[Paper Review] X-ray nanotomography reveals formation of single diamonds by block copolymer self-assembly

Kenza Djeghdi, Dmitry Karpov+15 more|arXiv (Cornell University)|Apr 24, 2023

Block Copolymer Self-AssemblyMaterials Science3 citations

TL;DR

This study uses X-ray nanotomography via ptychographic X-ray computed tomography to non-invasively image large volumes of block copolymer self-assembled nanostructures at ~11 nm resolution, revealing a previously elusive single diamond morphology in a triblock terpolymer system. The discovery demonstrates that high-resolution 3D imaging enables the identification of complex, long-range-ordered networks critical for photonic and metamaterial applications.

ABSTRACT

Block copolymers are recognised as a valuable platform for creating nanostructured materials with unique properties. Morphologies formed by block copolymer self-assembly can be transferred into a wide range of inorganic materials, enabling applications including energy storage and metamaterials. However, imaging of the underlying, often complex, nanostructures in large volumes has remained a challenge, limiting progress in materials development. Taking advantage of recent advances in X-ray nanotomography, we non-invasively imaged exceptionally large volumes of nanostructured soft materials at high resolution, revealing a single diamond morphology in a triblock terpolymer composite network. This morphology, which is ubiquitous in nature, has so far remained elusive in block copolymers, despite its potential to create materials with large photonic bandgaps. The discovery was made possible by the precise analysis of distortions in a large volume of the self-assembled diamond network, which are difficult to unambiguously assess using traditional characterisation tools. We anticipate that high-resolution X-ray nanotomography, which allows imaging of much larger sample volumes than electron-based tomography, will become a powerful tool for the quantitative analysis of complex nanostructures and that structures such as the triblock terpolymer-directed single diamond will enable the generation of advanced multicomponent composites with hitherto unknown property profiles.

Motivation & Objective

• To overcome the limitations of traditional 2D and electron-based imaging in characterizing large-volume, complex 3D nanostructures formed by block copolymer self-assembly.
• To identify and validate the presence of a single diamond network morphology in a triblock terpolymer system, which has remained elusive despite its potential for large photonic bandgaps.
• To demonstrate the utility of high-resolution X-ray nanotomography for quantitative, non-destructive 3D analysis of soft matter nanostructures with high structural fidelity.
• To enable the development of advanced multicomponent composites by providing accurate structural characterization of self-assembled networks.
• To assess sample stability during high-dose X-ray exposure using in-situ ptychographic reconstruction and phase registration techniques.

Proposed method

• Employed ptychographic X-ray computed tomography at beamline X12SA of the Swiss Light Source (SLS), using a 1.5M Eiger detector with 75×75 µm² pixels and 0.1 s exposure per projection.
• Acquired 8 subtomograms with 8× angular spacing using a non-sequential binary decomposition scheme to ensure uniform angular sampling and enable stability monitoring.
• Applied 500 iterations of a least-squares maximum-likelihood algorithm with compact set constraints for ptychographic phase reconstruction, achieving a 6.04 nm pixel size.
• Performed sub-pixel image registration across angular projections using advanced alignment algorithms to ensure structural consistency during acquisition.
• Conducted tomographic reconstruction via filter back projection with a Ram-Lak filter and 1.0 frequency cut-off, yielding a 3D volume with estimated resolution of ~11 nm (conservative) and ~7 nm (Fourier shell correlation).
• Used Fiji and Avizo™ for image processing, median filtering, trainable Weka 3D segmentation, and skeletonization to extract network topology, including strut length distribution and coordination number.

Experimental results

Research questions

• RQ1 Can X-ray nanotomography resolve long-range-ordered, complex 3D network morphologies such as the single diamond structure in block copolymer self-assembly?
• RQ2 What is the structural fidelity and resolution of ptychographic X-ray tomography when applied to soft, radiation-sensitive materials like block copolymer networks?
• RQ3 How do distortions and defects in the self-assembled network affect the identification of idealized morphologies such as the diamond phase?
• RQ4 Can high-dose X-ray exposure induce structural changes in the block copolymer network during acquisition, and can this be detected and corrected?
• RQ5 To what extent can the 3D topology of the self-assembled network—such as coordination number and bond angles—be quantitatively linked to its functional properties?

Key findings

• X-ray nanotomography successfully revealed a single diamond morphology in a triblock terpolymer composite network, a structure previously undetected in synthetic block copolymers despite its prevalence in nature.
• The 3D reconstruction achieved a conservative resolution estimate of approximately 11 nm, validated by line profiles across the dataset, with a Fourier shell correlation resolution of ~7 nm.
• Sample stability during high-dose exposure was confirmed via sub-pixel registration and phase consistency checks, indicating no significant structural changes occurred during acquisition.
• The network exhibited a well-defined strut length distribution and average coordination number consistent with a diamond-like topology, with bond and dihedral angles measured in Matlab from the skeletonized structure.
• The study demonstrates that ptychographic X-ray tomography enables non-destructive, high-resolution 3D imaging of large volumes (up to 12.5×3.7 µm²) of soft matter nanostructures, overcoming limitations of electron tomography.
• The discovery of the single diamond phase opens pathways for designing advanced multicomponent composites with tailored photonic and electronic properties based on precisely characterized self-assembled networks.