MXenes for Plasmonic Photodetection.

MXenes have recently shown impressive optical and plasmonic properties associated with their ultrathin-atomic-layer structure. However, their potential use in photonic and plasmonic devices has been only marginally explored. Photodetectors made of five different MXenes are fabricated, among which molybdenum carbide MXene (Mo2 CTx ) exhibits the best performance. Mo2 CTx MXene thin films deposited on paper substrates exhibit broad photoresponse in the range of 400-800 nm with high responsivity (up to 9 A W-1 ), detectivity (≈5 × 1011 Jones), and reliable photoswitching characteristics at a wavelength of 660 nm. Spatially resolved electron energy-loss spectroscopy and ultrafast femtosecond transient absorption spectroscopy of the MXene nanosheets reveal that the photoresponse of Mo2 CTx is strongly dependent on its surface plasmon-assisted hot carriers. Additionally, Mo2 CTx thin-film devices are shown to be relatively stable under ambient conditions, continuous illumination and mechanical stresses, illustrating their durable photodetection operation in the visible spectral range. Micro-Raman spectroscopy conducted on bare Mo2 CTx film and on gold electrodes allowing for surface-enhanced Raman scattering demonstrates surface chemistry and a specific low-frequency band that is related to the vibrational modes of the single nanosheets. The specific ability to detect and excite individual surface plasmon modes provides a viable platform for various MXene-based optoelectronic applications.

Anomalous Li Storage Capability in Atomically Thin Two-Dimensional Sheets of Nonlayered MoO2.

Since the first exfoliation and identification of graphene in 2004, research on layered ultrathin two-dimensional (2D) nanomaterials has achieved remarkable progress. Realizing the special importance of 2D geometry, we demonstrate that the controlled synthesis of nonlayered nanomaterials in 2D geometry can yield some unique properties that otherwise cannot be achieved in these nonlayered systems. Herein, we report a systematic study involving theoretical and experimental approaches to evaluate the Li-ion storage capability in 2D atomic sheets of nonlayered molybdenum dioxide (MoO2). We develop a novel monomer-assisted reduction process to produce high quality 2D sheets of nonlayered MoO2. When used as lithium-ion battery (LIB) anodes, these ultrathin 2D-MoO2 electrodes demonstrate extraordinary reversible capacity, as high as 1516 mAh g-1 after 100 cycles at the current rate of 100 mA g-1 and 489 mAh g-1 after 1050 cycles at 1000 mA g-1. It is evident that these ultrathin 2D sheets did not follow the normal intercalation-cum-conversion mechanism when used as LIB anodes, which was observed for their bulk analogue. Our ex situ XPS and XRD studies reveal a Li-storage mechanism in these 2D-MoO2 sheets consisting of an intercalation reaction and the formation of metallic Li phase. In addition, the 2D-MoO2 based microsupercapacitors exhibit high areal capacitance (63.1 mF cm-2 at 0.1 mA cm-2), good rate performance (81% retention from 0.1 to 2 mA cm-2), and superior cycle stability (86% retention after 10,000 cycles). We believe that our work identifies a new pathway to make 2D nanostructures from nonlayered compounds, which results in an extremely enhanced energy storage capability.

Non-Volatile ReRAM Devices Based on Self-Assembled Multilayers of Modified Graphene Oxide 2D Nanosheets.

2D nanomaterials have been actively utilized in non-volatile resistive switching random access memory (ReRAM) devices due to their high flexibility, 3D-stacking capability, simple structure, transparency, easy fabrication, and low cost. Herein, it demonstrates re-writable, bistable, transparent, and flexible solution-processed crossbar ReRAM devices utilizing graphene oxide (GO) based multilayers as active dielectric layers. The devices employ single- or multi-component-based multilayers composed of positively charged GO (N-GO(+) or NS-GO(+)) with/without negatively charged GO(-) using layer-by-layer assembly method, sandwiched between Al bottom and Au top electrodes. The device based on the multi-component active layer Au/[N-GO(+)/GO(-)]n /Al/PES shows higher ON/OFF ratio of ≈105 with switching voltage of -1.9 V and higher retention stability (≈104 s), whereas the device based on single component (Au/[N-GO(+)]n /Al/PES) shows ≈103 ON/OFF ratio at ±3.5 V switching voltage. The superior ReRAM properties of the multi-component-based device are attributed to a higher coating surface roughness. The Au/[N-GO(+)/GO(-)]n /Al/PES device prepared from lower GO concentration (0.01%) exhibits higher ON/OFF ratio (≈109 ) at switching voltage of ±2.0 V. However, better stability is achieved by increasing the concentration from 0.01% to 0.05% of all GO-based solutions. It is found that the devices containing MnO2 in the dielectric layer do not improve the ReRAM performance.

Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 µm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra.

Non-volatile organic memory with sub-millimetre bending radius.

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 µm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.