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Organic surface-grown nanowires for functional devices

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IOP PUBLISHING

REPORTS ON PROGRESS IN PHYSICS

Rep. Prog. Phys. 76 (2013) 126502 (24pp)

doi:10.1088/0034-4885/76/12/126502

Organic surface-grown nanowires for functional devices Jakob Kjelstrup-Hansen1 , Clemens Simbrunner2 ¨ and Horst-Gunter Rubahn1 1

NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark 2 Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstraße 69, A-4040 Linz, Austria E-mail: [email protected], [email protected] and [email protected]

Received 13 June 2013, in final form 18 September 2013 Published 22 November 2013 Online at stacks.iop.org/RoPP/76/126502 Abstract Discontinuous organic thin film growth on the surface of single crystals results in crystalline nanowires with extraordinary morphological and optoelectronic properties. By way of being generated at the interface of organic and inorganic materials, these nanowires combine the advantages of flexible organic films with the defectless character of inorganic crystalline substrates. The development of destruction-free transfer and direct growth methods allows one to integrate the organic nanowires into semiconductor, metallic electronic or photonic platforms. This article details the mechanisms that lead to the growth of these nanowires and exemplifies some of the linear as well as non-linear photonic properties, such as optical wave guiding, lasing and frequency conversion. The article also highlights future potential by showing that organic nanowires can be integrated into optoelectronic devices or hybrid photonic/plasmonic platforms as passive and active nanoplasmonic elements. (Some figures may appear in colour only in the online journal)

Contents 1. Growth 1.1. Organic molecular beam and hot wall epitaxial growth methods 1.2. Other growth methods 2. Integration 2.1. Transfer of individual entities and ordered arrays 2.2. Contacting 2.3. Coating 3. Organic molecular aggregate based optics 3.1. Optical waveguides 3.2. Amplifiers and lasers

3.3. Frequency converters 3.4. Surface plasmon generators and waveguides 4. Organic molecular aggregate based optoelectronic devices 4.1. Field-effect transistors 4.2. Organic nanowire-based light-emitting and light-sensing devices 5. Conclusions and outlook Acknowledgments References

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the nanowire in most cases has a shape slightly different from roundish (sometimes also called ‘nanoribbon’) and typically has nanoscale cross-sectional dimensions and lengths of a few to several tens of micrometres. The integration of nanowires made from inorganic semiconductors results in new devices with superior performance, such as field-effect transistors (FETs), bio-chemical sensors,

Introduction Quasi-one-dimensional crystals in the form of solid nanowires have great potential to act as microscopic components in nanotechnology applications. Nanowires might be called ‘nanofibres’ because of their optical waveguiding properties. In this article, we will solely use the term ‘nanowire’, where 0034-4885/13/126502+24$88.00

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Rep. Prog. Phys. 76 (2013) 126502

J Kjelstrup-Hansen et al

Figure 1. Variation of shapes of organic nanoaggregates, grown by organic molecular beam epitaxy on muscovite mica. Upper row (from left): fluorescence microscopy images of para-hexaphenyl (p-6P) (75 × 75 µm2 ), dimethoxy-p-quaterphenylene (200 × 200 µm2 ), dichloro-p-quaterphenylene (60 × 45 µm2 ) and quater-thiophene (15 × 15 µm2 ). Lower row: scanning force microscopy (SFM) images of aggregates of the same species, except for the SFM image of the ring on the right-hand side, which is constituted from p-6P molecules. Height scales 70, 50, 120 and 350 nm (from left). The base width of the nanowires is 250, 600 and 800 nm (from left). The ring has a width of about 100 nm. From [2–6].

light-emitting diodes, photodetectors and optical waveguides and resonators (see [1] and the references herein). In general, all advantages and possible applications that have already been pointed out for inorganic nanowires can also be affiliated to their organic counterparts, which consist of small molecules organized in a molecular crystal structure. However, organic materials exhibit some unique opportunities in terms of their morphological and optoelectronic properties. Using modern methods of synthetic chemistry, the molecular building blocks of the nanoaggregates can be tailored so as to exhibit the optical or electronic properties that are needed. Since the molecules assemble, e.g. on surfaces, into molecular crystals, elements with well-defined optoelectronic characteristics can be fabricated over a large size range, essentially from molecular clusters to macroscopic entities. These crystalline materials typically have a low number of defects, resulting in a low density of intrinsic losses and thus narrow spectra and high electronto-photon conversion efficiencies. In addition, the high crystallinity and 1D morphology observed in the nanowires provide improved electrical and special optical properties, such as polarized emission and 1D optical waveguiding, compared to thin films that typically have inferior molecular ordering. The surface growth can be directed via microstructure formation on the growth substrate, which enables one to ‘load’ surface circuitry with active elements by simple organic molecular beam growth (‘self growing photonic elements’). The crystalline nature of the aggregates results in distinct nanowire morphologies on the nanoscale (from ellipsoidal to pyramidal)—the exact shape being variable by exchanging the molecular building blocks and the growth contact plane, as shown in figure 1. Modifying the interaction between the growth substrate and the molecules results in bent wires such as rings or more complex curved structures. Morphology control on the nanoscale is interesting for shaping the electromagnetic near-field along the structures (e.g., for local Raman enhancement) or for coupling photonic excitations in the

wires to metallic surface excitations, such as surface plasmons. These techniques thus open up the wide field of hybrid organic/metallic photonics/plasmonics and nonlinearly responding ‘active’ aggregates. Several other types of organic nanowires have been reported, such as small molecule nanowires formed by selfassembly in a solution [7] and polymer nanowires made by template wetting [8] or electrospinning [9]; however, in this review, we focus primarily on surface-grown, crystalline nanowires from small organic molecules and refer to a recent review for other types of organic nanowires [10]. In the first part, we describe the epitaxial growth of nanowires from prototypical phenylene and thiophene oligomers. We then describe various strategies for integrating organic nanowires in devices, including examples of integration strategies demonstrated with other types of organic nanowires but applicable also to the surface-grown type. The last part reviews some of the nanowire applications, including optical and optoelectronic devices, with the main focus on devices based on surface-grown nanowires, but with a few examples of devices made from vapour transport-grown nanowires for comparison.

1. Growth 1.1. Organic molecular beam and hot wall epitaxial growth methods The epitaxial growth of small molecules on various substrate surfaces [11–20] has been identified as a powerful strategy to fabricate self-assembled organic nanostructures [21–26]. Especially rod-like molecules, e.g. para-phenylenes [23, 27–29], thiophenes [30, 31] and thiophene/phenylene cooliogomers [32–34] result in highly anisotropic needle-like crystallites. In general, those molecular systems crystallize in a herringbone structure [35–38] with parallel alignment of the long molecular axes (LMA) within a single crystallite. 2

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On the other hand, nanowire structures are found, which exhibit a length of several micrometres. X-ray diffraction (XRD) studies [31, 42, 50–52] reveal that such structures are formed from molecules, which are oriented parallel to the substrate surface. In the case of 6T on KCl (0 0 1) [42] or muscovite mica (0 0 1) [31], a (−4 1 1) contact plane is found. The obtained molecular stacking is sketched by a real space model in the bottom/left part of figure 2. The molecular alignment is indicated by white lines. Optical methods are sensitive to the molecular alignment of organic thin films, too. A fluorescence microscopy image after a normal incidence excitation of 4,4 -di(2,2 -bithienyl)biphenyl (TTPPTT), which has been deposited on muscovite mica (0 0 1), is depicted in the right panel of figure 2 [34]. Similarly to 6T on KCl, two crystal morphologies are observed. Whereas needle-like structures are characterized by bright fluorescence emission (the OTD is aligned along the surface plane), islands appear as dark dendritic structures (the OTD is aligned normal to the plane). For the sake of completeness, it should be noted that flat-lying molecular configurations can be achieved by the formation of various contact planes. Para-hexaphenyl (p6P) can nucleate with a (2 0 −3) [11, 13, 51, 53, 54], (1 1 −1) [50, 53, 55–57], (1 1 −2) [41, 50, 53], (2 1 −3) [58–61] and (−6 2 9) [13] contact plane. Whereas all configurations are characterized by nearly flat-lying molecules on the substrate surface, they differ considerably concerning their herringbone stacking angles relative to the substrate surface and surface unit cells. Both observed crystal types significantly differ concerning the molecular alignment relative to the substrate surface. Hence the initial adsorption process of single molecules on the substrate surface plays an essential role for the obtained sample morphology. For example, strongly interacting surfaces, e.g. Cu (1 1 0) [11, 62, 63], Au (1 1 1) [58] and TiO2 (1 1 0) [23, 64, 65], tend to force a flat-lying molecular arrangement. Island-like morphologies, consisting of upright standing molecules, are mainly observed on disordered and amorphous [45, 48, 65–68] substrate surfaces. Electrically insulating single crystalline substrate surfaces, e.g. KCl (0 0 1) or muscovite mica (0 0 1), typically show the co-existence of both morphologies [30, 31, 41, 42] or needle-like structures only [39, 50]. Another important parameter for the fabrication of nanowires is the substrate temperature. The surface area, which is covered by islands, decreases by increasing the substrate temperature for quarter-thiophene (4T) on muscovite (0 0 1) [30] and 6T on KCl (0 0 1) [42]. Standing p-6P configurations on KCl are more likely for elevated substrate temperatures [53, 69]. Moreover, the formation of nanowires is a thermally activated process. The values of needle-length (L), -width (W ), -height (H ) and -density for a nominally constant surface coverage of the organic layer, strongly depend on the chosen substrate temperature. All quantities, when analysed as a function of the substrate temperature, show an exponential behaviour with activation energies in the range of several 100 meV [39, 42]. Due to an enhanced molecular diffusion for elevated substrate temperatures, the number

Figure 2. In the left panel, the sample morphology obtained by SFM for sexi-thiophene deposited by HWE on KCl (0 0 1) is depicted. The sample surface is covered by two different crystal morphologies. Whereas island-like structures (1) are built up by approximately upright standing molecules, needle shaped crystallites have nucleated at flat-lying molecules. Real space models of the molecular packing and extracted SFM cross-sections are depicted below. The right panel indicates a fluorescence microscopy image of 4,4 -di(2,2 -bithienyl)-biphenyl (taken from [34]), which has been deposited on muscovite mica (0 0 1). Again, both film morphologies can be observed. Needle-like structures are characterized by strong fluorescence due to a parallel alignment of the OTD to the substrate. For dendritic islands, the OTD is aligned out-of-plane and fluorescence appears only weakly. Adapted with permission from [34]. Copyright 2009, American Chemical Society.

As the molecular optical transition dipole (OTD) is oriented along the LMA, the optical emission and adsorption properties of organic nanowires of that type are, in general, highly anisotropic [29, 33, 39, 40]. Standing versus lying molecular configurations. Upon deposition of the molecules on substrate surfaces by molecular beam [30, 34] or hot wall epitaxy (HWE) [31, 41, 42], the surface is typically covered by: (1) flat island-like structures, consisting of approximately upright standing molecules [43–46]; (2) needle-like crystal morphologies, which correlate with flat-lying molecules acting as nucleation centres [6, 46, 47]; (3) or a combination of both [30, 31, 41, 42, 48]. The observed growth mode and surface coverage critically depends on the chosen molecule–substrate combination and growth parameters [42, 46, 49]. As indicated in a SFM image of sexi-thiophene (6T) on KCl (0 0 1) (figure 2), on the one hand, flat island-like structures with stepped terraces corresponding to monolayers (MLs) of upright standing 6T molecules (2.2 nm) can be observed. These structures have nucleated with a (1 0 0) contact plane parallel to the substrate surface [30, 31, 42]. 3

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Figure 3. SFM images of para-hexaphenyl (p-6P) nanowires deposited on muscovite mica (0 0 1), phlogopite mica (0 0 1) and KCl (0 0 1). On all substrate surfaces, the p-6P molecules tend to form needle-like structures. The number of orientations scales with the degree of substrate symmetry. Whereas muscovite mica does not provide a rotational centre, phlogopite mica (KCl) is characterized by 3-fold (4-fold) rotational symmetry, as indicated by the representation of the corresponding wallpaper groups (bottom insets). By applying a 2D fast Fourier transform (FFT) (indicated in the top insets), the generated needle orientations can be visualized and it can be verified that all of them follow substrate surface symmetry. The indicated crystallographic orientations of the substrate surface have been deduced based on reported x-ray pole figure (XPF) analyses [41, 50, 57].

anisotropic morphology. The obtained SFM image, depicted in figure 3, reveals mutually parallel ordered nanowires. The azimuthal orientation of the substrate surface is indicated by a white arrow and has been deduced by comparing the sample morphology with reported x-ray pole figures (XPF) analyses [50]. In contrast, the phlogopite mica (KCl) substrate surface is characterized by 3-fold (4-fold) symmetry centres, which are responsible for the formation of multiple nanowire orientations. The corresponding representations of the substrate surface unit cell geometries are indicated in the bottom right part of the SFM images. The dominant needle orientations can be extracted by applying a 2D fast FFT on the SFM data, see insets in figure 3. Triangular nanostructures, which are oriented along [1 0 0], [1 1 0] and [1 −1 0] [57] of phlogopite mica (0 0 1), are indicated by three stripes (rotated by 120◦ relative to each other). The FFT image of p-6P on KCl (0 0 1) is characterized by two dominant orthogonal orientations (labelled A) along [1 1 0] of KCl, which are surrounded by two mirror symmetric needle directions ((1 1 −1),(−1 −1 1)) [41].

of nucleation sites decreases and thus the needle density decreases also. Additionally, the probability for adsorbed molecules to nucleate at an existing nanowire is significantly increased, which leads to an increase of L, W and H with the rising substrate temperature. As the obtained nanowire cross-section (A ≈ W × H ) plays an essential role for the fabrication of optical waveguides [70, 71], resonator structures, random lasers [40, 72] or hybrid plasmonic devices [73], the importance of substrate temperature control for the fabrication of optical devices is further underlined. Azimuthal order. Whereas organic crystallites on disordered and amorphous substrates, e.g. polymers [68, 74], SiO2 [45, 75, 76] and highly ordered pyrolytic graphite (HOPG) [77], are characterized by a fibre texture, the epitaxial growth on single crystalline surfaces leads to the formation of a welldefined azimuthal order. As inorganic epitaxy deals with quasi point-shaped atoms as the smallest building blocks, its successful implementation is mainly driven by a proper lattice match between the grown overlayer and the substrate crystal [78]. Contrarily, organic molecules are built up by two or more atoms, which are chemically bonded together. Consequently, they represent extended objects, thus the initial adsorption on the substrate surface and, in further consequence, the epitaxial growth is not only influenced by the crystal lattice match, but also by the molecular shape, symmetry and chemical composition—in particular during the initial adsorption process [57]. With increasing molecular density on the substrate surface, adsorbed molecules act as nucleation centres for the formation of organic crystallites [57]. This nucleation process is often accompanied by the formation of a wetting layer [11, 62, 79, 80] before molecules start to crystallize in their equilibrium bulk structures. As each molecular adsorption site can act as the nucleation centre for organic crystallites, epitaxially grown nanowires have to follow the symmetry of the chosen substrate surface too. Let us discuss, as an example, the growth of p-6P nanowires on muscovite-, phlogopite mica (0 0 1) and KCl (0 0 1). The muscovite mica (0 0 1) surface can be characterized by a cm space group; consequently, it does not provide any rotational centres and shows a highly

Nucleation and growth of nanowires. In the case of p-6P nanowires on KCl (0 0 1), XRD studies have verified a dominant fraction of (2 0 −3) and (1 1 −1) oriented p-6P nanowires [41, 81]. The obtained crystal contact plane in general strongly depends on the molecule–substrate interactions and consequently varies for different substrate surfaces. Beside the chemical composition of the substrate surface, the formation of the contact plane is also influenced by the dimensions of the surface unit cell. A slight azimuthal readjustment of the LMA with respect to the energetically most favourable adsorption site of a single molecule has frequently been reported [42, 50, 57, 58]. It results from an optimized lattice match [82] between the organic crystallites and the substrate surface [57]. For a better visualization, figure 4 depicts a top view of the molecular stacking of p-6P molecules in (2 0 −3) and (1 1 −1) type crystallites. The respective surface unit cells (indicated in figure 4 by polygons) differ, as does the alignment of the LMA relative to the generated long needle axis (LNA). The LNA coincides with the zone axis of the obtained contact 4

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[1 1 0]KCl . As the LMA of p-6P molecules is aligned orthogonal to [1 1 0]KCl , the LMA* of their mirror symmetric twins coincides [57]. In contrast, the LNA* of the mirror symmetric nanowires appears 15◦ tilted along [−1 1 0]KCl , which explains the observation of two mirror symmetric needle directions. Based on the above discussion, we not only deduce that the rotational symmetry of the substrate surface leads to the formation of energetically equivalent crystallites, but also that the reflection symmetry lowers the obtained anisotropy by the generation of mirror symmetric twin crystallites. Fabrication of nanowires by organic–organic heteroepitaxy. Among the huge variety of organic molecules, thiophenes and phenylenes are the most promising building blocks for the fabrication of nanowires because of their photonic and electronic properties, as well as their thermal stability [32]. As a consequence, beside para-phenylene oligomers, also thiophenes [30–32, 57], thiophene/phenylene co-oligomers [32, 34] and functionalized para-phenylene molecules [84–87] have been deposited on muscovite mica (0 0 1) to study their epitaxial growth. It has been shown that, by a proper selection of the molecular species, the optical properties and, in particular, the spectral emission can be efficiently tuned, which is summarized in figure 5. However, two main differences between thiophenes and thiophene/phenylene oligomers on the one hand and p-6P nanowires on the other hand are observed: flat island-like structures are formed that consist of upright standing molecules [30, 57] and the wires show lower macroscopic anisotropy [30–32, 57], which is sketched in figure 5 by rhombical and x-shaped structures. This reduces the possible length of the wires and also deteriorates the optical properties. The reason for this behaviour lies in the energetically preferred adsorption geometry of the individual molecular species on the muscovite mica substrate. Whereas p-6P tends to nucleate normal to the high symmetry direction of muscovite mica [50], 6T molecules are characterized by an angular tilt of approximately ±60◦ [31]. This results in a crystal nucleation at azimuthally diverging molecular adsorption sites [57] and thus an x-shaped structure. In order to utilize a wider spectrum of molecules for a precise colour tuning of self-assembled nanowires, heteroepitaxy of organic–organic thin films and nanostructures is a versatile approach. Crystalline and highly ordered heterostructures with different morphologies and molecular orientations are realized by heteroepitaxy starting from conjugated oligomers [88–92]. Organic–organic heteroepitaxy can also be utilized for the fabrication of p-6P/6T nanowires [72, 89, 93, 94]. Whereas 6T nanowires, when deposited on plain muscovite mica, are characterized by multiple nanowire orientations, which appear as an x-shaped morphology [30, 31, 57], the situation changes when 6T is deposited on a p-6P nanowire template. In contrast to 6T on plain muscovite mica [30, 31], no islands of upright molecules exist [89]. Moreover, p-6P crystallites force an azimuthal realignment of 6T molecules during adsorption parallel to the LMA of the organic template [89]. As indicated by a colour-coded SFM image (figure 6), highly parallel-oriented

Figure 4. (Top) Geometrical alignment of para-hexaphenyl molecules when nucleating with a (2 0 −3) and (1 1 −1) contact plane. The orientation of the long needle axis (LNA) is defined by the zone axis of the contact plane and low energy plane (0 0 1), yielding [0 −1 0]/[1 −1 0] for (2 0 −3)/(1 1 −1) crystallites. Moreover, the alignment of the LMA is crucially influenced by the obtained contact plane, yielding 90◦ /75.3◦ relative to the LNA orientation. (Bottom) Graphical representation of the observed LNA splitting for (1 1 −1)/(−1 −1 1) crystallites on KCl (0 0 1). Due to the mirror plane of KCl (0 0 1) along [1 1 0], the formation of a mirror symmetric (−1 −1 1) crystallite is energetically equivalent to its (1 1 −1) twin. Whereas the LMA orientations of both crystal types are aligned approximately parallel to each other, the oblique surface unit cell of (1 1 −1) crystallites is responsible for a doubling of the observed LNA.

plane and low energy plane (0 0 1), yielding [0 −1 0]/[1 −1 0] for (2 0 −3)/(1 1 −1) crystallites. By a preferred nucleation along these orientations, crystallites are able to maximize their low energy facet (0 0 1) [83]. A side view shows that p-6P molecules are aligned perfectly flat for (2 0 −3) orientations, whereas the LMA is inclined by ≈5◦ relative to the substrate surface in (1 1 −1) configurations. A real space model of a (1 1 −1) oriented p-6P crystallite on KCl (0 0 1) is sketched in the bottom part of figure 4. As deduced by x-ray analysis, the LMA of p-6P molecules is aligned along [1 −1 0], which leads to the formation of the LNA ≈15◦ tilted relative to [1 1 0]KCl . The herringbone stacking of p-6P molecules becomes nicely visible by an alternating assembly of edge-on and tilted molecules. The real space model presented further illustrates that the terminating hydrogen atoms of the p-6P molecules determine the alignment of the (0 0 1) side facet. Additionally, a mirror is indicated, which symbolizes the reflection plane of KCl (0 0 1) along 5

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Figure 5. Fluorescence microscopy images of selected molecular species belonging to the group of phenylenes, functionalized quater-phenylenes, thiophene/phenylene co-oligomers and thiophenes. Whereas the fluorescence can be efficiently tuned from the blue via green to the red spectral range by a proper molecular selection, morphological anisotropy and polarization is lost for green and red emitting nanowires. Fluorescence images are taken from the literature [34, 57, 84] and adapted with permission from [34] (Copyright 2009, American Chemical Society) and [57] (Copyright 2011, American Chemical Society).

p-6P template nanowires (cyan) and 6T crystallites (red) are obtained on top of the mica substrate (blue). The height histogram in figure 6 shows a sharp peak located at 16 nm, which is attributed to the bare muscovite mica substrate surface. The peak at 124 nm is attributed to p-6P template wires and the broad distribution at 500 nm to 6T crystallites. A cross-section of a single p-6P/6T nanowire, obtained by transmission electron microscopy (TEM), is depicted in the bottom left part of figure 6. The p-6P nanowire is visible as a rectangular shaped structure with a strongly tilted 6T entity on top. The contrast between the two is caused by the presence of the heavy sulfur atoms and it reveals that a sharp organic– organic interface exists in between p-6P and 6T crystallites. A real space model of the experimentally deduced p-6P (6T) crystal geometries is sketched in the bottom right of figure 6. The observation of strongly tilted and plate like 6T crystallites in the TEM image can be explained by a mismatch between the low energy facets of p-6P and 6T. The fabrication of p-6P/6T nanowires not only affects the nanowires’ morphology but also changes their optical properties. Whereas wires grown from p-6P are characterized by deep blue fluorescence [22, 28, 95], already the deposition of a sub ML coverage of 6T leads to a significant change towards green emission [89] (figure 7). Further 6T deposition results in red emitting 6T crystallites on top of p-6P nanowires [89]. The polarized fluorescence intensity of the red emitting clusters is much weaker than the green emission of interfacial 6T molecules, which can be explained by effective energy transfer with p-6P molecules [89, 94]. A structural analysis reveals a parallel orientation of p-6P and 6T molecules. Structural and optical analysis of p-6P/6T bi-layer heterostructures underlines the potential of the growth method to control the molecular alignment. A periodic deposition of sub ML 6T and p-6P spacing layers with controlled p-6P spacer thickness allows one to tune the spectral contribution of p-6P and green emitting 6T yielding blue-, green- and white

Figure 6. SFM image of para-hexaphenyl (p-6P)/sexi-thiophene (6T) bi-layers on muscovite mica (0 0 1). The height histogram, depicted beside, reveals the presence of three height levels that can be attributed to the bare muscovite mica substrate (A), p-6P template wires (B) and 6T crystallites (C), which have nucleated on top of p-6P. The drawn picture is further underlined by a transmission electron microscopy (TEM) image (bottom, left) showing the cross-section of a single p-6P/6T nanowire. Whereas the p-6P template is characterized by a rectangular cross-section, 6T nucleates as strongly tilted lamella like crystallite at a sharp organic–organic interface. The tilted morphology can be explained by the geometrical alignment of 6T’s low energy plane (1 0 0) when nucleating with a (−4 1 1) contact plane. 6

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Figure 7. Fluorescence microscopy images of para-hexaphenyl (p-6P)/sexi-thiophene (6T) nanowires. Whereas p-6P template wires (a) are characterized by a deep blue emission, a sub ML coverage of 6T (b) leads to green fluorescent nanowires. With continuing 6T deposition (c), green emission is accompanied by red emitting crystallites, which have nucleated on top of the p-6P template. By a subsequent deposition of sub ML 6T and p-6P spacing layers (d), precise control of the ratio of green 6T to blue p-6P emission can be achieved. The obtained colour can be tuned from the blue via white to the green spectral range.

Figure 8. (a) Schematics of the growth procedure. After the deposition of para-hexaphenyl (p-6P) template wires, 6T MLs and p-6P spacer layers are deposited periodically. The total thickness of the periodically deposited layers is kept constant (140 nm), whereas the number of periods n is varied. (b) Normalized fluorescence spectra for a series with n = 3, 13 and 25 periods ii). By increasing the number of 6T layers, the contribution of green 6T emission can be precisely tuned yielding blue, white and green fluorescent nanowires.

reported in figure 8(b). By increasing the number of 6T layers, the contribution of green 6T emission can be precisely tuned yielding blue, white and green fluorescent nanowires [78].

fluorescent nanowires (figure 7). The nucleation of red emitting 6T crystallites is avoided and nanowires are characterized by a homogeneous height level and approximately rectangular cross-sections [78], which makes them interesting candidates for waveguides and resonator structures. A graphical schematic of the growth process is indicated in figure 8(a). In the reported set of samples, 6T MLs and p-6P spacer layers have been deposited periodically on top of 33 nm thick p-6P template wires. Whereas the total nanowire height has been kept constant, the number of periods (n) introduced in a 140 nm thick layer has been varied. Photoluminescence (PL) spectra for nanowires grown with n = 3, 13 and 25 periods are

1.2. Other growth methods Organic–organic heteroepitaxy is a rather tedious way of modifying growth dynamics and it is bound to specific substrates, such as mica, that provides the initial substrate orientation. Another way of modifying the surface energetics and dynamics is the formation of nano- or microstructures on the growth substrate. 7

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Figure 9. (a) SEM image of lithographically fabricated ridge structures in silicon, coated with an ultrathin gold film. Nanowires have been grown on top of it at 435 K surface temperature. (b) Same as (a), but for nanowires grown in between 50 nm wide titanium pinning lines. From [96].

Figure 10. Measured current density of para-hexaphenyl nanowires (open circles) and nanoflakes (dots) grown on a transistor platform as a function of drain–source voltage. The nanowires show a significantly lower onset voltage due to a relatively larger contact area and thus smaller contact resistance. From [98]. The inset is an SEM image of in situ grown nanowires on a gold electrode structure on silicon.

The versatility of this approach has been demonstrated by using gold coated ridge structures on a silicon chip as the growth substrate for p-6P nanowires. Vapour deposition of p-6P molecules onto the heated substrate resulted in anisotropic molecular diffusion (enabled by the gold coating) and nucleation and thus in the formation of nanowire structures perpendicular to the ridge edges, as shown in figure 9(a). In comparison, unstructured surfaces lead to non-directed growth of nanoaggregates. The overall reason for the directed growth is the symmetry break at the ridges, leading to anisotropic diffusion as well as a local modification of surface free energy. A similar effect can be induced by even smaller perturbations of the surface free energy. For example, 50 nm wide and high titanium lines on a gold surface lead to preferred growth perpendicular to the lines (figure 9(b)) [97]. On nonmetallic surfaces, gold lines lead to preferred and directed growth in the area that is covered by the lines and perpendicular

to the main direction of the lines (see the inset in figure 10). That way lithographic structuring of the surface results in the control of organic nanowire growth and at the same time in the electric bottom contact to the nanowires. Hence measurements of the electrical properties of individual nanowires or of nanowires with different morphologies become possible [98]. An example is shown in figure 10. Mechanical methods can also be used to achieve a uniaxially aligned surface, which influences molecular orientation and anisotropy. It has been demonstrated that pretreatment of isotropic glass substrates by unidirectional rubbed poly-para-phenylene (PPP) leads to a parallel orientation of p-6P along the PPP chains [48, 99]. Analogous behaviour has also been reported for a unidirectionally rubbed 10 nm thick p-6P layer on glass, which has been used as an organic template layer, yielding a bi-axial texture of the subsequently deposited p-6P crystallites [100]. Beside mechanical treatments, it has 8

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Figure 11. (a) A sharp tip approaches a nanowire resting on top of a nanotube forest and is moved underneath the nanowire. (b) Schematic illustration of the technique: a sharp tip is moved beneath the nanowire slightly into the mechanically compliant nanotube forest. The nanowire may then be lifted up by pulling the tip vertically. From [107].

also been reported that using a thermal gradient can induce an azimuthal anisotropy during crystal nucleation [101].

A ‘carbon nanotube forest’ is a super-hydrophobic surface, from which individual nanowires can later be lifted up and placed on an arbitrary substrate [107], see figure 11. With this technique, the dense ‘forest’ of vertically aligned carbon nanotubes supports the nanowires in only a few discrete points and thereby has a much lower interaction with the nanowires. It also allows a sharp metal tip to penetrate into the forest and below the nanowire to gently lift it up without rupture. A related pick-and-place strategy uses a ‘mat’ of nanowires as the starting point [108]. An elastomeric stamp made from polydimethoxysilane (PDMS) is pressed into the mat causing a few of the nanowires to adhere to the stamp. When the stamp is then subsequently pressed against the receiver substrate, a few nanowires are transferred. Bao and co-workers have developed a filtration-andtransfer method to obtain an array of mutually parallel organic microwires on a silicon substrate [109]: a PDMS film with 200 µm-wide and several mm long holes was placed on a porous anodized aluminum oxide (AAO) membrane and the microwire suspension was filtered via vacuum suction. After the filtration process, the microwires remained on the AAO membrane inside the PDMS pattern, which caused the alignment of the microwires to the stripe pattern. Subsequent removal of the PDMS film, lamination against an OTS-coated SiO2 substrate and soaking in water enabled the removal of the AAO membrane and the successful transfer. Nanowires that are initially supported on a solid substrate, such as their growth substrate, can be transferred directly to the receiver via transfer printing under humid conditions [110, 111]. This method can efficiently transfer an array of nanowires and retain their initial mutual arrangement, however, it works best for transfer to a structured receiver substrate with platforms of an appropriate size (side length of a few hundred micrometres or less). A second method is based on roll printing, in which the substrate with the nanowires is initially attached to a cylinder that is then rolled towards the receiver substrate under appropriate conditions (high humidity) [112]. This method can similarly maintain the alignment that the nanowires had on the donor substrate and can be used to transfer to both unstructured and structured receiver substrates. In case the donor substrate is rigid and

2. Integration The application of organic nanowires in devices with new or improved functionalities requires the controlled positioning of either individual or an array of nanowires on a device platform including electrical or optical connections., Well-established integration methods exist for inorganic nanowires, such as in situ growth at pre-defined positions on the device substrate [102] or the transfer of individual nanowires or large arrays by flow alignment [103] or roll printing [104]. While the constituents of inorganic nanowires typically are covalently bonded atoms, organic nanowires consist of molecules that are bonded by van der Waals forces or similarly weak interactions. This results in a low mechanical aggregate strength and thus asks for new, non-destructive integration methods. In what follows, we explain methods for the integration of fragile organic nanowires. Direct growth on microstructured substrates, such as electrode platforms, is discussed in section 1.2 above. 2.1. Transfer of individual entities and ordered arrays The transfer of a single or several nanowires to a well-defined position on a substrate is different depending on whether the nanowires are supported on a solid substrate or have been suspended in a liquid. Surface-bound nanowires can be transferred to a receiver substrate by liquid transfer [105]. A liquid medium, in which the nanowires can be suspended, is initially placed on the nanowire substrate. A droplet of the nanowire suspension is sucked into a pipette and then spread on the receiver substrate [105]. If the interaction between the receiver substrate and the nanowires is sufficiently weak as compared to the intermolecular forces, the nanowire can be moved across the surface to a desired position, using a micro cantilever as a manipulation tool [106]. Typically, a substrate surface with an anti-stick coating (for example, fluorine-based to cause a hydrophobic surface) is required to prevent the rupture of the nanowires during the sliding process [106]. 9

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Figure 12. (a) Fluorescence microscope image of the luminescent nanowires on mica before transfer (the luminescence is excited via ultraviolet (UV) light and observed through a filter that cuts the excitation light). (b) The nanowires are transferred from the growth substrate (mica) to a receiver substrate via roll printing under humid conditions. (c) Fluorescence microscope image of nanowires transferred to a glass substrate. Notice that the mutually parallel alignment of the nanowires is maintained. (d) Top contacts to nanowires can be realized by metal deposition through a stencil. (e) Fluorescence microscope and (f ) white light microscope images of para-hexaphenyl nanowires contacted via nanostenciling. The gap between the source and drain electrodes is ∼2 µm. From [112, 113].

lift-off. However, the use of such conventional lithography techniques with organic materials is hindered by the processing steps (such as photo or electron beam exposure and liftoff in an organic solvent) that can potentially damage the organic nanowires. Instead, top contacts are often prepared by deposition of the metal electrodes using some form of shadow mask to define the contact area. A straight-forward technique involves the spreading of a nanowire suspension onto an insulating substrate and the deposition of top contacts using, for example, a TEM grid [114], a silicon nanowire [105] or a micrometre-sized gold wire [115] as a shadow mask during metal deposition. A more well-defined electrode pattern can be obtained using a nanostencil [113] consisting of a thin silicon nitride membrane through which structures of the desired electrode pattern geometry are etched out, as shown in figure 12(d). Figures 12(e)–(f ) show fluorescence and white light microscope images, respectively, of an array of p-6P nanowires contacted via nanostenciling. Since the metal–nanowire interface is typically created under vacuum conditions for top contact configurations, there are fewer restrictions on the choice of electrode material and essentially all conductive materials that can be evaporated or sputtered can potentially be used. The Zhu group has demonstrated a method where top contacts to a nanowire were realized by first depositing a thin gold film on a silicon substrate and then secondly peeling off a small piece (∼30 µm × 150 µm) of the gold film with the tip of a mechanical probe and transferring it onto the nanowires [116]. Using this method together with two different types of nanowires suspended between PMMA posts enabled the realization of a FET with an air-gap as the gate dielectric. In applications such as light-emitting diodes where ambipolar transport is required, it is often beneficial to be

cannot conform to the curved cylinder surface, it is also possible to transfer to a flexible substrate mounted on the roller. Figures 12(a)–(c) illustrates the method and demonstrates how the initial mutually parallel alignment is maintained for the transferred nanowires. 2.2. Contacting In many device applications including transistors and lightemitting and photovoltaic devices, the nanowires need to be electrically connected to metal electrodes. Two different configurations can be used: metal-on-wire (top contacts) or wire-on-metal (bottom contacts). With the latter geometry, the electrodes are often pre-fabricated on the device platform by a lithographic technique followed by the deposition of nanowires. The former method requires the preparation of the electrodes on top of the nanowires, which causes some additional fabrication restrictions as the nanowires are typically not compatible with environmental conditions used in conventional lithography techniques. In the bottom contact device geometry, the electrodes are exposed to ambient conditions prior to the deposition of the nanowires. This places some restrictions on the choice of electrode material, as many metals readily oxidize and form an insulating oxide layer, which would prevent the formation of a low-resistance electrical connection. The choice of electrode material is therefore often limited to high-work function metals that are inert under ambient conditions; therefore, bottom contact devices are almost exclusively made using gold as the electrode material [108, 113]. Top contacts to inorganic nanowires are often made by a lithography technique followed by metal deposition and 10

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process is mainly attributed to a photo-oxidation process suggesting that the corresponding effects can be minimized by coating the nanowires to protect them from ambient conditions [125]. Experiments have shown that deposition of a thin film of SiOx on top of luminescent nanowires slows down the bleaching reaction significantly [125]. However, at the same time the luminescence spectrum is significantly altered, which is caused by molecular defects introduced during the SiOx deposition via electron beam evaporation. A more suitable coating consists of a bi-layer of poly(methyl methacrylate) (PMMA) and SiOx [126]. The polymer film is applied by spin coating and does not cause any spectral changes in the luminescence output—however, it reduces deposition induced defect formation in the active crystalline material. Subsequent deposition of SiOx has the effect of encapsulating the nanowires against oxygen induced photoreactions.

able to deposit different contact materials on either end of the nanowire to realize anode and cathode contacts optimized for holes and electrons, respectively. One possibility is to use a double shadow-mask approach, in which two closely spaced masks shadow the first deposition to create a metal contact to only one end of the nanowire, followed by mechanical removal of one of the masks to uncover the other end and the second deposition [117]. An alternative and faster method is to use a shadow-mask strategy based on two electrode materials that are deposited under different angles onto a larger array of mutually parallel nanowires [118]. The use of a 5 µm diameter carbon fibre can, depending on the angle of evaporation, enable the realization of electrode gaps between several micrometres down to a few hundred nanometres. While the bottom contact configuration offers the easiest fabrication process, the top contact configuration has some benefits in terms of electrical performance. This is primarily due to the interface between the organic material and the electrode material (typically a metal). The interface in bottom contact devices is often formed under ambient conditions (for example by drop casting of the nanowire suspension followed by evaporation of the dispersion medium) causing the incorporation of water at the interface and thus resulting in a bad electrical contact. In contrast, top contacts are typically prepared by metal evaporation onto the organic material under high vacuum conditions resulting in metal penetration [119], which alters the electrical interface properties [120]. For otherwise similar devices, this results in top contact devices performing better than bottom contact devices [113, 121]. A second important aspect is the energy barrier for charge carrier injection that is formed between the electrode and the organic material. The organic material is characterized electronically by the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) that can support hole and electron transport, respectively. A significant difference to inorganic semiconductors is that charge carriers in inorganic semiconductors delocalize and energy band transport is observed, while the charge carriers typically localize on individual molecules in organic crystals and transport occurs by incoherent hopping. The second part of the interface is formed by the electrode, which often is a metal that is characterized by its work function. In a simplified picture (neglecting interface dipoles), efficient hole injection takes place when the metal work function lines up with the HOMO level, while efficient electron injection requires the work function to line up with the LUMO level. If this is not realized, an energy barrier will result, which the charge carriers must be injected over [122]. The situation for organic nanowires is similar to that for macroscale organic crystals; we refer to other publications for a more detailed review of the interfacial electronic structures between an organic material and a metal [123, 124].

3. Organic molecular aggregate based optics One of the key features of organic materials is the possibility to tune the optoelectronic properties of the molecular building blocks by well-established synthesis methods. This enables the rational design of molecules with certain properties, such as high charge carrier mobility in the crystalline phase or a desired absorption or luminescence spectrum. These properties are, to a large extent, maintained in the molecular crystals and organic nanowires, which also upon correct growth conditions exhibit defectless excitonic spectra (figure 13(a)). Temperature dependent optical spectroscopy has proven to be a viable tool to verify the crystalline nature of the nanowires (figure 13). 3.1. Optical waveguides Optical waveguides are the simplest approach to passive optical nanowire components in micro- and nanophotonic devices. Whereas most ‘conventional’ waveguides use an external light source that is far field coupled, organic nanowire waveguides can take advantage of the luminescence property of the organic material to realize a waveguide where the guided light is the luminescence that has been stimulated in the organic nanowire by an external light source. That way the light source is very effectively coupled in the near field to the waveguide. One of the first demonstrations of waveguiding in organic nanowires stems from the Morikawa group, who observed waveguided emission in p-6P nanowires grown on alkali halides [22]. By exposing the nanowires to UV light through an aperture, the blue S1–S0 luminescence (λ ≈ 425 nm) could be observed locally, both from within the area defined by the UV light spot but additionally also from the tips of the nanowires extending outside the UV illuminated area. These bright blue spots originate from luminescence stimulated by the UV light and guided along the nanowires to be scattered at the nanowire tips. Similar effects can also be seen from electroluminescence (EL) stimulated in the nanowires and guided through them (figure 14(c)) [128]. Further elaborate investigations of waveguided luminescence in individual p-6P nanowires grown on mica were used together with an analytical model to study the waveguiding conditions and damping

2.3. Coating A shortcoming of organic materials is that they suffer from a reduction in luminescence intensity upon extended exposure to ultraviolet (UV) light (‘photobleaching’). The bleaching 11

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Figure 13. (a) Temperature dependent luminescence spectra of para-hexaphenyl nanowires grown under conditions on mica that result in ‘perfect’ wires. (b) Same as (a), but grown under conditions that result in polymorphism. Note that the spectra at room temperature for both conditions are very similar, whereas at low temperatures and especially in the phase change regime around 135 K, the polymorphism becomes obvious. From [127].

the waveguiding behaviour of a 1,5-diaminoanthraquinone (DAAQ) nanowire grown vertically from a substrate and compared it to a horizontally arranged nanowire supported on glass [133]. As shown in figure 15, the optical loss is significantly higher for the supported nanowire as compared to the free-standing nanowire. Numerous other reports have studied various aspects of organic nanowire waveguides; we refer the reader to a recent review article for further details [134].

properties (figure 14(a)) [70, 129]. Only transverse magnetic (TM) modes can propagate and only if the wavelength is below a cut-off wavelength that depends on nanowire width. Whereas these studies investigated the waveguiding properties by studying the light scattered from the nanowire tip into the far field, the use of scanning near-field optical microscopy (SNOM) enabled the observation of waveguided luminescence in the near field [130] and thereby directly visualized the damping occurring along the nanowire (figure 14(b)). This damping is due to reabsorption of the blue light stimulated within the organic nanowires. The combined far- and near-field studies resulted in values for the dielectric functions of individual nanowires, which apparently critically depend on growth conditions and thus morphology. The Yao group demonstrated waveguiding of luminescence in the UV part of the spectrum [131]. The same group also demonstrated how the luminescence spectrum can be tuned by realizing binary nanowires that also support waveguiding [132]. Overall, it was found that the scattering during propagation in the waveguide is of minor importance, while an important loss channel is the coupling to the supporting substrate due to a weak difference in refractive indices between the nanowire and the substrate. Better confinement can be realized in a vertical nanowire configuration, in which the nanowire is surrounded by air. Huang and co-workers studied

3.2. Amplifiers and lasers Depending on the organic molecule of choice and on the optical pumping mechanism applied, strong gain and feedback processes take place in organic nanowires, which then function as nanoscale lasing elements. In order to reach a sufficiently high exciton density, the photoexcitation is typically performed with a short pulse-laser that can deliver a high pump fluence without thermal damage to the lasing material. Initial demonstrations of amplification and lasing were made on an ensemble of p-6P nanowires on mica [135]. The nanowires were excited at normal incidence by 150 fs long pulses at 380 nm and the resulting emission was detected normal to the substrate plane with a spectrometer. For low fluences (below ∼0.5 µJ cm−2 per pulse), the typical 12

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Figure 14. Waveguiding in para-hexaphenyl (p-6P) nanowires. (a) Far field luminescence image: p-6P nanowires are excited on the left-hand side and blue light is propagating along the nanowire to the right, being scattered at a break at distance of 90 µm. From [70]. (b) Topographical (left-hand side) and scanning near-field optical microscopy (SNOM, right-hand side) images (60 × 60 µm2 ) of p-6P nanowires on mica obtained at an emission wavelength λ ≈ 425 nm after UV excitation. The UV light spot boundary is positioned across the nanowires at the border of the scanning range of the SNOM. The SNOM image shows propagation of near-field intensity along some of the nanofibres, with rather strong damping. From [130]. (c) SFM (left-hand side) and far field EL image (right-hand side) of a p-6P nanowire spanning two electrodes. Emission is induced at the electrode edge, propagating along the nanowire and scattered at the break. From [128].

fluorescence spectrum for p-6P was observed. An increase in pump fluence resulted in randomly spaced spectral lines across the (0–1) emission peak near 425 nm with a resolution-limited width of 2 Å. By changing the spatial excitation position, the line spectral distribution changed significantly. This indicates that the lines are due to random lasing, in which a closed optical loop with net gain is formed via multiple scattering events caused by nanowire inhomogeneities. More detailed studies of lasing behaviour in individual p-6P nanowires have provided a better understanding of the influence of the nanowire morphology [136]. By preparing the nanowire to be segmented into many sections separated by small breaks, similar sharp lines could be observed above the threshold fluence with strong scattering and out-of-plane emission occurring at the breaks, as seen in figures 16(a)–(c). This corresponds to one-dimensional random lasing in which the end facets at the nanowire breaks cause efficient optical feedback. By preparing a break-less nanowire, the optical feedback could be inhibited. Instead, the peaks in the spontaneous emission spectrum were gradually narrowed for higher fluences and

the out-of-plane scattering intensity at the nanowire ends was strongly enhanced, demonstrating amplified spontaneous emission (ASE) and a high optical gain up to 103 cm−1 , as seen in figures 16(d)–(f ). A similar experiment demonstrated lasing in the UV part of the spectrum by stimulating an organic nanowire with a pulsed laser and with the nanowire forming a Fabry– Perot cavity [131]. Nanowires from thiophene/phenylene co-oligomers also result in lasing action [33]. Finally, the fabrication of periodically grown organic–organic heterostructures, as discussed above, provides a powerful technique to tailor the lasing properties [72]. 3.3. Frequency converters Optical second harmonic generation (SHG) occurs in materials with a non-zero first order non-linear susceptibility χ 2 , requiring molecules with a non-linear dipole moment that assemble in a non-centrosymmetric crystal structure. Since only a limited class of molecules can self-assemble to 13

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Figure 15. Bright field microscope image of (a) a 1,5-diaminoanthraquinone (DAAQ) nanowire supported on glass and (b) a free-standing DAAQ nanowire grown on a silicon wafer. (c) and (d) Microscope images of the same two nanowires with luminescence excited locally at four different positions (intense spot). The white rectangles at the left-hand side highlight the waveguided emission that is scattered at the nanowire tips. (e) and (f ) Spectra of the scattered light with the excitation spot located at the four different positions shown in (c) and (d), respectively. Inset shows the intensity versus the propagation length with exponential fits. Reprinted with permission from [133]. Copyright 2010, American Chemical Society.

Figure 16. Random lasing and amplified spontaneous emission (ASE) in para-hexaphenyl nanowires. Optical microscope images of organic nanowire emission when stimulated by fs pulses with a fluence of (a) 33 µJ cm−2 per pulse and (b) 170 µJ cm−2 per pulse. (c) Emission spectra of the nanowire placed in the centre of panels (a) and (b) for different values of pump fluence. (d)–(e) Optical microscope images similar to panels (a) and (b) but for a break-less nanowire excited with fluences of 75 µJ cm−2 and 370 µJ cm−2 per pulse, respectively. (f ) Emission spectra from the nanowire shown in panels (d) and (e) for different values of pump fluence. Reprinted with permission from [136]. Copyright 2006, American Institute of Physics. 14

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active materials is that the size of the non-linear optically active element is scalable from very small, cluster-like units of a few tens of nanometres in diameter to macroscopic entities of micrometre- or millimetre size. 3.4. Surface plasmon generators and waveguides The ability to generate structures on metal surfaces on a size scale that allows for the generation and support of elementary longitudinal electron density waves (surface plasmon polaritons (SPPs)) opens up new opportunities for future ultracompact photonic circuitry. An organic nanowire situated on a gold film can, under these conditions, act as a source of SPPs [140]. The efficiency of plasmon stimulation can be studied with high spatial resolution using photoemission electron microscopy (PEEM). Here, the surface of the sample is uniformly illuminated with a UV or laser light source in order to stimulate emission of photoelectrons. These photoelectrons are collected by an electrostatic lens system onto an imaging unit to create a map of the photoelectron emission intensity with a spatial resolution of the order of 20 nm. The photoelectron emission probability is influenced by various factors, such as surface topography and local work function. SPPs can be imaged as they propagate along the metal– vacuum interface since the polarization of the SPP mode locally modulates the work function and thus the photoelectron emission intensity. Often, the same laser pulse is used both to stimulate and to image the SPP; thus the resulting image does not show directly the spatial distribution of the SPP mode but rather a beating pattern arising from the interference of the stimulating laser pulse and the SPP mode (figure 19). Figure 19 shows a schematic drawing of the sample geometry and the resulting PEEM data. The organic nanowire is supported on a gold film perpendicular and in close proximity to the edge of the film, as shown in the left-hand side of figure 19. The sample is excited by a femtosecond laser (1.55 eV photon energy) from the right-hand side with an angle of incidence of 65◦ with respect to the surface normal. Since a single photon cannot stimulate a photoelectron, a two-photon process is necessary. With p-polarized light (polarized within the plane of incidence), the laser beam excites an SPP from the edge of the gold film propagating towards the left. The vertical, long-period pattern observed in the PEEM data in figure 19 corresponds to the beating pattern of this SPP mode interfering with the incident laser beam. The gradual decrease in contrast from right to left is due to the damping of the SPP as it propagates along the gold film. The horizontal, short-period beating patterns stem from the nanowire in the centre of the image. Closer analysis reveals that these patterns are due to an SPP mode originating from the nanowire and with a wave vector ß that forms an angle of 27◦ with respect to the surfaceprojected wave vector of the laser beam. It thus demonstrates that the local perturbation caused by the nanowire enables the excitation of a propagating SPP mode without the need of a k-vector matching unit, such as a grating or prism. This method can be extended towards the generation of an artificial pattern of plasmon waves on a metal surface by

Figure 17. SHG spectra from methoxy-chloro-p-quaterphenylene (MOCLP4) nanowires excited with different wavelengths. The dotted red line is the MOCLP4 nanowire single photon luminescence spectrum excited with UV light. From [138].

form nanowires of the form described in this article, reports on frequency doubling organic nanowires are limited. A successful route to SHG active nanowires is to use molecules composed of a para-quaterphenyl (p-4P) basis unit, which is then unsymmetrically functionalized with different end groups. Brewer et al demonstrated that nanowires made from methoxy-amino-p-quaterphenylene (MONHP4) molecules exhibit a large first order non-linear susceptibility and therefore generate a strong SHG signal upon excitation with femtosecond light pulses at 800 nm [137]. Figure 17 demonstrates the resonance enhancement of the SHG process for nanowires made from methoxy-chloro-p-quaterphenylene (MOCLP4) molecules: excitation with wavelengths of 770 nm, 790 nm or 830 nm results in peaks in the spectrum around 385 nm, 395 nm or 415 nm, respectively, with the strongest peak at 790 nm. Since the two-photon absorption edge is at 800 nm, excitation with wavelengths below 800 nm results in fluorescence spectra in addition to the SHG signals. Later studies have focused on a quantitative determination of χ 2 for nanowires made from six different molecules [138]. A systematic, wavelength-dependent SHG study using nanosecond laser pulses resulted in a value for the nonlinear susceptibility of 1-cyano-p-quaterphenylene (CNHP4) nanowires of 1.6 pm V−1 , which is comparable to that found for inorganic non-linear optical crystals [139]. For these experiments, molecules with a cyano electron pull group have been synthesized since the cyano group is highly electronegative and thus efficiently generates molecular dipole strength. Figure 18 compares emission spectra as a function of excitation wavelength for three differently unsymmetrically functionalized molecules: CNHP4, methoxy-cyano-pquaterphenylene (MOCNP4) and MOCLP4. SHG is observed (straight line) in all three cases, but the ratio between SHG and two-photon luminescence (TPL) emitted by the nanowires can be tuned by the molecular building block, the crystal structure and the excitation wavelength. Besides being tunable, another advantage of using nanowires for generating second harmonic 15

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Figure 18. Measured emission spectra as a function of nanosecond excitation pulses for nanowires made from 1-cyano-p-quaterphenylene (CNHP4), methoxy-cyano-p-quaterphenylene (MOCNP4) and methoxy-chloro-p-quaterphenylene (MOCLP4) molecules. The SHG line as well as the area for TPL are indicated. Reprinted with kind permission of Springer Science+Business Media from [139].

Figure 19. Left-hand side: geometry of the nanowire photoemission electron microscopy (PEEM) experiment: the nanowire is transferred onto a gold film that supports generation of SPPs after illumination with p-polarized near infrared femtosecond light pulses from the right-hand edge. Right-hand side: measured modulation of electron emission probability as a signature of SPPs propagating to the top and the bottom of the nanowire. The inset shows the k-vectors of SPP and projected laser, respectively. Reprinted with kind permission of Springer Science+Business Media from [140].

in good agreement with the leakage radiation microscopy investigations. A unique feature of two-photon PEEM is that pump–probe experiments become possible by delaying the femtosecond pulses with a sub-femtosecond temporal resolution. This allows one to obtain a real time view of the wave packet propagation in SPP waveguides, measure the group velocity and the characteristic damping lengths [142]. In order to tune the propagation properties, tailoring of the nanowire dimensions has been proven useful. PEEM data are nanowire selective and thus SPPs induced by nanowires with distinct variations in width and height can be wavelengthdependent investigated, thus resulting in SPP dispersion curves for individual nanowires [144, 145]. In figure 23, SPP dispersion curves are measured for three nanowires and compared with the dispersion curves for p-6P films of equivalent thickness. Nanowire 1 is flat and broad with a ratio ρ of height to width of 0.02 and rather film like, whereas nanowire 3 is more narrow (ρ = 0.17). As seen, with increasing ρ the SPP dispersion curves between films and nanowires start to disagree. The obvious tradeback of plasmonic waveguides are the damping losses due to scattering of the plasmons. Hence the guiding structure should be as defect-free as possible

depositing a multitude of nanowires (figure 20). Note that a classical simulation (right-hand side of figure 20) shows very good agreement with the measured PEEM pattern. Hence it is possible to calculate an arbitrary SPP pattern and then realize it via deposition of nanowires. Besides the stimulation of free propagation along the metal–air interface, the nanowires stimulate the directed propagation of SPPs along the interface between nanowire and the metal in the form of a dielectric loaded SPP waveguide (DLSPPW). This plasmonic waveguiding has been demonstrated via leakage microscopy (figure 21, [141]) and also via time-resolved pump–probe PEEM measurements [73, 142]. As shown in figure 21, plasmonic waveguiding by nanowires is induced both with the help of a grating structure (nanowire 1) [143] and without it (nanowire 2), albeit with different efficiencies. Presentation of the leakage microscopy image in the form of a Fourier transform image, as in figure 21, allows one to easily identify the propagating and counterpropagating modes and to determine the effective refractive index of the system. PEEM measurements provide additional information about the details of the SPP propagation along the nanowires on gold surfaces (figure 22); the overlapping information being 16

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Figure 20. (a) Fluorescence microscopy image induced by UV excitation of an array of para-hexaphenyl nanowires deposited on a thin gold film. (b) PEEM image after femtosecond excitation of the array from (a). (c) Classical simulation of the SPP pattern. Reprinted with kind permission of Springer Science+Business Media from [140].

Figure 21. (a) SFM image of three nanowires transferred onto a thin gold film. Nanowire 1 overlaps a grating structure, which is illuminated by an excitation beam. (b) Fourier transform image of the evanescent light, showing the SPP at the gold–air interface (circle) as well as directly transmitted light (central spot) and dielectric loaded SPP waveguides from nanowire 1 (propagating, ‘1’, and counterpropagating, ‘2’, mode) and nanowire 2 (labelled ‘3’). From [141].

Figure 22. (a) Two-photon PEEM image of a para-hexaphenyl nanowire (dielectric loaded SPP waveguide) on gold. (b) Beating pattern along the nanowire, showing strong damping. (c) FFT spectrum of the two-photon PEEM beating pattern; the beating period corresponds to a SPP wavelength of 784 nm. From [73].

and—in the optimum configuration—consist of active material that compensates for the losses. As shown above, organic nanowires can in fact act as amplifiers, lasing and non-linearly active materials, all depending on the functionalization of the

basic molecules. By using CNHP4 nanowires, coupling to SPPs of light that has been actively frequency doubled by the nanowires has been demonstrated with the help of leakage microscopy [146, 147]. 17

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copper phthalocyanine (CuPc) for p-type devices with a hole mobility up to around 0.5 cm2 V−1 s−1 [151] or copper hexadecafluorophthalocyanine (F16 CuPc) for n-type devices with an electron mobility of approximately 0.2 cm2 V−1 s−1 [152]. By placing CuPc and F16 CuPc nanowires on the same substrate, ambipolar transistor devices were realized with balanced transport characteristics [116]. In situ nanowire growth can also be realized directly on closely spaced, bottom electrodes by vapour deposition of p-6P molecules to fabricate a transistor without the need for any postprocessing after nanowire deposition (see section 1.2 above) [98]. The electrical performance of an organic nanowirebased transistor is in general inferior to that of inorganic nanowire-based devices due to a much lower charge carrier mobility. However, for optoelectronic device applications, such as light emitters and light detectors where charge carrier mobility is less important, organic semiconductors with high luminescence yields and efficient absorption constitute an attractive material class. Consequently, several studies have demonstrated miniaturized optoelectronic components from organic nanowires typically based on a transistor platform.

Figure 23. Measured SPP dispersion relations for individual nanowires (fibres 1–3) with heights of 20 nm (blue), 35 nm (green) and 75 nm (red) and different widths. The dotted grey lines are measurements for films of indicated thicknesses, while the yellow line is for a plain Au surface. From [144].

4. Organic molecular aggregate based optoelectronic devices

4.2. Organic nanowire-based light-emitting and light-sensing devices

4.1. Field-effect transistors

Organic nanowires made from luminescent molecules should be useful as electrically driven light-emitters when integrated with metal electrodes that enable injection of both holes and electrons. The first studies were made by Yanagi et al, who realized a structure of PPTTPP nanowires covered by a continuous film of p-6P molecules and sandwiched between an aluminum cathode and an indium tin oxide (ITO) anode in a diode configuration [153]. At low driving voltages, blue light emission could be observed from the p-6P film in the region between the PPTTPP nanowires where it was connecting to both electrodes and only a minor contribution in the green spectral region from the PPTTPP nanowires was detected. At higher driving voltages, when the carriers were transported through the p-6P film without recombining, holes would accumulate inside the PPTTPP nanowires and recombine with electrons transported through the p-6P layer. This resulted in green emission from the nanowires. Later studies have focused on a planar geometry, in which the lightemitting nanowire is connected to metal electrodes with an underlying gate electrode in a FET geometry, as shown in figure 25(a) [128]. The gate electrode is biased with an ac voltage of appropriate amplitude and frequency, while the ‘source’ and ‘drain’ electrodes are either dc biased or grounded. It should be noted that the device is not acting as a transistor in this configuration (no charge transport across the channel) but rather the holes and electrons are subsequently injected from the same electrode during one period of the ac cycle. This results in the stimulation of localized emission at the position where the nanowire connects to the metal electrode (figures 25(b) and 14(c)) with the same spectral features as observed in the PL emission (figure 25(c)) and with the emission being polarized due to the molecular ordering. The amount of EL can be modified by various factors, such as

The type of organic nanowires discussed in this article has semiconducting properties and numerous studies have therefore focused on the implementation of nanowires in FET devices. These consist of a pair of electrodes (source and drain) that connect to the nanowire, whose conductance can be modulated by the application of a suitable electrical potential to the transistor gate electrode. The first studies by Taniguchi and co-workers focused on nanowires made from a thiophene/phenylene co-oligomer (5,5 -di-4-biphenylyl-2,2 bithiophene or ‘PPTTPP’). A p-type transistor behaviour was found with a high hole mobility of up to 0.66 cm2 V−1 s−1 and on–off ratio of 4.4 × 105 [148]. Figure 24(a) shows an optical microscope image of the nanowire transistor device, while the electrical transfer characteristics, from which the hole mobility can be extracted, are provided in figure 24(b). The high hole mobility is presumably related to the molecular orientation. In such crystalline nanowires, the carrier mobility is in general anisotropic due to the difference in orbital overlap between neighbouring molecules along different crystal directions [149]. In the nanowire transistor devices reported in [148], the PPTTPP molecules are aligned perpendicular to the long nanowire axis resulting in improved transport along the nanowire. The same group later reported studies on a series of biphenyl-capped thiophene (PPTn PP, n = 1–4) nanowires and found that PPTTPP exhibited the highest hole mobility of the four materials, exhibited a higher effective mobility than a thin film of the same material and that the transport properties were dependent on the anisotropic arrangement of the molecules in the nanowire crystal structure [150]. Tang et al used a vapour transport technique to grow nanowires in situ on a silicon dioxide surface and later fabricate FETs by shadow-masked top contact deposition, using either 18

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Figure 24. (a) Optical microscope image of three 5,5 -di-4-biphenylyl-2,2 - bithiophene (PPTTPP or ‘BP2T’) nanowires connected between two gold electrodes on a FET device. (b) Transfer characteristics of the transistor device measured at a drain–source voltage of −30 V. Reprinted with permission from [148]. Copyright 2002, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

nanowire morphology, gate voltage and ac frequency. Since the emission is localized at the interface between the electrode and the nanowire, the spatial extension of the light source is given by the width and height of the nanowire and thus can be of sub-wavelength dimensions. By implementing two different types of nanowires with different emission spectra (p6P and PPTTPP) on the same transistor platform via a double roll-printing strategy, it is possible to realize multicolour light emission [154]. A similar light-emitting device structure can be realized with in situ grown nanowires by realizing the gold electrodes sufficiently close to enable the nanowires to grow across the gap during the deposition of the molecules [155]. This can enable a straight-forward method of colour tuning by sequential deposition of molecules with different emission spectra. The planar device configuration with a nanowire connected between two metal electrodes can also be used to form an organic photodetector, in which incoming light with above-band gap photon energy can generate electron–hole pairs and thus modulate the transistor conductance. Zhu and co-workers demonstrated an n-type organic phototransistor (OPT) based on F16 CuPc nanowires made by the vapour transport method [156]. By illuminating the nanowire OPT with light of variable wavelength, the spectral dependence of the generated photocurrent could be determined and was found to match the optical absorption spectrum, thus confirming that the induced current was due to optical carrier generation. A similar p-type device based on vapour transport-grown CuPc nanowires was recently demonstrated, although in a more simple two-contact configuration without a gate electrode [157]. Also epitaxially grown nanowires have been integrated in an OPT configuration to provide a high sensitivity [158]. Nanowires made by the evaporation of 5,5-bis(naphthyl)-2,2 bithiophene (NaT2) molecules were transferred onto gold electrodes via roll printing to form a FET and the performance was compared with a NaT2 thin film transistor. Upon illumination, the sensitivity of the nanowire-based transistor was found to exceed that of the thin film-device by roughly two orders of magnitude as a result of the better transport properties observed in the crystalline nanowires.

5. Conclusions and outlook In this article, we have detailed the mechanisms of thin film growth at the organic/inorganic interface under nonequilibrium conditions that lead to the formation of ordered arrays of crystalline organic nanowires, which after growth can be separated from the growth template and integrated into novel optoelectronic device platforms. These organic nanowires are scalable from nanometre-sized cluster-like aggregates to micrometre-sized classical wave guiding elements, their shapes being defined by the facetted crystalline growth process. Besides acting either as morphologically well-defined dielectric slabs in a passive wave guiding form or as surface plasmon inducing discontinuities in the dielectric function of the surface, the nanowires can also act as optical amplifying or light converting media. Their crystalline organic growth process results in a defectless optical response with low losses, while at the same time facilitating huge flexibility in the optical spectra due to the possibility to change the molecular building blocks via synthetic chemistry methods. Since it has become possible to integrate organic nanowires into plasmonic platforms as passive or active nanoplasmonic elements and since electric contact is also easily achievable, a significant potential of these aggregates for future hybrid photonics/plasmonics is expected. Similarly, organic nanowires can be implemented as components in optoelectronic devices to function as either light-emitting or light-sensitive elements with properties determined directly from the molecular constituents, which enables the realization of miniaturized devices with, for example, a desired emission or absorption spectrum.

Acknowledgments This work has been financially supported by the Danish Research Agency under various grants and by the Austrian Science Fund (FWF): P25154 as well as the federal government of Upper Austria (Project ‘Organische Nanostrukturen’). 19

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Figure 25. (a) Geometry of a para-hexaphenyl (p-6P) nanowire light-emitting device: a highly doped silicon substrate acts as gate electrode and a silicon dioxide layer as gate dielectric. On top of this, patterned gold electrodes connect to a p-6P nanowire. Also indicated is the biasing scheme, in which an ac voltage applied to the gate electrodes stimulates subsequent injection of holes and electrons, resulting in EL. (b) Microscope image of an array of nanowires connected to gold electrodes during biasing. Bright spots near the electrode edges are due to locally stimulated EL. (c) Spectrum of the emitted EL compared to the PL spectrum from similar p-6P nanowires. From [128].

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