Influence of Alloying on the Optical Properties of IV–VI Nanorods - The

Article pubs.acs.org/JPCC

Influence of Alloying on the Optical Properties of IV−VI Nanorods Anna Rubin-Brusilovski,† Georgy Maikov,† Dikla Kolan,† Roman Vaxenburg,† Jenya Tilchin,† Yaron Kauffmann,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,† †

Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute, Grand Technion Energy Program, Technion, Haifa 32000, Israel ‡ Department of Materials Engineering, Technion, Haifa 32000, Israel S Supporting Information *

ABSTRACT: The synthesis and structural and optical characterization of PbSexS1−x and PbSe/PbSexS1−x nanorods with a diameter between 2 and 4.5 nm and a length of 10 to 38 nm is reported. The energy band gap of the nanorods exhibits a pronounced variation upon the change in diameter and composition, with a minor influence on lengths beyond 10 nm. The photoluminescence spectrum of the nanorods is composed of a dominant band, accompanied by a satellite band at elevated temperatures. The dominant band shows an exceptionally small band gap temperature coefficient and negligible extension of the radiative lifetime at cryogenic temperatures compared with the photoluminescence processes in PbSe nanorods and in PbSexS1−x quantum dots with similar band gap energy. A theoretical model suggests the occurrence of independent transitions from a pair of band-edge valleys, located at the L points of Brillouin zone, related to the dominant and satellite emission processes. Each valley is four-fold degenerate and possesses a relatively small electron−hole exchange interaction. of II−VI ternary QDs25−27 and in a few examples of ternary IV−VI QDs.28,29 Exceptional alloyed core/shell QDs, such as PbSe/PbSexS1−x,17,30 CdxZn1−xSe/ZnSe,19 CuInS2/ZnS,31 and CdTe/CdTexSe1−x,32,33 have been developed recently, showing an exceptionally high chemical18 and spectral (blinking-free)33 stability as well as sustained biexciton lifetime over a nanosecond.34 Only a few prominent examples related to the development of ternary NRs35 and nanowires29 have been reported so far. The IV−VI elongated nanostructures are of particular interest due to their activity in the near-infrared spectral regime and efficient multiple exciton generation. Among these studies, the synthesis of pure PbSe NRs36 and PbSexS1−x wires of a few micrometers in length29 was reported. A theoretical study showed the electronic structure of pure PbSe and PbS nanowires (in the framework of a four-band effective-mass model) and suggested that the ground exciton binding energy in the elongated structures is substantially larger than in QDs of similar diameter, and the electronic levels are not affected by the surrounding medium because the selfinteraction energies of the electron and hole nearly cancel the Coulomb binding.37 The theory was confirmed by experimental steady-state photoluminescence (PL) spectroscopy.38,39 Recent magneto-optical measurements of binary PbSe NRs revealed a

1. INTRODUCTION Variation of electronic properties of semiconductor nanostructures by altering their size has been a topic of special interest for more than two decades, exploring numerous size-dependent properties, including tunable band gap energy1−3 and density of states,4 electron−hole Coulomb and exchange interactions,5 radiative lifetime,6 Auger relaxation,7 and multiple exciton generation.8 The engineering of electronic properties was also found to be of significant importance in the implementation of the nanostructures as absorbers or emitters in photovoltaic cells,9−11 light sources,12,13 photodetectors,14 and biological tags.15 Lately, additional degrees of freedom in the tunability of the electronic properties were explored by incorporation of alloy composition.16−19 Also, in front of interest is the development of elongated nanostructures such as wires, rods, or polypods that demonstrate polarization-sensitive light emission,20 significant absorption cross section,21 efficient charge separation, and transport along the major growth axis22 as well as possible efficient multiple exciton generation.23 Recent studies also examined the charge-transport properties across ordered self-assembled films of elongated nanostructures, with implementation in photovoltaic cells.24 This article demonstrates the development of colloidal elongated nanostructures − nanorods (NRs) − along with alloy composition from the IV−VI semiconductor compounds. A progressive effort in the synthesis of alloyed nanostructures has been demonstrated in the past, foremost in quantum dots (QDs).16 A major success has been attained in the development © 2012 American Chemical Society

Received: April 4, 2012 Revised: July 22, 2012 Published: August 1, 2012 18983

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relatively small (∼80 μeV) electron−hole exchange interaction.38 We recently published theoretical and experimental work exploring the properties of ternary IV−VI nanostructures, such as PbSexS1−x and PbSe/PbSeyS1‑y core/shell QDs, revealing the tunability of the electronic properties with variation of composition,30 with a unique spectral thermal stability and minimization of dark-bright exchange splitting whenever alloy composition was involved.30 Here we extend this endeavor to the synthesis, characterization, and modeling of ternary IV−VI NRs with general chemical formulas PbSexS1−x and PbSe/PbSexS1−x (0 ≤ x ≤ 1). The composition was controlled by employing high-temperature colloidal synthetic strategies with balancing of the precursors’ reactivity. The PL processes of the alloyed structures and their corresponding radiative lifetimes were monitored at various temperatures (from 1.4 to 300 K), revealing unique behavior with respect to the binary PbSe NRs and QDs of similar band gap energy. An electronic band theoretical model, using effective mass approximation, described the electronic properties of the alloyed IV−VI NRs, revealing lift of electronic degeneracy and relatively small electron−hole exchange interaction.

structures, all with similar diameter or equivalent band gap energy. The synthesis of the reference QDs followed the procedure reported in the literature.17,30 Characterization. The morphology and structure of NRs was characterized by the use of transmission electron microscopy (TEM, model FEI Tecnai G2 T20, operating at 200 keV), high-resolution TEM and high-angle annular darkfield scanning transmission electron microscopy (HR-TEM and HAADF-STEM, model FEI Titan, 80−300 keV, operating at 300 keV and equipped with an image Cs corrector), selected area electron diffraction (SAED), and X-ray spectroscopy. The overall composition of the NRs was determined by the use of energy-dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The local composition along the main axes of a single NR was measured using an EDX line scan in STEM mode with a spatial resolution of ∼1 nm. The progression of the NRs growth was monitored by immersing probe into the reaction flask, monitoring the absorption spectrum online instantaneously, using a spectrometer [Zeiss, model MCS-600]. A post-synthesis record of the absorption spectra was done by the use of a JASCO V-570 UV−vis-NIR spectrometer. The steady-state PL spectra were obtained by exciting the samples with a continuous-wave (cw) Ti:sapphire laser, Coherent 890 (Eexc = 1.48 to 1.80 eV). The cw-PL spectra of the materials studied were recorded at a temperature range of 1.4 to 300 K while immersing the samples in a variable temperature cryostat and detecting the emission with an Acton Spectrapro 2300i monochromator, which was equipped with a cooled InGaAs CCD. The transient PL (tr-PL) decay curves were recorded by exciting the samples immersed in the cryostat with a Nd:YAG laser, Continum Minilite II (Eexc = 1.17 eV). The measurements utilized a laser flux 1.5 min, accompanied by the formation of diameter fluctuation along the NR. An unusual twist of a crystallographic plane rarely appears along a small fragment of a NR (see Figure 1b), but the main crystallographic direction is preserved. It should be noted that the growth of length is delicately balanced by a change of the reaction temperature within ∼20°. A representative Fourier transform of a TEM image of ternary NRs is shown in the inset of Figure 1a, confirming a rock-salt crystallographic structure of the Fm3̅m space group. Similar rock-salt structures appeared in all of the investigated NRs. 18984

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1.) The discussion below compares samples exhibiting similar band gap energy, with different dimensions and composition. Figure 1e displays an image of a NR with PbSe/PbSexS1−x structure (typical length/diameter of 25 nm/3 nm). The Pb/Se/S atomic ratio within the NRs was measured by EDX, monitoring an ensemble of structures. The derived NRs’ compositions are listed in Table 1. The local composition along the main axes of a single NR was measured by acquiring EDX line scans with a spatial resolution of ∼1 nm (not shown here), revealing a homogeneous distribution of Pb, Se, and S atoms through the whole volume of an NR and confirming the formation of ternary PbSexS1−x compound in core NRs, under the synthesis conditions described in the Methods. The optical properties of the NRs were examined by monitoring the cw-PL and tr-PL curves at different temperatures. Representative cw-PL spectra of PbSexS1−x (x = 0.6) NRs with average length/diameter ratio of 10.4 nm/4.1 nm are shown in Figure 2a. The spectra were recorded at temperatures indicated in the legend. The orange line displays the absorbance curve of the NRs, with Eg of 0.925 eV, recorded at room temperature. Other NRs, listed in Table 1, showed similar cwPL spectra, and the discussion below represents the general trends found in all samples. The spectra in Figure 2a are shifted to higher energies with the increase in the temperature. The cw-PL band always has asymmetric appearance, which occasionally may be accompanied by a satellite band, depending on the NRs’ dimension. In general, the emission bands in the cw-PL spectra of NRs are Stokes shifted from the corresponding absorption edge by 40−60 meV, a substantially larger shift than that found in QDs of a similar Eg, suggesting a larger binding energy of an exciton in elongated structures. Figure 2b exhibits the plot of the energy change of the PL peak (EPL(T)) from that measured at 1.4K (EPL(1.4K)) versus the temperature of NRs and QDs mentioned in the legend. The symbols designate the experimental points, whereas the solid line is a specific example of a best fit curve to experimental points, revealing the temperature coefficient, dE/dT. This coefficient is commonly derived from the temperature dependence of the first excitonic absorption band, using Varshni relation.41 However, assuming that the emission Stokes shift is invariant under the temperature change (see Figure 2c), the coefficient derived in the present case should be relatively close to the band edge value, dEg/dT. Therefore, the best fit resulted in the band gap temperature coefficient values of 0.38, 0.35, 0.49, 0.45 meV/K for PbSexS1−x NRs, PbSe NRs, PbSe

Figure 1. Representative HR-TEM images of PbSexS1−x NRs with length/diameter ratio of 15.04 nm/3.86 nm (a) and 19.90 nm/2.90 nm (b). Fourier transform picture of the TEM image is shown in the inset of panel a, revealing rock-salt crystal structure. HAADF-STEM image of an ensemble of NRs shown in panel a, showing partial nematic self-assembly and relatively high size uniformity of the individual NRs (c). Representative absorbance curves measured online during NRs growth (d). HR-TEM image of PbSe/PbSexS1−x core/ shell NRs with x = 0.3 (e).

A TEM image of PbSexS1−x NRs of Figure 1a deposited on a TEM grid is displayed in Figure 1c, suggesting a partial selfassembly as a nematic assembly. Such an arrangement indicates the existence of relatively uniform dimensions, with an average size deviation of ∼10% in diameter and ∼15% in length of the NRs. Figure 1d, displays a sequence of absorbance curves recorded during the colloidal reaction, monitored one after the other with a time delay of one second. The spectra are initially characterized by the existence of an exciton band, related to the band-edge transition and considered as the band gap energy (Eg). This band shifts steadily to a longer wavelength with the growth progression; however, at a certain stage, the energy shift decelerates, together with a change in the curve profile, resembling typical rod-like absorbance and the additional appearance of a weak fine-structure. It should be noted that the value of Eg is dominantly controlled by the diameter of the NRs and mildly influenced by their length above 10 nm. (See Table

Table 1. Composition, Dimensions, Experimental and Calculated Energy Band Gap (Eg), Inter-Valley (ΔEv−v), and Exchange (Δex) Split Energy molecular formula PbSe0.3S0.7 PbSe0.6S0.4 PbSe0.7S0.3 PbSe0.6S0.4 PbSe/ PbSe0.7S0.3 PbSe/ PbSe0.6S0.4 PbSe PbSe PbSe a

Pb (%)

Se (%)

S (%)

diameter (2R) (nm)

length (nm)

Eg exp. (eV)

Eg calc. [110] (eV)

Eg calc. [100] (eV)

ΔEv‑v [110] (meV)

Δex (meV)

43 43 47 44 51

19 33 36 34 48

38 24 17 22 1

2.2 2.6 3.9 4.1 2.9

36.8 19.9 15.0 10.6 20.3

1.06 1.00 0.95 0.92 1.04

1.90 1.63 1.00 0.96

1.93 1.71 1.04 0.98

62 162 82 67

0.65 0.20 0.81 0.21

50

48

2

3.8

8.7

0.95

3.7 3.6 4.2

9.3 26.0 25.0

0.93 0.91 0.89

1.12 1.09 0.91

1.18 1.16 0.97

127 154 115

0.62 0.21 0.19

a a a

Nearly stoichiometric. 18985

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Figure 3. (a) Representative PL decay curves of PbSe0.6S0.4 NRs at various temperatures. (b) Plot of the measured lifetime versus temperature, of samples shown in Figure 2 and as listed in the legend of Figure 2b.

two exponent functions, always with a dominant decay component. The extracted lifetime (τmeas) of the dominant component deviates from the radiative lifetime (τrad) as 1/τmeas = 1/τrad(T) + 1/τnrad(T) when τnrad is the nonradiative contributions. The relation between τmeas and τrad was determined by examining the relative PL integrated intensity (IPL(T)) at various temperatures (T) (see Figure 2d) using the relation η(T) = IPL/I0 = τmeas/τrad. I0 corresponds to the exciting photon flux, and η(T) is the quantum efficiency at T. Figure 3b displays plots of the values of τrad versus the temperature of NRs and QDs, as given in the legend of Figure 2b. The plots display a drop of τrad from 6 to 1 μs between 1.4 and ∼150 K in QDs but only a mild change in NRs over the entire temperature range. As will be discussed below, the PL measured at the lowest temperature is related to a transition from a bright state, with minor nonradiative contribution, and thus can be considered at the most intense transition, closely supplying the value of I0. Also, η at room temperature determined by the given relation was found to be in close agreement with a direct measure, using an integrated sphere methodology.6,40 The relatively low values of τrad in NRs are in correlation with the theoretical consideration, suggesting reduction of the lifetime, due to an anisotropic dielectric screening in elongated structures.37 The influence of alloying in lifetime of NRs is negligible with respect to that of the reduced screening in NRs. In addition, the nearly independent behavior of τrad on temperature is related to the existence of extremely small exchange and dominant emission from bright exciton at a wide temperature range.

Figure 2. Representative cw-PL spectra of PbSexS1−x (x = 0.6) NRs with average length/diameter ratio of 10.4 nm/4.1 nm, recorded at the temperatures indicated at the legend. The orange curve displays the absorbance of the NRs, recorded at room temperature (a). Plots of emission peak energy at temperature T (EPL(T)) minus that recorded at 1.4 K (EPL(1.4 K)) versus temperature (b). Plot of fwhm of the dominant cw-PL emission band versus temperature (c) of various QDs and NRs. The representative best-fit curve (see text) is shown by a solid line. Plot of the integrated intensity versus the inverse of temperature (d). Panels b−d share a common legend: full red circles correspond to PbSe QDs, open blue circles to PbSe0.6S0.4 QDs, full orange squares to PbSe NRs, and open cyan squares to PbSe0.6S0.4 NRs. QDs and NRs have the same first excitonic absorption energy 0.925 eV.

QDs, and PbSexS1−x QDs, respectively, indicating a dominance reduction of dEg/dT in binary and ternary NRs with respect to the relevant QDs. Figure 2c exhibits plots of the full-width-half-maximum (fwhm) of the emission band versus the temperature (T) of the structures listed in Figure 2b. The symbols designate experimental points. Best-fit to these points was implemented using the relation Γ(T) = Γinh + σT + ΓLO/(eELO/kBT − 1), where Γ(T) is the emission band fwhm, Γinh is inhomogeneous broadening parameter at T = 0, σ is an acoustic phonon coupling, ΓLO is an optical phonon coupling, and ELO is the longitudinal optics phonon energy. It should be noted that the value of ELO varies with size,42 shape,43 and composition (unpublished results) of the NRs; however, a constant value 16.8 meV was used in the above relation for a set of NRs with relatively narrow size dispersion. Representative best-fit curve is shown by a solid line. The theoretical fit for all samples listed in the legend revealed the following values σ = 0.02, 0.04, 0.03, and 0.02 meV/K and ΓLO = 26, 22, 59, and 41 meV for PbSexS1−x NRs, PbSe NRs, PbSe QDs, and PbSexS1−x QDs, respectively. The values reflect reduction of optical phonon coupling in structures with alloy composition, suggesting localization of phonons in alloyed materials, as often found in bulk semiconductors.44 Figure 2d displays a plot of the cw-PL integrated intensity versus the inverse of the temperature (1/T (K)) for the samples discussed in Figure 2b (following the same color code). The plots are characterized by a drastic change of the intensity from >150 K but nearly a plateau behavior a ∑ Fc*′j(re)Fcj(re) ε(r , r )1|r − r | eh

0

j

e

∑ Fv*′ j ′(rh)Fvj ′(rh)d3red3rh j′

h

e

h

(2)

where c′, c and v′, v are the indices of the conduction band and valence band states. ε(re, rh) is a position-dependent microscopic dielectric function, evaluated by a mask function model that was proposed in ref 50. This model was altered in the present case for the asymmetric NRs, assuming a cylindrical shape, when ⎛1 1 1 1 ⎞ = +⎜ − ⎟m(re)m(rh) ε(re, rh) εout εout ⎠ ⎝ εin ⎧ 1 r ≤ R eff and z ≤ Leff m(r) = ⎨ ⎩0 otherwise 18987

(3)

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where r and z are the radial and longitudinal components of r in cylindrical coordinates, that is, r = (r, ϕ z), and the origin is taken at the NR’s center of mass. Reff and Leff define the inner region of the NR, where the dielectric constant is bulk-like. The values chosen are Reff = R −a0 and Leff = L − 2a0, where a0 is the lattice parameter. εin and εout are the high-frequency dielectric constants of the NR’s material and surrounding medium, respectively. The calculations estimated the Coulomb interactions of the NRs listed in Table 1 to vary between 50 to 70 meV, lowering the theoretical value of Eg and approaching the experimental values of the first exciton absorption band energy. (See Table 1.) Figure 4a shows a representative plot of calculated Eg (ignoring the Coulomb interaction) of the lowest energy valley in PbSexS1−x NRs with x = 0.5, grown in the ⟨110⟩ direction. The Figure reveals a large influence of the radius on the value of Eg; however, the contribution of the length becomes negligible above ∼10 nm. Figure 4b displays the dependence of Eg for NRs grown along the ⟨110⟩ direction versus the NRs’ radius and composition (with a fixed length of 10 nm). The plot exposes important flexibility in tailoring Eg with control of composition, when a fixed length is demanded. It should be mentioned that the current model is applicable mostly for NRs having a radius >3.0 nm, when evaluation by an effective mass model deviates from measured values for very small structures ( a0

∑ F *c′ j (re)Fvj(re) j

∑ Fcj ′(rh)Fv*′ j ′(rh)d3red3rh j′

(4)

using notations mentioned above. Calculated values of the exchange split Δex of the lowest energy excitons ([11̅1] and [1̅11] pair of valleys) are given in Table 1. This interaction split the exciton manifold into dark (forbidden) and bright (allowed) emission states. The typical exchange split in NRs is below 1 meV, one to two orders of magnitude smaller than the typical value of exchange splitting in IV−VI QDs.18,38 The relatively large exchange interaction in IV−VI QDs implies the occurrence of an emission process from a dark state at cryogenic temperatures but a dominant emission from a bright state at room temperature, rendering a large change in the radiative lifetime. (See Figure 3b.) In contrast, the current results showed that the variation of the emission lifetime is mild in NRs, in agreement with the small exchange split; hence, both dark and bright states are thermally populated over the wide temperature range under investigation and under off-resonance excitation.

5. CONCLUSIONS This work presented successful growth of ternary IV−VI NRs with varying length/diameter ratio and chemical composition. The cw-PL and tr-PL measurements, recorded at various temperatures, revealed a few distinctive properties of the ternary NRs with respect to binary NRs or binary/ternary QDs with similar energy band gap. They include thermal stability over a wide temperature range (small band-edge temperature coefficient); relatively small electron−phonon coupling, with significant merit when a cooling process of hot carriers should be avoided (enabling multiple exciton generation); and a negligible change in emission lifetime over a wide temperature range due to the extremely small exchange interaction so that radiation recombination is dominantly ruled by a bright exciton emission. The appearance of a satellite band in the cw-PL spectra was supported by the electronic band structure calculation, indicating the existence of emission from a pair of L-valleys. The current results revealed important benefits in ternary NRs, which could be significantly important when using them as active optical components in photovoltaic cells and gain devices.

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ASSOCIATED CONTENT

S Supporting Information *

Data of the local composition along the main axes of a single NR was measured using an EDX line scan in STEM mode with a spatial resolution of ∼1 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 18988

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Israel Science Foundation (Project Nos. 1009/07 and 1425/04), the USA-Israel Binational Science Foundation (No. 2006-225), and the European FP7 Nanomaterials for harvesting sub-band gap photons via upconversion to increase solar cell efficiencies (Nanospec) and self-assembled nanostructure system (SANS) projects.

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dx.doi.org/10.1021/jp303200b | J. Phys. Chem. C 2012, 116, 18983−18989