Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods

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Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods Wenbo Wei, Yuru Wang, Juanjuan Ji, Shanshan Zuo, Wentao Li, Feng Bai, and Hongyou Fan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01584 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods Wenbo Wei,1,2, # Yuru Wang,1,2, # Juanjuan Ji,1,2 Shanshan Zuo,1,2 Wentao Li,1,2 Feng Bai,1,2,* and Hongyou Fan3,4,* 1

Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Kaifeng 475004, China

2

Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, China 3

Department of Chemical and Biological Engineering, The University of New Mexico, Albuquerque, New Mexico 87131, United States 4

Sandia National Laboratories, Albuquerque, New Mexico 87106, United States

Corresponding author emails, phone numbers, and fax numbers: E-mail: [email protected], Tel: 86-15039024866, Fax: 86-0371-23883868 E-mail: [email protected], Tel: (505) 272-7128, Fax: (505) 272-7336

ACS Paragon Plus Environment

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ABSTRACT

Anisotropic nanoparticles such as nanorods and nanoprisms, enable packing of complex nanoparticle structures with different symmetry and assembly orientation, which result in unique functions. Despite previous extensive efforts, formation of large areas of oriented or aligned nanoparticle structures still remains a great challenge. Here we report fabrication of large-area arrays of vertically aligned gold nanorods (GNR) through a controlled evaporation deposition process. We began with a homogenous suspension of GNR and surfactants prepared in water. During drop casting on silicon substrates, evaporation of water progressively enriched the concentrations of the GNR suspension, which induces the balance between electrostatic interactions and entropically driven depletion attraction in the evaporating solution to produce large-area

arrays

of

self-assembled

GNR

on

the

substrates.

Electron

microscopy

characterizations revealed the formation of layers of vertically aligned GNR arrays that consisted of hexagonally close-packed GNR in each layer. Benefiting from the close-packed GNR arrays and their smooth topography, the GNR arrays exhibited a surface-enhanced Raman scattering (SERS) signal for molecular detection at a concentration as low as 10−15 M. Because of the uniformity in large area, the GNR arrays exhibited exceptional detecting reproducibility and operability. This method is scalable and cost effective and could lead to diverse packing structures and functions by variation of guest nanoparticles in the suspensions.

KEYWORDS: Gold nanorods, directional self-assembly, vertical alignment, depletion attraction, morphology, SERS


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Anisotropic nanoparticles such as nanorods and nanoprisms, enable packing of complex particle structures with different symmetry and assembly orientation.1-7 Assembly of these anisotropy colloidal nanoparticles into complex ordered structures is a sustained important topic in nanoscience because of their unique collective optoelectronic properties and integration in nanoeletronics and nanophotonics.1-3,

5-15

Successful integration of these nanoparticles into

reliable optoelectronic devices not only requires reproducible long-range ordered structure, but also macroscopic homogeneity of performance from the structure.1,

2, 16

Among the various

techniques used to fabricate the complex structures, droplet evaporation has been extensively investigated because of its operability, cost-effectiveness, and wide applicability to various nanomaterials.17,

18

However, drying droplet carrying colloidal particles typically leaves a

nonuniform solid ring on the substrate which is the well-known “coffee ring”.19 The formation of a solid ring structure is highly correlated to the balanced convection within the droplet, which is mainly caused by heat and mass loss during evaporation.20 The heat loss leads to evaporative cooling sufficient to create a surface tension gradient that can drive Marangoni flow and/or buoyancy-driven flow.21 The mass loss by evaporation leads to capillary flow outward from the center of the drop, which brings the suspended particles to the edge as the evaporation proceeds. After evaporation, the particles are left highly concentrated along the original drop edge forming a nonuniform solid ring.22 Recent research has been devoted to address the coffee-ring effect by regulating key condition factors such as the particle size and shape, particle charges, surface functionalization, and concentration of surfactants aiming to achieve large-area arrays of self-assembled particles.24, 8, 16, 23-34

These methods usually lead to local self-assembly either at the air-liquid surface or


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internal droplet. For example, Kim et al reported fabrication of honeycomb supracrystals of triangular nanoprisms by engineering the balance between electrostatic repulsion and entropically driven depletion attraction in the colloidal nanoprism system.8 Despite previous extensive efforts, it is still a big challenge to address key issues to achieve macroscopic area of complex structures of anisotropic nanoparticles with reliable, reusable, and homogenous performance. In this report, we use highly monodisperse GNR with controlled aspect ratios as the

building

block.

These

nanorods

are

surface

coated

by

binary

surfactants

cetylmethylammonium bromide (CTAB) and sodium oleate (NaOL) to render them electrostatically stabile.35 We developed an environmentally controlled self-assembly process to prepare large area self-assembled arrays. Essentially, we maintained constant system conditions such as temperature and relative humidity to suppress the convection caused by heat and mass loss during evaporation. We observed that self-assembled arrays are tunable depending on the substrate surface energy. Reproducible large-area arrays of self-assembled GNR were formed, which were vertically aligned to the substrates. We demonstrated these nanorod arrays exhibit exceptional detecting reproducibility and operability for molecular detection through surfaceenhanced Raman scattering (SERS) measurements. The GNR were synthesized by using a binary surfactant seed growth method (Figure S1).35 The surface of the GNR is protected by surfactants, which keep the GNR stable in the aqueous solution (Figure S2). The GNR stock suspension was prepared by centrifugation and redispersion of the as-prepared GNR in 1 mM CTAB solution twice. The final concentration of the GNR was adjusted to 10 nM. Si wafers with controlled surface hydrophobicity were used as the substrates. To form the GNR arrays, 10 µL GNR suspension was drop-casted on the Si substrates


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by a pipette at 30 ˚C. The evaporation was conducted in a controlled environment in which the humidity was maintained at 96% by using a saturated K2SO4 solution (see Figure S3). After

Figure 1. Representative images of vertically aligned GNR arrays self-assembled on silicon substrates under controlled evaporation. (A) A circular area of the self-assembled GNR arrays with ~ 2 mm diameter. (B) Magnified area of the self-assembled array, blue frame in (A). (C) Magnified side area of the self-assembled array, red frame in (A). (D) A shattered domain of the self-assembled array formed by uniformly aligned GNR. (E) A top view high resolution SEM image of the self-assembled GNR. Inset shows a cross-sectional image of the GNR array. (F) An AFM image of the self-assembled GNR arrays. (G) XRD spectra of the GNR array (red). The blue bars show the XRD peak positions and relative intensities of bulk gold. The yellow-colored rods show the relative alignment compared with bulk gold.

drying, a disk-like area with ~ 2 mm diameter was formed (Figure 1A). The deposited sample was then immersed in chloroform to wash away the free surfactants. Figure 1B and 1C show the


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center and edge area of the deposited GNR arrays on the Si substrate. The film comprises shattered domains (Figure 1D) with cracks within the film, and each shattered domain consists of uniformly aligned GNR (Figure S4). These cracks were probably created by the drying stress after removal of free surfactants. Surface functionalization has been demonstrated to avoid cracking.16 High resolution scanning electron microscopy (SEM) image (Figure 1E) revealed that the controlled self-assembly resulted in hexagonal close packing of the GNRs. Both plan view and cross-sectional SEM imaging confirmed that these GNR were vertically aligned to the substrate. The cross-sectional view of the array (Figure 1E inset) shows that the GNR arrays consist of two layers of the GNR with the nanorods close-packed between the layers, which create effective “hot spots” for SERS (shown below). The homogeneity of the arrays is confirmed by atomic force microscopy AFM imaging (Figure 1F) that shows the rather flat surface of the arrays with a height of approximately 180 nm. This height is very close to the length of two rods, which further confirms the uniformity of the ordered GNR arrays (see Figure S5). X-ray diffraction (XRD) patterns further confirm the vertical alignment of the nanorods. Similar large areas of complex structures could also be acquired with more flexible selfassembly conditions with the GNR concentration varying from 5 to 10 nM and the CTAB concentration varying from 0.5 to 1 mM (see Figure S6). In order to fabricate uniform self-assembled particle arrays, it is highly desirable to maintain the droplet evaporation in a steady state assembly process, which is mainly a thermodynamic process. Therefore, the environmental parameters of the evaporation process such as temperature, humidity, and the surface hydrophobicity of the substrates should be carefully controlled. The CTAB surfactant in the droplets will crystallize and precipitate from the solution when the temperature is below 25 ˚C. This definitely devastates the self-assembly


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process, leading to bad quality of the GNR arrays. On the other hand, the convection in the droplet becomes intense as the temperature increases to 50 ˚C, which leads to serious nonuniform accumulations (Figure S7).36 The evaporation temperature was finally set at 30 ˚C. To suppress the convention within the droplet caused by fast evaporation, we created a constant and high humidity environment for the evaporation. We employed a saturated K2SO4 solution to maintain the humidity as high as 96% at 30 ˚C. It was found that when the humidity was lower

Figure 2. Complex structures of the self-assembled GNR arrays formed on substrates with different contact angles. 10˚: (A, D, G), 30˚: (B, E, H), and 60˚: (C, F, I). A, B, C shows the surface profilers of the deposited arrays after evaporation taken by a 3D optical microscope.


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than 96%, (such as 91% when KNO3 solutions were used), random aggregated solids of GNR arrays were often formed (Figure S8).

After the evaporation conditions were optimized at the temperature of 30 ˚C and 96% humidity, we studied the influences of the substrate hydrophobicity on the formation of GNR arrays. Three substrates with water contact angles of 10˚, 30˚ and 60˚ were used for the deposition of the GNR arrays. The GNR suspensions with same GNR and surfactant concentrations were dropped on these Si substrates for evaporation under the controlled environment. When the droplet evaporated on the hydrophilic substrate with the contact angle of 10˚, an obvious “coffee ring” was formed (Figure 2A, D, and G). The ring was filled with beltlike clusters that consist of layers of ordered GNR. When the evaporation was conducted on the substrates with 60˚ water contact angle, the droplet shrunk into a circular area with ~ 2 mm diameter (Figure 2C, F, and I). The circular area was filled with round and semi-round clusters of ordered GNR arrays. On the substrate with 30˚ water contact angle, a film of ordered GNR was formed. It is comprised of shattered domains with cracks within the film. These cracks were probably created by the drying stress after removal of free surfactants. Benefiting from the close-packed GNR arrays and their smooth topography, the GNR arrays exhibited a SERS signal for molecular detection at a concentration as low as 10−15 M. SERS measurements were performed on the GNR arrays on a commercial Raman spectrometer with 633 nm excitation (Figure S9). Malachite green (MG) was used as a model analyte for SERS detection.37 Figure 3A shows the SERS spectra of MG for different concentrations from 10−6 to 10−10 M. The Raman spectra clearly show the characteristic Raman peaks of the MG


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(Figure 3A). When the MG concentration is as low as 10−15 M, we can still observe the characteristic Raman peaks of MG (Figure 3B). A common analyte Rhodamine 6G (R6G) was also used to evaluate SERS activity of the GNR arrays in comparison with MG. As a result of the excellent SERS activity, the spectra clearly show the characteristic Raman peaks of R6G at a concentration of 10−10 M (Figure 3C). The GNR arrays exhibit very good reusability as the SERS substrates allowing multiple SERS detections on the same substrate after cleaning the previous analytes by plasma etching.

Figure 3. SERS measurements on the vertically aligned GNR arrays. (A) Raman spectra of MG with different concentrations. (B) SERS spectra of MG with concentration of 10−15 M. (C) SERS spectra of MG and R6G with each of the concentration of 10−10 M. (D) SERS spectra of MG


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with concentration of 10−9 M (black curve), R6G (red curve) with concentration of 10−9 M after cleaning the first analyte (MG), and MG (blue curve) with concentration of 10−9 M after cleaning the second analyte (R6G).

Figure 3D shows the SERS results of MG and R6G measured on the same substrate of the GNR arrays after multiple usages. MG was first measured showing clear characteristic peaks of MG. Then, the GNR array substrate was cleaned by plasma etching for SERS measurement of R6G. After the first cleaning, no signal of MG was identified and the characteristic Raman peaks of R6G were observed clearly. After the second cleaning, the same GNR array substrate was used for measurement of MG again. The SERS spectra showed no signal of R6G, but clear Raman peaks of MG. This demonstrated the excellent reusability of the GNR arrays for SERS detection. Beyond ultrasensitive SERS detection, the purpose of fabricating ordered structure is to achieve good reliability and reproducibility for practical device integration. Through Raman mapping, we assessed the performance of the GNR arrays. The relative standard deviation (RSD) is used to represent performance stability with 20% as an acceptable threshold for practical applications.38 First, a 26 µm × 26 µm area in the arrays was mapped by point-by-point scanning with a step size of 0.5 µm (52 × 52 spots) during laser excitation. The signal intensity at 1616 cm−1 from the 2704 spots was measured and the RSD of these SERS intensities was calculated. The RSD of these signal intensities of the testing area of the arrays was ~ 11.8% and the intensity distribution was in strict conformity with array morphology (Figures 4A and 4B). To further assess the reproducibility of the array as SERS substrates, the spot-to-spot variation distribution of the SERS intensity of the 1616 cm−1 peak of MG along 130 µm of the array across several shatters is determined (Figure 4C). The RSD is 12.0%, which is very close to 11.8%


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derived from one shatter, indicating high reproducibility on a macroscopic scale. Then, the signal intensity at both 1616 cm−1 and 1172 cm−1 of a 95 µm × 89 µm area in the array was mapped with a step size of 1 µm from the 8640 spots and the RSD of these SERS intensities was calculated to be 15.8% and 16%, respectively (Figure 4D, E). In addition, there is no antenna effect of plasmatic observed on the array,1 so negative impact on device performance caused by cracks during drying process could be eliminated. At last, the signal intensity at 1616 cm−1 of a 1 mm × 1 mm area in the array was mapped with a step size of 20 µm from the 2601 spots was

Figure 4. SERS intensity maps of MG with the concentration of 10−6 M on the GNR arrays. (A) Raman mapping image at peak 1616 cm-1 of a 26 µm × 26 µm area with a step size of 0.5 µm. (B) Corresponding optical image of (A). (C) Line scanning of the GNR arrays along 130 µm length at peak 1616 cm-1. (D) and (E) are Raman mapping images at peak 1616 cm-1 and 1172 cm-1 of a 95 µm × 89 µm area with a step size of 1 µm, respectively. (F) Raman mapping image at peak 1616 cm-1 of a 1 mm × 1 mm area with a step size of 20 µm.


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measured and the RSD of these SERS intensities was calculated to be 18.6% (Figure 4F). The results show that the RSD is lower than the 20% threshold at all scales from micrometer to millimeter, showing excellent performance stability and structural uniformity. These results demonstrated very good reliability and reproducibility of our self-assembled GNR arrays for SERS applications.

Figure 5. SERS spectra of MG of five different samples of the GNR arrays with same concentration of 10−6 M, taken from center of an isolated shatter of the arrays.

The sample-to-sample reliability and reproducibility were also assessed. We used five different arrays made by our method as the SERS substrates for detecting MG with concentration


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of 10−6 M (Figure 5). The calculated RSD of the five samples is ~ 8.9% at the 1616 cm-1 peak, showing very good performance stability between samples. The results indicate high sample-tosample reliability and reproducibility of device performance fabricated by our GNR arrays. In summary, we fabricated large-area ordered arrays of vertically aligned GNR. The evaporation process was conducted in controlled self-assembly conditions of temperature, humidity, and hydrophobicity of substrates to suppress convection caused by heat loss and by mass loss. The resultant arrays exhibit consistent structural and functional homogeneity at a macroscopic scale. We demonstrated that the GNR arrays exhibited a surface-enhanced Raman scattering signal for detecting MG at a concentration of 10−15 M and R6G at a concentration as low as 10−10 M. The uniformity in large areas results in exceptional SERS detecting reproducibility and operability. Notably, the surface of the GNR was not functionalized using alkanethiols. The GNR structures rely only on the evaporating environment. Thus, this method can be readily extended to other nanoparticle systems for fabrication of macroscopic ordered arrays for device integrations.

ASSOCIATED CONTENT

Supporting Information. Additional experimental results and methods are available, including synthesis, fabrication and characterization of the GNR arrays, as well as Raman mapping. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author

Corresponding author emails, phone numbers, and fax numbers: E-mail: [email protected], Tel: 86-15039024866, Fax: 86-0371-23883868 E-mail: [email protected], Tel: (505) 272-7128, Fax: (505) 272-7336

Author Contributions #

These authors contributed equally. H.F. conceived the idea. W.W., Y.W., J.J., S.Z. W.L. and

F.B. performed the experiments. All authors commented on the manuscript and contributed to the writing of the manuscript.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. F.B. acknowledges the support from the National Natural Science Foundation of China (21422102, 21403054, 21771055, U1604139), Plan for Scientific Innovation Talent of Henan Province (No. 174200510019), and Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT_15R18). Research was carried out, in part, at the Center of Integrated Nanotechnology, a US Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the


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U.S. Department of Energy’s National Nuclear Security Administration under contract DENA0003525.

Notes

The authors declare no competing financial interest.

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Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods

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