Editorial for Themed Issue “Functional Nanostructures: Materials to Applications”

The nanoscience revolution that started in the early 1980s and culminated in a Nobel Prize in Chemistry in 2023 continues to shape the trajectory of current and emerging technologies across fields such as optoelectronics, spintronics, bioscience, and energy harvesting and conversion. Advances in the rational design and synthesis of functional nanostructures have not only enabled unprecedented control over matter at the atomic and molecular scale but have also revealed new regimes of physical phenomena governing light–matter interactions, magnetism, thermoelectricity, and beyond. As the field progresses, a need remains to push beyond established boundaries, exploring the fundamental properties, functionality, sustainability, and disruptive potential of nanomaterials. This special themed issue in Small Structures brings together pioneering contributions at the forefront of inorganic and hybrid nanomaterials research. This issue spans a broad spectrum of systems, from atomically precise structures to mesoscale assemblies, offering critical insights into structure–property relationships that translate into potential applications in optoelectronics and energy conversion. Surface plays a pivotal role in determining the functionality of nanostructures in many technological applications. However, the prominence of the surface presents a dual challenge: It requires developing precise characterization techniques while also understanding properties that directly result from surface chemistry and structure. Three contributions in this special issue address these aspects from different angles. In the work from the De Roo group [1], the authors develop two complementary protocols to quantify azide groups on hafnium oxide nanoparticle surfaces, crucial for efficient bioconjugation via click chemistry. Standard NMR methods fail at low coverages, so they employ fluorescence quenching and UV–Vis absorption to track surface ligands and reaction progress. These approaches provide accurate measurements of azide density, enabling precise control over nanoparticle valency and offering a robust framework for designing low toxicity nanocarriers with tailored surface functionality. In the work from Cerretani, Kondo, and Vosch [2], the authors elucidate the atomic structure of a 28-atom, near-infrared-emitting DNA-stabilized silver nanocluster using single-crystal X-ray diffraction and mass spectrometry. By solving the crystal structure with single-crystal X-ray diffraction and combining it with mass spectrometry, the authors provide an atomically precise picture of the cluster. Rod-shaped clusters exhibit emission at 960 nm in solution and at 1055 nm in the solid state, linked to differences in chlorido-ligand coordination. This highlights the close link between surface coordination and photophysical behavior. With brightness exceeding FDA-approved dyes, these clusters hold promise for deep-tissue imaging and biosensing. In a third contribution, from the Bacher group [3], the focus shifts to magic-sized cadmium selenide clusters consisting of just 13 CdSe units. These ultrasmall clusters sit at the boundary between molecules and quantum dots, enabling atomic-scale studies of structure–property relationships. At low temperatures, the photoluminescence from these clusters displays the expected features at the blue edge of the visible spectrum. Upon heating, sharp emission near the excitonic band edge emerges, attributed to polarons mediated by undercoordinated selenium defects at the cluster surface. Time-resolved spectroscopy and modeling reveal that defect-driven lattice rearrangements modulate photoluminescence energy. The study demonstrates how surface defects can stabilize novel emissive states, guiding the design of temperature-sensitive optical emitters. Self-assembly provides a powerful bottom-up route to functional nanomaterials, enabling nanoparticles to spontaneously organize into low-energy configurations during processes such as solvent removal or polymer encapsulation. This approach not only allows the integration of multiple components into hybrid systems but also gives rise to emergent structures whose properties differ from those of the individual building blocks. Two contributions in this special issue highlight complementary aspects of self-assembly. In the first contribution from the Vossmeyer group [4], the authors assemble titania nanoplates into submicron supraparticles via an emulsion-based approach, further stabilizing them with a tunable polystyrene shell through photocatalyzed polymerization. Oleylamine ligands mediate nanoparticle–polymer coupling, and polymerization enhances internal ordering. Functional versatility is demonstrated by modifying the shell with reactive groups and coassembling with magnetic nanocubes to create hybrid optical–magnetic supraparticles. This strategy enables multifunctional colloidal materials with tailored structural and responsive properties for photonic and biomedical applications. In a different direction, the Baranov group [5] explores the spontaneous self-assembly of quantum-confined perovskite nanocubes into diverse superlattice morphologies, including square, rhombic, and frame-like structures. Variations in nanocrystal size and shape dispersity dictate the assembly outcome, while solvent evaporation and diffusion govern complex structures. X-ray nanodiffraction reveals distinct packing orders linked to nanocrystal softness and surface chemistry. This work highlights how controlling superlattice formation can enable perovskite-based light-emitting materials with tunable optical properties. Further, three contributions focus on the rapid evolution of nanomaterials chemistry transforming the field of optoelectronics, where the interplay between structure, composition, and electronic properties enables advances in sensing, imaging, and energy conversion. These articles highlight complementary strategies: spanning narrow-bandgap semiconductors, hybrid perovskitoids, and quantum dots that collectively showcase how fundamental understanding and scalable synthesis converge to advance optoelectronic nanomaterials. The review by Cavallo et al. addresses one of the most mature colloidal nanomaterial platforms for infrared optoelectronics: HgTe NCs [6]. Long recognized for their unrivaled efficiency in mid- and far-infrared emission and detection, HgTe NCs nevertheless face challenges in precise carrier density control, a parameter central to device design and performance. By systematically analyzing carrier tuning across three interconnected scales, core composition, surface functionalization, and external gating, the authors provide a roadmap to manipulate doping and engineer p–n junctions at the nanoscale. This work identifies strategies to unlock higher device efficiencies, underlining the importance of electronic control beyond optical design. From another perspective, Olano-Vegas et al. explore zero-dimensional (0D) hybrid organic–inorganic perovskitoids, where structural isolation of [PbI6]4– octahedra gives rise to distinct optical phenomena [7]. Their study of the pyridinium-based compound 3 AMPy2PbI6 reveals how the choice of aromatic cations governs lattice anisotropy, exciton recombination pathways, and ultimately the photoluminescence properties. Polarization-resolved spectroscopy uncovers subtle orientational effects, while device tests confirm photoconductivity under visible excitation. Beyond structural characterization, this work emphasizes the role of organic–inorganic interactions in tailoring emission color, stability, and charge transport, qualities essential for the integration of perovskitoids in next-generation photodetectors and light-emitting devices. Complementing these insights into functional properties, Yadav et al. demonstrate progress in scalable synthesis, a critical bottleneck for the commercialization of colloidal semiconductors [8]. Focusing on InP/ZnS quantum dots, a cadmium-free alternative to conventional CdSe systems, the authors develop a continuous-flow synthesis that delivers gram-scale quantities of highly emissive nanocrystals with controlled size and photoluminescence quantum yields of 54%. Their combined synthetic innovation of using phosphorus and zinc precursors in noncoordinating solvents and mechanistic insights from solid-state NMR shed light on oxide intermediates that unexpectedly enhance luminescence. By addressing both scalability and surface chemistry, this study positions InP-based QDs as viable candidates for environmentally benign displays and lighting technologies. This themed issue further highlights the integration of nanomaterials into polymer matrices for the design of flexible and wearable electronic systems, where mechanical compliance must coexist with reliable electrical performance. The work by Roy et al. provides new insight into the dynamic network formation of 2D nanofillers within soft polymer hosts, elucidating how filler morphology, surface chemistry, and matrix polarity collectively shape the formation and stability of conductive pathways [9]. By systematically comparing 1D nanotubes and 2D graphene flakes, the authors demonstrate how dimensionality dictates the aggregation geometries of carbon nanofillers in both polar and apolar matrices. Using a combination of functionalized polyurethane and PDMS, they reveal that the presence of polar moieties in the host matrix strongly influences nanofiller interactions, guiding fractal-like aggregation patterns. Crucially, in situ USAXS measurements performed during simultaneous stretching and conductivity testing capture how loading fractions of functional fillers control the percolation threshold and network stability in real time. One of the most striking observations is the unique behavior of 2D nanoflakes under mechanical stress: Bending and unfolding events reshape the conductive architecture in ways unattainable with 1D fillers. This structural adaptability enhances percolation and provides new opportunities for tailoring conduction mechanisms in stretchable composites. This study not only clarifies the origin of dynamic network formation in flexible matrices but also provides a materials-by-design perspective for next-generation electronic skins, soft robotics, and wearable sensors. Moreover, Bergstrom and his group reported on their insights into cellulose-water integrations [10]. They used quasielastic neutron scattering to understand how hydration and temperature modulate hydrogen mobility and water diffusion dynamics in cellulose nanocrystals. Moving forward, this themed issue also illustrates how precise chemical control through cation substitution, heterostructure assembly, or interface engineering can reveal new functionalities and performance benchmarks in energy storage and conversion. The contributed articles exemplify the diverse yet convergent strategies for addressing the challenges in electrochemical performance of inorganic and hybrid nanomaterials. Liu and coworkers showcase a site-directed approach to engineer Pb-free layered double perovskite nanocrystals by substituting In3+ with Bi3+ and Sb3+ [11]. The resulting Cs4CoBi2Cl12 and Cs4CoSb2Cl12 broaden the compositional landscape of LDPs, with Cs4CoSb2Cl12 demonstrating exceptional stability over 100 days and Cs4CoBi2Cl12 delivering enhanced photoelectrochemical performance. This study underscores the promise of M(III) site engineering in designing stable, sustainable materials for solar energy conversion. Xu et al. introduce CuBSe2 nanocrystals, synthesized for the first time via a colloidal hot-injection route [12]. Morphology control through reaction kinetics, coupled with phase transformations upon lithiation, establishes these nanocrystals as a platform for studying superionic transitions near room temperature. Their findings position CuBSe2 as a versatile template for probing ion transport and developing solid-state electrolytes. Patil and colleagues report a one-pot hot-injection synthesis of In2O3@Cu and In2O3@Cu2.7P heterostructures [13]. By exploiting precursor reactivity and redox chemistry, they achieve sequential growth leading to oxide–phosphide hybrids with high catalytic activity in the oxygen reduction reaction. This mechanistic insight provides a predictive framework for constructing scalable heterostructures with tunable functionalities for energy applications. Finally, a study on Ni nanocluster/NiO nanosheet heterostructures highlights the importance of interface engineering in water splitting [14]. Incorporation of Ni nanoclusters significantly boosts hydrogen and oxygen evolution by enhancing charge transfer, with optimal activity at 20 wt% loading. Combined DFT and XPS analyses reveal that electronic interactions at the nanocluster/oxide interface facilitate oxygen desorption and intermediate formation, offering atomic-scale guidance for designing advanced electrocatalysts. All these contributions illustrate the breadth of current directions in functional nanomaterial research. Exploring structural design at multiple length scales from atomic substitution to nanoscale assembly and interfacial tuning, these articles demonstrate how cross-disciplinary collaborations is key to couple fundamental materials understanding with device-oriented metrics to accelerate translation into real-world technologies. The authors declare no conflicts of interest. Shalini Singh is an associate professor in the Department of Chemical Sciences at University of Limerick, where she leads the Functional Nanomaterial Research Group. She is also the lead for energy research pillar at the Bernal Institute, University of Limerick. Her work focuses on the design and synthesis of compositionally complex inorganic nanocrystals and nanocomposites, investigating their structural and surface characteristics and application in energy conversion, storage, and catalysis applications. She has authored more than 50 research articles. Her research is supported by Research Ireland and Horizon Europe grants, and she is a funded investigator in Research Ireland Centers AMBER and SSPC. Dmitry Baranov is an assistant professor at the Department of Chemistry, Lund University, where he leads the Nanochemistry and Spectroscopy Group. Supported by Horizon Europe and Swedish Research Council grants, his research focuses on the chemical synthesis, self-assembly, and collective photophysical phenomena in nanomaterials. He has authored more than 50 research articles and several reviews. He is also active in outreach, engaging students and the public in nanoscience through lectures and demonstrations. Loredana Protesescu is an associate professor specializing in the chemistry of nanomaterials at the Zernike Institute for Advanced Materials, University of Groningen. Her research focuses on inorganic and nanoscale materials, exploring their structural and surface chemistry to enable self-assembly and functional behavior. Her group designs semiconductor nanocrystals for applications in energy, extreme environments, and biotechnology, with notable contributions to lead and tin halide perovskite nanomaterials. She has published over 60 papers, held two patents, and advised AiLimiQ. Her honors include major NWO grants, the ERC Starting Grant, and the Nanomaterials Young Investigator Award. The data that support this editorial are available in the articles published in this themed collection as cited in the editorial.