Photoluminescence of Graphene Oxide Infiltrated into MesoporousSiliconIlaria Rea,*,† Lucia Sansone,‡ Monica Terracciano,†,§ Luca De Stefano,† Principia Dardano,†
Michele Giordano,‡ Anna Borriello,‡ and Maurizio Casalino†
†Institute for Microelectronics and Microsystems, and ‡Institute for Polymers, Composites and Biomaterials, National Council ofResearch, Naples, Italy§Department of Pharmacy, University of Naples Federico II, Naples, Italy
*S Supporting Information
ABSTRACT: Graphene oxide (GO) is a photoluminescent material whose application inoptoelectronics has been strongly limited due to its poor emission intensity. In this work, aGO−porous silicon (GO−PSi) hybrid structure is realized in order to investigate theemission properties of GO infiltrated into a porous matrix. GO−PSi is characterized byFourier transform infrared spectroscopy, spectroscopic reflectometry, and steady-statephotoluminescence. A photoluminescence enhancement by a factor of 2.5 with respect toGO deposited on a flat silicon surface is demonstrated. Photoluminescence measurementsalso show a modulation of the emitted signal; this effect is attributed to the interferencephenomena occurring inside the PSi monolayer.
1. INTRODUCTION
Graphene oxide (GO) is composed of graphene sheetsmodified with oxygen functional groups in the form of epoxyand hydroxyl groups on both the basal plane and edges.1,2 Inrecent years, GO has received great interest because of itssuperior dispersion ability in water and a finite electronic bandgap different with respect to graphene.3 Because of theseproperties, several applications ranging from electronic tobiomedical field have been proposed.4−6 Innovative biosensorsbased on GO surface functionalization with small molecules orpolymers through activation and amidation/esterification ofeither the carboxyls or hydroxyls have been demonstrated.7 Inaddition, new perspectives in optoelectronics have been openedby the discovery of the steady-state photoluminescence (PL)properties of GO. A broad PL emission from 500 to 800 nmhas been reported.8 Unfortunately, GO PL is very weak due tothe presence of oxygen functionalities (e.g., hydroxyl and epoxygroups) producing nonradiative recombination as a result oftransfer of their electrons to the holes present in sp2 clusters.9
Many approaches based on oxidation or reduction treatmentshave been followed to get higher PL emission from GO.10,11 Anew approach could be to infiltrate GO into large specificsurface area substrates, and porous silicon (PSi), realized byelectrochemical dissolution of crystalline silicon in a hydro-fluoridric-based solution, is a perfect candidate for this task. PSiis characterized by a sponge-like morphology with a specificsurface area up to hundreds of m2 cm−3. This property makesPSi an ideal support for sensing of different substances such asgases, liquids, and biological molecules.12−14 Changes of several
physical properties (e.g., dielectric constant and electricalconductivity) can be observed after infiltration of an analyteinside nanometric pores. Moreover, the PSi refractive index canbe widely tuned between that of Si and air (approximately).This is achieved by varying the porosity of PSi during theelectrochemical fabrication process. Owing to these character-istics, a lot of photonic structures, such as Fabry−Perotinterferometers, Bragg mirrors , and microcavities, have beenproposed.12,15,16
In this work, we investigated enhancement and wavelengthmodulation of photoluminescence signal of GO sheetsinfiltrated by spin-coating technique into silanized mesoporoussilicon. GO-modified PSi layer was characterized by spectro-scopic reflectometry, Fourier transform infrared spectroscopy,and steady-state photoluminescence. This hybrid structureshowed an intense and wavelength-modulated photolumines-cence signal on a broad range of optical frequencies.
2. MATERIALS AND METHODS
2.1. Preparation of Graphene Oxide. The grapheneoxide (GO) was purchased from Cheap Tubes, Inc. (USA).The GO was synthesized from purified natural graphite by themodified Hummers method.17−19 Ten milligrams of GOpowder was suspended in 5 mL of distilled water (D.I. waspurchased from Aldrich Chemical Co.) and sonicated by means
Received: July 1, 2014Revised: November 6, 2014Published: November 14, 2014
of an ultrasonic processor (Misonix Incorporeted Ultrasonicliquid Processors, U.S.A.) for 4 h at room temperature so as toproduce a stable yellow−brown colloidal suspension of GOsheets. Solution was then filtered with pore size of 0.22 μm(Millipore).2.2. Porous Silicon Layer Fabrication. PSi was fabricated
by electrochemical etching of p+ crystalline silicon (0.001 Ω cmresistivity, ⟨100⟩ oriented, 500 μm thick) in hydrofluoric acid(HF; 50% in weight)/ethanol = 1:1 solution, in dark and atroom temperature. Before the anodization process, the siliconsubstrate was immersed in HF solution for 2 min to remove thenative oxide layer. A current density of 200 mA/cm2 wasapplied for 15 s to produce a single layer of PSi with a porosityof 72% (n = 1.54) and a thickness of 2 μm. Porosity andthickness of PSi layer were estimated by spectroscopicellipsometry (data not shown here). After the electrochemicalprocess, pore dimension was increased to favor the infiltrationof graphene oxide by rinsing the “as-etched” PSi layer in aKOH−ethanol solution (1.5 mM) for 15 min.20
PSi sample was immersed in Piranha solution, a 4:1 mixtureof concentrated sulfuric acid (H2SO4, was purchased fromAldrich Chemical Co.) and 30% hydrogen peroxide (H2O2 30%w/w was purchased from Aldrich Chemical Co.), for 40 min inorder to ensure the formation of Si−OH bonds on its surface.After Piranha treatment, sample was rinsed with deionizedwater and dried in a nitrogen stream. Afterward, the PSi surfacewas silanized (i.e., functionalized by aminosilane) by incubatingthe chip in a 5% v/v solution of 3-aminopropyltriethoxysilane(APTES was purchased from Aldrich Chemical Co.) andanhydrous toluene (anhydrous 99.8% was purchased fromAldrich Chemical Co.) for 30 min at room temperature.21−23
After the reaction time, we washed three times the sample intoluene so as to remove any physisorbed APTES and baked itat 100 °C for 10 min.2.3. Graphene Oxide Infiltration into Porous Silicon
Layer. GO solution was infiltrated into aminosilane-modifiedPSi layer by spin-coating processing (Karl Suss Microtec Delta
80T) with a defined delay time, prior to spinning, necessary forGO infiltration. PSi substrate was completely covered withsufficient amount of the GO solution, allowed to stand for 60 s,after which the sample was allowed to spin at 600 rpm for 60 sto cause uniform spreading of the solution on the substrate,then at 800 rpm for 60 s to thin the solution layer, and finally at1600 rpm for 60 s to dry the film. After spin coating, the samplewas dried in desiccator overnight at room temperature.
2.4. Dynamic Light Scattering. Size distribution of GOsheets dispersed in water (pH = 7) was investigated by dynamiclight scattering (DLS) using a Zetasizer Nano ZS (MalvernInstruments, U.K.) equipped with a He−Ne laser (633 nm,fixed scattering angle of 173°, 25 °C).
2.5. Atomic Force Microscopy. A XE-100 AFM (ParkSystems) was used for the imaging of GO sheets deposited onsilicon substrate. Surface imaging was obtained in noncontactmode using silicon/aluminum coated cantilevers (PPP-NCHR10M; Park Systems) 125 μm long with resonance frequency of200 to 400 kHz and nominal force constant of 42 N/m. Thescan frequency was typically 1 Hz per line.
2.6. Scanning Electron Microscopy. The morphology ofGO infiltrated PSi layer was investigated by scanning electronmicroscopy (SEM) using a field emission instrument (Zeiss-Supra 35). Sample was mounted on a double-faced conductiveadhesive tape. Images were acquired at 5 kV acceleratingvoltage and 30 μm wide aperture.
2.7. Fourier Transform Infrared Spectroscopy. Chem-ical composition of PSi layer before and after GO infiltrationwas analyzed by Fourier transform infrared (FTIR) spectros-copy. Spectra were recorded by a Thermo-Nicholet NEXUSContinuum XL (Thermo Scientific) equipped with a micro-scope, at 2 cm−1 resolution.
2.8. Spectroscopic Reflectometry. The reflectivityspectra of PSi samples were measured at normal incidence bymeans of a Y optical reflection probe (Avantes), connected to awhite light source and to an optical spectrum analyzer (Ando,
Figure 1. Size distribution of graphene oxide in water, analyzed by DLS (A). AFM image of graphene oxide deposited on silicon and correspondingheight measurement (B).
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AQ6315B). The spectra were collected over the range 600−1200 nm with a resolution of 0.2 nm.2.9. Steady-State Photoluminescence. Steady-state
photoluminescence (PL) spectra were excited by a continuouswave He−Cd laser at 442 nm (KIMMON Laser System). PLwas collected at normal incidence to the surface of samplesthrough a fiber, dispersed in a spectrometer (PrincetonInstruments, SpectraPro 300i), and detected using a Peltiercooled charge coupled device (CCD) camera (PIXIS 100F). Along pass filter with a nominal cut-on wavelength of 458 nmwas used to remove the laser line at monochromator inlet.
3. RESULTS AND DISCUSSIONEven if GO sheets are nanometric in size, their infiltration in aporous material, such as a PSi layer, requires a carefulcharacterization of both the GO size distribution and the PSipores average dimension. Size distribution of GO nanosheetsdispersed in water (pH = 7) at a concentration of 2 mg/mL was
investigated by DLS analysis. Results, reported in Figure 1A,highlight the presence of two peaks corresponding to 30 ± 5and 110 ± 40 nm size populations (poly dispersion index; PdI= 0.211). It is worth noting that the heights of two peaks shownin Figure 1A are related to the scattering intensities and not tothe amount of small and big GO particles. The low value of PdIindicates that GO sample is constituted by only twopopulations, but we cannot say which is the most abundant.AFM image of GO deposited on flat crystalline Si andcorresponding height profile of GO sheets are reported inFigure 1B. Nanosheets with different sizes ranging from fewtens of nanometers to hundreds of nanometers can beobserved. Height measurement demonstrates the presence ofsheets up to 4 nm thick. Since a single-layered sheet is knownto be as high as 1 nm,24 the measured thickness is compatiblewith a multilayered morphology.The chemical nature of graphene oxide was confirmed by
Raman spectroscopy. Raman spectrum of GO deposited on
Figure 2. SEM image of bare aminosilane-modified porous silicon and corresponding histogram of pores size distribution (A). SEM image ofaminosilane-modified porous silicon after infiltration with graphene oxide (B). Cross-sectional SEM image of aminosilane-modified porous siliconafter infiltration with graphene oxide (C). Scale bar corresponds to 200 nm.
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crystalline silicon is reported in Figure S1, SupportingInformation: the broad G and D peaks and the low intensity2D and D + G bands characteristic of GO2,25 are clearly visible.In our experience, GO cannot be simply deposited in the PSi
sponge-like matrix by drop casting and spin coating: without aproper chemical modification of the PSi surface, we did notobtain any infiltration of GO into the PSi. On the contrary, GOstrongly interacts with a silanized surface. For this reason, GOsolution was spinned on PSi layer modified by APTES.Silanization procedure is crucial to guarantee interactionbetween GO and PSi surface; carboxyl and epoxy groups ofGO can interact with positively charged amine group of APTESforming a stable GO−PSi hybrid structure.26 Figure 2 showsSEM top view images of an aminosilane-modified PSi layerbefore (Figure 2A) and after (Figure 2B) GO infiltration. In thecase of bare silanized PSi, histogram of pores size distribution,reported in the inset of Figure 2A, reveals dimensions includedbetween 20 and 80 nm with an average value of 40 nm. Thismean size allows infiltration of smaller GO nanosheets (size ≈30 nm), while larger particles (size ≈ 110 nm) are accumulatedon PSi surface, as it can be observed in SEM image of Figure2B. The result is also evident from cross-sectional SEM imageof GO−PSi structure, reported in Figure 2C. The imagehighlights GO infiltrated along nanometric PSi pores as well asGO layer deposited on the chip surface. The thickness of thisGO layer is just of few nanometers, much smaller than theoverall PSi thickness (approximately 2 μm).Chemical composition of the hybrid structure was
investigated by FTIR analysis. Figure 3 shows FTIR spectra
of aminosilane-modified PSi (A), GO deposited on flatcrystalline silicon support (B), and GO infiltrated in amino-silane-modified PSi (C). FTIR spectrum of bare silanized PSi ischaracterized by the peak at 1100 cm−1, corresponding to Si−O−Si bond, and the peak at 955 cm−1, corresponding to Si−OH group, both due to Piranha treatment in strong acid.Surface aminosilane modification is demonstrated by thepresence of peaks at 1475 and 785 cm−1, indicating amineand SiOCH2CH3 groups, respectively.
27 In the spectrum of GOdeposited on crystalline silicon, a peak at 1740 cm−1
corresponding to CO carbonyl stretching of −COOH canbe observed; other peaks are present at 1640 cm−1 for CCand at 1460 cm−1 for CH2 deformation vibrations. The broadband located at about 970 cm−1 can be assigned to epoxygroup.28 FTIR spectrum of GO infiltrated aminosilane-modified PSi shows peaks characteristic of both silanized PSiand GO, demonstrating the formation of the hybrid structure.The optical spectrum of a PSi layer consists of a fringe
pattern due to interference occurring at air/PSi and PSi/bulkcrystalline silicon interfaces. In other words, the PSi layeroptically acts as a Fabry−Perot interferometer whosereflectance can be written as29
λπ
λ= + + ⎜ ⎟⎛
⎝⎞⎠R r r r r
n L( ) ( ) 2 cos
41 2 1 2
PSi
(1)
where r1 = (nAir − nPSi)/(nAir + nPSi) is the Fresnel reflectioncoefficient at air/PSi interface, r2 = (nPSi − nSi)/(nPSi + nSi) isthe Fresnel reflection coefficient at PSi/crystalline siliconinterface, L is the thickness of the PSi layer, λ is the wavelength,and nAir, nPSi, and nSi are the refractive indices of air, poroussilicon layer, and bulk silicon, respectively. The optical path(nPSiL) of the PSi layer can be calculated from the reflectivityspectrum just counting fringe maxima, which satisfy therelationship mλ = 2nPSiL, where m is an integer and λ is thewavelength of the incident light.30 A faster method forcalculating the optical path is based on the fast Fouriertransform (FFT) of PSi reflectivity spectrum. FFT of thespectrum displays a peak whose position along the x-axiscorresponds to two times the optical path (2nPSiL) of thelayer.30
In Figure 4A, normal incidence reflectivity spectra are shownfor aminosilane-modified PSi layer before and after GOinfiltration. The corresponding Fourier transforms, reportedin Figure 4B, show two peaks centered at 5800 and 6000 nm,before and after GO infiltration, respectively. Since thethickness L of the layer is fixed, the peak shift of about 200nm is ascribed to the increase of the PSi layer average refractiveindex. The GO solution penetrates into the porous siliconmatrix, and some nanosheets bind the silanized porous siliconsurface, partially filling the pores. Since air is displaced by GOinto the pores, the layer density increases, and as consequence,the average refractive index increases, too. Considering a PSithickness of 2000 nm, a refractive index increase of 0.05 afterGO infiltration can be calculated.Since SEM characterization showed the presence of a thin
GO layer on the top of PSi surface that could clog pores, weinvestigated their accessibility exposing GO−PSi hybrid deviceto ethanol vapors. It is well-known that vapors of volatilesubstances can condensate into nanometric pores by capillarycondensation inducing an increase of the average refractiveindex of the PSi layer and a consequent red shift of its opticalspectrum. Figure 5 reports the reflectivity spectra (A) of bare
Figure 3. FTIR spectra of aminosilane-modified porous silicon (A),graphene oxide on silicon (B), and graphene oxide infiltrated inaminosilane-modified porous silicon (C).
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PSi in air and on exposure to ethanol vapors together withcorresponding FFTs (B). We observed a FFT shift of 50 nmdue to ethanol condensation inside pores of the PSi device.29
The experiment was repeated in the case of GO−PSi device.Spectra are shown in Figure 5C, together with correspondingFFTs (Figure 5D): a FFT peak shift of 100 nm was measured.The larger shift is due to the presence of GO nanosheets that,coating the pores wall of the hybrid device, reduce theirdiameter making the structure more sensitive, according to theClausius−Kelvin equation, which rules the condensation.31
This experimental result demonstrates that, even if GOnanosheets are deposited onto and inside the PSi, the poresare still accessible to organic vapors, probably due to both thepresence of voids between the GO sheets and the partialpermeability of these extremely thin (just few nanometers)layers, whereas thicker and more compact GO coatings havebeen found to be quite impermeable.32
The PL signal emitted from GO nanosheets infiltrated in PSiwas investigated at an excitation wavelength of 442 nm. Resultsare reported in Figure 6A together with PL emission of bare
Figure 4. Normal incidence reflectivity spectra of aminosilane-modified porous silicon layer before (black line) and after (blue line) GO infiltration(A). Corresponding Fourier transforms (B).
Figure 5. Normal incidence reflectivity spectra of bare PSi structure in air (black line) and on exposure to ethanol vapors (blue line) (A).Corresponding Fourier transforms (B). Normal incidence reflectivity spectra of GO−PSi hybrid structure in air (black line) and on exposure toethanol vapors (blue line) (C). Corresponding Fourier transforms (D).
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silanized PSi and of GO spin-coated on silanized crystallinesilicon, for comparison. As it is well-known, any PL signalcannot be detected in the case of p+ PSi. A weak and broadphotoluminescence was measured in the case of GO spinnedon silanized Si in the wavelength range between 500 and 800nm. This very low signal is due to the presence of a thin GOlayer, with an average height of about 3 nm (see AFMcharacterization reported in Figure 1). After infiltration in PSi, astrong enhancement of the PL emitted from GO by a factor ofalmost 2.5 with respect to GO on crystalline silicon wasexperimentally measured. This strong enhancement in PLemission was attributed to the high GO concentration insidethe sponge-like PSi structure. Moreover, a modulation of thephotoluminescence signal can be observed, too.PL intensity modulation can be explained by considering the
theory of the Fabry−Perot interferometer.33 Among all thewavelengths, λem, emitted by GO infiltrated in PSi, only thosefullfilling the relationship L = m(λem/2nPSi), with L thickness ofthe PSi layer and m integer, can constructively interfere formingmaxima in the photoluminescence spectrum of hybridstructure. As it can be seen in Figure 6B, the distance betweentwo consecutive photoluminescence maxima is about 76 nm,exactly corresponding to the free spectral range (i.e., distancebetween two consecutive maxima/minima in the reflectivity/transmittivity spectrum of an interferometer) of the GO−PSihybrid structure. In order to exclude the possibility that theinterference fringes present in photoluminescence spectrum ofGO−PSi device could be due to the thin GO layer depositedon the PSi interferometer surface and not to the materialinfiltrated inside pores, GO was spinned on a layer ofaminosilane-modified amorphous silicon (aSi), 800 nm thick.The homogeneous layer (not porous) of aSi acts as aninterferometer with a free spectral range of about 55 nm. Thefabrication procedure of GO-deposited aSi (GO−aSi) isdescribed in the Supporting Information. We monitoredreflectivity spectra of aminosilane-modified aSi before and
after GO deposition. Spectra, reported in Figure S2, do notshow any difference since few nanometers of GO cannotcontribute to a measurable increase of the optical path.Reflectivity and photoluminescence spectra of GO−aSi werecompared in Figure S3. As in the case of GO deposited on flatcrystalline silicon, a very low and noisy signal was recorded.From this measurement, we can deduce that the presence of ahomogeneous interferometer under the GO layer slightlymodulates its photoluminescence, but not as much for the PSilayer. We also investigated where the luminescence of GOsheets deposited on PSi was occurring by confocal microscopyanalysis on the hybrid structure. Figure S1A,B clearly shows aluminescence signal coming from both surface and inside thechip.
4. CONCLUSIONSThis work demonstrates the enhancement and the modulationof the photoluminescence signal emitted by GO nanosheetsinfiltrated in silanized mesoporous silicon matrix. Theformation of GO−PSi hybrid structure is confirmed by severalspectroscopic techniques, such as FTIR spectroscopic reflec-tometry and steady-state photoluminescence. FFT analysisreveals a refractive index increase, due to the infiltration of GOinside the PSi layer, of 0.05. In addition, a strong enhancementof the PL emitted from GO by a factor of almost 2.5 withrespect to GO on crystalline silicon has been experimentallymeasured. Wavelength modulation of GO photoluminescenceemission is very attractive and opens new perspectives for GOexploitation in innovative optoelectronic devices and highsensitive fluorescent sensors.
■ ASSOCIATED CONTENT*S Supporting InformationRaman analysis, aSi optical characterization, and confocalmicroscopy imaging. This material is available free of charge viathe Internet at http://pubs.acs.org.
Figure 6. Photoluminescence spectra of aminosilane-modified PSi (solid line), GO on silanized Si (dashed line), and GO infiltrated in silanizedporous Si layer (short dashed line) at an excitation wavelength of 442 nm (A). Comparison between photoluminescence spectrum (upper graph)and reflectivity spectrum (lower graph) of GO-infiltrated PSi layer (B).
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■ ACKNOWLEDGMENTSThe authors thank Dr. M. Pirozzi of IBP-CNR for assistancewith confocal microscopy acquisition, and Dr. P. Musto and Dr.P. Lamanna of IPCB-CNR for Raman measurements.
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