Charge transport in Cu2S nanocrystals arrays: Effects of crystallite
size and ligand length
Yehonadav Bekenstein1,2,3, Orian Elimelech1,3, Kathy Vinokurov1,3, O. Millo2,3*,
U.Banin1,3*
1
Institute of Chemistry, the Hebrew University of Jerusalem, Jerusalem 91904, Israel
2
Racah Institute of Physics, the Hebrew University of Jerusalem, Jerusalem 91904,
Israel
3
Center for Nanoscience & Nanotechnology, the Hebrew University of Jerusalem,
Jerusalem 91904, Israel
Materials:
Cu(acac)2(99.99%),
1-dodecanethiol(≥98%),
dioctylether
(99%),
chloroform
(Anhydrous 99%), ethanol (Anhydrous 99%), isopropanol (Anhydrous 99.5%),
acetonitrile (Anhydrous 99.8%), 4-methylbenzenethiol (98%), 1,2-ethanedithiol
(≥98%),
1,6-hexaneditiol
(96%),
ethyl-3-mercaptopropionate
(99%)
and
hexamethyldisilazane (99.9%) All the above were purchased from Sigma Aldrich.
Si wafers were purchased from Virginia semiconductor inc.
Methods:
Synthesis of 14nm Cu2S NCs: Cu2S NCs, were synthesized following the procedure
described in Reference [1]. Briefly, Cu2S seeds are formed by the decomposition of
Cu(acac)2 in excess dodecanethiol under inert atmosphere at 200oC for 30 min. The
dodecanethiol acts as the solvent, surfactant and particularly as the sulfur source. The
crude seed solution is washed repeatedly with chloroform and the purified precipitate
is kept in the glove box. The washing process depletes the amount of the dodecanthiol
ligands on the NC surfaces enhancing the natural tendency of these highly faceted
NCs to self-assemble into ordered superlattices.
Synthesis of 3nm Cu2S NCs: Cu2S NCs, were synthesized following the procedure
described in the reference by Kuzuya et.al.[2]. Briefly, Cu2S seeds are formed by the
decomposition of Cu(acac)2 and dodecanethiol precursors in dioctylether which is in
excess. The reaction takes place under inert atmosphere at 200oC for 30 min. Both
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dodecanethiol and dioctylether act as ligands. Dioctylether has a dual role in the
reaction. In addition of to being a reaction solvent, it also acts as an inhibitor during
the reaction growth process, its presence in excess amount in the solution further
contributes for stabilizing the NCs even at smaller diameters. The crude seeds
solution was precipitated by the addition of ethanol and centrifuging the solution
repeatedly. The purified precipitate is further redispersed in chloroform and the seeds
solution is kept in a glove box. The NCs were characterized by TEM and HRTEM.
The observed average diameter of the seeds is 3.1 nm with standard deviation of 10%.
HRTEM selected area electron-diffraction (SAED) for the 3nm Cu2S NCs depicted
only broad rings, associated with the hexagonal crystal structure of low chalcocite
Cu2S NCs
Device fabrication: Au/Cr electrodes were patterned using standard optical
lithography methods onto highly p-doped SiO2/Si substrates (thermal oxide 100 nm).
Cr (2 nm adhesion layer) and Au (100 nm) electrodes were then thermally evaporated.
Multiple electrode separations were tested. Typical configuration of interdigiting
electrodes with a total overlap of 3000 μm gave the best results. Prior to the Cu2S NC
deposition, the substrates were cleaned with piranha solution and treated with
hexamethyldisilazane molecules to increase surface hydrophobicity. Tight size
distribution of the NCs and careful control over the ligands coverage on the NCs
surface were important parameters in the self-assembly process.
NCs deposition was performed at room temperature and under inert atmosphere, with
the ability to tune the evaporation rate of the solvent to achieve slow deposition of the
NCs for better coverage and ordering. Substrates were placed in a vial vertically to its
flat bottom. The vial was then filled with NC solution and the solvent was left to
slowly evaporate. The resulting film was washed with IPA and characterized using
XHR-SEM (Magellan). In few cases NC deposition resulted in highly ordered
superlattices.
Ligand exchange treatment: The electronic properties of devices with various
capping ligands were investigated. A solution 0.1M of certain ligand in anhydrous
acetonitrile was prepared under inert environment. The original capping ligands of the
Cu2S NC films were exchanged by dip coating technique. Practically a substrate was
dipped into a beaker containing the new ligand solution for several seconds. The
2
substrate was then slowly removed and washed with isopropanol, dried with N2 flow
and promptly inserted into the vacuum chamber of a variable temperature probe
station for electrical characterization.
Rational behind the selection of ligand: We have chosen the following ligands: 4methylbenzenethiol,
1,2-ethanedithiol,
1,6-hexaneditiol
and
ethyl-3-
mercaptopropionate. We have specifically chosen to use a thiol binding linker since
the original ligand, dodecanethiol, has a high adsorption affinity to the particle
surface. The thiol group in the new ligands ensures similar affinity to the surface of
the NCs. Moreover the use of same linker in the different ligands assures similar
surface chemistry, minimal surface restructuring and minor modification to intra NC
electronic properties after the exchange. The use of different carbon-chain lengths
serves a purpose of the investigation of the charge transfer in which inter NC effects
dominate. Therefore, one may consider the ligands in this study solely as spacers
between neighboring Cu2S NCs. 4-methylbenzenethiol which is different from the
other alkyl chains was chosen due to its aromatic ring, enabling the verification of the
ligand exchange in a complementary Fourier transform infrared spectroscopy (FTIR)
characterization.
Ligand exchange in solution: In addition to the increase in conductance of the
treated films we were interested in a complementary method for verifying the ligands
exchange. We developed a ligand exchange process which is performed in solution,
and is further studied by FTIR. For this purpose we have chosen an aromatic ligand 4methylbenzenethiol with vibrations that differ from the original dodecanethiol alkyl
chains. The experiment was carried out by mixing
1ml of Cu2S NCs solution
(~1mM) in excess of 4-methylbenzenethiol. Due to the Le Chatelier's principle the
surface ligands were exchanged after stirring the solution over night. The excess
ligands were cleaned by centrifugation with isopropanol. The FTIR spectrum of the
product confirms the presence of the 4-methylbenzenethiol ligand based on the
presented aromatic vibrations. (Fig S4).
Electrical properties of films that were self assembled after the ligand exchange,
matches the ones measured for films which were self assembled with the original
dodecanethiol and dipped into the exchange solution as mentioned above.
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Figures S1-S4.
1
0.9
a
c
a
0.7
0.6
0.5
0.4
0.3
0.2
Fig. S1: TEM image of 14nm and 3nm Cu2S. TEM image depicting the narrow size distribution of
0.1
the (left) 14nm and (right) the 3nm synthesis. (scale bar 50 nm for both images)
0
400
600
800
1000
1200
Wavelength (nm)
0.6
0.5
b
Scattering intensity (Arb.)
Absorbance (arb.u)
0.8
1400
1600
Cu1.96S (Djurleite)
0.4
d
0.3
0.2
0.1
Cu2S NCs
0
-0.1
-0.2
25
Cu2S (Low Chalcocite)
30
35
40
45
50
Angle 2
Fig. S2: XRD characterization. Powder XRD scattering curves of 14nm Cu2S NC films under inert
environment compared to standard diffraction spectra of stoichiometric bulk Cu 2S (chalcocite) and the
copper depleted Cu1.96S (djurleite) phases, lower and upper axis, respectively. The peak at 2θ=40.8º is
unique for the low chalcocite Cu2S phase (marked with arrows).
4
Fig. S3: Temperature dependence of conductance for 3nm NC arrays. Natural logarithms of the
conductance at 10 Vsd vs. the inverse of the temperature and an Arrhenius-like model fitted to the data
(red line). Data measured at temperatures of above ~160K (as discussed in the text) yields activation
energies with values of: (a) 1,2-ethanedithiol: 43±3 meV (b) 4-methylbenzenethiol: 47±7 meV (c) ethyl
3-mercaptopropionate: 69±2 meV (d) 1,6-hexanedithiol: 73±5 meV (e) dodecanethiol: 85±9 meV
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Fig. S4: Fourier transform infrared spectrum (FTIR) of Cu2S NCs after ligand exchange. Spectra
of Cu2S NC with the original dodecanethiol capping ligand (blue dots) and after ligand exchange in
solution (black curve). Clear indication of aromatic vibration is observed in the ligand exchanged
purified NCs precipitate (marked with blue line), at 800cm-1, 1000cm-1 and 1500cm-1 (C=C) stretches,
as can be compared to the pure ligand spectrum (dashed red curve). These peaks are not present in NCs
capped with the original ligands, further assuring the solution based ligand exchange process is
effective.
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References
1.
Han, W., Yi, L.X., Zhao, N., Tang, A.W., Gao, M.Y., Tang, Z.Y.: Synthesis
and shape-tailoring of copper sulfide/indium sulfide-based nanocrystals. J. Am.
Chem. Soc. 130, 13152–13161 (2008).
2.
Kuzuya, T., Yamamuro, S., Hihara, T., Sumiyama, K.: Water-free Solution
Synthesis of Monodisperse Cu2S Nanocrystals. Chem. Lett. 33, 352–353
(2004).
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