CrystEngComm PAPER
Cite this: CrystEngComm, 2013, 15, 10116
Enhanced photoluminescence from SiOx–Au nanostructures† Hu Luo,ab Rongming Wang,*a Yanhui Chen,b Daniel Fox,b Robert O'Connell,b Jing Jing Wangb and Hongzhou Zhang*b Highly efficient photoluminescence (PL) emission is obtained from flower-like silicon oxide (SiOx)–Au nanostructures that were synthesized by annealing silicon substrates coated with Au thin film under Ar–H2 atmosphere at 1100 °C via the vapour–liquid–solid process. The morphologies and structures of the SiOx–Au nanoflowers were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and helium
Received 22nd July 2013, Accepted 20th September 2013 DOI: 10.1039/c3ce41455h www.rsc.org/crystengcomm
ion microscopy. The results revealed that the silicon oxide–Au nanoflowers were amorphous with an Au nanoparticle core at the centre. As compared to simple SiOx nanowires, the SiOx–Au nanoflowers show a 5-fold enhancement in intensity of room temperature photoluminescence, which can be attributed to the unique nanostructures, the strong coupling between the photoluminescence emission of the SiOx nanoflowers and the local surface plasmon resonance of the Au nanoparticles.
Introduction Recently, much effort has been devoted to precisely controlling the size and morphology of nanostructures, which were considered to be the key parameters that determine their physicochemical properties.1,2 On account of their outstanding optical properties, silicon oxide (SiOx) nanostructures have attracted considerable research interest in the fields of photoluminescence (PL) emission, functional microphotonic devices and integrated optical devices.3–6 Up to now, a range of silicon oxide nanostructures, such as nanowires,7,8 octopus-like,9 lantern-like,10 and flower-like11 have been synthesized via the vapor–liquid–solid (VLS) or solid–liquid–solid (SLS) growth methods.12–15 The pursuit of high PL intensity of luminescent materials has become a focus in recent research.16–18 Several approaches for improving PL properties have been attempted. One is to synthesize luminescent materials with high surface to volume ratios.19,20 For silicon oxide, the research results showed that most of its PL emission resulted from defects in or on the surface of the nanostructures.21–23 SiOx nanostructures with a large surface to volume ratio tend to have a higher PL emission efficiency since the surface of the material is very active and easily forms defects, especially at the a
Department of Physics, Beihang University, Beijing 100191, PR China. E-mail:
[email protected] b School of Physics, Center for Research on Adaptive Nanostructures and Nanodevices (CRANN), and CRANN Advanced Microscopy Laboratory (CRANN AML), Trinity College Dublin, Dublin 2, Ireland. E-mail:
[email protected] † Electronic supplementary information (ESI) available: More SiOx nanoflower growth SEM images and EELS characterization spectra included. See DOI: 10.1039/c3ce41455h
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nanoscale.24 Owing to the large surface to volume ratio in dendritic nanostructures, more attention was paid to the synthesis of PL materials with dendritic structures.25–27 Another method is coupling the PL emission with metal surface plasmon resonance (SPR), such as the near-field enhancement around Au, Ag and Cu nanoparticles,28 which has turned out to be an effective means to improve the luminescent efficiency of optoelectronic semiconductor devices.18,29–31 When the emission energy of the PL material matches well with the SPR energy, the coupling can improve the spontaneous recombination rate of the luminescent materials and therefore enhance the PL emission. However, because of the differences in the microstructures and properties of metal particles and photoluminescent materials, it is a challenge to synthesize metal–photoluminescent materials as a whole with a heterostructure. In this work we synthesized flower-like silicon oxide–Au nanostructures by thermal annealing of silicon wafer deposited Au film in one step. The growth mechanism of the SiOx–Au nanoflowers was discussed based on the VLS model. An enhanced PL emission phenomenon was observed from the SiOx–Au nanoflowers compared with silicon oxide nanowire, and the enhancement mechanisms related to the microstructure and the Au nanoparticle surface plasmon resonance have been studied both experimentally and in theory.
Experimental P-type Si wafers with (110) orientation were used as the substrate for Au thin film deposition and SiOx nanostructure
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CrystEngComm growth. The Si wafers were cleaned successively by acetone, deionized water and alcohol and then dried at 50 °C in an oven. After that, a 10 nm thick Au film was deposited on the substrate by using a sputter coater. Then the Au deposited Si substrates were put in an alumina crucible and introduced into a horizontal quartz tube in a furnace. The synthesis process was carried out at 1100 °C in an Ar–H2 mixture gas (5% H2 in Ar) with a flow rate of 200 sccm for 3 min. After the furnace cooled down to room temperature, the grown SiOx products were found on the Si substrates and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron dispersive X-ray spectroscopy (EDS), and helium ion microscopy (HIM). The room temperature PL spectrum and lifetime of the SiOx products were also collected using a fluorescence microscopy Olympus IX 71 equipped with a 532 nm laser and a time correlated single photon counting system (PicHarp 300).
Results and discussion Structure and morphology characterization After annealing, many white dot products several hundred micrometers in diameter were formed on the Si substrate, see Fig. S1 in the ESI.† SEM observation shows that different areas, starting from the middle of the white dots and moving radially outwards, have different morphologies. The middle of the dot (area A) contained grass-like nanowires together with a big hollow, as shown in Fig. 1(b). Area B contained flower-like nanostructures (Fig. 1(c)), and area C contained nanodots (Fig. 1(d)). All of the products have an Au nanoparticle (NP) catalyst and etched pits can also be found on the Si
Paper substrate, marked by the red circle in Fig. 1(d). The high magnification SEM image reveals that each nanoflower consists of several nanobranches ~50 nm in diameter, which distribute around the Au catalyst (Fig. 1(e)). Fig. 1(f) is the HIM image of an individual nanoflower showing how the SiOx nanoflower grows from the Au nanoparticle catalyst surface. Further study finds that the nanoflowers in area B also have different morphologies, their morphologies change with distance from the middle hollow in area A. Fig. 2 (a)–(d) are the SEM images of SiOx–Au nanoflowers in different locations within area B. The distance to the middle nanowires (area A) decreases in the order of (a), (b), (c), (d). It can be seen from Fig. 2(a)–(d) that the size and branch number of the nanoflowers decreases with the increase in distance from area A. There are only some small SiOx dots attached to the Au NPs when the growth area is far away from the middle hollow in area A (Fig. 2(a)), however, visible branches can be found in the nearer areas as shown in Fig. 2(b) and (c). As the distance to area A continues to decrease, typical flowerlike SiOx nanostructures with an Au NP core are formed, as shown in Fig. 2(d), but a few nanowires appear among the nanoflowers, as shown by the arrows. Statistics were used to investigate the Au NP size distribution on the top of the nanowires and nanoflowers. The result (Fig. 2(e)) shows that the size of the Au NPs on top of the nanowires is below 50 nm, and most of them are 10–30 nm. However, the Au NPs on the nanoflowers are in the range of 60–170 nm, with most of the Au NPs in the range of 80–140 nm. It seems that the branch number of the nanoflowers depends on the size of the Au particles and the bigger Au NPs tend to have more branches.
Fig. 1 SEM images of the SiOx–Au nanostructures synthesized on a silicon substrate. (a) Low magnification SEM image of the as-grown products. (b), (c) and (d) show the typical morphologies in the different areas in Fig. 1(a). The areas A, B and C, distinguished by the red dotted lines, contain nanowires, nanoflowers and nanodots, respectively. (d) Lots of etched pits formed on the Si substrate, as shown in the red circle. (e) High magnification SEM image of the SiOx–Au nanoflowers showing that each nanoflower consists of several SiOx nanobranches with an Au NP catalyst on top. (f) Helium ion microscopy image of a single nanoflower.
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Fig. 2 SEM images of SiOx–Au nanoflowers within area B. The distance from the middle SiOx nanowires (area A) decreases in the order of (a), (b), (c), (d). It shows that the number and size of the silicon oxide dots attached to the Au nanoparticles decreases with increasing distance from area A. (e) The size distribution of Au NPs on top of the nanowires and nanoflowers.
Structural characterization and compositional analysis were performed on the samples, by TEM and EDS. The TEM sample of SiOx–Au nanoflowers was prepared by a focused ion beam (FIB)/SEM dual beam system. Fig. 3(a) displays the TEM image of a nanoflower. The diffraction pattern of the nanoflower's branch, inset on the upper right, shows a diffused ring, which is a result of its amorphous structure. The composition of the nanoflowers is confirmed by the EDS analysis attached to the TEM image. Fig. 3(b) and (c) show the EDS spectra of the nanobranch and the catalyst, respectively. The copper and carbon signals in the EDS spectra are due to the Cu grid and the supporting carbon film. The EDS
Fig. 3 (a) TEM image of a SiOx branch, the inset is the electron diffraction pattern. The TEM samples were prepared by a FIB/SEM dual beam system. Before cutting the SiOx lamella, a Pt protective layer was deposited on the sample, as shown in the figure. Between the Si substrate and the SiOx–Au nanoflower, there was a thin silicon oxide layer. (b), (c) EDS spectra of the SiOx branch and the Au particle, respectively. The O/Si ratio of the branch is 1.6. The Cu and C signals are from the TEM grid and the carbon support film.
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CrystEngComm analysis reveals that the catalyst is an Au nanoparticle without silicon. The main constituents of the nanobranches are Si and O, with a relative O/Si atomic ratio of 1.6. Therefore, the flower-like structures grown on the substrate are amorphous silicon oxide nanoflowers with an Au NP core, which was also confirmed by the EELS spectrum as shown in the ESI† (Fig. S2). Thus it can be obtained that the Au nanoparticles are crucial for the formation of SiOx nanostructures. No SiOx nanostructures can be observed from a bare silicon wafer in comparison experiments where the growth parameters were kept the same except for the deposition of the Au layer (see the ESI,† Fig. S3 (a)). As shown in Fig. S3,† the length of the structures and the number of branches growing from the Au particles increase with the synthesis time. This evidence strongly suggests the growth is through a VLS process. There have been previous reports on the synthesis of SiOx nanostructures by direct annealing of Si substrate covered metal catalyst film without an additional Si gas source.32,33 However, there was an argument about the gas source for SiOx growth in the VLS growth process: whether it is Si vapour evaporated from the Si substrate32,34 or the gas reactant of Si and O.8 Meanwhile, in our experiments, another very interesting phenomenon is that as well as the growth of single nanowires we also observed the growth of SiOx nanoflowers with several nanobranches from a single Au NP catalyst. This is different from the traditional VLS growth model, wherein one catalyst can only induce one dimensional structure growth.15,33 As a result, it is essential to further understand the VLS growth model in our experiment. According to the results presented above, we propose a growth process, sketched in Fig. 4, which can be divided into the following steps. Firstly, the pre-deposited Au film on the Si substrate began to agglomerate into small particles on the native oxide layer as the temperature was close to the Au–Si eutectic temperature of about 363 °C,35 as illustrated in Fig. 4(a). Secondly, as the annealing progressed, the Au particles started to react with the Si substrate to form liquid eutectic alloy droplets, which served as growth sites for the SiOx nanostructures. At the same time, because of the high solubility of the Si atoms in the Au–Si eutectic drops, a great number of Si atoms will diffuse into the liquid drops, which causes the etching of the Si substrate.15 Due to the very thin native oxide layer on the silicon substrate, the Au NPs were separated from the Si substrate and were prevented from reacting with it. However, the catalyst activity of Au based particles is size dependent, with smaller particles showing better activity due to them having a higher number of exposed atomic sites of high catalyst activity.36,37 That means that the smaller Au NPs have a higher possibility of etching the Si substrate compared with the larger ones. The smaller Au NPs therefore react with the Si substrate and form several etch pits on the substrate, as shown in Fig. 1(d), while the bigger Au NPs are still left on the top of native oxide layer, as shown in Fig. 4(b).
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Fig. 4 Schematic of the SiOx–Au nanoflower growth mechanism. The growth process is divided into three steps. (a) Au nanoparticles form on the silicon substrate at about 363 °C; (b) the formed Au particles react with the silicon substrate and form liquid Au–Si eutectic alloy as the temperature is increased, and several pits are etched into the Si substrate as Au NPs react continuously with the Si substrate; (c) the exposed Si surface of the etched pits reacts with the trace oxygen in the furnace to release SiO vapour, which causes the pits to expand into the hollows. The Au NPs absorb the released SiO vapour and form nanoflowers or nanowires around the etched hollows.
Thirdly, as indicated in Fig. 4(c), the formation of pits in the substrate exposes Si surfaces to the annealing atmosphere. These exposed Si surfaces can react with trace oxygen in the furnace. The oxidation of Si is known to fall within two distinct regimes depending on the annealing temperature and oxygen partial pressure:38,39 passive oxidation and active oxidation. SiðsÞ þ O2 ðgÞ ¼ SiO2 ðsÞ
ð1Þ
2SiðsÞ þ O2 ðgÞ ¼ 2SiOðgÞ
ð2Þ
Reactions 1 and 2 are passive and active oxidation, respectively. Passive oxidation happens at high oxygen partial pressure to from a passivated SiO2 layer. Active oxidation occurs in a vacuum or at a low oxygen partial pressure that is comparable to that employed in vacuum studies.40 For the case of the active oxidation reaction, the substrate becomes pitted due to the production of volatile SiO vapour. According to the phase diagram for the oxidation of Si as a function of temperature and oxygen concentration,40 active oxidation happens when the oxygen pressure is below 10 parts per million (ppm) and the annealing temperature is over 1050 °C. Our annealing process was carried out at 1100 °C in a protective atmosphere of Ar–H2 mixed gas. The residual O2 was estimated to be 3–10 ppm. So the Si oxidation reaction was active oxidation in our experiment. Consequently, the residual O2 in the furnace can react with the exposed Si surface of the
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etched pits to release SiO vapour into the atmosphere and cause the pits to expand into a hollow, as shown in Fig. 1(b). Meanwhile, the released SiO vapour can dissolve into the Au–Si liquid droplets, so SiO is the Si vapour source for the silicon oxide nanowire and nanoflower growth. Comparison experiments were carried out at temperatures of 890 °C and 1050 °C, and only Au nanoparticles, with no silicon oxide nanowires or nanoflowers, were found on the Si substrate after annealing because of the absence of SiO vapour in the atmosphere. These results are shown in Fig. S4 in the ESI.† Nanowire growth was catalyzed by small Au nanoparticles (60 nm) are more likely to form flower-like structures since there is enough surface area to support several nucleation sites growing at the same time.41 The distribution of these nanobranches is uniform and symmetric, as shown in Fig. 1(e) and (f). The nanowire growth rate in the VLS model mainly depends on the source concentration and the reaction temperature.42 Basically, high source concentration and high temperature lead to a high growth speed. In our experiment, the Si source for the silicon oxide nanostructure growth was supplied by the SiO vapour released from the etched pits. This resulted in a non-uniform distribution of SiO vapour concentration around the pits, which meant that the closer to the pits, the higher the concentration of SiO vapour. This uneven distribution of the growth source led to the different growth rates of nanoflowers in the area around the pits. This is the reason why the Au catalysts close to the pits grow into flower-like structures, whereas the further away ones just form some small silicon oxide dots, or even have nothing on the surface if sufficiently far from the pit, as displayed clearly in Fig. 2. Since the growth links with the etching of the substrate, the crystallographic orientation of the substrate may have some effects on the growth. The growth parameters may have to vary slightly if different types of silicon substrate are used for the growth.
Measurements of photoluminescence and lifetime The photoluminescence of the as grown silicon oxide nanostructures was measured at room temperature with an excitation laser of 532 nm, as shown in Fig. 5(a). The PL spectra show a high intensity emission from 550 nm to 650 nm peaked at 570 nm (2.17 eV). Nishikawa et al.21 obtained several luminescence bands in different types of silica, which have different peak energies from 1.9 eV to 4.3 eV under 7.9 eV excitation. They showed that the different emission energies were due to different defects in the silica structure, including the neutral oxygen vacancy, non-bridging oxygen hole centers, two-fold coordinated silicon lone-pair centers (Si–O–Si), and transitions at the neutral oxygen vacancies. According to the discussion of Yang et al.43 and Glinka et al.,44 the light emission at 570 nm can be attributed to the hydrogen-related species in composites of the silicon oxide nanostructures. In our annealing process, the two-fold
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Fig. 5 (a) The PL spectra of different areas on the as grown sample obtained under excitation with a 532 nm laser. The blue and red lines show the spectra collected from the SiOx nanowires and nanoflowers, respectively. All the PL spectra have a broad emission band with a peak centered at 570 nm (2.17 eV). (b) The PL decay curves of the SiOx nanoflowers and nanowires.
coordinated silicon lone-pair centers can be easily split into Si–O˙ and Si˙45 (here the dot denotes an unpaired spin), which coincides with the results reported by Lee et al.46 When these defects are exposed to a H2 atmosphere, they can react with hydrogen atoms and form hydrogen related species (Si–OH, Si–H) at the surface of the silicon oxide nanostructure. The PL spectrum of the silicon oxide nanoflowers shows a much stronger signal than the PL spectrum from the SiOx nanowires, and it is a ∼5-fold enhancement over the nanowires at the PL emission of 570 nm (Fig. 5(a)). Since PL often originates from the sample surface and is sensitive to the electronic states near the surface, it is conjectured that the wide peak and enhanced PL spectrum intensity is partly because of the large surface area of the silicon oxide nanoflowers.43 The obtained flower-like SiOx nanostructures have abundant corners and edges and thus possess very high surface areas and dramatically improved photoluminescence performance. Meanwhile, the excitation of surface plasmons of the Au NP in the middle of the silicon oxide nanoflower is another reason for the enhancement of the silicon oxide nanoflower's
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CrystEngComm PL emission. Metal surface plasmon resonance has attracted significant interest because light emission can be improved due to the coupling between SPR and the emitter.30,47,48 Under photoactivation, the plasmons couple with the excitation light and produce strong light scattering, a surface plasmon absorption band, and enhancement of the surface electromagnetic field.28,31 The optical properties of the luminescent material will be affected by the SPR, especially if the SPR energy of the metal matches well with the photoluminescence energy of the luminescent material. In our experiment, every silicon oxide nanostructure (nanoflowers and nanowires) has an Au nanoparticle at its top. When incident laser light irradiates on the SiOx nanostructure, the Au NP surface will form a local electromagnetic field. The photonic excitation energy of the SiOx structures can be strongly coupled to the Au SPR electromagnetic field. The intense coupling between the SiOx photon emission and the Au SPR will increase the radiative recombination efficiency of excited electrons in the SiOx nanostructures, consequently resulting in an improvement of the PL intensity. The average size of the Au NPs on the top of the silica nanoflowers is about 100 nm, which gives a SPR energy of ~2.17 eV (570 nm) according to previous reports.49,50 It can be seen that the SPR energy (2.17 eV) of the Au catalyst NPs matches well with the highest PL emission intensity energy of 2.17 eV (570 nm) of the SiOx nanostructures. Meanwhile, the SiOx nanoflowers grow around the surface of the Au NPs, which can offer a larger contact area with the Au NP compared to the SiOx nanowires. Both of these factors result in a much stronger enhancement of the PL emission of SiOx nanoflowers at 570 nm, as shown in the Fig. 5(a). Although there is also an Au NP on the tip of the SiOx nanowires, the contact area between the Au NP and the SiOx nanowire is too small to support the abundant coupling chance of the Au SPR and SiOx emission. Furthermore, as shown in Fig. 2(e), the average size of the Au NPs located on the tip of the nanowires is ~30 nm. The corresponding SPR energy is about 2.36 eV,50 which does not match with the SiOx PL emission energy. Thus, the coupling between the SiOx PL emission and Au SPR is very weak because of the mismatching of energy, which finally causes the low photoluminescence emission intensity from the SiOx nanowires. To further elucidate the metal surface plasmon enhanced phenomenon, PL lifetime measurements of the SiOx nanostructures were performed. The luminescence decay curves excited at 532 nm are illustrated in Fig. 5(b). The lifetimes were obtained by fitting the decay curves with an exponential model after proper deconvolution of the instrument response function.51 The PL lifetime for the SiOx nanoflowers is obtained to be 0.22 ns, while that of the nanowires is 1.39 ns. The reduction in lifetime together with the increase in PL intensity of the nanoflowers compared with that of the nanowires suggests that the observed PL enhancement derives from surface plasmons. What is more, a simple model was also used to simulate the electric field enhancement around the Au nanoparticle.
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Paper these SiOx–Au nanoflowers they could find potential applications in high intensity green light emitters and other silicon based optical devices.
Acknowledgements
Fig. 6 (a) A simple model for the PL enhancement of SiOx–Au nanostructures. An Au nanoparticle with a diameter of 100 nm is embedded into a SiOx layer. The refractive index for SiOx is n = 1.5. (b) The near-field intensity enhancement at point A as a function of the incident wavelength. Point A is 10 nm away from the metal surface. The incident wave vector (K) and polarization (E) are indicated by the red and green arrows in (a), respectively.
As shown in Fig. 6(a), a 100 nm Au nanoparticle was embedded into the SiOx. The refractive index of SiOx was adopted as 1.5 as reported in ref. 52. Fig. 6(b) shows the enhancement of the near-field intensity at point A, 10 nm away from the Au surface versus the excitation wavelength. The near-field intensity enhancement at 532 nm excitation is about 3 times. The emission enhancement at 570 nm, on the other hand, can be evaluated from the near-field enhancement (~8 times) according to the electromagnetic reciprocity. Then the PL enhancement will be 24 times, which is the same magnitude as the experimental result. Here we only demonstrate the enhancement ability at point A. In the experiment, the PL emission is contributed by the SiOx all around the Au particles. Then the predicted average PL enhancement will be smaller, which can explain the discrepancy between the simulation and experiment. Also, we have to admit the model is a rough approximation. The size of the Au particle, the refractive index of SiOx, and more importantly, the shape of the SiOx nanoflower will all cause further complication. Consequently, a full theoretical simulation will be worthy for future work.
Conclusions In summary, we synthesized flower-like silicon oxide–Au nanostructures by simply annealing a silicon substrate coated with thin Au film. A reasonable growth mechanism model based on the VLS model was proposed and employed to explain the synthesis of silicon oxide–Au nanoflowers. These novel nanostructures exhibit excellent photoluminescence properties, with a very high intensity emission at 570 nm. Because of the unique structure of the SiOx–Au nanoflowers, the PL intensity has a ∼5-fold enhancement factor compared with simple silicon oxide nanowires. This highly significant enhancement was attributed to the nanoflower's larger surface to volume ratio and the coupling of the silicon oxide branch emission with the core Au nanoparticle's surface plasmon resonance, which were confirmed by the PL lifetime measurement results and the theoretical simulation. Due to the outstanding photoluminescence properties of
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The authors gratefully acknowledge the support of the National Natural Science Foundation of China (11174023), Beijing Municipal research project for outstanding doctoral thesis supervisors (20121000603), Science Foundation Ireland (07/SK/I1220a) and the Chinese scholar council fellowship. The authors also thank Prof. Hong Wei and Qiang Li from Institute of Physics, Chinese Academy of Sciences for the PL lifetime measurement.
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