Assembly of mesoscale helices with near-unity enantiomeric excess and light-matter interactions for chiral semiconductors

Wenchun Feng; Ji-Young Kim; Xinzhi Wang; Heather A Calcaterra; Zhibei Qu; Louisa Meshi; Nicholas A Kotov

doi:10.1126/sciadv.1601159

Assembly of mesoscale helices with near-unity enantiomeric excess and light-matter interactions for chiral semiconductors

Sci Adv. 2017 Mar 1;3(3):e1601159. doi: 10.1126/sciadv.1601159. eCollection 2017 Mar.

Authors

Wenchun Feng¹, Ji-Young Kim², Xinzhi Wang¹, Heather A Calcaterra¹, Zhibei Qu¹, Louisa Meshi³, Nicholas A Kotov⁴

Affiliations

¹ Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
² Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
³ Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel.
⁴ Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.; Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA.

Abstract

Semiconductors with chiral geometries at the nanoscale and mesoscale provide a rich materials platform for polarization optics, photocatalysis, and biomimetics. Unlike metallic and organic optical materials, the relationship between the geometry of chiral semiconductors and their chiroptical properties remains, however, vague. Homochiral ensembles of semiconductor helices with defined geometries open the road to understanding complex relationships between geometrical parameters and chiroptical properties of semiconductor materials. We show that semiconductor helices can be prepared with an absolute yield of ca 0.1% and an enantiomeric excess (e.e.) of 98% or above from cysteine-stabilized cadmium telluride nanoparticles (CdTe NPs) dispersed in methanol. This high e.e. for a spontaneously occurring chemical process is attributed to chiral self-sorting based on the thermodynamic preference of NPs to assemble with those of the same handedness. The dispersions of homochiral self-assembled helices display broadband visible and near-infrared (Vis-NIR) polarization rotation with anisotropy (g) factors approaching 0.01. Calculated circular dichroism (CD) spectra accurately reproduced experimental CD spectra and gave experimentally validated spectral predictions for different geometrical parameters enabling de novo design of chiroptical semiconductor materials. Unlike metallic, ceramic, and polymeric helices that serve predominantly as scatterers, chiroptical properties of semiconductor helices have nearly equal contribution of light absorption and scattering, which is essential for device-oriented, field-driven light modulation. Deconstruction of a helix into a series of nanorods provides a simple model for the light-matter interaction and chiroptical activity of helices. This study creates a framework for further development of polarization-based optics toward biomedical applications, telecommunications, and hyperspectral imaging.

Keywords: Nanoparticle; cadmium telluride; chiral; circular dichroism; cysteine; enantiomeric excess; geometry; helix; self-assembly; semiconductor.