The preparation of oxygen-deficient ZnO nanorod arrays and their enhanced field emission

Vertical and uniform zinc oxide (ZnO) nanorod arrays (NRAs) with sharp tips were fabricated on Zn substrate by a straightforward hydrothermal method without the assistance of seed layer, template or surfactant. Whereafter, the as-synthesized ZnO NRAs were successfully doped with oxygen vacancies by sodium borohydride (NaBH₄) solution reduction, aiming to generate donor energy levels below the conduction band. More importantly, the doped concentration of oxygen vacancies could be effectively controlled by adjusting the reduction temperature, and we have ultimately achieved the purpose of controllable tailoring the energy band structure of ZnO NRAs. As with design, the oxygen-deficient ZnO NRAs present a lower turn-on field of 0.67 V/μm, higher field enhancement factor of 64601 and better field emission stability. Such excellent FE performance of the as-prepared emitter should originate from the optimization of geometry, the efficient electron transport, as well as the decreased work function.     

Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed polymer solar cells.     

Field Emission From Hydrothermally Grown ZnO Nanoinjectors

An injector-like nanostructure of ZnO fabricated by hydrothermal method was tested for field emission (FE) properties. The turn-on field of 3.8 V/mum was obtained at a current density of 0.1 muA/cm². Above the turn-on field, the emission current density quickly raised up to 1 mA/cm² at about 12.8 V/mum. The field enhancement factor beta was estimated to be 2556 and 1226 in the low and high field regions, respectively. The two-slope FE behavior was attributed to the adsorbates and defects which were introduced in the fabrication process in aqueous solution at low temperature. The good FE properties of the ZnO nanoinjectors indicate that they are applicable as low-cost and low-temperature cathode materials for FE-based devices.     

限流电阻层改善碳纳米管场发射显示器发光均匀度的研究

针对碳纳米管(CNT)场发射阴极薄膜中,CNT个体差异及其与衬底的不良接触对发光均匀性的影响,引入反馈限流电阻层以改善阴极薄膜的场发射发光均匀性.采用丝网印刷工艺在衬底上制备氧化锌作为电阻限流层,在其上制备CNT阴极薄膜.对CNT薄膜阴极的发射电流稳定性和均匀性进行了测试,给出了电阻限流层对场发射特性曲线的影响效果.SEM分析表明,氧化锌电阻层有利于消除CNT阴极的尖端屏蔽效应,并且使得CNT与衬底具有更加紧密的接触.场发射特性和场发射发光照片表明,虽然随着限流层厚度增加,阈值电压有所增加,发射电流有所减小,然而限流层的存在有效地改善了发射电流的稳定性,使得发射电流和场发射发光点分布更加均匀.     

Periodic TiO2 Nanorod Arrays with Hexagonal Nonclose‐Packed Arrangements: Excellent Field Emitters by Parameter Optimization

Periodic TiO₂ nanorod arrays with hexagonal nonclose‐packed (hncp) arrangements are synthesized by pulsed laser deposition (PLD) using polystyrene colloidal monolayers as templates and with subsequent annealing in air. The hncp‐array formation is governed by in situ volume shrinkage of amorphous TiO₂ nanorods in the crystallizing process during annealing. The array periodicity can easily be tuned by different sphere sizes of the colloidal template, whereas the distance between neighboring nanorods can be controlled by altering the background gas pressure during the PLD process, at a given periodicity for the nanorod array. Parameter‐controlled growth is helpful for investigating and optimizing the parameter‐dependent field‐emission properties. The hncp nanorod array exhibits an enhanced field‐emission (FE) performance compared to both particle films and nanorod arrays with top aggregation. With an increase in periodicity of a hncp nanorod array, the field‐enhancement factor decreases and the turn‐on FE field increases. FE characteristics can be further enhanced by increasing the distance between adjacent nanorods while maintaining the same periodicity. The parameter‐optimized results suggest that the arrays with a smaller periodicity and a larger distance display the best FE performance and could be highly valuable for designing field‐emission devices based on these periodic nanorod arrays.     

Luminescence Properties of Cobalt-Doped ZnO Films Prepared by Sol–Gel Method

Pure ZnO and Co-doped ZnO films have been deposited on coverslip substrates by sol–gel spin coating. The morphological, structural, and optical properties of the films were investigated. The microstructure of the ZnO films became increasingly fine and the crystalline size decreased with Co doping. Analysis of x-ray diffraction (XRD) and Raman spectra reveals that Co²⁺ ions are substituted for Zn²⁺ ions in the ZnO lattice without changing its wurtzite structure. Co doping induces a decrease of the band-gap energy and fluorescence quenching of the emission bands. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra. Photoluminescence (PL) spectra under different excitation energies and PL excitation spectra for the visible emissions suggest that the orange–red emission and green emission could be related to interstitial zinc (Zn_i) shallow donors and oxygen vacancy (V _O) deep donors, respectively. The red emission of Co-doped ZnO film could be assigned to the radiative transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal after band-to-band excitation. A consistent explanation for the pure and Co-doped ZnO films is that the red emission under the excitation energy below the band gap is probably associated with extended Zn_i states.     

Giant Red-Shifted Emission in (Sr,Ba)Y2O4:Eu2+ Phosphor Toward Broadband Near-Infrared Luminescence

Near-infrared (NIR) light-emitting diodes (LEDs) light sources are desirable in photonic, optoelectronic, and biological applications. However, developing broadband red and NIR-emitting phosphors with good thermal stability is always a challenge. Herein, the synthesis of Eu²⁺-activated SrY₂O₄ red phosphor with high photoluminescence quantum efficiency and broad emission band ranging from 540 to 770 nm and peaking at 620 nm under 450 nm excitation is designed. Sr/Ba substitution in SrY₂O₄:Eu²⁺ has been further utilized to achieve tunable emission by modifying the local environment, which facilitates the giant red-shifted emission from 620 to 773 nm while maintaining the outstanding thermal stability of SrY₂O₄:Eu²⁺. The NIR emission is attributed to the enhanced Stokes shift and crystal field strength originated from the local structural distortions of [Y1/Eu1O₆] and [Y2/Eu2O₆]. The investigation in charge distribution around Y/Eu provides additional insight into increasing covalency to tune the emission toward the NIR region. As-fabricated NIR phosphor-converted LEDs demonstration shows its potential in night-vision technologies. This study reveals the NIR luminescence mechanism of Eu²⁺ in oxide-based hosts and provides a design principle for exploiting Eu²⁺-doped NIR phosphors with good thermal stability.     

Selective growth and enhanced field emission properties of micropatterned iron phthalocyanine nanofiber arrays

In this study, we developed a simple method for the micropatterned growth of iron phthalocyanine (FePc) nanofiber arrays using a thermal evaporation process. By controlling the surface energy and the temperature of the substrate (T_sub), we obtained FePc films featuring a grain-like (in-plane) morphology on Si surfaces (higher surface energy) and a fiber-like (out-of-plane) morphology on Ag surfaces (lower surface energy) within a certain range of values of T_sub. On the Ag surfaces, these temperature-induced FePc nanofibers featured a high aspect ratio (AR) of 30.3 ± 3.6, with a mean length of 699 ± 216 nm and a mean radius of 22.2 ± 4.3 nm, as-prepared at a value of T_sub of 240 °C. The FePc films obtained at values of T_sub of 25, 120, 180, and 240 °C all possessed α-phase crystalline structures. Because the growth structures of the FePc molecules on the Si and Ag substrates were quite different, we could control the growth of micropatterned 1D FePc nanofiber arrays on previously patterned Ag/Si substrates. From the comparison of the field emission (FE) properties in different ARs of patterned devices, higher AR (30.3 ± 3.6) of devices (FE-240-P; T_sub of 240 °C) exhibited better FE performance than lower AR (6.0 ± 2.6) of devices (FE-180-P; T_sub of 180 °C). The FE current density of devices (T_sub of 240 °C) increased from 0.13 mA/cm² for the unpatterned device (FE-240-N) to 6.77 mA/cm² for the patterned device (FE-240-P) at an applied electric field of 12 V/μm. The turn-on electric fields required to produce a current density of 10 μA/cm² were 7.7 and 10.3 V/μm for the patterned and unpatterned FePc emitters, respectively. From the slopes of Fowler–Nordheim plots, we estimated the field enhancement factors (β) of FE-240-P and FE-240-N to be 314 and 329, respectively. Studies of the emission current stability revealed that the FePc nanofibers possessed outstanding anti-degrading capability. During stability tests, the micropatterned FePc emitter (FE-240-P) displayed an efficient emission current with fluctuations of less than 20%. Because this facile platform allows control over the morphologies of films of small organic molecules merely by tuning the surface energy of the substrates, such micropatterned-FePc nanofibers might have great applicability in practical field emitters.     

Nanostructured ZnO Films for Room Temperature Ammonia Sensing

Zinc oxide (ZnO) thin films have been deposited by a reactive dc magnetron sputtering technique onto a thoroughly cleaned glass substrate at room temperature. X-ray diffraction revealed that the deposited film was polycrystalline in nature. The field emission scanning electron micrograph (FE-SEM) showed the uniform formation of a rugby ball-shaped ZnO nanostructure. Energy dispersive x-ray analysis (EDX) confirmed that the film was stoichiometric and the direct band gap of the film, determined using UV–Vis spectroscopy, was 3.29 eV. The ZnO nanostructured film exhibited better sensing towards ammonia (NH₃) at room temperature (～30°C). The fabricated ZnO film based sensor was capable of detecting NH₃ at as low as 5 ppm, and its parameters, such as response, selectivity, stability, and response/recovery time, were also investigated.     

Template Deformation‐Tailored ZnO Nanorod/Nanowire Arrays: Full Growth Control and Optimization of Field‐Emission

Here, a facile and effective route toward full control of vertical ZnO nanorod (NR)/nanowire (NW) arrays in centimeter‐scale areas and considerable improvement of field‐emission (FE) performance is reported. Controlled deformation of colloidal crystal monolayer templates is introduced by heating near glass‐transition temperature. The NR/NW density, uniformity, and tapering were all adjusted through selection of template size and deformation, and electrolyte composition. In line with the adjustments, the field‐emission performance of the arrays is significantly improved. A low turn‐on electric field of 1.8 V µm^?1, a field‐enhancement factor of up to 5 750, and an emitting current density of up to 2.5 mA cm^?2 were obtained. These improved parameters would benefit their potential application in cold‐cathode‐based electronics.     

A Novel Buffering Technique for Aqueous Processing of Zinc Oxide Nanostructures and Interfaces,and Corresponding Improvement of Electrodeposited ZnO‐Cu2O Photovoltaics

A novel buffering method is presented to improve the stability of zinc oxide processed in aqueous solutions. By buffering the aqueous solution with a suitable quantity of sacrificial zinc species, the dissolution of functional zinc oxide structures and the formation of unwanted impurities can be prevented. The method is demonstrated for ZnO films and nanowires processed in aqueous solutions used for the selective etching of mesoporous anodic alumina templates and the electrochemical deposition of Cu₂O. In both cases, improved ZnO stability is observed with the buffering method. ZnO‐Cu₂O heterojunction solar cells (bilayer and nanowire cells) synthesized using both traditional and buffered deposition methods are characterized by impedance spectroscopy and solar simulation measurements. Buffering the Cu₂O deposition solution is found to reduce unwanted recombination at the heterojunction and improve the photovoltaic performance.     

溶胶-凝胶法制备Co,Cu共掺杂ZnO薄膜 结构及光学特性

采用溶胶-凝胶法在玻璃基片上制备了纯ZnO薄膜和高浓度Cu掺杂的Co,Cu共掺ZnO(Zn0.90CoxCu0.1-xO,x=0.01,0.03,0.05)薄膜。扫描电镜观察到无论是纯ZnO还是掺杂ZnO薄膜表面都有均匀分布的颗粒,但是在Cu含量较高时均匀性更好。X射线衍射揭示所有样品都具有纤锌矿结构,但是Cu掺杂量的增加使晶格常数略有减小,而晶粒尺寸却略有增大。XPS测试结果表明样品中Co离子的价态为+2价和+3价,Cu离子的价态为+2价和+1价共存。室温光致发光测量在所有样品中均观察到较强的紫外发光峰、蓝光双峰和较弱的绿光发光峰。     

Cathodoluminescence of ZnO/GaN/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> heteroepitaxial structures grown by chemical vapor deposition

The cathodoluminescent properties of ZnO films in ZnO/GaN/α-Al₂O₃ and ZnO/α-Al₂O₃ heteroepitaxial structures grown by chemical vapor deposition in a low-pressure flowing-gas reactor were studied and compared. A superlinear dependence of the excitonic-band intensity in the cathodoluminescence spectrum of the ZnO/GaN/α-Al₂O₃ structures on the electron-beam current is ascertained, which indicates that the emission is stimulated for relatively low thresholds of the excitation intensity. It is shown that the ZnO films grown on the GaN substrates exhibit a much more effective cathodoluminescence compared to the cathodoluminescence in the films grown on α-Al₂O₃. It was observed that the luminescent properties of ZnO layers in the ZnO/GaN/α-Al₂O₃ structures subjected to long-term heat treatment at 750°C in an oxygen atmosphere exhibit a high thermal stability.     

Enhancing Long-Term Thermal Stability of Non-Fullerene Organic Solar Cells Using Self-Assembly Amphiphilic Dendritic Block Copolymer Interlayers

Herein, interfacial engineering is demonstrated to improve the thermal stability of non-fullerene bulk-heterojunction (BHJ) OPVs to a practical level. An amphiphilic dendritic block copolymer (DBC) is developed through a facile coupling method and employed as the surface modifier of ZnO electron-transporting layer in inverted OPVs. Besides showing distinct self-assembly behavior, the synthesized DBC possesses high compatibility with plasmonic gold nanoparticles (NPs) due to the constituent malonamide and ethylene oxide units. The hybrid DBC@AuNPs interlayer is shown to improve device's performance from 14.0% to 15.4% because it enables better energy-level alignment and improves interfacial compatibility at the ZnO/BHJ interface. Moreover, the DBC@AuNPs interlayer not only improves the interfacial thermal stability at the ZnO/BHJ interface but also endows a more ideal BHJ morphology with an enhanced thermal robustness. The derived device reserves 77% of initial PCE after thermal aging at 65 °C for 3000 h and yields an extended T₈₀ lifetime of >1100 h when stored at a constant thermal condition at 65 °C, outperforming the control device. Finally, the device is evaluated to possess a T₈₀ lifetime of over 1.79 years at room temperature (298 K) when stored in an inert condition, showing great potential for commercialization.     

The optical properties of rare earth Gd doped ZnO nanocrystals

We present a study of the light emission properties, from UV to blue spectral region, of Gd doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The samples were grown from a mixed Zn/Gd source, with a molar percentage of Gd ranging from 0% (pure ZnO) to 5%, 10%, or 15%, in a constant O₂/Ar gas mixture flowing at 500° C. The pure ZnO nanocrystals exhibited a strong and predominant UV emission peaking at 375 nm. Besides the UV emission of ZnO nanocrystals, two strong blue emissions, located at 432 and 397 nm, are observed for the sample doped with 5% Gd. The strong blue emissions are mainly induced by the impurity levels of Gd introduced into the band gap of the ZnO nanocrystals. The UV emission of ZnO decreases as the doping concentration of Gd increases, and the blue emission is replaced by a broad defect emission due to the greater number of defects and impurities, as well as Gd₂O₃ on the surface. The results show that the optical properties of ZnO can be tuned by the doping concentration of Gd.     

Electron Field Emission of Geometrically Modulated Monolayer Semiconductors

Electron field emission, electrons emitted from solid surfaces under high electric field, offers significant scientific interests in materials sciences and potential optoelectronics applications. 2D atomic layers, such as MoS₂, exhibit fascinating properties for diverse applications in next‐generation nanodevices and rich physical phenomena for fundamental research. However, the study on field emission of semiconducting monolayers is lacking owing to its low efficiency and stability of electron emission. Here, electron field emission of the geometrically modulated monolayer semiconductors suspended with 1D nanoarrays is demonstrated. Chemical vapor deposition synthesis of two prototype monolayers of transition metal dichalcogenides (TMD), MoS₂ and MoSe₂, is presented and their diverse band structures offer an ideal platform to explore the fundamental process of the electron emission in the TMD. Geometrical modulation and charge transfer of the semiconducting monolayers can be significantly tuned with the structural suspension with the 1D ZnO nanoarrays. Possible mechanisms on the enhanced electron emission of the 2D monolayers are discussed. With geometrical control of the monolayers, a highly efficient and stable electron emission of TMD monolayers is achieved in low turn‐on electric fields, enabling applications on electrons sources and opening a new avenue toward geometrically tuned atomic layers.     

Thin SnO2 Nanowires with Uniform Diameter as Excellent Field Emitters: A Stability of More Than 2400 Minutes

The stability of a field‐emission event, i.e., the stability of the emission current over a long period of time, against thermal effects, etc., is one of the key factors for its application in real devices. Although nanostructures have the advantages of high aspect ratios and faster device turn‐on times, the small masses and large surface areas make them vulnerable to both chemical and physical damages and they have a lower melting point compared to bulk materials of same compositions. SnO₂, one of the most attractive oxide semiconductors, which has with a relatively low work function of 4.7 eV, has been a perspective candidate for field emitters. A highly stable field emitter based on thin and quasi‐aligned SnO₂ nanowire ensembles with uniform diameter is shown. Field‐emission measurements of these SnO₂ nanowire ensembles show low turn‐on and threshold voltages of 3.5 V μm^?1 and 4.63 V μm^?1, respectively, at an anode–sample distance of 200 μm and very long term scale stability of more than 2400 min, acquired at the electric field of 4.65 V μm^?1. Such values are not only better than those of the recently developed SnO₂ nanostructures with different morphologies and of randomly oriented SnO₂ nanowire ensembles with a similar diameter distribution, but also comparable with the most widely studied field‐emission materials, such as carbon nanotubes and ZnO nanostructures. The potential for using these thin SnO₂ nanowire ensembles with uniform diameter in field emitters is shown, with particular promise in those operated for long‐term real device applications.     

Theoretical and experimental investigations of the thermoelectric properties of Al-, Bi- and Sn-doped ZnO

In this paper, the thermoelectric properties of ZnO doped with Al, Bi and Sn were investigated by combining experimental and theoretical methods. The average Seebeck coefficient of Bi doped ZnO over the measured temperature range is improved from −90 to −497 μV/K. However, segregation of Bi₂O₃ in ZnO:Bi sample, confirmed by FESEM, lead to enormous grain growth and low electrical conductivity, which makes Bi is not a good dopant to improve ZT value of ZnO. As a 4+ valence cation, Sn doping actually show an increase in carrier concentration to 10²⁰ cm⁻³, further enhancing the electrical conductivity. Unfortunately, the Seebeck coefficient of ZnO:Sn samples is even lower than pure ZnO sample, which lead to a low ZT value. As for ZnO:Al sample, with nearly no change in lattice thermal conductivity, electrical conductivity and Seebeck coefficient were both enhanced. Threefold enhancement in ZT value has been achieved in ZnO:Al sample at 760 °C compared with pure ZnO.     

Synthesis and characterization of phosphorus-doped ZnO and (Zn,Mg)O thin films via pulsed laser deposition

The transport and optical properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied. The carrier type of as-deposited (Zn,Mg)O:P films converts from n-type to p-type with increasing oxygen partial pressure. All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O₃/O₂ mixture is two orders of magnitude higher than the films grown in oxygen and O₂/Ar/H₂ mixture. The room-temperature photoluminescence (PL) of the as-deposited films has been shown that growing in the O₂/Ar/H₂ mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The enhanced ultraviolet (UV) emission in the films grown in O₂/Ar/H₂ mixture may result from hydrogen passivation of the deep level emission centers. The annealed ZnO:P films are n-type with nonlinear dependence of resistivity on annealing temperature. The resistivity increases in the films with annealing at 800°C while decreasing with further increasing annealing temperature. Strong visible light emission is observed from the ZnO:P films annealed in oxygen.     

One step hydrothermal synthesis of SnO2-RGO nanocomposite and its field emission studies

Field emission (FE) properties of hydrothermally synthesized, SnO₂-RGO nanocomposite have been investigated at a base pressure of 1×10⁻⁸mbar. The results reveal that the SnO₂-RGO nanocomposite emitter prevails over the pristine RGO emitter. The values of turn-on field, defined at emission current density of 1 μA/cm², are found to be 1.8 and 2.2 V/μm for the SnO₂-RGO and pristine RGO emitters, respectively. Furthermore, the SnO₂-RGO emitter delivers maximum emission current density of ~800 µA/cm² at an applied field of 5 V/μm. The observed values of applied field corresponding to emission current densities of 1 μA/cm² and 10 µA/cm² are superior to those reported for various emitters due to SnO₂ nanostructures and their composites. The emission current at the pre-set value of 1 µA is found to be very stable over a period of 3hrs. The enhanced FE behaviour of SnO₂-RGO nanocomposite emitter has been attributed to synergic effect due to its nanometric dimensions offering high aspect ratio and modulation of electronic properties via formation of heterostructure. The results obtained herein propose the SnO₂-RGO nanocomposite as a prospective candidate for FE based vacuum microelectronic devices.