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K-12 Education & Outreach

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:

  • Part 1: Crystal Formation by Nucleation and Growth
    • Egg geode: egg shells, sodium tetraborate (borax), food coloring, glue, cups (one per egg shell), large glass dish (for microwaving borax solution), water, *microwave
    • Hot ice (sodium acetate) reusable handwarmers (see “Sources” section below)
  • Part 2: Materials in Bulk versus Nanoscale
    • gold nanoparticles in a vial, blue laser pointer, quantum dot samples in vials (red/green emitting)
  • Part 3: Mixing Light versus Mixing Color
    • red/green/blue glow sticks (emptied into vials shortly ahead of time), poster paint/kids washable paint, popsicle sticks, cups, paint mixing tray, plastic pipettes

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).
  • Examples of crystals include salt, sugar, gem stones, and snowflakes.
  1. Teach students what crystals are and how they grow (slide 3).
  • They are a solid material made up of atoms or molecules in a regularly repeating structure.
  • They are grown from a super-saturated solution (This means a very large amount of the atom or molecule that will make up the crystal are dissolved in the liquid. These atoms or molecules will find each other in solution and orient to form the crystal.)
  • Show the accompanying crystallization video (https://www.youtube.com/watch?v=d55JCDEv-UQ)
  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).
  • Show sodium acetate crystallization video, emphasizing focus on the way the crystals all grow outwards from a single initial point (https://www.youtube.com/watch?v=HnSg2cl09PI)
  • Hot Ice Hand Warmers Demo: Pass around the reusable hand warmers. They should be in their liquid state when passed out. Have the students all pop the seed in the center at the same time and they should see rapid crystallization and feel the temperature increase.
    • When the seed is popped a “nucleation” site is introduced to the super-saturated solution and crystals rapidly grow.
    • This is an exothermic process, which means that heat is released.
  • Egg Geode Demo (slide 5): Egg geodes should be prepared ahead of time. Pass around the egg geodes for the students to inspect the size and shape of the crystals. Discuss their observations.
    • If this is a multiday outreach event, students can set up their own egg geode recrystallization the day before and inspect their own results at this point. The crystals must grow on the egg shells overnight. See “extensions” section below.
    • See “sources” section below for detailed instructions on making the egg geodes.

Part 2: Materials in Bulk versus Nanoscale

  1. Hand out worksheet with diagram illustrating how small the nanoscale is (slide 6 – 7)
  • Slide 6: On the left is a photo of a bulk crystal, which is what people are used to seeing. On the right is a photo of nanocrystals which was taken using a Transmission Electron Microscope (TEM – a very strong microscope). Notice, all of the crystals are the same size! This is because scientists have worked very hard to understand nucleation and growth, which you just did too!
  • Slide 7: Analogy: a nanocrystal is to an ant, as the ant is to a football stadium (e.g. CenturyLink Field)
  • Slide 9 – 10: What are some examples of nanotechnology?
    • Spiderman got an upgrade! Today, Spiderman’s suit is made from nanotechnology.
    • Nanocrystals of different compositions and sizes give stained glass windows their vibrant colors!
    • They are also used for their colors in electronics displays
    • Silver nanoparticles are used in clothes because they are antimicrobial
    • The bike is made of carbon nanotubes, which are incredibly strong for their size.
  1. Let’s use gold as a comparison for bulk versus nano (Slide 11 – 12)
  • Demo: show/pass around bottle of gold nanoparticles suspended in solution (they appear red)
  • bulk gold is yellow and shiny
  • gold nanocrystals (suspended in solution) are red
  • this is because of the way the nanocrystals absorb and emit light
  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.
  • This property is used in for displays (slide 14)
  • Demo: Show vials of quantum dots with laser pointers. Teacher or Outreach lead should hold vials so they are not broken by the students.
    • Quantum dots are a specific type of nanocrystal.
    • When the blue laser pointer shines on the quantum dots, they in turn emit light and the color emitted changes with the size of the nanocrystal.
  • Demo (optional): show the deconstructed quantum dot TV (if accessible)
    • Quantum dots are a specific type of nanocrystal.
    • Lay the TV on the table, plugged in and turned on.
    • The screen is back lit with blue light and there is one capillary tube across the top of the screen filled with quantum dots (appears yellow from the color of the glass).
    • The part of the screen that has no capillary tube appears blue and the rest of the screen appears white.
    • The difference in color is due to the mixing of the blue light with the nanocrystals/quantum dots
    • These TVs use a combination of red, blue, and green emitters to generate white light.
  • Discussion: How do properties of crystals change from bulk to nanoscale?

Part 3: Mixing Light versus Mixing Color

  1. Is color the same thing as light? (slide 15)
  • Answer: No
  • Color (slide 16): The full visible light spectrum (a rainbow) hits an object. Some of the light is absorbed and some is reflected. The light that is reflected is the color you see. If all the light is absorbed and none is reflected, the object appears black. If none of the light is absorbed and all the light is reflected, the objects appears white.
  • Light (slide 17): Light is a type of energy that we can see. We say it is emitted from something (e.g., the sun emits light). Light that is higher or lower energy appears as different colors.
  1. What happens when we mix color versus when we mix light? (slide 18)
  • Paint Mixing Demo: Paint gets it color from absorbing and reflecting visible light.
    • Pass out cups, popsicle sticks, and red, green, and blue paint
    • Have the students mix the three colors together and make observations (the paint turns brown)
    • Discussion: What color did the paint turn? (A: brown) Why? (A: because more light is being absorbed and less is being reflected)
  • Glowstick Mixing Demo: Glowsticks get their color from a chemical reaction that results in the molecules emitting light of a specific energy.
    • Teacher or outreach leader should do this demo in front of the students.
    • Shortly ahead of time, empty the contents of the red, gree, and blue glowsticks into vials.
    • In the paint mixing plate, combine different colors using plastic pipettes to produce white light. The best ratio was 10:1:1 (blue : red : green).
    • Discussion: Why does the light turn white?
  • Discussion: What is the difference between light and color?

Extensions:

  1. Egg Geode Demo extension:
  • Students may make their own egg geodes if they are able to set it up the day before.
  • A microwave is required for making the borax solution.
  • Egg geodes must crystallize overnight.
  • During set up, emphasize how the borax crystals being glued on to the egg shells are going to act as seed crystals or “sites for nucleation”.
  • See “sources” section below for detailed instructions on making the egg geodes.

Resources:

  • Worksheet
    • document title: Nanocrystals_outreach_handout.pdf

Sources:

 

 

 

 

 

 

 

Molecular Engineering and Materials Center
Box 351700
Department of Chemistry
University of Washington
36 Bagley Hall
Seattle, WA 98195-1700
[email protected]
The University of Washington MRSEC is supported by the National Science Foundation under NSF Award Number DMR-2308979. Any opinions, findings, conclusions or recommendations expressed in this material are those of the PI(s) and do not necessarily reflect those of the National Science Foundation.