Optically active quantum dots in monolayer WSe2

Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons(1-3). In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature(4). Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large similar to 1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures(5) and to exploit the spin-valley degree of freedom(6) renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing.

Bandgap engineering in a nanowire: self-assembled 0, 1 and 2D quantum structures

Inherent to the nanowire morphology is the exciting possibility of fabricating materials organized at the nanoscale in three dimensions. Composition and structure can be varied along and across the nanowire, as well as within coaxial shells. This opens up a manifold of possibilities in nanoscale materials science and engineering which is only possible with a nanowire as a starting structure. As the variation in composition and structure is accompanied by a change in the band structure, it is possible to confine carriers within the nanowire. Interestingly, this results in the formation of local two, one and zero-dimensional structures from an electronic point of view within the nanowire. This novel palette of nanostructures paves the way toward novel applications in many engineering domains such as lasers, high-mobility transistors, quantum information and energy harvesting. In the present review we summarize and give an overview on recent achievements in the design and growth of advanced quantum structures starting from nanowire templates. The quantum structures presented have been grown by molecular beam epitaxy and correspond to different confinement approaches: quantum wells (2D), quantum wires (1D) and quantum dots (0D).

Elsevier2013

Grain boundaries and quantum transport in monolayer MoS2

Kolyo Marinov

The growing research on two-dimensional materials reveals their exceptional physical properties and enormous potential for future applications and investigation of advanced physics phenomena. They represent the ultimate limit in terms of active channel thickness as they consist only of few atomic layers and hold a great promise for future scaling of electronic devices. Further, their unique band structure provides a platform for the exploration of novel physics in the field of spin and valleytronics. The presence of a large band gap allows fully electrostatic control over doping and current flow, as well as the realization of novel optoelectronic devices. On of the most appealing and readily available 2D material is molybdenum disulphide (MoS2). This thesis is concentrated on the electronic properties of monolayers MoS2 ranging from the investigation of line defects in synthetically grown material to advanced study of the confinement of charge carriers at low temperature as function of magnetic field. A chapter is describing the details of each of the principal experiments. In Chapter 4 we study the properties of epitaxially grown MoS2 monolayers by means of electrical measurements and advanced scanning probe techniques. After confirming the high quality of the grown crystallites, we investigate the predominant types of occurring grain boundaries by means of Kelvin probe force microscopy. Due to the highly oriented growth, we find that the electronic properties of the film are homogeneous even across the line defects. This fact is confirmed by the observation of constant mobility even in devices over large area containing multiple grain boundaries. Chapter 5 presents different approaches for the fabrication of high-quality dual-gated MoS2 field effect transistors (FETs). Dielectric layers deposited by means of atomic layer deposition can guarantee good mobility and low contact resistance, but the observation of low-dimensional effects may be hindered due to the presence of charged impurities. On the other hand the use of hexagonal boron nitride as dielectric and few layer graphene as contact material is shown to yield very high mobility and low contact resistance. Using dual gated FETs with such material configuration, an electrostatically controlled quantum point contact (QPC) is realized in monolayer MoS2 and presented in Chapter 6. We find that the device conductance is quantized in multiples of e^2/h revealing the lifting of spin and valley degeneracies. Further, we perform bias spectroscopy at different magnetic fields and extract subband energy spacing from 0.8 (at 0T) to 2 meV (at 9.8T) and g-factor in the conduction band of 2.12±0.13. Finally, in Chapter 7 we propose several methods aiming the fabrication of a quantum dot in MoS2. The electrostatic definition of a small conductive island, weakly couples to two highly doped charge carrier reservoirs, requires similar advanced engineering of the dielectric material environment. Despite the low stability, we find possible regimes, in which controlled tunneling from and to a well defined quantum dot can be detected. The charging energy measured electrically is in very good agreement with the lithographically defined size of the island. In conclusion, this work provides insights about the electron transport in monolayer MoS2 exploring grain boundaries, dielectric environment and electron confinement. The results contribute to better understanding of its electronic properties.

EPFL2018

Investigation into the coupling of quantum dots to photonic crystal nanocavities at telecommunication wavelengths

Laurent Balet

Recently, the emission of single photons with emission wavelength in the 1.3 µm telecommunication window was demonstrated for InAs quantum dots. This makes them strong candidates for applications such as quantum cryptography, and in a longer term, quantum computing. However, efficient extraction of the spontaneous emission from semiconductors still represents a major challenge due to total internal reflection at the semiconductor/air interface. In particular, single photon sources based on quantum dots are plagued by low extraction efficiency and poor coupling to single-mode fibers, typically on the order of 10-3 ~ 10-4, which prevents their application to quantum communication. To seek a solution to this problem, this thesis work explores the integration of quantum dots, with emission at 1.3 µm, in photonic crystal microcavities. Photons emitted in a mode of the cavity are funneled out of the semiconductor, and thus bypass the total internal reflection. In addition, the modified density of electromagnetic states in the cavity affects the emission lifetime of a weakly coupled emitter: in resonance, we assist to an increase of the emission rate, known as the Purcell effect, that would allow faster data transmission. Photonic crystal microcavities conveniently address this objective as they provide modes with the required small volumes and high quality factors. They also allow the engineering of the farfield pattern of the cavity modes, and thus of the collection efficiency. In the following pages, after brie y reviewing single photon emitters, the Purcell effect, and photonic crystal cavities, we present our results on the coupling of quantum dots to photonic crystal cavities. We report on the different strategies we used to control the tuning between the cavity mode and the quantum dot emission frequency. We also show our efforts in improving the collection of coupled photons by engineering the shape of the microcavity. Finally, we present our time-resolved measurements demonstrating the Purcell effect under optical and electrical operation.