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Mehdi M. Yazdanapanh, PhD

2210 S. Brook St. Rm 253 BRB

University of Louisville, 40292

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Phone: (502) 619-5156

Cell: (502) 807-1199

Mehdi M. Yazdanpanah is Research Faculty Scientist of Department of Electrical Engineering, University of Louisville (UofL) and CEO/President of NaugaNeedles LLC. He has received his PhD degree with Honor in July 2006 from UofL, where he studied selective self-assembly of nanostructures via gallium reaction with metal thin films. He also holds B.S. degree in Physics from Sharif University of Technology (1998) and M.S. degree in Physics from the Beheshti University, Iran (2001), where he designed a scanning tunneling microscope (STM). Currently at UofL, he is studying liquid surface probing, formation of bio-active nanofibers as well as mechanical and electrochemical properties of individual cells using atomic force microscope (AFM). Dr. Yazdanpanah has published several journal papers devoted to the fabrication of nanostructures. One of his most fascinating researches is invention the method of crystallizing freestanding metal alloy nanoneedles at selected locations and chosen orientations to an AFM probe at room temperature. Based on this invention, He has established NaugaNeedles LLC to develop a platform for AFM-based biomaterial-to-electronic interface.

Selected Publications:
  • Rheological Measurements by AFM of the Formation of Polymer Nanofibers
The micro-Wilhelmy method is a well-established method of determining surface tension by measuring the force of withdrawing a tens of microns to millimeters in diameter cylindrical wire or fiber from a liquid. A comparison of insertion force to retraction force can also be used to determine the contact angle with the fiber. Given the limited availability of atomic force microscope (AFM) probes that have long constant diameter tips, force-distance (F-D) curves using probes with standard tapered tips have been difficult to relate to surface tension. In this report, constant diameter metal alloy nanowires (referred to as “nanoneedles”) between 7.2 to 67 microns in length and 108 to 1,006 nm diameter were grown on AFM probes. F-D and Q damping AFM measurements of wetting and drag forces made with the probes were compared against standard macroscopic models of these forces on slender cylinders to estimate surface tension, contact angle, meniscus height, evaporation rate and viscosity. The surface tensions for several low molecular weight liquids that were measured with these probes were between -4.2 % and +8.3 % of standard reported values. Also, the F-D curves show well-defined stair-step events on insertion and retraction from partial wetting liquids, compared to the continuously growing attractive force of standard tapered AFM probe tips. In the AFM used, the stair-step feature in F-D curves was repeatably monitored for at least one half hour (depending on the volatility of the liquid) and this feature was then used to evaluate evaporation rates (as low as 0.30 nm/s) through changes in the surface height of the liquid. A nanoneedle with a step change in diameter at a known distance from its end produced two steps in the F-D curve from which the meniscus height was determined. The step features enable meniscus height to be determined from distance between the steps, as an alternative to calculating the height corresponding to the AFM measured values of surface tension and contact angle. All but one of the eight measurements agreed to within 13 %. The constant diameter of the nanoneedle also is used to relate viscous damping of the vibrating cantilever to a macroscopic model of Stokes drag on a long cylinder. Expected increases in drag force with insertion depth and viscosity are observed for several glycerol-water solutions. However, an additional damping term (associated with drag of the meniscus on the sidewalls of the nanoneedle) limits the sensitivity of the measurement of drag force for low viscosity solutions, while low values of Q limit the sensitivity for high viscosity solutions. Overall, reasonable correspondence is found between the macroscopic models and the measurements with the nanoneedle-tipped probes. Tighter environmental control of the AFM and treatments of needles to give them more ideal surfaces are expected to improve repeatability and make subtle features more evident, that currently appear to be present on the F-D and Q damping curves.
  • Visual Force Sensing with Flexible Nanowire Buckling Springs
A calibrated method of force sensing is demonstrated in which the buckled shape of a long flexible metallic nanowire, referred to as a ‘nanoneedle’, is interpreted to determine the applied force. An individual needle of 157 nm diameter by 15.6 μm length is grown on an atomic force microscope (AFM) cantilever with a desired orientation. Using a nanomanipulator the needle is buckled in the chamber of a scanning electron microscope (SEM) and the buckled shapes are recorded in SEM images. Force is determined as a function of deflection for an assumed elastic modulus by fitting the shapes using the generalized elastica model. In this calibration the elastic modulus (68.3 GPa) was determined using an auxiliary AFM measurement, with the needle in the same orientation as in the SEM. Following this calibration the needle was used as a sensor in a different orientation than the AFM coordinates to deflect a suspended PLLA polymer fiber from which the elastic modulus (2.96 GPa) was determined. The practical value of the sensing method does depend on the reliability and ruggedness of the needle. In this study the same needle remained rigidly secured to the AFM cantilever throughout the entire SEM/AFM calibration procedure and the characterization of the nanofiber.
  • Selective self-assembly at room temperature of individual freestanding Ag2Ga alloy nanoneedles
Liquid gallium drops placed on thick Ag films at room temperature spontaneously form faceted nanoneedles of Ag2Ga alloy oriented nearly normal to the surface. This observation suggests that single nanoneedles can be selectively grown by drawing silver-coated microcantilevers from gallium. Needles from 25 nm to microns in diameter and up to 80 Micrometer long were grown by this method. These metal-tipped cantilevers have been used to perform atomic force microscopy, Force microscopy in liquids, AFM voltage lithography, etc.
  • Electrostatic deposition of graphene
Loose graphene sheets, one to a few atomic layers thick, are often observed on freshly cleaved HOPG surfaces. A straightforward technique using electrostatic attraction is demonstrated to transfer these graphene sheets to a selected substrate. Sheets from one to 22 layers thick have been transferred by this method. One sheet after initial deposition is measured by atomic force microscopy to be only an atomic layer thick (∼0.35 nm). A few weeks later, this height is seen to increase to ∼0.8 nm. Raman spectroscopy of a single layer sheet shows the emergence of an intense D band which dramatically decreases as the number of layers in the sheet increase. The intense D band in monolayer graphene is attributed to the graphene conforming to the roughness of the substrate. The disruption of the C–C bonds within the single graphene layer could also contribute to this intense D band as evidenced by the emergence of a new band at 1620 cm−1.

  • Formation of highly transmissive liquid metal contacts to carbon nanotubes
We have developed a method to produce liquid metal contacts to carbon nanotubes that allows direct measurement of the influence of the contact on the nanotube conductance. Gallium is deposited onto standard gold nanotube contacts, where it gradually spreads to coat the contact region. The two-terminal multiwall nanotube conductance increases by as much as 1.2e2 /h during the transition from gold to gallium contacts, and approaches 2e2 /h at room temperature, with a current density of 23108 A/cm2. Surprisingly, the conductance is independent of the contact area or contact separation, providing evidence that transport is ballistic in multiwall nanotubes.

  • Crystalline nano-structures of Ga2O3 with herringbone morphology
Highly crystalline b-Ga2O3 nanowires with two morphologies have been synthesized through physical evaporation of Te doped GaAs powder in Ar atmosphere. Growth is not based on VLS mechanism due to absence of Te. S in place of Te resulted in similar nanostructures. Some of the nanowires exhibit herringbone morphology with presence of hexagonal crystallites in regular spacing along the nanowire axis. The crystal planes of the nanowires were found to be parallel to one of the facets of the crystallites implying these crystallites may serve as the nucleation centers for the growth. Other dominant nanowire morphology is single crystalline nanoribbons.
  • Gallium-driven assembly of Gold nanowire networks
Nanowire networks of Au–Ga alloy are fabricated at temperatures between 220 and 300°C by application of small drops of liquid gallium to 10- to 100-nm-thick gold films. As the liquid gallium drop spreads and reacts with the gold film, lamellar segregation of gold-rich and gallium-rich regions form fractal-like networks of Au–Ga nanowires connected between gold-rich islands in specific zones concentric to the gallium droplet. The wires are subsequently suspended by wet chemical etching that undercuts the ,10-nm-thick chromium adhesion layer and the silicon substrate. Suspended nanowires as long as 6 mm and as narrow as 35 nm diameter have been produced using this method.
  • High aspect ratio etching of atomic force microscope-patterned intrided silicon
Silicon that is nitrided in a pure nitrogen plasma is patterned with voltage applied by an atomic force microscope (AFM). Wet chemical etching into AFM-patterned (110) silicon produced vertical trenches as narrow as 91 nm (for one 757 nm deep trench) and with aspect ratios as large as 8.9:1 (for a 95 nm by 849 nm trench). Compared to the ridge patterns resulting from AFM oxidation and wet etching of hydrogen-passivated silicon, a substantially higher applied voltage is required to pattern nitrided silicon.
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