Synthesis of metal selenide colloidal nanocrystals by the hot injection of selenium powder

Article abstract of DOI:10.1039/c3dt50757b

We describe the synthesis of metal selenide nanocrystals, including CdSe, ZnSe, CuInSe₂ and Cu₂(Zn,Sn)Se₄, by the hot injection of selenium powder dispersed in a carrier solvent. Since this results in a fast and high yield nanocrystal formation, we argue that the approach is well suited for the low cost, large volume production of nanocrystals.

Full text of DOI:10.1039/c3dt50757b

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Synthesis of metal selenide colloidal nanocrystals
by the hot injection of selenium powder
Cite this: DOI: 10.1039/c3dt50757b
a,b,c
a,b,d
a,b
c
Stijn Flamee,
Ruben Dierick,
Marco Cirillo, Dirk Van Genechten,
a,b
a,b
Tangi Aubert and Zeger Hens*
Received 20th March 2013,
Accepted 18th April 2013
We describe the synthesis of metal selenide nanocrystals, including CdSe, ZnSe, CuInSe
2 2
and Cu (Zn,Sn)-
4
Se , by the hot injection of selenium powder dispersed in a carrier solvent. Since this results in a fast and
DOI: 10.1039/c3dt50757b
high yield nanocrystal formation, we argue that the approach is well suited for the low cost, large
volume production of nanocrystals.
www.rsc.org/dalton
In the case of semiconductor materials, the combination of
synthetic control and a suitability for solution-based proces-
Introduction
The formation of sterically stabilized colloidal nanocrystals sing leads to roughly two types of applications. First, quantum
from metalorganic complexes in reaction mixtures containing confinement leads to a size-dependence of the opto-electronic
metalorganic precursors has seen a tremendous evolution over properties, most notably the band gap, of semiconductor
1
2
3
the last 20 years. Introduced by Murray et al. and Brust et al.
nanocrystals once their dimensions are reduced to ≈10 nm or
2
0
to make cadmium chalcogenide and gold nanocrystals, less. With size dispersions of 5% or less, this leads to
respectively, the approach was rapidly extended to the syn- materials having a well-defined onset wavelength for light
thesis of nanocrystals of various metals, metal oxides and absorption and a narrow light emission spectrum, both easily
4
semiconductor materials. Apart from this versatility, improve- tunable by the nanocrystal size. In the case of CdSe and InP,
ments of the method led to an excellent control over the nano- for example, saturated emission colors can be obtained
crystal size, resulting in colloidal suspensions with size throughout the visible spectrum, while the bandgap of PbS or
2
1
dispersions of 5% or less. Moreover, the use of specific ligands PbSe can be adjusted in the infrared, e.g., to reach the
during the synthesis enabled nanocrystals with a variety of optimum range for photovoltaic energy conversion. As a result,
5
,6
7,8
9,10
22,23
shapes to be grown, including cubes, rods, multipods
the use of these materials extends from lighting
and
photodetec-
Second, dispersions of semi-
Next to this, the synthetic conductor nanocrystals can be seen as precursors for the
approach resulted in dispersed nanocrystals with a well- formation of semiconductor thin films by solution-based pro-
1
1
17
15,24
and platelets, and it proved possible to form heterostruc- display applications to luminescent labeling,
tures, where, e.g., a second material is grown as a shell around tion
2
5,26
16,27
and photovoltaics.
8
,12
initial core nanocrystals.
1
3
defined and adjustable surface chemistry. This gives access cesses. Especially in the field of thin film photovoltaics,
to extensive post-synthesis processing, including the formation various studies have addressed the formation of absorber
of nanocrystal assemblies by solution-based deposition tech- layers, including CdTe, Cu(In,Ga)(S,Se) (CIGS) or Cu ZnSn-
2 2
2
8
29
1
4
30
niques, such as spin coating and inkjet printing, and the (S,Se) (CZTS) by depositing colloidal nanocrystals, followed
4
chemical functionalization of nanocrystals to make them by a heat treatment to densify the resulting nanocrystal film.
1
5
active towards biomolecules or analytes. As a result, appli- For both types of application, metal selenides are a widely
cations of colloidal nanocrystals are actively explored in used class of materials. In the case of light emission, CdSe-
1
5
diverse fields such as biolabeling and biosensing, photo- based heteronanocrystals have been the most widely used
1
6
17–19
voltaics and opto-electronics.
nanocrystals, while ZnSe forms a so-called type 2 heterostruc-
3
1
ture with CdS that is useful in lasing applications. The use of
CIGS and CZTS as absorber materials in thin film photo-
voltaics, on the other hand, raises interest in all metal selenide
nanocrystals from Cu, In, Ga, Zn and Sn or the relevant ternary
and quaternary combinations thereof.
a
Physics and Chemistry of Nanostructures, Ghent University, Krijgslaan 281-S3,
B-9000 Gent, Belgium. E-mail: Zeger.Hens@ugent.be; Fax: +32-9-2644983;
Tel: +32-9-2644863
b
With the increasing use of colloidal nanocrystals, the
Centre for Nano- and Biophotonics, Ghent University, Gent, Belgium
c
volume and concomitantly, the cost of production have
Umicore Group Research & Development, Kasteelstraat 7, B 2250 Olen, Belgium
d
32–35
Strategic Initiative Materials – Program SoPPoM, Gent, Belgium
become the subject of intense research.
Apart from issues
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of synthesis methodology, this involves a search for alternative UV-Vis absorption spectroscopy (CdSe) or X-ray fluorescence
precursors that can reduce various aspects of the synthesis spectroscopy (CuInSe ). The final reaction yield is obtained
2
cost, such as reagent cost, the need for protective atmosphere, from the amount of unreacted precursors as determined using
purification cost, waste management, etc. In the case of metal XRF in the supernatant after purification of the reaction
selenide nanocrystals, the initial CdSe synthesis for example product. For UV-Vis analysis (Perkin-Elmer lambda 950), pub-
4
5
was based on dimethyl cadmium and trioctylphosphine sele- lished sizing curves and absorption coefficients are used. For
nide (TOP-Se) or bis-trimethylsilyl selenide as the cadmium quantitative XRF analysis (Rigaku NEX CG), UltraCarry’s® have
2
and selenium precursors, respectively. While dimethyl been used to measure the liquid samples. A series of standard
cadmium was quickly replaced by cadmium carboxylate or samples, in the same range of concentrations as the super-
3
6,37
cadmium phosphonate complexes,
few alternatives have natant and the particle solutions, have been made making use
been developed for the selenium precursor. An example is the of chloride salts and selenious acid solutions. The X-ray fluo-
use of elemental selenium dissolved in a solvent such as octa- rescence intensities have been used to make a calibration
decene (ODE) at elevated temperature, either using a hot injec- curve and subsequently determine the concentration of the
3
8,39
tion or
a
heating-up approach.
Whereas elemental elements in the samples. X-ray diffraction (XRD) measure-
selenium is cheap and easy to handle, the formation of CdSe ments were carried out using a Thermo scientific ARL X’TRA
nanocrystals based on this so-called ODE-Se precursor is slow model on dried dispersions dropcast on silica substrates.
and does not allow – opposite to TOP-Se based syntheses – for Transmission electron microscopy (TEM) micrographs were
the full yield synthesis of CdSe NCs with different sizes. In acquired using a Cs-corrected Jeol 2200 FS microscope. Ruther-
fact, although expensive and oxidation prone, phosphine sele- ford backscattering spectrometry (RBS) was performed as
4
6
nides such as TOP-Se remain widely used Se precursors for the described before. RBS samples were made by spincoating a
1
synthesis of metal selenide nanocrystals, including – next to thin layer of nanocrystals on silicon. Quantitative H nuclear
4
0
41,42
43
CdSe – for example ZnSe, CuInSe
2
and Cu
2
(Zn,Sn)Se
4
.
magnetic resonance (NMR) spectra were taken as described
4
6
Here, we introduce the direct injection of Se powder – het- before. NMR samples were prepared by dissolving a known
erogeneously dispersed in a carrier solvent such as ODE – as amount of QDs in toluene-d
an alternative precursor for the synthesis of colloidal metal
8
.
selenide nanocrystals. Taking the synthesis of CdSe as a first
example, we show that this injection initiates a fast reaction
leading to the formation of CdSe nanocrystals with an 80–90%
reaction yield within 5–10 minutes. Moreover, the reaction can
Results
CdSe nanocrystals
be executed under air without compromising the quality of the For a standard CdSe nanocrystal synthesis, a mixture of CdO
end product. In a second step, we show how the same (0.4 mmol), a carboxylic acid (CA) such as myristic acid (MA,
approach can be extended to the formation of nanocrystals of 1.2 mmol) and ODE (10 mL) is heated up to 270 °C under air,
ZnSe, and various binary, ternary and quaternary selenides of resulting in the dissolution of CdO by the formation of a
the CIGS and CZTS materials family, including CuInSe
2
, the cadmium carboxylate complex. The reaction is initiated by the
binary compounds Cu Se and In Se and Cu ZnSnSe . Also injection of Se powder (0.2 mmol, 200 mesh), heterogeneously
2
2
3
2
4
here, we typically find that the injection of Se powder initiates dispersed in 1 mL of ODE. To uniformly distribute the Se
a fast (<10 min) and almost complete conversion of the precur- powder over the carrier liquid, the mixture is permanently
sors into nanocrystals. Using various literature examples, we stirred before injection, yet no attempt was made to dissolve
show that the typical features of the reaction following the hot the Se powder by heating. Upon injection, the black color of
injection of selenium powder, i.e., fast and high yield conver- the Se powder disappears and, leaving the reaction mixture at
sion, contrast with current synthesis methods where elemental 260 °C, it gradually changes color from yellow to orange to red
selenium is used. Based on this, we argue that the approach within a few minutes time. Fig. 1a shows a TEM micrograph of
presented here is well suited for the high volume, low cost syn- the product obtained after 5 minutes of reaction followed by
thesis of metal selenide nanocrystals.
successive precipitation/redispersion steps. Here, we first use a
methanol–isopropanol mixture as the non-solvent and a
toluene–oleic acid (OA) mixture as the solvent, followed by two
more precipitation/redispersion steps using methanol and
toluene as the respective non-solvent and solvent. The micro-
Experimental
All nanocrystal syntheses have been executed as described in graph clearly shows the formation of quasi-spherical nanopar-
the respective sections. After synthesis, nanocrystal purifi- ticles, which have according to X-ray diffraction (see Fig. 1b)
cation is achieved by standard precipitation methods using the crystal structure of zinc blende CdSe.
methanol or methanol–isopropanol mixtures as the non-
To follow the reaction development, we took aliquots of the
solvent and toluene as the solvent. The quantitative analysis of reaction mixture that were quantitatively analysed using
the reaction development is based on the weight of the reac- UV-Vis absorption spectroscopy to obtain the amount nCdSe of
4
4
tion mixture and of the different aliquots taken. The amount CdSe formed and the nanocrystal diameter dNC
.
As shown in
of product formed in the aliquots is either determined using Fig. 1c, the absorption spectra show the succession of peaks
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Fig. 1 (a) TEM micrograph of CdSe nanocrystals as formed by the injection of a Se powder into a solution of cadmium myristate, myristic acid and octadecene.
Scale bar: 10 nm. (b) X-ray diffractogram of the CdSe nanocrystals thus formed. The vertical lines indicate the most important reflections of zinc blende CdSe. (c) Nor-
malized absorption spectra of purified aliquots taken at the indicated time after starting the reaction by the injection of a Se powder. (d) Time development of the
amount of CdSe and the average nanocrystal diameter dNC as calculated from the absorption spectra shown. The dashed line indicates the 100% yield level, corres-
ponding to 0.2 mmol of the injected Se.
characteristic of size quantization in semiconductor nanocrys-
Over the last 5 years, a detailed picture of CdSe nanocrystals
tals. The red shift of these features – most notably the longest synthesized in the presence of carboxylic or phosphonic acids
wavelength peak related to the band gap transition – reflects has emerged, where a Cd excess in the nanocrystal core is
the nanocrystal growth as a function of time, from ≈2.6 nm charge balanced by anionic ligands, such as carboxylates or
1
3
after 5 s to ≈3.1 nm after 5 min (see Fig. 1d). Importantly, the phoshonates and phosphonic acid anhydrates.
Since a
red shift of the absorption features stops within 5 min of reac- different Se precursor is used here and the reaction is carried
tion time, where dNC attains a constant value for a reaction out under air, it makes sense to investigate whether the same
yield of ≈80%. Hence, we conclude that the injection of Se structural model applies to the CdSe nanocrystals formed
1
powder into a mixture of ODE, a cadmium carboxylate and here. Fig. 2a shows the solution H NMR spectrum of a dis-
excess CA leads to the rapid and almost complete conversion persion of CdSe nanocrystals in toluene-d , after a purification
8
of the Cd and Se precursors into CdSe nanocrystals. Moreover, process as mentioned above. In line with NMR spectra pub-
4
6,47
the size dispersion of the resulting nanocrystal suspensions – lished in the literature,
the broad resonances are character-
as determined from the full width at half maximum of the first istic of bound OA moieties, while no indication is given for
absorption feature and the diameter histogram as obtained residual free CA. By integrating the methyl resonance (labelled
from TEM micrographs – amounts to 5–6% or less. This is as 5 in Fig. 2a), we obtain a ligand density – OA plus possibly
comparable to the numbers obtained using TOP-Se as a Se pre- residual MA – of 3.4(2) nm− . To compare this with a possible
2
4
4
cursor, yet it is significantly better than the phosphine-free Cd excess, we use Rutherford backscattering spectrometry to
3
8,39
2
approaches described in the literature.
Similar results are determine the Cd : Se ratio. From the Z weighted intensity of
obtained when executing the reaction in a saturated hydro- the backscattered intensity on Se and Cd nuclei, we calculated
2
+
−
carbon such as hexadecane.
a Cd : Se ratio of 1.22(5). This leads to a Cd : CA ratio of
1
Fig. 2 H NMR spectrum of 3.1 nm CdSe nanocrystals synthesized as outlined in the text, followed by a purification using a toluene–oleic acid mixture as the
solvent in the first precipitation/resuspension step. The different resonances are attributed to OA protons as indicated. (b) Rutherford backscattering spectrum
showing the (blue) intensities and (red) integrated intensities related to backscattering with selenium and cadmium nuclei. (c) (red) Absorption and (blue) lumines-
cence spectra of 2.8 nm CdSe nanocrystals synthesized as outlined in the text yet using 0.2 mmol of Cd and 0.1 mmol of Se.
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1
.9(4), i.e., in line with literature results on CdSe nanocrystals function of the reaction time. It follows that the ZnSe nano-
synthesised in the presence of carboxylic or phosphonic acids, crystals grow larger with time, to reach a final diameter within
our approach yields non-stoichiometric nanocrystals where the 8 minutes. At that moment, the reaction yield, as determined
charge related to the Cd excess is balanced by the carboxylate by analyzing the supernatant after first purification of the reac-
ligands. To finish the analysis of the CdSe nanocrystals made tion mixture, amounts to ≈90%.
here, Fig. 2c shows a photoluminescence (PL) spectrum of
2
.8 nm CdSe nanocrystals formed as described here. In line
with the low size dispersion, the PL is spectrally narrow – a full
width at half maximum of 25–30 nm is typically obtained – For the synthesis of CuInSe
with a PL quantum yield of about 5%. We thus conclude that a mixture of Cu(acac) (2 mmol) and In(acac)
CuInSe and its binary compounds
2
2
(CISe) nanocrystals, we start from
(2 mmol) in
2
3
the synthesis method introduced here leads – even under air – oleylamine (OLA) (16 mL). After flushing the mixture for
to CdSe nanocrystals with state-of-the-art properties in terms 1 hour with nitrogen at 110 °C, its temperature is raised to
of surface chemistry and basic optical properties.
240 °C, leading to a color change from blue to brown to
brown-red at 240 °C. At this temperature, Se powder
(
4.1 mmol), heterogeneously dispersed in ODE (8 mL), is
ZnSe nanocrystals
swiftly injected, after which the reaction mixture turns black
The method developed for the synthesis of CdSe nanocrystals immediately. Fig. 4a shows a TEM micrograph and an XRD
is with minor changes extendable to the formation of ZnSe diffractogram of the product collected when stopping the reac-
NCs. Although ZnO can be used as the initial Zn precursor, we tion after 10 min. Clearly, quasi-spherical nanocrystals with a
obtained better results in terms of size dispersion using zinc mean diameter of about 7 nm are formed with a crystal struc-
5 3 2 6 2
carbonate basic, i.e., Zn (CO ) (OH) . In a typical synthesis, ture that corresponds to that of chalcopyrite CuInSe (Fig. 4b).
0
.08 mol of Zn (CO ) (OH) is dissolved in a mixture of OA A more detailed view on the reaction development follows
5
3 2
6
(1.2 mmol) and ODE at 270 °C, followed by the injection of Se from the analysis of aliquots, taken at different times after the
powder (0.2 mmol) using 1 mL ODE as a carrier liquid. Again, Se injection. The UV-Vis absorption spectra of the aliquots
after purification of the reaction mixture similar to the one taken in the first 5 min of the reaction clearly show the infra-
used for CdSe, a TEM analysis shows the formation of nano- red absorption band characteristic of the plasmon resonance
4
8
x y
crystals with an average diameter of 3.4 nm that correspond of Cu Se nanocrystals (see Fig. 4c). From 10 min on, the
according to XRD to zinc blende ZnSe (see Fig. 3a and 3b, infrared absorption is absent, and we notice no difference with
respectively). Although the major part of the nanocrystals thus subsequent absorbance spectra. In line with the TEM and XRD
formed appears as quasi-spherical, a fraction of them has a analyses, this indicates that the formation of CISe is finished
more elongated shape.
at about 10 min. Further analysis of the aliquots using XRF
Also for the ZnSe synthesis, the absorption spectra of reac- provides more information on the synthesis development
tion mixture aliquots are instrumental in analyzing the devel- (Fig. 4d). For these measurements, we first precipitated the
opment of the reaction. Although less pronounced than in the quantitative aliquots. Since for all aliquots, the supernatant
case of CdSe nanocrystals, all spectra show the absorption contained little Cu, In and Se, the precipitated powder was
peak characteristic of the band gap transition in ZnSe nano- redispersed and subjected to a second precipitation step using
crystals (see Fig. 3c). Since no sizing curve – linking the wave- a minimum amount of non-solvent (so-called size selective
length of the absorption peak to dNC – is available in the precipitation). This is meant to precipitate larger nanocrystals
literature for ZnSe, Fig. 3d represents the peak wavelength as a (denoted as NCs in Fig. 4d), leaving behind possible precursor
Fig. 3 (a) TEM micrograph of ZnSe nanocrystals as formed by the injection of Se powder into a solution of zinc oleate, oleic acid and octadecene. Scale bar: 10 nm.
(b) X-ray diffractogram of the ZnSe nanocrystals thus formed. The vertical lines indicate the most important reflections of zinc blende ZnSe. (c) Normalized absorp-
tion spectra of purified aliquots taken at the indicated time after starting the reaction by the injection of Se powder. (d) Time development of the position of the
first absorbance peak.
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Fig. 4 (a) TEM micrograph of CuISe nanocrystals formed by the injection of Se powder into a solution of Cu(acac)
2
and In(acac)
3
in oleylamine after 10 min of reac-
tion time. Scale bar: 10 nm. (b) X-ray diffractogram of the CISe nanocrystals thus formed. The vertical lines indicate the most important reflections of chalcopyrite
CuInSe . (c) Normalized absorption spectra of aliquots taken at the indicated time after the Se injection. The different spectra are offset for clarity. (d) Time develop-
2
ment of the amount of (blue dots) Cu and (red dots) In in the precipitate – indicated as NC – and of (red circles) In and (green circles) Se in the supernatant – indi-
cated as SN – after the second, size selective precipitation step of the respective aliquots.
complexes or small nanocrystals in the supernatant. Doing so, after which the reaction mixture is cooled down and the nano-
we find that the amount of Cu collected in the precipitate crystals are separated using a similar procedure as shown
remains constant, equal to the 2 mmol of Cu used in the reac- above. TEM analysis of the reaction product (Fig. 5a) shows
tion. On the other hand, most of the In is initially recovered in 8 nm nanocrystals with various shapes. In the XRD pattern, we
the supernatant. Since the supernatant contains little Se and no can see that the reflections occur at angles in between those
Cu, this is most likely as an In complex, and not as small indium expected for Cu Se and Cu1.8Se, indicating the formation of a
2
49
selenide nanocrystals, as previously observed by Kar et al.
Cu2−xSe phase (see Fig. 5b). By chemical analysis of the super-
During the first 5 minutes, the amount of In in the precipitate natant after precipitating the nanocrystals, a chemical yield of
gradually increases, at the expense of the In in the supernatant. 90–95% is obtained.
This points toward a gradual incorporation of In into the Cu
nanocrystals, with a concomitant transformation of the initially nanocrystals. To prevent the possible formation of indium
formed Cu Se into CISe. oxide, we start by dissolving 2 mmol InCl in 20 mL of OLA.
x y
Se
2 3
We follow a comparable approach to synthesize In Se
x
y
3
Cu2−xSe nanocrystals are obtained in a similar fashion. In This mixture is flushed with nitrogen at 110 °C for 1 hour,
this case, 2 mmol of Cu(acac) is dissolved in 20 mL of OLA. after which the temperature is increased to 200 °C. At this
2
This mixture is flushed with nitrogen at 110 °C during 1 hour, point, Se powder (3.1 mmol) dispersed in 3 mL of ODE is
after which the temperature is increased to 200 °C to obtain a injected into the reaction mixture. The color of the mixture
brown mixture. At this point, 1.5 mmol of Se dispersed in changes from yellow-red to dark red in the course of a minute.
1
.5 mL of ODE is injected, leading to a color change to green- After leaving the reaction mixture at 200 °C for 1 hour, the
brown. The mixture is allowed to react at 200 °C for 30 min, nanocrystals are collected by cooling down the synthesis and
Fig. 5 (a) TEM micrograph of Cu2−xSe nanocrystals formed by the injection of Se powder into a solution of Cu(acac)
see the text). Scale bar: 10 nm. (b) X-ray diffractogram of the Cu2−xSe nanocrystals thus formed. The vertical lines indicate the most important reflections of chalco-
pyrite Cu Se and Cu1.8Se as a reference. (c) TEM micrograph of In Se nanocrystals formed by the injection of Se powder into a solution of In(acac) in oleylamine
after 1 hour of reaction time (see the text). Scale bar: 10 nm. (d) X-ray diffractogram of the In Se nanocrystals thus formed. The vertical lines indicate the most
important reflections of In Se as a reference.
2
in oleylamine after 1 hour of reaction time
(
2
2
3
3
2
3
2
3
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using standard centrifugation methods. The TEM micrograph 30 s, all the injected Se has reacted to form nanocrystals poor
in Fig. 5c confirms the formation of nanocrystals with a size in Sn and Zn. As the reaction proceeds, first the amount of Sn
below 10 nm, yet the resulting particles are difficult to image. and next the amount of Zn incorporated into the nanocrystals
Nevertheless, the XRD pattern confirms the formation of crys- increase to reach their final levels after about 8 min. Overall,
talline In Se , with no impurity phases present (Fig. 5d). The we find that this results in CZTSe nanocrystals that are rich in
2
3
yield of the reaction, using the same procedure as for Cu2−xSe Sn and poor in Zn.
nanocrystals, was calculated as 80–85%.
Discussion
2 4
Cu ZnSnSe nanocrystals
We extended the method developed for the synthesis of CISe In the literature, elemental selenium has been used by various
nanocrystals with minor changes to the formation of kesterite authors for the synthesis of metal selenide nanocrystals as an
Cu
Cu, Zn and Sn salts in 10 mL of OLA where in this case, methylsilyl)selenide. In the case of CdSe, this involves the
Cu(acac) (3 mmol), zinc acetate [Zn(Ac) ·2H O, 1.5 mmol] and injection in a reaction mixture of a cadmium carboxylate and
tin chloride [SnCl ·5H O, 1 mmol] have been used – was excess carboxylic acid in ODE of a solution made by dissolving
heated up to 260 °C under vigorous stirring, after which selenium in octadecene (ODE-Se) by heating at ≈200 °C for up
2 4
(Zn,Sn)Se (CZTSe) nanocrystals. In this case, a solution of alternative for phosphine based selenium precursors or bis(tri-
2
2
2
4
2
3
8
3
mmol of Se powder is injected using 2 mL of OLA as a to 2 hours. Alternatively, CdSe nanocrystals can also be
carrier liquid. After the injection, the temperature is reduced formed by the gradual heating up of this entire reaction
3
9
to 240 °C and the reaction is stopped after 30 minutes. The mixture, including the selenium powder. However, both
final product is purified by successive precipitation/redisper- approaches result in relatively slow reactions. Injection of
sion steps. Here, we first use ethanol as the non-solvent and a ODE-Se for example results in the formation of CdSe nanocrys-
5
0
toluene–OLA mixture as the solvent, followed by two more pre- tals with a 20–40% yield after 30 minutes. Similarly, CuInSe2
cipitation/redispersion steps using ethanol and toluene as the nanocrystals have been synthesized using the combination of
respective non-solvent and solvent. The TEM micrograph in metal salts and elemental selenium as precursors and OLA as
Fig. 6a shows that this procedure results in relatively polydis- the ligand. Also here, all reagents are either mixed at low temp-
perse nanocrystals with irregular shapes and typical dimen- erature, followed by a gradual heating-up of the reaction
5
1,52
sions in the 10–30 nm range. The reflections in the X-ray mixture
diffractogram recorded on these nanocrystals correspond to by prolonged heating at elevated temperature.
those of kesterite Cu (Zn,Sn)Se approaches the reaction mixture is left at elevated temperatures
or selenium is dissolved prior to injection in OLA
5
3
In both
2
4
.
To analyze the time development of the reaction in more for one to four hours. Although little is known about the yield
detail, we took quantitative aliquots that were studied using or the yield development of these reactions, it indicates that
UV-Vis absorbance spectroscopy and elemental analysis. In this heating-up results – similar to the case of CdSe – in a rela-
this case, only the absorption spectrum of the first aliquots, tively slowly reacting Se precursor. A very similar approach,
taken after 15 and 30 s, shows the infrared absorbance indica- based on the injection of selenium dissolved in OLA, has been
4
8
48
x y
tive of Cu Se nanocrystal formation whereas little further described for the synthesis of Cu2−xSe nanocrystals. Although
changes in the spectra are observed after 2 minutes. This in this case, the reaction time was limited to 15 minutes, no
picture is confirmed by XRF analysis, which shows that within information is provided on the concomitant reaction yield.
Fig. 6 (a) TEM micrograph of CZTSe nanocrystals formed by the injection of Se powder into a solution of Cu, Zn and Sn salts in oleylamine after 32 min of reaction
time (see the text). Scale bar: 50 nm. (b) X-ray diffractogram of the CZTSe nanocrystals thus formed. The vertical lines indicate the most important reflections of chal-
copyrite CZTSe. (c) Normalized absorption spectra of aliquots taken at the indicated time after the Se injection. The different spectra are offset for clarity. (d) Time
development of the amount of (green circles) Se, (blue dots) Cu, (red squares) Sn and (red circles) Zn in the precipitate after the second, size selective precipitation
step of the respective aliquots.
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The results shown here indicate that for all these syntheses, CZTSe syntheses described here produce for example
state-of-the-art metal selenide nanocrystals can be obtained by 10–15 mg and 25–40 mg of nanocrystals per mL of solvent,
injecting the selenium powder into the reaction mixture at respectively.
elevated temperature using a carrier liquid. Opposite to
heating-up approaches or the injection of dissolved selenium
species, this typically initiates a rapid and high yield conver-
Conclusions
sion of the precursors into nanocrystals. Moreover, the pro-
We describe the hot injection of elemental selenium powder,
cedure can be extended to the formation of ZnSe, In Se and
2
3
heterogeneously dispersed in a carrier liquid for the synthesis
of various metal selenide nanocrystals. As compared to the
existing synthesis procedure that makes use of elemental sel-
CZTSe nanocrystals, where in all cases close to full yield is
obtained. It is worth mentioning that only a few synthesis pro-
cedures for In Se and CZTSe nanocrystals have been reported
2
3
2
enium, e.g., to form CdSe, CuInSe and Cu2−xSe, this leads to a
in the literature. It has been shown that prolonged heating of
InCl and elemental Se in OLA leads to cubic-phase InSe nano-
fast and close to full yield conversion of precursors into nano-
crystals. Moreover, as exemplified by the case of CdSe nano-
crystals, the materials thus formed have state-of-the-art
properties in terms of size dispersion, surface chemistry and
optical properties. In addition, the method can be extended to
3
5
4
wires, whereas In-Se nanosheets have been obtained using
5
5
2 3
selenourea as a selenium precursor and In Se nanocrystal-
5
6
lites have been obtained using solvothermal methods. In the
case of CZTSe, the use of a TOP-Se based selenium precursor
2 3
the formation of ZnSe, In Se and CZTSe nanocrystals –
4
3
has been shown to yield CZTSe nanocrystals, albeit with a
considerable Zn deficiency. A similar lack of Zn incorporation
materials for which little or no low-cost routes have been
described – which again results in a fast and close to full yield
reaction. Based on these results, we conclude that the hot
injection of selenium powder is a versatile approach for the
synthesis of metal selenide nanocrystals that allows for their
large volume, low cost production.
was found for an alternative approach based on SeO as a sele-
2
3
5
nium precursor.
In the context of the CdSe synthesis using Se dissolved in
octadecene, Bullen et al. have addressed the reactivity of this
selenium precursor as a function of the preparation con-
5
0
ditions. They found that dissolution at a lower temperature
and for a shorter time yields a more reactive precursor. This Acknowledgements
result was interpreted in terms of a vulcanization process
ZH acknowledges Ghent University (IOF StarTT project
Selene’), the FWO-Vlaanderen (research project G.0760.12)
and the Belgian Science Policy Office (BelSPo) (IAP7.35, photo-
nics@be) for funding this research. SF acknowledges the IWT-
Vlaanderen (Agency for Innovation by Science and Technology
in Flanders) for a scholarship.
where ODE molecules are first bridged by short Se chains and
later on by single Se atoms. Since we obtain similar results in
the case of the CdSe synthesis using either ODE or hexadecane
as a solvent, it appears that the reactive selenium species
formed after the injection of selenium powder is rather a dis-
solved selenium species that precedes these vulcanization
reactions. This conclusion is corroborated by the observa-
tion that when a twofold excess of selenium relative to
cadmium is used, 50% of the unreacted selenium is recovered
as selenium powder when stopping the reaction after
‘
Notes and references
1 D. V. Talapin, J.-S. Lee, M. V. Kovalenko and E. V.
Shevchenko, Chem. Rev., 2009, 110, 389–458.
2 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem.
Soc., 1993, 115, 8706–8715.
8
minutes. In the case of the CISe and CZTSe synthesis,
the highly reactive selenium species resulting from the injec-
tion of selenium powder forms a Cu Se compound in which
x
y
either In or Sn and Zn are incorporated in a later stage of the
reaction to form CISe and CZTSe, respectively. In the case of
3 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and
R. Whyman, Chem. Commun., 1994, 801–802.
CISe, this Cu
x
Se
y
formation has been described before by Kar
4 C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev.
Mater. Sci., 2000, 30, 545–610.
4
9
et al., who used a heating-up procedure. However, in their
case, the CuSe formation extends over a large part of
5 M. V. Kovalenko, M. I. Bodnarchuk, R. T. Lechner,
G. Hesser, F. Schaffler and W. Heiss, J. Am. Chem. Soc.,
2007, 129, 6352–6353.
the
1 hour heating-up time, while upon injection of
Se powder, we find that all CuSe is formed within the first
1
0–15 s of the reaction.
The combination of a fast and high yield reaction with low
6 T. L. Mokari, M. J. Zhang and P. D. Yang, J. Am. Chem. Soc.,
2007, 129, 9864–9865.
cost precursors makes the approach proposed here well suited
for larger scale nanocrystal synthesis. An additional asset of
the approach is the possibility to inject large quantities of sele-
nium using only a small volume of carrier solvent. This
enables synthesis conditions to be used where a large amount
of nanocrystals is produced using a relatively small amount of
solvent. Without attempting any optimization, the CISe and
7 X. G. Peng, L. Manna, W. D. Yang, J. Wickham, E. Scher,
A. Kadavanich and A. P. Alivisatos, Nature, 2000, 404, 59–61.
8 L. Carbone, C. Nobile, M. De Giorgi, F. D. Sala, G. Morello,
P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchini,
M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera,
R. Cingolani, R. Krahne and L. Manna, Nano Lett., 2007, 7,
2942–2950.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.

View Article Online
Paper
Dalton Transactions
9
X. G. Peng, Adv. Mater., 2003, 15, 459–463.
33 J. Park, K. J. An, Y. S. Hwang, J. G. Park, H. J. Noh,
J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat.
Mater., 2004, 3, 891–895.
1
1
1
0 L. Manna, E. C. Scher and A. P. Alivisatos, J. Am. Chem.
Soc., 2000, 122, 12700–12706.
1 S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. M. Lobo, 34 M. Protiere, N. Nerambourg, O. Renard and P. Reiss, Nano-
B. Dubertret and A. L. Efros, Nat. Mater., 2011, 10, 936–941. scale Res. Lett., 2011, 6, 472.
2 M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem., 1996, 35 F.-J. Fan, Y.-X. Wang, X.-J. Liu, L. Wu and S.-H. Yu, Adv.
00, 468–471. Mater., 2012, 24, 6158–6163.
3 Z. Hens and J. C. Martins, Chem. Mater., 2013, 25, 1211–1221. 36 L. H. Qu, Z. A. Peng and X. G. Peng, Nano Lett., 2001, 1,
1
1
1
4 M. Boberl, M. V. Kovalenko, S. Gamerith, E. J. W. List and
W. Heiss, Adv. Mater., 2007, 19, 3574–3578.
333–337.
37 W. W. Yu and X. G. Peng, Angew. Chem., Int. Ed., 2002, 41,
2368–2371.
38 J. Jasieniak, C. Bullen, J. van Embden and P. Mulvaney,
J. Phys. Chem. B, 2005, 109, 20665–20668.
1
1
5 I. L. Medintz, H. T. Uyeda, E. R. Goldman and
H. Mattoussi, Nat. Mater., 2005, 4, 435–446.
6 J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu,
L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, 39 Y. A. Yang, H. M. Wu, K. R. Williams and Y. C. Cao, Angew.
K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury and
E. H. Sargent, Nat. Mater., 2011, 10, 765–771.
7 T.-H. Kim, K.-S. Cho, E. K. Lee, S. J. Lee, J. Chae, J. W. Kim,
Chem., Int. Ed., 2005, 44, 6712–6715.
40 M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem. B, 1998,
102, 3655–3657.
1
D. H. Kim, J.-Y. Kwon, G. Amaratunga, S. Y. Lee, B. L. Choi, 41 J. Park, C. Dvoracek, K. H. Lee, J. F. Galloway, H.-e. C.
Y. Kuk, J. M. Kim and K. Kim, Nat. Photonics, 2011, 5, 176–182.
8 J. M. Caruge, J. E. Halpert, V. Wood, V. Bulovic and
M. G. Bawendi, Nat. Photonics, 2008, 2, 247–250.
Bhang, M. G. Pomper and P. C. Searson, Small, 2011, 7,
3148–3152.
42 H. Zhong, Y. Li, M. Ye, Z. Zhu, Y. Zhou, C. Yang and Y. Li,
Nanotechnology, 2007, 18, 025602.
1
1
9 S. Coe, W. K. Woo, M. Bawendi and V. Bulovic, Nature,
2
002, 420, 800–803.
43 A. Shavel, J. Arbiol and A. Cabot, J. Am. Chem. Soc., 2010,
132, 4514–4515.
2
2
0 H. Weller, Adv. Mater., 1993, 5, 88–95.
1 A. L. Rogach, A. Eychmuller, S. G. Hickey and S. V. 44 S. Abe, R. K. Capek, B. De Geyter and Z. Hens, ACS Nano,
Kershaw, Small, 2007, 3, 536–557. 2012, 6, 42–53.
2 A. L. Rogach, N. Gaponik, J. M. Lupton, C. Bertoni, 45 R. K. Capek, I. Moreels, K. Lambert, D. De Muynck,
2
D. E. Gallardo, S. Dunn, N. L. Pira, M. Paderi, P. Repetto,
S. G. Romanov, C. O’Dwyer, C. M. S. Torres and
A. Eychmuller, Angew. Chem., Int. Ed., 2008, 47, 6538–6549.
3 J. Ziegler, S. Xu, E. Kucur, F. Meister, M. Batentschuk,
F. Gindele and T. Nann, Adv. Mater., 2008, 20, 4068–4073.
4 J. K. Jaiswal, H. Mattoussi, J. M. Mauro and S. M. Simon,
Nat. Biotechnol., 2003, 21, 47–51.
5 T. Rauch, M. Boberl, S. F. Tedde, J. Furst, M. V. Kovalenko,
G. N. Hesser, U. Lemmer, W. Heiss and O. Hayden, Nat.
Photonics, 2009, 3, 332–336.
Q. Zhao, A. Van Tomme, F. Vanhaecke and Z. Hens, J. Phys.
Chem. C, 2010, 114, 6371–6376.
46 B. Fritzinger, R. K. Capek, K. Lambert, J. C. Martins and
Z. Hens, J. Am. Chem. Soc., 2010, 132, 10195–10201.
47 I. Moreels, B. Fritzinger, J. C. Martins and Z. Hens, J. Am.
Chem. Soc., 2008, 130, 15081–15086.
48 D. Dorfs, T. Haertling, K. Miszta, N. C. Bigall, M. R. Kim,
A. Genovese, A. Falqui, M. Povia and L. Manna, J. Am.
Chem. Soc., 2011, 133, 11175–11180.
49 M. Kar, R. Agrawal and H. W. Hillhouse, J. Am. Chem. Soc.,
2011, 133, 17239–17247.
2
2
2
2
6 G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond,
M. Bernechea, F. P. G. de Arquer, F. Gatti and 50 C. Bullen, J. van Embden, J. Jasieniak, J. E. Cosgriff,
F. H. L. Koppens, Nat. Nanotechnol., 2012, 7, 363–368.
7 J. M. Luther, M. Law, M. C. Beard, Q. Song, M. O. Reese,
R. J. Mulder, E. Rizzardo, M. Gu and C. L. Raston, Chem.
Mater., 2010, 22, 4135–4143.
2
2
2
3
R. J. Ellingson and A. J. Nozik, Nano Lett., 2008, 8, 3488–3492. 51 M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke,
8 J. Jasieniak, B. I. MacDonald, S. E. Watkins and
L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel,
P. Mulvaney, Nano Lett., 2011, 11, 2856–2864.
J. Am. Chem. Soc., 2008, 130, 16770–16777.
9 Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano 52 Q. Guo, S. J. Kim, M. Kar, W. N. Shafarman, R. W. Birkmire,
Lett., 2009, 9, 3060–3065.
0 Q. Guo, G. M. Ford, W.-C. Yang, B. C. Walker, E. A. Stach,
E. A. Stach, R. Agrawal and H. W. Hillhouse, Nano Lett.,
2008, 8, 2982–2987.
H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2010, 53 J. Tang, S. Hinds, S. O. Kelley and E. H. Sargent, Chem.
32, 17384–17386. Mater., 2008, 20, 6906–6910.
1 S. A. Ivanov, A. Piryatinski, J. Nanda, S. Tretiak, K. R. 54 K. H. Park, K. Jang, S. Kim, H. J. Kim and S. U. Son, J. Am.
1
3
Zavadil, W. O. Wallace, D. Werder and V. I. Klimov, J. Am.
Chem. Soc., 2007, 129, 11708–11719.
2 J. J. Li, Y. A. Wang, W. Z. Guo, J. C. Keay, T. D. Mishima,
Chem. Soc., 2006, 128, 14780–14781.
55 J. Ning, G. Xiao, C. Wang, B. Liu, G. Zou and B. Zou, Cryst-
EngComm, 2013, 14, 2139–2144.
3
M. B. Johnson and X. G. Peng, J. Am. Chem. Soc., 2003, 125, 56 H. I. Hsiang, L. H. Lu, Y. L. Chang, D. Ray and F. S. Yen,
2567–12575. J. Am. Ceram. Soc., 2011, 94, 3757–3760.
1
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