Zahra Fakhraai
Assistant Professor

Overview

Physical Chemistry, Materials Chemistry, Nanoscale Science and Engineering

Research

Our group is interested to study structure, dynamics and chemistry in nanometer length scales. Materials constrained in one or two dimensions can have different structural or dynamical properties than those of the bulk. A proper understanding of such differences is important in many technological applications where materials are constrained in nanometer size spaces, such as organic electronics, polymer applications and drug delivery. In biological systems, most of the dynamics happens in nanometer size proximity of surfaces and interfaces, and understanding the properties in confinement is key in predicting function. We focus our efforts on understanding the origins of such modified properties on a fundamental level as well as possible application of such phenomena in producing novel materials.

 

Specifically we look at properties of polymers and glassy materials in thin films and other confined geometries, properties of small aggregates of biological molecules, single molecule force spectroscopy and near-field enhancement of light close to nanostructures.

 

Single Molecule Force and Raman Spectroscopy:

 

We are interested to combine single molecule force spectroscopy with tip enhanced Raman spectroscopy to study structure and phase changes in single molecules. In single molecule force spectroscopy the molecule is confined between and atomic force microscopy (AFM) tip and a substrate and is pulled out of its equilibrium conformation by force applied to the AFM tip. The force drags the molecule up in the energy landscape and opens up the structure. Phase transitions in the molecule are observed as signatures in the force-elongation plots. The conformation of the molecule can strongly affect its functional properties. Studies can be carried out in various buffers and chemical conditions to learn details about molecular conformation.

 

Tipe enhanced Raman spectroscopy (TERS) utilizes surface plasmons to focus light in sub-diffraction limit and allows chemical imaging with nanometer resolution. TERS can be combined with force spectroscopy to obtain simultaneous structural and chemical. In order to achieve enhancement beyond the usual sensitivity of a TERS setup that is only a few nanometers, we take advantage of nanostructured substrates.

 

Enhanced Diffusion on the Surface of Glasses: 

 

It has been observed that a layer a few nanometers in thickness near the surface of an organic or polymeric glasses behave more like a liquid than a solid. The origins and properties of this mobile layer are not very well known. Various techniques can be used to investigate the properties of this liquid like layer. We use nanoparticle probes as well as other experimental techniques to study the temperature dependence of this enhanced surface mobility and understand the physical and chemical origins of this phenomenon.

 

When a nanoparticle is placed on the surface a meniscus forms around the nanoparticle that indicates the motion of molecules at temperatures well below the glass transition temperature, where the bulk is rigid. At longer times, or higher temperatures the nanoparticle sinks into the glass, to respond to viscoelastic relaxation of the bulk. These two distinct processes can be used to independently measure the surface and the bulk response of the material.

 

We are also using TERS to measure the diffusion of labeled polystyrene molecules patterned on the surface of polystyrene thin films to directly visualize the surface diffusion.

 

Stable Glasses: 

 

It has recently been shown that physical vapor deposition (PVD) can be used to produce amorphous materials with exceptional properties. Compared to glasses produced by cooling, PVD generates glasses with higher density, lower enthalpy and other interesting properties. For example glasses produced by PDV show structural anisotropy that is evident in measurements of index of refraction. We use various deposition and characterization techniques to understand the structure and properties of these glasses on a fundamental level. For example use ellipsometry to measure the optical anisotropy and density variations in these glasses and atomic force microscopy to study the early stage of film growth.

 

 

The reason that glasses with such interesting properties can be produced is the existence of the enhanced surface mobility at the surface of the glass. By slow deposition from the vapor phase one can take advantage of the enhanced surface mobility to drive the system towards a better equilibrium on much shorter time scales that are needed for the bulk material.

Optics in Nano-Scale:

We are interested to study non-linear optical properties of nano-structured materials, both to improve our TERS setup by utilizing various tip and substrate geometries and also to fundamentally study ways that light can be trapped and manipulated by nano-structures. This will enable prediction of properties of nano-particles, nano-structured surfaces and Raman substrates. We use Lumerical software to perform Finite-difference time-domain (FDTD) calculations, that solves Maxwell's equations in 3D to visualize how light is trapped by complicated nanostructures such as this core-sell nanoparticle.

Education

  • B.Sc. Physics, Sharif University of Technology, Iran 1999
  • M. Sc. Physics, Sharif University of Technology, Iran 2001
  • Ph.D. Physics, University of Waterloo, 2007
  • Post-Doctoral associate, Chemistry, University of Toronto, 2007-08.
  • NSERC Post-Doctoral Fellow, Chemistry, University of Wisconsin-Madison, 2009-11.

Selected Publications

 

1. Z. Fakhraai, S. Valadkhan, J. A. Forrest, "Qualitative Discrepancy between Different Measures of Dynamics in Thin Polymer Films", Eur. Phys. J. E. 18, 143-148 (2005).

 

2. Z. Fakhraai, J. A. Forrest, "Probing Slow Dynamics in Supported Thin Polymer Films", Phys. Rev. Lett. 95, 025701 (2005).

 

3. D. Qi, Z. Fakhraai, and J.A. Forrest, “Substrate and Chain Size Dependence of Near Surface Dynamics of Glassy Polymers”, Phys. Rev. Lett. 101, 096101 (2008). 

 

4. K. Mueller, X. Yang, M. Paulite, Z. Fakhraai, N. Gunari and G. C. Walker, „Chemical Imaging of the Surface of Self-Assembled Polystyrene-b-Poly (methyl methacrylate) Diblock Copolymer Films Using Apertureless-Nearfield IR Microscopy”, Langmuir, 24, 6946 (2008).

 

5. Fakhraai, J. A. Forrest, "Measuring the Surface Dynamics of Glassy Polymers", Science, 319, 600 (2008). 

 

6. M. J. Kofke, D. H. Waldeck, Z. Fakhraai, S. Ip, and G. C. Walker, “The Effect of Periodicity on the Extraordinary Optical Transmission of Annular Aperture Arrays”, Appl. Phys. Lett. 94, 023104 (2009).

 

7. M. Paulite, Z. Fakhraai, N. Gunari, A. Tanur, and G. C. Walker, “Imaging Secondary Struc-ture of Individual Amyloid Fibrils of a beta(2)-Microglobulin Fragment Using Near-Field Infrared Spectroscopy ”, J. Am. Chem. Soc. 133, 7376- 7383(2011).

 

8. Z. Fakhraai, T. Still, G. Fytas and M. D. Ediger, “Structural Variations of an Organic Glassformer Vapor-Deposited onto a Temperature Gradient Stage”, J. Phys. Chem. Lett. 2, 423–427 (2011). 

 

9. C. R. Daley, Z. Fakhraai, M. D. Ediger, and J. A. Forrest, “Comparing Surface and Bulk Flow of a Molecular Glass Former ”,Soft Matter, 8, 2206-2212(2012).

 

10. B. L. Sanchez-Gaytan, P. Zwanglap, T. J. Lamkin, Z. Fakhraai, S. Link, and S. J. Park, “Syn-thesis and Optical Properties of Spiky Gold Nanoshells”, J. Phys. Chem. C, (Accepted).

Department of Chemistry

231 S. 34 Street, Philadelphia, PA 19104-6323

215.898.8317 voice | 215.573.2112 fax | web@chem.upenn.edu