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natural dye core- shell quantum dots
Journal Pre-proof AN EXPLORATION INTO THE QUANTUM CONFINEMENT OF CTS/NATURAL DYE CORE- SHELL QUANTUM DOTS
Maya Mathew, K.C. Preetha PII:
S0921-4526(19)30792-6
DOI:
https://doi.org/10.1016/j.physb.2019.411913
Reference:
PHYSB 411913
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
28 October 2019
Accepted Date:
24 November 2019
Please cite this article as: Maya Mathew, K.C. Preetha, AN EXPLORATION INTO THE QUANTUM CONFINEMENT OF CTS/NATURAL DYE CORE- SHELL QUANTUM DOTS, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411913
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AN EXPLORATION INTO THE QUANTUM CONFINEMENT OF CTS/NATURAL DYE CORE- SHELL QUANTUM DOTS 1Maya
1Payyannur
2Sree
Mathew, 1,2*KC Preetha
College, Payyannur, India PIN- 670327
Narayana College, Kannur, India PIN- 670007
*Corresponding Author: [email protected]
Abstract: In this work, we have presented a simple way of changing the confinement energies of Copper Tin Sulphide (CTS) quantum dots using natural dyes as shell material. Tetragonal CTS quantum dots in the size range of 1.7nm- 2.2nm, of bandgaps of 2.48eV and 5.0 eV were prepared by means of a green colloidal synthesis technique. These quantum dots were treated with natural dyes such as onion and beetroot skin dyes. Pelargonidin and Betanin (pigments of onion and beetroot skin dye respectively) formed hydrogen bonding with the capping agent, thus forming a shell around the CTS quantum dots. The change in confinement due to the effect of dye as shell was studied from absorption, photoluminescence and infrared spectroscopic techniques. The transitions occurring were analysed using a theoretical approach. CTS quantum dots, with its high transmittance in a wide range of wavelengths find promising applications in the buffer layer of solar cells.
Keywords: Copper tin sulphide; quantum dots; quantum confinement; Pelargonidin; Betanin; colloidal synthesis
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1. INTRODUCTION Silicon solar cells, though of much demand commercially are being replaced by other semiconductor materials, one of them being ternary materials. Copper tin sulphide is one among such materials on which wide research has been made in bulk [1], nanorods [2] and in thin film [3] forms. It is a p-type material with a high absorption coefficient in the order of 105/cm suitable for photovoltaic applications. Photovoltaic band gaps in the range of 0.9eV- 1.35 eV have been achieved by changing the confinement of the material. Research is going on widely for increasing the efficiency of absorber layer material. An attempt in this direction is the development of tandem solar cells which can potentially absorb radiation from a wide range of the solar spectrum and thereby increase the efficiency of solar cells. Tandem solar cells have multiple absorber layers with high absorption coefficient in different regions of the electromagnetic spectrum as they are composed of different materials with different bandgaps. With quantum dots, such bandgap tuning becomes easier, simply by changing the size of the quantum dot. Santanu Pradhan et.al.have used PbS colloidal quantum dots as LEDs to gain an external quantum efficiency of 7.9% [4]. Quantum dots act as hole transporters in perovskite solar cells which increases the stability and efficiency [5,6].Use of quantum dots in bulk heterojunctions enable efficient charge extraction and in this way utilize the solar energy well [7]. Quantum dots are also efficiently used as electron collection centers in interfaces to prevent recombination of charge carriers upon irradiation [8]. The use of Copper Tin Sulphide quantum dots as photodetectors and in solar cells have been investigated by Sandra Dias et.al. [9,10].Works related to the synthesis of CTS core shell quantum dots are few in literature. Certain Cu based ternary quantum dots with ZnS as the shell material have been mentioned in a review paper by Xue et.al. ZnS is more lattice matched with
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these Cu based ternary materials. Berends and Stam have widely investigated upon these core shell quantum dots at different reaction temperatures and also by changing the reactivity of the precursors [11]. Rupam Dutta at.al. studied the interaction of carbon dots with fluorescent dyes and found that fluorescence intensity of the former gets quenched in the presence of the latter and photoinduced electron transfer was the cause found behind the quenching [12].Parul Bansal et.al. studied the electron transfer between perovskite nanocrystals and dyes, with green nanoparticles showing Forster Resonance Energy Transfer (FRET) and blue nanoparticles showing luminescence quenching [13].Monomethine cyanine dye aggregated onto the surface of CdS quantum dots by electrostatic force of attraction was found to enhance the photospectral response and photoconversion efficiency [14]. The study on CTS quantum dots is few and especially the studies with natural dyes are rare. So this paper intends to bring about an investigation into the properties of CTS quantum dots mixed with natural dyes so as to design an inorganic- organic core- shell quantum dot. The synthesis techniques used so far for CTS quantum dots are quite expensive. This paper also discusses an economic and green synthesis technique for the preparation of CTS quantum dots which yielded remarkable material properties.
2. EXPERIMENTAL 2.1. MATERIALS AND METHODS
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For the synthesis of copper tin sulphide (CTS) quantum dots, room temperature colloidal synthesis technique was adopted. Copper chloride (CuCl2.2H2O), Tin chloride (SnCl2.2H2O) and Sodium sulphide (Na2S) were used as precursors with cetly trimethylammonium bromide (CTAB) as the capping agent and water as the solvent. 2.2. SYNTHESIS OF CTS QUANTUM DOTS Millmolar solutions of copper chloride (CuCl2.2H2O) and Tin chloride (SnCl2.2H2O) were mixed followed by the addition of the capping agent. Millimolar solution of sodium sulphide (Na2S) was added drop wise to the mixture alongwith constant stirring. The whole mixture was stirred for about half an hour till a pale brown coloured solution was obtained. The sample was named CTS. For the study of interaction of CTS quantum dots with dyes, we have chosen natural dyes from onion skin and beetroot skin. CTS was added in 2ml, 3ml and 4ml to orange- red coloured solution of onion skin dye with pH 3.These samples were named as OCTS2, OCTS3 and OCTS4 respectively.The same procedure was repeated with scarlet red beetroot dye of pH 5 also. The set of samples were named as BCTS2, BTCS3 and BCTS4 respectively. 2.3. CHARACTERIZATION Diffraction pattern was obtained from SAED image; size and morphology from TEM images using JEOL/JEM Transmission Electron Microscope. Surface studies were made by Thermo Fisher Scientific Nicolet iS50 FT-IR Spectrophotometer and optical studies were made by Cary 5000 (Varian) UV- VIS- NIR Spectrophotometer.
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3. THEORY The behaviour of quantum dots in a strong confinement regime can be depicted as particles in a spherical quantum well. Owing to strong confinement, the columbic force of attraction between the charge carriers can be neglected and the Schrodinger equation for such a system can be written as [15]: ― ħ2 2 2𝑚 ∇
𝐻=
(1)
+ 𝑈(𝑟),
𝑟 = 𝑥2 + 𝑦2 + 𝑧2 The Hamiltonian can be rewritten in spherical polar coordinates as shown below:
𝐻=
― ħ2 1 𝑑 [ 2𝑚 𝑟2𝑑𝑟
(
∂𝛹 𝑟2 ∂𝑟
)+
𝑑 𝑟2𝑠𝑖𝑛𝜃𝑑𝜃 1
(𝑟
∂𝛹 𝑠𝑖𝑛𝜃 ∂𝜃
2
)+
1
∂2𝛹
𝑟2𝑠𝑖𝑛2𝜃 ∂𝜙2
(2)
And the wavefunction can be written as follows:
(3)
𝛹 = 𝑅(𝑟) 𝛩(𝜃) Ф(𝜙) 𝛹𝑛𝑙𝑚(𝑟,𝜃,𝜙) =
𝑢𝑛𝑙(𝑟) 𝑟
𝑌𝑙𝑚(𝜃,𝜙)
WhereYlm are the spherical Bessel functions and u(r) satisfies an equation: ― ħ2∂2𝑢 2𝑚 ∂𝑟2
+ [𝑈(𝑟) +
ħ2
𝑙(𝑙 + 1)]𝑢 = 𝐸𝑢
2𝑚𝑟2
Where Angular momentum quantum number, L=l(l+1), l=0,1,2,3... Magnetic quantum number Lz = mħ, m=0,±1, ±2, ±3… U(r)= 0,r
(4)
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=∞, r>a The energy eigenvalues are given by: ħ2𝜒2𝑛𝑙
𝐸𝑛𝑙= 2𝑚𝑎2
(5)
Where, Χnl2 are the roots of Bessel function, a is the diameter of the quantum dot and all other terms have their usual meaning. The energy of electron state: 𝐸𝑒=
The energy of the hole state: 𝐸ℎ=
ħ2𝜒2𝑛𝑙
― 2𝑚𝑎2 ħ2𝜒2𝑛𝑙
𝐸𝑔 + 2𝑚𝑎2
Depending upon the above mentioned eigenvalues, the energy levels of the quantum dot can be depicted as shown in figure1.
4. RESULTS AND DISCUSSION
Before going into discussion of CTS quantum dots with natural dyes, description on the structural and optical properties of CTS quantum dots have been described in the first subsection.
4.1. ANALYSIS OF CTS QUANTUM DOTS 4.1.1. Transmission Electron Microscopy of CTS quantum dots CTS quantum dots were prepared by a colloidal synthesis technique using appropriate precursors in a reaction medium of pH 8 which is found to be the optimum pH for its synthesis. Quantum dots of sizes ranging from 1.7nm- 2.2nm were formed as evident from the TEM
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image shown in figure2(a). The sizes of the quantum dots are much smaller than the Bohr radius for CTS [16] indicating strong confinement regime. Hence a non-interacting particle model, as described in the theory above, can be applied here, where the columbic force of attraction between the charge carriers can be neglected. The diffraction pattern of the quantum dots, from Selected Area Electron Diffraction (SAED) as depicted in figure2(b)) confirms that tetragonal Cu2SnS3 quantum dots have been formed, matching well with JCPDS 089-4714. The information on d-spacing and diffraction planes have been tabulated in table1.The size dispersity of the synthesized quantum dots is found to be less as indicated from the particle distribution curve in figure2(c).
4.1.2. Optical properties of CTS quantum dots
Bandgap of CTS quantum dots is direct in nature as evident from the absorption spectrum shown in figure 3(a). The absorptions are in the ultraviolet region indicating the formation of nanosized particles. Being a direct bandgap semiconductor, the bandgap can be found by drawing tangents from the absorption maximum to the x-axis. Quantum dots have discrete absorption spectrum and so there is a possibility of multiple bandgaps corresponding to 2.48eVand 5.0 eV. The transmittance curve of CTS quantum dots is shown in figure 3(b). It has almost 100% transmittance in the near ultra-violet, visible and near infra red regions. This property of CTS quantum dots can be used efficiently in the window layer of solar cells.
4.1.3. Study of surface properties by FTIR
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The confinement of quantum dots is achieved by means of a growth arresting agent called the capping agent. Here the capping agent used is cetyl- trimethyl ammonium bromide (CTAB). The confinement of the quantum dot depends on the degree of capping. Inorder to study the surface properties of CTS quantum dots, FTIR of the sample was recorded and is shown in figure 4(a). The spectrum gives the vibrations corresponding to that of CTAB (figure 4(b)) [17] which confirms the presence of CTAB as a shield over the quantum dots. The functional groups which caused the vibrations have been tabulated in table2. Large shifts in wavenumbers have been noticed compared to the vibrations of CTAB which is due to the presence of hydrogen bonding between the hydrogen atoms of the hydrocarbon chain of CTAB and the electronegative oxygen of water molecules in the solvent. CTAB, a cationic surfactant,has a positive hydrophilic head which binds the quantum dot and a hydrophobic tail which is in contact with the solution. The hydrophobic tail prevents agglomeration and dissolution of the quantum dots in the solvent. Due to the hydrophobic nature of the hydrocarbon tail, there is a possibility of curling at the quantum dot surface inorder to prevent its interaction with water. Shift in energies of characteristic vibrations of functional groups of the surfactant as shown in table2 may also be due to this curling. 4.2. STUDY OF INTERACTION OF CTS QUANTUM DOTS WITH NATURAL DYES Here a study is made on the change in optical and surface properties of CTS quantum dots when mixed with natural dyes.
4.2.1. Interaction of CTS quantum dots with Onion dye The absorption spectra of CTS, OCTS2, OCTS3 and OCTS4 have been plotted in figure 5(a). All the samples have multiple absorption peaks with the absorbance of each transition,
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decreasing as the dye in the mixture decreases. The transmittance curves have been plotted in figure 5(b). The transmittance of the samples goes on decreasing with increasing amount of dye in the quantum dot- dye mixture. The onset of constant transmittance has shifted to the visible region, compared to CTS, for the quantum dot- dye mixture. To get an apparent picture of the number of transitions in these quantum dots and bandgaps, photoluminescence spectra of these samples were recorded as shown in figure6.
All the dye treated samples have three PL peaks corresponding to three different transitions. The peaks appearing as shoulders or satellites have been shown by deconvolution. The wavelength corresponding to each PL peak gives the bandgap of the material. This indicates that there is a blueshift in the bandgap of the samples when the amount of dye in the mixture decreases. In other words, there is a variation in size of the quantum dots which is responsible for the bandgap change. One possibility is the formation of a core shell structure with CTS as the core and the onion dye molecules forming the shell. It must be the variation in shell size which is responsible for the change in overall size of the quantum dot, thereby changing the bandgap. The formation of a shell is possible only if there is an interaction of the onion dye molecules with the capping agent of the quantum dot. Taking the average size of CTS quantum dots to be 1.95nm, found from TEM image, the effective mass of electrons in CTS quantum dots is found to be 5.44 X 10-32 kg using Brus equation. Using this value of effective mass of electron, the thickness of the shell of onion dye formed around the CTS quantum dots is determined by simple mathematical calculations and is tabulated in table3.
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It is observed that lower the shell thickness, higher will be the band gap.Bandgap increases for highly confined quantum dots and so incorporation of dye molecules around the quantum dot decreases the confinement energy. The PL intensity of CTS/onion dye core shell quantum dots have also been tabulated in table3. PL intensity decreases with increasing shell thickness because PL intensity is directly proportional to confinement of charge carriers. So as the confinement of charge carriers to the core increases, this is the case for smaller core/shell quantum dots, with lower shell thickness, the PL intensity increases.
Figure7(a) shows the molecular structure of onion dye pigment called Pelargonidin. Pelargonidin is less polar compared to water. So the hydrophobic hydrocarbon chain of CTAB, capping CTS quantum dots, would prefer to bind with Pelargonidin by means of hydrogen bonding as shown in figure 7(b) thus acting as a shell material over the CTS quantum dots. This results in overall change in size of the quantum dot and thereby changes the optical properties. This binding of CTAB with Pelargonidin can be further confirmed from the FTIR spectra shown in figure 8. From table4 it is evident that all the four spectra have the same set of dips but with minor differences in the position of dips. Only the absorptions corresponding to –CH stretching have shifts in energy which indicates that hydrogen bonding has occurred only with the hydrogen atoms exposed to the solvent. Very few hydrogen bonds have been formed with the -NH functional group as there is almost no shift in energy. All the spectroscopic analyses reveal that the quantum dot- onion dye mixtures form core-shell quantum dots with CTS as the core and the onion dye molecules as the shell. Considering this, an investigation into the change in confinement of these core—shell CTS/onion dye quantum
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dots were made from PL data. The main objective being fulfilled in this study is the assignment of electronic transitions to each of the PL peaks and thereby to the absorption peaks as well. For this we have applied the model mentioned in the theory to these core shell quantum dots. The energy associated with each energy level has been calculated both for the experimental and theoretical cases and then the ratio between certain transitions were found. From the ratio, the transitions happening in the core- shell quantum dot is predicted. Consideringthe band gap due to consecutive transitions as A1, A2 and A3 the list of the bandgaps alongwith the differences, A3-A2 and A2-A1 and their ratios from experiment were found. These ratios were matched with theoretical ratios to determine the approximate electronic transitions. T1, T2, T3, T4, T5 and T6 are assigned to 1Se-1Sh,1Pe-1Ph, 1De-1Dh, 2Se-2Sh 1Fe-1Fh and 2P3-2Ph transitions respectively, from the model described in the theory above. So from table 5, it is evident that for OCTS2, the peaks A3, A2 and A1 correspond to the 1Fe-1Fh, 1De-1Dh and 1Pe-1Ph transitions respectively. For OCTS3, the peaks A3, A2 and A1 correspond to 2Pe-2Ph, 1De-1Dh and 1Pe-1Ph transitions respectively. For OCTS4, the peaks A3, A2 and A1 correspond to 1De-1Dh, 1Pe-1Ph and 1Se-1Sh transitions respectively. The Jablonski diagrams for transitions in OCTS2, OCTS3 and OCTS4 are shown in figure9.
As the amount of dye around the quantum dot decreases, lower level transitions predominate. A possible reason for this phenomenon could be related to the change in confinement of the quantum dot. At higher dye concentration, when the shell thickness is higher, there is a leakage of electron wavefunction into the shell material by which there is a decrease in confinement energy and so the energy levels are not too far apart which makes higher level transitions possible. When the amount of dye around the quantum dot decreases, the shell thickness is less
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and the electron wavefunctions are confined within the quantum dot. There is an increase in confinement energy by which the energy levels are wide apart, favouring lower level transitions alone. These have been depicted in figure10.
So apart from using inorganic materials and further chemical reactions to produce core shell quantum dots, it is possible to develop such quantum dots in a much economic way by using dyes as the shelling material. The only condition required is that there should be an interaction between the capping agent used and the dye molecule. It is possible to tune bandgap in a wide range or in other words it is possible to tune the electronic transitions by changing the quantity of dye added.
4.2.2. Interaction of CTS quantum dots with beetroot dye
The absorption spectra of CTS, BCTS2, BCTS3 and BCTS4 are plotted in figure11(a) and their transmittance spectra are given in figure11(b). The transmittance of CTS decreases with the increase of dye around the quantum dot. With the addition of dye, the constant transmittance onset shifts to the visible region. The PL spectra of BCTS2, BCTS3 and BCTS4 are plotted in figure12. PL in different ranges have been shown differently. The emission peaks give an idea of the bandgaps corresponding to each transition. The information on quantum dot bandgaps and PL intensities are given in table6. There is a clear blueshift in wavelength as the amount of dye decreases around the quantum dot which indicates that there is a change in size of the quantum dot. This is possible only by the formation of CTS/beetroot dye core- shell structure. From the overall size of the
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quantum dots (when treated with beetroot dye) and the size of the quantum dot alone from TEM image, the thickness of the shell has been calculated and tabulated in table 6. As the dye concentration decreases, the thickness of the shell also decreases. Contrary to the previous case, the PL intensity increases with increasing shell thickness. It is found that for shell thickness in the range of 0.2-0.36 nm, the PL intensity increases with increasing shell thickness after which the PL intensity starts to decrease.
Betanin pigment in beetroot dye is also non-polar in nature with donor electrons. This enables hydrogen bonding of the hydrocarbon tail of CTAB with betanin. Betanin is a large molecule and is likely have much stearic hindrance from the surrounding betanin molecules and so not many dye molecules will be able to form hydrogen bonds with CTAB. This point is evident from the thickness of the shell formed in this case. The extent of bonding of betanin dye molecules with CTAB can be understood from the FTIR spectrum shown in figure14. The degree of hydrogen bonding is weak in this case (tabulated in table7) as the energy shifts are narrow for CTS/beetroot dye quantum dots compared to CTS quantum dots alone. Since there are only two transitions taking place in CTS/beetroot dye quantum dots, it can be assumed that only 1Se-1Sh and 1Pe-1Ph transitions are taking place in this core shell quantum dot. This is because there is not much change in confinement energies compared to that of core only CTS quantum dots.
Conclusion: Thus through this work, we have prepared CTS/natural dye core shell quantum dots using onion skin dye and beetroot skin dye. The formation of a core/shell quantum dot was confirmed from the changes in confinement energies found from absorption, PL and infrared
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spectra. Pelargonidin has greater interaction with CTAB by means of hydrogen bonding compared to that of betanin, whose interaction was hindered by stearic effects. Thus the bandgap and confinement energies of charge carriers of CTS quantum dots can be tuned by changing the amount of natural dyes used. CTS core and CTS/natural dye core/shell quantum dots can be utilized effectively in the buffer layer of solar cells due to their high and constant transmittance in a wide range of wavelengths.
Acknowledgement The authors would like to thank STIC Cochin for the TEM analysis, TKM College of Arts and Science, Kollam for the absorption and infrared spectroscopic facilities and Nirmalagiri College, Kannur, for the PL facility. We also thank University Grants Commission for the Junior Research Fellowship for the completion of this work.
REFERENCES [1] Shen Y, Li C, Huang R, Tian R, Ye Y, Pan L, Koumoto K, Zhang R, Wan C, Wang Y. Ecofriendly p-type Cu 2 SnS 3 thermoelectric material: crystal structure and transport properties. Scientific reports. 2016 Sep 26;6:32501. [2] Xiao W, Xu G, Bi Y, Jiang J, Hu A, Shen K, Lu X, Zhu M. L-cysteine-assisted synthesis of capsule-like Cu2SnS3 nanostructures via solvothermal route. Materials Research Innovations. 2016 Jul 28;20(5):351-7. [3] Dias S, Krupanidhi SB. Solution processed Cu2SnS3 thin films for visible and infrared photodetector applications. AIP Advances. 2016 Feb 22;6(2):025217.
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[4] Pradhan S, Di Stasio F, Bi Y, Gupta S, Christodoulou S, Stavrinadis A, Konstantatos G. High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level. Nature nanotechnology. 2019 Jan;14(1):72. [5] Li Y, Wang Z, Ren D, Liu Y, Zheng A, Zakeeruddin SM, Dong X, Hagfeldt A, Grätzel M, Wang P. SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells. ACS Applied Energy Materials. 2019 May 3. [6] Akin S, Altintas Y, Mutlugun E, Sonmezoglu S. Cesiumlead based inorganic perovskite quantum-dots as interfacial layer for highly stable perovskite solar cells with exceeding 21% efficiency. Nano Energy. 2019 Jun 1;60:557-66. [7] Shi G, Kaewprajak A, Ling X, Hayakawa A, Zhou S, Song B, Kang Y, Hayashi T, Altun ME, Nakaya M, Liu Z. Finely Interpenetrating Bulk Heterojunction Structure for Lead Sulfide Colloidal Quantum Dot Solar Cells by Convective Assembly. ACS Energy Letters. 2019 Mar 28;4:960-7. [8] Wu T, Zhen C, Wu J, Jia C, Haider M, Wang L, Liu G, Cheng HM. Chlorine capped SnO2 quantum-dots modified TiO2 electron selective layer to enhance the performance of planar perovskite solar cells. Science Bulletin. 2019 Apr 30;64(8):547-52. [9] Dias S, Kumawat KL, Biswas S, Krupanidhi SB. Heat-up synthesis of Cu 2 SnS 3 quantum dots for near infrared photodetection. RSC Advances. 2017;7(38):23301-8. [10] Dias S, Kumawat K, Biswas S, Krupanidhi SB. Solvothermal synthesis of Cu2SnS3 quantum dots and their application in near-infrared photodetectors. Inorganic chemistry. 2017 Feb 9;56(4):2198-203.
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[11] Bai X, Purcell-Milton F, Gun’ko YK. Optical properties, synthesis, and potential applications of Cu-based ternary or quaternary anisotropic quantum dots, polytypic nanocrystals, and core/shell heterostructures. Nanomaterials. 2019 Jan;9(1):85. [12] Dutta R, Bhattacharya S, Pyne A, Datta PK, Sarkar N. Unveiling the interaction between carbon nanodot and IR light emitting fluorescent dyes inside the confined micellar environment. Journal of Photochemistry and Photobiology A: Chemistry. 2019 May 15;377:298-308. [13] Bansal P, Kar P. Probing the energy transfer process by controlling the morphology of CH3NH3PbBr3 nanocrystals with rhodamine B dye. Journal of Luminescence. 2019 Nov 1;215:116609. [14] Abdelbar MF, Fayed TA, Meaz TM, Subramani T, Fukata N, Ebeid EZ. Hybrid organic and inorganic solar cell based on a cyanine dye and quantum dots. Journal of Photochemistry and Photobiology A: Chemistry. 2019 Apr 15;375:166-74. [15] Gaponenko SV. Introduction to nanophotonics. Cambridge University Press; 2010 Apr 8. [16] Wang JJ, Liu P, Ryan KM. A facile phosphine-free colloidal synthesis of Cu 2 SnS 3 and Cu 2 ZnSnS 4 nanorods with a controllable aspect ratio. Chemical Communications. 2015;51(72):13810-3. [17] Shettigar RR, Misra NM, Patel K. Cationic surfactant (CTAB) a multipurpose additive in polymer-based drilling fluids. Journal of Petroleum Exploration and Production Technology. 2018 Jun 1;8(2):597-606.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
Author’s name DR. K.C. PREETHA
MAYA MATHEW
Affiliation SREE NARAYANA COLLEGE AND PAYYANNUR COLLEGE, KERALA, INDIA PAYYANNUR COLLEGE, KERALA, INDIA
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Figure1. Energy levels in a spherical quantum dot
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(a)
(b)
(c)
Figure2(a). TEM image of CTS quantum dots (b) SAED pattern of CTS quantum dots (c) Particle size distribution curve
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(a)
(b)
Figure3(a). Absorption spectrum of CTS quantum dots (b) Transmittance curve of CTS quantum dots
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(a)
(b)
(c)
Figure4(a). FTIR spectrum of CTS quantum dots (b) Representation of CTAB (cetyl- trimethyl ammonium bromide) (c) Interaction of CTAB with water molecules in the solvent
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(a)
(b)
Figure5(a).Absorption spectra (b) Transmittance spectra of OCTS2, OCTS3 and OCTS4
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Figure6. Photoluminescence spectra of OCTS2, OCTS3 and OCTS4
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(a)
(b)
Figure7. Molecular structure of Pelargonidin (b) Binding of Pelargonidin with CTS quantum dots through hydrogen bonds with CTAB
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Figure8. FTIR spectra of CTS, OCTS2, OCTS3 and OCTS4
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(a)
(b)
(c)
Figure9. Jablonski diagrams for OCTS2, OCTS3 and OCTS4.
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(a)
(b)
Figure10. Depiction of change in confinement of charge carriers in (a) core shell quantum dot and (b) core alone quantum dot
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(a)
(b)
Figure11. (a) Absorption spectra and (b) Transmittance spectra of CTS, BCTS2 BCTS3 and BCTS4.
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Figure12. PL spectra of BCTS2, BCTS3 and BCTS4
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Figure13. Structure of betanin
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Figure14. FTIR of CTS, BCTS2, BCTS3 and BCTS4
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Table1. Analysis of SAED pattern of Cu2SnS3 quantum dots 1/d (1/nm)
d-value (Å)
hkl
.6.54
1.53
(224)
4.96
2.02
(114)
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Table2. Characteristic vibrations of CTS quantum dots capped with CTAB Wavenumber (cm-1)
Functional group
1635.64
CN stretch
2171.85
CH bond stretch
2357.01
CH bond stretch
3327
NH stretch
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Table3. Determination of shell thickness of CTS/onion dye core- shell quantum dots. Sample
Bandgap of the quantum dot (eV)
PL intensity (in a.u.)
Overall size of the quantum dot (nm)
Thickness of the shell of the coreshell quantum dot(nm)
OCTS2
2.275
215
2.144
0.5309
OCTS3
2.441
224
2.025
0.2168
OCTS4
2.557
413
1.953
0.0090
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Table4. Comparison of characteristic vibrations of Cu2SnS3 quantum dots with Pelargonidin Functional group -CN stretch
Wavenumber of vibration (cm-1) CTS CTS with Pelargonidin 1635 1633
-CH stretch
2171
2160
-CH stretch
2357
2368
-NH stretch
3327
3327
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Table5. Comparison of experimental and theoretical ratios of electronic transitions in CTS/onion dye core—shell quantum dots. Sample
A1
A2
A3
A3-A2
A2-A1
Experimental Theoretical ratio
(eV)
(eV)
(eV)
(eV)
(eV)
Ratio
OCTS2
2.275
2.672 3.416 0.744
0.397
1.87
2.197
OCTS3
2.441
2.725 3.758 1.033
0.284
3.637
3.82
𝑇6 ― 𝑇3 𝑇3 ― 𝑇2
OCTS4
2.557
3.047 3.827 0.78
0.49
1.591
1.26
𝑇3 ― 𝑇2 𝑇2 ― 𝑇1
𝑇5 ― 𝑇3 𝑇3 ― 𝑇2
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Table6. Bandgap, PL intensity and thickness of shell in CTS/beetroot dye core shell quantum dots Sample
Bandgap (eV)
BCTS2
2.36
PL (a.u.) 290
intensity Overall (nm) 2.081
size Thickness of the shell (nm) 0.368
BCTS3
2.38
207
2.067
0.330
BCTS4
2.40
172
2.052
0.293
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Table7. Characteristic vibrations of CTS/ beetroot dye quantum dots
Functional group
CN stretch
Wavenumber (cm-1) CTS quantum CTS/beetroot dots dye quantum dots 1635 1631
CH stretch
2171
2160
CH stretch
2357
2355
NH stretch
3327
3336
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Table8. Electronic transitions in CTS/beetroot dye core—shell quantum dots. Sample
A1
A2
BCTS2
2.36
2.696
BCTS3
2.38
2.707
BCTS4
2.40
2.79
Possible transition
1Se-1Sh
1Pe-1Ph
Maya Mathew, K.C. Preetha PII:
S0921-4526(19)30792-6
DOI:
https://doi.org/10.1016/j.physb.2019.411913
Reference:
PHYSB 411913
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
28 October 2019
Accepted Date:
24 November 2019
Please cite this article as: Maya Mathew, K.C. Preetha, AN EXPLORATION INTO THE QUANTUM CONFINEMENT OF CTS/NATURAL DYE CORE- SHELL QUANTUM DOTS, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411913
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AN EXPLORATION INTO THE QUANTUM CONFINEMENT OF CTS/NATURAL DYE CORE- SHELL QUANTUM DOTS 1Maya
1Payyannur
2Sree
Mathew, 1,2*KC Preetha
College, Payyannur, India PIN- 670327
Narayana College, Kannur, India PIN- 670007
*Corresponding Author: [email protected]
Abstract: In this work, we have presented a simple way of changing the confinement energies of Copper Tin Sulphide (CTS) quantum dots using natural dyes as shell material. Tetragonal CTS quantum dots in the size range of 1.7nm- 2.2nm, of bandgaps of 2.48eV and 5.0 eV were prepared by means of a green colloidal synthesis technique. These quantum dots were treated with natural dyes such as onion and beetroot skin dyes. Pelargonidin and Betanin (pigments of onion and beetroot skin dye respectively) formed hydrogen bonding with the capping agent, thus forming a shell around the CTS quantum dots. The change in confinement due to the effect of dye as shell was studied from absorption, photoluminescence and infrared spectroscopic techniques. The transitions occurring were analysed using a theoretical approach. CTS quantum dots, with its high transmittance in a wide range of wavelengths find promising applications in the buffer layer of solar cells.
Keywords: Copper tin sulphide; quantum dots; quantum confinement; Pelargonidin; Betanin; colloidal synthesis
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1. INTRODUCTION Silicon solar cells, though of much demand commercially are being replaced by other semiconductor materials, one of them being ternary materials. Copper tin sulphide is one among such materials on which wide research has been made in bulk [1], nanorods [2] and in thin film [3] forms. It is a p-type material with a high absorption coefficient in the order of 105/cm suitable for photovoltaic applications. Photovoltaic band gaps in the range of 0.9eV- 1.35 eV have been achieved by changing the confinement of the material. Research is going on widely for increasing the efficiency of absorber layer material. An attempt in this direction is the development of tandem solar cells which can potentially absorb radiation from a wide range of the solar spectrum and thereby increase the efficiency of solar cells. Tandem solar cells have multiple absorber layers with high absorption coefficient in different regions of the electromagnetic spectrum as they are composed of different materials with different bandgaps. With quantum dots, such bandgap tuning becomes easier, simply by changing the size of the quantum dot. Santanu Pradhan et.al.have used PbS colloidal quantum dots as LEDs to gain an external quantum efficiency of 7.9% [4]. Quantum dots act as hole transporters in perovskite solar cells which increases the stability and efficiency [5,6].Use of quantum dots in bulk heterojunctions enable efficient charge extraction and in this way utilize the solar energy well [7]. Quantum dots are also efficiently used as electron collection centers in interfaces to prevent recombination of charge carriers upon irradiation [8]. The use of Copper Tin Sulphide quantum dots as photodetectors and in solar cells have been investigated by Sandra Dias et.al. [9,10].Works related to the synthesis of CTS core shell quantum dots are few in literature. Certain Cu based ternary quantum dots with ZnS as the shell material have been mentioned in a review paper by Xue et.al. ZnS is more lattice matched with
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these Cu based ternary materials. Berends and Stam have widely investigated upon these core shell quantum dots at different reaction temperatures and also by changing the reactivity of the precursors [11]. Rupam Dutta at.al. studied the interaction of carbon dots with fluorescent dyes and found that fluorescence intensity of the former gets quenched in the presence of the latter and photoinduced electron transfer was the cause found behind the quenching [12].Parul Bansal et.al. studied the electron transfer between perovskite nanocrystals and dyes, with green nanoparticles showing Forster Resonance Energy Transfer (FRET) and blue nanoparticles showing luminescence quenching [13].Monomethine cyanine dye aggregated onto the surface of CdS quantum dots by electrostatic force of attraction was found to enhance the photospectral response and photoconversion efficiency [14]. The study on CTS quantum dots is few and especially the studies with natural dyes are rare. So this paper intends to bring about an investigation into the properties of CTS quantum dots mixed with natural dyes so as to design an inorganic- organic core- shell quantum dot. The synthesis techniques used so far for CTS quantum dots are quite expensive. This paper also discusses an economic and green synthesis technique for the preparation of CTS quantum dots which yielded remarkable material properties.
2. EXPERIMENTAL 2.1. MATERIALS AND METHODS
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For the synthesis of copper tin sulphide (CTS) quantum dots, room temperature colloidal synthesis technique was adopted. Copper chloride (CuCl2.2H2O), Tin chloride (SnCl2.2H2O) and Sodium sulphide (Na2S) were used as precursors with cetly trimethylammonium bromide (CTAB) as the capping agent and water as the solvent. 2.2. SYNTHESIS OF CTS QUANTUM DOTS Millmolar solutions of copper chloride (CuCl2.2H2O) and Tin chloride (SnCl2.2H2O) were mixed followed by the addition of the capping agent. Millimolar solution of sodium sulphide (Na2S) was added drop wise to the mixture alongwith constant stirring. The whole mixture was stirred for about half an hour till a pale brown coloured solution was obtained. The sample was named CTS. For the study of interaction of CTS quantum dots with dyes, we have chosen natural dyes from onion skin and beetroot skin. CTS was added in 2ml, 3ml and 4ml to orange- red coloured solution of onion skin dye with pH 3.These samples were named as OCTS2, OCTS3 and OCTS4 respectively.The same procedure was repeated with scarlet red beetroot dye of pH 5 also. The set of samples were named as BCTS2, BTCS3 and BCTS4 respectively. 2.3. CHARACTERIZATION Diffraction pattern was obtained from SAED image; size and morphology from TEM images using JEOL/JEM Transmission Electron Microscope. Surface studies were made by Thermo Fisher Scientific Nicolet iS50 FT-IR Spectrophotometer and optical studies were made by Cary 5000 (Varian) UV- VIS- NIR Spectrophotometer.
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3. THEORY The behaviour of quantum dots in a strong confinement regime can be depicted as particles in a spherical quantum well. Owing to strong confinement, the columbic force of attraction between the charge carriers can be neglected and the Schrodinger equation for such a system can be written as [15]: ― ħ2 2 2𝑚 ∇
𝐻=
(1)
+ 𝑈(𝑟),
𝑟 = 𝑥2 + 𝑦2 + 𝑧2 The Hamiltonian can be rewritten in spherical polar coordinates as shown below:
𝐻=
― ħ2 1 𝑑 [ 2𝑚 𝑟2𝑑𝑟
(
∂𝛹 𝑟2 ∂𝑟
)+
𝑑 𝑟2𝑠𝑖𝑛𝜃𝑑𝜃 1
(𝑟
∂𝛹 𝑠𝑖𝑛𝜃 ∂𝜃
2
)+
1
∂2𝛹
𝑟2𝑠𝑖𝑛2𝜃 ∂𝜙2
(2)
And the wavefunction can be written as follows:
(3)
𝛹 = 𝑅(𝑟) 𝛩(𝜃) Ф(𝜙) 𝛹𝑛𝑙𝑚(𝑟,𝜃,𝜙) =
𝑢𝑛𝑙(𝑟) 𝑟
𝑌𝑙𝑚(𝜃,𝜙)
WhereYlm are the spherical Bessel functions and u(r) satisfies an equation: ― ħ2∂2𝑢 2𝑚 ∂𝑟2
+ [𝑈(𝑟) +
ħ2
𝑙(𝑙 + 1)]𝑢 = 𝐸𝑢
2𝑚𝑟2
Where Angular momentum quantum number, L=l(l+1), l=0,1,2,3... Magnetic quantum number Lz = mħ, m=0,±1, ±2, ±3… U(r)= 0,r
(4)
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=∞, r>a The energy eigenvalues are given by: ħ2𝜒2𝑛𝑙
𝐸𝑛𝑙= 2𝑚𝑎2
(5)
Where, Χnl2 are the roots of Bessel function, a is the diameter of the quantum dot and all other terms have their usual meaning. The energy of electron state: 𝐸𝑒=
The energy of the hole state: 𝐸ℎ=
ħ2𝜒2𝑛𝑙
― 2𝑚𝑎2 ħ2𝜒2𝑛𝑙
𝐸𝑔 + 2𝑚𝑎2
Depending upon the above mentioned eigenvalues, the energy levels of the quantum dot can be depicted as shown in figure1.
4. RESULTS AND DISCUSSION
Before going into discussion of CTS quantum dots with natural dyes, description on the structural and optical properties of CTS quantum dots have been described in the first subsection.
4.1. ANALYSIS OF CTS QUANTUM DOTS 4.1.1. Transmission Electron Microscopy of CTS quantum dots CTS quantum dots were prepared by a colloidal synthesis technique using appropriate precursors in a reaction medium of pH 8 which is found to be the optimum pH for its synthesis. Quantum dots of sizes ranging from 1.7nm- 2.2nm were formed as evident from the TEM
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image shown in figure2(a). The sizes of the quantum dots are much smaller than the Bohr radius for CTS [16] indicating strong confinement regime. Hence a non-interacting particle model, as described in the theory above, can be applied here, where the columbic force of attraction between the charge carriers can be neglected. The diffraction pattern of the quantum dots, from Selected Area Electron Diffraction (SAED) as depicted in figure2(b)) confirms that tetragonal Cu2SnS3 quantum dots have been formed, matching well with JCPDS 089-4714. The information on d-spacing and diffraction planes have been tabulated in table1.The size dispersity of the synthesized quantum dots is found to be less as indicated from the particle distribution curve in figure2(c).
4.1.2. Optical properties of CTS quantum dots
Bandgap of CTS quantum dots is direct in nature as evident from the absorption spectrum shown in figure 3(a). The absorptions are in the ultraviolet region indicating the formation of nanosized particles. Being a direct bandgap semiconductor, the bandgap can be found by drawing tangents from the absorption maximum to the x-axis. Quantum dots have discrete absorption spectrum and so there is a possibility of multiple bandgaps corresponding to 2.48eVand 5.0 eV. The transmittance curve of CTS quantum dots is shown in figure 3(b). It has almost 100% transmittance in the near ultra-violet, visible and near infra red regions. This property of CTS quantum dots can be used efficiently in the window layer of solar cells.
4.1.3. Study of surface properties by FTIR
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The confinement of quantum dots is achieved by means of a growth arresting agent called the capping agent. Here the capping agent used is cetyl- trimethyl ammonium bromide (CTAB). The confinement of the quantum dot depends on the degree of capping. Inorder to study the surface properties of CTS quantum dots, FTIR of the sample was recorded and is shown in figure 4(a). The spectrum gives the vibrations corresponding to that of CTAB (figure 4(b)) [17] which confirms the presence of CTAB as a shield over the quantum dots. The functional groups which caused the vibrations have been tabulated in table2. Large shifts in wavenumbers have been noticed compared to the vibrations of CTAB which is due to the presence of hydrogen bonding between the hydrogen atoms of the hydrocarbon chain of CTAB and the electronegative oxygen of water molecules in the solvent. CTAB, a cationic surfactant,has a positive hydrophilic head which binds the quantum dot and a hydrophobic tail which is in contact with the solution. The hydrophobic tail prevents agglomeration and dissolution of the quantum dots in the solvent. Due to the hydrophobic nature of the hydrocarbon tail, there is a possibility of curling at the quantum dot surface inorder to prevent its interaction with water. Shift in energies of characteristic vibrations of functional groups of the surfactant as shown in table2 may also be due to this curling. 4.2. STUDY OF INTERACTION OF CTS QUANTUM DOTS WITH NATURAL DYES Here a study is made on the change in optical and surface properties of CTS quantum dots when mixed with natural dyes.
4.2.1. Interaction of CTS quantum dots with Onion dye The absorption spectra of CTS, OCTS2, OCTS3 and OCTS4 have been plotted in figure 5(a). All the samples have multiple absorption peaks with the absorbance of each transition,
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decreasing as the dye in the mixture decreases. The transmittance curves have been plotted in figure 5(b). The transmittance of the samples goes on decreasing with increasing amount of dye in the quantum dot- dye mixture. The onset of constant transmittance has shifted to the visible region, compared to CTS, for the quantum dot- dye mixture. To get an apparent picture of the number of transitions in these quantum dots and bandgaps, photoluminescence spectra of these samples were recorded as shown in figure6.
All the dye treated samples have three PL peaks corresponding to three different transitions. The peaks appearing as shoulders or satellites have been shown by deconvolution. The wavelength corresponding to each PL peak gives the bandgap of the material. This indicates that there is a blueshift in the bandgap of the samples when the amount of dye in the mixture decreases. In other words, there is a variation in size of the quantum dots which is responsible for the bandgap change. One possibility is the formation of a core shell structure with CTS as the core and the onion dye molecules forming the shell. It must be the variation in shell size which is responsible for the change in overall size of the quantum dot, thereby changing the bandgap. The formation of a shell is possible only if there is an interaction of the onion dye molecules with the capping agent of the quantum dot. Taking the average size of CTS quantum dots to be 1.95nm, found from TEM image, the effective mass of electrons in CTS quantum dots is found to be 5.44 X 10-32 kg using Brus equation. Using this value of effective mass of electron, the thickness of the shell of onion dye formed around the CTS quantum dots is determined by simple mathematical calculations and is tabulated in table3.
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It is observed that lower the shell thickness, higher will be the band gap.Bandgap increases for highly confined quantum dots and so incorporation of dye molecules around the quantum dot decreases the confinement energy. The PL intensity of CTS/onion dye core shell quantum dots have also been tabulated in table3. PL intensity decreases with increasing shell thickness because PL intensity is directly proportional to confinement of charge carriers. So as the confinement of charge carriers to the core increases, this is the case for smaller core/shell quantum dots, with lower shell thickness, the PL intensity increases.
Figure7(a) shows the molecular structure of onion dye pigment called Pelargonidin. Pelargonidin is less polar compared to water. So the hydrophobic hydrocarbon chain of CTAB, capping CTS quantum dots, would prefer to bind with Pelargonidin by means of hydrogen bonding as shown in figure 7(b) thus acting as a shell material over the CTS quantum dots. This results in overall change in size of the quantum dot and thereby changes the optical properties. This binding of CTAB with Pelargonidin can be further confirmed from the FTIR spectra shown in figure 8. From table4 it is evident that all the four spectra have the same set of dips but with minor differences in the position of dips. Only the absorptions corresponding to –CH stretching have shifts in energy which indicates that hydrogen bonding has occurred only with the hydrogen atoms exposed to the solvent. Very few hydrogen bonds have been formed with the -NH functional group as there is almost no shift in energy. All the spectroscopic analyses reveal that the quantum dot- onion dye mixtures form core-shell quantum dots with CTS as the core and the onion dye molecules as the shell. Considering this, an investigation into the change in confinement of these core—shell CTS/onion dye quantum
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dots were made from PL data. The main objective being fulfilled in this study is the assignment of electronic transitions to each of the PL peaks and thereby to the absorption peaks as well. For this we have applied the model mentioned in the theory to these core shell quantum dots. The energy associated with each energy level has been calculated both for the experimental and theoretical cases and then the ratio between certain transitions were found. From the ratio, the transitions happening in the core- shell quantum dot is predicted. Consideringthe band gap due to consecutive transitions as A1, A2 and A3 the list of the bandgaps alongwith the differences, A3-A2 and A2-A1 and their ratios from experiment were found. These ratios were matched with theoretical ratios to determine the approximate electronic transitions. T1, T2, T3, T4, T5 and T6 are assigned to 1Se-1Sh,1Pe-1Ph, 1De-1Dh, 2Se-2Sh 1Fe-1Fh and 2P3-2Ph transitions respectively, from the model described in the theory above. So from table 5, it is evident that for OCTS2, the peaks A3, A2 and A1 correspond to the 1Fe-1Fh, 1De-1Dh and 1Pe-1Ph transitions respectively. For OCTS3, the peaks A3, A2 and A1 correspond to 2Pe-2Ph, 1De-1Dh and 1Pe-1Ph transitions respectively. For OCTS4, the peaks A3, A2 and A1 correspond to 1De-1Dh, 1Pe-1Ph and 1Se-1Sh transitions respectively. The Jablonski diagrams for transitions in OCTS2, OCTS3 and OCTS4 are shown in figure9.
As the amount of dye around the quantum dot decreases, lower level transitions predominate. A possible reason for this phenomenon could be related to the change in confinement of the quantum dot. At higher dye concentration, when the shell thickness is higher, there is a leakage of electron wavefunction into the shell material by which there is a decrease in confinement energy and so the energy levels are not too far apart which makes higher level transitions possible. When the amount of dye around the quantum dot decreases, the shell thickness is less
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and the electron wavefunctions are confined within the quantum dot. There is an increase in confinement energy by which the energy levels are wide apart, favouring lower level transitions alone. These have been depicted in figure10.
So apart from using inorganic materials and further chemical reactions to produce core shell quantum dots, it is possible to develop such quantum dots in a much economic way by using dyes as the shelling material. The only condition required is that there should be an interaction between the capping agent used and the dye molecule. It is possible to tune bandgap in a wide range or in other words it is possible to tune the electronic transitions by changing the quantity of dye added.
4.2.2. Interaction of CTS quantum dots with beetroot dye
The absorption spectra of CTS, BCTS2, BCTS3 and BCTS4 are plotted in figure11(a) and their transmittance spectra are given in figure11(b). The transmittance of CTS decreases with the increase of dye around the quantum dot. With the addition of dye, the constant transmittance onset shifts to the visible region. The PL spectra of BCTS2, BCTS3 and BCTS4 are plotted in figure12. PL in different ranges have been shown differently. The emission peaks give an idea of the bandgaps corresponding to each transition. The information on quantum dot bandgaps and PL intensities are given in table6. There is a clear blueshift in wavelength as the amount of dye decreases around the quantum dot which indicates that there is a change in size of the quantum dot. This is possible only by the formation of CTS/beetroot dye core- shell structure. From the overall size of the
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quantum dots (when treated with beetroot dye) and the size of the quantum dot alone from TEM image, the thickness of the shell has been calculated and tabulated in table 6. As the dye concentration decreases, the thickness of the shell also decreases. Contrary to the previous case, the PL intensity increases with increasing shell thickness. It is found that for shell thickness in the range of 0.2-0.36 nm, the PL intensity increases with increasing shell thickness after which the PL intensity starts to decrease.
Betanin pigment in beetroot dye is also non-polar in nature with donor electrons. This enables hydrogen bonding of the hydrocarbon tail of CTAB with betanin. Betanin is a large molecule and is likely have much stearic hindrance from the surrounding betanin molecules and so not many dye molecules will be able to form hydrogen bonds with CTAB. This point is evident from the thickness of the shell formed in this case. The extent of bonding of betanin dye molecules with CTAB can be understood from the FTIR spectrum shown in figure14. The degree of hydrogen bonding is weak in this case (tabulated in table7) as the energy shifts are narrow for CTS/beetroot dye quantum dots compared to CTS quantum dots alone. Since there are only two transitions taking place in CTS/beetroot dye quantum dots, it can be assumed that only 1Se-1Sh and 1Pe-1Ph transitions are taking place in this core shell quantum dot. This is because there is not much change in confinement energies compared to that of core only CTS quantum dots.
Conclusion: Thus through this work, we have prepared CTS/natural dye core shell quantum dots using onion skin dye and beetroot skin dye. The formation of a core/shell quantum dot was confirmed from the changes in confinement energies found from absorption, PL and infrared
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spectra. Pelargonidin has greater interaction with CTAB by means of hydrogen bonding compared to that of betanin, whose interaction was hindered by stearic effects. Thus the bandgap and confinement energies of charge carriers of CTS quantum dots can be tuned by changing the amount of natural dyes used. CTS core and CTS/natural dye core/shell quantum dots can be utilized effectively in the buffer layer of solar cells due to their high and constant transmittance in a wide range of wavelengths.
Acknowledgement The authors would like to thank STIC Cochin for the TEM analysis, TKM College of Arts and Science, Kollam for the absorption and infrared spectroscopic facilities and Nirmalagiri College, Kannur, for the PL facility. We also thank University Grants Commission for the Junior Research Fellowship for the completion of this work.
REFERENCES [1] Shen Y, Li C, Huang R, Tian R, Ye Y, Pan L, Koumoto K, Zhang R, Wan C, Wang Y. Ecofriendly p-type Cu 2 SnS 3 thermoelectric material: crystal structure and transport properties. Scientific reports. 2016 Sep 26;6:32501. [2] Xiao W, Xu G, Bi Y, Jiang J, Hu A, Shen K, Lu X, Zhu M. L-cysteine-assisted synthesis of capsule-like Cu2SnS3 nanostructures via solvothermal route. Materials Research Innovations. 2016 Jul 28;20(5):351-7. [3] Dias S, Krupanidhi SB. Solution processed Cu2SnS3 thin films for visible and infrared photodetector applications. AIP Advances. 2016 Feb 22;6(2):025217.
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[4] Pradhan S, Di Stasio F, Bi Y, Gupta S, Christodoulou S, Stavrinadis A, Konstantatos G. High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level. Nature nanotechnology. 2019 Jan;14(1):72. [5] Li Y, Wang Z, Ren D, Liu Y, Zheng A, Zakeeruddin SM, Dong X, Hagfeldt A, Grätzel M, Wang P. SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells. ACS Applied Energy Materials. 2019 May 3. [6] Akin S, Altintas Y, Mutlugun E, Sonmezoglu S. Cesiumlead based inorganic perovskite quantum-dots as interfacial layer for highly stable perovskite solar cells with exceeding 21% efficiency. Nano Energy. 2019 Jun 1;60:557-66. [7] Shi G, Kaewprajak A, Ling X, Hayakawa A, Zhou S, Song B, Kang Y, Hayashi T, Altun ME, Nakaya M, Liu Z. Finely Interpenetrating Bulk Heterojunction Structure for Lead Sulfide Colloidal Quantum Dot Solar Cells by Convective Assembly. ACS Energy Letters. 2019 Mar 28;4:960-7. [8] Wu T, Zhen C, Wu J, Jia C, Haider M, Wang L, Liu G, Cheng HM. Chlorine capped SnO2 quantum-dots modified TiO2 electron selective layer to enhance the performance of planar perovskite solar cells. Science Bulletin. 2019 Apr 30;64(8):547-52. [9] Dias S, Kumawat KL, Biswas S, Krupanidhi SB. Heat-up synthesis of Cu 2 SnS 3 quantum dots for near infrared photodetection. RSC Advances. 2017;7(38):23301-8. [10] Dias S, Kumawat K, Biswas S, Krupanidhi SB. Solvothermal synthesis of Cu2SnS3 quantum dots and their application in near-infrared photodetectors. Inorganic chemistry. 2017 Feb 9;56(4):2198-203.
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[11] Bai X, Purcell-Milton F, Gun’ko YK. Optical properties, synthesis, and potential applications of Cu-based ternary or quaternary anisotropic quantum dots, polytypic nanocrystals, and core/shell heterostructures. Nanomaterials. 2019 Jan;9(1):85. [12] Dutta R, Bhattacharya S, Pyne A, Datta PK, Sarkar N. Unveiling the interaction between carbon nanodot and IR light emitting fluorescent dyes inside the confined micellar environment. Journal of Photochemistry and Photobiology A: Chemistry. 2019 May 15;377:298-308. [13] Bansal P, Kar P. Probing the energy transfer process by controlling the morphology of CH3NH3PbBr3 nanocrystals with rhodamine B dye. Journal of Luminescence. 2019 Nov 1;215:116609. [14] Abdelbar MF, Fayed TA, Meaz TM, Subramani T, Fukata N, Ebeid EZ. Hybrid organic and inorganic solar cell based on a cyanine dye and quantum dots. Journal of Photochemistry and Photobiology A: Chemistry. 2019 Apr 15;375:166-74. [15] Gaponenko SV. Introduction to nanophotonics. Cambridge University Press; 2010 Apr 8. [16] Wang JJ, Liu P, Ryan KM. A facile phosphine-free colloidal synthesis of Cu 2 SnS 3 and Cu 2 ZnSnS 4 nanorods with a controllable aspect ratio. Chemical Communications. 2015;51(72):13810-3. [17] Shettigar RR, Misra NM, Patel K. Cationic surfactant (CTAB) a multipurpose additive in polymer-based drilling fluids. Journal of Petroleum Exploration and Production Technology. 2018 Jun 1;8(2):597-606.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
Author’s name DR. K.C. PREETHA
MAYA MATHEW
Affiliation SREE NARAYANA COLLEGE AND PAYYANNUR COLLEGE, KERALA, INDIA PAYYANNUR COLLEGE, KERALA, INDIA
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Figure1. Energy levels in a spherical quantum dot
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(a)
(b)
(c)
Figure2(a). TEM image of CTS quantum dots (b) SAED pattern of CTS quantum dots (c) Particle size distribution curve
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(a)
(b)
Figure3(a). Absorption spectrum of CTS quantum dots (b) Transmittance curve of CTS quantum dots
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(a)
(b)
(c)
Figure4(a). FTIR spectrum of CTS quantum dots (b) Representation of CTAB (cetyl- trimethyl ammonium bromide) (c) Interaction of CTAB with water molecules in the solvent
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(a)
(b)
Figure5(a).Absorption spectra (b) Transmittance spectra of OCTS2, OCTS3 and OCTS4
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Figure6. Photoluminescence spectra of OCTS2, OCTS3 and OCTS4
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(a)
(b)
Figure7. Molecular structure of Pelargonidin (b) Binding of Pelargonidin with CTS quantum dots through hydrogen bonds with CTAB
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Figure8. FTIR spectra of CTS, OCTS2, OCTS3 and OCTS4
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(a)
(b)
(c)
Figure9. Jablonski diagrams for OCTS2, OCTS3 and OCTS4.
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(a)
(b)
Figure10. Depiction of change in confinement of charge carriers in (a) core shell quantum dot and (b) core alone quantum dot
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(a)
(b)
Figure11. (a) Absorption spectra and (b) Transmittance spectra of CTS, BCTS2 BCTS3 and BCTS4.
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Figure12. PL spectra of BCTS2, BCTS3 and BCTS4
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Figure13. Structure of betanin
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Figure14. FTIR of CTS, BCTS2, BCTS3 and BCTS4
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Table1. Analysis of SAED pattern of Cu2SnS3 quantum dots 1/d (1/nm)
d-value (Å)
hkl
.6.54
1.53
(224)
4.96
2.02
(114)
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Table2. Characteristic vibrations of CTS quantum dots capped with CTAB Wavenumber (cm-1)
Functional group
1635.64
CN stretch
2171.85
CH bond stretch
2357.01
CH bond stretch
3327
NH stretch
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Table3. Determination of shell thickness of CTS/onion dye core- shell quantum dots. Sample
Bandgap of the quantum dot (eV)
PL intensity (in a.u.)
Overall size of the quantum dot (nm)
Thickness of the shell of the coreshell quantum dot(nm)
OCTS2
2.275
215
2.144
0.5309
OCTS3
2.441
224
2.025
0.2168
OCTS4
2.557
413
1.953
0.0090
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Table4. Comparison of characteristic vibrations of Cu2SnS3 quantum dots with Pelargonidin Functional group -CN stretch
Wavenumber of vibration (cm-1) CTS CTS with Pelargonidin 1635 1633
-CH stretch
2171
2160
-CH stretch
2357
2368
-NH stretch
3327
3327
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Table5. Comparison of experimental and theoretical ratios of electronic transitions in CTS/onion dye core—shell quantum dots. Sample
A1
A2
A3
A3-A2
A2-A1
Experimental Theoretical ratio
(eV)
(eV)
(eV)
(eV)
(eV)
Ratio
OCTS2
2.275
2.672 3.416 0.744
0.397
1.87
2.197
OCTS3
2.441
2.725 3.758 1.033
0.284
3.637
3.82
𝑇6 ― 𝑇3 𝑇3 ― 𝑇2
OCTS4
2.557
3.047 3.827 0.78
0.49
1.591
1.26
𝑇3 ― 𝑇2 𝑇2 ― 𝑇1
𝑇5 ― 𝑇3 𝑇3 ― 𝑇2
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Table6. Bandgap, PL intensity and thickness of shell in CTS/beetroot dye core shell quantum dots Sample
Bandgap (eV)
BCTS2
2.36
PL (a.u.) 290
intensity Overall (nm) 2.081
size Thickness of the shell (nm) 0.368
BCTS3
2.38
207
2.067
0.330
BCTS4
2.40
172
2.052
0.293
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Table7. Characteristic vibrations of CTS/ beetroot dye quantum dots
Functional group
CN stretch
Wavenumber (cm-1) CTS quantum CTS/beetroot dots dye quantum dots 1635 1631
CH stretch
2171
2160
CH stretch
2357
2355
NH stretch
3327
3336
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Table8. Electronic transitions in CTS/beetroot dye core—shell quantum dots. Sample
A1
A2
BCTS2
2.36
2.696
BCTS3
2.38
2.707
BCTS4
2.40
2.79
Possible transition
1Se-1Sh
1Pe-1Ph