Transcript

Universidad de Puerto Rico en Humacao Departamento de Física y Electrónica

Segundo Semestre 2010-2011

Número 6

RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64

Tabla de Contenido

Tunable Schottky diodes fabricated from electrospun crossed SnO2 /PEDOT –PSSA nanoribbons. Katherine V. Carrasquillo Mentor: Nicholas J. Pinto

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Schottky diodes and sensors fabricated from electrospun PEDOT-PSSA nanofibers and their characterization in toxic gaseous environments. Yarely Dávila, Danairé Rivera Mentor: Nicholas J. Pinto

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Fabrication and electrical characterization of Semiconducting ZnO thinfilms and nanofibers. Yarely C. Dávila, Joshua Robles Mentors: Nicholas J. Pinto and Idalia Ramos

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Study of nanofiber formation by injecting aqueos solutions containing polyethylene oxide and fluorescent latex beads inside electric fields. Jorge L. Castro Torre, Steven Arroyo Rivera Mentor: Rogerio Furlan

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Oriented micro and nanofibers containing polyethylene oxide and pectin. Gerardo González Rodríguez Mentor: Rogerio Furlan

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Definition of polymeric structures using an atomic force microscope. Godohaldo J. Pérez Medina, Mentor: Rogerio Furlan

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Observation Report 2008, Humacao University Observatory. D. Centeno, L. Rivera-Rivera, E, Franco, V. Maldonado, K. Morales, M. De Jesús, R. A. Rodríguez, A. J. Sosa, M. Rosario Mentor: Rafael J. Muller, Juan C Cersosimo

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Lock and Key Adsorption Chemistry: Preferential Absorption of an Isomer of Di-iodobenzene on Molecular Films of Quinonoid Zwitterions. Zhang, Zhengzheng, Alvira Jose, Barbosa Xenia Mentor: Luis G. Rosa, Routaboul Lucie, Braunstein Pierre, Doudin Bernard, Dowben Peter A.

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Franck-Condon Coupling in Anthracene Isomer Self-Assembled Layers and Symmetry Ef-fects on the High Resolution Ultraviolet Photoemission Spectra. Jiang Jinyue, Wong Freddy Li, Xiao Jie, Utreras Emmanuel, Lima Ocelio, Alvira Jose, Losovyj Ya. B., Tan Li Mentor: Luis G. Rosa

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Selective nanoshaving of self-assembled monolayers of 2-(4-pyridylethyl)triethoxysilane. Jiang Jinyue, Lima Ocelio V., Xiao Jie, Utreras Emmanuel, Dowben PA, Tan Li. Mentor: Luis G. Rosa

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UPRH Reválida Campeonato en “The Great Moon Buggy Race” 2011. José A. Tirado, Iro García, Luis Rivera, Francisco Serrano, Chary Molero, Iván Ortiz. Mentor: Abraham Ruiz

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RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64

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The kinematics of the Ionized Hydrogen in NGC 3372. Grace M. Fontánez Santana, Milennys Velázquez Flores, Clarissa Vázquez Colón, Milzaida Merced Sanabria. Mentor: Juan C. Cersosimo

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Engineering of Hybrid Organic-Inorganic Solar Cells. Melissa Dávila Santana, Earl Goodwin, David Kim, Damielle Reifsnyder. Mentor: Josee Vedrine-Pauleus, Chris B. Murray and Cherie Kagan

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Preparation of AlN nanofibers using Electrospinning. Xenia Barbosa. Mentor: Idalia Ramos, Eva Campo, Jorge Santiago-Avilés

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Study of Gallium Nitride Nanofibers using Transmission Electron Microscopy. Joshua L. Robles García, Lytzamed Santa, Anamaris Meléndez. Mentor: Idalia Ramos, Eva Campo

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Temperature dependence of the electrical conductivity of Sb-SnO2 nanofibers. Maritza Reyna. Mentor: Idalia Ramos, Nicholas J. Pinto

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Electrospun Polymer/Carbon Nanotubes Nanocomposites for Photoactuation. Sabrina Rosa, Jean P. Crespo. Mentor: Idalia Ramos, Eva Campo

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Thermal analysis study of polysterene-poly(methyl methacrylate) (PS-PMMA) diblock copolymer thin films morphologies when annealed and sheared under vacuum in inert atmosphere. Luis Pomales, Melissa Davila-Santana, Omar Vega, Mirna Rivera-Claudio. Mentor: Josee Vedrine-Pauleus

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A Computational Simulation of a Radiotherapy Clinic Procedure: Total Body Irradiation. Veronica de la Rosa Mentor: Ernesto P. Esteban

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Geometric Properties of Magnetized Kerr Black Holes Ergosurface and Event Horizons. John P. Colón Cordova Mentor: Ernesto P. Esteban

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On the Carcinogenic Effects of Space Radiation on the International Station’s Crew. Pierina Agosto Mentor: Ernesto P. Esteban

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p-doped conducting polymer PEDOT-PSSA is also stable in air and is widely used in flexible devices. We shall report on the electrospinning technique to fabricate in air Schottky diodes, by simply crossing n-doped SnO2 and p-doped PEDOT-PSSA nanoribbons. The device parameters could be tuned by a back gate bias and by shining UV light. The diode parameters were calculated using the standard thermionic emission model of a Schottky and was tested as a half wave rectifier.

TUNABLE SCHOTTKY DIODES FABRICATED FROM ELECTROSPUN CROSSED SnO2 /PEDOT-PSSA NANORIBBONS

Katherine V. Carrasquillo

Mentor: Nicholas J. Pinto Department of Physics and Electronics, University of Puerto Rico-Humacao

Hardware in most solid state devices contains at least one interface between a p-type and an n-type semiconductor. Such hetero-junctions are typically fabricated from all inorganic Si based materials. In the past two decades however, devices fabricated from organic-inorganic semiconductors that are not Si based, or from all organic semiconductors have been the focus of much research. Semicon-ducting n-doped metal oxides are also attractive for use in devices and of particular interest is tin oxide (SnO2) as it is stable in air and is optically transparent with a band gap of ~3.4 eV. The

SCHOTTKY DIODES AND SENSORS FABRICATED FROM ELECTROSPUN PEDOT-PSSA NANOFIBERS AND THEIR CHARACTERIZATION IN TOXIC GASEOUS ENVIRONMENTS

Yarely Davila1, Danairé Rivera2 Mentor: Nicholas J. Pinto2

1Department of Biology, University of Puerto Rico-Humacao 2Department of Physics and Electronics, University of Puerto Rico-Humacao

Sub 50nm diameter nanofibers of the commercially available p-doped conducting polymer poly(3,4-ethylenedioxythiophene) doped with (poly styrene sulfonic acid)-PEDOT-PSSA have been electrospun in air and used in the fabrication of Schottky diodes and sensors. The diodes were tested in the presence of several toxic gases like NO2 and aliphatic alcohols. The rectification ratio of the diodes were tunable and the response reversible in the presence of these gases with no damage to the device. Some of the fibers were also used in the fabrication of sensors. Due to the large surface to volume ratio and small quantity of active material used in their fabrication, these sensors have a similar or faster response time when compared to

alcohol sensors based on PEDOT. Increasing the size of the alcohol molecule leads to longer response times, which is attributed to slower diffusion of the larger molecule into the polymer. The same sensors were annealed in air at 70°C and used to sense NH3, HCl and NO2 gases and attest to its robust nature. The use of these fibers as diodes and as gas sensors makes them multi-functional, and the ability to easily fabricate true nanofibers of PEDOT-PSSA opens up a variety of new applications where size, speed and low power consumption are important performance criteria.

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FABRICATION AND ELECTRICAL CHARACTERIZATION OF SEMICONDUCTING ZnO THINFILMS AND NANOFIBERS

Yarely C. Dávila1, Joshua Robles2 Mentor: Nicholas J. Pinto2, and Idalia Ramos2

1Department of Biology, University of Puerto Rico – Humacao 2Department of Physics and Electronics, University of Puerto Rico – Humacao

The inorganic compound, ZnO, has been known for several favorable properties, one of them being its high electron mobility. We have fabricated fibers and thin films of this material using electrospinning. A solution was prepared using Cellulose Acetate (CA) as the base polymer, and mixing it with Zinc Chloride (ZnCl2) in Dymethylacetamide (DMA) and Acetone. The fibers were fabricated through the electrospin-ning method which consists of an electric field between the polymer solution and a collector plate. The samples were collected on silicon substrates and sintered at 600°C in air for two hours. A TEM grid

was placed over preselected fibers and used as a shadow mask, and 100 nm of gold were deposited by vacuum evaporation to create contact pads over the fibers. External contacts were made with silver paint and gold wires. Current-voltage (I-V) measurements on the fibers and films demonstrated n-doped behavior when a back gate bias was applied. This is very impor-tant as we plan to fabricate diodes and other electronic devices and sensors using this material. The fibers and films were also characterized using X-Ray diffraction (XRD) confirming the presence of Zinc Oxide.

RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64

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Nanofibers find several potential applications including engineered tissue for medicine, devices for electronics, sensors and filters for chemical detec-tion, etc. In this work we are are investigating the formation of nanofibers using aqueous solutions containing polyethylene oxide (PEO, bio compatible polymer, Sigma Aldrich, molecular weight of 2,000,000) and fluorescent latex beads (commercial aqueous solution containing carboxy-late modified polystyrene beads, 20 nm – 40 nm, fluorescent yellow-green, 2.5% solid, Sigma Al-drich). Polymeric solution is being injected between two electrodes, a process that allows oriented fibers to be formed. Initially, a polymer stream moves to wards the positive electrode. Then, after effective

charge rearrangement, polymer streams are ejected from the positive electrode and from the needle of the syringe. These streams travel towards the grounded electrode. At the end, oriented fibers are formed with the extremities attached to both electrodes. These fibers can be easily transferred to a substrate, keeping their orientation. This work is in an initial phase and the effects of the incorpora-tion of the nanometric beads in the fibers characteristics (diameter, morphology, orientation, beads distribution) are being investigated. Future work will explore the aspects of using fluorescent nanoparticles.

STUDY OF NANOFIBER FORMATION BY INJECTING AQUEOUS SOLUTIONS CONTAINING POLYETHYLENE OXIDE AND FLUORESCENT LATEX BEADS INSIDE ELECTRIC FIELDS

Jorge Luis Castro Torres, Steven Arroyo Rivera Mentor: Rogerio Furlan

Department of Physics and Electronics, University of Puerto Rico – Humacao

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ORIENTED MICRO AND NANOFIBERS CONTAINING

POLYETHYLENE OXIDE AND PECTIN

Gerardo González Rodríguez1

Mentor: Rogerio Furlan2 1Department of Biology, University of Puerto Rico – Humacao

2Department of Physics and Electronics, University of Puerto Rico – Humacao

The use of biocompatible materials in the formation of oriented fibers opens the possibility to obtain engineered tissues that can be used in medicine, environmental engineer, security and defense, among other applications. Pectin is a heteropolysaccharide that besides being biocompatible is biodegradable and bioactive. In this work the formation of oriented fibers using solutions containing different amounts of pectin, 0.15 g of polyethylene oxide (biocompatible polymer), and 10 ml of chloroform (solvent) is investigated. The injection of solution inside an intense electric field defined between two floating parallel electrodes was used to obtain the fibers. This approach is a modification of the conventional electrospinning process. The presence of pectin in the fibers was confirmed by FTIR analysis. Fibers with diameters of hundreds of nanometers can be collected. The incorporation of pectin leads to a higher variation of the fibers diameter, resulting in the collection of more fibers with an increased diameter, in the range of micrometers. This work was presented in the National Conference on Undergraduate Research 2011 (NCUR 2011).

Parallel fibers collected simultaneously (0.27 g of pectin, 0.15 g of PEO and 10 ml of chloroform).

DEFINITION OF POLYMERIC STRUCTURES USING AN ATOMIC FORCE MICROSCOPE

Godohaldo J. Pérez Medina Mentor: Rogerio Furlan

Department of Physics and Electronics, University of Puerto Rico – Humacao

The capability to direct-write and pattern polymeric materials with interesting electronic and electro-optical properties at the nanoscale creates a number of opportunities, since a large variety of monomers/polymers are available. In this prelimi-nary phase of the work an Atomic Force Microscope (AFM) model Dimension 3100 from Digital Instruments, operating in contact mode, is being used to establish conditions for this purpose. Initial condi-tions were established using a 0.5 mM supersaturated MHA (mercaptohexadecanoicacid) solution with ethanol. Images of squares with 1 µm2 could be observed using friction mode (scale of 0.2 V). These initial conditions are being changed for operation using solutions with polyacrylonitrile (PAN) and N, N

Dimethylformamide (DMF) as solvent. Friction images of squares with 1 µm2 we defined using a polymeric solution with 25 mg of PAN and 5 ml DMF. Writing conditions are: scan size 1 µm, scan angle 90°, f = 2 Hz, deflection set point: 4 V. The sample is scanned twice. Measurement conditions are: scan size of 5 µm, scan angle 90, f = 4 Hz, deflection set point: 1 V. After finishing this initial part of the study, nanometric structures will be formed using a Dip-Pen Nanolithography equipment.

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Friction image

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OBSERVATION REPORT 2008, HUMACAO UNIVERSITY OBSERVATORY

D. Centeno, L. Rivera-Rivera, E, Franco, V. Maldonado, K. Morales, M. De Jesús, R. A. Rodríguez, A. J. Sosa, M. Rosario

Mentor: Rafael J. Muller, Juan C. Cersosimo Department of Physics and Electronics, University of Puerto Rico – Humacao

We report the measurement of position angle and separation of 141 binary pairs. The data was obtained using the 31 inch NURO Telescope at the Anderson Mesa location of Lowell Observatory near Flagstaff, Arizona, in May and September 2008. We gathered the data using the 2K x 2K CCD camera - NASACAM - at the prime focus of the telescope. The data was transferred and analyzed at the Humacao University Observatory by undergraduate students undertaking research projects.

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LOCK AND KEY ADSORPTION CHEMISTRY: PREFERENTIAL ABSORPTION OF AN ISOMER OF DI-IODOBENSENE ON MOLECULAR FILMS OF QUINONOID ZWITTERIONS

Zhang, Zhengzheng, Alvira Jose, Barbosa Xenia,

Mentor: Luis G. Rosa, Routaboul Lucie, Braunstein Pierre, Doudin Bernard, Dowben Peter A.

Department of Physics and Electronics, University of Puerto Rico – Humacao

We have investigated the absorption and adsorption of three isomers of di-iodobenzene on molecular films of a zwitterionic p-benzoquinonemonoimine-type molecule which is characterized by a large intrinsic dipole of 10 D. Specifically, we compared the reversible adsorption and absorption of 1,2-di-iodobenzene, 1,3-di-iodobenzene, and 1,4-di-iodobenzene on molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C6H2((center dot center dot center dot) over bar NHR)(2)((center dot center dot center dot) over barO)(2) where R = n-C4H9. We provide unequivocal evidence that molecular adsorption and absorption of 1,3-di-iodobenzene are strongly favored at 150 K over that of the other isomers of di-iodobenzene.

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Franck-Condon Coupling to the electronic structure of anthracene silane isomer complexes self-assembled on a silicon oxide surface has been measured by high-resolution photoelectron emission spectroscopy. Our results are in strong agreement with theoretical semiempirical calculations and gas phase measurements. The Franck-Condon vibronic fine Structure observed in the high resolution photoemission spectra exhibits a strong dependence on the isomer, in particular, the anthracene functional group orientation with respect to the surface normal.

FRANCK-CONDON COUPLING IN ANTHRACENE ISOMER SELF-ASSEMBLED LAYERS SYMMETRY EFFECTS ON THE HIGH RESOLUTION ULTRAVIOLET PHOTOEMISSION

SPECTRA

Jiang Jinyue, Wong Freddy Li, Xiao Jie, Utreras Emmanuel, Lima Ocelio, Alvira Jose, Losovyj Ya. B., Tan Li

Mentor: Luis G. Rosa Department of Physics and Electronics, University of Puerto Rico – Humacao

RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64

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Source: JOURNAL OF PHYSICAL CHEMISTRY C Volume: 114 Issue: 2 Pages: 1015-1018 Published: JAN 21 2010

SELECTIVE NANOSHAVING OF SELF-ASSEMBLED MONOLAYERS OF 2-(4-PYRIDYLETHYL)TRIETHOXYSILANE

Jiang Jinyue, Lima Ocelio V., Xiao Jie, Utreras Emmanuel, Dowben PA, Tan Li.

Mentor: Luis G. Rosa Department of Physics and Electronics, University of Puerto Rico – Humacao

Single molecular layers of 2-(4-pyridylethyl)triethoxysilane have been deposited on native oxide surfaces of silicon, with the triethoxysilylethyl groups towards the silicon oxide interface and pyridine at the surface. It is possible to "shave" or mechanically break the Molecular bonds at the alkoxy-silane (Si-C) bond using scanning atomic force microscope, leaving large swaths of surface area cut to a depth of 0.64 +/- 0.06 nm, exposing the silicon of the alkoxy-silane ligand. Mechanical cleavage of the pyridine ligand alone is also possible, but more difficult to control selectively. (C) 2009 Elsevier B.V. All rights reserved.

Source: MATERIALS LETTERS Volume: 63 Issue: 12 Pages: 961-964 Published: MAY 15 2009

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Para esta competencia preparamos un buggy completamente nuevo. Perfeccionamos algunas ideas implementadas en los pasados 5 años que habían dado buenos resultados. Nos esforzamos para hacerlo simple y liviano. Gracias a esto ganamos por segundo año consecutivo, el “Featherweight Award”. Premio a la eficiencia por peso, lo cual es muy importante en la ingeniería aeroespacial. Este premio se le da al buggy cuyo tiempo en segundos multiplicado por su peso en libras tenga el menor valor. En nuestro caso el tiempo fue 321 segundos y el peso de 86 libras. El carro tenía suspensión independiente y tracción delantera. Al frente se le colocaron amortiguadores y gomas 26”, mientras que a l tren trasero (el cual soporta solo el 30% del peso) llevó bandas elásticas y gomas 24”. Por primera vez usamos como chasis un tubo cuadra-do de aluminio de 2” el cual simplifica la montura de muchos de los componentes, pero requiere mas solda-dura para darle la forma. Por primera vez pudimos hacer un buggy que entró en el cubo de 4’ por 4’ por 4’ con solo una articulación. La revista “Popular Mechanic” Publica algunos detalles de nuestro moon-buggy: http://www.popularmechanics.com/science/space/nasa/student-engineers-run-their-diy-lunar-rovers-at-the-great-moonbuggy-race. "Simplicity is sometimes better," University of Humacao lead dri-ver Ivan Ortiz said. Indeed, the design that led them to their second consecutive triumph at the Great Moonbuggy Race was simple but effective. The bug-gy sat low to the ground with large, widely spaced wheels, while the drivers both faced forward with one in front and the other in back. The lightweight design allowed them to pedal the buggy with great speed, while the larger wheels had no problem tearing through obstacles.”

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UPRH REVÁLIDA CAMPEONATO EN “THE GREAT MOON BUGGY RACE” 2011

José A. Tirado, Iro García, Luis Rivera, Francisco Serrano, Chary Molero, Iván Ortiz,

Supervisor: Abraham Ruiz García Departamento de Física y Electrónica , Universidad de Puerto Rico, en Humacao

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THE KINEMATICS OF THE IONIZED HYDROGEN IN NGC 3372

Fontánez Santana, Grace M.1, Velázquez Flores, Milennys2; Vázquez Colón, Clarissa2; and Merced Sanabria, Milzaida2

Mentor: Juan C. Cersosimo 1Petra Mercado Bougar High School

2Department of Physics and Electronics, University of Puerto Rico at Humacao

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In the southern sky the Carina Nebula (NGC 3372) has been observed at many frequencies including many transitions of radio recombination lines by Mc Gee and Gardner (1968), Mc Gee et al.(1969), Wilson et al. 1970; Gardner et at. (1970); Huchtmeier and Day (1975), Abraham et al. (1980), Mc Gee et al. (1975); Huchtmeier and Jay (1975).

Azcárate et al. (1981) show observa-tions of this region at the frequency corresponding to the H166a line tran-sitions. From these observations they calculate the electron temperature, assuming that the line is under Local Thermodynamic Equilibrium.

A typical characteristic of the Carina Nebula is its complex kinematical structure, which is observed in optical and radio lines. Due to the existence of many young and massive stars, there are considerable amounts of ionized gas. The aim of this work is to disentangle some behavior of the kine-matics and structure of the ionized gas in the nebula.

The region around l = 2870.5 and b = −00.5 was mapped at the H166α line frequency (1424.734 MHz). The receiver used was the Parkes multibeam, a 13-beam receiver package mounted at the prime focus of the Parkes adiotelescope. The inner 7 beams are packed in a hexagonal configuration with a beam separation on the sky of 29.1. On-the-fly mapping was performed by scanning the telescope at a rate of 1◦ per minute in R.A., and recording spectra every 5 s. In Fig. 1 a series of

Figure 1: contour at different interval of velocity. The gas be-tween --40 and -30 km s-1 is approaching (upper left). The gas between -10 and 0 km s-1 is receding (bottom right). The gas at intermediate velocities show annular shape like structures .

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brightness temperature contour maps are shown on l, b coordinates using a color scale. Each map corre-sponds to the integrated radial velocity over 10 km/s interval. The sequence is shown from -30 to +10 km/s. The color scale is shown on the right of each map. The velocity interval is indicated on the corner of each map.

The maps at -40 to -30 kms-1 and at -10 to 0 kms-1 show structures almost continuous. One of the structure shown by the velocities -40 to -30 kms-1 are approaching caps, meanwhile the structure shown by the velocities -10 to 0 km/s is a receding cap. And the other hand the maps at intermediate velocities, -30 to -20 km/s and at -20 to -10 km/s show a rela-tive gap of ionized hydrogen, about l=2870.5, b=-00.8.

We interpret this behavior as a shell like produced by some energetic event that is blowing the gas. A simple model of the region is represented in Figure 2.In this figure are represented the caps showing the characteristic signature of an expanding shell. At the left is showing the position of the observer. The velocities of the caps are taken respect to the barycenter velocity of the structure. The approaching and receding caps are moving at -20 and + 20 km/s respect to the barycenter. As hypothesis we suppose that the blow of the gas is caused by the stellar wind of the young and massive star of Trumpler 14.

Figure 2: A model of the expanding Shell.

ENGINEERING OF HYBRID ORGANIC-INORGANIC SOLAR CELLS

Melissa Dávila Santana1,Earl Goodwin2, David Kim2, Danielle Reifsnyder2,

Mentor: Josee Vedrine-Pauleus1,

Chris B. Murray2 and Cherie Kagan2

1Dept. of Physics and Electronics, UPR- Humacao 2Dept. of E.E./M.S.E., and Chemistry, UPENN

The application of organic-inorganic heterostructures would allow alternative photovoltaic solar cells that convert energy into current, low fabrication costs, and fabrication on flexible substrates. In this work we explore the combinations of P3HT (poly-3-hexyl-thiophene) with PCBM(penyl-C61-butyric acid methyl ester) or P3HT combined with QDs (quantum dots) to fabricate hybrid hetero-junctions. Our experimental results show that the solution containing P3HT:PCBM with chlorobenzene as solvent improved device efficiency, and the consequence of thickness on the efficiency yield of our hybrid so-lar cell devices.

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PREPARATION OF ALN NANOFIBERS USING ELECTROSPINNING

Xenia Barbosa1

Mentor: Idalia Ramos1, Eva Campo2, Jorge Santiago-Avilés3

1Dept. of Physics and Electronics, UPR-Humacao 2L.R.S.M., UPENN, 3Dept. of E.E. and M.S.E.; UPENN

Aluminum Nitride (AlN) has important applications for the development of optoelectronics devices, ultra-sonic transducers, surface acoustic wave devices and piezoelectric nano-actuators. In addition, AlN has become a very useful material as a barrier in superconductor-insulator –superconductor (SIS). We have attempted the fabrication of AlN nanofibers using electrospinning and a precursor containing aluminium nitrate nano-hydrate (Al (NO3)3·9H2O) and urea with cellulose acetate (CA) as the structural polymer. Both the precursor and the structural polymer were dissolved in a solution of acetone and dimethy-lacetamide (DMA). In electrospinning a high voltage source of 15 kilovolts (kV) is typically applied between the tip of a syringe containing a polymeric precursor and a grounded substrate, which is set at a specific distance to collect the fibers. Once the precursor was electrically charged, jets emerged from the syringe tip to the collector forming long and continuous nanofibers. After electrospinning, the nanofibers were subjected to a heat treatment under nitrogen (N2) and ammonia (NH3) atmospheres. Themogra-vimetric Differential Themal Analysis (TG-DTA) of the precursor solution showed chemical changes in the precursor solution as it was heated from room temperature to 600°C. This information was used to design the initial thermal treatment. The electrospun fibers were sintered in nitrogen (N2) at 12ºC/min until a temperature of 900ºC was reached. Then, the atmosphere was switched to ammonia (NH3) gas. Scanning Electron Microscopy (SEM) demonstrated the production of fibers with diameters ranging from a few nanometers to several micrometers. X-Ray Diffraction analysis confirms the production of α-Al2O3 fibers after heating for 90 minutes in ammonia. These results suggest electrospining is a promising technique in the production of AlN. Additional nitridation of electrospun fibers is expected by designing an appropriate heating treatment, namely by heating the fibers for longer periods and or at higher temperatures under an ammonia flow.

STUDY OF GALLIUM NITRIDE NANOFIBERS USING

TRANSMISSION ELECTRON MICROSCOPY

Joshua L. Robles García1, Lytzamed Santa1, Anamaris Meléndez1,

Mentor: Idalia Ramos1, Eva Campo2,

1Dept. of Physics and Electronics, UPR- Humacao 2L.R.S.M., UPENN

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Gallium nitride is one of most important semiconductors for electronic applications because it has a wide bandgap, high heat capacity and high thermal conductivity. GaN can crystallize in the hexagonal or in the cubic crystal system. The crystal is formed by competition of grains growing in different orientations. In this work, GaN nanofibers were fabricated through an economic, simple and fast method called electrospinning. To prepare the precursor solution, Gallium nitrate was dissolved in dimethylacetamide (DMA) and mixed with a viscous solution of cellulose acetate dissolved in DMA and acetone. Initially, the electrospun fibers were sintered in Nitrogen at 400°C for one hour, to eliminate the polymer. Subsequently, the sintering process was repeated using ammonia at 900°C during five hours. X-Ray Diffraction analysis showed the characteristic peaks of hexagonal wurtzite GaN with 101 preferential orientation. The fibers were characterized using a JEOL 2010F Transmission Electron Microscope (TEM) to study the atomic planes of the nanofibers. The space between atomic planes in the selected areas was approximately 0.5nm, as measured from transmission electron microscope images. This result suggests TEM imaging of these nanofibers offers enough contrast to determine lattice parameters in the as-grown nanocrystals. During the experiment it was observed that the fibers are fragile and soft. They tend to crumble if not treated carefully. Future work includes enhancing the recipe to produce stronger fibers.

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Maritza Reyna

Mentor: Idalia Ramos, Nicholas J. Pinto Dept. of Physics and Electronics, UPR-Humacao

TEMPERATURE DEPENDENCE OF THE ELECTRICAL CONDUCTIVITY OF Sb-SnO2 NANOFIBERS

Antimony-doped tin oxide (ATO) can be used for a variety of applications including the development of gas sensors, energy storage devices, and transparent electrodes. In previous work, ATO nanofibers were fabricated using the electrospinning method, which consists of producing nanofibers from a polymeric precursor solution using an electromagnetic field generated with a high voltage source. The polymeric solution was composed of tin (IV) chloride mixed with propanol and 2-propanol, and cellulose acetate dissolved in a mixture of acetone, dimethyl acetamide and antimonium (III) chloride. Using this method ATO nanofibers with sizes ranging from 200 nm to 600 nm and a bandgap of 4.4 e.V were produced. The XRD spectra of the nanofibers showed the characteristic peaks of Sb: SnO2 with rutile structure. In this research, the temperature dependent electrical properties of single ATO nanofibers were studied following a cycle of cooling from 295 to 15 K and then heating from 15 to 295 K. These measurements were done in cold finger (close cycle helium refrigerator) in a vacuum. The conductivity measured at room temperature is 4.3 S/cm. Compared to the literature this conductivity is in the higher end of the accepted range. The conductivity decreases monotonically from 295 to 15K. As the temperature increases an anomalous peak is observed in the range of 250 to 300 K. This anomaly has been attributed to the chemiabsorbed molecules on the surface of the fiber and could be reduced by improving the vacuum conditions.

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ELECTROSPUN POLYMER/CARBON NANOTUBES NANOCOMPOSITES FOR PHOTOACTUATION

Sabrina Rosa1, Jean P. Crespo2

Mentor: Idalia Ramos3, and Eva M. Campo4 1Petra Mercado H.S., 2Dept. of Biology, UPR-Humacao, 3Dept. of Physics

and Electronics, UPR-Humacao, 4L.R.S.M., UPENN

Research studies demonstrate that the photomechanical properties of carbon nanotubes (CNT) are important in the development of optical actuators. The integration of CNTs into polymers combines the good processability of polymers with the functional properties of CNTs. The understanding of photoac-tuation in these CNT/polymer composites can contribute to the development of nano-optical-mechanical systems for applications that include intracellular motors, artificial muscles, and tactile displays for the general public. CNT/polymer nanocomposites were fabricated in the form of fibers using the electrospinning technique. Electrospinning is a simple and economic method that can produce fibers with micro and nano dimensions in seconds. The precursor solution for electrospinning was prepared by adding Multiple Wall Carbon Nanotubes (MWCNT) to a combination of Poly(methylmethacrylate) (PMMA) and Polydimethylsiloxane (PDMS) in Dimethylformamide (DMF) and Tetrahydrofuran (THF). Before adding them to the precursor, the CNTs were dispersed in a Sodium dodecyl sulfate/water solution. Scanning Electron Microscopy (SEM) analysis of the electrospun mats shows the production of long and continuous fibers with diameters in the range of hundreds of nanometers. Atomic Force Microscopy (AFM) images of the fibers confirm the presence of CNTs in the fibers. Initial experiments show the photomechanical response of the fibers when exposed to visible light. Additional tests are being conducted to understand the mechanisms of the composites response to light stimulation and the effect of using different polymers and CNTs concentration.

Número 6 RESULTADOS DE INVESTIGACIONES SUBGRADUADA

REALIZADA EN LOS CURSOS FISI 4161-64 Page 17

THERMAL ANALYSIS STUDY OF POLYSTERENE-POLY(METHYL METHACRYLATE) (PS-PMMA) DIBLOCK COPOLYMER THIN FILMS MORPHOLOGIES WHEN ANNEALED

AND SHEARED UNDER VACUUM IN INERT ATMOSPHERE

Luis Pomales¹, Melissa Davila-Santana¹, Omar Vega¹, Mirna Rivera-Claudio²

Mentor: Josee Vedrine-Pauleus¹ ¹Dept. of Physics and Electronics, ²Chemistry Dept., UPR-Humacao

Diblock copolymers are made of two chemically bonded blocks, but contain a monomer distinct from the other. The incompatibility between monomers gives the block the property to phase separate at temperatures above the glass transition (Tg). This ability to self-assemble into different mesophase structures is important in nanolithography and nanofabrication. This research involves the study of the morphology of polysterene-poly(methyl methacrylate) (PS-PMMA) thin films annealed under inert atmosphere. Our objective is to determine the microstructure properties of the PS-PMMA diblock copolymer with varying film thickness, rotation speed and wt. %. The PS-PMMA thin film is spin casted onto silicon substrates, and annealed in vacuum under an inert atmosphere.

Figure 2: Graphs plotting the aver-age structural features of PS-b-PMMA thin films; highlighted markers measures dimensions of features-plot showing diameter of the reversed cylinders ~ 33 nm (left), plot showing the wall-to-wall separation between reversed cylin-ders ~20 nm (right).

Figure 1: Real image of the ordered reversed cylinders of PS-b-PMMA taken with the Atomic Force Microscopy (AFM). The image size is 856nm² and the reversed cylinders are the brighter things that look more alike spheres, that’s because they are perpendicular to the surface. So here the reversed cylin-ders are the PMMA and the dark spaces are the PS (left). The autocorrelation, which means that this is the PS-b-PMMA’s virtual image without the noise from the real image, and gives a better view of the reversed cylindrical system (right).

Page 18 Número 6 RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64

A COMPUTATIONAL SIMULATION OF A RADIOTHERAPY CLINIC PROCEDURE: TOTAL BODY IRRADIATION

Veronica De la Rosa1 Mentor: Ernesto P. Esteban2

1Biology Department, University of Puerto Rico-Humacao, Humacao 2Department of Physics and Electronics, University of Puerto Rico-Humacao

Leukemia, non-Hodgkin’s lymphoma and neuroblastoma patients prior to bone marrow transplants may be subject to a clinical radiotherapy procedure called total body irradiation (TBI). To mimic a TBI procedure, we modified the Jones model of bone marrow transplant cell kinetics by adding mutant and cancerous cell compartments. The modified Jones model is mathematically described by a set of n + 4 differential equations, where n is the number of mutations before a normal cell becomes a cancerous cell. Next, a computational simulation of a standard TBI radiotherapy treatment (total dose of 1320 cGy fractionated over four days) was carried out. Since a TBI procedure requires a high radiation dose rate, sub-lethal repair and repopulation were neglected. In this case, the proposed Jones model could be solved in a closed form. Exact solutions for n mutations have been found, and presented explicitly for the case n=2. After a graphical analysis, we found that the expected results of TBI treatment can be mimicked by the proposed bio-mathematical model. Therefore, the computational simulation (including the prior effects of chemotherapy procedure) suggests that the TBI standard treatment, is indeed correct, and it provides the needed conditioning treatment prior to bone marrow transplant .

GEOMETRIC PROPERTIES OF MAGNETIZED KERR BLACK HOLES ERGOSURFACE AND EVENT HORIZONS

John P. Colón Cordova Mentor: Ernesto P. Esteban

Department of Physics and Electronics, University of Puerto Rico-Humacao

Using the framework of general relativity we study a supermassive rotating black hole immersed in an external magnetic field. This compact object is located at most galaxies' centers, and it may play a fundamental role in galaxy formation.

In particular, we are interested in understand how the geometry of the black hole's event horizon and ergosurface is modified by the dimensionless parameter β (= MB), where M and B, are the supermassive black hole's mass, and the asymptotically external magnetic field, respectively.

Page 19

Four applications are considered. First, we prove the existence of a threshold value for β which assures the existence of two types of magnetized Kerr black holes. Namely, type I (β>2.3) when the black hole's angu-lar momentum is parallel to the magnetic field, and type II (β>1.3), when they are antiparallel. Second, we study the geometric shape of the magnetized Kerr black hole's event horizon and outer limit surface. In Type I, the black hole's event horizon could be an oblate spheroid (2.3< β<2.5), sphere (β=2.5) or a prolate spheroid (β>2.5). In the case of the outer limit surface, the shape will be similar to the event horizon except that other sphere could also be formed at β =2.3. For type II, it is shown that the event horizon and the ergosurface will always be prolate spheroids. Increasing β will decrease the Penrose process probabilities. Third, a graphical analysis showed that for type I, the Gaussian curvature of the event horizon is always positive while for the ergosurface there is a region around the poles (1.32<β<1.6) where is negative. For type II, the Gaussian curvature of the event horizon and ergosurface begin to become negative around the Equator when β>2.8 and β>1.7, respectively. Fourth, we calculated the areas for both the type I and type II magnetized Kerr black holes. Results show that increasing β in a fixed a/M will increase both type I and type II black holes. Results also show that increasing a/M at a fixed β will increase the area for the type I black hole and decrease the area for the type II black hole.

RESULTADOS DE INVESTIGACIONES SUBGRADUADA REALIZADA EN LOS CURSOS FISI 4161-64 Número 6

ON THE CARCINOGENIC EFFECTS OF SPACE RADIATION ON THE INTERNATIONAL STATION’S CREW

Pierina Agosto1 Mentor: Ernesto P. Esteban2

1Biology Department, University of Puerto Rico-Humacao, Humacao 2Department of Physics and Electronics, University of Puerto Rico-Humacao

The carcinogenic effects of Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE, April 21, 2002) on the International Space Station (ISS) crew are investigated. To mimic a carcinogenesis process, we have modified the mathematical Jones model of radiation-induced myelopoiesis by adding new compartments corresponding to mutated and cancerous cells populations. This new bio-mathematical model can be written in terms of a set of n+4 non-linear differential equations, where n is the number of mutations.

We assume n=2 mutations, and GCR as well as SPE dose rates measured by the STS 91 Shuttle-Mir bmission, and also calculated using BRYNTRN (a baryon transport computer code) to estimate ISS's as-tronauts survival curves and cancerous cell populations.

In this research, we have considered continuous cycles of five months missions at the ISS, and the following twelve months grounded at Earth. We found that the safest longest stay of the ISSs astronauts without increasing cancer risks is 9.5 years, and 2.43 hours when they are exposed to GCR and SPE, respectively.

Nota de los editores

La inclusión de la investigación y la labor creativa son fundamentales para el

desarrollo integral de los estudios subgraduados. Los miembros del Departa-

mento de Física y Electrónica reconocen la importancia de estos elementos

como parte fundamental de la labor académica. La aplicación del conoci-

miento en la búsqueda de nuevas verdades y la interpretación del mundo que

nos rodea, permite el desarrollo del pensamiento crítico; es una herramienta

académica innovadora para motivar y satisfacer la curiosidad intelectual del

los/las estudiantes. Además contribuye al desarrollo social y cultural

agilizando la inserción en el ciclo económico de Puerto Rico y del mundo.

La inmersión de estudiantes en investigación subgraduada se hace desde los

inicios del programa de Bachillerato de Física Aplicada a la Electrónica, en

1987, mediante el ofrecimiento del curso Investigación Subgraduada (FISI

4161). La usual contratación de Profesores con peritaje en investigación

capturó el interés de los estudiantes de Bachillerato para involucrarse en tan

digna tarea. El esmero y dedicación de los profesores ayudó a ellos mismos

a trasformar las técnicas de enseñanza debido al desafío que implica investi-

gar y formar a estudiantes en la disciplina.

La consecuencia de estos esfuerzos condujo a aumentar el ofrecimiento

debido al interés de los estudiantes en hacer investigación. En consecuencia

se atendió la demanda de los estudiantes, incluyendo los curso de Investiga-

ción Subgraduada (FISI 4162 FISI 4163 y FISI 4164). Los cuatro cursos

mencionados se ofrecen regularmente desde agosto de 1993. El compromi-

so permitió a la academia refinar las estrategias de enseñanza-aprendizaje,

que en esencia son el modelo tomado por el constructivismo, el método de

prueba y error.

Después de veinticuatro años de esfuerzo, el Departamento cuenta con una

decena de profesores que escriben propuestas, consiguen fondos externos y

hacen presentaciones en reuniones internacionales de ciencia, y publican

periódicamente en revistas de circulación internacional. En el Departamento

hay tres laboratorios dedicados a la investigación y un laboratorio de instruc-

ción equipado con instrumentos de primera línea en el que se ofrecen dos

cursos de concentración. También otras facilidades de investigación son los

recursos del Observatorio Astronómico de Humacao, en el cual la prepara-

ción obtenida por los estudiantes les facilita tener opción de seguir estudios .

Vale destacar el compromiso de la Institución (UPRH) que hace posible la

investigación subgraduada, es por ello que me complace reconocer a las

diferentes administraciones que están y han colaborando en este esfuerzo.

El reconocimiento es también extendido a los profesores por la labor de

adiestramiento a los estudiantes. La revista de investigación subgraduada fue

concebida para que sea un instrumento administrativo que permita la visibili-

dad de esta actividad, pero además es un documento dirigido a los estudian-

tes, profesores y la comunidad científica de Puerto Rico, con el propósito de

divulgar las actividades académicas que han sido pioneras en el Sistemas

UPR junto con otros Recintos. Este instrumento estará además disponible en

la Internet para permitir el acceso a todas las comunidades.

Proyectos Auspiciadores: 1. Puerto Rico Space Grant Consortium (NASA) 2. Puerto Rico Alliance for Minority Participation (LSAMP) 3. NSF - Penn-UPR Partnership for Research and Education in Materials (DMR-0353730) - Research in Undergraduate Institutions ( RUI) 5. RISE Program at UPRH 6. PRIDCO

Universidad de Puerto Rico en Humacao Departamento de Física y Electrónica

Call Box 860 Humacao PR 00792 www.uprh.edu/fisica

Tel. 787-850-9344-9381 Fax: 787-850-9308


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