Claudiu Teodor Fleaca

Optical response is commonly regarded as an intrinsic property of a photonic structure. Here we show that, in curved photonic systems, geometry plays a dual role: the photonic architecture defines the optical-state landscape, whereas the illumination–observation geometry determines which optical states become experimentally accessible. Self-assembled, millimetre-scale, free-standing curved silica opals coupled to spherical water droplets on Teflon or planar mirrors reveal simultaneous access to multiple Bragg diffraction bands, together with distance-controlled spectral redistribution, spatially organized halos, and pseudo-collimated beams under different illumination–observation geometries. Natural illumination further reveals optical responses relevant to biological and biomimetic photonic structures. Key aspects of this optical functionality are transferred to a graded curved photonic heterostructure formed by partial polystyrene infiltration of a curved silica opal, demonstrating that the underlying geometrical principle extends beyond liquid-confined systems. Curved photonic architectures therefore provide an experimental platform for geometry-controlled optical-state accessibility.

Optical response is commonly regarded as an intrinsic property of a photonic structure. Here we show that, in curved photonic systems, geometry plays a dual role: the photonic architecture defines the optical-state landscape, whereas the illumination–observation geometry determines which optical states become experimentally accessible. Self-assembled, millimeter-scale, free-standing curved silica opals coupled to spherical water droplets on Teflon or planar mirrors reveal simultaneous access to multiple Bragg diffraction bands, together with distance-controlled spectral redistribution, spatially organized halos, and pseudo-collimated beams under different illumination–observation geometries. Natural illumination further reveals optical responses relevant to biological and biomimetic photonic structures. Key aspects of this optical functionality are transferred to a graded curved photonic heterostructure formed by partial polystyrene infiltration of a curved silica opal, demonstrating that the underlying geometrical principle extends beyond liquid-confined systems. Curved photonic architectures therefore provide an experimental platform for geometry-controlled optical-state accessibility.

By combining hanging-drop self-assembly with melt infiltration and selective inversion, we fabricate millimetric and free-standing curved photonic heterostructures that integrate infiltrated-opal, inverse-opal, embossed, and white-scattering 2.5D metasurface domains within a single continuous body. These architectures enable configurations inaccessible to planar fabrication, including naturally formed concavities within convex inverse-opal films and alternating ordered/single-layer regions that preserve local coherence while introducing disorder at larger scales. Across these heterogeneous curved landscapes, we observe optical phenomena absent in flat photonic structures—spectrally selected lateral collimation, geometry-shifted ghost images, and transmission-derived valleys shaped by curvature-mediated Bragg extraction. Their origin lies in the geometric constraints inherent to curved assemblies, where spatially varying normals, non-parallel lattice orientations, and topologically required defects couple order and disorder into a distributed-coherence regime. This coupling expands the accessible photonic state space, establishing curvature as an active functional degree of freedom rather than a geometric constraint, positioning the self-assembled photonic heterostructures as a scalable route toward multifunctional 3D metasurfaces and new regimes of light–matter interaction.

Silicon–tin (Si–Sn) nanocomposites are viable anode substitutes for lithium-ion batteries (LIBs) as they have the high theoretical storage capacities of lithium (4200 mAh g ⁻¹ (Si) and 994 mAh g ⁻¹ (Sn)), which are significantly more than those of graphite (372 mAh g ⁻¹ ). The Si and Sn cannot be used widely due to their extreme volume expansion during cycling, which results in limited structural stability and quick capacity deterioration. Herein, a nanocomposite based on reduced graphene oxide and nanoparticles containing silicon and tin species (Sn/Si@reduced graphene oxide (RGO)) is presented as a potential solution to mitigate these challenges by incorporating rGO to enhance the electrochemical performance. This nanocomposite demonstrates a high discharge capacity of 1506.91 mAh g ⁻¹ at a current density of 0.55 A g ⁻¹ after 205 cycles. In contrast, the Sn/Si nanoparticles only kept 125.85 mAh g ⁻¹ after the same number of cycles. Sn/Si@rGO nanocomposite improved electrochemical performance due to the conductive network provided by rGO, which adapts to volume variations during cycling and facilitates charge transfer. Results from galvanostatic testing, cyclic voltammetry, and electrochemical impedance spectroscopy support the idea that Sn/Si @rGO nanocomposites could be a high-performance anode material for next-generation LIBs.

This study introduces novel experimental systems that facilitate the nucleation, growth, aggregation, and agglomeration of ionic salt solutions, leading to structurally and functionally distinctive crystal formations. Through evaporative crystallization in hanging drops—including layered binary solutions—a range of macroscopic agglomerates were produced, such as hollow spheroidal NaCl/NiSO₄ structures, octahedral NaCl films, pentagonally arranged CdSO₄ spherulites, and NH₄Cl dendritic shells. Additionally, NaCl spheroids were used as templates to fabricate carbon-based morphologies and colloidal photonic crystals with convex or concave geometries, which were subsequently analyzed optically. The study reveals that crystallization and self-assembly, whether independently or synergistically applied, can yield complex architectures with potential applications in advanced device manufacturing beyond conventional processing methods.

The laser pyrolysis technique was used in the synthesis of magnetic iron oxide nanopowders in the presence of ethanol vapors as a sensitizer. This technique uses the energy from a continuous-wave CO₂ laser operating at a 9.25 μm wavelength, which is transferred to the reactive precursors via the excited ethanol molecules, inducing a rapid heating of the argon-entrained Fe(CO)₅ vapors in the presence of oxygen. For a parametric study, different samples were prepared by changing the percentages of sensitizer in the reactive mixture. Moreover, the raw samples were thermally treated at different temperatures and their morpho-structural and magnetic properties were investigated. The results indicated a high degree of crystallinity (mean ordered dimension) and enhanced magnetic properties when high percentages of ethanol vapors were employed. On the contrary, at low ethanol concentrations, due to a decrease in the reaction temperature, nanoparticles with a very low size were synthesized. The raw particles have a dimension in the range of 2.5 to 10 nm (XRD and TEM). Most of them exhibited superparamagnetic behavior at room temperature, with saturation magnetization values up to 60 emu/g. The crystalline phase detected in samples is mainly maghemite, with a decreased carbon presence (up to 8 at%). In addition to the expected Fe-OH on the particles surfaces, C (and O) bearing functional groups such as C-OH or C=O that act as a supplementary hydrophilic agent in water-based suspension were detected. Using the as-synthesized and thermally treated nanopowders, water suspensions without or with hydrophilic agents (CMCNa, L-Dopa, chitosan) were prepared by means of a horn ultrasonic homogenizer at 0.5 mg/mL concentrations. DLS analyzes revealed that some powder suspensions maintained stable agglomerates over time, with a mean size of 100 nm, pH values between 4.8 and 5.3, and zeta-potential values exceeding 40 mV. All tested agents greatly improved the stability of 250–450 °C thermally treated NPs, with L-Dopa and Chitosan inducing smaller hydrodynamic sizes.

High-quality, 3D-shaped, SiO₂ colloidal photonic crystals (ellipsoids, hyperboloids, and others) were fabricated by self-assembly. They possess a quadratic surface and are wide-angle-independent, direction-dependent, diffractive reflection crystals. Their size varies between 1 and 5 mm and can be achieved as mechanical-resistant, free-standing, thick (hundreds of ordered layers) objects. High-quality, 3D-shaped, polystyrene inverse-opal photonic superstructures (highly similar to diatom frustules) were synthesized by using an inside infiltration method as wide-angle-independent, reflective diffraction objects. They possess multiple reflection bands given by their special architecture (a torus on the top of an ellipsoid) and by their different sized holes (384 nm and 264 nm). Our hanging-drop self-assembly approach uses setups which deform the shape of an ordinary spherical drop; thus, the colloidal self-assembly takes place on a non-axisymmetric liquid/air interface. The deformed drop surface is a kind of topological interface which changes its shape in time, remaining as a quality template for the self-assembly process. Three-dimensional-shaped colloidal photonic crystals might be used as devices for future spectrophotometers, aspheric or freeform diffracting mirrors, or metasurfaces for experiments regarding space-time curvature analogy.

In order to find solutions to current worldwide environmental problems, it is crucial to develop sustainable nanomaterials, ideally with multifunctional properties. Considering this, novel TiO₂-SnO₂@NMs (noble metals: Au and Ag) composites, for use as sustainable nanomaterials, were successfully prepared via a two-step synthesis process consisting of laser pyrolysis followed by the chemical impregnation of the collected materials with noble metals. The addition of SnO₂ favors the transformation of TiO₂ from a mixture with a majority Anatase phase to one with a Rutile phase majority. With consideration for their level of environmental toxicity, the features of the synthesized nanomaterials were structurally, morphologically, and optically described and assessed for environmental protection applications as gas sensors and photocatalysts. In the case of the Surface Acoustic Wave sensor, based on a pure TiO₂ nanopowder, a notable difference in the frequency shift was detected in comparison to the other examined sensors. All sensors responded to the CH₄ concentrations tested (0.02–0.1%). On the other hand, when methyl orange was photodegraded under visible light, the results obtained using NMs for decoration revealed that the photocatalytic activity of TiO₂-SnO₂@NMs was significantly improved compared to the TiO₂-SnO₂ binary composite, which already has an enhanced photocatalytic activity, compared to pure TiO₂. Overall, this work produces nanoparticles that exhibit better sensory and photocatalytic features, as well as higher levels of biocompatibility with skin cells, for use as eco-friendly nanomaterials for a sustainable future.

High-quality convex colloidal photonic crystals can be grown on the tip of an optical fiber by self-assembly using the hanging drop method. They are convex-shaped, produce the diffraction of reflecting light with high efficiency (blazing colors), and have a high curvature. The convex colloidal crystals are easily detachable and, as free-standing objects, they are mechanically robust, allowing their manipulation and use as convex reflective diffraction devices in imaging spectrometers. Currently, the same characteristics are obtained by using gratings-based structures. The optical fiber/colloidal crystal interface is disordered; thus, no light diffraction can be registered. The ordering at this interface was highly increased by forming a polystyrene spacer on the optical fiber tip, which served as a self-assembly substrate for silica colloid, as a mechanical bond between the fiber and the crystal, and as a filler reservoir for an inverse-opal synthesis. The silica opal-like grown on the optical fiber tip can be transformed into a high-quality polystyrene (blazing colors) inverse-opal by using the polystyrene spacer as a filler. We found that the colloidal crystal axisymmetric self-assembles onto the optical fiber tip only if a maximum volume of the colloid drop is settled on a flat end of the polystyrene spacer.

Silica and polystyrene spheres with a small size ratio (r = 0.005) form by sequential hanging drop self-assembly, a binary colloidal crystal through which calcination transforms in a silica-ordered concavity array. These arrays are capable of light Bragg diffraction and shape dependent optical phenomena, and they can be transformed into inverse-opal structures. Hierarchical 2D and 3D super-structures with ordered concavities as structural units were fabricated in this study.