November 2008 EPL, 84 (2008) 47001 doi: 10.1209/0295-5075/84/47001
www.epljournal.org
CdSe nanoparticles synthesized by laser ablation N. G. Semaltianos1(a) , S. Logothetidis2 , W. Perrie1 , S. Romani1 , R. J. Potter1 , M. Sharp1 , P. French1 , G. Dearden1 and K. G. Watkins1 1 2
Department of Engineering, University of Liverpool - Brownlow Hill, Liverpool, L69 3GH, UK, EU Department of Physics, Aristotle University of Thessaloniki - Thessaloniki, GR-54124, Greece, EU received 10 September 2008; accepted in final form 9 October 2008 published online 22 October 2008 PACS PACS PACS
79.20.Ds – Laser-beam impact phenomena 81.07.Ta – Nanoscale materials and structures: fabrication and characterization: Quantum dots 78.67.Hc – Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures: Quantum dots
Abstract – Nanoparticles of the II–VI semiconductor CdSe were synthesized by laser ablation (387 nm, 180 fs, 1 kHz, pulse energy = 4 µJ (fluence = 1 J/cm2 )) of the target material in methanol. The nanoparticles size distribution follows a log-normal function with statistical median diameter in the range of ≈ 5–6 nm. Their crystalline structure is the same as that of the bulk material and are Se rich (Cd : Se percentage ratio ∼ 0.8 : 1). Strong, spectrally “clean” and relatively narrow photoluminescence emission from the produced nanoparticles was detected in the green (∼ 552 nm). c EPLA, 2008 Copyright
Introduction. – Particles with dimensions in the nanometer range, made out of II–VI semiconductors are important nanomaterials for optoelectronic applications [1]. This is because size-dependent quantum confinement makes the properties of the nanoparticles tunable based on their size. When the size of the nanocrystallites is close to or smaller than the exciton Bohr radius within the corresponding bulk material they are termed as quantum dots and show very special physical and chemical properties [2]. The optical, electrical, mechanical and structural properties of organic polymers can be tuned by the incorporation into them of nanoparticles resulting in the manufacturing of nanoparticle polymer composites. Among the most common II–VI semiconductor nanoparticles, high-quality nearly monodisperse (< 5% rms in diameter) CdSe nanocrystals in solution have been made following synthetic chemical methods including co-dissolvation of dimethylcadmium Cd(CH3 )2 and Se powder in a tri-alkyl phosphine (–butyl or –octyl) and subsequent injection of the solution into hot coordinating solvent trioctyl phosphine oxide (TOPO) [3,4], the reaction between acetates or sulfates of Cd and Na2 SeSO3 in the presence of complexating agents using microwave irradiation [5] or the reaction of sodium citrate and (a) E-mail:
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cadmium perchlorate, the subsequent addition into the solution of dimethylselenourea and final solution heating also in a microwave oven [6]. Recently, CdSe nanoparticles have also been synthesized by laser ablation in liquid environments (usually using long-pulse laser sources at high fluences) [7]. In this letter we report for the first time the synthesis of CdSe photoluminescent nanoparticles by laser ablation of the target material in a liquid environment (methanol) using ultrashort laser pulses. Laser ablation offers an alternative method for nanoparticle synthesis as compared to chemical or other methods. Experimental details. – In this work a Ti:Sapphire femtosecond laser (Clark-MXR 2010) (wavelength (λ) = 387.5 nm produced by frequency doubling of the 775 nm main laser beam using a BBO crystal, pulse width = 180 fs, frequency = 1 kHz) based on the chirped pulse amplification (CPA) technique [8] was used for ablation of the target material. A schematic diagram of the experimental setup for the generation of nanoparticles by laser ablation of the target material in liquid, is shown in fig. 1. The target material in the form of a square with dimensions of ∼ 9 × 9 mm, thickness ∼ 2 mm, was positioned inside a cuvette (inner dimensions 10 × 10 × 35 mm) (Suprasil 300 quartz) and held firmly in place with a flexible thin teflon ring which was placed at the bottom of
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N. G. Semaltianos et al. Yvon HR800 system (pumping beam at λ = 514.48 nm) with a CCD detector. The colloidal solution was also characterized by UV-vis spectrophotometry.
Fig. 1: Schematic diagram (top view) of the experimental setup used for the synthesis of CdSe nanoparticles by laser ablation of the target material in a liquid environment.
the cuvette, acting as a “spring” and pushing the sample onto its vertical wall. The cuvette was filled with liquid and sealed. It was then placed onto an Aerotech moving stage which could be moved in the x, y and z directions with accuracy of ±0.5 µm. The laser beam was focused onto the target material surface using a lens (focal length 5 cm). Laser ablation was carried out by scanning the sample (and thus the beam on its surface) with speed of 1 mm/s in a rectangular pattern with dimensions of 5 × 3.3 mm and pitch of 0.005 mm (meander fashion scanning). No bubbles were observed to adhere onto the sample surface during ablation which would have otherwise shielded or scattered the laser radiation. The laser fluence on the material surface was kept low at 1 J/cm2 (laser pulse energy 4 µJ). Methanol was used as the liquid. The material was CdSe (Eg = 1.739 eV (∼ 713 nm)) bulk nominally undoped semiconductor single crystal with 1 : 1 ratio of the two elements in the binary alloy, grown by the vertical Bridgman method. Ablation was carried out for ∼ 1 hour and formation of nanoparticles in the solution could be confirmed by the slight change of the colour of the solvent during ablation. All measurements were performed one day after the preparation of the colloidal solution. Atomic force microscopy (AFM) imaging of the nanoparticles was performed by drying out droplets of the colloidal solution onto clean hydrophilic silicon substrates, using a Veeco CP-II instrument in non-contact mode with Si cantilevers with a given radius of curvature less than 10 nm (Tap300). Transmission electron microscopy (TEM) imaging of the nanoparticles was performed by drying out solution droplets onto carbon-coated copper grids and using a high-resolution JEOL JEM-3010 instrument equipped with EDS Genesis 4000 system to enable also acquisition of energy-dispersive X-ray spectroscopy (EDX) spectra. Photoluminescence spectra were measured from the nanoparticles colloidal solution using a Jobin
Results and discussion. – Investigations and data of femtosecond laser ablation and nanoparticle formation of CdSe are still lacking from the literature. Thus, in this paper we speculate that during our experiments the ablation of the material proceeds similarly to the ablation of other semiconductors such as Si, GaAs and InP for which detailed investigations of femtosecond laser ablation have already been well documented [9–11]. This will provide us with an intuitive understanding of the processes and mechanisms involved in the formation of CdSe semiconductor nanoparticles by laser ablation. Under the present conditions of ablation, the laser radiation which is focused onto the surface of the solid target passes through the liquid unaffected and the laser energy is first given to the carriers of the semiconductor via linear absorption which undergo interband transitions from the valence to the conduction band creating free electrons in the conduction band. The excitation of a highly dense electron-hole plasma leads to a destabilization of the lattice due to the rapid exchange of energy with the carriers (via electron-phonon coupling) in the form of mechanical work [10]. It is also possible that there might be an exchange of a small percentage of energy in the form of caloric heat. According to a model which has been developed recently, it is believed that the mechanism of formation of nanoparticles in the case of femtosecond laser ablation of a solid target is different than in the case of nanosecond laser ablation. This model predicts that the rapid transfer of energy from the carriers to the lattice due to the ultrashort pulse (isochoric lattice heating) has as a result that the material undergoes ultrafast non-thermal heating becoming a superheated fluid [9,12]. The buildingup of extreme thermoelastic pressure leads to material ejection directly in the form of nanoparticles (fragmentation model) [13–15]. However, there are observations of nanoparticles with elemental compositions which deviated from the stoichiometry of the target material (formed together with the ones which retained it) in thin films grown by femtosecond pulsed laser deposition of multielement targets (alloys) [16,17]. Besides, nanoparticles are observed with fundamentally different emission properties, formed during femtosecond laser ablation of single-element targets, as compared to the droplets directly emitted from the target during nanosecond laser ablation [18]. These lead to the conclusion that the mechanism of nanoparticle formation even in the case of femtosecond laser ablation might be similar to the case of nanosecond laser ablation. In the latter case the nanoparticles are thought to be formed out by condensation in the vapour phase due to the adiabatic cooling of the expanding plasma plume. In our experiments during laser ablation of the target materials a bright white colour “spot” could be clearly seen to be
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formed over the point of incidence of the laser beam onto the material surface. This spot corresponds to a plasma plume of ablated species from the material because of the laser beam-material interaction. The femtosecond pulse energy of 4 µJ which we use causes ablation of the material via sublimation. By taking into account a 5 × 3.3 mm ablated area, scanning speed of 1 mm/s (pitch 0.005 mm) and a measured ablation depth of ∼ 100 nm, the surface enthalpy per pulse of the material during ablation is estimated equal to ∼ 1.3 × 109 J/kg which is by three orders of magnitude larger than the sublimation (and therefore vaporization) enthalpy of 1.7 × 106 J/kg for CdSe [19]. We believe that in our experiments and under the present conditions of ablation the nanoparticles are formed out by nucleation in the vapour phase. This mechanism of nanoparticle formation —by nucleation in the vapour phase— is also confirmed by the elemental analysis of the synthesized nanoparticles as will be explained latter in the paper. Methanol as an organic solvent, upon undergoing extreme heating at the place of incidence of the laser beam onto the material surface may dissociate and the species produced may subsequently react chemically with the ablation plume and the produced nanoparticles [20]. Since the target material is immersed in the liquid, the liquid which is in contact with the material at the point of incidence of the laser beam onto the material surface is heated at the same very high temperature as the material surface. This results in the creation of vapours of the liquid. The loss of energy of the expanding plasma plume for the vaporization of the surrounding liquid and the additional pressure to the plume from the vapour of the liquid have as a result a decrease of the maximum expansion volume of the plume in a liquid environment as compared to air or vacuum expansion. This results in a higher nucleation rate resulting in the formation of a distribution of nanoparticles with a lower average diameter than in the case of air or vacuum expansion. The colloidal solution was first characterized by UV-vis spectrophotometry and its absorption curve is shown in fig. 2. Because of the so-called size quantization effect, the first excited electronic state (the fundamental absorption edge) of a quantum dot (Eg ) is expected to show a shift to lower wavelengths (higher energies) with respect to the band gap absorption edge (Egbulk ) of the corresponding bulk material [2]. This energy shift for a quantum dot with radius R is given by the relation 2 2 π 1.8e2 ∆E ≡ Eg − Egbulk ∼ − , (1) = 2m∗ R2 4πεε0 R
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Fig. 2: Optical absorption spectrum from nanoparticle colloidal solutions. The upper inset shows the theoretical dependence of the nanoparticle radius on the wavelength according to the size quantization model (eq. (1)).
was showing a weak absorption which, although it was starting at a wavelength below the one corresponding to the band gap of the bulk material, was however lying very close to it. The absorption curve resembled those measured from colloidal solutions of CdSe nanoparticles synthesized by long-pulse laser ablation [7]. This indicates somehow broad-band gap and therefore nanoparticle size distribution according to the size quantization model. The absorption was increasing very slowly in the visible region (an increase by ∼ 7% between 700 and 400 nm). Below ∼ 450 nm the absorption was exhibiting an increase with a much faster rate (increasing to 100% at 200 nm). By taking into account the graph of nanoparticle radius vs. wavelength, the broad band centered at ∼ 602 nm as well as the sudden increase of absorption at wavelengths below ∼ 450 nm imply nanoparticles with average diameters below 10 nm. These results about nanoparticle average diameters are also supported by TEM imaging of the produced nanoparticles. Detailed information about the size distribution of the nanoparticles in the produced colloidal solution was obtained by TEM images (fig. 3(a)). The histogram of particle size distribution (shown in fig. 3(b) by counting approximately 350 particles in the image of fig. 3(a) as well as in images of particles ensembles obtained on other areas on the grid) is described quite well by a log-normal function:
where m∗ = me mh /(me + mh ) is the electron-hole pair reduced mass [21]. R vs. wavelength described by the 1 ln2 (d/d0 ) 1 1 1 above theoretical model is plotted in the inset of fig. 2 (by exp − (2) h(d; d0 , σ) = √ considering: me = 0.119, mh ≡ mhh = 0.820 and ε = 10.2) 2 ln2 σ 2π d ln σ assuming electron (e) to heavy-hole (hh) transitions which are the most probable. The colloidal solution was not with median diameter of d0 ≈ 6.1 nm and geometrical exhibiting a “sharp” well-defined absorption onset but it standard deviation σ = 1. 47001-p3
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Fig. 3: TEM image of CdSe nanoparticles produced by laser ablation in methanol (a) and corresponding size histogram (b). The inset in (a) shows a typical EDX spectrum.
Fig. 4: AFM image of CdSe quantum dots, assembled onto a clean silicon substrate.
An AFM image of an area onto the substrate where nanoparticles only with sizes smaller than 10 nm have been accumulated, is shown in fig. 4 (this was inferred by measuring the height and apparent width of the nanoparticles in the image, accounting for the tip effect).
These nanoparticles with sizes smaller than the exciton length in the bulk material are behaving as quantum dots and are mainly responsible for photoluminescence emission in the green as will be analyzed below. During production of the nanoparticles by laser ablation the newly produced nanoparticles lie in the laser beam and their interaction with the laser radiation via “interpulse” absorption may contribute to a reduction of the width of the nanoparticle size distribution and further experiments to elucidate this effect are in progress and will be reported in a future paper. High-resolution TEM imaging of the nanoparticles reveals their crystalline structure. From fig. 5 the lattice spacing of the nanoparticles is measured equal to 3.34 ˚ A which corresponds to the distances between the {011} or {101} planes of the wurtzite structure (hcp lattice) of bulk CdSe within ∼ 1.5% error [22]. EDX analysis of the nanoparticles (a typical spectrum is shown in the inset of fig. 3(a)) indicated that they are slightly Se rich with stoichiometric ratio of Cd : Se ∼ 0.8 : 1. This can be understood by the fact that Se is more volatile than Cd since it has lower melting temperature and heat of vaporization [23]. Thus, during material vaporization by the laser radiation, the element with the higher volatility will vaporize easier (Se more easily than Cd) making the produced nanoparticles which are formed out by nucleation in the vapour phase, rich in that element. This also confirms the mechanism of formation of the nanoparticles, which was proposed earlier. Finally, photoluminescence emission (at room temperature) was detected from the nanoparticles in the colloidal solution. In fig. 6 the spectrum from the nanoparticles themselves was obtained by substracting the spectrum
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any trap emission as in the case of some nanoparticles synthesized chemically [1], indicating the high quality of the synthesized nanoparticles. This also indicates that 4000 any residual impurities or defect centers which might have been introduced into the target material during growth do not contribute to the introduction of any radiative defects 3000 or centers into the produced nanoparticles. However, these impurities may introduce non-radiative defects 2000 which are deleterious for the luminescence efficiency. Assuming that the luminescence spectrum from the nanoparticles ensemble which we measure is a convolution 1000 of Lorentzian luminescence spectra from each nanoparticle with amplitudes parametrized by their abundances which are determined by their size distribution, we can develop 0 500 550 600 650 700 750 800 a simple model to describe theoretically the luminescence wavelength (nm) spectrum of fig. 6. In this case the luminescence spectrum from a nanoparticle with diameter d, peaked at energy Fig. 6: Photoluminescence spectrum from CdSe nanoparticles, equal to its corresponding band gap energy Eg (d) (given bulk material and theoretical curve calculated numerically by eq. (1)) and width of ∆E, is described by the relation using eq. (5). ∆E 1 Inp (E; Eg (d), ∆E) = . (3) π (E − Eg (d))2 + (∆E)2 from the pure solvent (methanol) from the spectrum of the colloidal solution. The luminescence from the nanopar- This relation assumes that all nanoparticles are spherical ticles appears as an asymmetric band with FWHM ∼ = and have the same lattice constant. The probability 0.197 eV (∼ 50 nm), centered at the green ∼ 552 nm for radiative excitonic transitions from each nanoparticle (2.246 eV) and extending towards higher wavelengths is described by their oscillator strength [26]. Assuming (lower energies) due to the somehow broad size distrib- that the luminescence is emitted from the recombination ution of nanoparticles with radii above the exciton Bohr of bound excitons in the nanoparticles, in the strong radius. The width of the luminescence band from the CdSe confinement regime (where R 2α as in the present case) B nanoparticles synthesized here by laser ablation compares for spherical particles the oscillator strength is given by the well with the width of ∼ 30 nm of the luminescence relation [27] peak at room temperature from CdSe nanoparticles with 1 3 αB 3 diameters of < 3.2 nm [24] or ∼ 25 nm at room temperfex = 1232.2 3 fex , (4) f (d) ∼ = 4 R d ature from 3.5 nm nanoparticles [3] or ∼ 22 nm at 10 K from 6.21 nm nanoparticles [25], synthesized by chemical where f is the oscillator strength of the exciton in the ex methods. However, the luminescence emissions from those bulk material (f = 0.00225) [28] and α = 5.9 nm. The ex B nanoparticles ensembles appear as rather symmetric peaks luminescence emission from the nanoparticles ensemble is due to the narrower nanoparticle size distributions as then given by the relation compared to the laser ablation synthesized nanoparticles. ∞ The blue shift of the nanoparticles luminescence from the f (d)h(d; d0 , σ)Inp (E; Eg (d), ∆E) dd. (5) I(E) = luminescence of the bulk material (shown in the same 0 figure) is 0.507 eV. By taking into account in a first approxThis relation assumes that the absorption coefficient is imation the theoretical plot in the inset of fig. 2 (R vs. the same for all nanoparticles, independent of their size. wavelength), the wavelength of 552 nm implies nanopartiThe dotted line in fig. 6 corresponds to fitting eq. (5) cles with average diameter of 5.4 nm which compares well to the experimentally measured spectrum by taking into with the average diameter of ∼ 6.1 nm determined from account the nanoparticles size distribution described by the TEM images. The small difference might be due to an eq. (2) and the size-dependent band gap energy described overestimation of the average diameter of the nanoparby eq. (1). Fitting parameters are σ = 1.03, d0 = 5.4 nm ticles from TEM images because very small particles are and ∆E = 100 meV. not easily distinguished in the images (because they do Conclusions. – In conclusion, CdSe quantum dots not appear with high enough contrast) and thus they are not included in the statistical distribution. If we keep were synthesized by femtosecond laser ablation (387 nm, this in mind, the measurement of the photoluminescence 180 fs, 1 kHz, pulse energy = 4 µJ (fluence = 1 J/cm2 )) of spectrum of the nanoparticle colloidal solution seems as a the target material in methanol. They emit strong, specmore reliable method for estimating the average nanopar- trally “clean” and relatively narrow photoluminescence ticle diameter. The luminescence from the nanoparticles (at the green) and therefore are promising candidates for also appears spectrally “clean”, without the presence of optoelectronic applications. nanoparticles bulk material theory, Eq. 5
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N. G. Semaltianos et al. ∗∗∗ NGS acknowledges support by a Marie Curie Fellowship of the European Community, Grant No. MTKD-CT-2004517165. REFERENCES [1] Gaponenko S. V., Optical Properties of Semiconductor Nanocrystals (Cambridge University Press, Cambridge) 1998. [2] Brus L. E., J. Chem. Phys., 80 (1984) 4403. [3] Murray C. B., Norris D. J. and Bawendi M. G., J. Am. Chem. Soc., 115 (1993) 8706. [4] Peng X. G., Wickham J. and Alivisatos A. P., J. Am. Chem. Soc., 120 (1998) 5343. [5] Zhu J., Palchik O., Chen S. and Gedanken A., J. Phys. Chem., 104 (2000) 7344. [6] Rogach A. L., Nagesha D., Ostrander J. W., Giersig M. and Kotov N. A., Chem. Mater., 12 (2000) 2676. [7] Ruth A. A. and Young J. A., Colloids Surf. A: Physicochem. Eng. Aspects, 279 (2006) 121. [8] Strickland D. and Mourou G., Opt. Commun., 56 (1985) 219. [9] Cavalleri A., Sokolowski-Tinten K., Bialkowski J., Schreiner M. and von der Linde D., J. Appl. Phys., 85 (1999) 3301. [10] Stampfli P. and Bennemann K. H., Phys. Rev. B, 49 (1994) 7299. [11] Bonse J., Wiggins S. M. and Solis J., J. Appl. Phys., 96 (2004) 2628. [12] Perez D. and Lewis L. J., Phys. Rev. Lett., 89 (2002) 255504-1.
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