The Role of the Oxygen/Water Redox Couple in Suppressing Electron Conduction in Field‐Effect Transistors

Abstract

The back-gated field-effect transistor (FET) configuration has been central to the study of the electronic transport properties of organic and nanoscale materials. [1][2][3] Three-terminal transport measurements using this geometry are facilitated by using a degenerately doped silicon wafer covered with a thin silicon oxide layer as the substrate. It has been widely assumed that the substrate did not influence the measurement of the intrinsic properties of the material under study. However, Chua et al. [4] recently demonstrated that changing the nature of the dielectric led to ambipolar behavior in FETs made from organic semiconductors that were previously thought to be exclusively hole (p-type) conductors. Silanol groups present on the SiO 2 surface were singled out as the culprits for generating electron traps responsible for suppressing electron (n-type) conduction in these devices. Here we show that the substrate-induced suppression of n-type behavior is not unique to organic FETs, but influences the measurements of all devices fabricated on SiO 2 /Si substrates. By using carbon nanotubes as the testbed, we investigated the impact of the chemical nature of the substrate and of ambient adsorbates on the field-effect switching behavior of both nanoscale and thin-film FETs. Our study revealed that the reduction of n-type conduction occurs when an adsorbed water layer containing solvated oxygen is present on the SiO 2 surface. This finding demonstrates that an electrochemical charge transfer reaction between the semiconducting channel and the aqueous oxygen redox couple is the underlying phenomenon governing the suppression of electron conduction in carbon nanotube devices. This effect should be taken into account when interpreting three-terminal measurements conducted on SiO 2 /Si substrates. We anticipate that the design of electronic devices, [5] chemical sensors, [6] and biosensors [7] that are based on the FET configuration will be largely influenced by the charge transfer mechanism that has been brought to light by this study.Individual carbon nanotube field-effect transistors (CFETs) are the most extensively studied molecular-scale FETs to date.

Microwave purification of large-area horizontally aligned arrays of single-walled carbon nanotubes

“…The discussions above relate the resistance of the SWNTs to the heating power imparted into the SWNTs. Although s-SWNTs are generally more resistive than m-SWNTs, unintentional doping 51,52 and other effects can diminish these differences. Selectivity in such cases can be greatly enhanced by the Schottky barriers that form at the contacts between the antenna metal and the s-SWNT.…”

“…53). In practice, s-SWNT often are unintentionally doped (for example, oxygen or water) 17,51 , in a way that shifts the Fermi level towards the valence band, to favour hole transport (schematic of band diagram in Supplementary Fig. 15a).…”

Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks

“…The gate leakage current was always found to be at least two orders of magnitude lower than the channel ON current, while the small operating hysteresis is attributed to atmospheric oxidants/adsorbates that are present even within the nitrogen glovebox, albeit at relatively small concentrations (~ppm). [39,40] Finally, the resulted electron and hole mobility values extracted are ~0.03 cm 2 V -1 s -1 and ~0.02 cm 2 V -1 s -1 respectively.…”

“…The charge transport properties of the as-spun (7,5) SWNT networks were also characterized using a bottom-gate, bottom-contact transistor architecture fabricated on Si ++ /SiO2 (400 nm- Previous reports [39][40][41] have in fact shown that n-type conduction is suppressed in most organic semiconductor-based transistors due to charge traps from the SiO2 interface and especially due to charge transfer to the oxygen/water layer that is strongly bound to the SiO2 surface, where the H2O/O2 redox couple occurs. This is particularly important for SWNTs since the valence band position of the (7,5) SWNT (EV ≈ -5.2 eV) lies very close to the redox potential of oxygen (EREDOX ≈ -5.3 eV) dissolved in slightly acidic water (pH = 6) adsorbed on the SiO2 surface.…”

“…[39] This electrochemical charge transfer process is also likely to generate negative chemical intermediates, which in turn create and charge acceptor states on the substrate surface. [39] These surface states may then affect the operating hysteresis that is often seen in bottom-gate SWNTbased transistors made on bare SiO2. In addition to this, another possible cause for the hysteresis is the presence of residues of the cellulose membrane used to filter the nanotubes during the Finally, we remark that both the top-gate and bottom-gate (7,5) SWNT transistors shown in Figure 3 and Figure 4, respectively, have not been optimized for achieving high carrier mobility and low-voltage operation, but have been primarily used to study the impact of the device geometry on charge transport and for the current percolation analysis.…”