Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

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Gate-Variable Optical Transitions in Graphene

Electronic chirality near the Dirac point is a key property of graphene systems, which is revealed by the spectral intensity patterns as measured by angle-resolved photoemission spectroscopy under various polarization conditions. Specifically, the strongly modulated circular patterns for monolayer (bilayer) graphene rotate by ±90° (±45°) in changing from linearly to circularly polarized light; these angles are directly related to the phases of the wave functions and thus visually confirm the Berry's phase of π (2π) around the Dirac point. The details are verified by calculations.

https://absimage.aps.org/image/MAR12/MWS_MAR12-2011-001369.pdf

Visualizing Electronic Chirality and Berry's Phases in Graphene Systems Using Photoemission with Circularly Polarized Light

Post-graphene organic Dirac (PGOD) materials are ordered two-dimensional networks of triply bonded sp
 2 carbon nodes spaced by π-conjugated linkers. PGOD materials are natural chemical extensions of graphene that promise to have an enhanced range of properties and applications. Experimentally realised molecules based on two PGOD nodes exhibit a bi-stable closed-shell/multi-radical character that can be understood through competing Lewis resonance forms. Here, following the same rationale, we predict that similar states should be accessible in PGOD materials, which we confirm using accurate density functional theory calculations. Although for graphene the semimetallic state is always dominant, for PGOD materials this state becomes marginally meta-stable relative to open-shell multi-radical and/or closed-shell states that are stabilised through symmetry breaking, in line with analogous molecular systems. These latter states are semiconducting, increasing the potential use of PGOD materials as highly tuneable platforms for future organic nano-electronics and spintronics. The lack of band gap controllability in graphene severely restricts its use in nanoelectronics. Here, the authors predict that post-graphene organic Dirac materials should allow for exceptional electronic tunability between graphene-like semimetallicity and multi-radical and/or closed-shell semiconducting states.

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Existence of multi-radical and closed-shell semiconducting states in post-graphene organic Dirac materials.

This review examines the advantages of two-dimensional (2D) and thin film materials in the development of chemical sensor systems. More specifically, this paper focuses on the use of graphene, transition metal dichalcogenides (TMDs), and thin film metal-oxide semiconductors (MOX) in gasand liquid-phase chemical sensing applications. Key features in terms of material properties, device characteristics, as well as scalability for system development are examined. Key challenges associated with various sensing approaches (e.g. optical, electrochemical, FET/chemiresistive) are presented along with recent advances. Lastly, common methods for preprocessing and pattern recognition are summarized while highlighting the development of olfaction-inspired sensor systems to motivate the use of machine learning for data analysis. TOPICAL REVIEW 2020

https://iopscience.iop.org/article/10.1088/2053-1583/ab6e88/pdf

Chemical sensor systems based on 2D and thin film materials

Graphene, the newest member of the carbon’s family, has proven its efficiency in improving polymers’ resistance against photodegradation, even at low loadings equal to 1 wt% or lower. This protective role involves a multitude of complementary mechanisms associated with graphene’s unique geometry and chemistry. In this review, these mechanisms, taking place during both the initiation and propagation steps of photodegradation, are discussed concerning graphene and graphene derivatives, i.e., graphene oxide (GO) and reduced graphene oxide (rGO). In particular, graphene displays important UV absorption, free radical scavenging, and quenching capabilities thanks to the abundant π-bonds and sp2 carbon sites in its hexagonal lattice structure. The free radical scavenging effect is also partially linked with functional hydroxyl groups on the surface. However, the sp2 sites remain the predominant player, which makes graphene’s antioxidant effect potentially stronger than rGO and GO. Besides, UV screening and oxygen barriers are active protective mechanisms attributed to graphene’s high surface area and 2D geometry. Moreover, the way that graphene, as a nucleating agent, can improve the photostability of polymers, have been explored as well. These include the potential effect of graphene on increasing polymer’s glass transition temperature and crystallinity.

https://www.mdpi.com/2073-4352/11/1/3/pdf

A review on graphene’s light stabilizing effects for reduced photodegradation of polymers

Graphene emerged in 2004 as the first 2-D material with exotic properties. Since then the literature has been flooded with reports, with physicists, material scientists, and engineers grabbing their respective shares. Numerous reviews have also been published. While these reviews have done excellent works in their own ways, new reports are coming up faster than they could draw the attentions of researchers. The authors, therefore, feel that there is a demanding scope for a fresh review. Further, many aspects of graphene are not covered in the reviews so far. New concept devices are also entering into the arena of graphene day by day. The purpose of this paper is therefore to present a comprehensive review on the conventional as well as novel device applications of graphene. While we believe that graphene is the material which will transform the electron devices from the classical regime to the quantum world, it is difficult to believe that it will be a complete substitute to silicon in the near future.

Graphene for Electron Devices: The Panorama of a Decade

Graphene-based field-effect transistors (FETs)have received widespread attention as an alternative for conventional silicon devices. The purpose of this paper is to present a new simulation scheme for the I-V characteristics of graphene based FETs. Using the same, we studied the channel length and width dependence behaviour of the transfer characteristics of graphene based FETs (GFETs)with various channel lengths from 0.75 μm to 1.25 μm and widths from 1.75 μm to 2.25 μm, From our study, we observed that, the Dirac point is independent of the channel length and width for the range of values considered. However, the magnitude of the current progressively increases with increase in width of the channel.

Effect of Channel Dimensions on Transfer Characteristics of Graphene FET

Graphene based devices are impelling the innovations for the final implementation of flexible electronic devices within new perspectives of the future technological developments. Specially, GNRFET has placed in a superior position due to the existence of finite bandgap and the ability to tune the same by a simple way (of varying the width). Therefore, we have explored the properties of GNRFET by using some analytic approach with a view to assess its efficacy as a Field Effect Transistor (FET). Reduction of a 2-dimensional (2D) Poisson equation to a one-dimensional (1D) form in an effective manner is the main of focus of this method. The approach was used to study the influence of geometrical and electrical parameters on I-V characteristics of the GNRFET. The results of this work pave the way for the possibilities for device engineers to characterize GNRFETs based on the requirements of a specific device application. The resulting convergence of the inflection point is an interesting feature of our study.The existence of a critical voltage, beyond which the current rises in its value with temperature, is an pre-eminent feature of our GNRFET model.

Study of the influence of parameters to explore the Properties of Graphene Nanoribbon Field Effect Transistors

We have done a simulation study on the quantum capacitance and sheet carrier density of graphene MOSFETs following a quasianalytical model for graphene MOSFET with gapless large area graphene channel (Thiele et al. in JAP 107:094505, 2010 [1]). First, the simulation model has been validated by properly reproducing the values of quantum capacitance for a graphene MOSFET without back-gate and zero applied drain-source voltage. Next, we have investigated the effect of both top-gate and back-gate voltages on the quantum capacitance and sheet carrier density and the results are presented here.

Calculation of Quantum Capacitance and Sheet Carrier Density of Graphene FETs

The quality of contacts between 2D material and metal significantly affects the performance of devices based on 2D material. Therefore, we present an article that investigates the contact and channel resistance of graphene field-effect transistors. It is interesting to observe that channel and contact resistance efficiently affects the total resistance of graphene-based devices. In addition to the total resistance of the device, the channel and contact resistance are effectively influenced by applied bias. The source-drain resistance, the contact resistance inherently depends on the resistance of the graphene channel. We present a useful model for determining the channel resistance of a device. Our model explains, how the Gaussian peak values are same for the same contact resistances when RS <1000, which is a very characteristic study result. In addition, our model shows a multiplicity effect at high source resistance and low top gate voltage.

Sriyanka Behera

Papers

Graphene for Electron Devices: The Panorama of a Decade

Effect of Channel Dimensions on Transfer Characteristics of Graphene FET

Study of the influence of parameters to explore the Properties of Graphene Nanoribbon Field Effect Transistors

Calculation of Quantum Capacitance and Sheet Carrier Density of Graphene FETs

Exploring the Influence of Channel and Contact Resistance Variability in Graphene Field Effect Transistor