Controllable Two-Stage Droplet Evaporation Method and Its

Article pubs.acs.org/Langmuir

Controllable Two-Stage Droplet Evaporation Method and Its Nanoparticle Self-Assembly Mechanism Yong Xie,† Shengming Guo,† Chuanfei Guo,† Meng He,† Dongxue Chen,† Yinglu Ji,‡ Ziyu Chen,§ Xiaochun Wu,*,‡ Qian Liu,*,† and Sishen Xie∥ †

Laboratory for Nanodevices, National Center for Nanoscience and Technology, Beijing 100190, China CAS Key Laboratory of Standardization and Measurement for Nanotechnology, Beijing 100190, China § Department of Physics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China ∥ Beijing National Laboratory of Condensed Matter, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Bottom-up self-assembly is able to constitute a variety of structures and has been thought to be a promising way for advanced nanofabrication. Droplet evaporation, as the simplest method, has been used in various self-assemblies. However, the assembled area is not large enough and the order is still not well controlled. Here we show a facile and controllable two-stage droplet evaporation method by adjusting the humidity and temperature of the evaporating droplet. Taking the highly monodispersed gold nanorods (GNRs) as an example, large-area, self-assembly monolayer arrays are reproducibly achieved. To understand the self-assembly mechanism, we adopted simplified models to analyze the interactions between the nanorods. The results show that a metastable state of secondaryenergy-minimum exists, especially in the latter stage of the assembly process, leading to the ordered arrays. A large electrostatic barrier between the assembled arrays prevents the formation of the multilayer structures and thereby leads to the preferential monolayers. Moreover, we predict possibilities of different types of assemblies of the nanorods, and a schematic phase diagram is finally given. The results here may offer a way toward high-quality self-assembled nanoparticles superlattices for use in enhanced spectroscopy, sensors, or nanodevices.

1. INTRODUCTION Arrays of self-assembled nanoparticles have extensive applications in the fields of enhanced spectroscopies, sensors, information storage, and solar cells.1−4 In recent years, anisotropic nanoparticles, especially inorganic nanorods (exemplified by Au, CdS, or CdSe) have attracted increasing interest in aligning them into 2D or 3D ordered arrays. The controlled self-assembly of the nanorods on a suitable surface has many applications in science and technology, for example, for studying fundamental optoelectronic properties arising from collective interactions in an ordered state,5,6 and for incorporating nanomaterials into enhanced spectroscopies,7−10 biochemical sensors,11 and plasmonic devices.12−14 Currently, nematic or smectic phases, and standing superlattices of the nanorods have been experimentally obtained, especially, the standing superlattices have been mentioned to have potentially vectorial properties.9,15 Many techniques including Langmuir−Blodgett deposition, interfacial assembly, and droplet evaporation have been used to obtain the related assemblies.16−19 Among them, the droplet evaporation method is the simplest and the most popular method. However, progress on the order, assembly area, and reproducibility of the obtained self-assemblies has been unsatisfactory due to restriction of control accuracy of the droplet evaporation process. This, to © XXXX American Chemical Society

some extent, seriously hinders the development of the selfassembly technique. Here, we develop a facile and controllable two-stage droplet evaporation (T-DE) method by programmable adjustment of the humidity and temperature environment of the drying droplet. A suitable external environment and thereby a stable evaporation rate of the drying droplet is proved to be efficient for obtaining an ideal and reproducible self-assembly array. Taking the well-developed gold nanorods (GNRs) as an example, highly ordered and large-area self-assembly monolayer arrays are obtained on the surface of silicon substrate.

2. EXPERIMENTAL SECTION GNRs used in this study were stabilized by cetyltrimethylammonium bromide (CTAB) bilayer in aqueous dispersion, and were synthesized by the well-developed seed-mediated growth method.20,21 Rods with an average aspect ratio of 3.4 ± 0.4 (length = 59.2 ± 6.3 nm, diameter = 17.3 ± 1.5 nm) were used as the building block. After synthesis, GNRs were centrifuged (9000 rpm, 30 °C for 7 min) and redispersed in deionized water (18 MΩ·cm) to a concentration of ∼0.5 nM with a CTAB concentration of ∼1.0 mM, where the concentration of the Received: February 26, 2013 Revised: April 14, 2013

A

dx.doi.org/10.1021/la400736b | Langmuir XXXX, XXX, XXX−XXX

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GNRs was determined by Beer−Lambert law, A = εcl with a molar extinction coefficient ε = 4.6 × 109 M−1 cm−1,22 and the concentration of CTAB was cautiously estimated by the amount of CTAB powder and the volume of the aqueous solvent. Next, 1 mL of the GNRs solution was centrifuged again (9000 rpm, 30 °C for 7 min) and the supernatant was removed as much as possible. The precipitate (∼ 10 μL) was redispersed with as-prepared CTAB solutions to adjust the GNRs and CTAB concentration of the GNRs aqueous dispersion, e.g. adding 5 μL of the CTAB solution (5.5 mM) to the precipitate, the GNRs aqueous dispersion was adjusted to nCTAB ≈ 2.5 mM and nGNRs ≈ 33 nM. After a mild ultrasonic treatment, 15 μL of GNRs solution was dropped onto the clean silicon substrate (8 mm × 8 mm). The GNRs droplet on the substrate was first put into a Petri dish, and then the Petri dish was put into a programmable temperature and humidity chamber (STIK, CTHI-150B2) for the GNRs self-assembly. In the first stage of evaporation process, the droplet was evaporated for 30 min at the chamber with the environmental humidity of ∼50% and temperature of 25 °C for creating a pinned edge of the nanorods. In the second stage, the environmental humidity was continuously increased from ∼50% to ∼85% in about 90 min to create a near-equilibrium evaporation environment. Then, the humidity and temperature in the chamber was kept unchanged until the droplet dried out completely. Here, to obtain a reproducible assembly result, the increase of the humidity was controlled according to a designed curve (Supporting Information (SI) Figure S1). The whole T-DE process took about 4 h. Scanning electron microscopy (SEM) images were acquired with a Hitachi S-4800 microscope operating at 10 kV for secondary electron imaging. The low and high magnification scanning modes were used respectively. Each image was mapped with lowest scanning speed and maximum resolution in order to minimize the distortion of the sample surface. The assembled geometry of the GNRs was detected from both the SEM images and the corresponding fast Fourier transform images. Video material was made using screen recording software that records the real-time frames by an optical microscope (Leica DM2500, using 100× objective lens) under a reflective illumination mode. X-ray diffraction measurements were performed with a Bruker D8 focus diffractometer equipped with a copper rotating anode. The measurement was performed with a step of 0.02 deg and scan speed of 0.1 step/ sec. For a quantification of the number of rods aligned in a given direction, refinement of the XRD data was obtained using a whole profile fitting Rietveld-based program named FullProf. Atomic force microscope (AFM) images were acquired in tapping mode with a VEECO Dimensions 3100 instrument and a Nanoscope IV controller. The software package 6r13 was used for data analysis. UV−vis absorption spectra were measured by a UV−vis-NIR spectrophotometer (Cary50) under the conditions of wavelength scope of 200−1100 nm, scan rate of 600 nm/min, temperature range of 25−60 °C. and use of an ultrathin quartz colorimetric-ware (0.1 cm × 1 cm). ζ Potential was tested by a Zetasizer Nano Instrument under the following conditions: The dispersant was water, the temperature was set at 25 °C, the cell type was a disposable zeta cell. The number of measurement for one sample was 3 times and an average value was used as the final result of ζ potential.

which consists of a fast and a slow evaporation stage. The fast process offers the firmly pinned edge for preventing the shrinkage of the droplet, as shown in schematic Figure 1a. The slow evaporation

Figure 1. (a) Formation of pinned edge in the first stage and (b) formation of the near-equilibrium environment in the second stage.

process is to create a near-equilibrium environment to facilitate the formation of the large-area self-assembly arrays, as shown in schematic Figure 1b. Considering the gradually weakened convection in the droplet during the whole evaporation process, the near-equilibrium region would be expanded inside the droplet where the nanoparticles can assemble well into the ordered phase. Moreover, the slow evaporation rate at this stage prevents many nanoparticles from transferring to the edge and therefore ensures the effective assembly area within the solution. Figure 2 shows the GNRs assembled by the T-DE method. The low magnification SEM image (Figure 2a) shows the largearea, assembled arrays within the marked region (defined as our studied region). The periphery of the coffee stain is surrounded with the pinned edge (i.e., the coffee ring). Enlarged observation (Figure 2b) reveals that the assembled arrays are composed of many micrometer-scale domains (6−12 μm) and submicrometer-scale interstices of 0.2−0.4 μm. The inset in Figure 2b further shows that each domain is composed of standing GNRs monolayer on the substrate. Real-time observation of the evaporating droplet shows that a completely self-assembled monolayer is formed at first in the droplet, and then the interstices occur during the subsequent drying (see Video Material). In addition, an obvious dividing line (from red to yellow) in the evaporating droplet is found at the latest stage of the drying process, suggesting that the assembled monolayer arrays should form in the solution prior to falling on the substrate because the colors reflect the change of the GNRs states in the solution (Figure S3). XRD measurements (Figure 2c) for three different samples 1−3 prepared by the T-DE method, common room temperature droplet-evaporation method under a static condition, and a vibration condition, respectively, further show that the (200) peak of the face-centered cubic Au is significantly higher in sample 1 than in the other samples, and the (111) peak and (220) peak are not apparently different. For the GNRs, the growth direction is [001],27 therefore the strong (200) peak suggests that most of the GNRs in the sample 1 tend to be vertically aligned. By refining the XRD results (Figure S4), we also extract that ∼95% GNRs are in the vertical mode. Moreover, from optical microscopy and AFM measurements (Table 1 and Figure S5), the sample prepared by the T-DE method has the biggest domain size of ∼113 μm2, and the smallest surface roughness of 9.2 nm, indicating that the method can prepare a large-area GNRs self-assembled array with low density of defeat. The high magnification SEM image in Figure 2d and its fast Fourier transform pattern (inset) show that the obtained selfassembly has a hexagonally close-packed symmetry in a side-byside (SS) mode.

3. RESULTS AND DISCUSSION Droplet evaporation method has been widely recognized to be effective to assemble nanorods into ordered phases.23−25 Generally, the solvent evaporation process is most likely to form ordered arrays on a dense, localized “coffee ring” region. It is quite rare to get ordered arrays in the rest of the “coffee stain” region (Figure S2a). Slowing down the solvent evaporation rate can make the droplet shrink to a small spot (Figure S2b), not only restricting the assembled area but leading to too high nanoparticle density to keep long-range order of the assemblies. The shrinkage phenomena can be attributed to the absence of pinned edge of the nanoparticles and the effect of Marangoni flow in the solution.26 To avoid the droplet shrinkage and ensure the effective assembly area, we developed the T-DE method B


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Figure 2. (a) SEM image at low magnification shows the large-area self-assembled arrays on the silicon substrate. (b) SEM image at medium magnification shows the preferential monolayer structure. (c) XRD results of the different assembled samples. The rods in the blank space represent the preferential orientations of the assembled GNRs. The dashed circles mean that there is a certain probability to cause the specific Bragg diffraction due to the axial symmetry of the nanorods. (d) SEM image at high magnification shows the monolayer array with a separation of ∼21 nm, which approximates the diameter of GNRs with a CTAB bilayer (2−3 nm). The inset is the fast Fourier transform image of (d).

an equilibrium position and assemble together in the solution before the droplet dries out. The surfactant concentration nCTAB(0) and GNRs concentration nrod(0) of the as-prepared GNRs solution also play an important role in the formation of the GNRs assembly. Our experiment indicates that higher nCTAB(0) and higher nrod(0) result in higher percentage of the assemblies (Figure S2c). And a critical nCTAB(0) between 1 mM and 1.5 mM is found, below which the nanorods do not assemble into the ordered state. Likewise, a critical nrod(0) of about 10 nM is found. Considering that majority of GNRs in the solution are transferred to form the coffee ring due to the convection during the whole drying process, the critical nrod(0) should be the minimum amount, which can ensure the rest of the GNRs form an effective assembly in the studied region. The investigation of the aspect ratio of GNRs further indicates that low aspect ratio with narrow size-distribution is the most advantageous condition to form the selfassembly (Figures S7 and S8). Next, we also investigated the influence of substrates on the formation of the self-assembly. The result shows that hydrophobic or hydrophilic silicon substrates are not helpful to the 2D arrays (Figure S9), in contrast, the untreated silicon substrate with a contact angle of ∼25° is appropriate to the arrays. On the silica, quartz, and organic substrates, we also repeated the same experiments. The results show that some symmetrical island structures with standing GNRs multilayer are formed (Figure S10), which is different from the results on the silicon substrate. The cause may be attributed to the particle−substrate interactions and the contact angle hysteresis which affect the assembly state by forming different convection distributions inside the droplet (e.g., the inner coffee ring deposit modes).28

Table 1. Quality Comparison of Samples Obtained by Different Preparation Methods preferential orientations of the rods (200) sample 90° or 0° 1 2 3a

95% 70% 77%

(111)

(220)

67°

45° or 0°

domain areas (μm2)

surface roughness (Rq, nm)

3% 4% 2%

0 25% 19%

113 50 28b

9.2 18.3 26.4

a

All samples contain the same rods and surfactant. bMajority of the rods are disordered within the domains. Note that for the sample prepared by the one-stage slow evaporation, there was no way to get the large-area, monolayer self-assembly.

To understand the assembly mechanism, we investigated the influence of evaporation rate, surfactant concentration, GNRs concentration, aspect ratio of GNRs, and different substrates on the formation of the assembly. The evaporation rate, which can be affected by humidity and temperature, is found to have a profound effect on the formation of the assembly. Our experiment indicates that the rapid evaporation at high temperature and/or low humidity often leads to small random aggregations in the studied region besides the noticeable coffee ring, typically shown in Figures S2a and S6. In contrast, the slow evaporation at high humidity (nearly 99%) leads to the shrinkage of droplet, typically shown in Figure S2b. Obviously, optimal evaporation rate allows more GNRs to be at C


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Figure 3. Theoretical models of the (a) SS configuration and (b) EE configuration. (c) Typical potential curves for the SS configuration with separation x. (d) Influence of CTAB concentration on the interactive potential for the two parallel GNRs. The inset shows the changes of position and magnitude of the second EM with the increase of CTAB concentration. (e) Comparison of potential of EE configuration with that of SS configuration. (f) Interactive potential for two assembled monolayer islands with face-to-face configuration.

According to the estimate of Srinivasarao et al.,29 the electrostatic force and van der Waals force are the same order of magnitude, and both are much larger than the viscous force (electrostatic force/viscous force ≈ 10−3) and the gravitational

4. THEORETICAL BASIS AND CALCULATIONS In the assembly solution, the overall stability and phase behavior of the nanoparticles depends upon the interplay of the attractive or repulsive forces with the random Brownian forces. D


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force (electrostatic force/gravitational force ≈ 10−4). The depletion attraction among the nanorods caused by the CTAB micelles, especially at a high concentration, also cannot be ignored.30,31 Here, referring to the theory of Asakura and Oosawa,32,33 we give an approximate expression to describe the depletion force of the nanorods (Figure S11). Using the expression and existing formulas of the electrostatic force and van der Waals force, we derive the resultant force based on a simplified model of two parallel rods with any angle. The calculated results show that the SS configuration of the rods is more stable than the configurations of crossing and arbitrary angles (Figure S12). To more intuitively demonstrate the self-assembly mechanism, we calculated the potential energy curves of the typical SS configuration and the typical end-to-end (EE) configuration. The stability of other configurations, e.g., side-to-end (SE), should not have better stability than the two typical configurations.34 GNRs capped with CTAB bilayer are positively charged, providing sufficient repulsion against van der Waals force to prevent their aggregation. The typical electrical double layer (EDL), is shown in Figure 3a. The inner layer, called Stern layer, comprises CTA+ ions adsorbed directly onto the GNRs and the attracted negative ions (e.g., Br−). The surface charge of Stern layer is characterized by surface charge density (σd), and the electric potential on the external boundary of the Stern layer versus the bulk electrolyte is referred as Stern potential (ψd). The outer layer, called diffuse layer, is mainly composed of Br− ions attracted to the surface of Stern layer via the Coulomb force, electrically screening the first layer. The electric potential in the second layer exponentially decreases away from the surface to the bulk fluid. Since the diffuse layer is generally made of free-ions, there is an introduced slipping plane that separates the bulk fluid from the fluid that is still attached to the surface. The electric potential at this plane is denoted as ζ-potential. The characteristic thickness of the EDL is the Debye length λD( = κ−1), which is reciprocally proportional to the square root of the ion concentration in the GNRs dispersion. According to the EDL model, the dependence of electrostatic potential on the separation x can be given by35 Ue//(x) =

2

πR σd2l Li1/2(e−κx) 3/2

ε0εκ

The relationship between the van der Waals potential and the separation, x, for the two parallel GNRs is34 // Uvan (x ) ≈ −

Aeff l ⎛ R ⎞3/2 ⎜ ⎟ 24R ⎝ x ⎠

(2)

where Aeff is effective Hamaker constant of GNRs, which is about 10 × 10−20 J for Au in water.39 As a result, the potential is only determined by the composition and size of the nanorods. The attractive potential of the depletion forces with the two parallel rods can be given by integral Equation S5 from x1 to d′ + m′ 1 ⎡ // Udep (x1) = − l′P0⎢ −x1 (d′ + m′)2 − x12 + (d′ + m′)2 · 2 ⎣ ⎛ x1 ⎞⎤ ⎟ arccos⎜ ⎝ d′ + m′ ⎠⎥⎦ (3)

Here, d ≤ x1 ≤ d′ + m′, l′ = l + 2λD is the effective length of the rods, d′ = d + 2λD is the effective diameter of the rods, m′ = m + 2λD is the effective diameter of the CTAB micelles, and x1 = x + d is the center distance of the two rods. We use the effective sizes of the rods and micelles, which are equal to the physical size plus the electrostatic screening length, λD,40 for the following calculations. According to eq 3, the attractive potential of depletion force is also directly related to the CTAB concentration. The total potential can be expressed as // // (x + d) + Uvan (x ) U (x) = Ue//(x) + Udep

(4)

Therefore, the total potential is mainly determined by the CTAB concentration in our case. For the EE configuration (Figure 3b), the electrostatic repulsion potential and the attractive potentials of depletion force and van der Waals force can be expressed as eqs 5−8, respectively.34,36 ⎛ eψ ⎞2 Ue−−(x) = 64kTNAn ionκ −1tanh⎜ d ⎟ exp( −κx) ·πR2 ⎝ 4kT ⎠

(5)

⎛ d′ ⎞ 2 −− Udep (x − 2λD) = −π ⎜ ⎟ ·nmicellesRcT (m′ − x + 2λD) ⎝2⎠

(1)

0 ≤ x ≤ m′

where R is the radius of the rods (∼ 10 nm with the outer coated CTAB bilayer, tCTAB), l is the length of the rods (∼ 64 nm with the outer coated CTAB bilayer), ε0 is the vacuum permittivity, ε is the relative dielectric constant of solvent (80 for water),36 and Li1/2(z) is the polylogarithm function, defined by Li1/2(z) = ∑i(zi/i1/2). According to eq 1, the electrostatic potential is mainly determined by surface charge density σd and Debye length κ−1 of the double layer. The surface charge density is related to the functionalized surfactant, and is more reasonable to be treated as a constant value for GNRs with excess CTAB,37 which can be verified by the ζ-potential measurement (Figure S13). While κ−1 in the GNRs dispersion is expressed by38 κ−1 = ((ε0εkT)/(e2NA[2ncmc + (nCTAB− ncmc)·α]))1/2, where k is Boltzmann constant, T is the temperature in Kelvin, e is the charge of electron, NA is Avogadro constant, ncmc is the critical micelle concentration of CTAB defined as the concentration of surfactants above which micelles are spontaneously formed (∼ 0.9 mM),29 and α is the degree of dissociation of CTAB (about 25% at room temperature).38 Therefore, the electrostatic potential between the two parallel GNRs is directly related to the CTAB concentration.

−− (x ) = − U van

(6)

⎞ Aeff R2 ⎛ 1 1 2 − ⎜ 2 − 2 2⎟ 12 ⎝ (x) (x + l) (x + 2l) ⎠

−− −− U −−(x) = Ue−−(x) + U van (x) + Udep (x − 2λD)

(7) (8)

Figure 3c illustrates the calculated results for the SS configuration. A pretty high energy barrier (3.9 eV) at the separation of about 1 nm is found. Such an energy barrier should be the key factor to stabilize the GNRs dispersion and to prevent the GNRs from fast aggregation even at high nCTAB. More interestingly, a second-energy-minimum (second EM) of about −1.2 eV at a separation of ∼6.1 nm is exhibited, and its effect is much higher than that of the Brownian disturbance (kT ∼ 0.026 eV at room temperature), indicating the existence of a metastable state which plays a key role in the assembly process. Moreover, increasing nCTAB, the separation of the rods decreases and the potential well becomes deeper as shown in Figure 3d and its inset, leading to a more compact and stable nanorods assembly. This explains the result that high nCTAB and stable evaporating E


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Figure 4. Kinetic spectra for solution with (a) different GNRs concentration and (b) constant CTAB concentration of 0.5 M. The inset shows the kinetic spectra for solution with as-prepared CTAB concentration of 0.1 mM. A series of spectra obtained every 90 min for solution with (c) different CTAB concentrations and constant GNRs concentration of 2.1 nM, and (d) different GNRs concentrations and constant CTAB concentration of 0.5 M. The inset of panel c shows temperature dependence of the spectra obtained every 90 min, and the inset of panel d shows invertible effect by diluting the assembled GNRs solutions. The dashed line in all the panels shows the spectra of as-prepared monodispersed GNRs solution. All spectra are normalized by the extinction at 400 nm for facilitating comparison.

Brownian disturbance, the self-assembly of GNRs within the effective separation will be triggered. If the effective distance is smaller than 10 nm, the nCTAB to start the assembly should be larger than 0.2 M. It also indicates that the assembly should occur in a high nCTAB environment, corresponding to the latter stage of droplet drying. The next question is why the rods assemble into the preferential monolayer. Figure 3f shows that under the condition of high nCTAB, two assembled monolayer islands with face-to-face configuration exhibit an extremely high barrier due to the huge electrostatic repulsion, thereby hindering the formation of multilayer structures. This is consistent with the simulated results reported by Patti et al.,42 where they also considered the depletion force. And this result can also be supported by the fact that the monolayer islands formed at a relatively low nrod are evenly distributed on the substrate (Figure S2c).

rate in the experiments lead to the self-assembled array. It should be noted that the position and the magnitude of the second EM are changed with changing of the additives as well as the formed micelles. In the CTAB case, the effective micelles size is dependent on the micelles size plus the electrostatic screening length, i.e., Debye length, and the Debye length which is dependent on the electrolyte ion concentration is more essential to affect the energy position. At a high CTAB concentration (0.5 M), and thereby a high free ion concentration (the degree of dissociation is about 25%), the calculated Debye length is about 1.2 nm. According to our calculation, if decreasing the free ion concentration, corresponding to increasing the Debye length, e.g. up to 3 nm, the position of the second-energy-minimum is about 20 nm with a magnitude about −0.1 eV. This means that the interactions can be used to explain the existence of collective nanoparticles self-assemblies with intermediate range order.41 And in turn, it can also be used to estimate the free ion concentration and the micelles concentration for the self-assembly. The red dashed curve in Figure 3c presents the potential energy without considering the depletion force. Compared to the total potential curves in red in Figure 3c, we can conclude that the depletion force dramatically enhances the metastable state, and is an important interactive force for the assembly. In addition, Figure 3e reveals that for the EE configuration, GNRs prefer the EE assembly at a low nCTAB but they tend to the SS assembly at a high nCTAB. According to our calculations, as long as nCTAB increases to the value at which the second EM is larger than the

5. SPECTRAL VALIDATION The GNRs assembly process, especially in the near-equilibrium state at the latter stage of the drying, often needs a quite long duration time Δt, during which nCTAB(t) or nrod(t) can be a constant, i.e., nCTAB(t) ≈ nCTAB(t + Δt). Such a feature can be reproduced and investigated by the kinetic UV−vis absorption spectrum. Figure 4 demonstrates the measured results for the GNRs solutions. For nCTAB(t) = 0.5 M and nrod(t) = 2.1 nM (Figure 4a), the intensity of monomers (at 730 nm) gradually decreases with F


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Figure 5. (a)−(f) Schematic diagram of the standing GNRs monolayer by the T-DE method. The yellow cylinder represents the GNR and the gray layer around the cylinder represents the coated CTAB bilayer. The small cyan dot represents the CTAB molecule, and the bigger pink dot represents the CTAB micelle.

the time Δt, while the transverse peak (at 520 nm) has noticeable red-shift and band broadening together with a slight blue shift in the longitudinal peak (about 8 nm from Figure 4a), exhibiting typical features of the SS assembly in the solution.43 This indicates that the GNRs tend to assemble in the SS mode within the solution. This is consistent with our calculations. From Figure 4a, we can also tell that the longer Δt, i.e., the lower evaporation rate, allows more rods involved in the assembly. In other words, the near-equilibrium condition is essential for the self-assembly formation. When the duration time Δt reaches 55 min, the spectrum is not changed obviously, suggesting that the GNRs assembly in the solution approaches to finish. Comparing Figure 4a and 4b, the higher nrod(t) can speed up the assembly process. From the Figure 4a inset, it can be seen that no change is observed in the kinetic spectra within the investigated time of 50 min with nCTAB(t) = 0.1 mM and nrod(t) = 2.1 nM, indicating that the as-prepared GNRs dispersion is quite stable, even at high GNRs concentration. Figure 4c demonstrates the spectra after a sufficient time (∼ 90 min) with different nCTAB(t) from 0.3 to 0.5 M and constant nrod(t) = 2.1 nM. The spectrum with nCTAB(t) = 0.3 M is almost as same as that with nCTAB(t) = 0.1 mM in Figure 4a inset, indicating no assembly occurs. When nCTAB(t) > 0.4 M, the rods start to assemble effectively, confirming the existence of a critical CTAB concentration. Meanwhile, it is obvious that higher CTAB concentration leads to more stable assembly of the GNRs. This is also consistent with our calculation that higher CTAB concentration results in smaller separation distance and lower potential well in the SS mode. The inset of Figure 4c further illustrates the temperature dependence for nrod(t) = 2.1 nM and nCTAB(t) = 0.5 M. The temperature of 25 °C (close to the Krafft temperature of CTAB) is proven to be the optimum temperature for the solution assembly. Higher temperature leads the GNRs to be more dispersed. Lower temperature makes the surfactant CTAB seed out, and thereby affects the GNRs self-assembly (Figure S14). Figure 4d further demonstrates the spectra with different nrod(t) from 0.2 to 2.1 nM and constant nCTAB(t) = 0.5 M. It shows that the increase of GNRs concentration strengthens the

GNRs assembly, and it also confirms the existence of a critical nrod(t) of ∼0.4 nM. Here, it should be noted that the GNRs concentration is lower than the initial critical nrod(0), but the assembly still occurs as long as the nCTAB(t) is beyond the critical value of ∼0.4 M, indicating that the CTAB concentration plays a much more important role in the self-assembly process. This point can be supported by that the enhanced depletion attraction, and thereby the large second EM drives the GNRs self-assembly. The high nrod increases the probability of the selfassembly. Additionally, we find that the assembly behavior observed in all the spectra in Figure 4 is revertible by diluting the solution or treating the solution ultrasonically. A typical result is shown in the inset of Figure 4d, where we dilute the GNRs solutions from nCTAB(t) = 0.5 to 0.05 M, the spectrum is returned to the state of as-prepared GNRs solution, and no assembly exists. This observation can strongly support the result that the energy condition for the assembly is metastable and the driving force should be the secondary energy minimum.

6. SELF-ASSEMBLY MECHANISM AND PHASE DIAGRAM Based on above conclusions, we can describe the formation process for the self-assembly as sketched in Figure 5a−5f. In the first stage of our T-DE method, the appearance of the micelles will enhance the attractive potential when nCTAB increases to the critical micelle concentration. Then the second EM makes the rods stabilize within the effective separation, as illustrated from Figure 5a and 5b. In the second stage, continuous increase of the CTAB concentration further enhances the second EM, which drives GNRs to pack closer in the solution (Figure 5c). In addition, since the hexagonal phase transition of the CTAB micelle is around 25 °C, this would have a contribution to the GNRs hexagonal assembly. The low evaporation rate due to high humidity (∼ 85%) of the environment supplies enough time for more GNRs to find the metastable positions and assemble them into the monolayer islands; after that, the islands continue to grow up with the incoming free rods and prevent the formation of multilayers in the solution due to the huge electrostatic repulsion between the islands, as shown in Figure 5d. With the G


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same values as that used in Figure 3a. The assembly environment is set in the aqueous solution with a high micelles concentration. The calculation results are shown in Figure 6a. The gray narrow region at the bottom of the diagram is a prohibited zone, in which the GNRs cannot be stabilized due to the weak electrostatic repulsion. The region I represents the EE preferred assembly. The typical potential curve is similar to that in the inset of Figure 3e, where the EE configuration has a smaller energy. The need of such an assembly should have larger σd and smaller Aeff. And fading of the color means that the trend to the EE assembly weakens. The region II means monolayer assembly in the SS fashion, having the typical potential curves for the rods in Figure 3c and for the islands in Figure 3f. Such assembly needs an appropriate combination of σd and Aeff, for example, Aeff and zeta potential about 10 × 10−20 J and 56 mV, respectively, in the case of CTAB-coated GNRs (Figure S13). The region III stands for multilayer assembly in the SS mode. Their typical potential curves for the rods are similar to those in Figure 3c, i.e., the rods prefer to pack into the islands in the SS fashion, while the face-to-face configuration for the packed islands have smaller energy at a definite distance. For example, EG6OH-coated GNRs (Aeff ∼ 10 × 10−20 J and zeta potential −10 mV) belongs to this case.44 Besides, addition of extra electrolyte ions can lead to enlargement of the SS assembly region for either monolayer or multilayer, as shown in Figure 6b. In this situation, it can be predicted that with enough free ions, the CTAB-coated GNRs would be also possible to assemble into multilayer structures.

receding of the liquid surface, the nearby islands have to integrate mutually into a bigger monolayer and the residual defect can be repaired by the free rods, and finally a full monolayer film is formed. The decreased liquid surface at the latter stage of the drying compels the film to deposit parallel to the substrate, i.e., the assembled nanorods are vertical to the substrate, as shown in Figure 5e. At the latter moment of the droplet drying, cracking occurs in the film due to adhesion force with the substrate and capillary force in the formation of liquid vapor menisci among the rods, as sketched in Figure 5f. The cracking could be overcome by a fluid interfacial method with a solid-transfer procedure, however in that case additional defects caused in the transfer process need to be further addressed. Within Figure 5, three dominant factors determine the selfassembly: (1) near-equilibrium status is the prerequisite, (2) second EM is the key factor to form the close-packed assembly, and (3) huge barrier between the assembled islands with face-toface configuration forces them to the monolayer. Considering that the second EM can be tuned by changing the parameters such as materials and size of nanoparticles, the type and concentration of the surfactant as well as ionic strength or types of electrolyte, it is possible to sketch a phase diagram to predict the different self-assembly modes. For this purpose, we calculate the phase diagram under the near-equilibrium status. Here we just tune Hamaker constant Aeff (i.e., different composition of the rod or solvent media) and surface charge density σd (i.e., different surfactant coated on the rods), other parameters are set to be the

7. CONCLUSIONS To overcome the disadvantages of the ordering and repeatability of the method of droplet evaporation, we develop the T-DE method with well controlled droplet evaporation rates. The essence of the T-DE method is that the first stage can effectively control the assembly area, and the second stage can provide a key assembly condition of the near-equilibrium status. Using the method, the GNRs self-assembly monolayer has been achieved and demonstrated. Besides the evaporation rate, the high CTAB concentration and the suitable substrate are also important to achieve the assembly. Theoretical analyses further indicate that the second EM is the main cause for the nucleation and growth of the GNRs self-assembly. The huge electrostatic repulsion between the assembled islands with the face-to-face configuration results in the preferential monolayer. The existence and the role of the second EM are supported by the UV−vis absorption spectrum, in which we reproduce each scene of the assembly in the solution. In addition, a schematic phase diagram is obtained under the near-equilibrium condition, the EE and SS monolayer or multilayer assemblies are considered, offering us probability to delicately design and engineer different assembly structures. Although there are limitations to our approach, such as analytical precision, details for the kinetic processes, etc., it would also give a meaningful guide for exploration and application of the nanoparticles self-assembly superlattices.

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ASSOCIATED CONTENT

S Supporting Information *

The design curve for environmental humidity, the effect of evaporation rate, surfactant concentration, GNRs concentration, rod aspect ratio, size distribution, substrate and temperature on the self-assembly; XRD refined results; domain size and surface roughness analysis; the calculation of rod depletion force, interparticle forces analysis, and zeta potential. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. (a) Phase diagram for the GNRs assembly with parameters of surface charge density and Hamaker constant, and without considering the extra electrolyte ion. (b) Phase diagram considering extra electrolyte ion with concentration of 0.1 M. H


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(16) Bigioni, T. P.; Lin, X.-M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Kinetically Driven Self Assembly of Highly Ordered Nanoparticle Monolayers. Nat. Mater. 2006, 5, 265−270. (17) Zanella, M.; Gomes, R.; Povia, M.; Giannini, C.; Zhang, Y.; Riskin, A.; Van Bael, M.; Hens, Z.; Manna, L. Self-Assembled Multilayers of Vertically Aligned Semiconductor Nanorods on Device-Scale Areas. Adv. Mater. 2011, 23, 2205−2209. (18) Baker, J. L.; Widmer-Cooper, A.; Toney, M. F.; Geissler, P. L.; Alivisatos, A. P. Device-Scale Perpendicular Alignment of Colloidal Nanorods. Nano Lett. 2010, 10, 195−201. (19) Ahmed, S.; Ryan, K. M. Centimetre Scale Assembly of Vertically Aligned and Close Packed Semiconductor Nanorods from Solution. Chem. Commun. 2009, 6421−6423. (20) Gole, A.; Murphy, C. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed. J. Chem. Mater. 2004, 16, 3633−3640. (21) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. (22) Orendorff, C. J.; Murphy, C. J. Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (23) He, J.; Zhang, Q.; Gupta, S.; Emrick, T.; Russell, T.; Thiyagarajan, P. Drying Droplets: A Window into the Behavior of Nanorods at Interfaces. Small 2007, 3, 1214−1217. (24) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Dryingmediated Self-assembly of Nanoparticles. Nature 2003, 426, 271−274. (25) Xie, Y.; Jia, Y. F.; Liang, Y. J.; Guo, S. M.; Ji, Y. L.; Wu, X. C.; Chen, Z. Y.; Liu, Q. Real-time Observations on Crystallization of Gold Nanorods into Spiral or Lamellar Superlattices. Chem. Commun. 2012, 48, 2128−2130. (26) Hu, H.; Larson, R. G. Marangoni Effect Reverses Coffee-Ring Depositions. J. Phys. Chem. B 2006, 110, 7090−7094. (27) Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; DoanNguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R.; Murray, C. B. Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives. Acs Nano 2012, 6, 2804−2817. (28) Hguyen, T. H.; Hampton, M. A.; Nguyen, A. V. Evaporation of Nanoparticle Droplets on Smooth Hydrophobic Surfaces: The Inner Coffee Ring Deposits. J. Phys. Chem. C 2013, 117, 4707−4716. (29) Sharma, V.; Park, K.; Srinivasarao, M. Colloidal Dispersion of Gold Nanorods: Historical Background, Optical Properties, Seedmediated Synthesis, Shape Separation and Self-assembly. Mater. Sci. Eng., R 2009, 65, 1−38. (30) Baranov, D.; Fiore, A.; van Huis, M.; Giannini, C.; Falqui, A.; Lafont, U.; Zandbergen, H.; Zanella, M.; Cingolani, R.; Manna, L. Assembly of Colloidal Semiconductor Nanorods in Solution by Depletion Attraction. Nano Lett. 2010, 10, 743−749. (31) Marenduzzo, D.; Finan, K.; Cook, P. R. The Depletion Attraction: An Underappreciated Force Driving Cellular Organization. J. Cell Biol. 2006, 175, 681−686. (32) Asakura, S.; Oosawa, F. Interaction between Particles Suspended in Solutions of Macromolecules. J. Polym. Sci. 1958, 33, 183−192. (33) Asakura, S.; Oosawa, F. On Interaction between Two Bodies Immersed in a Solution of Macromolecules. J. Chem. Phys. 1954, 22, 1255−1256. (34) Russel, W. B.; Saville, U. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1999. (35) Ohshima, H.; Hyono, A. Electrostatic Interaction between Two Cylindrical Soft Particles. J. Colloid Interface Sci. 2009, 333, 202−208. (36) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in Self-Assembly. Small 2009, 5, 1600−1630. (37) Jones, M. The Relationship between Surface Charge Density and Double Layer Potential in Soap Films. Colloid Polym. Sci. 1969, 230, 236−240. (38) Pashley, R. M.; Ninham, B. W. Double-Layer Forces in Ionic Micellar Solutions. J. Phys. Chem. 1987, 91, 2902−2904.

A movie of the evaporating droplet is available in the HTML version of the manuscript.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +86-10-8254-5585 (Q.L.). E-mail: [email protected]; phone: +86-10-8254-5577 (X.C.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (10974037, 61006078), National Basic Research Programs of China (2010CB934102, 2011CB932802), International S&T Cooperation Program (2010DFA51970), and Eu-FP7 Project (247644).

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REFERENCES

(1) Stang, P. J. Abiological Self-Assembly via Coordination: Formation of 2D Metallacycles and 3D Metallacages with Well-Defined Shapes and Sizes and Their Chemistry. J. Am. Chem. Soc. 2012, 134, 11829−11830. (2) Kabashin, A. V.; Evans, P.; Pastkovsky, S.; Hendren, W.; Wurtz, G. A.; Atkinson, R.; Pollard, R.; Podolskiy, V. A.; Zayats, A. V. Plasmonic Nanorod Metamaterials for Biosensing. Nat. Mater. 2009, 8, 867−871. (3) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418−2421. (4) Philp, D.; Stoddart, J. F. Self-Assembly in Natural and Unnatural Systems. Angew. Chem., Int. Ed. 1996, 35, 1154−1196. (5) Khatua, S.; Chang, W.-S.; Swanglap, P.; Olson, J.; Link, S. Active Modulation of Nanorod Plasmons. Nano Lett. 2011, 11, 3797−3802. (6) Chirea, M.; Borges, J.; Pereira, C. M.; Silva, A. F. DensityDependent Electrochemical Properties of Vertically Aligned Gold Nanorods. J. Phys. Chem. C 2010, 114, 9478−9488. (7) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; WuDe, Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-isolated Nanoparticle-enhanced Raman Spectroscopy. Nature 2010, 464, 392−395. (8) Alvarez-Puebla, R. A.; Agarwal, A.; Manna, P.; Khanal, B. P.; Aldeanueva-Potel, P.; Carbo-Argibay, E.; Pazos-Perez, N.; Vigderman, L.; Zubarev, E. R.; Kotov, N. A.; Liz-Marzan, L. M. Gold Nanorods 3DSupercrystals as Surface Enhanced Raman Scattering Spectroscopy Substrates for the Rapid Detection of Scrambled Prions. Proc. Natl. Acad. Sci., U. S. A. 2011, 108, 8157−8161. (9) Guerrero-Martinez, A.; Perez-Juste, J.; Carbo-Argibay, E.; Tardajos, G.; Liz-Marzan, L. M. Gemini-Surfactant-Directed SelfAssembly of Monodisperse Gold Nanorods into Standing Superlattices. Angew. Chem., Int. Ed. 2009, 48, 9484−9488. (10) Ming, T.; Kou, X.; Chen, H.; Wang, T.; Tam, H.-L.; Cheah, K.-W.; Chen, J.-Y.; Wang, J. Ordered Gold Nanostructure Assemblies Formed by Droplet Evaporation. Angew. Chem., Int. Ed. 2008, 47, 9685−9690. (11) Li, F.; Josephson, D. P.; Stein, A. Colloidal Assembly: The Road from Particles to Colloidal Molecules and Crystals. Angew. Chem., Int. Ed. 2011, 50, 360−388. (12) Guerrero-Martínez, A.; Auguié, B.; Alonso-Gómez, J. L.; Džolić, Z.; Gómez-Graña, S.; Ž inić, M.; Cid, M. M.; Liz-Marzán, L. M. Intense Optical Activity from Three-Dimensional Chiral Ordering of Plasmonic Nanoantennas. Angew. Chem., Int. Ed. 2011, 50, 5499−5503. (13) Wu, C.; Khanikaev, A. B.; Adato, R.; Arju, N.; Yanik, A. A.; Altug, H.; Shvets, G. Fano-resonant Asymmetric Metamaterials for Ultrasensitive Spectroscopy and Identification of Molecular Monolayers. Nat. Mater. 2012, 11, 69−75. (14) Liu, N.; Tang, M. L.; Hentschel, M.; Giessen, H.; Alivisatos, A. P. Nanoantenna-enhanced Gas Sensing in a Single Tailored Nanofocus. Nat. Mater. 2011, 10, 631−636. (15) Petukhova, A.; Greener, J.; Liu, K.; Nykypanchuk, D.; Nicolaÿ, R.; Matyjaszewski, K.; Kumacheva, E. Standing Arrays of Gold Nanorods End-Tethered with Polymer Ligands. Small 2012, 8, 731−737. I


Langmuir

Article

(39) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370. (40) Sober, D. L.; Walz, J. Y. Measurement of Long Range Depletion Energies between a Colloidal Particle and a Flat Surface in Micellar Solutions. Langmuir 1995, 11, 2352−2356. (41) Hamon, C.; Postic, M.; Mazari, E.; Bizien, T.; Dupuis, C.; Hernandez, P.; Jimenez, A.; Courbin, L.; Gosse, C.; Artzner, F.; Artzner, V. Three-Dimensional Self-Assembling of Gold Nanorods with Controlled Macroscopic Shape and Local Smectic B Order. ACS Nano 2012, 6, 4137−4146. (42) Patti, A.; Dijkstra, M. Do Multilayer Crystals Nucleate in Suspensions of Colloidal Rods? Phys. Rev. Lett. 2009, 102, 128301. (43) Sun, Z.; Ni, W. H.; Yang, Z.; Kou, X. S.; Li, L.; Wang, J. F. pHControlled Reversible Assembly and Disassembly of Gold Nanorods. Small 2008, 4, 1287−1292. (44) Xie, Y.; Guo, S.; Ji, Y.; Guo, C.; Liu, X.; Chen, Z.; Wu, X.; Liu, Q. Self-Assembly of Gold Nanorods into Symmetric Superlattices Directed by OH-Terminated Hexa(ethylene glycol) Alkanethiol. Langmuir 2011, 27, 11394−11400.

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Controllable Two-Stage Droplet Evaporation Method and Its

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