Materials Horizons Blog

Researchers have long been interested in peering into the brain. Added to the inherent challenge of imaging through biological medium, the skull presents a major barrier that highly attenuates light.

To overcome this barrier, in a recent communication in Materials Horizons, Guo et al. have synthesized an organic nanoparticle for photoacoustic imaging with absorbance in the second near-infrared window. At this wavelength, there is relatively low scattering from tissue allowing for deeper penetration of light.

Photoacoustic images of a brain tumor after nanoparticle injection. The grey ultrasound image shows the skin and the skull margin, and the green signal indicates the nanoparticle distribution. Image adapted from Guo et al., Mater. Horiz., 2017, Advance Article with permission from The Royal Society of Chemistry. 

Nanoparticles were made from benzodithiophene-benzobithiadiazole donor-acceptor pairs co-polymerized and nanoprecipitated using biocompatible materials. When these imaging nanoparticles were applied to mice with orthotopic brain tumors, tumors 3.4 nm below the skull were resolved with a nearly 100-fold increase in photoacoustic signal compared to before intravenous administration of nanoparticles. The stable, high contrast photoacoustic imaging nanoparticle presented in this work offers a versatile platform for simple chemical modifications such as ligand targeting or drug loading.

Future work remains on the horizon to advance these materials for imaging through the ~5 mm thickness of human skulls.

Ester Kwon is a member of the Community Board for Materials Horizons. Currently, she works as an Assistant Professor in the Department of Bioengineering at University of California San Diego, USA. Check out her personal website here.

A new and highly controllable technique to manufacture functional gradient nanocomposites has been reported in a recent article, published in Materials Horizons. The technique enables smooth and programmable stiff-to-compliant (or compliant-to-stiff) transitions within micro-scale regions.

This technique, developed by Dr. Zhengzhi Wang and colleagues at Wuhan University, is based on a typical two-step process:

1. Use a magnetic field to generate a desired concentration gradient of magnetic-responsive nano-reinforcements inside a polymer matrix in liquid state.
2. Polymerize and solidify the redistributed polymer nanocomposites.

Using this technique, Wang et al. fabricated various biomimetic interfaces and surfaces and found that the functional gradient designs, with reduced stress concentrations, simultaneously improved the mechanical strength and durability over an order of magnitude compared with the traditional homogeneous counterparts.

The magnetically-actuated functional gradient nanocomposites can be further integrated into advanced additive manufacturing techniques to create a wide range of functional heterogeneous materials with unprecedented combinations of mechanical properties.

TEM image of functional gradient nanocomposites for compliant-stiff-compliant transitions

Mengye Wang is a member of the Community Board for Materials Horizons. Currently, she works as a postdoctoral fellow in the Department of Applied Physics at The Hong Kong Polytechnic University. She has a keen interest in advanced materials for environmental and energy applications, including photocatalysis and electrocatalysis.

Hybrid double perovskites have recently gained a vast amount of attention in the research area of photovoltaics as lead-free alternatives to the ground-breaking parent material [CH₃NH₃]PbI₃.¹ In double perovskites with the general formula A₂B’B’’X₆, two Pb²⁺ cations are effectively replaced with a monovalent B’ and a trivalent B’’ cation. Among the many fascinating properties of hybrid inorganic-organic perovskites, it is arguably the combination of strong light absorbance and long carrier lifetimes that make them so interesting for photovoltaic applications and light-emission devices. Recent experimental and theoretical studies on [CH₃NH₃]PbI₃ revealed a direct-indirect character of the bandgap, i.e. [CH₃NH₃]PbI₃ exhibits a direct band gap which is only approximately 47-60 meV higher in energy than the indirect band gap. Presumably, this is the origin of the paradox of strong absorption and long charge carrier lifetimes. When now turning our attention to lead free double perovskites, examples such as [CH₃NH₃]₂KBiCl₆ and [CH₃NH₃]₂AgBiBr₆ exhibit an indirect band gap,¹ hence unfavourable light absorption properties. The symmetry mismatch that leads to the indirect band gap in such materials was recently studied by D. O. Scanlon and A. Zunger theoretically.^2,3 Consequently, it is important to ask the question: is it possible to experimentally design a direct band gap in double perovskites?

In the recent article in Materials Horizons, ‘Designing Indirect-Direct Bandgap Transitions in Double Perovskites’,⁴ T. M. McQueen and co-workers have tackled this important question, studying the solid solution Cs₂AgIn_1-xSb_xCl₆ as a prototypical example. By wisely choosing B’ and B’’, a direct band gap in Cs₂AgInCl₆ has been achieved. The beauty lies in the simplicity of the concept – the understanding of band theory, i.e. symmetry and formation of bands with s-type and p-type character, see Figure 1. Going along the solid solution Cs₂AgIn_1-xSb_xCl₆, the valence band remains basically unchanged, whilst the character of the conduction band is continuously altered from s-type to p-type character. Clearly, the use of a chloride and in turn the ionic character of the solid with a band-gap larger than 3.5 eV limits the application of Cs₂AgInCl₆ in optoelectronics. However, the results depict a textbook example of how to manipulate properties in crystalline materials and open exciting opportunities for going forward in the field. For instance, one can easily envision a computational screening study of potential A₂B’B’’X₆ perovskites by using symmetry-based descriptors. Furthermore, it is important to note, that band engineering is a common concept in related areas of materials science, such as thermoelectrics and magnetic materials, and is a common tool for solid state chemists in general. Therefore, it is refreshing to see that band engineering now enters arguably one of the most fascinating developments of materials science within the last decade.

Figure 1. Schematic presentation of the orbital overlap (a) and the energy as a function of k for bands of s and p-σ orbitals (b) in a linear chain.

[1] F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe, A. K. Cheetham ‘The synthesis, structure and electronic properties of a lead-free hybrid inorganic-organic double perovskites (MA)₂KBiCl₆ (MA = methylammonium)’ Mater. Horiz. 2016, 3, 328.

[2] C. N. Savory, A. Walsh and D. O. Scanlon ‘Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?’ ACS Energy Lett. 2016, 1, 949.

[3] X.-G. Zhao, D. Yang, Y. Sun, T. Li, L. Zhang, L. Yu, A. Zunger ‘Cu-In Halide Perovskite Solar Absorbers’ J. Am. Chem. Soc. 2017, 139, 6718.

[4] T. Thao Tran, J. R. Panella, J. R. Chamorro, J. R. Morey, T. M. McQueen ‘Designing Indirect-Direct Bandgap Transitions in Double Perovskites’ Mater. Horiz. 2017, DOI: 10.1039/C7MH00239D.

Dr Gregor Kieslich is a Liebig-Fellow at Department of Chemistry, Technical University of Munich and is a member of the Community Board for Materials Horizons. He is an inorganic chemist focusing on crystal chemistry and structure–property relations in functional solids and hybrid frameworks: https://kieslichresearch.wordpress.com/