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Cite This: J. Phys. Chem. Lett. 2018, 9, 4451−4456
Optical Transparency Enabled by Anomalous Stokes Shift in Visible Light-Emitting CuAlS2‑Based Quantum Dots Biswajit Bhattacharyya, Triloki Pandit, Guru Pratheep Rajasekar, and Anshu Pandey* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
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S Supporting Information *
ABSTRACT: We observe and study the anomalous Stokes shift of CuAlS2/CdS quantum dots. While all known I−III−VI2 semiconductor core/shell quantum dots show Stokes shifts in excess of 100 meV, the shift associated with CuAlS2/CdS quantum dots is uniquely large, even exceeding 1.4 eV in some cases. CuAlS2/CdS quantum dots are thus associated with cross sections less than 10−17 cm2 under the emission maximum. We investigate this anomaly using spectroscopic techniques and ascribe it to the existence of a strong type-II offset between CuAlS2 and CdS layers. Besides their strong Stokes shift, CuAlS2/CdS quantum dots also exhibit high quantum yields (63%) as well as long emission lifetimes (∼1500 ns). Because of the combined existence of these properties, CuAlS2/CdS quantum dots can act as tunable, transparent emitters over the entire visible spectrum. As a demonstration of their potential, we describe the construction of a wide area transparent lighting device with waveguided optical excitation and a clear aperture of 7.5 cm2.
a typical synthesis, CuAlS2 QDs were prepared using a previously reported method28 (see details in the Supporting Information). Figure S1a shows the X-ray diffraction (XRD) pattern of CuAlS2 particles (red curve) along with the bulk powder pattern of CuAlS2 (black), which is consistent with phase-pure tetragonal chalcopyrite structured CuAlS2 QDs. The ensemble level structure is further confirmed at a singleparticle level by electron microscopy. In particular, Figure S1b exemplifies a high-resolution transmission electron microscopy (HRTEM) image of CuAlS2 QDs. Furthermore, a lowerresolution electron micrograph is shown in the inset of Figure S1b. The HRTEM image shows the existence of planes consistent with the ensemble XRD patterns. For example, the (112) planes are indicated by red lines in Figure S1b. To further verify phase purity of the CuAlS2 QDs, we analyzed these through scanning transmission electron microscopy (STEM). A low-resolution STEM image shown in Figure S1c further confirms the existence of a monodisperse (9.8%) phase-pure ensemble. The composition homogeneity was further verified through a high annular aperture dark field (HAADF) STEM methodology. These results are shown in Figure S1d−g. The individual images in Figure S1e−g correspond to copper (Figure S1e), sulfur (Figure S1f), and aluminum (Figure S1g) occurrences. As is clear from these images, all three elements are distributed uniformly across all QDs, implying a high degree of compositional purity and homogeniety. Additional confirmation of composition and compositional purity was obtained through an energy-
I−III−VI2 semiconductor quantum dots (QDs) have been the subject of numerous studies that probe both their applications as well as their fundamental properties. From the viewpoint of applications, I−III−VI2 semiconductors1−4 offer significant advantages over II−VI and III−V semiconductors5−9 particularly because of their benign composition.10,11 Besides the suitability of several I−III−VI2 materials for applications of scale,12−19 these semiconductors also play host to several interesting physical phenomena, such as ferroelectricity,20,21 unusual radiative emission lifetimes, etc.12,22,23 Here we study the anomalous optical properties of CuAlS2/ CdS core/shell structures. CuAlS2 is an attractive member of the I−III−VI2 family because of the earth abundance and benign nature of its constituent elements. Although bulk CuAlS2 is a wide gap semiconductor,24−26 it forms type-II heterojunctions with CdS where the electron resides largely in the latter semiconductor.24,27 This type-II character permits the tuning of the emission of CuAlS2/CdS structures across the visible spectrum, from 2.3 to 1.6 eV. Most notably, CuAlS2/ CdS structures show an exceptionally large Stokes shift, as high as 1.4 eV in certain cases. As an outcome of this Stokes shift, CuAlS2/CdS QDs show optical cross sections CuFeS2) follows the order expected from the conduction band offsets of these materials.24,25,41 It is further important to note that the above mechanism does not preclude the existence of valence band defects in I− III−VI2 semiconductors but rather provides an additional channel through which the Stokes shift may be enhanced in specific materials. To further confirm the above mechanism, and to preclude the existence of defects peculiar to CuAlS2, we studied the properties of these materials using transient absorption (TA) spectroscopy. Cleaned CuAlS2/CdS QDs were dispersed in hexane. Samples were illuminated with a pump wavelength of 400 nm derived from a Coherent Libra 100 fs, 4 mJ Ti:sapphire laser with a pump wavelength of 400 nm. Electron dynamics at the band edge was probed using a chirp-corrected white light probe derived from a sapphire plate. Samples were stirred to avoid degradation. The pump power was maintained at a level that leads to the formation of 0.56 excitons per QD. A strong pump-induced bleach feature is observed at 2.5 eV (Figure 4a, dashed green curve). This
feature is long-lived, and as shown in Figure 4b, it does not decay measurably over the course of the TA experiment. The presence of a long-lived bleach feature implies the absence of conduction band defects in CuAlS2/CdS and is entirely consistent with the large CuAlS2 and CdS band offset being the cause of the Stokes shift. The large Stokes shift as well as high PL QYs of these materials makes them exciting candidates for optoelectronics. Furthermore, as verified through our TA studies (Figure 4a), these materials do not show a measurable photoinduced absorbance, which ensures that their advantages persist even in situations demanding high-power excitation. Thus motivated, we examined the optical characteristics of CuAlS2/CdS films in greater detail. In particular, as shown in Figure 5a, CuAlS2/CdS QD films may be engineered to exhibit significant optical transparency in the visible region. This figure compares the light transmitted through a glass substrate (black) with the transmission of a QD-coated substrate (red). As shown in this figure, the deposition of the QD film causes a minor 7% change in transmission at 400 nm, while the mean change in transmission due to QDs beyond 450 nm is approximately 1.5%. Furthermore, these films show an absolute quantum yield of 40% upon excitation with 400 nm light. In addition, as shown in the inset of Figure 5b, the optical spectrum of the deposited films (black) is essentially identical to the emission spectrum of QDs in solution (red circles). The high film quantum yields and invariance of spectra are consistent with the low self-absorption losses in these materials. Other optical characteristics of these materials are also amenable for the intended usage as phosphors. In particular, the emission intensity increases linearly with power (Figure S10) up to an excitation energy of ∼90 mW/cm2. We further verified film stability in the ambient by using input light intensity of 38 mW/cm2 with a 405 nm laser light source (Figure S10). As shown in Figure 5b, the films are observed to be stable for long periods of time, in this case over 2 h. Motivated by these observations, we considered the possibility of utilizing these materials for transparent displays and lighting devices. Toward this end, we drop-cast films of these QDs on an 8 mm thick optically clear glass substrate. A set of eight 0.03 mW 405 nm LEDs are used as a pump source on the sides of the glass substrate. As shown in Figure 5c (top), the device has a clear aperture of 34 mm × 22 mm and appears transparent to the eye. This figure shows the device held above printed text under ambient lighting. The lower panel of Figure 5c shows the device in the same configuration in an “on” state in the absence of other light sources. In conclusion, we describe the properties of CuAlS2-based core/shell QDs. We find that CuAlS2/CdS core/shell QDs exhibit a gradient alloy structure, with the outer layers being more CdS enriched. Furthermore, CuAlS2/CdS QDs exhibit high quantum yields (as large as 63% in this work) and emission tunability in the visible−near-infrared region. Most significantly, these core/shell structures are associated with an unusually large Stokes shift that can be as high as 1.4 eV with respect to the most prominent absorption feature in some cases. This large Stokes shift is ascribed to the large band offset between CuAlS2 and CdS regions of the core/shell structure, which reduces the overlap of the lowest-energy electron and hole wave functions. Our spectroscopic data further rule out the possibility of occurrence of the Stokes shift due to conduction band defects. We also demonstrate the vast
Figure 4. (a) Absorbance (black line) and transient pump-induced changes in absorbance (Δα) (green dot) in CuAlS2/CdS QDs. (b) Bleach dynamics in CuAlS2/CdS QDs. Bleach is stable over the 100 ps duration of the experiment.
feature is energetically consistent with the CdS band edge and suggests the relaxation of photogenerated electrons to the CdS conduction band, in accord with the behavior expected from CuAlS2 and CdS band offsets. Furthermore, this bleach feature also has a large 0.8 eV separation from the emission band maximum (Figure 4a, red curve). It is observed that this 4454
DOI: 10.1021/acs.jpclett.8b01787 J. Phys. Chem. Lett. 2018, 9, 4451−4456
Letter
The Journal of Physical Chemistry Letters
Figure 5. (a) Transmittance spectra of a glass substrate (black) and a CuAlS2/CdS film (red) on the substrate. (b) Emission stability of CuAlS2/ CdS QD film on glass when irradiated with 38 mW/cm2 of 405 nm light. Inset: Emission spectra of QD solution (black) and QD on substrate (red circles). (c) Hand-held device with a 34 mm × 22 mm clear aperture. Top: device in an off state held over paper in the presence of ambient light. Bottom: device in the dark in an on state with the clear aperture emitting light. Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012−1057. (5) Yu, W. W.; Peng, X. Formation of High-Quality CdS and Other II−VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angew. Chem., Int. Ed. 2002, 41, 2368−2371. (6) Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection. Angew. Chem., Int. Ed. 2005, 44, 6712−6715. (7) Battaglia, D.; Peng, X. Formation of High Quality InP and InAs Nanocrystals in a Noncoordinating Solvent. Nano Lett. 2002, 2, 1027−1030. (8) Li, L.; Reiss, P. One-pot Synthesis of Highly Luminescent InP/ ZnS Nanocrystals without Precursor Injection. J. Am. Chem. Soc. 2008, 130, 11588−11589. (9) Cros-Gagneux, A.; Delpech, F.; Nayral, C.; Cornejo, A.; Coppel, Y.; Chaudret, B. Surface Chemistry of InP Quantum Dots: A Comprehensive Study. J. Am. Chem. Soc. 2010, 132, 18147−18157. (10) Sandroni, M.; Wegner, K. D.; Aldakov, D.; Reiss, P. Prospects of Chalcopyrite-Type Nanocrystals for Energy Applications. ACS Energy Lett. 2017, 2, 1076−1088. (11) Reiss, P.; Carrière, M.; Lincheneau, C.; Vaure, L.; Tamang, S. Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials. Chem. Rev. 2016, 116, 10731−10819. (12) Zhong, H.; Wang, Z.; Bovero, E.; Lu, Z.; van Veggel, F. C. J. M.; Scholes, G. D. Colloidal CuInSe2 Nanocrystals in the Quantum Confinement Regime: Synthesis, Optical Properties, and Electroluminescence. J. Phys. Chem. C 2011, 115, 12396−12402. (13) Park, J.; Dvoracek, C.; Lee, K. H.; Galloway, J. F.; Bhang, H. e. C.; Pomper, M. G.; Searson, P. C. CuInSe/ZnS Core/Shell NIR Quantum Dots for Biomedical Imaging. Small 2011, 7, 3148−3152. (14) Bhattacharyya, B.; Pandey, A. CuFeS2 Quantum Dots and Highly Luminescent CuFeS2 Based Core/Shell Structures: Synthesis, Tunability, and Photophysics. J. Am. Chem. Soc. 2016, 138, 10207− 10213. (15) Chen, B.; Pradhan, N.; Zhong, H. From Large-Scale Synthesis to Lighting Device Applications of Ternary I−III−VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters. J. Phys. Chem. Lett. 2018, 9, 435−445. (16) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. Synthesis of CuInS2, CuInSe2, and Cu(InxGa1‑x)Se2 (CIGS) Nanocrystal “Inks” for Printable Photovoltaics. J. Am. Chem. Soc. 2008, 130, 16770−16777. (17) Du, J.; Du, Z.; Hu, J.-S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; Wan, L.-J. Zn−Cu−In−Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201−4209. (18) Song, W.-S.; Yang, H. Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/ Shell Quantum Dots. Chem. Mater. 2012, 24, 1961−1967. (19) Ghosh, S.; Avellini, T.; Petrelli, A.; Kriegel, I.; Gaspari, R.; Almeida, G.; Bertoni, G.; Cavalli, A.; Scotognella, F.; Pellegrino, T.;
potential utility of this shift for practical applications by using CuAlS2/CdS QDs to demonstrate a wide area transparent light-emitting device with waveguided optical excitation and a 34 mm × 22 mm clear aperture.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01787. Detailed synthetic procedures with all modes of characterization, discussion about the degradation of pure CuAlS2 quantum dots, Figures S1−S10, and Table S1 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Biswajit Bhattacharyya: 0000-0003-2513-5676 Anshu Pandey: 0000-0003-3195-1522 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.B. acknowledges Indian Institute of Science for research fellowship. A.P. acknowledges DST Nano Mission [SR/NM/ NS-1117/2012] and ISRO-IISc-STC [ISTC/CSS/AP/0403] for funding. The spectroscopic studies were performed using facilities created under an IRHPA grant [IR/S2/PU-0005/ 2012].
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REFERENCES
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