pH sensitivity of edge-gated graphene field-effect devices with covalent edge-functionalization

We present here a new strategy for a field-effect device, termed graphene edge field-effect transistor (GrEdge-FET), where a micron-wide graphene monolayer is gated exclusively through its edge in an aqueous environment. This is achieved by passivating the basal plane selectively using photolithography. We observe a field-effect behavior in buffer solutions with an ON/OFF ratio of nearly 10 in a small gate-voltage range (+/- 0.5 V) without any need for complex nanofabrication or specialized electrolytes. We attribute this effect to the electrical double layer capacitance at the edge-electrolyte interface, which efficiently gates the entire graphene sheet although it acts only at the edge. We demonstrate that GrEdge-FET devices find applications as pH sensors. Through diazonium electrochemistry, the edges are functionalized persistently with substituted phenyl moieties, which renders the devices with a higher pH sensitivity than classical graphene FETs. Moreover, since only the edge is modified, the favorable field-effect behavior is preserved, despite the covalent nature of attachment of the functional groups

Size-Induced Enhancement of Carrier Density, LSPR Quality Factor, and Carrier Mobility in Cr–Sn Doped In₂O₃ Nanocrystals

Heterovalent dopant ions, such as Sn⁴⁺, in In₂O₃ nanocrystals (NCs) provide free electrons for localized surface plasmon resonance (LSPR). But the same heterovalent dopants act as electron scattering centers, both independently and by forming complexes with interstitial oxygen, thereby increasing LSPR line width. Also, such complexes decrease free carrier density. These detrimental effects diminish the figure-of-merit of LSPR known as the quality factor (Q-factor). Herein, we designed colloidal Cr–Sn codoped In₂O₃ NCs, where both high carrier density and low carrier scattering can be achieved simultaneously, yielding a high LSPR Q-factor of 7.2, which is a record high number compared to prior reports of doped In₂O₃ NCs. Q-factors increase systematically from 3.2 for 6.6% Sn doped In₂O₃ NCs to 7.2 for 23.8% Cr–6.6% Sn codoped In₂O₃ NCs by increasing the Cr codoping concentration, which is also accompanied by an increase in NC size from 6.7 to 22.1 nm. Detailed characterization and analysis of LSPR spectra using Drude model suggest that the increase in NC size (induced by Cr codoping) is mainly responsible for the enhanced LSPR Q-factor. Sn⁴⁺ dopants on the surface of NCs are more vulnerable to form irreducible complexes with interstitial oxide ions, compared to Sn⁴⁺ ions in the core. Therefore, an increase in the concentration ratio of [Sn_core]/[Sn_surface] (or [Sn]/[interstitial oxide]) by increasing the size of NCs, increases the carrier density. Furthermore, such increase in both NC size and Cr doping influences multiple factors reducing the scattering of charge carriers, thereby increasing the optical carrier mobility. This unique combination, which increases both the density and mobility of charge carriers, improves the LSPR Q-factor