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Aqueous Phase Synthesized CdSe Nanoparticles with Well-Defined Numbers of Constituent Atoms Yeon-Su Park,*,†,‡,§ Andriy Dmytruk,*,† Igor Dmitruk,† Atsuo Kasuya,† Yukihiro Okamoto,‡,§ Noritada Kaji,‡,§ Manabu Tokeshi,‡,§ and Yoshinobu Baba‡,§,| Center for Interdisciplinary Research, Tohoku UniVersity, Sendai 980-8578, Japan, Department of Applied Chemistry, Nagoya UniVersity, Nagoya 464-8603, Japan, FIRST Research Center for InnoVatiVe NanobiodeVice, Nagoya UniVersity, Nagoya 464-8603, Japan, and Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology, Takamatsu 761-0395, Japan ReceiVed: August 12, 2010; ReVised Manuscript ReceiVed: September 23, 2010
We report aqueous phase synthesized semiconductor nanoparticles with well-defined numbers of constituent atoms. Aqueous phase synthesis provides many advantages over organic phase synthesis for producing such high-quality semiconductor nanoparticles. We synthesized CdSe nanoparticles with excellent colloidal and optical stabilities directly in aqueous solution at room temperature and then identified them as selectively grown (CdSe)33 and (CdSe)34 magic-sized clusters. These clusters displayed extremely sharp excitonic absorption and emission peaks because of their practically monodispersed size distribution. Their X-ray diffraction pattern and Raman spectral features were considerably different from the corresponding pattern and features for typical crystalline CdSe nanoparticles. Growth of our magic-sized clusters was very slow and proceeded via the formation of different sizes of progressively larger CdSe nanoparticle intermediates with time. Our results demonstrated that aqueous phase synthetic routes could be successfully adopted for producing high-quality semiconductor nanoparticles. Introduction Semiconductor nanoparticles (NPs) stabilized with welldefined numbers of constituent atoms are very attractive because such NPs can be uniquely realized at those specific numbers and their atomic arrangement and associated optical and physical properties can be very different from those for their bulk crystalline phase. Such distinctive and high-quality semiconductor NPs may be very useful as building blocks for functional materials in nanoscience and nanotechnology. Presently many reports are available on relatively large semiconductor NPs (i.e., d g 2 nm) with a narrow size distribution that were identified mostly as fragments of the bulk crystalline structure by transmission electron microscopy (TEM) and/or by X-ray diffraction (XRD) spectroscopy.1-8 The literature on smaller semiconductor NPs (i.e., d < 2 nm) is comparatively scarce,1-5 and in most cases their structures have not been accurately determined. For fabricating such high-quality semiconductor NPs, much attention has been given to extremely small colloidal semiconductor NPs (typically within a size range of 1-2 nm) with single-sized ensembles and high stability.2,7-21 Those NPs, called semiconductor magic-sized clusters (MSCs), have a local minimum chemical potential because of their closed-shell configurations, and they may have well-defined chemical and physical structures.8,10,14 They are characterized by extremely sharp absorption peaks at well-defined energies due to their discrete size as well as with elevated stability against their neighboring sizes due to their energetically stable configurations. * To whom correspondence should be addressed. E-mail: yspman@ gmail.com (Y.-S.P.);
[email protected] (A.D.). † Tohoku University. ‡ Department of Applied Chemistry, Nagoya University. § FIRST Research Center for Innovative Nanobiodevice, Nagoya University. | National Institute of Advanced Industrial Science and Technology.
There have been several reports about semiconductor MSCs,2,7-21 although they are very limited compared with those for regular colloidal semiconductor NPs. The majority of the publications dealt with cadmium chalcogenide MSCs that were formed either as final reaction products or as intermediates for further growth to larger or different-shaped NPs.2,7-21 In 1998, Ptatschek et al.2 reported three different families of colloidal CdSe MSCs exhibiting extremely sharp first extinction peaks at 280, 360, and 410 nm, respectively. Their respective gyration sizes were 0.42, 0.85, and 1.7 nm. They attributed the MSCs with the first extinction peak at 410 nm to Cd34Se19Lig37.5 (Lig ) ligand) having a Kosh pyramid structure. Since then, there have been other publications about CdSe MSCs. In 2001, Soloviev et al.9 described CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands. Those clusters (gyration size ) 0.7-2 nm), crystallized from organic solution, were composed of a combination of adamanthane and barylene-like cages. Other different families of CdSe MSCs exhibiting extremely sharp bandgap absorption at 395, 463, and 513 nm, respectively, were synthesized in hot organic solution (120-240 °C) by a one-pot noninjection method.12 Their sizes were estimated to be in the range of 1.7-2.2 nm, based on diffusion ordered nuclear magnetic resonance (DOSY-NMR) spectroscopy measurements.12,13 A series of heterogeneous mixtures of CdSe MSCs were fabricated in organic solution at a relatively low temperature of 80 °C.7 The mixtures were composed of two or three different families of CdSe MSCs exhibiting a strong absorption at 330, 350-360, 384, 406, 431, or 447 nm. Wideangle XRD analysis estimated the sizes to be 1.5-2.0 nm for the MSCs with extinction peaks at 406, 431, and 447 nm, respectively. Reports about CdS MSCs and CdTe MSCs are also available. CdS MSCs were fabricated in organic solution using the one-pot noninjection method at elevated temperature (90-140 °C): they showed an extremely sharp absorption at
10.1021/jp107608b 2010 American Chemical Society Published on Web 10/18/2010
Aqueous Phase Synthesized CdSe Nanoparticles 378 nm, and their size was estimated to be ∼1.9 nm using DOSY-NMR spectroscopy.18 A mixture of CdS MSCs exhibiting strong absorption at 323 nm and regular CdS NPs exhibiting an absorption at 390 nm were prepared in a liquid paraffin/ glycerol biphasic system at 110-150 °C.19 Addition of alcohols induced transformation of those MSCs into MSCs exhibiting a strong absorption at 309 or 348 nm. CdTe MSCs showing very sharp absorption at 427 nm were fabricated in organic solution using a noninjection method (120-220 °C). Their size was estimated to be ca. 1.5 nm, using DOSY-NMR spectroscopy.20 CdTe MSCs exhibiting a distinctive absorption at 425 nm were prepared, using a hot injection method, as intermediates for further growth to larger-sized CdTe MSCs.21 (CdSe)33 and (CdSe)34 clusters have been found to be particularly stable among smaller CdSe NPs.22 Extremely elevated stability of those CdSe NPs was clearly evidenced from atomically resolved spectroscopic experiments: both laser ablation of bulk CdSe powder and laser ionization of selectively grown, decylamine-capped CdSe NPs produced distinctive mass spectroscopic peaks at the positions corresponding to (CdSe)33 and (CdSe)34. Macroscopic quantities of the decylamine-capped clusters were fabricated preferentially as a single species in a toluene phase of reverse micelles because of their extremely elevated stability against their neighboring sizes, like MSCs, and not because of the choice of their synthetic routes. Thus, this case is very similar to a compound version of carbon fullerenes with molecular-like structures never found before but different from fragments of bulk crystalline structure synthesized to a particular size or those stabilized rigidly by ligands such as Cd34Se19Lig37.5 and Cd32S14(SC6H5)36-DMF4.2,23 Aqueous-phase synthesis of semiconductor NPs is of great interest because it can provide a simpler, safer, more convenient, more reliable, more economical, and more eco-friendly route, as compared with organic phase synthesis, for fabricating highquality semiconductor NPs.24-33 However, the structural and optical properties of semiconductor NPs synthesized in aqueous media are typically much inferior to those of the NPs fabricated in organic media. Presently, to our best knowledge, no reports exist about aqueous phase synthesized, monodispersed semiconductor NPs with well-defined numbers of constituent atoms. Recently we introduced a novel aqueous phase synthetic route for fabricating stable CdSe NPs exhibiting an extremely sharp first extinction peak (fwhm ) 17.5-18 nm) at 420 nm.34,35 Details about those NPs (i.e., their physical and chemical structures, size, size dispersion, and emission properties) have not been uncovered, but their optical absorption features are very similar to those for organic phase synthesized, monodispersed CdSe MSCs except for a slight difference in the position of absorption peaks.22 Those MSCs stabilized in toluene showed an extremely sharp first extinction peak (fwhm ) 18 nm) at 415 nm. This spectral similarity signifies that monodispersed semiconductor NPs with well-defined numbers of constituent atoms could be fabricated in aqueous media. Herein, we report aqueous phase synthesized semiconductor NPs with well-defined numbers of constituent atoms as well as with excellent colloidal and optical stabilities. We synthesized CdSe MSCs directly in basic aqueous solution at room temperature. We studied their growth kinetics because of its importance to the development of synthetic chemistry for such high quality semiconductor NPs in aqueous media. Growth of our MSCs was very slow because of the low reaction temperature and low Cd precursor reactivity. The growth proceeded via the formation of different families of progressively larger CdSe NP intermediates with time. We also investigated their
J. Phys. Chem. C, Vol. 114, No. 44, 2010 18835 physical, chemical, and optical properties to confirm the fabrication of such high quality NPs and to acquire further details about them. Our CdSe MSCs were identified as selectively grown (CdSe)33 and (CdSe)34 MSCs. They exhibited extremely sharp excitonic absorption and emission because of their practically monodispersed size distribution and their spectroscopic features were considerably different from those of crystalline CdSe NPs. Experimental Methods Materials. Cadmium sulfate, sodium sulfite, selenium powder, sodium hydroxide (1M), and L-cysteine (cys) were purchased from Wako Pure Chemicals (Japan). 3-Aminopropyltriethoxysilane (APS) was acquired from Sigma-Aldrich (USA). All chemicals were used as received without further purification. All aqueous solutions were prepared with deionized H2O (Milli-Q water, R > 18.2 MΩ · cm). Synthesis of CdSe NPs in Aqueous Solution. The CdSe NPs were synthesized by sequential addition of desired amounts of NaOH (1M), cys, CdSO4 (0.15 M Cd2+, Cd precursor), and Na2SeSO3 (0.05 M Se2-, Se precursor) to glass vials filled with a predetermined volume of deionized H2O at room temperature with mild magnetic stirring. All aqueous solutions were used without inert gas bubbling. The vials were capped in an air atmosphere after Se precursor addition and kept at room temperature in the dark, while continuing the mild magnetic stirring. The initial synthesis solution (pH ∼12.3) contained 1.5 mM Cd precursor, 13.2 mM cys, 37.5 mM NaOH, and 0.375 mM Se precursor. Growth of CdSe NPs was very slow under our synthesis conditions: typically, a reaction period of 6-7 days was required to fabricate the NPs with matured optical absorption features. Resulting CdSe NPs were extremely stable during storage at room temperature in the dark. For our investigations, we used CdSe NPs grown for 7 days. Two important modifications were made from our previous synthesis methods34,35 to synthesize the highest-quality CdSe NPs in terms of absorption peak sharpness. First, freshly prepared, hot Se precursor (1-2 days aged; 90 °C) was used, without cooling it down to room temperature, to prevent decomposition of Na2SeSO3 to selenium oxides. Second, a 10-min or longer time interval was allowed between each chemical/solution addition to ensure better solution mixing. The long time interval adopted might be especially effective for the gradual and complete transformation of water-insoluble Cd-cys complexes (i.e., CdH2L2 and CdHL2-, major Cd-cys complexes in weakly acidic, neutral, and weakly basic conditions), formed tentatively along the Cd precursor (pH ∼5.2) injection trajectory, to water-soluble CdL34- (a major Cd-cys complex in synthesis solution at ncys/ nCd2+ of 8.8 and pH of ∼12.3).38 (For details, see Supporting Information). Characterization. UV-visible absorption spectra were recorded with a U-2000 (Hitachi) spectrophotometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained using a FP-750 (JASCO) spectrofluorometer. TEM images were obtained using a JEM-2000 (JEOL) microscope operated at 200 kV. Atomic force microscopy (AFM) images were obtained using a NanoScope IIIa (Digital Instruments) scanning probe microscope and DNP-S tips (Veeco Instruments) in a fluid tapping mode. For this, our CdSe NPs were immobilized on a fresh mica substrate, using APS as a linker molecule. Height profiles for a total of 61 spot images were used for NP size estimation. XRD profiles were recorded using a MSXHF22 (Rigaku) spectrophotometer. For Raman spectra, a triple monochromator T64000 (Jobin-Yvon) with a
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Figure 1. (a) Time-dependent evolution of UV-visible absorption spectra for cys-capped CdSe NPs. (b) The peak position and absorbance changes at an early stage of NP growth (trxn e 10 days). (c) Timedependent change in HWHM of the first extinction peak (P1) at the early stage.
micro Raman setup and an Ar-Kr mixed gas ion laser BeamLok 2060 (Spectra-Physics) were used (excitation at 413.1 nm of Kr+ line). Time-of-flight (TOF) mass spectra were recorded using a Reflex III (Bruker Daltonics) equipped with a nitrogen laser. Results and Discussion Growth and Stabilization. Figure 1a shows the timedependent development of absorption features for cys-capped CdSe NPs during their synthesis at room temperature. Immediately after final precursor (Se precursor) addition (reaction time, trxn ∼1 min), two very broad peaks appear around 370 nm (marked as P1) and 348 nm (marked as P2). As trxn increases until day 7, these peaks become more red-shifted and more distinctive and intense. A new tiny shoulder appears around 390 nm at trxn of 10 min, and it becomes more distinctive as trxn increases. All absorption features experience only a slight change after trxn of 7 days: the spectra for the NPs at trxn ) 7 and 77 days are almost identical. Close analyses (parts b and c of Figure 1) of the position, intensity, and sharpness of the extinction peaks in the spectra taken at an early NP growth stage (trxn e10 days) reveal three distinctive growth kinetic regimes (regime 1, trxn e 2 h; regime 2, 2 h < trxn < 7 days; regime 3, trxn g 7 days). Regime 1 is characterized by rapid changes in the position, intensity, and sharpness of the extinction peaks. As trxn increases in this regime, positions of P1 and P2 peaks are red-shifted rapidly; intensities of P1 and P2 peaks increase quickly; half width at half-maximum (HWHM) of P1 peak decreases rapidly. In regime 2, the red-shift, intensity enhancement, and sharpening of the peaks become gradually slow with increasing trxn. In regime 3, the position, intensity, and sharpness of the peaks experience little change. These observations can be interpreted
Park et al. as follows. In regime 1, upon Se precursor addition, CdSe monomers are initially formed; then, these monomers nucleate rapidly to form CdSe nuclei which subsequently grow into small CdSe NPs.17,36,37 It is also possible for the Cd and Se precursors to decompose directly on the monomers or on the growing small NP surfaces; the monomer formation and nucleation steps may not be completely separated. These kinetics-driven steps, in the initial growth stage, result in relatively unstable small CdSe NPs with broad size distributions; due to their low stability, the small CdSe NPs start growing quickly into more stable, larger ones by decomposition and coalescence, leading to a relatively narrow size distribution of the NPs (this is referred to as “focusing of the size distribution”).17,36,37 As a result, a series of different sizes of small but progressively larger CdSe NPs with narrower size distributions are formed with time. In regime 2, both growth of the NPs and focusing of the size distribution gradually slow down, due to the decreased concentrations of the monomers, nuclei, and small NPs necessary for further NP growth.17,36,37 In regime 3, growth of the NPs becomes extremely slow (practically no growth) due to the formation of the CdSe NPs with extremely high stability at trxn ≈ 7 days. Further detailed analyses of the results (parts b and c of Figure 1) reveal some interesting points. First, the growth of our CdSe NPs into those exhibiting their first extinction peak at 420 nm (i.e., fully grown or matured CdSe NPs) proceeds with monomer formation and nucleation, size focusing, and stabilization steps, but it lacks the Ostwald ripening step (or the defocusing step of the size distribution), which is typically observed from traditional NP growth.36,37 No observation of Ostwald ripening for our CdSe NPs suggests homogeneity in their size distribution as well as their extremely high stability. Second, the positions of each pair of peaks (P1 and P2 peaks) are concurrently redshifted with increasing reaction time; the P2 peak does not evolve at the expense of the P1 peak. These observations imply that each set of peaks corresponds to different sizes of CdSe NPs. The observations also indicate slow growth of progressively larger CdSe NP intermediates with time, to form extremely stable, matured CdSe NPs showing their first extinction peak at 420 nm. Third, P1 peaks are very sharp at trxn ) 30 min or longer; the sharpness of the peak increases exponentially with time. These findings could be attributed to the formation of a series of small CdSe NP intermediates whose size distribution becomes narrower as they grow to form progressively larger ones. The very sharp first extinction peaks (i.e., the formation of well-focused CdSe NP intermediates) can be mainly attributed to both low synthesis temperature and high stability (i.e., low reactivity) of Cd precursor. Our room temperature synthesis condition ensures both very slow nucleation and growth,7 favoring focusing of the size distribution. Under our synthesis conditions (ncys/nCd2+ ) 8.8, pH ∼12.3), Cd precursor exists as stable complexes with cys (H2L), mainly as CdL34-.38 This complex with a strong coordination between a Cd2+ ion and three ligands is relatively difficult to decompose and hence Cd2+ ions are liberated slowly,12,13,18 favoring focusing of the size distribution. Overall kinetics studies indicate very slow growth of cyscapped CdSe NPs in the current synthesis condition: their full growth takes about 7 days. During growth of CdSe NPs to form extremely stable, matured CdSe NPs, a series of relatively stable CdSe intermediates are formed as can be inferred from the very sharp first extinction peaks near the respective absorption edges. The slow growth of the NPs and the stability of the intermediates can be attributed to both low synthesis temperature and relatively low reactivity of Cd precursor (CdL34-) due to strong Cd2+-
Aqueous Phase Synthesized CdSe Nanoparticles
Figure 2. (a) UV-visible absorption, PL, and PLE spectra for cyscapped CdSe NPs. (b) Excitation wavelength dependence of PL spectra for the NPs. (c) Detection wavelength dependence of PLE spectra for the NPs.
cys ligand coordination. Under such conditions with a sufficiently long enough reaction time, thermodynamically stable, small species may form and survive leading to very stable CdSe NP families.12,13,18 In this regard, our fully grown CdSe NPs are extremely stable against long-term storage at room temperature. In addition, the high nCd2+/nSe2- ratio of 4 may lead to a condition at which the dissociation of the thermodynamically stable, fully grown CdSe NPs is hampered; it stabilizes the fully grown CdSe NPs and prevents their dissociation and consequently suppresses the formation of other families of CdSe NPs.12,13,18 Optical Properties. The red solid line in Figure 2a shows a typical absorption spectrum for our cys-capped CdSe NPs. The spectrum is very similar to spectra reported previously for aqueous phase synthesized, cys-capped CdSe NPs in that it displays an extremely sharp first extinction peak at 420 nm, a broad peak at 360 nm, and a tiny shoulderlike peak at 390 nm.34,35 The spectral features are also very similar to those for the colloidal mixture of decylamine-capped (CdSe)33 and (CdSe)34 MSCs synthesized in a toluene phase of reverse micelles,22 except for the slightly red-shifted peak positions for our NPs. The first extinction peak in the figure is extremely sharp as can be inferred from its fwhm value of 17 nm. This value is slightly smaller than fwhm values (17.5-18 nm) reported previously for cys-capped CdSe NPs.34,35 It is also slightly smaller than that of ca.18 nm reported for the colloidal mixture of monodispersed, decylamine-capped (CdSe)33 and (CdSe)34 MSCs synthesized in a toluene phase.22 To the best of our knowledge, this is the smallest fwhm value reported so far for aqueous phase synthesized colloidal semiconductor NPs,24-35 although sharper absorption and/or emission peaks have been observed from some organic phase synthesized colloidal semiconductor MSCs.11-13,17,18,20 The sharpness of the first extinction peak indicates an extremely narrow size distribution for our
J. Phys. Chem. C, Vol. 114, No. 44, 2010 18837 CdSe NPs, while the position of the peak suggests their ultrasmall size. We estimated their size as ca. 1.7 nm using an empirical size fitting formula3 based on the first extinction peak position. The formula is reliable for relatively large CdSe NPs (d g 2 nm) whose size has been measured directly for many samples by TEM, but it may not be so accurate for smaller CdSe NPs (d < 2 nm) because of instrumental limitations and difficulties in obtaining samples of desired sizes with a narrow size distribution. The samples used for smaller CdSe NPs were mostly ligand-stabilized clusters of extremely small bulk crystalline CdSe fragments, and their sizes were calculated from their chemical formulas and/or by utilizing X-ray diffraction data from periodic arrays prepared from the clusters. The corresponding emission spectrum (solid blue line in Figure 2a) consists of a sharp PL peak at 429 nm and a very broad longer-wavelength PL feature. The first PL peak at 429 nm can be ascribed to excitonic emission because the peak is extremely sharp (fwhm ) 20 nm); its Stokes shift is very small (62 meV); it does not show pinning39-41 that is related to surface state emission and typically observed for ultrasmall (d < 2 nm for CdSe) semiconductor NPs. The broad longer-wavelength PL feature can be attributed to the recombination of the excitons localized at various surface states with relatively low energies, considering its shape and position. Similar emission spectral features were observed previously for Cd-chalcogen MSCs: Ouyang et al.12 and Wang et al.20 reported organic phase synthesized CdSe and CdTe MSCs, respectively, showing both strong excitonic emission and weak surface trap emission. The emission spectral feature of our CdSe NPs is practically independent of excitation wavelength, as shown in Figure 2b. The spectra are very similar to each other regardless of the excitation wavelengths used (260-420 nm), if small Raman features appearing at a regular interval are excluded. This observation, together with the sharpness of the excitonic emission peak, indicates that the PL features originate from CdSe NPs with an extremely narrow size distribution. The shape and position of the PLE spectrum for our CdSe NPs are almost identical to those of the corresponding absorption spectrum, as shown in Figure 2a. Hence, the short-wavelength peaks on the absorption spectrum can be attributed to the excited states of excitons on the same CdSe NPs. The PLE spectra obtained at various fixed detection wavelengths (420-500 nm) are very similar to each other (Figure 2c). This observation suggests that our CdSe NPs have an extremely narrow size distribution and that the NPs producing the sharp excitonic PL peak are also responsible for the broad longer-wavelength PL feature. Chemical Structure. Mass spectra in Figure 3 reveal information about compositional structure of our cys-capped CdSe NPs synthesized in aqueous solution at room temperature. The spectrum obtained at 60% laser intensity attenuation level (AL) exhibits a broad feature composed of (CdSe)33 and (CdSe)34 peaks. This observation suggests that only (CdSe)33 and (CdSe)34 clusters were grown selectively in aqueous solution. As the laser ablation condition becomes more violent (AL ) 40%), these two peaks become more distinctive and two new peaks appear at the positions corresponding to (CdSe)13 and (CdSe)19. These findings signify that (CdSe)13 and (CdSe)19 clusters were produced by fragmentation of (CdSe)33 and (CdSe)34 clusters. Our current mass spectrometric observations are in good accordance with the previous reports about the selective stability of (CdSe)13, (CdSe)19, (CdSe)33, and (CdSe)34 clusters.22 Overall mass spectrometric observations indicate that our sample was originally composed of (CdSe)33 and (CdSe)34
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Figure 3. Time-of-flight (TOF) mass spectra for cys-capped CdSe NPs.
Figure 5. (a) TEM and (b) AFM micrographs for cys-capped CdSe NPs. (c) Height profile for the AFM images. TEM beam energy ) 200 keV. AFM image size )1 µm × 1 µm; fluid tapping mode.
Figure 4. (a) XRD profile and (b) resonance Raman spectrum (excitation at 413.1 nm of Kr+ line) for cys-capped CdSe NPs.
clusters, and these clusters were fragmented into smaller clusters of particularly high stability by the intense laser beam. XRD from the Periodic Arrays. Figure 4a shows an XRD profile for our CdSe NPs. The broad peaks centered at 28° (0.32 nm) and 45° (0.20 nm) can be attributed to interatomic (Cd-Se) reflections of the CdSe NPs. The broadness and center positions of these peaks suggest that our CdSe NPs are extremely small and have structures deviated considerably from the bulk crystalline fragments. The relatively sharp peak at 5.1° indicates ordering of the particles in arrays and hence a very narrow size distribution for our NPs. The angle indicates an average interparticle distance of 1.73 nm in the XRD sample. The peak might correspond to a self-assembled stack constructed from the CdSe NPs with cys capping molecules acting as a spacer.22,42 This suggests that our CdSe NPs are smaller than 1.73 nm. The XRD profile for our CdSe NPs is very similar to that reported for the decylamine-capped (CdSe)33 and (CdSe)34 MSCs fabricated in reverse micelles22 except at low diffraction angles. Those organic phase synthesized CdSe MSCs exhibited a series of five clear peaks at 2θ < 15°. The presence of only one peak, without a series of higher-order peaks, at low diffraction angles suggests the existence of only a short-range order of our NP arrays. The XRD investigation indicates that our CdSe NPs have an ultrasmall size (d < 1.73 nm) and a very narrow size distribution. Raman Scattering Characteristics. Raman spectroscopy is a powerful tool for characterizing NPs because it is sensitive to their structure and size. The resonance Raman spectrum for our CdSe NPs in Figure 4b has its main peaks near 190 cm-1 close to the bulk optical phonon frequency,43-48 together with many other peaks in the entire spectral range (down to the cutoff at
20 cm-1 set by our triple monochromator), as well as their overtones above 200 cm-1. It should be emphasized that the Raman spectrum is observed only if the wavelength of incident (413.1 nm of Kr ion laser) and scattered photons is closely resonated with the first excitonic absorption peak of the CdSe NPs. Other laser beam wavelengths produced no signal at all on top of the luminescence background. Thus, all Raman peaks are solely attributed to the CdSe NPs showing their first excitonic absorption peak at 420 nm, not to any byproduct or agglomerates of different kinds of NPs. The Raman spectral features in Figure 4b are fundamentally different from previous results on crystalline CdSe NPs with diameters down to 1.9 nm, where their Raman peaks appeared in two separated frequency regions.43 Those crystalline NPs showed relatively sharp peaks in a high frequency region near 210 cm-1 from bulk optical phonons originating from the stretching vibration of Cd-Se bonds and very broad peaks in a low frequency region near 30 cm-1 of the bulk acoustic phonons originating from the bending vibration of the Cd-Se bonds. The appearance of Raman peaks in two widely separated frequency regions is expected for NPs that are just bulk crystalline fragments showing vibrations originating from the crystal lattice of the translational symmetry.43-48 On the contrary, our Raman peaks appear in the entire spectral range, without separation, in Figure 4b. This fact indicates that the structure of our CdSe NPs is significantly different from the bulk crystalline lattice. Our CdSe NPs show relatively sharp Raman peaks near 190 cm-1, which are fairly close to the bulk optical phonon frequency. These peaks suggest that our CdSe NPs basically have sp3 bonding whose length and angle are fairly similar to the corresponding ones in crystalline CdSe.43-48 Many relatively sharp peaks appear in the middle frequency range. They may be thought of as discrete molecular vibrations allowed by the high point symmetry of the atomic arrangement rather than translational ones of bulk crystalline fragments. Overall Raman spectroscopic observations indicate that our cys-capped CdSe NPs have sp3 bondings, like bulk crystalline CdSe NPs, but their structure is significantly different from that of bulk crystalline CdSe NPs. Morphology. The TEM image in Figure 5a for our CdSe NPs shows a number of small particles (d < 2 nm) as well as
Aqueous Phase Synthesized CdSe Nanoparticles their bigger agglomerates (d ≈ 5 nm or larger). Agglomerate formation of our NPs was observed during TEM measurements, because of their melting-down caused by the instrument high energy electron beam (200 keV). Although the size estimated from the micrograph is
