505 research outputs found
Stratospheric Flight: Aeronautics at the Limit
No full textIn this book, Dr. Andras Sobester reviews the science behind high altitude flight. He takes the reader on a journey that begins with the complex physiological questions involved in taking humans into the "death zone." How does the body react to falling ambient pressure? Why is hypoxia (oxygen deficiency associated with low air pressure) so dangerous and why is it so difficult to 'design out' of aircraft, why does it still cause fatalities in the 21st century? What cabin pressures are air passengers and military pilots exposed to and why is the choice of an appropriate range of values such a difficult problem? How do high altitude life support systems work and what happens if they fail? What happens if cabin pressure is lost suddenly or, even worse, slowly and unnoticed? The second part of the book tackles the aeronautical problems of flying in the upper atmosphere. What loads does stratospheric flight place on pressurized cabins at high altitude and why are these difficult to predict? What determines the maximum altitude an aircraft can climb to? What is the 'coffin corner' and how can it be avoided? The history of aviation has seen a handful of airplanes reach altitudes in excess of 70,000 feet - what are the extreme engineering challenges of climbing into the upper stratosphere? Flying high makes very high speeds possible -- what are the practical limits? The key advantage of stratospheric flight is that the aircraft will be 'above the weather' - but is this always the case? Part three of the book investigates the extreme atmospheric conditions that may be encountered in the upper atmosphere. How high can a storm cell reach and what is it like to fly into one? How frequent is high altitude 'clear air' turbulence, what causes it and what are its effects on aircraft? The stratosphere can be extremely cold - how cold does it have to be before flight becomes unsafe? What happens when an aircraft encounters volcanic ash at high altitude? Very high winds can be encountered at the lower boundary of the stratosphere - what effect do they have on aviation? Finally, part four looks at the extreme limits of stratospheric flight. How high will a winged aircraft will ever be able to fly? What are the ultimate altitude limits of ballooning? What is the greatest altitude that you could still bail out from? And finally, what are the challenges of exploring the stratospheres of other planets and moons? The author discusses these and many other questions, the known knowns, the known unkonwns and the potential unknown unknowns of stratospheric flight through a series of notable moments of the recent history of mankind's forays into the upper atmospheres, each of these incidents, accidents or great triumphs illustrating a key aspect of what makes stratospheric flight aeronautics at the limit
Reconfigurable Diodes Based on Vertical WSe2Transistors with van der Waals Bonded Contacts
No full textNew device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate-tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic-liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe2transistor by only utilizing van der Waals bonded graphene and Co/h-BN contacts. Toward this, first the charge injection efficiencies into WSe2by graphene and Co/h-BN contacts are characterized. While Co/h-BN contact results in nearly Schottky-barrier-free charge injection, graphene/WSe2interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work-function values of graphene and Co/h-BN, vertical devices are constructed where different gate-tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts
Transmission Electron Microscopy and Nanoindentation Studies of Ultrathin Transition Metal Dichalcogenide Semiconductor Membranes
No full textUntil now, only two intrinsically two-dimensional materials have been isolated: graphene and boron nitride (BN). As a consequence, the knowledge of two-dimensional systems is still limited and other classes of two-dimensional materials are needed to enrich our understanding of these systems. In this thesis, we studied the structural and mechanical properties of two-dimensional transition metal dichalcogenide (TMD) crystals. Apart from fundamental interest, transition metal dichalcogenides have great appeal due to their possible applications in electronic. Here, the focus is on MoS2, while the main employed techniques are transmission electron microscopy and nano-indentation. The thesis is divided into three parts. The first part is devoted to summarize the general properties of transition metal dichalcogenide materials, to illustrate the instrumentation, and to describe the deposition techniques used in sample preparation. In particular, the transfer technique used to transfer selected flakes among different substrates is described. In the second part, we investigate single-layer MoS2 flakes obtained by mechanical exfoliation. We describe two methods to distinguish single-layer MoS2 flakes from thicker ones by electron diffraction. Ripples are observed over the flake surface. Thanks to diffraction pattern interpretation and high resolution transmission electron microscopy, their amplitude is estimated to be 0.6 - 1 nm in height. Nano-indentation is used to determine the Young’s modulus and the breaking strength of single-layer MoS2 membranes. The Young’s modulus is EY =270±100GPa while the breaking strength is σmax =23±5 GPa. In order to fully take advantage of the promising electronic properties of transition metal dichalcogenide, it is necessary to grow them over large scale. In this way, the pla- nar technology developed for silicon can be implemented and a large number of devices with uniform electrical properties can be realized. When comparing different growths, methods to quantify the quality of the films are mandatory. In this context, we show that transmission electron microscopy is a valuable method to characterize large area MoS2 and MoSe2 films. We are able to estimate the grain size and to have a direct view of the grain boundaries. In fact, grain size and boundary quality constitute the important parameters to qualify two-dimensional poly-crystalline films. Finally, we describe a novel in-situ experiment which allows us to simultaneously perform transmission electron microscopy and electrical characterization on a device based on a suspended MoS2 flakes. The effect of the current flow on the MoS2 flake are considered. These two topic constitute the last part of the thesis.LANE
Optoelectronic Devices Based on Monolayer Transition Metal Dichalcogenides
No full textMonolayer transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2), tungsten diselenide (WSe2) and tungsten disulfide (WS2), have shown in the last years remarkable physical properties. These direct badgap three-atoms thick monolayers, with a broken inversion symmetry, present a unique coupling between the spin and valley degrees of freedom originated from the relativistic spin¿orbit interaction. Together with their mechanical flexibility, result in materials promising for flexible, transparent and low power electronic, optoelectronic and spin/valleytronic applications. In this thesis, we investigate optoelectronic and spin/valleytronic devices based on monolayer MoS2, the most studied monolayer from the TMD familly, with 1.9 eV direct band gap and 6.5 Å thick. We show a monolayer MoS2 photodetector with 100,000-fold improved photoresponsivity from previous monolayer MoS2 phototransistors, first time reported entire junction light emitting diodes (LED) and solar cells based on monolayer MoS2/p-type silicon heterojunctions, which can work as an avalanche photodiodes too, and first spin-valley tunable light emitting diode made with monolayer MoS2/monolayer WSe2. Utilizing high-quality monolayer MoS2, we achieved a broad spectral range phototransistor with photoresponsivity of 880 A/W and low noise equivalent power (NEP) of 1.8 x 10^-15 W/Hz^1/2. Afterwards, we used the two-dimensional (2D) n-type monolayer MoS2 combined with three-dimensional (3D) p-type silicon to build vertical p-n junctions. The entire junction area of our 2D/3D heterostructures emitted light with a low emission threshold power density of 3.2 W/cm^2 and spectrum related to the direct band gap of monolayer MoS2. The heterojunction diode could operate as a solar cell with an external quantum efficiency (EQE) of 4.4% and a broad spectral response. With the use of large area chemical vapor deposition (CVD) grown monolayer MoS2, we scaled up the manufacturing process and showed the capability of these heterostructures to work as avalanche photodiodes (APD) with a multiplication exceeding 1000 for -10 V. Finally, we combined monolayer MoS2 with WSe2 or WS2 and made 2D/2D heterojunctions able to emit light with different characteristic spectrums related to the type of heterojunction. Monolayer WS2/monolayer MoS2 showed one of the emission peaks in the green region of the visible spectrum while all heterojunctions showed peaks in the red region. Spin-polarized charge carriers were injected across the Schottky barrier between a ferromagnetic electrode and the monolayer WSe2 of a monolayer WSe2/monolayer MoS2 heterostructure, resulting in valley polarization due to spin-valley locking. The degree of spin/valley polarization was controlled by a magnetic field between a polarization of ± 20%. A slope of 0.47 ± 0.08 meV/T was also seen related to the valley Zeeman effect.LANE
Disorder-induced electronic, magnetic, and optoelectronic properties of two-dimensional materials
No full textTechnological advancement has been in cadence with material development by improving the purity of single crystals and, at the same time, controlling their imperfections. These capabilities have been especially vital for developing new technolo-gies based on two-dimensional (2D) van der Waals (vdW) materials for future electronic and optoelectronic applications. This is because the inherent properties of 2D vdW materials is highly susceptible to the presence of intrinsic structural defects and ex-trinsic disorders due to large surface-area-to-volume ratio. The successful reduction of these disorders has significantly im-proved material properties and led to the discovery of novel physical phenomena in vdW materials. On the other hand, structural defects â for instance, 0-dimensional point defects â can induce completely new properties that are otherwise absent in the pre-fect lattice. To harness the full potential of vdW materials, it is thus essential to produce high-quality crystals and understand how the disorder affects their material properties, which is the central idea of this dissertation.
In this dissertation, we first present the work of high-quality epitaxial growth of NbS2. Based on atmospheric-pressure chemical vapor deposition, we have successfully synthesized the two polymorphs (2H and 3R) of NbS2 with the largest lateral size grown to date. Their distinct superconducting and metallic properties were examined under low-temperature charge transport, respectively. Our finding demonstrates the practical synthesis method for phase-controllable growth of 2D transition metal dichalcogenides and can benefit future studies in mesoscopic devices and large-area applications of 2D superconductors.
Secondly, we present the work of discovering defect-induced novel properties in ultrathin layers of PtSe2. Although bulk PtSe2 is non-magnetic, we observe the appearance of magnetism in monolayer and bilayer PtSe2. We were able to measure the magnetoresistance (MR) of mono- and bilayer PtSe2 under perpendicular magnetic fields using proximitized graphene, and found antiferromagnetic and ferromagnetic MR responses for mono- and bilayer, respectively. The appearance of such different magnetic states is theoretically explained by the first-principle density functional theory calculation, suggesting the origin of induced-magnetic moments from intrinsic Pt vacancies for both layers. Moreover, we also found that structural disorder in PtSe2 can induce bulk photovoltaic effect (BPVE). The second-order optical nonlinear effects, such as BPVE, require broken structural inversion symmetry and crystal symmetry can be reduced by the presence of structural defects. The broken local inversion symmetry from structural disorder in centrosymmetric PtSe2 is manifested by the generation of zero-biased photo-current under homogenous illumination. We observe linear and circular polarization-dependent photocurrents in defective PtSe2, which is largely absent in the pristine crystal. Our findings in PtSe2 emphasize the importance of the structural disorder for generating completely new properties and stress the need for defect-engineering for realizing the practical use of PtSe2 in spintronic and photovoltaic applications.LANE
Charge-transport properties of monolayer MoS2 at the interface with dielectric materials
No full textTwo-dimensional (2D) semiconductors, consisting of single-sheets of layered transition metal dichalcogenides (TMD), are attracting enormous interest from both fundamental science and technology. Monolayer molybdenum disulfide (MoS2), a typical example from this class of materials, is currently under intense research investigation because its direct band gap, atomic-scale thickness and mechanical flexibility could enable a wide range of novel technological applications, such as low-power flexible/transparent electronics, displays and wearable sensors. Critical to all these applications are the material mechanical strength, the mobility of charge-carriers in the 2D semiconductor and its interaction with the surrounding environment. This thesis describes experimental research conducted on these critical aspects and shows a proof-of-concept device application based on heterostructures of MoS2 and graphene. The main goal of the research was to assess experimentally the potential of monolayer MoS2 for application in flexible electronics. The thesis is based on three papers. The first paper describes the measurements of the in-plane stiffness and breaking strength of free-standing membranes of monolayer MoS2. Nanoindentation experiments were performed with the tip of an atomic force microscope (AFM) to extract the material's Young's modulus (E ~ 270 GPa) and breaking strength (Ï max ~ 23 GPa). Breaking occurred at maximum internal strain εint ~ 11%, which indicates that monolayer MoS2 is suitable for integration on flexible plastic substrates. The second paper represents the core work of this thesis. It explores the charge transport properties of monolayer MoS2 supported by different dielectric substrates, such as thin polymer films of parylene, atomically flat sapphire and 2D sheets of hexagonal boron nitride (h-BN). It was found that substrate surface corrugations do not represent a major limit to charge-carrier mobility in monolayer MoS2, which seems at this stage dominated by impurities and material's defects. Field-effect transistors with mobility ~ 100 cm2/Vs and on/off current ratio Ion/Ioff > 10^5 were successfully integrated on parylene substrates, whose root-mean-square roughness Rq can be more than two times larger than the thickness of the transistor channel itself. Because parylene is also a flexible material, this work showed a viable method for the realization of high-mobility and high performance flexible devices based on 2D semiconductors. Finally, the third paper describes a flash-memory cell fabricated using monolayer MoS2/graphene heterostructures and dielectric layers of HfO2 grown by atomic layer deposition (ALD). The architecture of the device resembles that of a floating-gate transistor. Monolayer MoS2 acts as the transistor channel, graphene electrodes are used to collect and inject the charge carriers, and a piece of multilayer graphene serves as ultrathin charge trapping layer. In this paper, it was shown that graphene contacts to monolayer MoS2 result in ohmic-like current-voltage characteristics at room temperature. Moreover, the use of graphene and 2D semiconductors in flash memory technology was indicated as a potential strategy for further scaling of memory cells and for their integration in flexible electronic devices.LANE
Single-layer MoS₂ : Electronics in Two Dimensions
No full textThis thesis explores the electronic properties of one layered transition-metal dichalcogenide – single-layer MoS2, and demonstrates the first transistors and integrated circuits with characteristics that outperform graphene electronics in many aspects and have comparable properties to nowadays silicon electronics. The first part describes the production methods of transition-metal dichalcogenides (TMDs) and optical detection of mechanically exfoliated TMDs. A simple optical model is used to calculate the contrast of nanolayers on SiO2 substrate. Atomic force microscopy is used for determination of the thickness of atomically flat layers. The optical contrast of thicker TMD layers is proposed for fast and ease differentiation among mono double and tri-layers. The second part is devoted to fabrication of filed-effect transistors based on single-layer MoS2. For the first time high current on/off ratio ∼ 108, subthreshold swing as low as 74 mV/dec and moderately high electron mobility ∼ 50 cm2/Vs are demonstrated. Subsequently, based on this platform we fabricated and demonstrated operation of the first logic gates, integrated circuits and small-signal analog amplifiers. The previous demonstrations in the first and the second part were based on the improved characteristics of filed-effect transistors with an HfO2 top gate dielectric that acts as a mobility booster in the same time. The physical mechanism of this improvement has been studied in the third section. The charge impurities and defects in SiO2 substrate are suggested to be the main reason of the mobility degradation in thin MoS2 layers. We performed Hall effect measurements in order to unambiguously determine the capacitance of the top gate dielectric and charge concentration in MoS2 channel. Magnetotransport measurements are presented as well demonstrating weak and strong localization in top-gated field-effect transistors based on single-layer MoS2.LANE
Two-Dimensional materials: from large-area growth to the performance in radio-frequency range
No full textThis thesis presents a collection of experimental results about the large-area growth of single-layer graphene and MoS2 films. In addition, we present the results of electrical characterisation of two-dimensional (2D) materials in direct current (DC) and radio-frequency (RF) range. The thesis is divided into four parts. In the first part we describe the recipe for the large-area growth of single-layer graphene on Cu substrate with the chemical vapor deposition (CVD) technique. In addition, we present two different methods for the transfer of the grown films onto arbitrary substrates. In order to investigate the intrinsic properties of the grown film we perform the electrical characterization in DC and RF range. The second part is devoted to the large-area growth of MoS2 single-layer films. We describe two different recipes for the growth: two-step thermolysis process of ammonium tetrathiomolybdate (NH4)2MoS4 in S atmosphere and CVD reaction ofMoO3 in S atmosphere. Additionally, we perform the electrical characterisation of the fabricated FETs based on grown MoS2 films. Apart from the grown materials, we also analysed the electrical properties of exfoliated MoS2. In the third part, we investigate the behaviour of 240 nm length top-gated MoS2 transistors operating in RF range. We fabricated MoS2 RF devices based on 1L-MoS2, 2L-MoS2, 3L-MoS2 and as well as multilayer MoS2 film with the thickness of » 5 nm. They exhibit current, power and voltage gain. The best DC and RF characteristics were obtained for 3L-MoS2 device showing a cutoff frequency fT = 6 GHz and a maximum oscillation frequency fmax = 8.2 GHz after the deembedding procedure. In order to boost the RF performance of MoS2 devices, firstly, we fabricated top-gated FETs with the gate length as short as 40 nm, secondly, we introduced the "edge-contacted" injection technique to our devices. Thus, we were able to obtain the record cutoff frequency for MoS2 devices of 6 GHz before deembedding and fT = 25 GHz after the deembedding procedure.LANE
Stretchable and Highly Bendable Thin Film Transistors Based on Atomically Thin Chemical Vapor Deposited Molybdenum Disulfide
No full textTwo-dimensional (2D) materials have received tremendous research attention recently, as they possess peculiar physical properties in their monolayer and few-layer forms, which further lead to novel applications. A wide range of 2D materials covers insulators, metals, superconductors and semiconductors with various bandgap, giving a fresh platform to reinvent conventional electronic and optoelectronic devices with new advantages. Molybdenum disulfide (MoS2), belonging to the group of transition metal dichalcogenides (TMDCs), has been widely studied for its particular physical properties and potential applications as a leading example among 2D semiconductors. In this thesis, we focus on the applications ofMoS2 in microelectronic and micro-electromechanical devices. The MoS2 we studied is grown specifically by chemical vapor deposition, which offers the highest potential in large area growth. Utilizing the flexible nature such as high in-plane fracture strain and low bending stiffness of atomically thinMoS2, we developed stretchable thin film transistors (TFTs) on polydimethylsiloxane (PDMS) and highly bendable thin film transistors on polyimide. The fabrication process is ameliorated and designed to provide good chemical and temperature compatibility to the polymer substrates. Electrical performance of the flexible devices under different degrees of bending and stretching has been studied. The stretchableMoS2 TFTs could sustain up to 4% of uniaxial tensile strain maintaining device function, and the failure occurs at 5% of strain due to the fracture in gate dielectric layer. The highly bendableMoS2 TFTs with parylene-C gate dielectric layer exhibit tolerance to the smallest bending radius of 0.625 mm. Different encapsulation layers such as HfO2, Cytop and SU8 have been implemented on the bendable devices to improve the performance represented by the increase in field effect mobility, reduction in hysteresis and better endurance to mechanical deformation. The resulting flexibleMoS2 TFTs show field effect mobility of 21.4 cm^2V^-1s^-1 and a large on-off ratio of 106 in air. The SU8 encapsulation layer is further demonstrated as controllable n-type doping strategy with low temperature process, which is ideal for flexible MoS2 devices as it additionally exhibits good stability in air and water.LANE
Nano-electromechanical systems based on ultra-thin semiconductors
No full textNowadays, the interest in 2D materials has gone far beyond graphene. Specially, monolayers of transition metal dichalcogenides (TMDs) offer a broad spectrum of electronic and optical properties, and show the potential to revolutionize the electronics industry. The promising electronic properties of 2D semiconductors combined with their mechanical strength and flexibility, makes them ideal candidates for nanoelectromechanical systems
(NEMS). This thesis focuses on realizing NEMS based on graphene and MoS2, which is one of the most appealing TMDs.
First, we present the realization of graphene NEMS by fabricating single and bilayer graphene transistors featuring a doubly clamped suspended channel. The electromechanical response of monolayer graphene nanoribbons show a strain-induced increase in their electrical resistance, making it possible to estimate an upper limit for the piezoresistive gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of the two graphene layers with respect to each other. Our results report on the rare observation of room temperature electronic interference in bilayer graphene.
Next, we investigate the static electromechanical response of atomically thin MoS2. MoS2 exhibits high youngâ s modulus and fracture strength. Furthermore, the bandgap of MoS2 is highly strain-tunable. This coupling between electrical and mechanical properties makes MoS2 a promising material for NEMS. Here we incorporate monolayer, bilayer and trilayer MoS2 in a suspended membrane configuration with the electrodes acting as mechanical clamps. Strain-induced bandgap tuning is detected via electrical conductivity measurements and the emergence of the piezoresistive effect in MoS2 is demonstrated. We observe a reversible bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element method (FEM) simulations are used to obtain the spatially varying bandgap profile on the membrane and to quantify the rate of bandgap change. The piezoresistive gauge factor is calculated for single layer, bilayer and trilayer MoS2. Our results reveal that monolayer and bilayer MoS2 show a piezoresistive effect which is comparable to the state-of-the-art silicon strain sensors and two orders of magnitude higher than in graphene.
Finally, we present the investigation of MoS2 NEMS resonators working in the VHF range and featuring piezoresistive transduction. The atomic thickness of monolayer MoS2 places it as a promising candidate for miniaturization of electromechanical devices to the limits of vertical scaling. While the small mass of MoS2 leads to an increased resonant frequency and a higher mass sensitivity, the presence of piezoresistivity offers a transduction mechanism in addition to the traditional capacitive transduction. Operating in the tension-dominated regime, monolayer MoS2 NEMS resonators not only allow tunability of the resonant frequency using an external voltage, but also show the strain-induced enhancement of their dynamic range. Furthermore, the resonators are driven into the nonlinear regime allowing the study of nonlinear effects. This work sheds light on the potential of TMD based NEMS as ultra-low power switches, sensors and resonators for applications in RF range.LANE
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