Phys. Status Solidi A, 1–4 (2012) / DOI 10.1002/pssa.201228309 p s sa

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applications and materials science

Photoluminescence enhancement of

porous silicon particles by microwave-assisted activation

Bing Xia*,1,2, Wenyi Zhang1, Weiyi Bao2, Chen Dong1, Junfeng Zhang3, and Jisen Shi**,1

1 Key Laboratory of Forest Genetics & Biotechnology (Ministry of Education of China), Nanjing Forestry University, Nanjing 210037,

P. R. China2 Advanced Analysis and Testing Center, Nanjing Forestry University, Nanjing 210037, P. R. China3 State Key Laboratory of Pharmaceutical Biotechnology, School of Life Sciences, Nanjing University, Nanjing 210093, P. R. China

Received 15 May 2012, revised 27 July 2012, accepted 31 July 2012

Published online 24 August 2012

Keywords microwave irradiation, nanoparticles, photoluminescence, porous silicon, wet etching

* Corresponding author: e-mail xiabing@njfu.edu.cn, Phone: þ86-25-85428960, Fax: þ86-25-85485496** e-mail jshi@njfu.edu.cn, Phone/Fax: þ86-25-85428711

Photoluminescence (PL) of porous silicon (PSi) particles can be

significantly enhanced in some organic solvents (i.e., ethanol or

dimethyl sulfoxide) under microwave irradiation. Fourier

transform infrared spectra, dynamic-light-scattering measure-

ments, and scanning electron microscopy had been adopted to

explore the mechanism of PL enhancement of PSi particles

under microwave irradiation, which is attributed to the

formation of higher porosity and the growth of silicon oxide

by microwave-assisted wet etching. Compared with that

fabricated by ultrasonication, smaller luminescent PSi nano-

particles (average size �60 nm) with stronger orange-red

fluorescence (PL quantum yield �14.8%) and higher dispersi-

bility can be large-scale prepared for cellular imaging and drug

delivery in biomedical applications.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Since the discovery of luminescentporous silicon (PSi) by electrochemically etching of single-crystalline silicon substrate in ethanolic HF solutions [1],micro- or nanoparticles of PSi have been fabricated foroptoelectronic and biological applications, which is due totheir strong photoluminescence (PL), biocompatibility,biodegradability, surface tailorability, tunable porous nano-structure, and compatibility with silicon technologies [2].Especially, luminescent PSi nanoparticles have exhibitedconsiderable potential for cancer treatments such as tumorimaging [3–5], anti-cancer drug delivery [3, 6–9], photo-dynamic therapy [10], or thermotherapy [11]. Generally,small PSi particles can be prepared by ultrasonication of PSilayers in a liquid (i.e., ethanol or water), whose average sizecan range from a few micrometers to a few nanometers,depending on the liquid used and the duration of thetreatment [12]. The resultant PSi particles can displayluminescence due to the quantum-confined effects of siliconnanocrystalline domains, and their luminescence can beactivated by oxidation of hydrogen-terminated PSi particlessurfaces in aqueous solution [3].

Microwave dielectric heating has been widely appliedfor synthesizing various kinds of functional nanomaterials,

because of its unique advantages such as rapid temperatureelevation, homogeneous heating, and high reaction selectiv-ity [13]. Noticeably, the luminescence of silicon nanowiresvia AgNO3/HF etching was generated in water under sealed-vessel microwave irradiation [14]. However, as far as weknow, similar researches focused on PSi particles have notbeen reported. In this work, by microwave-assisted acti-vation, PL enhancement of PSi particles in ethanol ordimethyl sulfoxide (DMSO) had been observed, but not inethylene glycol (EG) or water. The mechanism of PLenhancement is related to the porosity and oxidation of PSiparticles, which can be efficiently increased by microwave-assisted wet etching. By the use of this microwave method-ology, smaller luminescent PSi nanoparticles (sizes �60 nm)with higher dispersibility and stronger fluorescence (PLquantum yield (QY) �14.8%) can be large-scale fabricatedfor various optoelectronic or biological applications.

2 Experimental The single side polished (100)oriented, and p-type Si wafers (boron doped, 8–10 V cmresistivity, purchased from Hefei Kejing Materials Technol-ogy Co. Ltd., China) were boiled in 3:1 (v/v) concentratedH2SO4/30% H2O2 for 30 min and then rinsed copiously with

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 B. Xia et al.: PL enhancement of porous silicon particles by microwave-assisted activationp

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Figure 1 (online color at: www.pss-a.com) (a) Under microwaveheating, PL spectra of porous silicon particles in ethanol withincreasing heating time, and (b) histogram of PL intensity of PSiparticles in ethanol at different temperature.

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Figure 2 (online color at: www.pss-a.com) FTIR spectra of PSiparticles with 0 and 25 min of microwave treatment.

Milli-Q water (�18 MV cm resistivity). The PSi samples(1.54 cm2) with about 30 mm thick porous layer wereprepared by electrochemically etching in an ethanolic HFsolution (HF (40%)/ethanol (1:1, v/v)) at 20 mA cm�2 for45 min. Anhydrous ethanol (or other solvents) was freeze–pump–thaw degassed three times and handled under drynitrogen to exclude residual water and oxygen. After 3 hsonication in ethanol (water, EG, or DMSO) to detach theporous layer, PSi particles samples were then incubatedunder sealed-vessel microwave irradiation conditions. Thesingle-mode heating microwave system NOVA used wasmade by Preekem of Shanghai, China. UV–Vis adsorptionspectra were recorded by a Shimadzu UV-2450 spectropho-tometer. PL measurements were performed using a Perkin-Elmer LS55 fluorescence spectrometer. Fourier transforminfrared (FTIR) spectra were recorded using Bruker Vertex70 spectrometer at 0.25 cm�1 resolution. Analysis ofnanoparticles size was performed using Malvern ZetasizerNano ZS dynamic-light-scattering (DLS) measurements.Scanning electron microscopy (SEM) images were taken byHitachi s-4800 high resolution electron microscope. ThePLQY of PSi samples was determined via Rhodamine 101(PLQY¼ 100%) in ethanol as a reference standard, whichwas freshly prepared to reduce the measurement error [3].

3 Results and discussion During microwave treat-ment at 150 8C, PL spectra of PSi particles in ethanol wererecorded every 5 min in Fig. 1a. As seen in Fig. 1a, it displaysthat with increasing heating time, PL intensity of PSiparticles can be efficiently increased from 0 to 240 bymicrowave irradiation, and after 25 min of microwaveheating, these resultant PSi particles possess good orange-red PL properties (lem¼ 610 nm) under 370 nm excitation.Additionally, PL spectra of PSi particles under microwaveheating at different temperature were recorded in Fig. 1b,which presents that PL enhancement of PSi particles ismainly dependent on reaction time and temperature, andhigh microwave temperature (130–150 8C) and enough time(�15 min) are needed to activate significant luminescence ofPSi particles.

To explore the mechanism of PL enhancement, FTIRspectra were adopted to monitor the changes of oxidation ofPSi particles under microwave heating at 150 8C. Accordingto Fig. 2, FTIR spectra of PSi particles before microwavetreatment exhibit a ySi–OH stretching mode at around3434 cm�1, a dSi–OH bending mode at 1638 cm�1, and aySi–O–Si stretching mode at 1085 cm�1 [15]. After 25 minof microwave heating, three peaks at 3434, 1638 and1085 cm�1 become significantly stronger, which meansmicrowave irradiation can cause the growth of silicon oxideon the PSi surface. The oxidation mechanism suggestedis that CH3CH2O� radicals produced by field assisteddissociation of ethanol can oxidize PSi particles [16]. Duringthe activation step, silicon oxide grows on the PSi surface,generating significant luminescence due to quantum con-finement effects and to defects localized Si–SiO2 interface[17–19].

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Scanning electron microscopy was also used to observemorphological changes of PSi particles during microwaveheating. After filtration with a 0.45 mm filtration membrane,the typical SEM images of PSi particles in ethanol with 0 and25 min of microwave treatment were recorded in Fig. 3aand b, respectively. In contrast to minimum nanoparticles (redarrow, size �40 nm) in Fig. 3a, much smaller nanoparticles(size �10 nm) are formed after 25 min of microwavetreatment in ethanol (red arrow, in Fig. 3b). DLS measure-

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Phys. Status Solidi A (2012) 3

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Figure 3 (online color at: www.pss-a.com)SEMimagesofPSiparticles inethanolwith0 min(a)and25 min(b)ofmicrowave treatment, and DLS meas-urements of the distribution intensityfunction of LPSiNPs in ethanol with0 min (c) and 25 min (d) of micro-wave treatment.

ments were further utilized to confirm the generation ofsmaller nanostructures, which were shown in Fig. 3c and d.Comparing 0 min with 25 min of microwave treatments, wealso found that the mean hydrodynamic size of PSi particlesdecreases from �203 to �60 nm, which is also consistentwith the SEM measurements. The formation of much smallernanoparticles after microwave treatment can be attributed tomicrowave-assissted wet etching of CH3CH2O� radicals[20], which also result in the increasing of the pore size of PSiparticles. Because luminescence of PSi is a property of theporosity, higher porosity of PSi particles can result in PLenhancement of PSi particles, due to the increasing of thepore size [21].

As-prepared PSi nanoparticles were also characterizedby UV–Vis spectra, and PL spectra to calculate their PLQY(shown in Fig. 4a and b). The results show that the PLQY ofluminescent PSi nanoparticles in ethanol is determined to be�14.8% (relative to Rhodamine 101 standard), which ishigher than previously reported values for luminescent PSinanoparticles only by sonication (�10.2%) [3]. This kind ofluminescent PSi nanoparticles with smaller size and strongerluminescence is suitable for cellular imaging and moleculartransportation in biomedical applications.

Furthermore, PSi particles were incubated in othersolvents for microwave heating at 150 8C, such as DMSO,EG and water, and their PL spectra were recorded in Fig. 5.The results indicate that PL enhancement of PSi particlescan occur in ethanol or DMSO, but not in water or EG.Noticeably, in our experiments, the aqueous (or EG) solution

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of PSi particles was highly transparent and colorless after5 min of microwave heating, which demonstrates that PSiparticles without PL enhancement can be fast dissolved inwater (or EG) by microwave heating. The rate of microwave-assisted etching mainly depends on oxygen radicalsconcentration and solubility in solvent. In aqueous (or EG)solution, high concentration of oxygen radicals and excellentsolubility of reaction product can lead to high etching rate[19], thus most of PSi particles can be dissolved after 5 min ofmicrowave heating. Comparatively, yellow turbid solutiondispersed with PSi particles in ethanol (or DMSO) becamegradually transparent and remained yellow after even 25 minof microwave heating, which indicates there are still smallPSi particles with excellent dispersibility left in solution.From Fig. 3b, smaller nanoparticles (size �10 nm) can befound after 25 min of microwave treatment in ethanol, whichcan confirm the above discussion.

4 Conclusions In conclusions, PL of PSi particles canbe significantly enhanced in ethanol (or DMSO) undermicrowave irradiation. After filtration, luminescent PSinanoparticles fabricated in this way (sizes �60 nm) withexcellent dispersibility and strong fluorescence (QY�14.8%) can be large-scale fabricated, e.g., 4 mL solutioncontaining �4 mg of luminescent PSi nanoparticles arereadily prepared by 25 min of microwave heating. Themicrowave methodology is a promising strategy of preparingsilicon quantum dots for solar cells, fluorescent probes, anddrug carriers.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4 (online color at: www.pss-a.com) (a) UV–Vis andPL spectra of as-prepared PSi naoparticles, and its correspondingoptical image under 360 nm UV lamp (inserted picture). (b) Inte-grated PL intensity versus absorbance for PSi naoparticles in ethanoland for Rhodamine 101 under identical excitation condition.

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Figure 5 (online color at: www.pss-a.com) Histogram of PLintensity in different solvents at 150 8C.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Acknowledgements This work is funded by NationalNatural Science Foundation of China (No. 30930077 and No.31000164).

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