Study of Optoelectronic Properties of Quantum Emitter Assemblies Coupled to Graphene, Photonic and Plasmonic Materials

Abstract

Nano-photonics deals with different ways to control light-matter interactions beyond the diffraction limit. In contrast to optics, which is the concept of light rays, including absorption, transmission, and reflection, photonics is the interaction of light, including emission, detection, amplification, and propagation. The optical properties of semiconductor nanocrystals have been intensively studied over the last couple of decades due to their fascinating quantum confinement effect. The size-dependent quantized electronic energy levels make it possible to control the energy gap and other semiconductors' properties. In this thesis work, we have used a combination of the quantum emitters with graphene, photonic and plasmonic materials that can lead to broad applications of optoelectronic devices with enhanced sensitivity. The first chapter introduces the fundamental aspects of confined quantum materials, mostly describing the underlying physics of light-matter interaction. The discussion starts with the different coupling regimes of confined materials. We then discuss the exciton formation of the quantum dot, its band structure, the gold nano particle's plasmonic properties, and its interaction with exciton in a hybrid structure. This is followed by the discussion on optical and electrical properties of 2D material graphene, describing unique band structure based on solving the Hamiltonian in reciprocal space. Then we introduce a new 2D emitter, termed nanoplatelets of automatically controlled thickness and its remarkable properties and band structure. Second chapter deals with the details of experimental methods and techniques that we have used for the experiments. It contains different synthesis procedures of Semiconductor quantum dot (CdSe QD), nanoplatelets (CdSe NPL), and Gold nanoparticles (AuNP) with different capping and exchange methods. Then we discuss the Langmuir-Blodgett technique for monolayer film preparation. We have used different characterization techniques for thin film of atomic force microscopy (AFM), transmission electron microscopy (TEM). This is followed by the discussion on optical microscopy techniques as confocal, time-resolved photoluminescence measurements, two-photon absorption spectroscopy, back focal plane imaging, and electrical measurements of scanning photocurrent microscopy. In the third chapter, we have studied different interaction regimes of emitters through their coupling to the sources of localized radiation, typically enabled by plasmons in ultrasmall metal nanoparticles with very small emitter-particle separations. Here we obtain large radiative enhancements of quantum dot assemblies with extremely small metal nanoparticles and emitter-particle separations R of a few nanometers, where the Purcell effect would lead to either no enhancements or quenching. We also discuss the theoretical approach to understand the enhancement of radiative emission and the emergence of strong coupling of semiconductor quantum dot (QD) assemblies with metal nanoparticles using steady-state and time-resolved photoluminescence. Chapter four describes the study of optical properties of semiconductor quantum dots (QD) and single-layer graphene (SLG) FET devices. The optical properties of the resultant hybrid, the material is controlled by the interplay of energy transfer between QDs and charge transfer between the QDs and SLG. By studying the steady-state and time-resolved photoluminescence spectroscopy of hybrid QD-SLG devices, we observe a subtle interplay of short and long-range energy transfer between cadmium selenide (CdSe) QDs in a compact monolayer solid film placed in close proximity to an SLG and the charge transfer from the QD solid to SLG. The relative strength of energy and charge transfer can also be controlled by relative separation and electrostatic doping. Chapter 5 focuses on the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene (SLG). We have performed scanning photocurrent microscopy measurements to understand the electric field distribution and spatial profiles. We demonstrate high IQE and large enhancement of minority carrier diffusion length (LD). While the IQE ranges from 10.2%− 18.2% depending on QD-graphene separation, ds, the carrier diffusion length, LD, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5−8 as compared to pristine graphene. IQE and LD could be tuned by varying both back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. In chapter 6, we have explored the photoluminescence spectroscopic properties of two-dimensional nanoplatelets. It shows remarkable optical properties due to extended ultrathin structure. Here, we have studied the optical properties of CdSe nanoplatelets with single-photon and two-photon absorption spectroscopy in the presence of strong electromagnetic field confinement with Distributed Bragg Reflector (DBR). The non-linear emission from the CdSe nanoplatelets on DBR has been observed with the two-photon assisted excitation process. The selective transition for simultaneous photon absorption allows higher bandgap opening and enables light hole-conduction band transition. In Chapter 7, We discuss the summary of the experimental results presented in this thesis and related possible future experiments.