SnSe2 field-effect transistors fabricated using mechanical exfoliation are reported. Substrate-gated devices with source-to-drain spacing of 0.5 μm have been fabricated with drive current of 160 μA/μm at T = 300 K. The transconductance at a drain-to-source voltage of Vds = 2 V increases from 0.94 μS/μm at 300 K to 4.0 μS/μm at 4.4 K, while the field-effect mobility increases from 8.6 cm2/Vs at 300 K to 28 cm2/Vs at 77 K. The conductance at Vds = 50 mV shows an activation energy of only 5.5 meV, indicating the absence of a significant Schottky barrier at the source and drain contacts.

Understanding the interactions of ambient molecules with graphene and adjacent dielectrics is of fundamental importance for a range of graphene-based devices, particularly sensors, where such interactions could influence the operation of the device. It is well-known that water can be trapped underneath graphene and its host substrate, however, the electrical effect of water beneath graphene and the dynamics of how it changes with different ambient conditions has not been quantified. Here, using a metal-oxide-graphene variable-capacitor (varactor) structure, we show that graphene can be used to capacitively sense the intercalation of water between graphene and HfO2 and that this process is reversible on a fast time scale. Atomic force microscopy is used to confirm the intercalation and quantify the displacement of graphene as a function of humidity.Density functional theory simulations are used to quantify the displacement of graphene induced by intercalated water and also explain the observed Dirac point shifts as being due to the combined effect of water and oxygen on the carrier concentration in the graphene. Finally, molecular dynamics simulations indicate that a likely mechanism for the intercalation involves adsorption and lateral diffusion of water molecules beneath the graphene. § Equal contribution Keywords: graphene, sensor, varactor, capacitance, water 2 The successful exfoliation of single-layer graphene and subsequent development of chemical vapor deposition (CVD) for producing large-area graphene sheets has resulted in many interesting device applications. Its use in field-effect transistors, 1 mixers, 2 optical modulators, 3 photodetectors, 4 and a wide variety of chemical sensors is of particular note. [5][6][7][8][9] In nearly all of these device concepts, the intimate interactions between graphene and adjacent dielectrics is critical, yet has not been explored in detail. For instance, it has been shown previously, using density functional theory (DFT) simulations, that the equilibrium distance between graphene and HfO2 is 0.30 nm. 10 However, it has also been shown that this equilibrium distance can change in the presence of defects on graphene or in the adjacent dielectrics, as the defects create bonding sites that result in stronger coupling between graphene and HfO2. 10 The intimate surface interactions can become even more complex with the introduction of small molecules such as H2O, 11 which can often be trapped between the graphene and the adjacent surface. Previously, atomic-force-microscopy (AFM) studies have shown that exfoliated graphene on mica can visualize the trapped water underneath due to the displacement of graphene. 12 However, to date, these trapped molecules have only been probed using physical analysis techniques such as AFM. 12,13 It would be extremely useful if such molecular interactions could be probed using electrical techniques, as such methods could allow a greatly improved understanding of the dynamics of these processes.We have recently proposed a capacitanc...

A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 + 0.3 kHz / RH%. The capacitance values extracted from the wireless measurements agree with those determined from capacitancevoltage measurements, providing strong evidence that the sensing arises from the variable quantum capacitance in graphene. These results represent a new sensor transduction mechanism and pave the way for graphene quantum capacitance sensors to be studied for a wide range of chemical and biological sensing applications.

The concentration-dependent density of states in graphene allows the capacitance in metal-oxide-graphene structures to be tunable with the carrier concentration. This feature allows graphene to act as a variable capacitor (varactor) that can be utilized for wireless sensing applications. Surface functionalization can be used to make graphene sensitive to a particular species. In this manuscript, the effect on the quantum capacitance of noncovalent basal plane functionalization using 1-pyrenebutanoic acid succimidyl ester and glucose oxidase is reported. It is found that functionalized samples tested in air have (1) a Dirac point similar to vacuum conditions, (2) increased maximum capacitance compared to vacuum but similar to air, (3) and quantum capacitance "tuning" that is greater than that in vacuum and ambient atmosphere. These trends are attributed to reduced surface doping and random potential fluctuations as a result of the surface functionalization due to the displacement of H2O on the graphene surface and intercalation of a stable H2O layer beneath graphene that increases the overall device capacitance. The results are important for future application of graphene as a platform for wireless chemical and biological sensors.

The operation of multi-finger graphene quantum capacitance varactors fabricated using a planarized local bottom gate electrode, HfO 2 gate dielectric, and large-area graphene is described. As a function of the gate bias, the devices show a room-temperature capacitance tuning range of 1.22-1 over a voltage range of 62 V. An excellent theoretical fit of the temperature-dependent capacitance-voltage characteristics is obtained when random potential fluctuations with standard deviation of 65 mV are included. The results represent a first step in realizing graphene quantum capacitance varactors for wireless sensing applications. V C 2012 American Institute of Physics.Graphene, a two-dimensional sp 2 -bonded allotrope of carbon, has many unique properties that make it an interesting material for electronic device applications. 1-6 A particular characteristic that has been of interest lately is the quantum capacitance effect, which is especially prominent in graphene due to its very low density of states (DOS). This property, while being studied by numerous groups, 7-19 has only recently been proposed as the operational mechanism of an electronic device. 19 In Ref. 19, it was proposed that a metal-oxide-graphene capacitor could be utilized as the variable capacitor in a passive LC sensing circuit, where the high mobility in graphene would enable high quality factor (Q) to be achieved. Such sensors have the potential to be much smaller and have higher tuning range than resonators based upon micro-electro-mechanical systems (MEMS). 20,21 In order to create a practical graphene varactor, large-area graphene 22 must be transferred onto a local metal gate electrode, upon which thin, high-j dielectric material has been deposited. In addition, a multi-finger geometry must be utilized in order to allow high capacitances to be achieved while minimizing series resistance. In this paper, we report the fabrication and operation of a graphene quantum capacitance varactor fabricated using a planarized local bottom electrode, HfO 2 gate dielectric, and single-layer graphene grown by chemical vapor deposition (CVD).The device structure is shown in Fig. 1(a). The fabrication started by growing 90 nm of thermal SiO 2 on an n-type Si wafer. After patterning the gate resist using optical contact lithography, the SiO 2 was recessed using a buffered oxide etch, followed by evaporation and lift-off of Ti/Pd (10/40 nm). On top of the resulting planarized gate electrode, 20 nm of HfO 2 was deposited by atomic-layer deposition (ALD) at 300 C. Single-layer graphene grown by CVD on a Cu foil was then coated with PMMA and the Cu removed using FeCl 3 . The graphene was then transferred onto the wafer with the local bottom gate electrode using an aqueous transfer process, and the PMMA removed using solvent cleaning. The graphene was then patterned and etched using an O 2 plasma. Finally, contact electrodes to the graphene consisting of Ti/Au (10/100 nm) were patterned and lifted off completing the fabrication process. An optical micrograph of a compl...