Electronic Supplementary Data Realization of Thermally durable close-packed 2D gold nanoparticle arrays using Self-assembly and Plasma etching Thermally durable 2D nanoparticle arrays Sankar K. Sivaraman and Venugopal Santhanam* Department of Chemical Engineering, Indian Institute of Science, Bangalore, India 560 012. Email: venu@chemeng.iisc.ernet.in *Corresponding author 1
1. EXPERIMENTAL METHODS Materials Chloroauric acid, trisodium citrate, dodecanethiol, and tetra(4-pyridyl)porphyrin were purchased from Sigma-Aldrich,India. Tannic acid was purchased from Acros organics, India. Silver nitrate was purchased from Merck, India. Thiol-terminated polystyrene of different average molecular weights (3000, 12000 and 20000) were purchased from Polymer Source Inc., Canada. p-aminothiophenol (p-atp) was purchased from Alfa Aesar, India. Chloroform, dichloromethane, cyclohexane, ethanol, acetone and hexane were all of HPLC-grade and purchased from S. D. fine-chem, India. Photopaper (HP photo pack 57 series) was purchased from a retail outlet. Deionized water, obtained from a Millipore, Milli-Q system, was used in all the experiments. Sample Characterization A FESEM (Zeiss, Ultra55), typically operating at 10 kv, was used to characterize the ordering of the nanoparticle arrays on silicon substrates. A Tecnai F30 TEM (Transmission Electron Microscope) operating at 200 kv was used to obtain images of array transferred onto a silicon nitride grid, designed for TEM observation (DuraSiN, 100 nm thick membrane). The images obtained were analyzed to determine the histograms of particle size distributions using ImageJ (Rasband, W.S., http://imagej.nih.gov/ij/, 1997-2011). At least 250 nanoparticles, from images taken at different regions of the sample, were counted to determine the average diameter [μ Σd i /n] and standard deviation [σ (Σ(d i μ) 2 /(n-1)] of nanoparticle size distribution. The reported values represent equivalent circles having the same projected-area as the identified objects. The edge-to-edge interparticle spacing distributions within the array was determined using a customized code written in IgorPRO. Briefly, the code determines the centroids and diameters of particles within the array after appropriate thresholding. It then uses the Delaunay function in MATLAB to determine the nearest neighbors of particles within the array. This information is then used by the code to determine the corresponding edge-to-edge interparticle spacing distribution. Further description and the code are available either from the author upon request or on the web (http://chemeng.iisc.ernet.in/venu/temanalysis.pdf). A MFP-3D AFM (Asylum Research) was used to obtain the topography and cross sectional height of nanoparticle array. Olympus AC160TS cantilevers (nominal spring constant 40 N/m) were used for AFM imaging and AFM lithography. The average and standard deviation values for cross-section height reported were determined from differences between modes present in the histogram of heights for a region of interest, obtained after flattening of the raw data. The adhesion forces reported are based on the measured values of the probe s spring constant, which were determined using the thermal noise method [S1], available as a module in MFP-3D software (v080501+1016). The nanoparticle array printed on a quartz substrate was used for obtaining FTIR spectra in transmission mode using a Perkin-Elmer RX1 spectrometer. Thermo Gravimetric Analysis (TGA) was performed using a Perkin-Elmer Pyris 6000 instrument. Nanoparticle arrays transferred onto silicon wafers were used to obtain XPS spectra using a Thermo-Fisher Scientific Multilab 2000 instrument. Raman spectra were obtained using a Horiba LabRam spectrometer equipped with a 514 nm laser source. A 100 x lens (NA 0.9) was used for collecting the scattered signals. The laser power used was 4 mw, and the signal was integrated over 10 s. Integration over longer time or higher power degraded the adsorbed molecule. Typically, the preparations for imaging were carried out at one location on the sample, and then, just prior to the acquisition of the signal, shifted to a nearby location, to avoid artifacts due to probe molecule degradation/desorption. A total of 10 measurements were made on each sample, at different locations, to compute the mean and standard deviation values of the reported SERS intensities. Average SERS Enhancement Factor computation The average SERS Enhancement factor was estimated using the following formula [S2,S3] EF = (I SERS / I bulk ) (N bulk / N SERS ) N SERS is the number of adsorbed p-atp molecules on the substrate within the laser spot, and it is calculated using the formula N SERS = N d A laser A N /σ. The density of 2
nanoparticles per unit area (N d = 231 particles/μm 2 ) was estimated based on the measured average diameter and interparticle spacing of 30 nm and, respectively. The diameter of the laser spot was 1 μm. The surface area of the particles (A N ) was calculated using the average diameter of nanoparticles (30 nm); the entire surface of the nanoparticles, including the occluded areas, was considered for the purpose of computation. The surface density of p-atp (σ) was taken to be 0.20 nm 2 /molecule [S2]. Thus, the value of N SERS was calculated to be 1.16 x 10 6. N Bulk is the number of p-atp molecules within the illumination volume of laser in a bulk sample (i.e. p-atp spread in powdered form on a glass slide) and is calculated using the formula N Bulk = A laser hρ/m. The penetration depth of laser (h) was assumed as 16.5 μm, while the density (ρ) and molecular weight of p-atp are 1.16 g/ml and 125 g/gmoles, respectively [S2]. Thus, the value of N Bulk was calculated to be 7.24 x 10 10. I SERS is the measured Raman intensity at 1077 cm -1 in the SERS spectrum of p-atp. I Bulk is the Raman intensity at 1077 cm -1 of a bulk sample of p-atp. This wavenumber was chosen [S2,S4] to avoid the role of chemical enhancement effects and that of the extent of p-atp dimerization into DMAB on the computed average SERS EF values. The average value of the measured Raman intensity for the bulk sample was 250 (I bulk, a.u.). The average value of the Raman intensity in SERS spectra was 790 (I SERS, a.u.); this value corresponds to saturated coverage of p-atp molecules on the SERS substrate, i.e. the measured signal does not increase further upon dipping in more concentrated solutions. Thus, the average enhancement factor was calculated to be 1.972 x 10 5. [S3] Le Ru E C, Blackie E, Meyer M, and Etchegoin P G (2007) Surface enhanced raman scattering enhancement factors: A comprehensive study. J. Phys. Chem. C 111, 13794-13803. [S4] Huang Y -F, Zhu H -P, Liu G -K, Wu D -Y, Ren, B, and Tian Z-Q (2010) When the signal is not from the original molecule to be detected: Chemical transformation of para-aminothiophenol on Ag during the SERS measurement. J. Am. Chem. Soc. 132, 9244-9246. References [S1] te Riet J, Katan A J, Rankl C, Stahl S W, van Buul A M, Phang I Y, Gomez-Casado A, Schön P, Gerritsen J W, Cambi A, Rowan A E, Vancso G J, Jonkheijm P, Huskens J, Oosterkamp T H, Gaub H, Hinterdorfer P, Figdor C G and Speller S (2011) Interlaboratory round robin on cantilever calibration for AFM force spectroscopy. Ultramicroscopy 111, 1659-1669. [S2] Wang Y, Zou X, Ren W, Wang W, and Wang E (2007) Effect of silver nanoplates on Raman spectra of p-aminothiophenol assembled on smooth macroscopic gold and silver surface. J. Phys. Chem. C 111, 3259-3265. 3
Diameter (nm) 20 Untreated array Plasma treated array 10 0 0 100 200 300 400 500 Time (minutes) Figure S1 Plot showing that the measured particle size distributions for both plasma-treated and untreated arrays are independent of the duration of thermal annealing. The data presented in this exemplary plot are from samples of dodecanethiol coated gold nanoparticle arrays (initial average gold core size of 7 nm and interparticle spacing of 2 nm) that were annealed at 200 C for various durations. The symbols represent the mean value of nanoparticle size distribution, while the error bars represent one standard deviation. 4
RF power (W) RF power (W) a 30 30 b 25 25 20 20 15 1.0E-01 1.1E+00 2.1E+00 Pressure (mbar) 15 1.0E-01 1.1E+00 2.1E+00 Pressure (mbar) Figure S2 Effect of the reactor pressure on the minimum power required to ignite. (a) hydrogen, and (b) oxygen plasma. The symbols represent the mean value, while the error bars correspond to one standard deviation. These values are based on three runs at each condition. 5
20 W 30 W 0.15 mbar 0.5 mbar a b c 2 mbar d Figure S3 Representative FESEM images of gold nanoparticle arrays (nominally with 7 nm diameters and 2 nm interspacing) after hydrogen plasma treatment for 40 s at different pressure and RF power values. For the same process duration, use of lower RF power does not disturb the nanoparticle ordering. 6
Figure S4 Representative TEM image of a percolating nanoparticle array on a carbon film after excessive RF plasma treatment. (Gas - hydrogen and oxygen (3:1) Power 50 W. Duration 60 s.) 7
a b 1000 nm 100 nm Figure S5 Representative FESEM image of thiol-terminated polystyrene capped nanoparticle array after hydrogen plasma treatment at an RF power of 50 W for 60 s. (a) Low magnification image. (b) High magnification image. The formation of spherical aggregates of gold nanoparticles is attributed to the hydrogenation and cross-linking of polystyrene chains. 8
Height (nm) 10 5 0 2 4 6 8 10 Distance (µm) Figure S6 Averaged AFM cross sectional profile of oxygen plasma-treated nanoparticle array (formed using 7 nm gold nanoparticles coated with 20 kda thiol-terminated polystyrene ligands) after thermal annealing at 500 C. The measured difference in height of 7.1 ± 1.2 nm corresponds to that of the bare nanoparticle diameter. This suggests that all of the ligand molecules have been removed. 9
Figure S7 Representative FESEM images of dodecanethiol capped 7-nm size gold nanoparticle arrays (both plasma-treated and untreated) after heat treatment at different temperatures. 10
500 C 350 C 200 C 175 C 150 C a Plasma treated array b Untreated array c d e f g h i j Figure S8 Representative FESEM images of dodecanethiol capped 4-nm size gold nanoparticle arrays (both plasma-treated and untreated) after heat treatment at different temperatures. 11
500 C 350 C 200 C 175 C 150 C a Plasma treated array b Untreated array c d e f g h i j Figure S9 Representative FESEM images of dodecanethiol capped 10-nm size gold nanoparticle arrays (both plasma-treated and untreated) after heat treatment at different temperatures. 12
4 nm 7 nm 10 nm 150 C 175 C 200 C 350 C 500 C 0 10 20 30 Diameter (nm) 0 10 20 30 Diameter (nm) 0 10 20 30 Diameter (nm) 0 10 20 30 Diameter (nm) 0 10 20 30 Diameter (nm) 0 10 20 30 Diameter (nm) Untreated array Plasma treated array Untreated array Plasma treated array Untreated array Plasma treated array Figure S10 Histograms of nanoparticle particle size distribution (based on projected area) of dodecanethiol capped 4-, 7- and 10-nm size gold nanoparticles (plasma-treated and untreated) after annealing at different temperatures, corresponding to images shown in Fig. S-7 to S-9. Plasma treated arrays do not undergo drastic changes in particle size distribution up to 200 C. The differences in the evolution of particle size distributions for plasma-treated arrays at temperatures > 200 C is attributed to variations in initial number density of these arrays. 13
Figure S11 Representative FESEM images of dodecanethiol capped 7-nm size gold nanoparticle arrays on a 100 nm thick silicon nitride membrane (both plasma-treated and untreated) after heat treatment at 200 C. The insets are histograms of particle size distribution (counts vs. size in nm). 14
Figure S12 Effect of thermal annealing on plasma-treated and untreated 2D arrays of thiol-terminated polystyrene (20 kda) coated 7-nm size gold nanoparticles deposited on a sapphire substrate. (a) Plot shows variation of mean diameter of nanoparticles as a function of annealing temperature for plasma treated and untreated arrays. Representative TEM images of the plasma treated (b) and untreated arrays (c) on sapphire after thermal annealing at 800 C. The insets are histograms of particle size distribution (counts vs. size in nm). 15
Loss of weight (%) 100 75 50 25 0 100 200 300 400 500 Temperature ( C) Figure S13. TGA (Thermo Gravimetric Analysis) of thiol-terminated polystyrene capped 7 nm sized gold nanoparticles (MW 20 kda). This TGA curve is similar in character to that of bulk polystyrene samples. 16
nm a 25 C 275 C 350 C 15-15 b c 100 nm 100 nm Figure S14. (a) Topographic AFM images of thiol-terminated polystyrene capped nanoparticle array after heating to different temperatures. (b) Representative FESEM image of thiol-terminated polystyrene capped nanoparticle array (MW 20 kda) after heat treatment at 150 C. (c) Representative FESEM image of thiol-terminated polystyrene capped nanoparticle array after heat treatment at 275 C. At 275 C, the AFM and FESEM images clearly show that a phase separation of polymer rich domains from nanoparticles has occurred. 17
Figure S15. Effect of thermal annealing on plasma-treated and untreated 2D arrays of thiol-terminated polystyrene (3 kda) coated 7-nm size gold nanoparticles. Representative FESEM images at 500 C of (a) plasma-treated array and (b) untreated array on silicon substrate. The insets are histograms of particle size distribution (counts vs. size in nm). (c) Plot showing variation of mean nanoparticle diameter (corresponding to projected area in FESEM images) as a function of thermal annealing temperature for plasma-treated and untreated arrays on silicon substrate. 18
Absorbance (a.u) a b 0.1 c 0.05 200 nm 200 nm 0 530 nm 400 500 600 700 Wavelength (nm) d e 1 μm Figure S16. (a) Representative FESEM image of gold-silver core-shell nanoparticle array before plasma treatment. (b) Representative FESEM image of gold-silver core-shell nanoparticle array after argon plasma treatment. (c) UV visible spectra of an array of nanoparticles transferred onto a quartz substrate. (d) Photograph of a swabbing experiment and that of a nanoparticle array on paper (inset). (e) Representative FESEM image of nanoparticle array on paper. Sub-micron sized holes present on the surface of a photopaper, designed for wicking liquids away to an underlying absorbent layer, are also visible. Interestingly, the interparticle spacing in these arrays was found to be, which is much larger than that observed for the gold nanoparticle arrays. The reasons for this change are not clear; one possibility is the difference in the way thiol-terminated polystyrene adsorbs on gold and silver. 19