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| Author(s): |
Ricardo Javier Peña Román
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin |
| Defense date: | 2022-07-05 |
| Examining board members: |
Luiz Fernando Zagonel;
Daniel Mário Ugarte;
Abner de Siervo;
Luiz Gustavo de Oliveira Lopes Cançado;
Fernando Loureiro Stavale Junior
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| Advisor: | Luiz Fernando Zagonel |
| Abstract | |
This thesis presents a scanning tunneling microscopy (STM) study of atomically thin materials. We study exfoliated p-doped monolayers of tungsten disulfide (WSe2) on gold thin film substrates and epitaxial monolayers of hexagonal boron nitride (h-BN) on graphite. We combine STM measurements and related techniques with complementary characterizations employing atomic force microscopy and optical spectroscopy for investigating aspects such as point defects, sample doping, sample-substrate coupling, and their impact on the electronic and optical properties of the material. Firstly, we present the design and implementation of a new light collection device based on an off-axis parabolic mirror with 72% of collection efficiency to perform luminescence experiments in STM. The optical device can be used as an optical accessory of an adapted Pan STM, able to operate at low temperature and ultra-high vacuum (UHV) conditions without affecting the performance of the microscope. The potential of the device is demonstrated by performing STM-luminescence as well as in-situ Photoluminescence (PL)/Raman measurements on several systems. Secondly, we report the observation of excitons electrically generated on monolayers of TMD onto metallic substrates by using tunneling electrons in an STM. The as-transferred samples of WSe2 monolayers are optically active due to an interfacial water layer that decouples the material from the underlying substrate. The intrinsic sample-substrate decoupling allows exciting the local electroluminescence of the sample via STM-induced light emission (STM-LE) in ambient conditions. The presence of the interfacial water layer is due to air moisture, and it is a consequence of the sample preparation method. The STM-LE and PL spectra are similar, indicating that the luminescence is due to the recombination of neutral and charged (trions) excitons. The trion to exciton ratio is controlled with the tunneling current setpoint. Excitons can only be generated at sample bias voltage above 2.0 V, i.e., with tunneling electrons at energies equal to or above the electronic band gap of monolayer WSe2. STM images and scanning tunneling spectroscopy (STS) measurements under UHV conditions demonstrated the presence of intrinsic point defects and confirmed the p-type doping of the sample. The proposed STM- LE excitation mechanism is the elastic tunneling of electrons and the direct injection of carriers on the conduction band of the semiconductor. Finally, we present the measurement of the electronic band gap and the exciton binding energy of monolayer h-BN. The sharp interface in the van der Waals heterostructure of h- BN on graphite enables the employ of low-temperature STS measurements to determine the electronic band gap. We demonstrate that defect-free monolayer h-BN on graphite has an electronic band gap of 6.8±0.2 eV and an exciton binding energy of 0.7±0.2 eV. These values are about 1 eV lower than predicted for a free-standing monolayer. In addition, in some regions of the monolayer h-BN, we identified point defects by STM imaging, which have intragap electronic levels around (AU) | |
| FAPESP's process: | 18/08543-7 - Light emission by individual defects in 2D materials |
| Grantee: | Ricardo Javier Peña Román |
| Support Opportunities: | Scholarships in Brazil - Doctorate |