A.A. Balandin

Selective Sensing of Individual Gases Using Graphene Devices

Graphene chemiresistors have enabled gas and vapor detection with high sensitivity. However, changes in graphene resistivity under the equilibrium gas exposure cannot be used to determine both the gas concentration and its type, making the sensing selectivity with resistive detection one of the key barriers to overcome. In this paper, we report on using low frequency noise to define the new characteristic parameters, which, in combination with the resistance changes, form unique gas signatures. The noise measurements can also be used in combination with evaluating “memory step” effect in graphene under gas exposure. The “memory step” is an abrupt change of the current near zero gate bias at elevated temperatures T > 500 K in graphene transistors. The “memory step” in graphene under gas exposure can be also used for high-temperature gas sensors, and is attractive for harsh-environment applications.

High-temperature performance of MoS2 thin-film transistors: Direct current and pulse current-voltage characteristics

We report on fabrication of MoS2 thin-film transistors (TFTs) and experimental investigations of their high-temperature current-voltage characteristics. The measurements show that MoS2 devices remain functional to temperatures of at least as high as 500 K. The temperature increase results in decreased threshold voltage and mobility. The comparison of the direct current (DC) and pulse measurements shows that the direct current sub-linear and super-linear output characteristics of MoS2 thin-films devices result from the Joule heating and the interplay of the threshold voltage and mobility temperature dependences. At temperatures above 450 K, a kink in the drain current occurs at zero gate voltage irrespective of the threshold voltage value. This intriguing phenomenon, referred to as a “memory step,” was attributed to the slow relaxation processes in thin films similar to those in graphene and electron glasses. The fabricated MoS2 thin-film transistors demonstrated stable operation after two months of aging....

Selective Gas Sensing With

Owing to their ultimate surface-to-volume ratio two-dimensional (2D) van der Waals materials are candidates for flexible gas sensor applications. However, all demonstrated devices had relied on direct exposure of the active 2D channel to gases, which presents problems for their reliability and stability. We demonstrated, for the first time, selective gas sensing with molybdenum disulfide (MoS2) thin films transistors capped with a thin layer of hexagonal boron nitride (h-BN). The resistance change, AR/R, was used as a sensing parameter to detect chemical vapors. It was found that h-BN dielectric passivation layer does not prevent gas detection via changes in the current in the MoS2 channel. The detection without direct contacting the channel with analyte molecules was achieved with AR/R ratio as high as 103. In addition, we show that the use of h-BN cap layers (thickness H~10 nm) improves sensor stability and prevents degradation due to environmental and chemical exposure.

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We demonstrate, for the first time, selective gas sensing using MoS2 bilayer Thin Film Transistors (TFTs). The TFTs were fabricated using exfoliation from bulk crystals and transferring onto Si/SiO2 substrates. For control purposes, we used the same TFTs but with the surface covered by a 10 nm Al2O3 layer. The extracted field effect mobility varied from 0.1 to 7 cm2/V-s in different samples and was only a very weakly dependent on temperature in the range from room temperature to 220°C. The room temperature on-to-off ratio was ~ 104 and decreased to 103 at 220°C. Under the exposure to ethanol, acetonitrile, toluene, chloroform, and methanol vapors, the TFT current in uncovered samples changed with the increase amount strongly dependent on the kind of gas. The covered TFTs exhibited no change. We also report on 1/f noise measurement under the gas exposure and show that, just like for graphene transistors, the change in the noise spectra is also dependent on the kind of the sensed gas and could be used as component of the gas signature. The MoS2 devices demonstrate a much larger sensitivity than similar graphene devices.

-BN Capped MoS 2 Heterostructure Thin-Film Transistors

We demonstrate that graphene field effect transistors can operate as both plasmonic and bolometric THz sensors depending on polarization and bias configurations. The plasmonic mechanism of detection is consistent with the overdamped plasma wave response. The bolometric regime is caused by the shift of the Dirac point with temperature and reveals the asymmetry in the electron and hole response associated with long-lived impurity centers responsible for the shift of the Dirac voltage.

Selective gas sensing with MoS 2 thin film transistors

We review results of our studies of the low-frequency electronic noise in quasi-1D TaSe<inf>3</inf> nanowires of. The semi-metallic TaSe<inf>3</inf> is a quasi-1D van der Waals material with an exceptionally high breakdown current density. Our noise studies showed that TaSe<inf>3</inf> nanowires have lower levels of the normalized noise spectral density, S<inf>I</inf>/I², compared to carbon nanotubes and graphene. The temperature-dependent measurements revealed that the low-frequency 1/f noise becomes the 1/f²-type as temperature increases to ∼400 K, suggesting the onset of electromigration. Using the Dutta-Horn random fluctuation model of the electronic noise, we determined that the noise activation energy for quasi-1D TaSe<inf>3</inf> nanowires is approximately E<inf>P</inf>≈1.0 eV. From the empirical noise model for interconnects, the extracted activation energy, related to electromigration, is E<inf>A</inf>=0.88 eV, consistent with that for Cu and Al interconnects. Our results suggest that TaSe<inf>3</inf> nanowires and similar systems have potential for ultimately downscaled local interconnect applications.

Plasmonic and bolometric terahertz graphene sensors

The noise mechanisms in graphene and MoS<inf>2</inf> are quite different. The noise characteristics of MoS<inf>2</inf> transistors could be described by the McWhorter model. The range of the trap densities responsible for the 1/f noise in MoS<inf>2</inf> extracted from the noise measurements is from ∼10¹⁸ eV⁻¹cm⁻³ to ∼ 6×10²⁰ eV⁻¹cm⁻³. The smallest noise level and smallest trap density was found for multilayer MoS<inf>2</inf> transistors with the number of layer N>6. The noise level in graphene, in general, is much smaller than in MoS<inf>2</inf> transistors and does not comply with the McWhorter model. The low frequency noise in high quality graphene transistors might be relatively low and comparable to Si MOSFETs. The contacts have an important effect on the noise in graphene, but the dominant sources of noise in not aged devices is the channel. The gate voltage dependencies of noise in graphene indicate that the mobility fluctuations give the dominant contribution to noise. The noise measurements of electron irradiated graphene devices and measurements in magnetic field confirm this hypothesis. Both in MoS<inf>2</inf> and graphene, the noise decreases with the number, N, of the atomic layers. The 1/f noise in relatively thin graphene devices with N<7 is the surface phenomenon. In the thicker graphene devices the volume mechanisms of noise start to dominate. Gas environment changes the shape of the noise spectra in graphene. Therefore, the low frequency noise measurements, in combination with other sensing parameters, can be used for the selective gas sensing.

Low-frequency noise in quasi-1D TaSe 3 van der Waals nanowires

Raman spectroscopy was used to determine the number of atomic planes in the exfoliated graphene samples and verify the quality of the selected flakes. The optical microscopy images of the exfoliated BN and graphene flakes, and the resulting heterostructure are shown in Figure 2 (b-c). The current-voltage (I-V) characteristics of a representative BN-graphene-BN HFET are shown in Figure 3 (a). Both the effective and field-effect mobility extractions gave consistent results and the room-temperature (RT) mobility was determined to be ~30,000 cm2/Vs at 7·1011 cm-2. The normalized 1/f noise spectral density of the graphene encapsulated device is presented in Figure 3 (b). We found that the channel-area normalized noise spectral density in BN-graphene-BN HFET is factor of ×5 - ×10 smaller than that in typical reference graphene FETs without channel encapsulation. The observed strong noise reduction can be related to screening of the traps in SiO2 by the BN barrier. Other possible physical mechanisms and prospects of further noise suppression will be discussed at the presentation.

Low frequency noise in 2D materials: Graphene and MoS 2

Ballistic electron conduction offers a new possible approach to the development of low-power high-speed logic circuits [1-3]. Ultra-high mobility of graphene allows achieving the ballistic or near-ballistic transport regime at room temperature (RT) in devices with relatively long channels [4-5]. In this presentation, we show that heterostructure field-effect transistors (HFETs) with graphene channel encapsulated between two layers of hexagonal boron nitride (h-BN) combine extremely high electron mobility (~36,000 cm2/Vs) with a strongly reduced 1/f noise level (f is the frequency). Low-frequency 1/f noise hampers the operation of numerous devices and can be a major impediment to development of practical low-power low-voltage applications of graphene and other 2D van der Waals materials [6-8]. The low noise level is beneficial for the low-voltage electronic applications. The prototype h-BN-graphene-h-BN HFETs were fabricated using the mechanically exfoliated h-BN and graphene flakes on Si/SiO2 wafers. The viscoelastic materials adhered to glass slides were used as transparent stamps for the layer transfer. The stamps were spin coated with poly-propylene carbonate (PPC). A micromanipulator was used for careful positioning of h-BN flakes on the stamp over graphene flakes. Following this procedure, the h-BN layer was added to the stack resulting in a fully encapsulated graphene monolayer. The stack was then released onto target Si/SiO2 substrate by heating the stage to an appropriate temperature. The resulting heterostructure was etched to expose the edges of graphene and create one-dimensional Cr/Au (10/100 nm) electrical contacts. Figure 1 shows the schematics and an optical microscopy image of a representative device. Figure 2 (a) shows the current-voltage (I-V) characteristics of the representative HFET. Both the effective and field-effect mobility extractions gave consistent results showing the mobility of ~36,000 cm2/Vs at the carrier concentration of 7×1011 cm2. Figure 2 (b) presents the normalized 1/f noise spectral density of the graphene encapsulated device. The channel-area normalized noise spectral density in BN-graphene-BN HFET is factor of ×5-×10 smaller than that in typical reference graphene FETs without the channel encapsulation. The observed strong noise reduction can be explained by screening of the traps in SiO2 by the BN barrier. Other possible physical mechanisms and prospects of further noise suppression will be discussed at the presentation.

Reduced 1/f noise in high-mobility BN-graphene-BN heterostructure transistors

The studies of the low frequency noise in graphene transistors with the number of carbon layers from N=1 (single layer graphene) to N=15 showed that 1/f noise becomes dominated by the volume noise when the thickness exceeds approximately 7 atomic layers. We compare these results with the data on surface and volume noise in carbon nanotubes and Si MOS.