nanomachining regular noble metal/ceramic structures for catalytic applications
Hai Le-The is a PhD student in the BIOS Lab-on-a-chip group. His supervisors are prof.dr. J.C.T. Eijkel from the Faculty of Electrical Engineering, Mathematics and Computer Science and prof.dr.ir. B.M. Weckhuysen from Utrecht University.
The research work presented in this thesis was aimed at developing enabling techniques for patterning regular noble metal/ceramic nanostructures over large-areas at low-cost. The direct aim of this work was to apply such nanostructures in microfluidic devices for catalysis study of ceramic-supported noble metal nanoparticle arrays. In the course of the investigation a third aim arose, namely understanding the mutual interaction of noble metals with ceramic substrates.
Although many technologies used for top-down nanopatterning of regular nanostructures have been investigated intensively, fabrication of highly ordered nanostructures with sub-100 nm dimensions over large-areas at low-cost using a conventional ultraviolet (UV, 365 nm wavelength) light source has been considered challenging. To make this possible, we combined UV-based displacement Talbot lithography (DTL), which enables large-area nanostructures patterning (dots or lines) but with a dimensional limit of 100 nm, with a plasma shrink-etching technique that allowed tunable dimensions down to sub-30 nm (Chapter 2). We can transfer directly the DTL-patterned photoresist (PR) nanostructures into a bottom antireflection coating (BARC) layer at a 1:1 ratio using N2 plasma, or at smaller dimensions using O2/N2 plasma. It is highly remarkable that the verticality of the PR/BARC nanostructures remained preserved during the shrink-etching process. Combining the etching of an O2/N2 plasma with a N2 plasma allowed us to reproducibly pattern PR/BARC nanostructures with tunable dimensions from 110 nm to sub-30 nm. With the easy and flexible operation, our fabrication method is suitable for large-area mass production of sub-100 nm periodic nanostructure at low-cost.
Combining the DTL-based shrink-etching technique reported in Chapter 2 with dry etching, wet etching, and thin film deposition techniques, we reported a robust and high-yield fabrication method for wafer-scale patterning of high-quality arrays of dense gold (Au) nanogaps at low-cost (Chapter 3). Using the self-sharpening of <111>-oriented silicon crystal planes during the wet etching process, we fabricated silicon structures with extremely smooth nanogaps. Subsequent conformal deposition of a silicon nitride layer and a Au layer resulted in dense arrays of high-quality gold nanogaps. The gap spacing of the fabricated Au nanogaps could be easily tuned down to ~10 nm over full wafer areas by varying the thickness of deposited Au layers. Since the roughness of the template was minimized by the crystallographic etching of silicon, the roughness of the Au nanogaps depended almost exclusively on the roughness of the sputtered Au layers. Notably, our SiN-coated Si template can be reused by replacing the Au layer with another fresh one, without damaging the template material or altering its dimensions. Most importantly, the fabricated Au nanogaps showed a significant enhancement of surface-enhanced Raman scattering (SERS) signals of benzenethiol (BT) molecules chemisorbed on the nanogap surface, at an enhancement factor up to 3.0×108.
In Chapter 4, we reported a robust technique to large-scale fabricate highly ordered size-tunable noble metal nanoparticles, i.e. gold and platinum, supported on oxidized silicon substrates without metallic adhesion layers, combining UV-based DTL (see Chapter 2) with inclined argon (Ar) ion beam etching techniques. Upon applying this technique, we successfully fabricated 3×3 cm2 arrays of Au or Pt nanoparticles supported on cone-shaped silica features at various diameters. By tuning the inclined etching time, the particle diameters could be varied from sub-30 nm to 110 nm. Moreover, annealing such sub-30 nm metal nanoparticle arrays at high temperature resulted in sub-20 nm metal nanoparticle arrays with high uniformity in the particle diameter, and good particle adhesion. These well-ordered noble metal nanoparticles were used as model catalysts for catalysis study in combination with microfluidic technology mentioned in Chapter 7.
In another effort to explore the mutual interaction of sputtered Au with the surface of oxidized silicon (SiO2) substrates, we found that continuous Au films on SiO2 substrates upon treatment with ultraviolet (UV)-ozone reveal an excellent enhancement of adhesion to the SiO2 support (Chapter 5). Importantly, the enhancement is independent of the micro- or nanostructuring of such nanometer thick films. Continuous Au films with a thickness less than ~13 nm possess an extraordinarily strong adhesion after 5 min UV-ozone treatment. It is remarkable that this high adhesion effect vanishes when rising the samples with ethanol for 10 min. On the basis of this observation as well as other observations, we attributed the increased adhesion to the formation of an gold oxide layer. By depositing a second gold layer after UV-ozone treatment, thereby embedding the hypothesized gold oxide layer in between two sputtered Au layers, the adhesion increase becomes durable and independent of the influence of the surrounding environment, i.e. gases or solutions. The observed excellent enhancement of the adhesion can be tentatively explained as polarization-induced increased Au-SiO2 interaction at their interface due to the formation of the gold oxide on the Au surface. This novel technique enabled us to large-scale fabricate various SiO2-supported Au micro/nano-sized structures, i.e. lines and dots, at dimensions spanning from a few hundreds of nanometers to a few micrometers, without the use of additional adhesion layers. Our enabling technique thus opens new routes for patterning Au micro/nanostructures on SiO2 substrates, which are highly favored for biosensing and nanophotonic applications due to the absence of metallic adhesion layers.
In Chapter 6, we found that platinum (Pt) nanoparticles, upon annealing at high temperature of 1000oC, are engulfed into amorphous fused-silica or thermal oxide silicon substrates. Similar to the previously reported engulfment of Au nanoparticles, the engulfed Pt nanoparticles connect to the surface of the substrates through conical nanopores, and the size of the Pt nanoparticles decreases with increasing depth of the nanopores. We explain the phenomena as driven by the formation of volatile platinum oxide by reaction of the platinum with atmospheric oxygen, with the platinum oxide evaporating to the environment. We found that the use of Pt provides much better controllability than the use of Au. Due to the high vapor pressure of platinum oxide, the engulfment of the Pt nanoparticles into oxidized silicon (SiO2) substrates is faster than of Au nanoparticles. At high temperature annealing we also found that the aggregation of Pt nanoparticles on the substrate surface is insignificant, in contrast to the aggregation of Au nanoparticles. As a result, the Pt nanoparticles are uniformly engulfed into the substrates, leading to an opportunity for patterning dense nanopore arrays. Moreover, the use of oxidized Si substrates enables us to precisely control the depth of the nanopores since the engulfment of Pt nanoparticles stops at a short distance above the SiOx/Si interface. Using this method, we obtained, after subsequent etching steps, a membrane with dense nanopore through-holes with diameters down to sub-30 nm. With its simple operation and high controllability, this fabrication method provides an alternative for rapid patterning of dense arrays of solid-state nanopores at low-cost.
In Chapter 7, we applied microfluidic technology for catalysis study by fabricating microreactors integrated with noble metal nanoparticle arrays. The fabricated microreactor consists of a top-part made of glass containing a microchamber and two distribution channels anodically bonded to a bottom-part containing an array (3×3 cm2) of noble metal nanoparticles supported on an oxidized silicon substrate. The fabricated microreactors were used to perform the oxidation of carbon monoxide on chip. By using the two distribution channels, the flow was uniformly distributed inside the microchamber, thus providing homogenous reaction conditions. The use of our fabricated microreactors provides opportunities to precisely control the reactant flows, especially at very low flow rates, and to maximize the volume ratio of products to reactants.
In Appendix F, we reported a robust and simple technique for large-scale fabrication of free-standing and sub-μm thin PDMS through-hole membranes, combining soft-lithography with reactive plasma etching techniques. Using this fabrication technique, we successfully demonstrated the fabrication of free-standing PDMS membranes at various sub-μm thicknesses down to 600 ± 20 nm, and nanometer-sized through-hole (810 ± 20 nm diameter) densities, over areas as large as 3 cm in diameter. By its simple procedure, our fabrication technique provides an enabling technique for large-scale and rapid patterning of sub-μm PDMS through-hole membranes, which is suitable for high-yield production at low-cost. Furthermore, we demonstrated the potential of our membranes as cell-culture substrates for biomedical applications by culturing endothelial cells on the membranes in a Transwell-like set-up.
