A one pot organic/CdSe nanoparticle hybrid material synthesis with in situ π-conjugated ligand functionalization

Article abstract of DOI:10.1039/c2cc38544a

A one pot method for organic/colloidal CdSe nanoparticle hybrid material synthesis is presented. Relative to traditional ligand exchange processes, these materials require smaller amounts of the desired capping ligand, shorter syntheses and fewer processing steps, while maintaining nanoparticle morphology.

Full text of DOI:10.1039/c2cc38544a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
A one pot organic/CdSe nanoparticle hybrid material
synthesis with in situ p-conjugated ligand
functionalization†
Cite this: Chem. Commun., 2013,
49, 1321
Received 28th November 2012,
Accepted 20th December 2012
Katherine A. Mazzio,a Ken Okamoto,a Zhi Li,b Sebastian Gutmann,c
Elisabeth Strein,d David S. Ginger,d Rudy Schlafb and Christine K. Luscombe*a
DOI: 10.1039/c2cc38544a
www.rsc.org/chemcomm
A one pot method for organic/colloidal CdSe nanoparticle hybrid CdSe photoluminescence and exhibit shorter photoluminescence
material synthesis is presented. Relative to traditional ligand exchange lifetimes than that of tri-n-octylphosphine oxide (TOPO), a common
processes, these materials require smaller amounts of the desired insulating capping ligand used during the synthesis of CdSe nano-
capping ligand, shorter syntheses and fewer processing steps, while particles.6 They have also been shown to act as acceptors for
maintaining nanoparticle morphology.
photogenerated holes through the aromatic region with good charge
transport properties, and have been used to increase the short-
Functional II–VI and III–V colloidal semiconducting nanoparticles circuit current density relative to TOPO in photovoltaic devices.6
(NPs) have been extensively studied, primarily due to the ease of Some research has been done on the attachment of p-conjugated
controlling their photophysical properties and their potential for molecules to CdSe, but they have relied heavily on ligand exchange
replacement of traditional materials in many applications including processes or by the growth or attachment of conjugated polymers on
optoelectronics and biological labeling.1 The properties of the NPs, preexisting CdSe nanoparticles, often requiring two or three step
including their spectral response, reactivity, stability, and process- processes,8 and it is clear that a simpler and more efficient route to
ability, can be controlled by changing their size, shape, and surface CdSe surface functionalization is needed.
chemistry.2 Most synthetic routes for colloidal NPs use long chain
In this paper, we report a new method for synthesizing organic/
aliphatic ligands with coordinating head groups that act as diffusion CdSe NP hybrid materials where both the organic and inorganic
barriers during growth, and remain coordinated to the NPs as a components are introduced as reagents in the reaction. This method
surface monolayer when growth is complete.3 Efforts to control the has the advantage of being a fast and effective one-pot procedure that
surface chemistry of NPs have primarily relied on ligand exchange attaches ligands with aryl functionality while maintaining the size and
processes that use functional ligands with stronger binding head shape control of the colloidal CdSe NPs. The process is different from
groups to displace the native ligands,4 but these ligand exchange typical ligand exchanges in that it relies on using a precursor ligand to
processes often require a large excess of the desired capping ligands act as one of the reagents. As shown in Scheme 1, in a typical CdSe
and can take several days to exchange at elevated temperatures, synthesis, it is thought that the Cd coordinates to the Se on the
often resulting in incomplete native ligand displacement.5
trialkylphosphine selenide, and then the reaction is driven by the
For optoelectronic applications, it is often desirable to use formation of the PQO bond, resulting in the cleavage of the PQSe
capping ligands that facilitate electron transfer across the NP surface bond and providing CdSe.7 We anticipated that the replacement of
while maintaining the miscibility and solution processability of the some of the trialkylphosphine selenide in the reaction with aryl
material. To this end, the direct attachment of molecules containing
p-conjugated units is more attractive than using insulating aliphatic
ligands. p-conjugated thiols have been shown to effectively quench
a Department of Materials Science and Engineering and Molecular Engineering and
Sciences Institute, University of Washington, Box 352120, Seattle, WA 98195-2120,
USA. E-mail: luscombe@uw.edu
b Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, USA
c Department of Chemistry, University of South Florida, Tampa, FL 33620, USA
d Department of Chemistry, University of Washington, Box 351700, Seattle,
WA 98195-1700, USA
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterizations of the materials synthesized, including ArSP, ArSeP, ArSSAr,
ArSH, and CdSe; methods for NMR, XPS, PL, and TEM; various 1H and 31P NMR,
Scheme 1 CdSe synthesis reaction (top),7 and our proposed reaction (bottom).
and Se3d XPS spectra. See DOI: 10.1039/c2cc38544a
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1321--1323 1321

View Article Online
Communication
ChemComm
Scheme 2 Ligand structures.
phosphorochalcogenoates (ArSP or ArSeP) would allow for a similar
reaction, where the S or Se would coordinate to Cd, and cleavage of
the P–Se bond would achieve formation of CdSeAr, resulting in the
direct attachment of the aromatic group to the NP surface.
Fig. 1 1H NMR spectra of neat ArSP (bottom), neat ArSH (center), and ArSP after
attachment on CdSe nanocrystals (top).
CdSe nanoparticles were synthesized according to a modified
procedure of the CdO/amine route synthesis of Yu and Peng9 with the
inclusion of aryl phosphonate ligand precursors (ArSP or ArSeP) to the ArSP, followed by the cleavage of the P–S bond. A representative
designed in our lab (see ESI† for synthetic details), whose structure 1H NMR spectrum can be found in Fig. S2 of the ESI.†
is shown in Scheme 2. In our case, nanoparticles were grown from a
In order to determine if our ligands were attached to our NPs, we
mixture of Cd-oleate, tributylphosphine selenide (SeTBP) and ArSP or first looked at 1H NMR spectra in CDCl3 of the neat ArSP precursor
ArSeP precursors with octadecylamine (ODA) in 1-octadecene, which ligands and the spectra of the NPs synthesized in the presence of
1
was heated to 260 1C for 18 min under nitrogen to afford functiona- these precursors. Fig. 1 shows the H NMR spectra of neat ArSP
lized CdSe NPs. Control experiments were also performed in the (bottom), neat ArSH (middle), and ArSP after attachment on CdSe
absence of our precursor ligands. The resulting mixtures were nanocrystals. The spectrum of neat ArSP shows the protons from the
extracted into hexanes and washed with methanol, with reaction aryl group at 6.75 ppm, the ethoxy protons of the phosphonate as a
by-products being removed in the methanol. The hexanes were multiplet at 4.18 ppm, and the a-CH2 alkoxy protons off the aryl
removed by rotary evaporation and the NPs were then stored in the group at 3.94 ppm. The aryl proton signal shifts to 6.49 ppm for
dark. For comparison, ligand exchange reactions were also performed the ArSH ligand, with the a-CH2 aryl alkoxy protons remaining at
with the control NPs to replace the ODA ligands with the thiol (ArSH) 3.94 ppm. When CdSe nanoparticles are grown in the presence of
and disulfide (ArSSAr) ligands shown in Scheme 2 using protocols ArSP, several changes in the spectra are observed. The ethoxy protons
described in the literature.5b ODA coated CdSe NPs were dispersed in disappear from the spectrum while the aryl alkoxy protons remain at
CHCl3 and the desired exchange ligand (ArSSAr or ArSH) was added. 3.94 ppm, indicating the cleavage of the phosphonate group from
This mixture was stirred at 60 1C for 3 days under nitrogen and the precursor ligand. In addition, the aryl protons at 6.76 ppm
1
monitored each day by H NMR to determine the progress of the disappear, and three new peaks are developed, corresponding
ligand exchange (see Fig. S1 of the ESI†). 10 mL CHCl3 was added to ArSH at 6.49 ppm, ArSSAr at 6.69, and ArS–C2H5 at 6.57 ppm
daily to compensate for the CHCl3 lost overnight. With ArSSAr, no (See Fig. S3 in the ESI†). The ArSH is the primary and desired
ligand attachment was observed, as has been reported for other product. The formation of ArSSAr is attributed to the partial oxida-
disulfide ligands due to the weak affinity of the disulfide bond to the tion of ArSH due to the instability of the ArSH ligand, as similar
surface of CdSe.10 For ArSH, the ligand exchange process was very benzenethiol containing compounds have been shown to actively
slow and generally resulted in the oxidation of ArSH to ArSSAr. After photocatalytically oxidize on CdSe nanocrystal surfaces.11 The
one day, only ArSSAr was observed and no ArSH could be found by ArS–C2H5 comes from the partial thermolysis of ArSP as tempera-
NMR. After three days, some ArSH was observed, but three times as tures increase to the Se precursor injection temperature.12 The full
much ArSSAr was formed. The limited ligand exchange is consistent 1H NMR spectrum of ArS–CdSe is shown in Fig. S4 in the ESI.†
with the limited exchange of benzenethiol-type ligands experienced
Additional evidence for the cleavage of the phosphonate group
by other groups, and provides an example of the necessity of new from ArSP during nanocrystal synthesis comes from 31P NMR (Fig. S5
ligand attachment methodologies.6 We speculate that the ligand in the ESI†). As expected, the 31P NMR signal for neat ArSP does not
exchange was relatively unsuccessful where others have been more appear in either the methanol or hexanes wash spectra, and because
successful5b when using benzenethiol derivatives due to the steric of this we can assume that all of the ArSP precursor ligand added at
bulk of our ligands. Additionally, in order to show that attachment of the beginning of synthesis reacted. The methanol wash contains two
our ligands was not accomplished by the decomposition of our peaks that are assigned to unreacted Se precursor, SeTBP, and the
precursor ligands followed by a traditional thiol ligand exchange at sulfur analog, STBP, a reaction byproduct. Although we expect to
the reaction temperature, we injected ArSH at the same time as the Se see the formation of diethoxyphosphonic acid as a by-product of
precursor under standard growth conditions. This resulted in the phosphonate cleavage, it is not observed. We expect that this is
formation of standard nanoparticles without any attached thiol, because diethoxyphosphonic acid has a boiling point (200 1C) below
indicating that ligand attachment relies on the coordination of Cd that of the reaction temperature and thus is not observed because it
c
1322 Chem. Commun., 2013, 49, 1321--1323
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
Fig. 3 TEM images of ODA capped CdSe (left), ArS–CdSe (middle), and
ArSe–CdSe (right).
on the cleavage of a phosphonate moiety on the desired ligand
for direct attachment onto the nanoparticle. We showed that this
method is applicable for both sulfur and selenium containing
ligands where established ligand exchange processes with similar
traditional ligands are difficult. Use of this method for the attach-
ment of semiconducting polymers to CdSe NPs for use in photo-
voltaic cells is currently under investigation.
Financial support is greatly appreciated from the AFOSR
FA9550-10-1-0430 and the NSF SOLAR Award DMR 1035196.
Part of this work was conducted at the University of Washington
NanoTech User Facility, a member of the NSF National Nano-
technology Infrastructure Network (NNIN). The authors would
like to thank Ryan E. Toivola for experimental assistance.
Fig. 2 (a) XPS data for S2p binding energies for ArS–CdSe (top), ArSH (center),
and ArSP (bottom); (b) photoluminescence of CdSe NPs with octadecylamine
functionalization (solid line) and quenched photoluminescence with aryl function-
alization (dashed line).
vaporizes during NP synthesis. The hexanes wash containing
ArS functionalized NPs shows no NMR signal, which indicates
cleavage of the phosphonate group ArSP, although there is a
small chance that this is due to signal broadening due to an
inhomogeneous magnetic field on the nanoparticle surface.5b
As further evidence of actual ligand attachment to CdSe as
opposed to some form of decomposition, we looked at X-ray photo-
electron spectroscopy (XPS) and photoluminescence (PL). The bottom
spectrum in Fig. 2a was measured on neat, unreacted ArSP, while the
S2p core level spectrum (center) was measured on ArSH. Due to the
low photo-ionization cross section of the S atoms13 the signal-to-noise
ratio is low, resulting in a high residuum peak fitting. Yet, it is
apparent that the peak maxima differ only marginally by about 0.3 eV,
which is expected considering the similar electronegativity of H and P.
Upon formation of the ArS-capped CdSe nanoparticles, the S2p
spectrum shown at the top of Fig. 2a indicates a shift of the core
level by B0.8 eV to higher binding energy relative to the ArSH ligand.
Considering an overall resolution of Æ0.1 eV for such measurements,
this shift is significant and can be explained as the transfer of electron
density from the sulfur atoms to the Lewis acidic Cd metal ions,
indicating the bonding of the ligand to the surface of the CdSe.14 We
also observed PL quenching with ligand attachment relative to CdSe
nanocrystals synthesized without our ligands, as can be seen in
Fig. 2b. PL quenching is expected due to the attachment of conjugated
sulfur containing ligands to the surface of CdSe because the aromatic
p-electrons can act as efficient acceptors for photogenerated holes,
thus hindering the radiative recombination process.6
This in situ ligand attachment process is not limited to ligands
containing a sulfur–phosphonate bond. We found similar NMR and
XPS results when S was replaced by Se and ArSeP ligands were
attached to CdSe NPs in an analogous fashion, as can be seen in
Fig. S6 and S7 in the ESI.† However, with the attachment of the
ArSeP ligands, we found that we were not able to maintain size and
shape control as we were previously able to do with attachment of
the ArSP ligands under the same conditions, as can be seen in
Fig. 3. This loss of size and shape control is attributed to the greater
reactivity of the Se-containing ligands. Others have previously shown
a lack of control with highly reactive ligands.15
Notes and references
1 C. Bullen and P. Mulvaney, Langmuir, 2006, 22, 3007; I. L. Medintz,
H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005,
4, 435; C. R. Kagan, C. B. Murray, M. Nirmal and M. G. Bawendi,
Phys. Rev. Lett., 1996, 76, 1517.
2 Z. J. Ning, M. Molnar, Y. Chen, P. Friberg, L. M. Gan, H. Agren and
Y. Fu, Phys. Chem. Chem. Phys., 2012, 13, 5848; L. Manna, E. C. Scher
and A. P. Alivisatos, J. Am. Chem. Soc., 2000, 122, 12700;
D. H. Webber and R. L. Brutchey, J. Am. Chem. Soc., 2012, 134, 1085.
3 J. Park, J. Joo, S. G. Kwon, Y. Jang and T. Hyeon, Angew. Chem., Int.
Ed., 2007, 46, 4630.
4 A. M. Munro and D. S. Ginger, Nano Lett., 2008, 8, 2585;
A. M. Munro, I. Jen-La Plante, M. S. Ng and D. S. Ginger, J. Phys.
Chem. C, 2007, 111, 6220.
5 (a) M. V. Kovalenko, M. Scheele and D. V. Talapin, Science, 2009,
324, 1417; (b) M. Kuno, J. K. Lee, B. O. Dabbousi, F. V. Mikulec and
M. G. Bawendi, J. Chem. Phys., 1997, 106, 9869.
6 I. S. Liu, H. H. Lo, C. T. Chien, Y. Y. Lin, C. W. Chen, Y. F. Chen,
W. F. Su and S. C. Liou, J. Mater. Chem., 2008, 18, 675.
7 H. T. Liu, J. S. Owen and A. P. Alivisatos, J. Am. Chem. Soc., 2007,
129, 305.
8 J. S. Liu, T. Tanaka, K. Sivula, A. P. Alivisatos and J. M. J. Frechet,
J. Am. Chem. Soc., 2004, 126, 6550; L. Zhao, X. C. Pang, R. Adhikary,
J. W. Petrich, M. Jeffries-El and Z. Q. Lin, Adv. Mater., 2011, 23, 2844;
E. B. Pentzer, F. A. Bokel, R. C. Hayward and T. Emrick, Adv. Mater.,
2012, 24, 2254; L. Zhao, X. C. Pang, R. Adhikary, J. W. Petrich and
Z. Q. Lin, Angew. Chem., Int. Ed., 2011, 50, 3958; M. L. Steigerwald,
A. P. Alivisatos, J. M. Gibson, T. D. Harris, R. Kortan, A. J. Muller,
A. M. Thayer, T. M. Duncan, D. C. Douglass and L. E. Brus, J. Am.
Chem. Soc., 1988, 110, 3046.
9 W. W. Yu and X. G. Peng, Angew. Chem., Int. Ed., 2002, 41, 2368.
10 W. R. Algar and U. J. Krull, Langmuir, 2008, 24, 5514.
11 J. Aldana, Y. A. Wang and X. G. Peng, J. Am. Chem. Soc., 2001,
123, 8844.
12 K. Pilgram and F. Korte, Tetrahedron, 1965, 21, 1999.
13 J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976, 8, 129.
14 Z. Q. Liang, K. L. Dzienis, J. Xu and Q. Wang, Adv. Funct. Mater.,
2006, 16, 542; J. E. B. Katari, V. L. Colvin and A. P. Alivisatos, J. Phys.
Chem., 1994, 98, 4109.
In summary, we report here a novel procedure for organic/
CdSe NP hybrid material synthesis. The process developed relies
15 T. P. A. Ruberu, H. R. Albright, B. Callis, B. Ward, J. Cisneros,
H. J. Fan and J. Vela, ACS Nano, 2012, 6, 5348.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1321--1323 1323