88 research outputs found

    DFT calculations of the structure and stability of copper clusters on MoS2

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    Layered materials, such as MoS2, are being intensely studied due to their interesting properties and wide variety of potential applications. These materials are also interesting as supports for low-dimensional metals for catalysis, while recent work has shown increased interest in using 2D materials in the electronics industry as a Cu diffusion barrier in semiconductor device interconnects. The interaction between different metal structures and MoS2 monolayers is therefore of significant importance and first-principles simulations can probe aspects of this interaction not easily accessible to experiment. Previous theoretical studies have focused particularly on the adsorption of a range of metallic elements, including first-row transition metals, as well as Ag and Au. However, most studies have examined single-atom adsorption or adsorbed nanoparticles of noble metals. This means there is a knowledge gap in terms of thin film nucleation on 2D materials. To begin addressing this issue, we present in this paper a first-principles density functional theory (DFT) study of the adsorption of small Cun (n = 1–4) structures on 2D MoS2 as a model system. We find on a perfect MoS2 monolayer that a single Cu atom prefers an adsorption site above the Mo atom. With increasing nanocluster size the nanocluster binds more strongly when Cu atoms adsorb atop the S atoms. Stability is driven by the number of Cu–Cu interactions and the distance between adsorption sites, with no obvious preference towards 2D or 3D structures. The introduction of a single S vacancy in the monolayer enhances the copper binding energy, although some Cun nanoclusters are actually unstable. The effect of the vacancy is localised around the vacancy site. Finally, on both the pristine and the defective MoS2 monolayer, the density-of-states analysis shows that the adsorption of Cu introduces new electronic states as a result of partial Cu oxidation, but the metallic character of Cu nanoclusters is preserved

    Structural, Thermodynamic, and Electronic Properties of Mixed Ionic/Electronic Conductor Materials

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    Due to the mainstream CMOS technology facing a rapid approach to the fundamental downscaling limit, beyond CMOS technologies are under active investigation and development with the intention of revolutionizing and sustaining a wide range of applications including sensors, cryptography, neuromorphic and quantum computing, memory, and logic, among others. Resistive switching electronics, for example, are devices that can change their electrical resistance with an applied external field. Despite their simple metal-insulator-metal structure, resistive switching devices exhibit an intricate set of IV characteristics based on the chemical composition of the solid electrolyte that ranges from non-volatile bipolar and non-polar switching to volatile threshold switching (abrupt but reversible change in resistance). This rich variety of electrical responses offer new alternatives to traditional CMOS applications in the areas of RF-signal switching, relaxation oscillators, over-voltage protection, and notably, memory cells and two-terminal non-linear selector devices. With the aim of unraveling the physics behind two of such conduction mechanisms, filamentary and threshold, in electrochemical cells consisting solid mixed ionic-electronic conductor electrolytes, this work focused on using first-principles calculations to characterize the structural, thermodynamic, and electronic properties of copper-doped amorphous silicon dioxide and copper-doped germanium-based glassy chalcogenides. The Cu/a-SiO2 system is a promising candidate for resistive switching memory applications. The conduction mechanism in the low-resistance state is known to be filamentary, that is, a physical metallic filament bridges between the metallic electrodes through the amorphous silica. However, many fundamental materials processes that would aid the design and optimization of this devices, such the shape and size of stable metallic filaments, remain unknown. In the first part of this work, the morphology and diffusion of small copper clusters embedded in amorphous silicon dioxide were characterized by density functional theory calculations. The average formation energy of a single copper ion in the amorphous matrix is found to be 2.4 eV, about 50% lower than in the case of silicon dioxide in its cristobalite or quartz phases. The theoretical predictions show that copper clusters with an equiaxed morphology are always energetically favorable relative to the dissolved copper ions in a-SiO2; hence, stable clusters do not exhibit a critical size. The stochasticity in the atomistic structure of the amorphous silicon dioxide leads to a broad distribution activation energies for diffusion of copper in the matrix, ranging from 0.4 to 1.1 eV. Since ab initio molecular dynamics are prohibitively expensive to simulate the switching process in Cu/a-SiO2 electrochemical metallization cells, a multi-scale simulation approach based on electrochemical dynamics with implicit degrees of freedom and density functional theory was developed to study the electronic evolution during metallic filament formation cells. These simulations suggest that the electronic transport in the low-resistance configuration is attributed to copper derived states belonging to the filament bridging between electrodes. Interestingly, the participation of states derived from intrinsic defects in the amorphous SiO2 around the Fermi energy are negligible and do not contribute to conduction. Unlike the Cu/a-SiO2 system which only exhibits filamentary switching, copper-doped germanium-based glassy chalcogenides show filamentary or threshold type of conduction depending on the chemical composition of the glass and copper concentration. Ab initio molecular dynamics based on density functional theory is used to understand the atomistic origin of the electronic transport in these materials. The theoretical predictions show that glasses containing tellurium tend to favor the formation of copper clusters; hence, these materials exhibit filamentary conduction. Threshold conduction is predicted to be the transport mechanism for glassy sulfides and selenides due to the ability of copper to remain dissolved in the amorphous matrix even at high metal concentration. Furthermore, the charge carrier transport in sulfur and selenium glasses was found to be assisted by defective states derived from chalcogen atoms whose bonds exhibit a polar character. Finally, taking advantage of the van der Waals gap as intercalation sites and crystal order in molybdenum disulfide, a novel mixed ionic-electronic conductor material based on copper and silver intercalation of MoS2 is proposed. The theoretical predictions show that on average, the intercalation energy of copper into MoS2 is 1 eV, while intercalation of silver shows a strong dependence on concentration ranging from 2.2 to 0.75 eV for low and high concentrations, respectively. The activation energy for diffusion of copper and silver intercalated within the van der Waals gap of MoS2 is predicted to be 0.32 and 0.38 eV, respectively, comparable to other superionic conductors. Upon Cu and Ag intercalation, MoS2 undergoes a semiconductor-to-metal transition, where the in-plane and out-of-plane conductances are comparable and exhibit a linear dependence with metal concentration

    Beyond Graphene: Monolayer Transition Metal Dichalcogenides, A New Platform For Science

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    Following the isolation of graphene in 2004, scientists quickly showed that it possesses remarkable properties. However, as the scientific understanding of graphene matured, it became clear that it also has limitations: for example, graphene does not have a bandgap, making it poorly suited for use in digital logic. This motivated explorations of monolayer materials “beyond graphene”, which could embody functionalities that graphene lacks. Transition metal dichalcogenides (TMDs) are leading candidates in this field. TMDs possess a wide variety of properties accessible through the choice of chalcogen atom, metal atom and atomic configuration (1H, 1T, and 1T’). Similar to graphene, monolayer TMDs may be produced on a small scale through mechanical exfoliation, but useful applications will require development of reliable methods for monolayer growth over large areas. In this thesis, I report our group’s recent progress in the chemical vapor deposition (CVD) of high quality, large area, monolayer TMDs under a 1H atomic configuration, which were integrated into high-quality biosensor arrays. These devices were incorporated in a flexible platform and were used for electronic read out of binding events of molecular targets in both vapor and liquid phases. I also report our findings on the CVD growth of monolayer TMDs in the 1T’ atomic configuration and measurements of their physical properties. 1T’ TMDs have been labeled the holy grail of materials due to theoretical predictions that they are 2D topological insulators; however they remain relatively unexplored due to the difficulty of monolayer growth and their lack of stability in air. Through careful passivation techniques, we were able to stabilize the as-grown monolayer 1T’ TMD flakes and perform the first characterizations on the structure. Lastly, in-plane 2D TMD heterostructures are promising material systems that combine the unique properties of each TMD. I discuss our results on the synthesis and study of 1H TMD heterostructures and unique 1H/1T’ TMD heterostructures. TMDs, with its many different accessible physical properties, coupled with the large variety of applications, have been classified as the leading nanomaterials in the realm “beyond graphene”

    Enhanced Carrier Transport by Transition Metal Doping in WS2 Field Effect Transistors

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    High contact resistance is one of the primary concerns for electronic device applications of two-dimensional (2D) layered semiconductors. Here, we explore the enhanced carrier transport through metal-semiconductor interfaces in WS2 field effect transistors (FETs) by introducing a typical transition metal, Cu, with two different doping strategies: (i) a "generalized" Cu doping by using randomly distributed Cu atoms along the channel and (ii) a "localized" Cu doping by adapting an ultrathin Cu layer at the metal-semiconductor interface. Compared to the pristine WS2 FETs, both the generalized Cu atomic dopant and localized Cu contact decoration can provide a Schottky-to-Ohmic contact transition owing to the reduced contact resistances by 1 - 3 orders of magnitude, and consequently elevate electron mobilities by 5 - 7 times higher. Our work demonstrates that the introduction of transition metal can be an efficient and reliable technique to enhance the carrier transport and device performance in 2D TMD FETs.Comment: Under revie

    Study on Growth of Two-Dimensional Transition Metal Dichalcogenides on Graphene: The Interface-Driven Defects and Properties Relationship

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    Department of Materials Science and EngineeringVertically stacked heterostructures based on the different types of two-dimensional (2D) materials via van der Waals (vdW) interaction have been extensively researched due to their novel properties beyond the limitations of individual 2D materials. The development of chemical vapor deposition (CVD) method enables the fabrication of various vdW heterostructures with clean interface and mass production, compared to the conventional multiple transfer method. Given the 2D nature of these materials, the interface intrinsically plays an important role in modulating or modifying their properties. For example, graphene placed on hexagonal boron nitride shows high charge carrier mobility, but a non-negligible interaction leads to the observation of ???Hofstadter???s butterfly???. In addition, in the case of transition metal dichalcogenides (TMDs), the transition from the direct band gap to the indirect band gap is apparent when the thickness increases due to the interface effect. Therefore, a systematic understanding of the impact of the interface on the intrinsic characteristics and performances of the vdW heterostructures is required in order to design desirable properties and expand the scope of applications of the vdW heterostructure. It is well known that the structural features of the underlying substrate significantly affect the growth behavior and even the unique properties of the heterostructures. Therefore, in this dissertation, I studied novel defects in TMDs induced by an underlying graphene template with various structural features. For this research, I prepared 3 types of graphene templates: 1) Pristine, 2) wrinkle-rich, and 3) nanocrystalline graphene (ncG). Pristine graphene is a good substrate for synthesizing TMDs without dangling bonds and without friction. In addition, when TMDs grow on pristine graphene, the anti-phase boundaries (APBs) of the TMDs are generated more predominantly than the tilted grain boundaries (GBs) due to vdW epitaxial growth. Using this heterostructure, we discovered the anisotropic features of the APBs according to transition-metal-facing (saw-toothed) or chalcogen-facing (straight) APBs, and both types of APBs show metallic properties despite different in-plane charge mobility. Wrinkles in graphene cause significant friction due to out-of-plane deformation and result in AB/AC stacking boundaries (SBs) in epi-TMD layer driven by Shockley partial dislocations. AB/AC SB has a buckled structure for releasing in-plane strain and results in monolayer-like behavior by reducing interlayer coupling. Finally, ncG has lots of dangling bond based active sites for multilayer growth. Due to the diffusion limited growth regime on the ncG template, the synthesized WSe2 domain shows a fractal morphology with many Se-terminated edge states. The WSe2/ncG heterostructure shows a downshifted work function similar to the valence band maximum of WSe2, resulting in a small Schottky barrier height at the metal-semiconductor-junction. Interface-driven novel defects and their corresponding properties are mainly observed using multi-mode of transmission electron microscopy (TEM) and other surface analysis tools (e.g. Raman, x-ray spectroscopy, atomic force microscopy). Theoretical density functional theory (DFT) calculations and TEM image simulations were supported to identify thermodynamically stable defect configurations and to confirm the exact atomic structures of novel defects. These studies could provide a systematic understanding of defect engineering, especially interface-driven defect formation mechanisms, atomic configurations, and their corresponding properties. It could pave the way for achieving and expanding the manipulation and commercialization of 2D material-based devices via defect engineering.clos

    Synthesis, Characterization, and Device Applications of Two-Dimensional Materials

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    Two-dimensional (2D) materials have attracted tremendous research interest, as they offer novel physics, facile visualization by electron and scanning probe microscopy, and the potential to become next-generation electronic materials, all due to reduced dimensionality. Large-area 2D single crystals are needed for both fundamental scientific experiments and electronic device applications. New methods need to be developed to exploit state-of-the-art microscopy in the scientific investigation of 2D materials. Mechanisms behind the behavior of 2D-material based devices need to be resolved and new device concepts and applications need to be explored. This dissertation addresses these three aspects of 2D materials research.Using chemical vapor deposition growth of graphene on copper as a platform, the first part of my research in this thesis demonstrates a facile method involving a simple in-situ treatment of the copper catalytic substrate right before the growth that results in mm-sized graphene single crystals, elucidating the key factors of achieving large-area 2D single crystals.The second part of this work developed experimental methods to resolve important issues in 2D materials research by employing modern transmission electron microscopy. Here, a method has been developed to determine the edge orientation and termination without imaging the edge down to the atomic scale of monolayer hexagonal boron nitride (h-BN), enabling a direct comparison to theoretical predictions. Another important issue in 2D materials research is the determination of the layer count and its lateral spatial uniformity. In this work, a method is developed to map the layer count of a 2D material at nanometer-scale lateral resolution over extended areas, utilizing a combination of mass-thickness mapping offered by STEM and element-specific quantization afforded by electron energy loss spectrum (EELS) mapping.The last part of this thesis work unravels the multiple mechanisms behind the behavior of field effect transistors (FETs) based on PdSe2. The change in device behavior in early reports from ambipolar to n channel was puzzling. As commonly encountered in device research, many factors, including channel material properties, defects, contaminants, and contact effects, are almost always entangled. Here, I use multiple control devices to unravel various mechanisms and provide consistent explanations for device behvior variations

    Prediction of Co and Ru nanocluster morphology on 2D MoS

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    Layered materials, such as MoS2, have a wide range of potential applications due to the properties of a single layer, which often differ from the bulk material. They are of particular interest as ultrathin diffusion barriers in semiconductor device interconnects and as supports for low-dimensional metal catalysts. Understanding the interaction between metals and the MoS2 monolayer is of great importance when selecting systems for specific applications. In previous studies the focus has been largely on the strength of the interaction between a single atom or a nanoparticle of a range of metals, which has created a significant knowledge gap in understanding thin film nucleation on 2D materials. In this paper, we present a density functional theory (DFT) study of the adsorption of small Co and Ru structures, with up to four atoms, on a monolayer of MoS2. We explore how the metal–substrate and metal–metal interactions contribute to the stability of metal clusters on MoS2, and how these interactions change in the presence of a sulfur vacancy, to develop insight to allow for a prediction of thin film morphology. The strength of interaction between the metals and MoS2 is in the order Co > Ru. The competition between metal–substrate and metal–metal interaction allows us to conclude that 2D structures should be preferred for Co on MoS2, while Ru prefers 3D structures on MoS2. However, the presence of a sulfur vacancy decreases the metal–metal interaction, indicating that with controlled surface modification 2D Ru structures could be achieved. Based on this understanding, we propose Co on MoS2 as a suitable candidate for advanced interconnects, while Ru on MoS2 is more suited to catalysis applications

    First principle-based Research on properties of twisted bilayer of Graphene and GaN

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    Since quantum theory was founded in 1920s, it has developed quite rapidly and led people to a new field that totally different from the classical ones. As the size of an object decrease to a certain level, the classical physical law would be invalid and quantum mechanics would become the ruler of object behaviors. The property under quantum mechanics achieves functions that never been seen before. So, scientist would invent new materials and devices with properties that never found before. This is the reason that new materials and devices have been emerging due to vast developments in nanotechnologies. The project of my PHD research in four years is to research and analysis the property of nano semiconductor materials. In this thesis, there are mainly four parts. The first part is Introduction. In this part the basic concepts and related information would be listed. The second part is the first project that guide me to the further study. This project is set to simulate the electronic transport in quantum region based on the Schrodinger equation for random shaped energy barriers, which is very basic to quantum mechanics. The method of simulation is matlab coding. The simulation will help in understanding the concept of quantum field and applications in design and analysis of nanometer scale devices and systems. The third part is my first paper. In this paper I cooperate with my workmate Shuo Deng and we investigate the electron transport and thermoelectric property of twisted bilayer graphene nanoribbon junction (TBGNRJ) in 0o, 21.8o, 38.2o and 60o rotation angles by first principles calculation with Landauer-Buttiker and Boltzmann theories. It is found that TBGNRJs exhibit a strong reduction of thermal conductance compared with the single graphene nanoribbon (GNR) and negative differential resistance (NDR) in 21.8 o and 38.2 o rotation angles under ±0.2 V bias voltage. More importantly, three peak ZT values of 2.0, 2.7 and 6.1 can be achieved in the 21.8o rotation angle at 300K. The outstanding ZT values of TBGNRJs are interpreted as the combination of the reduced thermal conductivity and enhanced electrical conductivity at optimized angles. The fourth part is my second paper. In this paper I report electronic and optical properties of the GaN bilayer structures that are rotated in plane at several optimized rotation angles by using the density functional theory method. For the aim of maintaining the structural stability and using a small cell size, the twisting angles of the GaN bilayer structures are optimized to be 27.8°, 38.2° and 46.8° using the crystal matching theory. The last part is conclusion, possible further study and acknowledgement

    High Mobility N-Type Field Effect Transistors Enabled By Wse2/pdse2 Heterojunctions

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    Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have emerged as a promising candidate for post-silicon electronics. Few-layer tungsten diselenide (WSe2), a well-studied TMD, has sown high hole mobility and ON/OFF ratio in field effect transistor (FET) devices. But the n-type performance of WSe2 is still quite limited by the presence of a substantial Schottky Barrier. Palladium diselenide, (PdSe2) is a newly discovered TMD that is of interest because of its high electron mobility, and moderate ON/OFF ratios. However, despite its relatively small bandgap, the n-type performance of few-layer PdSe2 FETs has also been limited by a Schottky barrier, which is likely due to Fermi-level pinning. In this work, we report high performance n-type FETs enabled by a few-layer WSe2/PdSe2 heterojunction. We show that the current through few-layer WSe2 or PdSe2 alone is quite small, but across the heterojunction WSe2 serves as a “buffer layer” at the drain/source contacts for few-layer PdSe2 FETs. We observe a high ON/OFF ratio of 105, with an electron mobility of ~139 cm2 V-1 s-1. The mobility continues to rise at cryogenic temperatures, indicating a substantial reduction in the Schottky Barrier height. A heterojunction consisting of 3-layer PdSe2 and 3L WSe2 showed an ON/OFF ratio approaching 107, while still maintaining a moderate mobility of ~ 57 cm2 V-1 s-1. We believe the significantly improved device performance enabled by our contact engineering technique will facilitate further study of the intrinsic properties of few-layer 2D materials
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