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Nanopatterning Soluble Multifunctional Materials by Unconventional Wet Lithography By Massimiliano Cavallini,* Cristiano Albonetti, and Fabio Biscarini
control the effects of designed interactions across spatial length scales, as the emerging architecture depends on the interplay of thermodynamic and kinetics factors: the former are input by the materials design, the latter depend on the process used to cast, fabricate, or manufacture the material into a functional system. Quasi-equilibrium conditions are difficult to implement, and often are not technologically viable. Kinetics factors hamper the potential control across spatial length scales, introducing dispersion in order parameters, shape, dimensionality, and characteristic length scales. Mesoscale interactions, due to hydrodynamic and capillary forces, enter into play. It can therefore be envisioned that technological breakthroughs could be achieved from the integration of materials properties together with processes that optimize, and possibly exploit, their intrinsic self-organization into multiscale-defined architectures. Most processing techniques used for multifunctional materials, such as thin film techniques, either from vapor or liquid phase deposition, are not adequate for achieving such a control, as they are strongly out of equilibrium. A possible strategy to overcome the limitations of thin-film technology is to adopt additive manufacturing schemes, where only the functional amount of the material is deposited into spatially defined regions. In confined systems, characteristic time scales for self-organization change with the volume and dimensionality. Quasi-equilibrium conditions can be attained in shorter time scales, provided the fact that the physical dimension of the system where the materials self-organizes is reduced in size. This may imply, for instance, enhancement of surface nucleation effects with respect to bulk aggregation, or disappearance of diffusion-limited processes. Sustainability is another important factor pointing toward the direction of exploiting confinement for processing multifunctional materials, and implies the minimization of resources in the processing of the materials and the manufacturing of the functional system. An example is, for instance, the organic field-effect transistor: optimum device would comprise only a few monolayers of a suitably ordered material in the channel region. This configuration requires the minimum amount of material, and at the same time it encompasses the whole charge-transport layer, and minimizes the structural and morphological defects introduced by the growth of upper layers (e.g., orientational transitions, recrystallization).
Molecular multifunctional materials have potential applications in many fields of technology, such as electronics, optics and optoelectronics, information storage, sensing, and energy conversion and storage. These materials are designed exhibit enhanced properties, and at the same time are endowed with functional groups that control their interactions, and hence self-organization, into a variety of supramolecular architectures. Since most of the multifunctional materials are soluble, lithographic methods suitable for solutions are attracting increasing interest for the manufacturing of the new materials and their applications. The aim of this paper is to highlight some of the recent advances of solution-based fabrication of multifunctional materials. We explain and examine the principles, processes, materials, and limitations of this class of patterning techniques, which we term unconventional wet lithographies (UWLs). We describe their ability to yield patterns and structures whose feature sizes range from nanometers to micrometers. In the following sections, we focus our attention on micromolding in capillaries, lithographically controlled wetting, and grid-assisted deposition, the most used methods demonstrated to lead to fully operating devices.
1. Introduction Molecularly designed multifunctional materials are opening potential technological applications in electronics,[1] optoelectronics,[2] photonics,[3] photovoltaics,[4] spintronics,[5] permanent and erasable/rewritable memories,[6] sensing,[7] and, more recently, on functional scaffolds for cell and tissue growth.[8] The interest stems not only from the possibility to design new and enhanced properties, but also to combine different properties within the same material, else to integrate functionalities from libraries of materials that can be processed or fabricated on the same footings/technology. Chemical design can additionally impart noncovalent interactions (sterical, dispersion forces, hydrogen bonding, competing interactions) that give rise to supramolecular architectures.[9] The experimental control of architectures plays an important role in tailoring properties, since it is recognized that many relevant properties depend on size and dimensionality of the suitably organized material. Despite the potential offered by chemical design,[10] it is extremely difficult to [*] Dr. M. Cavallini, Prof. F. Biscarini, Dr. C. Albonetti Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-Bologna Via P. Gobetti 101 Bologna I-40129 (Italy) E-mail:
[email protected]
DOI: 10.1002/adma.200801979
Adv. Mater. 2009, 21, 1043–1053
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Massimiliano Cavallini, EURYI Awardee in 2006. Ph.D. in Chemistry, Cavallini is a researcher at CNR-ISMN Bologna since 2000, working in the interdisciplinary field of nanotechnology. He is author of more than 70 papers in highly qualified peer-reviewed international journals, several chapters of book and 10 international patents. He has been Principal Investigator of EU projects, and finalist of the Descartes Prize 2003. In 2005 he founded the spin-off company Scriba Nanotecnologie S.r.L., operating in the field of nanofabricated identification tags.
Cristiano Albonetti completed the Laurea in Physics at University of Bologna in 2000, and a Ph.D. in Physics in 2005. He is a researcher at CNR-ISMN Bologna since 2001. His research interests are on new scanning probe microscopies (SPMs), development of ultrahigh vacuum systems, growth of organic and inorganic materials, and surfaces nanostructuring.
Fabio Biscarini is senior scientist at CNR-ISMN Bologna and Head of Nanotechnology Division. He completed a Laurea in Industrial Chemistry at the University of Bologna and a Ph. D. in Chemistry at the University of Oregon, USA. He is coauthor of 130 publications and 14 patents. He is founder of Scriba Nanotecnologie srl, one of the first spin-off companies of CNR. He has been coordinator of EU projects, is Fellow of the Royal Society of Chemistry, and has been awarded the EU Descartes Prize 2007.
It is possible to envision different schemes for additive manufacturing guided by concepts such as multiscale control of self-organization through confinement, minimization of the amount of functional materials, enhancement of functionality. The aim of the present paper is to provide an overview, although not exhaustive, on some of the recent work performed along this direction with different fabrication approaches based on the use of solutions. We deliberately ignore important manufacturing techniques, such as inkjet printing and scanning probe lithography, as we have chosen to focus on large-area patterning. Soft stamps have been demonstrated as one of the most versatile
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and technologically viable tools for unconventional micro- and nano-manufacturing. However, they are often limited to polymers or to self-assembled monolayers. It is our specific purpose here to show how approaches developed from the broad family of unconventional fabrication can be effectively used to fabricate a vast number of multifunctional materials and to integrate them into operating devices, with optimized or even enhanced properties with respect to their bulk counterparts. In particular, we focus our attention on micromolding in capillaries[11] (MIMIC), lithographically controlled wetting[12] (LCW), and grid-assisted LCW[13] (GA-LCW), which are able to process solutions of functional materials and that can be used as explorative tests carried out in ambient laboratory environment. Our aim in this paper is to explain and examine the principles, processes, materials, and limitations of this new class of patterning techniques, and to describe their ability to form patterns and structures of a wide variety of materials with features that range from nanometers to micrometers in size.
2. Micromolding in Capillaries (MIMIC) 2.1. General Aspects MIMIC is a simple and versatile soft-lithographic method that forms complex microstructures on both planar and curved surfaces. It was introduced by Whitesides and coworkers in 1995.[11] MIMIC, whose scheme is shown in the Figure 1, can be considered the precursor of micro- and nanofluidics, and it can be used to pattern many soluble materials. The elastomeric stamp (or mold) is usually made of polydimethylsiloxane (PDMS) and is prepared by replica molding[14,15] of a structured master. The master can be fabricated by traditional lithographic methods (i.e., photolithography or electron beam lithography). Replica molding with an appropriate material such as PDMS enables highly complex structures in the master to be faithfully duplicated into multiple copies (replicas) with nanometer resolution in a reliable, simple, and inexpensive way. In particular, in the masters for MIMIC, the motif consists of protruding lines that become grooves in the replica. When the stamp is placed in contact with a surface, the grooves form the channels (capillaries). The size of the channel, together with the concentration of the solution and the self-organizing properties of the solute, determine the length scale that can be achieved with this process. Feature sizes ranging from millimeters to tens of nanometers have been demonstrated. When a solution is poured at the open end of the stamp, the liquid spontaneously fills the channels under the effect of capillary pressure.[16] The dynamics of infilling influence the final distribution of the solute. In order to apply MIMIC at a large scale, control over the experimental conditions is very important. In a laminar flow regime, the rate of infilling depends on the ratio between surface tension and the viscosity of the solution, the size of the channel, the length of the filled section of the channel, and the surface tension and friction between the solvent and the channel walls.[17] In particular, the rate of infilling is proportional to the cross-sectional dimension of the channel and inversely proportional to the length of the channel (capillary) that contains
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Figure 1. a,b) Scheme of microinject molding in capillaries and detail of last step of evaporation in c,d) concentrated regime, and e,f) dilute regime.
the liquid and to the viscosity of the liquid. Equating these contributions to pressure differences arising in the channel, it is possible to determine the local rate of flow, and thus the time necessary to fill the microchannel. In this manner, it is possible to know the distance travelled by the solution inside the channel:
Re f
hw 2s cosðuÞ z¼ 2 2 Dp Dp
(1)
where Re is the Reynolds number, f a geometrical factor, h the viscosity of the solution, w the velocity of the fluid along the main stream, s the surface tension, u the contact angle of the meniscus, and Dp the wet perimeter of cross-section. Applying Equation 1 on a finite-elements grid over the system domain, it is possible to calculate the velocity field, which depends on the presence and form of the PDMS mold. The mean velocity calculated for channels with a size in the range of 500–1000 nm is in the order of magnitude of a few millimeters per second. It must be noted that the rate of infilling of the microchannels by the solution is not constant, but tends to be faster in the center of the channel than at the edges. Furthermore, simulation results show a high vorticity during the infilling, indicating that the laminar flux is distorted by the geometry of the channel.[18] Although these effects are usually not intense enough to induce turbulence in the flux, they cause inhomogeneous distribution of the solute along the channels when the solvent shrinks. Depending on the concentration of the solution, two kinds of patterns can be obtained: i) Concentrated regime: If the solution reaches supersaturation when the microchannel is still full of solution, the pattern replicates the size of the microchannel (see Fig. 1c and d). ii) Dilute regime: If the solution reaches supersaturation when most of the solvent has evaporated and the volume of the
Adv. Mater. 2009, 21, 1043–1053
The self-organization of the solute enters into play at the later stages of shrinking when the solution reaches supersaturation, so that spatially organized nanodots, wires, or crystallites can be fabricated by exploiting dewetting,[19] ripening,[20] crystallite growth,[21] or more generally, self-organization. After the complete evaporation of the solvent, the stamp is gently removed (Fig. 1c) leaving the micro- and nanostructures on the surface. The capability of MIMIC has been demonstrated by the fabrication of patterned structures and devices from a variety of functional materials at micrometric and nanometric scales. Although MIMIC was used for the first time without solvent,[22] that is, using a prepolymer instead of solution, the most recent applications were developed using solutions of functional molecules. In fact, the only requirement is that the solvent not swell the polymeric stamp. The first successful application of MIMIC in the fabrication of heterostructured field-effect transistors using inorganic materials and a multistep procedure was reported by Hu et al. in 1997.[23] Recently, several other applications of MIMIC, such as nanopatterning using different classes of materials, that is, metallic[24] and magnetic nanoparticles (NPs),[25] nanoclusters,[26] organic semiconductors,[27,28] and biomolecules,[29] have been proposed.
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residual solution is not enough to fill the channel completely, the solution tends to accumulate on the boundaries of the channel, giving rise to some defects in the microstripes or split lines, as shown in Figure 1e and f.
2.2. Nanoparticles One of the most interesting and promising applications of MIMIC concerns the processing of nanoparticles (NPs). Blu¨mel et al. reported on the use of MIMIC to fabricate a test pattern with silver electrodes using a dispersion of silver NPs (CABOT AG-IJG-100-S1) followed by thermal annealing in order to compact the layer of particles. Using poly(3-hexylthiophene) as semiconducting active layer, they fabricated organic field-effect transistors (OFETs)[24] with performances comparable to corresponding devices based on sublined gold electrodes. Yu et al. applied MIMIC to fabricate stripes of rare-earth-iondoped LaPO4 nanocrystals sensitized by sol–gel process.[30] This class of NPs is particularly important, because they are employed in modern lighting and display flelds, such as fluorescent lamps, cathode-ray tubes, fleld-emission displays, and plasma display panels.[31] In their work, they prove the processability of NPs by MIMIC, controlling the pattern morphology, tuning the quantity of material, and the annealing conditions. They obtain stripes of doped LaPO4 nanocrystals with a resolution of 5 mm. Very recently, Cavallini et al. used the MIMIC to pattern magnetic NPs with sub-micrometer periodic features and a vertical resolution of a monolayer.[25] They demonstrated that the morphology of patterned NPs can be controlled simply by controlling the solution concentration. Exploiting confinement and competing interactions between the adsorbate and the substrate, they fabricated continuous or split stripes composed of
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Fe3O4 NPs. The particularity of that work is related to the effect of NP concentration on the printed stripes, controlling the volume of solution inside the channels. Working in a diluted regime, they reached a spatial resolution of a few tens of nanoclusters, depositing a single monolayer of NPs. Figure 2 shows the evolution of the morphology of Fe3O4 sub-micrometric stripes obtained using different quantities of Fe3O4 NPs dissolved in water. This approach represents a remarkable example of integrated top-down/bottom-up process.
2.3. Nanoclusters Conductive wires composed of platinum-carbonyl clusters with sub-micrometer width[32] have been fabricated by Greco et al.[26] They also show the possibility to transform microwires made of platinum-carbonyl clusters into ordered arrays of conductive platinum wires. This example illustrates the concept and application of a single-step bottom-up process to deposit sizedefined stripes of a soluble molecular conductor. In this case, a long-range molecular order was achieved by solute deposition in a confined environment, due to the slow deposition rate and the lateral confinement (quasi-equilibrium conditions) that depletes the number of stable nuclei. Depositing the platinum-carbonyl clusters in microchannels, they obtained parallel microstripes on different substrates, such as glass and silicon. The microstripes are crystalline and exhibit birefringence under polarized light in an optical microscope. The occurrence of light extinction at the same orientations for all the microstripes indicates that all the crystal domains inside them have the same
Figure 2. Evolution of the morphology corresponding to a variation of the solution filling fraction inside the microchannels: a) 100%; b) 75%; c) 50%; c) 25%. Adapted with permission from [26]. Copyright 2008 Institute of Physics.
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orientation. The confined deposition induces a coherent longrange order along the direction of the stripes. The electrical conductivity of the pattern is tuned and enhanced by controlling the size, shape, and order of the molecular domains. The [NBu4]2[Pt15(CO)30] microstripes exhibit an electrical conductivity of 2.1 102 S cm1, four orders of magnitude higher than the corresponding raw material (7.7 mS cm1). Thermal annealing can increase the conductivity of the microstripes due to the thermal decomposition of the [NBu4]2[Pt15(CO)30] that forms metallic platinum. The authors show that although the annealing process introduces morphological defects (grains boundaries) in the microstripes, the electrical conductivity is enhanced to 35 S cm1, a value comparable with other Pt nanowires reported in literature.[33] This work shows how the MIMIC process can be used to improve electrical conductivity, while exploiting the effect of a highly ordered aggregation of the soluble precursors upon solution confinement.
2.4. Biomolecules The possibility to work in solutions, including water, extends MIMIC capabilities to biological system,[34] and confers it high versatility into the patterning of biomolecules. Bystrenova et al. reported the multiple length scale patterning of DNA on a silicon surface using MIMIC and dewetting.[29] This approach yielded an array of DNA nanodots on a 1 mm2 area, where each dot consisted of a few molecules of DNA. The method exploits the selforganization of DNA molecules when the solution was confined between stamp and surface. The stamp size (few millimeters) controls the large-scale arrangement, while the self-organization of the confined molecules introduces the smaller characteristic length scale. The shape, size, and spacing of DNA nanostructures can be modulated by the choice of stamp features and the concentration of the DNA solution, due to the control of the wetting regime. This work demonstrated, for the first time, a viable route to fabricating patterned arrays of biomolecules by dewetting. Dimensional control can be achieved by controlling the stamp features (the periodicity and the volume of the capillary), the DNA concentration, and the volume of the infilling solution. For enough diluted solution of DNA (
