Gold Nanoparticle Monolayers with Tunable Optical and Electrical ...
Supporting Information
Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties Guang Yang, Longqian Hu, Timothy D. Keiper, Peng Xiong, Daniel T. Hallinan Jr.* *
Corresponding Author
Florida A&M University-Florida State University College of Engineering Department of Chemical and Biomedical Engineering 2525 Pottsdamer Street Tallahassee, FL 32310, USA [email protected]
Table of Contents Experimental ................................................................................................................................... 2 Materials ..................................................................................................................................... 2 GISAXS Experimental Setup ..................................................................................................... 3 IDE Fabrication ........................................................................................................................... 3 Results ............................................................................................................................................. 4 Amine Concentration Effect ....................................................................................................... 4 Summary of Key Parameters of Amine-Au NP Films................................................................ 5 Space Filling Model .................................................................................................................... 5 Comparison of Experimental SPR and Theoretical SPR ............................................................ 8 References ....................................................................................................................................... 8
Experimental Materials Gold (III) chloride trihydrate (HAuCl4∙3H2O, ≥99.9% trace metals basis), sodium citrate dihydrate (HOC(COONa)(CH2COONa)2∙2H2O ≥99%), ethanol (ACS reagent, ≥99.5%), nhexane (anhydrous, 95%), hexylamine (CH3(CH2)5NH2, 99%), nonylamine (CH3(CH2)8NH2, 98%), dodecylamine (CH3(CH2)11NH2, ≥99%), pentadecylamine (CH3(CH2)14NH2, 96%), and octadecylamine (CH3(CH2)17NH2, 97%) were purchased from Sigma-Aldrich and used as received. Deionized (DI) water (18.2 MΩ cm) was supplied by a Millipore water purification system. Gas-tight containers (10 × 10 × 5 cm3, Snapware) were used for interfacial ligand exchange and monolayer self-assembly. For all experiments, glassware was thoroughly cleaned with Piranha solution (Caution: Piranha solution is highly corrosive and reacts violently with organic matter!) at 60 ℃. Custom Teflon wells (with inner dimensions 5 × 2 cm2 and depth of 1.5 cm) and Teflon coated magnetic stir bars (VWR) were cleaned using acetone followed by THF. All glassware, stir bars, and Teflon wells were rinsed with DI water and oven-dried overnight at 100 ℃ before use.
2
GISAXS Experimental Setup
Figure S1. Schematic illustration of the experimental GISAXS setup. The incident beam impinges on the sample surface at a grazing angle, αi, and reflects off the sample surface at a set of in-plane angles, 2Θf, and normal angles, αf. The scattering wave vector has two components, qy and qz on a 2-D detector placed in the y-z plane.
IDE Fabrication A thin layer of light sensitive photoresist (AZ5214-E, AZ electronic material USA Co.) was spin-coated (5000 RPM for 30 s) on top of a SiO2 wafer (250 nm SiO2 on 500 µm Si, with resistivity on the order of 1014 Ω·cm). The photoresist-coated wafers were baked at 95 ℃ for 30 min. Then they were exposed to ultraviolet light (425 nm, Karl Suss MJB 3 mask aligner with a 350 W mercury lamp) for 8 s with a pre-defined mask. After developing the photoresist, all SiO2 wafers were placed in a thermal evaporator (EDWARDS Auto 306) to coat a 2 nm adhesive chromium layer followed by a 40 nm-gold layer in sequence. This was followed by 2 minuteultrasonication using acetone for liftoff. Au NP films were deposited on the IDEs using the method described previously. The SEM images of resulting IDEs is shown in Figure S2.
3
Figure S2. SEM image of an IDE (top panel, scale bar is 0.5 mm) and schematic of the IDE geometry (bottom panel). The light regions in the SEM image are gold. In the schematic, the gap, L, between two IDEs is 50 µm. The total length, W, is 13.650 mm. The thickness of the film, D, is the same as the average diameter of the Au NPs, 12.7 nm.
Results Amine Concentration Effect
Figure S3. TEM image of (a-d) C6-Au NP films and (e-h) C15-Au NP films fabricated using hexane/water interfacial selfassembly with various amine molar concentrations.C6-AuNP films prepared with C6 NH2/hexane molar concentration at (a) 0.01 mM, (b) 0.1 mM, (c) 1 mM and (d) 4 mM. A TEM image with high-magnification shown in the inset of (d) manifests the agglomeration of Au NPs. C15-AuNP films prepared with C15-NH2/hexane molar concentration at (e) 0.001 mM, (f) 0.005 mM, (g) 0.01 mM and (h) 0.1 mM. The scale bar is 50 nm (shown in (a)) for (a-d), 20 nm for inset of (d) and 100 nm (shown in (h)) for (e-h).
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Summary of Key Parameters of Amine-Au NP Films Table S1. Summary of experimental results for monolayer hexagonal-close-packed (HCP) alkylamine-stabilized Au NP films.
Au NP Film C6
Plane Spacing (d10, nm) 12.2
Lattice Constant (dc-c, nm) 14.0
Interparticle Gap (d, nm) 1.4
SPR Maximum (nm) 819.0
Conductivity (σ, µS/cm) 3.05
C9
12.4
14.3
1.7
735.8
1.81
C12
12.7
14.6
2.0
672.2
0.46
C15
13.1
15.2
2.6
662.2
0.18
C18
13.6
15.7
3.1
645.2
0.009
Space Filling Model
Figure S4. Schematic representation of the excess volume that can be occupied by an alkylamine ligand on the Au NP surface 2
5
The increased ordering of Au NP films with the increase of the alkylamine chain length can also be qualitatively rationalized by using the free-space filling model. Heath et al. 2 have shown that the phase behavior of the Langmuir monolayers of organically passivated Au NPs were mainly determined by the particle core size, r, and the structure of the surface ligand of the particle. Assuming a spherical shape of the nanoparticles, the variation of the Au NP phase behavior can be discussed in terms of the excess volume, Ve, between ligand cylinders that extend from the Au NP surface (Figure S4). The excess volume can be calculated by
Ve =
f ( ) 2 [( r + d l ) 3 − r 3 ] − πd l f 3 r
π
2
(S1)
where f is the alkyl ligand footprint radius and dl is the length of the alkyl chain on Au NP surface. Here we assume an all-trans (zig-zag) conformation of alkylamine chain on the Au NP surface, and the alkylamine chain length can be calculated using the formula listed in the main article. The footprint area of the alkylamine ligand is estimated at 0.21 nm2, corresponding to the crosssectional area of a hydrocarbon chain. 3 The calculated excess volume of each alkylamine ligand is listed in Table S2. Table S2. Excess volume of each alkylamine
Alkylamine C6-NH2 C9-NH2 C12-NH2 C15-NH2 C18-NH2
Ligand Length, (nm) 1.0 1.4 1.8 2.2 2.5
Excess Volume (nm3) 0.04 0.07 0.12 0.17 0.25
6
Three distinct phases of the Au NP monolayers have been identified.2 For Ve > 0.35 nm3 a twodimensional foam-like phase could be reached for the Langmuir monolayers at high compressed pressure. In the region between 0.15 nm3 and 0.35 nm3, the Langmuir monolayer particles condense into close packed structures (e.g. HCP structure). When Ve < 0.15 nm3, nanoparticles aggregate to form densely packed structures. From Table S2, C15-NH2 and C18-NH2 have an excess volume that falls between 0.15 nm3 and 0.35 nm3. This can be used to explain the better ordering of the HCP structure in C15-Au NP film and C18-Au NP film. However, for C12-NH2, reasonable ordering in the Au NP-film was also obtained, although the excess volume is slightly smaller than 0.15 nm3.The possible reason is, the free-space filling model was built based on the fact that all Au NPs have been thoroughly passivated by the organic capping ligands due to the Au NP synthesis method used in Heath’s study.4 However, in our study, since they were only partially passivated by alkylamine ligand at the water/hexane interface, Au NPs still remained negatively charged due to the remaining citrate ions. This would inevitably affect the phase behavior of the Au NPs in water/hexane interface. Since the electrostatic repulsive force provided by the negative charge facilitate Au NPs stay apart from one another, we would expect a smaller critical value of Ve than 0.15 nm3 for the Au NPs to aggregate. The existence of the remaining negative charge on Au NP surface can also be used to rationalize that, even for much smaller values of Ve, the Au NPs are still able to well disperse in the monolayer films (C6-Au NP film and C9-Au NP film), if the concentration of the alkylamine are properly controlled in hexane phase. However, the Au NPs would be more passivated by the short alkylamine ligands if higher amine concentrations were applied. In this case, the decreasing negative surface charge would result in inevitable agglomeration of Au NPs
7
(i.e. it falls into the third category of the phase behavior corresponding to Ve < 0.15 nm3 in freespace filling model). Comparison of Experimental SPR and Theoretical SPR
Figure S5. Comparison of experimental SPR maximum and the calculated values versus dielectric constant of each alkylamine.
References 1. Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389 (6653), 827829. 2. Heath, J. R.; Knobler, C. M.; Leff, D. V. Pressure/temperature phase diagrams and superlattices of organically functionalized metal nanocrystal monolayers: the influence of particle size, size distribution, and surface passivant. The Journal of Physical Chemistry B 1997, 101 (2), 189-197. 3. Petty, M. C. Langmuir-Blodgett films: an introduction; Cambridge University Press1996. 4. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of thiolderivatised gold nanoparticles in a two-phase liquid–liquid system. J. Chem. Soc., Chem. Commun. 1994, (7), 801-802.
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Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties Guang Yang, Longqian Hu, Timothy D. Keiper, Peng Xiong, Daniel T. Hallinan Jr.* *
Corresponding Author
Florida A&M University-Florida State University College of Engineering Department of Chemical and Biomedical Engineering 2525 Pottsdamer Street Tallahassee, FL 32310, USA [email protected]
Table of Contents Experimental ................................................................................................................................... 2 Materials ..................................................................................................................................... 2 GISAXS Experimental Setup ..................................................................................................... 3 IDE Fabrication ........................................................................................................................... 3 Results ............................................................................................................................................. 4 Amine Concentration Effect ....................................................................................................... 4 Summary of Key Parameters of Amine-Au NP Films................................................................ 5 Space Filling Model .................................................................................................................... 5 Comparison of Experimental SPR and Theoretical SPR ............................................................ 8 References ....................................................................................................................................... 8
Experimental Materials Gold (III) chloride trihydrate (HAuCl4∙3H2O, ≥99.9% trace metals basis), sodium citrate dihydrate (HOC(COONa)(CH2COONa)2∙2H2O ≥99%), ethanol (ACS reagent, ≥99.5%), nhexane (anhydrous, 95%), hexylamine (CH3(CH2)5NH2, 99%), nonylamine (CH3(CH2)8NH2, 98%), dodecylamine (CH3(CH2)11NH2, ≥99%), pentadecylamine (CH3(CH2)14NH2, 96%), and octadecylamine (CH3(CH2)17NH2, 97%) were purchased from Sigma-Aldrich and used as received. Deionized (DI) water (18.2 MΩ cm) was supplied by a Millipore water purification system. Gas-tight containers (10 × 10 × 5 cm3, Snapware) were used for interfacial ligand exchange and monolayer self-assembly. For all experiments, glassware was thoroughly cleaned with Piranha solution (Caution: Piranha solution is highly corrosive and reacts violently with organic matter!) at 60 ℃. Custom Teflon wells (with inner dimensions 5 × 2 cm2 and depth of 1.5 cm) and Teflon coated magnetic stir bars (VWR) were cleaned using acetone followed by THF. All glassware, stir bars, and Teflon wells were rinsed with DI water and oven-dried overnight at 100 ℃ before use.
2
GISAXS Experimental Setup
Figure S1. Schematic illustration of the experimental GISAXS setup. The incident beam impinges on the sample surface at a grazing angle, αi, and reflects off the sample surface at a set of in-plane angles, 2Θf, and normal angles, αf. The scattering wave vector has two components, qy and qz on a 2-D detector placed in the y-z plane.
IDE Fabrication A thin layer of light sensitive photoresist (AZ5214-E, AZ electronic material USA Co.) was spin-coated (5000 RPM for 30 s) on top of a SiO2 wafer (250 nm SiO2 on 500 µm Si, with resistivity on the order of 1014 Ω·cm). The photoresist-coated wafers were baked at 95 ℃ for 30 min. Then they were exposed to ultraviolet light (425 nm, Karl Suss MJB 3 mask aligner with a 350 W mercury lamp) for 8 s with a pre-defined mask. After developing the photoresist, all SiO2 wafers were placed in a thermal evaporator (EDWARDS Auto 306) to coat a 2 nm adhesive chromium layer followed by a 40 nm-gold layer in sequence. This was followed by 2 minuteultrasonication using acetone for liftoff. Au NP films were deposited on the IDEs using the method described previously. The SEM images of resulting IDEs is shown in Figure S2.
3
Figure S2. SEM image of an IDE (top panel, scale bar is 0.5 mm) and schematic of the IDE geometry (bottom panel). The light regions in the SEM image are gold. In the schematic, the gap, L, between two IDEs is 50 µm. The total length, W, is 13.650 mm. The thickness of the film, D, is the same as the average diameter of the Au NPs, 12.7 nm.
Results Amine Concentration Effect
Figure S3. TEM image of (a-d) C6-Au NP films and (e-h) C15-Au NP films fabricated using hexane/water interfacial selfassembly with various amine molar concentrations.C6-AuNP films prepared with C6 NH2/hexane molar concentration at (a) 0.01 mM, (b) 0.1 mM, (c) 1 mM and (d) 4 mM. A TEM image with high-magnification shown in the inset of (d) manifests the agglomeration of Au NPs. C15-AuNP films prepared with C15-NH2/hexane molar concentration at (e) 0.001 mM, (f) 0.005 mM, (g) 0.01 mM and (h) 0.1 mM. The scale bar is 50 nm (shown in (a)) for (a-d), 20 nm for inset of (d) and 100 nm (shown in (h)) for (e-h).
4
Summary of Key Parameters of Amine-Au NP Films Table S1. Summary of experimental results for monolayer hexagonal-close-packed (HCP) alkylamine-stabilized Au NP films.
Au NP Film C6
Plane Spacing (d10, nm) 12.2
Lattice Constant (dc-c, nm) 14.0
Interparticle Gap (d, nm) 1.4
SPR Maximum (nm) 819.0
Conductivity (σ, µS/cm) 3.05
C9
12.4
14.3
1.7
735.8
1.81
C12
12.7
14.6
2.0
672.2
0.46
C15
13.1
15.2
2.6
662.2
0.18
C18
13.6
15.7
3.1
645.2
0.009
Space Filling Model
Figure S4. Schematic representation of the excess volume that can be occupied by an alkylamine ligand on the Au NP surface 2
5
The increased ordering of Au NP films with the increase of the alkylamine chain length can also be qualitatively rationalized by using the free-space filling model. Heath et al. 2 have shown that the phase behavior of the Langmuir monolayers of organically passivated Au NPs were mainly determined by the particle core size, r, and the structure of the surface ligand of the particle. Assuming a spherical shape of the nanoparticles, the variation of the Au NP phase behavior can be discussed in terms of the excess volume, Ve, between ligand cylinders that extend from the Au NP surface (Figure S4). The excess volume can be calculated by
Ve =
f ( ) 2 [( r + d l ) 3 − r 3 ] − πd l f 3 r
π
2
(S1)
where f is the alkyl ligand footprint radius and dl is the length of the alkyl chain on Au NP surface. Here we assume an all-trans (zig-zag) conformation of alkylamine chain on the Au NP surface, and the alkylamine chain length can be calculated using the formula listed in the main article. The footprint area of the alkylamine ligand is estimated at 0.21 nm2, corresponding to the crosssectional area of a hydrocarbon chain. 3 The calculated excess volume of each alkylamine ligand is listed in Table S2. Table S2. Excess volume of each alkylamine
Alkylamine C6-NH2 C9-NH2 C12-NH2 C15-NH2 C18-NH2
Ligand Length, (nm) 1.0 1.4 1.8 2.2 2.5
Excess Volume (nm3) 0.04 0.07 0.12 0.17 0.25
6
Three distinct phases of the Au NP monolayers have been identified.2 For Ve > 0.35 nm3 a twodimensional foam-like phase could be reached for the Langmuir monolayers at high compressed pressure. In the region between 0.15 nm3 and 0.35 nm3, the Langmuir monolayer particles condense into close packed structures (e.g. HCP structure). When Ve < 0.15 nm3, nanoparticles aggregate to form densely packed structures. From Table S2, C15-NH2 and C18-NH2 have an excess volume that falls between 0.15 nm3 and 0.35 nm3. This can be used to explain the better ordering of the HCP structure in C15-Au NP film and C18-Au NP film. However, for C12-NH2, reasonable ordering in the Au NP-film was also obtained, although the excess volume is slightly smaller than 0.15 nm3.The possible reason is, the free-space filling model was built based on the fact that all Au NPs have been thoroughly passivated by the organic capping ligands due to the Au NP synthesis method used in Heath’s study.4 However, in our study, since they were only partially passivated by alkylamine ligand at the water/hexane interface, Au NPs still remained negatively charged due to the remaining citrate ions. This would inevitably affect the phase behavior of the Au NPs in water/hexane interface. Since the electrostatic repulsive force provided by the negative charge facilitate Au NPs stay apart from one another, we would expect a smaller critical value of Ve than 0.15 nm3 for the Au NPs to aggregate. The existence of the remaining negative charge on Au NP surface can also be used to rationalize that, even for much smaller values of Ve, the Au NPs are still able to well disperse in the monolayer films (C6-Au NP film and C9-Au NP film), if the concentration of the alkylamine are properly controlled in hexane phase. However, the Au NPs would be more passivated by the short alkylamine ligands if higher amine concentrations were applied. In this case, the decreasing negative surface charge would result in inevitable agglomeration of Au NPs
7
(i.e. it falls into the third category of the phase behavior corresponding to Ve < 0.15 nm3 in freespace filling model). Comparison of Experimental SPR and Theoretical SPR
Figure S5. Comparison of experimental SPR maximum and the calculated values versus dielectric constant of each alkylamine.
References 1. Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389 (6653), 827829. 2. Heath, J. R.; Knobler, C. M.; Leff, D. V. Pressure/temperature phase diagrams and superlattices of organically functionalized metal nanocrystal monolayers: the influence of particle size, size distribution, and surface passivant. The Journal of Physical Chemistry B 1997, 101 (2), 189-197. 3. Petty, M. C. Langmuir-Blodgett films: an introduction; Cambridge University Press1996. 4. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of thiolderivatised gold nanoparticles in a two-phase liquid–liquid system. J. Chem. Soc., Chem. Commun. 1994, (7), 801-802.
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