controlled synthesis of cdse nanowires by solution–liquid–solid method

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Controlled Synthesis of CdSe Nanowires by Solution–Liquid–Solid Method

By Zhen Li,* Ozgul Kurtulus, Nan Fu, Zhe Wang, Andreas Kornowski,

Ullrich Pietsch, and Alf Mews*

Semiconductor nanowires prepared by wet chemical methods are a relatively

new field of 1D electronic systems, where the dimensions can be controlled by

changing the reaction parameters using solution chemistry. Here, the

solution–liquid–solid approach where the nanowire growth is governed by

low-melting-point catalyst particles, such as Bi nanocrystals, is presented. In

particular, the focus is on the preparation and characterization of CdSe

nanowires, a material which serves a prototype structure for many kinds of

low dimensional semiconductor systems. To investigate the influence of

different reaction parameters on the structural and optical properties of the

nanowires, a comprehensive synthetic study is presented, and the results are

compared with those reported in literature. How the interplay between

different reaction parameters affects the diameter, length, crystal structure,

and the optical properties of the resultant nanowires are demonstrated. The

structural properties are mainly determined by competing reaction pathways,

such as the growth of Bi nanocatalysts, the formation and catalytic growth of

nanowires, and the formation and uncatalytic growth of quantum dots.

Systematic variation of the reaction parameters (e.g., molecular precursors,

concentration and concentration ratios, organic ligands, or reaction time, and

temperature) enables control of the nanowire diameter from 6 to 33 nm, while

their length can be adjusted between several tens of nanometers and tens of

micrometers. The obtained CdSe nanowires exhibit an admixture of wurtzite

(W) and zinc blende (ZB) structure, which is investigated by X-ray diffraction.

The diameter-dependent band gaps of these nanowires can be varied between

650 and 700 nm while their fluorescence intensities are mainly governed by

the Cd/Se precursor ratio and the ligands used.

1. Introduction

The adjustment of the structural, optical, electronic, andmagneticproperties of nanomaterials strongly relies on the development of

[*] Dr. Z. Li, Prof. Alf Mews, Dr. N. Fu, Z. Wang, A. KornowskiDepartment of Physical ChemistryUniversity of HamburgGrindelallee 117, 20146 Hamburg (Germany)E-mail: [email protected]; [email protected]

Dr. O. Kurtulus, Prof. U. PietschDepartment of PhysicsUniversity of SiegenWalter-Flex Str. 3, 57072 Siegen (Germany)

DOI: 10.1002/adfm.200900569

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

reaction schemes to prepare nanostruc-tures with distinct size and shape.[1]

Different wet and nonwet chemical techni-ques have been developed to generatevarious nanostructures of different dimen-sionality.[2] Compared with nonwet chemicalapproaches, such as molecular beamepitaxy[3] and chemical vapor deposition,[4]

wet chemical methods show advantages fortuning the size, shape, and properties of thenanostructures, especially in the prepara-tion of very small nanostructures.[5] Inaddition, wet chemical methods are attrac-tive in terms of cost and sustainability. Onetypical class of systems is semiconductornanostructures, where CdSe is chosen as aprototype material. Hence in this paper, wewill firstly summarize the controlled synth-esis conditions to prepare CdSe quantumdots (QDs) and rods through advancedcolloidal chemical methods, where theimportance of reaction parameters, such asprecursor concentrations, ligands, pre-cursor molar ratios, and reaction tempera-tures, will be emphasized.[6–17] Since theseparameters have been intensively studied inthe past, they serve as a basis for the synthesisof extended CdSe nanowires by the morerecently developed technique, namely thesolution–liquid–solid (SLS) approach,[18–20]

where the 1D wire growth is catalyzed by acatalyst particle with a low melting point. Inthis method, the range of parameters is even

broader in comparison to the unanalyzed growth of CdSenanostructures asmentionedabove, due to the additional presenceof the catalyst. Hence we will summarize the different syntheticapproaches from literature and compare it to our own results. Inparticular, wewill present a detailed investigation on the synthesis,characterization, and optical properties of CdSe nanowiresgenerated by the SLS method and finally we will provide anoutlook of this research direction.

Amilestone in the preparation of CdSe nanostuructures by wetchemical methods was presented in 1993 where it was shown thathigh quality CdSe QDs with a narrow size distribution can beprepared by thermal decomposition of organometallic precursorsin hot solvent.[6] Since then, several CdSe nanostructures such asdots, rods and tetrapods with tunable shapes and dimensionsin the nanometer or even subnanometer range have been

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reported.[6,7,9] It is interesting to note that only slightmodificationsof reaction parameters, such as ligands, reaction temperature, orprecursor concentration (ratios), can lead to the broad variety ofsizes and shapes as mentioned above.[7–9,13–15] For example, Penget al. demonstrated that the size and size distribution of CdSe dotscan be manipulated by the monomer concentration.[7] At highmonomer concentrations, the smaller nanoparticles grow fasterthan larger ones, which results in the size distribution being‘‘focused.’’ If the monomer concentration drops below a criticalthreshold, the smaller particles are depleted as larger ones grow(i.e., Ostwald ripening), and the size distribution becomes broaderor is ‘‘defocused.’’ Further attempts to adjust the growth kinetics ofthe CdSe nanomaterial incidentally lead to the development ofCdSe nanorods.[8,9] By using very high precursor concentrationsandadefinedadmixtureof alkylphosphonic acids and trioctylphos-phine oxide (TOPO), CdSe rods—and later even more complexstructures, such as arrows, teardrops or tetrapods—have beensynthesized.[9] Peng et al. could also show that the shape evolutionof CdSe nanorods goes through three stages upon decrease of themonomer concentration: i.e., 1D growth, 3D growth, and 1D/2Dripening.[11] On the other hand an extremely high monomerconcentration could also lead to complex 3D nanostructures, butonly after the formation of ‘‘magic-sized nuclei.’’[14] For instance,initial monomer concentrations as large as [Cd]¼ 433mmol(g TOPO)�1 resulted in tetrapod-shaped or branched CdSenanocrystals while successive lowering of the concentrationgenerated nanorods, rice-shaped nanorods, and finally sphericalnanocrystals.

In summary, it is possible to generate very complex CdSenanostructures by precisely controlling reaction parameters, suchas the precursors and their concentrations, the molar ratio of Cdand Se precursors, the molar ratio between alkylphosphonic acid-and TOPO-ligands, the chain length of alkylphosphonic acids, andthe reaction time and temperature.[14] Hence, the preparation andshape-evolution of CdSe nanorods is well investigated, but it is notpossible to grow high-yield CdSe quantum wires with a length ofmore than a few micrometers by controlling the reactionkinetics.[21] However, the preparation of very long 1D nano-structures is of particular importance, for example, electronicdevices.[22]

Therefore an alternative method to grow very long nanowireswith high yield has been established; this is the so-called SLSapproach.[18–20] Here low-melting-point (LMP) nanoparticles areused as catalysts,[19] and the reaction is performed at temperatureswhere the LMP nanoparticles are melted into nanodroplets. Thenanowire precursors are then adsorbed on or dissolved into thenanodropets and grow into 1D nanowires until the precursorsare consumed. Hence, in addition to the reaction parameters asmentioned above, the surface properties, solvating abilities, andreactivities of the nanocatalysts should also determine theformation of the nanowires.[19]

The first example of synthesizing CdSe nanowires by the SLSmethodwas reported by the Buhro group in 2003.[23] In contrast tothe recipes applied for the preparation of nanorods,[14] thesenanowires were prepared from cadmium stearate and n-R3PSe(R¼ butyl or octyl) in TOPO at 240–300 8C using bimetallicAu@Bi (diameter, D¼ 8.7 nm) or pure Bi (D¼ 21.2 or 23.6 nm)nanoparticles as catalysts. In addition the initial Cd precursorconcentrations ([Cd]¼ 3.8–29.7mmol (g TOPO)�1) and the molar

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ratios ofCd and Se precursor (Cd/Se¼ 0.01–0.1) weremuch loweras compared to the values used in nanorod preparation([Cd]¼ 267mmol (g TOPO)�1; Cd/Se¼ 2).[14] This indicates thatthe growth of CdSe nanowires can be already realized at lowprecursor concentrations and also at low reaction temperatures inthe presence of Bi or Au@Bi nanocatalysts. By adjusting thereaction parameters, the authors could even prepare CdSenanowires with diameters below the bulk exciton dimension, tocompare the respective band gaps (DEg) with theoretical values.

[23]

Unfortunately, the authorsdidnot present a systematic studyof thegrowth conditions at that time. So the individual chemicalparameters, which lead to the impressively narrow diameterdistributions of their nanowires remain unclear.

Following a similar strategy, the Kuno group prepared straightand branched CdSe nanowires from CdO and TOPSe (TOP: tri-n-actylphosphine) in TOPO in the presence of octanoic acids at330 8C using Au@Bi nanoparticles as catalysts.[24] The diametersof the nanocatalyst used (1.4–3.0 nm) were much smaller thanthose used byBuhro et al.(D¼ 8.7, 21.2, and 23.6 nm).[23] Also theyusedmuch higher Cd precursor concentrations ([Cd]¼ 64.7mmol(g TOPO)�1) and very different Cd to Se precursor ratios(Cd/Se¼ 7–1.7).[23] However, their observed nanowire diameterswere always much larger than the original catalyst diameter[24]

while in the work of the Buhro group the diameter of the CdSenanowires could be either smaller or bigger than the nanocatalystdimensions, depending on the reaction conditions.[23] In thisarticle, the authors empirically discussed synthetic considerations,such as reaction mixture concentration, reaction temperature,apparent Cd to Se ratio, catalyst Au@Bi size, catalyst volume, andTOP doping. They showed for example, that parameters such aslow concentrations, high temperatures, high Cd/Se ratios, largecatalyst sizes and lower TOP concentrations are beneficial to thegrowth of straight nanowires, in contrast to branched nanowiresHowever, the nanowire diameter and length distribution was notdiscussed.

It should be noted that this SLS approach has also been appliedto synthesize other semiconductor nanowires such as IV (Si,Ge),[25,26] IV–VI (PbS, PbSe, PbTe),[27] II–VI (CdS, CdTe, ZnS,ZnSe, ZnTe)[28–32] and III–V (InP, InAs, GaAs).[18,33–37] Thismethod can even be used to prepare diode-nanowires such asCdSe–CdS[38] and ZnSe–ZnTe.[39] In these reports, the nanowirediameters are larger than their corresponding exciton radius, andhow to control the nanowire diameter and length remains unclear,especially below the exciton dimension region.

Hence, the influence of the reaction parameters on thepreparation of nanowires and the details of the reactionmechanism are still not well understood. One of the main issuesin this context is the formation and function of the different Binanocatalysts.[23,25,26,30–32,35,36,38–40] For example, pure Bi parti-cles with diameters of approximately 20 nm were either preparedby thermal decomposition of Bi[N(SiMe3)2]3 in the presence ofNa[N(SiMe3)2] and poly(1-hexadecene)0.67-co-(1-vinylpyrrolidi-none)0.33,

[23,41] or by reduction of Bi(III) 2-ethylhexanoate withNaBH4 in presence of TOP.[35] The core–shell Au@Bi nanopar-ticles[23,24,27–29] were synthesized from Au seeds that weresuccessively covered by Bi.[23,24,42] By controlling the amount ofBi precursor, the particle size can be adjusted between 1.5 and8.7 nm.However, it is quite difficult to separate the coated from thenoncoated nanoparticles. In our previous study, we developed a

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simple way to prepare pure Bi nanoparticles (D¼ 3.3� 0.5 nm) bythe reduction of Bi[N(SiMe3)2]3 or BiCl3 with TOP at roomtemperature.[43] These nanocatalysts can also be in situ producedby mixing the Bi precursors and TOPSe solution (the Seconcentration is 1 or 2 M) without additional TOP. We showedthat the Bi nanocatalysts grow into large nanoparticles (40–80 nm)uponheating.[43]Alsowewereable toshowthat thegrowthof theBinanoparticles could be slowed down in the presence of Cd and Seprecursors upon formation of the CdSe nanowires.[43]

In summary, the growth mechanism of CdSe nanowires by theSLSmethod is evenmore complex than the kinetic reaction controlto form different CdSe nanostructures, due to the additionalpresence of the catalytic nanodroplets. In addition, the structuraland optical properties[44] of CdSe nanowires generated fromslightly different SLS conditions (e.g., different catalyst particles)can be different. Hence we performed a systematic variation ofreaction parameters to evaluate their influence on the CdSenanowires nucleation and growth during the SLS process. Wepropose that at least three parallel competing reactions are takingplace during the reaction; these include 1) the growth ofnanocatalysts, 2) the formation and catalytic growth of nanowires,and 3) the formation and noncatalytic growth of QDs. Thesecompetitive processes determine the diameter, length, shape, andyield of the nanowires produced. We will show that distinctchanges in reaction parameters allow guidance of the reactionpathway towards certain directions, and thus, allow adjustments ofthe structural and optical properties of the nanowires.

2. Results and Discussion

This part of the paper is organized as follows:firstly, we will describe the effect of differentreaction parameters on the nanowire morphol-ogy. Here, we will firstly change only one of thereaction parameters as described in the experi-mental section and discuss the effect on thediameter and length of the nanowires on thebasis of transmission electron microscopy(TEM) results. This is followed by an iterativevariation of several reaction parameters tohighlight their interactions. Secondly, we willshow synchrotron X-ray results of an investiga-tion of the crystallinity of the nanowires, andfinally, we will focus on the optical properties.

Figure 1. TEM images of CdSe nanowires prepared using Bi@OA as nanocatalysts. Images (a–c)

show the effects of reaction time on CdSe nanowires obtained from the molar ratio of 7/1 (Cd/Se)

after direct quenching through toluene injection: a) toluene and the mixture of Bi nanocatalysts

and TOPSe were co-injected, b) the mixture of Bi nanocatalysts and TOPSe was injected first,

followed by toluene, c) the mixture of Bi nanocatalysts and TOPSe was injected first, and toluene

was injected after 15 s. Images (d–f) show CdSe nanowires prepared from different Cd/Se

precursor ratios: d) 7/1, e) 1/1, and f) 1/7.

2.1. Nanowire Morphology

The individual reaction parameters which werechanged to adjust thediameter and lengthof thenanowires were: reaction time, precursor ratio,and nanowire and nanocatalyst ligands, nano-catalyst amounts, and reaction temperature.

2.1.1. Reaction Time

Reaction time is the most evident reactionparameter affecting the nanowire length

� 2009 WILEY-VCH Verlag GmbH &

because a longer reaction time should lead to longer nanowires.We performed experiments in which toluene was injected atdifferent time intervals after the precursor injection. Due to thehigh reaction temperatures, the toluene immediately evaporatedleading to a very fast temperaturedecrease, andhence, terminationof nanowire growth (Table S1, code 1–3, Supporting Information(SI)).[45] For example, Figure 1a–c shows TEM images of CdSenanowires quenchedwith toluene at different times. The diameterand length distributions of the resultant CdSe nanowires arepresented in Figure S1, SI. Relatively short rods (38.8� 10.0 nm)were produced when toluene and the mixture of Bi nanocatalystsand TOPSe were co-injected into the Cd precursor solution(Fig. 1a). When the mixture of Bi nanocatalysts and TOPSe wasinjected first, followed by toluene injection after about a second,pencil-shapedCdSenanorodswith a diameter of 16 nmand lengthof 150 nm were generated (Fig. 1b). If the growth was maintainedfor 15 s before quenching, the obtained nanowires exhibited asimilar diameter, but an increased lengthof about 625 nm(Fig. 1c).Further prolonging the reaction time to 3min resulted in partialprecipitation, probably due to a 3D ripening process similar to thecase of nanorods.[14] Therefore, we chose 1min as the standardreaction time for most of the investigations.

In comparison with other reports where a reaction time of3–5min was commonly used,[23] our short reaction time isassigned to the high reactivity of the small and pure Binanoparticles. As we have already shown in our previous report,these small (3 nm) particles grow very fast at high temperaturesforming larger Bi nanodroplets. Most likely, the Bi particle growthhappens simultaneously with the wire growth, which essentiallyexplains the tip-shape of the nanowire.[43]

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Figure 2. TEM images of CdSe nanowires prepared from Bi@HDA nano-

catalysts using a) oleic acid and b) octanoic acid as CdO ligands. The molar

ratio of Cd and Se precursor in these two cases is 7/1. Images (c–d) show

CdSe nanowires prepared from two different Cd/Se ratios, c) 7/1 and d) 1/

1, using Bi@TOP as nanocatalysts. The insets are the corresponding

histograms of diameter distributions.

2.1.2. Precursor Ratio

The initial Cd and Se precursor ratio also affects nanowiremorphology.This valuehasbeen found tobe an important factor ininfluencing the aspect ratio of CdSe nanorods during kineticgrowth control[9,14] or the branching CdSe nanowires in SLSgrowth.[24] Figure 1d–f shows the TEM images of CdSe nanowiresobtained from three different Cd/Se precursor ratios, i.e., 7/1, 1/1and 1/7 (Table S1, code 4–6, SI). With increasing Se precursorratio, the nanowires decrease in length from 1.5mm to 663 and334 nm with a respective slight decrease in diameter from 22 to17 nm (Fig. S2, SI). A decrease in length upon an increase of the Seratio has also been observed in the case of CdSe nanorods withoutadditional catalyst particles. Peng et al. attributed this to the lowconcentration of remaining monomers because more Seprecursor would lead to more ‘‘magic-sized nuclei.’’[14] In thiscontext, it isworthmentioning thatwe also observed the formationof a large quantity of noncatalytically grown nanocrystals, aspresented in Figure 1f.

On the other hand, for the catalytic growth of nanowires,different ratios have been reported for different nanocatalysts. Forsmaller Au@Bi nanocatalysts (D¼ 1.4–3.0 nm), it was observedthat under Cd-rich conditions a higher Cd/Se ratio (Cd/Se¼ 7)yields straight wires, and a lower ratio (Cd/Se¼ 1.7) generatesbranchednanowires.[24] For larger pureBi orAu@Binanocatalysts(D¼ 8.7, 21.2, and 23.6 nm), Se-rich conditions (Cd/Se< 1/10)were used, and the best Cd/Se ratio for producing uniform thinnanowires was around 1/30.[19,23] Obviously the nature of thedifferent catalyst particles used is of major importance for wiregrowth. For example, the mean diameter of the pure Bi catalystparticles used in this study is similar to the Au@Bi nanocatalysts(D¼ 1.4–3.0 nm) as used by the Kuno group. However, thediameter of the nanowires from the pure Bi particles is 22 and28 nmas shown inFigure S2 (SI), which is larger than the reporteddiameters of Au@Bi nanocatalysts (5–15 nm).[24] Additionly, inour case, we obtained almost straight nanowires even at lower Cd/Se ratios (1/1 and 1/7).

These differences are attributed to the different reactivities ofnanocatalysts. The Au@Bi nanocatalysts (D¼ 1.4–3.0 nm) typi-cally consist of Au cores approximately 1.5 nm indiameter coveredwith a Bi layer of 0.4–0.8 nm in thickness.[24,42] Such hybridnanocatalystsmay release the Bimore slowly, and the formation oflarge Bi particles is hampered, which also leads to the formation ofthinner nanowires. Hence in our case, the pure Bi particlesbecome 7–10 times larger in diameter after wire growth while theAu@Bi nanocatalysts used by Kuno et al. are around 10 nm,whichis only about 4–5 times the initial diameter.

2.1.3. Nanowire and Nanocluster Ligands

Different ligands covering the Bi catalyst or CdSe surface can alsoaffect the nucleation and growth of the CdSe nanowires. In thiscontext, it should be mentioned that the small bimetallic Au@Binanocatalysts (D¼ 1.4–3.0 nm) which grow during the nanowireformation are covered with TOP and oleic acid ligands.[24,42] Forthose particles, it was reported that the resulting nanowirediameter is always larger than theoriginal catalyst size.[24,42]On theotherhand, if largebimetallicAu@Bi (D¼ 8.7 nm)or large pureBinanocatalysts (D¼ 21.2 or 23.6 nm) covered with a polymer ligand(poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33) were used,


the resultant nanowire diameter can be either larger or smallerthan the original size of the nanocatalysts.[23] It might well be thatthe small particles fusemore easily due to thermodynamic effects,such as melting point depressions or differences in surfacetension. Another reason might be that the polymer shell exhibitsstronger ‘‘protective’’ effects on theBi particles in comparisonwiththe small organic molecules, which prevent the nanocatalyst fromgrowing. Finally the ligands might also influence the adsorptionkinetics and reactivity of the Cd and Se precursors with the Binanocatalyst surface.

To test the effects of ligands on the nanowire growth, wecompared the SLS reactivity of amine-covered Bi particles withthose covered with TOP only. Firstly, we investigated the influenceof different amines such ashexadecylamine (HDA) (Table S1, code7, SI) instead of oleylamine (OA; Table S1, code 4, SI). Figure 1dand 2a shows that both amine ligands lead to nanowires which areapproximately 20 nmindiameter.Only the lengthof thenanowiresresulting from Bi@HDA (3–5mm, Fig. S3, SI) is longer than thatof Bi@OA (1.5mm). Also, the use of different acids, such asoctanoic acid and oleic acid, had only a minor influence on theresulting nanowires (Table S1, code 8, SI). In both cases (Fig. 2a,b),themean diameter was in a range of 20–25 nm, and the lengthwasbetween 3 and 5mm (see also Fig. S3, SI).

On the other hand, if Bi@TOP nanoparticles are used, themorphology of the resulting nanowires is totally different (TableS1, code 9–11, SI). For example Figure 2c,d shows theTEM imagesand diameter histograms of CdSe nanowires prepared from 7/1and 1/1Cd/Se precursor ratios. Compared with those usingBi@OA or Bi@HDA as nanocatalysts (Fig. 1d, 2a), the nanowires


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obtained from Bi@TOP nanocatalysts (Fig. 2c) are noticeablythinner and shorter. Their mean diameter is only 14 nm, and theirlength is approximately 100 nm. It is interesting to note that fortheseBi@TOPparticles, the decrease ofCd/Se ratio from7/1 to 1/1 results in an increase of the nanowire length from 100 nm to1.7mm (Fig. 2d). This is in contrast to the Bi@OA particles asreported above, where a higher Se ratio leads to shorter nanowires(Fig. 1e). Further decrease of this ratio to Cd/Se¼ 1/7 producedeven thinner nanowires wires (7.5� 1.7 nm, Fig. S4, SI), whichhowever was more irregular in shape.

Obviously the ligands strongly affect the growth of nanowiresfrom Bi nanodroplets. In particular, an excess of HDA isdetrimental to catalytic growth of CdSe nanowires from the Binanocatalyst surface, and favors the noncatalytic growth ofnanocrystals. This can either be attributed to a strong affinity ofamines to theBi nanodroplet surface or to the amine complexationabilities of the formedCdSe species. Thedifferent affinities ofTOPand amines to the Bi catalyst can be explained in a veryfirst approximation from the viewpoint of electronegativities ofthe interacting Bi, P, and N atoms.[46] For example sincethe electronegative difference between Bi (2.02) and N (3.04) ismuch higher than that between Bi and P (2.19), the latter onemight be replaced more easily from the surface of the Biparticle. Another argument might be that the higher packing ofthe linear amines versus the branched TOP on the surfaceof the Bi nanodroplets leads to reduced surface activity inthe case of amines. The complexation abilities of amineswith the CdSe species on the other hand has been observedalready during the growth of CdSe QDs because an addition ofHDA to the TOPO–TOP system resulted in a more rapid‘‘focusing’’ of the size distribution during particle growth.[13]

Therefore it might well be that CdSe clusters are generated asintermediate species during the Bi-catalyzed nanowire growth,and that these CdSe clusters are stabilized by amines.

Figure 3. AFM images of CdSe nanowires prepared at different temperatures and Cd/Se ratios:

a–c) nanowires obtained from Bi@HDA nanocatalysts with high HDA/Bi molar ratio (10/1)

prepared at 150, 180 and 250 8C, respectively. d–f) Nanowires obtained from Bi@HDA nano-

catalysts with lowHDA/Bi molar ratio (5.5/1) prepared from Cd/Semolar ratios of 7/1, 1/1 and 1/

2, respectively.

While the details of adsorption and reactionare certainly much more complicated, thereseems to be a general competition between thegrowth of Bi nanodroplets, the formation andcatalytic growth of nanowires, and also theformation and noncatalytic growth of nano-crystals. For example, if the HDA/Bi ratiowas further increased to 20/1, 50/1, or 100/1(Table S1, code 12–14, SI), the averagediameters of the nanowires slightly decreased,and more CdSe nanocrystals were formed(Fig. S5, SI). It can be seen that the use ofBi@TOP particles leads to thinner, but moreirregular nanowires. On the other hand, thereaction can be more easily controlled uponaddition of HDA ligands (see below). Inaddition, we found that the nanowires fromBi@HDA nanocatalysts show higher fluore-scence intensities (see nanowire optical proper-ties, Section 2.3). Most likely, the HDAmolecules not only cover the surface of thenanocatalysts, but also attach to the surface ofthe nanowires and hence increase theirfluorescence.[13] Therefore, we also usedBi@HDA nanocatalysts to optimize the reac-


tion conditions with respect to the controlled growth of uniformand thin nanowires.

2.1.4. Nanocatalyst Volume

Since thin nanowires emerge from small nanocatalyst particles, asuppression of the Bi nanocatalysts fusionmight be accomplishedby using lower quantities of Bi nanoparticles. Hence differentvolumes of Bi@HDAcatalyst solutions ranging from 25 to 300mL(0.09–1.09mmol) were used for the growth of CdSe nanowires(Table S1, code 15–17, SI). For the smallest volume (25mL,0.09mmol), only a few nanowires with a majority of nanocrystalswere obtained. If the volume was increased to 50mL, several CdSenanowires 18 nm in diameter and 3mm in length were obtained(Fig. S6a,d, SI). When 100mL of nanocatalyst solution was used, amajority of nanowires with a diameter and length of 23 nm and 5–7mm, respectively, was formed (Fig S6b,e, SI). In comparison, thenanowires which were prepared using 200mL nanocatalystsolution (Fig. 2a) were similar in diameter, but slightly shorter(3–5mm). In general however, a further increase of the Bi catalystsolution volume to 300mL leads to an additional increase of thenanowire diameter (33 nm) and a decrease in the length (2–3mm;Fig. S6c,f, SI). Again this not only shows that the Bi particles growupon fusion, but also that a decrease of the nanocatalystconcentration is not sufficient to grow thin nanowires,because of the parallel nucleation of nanoparticles. Hence,we used a volume between 100 and 200mL, which relatesto a concentration 0.36–0.72mmol Bi for further optimizationstudies.

2.1.5. Reaction Temperature

The temperature dependence of the wire formation is importantbecause the reaction should only be catalyzed by molten Biparticles. It has been reported however, that themelting point of Bi


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nanoparticles depends on size, i.e., the bulk melting point of272 8C is only reached for particles as large as 20 nm in diameter,while particles of 4 nm in diameter show amelting point of approx150 8C.[47] Thismeans that any nanocatalyst particle will bemoltenat the high reaction temperature of 330 8C,which has been used sofar throughout this study. On the other hand, the small initial Biparticleswithadiameterof approximately 3 nmwouldalreadymeltat temperatures far below the bulk melting point. Hence weinvestigated the temperature-dependence of the growth ofnanowires at 150, 180, and 250 8C. For these studies we used alow CdO concentration (22.6mmol (g TOPO)�1) and a Cd/Seprecursor ratio of 1/1 (Table S1, code 18–20, SI). The atomic forcemicroscopy (AFM) images of the respective sample are shown inFigure 3a–c.

Most interestingly it can be seen from Figure 3a, that CdSenanowires can already be prepared if the reaction temperature isonly 150 8C. However, here we found that after a reaction time of1min, only QDs could be observed. On the other hand, if thereaction time was prolonged to 10min, a few nanowires with anaverage diameter of 12 nm and a length of 0.8mm were obtainedfrom the same solution. Again this shows that QDs might beformed as intermediates and successively transform intonanowires, if there is a sufficient interaction with suitablenanocatalysts. Also it shows that the reactivity of the nanocatalystis already given at temperatures far below the bulk eutectictemperature of Bi and CdSe (265 8C)[48]—yet to a somewhat lowerdegree. These results could be explained by the fact that there is adramatic decrease of the eutectic temperature upon decrease innanoparticle size. Another explanation would be that solid Binanoparticles might also catalyze the CdSe nanowire growth, amodel which has been recently proven in Ge–Au systemssynthesized by an vapor–liquid–solid (VLS) approach.[49]

With the reaction temperature increasing from 150 to 180 8C(Fig. 3b), the resulting nanowires already increase from 12 to22 nm in diameter and from 0.8 to 6.0mm in length. This is notmuch different upon further increase of the reaction temperaturefrom180 to 250 8C (Fig. 3c) where the diameter is also in the rangeof 21 nm and the average wire length is 7.0mm.

In comparing our results to literature values, it should bementioned that the synthesis of semiconductor nanowires fromBior Au@Bi nanoparticles was always reported for temperaturesabove 240 8C.[19,23–27,29–31,35–40,42] The lower reaction temperaturereported in this study is attributed to the low meltingtemperature[47] and high reactivity of the small and pure Binanocatalysts.

2.1.6. Cd Precursor Reactivity

The effects of Cd precursor reactivity on the formation of CdSenanowires is investigated by using dimethyl cadmium (CdMe2) asan alternative precursor of high reactivity. In contrast to the use ofCdOas precursor, the synthetic schemehad to be slightlymodifiedsuch that a Bi/CdMe2 mixture was injected into a TOPO solutioncontaining Se precursor (Table S1, code 21–23, SI). Figure S7a (SI)shows the TEM image of CdSe nanowires prepared from CdMe2using a Cd/Se precursor ratio of 1/2 and 100mL of Bi@HDAnanocatalysts. The mean diameter of the resultant nanowireswas 15 nm with a standard deviation of 4.4 nm. Upon furtherdecrease of the Cd/Se precursor ratio to 1/10, the nanowire


diameter decreased to 7.1� 3.0 nm (Fig. S7b, SI). However,the diameter distribution was broader compared to the wiresprepared using a highCd/Se ratio (7/1) and Bi@OAnanocatalysts(7.9� 1.9 nm).[43] Inall cases, thenanowiresprepared fromCdMe2precursor were shorter than those synthesized from CdOprecursor, most likely due to competitive growth of QDs, whichbecomes more significant due to the high reactivity of CdMe2.However, a large fraction of the nanowires was in the same sizerange as the size of the Bi nanocatalysts (�25% of D< 5 nm,Fig. S7c, SI), which indicates that very thin nanowires can beprepared by using highly reactive precursor.

So far we have investigated the effect of several reactionparameters, which have been changed individually. In summary,we could show that the reaction is very fast at high temperature,and short wires can only be prepared if the reaction isterminated within seconds. At temperatures far below the Bimelting point, the reaction becomes slow, and predominantlynanocrystals are formed. This competitive growth of nano-crystals happens also at low Bi volumes and high Cd precursorreactivities. The morphology of the nanowires on the otherhand is mainly affected by the interplay between the Cd/Seprecursor ratio and the ligands used. If only TOP is used as the Biligand, a low Cd/Se ratio results in thin and long nanowires,which become irregular in shape. This situation is reverse uponaddition of amines, where a high Cd/Se ratio results in longer andthinner wires, which are more uniform in shape.

2.1.7. Iterative Changes

All the above results show that the growth of Bi nanocatalysts andthe growth of CdSe nanowires cannot be individually controlled bychanging just one reaction parameter. Therefore we performediterative changes of the reaction parameters asmentioned above toachieve the goal of preparing long, homogeneous CdSe nanowireswith a uniform diameter below the bulk exciton dimension(DB¼ 11.2 nm). The general strategy is to enhance the growth ofCdSe nanowires and suppress the growth of the Bi catalyst as wellas the formation of CdSe QDs. Since it is rather difficult to slowdown the Bi particle growth, we combined several reactionparameters by which the growth of the nanowires can beaccelerated.

Since wire formation can be more easily controlled using CdOrather than CdMe2 as precursor, we optimized the reactionconditions using CdO by a stepwise change of the reactionparameters asmentioned above. In the first step, we decreased theHDA/Bi precursor ratio from 10/1 to 5.5/1 and used a high Cd/Seprecursor ratio of 7/1 to prepare nanowires at 330 8C (Table S1,code 24, SI). Figure 3d shows the AFM image of CdSe nanowiresobtained. Compared with nanowires prepared from the same Cd/Se ratio, but a high HDA/Bi ratio (10/1) as shown in Figure S6b(Table S1, code 16, SI), the diameter was reduced from23 to 16 nm(Fig. S8, SI), while the length of the nanowires remained almostunchanged (6mm).

In the second step,we decreased theCd/Seprecursor ratio from7/1 to 1/1 (TableS1, code25, SI). This resulted in a furtherdecreaseof nanowire diameter from 16 to 8.5 nm (Fig. 3e and Fig. S8, SI),which is already below the bulk exciton dimension. In addition,some of the nanowires become very long and exceeded 10mm inlength, and became more flexible. Further decrease of the Cd/Se


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Figure 4. TEM and AFM images of CdSe nanowires prepared using

different reaction times and several temperatures using Bi@HDA nano-

catalysts and a low HDA/Bi ratio (2.1/1). The temperature and reaction

time of nanowires shown in (a) and (c) are 330 8C and 30 sec, respectively;

while the temperature and reaction time in (b) and (d) are 250 8C and

1min. The height scale bar in the AFM images is 10 nm.

3656

ratio to 1/2 (Table S1, code 26) leads to even thinner (6.5 nm,Fig. S8, SI) and longer nanowires as shown in Figure 3f.

Hence in the third step,we started again fromaCd/Seprecursorratio of 1/1 (Fig. 3e) and decreased theHDA/Bi ratio from 5.5/1 to2.1/1 (Fig. 4a,c; Table S1, code 27, SI). The results show that even ata shorter reaction time of 30 s, the obtained nanowires are 8mm inlength and only 9.1 nm in diameter. This is somewhat larger than

Figure 5. a–c) HRTEM images of different sized core–shell Bi nanocatalysts attached to CdSe

nanowires. The particle sizes are 13.8, 19.7 and 29.4 nm, respectively. d–f) HRTEM images of

CdSe nanowires synthesized from different Cd precursors and Bi catalysts: CdMe2 and Bi@OA

(d–e); CdO and Bi@HDA (f).

the nanowire diameter observed using a smallCd/Se ratio, but the obtained nanowires aremore uniform in shape (Fig. S8c and S9a, SI).

In the next step, we decreased the reactiontemperature from 330 to 250 8C and kept thereaction time for 1min (Table S1, code 28, SI).Figure 4b shows the typical TEM image ofobtained nanowires, from which a meandiameter of 8.1 nm (�1.4 nm) and a nanowirelength of about 7.4mmcan be extracted. FigureS9 (SI) presents the low magnification TEMimages of these two samples prepared at 330and 250 8C. Further decrease of reactiontemperature to 180 8C and an increase ofreaction time to 2min produced thicker andshorter wires again (Table S1, code 29, SI).Therefore these results suggest that theoptimum temperature for growing thin uni-form nanowires is 250 8C.

As a final check, we also synthesized CdSenanowires using similar Bi@HDA nano-catalysts prepared from the BiCl3 precursor(Table S1, code 30, SI). Figure S10 (SI) showsthe TEM image of CdSe nanowires obtained.


The nanowire diameter (8.2� 1.3 nm) is similar to these shown inFigure 4, but the length of the wires is much shorter, which isattributed to the difference of Bi precursors.

2.2. Nanowire Crystal Structure

Before we discuss the crystal structure of the resulting nanowires,wewill briefly focus on the structure of theBi catalyst particles aftersolidification. Previouslywe investigated the size and compositionof the Bi nanoparticles before and after nanowire growth.[43] Theresults showed that within experimental error of the energy-dispersive X-ray spectroscopy (EDAX) method (<1%) the Binanoparticles only consist of bismuthdespite of thepresenceof theCd and Se precursor. Here, we want to emphasize that upon fastquenching of the reaction solution most of the Bi nanoparticlesshowed a typical core–shell structure, i.e., a crystalline core and anamorphous shell. The quenching can be either realized byinjection of toluene, or upon removal of the heating mantle andcooling the reaction flask with an air-blower or an ice-water bath.Figure 5a–c shows high-resolution TEM (HRTEM) images ofseveral typical Bi nanocatalysts attached to nanowires. Theamorphous shell thickness is in the range of 2.9–10.2 nm for Biparticles of 10–40 nm in diameter, respectively. Most likely thenanodroplet surface solidifies very quickly and forms anamorphous layerwhile the inner part of the catalyst can equilibrateto form a crystalline structure. For example, the contrast of theamorphous shell and the crystalline core is most obvious if thereaction is rapidly quenched upon toluene injection (see Fig. 5band Fig. S11a, SI).

In contrast, if the cooling rate is very slow (only several 8Cmin�1),almost no Bi particles can be found at the end of the nanowires,as shown in Figure S11b (SI). This might be due tothe crystallization of the entire Bi particle which leads to a large


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lattice mismatch between the rhombohedral Bi particle and thehexagonal (or cubic) CdSe nanowire. While more detailedexperiments are needed to investigate the Bi–CdSe interface,the slow temperature decrease provides a simple and effective wayto remove the Bi nanocatalysts from the CdSe nanowires.[37]

The crystal structure of the CdSe nanowires was investigated byHRTEM and synchrotron X-ray diffraction (XRD). Firstly, we willfocus on the HRTEM results of CdSe nanowires prepared fromdifferent precursors and using amines as Bi nanoparticle ligands.Figure 5d–e shows HRTEM images of CdSe nanowires obtainedusingCdMe2 asCdprecursor andoleylamine asBi ligand. It canbeseen that the lattice pattern was partially distorted along the wireaxis accompanied by a variations of contrast (Fig. 5d) and/orthickness (Fig. 5e). The different thickness along the wire axis canbe clearly seen in lowmagnification TEM images. If CdOwas usedas Cd precursor and HDA was used as the Bi nanocatalyst ligand,more uniform nanowires were obtained which exhibited coherentlattice fringes as shown in Figure 5f.

Besides the detailed morphology, the HRTEM images alsoreveal that all the CdSe nanowires exhibit a mixture of zinc blende(ZB) and wurtzite (W) sections, as has been previouslydescribed.[24] Here we performed a first attempt to estimate theratio ofZBandWon thebasis of powderXRD.Figure6a,bpresentsthe typical XRD patterns of four samples referring to the imagesshown inFigure3e–f, S8 (SI), 4, andS9 (SI), respectively; thesedatawere obtained at beamlines at the DELTA and the EuropeanSynchrotron Radiation Facility (ESRF). The angular values and

Figure 6. Four typical XRD patterns of CdSe nanowires. a) Patterns

obtained from DELTA beamline measurements using 15.5 keV radiation

resulting from an image plate with a beam size of 0.7mm� 0.7mm.

b) Patterns obtained from ESRF measurements using an X-ray energy of

25 keV and a point detector with a beam size of 0.2mm� 0.2mm.


background of the raw data were calibrated using a silicon powderas reference material. It can be seen that the measured peakpositions fit perfectly to the W structure of CdSe since thehexagonally close packed (hcp) Miller indices (hkl) are used toindex the reflections.

However, the intensity ratios between the integrated reflectionsdiffer considerably from a pure W phase, which is due to theadmixture of ZB and W. Hence, since several W reflections alsoexist for the ZB lattice, the intensity ratio of the pure phases can beused to estimate the W to ZB ratio within the nanowires, inprinciple. Therefore we calculated the CdSe structure factors forpureWand ZB structure, respectively, using the atomic scatteringfactors andDebye–Waller factors ofCdandSe fromliterature.[50,51]

Assuming thekinematical scattering lawwhere theBragg intensityis proportional to the square of the structure factors, somepeaks such as 100 or 103, for example, could be interpreted asresulting only from the W crystal structure (indexed by 100Wand 103W), while some others could be interpreted to be due to acombination of W and ZB. For example, it can be shownthat the 111ZB and 002W, 220ZB and 110W, 131ZB and 112W, 224Wand 300W, and 044ZB and 220W reflections coincide in peakposition. Taking into account the respective multiplicity factors,the relative contributions of pure ZB and W phases within thenanowires can be estimated from the ratios of intensities of thesereflections.[52]

It has to be mentioned however, that a reliable quantification ofZB andWphases in this way is possible only assuming ZB andWphaseswithperfect crystal structure, i.e.,without intrinsic stackingfaults. Although this is not the case (see HRTEM), we used thisapproximation to estimate the relative contribution ofZB fractionsbetween different samples. In any case, we could show that all thesamples crystallize predominantly in the W phase. Based on ourroughestimationdescribed above the contribution of theZBphaseforbothsamples,No. 1andNo.2, shouldonlybeof theorder of3%.However, this value is about twice as high for sampleNo. 3 andNo.4, where our model results in a ZB contribution of 6 and 8%,respectively. Even though the absolute values for the W and ZBfraction can be very different, the analysis of the diffraction patternclearly shows that the relativeZB contribution is higher for sampleNo. 3 andNo 4. This could be explained by the reaction conditions,which are summarized in Table S1, code 25–28, SI. Here it can beseen that the short reaction time used to prepare No. 3 (30 s)resulted in nanowires exhibiting a higher ZB ratio as compared tothe longer reaction time of No. 1 (1min). Also the lower reactiontemperature of No. 4 (250 8C) leads to a higher ZB ratio ascompared to a high temperature used forNo. 1 (330 8C).Obviouslya long reaction time as well as a high temperature leads to thethermodynamically more favored W phase. In contrast, alower reaction temperature at the initial stage of the wiregrowth leads to a somewhat larger amount of the kineticallyfavored ZB phase. However, it should be mentioned again thatthis model provides only a rough estimate of the coherent ZBsections within the nanowires, and further investigations andtheoreticalmodeling isneeded toquantify the amountofWandZBfractions.

Besides the intensity (ratios), the full width at half maximum(FWHM),Du, of thediffractionpeaks canbe analyzed todeterminethe coherence length of the diffracted X-rays. In principle thesevalues can be used to determine the crystalline domain size d and


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Table 1. Diameters and lengths of CdSe nanowires measured by TEM, AFM, and XRD.

Wire sample Diameter [nm] Length [mm] Estimated

ZB ratio [%]

TEM AFM XRD(100) TEM AFM XRD(002)

No. 1 8.5� 2.6 7.3� 2.9 8.0 >4 5.7� 2.3 0.018 3

No. 2 6.5� 2.5 6.4� 2.5 6.8 >6 7.1� 1.7 0.012 3

No. 3 9.1� 2.2 6.8� 1.8 7.6 >8 8.5� 2.9 0.022 6

No. 4 8.1� 1.4 6.7� 1.2 6.1 >6 7.4� 1.6 0.019 8

Figure 7. a) UV–vis absorption spectra of different sized CdSe nanowires.

b) Lowest-energy excitonic peaks are plotted as a function of nanowire

diameter. c) UV–vis absorption and photoluminescence spectra of CdSe

nanowires prepared from different molar ratios of Cd and Se precursor; the

AFM and TEM images of these samples are shown in Fig. 3d–f and Fig. S8,

SI. The excitation wavelength was 450 nm.

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the strain e acting in different directions of thenanowires, by usingEquation 1:[53]

Du ¼ l=d cos u þ " tan u (1)

where l refers to the beam wavelength. In order to extract Du, thepeak shape of a measured Bragg reflection was fit by a Gaussianprogram, and deconvoluted from the instrumental peak profilemodelled by another Gaussian program. In our previous paper,we have shown that the strain cannot be resolved from ourexperiment so far because of the low number of high-anglereflections, which are not additionally affected by stackingfaults.[52] Therefore strain effects shall be neglected at this point.This drawback can be overcome in future considering modernachievements in X-ray powder analysis, as shown for example formetals by Scardi et al.[54] Here we used the 002W reflection todetermine the length of the coherent blocks along the nanowireaxis, since the nanowires grow predominantly along thiscrystallographic direction. Likewise the perpendicular 100Wreflection is used to determine the (coherent) radial diameterof the nanowires. Table 1 compares the calculated diameters andlengths with those obtained from TEM and AFMmeasurements.It can be seen that the calculated diameters are in good agreementwith TEM and AFM data, but the calculated lengths are muchshorter (10–20 nm) than those determined by TEM and AFM(several micrometers).

This can be explained on the basis of the HRTEM results whichshowed that the coherence in length direction is relatively shortdue to a change of alternatingWandZBsections. Both sections areseparated by stacking faults. In addition,within a homogeneousWor ZB section additional stacking faults also could be a reason forthe discontinuity of the coherence. For example, a stacking faultwithin the W phase shows up if the sequence of layer stacking ischanged, such as ABABjCBCB.[55] In theZBphase a single changein stacking sequence as ABCjAjCBA is not a stacking fault, but atwin. In this case, the formation of a stacking fault requires at leasttwo sequential twin planes, such as ABCAjCjB.However, for XRDthe size of a coherently scattering domain is limited by any changein the stacking sequence. In both cases a single domain consists ofat least two or more complete stacking units such as ABAB forthe W phase or ABCABC for the ZB phase.

2.3. Nanowire Optical Properties

The optical properties of CdSe nanowire solution are investigatedby UV–vis absorption and photoluminescence spectroscopy.[23]


Figure 7a shows the UV–vis absorption spectra of different sizedCdSe nanowires. The energetic position of the lowest-energyabsorption band was plotted in Figure 7b. A blue-shift of theabsorption edge from 702 to 657 nm can be clearly observedwith adecrease of nanowire diameter from 33 to 6 nm, respectively. All


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Figure 8. Proposed scheme for the formation and growth of CdSe nano-

wires and CdSe nanocrystals. After injection at high temperatures, larger Bi

nanodroplets are formed upon coagulation and react with the Cd and Se

precursors or intermediately formed CdSe clusters to initiate the growth of

CdSe nanowires. Depending on the reaction conditions, this will lead to

extended CdSe nanowires or to the parallel growth of semiconductor

nanocrystals.

the absorption energies are higher than that of the bulk bandgap ofCdSe (712 nm, 1.74eV), which is marked by dashed line inFigure7b.[24] This shows that a quantumconfinement effect canbeobserved even for nanowires with a diameter of 33 nm, which isalmost 3 times the value of bulk Exciton Bohr diameter, which is11.2 nm for CdSe. These results are in good agreement withthose observed for CdSe nanorods[17] and nanowires[23] inprevious reports.

Besides the UV–vis absorption, the photoluminescence ofthe nanowire samples was investigated. Firstly it should benoted that every sample showed a weak bandgap luminescence,where the quantum yield was always lower than 1%, as comparedto standard dyes, which is in agreement with previous reports.[44]

Hence we compared the relative change of the fluorescenceintensity upon different reaction conditions. For example,Figure 7c shows the absorption and photoluminescence spectraof CdSe nanowires synthesized from different Cd/Se precursorratios using Bi@HDA as catalysts. Both UV–vis and fluorescencespectra were blue-shifted upon decrease of the Cd/Se precursorratios, due to the decrease of nanowire diameter as shown inFigure 3d–f and S8, SI. The fluorescence intensity reached amaximum when the Cd/Se ratio was 1/1 and decreased whenusing an excess of either precursor. This is in contrast to previousreports on CdSe QDs where it has been shown that the excess ofone of the precursors leads to a high fluorescence quantum yield,especially when excessive Se precursors were used.[15] Thisdifference could possibly be attributed to the stacking faults thatshow significant effects onnanowire optical properties,[44] but onlyslight effects on QDs.[6,13,15]

To further investigate the changeof thefluorescence intensity independence of the reaction conditions, we measured thephotoluminescence of CdSe nanowires synthesized fromBi@OAor Bi@TOP as nanocatalysts. Figure S12 (SI) presents the UV–visand photoluminescence spectra of CdSe nanowires, which wereintroduced in Figures 1d–e, 2c–d, and S4 (SI), respectively. Here itcan be seen that the nanowires prepared from Bi@TOPnanocatalysts show lower fluorescence intensities as comparedwith those synthesized fromBi@[email protected] result indicates that alkylamines play an important role inimproving the fluorescence intensity of CdSe nanowires. Sucheffects of surface ligands on the fluorescence intensity have beenwell-studied in CdSe QDs systems.[13,15] For example, thequantum yield (QY) of CdSe QDs prepared from the HDA–TOPO–TOP system can be as high as 40–60%, while TOPO-covered QDs exhibit only 5–15% QY.[13] Most likely, the HDAligands not only cover the surface of the Bi nanocatalyst, but alsopassivate the surface of the nanowires. However, as alreadymentioned above, a large excess of amines (OA and HDA) leadsto the formation of irregular nanowires. These results show thatthe role of the surface ligands on the formation and fluorescenceintensity of nanowires is very complex. Especially the surfaceproperties of the nanowires need to be further investigated tosynthesize nanowires with high fluorescence intensity.

3. Summary

In summary, we have presented a detailed investigation of thegrowth of semiconductor nanowires by systematic variation of


reaction parameters. The presence ofBi nanoparticles at the endofnanowires shows thatCdSenanowiresweregrownvia a typicalSLSmechanism.[19,20] Although this concept was proposed more than10 years ago,[18] the growth mechanism is still unclear since itdepends on several reaction parameters. In Figure 8, wesummarize how the formation of nanowires happens in solution.

When the mixture of Bi nanoprecursors, TOPSe and TOPO–CdO–alkyl acid (or other reactive species as mentioned in theExperimental section) are brought together at high temperature,Binanodroplets grow very rapidly by aggregation. At the same time,Cd and Se precursors or even intermediate CdSe clusters willdissolve into (or decompose on the surface) of these nanodroplets,and grow into nanowires. The parallel growth of Bi nanodropletsand growth of nanowires from those nanodroplets might explainthe tip-shape at the end of the nanowire. In any case, both thereactivity and concentration of the Bi precursors as well as thereactivity and concentration of the Cd precursors determinesthe morphology, and partially also the crystal structure of the finalnanowires.

At this point, we shall not repeat the different effects ofconcentration (ratios), reaction temperatures, and ligands used.However, the general strategy for a reproducible synthesis ofsemiconductor nanowires is control of the competition betweenthe growth of Bi nanocatalysts, the formation and catalytic growthof CdSe nanowires, and the formation and noncatalytic growth ofCdSe QDs. This general strategy can be extended to the SLScontrolled synthesis of other semiconductor nanowires belowtheir exciton dimensions. To this end, ultrathin (D< 5.0 nm)semiconductor nanowires which exhibit strong quantizationeffects as their counterpart QDs and rods will be one of thefuture directions.

Another direction will be the synthesis of highly fluorescentsemiconductor nanowires, since the highest-fluorescence quan-tum yield of nanowires reported is only 1%. One effectiveapproach to improve fluorescence is to passivate nanowiresurfaces with a wide bandgap shell to result in core–shellnanowires.[56] In addition, integration of multicomponents into asingle hetero-nanostructure represents another future direc-tion.[38–40,57] These homo- and hetero-nanostructures are excellentbuilding blocks for future nanometer-sized technologicaldevices.[22]


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4. Experimental

Materials: Cadmium oxide (CdO, 99.99%), Se powder (99%), HDA(98%), octanoic acid (99%), oleic acid (70%), OA (70%), and octyl ether(99%) were purchased from Aldrich. TOPO (98%) and TOP (90%) werereceived from Merck and Fluka, respectively. BiCl3 (99.99%) was obtainedfrom Acros. Dimethyl cadmium (CdMe2) was used as received (StremChemicals). Bi[N(SiMe3)2]3 was synthesized as described elsewhere[43,58]. TOPSe stock solutions were prepared by dissolving Se into TOPunder air free conditions.

Preparation of Bi Nanocatalysts: Bi nanocatalysts can be prepared eitherfrom Bi[N(SiMe3)2]3 or BiCl3 using TOP as reducing agent at roomtemperature [43]. Typically, 15.0mg Bi[N(SiMe3)2]3 was dissolved in 5mLoctyl ether, followed by a dropwise addition of 100mL of TOP. The mixturewas stirred for 30min and the resultant Bi nanocatalysts were onlystabilized with TOP (referred to as Bi@TOP). For comparison, the othertwo Bi nanocatalyst solutions, i.e., Bi@OA and Bi@HDA, were prepared byaddition of OA or HDA as stabilizing ligands. It should be noted that thesetwo nanocatalyst solutions also contain TOP, but are called Bi@OA andBi@HDA only for convenience. All the Bi nanocatalyst solutions werestored in a glovebox (H2O< 0.1 ppm, O2< 0.1 ppm, T< 5 8C) and useddirectly without further purification.

Preparation of CdSe Nanowires: The preparations of the CdSe nanowiresis in principle based on a standard TOPO/TOP SLS scheme as previouslyreported [24,43]. While the general procedure will be described in thefollowing, a summary of reaction parameters and quantities of reagentsused is given in Table S1 (SI) for each synthesis [45]. For all syntheses amixture of CdO, TOPO, and oleic acid (or octanoic acid) was loaded into a50-mL three-necked flask. This mixture was dried and degassed for 30minat 100 8C under vacuum (1mbar, 1 bar¼ 103 kPa). Then the flask was back-filled with Ar and the temperature was increased to the respective reactiontemperature (150–330 8C). For the syntheses operated at 150 and 180 8C,the temperature was firstly increased to 200 8C to completely dissolve CdOand then decreased to the corresponding temperature. Afterwards amixture of Bi nanoparticles and TOPSe was injected. The solution was keptat the reaction temperature for a certain time before cooling down. Whenthe temperature was decreased to 80 8C, 2–4mL of toluene was added tothe solution to prevent the TOPO from solidifying. Then an excess ofbutanol (or isopropanol) was added to precipitate the CdSe nanowires,which were separated by decantation after centrifugation at 4000 rpm for10min. The dark brown precipitate was washed with toluene/butanolseveral times and then redissolved in toluene or chloroform. It should benoted that butanol was used instead of short chain alcohols, only if thenanowires should be separated from nanocrystals, because the latter onesprecipitate only upon addition of methanol or ethanol [24]. An alternativeoption to separate nanowires from nanocrystals was the use of high-speedcentrifugation (18000 rpm, 10min).

Characterization: Low-resolution TEM images were obtained on aHitachi-8100 electron microscope operating at an acceleration voltage of200 kV. HRTEM images were collected on a Philips CM 300 UTmicroscopeoperating at an acceleration voltage of 200 kV. TEM samples were preparedby dropping a diluted toluene solution of CdSe nanowires onto carbon-coated copper grids. The AFM samples were prepared by putting onedroplet of a CdSe nanowire solution on a freshly cleaned Si/SiO2 substrate.The AFM images were recorded on a NanoScope III operated in tappingmode. The nanowire crystal structure was determined by XRD performed inBL9 at DELTA and ROBL beamlines at the ESRF with 15.5 and 25 keVradiation, respectively. The dried CdSe nanowires were ground and thenloaded into glass capillaries with a diameter of 0.3mm. The X-ray energy forESRF measurements was 25 keV and a point detector with a beam size of0.2mm� 0.2mm was used. In DELTA measurements, the X-ray energywas 15.5 keV and an image plate with a beam size of 0.7mm� 0.7mmwasadopted. The distance between the image plate and the sample was26.4 cm. UV–vis spectra were measured with a Cary 50 UV–vis spectro-meter. The fluorescence of nanowire solutions was investigated by a CaryEclipse fluorescence spectrometer. The excitation wavelength was 450 nmand both the emission and excitation slit width were 10 nm.


Acknowledgements

Z. Li gratefully acknowledges the Alexander von Humboldt Foundation forfinancial support. Supporting Information is available online from WileyInterScience or from the author.

Received: April 2, 2009

Revised: July 25, 2009

Published online: October 1, 2009

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controlled synthesis of cdse nanowires by solution–liquid–solid method

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