Transfer-free Graphene Research Articles

Transfer-free p-type graphene field-effect transistors with high mobility and on/off ratio

Graphene is considered a promising material because of the novel functionalities associated with its outstanding charge transport properties. However, because graphene has no bandgap, its electrical conductivity cannot be controlled as in semiconductors, at present. Although many attempts have been made to achieve a bandgap opening in graphene, a meaningful bandgap opening for p-type field-effect-transistors (FETs) still remains a challenge. In this study, boron-doping in transfer-free monolayer graphene was successfully demonstrated for digital logic devices. Our approach is highly versatile, as it allows the fabrication of p-type single-crystal graphene FETs having a mobility of ∼290 cm2V−1s−1, an on/off ratio of 1.9 × 105, and a subthreshold swing of 70 mVdec−1. The scalability and versatility of this transfer-free approach for the fabrication of p-type graphene FETs pave the way for high-performance p-type graphene-based digital logic circuits.

Graphene-Enhanced UV-C LEDs.

Light-emitting diodes in the UV-C spectral range (UV-C LEDs) can potentially replace bulky and toxic mercury lamps in a wide range of applications including sterilization and water purification. Several obstacles still limit the efficiencies of UV-C LEDs. Devices in flip-chip geometry suffer from a huge difference in the work functions between the p-AlGaN and high-reflective Al mirrors, whereas the absence of UV-C transparent current spreading layers limits the development of UV-C LEDs in standard geometry. Here it is demonstrated that transfer-free graphene implemented directly onto the p-AlGaN top layer by a plasma enhanced chemical vapor deposition approach enables highly efficient 275nm UV-C LEDs in both, flip-chip and standard geometry. In flip-chip geometry, the graphene acts as a contact interlayer between the Al-mirror and the p-AlGaN enabling an external quantum efficiency (EQE) of 9.5% and a wall-plug efficiency (WPE) of 5.5% at 8V. Graphene combined with a ≈1nm NiOx support layer allows a turn-on voltage <5V. In standard geometry graphene acts as a current spreading layer on a length scale up to 1mm. These top-emitting devices exhibit a EQE of 2.1% at 8.7V and a WPE of 1.1%.

Transfer-free robust n-type graphene field-effect transistors for digital logic electronic devices

Graphene has been hailed as wonder materials for electronics due to its outstanding charge transport properties. However, its lack of an electronic bandgap has hindered its application to large-scale field-effect transistor (FET) circuits, thereby making bandgap opening a key priority. Further, the fabrication of graphene devices to date conventionally requires the transfer of graphene from its growth substrate to a target substrate, which introduces defects and deteriorates the device performance. To overcome these challenges, we demonstrate a transfer-free approach for the low-temperature growth (∼100 °C) onto TiO2–x (∼10 nm) buffer layer and in-situ nitrogen doping of monolayer graphene, which enables stretchable nitrogen-doped graphene-based FETs with cutting-edge performance and stability. Hybridizing a TiO2–x channel with the nitrogen-doped graphene, nitrogen-doped graphene-based FETs could be achieved with an on-off ratio of ∼2 × 108 and an electron mobility of ∼1,500 cm2V−1s−1 for mainstream digital logic devices. Such devices exhibit high reproducibility and wafer-scale uniformity, thermal, bias-stress, long-term stability, and robust flexibility and stretchability. The scalability and versatility of this transfer-free approach for the flexible and stretchable FETs pave the way for high-performance nitrogen-doped graphene-based digital logic circuits.

Quasi van der Waals Epitaxy of Single Crystalline GaN on Amorphous SiO2/Si(100) for Monolithic Optoelectronic Integration.

The realization of high quality (0001) GaN on Si(100) is paramount importance for the monolithic integration of Si-based integrated circuits and GaN-enabled optoelectronic devices. Nevertheless, thorny issues including large thermal mismatch and distinct crystal symmetries typically bring about uncontrollable polycrystalline GaN formation with considerable surface roughness on standard Si(100). Here a breakthrough of high-quality single-crystalline GaN film on polycrystalline SiO2/Si(100) is presented by quasi van der Waals epitaxy and fabricate the monolithically integrated photonic chips. The in-plane orientation of epilayer is aligned throughout a slip and rotation of high density AlN nuclei due to weak interfacial forces, while the out-of-plane orientation of GaN can be guided by multi-step growth on transfer-free graphene. For the first time, the monolithic integration of light-emitting diode (LED) and photodetector (PD) devices are accomplished on CMOS-compatible SiO2/Si(100). Remarkably, the self-powered PD affords a rapid response below 250µs under adjacent LED radiation, demonstrating the responsivity and detectivity of 2.01×105 A/W and 4.64×1013Jones, respectively. This work breaks a bottleneck of synthesizing large area single-crystal GaN on Si(100), which is anticipated to motivate the disruptive developments in Si-integrated optoelectronic devices.

Magnetoresistance properties in nickel-catalyzed, air-stable, uniform, and transfer-free graphene

A transfer-free graphene with high magnetoresistance (MR) and air stability has been synthesized using nickel-catalyzed atmospheric pressure chemical vapor deposition. The Raman spectrum and Raman mapping reveal the monolayer structure of the transfer-free graphene, which has low defect density, high uniformity, and high coverage (>90%). The temperature-dependent (from 5 to 300 K) current–voltage (I–V) and resistance measurements are performed, showing the semiconductor properties of the transfer-free graphene. Moreover, the MR of the transfer-free graphene has been measured over a wide temperature range (5–300 K) under a magnetic field of 0 to 1 T. As a result of the Lorentz force dominating above 30 K, the transfer-free graphene exhibits positive MR values, reaching ∼8.7% at 300 K under a magnetic field (1 Tesla). On the other hand, MR values are negative below 30 K due to the predominance of the weak localization effect. Furthermore, the temperature-dependent MR values of transfer-free graphene are almost identical with and without a vacuum annealing process, indicating that there are low density of defects and impurities after graphene fabrication processes so as to apply in air-stable sensor applications. This study opens avenues to develop 2D nanomaterial-based sensors for commercial applications in future devices.

Highly-sensitive wafer-scale transfer-free graphene MEMS condenser microphones

Since the performance of micro-electro-mechanical system (MEMS)-based microphones is approaching fundamental physical, design, and material limits, it has become challenging to improve them. Several works have demonstrated graphene’s suitability as a microphone diaphragm. The potential for achieving smaller, more sensitive, and scalable on-chip MEMS microphones is yet to be determined. To address large graphene sizes, graphene-polymer heterostructures have been proposed, but they compromise performance due to added polymer mass and stiffness. This work demonstrates the first wafer-scale integrated MEMS condenser microphones with diameters of 2R = 220–320 μm, thickness of 7 nm multi-layer graphene, that is suspended over a back-plate with a residual gap of 5 μm. The microphones are manufactured with MEMS compatible wafer-scale technologies without any transfer steps or polymer layers that are more prone to contaminate and wrinkle the graphene. Different designs, all electrically integrated are fabricated and characterized allowing us to study the effects of the introduction of a back-plate for capacitive read-out. The devices show high mechanical compliances Cm = 0.081–1.07 μmPa−1 (10–100 × higher than the silicon reported in the state-of-the-art diaphragms) and pull-in voltages in the range of 2–9.5 V. In addition, to validate the proof of concept, we have electrically characterized the graphene microphone when subjected to sound actuation. An estimated sensitivity of S1kHz = 24.3–321 mV Pa−1 for a Vbias = 1.5 V was determined, which is 1.9–25.5 × higher than of state-of-the-art microphone devices while having a ~9 × smaller area.

Influence of deposition temperature and hydrogen on sustainable and transfer-free graphene transparent electrode for organic solar cells

The transfer-free graphene transparent conducting electrode (TCE) is a promising alternative to indium tin oxide (ITO) for organic solar cells (OSCs). In the present work, a comprehensive investigation on how deposition temperature and H2 flow rates affect the growth, structural, optical, and electrical properties of graphene produced by RF plasma enhanced chemical vapor deposition using sustainable sources was conducted. Inverted-geometry OSCs with P3HT: PCBM photoactive layer were fabricated on transfer-free graphene TCEs developed under different conditions. Moreover, the coupling of silver nanowires (AgNWs) with different graphene films was studied for hybrid graphene-AgNWs TCEs for OSCs. Devices based on graphene TCEs prepared at low or zero H2 flow have shown better performances than those at high flow of H2. Similarly, graphene TCEs prepared at high temperature (>700 °C, on quartz) led to a deteriorated device performance due to the highly increased growth of vertically oriented graphene nanosheets, which dramatically reduced film transmittance and increased surface roughness. The present work provides solid understanding of the growth mechanism of RF-PECVD graphene on glass from a sustainable carbon source. More importantly, the sustainable, ecofriendly, cost- and time-effective production of scalable transfer-free graphene TCEs for OSCs is optimized which paves the way towards ITO-free optoelectronics.

Recent advances in batch production of transfer-free graphene.

Large-area transfer-free graphene films prepared via chemical vapor deposition have proved appealing for various applications, with exciting examples in electronics, photonics, and optoelectronics. To achieve their commercialisation, batch production is a prerequisite. Nevertheless, the prevailing scalable synthesis strategies that have been reported are still obstructed by production inefficiencies and non-uniformity. There has also been a lack of reviews in this realm. We present herein a comprehensive and timely summary of recent advances in the batch production of transfer-free graphene. Primary issues and promising approaches for improving the graphene growth rate are first addressed, followed by a discussion of the strategies to guarantee in-plane and batch uniformity for graphene grown on planar plates and wafer-scale substrates, with the design of the target equipment to meet productivity requirements. Finally, potential research directions are outlined, aiming to offer insights into guiding the scalable production of transfer-free graphene.

A patterning technology of transfer-free graphene for transparent electrodes of near-ultraviolet light-emitting diodes

A technology for the fabrication of transfer-free, patterned graphene on semiconductor or weakly catalytic metal substrate is presented, and the graphene transparent electrodes on GaN-based LED with 398 nm wavelength is fabricated accordingly.

Highly responsive hydrogen sensor based on Pd nanoparticle-decorated transfer-free 3D graphene

H2 sensors play a crucial role in the development of hydrogen (H2) energy and safety monitoring. Therefore, efficiently and rapidly detecting low concentrations of H2 is a fundamental challenge that needs to be addressed in the field of H2 sensors. In this study, we present a highly sensitive H2 gas sensor based on Pd-nanoparticles (NPs)-decorated transfer-free three-dimensional (3D) graphene. 3D graphene provides an increased surface area for enhanced gas adsorption and reaction kinetics. The integration of PdNPs improved the electron transport properties, resulting in a higher sensitivity. The PdNP-decorated 3D graphene sensor exhibited a remarkable gas response of 41.9% under 3% H2 exposure. The transfer-free synthesis process ensures the excellent conductivity of 3D graphene to further enhance its overall sensing performance. These discoveries propel current gas-sensing technologies forward and introduce fresh opportunities for the development of dependable sensors that can effectively enhance industrial safety and public security.

Improved Transfer-Free Sustainable Graphene Electrode using Silver Nanowires for Organic Photovoltaics

Improved Transfer-Free Sustainable Graphene Electrode using Silver Nanowires for Organic Photovoltaics

Transfer-free and catalyst-free graphene thin films produced by plasma electron annealing at low temperatures

Research into graphene, an innovative material, has increased remarkably since Geim and Novoselov won the Nobel Prize in Physics in 2010 for forming single layers of graphene using Scotch tape. Indeed, despite being a remarkable material with excellent properties, graphene is not yet commercialized, owing to a lack of suitable direct-growth preparation methods. To address this issue, in this study we introduce a new method for growing graphene thin films directly on substrates by using a simple and clean plasma electron annealing (PEA) process involving a low processing temperature in a single chamber, which is advantageous. Specifically, we transform the carbon layer deposited on a silicon dioxide wafer into graphene at low temperature by using the electrons in inductively coupled plasma to transfer kinetic energy to the substrate. We expect that the method developed herein will provide new possibilities for future graphene research.

Recent Advances in Transfer-Free Synthesis of High-Quality Graphene.

High-quality graphene obtained by chemical vapor deposition (CVD) technique holds significant importance in constructing innovative electronic and optoelectronic devices. Direct growth of graphene over target substrates readily eliminates cumbersome transfer processes, offering compatibility with practical application scenarios. Recent years have witnessed growing strategic endeavors in the preparation of transfer-free graphene with favorable quality. Nevertheless, timely review articles on this topic are still scarce. In this contribution, a systematic summary of recent advances in transfer-free synthesis of high-quality graphene on insulating substrates, with a focus on discussing synthetic strategies designed by elevating reaction temperature, confining gas flow, introducing growth promotor and regulating substrate surface is presented.

Two-Step Thermal Transformation of Multilayer Graphene Using Polymeric Carbon Source Assisted by Physical Vapor Deposited Copper.

Direct in situ growth of graphene on dielectric substrates is a reliable method for overcoming the challenges of complex physical transfer operations, graphene performance degradation, and compatibility with graphene-based semiconductor devices. A transfer-free graphene synthesis based on a controllable and low-cost polymeric carbon source is a promising approach for achieving this process. In this paper, we report a two-step thermal transformation method for the copper-assisted synthesis of transfer-free multilayer graphene. Firstly, we obtained high-quality polymethyl methacrylate (PMMA) film on a 300 nm SiO2/Si substrate using a well-established spin-coating process. The complete thermal decomposition loss of PMMA film was effectively avoided by introducing a copper clad layer. After the first thermal transformation process, flat, clean, and high-quality amorphous carbon films were obtained. Next, the in situ obtained amorphous carbon layer underwent a second copper sputtering and thermal transformation process, which resulted in the formation of a final, large-sized, and highly uniform transfer-free multilayer graphene film on the surface of the dielectric substrate. Multi-scale characterization results show that the specimens underwent different microstructural evolution processes based on different mechanisms during the two thermal transformations. The two-step thermal transformation method is compatible with the current semiconductor process and introduces a low-cost and structurally controllable polymeric carbon source into the production of transfer-free graphene. The catalytic protection of the copper layer provides a new direction for accelerating the application of graphene in the field of direct integration of semiconductor devices.

Wafer-Scale Graphene Growth on Si/SiO2 Substrates via Metal-Free Chemical Vapor Deposition

The direct growth of graphene on an insulator plays a crucial role in various applications, such as optoelectronic devices, supercapacitors, sensors, and so on. However, how to synthesize large area, homogeneous monolayer graphene with high quality remains a primary challenge. How to evaluate the flake size of directly grown graphene from a macroscopic perspective is yet to be explored. Herein, a full monolayer wafer-scale graphene film covering a silicon/silicon oxide substrate was realized via atmospheric pressure chemical vapor deposition under an optimized temperature, growth time, and methane flow rate. Both the Raman mapping and AFM characterization confirm that a homogeneous monolayer graphene was obtained (I2D/IG = 4.89, FWHM2D = 19.08). The transfer-free graphene exhibited a Hall mobility of 1864.6 cm2·V–1·S–1. Moreover, an empirical formula has been developed by applying the Monte Carlo method with the measured results to reflect the dependence of the resistivity of the graphene film on its flake size distributions as ρ=ρ01−k(1d2)0.4835. This work is highly compatible with present semiconductor device processing techniques and is expected to be highly desirable for Si-based graphene applications.

Transfer-free graphene passivation of sub 100 nm thin Pt and Pt–Cu electrodes for memristive devices

Memristive switches are among the most promising building blocks for future neuromorphic computing. These devices are based on a complex interplay of redox reactions on the nanoscale. Nanoionic phenomena enable non-linear and low-power resistance transition in ultra-short programming times. However, when not controlled, the same electrochemical reactions can result in device degradation and instability over time. Two-dimensional barriers have been suggested to precisely manipulate the nanoionic processes. But fabrication-friendly integration of these materials in memristive devices is challenging.Here we report on a novel process for graphene passivation of thin platinum and platinum/copper electrodes. We also studied the level of defects of graphene after deposition of selected oxides that are relevant for memristive switching.

Optical saturable absorption of conformal graphene directly synthesized on nonlinear device surfaces

Optical saturable absorption (SA) in graphene has been intensively studied during the last decades to achieve graphene-based ultrafast pulse generation. We experimentally investigate the optical nonlinearities of transfer-free graphene synthesized by the atomic carbon spraying (ACS) method that is invented for conformal and uniform graphene coating on non-planar device surfaces to enhance the interaction of lasers with the nanostructures. Its successful implementation of nonlinear operations in practical photonic devices is confirmed. The SAs of two other different graphene structures including graphene prepared by chemical vapor deposition (CVD) and reduced graphene oxides (rGOs) by electro-spraying (ES) are quantitatively evaluated as references. All three nanocrystal structures are elucidated by transmission electron microscopy and Raman analysis comparatively and systematically. We confirm in X-ray photoelectron spectroscopy analysis that the contents of sp2 bonds in ACS graphene as an indicator of the nonlinearity is 73.94 %. The measured value reaches the level of CVD graphene (76.70 %) and is much higher than 58.75 % of ES rGOs. Importantly, the ACS graphene displays a steep SA slope with saturation intensity of 32.14 MW cm−2, an order of magnitude lower than conventional ES rGOs, verifying unimpaired nonlinear properties of graphene prepared by the suggested ACS method.

Laminar Flow-Assisted Metal Etching for the Preparation of High-Quality Transfer-Free Graphene

Laminar Flow-Assisted Metal Etching for the Preparation of High-Quality Transfer-Free Graphene

Direct Wafer-Scale CVD Graphene Growth under Platinum Thin-Films.

Since the transfer process of graphene from a dedicated growth substrate to another substrate is prone to induce defects and contamination and can increase costs, there is a large interest in methods for growing graphene directly on silicon wafers. Here, we demonstrate the direct CVD growth of graphene on a SiO2 layer on a silicon wafer by employing a Pt thin film as catalyst. We pattern the platinum film, after which a CVD graphene layer is grown at the interface between the SiO2 and the Pt. After removing the Pt, Raman spectroscopy demonstrates the local growth of monolayer graphene on SiO2. By tuning the CVD process, we were able to fully cover 4-inch oxidized silicon wafers with transfer-free monolayer graphene, a result that is not easily obtained using other methods. By adding Ta structures, local graphene growth on SiO2 is selectively blocked, allowing the controlled graphene growth on areas selected by mask design.

Synthesis of transfer-free graphene films on dielectric substrates with controllable thickness via an in-situ co-deposition method for electrochromic devices

The solutions and polymer supported materials in graphene transfer process would introduce lots of containments, defects and wrinkles, which weakens the performance of graphene. Herein, an in-situ co-deposition method is carried out to obtain transfer-free graphene films with controllable thickness on several dielectric substrates. The amorphous carbon (carbon source) and copper (catalyst) are co-deposited on dielectric substrates. Followed by an in-situ annealing process, the amorphous carbon is transformed to few-layer graphene. High co-deposition temperature could promote the decomposition of Cu(acac)2 precursors, leading to the controllable thickness of amorphous carbon layer in Cu@C films. Finally, 3-, 5-, 8- and 10- layers graphene films with transmittance of up to 93.5% and square resistance of 0.8 kΩ·sq−1 are obtained and a high-performance electrochromic device is fabricated using 3 layers graphene films as electrodes. The “color” and “bleach” time of the electrochromic device is 16.6 s and 6.8 s with the transmittance of 26.8% and 79.7% separately. This method paves an alternative way for the batch production of transfer-free graphene film as electrode materials.