Supplementary Materials for

www.sciencemag.org/cgi/content/full/336/6084/1011/DC1

Supplementary Materials for Real-Time Imaging of Pt3Fe Nanorod Growth in Solution Hong-Gang Liao, Likun Cui, Stephen Whitelam, Haimei Zheng*

*To whom correspondence should be addressed. E-mail: [email protected] Published 25 May 2012, Science 336, 1011 (2012) DOI: 10.1126/science.1219185 This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S10 References (30–39) Captions for movies S1 to S3 Other Supplementary Material for this manuscript includes the following: (available at www.sciencemag.org/cgi/content/full/336/6084/1011/DC1) Movies S1 to S3

Materials and Methods All chemicals including Pt(acetylacetonate)2 (99%, Aldrich), Fe(acetylacetonate)2 (99%, Aldrich), pentadecane (99%, Aldrich), oleylamine (70%, Aldrich) and oleic acid (99%, Aldrich) are used as received. Liquid cell fabrication and growth solution loading for TEM Liquid cells are fabricated by following the samilar process as described in a previous publication (17). We use ultra thin silicon wafers (100 μm, 4-inches, p-doped) purchased from Virginia Semiconductor (Fredericksburg, VA) and deposite low stress silicon nitride membranes with a thickness of only 13 nm on the silicon wafers. Here, the use of ultra thin silicon nitride membranes has effectively improved the spatial resolution of the liquid cell to sub-nanometer range. The subsequent fabrication processes include lithographic patterning, wet KOH etching of silicon, liquid cell bonding using an indium thin film spacer. The indium thin film is deposited by sputtering and it acts as a spacer as well as the sealing material for the liquid cell. 120 nm spacing is used for the current experiments, although different thickness can be achieved. All the fabrication processes are conducted at the Nanofabrication Lab of the University of California at Berkeley. The liquid loading is facilitated by a syringe and Teflon nanotube (purchased from Cole-Parmer, VH, IL) to control the size of liquid droplet. A droplet of 30 picoliters is directed into the liquid reservoir without contaminating the electron transmission window. After liquid is loaded into the liquid cell, we cover the cell using a single slot copper TEM grid (TEM grids are purchased from Ted Pella, Inc.) and seal the whole liquid cell using epoxy. Properly sealing the liquid cell can assist maintaining the liquid inside the liquid cell for an extended period of time, which is critical for enabling the nanoparticle interaction and Pt3Fe nanorod formation. Initiation of the growth of Pt3Fe nanocrystals The growth of Pt3Fe nanocrystals in a liquid cell is initiated by the electron beam illumination of the growth solution. The reduction of Pt(acetylacetonate)2 and Fe(acetylacetonate)2 precursor (Pt2+,Fe2+) to metal (Pt0,Fe0) can be from either (I) oleylamine-assisted metal ion reduction at elevated temperature, or (II) direct reduction by electron beam (17). Since the liquid temperature is low (see “electron beam effects” as below), we believe the reduction of Pt2+ and Fe2+ by electron beam, i.e., from primary electrons or the solvated electrons from elastic scattering (30), is predominant. It is noted that growth by monomer attachment becomes negligible during the later stage of growth by nanoparticle attachment. TEM imaging and image processing All movies are recorded using a Gatan SC200 camera (fiber optical charge-coupled device (CCD)), which allows a time resolution of 33 milliseconds. Images in the movies S1-S3 are recorded under 3010 JEOL TEM, which show spatial resolution of ~1 nm. All movies play 30 times faster than real time (one frame per second) and are compressed to larger pixel sizes to reduce the file sizes. Image contrast of all movies is used as-recorded. Snap shot images of in situ growth that are recorded under FEI monochromated F20 UT  

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Tecnai TEM show spatial resolution of ~0.2 nm. Those recorded under an aberration corrected TEM (TEAM0.5) reveal spatial resolution of better than 0.1 nm. Nanocrystals in the sequential images in Figs. 1-3 are highlighted in green by a false coloring process using Photoshop software. All original images can be retrieved, see Movies S1-S3. Supplementary Text Electron beam effects We investigate the electron beam effects on the growth of Pt3Fe nanorods by nanoparticle attachment. As discussed in the above, the earlier stage of growth is strongly mediated by electron beam reduction of Pt- and Fe-precursors in the growth solution. However, during the later stage of growth from nanoparticle building blocks, the contribution from the molecular precursor is minimum (it is likely monomers are depleted). Therefore, we estimate the effects of electron beam heating, momentum transfer and charges on the growth by nanoparticle attachment considering the individual nanoparticles maintain their sizes during their interaction with the electron beam. We have done extensive studies on heating induced by electron beam (17, 31, 32). The results show the minimum temperature rise (less than a few degrees) under a beam intensity of 1-8×105A/m2. The location temperature variations within the field of view are also negligible because of the uniform parallel illumination. We estimate the momentum transfer from electron beam and compare it with that of hydrodynamic liquid effect. Assume all the electron beam energy is transformed to the nanoparticle, the electron beam energy is estimated by (31, 33, 34)

Emax = where

2E(E + 2mec 2 ) Mc 2

(1)

E = mec 2 (1/ 1− β 2 −1), me and M are the (rest) mass of the electron and the

nanoparticle, respectively, c is the speed of light, β=v/c and v is the speed of the electron. The maximum momentum, Pe, transferred to the nanoparticle from an electron can be calculated by

Pe = 2MEmax

(2)

The effective momentum transfer from the electron beam (300 keV; beam intensity of 1-8×105A/m2) to a ~4nm nanoparticle is in the order of ~10-18 kg·m·s-1. When a liquid molecule collides elastically with the particle, the maximum transferable energy from the liquid is estimated by (31)

Emax =

4m L ME (m L + M )2

(3)

where E=3kBT/2, k is Boltzmann’s constant, T=300 K is applied, mL and M are the mass of  

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an liquid molecule and the particle, respectively. The momentum of a liquid molecule transferred to the particle can be estimated by

PL = 2MEmax

(4)

Taking into account of the total number of collisions from liquid molecules per second, we estimate the effective momentum transferred on the nanoparticle from liquids and P~10-15 kg·m·s-1 is achieved. The momentum transfer from the electron beam to the nanoparticles is about 3 orders of magnitude smaller than that of liquid contribution, thus can be neglected. We have also considered the effects of charges from electron beam on Pt3Fe nanorod growth by nanoparticle attachment. It has been reported that a non-uniform electron beam, such as a converged beam in STEM mode, introduces electromagnetic forces, which can pull two metallic nanoparticles moving toward each other or drive them apart (35). However, since in our in situ studies the electron beam is parallel and highly uniform within the field of view, we believe the above concern is unnecessary. In addition, the growth of Pt3Fe nanorods by nanoparticle attachment is similar to the results in flask synthesis of other metallic nanowires without electron beam (36). Therefore, although we can not exclude that small charges from inelastic scattering may play a role during nanoparticle interaction, it is not a governing factor during the later stage of growth. Drift velocity of nanoparticles and forces of interaction We measure the velocity of nanoparticle movements (v) when a spherical nanoparticle approaches another nanoparticle versus a nanoparticle chain. The velocity is expressed as

v = 0.5⋅ (dL / dt) for it approaching another nanoparticle and v = 0.61⋅ (dL / dt) for it approaching a nanoparticle chain (see estimation below), where the distance between nanoparticles (L) is defined as the nearest distance between the surfaces of two nanoparticles and t is time. We measure 4 pairs in each case and an average chain of 5 nanoparticles is used for the nanoparticle and nanoparticle chain interaction (Fig. S4). The measured pairs of nanoparticles or nanoparticle chains are relatively far away from other nanoparticles, thus can be treated as isolated pairs. The average velocity as a function of interparticle distance is plotted in Fig. 4A in the main text. For diffusion of a spherical nanoparticle in liquid with low Reynolds number as in this case, the Einstein–Stokes equation gives /6 (5) where kB is Boltzmann’s constant, η is viscosity of the solvent, and T is temperature, r is radium of the nanoparticle. For the diffusion of a one-dimensional nanorod, we assume it has translational motion only for a rough estimation. The translational diffusion coefficient of the nanorod can be estimated by (37)

D = (kBT / 3πη L)(ln p + γ ) (6)  

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where L is length of the nanorod, p=length/diameter, γ = 0.312 + 0.565p−1 − 0.100 p−2 . For example, the ratio of diffusion coefficient of a 4 nm nanoparticle (Dsphere) and that of a nanorod with diameter of 3.3 nm and length of 10 nm (Drod) is calculated to be Dsphere/Drod=1.57. Because forces applied on the two nanoparticles or nanorods attracted by each other are the same, we get the velocity of a nanoparticle (vsphere1) when it approaches another nanoparticle (vsphere2), vsphere1/ vsphere2=1 and the velocity of a nanoparticle (vsphere) when it approaches another nanorod (vrod), vsphere/vrod=1.57. The diffusion coefficient (D) of a nanoparticle is estimated by tracking the movements of an individual nanoparticle of the same size in the same growth solution and within the same field of view Fig. S6A shows the trajectory of two-dimensional movements of a 4 nm nanoparticle in the growth solution while Pt3Fe nanorods are formed. The selected nanoparticle is relatively far from other nanoparticles, thus the movements can be considered as not being biased by the interaction from other nanoparticles. From the above estimation of the electron beam effects, including local heating, momentum transfer, and electron charges, we believe the electron beam effects on the particle motion can be neglected at the current imaging condition. We plot the mean square displacement (MSD) as a function of time (Fig. S6B). The two-dimensional diffusion coefficient (D) is calculated using the equation 4 and by a linear fit of the plot in Fig. S6B. We 2 get D=0.11 nm /s. We believe that the much slower movements compared that in bulk liquid is because the nanoparticles can be weakly bound near the surface due to a potential of attraction between the surface and the particles (31, 38). We calculate the forces of interaction based on the measured velocity as a function of interparticle distance. The Stokes-Einstein equation (5) can be rewritten as (7) where kB is Boltzmann’s constant, T is temperature, and μ is the mobility of the nanoparticles. The consequence of fluctuation and dissipation theorem gives where

is the drift velocity of the nanoparticle and

/ ,

is the applied force. Therefore, the

interaction force is expressed as /

(8)

The calculated forces of the nanoparticle interacting with another nanoparticle or a nanoparticle chain are shown in Fig. 4A in the right axes. The long-range interaction between a nanoparticle and a nanoparticle chain suggests that there are additional forces other than van der Waals’ force. Estimation of dipolar interaction between nanoparticles We assume the nanoparticles as volume-excluding sphere with point electric dipoles at their centers and calculate the electrostatic dipole-dipole interaction. The nanocrystals  

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internal structure, e.g., local variations of dipole density, crystal structure, length of ligands on the surface, etc., is neglected. It further neglects the role of solvent in screening short-ranged attractions. We calculate the electric potential energy of the interaction between (I) a nanoparticle and a nanoparticle (II) a nanoparticle and a nanoparticle chain. We use the simple potential energy (26, 39). ̂

Ι

3 ̂

̂

̂

(9)

to represent the electrostatic interactions between two particles i and j, whose dipoles are oriented along the unit vectors ̂ and ̂ , respectively. The unit vector ̂ points from i to j, and I denotes the unit tensor. We express all distances in units of Dc, i.e., the diameter of a particle electric core is the distance between the centers of particle i to j relative to Dc. 1/ as a is a dimensionless measure of dipole-dipole interaction strength, we use 3 2 unit of energy, yielding λ=e /(4πε0Dc kBT), e is the magnitude of a particle’s net electric dipole moment. Fig. S7 shows the electrostatic interaction potential energy and the first derivative of the potential energy when two nanoparticles interact with each other plotted as a function of separation distance. The results show four different dipolar orientations, as indicated by the arrows within the circles. We also calculate the interaction energies of two nanoparticles separated by a distance ̂ =1.5Dc and plot it as a function of relative orientation θ (Fig. 8). The electrostatic potential energy of interaction between two nanoparticles of any distance and any orientation can be computed. We allow a nanoparticle free to rotate when it approaches to another nanoparticle and compute the minimum potential energy of interaction. Fig. S9A shows the contour map of the minimum potential energy exerted by a nanoparticle on an approaching nanoparticle. Negligible anisotropy is observed. We further compute the minimum potential energy when a nanoparticle approaches a nanoparticle chain. The chain consists of discrete nanoparticles with point dipoles aligned along the long axis. Fig. S9B shows the contour map of the minimum potential energy exerted by a nanoparticle chain on an approaching nanoparticle. Strong anisotropy with lower potential energy (more negative) at the end than in the middle if the chain is observed. In addition, stronger attractive interaction is also observed compared with that around a single nanoparticle. Although the direct comparison with the absolute experimental value requires more studies, these results suggest that end-to-end attachment is preferred. Fig. S9C shows the interaction potential energy when a nanoparticle approaches a nanoparticle chain along the long axis of the chain. The chain lengths of 1-7 nanoparticles are used. It shows a clear trend that there is stronger and longer range attractive interaction for a longer nanoparticle chain. We have also considered other interaction forces during Pt3Fe nanorod formation. Considering Pt3Fe nanoparticles might be magnetic, we calculated the magnetic dipolar interaction potential energy by following the same calculation as the above and ref. (26). We have achieved similar trend that favors the formation of a nanoparticle chain (Fig. S10).  

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Fig. S1. (A) Sequential images showing coalescence of nanoparticles and the relaxation into spherical nanoparticles (marked in squares) during the early stage of growth. Color images highlighted the nanoparticles corresponding to the left marked in squares. (B) HRTEM image of a twisted Pt3Fe nanowire showing the polycrystalline features. (C) Histograms of particle size distribution when the growth by nanoparticle attachment begins.

 

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Fig S2. TEM images of 3 sets of nanorods for the measurement of angle change during the straightening process.

 

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Fig. S3. Sequential images showing the growth of Pt3Fe nanocrystals follow different shape evolution when different surfactants were used. Nanoparticles are highlighted in green. A. 20% oleylamine. B. 30% oleylamine. C. 50% or above oleylamine (in the solvent mixture of oleylamine and pentadencane). The same growth precursor Pt(acetylacetonate)2 (20 mg/mL) and Fe(acetylacetonate)2 (20 mg/mL) were used. The same electron beam conditions (300 kV; an electron density of 1-8×105A/m2) were maintained in (A-B).

 

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Fig S4. Velocity of a nanoparticle when it approaches (A) another nanoparticle or (B) a nanoparticle chain. Error bars show the standard deviation.

 

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Fig S5. TEM images of pure Pt nanowires grown by nanoparticle attachment, which are from in situ TEM experiments in a liquid cell.

 

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Fig S6. Motion of a 4 nm particle in the growth solution. (A) Trajectories of the nanoparticle two-dimensional movements. (B) Mean square displacement vs time.

 

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Fig S7. (A) The electrostatic interaction energy as a function of interparticle distance. Each particle is assumed a point electrostatic dipole. (B) First derivative of the interaction energy in (A) as a function of the separation distance.

 

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Fig S8. The electrostatic interaction potential energy of two nanoparticles separated by a distance

 

=1.5Dc as a function of relative orientation θ.

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Fig S9. Electrostatic dipolar interaction. (A) Contour map of the minimum potential energy exerted by a nanoparticle on an approaching nanoparticle. (B) Contour map of the minimum potential energy exerted by a nanoparticle chain on an approaching nanoparticle. (C) The interaction energy when a nanoparticle approaches a nanoparticle chain along the long axis of the chain. The chain lengths of 1-7 nanoparticles are used.

 

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Fig S10. Magnetic dipolar interaction. (A) Contour map of the minimum potential energy exerted by a nanoparticle on an approaching nanoparticle. (B) Contour map of the minimum potential energy exerted by a nanoparticle chain on an approaching nanoparticle. (C) The interaction energy when a nanoparticle approaches a nanoparticle chain along the long axis of the chain. The chain lengths of 1-7 nanoparticles are used.

 

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Movie S1 Growth of Pt3Fe nanowires in a liquid cell by in situ TEM. Movie plays 30 times faster than real time. A growth solution of Pt(acetylacetonate)2 (20 mg/mL) and Fe(acetylacetonate)2 (20 mg/mL) dissolved in a solvent mixture of pentadecane and oleylamine (7:3 in volume ratio) is used. Movie S2 Growth of a short straight Pt3Fe nanorod in a liquid cell by in situ TEM. Movie plays 30 times faster than real time. A growth solution of Pt(acetylacetonate)2 (20 mg/mL) and Fe(acetylacetonate)2 (20 mg/mL) dissolved in a solvent mixture of pentadecane, oleylamine and oleic acid (6:3:1 in volume ratio) is used. Movie S3 Growth of a long straight Pt3Fe nanorod in a liquid cell by in situ TEM. Movie plays 30 times faster than real time. A growth solution of Pt(acetylacetonate)2 (20 mg/mL) and Fe(acetylacetonate)2 (20 mg/mL) dissolved in a solvent mixture of pentadecane, oleylamine and oleic acid (6:3:1 in volume ratio) is used.

 

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