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Confinement of Semiconductor ZnO Nanoparticles in Block-Copolymer Nanostructure Anna Malafronte, Finizia Auriemma, Rocco Di Girolamo, Carmen Sasso, Claudia Diletto, Angela Evelyn Di Mauro, Elisabetta Fanizza, Pasquale Morvillo, Antonio Manuel Rodriguez, Ana B. Munoz-Garcia, Michele Pavone, and Claudio De Rosa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05125 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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The Journal of Physical Chemistry

Confinement of Semiconductor ZnO Nanoparticles in Block-Copolymer Nanostructure Anna Malafronte,†,* Finizia Auriemma,† Rocco Di Girolamo,† Carmen Sasso,† Claudia Diletto,†,§ A. Evelyn Di Mauro,‡ Elisabetta Fanizza,‡,║ Pasquale Morvillo,┴ Antonio M. Rodriguez,† Ana B. Muñoz-Garcia,† Michele Pavone,† Claudio De Rosa.† †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Monte

S.Angelo, Via Cintia, I-80126 Napoli, Italy. ‡Istituto per i processi Chimico Fisici, Consiglio Nazionale delle Ricerche, Via Orabona 4, I-70126 Bari, Italy. ║Dipartimento di Chimica, Università di Bari, Via Orabona 4, I-70126 Bari, Italy. ┴ENEA Italian National Agency for New Technologies, Energy and Sustainable Development, SSPT-PROMAS-NANO Department, P.zza E. Fermi 1, I-80055 Portici, Napoli, Italy.

ACS Paragon Plus Environment

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ABSTRACT. The morphology and the electrical properties of hybrid nanocomposites characterized by the dispersion of ZnO nanoparticles (NPs), a n-type semiconductor, within an organic nanostructured matrix of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer are presented. A selective inclusion of NPs only into the lamellar PS nanodomains of the block copolymer matrix has been achieved by synthetizing ZnO NPs coated with nhexadecylamine (HDA) and tert-butylphosphonic acid (TBPA) molecules and subsequent thermal annealing of the nanocomposites ZnO NPs/PS-b-PMMA. Thermal treatments have allowed to obtain in one step the vertical orientation of the lamellar BCP nanodomains and the migration of the NPs to the PS domains, resulting in the formation of nanocomposites characterized by a precise control of the position of n-semiconductor ZnO NPs. Current-voltage measurements on the nanocomposites have indicated the presence of continuous path of charge carriers in the BCP films when the ZnO NPs content is above a threshold concentration. The experimental results have allowed to set-up and validate a theoretical protocol to study heterogeneous interfaces. In particular, the interaction of ZnO most stable non-polar surface with prototypical capping agents has been analyzed by using Density Functional Theory (DFT) calculations.


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Introduction Inorganic semiconductors, such as CdSe, TiO2, CdS and ZnO, usually have higher physical and chemical stability compared to that of organic semiconductors.1-3 Among the n-type inorganic semiconductors, ZnO-based materials are widely investigated due to the salient characteristics of zinc oxide such as low cost, easy synthesis, non-toxicity, high stability, and good optoelectronic properties.3,4-10 These peculiar properties have motivated applications in several technological devices such as electronics, optoelectronics, electrochemical and electromechanical devices, ultraviolet (UV) lasers,5 field emission devices,6 high performance chemical and gas sensors,7 piezoelectric generators8 and organic and hybrid solar cells.3,9,10 In particular, in the context of solar energy conversion devices, systems made of conjugated polymers and n-type ZnO NPs have showed promising performances in hybrid bulkheterojunction cells.3 The great interest in these applications inspired several experimental and theoretical studies that addressed the structure-property relationships in zinc oxides nanostructures.11-13 Several strategies have been reported in literature for the preparation of ZnO NPs, spanning from laser ablation, hydrothermal methods, electrochemical deposition, sol-gel methods, chemical vapor deposition, molecular beam epitaxy, common thermal evaporation method and soft chemical solution method.14 Among these approaches, the thermal decomposition of zinc precursors in hot coordinating solvents represents a solution-phase strategy useful for the low cost production of NPs, since the technique does not make use of harsh conditions. A variety of surfactants have been used including trioctylphosphine oxide (TOPO), oleic acid (OA), 1hexadecylamine (HDA), and tetradecylphosphonic acid (TDPA). These surfactants regulate the growth of nanoparticles by coordinating the NPs surface, affect the morphology, surface


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chemistry and physical-chemical properties of the final NPs14-16, and make the colloidal NPs soluble in apolar solvent. In addition, the selective coordination of capping molecules to specific crystallographic face can modulate the optoelectronic properties of the ZnO NPs so allowing the tuning of the properties toward specific applications such as in solar conversion devices. Understanding the main features of these interactions and their effects on the ZnO electronic structure is therefore key for the rational design of new devices. Beside this, the control of the spatial arrangement of the capped nanoparticles onto a solid support is a critical issue, since advantageous properties of NPs often emerge only when appropriate coupling and exchange phenomena between the NPs exist, thus requiring appropriate control over their inter-particle distance, regular ordering and location. Though various ZnO nanostructures have in fact been reported,5-8,17,18 the control of morphology and spatial arrangement of the nanostructures are common problems in conventional techniques such as hydrothermal or chemical vapor deposition method. An effective approach for controlling the spatial organization of NPs on solid supports consists in the use of nanostructured block copolymers (BCPs) as host for the selective inclusion of NPs in specific nanodomains according to well-defined geometries.19-30 BCPs consist of covalently linked chemically distinct macromolecules that tend to segregate into different microdomains due to their not favorable interaction, resulting in the spontaneous formation of periodic nanostructures by self-assembly.31,32 In particular, depending on the relative volume of the blocks, self-assembly of linear AB di-block copolymers may produces lamellar, bicontinuous gyroid, cylindrical or spherical phase separated morphologies.31 The use of block copolymers trapping nanoparticles has been proposed as a tool to prevent particles aggregation and control their spatial arrangement.19-30 Nanodomains of self-assembled BCPs may in fact act as hosts for


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sequestering nanofillers of appropriate chemical affinity according to prefixed periodic geometries, producing hybrid nanocomposites characterized by long-range order in the positioning of nanoparticles.19-30 The strategy of building ordered arrays of semiconductor nanoparticles exploiting selfassembly and selective inclusion of NPs into specific domains has received great attention so far.33-37 As an example, the role of ordered BCP morphology for photovoltaic applications has been demonstrated in ref. 35 by Cohen et al., where CdS NPs have been included both in a microphase-separated and non-microphase-separated triblock copolymer consisting of a polynorbornene block with pendant hole-transporting carbazole groups, a short mid-block able of binding semiconductor NPs and a polynorbornene block functionalized with electrontransporting groups. It has been shown that the selective inclusion of CdS NPs in the middle block domains of the microphase separated morphology enhances the performance of the photovoltaic device. Furthermore, it has been also demonstrated that energy transfer from carbazole moieties present in one block of the BCP could be transferred to CdS NPs sequestered in the middle block, indicating that the nanostructured polymeric substrate can be successfully used to tune energy-transfer processes.35,36 Similar concepts have been investigated by Schrock et al. by in situ synthesis of semiconductor NPs such as ZnS and CdS using nanostructured block copolymers derived from norbornenes as nano-rectors

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or of ZnSe nanodots, nanowires and

nanodiscs using micellar cubic, hexagonal and lamellar liquid-crystalline phases of amphiphilic BCPs, respectively. 38,39 Lamellar-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymer has been successfully used to selectively sequester and confine different surface-functionalized inorganic nanoparticles and nanorods (NRs) in lamellar PMMA domains, such as poly(methyl


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methacrylate) (PMMA)-grafted magnetite (FeO4) NPs40 and gold NRs modified with a poly(ethylene glycol) (PEG) brush.41 In this paper, semiconductor organic-capped ZnO nanoparticles (NPs) have been synthesized by thermal decomposition of Zn precursor in a hot coordinating solvent. In particular, nhexadecylamine (HDA) and tert-butylphosphonic acid (TBPA) have been purposely chosen as coordinating agents, resulting in HDA/TBPA-capped NPs. We report a simple method to control the relative arrangement of the organic-capped ZnO NPs by exploiting a lamellar nanostructured polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer as template. In particular, ZnO-based hybrid materials characterized by NPs selectively assembled and confined into vertically aligned PS lamellar nanodomains of the phase-separated PS-b-PMMA block copolymer have been obtained. The conductive properties of the hybrid nanocomposite have been evaluated by performing intensity-voltage measurements for different NPs loading and the effect of the NPs assembly and their nano-confinement in the BCP host matrix have been investigated. A computational study is also presented to envision the structural and electronic modification of the ZnO NPs when interacting with TBPA and the model aliphatic amine ethylamine (EA), in order to elucidate the effect of this capping in view of the application of the NPs as photoactive component in photovoltaic devices. The presented simple strategy for the preparation of PS-b-PMMA-based nanocomposites containing n-type ZnO NPs can represent a fundamental step for the preparation of promising materials for photovoltaic applications. For instance, using the surface of the BCP films characterized by selective inclusion of ZnO nanoparticles as substrate, the successive deposition of a thin film loaded with carriers of opposite charge ability may be the key to fabricate active layers for photovoltaics according to a layer-by-layer process.42,43


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Experimental Section Materials. Zinc acetate (C4H6O4Zn, 99.999%, ZnAc2), tert-butylphosphonic acid (C4H9PO3H2 or TBPA, 98%), n-hexadecylamine (C16H33NH2 or HDA, 98%) and toluene have been purchased from Aldrich. PS-b-PMMA has been purchased from Polymer Source, Inc. and used without further purification. The number-average molecular mass (Mn) of the PS and PMMA blocks in the copolymer are 25.0 and 26.0 Kg mol-1, respectively (polydispersity 1.06). The volume fraction of the PS block of 0.52 has been selected in order to obtain a lamellar microphase separated morphology (lamellar period ≈ 16 nm). The BCP sample is amorphous with glass transition temperatures equal to 108 °C for PS and 126 °C for PMMA.44 Indium thin oxide (ITO)-coated slides (nominal transmittance >85%, nominal coating thickness ≈ 130 nm) have been purchased from Delta Technologies. They have been cleaned with a 20 wt% water solution of ethanolamine at 80 °C in an ultrasonic bath before using as supports for the BCP films. Synthesis and characterization of ZnO nanocrystals. ZnO nanocrystals have been synthesized by thermal decomposition of ZnAc2 (0.8 mmol) in hot TBPA/HDA (0.5mmol/0.02 mol) mixture, by using the method described in ref. 16. All manipulations have been performed using standard airless techniques. Briefly, a mixture of ZnAc2, HDA and TBPA, with TBPA:ZnAc2 molar ratio fixed equal to ≈0.6, has been degassed under vacuum for 1 h at 110 °C under vigorous stirring. Then, the reaction vessel has been slowly heated up to 300 °C, under nitrogen flow to induce the decompositon of ZnAc2. Heating has been stopped when the solution became cloudy and the temperature has been dropped down to 80 °C. The NPs have been collected from the reaction mixture in air by addition of methanol (non-solvent) to the reaction mixture at 50 °C. The resulting precipitate has been isolated by centrifugation and has been washed twice with methanol to remove residual surfactants. The surfactant (i.e. HDA/TBPA)-coated ZnO


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nanoparticles have been then easily re-dissolved in toluene to give optically clear solutions. The absorption spectra of the synthesized materials have been recorded on a Cary Varian 5000 with UV-vis monochromator. Photoluminescence (PL) experiments have been carried out on a Fluorolog®-3 HORIBA Jobin Yvon with a Xe lamp of 450 W, equipped with double monochromators in excitation and emission and a TBX single photon counter detector. Samples for Transmission Electron Microscopy (TEM) have been prepared by casting a drop of a suitably diluted solution of the nanoparticles on the surface of a carbon-deposited copper grid. Energydispersed X-ray analysis (EDX) has been performed using a scanning electron microscopy (FEI Nova NanoSEM 650) equipped with an energy-dispersive X-ray spectrometer and a field emission gun (FESEM/EDX). Theoretical calculations. All the calculations have been performed with spin-polarized Density Functional Theory (DFT) within the generalized-gradient approximation of Perdew-BurkeErnzerhof (PBE),45-47 using the periodic super-cell approach as implemented in the VASP code.47-49 Dispersion correction has been taken into account with the semi-empirical approach of Grimme (D3BJ).50,51 Core electrons have been modeled using the projector augmented wave (PAW) method, while for valence electrons the plane-wave basis set has been expanded up to a kinetic-energy cut-off value of 750 eV.52 The surface models included a vacuum space larger than 15 Å along the direction perpendicular to the slab surfaces. For asymmetric slabs, we have used the self-consistent dipole correction implemented in VASP.52,53 Further details on the computational methods and structural models are reported in Supporting Information. Preparation and characterization of the hybrid nanocomposites. Toluene solutions containing the neat BCP (2 wt%) and BCP (2 wt%) + ZnO NPs at different concentrations (0.1, 0.2, 0.25, 0.3 and 0.35 wt%) have been prepared. Thin films have been obtained by spin coating (3000 rpm


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for 30 s) on ITO supports the toluene solutions at room temperature. Subsequently, the thin films have been annealed in vacuo at 150 °C for 6 h. From the initial solutions, BCP-based nanocomposites with ZnO NPs content of 5, 10, 12.5, 15 and 17.5 wt%, respectively, are obtained. Since the weight fraction of PS in the BCP is 0.52, the weight amount of ZnO relative to the sole PS block is 10, 19, 24, 29 and 34 wt%, respectively. The resultant nanocomposites are indicated with the symbol BCP-ZnOx, with x the weight content of ZnO referred to the PS blocks. We have checked that the 150°C annealing treatment for 6h of ZnO nanoparticles does not greatly impairs their energy gap and spectroscopic properties (Figure S1). The same procedure has been used to prepare nanocomposites based on polystyrene homopolymer (PS, Mn = 83.5 Kg mol-1, polydispersity 1.05). In this case, thin films have been prepared by spin coating toluene solutions of neat PS (2 wt%) and PS (2 wt%) + ZnO NPs (0.2 or 0.6 wt%) and no thermal annealing has been performed before the characterization of the films (vide infra). From the initial solutions, PS-based nanocomposites with ZnO NPs content of 10 and 30 wt%, respectively, are obtained, denominated PS-ZnOx with x the weight content of ZnO referred to the PS. TEM images of thin films have been obtained in bright field mode using a Philips EM 208S Transmission Electron Microscope (TEM) with an accelerating voltage of 100 kV. The thin films have been coated with carbon by using the Emitech K950X Turbo Evaporator. Carbon rods have been mounted in the vacuum system (5 10-5 mbar) between two high-current electrical terminals and heated to their evaporation temperature, allowing the deposition of a fine stream of carbon (thickness 3-4 nm) onto specimens. Then, drops of poly(acrylic acid) partial sodium salt solution (25 wt% in H2O) have been deposited on the surface of the carbon-coated films and the samples have been dried under a fume hood overnight. The dried poly(acrylic acid) drops have been


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knocked off with the use of a scalpel, so obtaining polymeric thin films backed with carbon and poly(acrylic acid). Poly(acrylic acid) has been completely dissolved by floating off the polymer/carbon/poly(acrylic acid) samples onto water (with the poly(acrylic acid) facing down the water) and, finally, the carbon-coated polymers have been fished with a 200 mesh TEM copper grid. In order to achieve a good contrast between the different BCP domains, some films have been stained with RuO4 by exposition the TEM grids to RuO4 vapors for 7 minutes at room temperature. TEM analysis has been repeated for different regions of the specimen, to check the uniformity of the morphology over the macroscopic area of the support, and for independent samples. The average lamellar thicknesses have been calculated from the TEM images using ImageJ software (National Institutes of Health, available free of charge at Web site rsb.info.nih.gov/ij/). At least 200 independent measurements have been taken at different locations of the TEM images of the samples. The measurements have been also confirmed by repeating the analysis on TEM images of independent samples. Current/voltage (I-V) measurements. Dark I-V measurements have been carried out using a Keithley 2400 source meter. The electrical behaviors have been tested on devices having a sandwich architecture ITO/polymer composite/Al. Aluminum top electrode has been evaporated under high vacuum (10-7 mbar) on the polymer composite surface film. In particular, thin films of the BCP nanocomposites loaded with ZnO nanoparticles at different concentrations (thickness of 70 nm) have been prepared by spin coating on glass slides (2.5 x 2.5 cm2) coated with two stripes of ITO with line width of 0.5 cm. In this configuration, the ITO acts as a bottom electrode. After in vacuo annealing (at 150 °C for 6 h), top electrodes have been fabricated by evaporating Al onto the polymer film surface, using a shadow mask in order to obtain 4 devices having 0.5 x 0.5 cm2 active area for each support (Scheme S1). IV measurements have been


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performed for each device of the support, repeating the measure using at least 3 independent solutions with identical composition, for a total of at least 3 x 4 = 12 IV measurements. The absence of de-wetting of the polymer films on ITO-coated glass slides has been verified on independent samples before Al deposition, acquiring optical microscopy images at different locations (data not shown). Similar IV measurements have been performed also on devices in which the BCP nanocomposites sandwiched between the two electrodes is replaced by PS nanocomposites. The reproducibility of the results obtained from different locations of the same films and from different samples and the absence of de-wetting of the polymer films exclude the possibility of shorting due to the infiltration of top electrode through the films.

Results and discussion Synthesis and characterization of ZnO nanocrystals. Semiconductor organic-capped ZnO NPs have been prepared by thermal decomposition of ZnAc2 in a high-temperature coordinating mixture of a long-chain n-hexadecylamine (HDA) and tert-butylphosphonic acid (TBPA). The coordination of surfactants to the NPs surface makes the final colloidal ZnO nanocrystals soluble in apolar media, which is advantageous for the subsequent preparation of the hybrid nanocomposite, since a common organic solvent (toluene) can be used to disperse both the block copolymer and the particles. The optically clear solution of diluted ZnO NPs has been characterized by UV-Vis absorption and photoluminescence (PL) spectroscopy (Figure 1A). The absorption spectrum (Figure 1A) shows a quite steep onset, indicating a narrow size distribution. The emission spectrum (inset of Figure 1A) has a main contribution in the visible region of the light spectrum, with a broad band centered at 584 nm that is commonly attributed to defect states in ZnO. In particular, theoretical


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and experimental studies have highlighted that emission in the yellow region of the electromagnetic spectrum (≈ 2.1 eV, nearly at 580 nm) originates from deep trap states related to oxygen interstitials of ZnO nanostructures.54 The protocol here used to prepare the ZnO colloidal sample usually results in NPs whose size can be tuned from 2 nm to 7 nm. In this regime, quantum size effects, and therefore energy gap dependence on the NP size, are expected. It is worth noting that, since ZnO is a direct band gap material, the absorption coefficient is proportional to the square root of the energy difference between the incoming photon (Eλ, energy of light with wavelength λ) and the band gap, Eg. According to the Lambert-Beer law, by plotting the square of the absorption (A2) against the energy of the incoming light, the intrinsic band gap can be calculated according to the equations 1 and 2. ∝ −

(1)

= lim →

(2)

The spectrum A2 versus hv in Figure 1B shows a clear band tailing, which, according to the literature,55 is due to non-idealities like traps. Therefore, the value of the band gap can be calculated from absorption measurements, as the intercept with the energy axis of a linear fit to the square of the absorption data in an energy interval slightly above the band gap, where the data is linear. For the synthesized sample the energy gap measured from absorption data is 3.51 eV, which is quite higher than that of the bulk ZnO (3.37 eV), and corresponds to nanoparticle size of 4.8 nm, as theoretically calculated (Viswanatha 2 approximation).55


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Figure 1. UV-Vis absorption (A) and photoluminescence (A, inset, λex = 350 nm) spectra of ZnO colloidal solution in toluene. Square of the absorption (A2) versus hv (B) and TEM image (C) of ZnO NPs. TEM image, FTIR, 1H NMR and FESEM-EDX spectra of the ZnO NPs have also been recorded in order to investigate the surface, chemistry and morphology of the nanocrystals,


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which are the crucial parameters for their distribution within the block copolymer matrix. ZnO nanocrystals exhibit a nearly spherical shape (Figure 1C) and a mean size of 4.3 ± 0.8 nm has been measured from the TEM image, in good agreement with the result obtained from the absorption measurements. The FTIR spectra in the regions 3100-2700 cm-1, 1800-1300 cm-1 and 1300-600 cm-1 of ZnO nanoparticles sample obtained in the preparative route making use of HDA and TBPA as coordinating solvent mixture are reported in Figure 2 A, B and C, respectively, in comparison with the FTIR spectra of pure HDA (Figure 2 D - F) and TBPA (Figure 2 G - I). The spectrum of ZnO exhibits a broad N-H bending band at about 1590 cm-1 (Figure 2B), as in pure HDA (Figure 2E). Notably, the N-H bending in the spectrum of ZnO (Figure 2B) results considerably broadened and slightly shifted to longer wavenumbers with respect to the position in the spectrum of the pure HDA (Figure 2E), suggesting some interaction of the amine group with the oxide surface. The spectrum of ZnO (Figure 2B) also exhibits an intense double band centered at 1470 cm-1 due to the C-H antisymmetric bending of tert-butyl groups in TBPA (Figure 2H), overlapping that of the methylene groups of HDA (Figure 2E). Moreover, in the spectrum of ZnO (Figure 2B) a double band peaking at 1390 and 1360 cm-1 assigned to the C-H symmetric bending of tert-butyl groups is also present, as in pure TBPA (Figure 2H). The broadness and the complexity of the signals in the fingerprint region below 1400 cm-1 (Figure 2C) make the unique interpretation of the peaks quite difficult. A residual POH stretching band is observable at 924 cm-1, together with a narrow band at 670 cm-1. In addition, the narrow band at 1006 cm-1 in TBPA (Figure 2I) is shifted to about 990 cm-1 in the spectrum of the ZnO NPs (Figure 2C). Therefore, the slight shift of the amine HDA bands and the remarkable changes of TBPA bands in the FTIR spectra of Figure 2 suggest that both species are well-coordinated to the


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nanoparticles and, in particular, that the phosphonate headgroup establishes good interactions with the atoms at the ZnO surface. The ratio between the amounts of the two ligands covering the surface of ZnO NPs has been quantitatively estimated by 1H NMR experiments (Figure S2 of Supporting Information). A relative amount of the two ligands HDA:TBPA covering the NPs surface equal to 1:0.6 has been estimated. The presence of TBPA as coating at ZnO surface is confirmed by FESEM-EDX analysis of the ZnO NPs (Figure S3 of Supporting Information). The analysis reveals that the relative amount of Zn, O and P are 39 ± 2 mol%, 55 ± 15 mol% and 6.0 ± 0.6 mol%, respectively (Table S1), whereas the nitrogen species are not detected due to the limitations of the FESEM-EDX technique in the detection of elements of low atomic number Z because of the absorption of Xray through the detector windows.56,57 The relative amount of oxygen species (≈ 55 mol%) includes ≈ 39 mol% belonging to ZnO, whereas the remaining ≈ 16 mol% is close to 3 times the relative amount of P species (≈ 6 mol%) belonging to TBPA capping, in agreement with the 3:1 stoichiometric ratio for O:P atoms in the phosphonate units. Assuming a spherical shape for the ZnO NPs, an average radius of ≈ 2.2-2.4 nm and density of 5.61 g/cm3 (equal to the bulk density of ZnO), a percentage of TBPA species coordinated to the surface of NPs equal to ≈ 6 mol % corresponds to a coverage density at NPs surface of ≈ 2 molecules/nm2. Since the ratio between the coordinating species HDA:TBPA is 1:0.6, the coverage density of HDA residues is ≈ 3 molecules/nm2.


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Figure 2. FTIR spectra in ATR mode in the regions 3100-2700 cm-1 (A, D, G), 1800-1300 cm-1 (B, E, H) and 1300-600 cm-1 (C, F, I) of organic-capped ZnO nanocrystals (A - C), HDA (D - F) and TBPA (G - I). The arrows indicate the common band ascribed to the asymmetric stretching of the –CH3 group. The reference bands at 2850 cm-1 and ≈ 924 cm-1 relative to HDA and TBPA ligands, respectively, are also indicated (stars).


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Computational study of aliphatic amines and phosphonic acid-capped ZnO NPs. The good conductive properties of the organic-capped ZnO nanoparticles confined in BCP domains is essential to project the development of nanoelements for the fabrication of photovoltaic devices of new conception. However, since the efficient inclusion of the NPs in the selective domain of the BCP depends on the NP surface chemistry and thus on the type of capping agents, the latter must be accurately selected, paying attention that they do not impair the ZnO NPs electronic performance of ZnO NPs. Therefore, the design of a photovoltaic devices based on conductive NPs dispersed in a nanostructured polymeric matrix requires a theoretical prediction of the electric properties of suitable modified NPs. In this context, we present a theoretical model using DFT calculations for the study of the performance of the surface-coated ZnO NPs as efficient ntype charge carrier. To this end, we focus on the interaction of ZnO non-polar surfaces with a model aliphatic amine (ethylamine, EA) and tert-butylphosphonic acid (TBPA), which are realistic model of the capping agents used in the experimental synthesis of ZnO NPs. For the sake of simplicity, our calculations have been performed on a 1:1 ratio between the two capping agents because our analysis aims at understanding the effects of the specific surface-molecule interactions on the overall ZnO electronic properties. In recent years many theoretical works have addressed the surface chemistry of ZnO: Brédas and co-workers have extensively studied the adsorption of phosphonic acids on ZnO58,59 focusing on the structural and electronic features of ZnO polar surfaces with molecular capping agents. As for the interaction of amine on ZnO, there are only few ab initio studies that have mostly focused on aromatic amines,60,61 while examples of small aliphatic amines with ZnO are scarce.62 Moreover, we found no theoretical analysis on the concurrent adsorption of amine and phosphonic acid on the ZnO NPs surfaces.


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Figure 3. Slab models for ZnO (10 1 0) (A) and (11 2 0) (B) surfaces. Surface energy (γ) values are reported in the insets. Grey and red spheres represent the zinc and oxygen atoms, respectively. Here, we address the ZnO non-polar surfaces that are known to be the most stable and the most probable to occur in ZnO nanostructures. In particular, we cleaved the (10 1 0) and (11 2 0) surface from the optimized ZnO bulk structure and the corresponding surface energy (γ) values have been computed according to Equation 3: =

∙ ∙

(3)

where Eslab corresponds to the energy of the slab model, Ebulk is the bulk energy, n represents the number of unit cells per slab model and S is the surface area. The two slabs models are reported in Figure 3. The obtained values of surface energy γ = 1.19 J m-2 and 1.33 J m-2 for (10


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1 0) and (11 2 0) surfaces are in good agreement with those of the literature on the stability of

ZnO apolar surfaces.

63

We have selected to study the structural and electronic modifications of

the most stable ZnO surface (10 1 0). This surface should be the most exposed surface of a ZnO NP when interacting with EA and TBPA (Figure 4). The adsorption energies (Eads) are calculated according to Equation 4: =

!"#$%&'(%&

− #$%&'(%& −

!

(4)

The total adsorption energy for both molecules is -4.58 eV (Figure 4), which is consistent with the sum of individual absorption energies reported for similar surface ligands in recent literature, with values ≈ -1 eV for amines64 and ≈ -2.5 eV for phosphonic acids58. The total adsorption energy confirms the convenient concurrent anchoring of both the capping agents to the NP surface. Other details on the ZnO surface and the adsorption of EA ant TBPA are reported in Table S2.


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Figure 4. EA and TBPA molecule adsorbed on the ZnO (10 1 0) surface. Perspective (A) and upper view (B) of the unit cell. Colour code: Zn (grey), O (red), C (green), N (blue), H (light pink), P (purple). The electronic structure has been characterized with atom- and angular momentum- projected density of states (PDOS). Figure 5 shows the PDOS plots of the ZnO bulk, the clean (10 1 0) surface and the system with EA and TBPA molecules adsorbed on the (10 1 0) surface, computed at the PBE level of theory. While the PBE band gap values are not quantitatively comparable with the experimental ones, several studies have proven that also for transition metal oxides the band gap centre and the nature of the band edges are well described with standard PBE.65 We observe only a negligible variation of the ZnO band gap when EA and TBPA molecules are adsorbed on the surface (Figure 5). Our results demonstrate that the adsorbed


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capping agents do not alter the main characters of both the valence and conduction band edges (Figure 5). Beside the band gap, it is also important to assess the absolute position of the band edges in order to design effective multicomponent photovoltaic devices. To this end, we have computed the work-function for the ZnO surface slab before and after the adsorption of the capping agents. The projection of electrostatic potential along the z-axis of the surface slabs for the clean (10 1 0) surface and for the same surface modified by EA and TBPA absorption is shown in Figure 6. The modification of the work-function contains the complete electronic response of the slab to the absorption of the capping agents.


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Figure 5. Projected density of states (PDOS) plots at the PBE level of theory for ZnO bulk, clean (10 1 0) surface and EA and TBPA adsorbed on (10 1 0) surface.


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Figure 6. Projection of electrostatic potential along the z-axis of the surface slabs for clean ZnO (10 1 0) surface (left) and for the (10 1 0) surface modified by EA and TBPA absorption (right). The Fermi level is also reported (EFermi) and the work-function is computed

as the difference between this level and the vacuum level (Evac).


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The variation of the computed work-function due to the molecular absorption ∆Φ on ZnO (10 1 0) surface is -0.38 eV, i.e. the work-function is slightly lowered by the adsorption of the EA and TBPA. This small but significant variation is in agreement with previous studies on ZnO58,59,65 and allows for an easier extraction of an electron from the capped NP. From the work-function position and the experimental band gap value, we can assess the absolute positions of the band edges, the conduction band minimum (CBM) and the valence band maximum (VBM).65 The values of CBM, VBM, band gap (BG) and band gap centre (BGC) for the clean (10 1 0) surface and for the same surface modified by EA and TBPA absorption are shown in Figure 7.

Figure 7. Absolute band edges positions at PBE level for clean ZnO (10 1 0) surface and for the (10 1 0) surface modified by EA and TBPA absorption. The positions are defined by the Valence Band Maximum (VBM), Conduction Band Minimum (CBM) and Band Gap Centre (BGC). The positions are confronted with the normal hydrogen potential scale (NHE). Our results show a ≈ 0.4 energy shift for the CBM and VBM upon adsorption of the capping agents. For applications in solar energy conversion devices, ZnO NPs are the n-type component (the electron acceptor) and their CBM should be lower in energy than the CBM (LUMO) of the


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p-type inorganic (organic) component (the electron donor). Thus, our results show that for a convenient photocurrent generation, the capped ZnO NPs should be coupled to electron-donor components with excited states that are higher in energy than those usually employed with clean ZnO NPs. Fine-tuning of the band edge position for capped ZnO NPs can be achieved by selecting a specific relative ratio between the capping agents. Preparation and characterization of hybrid nanocomposites. We have used a polystyreneb-poly(methyl methacrylate) (PS-b-PMMA) block copolymer with a volume fraction of the PS blocks equal to 0.52 in order to obtain a lamellar phase-separated morphology. Thin films of pure BCP and of the BCP-based nanocomposites have been prepared by spin coating toluene solutions containing 2 wt% PS-b-PMMA and different concentrations of ZnO NPs (0, 0.1 and 0.2 wt%) on ITO supports. From the initial solutions, BCP-based nanocomposites with contents of ZnO NPs relative to the sole PS blocks of 0, 10 and 19 wt%, respectively, are obtained. The nanocomposites are indicated with the symbol BCP-0, BCP-ZnO10 and BCP-ZnO19, respectively. The TEM image of a thin film (≈ 70 nm thick) of the neat BCP annealed at 150 °C for 6 h in vacuum and stained with RuO4 is shown in Figure 8A. The dark regions correspond to the stained PS lamellar microdomains, whereas the lighter regions to the PMMA domains. The image (Figure 8A) shows a disordered lamellar morphology with lamellar domains oriented perpendicular to the substrate. The vertical orientation of lamellae is in agreement with previous results44,66 demonstrating that the use of ITO support combined with thermal annealing treatments facilitates the vertically oriented morphology of the lamellar BCP domains. The average lamellar thicknesses are ≈ 14 ± 2 and ≈ 18 ± 2 nm for PS and PMMA, respectively. It is


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worth noting that the TEM image of PS-b-PMMA thin film obtained without performing the RuO4 staining procedure (Figure S4A) shows no contrast between PS and PMMA nanodomains. The TEM images of the films of the nanocomposite BCP-ZnO10 without staining and stained with RuO4 are reported in Figure 8B and C, respectively. The images of the nanocomposites (Figure 8B, C) also shows a disordered lamellar morphology with the vertical orientation of the lamellar domains, and a negligible increase of the average lamellar thickness is observed (≈ 16 ± 3 nm for PS and ≈ 20 ± 3 nm for PMMA). In the case of the image of Figure 8B obtained without performing any staining procedure, the dark regions correspond to the microdomains containing ZnO NPs. The achieved high contrast between the two different BCP lamellar nanodomains (Figure 8B) indicates that the ZnO NPs are not uniformly dispersed in the whole BCP, but they are selectively included only in one of the two blocks of the BCP (PS or PMMA), due to the chemical affinity of the surface-coated ZnO NPs for one of the blocks. This is confirmed by the absence of contrast between the two polymeric blocks in the TEM image of the neat BCP, when the RuO4 staining is not performed (Figure S3A). Since RuO4 selectively marks the PS domains (Figure 8A), the TEM image of Figure 8C of the nanocomposite film stained with RuO4 shows dark stained PS lamellar domains alternating with bright not-stained PMMA lamellae. This indicates that the ZnO NPs are selectively included only in the PS domains, thanks to the designed chemical modification of the nanoparticles surface, and practically all PS lamellae are filled with ZnO nanoparticles. The same result has been obtained by increasing the concentration of ZnO NPs (Figure S4B). It is worth noting that the TEM images collected without performing any thermal annealing of the nanocomposite films (Figure S5) show only the presence of large ZnO aggregates with no


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evidence of BCP phase separation. This indicates that the thermal annealing at a temperature above the BCP glass transition temperature (150 °C) triggers the migration of the ZnO NPs in the PS lamellar domains and simultaneously lead to the formation of the desired vertical morphology of the lamellar BCP domains (Figure 8 and S4B).


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Figure 8. Bright-field TEM images of thin films (≈ 70 nm thick) annealed at 150 °C in vacuum for 6 h of the neat PS-b-PMMA (BCP-0) stained with RuO4 (A) and of the PS-b-PMMA/ZnO nanocomposite BCP-ZnO10 (B,C) without staining (B) and stained with RuO4 (C).


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The role of BCP and thermal annealing in directing the distribution of NPs is confirmed by polarized optical microscopy (POM) and TEM image of films of a nanocomposite obtained by dispersion of ZnO NPs into a PS homopolymer matrix (Figure 9). The POM image of the film of PS-ZnO10 annealed at 150 °C of Figure 9A shows islands of polymer with size of 1-2 µm indicating that the thermal annealing results in the destruction of the thin films due to the occurrence of de-wetting of the polymer film on the ITO support. A TEM image of the nonannealed film of PS/ZnO nanocomposite of Figure 9B shows, instead, only large ZnO aggregates. These results demonstrate that the designed NPs are selectively confined in phase-separated lamellar domains of the nanostructured BCP, resulting in the formation of nanocomposites characterized by a precise control of the position of n-semiconductor ZnO NPs, and that the selfassembly of block copolymer during the thermal annealing procedure guides the location of the NPs and avoid the aggregation of NPs.


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Figure 9. Polarized optical microscope (POM) (A) and TEM (B) images of a PS-ZnO10 nanocomposite prepared by spin coating at room temperature on ITO support a toluene solution of PS (2 wt%) and ZnO (0.2 wt%). The thin film in A has been annealed at 150 °C for 6 h under vacuum, whereas the film in B has not been annealed. I-V measurements. The electrical characteristics of the designed hybrid nanocomposites based on the BCP (Figure 8B,C and S4 B) and PS (Figure 9B) matrices have been evaluated by current/voltage (I-V) dark measurements. The electrical behaviors have been tested on a device in a sandwich architecture ITO/BCP-ZnO (or PS-ZnO) film/Al and the results are reported in Figure 10.


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Figure 10. I-V measurements of ITO/BCP-ZnO/Al (A) and ITO/PS-ZnO/Al (B) devices. For the device A, films of neat BCP (BCP-0) and of BCP-based nanocomposites BCP- ZnO19, BCPZnO24, BCP-ZnO29 and BCP-ZnO34, containing a 19, 24, 29 and 34 wt% of ZnO NPs, respectively, relative to the sole PS block, have been tested. For the device B, neat PS (PS-0) and nanocomposites PS-ZnO10 and PS-ZnO30 containing 10 and 30 wt% of ZnO, respectively, have been tested. The relative error in density current is in the range 10-12%. I-V measurements have been cyclically performed on the BCP-based nanocomposites (Figure 10A). I-V data show almost null current density for ZnO NPs content lower than a critical value of ≈ 29 wt% (curves a-c of Figure 10A). For ZnO NPs concentrations of 29 and 34 wt%, the current density shows a linear increase with the voltage (curves d, e of Figure 10A). From the slope of the IV curves d, e of Figure 10A, (0.282 and 0.222 AV-1 cm-2, respectively) a


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conductivity of ≈ 7 10-6 S cm-1 can be roughly estimated, considering a global thickness of ≈ 300 nm for the device, where 300 nm is the sum of the thickness of the top and bottom electrodes (≈ 130 nm) and of the intermediate nanocomposite layer (≈ 70 nm). This indicates that an Ohm-like conductive regime of the BCP-based nanocomposites is established for NPs concentrations higher than a critical value c* comprised between 24 and 29 wt%. Only above this critical concentration the NPs included in the PS domains are able to form a continuous percolative path of charge carriers, which facilitates the current conduction. It is worth noting that the corresponding resistivity of ≈ 1.4 106 Ω cm (= (7 10-6 S cm-1)-1) is one-two orders of magnitude higher than bulk resistivity of undoped ZnO crystals (5 104 - 3 105 Ω cm),67 in agreement with a conduction where charge carriers move along a percolative path of NPs, rather than in a continuum matrix. Our data also indicate that the IV slope for the nanocomposites with 29 wt% NPs (curve d of Figure 10A) is higher than that of the nanocomposite with 34 wt% NPs (curve e of Figure 10A), indicating that the conductive ability tends to decrease for too high loadings of the NPs in the BCP domains. This is probably due to a damage in the lamellar nanostructured morphology of the BCP at high NPs loadings with consequent interruption of the percolative conductive path between the electrodes . These data are compared with the electrical properties of the PS-ZnO nanocomposites (Figure 10B) containing amounts of ZnO NPs (10 and 30 wt%) lower and higher than the threshold concentration c* able to guarantee an Ohm-like conductive regime for the BCP-based nanocomposites (24-29 wt%). It is apparent that the current density is negligibly small even at high loadings of ZnO NPs. These results confirm that the key for the obtainment of conductive properties resides in the formation of a continuous path of charge carriers, achieved through the confinement of the semi-conductive nanoparticles in the vertical BCP nanodomains (Figure 8B,


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C and S4B). Even a small concentration of NPs at nanoscale is able to produce a measurable effect for conductivity, providing that the nano-domains of the host matrix does not become too crowded. Conclusions ZnO nanoparticles with mean diameter of ≈ 4.5 nm, coated at the surface with a mixture of nhexadecylamine (HDA) and tert-butylphosphonic acid (TBPA), have been synthesized. A simple BCP-based method to control the spatial arrangement of the ZnO NPs on solid supports (ITOcoated glass slides) has been set-up, obtaining thin films of hybrid nanocomposites with promising electrical properties. In particular, thin films characterized by selective inclusion of the surface-modified ZnO NPs in the lamellar PS nanodomains have been successfully prepared by using self-assembly of a PS-b-PMMA block copolymer combined with thermal annealing treatments. The thermal treatment allows obtaining in one step the vertical orientation of the lamellar BCP nanodomains and the migration of the NPs to the PS domains, which is favored by the coating of the ZnO surface with HDA and TBPA molecules. The electric properties of the hybrid nanocomposites, tested by current-voltage measurements, confirm the presence of continuous path of charge carriers in the BCP films, when the ZnO NPs content is above a threshold concentration. The experimental results have allowed to set-up and validate a theoretical protocol to study heterogeneous interfaces. In particular, we have investigated the interaction of the ZnO most stable non-polar surface with prototypical capping agents, ethylamine (EA) and TBPA. We have tested the PBE density functional for describing structural and electronic features of clean and capped ZnO NP surface. The electronic band gap is not significantly affected by the adsorption of EA and TBPA, while the work-function variation drives a slight upward energy shift for the


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valence and conduction band edges of the capped ZnO NPs. This is an important feature to take into account in selecting the appropriate p-type component for the final device. These results confirm that the use of block copolymers as templates for the assembly of semiconductor nanoparticles can show promising results for applications in nanoelectronics. Furthermore, the presented computational study paves the route to more advanced simulations for considering also the effects of the chemical environment (solvent, polymer matrices) and to address complex charge transfer processes at the interfaces.

Supporting Information. Scheme of the electrical devices. Kubelka-Munk function versus photon energy plot for drop casted films of ZnO colloidal solution. 1H NMR spectrum of ZnO NPs. Quantitative analysis by FESEM-EDX measurements of the ZnO nanoparticles. Details the computational method and of results of calculations. TEM images of the PS-b-PMMA/ZnO nanocomposite at different ZnO NP concentrations and/or without thermal annealing. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Present Address §

ENEA Italian National Agency for New Technologies, Energy and Sustainable Development,

SSPT-PROMAS-NANO Department, P.zza E. Fermi 1, I-80055 Portici, Napoli, Italy. ACKNOWLEDGMENT


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A. M., C. D. R. and F. A. acknowledge funding from the University of Naples Federico II (project Blo-app-sun of Program Ricerca di Ateneo). A. M. R., A. B. M. G. and M. P. acknowledge funding from the Italian Ministry of University and Research (MIUR) under grant PRIN 2012NB3KLK. The computing resources and the related technical support used for this work have been provided by CRESCO/ENEAGRID High Performance Computing infrastructure and its staff;68 CRESCO/ENEAGRID High Performance Computing infrastructure is funded by ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development

and

by

Italian

and

European

research

programs,

see

http://www.cresco.enea.it/english for information. The authors thank Dr. Antonio Carella of the University of Naples Federico II for his assistance in performing NMR experiments and for useful discussions.

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For Table of Contents use only

Confinement of Semiconductor ZnO Nanoparticles in Block-Copolymer Nanostructure Anna Malafronte,* Finizia Auriemma, Rocco Di Girolamo, Carmen Sasso, Claudia Diletto, A. Evelyn Di Mauro, Elisabetta Fanizza, Pasquale Morvillo, Antonio M. Rodriguez, Ana B. MuñozGarcia, Michele Pavone, Claudio De Rosa.


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Confinement of Semiconductor ZnO Nanoparticles ... - ACS Publications

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