Efficient Heat Removal From Power Semiconductor Devices Using Carbon Nanotube Arrays And Graphene 165705 Or Thermal Management Solutions Via Carbon Nanomaterials

The downsizing of electronic devices and the consequent increasing power densities pose thermal management challenges for the semiconductor industry. Since the present thermal solutions limit their cooling capacity, developing new cooling methods for electronic devices has become important. This dissertation presents two types of novel methods for heat dissipation from integrated circuits: One is the use of advanced thermal interface materials, such as carbon nanotubes (CNTs), to increase heat dissipation between the solid and solid surface, such as a chip and heat sink. The second method is the use of a microjet impingement device to improve heat transfer between a liquid and solid in a heat sink. As advanced interface materials, vertically aligned carbon nanotube films are promising because of their unique mechanical and thermal properties. The first part of the dissertation describes the design, fabrication, and testing of CNTs using resonators to characterize their mechanical properties. Discussed in detail is the preparation of carbon nanotubes using different recipes, resulting in varied thicknesses of single-walled carbon nanotube films and multi-walled carbon nanotube films. The measurements of the resonant frequency shifts due to the presence of the CNT films using a laser Doppler vibrometer system result in extracted moduli of 0.5-220 [Mu]m-thick nanotube films varying from 1 to 370 MPa. To show how the physics between the effective modulus and thickness are connected, an analysis for the height dependence of the modulus is provided. After an image analysis is presented, a nanotube dynamics simulation based on tube properties and film morphology is introduced to predict mechanical properties. In addition to discussing the proposed interface materials, the second part of the dissertation describes the design, fabrication and testing of microjet impingement cooling, which display high heat capacities, as an advanced thermal management solution. The design of single-jet and multi-jet arrays with different numbers of diameters, locations, and spacing is discussed. Specifically, this part demonstrates how the microjet hydrodynamics are quantified using two-dimensional images by [Mu]PIV techniques, enabling the reconstruction of the three-dimensional flow field. The results indicate that CNT films offer a mechanical compliance that is suitable for TIM applications and that the microscale liquid jet devices provide quantified flow physics for heat sink applications.

This dissertation investigates the application of graphene and carbon nanotubes (CNTs) for thermal management of high-power batteries and interconnects. The research is focused on three applications: (i) thermal phase change materials (PCMs) with graphene fillers for thermal management of battery packs; (ii) CNTs incorporated in the battery electrodes; (iii) graphene coatings for copper (Cu) interconnects. In this study, lithium-ion (Li-ion) batteries were used for testing the proposed approaches. The graphene solutions for synthesis of graphene PCMs were obtained by the liquid-phase exfoliation. The graphene coatings on Cu films were grown by the chemical vapor deposition (CVD). In the first part of the dissertation, it is demonstrated that thermal management of Li-ion batteries can be drastically improved using PCM with graphene fillers. Incorporation of graphene to the hydrocarbon-based PCM allowed one to increase its thermal conductivity by more than two orders of magnitude while preserving its latent heat storage ability. A combination of the sensible and latent heat storage together with the improved heat conduction outside of the battery pack leads to a significant decrease in the temperature rise inside the Li-ion battery pack. I the second part of the dissertation, it is shown that thermal properties of Li-ion battery electrodes can be improved by incorporation of CNTs. The electrodes were synthesized via an inexpensive scalable filtration method , which can be extended to commercial electrode-active materials. The measurements reveal that the in-plane (cross-plane) thermal conductivity of the cathodes with the highest battery capacity was ~50 W/mK (3 W/mK) at room temperature. These values are up to two-orders-of-magnitude higher than those for conventional electrodes based on carbon black. The highest in-plane thermal conductivity achieved in the carbon-nanotube-enhanced electrodes was ~141 W/mK. In third part of the dissertation, it is demonstrated that graphene coating can strongly increase the thermal conductivity of Cu films as compared to the reference Cu and annealed Cu films. The observed improvement of thermal properties of graphene coated Cu films results primarily from the changes in Cu morphology during graphene CVD rather than from graphene's action as an additional heat conducting channel. The obtained results are important for thermal management of advanced batteries and downscaled computer chips.

This book comprehensively reviews recent and emerging applications of carbon nanotubes and graphene materials in a wide range of sectors. Detailed applications include structural materials, ballistic materials, energy storage and conversion, batteries, supercapacitors, smart sensors, environmental protection, nanoelectronics, optoelectronic and photovoltaics, thermoelectric, and conducting wires. It further covers human and structural health monitoring, and thermal management applications. Key selling features: Exclusively takes an application-oriented approach to cover emerging areas in carbon nanotubes and graphene Covers fundamental and applied knowledge related to carbon nanomaterials Includes advanced applications like human and structural health monitoring, smart sensors, ballistic protection and so forth Discusses novel applications such as thermoelectrics along with environmental protection related application Explores aspects of energy storage, generation and conversion including batteries, supercapacitors, and photovoltaics This book is aimed at graduate students and researchers in electrical, nanomaterials, chemistry, and other related areas.

Power dissipation is a major challenge in modern electronics, from battery-limited portable devices to cooling in massive data centers. The most often cited technological roadblock of nanoscale electronics is the textquotedblleft{}power problemtextquotedblright{} i.e. power densities and device temperatures reaching levels that will prevent their reliable operation. A majority of the power is dissipated during individual transistor switching, but a great deal of power is also dissipated through thermionic leakage. Therefore, a deeper understanding of power dissipation and energy efficiency at the individual device level could have a global impact, affecting all future electronics. In general, power in electronic devices is dissipated in the form of heat and is slowly lost to the environment. Given that all integrated circuits have a gate oxide (SiO$_2$) and several layers of inter-level-dielectric, the bulk of the heat is likely to be retained in the device because SiO$_2$ has a much lower thermal conductivity (1.4 W/m-K) than silicon (150 W/m-K). This leads to self-heating, mobility degradation and unreliable performance over time. This work examines the physics of energy relaxation in nanoscale transistors based on new popular materials like carbon nanotubes and graphene nanoribbons. The results hold significance for the design of future low power nanoscale devices and show that tunnel devices based on carbon nanotubes with a band gap less than 0.44 eV dissipate less than one sixtieth the power dissipated in traditional thermionic transistors. In addition, this thesis explores ways of efficient cooling of electronic devices by controlling heat flow and shows that thermal rectification in patterned graphene structures is possible.

The incorporation of graphitic compounds such as carbon nanotubes (CNTs) and graphene into nano-electronic device packaging holds much promise for waste heat management given their high thermal conductivities. However, as these graphitic materials must be used in together with other semiconductor/insulator materials, it is not known how thermal transport is affected by the interaction. Using different simulation techniques, in this thesis, we evaluate the thermal transport properties - thermal boundary conductance (TBC) and thermal conductivity - of CNTs and single-layer graphene in contact with an amorphous SiO2 (a-SiO2) substrate. First, the theoretical methodologies and concepts used in our simulations are presented. In particular, two concepts are described in detail as they are necessary for the understanding of the subsequent chapters. The first is the linear response Green-Kubo (GK) theory of thermal boundary conductance (TBC), which we develop in this thesis, and the second is the spectral energy density method, which we use to directly compute the phonon lifetimes and thermal transport coefficients. After we set the conceptual foundations, the TBC of the CNT-SiO2 interface is computed using non- equilibrium molecular dynamics (MD) simulations and the new Green-Kubo method that we have developed. Its dependence on temperature, the strength of the interaction with the substrate, and tube diameter are evaluated. To gain further insight into the phonon dynamics in supported CNTs, the scattering rates are computed using the spectral energy density (SED) method. With this method, we are able to distinguish the different scattering mechanisms (boundary and CNT-substrate phonon-phonon) and rates. The phonon lifetimes in supported CNTs are found to be reduced by contact with the substrate and we use that lifetime reduction to determine the change in CNT thermal conductivity. Next, we examine thermal transport in graphene supported on SiO2. The phonon contribution to the TBC of the graphene-SiO2 interface is computed from MD simulations and found to agree well with experimentally measured values. We derive the theory of remote phonon scattering of graphene electrons and compute the heat transfer coefficient dependence on doping level and temperature. The thermal boundary conductance from remote phonon scattering is found to be an order of magnitude smaller than that of the phonon contribution. The in-plane thermal conductivity of supported graphene is calculated from MD simulations. The experimentally measured order of magnitude reduction in thermal conductivity is reproduced in our simulations. We show that this reduction is due to the damping of the flexural (ZA) modes. By varying the interaction between graphene and the substrate, the ZA modes hybridize with the substrate Rayleigh modes and the dispersion of the hybridized modes is found to linearize in the strong coupling limit, leading to an increased thermal conductance in the composite structure.

In part two of this dissertation, I report the results of investigation of thermal conductivity of thin films made of a novel nanostructured graphene material, which consists of graphene nanoribbons encapsulated in single walled carbon nanotubes. The temperature dependent Raman spectrum of this material was measured in order to obtain the temperature coefficients of G + peak and 2D peaks. Using the Raman optothermal technique, I determine the local temperature rise due to laser heating from the shifts in Raman peak positions. A finite element analysis method was conducted to simulated heat dissipation in the samples and to determine their effective thermal conductivity. The obtained results suggest that this hybrid graphene-carbon nanotube material can be used as fillers in thermal interface materials.

Graphene is a one-atom thick planar monolayer of sp2 -bonded carbon atoms organized in a hexagonal crystal lattice. A single walled carbon nanotube (SWCNT) can be thought of as a graphene sheet rolled up into a seamless hollow cylinder with extremely high length-to-diameter ratio. Their large surface area, and exceptional optical, mechanical and electronic properties make these low-dimensional carbon materials ideal candidates for (opto-)electronic and sensing applications. In this thesis I studied the charge transfer processes that occur at their interface, and developed applications based on the discovered properties. When light is incident on a semiconducting SWCNT, it can excite an electron from the valence band to the conduction band, thereby creating a Coulombically bound electron-hole pair, also known as an exciton. Excitons can decay via radiative or non-radiative recombination or by colliding with other excitons. They can diffuse along the length of a SWCNT or hop from larger band gap SWCNTs to smaller band gap SWCNTs, a process known as exciton energy transfer (EET). We studied their behavior as a function of temperature in SWCNT fibers and showed that at room temperature the rate constant for EET is more than two orders of magnitude larger than that of each of the different recombination processes. This led us to construct a core-shell SWCNT fiber, which consists of a core of smaller band gap SWCNTs, surrounded by a shell of larger band gap SWCNTs, essentially forming what is known as a type I heterojunction. In agreement with a model that describes exciton behavior in the SWCNT fibers, we found that upon illumination all the energy (in the form of excitons) was quickly transferred from the shell to the core, faster than the excitons would otherwise recombine. The SWCNT fiber proved to be an efficient optical and energetic concentrator. We showed that SWCNTs and poly(3-hexylthiophene) (P3HT) form a type II heterojunction, which implies that excitons generated in the P3HT can easily dissociate into free charge carriers at the interface with the SWCNTs. Despite this, the efficiency of a P3HT/SWCNT bulk heterojunction (BHJ) photovoltaic is subpar. We developed a P3HT/SWCNT planar heterojunction (PHJ) and achieved efficiencies that were 30 times higher, which showed that the formation of bundled aggregates in BHJs was the cause: metallic SWCNTs can quench the excitons in an entire bundle. Another interesting feature of our SWCNT/P3HT PHJ is that a maximum efficiency was reached when -60 nm of P3HT was used, which is surprising since in a planar photovoltaic a maximum is expected for ~8.5 nm of P3HT, the value of the exciton diffusion length. A Kinetic Monte Carlo simulation revealed that bulk exciton dissociation was responsible for the lower efficiencies observed in devices with low P3HT thickness. Next we created and studied a junction between SWCNTs and a monolayer of graphene, an ideal one-dimensional/two-dimensional carbon interface. We used Raman spectroscopy to probe the degree of charge transfer at the interface and based on a shift in the G peak position of the graphene Raman signal at the junction deduced that a typical metallic (semiconducting) SWCNT dopes the graphene with 1.12 x 1013 cm-2 (0.325 x 101 cm-2) electrons upon contact, in agreement with the fact that the Fermi level of the SWCNTs is more shallow than that of the graphene. A molecular dynamics simulation ruled out that the observed Raman peak shifts are due to strain, although it did show that SWCNTs are being compressed radially by the graphene sheet, resulting in a widening of their Raman peaks. We studied charge transfer between diazonium molecules and graphene, to better inform transistor and sensor design. The reaction rate depends on the degree of overlap between the filled energy levels in graphene and the unoccupied ones in the diazonium molecule. We showed that with increasing degree of functionalization the charge transfer characteristics of a graphene field effect transistor (FET) alter in the following ways: the minimum conductivity decreases, the Dirac point upshifts, the conductivity plateau at high carrier density decreases and the electronhole conduction asymmetry increases. We developed a theoretical model of charge transport in graphene FETs that takes into account the effect of both short-range and long-range scatterers. Fitting it to the charge-transport data reveals quantitative information about the number of impurities in the substrate supporting the graphene, about the number of defects created as a result of the reaction, and about the degree of electron-hole conduction asymmetry. Graphene functionalization also affects the graphene Raman signal. After reaction, the D to G intensity ratio to increases, which is a sign of covalent modification of the graphene lattice. Additionally, the G peak and 2D peak positions increase while the 2D/G intensity ratio decreases, which are signs of hole-doping. Based on a Raman analysis, we were also able to show that the end group of the diazonium salt can affect both the degree of chemisorption (covalent modification) as well as the degree of physisorption (doping). Finally, we studied the effects of charge transfer between graphene and biological cells on the graphene Raman signal and designed a fundamentally new type of biosensor. Graphene can be thought of as a continuous array of information units (sensor units). The Raman signal collected in each unit can report on its local environment. In contrast to graphene FET biosensors, the graphene Raman biosensor offers subcellular spatial resolution. The graphene Raman signal was shown to display a strong dependence on pH. Metabolically active cells acidify their local environment; therefore, pH is a proxy for cellular metabolism. We placed both human embryonic kidney (HEK) cells that were genetically engineered to produce mouse antibodies and control HEK cells that were not genetically modified onto the graphene. Based on the change in the graphene Raman signal we deduced the former have a metabolic rate that is four times higher than that of the control cells. Increased cellular adhesion allows the cells to interact more closely with the graphene monolayer and intensifies the observed Raman effects.

As the electronic industry aggressively moves towards nanometer designs thermal issues are becoming increasingly important for the high-end electronic chips. One of the approaches to mitigate the self-heating problems is the high-heat-flux hot-spot removal via incorporation into the chip designs of materials with the high thermal conductivity. Graphene is found to be one of the best known heat conductors, thus it can be used in nanoelectronic and optoelectronic devices as a heat spreader component. Graphene, the latest isolated allotrope of carbon made of individual atomic sheets bound in two dimensions, shows many remarkable properties. A non-contact method of measuring G peak position of the Raman spectrum as a function of both the temperature of the graphene sample and the power of the heat source was used to measure the thermal conductivity of graphene. The samples in the experiment had approximately rectangle geometry and the assumption about the plane heat wave was used for the data extraction. In this dissertation research we propose to develop a model and numerical procedure for the (i) accurate modeling-based data extraction for the thermal conductivity measurements; and (ii) simulate heat propagation in semiconductor device structures with graphene layers incorporated as heat spreaders. To achieve the goals of this dissertation research we simulated the heat transport in graphene using the finite element method (FEM) with the help of COMSOL software package, which solves numerically the partial differential equations. The modeling based data extraction was necessary to determine thermal conductivity of the graphene flakes of arbitrary shape. It also substantially improved the accuracy of the measurements. The simulation of heat propagation in device structures with graphene heat spreaders allows one to assess the feasibility of the graphene high-heat-flux thermal management. We focused on understanding how thermal transport is influenced by a surface geometry of the sample and geometries of the heat sources. The simulation results showed that the size, shape and heat source geometry impact heat propagation in different ways and have to be included in the experimental data extraction. The simulation procedure provided a necessary input for next experiments on heat conduction in graphene structures e.g., graphene multi-layers and graphene-heat sink structures and other device-level thermal management applications. It was found that the incorporation of graphene or few-layer graphene (FLG) layers with proper heat sinks can substantially lower the temperature of the localized hot spots. The developed model and obtained results are important for the design of graphene heat spreaders and interconnects and lead to a new method of heat removal from nanoelectronic and 3-D chips.

Carbon is the most abundant material next to oxygen in terms of sustainability. The potential of carbon based materials has been recognized in recent decades by the discovery of fullerene (1996 Nobel prize in chemistry), carbon nanotubes (2008 Kavli prize in nanoscience) and graphene (2010 Nobel prize in physics). The synthesis of carbon materials with well controlled morphologies lead to their exploration in both fundamental research and industrial applications. Graphene also commonly referred to as a wonder material has been under extensive research for more than a decade, due to its excellent electronic, optical, thermal and mechanical properties. However, the realization of these applications for practical purposes require its large scale synthesis. The common method of graphene synthesis involves reduction of graphene oxide. Nevertheless, complete restoration of intact graphene basal plane destroyed by oxidation cannot be achieved, limiting the application of as synthesized graphene in flexible macro electronics, mechanically and electronically reinforced composites etc. Hence, research was pursued in regards to achieve controlled oxidation, sufficient enough to overcome the Vander-Waals forces and preserving the graphene domains. One such approach reported by our group is the solution processable graphene achieved via controlled oxidation, by the use of nitronium oxidation approach. However, toxic NOx gases and byproducts generated during the synthesis, limits the scalability of this approach. In this thesis, for the first time, we reported the synergy of piranha etching solution with intercalated graphite for the controlled oxidation of graphite particles via microwave heating in chapter 2. The controlled oxidation leads to rapid (60 seconds) and direct generation of highly conductive, clean low oxygen containing graphene sheets without releasing any detectable toxic gases or aromatic by-products as demonstrated by gas chromatography-mass spectrometry. These highly conductive graphene sheets have unique molecular structures, different from both graphene oxide and pristine graphene sheets. They can be dispersed in both aqueous and common organic solvents without surfactants/stabilizers producing "clean" graphene sheets in solution phase. "Paper-like" graphene films are generated via simple filtration resulting in films with a conductivity of 2.26 × 104 S m-1, the highest conductivity observed for graphene films assembled via vacuum filtration from solution processable graphene sheets to date. After 2-hour low temperature annealing at 300 C, the conductivity further increased to 7.44 × 104 S m-1. This eco-friendly and rapid approach for scalable production of highly conductive and "clean" solution-phase graphene sheets would enable a broad spectrum of applications at low cost. Irrespective of the vast applications of highly conductive graphene, it exhibits limited catalytic centers, is impervious, and limits the diffusion of ions. This inadequacy can be overcome by the hole generation on highly conductive graphene. Current approaches for large scale production of holey graphene require graphene oxide (GO) or reduced GO (rGO) as starting materials. Thus generated holey graphene derivatives still contain a large number of defects on their basal planes, which not only complicates fundamental studies, but also influences certain practical applications due to their largely decreased conductivity, thermal and chemical stability. This work reports a novel scalable approach exploiting the wireless joule heating mechanism provided by microwave irradiation of partially oxidized graphite intercalation compounds in chapter 3. The wireless joule heating mechanism affords region-selective heating, which not only enable fabrication of holey graphene materials with their basal plane nearly intact, but also engineers the edges associated with holes to be rich in zigzag geometry. The term pristine holey graphene was given, to differentiate from the holey graphene derivatives with basal plane defects, as reported in the literature. The pristine holey graphene with zigzag edges were studied and explored as a metal free catalyst for reduction reactions via hydrogen atom transfer mechanism. The pristine holey graphene nanoplatelets not only exhibited high catalytic activity and desired selectivity, but also provided excellent chemical stability for recyclability, which is very different from its counterpart holey graphene derivatives with basal plane defects. It was also reported that the reduction of nitrobenzene occurs via condensation pathway with this catalyst. To further provide insight into combustion of graphite in air with microwave irradiation, the stabilized intercalated graphene without point defects was used to generate holes in chapter 4. The co-intercalated O2 into graphite intercalated compound act as the internal oxidant, to oxidize the carbon, along with the surrounding air. High local temperatures were achieved via joule heating mechanism, hence promoting combustion of graphene to generate holes and edges. We observed that in combination to hole generation, higher conductivity was also observed in comparison to the holey graphene synthesized in chapter 3. The highly conductive holey graphene was tested for their electro-catalytic activity in the reduction of oxygen. The reduction of oxygen occurs via 2e- pathway, where peroxide with 90% yield was recorded. This opens path for onsite peroxide production in alkaline media, and therefore allowing its use in bleaching industries. In concern of carbon based materials being explored for catalysis, their high amount to facilitate the reaction, limits practicality of the catalyst for industrial applications. However, the immobilization of metal nanoparticles onto porous carbon supports, synthesized from sustainable and cheap biomass was widely pursued. It was widely reported that the doping of carbon support with N further improved their interaction with the metal and promoted higher catalytic activity. In chapter 5, for the first time, the influence of P doped carbon support on catalytic activity of Pd was reported. A single step microwave assisted fabrication of Pd embedded into porous phosphorous doped graphene like carbon was demonstrated. Structural characterization revealed that, the metal nanoparticles are in the range of 10nm with a surface area of 1133m2/g. The developed method is not only sustainable as it is synthesized from biomass and anti-nutrient molecule (phytic acid), but also energy efficient as microwave irradiation (50sec) is used for the catalyst synthesis. The as synthesized catalyst recorded 90% conversion with a TOF of 23000h-1 for benzyl alcohol oxidation, which remained constant even after 8 recycles indicating the stability of catalyst. Different wt% of Pd onto PGC was tested for their alcohol oxidation capacity and found that the 3% Pd-PGc which activates O2 more towards 4e- in ORR has the best conversion and selectivity. The biomass molecule phytic acid used for the synthesis of phosphorous doped carbon support was also used as a phosphorous source in the synthesis of tin phosphides in chapter 6. Current studies have shown that sodium, a low cost and naturally abundant metal, can act as a substituent for lithium in lithium ion batteries (LIB), hence, allowing their applications in real world. This transition towards the use of sodium ion batteries (SIB) has entailed research to improve the cycle stability and energy density of battery by introducing tin phosphides as anodes for batteries. Tin phosphides exhibit a self-healing mechanism, hence decreases the capacity decay as observed in the case of Sn metal. However, it was reported that the self-healing mechanism is not completely reversible with partial pulverization observed. Therefore, we pursued a time efficient method to synthesize tin phosphide in a phosphorous doped carbon matrix (SnP@PGc) via microwave irradiation. The SnP@PGc formed when tested as anode for SIBs, demonstrated superior capacity of 515 mAh/g after 750 cycles at a charge and discharge current of 0.2 C. The superior cycle stability can be attributed to the protection against volume expansion by phosphorous doped porous carbon shell during battery charge and discharge process and hence mitigating the pulverization of tin phosphides.