raman spectroscopy and photoluminescence mapping of … spectroscopy... · 2019-05-11 · raman...

“WE DREAM, WE DEVELOP, WE DELIVER.”

www.master-dynamic.com 852 - 3543 5898

Unit 103, 1/F, 10 Science Park West Avenue, Hong Kong Science Park, Shatin, N.T., HONG KONG

MD Technical Review Letter -

Raman Spectroscopy and Photoluminescence

Mapping

of Diamonds with Multiple Fluorescence Zones

Charis W.Y. LEE, J. CHENG, K. W. CHENG and Tony K. C. HUI

Master Dynamic Ltd.

Dr. Tim BATTEN

Renishaw PLC ·

Deep UV (DUV) excitation and Raman spectroscopy are the principle tools that help

gemologists identify the origin of diamonds and their authenticity. When illuminated with a DUV

source diamonds fluoresce in a range of colors, which are attributed to a specific optical defect

in the diamond allowing a real-time image to be displayed. To identify and observe the

distributions of these optical centers, it is necessary to not only pin-point the DUV source at a

spot on the sample, but to obtain a full screening over the surface of interest.

By applying correlated Raman spectroscopy and photoluminescence (PL) mapping to the same

region of the sample it is possible to directly characterize and precisely identify the optical

centers responsible for this fluorescence, allowing material properties of the diamond such as

unintentional doping to be determined.

2 Unit 103, 1/F, 10 Science Park West Avenue, Hong Kong Science Park, Shatin, N.T., H.K. 852 - 3543 5898

www.master-dynamic.com

Introduction

The techniques for lab-grown diamonds continues to advance and it is important to study how

they are grown, their characteristics and traits and how different treatment could change them

both internally and externally. From this information, the classification and identification of these

synthetic diamonds can be made easier. In this study, we focus on two techniques DUV

illumination and Raman/photoluminescence spectroscopy and demonstrate how they can be

used in tandem to characterize synthetic Diamonds.

Raman spectrometers and DUV illumination systems are fundamental instruments in the

identification process. Both provide photoluminescence (PL) information displayed as a

spectrum or as an image.

Raman and PL spectroscopy is a non-contact and non-destructive analytical method that can

detect defects at lower concentrations than other absorption spectroscopy [5]. A Raman

spectrometer can be equipped with different laser wavelengths to cover a wide range of the

spectrum and detect different optical defects with high sensitivity. On the other hand, a DUV

system exposes the samples to a short-UV radiation (~230 nm) and provides phosphorescence,

as well as, luminescence that results from the optical defects in the diamond.

The localized study of these techniques on the diamond (together with other analytical

instruments) is enough to identify origin, and a relatively standard measurement. However it is

also important to visualize the whole diamond in order to characterise more comprehensively

how these synthetic diamonds are grown And this can be achieved by carrying out correlated

PL mapping as demonstrated here.



Materials and Methods

The sample of interest is a 0.32ct Oval cut near-colorless CVD diamond.

Fig. 1. Sample examined is a 0.32ct Oval Cut CVD Diamond

The infrared absorption spectrum collected using FTIR shows the diamond to be Type IIa, as

there were no peaks present in the 0-1500cm-1 range (Fig. 2) which would be indicative of any

Nitrogen A- or B-aggregates exist.

Fig.2: FTIR spectra shows the CVD diamond examined to be Type IIa

Photoluminescence Spectra

PL measurement was conducted with Renishaw inVia Raman microscope and collected at

room and liquid nitrogen (LN) temperature prior mapping. 320 nm, 514 nm and 633 nm laser



excitation wavelengths were used to ensure the entire PL range of interest was covered A 20×

with 0.40 numerical aperture objective lens was used. In this case a Raman spectrometer was

used to collect PL data as the PL spectrum is weak and such spectrometers are significantly

more sensitive than standard PL instrumentation, in addition because the Raman spectrometer

chosen has a dispersive element it is able to provide excellent spectral resolution.

Fig. 3: PL spectrum under 320nm laser excitation at Room temperature (PL band centers are

marked).



Fig. 4: PL spectrum under 514nm laser excitation at LN temperature (Top) with zoom and

curve fit of 596/597nm doublet (Middle) and 736/737 doublet (Bottom)

Fig. 5: PL spectrum under 633nm laser excitation at LN temperature

Under 514 nm laser excitation at liquid-nitrogen (Fig.4), the PL spectra revealed nitrogen-

vacancy centers at 575 nm [NV0] and 638 nm [NV-], 596/597 nm doublet and Silicon-Vacancy

[SiV-]doublet at 736.7 and 737.1 nm.



It can be clearly observed that between both Nitrogen Vacancy centers, the intensity of [NV0]

at 575 nm is much higher than NV- at 638 nm.

[SiV-] doublet can be also be observed under 514 nm. This doublet is commonly introduced

during the growth of CVD diamonds and HPHT synthetic diamonds but rarely present in natural

diamonds [5][8].

The 596/597 nm doublet is a significant and reliable peak that can help in the identification of

as-grown CVD diamonds. This doublet is often removed and not observable if the diamond has

undergone post HPHT treatment. [7].

The presence of both doublets, although at a lower intensity compared to the NV centers, and

the absence of H3 at 503 nm and N3 at 415 nm center (frequently observed in HPHT treated

diamonds), suggests that the diamond is CVD synthesized. [1].

In the PL spectra obtained under 320 nm laser excitation (Fig. 3), the presence of 398 nm and

533 nm also suggests that the diamond is likely to be as-grown CVD diamond as these two

peaks can be removed if the diamond were subjected to HPHT high temperature annealing

(>1800oC) [3][8].

Photoluminescence Mapping

PL mapping of the aforementioned optical defects was conducted using the same 20× objective

lens under 514 nm and 633 nm laser excitation over the table of the diamond at room

temperature. The measurement parameters and conditions are summarized in (Table 1). The

observed PL maps are peaks area intensity profiles generated with the Renishaw spectroscopy

software (WiRE).

Laser Wavelength 514nm 633nm

Grating (l/mm) 2400 1200

Peaks measured (nm) 575, 638, [596/597] 738

Exposure Time (s) 0.1, 0.1, [5] 0.1

Pixel size (um) 39, 39, [73] 39

Total points 6732, 6732, [1980] 6732

Measuring Time (min) 12, 12, [33] 12



Table 1. PL Mapping measurement parameters and conditions under different laser

excitations. Data in brackets “[.]”, belongs to the 596/597nm doublets.

The 596/597nm doublets is less intense at room temperature than at liquid nitrogen

temperature, so it was necessary to measure for longer to achieve good signal to noise, in this

case 5 seconds.. A comparisons of the different signal to noise of the 596 nm peak, achieve

with different integrations times is shown in Fig 6. The spatial resolution of the 596 nm map is

slightly lower compared to the other maps due to the use of bigger pixel size to maintain a

comparable total measurement time.

[Fig. 6: PL spectra of 596 nm under 514 nm laser with Exposure time = 1s (Top) and 5s

(Bottom)]

DUV Imaging



The sample is examined under a DUV illumination system using 230 nm short-wave UV

radiation. Although there were no clear growth patterns, the sample exhibited an overall strong

reddish-orange fluorescence with dislocation areas of violet-blue fluorescence and a strong

strand of greenish blue fluorescence (Fig. 7). In addition, green-blue phosphorescence can be

observed at the same location (Fig. 8).

Fig. 7. Overall reddish-orange fluorescence with violet-blue dislocation bundles under D UV

illumination.



Fig. 8: A weak green-blue phosphorescence (Right Top & Bottom images) can be observed

under DUV at the same location as the strand of bright blue fluorescence (Left Top & Bottom

images).

The reddish-orange fluorescence is mainly attributed to the presence of [N-V] centers with

emission at 575 nm [NV0] and 638 nm [NV-]. The irregular violet-blue fluorescence is a typical

feature of as-grown CVD diamond [1][9]. Around the violet-blue dislocation bundle, some

pinkish fluorescence can be observed. This feature is associated to the 596/597 nm doublet

[7]., present in as-grown CVD diamonds.

The pronounced greenish-blue color observed as a strand in Fig. 8, is often related to H3

centers which are commonly present in CVD diamonds that has underwent HPHT treatment.

[1][9]. However, based on the PL spectra, H3 center characteristic peak at 503 nm is not

observed. On the other hand, the 596/597 nm doublet, an indicator that the diamond is as-

grown, is present.

In addition, this greenish-blue phosphorescence can also be found in boron-doped diamonds.

[8]. Yet, there is no sign of boron absorption peak in FTIR spectra (Refer to Fig.2).

This combination is rarely seen in as-grown CVD diamonds as during HPHT treatment, 389

nm, 533 nm and 596/597 nm doublet should have been removed [3][8].

So far, it can only suggest that there is a possibility that the diamond has undergo HPHT

annealing at a temperature lower than 1800oC. [1]

PL mapping and Results

Although the growth features cannot be observed clearly, the diamond display typical CVD

features: [N-V] centers at 575 nm and 638 nm, [Si-V] center at 736 nm and the 596/597 nm

doublets. H3 and N3 centers were not observed.

The whole table image is a result of the collection of numerous snaps along the diamond. This

montage (Fig.9) is done to facilitate the location of the mapping as the whole table cannot be

fully visualized using 20x objective lens in the microscope. Fig. 10, 11 and 12 are PL maps



corresponding to a different optical defect and its area intensity under different laser excitation

over the table of the diamond.

Fig. 9: Montage of numerous snaps along the table of the diamond

As observed in (Fig. 10), the NV centers are highly concentrated in one of the extremes of the

diamond and cut off by where the dislocation bundles are located. Under DUV illumination, one

of the extremes appear to be of a darker reddish/orange color than the rest of the table of the

diamond. After the bundles, the NV centers are also present in the rest of the diamond, but at

lower intensity. This shows that the NV centers are not evenly distributed along surface.

Low Intensity High Intensity



Fig. 10: PL Map of the NV0 located at 575 nm

Instead, the 596/597nm doublets often found near dislocation bundles, fills in the area where

the NV centers luminescence intensity is lower or absent, giving it a lighter purple-pinkish color

around where the dislocations are located (Fig. 11).


Fig. 11: PL Map of 596 nm under 514 nm laser

The violet-blue dislocation bundles were formed due to the stop/start cycles during its growth

process. [6]. This temporary change in chemical compositions and surface roughness could

lead to an increase incorporation of impurities. [4][10]. As shown in the mapping of 738 nm

(Fig. 12), it can be clearly observed that the [SiV-] are highly concentrated where the start of

the dislocation bundle is, visible under Deep UV illumination (Fig. 7). However, the decrease of

Si luminescence after the bundle could be due to the decrease in the silicon concentration

inside the growth chamber over time [2].




Fig. 12: PL Map of 738 nm under 633 nm laser

Discussion

Despite the fact that it is still unclear whether the CVD sample studied has been subjected to

post-growth HPHT annealing, it is certain that both Raman and Deep UV systems are

indispensable tools in the identification of products of continuously advancing synthesis

techniques (lab-grown diamond).

Photoluminescence mapping was successful in revealing the distribution of the different optical

defects found in a single spectrum along the diamond. It also allowed a better understanding

of the fluorescence colors displayed under DUV illumination. Both are reliable instruments that

help in the study and classification of the nature of the diamond. Additionally, with the

considerably fast screening of the defects at room temperature, PL mapping serves as a useful

method for the study of optical defects and its impurities uptake during the growth of both

synthetic and natural diamonds.



About the Authors

Miss Charis W.Y. LEE is Biology Engineer, Dr. J. CHENG is Principal Engineer (expertise in

Optical & Raman Spectroscopy), Mr. K.W. CHENG is Technical Manager and Mr. Tony K.C.

HUI is Senior Director of the NanoTechnology Development & Applications Centre (NTAC), at

Master Dynamic Limited in Hong Kong.

Dr. Tim BATTEN specializes in the study of carbon materials, semiconductors and

nanomaterials using optical spectroscopy and is a Senior Raman Spectroscopy Application

Scientist at Renishaw plc, UK.

References

[1] D’Alene’s-Johansson, U. et al. (2013) “Three CVD Synthetic Diamonds Submitted to Mumbai Laboratory”.

Gems & Gemology, Spring 2013, Vol. 49, No. 1

[2] Dieck C. et al (2015) “Two Large CVD-Grown Synthetic Diamonds Tested by GIA”. Gems & Gemology, Winter

2015, Vol. 51, No. 4

[3] Dobrinets I.A. et al (2013) “HPHT-Treated Diamonds”. Springer Series in Materials Science 181

[4] Eaton-Magaña, S, et al. (2014). “CVD Synthetic Diamond with Unusual DiamondView Image.” Gems &

Gemology, Spring 2014, Vol. 50, No. 1

[5] Eaton-Magaña, S, et al. (2016). “Observations on CVD-grown synthetic diamonds: A review”. Gems &

Gemology, Vol. 52, No. 3, pp. 222–245

[6] Law P.L, et al. (2016) “CVD Synthetic Diamond Over 5 Carats Identified by GIA”. Gems & Gemology, Winter

2016, Vol. 52, No. 4

[7] Martineau P.M, et al. (2004) “Identification of synthetic diamond grown

using chemical vapor deposition (CVD)”. Gem & Gemology, Vol. 40, No.1, pp. 2–25

[8] Nad S, et al. (2015) “Growth strategies for large and high quality single crystal diamond substrates”. Diamond

and Related Materials, Vol. 60, pp. 26–34,

[9] Wang W., et al. (2012) “CVD synthetic diamonds from Gemesis Corp”. Gem &Gemology, Vol. 48, No. 2, pp.

80–97

[10] Willems et al. (2011) “Exploring the origin and nature of luminescent regions in CVD synthetic diamond,” Fall

2011 G&G, pp. 202–207

raman spectroscopy and photoluminescence mapping of … spectroscopy... · 2019-05-11 · raman...

Documents