In Situ Visualization of Self-Assembly of Charged Gold Nanoparticles

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1 Supporting information for In Situ Visualization of Self-Assembly of Charged Gold Nanoparticles Yuzi Liu*, Xiao-Min Lin, Yugang Sun, Tijana Rajh Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinios 60439, United States List of contents Movie S1 shows the in-situ self-assembly of CTA-Au NPs with beam intensity of 10 pa/cm 2 Movie S2 shows the 8 CTA-Au NPs analyzed by particle tracking and statistics. Movie S3 shows the in-situ self-assembly of CTA-Au NPs with different size under beam intensity changing from 3 pa/cm 2 to 20 pa/cm 2. Movie S4 shows the contrast different between the area with and without liquid existing. Figure S1 Schematic of liquid cell. Figure S2 Schematic of the electron beam exposing liquid cell in TEM. Figure S3 UV-Vis absorption spectrum of the CTA + capped Au nanoparticles before (black curve) and after addition of different volumes of 1 M NaCl solution. Figure S4 TEM images of the Au nanoparticles after different volumes of 1 M NaCl solution Figure S5 TEM micrograph of Au NPs on dried Si3N4 membranes. Figure S6 The zeta-potential distribution (a) and the absorption spectrum (b) of CTA+ covered Au NPs under different dose irradiation by van de Graff non-focused beam accelerator (3 MeV). (c) is the morphology of CTA-Au NPs after 1 min irradiation. Figure S7 Si 3 N 4 memberane s temperature change under electron beam exposure with intensity of 70pA/cm2. Figure S8 The comparison of histograms of particle displacement with time step of 0.2s and 0.4s. Figure S9 Bright field image, electron diffraction pattern and the lattice fringes of Au NPs without liquid. S1

2 Materials and Methods: 1. Au nanoparticles (NPs) preparation: The CTA + covered Au NPs were prepared by seed mediated approaches following a gold nanorod synthesis, expect with no AgNO3 added to induce anisotropic growth. 1 The quasi-spherical shaped citrate-stabilized gold NPs with an average size of ~30 nm were synthesized following a kinetically controlled seeded growth strategy via the reduction of HAuCl4 by sodium citrate Liquid cell fabrication and TEM experiment: A schematic of the liquid cell design is shown in Fig. S1. Cells were fabricated by using one of liquid cell chip and one spacer (400nm) chip with 25nm Si3N4 membranes. (Hummingbird Scientific, WA). A droplet of suspension with Au NPs was transferred to the spacer and a blank chip was used to cover it as Fig S1 (a). Then the two chips will be clamped by using the quick sealing approach 3 and Fig S1 (b) shows the cross-sectional view of the liquid cell. Liquid cells were loaded to a TEM holder for real-time monitoring of the Au NPs self-assembly in JEOL 2100F equipped with high speed charge coupled device (CCD). The acceleration voltage was set at 200kV and the electron beam current intensity was varied from 1-30pA/cm 2. The thickness of the liquid layer is around 400nm. For movie recording, we moved the sample to a fresh area that has never been exposed to electron beam, and started the CCD capture. The CCD s exposure time was set as 0.4s which set the overall time resolution of our experiment. Figure S1 Schematic of liquid cell. (a) Stereo view of liquid cell. (b) Cross-sectional view of liquid cell. Electrostatic charging effect on liquid cell: The thin specimen in the TEM was at steady state condition. This means an electron current balance state in the specimen. Such state can be represented by equation 1 as following. 4 I + V s R s = I t + I η + I δ(v s ) (1) S2

3 Terms on the left-hand side of Eq. (1) represent negative current entering irradiated volume (electron beam exposed area). It includes the incident beam current, I; and electrons immigration from other un-exposed area, V s R s. V s is the surface potential developed by the electron beam, R s is an effective electrical resistance between the irradiated and surrounding regions of specimen. The right-hand side of Eq. (1) indicates the electrons escaped from electron beam exposed area. It includes the transmitted current, I t ; the backscattered electrons, I η, η is the backscattering coefficient; and secondary emission current, I δ(v s ), δ(v s ) is an effective secondary yield when the surface potential is +V s such that electrons of energy below ev s cannot escape from electron beam exposed area. I t approaches to I since the TEM specimen is thin and the incident electron energy is very high as 200KeV. Along with the increasing I t, the positive surface potential V s will be developed. 5 Cazaux calculated the surface potential by equation (2) under the postulate of the uniform distribution of positive charges inside a cylinder shown in Fig. S2(electron beam exposed area). 6 V(r) = Q 4πεd (1 r2 a 2 + 2ln r 0 a ) (0<r<a<r 0) (2) dv(r) da = Q 4πεd ( 2 a + 2r2 a 3 ) (3) where r = 0, we have the maximum surface potential like, V m = Q 4πεd (1 + 2ln r 0 a ) (4) Q = I 0(δ x +y x )ε (5) γ Where V m is the maximum value potential (at r =0). Q is total charge trapped in specimen. ε is dielectric constant. d is TEM specimen thickness. r is the radical distance from illumination center. Potential function to be zero at r 0. a is electron beam illumination radius. I 0 is incident current. δ x + y x is total effective yield for the secondary and Auger electron emission. γ is TEM specimen conductivity. For clarity, V(r), d, r 0, a are illustrated in Fig. S2. Figure S2 Schematic of the electron beam exposing liquid cell in TEM. The parameters of d, r 0, a are illustrated. S3

4 The Q, r 0 and TEM specimen thickness could be constant. From equation (3) we can see that the positive potential developed by electron beam is increasing with exposed area decreasing (0<r<a<r 0 ). In other words, the higher beam intensity will induce stronger positive potential. Such potential can be as big as 76V. In our experiment, the V m cannot be calculated since the unavailable value of δ x + y x. S4

5 Influence of Cl - anions on self-assembly of the CTA + capped Au nanoparticles: Influence of negative charges on the self-assembly of the CTA + capped Au nanoparticles was also evaluated by titrating the dispersion of Au nanoparticles with an aqueous solution of 1 M NaCl. When the NaCl solution was added to the dispersion of the Au nanoparticles, the negatively charged Cl - ions strongly interacted with the positively charged Au nanoparticles due to the electrostatic force between the free Cl - ions and the CTA + capping layers on the Au nanoparticles. This interaction reduced the repulsive force between individual Au nanoparticles, leading to attachment of nanoparticles. Such attachment resulted in the formation of aggregate assemblies corresponding to the appearance of a new absorption peak at longer wavelength as highlighted by the arrows in Figure S3. Meanwhile the absorption peak at 530 nm corresponding to the well dispersed individual Ag nanoparticles became weaker. The TEM images presented in Figure S4 clearly show the formation of aggregates of Au nanoparticles. Larger aggregates were formed as more NaCl solution was added, indicating that the degree of particle aggregation could be controlled by the concentration of NaCl. Figure S3 UV-Vis absorption spectrum of the CTA + capped Au nanoparticles before (black curve) and after addition of different volumes of 1 M NaCl solution: 120 µl (red curve) and 320 µl (blue curve). Figure S4 TEM images of the Au nanoparticles after different volumes of 1 M NaCl solution was added to the monodispersed Au nanoparticles: (a) 120 µl and (b) 320 µl. S5

6 Ex-situ characterization of the assembled Au naonparticles: The liquid cell used to record movie S3 was disassembled after the in-situ experiment. The Si3N4 membranes were examined with the regular TEM holder. Figures S5a and S5b show the formation of nanoparticle chains on both the top and bottom Si3N4 membranes. It is clear that the all the aggregates on the top Si3N4 membrane are made of smaller nanoparticles, whereas aggregates made of larger particles reside on the bottom Si3N4 membrane. We also observed some single smaller nanoparticles on bottom Si3N4 membrane (Fig S5b). These particles might migrated from the liquid solution to the bottom membrane when the liquid cell was disassembled. Figure S5 TEM micrograph of Au NPs on top (a) and bottom (b) Si3N4 membranes after in-situ assembly experiment. The average sizes of the nanoparticles NPs on the top and bottom membranes were determined as 20 nm and 45 nm, respectively. S6

7 Ex-situ characterization of the Au naonparticles under irradiation of van de Graff non-focused beam accelerator (e-beam energy 3 MeV) Irradiation of CTA-Au nanoparticle solution with 3 MeV electron beam from Van de Graaff accelerator (10 s e-beam exposure) results in the decrease in the positive charge of nanoparticles from 30±12 mv to 9±5 mv (data shown in Fig. S6 (a)) after irradiation while the charge of negatively charged (citrate coated) nanoparticles remains approximately the same (-20 mv to -30 mv). When the dose of irradiation was increased (1 min e-beam exposure) the Au-CTA zeta potential was further decreased to 7±4 mv and the absorption spectrum of the solution (Fig S 6 (b)) and bright field TEM image (Fig. S6 (c)) indicate clearly formation of the aggregates confirming that the cause of aggregation is radical species produced by e-beam. S7

8 Figure S6 The zeta-potential distribution (a) and the absorption spectrum (b) of CTA+ covered Au NPs under different dose irradiation by van de Graff non-focused beam accelerator (3 MeV). (c) is the morphology of CTA-Au NPs after 1 min irradiation. S8

9 Electron beam thermal effect on Si3N4 membranes: The temperature change on Si 3 N 4 under electron beam with intensity of 70pA/cm 2 was monitored by hummingbird heating holder as shown in Fig. S7. The temperature increases relatively quickly at the beginning of electron beam exposure. With the time going, the temperature became stable after 6000s. The maximum temperature increasing is 6.7 o C. Figure S7 Si3N4 memberane s temperature change under electron beam exposure with intensity of 70pA/cm 2. S9

10 Time step effects on histogram of distribution of NPs displacement: The frame rate was set at 5fps. So, the raw time step in the movies is 0.2 s. Although the frame rate can be future increased, enough CCD exposure time is very important to record high-quality images with better contrast. We post process the data on both original movie and the movie with frame decimation by 2, which means the time step is doubled as 0.4 s between frames. We compared the histograms of particle #7 displacement between the movies with time step of 0.2 s and 0.4 s as shown in Fig S8. There is no significant difference other than the counts at 0 displacement that is 126 with time step of 0.2 s. We checked the raw movie frame by frame and found the particles didn t move between successions of frames in many occasions. Figure S8 The comparison of histograms of particle #7 displacement with time step of 0.2s (left) and 0.4s (right). S10

11 Control experiment on liquid cell: It is extremely important to maintain the cell to be wet in viewing area during the in-situ observations. The nominal thickness of the liquid cell in our experiment is 400 nm. The real thickness could be larger especially in the center region of the Si 3 N 4 membrane windows because of the membrane bowing. In order to get better contrast, we performed the in-situ observations near the edge of the liquid cell. In this area, the liquid thickness is close to the nominal thickness of 400nm. By carefully analyzing Fig. 3, we found the contrast of particles in the upper left region is superior to those in the bottom right region. The upper left corner in Fig. 3 is very close to the edge of the liquid window with thinner liquid layer. The bottom right corner is a little further from the edge and is filled with a thicker liquid layer due to the bowing of the Si 3 N 4 membranes. The control experiments for imaging the dry Au nanoparticles were also performed and the images are shown in Fig. S9. The diffraction contrast in low magnification bright field image (Fig. S9 left) and the lattice fringes in HREM (Fig. S9 middle) can be clearly observed for the dry Au nanoparticles. The poly-crystal Au diffraction rings in electron diffraction pattern can also be recorded (Fig. S9 right). In contrast, the lattice fringes and diffraction rings cannot be observed for the wet Au nanoparticles. A movie clip (Movie S4) was recorded during our control experiment to show the contrast difference of the Au nanoparticles in the TEM before and after the liquid flooding. Figure S9 Bright field image of Au NPs without liquid (left) and corresponding electron diffraction pattern (right), the lattice fringes of Au NPs in liquid cell without liquid (middle). S11

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