Field Induced Ordering of Nanoparticles
Authors: Wilkins Ginger-Marie , Das Sanjana , Das Biswajit 
Published: February 2020

Nanoparticles of semiconductors and metals are promising for the implementation of a variety of photonic and electronic devices with new functionalities and improved performance. For the implementation of nanoparticle based devices it is essential that the nanoparticles are electrically isolated from each other, otherwise the benefits such as quantum confinement or Coulomb blockade, can be lost. A convenient way to achieve such isolation is by ordering the nanoparticles in a periodic fashion, which ensures inter-particle separation as well as improvement in reliability and device performance. There is thus significant interest in the fabrication of ordered nanoparticles. For most electronic and photonic devices, it is typically required that the nanoparticle dimensions be in the 1-20 nm range with size variations of 10% or less. Current lithographic techniques are not suitable for the implementation of such nanoparticles, and nonlithographic techniques are typically used for their fabrication. However, many of the nonlithographic techniques are based on natural self-organization processes and suffer from a lack of flexibility or a lack of engineering control. Among the various nonlithographic techniques, the predominant ones are solution based. While such solution based techniques can produce nanoparticles with the required dimensions and size distributions, and in some cases periodicity, they require complex surface passivation involving organic capping molecules. These capping molecules modify the electrical surface properties of the nanoparticles making charge injection/extraction difficult. In addition, the solution based synthesis techniques are not compatible with solid-state device technology, the primary manufacturing process for electronic and photonic devices. The problems associated with solution based methods can be addressed to some extent by using nonlithographic fabrication techniques based on physical vapor deposition such as Stransky-Krastanov growth by epitaxial process. However, most of these techniques have severe constraints in terms of nanoparticle size, material, periodicity and the choice of substrate. A versatile nanofabrication technique that can produce high purity nanoparticles with flexibility in terms of particle size, nanoparticle material, periodicity and the choice of substrate, together with the capability for in-situ deposition of ohmic contact and isolation dielectric materials, will be an important step towards the realization of a variety of nanoparticle based electronic and photonic devices.

Our research group has developed a cluster tool system that addresses the above issues. The system consists of a nanoparticle deposition unit, an electron-beam evaporation unit for the deposition of thin films, and a pulsed DC sputtering unit for the deposition of thick layers of metals and insulators. The nanoparticle deposition system is based on sputtering, thus allowing the deposition of nanoparticles of any material that can be sputtered. In addition, nanoparticle deposition can be carried out at room temperature, making it possible to deposit the nanoparticles on a variety of substrates including flexible substrates. Size selection of nanoparticles is achieved by using a quadrupole mass filter with the capability to provide as low as 2% size control. The nanoparticle deposition unit, the electron beam evaporation unit and the DC sputtering unit are all housed inside an ultra-high vacuum chamber to ensure high purity and good surface properties of the nanoparticles. We have used this technique to fabricate nanoparticles of a variety of metals and semiconductors with sizes ranging from 2 nm to 30 nm with less than 5% size variation and demonstrated the viability of the system for the fabrication of nanoparticle based electronic and photonic devices.

The nanoparticles deposited in this system are not uniformly distributed, however, most show physical separation from each other. This is due to the charged nature of the nanoparticles creating inter-particle repulsion. In order to create ordered nanoparticles, we have developed a system where the charged nanoparticles can be physically manipulated through the application of an external electric field. The system consists of an electrode mounted on a platform with x,y,z motion and rotation capability. The displacement in the z-direction (vertical) is of nanometer sensitivity which allows the probe to be placed in close proximity to the substrate. The ordering of the nanoparticles is achieved through x,y or rotational scanning of the electrode with an applied voltage, We have demonstrated various ordering of nanoparticles using this technique. We have also extended the technique to order neutral nanoparticles by inducing image charge on the nanoparticles through the electrode and then manipulating the particles. We have also demonstrated that the technique can be used to order magnetic nanoparticles by using a magnetic tip instead of an electrode. In this poster, we will present details of the nanoparticle ordering system, describe the mechanisms affecting the ordering and present experimental results.