Introduction
Recently small molecules have been widely employed in the applications of organic light-emitting diode (OLED), 1 organic photovoltaics (OPV), 2 organic field-effect transistor (OFET), [3] [4] [5] gas sensors, 6, 7 as well as photosensitizers for photodynamic therapy, etc. 8 Crystal structures and molecular orientations have a significant effect on the optoelectronic and magnetic properties in crystalline materials. One of the most significant advantages for organic materials is the ease to tune their properties by regulating crystal structures, in particular, the inter-molecular distance.
As an example, Metalphthalocyanines (MPcs), a π-conjugated system on an aromatic organic ring, exhibit excellent tunable physicochemical and thermal properties by controlling their crystal structures and morphologies. MPcs have two types of common crystal structures, namely the α-and β-forms, in which molecular chains are normally formed normally along the b axis. The lattice constant b, also the inter-metal distance along the molecular chain, is ~3.8 Å (~4.8 Å) for the α-(β-) phase, 9 which is the predominant difference between different types of phases. For instance, X. He et al. recently reported the improvement of free-carrier transport properties in highly crystalline α-ZnPc film by reducing the inter-Zn distance of ~0.1 Å. 10 Wu and co-workers revealed the relationship between the magnetic properties and the stacking geometry of the phthalocyanine molecules and found strong antiferromagnetic coupling (within organic semiconductors) with an exchange energy of ~ 100 K in the α-CoPc, even higher with reduced CoCo distances. 11, 12 In addition, a new stable form of CuPc with small lattice spacing and hence large electronic overlaps, namely the η-CuPc, has shown a broader optical absorption spectrum than the other crystal structures of CuPc. 13 Copper hexadecafluorophthalocyanines (F 16 CuPc), as airstable n-type organic semiconductors, have been widely applied in OLED, 14 OFET 15 and organic solar cells, 16 owing to their excellent electrical and optical properties, such as a relatively high mobility of 0.6 cm2 V-1s-1 and broad optical absorption spectrum compared to other organic semiconductors. [17] [18] [19] [20] What is more important is that, compared with CuPc, these outstanding properties mainly stem from a shorter π-π stacking inter-molecular-plane distance (~3.25 Å) in F 16 CuPc, yet that of MPc is ~3.4 Å. 9, 21 Recently much effort has been made to optimize the crystal structure and morphology of F 16 CuPc to improve the device performance. Oteyza, et al. discovered that the increasing of film thickness caused a structural transition of F 16 CuPc film from the β-bilayer to the β-form. 22 Yoon, et al.
determined the single-crystal structure of F 16 CuPc ribbons, which crystallized in triclinic space group rather than monoclinic. 15, 23 In the study by Pandey, et al., the structures of both the F 16 CuPc thin film on graphene oxide and the single crystal are resolved, which exhibited a brick-stacking structure.
Despite the success on the determination of F 16 CuPc structure, the previous research from different groups found a similar Cu-Cu distance, which is ~4.8 Å, although other parameters are different. We have been thus motivated to further explore the crystal structure of F 16 CuPc to achieve a smaller Cu-Cu distance, hence improving the photoelectric properties. Organic nanowires are one of the promising choices as shown in the previous work. 13, 24 Although F 16 CuPc nanowires have been synthesized in recent works, 16, 25 so far, none of their crystal structures have thoroughly been identified. In this work, a detailed investigation of F 16 CuPc nanowires, the η phase, is presented with the aim to address the above issues. F 16 CuPc nanowires were fabricated by organic vapor phase deposition (OVPD). Our X-ray diffraction (XRD) results reveal that F 16 CuPc nanowires (herringbone stacking) possess a monoclinic unit cell with a = 16.360 Å, b = 3.758 Å, c = 29.123 Å, β = 121.6 o . Hence, owing to the reduction of the Cu-Cu distance, a larger spatial π-π overlap is expected to be present in F 16 CuPc nanowires, which may improve the carrier mobility, facilitating a high performance in electrical devices. Moreover, we report the phase transition of F 16 CuPc crystal from the η-to β-phase by annealing treatment. Furthermore, the photodetector based on η-F 16 CuPc nanowires was fabricated and the performance to respond to visible and near-infrared (NIR) light was systematically investigated. The following discussions fall into three sections: in §II we introduce experimental and computational details, in §III we present and discuss our results, and finally in §IV, we draw some general conclusions.
Experimental and Theoretical Section
Fabrication of F 16 CuPc crystals and photodetector devices F 16 CuPc powders, purchased from Tokyo Chemical Industry, were loaded in a quartz tube. The quartz tube was then put in a single-zone heating furnace, keeping the powders in the middle of the furnace. Ambient air was flushed out by N 2 gas (99.999%) at a rate of 400 standard cubic centimeters per minute (sccm) for 30 min before heating the F 16 CuPc powders. After that, F 16 CuPc nanowires and needle-like F 16 CuPc crystals were prepared by vaporizing F 16 CuPc powders at a series of precursor temperatures, 450 °C, 460 °C, 480°C, 500 °C and 520°C, respectively, for 300 min, at a flow rate of 400 sccm. F 16 CuPc ribbons were deposited on the high -temperature area, while F 16 CuPc nanowires were deposited at the roomtemperature area. Annealing treatment on the as-prepared F 16 CuPc nanowire was carried out to demonstrate the phase transition. F 16 CuPc nanowires were annealed using a singlezone heating furnace under vacuum condition (~2 Pa) at a series of temperatures (60 °C, 70 °C, 80 °C, 100 °C, 200 °C, 250 °C, 260 °C, 270 °C, 280 °C, 300 °C to 400 °C). Photodetector devices were fabricated by attaching as-prepared nanowire cluster onto the gold interdigitated electrodes, which were wetted by a few drops of alcohol to improve the contact with nanowires. The devices were then placed in a vacuum drying oven at 40 °C for 12 hours to remove the solution.
Characterization of F 16 CuPc crystals and Photodetector devices
The morphology was measured using a scanning electron microscope (SEM, HITACHI 3400N). The analysis of the elemental compositions was obtained from energy-dispersive spectroscopy (EDS) (EDAX). The crystal structures of F 16 CuPc were measured by means of XRD using Cu Kα (λ = 0.154056 nm) (Rigaku, TTR III). X-ray photo-electron spectroscopy (XPS) measurements were performed by using a PHI5000 Versa Probe electron energy analyzer. The properties of chemical bonds were determined by a Fourier-transformed infrared (FTIR) spectrophotometer (Agilent Technologies, Cary 600 Series). To obtain the optical absorption spectra, F 16 CuPc nanowires, ribbons and powders were slightly milled on the quartz substrate and then measured by a UV-Vis spectrometer (Hitachi, U-4100). The phase transition of the F 16 CuPc nanowires was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (METTLER TOLEDO, STARe System). The I-V curves and photo response characteristics of the photodetector were measured using a Keithley 2400 source meter (Keithley Instruments Inc. USA).
Computational details
We have carried out first-principles calculations for the excited states of isolated F 16 CuPc molecule and dimers based on density-functional theory, using hybrid-exchange density functional, B3LYP, 26 with a 6-31G basis set in the Gaussian 09 code. 27, 28 The reviews of TDDFT formalism and applications can be found in Reference. 29 The advantages of B3LYP include a partial elimination of self-interaction errors and a balancing of the tendencies to delocalize and localize one-electron wavefunctions by mixing Fock exchange with that from a generalized gradient approximation (GGA) exchange functional. 26 B3LYP functional has previously been shown to provide an accurate description of the electronic structure and magnetic properties for both inorganic and organic compounds. 11, 12, 24, [30] [31] [32] [33] [34] Time-dependent density function theory (TDDFT) was used here to calculate the excitation energies of the single molecule and dimers (extracted from the β-and η-phase F 16 CuPc) to compare with the experimental spectra.
35, 36
Results and discussion Fig. 1 shows the morphology and transition of F 16 CuPc crystalline material. F 16 CuPc nanowires were obtained through OVPD process using ribbon-like powders. The average width of blue F 16 CuPc nanowires is ~60 nm and the length of the wires is approximately a few microns. Moreover, large size of purple ribbon-like F 16 CuPc crystal with the length of ~2 cm was also fabricated in the high-temperature zone, as shown in Fig. S1 (ESI †), which is similar to those in the previous studies. 19, 23 Morphology transition from nanowires to nanoribbons occurred when increasing the annealing temperature, as shown in Fig. 1 (c)-(f). When the annealing temperature is below 200 °C, the cluster is nanowires, whereas nanoribbon-Please do not adjust margins
Morphology and Chemical composition analysis
Please do not adjust margins like crystals can be found at 300 °C and became dominant as the temperature rises to 400 °C. This clearly indicates that F 16 CuPc crystals include two different morphologies, nanowires and nanoribbon, corresponding to the η-and β-phases; their crystal structures will be further discussed below. By systematically regulating experimental parameters, including heating temperature, the flow rate of carrier gas and especially the ambient and annealing temperatures the fabrication of F 16 CuPc nano/micron wires and ribbons can be controlled.
23, 37 16 CuPc nanowires, these identified peaks reinforce the effectiveness of our EDS analysis. For the high-resolution XPS spectra of Cu 2p ( Fig. 2(b) ), two danielle29@example.com eV and 955.5 eV are due to Cu 2p 3/2 and 2p 1/2 , respectively. The satellite peak appears at higher binding energy ∼944 eV, indicating the existence of Cu 2+ ions in F 16 CuPc molecule. 38, 39 By separating the core level of C 1s peak (see Fig. 2(c) ), four peaks can be drawn danielle29@example.com eV (P1), 285.7 eV (P2), 287.11 eV (P3) and 288.1 eV (P4), which indicated four different bonds of carbon atoms in F 16 CuPc molecule. The strongest peak, P1, can be attributed to the C-C bond in the benzene rings. P2 is assigned to the C-C bond of aromatic carbon atoms in the organic ring, 40 while P3 is most likely related to C-N bond. 41 P4 is due to the C-F bond. 42 Two danielle29@example.com eV and 399.91 eV, for the high-resolution spectra of N 1s in Fig. 2(d) , correspond to the bonds of the four inner most nitrogen atoms with the central copper ion and of outer most nitrogen with carbon atoms. 38, 43 Moreover, the FTIR spectra of F 16 CuPc nanowires, ribbons and powders are shown in Fig. 2(e) . The peaks at 1272 cm-1 and 1320 cm-1 correspond to the C=N-C at bridge sites while the peak at 1151 cm -1 can be assigned to Cu-N bonding. 44, 45 It should be noted that the peak shifts at 962 cm -1 , 1151 cm -1 , 1272 cm -1 , 1525 cm -1 are significantly affected by the structural change.
The characteristic peaks of as-prepared F 16 CuPc ribbons and nanowires are consistent with those of F 16 CuPc powders. Thus, by analyzing the XPS and FTIR spectra, these results manifest that the structures of individual molecules in F 16 CuPc ribbons and nanowires remain almost the same as that of the F 16 CuPc powders after high-temperature vapor deposition process. 
Crystal structure analysis
The XRD pattern of F 16 CuPc nanowires is shown in Fig. 3(a) . The corresponding parameters for the unit cell are listed in Table 1 Table S1 gives the comparison between the fitted and calculated diffraction peaks and the interplanar distances d, as well as the corresponding lattice planes for F 16 CuPc nanowires. The lattice parameters of F 16 CuPc nanowires reveal that the F 16 CuPc nanowires should possess a herringbone-type packing structure, as shown in Fig. 3(b) and (c). The shortest lattice constant in the F 16 CuPc nanowires, b = 3.758 Å, which closely matches that of the α-CuPc (b = 3.769 Å) 46 and the η-CuPc (b = 3.77 Å) 13 and Cl 16 CuPc (b = 3.76 Å).
47
The inter-plane distance between the adjacent molecules within one column is ~3.25 Å. 21, 23 Furthermore, the decrease on the lattice constant b indicated the increase of the stacking angle (ω = 60 o ) between the b axis and the molecular plane, comparing with the previous studies, 21, 23 which could result in a lager π-π overlap between the adjacent molecules within the chain. The b-axis in these F 16 CuPc nanowires was along the nanowires. 16 As a consequence, higher charge mobility in the nanowires is expected. 48, 49 In addition, the effects of precursor temperatures on the morphology and crystal structures of F 16 CuPc nanowires are further studied as shown in Fig. S4 (ESI †). The morphology of F 16 CuPc remains as flexible nanowire with precursor temperature ranging from 450 °C to 500 °C. But it is clear that the size of F 16 CuPc nanowires enlarges with increasing precursor temperature. The overall XRD pattern for each F 16 CuPc nanowire in Fig. S4 (e) remains the same as well, which implies they have identical crystal form. Moreover, as shown in Fig. S4(e) , when the precursor temperature increases, the three distinguishable peaks can be found and become sharper, suggesting that crystallinity of F 16 CuPc nanowires is significantly enhanced. According to the experimental results, the crystal structure of F 16 CuPc is most significantly influenced by the deposition area temperature. Thus, the possible formation mechanism of η-F 16 CuPc can be described as follows: F 16 CuPc molecules come out at the vaporization temperature and then flow downstream to crystallize into small nuclei acting as seeds for the growth of the η-F 16 CuPc and the β-F 16 CuPc in the hightemperature and low-temperature zone, respectively. When increasing the heating temperature for the powders and raising the concentration of the F 16 CuPc molecules, these nucleuses grow into nanowire and ribbon structure and the fast formation process of the η-F16CuPc nanowires and the β-F 16 CuPc ribbons happens thanks to the van der Waals forces between molecules.
Therefore, from these details, we can infer that F 16 CuPc nanowires with new crystal structure may exhibit enhanced electro-optical properties, thus leading to a high performance in the application of electronic devices such as organic photodetectors. 
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Structural transitions of F 16 CuPc nanowires
In order to further understand the structural transition dynamics of F 16 CuPc nanowires, the as-grown F 16 CuPc nanowires with annealing treatment are investigated by XRD and differential scanning calorimetry (DSC) analysis.
From the XRD patterns in Fig. 4(a) , we can clearly see three phase-transition steps in the annealing treatment process. When annealing below 70 °C, the F 16 (134), which can also be observed in the ribbon-like crystals, as shown in Fig. 1(c) and 1(d) . To illustrate the subtle change of phase transitions, DSC was employed for F 16 CuPc nanowires in Fig. 4(b) . A clear exotherm peak at 70 °C and an endothermic peak at 200 °C might be due to the variation of molecular orientations in the η-phase, in agreement with the two feature diffraction peaks at low-and high-temperature regions as revealed in Fig. S5 (ESI †). The rapid endothermic process at the temperature of 280 °C indicated the initial stage of the phase transition from the η-to the β-phase. An endothermic peak at ~498 °C can be ascribed to the phase transition into the β-phase, as shown in Fig. S6 (ESI †) . The XRD pattern of the remaining sample for F 16 CuPc nanowires after DSC measurements up to 500 °C is shown in Fig. S7 (ESI †) . This reinforces our analysis of the β-phase. Additional, from the thermogravimetric curve in Fig. S6 (ESI †), we can see that the F 16 CuPc crystal started to vaporize at 582 °C.
The above has systematically discussed the crystal structure of the η-F 16 CuPc nanowires and the transition process from the η to β-phase of F 16 CuPc. The η-F 16 CuPc nanowires should be a metastable stable, as can be proved from the DSC curves. The key factor for the formation of η to β-phase of F 16 CuPc is the growth temperatures. The β-phase is formed at a high temperature, while the η-phase can be formed as a metastable state with the extra energy as vapours phase rapidly crystallized into crystals at room temperature. 3 On the other hand, when sufficient energy is provided to the system, such as by annealing, the metastable η-phase can overcome the activation barrier and then transit to the stable β-phase.
3, 50

Time-Dependent Density-functional Theory Calculations
Our TDDFT calculations for the F 16 CuPc single molecule and the dimers (x-phase) of the η-and all the β-phases have been shown in Fig. 5(b) , along with the Uv-Vis optical absorption spectra for the η-phase. 9 In our TDDFT calculations for dimers, the Cu-Cu distances are set be the related lattice constants presented in Table. 1, and the inter-plane distance is set to be the experimental value, i.e., 3.25 Å. We have first performed a DFT calculation to compute the ground state, based on which a TDDFT calculation has been carried out to understand the experimental optical spectra. For the calculation of the single molecule (dimer), we have computed the first 100 (150) excited states. We then present all the normalized non-zero oscillator strength as a function of the excitation energies in Fig. 5(b) along with the experimental results. The computed lowest optically accessible state for an F 16 CuPc molecule is at λ = 600 nm, in a good agreement with the experimental results for the Q-band that arises from the transition between the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO). This is similar to those mentioned in our previous work. 24, 30 As compared with the calculation of an F 16 CuPc single molecule, those of the dimers show a widespreading optical spectrum from λ = 370 nm to λ = 760 nm. This arises from intra-molecular exciton and inter-molecular electron hopping, leading to a mixture between intra-molecule and charge-resonance excited states. By analyzing the transitions of these excited states, we found the peaks at λ=600 nm are dominated by the intra-molecular excitons (electron promotion happens within the same molecule), while the peaks at λ= ~750 nm can be predominantly attributed to the charge-resonance state (electrons are promoted to the adjacent molecule). 51 This charge-resonance state might be useful for enhancing optoelectronic devices by facilitating the charge movement via charge transfer.
Absorption spectrum of F 16 CuPc nanowires is shown in Fig. 5(b) . It is clear that the F 16 CuPc nanowires cover an even wider spectral range than that of CuPc, from the visible region with two peaks at ∼620 nm and ∼784 nm to the near-infrared light region (NIR, 900 nm). 52, 53 The blue shift in the Q-band, confirmed by our first-principles TDDFT calculations, might be understood as the effect of the interaction between excitons of the adjacent molecules within one column. From our calculations, we can see that the η-phase covers a slightly wider spectrum than the β-phases. Our TDDFT calculations are partially in agreement with the experimental results presented here. The remaining discrepancy can be identified between the wavelengths of 700 nm and 800 nm, where the experimental intensity is comparable to that at 600 nm, however, our TDDFT calculations show that the former is much weaker. This could be due to (i) the limited size of molecular cluster (which is only an approximation for a molecular chain) that could hinder lowering the energies, (ii) the quality of the chosen Gaussian basis set in our TDDFT calculations. 
Photodetector devices based on the η-F 16 CuPc nanowires
To explore the potential of the new form of F 16 CuPc nanowires crystal for device application, photodetector devices were investigated based on a facile approach, as shown in the schematic illustration in Fig. 5(a) . The photograph of photodetector and its SEM image are shown in Fig. S8 (ESI †) . Notice that the width of two Au electrodes is ~150 μm, which is wider than usual; this might cause the degradation of the photodetector devices, thus deteriorating the device performance. 54, 55 Fig . 5(c) shows the current-voltage (I-V) curves in the dark and under illumination of an AM 1.5G solar simulators (100 mW cm−2). NoƟce that the photocurrent increased drasƟcally under illumination. Meanwhile, the linear behavior of the curves suggests the ohmic contacts of Au electrodes with F 16 CuPc nanowires. The photocurrent responses of photodetectors were studied under illumination of an AM 1.5G solar simulators (100 mW cm−2) at a number of bias voltages: 0.5, 1, 2, 3, 5 and 10 V (Fig. 5(d) ). With the light on, the photocurrent of the device (~3 nA) rose by almost two orders of magnitude comparing with that in the dark (~0.01 nA) at a bias voltage of 10 V, which is much higher than photodetectors based on F 16 CuPc ribbons with I light /I dark ratio of 3 measured under similar conditions, in which the Cu-Cu distance is 4.796 Å. 23 Moreover, the photodetector based on η-F 16 CuPc nanowires exhibited a fast response to the pulsed incident light as well as an excellent stability and reproducibility with a rise and fall time of 448 ms. Several factors may result in the better performance. On one hand, the lager π-π overlap between the adjacent molecules within the chain may lead to high charge transport. Meanwhile, the η-F16CuPc nanowires server as charge transport channel. On the other hand, the interdigitated electrodes used in our work may be helpful for amplifying the output current and improve device performance in some degree. 55 In addition, the η-F16CuPc nanowires with high large surface-to-volume ratio might be another factor leading to better photon absorption to light. However, F16CuPc ribbon based phototransistor shows a rise/fall time of near 10 ms and 20 ms without the gate voltage and a maximum on/off ratio of 4.5 × 10 4 at gate voltage of -6 V, suggesting that the device structure will significantly influence the device performance. 56 Furthermore, there is still distinct photo-response for the device even at low-bias voltages of 0.5 V and 1 V.
To further investigate the properties of the photodetectors, the performance of the detector was studied under various irradiation wavelength from visible-light (589 nm) to near-infrared (940 nm). As can be seen in the I-V curves (Fig. S9, ESI †) , the photocurrent increased with the rising light intensity and bias voltage. The photocurrents may be influenced jointly by the different absorption coefficients of various wavelengths and by the different number of electronhole pairs excited under light illumination on the effective area of the device. The photoresponsivity (R) was calculated by adopting the following formula:
where R is the ratio of photocurrent to the incident light intensity; I light and I dark are the electrical currents with and without light, respectively; P 0 is the irradiance light intensity, and S is the effective illumination area.
The external quantum efficiency (EQE) was calculated as 57, 58 :
Please do not adjust margins Please do not adjust margins where h the Planck constant, c the velocity of light, e the elemental electron charge, and λ the wavelength of incident light.
The responsivities curves of the photodetector in Fig. 6 (a) reveal the strong dependence on the bias voltage, light intensity and irradiation wavelength. The responsivities clearly indicated incremental tendency when increasing the applied voltage. Meanwhile, the responsivity decreases as increasing the light intensity, intimating the sensitivity of photodetector to weak light. Fig. 6(b) and Fig. S10(a) show the values of R and EQE with the incident light density ranging from 0.01 to 1 mW cm -2 with the wavelengths of 589 nm, 628 nm, 850 nm and 940 nm, under a fixed bias of 10 V. R and EQE were found to increase with the irradiation wavelength of 589 nm, 628 nm and 850 nm, as the irradiance was lowered. The R and EQE for 940 nm are low, which may relate to the weak absorption as shown in Fig. 5(b) . Though the detector exhibited a low responsivity probably due to the device structure, its response was fast for all the wavelengths mentioned above with a rise and fall time of 448 ms, as shown in Thus, the high performance achieved in the η-F 16 CuPc nanowires-based photodetector than that of F 16 CuPc ribbon may be attributed to the smaller Cu-Cu distance. Given the material limitation such as the device structures and width of two Au electrodes, there is much room for optimization and improvement for the device performance. 
Conclusions
We report the new polymorph of F 16 CuPc nanowires fabricated by organic vapor phase deposition. The crystal structure of F 16 CuPc nanowires was determined, having a monoclinic unit cell and a herringbone stacking with a nearest inter-Cu distance (3.758 Å), comparing with previous studies on F 16 CuPc ribbons. The η-F 16 CuPc one-dimensional chain with the experimentally observed geometry in this work was predicted previously to possess a stronger exchange interaction. 59 Moreover, morphology and phase transitions were observed during the annealing treatment of F 16 CuPc crystals. The η-phase of F 16 CuPc is nanowire-like crystal, while β-phase is ribbon-like crystals. The phase transition from the η-to β-phase mainly happens between 300 °C and 500 °C. The TDDFT calculations were partially in agreement with UV-Vis experimental results, probably owing to the approximation made (by using molecular dimer rather chain). Photodetector devices based on the η-F 16 CuPc nanowires were investigated in the dark and under light illumination. Photodetector devices show fast response even at low bias voltage of 0.5 and 1V under a solar simulator with a power of 100 mW cm −2 . The photocurrent of detector largely depends on the bias voltage, illumination intensity, and irradiation wavelength. The response speed of photodetector devices is fast with a rise/fall time of 448 ms under visible light illumination at 589 nm and 628 nm and NIR light illumination at 850 nm and 940 nm. Herein, we provide a new type of organic nanowires formed by F 16 CuPc with large spatial π-π overlaps, which could potentially enhance electro-optical properties and electron transport, thus bringing up great interest in the application of F 16 CuPc nanowires to electronic devices.

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