A study of laser-induced blue emission with nanosecond decay of silicon nanoparticles synthesized by a chemical etching method

A study of laser-induced blue emission with nanosecond decay of silicon nanoparticles synthesized by a chemical etching method
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  A study of laser-induced blue emission with nanosecond decay of silicon nanoparticlessynthesized by a chemical etching method This article has been downloaded from IOPscience. Please scroll down to see the full text article.2009 Nanotechnology 20 355703( details:IP Address: article was downloaded on 26/01/2012 at 12:06Please note that terms and conditions apply.View the table of contents for this issue, or go to the  journal homepage for more HomeSearchCollectionsJournalsAboutContact usMy IOPscience  IOP P UBLISHING  N ANOTECHNOLOGY Nanotechnology  20  (2009) 355703 (7pp) doi:10.1088/0957-4484/20/35/355703 A study of laser-induced blue emissionwith nanosecond decay of siliconnanoparticles synthesized by a chemicaletching method Abdulaziz A Bagabas 1 , 4 , Mohammed A Gondal 2 ,Mohammed A Dastageer 2 , Abdulrahman A Al-Muhanna 3 ,Thaar H Alanazi 3 and Moath A Ababtain 3 1 Petroleum and Petrochemicals Research Institute (PAPRI), King Abdulaziz City for Scienceand Technology (KACST), PO Box 6086, Riyadh 11442, Saudi Arabia 2 Physics Department and Center of Excellence in Nano Technology (CENT), King FahdUniversity of Petroleum and Minerals, PO Box 5047, Dhahran 31261, Saudi Arabia 3 National Nanotechnology Center (NNC), King Abdulaziz City for Science and Technology(KACST), PO Box 6086, Riyadh 11442, Saudi ArabiaE-mail: Received 19 May 2009Published 12 August 2009Online at Abstract Silicon nanoparticles (Si NPs), exhibiting a strong visible photoluminescence (PL), have foundmany applications in optoelectronics devices, biomedical tags and flash memories. Chemicaletching is a well-known method for synthesizing orange-luminescent, hydride-capped siliconnanoparticles (H/Si NPs). However, a blueshift in emission wavelength occurs when reducingthe particle size to exciton Bohr radius or less. In this paper, we attempted to synthesize andcharacterize H/Si NPs that emit lower wavelengths at room temperature. We proved that ourmethod succeeded in synthesizing H/Si NPs with emission in the blue region. Thewavelength-resolved and time-resolved studies of the PL were executed for H/Si NPs inmethanol (MeOH), pyridine (py) and furan, using the 355 nm pulsed radiation from a Nd:YAGlaser. In addition, excitation wavelength-dependent and PL studies were executed using thespectrofluorometer with a xenon (Xe) broad band light source. We noticed solvent-dependentPL spectra with sharp peaks near 420 nm and a short lifetime less than 100 ns. The morphologyand particle size were investigated by high resolution transmission electron microscope(HRTEM). Particles as small as one nanometer were observed in MeOH and py suspensionswhile two-nanometer particles were observed in the furan suspension. 1. Introduction Silicon nanoparticles exhibit size-dependent PL quantumyields in the visible spectral region at room temperature,and hence Si NPs have gained an enormous significance.Due to this unique PL behavior, Si NPs finds an importantplace in a broad range of applications such as optoelectronic,light-emitting devices, sensors, optical communication, photo-pumped tunable lasers and bio-tagging (Chaabane  et al  2004). 4 Author to whom any correspondence should be addressed. As the Si NPs size is a crucial factor that controls theemission wavelength of the PL, the method of preparationof the Si NPs is of great importance. Numerous methodshave been employed for the synthesis of Si NPs. Theyare (i) chemical reduction of silicon tetrachloride, (Shirahata et al  2009, Baldwin  et al  2006, Liu and Kauzlarich 2002, Mayeri  et al  2001, Heath 1992), (ii) thermal decomposition of silanes (Holmes  et al  2001), (iii) laser photolysis of silaneprecursors (Cannon  et al  1982), (iv) carbon-dioxide-laser-induced pyrolysis (Li  et al  2004, Kirkey  et al  2004), (v) laservaporization-controlled condensation (LVCC) (Germaneko 0957-4484/09/355703+07 $ 30.00  ©  2009 IOP Publishing Ltd Printed in the UK 1  Nanotechnology  20  (2009) 355703 A A Bagabas  et al et al , 2001, 2000, Li  et al , 1998, 1997), (vi) electrochemical etching (Carter  et al  2005, Nayfeh  et al  2005, Belomoin et al  2002, Akcakir  et al  2000, Belomoin  et al  2000, Yamani et al  1997) and (vii) electroless chemical etching of siliconelement (Nielsen  et al  2007). The size and optical propertiesof the synthesized silicon nanoparticles strongly depend on theprocedure of synthesis.The chemical etching method has been a well-establishedmethod for the synthesis of Si NPs. However, this method onlygives orange-luminescent Si NPs, as reported earlier (Nielsen et al  2007). However, Si NPs of different particle sizes thatyield blue, green and yellow luminescence are also synthesizedby the electrochemical etching method (Nayfeh  et al  2005,Belomoin  et al  2002). The PL wavelength depends strongly onthe size of Si NPs and the emission wavelength shifts towardsthe blue end of the visible spectrum with a reduction of theparticle size: blue luminescence corresponds to ∼ 1 nm, greenluminescence to  ∼ 1 . 7 nm, yellow luminescence to  ∼ 2 . 2 nmand orange luminescence to ∼ 3 nm (Belomoin  et al  2002).In the present work, we use the chemical etching methodto synthesize and characterize H/Si NPs of different particlesizes, suspended in various solvents. The PL study of theseH/Si NPs shows a strong emission peaked near the blueregion of the visible spectrum at room temperature. Theexcitationspectruminourworkshowsanabsorptionmaximumat 355 nm, which corresponds to an emission maximum at420 nm of these Si NPs. This excitation peak wavelength gaveus the experimental convenience of using the third harmonic of the Nd:YAG laser as our excitation source for all the samples.We also carried out the time-resolved decay of the PL emissionusing a pulsed laser. We found that the decay time is of nanosecond scale and the lifetime of the emission depends onthe nature of the solvent (MeOH, py and furan). There aresome isolated reports on blue emissions (usually called ‘fastband’) from porous silicon nanocrystals and silicon nanowires,(Belomoin  et al  2000, 2002, Wolkin  et al  1999, Lee and Peng1993, Wang  et al  1993, Sato and Hirakuri 2006, Tischler et al  1992, ˇSvrˇcek   et al  2006), which are synthesized byother methods. However, some practical constraints, suchas degradation and instability properties when preserved inatmosphere under different solvents such as MeOH, py andfuran, are yet to be resolved. Besides understanding the srcinand nature of the PL srcinating from the H/Si NPs, this studyis important for potential applications of Si NPs in Si-basedoptoelectronic devices, biomedical tags and flash memories. 2. Experimental details 2.1. Materials All chemicals were commercially available and used withoutanyfurtherpurification. Thefollowinglistshowsthechemicalsand their purity: dihydrogen hexachloroplatinate (IV)hexahydrate (99.95% metal basis, Alfa Aesar), hydrofluoricacid (analytical reagent, 48%, BDH), hydrogen peroxide(Analar, 30%, BDH), methanol (for liquid chromatography,99.8%, Merck), furan (99 + %, Aldrich), pyridine (HPLCgrade, 99 . 9 + %, Sigma Aldrich), isopropanol (general purposereagent,  > 99 . 0%, BDH) and acetone (general purpose reagent,  99 . 0, BDH). Deionized water (18 . 2 M   cm) was obtainedfrom a Milli-Q water purification system (Millipore). Thesilicon wafers were of p-type,   100   orientation, 10 cm size,boron-doped, with a resistivity of 4–8    cm and thickness of 500–550  µ m (Siltron Inc., Korea). All reactions took placein pure Teflon ware. Pyridine, before its use, was dried overpotassium hydroxide (general purpose reagent, BDH). 2.2. Preparation of silicon pieces A silicon wafer was cut parallel to its  111  orientation, with adiamond scriber, into rectangular pieces of   ∼ 1 cm × 2 . 5 cmin size. The organic contaminants present on the siliconpieces were removed by immersing and ultrasonicating themin acetone in a Branson ultrasonic bath for 10 min. Aftersonication, the silicon pieces were air-dried over a Kimwipes ® wiper. 2.3. Preparation of treatment solution (solution-I) A 1% wt / vol solution of dihydrogen hexachloroplatinate (IV)hexahydrate was prepared in deionized water. This solutionwas mixed with hydrofluoric acid and hydrogen peroxide ina 3:10:27 volume ratio, respectively, resulting in a yellowsolution. 2.4. Preparation of etching solution (solution-II) The etching solution is a mixture of hydrofluoric acid,hydrogen peroxide and methanol in a 1:3:2 volume ratio,respectively. 2.5. Synthesis of silicon nanoparticles For synthesizing Si NPs, the air-dried silicon pieces wereimmersed in solution-I until the observation of a moderaterate of hydrogen bubbles. To remove residues of solution-I,we immersed the silicon pieces in deionized water for 1 min,washed them with isopropanol, and finally dried them gentlywith a highly pure nitrogen flush. The next step was to etchthem in solution-II until strong bubbling of hydrogen gas wasobserved. Solution-II residues were removed by following thesame procedure usedfor removingsolution-Iresidues. The laststep was to suspend the Si NPs, in a solvent of our choice, byimmersing and ultrasonicating the silicon pieces. The siliconpieces were then removed from the suspension medium for air-drying. Irradiation of the suspension, with long wavelength(365 nm) light from a commercial mercury lamp, makes theeventually produced Si NPs to luminescence, depending ontheir size. The above procedure was repeated six times. 2.6. Characterization of silicon nanoparticles The size and morphology of Si NPs were investigated byHRTEM (model: JEM-2100F (HR)) at a voltage of 200 kV.The Si NPs were mounted on a carbon-coated copper gridby immersing the grid in the silicon nanoparticle suspension.The PL measurements of Si NPs were performed using thethird harmonic (355 nm) of an Nd:YAG laser and using the2  Nanotechnology  20  (2009) 355703 A A Bagabas  et al Figure 1.  HRTEM images for Si NPs suspended in MeOH. spectrofluorometer (Shimadzu RF-5301 PC) equipped with a150 W Xe lamp as the excitation source.For PL studies, 2 ml of the Si NP suspension in a quartzcuvette was irradiated with excitation wavelength at the thirdharmonic of an Nd:YAG laser (355 nm). The cuvette wasplaced on a magnetic stirrer and gentle stirring was maintainedthroughout the experiment to keep the suspension even alongthe light path. Enough care was taken to avoid the residuallaser beam entering the monochromator by using an OG590filter near the entrance slit of the monochromator. The PLsignal was collected perpendicular to the excitation directionusing the appropriate collection lenses for optimum signalthroughput. The monochromator used in this work was a 0.5 mSpex (Model 1875) coupled with a Thorn EMI (Model 9558Bwith S20 photocathode) photomultiplier. The signal was fed tothe gated boxcar averager (EG&G, Model 4422) and the signalprocessor (EG&G, Model 4402). The Q-switch trigger out of the Nd:YAG laser triggered the boxcar. The gate delay andgate width of the boxcar were chosen for the maximum signal.The boxcar was used in two modes of operation: one withstatic gate with fixed delay and gate width for collecting the PLsignalwhen the monochromator is scanning. The second modeof operation was the waveform mode, where the time gateis moving along the life span of the PL signal while keepingthe monochromator wavelength at the peak of the signal. Theformer mode was used for wavelength-resolved studies of thePL, while the latter was used for the lifetime measurements. 3. Results and discussion Prior to the actual etching process, a platinum coating is madeon the silicon pieces, which serves as a catalyst in the etchingprocess. The following chemical reaction (equation (1)) takes place when immersing silicon pieces in solution-I and resultsin coating the silicon surfaces with platinum metal:H 2 [ Pt IV Cl 6 ] ( aq ) + Si 0 ( s ) + 6HF ( aq )  −→ Pt 0 ( s ) + H 2 [ Si IV F 6 ] ( aq ) + 6HCl ( aq ) .  (1)This type of coating technique is called electrodeless seedlayer formation (Nielsen  et al  2007). Equation (1) shows that the precipitation of a platinum atom is accompanied withthe dissolution of a silicon atom in the reaction solution in Figure 2.  HRTEM images for Si NPs suspended in furan. Table 1.  d-spacing calculated from HRTEM for Si NPs suspended inMeOH.d-spacingcalculated fromHRTEM, (˚A)d-spacing inbulk Si, (˚A)Millerindices ( hkl )assignment3.13 3.13 (111)1.88 1.92 (220)1.63 1.63 (311)1.32 1.35 (400)1.25 1.24 (331)1.11 1.11 (422) the form of a hexafluorosilicate ion. The platinum coatingcatalyzes the etching of silicon when immersed in solution-IIby facilitating the formation of hydrogen gas on its surface.The existence of hydrogen peroxide in the etching solutionleads to the formation of H/Si NPs, as shown in equation (2): Si 0 ( s ) + 6HF ( aq ) + H 2 O 2 ( aq ) −−−−−→ H 3 COH H 2 SiF 6 ( aq ) + H 2 ( g ) + H / Si NP ( s ) + 2H 2 O ( l ) .  (2)Moreover, hydrogen peroxide helps to produce Si NPswith a homogeneous surface where each silicon atom on thesurface is only capped with one hydride (Yamani  et al  1997).Figure 1 shows the HRTEM micrographs for the Si NPssuspended in MeOH. In figure 1, Si NPs clearly adopt a spherical shape and their diameters range between 1 and2 nm. The lattice plane fringes of Si NPs, in figure 1, appear as parallel arrangements with different orientations in eachparticle. The distance between lattice plane fringes in differentparticles was used to calculate the d-spacing values, whichare in excellent agreement with their corresponding values forbulk silicon crystals (table 1). Figure 2 shows spherical Si NPs having a size of 2 nm. Such observation agrees with theobserved yellow luminescence of Si NPs suspended in furan.Figure 3 shows spherical Si NPs, suspended in py, with 1 nmsize. However, some particles have a size range between 10and 15 nm. The 1 nm particles are responsible for the blueluminescence observed for py suspension. The lattice planefringes were also observed in different Si NPs, suspended inpy, for calculation of d-spacing values. Table 2 presents thed-spacing values calculated from HRTEM and compares themwith those of bulk silicon, indicating the formation of siliconnanocrystals with cubic lattice.3  Nanotechnology  20  (2009) 355703 A A Bagabas  et al Figure 3.  HRTEM images for Si NPs suspended in py. Table 2.  d-spacing calculated from HRTEM for Si NPs suspended inpy.d-spacingcalculated fromHRTEM (˚A)d-spacing inbulk Si (˚A)Millerindices  ( hkl ) assignment3.17 3.13 (111)1.85 1.92 (220)1.59 1.63 (311)1.32 1.35 (400)1.27 1.24 (331)1.06 1.11 (422) Bulk silicon is an indirect bandgap semiconductor, andhence does not exhibit efficient emission due to a lack of radiative recombination of the electron–hole pair. Moreover,for the bulk silicon the bandgap is nearly 1.1 eV, which fallsin the infrared region. In addition, the defects present insilicon act as an efficient quencher for the already feebleemission process. However, when the silicon particle size getsto the nanoscale, the probability of carriers to find structuraldefects becomes too small and quantum confinement relaxesthe selection rule, which in turn results in promotion of the radiative recombination process and prevention of thenon-radiative recombination process. These changes causebandgap (  E  g ) widening and emission shifting near the visibleregion. There are two distinct types of PL observed in thecase of silicon: one emission is bi-exponential long-livedluminescence of the order of tens of microseconds near the redregion and the second one shows a much shorter lifetime of the order of nanoseconds, near the blue region (Huisken  et al 2003).The dependence of the emission wavelength on theparticle size is evident when the particle size gets to thenanolevel. Mie scattering theory, which predominantlyoccurs when the particle size becomes much smaller than theinteracting light wavelength, could explain this size-dependentelectronic transition. In general, the electric dipole term isresponsible for the electronic transition and dominates in thelimit that  R /λ  →  0, where  R  is the particle size. In sucha situation, crystallites behave like a molecule as far as theelectromagneticradiationisconcerned. Inthequantumregime,the interaction Hamiltonian (  H  int ) between the radiation and Figure 4.  PL spectra of silicon nanoparticles (Si NPs) in methanolrecorded with a spectrofluorometer for five different excitationwavelengths starting from 315 to 355 nm. the electrons in the Si NPs can explain the size-dependentphenomena.  H  int  is the function of electric field vectorpotential and the momentum vector of the electrons. Whenthis  H  int  becomes small (due to the particle size), it will besmall enough to be treated with familiar perturbation theoryfrom which we deduce that the electric dipole transitions arevery strong as they appear in the first order (Brus 1986).The semiconductor silicon nanoparticles, with sizescomparable to or below their exciton Bohr radius, havedistinctive electronic and optical behaviors due to theexciton quantum confinement phenomenon. For such areason, ‘quantum dots’ suitably describe these semiconductornanoparticles, which absorb light at a specific wavelengthand emit it at a longer one. Therefore, recording the PLspectrum is of paramount importance for estimating the size of nanoparticles and their characteristics for various applications.To this end, we carried out PL studies of the synthesized SiNPs by using 355 nm radiation generated from an Nd:YAGlaser and the set-up described earlier. Furthermore, before thelaser-induced fluorescence studies, we used a conventional Xelamp spectrofluorometer technique to record PL spectra.Figure 4 depicts a typical PL spectrum, recorded witha spectrofluorometer at different excitation wavelengths, of the H/Si NPs suspended in methanol. As shown in figure 4,we varied the excitation wavelengths from 315 (3.93 eV) to355 nm (3.49 eV). The PL spectra for Si NPs in this figure havea broad band centered at 420 nm (2.94 eV) in the blue regionwhen excited with 355 nm (3.49 eV). Blue light emissions or4
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