Particles with a dimension that is 100nm or less in size are considered nanomaterials. But, size is not what provides the unique nanomaterial properties that are being observed. Consider the following: 80nm aluminum particles are dangerous as a possible inhalant. 30nm aluminum particles are very reactive (explosive) when they come in contact with air/oxygen. 70nm gold particles are fine dust, but below 20nm, these particles when added to glass change the color of the glass. Just because a particle is less than 100nm does not give it special properties just because it is less than 100nm.
There are other interesting changes in behavior in the sub-100nm range that can impact how materials behave. One example is that the adhesion forces of particles to surfaces change as the particles get smaller. Below roughly 70nm, van der Waals forces become the dominate adhesion mechanism. Most changes in material property and behavior start to occur somewhere in the 30nm to 70nm size region. (Part of this reason for change is that the size of a particle diminishes into the region where a significant portion of the atoms have “access” to the particle’s surface with the resultant increased opportunity to react with other material.)
Currently, research in the nanotechnology realm work with basic elements and combinations of nano-scaled materials, which has created some materials with interesting properties. Carbon has been the most researched with single walled nanotubes and multi-walled nanotubes having a lot of early interest. The ability of the carbon nanoteubes to increase strength of other materials with a decrease in weight has been utilized in automotive and sports industries to name a couple areas. Depending on the chirality of the carbon nanotubes, the resultant material can be either conducting or semi-conducting. “Unrolling“ a single walled carbon nanotube results in the material called graphene, which is called a “two-dimensional” material due to being only one atom thick. It is conductive. Contaminating the surface with oxygen produces a material called graphane, which is non-conducting (different name, different properties – graphane not graphene). [Ref. 1, 2, & 3]
What if we are not considering all the possibilities?
The current published research focuses on using various bulk materials to develop experiments and find the new properties of the nanomaterial. But, what if, nano-scale material is also different from bulk material due to the various isotopes of the material?
Bulk material containing various isotopes
Bulk material consists of various isotopes of the specific element(s) in a ratio that has been determined through various techniques and has been quantified. [Ref. 4] In general, one makes an assumption that the nano-scale material has the same isotope ratios as the bulk. One also assume that the different isotopes of the bulk material have identical properties when present together. Is it possible that individual isotopes are different from the bulk containing various isotopes? Is this possible?
One example that shows there is a different in isotopes is uranium. A specific form of the element (isotope) uranium is 235U, which makes up 0.07% of typical uranium in the mined ore. 238U is the predominant form on the element and has a half-life of 4.5 billion years, while 235U is more reactive and can be split to produce energy. [Ref. 5] The isotope that is useful, 235U, is separated from other isotopes of that material. It might be expensive and challenging, but if a specific isotope is useful, it will be obtained.
Another example of different properties of isotopes involves water. A water molecule consists of two hydrogen atoms and one oxygen atom. However, there are other forms (isotopes) of hydrogen that have one or two extra neutrons. Water molecules that consist of oxygen and deuterium (hydrogen with one extra neutron) are called heavy-water and are employed in damping nuclear radiation. So, one form of hydrogen has properties that the other does not.
It can be stated that in this case, the extra neutron causes a significant percentage increase in the atom’s mass. Work has been done that states the impact of additional neutrons are the greatest on the elements with the lowest mass. Some projections imply that the additional of extra neutrons does not have a significant, if any, impact on the material properties. Is this true? What about 235U?
Consider Lithium with two stable isotopes, 6Litium and 7Lithium, with 7Lithium accounting for over 92% of the material. There are also a number of short lived lithium isotopes. [Ref. 6 & 7] It is known that 6Lithium has a greater affinity than 7Lithium for the element mercury. This fact is used in the separation of the two isotopes of Lithium. How is this possible if the extra neutron does not change the material properties? Maybe it does!
Is it possible that we need to think about research that works with specific isotopes of “common” materials? If various isotopes have different reactions with other materials, is it possible that “lumping” all isotopes of an element into an experiment actually degrades the performance that a single isotope might have? At a minimum, research that works with specific isotopes of “common” materials should clearly state the isotopes used, including any percentage of other isotopes that may be present, which may impact performance.
Material purity
There are various levels of material purity that can be purchased. Very high purity copper can be obtained that at 99.9999% pure. That is one part per million pure material. Other materials are not available in purities of greater than 99.99% and some are not even close to 99%.
It is known that doping of semiconductors with a very small percentage of different elements can change the properties of the combined material. Doping in silicon (semiconductors to increase the charge carrier concentration) can range from lightly doped (parts per billions) to heavily doped (parts per thousands) of the doping material. Depending on the materials employed, other effects besides carrier concentrations can be impacted. Lithium can be employed for increasing the resistance of solar material (solar cells) to the sun’s radiation.
Has anyone examined the percentage impurity of isotopes in common materials, like carbon or copper? In order to conduct that experiment it would be necessary to have pure material and then add impurities. How difficult is that? Consider that conventional computer hard drives require more than 1 million atoms per bit and over ½ billion atoms per byte! Consider a 2µm copper sphere. Calculations yield that it should have 3.56 x 1011 atoms. If the material is six 9s pure, it contains 356,000 contaminant atoms! [Ref. 8 & 9] Do we really know the true properties of materials?
When the materials are in the nano-scale region, the total quantity of atoms is smaller. A 30nm aluminum particle has roughly 850,000 atoms in it. “Super pure” aluminum can be as much as five 9s pure. That would still leave 9 contaminant atoms. Is that enough to modify material properties?
Material isotope homogeneity
Another question is whether anyone has worked with a pure isotope of common materials. Granted that there are techniques for separating the isotopes, e.g., uranium, which can produce high purity materials. But, when the desired isotope exists as a very small percentage of the total material, it takes many passes through a process to achieve the desired concentration and that concentration is not 100%. It may be enough to be effective, but it is not 100%.
Copper has 29 isotopes with the two predominant being 63Cu (69%) and 65Cu (30.8%). That implies that the pure copper one can acquire will typically be 69% of one type and 31% of the other. Aluminum is interesting in the that for all practical purposes, 100% of aluminum is 27Al. So any changes to the properties would be due to contaminants and not isotopes of aluminum.
Thoughts
If we have found that different isotopes of materials may have different properties in the bulk, is it not reasonable to anticipate that there will be different properties in the nano realm? Maybe we should start to investigate the properties of various isotopes of nanomaterials? Are we missing some potentially important properties when we do not investigate the isotopes on various nanomaterials? Do we have the concept of nanotechnology research mis-focused or just misunderstood? Any thoughts? Send to: Ideas at nano-blog.com (The email address has been written with “at” in place of the “at symbol” to avoid spam filling the mail box.)
Acknowledgements:
Special thanks to Deb, Evelyn, and Harold for critical review and suggestions to improve this blog.
References:
- Carbon – http://www.rsc.org/periodic-table/element/6/carbon
- CNT – https://web.stanford.edu/group/cpima/education/nanotube_lesson.pdf
- CNT – https://www.nature.com/articles/ncomms5892
- Isotopes – https://en.wikipedia.org/wiki/Isotope
- Uranium – http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/what-is-uranium-how-does-it-work.aspx
- Lithium – https://en.wikipedia.org/wiki/Isotopes_of_lithium
- Lithium – http://www.rsc.org/periodic-table/element/3/lithium
- Atoms – http://gizmodo.com/5875674/ibm-figures-out-how-many-atoms-it-takes-to-hold-a-bit-hint-its-12
- # of atoms – https://socratic.org/questions/a-pure-copper-sphere-has-a-radius-0-929-in-how-many-copper-atoms-does-it-contain
