Exploring Crystallization and Color of Nanocrystals

Overview:

Crystals are solid materials that are composed of atoms or molecules arranged in a highly ordered, repeating pattern. We encounter crystals all around us; a few examples are salt, sugar, and snowflakes. Crystals like these are considered “bulk”. A bulk crystal will have uniform properties across the entire material. As the size of the crystal is reduced to the “nano” scale, the properties of the crystals begin to change, and scientists are learning how controlling the size of the crystals allows for unique interactions with light.

In this lesson, students will learn how crystals form through a process called nucleation and growth. They will then explore how materials behave differently in bulk versus at the nanoscale, specifically with regards to how they interact with light. Students with then investigate the difference between light and color (pigment).

Essential Question:

  1. What is a crystal?
  2. How do crystals form?
  3. How small is “nano” and how do crystals at nanoscale behave differently from the bulk (i.e. the crystals we are used to interacting with in everyday life)?
  4. What is the difference between light and color, and how can we take advantage of these differences to make useful things with nano-science?

Background:

Materials on the nanoscale behave very differently from bulk materials. We will focus on bulk vs nanoscale crystals, their growth, and their unique properties. The main concepts students should take away are (1) crystals (bulk and nanoscale) grow by a process called nucleation, where a single crystal causes atoms and/or molecules around it to self-organize into a regularly repeating pattern making a larger crystal; (2) that nanocrystals interact with light to produce pure colors; and (3) that the difference between light and color is that the source of light is emission, while the source of color is absorption and reflection.

Vocabulary:

Crystals: solid material made of atoms or molecules in a highly ordered arrangement

Nucleation: the first step in atoms or molecules self-organizing to form a crystal (a new phase/structure)

Nano-scale: dimension on the order of 1-100 nm (analogy: football stadium is to an ant, as an ant is to nanocrystal)

Absorb: light is taken in by material

Reflect: light is not taken in by material, and instead reaches our eyes

Emit: excited materials relax and give off light

Research Connection:

Nanocrystals are important materials for a wide variety of applications, including solar cells, device displays, and bio-medical imaging. Nucleation and growth are important concepts in understanding how nanocrystals are made.

NGSS Standards:

Standard Number Standard text
5-PS1-4. Conduct an investigation to determine whether the mixing of two or more substances results in new substances.

 

Materials:

Procedure:

Use accompanying google slide deck
https://docs.google.com/presentation/d/1gl88O4a5DhLLmPU5CSNiGhwA8Cgpf3e1ci-6qQoGzwo/edit?usp=sharing

Part 1: Crystal Formation by Nucleation and Growth

  1. Introduce where students may interact with crystals everyday (slide 2).
  1. Teach students what crystals are and how they grow (slide 3).
  1. Crystals from in a super-saturated solution around a “seed crystal” which serves as an initial point for crystallization to begin – this is nucleation! (slide 4).

Part 2: Materials in Bulk versus Nanoscale

  1. Hand out worksheet with diagram illustrating how small the nanoscale is (slide 6 – 7)
  1. Let’s use gold as a comparison for bulk versus nano (Slide 11 – 12)
  1. How do crystals form at the nanoscale? (slide 13)
  1. The property of nanocrystals that we are focused on is their ability to absorb and emit light.

Part 3: Mixing Light versus Mixing Color

  1. Is color the same thing as light? (slide 15)
  1. What happens when we mix color versus when we mix light? (slide 18)

Extensions:

  1. Egg Geode Demo extension:

Resources:

Sources: