Studio 3c

LEDs: Periodic Trends in Action

Light Emitting Diodes (LED)Periodic Trends in Practice

• World's Largest LED display:  8 stories tall; 19 million LEDs display in New York's Times Square

• Lighting consumes 20% of all energy; Use of LEDs would reduce this by 50% ($100 billion globally annually).

• Incandescent bulbs operate on the principle that any object heated to a high enough temperature will glow.  (Below 500oC, objects emit infrared radiation, but no visible light. A tungsten filament in incandescent bulbs is hot enough at 2500oC, to emit all the colors of visible light, even small amounts of blue and violet.)

LEDs• Traffic Lights: 10 million

in US; 3.5 terawatt-hours annually ($0.07 per kilowatt-hour)

• How much do we annually spend on traffic lights?

• LEDs last 100,000 hours (How often do LEDs have to be changed?)

LED materials taken from:  Breitzer, J.; Condren, S. M.; Lisensky, G. C.; Nordell, K. J.; Widstrand, C. G.; Ellis, A. B. LED Color Strip Kit Rehder, K. S., Ed.;  Institute for Chemical Education: Madison, WI, 2002.

Band Gap

Picture from: http://www.sysman.co.za/Xplo/Xplo_optical_principles.htm

Semiconductors

Semiconductors and LEDs

n-type

p-type

LED Activity 1

Take your LED strip out of the box and plug it into the 9-V battery provided.Do the LED’s feel hot after 5 seconds? After 15 seconds?Now attach the light bulb. Does it feel hot after 5 seconds? After 15 seconds? How is this related to the 50% better energy efficiency of the LED?

LED Activity 2

Briefly, touch the terminals of the 9-volt battery to the electrical connectors in the wrong orientation. What happens?Power-up all of the LED’s by touching the battery terminals to the electrical connectors.What happens when you remove battery?Do the LED’s dim at different rates? Why?

LED Activity 3

Power-up all of the LED’s and use a volt-meter to measure the voltages between TP1 and the TP beside each LED. Record your values.Which LED has the highest voltage? Which has the lowest?Do the voltages make sense with color of the LED?

White Light and LEDs• To replace household fluorescent and incandescent lights,

we would prefer a white-light source. • Fluorescent lamps accomplish this using mercury vapor

that emits ultraviolet light. A luminescent material on the glass absorbs the ultraviolet light and reemits a mixture of visible wavelengths that appear like white-light to us.

• Place the luminescent cap on the LEDs. What do you observe?

Band Gap and Atomic Size

The phase of Sn that exhibits a diamond structure is only stable below 13oC

How is atomic radius related to band gap? Explain this trend.

Are any of these wavelengths visible?

Diamond crystal lattice

Lattice Parameter (Å)

Band Gap kJ/mol

E (eV)

(nm)

C 3.57 346 5.3 230

Si 5.43 222 1.1 1100

Ge 5.66 188 0.66 1900

-Sn

6.49 146 <0.1 12000

LEDs as SemiconductorsIsovalent Semiconductors:  The total valence electron count

of elements having the diamond structure is preserved in compounds of AZ stoichiometry, where A and Z are elements that symmetrically flank the group 14 elements in the periodic table.

B C N O FAl Si P S Cl

Cu Zn Ga Ge As Se BrAg Cd In Sn Sb Te IAu Hg Tl Pb Bi Po At

Ge Ga-As Zn-Se CuBr

Isovalent semiconductors

• What periodic trend is observed?  Why?• Are any of these semiconductor materials emitting visible

light?

Lattice Parameter (Å)

Electro-negativity

E (eV) (nm)

Ge 5.66 0.0 0.66 1900Ga-As 5.65 0.4 1.4 890Zn-Se 5.67 0.8 2.7 460Cu-Br 5.69 0.9 2.9 430

LED materials taken from:  Breitzer, J.; Condren, S. M.; Lisensky, G. C.; Nordell, K. J.; Widstrand, C. G.; Ellis, A. B. LED Color Strip Kit Rehder, K. S., Ed.;  Institute for Chemical Education: Madison, WI, 2002.  Lisensky, G.C; Penn, R.; Geselbracht, M.J.; Ellis, A.B. "Periodic Properties in a Family of Common Semiconductors" J. Chem. Ed. 1992, 69, 151-156.

LED materials taken from:  Breitzer, J.; Condren, S. M.; Lisensky, G. C.; Nordell, K. J.; Widstrand, C. G.; Ellis, A. B. LED Color Strip Kit Rehder, K. S., Ed.;  Institute for Chemical Education: Madison, WI, 2002.  Lisensky, G.C; Penn, R.; Geselbracht, M.J.; Ellis, A.B. "Periodic Properties in a Family of Common Semiconductors" J. Chem. Ed. 1992, 69, 151-156.

GaPAs Semiconductor

Of the following, GaP or GaN, which one is most likely to produce green light (550 nm)? Ultraviolet light (300 nm)?

GaPxAs1-x

P 106 pm 2.1As 120 pm 2.0