Langmuir-Blodgett Matrix Research Articles

2D kinetic Monte-Carlo model of nanocrystal aggregation within a liquid matrix: Simulation and experimental study

A stochastic model of nanocrystals (NC) aggregation governing mobilities both of individual nanocrystals and its clusters is developed. The influence of the model parameters on the NC morphology is considered and an analysis is made of the applicability of the model to describe various real physical systems. The model is applied to simulate an aggregation of cadmium sulfide nanocrystals upon evaporation of the Langmuir–Blodgett matrix and as a result a comparison of simulations and experimental results is carried out. We give a comprehensive analysis of the patterns simulated by the model, and study an influence of the surrounding medium (solvent) on the aggregation processes. This system is a typical example from real life and is noteworthy in that the morphology of NC after evaporation of the matrix cannot be described exactly by a model based only on the motion of individual nanocrystals or by a cluster–cluster aggregation model.

A stochastic model, simulation, and application to aggregation of cadmium sulfide nanocrystals upon evaporation of the Langmuir–Blodgett matrix

Abstract A stochastic model of nanocrystals clusters formation is developed and applied to simulate an aggregation of cadmium sulfide nanocrystals upon evaporation of the Langmuir–Blodgett matrix. Simulations are compared with our experimental results. The stochastic model suggested governs mobilities both of individual nanocrystals and its clusters (arrays). We give a comprehensive analysis of the patterns simulated by the model, and study an influence of the surrounding medium (solvent) on the aggregation processes. In our model, monomers have a finite probability of separation from the cluster which depends on the temperature and binding energy between nanocrystals, and can also be redistributed in the composition of the cluster, leading to its compaction. The simulation results obtained in this work are compared with the experimental data on the aggregation of CdS nanocrystals upon evaporation of the Langmuir–Blodgett matrix. This system is a typical example from real life and is noteworthy in that the morphology of nanocrystals after evaporation of the matrix cannot be described exactly by a model based only on the motion of individual nanocrystals or by a cluster-cluster aggregation model.

Lipid-matrix effects on tyrosinase immobilization in Langmuir and Langmuir-Blodgett films.

The immobilization of the enzyme tyrosinase (Tyr) in lipid matrices can be explored to produce biosensors for detecting polyphenols, which is relevant for the food industry. Herein, we shall demonstrate the importance of the lipid composition to immobilize the enzyme tyrosinase in Langmuir-Blodgett (LB) films. Tyr could be incorporated into Langmuir monolayers of arachidic acid (AA), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (DPPG), having as the main effect an expansion in the monolayers. Results from polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) pointed to electrostatic interactions between the charged residues of Try and the lipid headgroups, in addition to changes in the order of lipid chains. The interaction between Tyr and DPPC in Langmuir monolayers can be correlated with the superior performance of DPPC/Tyr LB films used as biosensors to detect catechol by cyclic voltammetry. The molecular-level interactions assessed via PM-IRRAS are therefore believed to drive an immobilization process for Tyr in the lipid LB matrix and may serve as a general criterion to identify matrices that preserve enzyme activity.

Nature of luminescence of PbS quantum dots synthesized in a Langmuir–Blodgett matrix

The luminescence and structural properties of PbS quantum dots in a Langmuir–Blodgett matrix and after the removal of this matrix have been studied. It has been found that the photoluminescence peak of quantum dots in the matrix is near 1.6 eV, and luminescence disappears after the removal of the matrix. Beyond the matrix, quantum dots begin to luminesce at an energy of 0.8 eV only after aging in air lasting many days, which can be attributed to the passivation of surface defects because of oxidation. The size distribution of quantum dots in the initial Langmuir–Blodgett matrix and after its removal has been determined by transmission microscopy. Using the size distribution of quantum dots, the photoluminescence spectra have been calculated for quantum dots with different shells—Langmuir–Blodgett matrix/alkanes or oxide. It has been established that the shift of the photoluminescence spectrum of quantum dots toward lower energies is due to a decrease in the height of barriers and to an increase in the sizes of quantum dots.

Photoluminescence of the PbS quantum dots fabricated by the Langmuir-Blodgett technique

The optical and structure properties of the PbS nanoclusters in the Langmuir-Blodgett matrix as well as their properties after the matrix removal in the ammonium atmospheres have been studied. According to the electron microscopy data, it has been established that there are PbS nanoclusters formed on the surface of the samples. It has been found that the matrix removal results in a significant enlargement of the PbS nanocrystals accompanied by the shift of the luminescence into the infra-red range. It has been shown that the PL intensity of PbS quantum dots (QDs) in the matrix decreases with the temperature decrease due to the carrier transfer into defect QDs whereas the the luminescence intensity of the PbS nanocrystals without a matrix grows at the temperature increase.

Peculiarities of CdS nanocrystal formation at annealing of a Langmuir‐Blodgett matrix

AbstractFormation and assembling of CdS nanocrystals (NC) on highly‐ordered pyrolytic graphite (HOPG) and on oxidized silicon substrates have been investigated by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). NC were initially formed within a Langmuir‐Blodgett (LB) matrix of cadmium behenate. The LB matrix was then removed by annealing in an ammonia atmosphere. It is shown that the NC self‐assembling is mainly determined by the LB matrix wetting properties. In case of wettable HOPG substrate homogeneous matrix evaporation occurs that leads to coarsening of the NC arrays as the main self‐assembly process. Otherwise, NC prepared on non‐wettable SiO2‐Si substrate demonstrate arrays formation determined by the LB matrix dewetting process. For the both cases, the influence of kinetic limitations connected with matrix evaporation rate on the self‐assembly process is observed. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scanning Tunneling Spectroscopy of Free-Standing CdS Nanocrystals Fabricated by the Langmuir–Blodgett Method

The energy spectrum and tunneling transport through ensembles of closely packed CdS nanocrystals (NCs) obtained by the Langmuir–Blodgett (LB) method have been investigated using the scanning tunneling spectroscopy (STS) technique at room temperature. NC ensembles were obtained by annealing the solid LB matrix in a vacuum or in an ammonia atmosphere. The STS data indicate the formation of point defects attributed to an excess of Cd atoms in NC capturing tunneling electrons and the effect of the ammonia atmosphere on the defect formation process. A high density of defects arising as a result of sintering NCs that lost a protective organic shell during the migration on the substrate surface is likely to cause a band gap narrowing in NCs located near the ensemble edge. The analysis of a dependence of the zero-conduction gap value on the NC size gives evidence of a strong effect of the Coulomb interaction between the carriers located in NCs and their polarization charges in the shells on the NC energy spectrum. A dielectric constant of the shell (εout = 10–16.5) and the energy barrier height for electrons (V0 = 0.6–0.8 eV) were determined. The shell comprises ammonia and organic molecules, the ammonia molecules making a major contribution to the shell dielectric constant and the organic molecules determining the barrier height.

Effect of heat treatment on the photoluminescence of CdS nanocrystallites in cadmium-rich organic Langmuir Blodgett matrix

CdS nanocrystallites were grown within an organic layered matrix by partial sulphidation of precursor cadmium arachidate LB multilayers. The cadmium arachidate–arachidic acid composite multilayers containing CdS nanocrystallites were enriched by intercalation with Cd2+ ions in aqueous solution of CdCl2 and subsequently heat treated at different temperatures up to 300 °C, in air and in vacuum. CdS nanocrystallites within the composite multilayer have been found to exhibit treatment process dependent characteristic changes in optical absorption and luminescence. The optical data obtained at different stages/conditions of processing have been analyzed by considering changes in excitonic absorption and emission as well as contributions from surface and bulk defects related emission and a suitable energy level diagram has been proposed. The composition, microstructure and surface morphology of the composite multilayers were also studied at all stages of processing to develop a comprehensive understanding of the interaction of the organic matrix with CdS nanocrystallites and the consequent influence on the optical behavior of the nanocrystallites. These studies have shown that the organic moieties encapsulating the CdS nanocrystallites tend to restrict their growth and aggregation, while the presence of cadmium in the organic matrix is responsible for the passivation of surface defects as well as the reduction of bulk defects, and these factors have significant influence on the photoluminescence of CdS nanocrystallites.

Application of differential charging in XPS for structural study of Langmuir–Blodgett films

Differential charging in XPS was recently shown to be a novel technique for studying in-depth structural information of discrete cadmium layers in Langmuir–Blodgett (LB) multilayer films. Here we report structural modification in multilayer LB films after sulfidation using differential charging in angle-dependent XPS that are not observable by the x-ray reflectivity technique. An AFM study suggests less modification in compact LB films in comparison to the non-compact ones. The differential charging in the LB multilayers changes after sulfidation due to the formation of cadmium sulfide nanostructures in the cadmium arachidate LB matrix, which was reflected prominently in the differential charging of Cd 3d5/2 XPS peaks. It was found that, even after sulfidation, the compact multilayer LB films were differentially charged in the out-of-plane (in-depth) direction, whereas this kind of differential charging was not apparent in rough and non-compact films. Our results clearly indicate that for LB films with compact structure a partial layered structure survives the impact of particle formation, whereas for non-compact films the modification is large and no specific conclusion could be drawn. While x-ray reflectivity cannot provide specific information about the internal structure of the post-sulfidation films it shows that the total thickness of the films reduces in all cases.

SURFACE-ENHANCED RAMAN SCATTERING: METAL NANOSTRUCTURES COATED WITH LANGMUIR-BLODGETT FILMS

This review deals with surface-enhancved Raman scattering (SERS) employing Langmuir-Blodgett (LB) films, which serve as model systems for developing theoretical and experimental studies to elucidate the SERS effect. In addition, LB films have be used as integral parts of molecular architectures for SERS-active substrates. On the other hand, SERS and surface-enhaced resonant Raman scattering (SERRS) have allowed various properties of LB films to be investigated, especially those associated with molecular-level interactions. In the paper, emphasis is placed on single molecule detection (SMD), where the target molecule is diluted on an LB matrix of spectral silent material (low Raman cross section). The perspectives and challenges for combining SERS and LB films are also discussed.

Optical vibration modes in (Cd, Pb, Zn)S quantum dots in the Langmuir-Blodgett matrix

The structures with CdS, PbS, and ZnS quantum dots produced using the Langmuir-Blodgett method are investigated by infrared (IR) spectroscopy, Raman scattering, and ultraviolet (UV) spectroscopy. The quantum dot size estimated from the UV spectra and high-resolution transmission electron microscopy (HRTEM) falls in the range 2–6 nm. The longitudinal optical (LO) phonons localized in quantum dots and the surface optical vibration modes are revealed in the IR reflection and Raman scattering spectra of the structures under investigation. The frequencies of the surface optical modes are adequately described with allowance made for the effect of localizing optical phonons in the quantum dots.

Structure of CdS–arachidic acid composite LB multilayers

Abstract Langmuir–Blodgett (LB) multilayers of cadmium arachidate were used as precursors to grow semiconducting CdS nanoclusters. The formation of CdS in the multilayers was determined by Fourier transform-infrared (FT-IR), ultraviolet–visible (UV–vis) and Raman spectroscopy. The structural changes occurring as a consequence of CdS formation have been characterized using X-ray reflection (XR) and grazing incidence X-ray diffraction (GIXD) techniques. The CdS containing composite multilayers exhibit the presence of two types of molecular domains, one with close packed herringbone arrangement and the other with tilted molecular chains with no in-plane order. The structural and spectroscopic evidences together suggest that the CdS nanoclusters formed within the arachidic acid LB matrix are quasi two-dimensional in nature with lateral dimension ∼5–10 nm and thickness ∼1.1 nm.

Electron spin resonance characterization of Langmuir–Blodgett films containing functional molecules

The role of electron spin resonance (ESR) spectroscopy in the characterization of Langmuir–Blodgett (LB) films containing functional molecules is discussed. Unpaired electrons in LB films are associated with radical molecules produced by charge transfer, paramagnetic metallic ions such as Cu2+ and so on. ESR parameters such as g-value and hyperfine coupling provide information of local molecular orientation and electronic states. ESR can unambiguously determine the orientation distribution function of molecules through g-value anisotropy. The case of mixed LB films of a merocyanine dye and arachidic acid is discussed as a typical example. The analysis of the anisotropic ESR spectra of stable radicals existing in these films has revealed detailed in-plane orientation of radicals, which led to the discovery of flow orientation effect during LB dipping process. Light-induced ESR (LESR) is introduced to directly detect the dye radicals generated by photoinduced charge transfer. Observed LESR spectra, associated with a resolved 14N hyperfine structure, uniquely probe the orientation and electronic states of dye molecules forming the J-aggregates in highly ordered LB matrix. AFM observation of dye domains in these films is also discussed.

Photoelectrochemical behaviour of CdS “Q-state” semiconductor particles in 10,12-nonacosadiynoic acid polymer langmuir-blodgett films

CdS “Q-state” semiconductor particles from 2 to 10 nm diameter were nucleated and grown in 10,12-nonacosadiynoic acid (NCDA) polymer Langmuir-Blodgett (LB) films deposited on ITO plates. The polymerization process through exposure to UV-Visible light resulted in formation of the “blue form” followed by the final “red form” after 60 min. X-ray photoelectron spectroscopy (XPS) measurements confirmed the deposition of the NCDA cadmium salt and the formation of the CdS particles after exposure to H2S(g) in the LB matrix. A study of the photoelectrochemical behaviour of these systems was conducted through polarisation current-voltage (I-V) curves in the range of 0 to −1000 mV (Standard Calomel Electrode-SCE). An average open-circuit voltage (VOC) from −600 to −700 mV values was observed for photoelectrochemical (PEC) cells constructed for the undoped NCDA polymer LB film with 10 nm diameter CdS particles. The I2-doped NCDA polymer film presented an increase in the conductivity compared with the undoped film but with a deterioration of stability of the PEC system.

The temporal and thermal stability of pyroelectric calix[8]arene Langmuir–Blodgett films

Pyroelectric Langmuir–Blodgett (LB) films of calix[8]arene have been prepared using the alternate-layer technique. The technique allows the co-deposition of carboxyl and amino-substituted calix[8]arenes, and results in the alternating acid/amine/acid… layer structure. A pyroelectric coefficient of up to 15 μC m −2 K −1 has been obtained from a 13 alternate layer t-butyl calix[8]arene LB film. The pyroelectric response of this system has been shown to remain constant over ∼5000 continuous heating and cooling cycles. The introduction of metal ions into the LB matrix dramatically increases the temperature range over which the pyroelectric activity can be measured.

Photochemical Generation of Gold Nanoparticles in Langmuir−Blodgett Films

Gold nanoparticles were generated by ultraviolet irradiation of Langmuir−Blodgett (LB) films of octadecylamine (ODA), 4-hexadecylaniline (HDA), and benzyldimethylstearylammonium chloride monohydrate (BDSAC) deposited from aqueous HAuCl4 subphases. In contrast, no gold crystals were observed in irradiated LB films prepared from monolayers of dipalmitoyl-dl-α-phosphatidyl-l-serine (DPPS) and dipalmitoyl-l-α-phosphatidylcholine (DPPC). X-Ray photoelectron spectroscopy, UV−visible absorption spectroscopy, atomic force microscopy, and transmission electron microscopy measurements indicated the marked influence of the surfactants used to prepare the LB matrix on the shape of the gold particles. Particles formed in ODA and BDSAC LB films were grown with well-defined crystal faces, while particles generated in HDA LB films were irregular in shape.

Synthesis of size-quantized metal sulfides of Pb–Cd and Zn–Cd bimetallic systems in stearate Langmuir–Blodgett films

Synthesis of Pb–Cd and Zn–Cd bimetallic sulfides in stearate Langmuir–Blodgett (LB) matrices has been investigated using repeated cycles of sulfidation (S) by exposing a metal stearate LB film to H2S gas and the intercalation (I) of metal ions into the LB matrix by immersing the film into aqueous metal acetate. In a serial synthetic mode, one metal sulfide was synthesized in advance, and the metal sulfide LB composite film was subjected to the I–S cycles to synthesize the other sulfide. When CdS was pre-synthesized in both the PbS–CdS and ZnS–CdS systems, two sulfide phases coexisted in the stearate LB matrix, and the amount and size of each metal sulfide was controlled independently by the number of I–S cycles in the preparation of each sulfide. In the reverse sequence, on the other hand, the pre-formed PbS and ZnS were dissolved and CdS was formed during the immersion in aqueous Cd(CH3COO)2. These results of the serial synthesis are reasonable because the replacement reaction, NS + Cd2+ → CdS + N2+ (N = Pb, Zn), is thermodynamically favorable. When mixed metal stearate LB films of Zn and Cd were sulfurized, size-quantized solid solution phases of Zn1−yCdyS (0 < y < 1) were formed in the LB matrix, and the composition y could be controlled by that of each metal ion in the LB films.

Preparation and Characterization of Quantum-Sized PbS Grown in Amphiphilic Oligomer Langmuir−Blodgett Monolayers

Three kinds of amphiphilic oligomers, poly(maleic acid) with octadecanol ester (PMAO) with 2.0:1, 3.7:1, and 6.0:1 between the carboxylic groups and the hydrocarbon chains, were synthesized and have been used as a matrix to form and grown the quantum-sized PbS. Pure Y-type PbPMAO Langmuir−Blodgett (LB) films were prepared at a dipping speed of 4.0 cm/min and surface pressure of 18 mN/m. The results from Fourier tranform IR spectroscopy and X-ray diffraction measurements showed that an ordered structure was obtained from the PbPMAO LB films. The quantum-sized PbS particles were prepared by exposure of PbPMAO LB films to H2S and nanoparticulate PbS monolayers were formed without destroying the layered structure. The result from structured UV−vis absorption spectra shows that the PbS particles within the LB matrix are relatively monodispersed. It was found that the PbS within PMAO LB films indicated a larger blue shift of the optical absorption edge with the decrease of the ratio between the carboxylic groups and the hydrocarbon chains. Meanwhile, it can be deduced that the size of the aggregate of the sulfides within PMAO LB films is smaller than that formed in stearic acid LB films. The observed absorption peaks showed similarity with the monomolecular PbS.

Controlled growth of the size-quantized CdS at the interface of the stearic acid Langmuir–Blodgett films

A pure Y-type transfer form for cadmium stearate (CdSt 2) Langmuir–Blodgett (LB) films was obtained on a subphase of which the pH is suitable (6.4) as a result of using a higher surface pressure. The pressure dependence of the structure of the CdS produced within the LB matrix was observed. Monolayers of CdS were prepared by the exposure of CdSt 2 LB films deposited under higher surface pressure (37.5 mN/m) to H 2S gas at a pressure of 0.25 Torr. If the LB films deposited under lower surface pressure were used as the matrix, CdS would aggregate to the particles within the stearic acid (SA) LB films. And these phenomena suggest a novel means to control the microstructure of aggregates formed. We suggested that it should be ascribed to the larger interaction between the hydrocarbon chains of the LB films in the case of the higher surface pressure. This larger interaction can offer a sufficient position control for the guest sulphide monolayer. At the same time, the reaction speed of the LB films with the H 2S gas, as well as the form speed of CdS, was dependent on the layer number of the LB films and the surface pressure under which the LB films were deposited.

Molecular recognition of octadecanyl ester of 1-(2-carboxyethyl)thymine to adenine at the air/water interface and photodimerization in the Langmuir-Blodgett film matrix under ultraviolet irradiation

Abstract A new single-chain nuclei acid base-containing amphiphile, octadecanyl ester of 1-(2-carboxyethyl)thymine, was synthesized, and its surface behaviors on pure water and on aqueous solutions of adenine were investigated by the method of surface pressure-area (π-A) isotherms. Molecular recognition through complementary base pairing takes place at the air/water interface between the thymine moiety in the headgroup of the amphiphile and adenine, which results in the pick-up of the base-base complex onto quartz plate by the Langmuir-Blodgett (LB) technique. Photodimerization of thymine rings was also investigated in the multilayer LB film of the amphiphile deposited from pure water surface, and the results show that the orderly and regular packing of the molecules in the LB matrix provides favorable conditions for the formation of thymine dimers.