# Photophysical Properties of Multilayer Graphene–Quantum Dots Hybrid Structures

^{1}

^{2}

^{*}

## Abstract

**:**

^{8}s

^{−1}strongly inhibits photoinduced processes on the QD surfaces and provides photostability for QD-based structures.

## 1. Introduction

_{2}and graphene [21,22]. Hybrid systems based on graphene and semiconductor QDs are the most popular solutions for achieving high efficiency in the conversion of absorbed energy into useful signals for the final photoelectric device [23,24,25,26].

^{8}s

^{−1}should strongly inhibit photoinduced processes on the QDs’ surfaces and provide photostability for QD-based structures.

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Ligand Exchange Procedure

#### 2.3. Formation of MLG–QD Hybrid Structures Layered on Slides

#### 2.4. Characterization of the Structures

_{i}is the amplitude of the ith (1st or 2nd) decay component and τ

_{i}is the characteristic decay time of the ith component.

_{smpl}and φ

_{ref}are the quantum yields for QDs and rhodamine 6G luminescence, respectively; I

_{smpl}and I

_{ref}are the intensities at the maxima of the luminescence band for QDs and rhodamine 6G, respectively; D

_{smpl}and D

_{ref}are optical densities at the luminescence excitation wavelength for QDs and rhodamine 6G, respectively; n

_{smpl}and n

_{ref}are the refractive indices of the solvents toluene and ethanol, respectively [40].

## 3. Results and Discussion

#### 3.1. PL Kinetics of CdSe QDs in MLG–QD Structures

_{1}and τ

_{2}, are further referred to as fraction 1 and fraction 2, respectively.

_{ri}and k

_{nri}are radiative and nonradiative rates of exciton deactivation, respectively; τ

_{i}and φ

_{i}are the PL decay time and QY of the 1st and 2nd QD fractions, respectively.

_{ri}= k

_{r}= 4∙10

^{7}s

^{−1}), according to [42]. Therefore, the difference in PL decay time is caused by different nonradiative rates (k

_{nri}) only. In our QD monolayer samples, both QD PL fractions with τ

_{1}~2 ns and τ

_{2}~7 ns give approximately the same contribution to the PL signal. A schematic representation of various radiative and nonradiative relaxation pathways of the excited state in QDs for a MLG–QD hybrid structure system is shown in Figure 2.

_{0}are QD PL intensities after and before interaction with MLG; A

^{i}is the amplitude of the ith QD fraction; τ

^{i}is the PL decay time of the ith QD fraction; τ

_{r}is the radiative time of CdSe QDs, which is equal to 25 ns [32].

_{i}

^{G}) with MLG in the structures, as follows:

_{i}

^{G}is the PL decay time of the ith QD fraction in hybrid structures.

_{i}

^{G}) of their interactions with MLG (Q

_{i}

^{G}referring to Table 2, showing 98% and 97.5% for 1st and 2nd QD fractions, respectively). It should be noted that using Equation (7), we can estimate the minimal rate of QD interaction with MLG that totally quenches the PL of QDs in the hybrid structures, which is no less than 0.4 × 10

^{8}s

^{−1}.

#### 3.2. Photoelectric Properties of MLG–QD Hybrid Structures

_{i}is the amplitude and t

_{i}is the characteristic time for stage I. The fitting parameters of the photoresponses for MLG and hybrid structures are presented in Table 3.

#### 3.3. Photoactivation of MLG–QD Hybrid Structures

^{2}dose showed up to 1.5-fold growth of both the average PL decay time calculated using Equation (2) and the PL intensity. This means that irradiation of QD monolayer samples on dielectric slides by 72 J/cm

^{2}at 405 nm light leads to a decrease in the nonradiative rate in QDs from 6.9 × 10

^{7}s

^{−1}to 2.7 × 10

^{7}s

^{−1}, according to Equations (4) and (5). At the same time, as clearly seen from Figure 4a, there are no changes in the PL decay time or PL intensity of QDs in the MLG–QD hybrid structures. The efficiency of QD photoactivation processes (Q

^{PA}) calculated with Equations (S1) and (S2) clearly decreases by an order of magnitude for QDs located on MLG, in comparison with QDs located on dielectric substrates (i.e., Q

^{PA}of ~100% for QDs on dialectic substrates vs. Q

^{PA}of ~10% for MLG–QDs). This means that the rate of charge or energy transfer from QDs to MLG is much larger than the rate of nonradiative exciton relaxation processes in which trap states are involved. This allows us to estimate the minimal average charge or energy transfer rate from QDs to MLG in our hybrid structures as 〈k

^{G}〉 ≥ 1.0 × 10

^{8}s

^{−1}(according to Table 2). This value demonstrates an excellent agreement with the k

^{G}value estimated for the 1st QD luminescent fraction using PL quenching of QDs in the MLG–QD hybrid structures. This also implies that the k

^{G}value for both QD luminescent fractions is at least 1.0 × 10

^{8}s

^{−1}(see Table 2).

## 4. Conclusions

^{8}s

^{−1}is possible. We show that efficient interaction of QDs with MLG in our MLG–QDs hybrid structures enhances the photoresponse of these structures by a factor of up to 1.5 with respect to MLG. The analysis of the electric properties of the hybrid structures shows that the characteristic photoresponse time of the hybrid structures depends only on the electric properties of MLG and on the architecture of the hybrid structures. We demonstrate for the first time that QD photoactivation in hybrid structures can be an efficient tool for the estimation of the interaction rate of QDs–MLG.

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Photoluminescence (PL) decay curves of 5.5 nm CdSe–ZnS quantum dots (QDs) on a dielectric slide (green rectangles) and in the multilayer graphene (MLG)–QD structures (black triangles). A 405 nm pulse laser was used for PL excitation of QDs. Solid lines (green and black) are biexponential fits of the decay. Inset: table showing the fitting parameters of PL decay curves.

**Figure 2.**Schematic representation of radiative and nonradiative pathways of the excited state in QDs.

**Figure 3.**(

**a**) The photoresponse of MLG (black line) and MLG–QD hybrid structures (red line) with irradiation with a 405 nm laser (dashed regions). (

**b**) Enlarged parts of the curves (black and red circles) with biexponential fitting (green and purple line).

**Figure 4.**Photoactivation of QD monolayer and MLG–QD hybrid structures. (

**a**) Irradiation dose dependence of the photoresponse amplitude of MLG–QD structures (blue spots) and of the average PL decay time of QDs layered on dielectric slides (red triangles) and in the hybrid structures (green rectangles). Solid lines are guidelines for the eye. (

**b**) Photoluminescence (PL) spectra of QDs before photoactivation (black rectangles and triangles) and after exposure to 72 J/cm

^{2}of UV light (red circles, green rectangles) on dielectric slides (black rectangles, red circles) and in the hybrid structures (black triangles, green rectangles).

Samples | Characteristics |
---|---|

MLG | Multilayered graphene nanobelts with 30–40 nm thickness deposited on a glass slide with titanium contacts |

QDs | 5.5 nm core–shell CdSe–ZnS QD monolayer deposited on a glass slide from toluene solution |

MLG–QDs | Hybrid structures with 5.5 nm core–shell CdSe–ZnS QD monolayer deposited on the MLG on a glass slide with titanium contacts |

**Table 2.**The photophysical properties of CdSe–ZnS QD monolayers on glass slides and in hybrid structures based on MLG.

Samples | Parameters | Units | 1st QD fraction | 2nd QD fraction | Formula |
---|---|---|---|---|---|

CdSe–ZnS QDs monolayer | k_{r} | s^{−1} | 0.4 × 10^{8} | - | |

φ_{i} | % | 8.4 | 27.7 | (5) | |

τ_{i}^{QD} | ns | 2.1 ± 0.1 | 6.9 ± 0.1 | - | |

A_{i} | Counts | 1350 ± 50 | 1100 ± 50 | - | |

k_{nr} | s^{−1} | 4.3 × 10^{8} | 1.0 × 10^{8} | (4) | |

Hybrid structures | τ_{i}^{G} | ns | 1.2 ± 0.1 | 5.5 ± 0.3 | - |

A_{i} | Counts | 47 ± 2 | 34 ± 2 | - | |

k_{i}^{G} | s^{−1} | (3.4 ± 0.2) × 10^{8} | (0.40 ± 0.05) × 10^{8} | (7) | |

Q_{i}^{G} | % | 98 ± 2 | 97.5 ± 2 | (6) | |

Q^{G} | % | 98 ± 2 | (6) |

Samples | Stage II | Stage III | ||
---|---|---|---|---|

A_{II,}μA | t_{II,}s | A_{III,}μA | t_{III,}s | |

MLG | 2.5 ± 0.15 | 0.25 ± 0.03 | 5.9 ± 0.1 | 5.1 ± 0.3 |

MLG–QD Hybrid structures | 4.4 ± 0.15 | 0.31 ± 0.02 | 11.1 ± 0.1 | 5.3 ± 0.2 |

^{1}The fitting parameters A

_{i}and t

_{i}are obtained with Equation (8).

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**MDPI and ACS Style**

Reznik, I.; Zlatov, A.; Baranov, M.; Zakoldaev, R.; Veniaminov, A.; Moshkalev, S.; Orlova, A.
Photophysical Properties of Multilayer Graphene–Quantum Dots Hybrid Structures. *Nanomaterials* **2020**, *10*, 714.
https://doi.org/10.3390/nano10040714

**AMA Style**

Reznik I, Zlatov A, Baranov M, Zakoldaev R, Veniaminov A, Moshkalev S, Orlova A.
Photophysical Properties of Multilayer Graphene–Quantum Dots Hybrid Structures. *Nanomaterials*. 2020; 10(4):714.
https://doi.org/10.3390/nano10040714

**Chicago/Turabian Style**

Reznik, Ivan, Andrey Zlatov, Mikhail Baranov, Roman Zakoldaev, Andrey Veniaminov, Stanislav Moshkalev, and Anna Orlova.
2020. "Photophysical Properties of Multilayer Graphene–Quantum Dots Hybrid Structures" *Nanomaterials* 10, no. 4: 714.
https://doi.org/10.3390/nano10040714