Scalable Fabrication of Quasi-One-Dimensional Gold Nanoribbons for Plasmonic Sensing.

Plasmonic nanostructures have a wide range of applications, including chemical and biological sensing. However, the development of techniques to fabricate submicrometer-sized plasmonic structures over large scales remains challenging. We demonstrate a high-throughput, cost-effective approach to fabricate Au nanoribbons via chemical lift-off lithography (CLL). Commercial HD-DVDs were used as large-area templates for CLL. Transparent glass slides were coated with Au/Ti films and functionalized with self-assembled alkanethiolate monolayers. Monolayers were patterned with lines via CLL. The lifted-off, exposed regions of underlying Au were selectively etched into large-area grating-like patterns (200 nm line width; 400 nm pitch; 60 nm height). After removal of the remaining monolayers, a thin In2O3 layer was deposited and the resulting gratings were used as plasmonic sensors. Distinct features in the extinction spectra varied in their responses to refractive index changes in the solution environment with a maximum bulk sensitivity of ∼510 nm/refractive index unit. Sensitivity to local refractive index changes in the near-field was also achieved, as evidenced by real-time tracking of lipid vesicle or protein adsorption. These findings show how CLL provides a simple and economical means to pattern large-area plasmonic nanostructures for applications in optoelectronics and sensing.

Large-Area, Ultrathin Metal-Oxide Semiconductor Nanoribbon Arrays Fabricated by Chemical Lift-Off Lithography.

Nanoribbon- and nanowire-based field-effect transistors (FETs) have attracted significant attention due to their high surface-to-volume ratios, which make them effective as chemical and biological sensors. However, the conventional nanofabrication of these devices is challenging and costly, posing a major barrier to widespread use. We report a high-throughput approach for producing arrays of ultrathin (∼3 nm) In2O3 nanoribbon FETs at the wafer scale. Uniform films of semiconducting In2O3 were prepared on Si/SiO2 surfaces via a sol-gel process prior to depositing Au/Ti metal layers. Commercially available high-definition digital versatile discs were employed as low-cost, large-area templates to prepare polymeric stamps for chemical lift-off lithography, which selectively removed molecules from self-assembled monolayers functionalizing the outermost Au surfaces. Nanoscale chemical patterns, consisting of one-dimensional lines (200 nm wide and 400 nm pitch) extending over centimeter length scales, were etched into the metal layers using the remaining monolayer regions as resists. Subsequent etch processes transferred the patterns into the underlying In2O3 films before the removal of the protective organic and metal coatings, revealing large-area nanoribbon arrays. We employed nanoribbons in semiconducting FET channels, achieving current on-to-off ratios over 107 and carrier mobilities up to 13.7 cm2 V-1 s-1. Nanofabricated structures, such as In2O3 nanoribbons and others, will be useful in nanoelectronics and biosensors. The technique demonstrated here will enable these applications and expand low-cost, large-area patterning strategies to enable a variety of materials and design geometries in nanoelectronics.

Self-Collapse Lithography.

We report a facile, high-throughput soft lithography process that utilizes nanoscale channels formed naturally at the edges of microscale relief features on soft, elastomeric stamps. Upon contact with self-assembled monolayer (SAM) functionalized substrates, the roof of the stamp collapses, resulting in the selective removal of SAM molecules via a chemical lift-off process. With this technique, which we call self-collapse lithography (SCL), sub-30 nm patterns were achieved readily using masters with microscale features prepared by conventional photolithography. The feature sizes of the chemical patterns can be varied continuously from ∼2 µm to below 30 nm by decreasing stamp relief heights from 1 µm to 50 nm. Likewise, for fixed relief heights, reducing the stamp Young's modulus from ∼2.0 to ∼0.8 MPa resulted in shrinking the features of resulting patterns from ∼400 to ∼100 nm. The self-collapse mechanism was studied using finite element simulation methods to model the competition between adhesion and restoring stresses during patterning. These results correlate well with the experimental data and reveal the relationship between the line widths, channel heights, and Young's moduli of the stamps. In addition, SCL was applied to pattern two-dimensional arrays of circles and squares. These chemical patterns served as resists during etching processes to transfer patterns to the underlying materials (e.g., gold nanostructures). This work provides new insights into the natural propensity of elastomeric stamps to self-collapse and demonstrates a means of exploiting this behavior to achieve patterning via nanoscale chemical lift-off lithography.

Gap-directed chemical lift-off lithographic nanoarchitectonics for arbitrary sub-micrometer patterning.

We introduce a unique soft lithographic operation that exploits stamp roof collapse-induced gaps to selectively remove an alkanethiol self-assembled monolayer (SAM) on Au to generate surface patterns that are orders of magnitude smaller than structures on the original elastomer stamp. The smallest achieved feature dimension is 5 nm using a micrometer-scale structured stamp in a chemical lift-off lithography (CLL) process. Molecular patterns retained in the gaps between stamp features and their circumscribed or inscribed circles follow mathematical predictions, and their sizes can be tuned by altering the stamp structure dimensions, including height, pitch, and shape. These generated surface molecular patterns can function as biorecognition arrays or be transferred to the underneath Au layer for metallic structure creation. By combining CLL process with this gap phenomenon, soft material properties that are previously thought as demerits can be used to achieve sub-10 nm features in a straightforward sketch.

Narrower Nanoribbon Biosensors Fabricated by Chemical Lift-off Lithography Show Higher Sensitivity.

Wafer-scale nanoribbon field-effect transistor (FET) biosensors fabricated by straightforward top-down processes are demonstrated as sensing platforms with high sensitivity to a broad range of biological targets. Nanoribbons with 350 nm widths (700 nm pitch) were patterned by chemical lift-off lithography using high-throughput, low-cost commercial digital versatile disks (DVDs) as masters. Lift-off lithography was also used to pattern ribbons with 2 µm or 20 µm widths (4 or 40 µm pitches, respectively) using masters fabricated by photolithography. For all widths, highly aligned, quasi-one-dimensional (1D) ribbon arrays were produced over centimeter length scales by sputtering to deposit 20 nm thin-film In2O3 as the semiconductor. Compared to 20 µm wide microribbons, FET sensors with 350 nm wide nanoribbons showed higher sensitivity to pH over a broad range (pH 5 to 10). Nanoribbon FETs functionalized with a serotonin-specific aptamer demonstrated larger responses to equimolar serotonin in high ionic strength buffer than those of microribbon FETs. Field-effect transistors with 350 nm wide nanoribbons functionalized with single-stranded DNA showed greater sensitivity to detecting complementary DNA hybridization vs 20 µm microribbon FETs. In all, we illustrate facile fabrication and use of large-area, uniform In2O3 nanoribbon FETs for ion, small-molecule, and oligonucleotide detection where higher surface-to-volume ratios translate to better detection sensitivities.

Wafer-scale bioactive substrate patterning by chemical lift-off lithography.

The creation of bioactive substrates requires an appropriate interface molecular environment control and adequate biological species recognition with minimum nonspecific attachment. Herein, a straightforward approach utilizing chemical lift-off lithography to create a diluted self-assembled monolayer matrix for anchoring diverse biological probes is introduced. The strategy encompasses convenient operation, well-tunable pattern feature and size, large-area fabrication, high resolution and fidelity control, and the ability to functionalize versatile bioarrays. With the interface-contact-induced reaction, a preformed alkanethiol self-assembled monolayer on a Au surface is ruptured and a unique defect-rich diluted matrix is created. This post lift-off region is found to be suitable for insertion of a variety of biological probes, which allows for the creation of different types of bioactive substrates. Depending on the modifications to the experimental conditions, the processes of direct probe insertion, molecular structure change-required recognition, and bulky biological species binding are all accomplished with minimum nonspecific adhesion. Furthermore, multiplexed arrays via the integration of microfluidics are also achieved, which enables diverse applications of as-prepared substrates. By embracing the properties of well-tunable pattern feature dimension and geometry, great local molecular environment control, and wafer-scale fabrication characteristics, this chemical lift-off process has advanced conventional bioactive substrate fabrication into a more convenient route.

Large Area Nanoparticle Alignment by Chemical Lift-Off Lithography.

Nanoparticle alignment on the substrate attracts considerable attention due to its wide application in different fields, such as mechanical control, small size electronics, bio/chemical sensing, molecular manipulation, and energy harvesting. However, precise nanoparticle positioning and deposition control with high fidelity are still challenging. Herein, a straightforward strategy for high quality nanoparticle-alignment by chemical lift-off lithography (CLL) is demonstrated. This technique creates high resolution self-assembled monolayer (SAM) chemical patterns on gold substrates, enabling nanoparticle-selective deposition and precise alignment. The fabricated nanoparticle arrangement geometries and dimensions are well-controllable in a large area. With proper nanoparticle surface functionality control and adequate substrate molecular manipulation, well-defined nanoparticle arrays with single-particle-wide alignment resolution are achieved.

Finely Tunable Surface Wettability by Two-Dimensional Molecular Manipulation.

Local molecular environment governs material interface properties, especially the substrate's exposing behavior and overall functionality expression. Although current techniques can provide efficient surface property modification, challenges in molecule spatial distribution and composition controls limited the generation of homogeneous and finely tunable molecular environment. In this study, Au-thiolate rupturing operation in chemical lift-off lithography (CLL) is used to manipulate the substrate interface molecular environment. The creation of randomly distributed artificial self-assembled monolayer defects generates vacancies for substrate property modification through back-insertion of molecules with opposite functionalities. Surface wettability adjustment is utilized as an example, where well-controllable molecule distribution provides finely tunable substrate affinity toward liquids with different physical properties. The distinct property difference between two surface regions assists microdroplet formation when liquids flow through, not only water solution but also low-surface-tension organic liquids. These microdroplet arrays become a template to guide material assembly in its formation process and act as pH-sensitive platforms for high-throughput detection. Furthermore, the tunability of the molecular pattern in this approach helps minimize the coffee-ring effect and the sweet-spot issue in matrix-assisted laser desorption/ionization mass spectrometry. Two-dimensional molecular manipulation in the CLL operation, therefore, holds the capability toward controlling homogeneous material surface property and toward exhibiting behavior adjustments.

Analyzing Spin Selectivity in DNA-Mediated Charge Transfer via Fluorescence Microscopy.

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems.

Controlled DNA Patterning by Chemical Lift-Off Lithography: Matrix Matters.

Nucleotide arrays require controlled surface densities and minimal nucleotide-substrate interactions to enable highly specific and efficient recognition by corresponding targets. We investigated chemical lift-off lithography with hydroxyl- and oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers as a means to produce substrates optimized for tethered DNA insertion into post-lift-off regions. Residual alkanethiols in the patterned regions after lift-off lithography enabled the formation of patterned DNA monolayers that favored hybridization with target DNA. Nucleotide densities were tunable by altering surface chemistries and alkanethiol ratios prior to lift-off. Lithography-induced conformational changes in oligo(ethylene glycol)-terminated monolayers hindered nucleotide insertion but could be used to advantage via mixed monolayers or double-lift-off lithography. Compared to thiolated DNA self-assembly alone or with alkanethiol backfilling, preparation of functional nucleotide arrays by chemical lift-off lithography enables superior hybridization efficiency and tunability.

Fabrication of High-Performance Ultrathin In2O3 Film Field-Effect Transistors and Biosensors Using Chemical Lift-Off Lithography.

We demonstrate straightforward fabrication of highly sensitive biosensor arrays based on field-effect transistors, using an efficient high-throughput, large-area patterning process. Chemical lift-off lithography is used to construct field-effect transistor arrays with high spatial precision suitable for the fabrication of both micrometer- and nanometer-scale devices. Sol-gel processing is used to deposit ultrathin (∼4 nm) In2O3 films as semiconducting channel layers. The aqueous sol-gel process produces uniform In2O3 coatings with thicknesses of a few nanometers over large areas through simple spin-coating, and only low-temperature thermal annealing of the coatings is required. The ultrathin In2O3 enables construction of highly sensitive and selective biosensors through immobilization of specific aptamers to the channel surface; the ability to detect subnanomolar concentrations of dopamine is demonstrated.