2D materials based heterostructures : a lithography free method.

Many properties of Two-dimensional (2D) materials are vastly different from those of their 3D counterparts. A large family of 2D materials ranging from gapless graphene to metallic NbSe2 (also superconducting), semiconducting MoS2, and insulating hexagonal boron nitride (h-BN) possessing a broad range of exciting new properties have emerged in recent years. Moreover, 2D materials provide the perfect platform to create laterally and vertically stacked heterostructures with intriguing properties. The physics of 2D materials based heterostructures is extremely interesting and novel 2D-heterostructured devices including tunneling diodes, tunneling transistors, photovoltaic cells, and light-emitting diodes have started to emerge. In this work, we developed a novel lithography-free technique for the fabrication of 2D material-based electrical devices. We fabricated few-layer and multi-layer WS2 devices using a transmission electron microscope (TEM) grid as a shadow mask, and its transport characteristics were studied by electrical measurements. WS2 samples were synthesized by first depositing WO3 followed by sulfurization and characterized by scanning tunneling microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. Hydrazine adsorption on WS2 was studied by measuring the electrical resistances during adsorption (exposing to hydrazine vapor) and subsequent desorption (by pumping). WS2 sample consisting of two layers showed a decrease of resistance upon exposure to hydrazine vapor and showed complete reversibility upon pumping. WS2 sample with three layers showed a decrease of resistance during exposure but showed only partial recovery during desorption. In contrast, multi-layered (12 layers) WS2 sample showed an initial decrease followed by a continued increase of the resistance upon exposure to hydrazine with little or no reversibility upon pumping. The charge transfer from N2H4 to WS2 is believed to be responsible for the decrease of the resistance. Trapping of N2H4 molecules within the multilayers of WS2 causing charge redistribution and possible chemical reactions is believed to be responsible for the increase in resistance during the adsorption and complete irreversibility of resistance during desorption. The experimental results are explained with the help of computational calculations carried out by employing the density functional theory (DFT) framework, as implemented in the Vienna Ab-initio Simulation Package (VASP). Next, we extended our lithography-free technique for the fabrication of two-dimensional (2D) material based heterostructures. We fabricated graphene-WS2 heterostructured devices again using a TEM grid as a shadow mask. Graphene was directly deposited on a Si/SiO2 substrate by radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD). WS2 was synthesized as before. The temperature dependence of the resistance and magnetoresistance are measured for graphene, WS2, and graphene-WS2 heterostructure. At low temperatures, the transport was found to follow the variable-range hopping (VRH) process, where logarithmic R exhibits a �−1/3 temperature dependence, an evidence for the 2D Mott VRH transport. The measured low-field magnetoresistance also exhibits a quadratic magnetic field dependence ~�2, consistent with the 2D Mott VRH transport. Finally, a lithography-free technique was developed to fabricate Graphene/h-BN/Graphene tunnel junctions. Graphene and h-BN were directly deposited on a Si/SiO2 substrate by RF-PECVD using CH4 and ammonia borane as the precursors respectively. Tunnel diodes with varying barriers were fabricated by tuning the thickness of the h-BN layer thickness. The tunneling current was found to scale exponentially with the tunnel barrier thickness

A Perspective on Recent Advances in Phosphorene Functionalization and its Application in Devices

Phosphorene, the 2D material derived from black phosphorus, has recently attracted a lot of interest for its properties, suitable for applications in material science. In particular, the physical features and the prominent chemical reactivity on its surface render this nanolayered substrate particularly promising for electrical and optoelectronic applications. In addition, being a new potential ligand for metals, it opens the way for a new role of the inorganic chemistry in the 2D world, with special reference to the field of catalysis. The aim of this review is to summarize the state of the art in this subject and to present our most recent results in preparation, functionalization and use of phosphorene and its decorated derivatives. In particular, we discuss several key points, which are currently under investigation: the synthesis, the characterization by theoretical calculations, the high pressure behaviour of black phosphorus, as well as decoration with nanoparticles and encapsulation in polymers. Finally, device fabrication and electrical transport measurements are overviewed on the basis of recent literature and new results collected in our laboratories

Graphene Based Heterojunctions for Nano-Electronic and Sensing Applications

Graphene, an atomically thin and semi-metallic two dimensional material, has been extensively researched over the past decade due to its superior intrinsic carrier velocity, electrical and chemically tunable work function, ability to form layered heterostructure with other materials, and relevant potential applications in electronics, sensing, optoelectronics, energy storage, etc. However, the confinement of charge carriers within one atomic layer results in an electrical transport that is extremely sensitive to the surrounding environment, which is beneficial for sensing applications, but at times unfavorable for electronic applications due to scattering from extrinsic impurities. In addition, due to its rather delicate structure, engineering a high quality gate dielectric without altering its characteristic electronic structure while enabling optimal surface passivation and gate control is one of the major challenges for graphene device development. Hexagonal Boron Nitride (hBN) has emerged as a possible option to meet the challenges, and has been exploited to alter graphene electronic structure by intentional crystallographic misalignment between the layers at the time of transfer or synthesis. The variation in electronic structure by hBN is possible due to its unique properties such as inert surface, similar hexagonal and nearly lattice matched structure with graphene and high surface optical phonon modes. Low temperature Pulsed laser deposition (PLD) grown amorphous BN on SiO2/Si, phase transformed to hBN by forming gas annealing, has been employed for graphene device application. Graphene field effect transistor (FET) fabricated from layered heterostructure of graphene/hBN on SiO2/Si exhibited electrical performance enhancement over graphene on SiO2/Si substrate in terms of mobility, carrier inhomogeneity and extrinsic doping. In a parallel effort, taking advantage of graphene’s tunable work function, a novel genre of sensor based on noble metal nanoparticle functionalized graphene/Si heterojunction Schottky diode has been developed for sensing non-polar H2, and enhancing response for polar NH3 molecular species. Reverse bias operation of the diode sensor exhibited orders of magnitude higher response compared to graphene FET based sensors due to exponential change in reverse current originated from interface barrier height change. The reverse bias operation also allows low power operation and modulation of the Fermi level of graphene, which can lead to the tuning of sensitivity and expansion of the dynamic range. Impedance Spectroscopic analysis of the diode sensor has been carried out to understand the underlying current transport mechanism. Fitting the impedance spectra for different gaseous exposure conditions with an equivalent circuit model, the changes in junction resistance and capacitance have been extracted. Along with these two parameters, experimentally obtained 3-dB cut off frequency for each gas exposure has been utilized for multimodal sensing by the diode sensor. Finally, temperature dependent magneto-transport study of PdHx passivated graphene has been carried out to elucidate the effect of metal nanoparticle assisted doping and molecular adsorption on graphene electrical transport properties. It has been observed from the systematic study that, the dominant scattering mechanism in bilayer graphene switched from coulomb scattering to thermal excited surface optical phonon scattering after PdHx passivation, and Hall mobility exhibited significant enhancement at the measurement temperature range of 298 to 10 K. Due to recent interests in exploiting metallic nanoparticles as dopant for 2D crystals, as well as enhancing sensitivity of chemical sensors and photodetectors, the findings are significant and would pave the way for future research efforts in this area

Spin and magneto transport in van der Waals heterostructures of graphene with ferromagnets

The increasing demand for information and communication technologies has augmented the requirements of electronic devices with improved speed, sensitivity, and reduced power consumption. The utilization of novel electronic materials and the use of the spin degree of freedom as a state variable for information processing and storage are expected to fulfill these demands. In this direction, two-dimensional (2D) materials have attracted a significant research effort with the long-term goal of creating electronic devices with novel functionalities. Graphene has shown excellent potential for future device applications due to its outstanding electronic carrier mobility and spin coherence time at room temperature. Followed by the successful advent of graphene, a vast plethora of 2D materials with complementary electronic properties have been discovered, such as insulating hexagonal boron nitride (hBN), magnets and topological semimetals. We observed that engineering 2D material heterostructures by combining the best of different materials in one ultimate unit offers the possibility of the creation of new phases of matter and novel opportunities in device design. For example, graphene is shown to acquire magnetic properties because of proximity-induced interactions with a magnetic insulator in van der Waals heterostructure. On the other hand, topological semimetal candidates such as WTe2 and ZeTe5 allowed us to observe unconventional charge-to-spin conversion and anomalous Hall effects due to their enormous spin-orbit coupling, lower crystal symmetry, and larger fictitious magnetic field in the crystals. Furthermore, the performance of heterostructures comprised of graphene and hBN with one-dimensional ferromagnetic edge contacts and a path for optimizing such device geometry is outlined. These experimental findings on 2D materials and heterostructure device architectures can contribute to developing a new platform for spintronic as well as quantum science and technology

Silicon- and Graphene-based FETs for THz technology

[EN] This Thesis focuses on the study of the response to Terahertz (THz) electromagnetic radiation of different silicon substrate-compatible FETs. Strained-Si MODFETs, state-of- the-art FinFETs and graphene-FETs were studied. The first part of this thesis is devoted to present the results of an experimental and theoretical study of strained-Si MODFETs. These transistors are built by epitaxy of relaxed-SiGe on a conventional Si wafer to permit the fabrication of a strained-Si electron channel to obtain a high-mobility electron gas. Room temperature detection under excitation of 0.15 and 0.3 THz as well as sensitivity to the polarization of incoming radiations were demonstrated. A two-dimensional hydrodynamic-model was developed to conduct TCAD simulations to understand and predict the response of the transistors. Both experimental data and TCAD results were in good agreement demonstrating both the potential of TCAD as a tool for the design of future new THz devices and the excellent performance of strained-Si MODFETs as THz detectors (75 V/W and 0.06 nW/Hz0.5). The second part of the Thesis reports on an experimental study on the THz behavior of modern silicon FinFETs at room temperature. Silicon FinFETs were characterized in the frequency range 0.14-0.44 THz. The results obtained in this study show the potential of these devices as THz detectors in terms of their excellent Responsivity and NEP figures (0.66 kV/W and 0.05 nW/Hz0.5). Finally, a large part of the Thesis is devoted to the fabrication and characterization of Graphene-based FETs. A novel transfer technique and an in-house-developed setup were implemented in the Nanotechnology Clean Room of the USAL and described in detail in this Thesis. The newly developed transfer technique enables to encapsulate a graphene layer between two flakes of h-BN. Raman measurements confirmed the quality of the fabricated graphene heterostructures and, thus, the excellent properties of encapsulated graphene. The asymmetric dual grating gate graphene FET (ADGG-GFET) concept was introduced as an efficient way to improve the graphene response to THz radiation. High quality ADGG-GFETs were fabricated and characterized under THz radiation. DC measurements confirmed the high quality of graphene heterostructures as it was shown on Raman measurements. A clear THz detection was found for both 0.15 THz and 0.3 THz at 4K when the device was voltage biased either using the back or the top gate of the G-FET. Room temperature THz detection was demonstrated at 0.3 THz using the ADGG-GFET. The device shows a Responsivity and NEP around 2.2 mA/W and 0.04 nW/Hz0.5 respectively at respectively at 4K. It was demonstrated the practical use of the studied devices for inspection of hidden objects by using the in-house developed THz imaging system

Electronic and optoelectronic devices using grown 2-dimensional materials

Two-dimensional (2D) materials are a category of layered materials with atomic-level thickness. This thesis focuses on electronic and optoelectronic devices with both vertical and lateral configurations by using grown 2D materials, including graphene, tungsten disulfide (WS2), hexagonal boron nitride (h-BN), and platinum diselenide (PtSe2). Most of the 2D materials used in the project were synthesized by chemical vapor deposition (CVD). Chapter 4 demonstrates an asymmetrical vertical structure of graphene/WS2/h-BN/graphene (GrB/WS2/h-BN/GrT) using all CVD-grown 2D materials. The device arrays were fabricated by vertically stacking graphene electrodes, h-BN continuous film, and WS2 single-crystal domains via DI water/ IPA-based wet transfer techniques. The photovoltaic effect of the asymmetrical vertical structure is around 7 times improved than that of the symmetrical structure without the h-BN layer. By changing the sequence of the h-BN layer in the vertical stack, the electron flow direction can be delicately controlled. Chapter 5 focuses on high-performance electroluminescence (EL) devices with a vertical heterostructure of Gr/h-BN/WS2/h-BN/Gr by using all CVD-grown 2D materials. Long-lived persistent EL is demonstrated for more than 2 hours without significant degradation of the WS2. In the cycling test, the EL signal peak position and the intensity stay almost the same after several ON/OFF cycles under high bias, demonstrating good stability and durability when pulsed. Further investigation shows that the limiting factor for EL devices is not the degradation of the WS2 but the electroburning of the topmost graphene electrode exposed to the air. Chapter 6 explores a photodetector with a lateral structure of PtSe2-WS2-PtSe2, where semimetal multi-layered PtSe2 thin film acts as electrodes and monolayer WS2 acts as the photoactive material. PtSe2 thin film was synthesized by thermally assisted selenization. Direct laser patterning was applied for the device fabrication. As-fabricated devices exhibit a satisfactory ON/OFF ratio and fair photoresponse. A comparison study shows that the device with shorter channel width has better photoresponsivity. The back-to-back Schottky diodes model well estimates the barrier height of PtSe2-WS2 heterojunction. The device exhibits the lowering of barrier height with the increasing laser power, arising from the photogating effect

2D materials based heterostructure for quantum tunneling: a lithography free technique.

Two-dimensional electron gas (2DEG) systems have played a vital role in the development of superior electronic devices including tunnel junctions consisting of two such 2DEG systems. With the advent of the new 2D electronic material systems, it has opened a new route for 2D–2D tunneling in such extended systems. In this study, we have utilized a plasma enhanced chemical vapor deposition (PECVD) technique to directly deposit graphene (nanowalls) and h-BN on Si/SiO2 substrates to construct two-dimensional material based, vertically stacked electron tunneling devices free of expensive and cumbersome microfabrication steps. In the first study, we fabricated direct quantum tunneling devices by depositing atomically thin tunnel barriers of h-BN as the tunneling barrier with equally doped (p-doped under ambient conditions) graphene nanowalls as the active electrode layers (top and bottom) on Si/SiO2 substrates. Current-voltage (I-V) measurements for varying h-BN thicknesses of these single barrier tunneling devices showed linear I-V characteristics at low bias but an exponential dependence at higher bias. Our measurements of the electron tunnel current through the barrier demonstrated that the h-BN films act as a good tunnel barrier. The barrier thickness dependent tunneling current was in good agreement with the tunnelling currents computed using the Bardeen transfer Hamiltonian approach with equally doped top and bottom graphene electrodes. Presence of negative differential resistance (NDR) is characteristic of the current–voltage relationship of a resonant tunneling device, enabling many unique applications. NDR arises at a voltage bias corresponding to aligned band structures of the 2D systems, causing a sharp peak in the tunnelling current. The existence of devices with NDR has been reported since the late 1950\u27s in devices that contained degenerately doped p-n junctions with thin oxide barriers (tunnel diodes) and double barrier heterojunction devices where quantum tunneling effects are utilized. The NDR in the I-V characteristics of these devices has been used in many applications involving microwave/millimeter wave oscillators, high speed logic devices and switches. We investigated NDR phenomenon in our graphene/h-BN systems in two different routes. In the first case, graphene/h-BN/graphene single barrier device, the bottom and top graphene layers were unequally doped. One of the graphene layers was n-type doped using ammonia or hydrazine. Nitrogen doping using ammonia was accomplished during the growth by incorporating ammonia in the PECVD system. Hydrazine doping was accomplished by exposing the graphene to hydrazine vapor in vacuum. The unequal doping of graphene causes alignment of the band structures of graphene systems giving rise to NDR. The tunnelling devices consisting of unequally doped graphene with a single barrier shows resonant quantum tunneling with the presence of a pronounced peak in the current corresponding to NDC whose peak current value and the voltage value depend on the doping levels. The results are explained according to the modified Bardeen tunneling model. Next, resonant tunneling behavior was demonstrated in Graphene/h-BN/Graphene/h-BN/Graphene double barrier (DB) devices by directly depositing graphene and h-BN successive layers on Si/SiO2 substrates using PECVD. DB Tunneling junctions with various barrier widths were investigated (by varying the thickness of the second graphene layer). The I-V parameters of tunneling current at room temperature demonstrated resonant tunneling with negative differential conductance. A quantum mechanical double barrier tunneling model was used to explain the phenomenon, by solving the Schrödinger\u27s equation in either side of the system. A systematic behavior of the current peak values and the corresponding voltage values in I-V curves were seen to be in good agreement with the transmission coefficient calculated using a quantum mechanical model. Josephson tunneling is a different kind of tunneling phenomenon in superconductors, in which superconducting cooper pairs tunnel across a thin insulating barrier. A supercurrent can flow between two superconductors that are separated by a narrow insulating barrier. The current is influenced by the phase difference between the two superconductors. We fabricated Josephson junctions with atomically thin tunnel barriers by combining h-BN with magnesium diboride (MgB2) active electrode layers on a Si/SiO2 substrate using a PECVD (for h-BN) and a Hybrid Physical-Chemical Vapor Deposition (HPCVD) (for Mg ). The I-V characteristics were measured above and below the transition temperature Tc (37 K). A measurable supercurrent was detected below Tc

Fractional Quantum Hall Effect Under Tilted Magnetic Fields in GaAs/AlGaAs Heterostructure Hall Bar Devices, and the Effect of Current Annealing in CVD Graphene Devices

This dissertation presents two distinct research studies. The first study focuses on the fractional quantum Hall effect (FQHE) observed in two-dimensional electron systems subjected to high transverse magnetic fields. Specifically, we investigate the interplay between Zeeman spin splitting and correlation energy in a GaAs/AlGaAs 2D electron system. By tuning the spin energy, we observe transitions in the quantized Hall effect, resulting in changes in both the resistivity minimum (ρxx) and the Hall resistance (Rxy). We also uncover a size dependence in the tilt angle interval for the vanishing of certain resistance minima, with observable shifts in . These findings highlight the competition and crossover between different spin polarized states and distinct FQHE phenomena. The second study focuses on different methods of fabricating graphene devices, including exfoliation techniques such as dry transferring and creating heterostructures, as well as chemical vapor deposition (CVD). The main objective is to investigate the hysteresis effect in graphene devices. Graphene samples prepared using these methods often display p-type characteristics and significant hysteresis under ambient conditions. Current annealing has emerged as a promising in-situ approach for cleaning graphene samples. However, extended periods of current annealing may introduce defects in the underlying substrate. To address this concern, we examine the hysteresis behavior of a graphene Hall bar device before and after current annealing. The graphene sample, grown on copper foils via CVD, undergoes annealing with different current levels. Our experimental methodology involves studying electron/hole transport by cooling the sample from room temperature to 35 K while applying a back-gate bias. By analyzing the hysteresis characteristics, we obtain valuable insights into the impact of current annealing on the electrical properties of graphene. These two studies contribute to our understanding of complex electronic phenomena and the behavior of graphene under different experimental conditions. The findings presented in this dissertation offer valuable insights into the underlying physics and practical considerations for these systems, paving the way for further research and potential technological applications

2D Boron Nitride Heterostructures: Recent Advances and Future Challenges

Hexagonal boron nitride (h‐BN) is one of the most attractive 2D materials because of its remarkable properties. Combining h‐BN with other components (e.g., graphene, carbonitride, semiconductors) to form heterostructures opens new perspectives to developing advanced functional devices. In this review, the state‐of‐the‐art in h‐BN heterojunctions is highlighted. The preparation of high‐quality 2D h‐BN structures with fewer defects can maximize its intrinsic properties, such as thermal conductivity and electrical insulation, which are particularly important in 2D van der Waals electronics. On the other hand, the controlled introduction in 2D h‐BN of multiple defects creates new properties and advanced functions. In this last case, only through a better understanding of the nature and function of defects, it is possible to develop advanced applications based on h‐BN heterostructures. Engineering of the heterojunctions, such as the design of bonding at the interfaces, also plays a primary role. Several applications are proposed for h‐BN heterostructures, mostly in sensing and photocatalysis, and some new perspectives worth further studies are opened. Finally, the current challenges and the rising opportunities for the future developments of next‐generation h‐BN heterostructures are discussed

Nano-Bio Hybrid Electronic Sensors for Chemical Detection and Disease Diagnostics

The need to detect low concentrations of chemical or biological targets is ubiquitous in environmental monitoring and biomedical applications. The goal of this work was to address challenges in this arena by combining nanomaterials grown via scalable techniques with chemical receptors optimized for the detection problem at hand. Advances were made in the CVD growth of graphene, carbon nanotubes and molybdenum disulfide. Field effect transistors using these materials as the channel were fabricated using methods designed to avoid contamination of the nanomaterial surfaces. These devices were used to read out electronic signatures of binding events of molecular targets in both vapor and solution phases. Single-stranded DNA functionalized graphene and carbon nanotubes were shown to be versatile receptors for a wide variety of volatile molecular targets, with characteristic responses that depended on the DNA sequence and the identity of the target molecule, observable down to part-per-billion concentrations. This technology was applied to increasingly difficult detection challenges, culminating in a study of blood plasma samples from patients with ovarian cancer. By working with large arrays of devices and studying the devices\u27 responses to pooled plasma samples and plasma samples from 24 individuals, sufficient data was collected to identify statistically robust patterns that allow samples to be classified as coming from individuals who are healthy or have either benign or malignant ovarian tumors. Solution-phase detection experiments focused on the design of surface linkers and specific receptors for medically relevant molecular targets. A non-covalent linker was used to attach a known glucose receptor to carbon nanotubes and the resulting hybrid was shown to be sensitive to glucose at the low concentrations found in saliva, opening up a potential pathway to glucose monitoring without the need for drawing blood. In separate experiments, molybdenum disulfide transistors were functionalized with a re-engineered variant of a μ-opiod receptor, a cell membrane protein that binds opiods and regulates pain and reward signaling in the body. The resulting devices were shown to bind opiods with affinities that agree with measurements in the native state. This result could enable not only an advanced opiod sensor but moreover could be generalized into a solid-state drug testing platform, allowing the interactions of novel pharmaceuticals and their target proteins to be read out electronically. Such a system could have high throughput due to the quick measurement, scalable device fabrication and high sensitivity of the molybdenum disulfide transistor