Light-Mediated Contact Printing of Phosphorus Species onto Silicon Using Carbene-Based Molecular Layers.

The ability to deposit pattern-specific molecular layers onto silicon with either regional p-/n-doping properties or that act as chemoselective resists for area-selective deposition is highly sought after in the bottom-up manufacturing of microelectronics. In this study, we demonstrate a simple protocol for the covalent attachment and patterning of a phosphorus-based dopant precursor onto silicon(100) functionalized with reactive carbene species. This method relies on selective surface reactions, which provide terminal functionalities that can be photochemically modified via ultraviolet-assisted contact printing between the carbene-functionalized substrate and an elastomeric stamp inked with the inorganic dopant precursor. X-ray photoelectron spectroscopy (XPS) analysis combined with scanning electron microscopy (SEM) imaging was used to characterize the molecule attachment and patterning ability of this technique. XPS spectra are indicative of the covalent bonding between phosphorus-containing molecules and the functionalized surface after both bulk solution-phase reaction and photochemical printing. SEM analysis of the corresponding printed features demonstrates the effective transfer of the phosphorus species in a patterned orientation matching that of the stamp pattern. This simple approach to patterning dopant precursors has the potential to inform the continued refinement of thin-film electronic, photonic, and quantum device manufacturing.

Vapor-Phase Halogenation of Hydrogen-Terminated Silicon(100) Using N-Halogen-succinimides.

The focus of this study was to demonstrate the vapor-phase halogenation of Si(100) and subsequently evaluate the inhibiting ability of the halogenated surfaces toward atomic layer deposition (ALD) of aluminum oxide (Al2O3). Hydrogen-terminated silicon ⟨100⟩ (H-Si(100)) was halogenated using N-chlorosuccinimide (NCS), N-bromosuccinimide (NBS), and N-iodosuccinimide (NIS) in a vacuum-based chemical process. The composition and physical properties of the prepared monolayers were analyzed by using X-ray photoelectron spectroscopy (XPS) and contact angle (CA) goniometry. These measurements confirmed that all three reagents were more effective in halogenating H-Si(100) over OH-Si(100) in the vapor phase. The stability of the modified surfaces in air was also tested, with the chlorinated surface showing the greatest resistance to monolayer degradation and silicon oxide (SiO2) generation within the first 24 h of exposure to air. XPS and atomic force microscopy (AFM) measurements showed that the succinimide-derived Hal-Si(100) surfaces exhibited blocking ability superior to that of H-Si(100), a commonly used ALD resist. This halogenation method provides a dry chemistry alternative for creating halogen-based ALD resists on Si(100) in near-ambient environments.

Monolayer organic thin films as particle-contamination-resistant coatings.

Three organic monolayers coatings were developed and tested for their effectiveness to increase cleaning efficiency of attached microscale particles by air flows. The experiments were performed using silica substrates coated with these organic thin films and subsequently exposed to stainless-steel and silica microparticles as a model of contamination. Laser-induced-damage tests confirmed that the coatings do not affect the laser-induced-damage threshold values. The particle exposure results suggest that although the accumulation of particles is not significantly affected under the experimental conditions used in this work, the coated substrates exhibit significantly improved cleaning efficiency with a gas flow. A size-distribution analysis was conducted to study the adsorption and cleaning efficiency of particles of different sizes. It was observed that larger size (> 5-µm) particles can be removed from coated substrates with almost 100% efficiency. It was also determined that the coatings improve the cleaning efficiency of the smaller particles (≤ 5 µm) by 17% to 30% for the stainless steel metal and 19% to 38% for the silica particles.

Manufacturing-induced contamination in common multilayerdielectric gratings.

Contamination of pulse compression gratings during the manufacturing process is known to give rise to reduced laser damage performance and represents an issue that has not yet been adequately resolved. The present work demonstrates that the currently used etching methods introduce carbon contamination inside the etched region extending to a 50- to 80-nm layer below the surface. This study was executed using custom samples prepared in both, a laboratory setting and by established commercial vendors, showing results that are very similar. The laser-induced-damage performance of the etched and unetched regions in the grating-like samples suggest that contaminants introduced by etching process are contributing to the reduction of the laser-induced damage threshold.

Modulation of Interfacial Adhesion Using Semicrystalline Shape-Memory Polymers.

Semicrystalline shape-memory elastomers are molded into deformable geometrical features to control adhesive interactions between elastomers and a glass substrate. By mechanically and thermally controlling the deformation and phase-behavior of molded features, we can control the interfacial contact area and the interfacial adhesive force. Results indicate that elastic energy is stored in the semicrystalline state of deformed features and can be released to break attractive interfacial forces, automatically separating the glass substrate from the elastomer. Our findings suggest that the shape-memory elastomers can be applied in various contact printing applications to control adhesive forces and delamination mechanics during ink pickup and transfer.

Contact Printing of Multilayered Thin Films with Shape Memory Polymers.

This study describes a method for transfer printing microarrays of multilayered organic-inorganic thin films using shape memory printing stamps and microstructured donor substrates. By applying the films on the microstructured donor substrates during physical vapor deposition and modulating the interfacial adhesion using a shape memory elastomer during printing, this method achieves (1) high lateral and feature-edge resolution and (2) high transfer efficiency from the donor to the receiver substrate. For demonstration, polyurethane-acrylate stamps and silicon/silicon oxide donor substrates were used in the large-area transfer printing of organic-inorganic thin-film stacks with micrometer lateral dimensions and sub-200 nm thickness.

Label-Free Immunoassay Using Droplet-Based Brewster's Angle Straddle Interferometry.

We report the detection of antigen capture by immobilized antibodies using a simple, label-free version of monochromatic reflective interferometry. The technique is implemented on silicon with its native oxide and relies on choosing an incident angle between the Brewster angles for the air/oxide and oxide/silicon interfaces. We demonstrate sensitivity to anti-human and anti-rabbit immunoglobulin (anti-IgG) concentrations less than 100 nM using only 10 nL droplets of the analyte. We have introduced a protocol using a model sugar to reduce nonspecific binding and have been able to detect anti-IgG even in the presence of 100-fold larger concentrations of bovine serum albumin. The limit of detection is not yet associated with the optical method but is imposed by nonspecific binding. Evaluated in terms of pg/mm2, our sensors are comparable in sensitivity to surface plasmon resonance (SPR) but are advantaged with respect to SPR in the tolerance of the optical components and alignment, the low material usage, and the ability to exploit multiplex detection without modification. The simplicity and convenience of the method are promising for eventual application to portable diagnostic applications.

Second Generation Nanoporous Silicon Nitride Membranes for High Toxin Clearance and Small Format Hemodialysis.

Conventional hemodialysis (HD) uses floor-standing instruments and bulky dialysis cartridges containing ≈2 m2 of 10 micrometer thick, tortuous-path membranes. Portable and wearable HD systems can improve outcomes for patients with end-stage renal disease by facilitating more frequent, longer dialysis at home, providing more physiological toxin clearance. Developing devices with these benefits requires highly efficient membranes to clear clinically relevant toxins in small formats. Here, the ability of ultrathin (<100 nm) silicon-nitride-based membranes to reduce the membrane area required to clear toxins by orders of magnitude is shown. Advanced fabrication methods are introduced that produce nanoporous silicon nitride membranes (NPN-O) that are two times stronger than the original nanoporous nitride materials (NPN) and feature pore sizes appropriate for middle-weight serum toxin removal. Single-pass benchtop studies with NPN-O (1.4 mm2 ) demonstrate the extraordinary clearance potential of these membranes (105 mL min-1 m-2 ), and their intrinsic hemocompatibility. Results of benchtop studies with nanomembranes, and 4 h dialysis of uremic rats, indicate that NPN-O can reduce the membrane area required for hemodialysis by two orders of magnitude, suggesting the performance and robustness needed to enable small-format hemodialysis, a milestone in the development of small-format hemodialysis systems.

Modification of Nanoporous Silicon Nitride with Stable and Functional Organic Monolayers.

This study describes the formation of functional organic monolayers on thin, nanoporous silicon nitride membranes. We demonstrate that the vapor-phase carbene insertion into the surface C-H bonds can be used to form sub-5 nm molecular coatings on nanoporous materials, which can be further modified with monolayers of polyethylene glycol (PEG) molecules. We investigate composition, thickness, and stability of the functionalized monolayers and the changes in the membrane permeability and pore size distribution. We show that, due to the low coating thickness (~7 nm), the functionalized membrane retains 80% of the original gas permeance and 40% of the original hydraulic permeability. We also show that the carbene/PEG functionalization is hydrolytically stable for up to 48 h of exposure to water and that it can suppress nonspecific adsorption of the proteins BSA and IgG. Our results suggest that the vapor-phase carbenylation can be used as a complementary technology to the traditional self-assembly and polymer brush chemistries in chemical functionalization of nanoporous materials, which are limited in their ability to serve as stable coatings that do not occlude nanomembrane pores.

Vapor-Phase Carbenylation of Hard and Soft Material Interfaces.

This study describes the formation of functional organic monolayers on hard and soft interfaces via a vapor-phase carbene insertion into Si-H and C-H bonds. We demonstrate that functional diazirine molecules can be used to form monomolecular coatings on silicon, silicon nitride, and urethane-acrylate polymers under mild vacuum conditions and exposure to UV light. We investigate the molecular coverage and the long-term stability of the resulting monolayers in air, isopropanol, and water. Our results suggest that vapor-phase carbenylation can be used as a complementary technology to the traditional self-assembly, permitting functionalization of various passivated substrates with stable and functional molecular coatings under mild and scalable conditions.

High-Resolution Organic Light-Emitting Diodes Patterned via Contact Printing.

In this study, we report a contact printing technique that uses polyurethane-acrylate (PUA) polymers as the printing stamps to pattern electroluminescent layers of organic light emitting diodes (OLEDs). We demonstrate that electroluminescent thin films can be printed with high uniformity and resolution. We also show that the performance of the printed devices can be improved via postprinting thermal annealing, and that the external quantum efficiency of the printed devices is comparable with the efficiency of the vacuum-deposited OLEDs. Our results suggest that the PUA-based contact printing can be used as an alternative to the traditional shadow mask deposition, permitting manufacturing of OLED displays with the resolution up to the diffraction limit of visible-light emission.

Ultrathin silicon membranes for wearable dialysis.

The development of wearable or implantable technologies that replace center-based hemodialysis (HD) hold promise to improve outcomes and quality of life for patients with ESRD. A prerequisite for these technologies is the development of highly efficient membranes that can achieve high toxin clearance in small-device formats. Here we examine the application of the porous nanocrystalline silicon (pnc-Si) to HD. pnc-Si is a molecularly thin nanoporous membrane material that is orders of magnitude more permeable than conventional HD membranes. Material developments have allowed us to dramatically increase the amount of active membrane available for dialysis on pnc-Si chips. By controlling pore sizes during manufacturing, pnc-Si membranes can be engineered to pass middle-molecular-weight protein toxins while retaining albumin, mimicking the healthy kidney. A microfluidic dialysis device developed with pnc-Si achieves urea clearance rates that confirm that the membrane offers no resistance to urea passage. Finally, surface modifications with thin hydrophilic coatings are shown to block cell and protein adhesion.

Multicomponent inorganic Janus particles with controlled compositions, morphologies, and dimensions.

We report a new protocol for the preparation of shape-controlled multicomponent particles comprising metallic (Au and Ti), magnetic (Ni), and oxide (SiO2, TiO2) layers. Our method allows for a precise control over the composition, shape, and size and permits fabrication of nonsymmetrical particles, whose opposite sides can be orthogonally functionalized using well-established organosilanes and thiol chemistries. Because of their unique geometries and surface chemistries, these colloids represent ideal materials with which to study nonsymmetrical self-assembly at the meso- and microscales.

Multicomponent patterning of indium tin oxide.

We report a versatile functionalization and pattering technique that permits multicomponent pattern-specific modification of indium tin oxide (ITO) with organic species. The method relies on a bilayered molecular system that simultaneously protects ITO from degradation and provides uniform chemical functionality suitable for further elaboration. Pattern-specific modification is achieved via specific reaction between functionality on an elastomeric stamp and functionality of cognate reactivity at the surface of a bilayered molecular substrate. We demonstrate that a single molecular system in a combination with different printing approaches can be used to immobilize multiple organic functionalities with exquisite spatial control on a single ITO surface. Our study provides the first general approach that permits patterning and functionalization of ITO with different molecules using a single set of printing conditions and materials.

A general and efficient cantilever functionalization technique for AFM molecular recognition studies.

Atomic force microscopy (AFM) is a versatile technique for the investigation of noncovalent molecular associations between ligand-substrate pairs. Surface modification of silicon nitride AFM cantilevers is most commonly achieved using organic trialkoxysilanes. However, susceptibility of the Si−O bond to hydrolysis and formation of polymeric aggregates diminishes attractiveness of this method for AFM studies. Attachment techniques that facilitate immobilization of a wide variety of organic and biological molecules via the stable Si−C bond on silicon nitride cantilevers would be of great value to the field of molecular recognition force spectroscopy. Here, we report (1) the formation of stable, highly oriented monolayers on the tip of silicon nitride cantilevers and (2) demonstrate their utility in the investigation of noncovalent protein-ligand interactions using molecular recognition force spectroscopy. The monolayers are formed through hydrosilylation of hydrogen-terminated silicon nitride AFM probes using a protected α-amino-ω-alkene. This approach facilitates the subsequent conjugation of biomolecules. The resulting biomolecules are bound to the tip by a strong Si−C bond, completely uniform with regard to both epitope density and substrate orientation, and highly suitable for force microscopy studies. We show that this attachment technique can be used to measure the unbinding profiles of tip-immobilized lactose and surface-immobilized galectin-3. Overall, the proposed technique is general, operationally simple, and can be expanded to anchor a wide variety of epitopes to a silicon nitride cantilever using a stable Si−C bond.

Soft-lithographic approach to functionalization and nanopatterning oxide-free silicon.

We report a simple, reliable high-throughput method for patterning passivated silicon with reactive organic monolayers and demonstrate selective functionalization of the patterned substrates with both small molecules and proteins. The approach completely protects silicon from chemical oxidation, provides precise control over the shape and size of the patterned features in the 100 nm domain, and gives rapid, ready access to chemically discriminated patterns that can be further functionalized with both organic and biological molecules.

Patterning NHS-terminated SAMs on germanium.

Here we report a simple, robust approach to patterning functional SAMs on germanium. The protocol relies on catalytic soft-lithographic pattern transfer from an elastomeric stamp bearing pendant immobilized sulfonic acid moieties to an NHS-functionalized bilayer molecular system comprising a primary ordered alkyl monolayer and a reactive ester secondary overlayer. The catalytic polyurethane-acrylate stamp was used to form micrometer-scale features of chemically distinct SAMs on germanium. The methodology represents the first example of patterned SAMs on germanium, a semiconductor material.

Soft lithographic functionalization and patterning oxide-free silicon and germanium.

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. 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. 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. 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.

Inkless microcontact printing on SAMs of Boc- and TBS-protected thiols.

We report a new inkless catalytic muCP technique that achieves accurate, fast, and complete pattern reproduction on SAMs of Boc- and TBS-protected thiols immobilized on gold using a polyurethane-acrylate stamp functionalized with covalently bound sulfonic acids. Pattern transfer is complete at room temperature just after one minute of contact and renders sub-200 nm size structures of chemically differentiated SAMs.

Catalytic microcontact printing on chemically functionalized H-terminated silicon.

We report a novel inkless soft lithographic fabrication protocol that permits uniform parallel patterning of hydrogen-terminated silicon surfaces using catalytic elastomeric stamps. Pattern transfer is achieved catalytically via reaction between sulfonic acid moieties covalently bound to an elastomeric stamp and a Boc-functionalized SAM grafted to passivated silicon. The approach represents the first example of a soft lithographic printing technique that creates patterns of chemically distinctive SAMs on oxide-free silicon substrates.