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J. Phys. Chem. C 2009, 113, 8724–8729

Capping the Ball-Milled CdSe Nanocrystals for Light Excitation G. L. Tan*,† and X. F. Yu‡ Institute of New Materials, Wuhan UniVersity of Technology, Wuhan 430070, China, and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education and Department of Physics, Wuhan UniVersity, Wuhan 430072, China ReceiVed: January 22, 2009; ReVised Manuscript ReceiVed: March 27, 2009

CdSe nanocrystals have been synthesized by mechanically alloying Cd and Se elemental powders. XRD results show that pure CdSe compounds has been fabricated after mechanically alloying the elemental powders for more than 20 h. Subsequent capping of the surface of as-milled CdSe nanocrystals with organic-inorganic composite ligand of trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO /NC) has achieved colorful dispersion solution, which shows similar optical properties to those of CdSe nanocrystals prepared by the wet chemical process. The morphology and microstructure feature of the capped nanocrystals have been investigated by high resolution transmission electron microscopy. The grain sizes are within the range of 2∼8 nm for the capped CdSe nanocrystals being ball-milled for 40 h, averaging 3.1 nm. The wavelength of UV visible absorption and PL emission peaks for the dispersion solution of capped CdSe nanocrystals is tunable upon the grain size and dielectric properties of the capping ligand due to quantum confinement effects. The tunable emission wavelength ranges from 520 to 547 nm, and the average full width of half-maximum of the PL emission spectra is 74 nm. The PL quantum yield was measured to be 59%. 1. Introduction Fundamental spectroscopic and structural studies on CdSe nanocrystals by various chemical synthesis routes received increasing interest during the past years.1-3 The chemical synthesis process and optical properties of highly luminescent II-VI semiconductor nanocrystals have been extensively investigated ranging from pure basic research4 to the application of these nanomaterials in electrical and optoelectric devices.5-7 The synthesis of CdSe and CdTe nanocrystals by using Cd(CH3)2 as the cadmium precursor and trioctylphosphine oxide (Tech TOPO) as surface capping agent has led to the synthesis of high quality semiconductor nanocrystals for more than 20 years.8 Afterward, different kinds of safe, low cost inorganic compounds of CdO, Cd(AC)2, and CdCO3 had been been employed as cadmium precursors replacing the expensive Cd(CH3)2 to fabricate high quality CdSe nanocrystals.8 Optical properties of these monodispersed nanocrystals have also been widely studied to confirm the theoretical model of quantum confinement.9,10 Meanwhile, different kinds of shapes of (sphere, rods, disk, and tetrapod-branched)11-13 semiconductor nanocrystals have been recently fabricated through wet chemical methods for diversity application purposes. Zinc blende and wurtzite structures were found to coexist in the tetrapodbranched CdSe nanocrystals.12,13 Recently, the mechanical alloying (MA) process has been applied to synthesize CdTe nanocrystals14-17 and many other types of nanocrystals.18-20 The MA method provides a good pathway to produce surface clean semiconductor nanocrystals, which could be used to study the intrinsic valence electronic structure and optical excitation behavior of quantum dots. Surface clean samples may be able to exclude the influence of capping organic ligand on the physical properties of the semiconductor nanocrystals.14,16 This * Corresponding author. Tel.: +86-27-87870271. Fax: +86-27-87879468. E-mail address: [email protected]. † Wuhan University of Technology. ‡ Wuhan University.

is a safe, low cost, and easy way to fabricate the semiconductor nanocrystals. It can be easily extended to mass production. In our previous study, although CdSe nanocrystals have been successfully synthesized through mechanical alloying of the Cd and Se elemental powders at the atomic ratio 1:1 for more than 8 h, the diffraction peaks of elemental powders were still indexed in the XRD pattern of the final products, indicating that excess Cd coexisted with the as-milled CdSe nanocrystals. This paper is aimed at preparing pure CdSe nanocrystals through the mechanical alloying process by changing the atomic ratio of Cd and Se in the starting materials, so as to avoid the elemental powders remaining in the final product. Once the nanometer CdSe powders were fabricated by the mechanical alloying process, the surface of the nanocrystals was capped with organic-inorganic composite ligands to prepare colorful dispersion solution. The dispersion solution of capped CdSe nanocrystals exhibits similar optical behavior to the CdSe nanocrystals obtained by the wet chemical process. Only after the capping process has been done could the as-milled CdSe nanocrystals be considered for potential application in the fields of biolabeling, electronics, optoelectronics, and solar cells. 2. Experimental Procedure The mixtures of high purity cadmium (99.99%) and selenium (99.999%) elemental powders as the starting materials and stainless steel balls were sealed in a stainless steel vial. Cadmium and selenium powders were bought from a Shanghai chemical company. The atomic ratio of Cd and Se was set at 0.46:0.54. The raw materials (15 g) were weighted according to the above atomic ratio and then put into a stainless steel vial. The vial was then moved into a glovebox, where the lid of the vial was taken away, and the raw materials inside the vial were exposed to the inert gas atmosphere in the glovebox. In this way, the oxygen within the raw materials was removed. Afterward, the vial was sealed again and moved out of the glovebox. The ball milling process of the mixture powders was carried out on a

10.1021/jp900670x CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

Capping the Ball-Milled CdSe Nanocrystals

Figure 1. XRD pattern of the as-milled CdSe nanocrystals, which were ball milling for (a) 6 h, (b) 14 h, (c) 20 h, and (d) 40 h, and (e) positions of JCPDS X-ray diffraction peaks. Solid lines represent the zinc blende structure and short dashed lines the wurtzite structure.

SPEX 8000 M Mixer/mill using a ball-to-powder mass ratio of 10:1. The milling balls with different diameters (2-12 mm) were used. Within different periodic time, small amounts of as-milled powders were taken out of the vial inside the glovebox for structural and optical measurement. The structural evolution of as-milled powders upon different ball milling periods was detected by a Rigaku powder X-ray diffraction machine as well as JEOL 2100F high resolution transmission electronic microscopy (HRTEM). The surface of the ball-milled CdSe nanocrystals was subsequently capped with two kinds of organic-inorganic composite ligands. One is trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NC), and the other is mercaptopropionic acid (MPA)/ poly(sodium phosphate)/nitric acid (NC). The colloid solution of capped nanocrystals was then dispersed in pyridine to form colorful dispersion solution, which shows similar optical behavior to that being prepared by the wet chemical process. The dispersion solution exhibits excellent stability; its color did not change for months or even years. No particles deposit onto the bottom of the glass bottle. Therefore, it could be considered that the capped CdSe nanocrystals show extremely high dispersion stability in the solution. UV visible optical spectra and photoluminescence spectra were then measured upon the dispersion solution containing these capped CdSe nanocrystals. The luminescence spectra were recorded with a fluorescence spectrophotometer F-4500 made by HITACHI. The UV visible absorption spectra were measured upon the dispersion solution with a UV-vis-NIR spectrophotometer CARY 5000 made by VARIAN. 3. Results and Discussion 3.1. Structure Feature of As-Milled CdSe Nanocrystals. Cadmium and selenium elemental powders were mixed together at an atomic ratio of 0.54:0.46 in a stainless steel vial and mounted on a SPEX 8000 M mixer/mill machine. Mechanical alloying the mixture elemental powders leads to the formation of CdSe nanocrystals through the reaction Cd + Se f CdSe by implanting additional impact energy into the reaction system. A small amount of the specimens was taken out of the vial within the glovebox at different periods for structural evaluation. The crystal structure of the mechanically alloyed products was identified using a Rikagu X-ray diffraction machine.

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8725 The X-ray diffraction (XRD) patterns of the as-milled CdSe nanocrystals after ball milling the elemental powders for 6, 14, 20, and 40 h are demonstrated in Figure 1. It is seen from Figure 1 that the diffraction peaks of {111}, {200}, and {311} in zinc blende structure are observed in the pattern. The three cubic diffraction peaks may also be assigned to {002}, {110}, and {112} in wurtzite structure, since the two sets of the diffraction patterns overlapped together. Zinc blende structure absolutely dominates the phase of the 40 h as-milled product, because those important diffraction peaks of (101), (102), (103), (200), and (201) in the wurtzite structure are missing. However, the wurtzite structure may not be excluded in the as-milled CdSe nanocrystals, since the diffraction peaks from the {100} lattice plane of wurtzite structure are still observed in the XRD patterns. The as-milled CdSe nanocrystals are composed of mixture phases of zinc blende and wurtzite structures, among which zinc blende structure was the dominant phase. Extending the ball milling time from 6 to 40 h did not cause any change but broadens the diffraction peaks. This feature shows evidence of finite size broadening with longer ball milling time,21 which agrees with the broadening reflections of small CdSe nanocrystals less than 4.2 nm through the wet chemical method.5,8 The clean diffraction pattern in Figure 1d clearly shows that pure CdSe nanocrystals have been successfully fabricated through mechanical alloying of Cd and Se elemental powders for 40 h; no diffraction features of elemental powders or any other kinds of impurity are visible in the XRD pattern of Figure 1. 3.2. Microstructure Characterization of Capped CdSe Nanocrystals. The as-milled CdSe nanocrystals were subsequently capped with two kinds of organic-inorganic composite ligands to form dispersion solution for optical excitation. A mixture ligand of mercaptoproprionic (MPA)/poly(sodium phosphate)/nitric acid (NC) was first used to cap the surface of the 40 h ball-milled CdSe nanocrystals. A colloid solution was formed after 2 mL of the mixture MPA ligand was added into the as-milled powders (about 0.05 g) in a small glass bottle. Afterward, 3 mL of pyridine was added into the colloid solution. In this way, a colorful dispersion solution of capped CdSe nanocrystals was obtained. The capped CdSe nanocrystals show extremely high dispersion stability in the solution. A small amount of the dispersion solution was then dropped onto a carbon grid for TEM observation. The morphology of the capped CdSe nanocrystals in the dispersion solution has been demonstrated in Figure 2. It can be seen that the particles are homogeneously distributed in the dispersion solution. There is no aggregation of the nanocrystals. The size of the nanocrystals ranges from 2 to 8 nm. Without capping processing, the as-milled nanocrystals were seriously aggregated, and they are not be able to float in any kind of organic solution.14 The dispersion solution of uncapped CdSe nanocrystals is colorless, and most of the particles deposited onto the bottom of the bottle. The nanocrystals show much wider size distribution. However, after the surface of the as-milled CdSe nanocrystals was capped with MPA/poly(sodium phosphate)/NC and the addition of pyridine solution, the nanocrystals could stably float in the colloid solution, leading to the formation of colorful dispersion solution. The morphology and microstructure of the capped CdSe nanocrystals are shown in Figure 2, which is similar to that of the CdSe nanocrystals being prepared by wet chemical methods. In the second case of capping processing, a small amount (about 0.05 g) of 40 h as-milled powders was put into a small fresh glass bottle. Five grams of trioctylphosphine oxide (TOPO) was melted at 150 °C, and 5 mL of trioctylphosphine (TOP)

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Figure 2. TEM images for the morphology of capped CdTe nanocrystals by MPA, which was dispersed in pyridine; the nanocrystals have been ball-milled for 40 h.

Figure 3. TEM and HRTEM images of the capped CdSe nanocrystals by TOP/TOPO/NC, dispersed in the pyridine solution. The scale bar in image (b) is 5 nm.

and 0.5 mL of nitric acid were added into the melted TOPO. Then 3∼5 mL of TOPO/TOP/NC mixture liquid was poured onto the as-milled powders in the glass bottle. A red colloid dispersion solution was obtained. Afterward, 3 mL of pyridine was added into the colloid solution to prepare a semitransparent colorful dispersion solution for optical excitation. The dispersion solution exhibits excellent stability, the particles did not deposit onto the bottom of the bottle, and the color of the solution did not change for months. The resulting solution was dropped on a holey carbon TEM grid to enable study by transmission electron microscopy (TEM). The TEM measurement was carried out on a JEOL 2100F machine. The TEM and HRTEM images of capped CdSe nanocrystals are illustrated in Figure 3. It can be seen from Figure 3a that capped CdSe nanocrystals were homogeneously distributed on the chains of a polymer network, which was formed by the chemical reaction of TOPO/TOP/ NC. These particles are homogenously size distributed, and no aggregation was observed. When TOPO/TOP/HPA was used to stabilize CdSe nanocrystals, the long chain molecules were coated on the surface of the nanocrystals.8 The dispersive force among the polarized molecules stabilized the nanocrystals in the colloid solution. In this study, HPA was replaced by NC, and the chemical reaction of TOP + TOPO and NC leads to the formation of a polymer network, upon which the CdSe nanocrystals were pinned and supported within the dispersion

solution, as shown in the images of Figure 3a. This kind of polymer network has been extended into the whole dispersion solution; thus, the pinned CdSe nanocrystals exhibit excellent stability. The optical properties of the colloid CdSe solution did not change for months or even for years, which suggests that the dispersion solution of capped CdSe nanocrystals is stable enough for application in light emitting devices. Figure 3b shows a high resolution TEM image of the capped CdSe nanocrystals by TOPO/TOP/NC, lattice fringes of which are clearly seen in the image, indicating that the nanocrystals were well crystallized. Most of the lattice fringes are assigned to {111} and {200} lattice planes of CdSe nanocrystals in zinc blende structure. It can also be seen from Figure 3b that the particles are well separated and homogeneously distributed within the dispersion solution. The grain size is within the range of 2-8 nm, averaging around 3.1 nm. The statistical results of size distribution for capped CdSe nanocrystals are exhibited in Figure 4, where the columns represent the statistical count ratio corresponding to the grain size and the fitted line was calculated from the log-normal function. The CdSe nanocrystals have been ball-milled for 40 h, and a mixture of organic-inorganic ligand of TOPO/TOP/NC was subsequently capped on the surface of the as-milled nanocrystals, which were then dispersed in pyridine. The grain size of the individual nanocrystals was measured with TEM as

Capping the Ball-Milled CdSe Nanocrystals

Figure 4. Statistical size distribution of capped CdSe nanocrystals dispersed in pyridine.

well as HRTEM images of these capped CdSe nanocrystals dispersed in pyridine. Statistical size distribution was calculated from the summarized datum of the individual nanocrystal size, and the results are shown in Figure 4. It can be seen that the grain sizes are within the range of 2∼8 nm, and the capped CdSe nanocrystals are majorly distributed within the range of 2∼4 nm. The log-normal distribution model was centered at 3.1 nm, which could be considered as the average grain size of the capped CdSe nanocrystals. Therefore, it may be supposed that the capped CdSe nanocrystals are homogeneously distributed in the pyridine solution. 3.3. Optical Excitation Spectra for the Capped CdSe Nanocrystals. The as-milled CdSe powders exhibit dark colorization after ball milling for several hours. The as-milled CdSe nanoparticles were taken out of the vials within different intervals and then capped on the surface with long chain absorption ligand TOPO/TOP/NC. Afterward the capped CdSe nanopaticles were dispersed in organic solution of pyridine. The dispersion solution containing capped CdSe nanoparticles showed red color. In this way, the colorful dispersion solution containing the capped CdSe nanoparticles was obtained. In our previous study, the as-milled CdSe nanocrystals were capped by TOPO/TOP without NC, and the color of the dispersion solution could keep stable for only one or two days. After that, most of the particles deposit onto the bottom of the bottle, and the color of the dispersion solution turned colorless. In this study, we employed organic-inorganic composite ligand of TOPO/ TOP/NC to cap the surface of the as-milled CdSe nanocrystals, and thus a stable dispersion solution of the capped nanocrystals was obtained. Finally the measurement of UV visible and photoluminescence optical spectra was performed upon the stable dispersion solution of the capped CdSe nanocrystals. Figure 5a shows a typical absorption spectrum of CdSe nanocrystals being capped by MPA/poly(sodium phosphate)/ NC. It can be seen that there is a broad absorption peak at 391 nm. This absorption peak corresponds to the band gap energy of 3.18 eV for CdSe nanocrystals being capped by MPA and poly(sodium phosphate). There is a blue shift of 1.47 eV for the band gap energy of capped CdSe nanocrystals in comparison with that of bulk single crystal. This big blue shift was caused by the strong quantum confinement effect of small CdSe nanocrystals. The microstructure of capped CdSe nanocrystals was exhibited in Figure 2, where it can be seen that the particles were homogeneously distributed in the dispersion solution, and no aggregation was observed. The stable dispersion solution of

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8727 MPA-capped CdSe nanocrystals exhibits similar optical properties to the CdSe nanocrystals being prepared by wet chemical methods. However, the absorption peak in the UV visible spectrum of capped CdSe nanocrystals red shifts from 391 to 516 nm when the capping ligand of MPA/NC was replaced by the TOPO/TOP/NC organic-inorganic ligand. Figure 5b shows such a UV visible absorption spectrum for the dispersion solution of CdSe nanocrystals capped by TOPO/TOP/NC, the nanocrystals having been ball-milled for 40 h. It can be seen that the colloid dispersion solution containing CdSe nanocrystals capped by TOPO/TOP/NC exhibits two sharp absorption peaks at 516 and 550 nm, respectively. As mentioned above, the 40 h as-milled CdSe nanocrystals are composed of mixture phases of zinc blende and wurtzite structures, the phase of zinc blende structure dominating the nanocrystals. The optical excitation of the nanocrystals should be also contributed from two structure units; one is zinc blende structure, the other wurtzite structure. The band gap energy of zinc blende (cubic) CdSe crystal is unknown at the moment. However, cubic CdSe crystal has similar structure and lattice parameters to zinc blende CdTe crystal. Therefore, it may be assumed that the band gap energy of cubic CdSe crystal is comparable to that of cubic CdTe crystal, whose band gap energy is 1.475 eV (841 nm). Under this assumption, the sharp absorption peak at 550 nm in Figure 5b could be assigned to optical excitation from CdSe nanocrystals in zinc blende structure. The other absorption peak located at 516 nm may correspond to the contribution from CdSe nanocrystals in the wurtzite structure. This UV visible spectrum of the CdSe nanocrystals capped by TOPO/TOP/NC also demonstrates similar optical excitation properties to the CdSe nanocrystals prepared by wet chemical methods,8 which suggests that the as-milled CdSe nanocrystals could also be applied in light emission, solar cell, and biolabeling fields after the capping process. A huge red shift occurred between the absorption peaks of CdSe nanocrystals capped by MPA/NC and TOP/TOPO/NC, as shown in Figure 5a and b. This results from the change of the dielectric properties of the environment medium surrounding the CdSe nanocrystals. Supposing a quantum dot (QD) sphere of radius R and dielectric coefficient ε2 is surrounded by a medium of dielectric coefficient ε1, the shift of band gap energy of the spherical QD is expressed in terms of L. E. Brus’ model as in following equation22

∆E )

(

)

∞

S 1.8e2 1 e2 p2π2 1 1 1 + · + R 2 me mh R2 ε2 R R n)1 n R

∑

2n

()

(1)

where Rn ) (ε - 1)(n + 1)/[ε2(εn + n + 1)], ε ) ε2/ε1, and ∆E is the energy shift with respect to the bulk band gap. It can be seen from the third term of eq 1 that the band gap energy of the semiconductor nanocrystals would make a red shift if the dielectric coefficient of medium surrounding the nanocrystals is bigger than that of the nanocrystals itself. This kind of red shift is called the dielectric confinement effect. Obviously the dielectric constant of the composite medium of TOPO/TOP/NC is bigger than that of CdSe nanocrystals, while the dielectric coefficient of MPA/poly(sodium phosphate) is smaller than that of CdSe nanocrystals. In the first case, the band gap energy makes a red shift, while in the second case the band gap energy makes a blue shift in comparison with the uncapped CdSe nanocrystals. Therefore, the band gap energy of CdSe nanocrystals capped by TOPO/TOP/NC is smaller than that of nanocrystals capped by MPA/poly(sodium phosphate). The difference of the dielectric properties of the environment medium surrounding the nanocrystals changes the band gap

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Figure 5. UV visible spectra of the dispersion solution of capped CdSe nanocrystals. The CdSe nanocsrystals were ball-milled for 40 h and subsequently capped on the surface by (a) MPA/NC and (b) TOPO/TOP/NC.

4. Conclusion

Figure 6. Photoluminescence emission spectra of the dispersion solution of capped CdSe nanocrystals, which were ball-milled for (a) 6 h, (b) 14 h, (c) 20 h, (d) 40 h, and (e) 80 h.

energy of the semiconductor nanocrystals, which causes the huge red shift of the absorption peak in the optical spectra of Figure 5a and b. Figure 6 shows photoluminescence emission spectra for the dispersion solution of capped CdSe nanocrystals, which were ball-milled for (a) 6 h, (b) 14 h, (c) 20 h, and (d) 80 h; subsequently the surface of the as-milled CdSe nanocrystals was capped with TOPO/TOP/NC composite ligand. It can be seen from Figure 6 that the emission wavelength of the photoluminescence was tunable upon ball milling time. The tunable emission wavelength ranges from 520 to 547 nm. The wavelength of the emission peak for the capped CdSe nanocrystals being ball-milled for 6 h locates at 547 nm, which blue shifts to 520 nm for the capped CdSe nanocrystals being ball-milled for 80 h. The average full width of half-maximum (fwhm) of the PL emission spectra for the capped CdSe nanocrystals is 74 nm. The PL quantum yield was measured to be 59% for the dispersion solution of capped CdSe nanocrystals, which were ball-milled for 40 h. Since longer ball milling time improves the ratio of smaller nanocrystals, the average band gap energy of the as-milled CdSe nanocrystals increases with ball milling time. Therefore, photoluminescence emission spectra blue shift to the shorter wavelength side for the capped CdSe nanocrystals which were ball-milled for a longer period. The longer is ball milling time, the smaller is the size of the nanocrystals, which corresponds to shorter emission wavelength in the PL spectra of Figure 6.

CdSe nanocrystals have been synthesized by mechanically alloying the Cd and Se elemental powders. Zinc blende structural CdSe nanocrystals were produced after mechanically alloying the elemental powders for 40 h. The structural evolution of the elemental powders with the mechanical alloying time has been detected by X-ray diffraction patterns. Subsequent capping of thesurfaceoftheas-milledCdSenanocrystalswithorganic-inorganic composite ligand (TOPO/TOP/NC or MPA/NC) leads to the formation of colorful dispersion solutions. The capped CdSe nanocrystals show extremely high dispersion stability in the solution. TEM images demonstrate that the capped nanocrystals were homogeneously distributed in the dispersion solution. The grain size of the capped CdSe nanocrystals ranges from 2 to 8 nm, averaging 3.1 nm. UV visible optical and photoluminescence spectra have been measured upon these dispersion solutions. The absorption and PL emission peak positions were tunable upon ball milling time and surface-modified molecules due to the quantum confinement effect. Change of the capping ligand leads to the red shift of the band gap energy of the CdSe nanocrystals; this was caused by the change of the dielectric coefficient of the environment medium surrounding the CdSe nanocrystals. References and Notes (1) Resch, U.; Weller, H.; Henglei, H. Photochemistry and radiation chemistry of colloidal semiconductors. 33. Chemical changes and fluorescence in CdTe and ZnTe. Langmuir 1989, 5, 1015. (2) Piskach, L. V.; Parasyuk, O. V.; Olekseyuk, I. D.; Romanyuk, Y. E.; Volkov, S. V.; Pekhnyo, V. I. Interaction of argyrodite family compounds with the chalcogenides of II-b elements. J. Alloys Compd. 2006, 421, 98. (3) Pukowska, B.; Jaglarz, J.; Such, B.; Wagner, T.; Kisiel, A.; Mycielski, A. Optical investigations of the CdTeSe and CdMeTeSe (Me)Mn, Fe) semiconductors. J. Alloys Compd. 2002, 335, 35. (4) Tinjod, F.; Robin, I.-C.; Andre´, R.; Kheng, K.; Mariette, H. Key parameters for the formation of II-VI self-assembled quantum dots. J. Alloys Compd. 2004, 371, 63. (5) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P.; McEuen, P. L. The potential environmental impact of engineered nanomaterials. Nature (London) 1994, 370, 354. (6) Rogach, A.; Kornowski, A.; Gao, M.; Eychmueller, A.; Weller, H. Synthesis and characterization of a size series of extremely small thiolstabilized CdSe nanocrystals. J. Phys. Chem. B 1999, 103, 3065. (7) Beaulac, R.; Archer, P. I.; Liu, X.; Lee, S.; Salley, G. M.; Dobrowolska, M.; Furdyna, J. K.; Gamelin, D. R. Nano Lett., in press (10.1021/nl080199u).

Capping the Ball-Milled CdSe Nanocrystals (8) Peng, X. G.; Mannam, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadacanich, A.; Alisvisatos, A. P. Preparation of CdSe/ZnSe core-shell nanorystals in one step reaction. Nature (London) 2000, 404, 59. (9) Lippens, P. E.; Lannoo, M. Calculation of the bandgap for small CdS and ZnS crystallines. Phys. ReV. B 1989, 39, 10935. (10) Colvin, V. L.; Alivisatos, A. P. Valence-band photoemission from a quantum-dot system. Phys. ReV. Lett. 1991, 66, 2786. (11) Carbone, L. Synthesis and micrometer-scale assembly of colloidal CdSe/CdS nanorods prepared by a seeded growth approach. Nano Lett. 2007, 7, 2942. (12) Jun, Y. M.; Lee, S. M.; Kang, N. J.; Cheon, J. Controlled synthesis of multi-armed CdS nanorod architectures using monosurfactant system. J. Am. Chem. Soc. 2001, 123, 5150. (13) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled growth of tetrapod-branched inorganic nanocrystals. Nat. Mater. 2003, 2, 382. (14) Tan, G. L.; Hommerich, U.; Temple, D.; Wu, N. Q.; Zheng, J. G.; Louts, G. Synthesis and optical characterization of CdTe nanocrystals prepared by ball milling process. Scr. Mater. 2003, 48, 1469. (15) Tan, G. L.; Yang, Q.; Seo, J. T.; Hommerich, U.; Temple, D. Linear and non-linear optical properties of capped CdTe nanocrystals prepared by mechanical alloying. Opt. Mater. 2004, 27, 579.

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8729 (16) Tan, G. L.; Wu, N.; Zheng, J. G.; Hommerich, U.; Temple, D. Optical absorption and valence band photoemission from uncapped CdTe nanocrystals. J. Phy. Chem. B 2006, 110, 2125. (17) Camposa, C. E. M.; Grandi, T. A.; Ho¨hn, H.; Pizani, P. S. J. Alloys Compd. 2008, 466, 80. (18) Guimaraes, J. L.; Abbate, M.; Betim, S. B.; Alves, M. C. M. Preparation and characterization of TiO2 and V2O5 nanoparticles produced by ball-milling. J. Alloys Compd. 2003, 352, 16. (19) Balaz, P.; Takacs, L.; Ohtani, T.; Mack, D. E.; Boldizarova, E.; Soika, V.; Achimovicova, M. Preparation of nanosized antimony by mechanochemical reduction of antimony sulphide Sb2S3. J. Alloys Compd. 2002, 434-435, 773. (20) Bouad, N.; Marin-Ayral, R. M.; Nabias, G.; Tedenac, J. C. Phase transformation study of PbsTe powders during mechanical alloying. J. Alloys Compd. 2003, 353, 184. (21) Guinier, A. X-ray diffraction, Freeman, W. H., Ed.; San Francisco, 1963, p 122-149. (22) Brus, L. E. J. Chem. Phys. 1984, 80, 1984.

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