CdSe@ZnS

Article pubs.acs.org/JPCC

Hybrid Electrochromic Fluorescent Poly(DNTD)/CdSe@ZnS Composite Films Huige Wei,† Xingru Yan,† Yunfeng Li,†,‡ Shijie Wu,§ Andrew Wang,# Suying Wei,*,‡ and Zhanhu Guo*,† †

Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710, United States ‡ Department of Chemistry and Biochemistry, Lamar University, Beaumont, Texas 77710, United States § Agilent Technologies, Inc, Chandler, Arizona 85226, United States # Ocean NanoTech, LLC, 2143 Worth Lane, Springdale, Arkansas 72764, United States ABSTRACT: Hybrid electrochromic poly(DNTD)/CdSe@ZnS quantum dots (QDs) composite films were prepared using an electropolymerization technique at room temperature and their electrochemical properties at different temperatures were investigated by cyclic voltammetry (CV). At room temperature, the composite films exhibited asymmetry in the voltammograms arising from the heterogeneity in the polymer chains and the morphology change caused by the QDs. A more positive cathodic potential than that of the pure polymer films indicated that poly(DNTD) became more difficult to be oxidized as a consequence of the incorporated QDs. The CVs of these nanocomposite thin films were also carried out at 0 °C to evaluate the temperature effect on their electrochemical properties. The asymmetry in the oxidation/reduction process became more remarkable, and lower oxidation potentials were observed. Pure poly(DNTD) thin film and its composite thin film with 0.05 wt % QD loading were prepared at 0 °C as well. Their CVs were performed at both room temperature and 0 °C and were compared with those of the films prepared at room temperature. The morphology, optical, and spectroelectrochemistry (SEC) properties of the thin films were studied using atomic force microscopy (AFM), fluorescent microscopy, and a UV−visible spectrometer coupled with the potentiostat, respectively. The obtained uniform thin films turned out to be made of nanoparticles under high resolution AFM observation. Fluorescent microscopic observations revealed the fluorescent emission of the nanocomposites with the QDs embedded in the hosting polymer matrix. The nanocomposite thin films exhibited varying absorbance bands when different potentials were applied, and the film was observed to turn blue at an applied voltage of 1.4 V vs Ag/AgCl.

1. INTRODUCTION Semiconducting or conducting polymers with spatially extended p−p or p−n−p bonding systems have been intensively researched since they can serve as active components in photovoltaic cells, light emitting diodes, photodiodes, field effect transistors, and other electronic devices.1,2 Among the conjugated polymers, poly(thiophene),3 poly(pyrrole),4,5 and poly(aniline) (PANI),6,7 and their derivatives are widely studied. Significant efforts have been devoted to manipulate the physicochemical properties of the conjugated polymers by tailoring the molecular structure of either the main chain or pendant groups8−10 or through the formation of the blends, 11 laminates,12 or composites.13,14 One attractive area relating to conjugated polymers is their spectroelectrochemical properties, which occur upon the reduction (gain of electrons) or oxidation (loss of electrons), on the passage of an electrical current after the application of an appropriate electrode potential.15−17 The electrochromic properties of the conducting polymers have gained much popularity both from the academia and industry for their easy processability, rapid response time, high optical contrast, and the ability to modify their structures to create multicolor electrochromes.18−20 New electrochromic materials, © 2012 American Chemical Society

that is, thin films consisting of PANI and WO3 through molecular assembling synthesis21 and multilayer films of PANI and grapheme22 have been reported. Semiconductor nanocrystals (NCs) have attracted much interest due to their wide potential engineering applications such as biology and medication, biological fluorescence labeling and imaging.23−25 The primary focus has been on their optoelectronic properties, displaying a strong size-dependent quantum confinement effect, which takes place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called “energy subbands”.26−28 Among the various kinds of NCs, cadmium selenide (CdSe) quantum dots (QDs) are undoubtedly the most widely studied owing to their tunable emission in the visible range and the availability of precursors and the simplicity of crystallization.29−31 In order to increase the photostability of the core and the quantum yield, core− shell structured QDs, for example, CdSe coated with ZnS as a Received: December 7, 2011 Revised: January 10, 2012 Published: January 12, 2012 4500

dx.doi.org/10.1021/jp2117906 | J. Phys. Chem. C 2012, 116, 4500−4510

The Journal of Physical Chemistry C

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core−shell nanocomposite structure, have been achieved.32−35 In addition, the introduced ZnS shell can minimize the toxicity of the heavy metal cadmium.36 By virtue of the advantages of the conducting polymers and NCs, a strategy of combining these two components for applications in various domains of electronics, for examples, as active layers in light emitting diodes (LEDs), photodiodes and photovoltaic cells, has been developed.37−39 It turns out that these new hybrid materials may lead to polymer nanocomposites (PNCs) with properties unmatched by conventional materials. For both of the conjugated polymers and the NCs, their properties can be tuned individually to adapt to each other. This particularly applies to the positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, which determine their electronic, redox, and luminescent properties.40 Therefore, conjugated polymer/nanocrystal hybrids are much more superior to their corresponding individuals where the design flexibility is concerned. N,N′-Di[p-phenylamino(phenyl)]-1,4,5,8-naphthalene tetracarboxylic diimide (DNTD), a bis(diphenylamine)naphthalene diimide, belongs to the class of conjugated conducting polymers with a chemical structural motif of R−X−R, where R represents the diphenylamine (DPA) group and X represents naphthalene tetracarboxylic diimide (NTCDI) group as the electroactive center. Materials made from this monomer exhibit low dielectric, thermal/oxidative stability41 and increased bulk conductivity through π-stacking interactions.42 Though the electrochemical and optical properties of the pure poly(DNTD) polymer thin films have been widely investigated and well elucidated by cyclic voltammetry, UV−visible spectroelectrochemistry,43 and electrochemical quartz crystal microbalance (EQCM),44 the combination of the poly(DNTD) polymer with NCs in the nanocomposite thin films for using as electrochromic and fluorescent materials have been rarely explored. In this study, hybrid electrochromic nanocomposite thin films consisting of poly(DNTD) and core@shell CdSe@ZnS quantum dots (QDs) were fabricated by the cyclic voltammetry method. The physicochemical properties of the nanocomposite thin films are studied and compared with those of pure poly(DNTD) thin films. The electrochemical properties, optical, and spectroelectrochemistry (SEC) properties were studied using two different working electrodes, that is, platinum (Pt) disk with an area of 0.018 cm2 and indium tin oxide (ITO) coated glass slides. The effects of temperature on the film growth and electrochemical properties of the polymer and its nanocomposite thin films were also investigated. The composite films showed promising potential in the electrochromic device applications, while the fluorescent properties were introduced successfully.

Scheme 1. Chemical Structure of the Monomer DNTD

1 mL of 30.0 wt % hydrogen peroxide (both from Fisher), and 10 mL of deionized water before the usage. 2.2. Electrochemical Synthesis of Pure Polymer and Nanocomposite Thin Films. Pure poly(DNTD) and its QD nanocomposite thin films were prepared by oxidative electropolymerization from CH2Cl2 solutions containing monomer (0.5 mM), QDs and electrolyte TBAPF6 (0.1 M) with multiple cycles of cyclic voltammetry (CV) at room temperature. Figure 1a−d shows the different colors of the CH2Cl2 solutions

Figure 1. Images of DNTD (a) and QDs/DNTD dissolved in CH2Cl2 with QD loading of (b) 0.05 wt %, (c) 0.1 wt %, and (d) 0.2 wt %, respectively.

containing only monomer DNTD and QDs/DNTD with different QD loadings. CV was performed on an electrochemical working station VersaSTAT 4 potentiostat (Princeton Applied Research). A typical electrochemical cell consisting of a reference electrode, a working electrode, and a counter electrode was employed. An Ag/AgCl electrode saturated with KCl served as the reference electrode and a platinum (Pt) wire as the counter electrode. ITO coated glass slide and a platinum (Pt) disk with an area of 0.018 cm2 were used as the working electrode for optical characterizations and electrochemical characterizations, respectively. A long path length homemade spectroelectrochemical cell with Teflon cell body with front and rear windows clapped with two steel plates was employed when the ITO glass slide was used as the working electrode. A typical electrochemical polymerization was performed 10 cycles scanned back and forth from 0 to 1.8 V vs Ag/AgCl at a scan rate of 200 mV/s.42 To investigate the temperature effect on the performance of the thin films, pure poly(DNTD) and its nanocomposite thin films with 0.05 wt % of QDs at 0 °C were also prepared for comparison. 2.3. Characterizations. The electrochemical properties of pure poly(DNTD) and its nanocomposite thin films were investigated by CV scanned from −1.2 to 1.5 V vs Ag/AgCl using Pt disk as the working electrode. The three-electrode cell was bubbled with ultrahigh purity nitrogen (Airgas) for at least 15 min before the scan. The monomer-free solution is 0.1 M TBAPF6 in CH2Cl2. The CVs of all the thin films produced were performed at both room temperature and 0 °C. The morphology and thickness of the thin films grown on the ITO glass slides were characterized by atomic force microscopy (AFM, Agilent 5600 AFM system with multipurpose 90 μm scanner). Imaging was done in acoustic ac mode (AAC) using a silicon tip with a force constant of 2.8 N/m and a resonance frequency of 70 kHz.

2. EXPERIMENTAL SECTION 2.1. Materials. Methylene chloride (CH2Cl2, 99.9%) and tetrabutylammonium hexafluorophosphate (TBAPF6, 98%) were purchased from Alfar-Aesar. CdSe@ZnS quantum dots with octadecylamine ligand were supplied by Ocean NanoTech LLC (Springdale, Arkansas USA). The monomer DNTD was synthesized following the procedures in the literature,42 and Scheme 1 shows the chemical structure of the DNTD. The microscope glass slides and indium tin oxide (ITO) coated glass slides were purchased from Fisher and Delta technologies, Limited, respectively. ITO coated glass slides were degreased by a mixed aqueous solution containing 1 mL of 28.86 wt % ammonium hydroxide, 4501



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UV−vis spectra of the pure DNTD and DNTD/QDs CH2Cl2 solutions with the electrolyte were recorded with a 50 bio ultraviolet visible (UV−vis) spectrophotometer. Their photoluminescence (PL) spectra were recorded by a Varian CARY Eclipse fluorescence spectrophotometer. The solutions were put into a standard 10 mm path length quartz cuvette for testing. The solution of QDs was diluted to 1% of its original concentration with CH2Cl2 before measurement. The excitation wavelength is 350 nm. The films grown on ITO coated glass slide were further examined by an Olympus BX51 fluorescence microscope. Images were obtained using Olympus DP72 camera accompanied with Olympus CellSens software. The spectroelectrochemistry measurements were performed on a Varian Cary 50 Version 3 UV−visible spectrometer coupled with the potentiostat for applying electrochemical potentials. Solutions in the spectroelectrochemistry (SEC) cell were treated by the same manner bubbled with ultrahigh purity nitrogen for at least 15 min. Each spectrum was taken when the electrochemical cell current decayed to zero and the data was obtained in the mode of medium in Varian Cary WinUV Bio software. At each applied potential, a minimum of three spectra were taken to verify consistency.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Poly(DNTD) and Poly(DNTD)/QD Nanocomposite Films. Figure 2a,b shows the UV−vis absorption (a) and fluorescence spectra (b) of QDs and QDs/ DNTD in methylene chloride. In the UV−vis absorption, TBAPF6 and DNTD/TBAPF6 in the CH2Cl2 solutions show a flat curve without any absorption peaks. The QDs show two absorption peaks at 562 and 595 nm, corresponding to the absorption energy of 2.21 and 2.10 eV, respectively (ν = c/λ, E = h × ν, ν is light wave frequency, s−1; c is light speed, 3 × 108 m s−1; λ is wavelength, m; E is wave energy, eV; h is Planck’s constant, 4.135 × 10−15 eV·s) characteristic of a narrow-size distribution of CdSe@ZnS QDs.45 Cyclic voltammetry has been used widely to determine the HOMO and LUMO levels of QDs.46,47 The onsets of the first oxidation peak and reduction peak in the CV are assumed to correspond to the withdraw of an electron from the bonding HOMO and an addition of an electron to the antibonding LUMO, respectively.48 The onset potential is determined from the intersection of the two tangents drawn at the rising oxidation or reduction current and background current in the CV.49,50 Figure 3a,b shows the reduction voltammetry of the bare Pt (without QDs) (a) and 0.2 wt % QDs (b) in CH2Cl2 solution. The onset potential of −0.55 V vs Ag/AgCl, Figure 3b, could be obtained employing the graphical method. Therefore, the LUMO energy with respect to the vacuum value of QDs is calculated to be −3.82 eV on the basis of the assumption that the energy value of Ag/AgCl is −4.37 eV in a vacuum.51−53 The bandgap (energy difference between HOMO and LUMO) of the QDs is calculated to be 2.10 eV at 595 nm in the UV−vis spectra, Figure 2a, for the QDs in CH2Cl2 solutions. Correspondingly, the HOMO energy of the QDs with respect to the vacuum value is calculated to be −5.92 eV. Similar HOMO and LUMO energy values have been obtained by others.54 The measured bandgap value shows a blue shift relative to the band gap of the bulk cubic CdSe (1.77 eV),45 which is attributed to the small size and the effect of ZnS shell. The QDs/DNTD solutions show a weak absorption because of the low concentration of the QDs, and the absorption positions shift to longer wavelength, indicating an interaction between QDs and the monomer. The fluorescence spectra, Figure 2b, depict the

Figure 2. (a) UV−vis absorption and (b) fluorescence spectra of QDs, TBAPF6, QDs/TBAPF6, and QDs/DNTD with different loadings of QDs in CH2Cl2. The excitation wavelength for fluorescence spectra is 350 nm.

Figure 3. Reduction voltammetry recorded on Pt electrode in the (a) absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6 as the supporting electrolyte. The scan rate is 200 mV/s.

emission peak of the QDs solution at 612 nm at an excitation wavelength of 350 nm. However, the QD/DNTD solutions show a shifted emission peak at 616 nm, consistent with the result of UV−vis analysis, that some interaction between QDs and the monomer does exist. Figure 4a−d shows the cyclic voltammetric growth of pure poly(DNTD) films and its nanocomposite films with a QD loading of 0.05, 0.1, and 0.2 wt %, respectively, on a Pt electrode (0.018 cm2), scanned in CH2Cl2 solution containing 0.1 M TBAPF6 supporting electrolyte at room temperature. The scan 4502



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Figure 4. Films growth of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD, (c) 0.10 wt % QDs/DNTD, (d) 0.20 wt % QDs/DNTD at room temperature, and of (e) pure DNTD, and (f) 0.05 wt % QDs/DNTD at 0 °C, respectively, on a Pt disk electrode (area = 0.018 cm2) in CH2Cl2 solution containing 0.1 M TBAPF6 and 0.1 mM DNTD monomer at room temperature. Scan rate is 200 mV/s.

Scheme 2. Possible Cation (DPB+) and Dication (DPB2+) Chemical Structures

rate is 200 mV/s and scan range is from 0 to 1.5 V vs Ag/AgCl. Pure polymer and nanocomposite thin films exhibit similar CV shapes. During the first positive scan, an irreversible peak appeared, which is attributed to the irreversible oxidation of diphenylamine.55−57 For the first negative scan and the second positive scan, two sets of quasi-reversible peaks are observed, consistent with the formation of diphenylbenzidene (DPB) unit. These two peaks correspond to the oxidation of DPB to form cation (DPB+) and dication (DPB2+), respectively.42 The possible structures of DPB+ and DPB2+ are shown in Scheme 2.58 The current increased with increasing cycles, reflecting the growth of pure polymer and nanocomposite thin films. The polymerization occurs via the coupling of the para carbon on the terminal phenyl group, followed by proton loss and additional electron transfer reactions.43 Scheme 3 illustrates the proposed polymerization mechanism (a) and possible reactions between the dimer/oligmers and the ligands on the QD surface (b). The

ligands on the QD surface were oxidized to form radical cations and served as the terminating radicals to react with the initial radical cation of the diphenylamine group in the dimer/oligmers. There are also chances that QDs ligands formed dications to couple with or even more two dimer/oligmers. However, some differences between the pure poly(DNTD) polymer and the nanocomposite thin films can be observed 4503



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Scheme 3. (a) Polymerization Mechanism of Pure Polymer and (b) Possible Reactions between Oxidized QDs and Dimer/ Oligomers

upon closer observation. First, the irreversible anodic potential for pure polymer is around 1.34 V and is postponed to be 1.41, 1.42, and 1.45 V vs Ag/AgCl for the composite thin films with a QD loading of 0.05, 0.1, and 0.2 wt %, respectively. Figure 5a,b shows the oxidation voltammetry of bare Pt (a) and 0.2 wt % QDs (b) in CH2Cl2 solution. In the positive sweep, an obvious anodic peak at around +1.3 V vs Ag/AgCl was observed for QDs, corresponding to the oxidation of the QDs octadecylamine ligands. Therefore, these positive potentials for composite films are supposed to be caused by the combined irreversible oxidation of the QDs octadecylamine ligands with the diphenylamine in the monomer as shown in Scheme 3b. Second, for the growth of pure poly(DNTD) polymer thin film, the peak at 1.0 V vs Ag/AgCl corresponding to the 1 e− oxidation of DBP became less distinct upon repeated CV scans. However, for the nanocomposite thin films, the peak at 1.0 V vs Ag/AgCl appeared more pronounced and did not undergo appreciable changes in intensity with increasing the scans. This indicates that QDs favor the one-electron transfer process and stabilize the state of radical DPB+. At 0 °C, Figure 4e,f, DNTD monomers could still be electropolymerized. The thin films exhibit similar CV shapes compared to those at room temperature except the anodic current became comparatively lower, indicating a lowed polymerization rate.59

Figure 5. Oxidation voltammetry recorded on Pt electrode in the (a) absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6 as the supporting electrolyte. The scan rate is 200 mV/s.

However, both the anodic and cathodic current potentials exhibit negligible differences between the CVs at two different temperatures. This phenomenon suggests that a similar electropolymerization mechanism occurred for the thin films growing at room temperature and 0 °C even though the reaction rate was different. 4504



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Figure 6. Cyclic voltammograms of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD, (c) 0.10 wt % QDs/DNTD, and (d) 0.20 wt % QDs/DNTD, respectively, on Pt scanned in 0.1 M TBAPF6 CH2Cl2 DNTD-free solution with a scan rate of 200 mV/s at room temperature.

and the other at −0.82 V vs Ag/AgCl in the negative scan. These two reduction peaks are attributed to the formation of the naphthalene diimide anion and naphthalene diimide dianion, respectively.42 The possible chemical structures of those anions and dianions are shown in Scheme 4. For poly(DNTD)/QDs

3.2. Electrochemical Properties. After the electropolymerization, the electrode with a thin film on the surface of Pt disk was rinsed with fresh CH2Cl2 and then was placed in a degassed CH2Cl2 solution containing only 0.1 M TBAPF6 supporting electrolyte, and was cycled again. Figure 6a−d shows the CV of poly(DNTD) and poly(DNTD)/QDs nanocomposite thin films, respectively, on a Pt electrode. The scan rate is 200 mV/s and scan range is from −1.2 to +1.5 V vs Ag/AgCl. The CV data of pure polymer and nanocomposite films is summarized in Table 1. The half wave potentials of Eox

Scheme 4. Possible Anion and Dianion Chemical Structures of DNTD

Table 1. Electrochemical Data of Polymer Films Synthesized at Room Temperaturea polymer films

first Eox (V)

second Eox (V)

first Ered (V)

second Ered (V)

Eg,EC (eV)b

pure polymer 0.05 wt % of QDs 0.1 wt % of QDs 0.2 wt % of QDs

0.94 1.17 1.23 1.24

1.15

−0.39 −0.37 −0.37 −0.37

−0.82 −0.83 −0.83 −0.83

1.33 1.54 1.60 1.61

a b

nanocomposite thin films, Figure 6b−d, during the forward negative scan, the cathodic potential did not change too much compared to those of the pure poly(DNTD) thin films, one at about −0.37 vs Ag/AgCl and the other at −0.83 V vs Ag/AgCl, Table 1. Therefore, the same redox species are produced. Thus the QDs have negligible effect on the naphthalene diimide moiety in the polymer. When the potential began to sweep backward, asymmetry in the oxidation/reduction process was observed. The anodic peak at around 0.94 V vs Ag/AgCl disappeared and only one single prominent oxidation peak at 1.17 V vs Ag/AgCl appeared, and the oxidation potential shifted toward more positive positions with the increase of QD loading. Meanwhile, in the cathodic branch of the cyclic voltammogram, two distinguishable reduction peaks instead of one were observed. This asymmetric behavior might be attributed to the heterogeneity in the polymer during the eletropolymerization as shown in Scheme 3. The incorporated QDs gave rise to different polymers with varying conjugated chain length. In addition, the introduction of QDs in the polymer may also affect the morphology or crystallinity of the polymer

Potentials vs Ag/AgCl; 0.1 M TBAPF6, CH2Cl2, scan rate: 200 mV/s. Eg, EC=1st Eox − 1st Ered.

and Ered are obtained by taking the average of their anodic and cathodic peaks, i.e., Eox = (Epaox+ Epcox)/2 and Ered = (Epared+ Epcox)/2.48 For the pure poly(DNTD) thin films, Figure 6a, in either positive (potential larger than 0 V vs Ag/AgCl) or negative (potential lower than 0 V vs Ag/AgCl) scan, two pairs of well-defined redox peaks are observed. Therefore, poly(DNTD) can be oxidized and reduced, indicating its p-n bipolar properties. In the positive scan, the two sets of the redox peaks, one with an Eox of about 0.94 V vs Ag/AgCl and the other one of about 1.15 V vs Ag/AgCl, are associated with the oxidation of poly(DNTD) into DPB+ cation and DPB2+ dication.42 The pure polymer thin films also show two separated cathodic peaks, one at about −0.39 V vs Ag/AgCl 4505



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Figure 7. Cyclic voltammograms of (a) pure poly(DNTD), and QDs/poly(DNTD) nanocomposites with a QD loading of (b) 0.05, (c) 0.10, and (d) 0.20 wt %, respectively, on Pt scanned in 0.1 M TBAPF6CH2Cl2 DNTD-free solution with a scan rate of 200 mV/s at 0 °C.

Figure 8. Cyclic voltammograms of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD at room temperature, and (c) pure DNTD, (d) 0.05 wt % QDs/ DNTD at 0 °C.

increased with the increase of the loading of QDs. Eg,EC is calculated to be 1.33, 1.54, 1.60, and 1.61 eV, Table 1, for the pure polymer and the nanocomposite films with a QD loading of 0.05, 0.1, and 0.2 wt %, respectively. The phenomenon might be explained that the polymer became less planar and less conjugated in the hybrid films, thus giving rise to a widened band gap.48 Similar phenomena have been observed for the nanocomposites of the diaminopyrimidine functionalized

thin film, which also play a part in influencing the electrochemical properties of the nanocomposite films. The first Eox and Ered provide some valuable information on the HOMO and LUMO levels regarding the polymer and composite films.60 Given the higher first Eox for the nanocomposite films, Table 1, it can be inferred that poly(DNTD) became more difficult to be oxidized. On the other side, the bandgap obtained by the electrochemical method, Eg,EC, of the polymer 4506



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Figure 9. AFM images and thicknesses of (a) pure polymer and composite films with QDs loading of (b) 0.05 wt %, (c) 1.0 wt %, (d) 0.05 wt % at room temperature, and of (e) pure polymer at 0 °C, respectively, grown on ITO glass.

0.2 wt %, respectively, on a Pt electrode (0.018 cm2), scanned in CH2Cl2 solution containing 0.1 M TBAPF6 supporting electrolyte at 0 °C. Compared to those CVs at room temperature, these films showed lower oxidation peak potentials. Besides, the peaks of polymer became less symmetrical. This interesting phenomenon is also observed for the CVs of the polymers prepared at 0 °C, Figure 8. The polymer films appear to have more symmetrical CV shapes at 0 °C than at room temperature. Many factors such as interactions among charged sites,61,62

poly(3-hexylthiophene) and thymine-capped CdSe nanocrystals as a result of the formation of hydrogen bonding between the polymer and the nanocrystals.50 Cyclic voltammograms of poly(DNTD) and poly(DNTD)/ QDs nanocomposite films at 0 °C were also performed to investigate the effect of temperature on the electrochemical properties of these films prepared at room temperature. Figure 7a−d shows the CVs of pure poly(DNTD) and QDs/poly(DNTD) nanocomposite thin films with a QD loading of 0.05, 0.1, and 4507



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test. For example, the planar chain of the polymer film synthesized at room temperature is more subjected to be a twisted conformation when the CV test was conducted at 0 °C. For the nanocomposite thin films, the main reason might lie in the reduced conjugated length caused by the incorporation of the QDs in the polymer chains. 3.3. Morphology and Optical Properties. The polymer and nanocomposite thin films grown on the ITO coated glass slides were rinsed with CH2Cl2 and dried for AFM characterization. Figure 9 presents the morphology and thickness of the films grown at room temperature (a−e) and pure polymer film grown at 0 °C (f). At lower resolution, these films exhibit compact and continuous film structures. However, under high resolution, evenly sized and uniformly distributed nanoparticles are observed. For the films synthesized at room temperature, the thickness of the film increased with increasing the QD loading. The thickness is roughly estimated to be 17, 25, 30, and 35 nm for the pure polymer and nanocomposite films with a QD loading of 0.05, 0.1, and 0.2 wt %, respectively. Accordingly, the particle size increases with increasing the thickness of the film. This morphology change induced by the QDs may account for the changes in the electrochemical properties of the nanocomposite films compared to that of the pure polymer. At lower temperature 0 °C, the pure poly(DNTD) polymer film exhibits more orientated and smaller particles than that synthesized at room temperature. This phenomenon is reasonable for that at high temperature, the crystals grow fast, yielding bigger-sized particles and less-orderly structures.66 The films grown on ITO glass slides were further examined under the fluorescent microscope. Figure 10 shows the fluorescent microscope images of the pure poly(DNTD) polymer

Figure 10. Bright field fluorescent microscopy images of (a) pure polymer, and composite films with QDs loading of (b) 0.05 wt %, and (c) 2.0 wt % grown on ITO glass slides. a*−c* correspond to a−c in dark field.

differences in the properties of the oxidized and neutral polymer,63 or heterogeneity of the polymer film64,65 have been proposed for the asymmetry in the redox behavior of the polymers. Here the asymmetry of the pure polymer might be caused by the conformational change of the polymer backbone induced by the temperature difference between the film growth and CV

Figure 11. UV−vis spectra of (a) pure polymer, (b) 0.05 wt % QDs/DNTD, (c) 1.0 wt % QDs/DNTD, and (d) 2.0 wt % QDs/DNTD films, respectively, grown on ITO glass slide. 4508



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1.4 V vs Ag/AgCl, respectively. At 0 V vs Ag/AgCl, the nanocomposite film appeared nearly transparent; when applied to a potential of 1.4 V vs Ag/AgCl, the film turned to blue. The unique electrochromic properties promise potential applications of these hybrid nanocomposite thin films in the electrochromic devices.

and nanocomposite films. The pure polymer thin film shows no fluorescent emission signal as expected, Figure 10a. However, in the nanocomposite thin films with a QD loading of 0.05 and 2.0 wt %, uniform red emission signals are observed, indicating that QDs were fairly uniformly distributed in the hosting poly(DNTD) polymer matrix. 3.4. Spectroelectrochemistry of Polymer and Composite Films. Polymer and nanocomposite thin films can be both oxidized and reduced as revealed from the results of CVs; therefore, it is expected that the visible spectra shall be changed when subjected to different potentials.67,68 Figure 11 shows the spectroelectrochemistry of pure poly(DNTD) and nanocomposite thin films deposited on the ITO coated glass slides at different potentials. The spectrum of the neutral films of poly(DNTD) and nanocomposites show a less absorption band above 400 nm. When the potential was increased to 0.9 V vs Ag/AgCl, a peak appeared at 450 nm, which is assigned to be the formation of the cation (DPB+).42 As the potential was further increased to 1.4 V vs Ag/AgCl, the band at 450 nm disappeared, and a broad absorption at 613 nm was observed, corresponding to the oxidation of the cation (DPB+) to the dication (DPB2+). Correspondingly, the thin film turns to blue. At negative potential, for example, −0.5 V vs Ag/AgCl, a peak at 480 nm was observed, which corresponds to the naphthalene diimide anion absorption. The films remain in the anion state during the reduced potential. As for the nanocomposite thin films, similar absorption spectra were observed when the applied potentials were varied. Figure 12a,b shows the color change for the composite film with 0.05 wt % loading of QDs when subjected to 0 and

4. CONCLUSION Hybrid electrochromic composite films composed of poly(DNTD) and CdSe@ZnS quantum dots (QDs) have been synthesized by electrodeposition and were electrochemically and spectroelectrochemically characterized. QDs are observed to be fairly uniformly dispersed in the polymer matrix. The composite films showed bipolar properties and became less easily oxidized with a widened bandgap as a result of reduced conjugated length caused by the incorporation of QDs in the polymer chain. The spectroelectrochemistry (SEC) properties of the films grown on ITO exhibit varied absorption bands when applied to different potentials and the color turned blue at high positive potential, for example, 1.4 V vs Ag/AgCl. The effect of temperature on the composite films growth and their electrochemical properties were also studied by CVs, showing less asymmetry in the redox process and lowered oxidation potential at low temperature. These new hybrid nanocomposite thin films exhibit both unique fluorescent properties and novel electrochromic phenomena.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.G.); [email protected] (S.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Science FoundationNanoscale Interdisciplinary Research Team and Materials Processing and Manufacturing under Grant Number of CMMI 10-30755 is kindly acknowledged.

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REFERENCES

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Figure 12. Color change for composite film with 0.05 wt % loading of QDs when applied to (a) 0 V, and (b) 1.4 V, respectively. 4509



Article

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