Optical properties of MoSe2_2 monolayer implanted with ultra-low energy Cr ions

The paper explores the optical properties of an exfoliated MoSe$_2$ monolayer implanted with Cr$^+$ ions, accelerated to 25 eV. Photoluminescence of the implanted MoSe$_2$ reveals an emission line from Cr-related defects that is present only under weak electron doping. Unlike band-to-band transition, the Cr-introduced emission is characterised by non-zero activation energy, long lifetimes, and weak response to the magnetic field. To rationalise the experimental results and get insights into the atomic structure of the defects, we modelled the Cr-ion irradiation process using ab-initio molecular dynamics simulations followed by the electronic structure calculations of the system with defects. The experimental and theoretical results suggest that the recombination of electrons on the acceptors, which could be introduced by the Cr implantation-induced defects, with the valence band holes is the most likely origin of the low energy emission. Our results demonstrate the potential of low-energy ion implantation as a tool to tailor the properties of 2D materials by doping

Modification of monolayer-thick semiconductors by ultra-low energy ion implantation

Die vorliegende Doktorarbeit beschäftigt sich mit der ultra Niederenergie (ULE) Ionenimplantation zur Modifikation von 2D Materialien wie Graphen und Übergangsmetalldichalcogeniden (TMDs). Das bestehende Implantationssystem wird auf zwei Ebenen modifiziert. Durch die Entwicklung einer Sputter Ionenquelle lassen sich die zur Verfügung stehenden Elemente zur Implantation durch Elemente mit hohem Schmelzpunkt und niedrigem Gasdruck erweitern. Des Weiteren wird die Implantationskammer zusammen mit dem Probenhalter überarbeitet, sodass deponierte Elektroden auf der Probenoberfläche oder einfache Bronzeplättchen zur lateral kontrollierten ULE Ionenimplantation verwendet werden können. Anhand des Einbaus von Edelgasen in CVD gewachsenes Graphen wird der Einfluss der Implantationsenergie sowie Fluenz auf die induzierten Defekte in das Graphengitter untersucht. Außerdem wird Graphen mit Mn als Modellfall eines magnetischen Dotanden bestrahlt. Der Übertrag der Implantationsmethode von Graphen auf TMDs wird durch den Einbau von Se in MoS2 durchgeführt. Durch das Etablieren der Se Bestrahlung bei erhöhten Temperaturen wird ein reproduzierbarer Prozess zur TMD Modifkation entwickelt. Die Analyse von Cr implantierten MoS2 zeigt die Möglichkeit eines substitutionellen Austauschs im Übergangsmetalluntergitter.In this thesis, ultra-low energy (ULE) ion implantation is used to modify 2D materials such as graphene and transition metal dichalcogenides (TMDs). The existing implantation system is improved in two respects. By developing a sputter ion source, the list of implantable elements is expanded to include elements with high melting point and low vapour pressure. Further, the implantation stage and sample holder is extended to enable a lateral controlled ULE ion implantation using deposited electrodes on the sample or bronze plates. Based on the incorporation of noble gases into CVD grown graphene, the influence of implantation energy and fluence on the introduced defects in the host lattice is investigated. Moreover, graphene is irradiated with Mn as a model case of a magnetic dopant. It is shown that the implantation method for graphene is applicable to TMDs as well, by incorporating Se into MoS2. By establishing Se irradiation at elevated temperatures, a reproducible process for TMD modification is developed. The investigation of Cr implanted MoS2 demonstrates that substitutional exchange in the transition metal sublattice is possible.2022-03-1

Lateral Controlled Doping and Defect Engineering of Graphene by Ultra-Low-Energy Ion Implantation

In this paper, the effectiveness of ultra-low-energy ion implantation as a means of defect engineering in graphene was explored through the measurement of Scanning Kelvin Probe Microscopy (SKPM) and Raman spectroscopy, with boron (B) and helium (He) ions being implanted into monolayer graphene samples. We used electrostatic masks to create a doped and non-doped region in one single implantation step. For verification we measured the surface potential profile along the sample and proved the feasibility of lateral controllable doping. In another experiment, a voltage gradient was applied across the graphene layer in order to implant helium at different energies and thus perform an ion-energy-dependent investigation of the implantation damage of the graphene. For this purpose Raman measurements were performed, which show the different damage due to the various ion energies. Finally, ion implantation simulations were conducted to evaluate damage formation.Volkswagen FoundationDFGOpen Access Publication Funds of the Göttingen UniversityOpen-Access-Publikationsfonds 202

Lateral Controlled Doping and Defect Engineering of Graphene by Ultra-Low-Energy Ion Implantation

In this paper, the effectiveness of ultra-low-energy ion implantation as a means of defect engineering in graphene was explored through the measurement of Scanning Kelvin Probe Microscopy (SKPM) and Raman spectroscopy, with boron (B) and helium (He) ions being implanted into monolayer graphene samples. We used electrostatic masks to create a doped and non-doped region in one single implantation step. For verification we measured the surface potential profile along the sample and proved the feasibility of lateral controllable doping. In another experiment, a voltage gradient was applied across the graphene layer in order to implant helium at different energies and thus perform an ion-energy-dependent investigation of the implantation damage of the graphene. For this purpose Raman measurements were performed, which show the different damage due to the various ion energies. Finally, ion implantation simulations were conducted to evaluate damage formation

A comprehensive study of charge transport in Au-contacted graphene on Ge/Si(001)

We investigate the electronic transport properties of Au-contacted graphene on Ge/Si(001). Kelvin probe force microscopy at room temperature with an additionally applied electric transport field is used to gain a comprehensive understanding of macroscopic transport measurements. In particular, we analyze the contact pads including the transition region, perform local transport measurements in pristine graphene/Germanium, and explore the role of the semiconducting Germanium substrate. We connect the results from these local scale measurements with the macroscopic performance of the device. We find that a graphene sheet on a 2 μm Ge film carries approximately 10% of the current flowing through the device. Moreover, we show that an electronic transition region forms directly adjacent to the contact pads. This transition region is characterized by a width of >100 μm and a strongly increased sheet resistance acting as the bottleneck for charge transport. Based on Rutherford backscattering of the contact pads, we suggest that the formation of this transition region is caused by diffusion. © 2020 Author(s)

New single photon sources by optoelectronic tailoring of 2D materials using low energy ion implantation

Monolayer thick transition metal dichalcogenides (TMDCs) with the chemical formula MX2 (M=Mo, W; X=S, Se), constitute a new class of direct bandgap semiconductors. Their remarkable physical properties resulting from their two dimensional (2D) geometry and lattice symmetry make them an exciting platform for developing photonic devices with new functionalities [1]. Monolayer TMDCs can be easily incorporated into electrically driven devices, which in turn can be coupled to optical microcavities or photonic circuits [2]. This work constitutes a proof-of-principle study to incorporate implanted TMDCs into non-classical single photon emitting diodes [3]. The development of such devices has far-reaching implications for emerging technologies such as quantum cryptography and quantum metrology. In order to make such devices a reality, methods of material modification for these materials, such as ultra-low energy (10-25 eV) ion implantation, must be developed [4,5]. Post-growth doping [6] of TMDCs offers an expanded selection of possible dopants compared to the popular method of doping via CVD growth. The technique allows for highly pure, clean and selective substitutional incorporation of dopants [7] and is also compatible with standard semiconductor processing. Ultra-low energy ion implantation is carried out using the ADONIS mass-selected ion beam deposition system at the University of Gottingen [8]