Bruker AXS

D8 DISCOVER

N8 TITANOS

AFM image

GISAXS
2D mapApplication Note # AFM-01
AFM and GISAXS Study of Self-assembled FeO
Nanoparticles
Self-assembled iron oxide nanoparticles were investigatedusing atomic force microscopy (AFM) and grazing-incidence
small-angle X-ray scattering (GISAXS).
The atomic force microscopy was carried out using aBruker N8 TITANOS system and showed a dense particle
distribution across the sample surface. Its depth and lateralprofiling provides an estimate of the particle size and shapeand 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 toanalyze the correlation characteristics of the nanoparticles
position.

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

Sample Preparation
The iron oxide nanoparticles were synthesized througha high-temperature solution phase reaction of metal
acetylacetonates (Fe(acac) 3) with 1,2-hexadecanediol, oleic
acid and oleylamine in phenylether. Toluene was used asa 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 isattached 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 resonantfrequency. 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 theoscillation 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 noiselevel in Z below 0.05 nm.

AFM measurement results

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 fig. 1 leads to theassumption that they are of spherical shape and closely
packed. To determine the size of these particles a crosssection was drawn across the center of a number ofadjacent particles as indicated in fig. 1b. The profile shownin fig. 2 was extracted from this cross section. The blue
and red arrows (figs. 1b and 2) indicate the position ofheight maxima of two representative neighboring particles.
The distance of 6.44 nm between maxima leads to theconclusion that particles have a diameter of ca. 6.4 nm.
Fig. 3 shows a 3D representation of the scan. It confirms
both the assumptions that the particles are spherical andclosely packed.
Line profile evaluationa)Fig. 2 Line profile from fig. 1 b). The blue circles indicate the spherical particles, the arrows the positions of maximum height (particletops) used for size determination.
3D image of AFM scanb)

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

Fig. 3 3D representation of the 300 nm x 300 nm AFM scan. Thespherical 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 [2] as a novel technique for investigating
structures on or close to the surface. At grazing incidence, theincident beam undergoes total external reflection if the angle isbelow the critical angle. Scanning the angle of incidence frombelow 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 theother hand is sensitive to in-plane correlations in the surface andinterfaces. 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.
Today, GISAXS is a commonly used technique for investigationsof quantum dots, thin organic films, or nanomaterials arranged
on surfaces. Grazing incidence SAXS experiments require botha high primary beam intensity and a low beam divergence sountil now most GISAXS experiments have been performed atsynchrotrons. 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 bothcoherent and incoherent (diffuse) components:I(ai,af,2qi,2qf)=|c0 | 2|Ti| 2|Tf| 2 [Id + Ic],
Ti =kizs= k0

2kiz

2kfz, Tf =kiz + kizs

kfz + kfzs

1 + cs - cos2ai , kfzs = k0

1 + cs - cos2af

Id is diffuse scattering, caused by the fluctuations ofgeometrical sizes of nanoparticles, it depends on the oneparticle distribution functions near the average values:→→
Id = [<|Fl(q)| 2> - |<Fl(q)>| 2],
The coherent component Ic depends on the pair correlation
function gl(r) for distribution of nanoparticles within theplane, which is parallel to the surface:l0→

Ic = Σ |<Fl(q)>|2

l=1→→Sl (q⊥ ) = rs(l)d(q⊥ ) + 1 + rs(l)HFsph (q) = exp[iqz(H - R)]∫ dr [gl (r ) -1]exp[-iq r ]⊥∫ 2pR0→Sl(q⊥ )2z→⊥Jl(qlRz)
qlR z→ →⊥ ⊥D8 DISCOVER

Fig. 4 D8 DISCOVER with enclosure

Measurement Parameters
Source 					Sample holder 		
Laser video microscope
Detector – sample dist.
Detector 		
Angular Range 					Detector Resolution
Beam Size 		
Data collection timeMicro Focus X-ray Source (ImS)Cu Ka radiation, 45 kV / 650 mA
Eulerian cradle
Sample alignment & imaging
210 mmVÅNTEC-2000 (2-D Detector)
35° coverage in 2Q and g
at 200 mm detector distance2048 x 2048 pixels
covers the whole sample area
10 min/frame

Calculated scattering intensities
Spherical particles
FeO on Si substrate
R = 3 nm, DR = 0.3 nm,
Correlation model:
Hard sphere, L = 9 nm

Cylindrical particles
FeO on Si substrate
R = 3 nm, DR = 0.3 nm,
H = 6 nm, DH = 0.6 nm,
Correlation model:
Hard sphere, L = 9 nm

exp(iqzz)dz
Fig. 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 havebeen fitted simultaneously with fixed sample models (HardSphere correlations, Gaussian parameter distribution, Fullsphere particle shape), variable particle size values (diameter D) and interparticle distances (lateral correlation lengthL). The particular result obtained for a spherical particle
shape is:D = 6.4 ± 0.5 nm; L = 6.2 nm

Screenshots of the LEPTOS programc)

Fig. 6 a – c Left: a) 2D GISAXS map. b) Radial section from the map in a). c) Sample model used for the measurements

Conclusion

References

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 ofnanoparticles with dimensions down to the nanometerscale. The X-ray beam size makes it possible to evaluate thestatistically-averaged parameters over a large illuminatedarea. Precise data analysis, accounting for both coherent anddiffuse scattering, delivers a wide set of the nanoparticles’
parameters (shape, size, correlations, distributions).

[1] A. Ulyanenkov. 2004 Proc. of SPIE Vol. 5536 1 (SPIE,Bellingham, WA, 2004).
[2] J.R. Levine, J.B. Cohen, Y.W. Chung, P. Georgopoulos,
1989 J. Appl. Cryst. 22, 528-532

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

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Sample courtesy ofL. Chitu1, G. Maier 2, E. Majkova1, P. Siffalovic1,[1] Instute of Physics, Slovak Academy of Science, Slovakia[2] Department Materialphysik - Montanuniversität Leoben(DMP), AustriaAuthors
A. Thöne1, A.Ulyanenkov2[1] Bruker Nano, Herzogenrath, Germany[2] Bruker AXS GmbH, Karlsruhe, Germany

Bruker Nanowww.bruker-nano.com

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