Hole-mask Colloidal Lithography Research Articles

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.

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.

Tungsten nanodisc-based spectrally-selective polarization-independent thermal emitters

Thermophotovoltaic (TPV) cells convert thermally emitted photons into electrical power using photovoltaic (PV) detectors. To realize highly efficient thermal energy harvesting using TPV conversion, high-temperature stable spectrally-selective emitters are needed. The deployment of TPV technology lags behind conventional solar-PV technology due to the lack of large-scale fabrication of efficient thermal emitters, which would preferentially emit in the PV cell absorption band. In this work, we demonstrate a simple large-area nanofabrication method based on the hole-mask colloidal lithography and sputtering, which allows one to fabricate tungsten (W) nanodisc spectrally-selective emitters (consisting of a metal-insulator-metal configuration) with a high emissivity below the InGaAsSb PV-cell cut-off wavelength of 2.25 μm and a gradually decreasing emissivity (down to < 10%) in the mid-infrared region. Frequency-domain time-domain (FDTD) simulations reveal that the spectral selectivity is achieved due to the localized surface plasmon resonance of W nanodiscs strongly influenced by the insulator thickness. Importantly, the W emitters show thermal stability at temperatures of up to 1100 °C, and emissivity invariance to changes in polarization and incidence angles up to 65°. This work represents a significant step towards the realization of high-temperature stable efficient thermal emitters by a facile and cost-effective fabrication method, thereby promoting the implementation of photonic/plasmonic thermal emitters in the next-generation thermal energy harvesting systems. The method proposed in this study holds potential for scalability; however, empirical evidence to demonstrate this scalability has not yet been established. Subsequent studies are needed to confirm the scalability of the proposed method and its extensive applicability.

Spectrally selective emitters based on 3D Mo nanopillars for thermophotovoltaic energy harvesting

High-temperature stable emitters with spectral selective functionality are an absolute condition for efficient conversion of thermal radiation into electricity using thermophotovoltaic (TPV) systems. Usually, spectral selective emitters are made up of multilayered materials or geometrical structures resulting from complex fabrication processes. Here, we report a spectrally selective emitter based on a single metal layer coating of molybdenum (Mo) over a 3D dielectric pillar geometry. 3D Mo nanopillars are fabricated using large-area and cost-effective hole-mask colloidal lithography. These nanostructures show an absorptivity/emissivity of 95% below the cut-off wavelength of an InGaAsSb PV cell at 2.25 μm, and a sharp decline in absorptivity/emissivity in the near-infrared regions, approaching a low emissivity of 10%. The 3D Mo nanopillars show outstanding thermal/structural stability up to 1473 K for 24 h duration under Ar atmosphere and polarization and angle invariance up to 60° incidence angles. With a low-cost and scalable fabrication method, 3D Mo nanostructures provide tremendous opportunities in TPV and high temperature photonic/plasmonic applications.

Macroscopic magneto-chiroptical metasurfaces

Nanophotonic chiral antennas exhibit orders of magnitude higher circular dichroism (CD) compared to molecular systems. When the structural chirality is merged with magnetism at the nanoscale, efficient magnetic control over the dichroic response is achieved, bringing exciting prospects to active nanophotonic devices. Here, we devise macroscopic enantiomeric magnetophotonic metasurfaces of plasmonic-ferromagnetic spiral antennas assembled on large areas via hole-mask colloidal lithography. The simultaneous presence of 3D- and 2D-features in chiral nanoantennas induces large CD response, where we identify reciprocal and non-reciprocal contributions, respectively. Exploring further this type of magnetophotonic metasurfaces might allow the realization of high-sensitivity chiral sensors and prompts the design of advanced macroscopic optical devices operating with polarized light.

Fabrication of Monodisperse Colloids of Resonant Spherical Silicon Nanoparticles: Applications in Optical Trapping and Printing

We present a new method for making monodisperse highly spherical polycrystalline silicon (Si) nanoparticles dispersed in solution and a method for trapping, moving, and printing these nanoparticles on a substrate. Spherical Si nanoparticles with low dispersion in size (<3.5%) and diameter of 130 and 210 nm were fabricated using combined hole-mask colloidal lithography and laser-induced transfer. The particles are highly crystalline and possess electric and magnetic dipole resonances in the visible spectrum varying with the diameter. They could be trapped in 2D against a substrate using an optical tweezer and then printed onto the substrate by means of radiation pressure. The proposed method paves the way for the use of optical forces for assembling complex resonant dielectric nanostructures with engineered optical properties.

Oxidation controlled lift-off of 3D chiral plasmonic Au nano-hooks

Colloidal suspensions of plasmonic nanoparticles (NPs) are a well-established tool for biomedical applications and enhanced spectroscopy because of their strong optical response. The specific response is greatly dependent on the NP shape. The strong optical activity of chiral NPs has created special interest but fabrication of chiral NPs in solution remains challenging. Here, we present an approach whereby three-dimensional (3D) chiral Au nano-hooks, fabricated with the parallel hole-mask colloidal lithography (HMCL) method, can be lifted off from a glass substrate in a controllable manner by using a combined treatment with oxygen plasma oxidation and a reduction step in solution. This method has the advantage of being based on established techniques and not requiring strong acids or complex substrates as in etching based approaches. We furthermore demonstrate the integration of the hook NPs into reversibly cross-linked hydrogels inspired by mussel catechol chemistry but containing an oxidation resistant catechol analogue grafted onto poly(allylamine) crosslinked by coordination of Al3+ and how this facilitates the remote analysis of hydrogel microenvironment, e.g. the water content. The suspended particles are promising candidates for optically active surface-enhanced Raman spectroscopy (SERS), asymmetric photo catalysis or aggregation sensing. The integration into hydrogels to produce functional hydrogels holds benefits for applications of metamaterials in optics, sensing or activation in environmental remediation or drug delivery.

Increased Refractive Index Sensitivity by Circular Dichroism Sensing through Reduced Substrate Effect

An optical sensor based on the localized surface plasmon resonance (LSPR) of chiral Au nanohooks with increased refractive index (RI) sensitivity via circular dichroism (CD) measurements is presented. Programmed control of sample rotation combined with angled physical vapor deposition is applied to hole-mask colloidal lithography to provide in process modification of the hole-masks and generate arrays of chiral nanostructures with an adjustable optical response. Extinction spectra with unpolarized light as well as circular dichroism measurements are compared for left- and right-handed hook structures. Analysis of the LSPR peak shift of the substrate-attached nanostructures revealed the CD measurements to be twice as sensitive as the measurements with unpolarized light (304 and 146 nm RIU–1, respectively) and close to the maximum predicted for LSPR sensing at this spectral region (∼700 nm). Finite-difference time-domain simulations with different substrate materials show that the difference in RI sensitivi...

Single molecule protein patterning using hole mask colloidal lithography.

The ability to manipulate single protein molecules on a surface is useful for interfacing biology with many types of devices in optics, catalysis, bioengineering, and biosensing. Control of distance, orientation, and activity at the single molecule level will allow for the production of on-chip devices with increased biological activity. Cost effective methodologies for single molecule protein patterning with tunable pattern density and scalable coverage area remain a challenge. Herein, Hole Mask Colloidal Lithography is presented as a bench-top colloidal lithography technique that enables a glass coverslip to be patterned with functional streptavidin protein onto patches from 15-200 nm in diameter with variable pitch. Atomic force microscopy (AFM) was used to characterize the size of the patterned features on the glass surface. Additionally, single-molecule fluorescence microscopy was used to demonstrate the tunable pattern density, measure binding controls, and confirm patterned single molecules of functional streptavidin.

Colloidal lithography nanostructured Pd/PdOx core–shell sensor for ppb level H2S detection

In this work we report on plasma oxidation of palladium (Pd) to form reliable palladium/palladium oxide (Pd/PdOx) core–shell sensor for ppb level H2S detection and its performance improvement through nanostructuring using hole-mask colloidal lithography (HCL). The plasma oxidation parameters and the sensor operating conditions are optimized to arrive at a sensor device with high sensitivity and repeatable response for H2S. The plasma oxidized palladium/palladium oxide sensor shows a response of 43.1% at 3 ppm H2S at the optimum operating temperature of 200 °C with response and recovery times of 24 s and 155 s, respectively. The limit of detection (LoD) of the plasma oxidised beam is 10 ppb. We further integrate HCL, a bottom-up and cost-effective process, to create nanodiscs of fixed diameter of 100 nm and varying heights (10, 15 and 20 nm) on 10 nm thin Pd beam which is subsequently plasma oxidized to improve the H2S sensing characteristics. The nanostructured Pd/PdOx sensor with nanodiscs of 100 nm diameter and 10 nm height shows an enhancement in sensing performance by 11.8% at same operating temperature and gas concentration. This nanostructured sensor also shows faster response and recovery times (15 s and 100 s, respectively) compared to the unstructured Pd/PdOx counterpart together with an experimental LoD of 10 ppb and the estimated limit going all the way down to 2 ppb. Material characterization of the fabricated Pd/PdOx sensors is done using UV–vis spectroscopy and x-ray photoemission spectroscopy.

Nanofabricated Catalyst Particles for the Investigation of Catalytic Carbon Oxidation by Oxygen Spillover.

The catalytic oxidation of carbon by molecular oxygen was studied using C/Pt, Pt/C, Pt/Al2O3/C, Pt/CeO2/C, Al2O3/C, and CeO2/C model samples prepared by hole-mask colloidal lithography. By this technique, the degree of contact between platinum and carbon was controlled with high precision. The oxidation of carbon was monitored using atomic force microscopy and scanning electron microscopy. The results show that Pt in direct contact with carbon catalyzes the oxidation of carbon by spillover of dissociated oxygen from Pt to carbon. By physically separating Pt and carbon with a 10 nm thin spacer layer of Al2O3, the oxygen spillover was entirely blocked. However, through a corresponding spacer layer of CeO2, carbon oxidation was still observed, either by oxygen spillover from Pt to carbon or directly dissociated on the ceria, although at a slower rate compared to the case with no spacer layer between Pt and carbon.

Active magnetoplasmonic split-ring/ring nanoantennas.

Here we present a novel active system, which combines the plasmon resonance enhancement of the magneto-optical activity in magnetoplasmonic nanostructures and the strong electromagnetic field localization of split ring resonators. The structures consist of a gold split ring resonator placed on top of a gold nanoring in the section of which a Co nanodot is inserted. By placing the split ring gap on top of the nanodot, and continuously varying the split ring gap opening, we are able to tune and enhance the electromagnetic field intensity in the Co nanodot, as confirmed experimentally by EELS and numerically using DDA simulation methods. In this way we obtain structures with a magneto-optical activity, which is 3 times larger than that of equivalent magnetoplasmonic rings without a split ring on top. These enhanced performances are due to the better control of the positioning, dimensions, and shape of the different components of the system. Such improvements are achieved using hole-mask colloidal lithography technique combined with multiaxial evaporation of the different materials.

Electrochemical Characterization of Vertically Aligned Carbon Nanofiber Arrays Prepared by Hole‐mask Colloidal Lithography

AbstractNanoelectrode arrays consisting of vertically aligned carbon nanofibers were prepared through plasma enhanced chemical vapor deposition and patterned using hole‐mask colloidal lithography (HCL), a simple fabrication method employed as a cost‐effective patterning alternative to the conventional electron beam lithography. The density of the carbon nanofibers was easily altered by changing the concentration of the polystyrene spheres employed in HCL. Cyclic voltammetry and chronoamperometry were used to electrochemically characterize the arrays of different density. Results indicate that the density of the carbon nanofibers leads to differences in the macro/micro electroactive surface areas.

Nanopattern Gradients for Cell Studies Fabricated Using Hole-Mask Colloidal Lithography.

Culturing cells on gradient nanopatterns provides a useful tool to explore cellular adhesion to mimics of the extracellular matrix or screen for cellular responses to nanopatterns. A method is presented to fabricate complex gradient protein patterns based on hole-mask colloidal lithography, which can generate nanopatterns in multiple materials and of multiple shapes. Gradients of gold structures were functionalized to form gradients of protein nanopatterns of different shapes (bars, dot pairs, and rings), where a key parameter was systematically varied in each gradient. Cells were grown on vitronectin nanopatterns, showing differential adhesion (spread area/focal adhesion size) along the gradients.

High-yield fabrication of 60 nm Permalloy nanodiscs in well-defined magnetic vortex state for biomedical applications

Permalloy disc structures in magnetic vortex state constitute a promising new type of magnetic nanoparticles for biomedical applications. They present high saturation magnetisation and lack of remanence, which ease the remote manipulation of the particles by magnetic fields and avoid the problem of agglomeration, respectively. Importantly, they are also endowed with the capability of low-frequency magneto-mechanical actuation. This effect has already been shown to produce cancer cell destruction using functionalized discs, about 1 μm in diameter, attached to the cell membrane. Here, Permalloy nanodiscs down to 60 nm in diameter are obtained by hole-mask colloidal lithography, which is proved to be a cost-effective method for the uniform patterning of large substrate areas, with a high production yield of nanostructures. The characterisation of the magnetic behaviour of the nanodiscs, complemented with micromagnetic simulations, confirms that they present a very well defined magnetic vortex configuration, unprecedented, to our knowledge, for nanostructures of this size prepared by a high-yield method. The successful detachment of the gold-covered nanodiscs from the substrate is also demonstrated by the use of sacrificial layers.

Nanothermometry using optically trapped erbium oxide nanoparticle

A new optical probe technique using a laser-trapped erbium oxide nanoparticle (size ~150 nm) is introduced that can measure absolute temperature with a spatial resolution on the size of the trapped nanoparticle. This technique (scanning optical probe thermometry) is used to collect a thermal image of a gold nanodot prepared with hole-mask colloidal lithography. A convolution analysis of the thermal profile shows that the point spread function of our measurement is a Gaussian with a FWHM of 165 nm. We attribute the width of this function to clustering of Er2O3 nanoparticles in solution. The scanning optical probe thermometer is used to measure the temperature where vapor nucleation occurs in degassed water (555 K), confirming that a nanoscale object heated in water will superheat the surrounding water to the spinodal decomposition temperature. Subsequently, the temperature inside the vapor bubble rises to the melting point of the gold nanostructure (~1300) where a temperature plateau is observed. The rise in temperature is attributed to inhibition of thermal transfer to the surrounding liquid by the thermal insulating vapor cocoon.

Large-scale production of ferromagnetic nanorings by a modified hole-mask colloidal lithography: Controlled creation of flux-closure vortex state

Large arrays of ferromagnetic nanorings are produced by a modified hole-mask colloidal lithography and ring dimension can be modulated to create flux-closed vortex, known as a dipole-free magnetic state with a low crosstalk arising from neighboring entities.

Testing gold nanostructures fabricated by hole-mask colloidal lithography as potential substrates for SERS sensors: sensitivity, signal variability, and the aspect of adsorbate deposition.

Gold nanoplasmonic substrates with high sensitivity and spectral reproducibility are key components of molecular sensors based on surface-enhanced Raman scattering (SERS). In this work, we used a confocal Raman microscope and several types of gold nanostructures (arrays of nanodiscs, nanocones and nanodisc dimers) prepared by hole-mask colloidal lithography (HCL) to determine the sources of variability in SERS measurements. We demonstrate that significant variations in the SERS signal can originate from the method of deposition of analyte molecules onto a SERS substrate. While the method based on incubation of SERS substrates in a solution containing the analyte yields a SERS signal with low variability, the droplet deposition method produces a SERS signal with rather high variability. Variability of the SERS signal of a single nanoparticle was determined from the statistical analysis of the SERS signal in short-range Raman maps recorded using different sized laser spots produced by means of different objectives. We show that the number of nanoparticles located within the laser spot can be a source of substantial SERS signal variability, especially for high-magnification objectives. We demonstrate that SERS substrates prepared by HCL exhibit high SERS enhancement and excellent homogeneity (about 20% relative standard deviation from short-range maps). The nanocone arrays are shown to provide the highest SERS enhancement, the lowest relative level of fluorescence background, and also slightly better homogeneity when compared with arrays of nanodisc dimers or single nanodiscs.

Disordered nanostructures by hole-mask colloidal lithography for advanced light trapping in silicon solar cells

We report on the fabrication of disordered nanostructures by combining colloidal lithography and silicon etching. We show good control of the short-range ordered colloidal pattern for a wide range of bead sizes from 170 to 850 nm. The inter-particle spacing follows a Gaussian distribution with the average distance between two neighboring beads (center to center) being approximately twice their diameter, thus enabling the nanopatterning with dimensions relevant to the light wavelength scale. The disordered nanostructures result in a lower integrated reflectance (8.1%) than state-of-the-art random pyramid texturing (11.7%) when fabricated on 700 µm thick wafers. When integrated in a 1.1 µm thin crystalline silicon slab, the absorption is enhanced from 24.0% up to 64.3%. The broadening of resonant modes present for the disordered nanopattern offers a more broadband light confinement compared to a periodic nanopattern. Owing to its simplicity, versatility and the degrees of freedom it offers, this potentially low-cost bottom-up nanopatterning process opens perspectives towards the integration of advanced light-trapping schemes in thin solar cells.

Patterned Plasmonic Nanoparticle Arrays for Microfluidic and Multiplexed Biological Assays.

For applications ranging from medical diagnostics and drug screening to chemical and biological warfare detection, inexpensive, rapid-readout, portable devices are required. Localized surface plasmon resonance (LSPR) technologies show substantial promise toward meeting these goals, but the generation of portable, multiplexed and/or microfluidic devices incorporating sensitive nanoparticle arrays is only in its infancy. Herein, we have combined photolithography with Hole Mask Colloidal lithography to pattern uniform nanoparticle arrays for both microfluidic and multiplexed devices. The first proof-of-concept study is carried out with 5- and 7-channel microfluidic devices to acquire one-shot binding curves and protein binding kinetic data. The second proof-of-concept study involved the fabrication of a 96-spot plate that can be inserted into a standard plate reader for the multiplexed detection of protein binding. This versatile fabrication technique should prove useful in next generation chips for bioassays and genetic screening.