Prolonged reorganization of thiol-capped Au nanoparticles layered structures

Prolonged reorganization behaviour of mono-, di-, tri- and multi-layer films of Au nanoparticles prepared by Langmuir-Blodgett method on hydrophobic Si(001) substrates have been studied by using X-ray scattering techniques. Out-of-plane study shows that although at the initial stage the reorganization occurs through the compaction of the films keeping the layered structure unchanged but finally all layered structures modify to monolayer structure. Due to this reorganization the Au density increases within the nanometer thick films. In-plane study shows that inside the reorganized films Au nanoparticles are distributed randomly and the particle size modifies as the metallic core of Au nanoparticles coalesces.

Prolonged reorganization behaviour of mono-, di-, tri-and multi-layer films of Au nanoparticles prepared by Langmuir-Blodgett method on hydrophobic Si(001) substrates have been studied by using X-ray scattering techniques. Out-of-plane study shows that although at the initial stage the reorganization occurs through the compaction of the films keeping the layered structure unchanged but finally all layered structures modify to monolayer structure. Due to this reorganization the Au density increases within the nanometer thick films. In-plane study shows that inside the reorganized films Au nanoparticles are distributed randomly and the particle size modifies as the metallic core of Au nanoparticles coalesces. C  Metal nanoparticles exhibit interesting optical, 1-3 electrical, 3, 4 magnetic 5, 6 and catalytic 7,8 properties and for that reason can be used in nanotechnology by forming suitable architectures in different dimensions onto a chosen substrate. [9][10][11] Nanoparticles surrounded by dodecanethiol ligand shell have been used extensively for making such assembly 12 and their structures, patterns and morphologies on water and solid surfaces under different experimental conditions have been studied. [13][14][15][16] Monolayer formed by the Au nanoparticles exhibits reversible buckling on water surface up to a certain surface pressure (π ) but due to more compression irreversible monolayer-to-bilayer transformation occurs. 17,18 A trilayer structure has also been observed due to the irreversible transition of the monolayer 14 and with the compression of the monolayer the folding, wrinkling and then wrinkling to folding transitions have been observed. 19 Moreover, layer-by-layer assembly of thiol-capped Au nanoparticles has also been observed with the compression of the monolayer. 20 On solid substrate, deposition of such Au nanoparticles is easily possible by using Langmuir-Blodgett (LB) method. 13,21,22 During deposition from water to solid surface a two-dimensional short-range structural reordering of Au nanoparticles occurs and the packing symmetry changes from triangular to square-like. 23 Both in-plane and out-of-plane restructuring of monolayer of Au nanoparticles have been observed due to the evaporation of the trapped water. 16 Substrate surface conditions also play an important role in the growth and stability of any nanolayer on it. [24][25][26][27] For the organic capped nanoparticles, which are effectively hydrophobic in nature the growth, structure and stability on differently passivated Si surfaces have been studied. 28 Differently passivated Si surfaces have different hydrophilic/hydrophobic nature, which effectively controls the growth and stability of the nanoparticle films. Due to the reorganization for nearly two months, close packed layered structure has been observed. 29 In this paper, we have shown the prolonged reorganization of the thiol-capped Au nanoparticles LB films deposited on hydrophobic Si (001) substrates. The structures of the LB films and their evolution with time nearly for twelve months have been monitored using x-ray reflectivity (XRR) technique. After prolonged reorganization, the structure of the films has also been studied with the grazing incidence small angle x-ray scattering (GISAXS) techniques. Our study shows that after reorganization all layered structures become monolayer structure and inside the monolayer gold nanoparticles are distributed randomly. Due to such structural modification the Au density largely enhances within the nanometer thick films. Moreover, the particle size modifies and on the average nanoparticles of three different sizes form due to the coalescence of the metallic core of the Au nanoparticles.
Dodecanethiol-encapsulated Au nanoparticles were synthesized by a phase-transfer redox reaction mechanism using the Brust method. 30 Methanol was added to the toluene solution containing the capped nanoparticles to remove excess reagents and the nanoparticles were filtered out from the solution. The particles were then redispersed in toluene and the desired concentration of the nanoparticles (0.95 mg/mL) was obtained. Transmission electron microscopy (TEM) measurements were carried out with a FEI electron microscope model Tecnai G2 20S Twin operated at 200 kV with a resolution of 2 Å to obtain the average diameter of the nanoparticles. The average diameter of the metallic core of the nanoparticle was determined by the particle size distribution and found to be around 34 ± 7 Å. The metallic core is encapsulated by dodecanethiols of about 14 Å, so the average diameter of the encapsulated nanocrystal is about 60 Å.
Au nanoparticles were spread from a 0.95 mg/mL toluene solution (600 μL) using a micropipet on the surface of Milli-Q water (resistivity 18.2 M cm) in a Langmuir trough (Apex Instruments). It was kept undisturbed for 15 min to let the solvent evaporate. The π -A isotherm was recorded at 25 o C. π was measured with a paper Wilhelmy plate and the monolayer was compressed at a constant rate of 47.5 mm 2 /min. Films were deposited by the LB method on Si(001) substrates at three different surface pressures (π = 8, 16 and 21 mN/m) at room temperature (25 o C). Depositions were carried out using one down-up cycle, i.e., by using two strokes, where in the down stroke substrate goes from air to water and in the up stroke it goes from water to air through the nanoparticle monolayer. Two films were also deposited by the LB method at π = 8 mN/m of the monolayer at room temperature by using two and four down-up cycles. The speed for both up and down strokes were 2 mm/min. Prior to the deposition, Si(001) substrates were made H-terminated by keeping it in a solution of hydrogen fluoride (HF, Merck, 10%) for 3 min at room temperature (25 • C). 26,28 Immediately after the chemical treatment, all the substrates were kept inside the Milli-Q water until LB deposition.
XRR measurements were carried out by using both laboratory and synchrotron x-ray sources. In the laboratory, a versatile X-ray diffractometer (VXRD) setup was used. Details about the setup and instrumental resolution has been described earlier. 20,26 XRR and GISAXS measurements after prolonged reorganization were performed at ID 10B beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, using a high-energy (8 KeV, λ = 1.55 Å) synchrotron source. Data were taken in specular condition, i.e., the incident angle (θ ) is equal to the reflected angle (θ ) and both are in a scattering plane. Under such conditions, a nonvanishing wave-vector component, q z , exist which is equal to (4π /λ)sinθ . XRR technique essentially provides an electron-density profile (EDP), i.e., in-plane (x-y) average electron density (ρ) as a function of depth (z) in high resolution. 20 From EDP it is possible to estimate the film thickness, electron density and interfacial roughness. Analysis of XRR data has been carried out using Parratt's formalism. 31,32 For the analysis, each film has been divided into a number of layers including roughness at each interface. 20,32,33 In GISAXS measurements a 2D X-ray detector PILATUS 300K was used to record the scattered X-ray radiation. GISAXS images were taken at 0.2 • , 0.3 • and 1.55 • grazing angles of incidence. The direct beam was stopped and the specular reflected beam was attenuated to avoid the saturation of the detector.
XRR data and the corresponding analyzed curves of the Au nanoparticle LB films deposited by one down-up cycle at three different π values (8, 16 and 21 mN/m) and their time evolutions are shown in Fig. 1. EDPs obtained from the analysis are shown in the inset of the corresponding figures. EDPs obtained from the reflectivity analysis clearly show that only a monolayer of Au nanoparticles has been deposited at π = 8 mN/m which is shown in the inset of Fig. 1(a), whereas bilayer and trilayer has been deposited at π = 16 and 21 mN/m which are shown in the inset of Fig. 1(b) and Fig. 1(c)  adjacent particles occur. Time evolution EDPs obtained from the analysis show that with time, reorganization of the nanoparticles takes place and as a result the total film thickness decreases and the Au density inside the layer increases. EDPs shown in the inset of Fig. 1(a) implies that due to prolonged reorganization of the monolayer film, the Au layer density increases from ≈ 0.94 el/Å 3 to ≈ 1.25 el/Å 3 . EDPs shown in the inset of Fig. 1(b) and Fig. 1(c) show that for the bilayer and trilayer films two distinct reorganization process takes place. After ≈ 60 days, due to reorganization of bilayer film, Au layer density increases from ≈ 1.05 el/Å 3 (top) and ≈ 0.78 el/Å 3 (bottom) to ≈ 1.17 el/Å 3 and ≈ 0.96 el/Å 3 respectively and the total film thickness decreases, but the bilayer structure is maintained. However, in the next step, i.e., after prolonged reorganization (≈ 12 months) the bilayer structure modifies to monolayer structure where the Au layer density increases to ≈ 1.54 el/Å 3 . For the trilayer film, like bilayer, in the first step the layer density increases with time and the film thickness decreases maintaining the trilayer structure. However, after prolonged reorganization for ≈ 12 months, the trilayer structure collapses into monolayer structure like bilayer. EDPs also show that after prolonged reorganization the average monolayer, bilayer and trilayer film thickness decreases by ≈ 9Å, 22Å and 45Å respectively. XRR profiles and the corresponding analyzed curves of the Au nanoparticle LB films deposited by two and four down-up cycles at π = 8mN/m and their time evolutions are shown in Fig. 2. EDPs obtained from the analysis are shown in the inset of the corresponding figures which implies that three and seven layer structures have been formed from the two and four down-up cycles respectively. Time evolution (≈ 12 months) EDPs obtained from the analysis show that due to the reorganization the Au layered structures become monolayer and the Au density increases and the total film thickness decreases. EDPs shown in the inset of Fig. 2(a) implies that the Au layer density increases from ≈ 0.91 el/Å 3 to ≈ 1.7 el/Å 3 and the total film thickness decreases by ≈ 54Å. EDPs shown in the inset of Fig. 2(b) implies that for the seven layer film the Au layer density increases from ≈ 0.77 el/Å 3 to ≈ 1.8 el/Å 3 and the total film thickness decreases by ≈ 62Å. Thus, out-of-plane structural analysis shows that from the layered structures a denser monolayer like structure has formed. However, from the XRR study it is not clear whether the size of the nanoparticles is modified or not due to the time evolution. It is also not possible to know whether the particles make any equilibrium two dimensional patterns or not. To obtain the particle size and in-plane structural information from the prolonged reorganized films we did the GISAXS measurements. GISAXS data obtained from the monolayer, bilayer and trilayer films deposited by one down-up cycle are shown in Fig. 3(a)-3(c) respectively, while the GISAXS data obtained from the three and seven layer films deposited by two and four downup cycles are shown in Fig. 4(a) and 4(b) respectively. The beam-stopper stops the direct beam together with specular and Yoneda peaks, which are related with the incident angle and critical angle respectively. The absence of spots in the q x -q z plane implies that there is no in-plane ordering among Au nanoparticles after prolonged reorganization. The intensity variation as a function of q x at q z = 0.062 Å −1 are shown in the inset of the corresponding figures. The fitting of all the intensity profiles as a function of q x implies that after the reorganization there different Au sizes are present inside the five different films and their diameters are ≈ 26-28Å, ≈ 46-62Å and ≈ 118-162Å respectively. The errors of each diameter values are 5-9%. Thus after reorganization on the average two relatively bigger sizes Au nanoparticles are formed from the pristine nanoparticles and all these particles are distributed randomly on the solid surface.
The reorganized structures, two different reorganization processes and their final in-plane organization have schematically shown in Fig. 5. In the initial stage of reorganization the layered structures of Au nanoparticles are maintained, i.e, mono-, bi-and tri-layer structure is maintained but the film becomes more compact due to the interpenetration of the ligand shells and due to the filling of the defects that were formed during the film growth. These structures and reorganization process are shown in Fig. 5(a) and 5(b). However, due to prolonged reorganization for ≈ 12 months, the layered structure destroys and monolayer likes structure forms (shown in Fig. 5(c)). As the generated monolayer films are more compact so the Au density becomes very high inside such thinner films. The metallic core of Au nanoparticles takes on the average three different sizes, i.e., pristine, little bigger and more bigger size. As the surface energy, i.e., surface to volume ratio of the smaller nanoparticles are higher in comparison with the relatively bigger particle for the same total volume, so it is most probable that to minimize the surface energy the particles coalesce and becomes . Two reorganization processes are shown where in the first process layered structure is maintained but film thickness decreases (b) and in the second process layered structure collapses in to a monolayer structure having three different metal cores (c). bigger one. However, it depends upon the coating layer and the number of the available particles. Probably the room temperature and long enough time for reorganization helps the thiol molecules to redistribute on the surface of the modified Au nanoparticles. These modified Au nanoparticles are distributed randomly on the silicon surfaces, not forming any two-dimensional ordering, although in some other experimental conditions 2D ordering have been observed. 34,35 Thus, it is clear that multilayer to monolayer like transformation is the most probable trend of such organo-coated metallic nanoparticles irrespective of the substrate surface nature (hydrophilic, hydrophobic, etc) or initial layer numbers if sufficient time is given for the reorganization. In addition, the metallic core coalesces to form bigger size particles to minimize the surface energy.
In conclusion, reorganization behaviour of different layered structure LB films of Au nanoparticles deposited on hydrophobic Si(001) substrates have been studied by using XRR and GISAXS techniques to get out-of-plane and in-plane structural evolution with time. Structural information obtained from the studies show that at the initial stage the reorganization occurs through the compaction of the films keeping the layered structure unchanged but after prolonged reorganization all layered structure modifies to monolayer structure. Due to such reorganization the Au density inside the nanometer thick film increases. Moreover, the particle size modifies as the metallic core of the Au nanoparticles coalesces due to the reorganization and these particles are distributed randomly inside the monolayer film. Such reorganization behaviours and restructuring irrespective of substrate surface are very informative in nanotechnology.