayer GeS, and excitation with above-band gap photon energy (3.1 eV), which is absorbed within a sub-20 nm surface layer. We find that the photoexcited carriers in the bulk have significantly longer lifetimes and higher mobility, as they are less impacted by trap states that affect carrier behavior in the surface layer. These insights are important for designing GeS-based photodetectors, solar energy conversion devices, and sensors that leverage the sensitivity of surface-layer photoexcited carriers to trap states.

The mechanical properties of metal-organic frameworks (MOFs) are of high fundamental and practical relevance. A particularly intriguing technique for determining anisotropic elastic tensors is Brillouin scattering, which so far has rarely been used for highly complex materials like MOFs. In the present contribution, we apply this technique to study a newly synthesized MOF-type material, referred to as GUT2. The experiments are combined with state-of-the-art simulations of elastic properties and phonon bands, which are based on machine-learning force fields and dispersion-corrected density functional theory. This provides a comprehensive understanding of the experimental signals, which can be correlated to the longitudinal and transverse sound velocities of the material. Notably, the combination of the insights from simulations and experiments allows the determination of approximate values for the components of the elastic tensor of the studied material even when dealing with comparably small single crystals, which limit the range of accessible experimental data.

Whereas single crystals of organic compounds that respond to heat or light have been reported and studied in detail, studies on crystalline organic compounds that elicit an extreme mechanical response upon cooling to very low temperatures are relatively rare in the chemical literature. A tetrafluoro(aryl)sulfanylated bicyclopentane synthesized in our laboratory was discovered to exhibit such behavior; i.e., the crystals jumped and forcefully disintegrated upon cooling below ∼193 K. Accordingly, the origin of this low-temperature thermosalient effect was investigated through NMR, SC-XRD, PXRD, microscopy, DSC, Raman, and Brillouin experiments. To our surprise, NMR, SC-XRD, PXRD, and DSC experiments suggest the phenomenon can neither be attributed solely to a chemical transformation nor a phase transition of the entire material. Rather, XRD, Raman, and Brillouin experiments provide evidence that built-up strain released from the crystal upon self-destruction may be associated with crystal microstructure or a phase transition that occurs in another material (i.e., an impurity) in the crystal. This study demonstrates that molecular structural changes in organic material microstructure or impurity phases (which may not necessarily be visible by X-ray diffraction) can have a significant impact on the behavior of the bulk crystalline material. Thus, the role of microstructure may be considered more heavily in future mechanistic studies on mechanically responsive crystals.

The high-pressure behavior of silicon telluride (Si2Te3), a two-dimensional (2D) layered material, was investigated using synchrotron X-ray powder diffraction in a diamond anvil cell to 11.5 GPa coupled with first-principles theory. Si2Te3 undergoes a phase transition at < 1 GPa from a trigonal to a hexagonal crystal structure. At higher pressures (> 8.5 GPa), X-ray diffraction showed the appearance of new peaks possibly coincident with a new phase transition, though we suspect Si2Te3 retains a hexagonal structure. Density functional theory calculations of the band structure reveal metallization above 9.1 GPa consistent with previous measurements of the Raman spectra and disappearance of color and transparency at pressure. The theoretical Raman spectra reproduce the prominent features of the experiment, though a deeper analysis suggests that the orientation of Si dimers dramatically influences the vibrational response. Given the complex structure of Si2Te3, simulation of the resulting high-pressure phase is complicated by disordered vacancies and the initial orientations of Si-Si dimers in the crushed layered phase.

Intercalation of several elements (Ag, Bi, In, Mo, Os, Pd, Pt, Rh, Ru, Sb, and W) is used to chemically alter a wide range of properties of two-dimensional layered alpha-MoO3. Intercalation modifies acoustic phonons and elastic constants, as measured with Brillouin scattering. Intercalation alters electronic bandgaps, color, structure, Raman shifts, and electron binding energies. Optical chemochromism is demonstrated with intercalants changing the color of MoO3 from transparent to brilliant blue (In, Mo, Os, and Ru) and orange (Ag). Correlations are investigated among material properties. There is evidence that in-plane longitudinal stiffness c(11) correlates with changes in the bandgap, while various Raman modes appear to be connected to a variety of properties, including shear modulus c(55), Mo binding energies, lattice constants, and the preferred crystal structure of the intercalant. The results indicate a surprising degree of complexity, suggesting competition among multiple distinct mechanisms and interactions involving specific intercalant species.

Whereas single crystals of organic compounds that respond to heat ("thermosalient") or light ("photosalient") have been reported and studied in detail, crystalline organic compounds that elicit an extreme mechanical response at cryogenic temperatures ("cryosalient") are relatively rare in the chemical literature. During an attempt to measure an X-ray crystal structure of a tetrafluoro(aryl)sulfanylated bicyclopentane synthesized in our laboratory, we discovered that the crystals jump and forcefully disintegrate upon cooling below ~193 K. Accordingly, we investigated the origin of this cryogenic mechanical response (CMR) through NMR, SC-XRD, DSC, Raman, Brillouin, and microscopy experiments. To our surprise, NMR, SC-XRD, and DSC experiments suggest the phenomenon can neither be attributed to a chemical transformation nor a crystalline-crystalline or crystalline-amorphous phase transition. Rather, microscopy, Raman, and Brillouin experiments provide evidence that the built-up strain released from the crystal upon self-destruction may be associated with a phase transition that occurs in amorphous microstructure within the crystal. This study demonstrates that molecular structural changes within organic material microstructure – which may not necessarily be visible by SC-XRD – can have a significant impact on the thermodynamic behavior of the bulk crystal-line material. Thus, the role of microstructure may be considered more heavily in future mechanistic studies on mechanically responsive crystals.