Robin Nicholas

Abstract:

We report process improvements for the fabrication of single-walled carbon nanotube ethylene-vinyl acetate transparent conductive films. CNT:EVA films demonstrate high resilience against folding and can replace the external dopant in a spiro-OMeTAD based hole selective contact of n-i-p perovskite solar cells achieving a steady-state efficiency of 16.3%. The adapted process is fully scalable, and compared to previous reports (Mazzotta et al., 2018) lowers the material cost dramatically and improves DC to optical conductivity ratio by two orders of magnitude to σdc/σop = 3.6 for pristine and σdc/σop = 15 for chemically doped films. We analyse the microstructure of our films via small angle neutron scattering and find a positive correlation between the long range packing density of the CNT:EVA films and the σdc/σop performance. Increasing monomer ratio and chain length of the EVA polymer improves resilience against bending strain, whereas no significant effect on the CNT wrapping and electrical conductivity of resulting films is found.

Abstract:

Confining electric fields to a nanoscale region is challenging yet crucial for applications such as high resolution probing of electrical properties of materials and electric-field manipulation of nanoparticles. State-of-the-art techniques involving atomic force microscopy typically have a lateral resolution limit of tens of nanometers due to limitations in the probe geometry and stray electric fields that extend over space. Engineering the probes is the most direct approach to improving this resolution limit. However, current methods to fabricate high-resolution probes, which can effectively confine the electric fields laterally involve expensive and sophisticated probe manipulation, which has limited the use of this approach. Here, we demonstrate that nanoscale phase switching of configurable thin films on probes can result in high-resolution electrical probes. These configurable coatings can be both germanium-antimony-tellurium (GST) as well as amorphous-carbon, materials known to undergo electric field-induced non-volatile, yet reversible switching. By forming a localized conductive filament through phase transition, we demonstrate a spatial resolution of electrical field beyond the geometrical limitations of commercial platinum probes (i.e. an improvement of ~48%). We then utilize these confined electric fields to manipulate nanoparticles with single nanoparticle precision via dielectrophoresis. Our results advance the field of nanomanufacturing and metrology with direct applications for pick and place assembly at the nanoscale.

Abstract:

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ~200μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure observed in perovskite nanocrystals.

Abstract:

Double perovskite crystals such as Cs₂AgBiBr₆ are expected to overcome the limitation of classic hybrid organic–inorganic perovskite crystals related to the presence of lead and the lack of structural stability. Perovskites are ionic crystals in which the carriers are expected to strongly couple to lattice vibrations. In this work we demonstrate that the photoluminescence (PL) emission in Cs₂AgBiBr₆ is strongly influenced by the strong electron–phonon coupling. Combining photoluminescence excitation (PLE) and Raman spectroscopy we show that the PL emission is related to a color center rather than a band-to-band transition. The broadening and the Stokes shift of the PL emission from Cs₂AgBiBr₆ is well explained using a Franck–Condon model with a Huang–Rhys factor of S = 11.7 indicating a strong electron–phonon interaction in this material.