GISAXS application to a study of the nanoparticle self-assembly at the air-water interface

 

M. Jergel1, P. Šiffalovič1, K. Vegso1, M. Benkovičová1, E. Majková1, K. Nygård2, O. Konovalov3

 

1Institute of Physics SAS, Dúbravská cesta 9, 845 11 Bratislava, Slovakia

2Dept. Chemistry & Mol. Biology, University Göteborg, 412 96 Göteborg, Sweden

3European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France

matej.jergel@savba.sk

 

Keywords: nanoparticles, GISAXS, Langmuir film

 

Abstract

An in-situ study of the colloidal silver nanoparticle self-assembly into a close-packed monolayer at the air/water interface followed by a 2D to 3D transition was performed by a fast tracking GISAXS technique in a Langmuir-Blodgett trough. Monitoring the immediate response of the system to the barrier movement enabled us to identify all relevant self-assembly stages including those far from the equilibrium. A new non-equilibrium phase before the monolayer collapse was found that is inaccessible by the competing direct space imaging techniques such as the scanning and transmission electron microscopies.

Introduction

Self-assembled arrays of chemically synthesized monodisperse nanoparticles are attractive for many novel and emerging applications. For example, the nanoparticle templates of plasmonic gold or silver nanoparticles provide a key to enhanced power conversion efficiency of future solar cells or may serve as unique substrates for the surface enhanced Raman scattering spectroscopy. The self-assembly takes place on the water surface in the Langmuir-Blodgett (LB) trough where a nanoparticle Langmuir film is formed and subsequently transferred onto a solid substrate. Macroscopic physical quantities such as the surface pressure, refractive index or surface potential of the nanoparticle Langmuir film are readily measureable while the conventional direct space imaging methods such as the scanning and transmission electron microscopy or nanoscale scanning probe techniques are inapplicable at the air/water interfaces due to the high vapour pressure and surface tension of the water subphase. Here, the reciprocal space techniques based on the scattering of X-rays or neutrons, in particular the synchrotron based grazing-incidence small-angle X-ray scattering (GISAXS), prove to be unique in revealing the atomic or molecular structure details at the air/water interface. We report here on the principal stages of a plasmonic nanoparticle Langmuir film compression at the air/water interface followed by a pressure release. The immediate film response to the surface pressure evolution is monitored in situ by a fast tracking GISAXS technique in order to detect transient phenomena far from the equilibrium. The structure evolution at nanoscale is related to the macroscopic behaviour monitored by the surface pressure, Brewster microscopy and imaging ellipsometry.

Experimental details

We studied Langmuir film composed of spherical Ag nanoparticles capped with oleic acid and oleylamine surfactant. The chemical synthesis was published elsewhere [1]. The surfactant prevents the nanoparticles from agglomeration. The diameter of the Ag nanoparticle core of 7±0.7 nm was determined by SAXS. The Ag nanoparticle solution in chloroform of 0.2 mg/ml concentration was spread on the deionised water subphase (specific electrical resistance >18 MΩ.cm) in a custom designed LB trough. After spreading the nanoparticles, the solvent was let to evaporate for 15 minutes before the measurement started. The in situ GISAXS measurements at the air/water interface during the barrier movement in the LB trough were performed at ID10B beamline at ERSF, Grenoble. The X-ray beam of the 300x100 mm2 size and 8 keV energy hit the air/water interface at 0.35º grazing angle. A fast 2D X-ray detector PILATUS 300K was used. The reciprocal space calibration was done by the silver behenate standard. The Brewster angle microscopy and null ellipsometry images were obtained by an optical microscope with digital output (Hitachi) and imaging ellipsometer (Accurion), respectively.

Results and discussion

To track in situ the evolution of the nanoparticle order as a function of the surface pressure in the Langmuir trough during the compression and expansion at a constant barrier speed of 26 cm2/min, a continuous series of the GISAXS frames was recorded as a movie. The time between the two successive frames of 1.87 s was short enough not to miss any relaxation effect, hence, the immediate response was monitored. The Fig. 1 shows the GISAXS patterns of the nanoparticle Langmuir film before and after the compression. They consist of the nanoparticle interference function modulated by the nanoparticle form factor. The qz and qy are the normal and lateral (in-plane) scattering vector components with respect to the air/water interface. The 1st order Bragg rods suggest presence of an ordered nanoparticle monolayer while the null ellipsometry image (Fig. 2) shows that it is discontinuous at small surface pressure. Considering the hexagonally ordered close-packed spherical nanoparticles, the lattice spacing before compression reads  which for the first Bragg rod maximum position =0.75 nm-1 gives the nearest-neighbour interparticle distance =9.7 nm. Assuming the mean nanoparticle core diameter of 7 nm from SAXS, this gives some 1.4 nm for the surfactant shell thickness. The compression results in an intensity redistribution along qy of the central and 1st Bragg rods (Fig. 1). On the former, the maximum intensity shifts from the critical exit angle (Yoneda peak) to peak P1 (hidden by the specular beam stop) after compression which indicates the nanoparticle layering. On the latter, peak P2 evolves at the position that suggests formation of a vertically correlated second layer [2]. In particular, the AB stacking (hexagonal close-packed bilayer) is manifested by the diffraction peaks P1(qy, qz) and P2(qy, qz) located at (0, 2p/dz) and (2p/d10, p/dz), respectively, where the vertical lattice spacing dz is given as . Considering the in-plane interparticle distance D=9.7 nm, the calculated P2 peak position matches perfectly that one observed in the GISAXS pattern.

 


Figure 1. The GISAXS patterns of the nanoparticle Langmuir film at the surface pressure 0 mN/m (left) and 28.4 mN/m (right). The peaks P1 and P2 are associated with the AB stacked bilayer (see the text). The dead area between the detection modules of PILATUS 300K detector is hatched.

 

Based on the simultaneously measured pressure – area isotherm and GISAXS frames, we can identify four principal stages of the nanoparticle Langmuir film compression. In stage I, no measureable change in the surface pressure P is detected (Fig. 3a). We observe self-assembly of nanoparticles evidenced by the presence of Bragg rods in GISAXS (Fig. 1) which happens presumably within isolated self-assembled nanoparticle islands as the null ellipsometry shows (Fig. 2). The nanoparticle islands coalesce gradually into larger assemblies during the compression as indicated also by the Brewster microscopy (not shown). The surface elastic modulus E calculated from P derivative (Fig. 3a) has no physical meaning for the freely floating isolated islands here. The reduction of the Bragg rod width (Fig. 3c) is not observed because of a restricted size of the coherently scattering domains in the pristine islands controlled by the nanoparticle cumulative disorder. In stage II, we observe a steady increase in the surface pressure and elastic modulus

 

 


Figure 2. The null ellipsometry images of the nanoparticle Langmuir film before the compression (left) and after the expansion (right). The null imaging elipsometer was configured to stop the polarized light reflected from the nanoparticle monolayer.

 

 

 


Figure 3. The major compression stages of the nanoparticle Langmuir film. The arrows show the compression and expansion periods. For a closer explanation see the text.

 

(Fig. 3a) as the proceeding island coalescence gets gradually the larger assemblies into contact. This accumulates stress at the assembly boundaries that is relieved by the nanoparticle re-arrangements into a continuous close-packed monolayer. A nearly unchanged Bragg rod maximum position and width (Fig. 3b,c) suggest that the local hexagonal order from the original islands is preserved. The nanoparticle Langmuir film at the end of stage II is completely closed, being suitable for the transfer onto a solid substrate to deposit a high-quality nanoparticle monolayer. In stage III, we observe a steeper increase in the surface pressure followed by a maximum in the surface elastic modulus which precedes a bilayer formation [3]. A shift in the Bragg rod maximum position to the higher qy values suggests a decrease in the lattice spacing d10 by approximately 0.1 nm (Fig. 3b) while a simultaneous increase in the Bragg rod width (Fig. 3c) indicates a deterioration of the nanoparticle order due to the accumulated stress. Such a Bragg rod behaviour can be explained by a slight compressive deformation of the polymer surfactant capping the nanoparticles. This non-equilibrium compression phase has never been observed in the experiments under steady-state conditions. In stage IV, the peak P2 indicates a newly formed vertically correlated second nanoparticle layer allowing stress relief which results in the reversed shift of the Bragg rod maximum position and increase in the d10 lattice spacing. The Bragg rod width increases as well suggesting growing disorder. Such a simultaneous growth of the lattice spacing and disorder is typical for the paracrystal model [4]. In the limit of a highly disorder paracrystal, the Bragg rod maximum position  is controlled by the interparticle distance D instead the lattice spacing d10, i.e.  instead . The  at the end of stage IV is in between that suggests a partial paracrystalline disorder. The Bragg rod maximum position and width are not recovered during the film expansion suggesting a stable nanoparticle order in the bilayer. The drop in the P2 peak area may be explained by decreasing amount of nanoparticles in the probed X-ray volume during the film expansion when the bilayer islands are formed and go away from each other, leaving free subphase surface behind (see also Fig. 2).

Conclusions

We identified principal stages of the Ag nanoparticle Langmuir film formation using the fast in-situ GISAXS technique supported by Brewster angle microscopy and imaging ellipsometry. The nanoparticle monolayer formation takes place via coalescence of free self-assembled nanoparticle islands with the hexagonal close-packed order that persists in the coalesced assemblies up to a temporary squeezing of the lattice shortly before the monolayer. This transient phase has not been observed under steady-state conditions. The monolayer  collapse takes place by flipping up the nanoparticles and the second layer formation with the AB-like crystallographic stacking and enhanced paracrystalline-like disorder. The Langmuir film expansion runs irreversibly by decomposition into bilayer islands without observable changes in the nanoparticle order inside, leaving free subphase surface behind.

References

1.     K. Vegso, P. Šiffalovič, M. Weis, M. Jergel, M. Benkovičová, E. Majková, L. Chitu, Y. Halahovets, Š. Luby, I. Capek & A. Šatka, Phys. Stat. Sol., A 208, (2011), 2629-2634.

2.     M. Fukuto; R. K. Heilmann, P. S. Pershan, A. Badia & R. B. Lennox, J. Chem. Phys., 120, (2004), 3446-3459.

3.     S. Kubowicz, M. A. Hartmann, J. Daillant, M. K. Sanyal, V. V. Agrawal,  C. Blot, O. Konovalov & H. Möhwald, Langmuir, 25. (2008), 952-958.

4.     R. Lazzari, R., in X-ray and Neutron Reflectivity, edited by J. Daillant & A. Gibaud (Berlin-Heidelberg: Springer), 2009, pp 283-342.

 

Acknowledgement

This work was done during implementation of the project Research and Development Centre for Advanced X-ray Technologies, ITMS code 26220220170, supported by the Research and Development Operational Programme funded by the ERDF.