Category: Seminars

Infrared Photodetectors for Conformal Substrates

Friday, February 18, 2011 3:30 pm – 4:30 pm
Room G05, Rekhi Hall

Jeramy D. Zimmerman
Department of Electrical Engineering and Computer Science
University of Michigan

Abstract

Simple lens systems generally suffer from Petzval field curvature aberrations and focus on a curvedsurface; therefore, complicated lens systems are needed to create the flat focal surface required byconventional semiconductor fabrication techniques. To simplify the optics systems, we are designingimagers with curved imaging surfaces, which reduces other optical aberrations as well as system weight.The adoption of curved focal planes requires the development of new processing techniques and newmaterials for conformal surfaces. This talk will focus on two infrared-sensitive organic semiconductorphotodetector systems developed at the University of Michigan for use on conformal substrates.

Organic photodetectors are efficient (20-80% quantum efficient) in the visible region of the spectrum, butvery few organic materials exist with useful photoresponse beyond λ ≈ 1000 nm. Carbon nanotubes(CNTs) have band gaps that absorb in the λ ≈ 1000 to 2000 nm region, motivating our development of aprocedure to use single-wall CNTs wrapped with conjugated polymers as a photoactive component inphotodetectors. We have demonstrated that excitons on CNTs can be dissociated at CNTC60 interfaces, and have created the first photovoltaic detectors fabricated from bulk CNT films. Detectorspecific detectivities above D*=1010 cm-Hz½/W were demonstrated from λ ≈ 400 to 1400 nm, with peakexternal quantum efficiencies of approximately EQE=2% at λ ≈ 1155 and 1300 nm.[1]

More recently, we demonstrated a new porphyrin tape-based organic semiconductor materials systemwith the highest quantum efficiencies demonstrated to date at peak wavelengths greater than λ ≈ 1000nm. The porphyrin tapes consist of two porphyrin units triply linked to form a rigid tape with variousfunctional groups at the terminus of the tape, notably a pyrene group bonded in either one or twolocations. We have demonstrated quantum efficiencies of up to EQE=4% (D*=9×1011 cm-Hz½/W) at λ ≈1080 nm for a singlybonded pyrene end group and EQE=13% (D*=8×1010 cm-Hz½/W) at λ ≈ 1400 nm fora doubly-bonded pyrene end group.[2]

The presentation will discuss fabrication and analysis of devices and materials and conclude with afuture outlook and other applications for these materials.

  1. M. S. Arnold, J. D. Zimmerman, C. K. Renshaw, X. Xu, R. R. Lunt, C. M. Austin and S. R. Forrest, Nano Lett.9 (9), 3354-3358 (2009).
  2. J. D. Zimmerman, V. V. Diev, K. Hanson, R. R. Lunt, E. K. Yu, M. E. Thompson and S. R. Forrest, Adv. Mater.22 (25), 2780-2783 (2010).


Microstructural Engineering for Solar Photovoltaic Devices

Monday, February 14, 2011 2:00 pm – 3:00 pm
Room G06, Rekhi Hall

Dr. Joshua M. Pearce
Department of Mechanical and Materials Engineering
Queen’s University, Canada

Abstract

Although, solar photovoltaic (PV) electrical production is technologically feasible, growing rapidlyand a environmentally-benign solution to society’s energy requirements, its costs must declinefor deployment at the necessary TW scale. This presentation will review two fundamentalapproaches to reach this goal using microstructural engineering of PV devices. The firstapproach uses relatively inefficient, but proven hydrogenated amorphous silicon (a-Si:H)-basedsolar cells. Thin film cells from a-Si:H are currently the least expensive PV and possess anexcellent ecological balance sheet.  Utilizing AFM, TEM, and real time spectroscopicellipsometry to track the phase of Si:H, the evolutionary nature (protocrystallinity) and substratedependence of its growth were established. This enabled the contributions of the carrierrecombination from the p/i interface regions and the bulk to the dark and light current-voltage (IV) characteristics of a-Si:H p-i-n and n-i-p solar cells to be separated and identified. By applyingthis knowledge of both microstructure and recombination a-Si:H solar cell performance can beimproved to improve efficiency and the cost of electricity provided. The second approach usespotentially ultra-high efficiency indium gallium nitride (InGaN) PV. InGaN shows such incrediblepromise as a PV material due to the ability to modify its band gap by adjusting the ratio ofindium and gallium in the film. A multi-layered cell of InGaN can be made with band gapsranging from 0.7eV (InN) to 3.4eV (GaN), which nearly covers the entire range of the solarspectrum. InGaN has been grown by plasma enhanced evaporation in nanocolumn crystals,which provide optical enhancement and reduce strain during growth and defects. Thus, a welldesigned InGaN solar cell could absorb and convert a much higher fraction of solar energy intoelectricity. The presentation reports on the first stage of research on the characterization andmicrostructural engineering of InGaN nanocolumns. Conclusions are drawn from theexperimental evidence and a path is outlined for future research using both of these approachesto assist society move towards a sustainable energy system using solar photovoltaic devices.


Effect of Electric Field on Hydrogen Storage over Carbonaceous Sorbents at Ambient Temperature

Friday, February 4, 2011 3:00 pm – 4:00 pm
Room 610, M&M Building

Zheng Zhang
Graduate Student
Materials Science and Engineering
Michigan Technological University

Abstract

Storage and transportation of hydrogen in large quantities at small volume iscurrently a big obstacle on the way of hydrogen application. The primaryissue for hydrogen adsorption is weak interactions between hydrogen andthe surface of solid materials, which results in negligible sorption capacity atroom temperature. To solve this problem, electric field was introduced to theprocess of hydrogen adsorption at ambient temperature. For a certaincommercial activated carbon (NAC) with surface area of 1836m2/g, 12.5%and 18.5% enhancements were obtained at 80 bar under 1500V and 2000V.The enhancements were considered to be brought by strong orbitalinteractions between electrically charged sorbent and hydrogen. Moreover,dielectric phase, TiO2, was added to activated carbon to hold electricalcharges around carbon particles without distributing onto the body surface.Employing 2000V electric potential to the samples showed up to 100%enhancement.


Development of an Empirical Model of Rehydration/Rehydroxylation Kinetics for Archaeological Ceramics

Friday, January 21, 2011 3:00 pm – 4:00 pm (1 of 2 speakers)
Room 610, M&M Building

Patrick Bowen
Graduate Student
Materials Science and Engineering
Michigan Technological University

Abstract

Fired-clay ceramic rehydroxylation dating has recently been proposed as a newchronometric dating tool for use on archaeological ceramics.  The technique reliesupon the well-known characteristic of fired clay objects to take up water in a slowmanner, which has been shown to follow a (time)1/4 power law.  Experiments wereconducted in which the mass measurements taken from XIX-century ceramicartifacts revealed a deviation from the (time)1/4 power law over a wide range oftemperatures.  These findings have led to the formation of a general empiricalequation which describes the observed rehydration/rehydroxylation behavior.  This presentation will describe the development of an empirical model describingrehydration/rehydroxylation of ceramic artifacts.  It will also briefly discuss theapproach that can be used to date artifacts using this method, as well as issues ofprecision


A Systematic Investigation of Decomposition of Nano Zn4O(C8H4O4)3 Metal-Organic Framework

Friday, January 21, 2011 3:00 pm – 4:00 pm (1 of 2 speakers)
Room 610, M&M Building

Lei Zhang
Graduate Student
Materials Science and Engineering
Michigan Technological University

Abstract

Metal-Organic Framework (MOF) is a network in the appropriate topology via thecombination of inorganic and organic linker moieties from a wide range ofmultidentate ligands and metals or metal clusters secondary building units. Herein,the systematic investigation of thermal decomposition of the metal-organicframework Zn4O(BDC)4 (MOF-5) was carried out. It was found that thedecomposition of MOF-5, which could take place at 400 °C or above, was due tothe breaking of carboxylic bridges between benzene rings and Zn4O clusters. Thedecomposition produced CO2, benzene, and amorphous carbon besides crystalZnO. Furthermore, the ZnO was covered by amorphous carbon, resulting in theC/ZnO nanoparticles of about 10 nm. The removal of ZnO from the C/ZnOnanoparticles could generate mesoporous carbon with a large surface area of1844 m2/g.Metal-Organic Framework (MOF) is a network in the appropriate topology via thecombination of inorganic and organic linker moieties from a wide range ofmultidentate ligands and metals or metal clusters secondary building units. Herein,the systematic investigation of thermal decomposition of the metal-organicframework Zn4O(BDC)4 (MOF-5) was carried out. It was found that thedecomposition of MOF-5, which could take place at 400 °C or above, was due tothe breaking of carboxylic bridges between benzene rings and Zn4O clusters. Thedecomposition produced CO2, benzene, and amorphous carbon besides crystalZnO. Furthermore, the ZnO was covered by amorphous carbon, resulting in theC/ZnO nanoparticles of about 10 nm. The removal of ZnO from the C/ZnOnanoparticles could generate mesoporous carbon with a large surface area of1844 m2/g.


Crystallographic Domain Engineering of Ferroelectric Single Crystals

Friday, November 19, 2010 3:00 pm – 4:00 pm (1 of 2 speakers)
Room 610, M&M Building

Jie Zhou
Graduate Student
Materials Science and Engineering
Michigan Technological University

Abstract

Phase field modeling and computer simulation is performed to study the mechanisms ofcrystallographic domain engineering technique for ferroelectrics. It is found that both domainconfigurations and domain sizes in ferroelectric single crystals can be controlled throughsophisticated thermal and electrical conditions imposed on the materials during ferroelectricphase transformations. The simulations reveal that minimal domain sizes and highest domainwall densities are obtained with intermediate magnitude of electric field applied along non-polaraxis of ferroelectric crystals, while lower and higher fields produce coarser domains and lowerdomain wall densities. It is found that temperature also plays an important role in domain sizecontrol. The simulations show that selection of polar domain variants by external electric fieldduring nucleation stage of ferroelectric phase transition significantly affects subsequent domaingrowth and evolution kinetics and controls the formation and sizes of twin-related lamellardomains. It is also found that the long-range electrostatic and elastostatic interactions generateinsurmountable energy barrier to isolated ferroelectric nucleation, and independent nucleation offerroelectric phase in the context of classical nucleation theory is impossible. In order tocircumvent such an energy barrier, ferroelectric nucleation exhibits strong spatial correlation andself-organization behaviors, and ferroelectric phase transformation proceeds via spatial andtemporal evolution of self-accommodating domains that provide a low-energy kinetic pathwaythroughout the entire phase transformation process.


Particles at Fluid Interfaces and Effects on Coarsening Kinetics

Friday, November 19, 2010 3:00 pm – 4:00 pm (1 of 2 speakers)
Room 610, M&M Building

Tianle Cheng
Graduate Student
Materials Science and Engineering
Michigan Technological University

Abstract

A newly developed diffuse interface field approach (DIFA) is employed to perform simulationstudy of particles assembly at fluid interfaces. The model employs diffuse interface fields todescribe arbitrary particle shapes as well as multi-phase fluid. It takes into account capillaryforces associated with fluid interfaces, inter-particle short-range repulsions due to mechanicalcontact and/or steric force, hydrostatic force, and Brownian motion force. The model is appliedto simulate particle self-assembly in fluid that undergoes spinodal decomposition. It is shownthat particles stabilize the two-phase fluid morphology by slowing down the coarsening kinetics,and the fluid interfaces are finally arrested by particles closely packed at the interfaces. Inparticular, the effects of particles on the curvature of the fluid interfaces are discussed. It isfound that capillary force and pressure both play important roles in equilibrium of particles atfluid interfaces as well as the interface curvature. The latter is essential for coarsening kineticsof spinodal decomposition.


The Theory of Wetting Including the Fundamentals of Designing the Superlyophobic State of Materials

Tuesday, November 9, 2010 10:00 am – 11:00 am
Room 610, M&M Building

Prof. Ludmila Boinovich
Russian Academy of Science
Moscow, Russia

Abstract

The dominant role of the long-range surface forces in wetting phenomena is widely accepted. Theoretical grounds of the design of hydrophobic materials and coatings and the specificfeatures of the superhydrophobic state of the surface will be discussed on the basis ofthermodynamic and using the concept of surface forces which are characterized by the isothermof disjoining pressure. This concept allows one to explain the peculiarities of wetting and toshow that in general, bulk liquid (drop or concave meniscus) coexists with the substrate which isnot dry, but coated by wetting or adsorption film of liquid. The problems of ageing anddegradation of syperhydrophobic coatings will be outlined.

Biography

Scientific career of Ludmila Boinovich began in 1980s in the laboratory of famous Russian scientistB.V. Derjaguin. Her research activities, both theoretical and experimental, have been centered on molecular andsurface physics. She has discovered and studied several new physical mechanisms of surface forces including thephonon mechanism of surface forces, explaining the influence of confining phases on inter- and intramolecularinteractions in the intervening liquid layer. She has introduced the notion on dynamic structure of liquid innanosystems, characterized by the density of vibrational states. The analysis of the dispersion systems withmulticomponent dispersion phase allowed her to made pioneering advances at the theoretical level, for imagecharge mechanism of surface forces, associated with the polarization of confining phases by the electrostatic fieldof solute molecules. Her studies on phase transitions in nano-sized systems lead to establishing the physicochemical parameters determining the shift of melting/freezing temperatures at the interfaces, in wetting films,aerosols and porous matrices. Ludmila Boinovich and her team have developed a series of new spectroscopicmethods and devices for studying the structure of liquids in nanosize systems and have found experimentally thethickness dependent deviation of liquid structure in thin layers.

Among various awards and honors she received the Gold medal of ICEPEC (Institut Communautaire Europeenpour la Promotion des Enterprices Commerciales) for her contribution to the promotion of scientific results inengineering applications. She has earned a reputation as an outstanding lecturer and teacher who has motivatedand inspired the younger generations of Russian surface and colloid physicists. In 2006 Prof. Boinovich waselected to be a Corresponding Member of Russian Academy of Sciences.


The Modern State of the Theory of Surface Forces in Colloid Systems and Thin Liquid Films

Monday, November 8, 2010 3:00 pm – 4:00 pm
Room 610, M&M Building

Prof. Ludmila Boinovich
Russian Academy of Science
Moscow, Russia

Abstract

A modern physical theory, which describes the state and stability of liquid films and thedispersed and colloid systems, is based on the analysis of the surface forces acting across thinliquid films separating particles or macroscopic bodies. The major recent advances in the theorywill be considered. Limitations intrinsic in the conventional DLVO theory will be briefly outlined.Studies devoted to the analysis and calculations of non-DLVO interactions of different nature willbe reviewed. Particular attention will be paid to the forces caused by the inhomogeneity of liquidinterlayers. The applicability of the known approaches for solving the nanotechnologychallenges will be discussed.

Biography

Scientific career of Ludmila Boinovich began in 1980s in the laboratory of famous Russian scientistB.V. Derjaguin. Her research activities, both theoretical and experimental, have been centered on molecular andsurface physics. She has discovered and studied several new physical mechanisms of surface forces including thephonon mechanism of surface forces, explaining the influence of confining phases on inter- and intramolecularinteractions in the intervening liquid layer. She has introduced the notion on dynamic structure of liquid innanosystems, characterized by the density of vibrational states. The analysis of the dispersion systems withmulticomponent dispersion phase allowed her to made pioneering advances at the theoretical level, for imagecharge mechanism of surface forces, associated with the polarization of confining phases by the electrostatic fieldof solute molecules. Her studies on phase transitions in nano-sized systems lead to establishing the physicochemical parameters determining the shift of melting/freezing temperatures at the interfaces, in wetting films,aerosols and porous matrices. Ludmila Boinovich and her team have developed a series of new spectroscopicmethods and devices for studying the structure of liquids in nanosize systems and have found experimentally thethickness dependent deviation of liquid structure in thin layers.

Among various awards and honors she received the Gold medal of ICEPEC (Institut Communautaire Europeenpour la Promotion des Enterprices Commerciales) for her contribution to the promotion of scientific results inengineering applications. She has earned a reputation as an outstanding lecturer and teacher who has motivatedand inspired the younger generations of Russian surface and colloid physicists. In 2006 Prof. Boinovich waselected to be a Corresponding Member of Russian Academy of Sciences.


Electromechanical Molecular Actuators Based Quantum Mechanical Mechanisms

Thursday, November 4, 2010 11:00 am – 12:00 pm
Room 610, M&M Building

Prof. Miklos Kertesz
Department of Chemistry
Georgetown University
Washington DC

Abstract

Charged or neutral radicals often display π-stacking geometries that show signs ofintermolecular covalent bonding based on the partial occupancy of π-orbitals. This interaction issensitive to the number of π-electrons available for this bonding and goes beyond van derWaals interactions. We analyze known cases of intermolecular π-bonding in various radicaldimers and chains and connect the intermolecular bonding properties to observed properties.The simulations involve molecular and periodic first principles density functional theorycalculations. Various stacked chains are analyzed for their suitability as electrochemically drivenactuating molecules and compared with other mechanisms of electrochemical actuationincluding those based on polyacetylene and carbon nanotubes as well as those based onlocalized 2 center/3 electron bonds.

Relevant references

“Dimensional Changes as a Function of Charge Injection in Single Walled Carbon Nanotubes”Sun, G.Y.; Kürti, J.; Kertesz, M.; R. H. Baughman,  J. Am. Chem. Soc. 2002, 124, 15076-15080.

“Electronic Structure of Helicenes, C2S Helicenes, and Thiaheterohelicenes”Yong-Hui Tian, Gyoosoon Park, Miklos Kertesz, Chem Mater. 2008, 20, 3266-3277.

“Molecular Actuators Designed with S…N(sp(2)) Hemibonds Attached to a ConformationallyFlexible Pivot“ Tian, Y.-H.; Kertesz, M. Chem. Mater. 2009, 21, 2149-2157.

“Is There a Lower Limit to the CC Bonding Distances in Neutral Radical ?-Dimers? The Case ofPhenalenyl Derivatives”, Tian, Y.-H.; Kertesz, M. J. Am. Chem. Soc. 2010. 132, 10648-10649.