Cl-capped CdSe nanoc ys als ia in si u gene a ion
o chlo ide anions†
C is ina Palencia,‡*
a
Koen Lauwae ,
a
Leono de la Cue a,
a
Ma ´
ıa Aceb ´
on,
a
Julio J. Conde,
a
Michaela Meyns,
b
Ch is ian Klinke,
b
Jos´
e M. Gallego,
ac
Robe o O e o
ad
and Bea iz H. Ju´
a ez‡*
ae
Halide ions cap and s abilize colloidal semiconduc o nanoc ys al (NC) su aces allowing o NCs su ace
in e ac ions ha may imp o e he pe o mance o NC hin film de ices such as pho o-de ec o s and/o
sola cells. Cu en ways o in oduce halide anions as ligands on su aces o NCs p oduced by he ho
injec ion me hod a e based on pos -syn he ic ea men s. In his wo k we explo e he possibili y o
in oduce Cl in he NC ligand shell in si u du ing he NCs syn hesis. Wi h his aim, he effec o 1,2-
dichlo oe hane (DCE) in he syn hesis o CdSe od-like NCs p oduced unde diffe en Cd/Se p ecu so
mola a ios has been s udied. We epo a double ole o DCE depending on he Cd/Se p ecu so mola
a io (ei he unde excess o cadmium o selenium p ecu so ). Acco ding o mass spec ome y (ESI-
TOF) and nuclea magne ic esonance (
1
H NMR), unde excess o Se p ecu so (Se dissol ed in
ioc ylphosphine, TOP) condi ions a 265 C e hane-1,2-diylbis( ioc ylphosphonium)dichlo ide is
eleased as a p oduc o he eac ion be ween DCE and TOP. Acco ding o XPS s udies chlo ine ge s
inco po a ed in o he CdSe ligand shell, p omo ing e-shaping o od-like NCs in o py amidal ones. In
con as , unde excess Cd p ecu so (CdO) condi ions, DCE eac s wi h he Cd complex eleasing
chlo ine-con aining non-ac i e species which do no igge NCs e-shaping. The amoun o chlo ine
inco po a ed in o he ligand shell can hus be con olled by p ope ly uning he Cd/Se p ecu so mola a io.
In oduc ion
Colloidal semiconduc o nanoc ys als (NCs) show unable and
pho os able luminescence p ope ies along wi h b oadband
abso p ion and na ow emission, which makes hem sui able
o op oelec onic and pho o ol aic applica ions.
1,2
Fo hese
pu poses, ne con ol o e he syn he ic p ocedu e o ob ain
monodispe se, obus and highly emissi e semiconduc o NCs
is equi ed. The de elopmen o he ho -injec ion me hod
3
p o ides high con ol o e size, shape, shape dis ibu ion and
NCs su ace composi ion o semiconduc o ma e ials such as
CdSe,
4,5
PbSe
6,7
o PbS.
8,9
This me hod also allows ne con ol
o e he NCs g ow h, which is achie ed by he use o long alkyl
chain su ac an s ac ing as bo h me al complexing agen s and
ligands. These ligands o m an insula ing laye be ween NCs,
es ic ing he cha ge anspo be ween hem and, as a
consequence, de ices con aining such NCs (sola cells and
pho ode ec o s
10,11
) may show low con e sion efficiencies. A less
insula ing ligand shell is ob ained by he exchange o hese long
alkyl chain molecules o sho e ones, including hiols
12,13
o
amines,
14–16
in pos - eac ion ea men s. The use o halides o
pe o m ligand exchange eac ions has ecen ly been
p oposed.
11,17–19
Halide anions can be conside ed as he sho es
X ype (ionic) ligands, acili a ing cha ge anspo among NCs.
The passi a ing ole o halide anions in PbS sys ems has been
epo ed
11
and o he epo s ha e e idenced he passi a ion
effec o chlo ine in CdSe NCs, highligh ing he possibili y o
using halogena ed molecules o efficien ly displace bo h L
(coo dina i e) and/o X ype ligands.
18,19
Pos - eac ion ea -
men s wi h chlo ine-con aining solu ions seem o p o ide some
bene s o ce ain colloidal NCs, such as mo e obus op ical
a
IMDEA Nanoscience, c Fa aday 9, Campus de Can oblanco, 28049 Mad id, Spain.
E-mail: c is in[email p o ec ed]ambu g.de; bea iz.he nandez@
imdea.o g
b
Ins i u e o Physical Chemis y, Uni e si y o Hambu g, G indelallee 117, 20146
Hambu g, Ge many
c
Ins i u o de Ciencia de Ma e iales de Mad id, ICMM, CSIC, So Juana In´
es de la C uz
s n, 28049 Mad id, Spain
d
Dp o. de F´
ısica de la Ma e ia Condensada and Ins i u o Nicol´
as Cab e a, Facul ad de
Ciencias, Uni e sidad Au ´
onoma de Mad id UAM, A da. Fco. Tom´
as y Valien e 7,
28049 Mad id, Spain
e
Dp o. de Qu´
ımica F´
ısica Aplicada, Facul ad de Ciencias, Uni e sidad Au ´
onoma de
Mad id UAM, A da. Fco Tom´
as y Valien e 7, 28049 Mad id, Spain
†Elec onic supplemen a y in o ma ion (ESI) a ailable: Table S1 shows he mola
amoun s o he p ecu so used o p epa e CdSe NCs om diffe en Cd/Se
p ecu so mola a ios. Fig. S2 shows TEM images o he con ol expe imen s.
These consis o he syn hesis o CdSe NCs om Cd/Se p ecu so mola a ios
o 4, 2, 1, 0.5 and 0.25 in he absence o DCE. Table S3 shows he ela i e XPS
peak a eas o CdSe samples syn hesized om diffe en Cd/Se p ecu so mola
a ios. See DOI: 10.1039/c4n 00431k
‡Cu en add ess: Ins i u e o Physical Chemis y, Uni e si y o Hambu g,
G indelallee 117, 20146 Hambu g, Ge many.
Ci e his: Nanoscale,2014,6, 6812
Recei ed 22nd Janua y 2014
Accep ed 30 h Ma ch 2014
DOI: 10.1039/c4n 00431k
www. sc.o g/nanoscale
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p ope ies due o be e passi a ion.
20
Fu he mo e, be e
pe o mances o NC-based sola cells ha e been achie ed upon
chlo ine-based ea men s o he NCs ollowed by he mal
annealing.
2
On he o he hand, chlo ide anions a e known o o m
complexes o Cd chalcogenides and hus, nuclea ion and
g ow h kine ics can be manipula ed by he use o halides.
21
Halides can be also used as e ching agen s, al e ing he ini ial
shape o NCs.
22–24
The e-shaping o CdSe od-like in o py a-
midal NCs is ano he example in which he p esence o chlo ine
in he ligand shell also shows some bene s o p omo e in e -
ac ions wi h g aphi ic su aces, allowing o he ab ica ion o
composi es made o g aphi ic ca bon allo opes and semi-
conduc o NCs.
25–27
In his wo k we ha e pe o med a sys ema ic s udy on he
size, shape and su ace chlo ine con en o CdSe NCs syn he-
sized using diffe en Cd o Se p ecu so mola a ios in he
p esence o a chlo ina ed co-sol en (1,2-dichlo oe hane, DCE).
T ansmission elec on mic oscopy (TEM) and X- ay pho oelec-
on spec oscopy (XPS) ha e been used o co ela e he
mo phology and su ace composi ion. Unde excess Se
p ecu so condi ions, ini ial od-like NCs modi y hei shape
inuenced by chlo ide anions, which a e eleased h ough a
eac ion be ween TOP and DCE a he syn hesis empe a u e
(265 C). These anions ge inco po a ed in o he NCs ligand
shell and igge he abo e men ioned e-shaping om ods o
py amids. The eac ion be ween TOP and DCE has been e i-
denced by nuclea magne ic esonance (
1
H NMR) and mass
spec ome y (ESI-TOF). Densi y unc ional heo y (DFT) calcu-
la ions suppo he easibili y o he mechanism.
The inco po a ion o halogen anions as ligands du ing he
NCs syn hesis ep esen s a e sa ile p ocedu e, which may ha e
ad an ages in solu ion p ocessed op oelec onics.
Expe imen al sec ion
Ma e ials
Cadmium oxide (CdO, 99.998%) and oc adecylphosphonic acid
(ODPA, 97%) we e pu chased om Al a Aesa and i-
oc ylphosphine oxide (TOPO, 98%) om Me ck. Ald ich
supplied ioc ylphosphine (TOP, 90%), oluene (99.5%) and
me hanol (99.8%). 1,2-Dichlo oe hane (DCE, 99.5%) was
pu chased om Pan eac. All hese chemical eagen s and
sol en s we e used wi hou u he pu ica ion. XPS expe i-
men s we e pe o med on NCs co e ing Highly O ien ed Py o-
ly ic G aphi e subs a es (HOPG) ZYB quali y, wi h a hickness
o 2 mm pu chased om NT-MDT.
In si u chlo ine inco po a ion in o he CdSe NCs ligand shell
Syn hesis om diffe en Cd/Se p ecu so mola a ios. CdSe
NCs syn hesis was pe o med ollowing a a ia ion o he me hod
p e iously epo ed.
28
NCs ob ained om a Cd/Se p ecu so
mola a io o 0.5 we e p epa ed by s i ing and hea ing (unde
ni ogen) 0.025 g (2 10
4
mol) o CdO (cadmium p ecu so )
and 0.20 g (6 10
4
mol) o ODPA, using 2.9 g (8 10
3
mol) o
TOPO as eac ion medium. The solu ion u ned clea a a ound
250 C, e idencing he Cd(ODPA)
2
complex o ma ion. Ae -
wa ds he empe a u e was dec eased o 80 C and 4 mL(510
5
mol) o DCE we e injec ed wi h a Hamil on glass mic osy inge
701SN wi h an 11 cm needle o assu e eaching he le el o he
solu ion. Ae he addi ion o he chlo ina ed sou ce, he
empe a u e was inc eased o 265 C and 0.43 mL o Se dissol ed
in TOP (selenium p ecu so ) (1 M) (4 10
4
mol) we e injec ed
o nuclea ion. Fo he g ow h egime, he empe a u e was kep
a 255 C o 21h.Ae his, he eac ion was quenched by
dec easing he empe a u e o 70 C and adding 3 mL o oluene.
The p epa a ion o NCs om diffe en Cd/Se p ecu so mola
a ios was ca ied ou analogously bu wi h diffe en ela i e Cd,
Se and chlo ine p ecu so concen a ions. In o de o acili a e
he Cd(ODPA)
2
complex o ma ion, he ODPA/CdO mola a io
was xed o 3 in all cases. De ailed in o ma ion abou he co -
esponding amoun s can be ound in Table S1 (see ESI†). As
p e iously men ioned, CdSe NCs wi h chlo ine on hei su ace
a e p one o deco a e g aphi ic su aces.
25–27
Thus, he CdSe
syn he ic eac ions we e ca ied ou in he p esence o HOPG
subs a es (1 cm
2
) included in he po .
27
This me hodology
gua an ees he co e age o one single laye o NCs on he su ace,
which u ns ou o be essen ial o p e en cha ging effec s du ing
XPS measu emen s. Once he eac ion was nished, CdSe NCs
a ached o he HOPG subs a es we e pu ied by washing he
subs a es in oluene ba hs. NCs no a ached o he HOPG
subs a es we e pu ied by cen i uga ion/ edispe sion
p ocesses, using oluene as sol en and me hanol as non-sol en .
Finally, he pu ied NCs we e edispe sed in oluene and main-
ained in he da k a 4 C. CdSe NCs we e syn hesized om Cd/Se
p ecu so mola a ios o 4, 2, 1, 0.5 and 0.25 in he p esence o
DCE. Blank expe imen s we e also ca ied ou in he absence o
DCE o disca d he effec o hediffe en Cd/Se mola a ios on
he e-shaping o he NCs (images in Fig. S2 in he ESI†).
Chlo ine inco po a ion in o he CdSe NCs ligand shell in wo
s eps
Syn hesis om CdSe ods. P e iously p epa ed and pu ied
CdSe ods we e used in all he cases as s a ing samples. They ha e
been syn hesized analogously o he one-s ep CdSe NCs in he
absenceo DCE.CdSe odswe epu ied by cen i uga ion/ e-
dispe sion in oluene and me hanol and kep in he da k a 4 C.
The esul ing CdSe ods show an a e age diame e o 4.3 0.5 nm,
aleng ho 11.81.3 nm and an abso p ion maximum o 623 nm.
To pe o m he chlo ine inco po a ion eac ions, 2 mL o he CdSe
ods (op ical densi y 1.12) we e loaded wi h 2.9 g o TOPO unde a
ni ogen a mosphe e in o a h ee-neck eac ion ask and hea ed
un il 120 C. The mix u e was s i ed unde acuum a his
empe a u e o comple ely elimina e he oluene. To add he
equi ed amoun o DCE, he empe a u e was d opped un il 80 C.
Then, he mix u e was hea ed un il he TOP injec ion empe a u e
(265 C) was eached. Aliquo s we e aken a 2 h, 4 h, 5.3 h and 21 h
o ollow he e-shaping o he NCs wi h eac ion ime.
Expe imen s o he chlo ide gene a ion s udy
To p e en oxida ion o TOP, he di ec mixing o DCE and TOP
as well as he eac ion a 265 C and he samples p epa a ion o
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NMR and mass spec ome y expe imen s we e ca ied ou
unde ni ogen. In a h ee-neck ask coupled o a Schlenk line
an equimola mix u e o DCE and TOP was s i ed a oom
empe a u e o some minu es. Ae wa ds, a  s aliquo was
aken as a ep esen a i e sample o he di ec mixing o DCE
and TOP a oom empe a u e. A second aliquo was aken ae
keeping he empe a u e a 265 C o 21 hou s. NMR samples
we e p epa ed using deu e a ed chlo o o m (CDCl
3
) as sol en .
The NMR ubes, con aining he p ecise amoun o CDCl
3
we e
pu ged and lled unde ni ogen.
1
H NMR spec a we e eco -
ded on a B uke AMS-300 MHz.
1
H NMR spec a we e acqui ed
wi h a sufficien elaxa ion delay o allow comple e elaxa ion
be ween pulses. The samples o mass spec ome y we e
p epa ed analogously, using chlo o o m as he mobile phase.
Mass spec al da a we e acqui ed in an ESI-TOF (6224, Agilen )
mass spec ome e equipped wi h a 1200 Agilen HPLC. The
da a we e acqui ed in a mass ange o m/z110–3200.
Cha ac e iza ion
Abso p ion measu emen s we e ca ied ou in a Va ian Spec-
opho ome e Ca y 50. Pho oluminescence spec a we e
eco ded in a spec ouo ome e Ho iba Jobin Y on Fluo omax-
4. T ansmission elec on mic oscopy (TEM) images we e
ob ained in a JEOL 1010 mic oscope, ope a ing wi h an accel-
e a ion ol age o 100 kV. Size dis ibu ion da a we e ob ained
by measu ing he NC sizes om TEM images. A leas one
hund ed NCs we e measu ed in each sample. XPS measu e-
men s ha e been pe o med using a monoch oma ed Al-Ka
sou ce (hn¼1486 eV) and a PHOIBOS 150 elec on ene gy
analyze . Spec a o Cd 3d, Se 3d, and P 2p we e eco ded using
a pass ene gy o 10 eV, whe eas spec a o Cl 2p we e eco ded
wi h 20 eV. Using hese pass ene gies we ob ained a FWHM o
he Cd 3d
5/2
line o 1 eV and 1.3 eV, espec i ely. The binding
ene gies we e e e enced o Cd 3d
5/2
a 405.2 eV. Cha ge
neu aliza ion was deemed o be unnecessa y, since no mo e-
men o he Cd 3d
5/2
line wi h ime had been de ec ed. In o de
o e alua e he easibili y o he eac ion be ween TOP and DCE,
DFT calcula ions had been pe o med using he ORCA so-
wa e.
29
The simula ions we e pe o med using he hyb id
unc ional B3LYP wi h he Ahl ichs TZV basis se .
30
Resul s and discussion
Py amidally shaped CdSe NCs syn hesized in he p esence o
he chlo ina ed p ecu so DCE can be in e p e ed as a nge -
p in o he chlo ine inco po a ion in o he ligand shell.
27
To
su ey he co ela ion be ween Cd, Se and Cl p ecu so s in he
chlo ine inco po a ion in o he NC ligand shell, we de eloped a
se ies o expe imen s in which he Cd/Se p ecu so mola a ios
we e a ied om 4 (excess o Cd p ecu so , CdO) o 0.25 (excess
o Se p ecu so , Se@TOP), as s a ed in Table S1 (see ESI†).
Rep esen a i e TEM images o CdSe samples ob ained o Cd/Se
p ecu so mola a ios o 2, 1 and 0.5 a e depic ed in Fig. 1 along
wi h size his og ams bo h o he diame e and o he leng h o
he NCs.
When he NCs syn hesis is ca ied ou in he absence o DCE,
he shape o he NCs is od-like ega dless he Cd/Se p ecu so
mola a io (see TEM images o he con ol expe imen s in
Fig. S2, ESI†). As i can be obse ed in he TEM images in Fig. 1,
o s oichiome ic Cd/Se condi ions o unde Se excess
p ecu so condi ions he addi ion o DCE du ing CdSe syn hesis
leads o a e-shaping om ods in o py amids. Howe e , unde
excess Cd p ecu so condi ions he addi ion o DCE does no
igge he e-shaping. Thus, he inco po a ion o chlo ine
migh be d i en by a chlo ine-con aining species (diffe en om
he p ecu so , DCE) which mus be gene a ed in si u unde
ce ain eac ion condi ions, in which he Cd/Se p ecu so mola
a io migh play a c ucial ole. In o de o quan i y he effec o
DCE in he shape e olu ion, he size o od-like and py amidal
NCs has been measu ed o he samples shown in Fig. 1. As
clea ly seen in he his og ams, he mo phological aniso opy
obse ed o samples ob ained unde excess o Cd p ecu so
(Fig. 1a) u ns in o a mo e iso opic shape o samples wi h
s oichiome ic Cd/Se a ios o excess o Se p ecu so (Fig. 1b
and c). Aspec a ios a ound 3 a e obse ed o ods and close o
1 o py amids. The wid h o he size dis ibu ion o he NCs
does no exceed 10% o he mean adius o Cd/Se p ecu so
mola a ios anging om 2 o 0.5. The syn hesis wi h mola
a ios ou o his ange yields ei he bigge NCs and/o highly
polydispe sed ones (i.e. NCs ob ained om Cd/Se p ecu so
mola a ios o 4 and 0.25, images no shown). We a ibu e
such polydispe si y o he la ge diffe ence be ween Cd and Se
p ecu so concen a ions, which migh enable a high monome
sa u a ion egime du ing nuclea ion and  s g ow h s ages,
p omo ing u he se e e Os wald ipening.
28
Howe e , he Cl
inco po a ion can be con olled so as o ob ain smalle and
highly monodispe se NCs, as i will be demons a ed below.
Fig. 1 TEM images o CdSe NCs ob ained a e 21 hou s o eac ion om
Cd/Sep ecu so mola a ioso 2,1and0.5in hep esenceo DCE.The
inse in (b) shows a py amidal NC in he [001] di ec ion. The his og ams
co espond o he leng h (blue) and diame e s ( ed) o he NCs.
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In o de o ob ain quan i a i e su ace composi ion in o -
ma ion and o in es iga e he in eg a ion o Cl in he ligand
shell XPS analyses ha e been pe o med. Since he P 2p and Cl
2p XPS peaks a e ela ed exclusi ely o he NCs ligand shell,
27
we
calcula ed he no malized a eas Cl/Cd and P/Cd, be ween
elemen s comp ising he ligand shell (Cl o P) and a componen
o he NC co e (Cd). Fig. 2 shows he XPS Cl/Cd and P/Cd
no malized a eas plo ed as a unc ion o he Cd/Se p ecu so
mola a io in he syn hesis. The nume ic da a conce ning he
no malized a eas s udied o each sample a e ga he ed in Table
S3,†whe e he Se/Cd and he Cl/P no malized a eas ha e also
been included (ESI†). I is wo h no ing ha hese a ios do no
co espond o quan i a i e concen a ion alues o elemen s
p esen in he samples bu o quali a i e amoun s o diffe en
elemen s composing he shell ela i e o a gi en amoun o Cd.
As i can be obse ed in Fig. 2, he Cl/Cd and P/Cd XPS
no malized a eas show no iceable a ia ions as a unc ion o he
Cd/Se p ecu so mola a io. Fo low Cd/Se alues (Cd/Se < 1),
low P/Cd and high Cl/Cd alues a e obse ed. This end is he
opposi e o high Cd/Se alues (Cd/Se > 1). In a p e ious wo k we
p oposed ha he ligand shell o od-like CdSe NCs is composed
o oc adecylphosphonic acid (ODPA) ela ed species (bo h
double dep o ona ed oc adecylphosphonic acid and i s anhy-
d ides) whe eas inco po a ion o chlo ine as he ligand en ails
he subs i u ion o ODPA anhyd ides and u he e-shaping o
he ini ial ods.
27
The ac ha he Cl/Cd and P/Cd a ios ollow opposi e
ends as a unc ion o he Cd/Se p ecu so mola a io suppo s
his mechanism. Based on hese esul s, we obse e ha he
minimum NC chlo ine con en (as Cl/Cd) necessa y o pe o m
he e-shaping om ods o py amids is a ound 0.17. The ac
ha as he Cd/Se p ecu so mola a io dec eases (1 o 0.25), he
Cl/Cd a io ob ained om he XPS da a inc eases (0.17 o 0.22)
indica es a mo e effec i e Cl inco po a ion in o he ligand shell
unde excess Se p ecu so condi ions. A en a i e ligand dis i-
bu ion has been ep esen ed in he schemes o Fig. 2. Ou
esul s indica e ha while an excess o Cd p ecu so does no
a ou he inco po a ion o Cl in o he ligand shell, he excess o
Se p ecu so does, poin ing o diffe en pa hways o DCE du ing
he eac ion depending on he Cd/Se p ecu so mola a io.
CdSe NCs syn hesized unde ini ial excess Cd condi ions a e
od-shaped e en wi h he addi ion o ela i ely la ge DCE
amoun s (see Table S1†).
This can be a ionalized assuming a eac ion be ween he
excess o he Cd complex (Cd(ODPA)
2
) and he chlo ina ed
p ecu so , o ming compounds such as CdCl
2
, ha may modi y
bo h nuclea ion and/o g ow h p ocesses.
21,22,24,31
These chlo i-
na ed species, howe e , do no igge a shape modica ion o
he NCs (see Fig. 1). The eac i i y be ween he Cd–ODPA
complex and chlo ine is u he p o ed by a se ies o expe i-
men s in which DCE/CdO mola a ios exceeding 1.3 make he
Cd p ecu so unable o eac wi h Se@TOP. This esul
e idences he abili y o DCE o displace ODPA ligands om he
Cd complex, which may also lead o pa ial subs i u ion o
ODPA in he complex (i.e. Cl–Cd–ODPA). The e o e, unde
excess Cd p ecu so condi ions, he Cd complex migh mainly
ha e e ol ed o CdCl
2
(no igge ing he e-shaping), al hough a
small amoun may o m Cl–Cd–ODPA complexes. The NCs
ob ained om he la e complexes a e expec ed o ha e bo h
ODPA ela ed species and a small amoun o Cl in he ligand
shell. This could be he case o he low chlo ine con en ound
in od-like CdSe NCs syn hesized om Cd/Se p ecu so mola
a ios o 2 and 4 (see ligand dis ibu ion schemes in Fig. 2 and
nume ic da a in Table S3, ESI†).
In con as o excess Cd p ecu so condi ions, he inco po-
a ion o Cl in o he ligand shell unde excess Se p ecu so is
clea ly a ou ed. We pe o med a se ies o blank expe imen s o
conclude ha he eac ion be ween TOP (o TOP@Se) and DCE
is esponsible o he inco po a ion o Cl in o he NCs ligand
shell, igge ing he e-shaping o he NCs. As a esul o his
knowledge, con olled inco po a ion o chlo ine in o he ligand
shell o CdSe NCs can be ob ained a will, ei he in si u o in a
wo-s ep p ocess by pos - ea men s o p e iously syn hesized
NCs (see Expe imen al sec ion).
A ep esen a i e sample is shown in Fig. 3 whe e p e iously
syn hesized od-like CdSe NCs ha e been used o ca y ou
sys ema ic expe imen s. This wo-s ep syn hesis was designed
wi h he double aim o ob aining smalle and highly mono-
dispe se chlo ine-con aining CdSe NCs and o con m he
possibili y o pe o m he chlo ine inco po a ion o e p e i-
ously syn hesized CdSe NCs. Fig. 3a co esponds o he ini ial
4.3 nm od-like CdSe NCs used as he s a ing ma e ial and
Fig. 3b depic s he TEM images o he CdSe NCs ob ained ae
mixing he s a ing od-like CdSe NCs wi h DCE and TOP (see
Expe imen al sec ion). As e idenced by he images (Fig. 3b), his
p ocedu e leads o quasi-monodispe sed py amidal shaped
CdSe NCs ae 2 hou s o eac ion. Longe eac ion imes did
no p oduce obse able diffe ences in shape, size o size
dis ibu ion (images no shown). Howe e , DCE/TOP mola
a ios highe han 0.29 poin o a se e e e ching effec (disso-
lu ion o NCs ae 30 minu es o eac ion), d i en by a highe
concen a ion o he gene a ed chlo ide anions, which should
be e en ually esponsible o he NCs dissolu ion. The abso p-
ion and emission p ope ies o he ini ial ods and py amids
ob ained by he wo-s ep p ocedu e a e depic ed in Fig. 3. An
Fig. 2 Cl/Cd (ci cles) and P/Cd (da k squa es) no malized a eas
calcula ed om XPS da a and plo ed e sus he Cd/Se p ecu so
mola a ios. The ho izon al con inuous line indica es he minimum Cl
alue o p omo e he e-shaping o od-like o py amidal-like NCs. The
discon inuous cu ed lines a e only a guide o he eyes. The expe i-
men al e o associa ed wi h he ela i e peak a eas is a ound 10%.
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expec ed ed shiis obse able o py amids due o bigge sizes
(see his og am). Fu he mo e, an inc ease in he pho o-
luminescence (PL) in ensi y is appa en o py amids compa ed
o ods. Se e al wo ks ha e epo ed on he PL inc ease upon
passi a ion wi h chlo ine.
19,20,23
In he he e epo ed case, since
he wo-s ep me hod includes a new syn he ic p ocedu e in
ex a TOP and TOPO (known o con ain phosphonic-acid
impu i ies), he PL inc ease may also be ela ed o a be e
passi a ion due o phosphonic impu i ies and no o he p es-
ence o chlo ide on he su ace. Thus, he PL in ensi ies o ods
and py amids a e no di ec ly compa able. This opic will be he
ocus o u he esea ch.
In o de o ollow he eac ion and o unde s and he inu-
ence o his combina ion du ing he NCs syn hesis, Nuclea
Magne ic Resonance (
1
H NMR) and Mass Spec ome y (ESI-
TOF) ha e been employed o analyse he eac i i y be ween TOP
and DCE a RT and a 265 C. Upon mixing DCE and TOP a RT
no app eciable physical change was obse ed. Howe e , du ing
he cooling s ep om 265 C, a high inc ease o iscosi y o he
eac ion mix u e was e iden below 80 C, sugges ing he
o ma ion o new p oduc s. The
1
H NMR spec a o he ini ial
mix u e DCE/TOP ( op) and he esul ing eac ion p oduc ae
ea men a 265 C (bo om) a e shown in Fig. 4a. The peaks
obse ed in Fig. 4a ( op) a e he ones cha ac e izing he indi-
idual un eac ed species. A sha p single peak is obse ed a
a ound 3.7 ppm, which is assigned o he p o ons in he
me hylene g oups o DCE. A se ies o mul iple peaks wi h
diffe en in ensi ies can be obse ed in he ange be ween 0.9
and 1.4 ppm, which a e assigned o he p o ons in he alkyl
chains. The peak a a ound 0.9 ppm is in ag eemen wi h he
chemical shio me hyl g oups, while he peak a a ound 1.3 is
ela ed o he p o ons in me hylene g oups co esponding o C
4
,
C
5
,C
6
and C
7
. The small shoulde a a ound 1.4 ppm has been
assigned o he p o ons si ua ed in C
1
and C
2
in TOP, since hey
migh be mo e deshielded by P. The smalle peaks be ween 1.5
and 1.7 ppm a e in ag eemen wi h p o ons C
1
and C
2
in he
alkyl chains in TOPO. Thus, we canno disca d a sligh oxida-
ion o TOP in o TOPO du ing he eac ion due o impu i ies in
DCE o e en du ing he analysis p ocedu e. Besides ha , all
hese peaks co espond o he indi idual species, DCE and TOP,
disca ding any eac ion a oom empe a u e. The compa ison
wi h he
1
H NMR spec um ob ained a 265 C indica es ha he
main diffe ence is he absence o he peak assigned o DCE. On
he o he hand, he se ies o peaks obse ed be ween 0.9 and 1.7
do no allow us o es ablish a clea iden ica ion o he eac ion
p oduc beyond ha i migh ha e a s uc u e simila o i-
oc ylphosphines. The chemical shiobse ed a 1.1 ppm could
be a ibu ed o p o ons si ua ed close o chlo ine and oxygen
a oms.
In o de o be e in es iga e he eac ion be ween TOP and
DCE a high empe a u e, we ha e pe o med ESI-TOF mass
spec ome y analysis o he eac ion p oduc . The ESI-TOF
spec um o TOP (Fig. 4b) e idences he absence o signican
impu i ies, indica ing ha he main eac ion p oduc s should
p oceed om he eac ion be ween TOP and/o TOPO and DCE.
Fig. 3 Chlo ine inco po a ion in wo s eps: TEM images, size dis i-
bu ion da a and op ical measu emen s o he ini ial oluene dispe sed
od-like CdSe NCs (a) and he py amidal-like CdSe ones (b) esul ing
a e 2 h o eac ion wi h a DCE/TOP mola a io o 0.06 a 265 C.
Fig. 4 (a)
1
H NMR spec a o he di ec mixing (unde ni ogen) o DCE
and TOP ( op) and he p oduc ob ained a e eac ion a 265 C
(bo om). (b) ESI-TOF mass spec um o TOP, demons a ing no
ele an impu i ies. The inse shows a smalle spec al ange whe e he
(M + 1)
+
and i s co esponding iso opes a e obse ed. (c) ESI-TOF mass
spec um o he p oduc ob ained om he eac ion be ween DCE and
TOP a 265 C (e hane-1,2-diylbis( ioc ylphosphonium)dichlo ide).
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Fig. 4c shows he ESI-TOF spec um o he eac ion p oduc . The
main peak domina ing he spec um is si ua ed a m/z384.39.
This peak can be unambiguously a ibu ed o e hane-1,2-diyl-
bis( ioc ylphosphonium)dichlo ide. This assignmen is also
suppo ed by he ac ha he M + 1 and M + 2 peaks o e hane-
1,2-diylbis( ioc ylphosphonium)dichlo ide a e sepa a ed by a
diffe ence o 0.5, acco ding o a doubly cha ged p oduc
(a zoomed spec um can be ound in he ESI, Fig. S3†). The
in ensi y ela ions co espond o he abundance o he ca bon
iso opes. The peaks a 371.38 and 387.37 a e assigned o TOP
and TOPO, espec i ely, and co espond o he singly cha ged
species which ha e peaks o M + 1 and M + 2 sepa a ed by a
diffe ence o 1. The p esence o un eac ed TOP and a small
amoun o TOPO is also suppo ed by
1
H NMR. The peak in he
ESI-TOF spec um (m/z¼384.39, Fig. 4c), oge he wi h he
disappea ance o he DCE peak ae mixing wi h TOP a high
empe a u e (Fig. 4a) e idences he e hane-1,2-diylbis-
( ioc ylphosphonium)dichlo ide gene a ion. This eac ion may
occu h ough a subs i u ional mechanism, as depic ed in
Scheme 1. I is clea ha he p oposed eac ion mechanism in
Scheme 1 only occu s a ela i ely high empe a u es, since he
simple mixing o TOP and DCE a RT does no p oduce any
1
H
NMR peaks diffe en om he ones cha ac e izing he indi-
idual species. In o de o suppo he p oposed mechanism o
eac ion be ween DCE and TOP we employed DFT simula ions
(B3LYP/TZV). Fo ha , we calcula ed he en halpy o he eac-
ion shown in Scheme 1 by sub ac ing he calcula ed en halpy
o o ma ion o he p oduc s om he ones o he ini ial
eagen s. Two molecules o TOP eac wi h one molecule o DCE
o o m e hane-1,2-diylbis( ioc ylphosphonium)dichlo ide.
This esul s in a eac ion en halpy o 0.88 eV indica ing he
easibili y o he men ioned eac ion.
Acco ding o he li e a u e, chlo ide anion gene a ion could
be achie ed ei he by chemical o pho ochemical ou es.
Ande son and Owen ha e epo ed he possibili y o exchange
ca boxyla e ligands wi h chlo ide ligands in CdSe NCs by mixing
hem wi h chlo o ime hylsilane in Bu
3
P solu ion a RT.
19
In a
ecen pape by Lim e al.
23
he gene a ion o chlo ide anions is
sugges ed as he main d i ing o ce o he aniso opic e ching
o phosphonic capped CdSe NCs, which is in acco dance wi h
ou esul s. Howe e , hey did no obse e py amidal shapes,
which migh be due o he sho eac ion ime in hei case.
These wo s udies epo on he gene a ion o chlo ide anions a
oom empe a u e. Howe e , as shown in Fig. 4, only a high
empe a u es he up u e o he DCE molecule is appa en . The
ole o o he halogen-con aining sou ces (i.e., b omine, iodine)
in he shape e olu ion o he NCs has also been ecen ly
epo ed.
32
Conclusions
The addi ion o a chlo ina ed sol en (DCE) in he syn hesis o
CdSe NCs p oduces diffe en NC sizes and shapes depending on
he Cd/Se p ecu so s concen a ion. While NCs p oduced unde
excess Cd p ecu so condi ions show od-like shape and a
compa a i ely low amoun o Cl on hei su ace, NCs p oduced
unde excess Se p ecu so condi ions modi y hei shape om
ods o py amids con aining highe amoun s o he halogen on
hei su aces. This effec is p oduced by he p esence o chlo-
ide anions, eleased in si u upon eac ion be ween TOP and
DCE a 265 C. We p opose ha his eac ion p oceeds h ough
he o ma ion o e hane-1,2-diylbis( ioc ylphosphonium)
dichlo ide. The chlo ide anions gene a ion in si u allows high
e sa ili y o inco po a e halogen anions as ligands o a wide
ange o samples, which may ha e ad an ages in solu ion p o-
cessed op oelec onics.
Acknowledgemen s
CP acknowledges nancial suppo om he Eu opean
Commission unde he Se en h F amewo k P og am by means
o he g an ag eemen o he In eg a ed In as uc u e Ini ia-
i e N. 262348 Eu opean SoMa e In as uc u e (ESMI). CK
and MM acknowledge he Deu sche Fo schungsgemeinscha
(DFG) o nancial suppo (KL 1453/5-1). Financial suppo
om he Minis e io de Ciencia e Inno aci´
on (FIS2010-18847,
FIS2012-33011 and Consolide -Ingenio en Nanociencia Molec-
ula , e CSD2007-00010), Comunidad de Mad id (g an S2009/
MAT-1726) and EU (SMALL, PITN-GA-2009-23884) is g a e ully
acknowledged. The au ho s hank D Emilio M. P´
e ez, Albe o
de Juan and Alejand o L´
opez o ui ul discussions abou NMR
expe imen s. JJC acknowledges Fundaci´
on Ibe d ola o his
schola ship.
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