1. Introduction

Graphene Oxide (GO) is made of highly oxidated graphene sheets that exposes oxygen functional groups in form of epoxy and hydroxyl groups on both basal plane and edges [1]. In recent years, GO has received great interest because of its superior dispersion ability in water (since formation of hydrogen bonds between water molecules and polar functional groups on GO surface, stable colloidal suspensions can be formed), and a finite electronic band gap, different with respect to graphene [2]. Due to these properties, several applications ranging from electronic to biomedical fields have been proposed [3,4]. Innovative biosensors based on GO surface functionalization with small molecules or polymers through activation and amidation/esterification of either the carboxyls or hydroxyls have been demonstrated [5]. Moreover, new perspectives in optoelectronics have been recently envisaged by the discovery of the steady-state photoluminescence (PL) properties of GO: a broad PL emission from 500 to 800 nm has been reported on exposure to near UV radiation [6]. Unfortunately, GO PL is very weak due oxygen-related species (e.g. hydroxyl and epoxy groups) producing non-radiative re-combination due to electrons recombination with holes present in sp2 clusters [6]. Standard ways to make more efficient light generation in GO have been proposed, mainly based on oxidation or reduction treatments [7,8]. A new approach to get higher PL emission from GO could be the infiltration and collection of this material into large specific surface area substrates and porous silicon (PSi) is a perfect candidate for this task [9]. PSi is a nanostructured scaffold obtained by electrochemical anodization of crystalline silicon in hydrofluoridic acid with tunable pore size and morphology, which has been proposed for a huge number of technical and biomedical applications [10]. Since by changing etching time and current density multilayers with different pore characteristics can be easily fabricated, peculiar photonic PSi structures (optical microcavity, Bragg mirror, Thue Morse sequence) are available for utilization in fields from biosensing to telecommunications, in some cases alo integrated in complex microsystems [11–16]. Even if it is possible to obtain luminescent PSi [17] with high quantum yield in a wide range of wavelengths, this material is characterized by some drawbacks: it is very fragile, highly subject to environmental aging and complicated to stratify in order to obtain a resonant optical structure. PSi obtained by p⁺silicon is not luminescent but it allows to realize multilayered photonic structures with a good quality optical response that can be used as host matrix for emitting materials. Over the last few years, polymers, fluorescent dyes, quantum dots and other chemical substances, such as lanthanide, has been confined inside PSi for sharpening and amplification of emitted luminescence [18–21]. In our recent paper, we reported on the formation of GO-PSi hybrid organic-inorganic system by infiltrating GO nanosheets, homemade synthetized, in a homogeneous layer of silanized mesoporous silicon [22]. Enhancement of the PL emitted from GO by a factor of almost 2.5 with respect to GO on crystalline silicon was experimentally measured, without loosing gas and liquid sensing abilities.

In this work, we investigated enhancement and wavelength modulation of photoluminescence signal of commercial GO nanosheets infiltrated by spin-coating into silanized PSi aperiodic multilayer, a Thue-Morse (TM) sequence constituted by 64 layers. The hybrid GO-PSi TM structure was characterized by spectroscopic reflectometry, scanning electron microscopy equipped by energy dispersion spectrometer and steady-state photoluminescence. This hybrid device showed an intense and wavelength modulated photoluminescence signal on a broad range of optical frequencies that can be used for telecomm and biosensing purposes.

2. Materials and methods

2.1 Preparation and characterization of graphene oxide

GO was purchased by Biotool.com (Houston, TX, USA) as a batch of 2 mg/mL in water with a nominal sheets size included between 50 and 200 nm. GO solution was sonicated for 3 hours and left to decant for 2 days; supernatant was separated from precipitate and used for the experiment. Size and zeta-potential of GO sheets dispersed in water (pH = 7) were determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments, UK) equipped with a He− Ne laser (633 nm, fixed scattering angle of 173°, 25 ° C). GO was then deposited on a flat silicon support and characterized by atomic force microscopy (XE-100 AFM, Park System, KR); surface imaging was obtained in noncontact mode using silicon/aluminum coated cantilevers (PPP-NCHR 10M; Park Systems) 125 μm long with resonance frequency of 200 to 400 kHz and nominal force constant of 42 N/m, the scan frequency was typically 1 Hz per line. Infrared spectroscopy of GO deposited on silicon was also performed using a Thermo-Nicholet NEXUS Continuum XL (Thermo Scientific) equipped with a microscope, at 2 cm⁻¹ resolution.

2.2 Porous silicon multilayer fabrication and graphene oxide infiltration

PSi TM multilayered structure was fabricated by electrochemical etching of p⁺ crystalline silicon (0.001Ωcm resistivity, <100> oriented, 500µm thick) in hydrofluoric acid (HF; 50% in volume): ethanol = 1: 1 solution, in dark and at room temperature. TM fabrication and characterization was previously studied [9,20]. Briefly, if L is the label of a low refractive index (n_L = 1.54 at λ = 775 nm) PSi layer with thickness d_L = 108 nm (current density of 200 mA/cm² for 0.8 s), and H stays for the high refractive index (n_H = 1.78 at λ = 775 nm) layer with thickness d_H = 75 nm (current density equal to 100 mA/cm² applied for 1.0 s), the 2ⁿ layers T-M can be generated, starting from a LH bilayer, by following the substitution rules H→HL and L→LH into the 2^n-1 layers structure, as schematized in Fig. 1. In the experiment of GO infiltration, a 64 layers sequence, which can be expressly written as LHHLHLLHHLLHLHHLHLLHLHHLLHHLHLLHHLLHLHHLLHHLHLLHLHHLHLLHHLLHLHHL, was used. During the etching process of each layer, time breaks of 5 s were used in order to recover HF concentration at dissolution edge and start next layer formation with zero current density, so that variation current is always the same for all layers. The PSi etching area is 0.98 cm². After the electrochemical process, pores dimension was increased by rinsing the “as-etched” porous silicon structures in KOH-ethanol solution (1.5 mM) for 15 min [22]. PSi sample was immersed in Piranha solution for 40 min in order to assure the formation of Si–OH bonds on its surface. Afterwards, the PSi TM surface was silanized by incubating the chip in a 5% v/v solution of 3-aminopropyltriethoxysilane (APTES) and anhydrous toluene for 30 min at room temperature. The functionalization procedure was checked by infrared spectroscopy (data not showed here), as reported in our previous work [22]. GO solution was infiltrated into aminosilane-modified PSi TM structure by spin-coating [22]. After spin coating, the sample was dried in desiccator over night at room temperature.

 

Fig. 1 Schematic of PSi Thue-Morse sequences.

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2.3 Steady-state photoluminescence

Steady-state photoluminescence (PL) spectra were excited by a continuous wave He-Cd laser at 442 nm (KIMMON Laser System). PL was collected at normal incidence to the surface of samples through a fiber, dispersed in a spectrometer (Princeton Instruments, SpectraPro 300i), and detected using a Peltier cooled charge coupled device (CCD) camera (PIXIS 100F). A long pass filter with a nominal cut-on wavelength of 458 nm was used to remove the laser line at monochromator inlet.

2.4 Electron microscopy and energy dispersion spectroscopy

GO-PSi TM sample was examined by scanning electron microscopy (JEOL JSM-6400) equipped by energy dispersive x-ray analysis (EDAX) spectrometer (Noran Instruments Voyager Series IV).

2.5 Spectroscopic reflectometry

The reflectivity spectra of PSi sample were measured at normal incidence by means of a Y optical reflection probe (Avantes), connected to a white light source and to an optical spectrum analyzer (Ando, AQ6315B). The spectra were collected over the range 600−950 nm with a resolution of 0.2 nm.

3. Results and discussion

Being a commercial product, the GO from Biotool was characterized in order to assess its real features, independently from productor declaration on data sheet, i.e. dimension between 50 and 200 nm. After sonication, DLS analysis results, reported in Fig. 2(a), performed on the suspension diluted in ultrapure distilled water (pH = 7), highlighted the presence of two GO size populations of 70 ± 20 nm and 700 ± 80 nm (poly dispersion index; PdI = 0.295). AFM imaging, shown in Fig. 2(b), revealed GO nanosheets packed on the flat silicon surface with lateral dimension up to hundreds of nanometers and height up to one tenth of nanometers. Since PSi pores in the designed multilayer had dimensions in the range between 30 and 70 nm (analysed by scanning electron microscopy, image not reported here), only the smallest fraction of sonicated GO can penetrate inside it.

 

Fig. 2 (A) DLS analysis of GO nanosheets dispersed in water (pH = 7). (B) AFM image of GO deposited on silicon and corresponding hight measurement.

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The PL emission of GO under irradiation of He-Cd laser light (@442nm) is shown in Fig. 3(a): a bright luminescence characterized by a broad peak between 500 and 1000 nm was observed, with maximum value at 750 nm, when a drop of solution casted on flat crystalline silicon support dried at room temperature was shined by the laser light. Infrared spectroscopy on the same sample confirmed the presence of several oxygen functional groups exposed by GO as shown in Fig. 3(b): the graph showed the stretching of hydroxyl group at 3450 cm⁻¹, the C = O carbonyl stretching at 1728 cm⁻¹, the C–O epoxide group stretching at 1229 and 1061 cm⁻¹, and the epoxy vibration at 970 cm⁻¹. The reflectance intensities of the main peak we registered in this case were stronger than those in our previous study [22], confirming the presence of larger amount of oxygen functional groups, in case of commercial GO, of course due to different synthesis processes.

 

Fig. 3 (A) Photoluminescence of GO in water at an excitation wavelength of 442 nm. (B) FTIR spectrum of GO deposited on silicon.

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In a recent paper, scientists of Voelcker’s group used a PSi microcavity of 1.2 μm, with a scheme given by (HL)₂-HH-(LH)₂, to obtain luminescence enhancement of europium (Eu(III)) complex. They found that when the microcavity resonance wavelength overlapped with the maximum emission wavelength of the Eu(III) complex at 614 nm, there was an effective coupling between the confined light and the emitting molecules [19]. The structure proposed in [19] is essentialy a Fabry-Peròt microcavity very often used for different sensing and photonic applications [23,24]; however, concerning photoluminescence, a TM structure is more efficient than a Fabry-Peròt microcavity. Reflectivity spectra of TM sequences show multiple band gaps separated by very narrow transmission peaks corresponding to spatial regions of the photonic structures where the electromagnetic field strongly interacts with the surrounding matter. As consequence, the broad PL emission of GO can be enhanced in correspondence of specific multiple wavelengths after infiltration in the aperiodic porous silicon matrix as shown in Fig. 3(a). Moreover, TM sequences are characterized by a lower number of interfaces between layers with different (high and low) porosities compared to the corresponding periodic structures. For example, we recently demonstrated that a Bragg mirror of 64 layers (32 pairs of L and H refractice index layers) has 63 interfaces between the layers while the equivalent 64 layers TM sequence only 42 interfaces; the interfaces induce a vertical inhomogeneity that hinders the infiltration of substances in the pores network, thus reducing the filling [25]. In case of GO, an efficient infiltration in the porous silicon matrix narrows and increases its weak and broad PL emission. Even if the PSi stack was made of several tenths of porous layers, i.e. 64, the reflectivity spectrum was of very good quality in a wide range of wavelengths (600-950 nm), as shown in Fig. 4. Main photonic features of TM reflectivity spectrum were some sharp transmittance resonances in the middle of high reflectivity stop bands, whose nature was direct consequence of PSi high and low refractive index layers alternation, as demonstrated in our previous paper [15], where all theoretic optical calculation and argomentations were reported and discussed.

 

Fig. 4 Normal incidence reflectivity spectrum of S6 PSi TM multilayer.

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Figure 5 shows SEM lateral view images, together with EDS line scan result, of an aminosilane-modified PSi TMs multilayer after GO infiltration. As already observed, only smallest GO nanosheets can be infiltrated in porous structure, while larger particles were sparsely accumulated on PSi surface. EDS line scan, even with lowspatial resolution, clearly indicated an increase of O (oxygen) and C (carbon) signals from bulk silicon to PSi surface, just opposite to Si (silicon) one that decreased.

 

Fig. 5 SEM-EDS characterization of hybrid GO-PSi TM sequence S6. (A) SEM lateral view images of sample and (B) EDS signals of Si (green line), O (blue line) and C (purple line) measured from upper surface down to bulk silicon.

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The PL signal emitted from GO nanosheets infiltrated in PSi TM was investigated at an excitation wavelength of 442 nm: results are reported in Fig. 6 together with PL emission of bare silanized PSi and the same amount of GO spin-coated on silanized crystalline silicon, for comparison.

 

Fig. 6 PL emission from hybrid GO-PSi TM sequence compared to single materials.

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Figure 6 shows a very weak PL signal was measured in case of silanized p⁺ PSi in the wavelength range between 600 and 950 nm due to emission of silicon oxide (black curve): since this material was distributed inside the PSi multilayer, the PL weak emission was slightly modulated in wavelength. The PL signal from the GO drop casted and spinned on a silanized flat surface of crystalline silicon was nearly undetectable over the whole range of wavelengths considered as reported in Fig. 6 (red curve). After infiltration in PSi TM sequence, a strong enhancement of the PL emitted from GO by a factor of 32 with respect to GO on crystalline silicon was experimentally measured in correspondence with the maximum emission of hybrid structure around 720 nm, as shown in Fig. 6 (blue curve). This strong enhancement of PL emission was attributed to GO concentration inside the sponge-like PSi structure and electromagnetic field confinement in high refractive index layers that maximized light-GO interaction and emission. Moreover, a modulation of the photoluminescence signal could be observed, too.

A comparison between PL and reflectivity spectra recorded by GO infiltrated PSi TM sequence is reported in Fig. 7: since light emission comes mostly from inside the porous multilayers minima and maxima of both spectra are almost perfectly out of phase. This proves the presence of interferometers under the GO nanoflake layer that gives only a minimum contribution to the PL spectrum, as we have demonstrated in our previous work [22]. The correspondence was not perfect as we registered in case of PL and reflectivity spectra of GO modified PSi homogeneous layer presented in our previous work, since optical properties of TM sequence were dependent not only on the presence of local minima or maxima of the optical intensity, but rather on the collective shape of the whole spectrum, due to the fractal nature of its photonic stop bands [26]. These results can be analyzed also in terms of photon density of states (DOS) whose values are minimum inside the TM photonic band gaps [15]. Since PL light emitted from GO–PSi structure achieves its minima at the wavelengths corresponding to the device photonic band gaps, a proportional dependence of PL from DOS can be deduced, which is in agreement with the results discussed in [27].

 

Fig. 7 Comparison between PL and reflectivity spectra: minima and maxima are out of phase.

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4. Conclusions

In this work, we demonstrate that commercial GO nanoflakes can be infiltrated in silanized mesoporous silicon multilayer adding luminescent features to this photonic device. The hybrid GO-PSi TM structure shows an enhanced photoluminiscent signal, by a factor of 32 compared to the emission of the same amount of GO deposited on flat silanized silicon. Effective wavelength modulation in specific intervals of the 600-950 nm range explored can be obtained due to photonic properties of TM. An efficient photoemitting silicon-based structure could open a feasible new route for the fabrication of integrated, tunable and low-cost light source for telcomm and biophotonic applications due to the synergy between a multipurpose material such as PSi and a multifunctional one like GO.