Size-Dependent Optical Properties of Zinc Blende Cadmium TellurideQuantum DotsJohn Sundar Kamal,†,‡ Abdoulghafar Omari,†,‡ Karen Van Hoecke,§ Qiang Zhao,∥ Andre Vantomme,∥Frank Vanhaecke,§ Richard Karel Capek,†,⊥ and Zeger Hens†,‡,*†Physics and Chemistry of Nanostructures, Ghent University, Krijgslaan 281-S3, B-9000 Gent, Belgium‡Center for Nano and Biophotonics, Ghent University, B-9000 Gent, Belgium§Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, B-9000 Gent, Belgium∥Instituut voor Kern- en Stralingsfysica, K.U.Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

*S Supporting Information

ABSTRACT: We analyze the optical properties of CdTe quan-tum dots, including the sizing curve, the absorption coefficient,and the oscillator strength of the band gap transition, by com-bining absorption spectroscopy, elemental analysis, and electronmicroscopy imaging. At short wavelengths, the absorption coef-ficient spectrum is still affected by quantum confinement, yet alargely constant value, close to that of bulk CdTe, is found ataround 410 nm. At shorter wavelengths, remaining quantumconfinement effects on the CdTe E1 transition are present evenfor the largest quantum dots studied (11 nm). For the band gaptransition, we find an integrated absorption coefficient μgap that scales almost proportionally to the inverse of the quantum dotvolume. Especially for the smaller diameters, deviations up to a factor of 3 are found as compared to widely used literature values.The corresponding oscillator strength fgap is almost size-independent in the diameter range 3−7 nm. The correspondencebetween radiative lifetimes predicted based on fgap and literature values is discussed.

■ INTRODUCTIONOver the last 20 years, sterically stabilized colloidal nanocrystalshave emerged as unique and versatile building blocks of nano-structured materials that combine tunable opto-electronicproperties with a suitability for solution-based processing.1 Akey technique for the characterization of colloidal nanocrystaldispersions is absorption spectroscopy. In the case of semi-conductor nanocrystals or quantum dots (QDs), it gives accessto the average QD diameter dQD and the volume fraction f ofsemiconductor material in solution. Numerous examples showthat both quantities are essential for present day research oncolloidal QDs. Mapping the size dependence of electro-opticalproperties is widely used to understand QD properties and tolink them to theoretical modeling;2 the time development ofthe amount of QD material formedaccessible via the volumefractionis one of the starting points to analyze the kineticsand the mechanism of a QD synthesis.3 Furthermore, theknowledge of QD concentrations is needed for the rationaldevelopment of ligand exchange procedures1,4 or reproduciblecytotoxicity studies.5 As a result, the availability of reliable sizingcurveslinking dQD to the QD band gap Egand absorptioncoefficientsenabling the calculation of QD volume fractionsfrom an absorbanceis imperative for QD research.Establishing experimental sizing curves typically relies on the

analysis of transmission electron microscopy (TEM) images.Possible systematic errors related to the implementation of the

image analysis can be avoided by bringing together experimen-tal data of different groups, as was done for CdSe,6 CdTe,6

PbSe,7 and PbS.8 The determination of absorption coefficients(μ) is based on elemental analysis. This requires well-purifieddispersions, which can be verified by solution NMR.7 On theother hand, an absorption coefficient determination can bechecked for consistency since it was found for many materialsthat μ coincides with the value calculated for the bulk materialusing the Maxwell−Garnet model at short wavelengths.7−12

Using this consistency check, molar absorption coefficientsaquantity closely related to μfor CdSe QDs published by Yuet al.13, for example, were re-examined by Jasieniak et al.,14

leading to considerably improved values.Here, we present an analysis of the absorption coefficient of

CdTe QDs. Together with CdSe and CdS, CdTe is one of thefirst materials synthesized using the hot injection approach.15,16

By now, various approaches have been described to synthesizethem,17−19 control their size and shape,20 and incorporate themin heteronanocrystals.21 Both the sizing curve6,13 and size-dependent molar absorption coefficients13 have been published.Although less widespread than CdSe, CdTe QDs are nowadaysused in, for example, LEDs,22 bioimaging,23 and photovoltaics.24

Received: December 20, 2011Revised: January 25, 2012Published: January 31, 2012

Article

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Here, we combine UV−vis absorption spectroscopy, TEM im-aging, and elemental analysis to determine the absorptioncoefficients at short wavelengths and at the band gap transition.At short wavelengths, we find that size quantization persists, butstill, wavelength regions exist where the absorption coefficientis size-independent and close to the bulk value. At the band gaptransition, integrated absorption coefficients deviate markedlyfrom published values and lead to largely size-independentoscillator strengths in the range 10−13. These results willstrongly enhance the accuracy of volume fraction and concen-tration determination in future research on CdTe QDs.

■ EXPERIMENTAL SECTIONChemicals. Methanol and 2-propanol were purchased from

VWR BDH Prolabo and had Rectapur grade. Toluene wasalso purchased from VWR BDH Prolabo and had technicaland Normapur grade, respectively. Cadmium oxide (CdO,99.99+%) was purchased from Aldrich. Tellurium (99.999%) and1-octadecene (ODE, technical) were purchased from Alfa Aesar.Tetradecylphosponic acid (TDPA, 98%) was purchased fromPCI synthesis. Hexadecylamine (HDA, 90%) was purchasedfrom Merck.Synthesis. CdTe QDs were synthesized by adapting the

procedure of Dorfs et al.,25 which makes use of CdO, tetra-decylphosphonic acid (TDPA), elemental Te, trioctylphosphine(TOP), and hexadecylamine. Octadecene (ODE) is typicallyused as the solvent. The Cd-TDPA precursor was prepared bymixing CdO and TDPA in a 1:3 molar ratio, degassing undernitrogen flow for 1 h at 100 °C, followed by dissolving the CdOunder a nitrogen atmosphere between 250 and 300 °C until themixture became clear. TOP-Te was typically prepared bydissolving 1.5 mmol of Te in 10 mL of TOP at about 80 °C forone hour under a nitrogen atmosphere.In a typical synthesis (procedure 1), 0.3 mmol of the Cd-

TDPA precursor, 0.9 mmol of HDA, and 7.8 mL of ODE aredegassed under a nitrogen flow at room temperature and at100 °C for 1 h, respectively. The temperature is then raised to280 °C under a nitrogen atmosphere, and 2 mL of the TOP-Teprecursor is subsequently injected. After the injection, the tem-perature of the mixture is reduced and maintained at260 °C for the growth of nanocrystals. The reaction is stoppedby temperature quenching with a water bath. The productobtained is purified using toluene as the solvent and anisopropanol/methanol mixture as the nonsolvent. By control-ling the reaction time, CdTe QDs with a size of 4−6 nm areobtained.Smaller sizes (procedure 2) are obtained by adding stearyl

alcohol in a 12:1 ratio to the Cd precursor to the reaction mix-ture prior to injection.QDs with a diameter above 6 nm (procedure 3) are synthe-

sized using multiple precursor injections. For this, 4.5 mmol ofthe Cd-TDPA precursor is dissolved in 4 mL of ODE at 280 °Cfor 15 min and mixed with 1 mL of a 1.5 M TOP-Te solutionafter cooling. This precursor mixture is added to the reactionmixture with a syringe pump for 1 h at a rate 2 mL/h. Again,the temperature of the reacting mixture was quenched by awater bath, and the product was purified as described above.Size Determination. The mean diameter dQD of the CdTe

QDs was determined from bright-field TEM images recordedwith a Cs corrected JEM-2200FS transmission electron micro-scope. The samples for TEM were prepared by drop casting adilute suspension of CdTe QD on holey carbon film supportedby a copper TEM grid. To improve the contrast, a high contrast

aperture with Z-filter has been used. On the basis of the TEMimages, the size histogram, mean diameter, and size dispersionof the QDs are determined by analyzing 100−150 particles foreach sample.

Absorption Coefficient Determination. Absorptioncoefficients were determined by combining elemental analysiswith UV−vis absorption spectroscopy. The concentration ofCd was determined by inductively coupled plasma−mass spec-trometry (ICP−MS). Samples for ICP−MS analysis wereprepared by drying a known amount of CdTe QD suspensionin a nitrogen flow. The dried samples were digested in 500 μLof HNO3 to be analyzed with a Perkin-Elmer SCIEX ELAN5000 ICP-MS. The Cd:Te ratio R is determined using Ruth-erford backscattering spectrometry (RBS). Samples for RBSanalysis consisted of 10−15 nm thick films of CdTe QDs spincoated on Si substrates. The measurements were done with a 4MeV He2+ ion beam and an NEC 5SDH-2 Pelletron tandemaccelerator with a semiconductor detector at backscattering an-gles of 124° and 168°. The Cd:Te ratio is obtained from theratio of the backscattered intensity with Cd (ICd) and Te (ITe)nuclei after Z2 correction (Z: atomic number)

= ×RII

Z

ZCd

Te

Te2

Cd2

(1)

For the absorption measurements, an identical amount ofCdTe QDs as used for ICP−MS was dried under nitrogen flowand redispersed in toluene. The absorbance spectrum of theresulting dispersion was recorded with a Perkin-Elmer Lambda950 UV−vis spectrophotometer. We estimate that the pro-cedure leads to an error of about 10% on the determinedabsorption coefficients.12

■ RESULTS AND DISCUSSIONMaterial Characterization. Figure 1 gives the basic char-

acteristics of a sample of CdTe QDs synthesized following pro-cedure 1. The TEM overview image (Figure 1a) demonstratesthat the resulting CdTe QDs are quasi-spherical, in this casewith an average diameter of 5.7 nm and a narrow size disper-sion of around 4% (Figure 1b). According to the X-raydiffractogram (Figure 1c), the QDs used here have the zincblende crystal structure. Furthermore, from the ratios of theZ2-corrected integrated intensities of Cd and Te in the RBSspectrum (eq 1), it follows that the CdTe QDs studied here arecation rich, with a Cd:Te ratio R of 1.10 for the sample shownhere. A similar result was found for PbSe, PbS, and CdSeQDs.7,8,26−28 As shown in the Supporting Information, theseconclusions hold for all sizes. Only for the largest QDs (8−10 nm), crystal facets are more pronounced, and the size dis-persion increases to about 8%.Figure 2a shows the UV−vis absorption spectra of six dif-

ferent batches of CdTe QDs (see Supporting Information forTEM images and size histograms). For all samples, the wave-length λ1S−1S of the first exciton transition can be readily dete-rmined. The combination of the resulting λ1S−1S with TEMoverview images of the different samples enables us to con-struct the CdTe sizing curve, which relates the band gap Egascalculated from λ1S−1Sto dQD. Figure 2b represents the dif-ferent data points we obtain and compares them with sizingcurves previously published by Donega et al. and Yu et al.6,13 Agood correspondence is overall obtained with the former, withmaximum deviations of about 0.3 nm. On the other hand,deviations up to 0.6 nm are found with the sizing curve of Yu et al.,

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especially in the range dQD = 3.5−6.5 nm. Taking a value of 1.51eV for the band gap of bulk CdTe,29 we use the following best fitto our experimental data to link Eg to dQD (black line in Figure 2b)

= ++ −

E dd d

( ) 1.511

0.048 0.29 0.090 2 (2)

Intrinsic Absorption Coefficient at Short Wave-lengths. We quantify the absorbance of dispersed colloidalQDs in the first place by the intrinsic absorption coefficient μi,which relates the measured absorbance A of a QD dispersion tothe fraction f of the sample volume occupied by the QDs (theQD volume fraction)

μ = AfL

ln(10)i

(3)

An advantage of using μi is that its value does not depend onthe QD sizing curve, which means that f can be determinedirrespective of errors on the size determination. For CdTe QDs,f is related to the molar concentrations of Cd (CCd) and Te(CTe) according to (a: lattice parameter of the CdTe zincblende unit cell)

= + = +⎜ ⎟⎛⎝

⎞⎠f

aN C C

aN C

R8( )

81

13A Cd Te

3A Cd (4)

As outlined in the Experimental Section, we obtain CCd usingICP−MS and R using RBS measurements. Figure 3(a) shows

seven different μi spectra determined by combining elementalanalysis (yielding f) and absorption spectroscopy (yielding A)in toluene.Similar to previous studies on various QDs,7−12 we find that

the μi spectra tend to coincide at short wavelengths, especiallyin the wavelength ranges around 400 and below 340 nm. In theliterature, this has always been interpreted as an absence ofquantum confinement effects on higher-energy conduction andvalence band states. Moreover, excellent correspondence hasbeen demonstrated between the experimental μi and a theoreti-cal value μi,th calculated within the Maxwell−Garnett effectivemedium theory,30 using the bulk dieletric function εR + iεI ofthe QD material

μ = πλ

| | εn

f2

i ,ths

LF2

I(5)

Here, ns is the refractive index of the solvent, and f LF is the localfield factor, i.e., the ratio between the electric field outside andinside the QD. For spherical QDs, f LF is given by (εs = ns

2)

=ε

ε + ε + εf

i3

2LFs

R I s (6)

The bulk μi,th reference spectrum for CdTe in toluene isshown as the black line in Figure 3. It clearly reproduces theoverall increase of μi with decreasing wavelength, and the wave-length regions where all μi spectra tend to coincidearound410 and 320 nmcorrespond to wavelengths where the bulk

Figure 1. Basic characterization of 5.7 nm CdTe QDs synthesizedaccording to procedure 1. (a) TEM overview image. (b) Size histo-gram, yielding a size dispersion of 4%. (c) (black line) XRD pattern ofthe CdTe QDs and (red bars) the reference diffractogram of zincblende CdTe. (d) RBS spectrum indicating the backscattered intensityfor Cd (ICd) and Te (ITe), yielding a Cd:Te ratio of 1.10 after Z2

correction.

Figure 2. (a) Absorption spectra of QDs with sizes of (bottom to top)3.10, 3.65, 4.10, 5.00, 5.70, and 7.10 nm. (b) Relation between CdTeQD band gap Eg and diameter dQD, including (markers) experimentaldata obtained here, (full black) best fit according to eq 2, (dottedred) sizing curve of Yu et al.,13 and (dashed blue) sizing curve ofDonega et al.6

Figure 3. (a) Intrinsic absorption coefficient (μi) spectra of CdTeQDs of different sizes (left to right: 3.10, 3.65, 4.10, 5.00, 5.70, 7.10,and 11.0 nm) in toluene and (black line) intrinsic absorptioncoefficient (μi,th) calculated for bulk CdTe according to eq 5. (Inset)zoom of the short-wavelength range of the μi spectra. (b) μi at (bluemarkers) 410 nm and (red markers) 330 nm as a function of the CdTeQD diameter, together with (dashed lines) the respective average μiand (black lines) the corresponding bulk value. (c) Relative standarddeviation on μi as a function of wavelength calculated using the sevenspectra shown in (a).

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spectrum does not show particular features. Figure 3b showsμi,410 and μi,330 as a function of dQD. One sees that both valuesare largely size-independent with an average value lower thanμi,th by 16% and 24%, respectively. A similar deviation wasfound with zb-CdSe at wavelengths below 350 nm.12 As shown inFigure 3c, the standard deviation on μi is lowest in the wavelengthrange 405−420 nm. Therefore, we propose to use the averageabsorption coefficient μi,410 as obtained here for the determinationof CdTe QD volume fractions in toluene dispersions

μ = −75500 cmi ,4101

(7)

The intrinsic absorption coefficient of bulk CdTe shows apronounced feature at 365 nm, which corresponds to the E1transition that connects initial and final states along the Λ dir-ection in the Brillouin zone.31,32 Figure 3 shows that thisfeature is not present in the μi spectra of the smaller CdTeQDs. Only for the largest sizes, a shoulder develops in thiswavelength range, which becomes more pronounced and shiftsfrom shorter wavelengths toward 365 nm with increasing size.This is a clear indication of size quantization effects in this par-ticular region of the Brillouin zone, similar to what was foundfor the E1 transition along the Σ direction in PbSe QDs.33,34

Importantly, this persistence of size-quantization effects impliesthat normalization of CdTe QD spectra for relative comparisonis only viable at those wavelengths where size effects on the ab-sorption coefficient are minimal, i.e., around 410 or 320 nm,respectively.The molar absorption coefficient ε of dispersed colloidal

CdTe QDs is obtained as the product of μi and the volume of1 mol of QDs

ε =π

μd N

6 ln(10) iQD3

A

(8)

Since μi,410 is independent of dQD, we obviously find that ε410scales proportionally to dQD

3 (see Figure 4). Writing dQD in

nanometers, a best fit to the experimental data yields (ε410in cm−1/μM)

ε = ± × d(0.0106 0.0002)410 QD3

(9)

Intrinsic Absorption Coefficient at the Band Gap. Inthe literature, (molar) absorption coefficients are often re-ported at the band gap transition, typically in combination witha normalization procedure to account for peak broadening dueto size dispersion.13,35 A useful quantity in this respect is the

energy integrated absorption coefficient μi,gap of the first excitontransition. This can be obtained either by using μi spectra ob-tained by determining f using elemental analysis (see Figure 3)or by normalizing absorption spectra using the average value ofμi,410. In this way, μi,gap follows from the integrated absorbanceAgap of the first exciton absorbance peak

μ = μA

Ai i,gapgap

410,410

(10)

Typically, μi,gap is calculated by doubling the integrated low-energy half of the first exciton peak.7

Figure 5 shows the resulting μi,gap obtained with both pro-cedures. Opposite of μi,410, μi,gap shows a marked increase withdecreasing particle size. Since μi gives an absorbance per unitvolume of material, this shows that for the same volume of CdTesmall QDs absorb more light at their bandgap transition than largeQDs. The dependence of μi,gap on dQD in the size range envisagedhere (3−7 nm) can be well fitted to a power law. A best fit showsthat μi,gap is about proportional to the inverse of the QD volume

μ = ± × × − ± −d(2.68 0.41) 10 (eV m )i ,gap7 ( 2.76 0.12) 1

(11)

Molar absorption coefficients at the band gap for CdTe QDshave been published before by Yu et al.13 To compare theseresults with the data obtained here, we have combined themolar absorption coefficients and the sizing curve of theseauthors to calculate a μi,gap (see Supporting Information formore details). Figure 5 gives the resulting values as a functionof λ1S−1S (upper horizontal axis) and dQD calculated according tothe sizing curve proposed here (lower horizontal axis). Dependingon the actual size, the resulting μi,gap according to Yu et al. is up to3 times smaller than the values reported here. Since our valuescorrespond to a μi,410 that only deviates from the bulk value by16%, we conclude that the use of the molar absorption coefficientsproposed by Yu et al. for volume fraction and concentrationdetermination of CdTe QD dispersions may give considerableerrors and is therefore not recommended.

Oscillator Strength and Lifetime. The oscillator strengthof an electronic transition from a ground (g) to an excited (e)state compares the power absorbed by the electronic transition

Figure 4. Representation of (markers) the molar absorptioncoefficientʼ of CdTe QDs in toluene at 410 nm as a function of QDdiameter and (full line) the cubic fit according to eq 9.

Figure 5. Energy integrated intrinsic absorption coefficient μgap as afunction of QD diameter (bottom axis, calculated according to eq 2)and λ1S−1S (top axis), showing data obtained by (filled blue circles)elemental analysis and (open blue circles) normalization of μi at 410nm. The blue line is a best fit according to eq 11, and the red squaresrepresent the data according to Yu et al.13

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to that of a classical dipole oscillator with the same resonancefrequency. It is an intrinsic property of an electronic transitionclosely related to the g → e transition dipole matrix element.The oscillator strength fgap related to the first exciton transitioncan be calculated from the experimental μi,gap according to(ε0, permittivity of the vacuum; c, speed of light; me, electronmass; ℏ, Planck’s constant)7

=ε

πℏ | |

πμf

n cme f

d2 16 igap

0 s e

LF2

QD3

,gap(12)

Figure 6(a) shows the resulting fgap as a function of dQD calcu-lated using the bulk dielectric function of CdTe at the

wavelength of the respective band gap transition. As could beexpected from the almost cubic dependence of μi,gap on 1/dQD,fgap is largely constant in the diameter range 3−7 nm, varyingonly between 10 and 13. In this respect, CdTe QDs differ sig-nificantly from CdSe QDs, where the oscillator strength showsa pronounced size dependence and drops to values of around 5in the 2−4 nm size range.12 On the other hand, a similar size-independent oscillator strength was found for InAs QDs.10

The oscillator strength of an electronic transition is related tothe decay rate τ−1 of the excited state by spontaneous emission.Summing over all accessible exciton states jpresumed to bein thermal equilibriumτ−1 reads (kB, Boltzmann’s constant;T, absolute temperature)

τ =πε

| | ω∑

∑−

−Δε

−Δεe

c mn f

f e

e2

j ijk T

jk T

12

03

es LF

2 2/

/

ij

ij

B

B(13)

Here, Δεij denotes the energy difference between the excitonstate j and the lowest energy exciton state i. When Δεij ≪ kBT,the spontaneous emission rate can be rewritten as (g, totalnumber of exciton states accessible)

τ =πε

| | ω∑

=πε

| | ω

− ec m

n ff

g

ec m

n ff

g

2

2

j ij12

03

es LF

2 2

2

03

es LF

2 2 gap

(14)

Figure 6(b) plots the resulting predicted lifetime τ, calculatedtaking g = 8,37 together with experimental literature data.36 Inthe diameter range 3−7 nm, the predicted lifetimes slightly in-crease from 9 to 13 ns with increasing dQD. These lifetimes are

2 to 3 times smaller than the experimental ones, which rangefrom about 20 to 40 ns over the same diameter range. Althougha detailed interpretation of this difference is out of the scope ofthe present manuscript, a first explanation of this differencecould be that instead of an equal occupation probability of alleight exciton levels a dark exciton state is preferentially occu-pied. However, opposite of the data shown in Figure 6b, thisshould lead to a better correspondence between predicted andmeasured lifetimes for larger QDs, which have a concomitantlylower energy splitting between the different exciton levels.Alternatively, the measured lifetimes will exceed the predictedvalues when additional dark exciton states, not accounted for inthe g = 8 assumption, are accessible. This is not unlikely giventhe small energy spacing between the S3/2 and P3/2 hole states,

38

which would mean that a manifold of 16 instead of 8 excitonstates is readily accessible for the first exciton. As shown inFigure 6, the assumption of g = 16 leads to a good correspon-dence between measured and predicted lifetimes, especially forthe smaller sizes.

■ CONCLUSIONSWe have analyzed the basic optical propertiessizing curve,absorption coefficient, and oscillator strength of the band gaptransitionof CdTe QDs by combining UV−vis absorptionspectroscopy, TEM, ICP-MS, and RBS measurements. At410 nm, we retrieve a largely size-independent intrinsic absorp-tion coefficient μi. The resulting value is close to what is ex-pected for bulk CdTe and can be used for determining thevolume fraction of spherical CdTe nanocrystals in colloidaldispersions. Despite the size-independent μi at 410 nm, weobserve persistent quantum confinement effects on the E1transition, which appears less pronounced and blueshifted inCdTe QDs relative to its bulk value of 365 nm. Around theband gap, we find an integrated absorption coefficient μgap thatscales proportionally to the inverse of the QD volume. Espe-cially for the smaller diameters, significant deviations are foundas compared to widely used literature values. The correspond-ing oscillator strength fgap is almost size-independent in thediameter range 3−7 nm. Radiative lifetimes predicted based onfgap are in line with published values if a 16-fold degeneracy forthe first exciton is assumed.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional experimental details. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zeger.hens@ugent.be. Phone: 0032-9-2644863. Fax:0032-9-2644983.Present Address⊥Schulich Faculty of Chemistry, Russell Berrie NanotechnologyInstitute, Technion, Haifa 32000, Israel.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge BelSPo (IAP 6.10 photonics@be),EU-FP7 (ITN-Herodot) and the FWO-Vlaanderen (project(G.0794.10) for financial support. A.O. acknowledges the IWT-Vlaanderen for a research grant.

Figure 6. (a) Oscillator strength fgap of the first exciton transition inCdTe QDs, based on (filled blue circles) elemental analysis and (opencircles) normalization of μi at 410 nm. (b) Spontaneous emissionlifetime calculated according to eq 14 using (filled blue circles) g = 8and (open blue squares) g = 16 and (filled red triangles) experimentaldata according to Van Driel et al.36

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp212281m | J. Phys. Chem. C 2012, 116, 5049−50545053

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp212281m | J. Phys. Chem. C 2012, 116, 5049−50545054