This work was supported by the Physical Chemistry of Inorganic Nanostructures Program, KC3103, Director, Office of Science, Office of Basic Energy Sciences, of the United States Department of Energy under contract DE-AC02-05CH11231. The Center for Nanoscale Materials at Argonne National Laboratory is supported by a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357. K.M. and B.J.B. gratefully acknowledge the support of the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009 and administered by ORISE-ORAU under contract no. DE-AC05-06OR23100.

Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these syntheses are acutely toxic and carcinogenic. Nanomaterials may have additional hazards compared to their bulk counterpart. Please use all appropriate safety practices when performing a nanocrystal reaction including the use of engineering controls (fume hood, glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes). Portions of the following procedures involve standard air-free handling techniques15.
1. Preparation of Molecular Precursors
<ol>
	<li>Preparation of cadmium (II) myristate 
	Cadmium myristate can be synthesized in either an ex situ or in situ method, achieving similar results.
	<ol>
		<li>Ex situ preparation of cadmium myristate16
		<ol>
			<li>Dissolve 0.240 g sodium hydroxide and 1.370 g myristic acid in 240 ml&#160;methanol.</li>
			<li>Separately, dissolve 0.617 g cadmium nitrate tetrahydrate in 40 ml&#160;methanol.</li>
			<li>Add the cadmium nitrate solution drop-wise (~1 drop/sec) to the stirring (800 rpm) sodium myristate solution using an addition funnel.</li>
			<li>Filter the solution. Wash the resulting white precipitate with methanol three times. Dry the precipitate under vacuum overnight.</li>
		</ol>
		</li>
		<li>In situ preparation of cadmium myristate6
		<ol>
			<li>Combine 39 mg cadmium oxide (CdO) with 0.137 g myristic acid in 5 ml&#160;1-octadecene (ODE) in a 50 ml&#160;3-neck round bottom flask with a stir bar.</li>
			<li>Heat the solution to 250 &#176;C under inert gas while stirring (800 rpm) until the solution becomes a pale yellow color. Carefully swirl the solution as necessary to remove excess CdO from the sidewalls of the flask. Leaving this solution at elevated temperature beyond this point will cause it to become an undesired black color.</li>
			<li>Cool the solution to room temperature for use in subsequent reactions.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Preparation of TOP:Se precursor
	<ol>
		<li>Combine 58 mg selenium powder with 0.360 g tri-n-octylphosphine (TOP) under an inert atmosphere.</li>
		<li>Sonicate until the selenium powder is fully dissolved, yielding an optically clear solution. Alternatively, the solution can also be heated to 100 &#176;C in order to fully dissolve the selenium.&#160;Use proper glassware or a glove box to prevent exposure of the solution to air during sonication or heating.</li>
	</ol>
	</li>
	<li>Preparation of TOP:S precursor
	<ol>
		<li>Combine 0.160 g elemental sulfur with 1.853 g TOP.</li>
		<li>Heat to 50 &#176;C under an inert atmosphere until the solution is optically clear. Cool to room temperature for future use.</li>
	</ol>
	</li>
</ol>
2. Synthesis of CdSe Seeds
<ol>
	<li>Synthesis of wurtzite CdSe (w-CdSe) nanocrystals5
	<ol>
		<li>Combine 3.0 g trioctylphosphine oxide (TOPO), 280 mg n-octadecylphosphonic acid (ODPA), and 60 mg CdO with a stir bar in a 25 ml&#160;3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter or punctured through a rubber septum), a reflux condenser, and a rubber septum. Inspect all glassware for defects prior to use due to the high temperature of reaction and potential need for rapid cooling. Assemble all glass-to-glass joints with high temperature vacuum grease.</li>
		<li>Degas the mixture at 150 &#176;C under vacuum for 1 hr while stirring (800 rpm).</li>
		<li>Heat the solution to 300 &#176;C under inert gas to form an optically clear solution. Carefully swirl the solution to remove excess CdO from the sidewalls of the flask. Allow the solution to become optically clear again.</li>
		<li>Add 1.5 g TOP drop-wise to the solution and raise the temperature to 370 &#176;C.</li>
		<li>Rapidly inject TOP:Se solution using a syringe with a large diameter (14 G) needle. The rapid injection will create a sharp nucleation event, promoting a narrow size distribution.</li>
		<li>Let the reaction proceed for the desired duration, as the size of the w-CdSe nanocrystal can be tuned by adjusting the reaction time. Immediately cooling after the injection will yield nanocrystals of ~2-2.5 nm in diameter whereas reacting the solution for 3 and 5 min&#160;will yield ~3 nm and ~4 nm diameter nanocrystals, respectively.</li>
		<li>Remove the heating mantle and rapidly cool the flask using a stream of compressed air. Care should be taken when rapidly cooling a flask from elevated temperatures as the thermal shock may break the glassware. For this reason, the use of a water bath for rapid cooling is strongly discouraged.</li>
		<li>Add 10 ml&#160;of anhydrous methanol to the solution when the temperature is below 60 &#176;C to flocculate the nanocrystals. Transfer the solution to a vial under inert gas and centrifuge.</li>
		<li>Decant the solution. Discard the supernatant and redisperse the precipitate in anhydrous toluene. Add anhydrous methanol until the solution becomes turbid and centrifuge.</li>
		<li>Repeat a similar cleaning procedure (step 2.1.9) using a different solvent/nonsolvent pair: hexane/isopropanol. This helps prevent situations in which the nanocrystals and precursors precipitate simultaneously.</li>
		<li>Disperse the cleaned precipitate in a small volume (a few milliliters) of anhydrous toluene. The use of a small volume of solvent here will avoid the need to concentrate the particles as an additional step prior to their use in the seeded growth of CdS.</li>
		<li>Confirm the wurtzite crystal structure of the CdSe nanocrystals by X-ray diffraction. This step is strongly encouraged due to the polymorphism of CdSe and the strong dependence on the structure of the CdSe seed for the resulting heterostructures.</li>
		<li>Dilute a small aliquot of the w-CdSe stock solution by a known dilution factor and acquire a UV-Vis absorption spectrum.</li>
		<li>Determine the w-CdSe nanocrystal diameter and extinction coefficient based upon the first exciton absorbance according to the following formulas17: 
		D = (1.6122&#215;10-9)&#955;4&#160;- (2.6575&#215;10-6)&#955;3&#160;+ (1.6242&#215;10-3)&#955;2 - (0.4277)&#955; + 41.57 
		&#949;&#160;=&#160;5857(D)2.65 
		where D is the diameter (nm) of the w-CdSe nanocrystal, &#955;&#160;is the wavelength (nm) of the first excitonic absorption peak, and &#949; is the extinction coefficient (L/mol&#8226;cm)&#160;of nanocrystals at the first exciton.</li>
		<li>Determine the concentration of the w-CdSe nanocrystal stock solution using the absorbance at the first excitonic absorption peak in accordance with Beer-Lambert&#8217;s Law and the appropriate dilution factor.</li>
		<li>To prepare w-CdSe for the growth of seeded rods, combine 0.5 g TOP with a volume of the w-CdSe nanocrystal stock solution corresponding to 10-8 mol of w-CdSe nanocrystals. Sonicate briefly if needed to redisperse the w-CdSe nanocrystals in TOP. The use of a small volume of toluene for the w-CdSe stock solution in a prior step (2.1.11) should lead to only a small amount of toluene, on the order of microliters, in the seed solution used for the injection. The injection of larger volumes of volatile organic solvents, such as toluene, at temperatures substantially above their boiling point poses a safety hazard and will adversely affect the nanocrystal product due to large drops in reaction temperatures.</li>
	</ol>
	</li>
	<li>Synthesis of zinc-blende CdSe (zb-CdSe) nanocrystals16,18,19
	<ol>
		<li>Combine 0.17 g cadmium myristate and 37 ml&#160;ODE with a stir bar in a 50 ml&#160;3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter or punctured through a rubber septum), a reflux condenser, and a rubber septum. Inspect all glassware for defects prior to use due to the high temperature of reaction. Assemble all glass-to-glass joints with high temperature vacuum grease.</li>
		<li>Degas the solution under vacuum at 90 &#176;C for 1 hr while stirring (800 rpm). Subsequently, cool the solution to room temperature under inert gas.</li>
		<li>Briefly remove the rubber septum and add 33 mg selenium dioxide powder to the flask. Replace the septum. Degas the solution under vacuum at 50 &#176;C for 15 min.</li>
		<li>Heat to 240 &#176;C at a ramp rate of approximately 10 &#176;C/min under inert gas. As the temperature rises, the solution will begin to change color, progressing from yellow to red.</li>
		<li>Add a previously degassed solution of 1 ml&#160;oleic acid and 1 ml&#160;oleylamine in 4 ml&#160;ODE drop-wise when the solution reaches 240 &#176;C.</li>
		<li>React the solution for 30 min&#160;at 240 &#176;C. The size of the zb-CdSe nanocrystals can be tuned by modifying the reaction time. Take aliquots for absorption and photoluminescence spectra to monitor particle growth if desired.</li>
		<li>Remove the heating mantle and cool to room temperature. Transfer the crude solution to an inert atmosphere glove box and split the reaction mixture into 20 ml&#160;fractions in centrifuge vials.</li>
		<li>Add 10 ml&#160;anhydrous acetone to each vial and centrifuge solutions to isolate the nanocrystals.</li>
		<li>Pour off the supernatant and redisperse the precipitate in anhydrous toluene. Add acetone until the solution becomes turbid and centrifuge.</li>
		<li>Repeat a similar cleaning procedure (step 2.2.9) using a different solvent/nonsolvent pair: chloroform/methanol. This helps prevent situations in which the nanocrystals and precursors precipitate simultaneously.</li>
		<li>Disperse the cleaned precipitate in a small volume (a few milliliters) of anhydrous chloroform. The use of a small volume of solvent here will avoid the need to concentrate the particles as an additional step prior to their use in the seeded growth of CdS.</li>
		<li>After cleaning the particles, confirm the zinc-blende crystal structure of the CdSe nanocrystals by X-ray diffraction. This step is strongly encouraged due to the polymorphism of CdSe and the strong dependence on the structure of the CdSe seed for the resulting heterostructures.</li>
		<li>Dilute a small aliquot of the zb-CdSe stock solution by a known dilution factor and acquire a UV-Vis absorption spectrum.</li>
		<li>Determine the zb-CdSe nanocrystal diameter and extinction coefficient based upon the first exciton absorbance according to the following formulas20: 
		<img fo:content-width="2in" src="/files/ftp_upload/50731/50731eq1.jpg" width="240px" /> 
		&#949;&#160;=&#160;19300 (D)3 
		where D is the diameter (nm) of the zb-CdSe nanocrystal, Eg the band gap energy (eV) corresponding to the first excitonic absorption peak, and &#949; is the extinction coefficient (L/mol&#8226;cm)&#160;of nanocrystals at 340 nm.</li>
		<li>Determine the concentration of the zb-CdSe nanocrystal stock solution using the absorbance at 340 nm in accordance with&#160;Beer-Lambert&#8217;s Law and the appropriate dilution factor.</li>
		<li>Combine 0.5 g TOP with a volume of the zb-CdSe nanocrystal stock solution corresponding to 10-8 mol of zb-CdSe nanocrystals to prepare zb-CdSe injection for the growth of seeded tetrapods. Sonicate briefly if needed to redisperse the zb-CdSe nanocrystals in TOP. The use of a small volume of chloroform for the zb-CdSe stock solution in a prior step (2.2.11) should lead to only a small amount of chloroform, on the order of microliters, in the seed solution used for the injection. The injection of larger volumes of volatile organic solvents, such as chloroform, at temperatures substantially above their boiling point poses a safety hazard and will adversely affect the nanocrystal product due to large drops in reaction temperatures.</li>
	</ol>
	</li>
</ol>
3. Synthesis of CdSe/CdS Heterostructures
<ol>
	<li>Synthesis of CdSe/CdS seeded nanorods6
	<ol>
		<li>Combine 207 mg CdO, 1.08 g ODPA, 3.35 g TOPO, and 15 mg propylphosphonic acid (PPA) with a stir bar in a 25 ml&#160;3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter or punctured through a rubber septum), a reflux condenser, and a rubber septum. Inspect all glassware for defects prior to use due to the high temperature of reaction. Assemble all glass-to-glass joints with high temperature vacuum grease.</li>
		<li>Degas the solution under vacuum at 120 &#176;C for 30 min&#160;while stirring (800 rpm).</li>
		<li>Heat the solution to 320 &#176;C under inert gas to form an optically clear solution. Carefully swirl the solution to remove excess CdO from the sidewalls of the flask. Allow the solution to become optically clear again.</li>
		<li>Cool the solution to 120 &#176;C. Degas the solution under vacuum for 1 hr at 120 &#176;C.</li>
		<li>After degassing and complexing the reaction solution, heat the mixture to 340 &#176;C under inert gas. Inject 1.5 g TOP drop-wise and let the temperature recover to 340&#176;C.</li>
		<li>Rapidly inject 0.65 g TOP:S using a syringe with a large diameter (14 G) needle.</li>
		<li>20 seconds after the TOP:S injection, rapidly inject w-CdSe solution (10-8 mol w-CdSe nanocrystals in 0.5 g TOP) using a syringe with a large diameter (14 G) needle.</li>
		<li>Adjust the reaction temperature to 320 &#176;C. After 10 min&#160;at 320 &#176;C, remove the heating mantle and cool the solution.</li>
		<li>Add ~10 ml&#160;anhydrous toluene to the flask when the temperature is around 100 &#176;C to prevent solidification of the TOPO.</li>
		<li>Transfer the crude solution to a centrifuge vial. The purification of the crude solution of seeded rods can be performed in an inert atmosphere glovebox or ambient conditions, if desired, as the overgrowth of CdS enhances the environmental stability of the nanocrystals. Add an equal volume of anhydrous ethanol to flocculate the nanocrystals and centrifuge.</li>
		<li>Pour off the supernatant and redispersed the precipitate in hexane/octylamine (8:1 by volume). Add anhydrous ethanol until the solution appears turbid and centrifuge. Redisperse precipitate in anhydrous toluene.</li>
		<li>Occasionally, solutions of seeded rods will form a gel during the cleaning process. If this occurs, add a few drops of octylamine to dissolve the gel and continue the cleaning procedure. 
		Note: The length of the nanorods can be tuned by changing the concentration of injected CdSe seeds; higher seed&#160;concentrations lead to shorter rods. Adjusting the TOP:S concentration will vary the rod diameter and length; higher TOP:S concentration produces longer, thinner rods. See reference6 for further details.</li>
	</ol>
	</li>
	<li>Synthesis of CdSe/CdS seeded tetrapods6,17 
	<ol>
		<li>Combine 207&#160;mg CdO, 1.08 g ODPA, 3.35 g TOPO, and 50 mg PPA with a stir bar in a 25 ml&#160;3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter or punctured through a rubber septum), a reflux condenser, and a rubber septum. Inspect all glassware for defects prior to use due to the high temperature of reaction. Assemble all glass-to-glass joints with high temperature vacuum grease.</li>
		<li>Degas the solution under vacuum at 120 &#176;C for 1 hr while stirring (800 rpm).</li>
		<li>Heat the solution to 280 &#176;C under inert gas to form an optically clear solution. Carefully swirl the solution to remove excess CdO from the sidewalls of the flask. Allow the solution to become optically clear again.</li>
		<li>Cool the solution to 120 &#176;C. Degas the solution under vacuum for 1 hr at 120 &#176;C.</li>
		<li>After degassing and complexing the reaction solution, heat the mixture to 300 &#176;C under inert gas. Inject 1.5 g TOP drop-wise and let the temperature recover to its initial value.</li>
		<li>Rapidly inject 0.65 g TOP:S using a syringe with a large diameter (14 G) needle.</li>
		<li>40 seconds after the TOP:S injection, rapidly inject zb-CdSe solution (10-8 mol zb-CdSe nanocrystals in 0.5 g TOP) using a syringe with a large diameter (14 G) needle.</li>
		<li>Raise the reaction temperature to 315 &#176;C at a rate of 1 &#176;C/min. After 20 min&#160;at 315 &#176;C, remove the heating mantle and cool.</li>
		<li>Add anhydrous toluene to the flask when the temperature is around 100 &#176;C to prevent solidification of the TOPO.</li>
		<li>Transfer the crude solution to a centrifuge vial. The purification of the crude solution of seeded tetrapods can be performed in an inert atmosphere glovebox or ambient conditions, if desired, as the overgrowth of CdS enhances the environmental stability of the nanocrystals. Add anhydrous acetone until the solution becomes turbid and centrifuge.</li>
		<li>Pour off the supernatant and redisperse the precipitate in hexane/octylamine (8:1 by volume). Add anhydrous acetone until the solution becomes turbid and centrifuge.</li>
		<li>Repeat the purification of the tetrapods twice more using toluene and acetone as the solvent and nonsolvent, respectively. Store the tetrapods in toluene.</li>
		<li>Separate tetrapods from undesired nanorod byproducts by centrifuging toluene solutions at 12,000 x g for 30 min&#160;to develop a concentration gradient due to the mass difference between rods and tetrapods. Carefully remove and discard the upper layers, which contain mainly rods. Combine and dissolve the lower layers, which contain mostly tetrapods, in toluene. 
		Note: The length of the tetrapod arms can be tuned by changing the injection seed concentration; higher seed concentrations produce shorter arms. The growth time can be used to adjust the arm diameter; longer growth times increase the diameter of the arms. See reference6 for details. The yield of tetrapods can be varied by using different amounts of PPA in the initial reaction mixture. See reference19 for further details.</li>
	</ol>
	</li>
</ol>

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C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+of+CdSe+and+CdTe+Nanocrystals+without+Precursor+Injection.">Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection.</a> Angew. Chem. Int. Ed. 44, 6712-6715 (2005).</li><li>Yu, W. W., Qu, L., Guo, W., Peng, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+Determination+of+the+Extinction+Coefficient+of+CdTe,+CdSe,+and+CdS+Nanocrystals.">Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals.</a> Chem. 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K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+Properties+of+Zincblende+Cadmium+Selenide+Quantum+Dots.">Optical Properties of Zincblende Cadmium Selenide Quantum Dots.</a> J. Phys. Chem. C. 114, 6371-6376 (2010).</li><li>Qu, L., Peng, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+of+Photoluminescence+Properties+of+CdSe+Nanocrystals+in.">Control of Photoluminescence Properties of CdSe Nanocrystals in.</a> Growth. J. Am. Chem. Soc. 124, 2049-2055 (2002).</li><li>Manna, L., Scher, E. C., Alivisatos, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+Control+of+Colloidal+Semiconductor+Nanocrystals.">Shape Control of Colloidal Semiconductor Nanocrystals.</a> J. Cluster Sci. 13, 521-532 (2002).</li><li>Sitt, A., Sala, F. D., Menagen, G., Banin, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiexciton+Engineering+in+Seeded+Core/Shell+Nanorods:+Transfer+from+Type-I+to+Quasi-type-II+Regimes.">Multiexciton Engineering in Seeded Core/Shell Nanorods: Transfer from Type-I to Quasi-type-II Regimes.</a> Nano Lett. 9, 3470-3476 (2009).</li></ol>

We have nothing to disclose.

The optical spectra for the w-CdSe and zb-CdSe seeds exhibit clear excitonic absorption (Figures 1a and&#160;1b) as the seeds are highly monodisperse; the energy of the first exciton corresponds to diameters of 3.3 nm and 3.6 nm for w-CdSe and zb-CdSe, respectively. The seeds also exhibit strong photoluminescence, with quantum yields ranging from approximately 3-30%, depending on the method of preparation of the sample; for instance, a greater number of cleaning steps tends to remove the surface ligands and lead to lower quantum yields6. The narrow full width at half-maximum of the PL peak (31 nm and 33 nm for the w-CdSe and zb-CdSe nanocrystals, respectively) is consistent with size distributions of 5-10%21.
The X-ray diffraction patterns collected for the seeds (Figures 1c and&#160;1d) confirm that they are of the desired phase. The wurtzite phase has clear reflections at 41.1&#176; and 53.8&#176; that are absent for the zinc-blende phase. The peak broadening arises from Scherrer broadening due to finite crystal size.
The differences in the crystal structure of the CdSe seeds lead to dramatically different morphologies of the final heterostructure (Figures 2a and 2b)6. w-CdS nucleates and grows from the (001) and (00<img fo:content-width="0.07in" src="/files/ftp_upload/50731/50731bar.jpg" />)&#160;facets of the w-CdSe, leading to a final rod-shaped particle since there is uniaxial growth. In contrast, w-CdS nucleates and grows off the {111} facets of zb-CdSe. Of the eight {111} facets, four exhibit lower coordinative saturation than the others; as a result, we observe more rapid growth of w-CdS off of these four facets, which leads to a tetrapod morphology22. The seeded rods and tetrapods display high crystallinity, corresponding to wurtzite CdS as confirmed by XRD (Figures 2c and&#160;2d).
Because the CdS portion of the heterostructures is much larger compared to the CdSe seed, the absorption due to the seeds is a small percentage of the overall absorption. As a result, the heterostructures absorb weakly at wavelengths longer than 500 nm, where the majority of absorption is due to the CdSe seed (Figures 2e and&#160;2f). At wavelengths shorter than 500 nm, absorption is dramatically enhanced since the larger bandgap CdS begins to absorb significantly. The extinction coefficients for the seeds are on the order of 105 M-1cm-1, while the rods and tetrapods have significantly enhanced extinction coefficients6 of 107 M-1cm-1 and 108 M-1cm-1, respectively. For both the rods and tetrapods, the first excitonic transition for the heterostructures is red-shifted compared to the bare seeds (Figures 2e and 2f) since the electrons and holes are spread out over a larger particle.
Depending on the particular size of the components of the heterostructure and the resulting band structure, the hole tends to be somewhat confined to the seed while the electron is more free to move throughout the entire particle23. As a result, recombination occurs away from the particle surface, leading to quantum yields for rods above 75% and for tetrapods between 35-70%, which are significantly higher than that of the bare CdSe seeds6. Furthermore, the inorganic passivation from the CdS on the CdSe seeds provides enhanced environmental stability.
We have demonstrated a method for controlling the final morphology of a nanoscale heterostructure via control over the crystal structure of the seed. This same seeded-growth approach has already been applied to a wide range of structures, and we expect that the types of heterostructures that can be synthesized by this approach will continue to expand.

The synthesis of multicomponent nanostructures has developed rapidly over recent years1. A range of II-VI semiconductor nanoheterostructures has been synthesized, including spherical core-shell particles2,3, seeded nanorods4,5, and seeded tetrapods6. These heterostructures allow for sophisticated manipulation of photons and carriers at the nanoscale and exhibit many interesting phenomena, including extremely high extinction coefficients, polarized emission4&#160;and high quantum yields2,7.
The synthesis of multicomponent nanostructures requires precise control over the composition, phase, shape, size, and connectivity of each component. Control over these various factors relies largely on the ability to independently modulate the rate of nucleation and growth8,9,10. One strategy to achieve independent control over nucleation and growth is to separate these two processes into separate synthetic steps. Here, we will show how small seeds of CdSe generated under reaction conditions optimized for nucleation and growth of spherical nanocrystals can then be transferred to a second flask where the growth of CdS occurs under reaction conditions optimized for anisotropic growth of rod-shaped arms6. By tuning the crystal structure of the CdSe seed from wurtzite to zinc-blende, we will demonstrate how the morphology of CdSe/CdS heterostructures grown under similar conditions can be changed from a nanorod to a tetrapod, highlighting the precise control over phase, shape, and size afforded by state-of-the-art nanoparticle synthesis methods. Nanostructures of the cadmium chalcogenides have served as a useful model-system for the development of new synthetic techniques, as this material system has been intensively explored; although the synthesis of a particular nanostructure is shown in this video protocol, the same techniques are applicable to the synthesis of numerous other nanostructures.
Small differences in the techniques and reagents employed in nanoparticle syntheses have been shown to create dramatic differences in the final product. For instance, the temperature ramp rate11 and impurities12-14 present during the synthesis have been shown to dramatically impact the size and shape of the nanocrystals. With identification of the specific factors to which the synthetic procedures are highly sensitive, the reproducibility of nanocrystal syntheses continues to improve. This detailed video protocol is intended to help new practitioners in the field avoid many common pitfalls associated with the synthesis of nanoparticles.

<table><tbody><tr><td>Sodium hydroxide, 98%</td><td>Fisher</td><td>BP359</td><td></td></tr><tr><td>Myristic acid, 99%</td><td>Sigma Aldrich</td><td>M3128</td><td></td></tr><tr><td>Methanol, 99.8%</td><td>Fisher</td><td>A412</td><td></td></tr><tr><td>Cadmium nitrate tetrahydrate, 99%</td><td>Sigma Aldrich</td><td>20911</td><td>Highly toxic</td></tr><tr><td>Cadmium oxide (CdO), 99.99%</td><td>Sigma Aldrich</td><td>202894</td><td>Highly toxic</td></tr><tr><td>1-Octadecene (ODE), 90%</td><td>Sigma Aldrich</td><td>O806</td><td>Technical grade</td></tr><tr><td>Selenium (Se), 100 mesh, 99.99%</td><td>Sigma Aldrich</td><td>229865</td><td></td></tr><tr><td>Tri-n-octylphosphine (TOP), 97%</td><td>Strem</td><td>15-6655</td><td>Air sensitive</td></tr><tr><td>Sulfur (S), 99.9995%</td><td>Alfa Aesar</td><td>10343</td><td></td></tr><tr><td>Trioctylphosphine oxide (TOPO), 99%</td><td>Strem</td><td>15-6661</td><td></td></tr><tr><td>N-Octadecylphosphonic acid (ODPA), 99%</td><td>PCI Synthesis</td><td>104224</td><td></td></tr><tr><td>Methanol, anhydrous, 99.8%</td><td>Sigma Aldrich</td><td>322415</td><td></td></tr><tr><td>Toluene, anhydrous, 99.8%</td><td>Sigma Aldrich</td><td>244511</td><td></td></tr><tr><td>Chloroform, anhydrous, 99%</td><td>Sigma Aldrich</td><td>288306</td><td></td></tr><tr><td>Acetone, 99.9%</td><td>Fisher</td><td>A949</td><td>Dried over 4&amp;Aring; sieves</td></tr><tr><td>Hexane, anhydrous, 95%</td><td>Sigma Aldrich</td><td>296090</td><td></td></tr><tr><td>2-Propanol, anhydrous, 99.5%</td><td>Sigma Aldrich</td><td>278475</td><td></td></tr><tr><td>Selenium dioxide, 99.999%</td><td>Sigma Aldrich</td><td>204315</td><td></td></tr><tr><td>Oleic acid, 90%</td><td>Sigma Aldrich</td><td>364525</td><td>Technical grade</td></tr><tr><td>Oleylamine, 70%</td><td>Sigma Aldrich</td><td>O7805</td><td>Technical grade</td></tr><tr><td>Propylphosphonic acid (PPA), 95%</td><td>Sigma Aldrich</td><td>305685</td><td></td></tr><tr><td>Ethanol, anhydrous, 99.5%</td><td>Sigma Aldrich</td><td>459836</td><td></td></tr><tr><td>Octylamine, 99%</td><td>Sigma Aldrich</td><td>O5802</td><td></td></tr></tbody></table>

seeded synthesis of cdse/cds rod and tetrapod nanocrystals

Wir zeigen, ein Verfahren zur Synthese von Mehrkomponenten-Nanostrukturen, bestehend aus CdS und CdSe mit Stab und tetrapod Morphologien. Ein ausgesät Synthesestrategie eingesetzt wird, bei dem kugelförmige Samen von CdSe zunächst mit einem Heißinjektionstechnik hergestellt. Durch Steuern der Kristallstruktur des Samens entweder Wurtzit-oder Zinkblende, die anschließende Heißinjektions Wachstum von CdS aus den Samen führt entweder zu einem stabförmigen oder tetrapodförmigen Nanokristall betragen. Die Phase und Morphologie der Nanokristalle werden durch Röntgenbeugung und Transmissionselektronenmikroskopie bestätigt, die zeigen, dass die Nanokristalle sind phasenrein und haben eine einheitliche Morphologie. Der Extinktionskoeffizient und die Quantenausbeute der Nanokristalle werden mit UV-VIS-Absorptionsspektroskopie und Photolumineszenz-Spektroskopie ermittelt. Die Stäbe und tetrapods Ausstellungs Extinktionskoeffizienten und Quantenausbeuten, die höher ist als die der bloßen sindSamen. Dieses Verfahren zeigt die genaue Anordnung der Materialien, die im Nanometerbereich durch Verwendung einer geimpften Syntheseverfahren erreicht werden können.

Absorption spectra, photoluminescence (PL) spectra, X-ray diffraction (XRD) patterns, and transmission electron microscopy (TEM) images were collected for the seeds (Figure 1) and the final heterostructures (Figure 2). The absorption and PL spectra are used to calculate the extinction coefficient and quantum yield of the nanostructures. XRD and TEM are used to determine the phase and morphology of the nanostructures.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/50731/50731fig1highres.jpg" src="/files/ftp_upload/50731/50731fig1.jpg" width="500px" /> 
Figure 1. CdSe nanocrystal seeds of wurtzite and zinc-blende crystal structures. (a, b) Absorption (black) and PL (red) spectra with inset of TEM images of wurtzite and zinc-blende&#160;CdSe nanocrystals, respectively. (c, d) X-ray diffraction patterns of wurtzite and zinc-blende CdSe nanocrystals with the line patterns of corresponding bulk materials provided for reference. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/50731/50731fig1highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 2" fo:content-width="5in" fo:src="/files/ftp_upload/50731/50731fig2highres.jpg" src="/files/ftp_upload/50731/50731fig2.jpg" width="500px" /> 
Figure 2. CdSe/CdS seeded heterostructures of rod and tetrapod morphologies. (a, b)&#160; TEM images of CdSe/CdS nanorods and CdSe/CdS tetrapods, respectively. (c, d) XRD patterns of CdSe/CdS nanorods and tetrapods. Only reflections from wurtzite CdS are apparent. (e, f) Absorption (black) and PL (red) spectra of CdSe/CdS nanorods and tetrapods. Green line shows magnified absorption spectra to highlight absorption due to the CdSe seed. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/50731/50731fig2highres.jpg" target="_blank">Click here to view larger image</a>.

Watch this Scientific Journal Video about Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals at JoVE.com

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

Gesetzt Synthese von CdSe / CdS-Nanokristalle Rod und Tetrapod

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

seeded-synthesis-of-cdsecds-rod-and-tetrapod-nanocrystals

Universidad de Cartagena UDEC

Research

JoVE Journal

Chemistry

39.7K Aufrufe.  UC Berkeley. A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

Serie: Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Compact Quantum Dots für Einzelmolekül-Imaging

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Forschung

Ingenieurwesen

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Harvesting Solar Energy mittels Charge-Trennung Nanokristalle und ihre Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Synthese, Charakterisierung und Funktionalisierung von Hybrid Au / CdS und Au / ZnS Kern / Schale-Nanopartikel

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

Chemie

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

Photochemische oxidative Wachstum von Iridium-Nanopartikel auf CdSe @ CdS-Nanostäbchen

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Synthese von Ligand freien CdS-Nanopartikel innerhalb eines Schwefel-Copolymer-Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

Die Herstellung von Voll Lösung verarbeitet Anorganische Nanokristalline Photovoltaik-Anlagen

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Advanced Compositional Analysis of Nanoparticle-polymer Composites Using Direct Fluorescence Imaging

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise composition of these materials is essential, especially in the design of surface catalysts, where the surface concentration of the active component determines the activity of the material. Antimicrobial materials which utilize nanoparticles are a particular focus of this technology. Recently swell encapsulation has emerged as a technique for inserting antimicrobial nanoparticles into a host polymer matrix. Swell encapsulation provides the advantage of localizing the incorporation to the external surfaces of materials, which act as the active sites of these materials. However, quantification of this nanoparticle uptake is challenging. Previous studies explore the link between antimicrobial activity and surface concentration of the active component, but this is not directly visualized. Here we show a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of CdSe/ZnS nanoparticles can be accurately visualized through cross-sectional fluorescence imaging. Using this method, we can quantify the uptake of nanoparticles via swell encapsulation and measure the surface concentration of encapsulated particles, which is key in optimizing the activity of functional materials.

Here we present a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of cadmium selenide quantum dots can be accurately visualized through cross-sectional fluorescence imaging.

Erweiterte Analyse der Zusammensetzung von Nanopartikel-Polymer Composites mittels direkter Fluoreszenz-Imaging

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise ...

Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are composed of CdSSe branches grown on ZnO nanowires that are vertically aligned on a transparent sapphire substrate. The morphology was measured via scanning electron microscopy. The crystal structure was determined by X-ray powder diffraction analysis. Both the ZnO stem and CdSSe branches have a predominantly wurtzite crystal structure. The mole ratio of S and Se in the CdSSe branches was measured by energy dispersive X-ray spectroscopy. The CdSSe branches result in strong visible light absorption. Photoluminescence (PL) spectroscopy showed that the stem and branches form a type-II heterojunction. PL lifetime measurements showed a decrease in the lifetime of emission from the trees when compared to emission from individual ZnO stems or CdSSe branches and indicate fast charge transfer between CdSSe and ZnO. The vertically aligned ZnO stems provide a direct electron transport pathway to the substrate and allow for efficient charge separation after photoexcitation by visible light. The combination of the abovementioned properties makes ZnO/CdSSe nanotrees promising candidates for applications in solar cells, photocatalysis, and opto-electronic devices.

Here, we prepare and characterize novel tree-like hierarchical ZnO/CdSSe nanostructures, where CdSSe branches are grown on vertically aligned ZnO nanowires. The resulting nanotrees are a potential material for solar energy conversion and other opto-electronic devices.

Synthese von Hierarchical ZnO / CdSSe Heterostruktur Nanotrees

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are ...

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Facette für Facette Verknüpfung von Form-anisotropen kolloidales Cadmium Chalkogenid Nanostrukturen

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

Fabrication of Spherical and Worm-shaped Micellar Nanocrystals by Combining Electrospray, Self-assembly, and Solvent-based Structure Control

Micellar nanocrystals (micelles with encapsulated nanocrystals) have become an emerging major class of nanobiomaterials. We describe a method of fabricating micellar nanocrystals based on combining top-down electrospray, bottom-up self-assembly, and solvent-based structure control. This method involves first using electrospray to generate uniform ultrafine liquid droplets, each of which functions as a micro-reactor in which self-assembly reaction occurs forming micellar nanocrystals, with the structures (micelle shape and nanocrystal encapsulation) controlled by the organic solvent used. This method is largely continuous and produces high quality micellar nanocrystal products with an inexpensive structure control approach. By using a water-miscible organic solvent tetrahydrofuran (THF), worm-shaped micellar nanocrystals can be produced due to solvent-induced/facilitated micelle fusion. Compared with the common spherical micellar nanocrystals, worm-shaped micellar nanocrystals can offer minimized non-specific cellular uptake, thus enhancing biological targeting. By co-encapsulating multiple nanocrystals into each micelle, multifunctional or synergistic effects can be achieved. Current limitations of this fabrication method, which will be part of the future work, primarily include imperfect encapsulation in the micellar nanocrystal product and the incompletely continuous nature of the process.

The present work describes a method to fabricate micellar nanocrystals, an emerging major class of nanobiomaterials. This method combines top-down electrospray, bottom-up self-assembly, and solvent-based structure control. The fabrication method is largely continuous, can produce high quality products, and possesses an inexpensive means of structure control.

Herstellung von sphärischen und Wurm-förmigen Mizellen Nanokristalle durch die Kombination von Elektrospray, Selbstmontage und lösemittelhaltige Strukturkontrolle