Rodrigo Bernal - Academia.edu (original) (raw)

Papers by Rodrigo Bernal

EPJ Web of Conferences, 2010

One dimensional nanostructures, like nanowires and nanotubes, are increasingly being researched f... more One dimensional nanostructures, like nanowires and nanotubes, are increasingly being researched for the development of next generation devices like logic gates, transistors, and solar cells. In particular, semiconducting nanowires with a nonsymmetric wurtzitic crystal structure, such as zinc oxide (ZnO) and gallium nitride (GaN), have drawn immense research interests due to their electromechanical coupling. The designing of the future nanowire-based devices requires component-level characterization of individual nanowires. In this paper, we present a unique experimental setup to characterize the mechanical and electromechanical behaviour of individual nanowires. Using this setup and complementary atomistic simulations, mechanical properties of ZnO nanowires and electromechanical properties of GaN nanowires were investigated. In ZnO nanowires, elastic modulus was found to depend on nanowire diameter decreasing from 190 GPa to 140 GPa as the wire diameter increased from 5 nm to 80 nm. Inconsistent failure mechanisms were observed in ZnO nanowires. Experiments revealed a brittle fracture, whereas simulations using a pairwise potential predicted a phase transformation prior to failure. This inconsistency is addressed in detail from an experimental as well as computational perspective. Lastly, in addition to mechanical properties, preliminary results on the electromechanical properties of gallium nitride nanowires are also reported. Initial investigations reveal that the piezoresistive and piezoelectric behaviour of nanowires is different from bulk gallium nitride. 1 In situ testing of nanostructures Semiconducting nanowires (NWs) are potential building blocks for future electronic devices like logic circuits, transistors, and solar cells. NWs made of materials like zinc oxide (ZnO) and gallium nitride (GaN) are interesting for the research community due to their piezoelectric behaviour, offered by the non-symmetric wurtzite crystal structure. For example, ZnO NWs has been conceptually shown to generate electrical output when mechanically deformed [1-3]. To optimize the potential performance of piezoelectric NW-based devices, characterization of individual NWs at the component level is crucial. In this paper, a microelectromechanical system (MEMS)-based

Ultramicroscopy, 2015

MEMS and other lab-on-a-chip systems are emerging as attractive alternatives to carry out experim... more MEMS and other lab-on-a-chip systems are emerging as attractive alternatives to carry out experiments in situ the electron microscope. However, several electrical connections are usually required for operating these setups. Such connectivity is challenging inside the limited space of the TEM sideentry holder. Here, we design, implement and demonstrate a double-tilt TEM holder with capabilities for up to 9 electrical connections, operating in a high-resolution TEM. We describe the operating principle of the tilting and connection mechanisms and the physical implementation of the holder. To demonstrate the holder capabilities, we calibrate the tilting action, which has limits of ± 15°, and establish the insulation resistance of the electronics to be 36 GΩ, appropriate for measurements of currents down to the nano-amp (nA) regime. Furthermore, we demonstrate tensile testing of silver nanowires using a previously developed MEMS device for mechanical testing, using the implemented holder as the platform for electronic operation and sensing. The implemented holder can potentially have broad application to other areas where MEMS or electrically-actuated setups are used to carry out in situ TEM experiments.

Nano Letters, 2012

nanometers in diameter, nanowires are often considered one-dimensional. But researchers at Northw... more nanometers in diameter, nanowires are often considered one-dimensional. But researchers at Northwestern University have recently reported that individual gallium nitride nanowires show strong piezoelectricity-a type of chargegeneration caused by mechanical stress-in three dimensions.

Meccanica, 2014

MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscal... more MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscale materials. In a typical configuration, their design includes an actuator to deliver loads/displacements to a sample, and a sensing unit for load measurement. The sensing unit consists of a flexible structure, which deforms in response to the force imposed to the sample. Such deformation, while being necessary for the sensing function, may become a source of instability. When the sample experiences a load drop, as it may result from yield, necking or phase transitions, the elastic energy accumulated by the sensor can be released, thus leading to loss of the displacement-controlled condition and dynamic failure. Here, we report a newly-developed MEMS testing system where the sensor is designed to constantly keep its equilibrium position through an electrostatic feedback-control. We show design, implementation, and calibration of the system, as well as validation by tensile testing of silver nanowires. The implemented system allows capture of softening events and affords significant improvement on the resolution of stressstrain curves.

Advanced Materials, 2011

Utilizing the full mechanical capabilities of individual carbon nanotubes (CNT)-which can exhibit... more Utilizing the full mechanical capabilities of individual carbon nanotubes (CNT)-which can exhibit tensile strength and elastic modulus of up to 1TPa and 100 GPa, respectively [ 1-4 ]has motivated a great deal of interest in CNT based nanocomposite materials. [ 5-10 ] Despite this signifi cant scientifi c effort, the strength, modulus, and toughness of CNT based fi bers and composites are typically dominated by weak shear interactions between adjacent shells, tubes, bundles, and matrix materials, [ 2 , 4 , 10 , 11 ] which has limited their application to hierarchical macroscopic composite materials. Here we demonstrate that the mechanical performance of double-walled nanotube (DWNT) bundles is greatly enhanced through high-energy electron-irradiation-induced shell-shell and tube-tube crosslinking. The effective tensile strength and elastic modulus are found to increase by an order of magnitude as compared to un-crosslinked bundles. This enhancement is attributed to covalent bonds formed between outer and inner DWNT shells as well as adjacent DWNT outer tubes within the bundle. Distinct failure mechanisms were also identifi ed through in situ transmission electron microscopy (TEM) tensile tests of individual DWNT bundles revealing a sword-in-sheath like failure mechanism for low cross-linked bundles and complete fracture of all shells for highly cross-linked bundles. The optimized irradiation-induced cross-linking enhancements of DWNT bundles demonstraed here are predicted to translate to up to order-of-magnitude improvements in the mechanical behavior of advanced composites. Engineering lateral interactions through cross-linking has become an essential tool in the development of advanced hierarchical composite materials, many of which are inspired by natural interfaces. [ 12-15 ] Successful hierarchical designs typically require both cross-linking to strong/stiff reinforcement elements (e.g. mineral crystals in the collagen fi brils in bone [ 14 ]) coupled with soft-sacrifi cial cross-linking (e.g. hydrogen bonding between beta-sheet crystals in spider silk [ 16 ]) between elements to enhance toughness. In the case of individual CNTs and CNT bundles, one successful approach has been covalent cross-linking via high energy

Resumen Las palmas están entre los recursos más utilizados en los trópicos y por tanto muchas de ... more Resumen Las palmas están entre los recursos más utilizados en los trópicos y por tanto muchas de sus especies son susceptibles de agotarse o incluso desaparecer, si no se establecen prácticas adecuadas de uso sostenible. Estas prácticas deben derivarse de investigación rigurosa con un protocolo adecuado. Se describe aquí un protocolo de investigación para estudiar la sostenibilidad del manejo de palmas, resumido en los siguientes seis puntos: 1. Revisión bibliográfica; 2. Establecimiento de la oferta del recurso, que involucra estudios ecológicos para conocer la distribución, abundancia y fenología; 3. Caracterización de la cosecha, que está relacionado con la estructura de la palma y la parte de la palma que se usa; 4. Evaluación de la sostenibilidad de la cosecha actual, la cual dependiendo del tiempo y de los recursos, puede incluir estudios de la estructura de la población o de su dinámica, preferiblemente los dos combinados; 5. Modelación de la cosecha sostenible; y 6. Plan de ...

EPJ Web of Conferences, 2010

One dimensional nanostructures, like nanowires and nanotubes, are increasingly being researched f... more One dimensional nanostructures, like nanowires and nanotubes, are increasingly being researched for the development of next generation devices like logic gates, transistors, and solar cells. In particular, semiconducting nanowires with a nonsymmetric wurtzitic crystal structure, such as zinc oxide (ZnO) and gallium nitride (GaN), have drawn immense research interests due to their electromechanical coupling. The designing of the future nanowire-based devices requires component-level characterization of individual nanowires. In this paper, we present a unique experimental setup to characterize the mechanical and electromechanical behaviour of individual nanowires. Using this setup and complementary atomistic simulations, mechanical properties of ZnO nanowires and electromechanical properties of GaN nanowires were investigated. In ZnO nanowires, elastic modulus was found to depend on nanowire diameter decreasing from 190 GPa to 140 GPa as the wire diameter increased from 5 nm to 80 nm. Inconsistent failure mechanisms were observed in ZnO nanowires. Experiments revealed a brittle fracture, whereas simulations using a pairwise potential predicted a phase transformation prior to failure. This inconsistency is addressed in detail from an experimental as well as computational perspective. Lastly, in addition to mechanical properties, preliminary results on the electromechanical properties of gallium nitride nanowires are also reported. Initial investigations reveal that the piezoresistive and piezoelectric behaviour of nanowires is different from bulk gallium nitride. 1 In situ testing of nanostructures Semiconducting nanowires (NWs) are potential building blocks for future electronic devices like logic circuits, transistors, and solar cells. NWs made of materials like zinc oxide (ZnO) and gallium nitride (GaN) are interesting for the research community due to their piezoelectric behaviour, offered by the non-symmetric wurtzite crystal structure. For example, ZnO NWs has been conceptually shown to generate electrical output when mechanically deformed [1-3]. To optimize the potential performance of piezoelectric NW-based devices, characterization of individual NWs at the component level is crucial. In this paper, a microelectromechanical system (MEMS)-based

Ultramicroscopy, 2015

MEMS and other lab-on-a-chip systems are emerging as attractive alternatives to carry out experim... more MEMS and other lab-on-a-chip systems are emerging as attractive alternatives to carry out experiments in situ the electron microscope. However, several electrical connections are usually required for operating these setups. Such connectivity is challenging inside the limited space of the TEM sideentry holder. Here, we design, implement and demonstrate a double-tilt TEM holder with capabilities for up to 9 electrical connections, operating in a high-resolution TEM. We describe the operating principle of the tilting and connection mechanisms and the physical implementation of the holder. To demonstrate the holder capabilities, we calibrate the tilting action, which has limits of ± 15°, and establish the insulation resistance of the electronics to be 36 GΩ, appropriate for measurements of currents down to the nano-amp (nA) regime. Furthermore, we demonstrate tensile testing of silver nanowires using a previously developed MEMS device for mechanical testing, using the implemented holder as the platform for electronic operation and sensing. The implemented holder can potentially have broad application to other areas where MEMS or electrically-actuated setups are used to carry out in situ TEM experiments.

Nano Letters, 2012

nanometers in diameter, nanowires are often considered one-dimensional. But researchers at Northw... more nanometers in diameter, nanowires are often considered one-dimensional. But researchers at Northwestern University have recently reported that individual gallium nitride nanowires show strong piezoelectricity-a type of chargegeneration caused by mechanical stress-in three dimensions.

Meccanica, 2014

MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscal... more MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscale materials. In a typical configuration, their design includes an actuator to deliver loads/displacements to a sample, and a sensing unit for load measurement. The sensing unit consists of a flexible structure, which deforms in response to the force imposed to the sample. Such deformation, while being necessary for the sensing function, may become a source of instability. When the sample experiences a load drop, as it may result from yield, necking or phase transitions, the elastic energy accumulated by the sensor can be released, thus leading to loss of the displacement-controlled condition and dynamic failure. Here, we report a newly-developed MEMS testing system where the sensor is designed to constantly keep its equilibrium position through an electrostatic feedback-control. We show design, implementation, and calibration of the system, as well as validation by tensile testing of silver nanowires. The implemented system allows capture of softening events and affords significant improvement on the resolution of stressstrain curves.

Advanced Materials, 2011

Utilizing the full mechanical capabilities of individual carbon nanotubes (CNT)-which can exhibit... more Utilizing the full mechanical capabilities of individual carbon nanotubes (CNT)-which can exhibit tensile strength and elastic modulus of up to 1TPa and 100 GPa, respectively [ 1-4 ]has motivated a great deal of interest in CNT based nanocomposite materials. [ 5-10 ] Despite this signifi cant scientifi c effort, the strength, modulus, and toughness of CNT based fi bers and composites are typically dominated by weak shear interactions between adjacent shells, tubes, bundles, and matrix materials, [ 2 , 4 , 10 , 11 ] which has limited their application to hierarchical macroscopic composite materials. Here we demonstrate that the mechanical performance of double-walled nanotube (DWNT) bundles is greatly enhanced through high-energy electron-irradiation-induced shell-shell and tube-tube crosslinking. The effective tensile strength and elastic modulus are found to increase by an order of magnitude as compared to un-crosslinked bundles. This enhancement is attributed to covalent bonds formed between outer and inner DWNT shells as well as adjacent DWNT outer tubes within the bundle. Distinct failure mechanisms were also identifi ed through in situ transmission electron microscopy (TEM) tensile tests of individual DWNT bundles revealing a sword-in-sheath like failure mechanism for low cross-linked bundles and complete fracture of all shells for highly cross-linked bundles. The optimized irradiation-induced cross-linking enhancements of DWNT bundles demonstraed here are predicted to translate to up to order-of-magnitude improvements in the mechanical behavior of advanced composites. Engineering lateral interactions through cross-linking has become an essential tool in the development of advanced hierarchical composite materials, many of which are inspired by natural interfaces. [ 12-15 ] Successful hierarchical designs typically require both cross-linking to strong/stiff reinforcement elements (e.g. mineral crystals in the collagen fi brils in bone [ 14 ]) coupled with soft-sacrifi cial cross-linking (e.g. hydrogen bonding between beta-sheet crystals in spider silk [ 16 ]) between elements to enhance toughness. In the case of individual CNTs and CNT bundles, one successful approach has been covalent cross-linking via high energy

Resumen Las palmas están entre los recursos más utilizados en los trópicos y por tanto muchas de ... more Resumen Las palmas están entre los recursos más utilizados en los trópicos y por tanto muchas de sus especies son susceptibles de agotarse o incluso desaparecer, si no se establecen prácticas adecuadas de uso sostenible. Estas prácticas deben derivarse de investigación rigurosa con un protocolo adecuado. Se describe aquí un protocolo de investigación para estudiar la sostenibilidad del manejo de palmas, resumido en los siguientes seis puntos: 1. Revisión bibliográfica; 2. Establecimiento de la oferta del recurso, que involucra estudios ecológicos para conocer la distribución, abundancia y fenología; 3. Caracterización de la cosecha, que está relacionado con la estructura de la palma y la parte de la palma que se usa; 4. Evaluación de la sostenibilidad de la cosecha actual, la cual dependiendo del tiempo y de los recursos, puede incluir estudios de la estructura de la población o de su dinámica, preferiblemente los dos combinados; 5. Modelación de la cosecha sostenible; y 6. Plan de ...