Dense random packing of disks with a power-law size distribution in thermodynamic limit.

The correlation properties of a random system of densely packed disks, obeying a power-law size distribution, are analyzed in reciprocal space in the thermodynamic limit. This limit assumes that the total number of disks increases infinitely, while the mean density of the disk centers and the range of the size distribution are kept constant. We investigate the structure factor dependence on momentum transfer across various number of disks and extrapolate these findings to the thermodynamic limit. The fractal power-law decay of the structure factor is recovered in reciprocal space within the fractal range, which corresponds to the range of the size distribution in real space. The fractal exponent coincides with the exponent of the power-law size distribution as was shown previously by the authors of the work of Cherny et al. [J. Chem. Phys. 158(4), 044114 (2023)]. The dependence of the structure factor on density is examined. As is found, the power-law exponent remains unchanged but the fractal range shrinks when the packing fraction decreases. Additionally, the finite-size effects are studied at extremely low momenta of the order of the inverse system size. We show that the structure factor is parabolic in this region and calculate the prefactor analytically. The obtained results reveal fractal-like properties of the packing and can be used to analyze small-angle scattering from such systems.

Highly efficient lasing and thermal properties of Tm:Y2O3 and Tm:(Y,Sc)2O3 ceramics.

We report on thermal, spectroscopic, and laser properties of transparent 5 at.% Tm3+-doped yttria and "mixed" yttria-scandia ceramics fabricated by vacuum sintering at 1750°C using nanoparticles produced by laser ablation. The solid-solution (Tm0.05Y0.698Sc0.252)2O3 ceramic features a broadband emission extending up to 2.3 µm (gain bandwidth, 167 nm) and high thermal conductivity of 4.48 W m-1 K-1. A Tm:Y2O3 ceramic laser generated 812 mW at 2.05 µm with a slope efficiency η of 70.2%. For the Tm:(Y,Sc)2O3 ceramic, the output power was 523 mW at 2.09 µm with η = 44.7%. These results represent record-high slope efficiencies for any parent or "mixed" Tm3+-doped sesquioxide ceramics.

Isolated Spin-7/2 Species of Gadolinium (III) Chelate Complexes on the Surface of 5-nm Diamond Particles.

The magnetic characteristics of a system of triply charged gadolinium ions Gd3+ chelated with carboxyls on the surface of detonation nanodiamond (DND) particles have been studied. Gd3+ ions demonstrate almost perfect spin (S = 7/2) paramagnetism with negligible antiferromagnetic interaction between spins (Weiss temperature about -0.35 K) for a wide range of concentrations up to ~18 ions per 5 nm particle. The study of the concentration dependence of the electron paramagnetic resonance signal for DND intrinsic defects with spin ½ (g = 2.0027) shows that Gd3+ ions are located on average at a distance of no more than 1.4 nm from shallow subsurface defects with spin 1/2. At the same time, they are located (according to density functional theory calculations) at a distance of about or at least 0.28 nm from the particle surface. Magnetic studies also confirm the isolated nature of the gadolinium chelate complexes on the surface of DND particles. DND particles turn out to be an optimal carrier for high-spin 4f- ions (gadolinium) in a highly concentrated isolated state. This property makes DND-Gd particles a candidate for the role of a contrast agent for magnetic resonance imaging.

Dense random packing with a power-law size distribution: The structure factor, mass-radius relation, and pair distribution function.

We consider a dense random packing of disks with a power-law distribution of radii and investigate their correlation properties. We study the corresponding structure factor, mass-radius relation, and pair distribution function of the disk centers. A toy model of dense segments in one dimension (1D) is solved exactly. It is shown theoretically in 1D and numerically in 1D and 2D that such a packing exhibits fractal properties. It is found that the exponent of the power-law distribution and the fractal dimension coincide. An approximate relation for the structure factor in arbitrary dimensions is derived, which can be used as a fitting formula in small-angle scattering. These findings can be useful for understanding the microstructural properties of various systems such as ultra-high performance concrete, high-internal-phase-ratio emulsions, or biological systems.

A comprehensive model of nitrogen-free ordered carbon quantum dots.

We propose and demonstrate a novel range of models to accurately determine the optical properties of nitrogen-free carbon quantum dots (CQDs) with ordered graphene layered structures. We confirm the results of our models against the full range of experimental results for CQDs available from an extensive review of the literature. The models can be equally applied to CQDs with varied sizes and with different oxygen contents in the basal planes of the constituent graphenic sheets. We demonstrate that the experimentally observed blue fluorescent emission of nitrogen-free CQDs can be associated with either small oxidised areas on the periphery of the graphenic sheets, or with sub-nanometre non-functionalised islands of sp2-hybridised carbon with high symmetry confined in the centres of oxidised graphene sheets. Larger and/or less symmetric non-functionalised regions in the centre of functionalised graphene sheet are found to be sources of green and even red fluorescent emission from nitrogen-free CQDs. We also demonstrate an approach to simplify the modelling of the discussed sp2-islands by substitution with equivalent strained polycyclic aromatic hydrocarbons. Additionally, we show that the bandgaps (and photoluminescence) of CQDs are not dependent on either out-of-plane corrugation of the graphene sheet or the spacing between sp2-islands. Advantageously, our proposed models show that there is no need to involve light-emitting polycyclic aromatic molecules (nanographenes) with arbitrary structures grafted to the particle periphery to explain the plethora of optical phenomena observed for CQDs across the full range of experimental works.

Revised scattering exponents for a power-law distribution of surface and mass fractals.

We consider scattering exponents arising in small-angle scattering from power-law polydisperse surface and mass fractals. It is shown that a set of fractals, whose sizes are distributed according to a power law, can change its fractal dimension when the power-law exponent is sufficiently big. As a result, the scattering exponent corresponding to this dimension appears due to the spatial correlations between positions of different fractals. For large values of the momentum transfer, the correlations do not play any role, and the resulting scattering intensity is given by a sum of intensities of all composing fractals. The restrictions imposed on the power-law exponents are found. The obtained results generalize Martin's formulas for the scattering exponents of the polydisperse fractals.

Rational Synthesis of Solid-State Ultraviolet B Emitting Carbon Dots via Acetic Acid-Promoted Fractions of sp3 Bonding Strategy.

Carbon dots (CDs) have received tremendous attention for their excellent photoluminescence (PL) properties. However, it remains a great challenge to obtain CDs with ultraviolet (UV, 200-400 nm) emission in solid state, which requires strict control of the CDs structure and overcoming the aggregation-caused quenching (ACQ). Herein, a new sp3 compartmentalization strategy is developed to meet these requirements, by employing acetic acid to promote fractions of sp3 bonding during the synthesis of CDs. It markedly decreases the size of sp2 conjugating units in the CDs, and shifts PL emission to the ultraviolet B (UVB) region (λmax  = 308 nm). Moreover, sp2 domains are well spatially compartmentalized by sp3 domains and the ACQ effect is minimized, enabling the high quantum yield in solid state (20.2%, λex  = 265 nm) with a narrow bandwidth of 24 nm and environmental robustness. The solid-state UVB emissive CDs are highly desired for application in photonic devices. Hence, a demo of UVB light-emitting diodes is fabricated for plant lighting, leading to a 29% increase of ascorbic acid content in the basil. Overall, a rational and efficient way to construct solid UVB-CDs phosphors for wide applications is provided.

Trichoblastic carcinosarcoma in a 34-year-old woman with histopathologic and molecular analysis, including re-demonstration of a CDKN2A p.(R58*) mutation.

Trichoblastic carcinosarcomas are rare, adnexal-type cutaneous carcinosarcomas that are thought to be related histogenetically to trichoblastomas, yet in which both the epithelial and stromal components show features of malignancy. Ten cases have been described in the literature thus far, with a predilection for the head and neck of older males. We present a case of cutaneous carcinosarcoma in sun-damaged skin of a 34-year-old woman showing features of a trichoblastic carcinosarcoma, with histopathologic analysis along with targeted next-generation sequencing of 50 cancer-associated genes. Two pathogenic variants in TP53 were identified, p.(R158C), p.(R273P), along with a likely pathogenic variant CDKN2A, p.(R58*). In particular, it is noted that the CDKN2A p.(R58*) missense mutation has been described in two previous cases of cutaneous carcinosarcomas, including a case of trichoblastic carcinosarcoma.

High-Quality Green-Emitting Nanodiamonds Fabricated by HPHT Sintering of Polycrystalline Shockwave Diamonds.

We demonstrate a high-pressure, high-temperature sintering technique to form nitrogen-vacancy-nitrogen centres in nanodiamonds. Polycrystalline diamond nanoparticle precursors, with mean size of 25 nm, are produced by the shock wave from an explosion. These nanoparticles are sintered in the presence of ethanol, at a pressure of 7 GPa and temperature of 1300 °C, to produce substantially larger (3-4 times) diamond crystallites. The recorded spectral properties demonstrate the improved crystalline quality. The types of defects present are also observed to change; the characteristic spectral features of nitrogen-vacancy and silicon-vacancy centres present for the precursor material disappear. Two new characteristic features appear: (1) paramagnetic substitutional nitrogen (P1 centres with spin ½) with an electron paramagnetic resonance characteristic triplet hyperfine structure due to the I = 1 magnetic moment of the nitrogen nuclear spin and (2) the green spectral photoluminescence signature of the nitrogen-vacancy-nitrogen centres. This production method is a strong alternative to conventional high-energy particle beam irradiation. It can be used to easily produce purely green fluorescing nanodiamonds with advantageous properties for optical biolabelling applications.

Photoluminescence from NV- Centres in 5 nm Detonation Nanodiamonds: Identification and High Sensitivity to Magnetic Field.

The content of nitrogen-vacancy (NV-) colour centres in the nanodiamonds (DNDs) produced during the detonation of nitrogen-containing explosives was found to be 1.1 ± 0.3 ppm. This value is impressive for nanodiamonds of size < 10 nm with intentionally created NV- centres. The concentration was estimated from the electron paramagnetic resonance as determined from the integrated intensity of the g = 4.27 line. This line is related with "forbidden" ∆ms = 2 transitions between the Zeeman levels of a NV- centre's ground triplet state. Confocal fluorescence microscopy enables detection of the red photoluminescence (PL) of the NV- colour centres in nanoscale DND aggregates formed from the 5-nm nanoparticles. Subwavelength emitters consisting of NV- with sizes a few times smaller than the diffraction-limited spot are clearly distinguished. We have further observed an abrupt drop in the PL intensity when mixing and anti-crossing of the ground and excited states spin levels in NV- occurs under an applied external magnetic field. This effect is a unique quantum feature of NV- centres, which cannot be observed for other visible domain light-emitting colour centres in a diamond lattice.

The structure of deterministic mass and surface fractals: theory and methods of analyzing small-angle scattering data.

Small-angle scattering (SAS) of X-rays, neutrons or light from ensembles of randomly oriented and placed deterministic fractal structures is studied theoretically. In the standard analysis, a very few parameters can be determined from SAS data: the fractal dimension, and the lower and upper limits of the fractal range. The self-similarity of deterministic structures allows one to obtain additional characteristics of their spatial structures. In the present work, we consider models that can describe accurately SAS from such structures. The developed models of deterministic fractals offer many advantages in describing fractal systems, including the possibility to extract additional structural information, an analytic description of SAS intensity, and effective computational algorithms. The generalized Cantor fractal and few of its variants are used as basic examples to illustrate the above concepts and to model physical samples with mass, surface, and multi-fractal structures. The differences between the deterministic and random fractal structures in analyzing SAS data are emphasized. Several limitations are identified in order to motivate future investigations of deterministic fractal structures.

Comment on "Angstrom-scale probing of paramagnetic center location in nanodiamonds by 3He NMR at low temperatures" by V. Kuzmin, K. Safiullin, G. Dolgorukov, A. Stanislavovas, E. Alakshin, T. Safin, B. Yavkin, S. Orlinskii, A. Kiiamov, M. Presnyakov, A. Klochkov and M. Tagirov, Phys. Chem. Chem. Phys., 2018, 20, 1476.

Kuzmin et al. reported on the application of the 3He NMR relaxometry technique to localization of intrinsic paramagnetic defects in detonation nanodiamond (DND) particles. We found some inconsistencies in the DND characterization, the model of paramagnetic defects' allocation used in that work as well as the estimation of distances between 3He NMR probes and paramagnetic defects, which question the results obtained.

Multiscale wavelet decomposition of time-resolved X-ray diffraction signals in cyclohexadiene.

We demonstrate how the wavelet transform, which is a powerful tool for compression, filtering, and scaling analysis of signals, may be used to separate large- and short-scale electron density features in X-ray diffraction patterns. Wavelets can isolate the electron density associated with delocalized bonds from the much stronger background of highly localized core electrons. The wavelet-processed signals clearly reveal the bond formation and breaking in the early steps of the photoinduced pericyclic ring opening reaction of 1,3-cyclohexadiene, which are not resolved in the bare signal.

Macrocycle ring deformation as the secondary design principle for light-harvesting complexes.

Natural light-harvesting is performed by pigment-protein complexes, which collect and funnel the solar energy at the start of photosynthesis. The identity and arrangement of pigments largely define the absorption spectrum of the antenna complex, which is further regulated by a palette of structural factors. Small alterations are induced by pigment-protein interactions. In light-harvesting systems 2 and 3 from Rhodoblastus acidophilus, the pigments are arranged identically, yet the former has an absorption peak at 850 nm that is blue-shifted to 820 nm in the latter. While the shift has previously been attributed to the removal of hydrogen bonds, which brings changes in the acetyl moiety of the bacteriochlorophyll, recent work has shown that other mechanisms are also present. Using computational and modeling tools on the corresponding crystal structures, we reach a different conclusion: The most critical factor for the shift is the curvature of the macrocycle ring. The bending of the planar part of the pigment is identified as the second-most important design principle for the function of pigment-protein complexes-a finding that can inspire the design of novel artificial systems.

Multifocal diffractive lens generating several fixed foci at different design wavelengths.

We propose a method for designing multifocal diffractive lenses generating prescribed sets of foci with fixed positions at several different wavelengths. The method is based on minimizing the difference between the complex amplitudes of the beams generated by the lens microrelief at the design wavelengths, and the functions of the complex transmission of multifocal lenses calculated for these wavelengths. As an example, a zone plate generating three fixed foci at three different wavelengths was designed, fabricated, and experimentally investigated. The proof-of-concept experimental results confirm the formation of foci with fixed positions at the design wavelengths. The obtained results may find applications in the design and fabrication of novel multifocal contact and intraocular lenses with reduced chromatic effects.

Power Spectrum of Long Eigenlevel Sequences in Quantum Chaotic Systems.

We present a nonperturbative analysis of the power spectrum of energy level fluctuations in fully chaotic quantum structures. Focusing on systems with broken time-reversal symmetry, we employ a finite-N random matrix theory to derive an exact multidimensional integral representation of the power spectrum. The N→∞ limit of the exact solution furnishes the main result of this study-a universal, parameter-free prediction for the power spectrum expressed in terms of a fifth Painlevé transcendent. Extensive numerics lends further support to our theory which, as discussed at length, invalidates a traditional assumption that the power spectrum is merely determined by the spectral form factor of a quantum system.

Small-angle scattering from the Cantor surface fractal on the plane and the Koch snowflake.

The small-angle scattering (SAS) from the Cantor surface fractal on the plane and Koch snowflake is considered. We develop the construction algorithm for the Koch snowflake, which makes possible the recurrence relation for the scattering amplitude. The surface fractals can be decomposed into a sum of surface mass fractals for arbitrary fractal iteration, which enables various approximations for the scattering intensity. It is shown that for the Cantor fractal, one can neglect with good accuracy the correlations between the mass fractal amplitudes, while for the Koch snowflake, these correlations are important. It is shown that nevertheless, correlations can be built in the mass fractal amplitudes, which explains the decay of the scattering intensity I(q) ∼ qDs-4, with 1 < Ds < 2 being the fractal dimension of the perimeter. The curve I(q)q4-Ds is found to be log-periodic in the fractal region with a period equal to the scaling factor of the fractal. The log-periodicity arises from the self-similarity of the sizes of basic structural units rather than from correlations between their distances. A recurrence relation is obtained for the radius of gyration of the Koch snowflake, which is solved in the limit of infinite iterations. The present analysis allows us to obtain additional information from SAS data, such as the edges of the fractal regions, the fractal iteration number and the scaling factor.

Fluorescence correlation spectroscopy in thin films at reflecting substrates as a means to study nanoscale structure and dynamics at soft-matter interfaces.

Structure and dynamics at soft-matter interfaces play an important role in nature and technical applications. Optical single-molecule investigations are noninvasive and capable to reveal heterogeneities at the nanoscale. In this work we develop an autocorrelation function (ACF) approach to retrieve tracer diffusion parameters obtained from fluorescence correlation spectroscopy (FCS) experiments in thin liquid films at reflecting substrates. This approach then is used to investigate structure and dynamics in 100-nm-thick 8CB liquid crystal films on silicon wafers with five different oxide thicknesses. We find a different extension of the structural reorientation of 8CB at the solid-liquid interface for thin and for thick oxide. For the thin oxides, the perylenediimide tracer diffusion dynamics in general agrees with the hydrodynamic modeling using no-slip boundary conditions with only a small deviation close to the substrate, while a considerably stronger decrease of the interfacial tracer diffusion is found for the thick oxides.