Modifying surface properties using carbon nanomaterials [electronic resource]

Surfaces play an important role in a wide variety of applications and controlling surface properties like work function using novel materials is becoming ubiquitous in many electron emission and electron transfer applications. Many interesting materials have been used for this purpose most notably alkali metals, diamond and carbon based alkane monolayers. However with most of these materials at least one of chemical versatility, ease of functionalization to induce specific properties and air stability is an issue precluding them from practical applications. In this thesis we explore a new class of nanocarbon based materials called the diamondoids, a highly chemically versatile, both diamond and alkane like materials for potential applications in electron emission devices. Firstly we show that diamondoid monolayers act as extreme work function lowering coatings in field emission by reducing the work function of gold by as much as 3.31 eV, highest ever reduction achieved using organic monolayer coatings. Based on Ultraviolet Photoelectron Spectroscopy measurements and the hotspot dominated nature of field emission, we hypothesize that this effect results from formation of positively charged diamondoid radical cations. Secondly, we develop a simplified and computationally less expensive technique to model this diamondoid cation based work function lowering. We use Density Functional Theory in conjunction with transfer-matrix methodology and image charge approximation to show how this cation based process leads to a deviation from the well known Fowler Nordheim theory based triangular barrier approximation for field emission. We show that although this effect doesn't lead to change in the actual work function, it increases the electron tunneling probability which manifests as an "effective work function" lowering in field emission measurements. Calculations based on this theory are in excellent agreement with the field emission measurements performed using tetramantane-thiol monolayers on gold. Thirdly, we report on using atomic layer thick graphene as electron transparent, monolayer protecting coatings in diamondoid electron photoemission. Our photoemission experiments on graphene covered diamondoid photo-emitters show that graphene coatings preserve the monochromatic electron emission property of diamondoids, are electron transparent, slow down the monolayer degradation process and improve their lifetime significantly. This is an important step forward in developing practical techniques to improve lifetime of organic monolayer coatings in a multitude of applications. Finally, we explore atomic layer deposition (ALD) based aluminum oxide coatings on organic self-assembled monolayers for making molecular electronic devices. We study atomic layer deposition on co-adsorbed self-assembled monolayers, a model system for a general hydrophobic surface. Our study shows that the ALD based technique, typically only suitable to hydrophilic surfaces, can be extended to fabricate molecular electronic devices using molecular monolayers with water contact angles as high as 80 degrees.

Keywords: Materials Science and Engineering.
Advisor: Melosh Nicholas A. primary advisor. Thesis advisor
Salleo Alberto
Shen Zhi-Xun advisor. Thesis advisor