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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 6 — Jun. 1, 2012
  • pp: 799–813
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Single-step fabrication of luminescent GaAs nanocrystals by pulsed laser ablation in liquids

Turkka Salminen, Johnny Dahl, Marjukka Tuominen, Pekka Laukkanen, Eero Arola, and Tapio Niemi  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 6, pp. 799-813 (2012)
http://dx.doi.org/10.1364/OME.2.000799


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Abstract

Optical absorption and emission properties of gallium arsenide nanocrystals can be tuned across the visible spectrum by tuning their size. The surface of pure GaAs nanocrystals tends to oxidize, which deteriorates their optical properties. In order to prevent the oxidization, surface passivation has been previously demonstrated for GaAs nanocrystals larger than the Bohr exciton radius. In this paper, we study synthesis of small GaAs nanocrystals by pulsed laser ablation in liquids combined with simultaneous chemical surface passivation. The fabricated nanocrystals are smaller than the Bohr exciton radius and exhibit photoluminescence peaked near 530 nm due to quantum confinement. The photoluminescence properties are stable for at least six months, which is attributed to successful surface passivation. The chemical structure of the nanocrystals and changes caused by thermal annealing are elucidated with Raman spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy.

© 2012 OSA

1. Introduction

Fabrication of nanocrystals has gathered a lot of attention over the recent years due to the size dependent and hence tunable properties that can vary considerably from those of the bulk material. For semiconductor nanocrystals this allows adjustment of absorption and emission properties due to the change in the electron and the hole band-edge states and a blueshift of the band gap of the material when the particle size is smaller than the Bohr exciton radius of the material.

Gallium arsenide (GaAs), which is the base material for many optoelectronic devices, has a band gap of 1.42eV at 300K and the diameter of its Bohr exciton is about 19 nm [1

1. L. E. Brus, “Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state,” J. Chem. Phys. 80(9), 4403–4409 (1984). [CrossRef]

]. This allows tuning of the band gap of the GaAs nanocrystals across the visible spectrum, which makes nanocrystalline GaAs an interesting material for various applications. Partly due to the high quantum efficiency achieved with compounds of zinc, cadmium, sulfur and selenide and especially with their core/shell-structures, free-standing GaAs nanocrystals have not been studied as thoroughly as the II/VI semiconductors. Properties of the GaAs nanocrystals and quantum dots have been investigated within oxide and polymer matrices and as epitaxially fabricated heterostructures. However, synthesis of stable, free-standing GaAs nanocrystals, exhibiting luminescence at clearly shorter wavelengths than bulk GaAs, has been challenging.

GaAs nanocrystals have been produced through chemical methods from GaCl3 and As(SiMe3)3 in quinoline [2

2. M. A. Olshavsky, A. N. Goldstein, and A. P. Alivisatos, “Organometallic synthesis of gallium-arsenide crystallites, exhibiting quantum confinement,” J. Am. Chem. Soc. 112(25), 9438–9439 (1990). [CrossRef]

,3

3. H. Uchida, C. J. Curtis, and A. J. Nozik, “Gallium arsenide nanocrystals prepared in quinoline,” J. Phys. Chem. 95(14), 5382–5384 (1991). [CrossRef]

], from GaCl3 and (Na/K)3As in toluene, monoglyme and diglyme [4

4. S. S. Kher and R. L. Wells, “A straightforward, new method for the synthesis of nanocrystalline GaAs and GaP,” Chem. Mater. 6(11), 2056–2062 (1994). [CrossRef]

], from GaCl3 and As(NMe2)3 in 4-ethylpyridine [5

5. M. A. Malik, P. O’Brien, S. Norager, and J. Smith, “Gallium arsenide nanoparticles: synthesis and characterisation,” J. Mater. Chem. 13(10), 2591–2595 (2003). [CrossRef]

] and from a single organometallic precursor in hexadecylamine [6

6. M. A. Malik, M. Afzaal, P. O’Brien, U. Bangert, and B. Hamilton, “Single molecular precursor for synthesis of GaAs nanoparticles,” Mater. Sci. Technol. 20(8), 959–963 (2004). [CrossRef]

]. The reports include observations of blueshifted band gap absorption [2

2. M. A. Olshavsky, A. N. Goldstein, and A. P. Alivisatos, “Organometallic synthesis of gallium-arsenide crystallites, exhibiting quantum confinement,” J. Am. Chem. Soc. 112(25), 9438–9439 (1990). [CrossRef]

7

7. U. Uchida, C. J. Curtis, P. V. Kamat, K. M. Jones, and A. J. Nozik, “Optical properties of gallium arsenide nanocrystals,” J. Phys. Chem. 96(3), 1156–1160 (1992). [CrossRef]

] and visible photoluminescence [3

3. H. Uchida, C. J. Curtis, and A. J. Nozik, “Gallium arsenide nanocrystals prepared in quinoline,” J. Phys. Chem. 95(14), 5382–5384 (1991). [CrossRef]

7

7. U. Uchida, C. J. Curtis, P. V. Kamat, K. M. Jones, and A. J. Nozik, “Optical properties of gallium arsenide nanocrystals,” J. Phys. Chem. 96(3), 1156–1160 (1992). [CrossRef]

]. These methods allow a good size control and produce a relatively narrow particle-size distribution, which is typical for chemical synthesis methods. However, these processes are fairly complex, requiring careful fabrication of precursors and require several process steps. The fabrication process can also take several days to finish.

Pulsed lasers have been used to fabricate nanocrystals by ablation of GaAs targets in low-pressure gas atmosphere. Thereafter, the nanocrystals were transferred to ethanol by nitrogen flow [8

8. J. Perrière, E. Millon, M. Chamarro, M. Morcrette, and C. Andreazza, “Formation of GaAs nanocrystals by laser ablation,” Appl. Phys. Lett. 78(19), 2949–2951 (2001). [CrossRef]

]. In these crystals, visible luminescence was observed at a low temperature of 2 K. A straightforward technique to fabricate nanocrystal solutions is pulsed laser ablation in liquids (PLAL). In this technique the GaAs target is suspended in a solvent and ablated with several laser pulses [9

9. A. A. Lalayan, “Formation of colloidal GaAs and CdS quantum dots by laser ablation in liquid media,” Appl. Surf. Sci. 248(1-4), 209–212 (2005). [CrossRef]

,10

10. R. A. Ganeev, M. Baba, A. I. Ryasnyansky, M. Suzuki, and H. Kuroda, “Laser ablation of GaAs in liquids: structural, optical, and nonlinear optical characteristics of colloidal solutions,” Appl. Phys. B 80(4-5), 595–601 (2005). [CrossRef]

]. Lalayan reported visible luminescence from the samples fabricated using this technique, but the samples were not thoroughly characterized to confirm the source of the luminescence [9

9. A. A. Lalayan, “Formation of colloidal GaAs and CdS quantum dots by laser ablation in liquid media,” Appl. Surf. Sci. 248(1-4), 209–212 (2005). [CrossRef]

]. Ganeev et. al. reported gallium-rich (Ga:As-ratio 1.4:1) nanoparticles with increased extinction towards ultra-violet wavelength range, but no clear band-edge absorption was observed and photoluminescence was not studied [10

10. R. A. Ganeev, M. Baba, A. I. Ryasnyansky, M. Suzuki, and H. Kuroda, “Laser ablation of GaAs in liquids: structural, optical, and nonlinear optical characteristics of colloidal solutions,” Appl. Phys. B 80(4-5), 595–601 (2005). [CrossRef]

].

Pure GaAs surface tends to oxidize, which can be detrimental for the properties of the nanocrystals. The thickness of the oxide can be comparable to the particle size and in some cases the particle can be completely oxidized. Another issue is the high ratio of surface atoms in nanocrystals, which introduces defects that change the electronic and optical properties of the particles. Particularly, in the case of photoluminescence, the (surface) defects can enhance non-radiative transitions which reduce the photoluminescence. To reduce the harmful oxidation and to passivate the possible defects Traub et. al. demonstrated a process for oxide etching and subsequent surface passivation of solution suspended GaAs nanoparticles [11

11. M. C. Traub, J. S. Biteen, B. S. Brunschwig, and N. S. Lewis, “Passivation of GaAs nanocrystals by chemical functionalization,” J. Am. Chem. Soc. 130(3), 955–964 (2008). [CrossRef] [PubMed]

]. They reported improved luminescence intensity, especially after thermal annealing, but the produced nanoparticles were too large to exhibit change in the band gap due to quantum confinement.

In this paper, we report on a study of GaAs nanocrystals prepared with PLAL with simultaneous chemical surface passivation. The passivation is accomplished by using ammonium sulfide, a surface passivation agent widely studied for passivation of bulk GaAs [12

12. Y. Nannichi, J. Fan, H. Oigawa, and A. Koma, “A model to explain the effective passivation of the GaAs surface by (NH4)2Sx treatment,” Jpn. J. Appl. Phys. 27(Part 2, No. 12), L2367–L2369 (1988). [CrossRef]

], allowing a single-step fabrication of surface passivated nanocrystals. The nanocrystals exhibit visible photoluminescence and the prepared samples show no weakening of their luminescence during several months. We also elucidate the chemical structure of the prepared GaAs nanocrystals as a function of thermal annealing.

2. Experimental methods

2.1 Laser ablation

Laser ablation of solid GaAs targets was carried out in quartz and acrylic cuvettes using a high-repetition rate fiber laser (λ ≈1060 nm, pulse width 20 ps, repetition rate 1 MHz, pulse energy ≈1.6 μJ, FWHM of the focused beam ≈15 μm). Targets were submerged in ethanol, de-ionized water or water solutions containing ammonium sulfide. The laser beam was moved on the target surface with a 2D mirror scanner with a speed of 2 m/s to avoid crater formation and possible heat induced problems associated with a high number of overlapping subsequent laser pulses [13

13. T. Salminen, M. Hahtala, I. Seppälä, P. Laukkanen, and T. Niemi, “Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates,” Appl. Phys., A Mater. Sci. Process. 101(4), 735–738 (2010). [CrossRef]

].

2.2 Materials

The GaAs ablation target was an epitaxial grade, n-doped wafer manufactured by AXT. The dopant was silicon with atomic concentration of approximately 1.5·1018 atoms cm−3. De-ionized water with resistivity of 18 MΩ cm−1 was obtained from the Millipore Milli-Q-system. Ethanol (99.5%) was manufactured by Altia. Ammonium sulfide in a 20% water solution was obtained from Merck and it was used as received.

2.3 Sample preparation

The prepared nanoparticles were deposited on silicon substrates by drop casting. The substrate was placed on a hot-plate at a temperature of 170°C and drops of the nanoparticle solution were cast on the substrate until a total volume of 0.2 ml of the solution was consumed. Samples were then split in half; the first half was thermally annealed, while the second half was used as a reference.

Thermal annealing was performed in a nitrogen atmosphere. Annealing parameters were optimized to maximize the photoluminescence intensity of the samples while minimizing the changes to the shape of the luminescence spectra. A batch of 10 samples, prepared in ethanol, was used for the optimization procedure. Treatment at temperatures of 300°C and below caused no change in the photoluminescence of the samples when excited by laser pulses at 355 nm. The largest increase of the luminescence was observed at the temperature of 450°C, but the gain was accompanied by a considerable redshift in the wavelength of the maximum intensity. Parameters for the annealing experiment of the actual samples (375°C for 120 seconds) were selected as a compromise to maximize the PL with a minimal redshift.

2.4 Electron microscopy

To perform transmission electron microscopy (TEM) experiments, the nanoparticle samples dried on the silicon substrate were transferred to TEM-grids. This was achieved by first scratching the nanocrystals from the surface of the silicon substrate with a scalpel, then placing the grid on top of the sample and finally dropping a single drop of ethanol to facilitate the transfer of the crystals to the grid. After drying, the grids were examined with a high-resolution TEM (Jeol JEM-2200FS). The same device was also used for electron diffraction measurements and for energy dispersive spectrometry (EDS). Additional inspection by scanning electron microscopy (SEM) was performed with a Carl Zeiss Ultra-55.

2.5 X-ray photoelectron spectroscopy (XPS)

The photoemission spectra were measured using a spectrometer (Perkin-Elmer PHI 5400) with a monochromatized Al Kα x-ray source that was operated at 14 kV. The analyzer pass energy was 18 eV and the energy step was 0.1 eV. In spectral analysis (fitting), the Voigt function was used after Shirley's background subtraction. The minimum number of components, as deduced from the pure line shape was included in fittings. For results reported here, no sample charging was observed as the As 2p and Ga 2p emission components from the GaAs nanocrystal were located systematically at 1324-1325 eV and 1117-1118 eV binding energies, respectively.

2.6 Raman spectroscopy

Raman spectroscopy was carried out with a spectrometer (Andor Shamrock 303) and a cooled CCD-camera (Newton 940P). The excitation laser was a 532nm wavelength Cobolt Samba with a beam FWHM of 0.7 mm. The sample was illuminated with a collimated laser beam in an approximately 30 degree angle with respect to the sample surface. In order to avoid sample heating the beam was not focused. Indeed, varying the laser power induced no peak-shifts, confirming that all observed deviations from the bulk Raman spectra are due to the properties of the sample. The scattered light was collected with a microscope objective along the normal of the surface of the sample and Rayleigh scattered light was filtered out with a Semrock Razoredge filter.

2.7 Photoluminescence measurements (PL)

Room temperature photoluminescence with excitation wavelengths of 355nm and 532nm was measured using the same Raman spectrometry setup. The 355nm excitation source was a diode-pumped solid-state laser (Ekspla NL202). The low-temperature measurements were done using a scanning monochromator (Oriel DK480) and a photomultiplier tube connected to a lock-in amplifier. The laser emitting at 532nm was used for excitation in the low-temperature measurements. The FWHM of the focused excitation beam in the photoluminescence measurements was approximately 100 μm.

2.8 Zetapotential measurements

The stability of the solutions was studied using a Malvern Zetasizer Nano instrument to measure the zeta-potential of the nanoparticle-solutions.

3. Results

3.1 Nanoparticles in liquids

Nanoparticle solutions produced by PLAL in ethanol were reddish-brown in color and stable for several months. On the contrary, the samples prepared in water were much lighter in color with visible clusters. Within 24 hours of preparation the samples in water were completely clear with clusters in the bottom of the container.

Laser ablation in a water solution of ammonium sulfide produced solutions with visual appearance and stability very similar to samples prepared in ethanol when the molarity of ammonium sulfide was between 0.1 mmol/l and 10 mmol/l. As the molarity of ammonium sulfide is increased above 10 mmol/l, increasingly lighter color solutions are produced. The color of the solutions keeps getting lighter during the following days until the liquid is completely clear. We interpret, that instead of stabilization, excess sulfur leads to formation of sulfide compounds that are subsequently oxidized. This interpretation is supported by the analysis of the samples prepared with ammonium sulfide molarity of 100 mmol/l. The Raman measurements show the distinct spectra of arsenolite (As2O3), which is the oxidation product of arsenic sulfide. The SEM-images also show micrometer-sized octahedra which are typical for arsenolite.

The Zeta-potential of the samples prepared in ethanol, 0.1 mmol/l and 1 mmol/l ammonium sulfide was measured to be −41 mV, −20 mV and −48 mV, respectively. Measurement of samples prepared in water was impossible due to rapid agglomeration.

3.2 Transmission electron microscopy

The TEM analysis shows that the samples consisted of crystalline nanoparticles and additional amorphous material.

The TEM samples were prepared from nanoparticles that were deposited on silicon substrates and therefore the primary nanocrystals have mostly agglomerated into larger clusters with sizes from 20 nm up to over one hundred nanometers. This prohibits proper statistical analysis of the size distribution of the primary nanocrystals. The diameter of the observed primary crystallites varied between 2 and 10 nm, the typical diameter being between 3 and 6 nm. The primary nanocrystals are polyhedron shaped, but on the average their shape is close to a spherical form. Therefore, a relatively good estimation of the nanoparticle properties can be achieved by models developed for spherical nanoparticles. Few rod-like crystals were also observed.

Electron diffraction and Fourier-transforms of the HRTEM images confirm that the crystals have zinc-blende structure with visible (111), (220), (311) and (422) diffraction peaks. The lattice constant was calculated to be 0.555 nm, which is lower than that of bulk GaAs (0.565 nm).

To analyze the influence of ammonium sulfide, we performed an EDS analysis on samples prepared in ethanol and 1mmol/l ammonium sulfide. Analysis was performed on agglomerates that included both nanocrystals and amorphous material. In ethanol the material is gallium rich, with 2 to 2.5 times more gallium than arsenic and with 30% to 35% oxygen. The RTA-treatment reduces the oxygen content to 25% and slightly increases the relative amount of arsenic, which is to be expected if amorphous material is removed and GaAs nanocrystals remain mostly unaffected by the treatment, as suggested by the Raman measurements. Samples prepared in ammonium sulfide have gallium to arsenic ratio between 1.2 and 1.6 with 33% oxygen and 2% sulfur. Again the RTA treatment improves the overall gallium to arsenic ratio and nearly halves the amount of oxygen and sulfur in the sample.

To further investigate the distribution of elements in the samples, we measured the EDS in the center (Fig. 1
Fig. 1 HRTEM image of a large cluster of nanocrystals. The individual primary nanocrystals can be observed as separate crystalline regions within the cluster. The typical crystallite size is from 3 to 6nm. The EDS analysis was performed from regions A and B.
, region A) and on the edge (Fig. 1, region B) of a nanoparticle cluster for a sample prepared in 1 mmol/l of ammonium sulfide. The atomic percentages are listed in Table 1

Table 1. EDS Analysis of the Nanoparticle Cluster Shown in Fig. 1

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. The amorphous edge (region B) of the cluster is gallium rich with almost twice as much gallium as arsenic. The elemental composition suggests that the amorphous material attached to the nanocrystals is a mix of oxides and sulfides. The nanocrystals in the center (region A) are close to stoichiometric GaAs with gallium to arsenic ratio of 1.1 to 1. The amount of sulfur relative to oxygen is much higher than on the amorphous edge. Considering that the measurement of the center also includes some signal from the amorphous top layer, the results suggest that the surface of stoichiometric GaAs nanoparticles is covered with a compound with Ga:O:S ratio close to 1:1:1.

3.3 Raman spectroscopy

The peaks of both TO and LO phonons of GaAs can be observed in the Raman spectra of all of the samples. The peaks show considerable shifts towards lower energies and asymmetrical broadening, which is typical of GaAs nanocrystals [14

14. A. K. Arora, M. Rajalakshmi, and T. R. Ravindran, “Phonon confinement in nanostructured materials,” in Encyclopedia of Nanoscience and Nanotechnology, H. S. Nalwa, ed. (American Scientific Publishers, Los Angeles, 2004), Vol. 8, pp.409–512.

].

We used the Gaussian confinement model [20

20. H. Richter, Z. P. Wang, and L. Ley, “The one phonon Raman spectrum in microcrystalline silicon,” Solid State Commun. 39(5), 625–629 (1981). [CrossRef]

,21

21. A. K. Arora, M. Rajalakshmi, T. R. Ravindran, and V. Sivasubramanian, “Raman spectroscopy of optical phonon confinement in nanostructured materials,” J. Raman Spectrosc. 38(6), 604–617 (2007). [CrossRef]

] to estimate the size of the nanoparticles from the Raman spectrum. For a full derivation of the model we refer to the original article by Richter et al. [20

20. H. Richter, Z. P. Wang, and L. Ley, “The one phonon Raman spectrum in microcrystalline silicon,” Solid State Commun. 39(5), 625–629 (1981). [CrossRef]

], a brief explanation can be found in the paper by Arora et al. [21

21. A. K. Arora, M. Rajalakshmi, T. R. Ravindran, and V. Sivasubramanian, “Raman spectroscopy of optical phonon confinement in nanostructured materials,” J. Raman Spectrosc. 38(6), 604–617 (2007). [CrossRef]

]. The phonon dispersion curves were obtained from neutron scattering experiments performed at 12 K [22

22. D. Strauch and B. Dorner, “Phonon dispersion in GaAs,” J. Phys. Condens. Matter 2(6), 1457–1474 (1990). [CrossRef]

] and adjusted to room temperature using the temperature dependence measured by Chang et al. [23

23. R. K. Chang, J. M. Ralston, and D. E. Keating, in Light Scattering Spectra of Solids, G. B. Wright, ed. (Springer–Verlag, New York, 1969), p. 369.

] The original model by Richter et al. [19

19. I. H. Campbell and P. M. Fauchet, “CW laser irradiation of GaAs: Arsenic formation and photoluminescence degradation,” Appl. Phys. Lett. 57(1), 10–12 (1990). [CrossRef]

] used α = 2 to set the phonon amplitude at the nanoparticle boundary to 1/e, however the best fits to the experimental data have often been obtained by setting α = 8π2 as proposed by Campbell and Fauchet [24

24. H. Campbell and P. M. Fauchet, “The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors,” Solid State Commun. 58(10), 739–741 (1986). [CrossRef]

].

The best agreement with the experimental results, in very good agreement with the TEM measurements, was achieved for nanoparticle diameter of 4.0 nm, using α = 8π2 and including additional Gaussian peaks to model the contribution of amorphous GaAs and amorphous arsenic (Fig. 3). The broad peak centered at 247 cm−1 is attributed to amorphous GaAs and the peaks at 252 cm−1 and 204cm−1 to amorphous As. Calculations with α = 2 and α = 9.67, from the bond-polarizability model [25

25. J. Zi, K. Zhang, and X. Xie, “Comparison of models for Raman spectra of Si nanocrystals,” Phys. Rev. B 55(15), 9263–9266 (1997). [CrossRef]

], produced relatively poor fits.

The calculated spectrum agrees with the measurements surprisingly well considering that the size distribution of our samples is relatively wide. The nanoparticle diameter has a significant impact on the shape of the calculated spectrum in the size range observed in the TEM measurements. The broad size distribution was expected to produce considerably broadened Raman peaks and a greater mismatch between the fit and the experimental data. However, the results are very similar to the ones calculated for monodisperse nanocrystals [13

13. T. Salminen, M. Hahtala, I. Seppälä, P. Laukkanen, and T. Niemi, “Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates,” Appl. Phys., A Mater. Sci. Process. 101(4), 735–738 (2010). [CrossRef]

]. This can be partly explained by enhancement of the Raman signal by resonant excitation. Using a laser with a wavelength which is resonant with a transition in the studied material can increase the Raman signal by several orders of magnitude. In the particular case of 532 nm excitation wavelength, the signal from nanocrystals with transitions near the energy of 2.33 eV is enhanced. Nanocrystal size with corresponding band gap energy falls within the size range observed with TEM as discussed later and thus resonance enhancement could explain the observed strong Raman peaks.

For the samples prepared in the solutions of ammonium sulfide, the peak of the crystalline GaAs LO phonon is observed at 285 cm−1 and the peak of the TO phonon at 264 cm−1 (Fig. 4
Fig. 4 Raman spectrum of the samples prepared in the ammonium sulfide solution. In addition to the peaks of crystalline GaAs, a Raman signal from arsenic sulfides is observed.
). Two additional broad features are observed around 230 cm−1 and 345 cm−1. The broad peak around 230 cm−1 is in part attributed to amorphous gallium arsenide and arsenic that are also observed for the samples prepared in ethanol. The shape and the location of the peak is, however, slightly different suggesting formation of gallium and arsenic sulfides. The feature around 345 cm−1 is attributed to arsenic sulfide As2S3 (orpiment) [26

26. E. A. Rochette, B. C. Bostick, G. Li, and S. Fendorf, “Kinetics of arsenate reduction by dissolved sulfide,” Environ. Sci. Technol. 34(22), 4714–4720 (2000). [CrossRef]

], which is most likely in the amorphous form. This is suggested by the broadness of the peak and the absence of crystalline sulfides in the TEM analysis. Gallium sulfide, GaS, has Raman peaks near 345 cm−1 as well [27

27. G. Scamarcio, A. Cingolani, M. Lugarà, and F. Lévy, “Resonant Raman effects at the indirect band gaps of GaS,” Phys. Rev. B Condens. Matter 40(3), 1783–1789 (1989). [CrossRef] [PubMed]

], but gallium sulfide has a high melting point of 965 °C and the feature almost completely vanishes during RTA at 375 °C. Therefore the observed feature is most likely due to arsenic sulfide which has considerably lower melting point near 300 °C.

The RTA treatment decreases the signal from the sulfides and amorphous components, but at the same time increases the photoluminescence making detailed analysis of the peaks corresponding to GaAs difficult.

3.4 XPS

Before the RTA the samples prepared with the mild sulfur content appear to include also metallic Ga. Furthermore, the As XPS spectra reveal that the increased sulfur amount in the preparation decreases the formation of the highest oxidation state, As2O5, and enhances the As2O3 formation during the RTA. The increased sulfur concentration also decreases the amount of oxidized Ga. The signal corresponding to GaAs and amorphous As in the As 2p spectrum is exceptionally strong in the sample prepared in 5 mmol/l (NH4)2S which is reduced during the RTA suggesting that the sample has a considerable amount of amorphous material.

It is worth noticing that the possible emission from the lowest Ga oxidation state, Ga2O, overlaps with the GaAs emission. These XPS results are discussed later along with the PL results.

3.5 Photoluminescence

After drop casting on silicon substrate, the samples prepared in ethanol show weak photoluminescence when excited with a wavelength of 355 nm (Fig. 7A
Fig. 7 Photoluminescence of the samples under 355 nm excitation. (A) As-deposited samples. (B) Samples after thermal annealing for 120 s at 375 °C. Luminescence spectrum of the unannealed sample prepared in the 1mmol/l (NH4)2S solution is included as a reference.
). The spectrum is very broad and seems to have two underlying peaks centered at 530 nm and 650 nm. The RTA treatment with optimized parameters increases the intensity of the luminescence approximately by an order of magnitude (Fig. 7B). For this sample the enhancement is stronger near the peak at 530 nm creating an apparent blueshift for the total spectrum.

The samples prepared in pure water did not show luminescence in our measurements. However, addition of small amounts of ammonium sulfide produced samples with more intense photoluminescence than the samples prepared in ethanol. With 0.1 mmol/l molarity, the spectrum has the two features also observed in the case of ethanol; the biggest difference being that the shorter wavelength peak is dominant even before the RTA and the center is at a slightly shorter wavelength of 520 nm (Fig. 7A). Increasing the molarity further enhances the luminescence intensity and especially the peak at 520 nm until at molarities of 5 mmol/l and above the peak starts to slowly redshift and the maximum intensity starts to fall. With 1 mmol/l molarity of ammonium sulfide the intensity of the luminescence is roughly an order of magnitude stronger than for nanocrystals prepared in pure ethanol. Interestingly, a further gain of an order of magnitude is observed after the RTA treatment (Fig. 7B). The RTA treatment also induces a redshift, of the peak wavelength to approximately 550 nm. The luminescence of the RTA-treated samples is also visible to the naked eye.

For the RTA-treated samples prepared in ethanol the intensity is enhanced by a factor of 2 at low temperatures. The data and the fitted curve are plotted in Fig. 8A. A good agreement is achieved using two competing non-radiative decay mechanisms with activation energies of 2.5 meV and 50 meV. Unannealed samples did not have a strong enough PL signal to allow the measurement.

The measured temperature behavior and the fitted curves for samples prepared in 1mmol/l ammonium sulfide solution before and after annealing are plotted in Fig. 8C. Before annealing, the best fit is achieved using two terms with activation energies of 25 meV and 100 meV. The annealing seems to remove the low-energy decay route and a good fit for the RTA-treated sample is achieved using only one term yielding activation energy of 100 meV.

The behavior of the samples prepared in 5 mmol/l solution of ammonium sulfide is more complex before annealing (Fig. 8D). The integrated luminescence intensity starts to oscillate as a function of temperature at temperatures below 150 K. Due to these oscillations, the relevance of the fitting parameters of Eq. (1) becomes highly questionable. We cannot explain this behavior, but it is likely that the mechanism for luminescence quenching is more complicated for higher sulfur concentrations. After the RTA treatment a good fit is achieved also for this sample with activation energies of 5 meV and 81 meV.

4. Discussion

Laser ablation of the target induces formation of plasma. The expansion of the plasma is restricted by the surrounding liquid which causes nucleation of the evaporated material to form nanoparticles [28

28. G. W. Yang, “Laser ablation in liquids: Applications in the synthesis of nanocrystals,” Prog. Mater. Sci. 52(4), 648–698 (2007). [CrossRef]

]. In inert liquids this tends to lead to electrically charged nanoparticles repel each other and therefore form very stable solutions. In the case of GaAs in water and solutions of ammonium sulfide the liquid is clearly not inert and oxygen and sulfur can react with the formed GaAs nanoparticles leading to the observed thin oxide and sulfide shells. The reactive species in the solvent can also react directly with gallium and arsenic ions at the liquid-plasma boundary, which can lead to direct formation of gallium and arsenic oxides and sulfides. We suspect that this is the origin of the amorphous material observed in the TEM and Raman analysis.

The highest stability of the nanoparticle colloids is achieved in the intermediate molarity range between 0.1 mmol/l and 10 mmol/l of ammonium sulfide in water. The TEM and EDS analyses show that the most stable particles have a GaAs core and the surface is a gallium rich compound with both sulfur and oxygen. Therefore the stability is achieved by a combination of oxygen and sulfur and not by sulfur passivation alone. Particles produced in ethanol are stable and have a thin surface oxide layer, but it is unclear whether this oxide is produced in the liquid or after deposition in ambient air. High molarity of ammonium sulfide solution leads to destabilization and change in particle composition over time. All of these results support the proposed model for the PLAL in which the newly formed, very hot nanoparticles are reactive, but after cooling down the particles are stable unless the liquid is chemically reactive.

Due to the properties of the samples, statistical size distribution measurements were not possible. However, the different estimates for the average nanocrystal size agree relatively well. Based on the TEM analysis, the typical nanocrystals are between 3 and 6 nm in diameter and the diameter obtained from the fitted Raman spectra is 4 nm. The photoluminescence maximum is near to the photon energy of 2.34 eV. Using the commonly applied effective mass approximation [1

1. L. E. Brus, “Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state,” J. Chem. Phys. 80(9), 4403–4409 (1984). [CrossRef]

] a corresponding particle diameter of 2.3 nm can be calculated, however it is well known that this approximation does not work well for very small nanocrystals and yields too small size estimates. Calculations by Diaz et al. [31

31. J. G. Díaz and G. W. Bryant, “Electronic and optical fine structure of GaAs nanocrystals: The role of d orbitals in a tight-binding approach,” Phys. Rev. B 73(7), 075329 (2006). [CrossRef]

] based on an empirical sp3d5s* nearest neighbor tight-binding model developed by Jancu et al. [32

32. J. J. Jancu, R. Scholz, F. Beltram, and F. Bassani, “Empirical spds* tight-binding calculation for cubic semiconductors: General method and material parameters,” Phys. Rev. B 57(11), 6493–6507 (1998). [CrossRef]

] gives an estimated nanocrystal diameter of 3.4 nm which is in better agreement with the other size approximations. This is also in excellent agreement with the speculated resonance enhancement during Raman measurements.

The broad luminescence spectrum suggests that the nanocrystals have a relatively broad size distribution. The spectra for samples prepared in ethanol or in low ammonium sulfide molarities have two pronounced features, one centered at approximately 530 nm and another at 650 nm. This suggests that the size distribution is roughly bimodal with two separate and broad size distributions, which is typical to PLAL samples. The RTA treatment seems to enhance the luminescence from the smaller particles more efficiently. This is also the case when molarity of ammonium sulfide is increased, suggesting that the number of nanoparticles in the lower size-distribution mode is greater, but the smaller particles are also affected by defects more severely.

Roughly similar photoluminescence has been measured for porous GaAs [39

39. D. J. Lockwood, P. Schmuki, H. J. Labbé, and J. W. Fraser, “Optical properties of porous GaAs,” Physica E 4(2), 102–110 (1999). [CrossRef]

,40

40. N. Dmitruk, S. Kutovyi, I. Dmitruk, I. Simkiene, J. Sabataityte, and N. Berezovska, “Morphology, Raman scattering and photoluminescence of porous GaAs layers,” Sens. Actuators B Chem. 126(1), 294–300 (2007). [CrossRef]

]. This has also been attributed to quantum confinement in nanocrystals, although the evidence of existence of the nanocrystals in those cases has been ambiguous.

The wide application range of the nanocrystals include fluorescence labels in biodiagnostics [41

41. W. C. W. Chan and S. Nie, “Quantum dot bioconjugates for ultrasensitive nonisotopic detection,” Science 281(5385), 2016–2018 (1998). [CrossRef] [PubMed]

], luminescent media to convert and broaden the emission of blue LEDs to solid state white light source [42

42. J. M. Phillips, M. E. Coltrin, M. H. Crawford, A. J. Fischer, M. R. Krames, R. Mueller-Mach, G. O. Mueller, Y. Ohno, L. E. S. Rohwer, J. A. Simmons, and J. Y. Tsao, “Research challenges to ultra-efficient inorganic solid-state lighting,” Laser Photonics Rev. 1(4), 307–333 (2007). [CrossRef]

] and optical limiting by utilization of nonlinear optical properties of the particles [43

43. Q. Li, C. Liu, Z. Liu, and Q. Gong, “Broadband optical limiting and two-photon absorption properties of colloidal GaAs nanocrystals,” Opt. Express 13(6), 1833–1838 (2005). [CrossRef] [PubMed]

]. Our results could pave way for the GaAs nanocrystals as an alternative technology for generating various colors by pumping them with blue LEDs. On the other hand, solid state InGaN quantum wells have been intensively investigated and developed for the blue/green/yellow emitters for obtaining white light directly, and indeed great progress in this field has been made [42

42. J. M. Phillips, M. E. Coltrin, M. H. Crawford, A. J. Fischer, M. R. Krames, R. Mueller-Mach, G. O. Mueller, Y. Ohno, L. E. S. Rohwer, J. A. Simmons, and J. Y. Tsao, “Research challenges to ultra-efficient inorganic solid-state lighting,” Laser Photonics Rev. 1(4), 307–333 (2007). [CrossRef]

,44

44. C. Wetzel and T. Detchprohm, “Wavelength-stable rare earth-free green light-emitting diodes for energy efficiency,” Opt. Express 19(S4Suppl 4), A962–A971 (2011). [CrossRef] [PubMed]

46

46. H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express 19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]

]. An interesting future direction towards electrically pumped light source could rely on our nanoparticles uniformly layered on silicon substrates. At present, the properties of the crystals cannot compete with the properties of solid InGaN/GaN technology.

One factor that can limit the applications of the nanocrystals demonstrated in this work is the broad size distribution. The distribution can be narrowed by careful optimization of the process parameters, by modifying the liquids by additional chemicals, e.g. cyclodextrins [47

47. J.-P. Sylvestre, A. V. Kabashin, E. Sacher, M. Meunier, and J. H. T. Luong, “Stabilization and size control of gold nanoparticles during laser ablation in aqueous cyclodextrins,” J. Am. Chem. Soc. 126(23), 7176–7177 (2004). [CrossRef] [PubMed]

] prior to ablation and by further laser processing [48

48. A. Takami, H. Kurita, and S. Koda, “Laser-induced size reduction of noble metal particles,” J. Phys. Chem. B 103(8), 1226–1232 (1999). [CrossRef]

]. For size critical applications, further size-selective filtering and deposition techniques can be applied. Further work must also be done to improve the quantum efficiency of the nanocrystals. This could be done by further optimization of the surface passivation or by deposition of a suitable shell-layer to form core/shell-particles.

5. Conclusions

The combined information from the TEM, EDS, Raman and XPS analysis shows that we have successfully produced surface passivated GaAs-nanocrystals that form stable colloidal solutions. The as-prepared nanoparticle solutions are not luminescent. Nanocrystals deposited on silicon wafers show photoluminescence with a broad spectrum on visible wavelengths peaked near 530 nm. The luminescence shows no deterioration within six months. It is also evident that a considerable portion of the evaporated material reacts with the solvent and forms amorphous material. The PL intensity is considerably enhanced by the sulfur and oxygen containing surface passivation and by thermal annealing via the removal of non-radiative defects and possibly light-absorbing amorphous material.

Acknowledgments

The authors acknowledge Joel Salmi for performing the RTA-treatments, Antti Tukiainen and Janne Simonen for discussions that improved the quality of this research, Hua Jiang for the TEM measurements and Tapio Sorvajärvi and Juha Toivonen for the possibility to use the 355nm laser. We also acknowledge Jenni Leppiniemi and Vesa Hytönen for their help with the zeta-potential measurements.

This work was funded by European Regional Development Fund through Council of Pirkanmaa in project EA31189.

References and links

1.

L. E. Brus, “Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state,” J. Chem. Phys. 80(9), 4403–4409 (1984). [CrossRef]

2.

M. A. Olshavsky, A. N. Goldstein, and A. P. Alivisatos, “Organometallic synthesis of gallium-arsenide crystallites, exhibiting quantum confinement,” J. Am. Chem. Soc. 112(25), 9438–9439 (1990). [CrossRef]

3.

H. Uchida, C. J. Curtis, and A. J. Nozik, “Gallium arsenide nanocrystals prepared in quinoline,” J. Phys. Chem. 95(14), 5382–5384 (1991). [CrossRef]

4.

S. S. Kher and R. L. Wells, “A straightforward, new method for the synthesis of nanocrystalline GaAs and GaP,” Chem. Mater. 6(11), 2056–2062 (1994). [CrossRef]

5.

M. A. Malik, P. O’Brien, S. Norager, and J. Smith, “Gallium arsenide nanoparticles: synthesis and characterisation,” J. Mater. Chem. 13(10), 2591–2595 (2003). [CrossRef]

6.

M. A. Malik, M. Afzaal, P. O’Brien, U. Bangert, and B. Hamilton, “Single molecular precursor for synthesis of GaAs nanoparticles,” Mater. Sci. Technol. 20(8), 959–963 (2004). [CrossRef]

7.

U. Uchida, C. J. Curtis, P. V. Kamat, K. M. Jones, and A. J. Nozik, “Optical properties of gallium arsenide nanocrystals,” J. Phys. Chem. 96(3), 1156–1160 (1992). [CrossRef]

8.

J. Perrière, E. Millon, M. Chamarro, M. Morcrette, and C. Andreazza, “Formation of GaAs nanocrystals by laser ablation,” Appl. Phys. Lett. 78(19), 2949–2951 (2001). [CrossRef]

9.

A. A. Lalayan, “Formation of colloidal GaAs and CdS quantum dots by laser ablation in liquid media,” Appl. Surf. Sci. 248(1-4), 209–212 (2005). [CrossRef]

10.

R. A. Ganeev, M. Baba, A. I. Ryasnyansky, M. Suzuki, and H. Kuroda, “Laser ablation of GaAs in liquids: structural, optical, and nonlinear optical characteristics of colloidal solutions,” Appl. Phys. B 80(4-5), 595–601 (2005). [CrossRef]

11.

M. C. Traub, J. S. Biteen, B. S. Brunschwig, and N. S. Lewis, “Passivation of GaAs nanocrystals by chemical functionalization,” J. Am. Chem. Soc. 130(3), 955–964 (2008). [CrossRef] [PubMed]

12.

Y. Nannichi, J. Fan, H. Oigawa, and A. Koma, “A model to explain the effective passivation of the GaAs surface by (NH4)2Sx treatment,” Jpn. J. Appl. Phys. 27(Part 2, No. 12), L2367–L2369 (1988). [CrossRef]

13.

T. Salminen, M. Hahtala, I. Seppälä, P. Laukkanen, and T. Niemi, “Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates,” Appl. Phys., A Mater. Sci. Process. 101(4), 735–738 (2010). [CrossRef]

14.

A. K. Arora, M. Rajalakshmi, and T. R. Ravindran, “Phonon confinement in nanostructured materials,” in Encyclopedia of Nanoscience and Nanotechnology, H. S. Nalwa, ed. (American Scientific Publishers, Los Angeles, 2004), Vol. 8, pp.409–512.

15.

G. Burns, F. H. Dacol, C. R. Wie, E. Burstein, and M. Cardona, “Phonon shifts in ion bombarded GaAs: Raman measurements,” Solid State Commun. 62(7), 449–454 (1987). [CrossRef]

16.

I. D. Desnica, M. Ivanda, M. Kranjček, R. Murri, and N. Pinto, “Raman study of gallium arsenide thin films,” J. Non-Cryst. Solids 170(3), 263–269 (1994). [CrossRef]

17.

G. P. Schwartz, B. Schwartz, D. DiStefano, G. J. Gualtieri, and J. E. Griffiths, “Raman scattering from anodic oxide‐GaAs interfaces,” Appl. Phys. Lett. 34(3), 205–207 (1979). [CrossRef]

18.

Y. Takagaki, E. Wiebicke, M. Ramsteiner, L. Däweritz, and K. H. Ploog, “Spontaneous growth of arsenic oxide micro-crystals on chemically etched MnAs surfaces,” Appl. Phys., A Mater. Sci. Process. 76(5), 837–840 (2003). [CrossRef]

19.

I. H. Campbell and P. M. Fauchet, “CW laser irradiation of GaAs: Arsenic formation and photoluminescence degradation,” Appl. Phys. Lett. 57(1), 10–12 (1990). [CrossRef]

20.

H. Richter, Z. P. Wang, and L. Ley, “The one phonon Raman spectrum in microcrystalline silicon,” Solid State Commun. 39(5), 625–629 (1981). [CrossRef]

21.

A. K. Arora, M. Rajalakshmi, T. R. Ravindran, and V. Sivasubramanian, “Raman spectroscopy of optical phonon confinement in nanostructured materials,” J. Raman Spectrosc. 38(6), 604–617 (2007). [CrossRef]

22.

D. Strauch and B. Dorner, “Phonon dispersion in GaAs,” J. Phys. Condens. Matter 2(6), 1457–1474 (1990). [CrossRef]

23.

R. K. Chang, J. M. Ralston, and D. E. Keating, in Light Scattering Spectra of Solids, G. B. Wright, ed. (Springer–Verlag, New York, 1969), p. 369.

24.

H. Campbell and P. M. Fauchet, “The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors,” Solid State Commun. 58(10), 739–741 (1986). [CrossRef]

25.

J. Zi, K. Zhang, and X. Xie, “Comparison of models for Raman spectra of Si nanocrystals,” Phys. Rev. B 55(15), 9263–9266 (1997). [CrossRef]

26.

E. A. Rochette, B. C. Bostick, G. Li, and S. Fendorf, “Kinetics of arsenate reduction by dissolved sulfide,” Environ. Sci. Technol. 34(22), 4714–4720 (2000). [CrossRef]

27.

G. Scamarcio, A. Cingolani, M. Lugarà, and F. Lévy, “Resonant Raman effects at the indirect band gaps of GaS,” Phys. Rev. B Condens. Matter 40(3), 1783–1789 (1989). [CrossRef] [PubMed]

28.

G. W. Yang, “Laser ablation in liquids: Applications in the synthesis of nanocrystals,” Prog. Mater. Sci. 52(4), 648–698 (2007). [CrossRef]

29.

T. Fanaei and C. Aktik, “Passivation of GaAs using P2S5/(NH4)2S+Se and (NH4)2S+Se,” J. Vac. Sci. Technol. A 22, 874–878 (2004).

30.

D. Peide, “Main determinants for III–V metal-oxide-semiconductor field-effect transistors,” J. Vac. Sci. Technol. A 26, 697–704 (2007).

31.

J. G. Díaz and G. W. Bryant, “Electronic and optical fine structure of GaAs nanocrystals: The role of d orbitals in a tight-binding approach,” Phys. Rev. B 73(7), 075329 (2006). [CrossRef]

32.

J. J. Jancu, R. Scholz, F. Beltram, and F. Bassani, “Empirical spds* tight-binding calculation for cubic semiconductors: General method and material parameters,” Phys. Rev. B 57(11), 6493–6507 (1998). [CrossRef]

33.

J. S. Blakemore, “Semiconducting and other major properties of gallium arsenide,” J. Appl. Phys. 53(10), R123–R181 (1982). [CrossRef]

34.

G. E. Fenner, “Effect of hydrostatic pressure on the emission from gallium arsenide lasers,” J. Appl. Phys. 34(10), 2955–2957 (1963). [CrossRef]

35.

U. D. Venkateswaran, L. J. Cui, B. A. Weinstein, and F. A. Chambers, “Forward and reverse high-pressure transitions in bulklike AlAs and GaAs epilayers,” Phys. Rev. B Condens. Matter 45(16), 9237–9247 (1992). [CrossRef] [PubMed]

36.

C. L. Hinkle, M. Milojevic, B. Brennan, A. M. Sonnet, F. S. Aguirre-Tostado, G. J. Hughes, E. M. Vogel, and R. M. Wallace, “Detection of Ga suboxides and their impact on III–V passivation and Fermi-level pinning,” Appl. Phys. Lett. 94(16), 162101 (2009). [CrossRef]

37.

V. Polojärvi, J. Salmi, A. Schramm, A. Tukiainen, M. Guina, J. Pakarinen, E. Arola, J. Lång, I. J. Väyrynen, and P. Laukkanen, “Effects of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and thermal annealing on 1.3 μm GaInAsN/GaAs quantum well structures,” Appl. Phys. Lett. 97(11), 111109 (2010). [CrossRef]

38.

J. Dahl, V. Polojärvi, J. Salmi, P. Laukkanen, and M. Guina, “Properties of the SiO2- and SiNx-capped GaAs(100) surfaces of GaInAsN/GaAs quantum-well heterostructures studied by photoelectron spectroscopy and photoluminescence,” Appl. Phys. Lett. 99(10), 102105 (2011). [CrossRef]

39.

D. J. Lockwood, P. Schmuki, H. J. Labbé, and J. W. Fraser, “Optical properties of porous GaAs,” Physica E 4(2), 102–110 (1999). [CrossRef]

40.

N. Dmitruk, S. Kutovyi, I. Dmitruk, I. Simkiene, J. Sabataityte, and N. Berezovska, “Morphology, Raman scattering and photoluminescence of porous GaAs layers,” Sens. Actuators B Chem. 126(1), 294–300 (2007). [CrossRef]

41.

W. C. W. Chan and S. Nie, “Quantum dot bioconjugates for ultrasensitive nonisotopic detection,” Science 281(5385), 2016–2018 (1998). [CrossRef] [PubMed]

42.

J. M. Phillips, M. E. Coltrin, M. H. Crawford, A. J. Fischer, M. R. Krames, R. Mueller-Mach, G. O. Mueller, Y. Ohno, L. E. S. Rohwer, J. A. Simmons, and J. Y. Tsao, “Research challenges to ultra-efficient inorganic solid-state lighting,” Laser Photonics Rev. 1(4), 307–333 (2007). [CrossRef]

43.

Q. Li, C. Liu, Z. Liu, and Q. Gong, “Broadband optical limiting and two-photon absorption properties of colloidal GaAs nanocrystals,” Opt. Express 13(6), 1833–1838 (2005). [CrossRef] [PubMed]

44.

C. Wetzel and T. Detchprohm, “Wavelength-stable rare earth-free green light-emitting diodes for energy efficiency,” Opt. Express 19(S4Suppl 4), A962–A971 (2011). [CrossRef] [PubMed]

45.

R. M. Farrell, E. C. Young, F. Wu, S. P. DenBaars, and J. S. Speck, “Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices,” Semicond. Sci. Technol. 27(2), 024001 (2012). [CrossRef]

46.

H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express 19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]

47.

J.-P. Sylvestre, A. V. Kabashin, E. Sacher, M. Meunier, and J. H. T. Luong, “Stabilization and size control of gold nanoparticles during laser ablation in aqueous cyclodextrins,” J. Am. Chem. Soc. 126(23), 7176–7177 (2004). [CrossRef] [PubMed]

48.

A. Takami, H. Kurita, and S. Koda, “Laser-induced size reduction of noble metal particles,” J. Phys. Chem. B 103(8), 1226–1232 (1999). [CrossRef]

OCIS Codes
(160.4670) Materials : Optical materials
(250.5230) Optoelectronics : Photoluminescence
(350.3850) Other areas of optics : Materials processing
(160.4236) Materials : Nanomaterials

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: March 26, 2012
Revised Manuscript: April 20, 2012
Manuscript Accepted: May 10, 2012
Published: May 14, 2012

Citation
Turkka Salminen, Johnny Dahl, Marjukka Tuominen, Pekka Laukkanen, Eero Arola, and Tapio Niemi, "Single-step fabrication of luminescent GaAs nanocrystals by pulsed laser ablation in liquids," Opt. Mater. Express 2, 799-813 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-6-799


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References

  1. L. E. Brus, “Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state,” J. Chem. Phys.80(9), 4403–4409 (1984). [CrossRef]
  2. M. A. Olshavsky, A. N. Goldstein, and A. P. Alivisatos, “Organometallic synthesis of gallium-arsenide crystallites, exhibiting quantum confinement,” J. Am. Chem. Soc.112(25), 9438–9439 (1990). [CrossRef]
  3. H. Uchida, C. J. Curtis, and A. J. Nozik, “Gallium arsenide nanocrystals prepared in quinoline,” J. Phys. Chem.95(14), 5382–5384 (1991). [CrossRef]
  4. S. S. Kher and R. L. Wells, “A straightforward, new method for the synthesis of nanocrystalline GaAs and GaP,” Chem. Mater.6(11), 2056–2062 (1994). [CrossRef]
  5. M. A. Malik, P. O’Brien, S. Norager, and J. Smith, “Gallium arsenide nanoparticles: synthesis and characterisation,” J. Mater. Chem.13(10), 2591–2595 (2003). [CrossRef]
  6. M. A. Malik, M. Afzaal, P. O’Brien, U. Bangert, and B. Hamilton, “Single molecular precursor for synthesis of GaAs nanoparticles,” Mater. Sci. Technol.20(8), 959–963 (2004). [CrossRef]
  7. U. Uchida, C. J. Curtis, P. V. Kamat, K. M. Jones, and A. J. Nozik, “Optical properties of gallium arsenide nanocrystals,” J. Phys. Chem.96(3), 1156–1160 (1992). [CrossRef]
  8. J. Perrière, E. Millon, M. Chamarro, M. Morcrette, and C. Andreazza, “Formation of GaAs nanocrystals by laser ablation,” Appl. Phys. Lett.78(19), 2949–2951 (2001). [CrossRef]
  9. A. A. Lalayan, “Formation of colloidal GaAs and CdS quantum dots by laser ablation in liquid media,” Appl. Surf. Sci.248(1-4), 209–212 (2005). [CrossRef]
  10. R. A. Ganeev, M. Baba, A. I. Ryasnyansky, M. Suzuki, and H. Kuroda, “Laser ablation of GaAs in liquids: structural, optical, and nonlinear optical characteristics of colloidal solutions,” Appl. Phys. B80(4-5), 595–601 (2005). [CrossRef]
  11. M. C. Traub, J. S. Biteen, B. S. Brunschwig, and N. S. Lewis, “Passivation of GaAs nanocrystals by chemical functionalization,” J. Am. Chem. Soc.130(3), 955–964 (2008). [CrossRef] [PubMed]
  12. Y. Nannichi, J. Fan, H. Oigawa, and A. Koma, “A model to explain the effective passivation of the GaAs surface by (NH4)2Sx treatment,” Jpn. J. Appl. Phys.27(Part 2, No. 12), L2367–L2369 (1988). [CrossRef]
  13. T. Salminen, M. Hahtala, I. Seppälä, P. Laukkanen, and T. Niemi, “Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates,” Appl. Phys., A Mater. Sci. Process.101(4), 735–738 (2010). [CrossRef]
  14. A. K. Arora, M. Rajalakshmi, and T. R. Ravindran, “Phonon confinement in nanostructured materials,” in Encyclopedia of Nanoscience and Nanotechnology, H. S. Nalwa, ed. (American Scientific Publishers, Los Angeles, 2004), Vol. 8, pp.409–512.
  15. G. Burns, F. H. Dacol, C. R. Wie, E. Burstein, and M. Cardona, “Phonon shifts in ion bombarded GaAs: Raman measurements,” Solid State Commun.62(7), 449–454 (1987). [CrossRef]
  16. I. D. Desnica, M. Ivanda, M. Kranjček, R. Murri, and N. Pinto, “Raman study of gallium arsenide thin films,” J. Non-Cryst. Solids170(3), 263–269 (1994). [CrossRef]
  17. G. P. Schwartz, B. Schwartz, D. DiStefano, G. J. Gualtieri, and J. E. Griffiths, “Raman scattering from anodic oxide‐GaAs interfaces,” Appl. Phys. Lett.34(3), 205–207 (1979). [CrossRef]
  18. Y. Takagaki, E. Wiebicke, M. Ramsteiner, L. Däweritz, and K. H. Ploog, “Spontaneous growth of arsenic oxide micro-crystals on chemically etched MnAs surfaces,” Appl. Phys., A Mater. Sci. Process.76(5), 837–840 (2003). [CrossRef]
  19. I. H. Campbell and P. M. Fauchet, “CW laser irradiation of GaAs: Arsenic formation and photoluminescence degradation,” Appl. Phys. Lett.57(1), 10–12 (1990). [CrossRef]
  20. H. Richter, Z. P. Wang, and L. Ley, “The one phonon Raman spectrum in microcrystalline silicon,” Solid State Commun.39(5), 625–629 (1981). [CrossRef]
  21. A. K. Arora, M. Rajalakshmi, T. R. Ravindran, and V. Sivasubramanian, “Raman spectroscopy of optical phonon confinement in nanostructured materials,” J. Raman Spectrosc.38(6), 604–617 (2007). [CrossRef]
  22. D. Strauch and B. Dorner, “Phonon dispersion in GaAs,” J. Phys. Condens. Matter2(6), 1457–1474 (1990). [CrossRef]
  23. R. K. Chang, J. M. Ralston, and D. E. Keating, in Light Scattering Spectra of Solids, G. B. Wright, ed. (Springer–Verlag, New York, 1969), p. 369.
  24. H. Campbell and P. M. Fauchet, “The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors,” Solid State Commun.58(10), 739–741 (1986). [CrossRef]
  25. J. Zi, K. Zhang, and X. Xie, “Comparison of models for Raman spectra of Si nanocrystals,” Phys. Rev. B55(15), 9263–9266 (1997). [CrossRef]
  26. E. A. Rochette, B. C. Bostick, G. Li, and S. Fendorf, “Kinetics of arsenate reduction by dissolved sulfide,” Environ. Sci. Technol.34(22), 4714–4720 (2000). [CrossRef]
  27. G. Scamarcio, A. Cingolani, M. Lugarà, and F. Lévy, “Resonant Raman effects at the indirect band gaps of GaS,” Phys. Rev. B Condens. Matter40(3), 1783–1789 (1989). [CrossRef] [PubMed]
  28. G. W. Yang, “Laser ablation in liquids: Applications in the synthesis of nanocrystals,” Prog. Mater. Sci.52(4), 648–698 (2007). [CrossRef]
  29. T. Fanaei and C. Aktik, “Passivation of GaAs using P2S5/(NH4)2S+Se and (NH4)2S+Se,” J. Vac. Sci. Technol. A22, 874–878 (2004).
  30. D. Peide, “Main determinants for III–V metal-oxide-semiconductor field-effect transistors,” J. Vac. Sci. Technol. A26, 697–704 (2007).
  31. J. G. Díaz and G. W. Bryant, “Electronic and optical fine structure of GaAs nanocrystals: The role of d orbitals in a tight-binding approach,” Phys. Rev. B73(7), 075329 (2006). [CrossRef]
  32. J. J. Jancu, R. Scholz, F. Beltram, and F. Bassani, “Empirical spds* tight-binding calculation for cubic semiconductors: General method and material parameters,” Phys. Rev. B57(11), 6493–6507 (1998). [CrossRef]
  33. J. S. Blakemore, “Semiconducting and other major properties of gallium arsenide,” J. Appl. Phys.53(10), R123–R181 (1982). [CrossRef]
  34. G. E. Fenner, “Effect of hydrostatic pressure on the emission from gallium arsenide lasers,” J. Appl. Phys.34(10), 2955–2957 (1963). [CrossRef]
  35. U. D. Venkateswaran, L. J. Cui, B. A. Weinstein, and F. A. Chambers, “Forward and reverse high-pressure transitions in bulklike AlAs and GaAs epilayers,” Phys. Rev. B Condens. Matter45(16), 9237–9247 (1992). [CrossRef] [PubMed]
  36. C. L. Hinkle, M. Milojevic, B. Brennan, A. M. Sonnet, F. S. Aguirre-Tostado, G. J. Hughes, E. M. Vogel, and R. M. Wallace, “Detection of Ga suboxides and their impact on III–V passivation and Fermi-level pinning,” Appl. Phys. Lett.94(16), 162101 (2009). [CrossRef]
  37. V. Polojärvi, J. Salmi, A. Schramm, A. Tukiainen, M. Guina, J. Pakarinen, E. Arola, J. Lång, I. J. Väyrynen, and P. Laukkanen, “Effects of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and thermal annealing on 1.3 μm GaInAsN/GaAs quantum well structures,” Appl. Phys. Lett.97(11), 111109 (2010). [CrossRef]
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