Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes

This work was funded by the European Regional Development Fund (ERDF) project no. 1.1.1.1/19/A/138. A.S. and K. S. acknowledge the funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under grant agreement No. 739508, project CAMART2.Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) are known as a perspective material for a variety of applications in nanoelectronic devices, sensors, catalysts for carbon dioxide reduction, and flexible thermoelectrics. However, up to date most of the reports on the properties of N-MWCNTs are focused on a narrow niche of research, for example, a study of low-temperature magnetoresistance or room-temperature thermoelectrical properties. In this work, N-MWCNTs were synthesized using benzene:pyridine precursor in different ratios, and both magnetoresistance and thermoelectrical properties of the synthesized N-MWCNTs were systematically investigated in the temperature range 2-300 K and compared with the properties of undoped MWCNTs. Unexpected switching of the magnetoresistance of the N-MWCNTs at low temperatures from negative to positive values was observed, and the processes underlying this effect are discussed. The study of the thermoelectrical properties revealed n-type conductance in the N-MWCNTs, which was attributed to the impact of nitrogen defects incorporated in the MWCNT structure. Performed for the first-time investigations of the thermal stability of the Seebeck coefficient of N-MWCNTs in air revealed that the Seebeck coefficient retains its negative values and even increases after annealing of the N-MWCNTs in air at 500 °C. These findings illustrate the high potential of the presented in this work N-MWCNTs for applications in different devices in a wide range of temperatures. --//-- Jana Andzane, Mikhail V. Katkov, Krisjanis Buks, Anatolijs Sarakovskis, Krisjanis Smits, Donats Erts, Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes, Carbon Trends, Volume 13, 2023,100302, ISSN 2667-0569, https://doi.org/10.1016/j.cartre.2023.100302. (https://www.sciencedirect.com/science/article/pii/S2667056923000573). Published under the CC BY-NC-ND licence.ERDF project no. 1.1.1.1/19/A/138. A.S; European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under grant agreement No. 739508, project CAMART2

Dynamics and Limiting Mechanisms of Self-Aligned Carbon Nanotube Growth.

Carbon nanotubes (CNTs) are long, cylindrical molecules, which boast exceptional tensile strength and large thermal and electrical conductivities. Vertically aligned CNT “forests” have promising potential uses, including dry adhesives, electrical interconnects, light emitters, thermal interface materials, gas and liquid filters, composite reinforcements, and photonic crystals. Manufacturing indefinitely long CNTs may realize dreams of CNT-based cables and wires having stiffness, strength, and transport properties exceeding today’s best metal alloys and advanced fibers. However, the functional properties of CNT forests have so far fallen short of those of individual CNTs due to low packing fraction, polydisperse diameters, and relatively short lengths. Toward the eventual goal of bridging this structure-property relationship, my dissertation presents a novel set of in situ and ex situ characterization tools for CNT forest growth by chemical vapor deposition (CVD), as well as the use of these tools to investigate the limiting mechanisms thereof. In situ X-ray scattering reveals the dynamics of catalyst thin film dewetting into nanoparticle growth sites, the initial self-organization of the CNT forest, and the abrupt self-termination of growth. Quantification of catalyst and CNT sizes show that they are inevitably polydisperse, regardless of synthesis conditions. To overcome this, a novel method is introduced for templated dewetting of the catalyst film toward the formation of ordered, monodisperse particles using nanoporous anodic alumina. Further, a map of thermal conditions is explored by independently tuning the temperatures of the catalyst and gaseous precursors, thereby establishing a set of rules for engineering crucial characteristics of forest growth, including CNT diameter, structural quality, vertical alignment, as well as rate and lifetime of the reaction. Finally, aligned CNT ensembles are used as templates to direct the self-assembly of fullerene C60, creating hybrid films with high photoconductive gain, thereby demonstrating an immediate application of this exciting material. These studies represent many new insights into the so-called “birth, life, and death” of CNT growth, and they have important implications for future work in synthesis of advanced carbon materials, including CNTs, fullerenes, and graphene. Meanwhile, these results have immediate applicability to efficient CNT manufacturing, improved characterization, and new hybrid materials for energy conversion.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91427/1/emeshot_1.pd

Polymer - Carbon Nanostructures Composites:from Chemistry to Physics, to Material Science

Synthesis and characterization of polymer-based composites containing carbon nanostructures is the focus of this dissertation. The main polymer which has been used for this research is SU8. The motivation of the work is to overcome the drawbacks of SU8 e.g. (high electrical and thermal resistance, brittleness) by addition of the carbon-based nanofillers. In this thesis, three classes of carbon nanostructures have been used as composite fillers: 2 dimentional graphene, 1 dimentional carbon nanotubes and 0 dimentional onion like carbons. The idea behind this material selection is to study the influence of dimensionality of the fillers on the electrical transport properties of the composites. In addition, we have performed comprehensive characterizations of graphene composites and addressed some questions about carbon nanotubes and nano onions composites. SU8 graphene composites were prepared using solution mixing method. The microstructure analysis of the composites showed a homogeneous dispersion of the graphene flakes. The study of electrical properties of the composite as function of the filler loading exhibited superior electrical conductivity compared to other graphene-base polymer composites. The study of the viscoelastic behavior of the composites showed that the rheological percolation is very close to zero, which we attribute to the polymer chain restriction due to high aspect ratio graphene fillers. The mechanical properties were evaluated with nanoindentation technique. 67 percent enhancement for Youngâs modulus and 75 percent enhancement for hardness were acquired. The possibility of the linkage between the filler and the matrix was investigated by spectroscopy techniques including Photoluminescence, Raman and Fourier Transform Infrared spectroscopy. Our findings suggest that covalent bonds are formed between SU8 and the functional groups on the surface of the graphene flakes. SU8-CNT composites were synthesized using both randomly dispersed and well-aligned tubes. For composites with randomly dispersed CNTs, the effect of nanotube length and polydispersity was investigated with experimental approach, for the first time. We have shown that the conductivity in such composites is proportional to mean length of CNTs, Ln, rather than weighted average length, Lw, which is predicted by theory. For case of aligned CNTs, we have measured the thermal conductivity, kappa, parallel and perpendicular to the orientation of the tubes, which exhibited an anisotropy close to 20 . The study was motivated by thermal management applications. From the same composite, lamellas with thickness ranged 20-100 nm were prepared using ultramicrotomy technique, for proton channeling applications. The successful sample preparation and pioneering channeling experiments give an encouraging outlook for future investigations in this field. Composites containing Poly methyl methacrylate as matrix and onion-like carbon as fillers were prepared for transport studies. The temperature and pressure dependence of the conductivity were measured. Due to the complexities associated to non homogeneous structure of these composites, we do not have a unified model to describe the dependence of conductivity upon concentration, temperature and pressure, and this question has remained open

Characterisation and processing of carbon-based reinforced Al-MMCs for thermal management applications”

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonThis thesis examines the possibility of using multi-walled carbon nanotubes (MWCNTs) (as fillers), known for their unusual exceptional high thermal conductivity ( ~ 3000 to 3500 W m-1 K-1), to produce ultra-high thermal conductivity ( 400 W m-1 K-1) aluminium matrix composites (Al/MWCNTs). Composites were processed via a combination of rheocasting and equal channel angular extrusion (ECAE) techniques, for use in advanced thermal management applications such as in high power light-emitting diodes (HPLEDs). Al matrix composites reinforced with Cu-coated pitch-based carbon fibres (Al/Cu-CFs) were first produced to test the processing method selected. Rheocasting allowed the introduction and dispersion of 2 wt.% of Cu-CFs within the Al3Mg matrix. The subsequent ECAE processing of the composites reduced the porosity from 3 to 0.03 % and induced a high degree of fibres alignment (øED-DD ~ 2.69º) within the matrix. However, this resulted in considerable damage on the fibres. The rheocasting alone did not improve the of the composites as the addition of 2 wt.% of fibres showed a value of ,2 = 134.9 ± 4.1 W m-1 K-1, 9% lower in comparison with the matrix, = 148.4 ± 4.5 W m-1 K-1. After ECAE, for 6 iterations in the in-plane direction, composite with the highest degree of fibre alignment, showed a improvement of ~ 20 % (,1.5,6,1 = 153.7 ± 4.6 W m-1 K-1) with respect to the “as-rheocasted” composite (,1.5 = 128.5 ± 3.9 W m-1 K-1), and a 3.6% increase with respect to the matrix. The improvement is believed to be due to porosity reduction, fibre alignment and forced intimate contact of clean CF surfaces with the matrix. Rheocasting of the Al/MWCNTs allowed the introduction of up to 0.35 wt.% of MWCNTs (embedded in pure Cu) within the Al matrix. However, the MWCNTs were found in agglomerates. Their introduction within the matrix was aided by the pure Cu, which was further improved after the Cu solubility in Al was reached resulting in the formation of AlCuMg intermetallics which surrounded the agglomerated MWCNTs. ECAE processing reduced the composites porosity (from 1.5 % to 0.03%) and induced a high degree of nanotube bundle alignment (3.24º < øED-DD < 3.62º). Aligned individual nanotubes with a good nanotube matrix interface surface contact were also found. However, damage on the nanotubes was also observed. The SThM+FEM technique developed in this study allowed the acquisition of the of an individual MWCNT that resulted in a combined (in-and out-of-plane) thermal conductivity value of ,1,2 ~ 20 W m-1 K-1. The low value is due the long length and large outer diameter of the nanotube which increases the probability of an increase in the defect content and consequently a lower thermal conductivity. The results of the Al/MWCNTs composites processed via rheocasting+ECAE showed an improvement of ~ 5.7 % (1,4,1 = 156.9 ± 4.7 W m-1 K-1) for an addition of 0.3 wt.% of MWCNTs, in comparison to the matrix ( = 148.4 ± 4.5 W m-1 K-1). This finding may be related to porosity removal and MWCNTs bundle alignment forming a percolation network. Comparison with various thermal conductivity theoretical models, taking into account the models limitations, the characteristics of the microstructure of the composites, the MWCNTs quality and purity, and the SThM+FEM results, supports the hypothesis that the of the MWCNTs used for the composite processing is much lower than that claimed by the manufacturer (2000 Wm-1 K-1). However, its value is higher than , which is possible as the value obtained by the SThM+FEM is a combined value, and thus its in-plane value is higher than 20 W m-1 K-1. The theoretical models also showed that it should be possible to obtain values between ~ 351 W m-1 K-1 to ~ 497 W m-1K-1 for Al/MWCNTs composites processed via rheocasting+ECAE with a maximum filler volume content of = 0.3. Nevertheless, to process ultra-high ( 400 W m-1 K-1) thermal conductivity it is critical that the ,1 2000 W m-1 K-1.Korea Institute of Industrial Technology (KITECH), Incheon, South Kore

SWNT and Graphene Colloidal Dispersions: Phase behavior, Material Fabrication and Characterization

This dissertation explores the microstructural properties, flow, and phase behavior of aqueous suspensions of single-walled carbon nanotube (SWNT) and graphene. Liquid phase processing with scalable and industrially viable methods is used for fabrication of basic engineering materials like thin films, coatings and 1-D fibrillar structures. The electro-optical transport properties of these novel materials are characterized. A recipe for formulation of an aqueous colloidal suspension with high SWNT concentration is presented. A combination of two surfactants provides optimal rheological behavior for "rod coating" uniform transparent conductive SWNT thin films, with minimal dewetting, rupture and defects. "Doping" with acids and metallic nanoparticles yield SWNT films with electrical sheet resistance of 100 and 300 OJ sq for respective optical transparency of 70% and 90%. SWNT thin films with local nematic ordering and alignment are fabricated using a "slow vacuum filtration" process. The technique is successfully demonstrated on several aqueous SWNT suspensions, employing different ionic and non-ionic surfactants, as well as on dispersions enriched in metallic SWNTs, produced by density gradient ultracentrifugation. Scanning electron microscopy and image analysis revealed a local nematic order parameter of 8 "" 0.7-0.8 for the SWNT films. Aligned SWNT films and fibers are employed as templates, in a simple drop drying process, for large scale ordered (8 "" 0.7-0.9) assembly of plasmonic nanoparticles, like gold nanorods, micro-triangles and platelets. Colloidal self-assembly of surfactant stabilized SWNTs and AC dielectrophoresis (DEP) are combined in a novel two-step technique for fabrication of 1-D SWNT fibrils. The self-assembled SWNT-surfactant colloidal structures are 103 - 104 fold more responsive to external AC electrical fields. The DEP SWNT fibrils show significant Raman alignment ratio ("" 3-5) and good electrical conductivity. iii A detailed study on the phase behavior of giant graphene oxide flakes (aspect ratio > 104) suspended in water is presented. The lyotropic suspensions transition from an isotropic to a biphasic system and to a discotic nematic liquid crystal with increasing flake concentration. Polarizing optical microscopy and colloidal particle inclusions reveal the alignment and orientation flakes in the nematic phase, the nematic order parameter (8 f"V 0.43), low optical birefringence (~n = -0.0018), and an average Frank elastic constant (K f"V 100 pN) which is about 100 fold higher than previously studied discotic liquid crystals

Dry-transfer of chemical vapour deposited nanocarbon thin films

This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis for CNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-latticeintegration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and lowtemperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption

THE THERMOELECTRIC, THERMORESISTIVE, AND HYGRORESISTIVE PROPERTIES AND APPLICATIONS OF VAPOR PRINTED PEDOT-CL

Wearable electronics are a valuable tool to increase consumer access to real-time and long-term health care monitoring. The development of these technologies can also lead to major advancements in the field, such as self-charging systems that are completely removed from the electrical grid. However, much of the wearable technology available commercially contain rigid components, use unsustainable synthetic methods, or undesirable materials. The field has thus been moving towards wearables that mimic textiles or use textiles as a substrate. Herein, we discuss the use of oxidative chemical vapor deposition (oCVD) to produce textiles coated with poly(3,4-ethylenedioxythiophene) known as PEDOT-Cl. We evaluate the thermoelectric, thermoresistive, and hygroresistive properties of these PEDOT-Cl fabrics. We also explore the applications of these properties by creating humidity sensors, temperature sensors, and thermoelectric generators integrated with clothing. In general, we discuss the process of designing a wearable to best accommodate the desired application

Wearable Integrated Devices for Sustainable Energy: Self Powered e-Cloths

Nowadays, from lifestyle to sports and health to security, wearable technology is an inevitable trend that, through the human-machine interaction, has the capability of transforming businesses by making them smarter, more informative, and more communicative. In this study, commercial textile fibers have been functionalized with Polypyrrole (PPy) to achieve an electronic system that can convert external mechanical energy into electrical energy. PPy is a biocompatible π-conjugated polymer. The main principle behind the devices’ operation is the charge transfer mechanism that occurs between the π-conjugated polymer and the metal (electrode) layer when the system suffers mechanical stress. Furthermore, the PPy functionalized textile has been weaved to an e-cloth, through a custom-built weaving machine. This e-cloth can generate current under human-motion interaction. The best results achieved in this study, in terms of power density and current density, were 2.29 Wm-2 and 23.9 mA m-2 , respectively. Considering the best device, we were able to light up to 50 LEDs connected in series. With this device, we were also able to charge a 33F capacitor up to 1V, in 225 seconds. All the devices built have kept electrical stability during the six months of the work. The main application explored in this study was the detection of human movements through motion interactive energy harvesting technology

Fabrication of high aspect ratio silicon nanostructure arrays by metal-catalyzed etching

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Includes bibliographical references (p. 167-178).The goal of this research was to explore and understand the mechanisms involved in the fabrication of silicon nanostructures using metal-assisted etching. We developed a method utilizing metal-assisted etching in conjunction with block copolymer lithography to create ordered and densely-packed arrays of high-aspect-ratio single-crystal silicon nanowires with uniform crystallographic orientations. Nanowires with sub-20 nm diameters were created as either continuous carpets or as carpets within trenches. Wires with aspect ratios up to 220 with much reduced capillary-induced clustering were achieved through post-etching critical point drying. The size distribution of the diameters was narrow and closely followed the size distribution of the block copolymer. Fabrication of wires in topographic features demonstrated the ability to accurately control wire placement. The flexibility of this method will facilitate the use of such wire arrays in micro- and nano-systems in which high device densities and/or high surface areas are desired. In addition, we report a systematic study of metal-catalyzed etching of (100), (110), and (111) silicon substrates using gold catalysts with varying geometrical characteristics. It is shown that for isolated catalyst nanoparticles and metal meshes with small hole spacings, etching proceeded preferentially in the direction. However, etching was confined in the direction vertical to the substrate surface when a catalyst mesh with large hole spacings was used. This result was used to demonstrate the use of metal-assisted etching to create arrays of vertically-aligned polycrystalline and amorphous silicon nanowires etched from deposited silicon thin films using catalyst meshes with relatively large hole spacings. The ability to pattern wires from polycrystalline and amorphous silicon thin films opens the possibility of making silicon nanowire-array-based devices on a much wider range of substrates. Finally, we demonstrated the fabrication of a silicon-nanopillar-based nanocapacitor array using metal-assisted etching and electrodeposition. The capacitance density was increased significantly as a result of an increased electrode area made possible by the catalytic etching approach. We also showed that the measured capacitance densities closely follow the expected trend as a function of pillar height and array period. The capacitance densities can be further enhanced by increasing the array density and wire length with the incorporation of known self-assembly-based patterning techniques such as block copolymer lithography.by Shih-wei Chang.Ph.D