Materials Letters 146 (2015) 9–11
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Magnesium tetrakis dibenzoylmethide triethylammonium: A novel blue emitting phosphor Ross S. Fontenot a,b,n, Constance A. Owens a,1, Kamala N. Bhat a, William A. Hollerman b, Mohan D. Aggarwal a a b
Alabama A&M University, Department of Physics, Chemistry, and Mathematics, PO Box 1268, Normal, AL 35762, USA University of Louisiana at Lafayette, Department of Physics, PO Box 44210, Lafayette, LA 70504, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 7 November 2014 Accepted 28 January 2015 Available online 7 February 2015
Triboluminescence (TL) is the emission of cold light that is created when materials are fractured. Europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA) is one of the brightest triboluminescent materials that exist. However, due to the rising cost of europium, it is becoming very expensive to synthesize. As such, we have begun investigating other elements that can possibly replace europium as the luminescent center. After ruling out other lanthanides, we turned our attention to the alkaline earth metals. This paper will explore the luminescent properties of a new novel blue emitting phosphor: magnesium tetrakis dibenzoylmethide triethylammonium. & 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Organic Luminescence Optical materials and properties Crystal growth
1. Introduction Most phosphors (material that emits luminescence) are composed of a crystalline host or matrix and an activator, i.e., a small amount of intentionally added impurity atoms distributed in the host crystal [1]. It is these activators that are typically responsible for the luminescence. As such, they are called a luminescent center. Nearly four centuries after Vincentinus Casiarolo discovered the first phosphor, we have expanded our commercial use of phosphors to light sources for fluorescent lamps, cathode ray tubes for displays, X-ray screens and scintillators for detector systems, as well as luminous paints that exhibit long persistent phosphorescence [2]. However, it is the new applications such as sensor development and phosphor thermometry that is currently pushing phosphor research. One such example is in the area of triboluminescence (TL). TL is the emission of light caused by the fracture of crystals [3,4]. Currently about 50% of known crystals emit TL [3,4]. Recently, it has been suggested that triboluminescent materials be placed in composites to detect structural damage [3–7]. If these sensors are to be reality, however, they must be inexpensive and easy to integrate. One current challenge with triboluminescent sensors is that most triboluminescent materials are not very
n
Corresponding author. E-mail address:
[email protected] (R.S. Fontenot). Current address: Houston Baptist University, Department of Mathematics and Physics, 7502 Fondern Road, Houston TX, 77074. 1
http://dx.doi.org/10.1016/j.matlet.2015.01.141 0167-577X/& 2015 Elsevier B.V. All rights reserved.
bright. In 1966, Hurt et al. synthesized a material, i.e., europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA), that is bright enough to be easily seen in daylight [8]. This material with modified synthesis discovered by the authors in 2011 has 206% triboluminescent yield than the more commonly known inorganic manganese doped zinc sulfide (ZnS:Mn) [9–14]. While this material is sufficiently bright for sensors, the authors have been pushing the limits by investigating the effects of additives on the TL of EuD4TEA. These studies have determined that the best additive for EuD4TEA is dibutyl phosphate, which has a triboluminescent yield of 715% [15]. Unfortunately, due to the rising cost of europium and the need for other luminescent colors for sensors or lighting applications, we decided to investigate other non-lanthanide luminescent centers. This paper will report on the luminescent properties of (MgD4TEA) phosphor.
2. Material and methods 2.1. Synthesis of materials The synthesis of MgD4TEA was based on Ref. [11]. The process began by pouring 50 mL of 95% denatured ethanol into an Erlenmeyer flask. The ethanol was heated to boiling by setting the hot plate to 250 1C, and the stirring set to maximum. Then, 4 mmol of magnesium nitrate hexahydrate added to the hot solution. Once the magnesium salt dissolved, 13 mmol of 1,3-diphenyl-1,3-propanedione also known as dibenzoylmethane (DBM) was added to the
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R.S. Fontenot et al. / Materials Letters 146 (2015) 9–11
Fig. 1. Standard light pictures of MgD4TEA. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
hot solution. A funnel was then placed on top of the flask, and the solution was left for 20 min. After 20 min, the stirrer was removed and 14 mmol of triethylamine (TEA) was added. The solution was then kept aside to cool at ambient temperature. The MgD4TEA crystals that formed were light yellow in color and clumpy with no sparkles as shown in Fig. 1. 2.2. Triboluminescent testing Once the crystals were completely dried, they were placed inside a small clear round wide-mouth jar for storage. Using a custom built drop tower designed and fabricated by the authors and described in Ref. [10], the crystalline products were tested for their triboluminescent properties. The measurement began by placing a 0.10 g of sample powder on the Plexiglass plate. The powder 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 set distance of 42 in. above the material. The pin is pulled, and the ball falls and impacts with the sample material producing TL. After each test, the drop tube is removed, the ball is cleaned, and the sample powder is redistributed near the center of the target area [10]. To determine the triboluminescent emission yield for a given sample, a United Detector photodiode is positioned under the Plexiglass plate 2.25 cm below the sample. A MellesGriot large dynamic range linear amplifier set to a gain of 200 mA increases the signal amplitude. A Tektronix 2024B oscilloscope records the signal in single sequence mode with a 500 ms measurement time. Once the signal is acquired, it is analyzed using custom LabVIEW program that integrates the area under the curve and calculates the decay time for each emission [10].
Fig. 2. Photoluminescent spectrum of MgD4TEA with the inset showing the blue emission of MgD4TEA and red emission from EuD4TEA. Notice that they both emit a bright light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results Fig. 2 shows the resulting MgD4TEA photoluminescent emission spectrum. The resulting broadband peak is centered at 492 nm and has a FWHM of about 67 nm. Notice from the inset that MgD4TEA exhibits a bright blue emission that is comparable in intensity to that emitted from EuD4TEA, i.e., one of the brightest known triboluminescent materials. Unfortunately, MgD4TEA did not exhibit any measurable or observable TL. In addition, TL and photoluminescence were also not observed for calcium tetrakis dibenzoylmethide triethylammonium. Additional research is needed to determine the physical processes behind these results.
4. Conclusions It is apparent that MgD4TEA would be a suitable blue organic phosphor (492 nm) for photoluminescent applications owing to its high brightness. However, it was not found to be triboluminescent. More research is underway to determine if more colors can be synthesized by changing the luminescent center.
Acknowledgments This research was funded in part by NASA Alabama Space Grant Consortium fellowship under Training Grant NNX10AJ80H, NSFRISE Project HRD 0927644, and other grants from the State of Louisiana and Federal Agencies. One of the authors (MDA) wishes to acknowledge the support of NSF-MSP Project 1238192. References
2.3. Photoluminescent testing The photoluminescence of each material was excited using an UV transilluminator manufactured by UVP, Inc. The photoluminescence was recorded using an Ocean Optics USB4000-FL spectrometer, which has a wavelength range of 360–1000 nm and a resolution of 0.22 nm. The integration time was controlled using the SpectraSuites program. The integration time was increased by the software until the photoluminescent emission spectrum reached its maximum that was just below the saturation limit of the spectrometer. Once this was determined, one hundred (100) spectra were recorded and averaged.
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