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Transition metal dichalcogenides (TMDCs) are a promising class of two‐dimensional (2D) materials for flexible electronic applications due to their low integration temperature, good electronic properties, and excellent mechanical flexibility. Moreover, TMDCs offer the possibility of co‐integrating both n‐ and p‐type transistors on the same substrate, enabling the realization of complementary metal‐oxide‐semiconductor (CMOS) circuits. In this study, n‐type MoS2 field‐effect transistors (FETs), and p‐type WSe2‐FETs integrated on a flexible foil substrate fabricated by standard thin‐film technology are presented. These devices exhibit high stability in their electronic operation under strain and repeated bending cycles. A CMOS inverter based on these transistors is also successfully demonstrated, which shows excellent switching behaviour with high gain (up to 100), high noise margin (0.87 · VDD), and low average static power consumption (40 pW). [ABSTRACT FROM AUTHOR]

The feasibility of introducing optical nonlinearity in poly-dimethyl siloxane (PDMS) using organic solvent swelling was investigated. The third-order nonlinear refraction and absorption properties of the individual materials, as well as the PDMS/solvent compounds after swelling were characterized. The well-established Z-scan technique served as characterization method for the nonlinear properties under picosecond pulsed laser excitation at a 532 nm wavelength. These experiments included investigations on the organic solvents nitrobenzene, 2,6-lutidine, and toluene, which showed inherent optical nonlinearity. We showed that nitrobenzene, one of the most well-known nonlinear optical materials, has proven suboptimal in this context due to its limited swelling effect in PDMS and comparatively high (non)linear absorption, resulting in undesirable thermal effects and potential photo-induced damage in the composite material. Toluene and 2,6-lutidine not only exhibited lower absorption compared to nitrobenzene but also show a more pronounced swelling effect in PDMS. The incorporation of toluene caused a weight change of up to 116% of PDMS, resulting in substantial nonlinear optical effects, reflected in the nonlinear refractive index of the PDMS/toluene composite n 2 = 3.1 × 10 − 15   c m 2 / W . [ABSTRACT FROM AUTHOR]

Semiconducting transition metal dichalcogenides (TMDC) are 2D materials, combining good charge carrier mobility, ultimate dimension down‐scalability, and low‐temperature integration. These properties make TMDCs interesting for flexible electronics, where the thermal fabrication budget is strongly substrate limited. In this perspective, an overview of the state of TMDC research is provided by evaluating two scenarios, both with their own merit depending on the target application. First, high‐quality chemically grown 2D TMDCs are promising for nanoscale high‐performance and high‐frequency devices with excellent gate control and high current on/off ratios. Second, TMDC thin films can also be solution deposited from chemically exfoliated flakes allowing for moderate performance, but providing a path toward low‐cost production. A strong advantage of TMDCs is the possibility to realize p‐type and n‐type channels for complementary transistors having similar performance figures‐of‐merit. This aspect, as well as common transistor performance metrics are also compared with other flexible channel materials providing an overview of the state of the art of thin‐film transistors in the field of flexible electronics. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS2) has great potential as a two‐dimensional semiconductor for electronic and optoelectronic application, but its high sensitivity to environmental adsorbents and charge transfer from neighboring dielectrics can lead to device variability and instability. Aluminum oxide (Al2O3) is widely used as an encapsulation layer in (opto)‐electronics, but it leads to detrimental charge transfer n‐doping to MoS2. Here, this work reports a scalable encapsulation approach for MoS2 field‐effect transistors (FETs) where hexagonal boron nitride (h‐BN) monolayers are employed as a barrier layer in‐between each of the Al2O3 and MoS2 interfaces. These devices exhibit a significant reduction of charge transfer, when compared to structures without h‐BN. This benefit of h‐BN in the gate stack is confirmed by ab initio density functional theory calculations. In addition, the devices with h‐BN layers show very low hysteresis even under ambient operating conditions. [ABSTRACT FROM AUTHOR]

PtSe2 is one of the most promising materials for the next generation of piezoresistive sensors. However, the large‐scale synthesis of homogeneous thin films with reproducible electromechanical properties is challenging due to polycrystallinity. It is shown that stacking phases other than the 1T phase become thermodynamically available at elevated temperatures that are common during synthesis. It is shown that these phases can make up a significant fraction in a polycrystalline thin film and discuss methods to characterize them, including their Seebeck coefficients. Lastly, their gauge factors, which vary strongly and heavily impact the performance of a nanoelectromechanical device are estimated. [ABSTRACT FROM AUTHOR]

The deposition of dielectric materials on graphene is one of the bottlenecks for unlocking the potential of graphene in electronic applications. The plasma enhanced atomic layer deposition of 10 nm thin high quality aluminum oxide (Al2O3) on graphene is demonstrated using a monolayer of hexagonal boron nitride (hBN) as protection layer. Raman spectroscopy is performed to analyze possible structural changes of the graphene lattice caused by the plasma deposition. The results show that a monolayer of hBN in combination with an optimized deposition process can effectively protect graphene from damage, while significant damage is observed without an hBN layer. Electrical characterization of double gated graphene field effect devices confirms that the graphene does not degrade during the plasma deposition of Al2O3. The leakage current densities are consistently below 1 pA µm−2 for electric fields across the insulators of up to 8 MV cm−1, with irreversible breakdown happening above. Such breakdown electric fields are typical for Al2O3 and can be seen as an indicator for high quality dielectric films. [ABSTRACT FROM AUTHOR]

Platinum diselenide (PtSe2) is a 2D material with outstanding electronic and piezoresistive properties. The material can be grown at low temperatures in a scalable manner, which makes it extremely appealing for many potential electronics, photonics, and sensing applications. Here, the nanocrystalline structure of different PtSe2 thin films grown by thermally assisted conversion (TAC) is investigated and is correlated with their electronic and piezoresistive properties. Scanning transmission electron microscopy for structural analysis, X‐ray photoelectron spectroscopy (XPS) for chemical analysis, and Raman spectroscopy for phase identification are used. Electronic devices are fabricated using transferred PtSe2 films for electrical characterization and piezoresistive gauge factor measurements. The variations of crystallite size and their orientations are found to have a strong correlation with the electronic and piezoresistive properties of the films, especially the sheet resistivity and the effective charge carrier mobility. The findings may pave the way for tuning and optimizing the properties of TAC‐grown PtSe2 toward numerous applications. [ABSTRACT FROM AUTHOR]

Diodes made of heterostructures of the 2D material graphene and conventional 3D materials are reviewed in this manuscript. Several applications in high frequency electronics and optoelectronics are highlighted. In particular, advantages of metal–insulator–graphene (MIG) diodes over conventional metal–insulator–metal diodes are discussed with respect to relevant figures‐of‐merit. The MIG concept is extended to 1D diodes. Several experimentally implemented radio frequency circuit applications with MIG diodes as active elements are presented. Furthermore, graphene‐silicon Schottky diodes as well as MIG diodes are reviewed in terms of their potential for photodetection. Here, graphene‐based diodes have the potential to outperform conventional photodetectors in several key figures‐of‐merit, such as overall responsivity or dark current levels. Obviously, advantages in some areas may come at the cost of disadvantages in others, so that 2D/3D diodes need to be tailored in application‐specific ways. [ABSTRACT FROM AUTHOR]

This article presents a fully integrated radio frequency (RF) rectifier on a flexible polyimide (PI) substrate. This circuit is designed and implemented using a chemical-vapor-deposition (CVD) graphene monolithic microwave integrated circuit (MMIC) process, where a metal–insulator–graphene (MIG) diode is employed to obtain the necessary nonlinearity for the rectifier. The smooth surface enables good titanium dioxide (Tio2) barrier interface formation and a higher carrier transport performance of the graphene. Passive components are also developed on the same substrate. To carry out the circuit design, a large-signal diode model is created based on measured results on a test key. A rectifier is deliberately designed for eliminating the input matching network by properly sizing the MIG diode and the load impedance. Removing the matching network results in a compact size and reduces the loss of the input of the rectifier. Nevertheless, the measured input reflection coefficient at 2.4 GHz is −17.5 dB, and measured, peak output dc voltage is 93 mV at an input power of 15.7 dBm. To the best of the authors’ knowledge, this design achieves the highest output voltage of all graphene-based rectifiers on a flexible substrate at 2.4 GHz. Thanks to the CVD graphene MMIC process, a rectifier array could be realized and thus, low-energy devices such as wireless-sensor-network (WSN) and medical implants are potential applications. [ABSTRACT FROM AUTHOR]

A gate‐defined quantum dot in bilayer graphene is utilized as a sensitive probe for the charge density of its environment. Under the influence of a perpendicular magnetic field, the charge carrier density of the channel region next to the quantum dot oscillates due to the formation of Landau levels. This is experimentally observed as oscillations in the gate‐voltage positions of the Coulomb resonances of the nearby quantum dot. From the frequency of the oscillations, the charge carrier density in the channel is extracted, and from the amplitude the shift of the quantum dot potential. These experimental results are compared with an electrostatic simulation of the device and good agreement is found. [ABSTRACT FROM AUTHOR]

It has been shown that there can be a significant temperature increase in graphene field-effect transistors (GFETs) operating under high drain bias, which is required for power gain. However, the possible effects of self-heating on the high-frequency performance of GFETs have been weakly addressed so far. In this article, we report on an experimental and theoretical study of the effects of self-heating on dc and high-frequency performance of GFETs by introducing a method that allows accurate evaluation of the effective channel temperature of GFETs with a submicrometer gate length. In the method, theoretical expressions for the transit frequency (ƒT) and the maximum frequency of oscillation (ƒmax) based on the small-signal equivalent circuit parameters are used in combination with the models of the field- and temperature-dependent charge carrier concentration, velocity, and saturation velocity of GFETs. The thermal resistances found by our method are in good agreement with those obtained by the solution of the Laplace equation and by the method of thermo-sensitive electrical parameters. Our experiments and modeling indicate that the self-heating can significantly degrade the ƒT and ƒmax of GFETs at power densities above 1 mW/μm2, from approximately 25 to 20 GHz. This article provides valuable insights for further development of GFETs, taking into account the self-heating effects on the high-frequency performance. [ABSTRACT FROM AUTHOR]

We present a circuit-design compatible large-signal compact model of metal–insulator–graphene (MIG) diodes for describing its dynamic response for the first time. The model essentially consists of a voltage-dependent diode intrinsic capacitance coupled with a static voltage-dependent current source, and the latter accounts for the vertical electron transport from/toward graphene, which has been modeled by means of the Dirac-thermionic electron transport theory through the insulator barrier. Importantly, the image force effect has been found to play a key role in determining the barrier height, so it has been incorporated into the model accordingly. The resulting model has been implemented in Verilog-A to be used in the existing circuit simulators and benchmarked against an experimental 6-nm TiO2 barrier MIG diode working as a power detector. [ABSTRACT FROM AUTHOR]

In this letter, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency (${f}_{T}$) and maximum frequency of oscillation (${f}_{\text {max}}$) showed improved scaling behavior with respect to the gate length (${L}_{g}$). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from $0.5~\mu \text{m}$ to $\text {2}~\mu \text{m}$ have been characterized, and extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ frequencies of up to 34 and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ values of approximately 100 GHz at ${L}_{g}=\text {50}$ nm. Further optimization of the GFET technology enables ${f}_{\text {max}}$ values above 100 GHz, which is suitable for many millimeter wave applications. [ABSTRACT FROM AUTHOR]

We report on the fabrication and characterization of field-effect transistors (FETs) based on chemical vapor deposited (CVD) graphene encapsulated between few layer CVD boron nitride (BN) sheets with complementary metal–oxide–semiconductor (CMOS) compatible nickel edge contacts. Noncontact terahertz time-domain spectroscopy (THz-TDS) of large-area BN/graphene/ BN (BN/G/BN) stacks reveals average sheet conductivity >1 mS/sq. and average mobility of 2500 cm2/V $\cdot $ s. Improved output conductance is observed in dc measurements under ambient conditions, indicating the potential for radio frequency (RF) applications. Moreover, we report a maximum voltage gain of 6 dB from a low-frequency signal amplifier circuit. RF characterization of the GFETs yields an ${f}_{T}\times {L}_{g}$ product of 2.64 GHz $\cdot ~\mu \text{m}$ and an ${f}_{{\textsf {Max}}}\times {L}_{g}$ product of 5.88 GHz $\cdot ~\mu \text{m}$. This paper presents for the first time THz-TDS usage in combination with other characterization methods for device performance assessment on BN/G/BN stacks. The results serve as a step toward scalable, all CVD 2-D material-based FETs for CMOS compatible future nanoelectronic circuit architectures. [ABSTRACT FROM AUTHOR]

In this letter, we present for the first time a mixer circuit based on Metal–Insulator–Graphene (MIG) diodes fabricated with large-scale monolayer graphene grown by chemical vapor deposition. A small-signal model extracted from the diode physical structure is used together with a large-signal model extracted from the dc characteristics of the MIG diode to build a down-conversion mixer. The measured conversion loss at a local oscillator power ( ${P}_{\text {LO}}$ ) of 5 dBm is lower than 15 dB, while RF-to-IF isolation is 36 dB with an input return loss and RF-to-LO isolation better than 10 dB over the frequency band from 1.7–6 GHz. Promising mixer results in combination with the CVD-based process promote the MIG diode-based mixer to be used in low-power, low-cost, microwave, and millimeter-wave circuit applications. [ABSTRACT FROM AUTHOR]

The exploitation of the excellent intrinsic electronic properties of graphene for device applications is hampered by a large contact resistance between the metal and graphene. The formation of edge contacts rather than top contacts is one of the most promising solutions for realizing low ohmic contacts. In this paper the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e. Nickel and Aluminum is reported. Extraction of the contact resistance by Transfer Line Method (TLM) as well as the direct measurement using Kelvin Probe Force Microscopy demonstrates a very low width specific contact resistance down to 130 Ωμm. The contact resistance is found to be stable for annealing temperatures up to 150°C enabling further device processing. Using this contact scheme for edge contacts, a field effect transistor based on CVD graphene with a high transconductance of 0.63 mS/μm at 1 V bias voltage is fabricated. [ABSTRACT FROM AUTHOR]

Here in this work, we study the electro-refractive modulation of CVD grown single layer graphene placed on top of a silicon microring resonator biased using a polymer electrolyte gate. A voltage length product for a phase shift of π, VπL = 2.7 Vmm has been extracted, which is better compared to silicon depletion horizontal and interleaved pn junction type phase modulators and competitive with Silicon-Insulator-Silicon Capacitor modulators. [ABSTRACT FROM PUBLISHER]

The encapsulation of graphene-based Hall sensors on foil is shown to be an effective method for improving the performance in terms of higher sensitivity for magnetic field detection. Two types of encapsulation were investigated: a simple encapsulation of graphene with polymethyl methacrylate (PMMA) as a proof of concept and an encapsulation with mechanically exfoliated hexagonal boron nitride (hBN). The Hall sensor with PMMA encapsulation already shows higher sensitivity compared to the one without encapsulation. However, the Hall sensor with graphene encapsulated between two stacks of hBN shows a current and a voltage normalized sensitivity of up to 2270 V/AT and 0.68 V/VT, respectively, which are the highest reported sensitivity values for Hall sensors on foil so far. [ABSTRACT FROM AUTHOR]

We present spin transport studies on bi- and trilayer graphene non-local spin-valves which have been fabricated by a bottom-up fabrication method. By this technique, spin injection electrodes are first deposited onto Si [ABSTRACT FROM AUTHOR]

The encapsulation of graphene in hexagonal boron nitride provides graphene on substrate with excellent material quality. Here, we present the fabrication and characterization of Hall sensor elements based on graphene boron nitride heterostructures, where we gain from high mobility and low charge carrier density at room temperature. We show a detailed device characterization including Hall effect measurements under vacuum and ambient conditions. We achieve a currentand voltage-related sensitivity of up to 5700 V/AT and 3 V/VT, respectively, outpacing state-of-the-art silicon and III/V Hall sensor devices. Finally, we extract a magnetic resolution limited by low frequency electric noise of less than 50 nT/ Hz making our graphene sensors highly interesting for industrial applications. [ABSTRACT FROM AUTHOR]

The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments. [ABSTRACT FROM AUTHOR]

In this paper, we investigate with theory and experiments the performance improvements achievable using bilayer graphene as channel material in field effect transistors for analog applications. Bilayer graphene provides larger output resistance than monolayer graphene, which translates in both higher voltage gain and higher maximum frequency oscillation. To experimentally prove bilayer graphene potential as a channel material, simple circuits have been fabricated and tested, i.e., an amplifier and a frequency doubler. We show that they largely outperform similar circuits built with monolayer-graphene devices. [ABSTRACT FROM PUBLISHER]

In this work, we perform an experimental investigation of the saturation velocity in MoS2 transistors. We use a simple analytical formula to reproduce experimental results and to extract the saturation velocity and the critical electric field. Scattering with optical phonons or with remote phonons may represent the main transport-limiting mechanism, leading to saturation velocity comparable to silicon, but much smaller than that obtained in suspended graphene and some III-V semiconductors. [ABSTRACT FROM AUTHOR]

In this work, we present the fabrication and characterization of bulk-heterojunction solar cells on monolayer graphene (MLG) with nickel-grids (Ni-grid) as semitransparent conductive electrode. The electrodes showed a maximum transmittance of 90% (calculated in 300–800 nm range) and a sheet resistance down to 35 Ω/□. On these new anodes, we fabricated TCO free BHJ-SCs using PTB7 blended with PC70BM fullerene derivative as active layer. The best device exhibited a power conversion efficiency (PCE) of 4.2% in direct configuration and 3.6% in inverted configuration. The reference solar cell, realized on the ITO glass substrate, achieved a PCE of 6.1% and 6.7% in direct and inverted configuration respectively; for comparison we also tested OSCs only with simple Ni-grid as semitransparent and conductive electrode, obtaining a low PCE of 0.7%. The proposed approach to realize graphene-based electrodes could be a possible route to reduce the overall impact of the sheet resistance of this type of electrodes allowing their use in several optoelectronic devices. [ABSTRACT FROM AUTHOR]