Probing the Critical Role of Interfaces for Superior Performance in PbS Quantum Dot/Graphene Nanohybrid Broadband Photodetectors.

Colloidal quantum dots/graphene (QD/Gr) nanohybrids have been studied intensively for photodetection in a broadband spectrum including ultraviolet, visible, near-infrared, and shortwave infrared (UV-vis-NIR-SWIR). Since the optoelectronic process in the QD/Gr nanohybrid relies on the photogenerated charge carrier transfer from QDs to graphene, understanding the role of the QD-QD and QD-Gr interfaces is imperative to the QD/Gr nanohybrid photodetection. Herein, a systematic study is carried out to probe the effect of these interfaces on the noise, photoresponse, and specific detectivity in the UV-vis-NIR-SWIR spectrum. Interestingly, the photoresponse has been found to be negligible without a 3-mercaptopropionic acid (MPA) ligand exchange, moderate with a single ligand exchange after all QD layers are deposited on graphene, and maximum if it is performed after each QD layer deposition up to five layers of total QD thickness of 260-280 nm. Furthermore, exposure of graphene to C-band UV (UVC) for a short period of 4-5 min before QD deposition leads to improved photoresponse via removal of polar molecules at the QD/Gr interface. With the combination of the MPA ligand exchange and UVC exposure, optimal optoelectronic properties can be obtained on the PbS QD/Gr nanohybrids with high specific detectivity up to 2.6 × 1011, 1.5 × 1011, 5 × 1010, and 1.9 × 109 Jones at 400, 550, 1000, and 1700 nm, respectively, making the nanohybrids promising for broadband photodetection.

Metallic and Non-Metallic Plasmonic Nanostructures for LSPR Sensors.

Localized surface plasmonic resonance (LSPR) provides a unique scheme for light management and has been demonstrated across a large variety of metallic nanostructures. More recently, non-metallic nanostructures of two-dimensional atomic materials and heterostructures have emerged as a promising, low-cost alternative in order to generate strong LSPR. In this paper, a review of the recent progress made on non-metallic LSPR nanostructures will be provided in comparison with their metallic counterparts. A few applications in optoelectronics and sensors will be highlighted. In addition, the remaining challenges and future perspectives will be discussed.

Taurine deficiency as a driver of aging.

Aging is associated with changes in circulating levels of various molecules, some of which remain undefined. We find that concentrations of circulating taurine decline with aging in mice, monkeys, and humans. A reversal of this decline through taurine supplementation increased the health span (the period of healthy living) and life span in mice and health span in monkeys. Mechanistically, taurine reduced cellular senescence, protected against telomerase deficiency, suppressed mitochondrial dysfunction, decreased DNA damage, and attenuated inflammaging. In humans, lower taurine concentrations correlated with several age-related diseases and taurine concentrations increased after acute endurance exercise. Thus, taurine deficiency may be a driver of aging because its reversal increases health span in worms, rodents, and primates and life span in worms and rodents. Clinical trials in humans seem warranted to test whether taurine deficiency might drive aging in humans.

Defect-Polaron and Enormous Light-Induced Fermi-Level Shift at Halide Perovskite Surface.

Halide perovskites intrinsically contain a large amount of point defects. The interaction of these defects with photocarriers, photons, and lattice distortion remains a complex and unresolved issue. We found that for halide perovskite films with excess halide vacancies, the Fermi level can be shifted by as much as 0.7 eV upon light illumination. These defects can trap photocarriers for hours after the light illumination is turned off. The enormous light-induced Fermi level shift and the prolonged electron trapping are explained by the capturing of photocarriers by halide vacancies at the surface of the perovskite film. The formation of this defect-photocarrier complex can result in lattice deformation and an energy shift in the defect state. The whole process is akin to polaron formation at a defect site. Our data also suggest that these trapped carriers increase the electrical polarizability of the lattice, presumably by enhancing the defect migration rate.

MoS2 Nanodonuts for High-Sensitivity Surface-Enhanced Raman Spectroscopy.

Nanohybrids of graphene and two-dimensional (2D) layered transition metal dichalcogenides (TMD) nanostructures can provide a promising substrate for extraordinary surface-enhanced Raman spectroscopy (SERS) due to the combined electromagnetic enhancement on TMD nanostructures via localized surface plasmonic resonance (LSPR) and chemical enhancement on graphene. In these nanohybrid SERS substrates, the LSPR on TMD nanostructures is affected by the TMD morphology. Herein, we report the first successful growth of MoS2 nanodonuts (N-donuts) on graphene using a vapor transport process on graphene. Using Rhodamine 6G (R6G) as a probe, SERS spectra were compared on MoS2 N-donuts/graphene nanohybrids substrates. A remarkably high R6G SERS sensitivity up to 2 × 10-12 M has been obtained, which can be attributed to the more robust LSPR effect than in other TMD nanostructures such as nanodiscs as suggested by the finite-difference time-domain simulation. This result demonstrates that non-metallic TMD/graphene nanohybrids substrates can have SERS sensitivity up to one order of magnitude higher than that reported on the plasmonic metal nanostructures/2D materials SERS substrates, providing a promising scheme for high-sensitivity, low-cost applications for biosensing.

Probing the Origin of Light-Enhanced Ion Diffusion in Halide Perovskites.

Organic-inorganic hybrid halide perovskites have emerged recently as highly promising materials for optoelectronics such as photovoltaics and photodetectors. A unique feature of these materials is ion diffusion that directly impacts the optoelectronic process by affecting the charge transport and trapping. In order to shed light on the ionic diffusion behavior, the hybrid perovskites MAPbI3 and MAPbI3 with minor doping of phenyl-C61-butyric acid methyl-ester (MAPbI3-PCBM) thin-film capacitors were investigated in the presence of steady and dynamic visible illumination of different intensities. Light-induced capacitance, which increases monotonically with the increase of light intensity, was observed in the low-frequency range below 300 kHz of the electric field on both while differing quantitatively. Specifically, the large light-induced capacitance in the MAPbI3 capacitors can be obtained in the MAPbI3-PCBM ones in the dark. In addition, the increase of capacitance with light intensity is much less in the latter with electron trapping induced by PCBM. This result has revealed that the light-induced capacitance in MAPbI3 capacitors can be ascribed to the contribution of the additional charges across the capacitors associated with ionic diffusion activated by the illumination and that the effects on the capacitance will remain after the illumination is turned off due to residual photoexcited electrons trapped in the MAPbI3-PCBM sample.

Development of an ALD-Pt@SWCNT/Graphene 3D Nanohybrid Architecture for Hydrogen Sensing.

A nanohybrid architecture composed of single-wall carbon nanotube films and graphene heterostructures (SWCNT/graphene) was developed as a three-dimensional (3D) electrode. Atomic layer deposition (ALD) was used for conformal coating of catalytic Pt nanoparticles on the 3D ALD-Pt@SWCNT/graphene nanohybrid architecture for further enhancement of H2 sensing, taking advantage of the large sensing area and conformally coated nanostructures of the catalytic Pt. Remarkably, the H2 response was found to be improved by 50% in the SWCNT/graphene nanohybrid, indicating that graphene provides a more efficient charge transport. The ALD-Pt further enhances the H2 responsivity of the 3D ALD-Pt @SWCNT/graphene nanohybrids. By coating 10 cycles of ALD-Pt on the SWCNT/graphene nanohybrid, the H2 response (2.77%) is approximately twice that (1.4%) of its counterpart without the ALD-Pt. By further optimizing the 3D ALD-Pt@SWCNT/graphene nanohybrids with respect to the ALD-Pt cycle numbers and SWCNT film thickness, a H2 responsivity as high as 7.5% was achieved on the SWCNT/graphene nanohybrid sample with a 560 nm thick SWCNT film and 50 cycles of ALD-Pt.

Flexible Zinc Oxide Nanowire Array/Graphene Nanohybrid for High-Sensitivity Strain Detection.

A fully flexible strain sensor consisting of vertically aligned ZnO nanowires on graphene transferred on polyethylene terephthalate with prefabricated Au/Ti electrodes (ZnO-VANWs/Gr)/PET) has been obtained. The ZnO-VANWs were grown in solution using a seedless hydrothermal process and are single-crystalline of (0001) orientation that provides optimal piezoelectric gating on graphene when deformed mechanically. The change of the graphene channel conductance under such a piezoelectric gating through transduction of the mechanical deformation on the ZnO-VANWs/Gr was used to detect the strain induced by the deformation. Under applied normal forces of 0.30, 0.50, and 0.70 N in a dynamic manner, the ZnO-VANWs/Gr/PET strain sensors exhibited a high response and response times of ∼0.20 s to both force on and off were achieved. Under mechanical bending curvatures of 0.18, 0.23, 0.37, and 0.45 cm-1, high sensitivity of the gauge factors up to ∼248 and response times of 0.20 s/0.20 s (rise/fall) were achieved on the ZnO-VANWs/Gr/PET strain sensors. Moreover, the response changes polarity when the directions of bending alters between up and down, corresponding to the polarity change of the space charge on the ZnO-VANWs/Gr interface as a consequence of the compressive and tensile strains along the ZnO-VANWs. This result shows that the low-cost and scalable ZnO-VANWs/Gr/PET strain sensors are promising for applications in stress/strain monitoring, wearable electronics, and touch screens.

Localized Surface Plasmon Resonance Enhanced Light Absorption in AuCu/CsPbCl3 Core/Shell Nanocrystals.

Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all-inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl3 core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl3 shell of thickness in the range of 2-4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl3 core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl3 PNCs only (8-12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high-performance optoelectronics.

Ultrahigh Brightening of Infrared PbS Quantum Dots via Collective Energy Transfer Induced by a Metal-Oxide Plasmonic Metastructure.

We demonstrate that a solution-processed heterojunction interface formed via the addition of a thin buffer layer of CdSe/ZnS quantum dots (QDs) to a functional metal oxide plasmonic metastructure (FMOP) can set up a collective interquantum dot energy-transport process, significantly enhancing the emission of infrared PbS quantum dots. The FMOP includes a Schottky junction, formed via deposition of a Si layer on arrays of Au nanoantennas and a Si/Al oxide charge barrier. We show when these two junctions are separated from each other by about 15 nm and the CdSe/ZnS quantum dot buffer layer is placed in touch with the Si/Al oxide junction, the quantum efficiency of an upper layer of PbS quantum dots can increase by about 1 order of magnitude. These results highlight a unique energy circuit formed via collective coupling of the CdSe/ZnS quantum dots with the hybridized states of plasmons and diffraction modes of the arrays (surface lattice resonances) and coupling between such resonances with PbS QDs via lattice-induced photonic modes.

WormBot, an open-source robotics platform for survival and behavior analysis in C. elegans.

Caenorhabditis elegans is a popular organism for aging research owing to its highly conserved molecular pathways, short lifespan, small size, and extensive genetic and reverse genetic resources. Here we describe the WormBot, an open-source robotic image capture platform capable of conducting 144 parallel C. elegans survival and behavioral phenotyping experiments. The WormBot uses standard 12-well tissue culture plates suitable for solid agar media and is built from commercially available robotics hardware. The WormBot is controlled by a web-based interface allowing control and monitoring of experiments from any internet connected device. The standard WormBot hardware features the ability to take both time-lapse bright field images and real-time video micrographs, allowing investigators to measure lifespan, as well as heathspan metrics as worms age. The open-source nature of the hardware and software will allow for users to extend the platform and implement new software and hardware features. This extensibility, coupled with the low cost and simplicity of the system, allows the automation of C. elegans survival analysis even in small laboratory settings with modest budgets.

Plasmonic WS2 Nanodiscs/Graphene van der Waals Heterostructure Photodetectors.

Two-dimensional material van der Waals (vdW) heterostructures provide an excellent platform for design of novel optoelectronics. In this work, transition-metal dichalcogenide WS2 nanodiscs (WS2-NDs) of lateral dimension of 200-400 nm and layer number of 4-7 were synthesized on graphene using a layer-by-layer, transfer-free chemical vapor deposition. On this WS2-NDs/graphene vdW heterostructures, localized surface plasmonic resonance (LSPR) was achieved, resulting in remarkably enhanced light absorption as compared to the counterpart devices with a continuous WS2 layer (WS2-CL/graphene). Remarkably, the photoresponsivity of 6.4 A/W on the WS2-NDs/graphene photodetectors is seven times higher than that (0.91 A/W) of the WS2-CL/graphene vdW heterostructures at an incident 550 nm light intensity of 10 µW/cm2. Furthermore, the WS2-NDs/graphene photodetectors exhibit higher sensitivity to lower lights. Under 550 nm light illumination of 3 µW/cm2, which is beyond the sensitivity limit of the WS2-CL/graphene photodetectors, high photoresponsivity of 8.05 A/W and detectivity of 2.8 × 1010 Jones are achieved at Vsd = 5 V. This result demonstrates that the LSPR WS2-NDs/graphene vdW heterostructure is promising for scalable high-performance optoelectronics applications.

Using Silver Nanoparticles-Embedded Silica Metafilms as Substrates to Enhance the Performance of Perovskite Photodetectors.

Plasmonic metal nanostructures provide a promising strategy for light trapping and therefore can dramatically enhance photocurrent in optoelectronics only if the trapped light can be coupled effectively from plasmons to excitons, whereas the reverse transfer of energy, charge, and heat from excitons to plasmons can be suppressed. Motivated by this, this work develops a scheme to implement a metafilm with Ag nanoparticles (NPs) embedded in 10 nm thick silica (Ag NPs-silica metafilm) to the active device channel of a hybrid perovskite film/graphene photodetector. Remarkably, an enhancement factor of 7.45 in photoresponsivity, the highest so far among all the reports adopting plasmonic metal NPs in perovskite photodetectors, has been achieved on the photodetectors with the Ag NPs-silica metafilms. Considering that the synthesis of the Ag NPs-silica metafilms can be readily scaled up to coat both rigid and flexible substrates, this result provides a low-cost metaplatform for a variety of high-performance optoelectronic device applications.

Controllable Synthesis of Monodispersed Fe1- xS2 Nanocrystals for High-Performance Optoelectronic Devices.

The optical properties of stoichiometric iron pyrite (FeS2) nanocrystals (NCs) are characterized by strong UV-Visible (UV-Vis) absorption within the cutoff while negligible absorption beyond the cutoff in near-infrared and longer wavelengths. Herein, we show this bandgap limitation can be broken through controllable synthesis of nonstoichiometric Fe1- xS2 NCs ( x = 0.01-0.107) to induce localized surface plasmonic resonance (LSPR) absorption beyond the cutoff to short-wave infrared spectrum (SWIR, 1-3 µm) with remarkably enhanced broadband absorption across UV-Vis-SWIR spectra. To illustrate the benefit of the broadband absorption, colloidal LSPR Fe1- xS2 NCs were printed on graphene to form LSPR Fe1- xS2 NCs/graphene heterostructure photodetectors. Extraordinary photoresponsivity in exceeding 4.32 × 106 A/W and figure-of-merit detectivity D* > 7.50 × 1012 Jones have been demonstrated in the broadband of UV-Vis-SWIR at room temperature. These Fe1- xS2 NCs/graphene heterostructures are printable and flexible and therefore promising for practical optical and optoelectronic applications.

Surface plasmon assisted laser ablation of stainless steel.

Colloidal Au nanoparticles (NPs) were decorated on stainless steel for surface plasmon enhanced laser ablation. A comparative study of the laser ablation efficiency was carried out on stainless steel samples with and without the Au NPs decoration at a variable pulsed laser fluence and laser pulse number. Higher ablation efficiency was clearly demonstrated in the former as illustrated from the larger diameter, maximum depth and the cross-sectional area of the crater generated by the laser ablation under the same conditions. Additionally, both the maximum depth and efficiency enhancement were found to depend on the laser fluence and pulse number. The maximum enhanced ablation efficiency of 36% based on the cross-sectional area of the crater was obtained at 1 pulse number of laser fluence 1.53 J cm-2. The efficiency enhancement of laser ablation is attributed to the highly enhanced surface plasmon field at the interface between Au NPs and stainless steel.

High-Performance All-Inorganic CsPbCl3 Perovskite Nanocrystal Photodetectors with Superior Stability.

All-inorganic perovskites nanostructures, such as CsPbCl3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl3 NCs and facilitating high-efficiency charge transfer between the CsPbCl3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl3 NCs/graphene devices including a high ultraviolet responsivity exceeding 106 A/W, a high detectivity of 2 × 1013 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.

Ultrahigh-Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2 /Graphene/SnS2 p-g-n Junctions.

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next-generation optoelectronics since they can be stacked layer-by-layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice-mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h-BN/p-MoTe2 /graphene/n-SnS2 /h-BN p-g-n junction, fabricated by a layer-by-layer dry transfer, demonstrates high-sensitivity, broadband photodetection at room temperature. The combination of the MoTe2 and SnS2 of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built-in electric field for high-efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5-7-layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W-1 with fast photoresponse and specific detectivity up to ≈1013 Jones in the ultraviolet-visible-near-infrared spectrum. This result suggests that the vdW p-g-n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh-sensitivity and broadband photonic detectors.

High-Sensitivity Light Detection via Gate Tuning of Organometallic Perovskite/PCBM Bulk Heterojunctions on Ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 Gated Graphene Field Effect Transistors.

Organometallic perovskite (OMP) CH3NH3PbI3 doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been shown to form bulk heterojunction (OMP-PCBM BHJ) for improved charge separation. In this work, the OMP-PCBM BHJ photosensitizer is combined with graphene field effect transistors (GFETs) with a ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 gate of high gating efficiency. A remarkable gate tunability via shifting the Fermi energy of graphene with respect to the valence band maximum and conduction band minimum of the OMP was observed, which is critical for facilitating efficient charge transfer across the OMP-PCBM BHJ/GFET interface. The combination of the high-efficiency charge separation by BHJ and charge transfer by high gate tunability leads to achievement of high photoresponsivity up to 7 × 106 A/W and detectivity exceeding 7 × 1012 Jones at 550 nm at a small gate voltage of 1.0 V. These results represent almost 2 orders of magnitude improvement over that without a gate tuning under the similar experimental condition, illustrating the importance of the interface electronic structure in optimizing the optoelectronic performance of the OMP-PCBM BHJ/GFET devices.

Balancing silicon/aluminum oxide junctions for super-plasmonic emission enhancement of quantum dots via plasmonic metafilms.

We study the impact of structural features of Si/Al oxide junctions on metal-oxide plasmonic metafilms formed via placing such junctions in close vicinity of an Au/Si Schottky barrier. The emission intensity and dynamics of colloidal semiconductor quantum dots deposited on such metafilms are investigated, while the surface morphology and structural compositions of the Si/Al oxide junction are controlled. The results show the conditions wherein the Si/Al oxide junction can reshape the impact of plasmonic effects, allowing it to increase the lifetimes of excitons. Under these conditions, the plasmonic metafilms can quarantine excitons against the fluctuating trap environments of the quantum dots, offering super-plasmonic emission enhancement that includes enhancement of the spontaneous emission decay rate combined with the suppression of Auger decay.

Effects of rhenium dopants on photocarrier dynamics and optical properties of monolayer, few-layer, and bulk MoS2.

We report a comprehensive study on the effects of rhenium doping on optical properties and photocarrier dynamics of MoS2 monolayer, few-layer, and bulk samples. Monolayer and few-layer samples of Re-doped (0.6%) and undoped MoS2 were fabricated by mechanical exfoliation, and were studied by Raman spectroscopy, optical absorption, photoluminescence, and time-resolved differential reflection measurements. Similar Raman, absorption, and photoluminescence spectra were obtained from doped and undoped samples, indicating that the Re doping at this level does not significantly alter the lattice and electronic structures. Red-shift and broadening of the two phonon Raman modes were observed, showing the lattice strain and carrier doping induced by Re. The photoluminescence yield of the doped monolayer is about 15 times lower than that of the undoped sample, while the photocarrier lifetime is about 20 times shorter in the doped monolayer. Both observations can be attributed to diffusion-limited Auger nonradiative recombination of photocarriers at Re dopants. These results provide useful information for developing a doping strategy of MoS2 for optoelectronic applications.