Optical Transparency Enabled by Anomalous Stokes Shift in Visible

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Optical Transparency Enabled by Anomalous Stokes Shift in Visible Emitting CuAlS2 Based Quantum Dots Biswajit Bhattacharyya, Triloki Pandit, Guru Pratheep Rajasekar, and Anshu Pandey J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01787 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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The Journal of Physical Chemistry Letters

Optical Transparency Enabled by Anomalous Stokes Shift in Visible 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

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 lower 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). Due to 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.

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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 due to 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 due to the earth abundance and benign nature of its constituent elements. Although bulk CuAlS2 is a wide gap semiconductor24-26, it forms type-II heterojunctions with CdS where the electron resides largely in the latter semiconductor.24,

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This type-II character permits the tuning of the emission of CuAlS2/CdS

structures across the visible spectrum, from 2.3 eV to 1.6 eV. Most notably, CuAlS2/CdS


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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 CuInS2>CuFeS2) follows the order expected from the conduction band offsets of these materials.24-25, 41

Figure 4. (a) Absorbance (brown line) and transient pump induced changes in absorbance ∆ (green dot) in CuAlS2/CdS QDs. (b) Bleach dynamics in CuAlS2/CdS QDs. Bleach is stable over the 100 ps duration of the experiment.


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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. In order 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 energetically consistent with the CdS band edge and suggests the relaxation of photogenerated electrons to the CdS conduction band, in accord with the behaviour expected from CuAlS2 and CdS band offsets. Further, 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 feature is long lived, and as shown in Figure 4b, 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. Further, 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.


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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%. Further, these films show an absolute quantum yield of 40% upon excitation with 400 nm light. In addition, as shown in Figure 5b inset, the optical spectra of the deposited films (black) are essentially identical to the emission spectra 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.

Figure 5. (a)Transmittance spectrum of a glass substrate (black), and a CuAlS2/CdS film 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 spectrum of QD solution (black), and QD on substrate (red circles). (c) Hand held device with a 34 mm x 22 mm clear aperture. Top: device in an off state held over paper in presence of ambient light. Bottom: device in the dark in an on state with the clear aperture emitting light.


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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) upto 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 hours. Motivated by these observations, we considered the possibility of utilizing these materials for transparent displays and lighting devices. Towards 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 x 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 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. Further, 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, that reduces the overlap of the lowest energy electron and hole wavefunctions. Our spectroscopic data further rule out the possibility of occurrence of the Stoke’s shift due to conduction band defects. We also demonstrate the vast potential utility of this shift for practical


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applications by using CuAlS2/CdS QDs to demonstrate a wide area transparent light emitting device with waveguided optical excitation and a 34 mm x 22 mm clear aperture.

Associated Content Supporting Information. Detailed synthetic procedures with all mode of characterization. Discussion about the degradation of pure CuAlS2 quantum dots and Supporting figures (S1-S9) and Table S1.

Corresponding Author [email protected]

ACKNOWLEDGMENT 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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Optical Transparency Enabled by Anomalous Stokes Shift in Visible

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