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Abstract
It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize novel functions.
Once the 3D positioning is performed, the challenge arises how to non-destructively verify where the nanoparticles reside in the 3D nanostructure.
Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots.
The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP).
The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest.
Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, Pb.
The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator.
We conclude that X-ray fluorescence tomography is very well suited to non-destructively characterize 3D nanomaterials with photonic and other functionalities.
Supplementary materials
Title
S1: Scheme of the chemical strategy
Description
General reaction schemes to anchor the ATRP initiator 1 to silicon substrate and to perform ATRP of GMA 2. Lead sulfide quantum dots are attached to the PGMA brushes by using a poly(ethylene glycol)-amine ligand 3.
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S2: Gallium fluorescence throughout the 3D silicon photonic crystal
Description
The avi-movie shows the gallium fluorescence for each 50 nm slice depending on the Y-position. The top left number represents the slice number, the top right number represents the maximum amount of gallium atoms found for a voxel of 50x50x50 nm in the slice.
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S3: Bromine fluorescence throughout the 3D silicon photonic crystal
Description
The avi-movie shows the bromine fluorescence for each 50 nm slice depending on the Y-position. The top left number represents the slice number, the top right number represents the maximum amount of bromine atoms found for a voxel of 50x50x50 nm in the slice.
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S4: Lead fluorescence throughout the 3D silicon photonic crystal
Description
The avi-movie shows the lead fluorescence for each 50 nm slice depending on the Y-position. The top left number represents the slice number, the top right number represents the maximum amount of lead atoms found for a voxel of 50x50x50 nm in the slice.
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S5: Projection maps of the number of Ga atoms per pixel area
Description
Projection maps of the number of gallium atoms per pixel area (50 x 50 nm) and integrated along the propagation direction of the beam, for four sample orientations. The signal is an average of the Ga-K-alpha and the Ga-K-beta line. Color bars with the scales are given in each panel. The scale bar represents 40 pixels (2 um).
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S6: Projection maps of the number of bromine atoms per pixel area
Description
Projection maps of the number of bromine atoms per pixel area (50 x 50 nm) and integrated along the propagation direction of the beam, for four sample orientations. Color bars with the scales are given in each panel. The scale bar represents 40 pixels (2 um).
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S7: Attenuation maps of gallium fluorescence as a function of atomic position inside the Si beam.
Description
Attenuation maps of gallium fluorescence as a function of atomic position inside the Si beam. In this representation, the Si-beam orientation is fixed with the photonic crystal (small square) at bottom right, whereas the incident X-ray beam orientation is varied. All color bars run from 0 to 1.
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S8: Attenuation maps of bromine fluorescence as a function of atomic position inside the Si beam.
Description
Attenuation maps of bromine fluorescence as a function of atomic position inside the Si beam. In this representation, the Si-beam orientation is fixed with the photonic crystal (small square) at bottom right, whereas the incident X-ray beam orientation is varied. All color bars run from 0 to 1.
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S9: Attenuation maps of copper fluorescence as a function of atomic position inside the Si beam.
Description
Attenuation maps of copper fluorescence as a function of atomic position inside the Si beam. In this representation, the Si-beam orientation is fixed with the photonic crystal (small square) at bottom right, whereas the incident X-ray beam orientation is varied. All color bars run from 0 to 1.
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S10: Attenuation maps of chlorine fluorescence as a function of atomic position inside the Si beam.
Description
Attenuation maps of chlorine fluorescence as a function of atomic position inside the Si beam. In this representation, the Si-beam orientation is fixed with the photonic crystal (small square) at bottom right, whereas the incident X-ray beam orientation is varied. All color bars run from 0 to 1.
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