Purifying Colloidal Nanoparticles through Ultracentrifugation with

Invited Feature Article pubs.acs.org/Langmuir

Purifying Colloidal Nanoparticles through Ultracentrifugation with Implications for Interfaces and Materials Joseph B. Miller, John M. Harris, and Erik K. Hobbie* Department of Physics and Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, North Dakota 58108, United States ABSTRACT: Liquid-phase processing and colloidal selfassembly will be critical to the successful implementation of nanotechnology in the next generation of materials and devices. A key hurdle to realizing this will be the development of efficient methods to purify nanomaterials composed of a variety of shapes, including nanocrystals, nanotubes, and nanoplates. Although density-gradient ultracentrifugation (DGU) has long been appreciated as a valuable tool for separating biological macromolecules and components, the method has recently emerged as an effective way to purify colloidal nanoparticles by size and optical and electronic properties. In this feature article, we review our recent contributions to this growing field, with an emphasis on some of the implications that our results have for interfaces and materials. Through transient or isopycnic DGU performed in both aqueous and organic environments, we demonstrate some explicit examples of how the mechanical, electronic, and optical properties of thin films assembled from two specific colloidal nanomaterialssingle-walled carbon nanotubes and silicon nanocrystalscan be modified in response to fractionation.

1. INTRODUCTION The concept of polydispersity has its roots in colloid and polymer science, being defined as the ratio of weight-averaged to numberaveraged molecular mass or size. A size distribution with a standard deviation of zero thus has a polydispersity index (D) of exactly unity, implying that physical suspensions are always characterized by D > 1. For polymers, the distribution of chain lengths depends on a number of factors, such as the nature of the synthetic process, and tends to be relatively broad. Notable exceptions to this are certain forms of “living” polymerization and the natural polymerization that occurs in biological systems, both of which tend to produce size distributions that are relatively uniform. In colloidal science, the distribution of particle sizes has direct implications for the tendency of a sufficiently concentrated “hard-sphere” suspension to nucleate a close-packed crystal with long-range positional order. The simple dichotomy of the pair potential in the hard-sphere limit (zero at all finite separations but infinite upon contact) implies that the free energy and hence the driving force for crystallization are purely entropic. Colloidal suspensions with inhomogeneous size distributions that have interparticle potentials experimentally tuned to be short-range repulsive are generally not observed to crystallize on laboratory time scales.1 On the simulation side, Auer and Frenkel used kinetic Monte Carlo simulations to study the influence of size variance on colloidal crystallization and found that the free energy of the liquid−solid interface increases strongly with supersaturation in suspensions that have a broad distribution of sizes.2 As a general rule, rms variations in the particle size, (⟨δR2⟩)1/2, must be smaller than 12% of the mean size, ⟨R⟩, for entropic © 2014 American Chemical Society

crystallization to proceed, which corresponds to a critical D index of around 1.01. Quite recently, the concept of polydispersity has taken on a new and unique significance in the realm of nanotechnology. The fluid-based processing and self-assembly of conventional “hard” materials such as carbon and silicon represents one of the greatest promises that nanotechnology has to offer, and because the dispersion of a nanoparticle in a fluid implies a colloid, much of the formalism of colloidal science is immediately relevant. A familiar example closely linked to the discussion of the previous paragraph is the influence that size polydispersity typically has on superlattice formation in semiconductor nanocrystal suspensions.3,4 As a general rule, hard-sphere colloids do not exist on the nanoscale because of the relative importance of van der Waals (vdW) forces between metallic or semiconducting objects on nanoscopic length scales.3,4 The overall pair potential depends on both the nature of the core material, which defines the magnitude of vdW attraction, and the length and surface density of the capping ligand, which provides steric stabilization in solvated suspensions.3,4 Colloidal nanocrystals typically have an attractive potential on the order of a few kBT, but this attraction becomes much stronger in the absence of a solvent.3,4 Despite these attractive interactions, size polydispersity is still commonly viewed as a limiting factor for superlattice formation in colloidal Received: December 11, 2013 Revised: January 8, 2014 Published: January 13, 2014 7936

dx.doi.org/10.1021/la404675v | Langmuir 2014, 30, 7936−7946

Langmuir

Invited Feature Article

Figure 1. Size purification of colloidal nanoparticles based on transient DGU when the particles are (a) denser than the gradient medium (semiconductor nanocrystals) and (b) less dense than the gradient medium (single-walled carbon nanotubes). (c) Separation through isopycnic DGU where different colors denote different densities. In all cases, high-density impurities “pellet” out of suspension at the bottom of the tube.

concept of monodisperse takes on an entirely new meaning when viewed in this light. In this feature article, we review our recent contributions to this emerging field with an emphasis on some of the implications for colloidal materials and interfaces. Through transient or isopycnic DGU performed in both aqueous and organic environments, we demonstrate some unique examples of how the mechanical, electronic, and optical properties of thin films and coatings assembled from two specific types of colloidal nanomaterials single-walled carbon nanotubes and nanocrystalline siliconcan be modified, changed, and refined in response to fractionation. We conclude by examining the outlook for DGU as a large-scale purification scheme in light of other new approaches that have recently emerged for the same goals.

nanocrystal suspensions because entropy remains the driving force for long-range order.3,4 Other contemporary examples of polydispersity are entirely unique to the field of nanotechnology, and the most striking illustration of this involves single-walled carbon nanotubes (SWCNTs), the tubular allotropes of carbon approximately 1 nm in diameter and up to a million times larger in length.5 There are an infinite number of possible ways that a graphene sheet can be conceptually rolled into a SWCNT, and SWCNTs prepared from any given synthesis scheme thus typically contain a broad distribution of distinct nanotube species, each characterized by a chiral index (n, m) that describes the symmetry of the tube with respect to the underlying graphene lattice.5 When coupled with the semimetallic nature of graphene, the chiral index dictates the electronic band structure of the nanotube (semiconducting vs metallic),5 and this unique polydispersity problem represents one of the greatest hurdles to realizing the highly touted technological promise of SWCNTs. A tremendous amount of recent progress has been made toward resolving this issue, however, by relying on a variety of separation techniques directly borrowed from colloid and polymer science.4 In this expanded view of polydispersity, the ability to produce highly monodisperse suspensions of colloidal nanomaterials has significant ramifications for progress, and a particularly spectacular example of this comes from the recent integration of semiconducting SWCNTs into device arrays at densities of 109 nanotubes/cm2.6 The familiar surfactant sodium dodecyl sulfate (SDS) was used to disperse the SWCNTs in water, and densitygradient ultracentrifugation (DGU) was used to remove impurities and nanotube bundles.6 Purification by electronic type was then performed using column chromatography, where the semiconducting SWCNT fractions were collected, dialyzed, and deposited in trenches on silicon substrates through the ionic exchange of an iodide anion with the sodium cation of the surfactant.6 In this manner, nanotube transistors were assembled at a density of 104 transistors per chip in a conventional semiconductor fabrication line. Critical to the success of this breakthrough is a metallic SWCNT impurity concentration of 1 part in 104 or less. To cast this in the language of colloidal science, consider a binomial distribution with an outcome that can be either semiconducting (+) or metallic (−), where the latter has a probability of 10−4. In an ensemble of 109 nanotubes, the corresponding polydispersity index for the semiconducting SWCNTs would have to be D < 1 + 10−13, implying that the

2. BACKGROUND The simplest form of polydispersity encountered in nanotechnology is associated with particle size, and one of the most straightforward approaches for dealing with it is density-gradient ultracentrifugation (DGU), which is the method of primary interest here. DGU was first used to fractionate plant viruses in 1951 by Brakke,7 and it has been a powerful analytical tool in the field of biochemistry ever since. In its simplest transient form (sometimes referred to as rate-dependent or rate-zonal DGU), the method exploits the dependence of particle sedimentation rate on particle size and shape, being intimately linked to the effects of viscous hydrodynamics. A schematic of the transient DGU process is shown in Figure 1a,b, where separation by size occurs down or up the gradient depending on the density of the particle with respect to the solvent. A physical sense of the relevant parameters follows from an analysis of the underlying forces. For simplicity, we consider a monodisperse ensemble of spherical colloids of radius R and density ρc subjected to a spatially constant centrifugal field G along a direction z in a fluid of constant density ρs. The flux of particles in response to the centrifugal force is j(z, t) = c(z, t)v, where the terminal velocity v=

2GR2 Δρ 9ηs

(1)

depends on the density difference Δρ = ρc − ρs and is obtained by balancing the buoyant force with the low-Reynolds-number Stokes drag in a fluid of viscosity ηs. The continuity equation is 7937


Langmuir


∂c ∂c ∂ + ∇·j = + (cv) = 0 ∂t ∂t ∂z

(2)

subject to the initial condition c(z, 0) = c0δ(z), where c0 is the number of particles per unit area in a thin initial layer at z = 0. In the absence of diffusional broadening, the appropriate solution to eq 2 is c(z, t) = c0δ(z − vt), which corresponds to the layer moving down the tube at speed v. This solution will be strictly correct only for large Peclet numbers, Pe = 4πΔGR4/(3kBT), which will always be the case for large G provided the condition of Stokes flow remains valid. From this, we can extrapolate an expression for the particle size, R(z), along a medium of constant density: R (z ) =

9ηsz 2Gt Δρ

∝ z1/2 (3)

The profile of eq 3 suggests that a polydisperse suspension will fractionate by diameter along a centrifuge tube in response to both the density difference Δρ and the change in sedimentation velocity with size, where the latter effect is a combination of buoyant (R3) and hydrodynamic (1/R) effects. The z1/2 profile implies that the larger colloids will be less separated for larger z, but eq 3 also suggests that this tendency can be reduced by making Δρ decrease slightly with increasing z or by introducing a positive density gradient, ∂ρs/∂z, along the tube. The primary benefit of such a gradient, however, is to stabilize the sedimenting flow.8 Although somewhat oversimplified, the above arguments capture the essential physics of size fractionation through transient DGU, and they can be readily extended to nonspherical shapes such as rods and plates through the appropriate anisotropic extensions of Stokes’ law. Such an approach for separating nanotubes by length was first developed by Fagan et al.,9,10 and the same ideas have also been used to purify chemically modified graphene sheets.11 The transient DGU method has since been widely adapted to purify a variety of nanomaterials, including zeolite nanosheets,12 graphene oxide nanosheets,13 semiconductor nanocrystals, 14−16 semiconductor nanorods,17−19 and a range of other colloidal nanoparticles.20,21 For nanotubes in particular, the transient DGU approach has been used as an effective purification tool in a number of different studies, including SWCNT cellular uptake and interaction,22,23 SWCNT self-assembly and liquid crystallinity,24,25 the isolation and analysis of ultrashort SWCNTs,26 and the length purification of multiwalled carbon nanotubes (MWCNTs).27 An example of how length enrichment through transient DGU can enhance the optical properties of SWCNTs is shown in Figure 2. The longest fractions exhibit unique colors because of an enhanced oscillator strength in the interband optical resonances (Figure 2a,b), with an accompanying enhancement in the strength of the bandgap photoluminescence (PL) (Figure 2c,d).10 The increase in optical quality with increasing SWCNT length reflects a lower density of defects in the longer nanotubes, which has implications for the use of SWCNTs as nanoscale optical sensors.28 As another example of size separation, the influence of transient DGU on the bandgap PL of silicon nanocrystals (SiNCs) is shown in Figure 3, where the separations were performed in mixed solvents of chloroform and m-xylene using custom polyvinylidene fluoride ultracentrifuge tubes.15 A comparison of the fractions to the starting (AP) material is shown in Figure 3a. Quantum confinement in semiconductor nanocrystals implies a diameter dependence of the bandgap that leads to the well-known size dependence of nanocrystal PL, but for silicon, this effect is significantly modified by the indirect nature

Figure 2. (a) Length-enriched colloidal fractions of CoMoCat (blue/ left, purple/middle) and laser-ablation (green/right) SWCNTs show unique colors because of unique interband optical resonances coupled with a relative absence of impurities and defects. (b) Example of the UV−vis−NIR absorption spectra of length-enriched colloidal fractions of CoMoCat SWCNTs (F6 is the left/blue sample in a). (c) PL spectrum of a dilute initial (length-polydisperse) colloidal suspension of CoMoCat SWCNTs and (d) PL spectrum of the longest fraction, F6, in a and b. Adapted with permission from Langmuir 2008, 24, 13880−13889. Copyright 2008, American Chemical Society.

of the bandgap.29 Figure 3b shows the degree of SiNC PL line narrowing that can be achieved through DGU as referenced to both the parent (AP) material and a commercially available redemitting CdSe nanocrystal. The shift in the absorption spectra with changing size is shown in Figure 3c. An entirely different application of DGU involves running the separation to a steady state in which the solvated nanoparticle− surfactant complex reaches an isopycnic point in the density gradient (Figure 1c). From eq 1, the terminal velocity will be zero in a region where Δρ = 0, which defines a point of local equilibrium. In a density gradient, particles of different effective density will thus form equilibrium “bands” in regions of different solvent density. As noted above, the DGU method has its roots in biochemistry, where it has long been used for applications such as the rapid separation of lipoproteins for medical diagnostics. A wide variety of isopycnic, transient (rate-dependent), and mixed DGU schemes for lipoprotein purification can thus be found in the literature. The first application of isopycnic DGU to the purification of SWCNTs, however, was a novel scheme for separating DNA-wrapped nanotubes by electronic type (semiconducting vs metallic) developed by Arnold, Stupp, and Hersam.30 The same group extended the DGU approach to 7938


Langmuir


Figure 3. (a) PL spectra of solid films assembled from monodisperse SiNC fractions compared to the PL spectrum of the starting material (AP). (b) Typical fraction line shape (solid blue) compared to the parent population (dashed black) and a commercially available red-NIR-emitting metal chalcogenide (dashed red). (c) Solution extinction spectra and the corresponding emission spectra for typical SiNC fractions and the parent material (AP). Typical excitation lines are indicated in blue. Reproduced with permission from ACS Nano 2012, 8, 7389−7396. Copyright 2012, American Chemical Society.

surfactant-encapsulated SWCNTs shortly thereafter,31 and significant insight has since been gained into the mechanisms by which micelle formation in mixed-surfactant environments imparts subtle buoyancy differences to encapsulated SWCNTs of varied diameter, electronic type, and chirality as well as to SWCNT bundles and impurities.32,33 The practical implications of type separation through DGU have since been demonstrated for both semiconducting devices31 and transparent conducting thin films,34 and the utility of the approach for isolating doublewalled carbon nanotubes (DWCNTs) has also recently been established.35,36 Using nonlinear density gradients, Ghosh, Bachilo, and Weisman37 further extended the approach to achieve the isolation of single-chiral semiconducting species, where the nonlinear variation in density along the suspending gradient serves to enhance the resolution of slight buoyant shifts associated with small changes in the SWCNT chiral index. The DGU approach has also been used to isolate armchair metallic SWCNTs, which exhibit distinct colors in accordance with their unique diameter-dependent excitonic resonances.38 In the same manner that multiple centrifugation runs can be used to improve the isolation of specific lipoproteins, nanoparticle separations can also be further optimized through appropriately combined DGU schemes. Fagan et al.39,40 developed a novel approach that exploits both transient and isopycnic DGU by first isolating fully hydrated mixed-type SWCNTs through isopycnic DGU in a vertical rotor and then separating these into the two electronic types through DGU in a swinging-bucket rotor, where preliminary isolation of the waterfilled SWCNTs improves the yield of the final metallic and semiconducting SWCNT fractions in comparison to that of other DGU approaches.31,41 Dual-iteration DGU schemes can also be used to achieve narrow diameter distributions of metallic nanotubes.42 Looking again to previous work done with DGU in biochemistry, a newly emerging theme is the use of analytical ultracentrifugation to gain quantitative insight into surfactant structure and packing density for a variety of SWCNT−surfactant complexes, where such insight can then be used to better understand and further optimize the separation process.43 An example of the two-step separation scheme developed by Fagan et al.39,40 is presented in Figure 4, which shows the final distribution of the two fractions (semiconducting vs metallic) in the centrifuge tube as well as the colors of the parent and fractions before deposition on transparent poly(dimethylsiloxane) (PDMS) substrates. An initial narrow band of mixed-type SWCNTs separates into a broad upper (blue) band of the metallic species and a narrow (brown) band of the semi-

Figure 4. (a) Type-separated SWCNTs immediately after DGU, where the initial mixed-type suspension was layered near the position of the dark band. After separation, the metallic SWCNTs are broadly distributed over the top while the semiconducting SWCNTs remain in a brown band near the initial position. (b, top) Color of SWCNT solutions before separation (left) and the metallic (middle/blue) and semiconducting (right/brown) fractions compared to the transparent PDMS substrates (bottom). (c) SALS profiles from metallic and semiconducting films with a TEM image (inset) of the network morphology (scale = 250 nm). The scattering intensity has been reduced by the film thickness, and the greater scattering in the metallic film is due to the proximity of the laser wavelength to an interband optical resonance. Adapted with permission from Phys. Rev. B 2012, 85, 245439. Copyright 2012, American Physical Society.

conducting SWCNTs near the position of the initial parent layer. Such fractions can be assembled into thin films for applications in flexible electronics.44 Not surprisingly, the morphology of the films is independent of electronic type, as shown in Figure 4c, but subtle differences in nanotube diameter suggest modest differences in the strength of the vdW attraction, both between identical SWCNT types and between the nanotubes and a flexible PDMS substrate.45 Such empirical estimates are based on Lifshitz theory, which converts absorption data over a very broad spectral window into an attractive vdW potential.46 These effects are dominated by spectral overlap in the two higher-energy absorption features (plasmons),45 and although such estimates are approximate (most notably the attraction to the PDMS, which is likely overstated), the overall trends have been 7939


Langmuir


Figure 5. (a) Wrinkling in a compressed film of length-purified SWCNTs on PDMS (20 μm scale) and (b) the corresponding FFT (0.63 μm−1 scale). (c) Wrinkling state diagram in the plane of surface concentration and nanotube length. The lower solid curve is a fit of the conductivity percolation threshold, and the upper dashed curve is the rigidity threshold. The blue symbols denote films that remain flat up to 20% strain, the green symbols denote wrinkling by 2% strain, and the light-green markers are the mixed-length (unsorted) response. (d) NIR fluorescence micrograph (5% strain, L = 130 nm) showing percolated but unwrinkled regions (upper right, lower left) and an increase in the wrinkling length scale with thickness (20 μm scale). (e) TEM images of a compressed film showing SWCNT bundles aligned normal to the stress (20% strain, L = 130 nm, 5 μm scale, 100 nm for the inset). (f) FFT from optical micrographs and (g) the corresponding SALS pattern (10% strain, L = 130 nm, 3 μm−1 scale). Adapted with permission from J. Phys. Chem. C 2011, 115, 3973−3981, Phys. Rev. Lett. 2010, 104, 125505, and Phys. Rev. E 2013, 88, 032409. Copyright 2011, American Chemical Society and 2010 and 2013, American Physical Society, respectively.

substantiated by both ab initio calculations46 and recent experiments.47

The mean spacing of folds can still be used, however, to deduce the mechanical properties of the films,49−51 and what we find is a remarkable glimpse into the TPa mechanics of individual SWCNTs that is tempered by a very small yield strain indicative of extensive plastic rearrangement and change.49,50 Because of the disordered nature of the nanotube films themselves, the use of purified SWCNT materials with homogeneous characteristics is critical for isolating all of the variables that contribute to this type of plasticity. An example of this can be seen in Figure 5c, which shows a state diagram for wrinkling in the plane of surface density (or film thickness) and nanotube length. By separating an initial polydisperse SWCNT suspension into fractions of specific length, we can resolve the length-dependent curves that delineate connectivity percolation (solid curve, Figure 5c, I−II boundary) from rigidity percolation (dashed curve, Figure5c, II−II boundary).47,51 Transmission-electron microscopy (TEM) then reveals the mode of plastic change independent of nanotube length; compression of the film pushes the SWCNTs into vdW bundles oriented perpendicular to the direction of strain (Figure 5e−g).49,50 Purification by electronic type likewise offers insight into the structural mechanisms of plastic change in compressed SWCNT films but in an entirely different parameter space of polydispersity. The TEM images of a strained semiconducting SWCNT film shown in Figure 6a,b reveal the same mechanism of plasticity indicated in the previous paragraph for length-purified (mixed-type) films; the vertical dark bands indicate vdW bundles formed in response to horizontal compression.44 Our compression test is more stringent than other methods of evaluating mechanical flexibility for thin, stiff films, but it nonetheless allows for a detailed quantification of the response. Figure 6c,d shows how the strain response of the sheet resistance, RS, varies with film thickness, h, and applied strain, ε, for films assembled from metallic or semiconducting SWCNT fractions.44 Each measurement represents a SWCNT film of the indicated thickness under the indicated compressive strain, applied by releasing a prestrain held in the PDMS substrate, where the sheet resistance has been measured along the strain direction.44 In general, the junctions between contacted SWCNTs and bundles have been shown to

3. SOME IMPLICATIONS FOR INTERFACES AND MATERIALS Having reviewed several of the available DGU schemes currently being used for nanoparticle purification, we now turn our attention to some specific examples of how we have utilized a few of these approaches to improve, resolve, and differentiate the mechanical, electronic, and optical characteristics of some relevant colloidal materials and interfaces. We focus specifically on our recent collaborative work involving length-purified SWCNTs, type-purified SWCNTs, and size-purified SiNCs with an emphasis on applications for flexible nanotube electronics and the PL response of semiconductor nanocrystal ensembles. 3.1. SWCNT Purification for Flexible Conducting Films. Over the past decade, strain-induced wrinkling in thin, stiff films adhered to soft polymer substrates has emerged as a particularly powerful way of quantifying the mechanics of thin polymer films,48 and we have recently pioneered the use of this approach to study the mechanics of thin flexible films assembled from SWCNTs that have been purified by both length (through transient DGU)49,50 and electronic type (through isopycnic DGU).44,47 The wrinkling pattern that emerges in such coatings is intimately shaped by structural disorder,51 and the films themselves show considerable potential for applications as transparent conductors and flexible electronics.44 The insights gained from such studies can also be directly compared to atomistically informed coarse-grained simulations,52 with broad relevance to the mechanical behavior of carbon-nanotube polymer composites in general.53 Some generic examples of the pattern that emerges under compression for length-purified SWCNT films on PDMS substrates are shown in Figure 5. In contrast to the pure harmonic deformation exhibited by homogeneous thin polymer films under compression,48 the compressed SWCNT films show a nonperiodic arrangement of consecutive folds, where the disordered nature of the pattern is intimately linked to structural inhomogeneities and quenched variations in film thickness.51 7940


Langmuir


Figure 7. (a) Zero-strain modulus vs film thickness for each electronic type (blue/light, metallic; brown/dark, semiconducting) with percolation fits (solid curves). (b) Scaled modulus in terms of the rigidity percolation thresholds, where a factor of 2.4 is required to bring the metallic data up to the semiconducting data, in good agreement with a simple scaling argument and the independently determined Hamaker coefficients for the vdW pair potential between identical electronic types. Although the modulus increases strongly with increasing thickness, the yield strain decreases. Adapted with permission from Sof t Matter 2013, 9, 11568−11575. Copyright 2013, Royal Society of Chemistry.

Figure 6. (a) TEM image of a 35-nm-thick semiconducting film on PDMS subjected to a 10% compressive strain (10 μm scale) and (b) the same film viewed with TEM at higher magnification (300 nm scale). (c) Strain response of the sheet resistance for metallic films. (d) Strain response of the sheet resistance for semiconducting films. The dashed lines represent films with residual surfactant (film set A), and the solid lines are films that have undergone more extensive processing (film set B). Adapted with permission from ACS Nano 2012, 6, 881−887. Copyright 2012, American Chemical Society.

SWCNT−SWCNT potential is dictated by vdW forces, which are dictated by Hamaker constants.55 An independent estimate of the SWCNT−SWCNT Hamaker constants for the nanotubes in question suggests that the Hamaker constant for the semiconducting SWCNTs (195 zJ) is 20% larger than that for the metallic SWCNTs (160 zJ), where this difference primarily arises from differences in the oscillator strength of the high-energy plasmon resonances and a smaller mean diameter (and hence a higher effective charge density) for the semiconducting nanotubes.45 Simple scaling arguments then suggest that the lowstrain elastic moduli of the semiconducting films will be roughly a factor of 2.2 larger than those of the metallic films,45,47 in good accord with the scaling plot in Figure 7b. Although the differences in electronic and mechanical behavior between metallic and semiconducting SWCNTs cited above are specific to details of the particular DGU separation and assembly schemes employed, we anticipate that analogous changes would be indentified using different variations of the length and type purification protocols used here. A deeper understanding of these differences, in turn, will allow us to design new purification processes and assembly schemes that optimize and even enhance the mechanical response of the films while further optimizing how these materials perform in targeted device platforms. 3.2. Size Purification of SiNCs in Organic Solvents. An entirely different area where DGU can also have an impact on material performance is the PL response of SiNC solutions and films. Through the well-known phenomenon of quantum confinement, the bandgap of a semiconductor nanocrystal can be uniquely controlled via the size of the nanocrystal.3 This has led to a number of potential applications for semiconductor nanocrystals (quantum dots), ranging from quantum-dot transistors, photovoltaic devices, diode lasers, and LED displays to biomedical imaging contrast agents and pharmaceuticals.3,4 Although most of the focus to date has been on nanocrystals synthesized from binary alloys such as cadmium selenide, nanocrystalline silicon is gaining increasing attention because of its relative availability and its comparatively benign biological and environmental interactions. A number of different synthesis schemes have been devised to produce SiNCs with varying degrees of polydispersity, solubility, and doping.29,56−59 Working with plasma-synthesized SiNCs in the colloidal domain,29 we routinely achieve D values of less than 1.01 through

play a critical limiting role in the charge-transport characteristics of nanotube networks.54 For film set A, which was cleaned in acetone for 1 h, dried under vacuum, and then cleaned in ethanol for 30 min, the superior electronic durability of the metallic films is consistent with better interfacial charge transport for metallic SWCNT−SWCNT contacts, but it is also consistent with larger amounts of residual surfactant in the semiconducting SWCNTs acting to reduce the quality of interjunction electrical contacts.44 A larger amount of residual surfactant in the semiconducting nanotubes, in turn, is consistent with the DGU separation scheme because a higher-density micelle would have more tightly bound surfactant that would require extra processing to remove.44,45 Film set B, in contrast (solid curves, Figure 6c,d), was cleaned in acetone for 2 h, dried under vacuum, and then cleaned in ethanol for 2 h, and as a consequence, these films exhibit strain-stable sheet resistance to within 2% (solid curves, Figure 6c,d). This bodes well for the use SWCNTs in flexible electronic devices as long as stringent protocols are employed to remove as much residual surfactant as possible. Not surprisingly, such differences in strain response versus electronic type also emerge at the level of film mechanics. Although the percolation thresholds for conductivity and rigidity are distinct, differing by more than a factor of 2,47 they also vary with electronic type; percolation occurs at twice the film thickness in the metallic as compared to semiconducting SWCNTs.44,47 Because the percolation threshold scales inversely with the aspect ratio, this difference can be directly linked to a factor of 2 larger aspect ratio in the semiconducting SWCNTs because the potentials in both cases are so deep as to render the tubes irreversibly bonded on contact.44,47 This difference in aspect ratio, in turn, is also a higher-order consequence of the type purification scheme.44 As shown in Figure 7a, however, there are additional differences in mechanical response that go beyond the difference in percolation threshold. The attractive part of the 7941


Langmuir


yet been observed close to this important target SiNC size, although we are currently working to achieve it through novel variations of a variety of traditional solvent removal schemes.3,4 Although oxygen is detrimental to the PL stability of such films, recent experiments carried out under a nitrogen atmosphere show greatly improved performance.15 A striking trend exhibited by such data is a PL enhancement (PLE) where the intensity and lifetime both increase under continuous illumination, and we recently exploited this PLE to achieve photostable emission through an appropriately pulsed excitation scheme.15 Although an exact explanation for this enhancement is not yet clear, nanocrystal interactions are a likely candidate, which would also be consistent with the recent observation of enhanced luminescence through the interaction of a single SiNC with a proximate silicon nanowire.61 Such effects would have obvious implications for applications that seek to exploit the luminescence or light-harvesting capability of SiNC ensembles and films. As noted above, the exact mechanism by which reduced size polydispersity enables this type of PLE in SiNC ensembles is not fully understood, although similar effects are also observed in the metal chalcogenides.63 The most obvious influence is increased “band alignment” or a reduced polydispersity in the bandgap,15 which might allow more efficient charge transport throughout the ensemble and hence more efficient passivation of “dark” surface trap states by such charge carriers. Studies focused on amorphous clusters of SiNC’s assembled in SiNC−polymer mixtures, for example, suggest that PL enhancement is suppressed in this scenario, which could simply reflect residual polymer at the interface between neighboring nanocrystals acting to impede nanocrystal−nanocrystal interactions.15 Temperature-dependent PL measurements with and without polymer also reveal interesting and unanticipated trends in the PL lifetime as a function of SiNC size, even though there is no difference in the temperature dependence of the bandgap (Figures 9 and 10).63 Figure 9 shows the temperature dependence of the PL from both pure nanocrystal fractions and polymer nanocomposites as well as an example of how such nanocomposites can be assembled on the end of a coupled fiber to make an optical cryoprobe.63 Figure 10a shows how the extrapolated 0 K bandgap, E0, changes with nanocrystal size for both pure fractions and PDMS nanocomposites, along with TEM images of a typical fraction (upper inset) and a TEM image of an individual SiNC (lower inset). The curve in Figure 10a is the anticipated theoretical trend for quantum confinement in

transient DGU in organic density gradients of mixed chloroform and m-xylene,15 where an example of a typical separation is shown in Figure 2. Dried films appropriately cast from monodisperse SiNC suspensions with nanocrystal diameters close to 4 nm exhibit only very localized regions of ordered close packing, as shown in Figure 8a. True superlattice formation in nanocrystal-

Figure 8. (a) Raw TEM image of a dried fraction (left) with indicated grain boundaries and the corresponding Voronoi pattern (right, 10 nm scale). (b) PLE for a dried fraction under continuous illumination at 365 nm (115 mW/cm2), where the open circles denote the PL lifetime (normalized by the final value) before and after the illumination interval. Data represent an ensemble average of multiple spots under N2. The inset shows the brightening interval (BI) in units of 103 s as a function of nanocrystal diameter for fractions of comparable ensemble size and excitation power (50 μm and 80 mW/cm2). (c) PL and τ (normalized by the initial values) for the polydisperse starting material (bleaching) and for a fraction (f10, brightening). PL and τ were measured simultaneously under N2 with modulated pulsed excitation (375 nm, 1 kHz, 30 ps pulse width, 67 mW/cm2 mean excitation power). Adapted with permission from ACS Nano 2012, 6, 7389−7396. Copyright 2012, American Chemical Society.

line silicon has only recently been reported for nanocrystal diameters near 2 and 8 nm,60 both of which are well removed from the exciton Bohr radius of silicon (which is in the vicinity of 4 to 5 nm).29 To our knowledge, superlattice formation has not

Figure 9. (a) Low-temperature (80 K) PL spectra of pure SiNC films assembled from fractions, where the black curve is the (80 K) PL spectrum of the parent material. (b) Temperature dependence of the energy of peak PL for both pure SiNC fractions and PDMS nanocomposites, where the arrow indicates the direction of increasing nanocrystal size. (c) Normal and PL (inset) color images of a fiber-optic cryothermometer made from a middle SiNC fraction in PDMS. The excitation is introduced through the bottom fiber, and PL is collected through the top. Adapted with permission from ACS Appl. Mater. Interfaces 2013, 5, 4233−4238. Copyright 2013, American Chemical Society. 7942


Langmuir


carbon nanotubes and colloidal silicon nanocrystals. We continue to rely on DGU as an invaluable separation tool in our ongoing collaborative research, and we hope that we have made evident the utility of the method as a research tool in nanoscale colloid and interface science. Advances in the DGU approach applied to nanoparticle purification are ongoing, and such breakthroughs will undoubtedly continue to shape how we use the method in the future. Examples include a dual-iteration DGU scheme involving an initial linear density gradient applied to sodium cholatesuspended SWCNTs with subsequent ultracentrifugation in a cosurfactant environment of SDS to achieve the isolation semiconducting SWCNTs with a very narrow range of diameters.66 Similarly, the use of sodium cholate with added NaCl in a self-forming gradient leads to an enrichment of semiconducting SWCNTs with a slightly smaller diameter.66 Narrow distributions of semiconducting SWCNTs are of general interest for a range of technologically important applications such as fiber optic communications, biomedical imaging contrast agents, and field-effect transistors, and it will be interesting to see which SWCNT separation approaches ultimately triumph in the realm of commercial applications. Column chromatography has considerable appeal because it clearly scales in a high-throughput fashion,67,68 but column retention and column lifetime might be limitations in comparison to DGU, which can be tedious and time-consuming but is ultimately an otherwise robust and effective approach. More recently developed methods based on spontaneous SWCNT partitioning through liquid−liquid phase separation are quite promising, however, and also offer the promise of scalability with high yield at the level of single chirality.69 On the analytical side, there is still much to learn about the exact physical and chemical mechanism by which DGU works to achieve SWCNT separation by electronic type and chirality, much of it obviously dealing with the nature of surfactant packing around nanotubes in an aqueous mixed-surfactant environment.32,43 Recent work using amphiphilic block copolymers to disperse SWCNTs while monitoring solution pH during DGU, for example, suggests that linear copolymers are capable of extracting either metallic or semiconducting SWCNT fractions at greater than 99% purity, but such work also elucidates details of the SWCNT surface chemistry that ultimately templates copolymer adhesion.70 A better understanding of these underlying mechanisms has the potential to facilitate new and more efficient nanotube separation schemes. Similarly, the use of transient DGU in organic solvents as a sizeseparation scheme for colloidal nanocrystals is a relatively new approach that has considerable potential as a tool for clarifying the yield and polydispersity of new high-throughput synthesis schemes. This is particularly true for silicon. Although semiconductor nanocrystals synthesized in solution from precursors of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide can be readily produced in a manner that yields monodisperse colloidal suspensions, the scalable synthesis of silicon nanocrystals typically demands a different environment and hence a different approach. DGU in mixed organic solvents has the potential to isolate different sizes and densities of nanoscale colloids quickly into well-defined fractions in a manner that can then provide analytical resolution of the yield. Because of this, we anticipate that we will be using the method as an SiNC purification tool for the foreseeable future, and we look forward to what we will learn along the way.

Figure 10. (a) Extrapolated 0 K bandgap vs nanocrystal size shows no distinction between pure SiNCs or PDMS nanocomposites. The fit (solid curve) is the anticipated trend for quantum confinement in silicon, and the insets are TEM images of a typical fraction (upper, 5 nm scale) and a single nanocrystal (lower, 1 nm scale). (b) Lifetime as a function of nanocrystal size for pure SiNC’s and PDMS composites at 300 and 80 K. Error bars are the size of the markers. Adapted with permission from ACS Appl. Mater. Interfaces 2013, 5, 4233−4238. Copyright 2013, American Chemical Society.

silicon.63 Figure 10b shows the PL lifetime versus nanocrystal size for pure-fraction and nanocomposite films at both low and high temperature, with an unusual size dependence in the lowtemperature data for the pure nanocrystal films. Again, such results are suggestive of the emergence of many-body effects in the collective PL response.63 By comparison, recent studies of the ensemble PL from CdSe−polymer Langmuir−Blodgett films suggest that nanocrystal packing morphology might play a critical role,64 which leaves open the possibility that the influence of size polydispersity on ensemble morphology is an important factor in the PL response. Further study of such many-particle systems is clearly warranted, and our DGU purification schemes can be readily applied to other materials as well, such as germanium nanocrystals.65 Similar to the results we described in the previous section on nanotubes purified by length through DGU, what is striking about the nanocrystal data in Figures 8−10 is that all of the sizedependent results, for both pure films and polymer nanocomposites, have been obtained through DGU performed on a single parent SiNC suspension. Again, this speaks to the power of DGU as a separation tool in nanomaterials research. Significant further insight into these ensemble phenomena can likely be gained by studying the microscopic nature of PL enhancement from the scale of an individual “blinking” nanocrystal up to clusters containing more than a thousand SiNC’s, and we are currently engaged in such a study. As a research tool, DGU is again proving to be essential because it allows us to systematically and quantifiably narrow the range of particle diameters that occur within a given nanocrystal ensemble. DGU also allows us to focus our efforts on nanocrystal sizes close to the exciton Bohr radius of silicon.

4. OUTLOOK AND CONCLUSIONS We have reviewed our recent contributions to the emerging field of DGU fractionation applied to colloidal nanomaterials with an emphasis on how the method can be used as a research tool to advance the field of nanotechnology. Using the generic platform of transient and isopycnic DGU carried out in both aqueous and organic solvents, we have tried to offer a general overview of some unique examples of how we are using DGU to refine and better understand the mechanical, electronic, and optical properties of thin films and coatings assembled from colloidal single-walled 7943


Langmuir

■


AUTHOR INFORMATION

Erik Hobbie received his Ph.D. from the University of Minnesota and was a national research council postdoctoral fellow in polymer science at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. He was a senior research scientist in the Polymers Division at NIST for several years before joining the faculty at NDSU in the fall of 2009, where he is a professor in the Department of Physics and the Department of Coatings and Polymeric Materials. He directs the graduate program in materials and nanotechnology at NDSU. His research interests lie at the interface of soft materials, condensed matter physics, and nanotechnology, with an emphasis on engineering new materials from polymers and nanoparticles.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

■

ACKNOWLEDGMENTS We acknowledge the support of the NSF through CMMI0969155 and CBET-1133135 and the support of the DOE through DE-FG36-08GO88160. We thank Jeffrey A. Fagan for his invaluable insight into DGU and his continued support as a collaborator and provider of high-quality SWCNT materials. We also thank Uwe R. Kortshagen and Rebecca J. Anthony for providing us with their high-quality plasma-synthesized silicon nanocrystals as well as their valued expertise in the PL characteristics of semiconductor nanocrystals. Finally, we are indebted to our co-workers: Austin Vansickle, Christopher Moore, Matthew Semler, Steven Hudson, Ji Yeon Huh, Christopher Stafford, Scott Payne, Thomas Ihle, Daniel Kroll, Andrew Croll, Daneesh Simien, and Jan Obrzut.

Joseph B. Miller received an undergraduate degree in physics from North Dakota State University in 2010, where he is currently working toward a Ph.D. in the Materials and Nanotechnology Program. His research is directed at the purification, characterization, and self-assembly of silicon nanocrystals for applications in solid-state lighting, photovoltaics, and optical sensing.

■

REFERENCES

(1) Pusey, P. N. The Effect of Polydispersity on the Crystallization of Hard Spherical Colloids. J. Phys. (Paris) 1987, 48, 709−712. (2) Auer, S.; Frenkel, D. Suppression of Crystal Nucleation in Polydisperse Colloids Due to Increase of the Surface Free Energy. Nature 2001, 413, 711−713. (3) Vanmaekelbergh, D. Self-Assembly of Colloidal Nanocrystals as Route to Novel Classes of Nanostructured Materials. Nano Today 2011, 6, 419−437. (4) Miller, J. B.; Hobbie, E. K. Nanoparticles as Macromolecules. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1195−1208. (5) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1999. (6) Park, H.; Afzali, A.; Han, S.-J.; Tulevski, G. S.; Franklin, A. D.; Tersoff, J.; Hannon, J. B.; Haensch, W. High-Density Integration of Carbon Nanotubes via Chemical Self-Assembly. Nat. Nano 2012, 7, 787−791. (7) Brakke, M. K. Density-Gradient Centrifugation: A New Separation Technique. J. Am. Chem. Soc. 1951, 73, 1847−1848. (8) Jerri, H. A.; Sheehan, W. P.; Snyder, C. E.; Velegol, D. Prolonging Density Gradient Stability. Langmuir 2010, 26, 4725−4731. (9) Fagan, J. A.; Becker, M. L.; Chun, J.; Hobbie, E. K. Length Fractionation of Carbon Nanotubes Using Centrifugation. Adv. Mater. 2008, 20, 1609−1613. (10) Fagan, J. A.; Becker, M. L.; Chun, J.; Nie, P.; Bauer, B. J.; Simpson, J. R.; Hight Walker, A. R.; Hobbie, E. K. Centrifugal Length Separation of Carbon Nanotubes. Langmuir 2008, 24, 13880−13889. (11) Sun, X.; Luo, D.; Liu, J.; Evans, D. G. Monodisperse Chemically Modified Graphene Obtained by Density Gradient Ultracentrifugal Rate Separation. ACS Nano 2010, 4, 3381−3389. (12) Agrawal, K. V.; Topuz, B.; Jiang, Z. Y.; Nguenkam, K.; Elyassi, B.; Francis, L. F.; Tsapatsis, M.; Navarro, M. Solution-Processable Exfoliated Zeolite Nanosheets Purified by Density Gradient Centrifugation. AIChE J. 2013, 59, 3458−3467. (13) Chen, J. L.; Zhang, X. M.; Zheng, X. L.; Liu, C.; Cui, X. Q.; Zheng, W. T. Size Distribution-Controlled Preparation of Graphene Oxide Nanosheets with Different C/O Ratios. Mater. Chem. Phys. 2013, 139, 8−11.

John M. Harris holds an undergraduate degree in physics from Minnesota State UniversityMoorhead, and he is currently working toward a Ph.D. in physics at North Dakota State University. His research interests relate to the use of purified single-walled carbon nanotubes for applications in flexible electronics and photovoltaics.

7944


Langmuir


(14) Bai, L.; Ma, X. J.; Liu, J. F.; Sun, X. M.; Zhao, D. Y.; Evans, D. G. Rapid Separation and Purification of Nanoparticles in Organic Density Gradients. J. Am. Chem. Soc. 2010, 132, 2333−2337. (15) Miller, J. B.; Van Sickle, A. R.; Anthony, R. J.; Kroll, D. M.; Kortshagen, U. R.; Hobbie, E. K. Ensemble Brightening and Enhanced Quantum Yield in Size-Purified Silicon Nanocrystals. ACS Nano 2012, 6, 7389−7396. (16) Mastronardi, M. L.; Hennrich, F.; Henderson, E. J.; Maier-Flaig, F.; Blum, C.; Reichenbach, J.; Lemmer, U.; Kübel, C.; Wang, D.; Kappes, M. M.; et al. Preparation of Monodisperse Silicon Nanocrystals Using Density Gradient Ultracentrifugation. J. Am. Chem. Soc. 2011, 133, 11928−11931. (17) Ma, X. J.; Kuang, Y.; Bai, L.; Chang, Z.; Wang, F.; Sun, X. M.; Evans, D. G. Experimental and Mathematical Modeling Studies of the Separation of Zinc Blende and Wurtzite Phases of CdS Nanorods by Density Gradient Ultracentrifugation. ACS Nano 2011, 5, 3242−3249. (18) Song, S.; Kuang, Y.; Liu, J. F.; Yang, Q.; Luo, L.; Sun, X. M. Separation and Phase Transition Investigation of Yb3+/Er3+ Co-Doped NaYF4 Nanoparticles. Dalton Trans. 2013, 42, 13315−13318. (19) Sun, X. M.; Ma, X. J.; Bai, L.; Liu, J. F.; Chang, Z.; Evans, D. G.; Duan, X.; Wang, J. O.; Chiang, J. F. Nanoseparation-Inspired Manipulation of the Synthesis of CdS Nanorods. Nano Res. 2011, 4, 226−232. (20) Bonaccorso, F.; Zerbetto, M.; Ferrari, A. C.; Amendola, V. Sorting Nanoparticles by Centrifugal Fields in Clean Media. J. Phys. Chem. C 2013, 117, 13217−13229. (21) Zhang, C. L.; Luo, L.; Luo, J.; Evans, D. G.; Sun, X. M. A ProcessAnalysis Microsystem Based on Density Gradient Centrifugation and its Application in the Study of the Galvanic Replacement Mechanism of Ag Nanoplates with HAuCl4. Chem. Commun. 2012, 48, 7241−7243. (22) Boyer, P. D.; Holt, B. D.; Islam, M. F.; Dahl, K. N. Decoding Membrane- Versus Receptor-Mediated Delivery of Single-Walled Carbon Nanotubes into Macrophages Using Modifications of Nanotube Surface Coatings and Cell Activity. Soft Matter 2013, 9, 758−764. (23) Jin, H.; Heller, D. A.; Sharma, R.; Strano, M. S. Size-Dependent Cellular Uptake and Expulsion of Single-Walled Carbon Nanotubes: Single Particle Tracking and a Generic Uptake Model for Nanoparticles. ACS Nano 2009, 3, 149−158. (24) Hobbie, E. K.; Fagan, J. A.; Becker, M. L.; Hudson, S. D.; Fakhri, N.; Pasquali, M. Self-Assembly of Ordered Nanowires in Biological Suspensions of Single-Wall Carbon Nanotubes. ACS Nano 2009, 3, 189−196. (25) Puech, N.; Blanc, C.; Grelet, E.; Zamora-Ledezma, C.; Maugey, M.; Zakri, C.; Anglaret, E.; Poulin, P. Highly Ordered Carbon Nanotube Nematic Liquid Crystals. J. Phys. Chem. C 2011, 115, 3272−3278. (26) Kuang, Y.; Liu, J. F.; Sun, X. M. Ultrashort Single-Walled Carbon Nanotubes: Density Gradient Separation, Optical Property, and Mathematical Modeling Study. J. Phys. Chem. C 2012, 116, 24770− 24776. (27) Deng, X. Y.; Xiong, D.; Wang, H. F.; Chen, D. D.; Jiao, Z.; Zhang, H. J.; Wu, M. H. Bulk Enrichment and Separation of Multi-Walled Carbon Nanotubes by Density Gradient Centrifugation. Carbon 2009, 47, 1608−1610. (28) Fagan, J. A.; Simpson, J. R.; Bauer, B. J.; Lacerda, S. H. D.; Becker, M. L.; Chun, J.; Migler, K. B.; Walker, A. R. H.; Hobbie, E. K. LengthDependent Optical Effects in Single-Wall Carbon Nanotubes. J. Am. Chem. Soc. 2007, 129, 10607−10612. (29) Mangolini, L. Synthesis, Properties and Applications of Silicon Nanocrystals. J. Vac. Sci. Technol., B 2013, 31, 020801. (30) Arnold, M. S.; Stupp, S. I.; Hersam, M. C. Enrichment of SingleWalled Carbon Nanotubes by Diameter in Density Gradients. Nano Lett. 2005, 5, 713−718. (31) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nano. 2006, 1, 60−65. (32) Arnold, M. S.; Suntivich, J.; Stupp, S. I.; Hersam, M. C. Hydrodynamic Characterization of Surfactant Encapsulated Carbon Nanotubes using an Analytical Ultracentrifuge. ACS Nano 2008, 2, 2291−2300.

(33) Bonaccorso, F.; Hasan, T.; Tan, P. H.; Sciascia, C.; Privitera, G.; Di Marco, G.; Gucciardi, P. G.; Ferrari, A. C. Density Gradient Ultracentrifugation of Nanotubes: Interplay of Bundling and Surfactants Encapsulation. J. Phys. Chem. C 2010, 114, 17267−17285. (34) Green, A. A.; Hersam, M. C. Colored Semitransparent Conductive Coatings Consisting of Monodisperse Metallic Single-Walled Carbon Nanotubes. Nano Lett. 2008, 8, 1417−1422. (35) Green, A. A.; Hersam, M. C. Processing and Properties of Highly Enriched Double-Wall Carbon Nanotubes. Nat. Nano. 2009, 4, 64−70. (36) Huh, J. Y.; Walker, A. R. H; Ro, H. W.; Obrzut, J.; Mansfield, E.; Geiss, R.; Fagan, J. A. Separation and Characterization of Double-Wall Carbon Nanotube Subpopulations. J. Phys. Chem. C 2010, 114, 11343− 11351. (37) Ghosh, S.; Bachilo, S. M.; Weisman, R. B. Advanced Sorting of Single-Walled Carbon Nanotubes by Nonlinear Density-Gradient Ultracentrifugation. Nat. Nano 2010, 5, 443−450. (38) Haroz, E. H.; Duque, J. G.; Lu, B. Y.; Nikolaev, P.; Arepalli, S.; Hauge, R. H.; Doorn, S. K.; Kono, J. Unique Origin of Colors of Armchair Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 4461−4464. (39) Fagan, J. A.; Huh, J. Y.; Simpson, J. R.; Blackburn, J. L.; Holt, J. M.; Larsen, B. A.; Walker, A. R. H. Separation of Empty and Water-Filled Single-Wall Carbon Nanotubes. ACS Nano 2011, 5, 3943−3953. (40) Hobbie, E. K.; Fagan, J. A.; Obrzut, J.; Hudson, S. D. Microscale Polymer-Nanotube Composites. ACS Appl. Mater. Interfaces 2009, 1, 1561−1566. (41) Yanagi, K.; Miyata, Y.; Kataura, H. Optical and Conductive Characteristics of Metallic Single-Wall Carbon Nanotubes with Three Basic Colors; Cyan, Magenta, and Yellow. Appl. Phys. Express 2008, 1, 034003. (42) Tyler, T. P.; Shastry, T. A.; Leever, B. J.; Hersam, M. C. Narrow Diameter Distributions of Metallic Arc Discharge Single-Walled Carbon Nanotubes via Dual-Iteration Density Gradient Ultracentrifugation. Adv. Mater. 2012, 24, 4765−4768. (43) Fagan, J. A.; Zheng, M.; Rastogi, V.; Simpson, J. R.; Khripin, C. Y.; Batista, C. A. S.; Walker, A. R. H. Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation. ACS Nano 2013, 7, 3373−3387. (44) Harris, J. M.; Iyer, G. R. S.; Bernhardt, A. K.; Huh, J. Y.; Hudson, S. D.; Fagan, J. A.; Hobbie, E. K. Electronic Durability of Flexible Transparent Films from Type-Specific Single-Wall Carbon Nanotubes. ACS Nano 2012, 6, 881−887. (45) Hobbie, E. K.; Ihle, T.; Harris, J. M.; Semler, M. E. Empirical Evaluation of Attractive van der Waals Potentials for Type-Purified Single-Walled Carbon Nanotubes. Phys. Rev. B 2012, 85, 245439. (46) Rajter, R. F.; Podgornik, R.; Parsegian, V. A.; French, R. H.; Ching, W. Y. van der Waals-London Dispersion Interactions for Optically Anisotropic Cylinders: Metallic and Semiconducting Single-Wall Carbon Nanotubes. Phys. Rev. B 2007, 76, 045417. (47) Harris, J. M.; Huh, J. Y.; Semler, M. R.; Ihle, T.; Stafford, C. M.; Hudson, S. D.; Fagan, J. A.; Hobbie, E. K. Elasticity and Rigidity Percolation in Flexible Carbon Nanotube Films on PDMS Substrates. Soft Matter 2013, 9, 11568−11575. (48) Howarter, J. A.; Stafford, C. M. Instabilities as a Measurement Tool for Soft Materials. Soft Matter 2010, 6, 5661−5666. (49) Hobbie, E. K.; Simien, D. O.; Fagan, J. A.; Huh, J. Y.; Chung, J. Y.; Hudson, S. D.; Obrzut, J.; Douglas, J. F.; Stafford, C. M. Wrinkling and Strain Softening in Single-Wall Carbon Nanotube Membranes. Phys. Rev. Lett. 2010, 104, 125505. (50) Harris, J. M.; Iyer, G. R. S.; Simien, D. O.; Fagan, J. A.; Huh, J. Y.; Chung, J. Y.; Hudson, S. D.; Obrzut, J.; Douglas, J. F.; Stafford, C. M.; Hobbie, E. K. Structural Stability of Transparent Conducting Films Assembled from Length Purified Single-Wall Carbon Nanotubes. J. Phys. Chem. C 2011, 115, 3973−3981. (51) Semler, M. R.; Harris, J. M.; Croll, A. B.; Hobbie, E. K. Localization and Length-Scale Doubling in Disordered Films on Soft Substrates. Phys. Rev. E 2013, 88, 032409. (52) Ostanin, I.; Ballarini, R.; Potyondy, D.; Dumitrica, T. J. A Distinct Element Method for Large Scale Simulations of Carbon Nanotube Assemblies. Mech. Phys. Solids 2013, 61, 762−782. 7945


Langmuir


(53) Hahm, M. G.; Wang, H. L.; Jung, H. Y.; Hong, S.; Lee, S. G.; Kim, S. R.; Upmanyu, M.; Jung, Y. J. Bundling Dynamics Regulates the Active Mechanics and Transport in Carbon Nanotube Networks and Their Nanocomposites. Nanoscale 2012, 4, 3584−3590. (54) Znidarsic, A.; Kaskela, A.; Laiho, P.; Gaberscek, M.; Ohno, Y.; Nasibulin, A. G.; Kauppinen, E. I.; Hassanien, A. Spatially Resolved Transport Properties of Pristine and Doped Single-Walled Carbon Nanotube Networks. J. Phys. Chem. C 2013, 117, 13324−13330. (55) Wu, L.; Gao, B.; Tian, Y.; Munoz-Carpena, R.; Ziegler, K. J. DLVO Interactions of Carbon Nanotubes with Isotropic Planar Surfaces. Langmuir 2013, 29, 3976−3988. (56) Pan, G. H.; Barras, A.; Boussekey, L.; Qu, X. S.; Addad, A.; Boukherroub, R. Preparation and Characterization of Decyl-Terminated Silicon Nanoparticles Encapsulated in Lipid Nanocapsules. Langmuir 2013, 29, 12688−12696. (57) Sugimoto, H.; Fujii, M.; Imakita, K.; Hayashi, S.; Akamatsu, K. Codoping n- and p-Type Impurities in Colloidal Silicon Nanocrystals: Controlling Luminescence Energy from below Bulk Band Gap to Visible Range. J. Phys. Chem. C 2013, 117, 11850−11857. (58) Sugimoto, H.; Fujii, M.; Fukuda, Y.; Imakita, K.; Akamatsu, K. Allinorganic Water-Dispersible Silicon Quantum Dots: Highly Efficient Near-Infrared Luminescence in a Wide pH Range. Nanoscale 2013, 6, 122−126. (59) Yang, Z.; Dasog, M.; Dobbie, A. R.; Lockwood, R.; Zhi, Y.; Meldrum, A.; Veinot, J. G. C. Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability. Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201302091. (60) Yu, Y.; Bosoy, C. A.; Hessel, C. M.; Smilgies, D.-M.; Korgel, B. A. Silicon Nanocrystal Superlattices. Chem. Phys. Chem. 2013, 14, 84−87. (61) Kallel, H.; Arbouet, A.; Carrada, M.; Ben Assayag, G.; Chehaidar, A.; Periwal, P.; Baron, T.; Normand, P.; Paillard, V. Photoluminescence Enhancement of Silicon Nanocrystals Placed in the Near Field of a Silicon Nanowire. Phys. Rev. B 2013, 88, 081302. (62) Tice, D. B.; Frederick, M. T.; Chang, R. P. H.; Weiss, E. A. Electron Migration Limits the Rate of Photobrightening in Thin Films of CdSe Quantum Dots in a Dry N2(g) Atmosphere. J. Phys. Chem. C 2011, 115, 3654−3662. (63) Van Sickle, A. R.; Miller, J. B.; Moore, C.; Anthony, R. J.; Kortshagen, U. R.; Hobbie, E. K. Temperature Dependent Photoluminescence of Size-Purified Silicon Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 4233−4238. (64) Martín-García, B.; Paulo, P. M. R.; Costa, S. M. B.; Velázquez, M. M. Photo-luminescence Dynamics of CdSe QD/Polymer Langmuir− Blodgett Thin Films: Morphology Effects. J. Phys. Chem. C 2013, 117, 14787−14795. (65) Shirahata, N.; Hirakawa, D.; Masuda, Y.; Sakka, Y. Size-Dependent Color Tuning of Efficiently Luminescent Germanium Nanoparticles. Langmuir 2013, 29, 7401−7410. (66) Seo, J. T.; Yoder, N. L.; Shastry, T. A.; Humes, J. J.; Johns, J. E.; Green, A. A.; Hersam, M. C. Diameter Refinement of Semiconducting Arc Discharge Single-Walled Carbon Nanotubes via Density Gradient Ultracentrifugation. J. Phys. Chem. Lett. 2013, 4, 2805−2810. (67) Clar, J. G.; Silvera Batista, C. A.; Youn, S.; Bonzongo, J. J.; Ziegler, K. J. Interactive Forces between Sodium Dodecyl Sulfate-Suspended Single-Walled Carbon Nanotubes and Agarose Gels. J. Am. Chem. Soc. 2013, 135, 17758−17767. (68) Liu, H.; Feng, Y.; Tanaka, T.; Urabe, Y.; Kataura, H. DiameterSelective Metal-Semiconductor Separation of Single-Wall Carbon Nanotubes by Agarose Gel. J. Phys. Chem. C 2010, 114, 9270−9276. (69) Khripin, C. Y.; Fagan, J. A.; Zheng, M. Spontaneous Partition of Carbon Nanotubes in Polymer-Modified Aqueous Phases. J. Am. Chem. Soc. 2013, 135, 6822−6825. (70) Antaris, A. L.; Seo, J. T.; Brock, R. E.; Herriman, J. E.; Born, M. J.; Green, A. A.; Hersam, M. C. Probing and Tailoring pH-Dependent Interactions between Block Copolymers and Single-Walled Carbon Nanotubes for Density Gradient Sorting. J. Phys. Chem. C 2012, 116, 20103−20108.

7946


Purifying Colloidal Nanoparticles through Ultracentrifugation with

Recommend Documents