http://www.diva-portal.org
Postprint
This is the accepted version of a paper published in AIP Advances. This paper has been peer-reviewed
but does not include the final publisher proof-corrections or journal pagination.
Citation for the original published paper (version of record):
Lansåker, P., Hallén, A., Niklasson, G., Granqvist, C. (2014)
Characterization of gold nanoparticle films: Rutherford backscatteringspectroscopy, scanning
electron microscopy with image analysis, and atomic forcemicroscopy.
AIP Advances, 4(10): 107101
http://dx.doi.org/10.1063/1.4897340
Access to the published version may require subscription.
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-233494
Characterization of gold nanoparticle films:
Rutherford backscattering spectroscopy, scanning electron
microscopy with image analysis, and atomic force microscopy
Pia C. Lansåker,1,a) Anders Hallén,2 Gunnar A. Niklasson,1
and Claes G. Granqvist1
1
Department of Engineering Sciences, The Ångström Laboratory, Uppsala University,
P. O. Box 534, SE-751 21 Uppsala, Sweden
2
Royal Institute of Technology, KTH-ICT, Elektrum 229, Kista, SE-164 40 Stockholm,
Sweden
Gold nanoparticle films are of interest in several branches of science and
technology, and accurate sample characterization is needed but technically
demanding. We prepared such films by DC magnetron sputtering and recorded
their mass thickness by Rutherford backscattering spectroscopy. The geometric
thickness dg—from the substrate to the tops of the nanoparticles—was obtained
by scanning electron microscopy (SEM) combined with image analysis as well as
by atomic force microscopy (AFM). The various techniques yielded an internally
consistent characterization of the films. In particular, very similar results for dg
were obtained by SEM with image analysis and by AFM.
a)
Electronic mail address: [email protected]
1
I. INTRODUCTION
Gold nanoparticles (AuNPs) have numerous applications, especially in green
nanotechnology,1,2 and can be used for catalysis3 and in plasmonically enhanced devices such
as photovoltaic cells,4,5 light emitting diodes,6 photocatalytic reactors,7,8 and gas sensors.9,10
Thin films comprised of AuNPs have a geometric thickness dg—between the substrate and the
tops of the nanoparticles—that clearly is larger than the mass thickness dm for a hypothetical,
uniform layer containing the same number of atoms. Both of these thicknesses are of interest
for analyzing device performance. This paper reports on AuNP films prepared by sputter
deposition onto glass and analyzed by Rutherford backscattering spectroscopy (RBS),
scanning electron microscopy (SEM) combined with image analysis of the AuNP distribution
on the substrate, and atomic force microscopy (AFM). X-ray reflectivity measurements could
have served as an alternative to the RBS data, as shown recently by Kossoy et al.11 A major
result of our investigation is that very similar results for dg were obtained by SEM with image
analysis and by AFM.
II. SAMPLE PREPARATION
Gold was deposited onto glass by DC magnetron sputtering. The target was a 5-cmdiameter plate of 99.99% pure Au placed 13 cm above the substrate holder. The system was
first evacuated to ~1.6 × 10–5 Pa, and sputtering was then performed at 50 W in ~0.8 Pa of
99.98% pure Ar. The substrate holder was rotated to ensure even deposits. Samples were
prepared without deliberate substrate heating and also at a substrate temperature of 140 ± 10
ºC, as determined by a thermocouple. Deposits with 4.5 ≤ dg ≤ 10.4 nm and 1.7 ≤ dm ≤ 5.1 nm
are reported on below.
Film formation depends on substrate material and deposition conditions, as investigated
in earlier work of ours.12–14 Furthermore, detailed knowledge of the substrate is required for
RBS analysis, especially for determining accurate values of dm, since the signal from the
substrate is crucial for normalization of the number of atomic species incident onto the RBS
detector. Specifically, we used 1-mm-thick plates of glass (standard microscope slides,
supplied by Thermo Scientific, UK). According to RBS their composition was, in at.%, 59.4
O, 23.1 Si, 2.5 Ca, 0.5 K, 11.0 Na, 3.2 Mg, and 0.3 Al; this composition is consistent with
information by the glass supplier.
2
III. SAMPLE CHARACTERIZATION: TECHNIQUES AND DATA
A.
Rutherford backscattering spectroscopy
RBS data were taken at Uppsala University’s Tandem Laboratory, by use of 4He+ ions
backscattered at an angle of 170º, and were simulated with the SIMNRA code.15 Figure 1 is
an example of a RBS spectrum and shows a well-defined peak at high energies, caused by Au,
and onsets of scattering at lower energies due to the various constituents of the substrate. The
number NAu of Au atoms per area unit was determined via the simulation, and dm was then
derived from
dm 
M Au N Au
N A  Au
,
(1)
where MAu is the molar mass of Au, NA is Avogadro’s constant, and ρAu is the density of Au
and taken to be 19.31 g/cm3, i.e., the bulk value. The spectrum in Fig. 1 was recorded on
Sample D in Table I; the same table also contains data for three other samples that were
analyzed by RBS in the same way.
B.
Scanning electron microscopy and image analysis
A LEO 1550 FEG instrument with in-lens detection was used to obtain SEM images. The
samples were oriented perpendicular to the electron beam in order to obtain a picture of the
AuNP distribution on the substrate. The acceleration voltage was kept as low as 2–5 keV to
avoid charging effects, and the distance between lens and sample was 2–3 mm. Figure 2
shows data for Samples A–D and verifies that all of them are comprised of nanoparticles.
The SEM pictures were analyzed by a procedure based on an image processing tool
supplied with MATLAB’s Toolbox.16 Each image was converted to na pixels of equal size,
and a number np accounted for the pixels representing particles. The area fraction fSEM for the
substrate coverage by AuNPs was then obtained from
fSEM = np/na ,
(2)
and we finally derived the geometric thickness of the AuNP film by
dg = dm/fSEM ,
(3)
3
which assumes that the particles can be described as objects with top surfaces parallel to the
substrate. The image analysis also gave an average width lp of the particles. Table I reports
values of fSEM, lp and dg; it shows that 0.38 ≤ fSEM ≤ 0.49 and that lp is roughly twice as large
as dg.
C.
Atomic force microscopy
AFM measurements can estimate the average heights of the nanoparticles, denoted dAFM,
and thereby give information that is complementary to dg. We used a Nano-Scope III
instrument with a nominal tip radius of 8 nm and a nominal spring constant of 42 N/m. Figure
3 illustrates data for Sample C; upper panel depicts surface roughness for an area of 500 ×
500 nm and lower panel characterizes these data as a height histogram whose apex serves as a
definition of dAFM.
Data on dAFM are given in Table I for the various samples. It is interesting to note that
dAFM and dg are in very good agreement. The relationship between these parameters is
highlighted in Fig. 4, where the straight line signifies equality between dAFM and dg.
IV.
CONCLUDING REMARKS
The characterization of nanoparticle films is important in many applications, for example
in environmentally benign green nanotechnology,1,2 and is notoriously difficult. We reported
here on a comprehensive study of gold nanoparticles and applied RBS, SEM combined with
image analysis, and AFM to a set of AuNP films in order to determine their mass thicknesses,
geometrical thicknesses, and particle widths. The particle widths were typically twice the
geometrical thickness and about four times the mass thickness. A particularly interesting
result was that RBS and SEM with image analysis, and AFM provided almost identical data
on particle heights.
ACKNOWLEDGMENTS
Pär Lansåker is thanked for help with image analysis. Financial support was received
from the European Research Council under the European Community’s Seventh Framework
Program (FP7/2007–2013)/ERC, Grant Agreement No. 267234 (―GRINDOOR‖).
4
References
1
G. B. Smith and C. G. Granqvist, Green Nanotechnology: Solutions for Sustainability and
Energy in the Built Environment (CRC Press, Boca Raton, FL, 2010).
2
F. Pacheco-Torgal, M. V. Diamanti, A. Nazari, and C. G. Granqvist, editors,
Nanotechnology in Eco-Efficient Construction (Woodhead, Cambridge, UK, 2013).
3
A. S. K. Hashmi and G. J. Hutchings, Angew. Chem. Int. Ed. 45, 7896 (2006).
4
H. A. Atwater and A. Polman, Nature Mater. 9, 205 (2010).
5
S. Pillai and M. A. Green, Sol. Energy Mater. Sol. Cells 94, 1481 (2010).
6
Y. Xiao, J. P. Yang, P. P. Cheng, J. J. Zhu, Z. Q. Xu, Y. H. Deng, S. T. Lee, Y. Q. Li, and J.
X. Tang, Appl. Phys. Lett. 100, 013308 (2012).
7
S. T. Kochuveedu, D.-P. Kim, and D. H. Kim, J. Phys. Chem. C 116, 2500 (2012).
8
W. Hou and S. B. Cronin, Adv. Funct. Mater. 23, 1612 (2013).
9
N. Liu, M. L. Tang, M. Hentschel, H. Giessen, and A. P. Alivisatos, Nature Mater. 10, 631
(2011).
10
K. M. Mayer and J. H. Hafner, Chem. Rev. 111, 3828 (2011).
11
A. Kossoy, D. Simakov, S. Olafsson, and K. Leosson, Thin Solid Films 536, 50 (2013).
12
P. C. Lansåker, K. Gunnarsson, A. Roos, G. A. Niklasson, and C. G. Granqvist, Thin Solid
Films 519, 1930 (2011).
13
P. C. Lansåker, G. A. Niklasson, and C. G. Granqvist, Thin Solid Films 520, 3688 (2012).
14
P. C. Lansåker, P. Petersson, G. A. Niklasson, and C. G. Granqvist, Sol. Energy Mater. Sol.
Cells 117, 462 (2013).
15
M. Mayer, AIP Conf. Proc. 475, 541 (1999).
16
P. D. Kovesi, MATLAB and Octave Functions for Computer Vision and Image Processing
(Centre for Exploration Targeting, School of Earth and Environment, The University of
Western Australia, Perth, Australia, 2000);
http://www.csse.uwa.edu.au/~pk/research/matlabfns/
5
Table
TABLE I. Data for films of AuNPs sputter deposited onto glass at the shown substrate
temperature τs. Values are given on mass thickness dm determined from Rutherford
backscattering spectroscopy; area fraction of particles fSEM, particle width lp, and geometric
thickness dg determined from scanning electron microscopy combined with image analysis;
and particle height dAFM determined from atomic force microscopy.
Sample
τs [ºC]
dm [nm]
fSEM
lp [nm]
dg [nm]
dAFM [nm]
A
~25
1.7
0.38
8.7
4.5
4.5
B
140
1.7
0.39
11.4
4.4
4.7
C
140
3.4
0.46
15.6
7.4
7.1
D
140
5.1
0.49
22.0
10.4
9.9
6
Figure captions
FIG. 1. Experimental and simulated RBS data for a film of AuNPs (sample data are given in
Table I). The various features are associated with the shown elements. The deviation between
the two types of data for energies lower than ~400 keV is due to multiple scattering effects
and inaccurate stopping values.
FIG. 2. SEM images for films of AuNPs. Sample data are given in Table I.
FIG. 3. AFM data for a film of AuNPs (sample data are given in Table I). Upper and lower
panels show a three-dimensional rendition of sample roughness and a histogram of relative
heights, respectively. The peak in the histogram defines dAFM.
FIG. 4. Geometric thickness dg determined from SEM combined with image analysis, and
average particle height dAFM determined from AFM, for films of AuNPs (sample data are
given in Table I). The line represents equality between the two parameters.
7
Energy [keV]
Energy [keV]
200
2 300
400
400
800
800
600
1000
1200
1200
1400
1600
1600
1800
2 200
Experimental
Simulated
2 100
Au
Experimental
Simulated
2 000
2000
1 900
1 800
1 700
1 600
Sample D
Counts
Counts
1 500
1500
1 400
1 300
1 200
1 100
1 000
1000
900
800
700
Al
Mg
Na
Si
O
600
500
500
400
K Ca
300
200
100
0
0
50
100
150
200
200
250
300
350
400
400
450
500
Channel
550
600
600
650
700
750
800
800
850
900
Channel
Figure 1
8
Sample:
A
B
C
D
200 nm
Figure 2
9
Sample C
Frequency
dAFM
Height [nm]
Figure 3
10
Figure 4
11