Facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for promising solid-state lighting applications

Abstract

A novel strategy was introduced to produce large-scale pristine MgO nanostructures as a feasible candidate for light harvesting applications. Herein, MgO nanostructures with a nanoflakes-/nanofibers-like morphology were synthesized by a co-precipitation route at different calcination temperatures ranging from 500 to 1100 °C and well characterized by several standard experimental techniques, such as XRD, FTIR, SEM, EDX, and TEM, to confirm the formation of MgO nanostructures. Undoped MgO nanostructures obtained at 1100 °C exhibited a strong photoluminescence (PL) emission spectrum at 668 nm (hypersensitive red) at 466 nm excitation wavelength. Moreover, these nanostructures also showed strong blue (477 nm) and red (668 nm) luminescence emissions simultaneously at an excitation wavelength of 317 nm. Further investigations probed by PL mapping demonstrated the homogeneous distribution of PL intensity throughout the MgO surfaces and time-resolved photoluminescence spectroscopy results of these nanostructures indicated a decay time of less than 10 ns. Thus, the facile synthesis of these luminescent undoped MgO nanostructures provides a potential platform to harvest white light generation (a combination of blue and red emissions) as well as their potential use in LED applications.

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Introduction

Over the past few decades, a huge amount of luminescent nanomaterials/nanophosphors have been studied globally and several discoveries have emerged to provide alternate solutions for overcoming solid-state lighting problems in our daily life. Although, there are well-known materials that can easily emit light, such as semiconductor nanomaterials (CdS, ZnS), quantum dots, and rare earth based nanomaterials (Y₂O₃:Eu³⁺, YAG:Ce),¹ and even though these luminescent materials have found numerous applications, such as luminescent security ink, optoelectronic devices, solar cell, biological fluorescence labeling, light harvesting devices, and optical imaging,^2–5 they have some disadvantages. For instance, semiconductor materials, like sulfides, have issues of toxicity, low efficiency, and require the use of harmful solvents and additives.^2,6 Similarly quantum dot suffer from photobleaching effect as well as require a low temperature to control their size.⁷ Though, the rare earth materials have sharp transitions, anti-stokes emission, and better stability, they also suffer from some obvious drawbacks such as inconsiderate synthesis conditions, high cost, and limited availability on Earth.⁸ There are several investigations going on throughout the world to address these issues.⁹ In this series, alkaline earth metal oxides, such as CaO, BeO, MgO, and SrO, have been gaining considerable attention in material science as an inorganic phosphor for optoelectronics devices, sensors, and solid-state lighting applications due to their defect-induced luminescence properties.^10,11

Among these oxide nanostructures, MgO is one of the most important alkaline earth metal oxides that is attracting attention of many researchers due to its high economical availability, lower toxicity and ecofriendly nature. Nowadays, several studies are being carried out on MgO nanostructures like nanoparticles, nanorods, and nanoflakes^12–14 due to their excellent thermal, electrical, chemical, physical, and optical properties^15–18 and they are widely used in many applications such as adsorbents, sensors, catalysis, refractory material, paint, fluoride remover, optoelectronics, and luminescence devices.^19–24 There are, however, few reports on the synthesis of MgO nanostructures using various versatile methods such as sol–gel, thermal evaporation, combustion aerosol, chemical vapor deposition, hydrothermal, and co-precipitation.^25–30 Among these, the co-precipitation method has been most widely adopted mainly due to its advantages such as simplicity, inexpensive instrumental setup, low-reaction time, high yield production, and good control over the experimental conditions.³¹

The obtained MgO nanostructures have a wide band gap of 7.8 eV,³² and the photon excitation energy of rocksalt cubic structures of this material is significantly well below the band gap of transition materials because of the presence of intrinsic/extrinsic defect-induced absorption and emission.³³ The tailoring of the band gap of these nanostructures can be possible in the ultraviolet-visible range. The photoluminescence (PL) properties of these nanostructures have been well studied, both experimentally and theoretically, because the presence of defects at the surface sites, attracting significant interest among the scientific community.^11,34 Earlier, literature reports showed that the bare MgO nanostructures with intrinsic defects such as oxygen/magnesium ion vacancies (F-center defects and V center defects) mostly show a PL emission in the blue region at room temperature, while MgO with external defects (doped impurities), such as Cr, Fe, Mn or other transition metal ions doping, are responsible for red emissions.^35–40 It is interesting to note that this proposed study promises a strong red photoluminescence emission in undoped MgO nano-flakes/fibers at higher calcination temperature of 1100 °C, which provides a new analog for MgO-based luminescent materials in parallel to rare earth phosphors/nanophosphors.

In the present study, a simple and economical co-precipitation method was used for the synthesis of nano-sized MgO, which was calcined at different temperatures varying from 500 to 1100 °C. Furthermore, we analyzed the structural, morphological, and optical properties of MgO samples using different characterization techniques such as XRD, FTIR, SEM, TEM, and UV-Vis. Nano-sized MgO calcined at 1100 °C exhibited strong photoluminescence emissions in the blue and red regions upon excitations at 317 nm, whereas a strong red emission was observed at 466 nm with a fast decay time of less than 10 ns, which suggests its proposed legitimate use in white light generation as well as in solid-state lighting applications.

Experimental details

Chemicals used

MgO nano-flakes/fibers were prepared by a wet chemical co-precipitation method and calcined at different temperature ranges from 500 to 1100 °C. Magnesium nitrate hexahydrate [Mg(NO₃)₂·6H₂O] (99%, Qualigens) as a precursor and ammonium hydroxide [NH₄OH] (25% NH₃, Rankem) as a precipitating agent were used. Double distilled water was used during the entire synthesis process.

Synthesis of MgO nanostructures

In brief detail, NH₄OH solution (12%, 27 ml) was dropwise injected into a solution of Mg(NO₃)₂·6H₂O (20 g, 35 ml) at 50 °C with constant stirring and a resultant white color precipitate appeared after digesting in the mother liquor for 5 h, which indicated the formation of as-synthesized Mg(OH)₂. The reaction mixture was cooled to room temperature, then the precipitate was filtered, washed with double distilled water several times to remove the by-products and finally dried in an oven at 100 °C for 10 h. Subsequently, after preparing the as-synthesized Mg(OH)₂, MgO was obtained by the calcination of Mg(OH)₂ at higher temperature via thermal decomposition. In this experiment, the as-synthesized Mg(OH)₂ was thermally heated in a muffle furnace at 500, 700, 900, and 1100 °C at a constant heating rate of 10 °C min⁻¹ for 10 h dwell time to determine the effect of temperature on the properties of the MgO samples.

Details of instrumental analysis

XRD experiments were carried out to obtain the crystalline structure, phase formation, purity, and other information of synthesized material using a Bruker X-ray diffractometer, with monochromatic Cu-Kα₁ radiation (λ = 1.54059 Å) as a X-ray source in the scanning range from 10° to 80°. FTIR experiments were performed by a single beam Perkin Elmer instrument (Spectrum BX-500) to obtain structural information by determining the chemical group present in the sample. The morphological study of the nanostructure was carried out by scanning electron microscopy (Model: Zeiss EVO MA-10 SEM equipped). An energy dispersive X-ray spectrum of MgO was executed by EDS: Oxford Link ISIS 300. Transmission electron microscopy (TEM) was performed for high resolution morphology characterization and HRTEM was used for the calculation of lattice spacing (HR-TEM: FEI Tecnai G2 F30 STWIN at 300 kV). The UV-Vis absorption spectra of MgO samples were recorded by a UV-Vis spectrometer (Shimadzu UV-3101). Photoluminescence characteristics were observed using a photoluminescence spectrometer (Edinburgh Instruments, model FLSP-900) with a xenon lamp as the excitation source. The time-resolved PL decay profile of the spectra of MgO samples was recorded at room temperature by a time-correlated single-photon counting technique (Edinburgh Instruments, model FLSP-900) with a picoseconds diode laser as a source of fixed excitation wavelength of 375 nm. The PL mapping for a 2D view of the fluorescence PL intensity was performed with a WITec alpha 300R+ Confocal PL microscope system (WITec GnBH, Ulm, Germany), where a 375 nm diode laser was used as a source of excitation. To estimate the absolute luminescence quantum efficiency of the undoped MgO nanostructure, an integrating sphere equipped with an Edinburgh spectrometer (Model F900) instrument was used and the integrated fraction of luminous flux and radiant flux was measured with the standard method.

Results and discussion

Structural determination

Fig. 1(A) depicts the X-ray diffraction pattern of as-synthesized Mg(OH)₂ and MgO samples calcined at 500, 700, 900, and 1100 °C.

Fig. 1 (A) X-ray diffraction patterns of (a) as-synthesized Mg(OH)₂ (lower segment) and MgO (upper segment) calcined at (b) 500, (c) 700, (d) 900, and (e) 1100 °C. (B) Crystal structures of MgO and Mg(OH)₂, where the yellow arrows represent the (200) and (101) planes.

Fig. 1(a) displays the XRD pattern of the as-synthesized Mg(OH)₂ sample. All the diffraction peaks present in this sample were well indexed and closely matched with standard JCPDS card no. 84-2163 (a = b = 3.148, c = 4.779 Å) corresponding to the Miller indices (001), (100), (101), (102), (110), (111), (013), and (021), demonstrating the hexagonal close packed structure with lattice parameters a = b = 3.1384 Å and c = 4.7803 Å with sharp peaks at 37.97° and 18.59°. No other peaks for any impurity were found, signifying the formation of pure Mg(OH)₂. The average crystallite size using the FWHM value (0.40°) of the peak centered at 37.97° (calculated by gauss fit) was found to be ∼20 nm, as calculated by Debye–Scherrer equation.

Fig. 1(b)–(e) elucidate the XRD patterns of MgO samples produced by calcination of the as-synthesized Mg(OH)₂ at (b) 500, (c) 700, (d) 900, and (e) 1100 °C, where in all the cases the peak position remain unchanged and peak intensity continue to grow as the calcination temperature is increased. The peak positions corresponding to the (111), (200), (220), (311), and (222) planes could be indexed to the cubic structure of MgO with standard JCPDS card no. 87-0653 having lattice parameters and a unit cell volume as illustrated in Table 1. No other diffraction peak was present, thus concluding that there were no remnants. The average crystallite size was calculated to be ∼8, ∼15, ∼22, and ∼36 nm by using FWHM values of 0.94, 0.54, 0.37, and 0.23 with the peak locations at 42.83°, 43.06°, 43.01°, and 43.00°, respectively, for the products attained by calcination at 500, 700, 900, and 1100 °C. It is interesting to note that the cell volumes and cell parameters increase up to 1100 °C, but beyond this temperature there is a sharp decrease in cell volume as well as in cell parameters, due to the formation of secondary phases at the surface of MgO nanostructures, which is in good agreement with the obtained photoluminescence results.

Table 1 Least squares refined unit cell parameters and cell volume for MgO nanostructures at different calcinations temperature

Calcination temperature (°C)	Lattice parameters (Å)	V (m^3)
[Standard JCPDS no. 87-0653]	(a = b = c = 4.20)	(a^3)
500	4.2065	74.4325 × 10^−30
700	4.2076	74.4909 × 10^−30
900	4.2098	74.6078 × 10^−30
1100	4.2659	77.6304 × 10^−30

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The crystal planes (200) and (101) correspond to the highest intensity peaks in cubic MgO and hexagonal Mg(OH)₂, as shown in Fig. 1(B).

The transformation from the as-synthesized Mg(OH)₂ to MgO was further confirmed by the FTIR spectrum. The sharp and intense band at 3698 cm⁻¹ can be attributed to the –OH anti-symmetric stretching vibration,⁴¹ which vanishes with increasing calcination temperatures from 500 to 900 °C, as illustrated in Fig. S1 (ESI).† The bands present around 668–865 cm⁻¹ in the spectra of all the samples were assigned to the MgO stretching vibration mode³⁴ for both the uncalcined and calcined samples. The intensive bands at around 1550 cm⁻¹ occur due to the bending vibration of weakly bound molecular water.⁴¹ The emergence of absorption band at around 3463 cm⁻¹ was attributed to the stretching modes of hydroxyl group. The bands associated at around 1500 and 3463 cm⁻¹ were deformed into smaller peaks at a higher temperature of 900 °C, indicating the removal of moisture and complete transformation of the dense crystals of MgO from Mg(OH)₂. Interestingly, the bathochromic shift was also noticed with increasing temperature, as illustrated in Fig. S1,† from 870 (MgO: 500 °C) to 865 (MgO: 700 °C), and 858 (MgO: 900 °C) cm⁻¹ due to the dangling bonds present on the surface of the nanostructures.⁴²

Plausible mechanism for the formation of MgO nanostructures

The synthesis process for the formation of a solid precipitate of Mg(OH)₂ is described by following the chemical reaction between Mg(NO₃)₂·6H₂O as the precursor and NH₄OH as the precipitating agent.¹²

Mg(NO₃)₂·6H₂O + NH₄OH → Mg(OH)₂ (1)

The whole intermediate chemical reactions for the formation of Mg(OH)₂ can be explained in the following proposed steps.

Mg²⁺ + 2OH⁻ → Mg(OH)₂ (4)

The plausible mechanism is briefly described in three steps: among them, in the first step, the host material Mg(NO₃)₂·6H₂O was dissolved in deionized water and a clear solution of Mg(NO₃)₂ was formed. After dissolution, Mg(NO₃)₂ formed a complex intermediate cation [Mg(NO₃)(H₂O)_n]⁺ with water molecules,³⁷ where the legend Mg²⁺ has a bidentate binding motif and undergoes bond formation with Mg through its oxygen moiety. Here, n represents the number of water molecules, varying from 1 to 4. The intermediate cations at n = 1–3 have unstable structures, whereas at n = 4, Mg²⁺ forms a stable octahedral structure. It was found that n = 2 and 3 are more unstable than n = 1, because additional water molecules bind with (Mg²⁺), ion, which leads to a reduction of the average ion–water molecules interaction and it then becomes an unstable intermediate cation, which leads to the formation of magnesium bivalent cation Mg²⁺ according to eqn (3).⁴³ In the third step, the formed Mg²⁺ cation reacts with 2OH⁻ anion from NH₄OH and gives a white precipitate of Mg(OH)₂. Finally, a MgO rocksalt type sixfold coordinated cubic structure is obtained from the thermal calcination of as-synthesized Mg(OH)₂ at 500, 700, 900, and 1100 °C.

Surface morphology and microstructure analysis

The surface morphologies and microstructures of Mg(OH)₂ and MgO nanostructures were well characterized by SEM and TEM techniques. SEM micrographs of Mg(OH)₂ at low magnification revealed the nanoflower-like structure due to the agglomeration of several nanoflakes, as shown in Fig. 2(a₁).

Fig. 2 SEM images of (a₁, a₂) represent as-synthesized Mg(OH)₂. (b₁, b₂), (c₁, c₂), (d₁, d₂), (e₁, e₂) represent MgO nanostructures obtained at calcination temperatures of 500, 700, 900, and 1100 °C, respectively, at lower and higher magnifications. (f₁, f₂) show the EDX spectrum of MgO calcined at 900 and 1100 °C, respectively.

The probable reason behind this agglomeration could be the hydrogen bond formation among various hydroxyl species.⁴² The flakes are interconnected with adjacent ones, such that no clear boundaries exist between them. The corresponding high magnification image [Fig. 2(a₂)] identified nanofibers emerging out from the nanoflowers and show high pits and voids. Fig. 2(b₁–e₂) shows SEM micrographs at low and high magnification of the corresponding samples of MgO calcined at 500, 700, 900, and 1100 °C, respectively. SEM images of all the MgO samples were uniform, and heating the samples at higher temperatures resulted in the recovery of surface. It is clear from the micrographs shown in Fig. 2(b₂, c₂, d₂) that on increasing the calcination temperature from 500 to 900 °C, the aspect ratio as well as the density of nanoflakes that convert to nanofibers increase, indicating the bursting of the nanoflower-like structure due to the removal of moisture.³⁷ It can be interesting to note from Fig. 2(e₂) that the MgO nanostructures synthesized at 1100 °C exhibit an increase in the width of nanofibers, whereas the length becomes shortened and then the structure of highly packed nanofibers dominate. Such structures are highly desirable to produce a higher photoluminescence intensity due to the higher surface area. The EDS analysis [Fig. 2(f₁, f₂)] of MgO samples calcined at 900 and 1100 °C, respectively, reveal the presence of Mg and O concentrations of Mg : O of 54 : 46 and 49 : 51, respectively.

For investigation of the microstructures of nano-sized MgO obtained at various calcination temperatures, TEM was conducted. The bright field TEM image of the nanofiber calcined at 700, 900, and 1100 °C at low magnifications exhibited an irregular mix of morphologies of nanofibers with a flake-like structure, as shown in Fig. 3(a–c). High magnification micrographs of the nanofiber calcined at 700 and 900 °C [in the right inset of Fig. 3(a) and (b)] exhibit a width of about 19 nm and 75 nm, respectively. Fig. 3(d) shows the high magnification image of the nanofiber calcined at 1100 °C, which is approximately 95 nm in width and 400–500 nm in length, which reveals the enhancement of width with increasing temperatures. HRTEM represents the well-defined lattice fringes with a d-spacing of 0.24 nm, corresponding to the [111] plane for 700 °C, and that of 0.21 nm, corresponding to the [200] plane for MgO samples calcined at 900 and 1100 °C, respectively, which were in agreement with the XRD results represented in the left inset of Fig. 3(a–c), respectively. Bright and sharp dots obtained in the SAED pattern of the MgO sample calcined at 1100 °C also confirm the high crystallinity of the MgO sample, see left inset of Fig. 3(d).

Fig. 3 TEM images of MgO samples; (a), (b) low magnification images of MgO calcined at 700 and 900 °C, respectively. (c), (d) Low and high magnification images of MgO calcined at 1100 °C. Right and left inset in (a), (b) show high magnification images and lattice-resolved HRTEM images, respectively. Left inset of (c), (d) represent lattice-resolved HRTEM image and SAED pattern, respectively.

The electron microscopy results clearly reveal the formation of different types of nanostructure morphologies at various calcination temperatures. The nano-sized fiber-like morphology was found to be enhanced more than that of the nanoflakes as the calcination temperature increased, as represented in Scheme 1(i). PL intensity was observed to be highest at 1100 °C, as elucidated in Scheme 1(ii), which is probably due to the highly crystalline, well-defined, and uniform morphology of the obtained nanofibers.³⁵ The high crystallinity of the sample can be explained by the FWHM value of PL, as calculated and shown in the table in Scheme 1.

Scheme 1 Pictorial representation showing the relationship among crystallinity, structural morphology, and optical property. (i) Formation of as-synthesized nanoflowers of Mg(OH)₂ from a precursor and the variation in structural morphology of obtained MgO nanostructures with increasing calcination temperatures, (ii) PL emission at an excitation wavelength of 317 nm and a table representing the increase in crystallite size with FWHM of the MgO samples, (iii) schematic illustrating the red and orange emission from MgO nanofibers obtained at 1100 °C, and (iv) diagrammatic view showing the use of MgO nanostructure under a white LED.

The FWHM value of peak B was found to be the least for the highest temperature, in agreement with the sharp narrower peak leading to a high crystallite size (calculated by Debye–Scherrer equation) obtained from XRD. Moreover, the lattice spacing (d = 0.20 nm) and SAED pattern were also found to be in agreement with the FWHM values of the PL intensity peaks. Thus, we can say that high crystallinity points to the highest PL peak. On the blue excitation of the nanofibers obtained at 1100 °C, an orange and red emission was observed, as depicted in Scheme 1(iii), and on the combination of blue, orange and red, white light is produced that can be used for LED applications, as shown in Scheme 1(iv).

Optical properties

UV-Vis absorption spectroscopic analysis

The optical properties of MgO samples have been studied since many years due to their several defects formation and the presence of low coordination ions. Optical absorption has been studied for MgO samples to determine the existence of low coordination sites on the surface of oxides. MgO nanocrystals have three types of coordinated ions, namely, 3-coordinated, 4-coordinated, and 5-coordinated present at the corner, edges, and terrace sites, respectively.⁴⁴ UV-Vis absorption spectra of MgO samples calcined at 500, 700, 900 and 1100 °C are presented in inset of Fig. 4. The two UV-Vis absorption peaks were observed at about 220 and 250 nm for all the MgO samples. The same absorption bands for all the MgO samples concluded that they all have the same absorption sites at different temperatures. Absorption spectra at 220 nm were attributed to the optical excitation of 4-coordinated surface anions (O_4c²⁻) at the edges of the MgO nanocrystallite, while absorption spectra at 250 nm were attributed to the optical excitation of 3-coordinates surface anions (O_3c²⁻) at the corners of the MgO nanocrystallite.²³

Fig. 4 Tauc graph plotted for synthesized MgO samples at 500, 700, 900, and 1100 °C for optical band gap measurement; the inset demonstrates the UV-Vis absorption spectra for the MgO samples at calcination temperatures of 500, 700, 900, and 1100 °C.

The surface anions of MgO nanocrystals undergo reduction in their coordination number may be because of their low modulus potential and become less stable; as a result, the MgO sample exhibited unusual reactivity under the influence of UV-Vis excitation.²³ The Tauc plot of (αhν)² versus (hν) for the energy band gap detection of MgO samples calcined at 500, 700, 900, and 1100 °C are demonstrated in Fig. 4. The corresponding optical band gap of MgO calcined at 500 and 700 °C exhibited a higher band gap value of 3.44 eV, and optical band gap values of 3.30 eV and 3.10 eV were found for MgO samples calcined at 900 and 1100 °C, respectively. Fig. 4 clarifies that the energy band gap decreases as temperature increases. This might be due to the increase in the size of nano-flakes/fibers with temperature, which leads to decreased energy band gaps.^45,46

Photoluminescence spectroscopic analysis

In the present study, room temperature photoluminescence spectra were obtained for all the MgO samples calcined at different temperatures for a surface defect induced photoluminescence study using two excitation wavelengths (λ_ex) at 317 nm and 466 nm upon 668 nm fixed emission wavelength (λ_em), as shown in Fig. 5(a). The result of the PL emission spectrum exhibited two broad PL emission bands at 477 nm (blue region) and 668 nm (red region) at an excitation wavelength of 317 nm, as depicted in Fig. 5(b). The PL emission at 477 nm occurs due to F°-center oxygen vacancies and did not show a symmetry change with increase in calcination temperature.⁴⁷ Furthermore, the emission peak at 668 nm could be established because of the abundance of oxygen vacancies in terms of defects within the lattice via heating at a higher calcination temperature,⁴⁸ therefore the broad peak at 668 nm is significantly enhanced with increasing temperature and shows a very high intensity at 1100 °C. The range of the PL spectra of doped MgO is almost similar to undoped MgO because these types of emissions basically arise from defect-induced emission centers but the distinction in all the cases can be easily made through the peak shift of the photoluminescence spectra of MgO-doped nanostructures (as given in the literature evidence of the emission peaks of doped MgO-685, 703, 718, 750 nm compared to the present case (undoped MgO ∼668 nm)).^36,38–40 The emphasis of the study on photoluminescence emission at an excitation wavelength at 466 nm is due to its proposed application in integration with blue LEDs to convert into white LEDs. The PL spectrum of all the MgO samples was examined for use in LED applications because commercial blue LEDs exhibit electroluminescence around 470 nm. Fig. 5(c) shows strong emission from 580 to 750 nm, associated with two peaks at 593 nm and 668 nm under excitation wavelength of 466 nm, which is in the range of blue LED electroluminescence.

Fig. 5 (a) Photoluminescence excitation spectra at a fixed emission wavelength of 668 nm. (b) & (c) The emission spectra under the excitation wavelengths at 317 nm and 466 nm of MgO samples calcined at 500, 700, 900, and 1100 °C, respectively. (d) CIE coordinates at the white region of the full emission wavelength of MgO: 1100 °C sample at an excitation wavelength of 317 nm.

Fig. 5(d) presents the CIE color coordinates of MgO sample calcined at 1100 °C, corresponding to the photoluminescence emission at an excitation wavelength of 317 nm. Here, the blue emission (477 nm) of PL demonstrates the CIE coordinates at x = 0.13 and y = 0.37 (blue region), and the red emission (668 nm) shows CIE coordinates at x = 0.64 and y = 0.36 (red region). The combination of these two blue and red emissions or the PL spectrum of the full emission at the 317 excitation range estimate the CIE coordinates in the white region (x = 0.33 and y = 0.36), as shown in Fig. 5(d).

Time-resolved photoluminescence spectroscopic analysis

Time-resolved PL technique was performed to investigate the decay time of the luminescent MgO material. Here, TRPL was performed using a PL spectrometer at 375 nm excitation wavelength using a picoseconds diode laser as a source by a time-correlated single-photon counting spectrometer.

The resulting data demonstrate that the decay time was less than 10 ns at 477 nm emission, as represented in Fig. 6(a). The decay time data was fitted to a double-exponential function, as shown in eqn (6). The fitted curve is shown in Fig. 6(b)

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (6)

where I(t) is the PL intensity at time t and A₁ and A₂ are weighting parameters.

Fig. 6 (a) shows TRPL spectra in the nanosecond region and (b) represents the fitting curve and parameter obtained from this fitting curve. Here, τ₁ and τ₂ are two decay components called as fast and slow decay components, which represent the fast and slow recombination processes, respectively.

The exponential decay curve with a decay time less than 5 ns reveals direct radiative recombination without trapping and a short-lived emission of the F-center.^47,18 The average lifetime was also determined by the following eqn (7):^2,4,5,11

The average lifetime for the MgO obtained at 1100 °C was calculated to be τ_av = 133 ps, which is suitable for many potential applications such as solid-state lighting, white light generation, optical switching, or optical sensor applications, where it is essential for the decay time to be in the range of nanosecond to picoseconds.

Photoluminescence stability analysis

To evaluate the light-induced performance of MgO, we also performed photobleaching experiments on the MgO sample calcined at 1100 °C. Here, the obtained result represents the good photostability of the MgO material, as shown in Fig. S2 (ESI).†

PL mapping: spatial distribution of fluorescence intensity

Furthermore, the PL mapping was carried out by a confocal PL instrument with a 375 nm diode laser as a source of excitation wavelength for obtaining the optical image of the spatial distribution of the PL emission intensity at the surface of the MgO nanostructure. Here, PL mapping was carried out for the MgO sample calcined at 1100 °C. The optical micrograph images and 2D view of the fluorescence PL intensity mapping carried out in the blue region (477 nm) and red region (668 nm) of the MgO sample calcined at 1100 °C are presented in Fig. 7.

Fig. 7 (a) Optical micrograph images of MgO sample calcined at 1100 °C, (b) photoluminescence spectrum obtained using a picoseconds diode laser with excitation at 375 nm. (c, d) 2D micrographs of PL intensity mapping in the blue and red region for the MgO sample calcined at 1100 °C.

The obtained result reveals the uniform distribution of photoluminescence emission intensity on the MgO nanostructure surface.

Proposed futuristic application of MgO nanostructures in WLED

To propose futuristic applications of luminescent MgO nanostructures for WLED application, it is important to understand the basic concept of white-light generation in LEDs.

In the general concept for WLED, phosphor incorporated in the epoxy region of the blue LED (InGaN) generates white light, where some blue light emission from blue LED is converted into yellow light by the phosphor^49–51 and the remaining blue light results in white light by mixing with yellow light.^49,52,53 For example, YAG:Ce³⁺ is most commercially used phosphor in the field of solid-state lighting applications, due to their attractive optical properties, long lifetime, non-toxicity, chemical, and thermal stability.⁴⁹ YAG:Ce³⁺-based LEDs suffer from some drawbacks such as very low quantum efficiency and poor color rendering index (CRI) due to the deficiency of generating red light.⁴⁹ To improve the red emission of the YAG:Ce³⁺ phosphor, the effect of co-doping of red-emitting species, such as rare earth ions (Cr³⁺, Pr³⁺, and Sm³⁺), quantum dots, and nanophosphors, has been widely examined.^49,54–59 Recently another approach to improve the CRI and CCT was to incorporate plasmonic nanoparticles (such as Au) into the nanophosphors.⁴⁹ However, both the approaches suffer from the disadvantage of cost effectiveness. In the present investigation, MgO was proposed as a cost effective and environmentally friendly red-emitting luminescent material for WLED application. The spectroscopic result of this material satisfied the required benchmark. The schematic illustration in Scheme 2(a) depicts that by the integration of blue LED (λ_em = 470 nm) with a luminescent MgO material or via using a combination of blue light (emission of BLED), plus red and orange light (emitted from MgO luminescent material) one could engineer a system to generate a white light for WLED application.

Scheme 2 Plausible mechanism for the generation of white LED; (a) CIE coordinates for white color formation via the combination of red coordinates (from MgO: 1100 °C) and standard blue color coordinates; (b) PL emission of MgO: 1100 °C sample at an excitation of 466 nm (right inset) and standard blue emission of BLED (left inset) and their pictorial representation as white light emission.

The simulated CIE results is based on commercial blue LED electroluminescence emission with an observed red emission of MgO excited at 466 nm, as shown in Scheme 2(b). Briefly, in the present case, we observed a broad spectrum covering orange (593 nm) to a deep red region (668 nm) at an excitation of 466 nm, as shown in Scheme 2(b). The quantum efficiency of the present defect-induced undoped MgO nanostructure is ∼55%, which qualifies it as the almost required criteria for LED application. The spectroscopy results of MgO clearly prove that it could be integrated with blue LEDs for future white LED applications.

Conclusion

We have successfully demonstrated a new strategy for a high yield production of luminescent nanoflakes-/nano-fibers-like MgO nanostructure using a co-precipitation method. The structural/micro-structural and surface morphology of these nanostructures was confirmed by various characterization techniques such as XRD, FTIR, EDX, SEM, and TEM. The SEM and HRTEM images clearly demonstrate the nanoflakes-/nanofibers-like features throughout the sample. The photoluminescence spectroscopy results of the MgO sample calcined at 1100 °C clearly reveal that these nanofibers exhibit strong blue and hypersensitive red emission at 317 nm excitation wavelength. The combination of the CIE of blue and red emissions clearly demonstrates white color generation. In addition, the excitation at 466 nm emits a broad spectrum in the range of 580–750 nm (yellowish to red region), which is highly suitable for proposed white LED application after the integration of luminescent MgO with blue LED. Thus, luminescent MgO synthesized by a simple wet chemical method provides a new analog form of rare-earth-free luminescent material for solid-state lighting applications.

Acknowledgements

We thank the Director, NPL New Delhi, India, for providing the necessary experimental facilities. Dr N. Vijayen, Dr S. N. Sharma, Mr K. N. Sood, Mr J. S. Tawale, Dr Ritu Srivastava are gratefully acknowledged for providing the necessary instrumentation facilities for XRD, FTIR, SEM and UV-Vis. The Projects NanoSHE (BSC: 0112) and DST (GAP: 113932) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 and S2. See DOI: 10.1039/c5ra21150f

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