One of the main characterization tools in our lab is a scanning probe microscope with both scanning tunneling and atomic force microscopy capabilities. Here you can find a introductory description of how a scanning tunneling microscope and atomic force microscope operate. The scanning probe microscope allows us to probe nanostructure properties on many length scales from tens of microns down to the atomic level. Listed below are some current research topics:

Research Activities Overview

In the future, the capability to sense, communicate, and possibly even react, at the molecular level in electronic circuits will enable the creation of multifunctional devices and can also provide a greater understanding of how biological systems function. The integration of molecular systems with an electronic platform involves materials integration as well development of high throughput fabrication processes. The scientific projects pursued in the Ragan group at UCI involve the use of state of the art analytical techniques for fundamental understanding of assembly processes and how structure, interfaces and materials affect device performance and fundamental physical processes and  to understand signal transduction that occurs due to interactions between electronics systems and molecular systems; fabrication using self-assembly or directed self-assembly of biological/inorganic hybrid material systems that have immense potential for integration with manufacturing processes.

Scanning Probe Microscopy Studies of Metallic Nanostructures

As dimensions in electronic devices scale toward atomic dimensions it is necessary to understand how electrical properties evolve with size and when quantum size effects (QSE) become significant.  Theoretical studies provide insight into predicted behavior at the nanoscale, yet experimental realization of electrical properties as a function of nanoscale size is lacking due to difficulties of fabricating well-defined metallic structures on length scales approaching the Fermi wavelength. Rare earth disilicide nanostructures are a model system to understand electronic properties in quantized metallic structures since disilicide nanostructures are fabricated via self-assembly in ultrahigh vacuum conditions, thus nanometer feature sizes are achievable and the pristine surface can be probed in situ using both scanning tunneling microscopy and Kelvin probe force microscopy in order to correlate morphology and electronic properties. Furthermore, rare earth disilicides  nanostructure morphology can be controlled via growth conditions and thereby electronic properties can be measured as a function of feature size and film thickness. The surface electronic properties of a metal, especially the work function, are important factors in influencing functional properties such as catalytic activity in heterogeneous catalysis.

Self-Assembly of Nanoparticle Arrays on Block Copolymers

Metallic nanoparticle arrays are useful for the development of biosensors with single molecule detection. We use self-assembly techniques with the formation of thermodynamically stable copolymer templates and electrochemically stable spherical metal nanoparticles to form patterned nanoparticle arrays, in order to form commercially viable lab-on-chip molecular biosensors using cost efficient bottom-up self-assembly techniques. We have fabricated closely spaced noble metal nanostructures with surface enhanced Raman scattering (SERS) measuring up to 10⁹ signal enhancement and reproducible signal strength across the sample surfaces.

SERS Enhancement Factor (EF) measurements for benzenthiol on nanoparticle arrays, demonstrating increased signal with nanoparticle clustering.

Ab initio Calculations of Bimetallic Nanostructures

Ab initio calculations using a commercial software package are performed to gain insight into structural and electronic properties of bimetallic core shell nanostructures. 

Ultrahigh Vaccum Instrumentation

We use an ultrahigh vacuum system (UHV) maintained below a trillionth of atmospheric pressure to attain a contaminant-free experimental environment. We fabricate and characterize our nanostructure samples with the electron beam metal evaporator and scanning probe microscopes in this UHV system. We have also recently added a new chamber that allows for controlled vapor phase deposition of organic molecules, expanding our capability to study catalysis and molecular electronics.

Molecular Level Study of Biomolecular Assemblies on Metal Surfaces

Tethered phospholipid bilayer membranes (tLBMs) are an extraordinary lab based platform as biological membrane models and as a physiological matrix for studying the structure and function of membrane proteins and receptors.  While artificial membranes have less complexity than cell membranes, the artificial membrane system has the ability to alter variables individually to dissect their effects in a controlled setting.  Moreover, by assembling tLBM on a metal surface, interactions between proteins and membrane surfaces can be measured electronically, which gives potential application as biosensors.

Schematic of tLBM assembled on Au electrode surface.