Apparently it is polycrystalline. Too many refraction planes produce diffuse scattering. Grind the glass to powder and you'll have the same opaqueness

Apparently it is polycrystalline. Too many refraction planes produce diffuse scattering. Grind the glass to powder and you'll have the same opaqueness

It seems that you discuss the optical properties of polycrystalline diaomond and not of single crystals. For polycrystalline (and also micro- and nanocrystalline) diamond light scattering due to the small diamond grains in dimensions comparable to the wavelength is responsible for the optical appearance of the diamond films. Additionally, absorption caused by the grain boundary material will change the transparency. Of course, defects and impurities will change also the optical properties of diamond e.g nitrogen at substitutional lattice sites will give yellow diamond or boron (blue up to black). But usually for polycrystalline diamond light scattering at the grains and the grain boundaries are the main reason for the observed optical properties.

Hi there,

              Igor Goliney has explained very well. The size of the material matter much and if the grains are of nano size than their property as like band gap is different from the bulk. Your grains from CVD method are of nano-size and surface exposed (planes) too differ from bulk material. 

actually if it's dark it seems to me that these aren't grains specially if you say pcCVD diamond, depends what type of CVD diamond it is; size,what thickness, polished ?

intrinsic NCD samples if thin 10^22 are black..... intrinsic (very little atomic impurities) pcCVD not polished are opaque but not dark (rather 'milky), polished become transparent.... dark colour will be mainly associated with impurities including the grain boundaries - but black colour will be only for thick >>1 micron NCD samples

Can you tell us on what substrate (Si?, C? ) did you deposit your CVD diamond layer ? How thick ?  

To add to previous comments, you may see darkness  (even if homoepitaxial) due to the diamond pyramid structure on surface. 

I'm afraid is not an easy question... it depends a lot in the growing conditions of your PC diamond. As others told you the different planes of the grains creates some dispersion which accounts for the slightly "grey" coloration,  however in good PCD is almost negligible. If your diamond is dark grey it could be related to different factors like the dislocation density, the amount of extended defects (vacancies decorated with H atoms) and the impurities in the lattice. On the other hand the CH4 is not only related to the Sp2 but to the internal grain structure of the PCD; higher concentration of CH4 encreases the speed of growing but the grain structure consist in very small grains while concentrations of CH4 below 0.5% results in much bigger grains but at small growing rates... as a thump rule small grains means more defects and thus more light scattering. In short, you can have very transparent PCD, but there are a lot of factors which can increase the light scattering... Therefore obtaining transparent PCD is quite challenging because you should have under control all the growing parameters which can induce these defects in the diamond. 

Hey Ali BenMoussa,

Substrate is Silicon and polycrystalline diamond samples with thickness about 3 micra are composed by pyramidal grains.

Thank you for all for such good answers.

If its 3µm on silicon its scatter. It doesn't matter how pure your diamond is, it would be opaque at that thickness without polishing. This is due to the columnar growth regime, your roughness is huge at 3µm. Of course due to the small grains there is also some adsorption from non diamond carbon at the grain boundaries but this is a lesser effect, you will see a mirror finish if you polish it.

It is opaque because light can not transmit through the CVD grown diamond. Now if you ask why visible light can not pass through, although diamond is transparent, I would say that CVD growth process leads to many defects inside the lab-grown diamond. Depending on the density of defects, their nature (twining, dislocation, stacking fault etc.), the CVD diamond can be classified into from black to gray to white varieties. Black CVD diamond is heavily defective (growth defects, presence of graphite etc.), whereas white diamond has considerably less number of defects, which on polishing (reduces surface scattering of light) becomes transparent. So in general polishing of CVD diamond can transform them from opaque to transparent in visible light.

Please refer to:

1. J. X. Yang, C. M. Li, F. X. Lu, G. C. Chen, W. Z. Tang, Y. M. Tong, Microstructure and fracture strength of different grades of freestanding diamond films deposited by a DC Arc Plasma Jet process, Surface & Coatings Technology 192 (2005) 171– 176. doi:10.1016/j.surfcoat.2004.04.089

2. J. X. Yang, X. F. Duan, F. X. Lu, C. M. Li, T. C. Zuo, F. L. Wang, The influence of dark feature on optical and thermal property of DC Arc Plasma Jet CVD diamond films, Diamond & Related Materials 14 (2005) 1583 – 1587. doi:10.1016/j.diamond.2005.03.010.

In addition to all, Please polish the film from the surface side as growth side is already quite smooth due to polished Si surface (Substrate). Resulting a self standing film will be transparent in visible light but poly crystalline nature, grain size, structural defect and sp2 C related defect will reduce its transparency in visible region.