[I] Surface difference: Silica, as formed by the chemical reaction of silicon and oxygen can be either crystalline or amorphous. Depending upon the temperature, pressure and the rate of cooling, solid silica can take up different forms. Crystalline silica exists in seven different forms (polymorphs), four of which are very rare. The three major forms: quartz, cristobalite and tridymite, are stable at different temperatures. Even within these three major forms, there are subdivisions like α (trigonal; the most abundant) and ẞ quartz which mutually interchange as the temperature is changed. All are identical chemically (SiO2). But they attain different physical forms as their internal structures are very different due to different arrangements of Si, O, Si, O layers (like ABA--- and ABC-- packing of layers) but each Si is always surrounded by four O atoms (SiO4) to make up a three dimensional repeating pattern INDEFINITELY over whole bulk of the csystal.

The amorphous silica also possesses SiO4 units but not in whole of the bulk, i.e. exhibit only SHORT-RANGE ordering of their atoms.

The distinguishing feature of crystalline and amorphous silica is that you can take any portion of it and see the whole in the crystalline form but with a non repeating pattern, you can't do that in its amorphous form. Some short-range orderliness may exist, but no predictable order extends over whole range of the amorphous form. THE CHEMISTS CALL THIS STATE GLASSY.

[II] Difference in physical properties: The surface difference causes differences in their physical properties as: M. pt. Quartz (α=1670,ẞ= 1713 ; amorphous=1700 C), Dielectric constant (Quartz= 4.69-5.06 ; amorphous= 3.9) , Band gap ( Quartz= 9.65 e V ; amorphous≈9.0 e V).

In my opinion, the difference between a surface of amorphous silica and a stable surface of quartz (crystalline silica) is caused by the difference a number atoms on their surfaces. In the case of Si nanoparticles ratio of a number atoms on the surface to a number atoms in the volume increase with decrease of nanoparticles size. In this case a role of surface atoms in the determining of physical properties of nanoparticles increases.

What sort of surface properties are you interested in? If you're going to deposit an epitaxial film, for example, there's a substantial difference between the random organisation of atoms on an amorphous surface and the regular periodicity of a crystalline surface.

Dear Philip,

Thanks for your kind help.

I am trying to study the clean surfaces of amorphous silica. I'm interested in their local and overall chemical composition and structures, and related electronic surface states. Certainly such information might be also useful to understand their chemical properties, e.g. as acttion centers or so.

[I] Surface difference: Silica, as formed by the chemical reaction of silicon and oxygen can be either crystalline or amorphous. Depending upon the temperature, pressure and the rate of cooling, solid silica can take up different forms. Crystalline silica exists in seven different forms (polymorphs), four of which are very rare. The three major forms: quartz, cristobalite and tridymite, are stable at different temperatures. Even within these three major forms, there are subdivisions like α (trigonal; the most abundant) and ẞ quartz which mutually interchange as the temperature is changed. All are identical chemically (SiO2). But they attain different physical forms as their internal structures are very different due to different arrangements of Si, O, Si, O layers (like ABA--- and ABC-- packing of layers) but each Si is always surrounded by four O atoms (SiO4) to make up a three dimensional repeating pattern INDEFINITELY over whole bulk of the csystal.

The amorphous silica also possesses SiO4 units but not in whole of the bulk, i.e. exhibit only SHORT-RANGE ordering of their atoms.

The distinguishing feature of crystalline and amorphous silica is that you can take any portion of it and see the whole in the crystalline form but with a non repeating pattern, you can't do that in its amorphous form. Some short-range orderliness may exist, but no predictable order extends over whole range of the amorphous form. THE CHEMISTS CALL THIS STATE GLASSY.

[II] Difference in physical properties: The surface difference causes differences in their physical properties as: M. pt. Quartz (α=1670,ẞ= 1713 ; amorphous=1700 C), Dielectric constant (Quartz= 4.69-5.06 ; amorphous= 3.9) , Band gap ( Quartz= 9.65 e V ; amorphous≈9.0 e V).

Dear Dr. Manohar Sehgal,

Thank you for your kind help.

Best regards

Changming Fang

Just adding some comments. Amorphous silica may easily realize a surface which include more point defects, due to the short range order explained by M. Sehgal. Again, the type, depth distribution and density of defects depend on the growth and post-growth processes experienced by the material. You may expect a certain density of unsaturated bonds at the very surface, Oxygen vacancies in the near selvedge or even across the whole layer, as well as mismatching (geometrical) defects relating to the cristalline domains size and distribution. There is a huge literature on such topics. You may seek for papers by Pointdexter, DiMaria, and many others for electronic behaviour of structural/chemical defects in amorphous silica layer, widely used in Microelectronics.

Form the quantitative viewpoint, however, you should expect defect densities in the range 1E11 - 1E13 per square centimeter, that is an amont quite low to be reliably probed with spectroscopical tools (suc as XPS or Auger, or even EDX). Some work has been done by positron annihilation techniques, and perhaps something may be resolved by SIMS... But it is not that easy.

As a matter of facts electrical / morphological local probing methods may better help in identifying and also mapping structural/chemical defects in the near surface of a-Silica, such as EFM (Electrostatic Force Microp[robe) that is a modified version of typical AFM, or STM (Scanning tunneling Microscopy), or other similar techniques with nanometer side resolution.

Dear Giuseppe,

Thanks.

The information is very helpful.

Dear all,

Thank you for your kind kind help.

Merry Christmas and Happy New Year !!

dear Changming Fang,

I happened on your question, and one aspect you were interested is the difference in surface chemistry between amorphous and ordered SiO2.

There are significant differences between them for the amount and rate of contaminants absorption & desorption. As one would expect, a more disordered surface has more surface defects to interact with contaminants, while an ordered surface with fewer dangling bonds can more stable, adsorb fewer hydrocarbons and desorb them more easily. Therefore, surface ordering can decrease hydrocarbons adsorption, and surface contamination can be removed more easily, with chemical cleaning and lower temperatures.

To illustrate this effect, I attach here two Time-of-Flight SIMS analysis on ordered SiO2 surfaces we completed with Intel Corp, before and after thermal processing. They show that adsorption of hydrocarbons (a plague in micro-/nano-fabrication of gate oxides when there is exposure to atmosphere) is lower by several orders of magnitude on an ordered oxide surface.  In addition, on amorphous surfaces, thermal treatments results in further reacting the carbonaceous  contaminants, making them impossible to remove. In (b) of this figure,  the ease of desorption from the ordered surface leads to much lower incorporation of carbon in the oxides during thermal oxidation.

I attach a pdf of the ToFSIMS figure, the paper where the figure comes from, Mat.Sci. Eng B 2001, excerpt and another reference from Y. Chabal, APL 1998.

Article The formation of ordered, ultrathin SiO2/Si(100) interfaces ...

Dear All,

Thank you very much for your kind help. The answers from various aspects enhance my understanding very much.

These questions and answers might be, I hope, for you as well.

Best regards

changming