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Enhancement of Charge Transfer and Quenching of Photoluminescence of Capped CdS Quantum Dots.

Mehata MS - Sci Rep (2015)

Bottom Line: An external electric field of variable strength of 0.2-1.0 MV cm(-1) was applied to the sample of capped CdS Q-dots doped in a poly(methyl methacrylate) (PMMA) films.PL of capped CdS Q-dots is significantly quenched in presence of external electric field.Thus, understanding the CT character and field-induced PL quenching of CdS Q-dots is important for photovoltaic, LEDs and biological applications.

View Article: PubMed Central - PubMed

Affiliation: Laser-Spectroscopy Laboratory, Department of Applied Physics, Delhi Technological University, Bawana Road, Delhi 110042, INDIA.

ABSTRACT
Quantum dots (Q-dots) of cadmium sulfide (CdS) with three different capping ligands, 1-butanethiol (BT), 2-mercaptoethanol (ME) and benzyl mercaptan (BM) have been investigated. An external electric field of variable strength of 0.2-1.0 MV cm(-1) was applied to the sample of capped CdS Q-dots doped in a poly(methyl methacrylate) (PMMA) films. Field-induced changes in optical absorption of capped CdS Q-dots were observed in terms of purely the second-derivative of the absorption spectrum (the Stark shift), indicating an enhancement in electric dipole moment following transition to the first exciton state. The enhancement depends on the shape and size of the Q-dots prepared using different capping ligands. Field induced-change in photoluminescence (PL) reveals similar changes, an enhancement in charge-transfer (CT) character in exciton state. PL of capped CdS Q-dots is significantly quenched in presence of external electric field. The strong field-induced quenching occurs as a result of the increased charge separation resulting exciton dissociation. Thus, understanding the CT character and field-induced PL quenching of CdS Q-dots is important for photovoltaic, LEDs and biological applications.

No MeSH data available.


Related in: MedlinePlus

E-PL spectrum (shaded black line) of BT-capped CdS Q-dots embedded in a PMMA film observed with excitation at 367 nm with field strength of 0.2 MV cm-1.The simulated curve (dotted red line) and contribution of the zeroth and first-derivative (dotted lines) of PL spectrum used to reproduce the E-PL spectrum.
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f7: E-PL spectrum (shaded black line) of BT-capped CdS Q-dots embedded in a PMMA film observed with excitation at 367 nm with field strength of 0.2 MV cm-1.The simulated curve (dotted red line) and contribution of the zeroth and first-derivative (dotted lines) of PL spectrum used to reproduce the E-PL spectrum.

Mentions: To evaluate Stark shifts, the E-PL spectra of BT, ME and BM capped Q-dots were analyzed with the expression described by equations 2, 3, 4, 5, 6. In each case, the E-PL spectrum was reproduced by a linear combination of the zeroth- and first-derivative of the PL spectrum, the coefficient of the second-derivative spectrum is not necessary to be considered i.e., the contribution of second-derivative is negligible, as shown in Figs 7, 8, 9. The zeroth-derivative contribution measured the magnitude of field-induced quenching at 0.2 MV cm−1. However, the contribution of the first-derivative probably come from the orientation polarizability at room temperature because the emitting state has large electric dipole moment, and has been evaluated using the relation:where μe represents electric dipole moment at the emitting state and Δμ represents difference in the electric dipole moment between ground and emitting state, respectively. γ is the angle between the vectors of μe and Δμ. Considering the negligible ground state dipole moment the change in dipole moment between the emitting state to the ground state has been evaluated and presented in table 1. Note that the E-A spectra are purely the second-derivative of the absorption, i.e. there is no contribution of the polarizability or orientational polarizability, which is in agreement of the E-PL spectra. As shown in Table 1, there is a large difference in the contribution of and field-induced quenching obtained for ME as comparison to the BT and BM capped Q-dots. This could be explained by the quantum confinement effects, the PL which is originated from the surface state instead of direct recombination of e-h pair at the band gap and the capping effects. In all cases, the PL originated from the direct recombination of e-h pair is absent or negligibly small. This fact is also applicable to understand the difference in obtained by E-A and E-PL for capped Q-dots. The field-induced spectral broadening of E-A spectra originated due to enhancement of electric dipole moment in the excited state following the absorption, indicating CT character of Q-dots. Thus, both the E-A and E-PL spectra show the CT character of the Q-dots. The emitting state which is produced by the recombination of electron-hole is then dissociated into a carrier of hole and electron (charge-transfer state) in the presence of external electric field. The dissociation of e-h is also supported by the decrease in the initial population of emitting state57. In case of the ME capped Q-dots, the field-induced quenching is higher as compared to the BT and BM capped Q-dots indicating that the selection of the capping ligands are important to control the shape, size, stability and defects of Q-dots. Thus, understanding the CT character and field-induced quenching in Q-dots is equally important for using these Q-dots in LEDs, photovoltaic and biological applications. The field-induced quenching is important for the application of LEDs because electric field is always applied during the operation of LEDs.


Enhancement of Charge Transfer and Quenching of Photoluminescence of Capped CdS Quantum Dots.

Mehata MS - Sci Rep (2015)

E-PL spectrum (shaded black line) of BT-capped CdS Q-dots embedded in a PMMA film observed with excitation at 367 nm with field strength of 0.2 MV cm-1.The simulated curve (dotted red line) and contribution of the zeroth and first-derivative (dotted lines) of PL spectrum used to reproduce the E-PL spectrum.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4499802&req=5

f7: E-PL spectrum (shaded black line) of BT-capped CdS Q-dots embedded in a PMMA film observed with excitation at 367 nm with field strength of 0.2 MV cm-1.The simulated curve (dotted red line) and contribution of the zeroth and first-derivative (dotted lines) of PL spectrum used to reproduce the E-PL spectrum.
Mentions: To evaluate Stark shifts, the E-PL spectra of BT, ME and BM capped Q-dots were analyzed with the expression described by equations 2, 3, 4, 5, 6. In each case, the E-PL spectrum was reproduced by a linear combination of the zeroth- and first-derivative of the PL spectrum, the coefficient of the second-derivative spectrum is not necessary to be considered i.e., the contribution of second-derivative is negligible, as shown in Figs 7, 8, 9. The zeroth-derivative contribution measured the magnitude of field-induced quenching at 0.2 MV cm−1. However, the contribution of the first-derivative probably come from the orientation polarizability at room temperature because the emitting state has large electric dipole moment, and has been evaluated using the relation:where μe represents electric dipole moment at the emitting state and Δμ represents difference in the electric dipole moment between ground and emitting state, respectively. γ is the angle between the vectors of μe and Δμ. Considering the negligible ground state dipole moment the change in dipole moment between the emitting state to the ground state has been evaluated and presented in table 1. Note that the E-A spectra are purely the second-derivative of the absorption, i.e. there is no contribution of the polarizability or orientational polarizability, which is in agreement of the E-PL spectra. As shown in Table 1, there is a large difference in the contribution of and field-induced quenching obtained for ME as comparison to the BT and BM capped Q-dots. This could be explained by the quantum confinement effects, the PL which is originated from the surface state instead of direct recombination of e-h pair at the band gap and the capping effects. In all cases, the PL originated from the direct recombination of e-h pair is absent or negligibly small. This fact is also applicable to understand the difference in obtained by E-A and E-PL for capped Q-dots. The field-induced spectral broadening of E-A spectra originated due to enhancement of electric dipole moment in the excited state following the absorption, indicating CT character of Q-dots. Thus, both the E-A and E-PL spectra show the CT character of the Q-dots. The emitting state which is produced by the recombination of electron-hole is then dissociated into a carrier of hole and electron (charge-transfer state) in the presence of external electric field. The dissociation of e-h is also supported by the decrease in the initial population of emitting state57. In case of the ME capped Q-dots, the field-induced quenching is higher as compared to the BT and BM capped Q-dots indicating that the selection of the capping ligands are important to control the shape, size, stability and defects of Q-dots. Thus, understanding the CT character and field-induced quenching in Q-dots is equally important for using these Q-dots in LEDs, photovoltaic and biological applications. The field-induced quenching is important for the application of LEDs because electric field is always applied during the operation of LEDs.

Bottom Line: An external electric field of variable strength of 0.2-1.0 MV cm(-1) was applied to the sample of capped CdS Q-dots doped in a poly(methyl methacrylate) (PMMA) films.PL of capped CdS Q-dots is significantly quenched in presence of external electric field.Thus, understanding the CT character and field-induced PL quenching of CdS Q-dots is important for photovoltaic, LEDs and biological applications.

View Article: PubMed Central - PubMed

Affiliation: Laser-Spectroscopy Laboratory, Department of Applied Physics, Delhi Technological University, Bawana Road, Delhi 110042, INDIA.

ABSTRACT
Quantum dots (Q-dots) of cadmium sulfide (CdS) with three different capping ligands, 1-butanethiol (BT), 2-mercaptoethanol (ME) and benzyl mercaptan (BM) have been investigated. An external electric field of variable strength of 0.2-1.0 MV cm(-1) was applied to the sample of capped CdS Q-dots doped in a poly(methyl methacrylate) (PMMA) films. Field-induced changes in optical absorption of capped CdS Q-dots were observed in terms of purely the second-derivative of the absorption spectrum (the Stark shift), indicating an enhancement in electric dipole moment following transition to the first exciton state. The enhancement depends on the shape and size of the Q-dots prepared using different capping ligands. Field induced-change in photoluminescence (PL) reveals similar changes, an enhancement in charge-transfer (CT) character in exciton state. PL of capped CdS Q-dots is significantly quenched in presence of external electric field. The strong field-induced quenching occurs as a result of the increased charge separation resulting exciton dissociation. Thus, understanding the CT character and field-induced PL quenching of CdS Q-dots is important for photovoltaic, LEDs and biological applications.

No MeSH data available.


Related in: MedlinePlus