Groove-assisted solution growth of lead bromide perovskite aligned nanowires: a simple method towards photoluminescent materials with guiding light properties

Isabelle Rodriguez *, Roberto Fenollosa , Fernando Ramiro-Manzano , Rocío García-Aboal , Pedro Atienzar and Francisco J. Meseguer
Instituto de Tecnología Química (CSIC-UPV), Universitat Politècnica de València, Av. Los Naranjos s/n, 46022 Valencia, Spain. E-mail:

Received 2nd April 2019 , Accepted 17th June 2019

First published on 19th June 2019

High refractive index nanowires are very attractive because of their waveguiding properties and their multiple applications. In this sense, metal halide perovskites, an emerging and appealing optoelectronic material, have also been tailored into nanowire structures. Here, we present an easy, low-cost and versatile method that has made possible to achieve nanowires of controlled and uniform width. The method has been applied here to all-inorganic and hybrid lead bromide perovskite (CsPbBr3 and CH3NH3PbBr3 respectively) materials. The procedure is based on the spin coating of precursor solutions, at room temperature, on a PDMS replica of the periodic grooves and lands of commercially available Compact Disc (CD) or Digital Versatile Disc (DVD) polycarbonate plates. The method can be applied for the synthesis of other material nanowires before being transferred onto other substrates. The obtained CsPbBr3 and CH3NH3PbBr3 nanowires exhibit high photoluminescence and guiding light properties along the material.

1. Introduction

Nanowire-like nanostructured semiconductors have raised great interest since they were discovered, two decades ago, as they exhibit unique electrical, and optical properties, making them suitable for applications in optoelectronics, photonic devices or energy generation.1,2 In particular, high refractive index nanowires can be used for confining and guiding light at the nano- and micro-scale. That is the reason why they have become elements of paramount importance in photonics, where a technological revolution is expected to happen, similarly to microelectronics in the past century. So far, different base materials have been used to develop nanowires, namely II–VI (ZnO), III–V (GaAs, InP, InSb, GaN, etc.),2 and IV (Si, Ge)3–5 semiconductors, and more recently metal halide perovskites could be shaped into nanowire structures.6,7 That was a very important achievement because the metal halide perovskites (AMX3) have emerged as an appealing optoelectronic material,8 with a high photoluminescence efficiency,9–13 high optical absorption,14 and low charge carrier recombination coupled with high hole and electron mobility15 that make them very suitable for many optoelectronic applications16 such as in solar cell devices,17–19 light emitting diodes16,20–22 and optically pumped lasers.16,21,23–25 Moreover, they can be easily synthesized by low-temperature solution processing and the band gap can be tuned through halide substitution or halide mixtures26 and also by the divalent metal cation replacement.27 However, in any case, and particularly for the perovskite materials, the shape of the nanowires somehow follows the crystalline directions of growth, thus producing nanowires of a given dimension with triangular, or square-like shaped cross-sections, far from the cylindrical shape of conventional optical fibers. These geometrical parameters play a key role because they determine how light propagates through the nanowire: the losses, and the interaction of light with the nanowire itself and with the surrounding material. Therefore, in spite of the remarkable results obtained so far through chemical vapor deposition28–37 or via solution phase synthesis,23,38–51 it is very important to explore new routes that allow tuning of the shape and dimensions of 1D perovskite structures. In this way, recently MAPbX3 nanowire arrays have been grown using PDMS rectangular groove templates obtained from the replication of silicon masters previously prepared by photolithography.52 Herein, we propose, a simple and low-cost method to obtain metal halide perovskite nanowires based on the confinement of precursor solutions in submicrometer size grooves imprinted on the polycarbonate (PC) sheet of commercial recordable DVD or CD plates. Such method of preparation makes it possible to not only obtain nanowires with homogeneous width and shape but also choose between several width dimensions depending on the employed template. Indeed, as described below, DVD and CD structures can provide plates of different groove size and distribution.

2. Results and discussion

Recordable DVDs are made of two PC plates patterned with a spiral distribution of grooves and lands, the top one presenting the complementary geometry of the bottom one (see Fig. S1, ESI). Due to the incompatibility of PC with most of the employed solvents for the synthesis of perovskites, here, this kind of support cannot be used directly as a template. Therefore, patterned polydimethylsiloxane (PDMS) substrates were prepared via a molding procedure of the land and groove array of the DVD or CD polycarbonate masters. Fig. 1 shows an illustrative general scheme of the preparation and the features of the PDMS templates. The procedure to achieve the corrugated PC substrates is described in the experimental section. The PDMS replica was prepared by pouring the pre-polymer liquid (Sylgard 184 Silicone Elastomer Kit) over the PC plates and then letting it cure at 60 °C. After mechanically peeling it off from the master, the negative PDMS mold of the CD or DVD was obtained (Fig. S2, ESI). In order to enhance the wettability of the PDMS substrate surface, O2 plasma treatment was used before the next step. Then, precursor solutions of the metal halide perovskite were drop-casted and spin-coated on the patterned PDMS substrates previously hydrophilized. With an adequate spin speed and time, upon solvent evaporation, nanowires are formed in the grooves of the template. Fig. 2 summarizes the different stages of the procedure. Hence, it can be seen that it is a quite low cost and a versatile method that could be used for the preparation of nanowires of any other material from precursor solutions. We have applied this method for synthesizing both hybrid and all-inorganic lead halide perovskite nanowires. The first one, with the general structure MAPbX3 (MA = methylammonium, X = Cl, Br, I) has been largely studied and their efficiency has been proven.18,53 Although some halide derivatives seem to show higher stability, a drawback of the organic–inorganic halide perovskites is both, their sensitivity to moisture and O254,55 as well as their thermal instability.56,57 These disadvantages could be mitigated by using all-inorganic lead halide perovskites. In this sense, cesium-based lead halide perovskites seem to combine performance and stability58–60 and moreover they tend to show a high defect tolerance.61 Here, the CsPbBr3 perovskite nanowires were synthesized starting from a stoichiometric mixture of dissolution of precursors, CsBr and PbBr2, in dimethyl sulfoxide (DMSO).
image file: c9qm00210c-f1.tif
Fig. 1 Schematic illustration of the preparation and features of the PDMS template.

image file: c9qm00210c-f2.tif
Fig. 2 Preparation procedure of the MAPbBr3 and CsPbBr3 nanowires in the grooves of hydrophilized PDMS templates obtained by replication of the CD or DVD profiles.

Concerning methylammonium lead tribromide (MAPbBr3) nanowires, dissolutions of CH3NH2Br and PbBr2 in DMF were employed. A drop of the precursors solution was deposited onto a patterned PDMS stamp (1 cm2), previously hydrophilized by O2 plasma treatment. Then the stamp was spin-coated until solvent evaporation and formation of an orange/yellowish material occurred. Finally, the samples were kept at 50 °C overnight for the complete removal of any remaining solvent. For the sake of clarity, only results concerning the CsPbBr3 material will be shown here (for MaPbBr3 nanowires see ESI). As can be seen in the optical and electronic microscopy images in Fig. 3, the growth of the CsPbBr3 nanowires takes place in the well-aligned periodic grooves of the PDMS substrates, which correspond to the DVD and CD replica patterns. The precursor solutions fill the grooves of the templates resulting in shape-controlled nanowires upon solvent evaporation. Therefore, the widths are uniform, with values, in the examples shown in Fig. 3, of 500 nm and 930 nm for the DVD-like and CD-like PDMS templates, respectively. Nanowires of several dozen microns in length could be achieved. The cross-section area of the obtained nanowires is uniform and it is determined by the size and shape of the patterned grooves in the PDMS template. Fig. 3d and e show the Field-Emission Scanning Electronic Microscopy (FESEM) images of a transversal cut of a CsPbBr3 nanowire sample realized by FIB (focused ion beam) milling after sputtered Au or/and Pt coatings to reduce the sample charging and beam damage. It can be seen that the material has been conformed to the groove of the template by the spin process to form polycrystalline CsPbBr3 nanowires. A similar behavior is observed for the MAPbBr3 material in Fig. S3 (ESI). Therefore, the nanowire features could be tuned by changing the template (and the master) characteristics as well as the precursor solution employed.

image file: c9qm00210c-f3.tif
Fig. 3 Polycrystalline CsPbBr3 nanowire characteristics. The optical images of the CsPbBr3 perovskite nanowires obtained on (a) DVD and (b) CD replica PDMS substrates. (c) The XRD pattern of the CsPbBr3 nanowires and of a CsPbBr3 film structure. (d) The FESEM image of a lateral view of a sample of CsPbBr3 nanowires (obtained on the PDMS replica of the DVD structure) cut by FIB milling perpendicularly to the surface and coated with sputtered gold. (e) The FESEM image of the cross-section of a CsPbBr3 nanowire grown on the PDMS replica of the CD structure (sputtered Pt coating was used to cover the sample before the FIB milling).

The X-ray diffraction (XRD) pattern of the CsPbBr3 nanowires (Fig. 3c) shows 2 strong diffraction peaks with maxima at 2 theta angles of 15.46° and 31° that match respectively to the (110) and (220) lattice planes of an orthorhombic perovskite structure obtained at room temperature.38

In order to confirm the feasibility of the synthesized material to yield photoluminescence (PL) and see if the emitted signal could be guided along the 1D structures, we performed confocal PL spectroscopy on single nanowires and varied the distance between the collection and the excitation points. Specifically, the collection point was fixed at one end of the nanowire so as to acquire the scattered light at that position, while the excitation point was shifted along it. Collection and excitation could be accomplished by using 20 × 0.4 NA objectives in both cases, mounted in the forward configuration (Fig. S4 (ESI)). They allow focusing and collection of light in areas of about 1 or 2 micrometers in diameter. As the excitation source, we used the light of a 405 nm laser, with varying powers around 0.1 mW.

Fig. 4a–d show optical microscopy images, recorded by the camera of the set up of Fig. S4 (ESI), of two neighboring isolated CsPbBr3 nanowires supported on a PDMS replica of a DVD profile substrate (Fig. 3a). For the sake of simplifying the experimental procedure, we chose for the photoluminescence (PL) measurements the shortest one, which is about 12 μm in length (see the color optical image at the top right of Fig. 4). The images correspond to different collection–excitation distances (dE–C), which have been indicated under each picture. The white spots on the nanowire correspond to the PL signals produced by the excitation laser. The intensities between the images are not comparable due to both camera autofocus and laser intensity adjustment. The bottom end of the nanowire corresponds to the collection point. Therefore, at dE–C = 0 μm (Fig. 4a), only one spot appears at that point. The other images, corresponding to the dE–C values different from zero (Fig. 4b–d) show two spots, one coming from the direct excitation of the laser upwards and another one, which is less intense, at the collection point. We associated the last signal to that light which is emitted at the excitation point where the laser is focused and travels towards the end of the nanowire, where it is scattered in all directions. Of course, the light should travel to the other end of the nanowire as well, but it could not be always recorded by the camera, particularly for long distances from the excitation point. Fig. 4 shows as well the PL spectral evolution (blue, green, black and cyan curves) as a function of the collection–excitation distance (dE–C).

image file: c9qm00210c-f4.tif
Fig. 4 (a–d) Optical images recorded by the camera of the set up described in Fig. S4 (ESI) of the two neighboring isolated CsPbBr3 perovskite nanowires obtained in a groove of a DVD-based PDMS substrate. The chosen nanowire for the PL experiments is indicated by a black arrow in the optical microscopy image at the top right of the figure (scale bar: 10 μm). The white spots on the images (a–d) correspond to the PL signals produced by the excitation laser for several distances, dE–C, specified under each image, between excitation and collection (bottom end of the nanowire) points. The intensities between the images are not comparable because they were recorded with different sensitivity conditions of the camera. Down left of the figure: the solid color curves correspond to the PL spectra measured for each dE–C case (same color and label as camera image frames), and the dashed red curves correspond to fits according to a theoretical model described in ref. 65. As an example, the plot at the right side shows that the fitted spectrum “c” results from the overlap of two peaks, P0 and P1 (the blue and red dashed curves), corresponding respectively to the direct emission of the nanowire and the guided light emission from excitation to the collection area.

In general, the intensity of the light at the collection point decreases as the dE–C increases as previously reported in other studies.31,62–64 Therefore, we adjusted the integration time of the detector and the power of the excitation laser so as to achieve a reliable detection and to obtain a qualitative optical behavior of the material. For this reason and for the sake of comparison, we have normalized all the spectra. At dE–C = 0 (curve ‘a’, blue line), a Gaussian-like peak with some asymmetry appears with its maximum centered at about 513 nm. As the excitation beam moves away from the collection point, this shape changes substantially. The spectra broaden and new features appear at longer wavelengths. We decomposed each spectrum into two peaks and associated them to two distinct effects by means of a fitting procedure (red dashed curves). Firstly, the spectrum ‘a’ represents the direct-emission of a small portion of the material. The fit of this peak was achieved by using the model described in ref. 65, and assuming a thickness of 106 nm that corresponds to the nanowire height. This permits the bulk extinction coefficient of the synthesized material to be obtained. We call this peak P0. Secondly, the other spectra (b–d) include the incoherent addition of P0 with some attenuation coefficient and another peak, P1 from now on, centered at longer wavelengths. We hypothesize that P1 arises from the guiding phenomenon of light from the excitation to the collection spot. This peak is red-shifted in comparison to P0 because the absorption coefficient is higher for shorter wavelengths within the PL emission of the material. P1 was fitted to the model mentioned above taking into account the dE–C value for each case. The obtained extinction coefficient from this fit takes, however, a lower effective value than that of the bulk one, regardless of the dE–C and wavelength. We attributed this effect to the guiding mode field profile that probably forces the light to travel partially outside the nanowire core. It can be seen in Fig. 4 how the fitted spectrum “c” for example can be decomposed into two spectra, P0 and P1.

Finally, we would like to discuss the presence of P0 at dE–C's different from zero. Pazos-Outón et al.63 argued in a similar experiment that this kind of effect comes from a photon absorption–reemission phenomenon. However, we think that because the acquired signal is very weak, other effects such as a spurious acquisition of unguided PL from the unwanted reflections or spurious excitation by the tail of the laser spot should not be disregarded. In any case, P0 almost disappears for dE–C > 12 μm. The wider CsPbBr3 nanowires (Fig. 3b and e), obtained by using the CD replica as a template (Fig. 1, below right) yielded similar results to those of their narrower counterparts (Fig. S5, ESI). However, the peak P0, that corresponds to the PL at dE–C = 0 is centered in this case at about a slightly longer wavelength, λ = 525 nm, in spite of being the same material.

In fact, the origin of this redshift of the P0 peak comes from a higher thickness of the obtained nanowires and it is in accordance with previously published results.34 Indeed, from the fit of the spectrum, the thickness of the CsPbBr3 nanowires obtained on the PDMS replica of the CD substrate is found to be 110 nm. In the same way, photoluminescence of the CsPbBr3 and MAPbBr3 film samples is also redshifted in comparison with the nanowires of about 100 nm thickness (see Fig. S6, ESI).

Similar optical studies have been realized on the MAPbBr3 perovskite nanowires and can be seen in the supporting information file (Fig. S7 (ESI)).

Preliminarily tests to transfer the nanowires onto other substrates have been performed. Fig. S8 (ESI) shows an optical image of the MAPbBr3 nanowires on the ITO glass substrate.

3. Experimental section

DVD and CD polycarbonate substrates

Recordable DVDs are composed of two plates of polycarbonate that were mechanically separated. The bottom one PC sheet is patterned with a spiral periodic distribution of grooves (see Fig S1 of the ESI) coated with a photosensitive dye (where the data can be recorded by a laser beam) and a metallic film. The later one was peeled off by means of an adhesive tape. Then a mixture of water:ethanol (1[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) was employed to remove the dye coating as described in a previous work.66 The top plate of the DVD presents the complementary profile of the bottom one, providing, therefore, a polycarbonate template with wider grooves with less separation between them.

The CD structure is only composed of one plate of polycarbonate patterned with grooves periodically separated (Fig. S2, ESI) also coated with a dye and a reflective layer that was removed through a similar method to the one used for the DVD and described above.

PDMS replica molding

Sylgard 184 elastomer kit prepolymer was mixed with the curing agent (10[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio) and degassed under vacuum until all air bubbles were removed. The PDMS mixture was then poured on top of the CD or DVD polycarbonate substrates placed in a glass Petri dish. After a curing process of 2 h at 60 °C, the PDMS layer was mechanically peeled off from the PC plate.

CsPbBr3 and MAPbBr3 precursor solution preparation

CsBr, MABr, PbBr2, and the solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO) were obtained from Sigma-Aldrich. The CsPbBr3 precursor solution was prepared by dissolving a stoichiometric mixture of 0.45 M CsBr and 0.45 M PbBr2 in DMSO prepared at 50 °C. For the MAPbBr3 precursor solution, 1 M MABr and 1 M PbBr2 in DMF were employed and mixed in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio.67

Spin-coating experiments

They were realized on an Ossila spin coater system. Speeds of 2000 rpm (rotation per minute) and of 3000 rpm were used for MAPbBr3 and CsPbBr3, respectively.

Optical and electronic microscopy

Optical images were taken by a Nikon Eclipse LV100 microscope. The field emission scanning electron microscopy (FESEM) images were obtained on a Carl Zeiss Ultra 55 instrument and the Focused Ion Beam (FIB) milling experiments were performed on a Carl Zeiss AURIGA compact FIB-FESEM workstation.

X-ray diffraction

The X-ray diffraction patterns were recorded on a Bruker D8 Advance A25 X-ray diffractometer operating at 45 kV and 80 mA Cu Kα radiation (λ = 1.5406 Å) equipped with a LYNXEYE XE 1-D detector.

Photoluminescence measurements

The optical set-up scheme is represented in Fig. S4 of the ESI. A more detailed description of the components of the home built set-up can be found in ref. 68.

4. Conclusions

In conclusion, here we have shown a simple solution-based and low-cost method of fabrication of nanowires with controlled and uniform size at room temperature. The process includes a PDMS replica of DVD and CD profiles as substrates and it produces an array of aligned nanowires with a defined width. We have applied this method to obtain all-inorganic lead halide CsPbBr3 and hybrid MAPbBr3 perovskite nanowires starting from precursor solutions. However, this procedure can be employed to achieve other material nanowires that can also be transferred to other suitable substrates.

The optical studies of all inorganic and hybrid lead perovskite nanowires show the typical PL signal for these materials. Moreover, it can be excited at any point of the nanowire and guided along with it towards its ends. Transport of light has been observed along the material for more than 12 μm. Experimental results are supported by theoretical simulation.

Conflicts of interest

There are no conflicts to declare.


The authors would like to gratefully acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MIMECO) (Severo Ochoa (SEV-2016-0683), MAT2015-69669-P projects) and Generalitat Valenciana (Prometeo II/2017/026 Excellency project). P. A. acknowledges the Fundación Ramón Areces (XVII Concurso Nacional para la adjudicación de Ayudas a la Investigación en Ciencias de la Vida y de la Materia) for its funding. F. R.-M. thanks the financial contribution of the Spanish Ministry of Economy and Competitiveness (MIMECO) through the program for young researchers support (TEC 2015 2015-74405-JIN). Finally, IR also thanks the Electron Microscopy Service of the Universitat Politècnica de València for their support in FESEM image acquisition and FIB milling, as well as Ana Moreno for her help in template preparation.

Notes and references

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Electronic supplementary information (ESI) available: Fig. S1: FESEM images of DVD substrates and PDMS replica; Fig. S2: FESEM images of CD substrates and PDMS replica; Fig. S3: MAPbBr3 nanowires characterization: optical and FESEM images (top and cross-section view); Fig. S4: experimental optical set-up; Fig. S5: CsPbBr3 nanowires grown on PDMS CD-like substrate. PL evolution; Fig. S6: PL of CsPbBr3 and MAPbBr3 films; Fig. S7: optical properties of MAPbBr3 nanowires; Fig. S8: optical image of MAPbBr3 nanowires transferred onto ITO glass substrate. See DOI: 10.1039/c9qm00210c

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