1,721,005 research outputs found
Light effects on graphene/tungsten disulfide nanotubes/graphene heterostructure
No full textIn this study, we present a hybrid optoelectronic device consisting of a tungsten disulfide nanotubes deposited on graphene electrodes, forming ohmic contacts that enable efficient charge transport. The heterostructure is fabricated on a flexible polyethylene terephthalate substrate. Comprehensive electrical and optoelectronic characterizations are conducted under various environmental conditions, with a focus on photocurrent response and the photovoltaic effect. The device shows a broadband photoresponse from 405 to 900 nm, reaching its best performance at 880 nm, where it delivers a peak responsivity of 0.07 mA/W, a specific detectivity of 2.3 × 107 Jones and rise/decay constants of 1.6 s / 1.5 s, measured under 405 nm illumination at an incident power of 0.19 mW (there is also a long time tail of 23 s, attributed to trap-assisted processes). The long-wavelength cut-off (~ 880 nm) corresponds to an indirect bandgap of 1.4 ± 0.1 eV for the nanotubes. Under 520 nm illumination the heterostructure generates an open circuit photovoltage of ~ 15 mV and a short-circuit photocurrent of ~ 0.08 nA, confirming the presence of a photovoltaic effect. Illumination at 405 nm reveals a photocurrent response that is sensitive to changes in environmental pressure. These results highlight the multifunctionality of the heterostructure, which can be optimized for photovoltaic conversion, wearable photodetectors, and sensing applications
Electrical conduction and photoconduction in PtSe2 ultrathin films
No full textPlatinum diselenide (PtSe2) is one of the most studied materials of the emerging group-10 transition-metal dichalcogenides, with interesting chemical and physical properties as the semimetal-to-semiconductor transition when approaching the monolayer thickness.
In this work, we investigate the electrical conduction and the photoconduction in ultrathin films of PtSe2 synthesized by direct selenization of platinum deposited onto SiO2/Si substrates. The PtSe2 ultrathin films are exploited as the channel of back-gated field-effect transistors (FETs) and their electric conductance is investigated at different temperatures and pressures as well as under the irradiation of a super-continuous white light source. The increasing conductance with raising temperature confirms the semiconducting nature of the PtSe2 film, while the gate modulation reveals p-type conduction with hole field-effect mobility up to 40 cm2/(Vs). The PtSe2 conductivity is higher in air than in high vacuum due to the p-doping effect of oxygen. Moreover, electrical conduction measured along different directions shows isotropic transport ascribed to the polycrystalline structure of the film.
A reduction of the PtSe2 conductance (negative photoconductivity) is observed under exposure to light in air, while positive or negative photoconductivity is observed in vacuum depending on the intensity of the light and the pressure. Such a behavior can be explained by the combination of the photogating effect caused by photo-charge accumulation in the SiO2 dielectric and the adsorption/desorption of adsorbates
2D materials in field-effect transistors: effects of pressure, temperature and e-beam irradiation
No full text2D materials have been the subject of intense research in the past decade for their electrical behavior tunable by the number of layers, the strong interaction with light, the absence of dangling bonds enabling the formation of van der Waals heterostructures, the mechanical strength and the chemical stability.
In this study we fabricate back-gate field-effect transistors using nanosheets of layered materials such as MoS2, PdSe2 or GeAs, deposited on SiO2/Si substrate and contacted with different metals. We report the effect of gas pressure and e-beam irradiation on the electrical transport of MoS2 and PdSe2 nanosheets. We show that gas adsorption enhances the hysteresis in the transfer characteristics of the transistors and that gas pressure controls the polarity of the devices, making n-type conduction dominant in a high vacuum. We exploit the n-type conduction to extract a field emission current in high vacuum. We also investigate the temperature dependence of the electrical conductivity of GeAs nanosheets and report an anomalous peak in the 2D carrier density at temperature around 75 K, that we interpret as the manifestation of a 2D conduction phenomenon
Graphene–Silicon Device for Visible and Infrared Photodetection
Full text linkThe fabrication of a graphene-silicon (Gr-Si) junction involves the formation of a parallel metal-insulator-semiconductor (MIS) structure, which is often disregarded but plays an important role in the optoelectronic properties of the device. In this work, the transfer of graphene onto a patterned n-type Si substrate, covered by Si3N4, produces a Gr-Si device, in which the parallel MIS consists of a Gr-Si3N4-Si structure surrounding the Gr-Si junction. The Gr-Si device exhibits rectifying behavior with a rectification ratio up to 104. The investigation of its temperature behavior is necessary to accurately estimate the Schottky barrier height (SBH) at zero bias, φb0 = 0.24 eV, the effective Richardson's constant, A* = 7 × 10-10 AK-2 cm-2, and the diode ideality factor n = 2.66 of the Gr-Si junction. The device is operated as a photodetector in both photocurrent and photovoltage mode in the visible and infrared (IR) spectral regions. A responsivity of up to 350 mA/W and an external quantum efficiency (EQE) of up to 75% are achieved in the 500-1200 nm wavelength range. Decreases in responsivity to 0.4 mA/W and EQE to 0.03% are observed above 1200 nm, which is in the IR region beyond the silicon optical band gap, in which photoexcitation is driven by graphene. Finally, a model based on two parallel and opposite diodes, one for the Gr-Si junction and the other for the Gr-Si3N4-Si MIS structure, is proposed to explain the electrical behavior of the Gr-Si device
Characterization of the electric transport properties of black phosphorous back-gated field-effect transistors
Full text linkWe use thin layers of exfoliated black phosphorus to realize back-gated field-effect transistors in which the Si/SiO2 substrate is exploited as gate electrode. To prevent the detrimental effect of the air exposure the devices are protected by Poly(methyl methacrylate).
We report the observation of an improved contact resistance at the interface between the layered material and the metal contact by electrical conditioning. We also demonstrate the existence of a hysteresis in the transfer characteristics that improves by increasing the gate voltage sweep
range. Finally, we prove the suitability of such transistors as memory devices
3D porous laser-induced graphene coated sponges for field emission devices and temperature/pressure sensors
No full textRecent interest in flexible sensors, fueled by their affordability, wearability, lightweight design, and ease of fabrication, has driven advancements in applications and fundamental understanding. Herein, we explore the synthesis route of the three-dimensional (3D) graphene-coated sponges and investigate their mechanical and electronic transport properties. Tensile and compression tests on the graphene coated sponges demonstrate Young's modulus of around 0.075 MPa. Electrical measurements with ohmic contacts show DC conductivity as low as 0.5 S/cm. Bonding durability and wettability tests under water immersion and ultrasonic agitation confirmed the strong adhesion and enhanced hydrophobicity of the graphene coating, demonstrating its mechanical and chemical robustness. Temperature measurements reveal a non-monotonic behavior in the sponge's resistance as the temperature decreases. The resistance exhibits a pronounced peak around 250 K as the temperature drops from 295 K to 200 K, followed by a steady increase from 200 K to 77 K. Field emission measurements show a stable current and a reduction in turn-on voltage as the spacing between the anode and the emitting surface decreases, revealing a low turn-on voltage of about 13 V and a field enhancement factor of 286 at an anode-cathode distance of 300 nm. Experimental data are analyzed using the Fowler-Nordheim model, evidencing a non-monotonic dependence of the field enhancement factor on the cathode-anode separation distance in the range of 100–500 nm. The results show that such a flexible 3D graphene coated sponge can be utilized as a sensitive thermistor, a field emitter, and a pressure sensor
Temperature Dependence of Germanium Arsenide Field-Effect Transistors Electrical Properties
No full textIn this work, we report the fabrication of germanium arsenide (GeAs) field-effect transistors with ultrathin channel and their electrical characterizations in a wide temperature range, from 20 K to 280 K. We show that the p-type electrical conductivity and the field effect mobility of GeAs transistors increase with the temperature and that at lower temperatures the electrical conduction of the GeAs channel is dominated by the 3D variable range hopping but becomes band-type at higher temperatures, after the formation of a highly conducting two-dimensional (2D) channel. The presence of this 2D channel, limited to few interfacial GeAs layers, is confirmed by the observation of an unexpected peak in the temperature dependence of the carrier density per area at about 75 K. Such a feature is explained considering a model based on a temperature-dependent channel thickness. Indeed, at higher temperatures, the carrier injection from the contacts increase and the ionization of defects is favoured enabling the formation of a 2D highly conductive channel close to the dielectric interface, which screens the electric field from the gate and confine it to the first few layers of the material. The formation of the 2D channel is corroborated by numerical simulations, that show excellent agreement with the experimental data, and by the estimation of 0.4 nm Debye screening length at room temperature
Molybdenum Disulfide Field Effect Transistors under Electron Beam Irradiation and External Electric Fields
No full textIn this work, monolayer molybdenum disulfide (MoS2) nanosheets, obtained via chemical vapor deposition onto SiO2/Si substrates, are exploited to fabricate field-effect transistors with n-type conduction, high on/off ratio, steep subthreshold slope and good mobility. We study their electric characteristics from 10-6 Torr to atmospheric air pressure. We show that the threshold voltage of the transistor increases with the growing pressure.
Moreover, Schottky metal contacts in monolayer MoS2 field-effect transistors (FETs) are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is shown that e-beam irradiation lowers the Schottky barrier at the contacts due to thermally induced atom diffusion and interfacial reactions. The study demonstrates that electron beam irradiation can be effectively used for contact improvement though local annealing.
It is also demonstrated that the application of an external field by a metallic nanotip induces a field emission current, which can be modulated by the voltage applied to the Si substrate back-gate. Such a finding, that we attribute to gate-bias lowering of the MoS2 electron affinity, enables a new field-effect transistor based on field emission
Electron field emission of water-based inkjet printed graphene films
No full textSolution-processed graphene is extremely attractive for the realization of large area and patterned graphene films for field emitting devices. Previous studies have focussed only on the use of reduced graphene oxide; however, solution-processed graphene can also be produced by other approaches, giving rise to nanosheets with different surface chemistries and lateral and thickness distributions. Here, we report the field emission characterization of films made of water-based graphene ink, prepared by liquid phase exfoliation, and inkjet printed with an area of 2.5 mm2 on silicon (Si/SiO2) substrates. These films show excellent field emission properties, comparable to those measured on single flakes and carbon nanotubes with the same setup, and they show a remarkably high maximum current density (up to similar to 723 A cm-2), making them very attractive for field emission devices
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