Controlling the location and orientation of nanobuilding blocks in assemblies of nanodevices is a challenging task in "bottom-up" chemical approaches for nanodevice fabrication. Here, a strategy is described for in-situ fabrication of horizontal nanochannels that allows control over orientation and average pore size at an unprecedented range of ≈5 nm to ≈20 nm. Using this approach, nanochannels with controlled average pore sizes are fabricated at known positions on a surface. Equally important, the nanochannels are addressable photolithographically and integrable to microchannels without the use of any high resolution lithography. This technique uses horizontal ZnO nanowires (NWs) as a sacrificial template and affords the capability of fabrication of complex micro- and nanofluidic platforms. Cross-sections of nanowires and nanochannels are prepared via a focused-ion beam (FIB) and examined by electron microscopy (EM) and energy-dispersive X-ray spectroscopy (EDS).
In general, current technologies do not satisfy the requirements for high throughput detection and multiparameter analysis of biomolecules; nanofluidics combined with microfluidics is a promising platform for this purpose. The most important application areas/markets of the nanofluidic research are protein analysis and identification, medical diagnostics, and high-throughput screening. To this end, we demonstrate: horizontal NWs formation as a sacrificial template for formation of nanochannels, addressable nanochannels by microchannels on any size scale, and control of the average diameter of the nanochannels. The strategy for making nanochannels is schematically illustrated in Figure 1(a-d). In the first step, (a), horizontal ZnO NWs are grown selectively on a sapphire surface from gold nanodroplets that are deposited photolithographically. Next, the entire substrate is coated with a thick silicon oxide layer (gray color), followed by a photoresist coating (green color). In (b), patterns of microchannels are produced in the photoresist using a second round of photolithography. Fiducial marks are used to ensure that microchannels overlap with the two ends of the embedded horizontal NWs. This step is followed by reactive ion etching (c) to transfer the photopatterns to the oxide layer underneath the photoresist. After the photoresist removal (d), ZnO is etched out using a selective gas-phase process and nanochannels are formed at the interface of silicon oxide layer and sapphire substrate. Figure 1(e) shows a side view of two microchannels that are connected via a group of nanochannels. Since nanochannels are thin in diameter they are highlighted with the white arrows. The entrances of the nanochannels, marked with red arrows are shown in Figure 1(f). In order to confirm the complete release of ZnO from the silicon oxide nanochannels, EDS analysis was carried out on a group of NWs, coated with 1 μm thick oxide, before and after their etching. Results confirmed the successful removal of ZnO from the nanochannels. Nanochannels are further characterized by examining their cross-sections and comparing them with those of the overcoated ZnO NWs before the chemical etch. The electron transparent cross-sections are prepared using a DualBeam FIB. The electron micrographs of the cross-sections are obtained using EM. Figure 1(g), shows a cross-section of a group of ZnO NWs with 7 nm average diameter before ZnO etching in H2 /Ar atmosphere. The average height was measured by atomic force microscopy before NWs were coated with the thick oxide layer. The bright dots at the interface between sapphire (bottom layer) and silicon oxide (top layer) are ZnO NWs. Figure 1(h) shows a cross-section of a different group of NWs after the ZnO removal. One of the benefits of this method is that the nanochannels can be traced back exactly to their original NWs for their morphology and height information. By controlling the average size of the deposited gold nanodroplets, we have shown that the average nanochannel diameter can be controlled.
Figure 1(i) shows a side view of a group of microchannels that are fabricated exactly on the nanochannels. This method brings the opportunity of easily locating nanochannels on a large surface and using them for nanofluidic purposes. Figure 1: (a-d) Describes different steps in making the nano-microchannel combination. (e) Illustrates nanochannels, shown with arrows that are formed between two microchannels, scale bar: 400 nm (f) Shows the marked area in part (e), which is the entrance of the nanochannels that are shown in part (e) scale bar: 100 nm. (g-h) Cross-sections of ZnO nanowires before and after their transformation to the nanochannels, scale bar in (g): 200 nm, in (h): 300 nm. (I) Large view of integration of nanochannels to microchannels. In this picture, microchannels in each pair are connected by groups of nanochannels, scale bar: 3 μm.
• Nanochannels were fabricated at specified locations on a sapphire surface.
• Nanochannels where produced with known average diameters on desired locations.
• Nanochannels were connected to microchannels in a scalable fashion