Optimum Choice of Randomly Oriented Carbon Nanotube Networks for UV-Assisted Gas Sensing Applications.

We investigated the noise and photoresponse characteristics of various optical transparencies of nanotube networks to identify an optimal randomly oriented network of carbon nanotube (CNT)-based devices for UV-assisted gas sensing applications. Our investigation reveals that all of the studied devices demonstrate negative photoconductivity upon exposure to UV light. Our studies confirm the effect of UV irradiation on the electrical properties of CNT networks and the increased photoresponse with decreasing UV light wavelength. We also extend our analysis to explore the low-frequency noise properties of different nanotube network transparencies. Our findings indicate that devices with higher nanotube network transparencies exhibit lower noise levels. We conduct additional measurements of noise and resistance in an ethanol and acetone gas environment, demonstrating the high sensitivity of higher-transparent (lower-density) nanotube networks. Overall, our results indicate that lower-density nanotube networks hold significant promise as a viable choice for UV-assisted gas sensing applications.

Organic Vapor Sensing Mechanisms by Large-Area Graphene Back-Gated Field-Effect Transistors under UV Irradiation.

The gas sensing properties of graphene back-gated field-effect transistor (GFET) sensors toward acetonitrile, tetrahydrofuran, and chloroform vapors were investigated with the focus on unfolding possible gas detection mechanisms. The FET configuration of the sensor device enabled gate voltage tuning for enhanced measurements of changes in DC electrical characteristics. Electrical measurements were combined with a fluctuation-enhanced sensing methodology and intermittent UV irradiation. Distinctly different features in 1/f noise spectra for the organic gases measured under UV irradiation and in the dark were observed. The most intense response observed for tetrahydrofuran prompted the decomposition of the DC characteristic, revealing the photoconductive and photogating effect occurring in the graphene channel with the dominance of the latter. Our observations shed light on understanding surface processes at the interface between graphene and volatile organic compounds for graphene-based sensors in ambient conditions that yield enhanced sensitivity and selectivity.

Nature of the 1/f noise in graphene-direct evidence for the mobility fluctuation mechanism.

The nature of the low-frequency 1/f noise in electronic materials and devices is one of the oldest unsolved physical problems (f is the frequency). The fundamental question of the noise source-fluctuations in the mobility vs. number of charge carriers-is still debated. While there are several pieces of evidence to prove that the 1/f noise in semiconductors is due to the fluctuations in the number of the charge carriers, there is no direct evidence of the mobility fluctuations as the source of 1/f noise in any material. Herein, we measured noise in an h-BN encapsulated graphene transistor under the conditions of geometrical magnetoresistance to directly assess the mechanism of low-frequency electronic current fluctuations. It was found that the relative noise spectral density of the graphene resistance fluctuations depends non-monotonically on the magnetic field (B) with a minimum at approximately µ0B ≅ 1 (µ0 is the electron mobility). This observation proves unambiguously that mobility fluctuations are the dominant mechanism of electronic noise in high-quality graphene. Our results are important for all proposed applications of graphene in electronics and add to the fundamental understanding of the 1/f noise origin in any electronic device.

Graphene/AlGaN/GaN RF Switch.

RF switches, which use a combination of graphene and two-dimensional high-density electron gas (2DEG) in the AlGaN/GaN system, were proposed and studied in the frequency band from 10 MHz to 114.5 GHz. The switches were integrated into the coplanar waveguide, which allows them to be used in any system without the use of, e.g., bonding, flip-chip and other technologies and avoiding the matching problems. The on-state insertion losses for the designed switches were measured to range from 7.4 to 19.4 dB, depending on the frequency and switch design. Although, at frequencies above 70 GHz, the switches were less effective, the switching effect was still evident with an approximately 4 dB on-off ratio. The best switches exhibited rise and fall switching times of ~25 ns and ~17 ns, respectively. The use of such a switch can provide up to 20 MHz of bandwidth in time-modulated systems, which is an outstanding result for such systems. The proposed equivalent circuit describes well the switching characteristics and can be used to design switches with required parameters.

Double-Quantum-Well AlGaN/GaN Field Effect Transistors with Top and Back Gates: Electrical and Noise Characteristics.

AlGaN/GaN fin-shaped and large-area grating gate transistors with two layers of two-dimensional electron gas and a back gate were fabricated and studied experimentally. The back gate allowed reducing the subthreshold leakage current, improving the subthreshold slope and adjusting the threshold voltage. At a certain back gate voltage, transistors operated as normally-off devices. Grating gate transistors with a high gate area demonstrated little subthreshold leakage current, which could be further reduced by the back gate. The low frequency noise measurements indicated identical noise properties and the same trap density responsible for noise when the transistors were controlled by either top or back gates. This result was explained by the tunneling of electrons to the traps in AlGaN as the main noise mechanism. The trap density extracted from the noise measurements was similar or less than that reported in the majority of publications on regular AlGaN/GaN transistors.

Electrically Insulating Flexible Films with Quasi-1D van der Waals Fillers as Efficient Electromagnetic Shields in the GHz and Sub-THz Frequency Bands.

Polymer composite films containing fillers comprising quasi-1D van der Waals materials, specifically transition metal trichalcogenides with 1D structural motifs that enable their exfoliation into bundles of atomic threads, are reported. These nanostructures are characterized by extremely large aspect ratios of up to ≈106 . The polymer composites with low loadings of quasi-1D TaSe3 fillers (<3 vol%) reveal excellent electromagnetic interference shielding in the X-band GHz and extremely high frequency sub-THz frequency ranges, while remaining DC electrically insulating. The unique electromagnetic shielding characteristics of these films are attributed to effective coupling of the electromagnetic waves to the high-aspect-ratio electrically conductive TaSe3 atomic-thread bundles even when the filler concentration is below the electrical percolation threshold. These novel films are promising for high-frequency communication technologies, which require electromagnetic shielding films that are flexible, lightweight, corrosion resistant, inexpensive, and electrically insulating.

Graphene as a Schottky Barrier Contact to AlGaN/GaN Heterostructures.

Electrical and noise properties of graphene contacts to AlGaN/GaN heterostructures were studied experimentally. It was found that graphene on AlGaN forms a high-quality Schottky barrier with the barrier height dependent on the bias. The apparent barrier heights for this kind of Schottky diode were found to be relatively high, varying within the range of φb = (1.0-1.26) eV. AlGaN/GaN fin-shaped field-effect transistors (finFETs) with a graphene gate were fabricated and studied. These devices demonstrated ~8 order of magnitude on/off ratio, subthreshold slope of ~1.3, and low subthreshold current in the sub-picoamperes range. The effective trap density responsible for the 1/f low-frequency noise was found within the range of (1-5) · 1019 eV-1 cm-3. These values are of the same order of magnitude as reported earlier and in AlGaN/GaN transistors with Ni/Au Schottky gate studied as a reference in the current study. A good quality of graphene/AlGaN Schottky barrier diodes and AlGaN/GaN transistors opens the way for transparent GaN-based electronics and GaN-based devices exploring vertical electron transport in graphene.

Graphene Epoxy-Based Composites as Efficient Electromagnetic Absorbers in the Extremely High-Frequency Band.

We report on the synthesis of the epoxy-based composites with graphene fillers and test their electromagnetic shielding efficiency by the quasi-optic free-space method in the extremely high-frequency (EHF) band (220-325 GHz). The curing adhesive composites were produced by a scalable technique with a mixture of single-layer and few-layer graphene layers of few-micrometer lateral dimensions. It was found that the electromagnetic transmission, T, is low even at small concentrations of graphene fillers: T<1% at a frequency of 300 GHz for a composite with only ϕ = 1 wt% graphene. The main shielding mechanism in composites with the low graphene loading is absorption. The composites of 1 mm in thickness and a graphene loading of 8 wt% provide an excellent electromagnetic shielding of 70 dB in the sub-terahertz EHF frequency band with negligible energy reflection to the environment. The developed lightweight adhesive composites with graphene fillers can be used as electromagnetic absorbers in the high-frequency microwave radio relays, microwave remote sensors, millimeter wave scanners, and wireless local area networks.

Terahertz 3D printed diffractive lens matrices for field-effect transistor detector focal plane arrays.

We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type - diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.