Diamond-lattice photonic crystals assembled from DNA origami

Abstract

Colloidal self-assembly allows rational design of structures on the micrometer and submicrometer scale. One architecture that can generate complete three-dimensional photonic bandgaps is the diamond cubic lattice, which has remained difficult to realize at length scales comparable with the wavelength of visible or ultraviolet light. In this work, we demonstrate three-dimensional photonic crystals self-assembled from DNA origami that act as precisely programmable patchy colloids. Our DNA-based nanoscale tetrapods crystallize into a rod-connected diamond cubic lattice with a periodicity of 170 nanometers. This structure serves as a scaffold for atomic-layer deposition of high-refractive index materials such as titanium dioxide, yielding a tunable photonic bandgap in the near-ultraviolet.

Figure 1. Design and growth of the diamond cubic crystals.

(A) A model of two DNA origami tetrapods in the staggered configuration. Each gray cylinder in the model represents a double-stranded DNA helix. The binding sequence and the extensions of staples are shown in red and blue. The left inset shows the positions of extension on one of the end surfaces of the tetrapod. The right inset shows a TEM image of three tetrapods, with each showing only three arms, as the fourth arm is pointing out of the plane of the image. (B) Unit cell of the designed rod-connected diamond cubic lattice with a periodicity of 170 nm. (C) The unit cell, covered in layers of SiO₂ and ALD-grown high refractive index material. (D) Two DNA origami cubic diamond crystals, covered with a layer of SiO₂. (E) A 25 µm DNA origami diamond cubic crystal, covered with layers of SiO₂ and TiO₂. The octahedrons in the lower right corners of (D) and (E) illustrate the shape and orientation of the crystals.

Figure 2. Structure of silica-coated diamond crystals.

(A to- C) SEM images of {111} facets of the crystal viewed at (A) normal orientation or tilted by (B) 20° and (C) 40° from the normal as shown in the upper insets. The lower insets show the appearance of a model of the crystal at the same orientation. (D) A zoomed-in view of steps of {111} planes on a crystal facet. (E) A well-defined edge between two crystal facets. (F and G) An example of a twinned crystal (F) where the mirror plane is a single plane of hexagonal diamond, indicated with the red dashed lines in the zoomed-in SEM image and the model shown in the inset of panel (G).

Figure 3. Optical properties of DNA origami photonic crystals.

(A) Wide field and zoomed- in (inset) SEM images of a deposited film of silicified DNA origami diamond crystals, covered with a 13 nm layer of titania (TiO₂). (B) Calculated photonic band gaps for different thicknesses of TiO₂ cladding on the diamond cubic structure and the measured extinction coefficient (k) of ALD-grown TiO₂. The photonic band gap is calculated using measured values of the refractive index of ALD-deposited films. (C) Measured reflection spectra of DNA origami diamond crystal films with different thicknesses of titania coating.