GB2336901A

GB2336901A - Examining diamonds

Info

Publication number: GB2336901A
Application number: GB9809400A
Authority: GB
Inventors: Paul Martyn Spear; Philip Maurice Martineau
Original assignee: Gersan Ets
Current assignee: Gersan Ets
Priority date: 1998-04-30
Filing date: 1998-04-30
Publication date: 1999-11-03
Also published as: GB9809400D0; GB9812438D0

Abstract

A method of examining diamonds to, for example, distinguish natural from synthetic diamonds or to detect CVD/natural diamond doublets comprising the steps of irradiating the diamond with ultraviolet radiation having a wavelength of less than 225 nm, and observing the free exciton spectrum substantially at room temperature. A significant advantage of the present invention is that the diamond does not have to be held at liquid nitrogen temperatures in order to observe the free exciton emission spectrum. The free exciton emission is observed at room temperature. Therefore the method of the present invention is easy and convenient to perform.

Description

2336901 M&C Folio: 230P78160 Document k 211083 Examining Diamonds The

present invention relates to distinguishing natural diamond from synthetic diamond and, in particular, provides an accurate and convenient method and apparatus for distinguishing natural diamond from synthetic diamond quickly and simply.

The present invention is particularly useful for detecting high pressure/high temperature WHT) synthetic diamond and UD synthetic diamond, and for detecting doublets (natural diamonds on which synthetic diamond layers have been deposited).

Synthetic diamond material may be deposited on an uncut or part-worked natural diamond which is then worked, for example, into a round brilliant cut. Alternatively, a synthetic diamond coating may be deposited onto a fully fashioned brilliant stone after working of the stone. The thickness of the synthetic diamond material layer may be very thin - perhaps in the range 1 im to 0. 1 mm.

The value of a diamond is in part dependent upon its weight. Accordingly, synthetic diamond material may be deposited onto natural gem diamonds, before or after cutting of the diamond, to increase the weight of the finished product.

However, the value of a diamond also resides in its qualities of authenticity and uniqueness and in the fact that it is an entirely natural product. Thus, a diamond that has not been enlarged by deposition of synthetic diamond material has a value over a diamond that has.

Over the years, a number of methods of synthesising diamond material have been developed. One of these methods is the chemical vapour deposition (CVD) technique, which is a low pressure technique involving deposition of synthetic diamond (referred to as UD diamond material in this specification) onto a substrate from a gas. UD is the method which is most likely to be used to deposit synthetic diamond material onto a diamond.

A diamond artificially enlarged by deposition of UD diamond material is referred to in this specification as a "CVD/natural diamond doubleC.

2 CV1) diamond material may be deposited on a diamond substrate. The CVD diamond material can replicate the structure of the diamond substrate (referred to as "homoepitaxial growth"). The CVD/natural diamond doublet produced can be identical in appearance, density and other common physical properties to an entirely natural stone and there may be a problem in identifying such a CVD/natural diamond doublet, Methods of identifying CVD/natural diamond doublets have been proposed in the past. However, under certain conditions, such doublets may be passed as entirely natural diamonds. Thus, one object of the present invention to provide a method and apparatus for accurately detecting CVD/natural doublets.

It is also an object of the present invention to provide an effective and accurate method and apparatus for distinguishing natural diamonds from synthetic diamonds.

W086/07457 discloses a method for distinguishing diamond, by visually detecting the Raman signal emitted from a specimen which is irradiated with suitable exciting radiation.

Diamond simulant comprises dense non-diamond material (e.g. metal oxides, particularly zirconium dioxide) which has similar refractive properties to diamond. Synthetic diamond comprises diamond material (i.e. crystalline carbon) produced by an industrial process.

The technique disclosed by WO 86/07457 is only suitable for distinguishing diamond from diamond-like simulant. All diamonds, natural or synthetic, show the Raman emission when irradiated with suitable exciting radiation, and cannot be distinguished by this technique. Another object of the present invention, therefore, is to provide a method and apparatus for accurately distinguishing between natural diamond and synthetic diamond, particularly pure and high purity CV1) diamond and near colourless HPEIT synthetic diamond. Pure diamond exhibits phosphoresence but no 575 rim luminescence when irradiated. High purity diamond exhibits little or no phosphorescence and also no 575 nm luminescence when irradiated.

J. Appl. Phys. 77 (4), 15 February 1995, pp. 1729 - 1734 "Cathodoluminescence from high-pressure synthetic and chemical-vapordeposited diamond" (Lawson et al.) describes a method of performing spectroscopic studies of CV1) diamonds by observing 3 the exciton emission in a diamond using electron beam excitation or cathodoluminescence (CL).

The theory of exciton emission in diamond goes back to the early 1960s, and will now be briefly described. Free exciton (FE) luminescence is intrinsic to all diamond. It consists of a series of resolvable emission lines in the deep ultra-violet at a wavelength of around 235 rim. In order to generate FE luminescence, electric charge carriers (i.e. electrons) within the diamond must be excited across the indirect band gap, from the valence band to the conduction band. The indirect band gap is approximately 5.5 eV. Photons of this energy have wavelength of 225 rim.

When an electron reaches the lowest point of the conduction band there is a probability of this electron existing together with the hole it left behind in the valence band for a time of the order of 10 ns, during which time the electron exists in a level just under the conduction band. This is known as a free (or unbound) exciton. When the exciton recombines, i.e. the electron recombines with the hole, a photon is emitted. In order for the electron to return to the valence band, across the indirect band gap, a momentum-conserving phonon will also be emitted. There are three principal phonons. Therefore, the principal emission will be three lines around 234.5 rim, with the dominant emission being from a line at 234,5 rim.

Conventionally, as disclosed in the above-mentioned reference, exciton emission in diamond has been observed using electron beam excitation, or cathodoluminescence (CL) using custom made electron guns or Scanning Electron Microscopes (SEM). An BPHT diamond sample was irradiated with an electron beam having a diameter of approximately 2 im, with a typical beam current of around 10-8 A. An accelerating voltage of 15 kV was found to produce an efficient CL at around 2 pirn below the surface of the sample. The CL was then focused onto the entrance slit of a monochromator via an ellipsoidal mirror and the exciton emission spectra were observed using conventional spectroscopy techniques.

The FE emission was found to be very intense from pure and high purity CVD diamond, which does not exhibit other competing luminescence bands such as the nitrogen impurity related orange 575 rim emission. The FE emission is also very intense from near colourless HPHT synthetics. FE emission is quenched by the 4 presence of impurities such as nitrogen and the presence of lattice defects. However, pure and high purity CV1) diamonds contain these defects at concentrations that are ineffective at quenching the exciton emission. Therefore, the intensity of FE emission can also be used as a figure of merit for sample purity.

Exciton emission known as bound exciton emission can also be detected from natural semiconducting type 11b diamond.. the intensity of the emission can be used as a measure of the concentration of the uncompensated boron impurities present in these types of diamond.

In contrast, the FE emission from natural type la or type lIa diamond is very weak or undetectable even at liquid nitrogen temperatures. Thus, by observing or measuring the intensity of the FE emission, a M diamond or a near colourless EPHT synthetic diamond can be distinguished from a natural type Ia or type Ila diamond.

However, there are a number of disadvantages associated with the method described in the above-mentioned reference. Firstly, CL requires a vacuum. In the case of instruments which operate under a partial vacuum, there are critical issues of deposits building up on the diamond from the influence of the electron beam on the hydrocarbons from the vacuum pump, and the possibility of ion beam etching of the diamond, a symptom of the partial vacuum conditions. Therefore, high vacuum conditions must be maintained for' the best results.

Furthermore, FE emission under electron excitation must be measured with the sample at liquid nitrogen temperatures, principally to ensure that the intensity and resolution of the emission lines is sufficient. In the above-mentioned reference, the sample is held at a temperature of around 110 K by a liquid-nitrogen-cooled cold stage. This is because, at room temperature, the intensity of the FE emission is at least 100 times weaker than at liquid nitrogen temperatures, and is therefore extremely difficult to detect.

it is an object of the present invention to provide a method and apparatus of classifying diamond, distinguishing between natural and synthetic diamond, or assessing the purity of diamond which is simple to operate, such that it may be put into operation by a person with relatively little training, and which is reliable and relatively inexpensive.

In accordance with the present invention, there is provided a method of examining a diamond comprising the steps of:

irradiating the diamond with ultraviolet radiation having a wavelength which is less than 225 rim; and observing the resulting free exciton emission spectrum at substantially room temperature.

As used herein, the term "ultraviolet radiation" means electromagnetic radiation comprising UVA, UVB and LTVC radiation as defined by the International Commission on Illumination (CIE) in 1970 as follows:- UVA 315-400 nm UVB 280-315nm UW 100-280 nm Therefore, the ultraviolet radiation utilised by the present invention is in the LTVC range, specifically less than 225 nm but at least 100 nm.

In accordance with the present invention, there is also provided apparatus for performing the above method.

The ultraviolet radiation is preferably of wavelength less than 215 rim, and most preferably less than 200 rim. In a preferred embodiment, the ultraviolet radiation has a wavelength of around 193 rim, and is supplied from a LTV laser, although it is envisaged that the present invention could be operated using any convenient coherent or noncoherent ultraviolet source. It has been discovered that if ultraviolet radiation of less than 225 rim is used to irradiate the diamond, then the FE emission from pure and high purity CVD diamond and near colourless BPHT synthetic diamond is very intense at liquid nitrogen temperatures, and therefore, surprisingly, even sufficiently intense to be easily measured at room temperature using, for example, a spectrograph and a liquid nitrogen cooled CCD camera. In contrast, the FE emission is undetectable from natural diamonds or nitrogen-containing CVD and BPHT diamond.

The method of the present invention is simple and convenient to carry out, because it does not require the diamond to be held at liquid nitrogen temperatures.

In an alternative embodiment, it is envisaged that the ultraviolet laser could be replaced by an array of xenon flash lamps, for example. In this case, means are 6 preferably provided to remove any extraneous light at the FE wavelength, e.g. a filter means.

The method and apparatus of the present invention provides a very powerful way of discriminating high purity M diamond and near colourless EPHT synthetics from natural diamond. Moreover, it allows the detection of the pure or high purity CV1) diamond component in a loose diamond doublet. Some conventional detecting techniques may pass a CVD doublet as natural if the natural diamond component of the doublet exhibits absorption due to the N3 impurity centre and the CVI) component does not exhibit either 575 rim luminescence or phosphorescence. Thus, the present invention provides a method and apparatus for detecting doublets, prior to screening using other instruments for classification or further distinguishing between natural and synthetic diamonds, which even identifies a doublet having, for example, a natural diamond component exhibiting N3 absorption and a CV1) component which does not contain orange 575 nm luminescence or phosphorescence, such doublets being passed as natural by conventional detection instruments.

The apparatus of the present invention preferably comprises a static low bandpass filter between the diamond and the ultraviolet source, which is preferably a ultraviolet (LTV) laser, so as to allow only the laser wavelength and not the laser discharge to impinge on the diamond, thereby eliminating spurious light at the wavelength of the free exciton.

The FE emission may be detected using a spectrograph, or more preferably a static narrowband filter, e.g. of bandwidth lOnm at half maximum, or a rotating narrowband filter covering the dominant FE wavelength (234.5 rim). The thus modulated signal is then preferably detected using a photomultiplier (PMT).

In an alternative embodiment, the sample may be placed at the centre of an integrating sphere, effective in the ultraviolet, and the FE emission detected at a port on the integrating sphere by means of a static narrowband filter and PMT.

An embodiment of the present invention will now be described by way of example only, and with reference to the accompanying drawings which is a schematic block diagram of an apparatus for performing the method of the present invention.

7 Referring to the drawing, the apparatus comprises an ultraviolet laser radiation source 10, a static low bandpass filter 12 between the diamond sample 14 and the ultraviolet laser 10. The apparatus further comprises a rotating narrowband filter 16 and a photomultiplier 18.

The diamond sample 14 is irradiated with radiation of wavelength of around 193 nm by the source 10. The static low bandpass filter 12 is arranged to allow only the wavelength of the radiation source to impinge on the sample 14, thereby eliminating spurious light at the wavelength of free exciton.

The rotating narrowband filter which is centred at the dominant FE wavelength (234.5 nm) produces a modulated signal which is indicative of the FE emission from the sample. The modulated signal is detected using the photomultiplier 18.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention, which extends to the equivalents of the features described. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination.

8 CLAMS:

1. A method of distinguishing natural from synthetic diamond, comprising the steps of irradiating the diamond with ultraviolet radiation having a wavelength which is less than 225 rim, and observing the resulting free exciton emission spectrum substantially at room temperature.

2. A method according to claim 1, wherein said ultraviolet radiation has a wavelength less than 215 rim.

3. A method according to claim 1, wherein said ultraviolet radiation has a wavelength less than 200 rim.

4. A method according to claim. 1, wherein the wavelength of said ultraviolet radiation is substantially 193 rim.

5. A method according to any preceding claim, wherein said ultraviolet radiation is supplied by a ultraviolet (UV) laser.

6. A method according to any one of claims 1 to 5, wherein said ultraviolet radiation is supplied by an array of xenon flash lamps.

7. A method according to any preceding claim, comprising the step of providing a bandpass filter between the source of ultraviolet radiation and the diamond, so as to eliminate extraneous light at the wavelength of the free exciton.

8. A method according to any preceding claim, wherein said free exciton emission is observed using a spectrograph.

9 9. A method according to any one of claims 1 to 7, wherein said free exciton emission is observed using a rotating filter covering the dominant free exciton wavelength of substantially 234.5 rim.

10. A method according to any one of claims 1 to 7, wherein said free exciton emission is observed using a static filter and integrating sphere.

11. A method of distinguishing between natural and synthetic diamonds substantially as herein described with reference to the accompanying drawing.

12. An apparatus for distinguishing natural from synthetic diamond, comprising irradiating means for irradiating the diamond with ultraviolet radiation having a wavelength which is less than 225 rim, and means for observing the resulting free exciton emission spectrum substantially at room temperature.

13. An apparatus according to claim 12, wherein said irradiating means is a LT laser.

14. An apparatus according to claim 12, wherein the irradiating means comprises an array of xenon flash lamps.

15. An apparatus according to any one of claims 12 to 14, comprising a bandpass filter between said irradiating means and said diamond, so as to eliminate extraneous light at the wavelength of the free exciton.

16. An apparatus according to any one of claims 12 to 15, wherein said means for observing the free exciton emission spectrum is a spectrograph.

17. An apparatus according to any one of claims 12 to 15, wherein said means for observing the free exciton emission spectrum comprises a rotating filter covering the dominant free exciton wavelength of substantially 234.5 rim.

18. An apparatus according to any one of claims 12 to 15, wherein said means for observing the free exciton emission spectrum comprises a static filter and an integrating sphere.

19. An apparatus for distinguishing between natural and synthetic diamonds substantially as herein described with reference to the accompanying drawing.