Nanocrystal Films Research Articles

Multimodal, Convertible, and Chiral Optical Films for Anti‐Counterfeiting Labels

AbstractDesigning a sustainable security model that carries multilevel information with numerous optical states will significantly enhance anti‐counterfeiting abilities to deter counterfeits, ranging from artworks, currencies, and foods to medicines, but is enormously challenging. Herein, ambient‐friendly, large‐area, quadruple‐level, and chiral luminescent materials are prepared by incorporating lanthanide complexes wrapped in poly(ethylene glycol) matrix into chiral nematic cellulose nanocrystal (CNC) films through a co‐assembly strategy. Due to the regulation of the chiral nematic structures, the composite films enable full‐color structural colors and tunable fluorescence. Notably, the brightest fluorescent film radiates right‐handed circularly polarized luminescence with an asymmetric factor over −0.36, a lifetime up to 510 µs, and an absolute quantum yield up to 66.7%. More interestingly, a fascinating chiro‐optical behavior ranging from azure to khaki can be controlled by a polarizing filter at given 0° and 90° rotation angles. This anti‐counterfeiting system showcases the comprehensive properties of bright and responsive photoluminescence, quadruple and convertible color, and flexible and solvent‐resistant abilities. This CNC‐derived photonic material can act as the multimodal security label on a model banknote, which greatly facilitates its development for practical applications.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films

Although colloidal nanoparticles hold promise for fabricating electronic components, the properties of nanoparticle-derived materials can be unpredictable. Materials made from metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with internanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we assess the IMT as it depends on contact conductance and show how contact conductance is also key to predicting the temperature-dependence of conductivity in metallic films. The results establish a phase diagram for electron transport behavior that can guide the creation of metallic conducting materials from nanocrystal building blocks.

Nanocrystal Ordering Enhances Thermal Transport and Mechanics in Single-Domain Colloidal Nanocrystal Superlattices.

Colloidal nanocrystal (NC) assemblies are promising for optoelectronic, photovoltaic, and thermoelectric applications. However, using these materials can be challenging in actual devices because they have a limited range of thermal conductivity and elastic modulus, which results in heat dissipation and mechanical robustness challenges. Here, we report thermal transport and mechanical measurements on single-domain colloidal PbS nanocrystal superlattices (NCSLs) that have long-range order as well as measurements on nanocrystal films (NCFs) that are comparatively disordered. Over an NC diameter range of 3.0-6.1 nm, we observe that NCSLs have thermal conductivities and Young's moduli that are up to ∼3 times higher than those of the corresponding NCFs. We also find that these properties are more sensitive to NC diameter in NCSLs relative to NCFs. Our measurements and computational modeling indicate that stronger ligand-ligand interactions due to enhanced ligand interdigitation and alignment in NCSLs account for the improved thermal transport and mechanical properties.

Core-Shell Three-Dimensional Perovskite Nanocrystals with Chiral-Induced Spin Selectivity for Room-Temperature Spin Light-Emitting Diodes.

We developed type-II core-shell nanocrystals (NCs) with a chiral low-dimensional perovskite shell and an achiral 3D MAPbBr3 core. The core-shell NCs exhibit spin-polarized luminescence at the first excitation band of the achiral core, which is due to the chiral-induced spin selectivity (CISS) effect-governed spin-dependent shell-to-core electron transportation and the subsequent electron-hole recombination in the core. The preferred spin state of the transferred electrons is determined by the handness of the chiral shell. For the core-shell NCs film, a photoluminescence quantum yield (PLQY) of 54% and a circularly polarized luminescence (CPL) with a maximum |glum| of 4.0 × 10-3 are obtained at room temperature. Finally, we achieved a spin-polarized light-emitting diode (spin-LED), affording a circularly polarized electroluminescence (CP-EL) with a |gCP-EL|of 6.0 × 10-3 under ambient conditions.

Effect of a trace amount of deep eutectic solvents on the structure and optical properties of cellulose nanocrystal films

Effect of a trace amount of deep eutectic solvents on the structure and optical properties of cellulose nanocrystal films

Revealing the vertical structure of in-situ fabricated perovskite nanocrystals films toward efficient pure red light-emitting diodes

The development of efficient perovskite light-emitting diodes (PeLEDs) relies strongly on the fabrication of perovskite films with rationally designed structures (grain size, composition, surface, etc.). Therefore, an understanding of structure-performance relationships is of vital importance for developing high-performance perovskite devices, particularly for devices with in-situ fabricated perovskite nanocrystal films. In this study, we reveal the vertical structure of an in-situ fabricated quasi-two-dimensional perovskite film. By combining time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and low-temperature photoluminescence spectra, we illustrate that the resulting in-situ fabricated DPPA2Csn-1Pbn(Br0.3I0.7)3n+1 (DPPA+: 3,3-diphenylpropylammonium) film has a gradient structure with a very thin layer of ligands on the surface, predominantly small-n domains at the top, and predominantly large-n domains at the bottom owing to the solubility difference of the precursors. In addition, GIWAXS measurements show that the domain of n = 2 on the top layer has an ordered in-plane alignment. Based on the understanding of the film structure, we developed an in-situ fabrication process with ligand exchange to achieve efficient pure red PeLEDs at 638 nm with an average external quantum efficiency (EQE) of 7.4%. The optimized device had a maximum luminance of 623 cd/m2 with a peak EQE of 9.7%.

Two-Photon Absorption and Photoluminescence of Individual CsPbBr3 Nanocrystal Superlattices

Nonlinear optical responses such as two-photon absorption and photoluminescence with halide perovskites are promising for the implications of fluorescence microscopy, high-density data storage, and microfabrication. Since a high degree of structural coherence can help improve the nonlinear optical responses, this study experimentally demonstrates that for individual CsPbBr3 nanocrystal superlattices, the two-photon luminescence is enhanced by about 16 times compared with that of randomly oriented nanocrystal films, and the polarization-dependent luminescence spectra reveal that the nanocrystal superlattices possess a 4-fold symmetry, which is similar to the single-crystal film structures. Due to their broad nonlinear absorption responses, wide-range interband photoluminescence is observed below the band gap. Furthermore, the intensity-dependent absorption results demonstrate that the two-photon absorption coefficient can be as large as 0.644 cm/MW, which is comparable with that of the semiconductor materials such as WS2 and ZnO. These superior performances make the CsPbBr3 nanocrystal superlattices promising for the implications of nonlinear devices such as optical power limiting and optical switches.

Broadband Enhancement of Mid‐Wave Infrared Absorption in a Multi‐Resonant Nanocrystal‐Based Device (Advanced Optical Materials 9/2022)

The back-cover image depicts an infrared sensor based on HgTe nanocrystals. Incoming infrared light is focused into the nanocrystal film thanks to the presence of the photonic structure, which generates multiple resonances and enhanced sensitivity of the sensor over a broad optical band in the infrared. For further details, see article number 2200297 by Angela Vasanelli, Emmanuel Lhuillier, and co-workers.

Study on Controlling the Surface Structure and Properties of a Cellulose Nanocrystal Film Modified Using Alkoxysilanes in Green Solvents.

Film and sheet products made from naturally derived materials that exhibit high-performance surface functions are important as regards the environment. This study aimed to control the surface structure of a cellulose nanocrystal (CNC) film modified using methyltriethoxysilane and tetraethoxysilane coprecursors with environmentally friendly solvents (water and ethanol) during a spin-coating process. The surface-modified CNC film on the glass substrate was evaluated by microstructure analyses (Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM)) and water contact angle (hydrophobicity) measurements. Through FT-IR, NMR, and XPS, it was confirmed that the silane compounds were chemically bonded to the surface of the CNC. The AFM images suggested that the local surface structure of the silylation-modified CNC film was formed along with the rod-like shape of the CNC. The water contact angle was approximately 90°, owing to the silylation of the hydroxy group and increased surface roughness of the CNC layer enabled by the sol-gel reaction.

Combining ZnO and PDINO as a Thick Cathode Interface Layer for Polymer Solar Cells.

Cathode interface layers (CILs) are important for electron extraction in polymer solar cells (PSCs). Currently, the thickness of CILs is often below 15 nm due to their low electron mobility, which is not favorable for large-scale fabrication. Herein, we report a thick CIL for efficient PSCs by modifying the ZnO nanocrystals (NCs) film with perylene diimides functionalized with amino oxide (PDINO). The combined ZnO NCs/PDINO CIL inherits the high electron mobility of ZnO NCs and dense morphology of PDINO, affording higher power conversion efficiencies (PCEs) of its devices than the sole component controls. The PSCs with the ZnO NCs/PDINO CIL also exhibit good tolerance to the CIL thickness, and the PM6:Y6 and PM6:BTP-eC9 devices can achieve high PCEs of over 15% at the CIL thickness of 70 nm. Further, the ZnO NCs/PDINO devices show better stability than those with sole ZnO NCs or PDINO. Our results provide a new way to construct potential CILs for high performance PSCs.

Supramolecular modulation of the mechanical properties of amino acid-functionalized cellulose nanocrystal films

Cellulose nanocrystal (CNC) is a promising building block for the bottom-up assembly of novel lightweight renewable materials. The ability to engineer the interfacial properties of CNC is of paramount importance to develop assembled materials for various applications; yet it remains a challenge to manipulate the supramolecular interactions within the cellulosic nanomaterials in a controlled manner. In this context, we demonstrate in this article how the assembly and mechanical properties of cellulose thin films can be controlled by manipulating the interfacial interactions of TEMPO-oxidized cellulose nanocrystals (TOCNCs) grafted with two different amino acids, arginine and tryptophan. The amino acid grafting onto the cellulose scaffold was confirmed in aqueous solutions by 1H NMR spectroscopy and in solid form by FTIR and XPS techniques. The surface functionalization of TOCNCs with simple cationic and aromatic groups provides a strategy for tuning the assembly of the nanostructures based on different supramolecular cross-linking interactions, including hydrogen bonds, π–π stacking, and cation–π interactions. In particular, we show that nanocellulose chains cross-linked by the non-covalent cation–π interactions lead to the formation of a laminated superstructure with high deformation and load standing capabilities. Our work provides a versatile strategy for tuning the surface properties of nanocellulose and represents an important step toward the development of sustainable materials with tailored mechanical properties, which will enable a wider application range of this building block all the way from tissue-engineering scaffolds to flexible devices.

Iridescent chiral nematic papers based on cellulose nanocrystals with multiple optical responses for patterned coatings

Chiral nematic papers (CNPs) with mesopores structure based on cellulose nanocrystals (CNCs) were fabricated successfully via a swelling and freeze-drying method. The order of the original chiral nematic cellulose nanocrystals film was preserved in CNPs, which was proved by scanning electron microscopy (SEM), polarized optical microscopy (POM) measurements and circular dichroism (CD) spectra. The CNPs exhibited excellent optical responsive properties to different solvents. Inspired by this feature, a colorable ink containing amounts of gel particles was prepared by pulverizing CNPs/water mixture into a suspension. Patterns written in suspension ink with various colors can be formed when soaked with different solvents. Moreover, CNPs displayed an irreversible color response to compression. Additionally, the hydrophilicity of CNPs was tuned by polyethyleneimine. Modified CNPs exhibited different colors under the identical solvent environment when compared to the original one. Aqueous PEI can be used as an ink to depict responsive photonic patterns on CNPs.

Sensitive chemoselectivity of cellulose nanocrystal films

Cellulose nanocrystals (CNCs), self-assembled into a chiral nematic structure film, create an advanced platform for the fabrication of remarkable sensing, photonic and chiral nematic materials. Despite extensive progress in the knowledge of functions of CNCs, their chemoselectivity has rarely been reported. Here, we report a brand-new perspective of CNCs in chemoselectivity, which shows sensitive selectivity even between isomers of monosaccharides and disaccharides by generating discernible crystal patterns. This sensitive selectivity of glucose homologs is attributed to the selective carbohydrate–carbohydrate interactions (CCIs) through generating hydrogen bonds between CNC units and the glucose homologs, endowing a remarkable color variation of the CNC films. Moreover, the CCIs are distinct for different immersion times and concentrations of glucose homologs, verified through Fourier Transform Infrared Spectroscopy (FTIR) spectra. Based on the sensitive CCIs, pristine CNC films are then used as templates to generate chiral mesoporous carbon films with tunable specific surface areas by assembly of CNC suspensions and the glucose homologs. We envision that the sensitive chemoselectivity of CNC films as well as precise structure modulation could provide insights into the recognition of carbohydrates and the preparation of mesoporous carbon in numerous practical applications. A pristine chiral nematic cellulose nanocrystal film with sensitive chemoselectivity to glucose homologues and even isomers of monosaccharides and disaccharide are prepared through self-assembly. For different kinds of glucose homologs, distinct crystal morphologies are formed due to the selective carbohydrate–carbohydrate interactions. Besides, the highly ordered left-helical layered structure of pristine cellulose nanocrystal film could be used as templates for the precise construction of delicate nanostructures.

Carboxylated cellulose nanocrystal films with tunable chiroptical properties

Carboxylated cellulose nanocrystals prepared by TEMPO-mediated oxidation exhibit a distinct ability to form nematic order, however, their ability to form chiral nematic films remains relatively unexplored. In this study, bleached cotton pulp hydrolyzed with hydrochloric acid and oxidized by TEMPO-mediated oxidation produce carboxylated cellulose nanocrystals with different aspect ratios 33.1, 32.8, 30.9, 29.0 and 28.9, and surface charge densities 0.16, 0.56, 1.00, 1.25, and 1.42 e·nm−2. By tuning the aspect ratio and surface charge density, the optimal carboxylated cellulose nanocrystals producing left-handed chiral nematic films by evaporation-induced self-assembly are obtained. The left-handed chiral nematic films enable selective reflection of left-handed circularly polarized light with the peak wavelength tunable from the visible to the near-infrared regime by modifying the characteristics of nanorods and suspensions. Such carboxylated cellulose nanocrystal films transform spontaneous luminescence to right-handed circularly polarized luminescence with the peak luminescence dissymmetry factor of −0.51.

Achieving direct electrophoretically deposited highly stable polymer induced CsPbBr3 colloidal nanocrystal films for high-performance optoelectronics

Synthesizing colloidal nanocrystals (NCs) with long-term stable under ambient conditions via appropriate ligands capping is highly challenging. Herein, the rapid synthesis of CsPbBr3 colloidal NCs at room temperature using polyvinylpyrrolidone as the primary ligand is demonstrated. The addition of ethylene glycol stabilizes the colloidal NCs and enhance their photoluminescence by ∼ 5-fold. Colloidal NCs have a net negative surface charge, which enables the direct electrophoretic deposition of a polymer-induced all-inorganic CsPbBr3 NC film with ultra-high stability under ambient conditions at relative humidity of 55% for 240 days. As an example of the application of the CsPbBr3 NC film in an optoelectronic device, a hybrid MoS2/CsPbBr3 field-effect phototransistor is fabricated, which exhibits a detectivity of 9 × 1012 Jones and an ultra-high responsivity of 1.2 × 107 A W−1 under the illumination of 450-nm laser at 4.4 pW. The noble and practical methods applied in our CsPbBr3 NC films could be extended to produce various types of uniform NC films over a large geometrical area with outstanding long-term environmental stability, holding significant promise for use in a broad range of advanced optoelectronic devices.

Characterization and investigation of intermediate stages during anion-exchange reaction between CsPbBr3 and CsPbI3 nanocrystal solution and film via photoluminescence and electroluminescence spectroscopy

All-inorganic cesium-lead-halide CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals (NCs) have attracted great attention due to their outstanding photoelectric properties in recent years. However, the unsatisfactory optical stability and undesirable anion-exchange reaction between different halides of perovskite NCs seriously hinder their further development and application in optoelectronic devices. Herein, the intermediate stages during anion-exchange processes between as-prepared perovskite NCs after mixing into solution and film were respectively studied and characterized by using photoluminescence and electroluminescence (EL) spectroscopy. In our case, the physical collision and solvent bridge effect together affect the anion-exchange reaction of the mixed solution, while the physical collision mechanism is dominant in the mixed film. More importantly, a thin polystyrene layer was inserted between CsPbBr3 and CsPbI3 to efficiently prevent the halogen anion-exchange reaction. Color-tunable EL performance and stable white emission were successfully achieved by employing the optimized CsPbI3/PS/CsPbBr3 structures onto the blue-violet heterojunctions.

Luminescent Thin Films Enabled by CsPbX3 (X=Cl, Br, I) Precursor Solution.

Inorganic cesium lead halide perovskite nanocrystals are candidates for lighting and display materials due to their outstanding optoelectronic properties. However, the dissolution issue of perovskite nanocrystals in polar solvents remains a challenge for practical applications. Herein, we present a newly designed one-step spin-coating strategy to prepare a novel multicolor-tunable CsPbX3 (X=Cl, Br, I) nanocrystal film, where the CsPbX3 precursor solution was formed by dissolving PbO, Cs2 CO3 , and CH3 NH3 X into the ionic liquid n-butylammonium butyrate. The as-designed CsPbX3 nanocrystal films show high color purity with a narrow emission width. Also, the blue CsPb(Cl/Br)3 film demonstrates an absolute photoluminescence quantum yields (PLQY) of 15.6 %, which is higher than 11.7 % of green CsPbBr3 and 8.3 % of red CsPb(Br/I)3 film. This study develops an effective approach to preparing CsPbX3 nanocrystal thin films, opening a new avenue to design perovskite nanocrystals-based devices for lighting and display applications.

Broadband Enhancement of Mid‐Wave Infrared Absorption in a Multi‐Resonant Nanocrystal‐Based Device

AbstractLight management is one of the main challenges to address when designing a sensor from a nanocrystal (NC) array. Indeed, the carrier diffusion length, limited by hopping mechanism, is much shorter than the absorption depth. Several types of resonators (plasmon, Bragg mirror, guided mode, Fabry–Perot cavity) have been proposed to reduce the volume where light is absorbed. All of them are inherently narrow bands, while imaging applications focus on broadband sensing. Here, an infrared sensor in the short and mid‐wave infrared (SWIR and MWIR) that combines three different photonic modes is proposed to achieve broadband enhancement of the light absorption. Moreover, it is shown that these three modes can be obtained from a simple structure where the NC film is coupled only to a grating and a top metallic layer. The obtained device achieves a high responsivity of >700 mA W–1, a detectivity up to 2 × 1010 Jones at 80 K, and a short response time of 11 µs.

Self-biased high sensitivity near-infrared photodetector based on nanocrystalline indium nitride film

Most photodetectors based on nanoparticles have been designed to be powered by an external power source, which is used to simulate the photo-created carriers in order to generate a photocurrent. This clearly increases the weight and size of the device. As a consequence, their applications in areas like in-situ wireless environmental sensing and medical therapy monitoring are strictly limited. Constructing self-powered (0 V bias voltage) systems that can operate autonomously, wirelessly, and sustainably is an important research direction for next-generation nanodevices. In this study, a novel near-infrared self-powered photodetector based on indium nitride nanocrystals (InN-NCs) film that operates without an external power source is fabricated. A cost-effective and simple radio frequency technique was used to growth the film onto silicon substrate. The photodetector has good self-powered photo-response properties under NIR (850 nm) light, including high sensitivity (1300) and good response time (rise time 160 ms and decay time 210 ms).

Enhanced anodic electrochemiluminescence due to efficient exchange recombination and its sensing application

To obtain semiconductor nanocrystals (NCs) with strong anodic electrochemiluminescence (ECL) remains a challenge due to their insufficiency to resist following corrosion upon holes injection to the NCs. Herein, we report strong and stable anodic ECL generated from 0.8 at% Mn2+ doped CdS NC films on indium tin oxide (ITO) electrode in the presence of coreactant triethanolamine (TEOA) under potential scan to +1.5 V vs. saturated calomel electrode (SCE), which is much more negative than the valence band potential of host NCs. The Mn2+ doping leads to an 8-fold enhancement in anodic ECL intensity in comparison with undoped CdS NC films due to efficient Mn excitation via nonradiative recombination energy transfer, i.e., exchange recombination. The efficiency is dependent on Mn2+ doping amount as well as applied potential for holes injection with the maximum obtained when the charge transfer to Mn dopants is possible and the energy gap between conduction band and applied potential matches with Mn excitation. This anodic ECL was used to detect dopamine, a model analyte, showing a wide linear range from 5 nM to 1 μM with a detection limit of 1 nM at S/N=3. This work provides a new insight into ECL generation in doped semiconductor NCs. The Mn2+ doped CdS NCs passivated by carboxylic groups display great potential in future development of biosensors.