Atomic Structure of Ultrathin Gold Nanowires
Supporting Information For
Atomic Structure of Ultrathin Gold Nanowires Yi Yu,1,2 Fan Cui,1,2 Jianwei Sun,1,2 and Peidong Yang* 1,2,3,4 1
Department of Chemistry, University of California, Berkeley, California 94720, United
States 2
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, United States 3
Kavli Energy NanoScience Institute, Berkeley, California 94720, United States
4
Department of Materials Science and Engineering, University of California, Berkeley,
California 94720, United States Corresponding Author*: [email protected].
Ultrathin Au NW Synthesis In a typical reaction, 22 mg of HAuCl4·3H2O was mixed with 0.6 grams of oleylamine. Then, the mixture was diluted with 13 grams of hexanes. The solution was vigorous stirred at room temperature until it formed a homogeneous solution. 0.8 grams of triisopropylsilane was next added into the solution and mild stirring was applied to better mix these chemicals. The final solution was kept still at ambient temperature for 12 hours. The product was harvested by centrifugation at 6000 rmp for 30 mins. Then, the nanowires were washed repeatedly with toluene/ethanol (1:1 volume ratio) using centrifugation-redispersion cycles to remove excess oleylamine and silanes. Finally, the product was dispersed in toluene for further characterization. AC-HRTEM & Image Simulation The aberration-corrected HRTEM (AC-HRTEM) images of ultrathin Au NWs were collected using the negative spherical aberration (CS) imaging (NCSI) technique on TEAM 0.5, which is an aberration-corrected microscope equipped with a high-brightness Schottky-type field emission gun and a Wien-filter monochromator. The accelerating voltage was 80 kV. The lens aberrations were measured and compensated prior to the image acquisition by evaluating the Zemlin tableau of an amorphous carbon area close to the area of interest in the specimen. In order to rule out the possible effect of microscope instability, we measured the residual aberrations before and after our imaging experiment and checked it every 2 hour in between. No obvious changes of these parameters could be found, confirming the reliability of our experimental data. According to the measurements, the residual lens aberrations were listed below: CS ~ -9 µm, two-fold astigmatism A1 < 2 nm, three-fold astigmatism A2 < 40 nm, axis coma B2 < 30 nm. Image simulation was performed to compare with the AC-HRTEM experiments using the multislice method as implemented in the MacTempas software. The structure model of ultrathin Au NW was built up based on ideal bulk FCC Au structure, with the same diameter as that in the experimental image. Simulated image shown in Figure 1e was obtained for a defocus C1= +4.5 nm, two-fold astigmatism A1 = 1 nm, three-fold astigmatism A2 = 30 nm, coma B2 = 10 nm, and vibration of 30 pm.
Electron-specimen Interaction In TEM, the electron irradiation effects are mainly considered as knock-on damage, radiolysis damage, and e-beam heatingS1-S3. Knock-on damage, corresponding to the atom displacements, is attributed to the inelastic scattering of electrons at nuclei, dominates at high accelerating voltage. On the other hand, radiolysis damage, corresponding to the ionization, is attributed to the inelastic electron scattering of an electron beam at electrons in the specimen, dominates at low accelerating voltage. The ebeam heating dominates in materials with poor thermal conductivity. As metals are good conductor of heat, the heating effect could be ignored. To date, it is commonly believed that knock-on damage is the major (almost the only) source of radiation damage in metals. For Au bulk material, the threshold voltage for knock-on damage is 1320 kVS1, and the threshold for Au clusters and particles are 200 and 400 kV, respectively (Ref. 9 in the main text). In this manuscript, the accelerating voltage was chosen as 80 kV (below the threshold values) to reduce the knock-on damage. In this case, it seems that the damage could be govern by radiolysis damage. Radiolysis damage has been well recognized in insulators, in which the localized electrons are sensitive to electronic damage. However, for metals, the presence of conduction electrons normally quenches electronic excitations within an extremely short time so that the electronic damage should not occur. To the authors’ best knowledge, the complicated electron irradiation damage mechanism in metals as well as many other materials has not been clearly revealed yet, and it is still an on-going research topic in the electron microscopy community. Here, in our case of 80 kV, we propose the damage mechanism should still be the knock-on damage, and radiolysis damage (if there really is). Although 80 kV is below the above threshold values, as the materials scale down to nano-size and even atomic scale, such as ultrathin NWs, the binding strength of atoms might be weaker than the bulk counterpart; therefore the true threshold value might become even lower. Atom displacements and sputtering of surface atoms may occur under the circumstance. The observation of shearing of lattice planes and atom displacements in this manuscript is attributed to the above hypothesis. Apart from lowering accelerating voltage, we have also decreased the electron dose rate during the observation. In the acquisition of Fig.1c, the dose rate was controlled at 1900 eÅ-2s-1. With the exposure time of 1 s, the total dose in Fig.1c is 1900 eÅ-2. In contrast,
the electron dose rate commonly used in HRTEM is in the range of 104-105 eÅ-2s-1, and sometimes the needed dose rate is even larger to achieve better signal-to-noise ratio. Therefore, we believe we were in a relative safe condition to observe the as-synthesized structure. The structure evolution under this dose rate was relatively slow. In order to accelerate the breakdown process, later we deliberately increased the dose rate and kept the NW illuminated for a long time. The following after-breakdown-images (Fig.2c and Fig.2e) were taken with a dose rate of 104 eÅ-2s-1, approaching to the normal dose level of HRTEM. First-principle Calculation In first-principle calculations, the electronic wave functions were expanded via a planewave basis set with a cutoff energy of 1360 eV. The equilibrium lattice constants were obtained by a full optimization of the unit cell and the supercells were built up based on the interpolation method (Ref. 29 in the main text). The structural relaxations were carried out until the residual forces were less than 2.5×10-3 eV/Å.
REFERENCES (S1) Egerton, R. F.; Li, P.; Malac, M. Micron 2004, 35, 399-409. (S2) Egerton, R. F. Ultramicroscopy 2014, 145, 85-93. (S3) Banhart, F. Electron and Ion Irradiation. In In-situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science; Dehm, G., Howe, J. M., Zweck, J., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2012; pp 125-143.
Supporting Figures
Figure S1. Low magnification TEM images of ultrathin Au NWs deposited on lacey carbon grid covered with an ultrathin carbon supporting film (< 3 nm). In order to enhance the weak contrast of the ultrathin NWs, false color was applied.
Figure S2. In-situ observation of the breakdown of an ultrathin Au NW. The necks and breakdown positions are indicated by blue arrows. The selected-area images and corresponding fast Fourier transforms (FFT) in the lower part (yellow box) and upper part (red box) of the NW are compared. At beginning (Figure S2a), although the axial direction of the whole NW is , the lower part was in [011] zone axis (Figure S2c) while the upper part was out of zone axis (Figure S2b). After structure evolution for a while, the NW tended to break from the middle in Figure S2d. The upper part gradually rotated into [112] zone axis (Figure S2e), and the lower part was kept in [011] zone axis (Figure S2f). Such crystal orientation could be maintained after NW breakdown (Figure S2g, h, i). This is an example clearly showing the “twist mode” of ultrathin Au NW breakdown.
Figure S3. Morphology of the ultrathin Au NW after breaking up into a chain of Au segments/spheres. The TEM grid was kept at room temperature for 20 days after sample preparation. The imaged area shown here was not exposed to the electron beam before. It suggests that ultrathin Au NWs may automatically break up into thermodynamically stable segments/spheres (with minimum surface area) without e-beam effect. Electron beam induced radiation damage or heating plays a role in accelerating the breakdown process (Figure 2).
Figure S4. Image simulations of a single stretched oleylamine attached on ultrathin Au NW surface with (right) and without (left) underneath graphene supporting membrane. The ligand is still distinguishable in the case with graphene membrane but the contrast difference becomes small. This gives us a sense of the difficulty of extracting individual ligand information from the real system, which contains complex external factors such as membrane background noise and absorbed contaminations.
Figure S5. Another example of in-situ observation of evolution of surface ligands. After 200 s electron beam exposure, the morphology of surface chain-like feature evolved following with the NW breakdown and diameter expansion. Ligands, as well as possible surface carbon contaminations might even compressed and lay along the surface of the NWs.
Atomic Structure of Ultrathin Gold Nanowires Yi Yu,1,2 Fan Cui,1,2 Jianwei Sun,1,2 and Peidong Yang* 1,2,3,4 1
Department of Chemistry, University of California, Berkeley, California 94720, United
States 2
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, United States 3
Kavli Energy NanoScience Institute, Berkeley, California 94720, United States
4
Department of Materials Science and Engineering, University of California, Berkeley,
California 94720, United States Corresponding Author*: [email protected].
Ultrathin Au NW Synthesis In a typical reaction, 22 mg of HAuCl4·3H2O was mixed with 0.6 grams of oleylamine. Then, the mixture was diluted with 13 grams of hexanes. The solution was vigorous stirred at room temperature until it formed a homogeneous solution. 0.8 grams of triisopropylsilane was next added into the solution and mild stirring was applied to better mix these chemicals. The final solution was kept still at ambient temperature for 12 hours. The product was harvested by centrifugation at 6000 rmp for 30 mins. Then, the nanowires were washed repeatedly with toluene/ethanol (1:1 volume ratio) using centrifugation-redispersion cycles to remove excess oleylamine and silanes. Finally, the product was dispersed in toluene for further characterization. AC-HRTEM & Image Simulation The aberration-corrected HRTEM (AC-HRTEM) images of ultrathin Au NWs were collected using the negative spherical aberration (CS) imaging (NCSI) technique on TEAM 0.5, which is an aberration-corrected microscope equipped with a high-brightness Schottky-type field emission gun and a Wien-filter monochromator. The accelerating voltage was 80 kV. The lens aberrations were measured and compensated prior to the image acquisition by evaluating the Zemlin tableau of an amorphous carbon area close to the area of interest in the specimen. In order to rule out the possible effect of microscope instability, we measured the residual aberrations before and after our imaging experiment and checked it every 2 hour in between. No obvious changes of these parameters could be found, confirming the reliability of our experimental data. According to the measurements, the residual lens aberrations were listed below: CS ~ -9 µm, two-fold astigmatism A1 < 2 nm, three-fold astigmatism A2 < 40 nm, axis coma B2 < 30 nm. Image simulation was performed to compare with the AC-HRTEM experiments using the multislice method as implemented in the MacTempas software. The structure model of ultrathin Au NW was built up based on ideal bulk FCC Au structure, with the same diameter as that in the experimental image. Simulated image shown in Figure 1e was obtained for a defocus C1= +4.5 nm, two-fold astigmatism A1 = 1 nm, three-fold astigmatism A2 = 30 nm, coma B2 = 10 nm, and vibration of 30 pm.
Electron-specimen Interaction In TEM, the electron irradiation effects are mainly considered as knock-on damage, radiolysis damage, and e-beam heatingS1-S3. Knock-on damage, corresponding to the atom displacements, is attributed to the inelastic scattering of electrons at nuclei, dominates at high accelerating voltage. On the other hand, radiolysis damage, corresponding to the ionization, is attributed to the inelastic electron scattering of an electron beam at electrons in the specimen, dominates at low accelerating voltage. The ebeam heating dominates in materials with poor thermal conductivity. As metals are good conductor of heat, the heating effect could be ignored. To date, it is commonly believed that knock-on damage is the major (almost the only) source of radiation damage in metals. For Au bulk material, the threshold voltage for knock-on damage is 1320 kVS1, and the threshold for Au clusters and particles are 200 and 400 kV, respectively (Ref. 9 in the main text). In this manuscript, the accelerating voltage was chosen as 80 kV (below the threshold values) to reduce the knock-on damage. In this case, it seems that the damage could be govern by radiolysis damage. Radiolysis damage has been well recognized in insulators, in which the localized electrons are sensitive to electronic damage. However, for metals, the presence of conduction electrons normally quenches electronic excitations within an extremely short time so that the electronic damage should not occur. To the authors’ best knowledge, the complicated electron irradiation damage mechanism in metals as well as many other materials has not been clearly revealed yet, and it is still an on-going research topic in the electron microscopy community. Here, in our case of 80 kV, we propose the damage mechanism should still be the knock-on damage, and radiolysis damage (if there really is). Although 80 kV is below the above threshold values, as the materials scale down to nano-size and even atomic scale, such as ultrathin NWs, the binding strength of atoms might be weaker than the bulk counterpart; therefore the true threshold value might become even lower. Atom displacements and sputtering of surface atoms may occur under the circumstance. The observation of shearing of lattice planes and atom displacements in this manuscript is attributed to the above hypothesis. Apart from lowering accelerating voltage, we have also decreased the electron dose rate during the observation. In the acquisition of Fig.1c, the dose rate was controlled at 1900 eÅ-2s-1. With the exposure time of 1 s, the total dose in Fig.1c is 1900 eÅ-2. In contrast,
the electron dose rate commonly used in HRTEM is in the range of 104-105 eÅ-2s-1, and sometimes the needed dose rate is even larger to achieve better signal-to-noise ratio. Therefore, we believe we were in a relative safe condition to observe the as-synthesized structure. The structure evolution under this dose rate was relatively slow. In order to accelerate the breakdown process, later we deliberately increased the dose rate and kept the NW illuminated for a long time. The following after-breakdown-images (Fig.2c and Fig.2e) were taken with a dose rate of 104 eÅ-2s-1, approaching to the normal dose level of HRTEM. First-principle Calculation In first-principle calculations, the electronic wave functions were expanded via a planewave basis set with a cutoff energy of 1360 eV. The equilibrium lattice constants were obtained by a full optimization of the unit cell and the supercells were built up based on the interpolation method (Ref. 29 in the main text). The structural relaxations were carried out until the residual forces were less than 2.5×10-3 eV/Å.
REFERENCES (S1) Egerton, R. F.; Li, P.; Malac, M. Micron 2004, 35, 399-409. (S2) Egerton, R. F. Ultramicroscopy 2014, 145, 85-93. (S3) Banhart, F. Electron and Ion Irradiation. In In-situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science; Dehm, G., Howe, J. M., Zweck, J., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2012; pp 125-143.
Supporting Figures
Figure S1. Low magnification TEM images of ultrathin Au NWs deposited on lacey carbon grid covered with an ultrathin carbon supporting film (< 3 nm). In order to enhance the weak contrast of the ultrathin NWs, false color was applied.
Figure S2. In-situ observation of the breakdown of an ultrathin Au NW. The necks and breakdown positions are indicated by blue arrows. The selected-area images and corresponding fast Fourier transforms (FFT) in the lower part (yellow box) and upper part (red box) of the NW are compared. At beginning (Figure S2a), although the axial direction of the whole NW is , the lower part was in [011] zone axis (Figure S2c) while the upper part was out of zone axis (Figure S2b). After structure evolution for a while, the NW tended to break from the middle in Figure S2d. The upper part gradually rotated into [112] zone axis (Figure S2e), and the lower part was kept in [011] zone axis (Figure S2f). Such crystal orientation could be maintained after NW breakdown (Figure S2g, h, i). This is an example clearly showing the “twist mode” of ultrathin Au NW breakdown.
Figure S3. Morphology of the ultrathin Au NW after breaking up into a chain of Au segments/spheres. The TEM grid was kept at room temperature for 20 days after sample preparation. The imaged area shown here was not exposed to the electron beam before. It suggests that ultrathin Au NWs may automatically break up into thermodynamically stable segments/spheres (with minimum surface area) without e-beam effect. Electron beam induced radiation damage or heating plays a role in accelerating the breakdown process (Figure 2).
Figure S4. Image simulations of a single stretched oleylamine attached on ultrathin Au NW surface with (right) and without (left) underneath graphene supporting membrane. The ligand is still distinguishable in the case with graphene membrane but the contrast difference becomes small. This gives us a sense of the difficulty of extracting individual ligand information from the real system, which contains complex external factors such as membrane background noise and absorbed contaminations.
Figure S5. Another example of in-situ observation of evolution of surface ligands. After 200 s electron beam exposure, the morphology of surface chain-like feature evolved following with the NW breakdown and diameter expansion. Ligands, as well as possible surface carbon contaminations might even compressed and lay along the surface of the NWs.
