Chemistry
150
Please
have the following pages ready before class on Thursday, February 7. As usual, please write
an abstract
and paper-clip
it to the front of your individual writeup. The abstract and the carbon-copy
pages of the write-up is due in class on Thursday, February 14 ©
Lab 3: Conductors and semiconductors
Part 1. Purpose
Go to the Cal Poly website
titled "Conductors, Semiconductors, Superconductors". It can be found at:
http://chemweb.calpoly.edu/chem/124/124Experiments/LED/LEDExp.html
Note that there are four
procedure sections; you will be doing only the first three sections. Summarize each section's goal in
separate sentences.
Part 2. Materials and methods
List each procedure
section's equipment separately
(the procedure sections are quite good at telling you what you will need).
Ignore any references to "Discovery Slides" in procedure section 3. You
will not be dealing with superconductors for now.
Sketch each procedure
section's setup separately and
label the various pieces of equipment.
Part 3. Procedure
Print out and paste or
copy by hand the procedure for each section separately into your notebook. OMIT sample data tables and questions asked within the
procedure. The use of the second person voice in the procedure might make it preferable for you
to rewrite it in the first person
in your notebook.
In procedure section 1,
you will be measuring the resistance at dry ice/acetone slurry temperature as well.
In procedure section 2, measure
the wavelength of light emitted by each LED using the handheld spectroscope. Ignore the “Semiconductor Structure
Analysis” part in section 2.
For procedure section 3 (temperature
effect on LEDs), use, in order, the red, orange and green LEDs. You will be substituting a dry
ice/acetone slurry for the hot water
portion of section 3, and you will again be measuring the wavelengths using the
handheld spectroscope. Ignore any references to "Discovery Slides".
Warning: Liquid nitrogen is very cold and can cause
frostbite; for that reason, avoid prolonged contact with it on bare skin.
Part 4. Original data and preliminary analysis
Procedure section
1: Temperature Dependence of
Resistance for a Metal
1. Make a table that shows the temperature (in °C) and the resistance (in ohms) of the choke coil at room temperature, at dry ice/acetone slurry
temperature, and at liquid
nitrogen temperature. Sounds like
a 2 by 3 table!
2. Describe what you observed, i.e., how does resistance change with temperature?
Procedure section 2:
The Effect of Composition on the Band Gap of a Compound Semiconductor
LED
1. You may wish to have a table that looks
like this in your notebook:
2. Show a sample calculation for obtaining
the energy from the emission wavelength, and for obtaining the energy from the
measured voltage across the LED.
Procedure section
3: The Effect of Temperature on
LEDs
1. Generate three tables:
one each for the red, orange and green LEDs. Each table should show the temperature (in °C), the measured potential (in volts), the color, the measured wavelength (in nm) of the LED and the relative intensity of the LED (use “bright”, “dim”
and similar words). There will be three temperatures: room temperature, liquid nitrogen temperature and dry ice/acetone slurry temperature. Sounds like each will be a 4 by 3
table!
2. From either the
voltage or the measured wavelength, calculate the band gap energy in J for all three temperatures for each LED.
Oops, sorry, this sounds like another column for each of three tables.
Part
5. Calculated results
You will need three
graphs for this part. Please title and label axes appropriately for each so I
can tell them apart.
• Graph resistance as a function of temperature from procedure section 1.
• Graph energy as a function of phosphorus content from procedure section 2.
• Overlay three graphs in one for procedure section 3:
graph wavelength as a function
of temperature for each of the
three LEDs separately, but place all three curves on the same graph.
Attach all graphs to this
writeup.
Part 6. Questions
1. Is the relationship
between temperature and resistance in a metal linear? Granted, you have only three points, but make
your best guess. Explain why
the resistance of a metal changes as it does with temperature changes. Hint:
Does this have anything to do with the band gap?
2. Is the relationship between the energy of light
emitted by an LED and the phosphorus
content of an LED linear? Complete the phrase: As LED phosphorous content
increases, the energy emitted by the LED _____________ . Suggest an explanation for this observed behavior. Your explanation must
include the phrase "band gap".
3. What would the percent
phosphorous composition of a
blue light emitting diode be, according to your graph? Is this composition
possible? Then, since blue LEDs are real, if expensive, what materials must
they be made of? Hint: think of trends in the periodic table.
4. What is the
relationship between the wavelength of light emitted by an LED and the temperature of its environment? Answer this for the red and/or
the orange LEDs. Give an explanation for this behavior. Your explanation must
include the phrase "band gap".
5. Why is the green LED’s
behavior with temperature changes different from the others? At some point, a
graph should appear here that might explain things; ask me if it does not
appear on the Monday after the lab is completed.
6. Did you observe a
change in intensity of light
emitted from the LEDs as a result of differing temperatures? Suggest an
explanation for this behavior. Hint: Does this explanation have to do with the
band gap?
Part 8. Conclusion
Summarize each procedure
section's main trend in separate sentences.
For the error analysis in
procedure section 2, calculate the percent error between the energy calculated
from the voltage across the LED and the energy calculated from the measured wavelength.
In addition, go to the Radio Shack website (http://www.radioshack.com/)
and look up the actual wavelengths of each of the LEDs by using the product
search function. The red LED is 276-309, the orange LED is 276-306, the yellow
LED is 276-301, and the green LED is 276-304. Use their published wavelengths to
calculate the “true” energy (in J) of light emitted and determine
whether the value from the voltage or the value from the measured wavelength came
closer to the “true” energy.
Suggest a reason why the
other one differed more from the “true” value.
Abstract
Summarize each procedure
section’s main trend in separate sentences. Note any significant errors.