J. Jiao - Academia.edu (original) (raw)

Papers by J. Jiao

Research paper thumbnail of Characterization and Manipulation of Carbon Precursor Species during Plasma Enhanced Chemical Vapor Deposition of Graphene

Nanomaterials, Nov 11, 2020

To develop a synthesis technique providing enhanced control of graphene film quality and uniformi... more To develop a synthesis technique providing enhanced control of graphene film quality and uniformity, a systematic characterization and manipulation of hydrocarbon precursors generated during plasma enhanced chemical vapor deposition of graphene is presented. Remote ionization of acetylene is observed to generate a variety of neutral and ionized hydrocarbon precursors, while in situ manipulation of the size and reactivity of carbon species permitted to interact with the growth catalyst enables control of the resultant graphene morphology. Selective screening of high energy hydrocarbon ions coupled with a multistage bias growth regime results in the production of 90% few-to-monolayer graphene on 50 nm Ni/Cu alloy catalysts at 500 • C. Additionally, synthesis with low power secondary ionization processes is performed and reveals further control during the growth, enabling a 50% reduction in average defect densities throughout the film. Mass spectrometry and UV-Vis spectroscopy monitoring of the reaction environment in conjunction with Raman characterization of the synthesized graphene films facilitates correlation of the carbon species permitted to reach the catalyst surface to the ultimate quality, layer number, and uniformity of the graphene film. These findings reveal a robust technique to control graphene synthesis pathways during plasma enhanced chemical vapor deposition.

Research paper thumbnail of Low-temperature chemical vapor deposition growth of graphene films enabled by ultrathin alloy catalysts

Journal of vacuum science and technology, Mar 31, 2020

This report introduces a method for fabricating graphene at low temperatures via chemical vapor d... more This report introduces a method for fabricating graphene at low temperatures via chemical vapor deposition enabled by ultrathin (∼1 nm) nickel-gold (Ni-Au) catalysts. The unique combination of high carbon (C) solubility Ni, low C solubility Au, and an ultrathin layer of a catalyst demonstrates the effectiveness to produce graphene at 450°C with the layer number independent of growth duration. In contrast to grain-boundary defined catalyst morphology found in thicker (>20 nm) metal catalysts, the ultrathin catalyst morphology leads to the formation of nanoscale metal "islands" during the growth process, which results in curved graphene covering the catalyst. To test the effect of preactivation of the ultrathin catalyst for the formation of graphene, a preanneal process of the catalyst followed by the introduction of a carbon precursor was also investigated. The preanneal process resulted in the formation of carbon nanotubes (CNTs) in lieu of graphene, displaying the impact of the catalytic surface treatment in relation to the produced materials. The results and discussion presented here detail a low-temperature nanoscale manufacturing process that allows for the production of either graphene or CNTs on an ultrathin catalyst.

Research paper thumbnail of Spatial Identification of Defect Sites in Individual Carbon Nanotubes Using Micro-Raman Spectroscopy

Microscopy and Microanalysis, 2010

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Research paper thumbnail of Palladium and gold nanoparticles on carbon supports as highly efficient catalysts for effective removal of trichloroethylene

Journal of Materials Research, 2018

Palladium (Pd) and gold (Au) nanoparticles (NPs) hybridized on two types of carbon supports, grap... more Palladium (Pd) and gold (Au) nanoparticles (NPs) hybridized on two types of carbon supports, graphene and granular activated carbon (GAC), were shown to be promising catalysts for the sustainable hydrodehalogenation of aqueous trichloroethylene (TCE). These catalysts are capable of degrading TCE more rapidly than commercial Pd-on-GAC catalysts. The catalysts were synthesized at room temperature without the use of any environmentally unfriendly chemicals. Pd was chosen for its catalytic potency to break down TCE, while Au acts as a strong promoter of the catalytic activity of Pd. The results indicate that both graphene and GAC are favorable supports for the NPs due to high surface-to-volume ratios, unique surface properties, and the prevention of NP aggregation. The properties of NP catalysts were characterized using electron microscopy and spectroscopy techniques. The TCE degradation results indicate that the GAC-supported catalysts have a higher rate of TCE removal than the commercial Pd-on-GAC catalyst, and the degradation rate is greatly increased when using graphene-supported samples. I. INTRODUCTION Hydrogenation associated with palladium (Pd)-based catalysts has shown promise as a fast, efficient, and reliable technique for sustainable groundwater treatment, especially against volatile organic compounds (VOCs). Pd is more catalytically active, chemically stable, and reaction-selective in comparison to other metals, such as iridium, rhodium, zinc, etc. 1 It is especially potent for the decomposition of VOCs. However, Pd by itself is susceptible to some operational and environmental conditions that cause the catalyst to deactivate rapidly, especially in the presence of sulfide and chloride. 2,3 Particulate forms of Pd-based catalysts have been fabricated both individually and on a variety of supports, including alumina, 4 silica, activated carbon, zeolites, 1 titania, 5,6 etc., as well as with various metal promoters for the catalyst. These metals include copper (Cu), 7-9 tin (Sn), 9 iron (Fe), 10 magnesium (Mg), 10 and gold (Au). 11 Among these, Au is specifically known to boost the catalytic activity of Pd by increasing the adsorption of a)

Research paper thumbnail of Growth of Carbon Nanotubes with Controlled Morphologies

MRS Proceedings, 2002

The alignment, orientation and morphologies of multi-walled carbon nanotubes (MWNTs) can be tailo... more The alignment, orientation and morphologies of multi-walled carbon nanotubes (MWNTs) can be tailored by controlling catalyst deposition on porous silicon substrates. During the growth of MWNTs, H2 promoted the growth of carbon nanotubes and prevented the formation of amorphous carbon particles. With the introduction of H2, the average diameter of MWNTs decreased from 130 nm to 15 nm, and the average growth rate of nanotubes increased from 50 nm/s to 145 nm/s. The use of CH4 as the carbon source resulted in single-walled carbon nanotubes (SWNTs) with an average diameter of 2 nm, and the use of C2H2 as the carbon source resulted in MWNTs with an average diameter of 15 nm.

Research paper thumbnail of Measurement of impedance of individual carbon nanotubes

2007 IEEE Instrumentation & Measurement Technology Conference IMTC 2007, 2007

We describe impedance measurements of individual single wall carbon nanotubes (SWNTs) in the freq... more We describe impedance measurements of individual single wall carbon nanotubes (SWNTs) in the frequency range of 40 Hz to 100 MHz. The tubes were assembled on the active channel of field effect transistor (FET) structures from aqueous suspension using dielectrophoresis. The FET channels were made by using photo-lithography. We utilized a resistance-capacitance (RC) lumped element circuit model to describe the observed impedance of the tubes and the corresponding contact resistance. At the low frequency limit the impedance is frequency independent and equivalent to the real resistance. In the high frequency range we observe a sharp conductor-insulator transition at a crossover frequency, above which the circuit response becomes capacitive. The extracted SWNT capacitance, CSWNT, of about 4 10-14 F/mum, is independent on the total real resistance, however the CSWNT value is larger than that theoretically predicted quantum capacitance of a perfect tube. Our observations also indicate that the damping frequency is lower than the theoretically predicted in SWNTs.

Research paper thumbnail of Synthesis of SiO2 Nanowires and CdS/SiO2 Composite Nanowires and Investigation of Their Electron Field Emission Properties

MRS Proceedings, 2002

ABSTRACTA thermal evaporation method was used to obtain SiO2 and CdS/SiO2 nanowires by heating Si... more ABSTRACTA thermal evaporation method was used to obtain SiO2 and CdS/SiO2 nanowires by heating Si substrates coated with a gold thin film in a quartz tube furnace. During growth, pure CdS powder was placed at the heating zone in the furnace, serving as the CdS source for the CdS/SiO2 composite nanowires. It was found that both non-porous and porous Si substrates served as the Si source for the growth of SiO2 nanowires and the CdS/SiO2 composite nanowires. It was also found that the effect of the temperature gradient in the reaction chamber plays an important role in the density distribution of different nanowires (SiO2 or CdS/SiO2 nanowires). The electron field emission properties of these nanowires were investigated using an electron field emission microscope equipped with a Faraday cup.

Research paper thumbnail of Evaluation of a thermal interface material fabricated using thermocompression bonding of carbon nanotube turf

Nanotechnology, 2009

In this work a thermal interface material fabricated by thermocompression bonding of vertically a... more In this work a thermal interface material fabricated by thermocompression bonding of vertically aligned carbon nanotube turf (VACNT) to metallized substrates was characterized. The VACNT structure was fabricated onto silicon substrates using chemical vapor deposition. The structures were then transferred to metallized substrates using thermocompression bonding. The resulting structure consisted of VACNT turf sandwiched between two layers of Au. Two configurations of VACNT, full coverage and patterned, were fabricated and tested. In addition, the thermal interface resistance of structures at intermediate steps in the thermocompression bonding process were measured. For the full coverage turf a thermal interface resistance of 1.082 cm(2) degrees C W(-1) at an applied load of 1 N was measured, while a thermal interface resistance of 0.044 cm(2) degrees C W(-1) at a load of 1 N was measured for the patterned turf configuration.

Research paper thumbnail of Electron Microscopy Study of Exotic Nanostructures of Cadmium Sulfide

Microscopy and Microanalysis, 2005

In this article, two simple methods, evaporation-condensation and catalytic thermal evaporation, ... more In this article, two simple methods, evaporation-condensation and catalytic thermal evaporation, were used to investigate the synthesis of CdS nanostructures for nanoscale optoelectronic applications. To understand their growth mechanisms, various electron microscopy and microanalysis techniques were utilized in characterizing their morphologies, internal structures, growth directions and elemental compositions. The electron microscopy study reveals that when using the evaporation-condensation method, branched CdS nanorods and self-assembled arrays of CdS nanorods were synthesized at 800°C and 1000°C, respectively. Instead of morphological differences, both types of CdS nanorods grew along the [0001] direction. However, when using the catalytic thermal evaporation method (Au as the catalyst), patterned CdS nanowires and nanobelts were formed at the temperature region of 500–600°C and 600–750°C, respectively. Their growth direction was along the direction [1010] instead of [0001]. Ba...

Research paper thumbnail of Electron Microscopy Study of Carbon Nanotubes Synthesized by Plasma-Enhanced Chemical Vapor Deposition

Microscopy and Microanalysis, 2006

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois,... more Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Research paper thumbnail of Catalytic growth of CdS nanobelts and nanowires on tungsten substrates

Chemical Physics Letters, 2003

In this Letter, we report for the first time the observation of CdS nanobelts. Using a simple the... more In this Letter, we report for the first time the observation of CdS nanobelts. Using a simple thermal evaporation method, CdS nanobelts and nanowires with controllable growth positions were synthesized on tungsten substrates. To explore growth mechanisms of these nanostructures, we have examined the influence of temperature, substrate, and catalyst on their growth properties including morphology, composition, and internal structure. Compared with CdS nanowires, CdS nanobelts were formed in a higher temperature region. However, both growths were initiated by Au catalysts. When silicon or porous silicon was used as substrates, SiO 2 and SiO 2 /CdS composite nanobelts and nanowires were synthesized. The growth mechanisms of these nanostructures are discussed.

Research paper thumbnail of Local and non-local behavior and coordinated buckling of CNT turfs

Carbon, 2011

The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotub... more The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotubes (CNTs), referred to as turfs, have been measured using nanoindentation. The elastic properties appear to be non-linear; as noted in prior studies the observed tangent modulus decreases with increasing strain. Decreasing adhesion between the turf and probe lowers the perceived stiffness of the material. The elastic properties

Research paper thumbnail of Low-Temperature Nitrogen Doping in Ammonia Solution for Production of N-Doped TiO2-Hybridized Graphene as a Highly Efficient Photocatalyst for Water Treatment

ACS Sustainable Chemistry & Engineering, 2014

To facilitate the potential application of TiO 2 as an efficient photocatalyst, the modulation of... more To facilitate the potential application of TiO 2 as an efficient photocatalyst, the modulation of its band gap and electrical structure is of great significance. Herein, we report a very simple nitrogen (N)-doping method to obtain N-doped TiO 2 , which is hybridized with graphene sheets at a temperature as low as 180°C and using an ammonia solution as the N source and reaction medium. X-ray photoelectron spectroscopy analysis revealed that the atomic N level could reach 2.4 atomic percent when the reaction time was 14 h. Photoluminescence (PL) emission spectra indicated that Ndoped TiO 2 /graphene composites have drastically suppressed TiO 2 PL intensity when compared to undoped TiO 2 , confirming the lower recombination rate of electron−hole pairs in the N-doped TiO 2. Additionally, photodegradation data showed that the decomposition rate of methylene blue with our N-doped TiO 2 /graphene photocatalyst is twice as fast as a commercial Degussa P25 catalyst. Furthermore, density functional theory (DFT) calculations demonstrate that N doping creates empty states in the band gap of the TiO 2 that are below the Fermi energy of graphene. Thus, when N-doped TiO 2 is brought into contact with graphene, these states become filled by electrons from the graphene, shifting the TiO 2 bands upward relative to the graphene. Such a shift in band alignment across the TiO 2 /graphene heterojunction makes transfer of the photoexcited electron more energetically favorable. This work provides a very convenient chemical route to the scalable production of N-doped TiO 2 / graphene photocatalyst for potential applications in wastewater treatment.

Research paper thumbnail of Simulation to fabrication—understanding the effect of NiAuCu alloy catalysts for controlled growth of graphene at reduced temperature

Materials Research Express, 2019

It is a significant challenge to grow large-scale, high quality, monolayer graphene at low temper... more It is a significant challenge to grow large-scale, high quality, monolayer graphene at low temperature for the applications in industry, especially for the complementary metal oxide semiconductor fabrication process. To overcome this difficulty, we simulated the decomposition of acetylene (C2H2) on (100) surfaces of primarily nickel (Ni) catalysts with small mol fractions of gold (Au) and copper (Cu), using a 4 × 4 × 4 periodic supercell model. Based on the calculation of the reaction energies to decompose the C-H or C≡C bonds on different catalyst surfaces, a differential energy is proposed to clearly scale the decomposition difficulties such that larger differential energy leads to easier control of the monolayer growth. It is observed that on the NiAuCu alloy surface with a mol fraction 0.0313 of both Au and Cu, the differential energy of the C-H bonds and the C≡C bond are both positive, showing an obvious modulation effect on the decomposition of C2H2 and the catalytic activites...

Research paper thumbnail of Varying Phases of Alumina Nanowires Templated by Vertically Aligned Carbon Nanotubes Grown via Atomic Layer Deposition

Microscopy and Microanalysis, 2014

Research paper thumbnail of Thermocompression bonding of vertically aligned carbon nanotube turfs to metalized substrates

Nanotechnology, 2009

Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical car... more Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical carbon nanotubes, are being proposed for use as electrical and thermal contact materials. Issues in their implementation include high contact resistance, the van der Waals interactions of carbon nanotubes, and a low temperature limit during processing. One route for circumventing the 750 degrees C temperatures required for VACNT growth using chemical vapor deposition is for the VACNTs to be grown separately, and then transferred to the device. A method of mechanical transfer, using thermocompression bonding, has been developed, allowing dry mechanical transfer of the VACNTs at 150 degrees C. This method can be used for the construction of both a thermal switch or a permanent conducting channel. The conductivity of the bonded structure is shown to be independent of the imposed strain, up to strains in excess of 100%.

Research paper thumbnail of Annealing effect on Sb2S3-TiO2 nanostructures for solar cell applications

Nanoscale Research Letters, 2013

Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were pre... more Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were prepared on an F:SnO2 conductive glass by hydrothermal method and successive ionic layer adsorption and reaction method at low temperature. Sb2S3-sensitized TiO2 nanorod solar cells were assembled using the Sb2S3-TiO2 nanostructure as the photoanode and a polysulfide solution as an electrolyte. Annealing effects on the optical and photovoltaic properties of Sb2S3-TiO2 nanostructure were studied systematically. As the annealing temperatures increased, a regular red shift of the bandgap of Sb2S3 nanoparticles was observed, where the bandgap decreased from 2.25 to 1.73 eV. At the same time, the photovoltaic conversion efficiency for the nanostructured solar cells increased from 0.46% up to 1.47% as a consequence of the annealing effect. This improvement can be explained by considering the changes in the morphology, the crystalline quality, and the optical properties caused by the annealing tre...

Research paper thumbnail of Synthesis and Microanalysis of Aligned Carbon Nanotube Arrays

Microscopy and Microanalysis, 2005

Carbon nanotubes (CNTs) have been considered promising candidates for thermal devices due to thei... more Carbon nanotubes (CNTs) have been considered promising candidates for thermal devices due to their high thermal conductivity. Indeed, the multi-walled carbon nanotube (MWCNT) has demonstrated a thermal conductivity of more than 3000 W/mK at room temperature [1]. However, randomly aligned MWCNT films have displayed much lower thermal conductivities in the range of 25-35 W/mK [2]. This suggests that it is possible to exploit the much higher thermal conductivity of CNTs by using well-aligned arrays capable of improved thermal coupling.

Research paper thumbnail of Characterization of Silicon-Germanium Oxide Nanocomposites Fabricated by the Marine Diatom Nitzschia Frustulum

Microscopy and Microanalysis, 2005

Research paper thumbnail of Fabrication of Metal-Gated Carbon Nanotube Emitter Arrays Assisted by Focused Ion Beam and Chemical Vapor Deposition

Microscopy and Microanalysis, 2005

Carbon nanotubes (CNTs) have excellent electronic and structural properties. The high aspect rati... more Carbon nanotubes (CNTs) have excellent electronic and structural properties. The high aspect ratio and small tip radius of CNTs make them especially suitable as potential electron sources [1]. The objective of this study is to develop a technique for fabricating triode-type CNT field emitter arrays, where an integrated extraction gate is built between the cathode and the anode. The gate electrode is placed close to the cathode, making the gate to emitter distance small. As a result, the gate can attract or repel electrons with modest changes in gate voltage (positive or negative), enabling greater control over the emission current [2].

Research paper thumbnail of Characterization and Manipulation of Carbon Precursor Species during Plasma Enhanced Chemical Vapor Deposition of Graphene

Nanomaterials, Nov 11, 2020

To develop a synthesis technique providing enhanced control of graphene film quality and uniformi... more To develop a synthesis technique providing enhanced control of graphene film quality and uniformity, a systematic characterization and manipulation of hydrocarbon precursors generated during plasma enhanced chemical vapor deposition of graphene is presented. Remote ionization of acetylene is observed to generate a variety of neutral and ionized hydrocarbon precursors, while in situ manipulation of the size and reactivity of carbon species permitted to interact with the growth catalyst enables control of the resultant graphene morphology. Selective screening of high energy hydrocarbon ions coupled with a multistage bias growth regime results in the production of 90% few-to-monolayer graphene on 50 nm Ni/Cu alloy catalysts at 500 • C. Additionally, synthesis with low power secondary ionization processes is performed and reveals further control during the growth, enabling a 50% reduction in average defect densities throughout the film. Mass spectrometry and UV-Vis spectroscopy monitoring of the reaction environment in conjunction with Raman characterization of the synthesized graphene films facilitates correlation of the carbon species permitted to reach the catalyst surface to the ultimate quality, layer number, and uniformity of the graphene film. These findings reveal a robust technique to control graphene synthesis pathways during plasma enhanced chemical vapor deposition.

Research paper thumbnail of Low-temperature chemical vapor deposition growth of graphene films enabled by ultrathin alloy catalysts

Journal of vacuum science and technology, Mar 31, 2020

This report introduces a method for fabricating graphene at low temperatures via chemical vapor d... more This report introduces a method for fabricating graphene at low temperatures via chemical vapor deposition enabled by ultrathin (∼1 nm) nickel-gold (Ni-Au) catalysts. The unique combination of high carbon (C) solubility Ni, low C solubility Au, and an ultrathin layer of a catalyst demonstrates the effectiveness to produce graphene at 450°C with the layer number independent of growth duration. In contrast to grain-boundary defined catalyst morphology found in thicker (>20 nm) metal catalysts, the ultrathin catalyst morphology leads to the formation of nanoscale metal "islands" during the growth process, which results in curved graphene covering the catalyst. To test the effect of preactivation of the ultrathin catalyst for the formation of graphene, a preanneal process of the catalyst followed by the introduction of a carbon precursor was also investigated. The preanneal process resulted in the formation of carbon nanotubes (CNTs) in lieu of graphene, displaying the impact of the catalytic surface treatment in relation to the produced materials. The results and discussion presented here detail a low-temperature nanoscale manufacturing process that allows for the production of either graphene or CNTs on an ultrathin catalyst.

Research paper thumbnail of Spatial Identification of Defect Sites in Individual Carbon Nanotubes Using Micro-Raman Spectroscopy

Microscopy and Microanalysis, 2010

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Research paper thumbnail of Palladium and gold nanoparticles on carbon supports as highly efficient catalysts for effective removal of trichloroethylene

Journal of Materials Research, 2018

Palladium (Pd) and gold (Au) nanoparticles (NPs) hybridized on two types of carbon supports, grap... more Palladium (Pd) and gold (Au) nanoparticles (NPs) hybridized on two types of carbon supports, graphene and granular activated carbon (GAC), were shown to be promising catalysts for the sustainable hydrodehalogenation of aqueous trichloroethylene (TCE). These catalysts are capable of degrading TCE more rapidly than commercial Pd-on-GAC catalysts. The catalysts were synthesized at room temperature without the use of any environmentally unfriendly chemicals. Pd was chosen for its catalytic potency to break down TCE, while Au acts as a strong promoter of the catalytic activity of Pd. The results indicate that both graphene and GAC are favorable supports for the NPs due to high surface-to-volume ratios, unique surface properties, and the prevention of NP aggregation. The properties of NP catalysts were characterized using electron microscopy and spectroscopy techniques. The TCE degradation results indicate that the GAC-supported catalysts have a higher rate of TCE removal than the commercial Pd-on-GAC catalyst, and the degradation rate is greatly increased when using graphene-supported samples. I. INTRODUCTION Hydrogenation associated with palladium (Pd)-based catalysts has shown promise as a fast, efficient, and reliable technique for sustainable groundwater treatment, especially against volatile organic compounds (VOCs). Pd is more catalytically active, chemically stable, and reaction-selective in comparison to other metals, such as iridium, rhodium, zinc, etc. 1 It is especially potent for the decomposition of VOCs. However, Pd by itself is susceptible to some operational and environmental conditions that cause the catalyst to deactivate rapidly, especially in the presence of sulfide and chloride. 2,3 Particulate forms of Pd-based catalysts have been fabricated both individually and on a variety of supports, including alumina, 4 silica, activated carbon, zeolites, 1 titania, 5,6 etc., as well as with various metal promoters for the catalyst. These metals include copper (Cu), 7-9 tin (Sn), 9 iron (Fe), 10 magnesium (Mg), 10 and gold (Au). 11 Among these, Au is specifically known to boost the catalytic activity of Pd by increasing the adsorption of a)

Research paper thumbnail of Growth of Carbon Nanotubes with Controlled Morphologies

MRS Proceedings, 2002

The alignment, orientation and morphologies of multi-walled carbon nanotubes (MWNTs) can be tailo... more The alignment, orientation and morphologies of multi-walled carbon nanotubes (MWNTs) can be tailored by controlling catalyst deposition on porous silicon substrates. During the growth of MWNTs, H2 promoted the growth of carbon nanotubes and prevented the formation of amorphous carbon particles. With the introduction of H2, the average diameter of MWNTs decreased from 130 nm to 15 nm, and the average growth rate of nanotubes increased from 50 nm/s to 145 nm/s. The use of CH4 as the carbon source resulted in single-walled carbon nanotubes (SWNTs) with an average diameter of 2 nm, and the use of C2H2 as the carbon source resulted in MWNTs with an average diameter of 15 nm.

Research paper thumbnail of Measurement of impedance of individual carbon nanotubes

2007 IEEE Instrumentation & Measurement Technology Conference IMTC 2007, 2007

We describe impedance measurements of individual single wall carbon nanotubes (SWNTs) in the freq... more We describe impedance measurements of individual single wall carbon nanotubes (SWNTs) in the frequency range of 40 Hz to 100 MHz. The tubes were assembled on the active channel of field effect transistor (FET) structures from aqueous suspension using dielectrophoresis. The FET channels were made by using photo-lithography. We utilized a resistance-capacitance (RC) lumped element circuit model to describe the observed impedance of the tubes and the corresponding contact resistance. At the low frequency limit the impedance is frequency independent and equivalent to the real resistance. In the high frequency range we observe a sharp conductor-insulator transition at a crossover frequency, above which the circuit response becomes capacitive. The extracted SWNT capacitance, CSWNT, of about 4 10-14 F/mum, is independent on the total real resistance, however the CSWNT value is larger than that theoretically predicted quantum capacitance of a perfect tube. Our observations also indicate that the damping frequency is lower than the theoretically predicted in SWNTs.

Research paper thumbnail of Synthesis of SiO2 Nanowires and CdS/SiO2 Composite Nanowires and Investigation of Their Electron Field Emission Properties

MRS Proceedings, 2002

ABSTRACTA thermal evaporation method was used to obtain SiO2 and CdS/SiO2 nanowires by heating Si... more ABSTRACTA thermal evaporation method was used to obtain SiO2 and CdS/SiO2 nanowires by heating Si substrates coated with a gold thin film in a quartz tube furnace. During growth, pure CdS powder was placed at the heating zone in the furnace, serving as the CdS source for the CdS/SiO2 composite nanowires. It was found that both non-porous and porous Si substrates served as the Si source for the growth of SiO2 nanowires and the CdS/SiO2 composite nanowires. It was also found that the effect of the temperature gradient in the reaction chamber plays an important role in the density distribution of different nanowires (SiO2 or CdS/SiO2 nanowires). The electron field emission properties of these nanowires were investigated using an electron field emission microscope equipped with a Faraday cup.

Research paper thumbnail of Evaluation of a thermal interface material fabricated using thermocompression bonding of carbon nanotube turf

Nanotechnology, 2009

In this work a thermal interface material fabricated by thermocompression bonding of vertically a... more In this work a thermal interface material fabricated by thermocompression bonding of vertically aligned carbon nanotube turf (VACNT) to metallized substrates was characterized. The VACNT structure was fabricated onto silicon substrates using chemical vapor deposition. The structures were then transferred to metallized substrates using thermocompression bonding. The resulting structure consisted of VACNT turf sandwiched between two layers of Au. Two configurations of VACNT, full coverage and patterned, were fabricated and tested. In addition, the thermal interface resistance of structures at intermediate steps in the thermocompression bonding process were measured. For the full coverage turf a thermal interface resistance of 1.082 cm(2) degrees C W(-1) at an applied load of 1 N was measured, while a thermal interface resistance of 0.044 cm(2) degrees C W(-1) at a load of 1 N was measured for the patterned turf configuration.

Research paper thumbnail of Electron Microscopy Study of Exotic Nanostructures of Cadmium Sulfide

Microscopy and Microanalysis, 2005

In this article, two simple methods, evaporation-condensation and catalytic thermal evaporation, ... more In this article, two simple methods, evaporation-condensation and catalytic thermal evaporation, were used to investigate the synthesis of CdS nanostructures for nanoscale optoelectronic applications. To understand their growth mechanisms, various electron microscopy and microanalysis techniques were utilized in characterizing their morphologies, internal structures, growth directions and elemental compositions. The electron microscopy study reveals that when using the evaporation-condensation method, branched CdS nanorods and self-assembled arrays of CdS nanorods were synthesized at 800°C and 1000°C, respectively. Instead of morphological differences, both types of CdS nanorods grew along the [0001] direction. However, when using the catalytic thermal evaporation method (Au as the catalyst), patterned CdS nanowires and nanobelts were formed at the temperature region of 500–600°C and 600–750°C, respectively. Their growth direction was along the direction [1010] instead of [0001]. Ba...

Research paper thumbnail of Electron Microscopy Study of Carbon Nanotubes Synthesized by Plasma-Enhanced Chemical Vapor Deposition

Microscopy and Microanalysis, 2006

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois,... more Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Research paper thumbnail of Catalytic growth of CdS nanobelts and nanowires on tungsten substrates

Chemical Physics Letters, 2003

In this Letter, we report for the first time the observation of CdS nanobelts. Using a simple the... more In this Letter, we report for the first time the observation of CdS nanobelts. Using a simple thermal evaporation method, CdS nanobelts and nanowires with controllable growth positions were synthesized on tungsten substrates. To explore growth mechanisms of these nanostructures, we have examined the influence of temperature, substrate, and catalyst on their growth properties including morphology, composition, and internal structure. Compared with CdS nanowires, CdS nanobelts were formed in a higher temperature region. However, both growths were initiated by Au catalysts. When silicon or porous silicon was used as substrates, SiO 2 and SiO 2 /CdS composite nanobelts and nanowires were synthesized. The growth mechanisms of these nanostructures are discussed.

Research paper thumbnail of Local and non-local behavior and coordinated buckling of CNT turfs

Carbon, 2011

The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotub... more The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotubes (CNTs), referred to as turfs, have been measured using nanoindentation. The elastic properties appear to be non-linear; as noted in prior studies the observed tangent modulus decreases with increasing strain. Decreasing adhesion between the turf and probe lowers the perceived stiffness of the material. The elastic properties

Research paper thumbnail of Low-Temperature Nitrogen Doping in Ammonia Solution for Production of N-Doped TiO2-Hybridized Graphene as a Highly Efficient Photocatalyst for Water Treatment

ACS Sustainable Chemistry & Engineering, 2014

To facilitate the potential application of TiO 2 as an efficient photocatalyst, the modulation of... more To facilitate the potential application of TiO 2 as an efficient photocatalyst, the modulation of its band gap and electrical structure is of great significance. Herein, we report a very simple nitrogen (N)-doping method to obtain N-doped TiO 2 , which is hybridized with graphene sheets at a temperature as low as 180°C and using an ammonia solution as the N source and reaction medium. X-ray photoelectron spectroscopy analysis revealed that the atomic N level could reach 2.4 atomic percent when the reaction time was 14 h. Photoluminescence (PL) emission spectra indicated that Ndoped TiO 2 /graphene composites have drastically suppressed TiO 2 PL intensity when compared to undoped TiO 2 , confirming the lower recombination rate of electron−hole pairs in the N-doped TiO 2. Additionally, photodegradation data showed that the decomposition rate of methylene blue with our N-doped TiO 2 /graphene photocatalyst is twice as fast as a commercial Degussa P25 catalyst. Furthermore, density functional theory (DFT) calculations demonstrate that N doping creates empty states in the band gap of the TiO 2 that are below the Fermi energy of graphene. Thus, when N-doped TiO 2 is brought into contact with graphene, these states become filled by electrons from the graphene, shifting the TiO 2 bands upward relative to the graphene. Such a shift in band alignment across the TiO 2 /graphene heterojunction makes transfer of the photoexcited electron more energetically favorable. This work provides a very convenient chemical route to the scalable production of N-doped TiO 2 / graphene photocatalyst for potential applications in wastewater treatment.

Research paper thumbnail of Simulation to fabrication—understanding the effect of NiAuCu alloy catalysts for controlled growth of graphene at reduced temperature

Materials Research Express, 2019

It is a significant challenge to grow large-scale, high quality, monolayer graphene at low temper... more It is a significant challenge to grow large-scale, high quality, monolayer graphene at low temperature for the applications in industry, especially for the complementary metal oxide semiconductor fabrication process. To overcome this difficulty, we simulated the decomposition of acetylene (C2H2) on (100) surfaces of primarily nickel (Ni) catalysts with small mol fractions of gold (Au) and copper (Cu), using a 4 × 4 × 4 periodic supercell model. Based on the calculation of the reaction energies to decompose the C-H or C≡C bonds on different catalyst surfaces, a differential energy is proposed to clearly scale the decomposition difficulties such that larger differential energy leads to easier control of the monolayer growth. It is observed that on the NiAuCu alloy surface with a mol fraction 0.0313 of both Au and Cu, the differential energy of the C-H bonds and the C≡C bond are both positive, showing an obvious modulation effect on the decomposition of C2H2 and the catalytic activites...

Research paper thumbnail of Varying Phases of Alumina Nanowires Templated by Vertically Aligned Carbon Nanotubes Grown via Atomic Layer Deposition

Microscopy and Microanalysis, 2014

Research paper thumbnail of Thermocompression bonding of vertically aligned carbon nanotube turfs to metalized substrates

Nanotechnology, 2009

Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical car... more Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical carbon nanotubes, are being proposed for use as electrical and thermal contact materials. Issues in their implementation include high contact resistance, the van der Waals interactions of carbon nanotubes, and a low temperature limit during processing. One route for circumventing the 750 degrees C temperatures required for VACNT growth using chemical vapor deposition is for the VACNTs to be grown separately, and then transferred to the device. A method of mechanical transfer, using thermocompression bonding, has been developed, allowing dry mechanical transfer of the VACNTs at 150 degrees C. This method can be used for the construction of both a thermal switch or a permanent conducting channel. The conductivity of the bonded structure is shown to be independent of the imposed strain, up to strains in excess of 100%.

Research paper thumbnail of Annealing effect on Sb2S3-TiO2 nanostructures for solar cell applications

Nanoscale Research Letters, 2013

Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were pre... more Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were prepared on an F:SnO2 conductive glass by hydrothermal method and successive ionic layer adsorption and reaction method at low temperature. Sb2S3-sensitized TiO2 nanorod solar cells were assembled using the Sb2S3-TiO2 nanostructure as the photoanode and a polysulfide solution as an electrolyte. Annealing effects on the optical and photovoltaic properties of Sb2S3-TiO2 nanostructure were studied systematically. As the annealing temperatures increased, a regular red shift of the bandgap of Sb2S3 nanoparticles was observed, where the bandgap decreased from 2.25 to 1.73 eV. At the same time, the photovoltaic conversion efficiency for the nanostructured solar cells increased from 0.46% up to 1.47% as a consequence of the annealing effect. This improvement can be explained by considering the changes in the morphology, the crystalline quality, and the optical properties caused by the annealing tre...

Research paper thumbnail of Synthesis and Microanalysis of Aligned Carbon Nanotube Arrays

Microscopy and Microanalysis, 2005

Carbon nanotubes (CNTs) have been considered promising candidates for thermal devices due to thei... more Carbon nanotubes (CNTs) have been considered promising candidates for thermal devices due to their high thermal conductivity. Indeed, the multi-walled carbon nanotube (MWCNT) has demonstrated a thermal conductivity of more than 3000 W/mK at room temperature [1]. However, randomly aligned MWCNT films have displayed much lower thermal conductivities in the range of 25-35 W/mK [2]. This suggests that it is possible to exploit the much higher thermal conductivity of CNTs by using well-aligned arrays capable of improved thermal coupling.

Research paper thumbnail of Characterization of Silicon-Germanium Oxide Nanocomposites Fabricated by the Marine Diatom Nitzschia Frustulum

Microscopy and Microanalysis, 2005

Research paper thumbnail of Fabrication of Metal-Gated Carbon Nanotube Emitter Arrays Assisted by Focused Ion Beam and Chemical Vapor Deposition

Microscopy and Microanalysis, 2005

Carbon nanotubes (CNTs) have excellent electronic and structural properties. The high aspect rati... more Carbon nanotubes (CNTs) have excellent electronic and structural properties. The high aspect ratio and small tip radius of CNTs make them especially suitable as potential electron sources [1]. The objective of this study is to develop a technique for fabricating triode-type CNT field emitter arrays, where an integrated extraction gate is built between the cathode and the anode. The gate electrode is placed close to the cathode, making the gate to emitter distance small. As a result, the gate can attract or repel electrons with modest changes in gate voltage (positive or negative), enabling greater control over the emission current [2].