Abstract:
The method of rapid identification of natural and synthetic diamonds using a third-order Raman spectra is to make a diamond third order under large-scale comparative studies with synthetic diamond (CVD &amp; HPHT) to distinguish natural and synthetic diamonds with their differences in Raman peaks. This analysis of the differences in characteristic peak phenomenon can form the basis of a rapid identification and analytical technique.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for identifying natural and synthetic diamonds, and particularly to a method of rapid identification of natural and synthetic diamonds using a third-order Raman spectra. 
     2. Related Art 
     Raman spectroscopy is used as a diagnostic tool for diamonds because each carbon allotrope displays a clearly identifiable Raman signature, it is non-destructive, requires little or no specimen preparation. Raman scattering from diamond has been disclosed by prior literatures, which describe the fundamental properties of the Raman spectra of natural and synthetic diamonds. Most natural untreated diamonds exhibit nitrogen-vacancy crystallographic defects and exhibit a series of typical second and third order Raman peaks as shown in  FIG. 1 . Crystallographic defects causing the Raman peaks for natural untreated diamonds are not fully understood. Therefore, it is a pressing issue to find out a method for rapidly identifying natural and synthetic diamonds through the existing Raman technique. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a method of rapid identification of natural and synthetic diamonds using a third-order Raman spectra. 
     To achieve the above-mentioned object, the method comprises steps of: providing a Raman analyzer operating with a laser power at 50 mW, a laser beam of wavelength 532 nm, and spectral collection time of 0.1 seconds at an average of 20 times, for an average detection of surfaces of diamonds; utilizing a large spot scanning probe for sample screening and obtaining wave number and intensity of the diamonds; the wave number and intensity being automatically corrected via a software and a background fluorescence being masked automatically; a spectral range of the Raman analyzer used for data acquisition set to be 100-4200 cm −1  in order to facilitate a Raman spectrum with the correct strength and a flat baseline; and the spectrum being further normalized to identify treated samples with a third-order spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows Raman diamond 1 st  order, diamond 2 nd  order, and diamond 3 rd  order spectra; 
         FIGS. 2 a   -A to  2   a -F respectively show Raman spectra (A˜F) of natural untreated Ia and IIa diamonds; 
         FIG. 2 b    shows overlaid Raman spectra of DGA 01˜32 natural untreated Ia and IIa diamonds; 
         FIG. 2 c    shows overlaid Raman spectra of DGA 33˜64 natural untreated Ia and IIa diamonds; 
         FIGS. 3 a  to 3 d    show Raman spectra of CVD diamond A˜D; 
         FIG. 3-1  shows overlaid Raman spectra of synthetic CVD diamond A˜D; 
         FIGS. 4 a  to 4 d    show Raman spectra of HPHT diamond A˜D; 
         FIG. 4-1  shows overlaid Raman spectra of synthetic HPHT diamond A˜D; 
         FIG. 5-1  shows overlaid Raman spectra of natural diamond A &amp; E (3310, 3570, 3880 cm −1 ) and CVD diamond A &amp; B (3122, 3622 cm −1 ); 
         FIG. 5-2  shows overlaid Raman spectra of natural diamond A &amp; E (3310, 3570, 3880 cm −1 ) and HPHT diamond A &amp; B; and 
         FIG. 6  shows Raman spectra of HPHT treated natural diamond. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method of rapid identification of natural and synthetic diamonds using a third-order Raman spectra is to utilize Toptek-Enwave (TSI) G5 Raman analyzers, laser power at 50 mW, with spectral collection time of 0.1 seconds at an average of 20 times. It has a S/N&gt;16,000, denoting its high sensitivity, and a large spot scanning probe (&gt;100 um) for sample screening. The wave number and intensity is corrected automatically via dedicated software (Auto XY Calibration) and the background fluorescence is masked automatically (Auto-baseline). In order to facilitate the Raman spectrum, range is set with the correct strength and a flat baseline (100˜4200 cm-1), the spectrum is further normalized to identify the treated samples with the third-order spectrum. 
     In this preferable embodiment, the method of the present invention covers over a hundred type Ia natural and synthetic samples under 532 nm Raman Analyzer for individual testing. For illustration, the results from 6 units of natural diamonds ( FIGS. 2 a   -A, to  2   a -F, A˜F), 4 units of CVDs samples ( FIGS. 3 a  to 3 d   , A˜D), and 4 units of HPHT samples ( FIGS. 4 a  to 4 d   , A˜D) have been taken. Their Raman response is then matched with wave number and overlaid for analysis ( FIGS. 2 b   ˜ 2   c ,  FIG. 3-1 , and  FIG. 4-1 ). 
     The method is to overlay the 64 natural diamonds spectra for analysis. It is found that the spectra has interrelation between 3000˜4000 cm −1  wave number of the third-order spectrum positioned at 3310 cm −1 , 3570 cm −1 , and 3880 cm −1  as the three main Raman peaks (see  FIGS. 2 b  and 2 c   ); whereas the CVD only exhibits two fluorescence peaks close at 3120 cm −1  and 3620 cm −1  (as shown in  FIG. 3-1 ); HPHT synthetic diamond, in comparison, does not carry any fluorescent or Raman peaks (see  FIG. 4-1 ). 
     In addition, further place two outputs from each of the natural, CVD and HPHT samples under stacked comparative analysis (see  FIG. 5-1 ˜ 5 - 2 ). One can easily observe the differences between them with the evident characteristic Raman peaks; with these strong differences between the characteristics peaks, the method of the present invention can resolve that this described Raman identification technique is fast, accurate, and is entirely different from the traditional approach. 
     Most of the natural diamonds which underwent HPHT treatment for color adjustment, will find in addition to the third-order natural diamond Raman peak at 3310 cm −1 , also exhibit 3570 cm −1 , and 3880 cm −1  responses between the 3000 to 4000 wave numbers. It will also carry a 3122 cm −1  characteristic fluorescence peaks, of which the position is much similar to a lattice defects in CVD diamonds (see  FIG. 6 ). This is the result from the manufacturing process that the CVD diamond also typically undergo HPHT treatment to improve its color, so its lattice defect developing fluorescent peak (3122 cm −1 ) will be similar to the response from a HPHT treated natural diamonds. Because influenced by stress effect, the third-order natural diamond Raman peak will shift left a little after treated by HPHT. 
     Using Raman analyzer as a quick screening tool to identify natural diamonds and synthetic diamonds will give a strong competitive advantage over other comparable technologies. However, in the past the instrument function has technical limitations, especially the lack of sensitivity, so it was not likely to display the characteristic peaks necessary for modern identification. Relevant research literature is thus lacking (especially in diamond&#39;s third-order analysis), thus making the laboratory procedure difficult to evolve into a technical solution. The method of the present invention highlights the following: as long as the Raman analyzer is equipped with adequate performance, it is very quick and simple to determine the differences between the natural and synthetic diamonds. After all, all optimization techniques will leave noticeable characteristics through the analysis of micro-substances; on the prerequisite that the Raman analyzer high enough “sensitivity”. 
     Accordingly, when Raman analyzer is used with the method of the present invention to obtain enough scientific data so as to produce consistency and reproducibility of the result, there is no need to seek for other instruments for duplicated testing. Moreover, Raman analysis is simple and fast, and is able to perform “live rapid screening” without damaging the test samples, and the analytical outcome is fully supported by scientific data—these are the advantages of Raman analysis by means of the method of the present invention. 
     It is understood that the invention may be embodied in other forms within the scope of the claims. Thus the present examples and embodiments are to be considered in all respects as illustrative, and not restrictive, of the invention defined by the claims.