Versatile Bottom-Up Approach to Nanostructured Functional Materials for Optoelectronic Applications

A versatile strategy to load ordered mesoporous silica-based materials with functional molecular building blocks in order to obtain host-guest systems exhibiting specific functions is described. Optical microscope examination of the obtained material confirms that the addition of a chromophoric moiety to the reaction mixture is not detrimental in achieving the desired macroscopic morphology of the silica particles. A micro-photoluminescence investigation of the obtained material gave strong evidence that both surfactant micelles and the porous oxide matrix preserve the spectral features of the included molecular species in a nonconventional chemical environment.


INTRODUCTION
Since the discovery of conducting polymers, much effort has been put into the development of polymer-based optoelectronic devices (OEDs) [1][2][3].Organic polymers are ideal low-cost and high-versatility materials for the rapidly evolving markets of plastic-based information and energy technologies [4,5].Nevertheless, there are still problems associated with the application of organic materials in OEDs (e.g., aggregation-induced emission quenching, low quantum yield [6]).The idea described in this paper aims at overcoming some of these problems by developing photoresponsive organic/inorganic hybrid systems obtained by hierarchical organization of functional organic molecules in arrays of nanostructured host-guest compounds.Hybrid materials provide advantages which cannot be achieved with the single components.For example, the design of hybrid solar cells offers both, efficient light absorption of organic materials and efficient conversion of light to electric energy in inorganic materials [5,7].
The key idea underlying the present paper is the addition of functional molecular building blocks to the reaction mixture, in order to obtain a host-guest material exhibiting specific functions.This bottom-up approach will lead to many advantages: (i) the oxide matrix effectively shields the guest species form environmental moisture and chemical impurities (solvents, oxygen); (ii) the proper choice of the molecular components and host geometrical constraints produces a precise control of the supramolecular organization of the inserted molecules, thus tailoring the properties of the resulting system by preventing aggregation processes even at high concentration; (iii) the obtained nanostructured material should maintain the desired macroscopic morphology.The results that we will describe in this paper would open up the way to the preparation of active materials in various applications, such as optoelectronic devices and luminescent probes.
Accordingly to the literature [25], the length of CTACl alkane chains determines both micelle diameter and the pore size of the obtained material, which is of about 3.5 nm.Under quiescent condition, cylindrical micelles arrange themselves into gyroidal shapes containing concentric channels coiling around the main axis (see Figure 1(a)).
In a typical synthesis, 355 mg of CTACl were mixed in 15.52 mL of deionized water in a polypropylene bottle.The solution was kept under mild stirring at room temperature (RT) and 3.29 mL of HCl (37%W, Fluka Co., Buchs, Switzerland) was added dropwise as well as 0.29 mL of TEOS.2.06 mg of diphenylanthracene (DPA, ≥98.0%,Fluka Co., Buchs, Switzerland, see Figure 1(b)) was finally mixed to the mixture.After a few minutes of mild stirring, the polypropylene flask was closed and kept at RT for 3 hours.The obtained suspension was centrifugated 5000 rpm for 5 minutes, the supernatant removed, and the resulting white powder was extensively washed with 10 mL of deionized water and dried in air.The white powder was then placed between a microscope slide and the cover glass for optical

Optical data acquisition
Optical absorption and photoluminescence (PL) spectra on aqueous solutions were recorder using a Varian Cary 50 Scan UV-visible spectrophotometer (bandwidth 1 nm) and a Varian Cary Eclipse spectrofluorimeter (bandwidth 1 nm), respectively.
Reflected-light microimages were acquired using a doubled Nd:YAG laser (532 nm) as light source, coupled via single-mode optical fiber to an 80i Nikon confocal microscope, and a 100X oil-immersion objective (1.3 NA) to collect the light from sample.The signal is then amplified with a photomultiplier and a software-based scanning system reconstructs the overall image.Micro-PL spectra of silica particles were obtained with the same setup, using a filtered (3.26-3.76eV bandpass) high-pressure Hg lamp as excitation source and by coupling the microscope to a N 2 -cooled CCD camera through a 190 nm polychromator via a multimodal optical fiber.

RESULTS AND DISCUSSION
In order to test our approach, we test DPA as a suitable molecule for the inclusion in the silica matrix by means of spectroscopic investigation of its optical features in different environments.
In Figure 2, optical absorption spectrum of a DPA-water mixture (solid line) is reported, showing no significant bands or peaks in the range 3.0-4.0eV.
With DPA being water-insoluble, aggregates are probably formed and precipitation consequently takes place.Upon addition of surfactant (CTACl), a well-resolved vibronic progression becomes evident (see Figure 2, dashed line), with the 0-0 purely electronic transition centered at 3.15 eV.Both the shape and position of the spectrum are known in literature to be fingerprint of isolated molecule behaviour [26].PL spectrum of the DPA-water-CTACl ternary mixture is plotted in Figure 2 (dotted line), with the maximum at 2.85 eV, accordingly to the reported emission of isolated DPA in dilute solution [26].These findings suggest that the interior of micelles formed in the water-surfactant system is able to host DPA molecules, preventing aggregation process.Moreover, following the key idea described in Section 1, silica growth should take place around such all-organic host-guest systems.3(c) depicts a magnification of single silica particles: gyroidal shape confirms (see Figure 1(a) as reference) that the synthesis took place as expected and that the insertion of the chromophore does not affect the macroscopic morphology of the sample.
Micro-PL investigation of the particles in Figure 3(c) let us to inspect the emissive properties of the nanostructured material.The obtained PL spectrum is reported in Figure 4 (solid line) compared to emission spectrum of DPA-water-CTACl ternary system described above (dashed line).MCM-DPA emission shows a structured, quite resolved band spanning from 2.0 to 3.2 eV, with a maximum centered at 2.88 eV.The vibronic progression underlying this PL band is quite easily recognizable, with the purely electronic transition at about 3.0 eV and 0-1, 0-2, and 0-3 replica at 2.7, 2.5, and 2.3 eV, respectively.
The main difference between the two spectra reported in Figure 4 concerns the intensity of the 0-0 purely electronic transitions, being the shoulder in MCM-DPA spectrum about 70% of the corresponding peak in DPA-water-CTACl system emission.This is probably due to self-absorption phenomena, which is an evidence for the high concentration of the chromophore achievable within the silica microparticles.

CONCLUSIONS
In summary, an effective procedure to obtain hierarchicallyorganized nanostructured systems has been tested and described.The material prepared maintains the desired spectral features as well as the expected macroscopic morphology.A ∼30% of self-absorption observed in the silica microparticles emission spectrum demonstrates that a high concentration of the loaded chromophore can be achieved, producing very bright luminescent material by preventing aggregation processes (which are known to quench PL) that usually take place in more conventional systems.
These results therefore appear to be very promising for the inclusion of functional molecular and supramolecular units in inorganic host matrices as active materials in various applications, such as optoelectronic devices, luminescent probes, and hybrid solar cells.

Figure 1 :
Figure 1: (a) Sketch of a silica particle with the expected gyroidal morphology, showing the arrangement of surfactant micelles.(b) Molecular structure of DPA.

Figure 2 :
Figure 2: Comparison between optical absorption measurements of an aqueous solution of DPA alone (solid line) and after addition of a sample amount of CTACl (dashed line), along with PL spectrum of the resulting system (dotted line) excited at 3.15 eV.

Figure 3 :
Figure 3: Microscope images of MCM-DPA sample, (a) and (b) reflected and emitted light, respectively, from silica particles on glass substrate and (c) software-assisted zoom, showing silica powder morphology (scale bars 10 μm).

Figure 4 :
Figure 4: PL spectra of MCM-DPA sample (solid line) and DPAwater-CTACl ternary system (dashed line), excited with Hg lamp in the range 3.26-3.76eV and with Xe lamp at 3.15 eV, respectively.

Figure 3
Figure3reports microscope images of the sample prepared (MCM-DPA) accordingly to procedure described in Section 2.1.In Figure3(a), a layer of silica particles dried in air shows quite good homogeneity both in shapes and in dimensions.Figure3(b) reports the same portion of the material under UV light, showing that emitted light is coming entirely from the silica beads.Figure3(c) depicts a magnification of single silica particles: gyroidal shape confirms (see Figure1(a) as reference) that the synthesis took place as expected and that the insertion of the chromophore does not affect the macroscopic morphology of the sample.Micro-PL investigation of the particles in Figure3(c) let us to inspect the emissive properties of the nanostructured material.The obtained PL spectrum is reported in Figure4(solid line) compared to emission spectrum of DPA-water-CTACl ternary system described above (dashed line).MCM-DPA emission shows a structured, quite resolved band spanning from 2.0 to 3.2 eV, with a maximum centered at 2.88 eV.The vibronic progression underlying this PL band is quite easily recognizable, with the purely electronic transition at about 3.0 eV and 0-1, 0-2, and 0-3 replica at 2.7, 2.5, and 2.3 eV, respectively.The main difference between the two spectra reported in Figure4concerns the intensity of the 0-0 purely electronic transitions, being the shoulder in MCM-DPA spectrum about 70% of the corresponding peak in DPA-water-CTACl system emission.This is probably due to self-absorption phenomena, which is an evidence for the high concentration of the chromophore achievable within the silica microparticles.

Figure 3 (
Figure3reports microscope images of the sample prepared (MCM-DPA) accordingly to procedure described in Section 2.1.In Figure3(a), a layer of silica particles dried in air shows quite good homogeneity both in shapes and in dimensions.Figure3(b) reports the same portion of the material under UV light, showing that emitted light is coming entirely from the silica beads.Figure3(c) depicts a magnification of single silica particles: gyroidal shape confirms (see Figure1(a) as reference) that the synthesis took place as expected and that the insertion of the chromophore does not affect the macroscopic morphology of the sample.Micro-PL investigation of the particles in Figure3(c) let us to inspect the emissive properties of the nanostructured material.The obtained PL spectrum is reported in Figure4(solid line) compared to emission spectrum of DPA-water-CTACl ternary system described above (dashed line).MCM-DPA emission shows a structured, quite resolved band spanning from 2.0 to 3.2 eV, with a maximum centered at 2.88 eV.The vibronic progression underlying this PL band is quite easily recognizable, with the purely electronic transition at about 3.0 eV and 0-1, 0-2, and 0-3 replica at 2.7, 2.5, and 2.3 eV, respectively.The main difference between the two spectra reported in Figure4concerns the intensity of the 0-0 purely electronic transitions, being the shoulder in MCM-DPA spectrum about 70% of the corresponding peak in DPA-water-CTACl system emission.This is probably due to self-absorption phenomena, which is an evidence for the high concentration of the chromophore achievable within the silica microparticles.

Figure
Figure3reports microscope images of the sample prepared (MCM-DPA) accordingly to procedure described in Section 2.1.In Figure3(a), a layer of silica particles dried in air shows quite good homogeneity both in shapes and in dimensions.Figure3(b) reports the same portion of the material under UV light, showing that emitted light is coming entirely from the silica beads.Figure3(c) depicts a magnification of single silica particles: gyroidal shape confirms (see Figure1(a) as reference) that the synthesis took place as expected and that the insertion of the chromophore does not affect the macroscopic morphology of the sample.Micro-PL investigation of the particles in Figure3(c) let us to inspect the emissive properties of the nanostructured material.The obtained PL spectrum is reported in Figure4(solid line) compared to emission spectrum of DPA-water-CTACl ternary system described above (dashed line).MCM-DPA emission shows a structured, quite resolved band spanning from 2.0 to 3.2 eV, with a maximum centered at 2.88 eV.The vibronic progression underlying this PL band is quite easily recognizable, with the purely electronic transition at about 3.0 eV and 0-1, 0-2, and 0-3 replica at 2.7, 2.5, and 2.3 eV, respectively.The main difference between the two spectra reported in Figure4concerns the intensity of the 0-0 purely electronic transitions, being the shoulder in MCM-DPA spectrum about 70% of the corresponding peak in DPA-water-CTACl system emission.This is probably due to self-absorption phenomena, which is an evidence for the high concentration of the chromophore achievable within the silica microparticles.