Nanocrystal Films Research Articles

Mechanical Motion and Color Change of Humidity-Responsive Cellulose Nanocrystal Films from Sunflower Pith.

Nanocellulose has prompted extensive exploration of its applications in advanced functional materials, especially humidity-responsive materials. However, the sunflower pith (SP), a unique agricultural by-product with high cellulose and pectin content, is always ignored and wasted. This work applied sulfuric acid hydrolysis and sonication to sunflower pith to obtain nanocellulose and construct film materials with humidity-responsive properties. The SP nanoparticle (SP-NP) suspension could form a transparent film with stacked layers of laminated structure. Due to the tightly layered structure and expansion confinement effect, when humidity increases, the SP-NP film responds rapidly in just 0.5 s and completes a full flipping cycle in 4 s, demonstrating its excellent humidity-responsive capability. After removing hemicellulose and lignin, the SP cellulose nanocrystals (SPC-NC) could self-assemble into a chiral nematic structure in the film, displaying various structural colors based on different sonication times. The color of the SPC-NC film dynamically adjusted with changes in ambient humidity, exhibiting both functionality and aesthetics. This research provides a new perspective on the high-value utilization of sunflower pith while establishing a practical foundation for developing novel responsive cellulose-based materials.

Air-Stable Thin Films of Tin Halide Perovskite Nanocrystals by Polymers and Al2O3 Encapsulation.

Tin halide perovskites are promising for optoelectronics, although their sensitivity to ambient conditions due to Sn(II) oxidation presents a challenge. Encapsulation techniques can mitigate degradation and facilitate advanced studies of the intrinsic properties. To study and improve the ambient stability of CsSnBr3 and CsSnI3 nanocrystal (NC) thin films, we explored various encapsulation methods: organic, inorganic, and hybrid. We employed three methods for organic encapsulation: co-deposition with NCs, co-deposition with an additional top layer, and in situ polymerization with NCs. We synthesized thin layers of alumina by using atomic layer deposition for inorganic encapsulation. While individual methods offered marginal improvements, the hybrid approach provided the best results. By employing a hybrid heterostructured thin-film strategy, with the NC layer covered by a thin layer of poly(methyl methacrylate) followed by a 40 nm alumina layer, the stability in air was improved from a few seconds to a record period of 15 days, a crucial advancement for the further exploration of tin halide perovskites.

Prolonged Phase Segregation of Mixed-Halide Perovskite Nanocrystals in the Dark.

A critical issue hindering the potential applications of semiconductor mixed-halide perovskites is the phase segregation effect, wherein localized regions enriched with one type of halide anions would be formed upon continuous photogeneration of the excited-state charge carriers. These unexpected phases are capable of remixing again in the dark under the entropic driving force, the process of which is now being exclusively studied after the mixed-halide perovskites have arrived at the final stage of complete phase segregation. Here, we show that after the removal of laser excitation from a solid film of the mixed-halide perovskite nanocrystals (NCs) with partial phase segregation, the iodide- and bromide-rich regions can continuously grow in the dark for a prolonged time period of several minutes. We propose that this dark phase segregation is sustained by the local electric fields from the surface-trapped charge carriers, whose slow dissipation out of the mixed-halide perovskite NCs causes a delayed occurrence of the reversal phase remixing process.

Ultrabright and Efficient Green Perovskite Light-Emitting Diodes Enabled by Well-Crystallized Dense CsPbBr3 Nanocubes.

Perovskite light-emitting diodes (PeLEDs) are promising for next-generation high-definition displays. One of the keys to achieving high performance PeLEDs lies in how to fabricate crystalline and dense perovskite films. However, there exist challenges to directly grow well-crystallized CsPbBr3 nanocrystal thin films on transport layers due to low solubility in solvents and fast precipitation of all-inorganic CsPbBr3, and the corresponding bright, efficient, and stable green PeLEDs have rarely been reported. Herein, we report an efficient strategy to prepare well-crystallized and dense CsPbBr3 nanocubes for ultrabright and efficient green PeLEDs. We introduce sulfobetaine zwitterion as crystallization control agent and strontium fluoride nanocrystals as nucleation seeds to grow high-quality CsPbBr3 nanocube films. Eventually, the CsPbBr3 films enable green PeLEDs with a maximum luminance of 162 767 cd m-2 and a champion external quantum efficiency of 21.3% along with a narrow spectral line width of ∼14.7 nm, representing state-of-the-art performances in green PeLEDs.

Micrometer-Resolution Fluorescence and Lifetime Mappings of CsPbBr3 Nanocrystal Films Coupled with a TiO2 Grating.

Enhancing light emission from perovskite nanocrystal (NC) films is essential in light-emitting devices, as their conventional stacks often restrict the escape of emitted light. This work addresses this challenge by employing a TiO2 grating to enhance light extraction and shape the emission of CsPbBr3 nanocrystal films. Angle-resolved photoluminescence (PL) demonstrated a 10-fold increase in emission intensity by coupling the Bloch resonances of the grating with the spontaneous emission of the perovskite NCs. Fluorescence lifetime imaging microscopy (FLIM) provided micrometer-resolution mapping of both PL intensity and lifetime across a large area, revealing a decrease in PL lifetime from 8.2 ns for NC films on glass to 6.1 ns on the TiO2 grating. Back focal plane (BFP) spectroscopy confirmed how the Bloch resonances transformed the unpolarized, spatially incoherent emission of NCs into polarized and directed light. These findings provide further insights into the interactions between dielectric nanostructures and perovskite NC films, offering possible pathways for designing better performing perovskite optoelectronic devices.

Modulation of the Chirality and Dynamics of Self-Assembled Nanocellulose-Chiral C-Dot Film for Chiral Sensing Applications.

The detection and sensing of chirality using chiral biomaterials are growing areas of research in advanced bioelectronics. As a result, chiral-controlled biomaterials are crucial for advancing current technologies in chiral sensing applications within biosystems. A chiral carbon dot (C-dot) modulated self-assembled emissive cellulose nanocrystal (CNC) film is developed where the chirality of the CNC film can be tempered between left-handed and right-handed chirality after being doped with chiral L/D-C-dots in CNCs (C-dot-CNC film), transferring the chirality from C-dots to CNCs. The interaction between C-dots, CNCs, and carrier dynamics is investigated using a variety of steady-state and time-resolved PL spectroscopy techniques. The chiral C-dot enhanced the protonic conductivity across the CNC via the formation of hydrogen bonds with its surface functional groups and water molecules. Further, the chiral CNC-C-dots photoelectrodes demonstrate an excellent ability to distinguish between left-handed and right-handed small molecules. These findings on the underlying mechanism of spin selectivity between chiral CNC-C-dot and chiral ligand hold promise for the development of efficient chiral-sensing electronic devices.

Dimensionally integrated hybrid films of Ti3C2Tx MXene and cellulose nanocrystals for high-performance supercapacitors

The use of low-dimensional materials in electrode fabrication is gaining popularity due to their distinctive properties. This study introduces a straightforward method for creating a dimensionally integrated hybrid film that combines Ti3C2Tx MXene (2D) with cellulose nanocrystals (1D). The resulting freestanding hybrid film exhibits a lamellar structure, held together by weak intermolecular forces between the components. This structure leads to marked improvements in both structural and electrochemical properties. These enhancements are primarily due to the incorporation of 1D materials, which act as spacers to prevent the self-restacking typical of 2D materials and aid in the formation of electrochemically active sites and channels. This improvement stems from the synergistic effects of the combined materials, optimized at the ideal concentration ratio. This study may guide the development of electrodes integrated with redox-active materials, potentially advancing ideal energy storage systems.

Genetic algorithm driven optimization of a guided mode resonator coupled to an infrared nanocrystal film

Genetic algorithm driven optimization of a guided mode resonator coupled to an infrared nanocrystal film

Using Rheology and Image Processing to Study the Effects of Cellulose Nanocrystal Sedimentation.

The effect of sedimentation on lyotropic liquid crystalline dispersions is both an interesting subject in colloidal science and is of practical importance for understanding changes that can occur during dispersion storage. This research explored how the seemingly subtle changes in average length resulting from a single sedimentation step affected the rheological properties and self-assembly of aqueous dispersions of sulfated cellulose nanocrystals. Sedimentation of a primarily isotropic aqueous cellulose nanocrystal dispersion for 1 month at ambient conditions resulted in an isotropic top phase and a biphasic bottom phase, which were separated for further study. Both atomic force microscopy measurements and intrinsic viscosities determined via Fedor's equation showed preferential fractionation of longer cellulose nanocrystals (CNCs) to the bottom phase. The effects of this fractionation on dispersion rheology and phase behavior were studied by preparing a range of concentrations from each phase. As expected, the concentration for the isotropic-biphasic phase transition was significantly lower for the bottom phase than for the top phase or parent dispersion. Rheological changes were generally subtle, but significant differences were seen in the storage modulus at concentrations approaching rheological gelation. Most notably, quantitative image processing showed that even this simple, single-step fractionation process had a significant impact on the relative proportion of tactoids and chiral helices with planar and homeotropic anchoring in self-assembled cellulose nanocrystal films. These results highlight the impact that changes in polydispersity due to sedimentation can have on the self-assembly of nanomaterial mesogens.

Chiral Nematic Cellulose Nanocrystal Films for Enhanced Charge Separation and Quantum-Confined Stark Effect.

Efficient charge separation is essential in various optoelectronic systems, yet it continues to pose substantial challenges. Building upon the recent evidence that chiral biomolecules can function as electron spin filters, this study aims to extend the application of chirality-driven charge separation from the molecular level to the mesoscale and supramolecular scale. Utilizing cellulose nanocrystals (CNCs) derived from cellulose, the most abundant biomaterial on Earth, this research leverages their self-assembly into chiral nematic structures and their dielectric properties. A device is introduced featuring a chiral nematic hybrid film composed of CNCs and quantum dots (QDs), decorated with iron oxide nanoparticles. Using the quantum-confined Stark effect (QCSE) to probe charge separation, we reveal significant sensitivity to the circular polarization of light and the chiral nematic structure of the film. This approach achieves effective, long-lasting charge separation, both locally and across length scales exceeding 1 μm, enabling potential applications such as self-assembled devices that combine photovoltaic cells with electric capacitance as well as optical electric-field hybrid biosensors.

Liquid crystal phase behavior of oxalated cellulose nanocrystal and optical films with controllable structural color induced by centripetal force

Cellulose nanocrystal (CNC) is a sustainable bio-nanomaterial. The distinctive left-handed polarization properties render cellulose nanocrystal a promising candidate for optical film. Due to eco-friendliness, reliability, mildness and simplicity, the oxalate hydrolysis method stands out among various preparation methods for CNC. This study delved into the liquid crystal phase behavior of oxalated cellulose nanocrystal derived from pulp, and discovered the influences of CNC concentration and pH on suspension stability and phase transition, and evaluated its optical properties. The results demonstrated that oxalated CNC presented two different liquid crystal phases, the nematic phase and the cholesteric phase. The stability mechanism of CNC suspension and the regulatory principle of the liquid crystal phase transition were revealed. A novel CNC film-forming technology, the multilayer spin-coating technique, was developed for cellulose nanocrystal optical films. Driven by centrifugal force, cellulose nanocrystals were induced to self-assembly and formed the optical film with circular dichroism and structural color. This simple and efficient film-forming technology promised rapid processing (1 h) and controllable film structure and optical properties compared to traditional technologies. This work provided a theoretical understanding and practical prospects for integrating oxalated cellulose nanocrystal into sustainable advanced optical film materials.

Temperature-dependent Mn2+ emission in codoped CsPb2 (BrCl)5 perovskite nanocrystals

We prepared codoped CsPb2(BrCl)5 nanocrystal (NC) films by varying the Cu/Mn molar ratio using a facile two-step solution method. The Mn2+ emission peak is studied in the temperature range of 80–300 K. Strong electron–optical-phonon coupling causes an anomalous blueshift and linewidth broadening of the Mn2+ emission peak with increasing temperature. The average optical-phonon energy is approximately 50 meV. The activation energy reaches 79.32 meV, indicating that the introduction of Cu2+ in Mn-doped CsPb2(BrCl)5 NCs greatly enhances the energy transfer (ET) from the perovskite NC host to Mn2+, increasing the Mn2+ emission intensity. A clear understanding of the optical mechanism of Mn2+ emission is of great significance for the widespread application of codoped CsPb2(BrCl)5 NCs in optoelectronic devices.

Optically Ambidextrous Reflection Circularly Polarized Luminescence Film with High Dissymmetry Factor and On‐Demand Handedness

AbstractThe self‐assembled cellulose nanocrystal (CNC) film has a left‐handed layered structure, which makes the reflection of right‐handed circularly polarized (CP) light a challenge. Herein, a nematic phase layer is designed and inserted into the chiral organization, to work as a half‐wave retarder and make the ambidextrous CP light reflection. The maximum reflectivity exceeds 80%, which breaks the 50% limitations of single‐direction light reflection and is the current maximum in all the reported CNC‐based films. This “sandwich‐like” structure displays different optical properties on both sides, displayed as the chromatism and the inversion of the circular dichroism signals. The dual CP light reflection and direction‐dependent optical phenomena are reserved in the synthesized circularly polarized luminescence (CPL) film, with the dissymmetry factor (glum) of −0.4245. However, this luminescent intensity and single‐direction emission are not enough in advanced optical systems. This work designs a triple CPL amplification path and develops the handedness inversion strategy, with the glum of −1.0551 and 0.4082. Then, the dual‐directional CPL emission films are designed, where the chiral optics can be switched on‐demand. Finally, the photonic crystal films are applied in the anti‐counterfeit and chiral superstructure induction.

Visible and near-infrared modulating tungsten suboxide nanorods electrochromic films in acidic aqueous electrolytes

Proton-based aqueous electrolytes can be used to achieve high performance electrochromic nanocrystal thin films due to their small ion size. However, acidic aqueous electrolyte systems have not yet been explored in near-infrared (NIR) absorbing plasmonic tungsten oxide nanocrystal films. Here, we demonstrate tungsten suboxide nanorod films with excellent visible and NIR modulation performance in the H+-based aqueous electrolytes, thanks to their mesoporous structure, nanosized domains, and open tunnel structure. Colloidally synthesized WO2.83 nanorods with an average width of 6 nm and length of 48 nm were converted to WO2.90 nanorod film via annealing in air, while still preserving open tunnels. These films exhibit fast switching speed (tc = 0.9 s, tb = 2.1 s), excellent cycling stability over 2500 cycles, wide optical modulation up to ΔT = 53.8 % in the NIR region, and a high coloration efficiency (CE) of 167 cm2 C⁻1 at 1300 nm. Additionally, introducing a thin spacer (25 μm) reduced intrinsic NIR absorption from water, thereby enhancing the NIR modulation properties. These highly performing aqueous proton-electrolytes-based electrochromic devices open new possibilities for implementing visible and NIR electrochromism.

Effect of polymeric spherulite coatings on optical properties of chiral nematic cellulose nanocrystal films

AbstractCellulose nanocrystals (CNCs) have received an abundance of attention because of their distinctive chiral nematic structure and exceptional optical characteristics. However, the use of free‐standing films is constrained by the inherent stiffness and brittleness. In this work, CNC films with robust mechanical properties are prepared by simply coating them with polymeric spherulites. Maleic anhydride‐grafted polypropylene (PP‐g‐MA) is used to coat CNC films. The optical properties of coated films were also analyzed using polarized optical microscope. It is observed that the circular extinction patterns for the coated CNC film were created by the radial symmetry of banded spherulites, which were responsible for the perception of the optical transmission bands of the CNC film. Interfacial anchoring plays a crucial role in enhancing the overall performance and properties of the composite system. The future development of numerous portable functional components needing improved optical, mechanical, and thermal qualities of CNC films is made possible by this protective coating approach.

Tuning emittance in films of plasmonic metal oxide nanocrystals for daytime radiative cooling

Radiative cooling offers a passive, low-cost option for thermal management without expending energy on intensive active cooling mechanisms. Doped metal oxide nanocrystals are a promising option for maintaining high solar transparency while accessing tunable emission in the primary atmospheric transparency window from their localized surface plasmon resonance. Finite element method simulations of nanocrystal films are used to explore impacts of nanocrystal size, doping concentration, and film thickness on film emissivity. We reveal trade-offs between emission intensity and selectivity: Targeting selective emission results in unwanted reflectance from nanocrystal coupling limiting the maximum emissivity, while maximizing emissivity through increasing film thickness causes unwanted solar absorption from broadened emission. The trade-offs result in temperature-dependent design rules for optimizing radiated power. An optimized radiative cooler could provide 20 ° C of cooling, close to the ideal emitter. High emissivity is achievable in real nanocrystals which exhibit extinction peaks close to the best simulated case.

Thermal management towards ultra-bright and stable perovskite nanocrystal-based pure red light-emitting diodes

Despite the promising candidacy of perovskite nanocrystals for light-emitting diodes, their pure red electroluminescence is hindered by low saturated luminance, severe external quantum efficiency roll-off, and inferior operational stability. Here, we report ultra-bright and stable pure red light-emitting diodes by manipulating Joule heat generation in the nanocrystal emissive layer and thermal management within the device. Diphenylphosphoryl azide-mediated regulation of the nanocrystal surface synergistically enhances the optical properties and carrier transport of the emissive layer, enabling reduced Joule heat generation and thus lowering the working temperature. These merits inhibit ion migration of the CsPb(Br/I)3 nanocrystal film, promising excellent spectra stability. Combined with the highly thermal-conductive sapphire substrates and implementation of pulse-driving mode, the pure red light-emitting diodes exhibit an ultra-bright luminance of 390,000 cd m−2, a peak external quantum efficiency of 25%, suppressed efficiency roll-off, an operational half-life of 20 hours, and superior spectral stability within 15 A cm−2.

Impedance dynamics in tandem solar cells based on c-Si with upper layers of CsPbBr3 (CsPbI3) perovskite nanocrystals

The application of an additional nanoparticle layer is a common practice for enhancing the optical and electrical properties of third-generation solar cells. In this study, we present the results of impedance spectroscopy (IS) for modified solar cells using Nyquist and Bode diagrams. The structure investigated consists of a conventional double junction based on crystalline silicon (c-Si) coated with thin films of inorganic perovskite nanocrystals (NC) of lead halides CsPbI3 and CsPbBr3. The latter are characterized by a significant phonon disorder, which leads to unique electron-phonon interactions and dielectric responses. The IS results indicate that, under the same conditions, the measured Nyquist plots align well with the simulated ones. An equivalent circuit model is proposed, featuring ohmic resistance, recombination resistance, and geometric capacitance. These elements arise due to charge accumulation, charge transfer resistance, and/or additional interfacial electronic states. The study finds that the introduction of a CsPbI3 layer enhances the photoresponse under bias conditions, but this photoresponse leads to a decrease in DC conductivity. In contrast, the addition of a CsPbBr3 layer obstructs the photoresponse under bias while slightly improving the photoresponse in the absence of an applied voltage. The results obtained contribute to the improvement of tandem solar cell characteristics featuring top layers of perovskite nanocrystals.

Heterojunction functionalized sodium alginate/carboxylated cellulose nanocrystals film enhancing sterilization performance for wound healing

In the realm of natural polysaccharides, hydrogen bonding is a prevalent feature, yet its role in enhancing photocatalytic antimicrobial properties has been underexplored. In this paper, heterojunctions formed by graphene oxide (GO) and ZIF-8 were locked in sodium alginate/ carboxylated cellulose nanocrystals via hydrogen bonding networks, designated as SCGZ. The SCGZ films exhibit superior photocatalytic performance compared to either ZIF-8 or heterojunctions. This enhancement is primarily due to two key factors: firstly, the hydrogen bonding network significantly enhances the transfer of protons and holes, thereby improving the separation efficiency of photo-generated carriers; secondly, the hydrogen bonding between the layers facilitates a more efficient charge transfer, which expedites the movement of electrons from ZIF-8 to GO upon illumination. In vitro studies demonstrated that the SCGZ films possess remarkable antibacterial capabilities, achieving 99.75 % and 99.61 % inhibition rates against S. aureus and E. coli, respectively. In vivo animal experiments have shown that SCGZ films can significantly accelerate the healing process of damaged tissues, with a healing efficiency of up to 90.5 %. This research provides additional insights into the development of natural polysaccharide-based multi‑hydrogen bonded macromolecules with enhanced photocatalytic properties.

Effect of applied electrical potential and humidity on friction of Graphene-Based thin films

Understanding the nanofrictional characteristics of thin films in various conditions is important for fabricating nanoelectronic devices and micro-/nanoelectromechanical systems with high functionality and reliability. This study investigated the nanofrictional characteristics of composite thin films of graphene oxide (GO) and cellulose nanocrystals (CNCs) and of reduced graphene oxide (rGO) and CNCs, namely GO/CNC and rGO/CNC thin films, under an applied electrical potential and humidity by using a conductive atomic force microscope (AFM). For all films, the coefficient of friction (CoF) was affected by the applied electrical potential and enhanced mechanical properties resulting from strong interactions of GO and rGO with CNCs. Furthermore, the friction force increased with the electrical potential at low humidities (relative humidity (RH) < 40 %) because of the electrostatic force effect, while for RH>40 %, the friction force sensitively increased with the electrical potential because of the puckering effect. At high humidities (RH>40 %) and high electrical potentials (>4 V), the friction force decreased as the electrical potential increased, owing to the electron leakage phenomenon. This study also characterized the altered nanofrictional behavior influenced by interfacial phenomena in the films under an electrical potential and humidity, and it provides design guidelines for fabricating nanoelectronic devices and micro/nanoelectromechanical systems.