Electronic Changes Induced by Surface ... - ACS Publications

Article pubs.acs.org/JPCC

Electronic Changes Induced by Surface Modification of Cu2−xS Nanocrystals Willi Aigner,† Gergana K. Nenova,† Mahmoud A. Sliem,‡,§ Roland A. Fischer,‡ Martin Stutzmann,† and Rui N. Pereira*,†,∥ †

Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany Inorganic Chemistry II, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany § National Institute of Laser Enhanced Science, Cairo University, Gama St. 1, 11316 Cairo, Egypt ∥ Department of Physics and I3N, University of Aveiro, 3810-193 Aveiro, Portugal ‡

S Supporting Information *

ABSTRACT: Copper sulfide nanocrystals (Cu2−xS NCs) consisting of earth-abundant and nontoxic elements have attracted attention for optoelectronic and plasmonic applications due to their tunable light absorption and emission properties. In this work, we present a study of the electronic changes induced in organic-capped Cu2−xS NCs by surface modification treatments using charge transport and optical spectroscopy measurements. We have investigated surface treatments yielding ligand exchange and also ligand removal as well as changes in electronic defect density. The structural and morphological changes induced by the treatments were monitored by infrared spectroscopy, electron microscopy, and electron paramagnetic resonance. Untreated Cu2−xS NCs exhibit a strong absorption band arising from a localized surface plasmon resonance (LSPR). We found that using a ligand exchange procedure (ethanedithiol treatment), the electrical conductivity in films of Cu2−xS NCs can be enhanced by 5 orders of magnitude, while maintaining other electronic properties of the individual NCs like optical absorption and LSPR. The improvements in the electrical conductivity were attributed to the reduction of the inter-NC separation in the films, as revealed by the structural and morphological studies. We also have observed that ligand removal treatments such as thermal annealing and hydrazine treatment yield a LSPR red-shift, while the electrical conductivity increases by up to 5 and 7 orders of magnitude, respectively. We proposed a model for the surface reactions taking place during these treatments. Our work highlights the potential of simple chemical or thermal treatments in tailoring the electronic properties of Cu2−xS NCs, making thermally treated Cu2−xS NCs interesting for tunable plasmonic and optoelectronic applications.

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INTRODUCTION Semiconductor materials in the form of nanocrystals (NCs) have been intensively investigated as building blocks for electronic applications.1,2 In particular, transition metal sulfides, selenides, and tellurides have shown an impressive development, resulting in thin film transistors with excellent electronic properties3−8 as well as quantum dot solar cells with promising energy conversion efficiencies.9−13 However, most groups utilize Cd- and Pb-based materials, which have considerable disadvantages in terms of sustainability and toxicity. Copper sulfides are a promising alternative due to their earth-abundant and environmentally friendly constituents.14 Copper(I) sulfides Cu2−xS cover a stoichiometrical range between 0 ≤ x ≤ 0.2 with © XXXX American Chemical Society

three stable phases at room temperature, which are chalcosite (Cu2S), djurleite (Cu1.97−1.94S), and digenite (Cu1.8S), plus phases of mixed stoichiometry.15 In the past decade, the controlled formation of Cu2−xS NCs has been reported using solventless thermolysis,15−17 hydrothermal reaction,15 plasma synthesis,18 and solvent-based reaction methods.19−23 Many groups apply dodecanethiol (DDT) as a protecting ligand and/or sulfur source during growth.24 This is important as the ligands influence the Received: February 2, 2015 Revised: June 16, 2015

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DOI: 10.1021/acs.jpcc.5b01078 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Sigma-Aldrich) or EDT (C2H6S2, 98%, Sigma-Aldrich) at room temperature. After a well-defined reaction time, the samples were removed from the chemical. These Cu2−xS NCs will thus be referred to as “hydrazine-treated” and “EDT-treated”, respectively. In some cases the samples were subsequently rinsed with dry acetonitrile (referred to as “rinse”) and dried for at least 1 h. Another treatment consisted of annealing steps by heating films of untreated Cu2−xS NCs on a hot plate at 200 °C (“annealed” Cu2−xS NCs). Equipment and Sample Preparation. X-ray diffraction (XRD) measurements were performed using a Bruker AXS D8 Advance X-ray diffractometer in Bragg−Brentano geometry and Cu Kα radiation. The Cu2−xS NCs were inserted into Lindeman capillaries and flame-sealed under an inert atmosphere prior to the measurement. For Fourier transform infrared spectroscopy (FTIR) measurements, a Bruker Vertex 70v Fourier transform spectrometer equipped with a PIKE MIRacle attenuated total reflection (ATR) germanium crystal unit was used. The measurements were carried out with a resolution of 2 cm−1 at room temperature under vacuum conditions of approximately 1 mbar. Since Cu2−xS NC layers with several micrometers thickness were required to obtain a good signal-to-noise ratio, the NCs were drop-casted onto quartz glass substrates. Electron paramagnetic resonance (EPR) spectroscopy was performed at room temperature using a continuous-wave X-band spectrometer with a lock-in amplifier and a TE102 resonance cavity. Calibration of the g-factor was done by comparison with the signal of a 2,2-diphenyl-1picrylhydrazyl reference sample measured at the same temperature as the Cu2−xS NC samples. The EPR spin density was calibrated with a phosphorus-implanted silicon sample at liquid helium temperature. For EPR measurements, several microliters of NC suspension containing 0.9 ± 0.1 mg of Cu2−xS NCs were inserted into a Suprasil quartz tube, dried, and sealed under inert atmosphere with Teflon tape prior to the measurement at ambient conditions. Ultraviolet−visible absorption (UV−vis) measurements were performed using a PerkinElmer Lambda 900 spectrometer with an integrating sphere. Here, thin layers of Cu2−xS NCs (≈100 nm) were produced via spin-coating onto glass substrates. Photothermal deflection spectroscopy (PDS) was applied as a sensitive technique to measure low light absorption below the bandgap of the Cu2−xS NCs. We used a homemade setup with a tungsten−halogen light source and a spectral measurement range from approximately 0.4 eV to 5 eV. For this measurement, the Cu2−xS NCs were spin-coated onto quartz glass substrates and placed into a quartz glass vessel filled with perfluorohexane. For electrical measurements, interdigit gold contacts consisting of 49 digits of 20 μm width and 20 μm spacing were defined onto silicon substrates, covered with a 50 nm thick Si3N4 dielectric layer, by photolithography prior to deposition of a thin layer of Cu2−xS NCs by spin-coating. During measurements, the Cu2−xS NC layers were contacted using a homemade needle station setup in an argon atmosphere and connected to a Keithley 2400 source meter unit. Thermovoltage measurements were performed at ambient conditions using a homemade setup of two Peltier elements that create a temperature difference of 54 K. Thin layers of Cu2−xS NCs on Si3N4-covered Si wafers were put on the hot and cold sides of the Peltier elements and electrically contacted using silver paste. Scanning electron micrographs (SEM) of the Cu2−xS NC layers were obtained using a Zeiss NVision 40 microscope.

electronic properties of the NCs. They introduce an insulating barrier that increases the interparticle distance with increasing ligand size and, thus, prevent efficient electronic charge transport between neighboring NCs.25−27 Common ways to circumvent this electric insulation in NC systems are to either exchange the ligands with shorter molecules or to remove them by chemical treatments.1,3,4,6,27 The modification of the NCs surface results in changes in their electronic properties, which has been observed e.g. as a shift of the onset of absorption depending on the surface chemistry.28 Cu2−xS NCs usually exhibit p-type doping due to copper vacancies.15 For high carrier concentrations, reports have shown that localized surface plasmon resonance (LSPR) absorption bands arise, which are traditionally observed in noble metal nanostructures.29 In contrast to such metal NCs, in semiconducting NCs the frequency of the LSPR could be tuned over a large range from the terahertz regime to the near-infrared by variation of the NC doping. Recently, the plasmonic properties of stoichiometric CuS nanocrystals have been highlighted.30 Some works have also indicated that the frequency and intensity of LSPR may be affected by modification of the NC surface.15,29,31 In this article, we report on a comprehensive study of the surface modification of Cu2−xS NCs passivated using DDT ligands. We investigate chemical treatments using hydrazine and ethanedithiol (EDT) as well as thermal annealing treatments and measure the respective effect on the surface composition and defects and electrical conductivity of the Cu2−xS NCs. To gain a deeper understanding of the impact of ligand modification, we study optical absorption with respect to plasmonic features. We find that the electrical conductivity, the LSPR band, and thus the doping concentration of the Cu2−xS NCs can be tuned using postdeposition treatments. We propose thermal treatments as a simpler alternative to chemical treatments. Thermal treatments yield the largest changes in the LSPR band, making the thermally tuned Cu2−xS NCs interesting for plasmonic and electronic applications.

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EXPERIMENTAL SECTION Synthesis of Cu2−xS NCs. The Cu2−xS NCs were synthesized using a hot-injection microreactor. Copper oleate (Cu(O2C18H33)2) in 1-octadecene (CH3(CH2)15CHCH2) was heated to 230 °C, mixed with DDT (CH3(CH2)11SH), and continuously stirred for 1 h at a temperature of 215 °C. In the next step, the suspension was heated to 230 °C while copper olate, octadecene, and DDT were added, followed by stirring for 1 h at 215 °C. This step was repeated twice. The process led to the formation of DDT-capped Cu2−xS NCs with a typical diameter of 10 nm which appeared as a black precipitate. The NCs were washed with methanol and toluene and extracted with dry toluene. In the end, a clear, homogeneous suspension of Cu2−xS NCs with a concentration of approximately 10 mg/mL was obtained and stored under inert gas atmosphere. The as-prepared NCs without further treatment will be referred to as “untreated” Cu2−xS NCs. Surface Modification Techniques. Modification of the Cu2−xS NCs surface termination was performed by chemical and physical means using thin films of the Cu2−xS NCs deposited prior to these treatments. Sample preparation for measurements (described below) as well as the surface modification procedures were carried out on thin films under an argon atmosphere with both oxygen and water level below 1 ppm. For chemical modification, untreated Cu2−xS NC films were immersed in hydrazine (N2H4, 1 M in dry acetonitrile, B


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EXPERIMENTAL DATA Structural Characterization. XRD was used to investigate the crystal structure of the Cu2−xS NCs. This is especially important, as copper sulfides Cu2−xS exhibit several stable Curich phases such as chalcosite (Cu2S), djurleite (Cu1.97−1.94S), and digenite (Cu1.8S) each with different electronic properties.15,32 Figure 1 shows the XRD data of a sample of untreated

Figure 2. SEM images of (a) untreated Cu2−xS NCs, (b) Cu2−xS NCs treated with hydrazine (120 min, with rinsing), (c) Cu2−xS NCs treated with EDT (5 min, with rinsing), and (d) annealed Cu2−xS NCs (60 min at 200 °C).

Figure 1. (a) XRD pattern of an untreated Cu2−xS NCs sample (black dots). The red bars represent the fit to the spectrum using structure data from Evans.35 The inset (b) shows the fit of one of the peaks with a Gaussian function, which is used to determine the mean diameter of the Cu2−xS NCs.

Surface Characterization and Modification. The surface termination of the Cu2−xS NCs is studied using FTIR spectroscopy in ATR mode. Figure 3 shows the characteristic

Cu2−xS NCs. The spectrum consists of six main peaks at 2Θ = 26.3°, 29.5°, 37.6°, 46.3°, 48.6°, and 54.2°, which agree well with literature values at 2Θ ≈ 29°, ≈ 37°, 46.1°, 48.6°, and ≈54°.15 The peaks are frequently attributed to the djurleite phase,15 whereas other groups report similar peaks for nanowires in the chalcosite phase.16,19 We have simulated position and relative intensity of the peaks using the software MAUD33,34 considering the crystal structure of djurleite taken from Evans,35 which is shown in Figure 1. The red bars represent well the position and intensity ratios of the measured XRD pattern. However, the chalcosite phase produces a similar XRD pattern with only slightly shifted peak positions and intensity ratios.15,36 We conclude that our Cu2−xS NCs primarily consist of djurleite and/or chalcosite phases. However, the exact stoichiometry may not be significant for the surface modification studies presented below. The XRD spectra are also used to determine the mean diameter d of the Cu2−xS NCs using the Scherrer equation37 d=

0.94λ Λ cos(Θ)

Figure 3. ATR-FTIR spectra of untreated (black), hydrazine-treated (blue, 90 min), EDT-treated (red, 5 min), and annealed (green, 60 min at 200 °C) Cu2−xS NCs.

(1)

where λ is the X-ray wavelength, Λ is the full width at halfmaximum (fwhm) of the peak (in radian), and θ is the scattering angle. We fitted the main peaks of the spectrum using Gaussian peak functions to determine the position and fwhm of the peaks. From these, we calculate a mean diameter of the Cu2−xS NCs of d = 12 ± 2 nm. A top view SEM image of an untreated Cu2−xS NC thin film is shown in Figure 2a. The SEM image shows that the Cu2−xS NCs have a spherical shape and do not exhibit any sign of agglomeration. From the SEM image the diameter of the Cu2−xS NCs is determined to be d = 10 ± 4 nm, which is in good agreement with the values obtained from XRD data.

absorption bands of untreated Cu2−xS NCs as well as hydrazine-treated, EDT-treated, and annealed Cu2−xS NCs. To account for different thicknesses of the investigated Cu2−xS NC layers, the intensity of the peaks is normalized with respect to the absorption of the samples in the visible region at 800 nm measured using UV−vis spectroscopy (described below). The low absorption of the Cu2−xS NC layers at this wavelength further ensures that the absorption is a good measure of relative film thickness. We observe bands at 2962, 2920, 2850, and 1455 cm−1, which are attributed to the asymmetric stretching mode of CH3 (2962 cm−1), the asymmetric stretching mode of CH2 (2926 cm−1), the symmetric stretching mode of CH2 C


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The Journal of Physical Chemistry C (2853 cm−1), and the bending mode of CH2 (1465 cm−1).38 The width and position of the asymmetric and symmetric stretching mode of CH2 suggest a well-ordered layer of DDT ligands bound to the untreated Cu2−xS NCs.39 Other bands are found at 1260, 1090, 1017, and 796 cm−1. These peaks do not originate from vibrational modes involving carbon atoms of the organic ligands. This is supported by the fact that these signals do not significantly change after the treatments, unlike the case of the other modes due to CH2 and CH3, as will be discussed below. In the literature, we find a large number of sulfur− oxygen species with different vibrational modes. The band at 1260 cm−1 can be assigned either to the asymmetric stretching of O−S−O (1280 cm−1)40 or to the vibrational mode of the dimer (SO2)2− (1260 cm−1).41 The peak at 1090 cm−1 may originate from asymmetric stretching of SO2− (1086 cm−1)41 or symmetric stretching of O−S−O (1076 cm−1).40 For the band at 1017 cm−1, we find vibrational modes of the dimer (SO2)2− at 1015 cm−1 and the symmetric stretching mode of SO2− (991 cm−1).41 Furthermore, S−O stretching modes of dimethyl sulfoxide (CH3)2SO exhibiting four overlapping peaks with maxima at 1072, 1058, 1044, and 1027 cm−1 are reported, which resemble well the signal structure of the Cu2−xS NCs between 1100 and 1000 cm−1.42 We assign the peak at 796 cm−1 to the symmetric stretching of cyclic SSO, which is reported to be located at 800 cm−1.43 Thus, our FTIR data indicate that sulfur−oxygen species are present in the untreated Cu2−xS NCs. The presence of sulfur−oxygen bonds has further been supported by X-ray photoelectron spectroscopy (XPS) measurements which are shown in the Supporting Information. We find a small contribution of SO2− and SO3− species which increases upon annealing of the Cu2−xS NCs. We do not find evidence of copper−oxygen species as the Cu−O vibrational bands are reported at wavenumbers smaller than 600 cm−1, which is below the limit of our spectrometer.44 In conclusion, this means that oxidation has taken place already either during growth of the NCs or during subsequent extraction and/or storage. In the former case, we expect that the sulfur−oxygen species would be present in the Cu2−xS NC core and surface, whereas in the second case the oxidation primarily takes place at the surface. To ensure that the FTIR signals do not originate form residual reactants from the synthesis, we have compared our results with the FTIR spectra of 1-octadecene (main bands around 1640, 1460, and 915 cm−1),45 1-dodecanethiol (main band around 1460 cm−1),45 and copper oleate (main bands around 1590 cm−1, between 1470 and 1420 cm−1, and around 720 cm−1).46 As we do not find an overlap of these bands with our measured signals, we can exclude a significant amount of residual reactants. After treatment with hydrazine, EDT, and annealing, the alkyl signals around 3000 and 1455 cm−1 strongly decrease (see Figure 3). In contrast, we observe an increase of the sulfur− oxygen related signals in the range of 1300−600 cm−1. This means that the amount of CH3 and CH2 decreases, whereas the amount sulfur−oxygen species slightly increases. We do not detect any additional peaks after the treatment which allows us again to exclude bands originating from the hydrazine and EDT. The decrease of the alkyl-related signal can be explained by either the removal of the surface DDT or reduction of the length of the surface alkyl chain. Particularly, the EDT treatment is expected to decrease the length of the alkyl chain from approximately 18 to 4 Å.6,47 In both cases, the amount of surface hydrocarbons on the Cu2−xS NCs is reduced, which increases the possibility of reactions of the Cu2−xS NC

surface with surrounding chemicals; i.e., sulfur atoms located on the surface of the Cu2−xS NCs may oxidize. We would like to emphasize that although the treatment with hydrazine, EDT, and annealing is carried out under inert atmosphere, during mounting of the samples in the FTIR system the samples are exposed to air for a few minutes. To monitor the evolution of the surface modifications induced by the treatments, we measured the intensity of the CH3 and CH2 stretching vibration modes by integration of the signal between 3000 and 2800 cm−1 as a function of the treatment time. Figure 4 shows this dependence of the peak

Figure 4. Dependence of the FTIR peak intensities of CH3 and CH2 as a function of the treatment time with hydrazine (blue) and EDT (red) without and with rinsing (filled symbols/open symbols) as well as annealing (green, 200 °C).

intensities, normalized to the intensity of the corresponding untreated Cu2−xS NCs sample. We show data for samples which have only been treated as described above and also for samples that have been additionally rinsed after the treatment. As can be seen, the hydrazine treatment results in a decrease of the peaks intensity to about 30% within 120 min without a significant difference observed between the only treated sample and the sample that was also rinsed. We observe a logarithmic dependence of the peak intensity on the treatment time. The EDT-treated Cu2−xS NCs exhibit a strong decrease of the alkyl peaks to 20% already after a few minutes of treatment. The annealing of the sample at 200 °C reduces the alkyl peak intensity to 50% after a treatment time of 60 min. Our data indicate that the decrease of surface CH2 and CH3 units is most effective for the EDT treatment, whereas the hydrazine treatment and annealing of the Cu2−xS NCs only lead to a moderate reduction within the same period of time. We have also studied the effect of the treatments on the morphology of Cu2−xS NC thin films. Figures 2b−d show the SEM images of hydrazine- and EDT-treated and annealed Cu2−xS NC layers. Whereas the annealing does not lead to a significant change of the morphology, the hydrazine and EDT treatments partly remove the Cu2−xS NCs and thus lead to the formation of cracks. Closer statistical analysis of the distance between neighboring Cu2−xS NC shows that in the hydrazineand EDT-treated films the average distance can be up to about 1 nm smaller compared to the untreated film. Electrical, Spin Resonance, and Optical Absorption Studies. We also investigated the effect of the different D


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In addition thermovoltage measurements were performed on Cu2−xS NC thin films after the treatments. We observe a positive Seebeck coefficient of S ≈ +0.2 mV K−1 between the heated and cooled sides of the hydrazine and EDT-treated sample, which is evidence for hole conduction and thus p-type doping of the Cu2−xS NCs. In the literature, the p-type doping is assigned to Cu vacancies in the bulk copper sulfide lattice.29,36 From the Seebeck coefficient we can estimate the hole density P using

treatments on the electrical characteristics of the Cu2−xS NCs. Figure 5a shows the current−voltage (IV) curves of thin layers

S=

⎛ π ⎞2/3 ⎜ ⎟ mT ⎝ 3P ⎠ 3eh2

8π 2kB 2

(2)

where kB is the Boltzmann constant, h is the Planck, m* is the effective hole mass, and T is the temperature.48 Using m* = 0.8m0, where m0 if the electron mass, we find P ≈ 1019 cm−3 for the hydrazine and EDT-treated sample.29 Figure 5c shows the effect of illumination for a film of hydrazine-treated Cu2−xS NCs. We clearly see an enhancement of the current under illumination (photoconductivity). However, the ratio of photoconductivity to dark conductivity is small (≈1/100), which may result from the low illumination intensity and the high p-type doping of the Cu2−xS NCs. The Cu vacancies can be the result of migration and oxidation of the Cu+ to Cu2+, which could be detected by EPR measurements. A previous report on bulk copper sulfide has published a g-factor in the range of g = 2.09−2.61, depending on the stoichiometry and crystal phase of the copper sulfide crystal.49 In Figure 6a, the EPR spectra of untreated, hydrazineand EDT-treated, and annealed Cu2−xS NCs are shown. For all samples, we find an isotropic signal with a g-factor of g = 2.0036 ± 0.0005. The signal cannot originate from Cu2+ as its g-factor is well below literature values for Cu2+. We find that carbon dangling bonds (C-dbs) exhibit a g-factor of g = 2.0027−

Figure 5. (a) IV characteristics of untreated (black), hydrazine-treated (blue, 10 min), EDT-treated (red, 10 min), and annealed Cu2−xS NC layers (green, 60 min at 200 °C). (b) Conductivity of Cu2−xS NC layers as a function of the treatment time using hydrazine (blue), EDT (red), and annealing (green). (c) Current of a Cu2−xS NC layer (hydrazine-treated, 1 min) in darkness and under LED white light illumination (yellow shaded, intensity ≈10−5 W cm−2).

of untreated Cu2−xS NCs and treated with hydrazine (10 min), EDT (10 min), and annealing (60 min at 200 °C). The IV characteristics of all samples show an ohmic behavior in the measured voltage range, which is in agreement with previous electrical measurements on Cu2−xS NC networks.27 In Figure 5b, the corresponding data of the conductivity as a function of the treatment time are shown. The untreated Cu2−xS NC films exhibit a low conductivity around 10−9 Ω−1 cm−1. The chemical treatments with hydrazine and EDT increase the conductivity by a factor of 107 to 10−2 Ω−1 cm−1 and by 105 to 10−4 Ω−1 cm−1, respectively within 1 min. The increase of the conductivity after the treatments is smaller compared to results published by Brewer et al., who measured an increase of 12 and 7 orders of magnitude in conductivity after treatment of DDT-terminated Cu2−xS NCs using hydrazine and EDT.27 In contrast to our surface modification technique, Brewer et al. have used potentially thicker, multiple Cu2−xS NC layers treated with hydrazine and EDT for three times. Furthermore, the NC may also have a different stoichiometry which can influence conductivity of the samples. We have seen that longer treatments result in a further increase of the conductivity by approximately 1 order of magnitude (see Figure 5b). The annealing of the Cu2−xS NC films leads to an increase of the conductivity up to 10−4 Ω−1 cm−1.

Figure 6. (a) EPR spectra recorded with untreated Cu2−xS NCs (black), Cu2−xS NCs treated with hydrazine (blue, 120 min), and EDT (red, 10 min) as well as annealed Cu2−xS NCs (green, 60 min at 200 °C). (b) Number of C-dbs per NC determined from the EPR spectra in (a). E


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wavelength of the incident light.55 In semiconductor NCs, an appreciable free carrier concentration is required to give rise to LSPR, which has been previously reported for highly p-type doped chalcogenide NCs like Cu2−xS NCs.15,29,31,56 According to Luther et al. the hole concentration P can be calculated from the maximum of the LSPR

2.0038, which agrees very well with our experimental gvalue.50−52 Therefore, we conclude that our EPR signal arises from C-dbs of the surface ligands. The number of C-dbs can be calculated from the signal intensity, which is shown in Figure 6b. We find that on the average only every 10th untreated Cu2−xS NC has a C-db, whereas the number increases after hydrazine treatment (120 min) and annealing (60 min at 200 °C) by a factor of 3 and 4, respectively. Interestingly, the EDT treatment does not lead to a significant change in the amount of C-dbs with respect to the untreated NCs. This suggests that the EDT reaction mechanism is different from the hydrazine treatment and annealing which involve the formation of C-dbs. Optical absorption measurements on Cu2−xS-NC thin films were performed using PDS and UV−vis spectroscopy. The data are shown in Figure 7a. All samples show a similar increase of

ωsp =

ωp2 1 + 2ϵm

− γ2 (3)

and ωp =

Pe 2 ϵ0mh

(4)

where ωsp is the surface plasma oscillation frequency, ωp is the bulk plasma oscillation frequency, ϵm is the dielectric constant of the surrounding medium, γ is the LSPR line width, and mh = 0.8m0 is the effective hole mass (m0 is the free electron mass).29 In this way, we calculate a hole density for the untreated and EDT-treated Cu2−xS NCs of P ≈ 1021 cm−1. This does not agree well with the Seebeck measurements where a hole density of P ≈ 1019 were determined. However, the Seebeck measurements may produce lower values as the technique can be influenced by surface conductivity present under ambient conditions. In the literature, reported hole densities of Cu2−xS NCs are typically in the order of 1020−1021cm−1, which is comparable to our results.27,29 From Figure 7a it follows that the hydrazine and annealing treatments decrease ωsp to below our measurement range. This means that the hole density in the Cu2−xS NCs decreases with respect to the untreated NCs due to the applied treatments. The decrease is stronger for the annealed Cu2−xS NCs than for hydrazinetreated ones. Interestingly, the decrease of the hole density does not seem to lead to a shift of the bandgap within the accuracy of the measurement. In the literature, there is no clear trend in the bandgap of Cu2−xS NCs upon doping. Whereas Luther et al. considered a Burstein−Moss effect to explain a shift of the bandgap of Cu2−xS NCs upon doping,29 Krieger et al. did not observe a clear trend.31 Early studies of silicon and germanium revealed an interplay between band filling (Burstein−Moss effect) and band gap renormalization due to many-body effects.57 Therefore, it is a priori not clear if a decrease of the doping density leads to a decrease of the bandgap. Moreover, the size distribution and the discrete distribution of dopants in the Cu2−xS NCs do not allow taking more precise conclusions from our optical data.

Figure 7. (a) Absorption coefficient of untreated (black), hydrazinetreated (blue, 1 min), EDT-treated (red, 10 min), and annealed (green, 60 min at 200 °C) Cu2−xS NC layers. The data below 1.8 eV are obtained by PDS, and above this value UV−vis measurements are recorded. (b) Tauc plot for the data in (a) calculated using α2E with the absorption coefficient α and the energy E.

absorption above 1.5 eV. In contrast, we observe significant differences of the absorption coefficient between the samples at lower energies. Untreated and EDT-treated Cu2−xS NCs exhibit a strong increase of absorption for lower photon energies with a maximum around 0.6 eV. The intensity of this feature decreases, and the peak shifts to lower energies after hydrazine treatment for 1 min, which was sufficient to gain a large increase in conductivity, and annealing (60 min at 200 °C). As the absorption band at low energies disappears, a distinct onset of absorption can be seen, which becomes more visible using a Tauc plot shown in Figure 7b. We find a bandgap around 1.2 eV, which is in line with literature values for both chalcosite and djurleite.15,32 The nature of the bandgap has long been described as indirect;53,54 however, recent calculations propose a direct bandgap32 which agrees better with our observations. Since we do not measure a significant change of the absorption curves at high photon energies, we assume that the bandgap is not affected by the treatments applied in this study. Therefore, we conclude that the structure of the NCs is not affected by the treatments. The absorption band at low energies is usually related to LSPRs which arise due to the interaction of light with free charge carriers in conductive NCs that are smaller than the

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DISCUSSION The surface treatments of the Cu2−xS NCs performed in this study are different in terms of chemical modification, which will be discussed in the following. The untreated Cu2−xS NCs are terminated with DDT with its sulfur atom contributing to the crystal lattice of the NCs.24 In principle, the sulfur atom can be situated in a high or low coordination number site, which influences its chemical stability. 58,59 Being in a high coordination number site, the DDT is more inert and thus more resistant to ligand modification. Cu2−xS NCs growth using DDT usually leads to a ligand termination with sulfur in a high coordination number site,59 which may affect the efficacy of the treatments performed in our study. The DDT exhibits a length l of 18 Å, introducing a relatively large barrier between neighboring NCs.47 Wenyong et al. found a barrier of ≈1.4 eV F


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The Journal of Physical Chemistry C for DDT monolayers between gold contacts and tunneling to be the dominant charge transport process.47 For tunneling the current−voltage dependence at low electric fields scaled with I ∝ (V/d) · exp(−d), where d is the spacing between the electrodes. The IV characteristic was ohmic, which is similar to our results. In our case, d represents half the spacing of two neighboring NCs. EDT has been highlighted as a suitable molecule for a ligand exchange in thiol systems.6,60 The rational behind the EDT treatment is to keep the thiol-based surface chemistry, while decreasing the ligand diameter to 4 Å to enhance the charge transport in the NCs network. Hostetler et al. have reported that the rate of ligand exchange is solely driven by the concentration difference between entering and exiting ligands and proceeds via protonation of sulfur, leading to the desorption of DDT and bonding of EDT.61 In the FTIR measurements in Figure 4, we find a fast decrease of the CH3 and CH2 vibrational mode intensity, saturating within a few minutes of treatment time. We attribute this to a fast exchange of the DDT by the EDT molecules, which reduced the amount of CH2 units from 11 per DDT to two per EDT. At the same time, we measure a strong increase of conductivity by 5−6 orders of magnitude also saturating within few minutes, similar to the FTIR measurements, as shown in Figure 5. According to the model of Wenyong et al., an exchange of DDT with EDT should result in an increase of the tunneling current by 4−5 orders of magnitude, which is in good agreement with our results.47 In EPR and optical absorption measurements in Figures 6 and 7 no significant change compared to the untreated Cu2−xS NCs can be found. This means that the electronic properties of the Cu2−xS NCs are not affected by the ligand exchange. The treatment leads mainly to a replacement of DDT by EDT. In contrast to the EDT treatment, the annealing is a pure physical treatment without any chemicals involved. In Figure 4, the FTIR measurements show a decrease of the CH3 and CH2 vibrational mode intensity by ≈50% with respect to the untreated sample, indicating the removal of CH3 and CH2 groups from the Cu2−xS NCs surface. The conductivity of Cu2−xS NC thin films shown in Figure 5 increases by 5 orders of magnitude, which is comparable to the EDT-treated sample. According to the model of Wenyong et al., we also would expect a decrease of the average ligand length from 18 Å (DDT) to about 4 Å.47 The EPR measurements (Figure 6) show an increase of the C-db density by a factor of 4 after the annealing treatment. This indicates that the annealing leads to the cleavage of bonds involving C atoms. At the same time, we find a significant decrease of the LSPR peak intensity as well as a shift of its maximum to lower energies in the optical absorption measurements shown in Figure 7. According to Luther et al., this can be explained by the reduction of the hole density in the Cu2−xS NCs core.29 To explain these results, we propose a model for the surface reaction process taking place during the annealing treatment based on reports of Lai et al. and Turo et al.59,62 Figure 8 (step 1) shows a schematic illustration of the Cu2−xS NC surface with a DDT ligand and its sulfur atom with oxidation state −II situated in a high coordination number site. Increasing the temperature (step 2), the C−S bond is cleaved at the surface of the NC. This results in the formation of unpaired electrons situated at the carbon and sulfur atom (step 3). Accordingly, we measure that the number of C-dbs in the EPR measurements increases which can be explained by organic fragments present

Figure 8. Schematic illustration of the temperature-activated chemical reaction proceeding during annealing: (1) Surface of untreated Cu2−xS NCs; (2) temperature-activated cleaving of the C−S bond; (3) formation of unpaired electrons situated at the carbon and the sulfur atom, respectively; (4) desorption of dodecene and hydrogen while sulfur is oxidized and provides an electron to the NC.

in the samples after the cleavage of C−S bonds. We do not detect a signal from unpaired electrons situated at a sulfur atom which could be explained by a short lifetime of this transition state. In step 4, the ligand fragment may re-form to 1-dodecene. This plus hydrogen desorb from the NC surface. In the literature, a significant mass loss of Cu2−xS NCs terminated with DDT has been observed starting at 200 °C, which has been attributed to desorbing 1-dodecene by mass spectroscopy.59 Another study investigating 1-butanthiol on copper surfaces finds the desorption of 1-butene and hydrogen above 92 °C formed from β-hydrogen elimination.62 At the same time, the remaining sulfur oxidizes to atomic sulfur, which has been detected by X-ray photoelectron spectroscopy, proceeding at 200 °C.62 In this way, the sulfur provides a free electron to the Cu2−xS NC core (step 4). The model explains the decrease and red-shift of the LSPR peak due to recombination of holes with electrons provided by the oxidation of the sulfur, which decreases the average hole density of the Cu2−xS NC core. In this way, the hole density as well as the LSPR resonance can be tuned within a certain range by simply annealing the Cu2−xS NCs. An alternative explanation for the decrease of the LSPR band upon annealing could be structural changes affecting the vacancy defects. However, we do not expect this effect to be dominant as the nanocrystals have already been subjected to a temperature of 230 °C during synthesis, which is above the annealing temperature used in our study (200 °C), and no major changes could be detected by optical absorption and XPS measurements. Hydrazine is a common reducing agent and has been used to replace and/or remove ligands on chalcosite NCs.1,3,4,27 For our Cu2−xS NCs, FTIR measurements after the hydrazine treatment (Figure 4) show a logarithmic decrease of the CH3 and CH2 vibrational band with a reduction of the peak intensity of about 70% within 120 min. We do not find NH band appearing after the hydrazine treatment which indicates that hydrazine does not become chemically bound to the Cu2−xS NCs.4 The conductivity of the hydrazine-treated Cu2−xS NCs (Figure 5) increases by 7 orders of magnitude within only a few minutes of treatment time. We may speculate that for short G


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Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01078.

treatment times the DDT is already removed form the Cu2−xS NCs surface but remains within the pores of the NC film. The high conductivity cannot be explained by further decreasing the tunneling barrier according to Wenyong et al.; therefore, it is possible that other charge transport mechanisms resulting in an ohmic IV characteristic are dominant in the hydrazine-treated Cu2−xS NCs.47 The hydrazine treatment leads to an increase of the C-db density in the EPR measurements (Figure 6) similar to the annealing process, which implies that hydrazine may cleave C−S or C−C bonds. In the optical absorption spectroscopy (Figure 7), we only find a decrease and redshift of the LSPR band, which is however not as pronounced as after the annealing process. Therefore, we assume that the hydrazine treatment provides electrons to the Cu2−xS NCs, partly compensating for the high hole density. As the EPR and absorption measurements show similar trends after the hydrazine treatment and the annealing, we assume that the hydrazine also initiates a process which cleaves the C−S bonds and provides electrons to the system.1 Theoretically, hydrazine can reduce the Cu2−xS NCs either by saturation of dangling bonds or by the reduction of to Cu+I to Cu0.3 Comparing the redox potentials, we find Cu2−xS/Cu ≈ −1.1 eV and N2H4/N2 ≈ −0.9 eV.63,64 Therefore, a dominant contribution of this reaction is not likely but cannot be fully excluded.

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Corresponding Author

*E-mail: [email protected] (R.N.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work has been supported by the Bundesministerium für Bildung und Forschung project no. 03SF0402C, the Nano Initiative Munich, and by FCT through projects no. PTDC/ FIS/112885/2009.

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REFERENCES

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CONCLUSION In this study, we have characterized Cu2−xS NCs grown in a hot-injection microreactor and passivated with DDT. We found that the NC core primarily consists of the chalcosite and/or djurleite phase. We have studied surface modification treatments using EDT, hydrazine, and thermal annealing of the Cu2−xS NCs and investigated the effects on the surface composition, electrical conductivity, paramagnetic defects, and optical absorption. FTIR measurements showed that the hydrazine, EDT, and annealing treatments led to the decrease of the CH3 and CH2 signal intensity by a factor of 2 (annealing) to 5 (EDT treatment). At the same time, we found an increase of the conductivity by 5 (annealing and EDT treatment) to 8 orders of magnitude (hydrazine treatment). We assigned this to the improved direct charge transfer between neighboring Cu2−xS NCs. Thermovoltage measurements showed that the Cu2−xS NCs are p-type. The hydrazine and annealing treatments led to an increase of C-dbs by a factor of 3−4, reduced the intensity of the LSPR, and shifted its maximum to lower energies corresponding to a reduction of the hole concentration. The EDT treatment initiated a ligand exchange reaction, which maintains the surface chemistry of the Cu2−xS NCs but enables electronic transport between neighboring NCs. In contrast, the hydrazine and annealing treatments likely removed the DDT ligands by bond breaking. We proposed a model for the surface reaction proceeding during annealing involving the cleavage of the C−S bond and oxidation of sulfur, providing an electron to the NC core, which leads to an increase of C-dbs and the decrease of the LSPR band, compensating for the high hole density. The LSPR frequency could thus be tuned by a simple temperature treatment enabling selective plasmonic absorption.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

X-ray photoelectron spectroscopy measurements of untreated Cu2−xS NCs and annealed Cu2−xS NCs. The Supporting H


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