Date of Award

6-30-2016

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Scott R Crittenden

Abstract

The investigation of magnetic properties of thin films whose surfaces were modified by organic molecules shows that the addition of any functional group to the surface measurably changes the magnetic properties. The effect often scales with ligand strength and is not limited to surfaces with ferromagnetic properties. Improving upon the technique developed by Knaus et al. [1], a stable and sensitive device was developed to measure this magnetic effect using the planar Hall effect (PHE) in order to shed light onto controversial questions of purported paramagnetism of thiolated gold. There is a measureable non-diamagnetic response of thin gold layers when exposed to alkanethiols as well as other ligands such as alcohols, carboxylic acids, amines, and ketones.

This approach is not susceptible to contamination by external magnetic materials, unlike many of the previous measurements of anomalous gold-thiol magnetism. Significant efforts were devoted to exclude magnetic contaminations and are reported in chapters 4 and 5. The developed analysis system, a magnetic resistance measurement system (MaRMS), is stable, highly sensitive, and flexible. Our results and initial theoretical models open a door to a wide variety of magneto-chemical surface phenomena.

In the last few decades self-assembled monolayers (SAMs) on metal substrates have attracted attention for their ability to modify the physical properties of metal substrates [2, 3], for instance, the conductivity. The conductivity of gold is known to decrease significantly after the formation of SAMs on its surface. Recently, it has been suggested that the bulk diamagnetism of gold changed to paramagnetism after similar treatment [2, 3]. The measured values were extremely large and often ascribed to the susceptibility of non-diamagnetic contamination. The experimental techniques employed are very susceptible to contamination and hence it is difficult to rule it out. Therefore, a series of experiments not susceptible to contamination were performed and it was determined that the observed effects were due to magnetic changes in the gold or to systematic errors in the previous measurements. As presented in this dissertation, it was found that a change in magnetic properties of gold is indeed observable when the surface is modified via metal-organic bonds. In addition, it was demonstrated that a large number of different functional groups can cause this effect and that it is not exclusive to thiol ligands. Furthermore, the modification of magnetic properties for other metals was also shown.

The capability to make well characterized metal thin films has been developed, and the effect of a variety of organic surface modifications on the magnetic properties of metal thin films has been investigated. For these investigations a magnetic transport measurement technique (MTM) was used and a custom built instrument was developed and built.

First, measurements on cobalt, nickel, and Permalloy thin films will be discussed. The magnetic properties of these thin films were modified using the following molecules: ethanol, hexylamine, acetone, deionized water, 1-dodecylthiol, acetic acid, and hexane. The change in magnetism was recorded by measuring the planar Hall voltage of the sample.

For cobalt thin films in particular the reach of the PHE through the sample was determined by measuring thin films of varying thickness. It was demonstrated that the change of magnetic properties appears to reach much deeper into the metal sample than previously expected. An increasing effect was observed for up to 30 nm thickness of the thin film.

Also, the influence of the tail length was measured by applying different alcohols, ranging from methanol to hexanol. While methanol surprisingly had almost no effect on the magnetic properties, the longer alcohols showed a decreasing effect with increasing chain length. This shows that the choice of ligand for the surface modification cannot just be dictated by the head functional group, but one must consider the chain length as well.

Next, the change in magnetism of gold thin films and how it compares to other diamagnetic thin films, such as silver and copper, will be discussed. Induced magnetic moments in gold thin films and nanoparticles have been a popular topic in recent literature as will be discussed in chapter 3. Some of the reported magnetic moments for gold are extremely large, even when compared to the magnetic moment of iron. Although determining a quantitative value for the developed magnetic moment in gold is not possible with this technique, it was definitively determined that gold can be manipulated into changing the size of its magnetic moment and developing a measureable non-diamagnetic magnetization. It was found that not only molecules with a thiol functionality have an effect on the magnetism of gold thin films, but that other ligands can also cause a change in magnetism. This was demonstrated by using the same series of small organic molecules as used previously on ferromagnetic thin films. This showed that surface-modified magnetism is more common than previously thought and by using less expensive materials and simpler measurements it can become an attractive method for inexpensive, yet widely applicable sensors in the medical field as well as novel digital storage devices.

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