Time-resolved Grazing-incidence Small-angle X-ray Research Articles

Real-time observation of nucleation and growth of Au on CdSe quantum dot templates

Semiconductor quantum dot (QD) arrays can be useful for optical devices such as lasers, solar cells and light-emitting diodes. As the size distribution influences the band-gap, it is worthwhile to investigate QDs prepared using different solvents because each of them could influence the overall morphology differently, depending on the ligand network around individual QDs. Here, we follow the nucleation and growth of gold (Au) on CdSe QD arrays to investigate the influence of surface ligands and thereby realized interparticle distance between QDs on Au growth behaviour. We particularly emphasize on the monolayer stage as the Au decoration on individual QDs is expected at this stage. Therefore, we sputter-deposit Au on each QD array to investigate the morphological evolution in real-time using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). The growth kinetics - independent of the template - signifies that the observed template-mediated nucleation is limited only to the very first few monolayers. Delicate changes in the Au growth morphology are seen in the immediate steps following the initial replicated decoration of the QD arrays. This is followed by a subsequent clustering and finally a complete Au coverage of the QD arrays.

Interfacial and Activation Energies of Environmentally Abundant Heterogeneously Nucleated Iron(III) (Hydr)oxide on Quartz.

Poorly crystalline iron(III) (hydr)oxide nanoparticles are ubiquitous in environmental systems and play a crucial role in controlling the fate and transport of contaminants. Yet, the thermodynamic and kinetic parameters, e.g., the effective interfacial (α') and apparent activation (Ea) energies, of iron(III) (hydr)oxide nucleation on earth-abundant mineral surfaces have not been determined, which hinders an accurate prediction of iron(III) (hydr)oxide formation and its interactions with other toxic or reactive ions. Here, for the first time, we report experimentally obtained α' and Ea for iron(III) (hydr)oxide nucleation on quartz mineral surfaces by employing a flow-through, time-resolved grazing incidence small-angle X-ray scattering (GISAXS). GISAXS enabled the in situ detection of iron(III) (hydr)oxide nucleation rates under different supersaturations (σ, achieved by varying pH 3.3-3.6) and temperatures (12-35 °C). By quantitative analyses based on classical nucleation theory, α' was obtained to be 34.6 mJ/m2 and Ea was quantified as 32.8 kJ/mol. The fundamental thermodynamic and kinetic parameters obtained here will advance our fundamental understanding of the surface chemistry and nucleation behavior of iron(III) (hydr)oxides in subsurface and water treatment systems as well as their effects on the fate and transport of pollutants in natural and engineered water systems. The in situ flow-through GISAXS method can also be adapted to quantify thermodynamic and kinetic parameters at interfaces for many important solid-liquid environmental systems.

Investigation of Drying Process of Catalyst Ink for Polymer Electrolyte Fuel Cells by Grazing-Incidence X-Ray Scattering

The performance of polymer electrolyte fuel cells (PEFCs) depends on the structure of catalyst layer which is formed by coating catalyst ink. This study investigated the nanostructural formation of catalyst layer during ink drying process using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). GISAXS patterns measured for the catalyst ink coated with a 100 μm thickness on a silicon wafer suggest that Pt/C aggregation completed at drying time (t) of 250 sec although the ink was completely dried at t = 1600 sec. Thus, the Pt/C aggregate structure was mostly pinned in the early stage of the drying.

Time-Resolved Nanostructural Analysis of Catalyst Layer Formation Process by Synchrotron X-ray Scattering

The film formation process of blade-coated catalyst ink was investigated by in-situ time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) measurements utilizing synchrotron radiation. The GISAXS profiles were measured as a function of the time elapsed after coating the ink, t. As increasing t, the peak at around q z ~ 0.2 nm−1 shifted toward smaller q z. At t ~ 50 s, two peaks appeared at around q z ~ 0.5 nm−1 and then combined into one peak. These peak shifts would be ascribed to Pt/C agglomeration induced by solvent evaporation.

Periodic Mesoporous Organosilica Films with a Tunable Steady-State Mesophase.

The mesophase formation in spin-coated periodic mesoporous organosilica (PMO) films aged at a controlled ambient humidity is investigated by time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). The investigation demonstrates the existence of a tunable steady state in PMO spin-coated films. Thus, a film deposited at a relative humidity of 20 % has a lamellar mesophase, whereas a subsequent increase to 70 % leads to a phase transformation resulting in a P63 /mmc space group. On the other hand, an increase of the surfactant to organosilica molar ratio of between 0.26 and 0.31 results in films which at 70 % humidity form a mix of 2D and 3D hexagonal phases. A further increase of the surfactant amount leads to films with a 2D hexagonal phase. Finally, the different mesophases observed as a function of the solution aging emphasize the importance of the degree of polycondensation of the organosilica oligomers.

Role of Sputter Deposition Rate in Tailoring Nanogranular Gold Structures on Polymer Surfaces.

The reproducible low-cost fabrication of functional polymer-metal interfaces via self-assembly is of crucial importance in organic electronics and organic photovoltaics. In particular, submonolayer and nanogranular systems expose highly interesting electrical, plasmonic, and catalytic properties. The exploitation of their great potential requires tailoring of the structure on the nanometer scale and below. To obtain full control over the complex nanostructural evolution at the polymer-metal interface, we monitor the evolution of the metallic layer morphology with in situ time-resolved grazing-incidence small-angle X-ray scattering during sputter deposition. We identify the impact of different deposition rates on the growth regimes: the deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates are initial particle densities higher due to an increasing influence of random nucleation, and an earlier onset of thin film percolation occurs. The obtained results are discussed to identify optimized morphological parameters of the gold cluster ensemble relevant for various applications as a function of the effective layer thickness and deposition rate. Our study opens up new opportunities to improve the fabrication of tailored metal-polymer nanostructures for plasmonic-enhanced applications such as organic photovoltaics and sensors.

Toward an equilibrium structure in lamellar diblock copolymer thin films using solvent vapor annealing – An in-situ time-resolved GISAXS study

Solvent vapor annealing (SVA) is frequently used to improve the ordering in diblock copolymer thin films. An important question is which SVA protocol should be chosen to ensure thermodynamic equilibrium. Here, we investigate two thin films from a low molar-mass, lamellae-forming polystyrene-block-polybutadiene (PS-b-PB) diblock copolymer (28.0kg/mol). The films are prepared by spin-coating Si wafers from toluene solutions and have film thicknesses of 215nm and 332nm. The as-prepared films have mainly the parallel lamellar orientation with a lamellar thickness Dlam,par significantly lower than in the bulk. SVA cycles were carried out with cyclohexane, and the structural changes were followed in-situ using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). Before and after SVA, Dlam,par is significantly lower than in the bulk, i.e. the equilibrium value of Dlam,par in thin film geometry is different from the bulk value. Whereas the behavior of Dlam,par is different for the two films in the early stages of the first swelling, it is very similar in the late stages of swelling and during drying. During the first drying, the lamellae deswell, initially slowly and later, when PS becomes glassy again, affinely. During the second SVA cycle on the thin film, the scaling behavior of the lamellar thickness is identical to the one during the first drying and to the drying behavior of the thicker film. We conclude that one cycle of solvent vapor treatment with a degree of swelling of ca. 1.5 is sufficient to bring the PS-b-PB thin films studied into equilibrium and to create a nearly defect-free lamellar structure.

Time-resolved GISAXS and cryo-microscopy characterization of block copolymer membrane formation

Time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) and cryo-microscopy were used for the first time to understand the pore evolution by copolymer assembly, leading to the formation of isoporous membranes with exceptional porosity and regularity. The formation of copolymer micelle strings in solution (in DMF/DOX/THF and DMF/DOX) was confirmed by cryo field emission scanning electron microscopy (cryo-FESEM) with a distance of 72 nm between centers of micelles placed in different strings. SAXS measurement of block copolymer solutions in DMF/DOX indicated hexagonal assembly with micelle-to-micelle distance of 84–87 nm for 14–20 wt% copolymer solutions. GISAXS in-plane peaks were detected, revealing order close to hexagonal. The d-spacing corresponding to the first peak in this case was 100–130 nm (lattice constant 115–150 nm) for 17 wt% copolymer solutions evaporating up to 100 s. Time-resolved cryo-FESEM showed the formation of incipient pores on the film surface after 4 s copolymer solution casting with distances between void centers of 125 nm.

Structural Evolution of Perpendicular Lamellae in Diblock Copolymer Thin Films during Solvent Vapor Treatment Investigated by Grazing‐Incidence Small‐Angle X‐Ray Scattering

The structural evolution in poly(styrene-b-butadiene) (P(S-b-B)) diblock copolymer thin films during solvent vapor treatment is investigated in situ using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). Using incident angles above and below the polymer critical angle, structural changes near the film surface and in the entire film are distinguished. The swelling of the film is one-dimensional along the normal of the substrate. During swelling, the initially perpendicular lamellae tilt within the film to be able to shrink. In contrast, at the film surface, the lamellae stay perpendicular, and eventually vanish at the expense of a thin PB wetting layer. During the subsequent drying, the perpendicular lamellae reappear at the surface, and finally, PS blocks protrude. By modeling, the time-dependent height of the protrusions can be quantitatively extracted.

Real-time tracking of nanoparticle self-assembling using GISAXS

The self-assembling of iron oxide nanoparticles deposited on a substrate was studied employing the in situ time-resolved grazing incidence small-angle X-ray scattering with a temporal resolution of 28 ms. After the evaporation of the solvent, an ordered monolayer exhibiting a hexagonal close-packed arrangement of nanoparticles was found. The X-ray scattering data from the drop did not show a presence of ordered clusters in the drop volume and/or ordered islands at the drop surface. Three stages of the drop evaporation process were distinguished and analysed.

Self-assembly of iron oxide nanoparticles studied by time-resolved grazing-incidence small-angle x-ray scattering

Evaporation of colloidal nanoparticle solutions is known to produce ordered monolayers of nanoparticles, self-assembled arrays of magnetic nanoparticles being of special importance for applications. The in situ time-resolved grazing-incidence small-angle x-ray scattering with the temporal resolution down to $100\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$ was employed to study the self-assembling process of iron oxide nanoparticles after a colloidal drop was applied on a silicon substrate. The x-ray scattering contributions from the evaporating drop volume, drop surface, and substrate surface were monitored and separated. The x-ray scattering from the drop for the distances from the substrate surface larger than $\ensuremath{\approx}80\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ shows the absence of self-assembled clusters in the drop volume or self-assembled domains on the drop surface. These results indicate that the nanoparticle self-assembling occurs in the vicinity of the three-phase drop contact line. The ordered nanoparticle monolayer exhibits hexagonal close-packed arrangement.

Time-resolved grazing-incidence small-angle X-ray scattering studies of lipid multibilayers with the insertion of amyloid peptide during the swelling process

The β-amyloid peptide (Aβ) (1–40) is one of the major components that form Alzheimer's amyloid deposits. Studies of the membrane insertion of amyloid showed that amyloid is surface active and can insert into lipid monolayers [Ji et al. (2002). Biochemistry (Moscow), 67, 1283–1288; Ege & Lee (2004). Biophys. J. 87, 1732–1740]. The interaction between the peptide and a lipid monolayer or bilayer is critical to the understanding of the formation of amyloid peptide deposits on the membrane. In this paper, we have studied the structural transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multibilayers with the insertion of amyloid peptide 1–40 by grazing-incidence small-angle X-ray scattering at different bilayer hydration levels (changing the relatively humidity). We mixed the DPPC and amyloid peptide in an organic solvent at a 10 to 1 weight ratio, then cast it onto a silicon wafer to form the mixed lipid multibilayer film. In this study of the pure DPPC multibilayer film and the DPPC multibilayer film inserted with amyloid peptide, it was found that the hydration process was bimodal with a better-hydrated top layer and a less-hydrated bottom layer. The gel-to-ripple phase transition suffers a strong confinement effect due to the presence of the solid substrate. With the insertion of the amyloid peptide, the ripple phase of the membrane bilayers was suppressed at high humidity and the whole film can be swollen more uniformly at lower incubation time than the pure DPPC film supported on a silicon wafer. This means water vapor can penetrate more easily into the DPPC bilayers inserted with Aβ than into the pure DPPC bilayers. Amyloid peptides were found to form clusters in the bilayer and possess in-plane correlation. From analyzing the diffuse scattering around the Bragg peak in the lateral direction, the amyloid peptides are found to form clusters in the bilayer with a radius of about 9 nm. It is also estimated that the number of Aβ molecules in one cluster is about 12 and on average each Aβ molecule occupies an interface area of about 22 nm2. As the relative humidity exceeds about 94%, the Aβ clusters seem to develop an ordered structure with a spacing of about 300 Å.

Peering into the self-assembly of surfactant templated thin-film silica mesophases.

It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of nanostructures with feature sizes smaller than achievable with state of the art lithography and with a complexity approaching that of biological systems. For example, recent research has shown that silica/surfactant self-assembly combined with evaporation (so-called evaporation induced self-assembly EISA) can direct the formation of porous and composite thin-film mesostructures characterized by precise periodic arrangements of inorganic and organic constituents on the 1-50-nm scale. Despite the potential utility of these films for a diverse range of applications such as sensors, membranes, catalysts, waveguides, lasers, nano-fluidic systems, and low dielectric constant (so-called low k) insulators, the mechanism of EISA is not yet completely understood. Here, using time-resolved grazing incidence small-angle X-ray scattering (GISAXS) combined with gravimetric analysis and infrared spectroscopy, we structurally and compositionally characterize in situ the evaporation induced self-assembly of a homogeneous silica/surfactant/solvent solution into a highly ordered surfactant-templated mesostructure. Using CTAB (cetyltrimethylammonium bromide) as the structure-directing surfactant, a two-dimensional (2-D) hexagonal thin-film mesophase (p6mm) with cylinder axes oriented parallel to the substrate surface forms from an incipient lamellar mesophase through a correlated micellar intermediate. Comparison with the corresponding CTAB/water/alcohol system (prepared without silica) shows that, for acidic conditions in which the siloxane condensation rate is minimized, the hydrophilic and nonvolatile silicic acid components replace water maintaining a fluidlike state that avoids kinetic barriers to self-assembly.

Ion-induced pattern formation on Co surfaces: An x-ray scattering and kinetic Monte Carlo study

We report time-resolved grazing incidence small-angle x-ray scattering and atomic force microscope studies of the evolution of the surface morphology of the Co(0001) surface during low-energy ${\mathrm{Ar}}^{+}$ ion sputtering. At temperatures greater than 573 K, the surface is smooth, erosion proceeding in either a layer-by-layer mode or a step retraction mode. In contrast, at temperatures below 573 K, the surface develops a correlated pattern of mounds and/or pits with a characteristic length scale, $\ensuremath{\lambda}.$ At room temperature, the surface morphology is dominated by mounds, and coarsens as time progresses. The characteristic length scale obeys the apparent power law, $\ensuremath{\lambda}=A\ifmmode\times\else\texttimes\fi{}{t}^{n}$ with $n=0.20\ifmmode\pm\else\textpm\fi{}0.02.$ The rms roughness of the surface increases in time according to a similar power law with a slightly larger exponent $\ensuremath{\beta}=0.28\ifmmode\pm\else\textpm\fi{}0.02.$ Kinetic Monte Carlo simulations of a simple model of Cu(111) were also performed. These simulations suggest that mound formation and coarsening at low temperatures is due to the slow diffusion of sputter-created adatoms on step edges. The morphological transition from mounds to pits is associated with activation of kink diffusion. These simple simulations produce values for the scaling exponents that agree with the experimental measurements.