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Optically anisotropic substrates via wrinkle-assisted convective assembly of gold nanorods on macroscopic areas.

Tebbe M, Mayer M, Glatz BA, Hanske C, Probst PT, Müller MB, Karg M, Chanana M, König TA, Kuttner C, Fery A - Faraday Discuss. (2015)

Bottom Line: We characterise the optical response of the assemblies on ITO-coated glass via UV/vis/NIR spectroscopy and determine an optical order parameter of 0.91.The assemblies are thus plasmonic metamaterials, as their periodicity and building block sizes are well below the optical wavelength.The presented approach does not rely on lithographic patterning and provides access to functional materials, which could have applications in subwavelength waveguiding, photovoltaics, and for large-area metamaterial fabrication.

View Article: PubMed Central - PubMed

Affiliation: Physical Chemistry II, Universitätsstraße 30, 95440, Bayreuth, Germany. andreas.fery@uni-bayreuth.de.

ABSTRACT
We demonstrate the large-scale organisation of anisotropic nanoparticles into linear assemblies displaying optical anisotropy on macroscopic areas. Monodisperse gold nanorods with a hydrophilic protein shell are arranged by dip-coating on wrinkled surfaces and subsequently transferred to indium tin oxide (ITO) substrates by capillary transfer printing. We elucidate how tuning the wrinkle amplitude enables us to precisely adjust the assembly morphology and fabricate single, double and triple nanorod lines. For the single lines, we quantify the order parameter of the assemblies as well as interparticle distances from scanning electron microscopy (SEM) images. We find an order parameter of 0.97 and a mean interparticle gap size of 7 nm. This combination of close to perfect uni-axial alignment and close-packing gives rise to pronounced macroscopic anisotropic optical properties due to strong plasmonic coupling. We characterise the optical response of the assemblies on ITO-coated glass via UV/vis/NIR spectroscopy and determine an optical order parameter of 0.91. The assemblies are thus plasmonic metamaterials, as their periodicity and building block sizes are well below the optical wavelength. The presented approach does not rely on lithographic patterning and provides access to functional materials, which could have applications in subwavelength waveguiding, photovoltaics, and for large-area metamaterial fabrication.

No MeSH data available.


(A) High magnification SEM image of an example single nanorod line (top) and a schematic depiction of the parameter evaluation (bottom). The angle deviations θ of each single particle, along with the interparticle gap distances d, were determined with a semi-automated procedure (IGOR Pro, Wavemetrics). (B) Summary of the equations used for analysis. (C) Distribution of the deviation from the mean angle of individual particles (bin size 1°, Gaussian fit, σ〈θ〉 of 5.53°). (D) Distribution of the order of orientation as determined from the angle deviation (bin size 0.05, 〈S2D〉 = 0.97). (E) Gap size distribution (bin size 1.117 nm, LogNormal fit, 〈d〉 = 7.4 ± 6.2 nm). The filling factor was determined to be 88.2 ± 1.5%.
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fig5: (A) High magnification SEM image of an example single nanorod line (top) and a schematic depiction of the parameter evaluation (bottom). The angle deviations θ of each single particle, along with the interparticle gap distances d, were determined with a semi-automated procedure (IGOR Pro, Wavemetrics). (B) Summary of the equations used for analysis. (C) Distribution of the deviation from the mean angle of individual particles (bin size 1°, Gaussian fit, σ〈θ〉 of 5.53°). (D) Distribution of the order of orientation as determined from the angle deviation (bin size 0.05, 〈S2D〉 = 0.97). (E) Gap size distribution (bin size 1.117 nm, LogNormal fit, 〈d〉 = 7.4 ± 6.2 nm). The filling factor was determined to be 88.2 ± 1.5%.

Mentions: In order to study the homogeneity of the assembled gold nanorod arrays and structural parameters such as periodicity and filling factor, we recorded a series of SEM images with different magnifications at different points on the substrate. Representative SEM micrographs are shown in Fig. 4. The SEM micrograph with lower magnification confirms the high degree of selectivity. On the investigated 10 times 10 μm areas (Fig. 4A), no rods were identified outside the wrinkles. Furthermore, we found a high degree of filling along with well-defined orientation parallel to the wrinkles over the whole area. Fig. 4B shows an SEM image measured with higher magnification, highlighting the close packing of the nanorods organised in single lines. The AFM measurements shown in Fig. 4C and the cross-sections in Fig. 4D–E confirm a wrinkle amplitude smaller than the particle diameter. In order to quantitatively assess the quality of the arrays, we determined the overall order parameter S2D, the interparticle gap size and the one-dimensional filling factor using a semi-automated IGOR Pro procedure (see Experimental section). As shown in Fig. 5A for a representative higher magnification SEM image, SEM images were first converted to binary data, which allowed precise tracing of the boundaries of the particles. The preferred orientation and deviations from this, as well as the fill factor and interparticle distances, could thus be evaluated, as described in detail in the experimental section. Fig. 5B summarises the results for these parameters. The determined angle deviations for individual nanorods are plotted as a histogram in Fig. 5C. A Gaussian fit provides a standard deviation σ〈θ〉 of 5.53°. To quantify the degree of the order of orientation we determine the averaged two-dimensional order parameter 〈S2D〉 from the angle deviation θ of individual nanorods, according to S2D = 2 cos(θ)2 –1. For an ideal structure, one expects an order parameter of unity, whereas a completely disordered structure would result in an order parameter of zero. Our macroscopic assemblies allow us to evaluate 〈S2D〉 for the first time with high statistical significance. S2D was determined for more than 700 particles on a sample area larger than 15 μm.2 The resulting values are plotted in Fig. 5D. Consequently, the averaged two-dimensional order parameter 〈S2D〉 was determined to be 0.97. This further validates the high degree of structural control with our process. Interparticle gaps were measured to determine the mean gap size and the overall filling factor. The results for the gap sizes are plotted as a histogram in Fig. 5E. The applied LogNormal fit yields a mean gap size of only 7.4 ± 6.2 nm. The filling factor was determined as 88.2 ± 1.5% within the analysed area. The low average interparticle gap size will result in plasmonic coupling of adjacent nanoparticles as it is well below the dimensions of the individual constituents. At the same time, the high degree of filling should ensure that the effects are strong enough to be detectable in macroscopic UV/vis/NIR spectroscopy. Therefore, we proceeded with the transfer to flat substrates and spectroscopic characterisation.


Optically anisotropic substrates via wrinkle-assisted convective assembly of gold nanorods on macroscopic areas.

Tebbe M, Mayer M, Glatz BA, Hanske C, Probst PT, Müller MB, Karg M, Chanana M, König TA, Kuttner C, Fery A - Faraday Discuss. (2015)

(A) High magnification SEM image of an example single nanorod line (top) and a schematic depiction of the parameter evaluation (bottom). The angle deviations θ of each single particle, along with the interparticle gap distances d, were determined with a semi-automated procedure (IGOR Pro, Wavemetrics). (B) Summary of the equations used for analysis. (C) Distribution of the deviation from the mean angle of individual particles (bin size 1°, Gaussian fit, σ〈θ〉 of 5.53°). (D) Distribution of the order of orientation as determined from the angle deviation (bin size 0.05, 〈S2D〉 = 0.97). (E) Gap size distribution (bin size 1.117 nm, LogNormal fit, 〈d〉 = 7.4 ± 6.2 nm). The filling factor was determined to be 88.2 ± 1.5%.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4530594&req=5

fig5: (A) High magnification SEM image of an example single nanorod line (top) and a schematic depiction of the parameter evaluation (bottom). The angle deviations θ of each single particle, along with the interparticle gap distances d, were determined with a semi-automated procedure (IGOR Pro, Wavemetrics). (B) Summary of the equations used for analysis. (C) Distribution of the deviation from the mean angle of individual particles (bin size 1°, Gaussian fit, σ〈θ〉 of 5.53°). (D) Distribution of the order of orientation as determined from the angle deviation (bin size 0.05, 〈S2D〉 = 0.97). (E) Gap size distribution (bin size 1.117 nm, LogNormal fit, 〈d〉 = 7.4 ± 6.2 nm). The filling factor was determined to be 88.2 ± 1.5%.
Mentions: In order to study the homogeneity of the assembled gold nanorod arrays and structural parameters such as periodicity and filling factor, we recorded a series of SEM images with different magnifications at different points on the substrate. Representative SEM micrographs are shown in Fig. 4. The SEM micrograph with lower magnification confirms the high degree of selectivity. On the investigated 10 times 10 μm areas (Fig. 4A), no rods were identified outside the wrinkles. Furthermore, we found a high degree of filling along with well-defined orientation parallel to the wrinkles over the whole area. Fig. 4B shows an SEM image measured with higher magnification, highlighting the close packing of the nanorods organised in single lines. The AFM measurements shown in Fig. 4C and the cross-sections in Fig. 4D–E confirm a wrinkle amplitude smaller than the particle diameter. In order to quantitatively assess the quality of the arrays, we determined the overall order parameter S2D, the interparticle gap size and the one-dimensional filling factor using a semi-automated IGOR Pro procedure (see Experimental section). As shown in Fig. 5A for a representative higher magnification SEM image, SEM images were first converted to binary data, which allowed precise tracing of the boundaries of the particles. The preferred orientation and deviations from this, as well as the fill factor and interparticle distances, could thus be evaluated, as described in detail in the experimental section. Fig. 5B summarises the results for these parameters. The determined angle deviations for individual nanorods are plotted as a histogram in Fig. 5C. A Gaussian fit provides a standard deviation σ〈θ〉 of 5.53°. To quantify the degree of the order of orientation we determine the averaged two-dimensional order parameter 〈S2D〉 from the angle deviation θ of individual nanorods, according to S2D = 2 cos(θ)2 –1. For an ideal structure, one expects an order parameter of unity, whereas a completely disordered structure would result in an order parameter of zero. Our macroscopic assemblies allow us to evaluate 〈S2D〉 for the first time with high statistical significance. S2D was determined for more than 700 particles on a sample area larger than 15 μm.2 The resulting values are plotted in Fig. 5D. Consequently, the averaged two-dimensional order parameter 〈S2D〉 was determined to be 0.97. This further validates the high degree of structural control with our process. Interparticle gaps were measured to determine the mean gap size and the overall filling factor. The results for the gap sizes are plotted as a histogram in Fig. 5E. The applied LogNormal fit yields a mean gap size of only 7.4 ± 6.2 nm. The filling factor was determined as 88.2 ± 1.5% within the analysed area. The low average interparticle gap size will result in plasmonic coupling of adjacent nanoparticles as it is well below the dimensions of the individual constituents. At the same time, the high degree of filling should ensure that the effects are strong enough to be detectable in macroscopic UV/vis/NIR spectroscopy. Therefore, we proceeded with the transfer to flat substrates and spectroscopic characterisation.

Bottom Line: We characterise the optical response of the assemblies on ITO-coated glass via UV/vis/NIR spectroscopy and determine an optical order parameter of 0.91.The assemblies are thus plasmonic metamaterials, as their periodicity and building block sizes are well below the optical wavelength.The presented approach does not rely on lithographic patterning and provides access to functional materials, which could have applications in subwavelength waveguiding, photovoltaics, and for large-area metamaterial fabrication.

View Article: PubMed Central - PubMed

Affiliation: Physical Chemistry II, Universitätsstraße 30, 95440, Bayreuth, Germany. andreas.fery@uni-bayreuth.de.

ABSTRACT
We demonstrate the large-scale organisation of anisotropic nanoparticles into linear assemblies displaying optical anisotropy on macroscopic areas. Monodisperse gold nanorods with a hydrophilic protein shell are arranged by dip-coating on wrinkled surfaces and subsequently transferred to indium tin oxide (ITO) substrates by capillary transfer printing. We elucidate how tuning the wrinkle amplitude enables us to precisely adjust the assembly morphology and fabricate single, double and triple nanorod lines. For the single lines, we quantify the order parameter of the assemblies as well as interparticle distances from scanning electron microscopy (SEM) images. We find an order parameter of 0.97 and a mean interparticle gap size of 7 nm. This combination of close to perfect uni-axial alignment and close-packing gives rise to pronounced macroscopic anisotropic optical properties due to strong plasmonic coupling. We characterise the optical response of the assemblies on ITO-coated glass via UV/vis/NIR spectroscopy and determine an optical order parameter of 0.91. The assemblies are thus plasmonic metamaterials, as their periodicity and building block sizes are well below the optical wavelength. The presented approach does not rely on lithographic patterning and provides access to functional materials, which could have applications in subwavelength waveguiding, photovoltaics, and for large-area metamaterial fabrication.

No MeSH data available.