Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals

Review pubs.acs.org/cm

Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals Sarah M. Harrell,§ James R. McBride,† and Sandra J. Rosenthal*,†,‡,⊥,§ †

Department of Chemistry, Vanderbilt University, VU Station B Box 351822, Nashville, Tennessee 37235, United States Department of Physics and Astronomy, Department of Pharmacology, Department of Chemical and Biomolecular Engineering, and The Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States § Department of Interdisciplinary Materials Science, Vanderbilt University, Nashville, Tennessee 37235, United States ⊥ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Nanocrystals exhibit useful properties not found in their bulk counterparts; however, a subclass of nanocrystals that consist of diameters on the order of 2 nm or less further exhibit unique properties. As synthetic methodologies of nanocrystals have matured, greater emphasis has been made on controlling the early stages of the reaction in order to gain access to these sub-2 nm species. This review provides an overview of ultrasmall and magic-sized nanocrystals, and the diverse chemical means to obtain them. Due to their small size and their resultant properties, these ultrasmall and magic-sized nanocrystals have a distinct advantage in many applications including achieving renal clearance for the purpose of biological imaging, producing simple and high-quality white LEDs, and controlling the growth of nanocrystals to produce various morphologies. KEYWORDS: ultrasmall, magic-sized, nanocrystal, synthesis

■

INTRODUCTION Through the effect of quantum confinement, semiconductor nanocrystals have a size tunable absorption and emission as well as enhanced absorption per unit volume relative to their bulk counterparts.1 Taking advantage of these unique properties, a wide variety of technologies are currently being developed.2−13 What happens to these photophysical properties at the threshold of the nanocrystal to molecule transition? From a fundamental standpoint, the structure, growth, and optical properties of a nanocrystal in its nucleation and/or growth phase remains elusive. Practical synthesis of sub-2 nm nanocrystals could answer essential questions such as: (i) What are the underlying processes that control nanocrystal nucleation? (ii) What do the growth dynamics look like at the early stages of the reaction? (iii) What is the structure of a nanocrystal seed? To reach this regime, new chemical methods were necessary as the reaction rate of the Murray method is too fast to directly isolate very small nanocrystals.14 In 2001, two methods to obtain sub-2 nm CdSe nanocrystals were published. Soloviev et al. used a single-source molecular approach to build molecular clusters of phosphine capped CdSe containing up to 32 cadmium atoms.15 Alternatively, Landes et al. lowered the reaction temperature of the Murray method from 360 °C to ∼130 °C, slowing the reaction kinetics to produce nanocrystals with diameters as small as 1.6 nm.16 However, it would take a change of cadmium precursor to truly allow practical access to nanocrystal diameters below 2 nm. Prior to 2001, the predominant source of cadmium was the highly reactive, highly toxic dimethyl cadmium. Peng et al. © 2013 American Chemical Society

would change this standard with a demonstration of the importance of metal-phosphonate complexes formed during the decomposition of dimethyl cadmium in technical grade tri-nocytlphosphine oxide (TOPO).17,18 The authors showed that relatively benign CdO in conjunction with an alkylphosphonic acid could be used in place of dimethyl cadmium to synthesize high quality cadmium-based nanocrystals. Later, the reaction was also shown to work with carboxylate salts, such as cadmium oleate or cadmium stearate.19,20 These cheaper and air stable cadmium oxide precursors made synthesis accessible to a greater range of scientists. An important side effect of the switch to cadmium oxide -based precursors was the much slower nucleation time that precedes nanocrystal growth. For the first time, one could easily observe the nanocrystal solution evolve from colorless to black with increasing size of growing nanocrystals. Chemists could now access the earliest stages of nucleation and growth. Peng et al. were one of the first to capitalize on the slower reaction kinetics and investigate the progression of the absorption spectrum at the onset of nanocrystal nucleation.21 Utilizing cadmium oxide and tetradecylphosphonic acid (TDPA) as the cadmium source, very narrow absorption peaks were repeatedly observed to appear at 285 and 349 nm.18 Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: October 14, 2012 Revised: January 22, 2013 Published: February 15, 2013 1199

dx.doi.org/10.1021/cm303318f | Chem. Mater. 2013, 25, 1199−1210

Chemistry of Materials

Review

nm.24 Yu et al. achieved the most impressively narrow absorption fwhm of 8 nm and photoluminescent fwhm of 10 nm.25,26 In contrast to magic-sized nanocrystals, ultrasmall nanocrystals have a size distribution that is dependent on many parameters, particularly the speed at which the Se precursor is injected and nanocrystal growth is extinguished.27 The second distinguishing property is the method of growth of magic-sized nanocrystals vs ultrasmall nanocrystals. The growth of ultrasmall nanocrystals exhibits continuous growth seen as a continuous red shift of the band gap absorption peak; this type of growth is often called homogeneous growth and is seen in traditional nanocrystals.28 However, the growth observed for magic-sized nanocrystals involves the degradation of peaks and the creation of red-shifted peaks that grow in intensity over time. During the growth of magic-sized nanocrystals, the nanocrystals jump from one smaller discrete size to another larger discrete size; this is commonly termed heterogeneous growth.22,23,25,29 A good example of this type of growth is shown in Figure 2, where magic-sized nanocrystals

The stability of these peaks during the early stages of growth as well as the exceptionally narrow peak width suggested that these were a particularly thermodynamically stable cluster consisting of an exact number of atoms. Coined “magic size” in reference to metallic gas phase clusters, the appearance of these clusters was one of the first clues to the interesting science involved with nanoclusters. This review takes a look into the synthesis of these and other sub-2 nm nanocrystals as well as considers a comparison of their properties and corresponding applications.

■

ULTRASMALL VS MAGIC-SIZED NANOCRYSTALS A distinction needs to be made between ultrasmall nanocrystals and magic-sized nanocrystals. Although magic-sized nanocrystals can share similar properties to ultrasmall nanocrystals including composition and size, two fundamental properties put these nanocrystals in different classes, namely the full width half max (fwhm) of the band gap absorption peaks and the type of growth each type displays. Ultrasmall nanocrystals have generally broader fwhm’s and display homogeneous growth, whereas magic-sized nanocrystals have very narrow fwhm’s and display heterogeneous growth. The first property is the size distribution difference of ultrasmall nanocrystals compared with magic-sized nanocrystals. Figure 1 is an excellent example of this phenomenon;

Figure 1. Regular CdSe nanocrystals etched to magic-sized using butylamine providing an excellent example of band gap absorption fwhm difference between small, regular (labeled original nanoparticles), and magic-sized nanocrystals (labeled nanoparticles + amine). Reprinted with permission from ref 16. Copyright 2011 American Chemical Society.

Landes et al. found a way to turn a normal size distribution of nanocrystals with a band gap absorbance of around 445 nm displaying homogeneous growth into magic-sized nanocrystals with a band gap absorbance of 414 nm.16 The drastic difference in the fwhm of the absorbance peaks is obvious. The reason for the drastically narrow absorption peak for magic-sized nanocrystals is that they consist of only one size quantum dot. They have little size distribution in theory and, therefore, do not contribute to the fwhm of their band gap absorption peak. Some reported fwhm’s of band gap absorption peaks of magicsized nanocrystals are as little as 20 and 30 nm.22,23 Correspondingly, photoluminescence peaks can also be very narrow; Beri et al. synthesized magic-sized nanocrystals with a band gap photoluminescence peak with a fwhm of about 30

Figure 2. Absorption spectra of CdSe nanocrystals demonstrating heterogeneous growth. Reprinted with permission from ref 23. Copyright 2007 Wiley−VCH Verlag GmbH & Co. KGaA.

are growing only in discrete steps, 330, 360, 384, 406, 431, and 447 nm.22 As the intensity of one peak wanes, another redshifted peak begins to grow in intensity, usually starting as a shoulder on the peak before it. It is also common for several distinct sizes to grow at the same time. Yu reported families of nanocrystals; in this case, two different sized peaks start growing at the same time, and they are so thermodynamically 1200



Review

Table 1. Common Sizes of CdSe Magic-Sized Nanocrystals highest energy absorption peak (nm)

ligands

349, 350, 350 363, 364 372, 373 380, 383 389, 391, 395, 395 408, 410, 412, 413, 414, 414, 414, 415, 418 463, 463

TDPA, ODPA, decylamine acetic acid, oleic acid oleic acid, lauric acid decylamine, diisooctylphosphinic acid acetic acid, oleic acid, oleic acid, lauric acid TOP and TOPO, oleic acid, 1-napthoate, acetic acid, stearic acid, TOPO, diisooctylphosphinic acid, decylamine, benzoate CH3(CH2)20COOH, acetic acid

refs Peng et al.,18 Kasuya et al.,37 Owen et al.38 Liu et al.,36 Jose et al.39 Beri et al.,24 Ouyang et al.25 Kasuya et al.,37 Dukes et al.29 Liu et al.,36 Beri et al.,24 Jose et al.,39 Ouyang et al.25 Yu et al.,26 Jose et al.,39 Cossairt et al.,23 Liu et al.,36 Chen et al.,41 Landes et al.,42 Dukes et al.,29 Kasuya et al.,37 Ouyang et al.25 Jose et al.,39 Ouyang et al.25

drying the TOPO. The heating mantle was then removed for the injection of precursors, and the reaction was slowly heated back up to 130 to 140 °C for growth. In this way, nanocrystals with a diameter of about 1.6 nm were synthesized. After the discovery of CdO as an alternative Cd precursor, slower kinetics allowed for the reliable synthesis of sub-2 nm nanocrystals utilizing more conventional ligands terminated on the cation and yielding nanocrystals with higher cation:anion ratios, properties more similar to traditional nanocrystals. Then, it was discovered that Cd-fatty acid complexes could also be used as Cd precursors for nanocrystals because they also had slower kinetics than dimethyl cadmium.19,20 This allowed for a plethora of CdSe magic-sized nanocrystals to be discovered, many of them being synthesized with fatty acid ligands and some of them being synthesized with phosphonic and phosphinic acids.18,24−26,29,36−42 Later, a unique reverse micelle method of making stable nanoclusters similar to traditional semiconductor nanocrystals was also discovered by Kasuya et al. in 2004 in which cadmium nitrilotriacetate was mixed with decylamine and sodium selenosulphate.37 Upon addition of toluene, the micelles move up into the toluene layer, and CdSe magic-sized nanocrystals form. In 2005, Bowers et al. synthesized what was thought to be magic-sized nanocrystals with a band gap absorption peak at 414 nm.27,43 Later, it was clear that these nanocrystals were not magic-sized but displayed the same growth properties as the larger, more traditional nanocrystals. These 1.5 nm CdSe nanocrystals are synthesized by the exact same method as their larger counterparts except for the use of a “kill shot” of 20 mL of butanol to stop the reaction. After the injection of Se precursors, a color change from colorless to yellow is observed indicating nucleation and growth of nanocrystals, and the kill shot is rapidly injected reducing the reaction temperature in order to stop nanocrystal growth. These nanocrystals were notable for their white light emission.

stable that they grow only in intensity and do not increase in size; these nanocrystals can be considered to be a class of their own called magic-sized nuclei (MSN).26

■

SYNTHESIS TECHNIQUES Before the advent of the CdO precursor, nontraditional methods of making nanocrystals involving CdCl2 as the Cd precursor and ligands on the anion were used to synthesize molecular clusters of CdSe and other semiconductors, particularly CdS, ZnS, and ZnSe. Nanoclusters made by these methods were difficult to compare with larger, more traditional nanocrystals synthesized by the Murray method because of the major differences in the precursors.14 Choy et al. was the first to make chalcogenide molecular clusters. Traditionally used to make FeS, it was discovered that sulfophenol ligands along with selenophenol ligands could also be used to make CdS, CdSe, ZnS, and ZnSe nanoclusters. They claimed that with these ligands, the clusters are molecular and do not degrade to the metal chalcogenides. Their crystal properties were first studied by Dance et al.30 They were found to have crystal structures similar to their bulk counterparts making them seem more like nanopieces of bulk semiconductors rather than molecules. Vossmeyer et al. helped pioneer this front with the synthesis of nearly monodisperse CdS nanoclusters in 1994.31 With diameters of 13, 14, 16, 19, 23, and 39 Å, these nanocrystals were produced and stabilized with 1-thioglycerol ligands bound to the S atoms leading to a low Cd:S ratio. Similar to the traditional nanocrystal synthesis, the quantity and size were controlled by the addition of the S precursor, H2S, the reaction temperature, and the duration of heating. Different Cd and S precursors were needed for the formation of the 23 and 39 Å sizes, and the reaction had to be interrupted with water to obtain the 23 Å size. In 2000, Soloviev et al. was able to make CdSe nanoclusters having selenophenol ligands; this yielded clusters with a very low Cd:Se ratio, the opposite case as for traditional nanocrystals.15 Clusters were made with 4, 8, 10, 17, and 32 Cd atoms; the different sizes were made by changing the solvent and the Se precursor rather than stopping or trying to control nanocrystal growth as with traditional nanocrystals. The optical and electronic properties of these structures were analyzed, and it was found that the band gap dependence of these cluster molecules is somewhere between 1/r and 1/r2 as opposed to traditional nanocrystals having competing terms of 1/r2 scaling as the confinement energy and 1/r16 scaling as coulomb attraction.32 The idea for the synthesis of Soloviev et al.’s molecular clusters came from Choy et al. and Herron et al.33,34 Landes et al. did, however, find a way in 2001 to make nanocrystals in the 2 nm range using the Murray method.16,35 This was done by lowering the temperature to 120 °C after

■

MAGIC-SIZED NANOCRYSTALS SPECIES Table 1 shows common species of magic-sized CdSe nanocrystals identified by their primary absorption peak (defined as absorption peaks that have at least one similar absorption peak less than 4 nm) and the different ligands used to synthesize them. The ligands and sources are listed respective to the band gap wavelengths. The first and smallest common species of magic-sized nanocrystals with an average absorption band gap of 350 nm involved three different syntheses using three different ligands including two phosphonic acids and decylamine. The next two common species of magic-sized nanocrystals with band gap absorption peaks averaging at 364 and 373 nm all had carboxylic acid 1201



Review

Figure 3. Synthesis of CdSe nanoparticles with differing amounts of TOP replacing the fatty acid ligand in the reaction. At first, the TOP increases the yield of the same magic-sized nanocrystals. Then, the opposite effect occurs, and only homogeneous growth is observed at 13%. Once the TOP amount is increased to 65%, a new size of magic-sized nanocrystal is favored. The labels for the spectra on each graph refer to the growth periods and temperatures as follows: 0 min/120 °C (Sample 1), 60 min/120 °C (Sample 2), 0 min/140 °C (Sample 3), 0 min/160 °C (Sample 4), 0 min/180 °C (Sample 5), 0 min/200 °C (Sample 6), 0 min/220 °C (Sample 7), 0 min/240 °C (Sample 8), 20 min/240 °C (Sample 9), and 40 min/240 °C (Sample 10). Reprinted with permission from ref 26. Copyright 2010 American Chemical Society.

ligands. The next common species, averaging at 382 nm, was synthesized by amine and phosphonic/phosphinic acids. Four different syntheses, all involving carboxylic acids, resulted in nanocrystals with a band gap absorption averaging at 393 nm. Interestingly, one can see a pattern starting to develop. Each common species involving different syntheses either have

ligands with carboxylic acid functional groups or ligands with phosphonic/phosphinic acid and/or amine functional groups; for example, there are no common species that share ligands with both carboxylic acid functional groups and phosphonic acid functional groups until the next to last common species in Table 1. This is probably because the next common species of 1202



Review

growth temperatures seem to generally make larger magic-sized NCs, and the smaller chain fatty acids and lower temperatures seem to make smaller magic-sized NCs. Yu has predicted that the degree of supersaturation of the monomer precursor has a big affect on the size of the magic-sized nanocrystals formed as well as whether or not they will form at all.44 The supersaturation is connected to the CdSe monomer; a monomer that contains one acetic acid ligand and one fatty acid ligand has a higher degree of supersaturation and drives the reaction toward the formation of magic-sized nanocrystals as opposed to a CdSe monomer containing only fatty acid ligands. Yu was able to etch regular nanocrystals into magic-sized nanocrystals by the addition of (CH3−COO)2Cd. They also postulated that for a constant concentration of Cd-fatty acid complexes, as the chain length of the fatty acid and/or the growth temperature increases, the degree of supersaturation also increases. Higher supersaturation results in larger magicsized nanocrystals. According to Xie, above a certain degree of supersaturation, chemical kinetics drives the formation of nanocrystals rather than nucleation kinetics.45 Below a certain degree of supersaturation, homogeneous nucleation and growth occurs. Combining Yu’s and Xie’s insights, it would seem that magic-sized nanocrystal formation occurs at high supersaturation when chemical kinetics determines the product. Since supersaturation is determined by the solubility and concentration of the monomer precursor, we can think of the CdSe-ligand monomer as reactants and the different sizes of the nanocrystals with their ligands as products. In this regime, the ligands could have a large effect on the stability of the nanocrystal molecule produced. On the other hand by increasing or decreasing the activation barrier, coordinating solvents could affect the yield of the reaction. Dukes et al. found that one could not make magicsized nanocrystals with phosphinic acid ligands without an amine.29 Phosphinic acids bind more strongly to Cd than amines, so it is possible that the addition of an amine, a weaker binding ligand, decreased the activation barrier of the magicsized nanocrystals.46 Yu et al. found that the presence of a small amount of TOP as a coordinating solvent in a synthesis using fatty acids as ligands gave a higher yield of magic-sized NCs, but Xie et al. found that the addition of an amine gave a lower yield of magic-sized nanocrystals with fatty acid ligands.44,45 This could be possible because fatty acids bind more strongly than TOP on the nanocrystal surface but less strongly than amines.46 For carboxylic acid-capped nanocrystals, a high Cd:Se ratio is favored.24,25 The argument by Ouyang et al. is that a high concentration of Cd in the solution prevents the dissociation of the magic-sized nanocrystals that would provide the monomers to start the growth of regular quantum dots. However, Dukes et al. saw the opposite for phosphonic acid-capped nanocrystals; quantized growth was observed for a 1:5 ratio of Cd:Se but was not observed for a 5:1 ratio of Cd:Se. An even 1:1 ratio of Cd:Se also produced quantized growth with the Dukes synthesis.29

magic-sized nanocrystals seems to be especially stable. Nine different syntheses resulted in magic-sized nanocrystals with absorption band gaps around 413 nm. Five of these syntheses made nanocrystals with fatty acids as ligands; two of these syntheses were magic-sized nanocrystals made with TOPO as the primary ligand. The other two syntheses used phosphinic acid and amine ligands. The common species with the largest size of magic-sized nanocrystals had absorption band gaps at 463 nm, both syntheses involving carboxylic acids as ligands. It seems that there are two different sets of magic-sized nanocrystals, the sets formed by fatty acid ligands and the sets formed by phosphonic/phosphinic acid ligands (amines used as ligands seem to fall in the phosphonic/phosphinic acid ligands category). There are two possibilities that could cause these distinct sets of magic-sized nanocrystals. One possibility is that the exact size, i.e., the exact number of both Cd and Se atoms in the magic-sized cluster which is energetically favored is dependent on the nanocrystals’ ligands. The other possibility is that the same size nanocluster is being formed, but the exact energy of the band gap is dependent on the ligand. Cossairt et al. shows a change of 7 nm in the absorption maximum of magic-sized CdSe clusters for a ligand exchange from 1-napthoate-capped magic-sized nanoclusters to 1-benzoate-capped magic-sized nanoclusters.23 Landes et al. also saw a large absorption change (31 nm) because of ligand exchange from TOPO-capped nanocrystals to butylamine-capped magic-sized nanocrystals.16 However, this blue-shift in absorption was due to etching of the nanocrystal by the short chain primary amine. The difference between the bandgap energies is about 14 meV between most species of magic-sized nanocrystals in Table 1, and the fact that the amine-capped magic-sized nanocrystal band gap absorptions are so close to the phosphonic/phosphinic acid-capped magic-sized nanocrystal band gap absorptions makes the idea of a 14 meV difference between the same size carboxylic acidcapped magic-sized nanocrystals and phosphinic/phosphonic acid-capped nanocrystals seem unlikely. Further, Yu et al. would demonstrate how ligands can selectively favor one magic-size species over another.26 Yu et al. found that as an excess of TOP is added to a fatty acid-capped magic-sized nanocrystal synthesis, the original 433 and 463 nm magic-sized nanocrystals start to disappear and, eventually, a new family of nanocrystals at 408 and 356 nm starts to form with TOP/TOPO bound to the surface Cd atoms.26 Figure 3 shows that at first the addition of TOP enhanced the production of fatty acid-capped magic-sized nanocrystals with an optimal percentage of around 3% of TOP. Then, the magic-sized nanocrystals start to drop off until only homogeneous growth is seen in the 13−50% range. At 65% of TOP, the new family of magic-sized nanocrystals with band gap absorptions at 408 nm and 463 nm start to appear and increase in yield all the way up to 100% of TOP. Using DOSY−1H NMR, it was determined that the new family of magic-sized nanocrystals had TOP/TOPO ligands suggesting that a “Cd− P” complex forms differently sized magic-sized nanocrystals than the Cd-fatty acid complex. However, the different band gaps that can be obtained during magic-sized nanocrystal synthesis can not only be dependent on the functional group of the ligand that is used since different sizes of nanocrystals are made with the same functional group ligands as shown in Table 1. Chain length and growth temperature seem to also make a difference. For syntheses involving carboxylic acids, longer chain fatty acids and higher

■

ULTRASMALL NANOCRYSTALS Landes et al. were the first to synthesize sub-2 nm nanocrystals using a method similar to the traditional nanocrystal synthesis; this method produced nanocrystals displaying homogeneous growth.16 There are few sub-2 nm nanocrystals having this type of growth.27,47 The difference between Dukes et al.’s magicsized nanocrystal synthesis and the ultrasmall synthesis by Bowers et al. is the ligands.27,29 The same synthesis with 1203



Review

demonstrated method of brightening nanocrystals is called shelling and is done by synthesizing an inorganic shell of a wider band gap material to go over the nanocrystal; this confines the electron and hole pair to the core nanocrystal.51,52 Unfortunately, this method does not produce brighter white light because it modifies the surface of the nanocrystal to emit via the band gap which gives off a sharp peak with a much smaller Stokes shift in the ultraviolet/blue region.48,53 Basically, any extra passivation of the nanocrystals needs to enhance the emission peaks caused by the phosphonic acid passivation layer; this passivation layer cannot be replaced. Rosson et al. found that just by mixing the nanocrystals with formic acid with a little bit of heat, the three peaks in the white light emission could be enhanced to achieve a quantum yield as high as 45%.50 The enhancement of these peaks as well as an inset picture comparing treated ultrasmall nanocrystals with original ultrasmall nanocrystals is shown in Figure 4. It is still unclear

dodecylphosphonic acid instead of diisooctylphosphinic acid produced nanocrystals displaying continuous growth. Once again, it is the ligand that makes the difference and gives ultrasmall nanocrystals their unique properties. Another unique property of the ultrasmall nanocrystals by Bowers et al. is the presence of two emission peaks that exist in addition to the well-known broad deep-trap emission in the red wavelengths seen in sub-2 nm nanocrystals. These two peaks emit at 445 nm, referred to as the blue peak, and 488 nm, referred to as the green peak.28,48 They appear very suddenly in the emission spectrum when the nanocrystals are synthesized with an absorption band gap smaller than 420 nm. Schreuder et al. showed that the blue peak is directly associated with the ligand bound to the Cd at the surface of the nanocrystal.28 The exact wavelength as well as the ratio of the intensity of this peak compared with the intensity of the other peaks in the ultrasmall nanocrystals can be modified by using different phosphonic acids as ligands. The wavelength of the blue peak was correlated to the electronegativity of the ligand, and the intensity of the blue peak was correlated to the length of the carbon chain. It is very clear that this highest energy emission peak is not band gap emission for three reasons. First, the Stokes shift is 25 to 45 nm depending on the exact size of nanocrystals synthesized, too large to be due to the band gap. Second, the wavelength of this peak does not change with the nanocrystal size as Schreuder et al. showed but, instead, with the surface ligand used; however, the absorption peak of these nanocrystals does change with size as expected from the quantum confinement effect. Dukes et al. referred to these first two reasons as pinned emission; the absorption continues to change with size, but the emission is pinned.49 Third, ultrafast spectroscopy revealed that the decay kinetics of each of the white light emission features are similar to those measured for trap state emission, rather than true bandgap emission.48 A way to systematically modify the green peak has still not been found; changing the surface ligands does not appear to have an effect on this peak. From the myriad of nanocrystals that have been synthesized in this size regime with carboxylic acids, it is clear that the surface state causing this green peak does not exist for fatty acid-capped nanocrystals.16,24,25,36,39,41 Therefore, this extra surface state might have to do with the phosphonic/phosphinic acid functional group. The green peak was also present in the weak emission spectrum of the magicsized nanocrystals made by Dukes et al. with diisoocytlphosphinic acid.29 There are other sub-2 nm nanocrystals made with phosphonic acid ligands from Peng et al. which had no emission spectrum information; Owen et al.’s nanocrystal emission spectrum taken 30 s after nucleation did seem to have a very slight shoulder in the 490 nm region.18,38 Cossairt et al. reported magic-sized nanocrystals with emission affected by the ligands.40 However, the emission was phosphorescence from the ligands themselves rather than a surface trap on the nanocrystal. The ligands of the ultrasmall nanocrystals do not emit by themselves, so the blue and green peaks must be due to emission from the nanocrystal with strong evidence supporting that the emission is surface-related. The blue and green peaks combined with deep trap emission in the red wavelengths of the ultrasmall nanocrystal emission spectrum creates almost perfect white light.50 However, a major drawback of using these nanocrystals in LED applications is their low quantum yield of around 10%. Because these nanocrystals emit via surface-related states, conventional brightening methods cannot be used. A standard and

Figure 4. Dashed lines represent the absorption spectrum before (blue) and after (red) treatment. The solid lines show the emission spectrum of the nanocrystals before (red) and after (blue) treatment. Inset: vials containing a concentrated solution of untreated (left) and treated (right) nanocrystals. Reprinted with permission from ref 50. Copyright 2012 American Chemical Society.

whether this effect is caused by a ligand exchange or whether the formic acid is binding somewhere on the nanocrystal in addition to the dodecylphosphonic acid (DDPA). However, Rosson et al. claims that complete ligand exchange is unlikely because the blue and green peaks which seem to be related to the phosphonic acid ligand are still intact. From Figure 4, you can see that the absorption peak is slightly blue-shifted, indicating that the formic acid might have done a small amount of etching of the surface. Formic acid and toluene have low miscibility, so vigorous stirring is essential for the treatment to get sufficient contact between the nanocrystals and formic acid. Experiments were done using the same treatment method but in different solvents to see if better miscibility between formic acid and the solvent of the nanocrystals would make the treatment more effective.54 After synthesis, the ultrasmall nanocrystals were suspended in a variety of solvents including chloroform, methanol, butanol, isopropanol, mesitylene, and trichloroethylene. The treatment was then carried out with no other modifications with these different solvents. The use of cosolvents was also attempted in 1:6 ratios of isopropanol and butanol to toluene. Good miscibility was achieved with chloroform, trichloroethylene, and with the use of cosolvents. However, in all of these cases, the treatment had little effect on the nanocrystal’s emission. The only other solvent that could reproduce the same quantum yield enhancement after treatment was mesitylene, and none of 1204



Review

Figure 5. Z-contrast STEM images and the corresponding fluorescence from CdSe nanocrystals with diameters of (a, b) 7, (c, d) 5, and (e, f) 3 nm. (g) White light emission spectrum and (h−k) corresponding frames from a STEM movie showing the continuous motion and disorder. Scale bars are 1 nm. Images a, c, and e are the average over several consecutive images. Reprinted with permission from ref 61. Copyright 2012 American Chemical Society.

Se in the same injection volume of tributylphosphine (TBP).54 Figure S1 in the Supporting Information shows the change in the intensity of the emission peaks of the ultrasmall nanocrystals with varying Se concentrations. When the Se:Cd ratio was modulated from 1.0:0.8 to 1.0:2.8, the ultrasmall nanocrystals’ quantum yield increased from 9.0 to 15.3%, and the blue peak was enhanced giving a blue tint to the white light. For the treated nanocrystals, there was an optimum Se concentration of 0.5 M, and the white light for these nanocrystals also had more of a blue tint. All of the data points in Figure S1 in the Supporting Information are averaged data from at least three experiments for each concentration of Se. One can speculate whether this effect is caused by the production of a Se-rich surface by comparing it to other anionrich surfaces that have been produced in the literature. Jasieniak et al. used successive ion layer adhesion and reaction (SILAR)

the other solvents showed improvement over mesitylene and toluene. Table S1 in the Supporting Information shows the quantum yields that were obtained after treatments with alternate solvents. A possible explanation for this phenomenon is that the formic acid is drawn to the nanocrystal surface only when it prefers this environment over the environment of the solvent. Therefore, it only works when the miscibility between the solvent of the nanocrystals and the formic acid is sufficiently low. The effect of the anion to cation ratio of the precursors for the synthesis of treated and untreated ultrasmall nanocrystals was also explored.54 Kucur et al. found that he could get higher photoluminescence for conventional nanocrystals by increasing the Se:Cd ratio.55 Qu et al. found that by using a high Se:Cd precursor ratio, higher quantum yields that lasted over longer reaction times were possible.56 This effect was studied on the ultrasmall CdSe nanocrystals by changing the concentration of 1205



Review

obtain directly interpretable atomic images of nanocrystals with a degree of chemical sensitivity. Recently, we utilized a Nion UltraSTEM aberration corrected microscope to image ultrasmall CdSe nanocrystals. STEM images and the corresponding nanocrystal fluorescence are shown in Figure 5.61 Under classical imaging conditions, these nanocrystals looked like clumps of atoms. However, once images were obtained of these clusters in rapid succession, it became clear that these nanoclusters were dynamically fluctuating under the electron beam. This atomic motion was observed at several beam energies from 300 kV all the way down to 60 kV. From a practical stand point, this means that it is impossible to obtain the ground state crystal structure utilizing current electron microscopy techniques. Of broader implication, however, it was proposed that the energy imparted by the electron beam is on the same scale as that of the absorption of a UV photon. Therefore, as ultrasmall nanocrystals absorb a UV photon, it may be possible that their lattice is set in motion creating a dynamic palette of trap states for which electrons or holes can relax through. Broad luminescence is common for all ultrasmall nanocrystals suggesting this mechanism could be universal. Fluctuations were also observed for conventional nanocrystals, but were confined to the outermost layer, while the crystalline core remained stable. These observations could provide an alternative explanation to the complex nature of nanocrystal blinking memory and anomalous spectral diffusion.62−64 One promising method of characterizing these ultrasmall nanocrystals is by solid-state nuclear magnetic resonance (SSNMR). This was achieved for the first time by Berrettini et al. with 2 nm CdSe nanocrystals in 2004.65 They were able to find the location of the ligands on their nanocrystals, thiophenol and hexadecylamine (HDA). The thiophenol was far from surface Se atoms, and HDA was close to surface Se atoms; they postulated that thiophenol was binding at Se vacancies, and the long HDA ligand which binds on surface Cd atoms tilts over the nanocrystal to achieve close proximity to surface Se atoms. SSNMR is able to give valuable information on surface reconstruction and crystalline of the nanocrystals because SSNMR selectively studies the surface of the nanocrystal instead of the nanomaterial as a whole. It also gives information on the structure of ligands, their freedom of rotation, and how they are connected to the surface of the nanocrystal. XRD peaks are challenging to distinguish for very small nanocrystals due to the inherently broad peaks that result when nanomaterials smaller than a few nanometers are ananlyzed.43 Small-angle X-ray scattering (SAXS) can be used to find the shape and size of nanocrystals, but the structural coherence is not obvious from the peaks that result because the wavevector of the X-rays is much smaller than the inverse particle size. Pair distribution function (PDF) analysis is a way to use XRD peaks to estimate the atomic structure of a nanocrystal. The discrepancy of the size of the particle between PDF and SAXS will give information on the lattice distortion and structural coherence length. Gilbert found the PDFs for different sizes of nanoparticles.66 Masadeh et al. was able to elucidate many aspects of the CdSe nanocrystal structure including structural coherence length that was about equal to the nanocrystal diameter indicating little surface deviation from crystallinity.67 An increasing compressive strain of the Cd−Se bond length with decreasing particle size was also observed. Gateshki was able to explain the unusual magnetic properties of MgFe2O4 using this method; it was found that the particles

in order to study the effects of differing surfaces of the nanocrystals, specifically a Cd-rich surface vs a Se-rich surface.57 Because of the lack of terminated bonds on surface Se atoms, the nanocrystal with a Se-rich surface exhibited severely quenched emission. However, it was found that the addition of TOP to these Se-rich nanocrystals which binds to Se surface atoms led to a brighter emission than the nanocrystals that were synthesized before the addition of the Se layer. Recently, Wei et al. used a similar method to modulate the photoluminescence of CdS nanocrystals; adding a S-rich surface quenched the photoluminescence and adding a Cd-rich surface on a S-rich surface completely replenished the photoluminescence.58 From these findings, it is doubtful that the surface of the ultrasmall nanocrystals are Se-rich with high Se:Cd precursor ratios; it is more likely that the surface of the nanocrystals approaches a 1:1 ratio of Se:Cd as the Se:Cd precursor ratio increases. The original ultrasmall nanocrystals involving a precursor ratio close to 1:1 (corresponding to a Se:TBP concentration of 0.2 M) might have vacant Se surface sites which could lead to surface Cd atoms that the bulky phosphonic acid cannot completely passivate.50 Considering that the shortest carboxylic acid increases the quantum yield the most with an emphasis on the blue peak (the peak affected by the phosphonic acid surface ligands), the formic acid could be squeezing into these vacant Se sites to passivate these dangling Cd bonds that could otherwise be leading to nonradiative emission. As the Se:Cd ratio of precursors increases, the quantum yield of the untreated nanocrystals constantly increases but starts to saturate; this saturation could represent the point at which most Se sites are being filled. Once most of the vacant Se surface sites are filled, the formic acid treatment would not be as effective which could explain the reason for the optimum Se:Cd ratio for the treatment. Rutherford back scattering (RBS) could be employed to study the change in the Se:Cd ratio of ultrasmall nanocrystals with differing precursor ratios; however, these studies will be difficult because of the difficulty in purifying ultrasmall nanocrystals.

■

CHARACTERIZATION TECHNIQUES One of the challenges of working with these small clusters is the lack of a means to characterize them. The simplest way is to measure their absorption and emission. Optical characterization, particularly UV−visible spectroscopy (UV−vis), is certainly t he most used c h ara cteriza tion method.16,18,21,29,40,44,45,55,59 It is commonly the only characterization method used for estimating the size of very small nanocrystals. Using sizing curves from larger nanocrystals based off of transmission electron microscopy (TEM) and X-ray diffraction data (XRD) compared with the nanocrystal’s absorption peaks is the most practical way to estimate the size of a nanocrystal with a diameter below 2 nm. Below 2 nm, the peaks for XRD typically become too broad to interpret, whereas in conventional TEM, there is too little contrast to accurately measure the nanocrystal size. For example, for a 2 nm CdSe nanocrystal, there should be approximately 6 lattice fringes.60 However, because of the high background in conventional HRTEM imaging, the lattice fringes at the surface of a nanocrystal are typically lost, leaving only four visible lattice fringes for a 2 nm nanocrystal. Pennycook et al. have pioneered the use of aberrationcorrected atomic number contrast scanning transmission electron microscopy (Z-STEM) to directly image the atomic structure of nanocrystals. This technique allows for one to 1206



Review

nanocrystals with an absorption band gap of 349 nm were the seeds for nanorods and tetrapods.18 The size, shape, and size/ shape distributions were all dependent on the nucleation of these magic-sized nanocrystals as well as the concentration of the monomers in the solution after the formation of the magicsized nanocrystals. Liu et al. synthesized quantum CdSe belts which were found to really be made of (CdSe)13 magic-sized nanocrystals. It was also shown that the formation of quantum belts are reversible and that (CdSe)13 is released when the belts disassemble.36 CdSe nanoplatelets showcasing quantum confinement in one dimension have been synthesized using the opposite growth method from CdSe nanorods.70−72 By first terminating both sides of the c-axis with Cd and passivating these charged surfaces with carboxylic acids, controlled growth was accomplished in every direction except along the c-axis creating CdSe nanoplatelets.72 It was found that the seeds to these nanoplatelets were nanocrystals less than 2 nm in diameter.71 Figure 7 shows the TEM images of the first

have high lattice distortion, a structural coherence length of 5− 6 nm, and well-defined local atomic ordering similar to the spinel-type structure.68 Mass spectrometry (MS) is another way to determine the atomic structure of a nanocrystal by measuring its mass. Kasuya et al. was able to identify some very stable molecular clusters as (CdSe)13, (CdSe)33, and (CdSe)34 with time-of-flight MS, which measures the mass to charge ratio of a cluster or molecule.37 The top spectrum of Figure 6 is the spectrum of

Figure 7. Transition electron microscopy images (TEM) of the first batches of CdSe nanoplatelets synthesized. The scale bars in a−d are 10 nm and in e and f are 20 nm. Reprinted with permission from ref 70. Copyright (2008) American Chemical Society.

nanoplatelets synthesized.70 The different sizes and shapes of the nanoplatelets in images a−f could be controlled by the precursor injection and growth temperature. The nanoplatelets in image a wer obtained by the injection of cadmium acetate at 195 °C and heating at 240 °C for 10 min. Those in images b, c, and d were obtained by a similar synthesis but with a second precursor injection at 240 °C and heating for 20 min. Nanoparticles in image e were obtained in the same way as in a but with manganese acetate rather than cadmium acetate. Those in mage f were obtained with the injection of precursors at room temperature prior to heating. However, the formation mechanism of nanoplatelets remains unresolved. Nanocrystal seeds have also been found to aid in the quantum efficiency of photocatalysts for water splitting.73 The structure designed by Amirav et al. included a CdSe seed surrounded by a CdS nanorod with a Pt tip; when an electron and hole pair is created in the CdSe seed, the hole is confined to the seed, and the electron is drawn to the Pt tip. This nanostructure offered many advantages over other photocatalysts. The energy absorbed by the nanostructure could be modulated by changing the size of the CdSe seed, and the distance between the electron and hole could be modulated by changing the length of the rod. The best quantum efficiency

Figure 6. Top spectrum is time-of-flight mass spectrometry (MS) of nanoparticles of CdSe prepared by an inverse micelle method in toluene. The other three remaining spectra are MS of laser ablated bulk CdSe, CdS, and ZnS from top to bottom. Reprinted with permission ref 37. Copyright 2004 Macmillan Publishers.

nanoparticles that Kasuya et al. synthesized. These particles were so stable that they were able to survive the laser vaporization process before mass analysis. The bottom three spectra in Figure 6 are spectra of laser ablated bulk powders of CdSe, CdS, and ZnS; these bulk powders preferentially produced the same clusters as the synthesized nanoparticles shown in the top spectrum indicating extreme stability of these molecular formulas. Another MS technique called electrospray MS demonstrated by Gaumet et al. made it possible to isolate many more molecular clusters. This method of ionization is much gentler than the laser ablation Kasuya et al. used.69

■

APPLICATIONS Molecular clusters are the seeds of many different structures built on the nanoscale. Peng et al. found that magic-sized 1207



■

Review

SUMMARY AND CONCLUSIONS Although the nucleation process is still not understood, patterns that can be used to control the syntheses of magicsized nanocrystals are being found.26,29,45 Using particularly high degrees of supersaturation of monomers has allowed magic-sized nanocrystals to move out of the sub-2 nm regime including magic-sized nanocrystals with band gap absorptions up to 513 nm.25,29,44 With the knowledge that ligands and supersaturation seem to make the main difference on the size of nanocrystal produced, further experimentation could possibly lead to the control of the size of spherical nanocrystals with atomic precision. By revealing the growth kinetics of magicsized nanocrystals, other magic-sized morphologies and compositions may be possible. For ultrasmall CdSe nanocrystals, aspects of the emission spectrum and structure have been elusive largely due to the difficulty in characterizing this material. However, Rosson et al.’s treatment, Pennycook et al.’s observations, and experiments in this review provide more clues.50,54,61 The Se:Cd precursor ratio experiments might be implying the existence of dangling bonds on surface Cd atoms. The existence of these dangling bonds might be contributing to the fluxionality the CdSe nanocrystals are experiencing under an electron beam along with the high surface to volume ratio. Possibly, the Cd atoms are moving around on the surface and into interstitial sites of the interior of the nanocrystal due to the lack of passivation; Se atoms could be continually moving from the interior of the nanocrystal to the surface and back again to try to fill vacant positions. Further density functional theory calculations could be used to explore this possibility. Although it is challenging to characterize sub-2 nm materials, novel methods have recently been employed to better understand their structure and properties.35,37,61,65,67 Using these methods, it has been possible to probe these materials to finally better discern the implications of a nanocrystal made up almost entirely of surface atoms. Comparison of ultrasmall and magic-sized nanocrystals will give a better grasp of the effect of growth methods on structure and other properties of nanocrystals.

was achieved for the smallest CdSe seed tested at 2.3 nm and a rod length of 60 nm. Ultrasmall nanocrystals may be good candidates for applications in white LEDs.23,47,74,75 Ultrasmall CdSe nanocrystals have just the right balance of peaks to produce almost perfect white light with CIE coordinates (0.31, 0.33). Schreuder et al. was able to incorporate these nanocrystals into photoluminescent and electroluminescent solid state devices.74,75 The photoluminescence devices consisted of the nanocrystals encapsulated in a polymer, biphenyl-perfluorocyclobutanol (BP-PFCB), over a UV LED as shown in the inset in Figure 8.75 This polymer was chosen because it did not have

Figure 8. Absorption and emission of CdSe white light nanocrystals. The dashed lines show the absorption spectrum, and the continuous lines show the emission spectrum of nanocrystals encapsulated in the polymer (blue), BP-PFCB, and nanocrystals in solution (red). Inset: encapsulated nanocrystals over a LED to make a white LED. Reproduced with permission from ref 75. Copyright 2008 The Royal Society of Chemistry.

an effect on the white light spectrum of the nanocrystals. As can be seen in the graph of figure 8, there is very little difference between the spectrum of the nanocrystals in solution and the spectrum of the nanocrystals in the polymer. An efficiency of about 1 lm/W was achieved utilizing these devices. Using small nanocrystals for in vivo biological imaging is completely necessary to avoid toxicity since most nanocrystals do not degrade into biocompatible materials.76 Choi et al. found that spherical semiconductor nanoparticles achieve renal clearance (defined as the majority of nanoparticles being only in the bladder after 4 h) at a hydrodynamic diameter of only 5.5 nm by using different sizes of CdSe/ZnS core/shell quantum dots.76 The smallest nanocrystals at about 2.85 nm (this is the diameter with the shell but without the ligands) were vastly superior at transporting through the kidneys and into the bladder compared to their larger counterparts. The hydrodynamic diameter of the 2.85 nm nanoparticles is 4.4 nm once the ligands are added, and that diameter only includes the ligands needed to make them water-soluble and not targeting ligands. Therefore, the use of very small nanoparticles is essential. Choi et al. used the same ideas to apply renal clearance rules to tumor-targeted nanocrystals finding that only 5 to 10 ligands can be used for tumor-targeting if they are to be cleared from the body.77 Howarth et al. synthesized nanocrystals with only one targeting ligand greatly reducing the hydrodynamic diameter and the chance of binding to a serum protein also decreasing the hydrodynamic diameter.11

■

ASSOCIATED CONTENT

S Supporting Information *

A table showing the affects of quantum yield on using different solvents for the treatment of ultrasmall nanocrystals as well as a graph showing the affects of changing the Se:Cd precursor ratio on the quantum yield of untreated ultrasmall nanocrystals. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a grant from the National Science Foundation (EPS-1004083) (TN-SCORE). 1208



■

Review

(24) Beri, R. K.; Khanna, P. K. CrystEngComm 2010, 12 (10), 2762− 2768. (25) Ouyang, J.; Zaman, M. B.; Yan, F. J.; Johnston, D.; Li, G.; Wu, X.; Leek, D.; Ratcliffe, C. I.; Ripmeester, J. A.; Yu, K. J. Phys. Chem. C 2008, 112 (36), 13805−13811. (26) Yu, K.; Hu, M. Z.; Wang, R.; Piolet, M. l. L.; Frotey, M.; Zaman, M. B.; Wu, X.; Leek, D. M.; Tao, Y.; Wilkinson, D.; Li, C. J. Phys. Chem. C 2010, 114 (8), 3329−3339. (27) Bowers, M. J.; McBride, J. R.; Rosenthal, S. J. J. Am. Chem. Soc. 2005, 127 (44), 15378−15379. (28) Schreuder, M. A.; McBride, J. R.; Dukes, A. D.; Sammons, J. A.; Rosenthal, S. J. J. Phys. Chem. C 2009, 113 (19), 8169−8176. (29) Dukes, A. D.; McBride, J. R.; Rosenthal, S. J. Chem. Mater. 2010, 22 (23), 6402−6408. (30) Dance, I. G.; Choy, A.; Scudder, M. L. J. Am. Chem. Soc. 1984, 106 (21), 6285−6295. (31) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98 (31), 7665−7673. (32) Soloviev, V. N.; Eichhöfer, A.; Fenske, D.; Banin, U. J. Am. Chem. Soc. 2000, 122 (11), 2673−2674. (33) Choy, A.; Craig, D.; Dance, I.; Scudder, M. J. Chem. Soc., Chem. Commun. 1982, 21, 1246−1247. (34) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259 (5100), 1426−1428. (35) Landes, C.; Braun, M.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 363 (5−6), 465−470. (36) Liu, Y.-H.; Wang, F.; Wang, Y.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc. 2011, 133 (42), 17005−17013. (37) Kasuya, A.; Sivamohan, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R. V.; Sundararajan, V.; Kawazoe, Y. Nat. Mater. 2004, 3 (2), 99−102. (38) Owen, J. S.; Chan, E. M.; Liu, H.; Alivisatos, A. P. J. Am. Chem. Soc. 2010, 132 (51), 18206−18213. (39) Jose, R.; Zhanpeisov, N. U.; Fukumura, H.; Baba, Y.; Ishikawa, M. J. Am. Chem. Soc. 2005, 128 (2), 629−636. (40) Cossairt, B. M.; Juhas, P.; Billinge, S. J. L.; Owen, J. S. J.f Phys. Chem. Lett. 2011, 2 (24), 3075−3080. (41) Chen, X.; Samia, A. C. S.; Lou, Y.; Burda, C. J. Am. Chem. Soc. 2005, 127 (12), 4372−4375. (42) Landes, C.; El-Sayed, M. A. J. Phys. Chem. A 2002, 106 (33), 7621−7627. (43) Kodama, K.; Iikubo, S.; Taguchi, T.; Shamoto, S.-i. Acta Crystallogr., Sect. A 2006, 62 (6), 444−453. (44) Yu, K. Adv. Mater. 2012, 24 (8), 1123−1132. (45) Xie, R.; Li, Z.; Peng, X. J. Am. Chem. Soc. 2009, 131 (42), 15457−15466. (46) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. Nano Lett. 2004, 4 (12), 2361−2365. (47) Jose, R.; Zhelev, Z.; Bakalova, R.; Baba, Y.; Ishikawa, M. Appl. Phys. Lett. 2006, 89 (1), 013115−3. (48) Bowers, Ii, M. J.; McBride, J. R.; Garrett, M. D.; Sammons, J. A.; Dukes Iii, A. D.; Schreuder, M. A.; Watt, T. L.; Lupini, A. R.; Pennycook, S. J.; Rosenthal, S. J. J. Am. Chem. Soc. 2009, 131 (16), 5730−5731. (49) Dukes, A. D.; Iii; Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J. J. Chem. Phys. 2008, 129 (12), 121102−4. (50) Rosson, T. E.; Claiborne, S. M.; McBride, J. R.; Stratton, B. S.; Rosenthal, S. J. J. Am. Chem. Soc. 2012, 134 (19), 8006−8009. (51) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119 (30), 7019−7029. (52) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101 (46), 9463−9475. (53) Chen, X.; Lou, Y.; Samia, A. C.; Burda, C. Nano Lett. 2003, 3 (6), 799−803.

ABBREVIATIONS TOPO, tri-n-octlyphosphine oxide TDPA, tetradecylphosphonic acid ODPA, octadecylphosphonic acid TOP, tri-n-octylphosphine UV−vis, ultraviolet to visible spectroscopy TEM, transmission electron microscopy Z-STEM, aberration-corrected atomic number contrast scanning transmission electron microscopy SSNMR, solid-state nuclear magnetic resonance XRD, X-ray diffraction SAXS, small-angle X-ray scattering PDF, pair distribution function MS, mass spectrometry LED, light-emitting diodes DDPA, dodecylphosphonic acid TBP, tributylphosphine SILAR, successive ion layer adhesion and reactions BP-PFCB, biphenyl-perfluorocyclobutanol

■

REFERENCES

(1) Brus, L. E. J. Chem. Phys. 1984, 80 (9), 4403−4409. (2) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulović, V. Nano Lett. 2007, 7 (8), 2196−2200. (3) Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429 (6992), 642−646. (4) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66 (11), 1316−1318. (5) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420 (6917), 800−803. (6) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. J. Phys. Chem. Letters 2011, 2 (15), 1879−1887. (7) Kramer, I. J.; Sargent, E. H. ACS Nano 2011, 5 (11), 8506−8514. (8) Chang, J. C.; Tomlinson, I. D.; Warnement, M. R.; Ustione, A.; Carneiro, A. M. D.; Piston, D. W.; Blakely, R. D.; Rosenthal, S. J. J. Neurosci. 2012, 32 (26), 8919−8929. (9) Alivisatos, P. Nat. Biotechnol. 2004, 22 (1), 47−52. (10) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307 (5709), 538−544. (11) Howarth, M.; Liu, W.; Puthenveetil, S.; Zheng, Y.; Marshall, L. F.; Schmidt, M. M.; Wittrup, K. D.; Bawendi, M. G.; Ting, A. Y. Nat. Methods 2008, 5, 397−399. (12) Dahan, M.; Lévi, S.; Luccardini, C.; Rostaing, P.; Riveau, B.; Triller, A. Science 2003, 302 (5644), 442−445. (13) Rosenthal, S. J.; Tomlinson, I.; Adkins, E. M.; Schroeter, S.; Adams, S.; Swafford, L.; McBride, J.; Wang, Y.; DeFelice, L. J.; Blakely, R. D. J. Am. Chem. Soc. 2002, 124 (17), 4586−4594. (14) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115 (19), 8706−8715. (15) Soloviev, V. N.; Eichhöfer, A.; Fenske, D.; Banin, U. J. Am. Chem. Soc. 2001, 123 (10), 2354−2364. (16) Landes, C.; Braun, M.; Burda, C.; El-Sayed, M. A. Nano Lett. 2001, 1 (11), 667−670. (17) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2000, 123 (1), 183−184. (18) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124 (13), 3343− 3353. (19) Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1 (6), 333−337. (20) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41 (13), 2368−2371. (21) Qu, L.; Yu, W. W.; Peng, X. Nano Lett. 2004, 4 (3), 465−469. (22) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. Adv. Mater. 2007, 19 (4), 548−552. (23) Cossairt, B. M.; Owen, J. S. Chem. Mater. 2011, 23 (12), 3114− 3119. 1209



Review

(54) Ultrasmall nanocrystals were made with slight modifications from Rosson et al.60 Different solvents in place of toluene were used as mentioned, and the Se:TBP concentration was modulated from 0.3 to 0.7 M. (55) Kuçur, E.; Ziegler, J.; Nann, T. Small 2008, 4 (7), 883−887. (56) Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124 (9), 2049−2055. (57) Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2007, 129 (10), 2841−2848. (58) Wei, H. H.-Y.; Evans, C. M.; Swartz, B. D.; Neukirch, A. J.; Young, J.; Prezhdo, O. V.; Krauss, T. D. Nano Lett. 2012, 12 (9), 4465−4471. (59) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120 (21), 5343−5344. (60) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15 (14), 2854−2860. (61) Pennycook, T. J.; McBride, J. R.; Rosenthal, S. J.; Pennycook, S. J.; Pantelides, S. T. Nano Lett. 2012, 12 (6), 3038−3042. (62) Volkán-Kacsó, S. n.; Frantsuzov, P. A.; Jankó, B. r. Nano Lett. 2010, 10 (8), 2761−2765. (63) Cordones, A. A.; Bixby, T. J.; Leone, S. R. Nano Lett. 2011, 11 (8), 3366−3369. (64) Fernee, M. J.; Plakhotnik, T.; Louyer, Y.; Littleton, B. N.; Potzner, C.; Tamarat, P.; Mulvaney, P.; Lounis, B. J. Phys. Chem. Lett. 2012, 3 (12), 1716−1720. (65) Berrettini, M. G.; Braun, G.; Hu, J. G.; Strouse, G. F. J. Am. Chem. Soc. 2004, 126 (22), 7063−7070. (66) Gilbert, B. J. Appl. Crystallogr. 2008, 41 (3), 554−562. (67) Masadeh, A. S.; Božin, E. S.; Farrow, C. L.; Paglia, G.; Juhas, P.; Billinge, S. J. L.; Karkamkar, A.; Kanatzidis, M. G. Phys. Rev. B 2007, 76 (11), 115413. (68) Gateshki, M.; Petkov, V.; Pradhan, S. K.; Vogt, T. J. Appl. Crystallogr. 2005, 38 (5), 772−779. (69) Gaumet, J.-J.; Strouse, G. F. J. Am. Soc. Mass Spectrom. 2000, 11 (4), 338−344. (70) Ithurria, S.; Dubertret, B. J. Am. Chem. Soc. 2008, 130 (49), 16504−16505. (71) Ithurria, S.; Bousquet, G.; Dubertret, B. J. Am. Chem. Soc. 2011, 133 (9), 3070−3077. (72) Li, Z.; Peng, X. J. Am. Chem. Soc. 2011, 133 (17), 6578−6586. (73) Amirav, L.; Alivisatos, A. P. J. Phys. Chem. Lett. 2010, 1 (7), 1051−1054. (74) Schreuder, M. A.; Xiao, K.; Ivanov, I. N.; Weiss, S. M.; Rosenthal, S. J. Nano Lett. 2010, 10 (2), 573−576. (75) Schreuder, M. A.; Gosnell, J. D.; Smith, N. J.; Warnement, M. R.; Weiss, S. M.; Rosenthal, S. J. J. Mater. Chem. 2008, 18 (9), 970− 975. (76) Soo Choi, H.; Liu, W.; Misra, P.; Tanaka, E.; Zimmer, J. P.; Itty Ipe, B.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2007, 25 (10), 1165−1170. (77) Choi, H. S.; Liu, W.; Liu, F.; Nasr, K.; Misra, P.; Bawendi, M. G.; Frangioni, J. V. Nat. Nanotechnol. 2010, 5 (1), 42−47.

1210


Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals

Recommend Documents