CAREER: Understanding the Formation Mechanism of Binary SAMs to Create an Experimental Phase Diagram

In this project, funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Dr. L. Gaby Avila-Bront of the College of the Holy Cross is using a microscope capable of visualizing individual molecules to understand the fundamental driving forces that determine how a mixture of different molecules bound to a surface forms a two-dimensional pattern.

Although surfaces modified with molecular patterns find many applications including serving as chemical and physical sensors, it is currently not possible to precisely control the composition of a molecular pattern composed of dissimilar molecules on a surface. Furthermore, prior to mixing, there is no knowledge of what two-dimensional patterns may form, or what new properties of the surface will be observed.

Even though the behavior of mixtures has been thoroughly investigated in three-dimensions, the picture is not complete because current treatments do not properly consider chemical processes that occur on surfaces. This presents a challenge for understanding how molecules mix on surfaces, and predicting the pattern of a mixture of molecules on a surface.

Successfully addressing these issues could provide scientists with unprecedented top-down control over surface modification with molecular patterning. All of the research in this lab is conducted and driven by undergraduate students. These students will conduct the experiments, analyze and interpret the data, and will be encouraged to present their findings at scientific conferences, while working closely with Dr.

Avila-Bront to produce manuscripts for publication. In doing so, the students are developing and honing their problem-solving skills so that they can transfer these skills in the post-graduate fields of their choice.

The overall goal of this work is to understand the mechanism of two-dimensional mixing processes and construct the experimental phase diagram of two-component (binary) self-assembled monolayers (SAMs). An experimental phase diagram of a binary SAM would enable the enrichment or separation of two-dimensional materials based on thermodynamic conditions, as well as the design of surfaces with deliberate preformed patterns.

SAMs of organothiol compounds on noble metal surfaces are model two-dimensional systems that have been extensively characterized and utilized in numerous applications. However, as SAM functionalities have been extended, central questions about the mechanism of SAM phase behavior remain unanswered. An experimental phase diagram for a binary SAM has never been reported because it is very difficult to control the composition of a binary SAM.

Therefore, this project begins by creating a library of binary monolayers composed of aliphatic and aromatic organothiols on the (111) surface of gold. The structure of the SAM is to be molecularly resolved using STM (Scanning Tunneling Microscopy) in ambient conditions, and contact angle measurements will be made to quantify the ordering properties of the SAM.

Once this library is established, a model binary monolayer of known composition will be constructed. The composition of the monolayer will be interrogated using X-ray photoelectron spectroscopy and reductive desorption. An experimental phase diagram will then be constructed by correlating the phases present in this model binary monolayer to the chemical composition of the monolayer at different temperatures.

The accuracy of the resulting phase diagram will be tested by using it to predict the monolayer structure of novel SAMs. The broader impacts of this work will include the training and mentoring of undergraduate students in chemical research. In addition, Dr.

Avila-Bront is establishing a community outreach program and expanding remote outreach to elementary schools--including a program where undergraduate students work with elementary students on science-fair projects-- with virtual workshops aimed at encouraging critical assessment of science in the popular media.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.