1Department of Chemistry, Duke University, 2Hajim School of Engineering and Applied Sciences, University of Rochester, 3Department of Chemical Engineering, University of Rochester
Bowers, C. M., Toone, E. J., Clark, R. L., Shestopalov, A. A. Soft Lithographic Functionalization and Patterning Oxide-free Silicon and Germanium. J. Vis. Exp. (58), e3478, doi:10.3791/3478 (2011).
The development of hybrid electronic devices relies in large part on the integration of (bio)organic materials and inorganic semiconductors through a stable interface that permits efficient electron transport and protects underlying substrates from oxidative degradation. Group IV semiconductors can be effectively protected with highly-ordered self-assembled monolayers (SAMs) composed of simple alkyl chains that act as impervious barriers to both organic and aqueous solutions. Simple alkyl SAMs, however, are inert and not amenable to traditional patterning techniques. The motivation for immobilizing organic molecular systems on semiconductors is to impart new functionality to the surface that can provide optical, electronic, and mechanical function, as well as chemical and biological activity.
Microcontact printing (μCP) is a soft-lithographic technique for patterning SAMs on myriad surfaces.1-9 Despite its simplicity and versatility, the approach has been largely limited to noble metal surfaces and has not been well developed for pattern transfer to technologically important substrates such as oxide-free silicon and germanium. Furthermore, because this technique relies on the ink diffusion to transfer pattern from the elastomer to substrate, the resolution of such traditional printing is essentially limited to near 1 μm.10-16
In contrast to traditional printing, inkless μCP patterning relies on a specific reaction between a surface-immobilized substrate and a stamp-bound catalyst. Because the technique does not rely on diffusive SAM formation, it significantly expands the diversity of patternable surfaces. In addition, the inkless technique obviates the feature size limitations imposed by molecular diffusion, facilitating replication of very small (<200 nm) features.17-23 However, up till now, inkless μCP has been mainly used for patterning relatively disordered molecular systems, which do not protect underlying surfaces from degradation.
Here, we report a simple, reliable high-throughput method for patterning passivated silicon and germanium with reactive organic monolayers and demonstrate selective functionalization of the patterned substrates with both small molecules and proteins. The technique utilizes a preformed NHS-reactive bilayered system on oxide-free silicon and germanium. The NHS moiety is hydrolyzed in a pattern-specific manner with a sulfonic acid-modified acrylate stamp to produce chemically distinct patterns of NHS-activated and free carboxylic acids. A significant limitation to the resolution of many μCP techniques is the use of PDMS material which lacks the mechanical rigidity necessary for high fidelity transfer. To alleviate this limitation we utilized a polyurethane acrylate polymer, a relatively rigid material that can be easily functionalized with different organic moieties. Our patterning approach completely protects both silicon and germanium from chemical oxidation, provides precise control over the shape and size of the patterned features, and gives ready access to chemically discriminated patterns that can be further functionalized with both organic and biological molecules. The approach is general and applicable to other technologically-relevant surfaces.
1A. Primary Monolayer Formation on Silicon
1B. Primary Monolayer Formation on Germanium
2. NHS Substrate Functionalization on Silicon and Germanium
3. Small Molecule Functionalization
4. Acidic Polyurethane Acrylate Stamp (PUA) Preparation
5. Catalytic Printing and SEM/AFM Analysis
6. Protein Patterning and Fluorescent Microscopy
7. Protein Patterning and Fluorescent Microscopy
8. Representative Results:
An example of soft-lithographic catalytic nano patterning is shown in Figure 7. The approach creates chemoselective patterns on oxide-free silicon and germanium, which can be orthogonally functionalized with dissimilar chemical and biological moieties. The reaction between the NHS-functioanlized substrate and the catalytic patterned stamp leads to the hydrolysis of NHS moieties in areas of conformal contact, yielding a patterned bifunctional substrate bearing regions of NHS activated and free carboxylic acids. Due to the diffusion free nature of our method, we achieve resolution close to that of photolithography. For example, Figure 7 shows 125 nm features, which were uniformly reproduced across the entire silicon substrate surface. Remarkably, the catalytic stamp can be reused multiple times without losing efficiency.
Chemoselective functionalization of patterned semiconductors with biomolecules opens up the prospect of integrating traditional electronic materials with highly selective biological substrates for applications in sensing, diagnostic, and analytical areas of research. An example of such functionalization is shown in Figure 8, where NHS-patterned silicon was selectively functionalized with protein molecules. By exploiting the differential reactivities of activated and free carboxylic acids, we first affixed nitrilotriacetic acid-terminated (NTA) heterobifunctional linkers to the NHS-functionalized regions, and then used the resulting NTA-patterned surface as a template for the selective attachment of hexa-histidine-tagged GFP. Figure 8b clearly shows differential fluorescence intensity between GFP-modified and hydrolyzed free carboxylic acid regions. The size and shape of the replicated features are consistent between both NHS patterned surface (Figure 8a) and GFP-modified surface (Figure 8b), confirming the remarkable stability of carbon-passivated surfaces and the selectivity of the stamping approach. The protocol is not limited to His-tagged protein, and can be used to pattern other biomolecules including DNA and antibodies.
Figure 1. General scheme representing catalytic microcontact printing
Figure 2. Structure of bi-layered molecular system on Ge and Si. Primary alkyl monolayer forms stable Ge-C or Si-C bonds with the substrate and provides a chemically inert and close packed system that protects the underlying surface from degradation.(b) Secondary overlayer forms stable C-C bonds with primary protective layer and provides terminal functional groups
Figure 3. Reaction schemes representing formation of primary protective monolayers on Si (A) and Ge (B)
Figure 4. Chemical functionalization of the primary protective monolayer with a heterobifunctional carbene donor
Figure 5. Reaction scheme demonstrating small molecule modifications of NHS-functionalized substrates and the corresponding XPS spectra
Figure 6. Composition of the catalytic pre-polymeric mixture, polymerization conditions, and SEM images of the patterned sulfonic acid-modified stamp and the corresponding PMMA-Si master
Figure 7. SEM and AFM friction images of patterned SAMs on Si and Ge with an acidic stamp
Figure 8. Soft-lithographic patterning and functionalization of passivated silicon with organic and biological molecules. a: SEM image of the patterned NHS-modified substrate. b: Fluorescent micrograph of GFP modified substrate.
The presented protocol is a form or of inkless microcontact printing that can be universally applied to any substrate capable of supporting simple well-ordered monolayers. In this method, a stamp-immobilized catalyst transfers a pattern to a surface bearing corresponding functional groups. Because the process does not rely on ink transfer from stamp to surface the diffusive resolution limitation of traditional and reactive μCP is obviated, permitting routine manufacturing of nanoscale objects. The incorporation of a primary highly-ordered molecular system provides complete protection of the underlying semiconductor from oxidation damage. At the same time, the method supports immobilization of bulky reactive groups by utilizing a secondary reactive overlayer; together the system achieves both protection and functionalization.
The technique begins with the formation of stable carbon-surface bonds allowing for chemically inert primary monolayer which serves as an effective barrier to oxide formation. Formation of a secondary reactive overlayer provides terminal NHS functional groups that serve as attachment points for a variety of chemical and biological moieties. This stable bilayered molecular system is subsequently patterned using our catalytic μCP approach. The approach presented in this study offers a general method for patterning semiconductor substrates with a broad range of organic and biological materials. The ability to create patterned organic-semiconductor interfaces without expensive, complex instrumentation offers numerous opportunities in fields such as electronics, nanotechnology, biochemistry and biophysics.
We have nothing to disclose
We acknowledge the financial support of the NSF award CMMI-1000724.
|XPS spectrometer||Kratos Analytical|
|Atomic force microscope||Veeco Instruments, Inc.|
|Fluorescent microscope||Carl Zeiss, Inc.|
|Vacuum pump||BOC Edwards|
|Water purification system||EMD Millipore|
|TESP silicon probes||Veeco Instruments, Inc.|
|UV Lamp||UVP Inc.|
|Stamp Material||See references 20 and 18|
|PFTE syringe filters||VWR international|
|Nano Strip||Cyantek Corporation|
|Propenyl Magnesium Chloride||Sigma-Aldrich|
|Octyl Magnesium Chloride||Sigma-Aldrich|
|Boc protected ethylenediamine||Sigma-Aldrich|
|4N HCl solution in dioxane||Sigma-Aldrich|