# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 7 Mechanisms for Small Particles to Block Radiation
Greater than 3 nm gold nanoparticles exhibit a face-centered FCC crystal structure, whereas ultra-small gold nanoparticles often have non-FCC atomic packing structures. For ultra-small nanoparticles, even minor atomic changes in particle size can lead to significant property differences. Gold 38 is smaller than gold 40 by only two atoms but shows different Lewis acidities and the chelation of bidentate thiol ligands. Fluorescent properties can also be changed based on the size of the nanoparticle. Fermi energy levels can become quantized based on the size of the particle. Toxicity is also affected, such as quantum dots, which can have high toxicity. Spin quantum effects are described but not directly related to radiation shielding. Methods of synthesizing ultra-small nanoparticles are described, including how they can change the structural, optical, and electrical properties of materials. Forming stable cluster sizes is also described in the paper. Also described are ultra-small nanoparticles self-assembling via lateral extension to form nanoplatelets with large lateral dimensions. These may be useful in radiation shielding by providing thin material with different crystal structures but having a large lateral dimension to provide the self-assembly of materials into microscale materials. STEM images and combined density functional theory simulations showed that a predominance of surface atoms were destabilized in nanoparticles leading to a distorted structure. The three-dimensional atomic scale structure of gold ultra-small nanoparticles could have different geometries that may affect radiation blocking. In conclusion, the paper states that the properties of nanoparticles can be optimized and there is a need for the continuing development of new and better methods to control the size, shape, and composition of the ultra-small nanoparticles. Cost-effective large-scale methods of synthesis still need to be developed. It is pointed out again that the difference of only a few atoms per particle makes a difference between a functional and a non-functional product. Ultra-small nanoparticle-based medicine is considered a promising area. There is also non-clinical potential that is certainly apparent. The scale-up of nanoparticles to kilogram batches is needed [95]. In [96], it was found that the intensity of backward scattered radiation increases with an increasing grain size. The bulk density of the shielding material also affects the scattering. The scattering effect was observed for a pure matrix of sand and for samples that contained the heavy elements iron or barium. The geometry of the experimental setup to measure scattered radiation is shown in Figure 1 from [96]. The scattering results apply to the specific experimental setup used. Other radiation-shielding methods are presented in [97; 98; 99; 100; 101; 102]. These describe crystal plane engineering, nanoparticle-synthesis methods, metal squeezing, low-melting-point alloys for radiation shielding, and thin films. Radiation shielding aprons and materials for astronauts are described in [103; 104]. Other articles [105; 106; 107; 108; 109; 110; 111; 112; 113; 114] discuss the shielding properties of multi-component elements, multilayer materials, and nanocomposite shielding materials. Improved characterization methods are also needed to study the effects of the particle size and crystal structure on radiation blocking. In microscopy, for example, when an E-beam hits a nanoparticle sample, it can cause damage to the material and cause it to look amorphous. For ultra-high resolution on the atomic scale, a STEM TEM might be useful. A combination of both imaging and diffraction techniques may be necessary for characterization. Table 4 gives a summary of studies on the use of small particles for radiation shielding.