NANO LETTERS
Preparation of Ultrafine Chalcopyrite Nanoparticles via the Photochemical Decomposition of Molecular Single-Source Precursors
2006 Vol. 6, No. 6 1218-1223
Justin J. Nairn,† Pamela J. Shapiro,*,† Brendan Twamley,† Tyler Pounds,‡ Ray von Wandruszka,† T. Rick Fletcher,† Mark Williams,† Chongmin Wang,§ and M. Grant Norton‡ Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343, School of Mechanical and Materials Engineering, Washington State UniVersity, Pullman, Washington 99164-2920, and WR Wiley EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 Received March 23, 2006; Revised Manuscript Received May 1, 2006
ABSTRACT The synthesis and characterization of ultrafine CuInS2 nanoparticles are described. Ultraviolet irradiation was used to decompose a molecular single source precursor, yielding organic soluble ∼2 nm sized nanoparticles with a narrow size distribution. UV−vis absorption, 1H and 31P{1H} NMR, and fluorescence spectroscopies and mass spectrometry were used to characterize decomposition of the precursors and nanoparticle formation. The nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy energy dispersive X-ray spectroscopy, powder X-ray diffraction (XRD), electron diffraction, inductively coupled plasma analysis, UV−vis absorption spectroscopy, and fluorescence spectroscopy. They have a wurzite-type crystal structure with a copper-rich composition. The hypsochromic shift in their emission band due to quantum confinement effects is consistent with the size of the nanocrystals indicated in the HRTEM and XRD analyses.
Ternary chalcopyrite semiconductor materials of the type Cu(In,Ga)(Se,S)2 are receiving considerable attention as thin film absorbing materials in photovoltaic solar cells.1-3 Their favorable properties include a direct band gap that is well matched to the solar spectrum, a high absorption coefficient, and good thermal, environmental, and electrical stability. Furthermore, the optoelelectronic and conductive properties of these materials can be tuned by doping the materials or varying their In/Ga and Se/S ratios. CuInS2 is of particular interest because of its low toxicity and close to optimal band gap (Eg ∼ 1.5 eV). However, while impressive solar energy conversion efficiencies have been realized with Cu(In,Ga)Se2 thin films (∼19.2%),2j the efficiencies of Cu(In,Ga)S2 thin film solar cells to date have been below 13%.2e This is mainly because Cu(In,Ga)S2 thin film devices have yet to * To whom correspondence may be addressed. E-mail: shapiro@ uidaho.edu. † Department of Chemistry, University of Idaho. ‡ School of Mechanical and Materials Engineering, Washington State University. § WR Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory. 10.1021/nl060661f CCC: $33.50 Published on Web 05/10/2006
© 2006 American Chemical Society
achieve the expected linear gain in the open-circuit voltage compared to Cu(In,Ga)Se2.2e Solar cells based on quantum dots4 with radii smaller than that of the Bohr exciton of the material are predicted to have solar energy conversion efficiencies as high as 66%.4c These extraordinarily high predicted efficiencies are due to quantum size effects such as slower rates of exciton cooling and multiple exciton generation.4c Also, absorber layers consisting of ordered arrays of closely spaced nanoparticles are predicted to have intra-band-gap energy states that will allow sub-bandgap photons to be utilized.4b Reduction in dimensions also concentrates the photogenerated electrons and holes, which reduces entropy, thus improving energy conversion in a manner already observed in thin films.4d Numerous solar cell designs based on semiconductor nanoparticles have been investigated;5 yet few have employed chalcopyrite nanoparticles.6 A three-dimensional photovoltaic device based on a TiO2/CuInS2 nanocomposite7 exhibited a promising 5% photoconversion efficiency,7a and the printing and spraying of chalcopyrite nanoparticle precursors on various substrates, including flexible polymers, has been used to produce thin film solar cells,3 showing that alternative solar
Figure 1. Vials of samples from the irradiation of 20 mM solutions of precursor 1 in DOP for (from left to right) 0, 2, 4, 6, 8, 11, 21, 30, 50, 74, 124, and 218 h.
cell designs and fabrication approaches involving nanostructural chalcopyrite absorbers are worthy of further investigation. Different approaches to the synthesis of chalcopyrite nanoparticles have been reported;6,8 however, only a couple have achieved particle sizes small enough to exhibit quantum confinement (the Wannier-Mott bulk exciton radius of CuInS2 is 8 nm6f), and in most the nanoparticle size distribution is broad. The theoretical limit for quantum dot solar cells is based on an ordered array of nanoparticles with a narrow size distribution (e10% size variation). Compounds with the general formula [(LR3)2CuM(ER′)4] (L ) P, As; E ) S, Se; M ) In, Ga; R ) aryl, alkyl; R′ ) alkyl)1,9 have been used as single source precursors for chemical vapor deposition of chalcopyrite thin films. They have also been used to produce chalcopyrite nanoparticles with some measure of size control via their thermal decomposition in dioctyl phthalate (DOP) at 200-300 °C in the presence of a thiol capping agent.8c,d Herein, we describe the photochemical decomposition of related precursors as a means of producing even smaller, more crystalline chalcopyrite nanoparticles with a narrower size distribution. Detailed characterization of the particles by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy energy dispersive X-ray (SEM/EDX) analysis, powder X-ray diffraction (XRD), electron diffraction, inductively coupled plasma (ICP) spectroscopy, UV-vis absorption spectroscopy, and fluorescence spectroscopy has yielded some interesting phenomena associated with growth, size, composition, crystal structure, and concentration of the nanoparticles that are relevant to synthesis, characterization, and properties of semiconductor quantum dots in general. Although photolysis is most commonly used to make Ag, Au, Pd, Pt, and Cu nanoparticles,10 it has also been used to prepare nanoparticles of binary semiconductors, such as MS (M ) Cd, Zn, Pb),11 MSe (M ) Cd, Pb),12 and Bi2S3.13 To our knowledge, this is the first report of the photochemical synthesis of ternary semiconductor nanoparticles. Precursor Photolysis. Irradiation of 20 mM solutions of the molecular precursors [(TOP)2CuIn(SR)4] (TOP ) (octyl)3P; R ) n-Pr (1), t-Bu (2))14 in organic solvents such as DOP, benzene, toluene, and pentane with a medium pressure Hg Nano Lett., Vol. 6, No. 6, 2006
arc lamp (200 W/in.) causes a gradual color transition of the solutions from colorless to yellow to orange and ultimately orange-red (Figure 1). Similar color transitions were observed by Czekelius et al. during the reaction of bis(trimethylsilyl) sulfide with a mixture of Cu(I)-P(OPh)3 and In(III)-P(OPh)3 complexes to form CuInS2 colloids.6f In that case and in the thermal decomposition of (R3P)2CuIn(SR′)4 precursors,8c,d a deep red color is observed, which is never attained in the photochemical decomposition of these precursors, even with 218 h of irradiation. 1 H NMR studies showed that the single source precursor is completely decomposed within 4 h of irradiation. Nevertheless, the yellow solution continues to darken with further irradiation, indicating continued growth of the average size and concentration of the nanoparticles. Complete decomposition of the precursor occurred in 90% conversion 1219
Figure 2. UV-vis spectra of a 0.6 mM solution of precursor 1 in pentane irradiated for 0, 0.75, 1.5, 3, 6, 12, 24, 36, and 51 h as indicated. Inset: plot of calculated absorption onset as a function of photolysis time.
Figure 3. XRD pattern of nanoparticles (black) with calculated XRD pattern of bulk CuInS2 (roquesite)16 (blue).
after 16 h), as evidenced by a sharp 31P{1H} NMR resonance at δ 46.9. We have demonstrated that the phosphine sulfide can result from a photochemical side reaction between TOP and di-n-propyl sulfide, the major sulfur decomposition byproduct. After 14 h of photolysis the residual TOP appears as two very broad, overlapping signals at -24 and -20 ppm. We attribute these signals to a residual TOP coating on the nanoparticles that is responsible for their excellent solubility in organic solvents such as DOP, pentane, and toluene. Powder XRD characterization of the nanoparticles was problematic. Attempts at standard XRD (Cu KR, λ ) 1.5418 Å, Siemens D5000, SOLEX solid state detector) gave very poorly resolved patterns with extremely low counts. Using a molybdenum source (Mo KR, λ ) 0.71073 Å, Bruker SMART APEX, Debye-Scherrer method, APEX CCD detector, 3 × 3600 s scans) increased the signal-to-noise ratio, and the resulting powder pattern is shown in Figure 3. As expected for fine nanoparticulate material, the diffraction peaks are very broad compared to the calculated powder pattern for the bulk tetragonal CuInS2 (roquesite) phase16 using the same radiation. The peaks below 2θ ) 13° are due to organic material from the sample and amorphous scattering from the sample mounting (glass and glue). An average particle diameter of 1.4 ( 0.3 nm was estimated from the peak at 2θ ) 21.1° and 1.8 ( 0.3 nm from the peak at 2θ ) 24.5° using the Debye-Scherrer formula for 1220
spherical nanoparticles.17 However, due to significant peak overlap the accuracy of these calculations is limited. This calculation usually underestimates average particle diameters because it does not account for other sources of peak broadening.18 Furthermore, HRTEM images of the nanoparticles indicate that they may not be perfectly spherical. HRTEM analysis tends to overestimate the average particle size, since overlapping particles may be mistaken for a discrete particle. Also, larger particles are easier to resolve and are liable to dominate the micrograph.18 Nevertheless, there is reasonable agreement between particle size estimates from the two techniques. The HRTEM micrographs (Figure 4) reveal very small nanoparticles with a narrow size distribution. One of these micrographs (Figure 4a) is shown alongside a micrograph of CuInS2 nanoparticles prepared by thermolysis of the same precursor, (TOP)2CuIn(S-t-Bu)4 (2) (Figure 4b). Significantly, replacing the n-propyl thiolate group with a tert-butyl thiolate group reduces the thermolysis threshold temperature for the precursor from 200 to 170 °C; however, it does not appear to accelerate the photolytic decomposition of the precursor significantly. The nanoparticles formed by photolysis of 2 appear to be more crystalline, by virtue of their lattice fringes, than the ones formed by thermolysis of the precursor. The nanoparticles prepared by photolysis also have a narrower size distribution and appear ovoid rather than spherical. From HRTEM, we estimate the average size of the particles to be 2.2 ( 0.5 nm. The nanoparticles were found to be compositionally copper rich with an average 2.2:1:2.4 ((0.5) Cu:In:S stoichiometry. These results are repeatable and were independently confirmed by ICP and SEM-EDX analyses. This stoichiometry seems at odds with the powder XRD data and the electron diffraction data (vide infra), which confirm a tetragonal CuInS2 crystalline phase. However, it can be explained by assuming that the TOP capping ligands originating from our precursor favor particles with copper-rich surfaces due to the higher affinity of the phosphine ligand for copper over indium. Due to its large surface-to-volume ratio, a 2 nm diameter nanoparticle with a copper-rich surface could contain twice as many copper atoms as indium atoms, as shown by a computer-generated model of ∼2 nm diameter roquesite nanocrystal that has the formula Cu59In27S67. The crystallographic cell parameters determined from the electron diffraction pattern of the nanoparticles are consistent with that of bulk roquesite (CuInS2: a ) b ) 5.52279 Å, c ) 11.13295 Å, space group I-42d).16 Nanoparticles isolated after irradiating the precursor for 23 and 72 h exhibited cell parameters of 5.30, 5.30, 10.60 Å and 5.41, 5.41, 10.82 Å, respectively. Although we do not have enough data to establish a trend, we note that the cell dimensions of the 72 h photolysis sample are slightly larger than that of the 23 h photolysis sample and are approaching that of the bulk material. This difference is probably due to the release of strain in the crystal lattice as a result of particle growth, which has been reported previously.19 The photoluminescence (PL) behavior of the nanoparticles was monitored during the course of the photolyses to see if there were any changes that could be attributed to particle Nano Lett., Vol. 6, No. 6, 2006
Figure 4. TEM micrographs of CuInS2 nanoparticles formed from precursor 2 by (a) photolysis in DOP and (b) thermolysis in DOP at 170 °C.
Figure 5. (a) Photoluminescence spectrum of the photolysis sample at 350 nm excitation showing composite 450 and 550 nm emission bands. (b) Photoluminescence spectrum at 500 nm excitation showing 550 nm emission band. Note that the first excitation peak in the excitation spectrum of (b) corresponds to the contribution of the 450 nm emission tail to the 550 nm emission. (c) Logarithmic plot of fluorescence intensity vs precursor concentration. (d) Plot of fluorescence intensity of the emission peaks at 450 and 550 nm vs photolysis time.
growth. All samples exhibited a broad emission at 450 nm (fwhm ) 160 nm) and a second, weaker emission at 550 nm (fwhm ) 74 nm) at 350 nm excitation. These emission bands correspond to peaks at 350 and 511 nm, respectively, in the photoluminescence excitation (PLE) spectra (parts a and b of Figure 5). While the intensities of both emission bands increase with photolysis time up to 28 h and then level off (Figure 6d), their shapes and positions do not change. Initially, measurements were made on samples of the nanoparticles formed in DOP; however, it was found that the fluorescence of DOP, even at concentrations as low as 1 Nano Lett., Vol. 6, No. 6, 2006
× 10-5 mol L-1, was sufficiently enhanced by the nanoparticles that it interfered with their emission spectra. The fluorescence behavior of the nanoparticles was also influenced by their concentration; above ∼10-13 M nanoparticle concentrations, quenching of the two emissions occurred with the higher energy 450 nm emission quenched more than the 550 nm emission. (Figure 5c). The energies of these emission bands are significantly higher than those of bulk CuInS2 (>810 nm), which exhibits complex photoluminescence due to intraband gap levels as well as defect states.20 Castro et al. observed two broad 1221
ping ligands on the PL properties of the nanoparticles in order to elucidate the origins of their emission bands. The use of ultraviolet light to decompose single source precursors to form semiconductor nanoparticles is attractive because it does not require complicated equipment, it involves mild conditions, the amount of energy imparted to the precursor can be readily controlled, and the light source can be directed with high specificity. Potentially, this could be applicable to the in situ fabrication of solar cells or to the production of micropatterns of semiconducting materials. Figure 6. Effect of different capping agents on the emission intensities of nanoparticles generated from the photolysis of 1.
emission bands at 625 and 663 nm (fwhm ≈ 110 nm) from colloids formed by thermolysis of (PPh3)2CuIn(SEt)4 in DOP with hexanethiol at 200 °C.8c For comparison, we thermolyzed 1 in DOP at 200 °C and found that the resulting colloids also fluoresced at 660 nm. Castro et al. found that varying the capping groups on their nanoparticles affected the intensity of the 663 nm emission (hexanethiol > trioctylphosphine oxide (TOPO) > pyridine). The intensities PL emissions of the nanoparticles produced by photolysis of 1 also vary with capping group (Figure 6). The intensities of both bands decreased after precipitating and redissolving of the nanoparticles. This is probably due to partial surface ligand loss during the precipitation.21 Addition of more TOP to nonprecipitated nanoparticles enhanced the 450 nm emission and quenched the 550 nm band. Replacing the capping ligands with either TOPO or hexanethiol quenched both emissions significantly. The effect of hexanethiol contrasts with that found by Castro et al. for the nanoparticles formed by precursor thermolysis. We suspect that these differences are due to the different preparative conditions resulting in different nanoparticle surface compositions. Interestingly, capping the nanoparticles with thiolacetic acid greatly enhanced the intensity of the 450 nm band while reducing the intensity of the 550 nm emission. The chelating ability of the thiol acetic acid probably strengthens its interaction with the particle surface. Further experiments are in progress to determine the origins of these two emission bands. Castro et al. attributed the emission bands from their nanoparticles to broad-band, defect-related transitions. The higher energy PL from the nanoparticles generated by photolysis indicates that they are smaller than the nanoparticles produced by thermolysis. This is further supported by our preliminary findings that heating the photolysis products of 1 in DOP (4 and 24 h photolyses) for 5 min at 200 °C causes the yellow and orange solutions to turn deep red and develop an emission band at 660 nm. In summary, we have produced ultrafine CuInS2 nanoparticles from the photolytic decomposition of a molecular single source precursor. The nanoparticles produced by this method are ∼2 nm in diameter. HRTEM, powder XRD, and fluorescence measurements indicate that they are smaller and more crystalline than the particles formed by thermolysis of the precursor. The PL properties of the nanoparticles are complex. Further photophysical studies are planned along with a more detailed investigation into the effect of preparative conditions, nanoparticle surface composition, and cap1222
Acknowledgment. The authors are grateful to Idaho NSF EPSCoR, DOE EPSCoR, the NASA Idaho Space Grant Consortium, and the Electrical Power Research Institute for their support. We also thank Mr. Franklin Bailey (University of Idaho) for assistance with TEM and SEM/EDX analysis of our samples and Steven Hughes (University of California, Berkeley) for obtaining HRTEM images of our samples. We also thank Dr. Stephanie Castro for helpful discussions. References (1) (a) Hollingsworth, J. A.; Banger, K. K.; Jin, M. H.-C.; Harris, J. D.; Cowen, J. E.; Bohannan, E. W.; Switzer, J. A.; Buhro, W. E.; Hepp, A. F. Thin Solid Films 2003, 431-432, 63-67. (b) Banger, K. K.; Hollingsworth, J. A.; Harris, J. D.; Cowen, J.; Buhro, W. E.; Hepp, A. F. Appl. Organomet. Chem. 2002, 16, 617-627. (c) Banger, K. K.; Cowen, J.; Hepp, A. F. Chem. Mater. 2001, 13, 3827-3829. (2) (a) Lin, M.; Loh, K. P.; Deivaraj, T. C.; Vittal, J. J. Chem. Commun. 2003, 1400-1401. (b) Deivaraj, T. C.; Park, J.-H.; Afzaal, M.; O’Brien, P.; Vittal, J. J. Chem. Mater. 2003, 15, 2383-2391. (c) Yoshino, K.; Ikari, T.; Shirakata, S.; Miyake, H.; Hiramatsu, K. Appl. Phys. Lett. 2001, 78 (6), 742-744. (d) Weinhardt, L.; Fuchs, O.; Peter, A.; Umbach, E.; Heske, C.; Reichardt, J.; Baer, M.; Lauermann, I.; Kotschau, I.; Grimm, A.; Sokoll, S.; Lux-Steiner, M. C.; Niesen, T. P.; Visbeck, S.; Karg, F. J. Chem. Phys. 2006, 124 (7), 074705/ 1-074705/5. (e) Weinhardt, L.; Fuchs, O.; Grob, D.; Storch, G.; Umbach, E.; Dhere, N.; Kadam, A.; Kulkarni, S.; Heskw, C.; Appl. Phys. Lett. 2005, 86 (6), 062109/1-062109/3 (f) Terheggen, M.; Heinrich, H.; Kostorz, G.; Haug, F.-J.; Zogg, H.; Tiwari, A. Thin Solid Films 2002, 403-404, 212-215. (g) Nakada, T.; Hirabayashi, Y.; Tokado, T.; Ohmori, D.; Mise, T. Sol. Energy 2004, 77, 739747. (h) Lauermann, I.; Baer, M.; Ennaoui, A.; Fiedeler, U.; Fischer, Ch.-H.; Grimm, A.; Koetschau, I.; Lux-Steiner, M. Ch.; Reichardt, J.; Sankapal, B. R.; Siebentritt, S.; Sokoll, S.; Weinhardt, L.; Fuchs, O.; Heske, C.; Jung, C.; Gudat, W.; Karg, F.; Niesen, T. P. Mater. Res. Soc. Symp. Proc. 2003, 763, 175-180. (i) Hariskos, D.; Spiering, S.; Powalla, M. Thin Solid Films 2005, 480-481, 99-109. (j) Ramanathan, K.; Contreras, M. A.; Perkins, C. L.; Asher, S.; Hasoon, Falah, S.; Keane, J.; Young, D.; Romero, M.; Metzer, W.; Noufi, R.; Ward, J.; Duda, A. Prog. PhotoVoltaics 2003, 11, 225-230. (k) John, T. T.; Mathew, M.; Kartha, C. S.; Vijayakumar, K. P.; Abe, T.; Kashiwaba, Y. Sol. Energy Mater. Sol. Cells 2005, 89, 27-36. (l) Siebentritt, S. Thin Solid Films 2002, 403-404, 1-8 and references therein. (m) Shafarman, W. N.; Stolt, L. In Handbook of PhotoVotaic Science and Engineering; Luque, A., Hegedus, S., Eds.; John Wiley & Sons: Sussex, 2003; Chapter 13, and references therein. (3) (a) Basol, B. Thin Solid Films 2000, 361-362, 514-519. (b) Kapur, V. K.; Bansal, A.; Le, P.; Asensio, O. I. Thin Solid Films 2003, 431432, 53-57 (c) Kaelin, M.; Rudmanna, D.; Kurdesaua, F.; Meyerb, T.; Zogga, H.; Tiwaria, A. N. Thin Solid Films 2003, 431-432, 5862. (4) (a) Marti, A.; Cuadra, L.; Luque, A. Physica E 2002, 14, 150-157. (b) Raffaelle, R.; Castro, S.; Hepp, A.; Bailey, S. Prog. PhotoVoltaics 2002, 10, 433-439. (c) Nozik, A. Physica E 2002, 14, 115-120 (d) Queisser, H. Physica E 2002, 14, 1-10. (e) Aroutiounian, V.; Petrosyan, S.; Khachatryan, A. J. Appl. Phys. 2001, 89, 2268-2271. (5) (a) Chena, S.; Paulose, M.; Ruana, C.; Mora, G. K.; Varghese, O. K.; Kouzoudis, D.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 177, 177-184. (b) Sharma, G. D.; Kumar, R.; Sharma, S. K.; Roy, M. S. Sol. Energy Mater. Sol. Cells 2006, 90 (7-8), 933-943. (c)
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