A 20 × 20 pixel pressure sensor array based on a printed active‐matrix single‐wall carbon‐nanotube thin‐film transistor backplane is presented. Using a gravure printing process that is compatible ...with fully printed large‐area roll‐to‐roll processing, a 97% device yield is obtained on the 400‐transistor backplane. As a proof of concept, pressure sensors are integrated to map the applied tactile pressure across the array.
Near-unity photoluminescence quantum yield in MoS2 Matin Amani; Der-Hsien, Lien; Kiriya, Daisuke ...
Science (American Association for the Advancement of Science),
11/2015, Letnik:
350, Številka:
6264
Journal Article
Recenzirano
The confined layers of molybdenum disulphide (MoS2) exhibit photoluminescence that is attractive for optolectronic applications. In practice, efficiencies are low, presumably because defects trap ...excitons before they can recombine and radiate light. Amani et al. show that treatment of monolayer MoS2 with a nonoxidizing organic superacid, bis(trifluoromethane) sulfonimide, increased luminescence efficiency in excess of 95%. The enhancement mechanism may be related to the shielding of defects, such as sulfur vacancies. Science, this issue p. 1065 Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude. The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8 ± 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.
Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable ...their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
Gold‐mediated exfoliation of ultralarge optoelectronically perfect monolayers with lateral dimensions up to ≈500 μm is reported. Electrical, optical, and X‐ray photoelectron spectroscopy ...characterization show that the quality of the gold‐exfoliated flakes is similar to that of tape‐exfoliated flakes. Large‐area flakes allow manufacturing of large‐area monolayer transition metal dichalcogenide electronics.
Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe2 via NO x chemisorption at 150 °C is explored, ...with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe2–x–y O x N y species both on the surface and interface between layers upon chemisorption reaction. Ab initio simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe2:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼1019 cm–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe2 thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe2. More generally, the work presents a platform for manipulating the electrical properties and band structure of TMDCs using covalent functionalization.
Molybdenum disulfide (MoS2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS2 ...because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm2 of HER current density.
We report hysteresis-free carbon nanotube thin-film transistors (CNT-TFTs) employing a fluorocarbon polymer (Teflon-AF) as an encapsulation layer. Such fluorocarbon encapsulation improves device ...uniformity with excellent operation stability in air and even in water. The fluoropolymers possess high hydrophobicity for efficient removal of water molecules from the vicinity of nanotubes, which are known to induce charge trapping. In addition, the strong dipole associated with the carbon–fluorine bonds can provide effective screening of the charge carriers in nanotubes from various trap states in the substrate. We also report on the extended applications of encapsulation with Teflon-AF for the realization of dual-gate CNT-TFTs, where one gate is used to control the threshold voltage and the other to switch the device. The fluorocarbon encapsulation technique presents a promising approach for enhanced device reliability, which is critical for future system-level electronics based on CNTs.
Air-stable n-doping of carbon nanotubes is presented by utilizing SiN x thin films deposited by plasma-enhanced chemical vapor deposition. The fixed positive charges in SiN x , arising from +SiN3 ...dangling bonds induce strong field-effect doping of underlying nanotubes. Specifically, an electron doping density of ∼1020 cm–3 is estimated from capacitance voltage measurements of the fixed charge within the SiN x . This high doping concentration results in thinning of the Schottky barrier widths at the nanotube/metal contacts, thus allowing for efficient injection of electrons by tunnelling. As a proof-of-concept, n-type thin-film transistors using random networks of semiconductor-enriched nanotubes are presented with an electron mobility of ∼10 cm2/V s, which is comparable to the hole mobility of as-made p-type devices. The devices are highly stable without any noticeable change in the electrical properties upon exposure to ambient air for 30 days. Furthermore, the devices exhibit high uniformity over large areas, which is an important requirement for use in practical applications. The work presents a robust approach for physicochemical doping of carbon nanotubes by relying on field-effect rather than a charge transfer mechanism.
A matter of orientation: The nanofibers in cables that consist of assemblies of these nanofibers can be oriented parallel or perpendicular to the longitudinal axis by regulating the fluidic ...velocities of the core and sheath flows in coaxial microfluidic devices (see picture). Control of the internal morphology enables fabrication of cables with improved electrical conductivity and mechanical properties.
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