Date of Award

August 2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Nikolai Kouklin

Committee Members

Nikolai NK Kouklin, Chiu CL Law, Paul PL Lyman, Marvin MS Schofield, Devendra DM Misra

Keywords

2D γ-alumina, ALD, Luminescent defects, PL, Polaron, Quasi-2D material

Abstract

This study presents a facile high-yield bottom-up fabrication, morphology, crystallographic and optoelectronic characterization of free-standing quasi-2D γ-alumina, a non van der Waals 2D material. The synthesis comprises a multi-cycle atomic layer deposition (ALD) of amorphous alumina on a porous interconnected graphene foam as a growth scaffold and removed next by annealing and sintering the alumina/graphene/alumina sandwich at ~ 800 °C in air . The crystallographic and structural characteristics of the formed non-van der Waals quasi 2D γ-alumina were studied by X-ray diffraction (XRD), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). This analysis revealed the synthesized 2D nanosheet network to be polycrystalline cubic spinel γ-alumina with a close packed cubic texture in the <111> ALD induced growth direction. The 2D flakes observed using SEM and TEM, analyzed using a log ratio method, are shown to be comprised of sintered 2D nanosheets with a nominal thickness of the order of 10 nm as limited by the number of atomic layer deposition cycles. The optoelectronic properties of the 2D γ-alumina were probed by carrying out temperature dependent DC- charge transport conductivity, and photoluminescence (PL) measurements, wavelength dependent PL, intensity dependent PL, and photoconduction measurements done on engineered planar thin films and 3D-interconnected device prototypes made out of quasi-2D γ-alumina. The charge transport mechanism of the 2D-γ-alumina network was investigated by temperature dependent conductivity and wavelength dependent photoconduction measurements, and the charge transport was confirmed to be polaronic in nature, yet the analysis points to two independent activation energies and underlying mechanisms: a) thermally assisted hopping and b) quantum mechanical tunneling that dominates the low temperature range of T<303 K. At T>303 K the bound polarons are found to regulate the charge transport with the activation energy of ~1.5 eV. Furthermore, comparison and analysis of the fitting results for various charge transport models showed Arrhenius, Adiabatic Small Polaron Hopping (SPH), non-Adiabatic SPH, and Efros-Shklovskii Variable Range Hopping (ES-VRH) to be the charge transport mechanisms in 2D γ-alumina with hopping energies and hopping radiuses within the same order of magnitude within experimental error. The room temperature conductivity of the network, estimated to be ~23 nS/m, exceeds by up to four orders of magnitude previously reported for ~99.5% pure α-alumina. Bandgap energy reduction, formation of color centers, cation vacancies, and the presence of texture are believed to play a key role behind enhanced electrical conductivity of 2D γ-alumina. Furthermore, the results of the photocurrent measurements point to the intrinsic defects, i.e. color centers F, F2+, Ali+ , and cation vacancies, i.e. V centers, acting as trap and release centers of the charge carriers. Intensity dependent photocurrent measurements show that under excitation energy of 4.1 eV (300 nm), the current drops below the dark current, where a linear trend is observed in the reduction of the photoconduction as the excitation intensity increases. These results show that 2D γ-alumina is a promising material for application in photodetectors, optical gates, information storage, advanced photocatalysts, future radiation hard thermally stable light emitters, and nano-optoelectronic devices based on the low cost crystalline quasi-2D γ-alumina synthesized by the facile graphene assisted ALD synthesis.

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