Targeted Positioning of Quantum Dots Inside 3D Silicon Photonic Crystals Revealed by Synchrotron X-ray Fluorescence Tomography.

Andreas S Schulz,Jurriaan Huskens,Cornelis A M Harteveld,G Julius Vancso,Peter Cloetens,Willem L Vos

doi:10.1021/acsnano.1c06915

Andreas S Schulz, Jurriaan Huskens + Show 4 more

Open Access

https://doi.org/10.1021/acsnano.1c06915

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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 innovative functions. Once the 3D positioning is performed, the challenge arises how to nondestructively 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, and 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 nondestructively characterize 3D nanomaterials with photonic and other functionalities.

Highlights

In X-ray fluorescence tomography, the sample is illuminated with a finely focused X-ray beam to excite X-ray fluorescence that is characteristic for each element in the sample, see Figure 2a
The right detector will detect the fluorescence signal that is integrated along the incident X-ray beam At θ = −2°, the incident beam traverses the whole 500-μm-thick Si substrate, which is feasible on account of the Figure 1. 3D photonic crystal and chemical positioning of quantum dots inside of the nanostructure. (a) Schematic of an inverse woodpile photonic crystal on the edge of a silicon beam
Zoomed-in cross-section of one pore with targeted surface-chemistry: ATRP initiator layer, polymer chains forming brushes, and covalently attached PbS quantum dots on top of silicon. (c) Scanning electron microscope (SEM) image of a 3D photonic crystal viewed from 45° on the edge of the silicon beam showing the XY and the XZ surfaces; the scale bar indicates 2 μm

Summary

Introduction

Three-dimensional (3D) functionalized nanostructures are drawing fast-growing attention for their advanced applications in nanophotonics,[1−3] photovoltaics,[4,5] capacitors in electronics,[6] gas sensing,[7] materials for electrochemical energy conversion and storage,[8] and batteries.[9−12] The functionalization of these nanostructures is the result of the infiltration of active nanoparticles, e.g., fluorophores in nanophotonic light sources,[13−15] antibodies for biochemical sensors of diseases, or quantum dots for photovoltaics.[16,17] In most cases, the performance of the 3D functionalized nanostructure depends on the precise positioning of the nanoparticles inside the 3D nanostructure, with nanometer precision. We infiltrate functional quantum dot nanoparticles in these crystals that are precisely positioned relative to the silicon−air interfaces by means of polymer brushes[25,26] that grow from initiator molecules placed on the internal silicon interfaces, see Figure 1b.

Results

Conclusion