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Stabilization of 4H hexagonal phase in gold nanoribbons.

Fan Z, Bosman M, Huang X, Huang D, Yu Y, Ong KP, Akimov YA, Wu L, Li B, Wu J, Huang Y, Liu Q, Png CE, Gan CL, Yang P, Zhang H - Nat Commun (2015)

Bottom Line: These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions.Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface.Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.

ABSTRACT
Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

No MeSH data available.


SPR analysis of single 4H Au NRB.(a) Schematic illustration of monochromated EELS measurement on a single Au NRB. The focused electron beam is located next to the Au NRB to excite and measure its SPR. (b) EELS spectra acquired from an individual Au NRB excited at the positions indicated in the STEM image shown in g. (c,d) DFT-calculated dielectric function of 4H Au thin film with the limitation of 4 nm in its y direction. (e) Calculated EELS spectra of a 4H Au NRB based on dielectric function in c,d using the FEM with an electron beam. Inset: the theoretical model of an individual Au NRB, in which the excitation positions are marked. (f) Calculated EELS spectra of an fcc Au NRB with dielectric function taken from Palik at the same excitation positions as e. (g,h) HAADF-STEM image of single Au NRB (scale bar, 200 nm) and its corresponding EELS maps at different energy loss.
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f3: SPR analysis of single 4H Au NRB.(a) Schematic illustration of monochromated EELS measurement on a single Au NRB. The focused electron beam is located next to the Au NRB to excite and measure its SPR. (b) EELS spectra acquired from an individual Au NRB excited at the positions indicated in the STEM image shown in g. (c,d) DFT-calculated dielectric function of 4H Au thin film with the limitation of 4 nm in its y direction. (e) Calculated EELS spectra of a 4H Au NRB based on dielectric function in c,d using the FEM with an electron beam. Inset: the theoretical model of an individual Au NRB, in which the excitation positions are marked. (f) Calculated EELS spectra of an fcc Au NRB with dielectric function taken from Palik at the same excitation positions as e. (g,h) HAADF-STEM image of single Au NRB (scale bar, 200 nm) and its corresponding EELS maps at different energy loss.

Mentions: Single 4H Au NRB was further examined using the monochromated EELS in TEM (Fig. 3a, see Methods for details). The STEM-EELS measurement was performed by raster-scanning a focused electron beam with diameter of 1–2 nm in a rectangular region that included a single 4H Au NRB. The interaction of electron beam with the conduction electrons in Au NRB generated the localized SPR, which provided a fingerprint of the optoelectronic property of Au NRB. Figure 3b shows the EELS spectra of a single Au NRB collected from different positions (that is, I, II, III and IV), a few nanometres next to the Au NRB, as indicated in Fig. 3g. Two distinct SPR peaks were observed in all EELS spectra, that is, 0.82 and 1.75 eV at position ‘I', 0.52 and 1.72 eV at position ‘II', 0.36 and 1.93 eV at position ‘III' and 0.27 and 1.94 eV at position ‘IV'. The SPR peaks of the Au NRB at different excitation positions that appeared at low and high energy can be attributed to the eigenmodes longitudinally and transversely polarized with respect to the NRB geometry, respectively43. There is a remarkable red shift for the dominant SPR peak from excitation position ‘I' to position ‘IV'. As there is not any optical data available for 4H Au NRBs, density functional theory (DFT) calculations were used to determine the dielectric function of a 4H Au thin film (Fig. 3c,d, see Methods for details). In DFT modelling, we define the to be x direction, [001]4H to be y direction and [110]4H to be z direction (Fig. 1c,f). The 4H Au thin film is assumed to have 4 nm in its y direction and stack indefinitely in its x and z directions. The dielectric function of 4H Au thin film and that of fcc Au (obtained from literature, ref. 44) show different features in the perpendicular direction to the thin film (that is, the yy direction). Such difference comes from surface atoms with the formation of plasma frequency at low frequency. The contribution of surface atoms to the dielectric function is smaller with increasing of thin film thickness45. A red shift for the primary SPR peak from excitation position ‘I' to position ‘IV', which is consistent with the experimental result (Fig. 3b), was also observed in our finite-element-method (FEM) simulation of plasmon excitation with an electron beam (Fig. 3e, see Methods for details). Note that the Palik's frequency-dependent dielectric function, obtained from fcc Au (ref. 44), was also used in our FEM simulations to benchmark the difference of optical response between 4H Au and fcc Au (Fig. 3f). As compared with fcc Au (Fig. 3f), the optical response of 4H Au thin film shows a smaller number of SPR peaks in the range of 0–2.5 eV and red shift for all SPR peaks from position ‘I' to position ‘IV' (Fig. 3e). The experimental EELS maps further reveal that low-order harmonic resonances were formed using low-energy SPR eigenmodes and the harmonic order increased with the SPR energy (Fig. 3h). The critical insight gained here is that the simulations with DFT-calculated optical constants of 4H Au followed more closely to the trend of experiments. Since the optical responses of 4H Au and fcc Au are distinctive in terms of position and magnitude of SPR peaks, it is reasonable to conclude that the structure tuning of Au can significantly alter its optical properties.


Stabilization of 4H hexagonal phase in gold nanoribbons.

Fan Z, Bosman M, Huang X, Huang D, Yu Y, Ong KP, Akimov YA, Wu L, Li B, Wu J, Huang Y, Liu Q, Png CE, Gan CL, Yang P, Zhang H - Nat Commun (2015)

SPR analysis of single 4H Au NRB.(a) Schematic illustration of monochromated EELS measurement on a single Au NRB. The focused electron beam is located next to the Au NRB to excite and measure its SPR. (b) EELS spectra acquired from an individual Au NRB excited at the positions indicated in the STEM image shown in g. (c,d) DFT-calculated dielectric function of 4H Au thin film with the limitation of 4 nm in its y direction. (e) Calculated EELS spectra of a 4H Au NRB based on dielectric function in c,d using the FEM with an electron beam. Inset: the theoretical model of an individual Au NRB, in which the excitation positions are marked. (f) Calculated EELS spectra of an fcc Au NRB with dielectric function taken from Palik at the same excitation positions as e. (g,h) HAADF-STEM image of single Au NRB (scale bar, 200 nm) and its corresponding EELS maps at different energy loss.
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Related In: Results  -  Collection

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f3: SPR analysis of single 4H Au NRB.(a) Schematic illustration of monochromated EELS measurement on a single Au NRB. The focused electron beam is located next to the Au NRB to excite and measure its SPR. (b) EELS spectra acquired from an individual Au NRB excited at the positions indicated in the STEM image shown in g. (c,d) DFT-calculated dielectric function of 4H Au thin film with the limitation of 4 nm in its y direction. (e) Calculated EELS spectra of a 4H Au NRB based on dielectric function in c,d using the FEM with an electron beam. Inset: the theoretical model of an individual Au NRB, in which the excitation positions are marked. (f) Calculated EELS spectra of an fcc Au NRB with dielectric function taken from Palik at the same excitation positions as e. (g,h) HAADF-STEM image of single Au NRB (scale bar, 200 nm) and its corresponding EELS maps at different energy loss.
Mentions: Single 4H Au NRB was further examined using the monochromated EELS in TEM (Fig. 3a, see Methods for details). The STEM-EELS measurement was performed by raster-scanning a focused electron beam with diameter of 1–2 nm in a rectangular region that included a single 4H Au NRB. The interaction of electron beam with the conduction electrons in Au NRB generated the localized SPR, which provided a fingerprint of the optoelectronic property of Au NRB. Figure 3b shows the EELS spectra of a single Au NRB collected from different positions (that is, I, II, III and IV), a few nanometres next to the Au NRB, as indicated in Fig. 3g. Two distinct SPR peaks were observed in all EELS spectra, that is, 0.82 and 1.75 eV at position ‘I', 0.52 and 1.72 eV at position ‘II', 0.36 and 1.93 eV at position ‘III' and 0.27 and 1.94 eV at position ‘IV'. The SPR peaks of the Au NRB at different excitation positions that appeared at low and high energy can be attributed to the eigenmodes longitudinally and transversely polarized with respect to the NRB geometry, respectively43. There is a remarkable red shift for the dominant SPR peak from excitation position ‘I' to position ‘IV'. As there is not any optical data available for 4H Au NRBs, density functional theory (DFT) calculations were used to determine the dielectric function of a 4H Au thin film (Fig. 3c,d, see Methods for details). In DFT modelling, we define the to be x direction, [001]4H to be y direction and [110]4H to be z direction (Fig. 1c,f). The 4H Au thin film is assumed to have 4 nm in its y direction and stack indefinitely in its x and z directions. The dielectric function of 4H Au thin film and that of fcc Au (obtained from literature, ref. 44) show different features in the perpendicular direction to the thin film (that is, the yy direction). Such difference comes from surface atoms with the formation of plasma frequency at low frequency. The contribution of surface atoms to the dielectric function is smaller with increasing of thin film thickness45. A red shift for the primary SPR peak from excitation position ‘I' to position ‘IV', which is consistent with the experimental result (Fig. 3b), was also observed in our finite-element-method (FEM) simulation of plasmon excitation with an electron beam (Fig. 3e, see Methods for details). Note that the Palik's frequency-dependent dielectric function, obtained from fcc Au (ref. 44), was also used in our FEM simulations to benchmark the difference of optical response between 4H Au and fcc Au (Fig. 3f). As compared with fcc Au (Fig. 3f), the optical response of 4H Au thin film shows a smaller number of SPR peaks in the range of 0–2.5 eV and red shift for all SPR peaks from position ‘I' to position ‘IV' (Fig. 3e). The experimental EELS maps further reveal that low-order harmonic resonances were formed using low-energy SPR eigenmodes and the harmonic order increased with the SPR energy (Fig. 3h). The critical insight gained here is that the simulations with DFT-calculated optical constants of 4H Au followed more closely to the trend of experiments. Since the optical responses of 4H Au and fcc Au are distinctive in terms of position and magnitude of SPR peaks, it is reasonable to conclude that the structure tuning of Au can significantly alter its optical properties.

Bottom Line: These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions.Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface.Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.

ABSTRACT
Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

No MeSH data available.