Positioning and joining of organic single-crystalline wires

Organic single-crystal, one-dimensional materials can effectively carry charges and/or excitons due to their highly ordered molecule packing, minimized defects and eliminated grain boundaries. Controlling the alignment/position of organic single-crystal one-dimensional architectures would allow on-demand photon/electron transport, which is a prerequisite in waveguides and other optoelectronic applications. Here we report a guided physical vapour transport technique to control the growth, alignment and positioning of organic single-crystal wires with the guidance of pillar-structured substrates. Submicrometre-wide, hundreds of micrometres long, highly aligned, organic single-crystal wire arrays are generated. Furthermore, these organic single-crystal wires can be joined within controlled angles by varying the pillar geometries. Owing to the controllable growth of organic single-crystal one-dimensional architectures, we can present proof-of-principle demonstrations utilizing joined wires to allow optical waveguide through small radii of curvature (internal angles of ~90–120°). Our methodology may open a route to control the growth of organic single-crystal one-dimensional materials with potential applications in optoelectronics.


Stable BPEA wires grown through the GPVT process in air.
To evaluate the influence of atmosphere (especially oxygen) on the crystal growth, we performed the GPVT process in air at similar experimental conditions, followed by the characterizations of the stability and crystal structure.
Thermogravimetric analysis (TGA), infrared spectroscopy (IR) and ultraviolet (UV) absorption spectrum were performed to evaluate the stability of BPEA grown at the temperature of 200 °C in air. The TGA curve and the corresponding derivative thermogravimetry (DTG) analysis were carried out in air at a heating rate of 5 o C/min in Supplementary Fig. 7a. These results show that the BPEA start to degrade at about 350 o C, a temperature much higher than that during the GPVT process (200 °C). In other words, the BPEA molecules were stable without the degradation during the GPVT process. No obvious change of peak position and intensity in the UV absorption spectrum ( Supplementary Fig. 7b), indicating the aggregated structure of BPEA molecules are stable after heating at 200 o C in air. The results from IR spectroscopy also proved this point. Neither a significant band shift nor the appearance of any new band was observed in the IR spectra, which manifested that the molecular structure of BPEA has not been degraded after the heating process ( Supplementary Fig. 7c).
On the other side, SEM, TEM, SAED were carried out to investigate the effect of atmosphere on the morphology as well as crystallinity of as-prepared BPEA wires. The SEM and AFM images (Supplementary Fig. 8b-d) indicate that BPEA wires were uniformly aligned with smooth outer surface, which has no obvious difference compared to that grown in nitrogen (Fig.1). The TEM and SAED results (Supplementary Fig. 8e) reveal the single crystallinity of the as-prepared wires, which has the same crystal structure and growth orientation compared to that grown in nitrogen (Fig.1).
Based on those above results, it could be concluded that the BPEA molecules were stable and the wires exhibited single-crystalline structure under 200 o C through the GPVT process in air.

Calculation of transport loss in optical waveguide along organic straight wires
Under the irradiation of a focused laser (λ = 488 nm), the BPEA emitted intense broad-band PL. A part of the energy at the exciting spot was scattered into free space while the residual was confined and propagated along the wire. Supposing that the ratio of the light escaping from the excitation point and that of light propagating along the fiber is η, and the collection chance of total scattering by an objective is k, the confined PL intensity focused onto the wire (I ex ) can be expressed as: ( 1) where I in is PL intensity of the focused laser beam.
During the propagation of light, the energy loss is contributed by two fractions. The former one is the optical loss (α) along the increasing of propagation. The intensity of the tip emission decreases exponentially with the propagation distance. The latter one is caused by the broken points, which exist during the wire transfer or immature growth. These energy losses (I n ) could be represented directly by the intensity of the luminescence spots. Therefore, the propagation distance of the wires could be expressed as: Where n represents the number of broken points, I tip denotes the PL intensity at the final point.
The PL intensity is recorded from corresponding spectra. After a certain mathematic treatment, the equation would be simplified as: According to the equation (3), the transport loss in Figure 1h in manuscript can be calculated as 0.0131 ± 0.0004 dB/μm, which is a little higher than inorganic wire counterpart 3 yet smaller than that of nanoparticle linear assemblies 4 .

Restraining optical loss by 'hybrid plasmonics' approach
The loss in organic wires is due to the relatively low phase refraction index (n r ≈ 1.8) of BPEA nature 3 . Therefore, we introduce the 'hybrid plasmonics' approach to fabricate the plasmonic BPEA wires to eliminate this loss of propagation of the organic materials 5 . Firstly, the Ag (300 nm)/SiO 2 (5 nm) substrates were deposited. And then the BPEA wires were fabricated onto the Ag (300 nm)/ SiO 2 (5 nm) substrates. SiO 2 (5 nm) and Ag (100 nm) film were deposited on the surface of the BPEA wires. The configuration of SiO 2 and Ag encapsulated BPEA wires (BPEA@SiO 2 @Ag) is shown in Supplementary Fig. 17a. To characterize the surficial state and the structure of BPEA@SiO 2 @Ag, SEM observation and energy dispersive spectroscopy (EDS) linear scanning were performed. SEM image show a relatively smooth surface, which indicates that SiO 2 and Ag film are homogenously deposited ( Supplementary Fig. 17b). Further element distribution analysis by EDS linear scanning shows that Ag signal is stronger in margin of wires and C signal mainly exist in the area of core ( Supplementary Fig. 17c), which indicates that the BPEA wires are encapsulated by Ag film.
To evaluate the optical loss after the 'hybrid plasmonics' treatment, spatially resolved PL imaging and spectroscopy measurements were performed by locally exciting a single shown in Supplementary Fig. 17f, indicates that the intensity of the out-coupled light decays almost exponentially with the increase in propagation distance, which is a typical characteristic of active waveguides. The intensity at the excited site along the body of the wire (I body ) and at the emitting tip (I tip ) were recorded and the optical-loss coefficient (R) was calculated by single-exponential fitting I tip /I body = Aexp(-RD), where D is the distance between the excited site and the emitting tip.
Accordingly, R = 0.00121 dB µm -1 at 600 nm, which is much lower than the value for other fabricated pure organic optical waveguide. There are two prominent factors that contribute to the excellent optical waveguide behavior of the BPEA@SiO 2 @Ag wires. First, the smooth surface and high crystallinity minimized the optical loss caused by scattering. Second, the formation of surface plasmonics polarition (SPP) can largely reduces the optical loss during the propagation along the length direction of the wire.

Supplementary Note 4 Calculation of transport loss in waveguides along joined wire patterns
According to the description in Note 2 above, the transport loss along the straight wires can be calculated as: In order to calculate the optical loss caused by the joining of wires, I before and I after are defined as the PL intensity of the spots that appeared before and after the joining point of wires. In most case, the I before can be regarded as the PL intensity of the focused laser beam.
In this case, the equation (3) can be modified as: Where L total indicates the added lengths of wires that constitute the joining regions.