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Formation, Morphology, and Optical Properties of Electroless Deposited Gold Nanoparticles on 3C-SiC Rachela G. Milazzo,*,† Stefania Privitera,† Grazia Litrico,‡ Silvia Scalese,† Salvatore Mirabella,§ Francesco La Via,† Salvatore Lombardo,† and Emanuele Rimini†,§ †

CNR-IMM Institute for the Microelectronics and Microsystems, Z. I. VIII Strada 4, I-95121 Catania, Italy Laboratori Nazionali del Sud, via S. Sofia 62, 95123 Catania, Italy § Department of Physics and Astronomy, University of Catania, v. S. Sofia 64, I-95123 Catania, Italy ‡

ABSTRACT: 3C-SiC layers (7 and 15 μm thick), epitaxially grown on silicon, were covered with gold nanoparticles by immersion in a solution containing HF and KAuCl4. The surface of the layers played a crucial role in the morphology of the deposited metal network and large gold agglomerates developed on the pronounced antiphase domain boundaries of the thinner layer. Preferential growth was not observed on the smooth surface of the 15 μm thick layer. Rutherford backscattering spectrometry and electron microscopy outlined a progressive nucleation that takes place for less than 60 s immersion times under a kinetic control process. For longer deposition time each cluster grows under a diffusion control process, resulting in flower-like gold particles. However, the nucleation and growth processes can be strongly modified by multiple immersions in solution after rinsing in water. The adopted procedure allows the tailoring of the particles size and avoids the aggregation, therefore improving by about 1 order of magnitude the particle density (from 2 to 9 × 109 cm−2) while keeping their size systematically smaller (20 nm radius) than that obtained by a single immersion for the same total time (about 50 nm radius). Reflectivity measurements and micro Raman analyses evidenced the plasmonic effects produced by the clusters. photovoltaics, and optoelectronics devices.14−17 When exposed to light, the electromagnetic field of the incident radiation induces collective and coherent oscillations of the conduction electrons18 confined to the surface, at specific frequencies, called localized surface plasmon resonance (LSPR). Plasmonic properties depend on metal size, shape, and environment, so it is obviously desirable to ensure a good control of their fabrication to change the corresponding responses at will. A wide variety of techniques can be used to prepare supported NPs, and electrodeposition and electroless deposition on conducting supports are particularly attractive as they allow the direct growth of NPs on a substrate, ensuring electrical connection between the substrate and the NPs.19−22 Electroless deposition can offer control over the size and shape distribution, as well as the spatial distribution of NPs, by tuning the deposition parameters and electrolyte composition. In the recent years, electroless metal deposition has been extended also to the postsilicon materials, such as InP, GaN, and GaAs.23,24 In this paper we have studied the nucleation and growth mechanisms of gold nanoparticles (AuNPs) on 3C-SiC layers epitaxially grown on (100) silicon substrates with different thickness and hence with a different surface defect

1. INTRODUCTION Silicon carbide is a material with high mechanical-chemical stability and excellent properties for high voltage and/or high temperature electronic devices.1−3 Among its polymorphs the cubic, 3C-SiC, has the lowest band gap value of 2.4 eV and can be obtained by heteroepitaxial growth on silicon substrates, allowing low growing temperatures and a large-scale production, compared to the other polytypes.4 Thanks to these properties, 3C-SiC has found applications in the fabrication of microelectromechanical system (MEMS) devices, and to a minor extent, it has been also proposed for the realization of medium-high voltage MOSFET devices with higher mobility than 4H-SiC and lower cost.5−9 3C-SiC is also attractive for the use as a photoelectrode in photoelectrochemical (PEC) reactions, such as water splitting that allows hydrogen production using the solar light.10 However, the works related to this application indicate that the measured current is usually very low, and in order to improve the PEC processes, it has been shown that metallic nanoparticles (NPs) can give an advantage.11,12 More recently it has been also demonstrated that 3C-SiC is characterized by an extremely high level of bio- and hemocompatibility, which is the most important requirement for materials used in biomedical devices.13 Noble metal nanoparticles possess unique optical and electronic properties that make them suitable in sensors, © XXXX American Chemical Society

Received: November 20, 2016 Revised: January 27, 2017 Published: February 7, 2017 A

DOI: 10.1021/acs.jpcc.6b11638 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C density.25 Deposition was performed by immersion in a solution containing HF and KAuCl4. As expected26,27 gold particles agglomerate preferentially on the antiphase domain boundaries (APBs) present in a large amount on the surface of the thinner 3C-SiC layer. By increasing the SiC layer thickness, instead, the gold particles were homogeneously distributed all over the surface. The size and density of the particles can be tuned by varying the immersion time and the surface preparation. Particles with distinct plasmonic properties have been obtained, as checked by Raman spectroscopy and reflectivity measurements.

X40 objective onto the sample and then reflected onto a 1800 grooves/mm kinematic grating. At room temperature and under normal condition of temperature stability (±1 °C), the accuracy of the instrument is ±0.2 cm−1. The spatial resolution achievable with the motorized stage is 0.5 μm, while the spot laser diameter is about 3 μm. For each sample we collected several micro-Raman maps with size 30 μm × 30 μm and a spatial acquisition step of 5 μm. The power of the laser was 10 mW. The optical properties of electroless deposited AuNPs were studied by reflectance measurements in the range 300−1100 nm, using the PVE300 Bentham System, equipped with a 450 W xenon lamp and 300 mm focal length monochromator. The reflectance measurement is achieved by employing an integrating sphere, in order to collect reflected light at any angle.

2. EXPERIMENTAL SECTION Samples. The 3C-SiC films of a thickness of 7 or 15 μm were grown by chemical vapor deposition (CVD) in a lowpressure regime in a horizontal hot-wall reactor (LPE ACiS M10, sited in ETC, Epitaxial Technology Center). Trichlorosilane (SiHCl3 or TCS) and ethylene (C2H4) were used as the silicon and carbon precursors, respectively, while hydrogen (H2) was chosen as the gas carrier. The deposition was performed with a multistep process. First, carbonization was carried out to reduce the formation of voids and bubbles at the interface between 3C-SiC and Si. Then the temperature was increased up to the growth temperature of 1370 °C; during this heating ramp TCS was introduced into the chamber determining the growth of a very thin 3C-SiC film as a buffer layer. Finally the deposition of 3C-SiC film was carried out with distinct growth rates: 3 μm/h in the first step, 6 μm/h in the second step, and 30 μm/h in the last step. The deposition time was chosen to obtain the desired thickness. For the thinner 3C-SiC layer, the AFM (atomic force microscopy) analyses prior to Au deposition showed the presence of large domains with a size of about 10 μm. The typical surface steps are about 150 nm high, while the estimated RMS (root mean square) within the domains is about 0.63 nm. For more details see ref 28. The surface roughness of the 15 μm 3C-SiC layer instead is 0.4 nm; the size of single crystalline domains is slightly less than 20 μm; while the step height is about 40 nm. Gold Electroless Deposition. Prior to plating, the substrates were degreased in acetone, at 60 °C and for 6 min in an ultrasonic bath, and then they were dipped in DHF (HF:H2O 1:7) for 240 s. The substrates were then thoroughly washed with deionized water and blow-dried in air. Gold deposition was carried out by manually soaking the samples in a solution containing 1 mM KAuCL4 and 4.8 M HF at room temperature, for different times (30−240 s), in a lighting ambient and without stirring. Then each sample was rinsed in water to remove all the surfactants and the reaction products, and an additional cleaning in acetone was performed finally before drying in air. Measurements. The samples after deposition were analyzed by Rutherford backscattering spectrometry (RBS) with 2.0 MeV He+ ions scattered by target atoms at 165°, detected by a solid-state surface barrier detector with 15 keV resolution. The beam size was about 1 mm2 and the current a few nA. The morphological characterization was achieved by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Micro-Raman analyses were performed using an HR800 integrated system by Horiba Jobin Yvon in a backscattering configuration. The excitation source was supplied by a He−Cd laser with a wavelength of 325 nm. The laser was focused via an

3. RESULTS AND DISCUSSION 3.1. Deposition Kinetics. Gold deposition on the 3C-SiC layer 7 μm thick, characterized by smaller size of antiphase

Figure 1. SEM images of the 7 μm thick 3C-SiC film obtained after immersion in the plating solution (a) for 30 s and (b) 120 s. (c) RBS spectra of 2.0 MeV He+ ions impinging on a 15 μm 3C-SiC sample after immersion in the Au solution for 30, 60, and 120 s, respectively; the spectra report only the Au signal, i.e., the helium ions backscattered from the Au atoms. Low energy tails indicate the presence of thick gold particles (agglomerated at the APBs).

Figure 2. Diagram of the electron energy levels of (a) Si and (b) 3CSiC band edges (EF, Ec, and Ev are the Fermi level, the conduction, and valence bands, respectively) and the redox system Au/AuCl4− in HF solution.

domains and higher roughness, was investigated in the 30−120 s time range. We found that the Au deposition occurs preferentially on the surface defects, mainly APBs. This typical behavior is showed in the SEM micrograph of Figure 1a, obtained for a sample immersed for 30 s in the plating solution. Defects are preferential nucleation sites, and large Au agglomerates decorated the convex edges of the APBs. Prolonged immersion times are required in order to observe B

DOI: 10.1021/acs.jpcc.6b11638 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. SEM micrographs of AuNPs on 15 μm 3C-SiC obtained after immersion for (a) 30 s, (b) 60 s, (c) 120 s, and (d) 240 s in the plating solution.

edges of the 3C-SiC and the redox level of the Au/AuCl4− (Figure 2b), suggesting that charge transfer is mediated by surfaces states.29 The gold deposition can be then modulated by changing the surface states, by varying either the crystallographic orientation or the defect density at the surface. We therefore performed the electroless deposition on a 15 μm thick 3C-SiC film, characterized by larger antiphase domains and lower roughness. Figure 3a shows the Au signal in the RBS spectra of 15 μm thick 3C-SiC samples prepared with different deposition times. In this case no tails at low energy in the Au signal are detected, suggesting that there are no preferential nucleation sites and the gold particles are uniformly distributed on the surface. The integral of the RBS signal is proportional to the areal density of Au atoms. Figure 3b shows the (areal) density of gold atoms as a function of immersion time. The lowest deposition rate, defined as the first derivative of the atomic density versus time, occurs between 0 and 30 s. Then there is a peak at 60 s, corresponding to 7 × 1014 atoms cm−2 s−1, and after this time the deposition proceeds at a lower rate. The same experiment performed on the silicon substrate produced quite different results.30 Indeed, Au nucleation on silicon was instantaneous, and growth was diffusion limited throughout the same investigated time range. 3.2. Nucleation and Growth Processes. Further information on the morphology, taken over a large area, can be obtained by analyzing the shape of the RBS spectra. For a continuous layer, the height of the RBS signal is constant, while the width increases proportionally to the thickness. Since we do not have a continuous layer, by comparing the measured spectra with simulations assuming a thick continuous layer, we can evaluate (i) the covered surface fraction and (ii) the average particle thickness. The comparison between simulation (dark solid line) and experimental data (red line) is shown in Figure 3c for the case of 240 s immersion. The surface covered by gold nanoparticles is low, as expected, and increases with immersion times: it is 1%, 8%, and 13% after 30 s, 60 s, and 240 s, respectively. This trend is in agreement with the Volmer− Weber island growth mode.31

Figure 3. (a) RBS spectra of 2.0 MeV He+ ions impinging on a 15 μm 3C-SiC sample after immersion in the Au solution for 30, 60, 120, and 240 s, respectively; the spectra report only the Au signal. (b) Computed areal density of Au atoms on the 3C-SiC substrate. The error bar is of the same order of magnitude as the point marker. (c) RBS spectrum of the sample immersed for 240 s is overlaid to that simulated by assuming a gold uniform layer, 150 nm thick.

nucleation and growth of small particles also within the domains, as shown in Figure 1b, obtained after immersion for 120 s. This peculiar behavior is confirmed by RBS spectra of Figure 1c, where the signal produced by helium ions backscattered from the Au atoms is shown. This technique allows us to obtain information averaged over large areas, therefore avoiding errors due to the analysis of statistically deviating small regions, as observed by microscopy techniques. A pronounced peak at high energy, due to backscattering from a large amount of thin gold particles, is present only on the sample immersed for 120 s. For all the analyzed samples the tail of the Au signal extends to very low energy values, indicating the presence of thick agglomerates. Such a behavior can be explained by considering the energy levels of SiC in respect to the redox potential of gold. In Figure 2 we compare the redox potentials of Au/AuCl−4 with the band diagram of Si (a) and of 3C-SiC (b). The position of the valence band of Si with respect to the redox of Au/AuCl−4 ensured the electron transfer via the valence band (Figure 2a). On the contrary, there is a poor overlap between the band C

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Figure 5. (a) AFM analysis of the AuNPs obtained after immersion of a 15 μm 3C-SiC sample for 60 s in the plating solution; (b) z-profile taken along the white dotted line, and (c) height distribution of the particles.

The images were analyzed in order to obtain the cluster density and size distribution. After 30 s (Figure 4a) only 1.6 × 109 nuclei cm−2 are present on the surface, but the value doubles at 60 s (3.4 × 109 cm−2, Figure 4b). In the sample immersed for 120 s, shown in Figure 4c, the clusters density reduces to 1.9 × 109 cm−2 and maintains this value up to 240 s (Figure 4d). The presence of small grains even at longer time indicates that the nucleation is progressive, rather than instantaneous. The maximum particle density is obtained for an immersion time of 60 s, in very good agreement with the deposition rate evaluated using RBS. However, the cluster density is reduced after long immersion times since the nucleated particles may grow, and if they are close to each other they can coalesce into one. In particular, we also observe that gold particles tend to agglomerate one over each other, forming three-dimensional structures with the typical flower like shape.33−35 Indeed, by RBS we have evaluated a maximum thickness of 150 nm after longer immersion time, which is well explained with the formation of large agglomerates. The particle morphology was also probed by AFM (atomic force microscopy), as shown in Figure 5a,5b for the sample immersed for 60 s in the plating solution. The corresponding height distribution is instead reported in Figure 5c. The analyses pointed out the presence of a few agglomerates more than 100 nm high, together with a large number of very small particles with height in between 10 and 20 nm. The average value amounts to 33 nm in very good agreement with that determined by the RBS spectrum. The evolution of the particle density and size with immersion time indicates two distinct growth regimes. At the beginning the deposition rate is slow, as shown in Figure 3b, and it occurs under kinetic control due to the high barrier for nucleation. The island shape is dictated by the surface energy of the different crystallographic planes, leading to the formation of faceted particles with triangular, icosahedra, truncated tetrahedral, or hexagonal shapes,36 as those observed in the TEM micrograph of Figure 6a showing the sample immersed for 30 s in the plating solution. The corresponding SAED (selected area electron diffraction) shown in Figure 6b pointed out that gold crystalline particles do not show any preferential alignment with the substrate orientation. The Au diffraction rings are clearly

Figure 6. (a) TEM micrograph of AuNPs on 15 μm 3C-SiC after immersion for 30 s in the deposition solution and (b) the corresponding SAED showing the Au diffraction rings and the SiC patterns.

Figure 7. Log−log plot of the gold clusters mean radius versus the deposition time. The two dashed lines correspond, respectively, to the single cluster (R ∼ t1/2) and to the 1D diffusion (R ∼ t1/6) growth regimes.

Again, from the full width at half-maximum we can estimate the average height of the gold particles.32 It amounts to 20 nm for 30 s, 36 nm at 60 s, 54 nm at 120 s, and 58 nm at 240 s, respectively. Moreover, the Au signals in the samples obtained after long immersion time (120 s, 240 s) are characterized by a marked asymmetric shape with a long tail extending up to 1.5 MeV (similar to what we observed in Figure 1). Figure 4a−4d shows SEM micrographs of the 15 μm 3C-SiC substrate covered with AuNPs for increasing immersion times. D

DOI: 10.1021/acs.jpcc.6b11638 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 8. (a) SEM micrograph of the 15 μm 3C-SiC sample after a multistep (4 × 60 s) immersion. (b) The RBS spectra of samples after multi and single immersion; particle radius distribution of the (c) multi-immersed and (d) single-immersed samples, respectively.

Table 1. Comparison between the 15 μm 3C-SiC Samples after a Single 240 s and a Multi 4 × 60 s Immersion in the Deposition Solution

single 240 s multi (4 × 60 s)

radius [nm]

standard deviation [nm]

areal density [atoms cm−2]

cluster density [nuclei cm−2]

49 20

35 17

5.17E15 5.21E15

1.9E9 9.1E9

A simple model for electrodeposition predicts that, under diffusive control, the growth rate of a single semispherical cluster by direct ion attachment is described by eq 138,39 R(t ) = (2VmDC b)1/2 t

(1)

with R as the particle radius and Vm and Cb the molar volume and the bulk concentration of the solution, respectively. D is the diffusion coefficient of the ions in the solution. The model requires that the diffusion regions around each particle are hemispherical and not interacting. In reality the growth rate of each particle is influenced by the nuclei surrounding it,40−42 and at long times the diffusion flux is shielded; as a consequence the diffusion regions overlap. In this scenario, the growth rate is 1D diffusion limited, and the radius is given by eq 243,44 R(t ) ∝ (VmDC b)1/3 t 1/6

(2)

The two models are compared to our experimental data in Figure 7. In a Log−Log scale they are represented by the two straight lines with slope 0.5 and 0.16, respectively. The diffusion-limited regime takes over at or above 60 s immersion times, and the growth rate decreases with increasing immersion time, as expected from both models. Our experimental data in terms of the average radius, as obtained by the SEM analyses, lie in an intermediated region between the two regimes. Conceivably, the diffusion regions around each growing nucleus are not completely overlapped, and the clusters grow under the 3D ion supply.45 In addition, both models do not take into account coalescence and particle shape. The size distribution of the Au particles is very wide, and the standard deviation (see for instance Figure 8d that reports the radius distribution of the 15 μm 3C-SiC sample immersed for 240 s) for long deposition times (120 s and 240 s) is 35 nm; thus the present analysis is only qualitative.

Figure 9. Reflectivity measurements of the 15 μm 3C-SiC samples without and with Au particles deposited by single and multiple immersions.

distinguished from the substrate diffraction pattern. This was not the case, instead, for Au deposition on silicon substrates.37 At long immersion times instead, gold ions attach to the preexisting nuclei, resulting in flower-like gold particles with a diffusion-limited growth (Figure 3b). E

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Figure 10. Raman spectrum of the as-grown 15 μm 3C-SiC sample, of the sample immersed for 240 s single deposition, and of the sample after the multistep deposition: (a) transverse and (b) longitudinal modes.

3.3. Tuning of the Optical Properties. Au nanoparticles can exhibit plasmonic surface resonance in the visible range when the radius of the particles is around 50 nm.46 Therefore, in order to enhance these properties for practical applications we should obtain a radius distribution peaked at about 50 nm or less. Based on the proposed model for progressive nucleation in the 0−60 s time range and the corresponding kinetic-limited growth regime that produces nearly hemispheric particles, we implemented a multistep deposition with four subsequent immersions in the solution, each of 60 s. Samples were rinsed in deionized water after each step. Thanks to this procedure, deposition occurs always under kinetic control, and then the diffusion-limited growth of flower-like particles is prevented. Figure 8a shows the SEM micrograph of the sample after the multi-immersion, clearly indicating the presence of small particles. RBS spectra of the multi and single immersed samples are compared in Figure 8b. The areal density of gold atoms is nearly the same, indicating that we are not affecting the deposition rate. However, the shape of the Au signal is quite different, and the low energy tail present in the sample immersed for 240 s is quite reduced if the same time is obtained by four successive immersions, each of 60 s. Clearly, after each immersion the metal ions prefer to nucleate on bare 3C-SiC regions rather than to impinge on the already existing clusters. Probably the cleaning process in between the two consecutive immersion steps removes the passivation layer formed on the uncovered SiC surface during immersion.47,48 As a result, the cluster density is strongly increased by about one order magnitude. The size distribution in Figure 8c shows the continuous generation of small particles due to the multi-immersion process, while that of Figure 8d, corresponding to the single immersion step, demonstrates that the cluster growth is predominant. The data are summarized in Table 1. Once we have tailored the size distribution, we have studied the optical properties of the electroless-deposited Au particles. Figure 9 shows the reflectivity measured in 15 μm 3C-SiC samples without gold particles (black solid line) and immersed in 1 mM solution for 240 s either using a single immersion (red line) or a multi immersion process (blue). The fringes are due to the interference produced by the thin 3C-SiC film on silicon. In the case of multistep deposition, we observe a large increase in the reflectivity, with a peak at about 620 nm. A peak is also

detectable in the sample prepared by single immersion, but it is less intense and shifted to longer wavelengths, in agreement with the observed morphology (larger grain size and broader distribution). Another effect that can be ascribed to the LSPR is the modification of the Raman spectra. In 3C-SiC both transverse optical (TO) and longitudinal optical (LO) modes are Raman active, with peaks at 794 (Figure 10a) and 971.2 cm−1 (Figure 10b), respectively. The TO mode is forbidden, and it is therefore much less intense than the LO. A typical value of the intensity ratio, obtained in the reference sample, without gold NPs is ITO/ILO = 0.017. Under the effect of an electric field modulated by the plasmonic resonance of the gold nanoparticles, only the transverse mode is expected to be enhanced.49 This effect is clearly shown in Figure 10a,10b, which compares the Raman spectra acquired in the as-grown 3C-SiC film with those of the samples obtained after the singleand multistep immersion. After the 240 s gold deposition the ratio ITO/ILO increases to 0.065, and after the multistep deposition it reaches instead the highest value of 0.083, indicating a good coupling between gold plasmons and transverse vibrational modes of 3C-SiC.

4. CONCLUSION In this work, we have investigated the deposition mechanisms of gold on the 3C-SiC substrate. The deposition is strongly affected by the roughness and by the presence of defects at the surface. We have outlined two growing regimes. At the beginning the deposition is kinetic controlled, with progressive nucleation and formation of hemispherical clusters. Then the nucleation rate slows down, and the clusters grow according to a diffusion-limited aggregation model, generating large and complex 3D aggregates. The growth evolution can be strongly modified by rinsing in water and by multiple immersions, avoiding the aggregation and increasing by 1 order of magnitude the particle density while keeping their size systematically smaller than that obtained by a single immersion. Reflectivity measurements and micro Raman analysis evidenced the plasmonic effects produced by the nanoclusters. The proposed technique is therefore very promising and opens the field to several applications from the photocatalysis, enhanced by plasmonic particles, to the sensors, thanks to the high biocompatibility of the 3C-SiC. F

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AUTHOR INFORMATION

Corresponding Author

*Phone: +390955968244. Fax: +390955968312. E-mail [email protected]. ORCID

Rachela G. Milazzo: 0000-0002-3840-2297 Salvatore Mirabella: 0000-0002-9559-4862 Notes

The authors declare no competing financial interest.

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