Thermal Transport and Power Dissipation in Two-Dimensional (2D) Materials and Interfaces

Thermal and electrical transport characterization in devices based on two-dimensional (2D) materials have great implications in many areas such as nano- and optoelectronics, and energy generation, conversion, and storage systems. In many of these applications, the performance of the 2D-based devices is hindered by the heterogeneities such as grain boundaries (GBs) and interfaces. In the case of graphene, the effects of GBs on its electrical properties are well investigated. One of the goals of this dissertation is to establish an understanding on how individual graphene GBs affect the overall thermal properties of graphene films. Measurements on individual GBs are performed to identify the correlations between the thermal resistance imposed by the GBs and the crystallographic mismatch across the GB. Benchmarking the experimental results against theoretical predictions allows us to identify the governing mechanisms of the phonon scattering across GBs with different mismatch angles and morphological details. Next, the thermal transport in the through-plane direction is investigated which accounts for a major fraction of power dissipation from hot-spots in 2D-based devices. First, the interfacial thermal transport in graphene and MoS2 monolayers is characterized which may serve as the bottle-neck of dissipation in the through-plane direction. The effects of interface coupling and metal encapsulation are explored on thermal boundary conductance (TBC) across MoS2 and graphene monolayers. A system-level analysis of heat transport in the through-plane direction is also carried out to quantify the thermal dissipation limits in 2D-material-based structures on different technologically-viable substrates, e.g., diamond, aluminum nitride (AlN), sapphire, and silicon with different oxide types/thicknesses. The results highlight the importance of simultaneous optimization of the interfaces and the substrate and provides a route to maximize the heat removal capability of 2D-material-based devices. Another goal of this thesis was to develop a method for reliable fabrication of high-quality lateral interfaces between graphene and MoS2 monolayers for all-2D circuitry applications. The results show that the MoS2-graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to the conventional MoS2-metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts

Chemical Sensing in Two-Dimensional (2D) Nanomaterials

The development of highly sensitive portable chemical sensors is of a fundamental importance in environmental monitoring, medical diagnostics, and other areas. In this respect, two-dimensional (2D) nanomaterials have attracted great attention due to their excellent electrical and structural properties. As the main representative of the 2D materials, graphene has exhibited great performance in a wide range of sensing applications over the past decade. Particularly, it is unraveled that structural defects play a crucial role on the sensing performance of the graphene-based sensing devices. In this thesis, the roles of internal and external structural defects on the sensing performance of graphene-based chemical sensors have been elucidated. In the beginning, it is shown that the sensitivity (in terms of modulation in electrical conductivity) of pristine graphene chemical field-effect transistors (chemFETs) is not necessarily intrinsic to graphene, but rather it is facilitated by external defects in the insulating substrate, which can modulate the electronic properties of graphene. We disclose a mixing effect caused by partial overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of adsorbed gas molecules to explain graphene’s ability to detect adsorbed molecules. In the next phase, graphene grain boundaries are extensively studied as they are known to desirably alter the electronic properties and chemical reactivity of graphene structures. It is observed that an isolated graphene grain boundary has ~300 times higher sensitivity to the adsorbed gas molecules than a single crystalline graphene grain. Electronic structure and transport modeling reveal that the ultra-sensitivity in grain boundaries is caused by a synergetic combination of gas molecules accumulation at grain boundary, together with the existence of a sharp onset energy in the transmission spectrum of its conduction channels. Moving beyond the graphene-based devices, several other members of 2D nanomaterials are also tested for their potential in chemical sensing applications. In particular, the sensing characteristics of black phosphorus (BP) films of stacked atomic layers made by liquid exfoliation and vacuum filtration are studied. Interestingly, BP films are observed to exhibit an ultra-sensitive and selective response towards humid air with a trace-level detection capability and a negligible drift over time. Lifetime analysis predicts that BP film sensors can function stably for several years in highly humid environments. This study opens up the route for utilizing BP stacked films in many applications such as energy generation/storage systems, electrocatalysis, and chemical/bio-sensing. It is believed that the family of 2D materials consists of numerous unexplored members that can revolutionize the chemical and biological sensing industry

Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor Deposition

Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and “puckering” mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles

Stable and Selective Humidity Sensing Using Stacked Black Phosphorus Flakes

Black phosphorus (BP) atomic layers are known to undergo chemical degradation in humid air. Yet in more robust configurations such as films, composites, and embedded structures, BP can potentially be utilized in a large number of practical applications. In this study, we explored the sensing characteristics of BP films and observed an ultrasensitive and selective response toward humid air with a trace-level detection capability and a very minor drift over time. Our experiments show that the drain current of the BP sensor increases by ∼4 orders of magnitude as the relative humidity (RH) varies from 10% to 85%, which ranks it among the highest ever reported values for humidity detection. The mechanistic studies indicate that the operation principle of the BP film sensors is based on the modulation in the leakage ionic current caused by autoionization of water molecules and ionic solvation of the phosphorus oxoacids produced on moist BP surfaces. Our stability tests reveal that the response of the BP film sensors remains nearly unchanged after prolonged exposures (up to 3 months) to ambient conditions. This study opens up the route for utilizing BP stacked films in many potential applications such as energy generation/storage systems, electrocatalysis, and chemical/biosensing

Characteristic Work Function Variations of Graphene Line Defects

Line defects, including grain boundaries and wrinkles, are commonly seen in graphene grown by chemical vapor deposition. These one-dimensional defects are believed to alter the electrical and mechanical properties of graphene. Unfortunately, it is very tedious to directly distinguish grain boundaries from wrinkles due to their similar morphologies. In this report, high-resolution Kelvin potential force microscopy (KPFM) is employed to measure the work function distribution of graphene line defects. The characteristic work function variations of grain boundaries, standing-collapsed wrinkles, and folded wrinkles could be clearly identified. Classical and quantum molecular dynamics simulations reveal that the unique work function distribution of each type of line defects is originated from the doping effect induced by the SiO₂ substrate. Our results suggest that KPFM can be an easy-to-use and accurate method to detect graphene line defects, and also propose the possibility to tune the graphene work function by defect engineering

Power Dissipation of WSe₂ Field-Effect Transistors Probed by Low-Frequency Raman Thermometry

The ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry results in high power densities during device operation, which could cause a significant temperature rise within 2D channels. One challenge in Raman thermometry of 2D materials is that the commonly used high-frequency modes do not precisely represent the temperature rise in some 2D materials because of peak broadening and intensity weakening at elevated temperatures. In this work, we show that a low-frequency E_2g² shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in back-gated tungsten diselenide (WSe₂) field-effect transistors, whereas the high-frequency peaks (E_2g¹ and A_1g) fail to provide reliable thermal information. Our calculations indicate that the broadening of high-frequency Raman-active modes is primarily driven by anharmonic decay into pairs of longitudinal acoustic phonons, resulting in a weak coupling with out-of-plane flexural acoustic phonons that are responsible for the heat transfer to the substrate. We found that the TBC at the interface of WSe₂ and Si/SiO₂ substrate is ∼16 MW/m² K, depends on the number of WSe₂ layers, and peaks for 3–4 layer stacks. Furthermore, the TBC to the substrate is the highest from the layers closest to it, with each additional layer adding thermal resistance. We conclude that the location where heat dissipated in a multilayer stack is as important to device reliability as the total TBC

Cathode Based on Molybdenum Disulfide Nanoflakes for Lithium–Oxygen Batteries

Lithium–oxygen (Li–O₂) batteries have been recognized as an emerging technology for energy storage systems owing to their high theoretical specific energy. One challenge is to find an electrolyte/cathode system that is efficient, stable, and cost-effective. We present such a system based on molybdenum disulfide (MoS₂) nanoflakes combined with an ionic liquid (IL) that work together as an effective cocatalyst for discharge and charge in a Li–O₂ battery. Cyclic voltammetry results show superior catalytic performance for this cocatalyst for both oxygen reduction and evolution reactions compared to Au and Pt catalysts. It also performs remarkably well in the Li–O₂ battery system with 85% round-trip efficiency and reversibility up to 50 cycles. Density functional calculations provide a mechanistic understanding of the MoS₂ nanoflakes/IL system. The cocatalyst reported in this work could open the way for exploiting the unique properties of ionic liquids in Li–air batteries in combination with nanostructured MoS₂ as a cathode material

Bimodal Phonon Scattering in Graphene Grain Boundaries

Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs

Selective Ionic Transport Pathways in Phosphorene

Despite many theoretical predictions indicating exceptionally low energy barriers of ionic transport in phosphorene, the ionic transport pathways in this two-dimensional (2D) material has not been experimentally demonstrated. Here, using in situ aberration-corrected transmission electron microscopy (TEM) and density functional theory, we studied sodium ion transport in phosphorene. Our high-resolution TEM imaging complemented by electron energy loss spectroscopy demonstrates a precise description of anisotropic sodium ions migration along the [100] direction in phosphorene. This work also provides new insight into the effect of surface and the edge sites on the transport properties of phosphorene. According to our observation, the sodium ion transport is preferred in zigzag edge rather than the armchair edge. The use of this highly selective ionic transport property may endow phosphorene with new functionalities for novel chemical device applications