Triboelectric Nanogenerator as an Active UV Photodetector

Self‐powered nanosensors and nanosystems have attracted significant attention in the past decades and have gradually become the most desirable and promising prototype for environmental protection/detection because no battery is needed to power the device. Therefore, in this paper a design is proposed for a self‐powered photodetector based on triboelectric nanogenerator (TENG) configuration. 3D dendritic TiO₂ nanostructures are synthesized as the built‐in UV photodetector as well as the contact material of the TENG. The cost‐effective, robust, and easily fabricated TENG‐based photodetector presents superior photoresponse characteristics, which include an excellent responsivity over 280 A W^?1, rapid rise time (18 ms) and decay time (31 ms), and a wide detection range of light intensity from 20 μW cm^?2 to 7 mW cm^?2. In the last part of the paper, a stand‐alone and self‐powered environmental sensing device is developed by applying poly(methyl methacrylate) (PMMA) substrates and springs to assemble the TENG‐based photodetector. These results indicate that the new prototype sensing device based on the TENG configuration shows great potential as a self‐powered photodetector.     

Bisanthracene‐Based Donor–Acceptor‐type Light‐Emitting Dopants: Highly Efficient Deep‐Blue Emission in Organic Light‐Emitting Devices

Deep‐blue fluorescent compounds are particularly important in organic light‐emitting devices (OLEDs). A donor–accepotor (DA)‐type blue‐emitting compound, 1‐(10‐(4‐methoxyphenyl)anthracen‐9‐yl)‐4‐(10‐(4‐cyanophenyl)anthracen‐9‐yl)benzene ( BD3 ), is synthesized, and for comparison, a nonDA‐type compound, 1,4‐bis(10‐phenylanthracene‐9‐yl)benzene ( BD1 ) and a weak DA‐type compound, 1‐(10‐phenylanthracen‐9‐yl)‐4‐(10‐(4‐cyanophenyl)anthracen‐9‐yl)‐benzene ( BD2 ), are also synthesized. The twisted conformations of the two anthracene units in the compounds, confirmed by single crystal X‐ray analysis, effectively prevent π‐conjugation, and the compound shows deep‐blue photoluminescence (PL) with a high PL quantum efficiency, almost independent of the solvent polarity, resulting from the absence of an intramolecular charge transfer state. The DA‐type molecule BD3 in a non‐doped device exhibits a maximum external quantum efficiency (EQE) of 4.2% with a slight roll‐off, indicating good charge balance due to the DA‐type molecular design. In the doped device with 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP) host, the BD3 exhibits higher EQE than 10% with Commission International de L'Eclairge (CIE) coordinates of (0.15, 0.06) and a narrow full‐width at half‐maximum of 45 nm, which is close to the CIE of the high definition television standard blue.     

Highly Fluorescent and Color‐Tunable Exciplex Emission from Poly(N‐vinylcarbazole) Film Containing Nanostructured Supramolecular Acceptors

Highly fluorescent excited‐state charge‐transfer complexes (exciplexes) formed at the interfacial region between a polymeric donor matrix, here, poly(N‐vinylcarbazole), and embedded nanostructured acceptors are characterized for their photophysical properties. Exciplex‐to‐exciton emission switching is observed after solvent vapor annealing (SVA) due to the size evolution of the nanostructures beyond the exciton diffusion length. Color‐tunable exiplex emission (sky blue, green, and orange) is demonstrated for three different nanostructured acceptors with the same HOMO–LUMO gap (i.e., the same blue excitonic emission) but with different electron affinity. White‐emitting poly(N‐vinylcarbazole) film is also fabricated, simply by incorporating mixed supramolecular acceptors, which provide independent exciplex emissions. This study presents important insights into the excited‐state intermolecular interaction at the well‐defined nanoscale interface and suggests an efficient way to obtain multicolored exciplex emissions.     

Solution‐Processed Small‐Molecule Bulk Heterojunction Ambipolar Transistors

Solution‐processed small‐molecule bulk heterojunction (BHJ) ambipolar organic thin‐film transistors are fabricated based on a combination of [2‐phenylbenzo[d,d']thieno[3,2‐b;4,5‐b']dithiophene (P‐BTDT) : 2‐(4‐n‐octylphenyl)benzo[d,d ']thieno[3,2‐b;4,5‐b']dithiophene (OP‐BTDT)] and C₆₀. Treating high electrical performance vacuum‐deposited P‐BTDT organic semiconductors with a newly developed solution‐processed organic semiconductor material, OP‐BTDT, in an optimized ratio yields a solution‐processed p‐channel organic semiconductor blend with carrier mobility as high as 0.65 cm² V^?1 s^?1. An optimized blending of P‐BTDT:OP‐BTDT with the n‐channel semiconductor, C₆₀, results in a BHJ ambipolar transistor with balanced carrier mobilities for holes and electrons of 0.03 and 0.02 cm² V^?1 s^?1, respectively. Furthermore, a complementary‐like inverter composed of two ambipolar thin‐film transistors is demonstrated, which achieves a gain of 115.     

Topological‐Structure Modulated Polymer Nanocomposites Exhibiting Highly Enhanced Dielectric Strength and Energy Density

Dielectric materials with high electric energy densities and low dielectric losses are of critical importance in a number of applications in modern electronic and electrical power systems. An organic–inorganic 0–3 nanocomposite, in which nanoparticles (0‐dimensional) are embedded in a 3‐dimensionally connected polymer matrix, has the potential to combine the high breakdown strength and low dielectric loss of the polymer with the high dielectric constant of the ceramic fillers, representing a promising approach to realize high energy densities. However, one significant drawback of the composites explored up to now is that the increased dielectric constant of the composites is at the expense of the breakdown strength, limiting the energy density and dielectric reliability. In this study, by expanding the traditional 0–3 nanocomposite approach to a multilayered structure which combines the complementary properties of the constituent layers, one can realize both greater dielectric displacement and a higher breakdown field than that of the polymer matrix. In a typical 3‐layer structure, for example, a central nanocomposite layer of higher breakdown strength is introduced to substantially improve the overall breakdown strength of the multilayer‐structured composite film, and the outer composite layers filled with large amount of high dielectric constant nanofillers can then be polarized up to higher electric fields, hence enhancing the electric displacement. As a result, the topological‐structure modulated nanocomposites, with an optimally tailored nanomorphology and composite structure, yield a discharged energy density of 10 J/cm³ with a dielectric breakdown strength of 450 kV mm^–1, much higher than those reported from all earlier studies of nanocomposites.     

Mesostructured Arrays of Nanometer‐spaced Gold Nanoparticles for Ultrahigh Number Density of SERS Hot Spots

A novel one‐trough synthesis via an air‐water interface is demonstrated to provide hexagonally packed arrays of densely spaced metallic nanoparticles (NPs). In the synthesis, a mesostructured polyoxometalate (POM)‐silicatropic template (PSS) is first self‐assembled at the air‐water interface; upon UV irradiation, anion exchange cycles enable the free‐floating PSS film to continuously uptake gold precursors from the solution subphase for diffusion‐controlled and POM‐site‐directed photoreduction inside the silica channels. NPs ≈ 2 nm can hence be homogeneously formed inside the silica‐surfactant channels until saturation. As revealed via X‐ray diffraction, small‐angle X‐ray scattering (SAXS), grazing incidence SAXS, and transmission electron microscopy, the Au NPs directed by the PSS template are arrayed into a 2D hexagonal lattice with inter‐channel spacing of 3.2 nm and a mean along‐channel NP spacing of 2.8 nm. This corresponds to an ultra‐high number density (≈10¹⁹ NPs cm^?3) of narrowly spaced Au NPs in the Au‐NP@PSS composite, leading to 3D densely deployed hot‐spots along and across the mesostructured POM‐silica channels for surface‐enhanced Raman scattering (SERS). Consequently, the Au‐NP@PSS composite exhibits prominent SERS with 4‐mercaptobenzoic acid (4‐MBA) adsorbed onto Au NPs. The best 4‐MBA detection limit is 5 nm , with corresponding SERS enhancement factors above 10⁸.     

Localized Collection of Airborne Analytes: A Transport Driven Approach to Improve the Response Time of Existing Gas Sensor Designs

The detection of single binding has been a recent trend in sensor research introducing various sensor designs where the active sensing elements are nanoscopic in size. Currently, transport and collection of airborne analytes for gas sensors is either diffusion based or non‐localized and it becomes increasingly unlikely for analytes to interact with sensing structures where the active area is shrunk, trading an increased sensitivity with a slow response time. This report introduces a corona discharge based analyte charging method and an electrodynamic nanolens based analyte concentration concept to effectively transport airborne analytes to sensing points to improve the response time of existing gas sensor designs. Localized collection of analytes over a wide range, including microscopic particles, nanoparticles, and small molecules, is demonstrated. In all cases, the collection rate is several orders of magnitudes higher than in the case where the collection is driven by diffusion. The collection scheme is integrated on an existing SERS (surface‐enhanced Raman spectroscopy) based sensor. In terms of response time, the process is able to detect analytes at 9 ppm (parts per million) within 1 s. As a comparison, 1 h is required to reach the same signal level when diffusion‐only‐transport is used.     

Three‐Dimensional Branched Nanowire Heterostructures as Efficient Light‐Extraction Layer in Light‐Emitting Diodes

A facile method to fabricate three‐dimensional branched ZnO/MgO nanowire heterostructures and their application as the efficient light‐extraction layer in light‐emitting diodes are reported. The branched MgO nanowires are produced on the hydrothermally‐grown ZnO nanowires with a small tapering angle towards the tip (≈6°), by the oblique angle flux incidence of MgO. The structural evolution during the growth verifies the formation of the MgO nanoscale islands with strong (111) preferred orientation on very thin (5–7 nm) MgO (110) layer. The MgO nanobranches, then grown on the islands, are polycrystalline consisting of many grains oriented in specific directions of <200> and <220>, supported by the nucleation theory. The LEDs with the branched ZnO/MgO nanowire arrays show a remarkable enhancement in the light output power by 21% compared with that of LEDs with pristine ZnO nanowires. Theoretical calculations using a finite‐difference time‐domain method reveal that the nanostructure is very effective in breaking the wave‐guiding mode inside the ZnO nanowires, extracting more light especially in radial direction through the MgO nanobranches.     

Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.     

Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader

As a low dimensional crystal, graphene attracts great attention as heat dissipation material due to its unique thermal transfer property exceeding the limit of bulk graphite. In this contribution, flexible graphene–carbon fiber composite paper is fabricated by depositing graphene oxide into the carbon fiber precursor followed by carbonization. In this full‐carbon architecture, scaffold of one‐dimensional carbon fiber is employed as the structural component to reinforce the mechanical strength, while the hierarchically arranged two‐dimensional graphene in the framework provides a convenient pathway for in‐plane acoustic phonon transmission. The as‐obtained hierarchical carbon/carbon composite paper possesses ultra‐high in‐plane thermal conductivity of 977 W m^?1 K^?1 and favorable tensile strength of 15.3 MPa. The combined mechanical and thermal performances make the material highly desirable as lateral heat spreader for next‐generation commercial portable electronics.