Single nanocrystal spectroscopy of near and shortwave infrared emitters

Abstract

Optimizing material systems for their translation to the marketplace relies on a complete understanding of the underlying fundamental physical mechanisms governing the observed properties of the material. In semiconducting nanocrystals, coordinating the physical properties of the material with the synthetic procedures used to manipulate these properties has led to the successful proliferation of these systems throughout the display industry. Most of the high-profile applications of nanocrystals rely on the quality of these materials as emitters of light. Until recently, all of the applications have been limited to emitters of visible light due to ubiquitous silicon detector technology. If we look at longer wavelengths of light such as the near infrared and shortwave infrared (NIR and SWIR) we move toward regions that were historically exploited by the military and as such, all associated technology was heavily regulated.

Relatively recent deregulation of SWIR detector technology has opened up these technologies to academic researchers and commercial industries. This deregulation has been accompanied by significant advancements in both the detector technology and also the development and discovery of new materials that are active in this wavelength regime. Much of the success that nanocrystals have found in industry has relied on the understanding of the physical mechanisms that lead to an emission event. As spectroscopists and physical chemists we take snapshots of physical properties and we systematically manipulate our materials to develop a basic understanding of how the energy is transported and transformed in our material.

As we push further into the infrared, we are working with materials that are highly unoptimized and underdeveloped but which have incredible potential as material systems for in vivo high resolution biological imaging and single emitters for optical and secure quantum communications. Indium Arsenide (InAs) has long been exploited in the epitaxial quantum dot community for its emission throughout the SWIR and critically at the wavelengths where optical communication occurs. Currently, this is the leading technology for quantum-dot-based single photon and entangled photon emitters. These systems suffer, however, due to their difficult and heterogeneous fabrication procedures. Colloidal InAs has recently been synthesized with high quantum yields and tunability throughout the SWIR. In this thesis we explore some of the fundamental emission mechanisms that occur in colloidal InAs NCs. Colloidal NC synthesis aims for a homogeneous sample of emitters.

While InAs is far off from this goal, with new and sensitive SWIR single photon detector technology, we can study InAs at the single NC level to unravel some of the fundamental physical properties determining emission in this material. We find, strikingly, that while the ensemble properties of this material may be far from ideal, the single InAs NC properties approach some of the best visible-emitter systems that we have. This suggests that there is a path forward for implementation of these materials in high-profile applications. In this thesis I have translated a technique known as solution photon correlation Fourier spectroscopy to study the emission mechanisms in infrared emissive materials. I explore first lead sulfide quantum dots emissive in the NIR and conclude that the emission is mediated by multiple emissive states. I then translate this technique to the SWIR, overcoming several technical challenges with microscopy at longer wavelengths.

Finally I use this new technique to study InAs QDs at the single nanocrystal level. I conclude that single SWIR emissive InAs QDs have narrow emission linewidths but significant broadening due to heterogeneities at the ensemble level. While this is by no means a complete picture of the emission mechanisms in these materials, it is a demonstration of the types of experiments and the current technological capabilities available to us to understand these materials. At the end, I provide some suggestions and preliminary work to push our understanding even further.