Metal-free inorganic ligands for colloidal nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe -, TeS32-, OH-, and NH 2- as surface ligands

Article abstract of DOI:10.1021/ja2029415

All-inorganic colloidal nanocrystals were synthesized by replacing organic capping ligands on chemically synthesized nanocrystals with metal-free inorganic ions such as S^2-, HS^-, Se^2-, HSe^-, Te^2-, HTe^-, TeS₃^2-, OH^- and NH₂^-. These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange has been demonstrated for various semiconductor and metal nanocrystals of different size and shape. We showed that the key aspects of Pearsons hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.

Full text of DOI:10.1021/ja2029415

ARTICLE
pubs.acs.org/JACS
Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2ꢀ, HSꢀ,
Se2ꢀ, HSeꢀ, Te2ꢀ, HTeꢀ, TeS3 , OHꢀ, and NH2 as Surface Ligands
2ꢀ
ꢀ
Angshuman Nag,† Maksym V. Kovalenko,† Jong-Soo Lee,† Wenyong Liu,† Boris Spokoyny,† and
Dmitri V. Talapin*,†,‡
†Department of Chemistry and James Frank Institute, University of Chicago, Illinois 60637, United States
‡Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
S Supporting Information
b
ABSTRACT: All-inorganic colloidal nanocrystals were synthesized by
replacing organic capping ligands on chemically synthesized nanocrystals
with metal-free inorganic ions such as S2-, HSꢀ, Se2-, HSeꢀ, Te2-, HTeꢀ,
TeS32-, OHꢀ and NH2ꢀ. These simple ligands adhered to the NC surface
and provided colloidal stability in polar solvents. The versatility of such
ligand exchange has been demonstrated for various semiconductor and
metal nanocrystals of different size and shape. We showed that the key
aspects of Pearson’s hard and soft acids and bases (HSAB) principle,
originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal
surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport
between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.
the simplest, cheapest, and smallest ones for colloidal NCs and
they can find broad use in NC-based devices. In fact, similar
concept of colloidal stabilization was first reported more than a
century ago when Iꢀ ions were employed to stabilize AgI
colloids.12,13 We develop that “old” chemistry further by showing
that inorganic anions can displace the organic ligands from the
surface of various technologically important sub-10 nm
colloidal NCs.
1. INTRODUCTION
During past years, significant attention has been paid to the
applications of colloidal nanocrystals (NCs) in electronic and
optoelectronic devices.1ꢀ4 These require facile transport of
charge carriers through NCs. The efficiency of moving charges
in and out of NCs strongly depends on the NC environment.5ꢀ8
The surface of NCs prepared by traditional colloidal techniques
is capped with a layer of ligands with long hydrocarbon tails
which introduce insulating layers around each NC.8 As a result,
the benefits of quantum-confined materials are often outweighed
by inefficient injection (e.g., in LEDs) or extraction (in solar cells
and photodetectors) of electrons and holes.
2. EXPERIMENTAL SECTION
2.1. Nanocrystal Synthesis. Zinc blende phase CdSe NCs
capped with oleic acid and oleylamine were prepared following Cao
et al.14 Wurtzite phase CdSe NCs capped with n-octadecylphosphonic
acid (ODPA) were prepared from CdO and n-trioctylphosphine sele-
nide (TOPSe) following Manna et al.15 Large, 12 nm CdSe NCs capped
with n-tetradecylphosphonic acid (TDPA) were synthesized using Cd-
(CH3)2 and TOPSe as precursors.16 We also used CdSe and CdSe/ZnS
core/shell NCs capped with proprietary ligands obtained from Evident
Technologies, Inc. (Troy, NY). These CdSe/ZnS core/shell NCs
showed photoluminescence quantum efficiency about 65%.
To synthesize InP NCs, InCl3 solution (1.036 g of InCl3 in 3.44 mL of
TOP), 075 g of P(SiMe3)3, 7.4 g of TOP, and 0.9 g of TOPO were mixed
at room temperature. The mixture was heated to 270 °C for 2 days, and
cooled down to room temperature.17,18 Postsynthesis size selective
precipitation was carried out using toluene and ethanol as solvent and
nonsolvent, respectively.
We recently introduced the concept of inorganic molecular
metal chalcogenide (MCC) ligands that provided colloidal stabi-
lization without blocking the interparticle charge transport.9ꢀ11
The MCC ligands contain main group or transition metals bound
to the NC surface through chalcogenide bridges. The presence of
foreign metal ions in a close proximity to the NC surface could in
some cases affect the chemical and physical properties. For
example, metal ions from MCC ligands can participate in redox
processes, as in the case of Sn2S64ꢀ ligands that can switch between
SnIV and SnII oxidation states. These potential complications could
be avoided by introducing metal-free inorganic ligands.
In this work, we explored new surface chemistries for all-inorganic
colloidal NCs and introduced metal-free inorganic ligands, such as
chalcogenides and hydrochalcogenides (S2ꢀ, HSꢀ, Se2ꢀ, HSeꢀ,
Te2ꢀ, and HTeꢀ), mixed chalcogenides (TeS32ꢀ), as well as OHꢀ
and NH2ꢀ. These inorganic ions bind to the surface of semicon-
ductor and metal NCs and provide electrostatic stabilization for
colloidal dispersions in polar solvents. These ligands are probably
Received: April 10, 2011
Published: June 17, 2011
r
2011 American Chemical Society
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2-
2-
InAs NCs with TOP as capping ligands were prepared from InCl3 and
As[Si(CH3)3]3.19,20
yellow solution of TeS3 ions (with small amounts of Te2S5 as
confirmed by ESI-MS) at a concentration of about 0.25M.
Au NCs were synthesized following Stucky et al;21 0.25 mmol
AuPPh3Cl was mixed with 0.125 mL of dodecanethiol in 20 mL of
benzene forming a clear solution. Then, 2.5 mmol tert-butylamine-
borane complex was added to the above mixture followed by stirring at
80 °C.
2.3. Ligand Exchange. 2.3.1. Ligand Exchange with Chalco-
genide (S2ꢀ, Se2ꢀ, Te2ꢀ) and Hydrochalcogenide (HSꢀ, HSeꢀ, HTeꢀ)
Ions. The ligand exchange process was typically carried out under inert
atmosphere. Colloidal dispersions of different NCs with organic ligands
were prepared in nonpolar solvents like toluene or hexane, while
solutions of inorganic ligands were prepared in polar formamide (FA)
immiscible with toluene and hexane. For a typical ligand exchange using
S2ꢀ ions, 1 mL of CdSe NC solution (∼ 2 mg/mL) was mixed with 1 mL
of K2S solution (5 mg/mL). The mixture was stirred for about 10 min
leading to a complete phase transfer of CdSe NCs from toluene to the
FA phase. The phase transfer can be easily monitored by the color
change of toluene (red to colorless) and FA (colorless to red) phases.
The FA phase was separated out followed by triple washing with toluene
to remove any remaining nonpolar organic species. The washed FA
phase was then filtered through a 0.2 μm PTFE filter and ∼1 mL of
acetonitrile was added to precipitate out the NCs. The precipitate was
redispersed in FA and used for further studies. The NC dispersion in FA
was stable for months. Ligand exchange with Se2-, Te2-, HSꢀ, HSeꢀ, and
HTeꢀ ligands was carried out in a similar manner. In some cases, the
ligand exchange took longer time, up to several hours.
ZnSe NCs were prepared following ref 22 with some modifications.
Seven milliliters of oleylamine was degassed at 125 °C under vacuum for
30 min and then heated to 325 °C under N2 flow. The solution
containing 0.5 M Zn and Se precursors was prepared by co-dissolution
of diethylzinc and Se in TOP at room temperature. Two milliliters of the
precursors solution was added to degassed oleylamine at 325 °C. One
milliliter of additional precursors solution was added to the reaction
mixture after 1 h, followed by two successive injections of 1.5 mL of
precursors solution after 2.5 and 4 h. The reaction was continued for 1 h
after the final injection of the precursors and then cooled to room
temperature. The NCs were washed by precipitation with ethanol and
redispersed in hexane. The washing procedure was repeated three times.
CdS nanorods were prepared by slightly modified procedure
described in ref 23; 0.207 g of CdO, 1.08 g of n-ODPA, 0.015 g of
n-propylphosphonic acid, and 3.35 g of TOPO were heated at 120 °C
under vacuum for 1 h, followed by heating the mixture to 280 °C under
N2 until the formation of a clear solution. The mixture was degassed at
120 °C for 2 h before it was heated to 300 °C under N2. Two grams of
TOP was injected into the mixture at 300 °C and temperature was
immediately set to 320 °C. Then, 1.30 g of n-trioctylphosphine sulfide
(TOPS) was injected at 320 °C and heating was continued for 2 h. The
reaction mixture was cooled to room temperature, washed twice with
toluene/acetone, and redispersed in toluene.
The exchange of organic ligands with S2ꢀ and SHꢀ can be carried out
in air as well. Moreover, one can use concentrated aqueous solutions of
(NH4)2S, K2S, and Na2S as S2- source to carry out the ligand exchange.
As an example, 10 μL of (NH4)2S solution (Aldrich, 40ꢀ48 wt % in
water) was added to 1 mL of FA and mixed with NC dispersion in
toluene or hexane. The rest of the ligand exchange procedure was similar
to the above protocol. When handled in air, the solutions of S2--capped
NCs preserve their colloidal stability for only several days. Resulting
precipitates could be easily redispersed after addition of ∼5 μL (NH4)2S
solution; the dispersions stabilized with additional S2ꢀ remained stable
for several weeks in air. Similar to water and FA, the ligand exchange can
be carried out in DMSO.
In2O3 NCs were prepared using the recipe of Seo et al.24 by heating a
slurry of In(OAc)3 and oleylamine at 250 °C.
CdTe NCs were synthesized similar to the recipe published in ref 25.
PbS NCs capped with oleic acid were synthesized according to the
protocol developed by Hines et al.26
2.2. Synthesis of Inorganic Ligands. Alkali metal sulfides,
hydrogen sulfide, hydroxides, and amide ligands were purchased from
Aldrich and Strem and used as received.
K2Se was synthesized by the reaction of K (25.6 mmol) with Se (12.8
mmol) in about 50 mL of liquid ammonia.27,28 Special care was taken to
make sure that the chemicals and reaction environment were air and
moisture free. A mixture of dry ice and acetone was used to liquefy NH3
gas. Gray-white K2Se powder was stored inside a glovebox. In air, K2Se
turned red because of the formation of polyselenides. K2Te was prepared
in the same way as K2Se; 7.6 mmol of K and 3.8 mmol of Te were used.
The 0.05 M solutions of KHSe and KHTe in formamide (FA) were
prepared by titrating 25 mL of 0.05 M KOH solution with H2Se (or
H2Te) gas generated by the reaction of Al2Se3 (or Al2Te3) with 10%
H2SO4. A 1.7-fold molar excess of H2Se (or H2Te) was used. Rigorous
N2 environment was maintained while handling the KHSe and KHTe
solutions.
2.3.2. Ligand Exchange with TeS32ꢀ. The mixed chalcogenide TeꢀS
species are stable in basic solutions (we typically added NH4OH) and
are highly susceptible to oxidation. All ligand-exchange reactions were
carried out in a glovebox. In a typical ligand exchange for CdTe NCs,
0.4 mL of CdTe NCs capped with oleic acid in toluene (∼25 mg/mL)
was mixed with 3 mL of FA, 3 mL of toluene, and 0.4 mL of ∼0.25 M
(NH4)2TeS3 solution. Upon stirring for 2ꢀ10 h, CdTe NCs quantita-
tively transferred into FA phase. NC solution was rinsed 3 times with
toluene and mixed with 3ꢀ6 mL of acetonitrile. NCs were isolated by
centrifuging and redispersed in FA.
2.3.3. Ligand Exchange with OH ꢀ. A stock solution of KOH was
prepared by dissolving 135 mg of KOH in 0.4 mL of FA. For a typical
ligand exchange reaction, ∼2 mg of 5.5 nm CdSe NCs was dispersed in
1 mL of toluene. One milliliter of FA was added to the NC solution
followed by the addition of 20 μL of KOH solution and stirred for about
10 min. Red colored FA phase was separated out, washed three times
with toluene, and passed through a 0.2 μm PTFE filter. Acetonitrile was
added to precipitate the NCs, followed by centrifugation and redisper-
sion of NCs in FA. The process was carried out under inert atmosphere.
2.3.4. Ligand Exchange with NH2ꢀ. A total of 0.05 g of NaNH2 was
dissolved in 0.5 mL of FA to prepare a stock solution. Then, 0.1 mL of
the NaNH2 stock solution was diluted to 1 mL by adding FA and the
resulting solution was added to 1.5 mg of CdSe NCs dispersed in 1 mL of
toluene. The mixture was stirred for about 10 min. The NCs were
separated out to the FA phase, washed, precipitated, centrifuged, and
redispersed in FA under inert atmosphere.
(NH4)2TeS3 was synthesized using a modified literature method of
Gerl et al.29 2TeO2 HNO3 was prepared by dissolving 5 g of Te (pellets,
3
Aldrich) in 35 mL of HNO3 (65%, aqueous) diluted with 50 mL of H2O.
The solution was boiled in an open beaker until volume decreased to
about 30 mL. Upon cooling, 2TeO2 HNO3 was precipitated as a white
3
solid that was separated by filtering, rinsed with deionized water and
dried. To prepare (NH4)2TeS3, 2 g of 2TeO2 HNO3 was mixed with
3
40 mL of aqueous ammonia solution (∼30% NH3, Aldrich). Solution
was first purged with N2 for 5 min, then with H2S until all telluronitrate
2ꢀ
dissolved forming a yellow solution, characteristic for TeS3 ions.
2.3.5. Treatment with HBF4 and HPF6. A total of 1.5 mg of CdSe NCs
was dispersed in 1 mL of toluene followed by the addition of a solution of
HBF4 (prepared by mixing 25 μL of 50 wt % aqueous HBF4 with 1 mL of
FA). NCs completely transferred to the FA phase within 5 min. The
Solvents and (NH4)2S were removed by vacuum evaporation. The solid
was redispersed in 36 mL of H2O, 4 mL of NH4OH, and 0.1 mL of N2H4
(used as a stabilizer against oxidative decomposition) forming a clear
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colorless toluene phase was discarded followed by the addition of pure
toluene to wash out any remaining nonpolar organics. The washing was
repeated 3ꢀ4 times in about 30 min and finally the NCs dispersed in FA
were passed through a 0.2 μm PTFE filter. HPF6 treatment was carried
out in similar manner. All these steps were carried out under the nitrogen
environment inside a glovebox, and the FA dispersion of CdSe NCs
obtained after the ligand exchange was found to be stable for a few days,
after which the NCs precipitate out of the solution.
2.4. Characterization. UVꢀvis absorption spectra of NC disper-
sions were recorded using a Cary 5000 UVꢀvis-NIR spectrophotometer.
Photoluminescence (PL) spectra were collected using a FluoroMax-4
spectrofluoremeter (HORIBA Jobin Yvon). PL quantum efficiency of the
NC dispersions were measured using Rhodamine 6G (QE = 95%) as a
reference dye dissolved in ethanol. Fourier-transform infrared (FTIR)
spectra were acquired in the transmission mode using a Nicolet Nexus-
670 FTIR spectrometer. Samples for FTIR measurements were prepared
by drop casting concentrated NC dispersions on KBr crystal substrates
(International Crystal Laboratories) followed by drying at 90 °C under
vacuum. Transmittance of different NC samples was normalized by the
weight of NCs per unit area of the deposited film, assuming a uniform film
thickness across the KBr substrate. Transmission Electron Microscopy
(TEM) data were obtained using a FEI Tecnai F30 microscope operated
at 300 kV. Dynamic light scattering (DLS) and ζ-potential data were
obtained using a Zetasizer Nano-ZS (Malvern Instruments, U.K.). ζ-
potential was calculated from the electrophoretic mobility using Henry’s
equation in the Smoluchowski limit.30 Electrospray ionization mass
spectrometry (ESI-MS) was performed using Agilent 1100 LC/MSD
mass-spectrometer. The peak assignments were based on the comparison
of experimental mass-spectra with calculated isotope patterns.
2.5. Charge Transport Studies. 2.5.1. Device Fabrication. All
fabrication steps were carried out under dry nitrogen atmosphere.
Highly p-doped Si wafers (Silicon Quest, Inc.) with a 100 nm thick
layer of thermally grown SiO2 gate oxide were used as a substrate. The
oxide layer on the back side of the wafers was etched using 1% HF
aqueous solution, followed by careful rinsing with high purity deionized
water. The substrates were cleaned with acetone, isopropyl alcohol, and
ethanol, followed by drying in a flow of N2 gas. SiO2 surface was
hydrophilized by oxygen plasma or piranha (H2SO4/H2O2) treatments
immediately prior to NC deposition. The substrates were treated by
oxygen plasma (∼80W) for 10ꢀ15 min. Piranha treatment was usually
carried out at 120 °C for 10 min followed by multiple washings with high
purity deionized water. Piranha solution was prepared by adding
concentrated sulfuric acid to 30% H2O2 solution (2:3 volume ratio).
The NC films for electrical measurements were prepared by deposit-
ing thin 20ꢀ40 nm films by spin-coating (spread: 600 rpm, 6 s; spin:
2000 rpm, 30 s) from FA solutions at elevated temperatures (80 °C)
using a 500 W infrared lamp placed above the substrate. After deposition,
the NC films were dried at 80 °C for 30 min, followed by annealing at
200ꢀ250 °C for 30 min using a hot plate with calibrated temperature
controller. Such heating of NC films removes solvents and remaining
volatile ligands. The NC film thickness was measured using AFM or high
resolution SEM of device cross sections. The top source and drain
electrodes were thermally evaporated at 1.0ꢀ2.5 Å/s rate up to ∼1000 Å
total thickness using a metal evaporator located inside nitrogen-filled
glovebox. After deposition of Al electrodes, the devices were post
annealed at 80 °C for 30 min to improve the contacts.
Figure 1. (a) Red colored colloidal dispersion of CdSe NCs undergoes
the phase transfer from toluene to formamide (FA) upon exchange of
the original organic surface ligands with S2ꢀ. (b) Absorption and PL
spectra of 5.5 nm CdSe NCs capped with organic ligands and S2ꢀ ligands
dispersed in toluene and FA, respectively. (c) FTIR spectra of 5.5 nm
CdSe NCs with different combinations of ligands and solvents. (d) NC
size-distributions measured by Dynamic Light Scattering for ∼12 nm
CdSe NCs capped with organic ligands and S2ꢀ ions. Inset shows TEM
image of the NCs capped with S2ꢀ ligand. (e) TEM images of
S
2ꢀꢀDDAþ-capped CdS nanorods. (f) Absorption spectra of 5.5 nm
CdSe NCs capped with different ligands.
of the drain current (ID) versus VG measured when the gate voltage was
scanned in the forward direction (i.e., from negative to positive). For an
n-type FET, this measurement typically provides a conservative estimate
for the field effect mobility.8 Additional details on the mobility calcula-
tions provided in the Supporting Information.
3. RESULTS AND DISCUSSION
The NCs used in this work were synthesized following accepted
protocols14ꢀ26 using ligands with long (C12ꢀC18) hydrocarbon
tails to control NC size and shape. We did not notice substantial
differences in the behavior of NCs capped with the carboxylate,
amine, and phosphonate based organic ligands. For a typical
ligands exchange procedure, we combined solutions of organics-
capped NCs in a nonpolar solvent (toluene or hexane) with the
solution of inorganic ligands (K2S, KHS, (NH4)2TeS3, KOH,
KNH2, etc.) in formamide (FA). The two-phase mixture contain-
ing immiscible layers of FA and nonpolar solvent was stirred for
about 10 min, after which we observed complete transfer of NCs
from nonpolar solvent to FA (Figure 1a). Such exchange could be
carried out in different solvents, ruling out special role of FA in
colloidal stabilization. For example, S2ꢀ-capped CdSe NCs could
be easilypreparedusing (NH4)2S inaqueousmediumorNa2S and
K2S in dimethylsulfoxide (DMSO). However, we could not obtain
2.5.2. Electrical Measurements. All electrical measurements were
performed using Agilent B1500A semiconductor parameter analyzer.
The source electrode was grounded for the field-effect transistor
measurements. All electrical measurements were performed under dry
nitrogen atmosphere and under quasi-static conditions with slow scan of
source-drain or source-gate voltage. The transfer characteristics for the
field-effect transistors (FETs) were measured with the gate voltage (VG)
sweep rate 40 mV/s. The electron mobility was calculated from the slope
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colloidal dispersions of S2ꢀ-capped NCs in dimethylformamide
(DMF) that was chemically similar to FA. Our experimental data
showed that colloidal stability of NC dispersions is mainly
determined by the solvent dielectric constant; highly polar FA
(ε = 106) was found to be the best solvent, while water (ε = 80)
and DMSO (ε = 47) provided moderate stability. DMF, the least
polar in this series (ε = 36), did not stabilize the colloidal
dispersions. Below, we summarize and discuss the results obtained
for different classes of metal-free inorganic ligands.
3.1. Chalcogenide and Hydrochalcogenide Ligands (S2ꢀ
,
HSꢀ, Se2ꢀ, HSeꢀ, Te2ꢀ, and HTeꢀ). Figure 1b compares the
absorption and PL spectra of 5.5 nm CdSe NCs capped with
organic ligands in hexane and S2ꢀ in FA. The excitonic features in
the absorption spectrum of CdSe NCs remained unchanged after
the ligands exchange, implying no changes in size, shape, and
size-distribution of CdSe NCs. The NCs with organic ligands
showed ∼13% PL quantum efficiency (QE) which dropped to
about 2% QE for S2ꢀ-capped CdSe NCs in FA. The observation
of band edge PL from S2ꢀ-capped CdSe NCs was rather
unexpected since alkylthiols are well-known PL quenchers for
CdSe NCs.31 It was found that, in contrast to thiols, S2ꢀ ligands
do not introduce midgap states serving as fast nonradiative
recombination channels. CdSe NCs capped with HSꢀ ligands
also retained their band-edge PL (Supporting Information Figure
S1). S2ꢀ can replace different kinds of organic ligands such as
carboxylates, amines, phosphonic acids, and alkylphosphonic
oxide from surface of CdSe NCs.
Figure 1c shows FTIR spectra of 5.5 nm CdSe NCs taken
before and after the exchange of organic ligands with S2ꢀ. The
transfer of NCs from toluene to FA resulted in complete
disappearance of the bands at 2852 and 2925 cmꢀ1 correspond-
ing to CꢀH stretching in the original organic ligands. Other
bands in the FTIR spectra for S2ꢀ-capped NCs could be assigned
to the solvent and (NH4)2S (Figure S2). For example, FTIR
spectra of S2ꢀ-capped CdSe NCs prepared from water dispersion
showed no observable CꢀH bands. These results confirm the
efficacy of S2ꢀ ligands in complete removal of the original organic
ligands, forming all-inorganic colloidal NCs.
NC surface has numerous electrophilic sites, for example, the
surface of CdSe NCs is rich with undercoordinated Cd2þ sites.
These sites should favor adsorption of nucleophilic S2ꢀ ions
rather than positively charged counterions (e.g., Kþ or NH4þ).
In addition, in polar solvents, cations are typically more solvated
than anions.13 The ligands exchange can be described as a
nucleophilic substitution at the metal site. Because of sterical
crowding around the reaction center, one can expect the
dissociative reaction pathway, although detailed kinetic studies
would be required to clarify the mechanism of ligands exchange.
Colloidal stabilization of NCs in FA was achieved through
binding negatively charged S2ꢀ ions to NC surface, leading to the
formation of an electrical double layer around each NC. The
negative charging of CdSe NC surface resulted in a negative
(ꢀ40 mV) ζ-potential, sufficient for electrostatic stabilization of
the colloidal dispersion. Dynamic light scattering (DLS) studies
also confirmed single-particle populations in the solutions of
S2ꢀ-capped CdSe NCs (Figure 1d, for NCs with the TEM image
shown in the inset). The direct comparison of the volume
distribution curves calculated from DLS revealed that average
hydrodynamic diameter of S2ꢀ-capped NCs was smaller by
∼1.7 nm than that of the same NCs capped with n-tetradecyl-
phosphonic acid, which accounted for the effective length of the
hydrocarbon chains.
Figure 2. (a) Absorption and PL spectra of CdSe/ZnS core/shell NCs
capped with S2ꢀ ligands dispersed in FA; inset shows the luminescence
of NC dispersion upon illumination with 365 nm UV light. (b)
Absorption spectra of InP NCs capped with organic ligands and S2ꢀ
ligands dispersed in toluene and FA, respectively. Inset shows the optical
photograph of colloidal dispersion of InP NCs in FA. (c) Absorption
spectra of Au NCs capped with dodecanethiol and S2ꢀ ligands dispersed
in toluene and FA, respectively. Inset shows the optical photograph of a
colloidal dispersion of Au NCs in FA. (d) DLS size-distribution plots for
Au NCs capped with the organic and S2ꢀ ligands.
In polar solvents like FA, we used small inorganic cations Kþ,
Naþ, NH4þ to balance the negative charge of anions adsorbed on
the NC surface. These cations could be replaced with hydro-
phobic quaternary ammonium ions by treatment with didode-
cyldimethylammonium bromide (DDAþBrꢀ), rendering NC
insoluble in FA, but soluble in toluene.32 We applied this
technique to S2ꢀ-capped CdS nanorods. Figure 1e shows a
TEM image of S2ꢀꢀDDAþ-capped CdS nanorods. The shape
and dimensions of nanorods remained intact during the transfer
from toluene (ODPA-TOPO ligands) to FA (S2ꢀ ligands) to
toluene (S2ꢀꢀDDAþ ion pair ligands).
Stable colloidal solutions of CdSe NCs could be prepared in
FA using different chalcogenide (S2ꢀ, Se2ꢀ, and Te2ꢀ) and
hydrochalcogenide (HSꢀ, HSeꢀ, and HTeꢀ) ions. Figure 1f
shows the absorption spectra for colloidal solutions of the same
CdSe NCs stabilized with different ligands. In all cases of
chalcogenide and hydrochalcogenide ligands, we observed the
formation of stable colloidal solutions of negatively charged NCs.
It should be noted that selenide and telluride ions can be easily
oxidized in air and corresponding colloidal solutions should be
handled under inert atmosphere.
Chalcogenide ions can stabilize different nanomaterials in
polar solvents. For example, Figure 2a shows S2ꢀ-capped
CdSe/ZnS coreꢀshell NCs with a strong band edge PL with
QE about 25%. All-inorganic films of S2ꢀ-capped CdSe/ZnS
NCs also showed bright emission, even after annealing at 250 °C
for 30 min. S2ꢀ ligands worked well for InP NCs (Figure 2b)
providing the first example of all-inorganic colloidal IIIꢀV NCs.
The absorption spectra of S2ꢀ-capped InP NCs in FA were
similar to those of the organically capped NCs in toluene. The
ζ-potential of S2ꢀ-capped InP NCs was about ꢀ60 mV. The ability
of chalcogenide ions to provide colloidal stabilization was not
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Figure 3. (a) ESI-MS spectrum of (NH4)2TeS3 solution. The inset
Figure 4. (a) Absorption and PL spectra of OHꢀ and NH2ꢀ-capped
CdSe NCs dispersed FA. Inset shows a photograph of the colloidal
solution. (b) Absorption spectra of ZnSe NCs capped with oleylamine
and OHꢀ ligands dispersed in toluene and FA, respectively.
compares an experimental high-resolution mass-spectrum with simu-
ꢀ
lated spectra for TeS3Hꢀ and TeS3 ions. (b) Absorption spectra for
(NH4)2TeS3, for CdSe NCs capped with the organic ligands in toluene
and for CdSe NCs capped with TeS32ꢀ ions in formamide.
high energy part of absorption spectra (e450 nm) showing the
contribution from the TeS3 ions. Surface affinity of TeS3
limited to semiconductors; Figure 2c compares the absorption
spectra for spherical 7 nm Au NCs capped with dodecanethiol in
toluene and with S2ꢀ ions in FA. We observed an 11 nm red-shift
for surface plasmon band in S2ꢀ-capped Au NCs, consistent with
the difference in dielectric constants of toluene and FA.10 DLS
studies revealed that S2ꢀ-capped Au colloids retained size
distribution after the ligands exchange (Figure 2d), with no
agglomeration for both organic and S2ꢀ-capped NCs. The ζ-
potential of S2ꢀ-capped Au NCs was about ꢀ60 mV.
2ꢀ
2ꢀ
ions is rather similar to that of MCC ligands. For example, stable
colloidal solutions of PbS NCs could be formed with TeS32ꢀ ions
in FA, whereas capping with S2ꢀ could not provide stable
colloidal solutions for IVꢀVI NCs.
The presence of two chalcogens in different oxidation states
opens interesting possibilities for surface redox reactions. Te4þ
can oxidize its own ligand (S2ꢀ) releasing elemental Te and S,
which explains limited stability of (NH4)2TeS3 in the solid state,
under light or heat. Another redox pathway is the reaction of
Te4þ with the NC material. For example, metal telluride NCs
Mild oxidation of chalcogenide ions typically results in the
formation of polychalcogenide ions (Sn2ꢀ, Sen2ꢀ, Ten2ꢀ). In fact,
polychalcogenides are often present as an impurity in commer-
cial metal chalcogenide chemicals. As a simple test, aqueous
sulfide solutions should be colorless whereas polysulfides absorb
visible light and appear yellow to red. We used ESI-MS to identify
the presence and stability of polychalcogenides in different
solvents. The mass spectra of the red Na2Sn solution prepared
by adding an excess of sulfur to aqueous 0.7 M Na2S solution
showed the presence of polysulfides with n = 5, 6, and 7 unlike to
colorless Na2S solution (Figures S3ꢀS5). We found that both
(NH4)2Sn and Na2Sn solutions could stabilize colloidal NCs in
polar solvents, suggesting that polysulfides behave similarly to
sulfide ions. Survey of many samples led us to the conclusion that
polysulfides were weaker ligands than S2ꢀ. We also could not rule
out the possibility of NC passivation with sulfide ions that always
coexist in equilibrium with polysulfides in (NH4)2Sn and Na2Sn
solutions.
2ꢀ
such as CdTe and PbTe slowly reacted with TeS3 releasing
elemental Te (Te4þ þTe2ꢀ f Te0 comproportionation reaction)
and leading to partial or complete conversion to CdS and
PbS, respectively. For PbTe NCs, this reaction occurred within
minutes after the ligand exchange, while CdTe converted to CdS
only upon heating of NC films at 120 °C. CdSe NCs are
significantly more stable, and only partially react under the
thermal treatment (Figure S6). At the same time, PbS NCs do
not react with TeS32- as confirmed by powder XRD patterns for
dried and annealed samples (Figure S7). Therefore, the use of
mixed chalcogenide ligands can be advantageous for composi-
tional modulations in nanogranular solid materials such as
thermoelectric Pb and Sb chalcogenides.33
3.3. Other Metal-Free Anionic Ligands (OHꢀ and NH2ꢀ).
We also explored stabilization of colloidal NCs with other
inorganic anions. OHꢀ and NH2ꢀ successfully replaced organic
ligands at the surface of CdSe NCs and stabilized colloidal
solutions in FA. Figure 4a shows that absorption and PL spectra
of OHꢀ and NH2ꢀ-capped 5.5 nm CdSe NCs did not change
compared to original NCs (Figure 1b); no change was also
detected in XRD patterns of corresponding NCs (Figure S8).
Similar to S2ꢀ-capped NCs, OHꢀ-capped CdSe NCs exhibited
negative ζ-potentials in polar solvents, consistent with the
electrostatic stabilization. The value of ζ-potential for OHꢀ-
capped CdSe NCs in FA (ꢀ20 mV) was substantially lower than
that for S2ꢀ-capped NCs. Right after the ligand exchange,
colloidal solutions could be easily filtered through a 0.2 μm filter
but they slowly aggregated and precipitated in several days.
Unlike S2ꢀ-capped NCs, FTIR spectra of OHꢀ-capped CdSe
NCs (Figure S9) did not show complete removal of CH2 bands
at 2852 and 2925 cmꢀ1, but the intensity of CꢀH bands
decreased by >90% compared to the original NCs. Similar
3.2. Mixed Chalcogenide TeS32ꢀ Ligands. Having molecular
structure intermediate between MCCs and polychalcogenide
ions, mixed chalcogenide ions such as TeS32ꢀ represent another
interesting choice of metal-free ligands. For the proof-of-princi-
29
ple study, we used light-yellow aqueous (NH4)2TeS3 to
stabilize colloidal CdSe NCs via ligand exchange in FA. ESI-
MS spectra show that (NH4)2TeS3 aqueous solution contains
mainly TeS32ꢀ ions along with Te2S52ꢀ (Figure 3a). Adsorption
of submonolayer amounts of TeꢀS ions onto the surface of
4.4 nm CdSe NCs was confirmed by elemental analysis, indicat-
ing Te-to-Cd atomic ratio of 0.25. The atomic ratio of S-to-Te
equal to 3.67 suggests the adsorption of TeS32ꢀ ions, along with
possibility for some additional S2ꢀ incorporating onto the NC
surface. Note that Cd-to-Se ratio of 1.25 was similar to that of
original NCs. The integrity of CdSe NC core was also evident
from the similar excitonic absorption features in the optical
absorption spectra (Figure 3b). The main difference lies in the
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Scheme 1. (a) Typical Metal Complexes and (b) a Nano-
crystal with Surface Ligandsa
interaction with OHꢀ and HSꢀ ligands. Both HSꢀ and OHꢀ
bound to In2O3 NC surface resulting in similar ζ-potentials
(ꢀ40 mV), suggesting that the metal surface sites in In2O3 NCs
were harder than those in InP and InAs NCs. One should
understand that the HSAB principle has a limited predictive
power and does not necessarily dictate all possible ligand
exchange pathways. However, we found HSAB-based general-
ization of ligand exchange reactions very useful for predicting the
binding affinity between a given ligand and a given NC.
3.5. Comparison of Metal-Free Inorganic Ligands to MCCs.
Negatively charged metal-free ligands can be viewed as an
addition to the family of all-inorganic colloidal NCs that
started with the discovery of MCC ligands.9,10 In this section,
we summarize our observations comparing metal-free inorganic
ligands to MCCs.
a We want to emphasize similarities between metal-ligand (M-L) bond-
ing in both classes of compounds. The HSAB principle, originally
developed for metal complexes, should also apply to colloidal
nanomaterials.
The solutions of MCCs in polar solvent are expected to
contain a certain equilibrium concentration of free chalcogenide
ions. We explored the possibility of using chalcogenide ions to
prepare stable colloidal solutions of NCs in hydrazine, often used
with MCC ligands. It was found that chalcogenides could not
stabilize IIꢀVI, IVꢀVI, and IIIꢀV NCs in hydrazine. Probably,
the dielectric constant of hydrazine (ε = 51) and its solvation
ability were not sufficient to obtain stable NC colloids with
behavior and ζ-potentials have been observed for NH2ꢀ-capped
CdSe NCs. These observations point to somewhat lower affinity
ꢀ
of OHꢀ and NH2 toward CdSe surface compared to chalco-
genides. In contrast, ZnSe NCs did not form stable colloidal
solutions with S2ꢀ ligands but could be easily stabilized with
OHꢀ in FA (Figure 4b). The possible origin of such selectivity
will be discussed in the next section.
4ꢀ
3.4. Can the Hard and Soft Acids and Bases (HSAB)
Classification Be Applied to Colloidal Nanostructures? In
1963 Pearson proposed the HSAB principle that explained many
trends in stability of the complexes between Lewis acids and
bases by classifying them into “hard” and “soft” categories.34
Generally, soft acids form stable complexes with soft bases,
whereas hard acids prefer hard bases. The inorganic ligands used
in our work represent classical examples of soft (highly polarꢀiz-
chalcogenide ligands. In contrast, Sn2Se6 and other MCC
ligands provided long-term, up to several years, stability to CdSe
NC colloids in hydrazine. In FA, both chalcogenides and MCCs
generated stable colloidal solutions, for example, for Au and
CdSe NCs. Direct comparison revealed that CdSe NCs capped
with SnS44ꢀ or AsS33ꢀ MCC ligands exhibited higher ζ-poten-
tials, typically in the range of ꢀ50 to ꢀ80 mV. That probably
corresponded to a higher affinity of MCCs toward CdSe NCs.
On the other hand, S2ꢀ was superior for CdSe/ZnS NCs, for
which many MCCs showed poor affinity. MCCs and mixed
chalcogenide ligands were better for Pb chalcogenide NCs, for
which S2ꢀ capping failed. Compared to the MCCs, free chalco-
genide ions were somewhat more prone to oxidative degradation.
Both, sulfide-based MCC-capped NCs and S2ꢀ-capped nano-
materials can be prepared and handled in air, but S2ꢀ-capped
NCs were much longer stable if prepared and stored in a
glovebox (many months).
3.6. Electrostatic Stabilization of Colloidal Nanocrystals
in the Absence of Charged Ligands (“Ligand-Free”
Nanocrystals). So far we discussed inorganic ligands that
provided negative charge to the NCs. All-inorganic positively
charged NCs are rarer, presumably because NCs typically have
electrophilic metal-rich surfaces with preferential affinity toward
nucleophilic ligands. Recently, Murray and co-workers reported
colloidal NCs with positively charged surface.35 They used
nitrosonium tetrafluoroborate (NOBF4) and aryldiazonium tetra-
fluoroborate to perform ligand exchange on Fe3O4, FePt, Bi2S3,
and NaYF4 NCs, resulting in positively charged NC surface.
Unfortunately, this approach did not work with CdSe and other
semiconductor NCs.35 Nitrosonium salts are known as strong
one-electron oxidants,36 resulting in the etching of NC surface
immediately after the addition of NOBF4 (Figure S10).
able HSꢀ, S2ꢀ, Se2ꢀ) and hard (less polarizable OHꢀ, NH2
)
Lewis bases. We noticed that original ligands on Au and CdTe
NCs could be easily replaced with S2ꢀ, Se2ꢀ, or HSꢀ ligands, but
did not form stable colloidal solutions in the presence of OHꢀ. In
contrast, ZnSe (Figure 4b) and ZnO NCs could be stabilized
with OHꢀ but did not interact with S2ꢀ and HSꢀ. Other NCs,
including CdSe and CdS, formed stable colloidal solutions with
both hard and soft ligands. We attributed this difference in
behavior to the differences in chemical affinity between NCs
and Lewis bases and proposed that HSAB principle, originally
developed for molecular coordination compounds, could be
extended to the world of nanomaterials (Scheme 1). The ligands
used in our work bind primarily to the electrophilic sites at NC
surface. In agreement with HSAB, soft Au0 and Cd2þ sites
exhibited stronger affinity to soft S2ꢀ and HSꢀ ligands compared
to hard OHꢀ, whereas harder Zn2þ sites in ZnSe and ZnO NCs
had higher affinity to OHꢀ rather than to HSꢀ. More quantita-
tive information on the binding affinity could be obtained from
the comparison of ζ-potentials related to the charges on NC
surface (Table S1). The extension of HSAB principle to NCs
appears to fail for InP and InAs NCs. Free In3þ is a hard acid,
while InAs and InP behave as soft acids preferentially binding to
S2ꢀ and HSꢀ rather than to OHꢀ. This discrepancy, however,
could be explained by taking into account the character of
chemical bonding inside NCs. Small difference in electronega-
tivity between In and P or As led to reduced positive effective
charge on the In surface sites, much smaller than the charge on
isolated In3þ ions. Low charge density softens the acidic sites at
InP and InAs NCs surface. To further test this hypothesis,
we synthesized In2O3 NCs,24 where the bonding has a higher
ionic character compared to InP and InAs, and studied their
To cleave the bonds between CdSe NC and carboxylate
organic ligands without oxidizing the NCs, we used HBF4 and
HPF6 in polar solvents. This reaction resulted in the phase
transfer of NCs from nonpolar solvent (toluene) to a polar
solvent (FA). In the case of oleylamine and oleate capping
ligands, Hþ cleaved the CdꢀN and CdꢀO bonds, as schema-
tically shown in Figure 5a, leaving behind positively charged
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films. Strong electronic coupling provides an exciting opportu-
nity for the integration of colloidal NCs into electronic and
optoelectronic devices. Detailed investigations of the electronic
properties of NCs with these new capping ligands will be a
subject of a separate study. Here, we show our preliminary results
on the electron transport in the arrays of S2ꢀ-capped CdSe NCs
as well as CdSe/CdS and CdSe/ZnS coreꢀshell NCs.
The electron mobility in NC arrays can be conveniently
measured from transfer characteristics of a field-effect trans-
istor.6,8 For these studies, 20ꢀ40 nm thick close packed NC films
were deposited on highly doped Si wafers with 100 nm thick SiO2
thermal gate oxide. The NC films were spin-coated from FA
solutions at elevated temperatures (80 °C), followed by short
annealing at 200ꢀ250 °C. Source and drain Al electrodes were
patterned on the annealed NC films using a shadow mask.
Figure 6aꢀc shows sets of drain current (ID) versus drain-source
voltage (VDS) scans at different gate voltages (VG) for devices
made of (NH4)2S-capped 4.6 nm CdSe NCs (originally capped
with oleylamine and oleic acid), 10 nm CdSe/CdS, and 8 nm
CdSe/ZnS core/shell NCs. ID increased with increasing VG,
characteristic of the n-type transport. Both transfer and output
characteristics of our FETs were generally reproducible during
multiple voltage scans.
Figure 5. (a) Removal of the organic ligands from the surface of CdSe
NCs by HBF4 treatment. For simplicity, we showed monodentate
binding mode of carboxylate group. Proposed mechanism should hold
for other chelating and bridging modes as well. (b) Absorption and PL
spectra of ligand-free colloidal CdSe and CdSe/ZnS NCs obtained after
HBF4 treatment. (c) FTIR spectra of organically capped and ligand-free
CdSe NCs after HBF4 treatment.
The electron mobility corresponding to the linear regime of
FET operation (measured at VDS = 2 V across a 80 μm long
channel) for a film of CdSe NCs capped with (NH2)2S annealed
at 200 °C was μlin = 0.4 cm2 Vꢀ1 sꢀ1 with ION/IOFF ∼ 103
(Figure 6a,d). FETs assembled from CdSe/CdS (Figure 6b,e)
and CdSe/ZnS (Figure 6c,f) coreꢀshell NCs capped with
(NH4)2S ligands annealed at 250 °C showed μlin = 0.04 cm2
Vꢀ1 sꢀ1 with ION/IOFF ∼ 104 at VDS = 2 V and μlin = 6 ꢁ
10ꢀ5 cm2 Vꢀ1 sꢀ1 with ION/IOFF ∼ 102 at VDS = 4 V,
respectively. The electron mobility measured in the saturation
mode of FET operation (VDS = 30 V, Figure S12) was slightly
higher than μlin for CdSe and CdSe/ZnS NCs: 0.5 and 9 ꢁ
metal sites at the NC surface. HBF4 treatment was efficient for
NCs capped with carboxylate and alkylamine ligands whereas
CdSe NCs capped with alkylphosphonic acid remained in the
ꢀ
toluꢀene phase even after extended HBF4 treatment. BF4 and
PF6 are known as very weak nucleophiles.37 These weakly
coordinating anions did not bind to the NC surface, instead they
acted as counterions in the electrical double layer around NCs.
Figure 5b shows the absorption and PL spectra of 5.5 nm CdSe
NCs and CdSe/ZnS coreꢀshells after treatment with HBF4 in
FA. The size, size-distribution, and band-edge emission re-
mained, however, with a significant drop in the PL efficiency
(<0.5% for CdSe NCs and ∼2% for CdSe/ZnS NCs).
ζ-potentials of CdSe and CdSe/ZnS NCs in FA were measured
to be around þ30 mV. Similar values were obtained with HPF6.
The removal of organic ligands was not possible with NaBF4 and
NaPF6, emphasizing the role of Hþ ions in the reaction, which
was also suggested in previous reports.35 Figure 5c compares
FTIR spectra of CdSe NCs capped with organic ligands and
FTIR spectra of the same CdSe NCs after HBF4 treatment. The
absence of CꢀH bands at 2852 and 2925 cmꢀ1 constituted
complete removal of the original organic ligand. The band
centered at 2890 cmꢀ1 was from FA (Figure S2). Apart from
the FA related bands, we observed a new sharp band at
1083 cmꢀ1 corresponding to BF4ꢀ ions.
10ꢀ5 cm2 Vꢀ1 ꢀ1, respectively. The devices assembled from
s
CdSe/CdS coreꢀshell NCs showed similar electron mobility in
both linear and saturation regimes. Measured electron mobility
depends both on the intrinsic property of the material, and on a
number of parameters related to device characteristics (FET
structure, gate dielectric, film uniformity, contacts, etc.). There
are all reasons to expect that higher mobility will be obtained after
further optimization of FET devices.
The channel currents were orders of magnitude lower for CdSe
NCs capped with K2S ligands as compared to (NH4)2S ligands.
We attributedlow conductivity tothe presence ofKþ ionscreating
local electrostatic barriers around NCs and behaving as the
electron scattering centers. In contrast, when we used (NH4)2S
to stabilize the solution of CdSe NCs, the mild thermal treatment
resulted in the removal of residual solvent and surface ligands
and led to much higher carrier mobility in the NC array. The
devices assembled from CdSe NCs capped with (NH4)2TeS3
also showed n-type transport with μlin = 0.01 cm2 Vꢀ1 sꢀ1
(Figure S13).
Although the films of CdSe/ZnS coreꢀshell NCs showed
rather low carrier mobility, these NCs preserved bright band-
edge photoluminescence even after annealing at 250 °C (insets
in Figure 6c). Generally, metal-free ligands allowed obtaining
conductive NC layers without introducing foreign metal ions
that often introduce recombination centers. As a result, we were
able to observe for the first time efficient band-edge PL in
electronically conductive NC arrays (Figure 6c).
3.7. Colloidal NCs with Metal-Free Inorganic Ligands for
Device Applications. The exchange of bulky organic surfactants
with small inorganic ligands like S2ꢀ or Se2ꢀ should substantially
facilitate the electronic communication between individual NCs.
Indeed, when S2ꢀ-capped CdSe NCs were placed next to each
other in form of a close-packed film, we observed a red shift of the
excitonic peaks in the absorption spectra compared to those for
individual NCs dispersed in FA (Figure S11). Such red shift can
come from slight relaxation of the quantum confinement due to
partial leakage of the wave function into neighboring NCs. The
same CdSe NCs capped with the organic ligands showed almost
identical absorption spectra in solutions and in close-packed
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Figure 6. (aꢀc) Plots of drain current ID vs drain-source voltage VDS, measured at different gate voltages VG for the field-effect transistors (FETs)
assembled from colloidal NCs capped with (NH4)2S: (a) CdSe, (b) CdSe/CdS coreꢀshells and (c) CdSe/ZnS coreꢀshells. Insets to panel (c) show the
photoluminescence spectrum (left) and a photograph (right) for CdSe/ZnS NC film capped with (NH4)2S ligands annealed at 250 °C for 30 min.
(d and e) Plots of ID vs VG at constant VDS = 2 V used to calculate current modulation and linear regime field-effect mobility for FETs using CdSe and
CdSe/CdS NCs. (f) Plots of ID vs VG measured at constant VDS = 4 V in a FET assembled from CdSe/ZnS NCs. L = 80 μm, W = 1500 μm, 100 nm SiO2
gate oxide.
4. CONCLUSIONS
Laboratory at Argonne National Lab (ANL) for ICP-OES
elemental analysis and Evident Technologies, Inc. for providing
samples of high-quality CdSe and CdSe/ZnS NCs. The work was
supported by NSF CAREER under Award Number DMR-
0847535 and by NSF SBIR Award Number IIP-1047180. D.V.T.
thanks the David and Lucile Packard Foundation for their
generous support. The work at the Center for Nanoscale
Materials (ANL) was supported by the US Department of
Energy under Contract No. DE-AC02-06CH11357.
We have demonstrated that various metal-free inorganic
ligands like S2ꢀ, OHꢀ, and NH2ꢀ can behave as capping ligands
for colloidal semiconductor and metallic NCs. These ligands
significantly enrich the colloidal chemistry of all-inorganic NCs,
and represent a promising strategy for integration of colloidal
nanostructures in electronic and optoelectronic devices.
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