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Electrochimica Acta
jou rn al h om epa ge: www.elsev ier .com/ locate /e lec tac ta
ontrolled growth of catalyst assisted and catalyst free CdSe micro cactuses withharply pointed nanorods, their Photoluminescence (PL) and Photolectrochemical (PEC) properties
ulfiqar Ali, Chuanbao Cao ∗, Sajad Hussain, Waheed S. Khan, Faheem K. Butt, Ghulam Nabi,ahid Usman, Tariq Mahmood
esearch Centre of Materials Science, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
r t i c l e i n f o
rticle history:eceived 17 April 2012eceived in revised form 1 August 2012ccepted 3 August 2012vailable online 28 August 2012
We report here a thermochemical approach for controlling the catalyst assisted and catalyst free growthof CdSe cactus like structures with sharply pointed nanorods (SPNR). The cactuses were simultaneouslyobtained on the silicon substrate and in semicircular alumina boat using cadmium and selenium powdersas precursors under ammonia gas flow. The aqueous ammonia helps to control the growth of 1D structureand the formation of CdSe. Vapor–liquid–solid (VLS) growth mechanism is proposed for the catalystassisted while solid state chemical reaction is suggested for the catalyst free growth of microcactuses.The microcactuses obtained in the boat as powder form are hollow whereas solid on the silicon substrate.Room temperature photoluminescence (PL) studies exhibit sharp peak at 672 nm for catalyst assistedCdSe microcactuses. A broad peak at 596 nm was observed for catalyst free growth of CdSe SPNR. In thiswork we have demonstrated that PL emission from the catalyst free growth of nanostructured CdSe isbroad and intense. These two peaks are blue shifted from the bulk. Nanorods emerging from exterior
surface of cactus like structures are observed to have decreasing diameter along vertical-axis and endup in SPNR. Average diameter in the middle of nanorods is estimated in the range of 250–450 nm. Photoelectrochemical (PEC) solar cell is fabricated on ITO coated glass substrate with the help of PVDF inNMP solution. The efficiency and fill factor for as synthesized solar cell are 0.47% and 0.35 respectively.The as-prepared products were characterized using X-ray diffraction (XRD), energy dispersive X-ray
cann
spectroscopy (EDS), and s
. Introduction
The copious shape and size dependent characteristics that inor-anic nanostructures exhibit make them of great fundamental andechnological interest [1–9]. The great interest in these materials isypically due to the optical, electrical and photoredox properties ofhe semiconductor which can be tuned and manipulated in manyays, just by controlling the shape and dimensionality of the mate-
ial. Due to their photo electrochemical (PEC), electroluminescentnd photovoltaic applications group II–VI compound semi-onductors (CdSe, CdTe, ZnSe), etc. have received much attentionuring the recent years [10–13]. CdSe is predominantly interestingor photovoltaics [14,15], since its band-gap [16] favors absorption
ver a wide range of the visible spectrum. Cadmium Selenide hasistinctive physical properties as a semiconductor therefore it findsremendous applications to electronic devices [2,17,18].
With variation in the morphology of CdSe its optical propertiescan be tuned, due to quantum size confinement. Owing to this prop-erty it finds promising applications such as light emitting diodes[19,20], biological labeling [21,22], and transistors [23]. Moreover,it also finds applications in photoconductive devices, photovoltaicsolar cells and electro photographic photoreceptors [24] because ofthe excellent optical, electrical, magnetic and catalytic properties.CdSe nanocrystals have also been used to fabricate inorganic thin-film transistors with high field mobilities and hybrid polymer solarcells. Light emitting diodes (LEDs) have also been fabricated fromCdSe nanocrystals [25].
Diverse range of techniques such as hydrothermal, sol–gel,solvothermal and surfactant assisted approach [26] have beenemployed to synthesize CdSe nanostructures. Different precipita-tion techniques to prepare nanocrystals of CdSe [27] have beenreported, including photochemical [28], �-irradiation [29], sono-
chemical [30], and solvothermal [31]. All such techniques are eithercomplicated or use toxic compounds like H2Se.
In this paper we present a novel strategy to prepare CdSenanorods directly from commercially available Cd and Se powders
n a horizontal tube furnace without using any acids, toxic com-ounds or complicated techniques. Compared to other methodssing homogeneous liquid phase, the results reported here pointut a new synthetic direction for CdSe cactus like structure. Weave employed vapor–liquid–solid (VLS) approach to synthesizehe cactuses of CdSe having large sharply pointed nanorods (SPNR)merging from exterior surface. This facile route for the fabrica-ion of CdSe and its novel microcactuses with SPNR has not beeneported to the best of our knowledge.
Photoluminescence (PL) is a reputable technique to gain insightsn the optical quality of semiconductors. Size-dependent emissions doubtless the most eye-catching property of CdSe nanostruc-ures, and it is possible to synthesize diverse sized CdSe nanocrys-als that emit blue to red color with high purity. The intensitynd line width of the photoluminescence spectra are imperative,ecause they are very susceptible to the nature of particle’s sur-ace, size, impurities and defects. Due to quantum size effect, theand gap of CdSe nanocrystal increases as its size decreases, andhus band-edge emission color of nanocrystals shifts continuouslyrom red to blue. There have been a number of reports on the opticalehavior of CdSe nanocrystals [32–39]. In most of the reports, sele-ium vacancies and crystalline defects are held responsible for themission in CdSe. However, in order to minimize the contributionf Se vacancies, we have adopted such technique in which extra Ses present. Therefore, we suggest that emission in our case is dueo the size confinement. We have also presented the comparisonf PL from the catalyst assisted and catalyst free synthesis of theactuses having sharply pointed nanorods at exterior surface.
Due to the ever-increasing apprehension over the energy crises,eed for solar energy production and suitable materials for pro-ucing solar energy is becoming inevitable with every passing day.emiconductor liquid junction solar cells also possess great deal ofttention due to growing interest in solar energy conversion. Whenompared with solid–solid junction of conventional solar cells, aemiconductor electrode dipped in a suitable liquid electrolyte pro-ides the necessary charge transfer, a redox ionic species being usedo obtain photo voltage/photocurrent, which has in-built storageapability after a little modification. This interface between semi-onductor and electrolyte can be used for photo electrochemicalPEC) power conversion [40–43]. We fabricated a PEC solar celly using PVDF as a binder and calculated the power conversionfficiency and fill factor as 0.47% and 0.35 respectively.
. Experimental
Cadmium and selenium powders were used as starting materi-ls. These powders were mixed in (1:1) ratio and grinded in mortaror 30 min. About 0.5 g of the mixture was loaded in semicircu-ar long alumina boat mixed with few drops of aqueous ammonia.
silicon substrate ultrasonically cleaned in ethanol and rinsedith de-ionized water was coated with nickel chloride solution
nd placed on the semicircular alumina boat over the mixture ofowders. The distance between mixture of powders and the sili-on substrate was kept about ∼2 mm. This boat was loaded in theenter of quartz tube furnace. The furnace tube was pumped outsing a mechanical rotatory pump for removing the residual airontents and then also flushed heavily with high purity ammoniaas for about 30 min. Later on ammonia gas flow was adjusted at50 sccm (standard cubic centimeter per minute) and the furnaceas ramped at 10 ◦C/min to reach the target temperature of 600 ◦C.
his temperature was maintained for a reaction time of 120 minnd then furnace was cooled down naturally. Dark grey product
as observed on silicon substrate and alumina boat. These productsere preserved in airtight plastic boxes for further investigations.
he structure and the phase purity of the products were deter-ined by X-ray powder diffraction (XRD, Philips X’Pert Pro MPD)
Fig. 1. (a) XRD spectrum of catalyst assisted CdSe cactuses on silicon substrate (b)XRD spectrum of catalyst free CdSe cactuses in the powder form.
with Cu K� radiation (� = 0.15406 nm) whereas the morphologiesof the products were examined by scanning electron microscopy(SEM, Hitachi S-3500). The chemical composition of the productwas tested by EDS. PL characteristics were studied at room tem-perature by using Hitachi FL-4500 spectrofluorometer.
3. Results and discussion
Fig. 1(a) shows the XRD pattern of CdSe sample collectedon silicon substrate. From this figure, the characteristic peakslocated at 2� = 24.0465◦, 25.5307◦, 42.1422◦, 45.9978◦ and 49.8622◦
correspond to (1 0 0), (0 0 2), (1 1 0), (1 0 3) and (1 1 2) planes respec-tively. These peaks match well with the peaks of hexagonal CdSe.The lattice parameters calculated for CdSe are a = 4.2886 nm andc = 6.9781 nm, which are almost equal to the standard values ofJCPDS card No. 65-3436 for CdSe. The inset of Fig. 1(a) representsthe XRD patterns of all the samples with different morphologies on
silicon substrate.
Fig. 1(b) shows the XRD pattern of the CdSe sample collected inthe semicircular long alumina boat. This figure indicates the maincharacteristic peaks located at 2� = 23.9031◦, 25.4114◦, 27.1120◦,
124 Z. Ali et al. / Electrochimica A
Fo
4at
iasnA
tcs
iiddi
sponds to an energy decrease of the highest occupied molecular
ig. 2. (a) EDS results of CdSe whole cactus on silicon substrate (b) single nanorodbtained on silicon substrate (c) the product in the powder form.
1.9990◦ and 49.7106◦ correspond to (1 0 0), (0 0 2), (1 0 1), (2 0 0)nd (0 0 4) planes respectively. These peaks also match well withhe peaks of hexagonal CdSe.
The lattice parameters are calculated using the relation
1d2
= 43
(h2 + hk + k2
a2
)+ l2
c2. (1)
The calculated lattice parameters for hexagonal CdSe obtainedn semicircular long alumina boat in the powder form are
= 4.2987 nm and c = 7.0103 nm, which are almost equal to thetandard values of JCPDS card No. 77-2307 for CdSe. The sharp-ess of these peaks shows high crystallinity of as synthesized CdSe.ll the diffraction peaks are accounted for high purity of product.
Fig. 2(a) represents the result of energy dispersive X-ray spec-roscopic (EDS) investigation of the as-synthesized hexagonal CdSeactus with sharply pointed nanorods as a whole and Fig. 2(b)hows the EDS results of the single nanorod on the silicon substrate.
Fig. 2(c) represents the EDS results of the as synthesized productn the semicircular long alumina boat in the powder form. The sil-con peaks can be observed in Fig. 2(a) and (b) which are of course
ue to the substrate used, however in Fig. 2(a) the silicon peak isominant because large area of the product is selected for EDS stud-
es. In Fig. 2(c) there is no such silicon peak as the product is in the
cta 85 (2012) 122– 130
powder form. All these results indicate that our product consists ofpure CdSe compound without any impurity.
Morphology and shape evolution of the as synthesized productswere further studied using Scanning Electron Microscope (SEM).Fig. 3 shows SEM micrographs of as synthesized CdSe product atdifferent magnifications on silicon substrate. SEM micrograph inFig. 3(a) and (b) suggest that the as synthesized product consists ofsphere like cactuses. SEM micrographs at still higher magnificationsuggest that these spheres have sharply pointed nanorods whichare grown on the exterior surfaces of the cactuses.
To observe the product more clearly Field Emission ScanningElectron Microscopy (FESEM) was used and these micrographs arepresented in Fig. 3(c) and (d) which shows that at the bottom ofnanorods the diameter of nanorods is in the range of 550–900nm such as diameter of as grown nanorods decreased graduallyalong vertical direction while at the center of nanorods the diame-ter range remains 250–450 nm. These nanorods end up in sharplypointed tip.
Fig. 4 represents FESEM images of the product obtained in semi-circular long alumina boat in powder form. It can be observed fromimages that microcactuses obtained in boat are hollow and den-sity of nanorods on the exterior surfaces of cactuses is low, this isbecause product is in powder form and the nanorods are supposedto be broken from surfaces during handling process.
In order to control the morphology of CdSe we have performeddifferent experiments with different parameters and we have suc-cessfully controlled the morphology of CdSe. The parameters liketemperature, reaction time, ramp rate and gas flow rate are impor-tant in controlling the morphology in this facile technique. Table 1presents different morphologies of CdSe with different experimen-tal parameters.
Fig. 5 represents CdSe nanosheets fabricated at 500 ◦C. Fig. 5(a)shows product on silicon substrate at 50 �m scale which indicatesthat product is present almost on all the substrate. Fig. 5(b) and (c)represents micrographs of the product even at higher resolutions.
Fig. 5(d) represents SEM of the product at 10 �m scale. It alsoindicates that the sheets have diameter in range of 400–500 nm.
Fig. 6 shows product on silicon substrate at almost same condi-tions but the ammonia gas flow rate was increased from 100 sccm to200 sccm. With the change in gas flow rate the morphology of prod-uct is changed. Fig. 6(b) and (c) represents micrographs of productat higher resolutions. Fig. 6(d) represents SEM of product at 10 �mscale. It also indicates that the twisted belt like structures havethickness in the range of 400–550 nm, while width is in 1–2 �mrange.
Fig. 7 represents EDS analysis of product at 500 ◦C but withdifferent ammonia gas flow rates. Both the spectra indicate thatproduct is pure CdSe. The only difference is in intensity of siliconpeak. This difference is supposed to be due to the thickness of theproduct on different silicon substrates.
3.1. PL emission
Fig. 8 presents the room temperature photoluminescence mea-surements of CdSe cactus like structures with SPNR on a Si substrateand in powder form obtained from the boat. PL emission from theCdSe occurs over a wide energy range so different sets of peaksaccording to their spectral region can be grouped as they are likelydue to radiative states of similar nature [44]. PL emission from thecatalyst free synthesized product is presented in Fig. 8(a). This fig-ure indicates a broad and intense peak at 596 nm (2.08 eV) whichis blue shifted from bulk. From a molecular point of view it corre-
orbital (HOMO) and increase of the lowest unoccupied molecu-lar orbital (LUMO) due to spatial confinement of charge carrierwave functions. Therefore blue emission and broadening of the
Z. Ali et al. / Electrochimica Acta 85 (2012) 122– 130 125
as syn
btpsssf
TE
Fig. 3. SEM and FESEM micrographs of
and gap occurs. By changing the size of semiconductor one canune the color of nanocrystals as well as their oxidation-reductionroperties. The PL emission of the as obtained product on silicon
ubstrate which is catalyst assisted growth of microcactuses withharply pointed nanorods is shown in Fig. 8(b). This figure repre-ents a sharp peak at 672 nm (1.85 eV) which is also blue shiftedrom the bulk (711 nm). The energy range greater than 1.79 eV can
able 1ffect of experimental parameters on morphology of CdSe.
Morphology Temperature (◦C) Reaction time (min)
Microcactuses 600 120
Microcactuses 600 120
Nanosheets 500 240
Belt like twisted structures 500 240
thesized product on silicon substrate.
be referred to near band edge emission (NBE) which in our case isdue to free excitons. The decrease of particle size shifts the onsetof absorption from infrared to visible region of electromagnetic
spectrum as the band gap energy of semiconductor increases.
The difference in wavelengths between the two peaks obtainedfor catalyst assisted and catalyst free growth of microcactuseswith sharply pointed nanorods is 76 nm which corresponds to
Ramp rate (◦C/min) Gas flow rate (sccm) Type of product
10 150 On silicon substrate10 150 Powder in boat
5 100 On silicon substrate5 200 On silicon substrate
126 Z. Ali et al. / Electrochimica Acta 85 (2012) 122– 130
Fig. 4. FESEM micrographs of as synthesized microcactuses in the powder form.
Fig. 5. SEM micrographs of as synthesized sheets on silicon substrate.
Z. Ali et al. / Electrochimica Acta 85 (2012) 122– 130 127
Fig. 6. SEM micrographs of as synthesized tw
Fig. 7. EDS results of (a) nanosheets on silicon substrate (b) belt like twisted struc-tures on silicon substrate.
isted structures on silicon substrate.
∼0.074 eV. This energy difference between the energy bands ofCdSe microcactuses with and without catalyst assisted techniqueis due to the fact that impurities of the catalyst could incorporate inthe catalyst assisted growth of nanostructured materials. Anotherreason of this difference might be the difference of the morpholo-
gies of both products because microcactuses obtained in case ofcatalyst free growth are hollow.
It is also thought that the strong PL intensity from the CdSenanorods obtained from catalyst free synthesis can be attributed
Fig. 8. PL emission for the CdSe cactuses with sharply pointed nanorods (a) catalystassisted (b) catalyst free.
128 Z. Ali et al. / Electrochimica Acta 85 (2012) 122– 130
tpof[tdocsgtdwdfri
3
3
fAtIvseiaapt8fi
itbcmrUt
Fig. 9. I–V characteristics of CdSe thin film in dark and under illumination.
o their high crystallinity which is in good agreement with the XRDatterns discussed earlier and the presence of good surface statesn the nanorods, which effectively prevent the CdSe nanorodsrom having carrier-quenching defects and non-radiative traps45]. When the surface consists of many defect states, this leadso quenching of the band gap emission and appearance of weakeep trap long-wavelength emission. The low intensity in the casef catalyst assisted synthesis as compared to catalyst free synthesisan be attributed to carrier-quenching defects. However, it has beenhown that a good passivation of the surface by an organic or inor-anic shell gives rise to quantum yields approaching unity at roomemperature. The red band from 700 to 730 nm is thought to beue to surface defects but in our case there is no red band observedhich again illustrates that as synthesized product has very lowegree of surface defects. It is concluded that this approach for theabrication of CdSe nanorods is far better as compared to the resultseported in the literature [46] in which the degree of surface defectss too high.
.2. PEC properties
.2.1. Cell fabricationThe PEC solar cell was fabricated by using the product obtained
rom boat which is the catalyst free growth of cactus like structures. thin film was quoted on the ITO substrate having area 1 cm2 with
he help of PVDF in NMP solution. After pasting the product onTO coated glass substrate it was annealed at 120 ◦C for 8 h underacuum. Similarly same amount of PVDF solution was quoted onimple ITO coated glass substrate with same area. A standard threelectrode configuration was established to study the I–V character-stics of solar cell. Saturated calomel electrode (SCE) was used as
reference electrode with n-CdSe thin film as active photoanodend graphite as a counter electrode. The redox electrolyte was 1 Molysulfide (NaOH + Na2S + S). Photo electro chemical characteris-ics were measured using a solar simulator (incident light AM 1.5,0 mW/cm2). Fig. 9 represents the typical I–V plot for the CdSe thinlm in dark and under illumination.
Inset of Fig. 9 shows the I–V characteristics of only PVDF solutionn NMR on ITO quoted glass substrate with same area. It is observedhat in the dark the forward current increases rapidly with appliedias. The increase in forward current can be attributed to the smallontact height and increase in tunneling mechanism. The nonsym-
etrical nature of I–V curve in forward and reverse bias shows
ectification property of the semiconductor–electrolyte junction.nder illumination shift of I–V curve in IVth quadrant is indication
hat the cell can work as electricity generator.
Fig. 10. Power output curve for CdSe PEC solar cell.
Fig. 10 represents the power output curve for the CdSe PEC solarcell. Some critical parameters like Voc and Isc can be estimated bythis graph. Voc and Isc come out to be 320 mV and 3.412 mA/cm2
respectively.The fill factor (FF) was calculated by using the relation
FF = Vm × ImVoc × Isc
. (2)
where Vm and Im represent voltage and current obtained at themaximum power point in photovoltaic power output calculation.The fill factor from our results comes out to be 0.35
The photovoltaic efficiency was calculated using the relation
� = Vm × ImPinput
× 100 (3)
where Pinput is the power density of incident light, Voc is the opencircuit voltage and Isc is the short circuit current.
The fill factor and efficiency calculated are 0.35 and 0.476%respectively.
We have used PVDF as binder and the conversion efficiencyin our studies is not high enough, however, it is comparable tothe results in literature [47,48]. We think that PVDF is not a goodbinder; it reduces or affects the conversion efficiency of the PECsolar cell. We suggest that other binders can also be explored forimprovement of the conversion efficiency.
3.3. Growth mechanism
The ammonia environment plays an important role for theformation of spherelite structures of CdSe. Mixed with aqueousammonia, when the mixture of Cd and Se is heated under theammonia gas flow, the thermochemical reaction leads the forma-tion of CdSe spherelites. Above the melting temperature of CdSethese spherelites forms spherical clusters under ammonia environ-ment on the silicon substrate. These spherical clusters nucleate onthe nickel coated silicon substrate placed at ∼2 mm distance fromthe source. The VLS growth mechanism is proposed for the fab-rication of CdSe microcactuses with sharply pointed nanorods onexterior surface.
The growth mechanism for the product on silicon substratecan be understood in three different stages, firstly, the nucleatedeposition where dr/dt > 0 secondly transport to growth interfacewhere dr/dt = 0 and thirdly termination where dr/dt < 0. The nucle-
ate deposition, where the concentration of CdSe increases with thepassage of time on the silicon substrate, this increase in concen-tration forms spherical nucleate islands. In the growth interfacestage the nickel as catalyst grows nanorods in the axial direction.
Z. Ali et al. / Electrochimica Acta 85 (2012) 122– 130 129
F obtaib
TtltAaacfnanspap
fitaagppaSmcoc
eibc
4
e
ig. 11. (a) Growth mechanism for the CdSe cactuses with sharply pointed nanorodsoat in the powder form.
he nucleation process of the nanorods anywhere other than onhe catalyst is suppressed because growth only occurs at the cata-yst/nanorod interface. This is due to the reason that catalyst lowershe activation energy of nucleation at the axial growth interface.ccording to classical nucleation theory, there is an ample barrierssociated with the formation of the critical nucleation cluster at
random position on the substrate or nanorods. If the catalystan lower the nucleation barrier at the particle/nanowire inter-ace, then growth only occurs there [31]. Finally, termination ofanorods formation, “where tapering of CdSe nanorod diameternd ending of growth process” occurs. The reason for tapering ofanorod diameter is that the catalyst has been consumed. At thistage CdSe is not being supplied any more to the system or the tem-erature has become lower than the critical value. The SEM of thes grown cactuses reveals the fact that the nanorods have sharplyointed tips.
Fig. 11(b) shows the growth mechanism of the hollow cactusesormed in the semicircular long alumina boat. The solid state chem-cal reaction is suggested for the formation of hollow cactuses inhe powder form in semicircular long alumina boat. In the aqueousmmonia environment the spheres of the reactants are formed,s the temperature increases the reactants react in the ammoniaas flow to form CdSe while the selenium being low melting pointartner evaporates from the center leaving the spherical shapedroduct hollow. With the passage of time the nanorods grow axi-lly on the outer surface as the remaining product is purely CdSe.ince the product on the silicon substrate is also CdSe with the sameorphology but the difference is the presence of catalyst on the sili-
on substrate thus the adhesive force which prevents the formationf hollow spheres on silicon substrate while in the powder form theactuses grown are hollow.
The role of aqueous ammonia and ammonia gas flow in ourxperiments is to control the morphology and to control the chem-cal reaction between the elements with different melting pointsy lowering the formation energy. It only provides the favorableonditions but not take part in the chemical reaction.
. Conclusion
In summary, CdSe cactus-like structures having SPNR on thexterior surface have been synthesized successfully through a solid
ned on the silicon substrate (b) as synthesized product in semicircular long alumina
state chemical reaction by using a horizontal tube furnace at 600 ◦Cin ammonia environment. Structural characterizations throughXRD and EDS patterns show that the as synthesized nanostructuresare pure CdSe. Investigations also reveal that introduction of aque-ous ammonia and the ramp rate play a much important role in thefabrication of novel structures. These structures lead to an intenseblue-shifted emission in the room-temperature PL spectrum. It isconcluded that nanostructures grown without use of catalyst havehigh intensity of the peak as compared to catalyst assisted approachthus avoiding carrier quenching effects which arise due to the con-taminations of the catalyst. Moreover, catalyst free product consistsof hollow cactuses. There are a number of valuable properties whichhollow nanoparticles exhibit compared to solid inorganic nanopar-ticles such as decrease in their poisoning effects, larger adsorptioncapabilities, sensitive to interactions with their surroundings andtheir cost for technological applications. These are due to theirlower densities and their larger surface-to-bulk ratio. A supple-mentary advantageous property of hollow nanoparticles is theirability to encapsulate other substances within their interior cav-ity. These qualities make them potentially suitable materials forapplications in the fields of catalysis, sensing, and drug delivery.Hence it is suggested that catalyst free growth of nanostructuredmaterials is beneficial over the catalyst assisted approach. Wealso conclude that CdSe nanocrystals have great potential for PECsolar cell applications. Although the power conversion efficiency isnot high enough in our work but it can be increased by utilizingsome suitable binders. It is further concluded that our solid statechemical reaction technique for preparation of CdSe and new mor-phology of microcactuses with SPNR at exterior surfaces providesa new synthetic direction for CdSe nanocrystals and also providesCdSe as a good candidate for the production of nanodevices in thefuture.
Acknowledgements
This work was supported by National Natural ScienceFoundation of China (20471007, 50972017) and the ResearchFund for the Doctoral Program of Higher Education of China(20101101110026).
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