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Vibrational spectroscopy as a probe of molecule-based devices Anastasia B. S. Elliott, Raphael Horvath and Keith C. Gordon* Received 2nd August 2011 DOI: 10.1039/c1cs15208d This critical review discusses the applicability of vibrational spectroscopic techniques, specifically Raman and mid-infrared, to the study of molecule-based electronics through a series of examples. We focus on a number of devices currently of interest, such as solar cells, organic light emitting diodes, molecular junctions, switches and transistors. Infrared and Raman spectroscopic techniques and their variations, the main focus of this article, can be used to investigate properties such as crystallinity, multiphasic distributions in three dimensions, as well as lifetimes, structures and energetics of excited-states on ultrashort to very long timescales (210 references).

1

Introduction

In this review we address the uses of optical spectroscopic techniques, specifically infrared and Raman, in the examination of molecule-based devices and molecular electronics. One can consider molecular electronics in three ways (Fig. 1): firstly, the development of new materials that have the potential to be used in electronic devices; secondly, the understanding of these materials in pre-processed form; thirdly, the understanding of materials in working devices. Vibrational spectroscopy offers the ability to probe and understand molecules in each of these regimes and we will illustrate the use of spectroscopy in a series of examples from each of these. There are a large number of materials that have potential to be used in electronic or molecule based devices; fewer materials are studied in the film or pseudo-device states and even fewer are

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, Dunedin, New Zealand. E-mail: [email protected]; Tel: +64 3 479 7599

Fig. 1 Structure of investigation of molecular electronics.

actually deployed in devices. Over the last few decades, the ability to create smaller, more powerful and more energyefficient electronic devices has been the driving force for a global multi-billion dollar industry.1,2 Advancing miniaturisation has seen top-down approaches to manufacturing pushed to within a fraction of the wavelength of light, creating problems of quantum effects associated with this scale, as well

Anastasia (Stasi) Elliott completed her undergraduate study at the University of Otago, graduating in 2011 with a BSc(Hons), First Class, in Chemistry. She is currently studying as a PhD student at Otago University under the supervision of Professor Keith Gordon. Her interests include computational chemistry and Raman spectroscopy of various donor– acceptor type systems. Anastasia B. S. Elliott This journal is

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The Royal Society of Chemistry 2012

Raphael Horvath completed his BSc(Hons), First Class, in chemistry in 2008 at the University of Otago. He is now a PhD student at Otago, studying physical chemistry under Professor Keith Gordon. His ‘‘physical’’ interests include laser spectroscopy and computational modeling while his ‘‘chemistry’’ interests include metal complexes containing polypyridyl ligands. Raphael Horvath Chem. Soc. Rev., 2012, 41, 1929–1946

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as difficult manufacturing methods such as extreme-UV or electron-beam lithography. Molecular electronics offer several advantages over traditional complimentary metal-oxidesemiconductor (CMOS) devices. Their molecular nature enables individual circuits to be as small as possible, while smaller charges stored associated with this miniaturisation can lower the energy consumption. Quantum effects are inherent to properties on a molecular scale. While this requires new and possibly complicated designs, it also enables the creation of molecules that exhibit new properties lacking corresponding macromolecular effects, potentially enabling the design of radically different devices. Complex devices can be completely integrated within single molecules, in principal simplifying production. Finally, the material properties of molecular electronic devices are different to those of traditional silicon-based electronics and can potentially be controlled independently of electronic properties. This may allow for the use of very cost-effective production methods, such as inkjet printing, as well as the design of flexible or otherwise unusual devices.3–5 The field of molecular electronics can be divided into two distinct categories, these being ‘single molecule’ and ‘bulk material’ devices (Fig. 2a and b). Single molecule properties relate to how individual molecules interact with one another or between two electrodes. Perhaps the simplest single-molecule electronic system is the molecular wire. Generally conjugated

Fig. 2 Examples of the two paradigms investigated within the field of molecular electronics.

Keith Gordon completed his PhD at Queens University Belfast in 1989, under the supervision of Professor John J. McGarvey in transient spectroscopy of copper(I) polypyridyl complexes, he then worked for a year as a postdoc at Queens. He was then a Director’s postdoctoral fellow at Los Alamos National Laboratory studying timeresolved infrared spectroscopy of inorganic systems under the guidance of Professor W. H. Keith C. Gordon (Woody) Woodruff. In 1993 he joined the Chemistry department of the University of Otago and was awarded a personal chair in 2009. His research interests include the use of resonance Raman spectroscopy to probe excited states in metal complexes and the development of new materials for molecular electronics. 1930

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chains, they are envisaged to connect molecular devices; however, their study is also important from a fundamental standpoint as it can lead to insights into other charge-transfer systems.3 Steps above that, in terms of complexity, are molecular diodes and transistors. Vibrational spectroscopic methods have some significant advantages (as well as a number of shortcomings) when looking at molecular materials, whether they are in a device or present as bulk or pre-processed samples. Firstly, vibrational spectroscopic techniques are generally nondestructive and thus materials in a single device may be monitored over time and under differing stimuli. Secondly, spectroscopic methods may be time-resolved. In this regard it is possible to examine molecules in a device as the materials cycle through the operation of the device, for example are held under electrical potentials, or even examine short-lived excited or transient species. Thirdly, it is possible to spatially resolve spectroscopic measurements to micron length scales with comparative ease and to even shorter length scales in more involved and sophisticated experiments. In short, vibrational spectroscopy may be deployed, non-destructively, on operating devices and for measuring transient phenomena with micron spatial resolution. One type of molecule-based device that illustrates the potential of vibrational spectroscopy in interpreting and understanding materials and their operation in devices are dye-sensitized solar cells, or DSSCs, and we highlight a number of studies that have utilised vibrational spectroscopy in DSSCs. Studies examining bulk heterojunction solar cells, switches, memory, organic light emitting diodes (OLEDs), molecular wires and junctions are also discussed. With respect to vibrational spectroscopy, the focus of this review, bulk properties tend to be better suited for investigation. For example, the bulk-phase approach to organic field effect transistors (OFETs) essentially mimics traditional ‘‘inorganic’’ designs.6 These OFET designs show performances comparable to traditional a-Si:H thin film transistors, while being made from cheaper raw materials and opening up possibilities for novel properties, such as flexible devices. Manufacturing of plastic-substrate OFETs has also been achieved; only the electrodes are non-organic in these devices. Examination of the microscale of bulk materials is important for understanding a number of behaviours, including interfaces between phases, crystallinity, mixing of molecules in heterogeneous materials and layering of materials.

2

Techniques

Characterisation of molecule-based devices and the connections between them has remained a fairly difficult task, with many tools having been used to date. These include techniques to investigate the properties of both single-molecule systems and bulk systems. 2.1 Traditional techniques Single molecule techniques. A number of methods have been devised to measure electronic properties of single molecule systems, including: (1) Break junction method.7–9 A thin metal electrode is stretched until it breaks by mechanical or electrical means. The resultant sub-nanometre gap can then be spanned by one or more molecules. (2) Cross wire method.10 This journal is

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An electrode coated with a self-assembled monolayer (SAM) is brought into close proximity with an uncoated electrode and a current is passed through until contact is made. (3) Metal nano-particles method.11 SAM coated metal nano-particles are trapped between two electrodes. (4) Conductive probe atomic force microscopy (CP-AFM) and scanning tunnelling microscopic (STM) methods.12,13 A conducting AFM or STM tip is used to probe the current through a monolayer assembled on an electrode. Scanning tunnelling microscopy (STM) is a powerful technique,14,15 but is limited to well-defined surfaces. For this reason it is often implemented using a number of other techniques, for example ‘break-junctions’ (described above), to create a single molecule system. The size of the gap between electrodes can be controlled mechanically or by electron-beam lithography.16,17 Atomic force microscopy (AFM) typically has a lower resolution than STM, however unique information about bonding of the molecule–electrode contacts is provided by the concurrent force- and conductance-measurements. An extension of STM, ballistic electron emission microscopy (BEEM) uses a negatively biased tip to inject electrons into a thin metallic film, through the semiconductor layer to produce a current, which is measured at the semiconductor collector. This provides information on electron transport within the metal–molecule–collector system, with a spatial resolution of B1 nm. Inelastic electron tunnelling spectroscopy (IETS) is one of the few experimental techniques utilised to look at molecular electronic structures using vibrational spectroscopy. The method allows vibrational modes of adsorbates on metal oxides to be investigated. It is based on electron transfer through the molecule so is a fitting technique for investigation of molecular electronics.18,19 IETS yields high resolution and high sensitivity results but can be difficult to implement as it requires very low temperatures (liquid helium, B4 K) to avoid serious broadening of spectral features. Molecules ‘buried’ in a surface that may not be accessible by scanning probe microscopy (SPM) are able to be probed using this technique, making it suitable for investigating molecules sandwiched between two electrodes, as is often the case in molecular electronic systems. Troisi et al.20 have used IETS to investigate the electronic pathways of molecules. This was achieved by determining the vibrational modes associated with s and p orbital channels, using DFT calculations, and comparing these to the experimental IETS spectra. A significant amount of work has also been done on single molecules using IETS, inspired by a study published by Ho et al.21 The work showed that a substantial isotopic shift of the C–H stretch mode in acetylene could be seen upon deuteration with IETS, as well as investigating spatial imaging of the inelastic tunnelling channels. Bulk material techniques. Using X-ray reflectometry (XRR) thickness determination is achieved with a high degree of accuracy, down to A˚ resolution, by measuring the grazing incidence angle of X-rays reflected at the interface. Because of the high energy of X-rays this technique can be used to investigate SAMs present in multilayered structures. X-Ray photoelectron spectroscopy (XPS) provides information on the chemical composition of a sample as well as oxidation states of constituent elements.22–24 The technique involves This journal is

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irradiating the material with X-rays and measuring the number of electrons liberated from the surface as well as their kinetic energy. A challenge of the experimental procedure is the extremely high vacuum conditions required. An extension of this technique involves obtaining spectra at different ‘take-off angles’, that is the angle between the sample plane and the detector plane. As the take-off angle decreases a greater proportion of the signal detected is derived from the outermost surface species. Another technique that can be used to garner information on the composition of solid surfaces and thin films is Secondary Ion Mass Spectrometry (SIMS).25,26 Similarly to XPS, the surface is irradiated with a primary ion beam, the ejected secondary ions are subsequently collected and analysed using mass spectrometry. The technique is extremely sensitive, detecting elements in the range of parts per billion. Again the technique requires a high vacuum. Optical ellipsometry is a routinely utilised tool to establish the thickness of self-assembled monolayers (SAMs).27 Polarized light is reflected off a surface and the degree of rotation is measured to give information on layers as thin as a single molecule. As well as being able to determine the thickness of layers, ellipsometry can also reveal information on sample morphology, chemical composition and electrical conductivity. 2.2 Optical vibrational spectroscopic techniques The case studies discussed focus on mid-IR and Raman spectroscopic techniques; extensive literature exists for each of these, which focuses on the theory in depth.28–31 In contrast this article addresses applications. Vibrational spectroscopy involves a change in vibrational quantum number of an analyte, by direct absorption of a photon of appropriate energy (n1), or by non-elastic scattering of higher-energy photons (nex), as depicted in Fig. 3a. In both cases the vibrational signature of a molecule is probed, however different selection rules and experimental requirements make IR and Raman spectroscopic techniques complementary rather than competitive. Vibrational signatures are sensitive to electron-shifts and changes in environment, allowing for the investigation of effects such as polarisation, photoexcitation, thermal and solvent effects, intramolecular interactions such as hydrogen-bonding, amongst others. In many cases direct inferences relating to electronic and structural changes can be made with regard to such influences. Although Raman scattering is intrinsically a weak effect, signal-enhancement can be achieved in a number of ways. Perhaps the most common enhanced-Raman technique is surface enhanced Raman spectroscopy (SERS), encountered when an analyte is adsorbed onto an appropriate metallic surface. This has been explained using a combination of surface plasmon resonance and resonance with the Fermi-level of the metal.32,33 An extension of this is tip-enhanced Raman spectroscopy (TERS), where a SERS-active substrate is attached to an AFM-tip. Surface-enhancement occurs only at the tip position, allowing for the acquisition of spectra with detailed spatial resolution.34–37 It should be noted that selection rules are relaxed by analyteadsorption, making comparisons between surface enhanced and normal Raman spectra difficult. A second method of increasing Raman signal is resonance enhancement, which occurs if the Chem. Soc. Rev., 2012, 41, 1929–1946

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Fig. 3 (a) Energy-level diagram of infrared and Raman processes, depicting the ground state (GS, v = 0), the first vibrationally excited state (v = 1) of a sample as well as a virtual state (VS) that Raman scattering occurs from. (b) Energy-level diagram of resonance Raman and time-resolved infrared processes. Resonance Raman scattering occurs when nex is chosen to match the energy of the absorption manifold and thus provides a probe for the Frank–Condon (FC) state. Time-resolved infrared spectroscopy probes the thermally equilibrated excited (THEXI) state, which is accessed by absorption of a photon, followed by intersystem crossing (ISC) and vibrational relaxation (VR).

excitation wavelength (lex) is selected to coincide with an electronic transition (see Fig. 3b). As the scattering probability is dependent on the absorption manifold, the enhancement of bands observed is specific to vibrational modes that best mimic the structural changes incurred during the resonant photoexcitation. This can increase peak intensities by up to 106. Aside from higher-sensitivities, the systematic peak-enhancement also provides a tool to probe the initial state accessed by photoexcitation, that is, the Frank–Condon (FC) state. Comparison of resonance enhancement at a number of excitation wavelengths to the UV-Visible spectrum allows one to extract numerical information about the structural changes during excitation.38–41 A possible drawback of resonance Raman is that working in the visible region often leads to interference from fluorescence due to the relative weakness of the Raman signals, even with resonance enhancement. In order to probe states subsequent to the FC state, a pump– probe approach must be taken. Using optically or electronically coupled laser pulses, a compound can be excited and subsequently probed with very short time-delays. Fig. 3b shows the processes that occur upon the absorption of a photon that lead to the formation of a thermally equilibrated excited (THEXI) state, which can be probed by an infrared pulse. Although not shown in Fig. 3, Raman scattering can be effected from this state as well. Time-resolutions of femtoseconds have been achieved by both time-resolved infrared (TRIR) and time-resolved resonance Raman (TR3), which allows for the observation of extremely short-lived species while providing information about their structures. As it can be difficult to separate signals from the ground and excited states, it is often advantageous to probe so-called reporter groups with large IR or Raman cross-sections that occur in otherwise quiet parts of the spectra. Examples of such groups are carbonyl, carboxylate and thiocyanate (vide infra). A number of surface-specific techniques exist for the study of interfaces. Attenuated total reflection (ATR) IR spectroscopy42 relies on multiple total internal reflection of a probe beam 1932

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within a high refractive-index prism. An evanescent wave penetrates up to several micrometres above the surface at each reflection, allowing to selectively probe the region in close proximity to the prism. Even-order techniques such as sum frequency generation (SFG)43–47 or w4 Raman48 on the other hand rely on the fact that the optical processes involved require non-centrosymmetric analytes, as are found at interfaces, and are forbidden in bulk-media. SFG is a technique whereby two input beams are directed at a sample and the sum of the two frequencies is detected. In vibrational SFG (VSFG) one input beam (visible) remains at a constant frequency and the second beam (infrared) is scanned through a range of frequencies.49 The intensity of the sum-frequency generated varies as the infrared beam goes in and out of resonance with vibrational transitions. The key to the selectivity of this technique is in the selection-rule that requires sum-frequency active modes to be non-centrosymmetric (for the same reason, SFG modes must also be both IR and Raman active). Although SFG has in the past been applied mostly to liquid–air or liquid–liquid interfaces, it is also applicable to the various layers employed in many molecule-based devices, as discussed in Section 4.3.44,50,51 Vibrational spectroscopy is a powerful tool for investigation of molecular electronics because of its non-destructive nature, allowing live monitoring of working molecular junctions, and its utility for investigating buried interfaces by making the upper substrate transparent to the appropriate wavelength range. Achieving sufficient transparency of the electrode layer to allow IR or Raman analysis is often not a significant challenge.52,53 Another advantage lies in the possibility of coupling to optical microscopy. This allows Raman spectra to be acquired at mm resolution and thus the investigation of the microstructure of devices, as has been carried out for biological samples in the past.54,55 2.3 Computational modelling The advent of powerful and relatively inexpensive personal computers and simple to use graphical user interface-based software has made quantum chemical calculations very common. As such, we will be referring to density functional theory (DFT) and time-dependent (TD) DFT calculations; extensive literature has been published on this topic.41,56,57 It is often advantageous to supplement vibrational spectroscopic analysis with quantum chemical calculations. The reasons for this are two-fold. Firstly, ab initio (or pseudo ab initio) calculations provide an independent tool for assignment of vibrational modes observed in vibrational spectra. This is especially useful for larger systems with many vibrational modes, where peak assignment can be challenging. Resonance Raman spectra benefit from such an analysis since vibrational enhancement signifies electronic activity. Secondly, vibrational spectra can be used to verify the efficacy of calculations.41,58 By comparing calculated and measured frequencies and intensities one can begin to quantitatively assess the calculated spectra and thus gain some confidence in other calculated parameters, for example molecular orbital locations and energetics. Due to their high performance-toprice ratio and well-established literature, DFT and TD-DFT calculations become the de-facto standard for calculations of large systems, where the most commonly used functional is B3LYP. This journal is

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3.1

Case studies: materials Morphology

Vibrational spectroscopy lends itself to the study of sample morphology as vibrational modes can provide a great deal of information on structure and environment. For example, Raman spectroscopy has been used extensively in pharmaceutical settings to investigate the crystallinity of samples.59,60 The technique is not limited just to pharmaceutical samples though and has been employed in the study of, among other molecules, tris(8-hydroxyquinolinato)aluminium (Alq3), a fundamental and widely studied material in the fabrication of OLEDs. Further details of the study by Rajeswaran et al.61 can be found in Section 4.5. Fundamentally, the importance of crystallinity stems from its relationship with packing and therefore, in the context of molecular electronics, charge carrier mobility. Equally, mesophases can be studied to great effect using vibrational spectroscopy. Specifically, liquid crystal materials are an area of interest to molecular electronics for their unique properties and use in display technologies. Of these, polyaromatic hydrocarbons (PAHs) have been targeted as promising molecules as they often display self-assembling columnar mesophases allowing easy charge conduction between rings, along the columns. Carminati et al.62 discovered a clear discontinuity in spectral features when studying HBC-C12 (hexa-peri-hexabenzocoronene, hexa-dodecyl derivative) using Raman and IR, at the temperature where the molecules change from a crystalline mesophase to a liquid-crystal one, 107 1C. Specifically, a band at 1470 cm1 (arising from long trans alkanes displaying an ordered configuration) was seen to decrease in intensity with increasing temperature, inferring an increase in disorder (Fig. 4). Additionally, the shift from the previously determined herringbone crystalline structure to the planar stacking liquid-crystalline mesophase was confirmed by a softening of the aromatic C–C stretches (from 1610 to 1580 cm1). Vibrational spectroscopy may also be employed as a convenient non-destructive way of confirming what changes, if any, occur upon preparation of a self-assembled monolayer (SAM).63,64 SAMs are a useful construct in molecular electronics, as they allow a straightforward synthetic approach to fabricate multielectrode devices incorporating a molecular layer. Orientation and ordering are key properties of interest in such devices,

Fig. 4 The –CH2– deformation region of the IR spectrum of HBC-C12 taken at a range of temperatures: (a) 30 1C, (b) 50 1C, (c) 70 1C, (d) 100 1C, (e) 110 1C, (f) 130 1C, (g) 200 1C. Inset shows hexaperi-hexabenzocoronene. Adapted from ref. 62.

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and vibrational spectroscopy is well suited to highlighting such structural features. In particular, many studies have incorporated IR, whether it be attenuated total reflection (ATR)- or reflection–absorption (RA)-IR spectroscopy. In addition to crystalline properties, discussed in Section 4.5 with a Raman example, IR spectroscopy can inform on a number of other characteristics: (1) whether a molecule stays intact after adsorption onto a surface. For example, fluorinated oligo(phenylene ethynylenes) (OPEs) showed similar FT-IR features when comparing a SAM or a KBr pellet, both incorporating the molecule;65 (2) the difference in SAM coverage between different molecules. Alkanethiols and alkanethiocyanates have been compared and based on the shift in the asymmetric CH2 stretching mode (from 2922 cm1 for C12SCN to 2918 cm1 for C12SH) the authors concluded that the packing of the SAMs differed;66 and (3) the occurrence of disorder in a sample. Gomar-Nadal et al.67 investigated the packing of tetrathiafulvalene (TTF) derivatives on gold surfaces using IR. Several C–H stretching modes were monitored in the SAMs and compared to example spectra of crystalline, solid and liquid samples. The peak positions of the C–H modes displayed by the SAMs match more closely those observed for liquid-like n-alkylmonothiols than solid-state spectra of TTF samples, inferring a disordered or liquid-like packing in the SAMs.

4

Case studies: devices

There are a number of functioning devices in the literature related to molecular materials, at various stages toward commercialisation.68–73 In this section we look at a number of these in turn. 4.1 Dye sensitised solar cells An area of intense research in molecule-based devices is that of dye-sensitised solar cells (DSSCs), largely due to the immense potential impact of their successful mass-market application. Several in-depth reviews on the topic have recently been presented.74–82 A brief overview of the mechanisms within DSSCs follows. Fig. 5 shows a schematic of a DSSC with the processes active during operation. The cell consists of a sensitiser dye, denoted S in the ground state and S* and S+ in the photoexcited and oxidised states, respectively, a semiconductor nanoparticle film (typically TiO2) on a transparent electrode and a redox mediator in the electrolyte (typically I/I 3 ) to facilitate electronic communication with the counter-electrode. Binding of the dye to the semiconductor can occur through a number of linker groups, however we will focus on examples using carboxylate attachments. Successful operation involves absorption of a photon by the dye (S - S*, step 1), injection of the excited-state electron into the conduction band of the semiconductor (step 3) and channelling into the electrode (step 4). The redox couple in the electrolyte returns the oxidised dye back to the ground state (S+ - S, step 5), while being itself subsequently reduced at the counter-electrode (step 8). In black are shown processes that are detrimental to the efficiency and long-time stability of the cell; namely, decay of the excited state of the dye (step 2), recombination of the injected electron by interactions with the dye (step 6) and recombination by interaction with the electrolyte (step 7). Approaches taken to improving the efficiencies of DSSCs are Chem. Soc. Rev., 2012, 41, 1929–1946

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Fig. 5 Schematic of a DSSC showing relevant processes. CB is the conduction band of the semiconductor (traditionally TiO2), S, S* and S+ are, respectively, the ground, photoexcited and oxidised states of the sensitiser and I/I 3 is the redox mediator. Adapted from ref. 81, with modifications.

thus to promote processes 1, 3, 4, 5 and 8 and to hamper processes 2, 6 and 7. A widely used approach is to modify the dye in the hope of increasing the light-harvesting and charge-injection efficiencies. Although much work is done through empirical understanding, there have been several systematic efforts at understanding the functioning of the dyes. In detailed review, Hagfeldt et al.81 have identified a number of other key questions for further research into DSSCs. Amongst others: What is the mechanism for electron transport? Do electrons move through the conduction band or by hopping between traps? A detailed understanding of the microstructure of the semiconductor is required. What should the injection time be for high yields? How fast are electrons injected and when does injection become limiting? How relevant are currently used lifetime tests? This question is concerned with determining projected product-lifetimes. What happens to the oxidised dye after electron-injection? Are there pathways provided by the dye that facilitate reactions of injected electrons with the electrolyte? Is the reduction of S+ by the electrolyte a limiting step? Vibrational spectroscopy can address some of these issues. The most commonly used DSSC dyes are in the family of ruthenium polypyridyl complexes, most notably bis(2,20 -bipyridyl4,4 0 -dicarboxylate)ruthenium(II), N3 (Fig. 6a), which first helped DSSCs break the 10% efficiency mark83 and has been the de facto standard until recently. As such, a number of studies have investigated the electron transfer dynamics of N3 (and related compounds) in solution and bound to semiconductors using ultrafast time resolved infrared (TRIR) spectroscopy, both in in the mid- and near-IR regions.85–94 Although initial time-resolved studies were carried out using transient absorption measurements,95 the use of vibrational spectroscopy is advantageous since the probe windows can be selected to avoid interference from excited-state absorption changes; that is, the comparatively narrow linewidth of vibrational peaks aids accurate interpretation of spectra. Similarly, near-IR 1934

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Fig. 6 Examples of solar cell dyes. (a) The structure of bis(2,2 0 -bipyridyl-4,4 0 -dicarboxylate)ruthenium(II) (N3). (b) Porphyrin structures with varying b-substituents. Thiophenes with n = 1, 2, 3 were investigated by Lind et al.84

spectra have been shown to be less reliable than mid-IR spectra when monitoring electron-injection into semiconductors (vide infra). An advantage of N3 in terms of mid-IR spectroscopy is that is contains two thiocyanate (NCS) ligands that show IR-active stretching modes at ca. 2115 and 2140 cm1 (the latter as a shoulder), which are spectrally isolated from other infrared activity. The NCS groups can thus be used as a reporter group that provides a spectroscopic handle on the structural changes upon photoexcitation without the complexities of multiple signals arising from overlapping ground and excited state signals of the polypyridyl groups, found below ca. 1700 cm1. Similarly, vibrations from carbonyl groups in other metal complexes96–101 as well as carboxylate groups (or their ester-derivatives94) are also commonly used as reporters. As shown in Fig. 7, photoexcitation of N3 in solution results in bleaching of the peak at 2115 cm1 and shoulder at 2140 cm1 and a grow-in of new bands at 2040 and 2075 cm1. The timeconstant for the appearance of the new peaks has been reported around 50 25 fs, which is in agreement with transientabsorption studies.86,95 The change in vibrational frequency has been ratified based on the following resonance structures that are thought to exist in metal-bound NCS: Ru–NRC–S::: 2 RuQNQCQS:: A typical NQC bond, as found for example in acetonitrile, has a stretching-frequency of ca. 2250 cm1, while CQN bonds of imines are around 1650 cm1.102 From this it is evident that the structure of ground-state NCS is closer to the left-hand side of the equilibrium and photoexcitation causes a shift to This journal is

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Fig. 7 (a) The FT-IR spectra of N3 in ethanol (solid line) and on Al2O3 film (dotted line). (b) The TRIR spectra of N3 in ethanol (full circles) and on Al2O3 film (open circles). Adapted from ref. 85.

the right-hand side. This is achieved by an oxidation of the metal or NCS units and has been interpreted as the result of an MLCT transition. In solution, these signals persist for several nanoseconds, in consistency with luminescence studies.83 For N3 bound to Al2O3, a non-injecting semiconductor, the results are similar. In the ground-state the shoulder at 2140 cm1 is no longer observed while the excited-state peak at 2075 cm1 is very weak; however, the B50 fs rise-time and ns persistence of the excited-state signal is the same. Although the conductionband of Al2O3 is too high for electron injection, there is some injection into surface states, with a time-constant of B130 ps. Studies on TiO2 show significantly different behaviour since the conduction band of this semiconductor is of appropriate energy for electron-injection from the excited-state of N3. Photoexcitation of TiO2-bound N3 shows a band at ca. 1200 nm in the near-IR region, which was initially attributed to injected electrons;86 however, similar signals were later shown to be present in Al2O3-bound and free N3,87 possibly corresponding to excited-state absorption of the dye. In mid-IR experiments, the origins of the observed transients are more easily identified. They show a broad band, distinctly different to Al2O3bound or free N3, which can reliably be attributed to injected electrons. Again, the rise-time of the signal is ca. 50 fs. This shows that electron-injection can occur from the vibrationally hot state of N3, as cooling is on the picosecond timescale, and demonstrates that short excited state lifetimes (i.e. t E 1 ns) are not limiting to the injection efficiencies of these types of dyes. The broad band in the mid-IR experiments is thought to be due to shallow traps below the TiO2 conduction band, where the excited-state electrons reside. Potentially very important in understanding DSSC efficiencies are deeper traps, which require higher-energy probes and are thus missed by experiments probing only the mid-IR range.91 Further reading on charge-transfer of transition metal compounds bound to semiconductors can be found from Anderson and Lian103 as well as Ardo and Meyer.104 The electrolyte used in a vast majority of DSSCs is I/I 3, which is corrosive and undergoes complex two-electron redox reactions. This is likely to promote side-reactions and is thus This journal is

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detrimental to cell-lifetime, even with the most stable dyes (due to surface modification of TiO2, vide infra). A number of other electrolytes have been investigated, with some of them showing successful results recently;80,105–108 however, the majority of studies have so far been carried out on I/I 3 . Raman studies on cells is made possible despite low concentrations of dye by a combination of surface-enhancement and resonanceenhancement. Variations of the interfacial electron-transfer mechanism as well as the emergence of intermediate species on the photoelectrode are directly measurable using Raman, due to the effects these processes have on the vibrational frequencies and intensities. The reactions of I 3 with the oxidised dye have been followed spectroscopically using in situ and polarisation studies of DSSCs.109–111 Using low-frequency resonance Raman, bands at 112, 143 and 167 cm1 were found (Fig. 8), corresponding to I 3 , anatase TiO2 and an adduct of the oxidised state of the dye and iodide respectively. The S+/I 3 adduct is thought to be a crucial intermediate in the regeneration of the ground-state dye.112,113 It was possible to monitor the presence of I 3 and dye-adducts thereof for a number of polypyridyl dyes over a range of applied potentials (Fig. 8). After several cycles to negative potentials the band at 167 cm1 could no longer be recovered. In addition to the low-frequency stretches it is possible to observe NCS stretching modes and thus directly monitor coordinated, coupled to iodine species as well as free thiocyanate (NCS) in devices. Adsorption of the dye to TiO2 decreases the NCS stretching intensities and causes the emergence of new bands, in the polypyridyl stretching region, ca. 900–1700 cm1. Using these markers, degradation of DSSCs under prolonged exposure to light has been investigated.114 Only minor variations were observed in the resonance Raman spectra of the adsorbed pristine and stressed dyes, with no loss or substitution of the NCS ligands. However, broadening and intensity decreases of the TiO2 spectrum were found upon prolonged irradiation, consistent with surface modificaiton of the TiO2 by electron accumulation. It remains to be seen whether the new generation of electrolytes or modification of the dyes is able to prevent deterioration of the TiO2. The electrolyte solvent is also an important consideration, as it has influence over the ionic mobility, charge separation, TiO2 conduction-band energy as well as a host of other parameters.115–117 Stergiopoulos et al.118 have shown that the

Fig. 8 Low-frequency resonance Raman bands of a model of a DSSC at a number of potentials. From low to high Raman shift, we see peaks from I 3 , anatase TiO2 and a dye-I3 adduct. Adapted from ref. 109.

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differences in chemical interactions at the dye/semiconductor interface between a number of solvents results chiefly from a change in the donor number. Using resonance Raman, they monitored vibrational modes of carboxylate linker units and found that solvents with low donor numbers lead to stronger chemical binding, which has been correlated to surface coverage measurements measured by dye-I 3 scattering variations. Porphyrins have been used as sensitiser dyes77,119–122 with efficiencies of up to 7.1% reported. Fig. 6b shows examples of porphyrin dyes investigated using resonance Raman spectroscopy by Lind et al.84 A Zn-metalated porphyrin is b-substituted with a conjugated linker that attaches to the TiO2 semiconductor with a carboxyl group, which is a common motif. Since a number of conjugated linkers have been used in the past, this study attempts to gain an understanding of the interactions of the porphyrin core with b-substituents and the effect this has on device efficiencies. Fig. 9 shows the resonance Raman spectra complexes with one and three thiophene units (n = 1 and n = 3, see Fig. 6b) at a number of excitation wavelengths, where lex = 413.1, 457.9 and 514.5 nm represent the probing of different electronic transitions, termed the B-band, B 0 -band and T-band respectively. While the B-band is present in unsubstituted porphyrins, the B 0 -band and T-band are new and result from orbital mixing of the porphyrin core with the b-substituent. Looking at spectra of the n = 1 compound (Fig. 9, bottom), it is evident from the pattern of the enhanced peaks that different states are probed. Using DFT calculations the peaks of the lex = 413.1 nm spectrum have been assigned as vibrations centred on the porphyrin core, while peaks in the lower-energy spectra have been assigned as delocalised and linker-centred vibrations.

This shows that excitation of the B 0 -band and T-band results in population of molecular orbitals located, at least in part, along the thiophene linker. The lex = 413.1 nm spectrum of the compound with three thiophene units (Fig. 9, bottom) is different to that of the n = 1 compound; however, the vibrations can again be assigned as centred on the porphyrin core. The difference is evidence for the population of different molecular orbitals compared to the n = 1 compound. The spectrum consistent with probing of the T-band, acquired at lex = 514.5 nm, shows only two peaks at 1048 and 1453 cm1. These are assigned as based purely on the thiophene-linker, which means that at this wavelength the porphyrin core plays no part in the electronic excitation of this compound. TD-DFT calculations are consistent with these results as they show the LUMO of these complexes become more linker-based upon increasing the number of thiophene units in the linker, while higher-energy unoccupied molecular orbitals remain porphyrin-centred. Devices constructed with these complexes as dyes shows a trend of decreasing Z as well as total absorption cross-section with increasing number of thiophenes. This is surprising based on observations from previous designs of photosensitive organic dyes but can be explained by communication of the porphyrin core with the linker. Although both core and linker molecular orbitals of the n = 3 complex are accessible by visible-light excitation, they are too distant in energy to each other to allow for electron-transfer to take place from the core to the TiO2 during excitation of the B-band and the B 0 -band. On the other hand, even though excitation of the T-band leads to electroninjection into the TiO2, the oscillator strength of this transition is too low in energy without contribution from the porphyrin core for the complex to be an efficient photosensitiser. As the thiophene-chain is shortened, communication between the porphyrin and TiO2 becomes more efficient, which is demonstrated by the resonance Raman spectra. 4.2 Bulk heterojunction cells

Fig. 9 Resonance Raman spectra of porphyrins with a varying number of thiophene units (n) in the linker chain (see Fig. 6b). The top spectrum shows n = 1, the bottom spectrum shows n = 3. Adapted from ref. 84.

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A second prominent class of molecule-based solar cells is known as polymer-, bulk heterojunction-, polymer-fullerenecells or simply organic photovoltaic devices.123–127 Their design is based on light-absorbing conjugated polymers, such as poly(3-hexylthiophene) (P3HT); fluorene and carbazole derivatives are also used, amongst others. Photoexcitation of the polymer leads to formation of an exciton with a typical lifetime less than a nanosecond, which can travel along the polymer chain before undergoing radiative or non-radiative decay. In order to use the absorbed energy, charge-dissociation of the exciton has to be effected, which is achieved by blending the polymer with an electron-acceptor (usually phenyl-C61-butyric acid methyl ester or PCBM), creating an interpenetrating network. Increasing the interfacial contact minimises the diffusion-length required for efficient charge-separation. The ability to control and understand morphological changes are thus important to cell design. IR spectroscopy has been used in a number of studies to monitor bulk morphological changes.128–130 Ultrafast two-dimensional IR has been used to investigate conformational inhomogeneity of PCBM in thin films, involving the electron-donors P3HT and poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-(1-cyanovinylene)phenylene] (CN-MEH-PPV).131–134 It has been shown that the This journal is

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CQO modes of PCBM can be used as markers that indicate charge-transfer, as the position and linewidths of these are dependent on the environment of the PCBM.132 Thus, the location within domains of PCBM can be monitored using the CQO stretch, leading to a number of insights. It was found that domains of PCBM exhibit charge-separation only at their interfaces; furthermore, using the spectral evolution of the carbonyl bleach the radial velocity of electrons was estimated as 1–2 m s1. This radial velocity remains constant at a number of temperatures, which indicates that formation of free charge carriers in a CN-MEH-PPV:PCBM blend occurs on an activationless pathway.135–138 The timescale for this process is in the picosecond range and it is thought to occur by the distribution of excess vibrational excited-state energy. For a more thorough discussion of these studies, see an article by Pensack and Asbury.137 Gao et al.139,140 have recently used resonance Raman and photocurrent imaging (RRPI) as a method for spatially correlating film topography to photocurrent generation in P3HT/ PCBM-based devices. Using an excitation beam in resonance with P3HT absorption (lex = 488.0 nm), they monitored un Iagg CQC/ICQC (R), the intensity-ratio of CQC stretches at B1450 and B1470 cm1, assigned to be of aggregated and unaggregated P3HT respectively. Aggregated P3HT chains were found to possess high order and relatively long conjugation lengths while unaggregated P3HT is less ordered and conjugated. Due to intensity enhancements by the resonance effect, any observed features were assigned solely to P3HT. Since measurements are done on functional devices and the excitation beam serves to generate a photocurrent, it is possible to simultaneously characterise and correlate aspects of morphology and device operation. Confocal microscopy with a spatial resolution of 250 mm allowed the mapping of local variations of the total CQC intensity, the ratio R as well as photocurrent. These are shown of (as-cast, annealed) devices in Fig. 10(a, d), (b, e) and (c, f) respectively. Linescan graphs of annealed devices, as indicated by arrows on the corresponding maps, are shown in Fig. 10(g)–(i). In the as-cast devices the interdispersion of P3HT and PCBM is on a sub-resolution scale (see Fig. 10(a–c)), however the annealed devices show more definite results. Annealing causes the formation of PCBM

Fig. 10 (a, d, g) show the total Raman intensities of the CQC un stretching mode. (b, e, h) show the ratio Iagg CQC/ICQC. (d, f, i) show the photocurrent generated. The left and middle columns show as-cast and annealed devices, respectively, while the right column shows linescan graphs of the annealed devices. Adapted from ref. 139.

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microcrystallites as well as regions of high P3HT aggregation density, corresponding to very low and high Raman intensities respectively. Both of these types of regions show low photocurrents. While this is not surprising for the PCBM crystallite regions it is a new finding for P3HT aggregates, as it had previously been thought that high aggregation is important to attaining efficient charge transport. Interfaces also show low photocurrents and as with the P3HT aggregated regions this is due to lack of charge-generation, caused by more complete PCBM/P3HT separation on the nanoscale. Overall it was found that intermediate aggregation is most beneficial for photocurrent generation. More recently, Falke and co-workers141 have suggested that the peak found at B1470 cm1 is not due to unaggregated P3HT but instead originates from PCBM, which would significantly alter the interpretation of the earlier studies. This is supported by a strong peak in the PCBM spectrum at 1464 cm1, the absence of the 1470 cm1 peak in pristine P3HT as well as substructure in this peak that matches vibrations of C60. A way to resolve this would be to change the excitation energy away from the P3HT absorption peak, which should affect the relative intensities of the 1450 and 1470 cm1 peaks in the vibrational spectrum. Tip-enhanced Raman spectroscopy (TERS) also provides the opportunity to observe morphological structure with high spatial resolution while retaining the ability to vibrationally probe the sample. This has been successfully used in the analysis of thin film polymer blends,142,143 with a resolution of tens of nanometres, and some progress has been made with parabolic mirror-assisted TERS, which allows non-transparent samples to be probed, while retaining a high numerical aperture.35,144 Using CQC stretching modes from P3HT and photoluminescence from PCBM, the distribution of these materials in a blend has been imaged with a resolution of 10 nm. For a more thorough discussion of morphological characterisation of bulk heterojunction systems, we recommend a review by Nicholson and Castro.127 4.3 Switches and transistors One of the earliest molecule based junctions was made from benzene-1,4-dithiolate connected by two gold electrodes8 and calculations have revealed such a system to be a possible candidate for organic field-effect transistors (OFETs), if a gate electrode is introduced.4,70,145 The mechanism of electron transfer in this system is resonance-tunnelling, where the energy of the antibonding p* orbitals is modulated by the gate electrode to match those of the Fermi-levels of the source and drain electrodes.4,5 Single-molecule OFETs rely on the energetics and location of the unoccupied molecular orbitals; that is, these orbitals should provide a virtual ‘‘bridge’’ between source and drain, while being high enough in energy to provide a tunnelling barrier and low enough in energy to be pushed into the conduction regime by the gate electrode. Resonance Raman spectroscopy, especially when coupled with computational techniques,41 can be an excellent tool for ascertaining the location of electron-flow upon photoexcitation, thus providing information about molecular orbitals that are unoccupied in the ground state. Oligo- and poly-thiophenes have been heavily investigated with regard to single-molecule organic field-effect transistors.146–155 Several groups69,149,156 have probed charge-defects in oxidised thiophene-based oligomers. It was found that ethenyl bridges have Chem. Soc. Rev., 2012, 41, 1929–1946

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a low impedance to charge-transport in these systems, leading to increased conjugation-lengths. Further discussion about single-molecule detection and characterisation can be found in Sections 4.6 and 4.7. Readers interested in molecular transistors are also advised to consult the highly cited works of Liang et al.,157 Park et al.158,159 and Yu et al.160,161 These studies have some relevance to Raman and IR spectroscopy in the context of looking at differential conductance, Kondo effect (a measure of electrical resistivity with temperature) and Coulomb blockade. The bulk-phase approach to OFETs replicates traditional designs, such as the metal–insulator–semiconductor FET (MISFET), the metal–semiconductor FET (MESFET) and the thin-film transistor (TFT).6 Here, infrared and Raman spectroscopic techniques have provided useful tools for obtaining insights to mechanisms162–165 and characterisation.166,167 The interface between the organic semiconductor polymer and the dielectric substrate is particularly important, as the majority of charge carried in the ON state of the device is in a 1–2 nm thick layer in this region.168 Using attenuated-total-reflection (ATR) infrared spectroscopy, Chua et al.162 have probed interfacial charge-trapping states of a number of OFET devices using different semiconductor polymers. This was used to explain the lack of n-type charge-carrier mobilities, which had previously been elusive in OFETs. The surface-specific nature of ATR-IR makes this technique well suited to such an investigation. Using a similar methodology of manufacturing OFETs on the ATR prism, Kaake et al.164 were able to directly probe active layers under gate bias. Reversible electrostatic and irreversible electrochemical regions were observed under varying bias, the latter corresponding to diffusion of cations by excessive reduction of the dielectric layer. Several groups have used vibrational sum-frequency generation to investigate the interfaces of a number of dielectrics and silica in OFET devices.50,51,169–172 Anglin et al.50,51 have recently investigated the interface of P3HT and silica. Shown in Fig. 11 are the VSFG spectra of this interface at a number of gate biases, which shows a large non-resonant peak with superimposed resonant C–H stretching peaks. The non-resonant peak is due to a DC field created at the P3HT–SiO2 interface and it changes phase as a negative voltage is applied, causing the vibrational peaks to appear as negative features. Such enhancement is not observed in the control experiment, where insulating polymers are used in device construction. This is important as it implies field-enhancement by minority charge-carriers (i.e. electrons), which accumulate on the interface but not in high enough concentrations to be measured electronically. Further investigation using polarisation-dependent VSFG shed light on the interfacial orientation of P3HT. It was found that the surface-properties of the SiO2 substrate have a large influence on the orientations of the thiophene rings, which in turn determines the device performance, as p-stacking along the surface plane is facilitating of charge-migration.51 This explains earlier results linking device performance specifically to the surface functionalisation of the SiO2 substrate.173,174 It should be noted that both ATR-IR and VSFG have a significant advantage over small-angle X-ray scattering since they probe the entire substrate surrounding the interface, not just the crystalline portion. 1938

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Fig. 11 VSFG spectra of a P3HT OFET collected at a number of gate potentials (VG), acquired in increments of 10 V. Adapted from ref. 50.

4.4 Memory Write-once read-many-times (WORM) memory devices are used for disposable radiofrequency identification (RFID) tags, which need to be very inexpensive to produce.71,72,175,176 Poly(3,4-ethylenedioxythiophene) (PEDOT) is a promising candidate for memory applications due to its stability as well as ease of processing (it can be synthesized in a number of ways in aqueous media) and transparency when doped. Garreau et al.177,178 used the resonance Raman effect, in concert with computational modelling, to deduce the structural changes in the molecule as it is doped. The absorption spectra of the neutral and doped PEDOT show distinct peaks at 600 and 1200 nm respectively. Consequently, excitation wavelengths of 514 nm and 1064 nm can be used to selectively probe the two oxidation states (Fig. 12 and 13). The scheme in Fig. 14 was inferred from the Raman data as well as calculated force constants and cyclic voltammograms, which indicate that the doping process is a two-step oxidation. The most striking difference between the resonance Raman spectra of the neutral species between 514 nm and 1064 nm excitation is the shifting of the intense symmetric CaQCb peak from 1434 to 1423 cm1, indicating that this band is dependent on the exciting wavelength. It is unsurprising that with lex = 514 nm the observed peaks (which are due to the neutral species) decrease in intensity with increasing oxidation (Fig. 12). In comparison, with lex = 1064 nm (Fig. 13) the symmetric CaQCb peak (1423 cm1) does not change significantly in intensity but does shift in frequency, first down to 1411 cm1 (at 0.3 V) then up to 1431 (+0.3 V) and 1450 cm1 (+0.8 V). This weakening and subsequent strengthening of the CaQCb bond is consistent with the proposed shifting of electron density within the thiophene ring shown in Fig. 14. This journal is

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Fig. 14 Proposed scheme of the doping process of PEDOT. Adapted from ref. 177.

Fig. 12 Spectroelectrochemical resonance Raman spectra of PEDOT with excitation at 514 nm in 0.1 M (TBA)BF4/ACN. Asterisks indicate solvent bands. Adapted from ref. 177. Fig. 15 Raman spectra (lex = 633 nm) of PEDOT : PSS-based WORM memory device. Adapted from ref. 72.

ITO substrate. The switching mechanism of these devices was investigated using Raman spectra at 633 nm excitation. As shown in Fig. 15 there is a significant change upon switching, where a broad band at ca. 1420 cm1 is replaced with clear and sharp peaks at 1422 and 1516 cm1, corresponding to the symmetric and antisymmetric stretching modes of CQC. As these spectra are very similar to those observed by Garreau et al.,177,178 discussed above, the switching mechanism can be identified as transforming from p-doped to undoped PEDOT. This is also confirmed by reflectance measurements. 4.5 Organic light emitting diodes

Fig. 13 Spectroelectrochemical resonance Raman spectra of PEDOT with excitation at 1064 nm in 0.1 M (TBA)BF4/ACN. Asterisks indicate solvent bands. Adapted from ref. 177.

An organic device based on polyethylenedioxythiophene doped with polystyrene sulfonic acid (PEDOT:PSS) was demonstrated by Mo¨ller et al.175 As an extension of this, Wang et al.72,176 have recently shown entirely solution-processable WORM memory devices, using colloidal semiconductor ZnO nanoparticles as a hole-blocking layer that is spin-coated onto an This journal is

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Organic light-emitting diodes (OLEDs) have been of significant interest for a number of years because of their potential in next-generation display technologies.180,181 Vibrational spectroscopy can shed light on a number of properties of working OLEDs as well as promising materials for later inclusion into devices. With respect to working devices, Tsuji and Furukawa179 demonstrated how Raman spectroscopy can be used to monitor the temperature of an OLED incorporating, as the emitter layer, bis(2-(20 -benzothienyl)-pyridinato-N,C30 )iridium(acetylacetonate), (Btp2Ir(acac)). OLEDs work by injecting holes into the hole transport layer (HTL) and electrons into the electron transport layer (ETL) and subsequent recombination and emission of light at the middle layer when a bias is applied. Upon recombination, a large amount of energy is turned into heat, in addition to that converted to light, therefore monitoring the temperature of the OLED is an important consideration, especially in individual layers with regard to decomposition. The temperature was found Chem. Soc. Rev., 2012, 41, 1929–1946

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using the intensity ratio of anti-Stokes to Stokes Raman scattering, which is known to be related to the temperature of the thermally equilibrated material.182 532 nm excitation was used to probe the OLED. The Raman spectra at a number of different current densities, between 400 and 400 cm1 (Fig. 16), displays only bands attributed to Btp2Ir(acac), enhanced by the resonance Raman effect. This means that the temperature of the Btp2Ir(acac) layer specifically can be obtained and was found to be around 90 1C, as opposed to 26 1C for the glass layer, measured using a traditional thermocouple. A fairly fundamental property of OLEDs is their electrical conductivity. This was studied by Yokoyama et al.183 by showing that it could be correlated to the degree of C–H N hydrogen bonding and therefore molecular stacking and carrier mobility (Fig. 18). The experimental IR spectra of films of the molecules is compared to simulated spectra of individual molecules in Fig. 17. The observed blueshifting of the C–H stretching mode (3000 cm1) of B3PyMPM and B4PyMPM is well known to be associated with the formation of hydrogen-bonding. These molecules H-bond more easily because of the accessibility of the N atoms, and the IR spectrum corroborates this. This H-bonding results in an ordering of the molecules parallel to the surface (Fig. 18) and consequently an increase in charge-carrier mobility in the film. The results were analysed in terms of Ba¨ssler’s formalism where electron transport occurs by hopping through a manifold of localised states containing energetic (s) and positional (S) disorder. Based on this formalism the energetic disorder, s, is less for the B4PyMPM film than the other two, because of its smaller permanent dipole moment, inferred from the TOF experiments. Additionally the positional disorder of the B3 and B4PyMPM films is less than that of the B2 film, due to the horizontal ordering of molecules by the C–H N H-bonding. Fig. 18 summarises that the smaller disorder values lead to a greater charge mobility and hence can be correlated to the degree of H-bonding seen in the IR spectra of sample films. Another important characteristic in the consideration of OLED materials is that of crystallinity. As discussed above, Raman spectroscopy is an ideal tool for investigating structure and has been utilised in understanding one of the most seminal OLED materials, Alq3, tris(quinolin-8-olato)aluminium(III).

Fig. 16 Raman spectra of an OLED incorporating layers ITO/ a-NPD/CBP:Btp2Ir(acac)(15 wt%)/BAlq/Alq3/LiF-Al at current densities of (a) 0, (b) 100, (c) 200, (d) 300, and (e) 400 A m2. Inset shows Btp2Ir(acac). Adapted from ref. 179.

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Fig. 17 (a,b) diagrams of example C–H N bonding in films of B3PyMPM (a) and B4PyMPM (b) molecules, other interactions also exist for these molecules. (c) Simulated IR spectra of single molecules obtained by DFT (B3LYP/6-31G(d,p)) calculations. (d) Experimental IR spectra of vacuum-deposited films, note the blueshift of the main bands of B3PyMPM and B4PyMPM compared with B2PyMPM. Adapted from ref. 183.

Fig. 18 TOF measurements of the dependence of electron mobilities of films with respect to applied electric field (right). Schematics of the molecular orientations and permanent dipoles of films (middle). The greater degree of order in the B3 and B4PyMPM films is due to the H-bonding (observed in the IR spectra). Arrows represent the direction of the large permanent dipole moments of molecules. Adapted from ref. 183.

First reported by Tang and Van Slyke,184 the crystal phases of the molecule have been investigated more recently by Rajeswaran et al.61 Alq3 can exist in either a meridional or facial structure and has been found to exhibit five different crystal structures, termed b, a, e, d and g. Fig. 19 displays the Raman spectra obtained for Alq3 samples exhibiting different crystal phases. The phases can be grouped accordingly; b, a and e have meridional structures while d and g are facial and the Raman spectra corroborate these classifications. Within the meridional group the subtle differences in the Raman spectra of the b phase suggest that it packs differently to the a and e phases which pack more similarly. The effect of the thermal annealing process during OLED manufacture has also been studied using, as an example, a successful186 aromatic amine hole transport species N,N0 -diphenyl-N0 N0 -bis(1-naphthyl)1,1 0 -biphenyl-4,400 -diamine (NPB). Halls et al.185 incorporated not only vibrational spectroscopy but also DFT calculations and reflected absorption IR spectroscopy (RAIRS). This journal is

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Fig. 19 Raman spectra of Alq3 samples exhibiting different crystallinity. Adapted from ref. 61.

Fig. 20 shows the difference in RAIRS spectra upon annealing. The in-plane stretching vibrational mode region (1700 to 1000 cm1) exhibits a significant gain in intensity of the phenyl CQC/CQN stretching and C–H bending vibration at 1491 cm1 upon annealing as well as the CQC stretch (1592 cm1) and phenyl C–H bend (1293 cm1). The out-of-plane region (1000 cm1 and below) shows an intensity increase in the naphthyl C–H wag (772 cm1) and decrease in the phenyl C–H wag (821 and 751 cm1) vibrations. From these changes the authors suggested that upon annealing the naphthyl groups lie cis to one another (as opposed to trans in the gas phase global minimum), allowing a parallel orientation to the surface while the phenyl groups sit perpendicular to the surface. 4.6

potential applications including, among others, molecular wires and sensors.188–190 Functionalisation of the tubes allows electronic properties to be tailored to give desirable properties. Recently, Li et al.187 used Raman spectroscopy to verify the degree of functionalisation after ‘click’ reactions, by monitoring the increase in the number of sp3 hybridised carbons. A SWNT is essentially a rolled up graphite sheet, therefore it should consist of only sp2 carbons, these give rise to a characteristic vibrational mode at 1590 cm1. In a sample containing sp3 carbons, however, a peak at 1290 cm1, termed the disorder (or D) peak, with intensity depending on the number of sp2 carbon atoms was observed. Fig. 21 displays unsubstituted, alkyne-functionalised and polystyrene-functionalised SWNTs (A, B and C respectively), showing that upon functionalisation the D-peak increases significantly in intensity, indicating many sp2 hybridised carbons have been converted to sp3. IR spectroscopy (Fig. 22) was also able to inform on the functionalisation by revealing peaks due to the incorporated functional groups that are not picked up by Raman. The IR spectrum of the bare SWNTs shows no features except a peak at B3500 due to residual water in the KBr disc. The alkyne-functionalised nanotubes exhibit alkyne C–C and C–H stretches (2120 and 3280 cm1 respectively) while aromatic CQC and C–H stretches (1660 and 2920 cm1 respectively) are observed due to the aromatic linker in the polystyrene-functionalised tubes.

Molecular wires

Single-walled carbon nanotubes (SWNTs) are a chemical species that are of interest to molecular electronics for a number of

Fig. 21 Raman spectra of (A) SWNTs, (B) alkyne-functionalised SWNTs and (C) polystyrene-functionalised SWNTs. Note the increase in the disorder band at 1290 cm1. Adapted from ref. 187.

Fig. 20 RAIRS spectra of a NPB thin film before and after annealing at 125 1C for 30 minutes as well as the difference spectrum between the two. The top panel shows the wavenumber range in which in-plane stretching modes are observed, while the bottom plane displays the out-of-plane region. Adapted from ref. 185.

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Fig. 22 IR spectra of (A) SWNTs, (B) alkyne-functionalised SWNTs and (C) polystyrene-functionalised SWNTs. Adapted from ref. 187.

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TERS has been used to simultaneously probe the structure and chemical nature of a number of nanotube species.191–194 Cançado et al.191 detail how it is possible to determine the type, diameter, chirality and disorder in carbon nanotubes, amongst other properties, with spatial resolution typical of AFMs. The diameter can be determined from the topographical density-profile of the TERS spectrum as well as the phonon confinement effect,195 using the radial breathing mode (RBM), observed between 100–400 cm1, or the G mode, observed between 1540–1580 cm1.191,192 Similar analyses extend to other types of nanotubes, for example Ge nanowires.193 The nanowire-diameter is characterised by spectral changes effected by phonon confinement as well as topographic analysis. Furthermore, the ratio of Stokes and anti-Stokes scattering was used as a metric to determine the effect of heating by laser irradiation while crystalline and amorphous Ge contributions could also be resolved. 4.7

Molecular junctions

Molecular junctions are a significant part of many molecular electronic devices, therefore a detailed understanding of their properties is important. Vibrational spectroscopy is well suited to investigating junctions because the position and intensity of vibrational modes are sensitive to molecular structure and electrodes can often be made transparent to visible and IR radiation facilitating study of features below the electrode surface. McCreery and coworkers23,53,196–198 have undertaken a comprehensive study of metal/molecule/metal interfaces using the molecule 4-nitroazobenzene (NAB). A NAB monolayer chemisorbed onto glassy carbon was found to exhibit characteristic changes in intensity of certain Raman bands upon reduction. Specifically, the modes at 1334 and 1444 cm1 decrease in intensity while that at 1398 cm1 increases. The ratio of the 1398/1444 cm1 bands was subsequently used to study a carbon (pyrolyzed photoresist film, PPF)/NAB/titanium/gold interface where the Ti/Au film acts as the optically transparent electrode.196 Upon deposition of the Ti/Au layer during synthesis of the junction, the ratio between the analogous Raman bands at 1401/ 1450 cm1 increased, indicating a reduction of NAB by the Ti.23 Similarly, the real-time effect on the interface of a range of applied potentials were also investigated using this method. Changes in the Raman band intensities with applied bias infers a change in structure of the NAB layer. More specifically, when the PPF layer is positively biased, NAB is oxidised and the 1401/1450 ratio decreases and vice versa for a negative bias. This oxidation and reduction (down to 1 V bias) is reversible and shows hysteresis, but at potentials less than 1 V features at 1107 and 1340 cm1 are irreversibly lost, indicating a second reduction step involving a loss of the NO2 group. McCreery et al.198 have used resonance Raman spectroscopy to look at a number of junctions, involving nitroazobenzene (NAB) as the organic molecule. The nature of the molecular junction between copper and an (NAB) film bonded to pyrolysed photoresist film (PPF) was investigated by considering the intensity of Raman bands before and after Cu deposition. The NO2 stretching band was found to decrease while the intensity ratio between the 1400 (NQN stretch) and 1450 cm1 band increased, with additional results the molecular junction was concluded to involve Cu–N bonds and a 1942

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partial reduction of the NAB centres. These same NAB bands were also utilised to perform real-time monitoring of a PPF/ NAB/TiOx/gold junction subjected to a range of voltage biases. The structural changes occurring upon bias lead to changes in Raman peak intensities. These changes indicated whether the NAB was being reduced (negative bias at the PFF, loss of NO2 features) or oxidised (positive bias at the PFF, decrease in the 1400/1450 cm1 band ratio). A number of studies have also addressed simultaneous acquisition of electronic conductance and Raman spectral measurements199–204 in molecular junctions to garner information on charge transport and temperature. A review by Shamai and Selzer205 details a number of recent studies involving SERS as well as electroluminescence spectroscopy and conduction measurements to investigate charge transport. Specifically, the ratio between the Stokes and anti-Stokes Raman intensities can be used to estimate vibrational heating in molecules spanning metal surfaces.203,204 SERS has a natural advantage in this regime as gaps between metal particles provide very strong Raman enhancement. The intensity ratio relates to the non-equilibrium population of a particular mode upon applying a current and can be used to calculate an effective temperature of the mode.203 Liu et al.199 have examined a molecular junction of 4,4 0 -bipyridine between two gold electrodes. Using fishing-mode TERS, they were able to measure the conductance of a single molecule while simultaneously acquiring Raman spectra at a number of potentials. They found strong enhancement of the Raman signal in the ON state and voltage-dependent splitting of bipyridine vibrational modes. The signal-enhancement, which allows for signals from neighbouring molecules to be neglected, is due to a greater polarisability, induced by stronger chemical bonding when the electrode is switched. This is further resolved by the mode-splitting, as distortions of the pyridine in contact with the drain electrode compared to that attached to the source electrode induce spectroscopically measurable inequivalence along the single measured 4,4 0 -bipyridine molecule. This allows for direct spectroscopic investigation into switching mechanisms of single-molecule molecular junctions. Another recent study links geometry-changes of molecules bridging two electrodes with applied voltage and how this correlates with the junction conductance.201 Conductance has previously been shown to vary linearly with cos2 f, where f is the torsional angle of p-NH2-,206 p-SH-,207 and p-CN-substituted biphenyls207 bridging gold contacts. The assembly involved 4,4 0 -biphenyldithiol (BPDT) molecules sandwiched between gold nanoparticles. The authors compared the intensity of two bands, the CQC stretch of the phenyl rings which is dependent on the conjugation between rings and hence torsion angle (1580 cm1) and Cring–S stretch (1081 cm1) which is not dependent on either and can consequently be used as an internal standard. A linear correlation between the intensity ratio ICQC/ICring–S and cos2 f was established by plotting the intensity ratio versus cos2 f of a number of similar compounds with constrained (known) torsional angles (Fig. 23). The authors concluded that for BPDT (which is not constrained in f), as the potential becomes more negative, the torsional angle decreases and hence the conjugation increases, as does the ICQC/ICring–S ratio. Crossed-nanowire molecular junctions are another important area of interest, these can be incorporated in p–n junctions208 This journal is

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to test the quality of materials, medicines and foodstuffs. A number of the case studies touch upon the importance of the different crystal forms for the efficacy of the devices. Ultimately as these electronic materials come into the mainstream, to be manufactured into devices, the nature of the compounds and changes in morphology or crystalline state as the devices are fabricated will become an important focal point in which vibrational spectroscopy offers a powerful method in the analytical toolbox. In addition, to understand the operation of these devices the unambiguous assignment of intermediates that vary with the device structure both in terms of time and spatial extent may be addressed, at least in part, by vibrational spectroscopy. Fig. 23 Plot of ICQC/ICring–S vs. cos2 f for a number of related compounds with fixed torsional angles. Two points are also included for the 4,4 0 -biphenyldithiol(BPDT) molecule at applied biases of 0.2 and 0.8 V. Adapted from ref. 201.

and field effect transistors.209 In order to gain an understanding of the quantum transport between these wires Yoon et al.210 undertook IETS and SERS spectroscopy to study conductance– structure correlations with temperature variation. The paper investigated crossed-nanowire junctions that probe a SAM of oligo(phenylene-ethynylene)(OPE)dithio molecules. Coherent tunnelling was concluded to be the dominant transport mechanism due to the temperature-independent conduction observed. The vibrational spectroscopy on the other hand informed on the structure of the OPE molecules and was consistent with each other and theoretical calculations.

5

Conclusions and outlook

This article has presented a number of case studies that illustrate the use of vibrational spectroscopy in the examination of materials in devices as they become perturbed or altered by device operation or other external stimuli (heat, electrical current). The studies show how such spectroscopic techniques may be used to determine: the distribution of different materials in a film compared to a number of components; the presence of differing crystalline samples and the operating temperatures of OLEDs; the nature of absorbing chromophores and transient intermediates in solar cells, both DSSCs and bulk heterojunction; the nature of SWNTs as they are functionalised. The key strengths of vibrational spectroscopy are its specificity, the ability to unambiguously assign specific compounds or intermediate states, the spatial resolution (values in the mm range may be readily achieved) and the capability of capturing signals from intermediate states. The use of specialist techniques such as TERS, 2D-IR and SFG allow for the analysis of temperature versus conduction and other detailed properties of bulk materials and importantly junction type structures such as FETs. Another possible way forward for vibrational spectroscopic methods in regard to molecular electronic materials is in the area of materials quality control. For decades the food and pharmaceutical industries have utilised the relative ease of sample data acquisition that vibrational spectroscopy offers and have coupled this with chemometric techniques (multivariate analysis) This journal is

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