Printable Skin‐Driven Mechanoluminescence Devices via Nanodoped Matrix Modification

Mechanically driven light generation is an exciting and under‐exploited phenomenon with a variety of possible practical applications. However, the current driving mode of mechanoluminescence (ML) devices needs strong stimuli. Here, a flexible sensitive ML device via nanodopant elasticity modulus modification is introduced. Rigid ZnS:M²⁺(Mn/Cu)@Al₂O₃ microparticles are dispersed into soft poly(dimethylsiloxane) (PDMS) film and printed out to form flexible devices. For various flexible and sensitive scenes, SiO₂ nanoparticles are adopted to adjust the elasticity modulus of the PDMS matrix. The doped nanoparticles can concentrate stress to ZnS:M²⁺(Mn/Cu)@Al₂O₃ microparticles and achieve intense ML under weak stimuli of the moving skin. The printed nano‐/microparticle‐doped matrix film can achieve skin‐driven ML, which can be adopted to present fetching augmented animations expressions. The printable ML film, amenable to large areas, low‐cost manufacturing, and mechanical softness will be versatile on stress visualization, luminescent sensors, and open definitely new functional skin with novel augmented animations expressions, the photonic skin.     

Temporal and Remote Tuning of Piezophotonic‐Effect‐Induced Luminescence and Color Gamut via Modulating Magnetic Field

Light‐emitting materials have been extensively investigated because of their widespread applications in solid‐state lighting, displays, sensors, and bioimaging. In these applications, it is highly desirable to achieve tunable luminescence in terms of luminescent intensity and wavelength. Here, a convenient physical approach of temporal and remote tuning of light‐emitting wavelength and color is demonstrated, which is greatly different from conventional methods. It is shown that by modulating the frequency of magnetic‐field excitation at room temperature, luminescence from the flexible composites of ZnS:Al, Cu phosphors induced by the piezophotonic effect can be tuned in real time and in situ. The mechanistic investigation suggests that the observed tunable piezophotonic emission is ascribed to the tilting band structure of the ZnS phosphor induced by magnetostrictive strain under a high frequency of magnetic‐field excitation. Furthermore, some proof‐of concept devices, including red–green–blue full‐color displays and tunable white‐light sources are demonstrated simply by frequency modulation. A new understanding of the fundamentals of both luminescence and magnetic–optics coupling is thus provided, while offering opportunities in magnetic–optical sensing, piezophotonics, energy harvesting, novel light sources, and displays.     

Time resolved spectroscopic studies on some nanophosphors

Time resolved spectroscopy is an important tool for studying photophysical processes in phosphors. Present work investigates the steady state and time resolved photoluminescence (PL) spectroscopic characteristics of ZnS, ZnO and (Zn, Mg)O nanophosphors both in powder as well as thin film form. Photoluminescence (PL) of ZnS nanophosphors typically exhibit a purple/blue emission peak termed as self activated (SA) luminescence and emission at different wavelengths arising due to dopant impurities e.g. green emission for ZnS: Cu, orange emission for ZnS: Mn and red emission for ZnS: Eu. The lifetimes obtained from decay curves range from ns to ms level and suggest the radiative recombination path involving donor-acceptor pair recombination or internal electronic transitions of the impurity atom. A series of ZnMgO nanophosphor thin films with varied Zn: Mg ratios were prepared by chemical bath deposition. Photoluminescence (PL) excitation and emission spectra exhibit variations with changing Mg ratio. Luminescence lifetime as short as 10⁻¹⁰ s was observed for ZnO and ZnMgO (100: 10) nanophosphors. With increasing Mg ratio, PL decay shifts into microsecond range. ZnO and ZnMgO alloys up to 50% Mg were prepared as powder by solid state mixing and sintering at high temperature in reducing atmosphere. Time resolved decay of PL indicated lifetime in the microsecond time scale. The novelty of the work lies in clear experimental evidence of dopants (Cu, Mn, Eu and Mg) in the decay process and luminescence life times in II–VI semiconductor nanocrystals of ZnS and ZnO. For ZnS, blue self activated luminescence decays faster than Cu and Mn related emission. For undoped ZnO nanocrystals, PL decay is in the nanosecond range whereas with Mg doping the decay becomes much slower in the microsecond range.     

化学合成法制备ZnS基纳米荧光粉研究

描述了一种生产ZnS:M[M=Mn,Cu,Cu(Al)]纳米荧光粉的化学合成方法.运用本方法通过调节巯基乙酸与甲基丙烯酸的摩尔比,可在1.8～3.0nm范围内控制纳米粒子的尺寸.选择面积的电子衍射和X射线衍射证明ZnS:M纳米荧光粉具有闪锌矿结构.透射电镜图像表明ZnS:M纳米荧光粉具有较好的尺寸分布.ZnS:Mn的PL谱表明纳米尺寸的ZnS:Mn与体材料相比具有较高的发光效率。     

Effect of magnesium on the performance characteristics of ZnS:Cu,Mn phosphors

We have studied the effect of magnesium added to the starting mixture as MgS or MgCl₂ on the performance characteristics of synthesized ZnS:Cu,Mn phosphor powders. It has been shown that the incorporation of certain amounts of magnesium into the ZnS:Cu,Mn phosphor leads to an increase in photo- and electroluminescence brightness and is favorable for Mn incorporation into the structure of the phosphor. In particular, additional doping of a dc electroluminescent ZnS:Cu,Mn phosphor with 30 mol % Mg ensured not only a shift of its luminescence spectrum to shorter wavelengths, thereby extending the color range of light sources, but also a twofold increase in its brightness.     

Photoluminescent inkjet ink containing ZnS:Mn nanoparticles as pigment

An inorganic nanoparticle suspension, for use in an inkjet ink, has been prepared using chemically synthesised ZnS:Mn nanoparticles with acrylic acid (AA) as a dispersant. AA was also used as a polymeric binder for the jetted ink, by heat-initiated polymerisation of the AA monomer into solid poly(acrylic acid) (PAA). The AA/ZnS:Mn nanoparticle suspension was mixed with surfactant and two co-solvents to achieve the appropriate rheological properties for jetting. The AA suspension, inks, and jetted films of PAA and ZnS:Mn showed strong orange-red photoluminescence (PL) at 600?nm under ultraviolet laser excitation. The emission colour of the ZnS:Mn nanophosphors was tunable over a wide range of wavelengths by using different excitation wavelengths. The most intense PL was observed for jetted inks containing approximately 0.8?w/w% ZnS:Mn nanoparticles. Addition of a cross-linking agent to the inks significantly improved the mechanical resilience of the polymerised films. All suspensions, inks and films were prepared using simple wet chemical methods and low-temperature processing, making them an inexpensive alternative to semiconductive conjugated polymers and suitable for use on temperature-sensitive substrates, such as polymers and paper.     

A ZnS/CaZnOS Heterojunction for Efficient Mechanical-to-Optical Energy Conversion by Conduction Band Offset

Actively collecting the mechanical energy by efficient conversion to other forms of energy such as light opens a new possibility of energy-saving, which is of pivotal significance for supplying potential solutions for the present energy crisis. Such energy conversion has shown promising applications in modern sensors, actuators, and energy harvesting. However, the implementation of such technologies is being hindered because most luminescent materials show weak and non-recoverable emissions under mechanical excitation. Herein, a new class of heterojunctioned ZnS/CaZnOS piezophotonic systems is presented, which displays highly reproducible mechanoluminescence (ML) with an unprecedented intensity of over two times higher than that of the widely used commercial ZnS (the state-of-the-art ML material). Density functional theory calculations reveal that the high-performance ML originates from efficient charge transfer and recombination through offset of the valence and conduction bands in the heterojunction interface region. By controlling the ZnS-to-CaZnOS ratio in conjunction with manganese (Mn²⁺) and lanthanide (Ln³⁺) doping, tunable ML across the full spectrum is activated by a small mechanical stimulus of 1 N (10 kPa). The findings demonstrate a novel strategy for constructing efficient ML materials by leveraging interface effects and ultimately promoting practical applications for ML.     

Photoluminescence enhancement of synthesized ZnS-based-nanocrystals and aging effect on its emitting color

Various colors-emitting ZnS:Cu,Cl, ZnS:Cu,Cl,Mn and ZnS:Mn nanocrystals (NCs) which were shown to be about 3 nm sized-particle were synthesized by using a solution chemistry. And the luminescences of the synthesized ZnS-based NCs were investigated through photoluminescence excitation (PLE) and photoluminescence (PL) spectroscopy. The PLE and PL intensities of the ZnS-based NCs depends on their reflux time, and red shifted maximum PLE wavelengths of the synthesized NCs showed with increasing reflux time. The increased maximum PL intensity of NCs with increasing reflux time is due to the enhanced crystallinity of the NCs. And the shifted emitting colors of the NCs showed after aging treatment compared to those of refluxed NCs. The amount of shifted wavelength of Cu,CI doped ZnS, Cu,CI and Mn co-doped ZnS, only Mn doped ZnS NCs were -22 nm, +18 nm, and +14 nm, respectively.     

Triboluminescent properties of zinc sulfide phosphors due to hypervelocity impact

The emission of light due to crystal fracture, or triboluminescence (TL), is a phenomenon that has been known for centuries. One of the most common examples of TL is the flash created from chewing Wint-O-Green Lifesavers^®. From 2004 to 2006, research was completed using the two-stage light gas gun located at the NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama to measure the TL properties for zinc sulfide doped with both manganese (ZnS:Mn) and copper (ZnS:Cu). Results clearly show that hypervelocity impact-induced TL has been observed for both ZnS:Mn and ZnS:Cu. For ZnS:Mn, TL produced during 4.7 and 5.7 km/s impacts was statistically more luminous than was observed from similar data collected at 3.3 km/s. The TL decay time for ZnS:Mn was found to be 292 ± 58 μs, which is totally consistent with earlier measurements that did not use impact as an excitation source. Further, the emission of TL from ZnS:Mn undergoing hypervelocity impact has been demonstrated to have a significant component at the known peak emission wavelength of ZnS:Mn of 585 nm. Small TL emission generated as a result of hypervelocity impact was also observed from ZnS:Cu. The most intriguing conclusion from this research is that it may be possible to discriminate impact velocity by measuring the time-integrated luminosity of TL phosphors. An ability to measure the velocity of a hypervelocity impact is a significant indicator of the potential usefulness for this concept for use as an impact sensor in future spacecraft.     

Electroluminescent properties of chemically synthesized zinc sulfide nanocrystals doped with manganese

High-field electroluminescence (EL) of chemically synthesized ZnS:Mn nanocrystals with a crystallite size of 4 nm was investigated using a device consisting of glass substrate/indium–tin oxide/ZnS:Mn NC emission layer/Al. For electric fields over ca. 1 MV/cm, the current was turned on, and orange EL was observed. The maximum luminance was 0.45 cd/m² at a DC voltage of 42 V. The EL spectrum comprises a single peak with a peak wavelength of 593 nm, which is ascribed to the ⁴T₁–⁶A₁ transitions of Mn²⁺ ions. The excitation mechanism of the Mn²⁺ ions is discussed according to a scheme of impact excitation by hot-electrons.     

Luminescence characteristics of ZnS:Cu thin film electroluminescent devices fabricated by sputtering

A ZnS:Cu electroluminescent (EL) device was fabricated by sputtering and its luminescence properties were examined. The structure of the fabricated device was glass/Si_xN_y/ZnS phosphor/Si_xN_y/Al. The luminescence spectrum of the device showed two peaks, one blue and the other yellow. The blue peak is created by excitation and recombination of Cu atoms, and can be used for creating blue EL devices.     

Determination of the parameters of phosphor activators in thin-film electroluminescent capacitor structures

A method for determining the concentration and electron-impact excitation cross section of activated emission centers in the phosphor layer of a thin-film electroluminescent capacitor structure, based on measurements of the brightness as a function of the applied alternating voltage amplitude and frequency, is analyzed. The error of determination of the parameters of electroluminescence excited by alternating-sign ramp voltage is evaluated. The parameters of electroluminescent structures based on the ZnS:Mn, ZnS:TbF₃, and ZnS:SmF₃ films are presented.     

Parametric resonance in nanoelectromechanical single electron transistors

We show that the coupling between single-electron charging and mechanical motion in a nanoelectromechanical single-electron transistor can be utilized in a novel parametric actuation scheme. This scheme, which relies on a periodic modulation of the mechanical resonance frequency through an alternating source-drain voltage, leads to a parametric instability and emergence of mechanical vibrations in a limited range of modulation amplitudes. Remarkably, the frequency range where instability occurs and the maximum oscillation amplitude, depend weakly on the damping in the system. We also show that a weak parametric modulation increases the effective quality factor and amplifies the system's response to the conventional actuation that exploits an AC gate signal.     

Luminescent properties and excitation mechanism of ZnSe quantum dots embedded in ZnS Matrix

In this paper, we report alternative-current thin film electroluminescence structure with ZnSe quantum dots embedded in ZnS matrix as light-emitting center, i.e., ITO/SiO_x (100 nm)/[ZnS (10 nm)/ZnSe (1 nm)]₃₀/SiO_x (100 nm)/Al. Blue emissions at 390 and 477 nm are obtained in its alternative-current electroluminescent spectra. By studying its luminescent spectroscopy and brightness oscillogram of the device, we found that blue emission came from defect states at ZnSe/ZnS interface and the excitation mechanism was hot-electron impact.     

Role of Mn impurity in the deterioration of ZnS:Cu,Br electroluminescent phosphors

ZnS:Cu, Br powder EL phosphors showed 6-line EPR signal at 25°C whose intensity increases with Cu content and on annealing in Zn-vapour. The signal arises from native Mn impurity. The starting material does not show any EPR signal since Mn²⁺ acts as an affinity potential well for a hole in ZnS, forming Mn³⁺ - a chemically uncommon situation in sulfides. In doped ZnS, holes are trapped at Cu such that Mn²⁺ persists. Deterioration of EL brightness is accompanied by the decrease in EPR signal intensity due to field assisted hole transference to Mn²⁺. Intentional addition of Mn in ZnS:Cu, Br decreases the brightness and shortens life time. Stable phosphors require ZnS with Mn content less than 10¹⁴ cm^?3.     

Addressable and Color‐Tunable Piezophotonic Light‐Emitting Stripes

Piezophotonic light‐emitting devices have great potential for future microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) due to the added functionality provided by the electromechanical transduction coupled with the ability of light emission. Piezophotonic light‐emitting source based on Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) bulk is severely restricted by many challenges, such as high voltage burden, low integration density, and micromanufacturing complexity. Developing chip‐integrated devices or incorporating such photonic components onto a Si platform is highly sought after in this field. In this work, the authors overcome the abovementioned problems by introducing single‐crystal PMN–PT thin films on Si as central active elements. Taking advantage of mature microfabrication techniques, arrays of PMN–PT actuators with small footprints and low operation voltages have been implemented. Each actuator can be individually addressed, generating local deformation to trigger piezophotonic luminescence from ZnS:Mn thin films. Moreover, the authors have realized continuous and reversible color manipulation of piezophotonic luminescence on a bilayer film of ZnS:Cu,Al/ZnS:Mn. The color tunability promises an extra degree of freedom and distinctly suggests its great potential in developing a more compact and colorful piezophotonic light sources and displays related applications together with the “single pixel” addressability.     

Sputter deposition of ZnS:Mn/SrS:Ce multilayered thin film white phosphor

A full color thin film electroluminescent (TFEL) display can be fabricated by using color filters in combination with a high efficiency ‘white’ phosphor, such as a thin film multilayered stack of ZnS:Mn and SrS:Ce (denoted ZnS:Mn/SrS:Ce). To date, deposition of these multilayers has been limited to vacuum evaporation techniques and atomic layer epitaxy, both of which require different substrate temperatures for growth of high quality ZnS:Mn and SrS:Ce. This repeated thermal cycling during multilayer deposition can adversely affect electroluminescent (EL) performance and manufacturability. Sputter deposition of ZnS:Mn and SrS:Ce produces high quality phosphors for a wider range of substrate temperatures. We have determined a common set of radio frequency (rf) sputter deposition parameters for ZnS:Mn and SrS:Ce that result in high performance, multilayered white phosphors for use in TFEL devices. The EL performance of our samples is comparable to the best performance reported for evaporated multilayered samples. The major improvement is that the rf sputtered ZnS:Mn and SrS:Ce layers were deposited at the same substrate temperature. We report on the effects of sputter deposition parameters on the resultant composition and morphology of ZnS:Mn and SrS:Ce thin films and multilayers. Their EL performance was evaluated and correlated to composition and morphology.     

Effect of stacking fault energy on mechanical behavior of cold-forging Cu and Cu alloys

Cu, Cu–2.87 wt% Mn, Cu–4.40 wt% Mn and Cu-10.19 wt% Mn were prepared by cold-forging. The deformation behavior of Cu–Mn alloys is consistent with the Cu-Al alloys and Cu–Zn alloys but without lowering the stacking fault energy to simultaneously increase the strength and ductility. A series of analysis demonstrate that Cu–Mn alloys have a much smaller twin density than low stacking fault energy (SFE) metals, and dislocation strengthening is the major reason for the higher strength. The role of short range order (SRO) in promoting the mechanical properties has also been briefly discussed.     

Photoluminescence and field emission properties of ZnS:Mn nanoparticles synthesized by rf-magnetron sputtering technique

Nanoparticles of ZnS:Mn have been grown by radio frequency magnetron sputtering technique on glass and Si substrates at a substrate temperature 300 K. X-ray diffraction patterns and selected area electron diffraction patterns confirmed the nanocrystalline cubic ZnS phase formation. TEM micrographs of the films revealed the manifestation of ZnS:Mn nanoparticles with an average size 6 nm. UV–Vis–NIR spectrophotometric measurement showed that the films are highly transparent (90%) in the wavelength range 400–2600 nm. From the measurements of transmittance spectra of the films the direct allowed bandgap values have been calculated and they lie in the range 3.89–4.12 eV. The bandgap decreased with the increase of Mn concentration in the films. The Mn concentrations in the films have been varied from 0% to 8.9% and was measured by energy dispersive X-ray analysis. The photoluminescence of the Mn doped ZnS nanoparticles was measured. The intensity of the PL peaks at first increased with the increase of Mn concentration in the films up to 3.8% of Mn doping and at a Mn concentration higher than this, the intensity of PL peak decreased. Nanocrystalline ZnS:Mn showed good field emission property with a turn on field lying in the range 5.26–6.78 V/μm for a variation of anode to sample distance from 60 μm to 100 μm.     

Superelastic anisotropy characteristics of columnar-grained Cu–Al–Mn shape memory alloys and its potential applications

The columnar-grained (CG) Cu–Al–Mn shape memory alloy samples possess a strong < 001>-oriented texture along the solidification direction (SD) and straight low-energy grain boundaries fabricated by unidirectional solidification technique. When the angle between tensile direction and the SD ranged from 0° to 90° at the tensile tests, the superelasticity of samples changed in a “V” shape and showed a large anisotropy. Meanwhile, the martensite transformation critical stress of the CG Cu–Al–Mn samples increased from 258.5 MPa for 0° to 521.9 MPa for 45°, and then decreased to 324.3 MPa. The large anisotropy of the superelasticity was attributed to the combined effects of grain orientation and grain boundaries, wherein the influence of the grain boundaries had an obvious dependence on orientation. The potential applications of CG Cu–Al–Mn alloys as anisotropic shock isolators and dampers in high rise buildings and precision instruments were also proposed.