Quantum emitters in low dimensional van der Waals systems Von der Fakultät 8 Mathematik und Physik der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Vorgelegt von Natan Chejanovsky aus Jerusalem Hauptberichter: Prof. Dr. J. Wrachtrup Mitberichter: Prof. Dr. S. Loth Tag der mündlichen Prüfung: 09. 07. 2019 3. Physikalisches Institut der Universität Stuttgart, Stuttgart, Deutschland Max Planck Institut für Festkörperforschung, Stuttgart, Deutschland 2019 Contents Contents .................................................................................................................................................. 1 1 Summary ......................................................................................................................................... 9 2 Introduction and basic concepts ................................................................................................... 17 2.1 Optical excitation and detection ........................................................................................... 17 2.2 Single molecule excitation .................................................................................................... 18 2.3 Band-gap semiconductors and point defects ....................................................................... 20 2.3.1 Point defect orientation in crystals ............................................................................... 20 2.3.2 Optical excitation of single emitting point defects in semiconducting crystals ........... 21 2.3.3 Point defect charge states ............................................................................................ 23 2.3.4 Zero phonon line and phonon side band ...................................................................... 24 2.3.5 Cryogenic behavior of ZPL and PSB ............................................................................... 25 6.3.2 Well-studied 3D Crystals point defects for quantum applications ............................... 26 6.3.2 Quantum Yield .............................................................................................................. 30 2.4 Raman spectroscopy of 2D materials ................................................................................... 30 2.5 Nanomaterial - 2D Crystals ................................................................................................... 31 2.5.1 Van der Waals crystals .................................................................................................. 31 2.5.2 Excitonic effects in 2D materials ................................................................................... 35 2.5.3 Quantum dots ............................................................................................................... 36 2.5.4 Blinking .......................................................................................................................... 37 2.5.5 Quantum confinement.................................................................................................. 39 2.5.6 Candidates for intra-bandgap excitation ...................................................................... 39 2.6 Nanomaterials – van der Waals 1D Crystals ......................................................................... 41 2.6.1 Carbon based ................................................................................................................ 41 2.6.2 Boron nitride based ...................................................................................................... 42 2.6.3 TMDC based .................................................................................................................. 43 2.7 Hetero-structures .................................................................................................................. 43 2.7.1 Assembly ....................................................................................................................... 45 2.7.2 Band alignment ............................................................................................................. 45 2.7.3 Moiré engineering in 2D materials ............................................................................... 46 2.7.4 Interlayer coupling ........................................................................................................ 47 2.7.5 BN encapsulation and line width narrowing ................................................................. 48 3 Two dimensional hexagonal boron nitride (h-BN) ........................................................................ 51 1 3.1 Structure ............................................................................................................................... 51 3.2 DFT and defect calculation in h-BN ....................................................................................... 52 3.2.1 Boron vacancy ............................................................................................................... 53 3.2.2 Nitrogen anti-site defect (NBVN) .................................................................................... 54 3.2.3 Boron vacancy with carbon at nitrogen site (VBCN) ...................................................... 55 3.3 Band to band excitation ........................................................................................................ 56 3.4 Previous Intra-band excitation studies ................................................................................. 56 3.5 Common fabrication of BN ................................................................................................... 56 3.6 Transfer mechanisms of BN to substrate .............................................................................. 56 3.7 Emitter perimeter location tendency ................................................................................... 57 3.8 Spectral and autocorrelation properties exfoliated h-BN .................................................... 61 3.9 Spectral and autocorrelation properties of graphene supermarket CVD h-BN .................... 63 3.10 Emitter’s photo-physics ........................................................................................................ 64 3.11 Chemical testing and etching methods................................................................................. 67 3.11.1 Etching method I (H2O2:H2SO4 ).................................................................................. 67 3.11.2 Etching method II (+H3PO4:H2SO4) ............................................................................. 67 3.12 Ion irradiation ....................................................................................................................... 67 3.13 Emitter pair’s excitation polarization comparison ................................................................ 68 3.14 Chemical and gas environment treatments.......................................................................... 69 3.15 In-house grown CVD h-BN .................................................................................................... 70 3.16 Bulk excitation of an h-BN crystal ......................................................................................... 71 3.17 Paramagnetic point defects .................................................................................................. 72 3.18 Conclusions ........................................................................................................................... 73 4 One dimensional Boron nitride nanotubes (BNNT) ...................................................................... 79 4.1 Introduction .......................................................................................................................... 80 4.1.1 Structure and components ........................................................................................... 81 4.1.2 The role of oxygen in BN hexagonal systems ................................................................ 82 4.2 Results ................................................................................................................................... 83 4.2.1 Bulk BNNT ..................................................................................................................... 83 4.2.2 Raman analysis .............................................................................................................. 86 4.2.3 Power-dependent photo-dynamics for Figure 4.2.c ..................................................... 87 4.2.4 Isolating single emitters in BNNT .................................................................................. 88 4.2.5 SEM resolution of entwined BNNT SQEs ...................................................................... 91 4.2.6 BNNT Exfoliation ........................................................................................................... 94 4.2.7 SQE spectrum and ZPL analysis ..................................................................................... 94 2 4.2.8 SQE photo-dynamics analysis ....................................................................................... 95 4.2.9 PL spectra asymmetric fitting functions........................................................................ 96 4.2.10 Example of absorbed PSB for suspended material ....................................................... 96 4.2.11 SEM resolution of hybrid material SQEs ......................................................................