CdSeS Nanowires - American Chemical Society

Mar 11, 2014 - were mixed at the following molar ratios, Se:S = 0:1, 1:50, 1:25,. 1:15, 1:7, 1:5, 1:1, and 1:0. Subsequently, these CdSeS NWs are iden...
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CdSeS Nanowires: Compositionally Controlled Band Gap and Exciton Dynamics Jong-Pil Kim,†,‡ Jeffrey A. Christians,†,§ Hyunbong Choi,†,∥ Sachidananda Krishnamurthy,†,∥ and Prashant V. Kamat*,†,§,∥ †

Radiation Laboratory, §Department of Chemical and Biomolecular Engineering, and ∥Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Busan Center, Korea Basic Science Institute, Busan 618-230, Korea S Supporting Information *

ABSTRACT: CdS, CdSe, and ternary CdSexS(1−x) are some of the most widely studied II−VI semiconductors due to their broad range of applications and promising performance in numerous systems. Onedimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire, which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the [001] direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ∼10 ps with increasing Se; however, the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics that give them broad potential applications in the fields of optoelectronics, and photovoltaics. SECTION: Physical Processes in Nanomaterials and Nanostructures

O

can produce a series of solid solutions with tunable optical properties by adjusting the compositional ratio. This has been shown in a variety of CdSexS(1−x) morphologies, such as nanoparticles24−26 and various 1-D structures,27−32 where the band gap of the material can be tuned throughout the visible range. These CdSeS nanomaterials also offer high luminescence throughout the visible range, making them promising for applications in LEDs.25,26,31,33 The degree of control exhibited over its absorption and emission properties has sparked a great deal of recent interest in CdSeS nanomaterials.24−35 The merger of 1-D morphologies with the tunability of ternary semiconductors offers a new avenue for the optimization of these materials. To date, 1-D CdSeS nanomaterials have been made by various techniques such as evaporation,28,29 laser deposition methods,27,30,32 and patternassisted deposition,31 in contrast to the simple solution-based synthetic methods for CdSeS nanoparticles.24−26 In the present study, we report a hot injection synthesis for high quality CdSeS NWs using BiCl3 to catalyze growth. The solution-based synthetic preparation of these NWs is crucial to their future incorporation into devices. We show that the composition of

ne-dimensional (1-D) semiconductor nanomaterials show promise for use in fields ranging from photovoltaics and field effect transistors, to chemical and biological sensing.1−7 The anisotropic shape of nanowires (NWs) allows for tuning quantum effects by the wire diameter, while NWs are expected to provide better change transport properties than nanoparticle systems because of the continuous pathway for transport along the wire length.8−12 For example, in NW solar cells, the 1-D charge transport allows for the semiconductor NWs to serve as both photon absorber and the electron transporter.13 This eliminates the need for charge transport through a TiO2 nanoparticle network as in quantum dot solar cells,14,15 or exciton dissociation through a NW heterostructure.16 Therefore, these semiconductor NWs have shown great potential as building blocks for optoelectronic devices.2,13,14,17,18 For future implementation, it is desired that the band gap of these NW structures be widely tunable across the visible spectrum, more than is feasible by quantum confinement effects alone. This will allow for multifunctional devices that can better utilize and respond to all colors of visible light. Two of the most promising and widely studied II−VI semiconductors for optoelectronic applications are CdS and CdSe, which have been successfully incorporated into photovoltaics,19,20 sensors,21,22 and light-emitting diodes (LEDs).23 In addition, it has been shown that alloying of CdS and CdSe © 2014 American Chemical Society

Received: February 10, 2014 Accepted: March 11, 2014 Published: March 11, 2014 1103

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CdS NWs was measured to be approximately 12.1 ± 1.1 nm and 5.9 ± 0.5 nm for pure CdSe NWs. The ternary CdSeS (1:25) and CdSeS (1:5) NWs have a similar diameter measured as davg = 7.7 ± 0.6 nm. Previously, CdS and CdSe NWs have been reported to have various branched morphologies (vshapes, tripods, and y-shapes);39,40 however, in this study, transmission electron microscopy (TEM) images reveal the CdS, CdSeS, and CdSe NWs to all have a similar nonbranched morphology (Figure S1, Supporting Information). The atomic composition of the NWs was measured using energy dispersive X-ray spectroscopy (EDX). The atomic ratio of Cd, Se, and S for CdS, CdSeS (1:25), CdSeS (1:5), and CdSe NWs is reported in Table 1. As in the study by Jang et al.

these CdSeS NWs can be modulated from pure CdSe to CdS by simply adjusting the Se:S ratio of the injection precursor. Also, the absorption and emission properties of these NWs can be tuned across the visible spectrum without significantly changing their morphology or crystallinity. The control exerted over the optical properties as well as the high crystallinity of these NWs makes them ideal for photovoltaic and other optoelectronic applications. Nanowire Synthesis and Morphology. CdSexS(1−x) nanowires were synthesized by a hot injection synthesis similar to previously reported methods for CdSeS nanoparticles.25,33 Briefly, trioctylphosphine selenide/sulfide (TOPSe and TOPS) were mixed at the following molar ratios, Se:S = 0:1, 1:50, 1:25, 1:15, 1:7, 1:5, 1:1, and 1:0. Subsequently, these CdSeS NWs are identified by the Se:S ratio used in their synthesis (e.g., CdSeS synthesized with Se:S = 1:7 is denoted CdSeS (1:7)). In general, more sulfur was used than selenium on a mol/mol basis because the Cd precursor reacted faster with TOPSe than TOPS, as previously reported.33 Bismuth chloride (80 μL of 2 mM) was added to 0.5 mL of TOPSeS and the solution was injected into a solution of CdO in 1-octadecene and oleic acid at 300 °C. The reaction was allowed to proceed for approximately 2 min before being quenched by rapid cooling to room temperature. The resulting CdSeS NWs were washed 4 times with toluene to remove any residual nanoparticles. The BiCl3 is critical for promoting the formation of the large aspect ratio NWs, as evidenced by similar synthetic techniques without BiCl3 producing only CdSeS nanoparticles.25 The BiCl3 is thought to form Bi nanoparticles in situ, which is then able to catalyze the formation of NWs, as demonstrated previously.36−38 Control of the NW diameter was not attempted in this study, but should be possible by controlling the reactivity, reaction time, and/or concentration of the bismuth catalyst or cadmium precursor.37,38 Low magnification transmission electron microscopy (Figure 1) was used to investigate the structure and morphological details of the CdS, CdSeS, and CdSe NWs. The diameter of the

Table 1. Atomic Composition of CdSeS NWs Analyzed by Energy Dispersive X-ray Spectroscopy element

CdS

CdSeS (1:25)

CdSeS (1:5)

CdSe

Cd Se S

49.64 0 50.36

49.42 12.62 37.89

47.62 24.90 27.47

45.42 54.57 0

Units: atomic %.

for CdSeS nanocrystals,33 the cadmium precursor was found to react faster with TOPSe than TOPS for the formation of CdSeS NWs. The reactivity of S to Cd was also found to be nonlinear with respect to increasing sulfur content. Structural Characterization. CdS and CdSe have been known to crystallize with both wurtzite (hexagonal) and zinc blend (cubic) structures. For this reason, it is important to confirm the crystal structure of the synthesized NWs. The phase identification was carried out by X-ray diffraction (XRD) as shown in Figure 2A. For XRD measurements, a sample of the

Figure 2. (A) X-ray diffraction (XRD) pattern of (a) CdS, (b) CdSeS (1:25), (c) CdSeS (1:5), and (d) CdSe NWs. (B) Change in the (110) crystal plane d-spacing with increasing selenium content.

Figure 1. Low magnification TEM image of (A) CdS, (B) CdSeS (1:25), (C) CdSeS (1:5), and (D) CdSe nanowires. 1104

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lattice spacings of CdS (0.332 nm) and CdSe (0.349 nm) NWs correspond to the (002) lattice planes.42−44 This is in good agreement with the XRD results and further confirms that growth of all these CdSexS(1−x) NWs occurs along the [001] direction. Optical Characterization. The optical properties of the CdSeS NWs are important for optoelectronic applications. The steadystate absorption and emission properties were characterized by UV−visible absorption spectroscopy and photoluminescence measurements, respectively. The steady-state absorption spectra of the CdSeS NWs are shown in Figure 4A with the absorption of the NWs normalized at 400 nm. The optical band gap for the CdSeS NWs (Se:S = 0:1, 1:50, 1:25, 1:15, 1:7, 1:5, 1:1, and 1:0) can be estimated as a direct transition between ideal parabolic bands using the Tauc equation (eq 1),45

NWs was deposited onto an FTO substrate by electrophoretic deposition (EPD) from a mixed acetonitrile/toluene (1:2.5 v/ v) solution by applying 60 V DC for 10 min. The XRD patterns were indexed using JCPDS files (0020549 - CdS, and 0080459 CdSe). The XRD spectra show that all NWs crystallize in the Wurtzite phase. Figure 2A reveals that, as the composition of the CdSexS(1−x) nanowires is changed from x = 0 to 1, the peak arising from the (110) plane shifts to lower angle (2θ). From Bragg’s law, this peak shift to lower angle (2θ) indicates an increase in the lattice d-spacing, as shown in Figure 2B. The ionic radius of S2− is 0.17 nm, whereas the ionic radius of Se2− is 0.18 nm. Taken together, this indicates that the Se2− ions substitute at the S2− sites and cause a gradual shift from pure CdS to pure CdSe. This smooth shift occurs because of the good lattice parameter matching between the two crystals.41 The XRD analysis shows no evidence of phase segregation in the CdSeS NWs, confirming that it is possible to precisely tune the Se:S ratio of the resultant NWs by the Se:S ratio of the TOPSeS precursor solution. High-resolution TEM (HRTEM) images of CdSeS (1:25) and CdSeS (1:5) NWs are shown in Figure 3A,C. These typical

αhν = A(hν − Eg )(1/2)

(1)

where α is the absorption coefficient, hν is the photon energy in electron volts, A is a constant determined by the transition probability, and Eg is the band gap energy of the material. Therefore, to determine the band gap, a straight line was fit to a plot of (αhν)2 versus photon energy, as shown in Figure S2 in the Supporting Information, and the x-intercept of this fit was taken as the estimate of the optical band gap. The estimated optical band gap for each Se:S ratio is shown in Figure 4B. Similar to the crystal lattice spacing seen with XRD and TEM, the optical band gap can be tuned from 2.36 eV in pure CdS NWs to 1.79 eV in pure CdSe NWs simply by the stoichiometry of the TOPSeS precursor. In addition to the absorption spectra, the photoluminescence spectra of the CdS, CdSeS (1:25), CdSeS (1:5), and CdSe are shown in Figure 4C. Emission spectra were recorded using a quartz cuvette (1 cm optical path length) with a 420 nm excitation. The emission peak red shifts from 510 nm for pure CdS NWs to 680 nm for pure CdSe NWs. These results further confirm that the origin of the emission arises from band edge charge carrier recombination. As in CdSeS nanoparticles,25,33 the CdSeS NWs exhibit higher emission intensity than either CdS or CdSe NWs. Femtosecond Transient Absorption Spectroscopy. In combination with steady-state absorption measurements, femtosecond transient absorption spectroscopy is a powerful tool to probe the recombination dynamics of semiconductor systems.46,47 The excited state properties of these CdSeS NWs offer important insight into their optoelectronic properties. We carried out femtosecond transient absorption spectroscopy studies on the CdSeS NWs in degassed toluene solutions. Transient absorption spectra were recorded over a time window from 0 to 1600 ps following a 387 nm laser pulse excitation of CdSeS samples a−d. Photoexcitation by the 387 nm laser pulse forms an exciton which gives rise to a photobleaching of the CdSeS absorbance. This exciton can then decay through radiative and nonradiative processes resulting in a recovery of the photobleaching. Repeated measurements on the same sample gave reproducible traces, thus confirming the stability of these nanowires during the laser excitation. Femtosecond transient absorption spectra of samples a−d are shown in Figure 5A. The transient absorption spectra recorded immediately after the laser pulse excitation shows bleaching corresponding to the excitonic transitions. As is seen from these spectra, the photobleaching maxima of the CdSeS

Figure 3. High-resolution TEM images of (A) CdSeS (1:25) and (B) CdSeS(1:5). Insets show FFT electron diffraction images. Filtered lattice image of (C) CdSeS (1:25) and (D) CdSeS (1:5).

HRTEM images show the uniform lattice structure and single crystalline characteristic of the CdSeS NWs. The lattice spacing was analyzed using fast Fourier transform (FFT) electron diffraction (ED) patters of the CdSeS (1:25) and CdSeS (1:5) NWs as shown in the insets of Figure 3A,C, respectively. Figure 3B,D shows a filtered lattice image corresponding to the (001) planes of the FFT pattern in the red square shown in Figure 3A,C. From lattice images and FFT ED patterns, the lattice spacing of the synthesized CdS, CdSeS (1:25), CdSeS (1:5), and CdSe NWs were measured to be 0.332 nm, 0.333 nm, 0.336 nm, and 0.349 nm, respectively. The lattice spacing in the growth direction (axial direction) of the NWs was found, as expected, to increase with increasing Se content similar to the d-spacing of the (110) plane seen with XRD. The measured 1105

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Figure 4. (A) UV−visible absorption spectra and (C) The estimated optical band gap of CdSeS NW with varied Se:S ratio. (D) Emission spectra of (a) CdS at 494 nm, (b) CdSeS (1:25) at 499 nm, (c) CdSeS (1:5) at 532 nm, and (d) CdSe NWs 554 nm.

Figure 5. (A) Transient absorption difference spectra at a delay time of 70 ps after 387 nm laser pulse excitation, and (B) time-resolved kinetic decay monitored at 494, 499, 532, and 554 nm for (a) CdS, (b) CdSeS (1:25), (c) CdSeS (1:5), and (d) CdSe NWs, respectively.

NWs is red-shifted with increasing sulfur content, which is in agreement with the steady-state absorption spectra. With increasing time, the electron−hole pairs recombine, which is observed as the bleach recovery. Thus, the bleach recovery kinetics serve as a probe for the charge recombination processes. To investigate the dynamics of the charge carrier recombination, time-resolved kinetic traces were assembled from the absorption difference spectra at the photobleaching peak for each sample, as shown in Figure 5B. The recovery of the photobleaching was normalized and fit to a triexponential decay equation (eq 2).48

The shorter lifetime of the NWs with higher Se content presents a problem to overcome for eventual incorporation into optoelectronic devices. Previously, recombination in CdSe NWs has been attributed to either two or three body Auger recombination, depending on excitation intensity.47 Controlling these recombination processes will be integral in achieving the highest device performance. Toward this end, surface modification and doping are promising areas of further research.14,20,51 Currently, high rates of recombination limit the efficiency of NW solar cells compared to quantum dot solar cells.14,52 A significant challenge remains to incorporate NWs into devices and fully harness the advantages of their 1-D architectures. However, it remains to be seen how CdSeS NWs will ultimately perform in various optoelectronic devices. In conclusion, a single-step hot injection synthetic approach of designing CdSeS NWs offers a simple methodology for tuning the optical properties across the entire compositional range from pure CdS to pure CdSe. By varying the Se:S precursor ratio we were able to tune the optical band gap of the CdSeS NWs from 2.36 to 1.79 eV. These CdSeS NWs were found to be highly crystalline, Wurtzite phase, and grew along the [001] direction. In addition to changes in the optical band gap, we find that Se:S composition also influences the excited state dynamics of the NWs. Using transient absorption spectroscopy, it is shown that the contribution of the short, ∼10 ps, component of the photobleaching recovery increases from 8.4% to 57.7% with increasing Se content. Precise control of both the optical and excited state properties of these CdSeS NWs should lead to new uses in the fields of photovoltaics and optoelectronics.

k

y=

∑ Aie(−x /τ ) i

i

(2)

This fitting yields the decay lifetime (τi) and weighting coefficient (Ai) representing the contribution of each decay lifetime to the normalized photobleaching recovery. For these samples, the average short lifetime of the decay is similar for all samples (τ1 = 10.3 ± 3.9 ps). On the other hand, we observe an increase in the weighting coefficient of this lifetime with increasing Se content. This weighting coefficient is calculated as 8.4%, 42.9%, 50.3%, and 57.7% for CdS, CdSeS (1:25), CdSeS (1:5), and CdSe NWs, respectively. This increased contribution from the short lifetime component with increasing selenium content shows that the addition of Se effects both the optical properties and the exciton dynamics of the CdSeS NWs. A similar decrease in lifetime corresponding to increasing Se content has been observed in CdSeS nanoparticles.25,49,50 1106

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EXPERIMENTAL METHODS Materials. Bismuth chloride (BiCl3 , Aldrich, 99.999%), cadmium oxide (CdO, Alfa Aesar, 99.998%), hexadecylamine (Aldrich, 98%), methanol (Fisher Scientific, Certified ACS grade), 1-octadecene (Aldrich, 90%), oleic acid (Aldrich, 90%), selenium powder (Aldrich, 99.99%), sulfur powder (Alfa Aesar, 99%), toluene (Fisher Scientific, HPLC grade), and trioctylphosphine (TOP, Aldrich, 99%). All chemicals were used without further purification. Nanowire Synthesis. CdS, CdSe, and CdSeS NWs were prepared by the injection of a mixture of selenium and sulfur precursor into the cadmium precursor based on previously reported methods.25,33 First, a mixture of CdO (64 mg), oleic acid (0.8 mL), hexadecylamine (25.6 mg), and 1-octadecene (5 mL) were heated with stirring in a three neck flask at 80 °C, while purging continuously with nitrogen, for 1 h. The solution is then heated to 300 °C where it became clear. Selenium and sulfur precursors were made separately by dissolving Se and S powder in trioctylphosphine (TOP) under a nitrogen atmosphere, forming TOPSe and TOPS, respectively. These were then mixed with the desired ratios in TOP to form 0.5 mL of 1 M TOPSeS precursor used for injection. The selenium to sulfur molar ratios used for the TOPOSeS were 0:1, 1:50, 1:25, 1:15, 1:7, 1:5, 1:1, and 1:0. Subsequently, 80 μL of 2 mM BiCl3 in acetone was added to the 0.5 mL TOPSeS solution. This TOPSeS/BiCl3 solution was then injected into the hot CdO solution and maitained at temperature (275−280 °C) for ∼2 min following injection. The reaction mixture was then rapidly cooled to room temperature. The NWs were precipitated by adding a mixture of toluene and acetone (3:1 v/ v) to the solution followed by centrifugation. The NWs were then washed four times in toluene to remove residual nanoparticles and the products were collected by centrifugation and stored in toluene under an inert atmosphere. Material Characterization. X-ray diffraction (XRD) measurements were performed using a Bruker D8 X-ray diffractometer with a scan rate of 2° min−1 from 2θ values of 20° to 70° employing Cu Kα radiation (λ = 1.5406 Å). TEM images were obtained using an FEI Titan 80−300 transmission electron microscope operated at 300 kV. Samples for TEM were prepared by dipping a carbon coated copper grid (300 mesh) into a toluene solution of purified NWs. Energy dispersive Xray spectroscopy (EDX) was conducted using the built-in X-ray detector on the Titan TEM. The Cd, Se, and S content of the NWs were measured using a Costech Instruments ECS 4010. Optical Characterization. The optical properties of CdS, CdSeS, and CdSe NWs in the tolune solutions were characterized using UV−visible absorbance spectroscopy (Varian Cary 50 Bio Spectrophotometer) and photoluminescence measurements (Jobin Yvon Fluorlog-3, Horiba Fluorolog spectrophotometer). A 400 nm filter was introduced to exclude scattering from the excitation source. CdS, CdSeS, and CdSe NWs in the toluene solutions were purged using pure nitrogen gas for 10 min. Femtosecond Transient Absorption. Femtosecond transient absorption measurements were performed using a Clark MXR 2010 (775 nm fundamental, 1 mJ/pulse, fwhm = 130 fs, 1 kHz repetition rate) and an Ultrafast Systems (Helios) detection system. The fundamental laser output (775 nm) was split (95%/5%) into pump and probe beams, respectively. The probe beam passed through an optical delay stage to provide a time window of 1600 ps and focused on a Ti:sapphire crystal to

provide a white light continuum. The pump beam was directed through a second harmonic frequency doubler to produce the 387 nm pump beam used in all experiments. Pump fluence was attenuated to 17 μJ/cm2. Time-resolved absorption spectra were recorded using nitrogen purged toluene solutions of CdS, CdSe, and CdSeS NWs in quartz optical cells. Kinetic traces were assembled at the appropriate wavelengths from the timeresolved data.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images showing NW morphology, and Tauc plot of absorption edge used for determining optical band gap. This information 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Basic Science Institute (KBSI) Grant visiting researcher program and the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This is contribution number NDRL No. 5008 from the Notre Dame Radiation Laboratory. Dr. Jong-Pil Kim wishes to express his thanks to the University of Notre Dame for granting this research opportunity and to express his appreciation to Professor Kamat and the other colleagues with whom he has worked on this project.

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(52) Choi, H.; Kamat, P. V. CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter. J. Phys. Chem. Lett. 2013, 4, 3983−3991.

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