USING TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETRY TO INVESTIGATE BEHAVIOR OF VAPOR-DEPOSITED METAL ON ALKANETHIOL SELF-ASSEMBLED MONOLAYERS
Open Access
- Author:
- Zhu, Zihua
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 12, 2005
- Committee Members:
- Nicholas Winograd, Committee Chair/Co-Chair
David Lawrence Allara, Committee Member
Thomas E Mallouk, Committee Member
Carlo G Pantano, Committee Member - Keywords:
- ToF-SIMS
self-assembled monolayer
vapor metal deposition
surface
metal behavior
molecular electronics - Abstract:
- Metal behavior on alkanethiol self-assembled monolayers (SAMs) has been studied for more than ten years. In the beginning, this system was regarded as an idealized model for understanding the metal/polymer interfaces. In recent years, molecular electronics has developed rapidly. Electronic properties of these devices using a number of functional molecules have been characterized using metal substrate/organic thin film/metal top contact sandwich structures. Therefore, the study of metal/SAM interfaces is of great importance. Our group has employed time-of-flight secondary ion mass spectrometry (ToF-SIMS) to characterize the SAM/top metal contact for many years, and it has proven to be a very powerful technique. Analysis of the mass spectra yields direct information about chemical reactions, metal penetration, and metal nucleation. In this thesis, ToF-SIMS is applied to study the behavior of K, Au and Ti atoms on several alkanethiol SAMs. Au and Ti are the most commonly used contact metals in molecular electronic studies. The structure of the SAM/Ti interfaces and SAM/Au interfaces are important for measurements of electronic properties of functional molecules. K is an active metal and may be a potential reducing reagent to modify the surface of alkanethiol SAMs. It is also of interest to compare the difference between the chemical activity of K atoms on organic surfaces to that found in organic solvents. In Chapter 2, the K behavior on C15CH3 (-S(CH2)15CH3), C15CO2H, C15CO2CH3, C16OH, and C16OCH3 SAMs is investigated. It is found that K atoms react with -CO2H, -CO2CH3 and OH groups, forming -CO2K or OK. The K atoms only form complexes with OCH3 groups, whereas no chemical reaction is observed on the -CH3 surface. The activity of these organic functional groups can be ranked as CO2H > CO2CH3 > OH > OCH3 > CH3. Most K atoms remain at the vacuum interface with very weak penetration through the organic layer in the CO2CH3, CO2H, OH and OCH3 systems. At the same time, no chemical interaction between K and the (CH2)n chain is found, indicating that K dose not alter the basic structure of alkanethiol SAMs. In Chapter 3, the relationship between metal penetration and inter-molecular interactions is studied via the Au/SAM system. It is found that weak inter-molecular interactions lead to high penetration, and strong inter-molecular interactions lead to low penetration. For example, Au atoms continuously penetrate through C15CH3 and C15CO2CH3 films, forming smooth buried layers under organic thin films. Au atoms penetrate through the CO2H film, but form clusters due to H-bonding between CO2H groups. The K modified CO2CH3 or CO2H films are found to prevent Au atoms from leaking through the film. Apparently, the ionic interactions between adjacent film molecules block Au penetration while van der Waals forces and H-bonds do not. Thus, metal penetration can be controlled by adjusting inter-molecular interactions. In Chapter 4, SAM/Ti top contact interfaces are investigated. Ti/organic functional group interactions can be ranked as Ti/CO2H > Ti/CO2CH3 > Ti/CH3. Vapor deposited Ti atoms are so active that they damage all surface organic functional groups and the (CH2)n chain. At the same time, penetration of Ti atoms through the SAMs is weak. The Ti growth on active surfaces prefers a layer-by-layer mode, but the Ti growth on inactive surfaces prefers an island-enlargement process. The temperature effect on metal penetration through the SAMs is studied in Chapter 5. Decreasing sample temperature is found to reduce metal penetration. However, at low temperature, H2O molecules can be condensed on the sample surface. The effect of H2O molecules on the structure of metal overlayers is not clear. In this thesis, ToF-SIMS shows its unique ability to acquire information about chemical reactions between metal atoms and surface organic functional groups, penetration of metal atoms through the SAMs, growth modes of metal overlayers on top of the SAMs, and damage to organic molecules. All of these processes which are crucial for understanding the structure of SAM/top metal contact interfaces. It is found that the appearance of new characteristic peaks and the disappearance of initial peaks may indicate chemical reactions or damage of organic molecules. The relationship between metal dose and intensity of surface organic functional group-related peaks may suggest information about penetration or cluster-formation. At the same time, metal overlayers on the SAMs affect the intensities of SIMS signals. Removing the metal overlayers by chemical etching and then characterizing samples again is a complementary method that can supply useful information.