Biomedical Engineering Reference
In-Depth Information
color.centers.embedded.in.the.diamond.matrix.and.the.brightness.of.the.emission.scales.
with.the.number.of.the.color.centers.in.the.particles..In.the.following.section,.to.simplify.
the.presentation,.we.will.focus.our.discussion.on.the.nitrogen-vacancy.color.centers.only..
The.NDs.that.contain.a.high.concentration.of.color.centers.are.called.luorescent.nanodia-
monds.(FNDs).(Yu.et.al..2005).
25.2.1 Nitrogen-Vacancy Color Centers
Nitrogen.is.the.most.common.impurity.in.natural.and.synthetic.diamonds.(Davies.1994)..
The. impurities. are. incorporated. into. the. crystal. lattice. as. atomically. dispersed. entities.
or. aggregates. to. form. C-centers. (isolated. substitutional. nitrogen. atoms),. A-centers. (two.
nearest-neighbor.substitutional.nitrogen.atoms),.or.B-centers.(four.substitutional.nitrogen.
atoms. surrounding. a. vacancy).. These. centers. alone. do. not. luoresce. but. can. become.
brightly. luorescent. when. forming. stable. complexes. with. vacancies.. Table. 25.1. lists. the.
photophysical. properties. of. some. vacancy-related. defect. centers. in. diamond.. Among.
these.centers,.the.ones.of.particular.interest.for.optical.bioimaging.applications.are.(N-V)
−
,.
(N-V)
0
,.and.H3.(Hui.2010a)..These.three.centers.are.all.atom-like.and.sit.deep.within.the.
inert.crystal.lattice,.and.therefore.are.exceptionally.photostable.
The.negatively.charged.nitrogen-vacancy.center,.(N-V)
−
,.is.perhaps.the.best.character-
ized. color. center. in. diamond. (Santori. et. al.. 2010,. Aharonovich. et. al.. 2011b).. It. is. a. point.
defect. consisting. of. a. substitutional. nitrogen. atom. adjacent. to. a. carbon. atom. vacancy.
with.
C
3v
.symmetry.(Figure.25.1)..The.center.exhibits.a.zero-phonon.line.(ZPL).at.638.nm,.
accompanied. with. a. broad. phonon. sideband. peaking. at. ∼560.nm. (Davies. and. Hamer.
1976)..The.absorption.cross.section.of.the.center.at.532.nm.is.0.95.×.10
−16
.cm
2
.(Chapman.and.
Plakhotnik.2011)..When.excited.by.green.yellow.light,.the.center.emits.far-red.luorescence.
at.∼700.nm. with. a. near-unity. quantum. yield. (Rand. 1994).. Moreover,. the. luorescence. is.
perfectly. stable,. showing.no.sign.of.photoblinking.and.photobleaching. even.under. con-
tinuous.high-power.laser.excitation.at.room.temperature.(Gruber.et.al..1997)..Because.of.
these.outstanding.features,.the.(N-V)
−
.center.has.been.employed.as.a.single-photon.source.
for.quantum.information.application.(Aharonovich.2011a)..Fu.et.al..have.performed.pho-
tostability.tests.for.the.(N-V)
−
.centers.in.FNDs.(Fu.et.al..2007)..The.luorescence.intensities.
of.the.individual.FND.particles.of.size.of.35.and.100.nm.stay.nearly.the.same.over.a.time.
period. of. 300.s. (Figure. 25.2).. In. contrast,. organic. dye. molecules. such. as. Alexa. Fluor. 546.
photobleach.in.12.s.
Apart. from. (N-V)
−
,. another. type. of. N-V. defect. frequently. encountered. in. ND. is. the.
neutral.defect.center,.(N-V)
0
..Both.the.centers.can.be.readily.found.in.HPHT-synthesized.
type.Ib.ND,.which.typically.contains.100.ppm.of.atomically.isolated.nitrogen.as.impurity..
TABLe 25.1
Properties.of.Some.Vacancy-Related.Defect.Centers.in.Diamond
a
Defect Center
Point Group
ZPL (nm)
λ
em
(nm)
τ (ns)
φ
V
0
.(GR1)
T
d
741.2
898
2.55
0.014
(N-V)
−
C
3v
637.6
685
11.6
0.99
(N-V)
0
C
3v
575.4
600
—
—
N-V-N.(H3)
C
2v
503.5
531
16
0.95
N
3
.+.V.(N3)
C
3v
415.4
445
41
0.29
a.
Zero-phonon.lines.(ZPL),.emission.maxima.(λ
em
),.emission.lifetime.(τ),.and.quantum.
eficiency.(φ)..(Details.can.be.found.in.Davies.1994.)
