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Shining light on growth-dependent surface chemistry of organic crystals: a polarised Raman spectroscopic and computational study of aspirin
摘要： Understanding and controlling crystal surfaces is a critical problem in contemporary materials science. Organic crystal surfaces present the additional complexity of multiple terminations with different functional groups. These alternate terminations influence the surface chemistry and exert control on many material properties. While established tools for surface characterization exist, few provide the chemical information required to unambiguously identify functional groups. Polarized Raman spectroscopy is a versatile tool that can provide detailed chemical information on molecular materials, and, when used in a microscope configuration, can be used to map substrates on a micron scale. In this work, we demonstrate the use of polarized Raman to study the surface chemistry of aspirin. By analyzing crystals grown under a variety of conditions, we relate the growth solvent to the surface termination and reconcile the conflicting results in the literature on the nature of the <100> surface. Our results are supported by detailed first-principles modelling of the surfaces and their vibrational spectra. This study establishes the potential of polarized Raman microscopy as a tool for organic surface science that, when combined with predictive modelling, provides a powerful means to understand and ultimately control surface chemistry.
关键词： aspirin,first-principles modelling,crystal growth,polarized Raman spectroscopy,surface termination,lattice dynamics
更新于2025-09-23 15:23:52
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Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide
摘要： Hybrid organic–inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron–phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron–phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.
关键词： solar cells,inelastic neutron spectroscopy,lattice dynamics,hybrid perovskite,thermal conductivity
更新于2025-09-23 15:21:01
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with two-dimensional and three-dimensional local environments
摘要： ZnNi(CN)4 is a three-dimensional (3D) framework material consisting of two interpenetrating PtS-type networks in which tetrahedral [ZnN4] units are linked by square-planar [NiC4] units. Both the parent compounds, cubic Zn(CN)2 and layered Ni(CN)2, are known to exhibit 3D and 2D negative thermal expansion (NTE), respectively. Temperature-dependent inelastic neutron scattering measurements were performed on a powdered sample of ZnNi(CN)4 to probe phonon dynamics. The measurements were underpinned by ab initio lattice dynamical calculations. Good agreement was found between the measured and calculated generalized phonon density-of-states, validating our theoretical model and indicating that it is a good representation of the dynamics of the structural units. The calculated linear thermal expansion coefficients are αa = ?21.2 × 10?6 K?1 and αc = +14.6 × 10?6 K?1, leading to an overall volume expansion coefficient, αV of ?26.95 × 10?6 K?1, pointing towards pronounced NTE behavior. Analysis of the derived mode-Grüneisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localized rotational motions of the [NiC4] and/or [ZnN4] rigid units, echoing what has previously been observed in Zn(CN)2 and Ni(CN)2. However, in ZnNi(CN)4, modes below 10 meV have the most negative Grüneisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its square-planar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3D framework. Hence, although rigid-unit modes account for some of the NTE-driving phonons, the added degree of freedom compared with Zn(CN)2 results in modes with twisting motions, capable of inducing greater NTE.
关键词： framework compound,ZnNi(CN)4,negative thermal expansion,lattice dynamics,ab initio calculations,inelastic neutron scattering
更新于2025-09-19 17:15:36
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Attosecond Soft X-Ray Absorption Spectroscopy in Graphite
摘要： X-ray absorption fine-structure (XAFS) spectroscopy is a well-established technique capable to extract information on the electronic and lattice structure of a material with atomic resolution. Analysis of the XAFS spectrum in the near-edge region (XANES) provides information about the electronic configuration. Structural information is extracted from the extended XAFS (EXAFS) spectrum, consisting of several hundreds of eV above the absorption edge. Connecting the spectroscopic capabilities of XAFS technique to high temporal resolution would give the possibility to follow electronic and lattice dynamics in real time. Here, we present the simultaneously access to electronic and lattice parameters via XAFS spectroscopy using isolated attosecond soft X-ray (SXR) pulses covering the entire water window region (280 eV to 540 eV). We demonstrate the novel capabilities of attosecond XAFS (attoXAFS) by identifying the (cid:1)* and (cid:1)* orbital contributions to the density of states in highly oriented pyrolithic graphite (HOPG) simultaneously with the four characteristic bonding distances of graphite’s hexagonal lattice.
关键词： attosecond,graphite,soft X-ray,electronic and lattice dynamics,XAFS spectroscopy
更新于2025-09-11 14:15:04
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Ab initio Calculations of Phonon Dispersion in CdGa2S4
摘要： The phonon spectrum of CdGa2S4 has been experimentally investigated by Raman spectroscopy and theoretically analyzed using the density functional theory (DFT). Eleven active Raman modes (at 83, 135, 165, 219, 243, 260, 310, 322, 352, 361, and 392 cm–1) have been detected and identified. The identification of all vibrational modes is based on consideration of the point symmetry group. The results are compared with the existing experimental IR spectroscopy and Raman scattering data.
关键词： Raman spectroscopy,phonon dispersion,CdGa2S4,lattice dynamics,density functional theory
更新于2025-09-10 09:29:36
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-derived materials
摘要： Symmetry and dimensionality are essential factors defining lattice dynamics and conductivity (κ). Here, we critically examine these via ab initio Boltzmann transport applied to single chain and bulk electride Ba3N and Ba3NX (X = Sb,Bi). Chiral phonons in one-dimensional chains obey new symmetry-based scattering rules that limit thermal resistance. Weak chain coupling breaks these in the bulk, giving lower κ and large κ anisotropy. Curiously, intercalation of large X atoms binds chains more strongly, reducing the volume, yet gives lower κ and transforms the electronic behavior. Insights developed here can be more generally applied to other materials and provide avenues for predictive materials design.
关键词： symmetry,lattice dynamics,Ba3NX,dimensionality,thermal conductivity,Ba3N,chiral phonons,electride
更新于2025-09-09 09:28:46
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A Model on Heat Signal of Crystal Detector at Low Temperature
摘要： We present a model to calculate heat signal shapes from low-temperature bolometer attached to a crystal. This model is based on the elementary acoustic wave theory at low temperature and has been developed using modern Monte Carlo techniques. Physical processes in phonon propagation, such as transmission, scattering and re?ection are considered. Using our model, the calculated time dependence of signal agrees with real experimental data. This model has applications in low-temperature rare event particle detectors for dark matter and neutrinos.
关键词： Cryogenic calorimeter,Lattice dynamics,Phonon physics,Simulation,Acoustic wave
更新于2025-09-09 09:28:46