# Optoelectronic Properties of Monolayer Hexagonal Boron Nitride on Different Substrates Measured by Terahertz Time-Domain Spectroscopy

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## Abstract

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## 1. Introduction

^{−2}) was found to be much higher compared to ML MoS${}_{2}$ or graphene (about 5 mS cm

^{−2}). The electric transport through mono- and multi-layer hBN has also been examined [8]. It was found that the electron transport through the graphite/hBN/graphite heterostructure was also increased significantly when hBN was reduced to a monolayer. Hence, ML hBN has high conducting values compared to bulk or multi-layer hBN. More importantly, in recent years, ML hBN has been applied for the realization of van der Waals heterojunctions in combination with other 2D electronic systems such as graphene [9] and transition metal dichalcogenide (TMD) based 2D electronic systems [10]. ML hBN has been used as a dielectric substrate for graphene based electronic devices, owing to its unique characteristics like thermal stability and as an insulator [11]. Encapsulation of ML MoS${}_{2}$ by hBN has drastically altered the electronic and optical responses of ML MoS${}_{2}$ because of the hybridization of the electronic states between MoS${}_{2}$ and hBN [12]. For example, through the measurements of temperature dependent and time resolved photoluminescence (PL), it has been found that the momentum forbidden dark excitons can be observed with energy lower than 83 meV in an ML MoS${}_{2}$/hBN heterojunction. In recent years, the investigation of ML hBN based van der Waals heterojunctions has become a hot and fast-growing field of research in electronics and optoelectronics [13].

## 2. Samples and Experimental Measurements

#### 2.1. Sample Growth

#### 2.2. THz TDS Measurement System

## 3. Results and Discussion

## 4. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

ML | Monolayer |

hBN | Hexagonal boron nitride |

2D | Two-dimensional |

Si | Silicon |

SiO${}_{2}$ | Silicon dioxide |

PET | Polyethylene terephthalate |

THz | Terahertz |

TDS | Time-domain spectroscopy |

UV | Ultraviolet |

LEDs | Light-emitting diodes |

TMD | Transition metal dichalcogenide |

PL | Photoluminescence |

SOC | Spin-orbit coupling |

CVD | Chemical vapor deposition |

PMMA | Poly methyl methacrylate |

MOCVD | Metal organic chemical vapor deposition |

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**Figure 1.**The THz electric field strength transmitted through ML hBN on the quartz substrate (red curve) and through the bare quartz substrate (black curve), respectively, as a function of delay time at T = 280 K. The inset shows the corresponding amplitudes and phase angles of the THz electric field strengths transmitted through the ML hBN sample and the substrate in the frequency domain. The results for the phase angles coincide roughly.

**Figure 2.**(

**a**) Real ${\sigma}_{1}\left(\omega \right)$ and (

**b**) imaginary ${\sigma}_{2}\left(\omega \right)$ parts of the optical conductivity as a function of radiation frequency $f=\omega /2\pi $ at different temperatures for ML hBN on different substrates as indicated. Here, ${\Sigma}_{0}={e}^{2}/4\hslash =6.07\times {10}^{-5}$ S.

**Figure 3.**The experimental and fitted (through Drude–Smith formula) real ${\sigma}_{1}\left(\omega \right)$ and imaginary ${\sigma}_{2}\left(\omega \right)$ parts of optical conductivity as a function of radiation frequency $f=\omega /2\pi $ for ML hBN on PET, sapphire, quartz, and SiO${}_{2}$/Si substrates at 280 K, respectively. Here, the solid curves are obtained from the Drude–Smith formula, and the dots are experimental data. ${\Sigma}_{0}={e}^{2}/4\hslash $.

**Figure 4.**(

**a**) Electronic relaxation time, (

**b**) hole density, and (

**c**) electronic localization factor for ML hBN placed on sapphire, quartz, PET, and SiO${}_{2}$/Si substrates as a function of temperature from 80 K to 280 K.

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**MDPI and ACS Style**

Bilal, M.; Xu, W.; Wang, C.; Wen, H.; Zhao, X.; Song, D.; Ding, L.
Optoelectronic Properties of Monolayer Hexagonal Boron Nitride on Different Substrates Measured by Terahertz Time-Domain Spectroscopy. *Nanomaterials* **2020**, *10*, 762.
https://doi.org/10.3390/nano10040762

**AMA Style**

Bilal M, Xu W, Wang C, Wen H, Zhao X, Song D, Ding L.
Optoelectronic Properties of Monolayer Hexagonal Boron Nitride on Different Substrates Measured by Terahertz Time-Domain Spectroscopy. *Nanomaterials*. 2020; 10(4):762.
https://doi.org/10.3390/nano10040762

**Chicago/Turabian Style**

Bilal, Muhammad, Wen Xu, Chao Wang, Hua Wen, Xinnian Zhao, Dan Song, and Lan Ding.
2020. "Optoelectronic Properties of Monolayer Hexagonal Boron Nitride on Different Substrates Measured by Terahertz Time-Domain Spectroscopy" *Nanomaterials* 10, no. 4: 762.
https://doi.org/10.3390/nano10040762