Physics projects for Y3 and Y4 students

Project description

Nanosphere lithography: patterns emerging by self-assembly

(supervisor: Rudi Winter)

Nature of project: experimental, data analysis

Available to students on full-time physics degree schemes or joint students.

Project description and methodology

When small particles can move freely in a liquid, they will move erratically (Brownian motion) and equilibrate around an average velocity by repeatedly colliding with one another. If the liquid suspension medium is allowed to evaporate, the vapour molecules will remove energy from the system and gradually slow down the particles, while at the same time reducing the mean free path between collisions. Ultimately, the particles will settle into a densely-packed regular (crystalline) pattern. If a thin film of the particle suspension is deposited on a substrate, an monolayer array of regularly spaced particles will emerge.

Such a particle array can be used to "print" regular pattern of a more durable nature, e.g. by sputtering a thin layer of metal onto the layer of particles. Some of the deposited metal will go in the triangular gaps between the beads and form a regular hexagonal array of metal dots, which will remain on the substrate even when the particles themselves are removed by applying some solvent[1].

In materials science, such arrays of metal dots can be used to grow three-dimensional nanostructures by anchoring other materials to the metal dots[2].

However, even just the metal dot array can be useful in itself, e.g. as an optical element to scatter visible light under specific Bragg angles[3]. In this project, we will use nanosphere lithography to produce a calibration grid for the light-scattering instrument in the teaching lab.

The core part of the project consists of (a) making a regular array of micrometre-sized polystyrene beads by spin coating, (b) turning this into a metal dot array by sputtering with gold and dissolving the beads and (c) taking microsope images of the resulting pattern.

A successful project will develop beyond the above in one/some of the following directions:
(1) Characterise nano-particle arrays using microscopic techniques (visible, AFM).

(2) Investigate the self-assembly parameters (spin coating speed, amount and concentration of suspension used, suspension medium, temperature) systematically to determine the limits under which perfect arrays are formed.

(3) Extend systematically beyond those limits to produce arrays with crystal defects such as vacancies (holes), dislocations, grain boundaries or bilayer islands.

(4) Use the prepared metal dot array as a calibration sample on the light scattering instrument and compare its performance with that of a conventional optical grid.

(5) Investigate the light scattering from arrays with imperfections. What are the scattering features resulting from different types of defects?

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: A Y4 student would be expected to get as far as taking light scattering measurements from metal dot arrays produced. This should be accompanied by simulations of scattering patterns using software such as Mieplot.

Please speak to Rudi Winter if you consider doing this project.

Initial literature for students:

  1. JC Hulteen, RP Van Duyne; J Vac Sci Technol 13 (1995) 1553
  2. AX Wang, Kong XM; Materials 8 (2015) 3024
  3. HC van de Hulst, Light Scattering by Small Particles, New York: Dover 1981
  4. http://www.philiplaven.com/mieplot.htm

Novelty, degree of difficulty and amount of assistance required

The method is well studied but careful experimentation is needed to make good crystalline arrays. Light scattering of defect structures (development option 5) is difficult and not widely studied elsewhere.

Project milestones and deliverables (including timescale)

milestoneto be completed by
selection of type of spheres and solvents to be used based on literature studyend of November
particle arrays deposited and characterisedend of February
metal dot arrays prepared and characterisedmid-March
light scattering data taken and/or defect structures investigatedEaster

Students taking this project will have to submit a full risk assessment form