Article pubs.acs.org/JPCC
Sputtering Yields of Gold Nanoparticles by C60 Ions Li Yang,* Martin P. Seah, Emily H. Anstis, Ian S. Gilmore, and Joanna L. S. Lee Analytical Science Division, National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom ABSTRACT: The sputtering yields of nanoparticles are expected to be higher than those for traditional flat films because of the higher surface area and lower volume to dissipate the primary ion energy. In the present study, gold nanoparticles in the size range 10 to 100 nm, dispersed on a silicon wafer, are studied by secondary ion mass spectrometry (SIMS) during sputtering by 20 keV C++ 60 and by scanning electron microscopy. It is shown that early in the profile the particles melt and then resolidify. This is not mitigated by sample cooling. Profiling nanoparticles by SIMS may be very difficult without adequate heat sinking. The ways that the nanoparticle and substrate secondary ion intensities change during the profiling in a two-beam SIMS system are not simple to interpret, and so this situation is modeled in some detail. It is shown that the sputtering yield for primary ions on the axis of the nanoparticle, Y(0°), can be extracted from the data. For the first time, we measure a significant increase in this sputtering yield, Y(0°), for nanoparticles compared with bulk materials, in agreement with general expectations. At 9.3 nm diameter, the yield is 320 ± 35 compared with 96 ± 14 at 98.8 nm diameter, an increase of 3.3 times. For bulk Au films at 45° incidence, the yield, Y(45°), is 129 ± 2. The sputtering yield for SiO2 films at an angle of incidence of 45° may also be extracted, and this is found to be rather higher at 478 ± 112 atoms/primary ion.
1. INTRODUCTION Particles with nanometer dimensions are increasingly important for practical applications in many different fields, such as personal care products, high innovation products for biodiagnostics, drug delivery and medical imaging (contrast agents), catalysts, and as functional materials with special mechanical, optical, electric, or magnetic properties. There is also an increasing awareness of the need to understand the potential environmental, health, and safety risks of nanoparticles because novel and unusual properties in a particular environment, process, or application can occur.1 Robust, consistent methodologies for characterizing nanoparticle surface and bulk chemistries are urgently needed to support standardization, regulatory requirements and toxicology studies. Recent reviews by Baer et al.2,3 highlight the important role that surface chemical analysis methods can play in the characterization of nanoparticles. The information that can be obtained using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), low-energy ion scattering (LEIS), and scanning-probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are all briefly summarized there.3 Although there are many different surface analysis techniques, TOF-SIMS has great potential to obtain information about the molecular, elemental, and isotopic constituents at surfaces. The useful lateral resolution, excellent depth resolution, and sensitivity of SIMS have been applied in nanoparticle characterization.3−10 However, the fundamentals of the sputtering of nanoparticles are not yet well understood. The sputtering yield is expected to be significantly different for nanoparticles compared with bulk Published 2012 by the American Chemical Society
materials due to both the dimension of nanoparticles being comparable to the size of the primary ion collision cascade and the larger available surface area for secondary emission. Järvi et al.11 show, using molecular dynamics simulations, how the yield may be enhanced by a factor of 4 for isolated gold nanoparticles of ∼7 nm diameter sputtered by 25 keV Ga. Similarly, Zimmermann and Urbassek12 show, for 16 and 64 keV Au sputtering of 20 nm gold nanoparticles, that there are enhancements of 2.3 and 4.8 times, respectively, compared with the yield for flat films. Already, sputtering by atom and cluster impact has been extensively studied for planar targets;13−16 however, there are no experimental data on the sputtering yields of nanoparticles (that are, of course, residing in some way on a solid substrate). In this study, SIMS depth profiles were obtained for Au nanoparticles with diameters ranging from 10 to 100 nm, dispersed, and mounted onto a silanized silicon substrate. A silanized Si substrate was used following the recommendations of Liu et al.17 and of Hsieh et al.18 to ensure that the particles were suitably anchored and dispersed. The sputtering yields and profile shapes are evaluated using 20 keV C++ 60 primary ions for sputtering and 25 keV Bi3+ primary ions for analysis, with the nanoparticle shapes being characterized by scanning electron microscopy (SEM) both before and after sputtering. The SEM studies were made for separate samples sputtered for appropriate times rather than attempting to interrupt SIMS profiles. Because it is known that sputtering yields are affected Received: January 27, 2012 Revised: March 27, 2012 Published: April 3, 2012 9311
dx.doi.org/10.1021/jp300900j | J. Phys. Chem. C 2012, 116, 9311−9318
The Journal of Physical Chemistry C
Article
above. Two gold films, 29.5 and 86.3 nm thick were made with their final thicknesses measured by line traces at a scribed edge using an Asylum MFP-3D AFM in tapping mode. Clear steps were observed that could be measured with high accuracy. The standard deviations of repeat measurements along scribed edges were 0.26 and 0.89 nm, respectively. The calibration of the AFM height scale was better than 2%. 2.4. TOF-SIMS Depth Profiling. SIMS spectra and depth profiles were measured using a ToF-SIMS IV (IONTOF, Münster, Germany) time-of-flight secondary ion mass spectrometer. The instrument was equipped with a Bi liquid metal ion gun and a C60 ion gun mounted orthogonally to each other and at 45° to the sample surface. Depth profiles were generated in the “interlaced” mode, consisting of cycles of a short pulse of 25 keV Bi3+ primary ions for SIMS analysis, followed by a longer period of 20 keV C++ 60 sputtering, during which the analyzer extraction potential was switched off. Here, this is called the two-beam method (sometimes also referred to as dual beam). The time for each cycle (∼0.13 ms) determines the maximum detectable mass (∼1300 Da) during the experiment. Beam currents were measured using a Faraday cup on the sample holder both before and after each depth profile. During the profile, the C++ 60 ion beam, which was defocused to have a spot size of ∼30 μm diameter, was generally rastered over a 400 μm × 400 μm area with a pixel spacing of ∼3 μm. In this case, the ion dose should be constant within the central ∼150 μm × ∼150 μm area,22 and then, for safety, only the central 100 μm × 100 μm area was analyzed with the Bi3+ primary ion beam. This beam had a spot size of ∼5 μm diameter and was rastered in a 128 × 128 array. The beam currents were set so that the Bi3+ dose was ∼0.5% of the C++ 60 dose to keep the SIMS signal sufficiently high for good quality spectra while being sufficiently low that the sputtering attributed to bismuth ions could be ignored. The sputtering yield of Au by 25 keV Au3+ is estimated to be ∼100,23 but replacing the Au+3 primary ions by 25 keV Bi3+ is estimated to reduce this yield to ∼80. We see later that the C++ 60 yield is always greater than this figure and so the above limit of 0.5% is safe. In this instrument, the sample stage could be cooled with liquid nitrogen. A thermocouple just under the cooled samples recorded temperatures of −107 ± 9 °C. This value is given at appropriate places in the following text, but it should be remembered that the ion-impacted region of the sample may be warmer than this.
by temperature,19−21 a few tests were also made with samples cooled to ∼−107 °C. For comparison with the bulk sputtering yields, two Au films of ∼30 nm and ∼100 nm thick were also prepared on the same wafer substrates.
2. EXPERIMENTAL DETAILS 2.1. Pretreatment of the Si Substrates. Silicon wafers, 10 cm in diameter, with a (100) surface were purchased from Universitywafer. The thickness of silicon oxide was 7.86 nm, as measured directly from spectroscopic ellipsometry (M2000, Woollam, NE). The wafer was scribed and cleaved to ∼10 mm × 10 mm squares. Care was taken not to damage or touch the surface by using a specially designed, homemade cutting kit. After cutting the samples, to preclean them, we soaked the wafers overnight in isopropanol (IPA) and then agitated them ultrasonically in IPA solution for 2 min, followed by rinsing with copious amounts of distilled water and then by drying with an argon gas jet. After final cleaning with ultraviolet light and ozone for 30 min, the silicon wafers were immersed into a test tube filled with a 1:100 mL volume ratio of 3-aminopropyltriethoxysilane (APTES) to anhydrous toluene (99.8%) mixture (both from Sigma-Aldrich and used as received) for 1 h. The top of the tube was covered with an aluminum foil to reduce moisture ingress. After the silanization reaction was complete, the silicon wafers were removed from the tube and rinsed under flowing anhydrous toluene, followed by ultrasonic agitation in fresh anhydrous toluene for 10 min to remove any loosely physisorbed APTES molecules. The APTES deposits as a layer one monolayer thick.18 The wafers were rinsed in methanol and distilled water, then dried with the argon jet and placed in a clean oven at 120 °C for 30 min to complete the Si−O bond formation. 2.2. Preparation of the Au Nanoparticles. Four sizes (10, 20, 50, 100 nm) of gold colloid nanoparticles were purchased from BBInternational and used as received. BBInternational provided a precise value of the nanoparticle diameter from transmission electron microscope measurements; note that >95% are spherical and
