Vertical Single-Crystalline Organic Nanowires on Graphene: Solution

Letter pubs.acs.org/NanoLett

Vertical Single-Crystalline Organic Nanowires on Graphene: SolutionPhase Epitaxy and Optical Microcavities Jian-Yao Zheng,*,†,§,∥ Hongjun Xu,†,§,∥ Jing Jing Wang,§,∥ Sinéad Winters,‡,§,∥ Carlo Motta,†,§,∥ Ertuğrul Karademir,†,§,∥ Weigang Zhu,⊥ Eswaraiah Varrla,†,§,∥ Georg S. Duesberg,‡,§,∥ Stefano Sanvito,†,§,∥ Wenping Hu,⊥,# and John F. Donegan†,§,∥ †

School of Physics, ‡School of Chemistry, §Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), and Advanced Materials and BioEngineering Research Centre (AMBER), Trinity College Dublin, Dublin 2, Ireland ⊥ Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China # Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ∥

S Supporting Information *

ABSTRACT: Vertically aligned nanowires (NWs) of single crystal semiconductors have attracted a great deal of interest in the past few years. They have strong potential to be used in device structures with high density and with intriguing optoelectronic properties. However, fabricating such nanowire structures using organic semiconducting materials remains technically challenging. Here we report a simple procedure for the synthesis of crystalline 9,10-bis(phenylethynyl) anthracene (BPEA) NWs on a graphene surface utilizing a solution-phase van der Waals (vdW) epitaxial strategy. The wires are found to grow preferentially in a vertical direction on the surface of graphene. Structural characterization and first-principles ab initio simulations were performed to investigate the epitaxial growth and the molecular orientation of the BPEA molecules on graphene was studied, revealing the role of interactions at the graphene−BPEA interface in determining the molecular orientation. These free-standing NWs showed not only efficient optical waveguiding with low loss along the NW but also confinement of light between the two end facets of the NW forming a microcavity Fabry−Pérot resonator. From an analysis of the optical dispersion within such NW microcavities, we observed strong slowing of the waveguided light with a group velocity reduced to one-tenth the speed of light. Applications of the vertical singlecrystalline organic NWs grown on graphene will benefit from a combination of the unique electronic properties and flexibility of graphene and the tunable optical and electronic properties of organic NWs. Therefore, these vertical organic NW arrays on graphene offer the potential for realizing future on-chip light sources. KEYWORDS: Vertical, organic nanowire, graphene, solution, van der Waals epitaxy, optical microcavity hanks to their mechanical flexibility as well as their tunable optical and electronic properties, organic conjugated materials have opened new avenues of research in electronics, photonics, and solar energy harvesting.1,2 Singlecrystalline nanowires (NWs) of these materials are of particular interest, because their purity, high crystalline quality, and longrange order allow researchers to investigate the intrinsic properties of the material, along with their strong performance in devices.3 To achieve large area, high-density device arrays with high throughput, it is crucial to align these singlecrystalline NWs.4−6 Vapor-phase deposition methods have been widely used to realize the epitaxial growth of well-defined vertical single-crystalline NW arrays of semiconductor materials.7,8 However, these deposition techniques often suffer from the disadvantages of high cost and low efficiency due to the requirement of high temperature and high vacuum con-

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© XXXX American Chemical Society

ditions.9−11 Hence, a solution-phase synthetic strategy remains a competitive alternative method because of its low growth temperatures, easy operation, low-cost, and good potential for scale-up.12 In particular, the solution processability of organic semiconductors offers the possibility to fabricate large-area devices by as alternative methods including inkjet printing and spin coating at affordable cost. The mechanical properties of organic semiconductors are also beneficial for future photonic and optoelectronic applications.13,14 Despite the enormous potential benefits, several challenges remain in epitaxially growing well-defined vertical single-crystalline organic NW arrays from solution: (1) the difficulty of controlling the Received: February 5, 2016 Revised: July 1, 2016

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DOI: 10.1021/acs.nanolett.6b00526 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic drawing of solution-phase epitaxial growth of NWs on a graphene surface at room temperature. (b) SEM image of graphene on the copper substrate. Inset: a typical Raman spectrum of the graphene. (c) SEM image of BPEA nanowires grown on graphene by the vdW epitaxial growth technique. (d) XRD pattern of BPEA NW arrays (black curve) and the standard XRD of BPEA single crystals (red curve). (e) Top view fluorescent microscope image of the NW arrays. The numerous bright spots are from free-standing BPEA NWs. (f) TEM of BPEA NW. Inset: SAED pattern of the NW, showing the growth direction to be [001].

nanocrystal arrays on graphene and illustrated the advantages of having intimate contact quality for applications as a solar cell and a vertical conducting device, respectively.30,31 One of the attractive and unique features of organic materials as opposed to their inorganic counterparts is their superior optical properties such as high optical cross-section and quantum yield.32 Hence, a solution-based method for growing high optical quality, vertically oriented single-crystalline NWs is highly desirable to advance the current technology of organic nanophotonics in parallel with the rapid development in organic electronics and photovoltaics. In this work, we use 9,10-bis(phenylethynyl) anthracene (BPEA) as a prototype molecule (Figure 1a, inset), as well as a versatile material for various optoelectronic applications, and we describe a simple yet effective solution based technique for the fabrication of organic semiconductor NWs on graphene (Figure 1a). We observe arrays of crystalline NWs that possess excellent morphological and crystallographic orientation in

molecular orientation, packing, and crystallization at the substrate-molecule interface and (2) the inability to adapt inorganic synthesis techniques to organic materials. Recently, graphene has emerged as an excellent substrate for guiding the orientations of organic molecules and this has stimulated a number of studies in recent years.15−21 It has also been reported that the magnitude of the surface dipoles, as well as the electronic coupling between the molecules and graphene can affect both the energy level alignment at the molecule− graphene interface and the electron/hole injection in devices.22−25 The strong interaction has been shown to enhance device performance for applications such as thin film vertical field effect transistors (FETs).26,27 More generally, the graphene substrate can be utilized as a flexible, transparent electrode for fabricating many device architectures.28 To date, the reported organic/graphene heterostructures are mainly limited to thin films that are most suitable for two-dimensional (2D) devices.29 Two recent studies realized vertical organic B


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Nano Letters which the π−π stacking direction is found to be perpendicular to the graphene surface, a desirable feature for applications such as solar cells, FETs, and optical waveguides. Efficient waveguiding that showed low loss was demonstrated for freestanding NWs. Light within the nanowire can be confined between the two end facets of the NW, building Fabry−Pérot (FP) resonator modes as observed in the emission spectra of the NWs. An analysis of the optical dispersion of the waveguide modes showed a reduced group velocity of the waveguided light by as much as 90% to a value of c/10. These results are very appealing because graphene can then function as a transparent and flexible electrode, providing a facile way to fabricate on-chip light sources for applications such as displays, in optical communications and in electro-optic devices. High-quality graphene is critical for the subsequent NW growth throughout this work. Graphene was synthesized on a copper substrate by chemical vapor deposition using a 2:1 mixture of methane and hydrogen at a temperature of 1035 °C.33,34 The Raman spectrum of a typical sample is presented in the inset of Figure 1b, showing a 2D band located at ∼2679 cm−1, the intensity ratio of the 2D peak to the G peak is 2.7:1. The as-grown sample is predominantly single-layer graphene (SLG), which is demonstrated by fitting the 2D peak of Raman spectrum with a single Lorentzian line shape (fwhm = 29 cm−1).18,35−37 For the vertical growth of BPEA NWs, a BPEA solution in N,N′-dimethylformamide (DMF) with a concentration of 10 mM was drop-cast on the graphene surface as depicted in Figure 1a. Then the sample was left in air for 48 h at room temperature to evaporate the solvents from the samples. Before the optical and structural characterizations were carried out, the samples were put in vacuum for several hours to ensure complete solvent removal. In Figure 1c, a tilted-view scanning electron microscopy (SEM) image of nanowires grown perpendicularly on the surface of graphene is shown. The diameters of wires range from ∼100 nm to many microns, and lengths up to ∼80 μm are observed. As shown in Figure S1, the NW arrays can be fabricated in large quantities and at low cost making this approach a viable option for preparing NW-based devices. Figure 1d shows the X-ray diffractogram of a large-area NW array (∼1 cm2) grown on a graphene surface (black curve), which shows the peak corresponding to (002) crystal planes from BPEA and the (111) peak from the copper substrate. The peak from BPEA coincides with the single-crystalline structure of BPEA that has been reported (red curve in Figure 1d, a = 24.3 Å; b = 11.51 Å; c = 7.1 Å; α = β = γ = 90°),38,39 indicating that all BPEA molecules are uniformly π-stacked along the vertical direction in each crystal over a large area. Figure 1e shows the top view of the NW arrays with UV band excitation (330−380 nm) in an optical microscope; the large number of bright luminescent spots indicates the high vertical yield of the NWs. We verified the intermolecular distances by carrying out transmission electron microscopy (TEM) experiments on discrete NWs, as shown in Figure 1f. The spots from the selected area electron diffraction (SAED) indicate that the wires were composed of a single crystal, growing along the [001] direction. It is noteworthy that no catalyst is used in the growth process which reduces the potential for contamination that could cause deterioration of the performance of active semiconductors in any subsequent device operation.9 For a comparison, control experiments showed that growth of BPEA on the copper substrate alone could not form ordered NWs on the surface (see Figure S2).

We studied the effects of temperature and solution concentration on the length and the density of the nanowires. Figure 2a−c shows a collection of SEM images offering a tilted

Figure 2. Variation in NW formation on graphene with growth conditions. (a−c) SEM micrographs in tilted-view showing the different NW length and density at different solution concentrations (CBPEA) grown at room temperature. (d−f) SEM micrographs in tilted view illustrating the length and density control of the nanowires (nA) achieved by using different temperatures at a constant concentration of 8 mM. Scale bars are 50 μm. (g,h) Plot of length of the nanowires (L, blue columns) and the number-density of NWs (nA, black squares) with respect to the solution concentration (CBPEA) and growth temperature (T).

view of the NW arrays achieved with the indicated concentration of BPEA dissolved in DMF. The highest concentration (CBPEA) tested here is 12 mM, which is slightly below the saturation value of BPEA in DMF. The correlation between the length/density of NWs and the concentration is plotted in Figure 2g. As the solution concentration increases from 4 to 12 mM, the length of NWs was found to increase from ∼7 to ∼15 μm and the density of NWs increases from ∼7 × 105 to ∼12 × 105/cm2. The length and density of the NWs could also be controllably changed by varying the temperature of the solution (T), as shown in Figure 2d−f. A plot of nanowire length and density as a function of temperature is presented in Figure 2h. By increasing the growth temperature from 293 to 323 K, the NW density was substantially increased from ∼6.6 × 105 to ∼31 × 105/cm2, and the length of the NWs was found to be ∼7.6 μm, shorter than those at room temperature (293 K, ∼14 μm). We turn to crystal growth dynamics to understand these results. The chemical potential of the solution phase (μS) should be higher than the chemical potential of both the molecules binding to graphene (μG) and the crystal (μC) for growth to proceed. The growing phase, i.e., the crystal, should have the lowest chemical potential C


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Figure 3. SEM micrographs of the morphology evolution and crystallization kinetics of growth behavior. (a−e) Morphology evolution of BPEA films deposited on graphene with different solution volumes from 0 to 50 μL. (a) Vs = 0 μL. (b) Vs = 5 μL. (c) Vs = 15 μL. (d) Vs = 30 μL. (e) Vs = 50 μL. Scale bars are 20 μm. (f−i) A schematic diagram of the growth process of the vertical BPEA NWs. (j) Correlation between solution volume (Vs) and crystal nucleation density (nA) and NW length (L).

μC < μG < μS

the grains. Therefore, the diffusion of BPEA molecules on the graphene surface resulted in the reduction of the surface coverage while increasing the islands heights. If the cluster reaches a certain critical size, it becomes stable and forms a nucleus. The nucleus now provides both steps and kinks for continued growth (Figure 3d,e). This growth mode is often referred to as “birth-and-spread”.40,42 The nucleation density and length of the NWs with respect to solution volume are plotted in Figure 3j. The number density and the length of the NWs increased with the increase of solution volume (green and orange columns in Figure 3j). As shown in Figure 3f−i, three morphology evolution events are identified with increasing Vs. First, a wetting layer consisting of BPEA molecules is observed when the Vs = 5 μL, and the nucleation density is ∼7.8 × 105/ cm2. The formation of the 2D wetting layer is likely driven by the strong graphene−adsorbate interactions. Subsequent nucleation events follow, which initiate the 2D to 3D growth transition, and ultimately the nucleation density saturates (∼20.5 × 105/cm2) when Vs = 30 μL. Finally, the most obvious morphology evolution is observed when Vs > 30 μL, where the growth of individual single crystalline nanoparticles now becomes anisotropic along the growth axis. Vertical NW growth requires that the rate of crystal growth in the vertical (out-of-plane) direction be much higher than in the horizontal (in-plane) directions. One model discussed in the literature which describes 2D−3D morphology transitions in inorganic thin film growth is the Stranski−Krastanov (SK) growth model.43 According to this model, the first few layers exhibit commensurate epitaxy with respect to the substrate surface lattice thereby minimizing the interface energy and promoting 2D growth. However, the associated energy penalty within the growing film (strain) accumulates during the growth of subsequently deposited layers, providing an energetic driving force for the eventual 2D−3D transition. This has also been observed experimentally for many large aromatic molecules on graphite by thermal desorption spectroscopy that shows that the first monolayer is bound significantly stronger to a graphite substrate than the second and subsequent layers.44 The presence of a graphene surface exerts great control over the nucleation of BPEA molecules. To confirm this process, we have performed all-electron firstprinciples calculations with the FHI-aims code. Given the fact that the Cu substrate has a minor effect on BPEA growth and

From this equation, it follows that the solution-to-crystal supersaturation, ΔμSC = μS − μC, is always higher than the solution-to-graphene supersaturation, ΔμSG = μS − μG, that is ΔμSG < ΔμSC. Therefore, the BPEA molecules tend to pile up along the normal direction of graphene surface, leading to the one-dimensional growth.42 At higher temperature, kinetic factors need to be taken into account, the axial nanowire growth rate decreases due to the onset of competing growth on nanowire side facets and the substrate surface. Similar phenomena have been reported in inorganic nanowire growth.41 Interestingly, as shown in Figure 2e,f, by increasing the temperature, multiple nanowires from one single site merge together and form a thicker nanowire with the NW density decreasing from ∼31 × 105 to ∼24 × 105/cm2, probably because nanowires in close proximity are inclined to coalesce with each other. To understand the nucleation and growth behavior on graphene substrates, a systematic study on NW morphology evolution was conducted. Here we use the BPEA solution in dichloromethane and the concentration is fixed as 8 mM and the temperature is 293 K. The changes that occur in BPEA morphology on graphene with increasing starting solution volume (Vs) at room temperature are shown in Figure 3. With a 1 cm × 1 cm SLG/copper substrate, when we drop BPEA solution it will spread over the surface of graphene. Initially, we obtain a thin “wetting layer” grown with Vs = 5 μL, as shown in Figure 3b. When we increase Vs to 15 μL, a distinct morphology transition was observed in Figure 3c, the molecules diffusing on the surface come together to form clusters. In this process, a morphology transition from a 2D thin film to a 3D nanopillar morphology is observed. The shape of the crystals can be chosen arbitrarily because during this stage the nucleus undergoes dissolution and growth until the stable morphology is achieved. Approximately Vs ≥ 15 μL is required to reach a steady state at the graphene substrate. Above this volume, the discrete nuclei evolve into 3D nanoparticles as shown from the SEM micrographs in Figure 3d. It is also clear that the aspect ratio continuously increases with Vs > 30 μL. In this case, once the BPEA molecules diffuse on the initial islands, they prefer to pile on top of those islands, thereby inducing the vertical growth of D


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induced dipole moment (Figure 4c,d). As BPEA and graphene are oriented face to face with BPEA molecules stacked in a π−π orientation, the BPEA NWs grow vertically from the graphenecoated substrate (Figure 4a and b). It is well-known that if a specific plane of the nucleating phase matches the atomic structure of the substrate surface, the lattice strain can be reduced, consequently minimizing the enthalpic contribution to the interfacial free energy, which results in preferential nucleation of that crystal plane.45 As shown in the SEM and TEM images, most of the vertical BPEA NWs on the graphene substrate grew along the [001] direction, demonstrating the interface between the graphene and the BPEA nanostructures is the (001) plane (Figure 5a). The

that the SLG/Cu system is too large for the calculation, we use for simplicity a single graphene layer as the substrate. First, we have found that the intermolecular interaction of the stacking BPEA molecules (1.04 eV) is much weaker than the attraction between BPEA molecule and graphene (2.85 eV). We have further investigated the stability of different BPEA molecular orientations on the graphene substrate. Individual BPEA molecules have been placed on the graphene surface in both horizontal (face-to-face) and perpendicular (edge-to-face) orientations in order to understand the conditions of low molecular coverage and negligible intermolecular interactions. The face-to-face configuration is the most stable orientation (see Figure 4a). The adsorption energy is calculated to be 2.85

Figure 4. BPEA molecules on graphene: (a,c) top view and side view of the face-to-face orientation of BPEA on graphene substrate; (b,d) top view and side view of the edge-to-face orientation of BPEA on graphene substrate. The face-to-face BPEA molecules are strongly bound while the edge-to-face position is not stable. In fact, during structure relaxation, an initial edge-to-face orientation falls back to the face-to-face one. Final geometries are obtained after a full relaxation of the supercell with PBE + vdW. (e,f) Top view and side view of charge density difference isosurfaces, showing a small electron transfer from BPEA molecule to the graphene substrate.

Figure 5. (a) The predicted growth morphology based on the attachment energies, calculated using the software of Material Studio package. (b) Schematic illustration of the interfacial lattice mismatch showing the heteroepitaxial relationship of BPEA-(001)[100]∥SLG(0001)[11̅ 00]. (c) SEM image of a NW with (110) side facets. The tip is bounded by the slow-growing tip facets (low-index facets). (d) Tilt SEM micrograph showing the root of a vertical NW in contact with the graphene substrate. (e) SEM in top view of BPEA NWs grown on a graphene surface. (f) SEM in top-view of BPEA NWs grown on a highly ordered pyrolytic graphite (HOPG) surface.

eV, which indicates an off-centered overlap of the π-electrons of graphene and BPEA. Noticeably, the absorption energy is found to be small (0.23 eV) in the absence of vdW interactions, indicating the crucial role of vdW in stabilizing the configuration, as expected from π−π stacking. However, the perpendicular orientation (edge-to-face) is unstable on graphene (Figure 4b). During the structural relaxation process, the starting edge-to-face position falls back to the face-to-face one which is more stable (Figure 4a). This can be understood in terms of the strong vdW interactions between the two carbon-based systems. The calculations indicate that BPEA adsorbs on graphene favorably in a faceto-face orientation with the backbone oriented either along the zigzag or the armchair substrate directions. Both configurations optimize the overlap of the π-electron densities and display the same adsorption energy. A visualization of the charge difference of the coupled system with respect to the two neutral subsystems reveals that a slight amount of negative charge is transferred from the BPEA molecules to graphene, creating an

vertical growth of the BPEA nanowires is promoted as follows: First, as shown in Figure 5b, there is an epitaxial match between the hexagonal basal plane of the graphene and the (001) plane of the BPEA. The nearest carbon atom distance of the honeycomb lattice of graphene (d = 1.42 Å) is about one-eighth of the b-axis lattice constant of BPEA (b = 11.51 Å), the mismatch is 1.23%. The a-axis lattice constant of BPEA (a = 24.3 Å) is about 10 times the a-axis lattice constant of graphene (a = b = 2.46 Å), and the mismatch is of 1.22%. The strong vdW attraction provided by the graphene surface and the relationships of the lattice constants ensure the growth of vertical BPEA NWs.46 Second, during the growth, the planar π−π arrangement of the BPEA molecules gives rise to the E


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Figure 6. NW optical waveguide measurement. Optical microscope image of (a) a BPEA NW lying flat on a glass surface and (c) an individual vertical BPEA NW grown on a graphene substrate. (b,d) Fluorescent images of two NWs obtained by laser excitation at five different spots along each NW. (e,f) Spatially resolved emission spectra of the two NWs taken by collecting the out-coupled emission at the ends of NWs as marked in c,d with laser excitation at five different spots along the NWs. (g) Decay curve of light emission as a function of excited length in the NW, the fits show an exponential decrease. Vertical scale bars are 25 μm.

for the wire on glass and the vertical NW, respectively. The waveguided light can be seen as a bright spot at the far end of the NW as the light emerges into air. The positions along the NW are chosen so that the laser excitation and PL from the excited spot do not enter the optical system.6 As marked in Figure 6c,d, in both types of NWs we can clearly observe the light guided to the ends of NWs. As the laser is moved away from the top of the wire, we see that there is a strong reduction in the light emerging from this top position. Looking closely at the two cases, we see that the reduction in intensity at the NW top is much less for the vertical NW than the one on the glass substrate NW, demonstrating much lower loss waveguide loss of the light. Figure 6e,f shows the spectra of tip emission corresponding to Figure 6c,d, respectively. The reabsorption of the propagated light leads to the slight redshift of the tip emission of the NWs with the increase of waveguide distance.6,47 We can use the measurements in Figure 6c,d to determine the loss in the two cases. We expect the intensity to decrease exponentially with increasing length x according to I(x) = I(0)exp(−αx) where α is the attenuation coefficient or loss (Figure 6g). The attenuation constant of the NW can be calculated from the evanescent curves with a value for the NW on glass of 322 cm−1, and the value for the vertical NW is 71 cm−1. BPEA nanowires on graphene show much lower optical loss than that F


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in Figure 7d. The relation between the period (Δλ) and NW length (L) can be defined by the equation

of the previously reported free-standing DAAQ nanowires (1.74 × 103 cm−1) on the silicon substrate.6 The leakage of energy through the substrate underneath the NW is the main cause of large waveguide loss for the NW on the glass substrate. These BPEA on graphene NW samples can also exhibit striking FP resonances formed when the two ends of the wire confine the light within a microcavity. This result is important for potential optoelectronic and photonic applications of these structures. We use 488 nm excitation and collect the emission from the NWs in a confocal microscope system with micronscale optical resolution (see Figures 7a and S4). Very clear

Δλ =

λ2 2n(ω)L

where λ is the wavelength of guided light, n(ω) is the group refractive index, which is a function of light frequency ω, and L is the cavity length. The mode spacing Δλ is inversely proportional to the optical path length n(ω)L and displays a quadratic dependence on the wavelength λ. Therefore, the optical dispersion in NW microcavities could be profiled by calculating the group refractive index n(ω), as shown in Figure 7f. n(ω) increases with the increase of photon energy. Figure 7f shows the group refractive index of these modes with respect to the photon energy. The group refractive index of bulk organic materials is around 1.7. The group velocity for the waveguided modes is reduced by 35% compared with the bulk material at low emission energies, which can be ascribed to the confinement of the microcavity, close to 2.3 eV, where n(ω) ∼ 10, and the group velocity can be calculated as c/10, which clearly does not agree with the standard FP model. Considering the large exciton binding energy of organic materials, we expect that the flattening of the dispersion and subsequent slowing of light can be attributed to the strong light-matter coupling between photons and excitons.51,52 Generally, the excited exciton-polariton and charge carrier transport are facilitated along the π−π stacking direction. The controlled morphology and crystallinity of highly oriented organic semiconductor NWs on graphene would be highly desirable for the vertical diodetype optoelectronic devices in which the electrons and the excitons necessarily flow along the vertical direction with respect to the substrate. Therefore, electrically pumped excitonpolariton lasing in NWs becomes feasible with a suitable topcontact electrode and graphene as bottom electrode.53,54 In summary, we have developed a simple solution-based technique for fabricating vertical organic semiconductor NW arrays by using graphene as a guiding substrate for the growth. Factors controlling the formation of nanowires are investigated. The resulting crystals are single crystalline in nature and exhibit uniform crystallographic and morphological orientations. Excellent optical microcavities are realized for the BPEA NWs upon excitation, which can be attributed to the ordered packing and their high crystalline quality. By analyzing the optical modes of the build-in FP resonator, we demonstrated strong optical dispersion in NW microcavities. Because of the strong light-matter coupling in the nanowire, we observed substantial slowing of the propagating light with group velocity of only c/10 the speed of light. This work will shed new light on the crystallization of organic semiconductor and advance the knowledge of fabrication of NW-based devices such as nanolasers, solar cells, and vertical transistors.

Figure 7. (a) Schematic of an optically pumped NW cavity. NW is excited on the tip by the focused laser. (b) SEM image of typical BPEA NWs viewed at 30° with respect to the wire longitudinal axis. (c) PL spectrum from the end of a 28.6 μm long NW shows FP modes of the microcavity. Inset: magnification of the trace near 2 eV. (d) PL spectra with a discrete set of optical modes obtained from the ends of three BPEA NWs with different lengths. (e) Free spectral range at 2.2 eV as a function of reciprocal cavity length for NWs of comparable diameters exhibiting a linear relationship. (f) Group refractive index n(ω) determined from the FP modes for wires with different lengths.

Fabry−Pérot fringes are observed in the emission spectra as has been found in other systems.48 It is clear that the simple fabrication allows for very high quality optical microcavities which show no contamination from the graphene (Figure 7b,c). The energy spacings between neighboring modes become smaller with increasing photon energy. This is particularly clear for the case of the 2.8 μm long sample. This result shows that the modes do not correspond to classical FP modes in the wire resonator. To understand the mode spectrum, the coupling between excitons and confined light needs to be taken into account.49,50 Figure 7d shows the spectra at the end of NWs with lengths ranging from 2.8 to 28.6 μm with the mode spacing in inverse proportion to NW length (Figure 7e), signifying the formation of an optical F−P resonator in the NW microcavity. The modes number increases with the increase of the cavity length in the PL spectra as shown

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00526. Description of the materials and methods, photographs, and SEM images of the large area of BPEA NWs, SEM and fluorescent microscope images of BPEA NWs on copper substrate, and schematic illustration of the G


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experimental setup for the NW optical characterization. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(E.V.) ERI@N, Research Techno Plaza, X-Frontier Block, Level 5, 50 Nanyang Drive, Singapore 637553, Singapore. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from Science Foundation Ireland (SFI) under Grant 14/IF/2499 and SFI/12/RC/ 2278. This work was partly funded by Nokia Bell Labs. S.W. and G.S.D. acknowledge SFI under PI_10/IN.1/I3030. C.M. and S.S. acknowledge the European Research Council (QUEST project) for additional financial support. W.Z. and W.H. acknowledge the financial support from National Natural Science Foundation of China (21473222, 51303185, 51033006, 51222306, 61201105, 91222203, and 91233205), the Ministry of Science and Technology of China (2011CB808400, 2013CB933500, and 2014CB643600) and the Chinese Academy of Sciences.

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