Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are ideal candidates to create ultra-thin electronics suitable for flexible substrates. Although optoelectronic devices based on TMDs have demonstrated remarkable performance, scalability is still a significant issue. Most devices are created using techniques that are not suitable for mass production, such as mechanical exfoliation of monolayer flakes and patterning by electron-beam lithography. Here we show that large-area MoS grown by chemical vapor deposition and patterned by photolithography yields highly sensitive photodetectors, with record shot-noise-limited detectivities of 8.7 × 10 Jones in ambient condition and even higher when sealed with a protective layer. These detectivity values are higher than the highest values reported for photodetectors based on exfoliated MoS. We study MoS devices with gold electrodes and graphene electrodes. The devices with graphene electrodes have a tunable band alignment and are especially attractive for scalable ultra-thin flexible optoelectronics.
Atomically thin transition metal dichalcogenides (TMDs) are ideal candidates for ultrathin optoelectronics that is flexible and semitransparent. Photodetectors based on TMDs show remarkable performance, with responsivity and detectivity higher than 10 3 AW -1 and 10 12 Jones, respectively, but they are plagued by response times as slow as several tens of seconds.Although it is well established that gas adsorbates such as water and oxygen create charge traps and significantly increase both the responsivity and the response time, the underlying mechanism is still unclear. Here we study the influence of adsorbates on MoS2 photodetectors under ambient conditions, vacuum and illumination at different wavelengths. We show that, for wavelengths sufficiently short to excite electron-hole pairs in the MoS2, light illumination causes desorption of water and oxygen molecules. The change in the molecular gating provided by the physisorbed molecules is the dominant contribution to the device photoresponse in ambient conditions.
At high phonon temperature, defect-mediated electron-phonon collisions (supercollisions) in graphene allow for larger energy transfer and faster cooling of hot electrons than the normal, momentum-conserving electron-phonon collisions. Disorder also affects the heat flow between electrons and phonons at very low phonon temperature, where the phonon wavelength exceeds the mean free path. In both cases, the cooling rate is predicted to exhibit a characteristic cubic power law dependence on the electron temperature, markedly different from the T 4 dependence predicted for pristine graphene. The impact of defect-induced cooling on the performance of optoelectronic devices is still largely unexplored. Here we study the cooling mechanism of hotelectron bolometers based on epitaxial graphene quantum dots where the defect density can be controlled with the fabrication process. The devices with high defect density exhibit the cubic power law. Defect-induced cooling yields a slower increase of the thermal conductance with increasing temperature, thereby greatly enhancing the device responsivity compared to devices with lower defect density and operating with normal-collision cooling.
The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 10 −15 W Hz −0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.
With the intense electron beams used for cooling, matching of the ion and electron velocity over the la rgest possible fraction of the beam profile becomes important. In this situation, a finite dispersion from the ring in the cooling section can lea d to an appreciable gain in the transverse cooling speed. Based on a simple model of the cooling force, an expression for the "optimum" dispersion as a function of the electron beam intensity, the momentum sprea d and other properties of the ion beam will be derived. This simple theory will be compared to mea surements made on the Low Energy Ion Ring (LEIR) at CERN during 1997 . With the intense electron beams used for cooling, matching of the ion and electron velocity over the largest possible fraction of the beam profile becomes important. In this situation, a finite dispersion from the ring in the cooling section can lead to an appreciable gain in the transverse cooling speed. Based on a simple model of the cooling force, an expression for the "optimum" dispersion as a function of the electron beam intensity, the momentum spread and other properties of the ion beam will be derived. This simple theory will be compared to measurements made on the Low Energy Ion Ring (LEIR) at CERN during 1997. CERN/PS 99-045
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