Controlled photochemical particle growth in two-dimensional ordered metal nanoparticle arrays

Process Window for Seeded Growth of Arrays of Quasi-Spherical Substrate-Supported Au Nanoparticles

The controlled growth of surface-supported metal nanoparticles (NPs) is essential to a broad range of applications. To this end, we explore the seeded growth of highly ordered arrays of substrate-supported Au NPs through a fully orthogonal design of experiment (DoE) scheme applied to a reaction system consisting of HAuCl₄, citrate, and hydrogen peroxide. Scanning electron microscopy in combination with digital image analysis (DIA) is used to quantitatively characterize the resultant NP populations in terms of both particle and array features. The effective optical properties of the NP arrays are additionally analyzed using spectroscopic ellipsometry (SE), allowing characteristics of the localized surface plasmon resonances (LSPRs) of the arrays to be quantified. We study the dependence of the DIA- and SE-extracted features on the different reagent concentrations through modeling using multiple linear regression with backward elimination of independent variables. A process window is identified for which uniform arrays of quasi-spherical Au NPs are grown over large surface areas. Aside from reagent concentrations the system is highly sensitive to the hydrodynamic conditions during the deposition. This issue is likely caused by an Au precursor mass-transport limitation of the reduction reaction and it is found that agitation of the growth medium is best avoided to ensure a macroscopically even deposition. Parasitic homogeneous nucleation can also be a challenge and was separately studied in a full DoE scheme with equivalent growth media but without substrates, using optical tracking of the solutions over time. Conditions yielding quasi-spherical surface-supported NPs are found to also be affiliated with strong tendencies for parasitic homogeneous nucleation and thereby loss of Au precursor, but addition of polyvinyl alcohol can possibly help alleviate this issue.

Seeded Growth of Large-Area Arrays of Substrate Supported Au Nanoparticles Using Citrate and Hydrogen Peroxide

While seeded growth of quasi-spherical colloidal Au nanoparticles (NPs) has been extensively explored in the literature, the growth of surface supported arrays of such particles has received less attention. The latter scenario offers some significant challenges, including the attainment of sufficient particle-substrate adhesion, growth-selectivity, and uniform mass-transport. To this end, a reaction system consisting of HAuCl₄, citrate, and H₂O₂ is here investigated for the growth of supported arrays of 10 nm Au seeds, derived via block copolymer (BCP) lithography. The effects of the reagent concentrations on the properties of the resultant NPs are evaluated. It is found that inclusion of citrate in the growth medium causes substantial particle desorption from Si surfaces. However, the presence of citrate also yields NPs with more uniformly circular top-view cross sections ("quasi-circular"), motivating the exploration of particle immobilization methods. We demonstrate that atomic layer deposition (ALD) of a single cycle of HfO₂ (∼1 Å), after the seed particle formation, promotes adhesion sufficiently to enable the use of citrate without the added oxide noticeably affecting the shape of the resultant NPs. The presented ALD-based approach differs from the conventional sequence of depositing the adhesion layer prior to the seed particle formation and may have advantages in various processing schemes, such as when surface grafting of brush layers is required in the BCP lithography process. A proof-of-concept is provided for the growth of large-area arrays of supported "quasi-circular" Au NPs, in a rapid one-step process at room temperature.

Self-assembled quasi-hexagonal arrays of gold nanoparticles with small gaps for surface-enhanced Raman spectroscopy

The fabrication and optical characterization of self-assembled arrangements of rough gold nanoparticles with a high area coverage and narrow gaps for surface-enhanced Raman spectroscopy (SERS) are reported. A combination of micellar nanolithography and electroless deposition (ED) enables the tuning of the spacing and size of the noble metal nanoparticles. Long-range ordered quasi-hexagonal arrays of gold nanoparticles on silicon substrates with a variation of the particle sizes from about 20 nm to 120 nm are demonstrated. By increasing the particle sizes for the homogeneously spaced particles, a large number of narrow gaps is created, which together with the rough surface of the particles induces a high density of intense hotspots. This makes the surfaces interesting for future applications in near-field-enhanced bio-analytics of molecules. SERS was demonstrated by measuring Raman spectra of 4-MBA on the gold nanoparticles. It was verified that a smaller inter-particle distance leads to an increased SERS signal.

Nanoporous silicon nitride-based membranes of controlled pore size, shape and areal density: Fabrication as well as electrophoretic and molecular filtering characterization

A new route will be presented for an all-parallel fabrication of highly flexible, freestanding membranes with well-defined porosity. This fabrication is based on arrays of well-defined Au nanoparticles (NPs) exhibiting a high degree of hexagonal order as obtained in a first step by a proven micellar approach. These NP arrays serve as masks in a second reactive ion etching (RIE) step optimized for etching Si and some important Si compounds (silicon oxide, silicon nitride) on the nanoscale. Application to commercially available silicon nitride membranes of well-defined thickness, delivers a diaphragm with millions of nanopores of intended and controlled size, shape, and areal density with narrow distributions of these parameters. Electrophoretic transport measurements indicated a very low flow resistance of these porous membranes in ionic solutions as expected theoretically. Size-selective separation of protein molecules was demonstrated by real-time fluorescence microscopy.

Electrochemically-Driven Insertion of Biological Nanodiscs into Solid State Membrane Pores as a Basis for "Pore-In-Pore" Membranes

Nanoporous membranes are of increasing interest for many applications, such as molecular filters, biosensors, nanofluidic logic and energy conversion devices. To meet high-quality standards, e.g., in molecular separation processes, membranes with well-defined pores in terms of pore diameter and chemical properties are required. However, the preparation of membranes with narrow pore diameter distributions is still challenging. In the work presented here, we demonstrate a strategy, a "pore-in-pore" approach, where the conical pores of a solid state membrane produced by a multi-step top-down lithography procedure are used as a template to insert precisely-formed biomolecular nanodiscs with exactly defined inner and outer diameters. These nanodiscs, which are the building blocks of tobacco mosaic virus-deduced particles, consist of coat proteins, which self-assemble under defined experimental conditions with a stabilizing short RNA. We demonstrate that the insertion of the nanodiscs can be driven either by diffusion due to a concentration gradient or by applying an electric field along the cross-section of the solid state membrane. It is found that the electrophoresis-driven insertion is significantly more effective than the insertion via the concentration gradient.

Large-scale fabrication of highly ordered sub-20 nm noble metal nanoparticles on silica substrates without metallic adhesion layers

Periodic noble metal nanoparticles offer a wide spectrum of applications including chemical and biological sensors, optical devices, and model catalysts due to their extraordinary properties. For sensing purposes and catalytic studies, substrates made of glass or fused-silica are normally required as supports, without the use of metallic adhesion layers. However, precise patterning of such uniform arrays of silica-supported noble metal nanoparticles, especially at sub-100 nm in diameter, is challenging without adhesion layers. In this paper, we report a robust method to large-scale fabricate highly ordered sub-20 nm noble metal nanoparticles, i.e., gold and platinum, supported on silica substrates without adhesion layers, combining displacement Talbot lithography (DTL) with dry-etching techniques. Periodic photoresist nanocolumns at diameters of ~110 nm are patterned on metal-coated oxidized silicon wafers using DTL, and subsequently transferred at a 1:1 ratio into anti-reflection layer coating (BARC) nanocolumns with the formation of nano-sharp tips, using nitrogen plasma etching. These BARC nanocolumns are then used as a mask for etching the deposited metal layer using inclined argon ion-beam etching. We find that increasing the etching time results in cone-shaped silica features with metal nanoparticles on the tips at diameters ranging from 100 nm to sub-30 nm, over large areas of 3×3 cm². Moreover, subsequent annealing these sub-30 nm metal nanoparticle arrays at high-temperature results in sub-20 nm metal nanoparticle arrays with ~10¹⁰ uniform particles.

Bottom-Up Fabrication of Hybrid Plasmonic Sensors: Gold-Capped Hydrogel Microspheres Embedded in Periodic Metal Hole Arrays

The high potential of bottom-up fabrication strategies for realizing sophisticated optical sensors combining the high sensitivity of a surface plasmon resonance with the exceptional properties of stimuli-responsive hydrogel is demonstrated. The sensor is composed of a periodic hole array in a gold film whose holes are filled with gold-capped poly(N-isoproyl-acrylamide) (polyNIPAM) microspheres. The production of this sensor relies on a pure chemical approach enabling simple, time-efficient, and cost-efficient preparation of sensor platforms covering areas of cm². The transmission spectrum of this plasmonic sensor shows a strong interaction between propagating surface plasmon polaritons at the metal film surface and localized surface plasmon resonance of the gold cap on top of the polyNIPAM microspheres. Computer simulations support this experimental observation. These interactions lead to distinct changes in the transmission spectrum, which allow for the simultaneous, sensitive optical detection of refractive index changes in the surrounding medium and the swelling state of the embedded polyNIPAM microsphere under the gold cap. The volume of the polyNIPAM microsphere located underneath the gold cap can be changed by certain stimuli such as temperature, pH, ionic strength, and distinct molecules bound to the hydrogel matrix facilitating the detection of analytes which do not change the refractive index of the surrounding medium significantly.

Au nanocluster arrays on self-assembled block copolymer thin films as highly active SERS substrates with excellent reproducibility

We demonstrate the fabrication of uniform Au nanocluster arrays utilizing a self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin film as the template and their application as a surface-enhanced Raman scattering (SERS) substrate.

Fabrication of periodic arrays of metallic nanoparticles by block copolymer templates on HfO2 substrates

Block copolymer-based templates can be exploited for the fabrication of ordered arrays of metal nanoparticles (NPs) with a diameter down to a few nanometers. In order to develop this technique on metal oxide substrates, we studied the self-assembly of polymeric templates directly on the HfO₂ surface. Using a random copolymer neutralization layer, we obtained an effective HfO₂ surface neutralization, while the effects of surface cleaning and annealing temperature were carefully examined. Varying the block copolymer molecular weight, we produced regular nanoporous templates with feature size variable between 10 and 30 nm and a density up to 1.5 × 10¹¹ cm⁻². With the adoption of a pattern transfer process, we produced ordered arrays of Pt and Pt/Ti NPs with diameters of 12, 21 and 29 nm and a constant size dispersion (σ) of 2.5 nm. For the smallest template adopted, the NP diameter is significantly lower than the original template dimension. In this specific configuration, the granularity of the deposited film probably influences the pattern transfer process and very small NPs of 12 nm were achieved without a significant broadening of the size distribution.

Arrays of quasi-hexagonally ordered silica nanopillars with independently controlled areal density, diameter and height gradients

A consecutive fabrication approach of independently tailored gradients of the topographical parameters distance, diameter and height in arrays of well-ordered nanopillars on smooth SiO2-Si-wafers is presented. For this purpose, previously reported preparation techniques are further developed and combined. First, self-assembly of Au-salt loaded micelles by dip-coating with computer-controlled pulling-out velocities and subsequent hydrogen plasma treatment produce quasi-hexagonally ordered, 2-dimensional arrays of Au nanoparticles (NPs) with unidirectional variations of the interparticle distances along the pulling direction between 50-120 nm. Second, the distance (or areal density) gradient profile received in this way is superimposed with a diameter-controlled gradient profile of the NPs applying a selective photochemical growth technique. For demonstration, a 1D shutter is used for locally defined UV exposure times to prepare Au NP size gradients varying between 12 and 30 nm. Third, these double-gradient NP arrangements serve as etching masks in a following reactive ion etching step delivering arrays of nanopillars. For height gradient generation, the etching time is locally controlled by applying a shutter made from Si wafer piece. Due to the high flexibility of the etching process, the preparation route works on various materials such as cover slips, silicon, silicon oxide, silicon nitride and silicon carbide.

Single core-shell nanoparticle probes for non-invasive magnetic force microscopy

We present an easy, fast and reliable method for the preparation of magnetic force microscopy (MFM) probes based on single Co nanoparticles (NPs). Due to their dipolar character, these magnetic probes open up a new approach for quantitative and non-invasive MFM measurements on the nanometer length scale. To guarantee long-term stability of these tips under ambient conditions, an ultrathin protecting Au shell was grown around the Co NPs through photochemical deposition. Single magnetic particles were firmly attached to standard silicon AFM tips using bifunctional self-assembling molecules. Such probes were tested on longitudinal magnetic recording media and compared to the results as recorded with conventional thin-film MFM tips. Easy data interpretation of the magnetic nanoparticle probes in a point dipole model is shown. Our nanoparticle tips provide excellent endurance for MFM recording, enable non-invasive probing while maintaining a high sensitivity, resolution, and reproducibility.

Freeze fracture approach to directly visualize wetting transitions on nanopatterned superhydrophobic silicon surfaces: more than a proof of principle

Freeze fracturing is applied to make the wetting behavior of artificially nanopatterned Si surfaces directly visible. For this purpose, almost hexagonally arranged nanopillars of fixed areal density (127 μm(-2)) and diameters (35 nm) but varying heights (40-150 nm) were fabricated on silicon. Measurement of contact angles (CAs) including hysteresis allowed to distinguish between the Wenzel (W) and the Cassie-Baxter (CB) states with droplets completely wetting the pillars or residing on top of them, respectively. Providing additional depth contrast by evaporating the ice replica with thin carbon and (typically 3 nm) platinum layers under 45° allowed resolving 3D features of 5 nm within the ice replica. In this way, laterally sharp transitions from CB- to W-states could be revealed, indicating the formation of zero-curvature water surfaces even on the nanoscale.

Cyclic photochemical re-growth of gold nanoparticles: Overcoming the mask-erosion limit during reactive ion etching on the nanoscale

THE BASIC IDEA OF USING HEXAGONALLY ORDERED ARRAYS OF AU NANOPARTICLES (NP) ON TOP OF A GIVEN SUBSTRATE AS A MASK FOR THE SUBSEQUENT ANISOTROPIC ETCHING IN ORDER TO FABRICATE CORRESPONDINGLY ORDERED ARRAYS OF NANOPILLARS MEETS TWO SERIOUS OBSTACLES: The position of the NP may change during the etching process and, thus, the primary pattern of the mask deteriorates or is completely lost. Furthermore, the NP are significantly eroded during etching and, consequently, the achievable pillar height is strongly restricted. The present work presents approaches on how to get around both problems. For this purpose, arrays of Au NPs (starting diameter 12 nm) are deposited on top of silica substrates by applying diblock copolymer micelle nanolithography (BCML). It is demonstrated that evaporated octadecyltrimethoxysilane (OTMS) layers act as stabilizer on the NP position, which allows for an increase of their size up to 50 nm by an electroless photochemical process. In this way, ordered arrays of silica nanopillars are obtained with maximum heights of 270 nm and aspect ratios of 5:1. Alternatively, the NP position can be fixed by a short etching step with negligible mask erosion followed by cycles of growing and reactive ion etching (RIE). In that case, each cycle is started by photochemically re-growing the Au NP mask and thereby completely compensating for the erosion due to the previous cycle. As a result of this mask repair method, arrays of silica nanopillar with heights up to 680 nm and aspect ratios of 10:1 are fabricated. Based on the given recipes, the approach can be applied to a variety of materials like silicon, silicon oxide, and silicon nitride.

Ordered arrays of gold nanostructures from interfacially assembled Au@PNIPAM hybrid nanoparticles

In this Article, we report on the assembly of hybrid Au@PNIPAM core-shell particles at the air/water interface, their transfer onto solid substrates, and the controlled combustion of the organic material to produce arrays of gold nanoparticles. A detailed investigation on the assembly behavior of such soft hybrid colloids at the air/water interface was performed by correlating the surface pressure-area isotherms with SEM and AFM images from samples transferred at different surface pressures. The hybrid particles display a complex behavior at the interface, and we could distinguish three distinct phases with varying interparticle spacings at different compression. The transfer process presented enables the decoration of topologically structured substrates with gold nanoparticle arrays, and the order of the initial monolayers is retained in the arrays of inorganic gold nanoparticles. The change in monolayer morphology upon compression can therefore be used to tailor the interparticle distance between approximately 650 and 300 nm without exchanging the colloids. More sophisticated gold nanostructures can be patterned into symmetric arrays using a similar protocol, which we demonstrate for nanostars and nanorods.

Arrays of size and distance controlled platinum nanoparticles fabricated by a colloidal method

Based on emulsion polymerization in the presence of a Pt complex, polystyrene (PS) particles were prepared exhibiting a well defined average diameter with narrow size-distribution. Furthermore, the colloids contain a controlled concentration of the Pt precursor complex. Optimized coating of Si substrates with such colloids leads to extended areas of hexagonally ordered close-packed PS particles. Subsequent application of plasma etching and annealing steps allows complete removal of the PS carriers and in parallel nucleation and growth of Pt nanoparticles (NPs) which are located at the original center of the PS colloids. In this way, hexagonally arranged spherical Pt NPs are obtained with controlled size and interparticle distances demonstrating variability and precision with so far unknown parameter scalability. This control is demonstrated by the fabrication of Pt NP arrays at a fixed particle distance of 185 nm while systematically varying the diameters between 8 and 15 nm. Further progress could be achieved by seeded emulsion polymerization. Here, Pt loaded PS colloids of 130 nm were used as seeds for a subsequent additional emulsion polymerization, systematically enlarging the diameter of the PS particles. Applying the plasma and annealing steps as above, in this way hexagonally ordered arrays of 9 nm Pt NPs could be obtained at distances up to 260 nm. To demonstrate their stability, such Pt particles were used as etching masks during reactive ion etching thereby transferring their hexagonal pattern into the Si substrate resulting in corresponding arrays of nanopillars.

Light-induced selective deposition of Au nanoparticles on single-wall carbon nanotubes

Novel applications of single-walled carbon nanotubes (SWNT) rely on the development of new strategies to make them easier to handle without affecting their structural properties. In this work, we have selectively deposited Au nanoparticles (Au NP) on SWNT assisted by UV light irradiation. XPS analysis and UV-vis spectroscopy indicate that the deposition occurs at the defects generated after oxidation of the SWNT. By addition of n-dodecylthiol, the separation of oxidized tubes with Au NP (Au-ox-SWNT) from tubes devoid of Au NP (bare tubes, b-SWNT) was achieved. Raman and UV-vis-NIR spectra indicate that UV irradiation induces a faster nucleation of Au NP on metallic SWNT. This new technique can be useful for the preparation of nanohybrid composites with enhanced properties, as increased thermal stability, and to obtain purified SWNT.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L1(0) phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.