Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+‐ and Tb3+‐doped Zeolite X

The optical properties of Tb³⁺/Ce³⁺ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce³⁺ sensitized Tb³⁺ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce³⁺ concentrations or low Ce³⁺/Tb³⁺ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg‐based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite → Ce³⁺ (5d¹) → Tb³⁺ (4f⁷5d¹) energy transfer channel, which may be exploited for the down‐conversion of the Xe₂ excimer emission.     

Ligand‐Driven Wavelength‐Tunable and Ultra‐Broadband Infrared Luminescence in Single‐Ion‐Doped Transparent Hybrid Materials

Here, tuning of the optical properties of emission centers by tailoring the ligand fields is investigated. Experimentally, it is demonstrated that Ni²⁺ can act as a single emission species in multiple octahedral local environments. Nanocrystal‐embedded hybrid materials are employed as hosts in order to take advantage of their convenience in local environment design for practical applications. Novel composite gain materials with high transparence are successfully made, and show interesting wavelength‐tunable and ultra‐broadband infrared luminescence covering the whole near‐infrared region from 1 100 to 1 800 nm. The infrared luminescence peak positions can be finely tuned from 1 300 to 1 450 and to 1 570 nm, with the largest full width at half maximum being about 400 nm and covering the telecommunication bands at 1 200–1 500 nm. According to the results of characterization, the unusual luminescence, interestingly, originates from Ni²⁺ in nanocrystals and the doping efficiency of Ni²⁺ is surprisingly high. The results demonstrate that the method presented may be an effective way to fabricate multifunctional light sources with various fundamental multifunctional applications from efficient broadband optical amplifiers to bio‐imaging.     

Surface Structural Transition Induced by Gradient Polyanion‐Doping in Li‐Rich Layered Oxides: Implications for Enhanced Electrochemical Performance

Lithium‐rich layered oxides (LLOs) exhibit great potential as high‐capacity cathode materials for lithium‐ion batteries, but usually suffer from capacity/voltage fade during electrochemical cycling. Herein, a gradient polyanion‐doping strategy is developed to initiate surface structural transition to form a spinel‐like surface nanolayer and a polyanion‐doped layered core material in LLOs simultaneously. This strategy integrates the advantages of both bulk doping and surface modification as the oxygen close‐packed structure of LLOs is stabilized by polyanion doping, and the LLO cathodes are protected from steady corrosion induced by electrolytes. A LLO material modified with 5 at% phosphate (5%P@LLO) shows a high reversible discharge capacity of ≈300 mAh g^?1 at 0.1 C, excellent cycling stability with a capacity retention of 95% after 100 cycles, and enhanced electrode kinetics. This gradient doping strategy can be further extended to other polyanion‐doped LLO materials, such as borate and silicate polyanions.     

Tris(8‐hydroxyquinoline‐5‐sulfonate)aluminum Intercalated Mg–Al Layered Double Hydroxide with Blue Luminescence by Hydrothermal Synthesis

Blue luminescent hybrid materials (DDS–AQS(x%)/LDH) are successfully prepared by co‐intercalating tris(8‐hydroxyquinoline‐5‐sulfonate)aluminum anions (AQS^3?) and dodecyl sulfonate (DDS^?) with different molar ratios into Mg–Al layered double hydroxides (LDHs) by the hydrothermal and solution co‐precipitation methods. A film of the material on a quartz substrate is obtained by the solvent evaporation method. The results show the blue luminescence is remarkably different from the pristine Na₃AQS, which has cyan luminescence (ca. 450–470 nm vs. 495 nm). Furthermore, the hydrothermal product of DDS–AQS(66.67%)/LDH exhibits optimal luminous intensity and a significantly enhanced fluorescence lifetime. Nuclear magnetic resonance and Fourier‐transform infrared spectroscopy indicate that the cyan–blue luminescence transition is due to the isomerization of meridianal to facial AQS via ligand flip caused by a host–guest electrostatic interaction, in combination with the dispersion and pre‐intercalation effect of DDS. The hydrothermal conditions can promote a more ordered alignment of the intercalated fac‐AQS compared with alignment in the solution state, and the rigid LDHs environment can confine the internal mobility of AQS to keep the facial configuration stable. This stability allows a facile preparation of large amounts of blue luminous powder/film, which is a new type of inorganic–organic hybrid photofunctional material.     

Calcination‐ and Etching‐Free Photolithography of Inorganic Phosphor Films Consisting of Rare Earth Ion Doped Nanoparticles on Plastic Sheets

It is demonstrated that patterned inorganic phosphor films consisting of rare earth ion doped nanoparticles (RE‐NPs) can be fabricated on plastic sheets using calcination‐ and etching‐free photolithography. Green up‐conversion luminescence and near‐infrared (NIR) fluorescence appears from the RE‐NPs that are prepared from Y₂O₃ doped with 1 mol% Er³⁺ and 0.85 mol% Yb³⁺. The diameter of the RE‐NPs is estimated to be about 300 nm using dynamic light scattering. Visible transmittance of the RE‐NP film fabricated by dip‐coating is more than 90%. Patterned RE‐NP films are obtained by dip‐coating the RE‐NPs on patterned photoresist films fabricated by UV exposure through a photomask, followed by selective removal of the photoresist. Optical, fluorescence, scanning electron, atomic force, and Kelvin probe force microscopies are used for the characterization of the patterned RE‐NP films. The present methodology enables fabrication of patterned RE‐NP films, not only on inorganic substrates but also on plastic sheets, with low cost and material consumption.     

An Integrated Multifunctional Nanoplatform for Deep‐Tissue Dual‐Mode Imaging

The combination of biocompatible superparamagnetic and photoluminescent nanoparticles (NPs) is intensively studied as highly promising multifunctional (magnetic confinement and targeting, imaging, etc.) tools in biomedical applications. However, most of these hybrid NPs exhibit low signal contrast and shallow tissue penetration for optical imaging due to tissue‐induced optical extinction and autofluorescence, since in many cases, their photoluminescent components emit in the visible spectral range. Yet, the search for multifunctional NPs suitable for high photoluminescence signal‐to‐noise ratio, deep‐tissue imaging is still ongoing. Herein, a biocompatible core/shell/shell sandwich structured Fe₃O₄@SiO₂@NaYF₄:Nd³⁺ nanoplatform possessing excellent superparamagnetic and near‐infrared (excitation) to near‐infrared (emission), i.e., NIR‐to‐NIR photoluminescence properties is developed. They can be rapidly magnetically confined, allowing the NIR photoluminescence signal to be detected through a tissue as thick as 13 mm, accompanied by high T₂ relaxivity in magnetic resonance imaging. The fact that both the excitation and emission wavelengths of these NPs are in the optically transparent biological windows, along with excellent photostability, fast magnetic response, significant T₂‐contrast enhancement, and negligible cytotoxicity, makes them extremely promising for use in high‐resolution, deep‐tissue dual‐mode (optical and magnetic resonance) in vivo imaging and magnetic‐driven applications.     

Structural and Electrical Properties of Atomic Layer Deposited Al‐Doped ZnO Films

Structural and electrical properties of Al‐doped ZnO (AZO) films deposited by atomic layer deposition (ALD) are investigated to study the extrinsic doping mechanism of a transparent conducting oxide. ALD‐AZO films exhibit a unique layer‐by‐layer structure consisting of a ZnO matrix and Al₂O₃ dopant layers, as determined by transmission electron microscopy analysis. In these layered AZO films, a single Al₂O₃ dopant layer deposited during one ALD cycle could provide ≈4.5 × 10¹³ cm^?2 free electrons to the ZnO. The effective field model for doping is suggested to explain the decrease in the carrier concentration of ALD‐AZO films when the interval between the Al₂O₃ layers is reduced to less than ≈2.6 nm (>3.4 at% Al). By correlating the electrical and structural properties, an extrinsic doping mechanism of ALD‐AZO films is proposed in which the incorporated Al atoms take oxygen from the ZnO matrix and form doubly charged donors, such as oxygen vacancies or zinc interstitials.     

Self‐Assembled Injectable Nanocomposite Hydrogels Stabilized by Bisphosphonate‐Magnesium (Mg2+) Coordination Regulates the Differentiation of Encapsulated Stem Cells via Dual Crosslinking

Nanocomposite hydrogels consist of a polymer matrix embedded with nanoparticles (NPs), which provide the hydrogels with unique bioactivities and mechanical properties. Incorporation of NPs via in situ precipitation in the polymer matrix further enhances these desirable hydrogel properties. However, the noncytocompatible pH, osmolality, and lengthy duration typically required for such in situ precipitation strategies preclude cell encapsulation in the resultant hydrogels. Bisphosphonate (BP) exhibits a variety of specific bioactivities and excellent binding affinity to multivalent cations such as magnesium ions (Mg²⁺). Here, the preparation of nanocomposite hydrogels via self‐assembly driven by bisphosphonate‐Mg²⁺ coordination is described. Upon mixing solutions of polymer bearing BPs, BP monomer (Ac‐BP), and Mg²⁺, this effective and dynamic coordination leads to the rapid self‐assembly of Ac‐BP‐Mg NPs which function as multivalent crosslinkers stabilize the resultant hydrogel structure at physiological pH. The obtained nanocomposite hydrogels are self‐healing and exhibit improved mechanical properties compared to hydrogels prepared by blending prefabricated NPs. Importantly, the hydrogels in this study allow the encapsulation of cells and subsequent injection without compromising the viability of seeded cells. Furthermore, the acrylate groups on the surface of Ac‐BP‐Mg NPs enable facile temporal control over the stiffness and crosslinking density of hydrogels via UV‐induced secondary crosslinking, and it is found that the delayed introduction of this secondary crosslinking enhances cell spreading and osteogenesis.     

Intercalation‐Induced Tunable Stimuli‐Responsive Color‐Change Properties of Crystalline Organic Layered Compound

Layered structures accommodate guest molecules and ions in the interlayer space through intercalation. Organic layered compounds, such as layered polymers, have both intercalation and dynamic properties. Here intercalation‐induced tunable temperature‐ and mechanical‐stress‐responsive color‐change properties of crystalline layered polydiacetylene (PDA) as an organic layered compound are reported. In general, organic materials with stimuli responsivity are developed by molecular design and synthesis. In the present work, intercalation of guest metal cations in the layered PDA directs tuning of the stimuli‐responsive color‐change properties, such as color, responsivity, and reversibility. Whereas PDA without intercalation of metal ions distinctly changes the color from blue to red at the threshold temperature, the PDA with intercalation of the divalent metal ions (PDA‐M²⁺) shows a variety of color‐change properties. The present study indicates that intercalation has versatile potentials for functionalization of organic layered compounds.     

Visible and NIR Upconverting Er3+–Yb3+ Luminescent Nanorattles and Other Hybrid PMO‐Inorganic Structures for In Vivo Nanothermometry

Lanthanide‐doped luminescent nanoparticles are an appealing system for nanothermometry with biomedical applications due to their sensitivity, reliability, and minimal invasive thermal sensing properties. Here, four unique hybrid organic–inorganic materials prepared by combining β‐NaGdF₄ and PMOs (periodic mesoporous organosilica) or mSiO₂ (mesoporous silica) are proposed. PMO/mSiO₂ materials are excellent candidates for biological/biomedical applications as they show high biocompatibility with the human body. On the other hand, the β‐NaGdF₄ matrix is an excellent host for doping lanthanide ions, even at very low concentrations with yet very efficient luminescence properties. A new type of Er³⁺–Yb³⁺ upconversion luminescence nanothermometers operating both in the visible and near infrared regime is proposed. Both spectral ranges permit promising thermometry performance even in aqueous environment. It is additionally confirmed that these hybrid materials are non‐toxic to cells, which makes them very promising candidates for real biomedical thermometry applications. In several of these materials, the presence of additional voids leaves space for future theranostic or combined thermometry and drug delivery applications in the hybrid nanostructures.     

The Active‐Core/Active‐Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide‐Doped Nanoparticles

Nanoparticles of NaGdF₄ doped with trivalent erbium (Er³⁺) and ytterbium (Yb³⁺) are prepared by a modified thermal decomposition synthesis from trifluoroacetate precursors in 1‐octadecene and oleic acid. The nanoparticles emit visible upconverted luminescence on excitation with near‐infrared light. To minimize quenching of this luminescence by surface defects and surface‐associated ligands, the nanoparticles are coated with a shell of NaGdF₄. The intensity of the upconversion luminescence is compared for nanoparticles that were coated with an undoped shell (inert shell) and similar particles coated with a Yb³⁺‐doped shell (active shell). Luminescence is also measured for nanoparticles lacking the shell (core only), and doped with Yb³⁺ at levels corresponding to the doped and undoped core/shell materials respectively. Upconversion luminescence was more intense for the core/shell materials than for the uncoated nanoparticles, and is greatest for the materials having the “active” doped shell. Increasing the Yb³⁺ concentration in the “core‐only” nanoparticles decreases the upconversion luminescence intensity. The processes responsible for the upconversion are presented and the potential advantages of “active‐core”/“active‐shell” nanoparticles are discussed.     

Multivalent‐Ion‐Mediated Stabilization of Hydrogen‐Bonded Multilayers

Hydrogen‐bonding interactions are an important alternative to electrostatic interactions for assembling multilayer thin films of uncharged components. Herein, a new method is reported for rendering such films stable at pH values close to physiological conditions. Multilayer films based on hydrogen bonding are assembled by the alternate deposition of poly[(styrene sulfonic acid)‐co‐(maleic acid)] (PSSMA) and poly(N‐isopropylacrylamide) (PNiPAAm) at pH 2.5. The use of PSSMA results in multilayers that contain free styrene sulfonate groups, as these moieties do not interact with the PNiPAAm functional groups. Subsequent infiltration of a multivalent ion (Ce⁴⁺ or Fe³⁺) leads to an increase in the total film mass, with little impact on the film morphology, as determined by using atomic force microscopy. To examine the film stability, the resulting films have been exposed to elevated pH (7.1). While there is substantial swelling of the multilayers (25 % and 55 % for Ce⁴⁺‐ and Fe³⁺‐stabilized films, respectively), film loss is negligible. This provides a stark contrast with non‐stabilized films, which disassemble almost immediately upon exposure to pH 7.1. This method represents a simple and effective strategy for stabilizing hydrogen‐bonded structures non‐covalently. Further, the multivalent ions also render the films responsive to changes in the local redox environment, as demonstrated by film disassembly after exposure of Fe³⁺‐treated films to iodide solutions.     

Assembly of Layered Rare‐Earth Hydroxide Nanosheets and SiO2 Nanoparticles to Fabricate Multifunctional Transparent Films Capable of Combinatorial Color Generation

Colloidal solutions of layered rare‐earth hydroxide nanosheets provide a simple route to deposit ultra thin luminescence films. The antireflection and antifogging properties were integrated into transparent luminescent films by the layer‐by‐layer assembly of Eu³⁺, Tb³⁺, Dy³⁺ doped‐hydroxocation nanosheets and negatively‐charged SiO₂ nanoparticles. Resulting multifunctional films exhibited efficient red, green, and blue emissions with controllable intensity. Highly improved transmittance enabled us to display combinatorial color luminescence, which can be achieved by multiply overlapping individual films with different combinations, without significant loss of transparency. Triple overlap of red/green/blue films generated an excellent white‐light under 254 nm UV irradiation.     

Interfacial Reaction‐Directed Synthesis of Ce–Mn Binary Oxide Nanotubes and Their Applications in CO Oxidation and Water Treatment

Interfacial oxidation–reduction reaction is herein developed to prepare hollow binary oxide nanostructures. Ce–Mn nanotubes are fabricated by treating Ce(OH)CO₃ templates with KMnO₄ aqueous solution, where MnO₄^? is reduced to manganese oxide and the Ce³⁺ in Ce(OH)CO₃ is simultaneously oxidized to form cerium oxide, followed by selective wash with HNO₃. The resulting Ce–Mn binary oxide nanotubes exhibit high catalytic activity towards CO oxidation and show significant adsorption capacity of Congo red. Moreover, guided by the same interfacial‐reaction principle, binary oxide hollow nanostructures with different shapes and compositions are synthesized. Specifically, hollow Ce–Mn binary oxide cubes, and Co‐Mn and Ce‐Fe binary oxide hollow nanostructures are achieved by changing the shape of the Ce(OH)CO₃ templates from rods to cubes, by changing the tempates from Ce(OH)CO₃ nanorods to Co(CO₃)_0.35Cl_0.20(OH)_1.10 nanowires, and by replacing the oxidant of KMnO₄ with another strong one, K₂FeO₄, respectively. This work is expected to open a new, simple avenue for the general synthesis of hollow binary oxide nanostructures.     

Ru Species Supported on MOF‐Derived N‐Doped TiO2/C Hybrids as Efficient Electrocatalytic/Photocatalytic Hydrogen Evolution Reaction Catalysts

The development of efficient catalysts is of great importance for hydrogen evolution reaction (HER) of water splitting via electrocatalytic/photocatalytic processes to remediate the current severe environmental and energy problems. By aid of the stabilization effects of uncoordinated groups and inherent pore‐confinement of amine‐functionalized metal–organic frameworks (NH₂‐MIL‐125), two forms of Ru species including nanoparticles (NPs) and/or single atoms (SAs) can be firmly embedded in NH₂‐MIL‐125 derived N‐doped TiO₂/C support (N‐TC), and thus obtain two kinds of samples named Ru‐NPs/SAs@N‐TC and Ru‐SAs@N‐TC, respectively. In the synthetic process, the initial feeding amount of Ru³⁺ ions not only strongly determines the final size and dispersion states of Ru species but also the morphology and defective structures of N‐TC support. Impressively, Ru‐NPs/SAs@N‐TC exhibit superior catalytic activities to Ru‐SAs@N‐TC for either electrocatalytic or photocatalytic HER. This should be attributed to its larger specific surface area and benefiting from synergistic coupling of Ru NPs and Ru SAs. It is envisioned that the present work can provide a new avenue for development of high‐efficiency and multifunctional hybrid catalysts in sustainable energy conversion.     

Room‐Temperature Ion‐Exchange‐Mediated Self‐Assembly toward Formamidinium Perovskite Nanoplates with Finely Tunable,Ultrapure Green Emissions for Achieving Rec. 2020 Displays

Delicate engineering of chromaticity is required to faithfully reproduce colors in a backlit display, this is extremely difficult for green downconverters because the human eye is highly sensitive to green colors. The central challenge is to achieve finely tunable green emissions in the narrow range of 525–535 nm while keeping the full width at half maximum (FWHM) <25 nm at the same time. Here, a room‐temperature ion‐exchange‐mediated self‐assembly strategy for preparing FAPbBr₃ (FA = CH(NH₂)₂⁺) nanoplates (NPs) to fulfill this goal is introduced. 2D layered OA₂PbBr₄ (OA is octadecylamine) NPs are first synthesized by spontaneous reprecipitation, and are then transformed into FAPbBr₃ NPs through a OA⁺‐to‐FA⁺ exchange induced self‐assembly of HP monolayers. A c‐axis contraction in this process makes a relative large thickness variation in OA₂PbBr₄ NPs, which can be realized by simply varying the precursor concentration, only result in a small thickness change in subsequent FAPbBr₃ NPs, thereby enabling finely tunable emissions in the range of 525–535 nm along with FWHM <25 nm and a quantum yield up to 85%. As a downconverter, the FAPbBr₃ NPs realize an ultrapure green backlight that covers ≈95% Rec. 2020 standard in the CIE 1931 color space.     

Etching‐Doping Sedimentation Equilibrium Strategy: Accelerating Kinetics on Hollow Rh‐Doped CoFe‐Layered Double Hydroxides for Water Splitting

Exploring highly active and inexpensive bifunctional electrocatalysts for water‐splitting is considered to be one of the prerequisites for developing hydrogen energy technology. Here, an efficient simultaneous etching‐doping sedimentation equilibrium (EDSE) strategy is proposed to design and prepare hollow Rh‐doped CoFe‐layered double hydroxides for overall water splitting. The elaborate electrocatalyst with optimized composition and typical hollow structure accelerates the electrochemical reactions, which can achieve a current density of 10 mA cm^?2 at an overpotential of 28 mV (600 mA cm^?2 at 188 mV) for hydrogen evolution reaction (HER) and 100 mA cm^?2 at 245 mV for oxygen evolution reaction (OER). The cell voltage for overall water splitting of the electrolyzer assembled by this electrocatalyst is only 1.46 V, a value far lower than that of commercial electrolyzer constructed by Pt/C and RuO₂ and most reported bifunctional electrocatalysts. Furthermore, the existence of Fe vacancies introduced by Rh doping and the typical hollow structure are demonstrated to optimize the entire HER and OER processes. EDSE associates doping with template‐directed hollow structures and paves a new avenue for developing bifunctional electrocatalysts for overall water splitting. It is also believed to be practical in other catalysis fields as well.     

Electronic Structure Regulation of Layered Vanadium Oxide via Interlayer Doping Strategy toward Superior High‐Rate and Low‐Temperature Zinc‐Ion Batteries

Currently, development of suitable cathode materials for zinc‐ion batteries (ZIBs) is plagued by the sluggish kinetics of Zn²⁺ with multivalent charge in the host structure. Herein, it is demonstrated that interlayer Mn²⁺‐doped layered vanadium oxide (Mn_0.15V₂O₅·nH₂O) composites with narrowed direct bandgap manifest greatly boosted electrochemical performance as zinc‐ion battery cathodes. Specifically, the Mn_0.15V₂O₅·nH₂O electrode shows a high specific capacity of 367 mAh g^?1 at a current density of 0.1 A g^?1 as well as excellent retentive capacities of 153 and 122 mAh g^?1 after 8000 cycles at high current densities up to 10 and 20 A g^?1, respectively. Even at a low temperature of ?20 °C, a reversible specific capacity of 100 mAh g^?1 can be achieved at a current density of 2.0 A g^?1 after 3000 cycles. The superior electrochemical performance originates from the synergistic effects between the layered nanostructures and interlayer doping of Mn²⁺ ions and water molecules, which can enhance the electrons/ions transport kinetics and structural stability during cycling. With the aid of various ex situ characterization technologies and density functional theory calculations, the zinc‐ion storage mechanism can be revealed, which provides fundamental guidelines for developing high‐performance cathodes for ZIBs.     

Dual Emission of Water‐Stable 2D Organic–Inorganic Halide Perovskites with Mn(II) Dopant

Moisture‐delicate and water‐unstable organic–inorganic halide perovskites (OI‐HPs) create huge challenges for the synthesis of highly efficient water‐stable light‐emitting materials for optoelectronic devices. Herein, a simple acid solution–assisted method to synthesize quantum confined 2D lead perovskites through Mn doping is reported. The efficient energy transfer between host and dopant ions in orange light‐emitting Mn²⁺‐doped OI‐HPs leads to the most efficient integrated luminescence with a photoluminescence quantum yield over 45%. The Mn²⁺ substitution of Pb²⁺ and passivation with low dielectric constant molecules such as phenethylamine, benzylamine, and butylamine enhance water resistivity, leading to water stability. The dual emission process of this water‐stable 2D Mn‐doped perovskite will help in developing highly efficient 2D water‐stable perovskites for practical applications.     

Wavelength‐Emission Tuning of ZnO Nanowire‐Based Light‐Emitting Diodes by Cu Doping: Experimental and Computational Insights

The band‐gap engineering of doped ZnO nanowires is of the utmost importance for tunable light‐emitting‐diode (LED) applications. A combined experimental and density‐functional theory (DFT) study of ZnO doping by copper (Zn²⁺ substitution by Cu²⁺) is presented. ZnO:Cu nanowires are epitaxially grown on magnesium‐doped p‐GaN by electrochemical deposition. The heterojunction is integrated into a LED structure. Efficient charge injection and radiative recombination in the Cu‐doped ZnO nanowires are demonstrated. In the devices, the nanowires act as the light emitters. At room temperature, Cu‐doped ZnO LEDs exhibit low‐threshold emission voltage and electroluminescence emission shifted from the ultraviolet to violet–blue spectral region compared to pure ZnO LEDs. The emission wavelength can be tuned by changing the copper content in the ZnO nanoemitters. The shift is explained by DFT calculations with the appearance of copper d states in the ZnO band‐gap and subsequent gap reduction upon doping. The presented data demonstrate the possibility to tune the band‐gap of ZnO nanowire emitters by copper doping for nano‐LEDs.