quantum dots and forms of nanomaterials.ppt

Anonymous

1. DIFFERENT FORMS OF NANOMATERIALS
DEFINITION
A material with at least one of its dimension in the order of few
nanometers is known as nanomaterial.
Based on the number of electron confinement directions , their individual
shapes and size reduction in various directions, nano materials can be divided into
the following classes
Two dimensional nanomaterials
One dimensional nanomaterials
Zero dimensional nanomaterials

2. DIFFERENTFORMS OF NANOMATERIALS
Two dimensional (2D) materials
one dimension at nanoscale, other two dimension at macroscale.
Hence electrons are confined in one dimensional and are free to move in two
dimensions parallel to the structure.
Definition:
Materials whose thickness is at nano scale and length and breadth are at
macro scale are known as two dimensional nanomaterials.
Example:
Quantum well, thin films, nano coatings, nano layers.

3. DIFFERENTFORMS OF NANOMATERIALS
One dimensional (1D) materials
two dimensions at nanoscale, other one dimension at macroscale.
Hence electrons are confined in two dimensions and are free to move in one
dimension along the structure.
Definition:
Materials whose diameter is at nanoscaleand length typically at macro scale.
Example:
Quantum wire cylinder, nanowires, nanofibers.

4. DIFFERENTFORMS OF NANOMATERIALS
Zero dimensional (0D) materials
All dimensions at nanoscale, hence electrons are confined in all directions
Definition:
Materials in which electron motion is confined in all the three dimension are
called zero –dimensional nanomaterials.
Example:
Quantum dot, particles, hollow spheres microcapsules.

5. Forms of
Nanomaterials

6. Quantum dots
•Quantumdots(QDs)aretinysemiconductorparticles2-10nm
(nanometers,10^-9)indiameter.Becauseoftheirsmallsize,these
particleshaveuniqueopticalandelectricalproperties.Forexample,
whenexposedtolight,quantumdotcrystalsemitlightofparticular
frequencies.
•Thesizeandshapeofquantumdotscanbepreciselycontrolledby
adjustingreactiontimeandconditions,thusmakingthis
nanotechnologyscalableandusefulfordisplayapplications.

7. Quantum dots -
Working
TheprocessoflightemissioninQDsiscalledphotoluminescence
(abbreviatedasPL),asitoccursbecauseoftheexcitationbyphotons.
Undertheinfluenceoflight,photonsgetexcitedand“jump”uptoa
higherenergyband.Thisisfollowedbytheprocessofrelaxation,
duringwhichphotonscanrelaxnon-radiatively(“fallback”)intoa
lower-lyingstateorrecombineandre-radiate.

8. Quantum dots –Unique property
Inregularsemiconductorslikesilicon(alsoknownasbulk
matter),thebandsareformedbythemergerofadjacent
energylevelsofaverylargenumberofatomsand
molecules.However,astheparticlesizereachesthe
nano-scaleandthequantityofatomsandmolecules
decreasessubstantially,thenumberofoverlappingenergy
levelsdecreases,causingthewidthofthebandto
increase.AsQDsaresotiny,theyhaveahigherenergy
gapbetweenthevalenceandconductionbands,
comparedtothebulkmatter.

9. Quantum confinement effect
Quantumconfinementeffectisthechangeintheatomicstructureoftheparticleobservedwhenthe
energybandisaffectedbytheshiftsintheelectronicwaverange.Becausethewaverangeis
comparabletotheparticle'ssize,electronsareconstrainedbythewavelengthboundaries.Hence,
quantumdots'propertiesaresize-dependent,andtheirexcitationsareconfinedinallthreespatial
dimensions.
ConfinementenergyisthekeypropertyofaquantumdotthatexplainstherelationshipbetweenQDs
sizeandthefrequencyoflighttheyemit

10. Quantized (or discrete) electronic states of QD
•BecauseofthesmallsizeofQDparticles,thequantumconfinement
effectcausesalargebandgapwithobservablediscreteenergylevels.
Suchquantizedenergylevelsinquantumdotsleadtoelectronic
structuresthatareinbetweensinglemolecules,whichhaveasingle
gap,andbulksemiconductors,whichhavecontinuousenergylevels
withinbands

12. Unique properties of quantum dots -caused by their unusually high surface to volume ratio
Asthesizeofthecrystaldecreases,thedifferenceinenergybetweenthehighestvalencebandandthe
lowestconductionbandincreases.Moreenergyisthenneededtoexcitethedot,andatthesametime,
moreenergyisreleasedwhenthequantumdotreturnstoitsoriginalrelaxedstate.
Becauseofthisphenomenon,quantumdotscanemitanycoloroflightfromthesamematerialiftheirsize
isaltered.