Date of Award

Summer 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering - (Ph.D.)

Department

Electrical and Computer Engineering

First Advisor

Grebel, Haim

Second Advisor

Tsybeskov, Leonid

Third Advisor

Sosnowski, Marek

Fourth Advisor

Rajendran, Bipin

Fifth Advisor

Federici, John Francis

Abstract

Previous research has elucidated the remarkable electrical and optical characteristics of graphene and pointed to the various applications of graphene-based devices. One of such applications is electro-optical graphene-based elements. In this work, the optoelectronic properties of field-effect transistors are explored. These are composed of surface graphene guides, which are interfaced with an array of individual semiconductor quantum dots. The graphene guide also serves as a channel for the field-effect transistor (FET) while the dots provide for fluorescence markers. They may be placed either within the capacitor formed between the graphene and the gate electrode, or on top of the graphene. Electrical characteristics under white light illumination and the device’s photoluminescence (PL) properties at various biasing conditions are studied.

The graphene's channel conductivity as a function of gate bias and drain-source bias under illumination are obtained. A minimum in source-drain current signifies the Dirac point. Under a low intensity of white light, the photocurrent changes signs as a function of gate bias, which suggests that the photocurrent may have originated from the graphene channel rather than the QDs. Negative differential photo-conductance is observed under illumination at large negative gate voltages. Changes in the fluorescence are noted as a function of both the drain-source and gate-source potentials. The fluorescence is more pronounced when the incident or the emission wavelengths are coupled to surface modes.

Luminescence lifetimes and linewidths from an array of individual quantum dots (QDs) that are either interfaced with graphene surface guides or dispersed on

aluminum electrodes are studied. The observed fluorescence quenching is consistent with screening by charge carriers. Fluorescence quenching is typically mentioned as a sign that chromophores are interfaced with a conductive surface (metal or graphene). The QDs interfaced with the metal film indeed exhibits shorter lifetime and line-broadening compared to QDs on a dielectric substrates but not necessarily fluorescence quenching; the latter may be impacted by molecular concentration, reflectivity considerations and conductor imperfections.

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