Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring.

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

Direct Observation of Off-Stoichiometry-Induced Phase Transformation of 2D CdSe Quantum Nanosheets.

Crystal structures determine material properties, suggesting that crystal phase transformations have the potential for application in a variety of systems and devices. Phase transitions are more likely to occur in smaller crystals; however, in quantum-sized semiconductor nanocrystals, the microscopic mechanisms by which phase transitions occur are not well understood. Herein, the phase transformation of 2D CdSe quantum nanosheets caused by off-stoichiometry is revealed, and the progress of the transformation is directly observed by in situ transmission electron microscopy. The initial hexagonal wurtzite-CdSe nanosheets with atomically uniform thickness are transformed into cubic zinc blende-CdSe nanosheets. A combined experimental and theoretical study reveals that electron-beam irradiation can change the stoichiometry of the nanosheets, thereby triggering phase transformation. The loss of Se atoms induces the reconstruction of surface atoms, driving the transformation from wurtzite-CdSe(11 2 ¯ $\bar{2}$ 0) to zinc blende-CdSe(001) 2D nanocrystals. Furthermore, during the phase transformation, unconventional dynamic phenomena occur, including domain separation. This study contributes to the fundamental understanding of the phase transformations in 2D quantum-sized semiconductor nanocrystals.

Integration of synaptic phototransistors and quantum dot light-emitting diodes for visualization and recognition of UV patterns.

Synaptic photodetectors exhibit photon-triggered synaptic plasticity, which thus can improve the image recognition rate by enhancing the image contrast. However, still, the visualization and recognition of invisible ultraviolet (UV) patterns are challenging, owing to intense background noise. Here, inspired by all-or-none potentiation of synapse, we develop an integrated device of synaptic phototransistors (SPTrs) and quantum dot light-emitting diodes (QLEDs), facilitating noise reduction and visualization of UV patterns through on-device preprocessing. The SPTrs convert noisy UV inputs into a weighted photocurrent, which is applied to the QLEDs as a voltage input through an external current-voltage-converting circuit. The threshold switching characteristics of the QLEDs result in amplified current and visible illumination by the suprathreshold input voltage or nearly zero current and no visible illumination by the input voltage below the threshold. The preprocessing of image data with the SPTr-QLED can amplify the image contrast, which is helpful for high-accuracy image recognition.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production.

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe2 (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO2. QDs with monofunctional ligands are directly attached on TiO2 through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO2. In contrast, bifunctional ligands bridge QDs and TiO2, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm2, while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm2 (at 0.6 VRHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Several Shapes of Single Crystalline Gold Nanomaterials Prepared in the Surfactant Mixture of CTAB and Pluronics.

Twin structures in gold nanomaterials are destined because they reduce the severe strains in the misfit region of nanostructures. Defect-free single crystalline plasmonic nanomaterials gain interests these days as the integration of plasmonic materials or plasmons into electronic devices and circuits becomes more common. In this study, without subtle experimental adjustments, such as pH or halide additives, several shapes of single crystalline gold nanoparticles (NPs) are prepared in the surfactant mixture of cetyltrimethylammonium bromide (CTAB) and Pluronic triblock copolymers. The synthesized NPs are primarily composed of {100} planes with small numbers of particles possessing a [110] zone axis. Pluronic copolymers with low number average molecular weights (M n), such as L-31 (M n ≈ 1100) and L-64 (M n ≈ 2900), prefer anisotropic nanorods with the aspect ratios of 4.3 and 3.0, respectively, while Pluronics with high M n values, such as F-68 (M n ≈ 8400) and F-108 (M n ≈ 14 600), favor more concentric and isotropic cube-like NPs. Extended micelles are believed to form in Pluronics with low M n values in which hydrophobic cores are merged with the increase of temperature, while the corona regions that are composed of long tails of PEO prevent the merge of hydrophobic cores, and the growth of the micelles is limited in Pluronic copolymers with high M n values. The catalytic degradation reactions of methyl orange are conducted, and rather than isotropic particles, gold nanorods exhibit better catalytic performances. More hydrophilic environment and the steric alignment of rigid aromatic structures of methyl orange along the nanorods are thought to contribute to the catalytic activities. Overall, highly confined geometries of the appropriately swollen micellar templates of Pluronics and CTAB, which is not so hydrophobic as for the formation of contracted deswollen templates and for the inhibition of the growth of NPs, and which is not so hydrophilic as for the formation of coarse templates and for the formation of isotropic spheres with varying sizes, are believed as the main factor for the formation of single crystalline gold NPs.

Synthesis of Prolate-Shaped Au Nanoparticles and Au Nanoprisms and Study of Catalytic Reduction Reactions of 4-Nitrophenol.

The growth into anisotropic one or two dimensions is important in plasmonic gold nanomaterials because extinction occurs along multiple axes and sometimes the resonance extends to the near-infrared region. The surfactant mixture of cetyltrimethylammonium bromide (CTAB) and Pluronic triblock copolymers has been recently demonstrated to be efficient anisotropic soft templates for the growth of noble metal nanomaterials. Seed-mediated growth of two types of anisotropic Au nanoparticles is achieved in this study. One is one-dimensional prolate-shaped Au nanoparticles with the average aspect ratios of 2.29 and 2.59, and the other is two-dimensional gold nanoprisms with the average edge length of 50.4 nm. These anisotropic structures are believed to be produced by the tendency of Pluronic copolymers to be micellized anisotropically at the elevated temperatures and by the preference for being lamellar mesophases in the phase diagrams when the concentration is highest. When prepared in the surfactant mixture of CTAB and L-64 (17.9%), Au nanoparticles containing spherical particles (27.9 nm) as the major products show the best catalytic performances in the reduction reactions of 4-nitrophenol.