Electronic Properties of Cu2âxSe Nanocrystal Thin Films Treated with

Subscriber access provided by The University of British Columbia Library

Article 2-x

Electronic Properties of Cu Se Nanocrystal Thin Films Treated with Short Ligand (S , SCN, and Cl) Solutions 2-

-

-

Juhee Lee, Jee Hye Yang, Chanil Park, Jung Hyun Kim, and Moon Sung Kang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03214 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Electronic Properties of Cu2-xSe Nanocrystal Thin Films Treated with Short Ligand (S2-, SCN-, and Cl-) Solutions Juhee Lee†,#, Jeehye Yang†,#, Chanil Park‡, Jung Hyun Kim‡, and Moon Sung Kang †,* †

Department of Chemical Engineering, Soongsil University, Seoul, 156-743, Korea

‡

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 120-749, Korea

#These

authors contributed equally.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. To exploit interesting electronic properties of colloidal semiconductor nanocrystals (NCs) in thin film devices, replacement of the original bulky ligands attached on the NC surface to short ones is essential. Here, we investigate the electronic properties of thin films of Cu2-xSe NCs treated chemically with short sulfide (S2-), thiocyanate (SCN-), and chloride (Cl-) ligands that are known to yield superior physical properties compared to the first-generation short ligand systems including amines and thioles. Specifically, the study focuses on the impact of ligand-treatment on their direct/indirect band-gap and NIR-localized surface plamson responance (LSPR) in the near-IR regime as well as their electrical conductivity and thermoelectric properties. While the application of S2- solution resulted in exchange of the original oleylamine (OAm) on NC surface with S2- ligands, using of SCNand Cl- solutions only removed the original ligands. The different ligands consistently led a red-shift of the direct and indirect band-gap. The LSPR was also red-shifted after applying solutions with SCN- and Cl- but was blue-shifted after applying solutions with S2- which we attribute to the formation of sulfur shell on NC surface. Conductivity as high as 442 S/cm and Seebeck coefficient of 13 µV/K could be obtained from the NC films with Cl- and SCNligands, respectively. We believe that the understanding on Cu2-xSe NCs will expand the materials library for electronic applications of copper chalcogenide NCs.


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ Introduction Colloidal semiconductor nanocrystals (NCs) have attracted significant attention due to their intriguing physical properties based on quantum confinement effect1, 2 and their applicability to solution processes for large area electronic devices such as photovoltaics3,4, logic units5, 6, and light-emitting devices7, 8, 9, 10. While the cadmium and lead chalcogenides (CdE and PbE, where E = S, Se, or Te) constitutes the benchmark research NC systems, these materials are based on toxic elements which restricts their universal applications. Alternatively, colloidal NCs based on environmentally benign elements, including indium, phosphine, copper, silver, and etc. have been synthesized and investigated heavily in the recent few years11,

12, 13, 14, 15

.

Among them, copper chalcogenide NCs are interesting series of materials. Not to mention the non-toxic elements, these materials exhibit interesting electronic characteristics. Not only the absorbance in the ultraviolet-visible (UV-Vis) regime based on typical excitonic processes in NCs, these materials exhibit localized surface plasmon resonance (LSPR) effect in the nearinfrared (NIR) regime, which are commonly observed in the visible regime from metal nanostructures16, 17. In fact, the LSPR effects are attained for copper chalcogenides that have copper deficiencies (Cu2-xE NCs, where E = S, Se, or Te) which generate plentiful of free holes in the NCs18,

19, 20

. Also, the free holes in these NCs allow one to attain thin-films of

NCs exhibiting p-type conduction21, 22. Moreover, the materials both in bulk and in quantized dimensions are known to yield good thermoelectric properties23,

24

. Accordingly, various

synthetic methods capable of controlling the copper deficiency have been developed. For example, using of different combinations of ligand molecules (acids and amines) during the colloidal synthesis of Cu2-xSe NCs yields crystals with variable stoichiometry25. Also, numbers of post-synthetic treatments based on oxidation/reduction processes19,

26, 27

and

surface ligand modifications28 have been demonstrated that allow one to control the hole density in copper chalcogenide NCs in dispersions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To fully exploit these characteristics of Cu2-xE NCs in thin-film electronic and optoelectronic devices, good understanding on the electronic properties of the thin-films is essential. In particular, investigating the influence of the NC surface ligands on the electronic properties of Cu2-xE NCs films sets the fundamental for applications. This is because various electronic processes involving NCs rely heavily on the nature of the ligands attached onto the NC surface29,

30, 31

. While the surface ligand molecules are critical to form stable colloidal

dispersion of NCs during the synthesis and storage, these molecules (e.g., oleic acid, oleylamine, or trioctyl phosphine) typically contain bulky and lengthy alkyl chains that are electrically insulating32. Therefore, either the charge hopping processes between neighbouring NCs or the charge transfer processes involving NCs is greatly suppressed. To harness the intriguing physical characteristics of NC assemblies, therefore, the surface ligand molecules of NC films have been commonly exchanged with smaller ligands (e.g. short thiols33, short amines34, halides35,36, or sulfur34, 37) through chemical treatment. We herein report a comprehensive study on the electronic properties of Cu2-xSe NC thin films that are treated with short ligand solutions. In particular, the influence of ligand treatment for Cu2-xSe NC thin films on their UV-Vis-NIR absorbance, electrical conduction, and thermoelectric properties were investigated thoroughly. The representative short ligands included sulfide (S2-), thiocyanate (SCN-), and chloride (Cl-). This selection of ligands was made according to the recent studies35,

38, 39

demonstrating the superior NC device

performances when using thiocyanate, chalcogenide, and halide ligands, compared to those obtained from the early generation ligand systems (i.e., short amines and thiols23, 33, 40, 41). In parallel with the recent studies on films of Cu2-xS NCs, a sister copper chalcogenide NC system, the results presented here on Cu2-xSe NCs will expand the materials library for electronic applications of copper chalcogenide NCs, similar to what has been done extensively with CdS and CdSe NCs or with PbS and PbSe NCs.


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ Experimental Methods Materials. Copper chloride (99.995%), elemental selenium (99.5%), 1-octadecene (technical grade 90%), oleylamine (technical grade 70%), hexane (95%), toluene (99.9%), oleic acid (90%), ammonium thiocyanate (97.5%), sodium sulfide, and acetonitrile (99.8%) were purchased from Sigma Aldrich. Ammonium chloride was purchased from Daejung Chemicals. Ethanol and chloroform were purchased from Samjung Chemicals. Glass substrate was purchased from Marienfeld. Characterization. Transmission electron microscopy (TEM) images of the Cu2-xSe NCs were collected using a LIBRA 120 (Carl Zeiss) operating at 120 keV. Scanning electron microscope (SEM) images of the NC films were obatined using a Supra 55VP (Carl Zeiss) equipted with a energy-dispersive X-ray spectroscopy (EDS) setup. XRD spectra were obtained using a D2 Phaser (Bruker). Average crystalline size within the nanoparticle samples were determined from the XRD spectra using the Scherrer equation. UV-Vis-NIR absorption spectra of Cu2-xSe NCs were collected using a Shimadzu UV-3600. FT-IR spectra were collected using a FT-IR spectroscopy (Thermo Fisher Scientific, iS10). Inductively coupled plasma mass microscopy (ICP-MS) was carried out using a NexION 350D (PerkinElmer SCIEX). Electrical conductivity of the Cu2-xSe NC films was measured using a homebuilt 4-probe measurement set up. The Seebeck coefficient of the samples was measured using another home-built setup (the temperature difference of 5 K was maintained with two Peltier devices and the thermovoltage was measured from a Keithley 2400 source-measure unit). For the electrical contact, silver paste was gently applied to the Cu2-xSe NCs films.

■ Results and Discussion



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu2-xSe NC synthesis. The synthesis of Cu2-xSe NCs was done based on a hot injection method using a standard Schlenk line technique according a modified recipe that was originally developed by Talapin et al.26 Briefly, 1.2 mmol (94.8 mg) of selenium (Se) powder was added in a three-neck round-bottom flask along with 9 mL of 1-octadecene (ODE) and 6 mL of oleylamine (OAm). The Se solution was first heated to 110°C and kept for 30 min under vacuum to remove residual moisture. Subsequently, the temperature was raised to 310°C and was held at the temperature for 30 min to prepare orange and transparent Se precursor solution. Separately, 2 mmol (198 mg) of copper chloride (CuCl) was mixed with 2 mL of OAm and 3 mL of ODE. The mixture was heated to 110°C and kept for 30 min at the temperature under vacuum, which yielded a transparent green Cu precursor solution (the color of the solution sometimes changed into blue or brown depending on the temperature that the precursor was prepared). Following, the Cu precursor solution was rapidly injected into the Se precursor solution at 310°C and the temperature of the reaction mixture was held at 300°C. After 20 min of nucleation and crystal growth, the reaction solution was cooled quickly down to 80°C. To purify the as-synthesized NCs, the reaction solution was transferred to a 50 mL conical centrifuge tube and added with 15 mL of ethanol. The mixture was centrifuged for 5 min at 4000 rpm. After removing the supernatant, the precipitates were re-dispersed in a mixture of toluene (1 mL), hexane (0.5 mL), and oleic acid (few drops). The solution was again centrifuged for 10 min at 4000 rpm to precipitate away the undesired impurities. The collected supernatant was precipitated with minimal amount of ethanol and then was centrifuged for 5 min at 4000 rpm. This yielded pellets of Cu2-xSe NCs capped with OAm. The NCs were stored in hexane before use. Cu2-xSe NC characterization. Figure 1a displays an X-ray diffraction (XRD) pattern of the as-prepared NC solids. The pattern reveals that the samples are cubic Cu2-xSe NCs containing copper deficiencies. From the sharpness of the most intense (220) peak of the


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XRD pattern, an average crystalline size of 16 nm was estimated using the Scherrer equation. Slightly a smaller value (14.5 ± 1.4 nm, the error equals one standard deviation determined from 500 particles) was attained from analyzing the NC size from a TEM image (Figure 1b). Moreover, due to the copper deficiencies that are known to produce free holes in Cu2-xSe NCs or to dope Cu2-xSe NCs in p-type, the characteristic LSPR peak at NIR wavelengths was obtained. Figure 1c shows an UV-Vis-NIR absorbance spectrum of the cubic Cu2-xSe NCs dispersed in tetrachloroethylene yielding its maximum LSPR peak position at 1090 nm; the sharp peak at 1700 nm is assigned to the absorbance of the solvent (tetrachloroethylene). Cu2-xSe NC thin-film formation and ligand-treatment methods. To exploit the interesting physical properties of Cu2-xSe NCs in solid-state devices, thin-films of NCs had to be prepared. The original OAm ligands of Cu2-xSe NCs in these films were treated with representative S2-, SCN-, or Cl- ligand solutions. Recent studies25,

29, 34

have demonstrated

that these small ligands yield superior electronic characteristics compared to the first generation short ligand systems including amines and thiols. The ligand treatment process is described schematically in Figure 1d. This was done by first preparing thin-films using the as-synthesized Cu2-xSe NCs (capped with the original OAm ligands) through spin-coating and then applying subsequent rounds of spin-coating of a solution containing the short ligands directly onto the as-prepared NC solid. A typical procedure was as follows. First, a dispersion of Cu2-xSe NCs (20 mg/mL) in a hexane and toluene mixture (3:7 volume ratio) was spin-coated (60 sec at 2500 rpm) onto glass substrates that were pre-cleaned using acetone, isopropyl alcohol and deionized (DI) water. Note that using a mixture solvent containing more toluene (e.g. hexane : toluene = 3 : 7 volume ratio) often yielded discontinuous films with islands. After the spin-coating, the substrate was heated at 65°C using a hot plate for 30 min under nitrogen flow. The thickness of the resulting spin-coated NC films was ~30 nm. Separately, 0.064 M of three different short ligand solutions were



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

prepared; sodium sulfide (Na2S) in ethanol, ammonium thiocyanate (NH4SCN) in acetonitrile, and ammonium chloride (NH4Cl) in methanol. These short ligand solutions were spin-coated (15 sec at 2500 rpm) respectively onto the Cu2-xSe NC film. Because the exchange was thought to be proceeded while the ligand solution was in contact with the NC solid, the dwell time of the coating solution on the NC film before the actual spinning was held constant for a period of time (2 min). From ICP-MS elemental analysis on the solutions used for the ligandtreatment (Figure S1), it was found that small numbers of Cu cations can leak out during the given dwell time. In fact, it is known that copper chalcogenides are very labile systems that can release Cu ions from their lattice27. The influence of the release of Cu ions and the resulting increase of mismatch in the stoichiometry between Cu and Se on the electronic properties of the ligand-treated Cu2-xSe NC films will be discussed below. Subsequently, the ligand-treated NC films were rinsed with the pristine solvent involved in the respective short ligand solution (acetontrile, ethanol, or methanol, respectively) to remove both the weakly attached OAm and any residual short ligand chemicals. The films were then heated at 100°C (for SCN-) or 80°C (for S2- and Cl-) using a hot plate for 3 min under nitrogen atmosphere. Unlike the S2- and SCN- ligands, spin-coating the Cl- ligand solution could not remove the NC ligands sufficiently at a single attempt. Therefore, the Cl- ligand solution had to be applied multiple times. Figure 2 displays FT-IR spectra of the ligand-treated films over the wavenumbers between 2800 and 3000 cm-1. The peaks in the spectra correspond to stretching vibration of C-H bond. From the area under the peaks, relative degree of the ligand-removal could be quantified. Despite the removal of the original OAm ligands, the presence of short ligands after the treatment was only confirmed for the films treated with S2- but not for those treated with SCN- and Cl- ligands (See Figure S2). These results indicate that the exchange of ligand has only taken place when applying the Na2S solution. Meanwhile, the application of NH4SCN and NH4Cl solutions have only removed the OAm ligands. Also, we noticed the


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formation of cracks in the films owing to the reduced inter-NC distance during the ligandtreatment processes. These cracks had to be filled to form fully-percolated conductive NC films. Therefore, NC dispersions and ligand solutions were spin-coated in a set for multiple cycles. SEM images of the resulting films after 3 cycles of NC-deposition and subsequent ligand-treatment steps are shown in Figure 3. The entire film formation and ligand-treatment processes were done under ambient condition. UV-Vis-NIR absorbance characteristics. Figure 4a displays the UV-Vis-NIR absorbance (A) spectra of the ligand-treated Cu2-xSe NC films. The absorbance spectrum of a Cu2-xSe NC film (containing original OAm ligands) that did not underwent the ligand-treatment step is also shown as a reference (black curve). Resembling the feature obtained from the solution (Figure 1c), the spectra show two distinct absorbance modes. The one in the UV-Vis range is associated with the band-to-band transition. The other one in the NIR range is associated with the LSPR effect of the Cu2-xSe NCs.42 Consistent with the observation from sister Cu2-xS NC films26, 42, our Cu2-xSe NC films also exhibited both direct and indirect bandgap. The direct band gap could be estimated from the y-intercept of an extrapolated line from A2 versus photon energy (E) plot (Figure 4b). The indirect band gap could be obtained from A0.5 versus E plot (Figure 4c). The pristine Cu2-xSe NC film containing the original OAm ligands yielded direct band gap of 2.4 eV and indirect band gap of 1.7 eV. Both these values were reduced after applying the ligand-treatment. The direct bandgap of the Cu2-xSe NC films with S2-, SCN-, and Cl- ligands were slightly reduced to 2.2, 2.3, and 2.3 eV, respectively. Meanwhile a more pronounced reduction in the indirect bandgap was observed; the indirect bandgap of Cu2-xSe NC films with S2-, SCN-, and Cl- ligands was 1.2, 1.3, and 1.3 eV, respectively. Such a red-shift in the bandgap for the ligand-treated NC films is typically attributed to the enhanced electron coupling between NCs after replacing/removing the original bulky and long ligands.29, 37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The absorbance peak in the NIR associated with the LSPR also exhibited an energy-shift after treating the films with small ligand solutions (Figure 4d). For a closely packed array of plasmonic nanoparticles with quite small interspacing compared to the size of nanoparticles, the dipole-dipole interaction is known to cause a red-shift in LSPR effect, as the interparticle distance is reduced43. Also, the variation in the dielectric environment of the NC surroundings after ligand treatment could affect the energy-shift. As the inter-NC spacing is reduced and the inorganic bodies approach to each other, the effective dielectric constant of the films will be increased and contribute to the red-shifts the resonant frequency. This is, indeed, what was observed from Cu2-xSe NC films treated with solutions containing SCNand Cl- ligands, which only removed the original bulky OAm ligands. Beyond the contributions from the reduced inter-NC spacing, the electron-donating nature of Cl- ligands44 could have made addtional contribution in red-shifting the LSPR by suppressing the effective density of free holes in Cu2-xSe, if they were attached on NC surface. However, this explanation would not be valid in our case, because the presence of Cl- ligands on NC surface was found to be marginal. For Cu2-xSe NC films with S2- ligands, an opposite outcome was observed; a blue-shift in LSPR was observed after replacing the ligands. The blue-shift can be first attributed to the increased mismatch in the stoichiometry between Cu and Se as a consequence of Cu leakage during the ligand-treatment process. However, this effect cannot be the sole origin of the observed blue-shift of the LSPR. The films treated with SCN- and Cl- ligands also exhibited similar leakage of Cu ions during the treatment (Figure S1) but yielded a red-shift in LSPR. We consider that the presence of S- ligands on NC surface or the sulfurization of the NC shell makes important role in LSPR for Cu2-xSe NC films. Despite the reduction of the inter-NC spacing after the S2- ligand treatment, the attachment of S2- ligands on the surface of Cu2-xSe NC will enlarge the effective mismatch in the stoichiometry between the copper and


Page 10 of 23

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chalcogenides (Se and S in this case), which would increase the apparent copper vacancies in copper chalcogenide NCs. The resulting increased effective density of free holes in the plasmonic NCs would then lead to the blue-shift of the LSPR. Broadening of the LSPR was also found after the ligand treatment process. The broadening of the peak in the lower energy side was more pronounced than that in the higher energy side of the peak. The red-axis in Figure 4d summarizes the broadening of the LSPR peak. Conventional full-width half-maximum value could not be extracted from the given peak because the broadening on the low energy-side was substantial. Thus, we estimated the width of the peak at its half maximum between the wavelength of the peak position and the wavelength at the high energy-side of the half maximum intensity, which we referred to as the (left) half-width half-maximum. Such an asymmetry has been also observed from the LSPR of metallic nanoparticles after exchanging their surface ligands16. The broadening of the peak can be understood primarily by the change in the distribution of holes at the NCligand interface that dephases the plasmonic resonance. The presence of short ligands around the NCs can accelerate the loss of coherence of the collective excitation45,

46

. Secondly, the

reduced inter NC-distance after exchanging the long ligands to short ones would also contribute to the broadening of the LSPR. This is because the enhanced electronic coupling between the NCs resulting from the reduced spacing would suppress the coherence of the plasmon resonance. In addition, the cracks or defects formed in the films upon the ligandtreatment process are also known to reduce the coherence of NC’s resonance47, and thus the peak would broaden. Electrical conductivity and thermoelectric properties. The electrical conduction and the associated thermoelectric properties of the ligand-treated Cu2-xSe NC films were investigated. The highly resistive Cu2-xSe NC films with the original OAm ligands became fairly conductive after treating the films with short ligand solutions. For examples, the ligand-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

exchanged films prepared through the post-treatment method with S2-, SCN-, and Cl- ligands became 318, 319, and 442 S/cm, respectively, after carrying out three repetitive cycles of NC deposition and ligand treatment processes. The average values are summarized in Table 1. The errors in the table equal standard deviations of data collected from more than 3 samples prepared independently. We note that film conductivity, which should be an intrinsic property of a film in ideal, turned out to be thickness-dependent. This is perhaps due to the filling of voids and cracks between NC domains (which were formed inevitably as a result of reduced inter-NC spacing during the ligand-treatment process) upon repetitive deposition of NCs. The thermovoltage of the conductive films were obtained at ambient condition using a home-built setup with two Peltier devices that generated a temperature difference (∆T) of 5 K near room temperature. The Seebeck coefficient (S) of the NC films were all positive values indicating that holes are involved in the transport, and these values were 11, 13, and 11 µV/K for films with S2-, SCN-, and Cl- ligands, respectively. Accordingly, the power factors of 6.6 × 10-6, 9.3 × 10-6, and 8.1 × 10-6 W/mK2 could be estimated for films with S2-, SCN-, and Clligands, respectively. The conductivity, the Seebeck coefficient, and the power factor of these films are comparable to the values obtained from Cu2-xSe NCs treated with thiols, amines, and acids23. In addition, the hole density (p) of the NC films could be estimated from the Seebeck coefficient using a relation48, S = (8π2kB2/3eh2)mhT(π/3p)2/3, where, kB is the Boltzmann constant, e is the elemental charge, h is the Planck constant, and mh is the effective hole mass of the material (1.0 m0)49. The hole density was 2.5 × 1021, 2.4 × 1021, and 2.7 × 1021 #/cm3 for NC films treated with S2-, SCN-, and Cl- ligands, respectively. This confirms the heavy doping density of Cu2-xSe NCs, consistent with previous results18, 49. Note that these hole concentrations are higher by two orders of magnitude than those obtained from Cu2-xS NC films with hydrazine or ethanedithiol ligands40.


Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ Conclusions We investigated the electronic properties of Cu2-xSe NC films that were treated chemically with short ligand solutions containing S2-, SCN-, or Cl- ligands. The ligand-treatment was found to enhance electronic coupling between NCs consistently for all the three short ligands, causing a red-shift in the direct/indirect bandgap. The ligand-treatment made more complicated influence on the LSPR of the Cu2-xSe NC film, which led a red-shift in the resonance for the films with SCN- and Cl- ligands but a blue-shift for those with S2- ligands. The results were interpreted based on different contributions of the following factors on the resonance for different ligands; the reduced inter-NC spacing and the change in the dielectric environement, as well as the chance of creating extra Cu vacancies. In addition, electrical conductivity and the thermoelectric properties of the ligand-treated Cu2-xSe NC films were investigated. A high conductivity value of 442 S/cm and a Seebeck coefficient of 13 µV/K could be obtained from Cu2-xSe NC films treated with Cl- and SCN- ligands, respectively, perhaps due to the heavily-doped nature of the NCs. Also, a superior thermoelectric power factor compared to the sister Cu2-xS NC films was obtained. The efforts to harness the intriguing physical characteristics of the copper chalcogenide NCs in electronics have just begun. The results presented here should provide fundamental guidance along the line.

Supporting Information. Compositional characterization. This material is available free of charge via the Internet at http://pubs.acs.org

ACKNOWLEDGMENT



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A1A2058531).

Corresponding Author [email protected] (M.S.K)

■ REFERENCES

1. 2.

3. 4.

5.

6.

7.

8.

9.

10.

Klimov, V. I., Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties. Taylor & Francis: 2003. Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J., et al. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012-1057. Carey, G. H.; Abdelhady, A. L.; Ning, Z.; Thon, S. M.; Bakr, O. M.; Sargent, E. H., Colloidal Quantum Dot Solar Cells. Chem. Rev. 2015, 115, 12732-63. Lim, J.; Lee, D.; Park, M.; Song, J.; Lee, S.; Kang, M. S.; Lee, C.; Char, K., Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based on Breakwater-like CdSe Tetrapod Nanocrystal Network Infused with P3HT. J. Phys. Chem. C 2014, 118, 3942-3952. Saari, J. I.; Krause, M. M.; Walsh, B. R.; Kambhampati, P., Terahertz Bandwidth All-Optical Modulation and Logic Using Multiexcitons in Semiconductor Nanocrystals. Nano Lett. 2013, 13, 722-727. Kim, D. K.; Lai, Y.; Diroll, B. T.; Murray, C. B.; Kagan, C. R., Flexible and LowVoltage Integrated Circuits Constructed from High-Performance Nanocrystal Transistors. Nat. Commun. 2012, 3, 1216. Coe-Sullivan, S.; Steckel, J. S.; Woo, W. K.; Bawendi, M. G.; Bulović, V., LargeArea Ordered Quantum-Dot Monolayers via Phase Separation During Spin-Casting. Adv. Funct. Mater. 2005, 15, 1117-1124. Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P., Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 370, 354-357. Kwak, J.; Lim, J.; Park, M.; Lee, S.; Char, K.; Lee, C., High-Power Genuine Ultraviolet Light-Emitting Diodes Based On Colloidal Nanocrystal Quantum Dots. Nano Lett. 2015, 15, 3793-3799. Lim, J.; Jeong, B. G.; Park, M.; Kim, J. K.; Pietryga, J. M.; Park, Y.-S.; Klimov, V. I.; Lee, C.; Lee, D. C.; Bae, W. K., Influence of Shell Thickness on the Performance of Light-Emitting Devices Based on CdSe/Zn1-XCdXS Core/Shell Heterostructured Quantum Dots. Adv. Mater. 2014, 26, 8034-8040.


Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11.

12. 13. 14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

Guo, Q.; Kim, S. J.; Kar, M.; Shafarman, W. N.; Birkmire, R. W.; Stach, E. A.; Agrawal, R.; Hillhouse, H. W., Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells. Nano Lett. 2008, 8, 2982-2987. Allen, P. M.; Bawendi, M. G., Ternary I−III−VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240-9241. Kolny-Olesiak, J.; Weller, H., Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221-12237. Garcia, G.; Buonsanti, R.; Runnerstrom, E. L.; Mendelsberg, R. J.; Llordes, A.; Anders, A.; Richardson, T. J.; Milliron, D. J., Dynamically Modulating the Surface Plasmon Resonance of Doped Semiconductor Nanocrystals. Nano Lett. 2011, 11, 4415-4420. Lim, J.; Park, M.; Bae, W. K.; Lee, D.; Lee, S.; Lee, C.; Char, K., Highly Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes Enabled by the Direct Formation of Excitons within InP@ZnSeS Quantum Dots. ACS Nano 2013, 7, 9019-9026. Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T., Solvent and Ligand Effects on the Localized Surface Plasmon Resonance (LSPR) of Gold Colloids. J. Phys. Chem. B 2004, 108, 13963-13971. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C., The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668-677. Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P., Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361-366. Dorfs, D.; Härtling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L., Reversible Tunability of the Near-Infrared Valence Band Plasmon Resonance in Cu2–xSe Nanocrystals. J. Am. Chem. Soc. 2011, 133, 1117511180. Faucheaux, J. A.; Stanton, A. L. D.; Jain, P. K., Plasmon Resonances of Semiconductor Nanocrystals: Physical Principles and New Opportunities. J. Phys. Chem. Lett. 2014, 5, 976-985. Xie, Y.; Carbone, L.; Nobile, C.; Grillo, V.; D’Agostino, S.; Della Sala, F.; Giannini, C.; Altamura, D.; Oelsner, C.; Kryschi, C.; Cozzoli, P. D., Metallic-like Stoichiometric Copper Sulfide Nanocrystals: Phase- and Shape-Selective Synthesis, Near-Infrared Surface Plasmon Resonance Properties, and Their Modeling. ACS Nano 2013, 7, 7352-7369. Otelaja, O. O.; Ha, D.-H.; Ly, T.; Zhang, H.; Robinson, R. D., Highly Conductive Cu2–xS Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition. ACS Appl. Mater. Interfaces 2014, 6, 18911-18920. Lynch, J.; Kotiuga, M.; Doan-Nguyen, V. V. T.; Queen, W. L.; Forster, J. D.; Schlitz, R. A.; Murray, C. B.; Neaton, J. B.; Chabinyc, M. L.; Urban, J. J., Ligand Coupling Symmetry Correlates with Thermopower Enhancement in SmallMolecule/Nanocrystal Hybrid Materials. ACS Nano 2014, 8, 10528-10536. He, Y.; Day, T.; Zhang, T.; Liu, H.; Shi, X.; Chen, L.; Snyder, G. J., High Thermoelectric Performance in Non-Toxic Earth-Abundant Copper Sulfide. Adv. Funct. Mater. 2014, 26, 3974-3978. Liu, X.; Wang, X.; Zhou, B.; Law, W.-C.; Cartwright, A. N.; Swihart, M. T., SizeControlled Synthesis of Cu2-xE (E = S, Se) Nanocrystals with Strong Tunable Near-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

26.

27.

28.

29.

30. 31.

32.

33.

34.

35. 36.

37.

38.

39.

Infrared Localized Surface Plasmon Resonance and High Conductivity in Thin Films. Adv. Funct. Mater. 2013, 23, 1256-1264. Kriegel, I.; Jiang, C.; Rodríguez-Fernández, J.; Schaller, R. D.; Talapin, D. V.; da Como, E.; Feldmann, J., Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 1583-1590. Xie, Y.; Riedinger, A.; Prato, M.; Casu, A.; Genovese, A.; Guardia, P.; Sottini, S.; Sangregorio, C.; Miszta, K.; Ghosh, S., et al. Copper Sulfide Nanocrystals with Tunable Composition by Reduction of Covellite Nanocrystals with Cu+ Ions. J. Am. Chem. Soc. 2013, 135, 17630-17637. Balitskii, O. A.; Sytnyk, M.; Stangl, J.; Primetzhofer, D.; Groiss, H.; Heiss, W., Tuning the Localized Surface Plasmon Resonance in Cu2–xSe Nanocrystals by Postsynthetic Ligand Exchange. ACS Appl. Mater. Interfaces 2014, 6, 17770-17775. Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M., Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960-1969. Talapin, D. V.; Murray, C. B., PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors. Science 2005, 310, 86-89. Vanmaekelbergh, D.; Liljeroth, P., Electron-Conducting Quantum Dot Solids: Novel Materials based on Colloidal Semiconductor Nanocrystals. Chem. Soc. Rev. 2005, 34, 299-312. Koh, W.-k.; Koposov, A. Y.; Stewart, J. T.; Pal, B. N.; Robel, I.; Pietryga, J. M.; Klimov, V. I., Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene. Sci. Rep. 2013, 3, 2004. Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J., Structural, Optical, and Electrical Properties of Self-Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol. ACS Nano 2008, 2, 271-280. Draguta, S.; McDaniel, H.; Klimov, V. I., Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2–x Quantum Dots. Adv. Funct. Mater. 2015, 27, 1701-1705. Zhang, H.; Jang, J.; Liu, W.; Talapin, D. V., Colloidal Nanocrystals with Inorganic Halide, Pseudohalide, and Halometallate Ligands. ACS Nano 2014, 8, 7359-7369. Sayevich, V.; Gaponik, N.; Plötner, M.; Kruszynska, M.; Gemming, T.; Dzhagan, V. M.; Akhavan, S.; Zahn, D. R. T.; Demir, H. V.; Eychmüller, A., Stable Dispersion of Iodide-Capped PbSe Quantum Dots for High-Performance Low-Temperature Processed Electronics and Optoelectronics. Chem. Mater. 2015, 27, 4328-4337. Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V., Metalfree Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 1061210620. Nag, A.; Chung, D. S.; Dolzhnikov, D. S.; Dimitrijevic, N. M.; Chattopadhyay, S.; Shibata, T.; Talapin, D. V., Effect of Metal Ions on Photoluminescence, Charge Transport, Magnetic and Catalytic Properties of All-Inorganic Colloidal Nanocrystals and Nanocrystal Solids. J. Am. Chem. Soc. 2012, 134, 13604-13615. Fafarman, A. T.; Koh, W.-k.; Diroll, B. T.; Kim, D. K.; Ko, D.-K.; Oh, S. J.; Ye, X.; Doan-Nguyen, V.; Crump, M. R.; Reifsnyder, D. C., et al. Thiocyanate-Capped Nanocrystal Colloids: Vibrational Reporter of Surface Chemistry and Solution-Based Route to Enhanced Coupling in Nanocrystal Solids. J. Am. Chem. Soc. 2011, 133, 15753-15761.


Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40.

41.

42.

43.

44.

45.

46.

47.

48. 49.

Aigner, W.; Nenova, G. K.; Sliem, M. A.; Fischer, R. A.; Stutzmann, M.; Pereira, R. N., Electronic Changes Induced by Surface Modification of Cu2–xS Nanocrystals. J. Phys. Chem. C 2015, 119, 16276-16285. Zarghami, M. H.; Liu, Y.; Gibbs, M.; Gebremichael, E.; Webster, C.; Law, M., pType PbSe and PbS Quantum Dot Solids Prepared with Short-Chain Acids and Diacids. ACS Nano 2010, 4, 2475-2485. Liu, L.; Zhou, B.; Deng, L.; Fu, W.; Zhang, J.; Wu, M.; Zhang, W.; Zou, B.; Zhong, H., Thermal Annealing Effects of Plasmonic Cu1.8S Nanocrystal Films and Their Photovoltaic Properties. J. Phys. Chem. C 2014, 118, 26964-26972. Jain, P. K.; Huang, W.; El-Sayed, M. A., On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7, 2080-2088. Tanaka, T.; Yamaguchi, T.; Ohshima, T.; Itoh, H.; Wakahara, A.; Yoshida, A., Effect of Cl Ion Implantation on Electrical Properties of CuInSe2 Thin Films. Sol. Energ. Mat. Sol. Cells 2003, 75, 109-113. Aruda, K. O.; Tagliazucchi, M.; Sweeney, C. M.; Hannah, D. C.; Schatz, G. C.; Weiss, E. A., Identification of Parameters through Which Surface Chemistry Determines the Lifetimes of Hot Electrons in Small Au Nanoparticles. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 4212-4217. Bauer, C.; Abid, J.-P.; Fermin, D.; Girault, H. H., Ultrafast Chemical Interface Scattering as an Additional Decay Channel for Nascent Nonthermal Electrons in Small Metal Nanoparticles. J. Chem. Phys. 2004, 120, 9302-9315. Mock, J. J.; Hill, R. T.; Degiron, A.; Zauscher, S.; Chilkoti, A.; Smith, D. R., Distance-Dependent Plasmon Resonant Coupling between a Gold Nanoparticle and Gold Film. Nano Lett. 2008, 8, 2245-2252. Snyder, G. J.; Toberer, E. S., Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105-114. Mansour, B. A.; Demian, S. E.; Zayed, H. A., Determination of the Effective Mass for Highly Degenerate Copper Selenide from Reflectivity Measurements. J. Mater. Sci. Mater. 1992, 3, 249-252.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) XRD pattern of Cu2-xSe NCs. (b) TEM image of Cu2-xSe NCs (scale bar = 100 nm). (c) Absorption spectra of Cu2-xSe NCs in tetrachloroethylene. (d) Schematic description of the ligand-treatment process.


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. FT-IR spectra for ligand treated Cu2-xSe NC films. (a) NC films treated with S2ligands. (b) NC films treated with SCN- ligands. (c) NC films treated with Cl- ligands (multiple applications of the short ligand solution were necessary to remove more than 70% of the original OAm ligands.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. SEM images of Cu2-xSe NC films treated with different ligand solutions. Cycles of film deposition and ligand treatment processes were repeated 3 times. (a) Cu2-xSe NC films treated with S2- ligands. (b) Cu2-xSe NC films treated with SCN- ligands. (c) Cu2-xSe NC films treated with Cl- ligands (left: high resolution images, right: low resolution images).


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) UV-Vis-NIR spectrum for Cu2-xSe NC films treated with different ligands. (b) Absorbance2 vs. Energy plots to extract the direct bandgap of the Cu2-xSe NC films treated with different ligands. (c) Absorbance1/2 vs. Energy plots to extract the indirect bandgap of the Cu2-xSe NC films treated with different ligands. (d) Summary of the LSPR position and broadening for the Cu2-xSe NC films treated with different ligands.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

Table 1. Summary of the electrical conduction and associated thermoelectric properties of the ligand-treated Cu2-xSe NC films. Average values were obtained from measurements on 3 separate samples prepared independently. Errors in the table are standard deviation of the

S/cm

Seebeck Coefficient µV/K

318 (±243)

SCNCl-

Ligands S

2-

Conductivity

Power factor

Hole density

W/mK 2

#/cm3

11 (±2)

6.6 (±3.4) x 10

-6

2.5 (±0.6) x 1021

319 (±72)

13 (±6)

9.3 (±6.5) x 10-6

2.4 (±1.3) x 1021

442 (±145)

11 (±4)

8.1 (±3.6) x 10-6

2.7 (±1.1) x 1021

data.


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Image


Electronic Properties of Cu2âxSe Nanocrystal Thin Films Treated with

Recommend Documents

Electronic Properties of Cu2âxSe Nanocrystal Thin Films Treated with