TY - THES

T1 - Exfoliation and characterization of natural two-dimensional magnetic flakes

AU - Gradwohl, Kevin-Peter

N1 - no embargo

PY - 2018

Y1 - 2018

N2 - Since the discovery of graphene, the field of two-dimensional (2D) materials is rapidly growing, due to their extraordinary physical properties. However, 2D materials with intrinsic magnetism are to some extend prohibited by the Mermin-Wagner theorem and so far no stable 2D magnetic materials - at least under ambient conditions - have been reported. Here, a mineral aggregate from a private mineral collection, exhibiting macroscopic magnetic properties and obvious cleavage, was examined towards 2D magnetism. Analysis of the mineral aggregate revealed that major present phases are sulfidic and oxidic iron compounds such as hematite, magnetite, pyrite, and chalcopyrite. Micromechanical exfoliation was used to prepare nanometer thin flakes from the mineral aggregate. Best flake output and quality was achieved by using only hematite rich parts of the mineral, indicated by their red and transparent appearance. The flakes were transferred onto a Si-wafer with a 300 nm thick SiO2 layer. Fabry-Pérot interference was utilized to identify the few nanometer thick flakes on the wafer in the optical microscope. The flakes were further investigated by Atomic Force Microscopy. Their lateral dimension reached up to 15 µm x 15 µm, while their thickness was reduced by the exfoliation process to only 1.5 nm. Energy dispersive x-ray spectroscopy revealed Magnesium as a major component of the flakes. Raman spectroscopic measurements revealed presents of hydroxide groups, which points towards the monoclinic Talc Mg3Si4O10(OH)2 crystal structure. Two further Raman peaks, with a Raman shift associated to the vibrational modes of a metal-oxygen bond were found. They are in good agreement with the Raman modes of talc (steatite). Magnetic Force Microscopy in dual-pass as well as plane scan modes was applied to observe magnetic properties of the flakes. The effect of different magnetization of the tip as well as the flakes was studied. This led us to the conclusion that magnetic properties arise from a substitution of Mg atoms with magnetic transition metal atoms - most probably iron atoms - due to the chemistry of the mineral aggregate. The outstanding flake quality, their stability under ambient conditions, as well as their magnetic properties make them interesting for application not only in data storage devices and spintronics but also as a substrate in organic electronics. Therefore, the magnetic flakes were used as a substrate for growth of rod-like organic molecules such as DHTA7 and 6P by hot wall epitaxy. Their complex growth modes need further experimental investigation to be fully understood.

AB - Since the discovery of graphene, the field of two-dimensional (2D) materials is rapidly growing, due to their extraordinary physical properties. However, 2D materials with intrinsic magnetism are to some extend prohibited by the Mermin-Wagner theorem and so far no stable 2D magnetic materials - at least under ambient conditions - have been reported. Here, a mineral aggregate from a private mineral collection, exhibiting macroscopic magnetic properties and obvious cleavage, was examined towards 2D magnetism. Analysis of the mineral aggregate revealed that major present phases are sulfidic and oxidic iron compounds such as hematite, magnetite, pyrite, and chalcopyrite. Micromechanical exfoliation was used to prepare nanometer thin flakes from the mineral aggregate. Best flake output and quality was achieved by using only hematite rich parts of the mineral, indicated by their red and transparent appearance. The flakes were transferred onto a Si-wafer with a 300 nm thick SiO2 layer. Fabry-Pérot interference was utilized to identify the few nanometer thick flakes on the wafer in the optical microscope. The flakes were further investigated by Atomic Force Microscopy. Their lateral dimension reached up to 15 µm x 15 µm, while their thickness was reduced by the exfoliation process to only 1.5 nm. Energy dispersive x-ray spectroscopy revealed Magnesium as a major component of the flakes. Raman spectroscopic measurements revealed presents of hydroxide groups, which points towards the monoclinic Talc Mg3Si4O10(OH)2 crystal structure. Two further Raman peaks, with a Raman shift associated to the vibrational modes of a metal-oxygen bond were found. They are in good agreement with the Raman modes of talc (steatite). Magnetic Force Microscopy in dual-pass as well as plane scan modes was applied to observe magnetic properties of the flakes. The effect of different magnetization of the tip as well as the flakes was studied. This led us to the conclusion that magnetic properties arise from a substitution of Mg atoms with magnetic transition metal atoms - most probably iron atoms - due to the chemistry of the mineral aggregate. The outstanding flake quality, their stability under ambient conditions, as well as their magnetic properties make them interesting for application not only in data storage devices and spintronics but also as a substrate in organic electronics. Therefore, the magnetic flakes were used as a substrate for growth of rod-like organic molecules such as DHTA7 and 6P by hot wall epitaxy. Their complex growth modes need further experimental investigation to be fully understood.

KW - magnetisch

KW - Magnetismus

KW - 2D

KW - Graphen

KW - Mermin

KW - Wagner

KW - Spaltbarkeit

KW - Exfolierung

KW - Eisen

KW - Eisenoxid

KW - Hämatit

KW - Fabry

KW - Pérot

KW - Interferenz

KW - EDS

KW - EDX

KW - AFM

KW - Rasterkraftmikroskopie

KW - Phyllosilikat

KW - Talk

KW - Steatit

KW - MFM

KW - Magnetkraftmikroskopie

KW - Magnesium

KW - organische Elektronik

KW - Datenspeicherung

KW - 6P

KW - DHTAP

KW - DHTA7

KW - Hot-Wall

KW - Epitaxy

KW - magnetism

KW - exfoliation

KW - characterization

KW - 2D

KW - iron

KW - talc

KW - hematite

KW - Fabry

KW - Pérot

KW - interference

KW - AFM

KW - atomic force microscopy

KW - EDS

KW - EDX

KW - steatite

KW - Raman

KW - spectroscopy

KW - magnetic force microscopy

KW - MFM

KW - data storage

KW - organic electronics

KW - DHTAP

KW - DHTA7

KW - 6P

KW - hot wall epitaxy

M3 - Master's Thesis

ER -