Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns
© Liu et al.; licensee Springer. 2012
Received: 7 July 2012
Accepted: 15 August 2012
Published: 3 September 2012
The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor–liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.
In recent years, nanomaterials with diverse configurations have been investigated actively to explore their different properties which are strongly dependent on the internal and external structural features [1–3]. Various silica nanostructures and their assemblies have demonstrated unique physical, chemical, and optical properties [4–7], which can be used for a wide range of applications. For example, silica nanoparticles with solid or porous structures have been applied for biomedical imaging or theranostic applications as nanoprobes after introducing new functional groups . Silica nanotubes with tubular hollow structures would not only realize easier surface functionalizations on both the outer and inner walls, but would also show potential applications in bioanalysis , biocatalysis,  and optical devices . Rope-like silica nanowire assemblies showing reversible blue light emission behavior in photoluminescence and infrared analysis could be used to fabricate effective optoelectronic devices and optical signal humid sensors . Theoretical calculations have also been conducted to explore some interesting optical properties of silica nanoclusters [13, 14]. Up to the present, many approaches have been introduced to synthesize silica nanostructures [15–19], and various morphologies were reported, such as nanowire assemblies [17, 20], nanoflowers , nanotubes , and nanosprings,  etc. The vapor–liquid-solid (VLS) mechanism has been widely applied to explain the growth process above-mentioned . The critical process parameters such as temperature and the growing sources of Si and O elements are always considered to be important factors affecting the synthesis of silica nanostructures. Temperature-dependent growth of silica nanostructures with different morphologies has been reported [17, 20]. The growth of silica nanowires under the effect of oxygen-to-Ar carrier gas ratio has been investigated . However, the growth dependence of Si vapor source concentration is seldom reported.
Recently, we reported that the Si vapor source can be generated through high temperature annealing of silicon substrate coated with a thin copper layer in a quartz tube furnace . Different from our previous work, where a method was introduced to selectively grow silica nanowires on the photoresist-derived three-dimensional (3D) carbon posts at the annealing of high temperature, we mainly investigate the effects of local Si vapor source concentration gradient on the growth behavior of silica nanostructures in the present study. It has also been shown in our previous work that copper is a good candidate to catalyze the growth of silica nanowires on photoresist-derived carbon microstructures; therefore, Cu is further to be applied as the catalyst in the present work. Through the design of experiments, silica nanostructures with diverse morphologies of evolution induced by the concentration gradient are obtained, and the growth mechanisms such as VLS and vapor-quasiliquid-solid (VQS) are discussed. The study is meaningful in both the development of controllable synthesis of nanostructures and the integration of nanostructures on microstructures for a variety of electrochemical and bio-nanotechnology applications .
The typical process flow of our method has been reported earlier, involving main processes of photolithographic patterning of negative photoresist on silicon substrate, magnetron sputtering of thin copper layer, and thermal heating . An SU-8 photoresist (MicroChem Nano SU-8 100, MicroChem Corp, Newton, MA, USA) was applied in this work with phosphorus-doped n-type silicon <100 > wafer as the substrate. Ultrahigh-purity N2and H2were used as carrier gases during the thermal heating process to carbonize the patterned photoresist.
Firstly, thick photoresist was spin-coated on a cleaned Si/SiO2 (5-nm native oxide layer) wafer. Secondly, a soft bake process at 40°C for 30 min and 120°C for 2 min was carried out, followed by exposure performed by a Karl Suss MJB3 contact aligner (SUSS MicroTec, Garching, Germany) for 80 s at 150 mW/cm2. A post bake process was then carried out at 95°C for 30 min. Following a 1-h delay time, development was carried out using an SU-8 developer from MicroChem to obtain a patterned 3D photoresist sample. A Cu layer of about 30 nm was then sputtered on the sample, followed by a carbonization process in an alumina tube furnace. Before heating, the furnace was evacuated to 10−3 Torr; N2 gas flow was introduced (2,000 sccm). In the carbonization process, the furnace was firstly heated to 300°C at the rate of 15°C/min for 40 min with the introduced 2,000 sccm of N2 gas flow as the atmosphere; then the sample was heated up to the growth temperature at the rate of about 15°C/min in the atmosphere of 150 sccm of H2 gas flow and maintained at the growth temperature for 120 min with the atmosphere changed to 2,000 sccm forming gas (5% H2 in N2). After the carbonization process, the heater was turned off, and the samples were naturally cooled down in N2 atmosphere to room temperature. The temperature was set as 950°C and 1,000°C, respectively, to investigate the growth of silica nanostructures. Special configurations were designed to investigate the effect of Si vapor concentration variations on the silica nanostructure growth. In the configuration, the downstream part of each sample was with patterned SU-8 photoresist micro-post array and sputtered Cu layer, while the upstream part was Si substrate-coated with only a thin layer of Cu, which was designed to generate local concentration gradient at a microscale of Si vapor source for the downstream part. It has been shown that Si vapor source could be generated from thin Cu layer-coated Si substrate at the annealing of high temperature ; therefore, the Si vapor source could be controlled by the size of bare Cu-coated Si substrate, which would flow gradually down to the region of 3D carbon-post-patterned Si substrate with the flow of carrier gas. Since the growth of silica nanostructures on 3D carbon posts will consume the Si vapor, Si vapor concentration gradient will be generated along the flow from the upstream to the downstream. It is obvious that the area of the Cu-coated Si substrate, the distance from the upstream to the downstream, and the density of the 3D carbon posts will affect the local Si vapor concentration gradient. Moreover, the overall Si vapor source concentration could also be controlled by the annealing temperature since the generation process of Si vapor source is influenced greatly by the temperature. In general, the upstream is with a higher concentration of Si vapor, while the downstream is with a lower one due to the consumption. During the experiment, the whole length of the sample was about 10 mm along the gas flow, while the length of the 3D post-patterned region was about 7 mm.
The morphologies of the obtained products were investigated by scanning electron microscopy (SEM) (Quanta 200, FEI Company, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray (EDX), and the nanowires were further characterized and analyzed by transmission electron microscope (TEM) and high-resolution TEM (HRTEM) (Tecnai 12,Philips Tecnai, Amsterdam, The Netherlands) equipped with an EDX.
Results and discussion
Concentration gradient induced results at a temperature of 1,000°C
Concentration gradient induced results at a temperature of 950°C
In general, Si vapor source is mainly from the active oxidation of the silicon substrate in the presence of copper at high temperature, and O vapor source might come from the leakage of the vacuum system . The copper layer breaks to small droplets at the heating process, then Cu-Si eutectic catalysts can be formed to initiate the nanostructure growth on the 3D carbon posts above the eutectic temperature, which matches with the photoresist carbonization process . By dissolving Si and O source onto the catalyst droplet continuously, the silica nanostructures grow mainly through VLS and VQS mechanisms. The catalyst droplets with smoothly curved surfaces confirm the VLS process, as shown in Figures 2, 4, and 5. Combining the results of the SEM and TEM, it is concluded that the Si vapor source concentration gradient would lead to unique morphology changes of nanostructures. The underlying reason could be due to the variations of reactive source concentration gradient inside different catalyst droplets caused by the local source concentration gradient in the environment, which is a key factor to determine the nanowire growth behavior and could result in diverse morphologies [32, 34]. Furthermore, at a relatively lower temperature of 950°C, the active smaller catalysts in each catalyst might be concentric to initiate tubelike structures due to thermodynamic imbalance, which could be confirmed by Figure 4f .
It is also noticed that silica nanostructures are hard to observe at an annealing temperature below 900°C, while abundant silica nanowires are found all over the substrate when the temperature is above 1,050°C, which reminds us that temperature is another critical parameter to determine the growth behavior of silica nanostructures.
In summary, the morphology evolution of silica nanostructures is induced by local Si vapor source concentration gradient as grown on a photoresist-derived 3D carbon microelectrode array at both 950°C and 1,000°C. Different silica nanostructure morphologies and their assemblies such as bamboo-, tube-, wire-, feature-, string-, and rope-like were obtained. Nanostructure branching, connecting, and batch growth phenomenon were also observed, which involved both VLS and VQS growth mechanisms. The study would help further understand the process of the nanowire formation and offers a potential method to design and synthesize nanostructures with controlled features. Meanwhile, it promotes an effective and controllable route to grow and integrate nanostructures on microstructures for applications in diverse fields.
This work is financially supported by the National Science Foundation of China (no. 90923019, 51175210) and the National Key Basic Research Special Fund of China (grant no. 2009CB724204).
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