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Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties Guang Yang, Longqian Hu, Timothy D Keiper, Peng Xiong, and Daniel Hallinan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00347 • Publication Date (Web): 27 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016

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Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties Guang Yanga,b, Longqian Huc, Timothy D. Keiperc, Peng Xiongc, Daniel T. Hallinan Jr.a,b,* a

Florida State University — Aero-propulsion, Mechatronics & Energy Center — 2003 Levy Avenue, Tallahassee, FL 32310, USA b

Florida A&M University-Florida State University College of Engineering, Department of Chemical and Biomedical Engineering, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA c

Florida State University, Department of Physics, Tallahassee, FL 32306, USA

ABSTRACT Centimeter-scale gold nanoparticle (Au NP) monolayer films have been fabricated using a water/organic solvent self-assembly strategy. A recently-developed approach, “drain to deposit”, is demonstrated most effective to transfer the Au NP films from the water/organic solvent interface to various solid substrates while maintaining their integrity. The interparticle spacing was tuned from 1.4 nm to 3.1 nm using different length alkylamine ligands. The ordering of the films increased with increasing ligand length. The surface plasmon resonance and the in-plane electrical conductivity of the Au NP films both exhibit an exponential dependence on the interparticle spacing. These findings show great potential in scaling up the manufacturing of high-performance optical and electronic devices based on two-dimensional metallic nanoparticle superlattices.

INTRODUCTION

ACS Paragon Plus Environment

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The unique physical and chemical properties of most traditional materials are often to a large extent determined by the spatial arrangement of the constituent building blocks (i.e. atoms)

relative to one another.1 When the scale of the building blocks extend to the range outside that of atomic elements (e.g. nanoparticles), the ‘artificial solids’ composed of such nanoparticles exhibit unique properties different from their bulk counterparts.2,

3, 4

In particular, monolayer

two-dimensional (2D) artificial solids, serving as the structural basis for more complicated nanostructures, display distinct collective optical,5,

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electrical,7 and catalytic properties,8 thus

finding vast prospective applications in high-performance solar cells,9 electrogenerated chemilumines,10 chemical sensors,11 transistors,12 integrated microcircuitry,13 batteries, capacitors, and thermolectrics.14 Akin to traditional materials, the physical and chemical properties of artificial solids not only depend on the elementary nanoparticle size and shape, but as importantly on the interparticle separation and the periodic arrangement of the constituents.3, 4 For instance, for metal nanoparticles, most of the optical properties are based on their strong optical resonance phenomenon,5, 6, 15, 16 which arises from coherent surface oscillations of free (valence) electrons in the metal nanoparticle surface that are in resonance with incident light of a certain frequency, i.e. the surface plasmon resonance (SPR) frequency. The SPR results in a strongly enhanced electric field localized at the particle surface.17 In ordered nanoparticle assemblies, the electron wave function of a nanoparticle can overlap with adjacent nanoparticles. This near-field interaction is thought to be responsible for the SPR frequency being shifted with respect to that of a single nanoparticle. The magnitude of the SPR frequency shift exponentially depends on the strength of interparticle coupling, and has been demonstrated tunable by interparticle distance.18, 19, 20 Long-range dipole interactions have also been shown to affect SPR frequency in larger, lithographically prepared nanostructures separated by distances much larger


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than those commonly observed in NP monolayers.21 Long-range dipole effects will be negligle in Au NP monolayers with particle size on the order of 10 nm and separation distance on the order of 1 nm, as is used in this study. Electron transfer to/from metal nanoparticles and through monolayer and multilayer assemblies is another area of both fundamental and practical interest.22,

23, 24

Electrons transfer between

adjacent nanoparticles through interparticle gaps. The electron transfer is governed by the overlap of the electron wave functions of adjacent nanoparticles.25 Thus, it can be envisioned that, the quantum and classical particle-particle interaction, and hence the interparticle charge transfer, can be manipulated by tuning the interparticle separation.4,

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To the best of our

knowledge, there is only one report investigating both the SPR shift and the electrical conductivity of metal nanoparticle assemblies as a function of nanoparticle separation.24 The particle separation was controlled by compressing a Langmuir monolayer of silver nanoparticles with alkylthiol ligands. The disadvantage of this approach is that trace ions in the water subphase of Langmuir monolayers can adversely affect the measurements.22 However, studying both SPR shift and electrical conductivity of nanoparticle monolayers having the same structure prepared by the same technique is important for understanding the closely related mechanisms of the optical and electrical properties of metal nanoparticle assemblies. Many groups have studied alkylthiol-stabilized gold nanoparticle (Au NP) superlattices that were deposited on substrates using solvent casting,26, Langmuir-Schaefer23,

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adsorption,28,

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Langmuir-Blodgett,30,

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or

techniques. However, it is quite challenging to generate well-ordered,

large-scale (> 1 cm2) monolayers on solid substrates using these techniques. One of the best known methods is to drop-cast nanoparticle dispersions on a solid wafer and to rely on selfassembly to form ordered structures.5, 33, 34 By varying the length of alkyl chain ligands on the


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nanoparticles, interparticle separation can be adjusted.33 It has been found, however, that the nanoparticle ensembles formed in this way usually lack long-range order and reasonable reproducibility.35 Several post-treatment approaches have been used to tune interparticle spacing after the formation of the 2D NP superlattices. These include ligand exchange36, 37, 38, 39, 40, 41, 42 and thermal annealing.43, changes.41,

42, 45

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The former approach causes severe film cracking due to volume

The latter approach can decrease interparticle spacing, but also results in

significant decrease of long-range order.43, 44 Interfacial self-assembly is powerful due to the efficiency of fabrication, possibility for largescale application, and development of small interparticle gaps that are difficult to achieve with lithography.46 In particular, self-assembly of nanoparticles at an organic/aqueous interface has been found to achieve nanostructures over larger areas than is possible with the aforementioned techniques.47,

48, 49, 50, 51

As-synthesized nanoparticles are charged and thus electrostatically

stabilized in water. Addition of a low-dielectric solvent such as ethanol to the aqueous colloidal suspension and an organic solvent on top of the aqueous phase induces the NPs to migrate to the organic/aqueous interface, forming 2D arrays.47, 49, 51 This process appears to be dependent on the contact angle between the particles and the liquids being close to 90°.50 This condition seems to be achievable for a variety of NP materials.48 The resulting NP films have been reported to have poor order due to electrostatic repulsions counteracting van der Waals attractions.47, 49, 51 Park et al.52 reported that a well-ordered nanoparticle film can be formed at an organic/water interface by adding 1-dodecanethiol to the organic phase. The electrostatic stabilization in the aqueous phase is (at least partially) replaced by steric stabilization at the interface due to 1-dodecanethiol displacing sodium citrate on the nanoparticle surfaces. The fluid nature of the organic/water interface allows facile NP rearrangement and can thus be used to form monolayers of Au NPs


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covering large areas (cm2).52, 53 There remains a big challenge of transferring such monolayers onto solid substrates, especially if excess ligand is not desired.52 We have recently demonstrated that a “drain-to-deposit” technology was most effective in depositing large-scale crack-free Au NP monolayers onto various substrates.54 This method involves slowly draining the bottom, aqueous phase so that the Au NP film at the organic/aqueous interface deposits on substrates initially contained in the aqueous phase. Since the liquid is removed by pumping (rather than evaporation) excess ligand and ions from ligand exchange are removed. Removal of the organic phase by evaporation has the added disadvantage that it destabilizes monolayers of Au NPs smaller than 4 nm.55 The drain-to-deposit technique deposits NPs on a solid substrate in the presence of both liquid phases. Therefore, it is only limited by the ability to self-assemble NPs at a liquid-liquid (or liquid-air) interface. Park et al. have demonstrated that Au NPs as large as 100 nm can be assembled at a liquid-liquid interface.56 In our previous study, dodecylamine (denoted as C12) and octadecylamine (C18) were used as spacers to change the interparticle spacing of the Au NP films.54 Here, we extend this study to alkylamine ligands of even shorter chain lengths, and present a comprehensive study of the structure of the monolayers as well as their optical and electrical properties. Although it is an apparently minor modification, we consider the use of alkylamine rather than alkylthiols ligands as important. Thiol-gold bonds have an inner shell energy barrier to electron tunneling/hopping.22 In addition, they can take many conformations that significantly affect measured electrical conductivity; whereas, the amine-gold bond is well defined resulting in consistent measurements across single-molecule junctions.57,

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The stability of amine-gold

nanocrystals and nanoparticles is comparable to their thiol-capped counterparts.59,

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Unlike

alkylthiols, alkylamines have received relatively sparse attention in functionalizing gold


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nanoparticles. This report demonstrates that 2D Au NP monolayers can be deposited with unprecedented film integrity for amine ligands of various alkane lengths. The Au NP interparticle spacing was tunable due to different alkylamine lengths. The resulting monolayers exhibit a surface plasmon resonance (SPR) with a pronounced dependence on the interparticle spacing. Electrical conductivity of the films also exhibits a ligand-length dependent behavior. To the best of our knowledge, this is the first systematic study of the SPR and electrical conductivity of monolayer 2D Au NPs functionalized with primary alkylamine ligands. This approach allows the precise control over 2D artificial NP crystal lattice with alkylamine ligands, opening up a new avenue in scaling up the manufacture of high-performance, 2D-superlattice-based photonic and electronic devices. EXPERIMENTAL SECTION Gold nanoparticle synthesis The detailed information of materials used in this study can be found in supporting information. Citrate-stabilized Au NPs (diameter 12.8 ± 1.2 nm) were prepared using a modified Turkevich method.

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Aqueous solutions of HAuCl4 (200 mL, 0.5 mM) and sodium citrate (10 mL, 38.8

mM) were brought to boiling separately. Then the latter was rapidly added to the former under vigorous stirring. A gradual visual color change from light purple to red was observed. The mixture was kept boiling for 20 minutes until the color remained unchanged. Full conversion of the Au (III) to Au NPs is expected due to the large stoichiometric excess of sodium citrate, which acts as reducing agent. All samples used in this study were diluted to 75 vol% of the assynthesized Au NP colloid with DI water. 2D Au NP array preparation


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Detailed process of preparing monolayer Au NP films can be found in our previous report.

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Briefly, aqueous NP colloid was injected into a gas-tight Teflon well, and alkylamine/hexane solution was floated on top of the aqueous colloid. After dropwise adding ethanol (75 vol% in DI water), Au NP monolayer films self-assembled at the organic/aqueous interface. The aqueous subphase was then drained slowly using a syringe. During this procedure, the Au NP film moved with the water/hexane interface, until it deposited on the bottom substrates. The draining process continued until the remaining organic solution was removed. Residual liquid retained in the film by capillary force was allowed to slowly dry over the course of 48 hours. Characterization Both neat alkylamines and the corresponding alkylamine-Au NP films were characterized at room temperature with Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, Frontier) with a diamond attenuated total reflection (ATR) accessory (Specac, Golden Gate), in the spectral range from 4000 to 650 cm-1. Wet alkylamine-Au NP films were transferred onto the diamond crystal with nitrogen flow immediately after being deposited on the glass slide Measurements were taken after the films were thoroughly dried. Transmission electron microscopy (TEM, JEOL JEM-2011) micrographs were collected using an accelerating voltage of 200 kV. Each sample was deposited on a carbon coated copper grid (200 mesh, Ted Pella). Interparticle spacing of each alkylamine-Au NP film was analyzed with ImageJ software.62 At least 500 Au NPs on five different locations were analyzed for each film. Grazing incidence small angle X-ray scattering (GISAXS) measurements were performed at beam line 8-ID-E, Advanced Photon Source at Argonne National Laboratory, with a monochromatic X-ray beam (at a photon energy of 7.35 keV). Au NP monolayers were


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deposited on silicon wafers. These were tilted at an angle of 0.14° with respect to the beam. Scattering was collected by an X-ray charge-coupled device (CCD) area detector at a distance of 1474 mm from the sample. The experimental geometry is schematically shown in Figure S1. Later, the data was reduced using the ‘NIKA’ 63 package for Igor pro. A horizontal line cut was taken at a fixed qz = 0.5 nm-1 in order to reduce the GISAXS data to a one-dimensional scattering profile. Other horizontal cuts at different qz values (from 0.3 nm-1 to 0.8 nm-1) were also tested, and the primary peak position remained unchanged for each of the Au NP films. A UV-vis-NIR spectrophotometer (Agilent, Cary 5000) was used to optically analyze the 2D Au NP films deposited on glass slides. The glass slide was mounted on a sample holder supplied with the instrument such that the incident beam was perpendicular to the sample. The measured wavelength was between 350 nm and 1200 nm.

Before each UV-vis-NIR experiment, a

background was collected from a blank glass slide. UV-vis-NIR spectra were calculated from 3-5 locations on each film. Interdigitated electrodes (IDEs) for electrical conductivity measurements are shown in Figure S2. They consist of 6 pairs of gold fingers with a 50 µm gap (L) and total length of 13.650 mm (W). They were fabricated using photolithography (see supporting information for preparation details). Au NP films were deposited on the IDEs using the method described previously. Resistance measurements were performed at 25 ℃ using a two-electrode configuration (Keithley 2401 SourceMeter). A custom Faraday cage composed of aluminum foil was connected to ground with an aluminum wire in order to prevent interference from stray electromagnetic fields. At least 3 films were measured for each ligand length. RESULTS AND DISCUSSION


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Shown in Figure 1, centimeter-scale Au NP films free of visible cracks could be fabricated using all five alkylamine ligands, by a “drain-to-deposit” strategy previously reported.61 Large area films can be generated on liquid interfaces, but upon transfer to solid substrates using Langmuir techniques the macroscopic quality of the film is somewhat compromised.64 Conversely, the drain-to-deposit approach results in Au NP monolayer films of exception quality. For example, the C15 and C18 films in Figure 1 have a mirror-like appearance across the entire surface of the glass slide.

Figure 1. Photograph of five alkylamine-Au NP films deposited on glass slides (18×18 mm2).A blank piece of glass slide is shown as a reference.

The molecular configuration of the alkylamine molecules on the Au NP surfaces was studied by FTIR-ATR spectroscopy. The regions containing C-H vibrations are shown in Figure 2(a). All


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spectra were normalized by the IR absorption maximum of methylene C-H asymmetric stretching band at around 2920 cm-1. The peak position of this vibration is listed in Table 1 for each neat alkylamine and for each amine-protected Au NP film. It has been reported that IR spectroscopy provides the most complete evidence for the structure of the polymethylene chains,65 with the position of the asymmetric CH2 stretching vibration (2918 cm-1) indicating that the chains are fully extended in an all-trans configuration.66 The shift of this band to higher frequency suggests that either significant numbers of gauche defects exist or that the chains are not tightly packed. Shown in Figure 2(a) and Table 1, the C-H stretching vibration of the neat alkylamines shifts to higher frequency with decreasing carbon number, from 2917 cm-1 for C18NH2 to 2923 cm-1 for C6-NH2. Furthermore, the abrupt change between C12-NH2 and C9-NH2 is corroborated by the fact that C12-NH2 is solid at room temperature whereas C9-NH2 is liquid. However, once the alkylamines attach to the Au NP surface, the asymmetric CH2 stretch shifts to lower frequency for C6-NH2 and to higher frequency for the rest of the four alkylamines. Possibly, C6-NH2 chains become more densely packed once adsorbed to Au NP surface. For the other four alkylamines, after attaching to Au NP surface, they are less densely packed and less well-ordered than their neat counterparts.


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Figure 2. FTIR-ATR spectra of neat amines and corresponding alkylamine-Au NP films. Spectra are offset for clarity. (a) The wavenumber axis is broken from 2700 cm-1 to 1600 cm-1, and from 1200 cm-1 to 800 cm-1. (b) The wavenumber axis is broken from 3000 cm-1 to 1800 cm-1.

Figure 2(a) also shows that, for the alkylamines with long aliphatic chains (Cn-NH2, n ≥ 12), splitting was observed for the methylene/methyl C-H bending vibrations (around 1470 cm-1) and the methylene rocking band (725 cm-1-720 cm-1). Splitting is an indication of crystallinity or a high degree of ordered packing of the long-chain backbone.67 The splitting disappeared after the alkylamine ligands attached to Au NP surface indicating a loss of chain order. This could be due to less dense alkane-chain packing and/or an increased presence of gauche conformations in the alkane chains.65 The IR absorbances characteristic of the amine functional groups are presented in Figure 2(b). These include the N-H stretching band (3370 cm-1-3250 cm-1), N-H bending band (1650 cm-11580 cm-1), aliphatic amine C-N stretching band (1050 cm-1-1150 cm-1) and N-H wagging band (909 cm-1-666 cm-1). All three bands related to N-H are broadened and the intensities decrease for alkylamine-Au NPs compared with those of their neat amine counterparts. This indicates that the amine is bonded to the metal surface,59 and verifies that the Au NPs have been modified with alkylamines. Table 1. Summary of the IR asymmetric CH2 stretching band maxima for neat alkylamines and alkylamine-Au NP films.

Sample

Asymmetric C-H Stretch (cm-1)

Sample

Asymmetric C-H Stretch (cm-1)

Neat C6-NH2

2923

C6-Au NP Film

2922

Neat C9-NH2

2922

C9-Au NP Film

2923

Neat C12-NH2

2919

C12-Au NP Film

2924

Neat C15-NH2

2917

C15-Au NP Film

2924

Neat C18-NH2

2917

C18-Au NP Film

2923


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The local NP order of each film was characterized by TEM (Figure 3). By carefully adjusting the ligand molar concentrations in hexane, a single layer Au NP film can be formed with each alkylamine ligand. The alkylamine molar concentrations were optimized at 1 mM for C6-NH2, 0.1 mM for C9-NH2, 0.01 mM for C12-NH2, 0.001 mM for C15-NH2 and 0.0002 mM for C18NH2. Concentrations below the optimized concentration resulted in voids in the Au NP films possibly due to an excess of residual electrostatic repulsion. As shown in Figure S3, concentrations greater than the optimized concentration resulted in multilayers for long ligands (n ≥ 12) or agglomeration for short ligands (n < 12). The effect of the ligand concentration in the hexane phase can be reasoned as follows. For the oil/water interfacial self-assembly of NPs with ethanol but no ligands, Reincke et al.68 derived an equation relating the NP population density, ρNP, at the oil/water interface to the NP surface charge density, ρe, in which ρNP increases with decreasing ρe. However, there is a critical upper limit for ρNP, above which further addition of ethanol has no effect. Thus, NP film formation is limited to a single layer with significant voids.47, 68 Park et al.52 proposed that passivating the NP surface with alkylthiols when the NPs were entrapped in oil/water interface could further decrease ρe. By reducing the electrostatic repulsive force, voids could be eliminated from the NP film. However, when the amount of alkylthiols was greater than that required for the 2D NP arrays, the attractive interaction between the NP and hexane would increase due to the larger NP hydrophobicity, resulting in the movement of the center of NP to the upper hexane phase. The second NP layer could fill in the vacancies due to the center shift of the first layer of NP, so that multiple-layer NP film could be formed.52, 56 In our study, similar concentration effects were observed when using primary alkylamines as passivating agent (Figure S3). However, for C6-NH2, the increase of ligand molar concentration


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above the optimal value at which a closely packed monolayer film forms, led to the agglomeration of NPs, instead of the formation of a multiple-layer film (Figure S3(d)). For alkylamine ligands with n greater than or equal to 9, sufficient steric repulsion can be provided by the surface ligand to prevent agglomeration at the interface. However, for alkylamine ligands with n less than 9, once ρ is decreased to a critical value, the residual electrostatic repulsion and ligand-induced steric repulsion become weaker than van der Waals attractions, thus triggering the agglomeration of NPs. High-magnification TEM images (Figure 3, upper right insets) and the corresponding fast Fourier transforms (FFTs) (Figure 3, lower right insets) clearly show that the ordering of Au NP films increases with increasing alkylamine chain length. The FFT patterns of C6-Au NP film and C9-Au NP film show diffuse rings, indicating the presence of multiple grains in the 2D NP crystal. From C12-Au NP film to C18-Au NP film, the presence of Bragg spots instead of diffuse rings are apparent, demonstrating enhanced long-range order.

Figure 3. TEM micrographs of single layer (a) C6-Au NP film, (b) C9-Au NP film, (c) C12-Au NP film, (d) C15-Au NP film and (e) C18-Au NP film. Lower right inset shows the Fourier transformation of the corresponding upper right inset. The scale bars for the main images are 0.2 µm, and those for the upper right insets are 20 nm.

GISAXS was employed to evaluate the bulk ordering of the Au NP film and quantify interparticle separation for each of the alkylamine-protected Au NP films (Figure 4). The 2D GISAXS patterns of C6-Au NP (Figure 4 (a)) and C9-Au NP films (Figure 4 (b)) display broad,


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symmetric diffraction spots at around 0.05 nm-1, which indicates a NP assembly without long-range order, but with localized order, consistent with TEM observations in Figures 3(a) and 3(b). Figures 4(c), 4(d), and 4(e) show 2D GISAXS patterns of C12-Au NP, C15-Au NP and C18-Au NP films, respectively. Bragg rods and higher-order peaks are apparent in all three figures, and the intensity increases with increasing alkylamine length. This indicates an improvement in the long-range Au NP monolayer order with increasing ligand length.

Figure 4. 2-D GISAXS patterns for single layer (a) C6-Au NP film, (b) C9-Au NP film, (c) C12-Au NP film, (d) C15-Au NP film and (e) C18-Au NP film. Color bar indicates the intensity of the scattered beam (of arbitrary unit).

An explanation of the chain length dependence of alkylamine-Au NP monolayer ordering follows. Au NPs were only partially passivated by alkylamine ligands, i.e., there are residual negative citrate ions on the surface of the Au NPs.60 There is more residual charge on the Au NPs with shorter ligands and therefore larger electrostatic repulsion leading to more voids. An additional consideration is that of the free-space filling model69 (See supporting information Figure S4). The idea of space filling can be logically extended to consider that the longer ligands


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fill more of the space created by imperfect packing due to finite NP size dispersity that would otherwise lead to some disorder in the superlattice. Electrostatic repulsion is longer range than the steric term. This coupled with space filling results in more voids being present in the Au NP monolayers with short alkylamine ligands. It seems that the interaction/intercalation of chains between different nanoparticles results in an inverse relationship between ligand order and NP order. Although the curvature is mild in these nanoparticles, the Au NP lattice is such that ligands from different particles cannot pack together efficiently. The slightly higher order of the shorter ligands may be possible because they do not completely fill the space between particles, which drives a decrease of the order of the Au NP lattice. In other words, Au NP rearrangement in monolayers with shorter ligands allows the ligands from different particles to pack more efficiently, while the regular arrangement of NPs in monolayers with longer ligands restricts the efficiency of ligand packing. More quantitative structural information is presented in Figure 5 where the 1D GISAXS profile of each film is presented as the ℓn(intensity) vs qy. The primary peak of each 1D scattering profile corresponds to the (10) plane of a 2D hexagonal lattice. The (10) plane spacing, d10, the Au NP center-to-center distance, dc-c, and the interparticle spacing , d, can be calculated using the method described previously.61 The order of the C12 through C18-AuNP films is demonstrated by the existence of higher order peaks. The broad higher order peak centered at about 0.8 nm-1 is due to scattering from the (11) lattice plane. The shoulder at approximately 0.9 nm-1 is due to scattering from the (20) lattice plane.


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Figure 5. (a) Representative horizontal line cuts along the qy axis. The primary peak indicates the distance between two neighboring (10) planes in each single layer Au NP film. (b) Magnified 1-D GISAXS plots of C6-Au NP film (black curve) and C9-Au NP film (red curve) corresponding to those in (a) after background correction. The arrow indicates the primary peak for each plot.

Figure 6. Interparticle gap of each Au NP film calculated from GISAXS and TEM as a function of the alkyl chain length. In addition, two theoretical predictions and trend lines are shown. The error bar in 1D GISAXS data is estimated by the standard deviation of the Gaussian fit performed on the 1D GISAXS primary peak, and the error bar in TEM data is from the standard deviation of the size distribution analysis.

As shown in Figure 6, the calculated interparticle separation for each alkylamine-Au NP film from GISAXS agrees well with the results from statistical analysis of TEM images. The interparticle distance shows a roughly linear dependence on the number of carbon atoms in different alkyl chains (Table S1). The length, L, of a single alkylamine chain with an all-trans conformation can be evaluated as a function of the number of carbon atoms per alkyl chain, n, as


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L(nm) = 0.127n + 0.25,

(2)

so that the theoretical interparticle spacing for two alkylamine ligands is70 d theory = 2 L = 0.254 n + 0.5 .

(3)

Obviously, the interparticle gap is smaller than the theoretical prediction (red dashed line in Figure 6) for each of the alkylamine-Au NP films. There are three possible contributions to this discrepancy. First and foremost, alkylamine chains on the neighboring particles might interdigitate, as suggested by other studies with alkylthiols5,

33

. In agreement with our study,

these studies found the particle spacing to be slightly larger than the length of a single ligand (black dotted line in Figure 6). Further evidence for interdigitation comes from recent Monte Carlo simulations using an overlapping cone model, in which the NP surface ligands were considered to fill a cone-shaped volume on average. 71 These cones were found to overlap when two or more NPs were in close proximity. Examining C6 through C12, in which the monolayer lacks significant long-range order, the interparticle gap appears to scale with the same slope as for a single ligand. For the monolayers with long-range order (C12 through C18), the interparticle gap trends with a slope similar to the theoretical predictions for two ligands (red dashed line). This is in qualitative agreement with studies of self-assembled monolayers of alkylthiols on planar gold 72 that found a weaker scaling for short chains (n < 7) than for long chains (n > 7). Despite the apparently different scaling, within the bounds of the error of our measurements no apparent phase transition was detected with increasing alkylamine chain length, in contrast to a recent study by Gwo et al.5 The apparent phase transition was attributed to the alkylthiol chain conformations on the Au NP surface, i.e. for long-chain thiolates (carbon number >16), the alkyl chains were mainly in an extended all-


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Langmuir

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Page 18 of 36

trans-ordered conformation, while for shorter chains, significant chain disorder existed. However, FTIR analysis of the C-H vibrations of the alkylamine ligands (discussed above) indicates that ligands of all lengths are disordered (Figure 2(a) and Table 1). This could be due to the different ligand type and different concentrations during ligand exchange, or different 2D superlattice fabrication methods between these two studies. UV-vis-NIR absorbance spectra were collected in order to investigate the dependence of the surface plasmon resonance (SPR) on interparticle spacing. UV-vis-NIR absorbance spectra of the monolayer alkylamine-Au NP films are shown in Figure 7(a). The SPR maximum of these films gradually red-shifts from 645 ± 3 nm for C18-Au NP film to 819 ± 5 nm for C6-Au NP film (detailed SPR position for each alkylamine-Au NP film is summarized in Table S1). Figure 7(b) shows a plot of the SPR maximum, λmax, as a function of the interparticle gap, d, between NPs. Note that the SPR maximum for these Au NPs (stabilized with sodium citrate) dispersed in water was measured as 519 nm in this study and the plasma resonance for bulk gold in vacuum (at 273 K) is 219 nm. The following exponential decay model was regressed to the data in Figure 7(b):5, 20

λmax = λ0 + λ A exp(− βd ) ,

(4)

4 where λ0 is 652 nm, λA is 1.7 ×10 nm, and β is 3.2 nm-1 (β-1 = 0.31 nm). λ0 is significantly red-

shifted with respect to the same particles in water. This is to be expected due to the difference in dielectric constant of the media. Using the analysis results of Equations 8 and 9 (discussed below), the effective dielectric constant of the aqueous electrolyte around the Au NPs is 3.07. A decrease of more than an order of magnitude from the dielectric constant of pure water at 25 °C (78.304).73 This means that the aqueous environment around the Au NPs is highly charged.74 λA represents the SPR maximum for zero interparticle gap. Using the same analysis the value of λA


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Langmuir

at d = 0 corresponds to an effective dielectric constant around the nanoparticles (εm = 11.54) that is of similar order of magnitude to that of methylamine.75

Figure 7. (a) UV-vis-NIR absorbance spectra of monolayer Au NP films functionalized with each ligand type used in this study. (b) SPR maximum versus the interparticle gap. The red curve is the best fit of an exponential decay model.

In essence, two types of electromagnetic interactions can be employed to manipulate the collective surface plasmon resonance of arrays of nanoscale objects, namely far-field dipoledipole interactions and near-field coupling.76,

77

The former is significant for arrays of large

diameter (D > 20 nm) NPs and large interparticle gap (d ~ D), in which case the SPR maximum exhibits a linear blue shift with decreasing lattice spacing.76 While the latter is especially related to nearly touching (d

Gold Nanoparticle Monolayers with Tunable ... - ACS Publications

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