Single electron quantum dot in two-dimensional transition metal dichalcogenides

In recent years, zinc oxide (ZnO) quantum dots (QDs) have gained much attention due to their quantum and piezoelectric properties, which make them viable candidates for optoelectronic applications and nano-sensing devices. The piezoelectric properties of ZnO QDs are intimately linked to their optoelectronic behavior in response to mechanical stress and applied external forces. First-principle calculations of the electronic properties of the QDs have been performed using density functional theory (DFT) to compute their electronic structure, density of states (DOS), charge density, phonon behavior, and the photoluminescence properties of the material. Moreover, the size-dependent quantum confinement effect can be seen through the discrete energy levels in the band structure, which results in defined peaks in the DOS and blue shift in the photoluminescence. The bandgap energy of ZnO QDs determines the conductivity of the material and is dependent on the size of the QDs. This property in turn allows for tunability of the electronic properties of the QDs as a function of their size and is thereby appropriate for optoelectronic, photonic, and sensing applications. The nano-scale sizes and quantum properties of ZnO QD qubits also make them suitable for applications in quantum computing and data storage.

In this serie of lectures we present some of the basic issues in semiconductor spintronics at an introductory level. We start with a short overview of low dimensional semiconductor structures and their applications. We then move on to discuss the notions of spin generation, injection, relaxation and detection in spintronics, with an emphasis in the application of diluted magnetic semiconductors for spin generation and injection. Finally, we discuss the manipulation of spin in semiconductor nanostructures like quantum dots, and its applications in quantum computation.

Hexagonal boron nitride (h-BN) and semiconducting transition metal dichalcogenides (TMDs) promise greatly improved electrostatic control in future scaled electronic devices. To quantify the prospects of these materials in devices, we calculate the out-of-plane and in-plane dielectric constant from first principles for TMDs in trigonal prismatic and octahedral coordination, as well as for h-BN, with a thickness ranging from monolayer and bilayer to bulk. Both the ionic and electronic contribution to the dielectric response are computed. Our calculations show that the out-of-plane dielectric response for the transition-metal dichalcogenides is dominated by its electronic component and that the dielectric constant increases with increasing chalcogen atomic number. Overall, the out-of-plane dielectric constant of the TMDs and h-BN increases by less than 15% as the number of layers is increased from monolayer to bulk, while the in-plane component remains unchanged. Our computations also reveal that for octahedrally coordinated TMDs the ionic (static) contribution to the dielectric response is very high (4.5 times the electronic contribution) in the in-plane direction. This indicates that semiconducting TMDs in the tetragonal phase will suffer from excessive polar-optical scattering thereby deteriorating their electronic transport properties.

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy (EF) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to EF of the graphene layer and outline a new method to estimate the excitonic binding energies (Eb) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

As the semiconductor industry approaches the physical limits of silicon-based technologies, the quest for alternative materials and innovative technologies has become paramount to continuing Moore's Law and addressing the growing demands for computing power and energy efficiency. This paper explores the frontier of semiconductor innovation, highlighting the emergence of novel materials and technologies poised to transcend the limitations of traditional silicon. The paper examines the properties and potential of materials such as graphene, transition metal di-chalcogenides (TMDCs), and topological insulators, alongside advancements in device architectures like quantum dots and spintronic. The methodology combines a systematic review of recent literature with comparative analysis of material properties, device performance, and scalability challenges. The integration of these emerging materials with current semiconductor manufacturing ecosystems is also discussed. Findings indicate that these materials offer significant advantages over silicon, including higher electron mobility, flexibility, transparency, and superior thermal properties, which could revolutionize the semiconductor industry by enabling faster, smaller, and more energy-efficient devices. These materials are showcasing their potential to enhance computational capabilities, sensor sensitivity, and connectivity for a wide range of applications, from wearable electronics to quantum computing. However, challenges related to synthesis, large-scale production, and integration into current systems persist. Addressing these hurdles requires interdisciplinary research and collaboration between academia, industry, and regulatory bodies to develop standardized methods for material synthesis, characterization, and device fabrication. The implications of findings extend beyond the semiconductor industry, offering insights into the future of electronics, telecommunications, and computing. As we stand on the brink of a new era in semiconductor technology, this paper calls for increased investment in research and development, policy support, and international cooperation to harness the full potential of these emerging materials and technologies, ensuring a sustainable and innovative future for the global semiconductor market.

Background. Quantum computing is a technology that has great potential to revolutionize various scientific disciplines with its extraordinary computing capabilities. Multiple fields, such as cryptography, optimization, and complex simulation, have begun to explore quantum computing applications. However, a deep understanding of how these technologies can be used effectively in cross-disciplinary informatics still needs to be improved. Purpose. This research aims to develop a theoretical framework that explains the role of quantum computing in advancing interdisciplinary informatics. The main focus is to identify potential applications and challenges faced in applying this technology in various fields and evaluate its impact on the development of science and technology. Method. This research uses a qualitative approach, a literature review, and theoretical analysis methods. Data were collected from various academic sources, including journals, books, and conferences relevant to quantum computing and interdisciplinary informatics. Analysis was conducted to identify patterns, themes, and relationships between quantum computing and cross-disciplinary applications. Results. The research results show that quantum computing has the potential to solve complex computational problems in various fields, including cryptography, optimization, and physical simulation. Quantum computing can provide more efficient and faster solutions than classical computing. However, the research also identified several significant challenges, such as the need for quantum-specific algorithms and sophisticated technological infrastructure. Conclusion. Quantum computing is essential in advancing interdisciplinary informatics by providing superior computing capabilities. The theoretical framework developed in this research can be used to further study and develop quantum computing applications in various fields. The identified challenges require special attention to ensure effective and efficient implementation of this technology.

Direct detection of electronic states for individual indium arsenide (InAs) quantum dots grown by molecular beam epitaxy

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS2 QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS2 QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS2 QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments.

Semiconducting transition metal dichalcogenides (TMDC) have been growing interest for both scientific interest and practical applications because of their various material properties. Potential applications and fundamental understanding in new classes of semiconductor materials motivates the fabrication of field-effect transistors (FETs) that are indispensable for probing of carrier transport properties, accumulation of charge and interfacial phenomena. This paper reports a fabrication and characterization of TMDC FETs with heavily-doped silicon-on-insulator substrate. The silicon processing is sophisticated technology that can be extended to the reliable operation of TMDC FETs with high reproducibility. In this presented approach, the heavily-doped SOI can work as a gate electrode in FETs, while the carrier injection into TMDC layer is possible by preparing overlap structure between gate electrode and source/drain contacts. The effectiveness of our approach was demonstrated through the fabrication and characterization of FETs.P-type SOI(88 nm)/BOX(145 nm) wafer was cleaned with SPM and HF solutions. For heavily-doped n-type SOI (n+ SOI), phosphorus-doped spin-on-glass (SOG) was deposited with spin coating and baked on a hot plate at 180 oC. The substrate was annealed at 1000 oC in N2 for 30 min to perform solid phase diffusion. Then, SOG was removed by 1 % HF for 30min. For heavily-doped p-type SOI (p+ SOI), B+ ions were implanted into SOI layer. Activation was performed at 1000 oC in Ar for 30 min. SOI layer was patterned by chemical dry etching with CF4/O2. Thermal SiO2 was grown by dry oxidation at 900 oC for 10 min. Next, AlOx was deposited by RF sputtering. After contact opening with 1 % HF, Al (20 nm)/TiN (20 nm) were deposited by RF sputtering and lift-off for electrical contact to SOI. The substrates were subjected to annealing in forming gas at 420 oC for 30 min to improve the quality of gate dielectrics and electrical contact of gate pad. Al (10 nm)/Au (40nm) was deposited by thermal evaporation and lift-off for contact electrodes. Subsequently, the substrate was exposed to oxygen plasma to form hydroxyl groups on the surface of AlOx. Then, the substrate was immersed into 2-propanol containing 5 mM n-octadecylphosphonic acid (ODPA) for 6 hours at room temperature. Annealing was conducted at 100 oC in N2 for 30 min to stabilize ODPA. The gate dielectrics consists of hybrid ODPA/AlOx/SiO2. Mechanically exfoliated WS2 was transferred to the substrate with the PDMS elastomer. Finally, devices were annealed in N2 at 150 oC for 30 min to improve source/drain contact.The FET operation was observed for both n+ and p+ SOI gate. The threshold voltage (Vth) was evaluated by linear extrapolation method with the drain current measured as a function of gate voltage at a low drain voltage of 0.05 V. In the case of n+ SOI gate electrode, the Vth of -0.71 V, On/Off ratio of 103～104 and subthreshold slope (SS) of 150 mV/dec were evaluated. On the other hand, the Vth of -0.13 V, On/Off ratio of 103～104 and subthreshold slope (SS) of 107 mV/dec were obtained for p+ SOI gate electrode. The difference in Vth between n+ and p+ SOI gate electrodes is derived from the difference in the Fermi level of SOI. The proposed method is suited for research in electrical characteristics of various TMDC semiconductors.

Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

Contact engineering on monolayer layer (ML) semiconducting transition metal dichalcogenides (TMDs) is considered the most challenging problem toward using these materials as a transistor channel in future advanced technology nodes. The typically observed strong Fermi-level pinning induced in part by the reaction of the source/drain contact metal and the ML TMD frequently results in a large Schottky barrier height, which limits the electrical performance of ML TMD field-effect transistors (FETs). However, at a microscopic level, little is known about how interface defects or reaction sites impact the electrical performance of ML TMD FETs. In this work, we have performed statistically meaningful electrical measurements on at least 120 FETs combined with careful surface analysis to unveil contact resistance dependence on interface chemistry. In particular, we achieved a low contact resistance for ML MoS2 FETs with ultrahigh-vacuum (UHV, 3 × 10-11 mbar) deposited Ni contacts, ∼500 Ω·μm, which is 5 times lower than the contact resistance achieved when deposited under high-vacuum (HV, 3 × 10-6 mbar) conditions. These electrical results strongly correlate with our surface analysis observations. X-ray photoelectron spectroscopy (XPS) revealed significant bonding species between Ni and MoS2 under UHV conditions compared to that under HV. We also studied the Bi/MoS2 interface under UHV and HV deposition conditions. Different from the case of Ni, we do not observe a difference in contact resistance or interface chemistry between contacts deposited under UHV and HV. Finally, this article also explores the thermal stability and reliability of the two contact metals employed here.

Transition metal dichalcogenide (TMDs) quantum dots (QDs) have attracted widespread attention among scientific community as fluorescent nanomaterial. Among TMDs, MoS2 has received particular interest due to its novel and intriguing optical properties. In this work hydrothermal method is used to prepare aqueous soluble MoS2 QDs using Molybdenyl acetylacetonate and thiourea as Mo and S precursors, respectively. The solubility of unfunctionalized MoS2 is poor in water and organic solvent, therefore thioglycolic acid is used as surfactant for surface functionalization and to enhance the aqueous solubility & stability. These functionalized QDs have been synthesized at pH 11 using hydrothermal method and explored their structural and optical properties. The as synthesized QDs are characterized using TEM, XRD, FTIR, UV-Vis and PL spectroscopy. The morphological shape of as prepared QDs is spherical with size 4-6.5 nm. The MAA encapsulated MoS2 QDs are further explored as fluorescent probe for the sensitive sensing of flavonoid Quercetin.

Semiconductor quantum dots (QDs) are promising candidates to fulfill a wide range of applications in real-world quantum computing, communication, and networks. Their excellent optical properties such as high brightness, single-photon purity, and narrow linewidths show potential utility in many areas. In order to realize long term goals of integration into complex and scalable quantum information systems, many current challenges must be overcome. One of these challenges is accomplishment of all necessary computing operations within a QD, which might be enabled by coherent manipulation of single QD energy level structures. In the realm of scalability for quantum devices, a way to address problems posed by the inhomogeneous distribution of individual QD spectral properties is necessary. This dissertation presents record large AC Stark shifts applied to the energy level structure of a single semiconductor QD, as well as an automated spectroscopy method for efficient characterization of many QDs. The AC Stark shifts are spin-selective and fast, which are highly desirable qualities in a mechanism for energy level reconfiguration. The AC stark shifts are measured by resonant excitation spectroscopy; a coherent technique which explicitly shows the change in energetic structure. A novel optical filtering scheme is developed to discriminate the high-powered laser inducing the shifts from the weak QD photoluminescence, and a polarimeter device is constructed to accurately measure the polarization state of a laser beam. Machine vision is used to automate spectroscopy of many QDs in bulk by acquiring high-resolution emission spectra, which allows characterization of individual QD energy level structures prior to device fabrication.

The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband transitions generate an unbalanced nonequilibrium population of electrons and holes in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows through the system. This is the so-called valley Hall effect that has recently been observed experimentally. Here, we show that this effect is mediated by photogenerated neutral excitons and charged trions and not by interband transitions generating independent electrons and holes. We further demonstrate an experimental strategy, based on wavelength dependent spatial mapping of the Hall voltage, which allows the exciton and trion contributions to the valley Hall effect to be discriminated in the measurement. These results represent a significant step forward in our understanding of the microscopic origin of photoinduced valley Hall effect in semiconducting transition metal dichalcogenides and demonstrate experimentally that composite quasi-particles, such as trions, can also possess a finite Berry curvature.

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.