Photoluminescence of Natural Diamonds

Journal of the Korean Physical Society, Vol. 48, No. 6, June 2006, pp. 1556∼1559

Photoluminescence of Natural Diamonds Hyunjin Lim, Sooyoun Park and Hyeonsik Cheong∗ Department of Physics, Sogang University, Seoul 121-742

Hyun-Min Choi and Young Chool Kim Hanmi Gemological Institute Laboratory, Seoul 110-390 (Received 21 November 2005) The optical properties of type-Ia and type-IIa natural diamonds were studied using IR absorption spectroscopy and photoluminescence spectroscopy. Type-Ia and type-IIa diamonds have very different IR spectra, but IR spectra of all type-IIa diamonds are similar. However, significant differences were found in the photoluminescence spectra of type-IIa diamonds, which demonstrates that photoluminescence spectroscopy can be used as an accurate grading method of diamonds. PACS numbers: 81.70.Fy, 81.05.Uw, 78.55.Ap Keywords: Photoluminescence spectroscopy, IR absorption spectroscopy, Natural diamond

I. INTRODUCTION Diamonds formed in high-temperature and highpressure conditions in nature or in artificial conditions are commercially and industrially useful due to their extreme physical properties, such as hardness, large energy gap, large thermal conductivity, chemical and radiation hardness, and high carrier saturation velocity. These properties may be utilized in developing semiconductor devices operating in harsh environments. [1–7] In addition to these industrial applications, diamond continues to be the most important jewel stone, and accurately determining the value of a given diamond stone remains an important and difficult task. An ideal diamond is made up of carbon atoms in the diamond structure and is transparent to light in the visible to near-uv range of the electromagnetic wave spectrum due to its large band-gap energy of 5.49 eV. However, most natural and synthetic diamonds have impurities, such as nitrogen and boron, as well as inclusion and dislocations, which result in various colors and clarity levels [8,9]. The color scale of a diamond is an alphanumeric scale starting with color D for the most colorless one. The clarity is an indication of the diamond’s purity, and internal clarity grades range from internally flawless (IF) to fairly included (I3 ). In this study, we used SI (slightly imperfect) and VVS (very very slightly imperfect) grades. Since the impurity content and distribution are crucial criteria for determining the value of a diamond, it is very important to measure the amount and the distribution of impurities accurately. IR absorption spectroscopy is generally used in determining the dia∗ Corresponding

Author: [email protected]

mond type according to the nitrogen content [10,11], but this method can detect nitrogen only for concentrations higher than 10 ppm. In addition, recently, it has become possible to improve colored diamonds to high-quality D ∼ G grade diamonds by using high-temperature and high-pressure (HPHT) treatment [12–15]. Usual IR absorption measurements cannot discern such treated diamonds from untreated high-quality ones. This has made it extremely difficult to correctly determine the value and the cost of a given diamond stone based on traditional technologies alone. Here, we demonstrate that photoluminescence spectroscopy can be an easier and a more accurate method of grading diamonds. Photoluminescence spectroscopy is usually used to measure the band-gap energy of semiconductors. The band-gap energy of diamond is 5.49 eV [9,16], but there are impurity levels in the band-gap, which are more important in determining the color of a diamond stone. Therefore, we focus on the photoluminescence spectrum of impurity levels in this study. The Raman signal at 1332 cm−1 [17,18] was used as a common reference. The photoluminescence of diamond has been studied by other groups [19,20], but a systematic study of a large number of samples has not been reported.

II. EXPERIMENTS We measured 30 round brilliant-cut diamond stones and chose one type-Ia and three type-IIa diamonds, which are representative of all diamonds measured (Table 1). Other diamonds have photoluminescence spectra similar to that of one of these four stones. The photoluminescence spectra were measured with the samples

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Photoluminescence of Natural Diamonds – Hyunjin Lim et al.

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Table 1. Basic characteristics of the diamond samples used in this study as graded by the Hanmi Gemological Institute Laboratory. Sample A B C D

Type IIa Ia

Weight

Color

Clarity

1.02 ct 1.26 ct 0.117 ct 0.180 ct

E E M Fancy yellow

SI1 SI2 VVS1 SI2

Fig. 2. Photoluminescence spectra of sample A, B, C (type-IIa), and D (type-Ia) recorded at 8 K under 488-nm excitation.

Fig. 1. IR absorption spectra of natural diamonds. The square region is related to the nitrogen content.

placed in a closed-cycle He cryostat at 8 K. The 488-nm and 514.5-nm lines of an Ar ion laser, operating at 10 to 20 mW, focused to a spot of about 50 µm in diameter, were used as the excitation source. Since absorption of laser light in diamond is minimal, the laser irradiation does not heat the sample. The luminescence signal was filtered with a holographic edge filter, dispersed by a Spex 0.55-m spectrometer, and detected with a liquid-nitrogen-cooled charge-coupled-device (CCD) detector array. We devised a holder especially for round diamonds to hold samples in good thermal contact with the cold finger of the cryostat system without interfering with the luminescence measurements. Mid- and near- infrared absorption spectra were measured in the range of 6000 cm−1 to 450 cm−1 at room temperature by using a Perkin-Elmer Spectrum One Fourier transform infrared spectrometer with a resolution of 4.00 cm−1 .

III. RESULTS AND DISCUSSION Figure 1 shows the IR absorption spectra of the samples. The signal in the range from 1000 cm−1 to 1400 cm−1 , marked by a rectangle, is known to be caused by the nitrogen impurities, and the other features are re-

Fig. 3. Photoluminescence spectra of sample A, B, C (type-IIa), and D (type-Ia) recorded at 8 K under 514.5-nm excitation.

lated to carbon or other impurities [16,21]. In the range from 1000 cm−1 to 1400 cm−1 , the spectra of the typeIIa diamonds are very similar to each other and show little absorption, but that of the type-Ia diamond has sharp peaks due to absorption by nitrogen impurities. Generally, the type of diamond can be determined using IR absorption spectroscopy because it detects nitrogen contents over 10 ppm and the nitrogen contents of typeIa diamonds range up to 5000 ppm. However, since the nitrogen contents in type-IIa diamonds are less than 5 ppm [16], it is impossible to discriminate different typeIIa diamonds by using IR absorption spectroscopy alone. Figure 2 and 3 show the photoluminescence spectra

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Journal of the Korean Physical Society, Vol. 48, No. 6, June 2006

ure 1, the color is not caused by nitrogen impurities [10], but comes from structural factors, such as dislocations [19]. Its luminescence spectrum for wavelengths shorter than 575 nm is similar to that of the type-Ia sample A, especially for the 488-nm excitation. However, there are no appreciable peaks for wavelengths longer than 575 nm. Figure 4 shows a more detailed comparison of the spectra for samples A, B, and D in the range from 493 nm to 515 nm. Differences among the samples are clearly seen. All three samples have common luminescence lines at 496.3 nm and 503.8 nm, which can be attributed to oxygen-vacancy pairs and V-C-C-V complexes, respectively [16, 23, 24]. In addition, there are distinct lines at 505.3 nm and 512.2 nm, which have not yet been reported. More studies are needed to identify these newly observed peaks. Fig. 4. Detailed comparison of the photoluminescence spectra of samples A, B, and D taken with 488-nm excitation

of the samples measured at 8 K with 488-nm (Figure 2) and 514.5-nm (Figure 3) excitation. The sharp lines at 522 nm (Figure 2) and 552 nm (Figure 3) are due to Raman scattering (1332 cm−1 ) [16,18], and the very sharp lines at 575 nm and 637 nm are due to emission from the neutral nitrogen-vacancy complex (N-V)0 and negatively charged nitrogen-vacancy complex (N-V)− , respectively [8,12]. Nitrogen impurities can be located next to vacancies creating nitrogen-vacancy complexes. Unlike the IR absorption spectra, the photoluminescence spectra show clear differences although sample A-C are all type-IIa diamonds. The photoluminescence spectra of all the diamond stones can be classified into one of these types. Sample D, a type-Ia diamond, has a strong and rich luminescence spectrum for both excitation wavelengths due to impurities, such as nitrogen, which gives color to type-Ia diamonds. Sample A, on the other hand, show very little luminescence, except for some peaks due to nitrogen-vacancy centers, sharp lines at 610 nm and 741 nm. Sample B is very interesting because it shows unusually strong red luminescence. Only a few other stones showed similar properties. The red emission from the diamond could be observed with the naked eye when the laser beam was incident on this sample. Diamonds of this kind can be exploited in optoelectronic devices using defect-level engineering [22]. More studies are needed to identify the luminescence centers and their physical properties. One should note here that samples A and B have basically identical grades based on traditional methods (Table 1), but show dramatically different luminescences. Sample C is a type-IIa natural diamond with brown color. Since this sample does not contain enough nitrogen, as can be seen in the IR absorption spectrum in Fig-

IV. SUMMARY IR absorption spectra and photoluminescence spectra of type-Ia and type-IIa diamonds were measured in an effort to develop a more reliable method of grading diamonds. IR absorption spectra of diamonds can be used to classify diamonds into different types based on nitrogen content, but photoluminescence spectra show distinct differences even among diamonds of the types and grades. This demonstrates that photoluminescence spectroscopy can be a powerful method for grading diamonds.

ACKNOWLEDGMENTS This work was supported by a Korea Research Foundation grant (KRF-2004-005-C00002) and by the Hanmi Gemological Institute Laboratory.

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