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The nanoparticles of a prominent luminescent material, cadmium selenide (CdSe) have been prepared by simple wet chemical route using starch as capping agent. The X-ray diffraction of the synthesized CdSe nanocrystals exhibits hexagonal phase with average crystallite size decreasing with capping agent concentration. Surface morphology of CdSe nanoparticles has been studied using scanning electron microscopy. The UV–Vis absorption shows blue shift in absorption edge indicating increased band gap with decreasing the particle size due to quantum confinement. The photoluminescence and electroluminescence (EL) measurement show single peak at about 650 nm with increasing intensity for smaller nanocrystals, however emission peak is quite broad in case of EL indicating involvement of impurity states. The luminescence intensity increases because of increased oscillator strength and the better passivation of the surface defects responsible for non-radiative relaxation process. The light emission starts at lower threshold voltage when the concentration of capping agent is increased.

Impact of Quantum-size effects of an electron-hole system confined in semiconductors is studied theoretically by computational modeling. Here we proposed a formula to relate particle size with decay time and oscillator strength using the concept of transition probability. The theory modeled is for CdSe and CdS semi-conductor quantum dots in the strong confinement region only where the particle size is less than Bohr's radius and the oscillator strength per volume is expected to increase with 1/r3 for the first excited state, r being the radius of spherical nano particle. As the experimental data needed to co-relate the modeled theory is available only for CdSe and CdS quantum dots only, therefore the theory is modeled for these two quantum dots only. The decay time for such spherical quantum dots is reduced thereby causing faster decay of luminescence. On the other hand it is also observed from the calculations that for same size of two different spherical quantum dots the decay time as well as oscillator strength varies depending upon the bulk oscillator strength of the material.

Zinc sulfide (ZnS) doped with transition metal has been used as phosphor for various optoelectronic applications. In the present report, ZnS:Mn doped and ZnS:Mn,Ni co-doped were prepared using chemical co-precipitation method using polyvinyl pyrolidone as a surfactant. The structural studies were carried out using an X-ray diffraction technique; optical studies have been performed using ultraviolet-visible light absorption and photoluminescence (PL) spectroscopy. The presences of functional groups were confirmed using Fourier transform infrared spectroscopy. The X-ray diffraction study and Reitveld analysis confirms the formation of cubic phase with crystalline size 2-3 nm for undoped and doped ZnS nanoparticles. A novel and enhanced luminescence characteristic have been observed in PL spectra. The luminescence intensity of Mn,Ni co-doped ZnS in the blue region is much higher of that of ZnS. The PL results indicate that the doping of Ni creates shallow trap states or luminescence centres in the forbidden energy gap, which quenches the Mn states emission. Concentration quenching has been observed in Mn-doped ZnS nanoparticles. From CIE coordinates, it is seen that the yellow and blue light emission of ZnS:Mn,Ni co-doped nanophosphor may be a promising candidate for display devices and phosphor converted light-emitting diode applications.

We developed novel phosphors based on Y6Ba4(SiO4)6F2(YBSF) apatite doped with Dy3+. Phosphors with different doping concentrations of Dy3+ were prepared and their morphological, structural and spectral properties were analyzed. XRD pattern confirms the formation of Y6Ba4(SiO4)6F2 apatite structured phosphors with space group P63/m and Debye scherrer formula gives the average crystalline size 33.4 nm. The morphological and topographical studies were done by FESEM and HRTEM analysis. EDX spectra show presence of all initial reactants in the final product. Photoluminescence spectra show two prominent peaks at 487 nm and 573 nm corresponding to magnetic dipole transitions (4F9/2 → 6H15/2) and electric dipole transitions (4F9/2 → 6H13/2) for 350 nm excitation wavelength. The chromaticity coordinate of the prepared phosphors was found to be in the region of near white light which shows that prepared phosphor are potential single phased phosphor for white light emitting diodes.

The outcome of particle size effect on Photoluminescence has been investigated theoretically for CdS, ZnS, CdSe and ZnSe quantum dots. The theory is based on computational modeling in the strong confinement region only where the particle size is less than Bohr’s radius. The Photoluminescence emission line shape function which depends on band gap was found to be strongly dependent on particle size. The PL spectra of bulk material is similar for the quantum dots where similar vibronic coupling and normalized intensity is considered, only the peak wavelength changes and thus shifts the band edge luminescence peak to higher energies for the quantum dots corresponding to their band gap with no broadening. The PL spectra of monodisperse dots revels the fact that the shape of the emission peak is same, only it shift towards higher energy or smaller wavelength for decreasing size of quantum dots.

Over the past few decades, many efforts have been put to optimize the properties of electroluminescent devices such as electroluminescence (EL) panel, organic light emitting diode (OLED), flat panel display, EL lamps, etc. In this chapter, the authors provide a comprehensive review of the state-of-the-art research activities related to EL phenomena, the principle of electroluminescence, different types of EL, fabrication of devices, studies on novel electroluminescent materials, their characteristics and potential applications. The authors begin with a historical background of electroluminescence, description of its structure, working principle and parameters with mechanism. They discuss the experiments determining the electrical, optical, and physical properties of the powder as well as thin film EL cells. The recent progress on the improvement of their characteristics and finding novel structures, such as the latest achievements in using various semiconductor nanostructures in polymer matrix as emitting layer in EL devices, are summarized.

When II-VI semiconductors are fractured, initially the mechanoluminescence (ML) intensity increases with time, attains a maximum value Im at a time tm, at which the fracture is completed. After tm, the ML intensity decreases with time, Im increase linearly with the impact velocity v0 and IT initially increase linearly with v0 and then it attains a saturation value for a higher value of v0. For photoluminescence, the temperature dependence comes mainly from luminescence efficiency, ηo; however, for the ML excitation, there is an additional factor, rt dependent on temperature. During fracture, charged dislocations moving near the tip of moving cracks produce intense electric field, causes band bending. Consequently, tunneling of electrons from filled electron traps to the conduction band takes place, whereby the radiative electron-hole recombination give rise to the luminescence. In the proposed mechanism, expressions are derived for the rise, the time tm corresponding to the ML intensity versus time curve, the ML intensity Im corresponding to the peak of ML intensity versus time curve, the total fracto-mechanoluminescence (FML) intensity IT, and fast and slow decay of FML intensity of II-VI semiconductors. The FML plays a significant role in understanding the processes involved in biological detection, earthquake lights and mine failure.

Sulfide based luminescent materials have attracted a lot of attention for a wide range of photo-and electroluminescence applications. Among the sulfides, ZnS is promising host material for development of phosphors in different visible emission bands. Doping of Ag can affect the electroluminescence of the host material. Incorporation of host ZnS into the polymer matrix is one of the best method to display their special functions, which stabilize the nanoparticles. Here we report a synthesis and electroluminescence of silver doped ZnS/PVK nanocomposites thin films. Reported films were prepared by using chemical route with varying Ag doping and ZnS loading in the composite. Structural and morphological characterization were carried out through XRD and SEM techniques, which confirmed the particles in nanoregime. Though optical absorption spectra and band gap of ZnS semiconductor nanoparticles in ZnS:Ag/PVK matrix were estimated, and using EMA model, particle size was calculated which supports the results of XRD. Electroluminescence of nanocomposite samples was studied and it was found that threshold voltage depends on doping of Ag and also on loading of ZnS. Voltage brightness characteristics support the production of EL by acceleration-collision mechanism.Contents of Paper

Currently there is a great interest in II–VI semiconductor nanoparticles, particularly organically capped soluble particles of cadmium or zinc sulphide and selenide, for their ready to use application in devices. For electroluminescence (EL) devices, it is expected to cover a broad spectrum and to tune various specific colours by preparing Cd1-xZnx Se instead of CdSe and ZnSe. Ternary alloys have composition dependent properties; therefore Cd1-xZnxSe has attracted much attention in the fields of luminescence and optoelectronic devices. It has wide optical band-gap and good stability with respect to environment. In this study, Cd1-xZnxSenanoparticles have been synthesized by using starch as a capping agent through a chemical synthesis route at room temperature. Samples have been prepared varying composition factor ‘x’ in ternary alloy Cd1-xZnxSe. Cubic structure of all has been confirmed by XRD. Crystallite size calculated from XRD was found in the range of 3-5 nm and it was observed that size reduces on increasing Zn content in ternary compound. Optical absorption spectra showed the blue shift in absorption edge with increasing Zn content. Band gap has been obtained by absorption studies and increase in band gap observed on increasing Zn content in the compound. Electroluminescence studies reveal that lower threshold voltage is required for samples with lower ‘x’ value. The Brightness increases on increasing the voltage above threshold voltage and the variation pattern is almost exponential for all samples. The voltage-current curve represents ohmic nature of the EL cell. Impedance was found to increase on increasing ‘x’ value. The increase in EL intensity is faster for higher frequency. EL spectra revealed that light emission is in violet-green region corresponding to band gap for both Cd0.75 Zn 0.25Se and Cd0.5 Zn 0.5Se, with a slight blue shift on increasing Zn content. A ternary system Cd1–xZnxSe, may be engineered better for application purpose by suitably choosing the composition parameter ‘x’.Contents of Paper

Several research groups have reported that nanocrystalline II-VI semiconductors show enhanced luminescence, increased oscillator strength and shorter response time. Nanocrystalline powder samples of CdS, CdSe, ZnS and ZnSe nanocrystals and their composites with PVA and PVK have been prepared by chemical route. SEM. TEM and AFM images indicate agglomeration of particles. XRD reveal the crystal structure and size in nanometer range and absorption spectra show increased band gap due to quantum confinement.The EL studies on nanocrystalline powder samples and nanocrystal/polymer composites have shown that the light emission starts at certain threshold voltage, different for different specimens and then increases with increasing voltage. It is found that smaller nanocrystals have lower threshold voltage and higher EL brightness. It is observed that nanocomposite give much higher electroluminescence starting at lower voltage and increasing very fast with the voltage as compared to nanocrystalline powder. The emission spectra are found to depend on the material, crystalline size and doping. Electroluminescence in undoped and doped chalcogenide nanocrystals and nanocomposites is reviewed in this paper. In nanosize regime, electroluminescence (EL) is governed by the size quantization effect. Contents of Paper

In the present work, the thin films of CdSe/PVK nanocomposites with various concentration of CdSe have been synthesized by chemical route. Polyvinyl carbazole (PVK) was used as a stabilizer as well as host matrix to prevent agglomeration of nanoparticles. The synthesized CdSe/PVK films were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), UV–Visible absorption spectra. The crystalline size was calculated from the XRD while FESEM images confirm the surface morphology of CdSe/PVK nanocomposites. CdSe nanocrystals have hexagonal phase and size increases with increasing CdSe content, Single PL peak at 498 nm for all the samples is caused by luminescence centres or surface/defect states. The EL spectra are quite broad as compared to PL spectra and the peaks are obtained at nearly same wavelength. The EL peak intensity increases slightly and shifts towards red by increasing concentration of CdSe in polymer matrix.

Copper doped zinc sulfide nanoparticles were prepared by chemical precipitation method. The size of the particles was varied by changing the concentration of capping agent. The XRD studies indicate that most of the samples are cubic in nature. The broadening of peaks tends to increase with increasing capping agent concentration showing decrease in particle size. The crystalline size computed using Scherrer formula is found to be in range of 3–10 nm. Absorption spectra show absorption edge in UV region. The edge was found to shift towards shorter wavelength as the capping agent concentration is increased. This indicates increased effective band gap and hence reduced particle size. The nanoparticle size has been estimated in the range 5–10 nm using effective mass approximation model. For electroluminescence (EL) study of ZnS:Cu nanocrystals, the EL cells were prepared by placing ZnS:Cu nanoparticles between SnO 2 coated conducting glass plate and aluminum foil. Alternating voltage of various frequencies was applied and EL brightness ( B ) at different voltages ( V ) was measured and reported in this paper.

Copper doped Zinc Sulfide (ZnS:Cu)) is a known green light emitter. Present paper reports luminescence of ZnS:Cu nanoparticles and nanocomposites. Three different nanostructures: mercaptoethanol capped ZnS:Cu nanoparticles, polyvinyl alcohol (PVA) capped ZnS:Cu nanoparticles and ZnS:Cu/PVA nanocomposites have been prepared by chemical route. X-ray diffraction (XRD) revealed cubic zinc blende structure of ZnS:Cu nanocrystals of size below 20 nm. The particle size is found to decrease with increasing capping agent concentration or ZnS loading in PVA matrix. Optical absorption spectra show blue shift in the absorption edge indicating quantum size effect. Photoluminescence (PL) of all the samples was studied by exciting with 212 nm light. The PL spectra of ZnS:Cu/ PVA nanocomposite films show quite broad emission peak at 415 nm where as the PL spectra of mercaptoethanol capped and PVA capped nanoparticles show a very narrow peak at 426 nm and 403 nm respectively. It seems that the nature of passivation of surface states affects the position of surface states. Electroluminescence (EL) studies have shown that light emission starts at a threshold and then increases with voltage. Higher EL intensity and lower threshold voltage is obtained in case of smaller particles. The EL spectra of all the samples are found to be broad with peak at about 420 nm. The EL intensity of ZnS:Cu/PVA nanocomposites is much larger than the ZnS:Cu nanoparticles. The high efficiency EL devices for display and lighting can be fabricated using ZnS:Cu nanocomposites with PVA matrix giving violet emission.

The powder of ZnS nanoparticles were prepared by using chemical deposition technique and characterized by electroluminescence techniques are reported in this paper. The estimated size of ZnS:Cu nanocrystals with change in capping agent concentration and ZnS:Cu/PVA nanocomposites and no effect of doping has been observed on the absorption spectra. Electroluminescence (EL) investigations of nanocrystalline powder as well as nanocomposites, it is seen that Log B vs. 1/V curve is a straight line with negative slope. This indicates that EL is produced by acceleration-collision mechanism. The detail EL characterization and application in display devices of these materials are reported in this paper. Copyright © 2013 VBRI press.

Nanocrystalline cadmium selenide(CdSe) powder as well as nanocomposites of CdSe in poly vinyl alcohol (PVA) matrix were prepared by chemical method and characterized by TEM and XRD. Hexagonal phase has been observed with size depending on preparation conditions. Absorption spectra of powder samples give a peak and then monotonous increase in absorption at shorter wavelengths where as nanocomposites show only absorption edge. Powder samples were excited by 300 nm UV light and photoluminescence (PL) spectra show single peak between 345 and 400 nm. It is seen that as the particle size is reduced the intensity of luminescence increases. In case of nanocomposites, PL was excited by 475 nm light and emission spectra show two peaks at 525 and 575–585 nm. Electroluminescence (EL) study of CdSe nanopowder and CdSe/PVA nanocomosite show that the emission starts at a threshold voltage and then increases with voltage. Lower threshold and higher brightness have been observed for smaller particles. Current is found to increase linearly with voltage indicating ohmic nature. The EL spectrum of CdSe/PVA nanocomposites shows only one broad peak at 580 nm similar to PL. The relation between brightness and voltage indicates that EL is produced by acceleration–collision mechanism. The investigations have revealed that intensity of photo- as well as electro-luminescence can be increased by reducing the size of CdSe nanocrystals. Nanocrystalline CdSe powder shows better photoluminescence but for electroluminescence nanocomposite is beneficial.

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time tm and intensity Im corresponding to the peak of EL overshoot, total EL intensity It and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio Im/Is decreases with increasing value of the applied pulse voltage because Im increases linearly with the amplitude of applied voltage pulse and Is increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τt of electrons at the interface decreases with increasing temperature whereby the dependence of τt on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.

Nanocrystalline thin films of CdS have been grown onto flexible plastic and titanium substrates by a simple and environmentally benign chemical bath deposition (CBD) method at room temperature. The films consist of clusters of CdS nanoparticles. The clusters of CdS nanoparticles in the films were successfully converted into nanowire (NW) networks using chemical etching process. The possible mechanism of the etching phenomenon is discussed. These films were examined for their structural, surface morphological and optical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV–vis spectrophotometry techniques, respectively. Photoelectrochemical (PEC) investigations were carried out using cell configuration as n-CdS/(1 M NaOH + 1 M Na 2 S + 1 M S)/C. The film of nanowires was found to be hexagonal in structure with the preferential orientation along the (0 0 2) plane. The nanowires have widths in the range of 50–150 nm and have lengths of the order of a few micrometers. Optical studies reveal that the CdS nanowires have value of band gap 2.48 eV, whereas it is 2.58 eV for nanoparticles of CdS. Finally, we report on the ideality of junction improvement of PEC cells when CdS nanoparticles photoelectrode converted into nanowires photoelectrode.

Polyvinyl carbazole (PVK) film has been prepared by solution method. Its absorption, photoluminescence (PL) and electroluminescence (EL) have been studied. For PL and absorption studies, film of PVK was prepared by spreading PVK solution on clean glass plate. The dried film was taken out of the glass plate and used for absorption, thickness measurement and PL studies. The film is transparent in the visible region and absorption starts at 340 nm wavelength. The absorption peaks are obtained at 280, 250 and 220 nm, indicating that optical gap of film is 3.65 eV and molecular orbitals exist at 4.43, 4.96 and 5.64 eV. PL studies reveal that excitation by violet light gives luminescence at 430, 480 and 690 nm. For EL studies, cell is prepared by depositing PVK film on a portion of conducting glass plate and taking aluminum foil as second electrode. It is observed from the characteristics that the current varies linearly, where as EL intensity varies non-linearly with increasing voltage. Higher brightness has been observed at higher frequencies. EL spectrum shows a sharp peak at 400 nm and a broad and less intense peak at 700 nm, which are attributed to radiative decay of singlet exciton and defect centers, respectively.

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.

The physical properties of semiconductors define to a great extent the performance of photoelectrochemical (PEC) cells. With a band gap of 1.7 eV, cadmium selenide is an attractive candidate as photoelectrode in PEC cells for the conversion of light into electricity. Polycrystalline films are used for low-cost cells. CdSe films are prepared on a titanium substrate by the electrocodeposition technique from a solution of SeO 2 , CdSO 4 and H 2 SO 4 in galvanostatic conditions, then annealed at 400 °C. Chemical modification of semiconductor surfaces by metal ions has been found to improve the performance of photoelectrodes. Zn ion incorporation on the CdSe film surface was achieved by dipping the film in aqueous solution of ZnSO 4 followed by heating in air. The preparation conditions were optimized to give the best performance. The photovaltaic activity of the films was studied with a polysulphide electrolyte using graphite as a counter electrode and a 200 Watt tungsten lamp as the light source. Addition of Zn ions at the surface produces favourable states in band gap, enhancing charge transfer kinetics at the interface. On heating, Zn diffuses through grain boundaries, reducing recombination centres for majority carriers, thus increasing diffusion length and so photocurrent. In the CdSe lattice, Zn replaces Cd ions which causes an increase in band gap since ZnSe has a band gap higher than CdSe. Hence a small amount of Zn reduces I sc due to an increase in band gap, but as the Zn concentration is increased, greater band bending is obtained, increasing V oc and improving overall performance.

During impulse excitation of mechanoluminescence (ML) in X- or γ-irradiated alkali halide crystals, two peaks are observed in the ML intensity versus time curves. The first peak lies in the deformation region and the second peak in the post-deformation region. The ML in the deformation region is due to the recombination of dislocation-trapped electrons with the holes in defect centres. The ML in the post-deformation region is due to the transient thermostimulated luminescence from shallow traps which get populated due to the Auger process occurring during the transfer of dislocation-trapped electrons to deep traps. In the framework of the mechanism proposed, an equation is derived which is able to explain the ML in the deformation and post-deformation regions of the crystals. The equation is also able to explain the occurrence of two peaks in the ML kinetics. Several other facts related to the ML kinetics of coloured alkali halide crystals are also discussed. Bei Impulsanregungen der Mechanolumineszenz (ML) in rontgen- oder γ-bestrahlten Alkalihalogenidkristallen werden zwei Maxima in der Zeitabhangigkeit der ML-Intensitat beobachtet. Das erste Maximum liegt im Deformationsgebiet, das zweite im Nachdeformationsgebiet. Im ersten beruht die ML auf der Rekombination von Elektronen an Versetzungen mit Lochern aus Defekten. Im Nachdeformationsgebiet ruhrt die ML aus thermisch stimulierter Lumineszenz aus flachen Haftstellen, die durch den Augerprozes beim ubergang von Elektronen von Versetzungen zu tiefen Haftstellen besetzt wurden, her. Im Rahmen dieses Mechanismus wird eine Gleichung zur Erklarung der ML abgeleitet, insbesondere fur das Auftreten der zwei Maxima in der ML-Kinetik. Einige andere Probleme der ML-Kinetik in verfarbten Alkalihalogenidkristallen werden diskutiert.

The present paper reports on a statistical model of the mechanoluminescence (ML) excitation in crystals where attempts have been made to explain stress, temperature-, strain rate-, activator concentration dependence and several other aspects of ML. It is found that ML emission should take place only during the period at which stress will change with time. The total ML intensity should be directly proportional to the square of the stress and there should be phase difference between the ML pulse and the applied stress pulse. It is found that the ML intensity should decrease faster with temperature as compared to the corresponding photoluminescence intensity. within the limit of concentration quenching the ML intensity should increase directly with the defect centre concentration. The ML intensity should increase with the stress rate or strain rate of the solids. Furthermore, the concept of mechanoluminescent and non-mechanoluminescent materials is explored.

During the process of deforming a crystal, a high pressure is developed near the tip of mobile cracks, which may in turn produce a new ground state by thermal electron transfer. Upon sudden release of pressure, the electron can either relax to one atmosphere ground state or remain in the excited state potential well long enough to relax to one atmosphere and radiatively transfer back to the ground state. For analysing the pressure induced thermal population of the excited state, the mechanoluminescence(ML) and high pressure photoluminescence(PL) of several organic and inorganic crystals were measured. The study indicated that usual pressure coefficient of energy shift of the order of 50–100 cm−1/kbar and the stress at the crack-tip of the order of 5–10 kbar, are not sufficient to cause the thermal population of the excited state. If by any means the product of pressure coefficient and stress at the mobile crack-tip can be increased by 50 to 100 times, then the thermal population of the excited states may take place. Using the pressure coefficient of energy shift and the difference in ML and PL spectra, and using independently the change in relative intensities of the vibronic peaks, the pressure at the emitting mechanoluminescent crystal sites is evaluated and it is found to be of the order of several kbar which varies from crystal to crystal.

The paper reports that the mechanoluminescence (ML) is not an inherent property of only the non-centrosymmetric crystals. The ML may appear in number of centrosymmetric crystals due to variety of processes. The ML of 82 centrosymmetric crystals are reported and different models are proposed for the ML excitation. The models proposed are: space charge electrification model, triboelectrification model, phase transformation model, gas adsorption model, chemical reaction model, thermal population model, molecular deformation model, cleavage electrification model, defective piezoelectric phase model, dislocation defect stripping model, dislocation unpinning model, dislocation annihilation model, charged dislocation model and incandecent light emission model. It is shown that on the basis of the proposed model, intense mechanoluminescent materials with desired nature and characteristics may be prepared.

A phenomenological theory of mechanical excitation of luminescence centres in crystals is presented, and the suitability of the proposed model explored. The phenomenon of mechanoluminescence, (ML) may have its origin, as other kinds of the luminescence, in the radiating transition between two energy levels E1 and E2 > E1 and may be a three step cyclic process, namely excitation of charge carriers, their transport through material, and recombination. An expression for the stress dependence of ML is derived and simplified for low and high stress. It is found that the ML intensity should be maximum for a particular temperature, and that the ML excitation will be higher for materials which have a higher value of the stiffness constant and of the transport probability. Other factors responsible for efficient ML emission are also discussed. Eine phanomenologische Theorie der mechanischen Erregung von Lumineszenz in Kristallen wird gegeben, und die Eignung des vorgeschlagenen Modells untersucht. Die Erscheinung der Mechanolumineszenz (ML) kann, wie andere Arten der Lumineszenz, ihren Ursprung haben in dem strahlenden Ubergang zwischen zwei Energieniveaus E1 und E2 > E1, und uber einen zyklischen Dreistufen-Prozes ablaufen: Anregung von Ladungstragern, deren Transport durch das Material und Rekombination. Ein Ausdruck fur die Abhangigkeit der ML von der mechanischen Spannung wird abgeleitet und fur kleine und grose Spannungen vereinfacht. Es ergibt sich, das die Intensitat der ML fur eine bestimmte Temperatur ein Maximum haben sollte, und das fur gegebene Spannung der ML-Erregung hoher ist fur Materialien mit hoherer Steifheit und hoherer Transport-Wahrscheinlichkeit. Auch andere Einflusse auf die ML werden diskutiert.

Flat bones of human skeleton were subjected to dynamic indentation with ball indenters. The impacted surface was studied under high magnification and also by using the technique of multiple beam interferometry. The impulse caused the pile up of material at a little distance from the edge of the indent. The diameter of indent is found to increase as fourth root of the energy of impact. Bone structure also has the tendency to minimize the damage caused by external forces. There was about 90% recovery in deformation in the depth of indents due to internal stresses created inside the bone by the impact.

Specimens of dry human bone were subjected to gradual or sudden heating up to different temperatures. After heating they were cooled suddenly or gradually to room temperature. Their hardness was studied by the Vickers and ball indentation tests. The deformation produced was studied by interferometric and phase-contrast techniques. Heating caused the bone to become brittle without any considerable changes in the surface features. Gradual cooling induced surface hardening. Heating was not found to affect indentation-induced surface distortion.

Bones from different portions of human skeleton are polished and cut to suitable sizes and then subjected to micro-indentation at various loads using Vicker’s diamond pyramidal indenter. The Vicker’s hardness number is found to differ slightly from portion to portion of the skeleton. Interferometric studies of the indentation reveal that the applied stress of indentation causes a ‘pile-up’ of material near the corners of the pyramidal indents.