Prof. Pedro Alpuim
Braga, Portugal
P. Alpuim holds a Ph.D. in Materials Engineering from IST-Lisbon (2003), where he worked on thin-film silicon devices on plastic substrates for flexible electronics. Since then, he has been a professor at the Physics Department of the University of Minho. He is also with the International Iberian Nanotechnology Laboratory (INL), Braga, where he has been a group leader since 2016. His group grows graphene and other 2D materials by CVD and LPE, fabricating bio-sensing devices at the wafer scale, different environmental sensors, and optoelectronic devices. His research interests include single photon emitters from hBN, and devices based on light-matter interactions.
Talk title: Advantages and challenges of graphene transistors for biosensing
Biosensing graphene transistors, with their promising potential, are a significant area of research. Their inherent 2D nature, high carrier field-effect mobility and ambipolar transport, chemical inertness and robustness, and the possibility of surface functionalization make them a compelling choice. When operated as liquid-gate transistors (LGFET), they open a new era in label-free device sensitivity and limit of detection, reaching concentration values inaccessible to other technologies [1]. Their integration with microfluidic platforms or other sample-delivering methods enables point-of-care and point-of-use platforms with unprecedented sensitivity in low power consumption real-time measurements.
The sensing principle, local gating by analyte molecules whenever they attach to the graphene channel, modulates its Fermi energy (EF), causing a shift in the transistor transfer curve, typically detected by measuring the Dirac point voltage, the point of minimum conductance in the curve [1]. However, electronic devices based on polycrystalline 2D materials often suffer from electrical instability due to the interaction of their charge carriers with the defects in the surrounding insulator layers [2]. In LGFETs, the charges trapped in the SiO2 layer underneath the graphene channel can provoke hysteresis and drift in the transistor output due to uncontrolled doping [2], hindering biodetection at low analyte concentrations.
Here, we present a complete model for the response of an LGFET, showing that, at the transistor operating voltages, the graphene EF sits within the oxide bandgap and between the two SiO2 defect bands [3]. The electron capture and emission rates from and to the defect bands depend on the energy barrier between the graphene EF and the defect, the distance of the defect from the surface, and the strength of the electron-phonon coupling. The model is validated against experiments with devices immersed in a 10 mM phosphate buffer solution. We also show that operating the LGFET in AC mode is another way to minimize the drift. Examples of neurotransmitter detection in the brain of animal models will be shown.
[1] R. Campos et al, ACS Sensors, 4 (2019) 286-293
[2] Y. Y. Illarionov et al., Nature Communications, 11 (2020) 3385
[3] T. Knobloch et al, Nature Electronics, 5 (2022) 356-366
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