Nanocrystal Films Research Articles

Electrochemiluminescence and stability of cesium lead halide perovskite nanocrystals

This work is devoted to the synthesis and investigation of the stability of cesium lead halide CsPbBr3 perovskite nanocrystals for the problems of electroanalytical chemistry and in particular for the development of electrochemiluminescent sensors. The photoactivation and photodegradation of nanocrystals in solutions and polymer thin films are studied. Electrodes based on glassy carbon and modified by thin films containing CsPbBr3 nanocrystals are developed. The possibility of electrochemiluminescence signal generation with modified electrodes during the reaction with aqueous solutions of coreactant is studied. The important role of photoactivation of CsPbBr3 containing thin films of nanocrystals for obtaining the effect of electrochemiluminescence in them is revealed.

Numerical Model to Simulate Electrochemical Charging of Nanocrystal Films.

Electrochemical charging of nanocrystal films opens up new possibilities for designing quantum dot-based device structures, but a solid theoretical framework of this process and its limitations is lacking. In this work, drift-diffusion simulations are employed to model the charging of nanocrystal films and gain insight into the electrochemical doping process. Through steady state simulations it is shown that the Fermi level and doping density in the nanocrystal film depend on the concentration of the electrolyte in addition to the value of the applied potential. Time-resolved simulations reveal that charging is often limited by transport of electrolyte ions. However, ion transport in the film is dominated by drift, rather than diffusion, and the concentration profile of ions differs substantially from concentration profiles of diffusing redox species at flat electrodes. Classical electrochemical theory cannot be used to model this type of mass transport limited behavior in films of nanocrystals, so a new model is developed. We show that the Randles-Ševčík equation, which is derived for electrochemical species diffusing in solution, but is often applied to films as well, results in a significant underestimation of the diffusion coefficients of the charge compensating electrolyte ions.

Slipping-Free Halide Perovskite Supercrystals from Supramolecularly-Assembled Nanocrystals.

Supramolecularly assembled high-order supercrystals (SCs) help control the dielectric, electronic, and excitonic properties of semiconductor nanocrystals (NCs) and quantum dots (QDs). Ligand-engineered perovskite NCs (PNCs) assemble into SCs showing shorter excitonic lifetimes than strongly dielectric PNC films showing long photoluminescence (PL) lifetimes and long-range carrier diffusion. Monodentate to bidentate ligand exchange on ≈ 8nm halide perovskite (APbX3 ; A:Cs/MA, X:Br/I) PNCs generates mechanically stable SCs with close-packed lattices, overlapping electronic wave functions, and higher dielectric constant, providing distinct excitonic properties from single PNCs or PNC films. From Fast Fourier Transform (FFT) images, time-resolved PL, and small-angle X-ray scattering, structurally and excitonically ordered large SCs are identified. An Sc shows a smaller spectral shift (<35 meV) than a PNC film (>100 meV), a microcrystal (>100 meV), or a bulk crystal (>100 meV). Also, the exciton lifetime (<10ns) of an SC is excitation power-independent in the single exciton regime 〈N〉<1, comparable to an isolated PNC. Therefore, bidentate-ligand-assisted SCs help overcome delayed exciton or carrier recombination in halide perovskite nanocrystal assemblies or films.

Co-assembly of stearoylated cellulose nanocrystals and GO (or CNTs) for the construction of superhydrophobic hierarchical structure with enhanced photothermal conversion

The development of photothermal materials with high photothermal-conversion efficiencies is important in a range of applications, such as power generation, sterilization, desalination, and energy-production. To date, a few reports have been published related to improving the photothermal conversion performances of photothermal materials based on self-assembled nanolamellae. Herein, hybrid films of co-assembled stearoylated cellulose nanocrystals (SCNCs) and polymer-grafted graphene oxide (pGO)/polymer-grafted carbon nanotubes (pCNTs) were prepared. The chemical compositions, microstructures, and morphologies of these products were characterized, and it was found that the self-assembled SCNC structures exhibited numerous surface nanolamellae due to crystallization of the long alkyl chains. The hybrid films (i.e., SCNC/pGO and SCNC/pCNTs films) consisted of ordered nanoflake structures, confirming the co-assembly behavior of the SCNCs with pGO or pCNTs. The melting temperature (~65 °C) and latent heat of melting (87.87 J/g) of SCNC1.07 indicate its potential to induce the formation of nanolamellar pGO or pCNTs. Under light irradiation (50–200 mW/cm2), the pCNTs exhibited a higher light absorption capacity than pGO, and as a result, the SCNC/pCNTs film exhibited the best photothermal performance and electrical conversion, ultimately demonstrating its potential for use as a solar thermal device in practical applications.

Hexagonal Lead-Free Cs2AgBiI6 Perovskite Nanocrystals: A Promising Material for Solar Cell Application

Lead halide perovskite (LHP) nanocrystals (NCs) have extensive applications in optoelectronics. However, their high toxicity and low stability inhibit their commercialization. Herein, lead-free Cs2AgBiI6 NCs with a hexagonal phase are synthesized using a hot-injection process, which exhibit a band gap of 2.29 eV, a high absorption coefficient (∼2.8 × 105 cm–1), and a symmetric and narrow photoluminescence peak (full width at half-maximum = 10 nm). In addition, the hexagonal Cs2AgBiI6 NC films exhibit high resistance to heat, humidity, and illumination in air. Solar cells based on hexagonal Cs2AgBiI6 NCs are fabricated for the first time, achieving a power conversion efficiency (PCE) of 0.076%, which confirms its promising potential for photovoltaic applications.

Lasing properties and carrier dynamics of CsPbBr3 perovskite nanocrystal vertical-cavity surface-emitting laser

Abstract All-inorganic lead halide perovskite nanocrystals (NCs) have been widely investigated as highly promising optical gain materials due to their compelling electrical and optical properties. Although many efforts have been carried out, a deep understanding of perovskite NC vertical-cavity surface-emitting lasers (VCSELs) is elusive, which is very important in the development of photoelectronic integrated circuits. Along these lines, in this work, a low lasing threshold (22 μJ/cm2) single-mode VCSEL consisting of CsPbBr3 NCs film and two distributed Bragg reflectors was successfully constructed. The CsPbBr3 NCs were synthesized by using the supersaturated recrystallization method. Interestingly, benefiting from the strong coupling between the active layer and the optical field in the cavity, a single-mode lasing at 527 nm was demonstrated under femtosecond optical pumping. The carrier dynamics of the perovskite NC VCSEL was also thoroughly investigated by performing pump intensity-dependent time-resolved photoluminescence measurements. The typical gain-switching phenomenon was observed with an ultrafast decay of the laser pulse of ∼10 ps. Our work provides valuable insights for the implementation of the CsPbBr3 NC VCSEL for various optoelectronic applications.

Bias Reconfigurable Photoresponse of an Infrared Nanocrystal Film Integrated into a Coupled Fabry-Perot Resonator

The coupling of a photonic structure to a nanocrystal film enables light focusing over a distance compatible with the charge diffusion length. This strategy has been implemented in infrared sensors based on nanocrystal films to maximize their absorption. Here, we demonstrate that the photonic structure can be further exploited to generate a bias-reconfigurable photoresponse. To reach this goal, we use a coupled Fabry-Perot structure that can be seen as a periodic grating with two different cavities per period. The two cavities are coupled and a resonance appears, increasing the effective absorption coefficient by a factor of 30 compared to standard interdigitated electrodes. When connected in the appropriate configuration, this device enables the turning-on and -off of part of the spectral response (over a factor of 25 in magnitude) simply by tuning the applied bias (<1 V). Finally, we also demonstrate that this resonator geometry is promising for integration at the focal plane array level since the resonance is maintained for a pixel size as small as 10 μm.

Enhanced electron field emission of two-dimensional boron-doped ZnO nanoflakes on nanocrystal diamond films

Enhanced electron field emission of two-dimensional boron-doped ZnO nanoflakes on nanocrystal diamond films

All bio-based chiral nematic cellulose nanocrystals films under supramolecular tuning by chitosan/deacetylated chitin nanofibers for reversible multi-response and sensor application

Application of green and renewable all bio-based resources on functional nanomaterials has been of much interest, but often limited by their complex supramolecular construction. Herein, two kinds of most abundant biopolymers on earth, cellulose and chitin were selected as raw materials to prepare all bio-based iridescent films. Cellulose nanocrystals (CNCs) were cooperated with chitosan (CS) or deacetylated chitin nanofibers (DChN) at various content ratios to fabricate chiral nematic films via evaporation-induced self-assembly (EISA). The iridescent films showed increased crystallinity (80%∼ 90%) than pristine CNCs as well as lowered crystallite sizes, due to hydrogen bonding linkages and hydrophobic interactions between CNCs and amphiphilic CS/DChN with diverse degrees of acetylation. Obviously enlarged helical pitches were achieved by intercalation of CS/DChN, which reflected optical light with red-shifted wavelength bands. CS and DChN also improved the hydrophobicity of the nanocomposite films with various degrees, but both maintaining good biocompatibility proved by cell viability assay. The films showed wide-range/fast humidity and pH stimuli response capabilities via visible colorimetric changes. Moreover, the materials were equipped with satisfying reversible response, which was confirmed by repeated water ink writing-drying and acidic/alkaline vapor evaporation tests for 10 cycles. A food freshness sensor device was assembled on quick response (QR) code for sensitive and convenient recognition by smart phone, exhibiting high potential on multi-functional sensors of the all bio-based sustainable materials.

Improvement of Chemical Stability of Perovskite Nanocrystals as a Photoelectrochemical Catalyst for Hydrogen Evolution Reaction

Recent advances in the synthesis and processing of perovskite materials have led to significant improvements in their stability under harsh conditions, making them increasingly attractive for use as photo and photoelectrochemical catalysts. In particular, core-shell structured perovskite nanocrystals have greatly enhanced chemical stability, enabling their use in aqueous environments; however, their low conductivity remains a challenge. To address this issue, this study developed two-time cross-linkable core-shell perovskite nanocrystals using two types of silanes, resulting in excellent chemical stability and high conductivity. The first cross-linking reaction was spontaneously initiated in the solution state, forming an ultrathin Si-O-Si amorphous matrix on the surface of the perovskite nanocrystals. The second cross-linking reaction was intentionally induced in the film state by exposing it to UV light. The resulting cross-linked perovskite/SiO2 nanocrystal films exhibited a high packing density, and among four different dopants, the Ag-doped film demonstrated the lowest onset potential for HER in a 0.5 M H2SO4 aqueous solution due to effective band modulation. These findings suggest that the two-time cross-linking approach can significantly enhance the performance of perovskite nanocrystals in photoelectrochemical applications.

Visualizing the Shear Flow-Modulated Alignments in Cellulose Nanocrystal Films by the Mueller Matrix

Visualizing the Shear Flow-Modulated Alignments in Cellulose Nanocrystal Films by the Mueller Matrix

CORRELATION BETWEEN ELECTRICAL AND OPTICAL PROPERTIES OF DOPED SILICON NANOCRYSTALS

We propose through this work a correlation method leading to a determination of a semi-empirical relationship between optical and electrical properties in terms of refractive index and dark conductivity of doped silicon nanocrystals based on experimental data published in literature. First, an analytical model relating the conductivity and bandgap of doped silicon nanocrystals was derived. Using an empirical expression relating the refractive index to the bandgap energy, we correlated the electrical and optical parameters of N-type nanocrystalline silicon with a semi-empirical expression. The semi-empirical relationship was found to account correctly for the experimental results and yield a reasonably good agreement in an interval of the bandgap energy variation of N-type silicon nanocrystal films. The values of the fitting parameters were calculated for the N-type silicon nanocrystal films having their bandgap energy between 1. 7 eV and 2.2 eV.

Lithium-Ion Glass Gating of HgTe Nanocrystal Film with Designed Light-Matter Coupling.

Nanocrystals' (NCs) band gap can be easily tuned over the infrared range, making them appealing for the design of cost-effective sensors. Though their growth has reached a high level of maturity, their doping remains a poorly controlled parameter, raising the need for post-synthesis tuning strategies. As a result, phototransistor device geometry offers an interesting alternative to photoconductors, allowing carrier density control. Phototransistors based on NCs that target integrated infrared sensing have to (i) be compatible with low-temperature operation, (ii) avoid liquid handling, and (iii) enable large carrier density tuning. These constraints drive the search for innovative gate technologies beyond traditional dielectric or conventional liquid and ion gel electrolytes. Here, we explore lithium-ion glass gating and apply it to channels made of HgTe narrow band gap NCs. We demonstrate that this all-solid gate strategy is compatible with large capacitance up to 2 µF·cm-2 and can be operated over a broad range of temperatures (130-300 K). Finally, we tackle an issue often faced by NC-based phototransistors:their low absorption; from a metallic grating structure, we combined two resonances and achieved high responsivity (10 A·W-1 or an external quantum efficiency of 500%) over a broadband spectral range.

Remarkable Electrical Conductivity Increase and Pure Metallic Properties from Semiconducting Colloidal Nanocrystals by Cation Exchange for Solution-Processable Optoelectronic Applications.

The authors report a strategic approach to achieve metallic properties from semiconducting CuFeS colloidal nanocrystal (NC) solids through cation exchange method. An unprecedentedly high electrical conductivity is realized by the efficient generation of charge carriers onto a semiconducting CuS NC template via minimal Fe exchange. An electrical conductivity exceeding 10 500 S cm-1 (13 400 S cm-1 at 2 K) and a sheet resistance of 17 Ω/sq at room temperature, which are among the highest values for solution-processable semiconducting NCs, are achieved successfully from bornite-phase CuFeS NC films possessing 10% Fe atom. The temperature dependence of the corresponding films exhibits pure metallic characteristics. Highly conducting NCs are demonstrated for a thermoelectric layer exhibiting a high power factor over 1.2mW m-1 K-2 at room temperature, electrical wires for switching on light emitting diods (LEDs), and source-drain electrodes for p- and n-type organic field-effect transistors. Ambient stability, eco-friendly composition, and solution-processability further validate their sustainable and practical applicability. The present study provides a simple but very effective method for significantly increasing charge carrier concentrations in semiconducting colloidal NCs to achieve metallic properties, which is applicable to various optoelectronic devices.

Enhanced luminescence sensing performance and increased intrachain order in blended films of P3HT and cellulose nanocrystals

Blended films comprising poly(butyl acrylate) (PBA)-grafted cellulose nanocrystals (CNCs) and poly(3-hexylthiophene) (P3HT), exhibited more intense photoluminescence (PL) and longer PL emission lifetimes compared to pristine P3HT films. Optical absorption and photoluminescence spectra indicated reduced torsional disorder i.e. enhanced backbone planarity in the P3HT@CNC blended composites compared to the bare P3HT. Such molecule-level geometrical modification resulted in both smaller interchain and higher intrachain exciton bandwidth in the blended composites compared to the bare P3HT, because of reduced interchain interactions and enhanced intrachain order. These results indicate a potential switch of the aggregation behavior from dominant H-aggregates to J-aggregates, supported by Raman spectroscopy. The reorganization of micromolecular structure and concomitant macroscopic aggregation of the conjugated polymer chains resulted in a longer conjugation length for the P3HT@CNC blended composites compared to the bare P3HT. Additionally, this nanoscale morphological change produced a reduction in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap of the blends, evidenced from optical absorption spectra. Classical molecular dynamics simulation studies predicted the probability of enhanced planarity in the polymer backbone following interactions with CNC surfaces. Theoretical results from density functional theory calculations corroborate the experimentally observed reduction of optical bandgap in the blends compared to bare P3HT. The blended composite outperformed the bare P3HT in nitro-group PL sensing tests with a pronounced difference in the reaction kinetics. While the PL quenching dynamics for bare P3HT followed Stern–Volmer kinetics, the P3HT@CNC blended composite exhibited a drastic deviation from the same. This work shows the potential of a functionalized rod-like biopolymer in tuning the optoelectronic properties of a technologically important polymeric organic semiconductor through control of the nanoscale morphology.

Design and fabrication of superhydrophobic cellulose nanocrystal films by combination of self-assembly and organocatalysis

Cellulose nanocrystals, which have unique properties of high aspect ratio, high surface area, high mechanical strength, and a liquid crystalline nature, constitute a renewable nanomaterial with great potential for several uses (e.g., composites, films and barriers). However, their intrinsic hydrophilicity results in materials that are moisture sensitive and exhibit poor water stability. This limits their use and competitiveness as a sustainable alternative against fossil-based materials/plastics in packaging, food storage, construction and materials application, which cause contamination in our oceans and environment. To make cellulose nanocrystal films superhydrophobic, toxic chemicals such as fluorocarbons are typically attached to their surfaces. Hence, there is a pressing need for environmentally friendly alternatives for their modification and acquiring this important surface property. Herein, we describe the novel creation of superhydrophobic, fluorocarbon-free and transparent cellulose nanocrystal films with functional groups by a bioinspired combination of self-assembly and organocatalytic surface modification at the nanoscale using food approved organic acid catalysts. The resulting film-surface is superhydrophobic (water contact angle > 150°) and has self-cleaning properties (the lotus effect). In addition, the superhydrophobic cellulose nanocrystal films have excellent water stability and significantly decreased oxygen permeability at high relative humidity with oxygen transmission rates better than those of commonly used plastics.

A Hybrid Stainless-Steel SPME Microneedle Electrode Sensor for Dual Electrochemical and GC-MS Analysis.

A mechanically robust in-tube stainless steel microneedle solid phase microextraction (SPME) platform for dual electrochemical and chromatographic detection has been demonstrated. The SPME microneedle was fabricated by layer-by-layer (LbL) in-tube coating, consisting of carbon nanotube (CNT)/cellulose nanocrystal (CNC) film layered with an electrically conductive polyaniline (PANI) hydrogel layer (PANI@CNT/CNC SPME microneedle (MN)). The PANI@CNT/CNC SPME MN showed effective analysis of caffeine by GC-MS with an LOD of 26 mg/L and excellent precision across the dynamic range. Additionally, the PANI@CNT/CNC SPME MN demonstrated a 67% increase in sensitivity compared to a commercial SPME fiber, while being highly robust for repeated use without loss in performance. For electrochemical detection, the PANI@CNT/CNC SPME MN showed excellent performance for the detection of 3-caffeoylquinic acid (3-CQA). The dynamic range and limits of detection (LOD) for 3-CQA analysis were 75-448 mg/L and 11 mg/L, respectively. The PANI@CNT/CNC SPME MN was demonstrated to accurately determine the caffeine content and 3-CQA in tea samples and dark roast coffee, respectively. The PANI@CNT/CNC SPME MN was used for semiquantitative antioxidant determination and composition analysis in kiwi fruit using electrochemistry and SPME-coupled GC-MS, respectively.

Aging Effects on the Exciton Relaxation and Diffusion Processes in CsPbBr3 Nanocrystals

AbstractFully inorganic perovskite nanocrystals (NCs) have been widely investigated due to their potential as very interesting active materials for several types of photonic and optoelectronic devices. Despite several experiments designed to investigate the basic emission properties of these NCs, a clear and complete understanding of their photophysics is still missing. In this work, temperature‐dependent steady state and time‐resolved photoluminescence (PL) measurements are used to investigate the nature of the emitting states, the origin of the excitation relaxation dynamics and the effects of aging upon long exposure to wet air for thin films of CsPbBr3 NCs prepared by coprecipitation. It is demonstrated that both free excitons and localized excitons contribute to the NC emission and that electron traps within the conduction band, ≈14 meV and ≈80 meV above the band edge, determine thermal emission quenching. Moreover, it is shown that the non‐exponential PL relaxation dynamics are due to short‐range energy migration within a disordered distribution of localized states between 10 and 100 K, with activation of a second, long‐range diffusion process at higher temperatures. It is also demonstrated that aging determines the variations in defect levels, exciton‐phonon coupling and exciton relaxation dynamics. The results substantially improve the current understanding of the basic photophysics of CsPbBr3 NC films and of the aging effects and are expected to provide a useful guide for future characterization of other similar materials.

Organic Acid-Doped Cellulose Nanocrystals in Biocomposite Laminates as Chiral Nematic Photonic Films: Implications for Intelligent Packaging

The organic photonic film is one of the important materials in optical, bioengineering, and sensing applications. It is a task to achieve excellent mechanical properties and foldability of the biopolymer composites having chiral nematic structures without sacrificing their iridescence. Herein, we have proposed an approach to preserve and lock the self-assembled cellulose nanocrystal (CNC) films based on agro-waste between the layers of a poly(lactic acid) (PLA)/polycaprolactone (PCL) biocomposite. The photonic properties of the composites were tweaked by doping with different organic acids (OAs). The addition of OAs into CNCs has resulted in the generation of flexible films. The multilayer films were studied for interfacial interactions in the form of spherulite interlocking between CNCs and polymer blends through polarized optical microscopy and differential scanning calorimetry analysis. Composite films exhibited high mechanical strength and also showed reflection from the UV region to the near-IR region. The composite films displayed good visual color-sensing properties with moderate antibacterial activity for Escherichia coli bacteria. Such ordered nanocellulose–polymer biocomposites fabricated by a facile layer-by-layer approach may assist the growth of obtained photonic films as candidate materials having implications in bio-optical engineering and packaging.

Controlling the surface structure and functionalization of a cellulose nanocrystal film modified by using glycidyloxypropylsilane in a coating process

Controlling the surface structure and functionalization of a cellulose nanocrystal film modified by using glycidyloxypropylsilane in a coating process