1
Diamond Material
This invention ,relates to changes to the absorption characteristics of some types of
nitrogen-containing diamond material that may occur when that diamond material is
exposed to certain conditions. In particular the invention relates to a method involving
controlled irradiation of that diamond material to minimise the changes. Preferred
methods according to the invention relate to controlled irradiation of diamond material
made by chemical vapour deposition (so-called CVD synthetic diamond material}. The
invention also relates to diamond material per se. Preferred embodiments of diamond
material according to the invention relate to CVD synthetic diamond material which has
been subjected to controlled irradiation.
A particular type of rare fancy-coloured, naturally occurring diamond, known as
chameleon diamond, is known, which shows changes to its absorption characteristics
under certain conditions. Being rare and deemed valuable, there are not known to be
any reports of non-reversible treatments on these diamonds.
The term "fancy-coloured diamond" is a well-established gem trade classification of
stronger and more unusual colours in diamond material.
Pink type II a natural diamonds also form another important class of diamonds that exhibit
colour changes. These have been shown to change colour from pink to brown upon
ultraviolet (UV) illumination as described by de Weerdt and van Royen (Diamond and
Related Materials, 10 (2001), 474-479). The pink colour is a result of a broad optical
absorption feature at 550 nm that is bleached by ultraviolet radiation and also diminishes
on cooling.
In the manufacture of synthetic diamond material, in particular CVD synthetic diamond
material, it is krnown to dope the diamond material by the addition of low concentrations
of nitrogen to the gases fed into the synthesis chamber during the CVD process. · It is
known in the artt that this can be advantageous, the nitrogen increasing the growth rate of
the CVD synthetic diamond material. It is also known that the presence of the low
concentrations of nitrogen in the CVD growth environment can affect the nature and
concentration of the defects that are incorporated as the material grows.
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As mentioned above, this invention relates to controlled irradiation of diamond material to
minimise changes to the absorption characteristics of some types of nitrogen-containing
diamond material that may occur when that diamond material is exposed to certain
conditions. Irradiation of diamond material is known in the prior art. For example.
EP 0615954 and EP 0316856 describe irradiation of synthetic diamond material with an
electron bean or a neutron beam to fonn lattice defects in the crystal. Thereafter the
diamond crystal is annealed in a prescribed temperature range, so that nitrogen atoms
are bonded with the lattice defects to fonn colour centres, such as that comprising a
substitutional nitrogen atom adjacent to a vacancy, referred to as an "[N-V] colour centre"
or just "N-V", which can give the diamond material a desirable colour, such as purple or
red/pink, i.e. they introduce a desired fancy colour into the diamond material. Similarly
"Optical Absorption and Luminescence" by John Walker in "Reports on Progress in
Physics", Volume 42, 1979, describes the steps of fonning lattice defects in crystals by
electron beam irradiation, and if necessary annealing to cause the lattice defects to
combine with nitrogen atoms contained in the crystals. In the prior art the whole purpose
of the irradiation and annealing steps is to produce so-called colour centres, that is to
introduce colour into the diamond material. The irradiated and annealed diamond
material of EP 0615954A and EP 0316856A contains so-called N-V defects as identified
by significant absorptions at about 570 nm.
Similarly ·cvo Grown Pink Diamonds• by Wuyi Wang, of GIA Laboratories, published 30
April 2009 on www.gia.edu/research-resources describes CVD synthetic diamond
material where some significant amount of the nitrogen present is in the form of N-V
centres which strongly absorb yellow and orange light, thus creating a pink to red colour.
These diamond materials, like those described in the two EP publications (EP 0615954
and EP 0316856) contain N-V centres as evidenced by inter alia strong absorptions at
about 570 nm.
We have observed that some nitrogen doped synthetic diamond material, particularly
some nitrogen doped synthetic diamond material when made by chemical vapour
deposition (CVD synthetic diamond material), also show changes to their absorption
characteristics, which can be observed as a colour change, under certain conditions.
Thus, like the chameleon diamonds, we have noticed that the colour of such synthetic
diamond material may vary across a colour range, the colour being unstable across this
colour range, and the observ.ed colour at any ·particular time depending on the recent
history of the diamond material. For example exposure to radiation, particularly to
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radiation with an energy greater than 2.0 eV, for example exposure to ultraviolet
radiation, tends to change the diamond material to a lower grade colour, which is
generally considered less desirable, whereas thermal treatment at elevated temperatures
tends to change the diamond material to a higher grade colour, which is generally
considered more desirable. Low and high grade colours are defined later in the
specification. Since for most applications the usual state for diamond material is one in
which it is exposed to radiation in the form of light, the usual colour of the diamond
material tends towards the lower, less desirable colour grades of their unstable colour
range. In this specification we shall refer to this usual colour for the diamond material,
i.e. the colour of the diamond material when it is exposed to radiation, or has recently,
been exposed to radiation, particularly to radiation with an energy greater than 2.0 eV, as
its "equilibrium colour", and the usual state of diamond material, i.e. the state where it is
exposed to radiation, or has recently been exposed to radiation, as its "equilibrium state"
or "equilibrium condition".
As mentioned above, it is known that the presence of the low concentrations of nitrogen
in a CVD growth environment can affect the nature and concentration of the defects that
are incorporated in a CVD synthetic diamond material as the diamond material grows.
Electron paramagnetic resonance (EPR) and optical absorption spectroscopy have been
used to study the nature of defects introduced into the diamond material as a direct
consequence of the CVD growth process in which low concentrations of nitrogen are
introduced. The presence of the nitrogen in the grown CVD synthetic diamond material
can be evidenced by looking at absorption spectra for the grown diamond material, and
while the exact nature of all the defects in this diamond material are not fully understood,
analysis of these spectra gives some indication of the relative proportions of different
types of defect present in the grown CVD synthetic diamond material as a result of the
presence of low concentrations of nitrogen during the growth process. A typical
spectrum for grown CVD synthetic diamond material grown with nitrogen added to the
synthesis environment shows a peak at about 270 nm, which is generated by the
presence of neutral single substitutional nitrogen (SSN) atoms in the diamond lattice.
Additionally peaks have been observed at about 350 nm and approximately 510 nm
corresponding to other defects characteristic and unique to CVD synthetic diamond
material, and ·furthermore .a so-called "ramp", that is a rising background of the form
c x A.-3 ·has .been observed, where c is a constant and A. is the wavelength. The
combination of these features (SSN, 350 nm peak, 510 nm peak and ramp) affect the
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colour in the visible part of the electromagnetic spectrum (which is generally considered
to cover the wavelength range 380 nm to 750 nm), and are believed to be responsible for
the brown colour usually seen in nitrogen doped CVD synthetic diamond material.
In practice it has been found that the precise width and position in the spectrum of these
characteristic absorption bands can vary. The position of peak maxima is most easily
ascertained by using the second differential of the spectrum. It has been found that
absorption spectra can generally be deconstructed into a number of approximate
components, and it is useful to carry out this deconstruction, since when the well
understood components are subtracted from any recorded spectrum, then the
contribution of other, less well understood, components can more easily be seen. For
example, it is known to subtract the spectral component of standard synthetic diamond
material from any observed spectrum. In the present case, for diamond material referred
to in the present invention, we have used the spectral decomposition set out in the
numbered paragraphs below and shown in Figure 1. For ease of comparison the
following limits are measured from 0 cm-1 on the individual spectral component, with the
spectrum being made to be 0 at 800 nm.
1) Single substitutional nitrogen component with an absorption coefficient at 270 nm
that is generally within the range 0.05 cm-1 and 20 cm-1 and an absorption
coefficient at 425 nm that generally lies between 0.04 cm-1 and 1 cm-1.
2) An absorption band centred at 3.54 eV (350 nm) ±0.2 eV with a full width at half
maximum (FWHM) of approximately 1 eV and a maximum contribution to the
absorption spectrum generally between 0.05 cm-1 and 8 cm-1 at its centre.
3) An absorption band centred at 2.43 eV (510 nm) ±0.4 eV with a FWHM of
approximately 1 eV and a maximum contribution to the absorption spectrum
generally between 0.02 cm-1 and 4 cm-1 at its centre.
4) A small residual wavelength dependent component of the measured absorption
coefficient (in cm-1
) that is found to have a wavelength dependence of the following
approximate form: c x (wavelength, A., in (.Jm)-3 where c <0.2 such that the
;; contribution of this component at 510 nm is generally less than 1.5 cm·1•
5
Figure 1 shows the absorption spectrum of a typical CVD synthetic diamond layer (curve
B) which has been removed from its growth substrate, and the components into which it
can be deconvoluted. The first step in the spectral decomposition is the subtraction of
the spectrum of a type lb high pressure high temperature (HPHT) synthetic diamond
material (curve A), scaled so that the residual shows no 270 nm feature. The residual
spectrum is then be deconvoluted into a c x A.-3 component (curve C) and two
overlapping bands, one centred at 350 nm and the other centred at 510 nm of the kind
described above in numbered paragraphs (2) and (3)above. The two overlapping bands
are shown as curve D in Figure 1.
It is known that the form of UV/visible optical absorption spectra of CVD synthetic
diamond material grown using a range of different processes can be specified by sums
of the components described above, with different weighting factors for the components
in different cases. For the purposes of specifying the shape of the spectrum the
contributions of the different components are given in the following ways.
270 nm: The peak 270 nm absorption coefficient of the type lb component is measured
from a sloping baseline connecting the type lb spectrum either side of the 270 nm feature
that extends over the approximate range from 235 nm to 325 nm.
350 nm band: The peak absorption coefficient contribution of this band.
510 nm band: The peak absorption coefficient contribution of this band.
Ramp: The contribution of the c x A.-3 component to the absorption coefficient at 510 nm.
In the present specification, for all quoted absorption coefficients, the absorption
coefficients have been measured by normalising the spectra so that they start at 0 cm·1
at 800 nm.
The method according to the present invention is particularly applicable to synthetic
diamond material, more particularly to CVD synthetic diamond material.
As :noted· above,. diamond materials that exhibit a pronounced -amount of colour are
known· as "fancy" coloured diamonds in the field. Other diamond materials that do not
show such pronounced colour may be graded using the Gemological Institute of America
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(GIA) scale. That scale grades diamond materials alphabetically from D to Z. The GIA
scale is well known. D represents the highest grade and most colourless diamond
material on the GIA scale, and Z represents the lowest grade on the GIA scale, a
diamond material of grade Z appearing light yellow to the naked eye. Higher grade
diamond materials (those on the GIA scale nearer to grade D) are generally considered
more desirable than lower grade diamond materials (those nearer to grade Z), both in the
gem trade and for industrial applications. When a diamond material's colour is more
intense that the Z grading it enters the realm of "fancy" diamond of which chameleon
diamonds are a sub set.
There are three visual attributes to colour, these being hue. lightness and saturation.
These terms are well understood by the person skilled in the art. Briefly: hue is the
attribute of colour that allows it to be classified as red, green, blue, yellow, black or white,
or and intermediated thereof; lightness is the attribute of colour that is defined by the
degree of similarity with a neutral achromatic scale starting with white and progressing
through grey to black; saturation is the attribute of colour that is defined by the degree of
difference from an achromatic colour of the same lightness. Saturation is also a
descriptive term corresponding to the strength of the colour. A system for quantifying
colour, known as the "CIE L *a*b* colour space" or more simply "CIELAB", exists which
defines the saturation by a parameter known as C*. Where the colour of a diamond
material falls within the GIA scale, the correlation between C* values and GIA grades
respectively is 0=0, 0.5=E. 1.0=F. 1.5=G, 2.0=H etc for a round brilliant cut (RBC)
diamond and where the saturation (C*) is determined from the absorbance spectrum
measured along the axis of the stone through the complete depth of the material. In
practice, all measurements on the GIA scale in this document are for an equivalent 0.5 ct
round brilliant cut gemstone. C* values are also applicable to fancy coloured diamonds.
Our invention is applicable to all colours of diamond materials, including diamond
materials whose colour falls only within the GIA scale of D to Z, and fancy diamonds
whose colour may be outside the GIA scale of D to Z.
The perceived colour of an object depends on the transmittance/absorbance spectrum of
the object, the spectral power distribution of the illumination source and the response
curves ·of ·the observer's e9es. The CIELAB chromaticity coordinates .quoted in this
·· patent application have been ·aerived in the way. described below. ·Using a standard 065
illumination spectrum and standard (red, green and blue) response curves of the eye (G.
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Wyszecki and W. S. Stiles. John Wiley, New York-London-Sydney, 1967), CIE L*a*b*
chromaticity coordinates of a parallel-sided plate of diamond material have been derived
from its transmittance spectrum using the relationships below, between 350 nm and
800 nm with a data interval of 1 nm:
SA = transmittance at wavelength 'A
LA = spectral power distribution of the illumination
XA = red response function of the eye
YA = green response function of the eye
z~. = blue response function of the eye
L * = 116(Y/Y0)
113
- 16 = Lightness (for Y/Y0 > 0.008856)
a*= 500[(X/Xo)113
- (Y/Y0)
113
] (for X/Xo > 0.008856, Y/Yo > 0.008856)
b* = 200[(Y/Yo)113
- (Z/Zo)113
) (for Z/Zo > 0.008856)
C* = (a*2 + b*2f 12 = saturation
hab = arctan(b*/a*) = hue angle
Modified versions of these equations must be used outside the limits of Y/Y0, X/Xo and
ZfZ.o. The modified versions are given in a technical report prepared by the Commission
lnternationale de L'Eclairage (Colorimetry (1986)).
It is possible to predict how the a*b* coordinates of diamond material with a given
absorption coefflcient.sp~ctrum ·will = change as the optical .path length "is varied. In order
to do this. the reflection loss must first be subtracted from the measured absorbance
spectrum. The absorbance is then scaled to allow for a different path length and then
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the reflection loss is added back on. The absorbance spectrum can then be converted to
a transmittance spectrum which is used to derive the CIELAB coordinates for the new
thickness. In this way the dependence of the hue, saturation and lightness on optical
path length can be modelled to give an understanding of how the colour of diamond
material with given absorption properties per unit thickness will depend on the optical
path length.
The C* (saturation) numbers can be divided into saturation ranges of 10 C* units and
assigned descriptive terms as below.
0-10 weak
10-20 weak-moderate
20-30 moderate
30-40 moderate-strong
40-50 strong
50-60 strong-very strong
60-70 very strong
70-80+ very very strong
As mentioned above, we have observed that the colour of some CVD synthetic diamond
material may vary across a colour range. the colour being unstable across this colour
range, and the observed colour at any particular time depending on the recent history of
the diamond material.
We have surprisingly found that irradiation (performed in a controlled manner) can be
used, not to introduce colour as has been used in the aforementioned prior art, but to
reduce the colour range over which the diamond material can exist. This means that the
controlled irradiation tends to stabilise the colour of the diamond material regardless of
its environmental conditions or its recent thermal/illumination history. In preferred
embodiments we have also found that not only can the colour be stabilised, but also the
equilibrium colour (as hereinbefore defined) can be improved, by which we mean that the
equilibrium colour of the diamond material that has been subjected to the controlled
irradiation treatment is a higher colour grade (nearer to D on the GIA scale and/or has a
lower C* value), i.e ... it.fias what'is generally considered to be a mere. desirable·colour (or
lack of colour) than the·· equilibrium ·colour of the same diamond material prj or to the
controlled irradiation.
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Our observations have shown that, for a nitrogen containing CVD synthetic diamond
material round brilliant with a colour grade on the GIA scale, and in particular for a
nitrogen containing CVD synthetic diamond round brilliant with a starting colour grade in
the range F to Z (corresponding to 1 to 11 on the C* scale especially for a nitrogen
containing CVD synthetic diamond material with a starting colour grade in the range I to
K on the GIA scale (corresponding to 2.5 to 3.99 on the C* scale), exposure to radiation,
and in particular to UV radiation results in a colour change of the diamond material to
lower colour grades (i.e. away from D grade on the GIA colour scale), while thermal
treatment, for example at a temperature of 525°C for one hour, causes the diamond
material to change colour in the opposite direction to the higher, generally more
desirable, (i.e. towards D grade on the GIA colour scale) colour grades. For example for
a CVD synthetic diamond round brilliant of starting colour grade K (C* 3.5-3.99) it is
observed that exposure to UV radiation for 20 minutes results in a colour change to a
lower colour grade L (C* 4-4.49), while subsequent heating to 525°C for 1 hour results in
a colour change to higher grade I (C* 2.5-2.99) , i.e. a total colour grade difference of L to
I (i.e. 3 colour grade difference on the GIA scale, corresponding to a C* change of 1.5).
Overall we have observed that for any given CVD synthetic diamond sample the overall
colour grade change when exposed to radiation and then thermally treated may vary by
up to 1, up to 2, up to 3, up to 4, up to 5, even up to 6 colour grades as measured on the
GIA scale (corresponding to a C* change of up to 0.5, 1, 1.5, 2, 2.5 or even up to 3).
Similarly for fancy diamond (C*>11 ), a change of at least 0.5, or for some embodiments
a change of at least 1 or 1.5 or 2, up to 4, 5 or even 6 in the value of C* may result from
exposure to radiation and then thermal treatment.
This variation in colour, and the associated variation in the absorption characteristics of
the electromagnetic spectrum of the synthetic diamond material, on exposure to radiation
(e.g. light) and heating is undesirable for some applications. As will be elucidated
further, while the observed colour of the synthetic diamond material is associated with
specific defects (with specific electronic configuration) within the diamond material which
give rise to absorption, colour instability can be associated with charge transfer between
these same defects in the crystal lattice, these charge transfer effects modifying the
·.absorption spectrum . . ln. practice.: . il: has been found that · expo~ure .of.;the. diamond
.. material to any radiation w.ith .energy greater than 2.0 eV leads•to an. increase in ·the .
absorption in the visible part of the spectrum, causing the diamond material colour to
• 0 • •••
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change to a lower (generally considered less desirable} colour grade (nearer to Z on the
GIA scale if applicable, and/or higher C* value). While this increase can be removed by
a simple low temperature (e.g. -soooC} thermal treatment, for most usual applications
the diamond material will ordinarily be exposed to radiation with energy greater than
2.0 eV (for example sun light, fluorescent room lights etc). This means that for these
usual applications the diamond material is always pushed towards a state where it
contains those specific defects and associated specific electronic configurations that
increase the absorption of the diamond material in the visible part of the spectrum,
leading to lower, generally considered less desirable colour grades (nearer to Z if the
GIA scale is applicable, and/or to higher C* values}. As mentioned above we refer to this
"usual" state of a diamond material, when the diamond material is, or has recently been
exposed to radiation, particularly to radiation with energy greater than 2.0 eV, as the
"equilibrium state" or "equilibrium condition" and the colour of the diamond material in this
state as the "equilibrium colour".
When a diamond material exhibits colour instability this may be undesirable for a number
of applications. Similarly the fact that the "equilibrium condition" corresponds to the
highest absorption across the colour range over which the diamond material is unstable
is also undesirable for a number of applications.
Also where a diamond material exhibits colour instability, this, as explained previously,
can also impact the materials carrier properties (for example carrier mobility) and thus
the material can show variable carrier mobility depending on the history of the diamond
material. For some applications carrier mobility instability and, for example, the related
instability in other electronic properties of the diamond material may be undesirable.
As examples of applications where the colour instability and/or the carrier property
instability, and/or the tendency for the equilibrium condition of the diamond material to
correspond to the highest absorption across the colour range over which the diamond
material is unstable is undesirable, there may be mentioned the following:
(i) For diamond material used in certain optical applications such as Raman lasers or as
a passive optical element in a high power laser operating the range from 225 nm to
.. 1600 nm it is desirable that the absorption·is minimized and stable;
'# · .. r. •
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(ii) In electronic devices made of diamond material, a key performance indicator is
related to the carrier mobility. The mobility is reduced in materials which contain a high
concentration of ionized (charged) impurities through carrier scattering. Any process
which can lead to the number and type of . these ionized impurities changing is
undesirable as it can lead to an unstable electronic device.
(iii) While there are some exceptions in the field of fancy natural diamonds, it is
undesirable for a diamond used in ornamental purposes to show colour instability, not
least related to difficulties associated with grading such a diamond.
The colour change observed in the CVD synthetic diamond material on exposure to
radiation and/or thermal treatment is thought to be due to a change in the charge state of
one or more of the defects in the diamond material. This change in charge state is
thought to arise from an electron or other charge transfer effect taking place within the
diamond lattice. Thus the colour instability that we observe is thought to result from
these electron or other charge transfers.
Without limiting the invention in any way, a possible explanation for the charge transfer
effects which give the colour changes on exposure to radiation and heating is as follows.
Neutral single substitutional nitrogen atoms (N8°) act as electron donors. Thermal or
optical excitation of an electron from Ns 0 sites occurs as a result of either heating or
radiation exposure respectively, the excited electrons being captured in some other kind
of defect in the diamond structure, which defect sites we shall call "X". This charge
transfer effect can be written as follows:
(Equation 1)
We believe that exposure of the diamond material to radiation, particularly to radiation
with energy greater than 2.0 eV, and more specifically UV radiation, causes the equation
to move to the left, resulting in changes to the absorption spectrum of the diamond
material that result in a lower colour grade (for example nearer to Z on the GIA scale and
corresponding to a higher C* value); while thermal treatment of the diamond material
causes the equation to move to the right, resulting in changes to the absorption spectrum
.of the diamond-material that result in a higher colour grade-{for example nearer to Don. . ..
the GIA scale 'and ·corresponding .to a lower C* value). It is thought that the colour that is
observed in diamond material is associated with the presence of defect X in the diamond
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lattice, the charged defect x- being associated with a colourless state, or at least more
colourless state.
Specifically we use X here to cover the unknown defect(s) in nitrogen doped CVD
synthetic diamond material which take place in the charge transfer process. These
defect(s) are thought to be responsible for the visible absorption features characterized
by one or some combination of the 350 nm, 510 nm and the Ramp components.
A first aspect of the present invention provides a method comprising:
a) providing a nitrogen-containing diamond material which shows a measurable
difference in at least one of its absorption characteristics in first and second states, the
first state being after exposure to radiation having an energy of at least 5.5 eV, and the
second state being after thermal treatment at 798 K (525°C),
b) treating the said nitrogen containing diamond material by controlled irradiation of the
said nitrogen-containing diamond material so as to introduce sufficient defects in the
diamond material so as to produce one or both of:
(i) an absorption coefficient measured at 77 K of at least 0.01 cm-1 and at most
1 cm·1 at a wavelength of 741 nm: and
(ii) an absorption coefficient measured at 77 K of at least 0.01 cm·1 and at most 0.5
cm-1 at a wavelength of 394 nm;
whereby the measurable difference in the said absorption characteristics of the
irradiation treated diamond material in the said first and second states, having been
exposed to the same radiation and thermal treatment as the provided diamond, is
reduced relative to the measurable difference in the said absorption characteristics of the
provided diamond material in the said first and second states.
When we say the diamond material shows a measurable difference in its absorption
characteristics we include any differences that can be recorded. This includes for
example numerical absorption coefficients of the diamond material at various
wavelengths, and visible changes, e.g. colour changes that may be observed by the
nak~ -eye, or with.the aid of magnification equipment. .... . . . ·· ... · .
. .
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The step of actually growing the CVD diamond material may or may not form part of the
method of embodiments of the invention. Providing a CVD diamond material may mean
growing the CVD diamond material, or may simply mean, for example, selecting a pregrown
CVD diamond material.
By "controlled irradiation" we mean applying an amount of irradiation so as to introduce
the said sufficient defects in the diamond material so as to produce one or both of:
(i) an absorption coefficient measured at 77 K of at least 0.01 cm·1 and at most
1 cm·1 at a wavelength of 7 41 nm; and
(ii) an absorption coefficient measured at 77 K of at least 0.01 cm·1 and at most
0.5 cm·1 at a wavelength of 394 nm.
We have observed that for the diamond materials of interest to the present specification
exposure to radiation changes at least one of the absorption characteristics of the
diamond material, the change increasing as the period of exposure is increased, and
then levelling of after a certain period of exposure. The time at which this "levelling off'
occurs depends on the intensity of the radiation and its energy. Preferably, in order to
make worthwhile comparisons of diamond materials treated by methods according to the
invention, the period of exposure to the said radiation is until substantially no further
measurable change in the said absorption characteristics are found to occur. For
example, for a diamond material whose colour is one on the GIA scale, the period of
exposure is preferably until the diamond material shows no further change in colour
grade on the GIA scale, for example such that sequential measurements show no further
change in colour on the GIA scale. As another measure, the period of exposure is
preferably until the diamond material shows substantially no further change in its C*
value, e.g. such that sequential measurements on the diamond material show a C*
change of less than 0.5, preferably less than 0.4, more preferably less than 0.3, and
even less than 0.2 or 0.1. Sequential measurements may be taken, for example, every
hour, or every 20 minutes, or every two minutes, or every minute or every 30 seconds.
In use, the diamond material may be exposed to radiation with an energy less than 5.5
eV, e.g. less than 4.5 eV or less than 3.5 eV or even less than 2.5 eV, and for some
diamond materials this lower wavelength exposure will also cause changes to its
absorp.tion. 'cha.racter:istics, .. which it is ·desirabl~ . to ·stabilise:, Such .diamond material
wQuld also · show changes to i.ts absorption characteristics when exposed to radiation
having an energy greater than 5.5 eV. The value of 5.5 eV is defined in this specification
14
to encompass the broadest scope of diamond material that show changes to its
absorption characteristics when exposed to radiation.
Those skilled in the art will realize that charged particles other than photons of
electromagnetic radiation can also introduce electron hole pairs with energy >5.5 eV into
the diamond lattice and hence affect the absorption characteristics. As examples there
may be mentioned beta and alpha particles. The rest of this discussion focuses on the
use of photons but this excitation source should not be seen to limit the invention, and
reference in the invention to radiation of energy of at least 5.5 eV includes in its broadest
scope charged particles of electromagnetic radiation other than photons with an energy
greater than 5.5 eV.
Similarly we have observed that for the diamond material of interest to the present
specification thermal treatment changes at least some of the absorption characteristics,
the change increasing as the period of thermal treatment is increased, and then levelling
off after a certain period of thermal treatment. The time at which this "levelling off'
occurs depends inter alia on the temperature of the thermal treatment, and the nature of
the diamond material. Preferably the perio~ of thermal treatment is until substantially no
further measurable changes to the said absorption characteristics are found to occur.
For example, for a synthetic diamond 0.5 ct round brilliant with measured C*=3, the
period of thermal treatment is preferably until the diamond shows no further change in
colour grade on the GIA scale, for example such that sequential measurements show no
further change in colour on the GIA scale. As another measure, the period of thermal
treatment is preferably until the diamond material shows substantially no further change
in its C* value, e.g. such that sequential measurements on the diamond material show a
C* change of less than 0.5, preferably less than 0.4, more preferably less than 0.3, and
even less than 0.2 or 0.1 . As before, sequential measurements may be taken, for
example, every hour, or every 30 minutes, or every 10 minutes, or every two minutes, or
every minute or every 30 seconds.
In use, the diamond material may be exposed to elevated temperatures for various
lengths of time, and these elevated temperatures may be less than 525°C, but
nonetheless result in absorption characteristic changes to the diamond material. For
example exposure. to eleva.ted . temperatures of at least 150°C, .at leasr 200~C. :at least
250°C, .at least 400°C, or at least'.450°C for periods of 30 minutes; 1 hour, 5 hours, 10
hours or 24 hours, or even a week may result, for some diamond material in changes to
15
its absorption characteristics, which it is desirable to stabilise. Such diamond material
would also show changes to its absorption characteristics when thermally treated at
525°C. The recitation of thermal treatment at 525oC is defined in this specification to
encompass the broadest scope of synthetic diamond material that show changes to its
absorption characteristics when exposed to elevated temperatures.
Thus if diamond material is shown to exhibit a change in at least one absorption
characteristic when exposed to radiation of energy of at least 5.5 eV and a thermal
treatment at 525°C, then it is diamond material which would benefit from controlled
irradiation according to the method of the invention, even if in use it would never be
exposed to such high temperatures or to light or other electromagnetic radiation of such
high energy.
It is not fully understood why the irradiation step reduces the differences in the
absorption characteristic of the diamond material in its first and second states (e.g.
stabilises the colour), but it is thought that it may be because the controlled irradiation
introduces specific defects into the diamond's crystal lattice which act as alternative
donors/acceptors for electrons/holes (alternative to the X defects) subsequently excited
by UV radiation or thermally treated, thereby substantially preventing, or at least reducing
or minimising charge transfer to the optical sites that were active in the diamond material
prior to its treatment by controlled irradiation.
According to the invention the provided nitrogen-containing diamond material shows a
measurable difference in at least one of its absorption characteristics in first and second
states. Preferably it is the majority volume of the diamond material which shows this
measurable difference, by which we means at least 55% and preferably at least 80% and
most preferably at least 95% of the whole volume of the diamond material.
Defects can be introduced by any means that lead to the production of interstitials and
vacancies within the diamond lattice. Those skilled in the art will be aware of a number
of different routes to achieve this. The most common route is to use one or some
combination of electrons, neutrons or gamma photons. Therefore in a preferred method
according to the invention the controlled irradiation is provided by one or more of
electrons, neutrons and gamma.photons. . .. . .. ..... · .. ._..:.,·
: •' .
16
When energy is imparted to the diamond lattice there are four possible outcomes. Firstly
the lattice may be undamaged and the energy transferred dissipated in the form of a
phonon. Secondly, a carbon atom may be temporarily displaced from its lattice position
but without sufficient energy to remain free from its original position, recombination
occurring. Thirdly, the carbon atom may be displaced to a new interstitial site thus
creating a vacancy. This vacancy-interstitial pair is known as a Frenkel defect. Fourthly,
a carbon atom may receive enough energy to be moved out of its lattice position and
knock other carbon atoms out of their lattice positions, producing a cascade of damage
resulting in numerous interstitials and vacancies. Neutral vacancies are referred to in the
field as VO and negatively charged vacancies as v-.
Factors which affect the type, concentration and depth of damage include (a) the
irradiation/implantation dose which has a primary impact on the density of defects
produced, (b) the damaging particle's energy which affects the depth of damage and
whether there is point damage or extended cascade damage, (c) the type of sample, for
example the damage production rate in type lb diamond material is -7x times higher
than for type Jla diamond material, (d) the temperature at which irradiation is carried out,
this impacting both the type of defects formed (e.g. if the sample temperature exceeds
500 K) and the concentration of defects. The irradiation process itself can lead to in-situ
annealing processes if the sample temperature is not carefully controlled. This is
particularly an issue when using large beam currents/fluxes.
During the irradiation the sample temperature is preferably maintained at temperature
below 400°C. For some preferred embodiments it is preferably maintained at a
temperature less than 300°C, or less than 250°C, less than 200°C, less than 150°C, less
than 100°C, less than 80°C, less than 50°C, less than 30°C. For some preferred
embodjments it is preferably maintained at a temperature no lower than - 200°C (i.e.
minus 200°C, preferably no lower than -150°C (i.e. minus 150°C).
For diamond material, the absorption characteristic at 7411 nm and/or at 394 nm are
characteristic of defects introduced by irradiation, and are discussed in more detail later
in the specification. In general, when a diamond material is irradiated the higher the
irradiation dose the higher the absorption coefficients at 7 41 nm and/or 394 nm.
:-. . ; . i.. ... • : ·... . . - ~·
We have found that the required controlled amount of irradiation is one that· introduces
sufficient interstitial defects in the diamond material as to produce an absorption
17
coefficient measured at 77 K of at least 0.01 cm-1 and at the most 1 cm-1 at the
wavelength of 741 nm (this is known in the field as the GR1 characteristic wavelength),
and/or an absorption of at least 0.01 cm-1 and at the most 0.5 cm-1 at a wavelength of
394 nm (this is known in the field as the ND1 characteristic wavelength).
The maximum values of these coefficients recited in this specification, define the point
when in themselves they add measurable absorption to distort the colour of the diamond
object, and the minimum value is that when enough additional defects have been created
on irradiation to compete with the charge transfer process in Equation 1 and yield
diamond material whose measurable difference in its absorption characteristics in its first
and second states is reduced.
An absorption coefficient of at least 0.01 cm-1 and at the most 1 cm-1 at the wavelength of
7 41 nm (the so-called GR 1 characteristic wavelength), corresponds to a concentrations
of \f of at least 4 parts per billion (ppb) and at the most 0.15 parts per million (ppm). An
absorption of at least 0.01 cm-1 and at the most 0.5 cm-1 at a wavelength of 394 nm (the
so-called ND1 characteristic wavelength), corresponds to a concentration v-of at least 1
ppb and not more than 0.2 ppm. Concentrations of vacancies in ppm are calculated in a
known standard manner by integrating the area of peaks from the absorption spectrum of
the diamond material, and using published coefficients for comparison to calculate
concentration. The coefficients that are used for the calculations of concentrations of
vacancies in the present specification are those set out by G. Davies in Physica B, 273-
274 (1999), 15-23, as detailed in Table A below.
Table A
Defect Calibration
'V Ar-D1 = (4.8 ± 0_2) X 10-1b(Vl
\f Ac.R1: (1 .2 ± 0.3) X·10-H'(V']
NV .Ar..tv = (1.4 i 0.35} X 10-1b{N- V]
In Table A, "A" is the integrated absorption (meV cm-1
) in the zero phonon line of the
transition, measured at 77 K, with the absorption coefficient in cm-1 and the photon
energy in meV. The concentration of the defect is in cm-3.
A preferred irradiation dose· of electrons corresponds to an electron fluenee of aflea·st
1 x 1015 electrons/cm2
, and/or preferably of at most 2 x 1017 electrons/cm2
, more
18
preferably to an electron fluence of at least 5 x 1015 and/or preferably of at most 4 x 1016
electrons/cm2 {where "electrons/cm2
" is sometimes abbreviated to ·e-/cm2
"). One skilled
in the art will realize that the required minimum and maximum dose will depend on the
starting characteristics of the diamond material. By simple way of illustration a diamond
sample containing a higher concentration of N and X may need a greater dose than a
sample containing a lower concentration. Typically a 4.5 MeV electron beam can be
used to provide the irradiation. It may provide a current in the range 0.5 rnA to 400 rnA
e.g. 20 rnA. It may be applied for a period of 1 0 seconds to 100 hours, e.g. about 2
minutes. As an example a 4.5 MeV electron beam with a current of 20 rnA applied for 2
minutes provides a dose of 3.2 x 1016 electrons/cm2
. It is preferred that the electrons are
sufficiently energetic to impart a substantially uniform distribution of damage through the
thickness of the diamond material. All doses/energies quoted in this specification are
based on this presumption, but in principal the invention can be enacted using lesser
energies, particularly a distribution of lesser energies.
We have found that the time for irradiation to achieve the desire electron fluence is
preferably in the range 10 seconds to 10 hours, more generally in the range 10 seconds
to 2 or 3 hours.
The irradiation treatment may be carried out at any suitable pressure, and is conveniently
carried out at or near atmospheric pressure.
As mentioned above the measurable difference in at least some of the absorption
characteristics of the treated (irradiated) diamond material in the said first and second
states is reduced relative to that of the provided diamond material in the said first and
second states by the method of the invention. What is typically desired is that the
variation in the absorption characteristics leading to variation in colour in the first and
second states is reduced by the controlled irradiation treatment. Other features of the
absorption spectrum, e.g. the absorption coefficient at 741 nm or 394 nm would be
expected to rise due to the irradiation.
In preferred embodiments according to the invention (i) the absorption spectrum of the
provided diamond material in one or both of its first and second states has (a) an
a_bsorption coefficient of at least 0.1 cm-1· at 270 nm, and one or both of (b) an absorption
· coefficient of-at least 0.05 cm-1 at 350 nm .and (c} an absorption· coefficient of at least-
0.02 cm-1 at 510 nm; (ii) the measurable difference in the absorption characteristics of
19
the provided diamond material in its first and second states Is a difference in the
absorption coefficient at one or both or 350 nm and 510 nm is at least 0.15 cm- 1
; and (iii)
the controlled irradiation treatment step reduces the said difference between the
absorption coefficients in the first and second states at one or both of 350 nm and
510 nm by at least 0.05 em-\ preferably by at least 0.1 cm-1, preferably at least
0.15 cm-1
• Thus in these embodiments precise measurement of the absorption
coefficients can be used as the measure to determine the colour stabilisation achieved
by the controlled irradiation.
In other embodiments according to the invention, the measurable difference in the
absorption characteristics of the provided diamond material in its first and second states
is a difference in the colour grade saturation value C* of the provided diamond material in
its first and second states of at least 0.5, and in some cases may be up to 1, 1.5, 2, 2.5,
3, 3.5, 4, 5, 6, 7 o:r 8.
C* is dependent on the size and geometry of the diamond material since it depends on
the path length through the diamond material. Where C* values are given in the present
specification they are based on a size and geometry for a diamond material that is a 0.5
carat (ct) round brilliant cut (RBC) stone. Where the diamond material used is actually a
different size and geometry from a 0.5 ct RBC stone, then the measured C* value is
adjusted. Therefore throughout this specification quoted c• value are those for an
equivalent 0.5 ct RBC stone.
The difference in colour saturation value C* is preferably reduced by 0.25, more
preferably by 0.5, even by 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7. or even 8 by the controlled
irradiation step. Preferably the treated diamond material has an absolute C* range
between its first and second states of at most 0.5, or at most 1. 1.5, 2, or 3.
Thus in these embodiments measurement of C* is used as the measure to determine the
absorption characteristic stabilisation. e.g. the colour stabilisation achieved by the
controlled irradiation.
In other embodiments according to the invention, the measurable difference in the
absorption characteristics of the provided diamond material: in its first and second states
is a difference 'Of at least· two colour grades. or even 3, 4,. 5; 6, 7 . .--or 8 colour grades, as.
measured on the GIA scale for diamond material.
20
As with C*, GIA scale measurements depend on path length, and as used in this
specification all quoted GIA grades are for diamond material made into an equivalent 0.5
ct round brilliant cut stone.
The stated difference in GIA colour grade is preferably reduced by the controlled
irradiation step by at least one grade, and in some cases by preferably at least two
grades, three grades, four grades five grades, six grades, seven grades, or even eight
grades, as measured on the GIA scale. Preferably the difference is colour grades
between the diamond material in the first and second states is reduced so that the
diamond material has the same colour grade in its first and second states, or has a
colour grade difference in its first and second states that differs by no more than one
grade, two grades, or three grades.
Thus in these embodiments optical analysis of the colour and grading on the GIA scale is
used as the measure to determine the colour stabilisation achieved by the controlled
irradiation.
Preferably the colour grade on the GIA scale of the irradiated diamond material in its
equilibrium condition (as hereinbefore defined) is the same colour grade or higher (i.e.
nearer to D) than the colour grade of the provided diamond material in its equilibrium
condition, higher grades generally being considered preferable for most applications.
Thus, in these cases the controlled irradiation can be seen not only to stabilise the colour
against changes due to exposure to radiation and temperature, but also to improve the
absolute colour of the diamond material. This is a significant difference to the prior art,
with irradiation resulting in a higher colour grade (lower C*) in the equilibrium condition,
in addition to the colour stabilisation effect. In contrast in the prior art irradiation (which is
not in the limited controlled manner of the present invention) results in more colour
(higher C*). More preferably the colour grade of the irradiated diamond material in its
equilibrium condition is at least one, preferably at least two, or even at least 3 or 4
grades higher than that of the provided diamond material in its equilibrium condition.
This preferred coLour improvement achieved by irradiation may also be defined in terms
of the change -in: C~~value of the diamond material. The preferred··colour improvement
achieved bydtradiatien in .terms·of the C* value ·of the ir:radfated diamond: material in its •:
equilibrium condition (adjusted to be that of an equivalent 0.5 ct RBC stone) is the same
21
as, or preferably at least 0.5, 1. 1.5 or even 2 lower than the C* value of the provided
diamond material in its equilibrium condition.
In preferred methods according to our invention the irradiated diamond material has an
absorption coefficient at 570 nm that is less than 0.04 cm-1
• Preferably the irradiated
diamond material has an absorption coefficient at 570 nm that is less than 0.02 cm-1
,
more preferably less than 0.01 cm-1, especially preferably less than 0.05 cm-1
• In
general measurable levels of noise in spectra are of the order of 0.05 cm-1
, so it is
difficult to measure peak absorption coefficients that are less than 0.05 cm-1
• The low
absorption coefficient at 570 nm is evidence that the irradiated diamond material has no,
or limited numbers of NV defects, as distinguished from the prior art referred to
hereinbefore which irradiates and anneals diamond material to introduce colour and
which has significant numbers of NV defects.
Methods according to the invention find particular application where the nitrogen
concentration in the diamond material is such that there is a measurable difference in the
absorption spectra of the provided diamond material after UV exposure and thermal
treatment, that is in situations where there is typically a colour change to stabilise. In
practice this is found for diamond samples which contain SSN concentrations >0.01
ppm, preferably >0.03 ppm, preferably >0.05 ppm, preferably >0.08 ppm, preferably
>0.1 0 ppm preferably >0.15 ppm preferably >0.20 ppm, preferably >0.30 ppm
preferably >0.40 ppm preferably >0.50 ppm preferably >0.8 ppm, preferably >1 ppm,
preferably >1 .5 ppm, preferably >2.0 ppm, preferably >3.0 ppm, preferably >4.0 ppm,
preferably >5.0 ppm, preferably >8.0 ppm, preferably >10 ppm, or preferably >20 ppm.
Methods according to the invention are preferably applied to diamond material not
dominated by high SSN. The methods are preferably applied to diamonds with SSN
concentrations <150 ppm, preferably <100 ppm, preferably <75 ppm, preferably <50
ppm.
A second aspect of the present invention provides diamond material which has an
absorption spectrum with one or both of the following characteristics:
Designation Peak Absorption coefficient (at peak)
V'! .. ' · ·. J " 741 nm 0.01 cm-1 - ·1 an:-1 (at ·77 K.) -· ~ -: , .. · .·
v- ~ ·: . .. ·394 nm . 0.01 cm-1
• I"' . - 0.5 em_, (at·1-7K)· ..... .
22
These characteristics are an indication that controlled irradiation of the diamond material,
as hereinbefore described with reference to the method according to the first aspect of
the invention, has taken place.
The diamond material preferably has both of the characteristics designated VO and v- set
out above.
Preferably the diamond material is a synthetic diamond material.
Preferably the synthetic diamond material is CVD diamond material and has an
absorption spectrum with the following additional characteristics
(i)
Designation Starts Ends Peak Absorption coefficient (at peak)
270nm 220nm 325nm 270 nm 0.05 cm-1
- 20 cm-1
and
(ii) one or more of
Designation Starts Ends Peak Absorption coefficient (at peak)
350 nm band 270 nm 450nm 350 nm 0.05 em -1
- 1 0 cm-1
±10 nm
510 nm band 420 nm 640 nm 510 nm 0.02 cm-1
- 10 em
±50 nm
Designation Form of Curve Absorption Coefficient
Ramp Rising background of form Contribution at 510 nm is:
Absorption coefficient (cm-1
) = < 1.5 cm-1
C x ;.-3 (C=constant, A in IJm)
Preferably the CVD synthetic diamond material has any two, or more preferably all three,
of the characteristics set out in (ii) above.
Preferred values of the absorption characteristics are shown in the table below.
23
Designation Starts Ends Peak Absorption coefficient (at peak)
270 nm 220nm 325nm 270nm 0.05 cm- 1
- 20 cm-1
,
preferably 0.1 cm-1
- 8 cm- 1
,
more preferably 0.2 cm-1
- 5 cm-1
350 nm band 270nm 450nm 350 nm 0.05 cm-1
- 10 cm-1
,
±10nm preferably 0.10 cm-1
- 5 em-more preferably 0.20 cm- 1
- 3 cm-1
510 nm band 420 nm 640 nm 510 nm 0.02 em_, - 10 cm- 1,
±50nm preferably 0.03 cm-1
- 4 cm-1
,
more preferably 0.1 cm-1 - 2 em_,
\jJ 741 nm 0.01 em _, - 1 cm-1
preferably 0.05 cm-1
- 0.5 cm-1
more preferably 0.1 cm-1
- 0.3 cm·1
(all at 77 K)
v- 394 nm 0.01 em_, -·o.s em_,
preferably 0.015 em_, - 0.4 cm-1
more preferably 0.02 cm-1
- 0.2 cm-1
(all at 77 K)
Designation Form of Curve Absorption Coefficient
Ramp Rising background of form Contribution at 510 nm is: < 1.5 cm-1,
Absorption coefficient (cm-1
) preferably < 1.0 cm-1
,
= C x A.-3 (C=constant, A. in more preferably < 0.5 em_,
~m)
Preferred diamond materials according to the invention may have one or more of the
preferred absorption coefficient characteristics, in any combination.
Where a range of preferred absorption coefficients are given, these are to be interpreted
to represent separate preferred upper and lower limits. For example for the 270 nm
designation band, the preferred absorption coefficient range from 0.15 cm-1 to 8 cm-1
represents a preferred minimum coefficient of 0.15 cm-1 and a preferred maximum
coefficient of 8 cm- 1•
The ·provided diamond material ·in the method according to the first aspect of the
invention, and the diamond material of the second aspect of the invention are preferably
24
single crystals. In certain embodiments according to the first and second aspects of the
invention, where the diamond material is a single crystal, the single crystal is in the form
of a gemstone. As an alternative the diamond material may be polycrystalline.
Polycrystalline diamond materials lead to light scatter in the visible part of the spectrum.
The invention is therefore likely to find most application for polycrystalline diamond
material when the stabilization that is required is related to properties other than optical
absorption, for instance dielectric loss and carrier mobility.
Embodiments and examples of the invention will now be described, by way of example,
with reference to the following figures, wherein:
Figure 1, which has been referred to above, shows a spectral decomposition of the
UV/visible absorption spectrum of an orangish brown CVD synthetic diamond layer,
Figure 2a shows a room temperature absorption spectrum for a CVD synthetic diamond
material according to Example 1, as-grown (curve A), after UV exposure (curve B), and
after thermal treatment at 525°C (curve C), but prior to the controlled irradiation
treatment of the method of the present invention;
Figure 2b shows a "difference" absorption spectrum for the CVD synthetic diamond
material according to Example 1 after exposure to ultraviolet radiation and thermal
treatment at 525°C after an electron irradiation dose according to a method of the
present invention (curve B) and in the as-grown provided state (curve A).
Figure 3 shows an absorption spectrum, measured at 77 K, of the grown CVD synthetic
diamond material of Example 1 after controlled irradiation treatment;
Figure 4 shows a room temperature absorption spectrum for a CVD synthetic diamond
sample according to Example 3, as grown (curve A), after UV exposure (curve B) and
after thermal tr~atment (curve C), but prior to the controlled irradiation treatment of the
method of the present invention;
Figure 5 shows the room temperature absorption difference spectra for CVD synthetic
diamond material according to examples 2 and 3, deduced by subtracting the absorption
·coefficient values as a function of wavelength measure after thermal treatment frori'f ·
those measured after UV exposure.
25
Figures 6a and 6b are optical micrographs showing example 2 after thermal treatment
and UV radiation exposure respectively;
Figures 6c and 6d are optical micrographs showing example 3 after thermal treatment
and UV exposure respectively; and
Figure 7 shows the absorption spectra measured at 77 K for examples 10-15 which have
been irradiated with different doses.
Example 1
A CVD diamond material sample was grown on an HPHT substrate in the manner
described in W02003/052177 as set out below.
HPHT diamond substrates suitable for synthesising single crystal CVD synthetic
diamond material of the invention were laser sawn, lapped into substrates, polished to
minimise subsurface defects such that the density of defects is below 5 x 103 /mm2
, and
generally is below 102 /mm2
• Polished HPHT plates 3.6 mm x 3.6 mm laterally by 500
1Jm thick, with all faces substantially {100} and having a surface roughness Ra (also
known as the root mean square roughness) of less than 1 nm on the surfaces where
homoepitaxial diamond growth will subsequently take place, were mounted on a
molybdenum disk, and introduced into a CVD synthetic diamond growing reactor. By
substantially {100} faces we means faces that are exactly {100} faces and also faces that
deviate from this by up to 1 o·.
Growth stages
1) The CVD synthetic diamond reactor was pre-fitted with point of use purifiers,
reducing unintentional contaminant species in the incoming gas stream to below
80 ppb.
2) An in situ oxygen plasma etch was performed using 50/40/3000 seem (standard
cubic centimetre per second) of 0 2/Ar/H2 and a substrate temperature of 76o·c.
3) This moved without interruption into a hydrogen etch with the removal of the 0 2
from the gas flow. .· ·
· ': A}· This moved. into the growth process, at a: suitable pressure,· by the addition of the :::
carbon source (in this case CH4) and dopant gases. In this instance was CH4
26
flowing at 165 seem and 0. 7 ppm N2 was present in the process gas mix. The
temperature at this stage was 875°C.
5) On completion of the growth period, the substrate was removed from the reactor
and the CVD synthetic diamond layer removed from the substrate by laser
sawing and mechanical polishing techniques.
This grown CVD synthetic diamond material is the "provided diamond material" defined
by the claims of the present specification.
The room temperature absorption spectrum of the grown sample was then measured
using a Perkin Elmer Lambda 19 spectrophotometer. The recorded spectrum is shown
as trace A on Figure 2a. The sample was subsequently exposed to 20 minutes UV
radiation from a deuterium lamp source set at 230 rnA and 76 V and the room
temperature absorption spectrum was then re-measured. The deuterium lamp source
was an EEO pic lamp source, ,catalogue number 37-4702, serial number 246/031. The
re-measured room temperature absorption spectrum is trace B on Figure 2a. The
sample was then removed from the spectrometer and heated in an Elite Thermal System
tube furnace at 798 K for 1 hour in the dark under vacuum, and the room temperature
optical absorption spectrum was retaken. This is shown as trace C on Figure 2a.
It will be seen from Figure 2a that the spectrum showed typical CVD characteristic
features at 350 nm and 510 nm as well as a broad band at 270 nm, the latter being
associated with single substitutional nitrogen. The absorption coefficients at each of
these wavelengths, as measured from the spectra, are shown in Table 1 below:
Table 1
270 nm 350 nm 510 nm
As grown = "provided 1.16cm_, 0.78 cm-1 0.30 cm-1
diamond material"
Post 20 minutes UV 1.46 cm·1 0.87 cm-1 0.34 em_,
exposure
Post 1 hour thermal 0.92 cm-1 0.62 em_, 0.18 em_,
treatment
· From measurements taken ·from the 270 nm peak· the ·sample· -wai-' found to have ' -
nominally 0.1 ppm neutral single substitution -nitrogen.
27
CIELAB C* values for the as grown (or "provided") CVD synthetic diamond material, the
UV exposed diamond material, and the thermally treated diamond material were derived
from traces A B and C respectively of the absorption spectra of Figure 2. The method for
obtaining these is well known, and is described for example in US patent application
2004/0194690. The derived C* values are shown in Table 2, alongside the equivalent
GIA scale colour grade letter.
Table 2
As grown Post UV Post thermal Change
exposure treatment
c· 3.72 4.37 2.97 1.4
GIA equivalent K L I 4 grade
colour grade (0.5 ct range
RBC stone)
From Figure 2a and the data shown in Table 2, it can be seen that the grown CVD
synthetic diamond material's absorption spectrum is not stable to exposure to these
different conditions (UV exposure then thermally treated). The change in c· is 1.4, and
in terms of the GIA colour grading system this change in C* (Table 2) is equivalent to a
range of 4 colour grades. The GIA grading of the CVD synthetic diamond material is K
post growth, L following the UV exposure and I following the thermal treatment (a colour
range of I, J, K, L i.e. 4 colour grades).
In a known manner an absorption spectrum was then recorded at 77 K (not shown) to
determine the upper limits of the concentration of vacancy (VO and v -) and nitrogen
vacancy defects ([N-V]0 and [N-V]-) in the diamond material. These concentrations were
determined in a known manner by integrating the areas of the peaks from the absorption
spectrum at the wavelengths known to be characteristic of these defects. The UV-Visible
spectra were taken at a scan speed of 60 nm/min, at 0.2 nm data intervals and the
baseline subtracted before the peaks areas were integrated. These upper limits are
shown in Table 3. As is known, it is usual to carry out this analysis of defect
concentrations associated with vacancy and nitrogen-vacancy defects at low
temperatures, · ·e.g. 77· K, since at higher temperatures the peak in the absorption
spectrum that is·associated with them defect is smoothed and therefore less visible.
28
Table 3
Absorption Peak Defect Model Defect concentration cm-3 [ppm]
741 nm y; <6x1014 [<0.004]
394 nm v- <2x1014 [<0.001]
637 nm [N-V]- <5x1014 [<0.003)
575 nm [N-V]u <5x1014 [<0.003]
The sample was then exposed to daylight for six hours. Daylight exposes the diamond
material to radiation with an energy greater than 2.0 eV. The absorption spectrum was
re-measured and was characterized by a c· value of 3.50 with an equivalent GIA colour
grade of K. Further exposure to daylight was shown not to significantly alter the
properties of the diamond. After a second UV treatment similar to the first the absorption
spectrum returned to that shown in Figure 2a (curve B). The spectrum was re-measured
after a period of 24 hours and did not change from that shown in Figure 2a (curve B).
Following a second thermal treatment at 798 K the spectrum was re-measured and
found to be identical to that in Figure 2a (curve C). This cycling between the extreme
colour grades following thermal treatment/UV radiation procedure was repeated three
further times. Each time the absorption properties were characterized by the same
characteristics as the first time. This indicates that even after repeated cycling of UV
exposure and thermally treating the sample's absorption properties in each state are
consistent but that they are not stable.
The sample, uncoated, un-mounted and clean, was then subjected to a controlled
irradiation treatment according to the method of the invention. To do this the sample was
treated using 4.5 MeV electrons at a beam current of 20 rnA for 2 minutes,
corresponding to an approximate dose of 3.2 x 1016 electrons/cm2
.
Following this treatment the UVNisible spectra of the sample in the irradiated form, after
subsequent exposure to UV radiation and after subsequent heating were plotted (not
shown) and CIELAB C* values were derived from the absorption spectra. The derived
C* values are shown in Table 4.1:
29
Table 4.1
Post Post electron Post electron Change
electron irradiation and irradiation
irradiation UVexposure and thermal
treatment
C* 3.99 3.96 3.32 0.67
GIA equivalent colour K K J 2 Grade
grade (0.5 ct RBC range
stone)
Comparing the data in Table 2 and Table 4, it is clear that the impact of the electron
irradiation has led to a significant reduction in the variation in the sample's absorption
properties compared with the pre-treated sample.
Table 4.2 below shows the reduction in change of absorption between the two states
following the addition of a short irradiation. In some parts of the spectrum the absorption
can increase, but the difference is reduced and the colour in terms of C* and GIA is
improved because of the flattening of the spectrum. In table 4.2 the measured
absorption coefficients post the electron irradiation are shown. When these are
compared with those in Table 1 it can be seen that the difference between absorption
coefficients measured after UV exposure and heat treatment reduces after irradiation as
follows:, at 350 nm from 0.25 cm-1 to 0.13 cm-1
, at 510 nm from 0.16 cm-1 to 0.01 cm-1
•
Table 4.2
270 nm 350 nm 510 nm
Post 2 minutes 1.74 1.19 0.39
electron irradiation
Post 20 minutes 1.70 1.13 0.36
UVexposure
Post 1 hour 0.95 1.26 0.37
thermal treatment
Thus it can be seen from the comparison of the results in Table 4.2 and Table 1 that the
· difference in absorption coeffiCient after exposure to ultraviolet radiation and therl' subject
to thermal treatment is reduced by the step of applying a controlled irradiation to the·
30
diamond material. This reduction is similarly illustrated in Figure 2b which is an
absorption difference spectrum which plots the difference in the absorption coefficient at
any given wavelength calculated by subtracting the absorption coefficient after thermal
treatment from the absorption coefficient after exposure to UV radiation. In Figure 2b
curve B shows the absorption difference spectrum for the diamond material after the
controlled irradiation treatment, and curve A shows the absorption difference spectrum of
the as-grown provided diamond material.
In a known manner, as described earlier, a UV/visible spectrum was then taken at 77 K
of the irradiated sample to investigate defect concentrations within the sample. The
spectrum is shown in Figure 3 and shows that in addition to improving the sample's
absorption stability, a number of features are present Which are characteristic of the
electron irradiation treatment. Specifically these include the vacancy related absorptions
at 394 nm (ND1) and 741 nm (GR1} corresponding to the negative and neutral charge
states respectively of the single vacancy in diamond material. From these absorptions
the concentration of characteristic irradiation damage defects was derived and these are
shown in Table 5. These derivation techniques are known in the art and described for
example in G. Davies, Physica B, 273-274 (1999), 15-23.
Table 5
Absorption Peak Defect Model Peak Height (em_,} Defect concentration
cm-3 [ppm]
741 nm \fl 0.21 5.13 X 101:> (0.029]
394 nm v- 0.05 2.02 X 10':> (0.012]
637 nm IN-Vf 0 <5x1014 [<0.003]
575 nm [N-V)0 0 <5x1014 [<0.003]
Further photoluminescence recorded at 77 K using 458 nm excitation showed features at
533 and 467 nm which are thought to be unique and characteristic of CVD synthetic
diamond material. In addition, to more usual irradiation damage features (for example
TR12 at 471 .2 nm} features at 512.6 nm, 526.4 nm and 486.2 nm with Raman
normalized intensities of 0.095, 0.001 and 0.01 respectively were also present post the
electron irradiation but not prior.
31
Examples 2 and 3
Example 2 and 3 were grown using the method of Example 1 with the exception the
concentrations of nitrogen (measured as N2 equivalent) in the gas phase were increased
to 7 ppm and 11 ppm respectively. These samples were processed into polished single
crystal CVD plates with dimensions 3.4 mm x 3.5 mm x 2.2 mm (Example 2) and 3.7 mm
x 3.6 mm x 1.1 mm (Example 3).
Room-temperature UV/visible optical absorption data between 200 nm and 800 nm for
Example 3 in its initial grown provided state (A, solid line), following heating up to 798 K
(8, dashed line) and following exposure to ultraviolet radiation for a duration of 40
minutes (C, dotted line) is shown in Figure 4.
Using the optical absorption spectra for the grown ("provided") diamond but unirradiated
sample of Example 3, following ultraviolet illumination and following heating,
the concentration of N5 ° centres were derived using the 270 nm absorption peak and the
absorption coefficients of the 350 nm and 510 nm absorption bands were noted. These
are shown in Table 6:
Table 6
Before Following UV Following heating
treatment irradiation
Ns u [270 nm band] 0.50 0.30 0.65
(ppm)
350 nm band (em _,) 0.4 0.2 0.5
510 nm band (em _,) 0.3 0.1 0.3
UVNisible/NIR (near-infrared) absorption difference spectra were then derived for
Example 2 and Example 3. These are shown in figure 5, deduced by subtracting the
absorption coefficient values as a function of wavelength measured after ultraviolet
illumination (40 minutes duration) with those values measured after heating (to 798 K).
More positive values indicate a feature has increased in strength after illumination, and
more negative values indicate that it has increased after heating. From these the
changes in at;>sorption coefficient at 270 nm, 350 nm c:md 510 nm after the UV ·
exposure/thermal treatment are evident.· In Figure 5. the "sample 1" curve corresponds
to Example 2, and the "sample 2" curve corresponds to Example 3.
32
Optical micrographs were taken of Example 2 and are shown in Figures 6a and 6b.
Figure 6a shows the example after heating to 798 K, Figure 6b shows the example
following 40 minutes of ultraviolet irradiation, Figure 6c shows Example 3 after heating to
823 K and Figure 6d shows example 3 following 40 minutes of ultraviolet irradiation. All
micrographs were taken at room temperature using a transmission microscope. From
the micrographs it can be seen that UV exposure tends to intensify the colour of the
samples, whereas thermal treatment tends to de-intensify the colour of the samples. In
this specification the micrographs are shown in greyscale. In fact the true colours are
clear (Figure 6a), light pink (Figure 6b), light brown (Figure 6c) and dark brown (Figure
6d).
c• (scaled to give values for thickness equal to the depth of a 0.5 ct round brilliant cut
stone) values were calculated from the absorption spectrum shown in Figure 5 for
Example 3 and are shown in Table 7 below.
Table 7
As grown Post UV exposure Post heating Change
C"' 4.85 5.43 1.64 3.79
As with Example 1, this example shows a clear change in C* when exposed to UV and
then thermally treated. Similarly to Example 1, it was found that the absorption spectrum
could be repeatedly driven between the two extremes on UV exposure/thermal treatment
indefinitely clearly indicating colour instability.
Extrapolating from our experimental testing on Example 1, it is believed that the diamond
materials of Examples 2 and 3 when electron irradiated to a total dose of 4 x 1016 cm-2
using similar conditions to that for Example 1 would result in a reduction in the change in
C* of the diamond materials when measured in their first and second states of at least
20%. That is we would predict that the C* value change in its first and second state
would reduce by about 0.5 after the irradiation.
Examples 4-7
Four more CVD synthetic diamond samples with the same nitrogen content as Example
1, and grown and prepared into plates in identical fashion to Example 1 were subjected
33
to UV radiation/thermal treatment in the same manner as for Example 1, in order to
ascertain the repeatability of the method of the invention. In all four examples, the colour
grade was again shown to change by 3-4 colour grades when measured after exposure
to UV radiation and then after the thermal treatment. These CVD synthetic diamond
samples were subsequently electron irradiated in the same manner as the sample of
Example 1. The results after electron irradiation are summarized in Table 8. The C*
values and colour grades are derived from the measured absorption spectra according to
the method described above.
Table 8
Example After 20 minutes After 1 hour thermal Colour Grade range
UV exposure - C* treatment at 525°C - C*
(GIA colour grade) (GIA colour grade)
Ex4 3.83 (K) 3.06 (J) 2 grades
Ex5 3.02 (J) 3.00 (J) 1 grade (i.e. no change)
Ex6 2.92 (I) 2.55 (I) 1 grade (i.e. no change)
Ex7 1.96 (G) 1.96 (G) 1 grade (i.e. no change)
From table 8 it can be seen that irradiation causes the colour variation between the two
extremes (after UV exposure and thermal treatment) to reduce to less than a grade (on
the GIA scale). corresponding to a change in C* of less than 0.8. The examples
therefore show stabilisation of colour after irradiation similar to that illustrated for
Example 1. C* values and colour grades are derived from the absorption spectra using
the methods described hereinbefore.
Example 8 (comparative)
A CVD sample prepared with low nitrogen concentration was prepared by a manner
similar to that described for Example 1, but with nitrogen concentration in the gas phase
that was nominally 92 ppb giving rise to a concentration of 0.01 ppm in the solid. The
modelled c• and GIA grades for a 0.5 ct round brilliant produced from the sample after
UV exposure and thermal treatment are shown in Table 9 below.
34
Table 9
Example Starting colour Colour after 20 Colour after 1 Colour grade
C* (GIA grade) minutes UV C* hour at 525°C range
(GIA grade) C*(GIA grade)
8 0.59 (E) 0.57 (E) 0.72 (E) 1 grade (i.e. no
change)
The results for the comparative Example 8 show that when the Ns 0 and X concentrations
are low enough the colour change affect is not seen on exposure to UV
irradiation/thermal treatment. This is a comparative example since there is not
measurable difference in the absorption characteristics in the first and second states
(after UV exposure and after thermal treatment). While there are slight differences in the
c· values these are too small to be significant. There is no change in the GIA colour
grades in the first and second states.
This sample was not irradiated since it is a comparative example and there was no
colour change to stabilise.
Examples 9-15
CVD grown synthetic diamond samples of the same composition and grown in the same
manner as the sample of Example 1 were irradiated for different doses as shown in
Table 10. Figure 7 shows the absorption spectra for each example, with curves A , B, C,
D, E corresponding to Examples 10, 11, 12, 13, 14, 15 respectively.
35
Table 10
Example Dose 4.5 MeV, \jJ v- Comment
Number 20 rnA (e-/cm2
) concentration concentration
(ppm) (ppm)
9 1.3x 101
:. 0.0027 0.0005 No visible colour
10 8.2 X 101
" 0.0072 0.003 No visible colour
11 3.7 X 1010 0.029 0.012 No visible colour
12 6.5 X 10"' 0.050 0.014 No visible colour
13 1.3x1017 0.10 0.022 No visible colour
14 2.6 X 1011 0.17 0.033 Pale blue - dose too
high
15 1.95 X 10111 0.85 0.20 Vivid blue - green
From table 10 and Figure 7 it can be seen that while a dose as small as 1.3 x 1015
electronslcm2 is enough to stabilize the colour, if the dose is too high (e.g. when it was
2.6 x 1017 electrons/cm2
) the concentration of irradiation damage defects is sufficient to
introduce its own added absorption features. These absorption features are evident as a
blue colour (see table 10) , and as the ND1 and GR1 peaks on the absorption spectra
(Figure 7) which are undesirable in near-colourless faceted stones for jewellery or in
material tailored for a high power laser application requiring low absorption coefficient.
For easy comparison, the characteristics of examples that have different compositions or
have been irradiated different amount (1, 3, 8 (comparative), and 9-15) are set out in
Table 11 below.
Table 11
Example I State
provided diamond material
provided and post UV
provided and post thermal
treatment 798K for 1 hour
Difference in absorption
characteristic of provided
diamond material after UV
and thermal treatment
Post irradiation
Post irradiation and UV
Post irradiation and thermal
treatment
Difference in absorption
characteristic of irradiated
diamond material after UV
/thermal treatment
~
-~
.!!l
~
."t::'.
(.)
"&'
"' ~ Q. ·=
& Q. .....
c:i
z
Colour Absorption
apparent coefficient of
to eye broad 270 nm
peak indicative
ofN:. em·•
S"'l
'C ~"
"8l'
'C
5
'S
1.16
1.46
0.92
1.46-0.92 = 0.54
1.74
1.70
0.95
1.70-0 .95 = 0.75
Proportion N;"
calculated from
270nm peak,
ppm
0.1
j
8 E
"" c e- ~ ~ ~
0.78
0.87
0.62
0.87-
0.62 =
0.25
1.19
1.13
1.26
1.26-
1.13 "
0.13
'E i
..8 ,. cE e- 0
~ ~
0.30
0.34
0.18
0.34-
0.18"
0.16
c·
3.72
4.37
2.97
4.37-
2.97
=1 .4
0.39 I 3.99
0.36 I 3.96
0.37 I 3.32
o.37- 1 o.67
0.36"
O.D1
GIA
grade
K
L
t toL =
4
gradeS
K
K
J
J to K
=2
gradeS
1/'
cm·3
<5x
1014
5.13 X
10'5
V'
cm·3
<5x
1014
2.02x
10'5
NV"
cm-3
<5x
1014
<5 X
1014
NV'
cm-3
<5x
1014
<5x
10'"
w
0>
Example I State
3
3
3
3
Provided diamond material
provided and post UV
provided and post thermal
treatment 798 K for 1 hour
Difference in absorption
characteristic of provided
diamond material after UV
and after thennal treatment
.~
.!!!
~
~
r".' u
.s - "t/)' N C: 0) z .S! If)
E "' g) 8: ·~ 8
... i a
Colour Absorption
apparent I coefficient of
to eye broad 270 nm
peak indicative
of N.O. em·•
Dark
brown
Not measured
3 Post irradiation Light
3 Post irradiation and UV brown
3 Post irradiation and thermal
treatment
3 Difference in absorption
characteristic or irradiated
diamond material after UV
and thennal treatment
(predicted value}
Proportion N,
calculated from
270 nmpeak,
ppm
0.5
0.3
0.65
Not measured
c Qj
~ "' Qj §
8 .!0 g E
"" c e- ~
~ ..,
~ 1ii
0.4
0.2
0.5
0.5-
0.2 =
0.3
c
.'!!
~ ~ 8 c:
c: ·e g c:
~* ~ 1ii
0.3
0.1
0.3
0.3-
0.1 =
0.2
c·
4.85
5.43
1.64
3.79
Not
me as
ured
0.75
GIA
grade
v
cm.J
Not measured
v
cm'3
NV'
cm·3
NV
cm·3
w....... ,
Example State Colour Absorption Proportion N.•
apparent coefficient of broad calculated from
~ to eye 270 nmpeak 270 nm peak
·c: indicative of N." ppm
0
(!! em·•
~
u
a· Provided 92 ppb N2 colour1ess Not measured 0.01
8• Provided post UV equivalent in
8• Provided post thermal process gas
treatment
8" Difference In absorption
Characteristic after
UV/thermal treatment
8". Irradiated sample Not tested - no colour change to stabilise
9 As example 1, but 1.3x10'" Not measured
10 irradiated 4.5 MeV, 20 8.2 X 10"
11 mA for dose specified in 3.7 X 10 ""'' Characteristics column ~
12 6.5x10'" " ce·/cm2
)
0
13 1.3 X 10 -g
14 2.6 X 10" Pale blue
15 1.95 X 10 Vivid
bluelgreen
. Example 8 is a comparative example
Absor Absor c· GIA
ption ption grade
co effie coeffic
ientat ientat
350 510
nm nm
Not measured 0.59 E
0.57 E
0.72 E
0.72- No
0.57= chang
0.05 e
I/' vppm
ppm
Not measured
0.0027 0.0005
0.0072 0.003
0.029 0.12
0.050 0.14
0.1 0.022
0.17 0.033
0.85 0.2
NV' NV'
cm-3 cm·3
Not measured
w
00

39
Claims
1. A method comprising:
a) providing a nitrogen-containing diamond material which shows a measurable
difference in at least one of its absorption characteristics in first and second states,
the first state being after exposure to radiation having an energy of at least 5.5 eV,
and the second state being after thermal treatment at 798 K (525°C),
b) treating the said nitrogen containing diamond material by controlled irradiation of
the said nitrogen-containing diamond material so as to introduce sufficient defects
in the diamond material so as to produce one or both of:
(i) an absorption coefficient measured at 77 K of at least 0.01 cm·1 and at most
1 cm·1 at a wavelength of 7 41 nm; and
(ii) an absorption coefficient measured at 77 K of at least 0.01 cm·1 and at most
0.5 cm·1 at a wavelength of 394 nm;
whereby the measurable difference in the said absorption characteristics of the
irradiation treated diamond material in the said first and second states, having been
exposed to the same radiation and thermal treatment as the provided diamond, is
reduced relative to the measurable difference in the said absorption characteristics
of the provided diamond material in the said first and second states.
2. A method according to claim 1, wherein the · measurable difference in the
absorption characteristics of the treated diamond material in its first and second
state, and the reduction in said measurable difference after the irradiation treatment
are visible colour changes.
3. A method according to claim 1 or 2, wherein:
(a} the absorption spectrum of the provided diamond material in one or both of its
first and second states has (i) an absorption coefficient of at least 0.05 cm-1 at 270
nm, and one or both of (ii) an absorption coefficient of at least 0.05 cm-1 at 350 nm
and (iii) an absorption coefficient of at least 0.02 cm-1 at 510 nm;
40
(b) the measurable difference in the absorption characteristics of the provided
diamond material in its first and second states is a difference in the absorption
coefficient at one or both of 350 nm and 510 nm is at least 0.15 cm-1
; and
(c) the controlled irradiation treatment step reduces the said difference between the
absorption coefficients in the first and second states at one or both of 350 nm and
510 nm by at least 0.05 cm-1
•
4. A method according to any preceding claim, wherein the measurable difference in
the absorption characteristics of the provided diamond material in its first and
second states is a difference in the colour grade saturation value C* of the provided
diamond material in its first and second states of at least 1, which difference in
colour saturation value C* is reduced by at least 0.5 by the controlled irradiation
step.
5. A method according to any preceding claim, wherein the measurable difference in
the absorption characteristics of the provided diamond material in its first and
second states is a difference of at least two colour grades as measured on the GIA
scale in the form of an equivalent 0.5 ct RBC stone, which difference in colour
grade is reduced by at least one grade as measured on the GIA scale by the
controlled irradiation step.
6. A method according to claim 5, wherein the colour grade of the irradiated diamond
material in its equilibrium condition is the same colour grade or higher than the
colour grade of the provided diamond material in its equilibrium condition.
7. A method according to claim 4, wherein the C* value of the irradiated diamond
material in its equilibrium condition is numerically lower than the C* value of the
provided diamond material in its equilibrium condition.
8. A method according to any preceding claim, wherein the irradiated diamond
material has an absorption coefficient at 570 nm that is less than 0.01 cm-1
.
9. A method according to any preceding claim, wherein the irradiation is provided by
one· or more of electrons, neutrons or gamma photons.
41
10. A method according to claim 8, wherein the irradiation provides an electron fluence
in the range 1 x 1015 to 2 x 1017 electrons/cm2
•
11. A method according to any preceding claim, wherein the provided diamond material
has been made by a CVD synthesis process.
12. A method according to any preceding claim wherein the diamond material is a
single crystal.
13. A method according to claim 12, wherein the single crystal is in the form of a
gemstone.
14. Diamond material which has an absorption spectrum with one or both of the
following characteristics:
Designation Peak Absorption coefficient (at peak)
'<._/) 741 nm 0.01 em_, - 1 cm-1 (at 77 K)
v- 394 nm 0.01 em_, - 0.5 em_, (at 77 K)
15. Diamond material according to claim 14, which is a synthetic diamond.
16. Diamond material according to claim 15, which is CVD diamond material and which
has an absorption spectrum with the following additional characteristics:
(i)
Designation Starts Ends Peak Absorption coefficient (at peak)
270 nm 220 nm 325 nm 270nm 0.05 cm-1
- 20 em ·1
and
42
(ii) one or more of
Designation Starts Ends Peak Absorption coefficient (at peak)
350 nm band 270 nm 450 nm 350nm 0.05 em- -10 em
±10 nm
510 nm band 420 nm 640 nm 510 nm 0.02 cm-1
- 10 cm-1
±50 nm
Designation Form of Curve Absorption Coefficient
Ramp Rising background of form Contribution at 510 nm is:
Absorption coefficient (cm-1
) = < 1.5 cm-1
C x "A-3 (C=constant, "A in 1Jm)
17. Diamond material according to any of claims 14 to 16, which has an absorption
coefficient at 570 nm that is less than 0.04 cm-1
•
18. Diamond material according to any of claims 14 to 17, which has a
photoluminescence spectrum in its equilibrium state showing one or more of the
following features:
Wavelength (nm) Raman normalized intensity
512.6 0.095
526.4 0.001
486.2 0.01
19. Diamond material according to any of claims 14 to 18, which is a single crystal.
20. Diamond material according to claim 19, wherein the single crystal is in the form
of a gemstone.
21. Diamond material that has been made by a method according to any one of claims
1 to 13.
Dated this 19th day of December 2011 ofu~~Advocates Agents for the Applicant