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J. Phys. Chem. C 2010, 114, 17374–17384

Optical Properties of Selenium Quantum Dots Produced with Laser Irradiation of Water Suspended Se Nanoparticles S. C. Singh,*,† S. K. Mishra,‡ R. K. Srivastava,‡ and R. Gopal§ National Centre for Plasma Science & Technology, Dublin City UniVersity, Dublin-9, Ireland, Department of Electronics & Communication, UniVersity of Allahabad, Allahabad-211002, India, and Laser Spectroscopy and Nanomaterials Lab., Department of Physics, UniVersity of Allahabad, Allahabad-211002, India

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ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: August 25, 2010

Semiconductor quantum dots (QDs) and their assemblies have shown potential research interest due to their size dependent optical and electronic properties. Laser irradiation of larger sized semiconductor nanoparticles (NPs) suspended into liquid media is an easy, quick, versatile, environmental friendly, and rapidly growing method for the synthesis of QDs through melting/vaporization and fragmentation mechanisms. Most of the available reports in this field are related to the laser induced modification of shape, size, and morphology of noble metal nanoparticles, while only few exist for semiconductors. Synthesis of selenium QDs using laser induced melting/vaporization of water suspended selenium NPs of larger size and studies of their irradiation time or size dependent optical properties are subjects of current investigation. The fundamental wavelength of a pulsed nanosecond Nd:YAG laser is used for the irradiation of water suspended 69 nm average sized NPs for different times of irradiation. UV-visible absorption, XRD, TEM, and PL spectroscopic methods are utilized for the characterization of as synthesized QDs and raw NPs. Size and hence optical properties of produced selenium QDs are found to be highly dependent on the time of irradiation. The size of the produced selenium QDs follows a second order exponential decay function of irradiation time, while the rate of size reduction, da/dt, is directly dependent on the diameter, a, of the instantaneous QDs, very similar to the radioactive decay model. Laser irradiation causes transformation of β-Se NPs of 69 nm diameter to R-Se QDs of different sizes depending on the time of irradiation. We have achieved a minimum 2.74 ( 2.32 nm diameter of selenium QDs for 15 min laser irradiation and reported that almost 3.75 ( 0.15 nm size may be the quantum confinement limit for Se QDs. Surface defect density of the selenium QDs increases, while defect/electron trap level energy decreases, with the time of laser irradiation. 1. Introduction Semiconductor nanocrystals, also known as quantum dots (QDs), have attracted researchers and scientists in the past few decades due to their unique size dependent electronic and optical properties. Semiconductor QDs are very attractive as biological labels,1,2 due to their small size, emission tunability, superphotostability, and longer PL decay times in comparison to dyes. One of the major challenges is to obtain colloidal solution of QDs having surfaces free from chemical contamination for their biological and medical applications. Laser ablation of solid targets at the solid-liquid interface has attracted the research community and provided a green, cheap, easy, and versatile route for the synthesis of various metallic,3 alloy,4 and metal oxides/hydroxide5-8 nanostructures in recent times. Laser ablation at the solid-liquid interface has advantages over that of the vacuum or gas phase ablation because (a) liquid provides strong confinement of expanding plasma plume, which induces excess temperature and pressure inside the plasma plume and synthesis of smaller size of nanoparticles, and (b) suitability to use surfactants or polymers as capping agent to prevent aggregation and agglomeration to get a smaller size of nanoparticles and nanocomposites. Laser induced melting and fragmentation of bulk or larger sized particles dispersed into liquid media was * Author to whom any correspondence should be addressed. Phone: +353-1-7007787. E-mail: [email protected]. † Dublin City University. ‡ Department of Electronics & Communication, University of Allahabad. § Department of Physics, University of Allahabad.

recently used for resizing and reshaping of materials. Link et al. have used laser pulses to change gold nanorods into nanoparticles using this approach.9 Size reduction of metal nanoparticles,10 synthesis of AuAg core shell particles from their colloidal solutions,11 conversion of Ag nanosphere into nanoprisms,12 melting of network gold nanostructures and twisted nanorods,13 change in the structure of the Au/Ag core/shell nanoparticles,14 synthesis of Au/Pd and Ag/Pd alloy from mixtures of their colloidal solutions,15 and nanoscale soldering of nanoparticles16 are some recent achievements of this approach. In spite of several inspiring and exciting reports on noble metal nanomaterials, there are very few works on the size, shape, and surface modifications of semiconductor particles. Recently, Usui et al. have used this approach for the size, surface, and optical property modifications of indium tin oxide nanoparticles.17 Selenium, Se, is a promising low dimensional semiconductor material with direct band gap of 1.7 eV, low melting point (∼490 K) with catalytic activity toward organic hydration and oxidation reaction, intrinsic chirality, high refractive indices, and large birefringence. It has interesting and considerable properties such as photovoltaic effect, high photoconductivity in the entire visible range (∼8 × 104 S cm-1), relatively large piezo- and thermoelectricity, and nonlinear optical responses.18-20 Selenium has also been used as a key material for photographic exposure meters, rectifiers, and xerography.21,22 Owing to its potential scientific and technological applications, synthesis of Se nanoparticles and study of its size dependent optical properties are greatly demanded nowadays.

10.1021/jp105037w  2010 American Chemical Society Published on Web 09/16/2010

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Figure 1. Experimental arrangement and photographs of colloidal solution of selenium nanoparticles produced after different times of ablation in the inset (from left to right: pure water, 2, 4, 6, 8, and 10 min).

Nanometer-sized Se particles have been synthesized by several wet chemical routes such as acid induced,23surfactant assisted,24 decomposition,25 microwave irradiation,26 and hydrothermal routes27 yielding larger (>30 nm) sized particles, while there are comparatively fewer reports on the physical synthesis of Se nanomaterials. Recently, Jiang et al.28 have synthesized selenium nanorods and particles using thermally controlled pulsed laser ablation in a furnace. Similar to the noble metal NPs, melting and fragmentation of larger sized selenium particles dispersed into liquid media can quickly generate smaller sized, high quality selenium NPs without any chemical contamination in a simple and one step process. Present communication reveals laser irradiation of 69 nm average sized β-Se NPs suspended into double distilled water for different irradiation times to modify their size, phase, and surface morphology. We have observed several exciting novel findings and proposed a possible synthesis mechanism. It is found that the rate of decrease of particle size at any time is directly proportional to the size of the particle at that instant, which is one of the new physical insights of the current work in the interest of physical chemists and chemical physicists. To the best of our knowledge, this is the first report on the synthesis of selenium QDs using laser irradiation of larger sized particles suspended into liquid media. Variation of the optical properties, size, band gap of synthesized particles with irradiation time, and their intercorrelation are discussed. 2. Experimental Section The experimental arrangement for the synthesis of selenium QDs using laser irradiation of water suspended particles is depicted in Figure 1. Laser irradiation of selenium (average size 69 nm, SpecPure, Johnson Matthey) nanopowder dispersed into double distilled water (Merck & Co., Inc.) was done for different times of irradiation. In a typical experimental procedure, 2 g of selenium powder was dispersed into 100 mL of double distilled water contained in a 200 mL boiling tube (Borosil glass), which was equivalent to 25.3 × 10-2 mol/L. Fundamental (1064 nm)

output from a pulsed Nd:YAG (Spectra Physics, Quanta Ray, U.S.A.) laser operating at 20 mJ/pulse energy, 10 ns pulse width, and 10 Hz repetition rate was focused at the center of the solution column using a 10 cm focal length quartz lens through 7.5 cm of liquid media. Energy of the laser was measured before and after the solution in order to calculate absorbed laser energy. To prevent gravitational settlement of the raw powder, a magnetic stirrer was simultaneously used during irradiation. The samples are collected in the sealed bottle after 2, 4, 6, 8, 10, and 15 min of laser irradiation. UV-visible absorption spectra of raw Se powder and as synthesized colloidal solution obtained after different times of irradiation were recorded with a PerkinElmer Lambda-35 UV-visible double beam absorption photometer, while a Perkin-Elmer LS-55 spectrofluorometer was utilized for PL investigations. A drop of colloidal solution of selenium NPs was placed on the carbon coated copper grid and dried by placing it below the tungsten lamp. Transmission electron microscopic images of solution of QDs obtained after 6 min of ablation were recorded with a Technai G20 Stwin transmission electron microscope. X-ray diffraction patterns for raw selenium powder and those obtained after 10 min of laser irradiation were recorded using a CuKR line (λ ) 1.5406 Å) of a Rigaku D/Max 2200 X-ray diffractometer. 3. Results 3.1. Physical/Visual Observations during Laser Irradiation of Water Suspended Se Particles. Strong focusing of pulsed laser beam at the center of the liquid column, containing bulk selenium powders, causes optical breakdown of water molecules as well as melting/vaporization and may be the fragmentation of selenium particles with the generation of intense sound and light emission close to the focal point. Intensities of sound as well as light emission drastically decrease with the passage of ablation time. It is assumed that sound comes from the fragmentation of selenium powder and collapse of the cavitation bubbles, while the light emits from the laser produced

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Figure 2. (A) XRD pattern and (B) pictorial representation of atomic arrangement of selenium nanoparticles before and after laser irradiation.

selenium plasma. To confirm this, we have done the same experiments in the absence of the selenium powder and observed comparatively low sound and light emissions. In the absence of Se powder, sound comes from the collapse of water bubbles. The sound and light intensities produced during the experiment increased with the increase of the bulk Se powder density in the focal volume, by just increasing the stirring speed, which was further decreased similarly with the passage of ablation time. It shows that when the particle size is larger, there is higher interaction between laser light and selenium powder resulting in comparatively elevated absorption of laser photons and consequently melting and fragmentation of a larger sized particle into several smaller ones. Intensities of sound and light emissions are directly related to the collision cross section between photons and raw Se particles. With the decrease of the size of particles, the cross section of laser matter interaction decreases, which causes a decrease in the sound and light intensities. The rate of decrease of the particle size depends on the absorption of laser photons by the particles, which itself depends on the particle dimensions. Intensities of light and sound at a particular instance for given laser irradiance can provide direct evidence of rate of ablation at that instance. After a long time, 15 min in this case, it is observed that there are almost no sound and light emissions, which suggests that there is almost negligible interaction between laser and particles. Now it can be assumed that almost all the particles in the solution have reached the critical particle dimension. After 15 min of ablation, when we further increased the stirring speed, larger particles settled at the bottom due to larger gravity, came to the focal volume, and resulted in an increase of sound as well as light intensities. Optical photographs of colloidal solution of selenium nanoparticles obtained after different times of laser irradiation are illustrated in the inset of Figure 1. For the better comparison of

color, the photograph of distilled water is also placed along side the colloidal solution of Se nanoparticle samples. The color of the colloidal solution of produced selenium nanoparticles changes from slight reddish to deep orange with the increase of irradiation time. Variation in the color with ablation time suggests that the critical size of nanoparticles is down to quantum confinement regime of the selenium nanoparticle. It is well-known that the size dependent color of colloidal nanoparticles is observed, if sizes of the nanoparticles are below the quantum confinement regime of the particular material. As the produced particles are below the quantum confinement regime, we will refer to them as quantum dots (QDs) hereafter rather than nanoparticles (NPs). 3.2. Structural and Microscopic Characterization of Raw Se NPs and Laser Produced Se QDs. X-ray diffraction patterns of selenium nanoparticles, before and after laser irradiation, are displayed in Figure 2. Selenium nanoparticles before laser irradiation have diffraction peaks having 2θ values at 23.523, 29.87, 41.36, 43.89, 45.40, 51.66, 56.13, and 61.66, which are assigned to (310), (122j), (114), (430), (611), (242), (315), and (235j) planes of β selenium having lattice parameters a, b, and c of 9.05, 9.07, and 11.61, respectively, with crystallographic orientation R, β, and γ values of 90°, 93.13°, and 90°, respectively.29 Diffraction peaks, 2θ ) 20.54, 21.9, 24.7, and 28.8°, of the selenium quantum dots produced after 10 min of laser irradiation are indexed for (112j), (120), (202j), and (221j), respectively, planes of the R-Se with comparatively lower degree of crystallinity29 and the same lattice parameters as β-Se but with a slight change in one of the three (β ) 90.75) crystallographic values. Molar volume of the R-Se is 0.21 cm3 lower, while its lattice energy is 0.8 kcal/mol higher28,29 than that of the β-Se, which illustrates that laser irradiation induces synthesis of higher stability of selenium quantum dots with enhanced packing fraction (Figure 2B). The size of raw β-Se

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Figure 3. TEM images of selenium nanoparticles (A, B) before and (C, D) after 6 min of laser irradiation. White circles in C and D represent individual QDs. The inset of (C) shows TEM image of QDs with better visibility, while the inset of (D) illustrates the histogram of size distribution of QDs shown in the inset of (C).

nanoparticles is calculated using Scherrer’s formula, D ) 0.9λ/(β cos θ). TEM images of selenium nanoparticles before (Figure 3A, B) and after laser irradiation (Figure 3C, D) are presented in Figure 3. Using the size calculation of all the particles having 65-80 nm diameters for the raw selenium nanoparticles, we obtained a 69 nm average diameter. High resolution TEM images of selenium nanoparticles produced after 6 min of laser irradiation depict synthesis of 3.17 ( 0.51 nm sized selenium QDs (inset Figure 2D). Interplanar spacing between two planes of a single selenium quantum dot is 0.308 nm, which corresponds to the (221) planes of the R-Se.29 3.3. Optical Characterization of Se QDs Obtained after Laser Irradiation. 3.3.1. UV-Visible Absorption Spectroscopy. UV-visible absorption spectra of raw selenium particles and colloidal solutions of Se QDs obtained after different times of laser irradiation are depicted in Figure 4. The absorption spectrum of the bulk Se powder has a sharp peak at 273 nm and a wide peak above 700 nm, which gets shifted to 260 and 558 nm, respectively, for just 2 min laser irradiation of raw selenium particles. The absorption peak in the UV (260 nm) may be the consequence of the interband and core electronic transitions (transitions from the bands corresponding to 3s2p6d10 or lower energy levels to conduction or higher bands), while the peak in the visible region is a consequence of coherent

oscillation of free electrons from one surface of the particle to other known as surface plasmon resonance (SPR) absorption. The SPR as well as UV peak shifted toward the shorter wavelength side with the increase of ablation time, illustrating a decrease in the size of the synthesized QDs in the solution with time of irradiation. A shorter wavelength shift of the SPR peak with the decrease of the size of QDs is the well-known quantum confinement effect. 3.3.2. Quantum Mechanical Treatments for Band Gap and Size Determinations. Employing absorption data, the absorption coefficient, R, of the colloidal solution of QDs under Beer’s law, is related to its band gap energy by R ) A(hν - Eg)n/hν, where A is a constant, Eg is the band gap of the material, and the exponent n may have the values 1/2, 2, 3/2, and 3 corresponding to allowed direct, allowed indirect, forbidden direct, and forbidden indirect semiconductor, respectively 24. The region of fundamental absorption, which corresponds to the electronic transition from the top of the valence band to the bottom of conduction band, can be utilized to determine the band gap energy of the material using the above relation. Selenium is a direct allowed band gap semiconductor, which is justified by the work of Mehta et al.,24 even for the nanoparticles of 10-30 nm diameters. Therefore, it is quite safe to use n ) 1/2 in the present study. The values of the band gap energies,

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Figure 4. (A) UV-visible absorption spectra of Se powder (a) before ablation; (b) modified image of (a) for clarity; (c) after 2 min of ablation. (B) UV-visible absorption spectra of Se powder for different times of ablation from 0 to 15 min.

Figure 5. Tauc plots of UV-visible spectra shown in Figure 4B for the calculation of band gap energy. The inset shows the Tauc plot corresponding to 2 min of laser irradiation for clarity.

TABLE 1: Variation of Size and Band Gap Energy with Time S. no.

ablation time (min)

band gap, Eg (eV)

∆Eg ) Eg - Eb (eV)

diameter (nm)

1 2 3 4 5 6 7

0 2 4 6 8 10 15

1.72 ( 0.04 1.82 ( 0.08 2.50 ( 0.10 2.63 ( 0.05 2.76 ( 0.12 2.98 ( 0.06 3.19 ( 0.08

0.00 ( 0.04 0.10 ( 0.12 0.78 ( 0.14 0.91 ( 0.09 1.04 ( 0.16 1.26 ( 0.10 1.47 ( 0.12

69.0 ( 1.5a 10.5 ( 3.62 3.76 ( 1.13 3.48 ( 0.336 3.25 ( 0.26 2.96 ( 1.07 2.74 ( 2.32

a Based on the SD in fwhm of the Gaussian curve fitted XRD peak of bulk Se powder.

for the QDs obtained after different times of laser irradiation, were thus obtained by extrapolating the straight line portion of the (Rhν)2 vs hν graph, well-known as the tauc plot, to the hν axis, which are illustrated in Figure 5. The plot for the solution obtained after 2 min of ablation is presented in the inset only due to the clarity. The values of the band gap energies are 1.72, 1.82, 2.50, 2.63, 2.76, 2.98, and 3.19 eVs for the colloidal solution of Se nanoparticles obtained after 0, 2, 4, 6, 8, 10, and 15 min of laser irradiation, respectively (Table 1). The size of the nanoparticles, obtained after different times of laser irradiation, could be obtained from the shift in their band gap energies compared to that of the raw Se (1.72 eV) particles, using the effective mass model, ∆Eg ) [p2π2/2µR2] - 1.8e2/4πKε0R, of

the Brus,30 where ∆Eg is the shift in the band gap energy due to the quantum confinement, R is the radius of the particle, K is the permittivity of the material, and µ ) memh/me + mh is the effective mass of an exciton in the solid. The second term of the effective mass model is only 1% of the first and, therefore, can be safely ignored. As the hole has very large effective mass compared to the electron (mh . me), it can be omitted in the calculation for µ. The effective mass of the electron for m-Se is almost 0.25m0, where m0 is free electron mass. Diameters of selenium nanoparticles produced after different times of laser irradiation, and corresponding shift in band gap energy, are displayed in Table 1, and variation in the size and band gap energy of selenium nanoparticles with time of irradiation are illustrated in Figure 6A. The best fitting of size vs time data in the region of 0-15 min using second order exponential decay function shows that size decreases very fast initially (time constant 0.89 ( 0.02 min) and attains comparatively much slower size reduction rate (time constant 68.28 ( 47.03) after 6.5 min of laser irradiation. It can be safely concluded that, at this particular laser irradiance, the ablation process attains nearly stationary state after almost 15 min of irradiation and produces 2.74 nm, critical diameter, of selenium nanoparticles. Experimental and fitted data points for the variation in band gap energy of selenium QDs with size are illustrated in Figure 6B. Band gap energy, Eg, relates to the size, a, of the QDs as Eg ) C + A1 exp(-t1a) + A2 exp(-t2a), where C ) 1.746 56 ( 0.008 39, A1 ) 1.870 78 ( 0.636 57, t1 ) -3.123 41 ( 0.423 88, A2 ) 18.479 5 ( 9.200 21, and t2 ) 0.823 22 ( 0.187 61. This expression is valid in the size range of 2-70 nm and 99.99% close to the experimental data. 3.3.3. Laser Induced Controlling of Surface Plasmon Resonance Absorption. The systematic tunability of optical properties of semiconductor quantum dots (QDs) has achieved increasing fundamental and technological interest in recent times, owing to their applications in nanophotonics, imaging labels, biological and optical sensing,31 and biomedicine. One of the most interesting optical properties of the quantum dots is their surface plasmon resonance (SPR), which is the basis of several advanced technological applications including color based biosensor,32 measuring extent of adsorption, and electromagnetic field enhancement.33 SPR is governed by coherent oscillation of free electrons under the resonance of the incident light, which is highly dependent on the size and shape of the quantum dots, especially when these are below the quantum confinement limit. SPR becomes more interesting in the field enhancement and light amplification, when resonance frequency matches the wavelength of the incident laser

Optical Properties of Selenium Quantum Dots

Figure 6. (A) Variation of band gap energy and size of the Se particles with ablation time. (B) Experimental and curve fitted data points of band gap energy of Se QDs with size.

beam; therefore, tunability of SPR in the visible range is extremely demanding. In principle, SPR absorption for semiconductor quantum dots is similar to that of the noble metal nanoparticles, in spite of having almost 100 times lower carrier density than metals. The SPR absorption wavelength, λ, relates to the bulk plasma frequency as λ2 ) λp2(ε∞ + 2εm), where λp is the bulk plasma frequency of the material, ε∞ is the high frequency dielectric constant, and εm is the dielectric constant of the surrounding medium. In the case of our study, the medium’s dielectric constant is the same for all the samples, and according to most of the models, the dielectric constant of the material depends on the size of particle below the quantum confinement limit. The SPR frequency can be tuned easily by tuning the size of QDs below the limit of the quantum confinement. Figure 7 illustrates SPR and UV peak positions, their area, and variation of the SPR peak position with the size of the QDs. UV and SPR peaks of absorption spectra (Figure 4B) of every colloidal solution of Se nanoparticles were best fitted using a Gaussian function (Supporting Information Figure 7) and are plotted in Figure 7A with corresponding data in Table 2. SPR peak position shifted toward shorter wavelength side, and peak intensity increases with increase in the time of irradiation, which depicts that SPR frequency and intensity are highly dependent on the particle size. According to the above-mentioned equation of SPR peak position, it can be concluded that synthesized particles are below the quantum confinement limit. The position of UV absorption corresponding to the interband and deep level electronic transitions also shifted toward the shorter wavelength

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17379 side with increase in irradiation time, but with slower rate compared to the SPR (Figure 7B). A shorter wavelength shift of UV and SPR peak positions with increase in the time of irradiation states that time of laser irradiation induces more and more confinement of electronic motion. The width of the SPR and UV peak almost remains constant with irradiation time, which illustrates that the duration of laser irradiation does not affect the fwhm of the SPR oscillations and hence the size distribution of QDs. Laser induced tuning of Se SPR peak position, while keeping its fwhm constant, provides an excellent method for the plasmonic application of selenium QDs with a wide range of available light sources for resonance. SPR as well as UV peak area and hence intensity increase with the decrease in the size of quantum dots (Figure 7C), which is also a well-known property of quantum dots. Increase of the selenium concentration in the solution with the increase of irradiation time also may contribute to the increase of the SPR peak intensity. The position, λp, of the SPR peak shifted sharply toward the shorter wavelength side (Figure 7D) when the size of the selenium quantum dots came down to 3.7 nm. This size may be treated as the quantum confinement limit for selenium. In other words selenium nanoparticles with a size down to 3.7 nm should qualify as quantum dots. 3.4. Photoluminescence Studies of Se QDs Produced after Different Times of Irradiation. Photoluminescence spectra of selenium QDs produced after different times of laser irradiation are presented in Supporting Information Figure 10 and have green and orange bands in the spectral region of 520 and 580 nm, respectively. PL spectra for all the samples are curve fitted for green and orange bands using a Gaussian function and are separately illustrated in Figure 8A with corresponding data in Table 3. The position of the green peak exhibits a slight shift toward blue, while the orange peak shows more irradiation time, i.e., size dependency and shifted toward red with the increase of time of irradiation. The green peak35 may be the consequence of excitonic decay, while the orange peak corresponds to surface defects, i.e., excess or deficiency of Se atom. Intensity, width, and area of both the PL peaks increase with the increase of irradiation time. Two factors may be responsible for affecting the PL peak characteristics of colloidal solution of Se QDs obtained after different times of laser irradiation. The first is the increase of the concentration of selenium QDs in the solution with the increase of time of irradiation, while the second is related to the intrinsic properties, i.e., size, phase, and surface defects, of the QDs. The first factor should equally influence both the PL peak characteristics, while intrinsic properties should have different behaviors for excitonic and defect related peaks. In order to determine whether the change in the PL peak characteristic is the effect of variation in particle size or surface defects, we studied change in the behaviors of the ratios of green to orange peak characteristics with irradiation time. Variations in the ratios of intensity, area, and width of the green and orange peaks are plotted with time and are depicted in Figure 8C, D. These plots states that intensity, width, and area of the orange peak increase with time at a higher rate compared to that of the green peak. These observations conclude that laser irradiation not only reduces the size of the selenium QDs but also creates surface defects, and density of these defects increases with reduction in the size of QDs. Increase in the quantum confinement, i.e., reduction in the size, may either shift the surface defect/ electron trap level toward lower energy or shift the hole trap level toward higher energy, or both, which causes a red shift of the defect related peak with irradiation time. An increase

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Figure 7. (A) Gaussian curve fitted SPR and UV absorption spectra of colloidal solution of Se QDs produced after different times of laser irradiation. Irradiation time dependence of UV and SPR (B) peak position and (C) peak area. (D) Variation of SPR peak position with size of QDs shows confinement at 3.75 ( 0.15 nm.

TABLE 2: Gaussian Curve Fitted Data, Position, Width, and Area of SPR and UV Peaks for the Colloidal Solution of Se QDs Produced after Different Times of Laser Irradiationa time

P1 (nm)

P2 (nm)

W1 (nm)

W2 (nm)

A1 (arb)

A2 (arb)

Rˆ2

Chıˆ2/DoF

2 4 6 8 10 15

248 ( 0.55 228 ( 0.58 226 ( 0.48 224 ( 0.67 223 ( 0.50 204 ( 0.72

473 ( 2.21 464 ( 2.63 448 ( 2.87 440 ( 3.29 430 ( 3.24 389 ( 5.1

166 ( 2.08 187 ( 2.12 179 ( 2.04 200 ( 2.50 172 ( 2.03 174 ( 2.33

348 ( 6.02 340 ( 5.59 361 ( 6.09 327 ( 5.92 372 ( 6.46 350 ( 7.09

18 ( 0.56 65 ( 1.74 81 ( 2.37 123 ( 3.40 150 ( 3.24 215 ( 7.76

51 ( 1.55 113 ( 3.25 161 ( 4.76 180 ( 4.87 216 ( 6.55 285 ( 10.55

0.997 0.999 0.999 0.999 0.999 0.999

3 × 10-6 10 × 10-6 20 × 10-6 25 × 10-6 30 × 10-6 90 × 10-6

a

P1/P2, W1/W2, A1/A2 are peak position, peak width, and peak area of the UV/SPR peak, respectively.

in the PL peak width also suggests that defect level width may increase with the increase of quantum confinement.

∞

σSCA ) πa2(2/q2)

∑ (2j + 1){|aj|2 + |bj|2}

(2)

j)1

4. Discussion 4.1. Laser Induced Controlling of Surface Plasmon Resonance Absorption. According to the Mie theory,36,37 the extinction and scattering cross sections of a sphere with radius a and complex refractive index m relative to the external medium, water in this case, are given by

∞

σEXT ) πa2(2/q2)

∑ (2j + 1){Re(aj + bj)} j)1

(1)

where q is the dimensionless size parameter defined by 2πa/λ, with λ as the wavelength of incident light in the external medium. The expansion coefficients, aj and bj, are given by complex functions of q and mq and represent oscillating electric and magnetic multipoles, respectively. For the values of q , 1, the expansion series converges very rapidly and results in almost the same values for extinction and absorption coefficients. In the case of the present study, the average size of raw selenium powder is 69 nm with refractive index ∼1, and the wavelength (λ) of light used for irradiation is 1064 nm, which resulted in values of q, m, and mq less than unity. With the decrease of the particle size, scattering of 1064 nm laser light may dominate

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over that of its absorption, which may be one of the causes of attaining the critical size limit in the process of laser induced size reduction. When the sizes of the nanoparticles are much smaller than the wavelength of laser light and the extinction and absorption coefficients have the same values, the absorption cross section in terms of the angular frequency is estimated by the following expression 3/2 ω σ(ω) ) 9εm V

ε2(ω) c[ε1(ω) + 2εm]2 + ε22(ω)

(3)

where V is the particle volume, c is the velocity of light, εm is the frequency independent medium dielectric constant, and ε1(ω) and ε2(ω) are the real and imaginary parts, respectively, of the dielectric constant of the particle and have interband and free

electronic contributions. The contribution of interband (3d electrons of Se) electronic transitions in the absorption cross section of nanomaterials is almost the same as that for bulk materials; therefore, the size dependence of σ(ω) in QDs mainly incorporates free electronic, Drude term, contribution, which itself depends on the contribution from inelastic scattering of free electrons38 by the particle surface and free electron plasma frequency, defined by (Ne2/ε0meff)1/2, where N is the free electron density, e is the electronic charge, ε0 is the permittivity of vacuum media, and meff is the effective mass of the electron in the nanoparticle system. The inelastic scattering can be expressed by increase of scattering frequency from ω0 for the bulk to ∼VF/a for the QDs, where VF is the Fermi velocity and a is the particle diameter. Incorporation of the Drude term and free electron plasma frequency contributions with the basic expression (eq 3) for the absorption cross section are termed as mean

Figure 8. (A) Gaussian curve fitted curves of PL spectra (illustrated in Supporting Information Figure 10) of colloidal solution of Se nanoparticles produced for different times of laser irradiation. (B) Variation in the position of green and orange PL bands with time. (C and D) Irradiation time dependence of PL peak characteristics ratios (C) intensity and area and (D) width of green (524 nm) to yellow (580 nm) bands. Schematic representation of the energy level diagram is presented in the inset.

TABLE 3: Gaussian Curve Fitted Data, Peak Position, Width, and Area of Green (G) and Orange (O) PL Bands of the Colloidal Solution of Se QDs Produced after Different Times of Laser Irradiationa time

P1 (nm)

P2 (nm)

W1 (nm)

W2 (nm)

A1 (arb)

A2 (arb)

Rˆ2

Chıˆ2/DoF

2 4 6 8 10 15

523.4 ( 0.07 523.2 ( 0.07 523.0 ( 0.08 522.8 ( 0.07 522.6 ( 0.07 522.2 ( 0.08

558.6 ( 1.19 559.5 ( 0.98 560.0 ( 1.13 561.0 ( 0.98 563.3 ( 0.78 565.6 ( 0.60

28.3 ( 0.28 28.8 ( 0.28 29.0 ( 0.32 28.6 ( 0.31 28.6 ( 0.26 27.9 ( 0.24

57.6 ( 1.34 60.5 ( 1.16 58.9 ( 1.26 63.8 ( 1.16 60.6 ( 1.03 62.2 ( 0.86

453.4 ( 11.74 486.3 ( 11.59 510.7 ( 14.74 544.8 ( 13.6 610.3 ( 12.9 714.2 ( 13.4

348 ( 12.69 444 ( 12.65 466.8 ( 15.9 569.0 ( 15.2 630.4 ( 14.2 890 ( 14.9

0.997 0.996 0.995 0.999 0.996 0.996

4.9 × 10-2 5.8 × 10-2 10.2 × 10-2 10.2 × 10-2 10.3 × 10-2 16.9 × 10-2

a

P, W, and A are peak position, width, and area of PL peaks. G and O bands are assigned by 1 and 2, respectively.

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free path and free electron density corrections, respectively in the position and shape of absorption spectrum of nanoparticles. With the decrease of particle diameter, a, both the contributory factors of the Drude term, i.e., free electron plasma frequency (free electron density, N, increases due to decrease of the particle volume) as well as scattering frequency (∼VF/a) increase, which causes increase of the real and imaginary parts of the dielectric constant and hence decrease of the absorption cross section, σ(ω), and extinction coefficient with decrease of the particle size. Combining this discussion with the recorded SPR absorption in the present study, we can say that the increase of the SPR peak intensity is due to the increase of the concentration of QDs in the colloidal solution with the increase in the time of irradiation. Increase of the plasma frequency with decrease of the size is responsible for the shift of the size dependent absorption peak, i.e., SPR, toward the higher frequency (shorter wavelength) side with size reduction. Therefore, the tuning size of the selenium QDs using time of laser irradiation can indirectly tune position and intensity of the SPR absorption of the colloidal solution of selenium QDs and their derived scientific and technological applications. 4.2. Mechanism of Laser Induced Size Reduction. Two mechanisms are proposed for the resizing and/or reshaping of the nanostructures and/or their surface modification using laser irradiation. The first is the laser induced melting and/or vaporization of larger size particles and rearrangements of atomic species into smaller or larger sized nanostructures with the same or different shapes depending on the experimental conditions. The second is the laser induced fragmentation, which involves ejection of photoelectrons from the surfaces of target nanostructures leaving positive charges behind them on the surface. These induced surface charges derive electrostatic repulsion between different parts of the surfaces, and consequently fragmentation of a single larger sized particle into several smaller ones with different shapes than their mother targets, where the latter may become spherical to minimize their surface energy. Experimental conditions to follow these two mechanisms are quite different. The first requires high laser irradiance which can make the temperature of the mother target higher than its boiling point to vaporize the surface atoms/ molecules, while the latter requires matching of the laser wavelength to the work function of target material to eject photoelectrons. Laser induced fragmentation may also be achieved with the laser wavelength having a photon energy much less than the work function, through multiphoton ionization, which is a nonlinear process and requires very high laser intensity. We have made some analytical calculations in order to ensure which one of the two mechanisms takes place in the present case of study. 4.2.1. Temperature Estimation of the Selenium Nanoparticles. To determine whether the laser induced size reduction is achieved by melting/vaporization of the particles, an attempt is made to estimate the temperature of the raw selenium nanoparticles using laser energy absorbed by them. Energy absorbed by Se particles per unit mass of the selenium atoms and per pulse, Q (J g-1 pulse-1), is calculated by

Q ) Eabs /RCV

T ) (Q - ∆Hmelt - ∆Hvap)/Cp + 293

(5)

T ) (Q - ∆Hmelt)/Cp + 293

(6)

Here ∆Hmelt is the heat of fusion (6.694 kJ/mol), ∆Hvap is the heat of vaporization (37.7 kJ/mol), and Cp is the specific heat (0.32 J/g K) at constant pressure for selenium. Estimated temperature of the raw selenium nanoparticles at this particular laser irradiance and experimental condition is of the order of 105 K, which is higher than the boiling point, 958 K, of the bulk selenium. In order to investigate the possibility of the second mechanism to occur, we have to think about the possibility of photoejection of electrons from the surface of selenium raw NPs. The electron work function of the bulk selenium is 5.9 eV, while the energy (hc/λ) of a single, 1064 nm wavelength, laser photon is 1.16 eV. Ejection of a single electron from the surface of selenium nanoparticles requires absorption of almost five laser photons through multiphoton absorption process, which requires very high intensity (1018-1019 W/cm2) of the laser.39 Use of shorter wavelength laser with picosecond (10-12 s) or femtosecond (10-15) pulse widths may cause multiphoton absorption and fragmentation of the particles as reported previously.40 These calculations suggest that the first mechanism may be the principle and dominating factor for the synthesis of smaller sized selenium QDs from larger ones in the present investigation. For most of the materials, binding energy per atom and melting and boiling points of their quantum dots or clusters are proportional to their sizes. Decrease of the particle size down to the 10 nm scale may cause a decrease in its melting and boiling34 points and decrease in the extinction/absorption coefficients. Decrease of the particle size up to a certain value may also cause a decrease in its light absorption at 1064 nm, thus raising the particle temperature above the melting/boiling point even after a decrease of melting/boiling temperature with size, which stops the rate of size reduction process after attaining the critical size limit. 4.2.2. Calculation of Thermal Diffusion Length of Selenium Nanoparticles. Thermal diffusion length, lth, of the selenium nanoparticles due to laser heating is estimated using bulk physical constants of the selenium employing following equation

lth ) (Rτ)1/2

(7)

where R stands for thermal diffusivity of the selenium, while τ represents the pulse width (10 ns) of the fundamental wavelength of the pulsed Nd:YAG laser used in this study.

(4) R ) κ/Fcp

(8)

-1

where E (5 × 10 J s ) is the laser energy absorbed by the solution per unit time, R is the repetition rate (10 Hz) for pulsed laser, C is the concentration (2 × 10-2g/cm3) of β-Se in the solution, and V (4.86 cm3) is the irradiated volume of the solution. Utilizing these data, energy absorbed by the unit mass of the selenium (Q) is estimated as 5.15 × 105 J g-1 pulse-1. The temperature T(K) of the selenium particles can be estimated 5

on the basis of the absorbed laser energy using eqs 5 and 6, for temperatures higher than that of the boiling point and melting point, respectively, and employing bulk physical constants for the selenium. The initial temperature, 293° K, was considered to be the room temperature.

where k is the thermal conductivity, F is the density, and cp is the specific heat capacity of selenium. As thermal conductivity and specific heat are temperature dependent physical constants, the calculation of diffusion length of the selenium during laser heating at the temperature of its boiling point (958 K) is required. Unfortunately, due to the unavailability of these

Optical Properties of Selenium Quantum Dots thermal data of selenium at high temperature, we have estimated diffusion length using the RT thermal constants. At 300 K temperature, cp ) 0.32 J/g K, k ) 0.0204 W/cm · K, with FSe ) 4.7 g/cm3, and the thermal diffusion length can be estimated to be 12.5 µm, which is larger than the size of raw selenium nanoparticles used as a target for the synthesis of smaller sized selenium QDs in the present investigations. This suggests that the whole volume of raw selenium NPs (a < lth) can vaporize into selenium atoms in the time duration of a single laser pulse. SPR absorption of raw selenium nanoparticles (69 nm diameter) above 700 nm wavelength (curve b in Figure 4A) causes significant absorption of 1064 nm of laser photon. These photons excite the raw selenium nanoparticle electronically to its excited state through photon-electron interaction. These energetic free electrons interact with the selenium lattice through the electron-phonon interaction in picoseconds and cause generation of phonon waves, through phonon-phonon interaction in nanoseconds, which get reflected from the surfaces of nanoparticles with the oscillation of electrons and phonons inside the particle. However, these excited particles undergo a rapid relaxation to their electronic ground state through the strong electron-electron and electron-phonon coupling. There is sufficient absorption of laser pulse by the raw selenium particles, and the pulse duration (10 ns) is longer than the period of a single excitation and relaxation cycle. The photon energy absorbed by the selenium raw nanoparticles is converted into heat, causing their temperature to rise above the boiling temperature as estimated above. As the diffusion length of the selenium for one laser pulse duration is much larger compared to the average size of raw selenium particle, the whole volume of selenium NPs lying in the spatial volume of laser irradiation may get vaporized into selenium atoms in a single laser pulse. Some of the selenium atoms may act as embryos, while the others present in the solution get deposited on surfaces of embryos to make smaller sized NPs and QDs. On the basis of XRD data, conversion of β-Se NPs into more closely packed R-Se QDs supports rearrangement of Se atoms after laser vaporization of raw β-Se NPs. Now, raw as well as newly evolved nanoparticles may act as the targets for the synthesis of smaller sized QDs. In the first 2 min of laser irradiation, 1200 laser pulse irradiate almost all the nanoparticles in the solution at a particular stirring speed, and conversion of all these raw β-Se NPs of 69 nm sizes into R-Se NPs of 10.5 nm size may be achieved. Now these newly synthesized selenium nanoparticles acts as targets for secondary processes of laser induced melting/vaporization for the synthesis of smaller sized QDs. Rate of size reduction, da/dt, is directly proportional to the initial diameter of the particle and therefore decreases linearly with the decrease of the size of target selenium particles (Figure 9) very similar to the radioactive decay model. There are several factors: (a) decrease of absorption coefficients at 1064 nm, (b) increase of the surface/volume ratio and hence heat loss from the particle surface to the surrounding environment through conduction, convection, as well as radiation, and (c) increase of surface adsorption coefficients, due to the increase of dangling bond on the surfaces of NPS/QDs present in the solution. These factors significantly increase with the reduction of particle size and play a vital role in attaining very small, almost negligible, reduction rate and resulting critical size of the particle. Increase of surface adsorptive capability with the decrease of particle size causes adsorption/deposition of selenium atoms on the target particles or newly synthesized smaller sized selenium QDs rather than making embryos for the synthesis of new smaller QDs. Critical size is achieved when rate of vaporization becomes almost equal to the rate of adsorption/deposition on the same or other particles. Based on the slope of the Figure 9, we have tried

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Figure 9. Plot of rate of size decrease, da/dt, with size, a, of the particles. Slow reduction rate for very small QDs is displayed in the inset. See Supporting Information Figures 9 and 10.

to develop a time dependent size, a(t), function for the QDs at any time t for particular laser irradiance as follows

a(t) ) a0e- /τ t

(9)

where a0 and τ are initial diameter of the raw selenium particles and size reduction constant, respectively. Size reduction constants are 2.27 ( 0.12 and 8.33 ( 0.2 with corresponding T1/2 ) (τ ln 2) values (time required to get half diameter of its initial value) are 1.57 ( 0.12 and 5.75 ( 0.2 min for sizes above and below the quantum confinement value (≈3.75 ( 0.15 nm), respectively. Almost 6.5 ( 1.5 min is required to attain this size value, after which size reduction rate becomes very slow and attains a final size of 2.55 ( 1.1 nm at this particular experimental condition. At this size value, rate of reduction, da/dt, becomes zero and the size reduction process completely stops (Supporting Information Figures 9 and 10). 5. Conclusion and Future Scenario Laser irradiation of water suspended β-selenium spherical nanoparticles with 69 nm average diameter, using 20 mJ/pulse energy of 1064 nm laser line for 15 min, not only causes efficient size reduction below 3 nm but also converts phase from β-Se NPs to more closely packed R-Se QDs. The size the nanoparticles get reduced with second order exponential decay function of time and rate of size reduction is proportional to the initial size of the target nanoparticles, which is similar to the decay model of the radioactive nuclei. Surface defect density, electron trap level, of QDs increases and its energy level decreases with the increase of time of laser irradiation. Tuning of the time for laser irradiation of water suspended selenium nanoparticles is an efficient and simple way of controlling size, surface defect, and SPR absorption of selenium QDs. On the basis of analytical calculations of the temperature of selenium nanoparticles and diffusion length, retainment of the spherical shape of the synthesized QDs, low photon energy and intensity of laser light, high work function and ionization potential of selenium atoms, etc., we have concluded that laser induced vaporization of larger sized target particles followed by nucleation/ embryonic starting and adsorption/deposition based growth is the dominant mechanism of the laser induced size and phase conversion in the present case of investigation.

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The present investigation may become a future landmark in the field of size, shape, surfaces modifications, and transformation of phase of nanoscale materials to enhance efficiencies of their scientific and technological applications. Laser induced size, shape, and phase conversion of water suspended semiconductor nanoparticles may open new avenues for the creation of surface defects for electronic rearrangement to produce magnetic moment in nonmagnetic materials as well as tuning of optical properties and advancement in the field of laser based nanomaterials processing such as welding, melting and fragmentation, annealing, alloying, shape and size conversion, and medical applications of laser heating of nanomaterials suspended in the media of cancerous cells for targeted cell killing, etc. As this is a simple, quick, and green approach, one can obtain NPs/QDs having desired shape, size, and surface properties without any chemical contaminations, which may be useful for biological and medical applications, where purity of QDs and their surface properties play important roles. Acknowledgment. Dr. S. C. Singh is grateful to IRCSET, Ireland, for providing an EMPOWER postdoctoral fellowship for financial assistance during compilation and analysis of this work. We are thankful to Nanophosphor Application Centre, Allahabad University, India for TEM, PL, XRD, etc., characterization facilities. Supporting Information Available: Crystallographic data of R-Se and β-Se, optical photograph of colloidal solution, some calculations, and several other sources of information and graphs. The information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor nanocrystals as fluorescent biological labels. Science 1998, 281, 2013–2016. (2) Chan, W. C. W.; Nie, S. Quantum dot bioconjugates for ultrasensitive nonisotopic detection. Science 1998, 281, 2016–2018. (3) Mafune´, F.; Kohno, J.; Takeda, Y.; Kondow, T. Formation of stable platinum nanoparticles by laser ablation in water. J. Phys. Chem. B 2003, 107, 4218–4223. (4) Yan, Z.; Bao, R.; Huang, Y.; Caruso, A. N.; Qadri, S. B.; Dinu, C. Z.; Chrisey, D. B. Excimer laser production, assembly, sintering, and fragmentation of novel fullerene-like permalloy particles in liquid. J. Phys. Chem. C 2010, 114, 3869–3873. (5) Singh, S. C.; Swarnkar, R. K.; Gopal, R. Synthesis of titanium dioxide nanomaterial by pulsed laser ablation in water. J. Nanosci. Nanotech. 2009, 9, 5367–5371. (6) Singh, S. C.; Gopal, R. Synthesis of colloidal zinc oxide nanoparticles by pulsed laser ablation in aqueous media. Physica E 2008, 40, 724– 730. (7) (a) Singh, S. C.; Gopal, R. Laser irradiance and wavelengthdependent compositional evolution of inorganic ZnO and ZnOOH/organic SDS nanocomposite material. J. Phys. Chem. C 2008, 112, 2812–2819. (b) Zeng, H.; Cai, W.; Li, Y.; Hu, J.; Liu, P. Compositional/structural evolution and optical properties of ZnO/Zn nanoparticles by laser ablation in liquid media. J. Phys. Chem. B 2005, 109, 18260–18266. (8) (a) Singh, S. C.; Swarnkar, R. K.; Gopal, R. Laser ablative approach for the synthesis of cadmium hydroxide-oxide nanocomposite. J. Nanopart. Res. 2009, 11, 1831–1838. (b) Singh, S. C.; Gopal, R. Nanoarchitectural evolution from laser-produced colloidal solution: Growth of various complex cadmium hydroxide architectures from simple particles. J. Phys. Chem. C 2010, 114, 9277–9289. (9) (a) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. Laserinduced shape changes of colloidal gold nanorods using femtosecond and nanosecnd laser pulses. J. Phys. Chem. B 2000, 104, 6152–6163. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. How does a gold nanorod melt? J. Phys. Chem. B 2000, 104, 7867–7870. (10) Takami, A.; Kurita, H.; Koda, S. Laser-induced size reduction of noble metal particles. J. Phys. Chem. B 1999, 103, 1226–1232. (11) Hodak, J. H.; Henglein, A.; Giersig, M.; Hartland, G. V. Laserinduced inter-diffusion in AuAg core-shell nanoparticles. J Phys. Chem. B 2000, 104, 11708–11718.

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