Investigation of Self-Assembled Iron Oxide Nanoparticles using N8 TITANOS and D8 Discover from Bruker Nano

Topics Covered

About Bruker Nano
Introduction
Sample Preparation
Experimental Setup for AFM (N8 TITANOS)
AFM Results
Experimental Setup for GISAXS (D8 DISCOVER)
GISAXS Theory
Measurement Parameters
GISAXS Results
Conclusion

About Bruker Nano

Bruker Nano provides Atomic Force Microscope/Scanning Probe Microscope (AFM/SPM) products that stand out from other commercially available systems for their robust design and ease-of-use, whilst maintaining the highest resolution. The NANOS measuring head, which is part of all our instruments, employs a unique fiber-optic interferometer for measuring the cantilever deflection, which makes the setup so compact that it is no larger than a standard research microscope objective.

The firm basis for the quality of our microscopes is a team of experienced scientist and engineers with a background of more than 15 years in the AFM business.

Introduction

Self-assembled iron oxide nanoparticles were investigated using atomic force microscopy (AFM) and grazing-incidence small-angle X-ray scattering (GISAXS).

The atomic force microscopy was carried out using a Bruker N8 TITANOS system and showed a dense particle distribution across the sample surface. Its depth and lateral profiling provides an estimate of the particle size and shape and preliminary information on the ordering of the objects. The X-ray measurements were performed with a D8 DISCOVER diffractometer with VÅNTEC-2000 2-D area detector. This permits a long-range, statistically-averaged analysis over the whole surface of the investigated FeO nanoparticle sample. The shape and size of the particles as well as the inter-particle distance were evaluated using distorted-wave Born approximation (DWBA) implemented in the LEPTOS software. Several models were verified to analyze the correlation characteristics of the nanoparticles position.

The results obtained demonstrate a consistency of the nanoparticle dimensions as measured with AFM and X-ray methods. The conventional non-synchrotron X-ray diffraction setup provides sufficient data quality for comprehensive evaluation of the investigated samples.

Sample Preparation

The iron oxide nanoparticles were synthesized through a high-temperature solution phase reaction of metal acetylacetonates (Fe(acac)₃) with 1,2-hexadecanediol, oleic acid and oleylamine in phenylether. Toluene was used as a solvent. The FeO nanoparticles are super-paramagnetic at room temperature (the blocking temperature T is 22 K). For self-assembling studies, 5 µL drops of a colloid solution were deposited manually onto Si substrates with a native SiO₂ layer over an area of 1 cm². The drops were dried in air at room temperature.

Experimental Setup for AFM (N8 TITANOS)

Atomic force microscopy (AFM) is a surface inspection technique. A very sharp tip (radius < 10 nm) which is attached to a cantilever is scanned along the sample surface and detects the topography.

Scanning can be done either in contact or dynamic mode. In dynamic mode the cantilever oscillates near its resonant frequency. The oscillation amplitude and the damping determine whether the measurement is done in intermittent contact or non-contact mode. With the Bruker Nano AFM these values can be adjusted very accurately as the oscillation amplitude is automatically calibrated in nm. The N8 TITANOS is a large sample AFM for analyzing samples up to 300 mm x 300 mm with a very low noise level in Z below 0.05 nm.

Figure 1. a) 300 nm x 300 nm topography scan of FeO particle sample. b) Zoom into a), scan size 85 nm x 85 nm. The white line with arrows indicates position of a line profile.

AFM Results

The results shown in figures 1 to 3 were achieved using regular cantilevers measuring in intermittent contact mode (8 nm free amplitude, 39% damping).

The appearance of the particles in figure 1 leads to the assumption that they are of spherical shape and closely packed. To determine the size of these particles a cross section was drawn across the center of a number of adjacent particles as indicated in figure 1b. The profile shown in figure 2 was extracted from this cross section. The blue and red arrows (figures 1b and 2) indicate the position of height maxima of two representative neighboring particles. The distance of 6.44 nm between maxima leads to the conclusion that particles have a diameter of ca. 6.4 nm. Figure 3 shows a 3D representation of the scan. It confirms both the assumptions that the particles are spherical and closely packed.

Figure 2. Line profile from figure 1b). The blue circles indicate the spherical particles, the arrows the positions of maximum height (particle tops) used for size determination.

Figure 3. 3D representation of the 300 nm x 300 nm AFM scan. The spherical shape and close packing of particles is clearly visible.

Experimental Setup for GISAXS (D8 DISCOVER)

Grazing-Incidence Small Angle X-Ray Scattering (GISAXS) was first introduced in 1989 as a novel technique for investigating structures on or close to the surface. At grazing incidence, the incident beam undergoes total external reflection if the angle is below the critical angle. Scanning the angle of incidence from below to above the critical angle is therefore a kind of nondestructive depth profiling. This is used for conventional X-ray reflectivity (XRR) measurements, which are sensitive to electron density differences along the surface normal. GISAXS on the other hand is sensitive to in-plane correlations in the surface and interfaces. Therefore periodically distributed electron density variations (height-height correlations e.g.) on or slightly below the surface can be investigated. Additionally the GISAXS signal is very sensitive to surface roughness.

Figure 4. D8 DISCOVER with enclosure

Today, GISAXS is a commonly used technique for investigations of quantum dots, thin organic films, or nanomaterials arranged on surfaces. Grazing incidence SAXS experiments require both a high primary beam intensity and a low beam divergence so until now most GISAXS experiments have been performed at synchrotrons. Now the D8 DISCOVER using the Micro Focus X-ray Source (IµS) and the VÅNTEC-2000 2-D detector opens up this exciting field to laboratory instruments.

GISAXS Theory

Scattered X-ray Intensity within the DWBA includes both coherent and incoherent (diffuse) components:

I_d is diffuse scattering, caused by the fluctuations of geometrical sizes of nanoparticles, it depends on the oneparticle distribution functions near the average values:

The coherent component I_c depends on the pair correlation function g_l(r) for distribution of nanoparticles within the plane, which is parallel to the surface:

Measurement Parameters

The measurement parameters are given in the table.

Figure 5. Scattering intensities for spherical and cylindrical particles, calculated with LEPTOS G

GISAXS Results

The software package LEPTOS G was used for the evaluation of the GISAXS maps.

Several GISAXS 2D maps have been recorded at different incidence angles. For each angle, several map sections have been fitted simultaneously with fixed sample models (Hard-Sphere correlations, Gaussian parameter distribution, Full sphere particle shape), variable particle size values (diameter D) and interparticle distances (lateral correlation length L). The particular result obtained for a spherical particle shape is:

D = 6.4 ± 0.5 nm; L = 6.2 nm

Figure 6. 2D GISAXS map

Conclusion

The present work proves the GISAXS technique as a reliable method for comprehensive evaluation of nanoscale objects. The wavelength of X-rays allows the characterization of nanoparticles with dimensions down to the nanometerscale. The X-ray beam size makes it possible to evaluate the statistically-averaged parameters over a large illuminated area. Precise data analysis, accounting for both coherent and diffuse scattering, delivers a wide set of the nanoparticles’ parameters (shape, size, correlations, distributions).

Modern X-ray sources and detectors permit in-house evaluation without recourse to external laboratories. AFM results confirm the GISAXS data, and supply short-range structural information to complement the long-range GISAXS results.

Source Bruker AXS - AFM and SPM

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