†Department of Materials Science and Engineering, ‡Frederick Seitz Materials Research Laboratory, and §Department of Chemistry,University of Illinois, Urbana, Illinois 61801, United States∥Institute of Functional Nano and Soft Materials (FUNSOM), Collaborative Innovation Center for Suzhou Nano Science andTechnology (NANO-CIC), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University,Suzhou, Jiangsu 215123, People’s Republic of China⊥Wheeler High School, Marietta, Georgia 30068, United States
*S Supporting Information
ABSTRACT: The shape anisotropy of nanoparticle buildingblocks is of critical importance in determining their packingsymmetry and assembly directionality. While there has beenextensive research on the effect of their overall geometricshapes, the importance of nanometer morphology details is notwell-recognized or understood. Here we draw on shape-anisotropic gold triangular nanoprism building blocks synthe-sized based on a method we recently developed; besides the“large-scale” triangular prism shape (79.8 nm in side length and22.0 nm in thickness), the prisms are beveled with their sidesconvexly enclosed by two flat {100} facets. We engineer thebalance between electrostatic repulsion and entropically drivendepletion attraction in the system to generate self-assemblies without or with the effect of the nanoscale beveling detail. Aconventional, planar honeycomb (p-honeycomb) lattice forms with the triangular basal planes packed on the same plane at lowdepletion attraction, whereas an unexpected interlocking honeycomb (i-honeycomb) lattice and its “supracrystal” forms areassembled with additional close-paralleling of side facets at high depletion attraction. The i-honeycomb lattice renders all themetallic surfaces in close proximity and leads to a surface-enhanced Raman scattering signal nearly 5-fold higher than that in thep-honeycomb lattice and high sensitivity for detecting the model molecule Rhodamine 6G at a concentration as low as 10−8 M.Our study can guide future work in both nanoparticle synthesis and self-assembly; nanoscale geometrical features in anisotropicnanoparticles can be used as an important handle to control directional interactions for nonconventional ordered assemblies andto enrich diversity in self-assembly structure and function.
Shape-anisotropic nanoparticles are excellent building blocksfor engineering collective properties through self-assembly,
partially due to their unique functions from asymmetricquantum-confinement and partially due to their greatlyenriched self-assembly structures from directional interac-tions.1−6 Shape anisotropy encodes intricate symmetries inthe final assemblies. For example, faceted anisotropic nano-particles often preferentially self-assemble to maximize facet−facet contact,7 such as the cubic lattice for nanocubes8,9 andMinkowski lattice for nanosized octahedra.10,11 Regardingnanoparticles of high aspect ratio, such as plate- and rodlikeshapes, the dominantly large facets often determine theassembly symmetry. For instance, nanoplates regardless ofcomposition have been shown to entropically favor self-assembly into columnar stacks in the face-to-face config-uration,12−16 and nanorods assemble via side-by-side attach-ment.17,18 The facets with smaller surface areas are meanwhilenot well-recognized or utilized in the self-assembly structures.
Selective chemical modifications can sometimes augment therole of small surfaces by rendering them strongly attrac-tive,19−21 but there are limited chemistry options for preciselyselective ligand coating.Here, we use highly monodisperse beveled gold triangular
prisms (side = 79.8 ± 3.7 nm, thickness = 22.0 ± 1.7 nm) withtwo types of interaction surfaces, the basal plane and thebeveled side, as the building blocks with high aspect ratio andnanometer morphology details (Figure 1a).22 These prisms arecoated by positively charged cetyltrimethylammonium chloride(CTAC) ligands to render them electrostatically repulsive.They are geometrically intriguing with two protruded {100}facets intersecting at an angle of 120−126° on the sides(Figures 1a, S1, and S2). The two intersecting facets yield
Received: March 6, 2017Revised: April 16, 2017Published: April 26, 2017
additional interacting surfaces and directionalities on the prismside when interparticle interaction takes effect, which is distinctfrom triangular prisms with flat sides reported in litera-ture.13,23,24 We use depletion attraction (CTAC micelles as thedepletant) as the tunable driving force for self-assembly, whichis a nonspecific entropic effect independent of particle surfacechemistry. We observe that self-assembled structures areselected thermodynamically, based on whether the area of thebeveled prism side is negligible or not, an effect that is notobserved with thin triangular prisms with flat sides13 orequilateral polygonal shapes.11
The prisms are self-assembled on silicon substrates via adroplet evaporation method in the presence of depletants. Ourassembly method enables facile control and use of theassembled architectures for later surface-enhanced Ramanscattering (SERS) measurements. The conventional dropletevaporation method generates closely packed assemblies thatare kinetically formed by capillary forces and convective liquidflow, which move particles in the droplet until they settle onsubstrates.25,26 In contrast, we optimize parameters to make theself-assembly process thermodynamically controlled. Specifi-cally, a drop (10 μL) of the prism solution is placed on a siliconwafer. The prism solution contains CTAC molecules of variousinitial concentrations (CTAC concentration (cCTAC) = 1.5−10mM); the CTAC molecules form into small micelles (diameter= 5−6 nm13) in water and serve as the depletants (Figure 1a).The droplet evaporates slowly while covered by a lid at roomtemperature. The CTAC micelle concentration and thedepletion attraction strength are thus gradually increasedduring solvent drying to overrun decreasing electrostaticrepulsion (due to more concentrated counterion screening),which produces self-assembly structures close-packed todifferent extent on the substrate. We investigate changes inthe assembly configuration with respect to various parameters
such as cCTAC, temperature, and surface ligand chemistry toelucidate the depletion-driven assembly mechanism.The nonconventional interlocking honeycomb (i-honey-
comb) lattice is observed to form in a relatively wide windowof cCTAC and prism concentration (cprism) (cCTAC = 5−10 mMand cprism with a peak value of 8−16 in extinction at 665 nm,Figure S3). As shown in Figure 1, each prism stands verticallyon the substrate and is closely packed side-by-side (side view).The top view schematics and scanning electron microscopy(SEM) images clearly show the interlocked secondary stackingof columns (Figure 1b−d), where the beveled sides alternatelystack. In this packing, even the tiny interstices between columnscaused by the beveled sides are completely filled. The gapdistance between adjacent prisms is measured as 5−6 nm,consistent with CTAC ligand thickness on the prismsurface.13,27 At low magnifications (Figure 1d), we even seestaircases of the interlocked layers, highlighting the three-dimensional (3D) ordering of the i-honeycomb lattice. Becauseof the beveled sides, the i-honeycomb lattice renders a moreefficient tessellation of 3D space than the commonly observedplanar honeycomb lattice (p-honeycomb).24,28,29 Namely, thegeometry packing density is 100% for the i-honeycomb lattice ifnot considering ligand layers and only 81% for the p-honeycomb lattice where the beveled geometry gives rise tovoids between prisms (Figure S4). We show that a seeminglytrivial nanoscopic detail of beveled side matters to the packinggeometry of the final assembled structure.We attribute the formation of the i-honeycomb lattice, a
structure with maximized surface contact given the buildingblock geometry, to an intricate interplay between depletionattraction (by CTAC micelles) and electrostatic repulsion.Depletion attraction has a purely entropic origin and arises bymaximizing the translational entropy of small, nonadsorbingdepletants, such as the CTAC micelles here, in the particlesuspension.13,30 In such systems, assembling particles possess
Figure 1. Beveled gold triangular prism assembly via droplet evaporation. (a) Schematics of the assembly process. Left: A water droplet (light blue)on a silicon wafer consists of individual prisms (blue) and CTAC micelles (green sphere). The zoomed-in views show the beveled geometry of aprism, including a schematic on the left and an SEM image on the right, and the structure of a CTAC micelle. Right: Schematics of the depletionattraction-driven assembly process and the self-assembled i-honeycomb lattice. The top view schematic of the i-honeycomb lattice on the siliconwafer features the staggered alignment between two adjacent prisms by half of the single prism thickness. The side view schematic shows the closelypacked configuration as side-by-side attachment. (b−d) SEM images of the i-honeycomb lattice (from left to right: a high-resolution image, a single-layer, and multilayers). The orange lines highlight the beveled sides in the i-honeycomb lattice. Scale bars: 10 nm for (a), 40 nm for (b), and 100 nmfor (c,d).
Nano Letters Letter
DOI: 10.1021/acs.nanolett.7b00958Nano Lett. XXXX, XXX, XXX−XXX
an exclusion layer wrapping their surface with a thickness equalto the radius of depletants. The total overlapping volume ofexclusion layers through particle self-assembly lowers the freeenergy of the system by making more space available to thedepletants in the solution following Edep = −nkBTΔV, where nis the number density of the depletant, kB is the Boltzmannconstant, T is the temperature, and ΔV is the volume gained bythe overlap of exclusion layers. From this relation, depletionattraction favors directional attachments of flat facets, more soas it overruns the competing facet−facet electrostatic repulsion.This clean geometry preference makes depletion attractionespecially suitable for controlling faceted nanoparticle self-assembly.12−14 Still, the use of depletion in previous nano-particle self-assembly studies is different from ours in that theeffect of depletion attraction on utilizing nanoscopic morphol-ogy details is not well demonstrated.The prism self-assembly is shown to indeed depend on the
initial cCTAC and the consequent depletant concentration in thedroplet (Figure S5). As cCTAC increases, the depletion attractionincreases and gradually wins over the interparticle electrostaticrepulsion; the former favors facet−facet packing to maximizeexclusion layer overlap while the latter tends to minimizefacet−facet packing.16 As shown in Figure 2a, given otherconditions the same, disordered structures are collected on thesubstrate at a very low cCTAC (0.1 mM), at which the CTACmolecules do not even form into depletant micelles to beginwith (critical micelle concentration = 1 mM31). Electrostaticrepulsion thus dominates and leads to no assemblies butrandom arrangements sitting on the substrate. At a higher cCTACwhere depletion attraction comes into play, assemblies withincreasing facet−facet packing are favored. We calculate thevolume overlap ΔV of exclusion layers for four possible self-assembled structures (Figure 2b,c). The derived ΔV diagram(Figures 2c and S6) shows that ΔV increases sequentially fromthe case of single-layer p-honeycomb, multilayer p-honeycomb,single-layer i-honeycomb, to multilayer i-honeycomb. This
trend matches with our experimental observations of the fourassemblies following an increase in cCTAC and correspondinglyin depletion attraction strength (Figure 2a), each in extendedregions on the substrate. This consistency shows that ourprecise control of depletion attraction strength distinguishesdifferent levels of close-packing in the final structures.To corroborate the assembly mechanism based on the
interplay of solution-mediated interactions, we directlycharacterize the assemblies formed in the solution andeliminate possible effects from the substrate and the capillaryeffect during solvent drying. The hypothesis is as follows. In thedroplet evaporation process, gradual solvent drying concen-trates CTAC micelles (increasing n) so that depletion attractionoverruns the decreasing electrostatic repulsion, which drives theself-assembly of the prisms. The increased mass of theassembled structures causes them to fall down on the substrate.To validate this hypothesis, we keep a prism suspensioncontaining high cCTAC (100 mM) for a prolonged period(without solvent evaporation) at room temperature. Thisprocess allows individual prisms to self-assemble and reachthermodynamic equilibrium based on internanoparticle inter-actions, and generates eventual formation of black sediments atthe bottom of the solution (Figure S7). The sediments arecarefully transferred on a silicon wafer and flash-dried undervacuum. The SEM images of the sediments consist of micron-sized 3D supracrystals with a few micrometers in size lying onthe substrate (Figure 3a and see more examples in Figure S8).In essence, the enlarged view of the supracrystal (Figure 3a)
clearly shows prisms closely packed and interlocked with eachother, which is identical to the prism arrangements in the i-honeycomb lattice formed via the droplet evaporation (Figure2a). The long-range order of the prisms is shown throughoutthe surface facets of the supracrystal in the long axis, and thetilted view also demonstrates the long-range periodicity in theinterior of the supracrystal (Figures 2a and S8). In addition, anensemble analysis of the supracrystals through small-angle X-
Figure 2. CTAC concentration effects on depletion attraction-driven self-assembly. (a) SEM images of prism assembly structures under differentinitial cCTAC (from left to right: 0.1, 1.5, 3.5, 5, and 10 mM). The schematics represent the corresponding self-assembled structures (0.1 mM,disordered prisms; 1.5 mM, single-layer p-honeycomb; 3.5 mM, multilayer p-honeycomb; 5 mM, single-layer i-honeycomb; and 10 mM, multilayer i-honeycomb). The prisms stand vertically on the substrate with cCTAC > 3.5 mM. (b) Schematics of the volume overlap of exclusion layers betweentwo prisms in different configurations. The dark purple shapes are the prisms, the light purple layer shows the exclusion layer surrounding a prism,and the middle purple layer shows the volume overlap. The prism has three elemental overlapping configurations for exclusion layers, the parallelside-by-side ΔVp−side, the interlocking side-by-side ΔVi−side and the face-to-face ΔVface. (c) A graph showing the relative volume overlap ΔV (red barin arbitrary unit) as a function of configuration in different assembled structures. The ΔV values are normalized with that of the multilayer i-honeycomb lattice as 1. Scale bars: 100 nm.
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ray scattering displays 1D lamellar structure consistent with theface-to-face stacking of the prisms (Figure S9). The signal fromlateral prism ordering is not detected clearly, presumably due torelatively smaller occurrence of the lateral ordering than theface-to-face stacking direction. The formation of the supra-crystal in high cCTAC indicates the i-honeycomb lattice is athermodynamically stable configuration, consistent with ourhypothesis. The supracrystals adopt their own surface “facets”as elongated hexagonal prisms in two types about micron insize, a rectangular facet with the beveled prism sides and tipsexposed, and a hexagonal facet with the planar prism faces. Themajority of the intact supracrystals are measured as ∼8 μm inlength and ∼2 μm in width. To the best of our knowledge, thisis the first supracrystal composed of triangular-shapedcomponents and with micron-sized facets in high aspect ratio.The free-standing supracrystals are likely to have uniqueplasmonic properties because they are an ensemble ofindividual nanoparticle building blocks (hot spots) in 3D, andthe mesoporosity between particles may enable their use fordelivering small molecules.32,33
Unlike most enthalpic interactions that are weakenedrelatively as temperature increases, depletion is more prominent
as the temperature increases, in that the entropic effect isweighted higher in the final free energy. To illustrate this effect,two droplets with the same cCTAC and cprism are dried underdifferent temperatures given the same drying speed (Figure 3b).The one at room temperature (25 °C) generates the p-honeycomb lattice (cCTAC = 3.5 mM), while the other kept in aclosed oven at 60 °C gives rise to the i-honeycomb lattice. Thisobservation is consistent with the more favored depletionattraction at higher temperature according to literature.13
Specifically, based on Edep = −nkBTΔV, the temperature termalone contributes to 11% increase in depletion attraction (from25 to 60 °C). In addition, it has been reported that the micelleconcentration n also increases up to 44% with this muchtemperature increase, which further enhances the role ofdepletion attraction.13
Depletion attraction does not concern surface chemistry ofnanoparticles, which can serve as a versatile control handle innanoparticle self-assembly. We demonstrate this feature byusing prisms of different surface chemistry. The native CTACligands due to synthesis are exchanged with thiol ligands (2-(boc-amino)ethanethiol) (see more details in the SI), which areshorter than the CTAC ligands while also being positivelycharged. Uniform distribution of sulfur elements on the prismbasal plane in the EDS elemental mapping (Figure 3c) showsthat the ligand exchange is successful. It is remarkable that thethiol-modified prisms also self-assemble into the i-honeycomblattice under the same condition as that for the CTAC-coatedprisms in the presence of the same concentration of depletants(Figure 3c, and see detailed assembly conditions in the SI).Besides 2-(boc-amino)ethanethiol, we find that the prismsligand-exchanged with 11-mercaptoundecanoic acid, a repre-sentative negatively charged ligand, are also assembled into thei-honeycomb lattice (Figure S10), supporting the versatility ofour method. Note that the excess CTAC ligands added afterligand exchange do not trigger further ligand exchange with thethiols already bound on the prism surface due to the muchstronger bonding between sulfur and gold atoms. Thisobservation is distinctive from conventional droplet evapo-ration-driven self-assembly, where an exchange in ligands canlead to very different final self-assembled structures due todifferences in ligand length, charge density, and packinggeometry.20,24,34,35
The different packing density results in distinctive SERSresponse; the i-honeycomb lattice has a 5-fold increase in SERSperformance compared to the p-honeycomb lattice (Figure 4).The two superlattice configurations with multilayers areprepared separately on the silicon wafer, and their SERSbehaviors are investigated under an incident laser source with632 nm wavelength, which is close to the extinction wavelengthfor a single prism in solution (Figure S3). The R6G moleculesas a probe are added to the prism solution and adsorbednaturally onto prism surfaces during the droplet evaporationself-assembly. R6G has characteristic features for carbonskeleton stretching modes at 1180, 1314, 1364, 1510, 1575,and 1650 cm−1.22 We hypothesize the SERS signals originatefrom R6G on ordered prisms on the top layer in that themultilayers below the top layer do not affect the SERS. Largeassembled areas of both configurations are scanned using aconfocal Raman spectrometer and show uniform distribution ofscattering intensity (insets in Figure 4a,b), indicatinghomogeneously distributed hot spots and R6G molecules(see SEM images of the assemblies in Figure S11 andconsistent signal enhancement on other large areas of the i-
Figure 3. Versatility of the depletion attraction-driven assembly. (a)SEM images of supracrystals that are assembled in the solution phasecontaining 100 mM of CTAC. Different magnifications (high to low)show the structural details of supracrystals from nanoscale to micron-scale. (b) Schematics of the drying condition at different temperatures(T = 60 °C and room temperature), and SEM images of i-honeycomblattices (T = 60 °C) and p-honeycomb lattices (room temperature).(c) Schematic of ligand exchange from CTAC to 2-(boc-amino)-ethanethiol on the prism surface (top) and energy dispersivespectroscopy (EDS) maps of a single prism after the ligand exchange(bottom: green for sulfur and red for gold). The SEM image showsthat the i-honeycomb lattice is obtained using the thiolated prisms.Scale bars: 100 nm, 200 nm, and 1 μm for a (from left to right), 100nm for b, and 50 nm for EDS and 100 nm for the SEM image in c.
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DOI: 10.1021/acs.nanolett.7b00958Nano Lett. XXXX, XXX, XXX−XXX
honeycomb lattice in Figure S12). The corresponding spectraclearly show the intensity difference at 1510 cm−1, 5.5 × 104
counts for the i-honeycomb lattice and 1.1 × 104 counts for thep-honeycomb lattice (Figure 4c). Although both configurationsshare similar interparticle spacing (∼5 nm) owing to thepresence of CTAC surface layers, the degree of plasmoniccoupling between adjacent prism tips/edges differs from eachother. To further investigate the enhancement property of the i-honeycomb lattice, we decrease the probe molecule concen-tration from 10−4 to 10−8 M (Figure 4d). The i-honeycomblattice detects concentrations of R6G as low as 10−8 M. Inaddition, the i-honeycomb lattice shows polarization-dependentSERS activity (Figure S13), which may be inherent in the hotspots from prism tips and beveled edges ordered in the i-honeycomb lattice.36
We attribute the superior sensitivity to enriched hot spotsfrom the interlocked prism tips and bevels. From our FDTDsimulations (see details in the SI), the beveled prisms retain sixbeveled side facets containing additional edges and three tipsrelatively sharpened by the bevels, compared with traditional
gold triangular prisms. These morphological details can giverise to a unique localized electric field distribution andplasmonic coupling. Concentration of the electric field occursmore intensely on the tip areas than the side areas within asingle prism (Figure 4e) and the electric field intensity increasesdramatically with the ordered lattices (7.8 for the single prism,15.4 for the p-honeycomb lattice, and 63.1 for the i-honeycomblattice). Specifically, the electric field intensity is four timesstronger with the i-honeycomb lattice than that of the p-honeycomb lattice, which is in good agreement with ourexperimental observations in SERS. The sharp tips and bevelsin the superlattices promote high intensity electric fields as hotspots.37,38 It appears that the stronger electric field enhance-ment with the i-honeycomb lattice originates from twoplausible factors: (i) higher density of ordered hot spots thatcontribute to a plasmonic antenna effect39−41 and (ii) beveledsides that produce intense electromagnetic radiation through a“lightning rod” effect.42−44
In this work, we show using the examples of beveled goldtriangular prisms whose nanoscopic morphology details caninduce unexpected self-assembly structures of superiorfunctions for applications. We see both an emphasis on shapedetails and a depletion-based control of geometric packing asapplicable to other nanoparticles with different morphologicaldetails45−48 and elemental compositions such as silver,49,50
which will broaden diversity of anisotropic nanoparticle self-assembly structures and functions. In addition, our approachcan be applied to other assembly methodologies as a controlhandle for the assembly structure, such as pressure-directedself-assembly,51,52 droplet evaporation on patterned sub-strates53,54 and field-directed assembly.55 Beyond nanoparticleself-assembly, our study draws interesting parallels with theeffects of shape complementarity observed in the interactions of“living” matter, where adaptive nanoscopic morphology detailscan modulate protein−protein or protein−DNA interac-tions.56−58
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.7b00958.
Materials and Methods, Table S1, and Figures S1−S13(PDF)
■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Zhang: 0000-0001-9682-3295Qian Chen: 0000-0002-1968-441XAuthor ContributionsJ.K. and X.S. contributed equally. The manuscript was writtenthrough contributions of all authors. All authors have givenapproval to the final version of the manuscript.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSWe thank Cong Xu at the University of Illinois for help withthe EDS element mapping. The nanoparticle synthesis,
Figure 4. SERS measurements for p-honeycomb and i-honeycomblattices. (a,b) Optical microscopy images of p-honeycomb lattices andi-honeycomb lattices on silicon wafer substrates, respectively (see SEMimages in Figure S11). The bright gray areas correspond to locationsof assembled prisms. The dark gray areas are bare silicon wafer. Theinset images show Raman signal map of the black boxed area measuredby a confocal Raman spectrometer (excited source wavelength = 632nm, [R6G] = 10−4 M). (c) Raman spectra of different assemblystructures prepared on a silicon wafer separately with [R6G] = 10−4 M(red curve, i-honeycomb lattice; blue curve, p-honeycomb lattice; graycurve, scattered single prisms). (d) Raman spectra of the i-honeycomblattice with different dye concentrations ([R6G] = 10−4 M (red), 10−5
M (brown), 10−6 M (yellow), 10−7 M (green), and 10−8 M (blue)).The inset graph shows the rescaled signal with [R6G] = 10−8 M, whichis detectable over the level of the noise. (e) Electric field distribution ofa single prism, p-honeycomb lattice, and i-honeycomb latticecalculated by finite-difference time-domain (FDTD) simulationsunder an incident laser source with 632 nm wavelength. Theconfigurations are shown from the top view. Scale bars: 10 μm.
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DOI: 10.1021/acs.nanolett.7b00958Nano Lett. XXXX, XXX, XXX−XXX
characterization (TEM, SEM, and SAXS), and self-assemblywere supported by the U.S. Department of Energy, Office ofBasic Energy Sciences, Division of Materials Sciences andEngineering under award no. DE-FG02-07ER46471, throughthe Frederick Seitz Materials Research Laboratory at theUniversity of Illinois. The SERS measurement and the FDTDcalculations were supported by National Science Foundation,Grant NSF CHE 13-03757. N.F.I. acknowledges the NationalScience Foundation under Grant NSF EEC 14-07194 RET aspart of the nano@illinois Research Experiences for Teachers(RET) project.
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Nano Letters Letter
DOI: 10.1021/acs.nanolett.7b00958Nano Lett. XXXX, XXX, XXX−XXX
