Liquid-Phase Beam Pen Lithography

Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s-1

We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20-50 nm are achieved repeatably and controllably at writing rates up to 1 mm s^-1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π-π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N-N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size.

Plasma-Assisted Microcontact Printing

Microcontact printing, polymer pen lithography, and their variations have attracted interests from a broad spectrum of research fields as a result of the feasibility of defining patterns in micro- and nanoscales. In this work, we have proposed and demonstrated a novel lithography method, named plasma-assisted microcontact printing (PA-μCP). Unlike the previous printing methods, where a direct contact is normally required for the transport of liquid-phase inks, plasma-deposited fluorocarbon (FC) has been employed in PA-μCP as the ink material, which can be transferred from the stamp to substrates through a thermal evaporation process. The geometry of the patterns can be modified by adjusting the design of stamp patterns and the contact time, and transferred FC patterns can be used directly as an etch mask to create microstructures in the substrate materials. We have demonstrated the possibility of performing multi-patterning with PA-μCP, where FC patterns can be generated conformally on structured substrates. Because the height of FC patterns is closely related to the local pattern designs, PA-μCP can be used for grayscale patterning. As a proof of concept, Fabry-Perot planar cavities are fabricated with grayscale PA-μCP for structure color printing. We believe PA-μCP is distinguished from conventional techniques by its printing mechanism, which can pave the way for convenient fabrication of photonic, electronic, and biological devices.

Photopolymerized Features via Beam Pen Lithography as a Novel Tool for the Generation of Large Area Protein Micropatterns

A cantilever-free scanning probe lithography (CF-SPL)-based method for the rapid polymerization of nanoscale features on a surface via crosslinking and thiol-acrylate photoreactions is described, wherein the nanoscale position, height, and diameter of each feature can be finely and independently tuned. With precise spatiotemporal control over the illumination pattern, beam pen lithography (BPL) allows for the photo-crosslinking of polymers into ultrahigh resolution features over centimeter-scale areas using massively parallel >160 000 pen arrays of individually addressable pens that guide and focus light onto the surface with sub-diffraction resolution. The photoinduced crosslinking reaction of the ink material, which is composed of photoinitiator, diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide, poly(ethylene glycol) diacrylate, and thiol-modified functional binding molecules (i.e., thiol-PEG-biotin or 16-mercaptohexanoic acid), proceeds to ≈80% conversion with UV exposure (72 mW cm^-2 ) for short time periods (0.5 s). Such polymer patterns are further reacted with proteins (streptavidin and fibronectin) to yield protein arrays with feature arrangements at high resolution and densities controlled by local UV exposure. This platform, which combines polymer photochemistry and massive arrays of scanning probes, constitutes a new approach to making biomolecular microarrays in a high-throughput and high-yielding manner, opening new routes for biochip synthesis, bioscreening, and cell biology research.

"Writing biochips": high-resolution droplet-to-droplet manufacturing of analytical platforms

The development of high-resolution molecular printing allows the engineering of analytical platforms enabling applications at the interface between chemistry and biology, i.e. in biosensing, electronics, single-cell biology, and point-of-care diagnostics. Their successful implementation stems from the combination of large area printing at resolutions from sub-100 nm up to macroscale, whilst controlling the composition and volume of the ink, and reconfiguring the deposition features in due course. Similar to handwriting pens, the engineering of continuous writing systems tackles the issue of the tedious ink replenishment between different printing steps. To this aim, this review article provides an unprecedented analysis of the latest continuous printing methods for bioanalytical chemistry, focusing on ink deposition systems based on specific sets of technologies that have been developed to this aim, namely nanofountain probes, microcantilever spotting, capillary-based polymer pens and continuous 3D printing. Each approach will be discussed revealing the most important applications in the fields of biosensors, lab-on-chips and diagnostics.

Evolution and applications of polymer brush hypersurface photolithography

Hypersurface photolithography creates arbitrary polymer brush patterns with independent control over feature diameter, height, and spacing between features, while controlling composition along a polymer chain and between features.

Recent progress in creating complex and multiplexed surface-grafted macromolecular architectures

Surface-grafted macromolecules, including polymers, DNA, peptides, etc., are versatile modifications to tailor the interfacial functions in a wide range of fields. In this review, we aim to provide an overview of the most recent progress in engineering surface-grafted chains for the creation of complex and multiplexed surface architectures over micro- to macro-scopic areas. A brief introduction to surface grafting is given first. Then the fabrication of complex surface architectures is summarized with a focus on controlled chain conformations, grafting densities and three-dimensional structures. Furthermore, recent advances are highlighted for the generation of multiplexed arrays with designed chemical composition in both horizontal and vertical dimensions. The applications of such complicated macromolecular architectures are then briefly discussed. Finally, some perspective outlooks for future studies and challenges are suggested. We hope that this review will be helpful to those just entering this field and those in the field requiring quick access to useful reference information about the progress in the properties, processing, performance, and applications of functional surface-grafted architectures.

Nanostructured High-Performance Thin-Film Transistors and Phototransistors Fabricated by a High-Yield and Versatile Near-Field Nanolithography Strategy

Thin-film transistors (TFTs) and field-effect transistors (FETs) are basic units to build functional electronic circuits and investigate transport physics. In conventional TFTs or FETs, performance in terms of current level, on-off ratio, and the sensitivity of detection is limited by homogeneous semiconducting layers. In this paper, we develop TFTs with submicron heterostructures by using a strategy based on near-field photolithography. We use an array of total-reflective polydimethylsiloxane pyramids or trenches as a soft photomask in photolithography to induce multiple reflections and diffractions to focus the light. The textured feature enables the generation of gaps, dots, and grids at the nanoscale, with dimensions as small as sub-100 nm on substrates at the centimeter scale. We demonstrated the very high performance oxide TFTs on the nanoscale and periodic degenerately doped heterojunctions, and they yielded a nearly 20-fold increase in transconductance and apparent device mobility. The on-off ratio was higher than 10⁹, with notably enhanced output current and clear scaling effect with channel length. We also built nanostructured wide-gap/narrow-gap heterojunctions to balance the high on-off ratio and sensitive photoresponse in a unidirectional phototransistor. This study shows the viability of programming a variety of nanoscale submicron patterns or interfaces in TFTs and FETs to significantly enlarge the scope of research on multifunctional TFTs and FETs.

Nanocombinatorics with Cantilever-Free Scanning Probe Arrays

The effectiveness of combinatorial experiments is determined by the rate at which distinct experimental conditions can be prepared and interrogated. This has been particularly limiting at the intersection of nanotechnology and soft materials research, where structures are difficult to reliably prepare and materials are incompatible with conventional lithographic techniques. For example, studying nanoparticle-based heterogeneous catalysis or the interaction between biological cells and abiotic surfaces requires precise tuning of materials composition on the nanometer scale. Scanning probe techniques are poised to be major players in the combinatorial nanoscience arena because they allow one to directly deposit materials at high resolution without any harsh processing steps that limit material compatibility. The chief limitation of scanning probe techniques is throughput, as patterning with single probes is prohibitively slow in the context of large-scale combinatorial experiments. A recent paradigm shift circumvents this problem by fundamentally altering the architecture of scanning probes by replacing the conventionally used cantilever with a soft compliant film on a rigid substrate, a substitution that allows a densely packed array of probes to function in parallel in an inexpensive format. This is a major lithographic advance in terms of scalability, throughput, and versatility that, when combined with the development of approaches to actuate individual probes in cantilever-free arrays, sets the stage for scanning-probe-based tools to address scientific questions through nanocombinatorial studies in biology and materials science. In this review, we outline the development of cantilever-free scanning probe lithography and prospects for nanocombinatorial studies enabled by these tools.

Towards scanning probe lithography-based 4D nanoprinting by advancing surface chemistry, nanopatterning strategies, and characterization protocols

Biointerfaces direct some of the most complex biological events, including cell differentiation, hierarchical organization, and disease progression, or are responsible for the remarkable optical, electronic, and biological behavior of natural materials. Chemical information encoded within the 4D nanostructure of biointerfaces - comprised of the three Cartesian coordinates (x, y, z), and chemical composition of each molecule within a given volume - dominates their interfacial properties. As such, there is a strong interest in creating printing platforms that can emulate the 4D nanostructure - including both the chemical composition and architectural complexity - of biointerfaces. Current nanolithography technologies are unable to recreate 4D nanostructures with the chemical or architectural complexity of their biological counterparts because of their inability to position organic molecules in three dimensions and with sub-1 micrometer resolution. Achieving this level of control over the interfacial structure requires transformational advances in three complementary research disciplines: (1) the scope of organic reactions that can be successfully carried out on surfaces must be increased, (2) lithography tools are needed that are capable of positioning soft organic and biologically active materials with sub-1 micrometer resolution over feature diameter, feature-to-feature spacing, and height, and (3) new techniques for characterizing the 4D structure of interfaces should be developed and validated. This review will discuss recent advances in these three areas, and how their convergence is leading to a revolution in 4D nanomanufacturing.

And There Was Light: Prospects for the Creation of Micro- and Nanostructures through Maskless Photolithography

In photolithographic processes, the light inducing the photochemical reactions is confined to a small volume, which enables direct writing of micro- and nanoscale features onto solid surfaces without the need of a predefined photomask. The direct writing process can be used to generate topographic patterns through photopolymerization or photo-cross-linking or can be employed to use light to generate chemical patterns on the surface with high spatial control, which would make such processes attractive for bioapplications. The prospects of maskless photolithography technologies with a focus on two-photon lithography and scanning-probe-based photochemical processes based on scanning near-field optical microscopy or beam pen lithography are discussed.

Solution-Phase Photochemical Nanopatterning Enabled by High-Refractive-Index Beam Pen Arrays

A high-throughput, solution-based, scanning-probe photochemical nanopatterning approach, which does not require the use of probes with subwavelength apertures, is reported. Specifically, pyramid arrays made from high-refractive-index polymeric materials were constructed and studied as patterning tools in a conventional liquid-phase beam pen lithography experiment. Two versions of the arrays were explored with either metal-coated or metal-free tips. Importantly, light can be channeled through both types of tips and the appropriate solution phase (e.g., H₂O or CH₃OH) and focused on subwavelength regions of a substrate to effect a photoreaction in solution that results in localized patterning of a self-assembled monolayer (SAM)-coated Au thin film substrate. Arrays with as many as 4500 pyramid-shaped probes were used to simultaneously initiate thousands of localized free-radical photoreactions (decomposition of a lithium acylphosphinate photoinitiator in an aqueous solution) that result in oxidative removal of the SAM. The technique is attractive since it allows one to rapidly generate features less than 200 nm in diameter, and the metal-free tips afford more than 10-fold higher intensity than the tips with nanoapertures over a micrometer propagation length. In principle, this mask-free method can be utilized as a versatile tool for performing a wide variety of photochemistries across multiple scales that may be important in high-throughput combinatorial screening applications related to chemistry, biology, and materials science.

Tip-Based Nanofabrication for Scalable Manufacturing

Tip-based nanofabrication (TBN) is a family of emerging nanofabrication techniques that use a nanometer scale tip to fabricate nanostructures. In this review, we first introduce the history of the TBN and the technology development. We then briefly review various TBN techniques that use different physical or chemical mechanisms to fabricate features and discuss some of the state-of-the-art techniques. Subsequently, we focus on those TBN methods that have demonstrated potential to scale up the manufacturing throughput. Finally, we discuss several research directions that are essential for making TBN a scalable nano-manufacturing technology.

Optimization of 4D polymer printing within a massively parallel flow-through photochemical microreactor

Brush polymer patterns, where the position (x,y), height (z), and chemical composition of each feature in an array were controlled independently, were prepared by combining massively parallel tip-based photolithography, microfluidics, and photochemical radical polymerizations.