Shape Transformation of Constituent Building Blocks within Self

Jan 5, 2018 - Self-assembly of nanoparticles represents a simple yet efficient route to synthesize designer materials with unusual properties. However...
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Shape Transformation of Constituent Building Blocks within Self-Assembled Nanosheets and Nano-origami Qianqian Shi,†,‡ Dashen Dong,†,‡ Kae Jye Si,†,‡ Debabrata Sikdar,§,∥,⊥ Lim Wei Yap,†,‡ Malin Premaratne,§ and Wenlong Cheng*,†,‡ †

Department of Chemical Engineering, Faculty of Engineering, Monash University, Clayton 3800, Victoria, Australia The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton 3168, Victoria, Australia § Advanced Computing and Simulation Laboratory (AχL), Department of Electrical and Computer Systems Engineering, Faculty of Engineering, Monash University, Clayton 3800, Victoria, Australia ∥ Faculty of Natural Sciences, Department of Chemistry, Imperial College London, South Kensington, London SW72AZ, United Kingdom ⊥ Department of Electronics and Electrical Engineering, Indian Institute of Technology Guwahati, Guwahati, India 781039 ‡

S Supporting Information *

ABSTRACT: Self-assembly of nanoparticles represents a simple yet efficient route to synthesize designer materials with unusual properties. However, the previous assembled structures whether by surfactants, polymer, or DNA ligands are “static” or “frozen” building block structures. Here, we report the growth of transformable self-assembled nanosheets which could enable reversible switching between two types of nanosheets and even evolving into diverse third generation nanosheet structures without losing pristine periodicity. Such in situ transformation of nanoparticle building blocks can even be achieved in a free-standing two-dimensional system and three-dimensional origami. The success in such in situ nanocrystal transformation is attributed to robust “plant-cell-wall-like” ion-permeable reactor arrays from densely packed polymer ligands, which spatially define and confine nanoscale nucleation/growth/etching events. Our strategy enables efficient fabrication of nanocrystal nanosheets with programmable building blocks for innovative applications in adaptive tactile metamaterials, optoelectronic devices, and sensors. KEYWORDS: nanoparticle, self-assembly, transformation, nanosheet, nano-origami

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manipulate at different temporal and spatial scales, which are essential for achieving high-quality nanoassemblies, especially for complex shapes (such as those shown in the so-called nanoparticle periodic table).23−26 Importantly, it has to be noted that all the previous strategies lead to single generation of

elf-assembly of nanocrystals via molecules, DNA, or polymer has emerged as a facile yet efficient approach to fabricate ordered nanoassemblies with finely tunable properties.1−13 To date, progress has been made to achieve large-scale synthesis of superlattices,14,15 free-standing sheets/ membranes,4,8,16 binary systems,1,15,17,18 orientation control,19−22 and origami.8 For all these examples in general, nanoparticle−ligand conjugation chemistry and nanoscale forces among nanoparticle shapes are notoriously difficult to © XXXX American Chemical Society

Received: November 23, 2017 Accepted: December 29, 2017

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DOI: 10.1021/acsnano.7b08334 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano “frozen” assembled structures, in which constituent nanocrystal building blocks are not transformable. Here, we report a powerful strategy for fabricating sheet-like nanoassemblies, in which the constituent nanocrystal building blocks can be in situ transformed into various sizes/shapes and elements by defining and confining nanoscale crystallization events. This leads to the evolvement of nanoassemblies without destroying the original structural integrity. In such a way, different types of building blocks can be produced within the predesigned framework, avoiding the control of complex force between anisotropic structures. Using core−shell nanocrystals as the model system, we show that first generation (1st Gen) assemblies could evolve into second generation (2nd Gen) in a reversible manner, which could even be further transmuted into third generation (3rd Gen) with new shapes/elements (Figure 1). This general strategy does not discriminate the shape,

via precisely adjusting nanoscale nucleation/growth/etching events in substrate-supported, free-standing nanosheets as well as origami structures.

RESULTS AND DISCUSSION We begin with 1st Gen nanosheet assembly using a Au nanorod core and Ag shell nanocuboids (NBs) as model building blocks (Figure S1a,b). Similar to our previous reports, thiolated polystyrene (PS) was used as a capping ligand to assemble nanocrystals at the air−water interface through a two-stage drying-mediated self-assembly process.8,19 Upon drying, the densely packed PS brushes collapsed to form robust nanosheets with quasicrystalline NB arrays embedded, as shown from scanning electron microscopy (SEM) images in Figure 2a. This 1st Gen nanosheet could be transformed into 2nd Gen structures by a selective etching process in which the mixture solution of H2O2 and NH3·H2O is used as a gentle etchant to etch away the Ag shell while retaining the Au seed (Figure 2b).27−29 Notably, the gold rods resided in their original locations, remaining trapped inside PS nanocavities. This shape transformation process could be reversed back to their original shapes to recover 1st Gen brick-like structures (Figure 2c). Similar in situ reversible shape transformation could be achieved for vertically aligned nanocuboid (V-NB) assemblies (Figure 2d−f). Clearly, vertical orientation was essentially retained during the entire reversible process. This shape transformation process did not discriminate sizes of NB building blocks (see details for three different sizes in Figure S2 and statistical analysis in Table S1). We further show that the reversible shape transformation is extendable to other shapes, as demonstrated by a Au nanobipyramid core and Ag shell nanorods (NBP-NRs) (Figure 2g−i) and a sphere-like Au core and Ag shell nanocube (NC) system (Figure 2j−l). Removal and regrowth of the Ag shell for all types of building blocks are confirmed by energy-dispersive X-ray spectroscopy (EDS) analysis (Figure S3). Our results on NB orientation, NB sizes, other shapes, and elemental changes demonstrate that our reversible in situ shape transformation is robust and general for the core−shell nanocrystal-based systems. We further show that constituent building blocks in the 2nd Gen structure could be transmuted into new shapes using the powerful and controllable seed-mediated method.30−32 For example, NB nanocrystals could evolve into gold nanorods and then into a Au@Ag nanorod in the 3rd Gen (Figure 2m−o); gold NBP-embedded silver nanorods could be converted into gold NBP nanocrystals and then 3rd Gen Au@Ag nanorices (Figure 2p−r) by increasing the concentration of cetyltrimethylammonium chloride solution. In this way, a mass of halide− surfactant complexes coated along the NRs/NBPs instead of selective location-specific adsorption on their sides and ends,33,34 resulting in a conformal coating of Ag along Au seeds. It is worth mentioning that both nanocrystal shapes and elements could be controlled for the transmutation stage. For example, for the 2nd Gen, gold seeds derived from 1st Gen Au@Ag nanocubes could lead to wrinkled gold nanocrystals (Figure 2s−u) when silver is replaced by gold precursors during the growth process. Moreover, the 2nd Gen can be transmuted to 3rd Gen with other new elements such as palladium (Pd) (Figure 2v−x and Figure S4), indicating the modularity and versatility of our fabrication strategies. The in situ shape transformation process was so robust that it could even be achieved in suspended systems including a freestanding system (Figures S5−S7) and 3D origami (Figure 3a−c

Figure 1. Schematic of fabrication of transformable sheet-like nanoassemblies from core−shell nanocrystals. Core−shell nanocrystals are functionalized by densely packed ligands and then selfassembled into 1st Gen sheet-like nanoassemblies (either twodimensional or three-dimensional). 1st Gen structures can convert into 2nd Gen core nanosheets via shell removal. The 2nd Gen nanosheets can revert back to 1st Gen solids again or even be further transmuted into 3rd Gen nanosheets with new shapes or elements. The overall structural integrity can be well maintained for all the transformation processes due to the protection of a soft ligand.

orientation, and size of constituent nanocrystals in the nascent 1st Gen assemblies. Such in situ programing of nanoparticle building blocks can be achieved in a free-standing twodimensional (2D) system and even three-dimensional (3D) origami. In comparison to previous nanoassemblies mediated by alkyl molecules,3,6,10,14,21 DNA,4,9 polymers,18,19 or proteins,16 this study adds the following to the field: (i) confined and defined shape transformation in soft plant-cellwall matrix; (ii) reversible constituent building block shape switching in nanoassemblies without destroying overall structural integrity; (iii) programming plasmonic metamaterials B

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We further attempted to monitor shape transformation on the same position in a particular nanosheet sample by SEM (Figure S11). Comparison between Figure S11a and 11b clearly demonstrates the successful etching of the silver shell, leading to the conversion of 1st Gen to 2nd Gen NB nanosheets at the same position. However, we could not observe the further shape transformation at this same position (Figure S11c), but we still observed successful transformation in low or nonelectron-beam exposure areas in the same sample (Figure S11d−g). This result demonstrates that high-electron-beam exposure during the imaging process may likely damage PS nanocavity structures due to charging effects. When silver shells are supporting the PS corona, electrons can easily dissipate in closely spaced metallic particle arrays without destroying the PS corona. However, upon silver removal, incident electrons will build up on PS cavity walls due to inefficient electron escape pathways in the nanocavity dominant matrix. This may lead to PS cavity wall collapse onto core particle surfaces, rendering it impossible for further seed-mediated growth. We elucidated that the soft PS corona layer plays critical roles for the shape transformation process. During the two-stage drying-mediated self-assembly process, PS brush chains will collapse significantly to entangle each other, forming robust plastic-like structures (Figure S12). The strongly entangled PS corona networks are typically 3−6 nm in thickness (according to our atomic force microscopy (AFM) results and literature8,35) yet mechanically strong enough to support suspended embedded metallic nanoparticles and maintain their overall frameworks in all types of following etching and regrowth processes. The hydrophobicity nature of PS chains prevents the cavity frameworks from being dissolved in hydrophilic reaction solutions. Structural integrity of PS nanoreactors is evident from the well-defined shapes, as seen in the suspended free-standing nanosheets after the etching process (Figure 3d−f). As shown in the TEM images, the shape of the nanoreactor is predetermined by the shape of the first building blocks. It has to be noted that PS cavity walls must be porous, allowing access of chemical reagents in and out, similar to previous hydrophobic carbon nanotube membranes36 or silica nanocages.37 Ion-permeable polystyrene thin films have been demonstrated in both our nanoparticle system38 and other systems.39 Our PS cavity walls are only a few nanometers,35 enabling efficient transport of hydrated silver ions in and out. This is analogous to the cell wall framework in typical plant tissues. In order to further prove the presence of the PS wall after the silver etching step, we sputter-coated a ∼3.5 nm thick gold layer on the pristine 2nd Gen nanosheet and the one after oxygen plasma treatment. SEM characterizations revealed that the sputtered particles floated on the top of holey PS layers, and the Au seeds resided inside the PS corona (Figure 4a,b). In contrast, the sputtering step on a control sample with oxygen plasma treatment led to gold nanoparticle deposition on top of the Au seed nanocrystal and supporting substrate due to the absence of PS layers (Figure 4c,d). These results clearly demonstrate that the PS layer is strong enough to maintain the nanoscale structural integrity in both wet and dried state. This was even true for special areas such as edges and regions where the nanosheet was folded or double layered (Figures S13 and S14) and suspended systems (Figure 3). Densely packed ion-permeable PS nanocavity reactor arrays define where dissolution or growth occurs and confine the dimension of nanocrystals to be grown. This ensures

Figure 2. Reversible and transmutable transformation of core−shell nanocrystal sheets. SEM images of the 1st Gen nanosheets that are assembled from building blocks of (a,m,v) NB (horizontal alignment), (d) NB (vertical alignment), (g,p) NBP-NR, (j,s) NC, SEM images of their 2nd Gen nanosheets formed by silver etching (b,e,h,k and n,q,t,w), and SEM images of their corresponding reversed 1st Gen nanosheets by silver regrowth (c,f,i,l), and SEM images of their 3rd Gen nanosheet structures from (o) core−shell NRs, (r) core−shell rice-like nanocrystals, (u) wrinkled gold nanocrystals, and (x) core−shell Au@Pd NBs. Inset on the top of each image illustrates its corresponding structure. Note that the soft yet robust PS nanoreactor defines and confines nanoscale silver dissolution and crystallization events. The scale bars are 100 nm.

and Figure S8). The morphological/material transformation is confirmed by the statistical size/shape analysis of constituent building blocks (Figure S9) and chemical elemental mapping (Figure S10). The ability to maintain the internal and overall structural integrity during transforming building blocks is clearly demonstrated for the 3D origami system. As shown in Figure 3a−c and Figure S8, the origami nanostructure could retain its original 3D shape at all generations without structural collapse. C

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Figure 3. Transformation of building block shapes in free-standing origami and free-standing nanoreactors. Transmission electron microscopy (TEM) images of the NB nanosheet origami suspended in the holes of free-standing nanosheets for (a) 1st Gen, (b) 2nd Gen, and (c) reversed 1st Gen. TEM images of free-standing nanoreactors from 2nd Gen (d) NB, (e) NBP-NR, and (f) NC. The red dashed lines in (d−f) highlight the rectangular/rod/cube-like PS nanoreactors with one NR/NBP/sphere inside. The scale bars are 100 nm in a−c and 10 nm in d− f.

(Figure 5a−c). The 1st Gen NB and NC nanosheets display one characteristic peak located at ∼638 and 483 nm, respectively, which stemmed from the dominant gap mode plasmons confined within interparticle spaces,8 whereas the 1st Gen NBP-NR nanosheet shows one main peak at ∼571 nm and a shoulder around 878 nm. The 2nd Gen nanosheet solids did not contain any silver elements (Figure 2n,q,t), hence they displayed characteristic plasmonic properties of gold nanocrystals, namely, dipolar transverse and longitudinal plasmonic resonance modes for pure gold NR and NBP (solid red curves in Figure 5a,b),27,40 and weak plasmonic response for small gold nanoseeds (solid red curve in Figure 5c). The 2nd Gen structures still exhibit plasmon coupling within the array. Interestingly, we found two different coupling modes between the particles within 2nd Gen NB and NBP-NR from both experimental data and simulation results. For 2nd Gen NB, due to interparticle plasmonic coupling, both long and short wavelength peaks exhibit shifts toward red. These red shifts can be attributed to bonding-type interactions between the NRs that lower plasmon resonance energy at both longitudinal and transverse modes, respectively (Figure 5d−g). However, for 2nd Gen NBP-NR, the peak at long wavelength undergoes a blue shift, which can be attributed to an antibonding-type interaction that increases plasmon resonance energy (Figure 5h−j). With the reduction in spacing between the NBPs, this mode further shifts toward blue as interparticle coupling gets stronger (Figure 5k). On the other hand, the peak at the short wavelength can be attributed to a bonding-type interaction. Therefore, this transverse peak shifts toward red as interparticle coupling gets stronger (Figure 5k), indicating a decrease in plasmon resonance energy, with a reduction in spacing between Au NBPs. Corresponding to the nanoparticle building block shape reversal from 2nd Gen to 1st Gen, characteristic plasmonic spectra recover, as shown in Figure S18. The

maintenance of structural integrity at the nanoscale for individual particle shape/size control (Figure 2 and Figure S2), as well as the overall integrity at the macroscopic scale for the entire nanosheet, as evidenced by the optical images (Figure S15) and AFM images (Figure 4e−j) of different generations of nanosheets. The film thickness of 1st Gen (Figure 4h−j) decreases from 35.6 ± 0.7 to 21.6 ± 0.9 nm after transforming into 2nd Gen but increases to 34.1 ± 1.1 nm again after growing back to 1st Gen. The PS is robust to support a free-standing structure, and meanwhile, it is flexible owing to the soft nature of PS. Therefore, rather than collapsing when silver is etched away, the PS still holds the same rectangular shape. However, the PS structures are soft and resilient (Figure S16), which will be deformed by the AFM probe during the imaging, resulting in the decreased thickness in 2nd Gen (Figure 4i) and showing clear particle morphologies (Figure 4f). Nevertheless, thin PS reactor walls were still elastic and robustly guided the shape recovery to 1st Gen structures (Figure 4g,j). The shape of Au seeds and growth solution determine the type of nanocrystal, whereas the PS nanoreactor defines the position of the Au seeds (and finally determines the position of the obtained nanocrystal) and confines the final size of the nanocrystal. In the control experiment, PS reactor walls were deliberately removed by oxygen plasma, and then only polydispersed sizes with poorly defined locations were obtained (Figure S17). This further verifies our hypothesis that the robust PS nanocavity walls act as ion-permeable nanoreactor arrays (analogous to plant tissues comprising densely packed cell walls) that define and confine dissolution/growth events across the entire nanosheets, which are crucial for in situ transformation. Transformable core−shell nanosheets allow for precisely tuning and transforming their collective plasmonic properties D

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Figure 4. Evidence of the presence the PS layer after silver etching. Top left: schematic of gold nanorods caged by the free-standing PS layers after sputter coating a layer of ∼3.5 nm gold. The PS layer is strong enough to survive during the silver etching process, as demonstrated by SEM images of a (a) normal and (b) tilted (52°) view of 2nd Gen solid films. Clearly, core particles are present inside the PS cavities. Bottom left: schematic of gold nanorods without a caged PS layer after sputter coating a layer of ∼3.5 nm gold (PS was removed by plasma treatment). Without PS layers, sputter-coated gold particles sit directly on gold nanorods and substrates, as shown from the (c) top and (d) side view of the particle films. AFM height images (scan size, 1 μm × 1 μm) of the (e) 1st, (f) 2nd, and (g) reversed 1st Gen NBP-NR. (h−j) Corresponding AFM height profile marked by the horizontal line in (e−g). The scale bars are 100 nm in (a−d); black lines under (e−g) represent 1 μm.

features of reversed plasmonic peaks resemble the original ones in 1st Gen albeit with some blue shifts. This is due to smaller recovered particle sizes (see Figures S3 and S9 for elemental and statistical analysis), which may be attributed to the possible PS shrinkage during silver shell removal process from 1st Gen to 2nd Gen. Smaller particle building blocks recovered on the same locations also mean larger interparticle spacing and weaker plasmonic coupling, hence, blue-shifted peaks were observed. Corresponding to the 3rd Gen structures shown in

Figures 2o,r,u, transmutation led to entirely new plasmonic features (solid blue curves in Figure 5a−c). As expected, two characteristic dipolar resonance peaks for NR- and NBP-based 2nd Gen nanosheets shift to blue due to the growth of Ag shells around the Au cores. In contrast, the nanosphere-evolved 3rd Gen wrinkled nanocrystal nanosheets (Figure 2u) led to significantly enhanced and red-shifted plasmonic resonance modes. The spectral evolution is consistent with the transE

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Figure 5. Plasmonic properties of transformed nanosheets. Extinction spectra acquired from the 1st (solid black curves), 2nd (solid red curves), and 3rd (solid blue curves) Gen nanosheets from building blocks (a) NB, (b) NBP-NR, and (c) NC. The dashed lines represent the extinction spectra from corresponding Au seed solution. (d,h) Extinction cross-section (normalized) spectra obtained from full-wave simulations of individual NRs/NBPs (dispersed in aqueous solution) and a 2nd Gen NB/NBP-NR nanosheet. Schematic of the simulation model, where the inner pink rectangle highlights the unit cell that repeats periodically in both lateral dimensions to emulate a large (e) 2nd Gen NB and (i) NBP-NR nanosheets. Periodicities along ends and sides of NRs/NBPs are denoted by Lee and Lss, respectively. (f,g,j) Simulated patterns of the normalized electric (E)-field distribution corresponding to the peaks at long (⧫) and short (▼) wavelengths, calculated for light polarization along X and Y axes (denoted as X-pol and Y-pol), respectively. The maximum E-field magnitudes (in V/m) are mentioned in dashed boxes. Intense E-field confinement between NRs/NBPs takes place in the gap along the length and width of the NRs/ NBPs for X-pol and Y-pol, respectively. Schematic illustrating the origin of (g) bonding interaction between NRs resulting in strong plasmonic coupling both at long wavelength (as longitudinal mode) and at short wavelength (as transverse mode) and (j) antibonding interaction between longitudinal modes of NBPs for Y-pol at long wavelength; bonding interaction between transverse modes of NBPs takes place for X-pol at short wavelength. (k) Extinction cross-section spectra of 2nd Gen NBP-NR sheets simulated with different interparticle spacing. With the reduction in Lss, interparticle coupling gets stronger. Hence, the transverse (longitudinal) mode peak at short (long) wavelength originating from bonding (antibonding) interaction exhibits red (blue) shift.

fabrication strategy demonstrated here, together with the welldeveloped wet chemical methods that can produce ensembles of core−shell nanoparticles,41−44 may act as a basis to design a rich library of switchable metamaterials and devices.

mutation of noncoupling of tiny gold nanoseed arrays to strongly coupling enlarged wrinkled gold nanocrystal arrays.

CONCLUSIONS In summary, we demonstrate a general paradigm in growing ordered nanosheet assemblies, in which constituent particle building blocks can be in situ transformed into new shape/ element without destroying internal and overall structural integrity. This spatially defined transformation is attributed to the robust “plant-cell-wall-like” PS nanocavity structures, which serve as nanoscale size/shape-controllable ion-permeable reactors that spatially define and confine nanoscale crystallization events. Our results clearly demonstrate the feasibility of performing wet chemical reactions within dried nanoreactor arrays, enabling the creation of different generation nanosheets with finely tunable plasmonic properties. The transformable

METHODS Synthesis of Core−Shell Building Blocks. Synthesis of Au@Ag NB. The Au NRs were prepared using a two-step procedure.20 Typically, a brownish seed solution was made by reducing gold(III) chloride trihydrate (HAuCl4·3H2O) (5 mL, 0.5 mM) with ice-cold sodium borohydride (NaBH4) (0.6 mL, 0.01 M) in the presence of 5 mL of 0.2 M hexadecyltrimethylammonium bromide (CTAB). Then a growth solution was prepared by mixing silver nitrate (AgNO3) (0.2 mL, 4.0 mM), CTAB solution (5 mL, 0.2 M), HAuCl4 (5 mL, 1.0 mM), and ascorbic acid (AA) (0.08 mL, 0.08 M). To grow Au NRs, 12 μL of prepared seed solution was added into the above growth solution and aged for 2 h at 30 °C without disturbing. The final F

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of AgNO3 (7 mM) and 300 μL of AA (0.1 M) were added into a solution containing 400 μL of 25 mM HAuCl4 in 40 mL of 0.05 M CTAB. Stock solution ⑦: 2.2 mL of H2Pd Cl4 (0.4 mM), 40 μL of AA (0.1 M). Etching Process. The resulting nanosheet was subsequently etched by adding stock solution ①. NH3·H2O was dropped on top of the nanosheet, followed by an addition of H2O2. A pipet was used for promoting mixing, and the reaction was maintained for 4 h. During this process, the Ag elements were gradually etched away. After this step, the substrate was washed with Milli-Q water three times and dried under N2, leaving the clean (2nd Gen) nanosheet for further use. Regrowth of Substrate-Supported Nanosheets. For the reversing process of NB and NBP-NR nanosheets, the samples on the silicon wafer plate were first fixed on the inner side of a 10.5 mL glass vial using carbon tape, then 2.5 mL of Milli-Q water was added. The growth process was followed by injecting stock solution ② with the injection rate of 0.05 mL/min. The reaction was kept in a 60 °C water bath under stirring at 500 rpm for 25 min/20 min for NB/NBP-NR. To reverse the 2nd Gen NC nanosheets back to the original 1st Gen structures, the 2nd Gen film was immersed into a solution of 1.205 mL of Milli-Q water and 33 μL of CTAC, followed by injection of stock solution ③ with the injection rate of 0.2 mL/min. Then the glass vials were put into a 60 °C water bath under stirring at 500 rpm for 3.5 h. For the growth of the 3rd Gen NB nanosheets, 15 mL of 80 mM CTAC solution was added into a 20 mL vial that contained the 2nd Gen sample, followed by injection of stock solution ④ with the injection rate of 0.05 mL/min in a 60 °C water bath and stirring for 6 h. The 3rd Gen Au@Ag nanorice nanosheet was fabricated by immersing 2nd Gen NBP-NR solid films into 45 mL of 80 mM CTAC solution, followed by injecting stock solution ⑤ at a rate of 0.05 mL/ min. The reaction was kept in a 60 °C water bath under stirring for 6 h. The 3rd Gen wrinkled gold nanocrystal nanosheet was fabricated according to literature with modifications.30 The growth process started with immersing the 2nd Gen NC solid film into the stock solution ⑥ and was then kept undisturbed at 30 °C for 30 min to 5 h. The obtained samples were finally sonicated in 80 mM CTAC for 20 min at 60 °C and rinsed with Milli-Q water three times, followed by drying under N2. For the growth of Pd nanoparticles, 5 mL of 0.1 M CTAB solution was added into a 20 mL of vial that contained the 2nd Gen NB sample, followed by dropping of stock solution ⑦ and reacting at 60 °C for 2 h. The obtained samples were finally sonicated in 0.1 M CTAB for 20 min at 60 °C and rinsed with Milli-Q water three times, followed by drying under N2. Regrowth of Free-Standing Samples. The regrowth of freestanding nanosheets followed the same steps as the substratesupported samples. The resultant nanosheets were washed by gently dropping Milli-Q water on top of them to eliminate the fluctuating effect to the films. Fabrication of Origami. Origami was fabricated according to ref 8. The 1st Gen free-standing nanosheet on a holey silicon nitride membrane was bonded to an indium-tin oxide glass on an aluminum sample holder. Then FEI Helios Nanolab 600 FIB-SEM was used to generate gallium ions with an accelerating voltage of 30 kV to fold 2D nanosheets into 3D origami. The ion beam current was 9.7 pA, and the dwell time was 100 μs/1200 μs in the experiments. The reverse of origami followed the same method as the regrowth of free-standing samples. Characterization. The morphology of nanosheets was observed through SEM (FEI Helios Nanolab 600 FIB-SEM operating at 5 kV) and TEM (FEI Tecnai G2 T20 TWIN LaB6 TEM operating at 200 kV). The absorption spectra of the nanocrystal solution were measured using an Agilent 8453 UV−vis spectrophotometer. The extinction spectra measurements of the plastic-slide-supported nanosheets were performed by a J&M MSP210 microscope spectrometry system under a 20× objective. The elemental mapping was carried out on a FEI Tecnai G2 F20 FEGTEM equipped with a Bruker 30 mm2 123 eV windowless SDD and Quantax analysis system operated at 200 kV. EDS analysis was finished using a FEI Helios Nanolab 600 FIBSEM equipped with a Genesis EDX detector operating at 5 kV, 1.4 nA.

solution was washed with water twice at 8000 rpm for 20 min, followed by being redispersed in 10 mL of 80 mM cetyltrimethylammonium chloride (CTAC) solution. Au@Ag NBs were prepared by adding 1 mL of AgNO3 (10 mM) and 0.5 mL of ascorbic acid (0.1 M) into the prepared CTAC-capped Au NR solution at 60 °C. The reaction was allowed to keep at 60 °C for 4 h under continual stirring. The final product was collected by centrifugation at 7000 rpm for 10 min and redispersed in 10 mL of Milli-Q water for further use. Synthesis of Au@Ag NBP-NR. Synthesis of Au NBPs. A seed solution was first prepared by adding ice-cold NaBH4 solution (1.0 mL of 100 mM) into a 40 mL solution containing HAuCl4 (0.25 mM) and trisodium citrate (0.25 mM) under vigorous stirring. The solution was then aged at room temperature for 2 h. To grow Au NBPs, 0.8 mL of as-prepared seed solution was added into a mixture of HAuCl4 (2 mL, 25 mM), CTAB (98 mL, 0.1 M), AgNO3 (1 mL, 10 mM), HCl (32 wt %) (2 mL, 1.0 M), and AA (0.8 mL, 0.1 M), followed with aging at 30 °C in a water bath overnight. Then, a three-step purification method was involved to improve the Au NBPs’ purity as reported.19 The final Au NBPs were redispersed in a CTAB solution (50 mM, 25 mL). Au@Ag NBP-NRs were prepared by using the same method as the synthesis of Au@Ag NBs,43 except adding 1.1 mL of AgNO3 (10 mM) and 0.55 mL of ascorbic acid (0.1 M) into 10 mL of prepared CTAC− Au NBP solution. Synthesis of Au@Ag NC. The Au cores were prepared by a seed growth method, which is the same as the seed synthesis of the NR section. The prepared seed solution was aged at 27 °C for 3 h. The CTAC-capped Au cores were then obtained by adding 0.3 mL of asprepared seeds into the growth solution containing 6 mL of 0.5 mM HAuCl4, 6 mL of 0.2 M CTAC, and 4.5 mL of 0.1 M ascorbic acid. The red solution was then aged at 27 °C for 1 h and washed twice with Milli-Q water by centrifugation at 14 500 rpm for 15 min. Au@Ag NCs were synthesized by growing a Au shell over the Au cores.8 The first step involved adding 0.1 mL of the prepared Au cores into a mixture of 4.9 mL of 20 mM CTAC solution followed by heating at 60 °C for 20 min under vigorous stirring. To grow Ag shell on Au cores, 5 mL of 2 mM AgNO3 and 5 mL of AA−CTAC mixture solution (50 mM ascorbic acid + 40 mM CTAC) were simultaneously injected into the above solution with a rate of 0.2 mL/min. The final solution was allowed to age at 60 °C for 4 h followed by centrifugation at 14 500 rpm for 30 min. The product was finally redispersed in MilliQ water for further use. Fabrication of Nanosheets and the Transformation Process. Fabrication of Nanosheets. The nanosheets were fabricated by our recently developed approach.8 In a typical assembly procedure, the CTAB/CTAC ligands were first replaced by thiolated-polystyrene (Mn = 50 000 g/mol for NB and NC, Mn = 20 000 g/mol for NBP-NR) through a two-step ligand exchange method. Then, the concentrated core−shell nanocrystal/chloroform solution was dropped upon the surface of a sessile water droplet on silicon wafers (or on plastic slides for plasmonic characterization, on silicon nitride membrane (with 2.5 μm holes, TED PELLA) for free-standing samples). After the water droplet evaporated, monolayered nanosheets (1st Gen) were formed. Vertical aligned NB nanoparticles can be found in the NB nanosheets from both substrate-supported and free-standing structures (similar to the results reported in ref 19); these regions are used for the vertical NB characterization. Removal of the PS Layer. The PS layer could be removed by oxygen plasma treatment. The nanosheets assembled on a silicon substrate were placed in a UV-ozone chamber at an oxygen flow rate of 0.5 L/min. The duration of treatment was varied at 7 min for 2nd Gen NB nanosheets and 4 min for 2nd Gen NC nanosheets. Preparation of Stock Solution. Stock solution ①: 50 μL of ammonium hydroxide (NH3·H2O, 25 wt %), 25 μL of hydrogen peroxide (H2O2, 30 wt %). Stock solution ②: 0.1 mL of 5 mM AgNO3 solution, 0.1 mL of a mixture solution of 0.1 M AA, and 80 mM CTAC. Stock solution ③: 1.25 mL of 2 mM AgNO3, 1.25 mL of a mixture solution of 50 mM AA and 40 mM CTAC. Stock solution ④: 1.5 mL of 10 mM AgNO3, 1.5 mL of 0.1 M AA. Stock solution ⑤: 1.65 mL of 10 mM AgNO3, 1.65 mL of 0.1 M AA. Stock solution ⑥: 1.2 mL G

DOI: 10.1021/acsnano.7b08334 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano The thickness of the 2D film was recorded by a Veeco Dimension Icon AFM in tapping mode using Bruker silicon probes (MPP-11120-10). The spring constant of the cantilever was 40 N m−1. The AFM data were characterized using NanoScope Analysis software. Numerical Simulations. The numerical simulations presented in this work were performed using the CST Microwave Studio Suite. For nanoparticle sheets, a unit cell is repeated in both lateral directions using periodic boundary conditions to model optical extinction properties of large nanosheets under plane-wave excitation. The dimensions of nanoparticles and their periodicity parameters (along nanoparticles length and width) are taken from the mean values as measured from the SEM/TEM images of the nanosheets. The permittivity values of gold and silver for the nanoparticles were taken from the literature.45 For the tiny gold nanospheres in the core of Au@Ag NCs, additional size-dependent corrections were also incorporated to the bulk gold permittivity.46 Tetrahedral meshing with automatic mesh refinement was chosen to be fine enough for the frequency-domain simulations of the extinction spectra over the wavelength range of interest. To identify the modes and nature of interparticle coupling associated with the extinction peaks,47 we calculated electric-field distribution patterns (presented along the plane passing through centers of the nanoparticles in a unit cell). For the sheet, we obtained periodicities Lee and Lss between NRs of 62 and 34 nm, respectively; Lee and Lss of NBPs of 125 and 34 nm, respectively. Though the unit cell comprises four NBPs, Lee is too large to allow any coupling between NBPs along that direction. Hence, the E-field distributions are shown simply using two NBPs. For NRs/ NBPs in solution, we consider relative permittivity of the surrounding medium to be εm = 1.78. For the 2nd Gen NB/NBP-NR nanosheets, we considered εm = 1.9. This is an estimate, to closely match the experimental spectrum, made using effective medium theory while assuming the fractions of NR/NBP volume in the sheet covered by PS (εPS = 2.4) and exposed to air are 0.7 and 0.3, respectively.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08334. Detailed characterization of the structural, elemental, and plasmonic properties changes of nanoparticles during transformation procedures; structural features of freestanding systems; E-beam effect and the role of PS ligands (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Lim Wei Yap: 0000-0003-3072-6307 Wenlong Cheng: 0000-0002-2346-4970 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Australian Research Council for financial support via Discovery Grant schemes DP140100052 and DP170102208. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors also gratefully acknowledge the use of facilities at the Monash Centre for Electron Microscopy. We thank Tim Williams for technical support in elemental mapping. H

DOI: 10.1021/acsnano.7b08334 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

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DOI: 10.1021/acsnano.7b08334 ACS Nano XXXX, XXX, XXX−XXX