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13Electron-microscope study of synthetic diamonds formed under mild
conditions
1*1 2 I.V. Fedoseyev , N.S. Gerasimova ([email protected]) , V.I.Petrov , M.A.
Stepovich1
(1) Moscow Bauman State Technical University, Kaluga Branch (2) Moscow Lomonosov State University
It is known that in the graphite-diamond system thermodynamically resistant bands of the latter have very high temperatures and high pressure. Therefore the diamond synthesis using graphite is performed at t = 1700 - 2000 0C and p « 5-109 Pa.
At the same time the methods of diamond synthesis in the area of its instability by means of the gas-phase decomposition of the carbon containing compounds, e.g. CH4 [1], are also known.
Here we consider the possibility of using carbon monoxide as carbon containing substance for diamond synthesis.
The calculations show that CO molecules must decompose under normal conditions according to reaction:
2CO ^ C + CO2 (1)
since AG0 for (1) equals - 117,3 kJ if diamond formation takes place. But high strength of the carbon-oxygen bond and therefore high activation energy make the process (1) forbidden at low temperatures.
CO molecules activity increases sharply when they enter carbonyl complexes which is indicated by considerable lowering of the frequencies of vCo in IR-spectra of these compounds [2-5].
It is known that complex formation results in rapprochement of the reagents and this, if they are properly oriented, may increase the rate of reaction between the molecules-ligands up to 106 times.
Thus the conditions for actual proceeding of the thermodynamically allowed reactions for free molecules are created but they cannot be realized because of the kinetic restrictions.
Thus reaction (1) may proceed even at 250 0C as a result of osmium car-bonyl cluster thermolysis [6]:
[Os8(CO)23] ^ [Os8(CO)21C] + CO2 (2)
In reaction (2) formation of carbide carbon may only result from the dis-proportionation of coordinated CO molecules according to (1).
Reaction (2) indicates that with the increase of the cluster size steric destabilizing interaction between the legands also increases.
It is known that when CH4 is synthesized from CO + H2 involving supported metal catalysts, e.g. Ni/Al2O3, the first stage of the synthesis will be dissociation of a chemisorped CO molecule:
CO ^ C + O
Therefore, most of CO molecules on the catalysts surfaces are either in dissociated form or close to it [7].
Low frequency of vCO in carbonyl clusters indicates according to [5] the formation of the particles containing C- and O-bound carbonyls. This kind of coordination first of all occurs in binuclear complexes having metal bonds, which can be presented as follows: M - C
I / I
M' - O
Such structures may be considered molecular analogues of CO molecules which are chemisorped on the metals surfaces in the state preceding their dissociation.
CO coordinated molecules as the source of carbon for diamond synthesis were first used in the URSS. Vapours of Re2(CO)io at t=920 0C and pressure of 1.3 Pa were subjected to thermolysis in a special reactor which resulted in the growth of the diamond crystals by 0.5 - 1 mg for the period of 10 hours.
Thus the possibility of coordinated CO molecules disproportionation according to (1) with the diamond formation was shown.
While studying the properties of different homo- and heteronuclear clusters of platinum metals it was found that in the process of their spontaneous decomposition some of them form diamonds - both "grainlike" and "threadlike" [9].
For example, this is characteristic of heteronuclear platinum-palladium carbonyl clusters having general composition of PtxPdy(CO)Z. When their decomposition occurs, reaction (1) may be presented as follows:
PtxPdy(CO)z ^ xPt + yPd + (z-n)C + S (CO2, CO)n (3)
Formation of a new system of the generalized orbitals in the clusters and their high energy predetermine occurrence of the metastable structures in them which allows reaction (3), and formation of diamondlike carbon seems to depend on the properties of the carbonyl cluster frame structure, which, for example, includes carbon-carbon bonds.
It is know that some of synthetic diamonds are semiconductors, boron doping improving their electrophysical characteristics. As a dopant boron may be introduced at the synthesis stage of the platinum metals carbonyl complexes, there-
fore there is a good reason to believe that diamonds produced by means of decomposition of these compounds will show semiconducting properties.
Semiconductors in the state of excitation may produce electromagnetic radiation, called luminescence. Such state of the substance can be obtained in different ways. In the case of photoluminescence (FL) the radiation is caused by the absorbed light energy. Luminescence resulting from bombardment of the sample by electrons is refferred to as cathodoluminescence (CL).
All impurity defects have additional luminescent centers, some of them being derivatives of the basic defects. When diamonds are X-ray excited only a wide A-band with the maximum of about 420 nm (in Ila type crystals), 470 nm (Ia), 520-550 nm (Ib) can be observed. The same A-band is present in CL with superposition of the narrow lines system belonging to N3, H3, H4, 575 nm, GR1 and other defects caused by radiation.
Certain combinations of FL centers with impurity defects make in possible to identify impurity defects by means of luminescence.
Thus, for example, in [10] it was mentioned that boron doped diamond layers in IIa type diamonds showed ultraviolet CL. It was found that acceptors activation after their proper annealing results in formation of 2 UV luminescence bands of about 3.5 and 4.6 eV. The same bands were previously observed in diamonds of IIb type, including synthetic ones.
Diamond samples produced according to equation (3) were studied using ISM - 50 A electron microscope which had special attachment for making a local analysis of CL radiation.
In the course of the experiments with scanning electron microscope the sample was placed into the focus of a parabolic mirror through which CL radiation got to the outlet slot of the MDR-12 monochromator and was recorded there by the secondary emission photocell. Acceleration voltage and the probe current in the
o
microscope were 25 kV and 10 A correspondingly.
The data obtained were computer processed using determinated method of the data processing [11]. As a result CL spectra have been obtained (see fig. 1 and 2).
CL-spectra of the studied samples were in the range of 340 to 430 ^ 590 nm (for different samples). Intensive radiation was observed in the range of about 340 ^ 380 nm. The characteristic feature of some samples was one band of intensive radiation having the maximum of about 362 nm and half width of about 22 nm, others had 2 bands, the first of them having the maximum of 361 nm and the second me that of about 378 nm, its intensivity being 2 times less than that of the first one.
Comparison of the spectra obtained by us with those of synthetic diamond CL (see fig. 3) [12] gives every reason to assume that they are identical in their characteristics. One can observe a characteristic band with a strongly pronounced maximum.
Proceeding from our study of the spectra character we may assume that the diamond samples produced correspond to Ia type diamonds containing nitrogen
impurity with different types of nitrogen entering the crystal lattice, having A, B1, B2 type defects according to the conventional classification.
Electronic journal "INVESTIGATED IN RUSSIA" 935 http://zhurnal.ape.relarn.ru/articles/2004/084e.pdf
Experimental results of the CL produced samples study
Ê 0
V)
c
c 0
0 0 -0
20 00 80 60 40 20 00
3
20
Af
?;
»
Xv •• •
kw* ''
■00» 350 400 450 500 550 600 •
wave length
• Initial data
The spectrum of the CL studied samples obtaned as a result of initial experimental data processing
¡¡^
w c a
450 500
wave length —Smoothed
1
A"!/
.a-.,
0,00
13001
; ;, I i _ 1 ! . J. , ,1 '
^ 350
400
450
500
<$) O oO 550
600
o Initial data
—Smoothed......Error
Fig. 1.
Experimental results of the CL produced samples study
Ê 0
m
c
1= 0
,00 ,80 ,60 ,40 ,20 ,00
3
,20 ,40
• •
•• M
' •• « V
0» *340
360
380
400
420
440
wave length • Initial data
The spectrum of the CL studied samples obtaned as a result of initial experimental data processing
0,70 0,60 0,50 0,40 0,30 0,20 0,10 0,00 -0,10
320
380
wave length -Smoothed
440
-0,40
o Initial data
■Smoothed......Error
Fig 2
Typical Cl spectra for diamond polycrystal films having low (1) and relatively high (2) content of nondiamond carbon phase for nanocrystal diamond films (3) and graphitelike carbon films (4). All the spectra were obtained at the room temperature and electron accelerating voltage of 30 kV.
O
200 300 400 500 600 700 800 X
Fig.3.
Literature
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11. Stepanov S.E. The Development optimum on order of capacities of method instrumentation and data processing // Auto.ref. Obninsk. OIAT. - 2000.
12. The Samples А.Н., Wolfs А.П., Pavlovski I.Y. // Letters in GITF. - 1998. - т. 68. С.55.
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