Graphene-on-diamond devices with increased current-carrying capacity: carbon sp2-on-sp3 technology

Chitosan-Based Flexible Memristors with Embedded Carbon Nanotubes for Neuromorphic Electronics

In this study, we propose high-performance chitosan-based flexible memristors with embedded single-walled carbon nanotubes (SWCNTs) for neuromorphic electronics. These flexible transparent memristors were applied to a polyethylene naphthalate (PEN) substrate using low-temperature solution processing. The chitosan-based flexible memristors have a bipolar resistive switching (BRS) behavior due to the cation-based electrochemical reaction between a polymeric chitosan electrolyte and mobile ions. The effect of SWCNT addition on the BRS characteristics was analyzed. It was observed that the embedded SWCNTs absorb more metal ions and trigger the conductive filament in the chitosan electrolyte, resulting in a more stable and wider BRS window compared to the device with no SWCNTs. The memory window of the chitosan nanocomposite memristors with SWCNTs was 14.98, which was approximately double that of devices without SWCNTs (6.39). Furthermore, the proposed SWCNT-embedded chitosan-based memristors had memristive properties, such as short-term and long-term plasticity via paired-pulse facilitation and spike-timing-dependent plasticity, respectively. In addition, the conductivity modulation was evaluated with 300 synaptic pulses. These findings suggest that memristors featuring SWCNT-embedded chitosan are a promising building block for future artificial synaptic electronics applications.

Mixed sp2-sp3 Nanocarbon Materials: A Status Quo Review

Carbon nanomaterials with a different character of the chemical bond-graphene (sp2) and nanodiamond (sp3)-are the building bricks for a new class of all-carbon hybrid nanomaterials, where the two different carbon networks with sp3 and sp2 hybridization coexist, interacting and even transforming into one another. The extraordinary physiochemical properties defined by the unique electronic band structure of the two border nanoallotropes ensure the immense application potential and versatility of these all-carbon nanomaterials. The review summarizes the status quo of sp2 - sp3 nanomaterials, including graphene/graphene-oxide-nanodiamond composites and hybrids, graphene/graphene-oxide-diamond heterojunctions, and other sp2-sp3 nanocarbon hybrids for sensing, electronic, and other emergent applications. Novel sp2-sp3 transitional nanocarbon phases and architectures are also discussed. Furthermore, the two-way sp2 (graphene) to sp3 (diamond surface and nanodiamond) transformations at the nanoscale, essential for innovative fabrication, and stability and chemical reactivity assessment are discussed based on extensive theoretical, computational and experimental studies.

The stability, structural, electronic, and optical properties of hydrogenated silicene under hydrostatic pressures: a first-principle study

The structural, electronic, and optical properties of hydrogenated silicene have been studied under different hydrostatic pressures using first-principle calculations. The binding energy and band structure have been calculated for chair (C-) and boat (B-) structures, which are having good stability at 0 GPa, 3 GPa, 6 GPa, 9 GPa, 12 GPa, 15 GPa, and 18 GPa hydrostatic pressures. Stability has been verified using binding energy and phonon calculations. The C- and B-structures have become metallic and unstable at 21 GPa. The optical properties of B-configuration have been studied in the energy range of 0-20 eV. Five optical parameters such as conductivity threshold (σ_th), dielectric constant ε(0), refractive index n(0), birefringence Δn(0), and plasmon energy (ħω_p) have been calculated for the first time under different hydrostatic pressures. The calculated values are in good agreement with the reported values at 0 GPa.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000 nm) and ultra-thin (≤100 nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Deep learning-enabled prediction of 2D material breakdown

Characterizing electrical breakdown limits of materials is a crucial step in device development. However, methods for repeatable measurements are scarce in two-dimensional materials, where breakdown studies have been limited to destructive methods. This restricts our ability to fully account for variability in local electronic properties induced by surface contaminants and the fabrication process. To tackle this, we implement a two-step deep-learning model to predict the breakdown mechanism and breakdown voltage of monolayer MoS₂devices with varying channel lengths and resistances using current measured in the low-voltage regime as inputs. A deep neural network (DNN) first classifies between Joule and avalanche breakdown mechanisms using partial current traces from 0 to 20 V. Following this, a convolutional long short-term memory network (CLSTM) predicts breakdown voltages of these classified devices based on partial current traces. We test our model with electrical measurements collected using feedback-control of the applied voltage to prevent device destruction, and show that the DNN classifier achieves an accuracy of 79% while the CLSTM model has a 12% error when requiring only 80% of the current trace as inputs. Our results indicate that information encoded in the current behavior far from the breakdown point can be used for breakdown predictions, which will enable non-destructive and rapid material characterization for 2D material device development.

The structural, electronic, and optical properties of hydrofluorinated germanene (GeH1-xFx): a first-principles study

The structural, electronic, and optical properties of hydrofluorinated germanene have been studied with different occupancy ratios of fluorine and hydrogen. The hybridization of H-1 s and Ge-4p orbitals in hydrogenated germanene and F-2p and Ge-4p orbitals in fluorinated germanene plays a significant role in creating an energy bandgap. The binding energy and phonon calculations confirm the stability of hydrofluorinated germanene decreases with the increase of the F to H ratio. The value of the energy bandgap decreased by increasing the ratio of F and H. The optical properties have been studied in the energy range of 0-25 eV. Six essential parameters such as energy bandgap (E_g), binding energy (E_b), dielectric constant ε(0), refractive index n(0), plasmon energy (ћω_p), and heat capacity (C_p) have been calculated for different occupancies of H and F in hydrofluorinated germanene for the first time. The calculated values of structural parameters agree well with the reported values.

High Current Density in Monolayer MoS2 Doped by AlOx

Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low-temperature (<200 °C) substoichiometric AlO_x provides a stable n-doping layer for monolayer MoS₂, compatible with circuit integration. This approach achieves carrier densities >2 × 10¹³ cm^-2, sheet resistance as low as ∼7 kΩ/□, and good contact resistance ∼480 Ω·μm in transistors from monolayer MoS₂ grown by chemical vapor deposition. We also reach record current density of nearly 700 μA/μm (>110 MA/cm²) along this three-atom-thick semiconductor while preserving transistor on/off current ratio >10⁶. The maximum current is ultimately limited by self-heating (SH) and could exceed 1 mA/μm with better device heat sinking. With their 0.1 nA/μm off-current, such doped MoS₂ devices approach several low-power transistor metrics required by the international technology roadmap.

Mechanical Pressure Characterization of CNT-Graphene Composite Material

Carbon nanotubes (CNTs) and graphene are extensively studied materials in the field of sensing technology and other electronic devices due to their better functional and structural properties. Additionally, more attention is given to utilize these materials as a filler to reinforce the properties of other materials. However, the role of weight percentage of CNTs in the piezoresistive properties of these materials has not been reported yet. In this work, CNT-graphene composite-based piezoresistive pressure samples in the form of pellets with different weight percentages of CNTs were fabricated and characterized. All the samples exhibit a decrease in the direct current (DC) resistance with the increase in external uniaxial applied pressure from 0 to 74.8 kNm⁻². However, under the same external uniaxial applied pressure, the DC resistance exhibit more decrease as the weight percentage of the CNTs increase in the composites.

Plasmonic Nonreciprocity Driven by Band Hybridization in Moiré Materials

We propose a new current-driven mechanism for achieving significant plasmon dispersion nonreciprocity in systems with narrow, strongly hybridized electron bands. The magnitude of the effect is controlled by the strength of electron-electron interactions α, which leads to its particular prominence in moiré materials, characterized by α≫1. Moreover, this phenomenon is most evident in the regime where Landau damping is quenched and plasmon lifetime is increased. The synergy of these two effects holds great promise for novel optoelectronic applications of moiré materials.

Graphene-diamond junction photoemission microscopy and electronic interactions

Polycrystalline graphene was transferred onto differently terminated epitaxial layers of boron-doped diamond deposited onto single crystal substrates. Chemical and electronic characterisation was performed using energy-filtered photoemission electron microscopy and angle-resolved photoemission spectroscopy. Electronic interaction between the diamond and graphene was observed, where doping of the graphene on the hydrogen and oxygen terminated diamond was n-doping of 250 meV and 0 meV respectively. We found that the wide window of achievable graphene doping is effectively determined by the diamond surface dipole, easily tuneable with a varying surface functionalisation. A Schottky junction using the graphene-diamond structure was clearly observed and shown to reduce downward band bending of the hydrogen terminated diamond, producing a Schottky barrier height of 330 meV.

Enhanced Peltier Effect in Wrinkled Graphene Constriction by Nano-Bubble Engineering

Inspired by the promising applications in thermopower generation from waste heat and active on-chip cooling, the thermoelectric and electrothermal properties of graphene have been extensively pursued by seeking ingeniously designed structures with thermoelectric conversion capability. The graphene wrinkle is a ubiquitous structure formed inevitably during the synthesis of large-scale graphene films but the corresponding properties for thermoelectric and electrothermal applications are rarely investigated. Here, the electrothermal Peltier effect from the graphene wrinkle fabricated on a germanium substrate is reported. Peltier cooling and heating across the wrinkle are visualized unambiguously with polarities consistent with p-type doping and in accordance with the wrinkle spatial distribution. By direct patterning of the nano-bubble structure, the current density across the wrinkle can be boosted by current crowding to enhance the Peltier effect. The observed Peltier effect can be attributed to the nonequilibrium charge transport by interlayer tunneling across the van der Waals barrier of the graphene wrinkle. The graphene wrinkle in combination with nano-bubble engineering constitutes an innovative and agile platform to design graphene and other more general two-dimensional (2D) thermoelectrics and opens the possibility for realizing active on-chip cooling for 2D nanoelectronics with van der Waals junctions.

High current carrying and thermal conductive copper-carbon conductors

The surging demand for miniaturized compact devices has generated the need for new metal conductors with high current carrying ampacity, electric and thermal conductivity. Herein, we report carbon-metal conductors that exhibit a high breakdown current density (39% higher than copper) and electrical conductivity (e.g. 63% higher than that of copper at 363 K) in a broad temperature range. The mechanistic studies of thermal conductivity through first-principle modeling show that the multilayer graphene percolation networks efficiently decrease the electron-phonon coupling in the copper-graphene composites, even if phonon modes are activated at a high temperature. These results imply that the copper-based composites have the potential to be the next generation metal conductor with high electrical and thermal conductivity, as well as excellent current-carrying ampacity. More importantly, the developed composite can be deployed in the ink form, making it possible to be utilized by the microelectronic fabrication process.

Electrical transportation mechanisms of molybdenum disulfide flakes-graphene quantum dots heterostructure embedded in polyvinylidene fluoride polymer

In the interest of the trend towards miniaturization of electronic gadgets, this study demonstrates a high-density data storage device with a very simple three-stacking layer consisting of only one charge trapping layer. A simple solution-processed technique has been used to fabricate the tristable non-volatile memory. The three-stacking layer was constructed in between two metals to form a two-terminal metal-insulator-metal structure. The fabricated device showed a large multilevel memory hysteresis window with a measured ON/OFF current ratio of 10⁷ that might be attributed to the high charge trapped in molybdenum disulphide (MoS₂) flakes-graphene quantum dots (GQDs) heterostructure. Transmission electron microscopy was performed to examine the orientation of MoS₂-GQD and mixture dispersion preparation method. The obtained electrical data was used further to speculate the possible transport mechanisms through the fabricated device by a curve fitting technique. Also, endurance cycle and retention tests were performed at room temperature to investigate the stability of the device.

Recent Advances in Transparent Electronics with Stretchable Forms

Advances in materials science and the desire for next-generation electronics have driven the development of stretchable and transparent electronics in the past decade. Novel applications, such as smart contact lenses and wearable sensors, have been introduced with stretchable and transparent form factors, requiring a deeper and wider exploration of materials and fabrication processes. In this regard, many research efforts have been dedicated to the development of mechanically stretchable, optically transparent materials and devices. Recent advances in stretchable and transparent electronics are discussed herein, with special emphasis on the development of stretchable and transparent materials, including substrates and electrodes. Several representative examples of applications enabled by stretchable and transparent electronics are presented, including sensors, smart contact lenses, heaters, and neural interfaces. The current challenges and opportunities for each type of stretchable and transparent electronics are also discussed.

Alignment of semiconducting graphene nanoribbons on vicinal Ge(001)

Chemical vapor deposition of CH4 on Ge(001) can enable anisotropic growth of narrow, semiconducting graphene nanoribbons with predominately smooth armchair edges and high-performance charge transport properties. However, such nanoribbons are not aligned in one direction but instead grow perpendicularly, which is not optimal for integration into high-performance electronics. Here, it is demonstrated that vicinal Ge(001) substrates can be used to synthesize armchair nanoribbons, of which ∼90% are aligned within ±1.5° perpendicular to the miscut. When the growth rate is slow, graphene crystals evolve as nanoribbons. However, as the growth rate increases, the uphill and downhill crystal edges evolve asymmetrically. This asymmetry is consistent with stronger binding between the downhill edge and the Ge surface, for example due to different edge termination as shown by density functional theory calculations. By tailoring growth rate and time, nanoribbons with sub-10 nm widths that exhibit excellent charge transport characteristics, including simultaneous high on-state conductance of 8.0 μS and a high on/off conductance ratio of 570 in field-effect transistors, are achieved. Large-area alignment of semiconducting ribbons with promising charge transport properties is an important step towards understanding the anisotropic nanoribbon growth and integrating these materials into scalable, future semiconductor technologies.

Graphene-Bridged Multifunctional Flexible Fiber Supercapacitor with High Energy Density

Portable fiber supercapacitors with high-energy storage capacity are in great demand to cater for the rapid development of flexible and deformable electronic devices. Hence, we employed a 3D cellular copper foam (CF) combined with the graphene sheets (GSs) as the support matrix to bridge the active material with nickel fiber (NF) current collector, significantly increasing surface area and decreasing the interface resistance. In comparison to the active material directly growing onto the NF in the absence of CF and GSs, our rationally designed architecture achieved a joint improvement in both capacity (0.217 mAh cm^-2/1729.413 mF cm^-2, 1200% enhancement) and rate capability (87.1% from 1 to 20 mA cm^-2, 286% improvement), which has never been achieved before with other fiber supercapacitors. The in situ scanning electron microscope (SEM) microcompression test demonstrated its superior mechanical recoverability for the first time. Importantly, the assembled flexible and wearable device presented a superior energy density of 109.6 μWh cm^-2 at a power density of 749.5 μW cm^-2, and the device successfully coupled with a flexible strain sensor, solar cell, and nanogenerator. This rational design should shed light on the manufacturing of 3D cellular architectures as microcurrent collectors to realize high energy density for fiber-based energy storage devices.

Study of Ni-Catalyzed Graphitization Process of Diamond by in Situ X-ray Photoelectron Spectroscopy

Graphene on diamond has been attracting considerable attention due to the unique and highly beneficial features of this heterostructure for a range of electronic applications. Here, ultrahigh-vacuum X-ray photoelectron spectroscopy is used for in situ analysis of the temperature dependence of the Ni-assisted thermally induced graphitization process of intrinsic nanocrystalline diamond thin films (65 nm thickness, 50-80 nm grain size) on silicon wafer substrates. Three major stages of diamond film transformation are determined from XPS during the thermal annealing in the temperature range from 300 °C to 800 °C. Heating from 300 °C causes removal of oxygen; formation of the disordered carbon phase is observed at 400 °C; the disordered carbon progressively transforms to graphitic phase whereas the diamond phase disappears from the surface from 500 °C. In the well-controllable temperature regime between 600 °C and 700 °C, the nanocrystalline diamond thin film is mainly preserved, while graphitic layers form on the surface as the predominant carbon phase. Moreover, the graphitization is facilitated by a disordered carbon interlayer that inherently forms between diamond and graphitic layers by Ni catalyst. Thus, the process results in formation of a multilayer heterostructure on silicon substrate.

Ultrastrong Graphene-Copper Core-Shell Wires for High-Performance Electrical Cables

Recent development in mobile electronic devices and electric vehicles requires electrical wires with reduced weight as well as enhanced stability. In addition, since electric energy is mostly generated from power plants located far from its consuming places, mechanically stronger and higher electric power transmission cables are strongly demanded. However, there has been no alternative materials that can practically replace copper materials. Here, we report a method to prepare ultrastrong graphene fibers (GFs)-Cu core-shell wires with significantly enhanced electrical and mechanical properties. The core GFs are synthesized by chemical vapor deposition, followed by electroplating of Cu shells, where the large surface area of GFs in contact with Cu maximizes the mechanical toughness of the core-shell wires. At the same time, the unique electrical and thermal characteristics of graphene allow a ∼10 times higher current density limit, providing more efficient and reliable delivery of electrical energies through the GFs-Cu wires. We believe that our results would be useful to overcome the current limit in electrical wires and cables for lightweight, energy-saving, and high-power applications.

Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium

It was recently discovered that the chemical vapor deposition (CVD) of CH₄ on Ge(001) can directly yield long, narrow, semiconducting nanoribbons of graphene with smooth armchair edges. These nanoribbons have exceptional charge transport properties compared with nanoribbons grown by other methods. However, the nanoribbons nucleate at random locations and at random times, problematically giving rise to width and bandgap polydispersity, and the mechanisms that drive the anisotropic crystal growth that produces the nanoribbons are not understood. Here, we study and engineer the seed-initiated growth of graphene nanoribbons on Ge(001). The use of seeds decouples nucleation and growth, controls where growth occurs, and allows graphene to grow with lattice orientations that do not spontaneously form without seeds. We discover that when the armchair direction (i.e., parallel to C-C bonds) of the seeds is aligned with the Ge⟨110⟩ family of directions, the growth anisotropy is maximized, resulting in the formation of nanoribbons with high-aspect ratios. In contrast, increasing misorientation from Ge⟨110⟩ yields decreasingly anisotropic crystals. Measured growth rate data are used to generate a construction analogous to a kinetic Wulff plot that quantitatively predicts the shape of graphene crystals on Ge(001). This knowledge is employed to fabricate regularly spaced, unidirectional arrays of nanoribbons and to significantly improve their uniformity. These results show that seed-initiated graphene synthesis on Ge(001) will be a viable route for creating wafer-scale arrays of narrow, semiconducting, armchair nanoribbons with rationally controlled placement and alignment for a wide range of semiconductor electronics technologies, provided that dense arrays of sub-10 nm seeds can be uniformly fabricated in the future.

High-Velocity Saturation in Graphene Encapsulated by Hexagonal Boron Nitride

We measure drift velocity in monolayer graphene encapsulated by hexagonal boron nitride (hBN), probing its dependence on carrier density and temperature. Due to the high mobility (>5 × 10⁴ cm²/V/s) of our samples, the drift velocity begins to saturate at low electric fields (∼0.1 V/μm) at room temperature. Comparing results to a canonical drift velocity model, we extract room-temperature electron saturation velocities ranging from 6 × 10⁷ cm/s at a low carrier density of 8 × 10¹¹ cm^-2 to 2.7 × 10⁷ cm/s at a higher density of 4.4 × 10¹² cm^-2. Such drift velocities are much higher than those in silicon (∼10⁷ cm/s) and in graphene on SiO₂, likely due to reduced carrier scattering with surface optical phonons whose energy in hBN (>100 meV) is higher than that in other substrates.

Electrical Breakdown of Suspended Mono- and Few-Layer Tungsten Disulfide via Sulfur Depletion Identified by in Situ Atomic Imaging

The high-bias and breakdown behavior of suspended mono- and few-layer WS₂ was explored by in situ aberration-corrected transmission electron microscopy. The suspended WS₂ devices were found to undergo irreversible breakdown at sufficiently high biases due to vaporization of the WS₂. Simultaneous to the removal of WS₂ was the accompanying formation of few-layer graphene decorated with W and WS₂ nanoparticles, with the carbon source attributed to organic residues present on the WS₂ surface. The breakdown of few-layer WS₂ resulted in the formation of faceted S-depleted WS₂ tendrils along the vaporization boundary, which were found to exhibit lattice contraction indicative of S depletion, alongside pure W phases incorporated into the structure, with the interfaces imaged at atomic resolution. The combination of observing the graphitization of the amorphous carbon surface residue, W nanoparticles, and S-depleted WS₂ phases following the high-bias WS₂ disintegration all indicate a thermal Joule heating breakdown mechanism over an avalanche process, with WS₂ destruction promoted by preferential S emission. The observation of graphene formation and the role the thin amorphous carbon layer has in the prebreakdown behavior of the device demonstrate the importance of employing encapsulated heterostructure device architectures that exclude residues.

High-Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium

The growth of graphene on Ge(001) via chemical vapor deposition can be highly anisotropic, affording the facile synthesis of crystallographically controlled, narrow, long, oriented nanoribbons of graphene that are semiconducting, whereas unpatterned continuous graphene is semimetallic. This bottom-up growth overcomes long-standing challenges that have limited top-down ribbon fabrication (e.g., inadequate resolution and disordered edges) and yields ribbons with long segments of smooth armchair edges. The charge transport characteristics of sub-10 nm ribbons synthesized by this technique (which are expected to have band gaps sufficiently large for semiconductor electronics applications) have not yet been characterized. Here, we show that sub-10 nm nanoribbons grown on Ge(001) can simultaneously achieve a high on/off conductance ratio of 2 × 10⁴ and a high on-state conductance of 5 μS in field-effect transistors, favorably comparing to or exceeding the performance of nanoribbons fabricated by other methods. These promising results demonstrate that the direct synthesis of nanoribbons on Ge(001) could provide a scalable pathway toward the practical realization of high-performance semiconducting graphene electronics, provided that the width uniformity and positioning of the nanoribbons are improved.

A Solar-Blind UV Detector Based on Graphene-Microcrystalline Diamond Heterojunctions

An ultraviolet detector is demonstrated through a whole-wafer, thin diamond film transfer process to realize the heterojunction between graphene and microcrystalline diamond (MCD). Conventional direct transfer processes fail to deposit graphene onto the top surface of the MCD film. However, it is found that the 2 µm thick MCD diamond film can be easily peeled off from the growth silicon substrate to expose its smooth backside for the graphene transfer process for high-quality graphene/MCD heterojunctions. A vertical graphene/MCD/metal structure is constructed as the photodiode device using graphene as the transparent top electrode for solar-blind ultraviolet sensing with high responsivity and gain factor. As such, this material system and device architecture could serve as the platform for next-generation optoelectronic systems.

Intercalation Doped Multilayer-Graphene-Nanoribbons for Next-Generation Interconnects

Copper-based interconnects employed in a wide range of integrated circuit (IC) products are fast approaching a dead-end due to their increasing resistivity and diminishing current carrying capacity with scaling, which severely degrades both performance and reliability. Here we demonstrate chemical vapor deposition-synthesized and intercalation-doped multilayer-graphene-nanoribbons (ML-GNRs) with better performance (more than 20% improvement in estimated delay per unit length), 25%/72% energy efficiency improvement at local/global level, and superior reliability w.r.t. Cu for the first time, for dimensions (down to 20 nm width and thickness of 12 nm) suitable for IC interconnects. This is achieved through a combination of GNR interconnect design optimization, high-quality ML-GNR synthesis with precisely controlled number of layers, and effective FeCl₃ intercalation doping. We also demonstrate that our intercalation doping is stable at room temperature and that the doped ML-GNRs exhibit a unique width-dependent doping effect due to increasingly efficient FeCl₃ diffusion in scaled ML-GNRs, thereby indicating that our doped ML-GNRs will outperform Cu even for sub-20 nm widths. Finally, reliability assessment conducted under accelerated stress conditions (temperature and current density) established that highly scaled intercalated ML-GNRs can carry over 2 × 10⁸ A/cm² of current densities, whereas Cu interconnects suffer from immediate breakdown under the same stress conditions and thereby addresses the key criterion of current carrying capacity necessary for an alternative interconnect material. Our comprehensive demonstration of highly reliable intercalation-doped ML-GNRs paves the way for graphene as the next-generation interconnect material for a variety of semiconductor technologies and applications.

Ultrafast electronic response of graphene to a strong and localized electric field

The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 10¹² A cm⁻², the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.

Graphene has so far demonstrated remarkable properties, making it increasingly interesting for ultrafast electronic applications. Here, the authors show that, when probed by a highly charged ion, freestanding graphene is able to provide dozens of electrons for ion neutralization within a few femtoseconds.

Metal-induced rapid transformation of diamond into single and multilayer graphene on wafer scale

The degradation of intrinsic properties of graphene during the transfer process constitutes a major challenge in graphene device fabrication, stimulating the need for direct growth of graphene on dielectric substrates. Previous attempts of metal-induced transformation of diamond and silicon carbide into graphene suffers from metal contamination and inability to scale graphene growth over large area. Here, we introduce a direct approach to transform polycrystalline diamond into high-quality graphene layers on wafer scale (4 inch in diameter) using a rapid thermal annealing process facilitated by a nickel, Ni thin film catalyst on top. We show that the process can be tuned to grow single or multilayer graphene with good electronic properties. Molecular dynamics simulations elucidate the mechanism of graphene growth on polycrystalline diamond. In addition, we demonstrate the lateral growth of free-standing graphene over micron-sized pre-fabricated holes, opening exciting opportunities for future graphene/diamond-based electronics.

Direct growth of large-area graphene on dielectric substrates is a promising route to wafer scale integration. Here the authors use a rapid thermal annealing process to grow graphene layers on four-inch diameter polycrystalline diamond, eliminating the need for transfer.

Hybrid Quantum Device with Nitrogen-Vacancy Centers in Diamond Coupled to Carbon Nanotubes

We show that nitrogen-vacancy (NV) centers in diamond interfaced with a suspended carbon nanotube carrying a dc current can facilitate a spin-nanomechanical hybrid device. We demonstrate that strong magnetomechanical interactions between a single NV spin and the vibrational mode of the suspended nanotube can be engineered and dynamically tuned by external control over the system parameters. This spin-nanomechanical setup with strong, intrinsic, and tunable magnetomechanical couplings allows for the construction of hybrid quantum devices with NV centers and carbon-based nanostructures, as well as phonon-mediated quantum information processing with spin qubits.

"Freeing" Graphene from Its Substrate: Observing Intrinsic Velocity Saturation with Rapid Electrical Pulsing

Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene's carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 10(9) A/cm(2), similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to "free" graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors.

Superior Current Carrying Capacity of Boron Nitride Encapsulated Carbon Nanotubes with Zero-Dimensional Contacts

We report fabrication and characterization of hexagonal boron nitride (hBN)-encapsulated carbon nanotube (CNT) field effect transistors, which are coupled to electrical leads via zero-dimensional contacts. Device quality is attested by the ohmic contacts and observation of Coulomb blockade with a single periodicity in small bandgap semiconducing nanotubes. Surprisingly, hBN-encapsulated CNT devices demonstrate significantly enhanced current carrying capacity; a single-walled CNT can sustain >180 μA current or, equivalently, a current density of ∼2 × 10(10) A/cm(2), which is a factor of 6-7 higher than devices supported on SiO2 substrates. Such dramatic enhancement of current carrying capacity arises from the high thermal conductivity of hBN and lower hBN-CNT interfacial thermal resistance and has implications for carbon electronic applications.

Current-limiting challenges for all-spin logic devices

All-spin logic device (ASLD) has attracted increasing interests as one of the most promising post-CMOS device candidates, thanks to its low power, non-volatility and logic-in-memory structure. Here we investigate the key current-limiting factors and develop a physics-based model of ASLD through nano-magnet switching, the spin transport properties and the breakdown characteristic of channel. First, ASLD with perpendicular magnetic anisotropy (PMA) nano-magnet is proposed to reduce the critical current (I_c0). Most important, the spin transport efficiency can be enhanced by analyzing the device structure, dimension, contact resistance as well as material parameters. Furthermore, breakdown current density (J_BR) of spin channel is studied for the upper current limitation. As a result, we can deduce current-limiting conditions and estimate energy dissipation. Based on the model, we demonstrate ASLD with different structures and channel materials (graphene and copper). Asymmetric structure is found to be the optimal option for current limitations. Copper channel outperforms graphene in term of energy but seriously suffers from breakdown current limit. By exploring the current limit and performance tradeoffs, the optimization of ASLD is also discussed. This benchmarking model of ASLD opens up new prospects for design and implementation of future spintronics applications.

Graphene-Nanodiamond Heterostructures and their application to High Current Devices

Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach.

Room-temperature negative differential resistance in graphene field effect transistors: experiments and theory

In this paper we demonstrate experimentally and discuss the negative differential resistance (NDR) in dual-gated graphene field effect transistors (GFETs) at room temperature for various channel lengths, ranging from 200 nm to 5 μm. The GFETs were fabricated using chemically vapor-deposited graphene with a top gate oxide down to 2.5 nm of equivalent oxide thickness (EOT). We originally explain and demonstrate with systematic simulations that the onset of NDR occurs in the unipolar region itself and that the main mechanism behind NDR is associated with the competition between the specific field dependence of carrier density and the drift velocity in GFET. Finally, we show experimentally that NDR behavior can still be obtained with devices of higher EOTs; however, this comes at the cost of requiring higher bias values and achieving lower NDR level.

Three-dimensional bicomponent supramolecular nanoporous self-assembly on a hybrid all-carbon atomically flat and transparent platform

Molecular self-assembly is a versatile nanofabrication technique with atomic precision en route to molecule-based electronic components and devices. Here, we demonstrate a three-dimensional, bicomponent supramolecular network architecture on an all-carbon sp(2)-sp(3) transparent platform. The substrate consists of hydrogenated diamond decorated with a monolayer graphene sheet. The pertaining bilayer assembly of a melamine-naphthalenetetracarboxylic diimide supramolecular network exhibiting a nanoporous honeycomb structure is explored via scanning tunneling microscopy initially at the solution-highly oriented pyrolytic graphite interface. On both graphene-terminated copper and an atomically flat graphene/diamond hybrid substrate, an assembly protocol is demonstrated yielding similar supramolecular networks with long-range order. Our results suggest that hybrid platforms, (supramolecular) chemistry and thermodynamic growth protocols can be merged for in situ molecular device fabrication.

Carbon nanotubes and graphene towards soft electronics

Although silicon technology has been the main driving force for miniaturizing device dimensions to improve cost and performance, the current application of Si to soft electronics (flexible and stretchable electronics) is limited due to material rigidity. As a result, various prospective materials have been proposed to overcome the rigidity of conventional Si technology. In particular, nano-carbon materials such as carbon nanotubes (CNTs) and graphene are promising due to outstanding elastic properties as well as an excellent combination of electronic, optoelectronic, and thermal properties compared to conventional rigid silicon. The uniqueness of these nano-carbon materials has opened new possibilities for soft electronics, which is another technological trend in the market. This review covers the recent progress of soft electronics research based on CNTs and graphene. We discuss the strategies for soft electronics with nano-carbon materials and their preparation methods (growth and transfer techniques) to devices as well as the electrical characteristics of transparent conducting films (transparency and sheet resistance) and device performances in field effect transistor (FET) (structure, carrier type, on/off ratio, and mobility). In addition to discussing state of the art performance metrics, we also attempt to clarify trade-off issues and methods to control the trade-off on/off versus mobility). We further demonstrate accomplishments of the CNT network in flexible integrated circuits on plastic substrates that have attractive characteristics. A future research direction is also proposed to overcome current technological obstacles necessary to realize commercially feasible soft electronics.

Thermal properties of graphene-copper-graphene heterogeneous films

We demonstrated experimentally that graphene-Cu-graphene heterogeneous films reveal strongly enhanced thermal conductivity as compared to the reference Cu and annealed Cu films. Chemical vapor deposition of a single atomic plane of graphene on both sides of 9 μm thick Cu films increases their thermal conductivity by up to 24% near room temperature. Interestingly, the observed improvement of thermal properties of graphene-Cu-graphene heterofilms results primarily from the changes in Cu morphology during graphene deposition rather than from graphene's action as an additional heat conducting channel. Enhancement of thermal properties of graphene-capped Cu films is important for thermal management of advanced electronic chips and proposed applications of graphene in the hybrid graphene-Cu interconnect hierarchies.

Theoretical investigation of the electronic structure and quantum transport in the graphene-C(111) diamond surface system

We investigate the interaction of a graphene monolayer with the C(111) diamond surface using ab initio density functional theory. To accommodate the lattice mismatch between graphene and diamond, the overlayer deforms into a wavy structure that binds strongly to the diamond substrate. The detached ridges of the wavy graphene overlayer behave electronically as free-standing polyacetylene chains with delocalized π electrons, separated by regions containing only sp(3) carbon atoms covalently bonded to the (111) diamond surface. We performed quantum transport calculations for different geometries of the system to study how the buckling of the graphene layer and the associated bonding to the diamond substrate affect the transport properties. The system displays high carrier mobility along the ridges and a wide transport gap in the direction normal to the ridges. These intriguing, strongly anisotropic transport properties qualify the hybrid graphene-diamond system as a viable candidate for electronic nanodevices.

Nonvolatile memory cells based on MoS2/graphene heterostructures

Memory cells are an important building block of digital electronics. We combine here the unique electronic properties of semiconducting monolayer MoS2 with the high conductivity of graphene to build a 2D heterostructure capable of information storage. MoS2 acts as a channel in an intimate contact with graphene electrodes in a field-effect transistor geometry. Our prototypical all-2D transistor is further integrated with a multilayer graphene charge trapping layer into a device that can be operated as a nonvolatile memory cell. Because of its band gap and 2D nature, monolayer MoS2 is highly sensitive to the presence of charges in the charge trapping layer, resulting in a factor of 10(4) difference between memory program and erase states. The two-dimensional nature of both the contact and the channel can be harnessed for the fabrication of flexible nanoelectronic devices with large-scale integration.

A review of carbon nanotube- and graphene-based flexible thin-film transistors

Carbon nanotubes (CNTs) and graphene have attracted great attention for numerous applications for future flexible electronics, owing to their supreme properties including exceptionally high electronic conductivity and mechanical strength. Here, the progress of CNT- and graphene-based flexible thin-film transistors from material preparation, device fabrication techniques to transistor performance control is reviewed. State-of-the-art fabrication techniques of thin-film transistors are divided into three categories: solid-phase, liquid-phase, and gas-phase techniques, and possible scale-up approaches to achieve realistic production of flexible nanocarbon-based transistors are discussed. In particular, the recent progress in flexible all-carbon nanomaterial transistor research is highlighted, and this all-carbon strategy opens up a perspective to realize extremely flexible, stretchable, and transparent electronics with a relatively low-cost and fast fabrication technique, compared to traditional rigid silicon, metal and metal oxide electronics.

Graphene field-effect transistors with gigahertz-frequency power gain on flexible substrates

The development of flexible electronics operating at radio-frequencies (RF) requires materials that combine excellent electronic performance and the ability to withstand high levels of strain. In this work, we fabricate graphene field-effect transistors (GFETs) on flexible substrates from graphene grown by chemical vapor deposition (CVD). Our devices demonstrate unity-current-gain frequencies, f(T), and unity-power-gain frequencies, f(max), up to 10.7 GHz and 3.7 GHz, respectively, with strain limits of 1.75%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%. As such, they demonstrate the potential of CVD graphene to enable a broad range of flexible electronic technologies which require both high flexibility and RF operation.

Breakdown of high-performance monolayer MoS2 transistors

Two-dimensional (2D) materials such as monolayer molybdenum disulfide (MoS(2)) are extremely interesting for integration in nanoelectronic devices where they represent the ultimate limit of miniaturization in the vertical direction. Thanks to the presence of a band gap and subnanometer thickness, monolayer MoS(2) can be used for the fabrication of transistors exhibiting extremely high on/off ratios and very low power dissipation. Here, we report on the development of 2D MoS(2) transistors with improved performance due to enhanced electrostatic control. Our devices show currents in the 100 μA/μm range and transconductance exceeding 20 μS/μm as well as current saturation. We also record electrical breakdown of our devices and find that MoS(2) can support very high current densities, exceeding the current-carrying capacity of copper by a factor of 50. Our results push the performance limit of MoS(2) and open the way to their use in low-power and low-cost analog and radio frequency circuits.

Graphene: an emerging electronic material

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

Preferentially grown ultranano c-diamond and n-diamond grains on silicon nanoneedles from energetic species with enhanced field-emission properties

The design and fabrication of well-defined nanostructures have great importance in nanoelectronics. Here we report the precise growth of sub-2 nm (c-diamond) and above 5 nm (n-diamond) size diamond grains from energetic species (chemical vapor deposition process) at low growth temperature of about 460 °C. We demonstrate that a pre-nucleation induced interface can be accounted for the growth of c-diamond or n-diamond grains on Si-nanoneedles (Si-NN). These preferentially grown allotropic forms of diamond on Si-NN have shown high electron field-emission properties and signify their high potential towards diamond-based electronic applications.

Transport in nanoribbon interconnects obtained from graphene grown by chemical vapor deposition

We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ∼2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ∼5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.

Current saturation in submicrometer graphene transistors with thin gate dielectric: experiment, simulation, and theory

Recently, graphene field-effect transistors (FET) with cutoff frequencies (f(T)) between 100 and 300 GHz have been reported; however, the devices showed very weak drain current saturation, leading to an undesirably high output conductance (g(ds)= dI(ds)/dV(ds)). A crucial figure-of-merit for analog/RF transistors is the intrinsic voltage gain (g(m)/g(ds)) which requires both high g(m) (primary component of f(T)) and low g(ds). Obtaining current saturation has become one of the key challenges in graphene device design. In this work, we study theoretically the influence of the dielectric thickness on the output characteristics of graphene FETs by using a surface-potential-based device model. We also experimentally demonstrate that by employing a very thin gate dielectric (equivalent oxide thickness less than 2 nm), full drain current saturation can be obtained for large-scale chemical vapor deposition graphene FETs with short channels. In addition to showing intrinsic voltage gain (as high as 34) that is comparable to commercial semiconductor FETs with bandgaps, we also demonstrate high frequency AC voltage gain and S21 power gain from s-parameter measurements.