Graphenea (Spain)

Proper physicochemical characterization of advanced materials and complex industrial composites remains a significant challenge, particularly for nanomaterials, whose nanoscale dimensions and mostly complex chemistry challenge the analysis. In this work, we employed a correlative analytical approach that integrates atomic force microscopy (AFM), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), Auger electron spectroscopy (AES), and Raman spectroscopy. This combination enables detailed chemical and structural characterization with sub-micrometer spatial resolution. Three commercial graphene-based materials of varying complexity were selected and investigated to test the analytical performance of this approach. Furthermore, one of the commercial graphene oxide samples was chemically functionalized via amination and fluorination. This allowed us to assess how surface modifications influence both the material properties and the limits of the applied analytical techniques.

This work proposes a methodology entirely based on processing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i>-parameters to determine the gate-to-source voltage-dependent mobility of charge carriers in the graphene field-effect transistor channel, without requiring any information about the material properties. Furthermore, regressions from experimental data of transistor arrays with different geometries are not required, thus avoiding uncertainties related to process variations and device-to-device measurement conditions. Hence, one key advantage of this method is its applicability to analyze the performance of different devices under the same operating conditions, or the performance of a single device under varying conditions. As part of the methodology, the effects of the parasitic series resistances associated with the source and drain access paths are considered, thereby overcoming the well-known disadvantage of direct-current methods, where the corresponding de-embedding is cumbersome. This method has been used to determine the carrier mobility in graphene within a gate-to-source voltage range, starting from the Dirac voltage and extending to the region where electron conduction dominates. A small-signal model with the extracted parameter values shows an excellent agreement with the experimental <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i>-parameters up to 20 GHz for the dynamic response of different devices, including two devices that have not been used during parameter extraction. Throughout the development and application of the proposal, a mobility model accounting for the degradation caused by the transverse electric field has been considered.

This work reports, for the first time, the use of spark ablation with impaction printing to selectively deposit silver (Ag) and gold (Au) nanoparticle (NP) functionalization on single-layer graphene (SLG) based gas sensors. This method avoids lithography and chemical processes, maintaining the device's quality while potentially lowering the fabrication costs. Ag-decorated sensors reveal a three-fold improvement in nitrogen dioxide (NO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) gas response over pristine-SLG sensors. We demonstrate detection capabilities down to 50 ppb at room temperature, negating the requirement for external thermal or photoactivation. In contrast to pristine or Au-decorated SLG sensors, Ag-decorated devices exhibit 96% recovery at room temperature (RT). These results highlight the potential of using spark ablation with impaction printing for functionalizing graphene-based sensors.

The commercialization of electronic devices based on graphene has not yet been successful, even 20 years after its first isolation. To this end, the European Commission is supporting research toward establishing a European experimental pilot line for electronic and optoelectronic devices based on graphene and related two-dimensional (2D) materials, namely the Experimental Pilot Line (2D-EPL) project. Here, we report the results obtained during the first and third multi-project wafer (MPW) runs completed at the end of 2022 (MPW run 1) and 2023 (MPW run 3) as an outcome of the 2D-EPL. Test devices were measured across the wafers to assess the device quality and variability before delivering the fabricated dies to the customers. Raman spectroscopy confirmed minimal structural changes in the graphene caused by the fabrication process, while electrical measurements of two different device types verified the device specifications defined in the process design kit.

Two-dimensional transition metal dichalcogenides (2D TMDC’s) hold a wide variety of applications, among which microelectronic devices. 2D TMDC’s are promising alternatives for today’s silicon-based technology but suffer from various integration challenges. Among others, direct dielectric growth on the 2D surface occurs poorly due to their self-passivated surfaces resulting in island-like growth initiated at defect sites. This work focuses on enabling uniform high-k dielectric nucleation and growth on 2D TMDC’s via a sacrificial, graphene oxide-based buffer layer. Essentially, the role of the sacrificial graphene buffer layer is twofold: 1) serve as a passivation layer, protecting the underlying 2D TMDC (i.e., WS2) during processing and 2) act as a nucleation layer, enabling uniform atomic layer deposition (ALD) high-k dielectric (i.e,. HfO2) growth. A graphene layer is transferred on monolayer WS2, after which polymeric transfer residues are cleaned via a combination of wet treatments and a dry hydrogen downstream plasma exposure. The cleaned graphene is functionalized via a dry UV/O3 oxidative treatment, which enriches its basal plane with carbon-oxygen single bond functionalities by sacrificing its π-network. This way, the graphene carbon lattice itself remains intact and retains its passivation properties. Moreover, a substrate dependency is observed where graphene transferred on WS2 requires longer UV/O3 exposure to ensure functionalization compared to a SiO2 substrate and can be explained by UV-light induced, ultrafast charge transfer between the graphene and WS2 monolayer. The carbon-oxygen groups formed on graphene’s basal plane act as nucleation sites during a subsequent HfO2 ALD process. A constant roughness is noted for the HfO2 film grown on the graphene oxide capped WS2, indicating uniform material deposition even in the early stages of the ALD process. This is in sharp contrast when HfO2 is directly grown on the WS2 monolayer, whose roughness stabilizes only after many ALD cycles (up to 256), demonstrating the poor, island-like growth mechanism. In addition, a similar hafnium areal density is measured for the graphene oxide capped WS2 compared bare SiO2, which was selected to represent a well-functionalized surface and thus confirming the efficiency of the graphene oxide-based nucleation layer. For devices incorporating the graphene oxide buffer layer concept, a crucial question is to what extent the graphene oxide layer gives rise to leakage currents by providing a conductive path between source and drain. Electrical evaluation of a GFET by means of I-V measurements reveal a significant shift in Dirac point towards positive gate voltage for transferred graphene, which is typically associated with p-type doping originating from the polymeric transfer residues still present on the surface. The Dirac point shifts towards 0-point voltage after final hydrogen downstream plasma cleaning, confirming the effectiveness of the established cleaning protocol. Moreover, Graphene’s conductivity is suppressed after UV/O3 to the nA range, as caused by the formation of carbon-oxygen functionalities and associated sacrifice of its π-network. The residual current may be further suppressed by extending the UV/O3 treatment, but nevertheless demonstrates that the graphene oxide buffer layer will not give rise to significant leakage currents.

In the current work we will present the transfer of graphene pixels and arrays. The process we used to accomplish the transfer was the Laser Induced Transfer technique. We will exhibit the advantages of the certain technique, the resolution of the transferred pixels and the characterization of the samples. Also, we will demonstrate the transfer of graphene arrays on both flexible polymeric and rigid substrates. The transfer of the graphene pixels was accomplished for resolution from 40 μm to 15 μm, whereas the digital manipulation of the transfer enables the deposition of arrays with dimension up to 1 mm2. Furthermore, in this work we will present the fabrication of a flexible capacitance touch sensor. The device is composed by LIFT transfer bottom electrodes and top electrodes and between them we deposit a insulating layer with the assistance of a spin-coater. For bottom electrodes we used silver nanoparticles ink, which was LIFT transferred and laser sintered in order to be conductive and form two pads with a line to connect them. The top electrode is a transferred graphene array with dimension at 1 mm2 and the insulating layer we used is an in-house fabricated PDMS, which was deposited on the half of the bottom electrodes. The quality and quantity of graphene layers were measured and characterized with Scanning Electron Microscopy and micro-Raman spectroscopy. Electrical measurements were conducted on the same samples in order to receive the sheet resistance and the capacitance values. The capacitance measurements were conducted at the top electrode of the flexible touch sensor. Finally, we will introduce the next step in the demonstration of the touch sensor, which is the application of tensile or compressive stress on the flexible touch sensor, which will result in accumulation of different capacitance.

Graphene field-effect transistor (GFET) biosensors exhibit high sensitivity due to a large surface-to-volume ratio and the high sensitivity of the Fermi level to the presence of charged biomolecules near the surface. For most reported GFET biosensors, bulky external reference electrodes are used which prevent their full-scale chip integration and contribute to higher costs per test. In this study, GFET arrays with on-chip integrated liquid electrodes were employed for COVID-19 detection and functionalized with either antibody or aptamer to selectively bind the spike proteins of SARS-CoV-2. In the case of the aptamer-functionalized GFET (aptasensor, Apt-GFET), the limit-of-detection (LOD) achieved was about 103 particles per mL for virus-like particles (VLPs) in clinical transport medium, outperforming the Ab-GFET biosensor counterpart. In addition, the aptasensor achieved a LOD of 160 aM for COVID-19 neutralizing antibodies in serum. The sensors were found to be highly selective, fast (sample-to-result within minutes), and stable (low device-to-device signal variation; relative standard deviations below 0.5%). A home-built portable readout electronic unit was employed for simultaneous real-time measurements of 12 GFETs per chip. Our successful demonstration of a portable GFET biosensing platform has high potential for infectious disease detection and other health-care applications.

Abstract We report real-space near-field images of mid-infrared (IR) surface plasmon-polariton (SPP) waves in the insulating/metal/insulating (IMI) heterostructure: hexagonal boron nitride/gold/silicon dioxide (hBN/Au/SiO2). The SPPs are observed in the 750 – 1500 cm-1 (~13.3 – ~6.7 μm) range, feature micrometer-sized wavelengths and propagation lengths (LSPP) exceeding 20 μm at room temperature. Comparatively, real-space mapping of SPP waves in the mid-IR has been shown only in graphene, but with nanometer sized-wavelength and LSPP ~ 10 μm at cryogenic temperatures. Interestingly, we show interference between different polariton types in the IMI since the lower momenta SPPs in the metal surface interfere with higher momenta hyperbolic phonon polaritons (HPhP) in the hBN top layer creating SPP-HPhP overlapped waves. In agreement with theory, we quantify momentum and damping governing the SPP waves. Tunability is discussed upon changing the IMI heterostructure. Our theory predicts SPP group velocities reaching 20 % of the light velocity in vacuum and 0.2 – 0.4 ps lifetimes. We further demonstrate that the SPP waves interact with SiO2 and hBN phonons in the strong coupling regime. As a general effect of the metal/dielectric interface, the mid-IR SPP waves can be compelling for fast metal-based plasmonics, whilst their ability to strongly couple to phonons can be further explored for enhanced sensing in the mid-IR.