Background

The generalized process of ‘cold light’ emission is called luminescence, and it refers to the absorption of energy by a material with the subsequent emission of photons[1]. It is a phenomenon distinct from blackbody radiation, incandescence, or other such effects that cause materials to glow at high temperatures[1–4]. There are various forms of luminescence, usually denoted by the method from which the electrons were excited. For example, electroluminescence happens when an electrostatic field excites the electrons. Photoluminescence is the excitation of electrons due to electromagnetic radiation. Likewise, triboluminescence (TL) is light produced by the fracture of crystals[4, 5].

The word triboluminescence comes from the Greek tribein, meaning ‘to rub,’ and the Latin prefix lumen, meaning ‘light.’ TL is a specific form of mechanoluminescence, which is the production of non-thermal light from any type of mechanical action or application of stress[4, 5]. The mechanisms of TL light are still not fully understood, but it is believed that TL is associated with asymmetric crystal structure. Crystal bonds are broken along planes with opposing charge, and when they re-connect, light is emitted as the charges pass through the separations created from the fracture. A classic example of TL light is found in the crystals used for real wintergreen-flavored Lifesavers®[6, 7]. The green/blue sparks seen when chewing the candy is TL light being emitted from the crystal breakage in the sucrose[6, 7]. TL can also be produced by the peeling of tape in a vacuum[8] and as a result of plate stresses during and just prior to an earthquake[4, 9, 10].

When a TL crystal is fractured, electrons are torn away from their parent atoms, resulting in a static discharge across the gap of the fracture[11, 12]. The light is emitted by several distinct, material-dependent mechanisms. The emission spectrum for sugar indicates that the light comes from the atmospheric nitrogen that fills the gap during fracture. This is the same source of light as that from lightning or touching a doorknob on a winter day. Spectra for other samples show emission characteristic of the material as well as nitrogen lines. Such spectra suggest a secondary energy process[11, 12]. Other substances exhibit a spectrum characteristic of the material alone. While abrupt charge separation is the same in all cases, emission mechanisms depend on the material[11, 12].

In 1999, Sage and Geddes used the property of TL to patent a design for a sensor capable of discerning the locations of impact[13–15]. Their design involved coating a material with a triboluminescent crystal or creating a composite triboluminescent object[13–15]. A sensor would then be embedded within the structure or mounted on its surface[14]. Impacts to the structure would produce light which would be recorded and analyzed to determine the location[14]. In addition, Sage et al. proposed that several different triboluminescent materials could be used and arranged at various locations[13–15]. The advantage is that when an impact takes place, its location could be determined by the wavelength emitted[14]. For example, by placing two different triboluminescent materials at known distances from the detector, it is possible to determine the approximate location of the impact by measuring the emitted wavelength.

However, the triboluminescent emission yield for most materials is small, and the resulting light can be difficult to detect. For the last few years, Fontenot and Hollerman et al. have been working with europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA)[16–18]. This material was found to have 206% of the triboluminescent emission yield compared to ZnS:Mn when subjected to low-energy impacts[17, 19]. Both EuD4TEA and ZnS:Mn emit strong TL when their crystals are crushed, scratched, or struck[20].

The first practical EuD4TEA material was synthesized by Hurt et al.[18]. In 1987, Sweeting et al. studied the crystal form for EuD4TEA[21] using the synthesis described by Hurt et al. Purification by recrystallization was accomplished by room temperature evaporation of methanol and dichloromethane[21]. After testing both materials for triboluminescence by grinding each of the crystals with a glass rod or steel spatula, it was discovered that the methanol-based material exhibited TL while the dichloromethane lacked TL[21]. In order to understand the structure of EuD4TEA, the final products were analyzed using X-ray diffraction. From these results, Sweeting et al. determined that the material made with methanol showed no evidence of solvent incorporation, was monoclinic, and had a centric symmetry belonging to the I2/a space group[21]. However, in 2001, Cotton et al. showed that EuD4TEA does not belong to the I2/a space group, but instead to the non-centric (and polar) space group Ia[22]. In 2011 and 2012, the synthesis process was modified to optimize the TL of EuD4TEA[16, 17, 23, 24]. In order to create a sensor as described by Sage and Geddes, another material that emits intense TL of a different color is required.

Lanthanide (Ln) compounds have distinctive optical properties, which include long luminescence lifetimes that range from microseconds to milliseconds and sharp emission bands with a full width half maximum that rarely exceeds 10 nm. These properties make lanthanides useful for organic light-emitting diodes, laser materials, and sensors[25]. In order to increase luminescence excitation, the Ln ion is usually coordinated to the ligands of β-diketone and aromatic amine derivatives. This typically increases the quantum efficiency through ‘synergistic effects’ and prevents the coordination of solvent molecules that quench the emissions[25].

Besides their outstanding photoluminescent properties, lanthanide β-diketone compounds containing chiral groups have additional properties that include second harmonic generation, ferroelectricity, TL, chirality sensing, circularly polarized luminescence, symmetric catalysis, and enantiomer selective synthesis[25]. In this paper, we investigate the luminescent properties of dibenzoylmethide triethylammonium compounds synthesized using a selection of Ln nitrates.

Methods

Synthesizing lanthanide-based dibenzoylmethide triethylammonium compounds

The synthesis of each lanthanide dibenzoylmethide triethylammonium compound was based on the procedures and methods used for EuD4TEA as shown in[17]. The synthesis began by dissolving 4 mmol of the lanthanide nitrate in a hot solution of 35 mL anhydrous denatured ethyl alcohol. Then, 13 mmol of 1,3-diphenyl-1,3-propanedione also known as dibenzoylmethane (DBM) and 14 mmol of triethylamine (TEA) was added to the hot solution. The solution was then kept aside to cool at ambient temperature. Each Ln compound that formed was filtered and air-dried at room temperature. After formation, the synthesized compounds were shown to be very stable with no degradation due to humidity. Table 1 shows the color of the resulting solution or product at each step of the synthesis process. Table 1 also shows the color of the photoluminescent emission when the synthesized crystallites were exposed to ultraviolet light. The Ho, Ce, and La compounds do not emit fluorescence when excited with ultraviolet irradiation.

Table 1 Color of resulting solution and products of the Ln dibenzoylmethide triethylammonium compounds
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All lanthanide elements form trivalent cations whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium. For this reason, the notation LnD4TEA can be used to represent a generic lanthanide dibenzoylmethide triethylammonium compound. For individual compounds, Ln will be replaced with the symbol used for the specific lanthanide element. For example, TbD4TEA is shorthand for the terbium-based dibenzoylmethide triethylammonium compound.

Notice from Table 1 that EuD4TEA remained clear when the europium nitrate is added, then turns dark yellow with the addition of DBM and TEA. This synthesis creates a light yellow crystalline structure that sparkles. However, this is not true for all Ln compounds. The holmium nitrate compound turned the ethanol solution pink. When DBM is added to this solution, it turns light orange. When TEA is added, this solution turns dark orange. Figure 1 shows that the holmium compound is flaky and exhibits a color that depends on the nature of ambient light. When viewed under indoor fluorescent light, the holmium compound flakes are light orange in color as shown in Figure 1b. However, when the fluorescent lights are turned off and the sample is exposed to standard daylight, the holmium compound crystallites are light yellow as observed in Figure 1a. The holmium compound crystallites emit no luminescence when excited.

Figure 1
figure 1

Pictures of the flaky holmium dibenzoylmethide triethylammonium compound when illuminated. (a) By daylight. (b) By fluorescent light.

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The cerium dibenzoylmethide triethylammonium compound also showed some interesting properties. The cerium nitrate and ethanol solution is completely clear. The addition of DMB turns this solution dark yellow; subsequently, when TEA is added to complete the reaction, the solution becomes blood red as shown in Figure 2a. Figure 2b shows that small black crystallites are produced during synthesis. Due to the limitations of the PTI spectrometer (Photon Technology International, Birmingham, NJ, USA) in measuring the 60-ns decay of cerium compounds, no observed luminescence was observed when excited. As a result, we are unable to say conclusively that CeD4TEA emits no photoluminescence when excited.

Figure 2
figure 2

Pictures taken during the cerium dibenzoylmethide triethylammonium compound synthesis process showing the resultant. (a) Blood red solution after TEA is added. (b) Black crystalline structure.

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Measuring triboluminescence

The relative triboluminescent emission yield from each LnD4TEA compound was measured using a custom-built drop tower as shown in Figure 3 and described in[19]. The visible emission spectrum for a candidate Ln compound can be made using a fiber optic spectrometer connected in place of the photodiode. The test begins by placing a small 0.1-g pile of each sample material on a plexiglass plate. The material is arranged so that it is positioned around the center of the tube with a minimum height. A 130-g steel ball is positioned on a pull pin at a distance of 1.06 m (42 in.) above the pile. The pin is pulled and the ball falls, producing TL at impact with the sample material. After each drop, the tube is removed, the ball is cleaned, and the sample powder is redistributed near the center of the target area[19, 23].

Figure 3
figure 3

Schematic diagrams of the specially designed drop tower used to measure integrated triboluminescent light yield[19].

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To determine the triboluminescent yield for a given sample, a United Detector photodiode (United Detector Technology, Hawthorne, CA, USA) is positioned under the plexiglass plate 2.25 cm below the sample. A Melles Griot large dynamic range linear amplifier (Melles Griot, Rochester, NY, USA) set to a gain of 200 μA increases the signal amplitude. A Tektronix 2024B oscilloscope (Tektronix, Beaverton, OR, USA) records the signal in single sequence mode with a 500-μs measurement time. Once the signal is acquired, it is analyzed using custom LabVIEW software that integrates the area under the curve and calculates the decay time for the particular emission[19, 23]. In addition, five drops were made on each Ln compound sample so an average TL emission yield could be calculated.

Measuring photoluminescent spectra

For most lanthanide compounds, their ligand absorbs energy, undergoes intersystem crossing into a triplet state, and then transfers its energy to the Ln3+ ion[26]. The excitation and emission phosphorescence from each compound was measured using a PTI QuantaMaster™ 14 spectrofluorometer located at Oak Ridge National Laboratory in Oak Ridge, TN, USA. The QuantaMaster™ was equipped with a Xe pulsed light source that allowed for a continuously tunable repetition rate of up to 300 Hz. Each spectra measurement was completed using a step size of 0.25 nm, five samples per average, five shots per wavelength, and a frequency of 100 Hz. In addition, the integration time and delay were varied depending on the decay time of each material. EuD4TEA for example had a delay of 40 μs with respect to the flashlamp firing and an integration time of 500 μs.

Results and discussion

Triboluminescent light yield results

Results of the TL drop tower measurements for the synthesized lanthanides are shown in Figure 4. Since this is a relative measurement, the resulting emission light yield for each compound was normalized to the previously measured EuD4TEA value. In other words, the relative light yield for EuD4TEA is equal to one. The error in emission yield was estimated to be 7% which is the total uncertainty for the experiment. It is from combination of the uncertainties from material synthesis and drop tower operation[27]. As shown in Figure 4, it is evident that no tested Ln compound produces as much TL as EuD4TEA. The next largest TL yield was emitted by the Sm compound, which is 1.8% of that measured for EuD4TEA. More research is needed to fully understand these results.

Figure 4
figure 4

Comparison of the average normalized triboluminescent light yield for the synthesized Ln compounds. All the collected results were normalized to the EuD4TEA light yield.

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Once the average light yields were determined, the triboluminescent decay times for each compound were determined as shown in Table 2. This was accomplished using the decay time software as described in[19]. It is evident that the triboluminescent decay time is unique for each Ln compound. EuD4TEA has the longest decay time, while the Yb compound has the shortest decay time. Interestingly, it appears as though the decay time and triboluminescent emission yield are not correlated. While EuD4TEA has the longest decay time and is the brightest material, the Tb compound has the second longest decay time and has one of the smallest TL emission yields. Likewise, the Sm compound has the second highest TL yield, but it has one of the fastest decay times.

Table 2 Decay time (1/ e ) results for the Ln dibenzoylmethide triethylammonium compounds
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Photoluminescent spectra

Europium dibenzoylmethide triethylammonium

The measured photoluminescent emission and absorption spectra for EuD4TEA are shown in Figure 5. The excitation spectrum was measured using the 612-nm (2.03-eV) emission peak, while the emission spectrum was measured using the 412-nm (3.01-eV) excitation wavelength. The spectrum indicates that the luminescence from the europium compound is due to its Eu3+ ion that produces the excited-state Eu3+-centered transitions from the 5D0 levels to the lower 7F0-4 levels[28, 29]. Due to overwhelming numbers, the wavelengths corresponding to each measured transition are shown in Table 3.

Figure 5
figure 5

Plot of the excitation and emission spectra for EuD 4 TEA with its associated electronic transitions.

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Table 3 The electronic transitions for EuD 4 TEA with the associated wavelengths and energies
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Notice that in Figure 5, the excitation energy is always from the ground 7FJ levels to the higher excited states. In order to save space and make the graph more readable, each energy transition was color coded. Thus, the 3.01-eV (312-nm) energy transition would be the blue 7F1→5F2. Also, notice that the majority of the excitation electronic transitions are from the 7FJ→5DJ and 5GJ levels. The most probable excitation transition is the 3.97-eV 7F4→5G3 transition, while the most favorable emission transition is the 2.03-eV 5D0→5F2 transition. This indicates that the europium compound can only be excited by ultraviolet to blue light, which yields a red luminescence. The energy level diagrams corresponding to the Figure 5 transitions that are listed in Table 3 are shown in Figure 6. The energy levels were determined from the measured values in Figure 5 and from the data shown in[28].

Figure 6
figure 6

Excitation and emission energy level diagrams for the EuD 4 TEA ions.

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Terbium dibenzoylmethide triethylammonium

The measured photoluminescent emission and absorption spectra for TbD4TEA are shown in Figure 7. The excitation spectrum was measured using the 540-nm (2.30-eV) emission peak, while the emission spectrum was measured using the 297-nm (4.17-eV) excitation wavelength. The spectrum indicates that the luminescence from the terbium compound is due to its Tb3+ ion that produces the excited-state Tb3+-centered transitions from the 5D4 levels to the lower 7F6-4 levels[29, 30]. Due to the large number of measured transitions, the corresponding wavelengths are shown in Table 4.

Figure 7
figure 7

Plot of the excitation and emission spectra for TbD 4 TEA with its associated electronic transitions.

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Table 4 The electronic transitions for TbD 4 TEA with the associated wavelengths and energies
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Notice in Figure 8 that the excitation energy is always from the ground 7FJ levels to the excited states. Similar to Figure 5, the transitions were color coded in order to make Figure 7 more readable. Notice that a majority of the excitation transitions are from the 7F6 ground level. The most probable excitation transition for Tb3+ is the 4.76-eV 7F6→5G6 transition, while the most favorable emission transition is the 2.30-eV 5D4→7F5. This indicates that TbD4TEA can only be excited by middle ultraviolet (MUV) irradiation and will yield a green luminescence. The energy level diagrams corresponding to the Figure 7 transitions that are listed in Table 4 are shown in Figure 8. The energy levels were determined from the measured values in Figure 7 and from the data shown in[29, 30].

Figure 8
figure 8

Excitation and emission energy level diagrams for the TbD 4 TEA ions.

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Samarium dibenzoylmethide triethylammonium

The measured photoluminescent spectrum for SmD4TEA is shown in Figure 9. The excitation spectrum was measured using the 650-nm (1.91-eV) emission peak, while the emission spectrum was measured using the 485-nm (2.56-eV) excitation wavelength. The spectrum indicates that the luminescence from samarium compounds is due to their Sm3+ ion that produces the excited-state Sm3+-centered transitions from the 4G5/2 levels to the lower 6H5/2–11/2 and 4I9/2 to 6H13/2 levels[29, 31]. Due to the large number of measured transitions, the corresponding wavelengths are shown in Table 5.

Figure 9
figure 9

Plot of the excitation and emission spectra for SmD 4 TEA with its associated electronic transitions.

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Table 5 The electronic transitions for SmD 4 TEA with the associated wavelengths and energies
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Notice in Figure 9 that the excitation energy is always from the ground 6HJ levels to the excited states. Notice that a majority of the excitation transitions are from the 6H5/2 and 6H7/2 ground level. The most probable excitation transition for Sm3+ is the 2.77-eV 6H5/2→4G9/2 and 2.55-eV 6H5/2→4I9/2 transitions, while the most favorable emission transition is 1.91-eV 4I9/2→6H13/2. This indicates that the Sm compound has a broad excitation energy ranging from UV to short wavelength green light. The energy level diagrams corresponding to the Figure 9 transitions that are listed in Table 5 are shown in Figure 10. The energy levels were determined from the measured values in Figure 9 and from the data shown in[29, 31].

Figure 10
figure 10

Excitation and emission energy level diagrams for the SmD 4 TEA ions.

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Neodymium dibenzoylmethide triethylammonium

The measured photoluminescent spectrum for NdD4TEA is shown in Figure 11. The excitation spectrum was measured using the 469-nm (2.64-eV) emission peak, while the emission spectrum was measured using the 363-nm (3.42-eV) excitation wavelength. The spectrum indicates that the luminescence from the neodymium compound is due to its Nd3+ ion that produces the excited-state Nd3+-centered transitions from the 4GJ levels to the lower 4IJ and 4H11/2 to 4I9/2 levels[29, 31]. Due to the large number of measured transitions, the corresponding wavelengths are shown in Table 6.

Figure 11
figure 11

Plot of the excitation and emission spectra for NdD 4 TEA with its associated electronic transitions.

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Table 6 The electronic transitions for NdD 4 TEA with the associated wavelengths and energies
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Notice in Figure 11 that the majority of the excitation energy is from the ground 4IJ levels to the excited states. However, unlike the Eu3+ and Sm3+ excitation transitions, Nd3+ has transitions from two different excited states. These transitions include 4F3/2 to 2G7/2 and 2H9/2 to 2G9/2. The most probable excitation transition for Nd3+ is the 4.77-eV 4I9/2→2F5/2 transition, while the most favorable emission transition is 1.91-eV 4G11/2→4I9/2. This indicates that the Nd compound has broad excitation energy in the MUV area with sharp bands in the 350- to 450-nm range. In addition, when the neodymium compound is excited, it produces a blue light. The energy level diagrams corresponding to the Figure 11 transitions that are listed in Table 6 are shown in Figure 12. The energy levels were determined from the measured values in Figure 11 and from the data shown in[29, 31].

Figure 12
figure 12

Excitation and emission energy level diagrams for the NdD 4 TEA ions.

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Dysprosium dibenzoylmethide triethylammonium

The measured photoluminescent spectrum for DyD4TEA is shown in Figure 13. The excitation spectrum was measured using the 519-nm (2.39-eV) emission peak, while the emission spectrum was measured using the 285-nm (3.35-eV) excitation wavelength. The spectrum indicates that the luminescence from the dysprosium compound is due to its Dy3+ ion that produces the excited-state Dy3+-centered transitions from the 4F7/2,9/2 to 6H11/2,13/2 levels[29, 31]. Due to the large number of measured transitions, the corresponding wavelengths are shown in Table 7.

Figure 13
figure 13

Plot of the excitation and emission spectra for DyD 4 TEA with its associated electronic transitions.

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Table 7 The electronic transitions for DyD 4 TEA with the associated wavelengths and energies
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Notice in Figure 13 that the majority of excitation energy is from the ground 4H15/2 levels to the excited states. Interestingly, it seems that the 4F3/2 splits into two different levels as shown in[31]. This splitting causes two different excitation energies as shown in Table 7. The most probable excitation transition for Dy3+ is the 4.76-eV 6H15/2→4F3/2 transition, while the most favorable emission transition is 2.39-eV 4F7/2→6H11/2. Figure 13 indicates that the Dy compound has broad excitation energy in the MUV region with sharp bands in the 390- to 430-nm range. Using these excitation energies will cause the dysprosium compound to produce a green light. The energy level diagrams corresponding to the Figure 13 transitions that are listed in Table 7 are shown in Figure 14. The energy levels were determined from the measured values in Figure 13 and from the data shown in[29, 31].

Figure 14
figure 14

Excitation and emission energy level diagrams for the DyD 4 TEA ions.

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Conclusions

Previous research has shown that EuD4TEA can easily be synthesized with 206% of the triboluminescent emission yield compared to ZnS:Mn. The research completed here shows that lanthanide-based dibenzoylmethide triethylammonium compounds other than EuD4TEA can also be successfully synthesized and characterized. However, none of the other synthesized lanthanide-based compounds has an emission yield near the values achieved by EuD4TEA. In fact, SmD4TEA had the next largest triboluminescent emission yield over all the tested lanthanide compounds. However, the SmD4TEA emission yield was only 1.8% of what was measured for EuD4TEA.

Authors’ information

RSF received his applied physics Ph.D. in May 2013 from Alabama A&M University. WAH holds the position of Dr. and Mrs. Sammie W. Cosper/BORSF endowed associate professor of physics at the University of Louisiana at Lafayette. KNB holds the position of associate professor of chemistry at Alabama A&M University. SWA holds the position of chief scientist at Emerging Measurements Co. (EMCO). MDA holds the position of professor of physics and chairman of the Department of Physics, Chemistry, and Mathematics at Alabama A&M University.