Добірка наукової літератури з теми "Metal-graphene Junction"

Reported here is a new method of fabricating the graphene/silicon schottky junction. Using a femtosecond laser, graphene oxides are reduced to graphene and behave a metal. The junction of reduced GO and Si shows rectifying behavior indicating that the junction is schottky junction. Take advantage of the laser fabricating method, one can get reduced GO at any position on the substrate. Xps spectra shows that the reduced GO has only 12% oxygen content, and it is truly have a good conductivity similar to metal. This method opens a new effective way to graphene-based micro nano electronics.

Thermoelectric readout in a graphene terahertz (THz) radiation detector requires a p- n junction across the graphene channel. Even without an intentional p- n junction, two latent junctions can exist in the vicinity of the electrodes/antennas through the proximity to the metal. In a symmetrical structure, these junctions are connected back-to-back and therefore counterbalance each other with regard to rectification of the ac signal. Because of the Peltier effect, a small dc current results in additional heating in one and cooling in another p- n junction, thereby breaking the symmetry. The p- n junctions then no longer cancel, resulting in a greatly enhanced rectified signal. This allows simplifying the design and controlling the sensitivity of THz radiation detectors.

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

A fully metal-free molecular junction by using carbon fiber as the top electrode, and graphene as the bottom electrode was demonstrated for single molecular junctions, which is retaining long-lived charge excited states and potential for charge storage and manipulation.

In this research work we have modeled, simulated and compared the electronic charge transport for Metal-Semiconductor-Metal interfaces of Graphene Nano Ribbons (GNR) with different geometries using First-Principle calculations and Non-Equilibrium Green's Function (NEGF) method. We modeled junctions of Armchair GNR strip sandwiched between two Zigzag strips with (Z-A-Z) and Zigzag GNR strip sandwiched between two Armchair strips with (A-Z-A) using semi-empirical Extended Huckle Theory (EHT) within the framework of Non-Equilibrium Green Function (NEGF). I-V characteristics of the interfaces were visualized for various transport parameters. The distinct changes in conductance and I-V curves reported as the Width across layers, Channel length (Central part) was varied at different bias voltages from -1V to 1 V with steps of 0.25 V. From the simulated results we observed that the conductance through A-Z-A graphene junction is in the range of 10-13 Siemens whereas the conductance through Z-A-Z graphene junction is in the range of 10-5 Siemens. These suggested conductance controlled mechanisms for the charge transport in the graphene interfaces with different geometries is important for the design of graphene based nano scale electronic devices like Graphene FETs, Sensors.

Graphene-metal contact is crucial to fabricate high-performance graphene photodetectors since the external quantum efficiency (EQE) of the photodetector depends on the contact properties, and the influence of the contact properties is particularly dominant in short channel devices for high-speed applications. Moreover, junction properties between the channel graphene and graphene near the contact are also important to analyze the photoresponse because the built-in electric field in the junction determines the EQE of the photodetector. In this study, we investigated a relation between the photoresponse and the built-in electric field induced from the doping level difference in the junction between the channel graphene and graphene near the contact. The photoresponse could be enhanced with a high junction barrier height that is tuned by the doping level difference. In addition, we observed that the improved electrical characteristics of channel graphene do not guarantee the enhancement of the photoresponse characteristics of graphene photodetectors.

In recent years, the characteristics of the graphene/crystalline silicon junction have been frequently discussed in the literature, but study of the graphene/polycrystalline silicon junction and its potential applications is hardly found. The present work reports the observation of the electrical and optoelectronic characteristics of a graphene/polycrystalline silicon junction and explores one possible usage of the junction. The current–voltage curve of the junction was measured to show the typical exponential behavior that can be seen in a forward biased diode, and the photovoltage of the junction showed a logarithmic dependence on light intensity. A new phototransistor named the “photodiode–oxide–semiconductor field effect transistor (PDOSFET)” was further proposed and verified in this work. In the PDOSFET, a graphene/polycrystalline silicon photodiode was directly merged on top of the gate oxide of a conventional metal–oxide–semiconductor field effect transistor (MOSFET). The magnitude of the channel current of this phototransistor showed a logarithmic dependence on the illumination level. It is shown in this work that the PDOSFET facilitates a better pixel design in a complementary metal–oxide–semiconductor (CMOS) image sensor, especially beneficial for high dynamic range (HDR) image detection.

Cette thèse présente une étude des propriétés de transport à basses températures de matériaux hybrides composés de nano-clusters de métaux supraconducteurs (Sn et Pb) auto-assemblés à la surface d'une feuille de graphène. L'auto-assemblage du métal réalise un réseau bi-dimensionnel désordonné de jonctions Josephson. La caractérisation des propriétés supraconductrices révèle une transition de type 'BKT' avec une température de transition dépendant de la morphologie de la surface. Les propriétés supraconductrices de ce système sont fortement influencées par la grille arrière, qui contrôle la résistance dans l'état normal du graphène. Le résultat le plus marquant de cette thèse a été obtenu en utilisant du graphène désordonné. La présence de défauts structuraux dans la maille de graphène induit un régime de localisation forte à basses températures. En faisant varier le voltage de grille, la résistance de tels échantillons peut varier de 3 ordres de grandeurs. Cette grande dynamique a été mise à contribution pour la réalisation d'une transition de phase supraconducteur-isolant dans des échantillons décorés à l'étain. L'étude de cette transition de phase quantique révèle un comportement de type percolatif et une résistivité universelle prédite par la théorie à la transition. Enfin, un travail préliminaire visant à réaliser des résonateurs mécaniques supraconducteurs à l'aide des ces matériaux hybrides est également présenté.

Condensed matter physics provides us with an opportunity to explore a large variety of systems with diverse properties. Central to the understanding of these systems is a characterization of the nature of their ground states and low energy excitation. Often, such systems show various forms of emergent properties that are absent in the microscopic level. Identification of such emergent phases of condensed matter form an important avenue of research in the field. In this thesis example of such phases and their associated phase transitions have been studied. The work presented here may be broadly divided into two themes: construction of the theoretical framework for understanding materials already studied experimentally, and, trying to provide new theoretical avenues which may be relevant for understanding future experiments. In these studies we shall explore some unconventional phases and phase transitions that may occur in condensed matter systems. A comprehensive understanding of the properties of such unconventional phases and phase transitions is important in the context of the large array of experimentally studied materials that regularly defy conventional wisdom in more than one way. The thesis consists of two distinct parts. In the first part we study three problems in frustrated magnets. The second part consists of studies of the tunnelling spectroscopy of metal-insulator-superconductor junctions in graphene. Studies in frustrated magnets have opened up the possibility of existence of a whole range of phases beyond the already known magnetically ordered ones. Some of these new phases, like the spin nematic or the valence bond solid, display some other conventional order themselves. Others, like the much sort after spin liquid phases displays a whole new kind of order that cannot be captured through the celebrated Landau’s classification of phases on the basis of symmetry breaking and associated order parameters. The phase transitions in these systems are also equally interesting and lead to intriguing possibilities that demand new modes of analysis. In this part of the thesis we shall study the different properties of three magnets with spin-1/2, 1 and 3/2 respectively. We start by providing an introduction to frustrated spin systems in Chapter [1]. The origin of antiferromagnetic interactions in Mott insulators is discussed and the concept of frustration of magnetic interaction is explained. We also point out the causes that may destroy magnetic order in spin systems, particularly the role of quantum fluctuations in presence or absence of magnetic frustration. This is followed with a brief outline of various magnetically ordered and disordered ground states with particular emphasis on the description of the later. We also give a brief outline of various properties of such phases and associated quantum phase transitions particularly noting the influences of quantum interferences encoded in the Berry phase terms. A brief description of the finite temperature properties is also provided. We end an outline of various experimentally relevant compounds that requires comprehensive understanding, some of which have been addressed in this thesis. In Chapter [2] we study the properties of a spin-nematic state in context of the recently discovered spin-1 Mott insulator Nickel Gallium Sulphide (NiGa2S4). This isotropic triangular lattice compound shows no spin ordering till low temperatures. We propose that it may have a particular type of spin-nematic ground state and explain the experimentally observed properties of the compound on the basis of our proposal. Starting from a two band Hubbard model description, relevant for the compound, we derive the Bilinear Biquadratic spin Hamiltonian. We then show, within mean field theory, that this Hamiltonian describes a transition from the spiral state to a ferro-nematic state as a function of the ratio of bilinear and biquadratic couplings. We also study the possible effects of small pinning disorder andmagnetic field and suggest experiments that can possibly distinguish the proposed nematic state from others. In Chapter [3] we explore the effects of the magneto-elastic coupling in the spin-3/2 B-site chromite spinel Cadmium Chromite (CdCr2O4). In this compound the spins form a pyrochlore lattice. Nearest neighbour spins interact antiferromagnetically. Due to frustration the system does not order at low temperatures and instead goes into a classical spin liquid state. Such a cooperative paramagnet is very susceptible to external perturbations which may relieve their frustration. In CdCr2O4, at lower temperatures the magnetic frustration is relieved by distorting the lattice through a first order magnetoelastic transition. Thus the compound presents a case where the relevant perturbation to the frustrated spin interactions is provided by spin-phonon coupling. An effect of such perturbations on a cooperative paramagnet is of general interest and all aspects of this are not understood presently. We take the initial step of characterizing the spin-phonon interaction in detail. Based on recent sound velocity experiments, we construct a microscopic theory for the sound velocity renormalization due to the spin-phonon coupling and explain the recent experimental data obtained by S. Zherlitsyn et al. using our theory we can explain the dependence of the sound velocity on temperature as well as magnetic field. We also construct a Landau theory to explain (qualitatively) the behaviour of sound velocity across the magneto-structural transition. Further, we discuss the effects due to the small Dzyaloshinskii-Moriya interaction that may be present in these compounds. In Chapter [4] we study the possibility of a direct second order quantum phase transition from spiral to dimer phase in two dimensional antiferromagnets. Such transitions between phases with incompatible symmetries are forbidden within conventional Landau Ginzburg-Wilson paradigm of critical phenomena. Early works showed that when the spiral is destroyed by long wavelength fluctuations a fractionalized Z2 spin liquid is obtained. In this work we show an alternative way–the quantum destruction of the spiral magnet. We argue that, when the defects of the spiral phase proliferate and condense, their associated Berry phase automatically leads to dimerization. We apply our theory to study concrete lattice models where such transitions may be observed. This transition is an example of a Landau forbidden deconfined quantum phase transition. The proposed critical theory is naturally written in terms of fractional degrees of freedom which emerge right at the critical point. These fractional particles interact with each other through emergent gauge fields and are deconfined right at the critical point (but are confined in either of the two adjoining phases). We argue, based on existing results, that the monopoles of the gauge field are dangerously irrelevant right at the critical point rendering the later noncompact. The critical point is characterized by an emergent global U (1) conservation law that is absent in the microscopic model, a typical feature of a deconfined quantum critical point. The resultant field theory belongs to the class of anisotropic NCCP3 class which may be studied numerically in future to understand its critical properties. In modern condensed matter physics the emergence of new and novel phases of matter have often been associated with the presence of strong correlations. Indeed, strongly correlated systems seem to harbour in them the potential to realize some of the most unconventional and exotic emergent phases of matter. However in graphene, which is a single layer of graphite, the emergence of novel properties, as present experiments suggest, is due to its unique band structure and not a fallout of intricate correlation effects. Band structure studies of graphene suggest that the material is a zero gap semiconductor with the low energy excitations resembling massless Dirac quasi-particles. The consequence of this is immediate and interesting. It has lead to the possibility of exploring the physics of relativistic fermions in two spatial dimensions and much of this has been studied with great vigour in the last five years. In our studies, presented in Chapter [5], we explore one of the many consequence of this emergent Dirac structure of the low energy quasi-particles, namely the properties of metal-insulator-superconductor junctions of graphene. The twin effect of Klein tunneling of Dirac fermions (and associated transmission resonances) and Andreev reflection (both specular and retro) sets them aside from their conventional counterparts. The graphene normal metal-insulator-superconductor (NIS) junctions show strikingly different properties like oscillations in the sub-gap tunneling conductance as a function of both barrier strength and width. We make a detailed study of this for arbitrary barrier strengths and widths with and without Fermi-surface mismatch between the normal and the superconducting sides. The amplitude of these oscillations are maximum for aligned Fermi surface and vanishes for large Fermi surface mismatch. We provide an understanding for this unconventional behaviour of graphene NIS junctions. We also suggest experimental tests for our theory. Such experimental verification will reveal one more remarkable emergent property in a condensed matter system.

2D materials like transition metal dichalcogenides, black phosphorous, silicene, graphene are at the forefront of being the most potent 2D materials for optoelectronic applications because of their exceptional properties. Several application-specific photodetectors based on 2D materials have been designed and manufactured due to a wide range and layer-dependent bandgaps. Different 2D materials stacked together give rise to many surprising electronic and optoelectronic phenomena of the junctions based on 2D materials. This has resulted in a lot of popularity of 2D heterostructures as compared to the original 2D materials. This chapter presents the progress of optoelectronic devices (photodetectors) based on 2D materials and their heterostructures.

Abstract In this study, first-principles calculations based on density functional theory (DFT) were performed to analyze in detail the electronic transport properties of dumbbell-shape graphene nanoribbons (DS-GNRs) and their strain dependence as a strong function of the combination of the basic structure of GNRs in the narrow segment and the structures at both ends of DS-GNRs. Then, the current-voltage characteristics (I-V characteristics) and orbital distributions of DS-GNRs were investigated to develop a highly sensitive DS-GNR-based strain sensor. By combining two GNRs with metallic and semiconducting electronic properties, a non-negligible transition layer (gradient Schottky barrier) was formed near the junction. The length of the transition layer was about five six-membered rings of carbon atoms. The formation of this transition layer is considered to be due to the exudation of the wave function from the wide segment to the narrow segment in DS-GNR. In the DS-GNR with metal-semiconductor interfaces, the strain dependence of the electronic transport properties was very complicated due to the presence of the transient Schottky barrier. On the other hand, in a DS-GNR consisting of two metallic GNRs, the Schottky barrier and the transition layer disappeared, and stable current-voltage and piezoresistive characteristics close to those of a single GNR were observed. The predicted gauge factor of DS-GNRs was larger than that of conventional metal foils (gauge factor 2–5) and close to that of conventional polysilicon (gauge factor ±30). These results indicate that DS-GNRs have the potential to produce highly sensitive and reliable strain sensors.

Carbon foam is recognized as having the greatest potential to replacement for metal fins in thermal management systems such as heat exchangers, space radiators, and thermal protection systems [1–5]. Carbon foam refers to a broad class of materials that include reticulated glassy, carbon and graphitic foams that are generally open-cell or mostly open-cell. They can be tailored to have low or high thermal conductivity with a low coefficient of thermal expansion and density. These foams have high modulus but low compression and tensile strength. Among the carbon foams, the graphitic foam offers superior thermal management properties such as high thermal conductivity. Graphitic foams are made of a network of spheroidal shell segments. Each cell has thin, stretched ligaments in the walls that are joined at the nodes or junctions. The parallel arrangement of graphene planes in the ligaments confers highly anisotropic properties to the walls of the graphitic foams. The graphene planes tend to be oriented with the plane of the ligaments but become disrupted at the junctions (nodes) of the walls. Since conduction is highest along parallel graphene planes, the thermal conductivity is highest in the plane of the ligaments or struts, and much lower in the direction transverse to the plane of these ligaments. In a previous study [6] extensive mechanical and thermal property characterization of carbon foams from Kopper Inc. (L1) and POCO Graphite, Inc. (P1) were reported. These foams were graphitic ones that are expected to have high thermal conductivity. Figure 1 shows sections of light microscopy images of the three foams of four foams. The most important thing to notice is that the images were not at the same magnification. The large cells in the GrafTech foam have an average diameter of only ∼100 μm but have a bimodal distribution cells with many small closed-cells few micrometers in diameter. Changes in density in the GrafTech foam was accompanied by a change in the large cells’ diameter — larger diameter giving greater porosity and lower density without changing the smaller cells’ sizes that filled the solid phase between the larger bubbles. The POCO foam has a fairly uniform size cell distribution of a few hundred micrometers. The Koppers’ foams show larger cells yet with the left (“L” precursor) having a uniform size while the right-hand (“D” precursor) is a less uniform and lower porosity.