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*

1Institute of Chemistry, the Hebrew University of Jerusalem, Jerusalem 91904, Israel

2Racah Institute of Physics, the Hebrew University of Jerusalem, Jerusalem 91904,

Israel

3Center 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

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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: 4-

methylbenzenethiol, 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 4-

methylbenzenethiol 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.

400 600 800 1000 1200 1400 16000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Abso

rban

ce (a

rb.u

)Wavelength (nm)

a

db

ca

Fig. S1: TEM image of 14nm and 3nm Cu2S. TEM image depicting the narrow size distribution of

the (left) 14nm and (right) the 3nm synthesis. (scale bar 50 nm for both images)

25 30 35 40 45 50-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

Angle 2

Scat

terin

g in

tens

ity (A

rb.)

Cu1.96S (Djurleite)

Cu2S (Low Chalcocite)

Cu2S NCs

Fig. S2: XRD characterization. Powder XRD scattering curves of 14nm Cu2S NC films under inert

environment compared to standard diffraction spectra of stoichiometric bulk Cu2S (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).

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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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