RESUMO
Electrical synaptic devices are the basic components for the hardware based neuromorphic computational systems, which are expected to break the bottleneck of current von Neumann architecture. So far, synaptic devices based on three-terminal transistors are considered to provide the most stable performance, which usually use gate pulses to modulate the channel conductance through a floating gate and/or charge trapping layer. Herein, we report a three-terminal synaptic device based on a two-dimensional molybdenum ditelluride (MoTe2)/hexagonal boron nitride (hBN) heterostructure. This structure enables stable and prominent conductance modulation of the MoTe2channel by the photo-induced doping method through electron migration between the MoTe2channel and ultraviolet (UV) light excited mid-gap defect states in hBN. Therefore, it is free of the floating gate and charge trapping layer to reduce the thickness and simplify the fabrication/design of the device. Moreover, since UV illumination is indispensable for stable doping in MoTe2channel, the device can realize both short- (without UV illumination) and long- (with UV illumination) term plasticity. Meanwhile, the introduction of UV light allows additional tunability on the MoTe2channel conductance through the wavelength and power intensity of incident UV, which may be important to mimic advanced synaptic functions. In addition, the photo-induced doping method can bidirectionally dope MoTe2channel, which not only leads to large high/low resistance ratio for potential multi-level storage, but also implement both potentiation (n-doping) and depression (p-doping) of synaptic weight. This work explores alternative three-terminal synaptic configuration without floating gate and charge trapping layer, which may inspire researches on novel electrical synapse mechanisms.
RESUMO
In the past decade, organic-inorganic perovskite solar cells (PSCs) have received significant attentions due to their high efficiencies and low costs. However, the commercialization of PSCs is stilled hindered by several issues such as device performance (especially for large-area cells) and stability. Recently, two-dimensional (2D) transition metal disulfides (TMDs) show great potentials in solving aforementioned problems due to their unique morphological structure and electrical properties. Herein, we summarize the advancements in the recent applications of various TMDs materials as charge transport layers in PSCs. Although some progress have been made, there are considerable issues to be tackled in this field. The challenges and development directions of these 2D TMDs materials for PSCs are also clarified. Lastly, the most recent advancements about TMDs materials in some other electronic (or optoelectronic) fields are also summarized and discussed.
RESUMO
Tensile strain is intrinsic to monolayer crystals of transition metal disulfides such as Mo(W)S2 grown on oxidized silicon substrates by chemical vapor deposition (CVD) owing to the much larger thermal expansion coefficient of Mo(W)S2 than that of silica. Here we report fascinating fluorescent variation in intensity with aging time in CVD-grown triangular monolayer WS2 crystals on SiO2 (300 nm)/Si substrates and formation of interesting concentric triangular fluorescence patterns in monolayer crystals of large size. The novel fluorescence aging behavior is recognized to be induced by the partial release of intrinsic tensile strain after CVD growth and the induced localized variations or gradients of strain in the monolayer crystals. The results demonstrate that strain has a dramatic impact on the fluorescence and photoluminescence of monolayer WS2 crystals and thus could potentially be utilized to tune electronic and optoelectronic properties of monolayer transition metal disulfides.
RESUMO
This study set out to calculate the full-potential linearized augmented plane wave (FPLAPW)-based energy loss near sulfur K and L2,3 edge structures of group V layered transition metal disulfides MS2 (M=Ta, Nb, and V) in octahedral (1T) as well as trigonal prismatic (2H) structures. The calculations showed that, consistent with other calculations, all the studied materials were metallic due to the partially filled d bands in their configurations. Furthermore, the calculated ELNES spectra revealed a good agreement with the available experimental XANES analogues. The d-like and p-like transitions of M and sulfur atoms were the dominant electron transitions in K edge spectra. Spectrum characteristics of the sulfur L2,3 edge of ELNES indicated the transition of sulfur-p electrons to the unoccupied s or d states. These spectra reflect the electronic band structures of materials, as well. As the focus shifts from bulk to monolayer, substrate hybridization becomes stronger. In 2H phases, the dominant peaks of sulfur K edge spectra originate from unoccupied d bands. Further, the broad peaks at higher energy ranges are due to the transitions to sulfur p hybridized with M-s and p states. For energies below 7eV, M-d state is the target state of most of the transitions in both 1T and 2H phases. For above 10eV energies, however, sulfur-d is the target state. Moreover, density of states of sulfur-p (d) is very similar in shape to that of sulfur K (L2,3) edges spectra. For the sulfur L2,3 edges, from 2H-TaS2 to 2H-VS2 and also from bulk to monolayer, the number of transitions to M-d state increases.
RESUMO
Photoacoustic activity is the generation of an ultrasonic signal via thermal expansion or bubble formation, stimulated by laser irradiation. Photoacoustic nanoplatforms have recently gained focus for application in bioelectric interfaces. Various photoacoustic material types have been evaluated, including gold nanoparticles, semiconductive π-conjugating polymers (SP), etc. In this study, surfactant-free methoxy-polyethylene glycol-co-polypyrrole copolymer (mPEG-co-PPyr) nanoparticles (NPs) and mPEG-co-PPyr NP/molybdenum disulfide (mPEG-co-PPyr/MoS2) nanocomposites (NCs) were prepared and their photoacoustic activity was demonstrated. The mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS2 NCs both showed photoacoustic signal activity. The mPEG-co-PPyr/MoS2 NCs presented a higher photoacoustic signal amplitude at 700 nm than the mPEG-co-PPyr NPs. The enhanced photoacoustic activity of the mPEG-co-PPyr/MoS2 NCs might be attributed to heterogeneous interfacial contact between mPEG-co-PPyr and the MoS2 nanosheets due to complex formation. Laser ablation of MoS2 might elevate the local temperature and facilitate the thermal conductive transfer in the mPEG-co-PPyr/MoS2 NCs, amplifying PA signal. Our study, for the first time, demonstrates enhanced PA activity in SP/transition metal disulfide (TMD) composites as photoacoustic nanoplatforms.
RESUMO
Metal sulfides (MeS2) such as MoS2 and WS2 were used as charge transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells in order to enhance the stability in air comparing to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS). MeS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. The ultraviolet-ozone (UV-O3) treatment on MeS2 leads to the removal of the surface contaminants produced by the transfer process, resulting in a uniform surface and an increase of the work function. The maximum luminance efficiencies of the OLEDs with UV-O3-treated MoS2 and WS2 were 9.44 and 10.82 cd/A, respectively. The power conversion efficiencies of OPV cells based on UV-O3-treated MoS2 and WS2 were 2.96 and 3.08%, respectively. These values correspond to over 95% of those obtained with ( PEDOT: PSS) based devices. Furthermore, OLEDs and OPV cells based on MeS2 showed two to six times longer stability in air compared with PEDOT: PSS based devices. These results suggest that UV-O3-surface-treated MeS2 could be a promising candidate for a charge transport layer in optoelectronic devices.