Facile LaOF: Sm³⁺ based labeling agent and their applications in residue chemistry of latent fingerprint and cheiloscopy under UV–visible light

1 Introduction

Fingerprints (FPs) were the universal investigative protocol for the identification of individuals and imparting added records of the individuals in forensic field. Certain FPs can be visualized without hindrance, when the fingers were stained with extrinsic additives including, blood, food contaminants, paint, moisture, etc. Such FPs were referred to as latent fingerprints (LFPs). Nevertheless, the majority of FPs on many surfaces have been due to sweat accommodates with the help of epidermis, secretory glands (eccrine, apocrine, apoeccrine and sebaceous), etc. These FPs were invisible to bare eyes; therefore, such FPs were referred to as LFPs (Zhao et al., 2016; Basavaraj et al., 2017a; Wang et al., 2017; Chen et al., 2015). Till date, numerous methods (chemical & optical) have been established to visualize LFPs. However, the recognition was restricted only to level 1 and level 2 ridge details due to the poor image qualities. Further, the level 1 and 2 features were easily forged by the use of artificial skin imprints. However, the analysis of level 3 features requires some efficient nano powders with uniform particle size and morphology (Darshan et al., 2016a; Saif, 2013; Champod et al., 2004). The various phosphor nanomaterials used for visualization of LFPs by various authors were listed in Table 1. By focused investigation of FPs, exhibits permanent, immutable and unique pores which were scattered on the ridges. For individual recognition, 20–40 pores were quite sufficient to establish the pore investigation (Labati et al., 2017). When a finger touches various surfaces, nonvolatile sweat constituents from the fingertip becomes immobilized on the surfaces, due to its highly networked structure. This confines the free migration of the constituents. Consequently, dot description of the pores rather than complete ridge patterns corresponds to fingertip ridges deposited on the surface. Unfortunately, sweat pores were the most neglected part due to non-availability of rapid, reliable and affordable FPs visualization methods (Labati et al., 2017). Hence, there is an urgent need to find a simple and efficient route for visualize LFPs with superior sensitivity, little background hindrance, extraordinary efficiency and lower toxicity, which are the essential requirements for forensic investigators (Darshan et al., 2016b; Rakesh et al., 2011; Dhanalakshmi et al., 2017).

Powder dusting approach became notably easy and flexible method for the LFPs visualization with superior visibility and detailed information. The visualization of LFPs via powder dusting technique includes the relevance of exceptional powder particles which shows higher affinity to moisture and adhering to the chemical compositions within the LFPs residue on numerous surfaces (Darshan et al., 2016c; Li et al., 2012). The form, size and surface vicinity of powder particles play a primary position for reinforcing LFPs. Labeling powders utilized for visualization of LFPs can categorized into three kinds; light, dark and phosphorescence powders. Sometimes, FPs visualized by using light and darkish powders was faint and exhibits low sensitivity on selected dark/light surfaces. However, combination of powders to gain effectiveness on each light and darkish surface also suffers for extremely faint impressions. In contrast, phosphorescence nanopowders were highly sensitive abounding with appropriate gentle property to visualize LFPs on porous and non-porous surfaces due to their exceptional physical and chemical properties together with massive surface region and high phosphorescence depth. Consequently, fluorescent/phosphorescence nanomaterials were the new agents for visualization of LFPs with high clarity on porous and non-porous surfaces (Darshan et al., 2017).

Recently, rare earth doped nanophosphors were attracting considerable attention owing to their superior brightness, high efficiency, and long life time, as a result, they were useful in plasma display panels (PDP), field emission displays (FEDs) and white light-emitting diodes (WLEDs) (Paulo et al., 2015; Chen et al., 2012; Shang et al., 2012; Zhang et al., 2015; Vijayakumar and Marimuthu, 2015; Xia et al., 2013a; Xia et al., 2013b; Shi et al., 2014; Guogang et al., 2014). In recent times, oxyfluoride phosphors for WLED’s have expanded much attention due to their improved properties such as thermal and chemical stability when compared to other hosts such as oxides, sulphides, molybdate, tungstate, etc. (Dai et al., 2016; Chena et al., 2016; Fang et al., 2016; Chen et al., 2016; Xia et al., 2012; Xia et al., 2014; Kim et al., 2016).

Normally, phosphors with spherical morphology, narrow size distribution and free from agglomeration was highly essential because they exhibit low scattering of light and higher packing densities. Consequently, a superior method was required to develop above kind of phosphor materials for solid state lighting and forensic applications (Venkataravanappa et al., 2017; Venkatachalaiah et al., 2017a,b, Basavaraj et al., 2017b). Rare earth (Sm³⁺) ions were found to be essential activator for generating strong amber light emission. Sm³⁺ doped in stable luminescent host was more concern presently due to bright emission in orange or red regions (Xu et al., 2013; Wang et al., 2015; Luitel et al., 2012).

The rare earth doped LaOF materials were synthesized by various wet chemical routes including sol-gel, combustion, co-precipitation, etc. (Rakov et al., 2015; Bayarta et al., 2016; Li et al., 2015). From the numerous synthesis techniques stated above, ultrasound sonochemical synthesis method was advantageous over other routes due to fast production, relatively low temperature, low cost, and the easy way to accomplish high purity and single phase nanopowders. Sonochemistry comprises cavitation via energetic sound waves followed by formation of froths and bubbles in liquid which benefits for active nucleation and the evolution of nano/micro/superstructures (Suresh et al., 2018; Venkatachalaiah et al., 2017a,b; Som et al., 2016).

In the present work, Sm³⁺ doped LaOF NPs were prepared by ultrasound assisted sonochemical synthesis route using different surfactants such as Polyethylene glycol (PEG), Cetrimonium bromide (CTAB), Sodium dodecyl sulfate (SDS) and starch. The products were characterized by powdered X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectra (DRS) and photoluminescence (PL). The optimized NPs were utilized to visualize the LFPs on non-porous surfaces. The photometric properties viz., Commission International de I ‘Eclairage (CIE), correlated color temperature (CCT) and color purity were also evaluated and discussed in detail.

2 Experimental

2.1

2.1 Material and methods

Pure and Sm³⁺ (1–11 mol%) doped LaOF NPs were prepared via ultrasound assisted sonochemical method. The precursors used to prepare LaOF: Sm³⁺ NPs were of analytical grade with high purity and were purchased from Sigma Aldrich. The stoichiometric amount of Lanthanum nitrate [La (NO₃)₃ 4H₂O (99.9%)], Ammonium fluoride [NH₄F (99.9%)] and Samarium nitrate [Sm (NO₃)₃ (99.9%)] were used in molar ratio of 1:2:1 and PEG, CTAB, SDS and Starch were used as surfactants. The detailed list of chemicals and surfactants used in the present study was given in Table 2. Initially, the precursor solutions were prepared by dissolution of La (NO₃)₃ and NH₄F in 50 ml of distilled water. Further, the required quantity of dopant ions and surfactants were added to the reaction mixture. The resulting final solution was subjected to ultrasound treatment (frequency of ∼20 kHz, power of ∼300 W and sonication time of ∼1–6 h at a fixed temperature of 80 °C). The precipitate obtained at the end of the reaction was filtered using distilled water and alcohol. The powder was dried at 80 °C for 3 h in a hot air oven and further heat treated at ∼700 °C for 3 h. Fig. 1. shows the schematic diagram of the ultrasonic synthesis of incorporated LaOF: Sm³⁺ NPs.

Table 2 List of chemicals and surfactants used in the present study.

Chemicals	Linear formula	Molecular weight (g/mol)	Details of surfactant
			Surfactant	Linear formula	Structure	Molecular weight (g/mol)
Lanthanum nitrate	La (NO3)3·6H2O	433.01	Starch	(C6H10O5)n		342.30
Ammonium fluoride	NH4F	37.04	Polyethylene glycol (PEG)	H(OCH2CH2)nOH		200
Samarium nitrate (III)	Sm(NO3)3·6H2O	444.47	Cetrimonium bromide (CTAB)	CH3(CH2)15N (Br)(CH3)3		364.45
			Sodium dodecyl sulfate (SDS)	CH3(CH2)11OSO3Na		288.38

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Fig. 1

Schematic illustration of ultrasound assisted sonochemical synthesis of LaOF: Sm³⁺ NPs.

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3 Characterization

Shimadzu 7000 instrument was utilized to analyze the phase purity and crystallinity of the samples. Morphology was studied using Hitachi TM-3000 scanning electron microscope (SEM). Perkin Elmer (Lambda-35) spectrometer was used to record the diffused reflectance (DR) of the powder. PL spectra were recorded using Jobin Yvon Spectroflourimeter Fluorolog-3 attached with xenon lamp (450 W) as a source of excitation. The Hitachi H-8100, Transmission electron microscopy (TEM) was utilized for crystallite size analysis and for confirmation of chemical composition.

3.1

3.1 Visualization of LFPs and lip prints

LFPs were collected from the thoroughly washed and clean hands of healthy person and gently wiped and pressed on the surfaces of different non-porous substrates surfaces. High PL intensity emitted composition was LaOF: Sm³⁺ (5 mol%) NPs which was used for LFPs visualization. Carefully these compounds were stained on the surfaces of the substrates with a light brushing action and gently removed the excess powder until a fingerprint image was developed (Bandey, 2004). Stained fingerprints were photographed by a Nikon D3100/AF-S Nikkor 50 mm f/1.8G ED lens digital camera under normal light. The schematic representation for the visualization of LFPs was shown in Fig. 2.

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Fig. 2

Schematic representation shows visualization of LFPs by powder dusting method.

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In order to analyze the details of the lips print of the individuals, lips of healthy donor was washed thoroughly with water and cleaned with the cotton ball dipped in lipstick remover. Clean and washed glass substrate on which lips impressions were taken. In the beginning, the glass slab was gently pressed starting from center and moving laterally in a single motion. After the proper lip impression on the glass slab, LaOF: Sm³⁺ (5 mol%) labeling agent was gently stained on the latent lip print with feather brush, thereafter the stained lip print was photographed with the camera under normal light.

4 Results and discussion

In the present studies, undoubtedly visualized all the three levels of LFPs, i.e., level I (core), level II (bifurcation, bridge, ridge ending, crossover, island, enclosure, scar, eye and delta) and level III (sweat pores) by using LaOF: Sm³⁺ (5 mol%) labeling agent and observed under normal light (Fig. 3). Further, the minutiae ridge details as well as powder distribution on the ridges were studied by SEM studies (Fig. 3). The obtained images clearly demonstrated that the powder particles were uniformly distributed on the ridges due to identical sized particles with superior adhesive ability.

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Fig. 3

FPs visualized by using LaOF: Sm³⁺ (5 mol%) NPs on aluminum foil under normal light and SEM micrographs of ridge patterns. (Highlighted parts clearly displayed the various ridge details).

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Fig. 4 depicts the enhanced LFPs on various non-porous surfaces such as remote, wooden bar, steel plate, metal shield, metal scale and marble. From the figure, it was observed that the FPs was clearly visible with level I and level II ridge details. Whereas, the sweat pores were clearly visible on the surface of wooden sheet and stainless steel plate (Fig. 4(b & c)). Further, the LFPs were also tested on the other surfaces namely; glass slab, cell phone, glass bottle, calculator, compact disk, aluminum foil and etc. and obtain the requisite information for forensic investigators (Fig. 5(a–f)). The post processed FP images of level I, level II and level III minutiae ridges on aluminum foil were studied and depicted in Fig. 5(g–j). LFPs images of different donors were also investigated as shown in Fig. 6. FPs of four different healthy donors were collected on the surface of aluminum foil and dusted with the optimized NPs. It was observed from the figures that all the FPs was unique and easily identifies the ridge details. As it was observed from the Fig. 6(a & d), the FP images of the first and fourth donors resemble central and pocket whorl type. However, the second and third donors resemble plain and ulnar arch (Fig. 6(b & c)). In order to check the efficiency of the prepared NPs, the overlapped fingerprints were studied on aluminum foil surface (Fig. 7). From the figure it was evident that, all the three levels minutia ridge patterns were visible. This indicates that the present powder can also be used effectively as a labeling agent for visualize complex FP patterns.

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Fig. 4

FPs images visualized by LaOF: Sm³⁺ (5 mol%) NPs on non-porous surfaces such as (a) remote, (b) wooden sheet, (c) stainless steel, (d) metal shield, (e) metal scale and (f) marble under normal light.

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Fig. 5

FPs images stained by LaOF: Sm³⁺ (5 mol%) NPs on non-porous surfaces namely (a) glass slab, (b) cell phone screen, (c) glass bottle, (d) calculator screen, (e) compact disk, (f) aluminum foil and (g–j) post processed fingerprint exhibits various minutiae ridges (sweat pores, bifurcation, eye and island).

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Fig. 6

Different donors LFP images on the aluminum foil visualized by using LaOF: Sm³⁺ (5 mol%) NPs under visible light. (Right column images show standard ridge patterns).

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Fig. 7

Overlapped LFPs stained by LaOF: Sm³⁺ (5 mol%) NPs collected from different donors on the aluminum foil surface under normal light. (Highlighted parts clearly displayed the various ridge details).

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Cheiloscopy also known as lip print technique was one of the forensic investigation for identifying the individual humans based on lip traces. Characteristics of lip pattern of any individual were due to sulci laborium glands of labial mucosa on wrinkles and grooves. Observed wrinkles and grooves were named as sulci laborium rubrorum (Kaul et al., 2015). The continual moisturizing by the tongue due to occasional sebaceous glands present on the lips leaves behind the latent lip prints on various material surfaces such as glasses, cigarettes, straws, food items etc. Normally, the lip on these surfaces were not visible, some extra effort has to be paid to make the lip prints visible. In the present work, the powder dusting technique has been utilized to visualize lip print on glass surface. Fig. 8 shows the lip print dusted with the LaOF: Sm³⁺ (5 mol%) NPs. The detailed classifications of the lip prints as indicated by Tsuchihashi were studied. All the five types of grooves (Type I to V) were clearly observed due to the smaller and uniform sized NPs.

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Fig. 8

(a) Lips print visualized by LaOF: Sm³⁺ (5 mol%) NPs on glass surface and (b–f) various lip print grooves. (Left column images show standard ridge grooves).

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Fig. 9 shows the PXRD patterns of LaOF NPs prepared via conventional stirring and ultrasonication method. It was observed from the figure that a tetragonal phase along with some byproducts was formed by the conventional stirring method. However, in the case of ultrasonication, the highly crystalline and impurity - free phase was observed and it well matches with the JCPDS, NO: 89-5168 with space group P4/nmm (No.129) (Luo et al., 2012). The effects of different surfactants namely PEG, CTAB, SDS and starch on Sm³⁺ doped LaOF NPs were studied, as shown in Fig. 10. All the PXRD profiles were well matched with standard JCPDS, NO: 89-5168 without any impurities. The average crystallite size (D) was estimated using Scherrer’s relation (Suresh et al., 2018).

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Fig. 9

PXRD patterns of LaOF fabricated by (a) conventional stirring and (b) ultrasonication method.

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Fig. 10

PXRD patterns of LaOF:Sm³⁺ NPs synthesized with different (a) surfactants, (b) PEG concentrations, (c) Sm³⁺ (1–11 mol%) and (d) W-H plots of LaOF: Sm³⁺ (1–11 mol%) NPs.

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To evaluate the structural parameters, the PXRD profiles were refined for various lattice parameters, Pseudo-Voigt profile function (u, v and w), isothermal temperature factors (B_iso), by considering backgrounds scale factor and atomic co-ordinates (Dhananjaya et al., 2016). The recorded, calculated and the difference PXRD patterns of pure and Sm³⁺ doped LaOF were shown in Fig. 11(a, c–h). The experimental and calculated data of PXRD profiles showed nearly to zero in the intensity scale as illustrated by a line (Y_obs–Y_calc). The packing diagram of LaOF: Sm³⁺ (5 mol%) was shown in Fig. 11(b). The structural parameters refined by Rietveld analysis for LaOF: Sm³⁺ (1–11 mol%) NPs were summarized in Table 4. The unit cell parameters decrease after the substitution of Sm³⁺ ion (R_d = 1.066 Å) for 8 coordinated La³⁺ (R_s = 1.160 Å) site in the LaOF lattice. The acceptable percentage difference (D_r) between dopant Sm³⁺ and La³⁺ should not exceed 30%. The value of D_r between the doped ions (Sm³⁺) and substituted ions (La³⁺) in LaOF: Sm³⁺ NPs was estimated based on the formula (Darshan et al., 2016a):

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Fig. 11

Rietveld refinement of (a) pure LaOF, (c–h) LaOF: Sm³⁺ (1–11 mol%) NPs and (b) packing diagram of LaOF: Sm³⁺ (5 mol%) NPs.

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The diffuse reflectance (DR) spectra of LaOF: Sm³⁺ (1–11 mol%) NPs recorded at RT in the range of 200–1100 nm was shown in Fig. 12(a). The spectra exhibited dominant peaks at 357, 405,446, 478, 529, 586, 939 and 1070 nm due to the transitions of the 4f electrons of Sm³⁺ from the ground-state to ⁶H_5/2 → ⁴H_7/2, ⁶H_5/2 → ⁴F_7/2, ⁶H_5/2 → ⁴G_9/2, ⁶H_5/2 → ⁴I_11/2, ⁶H_5/2 → ⁴F_3/2, ⁶H_5/2 → ⁴G_5/2, ⁶H_5/2 → ⁴F_11/2 and ⁶H_5/2 → ⁴F_9/2 respectively (Shang et al., 2012; Xie et al., 2012). Further, the energy gap (E_g) of the prepared NPs was estimated by applying the Kubelka–Munk (K-M) theory to DR spectra. The plots of ${[F(R_{\infty })h\nu ]}^{1/2}$ versus photon energy ( $h\nu$ ) were shown in Fig. 12(b). The Kubelka–Munk function F (R_∞) and photon energy ( $h\nu$ ) were estimated using the relations (Amith et al., 2017; Xiao et al., 2010):

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Fig. 12

(a) DR spectra and (b) energy band gap spectra of pure & Sm³⁺ (1–11 mol%) doped LaOF NPs.

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The estimated E_g and R.I values were tabulated in Table 5. From the table, it was clear that E_g and R.I values increases with increase of dopant concentration. The variation in E_g and refractive index was associated with quantum confinement effect in nano-size particles i.e. the squeezing of electron hole pair below the dimensions approaching exciton Bohr radius and with the intermediate localized states in the band gap due to structural defects.

Fig. 13 shows the SEM micrographs of LaOF: Sm³⁺ (5 mol%) NPs prepared with four different surfactants namely PEG, CTAB, SDS and starch under 3 h sonication time. The cubic and hexagonal discs were observed for PEG used sample (Fig. 13(a)). The agglomerated flake - like structures with indefinite shape were identified in CTAB and starch assisted samples (Fig. 13(b & d)). However, the SDS assisted products exhibited agglomerated particles with pores and voids (Fig. 13(c)). From these results, it was evident that the PEG surfactant one could able to obtain various shapes and sized particles. Fig. 14 shows the mechanism for the formation of spherical shaped nanostructures in the presence of PEG.

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Fig. 13

SEM micrographs of LaOF: Sm³⁺ (5 mol%) NPs prepared with different surfactants (a) PEG, (b) CTAB, (c) SDS and (d) Starch under 3 h sonication time.

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Fig. 14

Schematic illustration to show formation mechanism of LaOF: Sm³⁺ (5 mol%) NPs in the presence of PEG surfactant.

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Fig. 15(A) shows the SEM micrographs of LaOF: Sm³⁺ (5 mol%) NPs synthesized with different sonication time (1–6 h) while PEG surfactant was fixed to 30 ml. Initially, when the sonication time was 1–2 h, that elongated particles attached each other (Fig. 15(A) (a & b)). Further, when the sonication time was increased to 3–5 h, these particles were separated and to form a network - like structure (Fig. 15(A) (c–e)). Further, when the sonication time was extended to 6 h, plate - like structures were noticed (Fig. 15(A), (f)). This confirms that the sonication time influences considerably in the modification of morphology of the product.

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Fig. 15

SEM micrographs of LaOF: Sm³⁺ (5 mol%) NPs prepared with different (A) sonication time (a) 1 h, (b) 2 h, (c) 3 h, (d) 4  h, (e) 5 h and (f) 6 h with 30 ml of PEG surfactant and (B) concentrations of PEG (a) 5 ml, (b) 10 ml, (c) 15 ml, (d) 20 ml, (e) 25 ml and (f) 30 ml with 3 h of sonication time.

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The effect of PEG concentration (5–30 ml) on the product morphology was studied, as shown in Fig. 15(B). When PEG surfactant volume was ∼5 ml, prisms like microstructures were observed (Fig. 15(B) (a)). When PEG volume was increased from ∼5 to 10 ml, almost spherical shaped particles along with mesh like morphology were noticed (Fig. 15(B) (b)). However, when the PEG volume was ∼15 ml, the hexagonal prism-like structures were obtained Fig. 15(B) (c)). The smooth edged dice - like network particles were formed when the PEG volume was ∼20 ml (Fig. 15(B) (d)). When the PEG volume was increased to ∼25 ml, the elongated dumbbell shaped structures were obtained (Fig. 15(B) (e)). Finally, a spindle shaped network was formed when the PEG volume was increased to 30 ml (Fig. 15(B) (f)). The results clearly indicate that the surfactant volume plays a vital role in product morphology engineering. The effects of pH and sonication power on the product morphology were also studied, as shown in Fig. 16(A) & (B). From the figure, it was clear that the nest - like structure was commonly observed for all pH and sonication power.

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Fig. 16

SEM micrographs of LaOF: Sm³⁺ (5 mol%) NPs prepared with different (A) pH values (a) 1, (b) 3, (c) 5, (d) 9, (e) 11 and (f) 13 and (B) sonication power (a) 20 kHz, (b) 22 kHz, (c) 24 kHz, (d) 26 kHz, (e) 28 kHz and (f) 30 kHz with 3 h of sonication time and 30 ml of PEG surfactant.

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Fig. 17 shows the TEM, HRTEM images, SAED patterns and EDX analysis of LaOF: Sm³⁺ (5 mol%) NPs prepared with PEG, CTAB and SDS surfactants under 3 h sonication time. The particles were obtained as hexagonal and flake like structures, which was in good agreement with the SEM results. Further, the particles sizes for all the surfactants lie in the range of ∼20–35 nm. Further, the SAED pattern the product exhibits highly crystalline nature. The interplanar spacing values (d) were estimated to be ∼0.32, 0.30 and 0.28 nm. In Fig. 17(j), the EDAX spectrum shows the strong La line at ∼4.954 keV, O line at ∼0.827 keV, F line at ∼0.954 keV, and small traces of Sm line showing that there was no trace of other by products. In other words, no elements other than LaOF: Sm³⁺ and oxygen were detected which confirm the purity of the prepared NPs synthesized in the present work.

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Fig. 17

(a, d & g) TEM images, (b, e & h) SAED patterns, (c, f & i) HRTEM images of LaOF: Sm³⁺ (5 mol%) NPs prepared with PEG, CTAB and SDS surfactants and (j) EDAX and elemental analysis.

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The PL excitation spectrum of doped LaOF: Sm³⁺ (5 mol%) NPs monitored at 607 nm emission wavelength were shown in Fig. 18(a). The spectrum exhibits peaks at ∼343, 361, 377, 406, 414, 449, 467 and 480 nm corresponding to ⁶H_5/2 → ⁴K_17/2 + ⁴L_17/2, ⁶H_5/2 → ⁴D_15/2 + ⁶P_15/2, ⁶H_5/2 → ⁴L_17/2, ⁶H_5/2 → ⁴K_11/2, ⁶H_5/2 → ⁶P_5/2 + ⁶M_19/2, ⁶H_5/2 → ⁴G_9/2 + ⁴I_15/2, ⁶H_5/2 → ⁶P_5/2 + ⁶M_19/2, ⁶H_5/2 → ⁴G_9/2 + ⁴I_15/2, ⁶H_5/2 → ⁴F_5/2 + ⁴I_13/2 and ⁶H_5/2 → ⁴I_11/2 + ⁴M_15/2 transitions of Sm³⁺ ions respectively (Kang et al., 2011). Fig. 18(b) shows the PL emission spectra of LaOF: Sm³⁺ (1–11 mol%) NPs excited at 406 nm. The spectra exhibit strong emission peaks at ∼566, 607, 653 and 708 nm attributed to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁴H_7/2, ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_11/2 intra - 4f orbital transitions of Sm³⁺ ions respectively (Zou et al., 2017; Atuchin et al., 2015). Among these transitions, the peaks at ∼566 nm (⁴G_5/2 → ⁶H_5/2), ∼607 nm (⁴G_5/2 → ⁶H_7/2) and ∼653 nm (⁴G_5/2 → ⁶H_9/2) were magnetic-dipole (MD), partly forced electric-dipole (ED) transition, and purily ED transition, respectively (Kodaira et al., 2007). Notably, the peak at ∼607 nm (⁴G_5/2 → ⁶H_7/2 transion) has the strongest intensity and it was applied to orange-red emitting display materials. The inensity ratio (Asymmetry ratio (A₂₁)) of ED and MD trasitions can be used to understand the distortion degree from the inversion symmetry of the Sm³⁺ local environment in the host (Shi et al., 2016). In the present case, the value of A₂₁ increases with increase of Sm³⁺ concentration up to 3 mol% and then decreases. This variation of A₂₁ values can influence the luminescent property of a sample. It was evidence that the PL intensity increased up to 5 mol% of Sm³⁺ and after this, it diminished. The variation of PL intensity and A₂₁ value with Sm³⁺ ion concentration was shown in Fig. 18(c). The PL emission intensity decrease was due to well-known self-concentration quenching phenomenon attributed to energy transfer among the Sm³⁺ ions in the LaOF host (Tian et al., 2012).

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Fig. 18

(a) PL excitation (b) Emission spectra (c) Plot of PL emission intensity, asymmetric ratio v/s dopant Sm³⁺ concentration, (d) Logarithmic plot of (I/x) v/s (x), (e) CIE and (f) CCT diagram of LaOF: Sm³⁺ (1–11 mol%) NPs prepared with 30 ml PEG under 3 h sonication time.

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To know more about concentration quenching, it was significant to elucidate the interaction mechanism, which usually happens due to exchange interaction, re-absorption or a multiple – multiple interaction. Therefore, it was essential to calculate the critical distance (R_c) between the two Sm³⁺ ions in LaOF host by using following relation (Li et al., 2009);

According to Van Uitert model, the type electric multipolar interaction was estimated using the following relation (Shi et al., 2015);

The Judd-Ofelt (J – O) intensity parameters (Ω₂ & Ω₄) and different radiative properties such as radiative transition probability (A_T), radiative (τ_rad) lifetime, branching ratio (β_R) and asymmetric ratio (A₂₁) were calculated for detailed investigation of site symmetry as well as luminescence dynamics of Sm³⁺ ions in LaOF matrix (Judd, 1962; Ofelt, 1962). Although J – O analysis was a influential tool, which efficiently defines the spectral behavior in a specific coordination location of rare earth ion incorporated single and polycrystalline materials, glasses and solutions. Detailed physical interpretation J-O parameters were given by Jorgensen and Reisfeld (1983). These parameters were estimated from the PL emission spectra considering magnetic dipole allowed transition (⁵G_5/2 → ⁶H_7/2) as the reference, since it was unchanged by the neighboring crystal field environment. The rates of parity forbidden dipole transitions among various electronic states of the rare earths, local structural environment surrounding them and covalent bonds associated ligands of them can easily evaluated by this J-O analysis (Som et al., 2015). The relation between radiative emission rates and the integrated emission intensities were calculated by the relations (Feng and Zhang, 2013);

The radiative emission rates (A_0-J) of electric dipole related with J-O parameters was expressed as (Kodaira et al., 2003);

The radiative transition probability (A_T) was determined by using the relation;

The radiative lifetime (τ_rad) of an excited state can be expressed as

The branching ratio $\beta ({\psi }_J)$ corresponding to the emission from an excited level to its lower level was given by (Som et al., 2015);

The radiative properties, namely radiative transition probabilities (A_T), radiative lifetimes (τ_rad) and branching ratios (β_R), for the excited states of Sm³⁺ ion were estimated using J–O parameters and tabulated in Table 5. Since the branching ratio (β) of the optimized phosphor was found to be higher than 0.50, it was potentially useful in solid state lightning applications.

The CIE chromaticity diagram (Smith and Guild, 1931) of LaOF: Sm³⁺ (1–11 mol%) NPs was presented in Fig. 18(e). It was evident that the CIE chromaticity coordinates were located in the orange-red region. To identify applicability of this orange-red emission, the CCT was estimated from CIE coordinates. Fig. 18(f) shows the CCT diagram of LaOF: Sm³⁺ (1–11 mol%) NPs. The CCT was a specification of the color appearance of the light emitted by a light source, relating its color to the color of light from a reference source when heated to particular temperature. The correlated color temperature (CCT) was one of the essential parameter to know the color appearance of the light emitted by a light source with respect to a reference light source when heated up to a specific temperature, in Kelvin (K). CCT was estimated by transforming the (x, y) co-ordinates of the light source to (U₀, V₀) by using the eqn. (14) and (15), and by determining the temperature of the closest point of the Planckian locus to the light source on the (U₀, V₀) uniform chromaticity diagram;

Also, the quality of white light in terms of color correlated temperature (CCT) was given by McCamy empirical formula $\mathit{CCT}=-437n^3+3601n^2-6861n+5514.31$ (theoretical) where $n=(x-x_c)/(y-y_c)$ ; the inverse slope line and chromaticity epicenter was at x_c = 0.3320 and y_c = 0.1858. Generally, CCT value was greater than 5000 K indicates the cold white light used for commercial lighting purpose (Meetei and Singh, 2014). The estimated CIE co-ordinates (x, y), (U₀, V₀) and CCT values of LaOF: Sm³⁺ (1–11 mol%) NPs were tabulated in Table 6. Thus, the present phosphor can be useful for artificial production of white light in illumination devices.

The Quantum efficiency (Q_E) of the optimized LaOF: Sm³⁺ (5 mol%) NPs was estimated using the relation reported elsewhere (Mello et al., 1997). The estimated value of Q_E was found to be ∼88.14%. Colour purity (CP) of the prepared sample was also checked using following relation (Chhajed et al., 2005);

The luminescence decay of prepared LaOF: Sm³⁺ (1, 5, & 11 mol%) NPs was realized by fitted with following bi-exponential decay equation;

The average lifetime in case of a bi-exponential decay can be calculated using the equation;

The multi exponential decay may be explained as: (i) Difference in the nonradiative probability of decays for lanthanide ions at or near the surface and lanthanide ions in the core of the particles. (ii) Inhomogeneous distribution of the doping ions in the host material leading to the variation in the local concentration. (iii) The transfer of excitation energy from donor to lanthanide activators (Som et al., 2014). In the present work, the variation in the luminescence decay of prepared LaOF: Sm³⁺ (1, 5, & 11 mol%) NPs (Fig. 19) due to structural defects and the presence of several Sm³⁺ sites in the matrix may impact the emission. This indicates that the prepared phosphor might be excellent materials for the WLED applications.

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Fig. 19

Luminescence decay curve of LaOF: Sm³⁺ NPs.

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5 Conclusions

In summary, preparation of series of LaOF: Sm³⁺ (1–11 mol%) NPs using non-toxic, eco-friendly ultrasound assisted sonochemical method using four different surfactants was reported. Particles morphology can be greatly engineered by varying experimental parameters, namely sonication time, volume of the surfactant, pH value, temperature of sonication bath and sonication power. PXRD and TEM/HRTEM analysis revealed the crystallite size ∼20 nm and interplanar spacing ∼0.32 nm. Prepared NPs exhibit excellent photometric characteristics and they can be used to fill amber gap in LEDs under near-UV excitation. Optimized NPs have been effectively used as labeling agent to visualize LFPs on various non-porous surfaces under normal light. LFPS were visualized with high sensitivity, low background hindrance, high efficiency and low toxicity due to the quality of prepared nanophosphor materials adherence efficiency. These results clearly indicate the utility of LaOF: Sm³⁺ (5 mol%) NPs in visualization of LFPs, lips print as well as useful component in solid state lighting applications.