Research in Ragan Group

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.

 

Characterization of 3D Graphene Porous Structures 

Adrian E. Garcia, Chen Wang, Robert Sanderson

In this collaborative effort, we attempt to combine the naturally extraordinary properties of graphene with a 3-dimensional structure made through a novel method involving the derivation of a sacrificial solid from a fluid-fluid-colloid mixture. This essentially allows us to "wrap" layers of defect-free graphene around a structure with uniform open-pore morphology. These 3D graphene-based structures reduce the tortuousity typically associated with mass transport in other 3D graphene structures, which are typically made of self-assembled reduced graphene oxide or metal foams with random pathways. 

 

Graphene Bijel Schematic
Schematic of the process we take to create our freestanding 3D graphene bijels.

SEM images of the bijel-templated sample (a,d) at Ni deposition (b,e) after graphene growth, and (c,f) after Ni etch. Red circle in (a-c) tracks the same pore throughout the stages.

Self-Assembly of Nanoparticle Arrays

Will Thrift

Nanoparticle arrays are useful in current development of molecular biosensors with the capacity for single molecule detection. Field enhanced chemical and biological detection can be achieved through Surface Enhanced Raman Scattering (SERS) associated with closely spaced noble metal nanostructures, with which the surface plasmon resonance is observed as a dipole due to the collective electronegative nature with respect to the positive ionic background. This project aims to develop commercially viable lab-on-chip molecular biosensors using cost efficient bottom-up self-assembly techniques comparable to designs manufactured with costly lithographic methods at nanoscale dimensions. Cost efficient molecular biosensors would be of great use in diagnostic medical applications through SERS measurement for the presence of antibodies and other biological molecules as well as a method to identify the presence of contamination in agricultural materials and water sources. Furthermore this enhanced electrical field design, using the strong scattering from the interacting surface plasmons of metal nanoparticles in a patterned array, is also applicable to enhancement of photovoltaic technology, in which the dipole interactions increase the optical path of incident light in the absorber layers of photovoltaic solar cells.

Cuong Nguyen

Surface-enhanced Raman scattering (SERS) spectroscopy offers the ability to perform label-free biosensing near parts-per-trillion detection limits. Employing this technique can produce low-cost biological and chemical sensor with unprecedented performance. Specifically, this project seeks to study bacterial interactions in complex biological system using self-assembled SERS-active nanostructures. Integrating microfluidics enables longitudonal real-time studies that require low sampling volumes. We also utilize advanced data analysis algorithms from machine learning for identification and quantification of biomolecules.

Hong Wei

Hong is working in Dr. Regina Ragan’s lab to design and fabricate optical waveguides for integrated SERS biosensors. The optical nanoantennas are integrated with waveguides that provide an efficient excitation and collection mechanism, enabling more efficient SERS response and a less expensive SERS system.

Analysis of Metal Nanocatalysts on Modified Graphene Supports

Bobby Sanderson and Chen Wang

Our group uses a UHV system, manufactured by Omicron Nanotechnology, which houses a scanning tunneling microscope (STM) and a Kelvin probe force microscope (KPFM). It also includes a sample preparation chamber equipped with an electron-beam metal evaporator. An ionization pump, occasionally backed by a titanium sublimation pump, maintains the system at a pressure of about 10-11 torr. This research project seeks to investigate how a modified graphene support can change the electronic properties of metal nanoclusters. Under UHV conditions, 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. Additionally, we plan to characterize the electronic properties of noble metal nanostructures and their interaction with various adsorbates of organic molecules. 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.

 STM of CVD-grown graphene on an electroless nickel substrate.

These experiments are complimented with theoretical work, using a commercial ab initio software package known as VASP (Vienna ab initio Simulation Package) to perform desnity functional theory (DFT) calcuations so that STM simulations and electronic properties such as density of states and work function can be predicted. Using first principle methods, we currently simulate the optimal structure as well the electronic and chemical characateristics of supported metal nanoclusters. Such research has novel implications in either reducing the amount of expensive platinum necessary required to catalyze reactions or replacing the standard platinum catalyst with more earth-bundant and cost-efficient efficient substitutes. This project is in collaboration with Dr. Ruqian Wu from UCI Department of Physics and Astronomy.

 Comparison of charge density difference plots for various metals supported on pristine and single vacancy graphene.