Colloidal Lithography Research Articles

Large-Scale Formation of a Close-Packed Monolayer of Spheres Using Different Colloidal Lithography Techniques.

The possibility of using colloidal lithography at the industrial level depends on the ability to form defect-free coatings over large areas. The spin-coating method has not yet shown acceptable results, but a more detailed studying of the regularities of this process may improve the quality of masks. The Langmuir-Blodgett method is expected to be the most preferable for forming high-quality large-scale monolayers. Real-time controlling the surface pressure of the monolayer can allow to obtain close-packed arrays with long-range order. In this work, to develop the spin-coating technology, the influence of technological parameters (spin-coating speed and time, concentrations of components in suspension) on the substrate coverage area with a monolayer of polystyrene spheres (1.25 μm) was studied. An original automated Langmuir-Blodgett system was developed to study the influence of the monolayer surface pressure on its quality using polystyrene spheres (1.25, 1.8, 2.1 μm). The developed spin-coating technology resulted in a record coverage area (90%) of Si substrate (76 mm) and a defect-free hexagonally ordered domain area of 500 μm2. As a result of the developed Langmuir-Blodgett technique, a close-packed monolayer coating was obtained over the entire substrate area (coverage area 99.5%, defect-free domain area 3000 μm2) without the use of any surfactants.

Ultrafast Metaphotonic PCR Chip with Near-Perfect Absorber.

Polymerase chain reaction (PCR) is the gold standard for nucleic acid amplification and quantification in diverse fields such as life sciences, global health, medicine, agricultural science, forensic science, and environmental science for global sustainability. However, implementing a cost-effective PCR remains challenging for rapid preventive medical action to the widespread pandemic diseases due to the absence of highly efficient and low-cost PCR chip-based POC molecular diagnostics. Here, this work reports an ultrafast metaphotonic PCR chip as a solution of a cost-effective and low-power-consumption POC device for the emerging global challenge of sustainable healthcare. This work designs a near-perfect photonic meta-absorber using ring-shaped titanium nitride to maximize the photothermal effect and realize rapid heating and cooling cycles during the PCR process. This work fabricates a large-area photonic meta-absorber on a 6-inch wafer cost-effectively using simple colloidal lithography. In addition, this work demonstrates 30 thermocycles from 65 (annealing temperature) to 95°C (denaturation temperature) within 3min 15s, achieving an average 16.66°Cs-1 heating rate and 7.77°Cs-1 cooling rate during thermocycling, succeeding rapid metaphotonic PCR. This work believes a metaphotonic PCR chip can be used to create a low-cost, ultrafast molecular diagnostic chip with a meta-absorber.

Accessible Double Nanohole Raman Tweezer Analysis of Single Nanoparticles.

Raman spectroscopy allows for material characterization of nanoparticles; however, probing individual nanoparticles requires an efficient way of isolating and enhancing the signal. Past works have used optical trapping with nanoapertures in metal films to measure the Raman spectra of individual nanoparticles; however, those works required custom laser tweezer systems that provided a transmission signal to verify trapping events as well as costly top-down nanofabrication. Here, we trapped Titania nanoparticles in a commercial Raman system using double nanoholes (DNH) and measured their spectra while trapped. The microscope camera allowed for measuring the trapping event in reflection mode, and a simultaneous Raman spectrum was recorded to allow for material characterization. The Raman signal was comparable to a past work that used particles a million times larger in volume without utilizing double nanoholes, and all other features were similar. The DNHs were created with a colloidal lithography technique and identified in the microscope, as confirmed by electron microscopy registration. Therefore, this approach allows a simple way of characterizing the Raman signal of individual nanoparticles while in solution by using existing commercial Raman systems.

Swiss roll nanoarrays for chiral plasmonic photocatalysis

Manipulating the chirality at nanoscale has drawn great attention among scientists, considering its pivotal role in various applications of current interest, including nano-optics, biomedicine, and photocatalysis. In this work, we delve into this arena by fabricating chiral Swiss roll nanoarray (SRNA) continuous films employing colloidal lithography. The technique permits the dimension of Swiss roll metamaterials to reduce to nanoscale, thus achieving chiroptical response (circular dichroism (CD)) in the visible region. The interplay between the CD signals and plasmon resonance modes is revealed through theoretical simulations, enabling a deep understanding of chiral plasmonic metamaterials. The polarization-sensitive photocatalytic activity of chiral SRNAs is investigated, noting a marked increase in the reaction rate when the chirality of SRNAs matches with the handedness of circularly polarized light (CPL). Notably, the SRNA continuous films based on substrate possess integration and reusability without complex recycling process, enhancing their practicality in applications like sensing and plasmonic nanochemistry, particularly toward polarization-dependent photocatalysis.

Unlocking Predictive Capability and Enhancing Sensing Performances of Plasmonic Hydrogen Sensors via Phase Space Reconstruction and Convolutional Neural Networks.

This study innovates plasmonic hydrogen sensors (PHSs) by applying phase space reconstruction (PSR) and convolutional neural networks (CNNs), overcoming previous predictive and sensing limitations. Utilizing a low-cost and efficient colloidal lithography technique, palladium nanocap arrays are created and their spectral signals are transformed into images using PSR and then trained using CNNs for predicting the hydrogen level. The model achieves accurate predictions with average accuracies of 0.95 for pure hydrogen and 0.97 for mixed gases. Performance improvements observed are a reduction in response time by up to 3.7 times (average 2.1 times) across pressures, SNR increased by up to 9.3 times (average 3.9 times) across pressures, and LOD decreased from 16 Pa to an extrapolated 3 Pa, a 5.3-fold improvement. A practical application of remote hydrogen sensing without electronics in hydrogen environments is actualized and achieves a 0.98 average test accuracy. This methodology reimagines PHS capabilities, facilitating advancements in hydrogen monitoring technologies and intelligent spectrum-based sensing.

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Opto-Thermal-Tension Mediated Precision Large-Scale Particle Manipulation and Flexible Patterning.

Large-scale particle manipulation with single-particle precision and further flexible patterning into functional structures is of huge potentials in many fields including bio-optoelectronic sensing, colloidal lithography, and wearable devices. However, it is very challenging for the precision manipulation and flexible patterning of particles on complicated curved and functional substrates. In this work, opto-thermal-tension (OTT) mediated precision large-scale particle manipulation and flexible patterning based on soap film are reported. Flexible manipulation and subsequent patterning of particles with single-particle resolution is realized by optothermal regulated surface tension on soap films. Reconfigurable patterning of particle structures with different shapes as well as large-scale ordered structures (up to 2000 particles) with particle sizes spanning two orders of magnitude (0.5-20µm) is realized using this OTT mediation method. Importantly, due to the high flexibility of soap films, the patterned large-scale particle structures can be non-destructively transferred to curved and rough substrates, including rough iron pipe surface, leaf and skin surface. This OTT mediated method provides a new method for precision large-scale particle manipulation and flexible patterning with high versatility on complicated functional substrates, with great potentials for optoelectronic and biophotonic sensing and wearable device design on different curved and rough functional substrates.

Versatile Nanoring Fabrication Assisted by Hole-mask Colloidal Lithography.

Nanomaterials shaped as rings are interesting nanostructures with control of the materials properties at the nanoscale. Nanoring plasmonic resonators provide tunable optical resonances in the near-infrared with application in sensing. Fabrication of nanorings can be carried out via top-down approaches based on electron beam lithography with high control of the ring size parameters but at high cost. Alternatively, fabrication via self-assembly approaches has a higher speed/lower cost but at the cost of control of ring parameters. Current colloidal lithography approaches can provide nanoring fabrication over large areas but only of specific materials and a select set of rings (large ring diameters or small rings with ultrathin walls). We extend Hole-mask Colloidal Lithography to use ring shaped holes, allow the deposition of arbitrary materials, and allow the independent tuning of ring-wall thickness over a large range of values. We present a generic approach for the fabrication of nanorings formed from a range of materials including low cost (e.g., Cu, Al) and nonplasmonic (e.g., W) materials and with control of ring wall thickness and diameter allowing tuning of ring parameters and materials for applications in nanooptics and beyond.

Phonon–Polaritonic Resonances on Nanopillars of Hexagonal Boron Nitride for Surface‐Enhanced Infrared Absorption

Phonon polaritons (PhPs) in hexagonal boron nitride enable sharp midinfrared optical resonance with strong spatial confinement, making them promising for surface‐enhanced infrared absorption (SEIRA) spectroscopy. Here, using colloidal nanosphere lithography, hBN nanopillar antennas are fabricated and their PhP resonances in a cost‐effective way are demonstrated. By varying the diameters of the hBN nanopillars, the PhP resonance can be readily tuned to match the molecular vibrations of CBP (4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl) molecules. Upon frequency matching, the coupling between the PhP resonance and the molecular vibration shows pronounced mode splitting, illustrating the SEIRA behavior with a coupling strength approaching the strong coupling regime. However, with slight frequency mismatching around 10 cm−1, the coupling strength decreases significantly, indicating a high sensitivity of the SEIRA activities to the resonance frequency of hBN nanopillar antennas. The findings provide a new method for the fabrication of PhP nanoantennas and may promote the development of PhPs in SEIRA‐based midinfrared sensing applications.

Smart Biointerfaces via Click Chemistry-Enabled Nanopatterning of Multiple Bioligands and DNA Force Sensors.

Nanoscale biomolecular placement is crucial for advancing cellular signaling, sensor technology, and molecular interaction studies. Despite this, current methods fall short in enabling large-area nanopatterning of multiple biomolecules while minimizing nonspecific interactions. Using bioorthogonal tags at a submicron scale, we introduce a novel hole-mask colloidal lithography method for arranging up to three distinct proteins, DNA, or peptides on large, fully passivated surfaces. The surfaces are compatible with single-molecule fluorescence microscopy and microplate formats, facilitating versatile applications in cellular and single-molecule assays. We utilize fully passivated and transparent substrates devoid of metals and nanotopographical features to ensure accurate patterning and minimize nonspecific interactions. Surface patterning is achieved using bioorthogonal TCO-tetrazine (inverse electron-demand Diels-Alder, IEDDA) ligation, DBCO-azide (strain-promoted azide-alkyne cycloaddition, SPAAC) click chemistry, and biotin-avidin interactions. These are arranged on surfaces passivated with dense poly(ethylene glycol) PEG brushes crafted through the selective and stepwise removal of sacrificial metallic and polymeric layers, enabling the directed attachment of biospecific tags with nanometric precision. In a proof-of-concept experiment, DNA tension gauge tether (TGT) force sensors, conjugated to cRGD (arginylglycylaspartic acid) in nanoclusters, measured fibroblast integrin tension. This novel application enables the quantification of forces in the piconewton range, which is restricted within the nanopatterned clusters. A second demonstration of the platform to study integrin and epidermal growth factor (EGF) proximal signaling reveals clear mechanotransduction and changes in the cellular morphology. The findings illustrate the platform's potential as a powerful tool for probing complex biochemical pathways involving several molecules arranged with nanometer precision and cellular interactions at the nanoscale.

Enhanced photoelectron emission in a large area aluminum nanohole array via a deep-UV surface plasmon

We measured the photoelectron emission efficiency of aluminum (Al) nanohole arrays fabricated by colloidal lithography and demonstrated the enhancement of photoelectron emission in the deep-UV region via surface plasmon resonances. The Al nanohole arrays for increasing absorption in the deep-UV region were designed using the finite-difference time-domain method and used as photocathodes to enhance the photoelectron emission efficiency. The enhancement factor improved by up to 3.5 times for the optimized nanohole array. Using a two-dimensional mapping system, we demonstrated that the photoelectron emission depended on the uniformity of the sample and diameter of the nanohole arrays. Al nanohole arrays fabricated by colloidal lithography can be used to develop highly sensitive surface-detecting optical sensors and highly efficient surface-emitting electron sources. The two-dimensional mapping system can facilitate the development of highly efficient photocathodes.

A Novel Approach for Colloidal Lithography: From Dry Particle Assembly to High-Throughput Nanofabrication.

We introduce a novel approach for colloidal lithography based on the dry particle assembly into a dense monolayer on an elastomer, followed by mechanical transfer to a substrate of any material and curvature. This method can be implemented either manually or automatically and it produces large area patterns with the quality obtained by the state-of-the-art colloidal lithography at a very high throughput. We first demonstrated the fabrication of nanopatterns with a periodicity ranging between 200 nm and 2 μm. We then demonstrated two nanotechnological applications of this approach. The first one is antireflective structures, fabricated on silicon and sapphire, with different geometries including arrays of bumps and holes and adjusted for different spectral ranges. The second one is smart 3D nanostructures for mechanostimulation of T cells that are used for their effective proliferation, with potential application in cancer immunotherapy. This new approach unleashes the potential of bottom-up nanofabrication and paves the way for nanoscale devices and systems in numerous applications.

Large-scale SU-8 Micro-Pole-Arrays decorated with Hierarchical Metal-Dielectric-Metal nanostructures as sensitive 3D SERS substrates

Dielectric layer-mediated surface-enhanced Raman scattering (SERS) substrates are currently explored for ultrasensitive detection and expected to provide uniform SERS response by the virtue of three-dimensional micro-nanostructures, dielectric layer, and plasmonic metal nanostructures. For this purpose, the present work reports on the development of the metal-dielectric-metal (MDM) three-dimensional (3D) micro-pole-arrays structure based on SU-8 colloid lithography. By utilizing the large-scale periodic arrangement characteristics of SU-8 colloidal lithography, along with the hot spot effect created by the nano-gap between MDM, we successfully prepared an array of SERS substrates that exhibit both periodicity and multi-dimensional plasmonic coupling effect. Compared to a single metal nanoparticle substrate, the SERS activity of the substrate is significantly enhanced, and the substrate has trace detection ability (LOD of detection of 10−12 mol/L for R6G). The enhancement factor (EF) is 5.72 × 107. Periodicity ensures that the prepared substrate has good reproducibility (RSD < 8 %) and stability. Furthermore, the substrate modified with HS-β-CD demonstrated potential for rapid trace SERS detection of polychlorinated biphenyls (PCB-77) (LOD of 10−8 mol/L for PCB-77). The results indicate that the proposed substrate has the potential for application in the preparation of SERS sensors with high sensitivity and uniformity.

Tuning the bulk behavior and 2D interfacial self-assembly of microgels by keggin-type polyoxometalate ionic specificity

Finding new ways to tune the behavior of thermoresponsive microgels in bulk and confined at 2D liquid interfaces is key to achieve a deeper understanding and control of these smart materials. We studied the interaction of positively and negatively charged pNIPAM microgels with the Keggin-type polyoxometalates Na3PW12O40 (POM3−) and H4SiW12O40 (POM4−). In bulk, we observed charge inversions of the positively charged microgels below and above the volume phase transition temperature (VPTT) at significantly low POM concentrations as 5⋅10−5 M. In the presence of POM, both microgels exhibited a deswelling-swelling-deswelling behavior below the VPTT, and a two-step further deswelling above the VPTT for the positively charged microgels. When the later were confined at 2D water/air interfaces, adding 10−5 M of POM3− below the VPTT was equivalent to heating above the VPTT and compressing the monolayer from 5 to 20mN m−1. Above the VPTT, the diameter at the interface did not change while the portion immersed in the subphase further deswelled, in agreement with the behavior in bulk. Adding more POM3− did not change the diameter at the interface nor the height of the microgels, showing a saturation effect in 2D. The restructuring of the pNIPAM polymeric network by the POM3− was characterized by EDS mapping and XPS. The microgel monolayers with POM3− improved their resistance to plasma etching, which could be useful for soft colloidal lithography.

Collective motion of Nafion-based micromotors in water.

Ion exchange is one of the most interesting processes occurring at the interface between aqueous solutions and polymers, such as the well-known Nafion. If the exchanged ions have different diffusion coefficients, this interchange generates local electric fields which can be harnessed to drive fluid motion. In this work, we show how it is possible to design and fabricate self-propelling microswimmers based on Nafion, driven by ion-exchange, and fueled by innocuous salts. These Nafion micromotors are made using colloidal lithography by micro/nanostructuring Nafion in the form of asymmetric rods. These microswimmers exhibit fascinating collective motion in water driven by the interplay of their self-generated chemical/electric fields and their capability to pump matter nearby towards the collective motile structure. The pumping activity of the microswimmers induces the formation of growing mobile clusters, whose velocity increases with size. Such dynamic structures are able to trap nearby micro/nano-objects while purifying the liquid, which acts both as the transport media and as fuel. Such phenomenology opens the door to potential applications in water remediation that are currently under development.

Tuning of the Effective Refractive Index of Crystalline Si Thin Films with Controlled Modification of Nanohole Dimensions By Dry-Etching

Silicon thin films with holes are composite materials with noteworthy optical properties that can be controllably fabricated by state-of-the-art Si process technologies. For light with wavelengths larger than the hole sizes and spacing, typically by a factor of 10, the perforated Si thin films behave like an effective (refractive index) medium. Under these conditions, the optical properties of the films can be modelled by the effective medium approximation (EMA). By manipulating the volumetric air fraction (holes) of such films, the refractive index can be modified accordingly. Such a property can be exploited to tune the index contrast in various photonic devices such as filters and waveguides.This work presents a novel scalable method that combines charged sphere colloidal lithography (CSCL) and dry-etching to pattern Si thin films. We demonstrate controlled tuning of the effective refractive index by controlled lateral dry-etching of the holes. The fabricated nanoholes are spatially disordered in a 220 nm thick crystalline Si film. CSCL is performed using polystyrene particles of 60 (P60) and 100 (P100) nm diameter. The CSCL step results in a disordered arrangement of the particles with average spatial separations of 2-4 times their diameter. Using Cr as a mask, cylindrical holes are etched into Si by inductively coupled plasma-reactive ion-etching (ICP-RIE) using a pseudo-Bosch process. An additional isotropic ICP-RIE dry-etch step is introduced to widen the holes in a controlled manner, utilizing CF4 and O2 gas mixture. Importantly, this hole-widening step is time-controlled to ensure that the holes do not overlap laterally. Finally, the Cr mask is removed by wet-etching.By analyzing scanning electron microscopic (SEM) images, we estimate the lateral etch rates for the P60 and P100 samples are approximately 0.76 nm/s and 0.52 nm/s, respectively. Using this process, the average hole diameter can be increased systematically up to circa 68-97%, compared to their original diameters, while avoiding overlap. The effective refractive index of the perforated Si thin films is determined using the optical air fill factor obtained from modelling the spectroscopic ellipsometry data with Bruggeman effective medium approximation. The determined optical air fill factor varies from circa 20% to 50% with hole widening. The ellipsometry data provides the true air fill factor of the samples directly; SEM analysis could also be used, however, it is complex and cannot determine the actual porosity of the films. The ellipsometry results show that the determined refractive index is constant in the 1500 - 4000 nm wavelength range. The refractive index decreases from 3.42 for unstructured Si film to 2.9 and 2.7 for the as-etched perforated Si films of P60 and P100 samples, respectively. With controlled lateral etching of the holes, the effective refractive index systematically decreases further to approximately 2.1 for the P60 and P100 samples, which is a 39% reduction compared to Si refractive index. The determined effective indices represent averaged values accounting for possible structural birefringence, which can be used as a guideline to design photonic devices such as filters.The presented findings indicate that the effective refractive index of perforated Si thin films can be engineered by controlling the lateral dimensions of holes. While this conclusion relates specifically to crystalline Si thin films, the same concept may also be applied to other perforated dielectric thin films. Figure 1

Versatile Deposition of Complex Colloidal Assemblies from the Evaporation of Hanging Drops.

Existing strategies designed to produce ordered arrangements of colloidal particles on solid supports are of great interest for their wide range of applications, from colloidal lithography, plasmonic and biomimetic surfaces to tags for anti-counterfeiting, but they all share various degrees of complexity hampering their facile implementation. Here, a drastically simplified methodology is presented to achieve ordered particle deposition, consisting in adding micromolar amounts of cationic surfactant to a colloidal suspension drop and let it evaporate in an upside-down configuration. Confinement at the air/water interface enables particle assembly into monolayers, which are then transferred on the substrate producing highly ordered structures displaying vivid, orientation-dependent structural colors. The method is compatible with many particle types and substrates, while controlling system parameters allows tuning the deposit size and morphology, from monocrystals to polycrystalline disks and "irises", from single-component to crystal alloys with Moiré patterns, demonstrating its practicality for a variety of processes.

Stepwise colloidal lithography toward scalable and various planar chiral metamaterials

Stepwise colloidal lithography toward scalable and various planar chiral metamaterials

Effect of nanoscale roughness on four different atomic force microscopy probes in aqueous solutions using adhesion force measurement

Despite substantial theoretical approaches explaining the effect of nanoscale roughness (NR) on particle deposition, experiments are rarely performed. Fabricating the sophisticated rough surfaces and obtaining the experimental data for interaction between the colloids and surfaces are crucial to understand colloidal adsorption onto the rough surfaces. We investigated the adhesion of different particle types of atomic force microscope (AFM) probes (different shapes, i.e., sphere and plateau; diameters, i.e., 2 μm–15 μm) onto smooth and rough fabricated surfaces. We successfully fabricated well-organized rough surfaces on Silicon (Si) using colloidal lithography and metal-assisted chemical etching using a cylindrical shape of constant nanoscale height and diameter. The properties of roughness (i.e., height, diameter, and fraction) were quantitatively analyzed, and the corresponding. The force-distance curves measured with the AFM revealed that, for the four types of probes, contact area varied with the surface roughness, and their adhesion forces differed accordingly. Expanding the contact area tended to increase the repulsive force possibly because of the hydration force. This study effectively devised a simplified experimental system that explains the influence of NR on particle deposition using previous theoretical data. The findings should be considered in designing NR for subsequent colloidal deposition.

Development of nanostructured silver layers via colloidal lithography for AZO/Ag/AZO transparent conductive oxide applications

Development of nanostructured silver layers via colloidal lithography for AZO/Ag/AZO transparent conductive oxide applications