Solid StateElectrical Conductivity & Reactivity

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

Solid State Electrical Conductivity

Solid ionic compounds are poor electrical conductors.

Mobile charges (ions or electrons) are needed for electrical conductivity.

The difference in energy of bonding and antibonding orbitals in a solid can explain many electrical properties.

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BondingBand Theory

directionallocalized bonds

Si 4 Si atoms

Si

Si

SiSi

Si

silicon(diamond structure)

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BondingBand Theory

band gap

valence band

conduction band

for siliconthe valence band is full

the conduction band is emptydirectional

localized bonds

Si 4 Si atoms

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Band Gap

carbon(diamond)

silicon germanium tin

Smaller gap for heavier elements

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Band Gap

Smaller gap for heavier elements

C

Si

Ge

Sn

diamond - insulator

semimetal, semiconductor

grey tin - metallic, conductor

graphite has a differentstructure than diamond

and is a conductor

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BondingCovalent and Metallic Bonding

directionallocalized bonds

C 4 C atoms

no band gap

valence band

conduction band

Insulator Metallic Conductor

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Density of States

valence band

conduction band

Insulator MetalSemiconductor

overlapping mo’s inextended structure

levels arenot uniformly spaced

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Conductivity

metal

semiconductorinsulator

T

e-

Band Gap

T2 > T1

average

Energy

num

ber

of

ele

ctro

ns

free electrons or holes move charge

higher temperature puts more e- in conduction band

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Conductivity

metal

semiconductorinsulator

T

e-

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Elements in Semiconductors

N

P

As

Sb

O

S

Se

Te

B

Al

Ga

In

C

Si

Ge

Sn

Zn

Cd

Alloys: GaP, GaAs, ZnS, CdS, CdSe, SiC

Intrinsic: Si, Ge, Fe3O4

Cu

Ag

F

Cl

Br

I

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Doped Semiconductors

valence band

conduction band

n-typesemiconductor

1% As in Ge

excess mobileelectrons

p-typesemiconductor1% Ga in Ge

excess mobileholes

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Diodea combination of an

n-type semiconductorand a

p-type semiconductorthat allows current flowin a preferred direction

n-type p-type

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Diodee- flow can occurwith e- moving to

more stableenergy levels

- +

Both conductbecause there are

mobile electrons or holesand locations to move to.

n-type p-type

e- e- Battery provides e- on one side and

drain on the other side.

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Diode

-+

n-type p-type

e- e-

Current flow in thereverse direction requires e- move to higher energy levels,

and occurs only withlarge applied potentials(breakdown voltage).

The semiconductors arecharge neutral,

and additional chargewill build up in the valence band

preventing significantcurrent flow.

Diode

Curr

en

t

Reverse Bias Forward Bias Applied Voltage

Light Emitting Diodes

= Eh =

c

h = 6.62 x 10-34 J·s·molecule-1

c = 3.00 x 108 m·s-1

band gap

wavelength(color)

Solid State Photoreactions

Ag+ Br- Ag+ Br- Ag+ Br-

Br- Ag+ Br- Ag+ Br- Ag+

Ag+ Br- Ag+ Br- Ag+ Br-

Br- Ag+ Br- Ag+ Br- Ag+

Ag+ Br- Ag+ Br- Ag+ Br-

Br- Ag+ Br- Ag+ Br- Ag+

h

Solid State Photoreactions

Ag+ Br- Ag+ Br- Ag+ Br-

Br- Ag+ Br- Ag+ Br- Ag+

Ag+ Br- Ag+ Br Ag Br-

Br- Ag+ Br- Ag+ Br- Ag+

Ag+ Br- Ag+ Br- Ag+ Br-

Br- Ag+ Br- Ag+ Br- Ag+

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AgBr(s) Ag(s) + ½ Br2(l)

Ag(s) + Br2(l)

Ag(l)

Ag(g)

½ Br2(g)

AgBr(s)

Ag+(g)Br-(g)

325 kJ·mol-1

100 kJ·mol-1

major energy requirement is reverse of EA of Br-

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AgBr(s) Ag(s) + ½ Br2(l)

E = 325,000 J·mol-1

= Eh =

c

= hcE

= 3.68 x 10-7 m = 3680 Å

h = 6.62 x 10-34 J·s·molecule-1

c = 3.00 x 108 m·s-1

mol = 6.02 x 1023 molecules

(near uv)

1-2-3 SuperconductorYBa2Cu3O7

barium

yttrium

copper

oxygen

Resistivityresistivity

temperatureresistivity

temperature

metal superconductor

Tc

Superconductivity

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Heavier Members of a Familytend to form single bonds

C OO

carbon dioxide Si

O O

Si

O

Si

O

OSi

O

O

Si

O

O

silicon dioxidequartz, glass, sand

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Si

Si

SiSi

Si

silicon