Journal of Materials Science

, Volume 48, Issue 2, pp 920–928

Microstructure analysis of ion beam-induced surface nanostructuring of thin Au film deposited on SiO2 glass

Authors

  • Xuan Meng
    • Graduate School of EngineeringHokkaido University
    • School of Nuclear Science and TechnologyLanzhou University
    • Center for Advanced Research of Energy Conversion Materials, Faculty of EngineeringHokkaido University
  • Ruixuan Yu
    • Graduate School of EngineeringHokkaido University
  • Shinya Takayanagi
    • Graduate School of EngineeringHokkaido University
  • Seiichi Watanabe
    • Center for Advanced Research of Energy Conversion Materials, Faculty of EngineeringHokkaido University
Article

DOI: 10.1007/s10853-012-6816-1

Cite this article as:
Meng, X., Shibayama, T., Yu, R. et al. J Mater Sci (2013) 48: 920. doi:10.1007/s10853-012-6816-1

Abstract

Effects of the irradiation dose on surface nanostructuring accompanied with the dewetting process of Au films deposited on SiO2 glass were examined using an atomic force microscope and a scanning electron microscope. In addition, the microstructural evolution and the chemical concentration of Au films were investigated using a transmission electron microscope equipped with an energy-dispersive spectrometer. As increasing the Ar ion irradiation dose, the lattice expansion of Au nanoscale islands sustained on the SiO2 glass was observed and irradiation-induced lattice defects together with irradiation-induced interface ion mixing were accounted for this lattice expansion. Finally a layer of photosensitive Au nanoballs with highly spherical shape embedded in a SiO2 substrate was obtained after Ar ion irradiation to 10.0 × 1016/cm2 and some of Au nanoballs were found to be single crystals. As the irradiation energy of the Ar ions increased from 100 to 150 keV, the average diameter of the Au nanoballs in the substrate increased and the red shift of the SPR peak was observed. This tendency of the experimental SPR peaks corresponded with that of the theoretically calculated SPR peaks using Mie solution.

Abbreviations

UV

Ultraviolet

AFM

Atomic force microscope

TEM

Transmission electron microscope

SEM

Scanning electron microscope

EDS

Energy-dispersive spectrometry

SPR

Surface plasmon resonance

FCC

Face-centered cubic

Introduction

The physical and chemical properties of low-dimensional solid-state systems have attracted considerable attention because of their technological importance. In the past decade, the optical properties of metal nanoparticles have been extensively studied experimentally as well as theoretically [17]. Most of the studies have focused on surface plasmon excitation, which by far dominates the photoabsorbance spectra in the near UV/visible range for metal particles with diameters much lower than the wavelength of light. Metal-dielectric nanoparticles show nonlinear and rapid optical responses near the surface plasmon resonance (SPR) frequency, especially for noble-metal nanoparticles such as Au, Ag, and Cu [4]. In recent years, there are many interests in synthesizing silica glass-based metal–silica nanocomposites for their considerable applications in nano-optical devices, which have been widely used in ultra-fast optical nonlinear device [8], plasmonic [5, 6], biosensor and biomedicine [2, 7], catalysis [9, 10], solar energy utilization [11], and so on. For example, nanofloating gate memory with Au nanoparticles is considered to be an ideal candidate for next generation memory devices and nanoparticle-enhanced SPR biosensors may improve their detection limitation by 1–2 orders of magnitude [6, 7]. From the viewpoint of practical applications, controlling in the size, shape, and volume fraction of the embedded Au nanoparticles still remains a challenge.

Various techniques have been applied to fabricate nano-dielectric composite such as ion implantation and afterward thermal annealing [1, 5, 8, 12], ion beam irradiation [4, 1315], electron beam irradiation [16], and laser irradiation [17, 18]. Ion beam process has the pronounced advantages in controlling size distribution and depth in matrix [13], and also in improving the adhesion of deposited metal films on insulating substrates [1922]. Ion irradiation has been considered to be an effective approach in introducing nanoparticles in a substrate, and their optical properties have been emphasized [4, 13]. In addition, ion irradiation is a pronounced approach in surface nanostructuring, which is the basic knowledge of understanding the ion beam interaction with the surfaces [23]. Surface-interface modification in metal–dielectric systems can be obtained using low-energy ion irradiation. Low-energy ions up to few 100 keV undergo nuclear stopping and the energy deposition is dominated by nuclear energy loss, which effectively induces mass transfer and so-called ion mixing and therefore effectively introduces surface structuring [1922]. On the other hand, ion irradiation resulted in lattice damage and the microstructure evolution of the thin metal films on dielectric surface.

In this paper, ion-induced surface nanostructuring of Au films deposited on the SiO2 glass substrates was examined, and ion-induced lattice evolution and the chemical concentration of thin Au films were investigated. Effects of nanostructuring of Au films sustained on the SiO2 glass substrates on photo absorption properties were also discussed.

Experimental procedure

A detailed study of the surface nanostructure of Au films on SiO2 glass was carried out. Au thin films were evaporated on mirror polished SiO2 substrates with an O–H density of 80–100 ppm (Shin–Etsu Chemical Co., Ltd.) at room temperature by electrically heating the Au source (purity, 99.9 %) in a 6.0 × 10−5 Torr vacuum. The thickness of the Au films was estimated by comparing the colors with that of the standard sample, which had been previously calibrated. Also, the film thickness was verified by cross-sectional transmission electron microscope (TEM; JEOL JEM–2010FE) observation. After the deposition, the surface morphology was analyzed using an atomic force microscope (AFM; KEYENCE VN–8000) working in the tapping mode. In addition, a field emission scanning electron microscope (SEM; JEOL JSM–7001FA) equipped with an energy dispersive spectrometer was used to verify the surface morphology and the chemical concentration.

Subsequently, as-deposited samples were irradiated by 100 keV Ar ions at ambient temperature with doses of 1.0 × 1016/cm2, 2.0 × 1016/cm2, 6.0 × 1016/cm2, and 10.0 × 1016/cm2. In addition, the Ar ion irradiation at 150 keV was also carried out to a dose of 10.0 × 1016/cm2. Energies of the ions have been chosen in such a way that the range is wider than the Au film thickness, as calculated by the SRIM 2011 code [24]. Ar ion irradiation perpendicular to the specimen was performed using the 400 keV ion accelerator at Center for Advanced Research of Energy Conversion Materials, Hokkaido University [25]. A low pressure of 4.3 × 10−4 Pa was maintained inside the irradiation chamber. To insure uniform irradiation, Ar beam was scanned and the current was kept around 2.0 μA/cm2. Also, the target was continuously monitored by thermocouple and infrared thermal detectors, and the temperature of the beam spot was maintained below 120 °C.

After the irradiation, the photoabsorbance spectra were obtained using the Spectrophotometer (JASCO FP–6200), equipped with a UV measurement attachment (model FUV–420), at a wavelength range between 280 and 725 nm. After that, the AFM and the SEM were used to examine the surface modification. Microstructural characterization was performed by the TEM operated at 200 keV, and the elemental concentration was obtained in the nanoscale by operating the TEM in the scanning mode, accompanied with an energy-dispersive spectrometer (Noran, Thermo Fischer Scientific). Cross-sectional TEM specimens were prepared both by precision ion polishing system (PIPS; JEOL AT–12310) and focused ion beam facility (JEOL JEM–9320FIB).

Results and discussion

The process of the ion beam-induced surface nanostructuring on a dielectric substrate depends on the variation of irradiation parameters such as dose, current, and energy of the ion beam. In this paper, the fabrication of Au nanoparticles with controlled size and shape in a 2D distribution can be achieved by applying the appropriate irradiation dose, beam energy, and Au foil’s thickness.

Surface nanostructuring after Ar ion irradiation

Surface morphology of the samples was examined before and after irradiation using AFM and SEM. Figure 1 gives the AFM images of samples irradiated with 100 keV Ar ions at the dose of (b) 1.0 × 1016/cm2, (c) 6.0 × 1016/cm2, and (d) 10.0 × 1016/cm2. The corresponding SEM images are given in Fig. 2, in which the bright parts represents the Au films and the dark parts represents the exposed substrate. Therefore, the process of Au film’s dewetting under Ar ion irradiation was clearly distinguished and finally the formation of nanodots was observed. While the surface of the as-deposited SiO2 glass substrate was smooth with homogenous cracks (shown in Fig. 2a), which can be confirmed by cross-sectional TEM image shown in Fig. 3a, the holes and partially connected nanoscale islands were formed by the lateral transport of Au atoms after irradiated to a dose of 1.0 × 1016/cm2. It is therefore concluded that the cracks are the triggers of the holes formation and therefore starting the dewetting process, which is enhanced by the Ar ion irradiation, taking into the effect of surface sputtering. As the dose increased to 2.0 × 1016/cm2, the nanoscale Au islands on the surface became discontinuous. Similar features of ion induced dewetting have been reported by 800 keV Kr2+-ion irradiation of thin Pt films on SiO2 substrate and 150 keV Ar-ion irradiation of thin Au films on carbonaceous substrate, and the mechanism of the irradiation-enhanced dewetting was ascribed to be the radiation-enhanced diffusion [14, 15, 26].
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Fig. 1

AFM micrographs (2.0 × 1.5 μm2) with the Z scale of nm of: a as deposited SiO2, samples irradiated with 100 keV Ar ions at doses of b 1.0 × 1016/cm2, c 6.0 × 1016/cm2, d 10.0 × 1016/cm2, and e the sample irradiated with 150 keV Ar ions at a dose of 10.0 × 1016/cm2

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Fig. 2

SEM images of a as deposited SiO2, samples irradiated with 100 keV Ar ions at doses of b 1.0 × 1016/cm2, c 2.0 × 1016/cm2, d 6.0 × 1016/cm2, e 10.0 × 1016/cm2, and f the sample irradiated with 150 keV Ar ions at a dose of 10.0 × 1016/cm2

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Fig. 3

Bright field cross-sectional TEM images with selected area diffraction pattern of a as deposited SiO2, samples irradiated with 100 keV Ar ions at doses of b 1.0 × 1016/cm2, c 2.0 × 1016/cm2, d 6.0 × 1016/cm2, e 10.0 × 1016/cm2, and f the sample irradiated with 150 keV Ar ions at a dose of 10.0 × 1016/cm2

At the higher dose of 6.0 × 1016/cm2, the spherical nanodots on the substrate were observed. Compared with the surrounding areas, the aggregation of Au atoms in the nanodots was identified by SEM–EDS spectra, and these nanodots were therefore entitled Au nanoballs. When the irradiation dose increased to 10.0 × 1016/cm2, the nanoballs were modified to be highly spherical nanoballs. In addition, 2D autocorrelation function image of the SEM image indicate the nanoballs has a homogenous distribution along their radius. Figure 5a gives the size distribution of these nanoballs (NBs), reproduced by a Gaussian fit (black line). The mean diameter <D> was deduced as the position of the Gaussian peak and the error on <D> was evaluated as the standard deviation of the Gaussian fit. A mean diameter of 33.9 ± 8.8 nm was obtained with a number density of approximately 27.9/μm2. However, according to the AFM observation, the average height of the nanoballs was much smaller than their lateral dimension. This can be clarified by the following cross-sectional TEM analysis: nanoballs with exposed height lesser than their lateral size would be embedded in the substrate.

Microstructure evolution after Ar-ion irradiation

Figure 3 gives the bright field cross-sectional TEM images together with selected area diffraction pattern for the specimens irradiated with 100 keV Ar ions at the dose of (b) 1.0 × 1016/cm2, (c) 2.0 × 1016/cm2, (d) 6.0 × 1016/cm2, and (e) 10.0 × 1016/cm2. While the diffraction pattern for the SiO2 substrate showing diffused rings indicated the substrate was amorphous before and after the Ar-ion irradiation, diffraction patterns with bright spots for the Au layers or nanoballs indicates a crystalline nature. Furthermore, diffraction patterns for several nanostructures with FCC structure for each irradiation dose were obtained, and each image gives a typical diffraction pattern. These diffraction patterns enable us to study the lattice evolution under Ar-ion irradiation. The lattice parameter at each irradiation dose is given in Table 1, and an increase of lattice parameter with the irradiation dose was observed.
Table 1

The expansion of the lattice parameter after Ar-ion irradiation

Specimen

Irradiation dose (1016/cm2)

aAu (Å)a

aM (Å)b

Expansion (%)

100 keV Ar+

1.0

4.0786

4.04

2.3

2.0

4.0786

4.24

3.9

6.0

4.0786

4.30

5.4

10.0

4.0786

4.72

15.7

150 keV Ar+

10.0

4.0786

5.00

22.6

aaAu represents the lattice parameter from the documented values [27]

baM represents the lattice parameters from measurement

From the TEM observation, the Au nanoscale islands due to irradiation remained on the surface of the substrate at a dose of 1.0 × 1016/cm2, while a lattice expansion was observed from several Au nanoscale islands. Therefore, lattice expansion (E) was defined (Eq. 1) to characterize the effects of ion irradiation:
$$ E = (a_{\text{M}} - a_{\text{Au}} )/a_{\text{Au}} \times 100\,\% , $$
(1)
where aM is the lattice parameters of Au nanoscale islands after Ar ion irradiation and aAu is the documented value corresponding to bulk Au [27]. A lattice expansion of 2.3 % was observed and irradiation-induced defects like vacancies and interstitials have been ascribed to account for this lattice expansion.
When the dose of the irradiation increased to 2.0 × 1016/cm2, the interface between the Au nanoscale islands and the surface of the substrate became blurring, indicating the Au nanoscale islands began to burrow into the SiO2 substrate. In addition, a lattice expansion of 3.9 % was observed. When the dose increased to 6.0 × 1016/cm2, nanostructures with fine particles surrounding the larger nanoballs partially embedded in the near surface with a layer distribution was identified, and a lattice expansion of 5.4 % was obtained. When the dose increased to 10.0 × 1016/cm2, the Au nanoballs were modified to be highly spherical shape and deeply embedded in the substrate with a mean diameter of 33.3 ± 9.0 nm (given in Fig. 5c), and a lattice expansion of 15.7 % was obtained. In order to confirm the element concentrations of the nanostructures, a 2D EDS elemental mapping was obtained (shown in Fig. 4c) and the corresponding STEM image is given in Fig. 4a. The upper yellow area shows carbon deposition, which was used to protect the specimen during FIB sample preparation and the bottom area represents the SiO2 glass substrate. The red nanoballs in the middle represent the Au atoms assembled in the form of spherical nanoballs and exhibit a single layer distribution. According to the primary measurement, around 50 % of these nanoballs were found to be single crystals.
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Fig. 4

Cross-sectional STEM image a with EDS mapping c of Au nanoballs embedded in SiO2 substrates after 100 keV Ar ion irradiation to a dose of 1.0 × 1017/cm2; STEM image b with EDS mapping d of Au nanoballs embedded in SiO2 substrates after 150 keV Ar-ion irradiation to a dose of 1.0 × 1017/cm2

The kinetic energy deposition due to continued Ar-ion irradiation resulted in greater defects movement than that expected with ambient temperature, which can be 100 times faster [1]. This is referred as radiation-enhanced diffusion, which is thought to dominate the mass transport in ion mixing [1922]. Therefore, the new phase of Au–Si system might be formed under this non-equilibrium process, in which the existence of the Si in Au nanoballs was confirmed by the EDS spectra, and the expanded lattice parameter is corresponding to this new phase. This is similar to the studies that metastable Au–Si (hexagonal structure) alloys with larger lattice parameters were obtained by ion–beam interface mixing of Au–Si multilayers [2022]. However, the Au nanostructures showed FCC structure in this study. As increasing the irradiation dose up to 6.0 × 1016/cm2, the lattice expansion showed linear tendency. After the irradiation to a dose of 10.0 × 1016/cm2, the lattice expansion deviated from the linear tendency. Because the sample of cross-sectional TEM specimen after 10.0 × 1016/cm2 irradiation was prepared by FIB system, the Ga irradiation could also have a contribution to the lattice expansion. The Au nanoballs embedment in the SiO2 substrate can be interpreted as irradiation induced burrowing. The mechanism of the nanoballs burrowing was particularly investigated [28, 29]. Thermodynamic driving force, which related to the surface and interface energies of nanoballs and substrate, is account for this burrowing effect, and the sputtering effect of nanoballs was also taken into consideration. In addition, irradiation-induced viscosity of the SiO2 substrate was sufficient enough to accomplish this burrowing process [29]. In the following part, a detailed discussion of sputtering effect was given.

Sputtering effects under Ar-ion irradiation

For the specimens before and after irradiation up to 6.0 × 1016/cm2, the covered surface fractions of Au films can be obtained by integrate the bright parts in the SEM images (Fig. 2). In addition, the thickness of the Au films can be estimated using the cross-sectional TEM images (Fig. 3). Then the volume of the retained Au atoms within unit area on the SiO2 substrate corresponding to each irradiation dose can be obtained by simply multiply the Au covered region with the film thickness, and the uncertainty was also estimated.

For the highest irradiation dose of 10.0 × 1016/cm2, the nanoballs were modified to be perfectly spherical nanoballs partially embedded in the substrate. Each Au nanoballs was treated as a sphere, and the volume of the Au nanoballs within unit area can be obtained by multiply the number density. Finally, Au atoms within unit area can be calculated by assuming the density of retained Au after ion irradiation kept the same as bulk Au, and the results are summarized in Fig. 6. Figure 6 shows the plot of concentration of Au varying with irradiation dose which reveals sputtering effects of Au under ion irradiation. It shows that at the initial irradiation dose of 1.0 × 1016/cm2, sputtering yield is about 5.4 atoms/ion. Afterwards, the retained Au atoms decreases with the irradiation dose with a deviation from the linear tendency indicating the sputtering yields decreases with the irradiation dose. At the highest irradiation dose of 10.0 × 1016/cm2, the sputtering rate decreases to less than 0.6 atoms/ion. The reason for this low sputtering rate is the small fraction of surface area coverage by Au film which decreases with increasing irradiation dose as observed in SEM images.

Beam energy dependence on the nanoballs formation

Figure 1e gives the AFM image of 150 keV Ar ions irradiation of the same sample to a dose of 10.0 × 1016/cm2. The corresponding SEM image is given in Fig. 2f. The homogeneous distribution of Au nanoballs on the substrate was identified, and a mean diameter of 55.8 ± 16.1 nm was obtained (Fig. 5b) with a number density of approximately 26.3/μm2. Figure 3f is the bright field cross-sectional TEM image, and the EDS elemental mapping is shown in Fig. 4d together with the corresponding STEM image in Fig. 4b. A layer of Au nanoballs embedded in the substrate was identified with a mean diameter of 50.5 ± 8.0 nm (Fig. 5d), and a lattice expansion of around 22.6 % was obtained. Note that nanoballs produced by 100 keV Ar-ion irradiation has a mean diameter of 33.9 ± 8.8 nm with a number density of around 27.9/μm2. It is therefore concluded that the higher energy ion irradiation results in a sparser but larger nanoballs distributed on the SiO2 glass substrate. In addition, the retained Au concentration under 150 keV Ar-ion irradiation to a dose of 10.0 × 1016/cm2 is also given in Fig. 6, shown in red solid circle. It is obvious that the higher energy ion irradiation results in a larger Au concentration retained on the SiO2 substrate, and the reason is the lower nuclear energy loss in the Au layer resulted in lower sputtering yield by higher energy ion irradiation.
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Fig. 5

Each figure shows the diameter distribution (N is the portion of Au nanoballs embedded in SiO2 substrate, and <D> is the mean diameter of nanoballs) of the acquired nanoballs formed by Ar-ion irradiation to a dose of 10.0 × 1016/cm2: a SEM images exhibits a mean diameter of 33.9 ± 8.8 nm for the nanoballs under 100 keV Ar-ion irradiation, and the corresponding TEM images exhibits a mean diameter of 33.3 ± 9.0 nm (c); b SEM images exhibits a mean diameter of 55.8 ± 16.1 nm for the nanoballs under 150 keV Ar-ion irradiation, and the corresponding TEM images exhibits a mean diameter of 50.5 ± 8.0 nm (d)

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Fig. 6

Retained Au concentrations on SiO2 substrate as a function of irradiation dose

Figure 7 shows the photo absorbance spectra of 100 keV (dashed line) and 150 keV (solid line) Ar-ion irradiated samples to a dose of 1.0 × 1017/cm2. Compared with the photo absorbance spectra for the 100 keV Ar-ion irradiation of the pure SiO2 to a dose of 10.0 × 1016/cm2 (dotted line), the absorbance band located around 550 nm corresponds to the surface plasmon resonance (SPR) peak possessed by Au nanoballs embedded in SiO2 [5], while the 150 keV irradiation resulted in a broadening peak with increased intensity. In general, the intensity and position of the SPR peak are closely related to the size and volume fraction of the nanoballs embedded in the dielectrics. As the irradiation energy of Ar ions increased from 100 keV to 150 keV, the average diameter of the Au nanoballs increased and the red shift of the SPR peak was observed.
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Fig. 7

Photoabsorbance spectra of the samples irradiated by Ar ions at 100 keV (dashed line) and 150 keV (solid line) to a dose of 10.0 × 1016/cm2. The photoabsorbance spectra of the pure SiO2 irradiated with 100 keV Ar ions to a dose of 10.0 × 1016/cm2 was also obtained (dotted line). The top scale represents the photo energy in eV

This tendency of the experimental SPR peaks corresponds to those of the theoretical calculated SPR peaks using the code for electromagnetic scattering by spheres, in which Mie solution was applied to nanoballs with a unique size [2]. However, it is hard for nanoballs fabricated by ion irradiation to keep a unique size and pure chemical concentration because of ion mixing, in which the existence of the Si in the Au nanoballs was observed. In the case of the sample after 100 keV Ar-ion irradiation to 10.0 × 1016/cm2, the experimental SPR peak was 550 nm and the calculated SPR peak was 520 nm by using Jain’s calculation [2]. On the other hand, the experimental SPR peak for the sample after 150 keV Ar-ion irradiation was 590 nm and the calculated SPR peak was 550 nm. Jain’s calculation was considered on the assumption of spherical Au nanoparticles with unique diameter dispersed in the solution homogenously. However, the red shift tendency of the SPR peak depending on the particle size was consistent.

Conclusions

In this paper, to study the process of ion irradiation-induced surface nanostructuring of Au film deposited on SiO2 glass substrate and its optical properties, the surface morphology was examined using an AFM and a SEM, and the microstructural evolution and the chemical concentration of Au films were investigated using a TEM equipped with an energy dispersive spectrometer. The following conclusions were obtained;
  1. 1.

    With the irradiation dose increased from 1.0 × 1016/cm2 to 10.0 × 1016/cm2, the dewetting of the Au films on the SiO2 glass substrate was occurred and the Au nanoscale islands were formed on the substrate. Finally a single layer of photosensitive gold nanoballs with highly spherical shape embedded in a SiO2 substrate was obtained.

     
  2. 2.

    The lattice expansion was observed as increasing irradiation dose. These lattice expansions could be caused by irradiation-induced lattice defects and the irradiation induced interface ion mixing. However, the Au nanoparticles and the Au nanoballs in this study showed FCC structure instead of the hexagonal metastable phase in the previous study. Half amounts of the Au nanoballs were also found to be single crystals.

     
  3. 3.

    As increasing the irradiation energy from 100 to 150 keV, the average diameter of the Au nanoballs increased and a red shift of the SPR peak was observed. Ion beam-induced nanostructuring method could be useful in controlling the SPR properties of the Au film deposited on SiO2 glass substrate.

     

Acknowledgements

This study was partly supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (A) #21241025. Mr. Xuan MENG thanks Chinese Scholarship Council’s stipend support to carry out this work in Hokkaido University. The authors thank Prof. S. Yatsu, Mr. K. Ohkubo, Dr. Z. Yang, Dr. Y. Yoshida, and Mr. J. Wajima, for their experimental assistance and helpful discussions.

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