Strategies for Controlled Placement of Nanoscale Building Blocks
© to the authors 2007
Received: 2 June 2007
Accepted: 20 August 2007
Published: 9 October 2007
The capability of placing individual nanoscale building blocks on exact substrate locations in a controlled manner is one of the key requirements to realize future electronic, optical, and magnetic devices and sensors that are composed of such blocks. This article reviews some important advances in the strategies for controlled placement of nanoscale building blocks. In particular, we will overview template assisted placement that utilizes physical, molecular, or electrostatic templates, DNA-programmed assembly, placement using dielectrophoresis, approaches for non-close-packed assembly of spherical particles, and recent development of focused placement schemes including electrostatic funneling, focused placement via molecular gradient patterns, electrodynamic focusing of charged aerosols, and others.
KeywordsPlacement Array Alignment Nanoscale building blocks Nanoparticle Nanocrystal Quantum dot Nanowire DNA Protein Carbon nanotube Template Electrostatic SAMs Dielectrophoresis Capillary force Growth
There has been a lot of interest recently in fabricating electronic, optical, and magnetic devices/sensors that are built on nanoscale building blocks such as nanoparticles, nanowires, carbon nanotubes, DNA, proteins, etc. Over the past decade, very promising performances have been demonstrated at the single device level or in a collection of a few single units [1–14]. Despite these successes, a major challenge remains: for the individual functional units to be incorporated into practical devices and sensors, they must be placed onto exact substrate locations so that they can be addressed and connected among themselves and to the outside world. This, i.e., the precise placement of nanoscale building blocks on exact substrate locations, is an extremely challenging goal. This article reviews recent progress in a variety of placement strategies, some of which are nearing maturity, while others are in their infant stages . Specifically, this review will discuss the following: (1) Placement using physical templates, employing capillary forces, spin-coating, surface steps, and others. This section also discusses template-assisted growth of quantum dot arrays; (2) Placement using molecular templates, employing patterned self-assembled monolayers (SAMs), whose specific terminal groups are functionalized to selectively interact with the building blocks; (3) Placement using electrostatic templates, employing localized charges on the substrate surface to attract charged building blocks; (4) DNA-programmed placement, employing 2D DNA crystals as scaffolds; (5) Placement using dielectrophoresis; (6) Non-close-packed assembly of spherical particles; and (7) Focused placement, employing focusing mechanisms to guide nanoscale building blocks to substrate locations which are smaller than the template guiding them.
The strategies that we will discuss in this article are not limited toabsolute placement in the fixed substrate coordinates, but includerelative positioning of nanoscale entities with respect to each other or to some reference structures. An example is a formation of 2D nanoparticle or protein arrays using a scaffold of 2D DNA crystal;relative positions between nanoparticles or proteinswithin the 2D DNA scaffold are well defined, although placement of DNA scaffolds themselves on the substrate is not easily controlled. We will also cover thegrowth orformation(rather thanplacement) of nanoscale entities that organize into an ordered form in one- or two-dimension. Formation of 2D quantum dot arrays using physical templates and growth of nanowires along the step edges belong to this category.
Placement Using Physical Templates
Physical templates can be utilized for controlled placement of nanoscale or microscale building blocks. Examples of physical templates include holes and trenches that can be fabricated on a substrate surface using lithography and etching/lift-off techniques, surface steps that naturally exist on crystalline metal and semiconductor surfaces, corrugation of substrate surfaces, and channels formed in a microstamp for molecular printings. In this section, we will review several strategies to position nanoscale and microscale building blocks using these physical templates.
Capillary Force Driven Placement into Physical Templates
Capillary force has been successfully exploited to place individual nanoscale/microscale building blocks into trenches or holes pre-defined on the substrate. In this approach [16–20], the substrate is immersed into a colloidal solution, and then slowly pulled out or slowly dried by solvent evaporation through heating. In both cases, the solution–air interface slowly recedes. At the front of the receding interface, the thickness of solution becomes smaller than the diameter of nanoparticles (for non-spherical shape, the height of the building blocks) and a three-phase solution–air–nanoparticle interface is formed around the nanoparticle surfaces. This three-phase interface creates capillary forces on the nanoparticles. The direction of the capillary force depends on thickness of the solution layer, which depends on the substrate pattern, thicker in the trenches or holes. The net result is that the nanoparticles are pushed into the trenches or holes while they pass through other areas without any deposition.
Formation of Quantum Dot (QD) Arrays Using Physical Templates
Quantum dots (QDs) are nanoscale objects in which electrons are confined in a dimension that is smaller than their de Broglie wavelength, resulting in the change of energy gaps or creation of quantized energy levels much like individual atoms (therefore, QDs are sometimes called artificial atoms) [22–24]. QDs have been of great interest due to their promising applications such as quantum electronic/optical devices [25–27], single electron devices , and single photon sources [28–30]. Among many methods, the formation of QDs in the heteroepitaxial growth of thin films using molecular beam epitaxy (MBE) has been most extensively studied. The usual growth mode is Stranski-Krastanow (SK) growth, in which self-assembled QDs are formed via 2D to 3D transition of epitaxial films in heteroepitaxial growth of lattice mismatched materials. This transition occurs spontaneously to reduce the misfit strain in the 2D strained heteroepitaxial wetting layers by forming dislocation-free 3D islands (QDs). Various QD systems have been grown using SK growth mode including Ge QDs on Si (001), GaAs QDs on GaAs (001), and InAs/InGaAs QDs on GaAs (001) [31–33].
The QDs produced as above, however, are randomly distributed over the surface and control of positioning has been difficult. For practical applications where individual QDs must be addressable, including integrated systems on single chips and single QD devices, it is required to grow QDs at exact locations. Among many strategies to grow QDs with precise position control, the template-assisted SK growth (specifically, the SK growth of QDs on pre-patterned substrates) has been shown to be very promising as demonstrated by many recent studies [30, 34–44]. This section briefly reviews recent advances in this strategy.
One method to grow well-ordered QD arrays is to use selective epitaxial growth (SEG) on a patterned substrate. In this approach, the substrate surface is masked with a material different from the substrate, and upon exposure to source gases, QDs grow only on the unmasked exposed surface, leading to a QD array in the original mask pattern. For example, well-ordered Ge QD arrays were grown on Si (001) by Kim et al. . They first made a square array of windows in SiO2 film (thickness 50 nm) on a Si (001) substrate. After selective deposition of a Si buffer layer on the exposed Si substrate, selective SK growth of Ge on the Si buffer layer was carried out. With a window size of 300 nm or below, they were able to grow exactly one Ge QD at the center of each window with excellent size uniformity, which was attributed to nucleation and diffusion kinetics, and/or strain energetics. Importantly, using this method, the size of the QDs can be made smaller than that of exposed windows.
The formation of precisely positioned QD arrays is not limited to 2D arrays, but can be realized for 1D and 3D arrays as well. One-dimensional QD arrays were formed utilizing modulated strain fields created by underlying pre-patterned trenches [39, 45, 50, 51]. The capability of forming ordered 2D QD arrays can be utilized to form 3D QD crystals through stacking of 2D QD arrays. Formation of 3D QD crystals was demonstrated for InAs/GaAs QDs on patterned GaAs (001) [36, 46] and for Ge QDs on patterned Si (001) . The capability of growing QDs on exact substrate locations has significant implications for the realization of practical quantum devices. For example, Kiravittaya et al. grew ordered GaAs QD arrays on GaAs (001) and demonstrated single photon emission from the ordered QDs . The formation of addressable QDs could lead to fabrication of integrated single QD devices.
Other Placement Schemes Utilizing Physical Templates
In addition to the capillary force assisted method and formation of QD arrays using pre-patterns, spin-coating assisted placement, assembly along the step edges of the surface, and sonication-assisted solution embossing are examples of other placement schemes using physical templates. Brueck and co-workers explored spin-coating to place sub-100 nm silica particles into holes and grooves patterned on silicon oxide film or a silicon wafer . They showed that the controlled placement of spherical particles can be achieved by choosing appropriate spin speed, the pH, and the geometries of grooves and holes (width, depth, diameter, and the sidewall slope). By adjusting these parameters, they formed one-particle wide linear chains, zigzag chains (1.5 particle wide), and two-column arrays of ∼80 nm silica nanoparticles inside pre-defined grooves.
Electrodeposition of atoms along the surface step edge is another useful method for positioning of nanowires on the substrate. For example, Penner and co-workers utilized step edges on a graphite surface to produce metallic molybdenum nanowires . Their approach involved two steps; first electrodeposition of molybdenum oxide (MoO x ) along step edges and reduction of MoO x to metallic Mo wires by hydrogen treatment. Mo wires with diameters ranging from 15 nm to 1.0 μm and lengths up to 0.5 mm were produced along the step edges. A similar approach allowed nanowire formation along the step edges with other materials such as Fe2O3, Cu2O, and Pd [60, 61]. The parallel alignment of Pd nanowires formed along the step edges was utilized by Penner and co-workers to fabricate hydrogen sensors .
Sonication-assisted solution embossing, recently reported by Stupp and co-workers, is a useful way for a simultaneous self-assembly, orientation, and patterning of one-dimensional nanostructures as demonstrated for the nanofibers of peptide-amphiphile molecules . In their approach, a stamp made of polydimethylsiloxane (PDMS) was pressed and held onto a glass or silicon substrate in a beaker containing peptide-amphiphile nanofibers in water, trapping the nanofibers between the channels of the stamp and the substrate. The combined effect of solvent evaporation, ultrasonic agitation, and confinement within the channels of the PDMS stamp resulted in alignment of peptide-amphiphile nanofibers parallel to stamp channels. Its capability of simultaneously orienting and patterning macromolecules may find many useful applications.
Placement Using Molecular Templates (SAMs)
Self-assembled monolayers (SAMs) are ordered assembly of organic molecules that spontaneously form on the surface of metals, metal oxides, and semiconductors [63–68]. The surface properties of SAMs can be engineered by selecting an appropriate tail group of the organic molecules comprising SAMs or modifying the tail group of existing SAMs with various techniques. Then, the substrate surface functionalized with localized patterns of SAMs can serve as templates onto which nanoscale or microscale building blocks are selectively attracted. There are many approaches for producing or modifying SAMs patterns and subsequent organization of the building blocks into the pattern areas. Since these are extensively reviewed by others [69–77], only some major approaches will be briefly described here.
The techniques for creating patterned SAMs can be categorized into three themes [69, 73]. First is to locally attach SAMs molecules onto desired substrate locations. This scheme includes microcontact printing (μCP) [78, 79], dip-pen nanolithography (DPN) [74, 80], and selective adsorption of specific SAMs molecules onto pre-defined substrate patterns [81, 82]. Second approach is to locally remove SAMs molecules from existing SAMs layer. This includes selective removal of SAMs using UV light [83, 84], STM-induced localized desorption of SAMs [73, 85, 86], and AFM-assisted localized removal of SAMs [73, 74, 77, 87–89]. For both themes, the exposed surface area having no SAMs can either be backfilled with other SAMs molecules or left bare. The third approach is to locally modify the terminal group of SAMs molecules, followed by selective functionalization and/or selective attachment of nanoscale building blocks [73, 88, 90–95].
An example of the first theme (patterning via attaching SAMs) is the μCP method [78, 79]. In this approach, organic molecules are inked onto an elastomeric stamp (typically made of polydimethylsiloxane (PDMS)) and transferred to the substrate surface by stamping. For example, alkanethiol molecules can be printed to form patterned SAMs on gold surfaces. Micrometer or sub-micrometer resolution patterns can be routinely obtained with this method. Selective placement of nanoscale or microscale building blocks onto the SAMs patterns were demonstrated for nanoscale or microscale particles, carbon nanotubes, nanowires, proteins, and DNA [96–100]. In another approach, target molecules (to-be-deposited molecules) themselves are inked onto the stamp and directly printed onto the SAMs-coated substrate surface utilizing specific binding between the target molecules and tail groups of SAMs molecules. For example, Whitesides and co-workers demonstrated patterned placement of biotin and benzenesulfonamide ligands onto SAMs of alkanethiolates on gold . The merit of μCP is that it is a parallel process and allows placement of nanoscale objects over a large area in very short time. Another merit is that placement of building blocks is possible for flexible or even curved substrates .
An example of the second theme (patterning via removal of SAMs) is STM-assisted patterning [73, 85, 86]. There are several mechanisms for the STM-assisted removal of SAMs (or combinations of these) including mechanical removal by tip-surface interactions, electron-beam-induced degradation or desorption, field ionization, and field-enhanced surface diffusion. For example, Kim and Bard demonstrated patterning SAMs of n-Octadecanethiol (ODT) on a gold surface through mechanical removal by bringing the STM tip closer to the substrate and employing a low bias (10 mV) and high tunneling current (10 nA) . Crooks and co-workers showed patterning of ODT SAMs with a resolution of 25 nm × 25 nm . AFM can also be utilized to locally remove SAMs [73, 74, 87–89]. The SAMs can be mechanically removed by the AFM tip, a process sometimes called nanoshaving. For example, Liu and co-workers demonstrated AFM-assisted removal of alkanethiol SAMs on a Au surface, followed by selective attachment of thiol-passivated Au nanoparticles onto exposed SAMs patterns . Another type of AFM-assisted patterning involves removal of SAMs and simultaneous oxidation of the exposed substrate surface, named local oxidation nanolithography (LON) [77, 87]. LON is based on localized oxidation reaction that occurs within a water meniscus formed between an AFM tip and the substrate surface. Lateral resolution of several tens of nanometers can be obtained with LON . The localized oxide pattern was utilized as templates to place nanoscale objects such as single-molecule magnets .
As a final note for this section, it is appropriate to point out that the scanning probe techniques, like other scanning techniques (e.g. e-beam and ion beam), have a limited throughput because they are serial processes. Nevertheless, recent studies employing a large number of probe tips have demonstrated the practicality of higher throughput processing [74, 106, 115–121]. For example, Mirkin and co-workers designed and fabricated a 55,000-pen 2D array, with a pen spacing of 90 and 20 μm in the x and y directions, respectively, occupying an area of 1 cm2 [115, 118]. With this parallel approach, they constructed a 2D array composed of 88 million gold dots on silicon wafer . A massive array of phospholipids has been constructed as well with a lateral resolution of ∼100 nm and a throughput of 5 cm2/min .
Placement Using Electrostatic Templates
Electrostatic interactions between a charged substrate surface and nanoscale building blocks can be utilized for controlled placement. This is done by creating charge patterns, i.e. electrostatic templates, on the substrate surface and letting the building blocks interact with the charge patterns. Electret materials such as poly(methylmethacrylate) (PMMA), poly(tetrafluoroethylene) (PTFE), silicon dioxide, and silicon nitride can hold trapped charges or polarization for a long time, and charge patterns can be created on the electret film through direct injection of electrons, holes, or ions [122–128]. Several methods have been developed to locally charge the electret surface and then place the building blocks selectively on the charged areas. These include methods using electrical microcontact printing (e-μCP), electron beams, ion beams, and scanning probe microscopes such as AFM. These techniques will be reviewed one by one.
Creating Charge Patterns Using Electrical Microcontact Printing (e-μCP)
Creating Charge Patterns Using Electron Beams
Controlled placement of biological molecules, such as DNA and proteins, was made by exploiting electron beam induced charge trapping [127, 134]. For example, by selecting an appropriate electron beam irradiation energy on glass substrate, Chen and co-workers created a layer (5–20 nm) of highly localized positive charges at the irradiated spot even though the net charge in the region as a whole was negative . This effect was due to the escape of secondary electrons, which varies with the incident electron beam energy [135, 136]. When the glass substrate with positively charged pattern was immersed in the DNA solution, the DNA, which are negatively charged, were selectively attracted onto the positively charged area. Using this procedure, they demonstrated the placement of DNA on a glass substrate with lateral resolution of ∼50 nm.
Creating Charge Patterns Using Ion Beams
Ion beams are also used as charge sources for creating patterns on electret films. Once the charged pattern is produced, oppositely charged nanoscale building blocks can be selectively adsorbed by immersing in a colloid containing charged particles, spraying the building blocks from the gas phases, or attracting them from the solid state powder form. For example, Fudouzi et al. used a Ga+-focused ion beam (FIB) to draw a charge pattern on a CaTiO3 substrate . They made a charged dot array (dot diameter: ∼6 μm), with the electric field from the charged dots being controlled by the Ga+ ion dose. Using an appropriate ion dose and choosing appropriate size microspheres (10 μm polymer spheres), they were able to place only one particle onto each charged dot. They attributed this one-particle-per-dot deposition to the shielding effect: once one particle occupies a charged dot, it shields the electric field coming from the charged dot, reducing the effective electric field.
Creating Charge Patterns Using AFM
Atomic force microcopy (AFM) offers another way to deposit localized charges on electret films [124, 125, 138, 139]. In this approach, a conducting AFM tip is positioned on the surface of a thin electret film which is deposited on a conducting substrate. When voltage pulses are applied between the conducting AFM tip and the substrate, localized charges can be deposited in the electret film. Depending on the polarity of the voltage pulses, either positive or negative charges can be deposited. This is a very attractive feature of AFM assisted patterning since it can create a combination of positively and negatively charged patterns on a same substrate by just varying the voltage pulse polarity. The amount of charge deposited and the area of the localized charge can be controlled by varying the height of the voltage pulses; with increasing pulse height, the amount of deposited charge and charged area increases . The charge area also depends on the tip geometry and quality. With their best tips, Mesquida and Stemmer obtained a lateral resolution of ∼100 nm using poly(tetrafluoroethylene) (PTFE) as an electret, as verified by the surface potential image acquired with KFM . On the charge patterns created with AFM, they were able to selectively deposit 290 and 50 nm silica beads.
DNA is a remarkable molecule that stores all the genetic information required for proper functioning and reproduction of living organisms. The important feature of DNA is the capability of molecular recognition through the Watson–Crick base paring, in which, through hydrogen bonding, Adenine (A) binds specifically to Thymine (T) and Guanine (G) to Cytosine (C). In addition, the DNA is a nanoscale molecule; for double-helical B-DNA, the diameter is about 2 nm and its helical pitch is about 3.4 nm [140–142]. The molecular recognition capability of DNA as well as its nanoscale dimension has been utilized as a powerful tool for programmed arrangement of various nanoscale building blocks. The key to this approach is to design DNA motifs that contain molecular recognition parts which can specifically combine with other DNA motifs in a selective and programmable manner. Conjugating nanoscale building blocks such as nanoparticles, proteins, ions, and organic/inorganic molecules with the DNA motifs can lead to the well-defined arrangement of nanoscale building blocks. This DNA-programmed assembly of nanoscale building blocks is a fascinating emerging field with high potential for bottom-up construction of nanoscale devices and sensors. Here we present several examples of recent successful studies. The interested reader may also look at excellent reviews and the references therein [140–150]. In this section, we first introduce DNA-assisted assembly using single-stranded DNA (ss-DNA), which leads to formation of linear arrays of nanoscale building blocks. We then briefly describe the key aspects of artificial DNA motifs (DNA tiles), which are more rigid than ordinary DNA, can be assembled into crystals, and are suitable as scaffolding for nanoscale building blocks. We then review programmed assembly of nanoscale building blocks that utilize DNA crystals as scaffolds. Several successful studies will be presented as examples.
Because ss-DNA is topographically of one-dimension, it is natural to try to utilize it for assembly of linear arrays of nanoscale building blocks. Many studies over the last decade have demonstrated that this approach is successful. For example, Niemeyer et al. used DNA–protein conjugate motifs to form linear protein arrays [148, 151–153]. They first made STV–ssDNA (streptavidin–single-stranded DNA) conjugates through covalent coupling between STV and thiol terminated short ss-DNA. These STV–ssDNA motifs were then hybridized with a long ss-DNA that contains sections with sequences complementary to those of the short DNA in STV–ssDNA. This led to the programmed formation of a linear streptavidin array along the long ss-DNA. This approach is not limited to streptavidin, but can be applied to many nanoscale objects that can bind to ss-DNA. For example, Matsuura et al. demonstrated one-dimensional assembly of galactose , Waybright et al. showed the assembly of organometallic compound arrays , and various nanoparticle arrays were also demonstrated by other groups [156–158].
Programmable DNA tiles and their assembly into crystals can be exploited to construct arrays of various nanoscale building blocks. This has been accomplished by employing the DNA crystals as scaffolds onto which nanoscale building blocks systematically attach. This may be done either by post-attachment of the building blocks on the pre-existing DNA scaffolds or by pre-attachment of the building blocks to DNA tiles, forming DNA-building block conjugates, followed by the DNA-programmed assembly of the DNA-building block conjugates. Using these approaches, various building blocks were controllably assembled, including arrays of proteins [165, 167, 169–171] and nanoparticles [168, 171–173]. A few examples of these recent studies are presented below.
In Fig. 15C, the DX tile B (red) contained an extended single-stranded DNA feature onto which a DNA–Au nanocomponent was able to bind. The DNA–Au nanocomponents were separately prepared by functionalizing 5 nm Au nanoparticles with thiolated single-stranded DNA via well-known thiolate-Au conjugation . When a droplet containing DNA–Au nanocomponents was deposited onto a pre-assembled 2D DNA crystal, the DNA–Au nanocomponents were selectively attached to DX tile B’s via DNA hybridization, leading to self-assembly of 5 nm Au nanoparticles as evidenced by AFM and TEM images in Fig. 15A and B, respectively.
Previous examples demonstrate assembly of nanoscale building block arrays that were made through post-placement of building blocks onto pre-assembled DNA crystal scaffolds. An alternative scheme is to prepare ss-DNA-building block conjugates first, followed by incorporation of the conjugates into DNA tiles and eventually into a DNA crystal. This leads to programmed placement of nanoscale building blocks onto specific sites in a DNA crystal. For example, Xiao et al. demonstrated self-assembly of metallic nanoparticle arrays using ss-DNA–nanoparticle conjugates . They designed 22 different types of ss-DNA which form four types of DX tiles (referred to A, B, C, and D). Au nanoparticles of 1.4 nm in diameter were used to form DNA–nanoparticle conjugates through covalent bonding between the Au nanoparticles and one type of ss-DNA which was modified with a thiol. These conjugates were then specifically incorporated into tile B during tile formation. When the DNA tiles self-assembled to a 2D DNA crystal, a 2D Au nanoparticle array having programmed nanoscale separations (4 and 64 nm in x and y direction, respectively) was constructed. Other examples include recent demonstration of programmed assembly of 5 and 10 nm Au nanoparticles into a 2D rhombic pattern by Zheng et al. . They designed and constructed triangular DNA motifs (termed 3D DX triangles) that were composed of three DX molecules forming a triangle. For each motif (3D DX triangle), two DX molecules were designed to form a rhombic DNA crystal, with one remaining DX molecule being used for attachment of a 5 or 10 nm Au nanoparticle. Self-assembly of the 3D DX triangles into a DNA crystal led to a formation of a precisely positioned nanoparticle array in rhombic pattern.
Placement Using Dielectrophoresis
Dielectrophoresis is the movement of uncharged objects in a liquid dielectric medium under the influence of a non-uniform electric field . Dielectrophoresis originates from the induced dipole moment of an object, whose value depends on the dielectric and electrical properties of both the object and the surrounding medium. With an appropriate design of the non-uniformity of the electric field, the movement of an object can be manipulated, allowing controlled placement onto specific locations and/or alignment in a particular direction. Dielectrophoresis has been extensively studied as a promising tool for manipulating various nanoscale or microscale objects such as nanowires, carbon nanotubes, nanoparticles, DNA, proteins, cells, bacteria, and viruses [176–188]. Recently, a lot of effort has been made to utilize dielectrophoresis for controlled placement/alignment of nanoscale building blocks for fabrication of nanoelectronic devices or sensors, where precise placement of the building blocks onto addressable locations is required on a large scale. A brief review of these advances is given here.
Equations 1–3 provide the fundamental basis for controlling the dielectrophoretic forces exerted on the objects. With appropriate choice of parameters (electric field gradient, frequency, dielectric medium, etc.), controlled placement and/or alignment of nanoscale and microscale building blocks have been accomplished.
Beyond the capability of positioning carbon nanotubes or nanowires between electrode pairs, for practical realization of nanoscale devices and sensors, more challenging requirements must be met. First, every electrode pair should be bridged by only a single nanotube/nanowire. Second, positioning of single nanotubes/nanowires over electrode pairs should be done simultaneously over a large area in parallel processing. A lot of effort, and with significant progress, has been made to meet these challenges over the past few years. For example, Chung et al. explored placing multi-walled carbon nanotubes (MWNTs) between a pair of opposing electrodes separated by a gap . They studied the effect of combining DC and AC electric fields on positioning of MWNTs and found that the ratio of DC versus AC field affects the degree of alignment, the separation between adjacent MWNTs deposited between electrodes, and the degree of contaminant deposition. With an appropriate electrode design and an optimized DC/AC ratio (AC frequency fixed at 5 MHz), they were able to place a single MWCT onto each electrode pair with a 90% yield as demonstrated for an array of 100 electrode pairs. They attributed this controlled placement of single MWNTs to a combined result of a dielectrophoretic force, an electrophoretic force, and a mechanical flow of ions generated by electrokinetic force. Upon bridging of an electrode pair by a single MWNT, these forces dramatically change and prevent the approach of other MWNTs, leading to single MWNT placement per electrode pair.
Krupke and co-workers utilized dielectrophoretic forces for simultaneous and site-selective placement of single bundles of SWNTs onto an array of electrode pairs . With AC bias, typically V p-p = 1 V and frequency at 1 MHz, they showed that ∼70% of the electrode pairs were bridged by SWNT bundles, of which more than 50% were by single bundles. This self-limiting positioning of single bundles was attributed to the change of electric field upon bridging of an electrode pair . An additional important finding of this study is that only metallic or quasi-metallic SWNTs were attracted to the electrodes, whereas semiconducting SWNTs were repelled. This was attributed to the fact that at high AC frequency the dielectrophoretic force is proportional to the difference of dielectric constants of carbon nanotubes and solvent medium, εCNT − εm, where εCNT is the dielectric constant of nanotubes and εm that of the solvent medium (see Eqs. 1–3). They used N,N-dimethylformamide (DMF) as solvent, whose dielectric constant εm is 39. At the 1 MHz frequency they used, the dielectric constant εCNT for metallic SWNTs is much larger than 39, whereas it is less than 5 for semiconducting SWNTs . This led to the attraction of metallic SWNTs to the electrodes, but repulsion of semiconducting SWNTs from the electrodes. Combined with a technique to well disperse individual SWNTs , this capability of dielectrophoretic forces to selectively position metallic SWNTs was exploited to separate metallic SWNTs from the usual mixture of metallic and semiconducting SWNTs .
As we briefly discussed, dielectrophoresis is emerging as a powerful tool to manipulate and position individual nanoscale objects, especially one-dimensional entities such as nanowires and carbon nanotubes. In particular, the capability of self-limiting deposition and that of large area positioning in parallel processing are important characteristics of this method, making it a candidate for practical fabrication of nanoelectronic devices or sensors.
Non-close-packed (ncp) Patterns of Spherical Particles
Self-assembly of nanoscale and microscale spherical particles into two-dimensional ordered form has been extensively explored by many researchers exploiting capillary forces, spin coating, and controlled solvent evaporation [16, 197–203]. In these types of self-assembly, the structure is usually limited to the hexagonal close-packed (hcp) structure. For many applications, it is desirable to have non-close-packed (ncp) arrays. Yang and co-workers have developed a method which can form ncp arrays of colloidal spheres by controllably deforming the substrates supporting the spheres [204, 205]. In their approach, they first fabricated a three-dimensional hcp array of silica spheres via controlled solvent evaporation of a silica suspension. Then, by using lift-up soft lithography, a top single layer of hcp spheres was transferred to the surface of a PDMS film. This PDMS film was subsequently swollen with a solution of toluene in acetone, transforming the hcp array of silica spheres on the PDMS surface into a ncp array. The lattice spacing of this ncp array was readily tuned by varying toluene concentration. A ∼50% increase in the lattice spacing was demonstrated using pure toluene. Finally, using the μCP (micro contact printing) technique, the two-dimensional ncp array of silica spheres on the deformed PDMS film was then transferred to the surface of a substrate that was spin-coated with a thin film of poly(vinyl alcohol) (PVA), producing an ncp array on the PVA-coated substrate.
For the various placement strategies discussed thus far, the placement precision is, at best, determined by the precision with which the templates (physical, molecular, or electrostatic) are defined on the substrate. Recently, there has been a lot of effort to develop new strategies that enable placement with much higher precision than the templates are defined . These strategies have a common theme, which may be termed “focused placement”, since the nanoscale building blocks are guided or focused onto targeted locations via electrostatic or mechanical forces. An analogy may be found in the operation of an electron microscope, where electron beam can be focused with sub-nanometer resolution although the guiding electromagnetic lenses are defined on the centimeter scale. In this section, we will review some recent advances in this approach including (1) electrostatic funneling, (2) directed assembly using molecular gradient patterns, (3) electrodynamic focusing of charged aerosols, and (4) guided placement combining capillary effect and electrostatic forces. In addition, precision placement of nanoparticles utilizing polymer micelles via template-assisted placement will be presented. Although this scheme might not be considered as truly focused placement, we will discuss this approach in this section because the nanoparticle placement is confined to a small area in the center of a template, allowing precision placement using much coarser templates.
This electrostatic funneling scheme works for other geometries as long as appropriate guiding structures are created. For example, when the guiding structures are changed from lines to dots, it is possible to place individual nanoparticles onto targeted locations, one nanoparticle per dot. Figure 24B shows an SEM image where ∼20 nm Au nanoparticles were funneled into the center area of square-shaped guiding patterns, one Au nanoparticle per square-shaped pattern. The electrostatic funneling method is not constrained to rely on surface patterns but is also effective for three-dimensional structures having appropriate guiding geometry. An example is shown in Fig. 24C, where ∼50 nm Au nanoparticles were placed along the centers of the exposed silicon oxide stripe made in a three-dimensional step structure.
Molecular Gradient Patterns
Electrodynamic Focusing of Charged Aerosols
Combination of Electrostatic Forces and Capillary Forces
Focused placement of spherical particles also can be achieved by exploiting both electrostatic forces and capillary forces. Using microscale polystyrene (PS) spheres as model systems, Aizenberg et al. have demonstrated a focused assembly of PS spheres (∼1 μm in diameter) onto small targeted spots using a two-step process; first, electrostatic attachment of the particles onto functionalized surface patterns of circular shape, then rearrangement of the attached particles toward the center area of the circular patterns . The functionalized surface patterns were made using μCP , which created patterns with negatively charged SAMs, and the remainder with positively charged SAMs. When the sample was immersed into the PS colloid in water, the PS particles (positively charged with amidine termination) selectively attached onto the negatively charged areas. In this process, the long-range electrostatic interaction (the electrical double-layer interaction [211, 212]) repelled the particles away from the positively charged SAMs and pushed them toward the center region of the pattern that was charged negatively, resulting in the first stage focusing effect. Second stage focusing effect, which is more dominant, was achieved during the drying process when the immersed sample was pulled from the colloid, rinsed, and allowed to dry. While the sample was drying, they monitored the movement of PS particles in real time using an optical microscope, which revealed dynamic rearrangement of particles toward to the center region of the circular patterns. This rearrangement, the second focusing effect, was attributed to lateral capillary forces that were created when water–air interfaces formed asymmetrically on the particle surfaces during the drying process.
Precision Placement Using Polymer Micelles
We have reviewed recent advances in various strategies for the controlled placement/growth of nanoscale building blocks. These were discussed in the context of seven categories; (1) placement using physical templates, (2) placement using molecular templates, (3) placement using electrostatic templates, (4) DNA-programmed placement, (5) placement using dielectrophoresis, (6) self-assembly of non-close-packed structure, and (7) focused placement. For the placement scheme using physical templates, we reviewed various approaches utilizing capillary forces, spin-coating, step-edges of crystalline metal and semiconductor surfaces, and sonication-assisted solution embossing. These methods allow controlled placement of spherical particles in the range of a few nanometers to several micrometers, nanometer scale non-spherical shape building blocks, nanofibers, and metal/metal-oxide nanowires. We also reviewed the formation/growth of 2D QD arrays utilizing underlying pre-defined patterns. A near-perfect yield of QD arrays has been demonstrated over a large area with a narrow size distribution.
Molecular templates (patterned SAMs) allow controlled placement of various nanoentities including nanoscale and microscale particles, carbon nanotubes, nanowires, proteins, viruses, and DNA. Patterning SAMs can be realized via selective attachment, removal, and/or modification of SAMs molecules. The associated techniques for SAMs patterning include microcontact printing (μCP), dip-pen nanolithography (DPN), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and irradiation with deep UV light or electron-beams. Placement precision of up to a few tens of nanometers was achieved using these approaches.
Placement using electrostatic templates utilizes electret materials to create charge patterns onto which charged building blocks are selectively attracted. The charging methods include electrical microcontact printing (e-μCP), electron-beam writing, ion-beam writing, and writing using a conductive AFM tip. Placement precision of a few tens of nanometers has been demonstrated using these approaches.
The molecular recognition capability of DNA has been utilized to form one- or two-dimensional arrays of nanoscale building blocks. Single-stranded DNA has been used to form one-dimensional arrays of nanoparticles, proteins, and organometallic compounds. Rigid artificial DNA motifs (DNA tiles) have been synthesized via reciprocal exchanges. The DNA tiles have been built into 2D DNA crystals through programmed matching of sticky ends in DNA tiles. Utilizing DNA crystals as scaffolds, 2D arrays of nanoparticles, proteins, and peptide–antibodies have been constructed.
Dielectrophoresis has been exploited to manipulate uncharged nanoscale objects in dielectric liquid medium. The control of dielectrophoresis with many parameters, such as dielectric constants of an object and its surrounding medium, magnitude and frequency of applied electric field, and electric field gradient, has been discussed. Special attention has been paid to utilizing dielectrophoresis to place one-dimensional objects (such as nanowires and carbon nanotubes) between two electrodes, which is essential for fabrication of nanoelectronic devices and sensors. A recent advance has demonstrated self-limiting deposition of single SWNTs across electrode pairs with more than 90% yield over a large area.
Self-assembly of spherical particles to non-close-packed (ncp) structure provides an important pathway to large-scale placement of nanoscale or microscale particles with a variety of spatial configuration and varying lattice parameters. Two advances were discussed in this article; one is based on geometrical change of PDMS films either by expansion in solvent or mechanical stretching. The other uses polymer micelles whose core either contains or reduces to metal or metal-oxide nanoparticles. Control of nanoparticle diameter and lattice spacing of nanoparticle arrays was demonstrated by appropriate selection of block copolymers.
Focused placement approaches allow placement of nanoscale building blocks with precision much higher than the precision with which guiding templates are defined. This approach includes electrostatic funneling, placement using molecular gradient patterns, electrodynamic focusing of charged aerosols, guided placement using the synergy of electrostatic force and capillary force, and precision placement using polymer micelles. The important merit of these focused placement approaches is that large scale placement with nanoscale precision can be accomplished because the guiding structures can be defined on the scale of a few hundreds nanometers using conventional lithography. Placement precision of less than 10 nm was demonstrated over large areas using guiding structures on the scale of ∼100 nm.
Although the materials covered in this review are only a small portion of vast research effort on-going or performed over the last decade or so, it is quite clear that there are already many techniques that are maturing and have potential for practical implementation. Considering the accelerating speed of new discoveries and developments in this field, we may anticipate practical devices or sensors based on nanoscale building blocks being a reality in the near future.
The author gratefully acknowledges Dr. Nancy Michael for valuable discussions. This work was supported in part by the Office of Naval Research (N00014-05-1-0030), National Science Foundation CAREER Grant (ECS-0449958), and Advanced Research Program of Texas Higher Education Coordinating Board (003656-0014-2006).
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