Fabrication of Ordered Nanostructure Arrays

Surface nanopatterning of periodic ordered nanostructures using colloidal nanoparticles

Two-dimensional patterned substrates with nanosize features have attracted great interest due to the necessity for high-resolution devices in biosensor research. The manufacturing of periodically ordered Au and Ag nanostructures has obvious advantages in comparison with disordered nanoarrays due to higher possibility for cooperative oscillation of free electrons in the ordered nanoparticle array, so-called “cooperative plasmon resonance.” Different methods have been demonstrated to fabricate periodic plasmonic nanostructures. Electron beam lithography with subsequent evaporation and lift-off is commonly used to produce one- or two- dimensional ordered nanoarrays. The obvious disadvantages of this method are small working range (<200 pm), low speed, and high costs. Interference lithography is a high-speed method for fabrication of large-area nanostructures with possibilities to change the periodicity of array. A drawback is the necessity to perform the complicated nanomanufacturing lift-off procedure.

Zhang et al. [55] proposed an interesting and one of the simplest methods of manufacturing similar structures based on high-speed combination of colloidal gold synthesis and interference lithography.

In this interesting work, the authors proposed an alternative method for fabricating periodic ordered nanostructures, demonstrating advantages of simplicity, high throughput, and low cost. In this technique, a one-dimensional photoresist (PR) grating on top of an indium tin oxide (ITO) glass substrate is produced initially by interference lithography. Subsequently, a gold nanoparticle colloidal suspension is spin-coated onto the PR mask. The average size of the nanoparticles was about 1.5 nm in diameter, and the ITO layer has a thickness of about 210 nm. Bearing in mind the low melting temperature of the gold nanoparticles, the sample was heated to above 200°C for about 5 min and subsequently cooled to room temperature. With temperature higher than the melting point, the gold colloidal solution aggregated into the grating grooves of the ITO substrate due to the strong wetting on the ITO surface and the constraint by the groove structures. The processes of annealing of gold nanoparticles and their accumulation in the grooves only results in the formation of nanowires, which are distributed periodically over an area of up to 20 mm². The mask-determined parameters of the gold nanowire structure, such as a period below 500 nm and a width from 100 to 300 nm, were obtained (Fig. 3.8). The height of the nanowires was dependent on the concentration of the gold colloid and the depth of the mask grating. The main advantage of this promising method is the ordering of nanostructures and the absence of lift-off process; however, the rough surface of the prepared nanowires due to the nanocrystal size distribution broadens the plasmon resonance spectrum.

Figure 3.8 (a) Measurements of contact angles of water on the ITO and on the PR surfaces: 0₂ = 44-45°, 0₂ = 73-75°. (b) Mechanisms for the confinement of gold nanoparticles into the grating grooves when the PR channel is small enough, (c) Two gaps will form for structures with large PR channels. Reprinted with permission from Ref. [55]. Copyright (2006) American Chemical Society.

Anodic porous alumina membranes

The usage of well-known anodic porous alumina (A1₂0₃) nanomembranes was established five decades ago as a convenient and powerful tool for nanomanufacturing, starting from the pioneer works of Possin with the preparation of nanowires within mica template [56]. The subsequent works of Martin [57] and Moscowits [58] using anodized aluminum oxide (AAO) as a template for metal nanowire fabrication initiated wide utilization of this method for different applications. Among the advantages of utilization of such AAO template are simplicity, possibility to use cheap equipment, tunability in preparation of nanostructures with different size and interparticle distance, and possibility to work with different materials [59,60]. The AAO membrane method allows better control of nanowire parameters than other protocols for the synthesis of similar nanostructures. Because AAO is also highly suitable for manufacturing nanostructure arrays that provide high surface- to-volume ratio and good access for biomolecules due to vertical alignment [61-64], we will consider this method in more detail.

The main approaches for the preparation and utilization of AAO were presented in an excellent review by Lei et al. [65]. The nanotemplate from anodic porous alumina is a highly ordered nanopore array with controllable geometry. Using different acid solutions (sulfuric for small (10-30 nm], oxalic for medium (30-80 nm), and phosphoric for larger than 80 nm pore sizes) in the electrochemical treatment of thin Al foil (or evaporated film), careful control of pore diameter is possible [65, 66]. The size of pores can be regulated also by anodization voltage in the range of 10-200 nm [65]. It should be noted that good tuning of nanopore diameter is observed for small and medium pore size, whereas difficulties occur for large pores. Usually, the quality of nanopore array ordering is better on the bottom side of the membrane, so Masuda and Satoh [67] proposed that the anodization process be performed in two steps, in which the top alumina layer is removed from the Al foil, leaving the highly ordered bottom layer. The next stage of the anodization process results in a pore array with high regularity for both sides of the AAO membrane. Subsequent annealing and polishing improves pore regularity and membrane surface smoothness. The area of pores free of defects typically does not exceed several square micrometers, so the "pretexturing" procedure was proposed by Masuda et al. [67], in which a single silicon carbide (SiC) mold was used as a stamp before anodization procedure, which allows to extend the defect-free ordered areas up to millimeter size. Disadvantages of thick (more than 1 pm) AAO membranes are difficulties in manufacturing zerodimensional nanostructures and preparing nanostructures directly on the substrate. To overcome this problem, a novel nanopatterning technology for the preparation of nanostructures using ultra-thin alumina mask (UTAM) was developed [68, 69]. Nanoparticle or nanohole arrays using AAO can be prepared on almost any smooth substrate (Fig. 3.9). Two different types of membranes—attached (which is placed on the surface) and connected (fabricated on the surface)—can be used. An advantage of attached membranes is higher regularity of pore arrays; however, if the attachment with the substrate is not in good condition, detachments are possible. A thin (5 nm) Au layer covering the backside of the AAO usually serves as a working electrode and as an initial substrate for the deposition of materials within pore array. Due to the direct contact of electrolyte solutions with gold, deposition takes place mainly in the pores [70]. The geometry of AAO template defines the diameter of nanowires and the distance between them, whereas the charge passed during the deposition procedure is proportional to the length of the prepared nanowires. The process of preparation of nanowires is finished by dissolving the AAO template in 1 M acidic or basic solution for several hours [70]. The total area of such a mask can be up to a few square centimeters. An advantage of the AAO method, especially in biosensing, is the possibility to prepare nanostructures with regulated interparticle gap, which is of importance due to the role of the so-called "hot spots” (interspaces in the vicinity of plasmonic nanoparticles, where a strong electric field is generated). Decreasing the distance between nanoparticles increases the electric field and, mainly, results in higher response of a biosensor. Using AAO membranes, Mirkin et al. [71] developed the on-wire lithography (OWL) technique, which allows production of segmented gold nanowires with unique narrow regulated gaps (from 2.5 nm). By periodically changing the composition of electrolyte and the time of deposition, segmented nanowires were prepared. After membrane dissolution, an oxide (e.g., Si0₂) or metal backing was deposited on the wires by thermal evaporation. The etching procedure was performed to dissolve the segments between gold segments, whereas backing maintains spacing between them. Similar structures enhance, for instance, the SERS signal up to 10^s times. Using the OWL technology, it is possible to produce nanostructures with distance between segments especially for biomolecules with known length. Besides the fabrication of one-dimensional nanostructures, AAO membranes can be used as original in the preparation of metal-replicated membranes [72, 73], and at present it is an excellent low-cost alternative method for manufacturing large-scale ordered arrays.

Figure 3.9 Schematic outline of the general fabrication processes of nanoparticles (a—bl—cl) and nanoholes (a-b2-c2). dl and d2 are highly ordered nanoparticle (Pd) and nanohole arrays (etched using focused ion beam) on Si substrates, respectively. Reprinted from Ref. [65], Copyright 2007, with permission from Elsevier.

Scanning beam lithographies

Scanning beam techniques, such as electron beam lithography (EBL) and focused ion beam (FIB) lithography, are the most common techniques for producing metal nanostructures on the solid surface. The main advantage of these methods is the ability to fabricate nearly arbitrary one- and two-dimensional patterns with precise control of the size, shape, and interparticle distance of surface-bound plasmonic nanoparticles. This type of control is needed to meet the requirements of sensor chip reproducibility and optical properties tunability for biosensing, SERS, and SEF applications.

EBL uses a highly focused electron beam to scan across a thin layer of radiation-sensitive resist that is deposited on a substrate, which is subsequently used as a mask for etching and metal deposition procedures to yield the desired metallic nanopattern. EBL provides outstanding resolution down to sub-10 nm range, which is attainable through the exploitation of specialized resists such as hydrogen silsesquioxane [74, 75] or traditional poly(methyl methacrylate) resist with ultrasonically assisted development [76]. The EBL technique is often used when dense gold and silver nanoparticle arrays of a particular geometry and size are needed. This method has been used to fabricate highly uniform arrays of monomers [77-79], dimers [80], and oligomers [81] of discs (Fig. 3.10), elliptical discs [82], trigonal prisms [77, 83], spheroids [84], half spheroids [85], rods [86], split rings [87], L-shaped [88] and almost spherical particles [89] with varying size and interparticle distance. EBL has been also applied for the creation of gold nanogratings [90-92] and subwavelength apertures [93] in thin gold films. Such nanoparticle arrays have been exploited to achieve low detection limits for anti-biotin and streptavidin in LSPR biosensor (3 nM and 7 pM, respectively) [79], to create a multianalyte DNA sensor based on surface-enhanced resonance Raman spectroscopy [87], and to enhance the dye fluorescence both on periodic nanoantenna [82] and subwavelength aperture [93] arrays. Another important application of EBL is the preparation of reusable masks and master stamps for other lithographic techniques such as nanostencil lithography [94] and nanoimprint lithography [95-97]. However, limitations of fabrication area (<200x200 pm²), low speed due to serial processing, and high costs entail the low throughput sample preparation with the EBL method.

Figure 3.10 SEM images of a typical gold heptamer sample fabricated by EBL. (Top) A normal view of the sample. The interparticle gap distance is 20 nm. The thickness of the gold nanoparticles is 80 nm. Reprinted with permission from Ref. [81]. Copyright (2010) American Chemical Society.

Another scanning beam lithography technique, FIB, allows writing both subtractive and additive patterns into a resist or directly onto the thin metal film by a highly focused beam of ions through the milling, ion-assisted etching, and ion-induced deposition processes [98, 99]. The attainable resolution of the FIB lithography method is <10 nm [100, 101], which is comparable to that of EBL. FIB techniques usually employ Ga⁺ ions to form a scanning beam [102- 104]; however, the use of other ions such as Ar⁺ for ion-assisted lithography has also been reported [105]. In principle, FIB can be used to fabricate arbitrary two-dimensional metal nanostructures, but due to its relatively slow processing capabilities in common with EBL, it is mostly used for the production of holes in metal films rather than isolated metal nanoparticles. Various metallic (i.e., Au, Ag, Al) nanostructures such as single and arrayed circular [102-104,106- 109] and rectangular [108] nanoholes, circular double-holes [110], slits [90, 111, 112], circular slits [113], and V-grooves [114] with varying size and periodicity have been fabricated by means of FIB lithography for research in the field of plasmonics and, especially, for biosensor applications [102-104, 106, 107, 110]. Namely, biosensors based on nanohole arrays demonstrated detection of attomolar streptavidin concentrations [106], 6-fold improvement in response time for surface adsorption of mercaptoundecanoic acid when operated in flow-through format [103], and up to 6.5- fold enhancement of Rhodamine 6G single-molecule fluorescence detection as compared to the open solution [102].

Colloidal lithographies

NSL is a type of colloidal lithography that employs a densely packed two-dimensional colloidal crystal as a deposition/etching mask. As opposed to SCL and HCL techniques, NSL is capable of producing ordered nanoparticle arrays with well-controlled shape, size, and interparticle spacing [115]. Such a possibility is achieved by charging the substrate surface with the same charge sign that is carried by polystyrene nanospheres. When polystyrene colloidal solution is deposited on such a pretreated substrate and dried, capillary forces drag the nanospheres together and they crystallize in a hexagonally packed pattern (Fig. 3.11). This colloidal mask can be readily used for both additive and subtractive lithographic processes to produce metallic nanostructures with sizes down to 20 nm [116]. In one of the NSL approaches, a metal film is deposited through the mask to produce a metallic "film over nanosphere” (FON) structure. Enhanced electromagnetic fields generated by the surface roughness of the Ag FON structure have been employed to excite strong SERS signals for detection of Bacillus subtilis (a harmless analog of Bacillus anthracis, an anthrax causative agent) with limit of detection of 2.lx 10'¹⁴ M [117], for in vivo glucose detection using an implantable SERS sensor [117] and for quantitative lactate detection in the clinically relevant range 0.5-22 mM [118]. Alternatively, if the nanosphere mask is removed after the metal deposition, typically by sonicating the sample in a solvent, a substrate can be obtained, which is covered with surface-confined metallic nanoparticles having triangular cross section, which corresponds to the shape of voids between polystyrene nanospheres. Such nanostructures, mostly made of Ag, have been widely exploited in LSPR and SERS biosensing [8, 119-121]. Ordered arrays of nanovoids in gold films have also been prepared by the electrochemical deposition of Au through the colloidal mask and subsequent removal of polystyrene nanospheres [122] and have been used as substrates for NIR-SERS [123]. Other types of nanostructures that have been prepared using NSL-based techniques include rhombic nanoparticles [124], nanopillars [125], anchored nanoparticles (Fig. 3.11) [126], "film over nanowell” structures [127], nanodiscs [128, 129], nanoholes [128], split rings [130], and micropatterned metallic films [131] for applications in LSPR biosensing and surface-enhanced spectroscopies. The main disadvantage of NSL is a variety of defects that arise in the process of crystallization as a result of nanosphere polydispersity, site randomness, point and line defects; typical defect-free domains have sizes in the 10-100 pm range. Dense nanosphere mask also imposes geometric constraints on the shape of the produced nanoparticles; specifically, only triangular or hexagonally shaped metal nanoparticles can be fabricated by metal deposition through the stacked layers of close-packed nanospheres. In order to tune the nanoparticle shape, additional processing is required, such as moving the sample or changing an angle during metal deposition, annealing, and electrochemical growth.

Figure 3.11 Schematic representation of the anchored Ag nanoparticle array fabrication using NSL. Reprinted with permission from Ref. [126]. Copyright (2007) American Chemical Society.

Nanoimprint lithography

Nanoimprint lithography is a nanopatterning technique that provides a tradeoff between fabrication throughput, precision, costs, and sample area, which allows eliminating drawbacks of scanning beam and colloidal lithographies for specific applications. This fast-developing technology provides means to replicate features on a hard or soft stamp in a thermoplastic or photocurable resist by embossing or molding. Subsequent deposition of a metal film on such a replica with or without further resist lift-off can be used to produce plasmonic structures resembling the stamp nanorelief for applications in chemical and biosensing. Among the advantages of NIL is the capability to create nanoreliefs over large sample areas in parallel mode with high pattern uniformity and low defect densities, which is crucial for mass production of reproducible sensor chips for biosensing instruments. Namely, NIL is capable of molding a variety of materials and pattern features with a sub-10 nm resolution on the cm² area scale [132-134]. Another advantage of NIL is a wide range of nanoparticle shapes that can be fabricated using specially prepared reusable NIL stamps (Fig. 3.12) [135], which is important in biosensing applications to optimize the performance of plasmonic nanoparticle arrays.

NIL has been employed to produce uniformly oriented and homogenous noble metal nanoparticle arrays using "nanoblock" molds fabricated from a collection of one-dimensional grating molds with different profiles (Fig. 3.13) [135]. Such anisotropic Ag and Au nanoparticle arrays have shown light polarization- and dimension- dependent plasmonic properties [135]. A similar approach has been applied to produce arrays of Au nanorectangles, which were shown to be a suitable basis for LSPR biosensor by detecting bovine serum albumin (BSA)-anti-BSA specific interaction [136]. High-density square arrays of free-standing cylindrical gold-coated polymer "nanofingers” fabricated using NIL have been demonstrated to be an efficient SERS substrate providing enhancement factors up to 2x10¹⁰ [137]. A two-dimensional array of cavities fabricated by NIL using a soft elastomeric mold in polyurethane, coated by 50 nm Au film, has demonstrated its sensing potential in light transmission experiments during the formation of a self-assembled monolayer of hexadecanethiol [138]. Similar quasi-3D plasmonic crystals have

Figure 3.12 (Top) Illustration of the general process used to fabricate "nanoblock" NIL molds possessing different lattice and particle geometries. (Bottom) Matrix illustrating possible nanoparticle array configurations from nanoblock molds produced using a collection of one-dimensional grating molds with different profiles. The array bordered by a solid line is produced using the inverse profile of grating A. Similarly, the arrays bordered by broken lines are achieved using the inverse of grating B. Reprinted with permission from Ref. [135], Copyright 2008, John Wiley and Sons.

shown a refractive index sensitivity of 700-800 nm/RIU [139] and enabled full multi-wavelength spectroscopic and spatially resolved detection of biomolecular binding events with sensitivities that correspond to small fractions of a monolayer [139-142], for example, 400 pM limit of detection for anti-IgG/IgG binding [142]. Au dot and ring arrays produced by NIL were used to detect the binding of streptavidin to biotin, and it was shown that the sensitivity of the LSPR spectra to the binding of the biomolecules was enhanced as the ring width of Au rings was decreased [143]. Arrays of gold-coated nanodomes fabricated on glass substrates using a soft NIL revealed complex plasmonic resonances highly sensitive to the array dimensions, the thickness of the gold layer, and the refractive index of the surrounding medium [144]. Other metallic nanostructures fabricated using NIL, which are promising for biosensing applications, include arrays of Pt nanowires [145], Ag island films deposited on grated and pillared Si substrates [146], elliptical Au nanodisc arrays [147], and Au nanodot and nanowire arrays [148,149].

Figure 3.13 SEMs of nanoblock molds derived from one-dimensional gratings. Reprinted with permission from Ref. [135], Copyright 2008, John Wiley and Sons.

However, this attractive nanolithographic technique is still under development and has several challenges to overcome. One of the important problems to be solved is the useful lifetime of the mold; at present, nanoimprint stamps require replacement after about 50 imprints made [150] due to the wear produced by temperature variations and high pressures applied during the imprinting process. A possible way to partially solve this problem is to avoid high temperature by using special resist treatment, which allows imprinting at room temperature [151]. Thermally assisted imprinting also has limited throughput caused by time requirements of thermal cycling; however, employment of photocurable resist instead of thermoplastic or imprinting at room temperature and high pressures [152] can speed up the nanofabrication process. Other conditions that should be maintained in order to enable uniform pattern transfer over large areas include absence of air bubbles between the mold and resist, parallelism of rigid mold and resist surfaces, and applied pressure uniformity.