Energy and charge transfer in nanoscale hybrid materials.

Macromolecular Rapid Communications

Feature Article

Energy and Charge Transfer in Nanoscale Hybrid Materials Thomas Basché,* Anne Bottin, Chen Li, Klaus Müllen, Jeong-Hee Kim, Byeong-Hyeok Sohn, Prem Prabhakaran, Kwang-Sup Lee

Hybrid materials composed of colloidal semiconductor quantum dots and π-conjugated organic molecules and polymers have attracted continuous interest in recent years, because they may find applications in bio-sensing, photodetection, and photovoltaics. Fundamental processes occurring in these nanohybrids are light absorption and emission as well as energy and/or charge transfer between the components. For future applications it is mandatory to understand, control, and optimize the wide parameter space with respect to chemical assembly and the desired photophysical properties. Accordingly, different approaches to tackle this issue are described here. Simple organic dye molecules (Dye)/quantum dot (QD) conjugates are studied with stationary and time-resolved spectroscopy to address the dynamics of energy and ultra-fast charge transfer. Micellar as well as lamellar nanostructures derived from diblock copolymers are employed to fine-tune the energy transfer efficiency of QD donor/dye acceptor couples. Finally, the transport of charges through organic components coupled to the quantum dot surface is discussed with an emphasis on functional devices.

1. Introduction Hybrid materials composed of colloidal inorganic semiconductor quantum dots (QDs) and π-conjugated organic molecules are being increasingly considered for optoelectronic Prof. T. Basché, A. Bottin Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10–14 , 55099 Mainz, Germany E-mail: [email protected] Dr. C. Li, Prof. K. Müllen Max Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany J.-H. Kim, Prof. B.-H. Sohn Department of Chemistry, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 151–747, South Korea Dr. P. Prabhakaran, Prof. K.-S. Lee Department of Advanced Materials, Hannam University, Daejeon 305–811, South Korea

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and sensing applications.[1,2] QDs have received broad and multidisciplinary research interest, which ranges from the elucidation of their fundamental photophysical properties such as blinking[3] and exciton relaxation dynamics[4] to applications such as photovoltaic[5,6] and light-emitting devices,[7] lasers,[8] and fluorescent labels for bioimaging.[2,9] What makes QDs particularly appealing are their size-dependent optical properties leading, e.g., to the well-known tunability of their absorption and emission spectra.[10] Moreover, the gram scale synthesis of QDs is well established[11,12] and QDs are available for a broad variety of semiconductor materials spanning the spectral region from the ultraviolet to the near infrared and beyond.[13] Recently, it turned out that a rational combination of the favorable properties of the QDs with the rich variety of suitable π-conjugated organic and polymeric materials gives rise to hybrid nano-composites that are potentially useful for a number of purposes.[14,15] With respect to sensing, conjugates of QDs and organic dye

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DOI: 10.1002/marc.201400738


Energy and Charge Transfer in Nanoscale Hybrid Materials

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molecules (Dye) have been assembled to provide electronic coupling between the two moieties. In a common design for fluorescence resonance energy transfer (FRET), electronic excitation energy is transferred form a QD donor to a Dye acceptor.[16] A modulation of the FRET efficiency, e.g., by analyte binding, typically provides the basis of the sensing capability. Hybrids of QDs and functional π-electron systems play also an important role in contemporary photovoltaic research.[17–19] Besides their size tunability and photochemical stability, a potential advantage of QDs over organic materials is charge carrier multiplication,[20] which is not possible in organic dyes and leads to the formation of multi-excitons in QDs. These unique optical properties of QDs can potentially redefine the efficiencies of photodetectors and photovoltaic devices.[21] In applications envisioned, electronic coupling between the QDs and their π–conjugated electronic counterpart is of prime importance. Manifestations of this coupling are energy and/or charge transfer between the components in the nanohybrid. Which of the processes prevails depends on a number of factors such as the spatial distance between the components, the energetics, and the absence/presence of barriers. For any application it is of utmost importance to understand the mechanism and dynamics of these processes. In our presentation, we focus on the work done by the contributing groups rather than giving an exhaustive overview of the growing and promising field of nanohybrids. In the first part of this article we will present investigations of Dye/QD hybrids which served as simple model systems to address energy and electron transfer. Rylene dyes furnished with carboxylic groups gave rise to extraordinarily stable complexes with II-VI core/shell QDs. By employing stationary and time-resolved spectroscopy, energy transfer (FRET) and electron transfer have been studied thoroughly in various assemblies. FRET in the meaning of fluorescence resonance energy transfer will be used throughout this article to denote singletsinglet excitation energy transfer. This notation avoids an interpretation in terms of Förster theory, since for some of the cases discussed it is not clear yet whether electronic excitation energy transfer can be fully described in terms of dipole-dipole coupling.[22] In the simple Dye/QD model systems, both the mechanism and the time scales of energy and electron transfer have been addressed providing fundamental insight into these processes. The observation of ultrafast charge separation revealed that charge carrier multiplication in QDs indeed can compete with fast Auger recombination. An important parameter in FRET applications is the distance between donor(s) and acceptor(s) which is typically on the nanometer scale. In the second part of this paper it will be shown that diblock copolymers, composed of two chemically different polymers, spontaneously self-assemble into

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Thomas Basché received his doctoral degree in 1990 from the Ludwig-Maximilians-Universität in München. From 1991-92 he was an IBM World Trade Visiting Scientist at the Almaden Research Center, San Jose, working with Nobel Laureate W. E. Moerner in the burgeoning new field of single molecule spectroscopy. After his return to München he finished his habilitation in physical chemistry in 1995. Since 1997 he holds a chair in Physical Chemistry at the Johannes GutenbergUniversität, Mainz. His current research interests include single molecule studies of electronic coupling in aggregates, correlative microscopy, and preparation and spectroscopy of semiconductor nanoparticles. Klaus Müllen received his PhD in 1972 at the University of Basel in Switzerland. He was a postdoctoral fellow at ETH Zürich where he obtained his habilitation in 1977. After working as a Professor of Organic Chemistry at the Universities of Cologne and Mainz, he became the Director of the Department of Synthetic Chemistry at the Max Planck Institute for Polymer Research in Mainz (Germany) in 1989. His research interests include macromolecular and supramolecular chemistry as well as the design, synthesis, and characterization of novel organic semiconductors and graphenes for electronic and optoelectronic applications. Byeong-Hyeok Sohn received a PhD degree in Polymer Science and Technology from Massachusetts Institute of Technology, USA, in 1996. After postdoctoral work at M.I.T. and at the University of Wisconsin–Madison, USA, he joined Pohang University of Science of Technology, Korea, in 1998. He worked as a faculty member in the Department of Materials Science and Engineering. In 2004, he moved to Seoul National University, and since then has worked in the Department of Chemistry. His main research interest is polymeric and soft nanomaterials for nanotechnology applications. Kwang-Sup Lee is Professor of the Department of Advanced Materials at Hannam University, Korea. He received his PhD degree in polymer science from the Freiburg University, Germany, in 1984. He was a postdoctoral fellow at MaxPlanck-Institute for Polymer Research from 1985 to 1986 and a visiting professor at the Naval Research Laboratory, USA, in 1998. His research interests lie in the field of photofunctional polymers, organic-inorganic hybrid materials, and their application in devices. Prof. Lee is a Fellow of SPIE and EM Academy.

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Scheme 1. Synthesis of PDI 1 i) KOH, 2-propanol, H2O, reflux, 12 h, 24%; ii) β-glutamic acid, NMP, 130 °C, 24 h, 53%.

periodic nanostructures[23] which allow precise control of QD-Dye distances. Micellar as well as lamellar nanostructures have been considered to control the FRET efficiency of QD donor/Dye acceptor couples. The additional presence of metal nanoparticles was utilized to further manipulate the FRET efficiency or to enhance the photoluminescence yield of emitters which were decoupled by micellar nanostructures. The transport of charges through organic components associated with QDs in devices is a central issue of the third section of this article.[24,25] First, hybrids between QDs and carbon nanotubes or polymeric nanowires are considered. In addition to their photophysical characterization, the QD-nanotube hybrids were used in photodetector devices and crucial parameters for this application are discussed. QDs decorated with a photoresponsive ligand shell, which allows for creating patterns in QD films and for controlling the extraction and transport of charges, were employed in electroluminescent devices. The paper closes with studies of charge transport through functional ligands directly attached to the QD surface.

thiols, and oxides, which have different affinities to QD surfaces. Carboxylic groups have recently attracted attention as anchor groups, and it was shown that they firmly bind organic chromophores to the surfaces of metal oxide semiconductor nanoparticles.[26] Such constructs have been used as light-harvesting systems, which can convert light to current in dye-sensitized solar cells. Carboxylic groups have also been used as surface ligands of colloidal QDs. As an example, CdSe/CdS core-shell nanocrystals covered with dendron ligands carrying two carboxylic acids have been found to be significantly more luminescent and photostable against UV light than particles which have been stabilized by thiolated dendron ligands.[27] In the following, we briefly summarize the preparation of hybrids from core/ shell QDs and rylene dyes furnished with carboxylic groups which were realized over the last years. While the preparation of the QD component basically followed standard procedures described in the literature,[28,29] we will concentrate here on the discussion of the synthesis of rylene diimide dyes and the subsequent complex formation with QDs. Numerous other approaches for the preparation of Dye/QD complexes have been reported in the literature.[30–37]

2. Hybrids of Colloidal Inorganic Nanoparticles and π-Conjugated Organic Molecules

2.1.1.1. Functionalized Rylene Dyes: The synthesis of rylene diimide dyes has become a versatile and facile process due to the various, readily available building blocks.[38] While selecting an appropriate synthetic method, a compromise between solubility and reactivity must be made.[39] Generally, imidization of rylene anhydrides offers the possibility for different imide functions and is the easiest way to rylene dyes bearing carboxylic acid groups.[40,41] In order to tailor the photo-physical properties, including absorption and emission wavelengths, perylene diimide (PDI 1), and its higher homologous terrylene diimide (TDI 2 and 3), are chosen as suitable candidates to form Dye/QD hybrids (Scheme 1, 2). PDI 1 and TDI 2 possess the same number of acid groups, providing them with similar binding affinity to colloidal QD nanocrystals, while TDI 3 acts differently since it is furnished with two carboxylic groups. Depending

2.1. Dye/Quantum Dot Model Systems for Energy and Electron Transfer Studies 2.1.1. Preparation of Dye/QD Complexes The various steps involved in the preparation of hybrids from colloidal semiconductor quantum dots and organic dye molecules comprise the synthesis of each of the components as well as their subsequent conjugation. In order to realize a stable and reproducible model system of Dye/ QD hybrids for energy and electron transfer studies, a suitable linkage between the dye molecule and QD is of utmost importance.[16] Typical anchor groups include amines,

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Scheme 2. Synthesis of TDI 2: 2-i) Br2, acetic acid, iodine, r.t., quantitative; 2-ii) bis(pinacolato)diboron, potassium acetate, [PdCl2(dppf)2], toluene, 60 °C, 48 h, 48%; 2-iii) 4-bromo-1,8-naphthalic anhydride, [Pd(PPh3)4], K2CO3/H2O, ethanol, toluene, 80 °C, 15 h, 80%; 2-iv) β-glutamic acid, zinc acetate, NMP, without purification; 2-v) ethanolamine, K2CO3, 160 °C, 3 h, 50%. Synthesis of TDI 3: 3-i) sodium tert-butoxide, diglyme, DBN, 130 °C, 3 h, 36%; 3-ii) Br2, chloroform, reflux, 12 h, 75%; 3-iii) tert-octylphenol, K2CO3, NMP, 80 °C, 8 h, 86%; 3-iv) KOH, 2-methyl2-butanol, 80 °C, 3 h, without purification; iii) β-glutamic acid, NMP, 12%.

on the particle size, a given QD particle can bind a number of PDI 1 or TDI 2 molecules on the surface,[40,42] whereas a few QD particles can integrate with TDI 3 ‘bridges’ to form QD oligomers, such as dimers or trimers.[41] In the case of PDI 1 (Scheme 1), commercially available PDI 1-a is used as the starting compound, which is heated with a base to yield 1-b. With an asymmetric structure containing one anhydride group, 1-b is reacted with β-glutamic acid under imidization conditions to generate the designed PDI 1. Both TDIs 2 and 3 are obtained via perylene monoimide (PMI) 4 through two different synthetic pathways (Scheme 2).[43] Treating PMI 4 with bromine and catalytic amounts of iodine leads to the monobromo 2-a almost quantitatively. Under Miyaura borylation conditions through the reaction with bis(pinacolato)diboron in the presence of palladium catalyst, 2-a is converted to the important intermediate 2-b, which further reacts with 4-bromo-1,8-naphthalic anhydride to give 2-c with reasonable solubility. Imidization of 2-c with β-glutamic acid results in 2-d, which is then cyclo-dehydrogenated with a base to give the target TDI 2.


Alternatively, by heating N-(2,6-diisopropylphenyl)1,8-naphthalene monoimide (NMI) in a “one-pot” fashion with PMI 4 in the presence of a strong base, here, sodium tert-butoxide in a solvent mixture of diglyme and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), TDI 3-a is obtained. In order to favor the hetero-coupling and suppress the homo-coupling of PMI 4 to the undesired quaterrylene diimide (QDI), which cannot easily be separated, a four-fold excess of NMI is required. After bromination and subsequent phenoxylation with 4-t-octylphenol, TDI 3-a is transformed to the soluble TDI 3-c, which then undergoes a saponification to give dianhydride 3-d used for the next reaction without further purification. Eventually, upon heating the mixture of the crude 3-d and β-glutamic acid in N-methyl-2-pyrrolidon (NMP), the double-functionalized TDI 3 is formed. 2.1.1.2. Dye/Quantum Dot Complexes: CdSe as well as core/ shell CdSe/CdS, or CdSe/CdS/ZnS colloidal nanoparticles, served as the QD component in Dye/QD conjugates. The core-shell particles used in the studies discussed here represent type-I structures, in which the photo-generated


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electrons and holes are largely confined in the core. Complexes between PDI 1 and TDI 2 and CdSe/CdS/ZnS QDs were prepared by a simple procedure described by Ren et al.[40] After formation in solution, the Dye/QD complexes were precipitated, washed, and re-dispersed in chloroform. This specific procedure yielded samples in which, after re-dispersion of the complexes in chloroform, the amount of uncoupled PDI 1 was negligible. These studies demonstrated that rylene dyes furnished with dicarboxylate anchors provide a versatile route for assembling extraordinarily stable Dye/QD complexes. Complexes between QDs and methylviologen (MV2+) were prepared by simply adding a defined quantity of MV2+ dissolved in methanol to a certain amount of QDs dissolved in chloroform.[44] A critical issue in the investigation of Dye/QD conjugates is the exact knowledge of the Dye/QD ratio, since typically the concentration of the QD solution is not known very accurately.[45] As a first guess, it is commonly assumed that the Figure 1. a) Distribution of QDs in the tube after cross-linking with TDI 3 and density graconcentration of the core/shell particles dient ultracentrifugation. TEM images of the QD monomers (b), TDI linked QD dimers (c), and trimers (d) taken from the corresponding bands in (a). Adapted with permission.[41] (i.e. CdSe/CdS/ZnS) is the same as for Copyright 2011, American Chemical Society. the cores (CdSe) which is prone to error within an order of magnitude. As will convincing evidence that the QDs were linked by the TDI be shown in the next section, transient absorption spec3 bridge.[41] troscopy has been successfully used to accurately determine the actual Dye/QD ratio in PDI 1/QD complexes.[42] It is important to note that the presence of a passivating 2.1.2. Steady State and Time-Resolved Spectroscopy shell in the core/shell structures mediates the impact for the Study of Energy and Electron Transfer of ligands on the photophysics of the QDs. The absence or presence of such a shell largely determines whether In a collaboration of the Wachtveitl and Basché groups, besides energy transfer, electron transfer between the Dye/QD complexes have been investigated by a number of QD and attached dye molecules takes place or not. Charge different spectroscopic techniques to address energy and transfer and transport in nanohybrids will also be considelectron transfer in such systems. Complexes from PDI 1 ered in Section 2.3. and CdSe/CdS/ZnS QDs are the first system to be addressed Xu et al.[41] employed the doubly functionalized TDI 3 here. By increasing the amount of dye bound to the QDs, the QD emission was successively quenched while the (Scheme 2) to prepare QD oligomers. In this work density dye emission increased (Figure 2). Such a behavior clearly gradient ultracentrifugation appeared as an appropriate indicated efficient energy transfer from QD to Dye, which tool to separate the various oligomers (dimers, trimers, in the particular case of PDI 1/QD complexes could be and higher oligomers) formed in solution. In Figure 1a satisfactorily modeled within the frame work of Förster distinctive bands can be recognized after ultracentrifutheory.[40] While these results from steady state spectrosgation of the reaction mixture. TEM images of samples taken from these bands showed that monomers, dimers, copy gave first insights into the energy transfer process in and trimers could be separated to a significant extent. The the complexes, deeper knowledge was gained by transient corresponding images of particles from the three bands absorption spectroscopy by probing the energy transfer on marked in Figure 1a are shown in Figure 1b–d. High-resits intrinsic time scale. A pump-probe set-up with a timeolution TEM as well as spectroscopic investigations gave resolution of 70–100 fs has been used for the transient

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Figure 2. Fluorescence spectra of PDI 1/CdSe/CdS/ZnS-QD complexes in chloroform as a function of the Dye/QD ratio given in the inset.

absorption (TA) measurements.[42] If not stated otherwise, a low excitation energy was chosen to ensure that after light absorption mainly singly excited species were generated. TA spectra of pure PDI 1, QD, and the PDI 1/QD complexes are displayed in Figure 3. Dye/QD complexes with different Dye:QD ratios have been investigated, whereby the relative amounts of PDI-1 within the samples (s1, s2, s3) had been determined by absorption spectroscopy to be 1:2:5.4 (see Table 1). The TA spectrum of pure PDI 1 (Figure 3a) exhibited a long-lived negative absorption at 500–650 nm, which was attributed to the ground state bleach and the stimulated emission as well as a positive contribution above 650 nm, which was assigned to excited state absorption of the dye.[42] The TA signals of the QDs (Figure 3b) were interpreted in terms of Coulomb interactions of the photo-induced charge carriers and state filling.[42] When comparing the TA spectra of QD and PDI 1/QD (s3) presented in Figure 3c, it was seen that in the spectra of the coupled system additional negative absorption changes emerged at 575–675 nm and positive absorption changes at wavelengths >675 nm. These contributions, which were also observed for samples s1 and s2, were due to the electronically excited PDI. Since at the excitation wavelength of 388 nm, pure PDI did not give rise to a signal, it was evident that energy transfer from QD promoted PDI into its electronically excited state. In addition, the energy transfer also led to a faster decay of the QD signal in the complexes. Importantly, the data analysis relied on the TA dynamics of the electronically excited PDI 1 acceptor induced by the energy transfer process and was assumed to be insensitive to QD intrinsic relaxation pathways. It was concluded that focusing on the acceptor appears to be the most reliable way to investigate the FRET dynamics in Dye/QD complexes.[42]


Figure 3. Transient absorption spectra. a) Pure PDI 1 in methanol after excitation at 550 nm. b) Pure CdSe/CdS/ZnS-QDs and c) PDI 1/QD complexes (sample s3) in chloroform after excitation at 388 nm. Blue color represents negative and red color positive absorption changes. Reproduced with permission.[42] Copyright 2014, American Chemical Society.

In Table 1, the FRET rates as determined from the TA analysis are given, and it was found that with increasing dye coverage of the QDs the rate increased. The energy transfer was quite efficient leading to time constants in the range of 100 ps. The rates, however, did not increase linearly with the relative PDI amount. Obviously, and as was shown in earlier investigations,[40,46–49] the number of acceptors per QD has to be approximated by an appropriate distribution function. Taking into account a Poisson distribution, straightforward analysis gave access Table 1. Relative PDI amount (PDIrel) obtained from the fit of the absorption spectra of s1,s2 and s3; amplitude weighted average of the FRET rate (); absolute molar ratios between PDI and QD (xabs) of s1,s2 and s3; mean value of PDI molecules per QD (QD).

PDIrel

/109 s−1

xabs/QD

QD

1

5.5

0.85

1.5

s2

2

7.7

1.7

2.1

s3

5.4

17

4.6

4.6

sample s1


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to the absolute molar ratios between PDI and QD (xabs, see Table 1). At these ratios, the average number of bound PDI molecules in the PDI-1/QD complexes were 1.5, 2.1, and 4.6 (Table 1). These numbers were found to be larger than the nominal values used for sample preparation, indicating that the QD concentration of the starting solution was lower than assumed. Being in possession of the energy transfer rate, Dworak et al.[42] used a Fermis Golden rule type expression to calculate the magnitude of the electronic coupling in the weak coupling limit. For PDI 1/QD a value of 3.1 cm−1 was obtained in good agreement with very recent quantum chemical calculations for similar systems.[50] Earlier analysis of the PDI 1/QD system had indicated that Förster theory (weak coupling limit) gives a satisfactory description of the FRET process.[40] The advantage of the novel TA analysis described here is that it did not invoke parameters intrinsic to Förster theory as, e.g., the Förster radius, which was needed in former approaches.[40,45] Moreover, the TA approach did not need “a priori” information about further details of the energy transfer mechanism, and Dworak et al.[42] concluded that it should also work in cases, where the dipole approximation does not hold; i.e., under conditions were a description in terms of Förster theory is not appropriate. The electronic coupling between CdSe/CdS/ZnS QDs and PDI in the PDI 1/QD complexes was clearly dominated by fast energy transfer from the excited QD donors to the PDI acceptors. Notably, recent preliminary experiments with core CdSe QDs and PDI 1 gave first indications that with increasing dye amount adsorbed to the CdSe nanoparticles, electron transfer starts to compete with energy transfer. Obviously, by omitting the CdS and ZnS shells, which in type-I structures confine the charge carriers to the core region, a barrier for electron transfer has been removed. Electron transfer from excited CdSe QDs to a different kind of acceptor, methylviologen (MV2+),[51] has recently been investigated by Matylitsky et al.[44] In a subsequent work the influence of a CdS shell on the electron transfer dynamics has been studied.[52] Upon mixing of MV2+ with CdSe QDs at ratios of 7:1 the fluorescence of the QDs was completely quenched, which was attributed to electron transfer from the conduction band of CdSe QDs to MV2+.[44] The TA kinetics of MV2+/ CdSe complexes probed at 539 nm, where the transient bleach signal of the QDs is found, is displayed in Figure 4. The presence of MV2+ led to an ultrafast recovery of the lowest optical 1S(e)-1S3/2(h) absorption band of the QDs. A global analysis of the complete set of TA data yielded an ultrafast electron transfer time of 70 fs in the MV2+/ CdSe complexes.[44] At the time of publication this value has been the fastest electron transfer time that had been measured in QDs coupled to a molecular electron acceptor. The ultrafast time scale of the process suggested

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Figure 4. Transient absorption kinetics of CdSe QDs (black curve) and MV2+/CdSe QD complexes (red curve) probed at 539 nm which corresponds to the bleach of the lowest optical 1S(e)-1S3/2(h)-transition of the QDs. Symbols are experimental data points; the solid lines are the result of a multi-exponential fit.[44] The fast component of the fit yields a time constant of 70 fs for electron transfer from excited QD to MV2+. Reproduced with permission.[44] Copyright 2009, American Chemical Society.

the possibility for efficient dissociation of multiple electron-hole pairs (excitons) generated in the QDs, since such fast electron transfer was expected to successfully compete with Auger recombination of the excitons.[44,53] Different average numbers of electron-hole pairs ranging from 0.5 to 3.8 were prepared in the QDs by increasing the excitation intensity in the TA experiments. The TA traces of the MV2+/CdSe QD complexes were analyzed at a probe wavelength of 403 nm, which matched the absorption band of the reduced acceptor, i.e., the MV•+ radical. The dependence of this TA signal on the average number of electron-hole pairs in the QDs contained the information how many electron-hole pairs could be separated by electron transfer to the MV2+ acceptors. The corresponding experimental curve could be nicely reproduced by a simple theoretical description based on the assumption that only Auger and electron transfer processes contribute to the relaxation of the multiple excitons. For the given MV2+:CdSe QD ratio is was shown that at least 4 electron-hole pairs could be separated by ultrafast electron transfer to the MV2+ acceptor. This study convincingly demonstrated that electron transfer can be fast enough to compete with Auger relaxation which is an important requirement for the successful application of charge carrier multiplication in QD-sensitized solar cells.[54,55] To study the influence of a passivating shell on the electron transfer from QD to MV2+, CdSe nanoparticles were covered by different numbers of CdS layers





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(x = 0.5, 1.0, 1.5; x = number of nominal CdS monolayers).[52] Steady state absorption spectra of the core/ shell QDs showed a weak red shift of the 1S(e)-1S3/2(h)transition (ground state exciton) compared to pure CdSe QDs, which was explained by partial leakage of the electronic wave function into the CdS shell.[56,57] Complexes of the various QDs and MV2+ were prepared at a MV2+: QD ratio of 10:1. At this ratio, the fluorescence of all core/ shell QDs in the complexes was efficiently quenched compared to the corresponding pure QD samples. To study the dynamics of the electron transfer process, again TA spectroscopy was employed by Dworak et al.[52] As mentioned in the section about the MV2+/CdSe-QD system, the bleach recovery of the QDs was utilized to investigate the time scale of the relaxation dynamics. The decay kinetics of the MV2+/QD complexes was strongly dominated by the electron transfer process although other mechanisms contributed to the signal.[52] In addition, the decays were multi-exponential and amplitudeweighted average rates were used for further analysis. Notably, when the electron transfer rates were compared for complexes from QDs with different number of CdS layers, a clear dependence on the number of layers was observed. It was found that the increase of the shell thickness x led to an exponential decrease of the electron transfer rate . In particular, the following values were determined: x = 0.5: ET = 3.5·1011 s−1; x = 1: ET = 1.9·1011 s−1; x = 1.5: ET = 1.2·1011 s−1. Employing the frequently used expression for the transfer rate, kET = k0 exp (–ß rDA),[58] where k0 is a kinetic prefactor and rDA the donor-acceptor distance, a value of 0.33 Å−1 was derived for ß. The factor ß depends on the nature of the bridge between donor and acceptor, which in the present case was the CdS shell. The value found for ß was in good agreement with a previous study on similar Dye/QD complexes.[59] On the basis of the above findings Dworak et al. concluded that the CdS shell can be seen as an electronic barrier and the retardation of the electron transfer rate with growing shell thickness interpreted by a loss of overlap between the donor (QD) and acceptor (MV2+) electronic wave functions.[52] Besides the thickness of the shell, also the size of the core significantly influenced the electron transfer rate. Being another manifestation of the quantum size effect in semiconductor quantum dots, this phenomenon was explained by an increase of the free enthalpy ∆G for the electron transfer reaction when the core size of the QDs shrinks.[52] 2.2. Nanostructured Diblock Copolymers to Control Emission and Energy Transfer of Dye and Nanoparticle Assemblies Successful applications of energy transfer require precise control of the spatial distribution of multiple fluorophores


because the energy-transferring process is strongly dependent on the distance and orientation between lightabsorbing donors and energy-taking acceptors at the nanometer scale.[60] There have been many reports on the utilization of various nanostructures to mimic light-harvesting systems in nature,[61] which fundamentally share the principle of the accurate positioning of fluorophores and controlling of the distance between them by the nanostructure.[62] Diblock copolymers, composed of two chemically different polymers, spontaneously self-assemble into periodic nanostructures.[23] Since they offer precise control over self-assembled nanostructures and their functionalities,[63] diblock copolymers are promising candidates to adjust energy transfer processes among multiple fluorophores.[64] This section will highlight some recent developments on this topic. Sohn et al. introduced nano-encapsulation in the cores of diblock copolymer micelles to control FRET between fluorophores.[65,66] Diblock copolymer micelles are capable of isolating fluorescent dyes in the cores and can be easily coated to form a thin film with a controlled thickness. To demonstrate nano-encapsulation of fluorophores in copolymer micelles for controlled FRET, micelles of polystyrene-poly(4-vinylpyridine) (PS-PVP) diblock copolymers, which spontaneously associate into spherical micelles with soluble PS coronas and insoluble PVP cores in toluene, were employed. Then, rhodamine 123 (R123) as a donor and sulforhodamine 101 (S101) as an acceptor were selected. They are a good pair for FRET with the appropriate spectral overlap of R123 emission and S101 absorption and can be effectively incorporated into the polar PVP core. Concurrent loading of R123 and S101 in the same micellar core generated single emission from a spin-coated film of PS-PVP micelles (Figure 5a). Since the distance between the two dyes in the same core of a micelle was ≈2.1 nm, which was shorter than their estimated Förster radius (≈4.1 nm), the emission of S101 was dominant by FRET from R123 to S101. In contrast, simultaneous emission of R123 and S101 was created when the two dyes were isolated in independent micellar cores (Figure 5b), because FRET was effectively prohibited by the gap between the cores (≈9.5 nm). Thus, the light emission of nano-encapsulated fluorophores in diblock copolymer micelles was successfully controlled by utilizing the nanometer-sized micellar structure, which enabled or restricted the FRET between fluorophores. Based on the concept of nano-encapsulation, full utilization of the micellar nanostructures, i.e., the core as a place for FRET and the corona as a barrier for FRET, was demonstrated to achieve concurrent emission of multiple fluorophores with FRET-enhanced intensities from a single emitting layer.[65,67] Sohn et al. also demonstrated the nanoscale arrangement of QD donors and dye acceptor fluorophores in a


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Figure 5. Schematic illustration and PL spectra of thin films of PS-PVP micelles with fluorophores of R123 and S101: a) concurrent encapsulation of R123 and S101 in the same micelle; b) independent encapsulation of R123 and S101 in separated micelles. Reproduced with permission.[66]

single-layered film of diblock copolymer micelles to control FRET.[65,68] Green-emitting CdS/ZnS QDs (≈3.8 nm in diameter) were selected as donors. These QDs were added to a toluene solution of PS-PVP micelles with S101 acceptors. In the micellar film spin-coated from this solution, S101 acceptors were located in the cores of the micelles, whereas QD donors were arranged around their periphery. To control FRET between QDs in the periphery and S101 in the core of micelles, the PS gap between QDs and S101, i.e., the corona of PS-PVP micelles, was adjusted with micelles of different molecular weights of PS-PVP copolymers as shown in Figure 6. As visible in the TEM images, the QD donors got closer to the core containing the S101 acceptors by employing micelles with smaller coronas. The increased emission of S101 and the decreased emission of QDs gave evidence that the energy transfer proceeded. With large micelles, the dominant emission of QDs was observed because FRET between QDs and dyes was prohibited due to the size of the micellar coronas (≈11 nm) which was larger than the Förster radius of QD donors and S101 acceptors (≈5.2 nm). By employing micelles with smaller coronas, however, FRET from QD to S101 was enabled, resulting in decreased emission of QD donors and increased emission of S101 acceptors. Therefore, the nanoscale positioning of QD donors and dye acceptors by the nanostructured diblock copolymer micelles enabled the effective control of the energy transfer. Since surface-plasmon-coupled FRET has a fundamental and practical importance in many fields of science,[69] it is desirable to realize highly organized nano- or mesoscopic structures consisting of metal nanoparticles (NPs) and (dyes) fluorophores to engineer their interactions. By applying self-assembling techniques of diblock copolymer micelles to organize the spatial location of quantum dots, fluorescent dyes, and metal NPs, FRET between QD and dye in this hybrid assembly was

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switched off by plasmonic effects.[70] A single-layered film of PS-PVP micelles was coated with both R123 acceptors in the core and QD donors in the periphery onto an Ag NP film. The micellar film with both QDs and R123 showed efficient FRET from QD donors to R123 acceptors on a silicon wafer (or quartz) without Ag NPs. In stark contrast, QD emission was quenched and R123 emission was enhanced with the same micellar layer on an Ag NP film. The emission intensity of R123 was almost equal to that from the micellar layer with only R123 on the Ag NP film. In other words, Ag NPs completely switched off the FRET from QDs to R123 in the micellar hybrid (Figure 7). The inhibited FRET from QDs to R123, in the presence of Ag NPs, was explained by fast energy transfer from QD to the surface of the metal nano-particles due to the closeness of QDs and Ag NPs as shown in the schematic illustration of Figure 7. In this regard, metal NPs can be considered as an optical switch to control FRET. It is noted that charge transfer from QDs to Ag NPs is negligible because of the capping agents on both QDs and Ag NPs. Therefore, the near-field interactions among QDs, dyes, and metal NPs were engineered to switch-off FRET by their nanoscale organization in a single-layer film of diblock copolymer micelles. Separation of energy donors and acceptors beyond their Förster radius can inhibit FRET, allowing simultaneous fluorescence from both fluorophores. If multiple distant fluorophores are coupled with the surface plasmon of metal NPs, their fluorescence can be enhanced simultaneously. In this manner, Sohn et al. also applied the self-segregating property of diblock copolymer micelles to organize the spatial location of fluorophores in the proximity of metal NPs.[71] PS-PVP micelles with R123 and micelles with S101 were independently prepared and mixed in a 1:1 ratio. Then, a single-layer of mixed micelles containing R123 and S101 in different cores was spin-coated on a Ag NP substrate as well as on a silicon substrate (or




Macromolecular Rapid Communications www.mrc-journal.de

would have occured in the micellar film, the lifetime of R123 in the mixed film would have decreased in the presence of S101. It appears that the spatial separation of R123 and S101 by different micellar cores beyond their Förster radius is responsible for FRET inhibition, resulting in independent fluorescence enhancement in each micellar core. Therefore, simultaneous surfaceplasmon-coupled fluorescence without FRET was achieved by arranging both fluorophores in the vicinity of metal NPs with the micellar nanostructure of diblock copolymers. Besides nano-encapsulation of fluorophores in diblock copolymer micelles, nanoscale confinement of fluorophores in periodic self-assembled nanostructures of diblock copolymers can be utilized for the control of FRET. The selective incorporation of donors and acceptors into nanoscale domains can determine their spatial distance. Sohn et al. demonstrated the nanoscale confinement of fluorophores with nanostructured diblock copolymers by the simultaneous incorporation of two functionalities into the same nanostructure.[72] Two diblock copolymers which had the same basic chemical structure for each block but differed in small amounts of functional moieties in one of the blocks were blended so that they went into the same nanostructure. Thus, a diblock copolymer of Figure 6. TEM images and PL spectra of single-layered PS-PVP micelles with S101 in the poly(methylmethacrylate)-poly(pencores and QDs in the periphery of micelles: a,d) PS(93)-PVP(33); b,e) PS(48)-PVP(21); c,f) PS(32)tafluorophenylmethacrylate) (PMMAPVP(13). PL spectra of PS-PVP micellar film with only QDs in the periphery (green dotted PPFPMA) was synthesized, which has line) and with only S101 in the cores (red dotted line) are displayed together. The number in parentheses next to each block is the number-average molecular weight of the block in a reactive block with activated esters capable of anchoring thiol and fluokg/mol. Reproduced with permission.[68] Copyright 2008, American Chemical Society. rophore functional groups. From this copolymer, thiol-functionalized PMMAP(PFPMA-SH) was obtained for selective incorporation quartz) without Ag NPs. Steady-state fluorescence and of green-emitting QDs into lamellae of the PPFPMA-SH Streak Camera images clearly show that the fluorescence block in the alternating lamellar nanostructures. In addiof R123 and S101 are simultaneously enhanced on the Ag tion, by synthesizing a fluorophore-anchored diblock NP substrate compared to the same mixed micelles on copolymer, PMMA-P(PFPMA-TAMRA), the lamellar nanothe substrate without Ag NPs (Figure 8). PL decays of R123 structures were also functionalized by the TAMRA fluoat 540 nm and S101 at 600 nm on the Ag NP substrate rophore. Then, with this blending strategy, dual funcbecame much faster than those on the substrate without tionalization of lamellar nanostructures of diblock Ag NPs as shown in Figure 8. Average life times of R123 copolymers was achieved by simultaneous incorporaand S101 are virtually similar to those independently tion of QDs and TAMRA into the lamellar nano-domains obtained from the micellar system only with a single (Figure 9). Enhanced fluorescence of TAMRA at 568 nm and type of dye (R123 or S101) on the Ag NP substrate. If FRET



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Figure 7. a) Steady-state PL spectra from a single layer of PS-PVP micelles on an Ag NP film with only QDs (blue), with only R123 (green), and with QDs and R123 (pink); b) Time-resolved PL monitored at 500 nm from a single layer of micelles on the Ag NPs film with only QDs (pink) and with QDs and R123 (gray); c) schematic illustration of near-field interactions among QDs, R123, and Ag NPs in the micellar hybrid (NSET: Nano-metal surface energy transfer). Reproduced with permission.[70] Copyright 2012, American Chemical Society.

a reduction of the QD emission at 502 nm were observed, indicating FRET from the QD donor to the TAMRA acceptor. Therefore, FRET by nanoscale confinement of both donors and acceptors in self-assembled lamellar nanostructures of diblock copolymers was effectively induced. The selective incorporation of fluorophores into nanoscale domains determined their spatial positions. 2.3. Materials for Optoelectronic and Photonic Applications Functional nanomaterials like polymers, QDs, and inorganic as well as organic nanotubes and nanowires can be assembled into useful nano-hybrids through chemical functionalization strategies.[73,74] The efficiency of energy transfer or charge transfer phenomena in hybrid composites comprising the above materials depends on the relative energy levels of the constituents, as well as the morphology of the composites.[75] 2.3.1. Quantum Dot Coupled Nanotubes and Nanowires Single wall carbon nanotubes (SWCNTs) in their metallic state show ballistic n-type conduction which is highly

desirable in optoelectronic devices. An effective functionalization scheme was developed for the attachment of QDs onto SWCNTs by generating an abundance of thiol groups on the SWCNT surface. These thiol groups were then used to attach PbSe QDs onto the SWCNTs.[76] A graphics depicting the nanohybrid is displayed in Figure 10a. The PbSe-SWCNT nanohybrid seen in the TEM images in Figure 10b–d is optically active in the IR region. The large Bohr radius of PbSe allows the size of the nanoparticles to be tuned to match the targeted IR wavelength. The hybrid can be combined with hole-conducting polymers like poly(N-vinyl carbazole) (PVK) to fabricate films that are photo-conducting. The structure of a photoconductor device fabricated with PVK-PbSe-SWCNT nanocomposite films is shown in the inset of Figure 10e. The photoactive layers in the devices consisted of only PVK-PbSe QD or PVK-PbSe-SWCNT. The alignment of the energy bands of the various components in the photodetector device can be seen in Figure 10f. The higher electron affinity of the PbSe QD (–4.4 eV) allows non-activated electron transfer from the QD to the SWCNT. The ionization potential of PVK polymer close to vacuum favors the transfer of holes from the QD to the polymer. The indium tin oxide (ITO) (anode) and aluminum layers (cathode) form the electrodes, while

Figure 8. Spectroscopic results from a single-layer of mixed PS-PVP micelles containing R123 and S101 in different cores. a) Steady-state PL spectra from mixed micelles on a substrate without Ag NPs (gray dashed) and with Ag NP (pink solid); b,c) Streak Camera images from mixed micelles on the substrate without Ag NPs (b) and with Ag NP (c); d) normalized PL decays of R123 at 540 nm from mixed micelles on the substrate without Ag NPs (gray circles) and with Ag NPs (green circles); e) normalized PL decays of S101 at 600 nm from mixed micelles on the substrate without Ag NPs (gray circles) and with Ag NPs (red circles). Reproduced with permission.[71] Copyright 2012, Royal Society of Chemistry.

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Figure 9. Schematic illustration (left), cross-sectional TEM image (center), and PL spectrum (right, red line) of a thin film consisting of QDs, PMMA-P(PFPMA-SH), and PMMA-P(PFPMA-TAMRA). The scale bar in the image is 100 nm. PL spectra of thin films with QDs only (green dashed line) and with TAMRA only (red dashed line) are also displayed. Reproduced with permission.[72]

poly(3,4-ethylenedioxythiophene) (PEDOT) acts as a selective hole transport layer. The SWCNTs establish a conducting percolation network in the PVK-PbSe-SWCNT films due to their high aspect ratios even at low loadings. Such a network leads to high electron mobility and enhanced photocurrents in the films due to ballistic transport.[77]

The devices were tested at a working wavelength of 1340 nm. Since pure PVK has no absorption at wavelengths longer than 370 nm, no photoresponse was obtained by exciting it at 1340 nm. This means that any response in PVK-PbSe QD devices arose out of the QD absorption. By increasing the fraction of PbSe-SWCNT in

Figure 10. a) Graphics depicting the QD functionalized SWCNT. b-d) TEM images of PbSe QDs with different magnifications. e) Photocurrent density-voltage (I–V) curves of PbSe-QD/PVK and SWNT–PbSe/PVK devices in the dark and under illumination (inset: structure of the IR photodetector device employed in this study), and f) energy band diagram for the composite photodetector, indicating the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) levels of different constituent materials. Reproduced with permission.[76]



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the photoactive films, different devices were fabricated. The devices containing PbSe-SWCNT nanohybrids showed larger photocurrent per effective load fraction of PbSe QDs than PbSe-PVK QD devices. In Figure 10e it can be seen that the current density increased with increasing fraction of PbSe-SWCNT. A photodetector device with 20 wt% of PbSe-SWCNT showed an external quantum efficiency (EQE) of 2.6% compared to 1.2% found for the PbSe-PVK QD device with an equivalent loading of free QDs, which meant an increase of about 100%. The presence of QDs in close proximity to the SWCNTs because of their linkage was a key factor in achieving these results. The close proximity of QD and SWCNT due to the short thiol spacer group aiding their attachment could facilitate charge transfer between these components.[78] Surprisingly, there was little evidence of any short circuit behavior by the high aspect ratio SWCNTs which had an average length larger than the average thickness of the device. Besides SWCNTs, nanowires (NWs) can be combined with QDs or noble metal nanoparticles to give optically and electronically active nanomaterials.[79,80] The optical and electronic properties of nanomaterials show significant divergence with increasing aspect ratios.[81–83] QDs stabilized either by oleic acid (OA) or by 11-mercapto1-undecanol (MUD) were used to decorate the surface of NWs, or were incorporated into NWs during formation.[84–87] The nanowires were fabricated using anodized alumina templates.[88] The QDs were coupled with p-type polymer nanowire, plasmonic nanowire, as well as hybrid nanowires containing bulk heterojunctions of p-type polymer and fullerene derivatives to study charge transfer in the resulting hybrids. Laser confocal microscopy photoluminescence (LCM PL) measurements were used to characterize the photoluminescence properties of single QD-NW hybrids. This technique was used to record spectra as well as mapping images of a single nanowire or nanohybrid under comparable experimental conditions. A large number of spectra (about 100) were recorded along a nanowire/nanohybrid to derive mean values and standard deviations of intensities before comparisons were made among different materials.[84] Exciton dynamics of the hybrids were studied using time resolved confocal PL measurements. This technique is very adept at looking at exciton dynamics of the QD-NW hybrid material as a whole or its individual components, i.e., QD or NW.[89] The intensity weighted average exciton lifetimes (τavg) measured for the nanowire hybrid and its individual components were used to derive conclusions about their charge transport properties.

τ avg = Σ i Aiτ i /Aiτ i2

(1)

In Equation (1), Ai and τi represent the amplitude and the lifetime of the ith exciton component.

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Further the energy transfer efficiency can be defined as follows. E = 1 − τ DA /τ D

(2)

τDA and τD are the exciton life times associated with the donating system (QDs for most cases discussed in this section) in its hybridized state and un-hybridized state, respectively. Different NW-QD hybrid systems that were studied are listed along with their τavg values in Table 2. High-resolution scanning transmission electron microscopy (HR-S/TEM) images of poly(3-hexylthiophene) (P3HT) NW decorated on the surface with 11-mercapto1-undecanol (MUD) stabilized CdSe/ZnS core-shell QDs (NW-QD1 in Table 2) are presented in Figure 11a. The P3HT NW was about 30 μm long and QDs are seen as the bright spots on top of the nanowire. Energy and charge transfer processes in NW-QD1 were compared with data from polystyrene nanowires (PS NW) decorated with MUD stabilized CdSe/ZnS core-shell QDs.[84] The spectral overlap of the emission of MUD-QD with the absorption of the P3HT NW shown in Figure 11d suggests a strong possibility of energy transfer from QD to NW. The PL spectrum of the PS NW-QD hybrid in Figure 11e (green curve) is dominated by the QD fluorescence (ca. 532 nm) due to the lack of overlap between the QD emission and the PS NW absorption. The PL spectrum of NW-QD1 (orange curve) showed peaks characteristic of both QD at 532 nm and P3HT at ≈640 as well as ≈700 nm. The PL peak of the QD at 532 nm exhibits a significant decrease of intensity in the spectrum of NW-QD1. The intensity of the P3HT peaks in the hybrid (orange curve) was higher than in the spectrum for individual P3HT NW (red curve). Meanwhile, the PL peak of P3HT at 640 nm grows in intensity indicating FRET. For efficient FRET the interacting components should ideally be separated by a few nanometers.[90] Due to the surface ligands the distance between QDs and the surface of the NWs amounts to ≈1.5 nm. Such close proximity combined with the spectral overlap ensures FRET from QD to NW. The PL decays of the QDs, P3HT NW, and the QD-P3HT hybrid (NW-QD1) can be seen in Figure 11f. The τavg values for P3HT and the MUD-CdSe/ZnS QD were estimated to be 0.4 and 32.9 ns, respectively. For the NW-QD1 hybrid a value of 4.7 ns was found. The QD part of NW-QD1 was measured to be 7.7 ns. This value was very small compared to the τavg value for the MUD-CdSe/ZnS QD (32.9 ns). The decrease in the QD lifetime along with the decrease in its PL intensity in the hybrid indicates FRET between the QD donor and the P3HT acceptor.[91] The transfer efficiency can be approximately calculated as 0.76 taking τDA as 7.7 and τD as 32.9 ns. Thus, the exciton life times demonstrate energy transfer from the excited QD to the p-type P3HT NW.





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Table 2. Exciton lifetime values obtained from time-resolved PL measurements for different hybrid materials.

Nanowire

quantum dots

τavga) ns

-

P3HT

−

0.40

Nanowire

-

-

c)MUD-CdSe-ZnS

32.9

Quantum dot

-

-

d)OA-CdSe-ZnS

45.8

Quantum dot

P3HT

−

0.40

Nanowire

-

P3HT-PCBM

−

0.24

650 nm for Nanowire

-

P3HT-PCBM

−

0.28

700 nm for Nanowire

P3HT

MUD-CdSe-ZnS-on surface

4.7

Hybrid

P3HT


7.7

Quantum dot part of hybrid

P3HT-PCBM


0.22

Hybrid at 650 nm

P3HT-PCBM


0.26

Hybrid at 700 nm

P3HT-PCBM


3.57


P3HT-PCBM

OA-CdSe-ZnS-incorporated

0.18

Hybrid at 650 nm

P3HT-PCBM


0.21

Hybrid at 700 nm

P3HT-PCBM


0.77


Hybrid

-

NW-QD1

NW-QD 2

NW-QD3

a)Intensity

Materialb)

weighted average exciton lifetime; b)the material for which τavg is measured; c)11-mercapto-1-undecanol; d)oleic acid.

Two different approaches were adapted to conjugate QDs with the donor-acceptor combination of P3HT:PCBM (1:1). The first method involved the attachment of MUD functionalized CdSe/ZnS QD to the surface of P3HT:PCBM NW (NW-QD2 in Table 2). The second method involved mixing OA functionalized CdSe/ZnS QD into the P3HT:PCBM prior to fabrication of the NW (NWQD3 in Table 2). The HR-S/TEM images of NW-QD2 and NW-QD3 can be seen in Figure 11b,c, respectively. The PL spectra of pristine P3HT:PCBM NW (red curve), NW-QD2 (blue), and NW-QD3 (black) can be seen in Figure 11g. The pristine P3HT:PCBM NW showed PL peaks at 647 nm and 691 nm (red curve) due to P3HT and PCBM, respectively. The OA-QD infiltrated NW-QD3 (black curve) showed a dramatic reduction in PL intensity compared to the peaks of pristine P3HT:PCBM NW. This is an indication of charge transfer from the p-type P3HT to both the n-type QD and PCBM. The close contact between the QDs and P3HT:PCBM in NW-QD3 might be the reason for the observed PL quenching due to charge transfer effect. The NW-QD2 showed emissions at 532 nm, 647 nm, and 703 nm due to QD, P3HT, and PCBM, respectively. A reversal in intensity was observed between the peaks


corresponding to P3HT and PCBM in the QD decorated NW (647 nm, 703 nm) compared to pristine P3HT:PCBM NW (647 nm, 691 nm). This implies that the decoration of the P3HT:PCBM surface with QDs resulted in an increase in intensity of the PCBM peak due to transfer of excitons. Such a reversal in intensity was not observed for NW-QD3 indicating the presence of other exchanges at its heterojunctions. A schematic energy band diagram of possible transfer phenomena in NW-QD2 and NW-QD3 is presented in Figure 11h. In the case of NW-QD3 the QDs are located close to the P3HT chains facilitating easy charge transfer. In contrast, QDs decorated at the NW surface (NW-QD1) are 1.5 nm away from the NW because of the functional groups on the QD surface,[92] leading to energy transfer between the QD and the NW rather than charge transfer. Exciton lifetimes of the hybrids and their individual components are shown in Figure 11i. The pristine P3HT:PCBM NW (in Table 2: P3HT-PCBM) showed τavg values of 0.24 ns and 0.28 ns at 650 nm and 700 nm, respectively. In NW-QD2 the τavg values at these wavelengths were 0.22 ns and 0.26 ns, respectively. For QD incorporated NW-QD3, τavg values of 0.18 ns and 0.21 ns,


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Figure 11. The HR-S/TEM images of a) NW-QD1, b) NW-QD2, and c) NW-QD3. d) Overlap of the UV absorption of P3HT (red) with the PL emission of MUD-QD (green), e) LCM PL spectra of NW-QD1 (orange), P3HT NW (red) and QD decorated polystyrene NW (green), f) trPL spectra of P3HT NW and MUD-QD (blue) compared to NW-QD1 (red). g) LCM PL spectra of the hybrids NW-QD2 (blue) and NW-QD3 (black) compared to the corresponding spectrum of P3HT:PCBM nanowire (red), h) schematic energy band diagram showing transfer phenomena corresponding to NW-QD2 and NW-QD3 (i) normalized trPL spectra of the OA-QDs (black) dispersed film, MUD-QDs dispersed film (blue), NW-QD3 and NW-QD2, the NWs are described in the same color as their associated QDs. (a,d,e,f) Reproduced with permission.[84] Copyright 2013, RSC. (b,c,g,h,i) Reproduced with permission.[86] Copyright 2013, Elsevier.

respectively, were obtained. The lower τavg values for the QD incorporated NW indicate a slightly higher tendency for charge transfer between the incorporated QDs and P3HT:PCBM. Films consisting of MUD-QDs or OA-QDs

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showed τavg values of 32.9 ns and 45.8 ns, respectively. The τavg value for the MUD-QD in NW-QD2 decreased to 3.6 ns. The OA-QDs incorporated into NW-QD3 showed an even greater decrease at 0.77 ns. The shorter exciton lifetimes





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in the case of NW-QD3 indicate that the infiltration of QDs into P3HT:PCBM NWs increases the probability of charge transfer between the nanowire and QD components as seen in Figure 11h. The QD infiltration can also have an effect on the orientation and ordering of P3HT facilitating better hole transport. The nanoscale photo-responsive electrical characteristics of NW-QD3 can be explained on the basis of Fowler–Nordheim tunneling model under conditions of high bias, an effect which was not observed for the QD decorated NW-QD2.[93] Thus charge conduction and formation of charge transfer excitons can be enhanced by the infiltration of n-type CdSe/ZnS semiconductor nano-crystals in p-n hetero-junction organic nanowires. This makes them suitable for future testing in photovoltaic devices. The CdSe/ZnS decorated p-n heterojunction nanowires may be applied to lighting applications due to their PL at three different wavelengths. The efficiency of charge transfer interaction in NW-QD nano-hybrids might be further increased by coupling them with plasmonic nanomaterials. It has been demonstrated recently that donor-acceptor charge transfer is enhanced by coupling to plasmonic nano-structures.[94–96] The fluorescence enhancement of the donor due to plasmonic coupling leads to greater donor-acceptor interaction.[84,87] 2.3.2. Effect of QD Surface Functionalization on Charge Transport The functionalization of QDs with photo-responsive ligands provides a versatile method for controlling the structure and properties of their films. Two different methods shown in Figure 12a were adapted to synthesize photo-patternable QDs (PQDs). The first method involved the stabilization of the QDs with a ligand containing thiol at one end and a t-butoxycarbonyl (t-BOC) group at the other end to give PQD1.[97] t-BOC can be thermally or photochemically cleaved to yield an amine-terminated outer ligand layer. The functionalization strategy could be easily applied to PbSe, CdSe, and CdTe QDs. Films formed by dispersion of PQD1 containing an added photoacid generator could be easily patterned on a glass substrate. To this end, spin coated films of PQD1 were exposed to UV light through a desired photomask, followed by heating for 90 s at 100 °C. A photomask is used to define the desired pattern during UV irradiation. The UV light induces localized photoacid generation in the exposed areas. The photogenerated acid cleaves the tBOC groups on the QDs during a subsequent heating step. The cleavage of t-BOC groups leads to more hydrophilic amino surface functionalized QDs. After exposure, the pattern can be developed as positive (exposed areas are washed away) or negative (unexposed regions are washed


away) depending on the solvent used. A negative pattern formed from photoexposure of a CdSe QD film can be seen in Figure 12b. Films made from PQD1 generated photocurrent on illumination with white light (100 mW cm−2). Tests were carried out with films that were exposed to UV and those that were not exposed to UV. Both films showed negligible dark current (0.1 nA). Upon illumination with white light, the patterned films showed photocurrents an order of magnitude higher than that of unpatterned film as seen in Figure 12d. The removal of the t-BOC groups upon photoexposure leads to an increase in the photocurrent, clearly indicating the role of surface ligands on the charge transfer between QDs. Alternatively, the t-BOC group could also be cleaved by thermal treatment of PQD1. The influence of thermal curing upon the deprotection of t-BOC groups was investigated with CdSe nanorods of length of 12–18 nm (λmax at 627).[98] The t-BOC layers remained intact on heating up to 100 °C. Temperatures between 150–200 °C were found to be best to cleave the t-BOC groups to yield an amineterminated ligand layer on the QDs. The t-BOC functionalized PQD1 was blended in different ratios with the p-type polymer P3HT to serve as active layers in photovoltaic devices. The I–V characteristics of the devices were measured in dark and under AM 1.5 G at an intensity of 90 mW cm−2. For devices annealed at 150 °C the best performance was given by a device with P3HT:CdSe PQD1 ratio 10:90 wt%. The increase in device efficiency with increased ratio of CdSe PQD1 could be due to the formation of improved bulk heterojunctions. The I–V characteristics of the device containing this best performing combination can be seen in Figure 12e. The dark current and photocurrent in two different devices are also compared in Figure 12e. In the first device the active QD film was annealed at 100 °C, and in the second device the active QD film was annealed at 200 °C. Annealing at 100 °C served only to remove solvents or other smaller molecules in the films and did not lead to the cleavage of t-BOC groups. On the contrary, annealing the active QD film at 200 °C led to the removal of t-BOC group effecting a change in the transport properties in the QD-polymer films. The first device gave an open circuit voltage (Voc) = 0.86 V, short circuit current Jsc = 0.019 mA cm−2, and fill factor (FF) = 22.9%; while the second devices showed Voc = 0.85 V, Jsc = 1.17 mA cm−2, and FF = 34.5%. The monochromatic power conversion efficiency (PCE) of a photovoltaic device is PCE = FF(Voc/Isc)/Pinput, where Pinput is the power of the incident light. The increase in current density coupled with the increased fill factor in the second device (heated at 200 °C) yielded a PCE value 90% higher than that of the first device (heated at 100 °C). It was found that the PCE of the device increased when the hybrid films were thermally cured between 150–260 °C. Beyond 260 °C the PCE values decreased on increasing the temperature.


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Figure 12. Patternable QDs. a) The graphic depicts the functionalized QD surface, t-BOK terminated ligands are used in PQD1 and methacrylate terminated ligand was used in PQD2, inset of (a) shows red CdSe-ZnSe, blue CdS-ZnS and green CdSe-ZnS type PQD2. b) Negative patterns fabricated by CdTe PQD1 (inset displays patterned 5 μm discs). c) PQD2 patterned on a glass substrate. d) Current-voltage curves for PQD1 films under dark (black solid) as well as illuminated conditions (red dash). The blue dotted curve is given by the patterned PQD1film. e) Current density-voltage characteristics of the photovoltaic devices consisting of ITO/PEDOT:PSS/P3HT:CdSe-ZnS PQD1 (10:90 wt%)/ Al at heat treatment temperatures 100 °C and 200 °C. f) Electroluminescence (EL) efficiencies of EL devices containing green PQD2 with (red closed circles) and without (blue open circles) photoexposure, inset shows the structure of the device. (a) Reproduced with permission.[99] Copyright 2012, Optical Society of America. (b,d) Reprodcued with permission.[97] Copyright 2008, American Chemical Society. (c,f) Reproduced with permission.[100] Copyright 2010, American Chemical Society. (e) Reproduced with permission from.[98] Copyright 2010, American Institute of Physics.

Considering the reported melting point of P3HT at around 230–240 °C, the high PCE might be related to this phase transition in the polymer and an increased interaction between QDs in the composite films to form conducting domains. Thus, the electronic properties of the hybrid films are governed by the close proximity of the QDs as well as their surface functionalization. In case of PQD2 the patternable ligand contains a thiol group belonging to a 11-mercapto-1-undecanol molecule to interact with the QD. The hydroxyl group at the other end is reacted with 3-(trimethoxysilane)propyl methacrylate to give a ligand unit terminated with a

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methacrylate group and containing an inner siloxane group (Figure 12a). Red CdSe/ZnSe core shell, green CdSe/ ZnS core-shell, as well as blue CdS/ZnS core-shell QDs could be functionalized by this ligand. Solutions of red (R), green (G) and blue (B) type PQD2 can be seen in the inset of Figure 12a.[99,100] To pattern the QDs on glass substrates, they were spin coated on glass followed by a soft bake (90 °C for 1.5 min) and UV exposure (33 mW cm−2 at 360 nm). The patterns were then developed with ethanol. The CdSe/ZnS green PQD2 patterns on glass can be seen in Figure 12c (under UV illumination). The polymerizable layer of ligands on the nanoparticles could also be





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used for controlling the packing in the QD films. During photocuring and the subsequent baking process the polymerization of methacrylate groups and the crosslinking of the siloxane groups on the QDs lead to a densification of the QD films. The implications of such a densification on the device performance of this material were tested by fabricating an electroluminescent (EL) device with green PQD2 as the active layer. Two different types of devices were prepared, one with UV exposed green CdSe/ ZnS PQD2 active layers and the other with unexposed green CdSe/ZnS PQD2 films. The devices with UV exposed films showed a greater EL efficiency as seen in Figure 12f. The device with a UV cured film gave an EL efficiency of 2.40 cd A−1 corresponding to a luminance of 4384 cd m−2 at a drive voltage of 13 V. The maximum external quantum efficiency was found to be 0.62%. This device also showed a small turn-on voltage compared to the devices with uncured PQD2 films as active layer. The uncured device showed an efficiency of 1.57 cd A−1 corresponding to a luminescence of 2996 cd m−2 at a drive voltage of 13 V. The maximum external quantum efficiency was found to be 0.53%. The photo-induced ordering of the QDs led to an increase in EL efficiency in the devices with UV cured PQD2 films as active layers. Hence, the ability to pattern the QD films not only introduces the possibility of new interfaces in a device but also allows a handle over the film structure and transport properties. The role of the surface functionalization on the charge transfer properties of QDs has already been established through the study of PQD1 and PQD2. The previous sections have also emphasized the importance of forming robust interfaces between polymers and QDs. Conjugated molecules with thiol-functional groups shown in Figure 13a were synthesized to study the transport through functional ligands directly attached to the surface of QDs. The general structure of the QD-organic hybrid is presented along with two different thiol-terminated organic functional ligands.[101] The first hybrid consisted of π-conjugated macromolecular dioctyloxybenzodithiophene-based polythiophene of molecular weight 3 kDa (P3000) coupled with green CdSe/ZnS core-shell QDs (QD-P3000). The second hybrid consisted of 11-(9H-carbazol-9-yl) undecane-1-thiol (CB) stabilized red CdSe/ZnS core-shell QDs (QD-CB). The TEM images of QD-P3000 and QD-CB are displayed in Figure 13b and 13c, respectively. Using atomic force microscopy, the diameter of QD-P3000 was found to be 25 nm which agreed well with the calculated estimate employing a QD size of 7 nm and adding two times the length of the polymer (7 nm). The total diameter of the QD-CB was found to be 11 nm from AFM measurements. In Figure 13d, the PL spectrum of single QD-P3000 hybrid nanoparticle (red curve) is presented together with the PL of green MUD-QD (green curve). The blue


and green dashed curves are the decomposed components of the red curve corresponding to the hybrid QD-P3000. The low intensity of the blue curve centered at 530 nm indicates a strong quenching of QD PL in QD-P3000. The PL spectrum of QD-CB (black curve) in Figure 13e was dominated by the PL of the red QDs. This spectrum compares well with the red QD PL given by the red curve in Figure 13e. The PL intensity of the red QDs coupled with CB was not reduced to the same extent as was the reduction in the case of green QD when coupled with P3000. The reduction ratio of maximum photon count of QD-P3000 and QD-CB was estimated to be 97.5% and 45.4%, respectively.[101] This suggests a less efficient charge transfer in the latter. The long insulating alkyl chain between the QD and the carbazole molecule in CB acts as a deterrent for effective charge transfer between the two. The τavg values of the QD part of QD-P3000 and QD-CB were compared with their unfunctionalized QD counterparts in Figure 13f,g, respectively. The exciton life time of the QD part in QD-P3000 was very small with a τavg of 1.2 ns compared to 37.8 ns of the unfunctionalized green CdSe/ZnS QD. The difference in life times between the unfunctionalized red QD and QD-CB was less sizeable with lifetimes of 6.5 ns and 4.3 ns, respectively. These results clearly indicate efficient charge transfer between the QD and the conjugated polymer attached to it in QD-P3000. In terms of photocurrent generation the nanoscale I–V characteristics of the QD-P3000 and QD-CB hybrids in dark and light conditions could be described by Fowler-Nordheim and direct tunneling models in forward bias.[101] The presence of the conjugated P3000 on the QD surface led to better transfer of charge from the QD. It was recently reported that charge transfer from a QD through surface attached conjugated polymer ligand increases with increasing molecular weight of the ligand. This phenomenon was attributed to the formation of crystalline domains in the high molecular weight ligand leading to faster transport.[102]

3. Conclusions The contributions collected in this article have revealed that energy and charge transfer in nanohybrids composed of semiconductor quantum dots and π-conjugated organic molecules can be achieved and controlled in several ways, and a number of possible applications have been discussed along with preliminary results obtained with straightforward device structures. While the focus has been on research done by the authors and their collaborators, the growing importance of organic-inorganic hybrid nanostructures is high-lighted by other very recent compilations dealing with similar topics.[103,104]


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Figure 13. a) Thiol-terminated functional organic ligands used for modifying the surface of core-shell green CdSe-ZnS QD. TEM images of b) QD-P3000 and c) QD-CB. d) Comparison of LCM PL spectra of the green CdSe-ZnS (green) and nanohybridQD-P3000(red); the dashed blue and green curves show the decomposed contributions to the QD-P3000 spectrum from its components. (e) LCMPL spectrum comparing the red CdSe-ZnS QDs with the QD-CB hybrid. f) The trPL decay curve of the green QD(green) compared to QD-P3000 (red curve). g) The trPL decay curve of the red QD (red curve) compared to QD-CB (black curve), insets in (f) and (g) plots the log scale along Y-axis. Reproduced with permission.[101] Copyright 2014, Nature Publishing Group.

The important class of rylene diimide dyes furnished with dicarboxylate anchors turned out to yield stable complexes with QDs and allowed for assembling defined QD oligomers. The rate constant of energy transfer was measured by femtosecond transient absorption spectroscopy and its increase with increasing amount of dye coupled to QD allowed for determining the Dye/QD ratio which often is difficult to assess. Ultrafast electron transfer of about 70 fs was observed for QDs coupled to appropriate molecular electron acceptors. By virtue of this fast time scale, several excitons in the QDs could be dissociated by electron transfer demonstrating that charge carrier multiplication in QDs indeed can compete with fast Auger recombination. By growing shells of different thickness of a higher band-gap material on the QD cores, the electron transfer was substantially slowed down. Nanostructures of diblock copolymers offer precise control over nanoscale positioning of fluorophores including

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QDs in thin films to adjust FRET or metal nanoparticles to induce surface energy transfer for tuning emissions from an emitting layer. Encapsulation or confinement of light-harvesting donors and light-emitting acceptors into the same micellar or lamellar nanostructure of diblock copolymers gave rise to efficient FRET. In contrast, the coronas of diblock copolymer micelles were shown to act as an effective barrier for FRET, when donors and acceptors were located in independent micelles. In addition, near-field interactions among fluorophores, QDs, and metal NPs were engineered by nanoscale organization in a single-layer of diblock copolymer micelles. Surface functionalization of QDs was carried out to control the composition, structure, and density of the interface as well as to govern the extraction and transport of charges. Semiconducting polymer nanowires infiltrated with QDs were found to perform better than nanowires decorated with QDs because of a more intimate contact





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between the components in the former. Conjugated molecules directly attached to the surface of the QDs led to greater charge transfer. Regarding device performance, the specific molecular structures as well as the bulk interaction of the hybrid materials appeared as key factors. With the domain of optoelectronic devices pushing towards the molecular scale, understanding the dynamics of energy and charge transfer at interfaces of nanohybrids with space- and time-resolved techniques is a crucial issue for the future design of such devices. The compilation presented here may have revealed that variations of the same combination of materials offer a rich variety of photophysical processes and electronic interactions to be exploited. The unique properties of colloidal semiconductor quantum dots, such as size dependent electronic transition energies and a high surface-to-volume, render them an appealing and promising component for applications in photonics, optoelectronics, and sensors. Acknowledgements: The authors acknowledge financial support from the IRTG 1404/2006-IRTG-001 (Self-Assembled Materials for Optoelectronic Applications) jointly funded by the Deutsche Forschungsgemeinschaft (DFG) of Germany and by the National Research Foundation (NRF) of Korea. B.H.S. acknowledges the support by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A2A01002290). K.-S. L. acknowledges the funding from the NRF through the APCPI (ERC R11–2007–050–01002–0). T. B. acknowledges the collaboration with J. Wachtveitl. Received: December 26, 2014; Revised: February 10, 2015; Published online: March 11, 2015; DOI: 10.1002/marc.201400738 Keywords: nanostructures; quantum dots; dye/pigments; charge transfer; energy transfer

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