Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

In this work, we demonstrate the suitability of Reconfigurable Ferroelectric Field-Effect- Transistors (Re-FeFET) for designing non-volatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe$_2$) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP$_2$S$_6$) layer. Controlling the state encoded in the Program Gate enables switching between p, n and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 10$^6$ and hysteresis windows of up to 10 V width. The homojunction can change from ohmic-like to diode behavior, with a large rectification ratio of 10$^4$. When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with long retention time exceeding 10$^4$ seconds. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key non-volatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates remarkable compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit groundbreaking potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.Comment: 23 pages, 5 figures; Supporting Information: 12 pages, 6 figure

Recent advances in electronic and optoelectronic Devices Based on Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs

Two-Dimensional Materials for Advanced Solar Cells

Inorganic crystalline silicon solar cells account for more than 90% of the market despite a recent surge in research efforts to develop new architectures and materials such as organics and perovskites. The reason why most commercial solar cells are using crystalline silicon as the absorber layer include long-term stability, the abundance of silicone, relatively low manufacturing costs, ability for doping by other elements, and native oxide passivation layer. However, the indirect band gap nature of crystalline silicon makes it a poor light emitter, limiting its solar conversion efficiency. For instance, compared to the extraordinary high light absorption coefficient of perovskites, silicon requires 1000 times more material to absorb the same amount of sunlight. In order to reduce the cost per watt and improve watt per gram utilization of future generations of solar cells, reducing the active absorber thickness is a key design requirement. This is where novel two-dimensional (2d) materials like graphene, MoS2 come into play because they could lead to thinner, lightweight and flexible solar cells. In this chapter, we aim to follow up on the most important and novel developments that have been recently reported on solar cells. Section-2 is devoted to the properties, synthesis techniques of different 2d materials like graphene, TMDs, and perovskites. In the next section-3, various types of photovoltaic cells, 2d Schottky, 2d homojunction, and 2d heterojunction have been described. Systematic development to enhance the PCE with recent techniques has been discussed in section-4. Also, 2d Ruddlesden-Popper perovskite explained briefly. New developments in the field of the solar cell via upconversion and downconversion processes are illustrated and described in section-5. The next section is dedicated to the recent developments and challenges in the fabrication of 2d photovoltaic cells, additionally with various applications. Finally, we will also address future directions yet to be explored for enhancing the performance of solar cells

Two-Dimensional Electronics and Optoelectronics

The discovery of monolayer graphene led to a Nobel Prize in Physics being awarded in 2010. This has stimulated further research on a wide variety of two-dimensional (2D) layered materials. The coupling of metallic graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) and black phosphorus have attracted a tremendous amount of interest in new electronic and optoelectronic applications. Together with other 2D materials, such as the wide band gap boron nitride nanosheets (BNNSs), all these 2D materials have led towards an emerging field of van der Waal 2D heterostructures. The papers in this book were originally published by Electronics (MDPI) in a Special Issue on “Two-Dimensional Electronics and Optoelectronics”. The book consists of eight papers, including two review articles, covering various pertinent and fascinating issues concerning 2D materials and devices. Further, the potential and the challenges of 2D materials are discussed, which provide up to date guidance for future research and development

Substrate-Dependent Photodetection with Functional Nanomaterials

Dielektrische Einflüsse auf nanostrukturierte Materialien sind bekannt, jedoch in Bezug auf die Geschwindigkeit der Photodetektion noch kaum erforscht. Diese Arbeit behandelt den Einfluss des Substrates auf die Schaltgeschwindigkeit funktioneller Nanomaterialien am Beispiel von CdSe Quantenpunkten und WSe2 Kristallen und berücksichtigt dabei sowohl die Zeit, die bis zum Erreichen des Gleichgewichtszustandes benötigt wird, als auch das Verhalten des Detektors im instationären (Nicht-Gleichgewichts) Zustand. Ersteres kann Informationen bezüglich des geschwindigkeitsbestimmenden Faktors des Photodetektors liefern, während letzteres die im Detektor vorliegenden Abklingmechanismen aufzeigen kann.Dielectric influences on nanostructured materials are widely known but hardly explored in terms of the speed of photodetection. This work deals with the influence of the substrate on the speed of response of functional nanomaterials considering CdSe quantum dots and WSe2 crystals as examples, and takes into account both the time required to reach the steady state and the performance of the detector in the transient (non-steady state) condition. The former can provide information regarding the speed limiting factor of the photodetector, while the latter can reveal the decay mechanisms present in the detector

Two-dimensional electronics and optoelectronics

The discovery of monolayer graphene has led to a Nobel Prize in Physics in 2010. This has stimulated research on a wide variety of two-dimensional (2D) layered materials. The coupling of metallic graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) and black phosphorus has attracted tremendous amount of interest in new electronic and optoelectronic applications. Together with other 2D materials such as the wide band gap boron nitride nanosheets (BNNSs), all these 2D materials have led towards an emerging field of van der Waal 2D heterostructures. This book is originally published in Electronics (MDPI) as a special issue of “Two-Dimensional Electronics and Optoelectronics”. The book consists of a total of eight papers, including two review articles, covering important topics of 2D materials. These papers represent some of the important topics on 2D materials and devices. Promises and challenges of 2D materials are discussed herein, which provide a great recent guidance for future research and development

Beyond Graphene: Monolayer Transition Metal Dichalcogenides, A New Platform For Science

Following the isolation of graphene in 2004, scientists quickly showed that it possesses remarkable properties. However, as the scientific understanding of graphene matured, it became clear that it also has limitations: for example, graphene does not have a bandgap, making it poorly suited for use in digital logic. This motivated explorations of monolayer materials “beyond graphene”, which could embody functionalities that graphene lacks. Transition metal dichalcogenides (TMDs) are leading candidates in this field. TMDs possess a wide variety of properties accessible through the choice of chalcogen atom, metal atom and atomic configuration (1H, 1T, and 1T’). Similar to graphene, monolayer TMDs may be produced on a small scale through mechanical exfoliation, but useful applications will require development of reliable methods for monolayer growth over large areas. In this thesis, I report our group’s recent progress in the chemical vapor deposition (CVD) of high quality, large area, monolayer TMDs under a 1H atomic configuration, which were integrated into high-quality biosensor arrays. These devices were incorporated in a flexible platform and were used for electronic read out of binding events of molecular targets in both vapor and liquid phases. I also report our findings on the CVD growth of monolayer TMDs in the 1T’ atomic configuration and measurements of their physical properties. 1T’ TMDs have been labeled the holy grail of materials due to theoretical predictions that they are 2D topological insulators; however they remain relatively unexplored due to the difficulty of monolayer growth and their lack of stability in air. Through careful passivation techniques, we were able to stabilize the as-grown monolayer 1T’ TMD flakes and perform the first characterizations on the structure. Lastly, in-plane 2D TMD heterostructures are promising material systems that combine the unique properties of each TMD. I discuss our results on the synthesis and study of 1H TMD heterostructures and unique 1H/1T’ TMD heterostructures. TMDs, with its many different accessible physical properties, coupled with the large variety of applications, have been classified as the leading nanomaterials in the realm “beyond graphene”

Atomic and electronic structure of random and ordered 2D alloys

Recently, transition metal dichalcogenides (TMDs) have attracted increasing research interest as promising two-dimensional materials for a range of electronic and optoelectronic applications and for the fundamental science that can be studied in them. This interest stretches beyond the properties of pure TMD materials, with an increasing number of reports on doped-TMD and TMD alloys to increase the functionality of these materials. For example, by substituting or alloying the metals or chalcogens of the TMDs, it is possible to tune their electronic structure. In this thesis, single crystals of pure TMDs and of TMD alloys have been synthesised via chemical vapor transport (CVT). Techniques for the fabrication of van der Waals stacks with artificial sequences have been developed, specifically to allow the direct measurement of atomic and electronic structure in the alloys and to correlate these properties to optical spectroscopy measurements. The atomic structure of Mo1-xWxS2 alloys are visualised by annular dark field (ADF) scanning transmission electron microscopy (STEM). Statistical analysis of the images allows the atomic distributions to be compared to those from Monte Carlo simulations and first principles calculations. Mo1-xWxS2 alloys show random distributions as expected from thermodynamic considerations. The evolution of the band structure of the Mo1-xWxS2 alloys is determined by angle-resolved photoemission spectroscopy (ARPES) measurements and compared to first principles calculations. Combined, these results demonstrate that TMD alloying is a powerful approach for band structure engineering. In contrast to the Mo1-xWxS2 alloys, short-range ordering is found in Nb0.1W0.9S2, with the Nb atoms forming atomic lines along one of the equivalent crystallographic directions. This ordering is confirmed by quantitative statistical analysis through the Warren-Cowley short-range order (SRO) parameters. Meanwhile, density function theory (DFT) calculations have been applied to explain this ordering, revealing this structure results from a combination of thermodynamic and kinetic considerations

Ion-controlled electronics enabled by electric double layer gating of two-dimensional materials

An electric double layer transistor (EDLT) is a type of emerging, ion-controlled electronic device that uses ions in an electrolyte to induce charge in the transistor channel by field-effect. Because the EDL formed at the electrolyte/channel interface under an applied field acts as an interfacial capacitor (thickness < 1 nm), large capacitance densities, corresponding to sheet carrier densities exceeding 10^14 cm^-2, can be induced in two-dimensional (2D) crystals. This dissertation presents efforts to both improve EDL gating performance and add new functionality to 2D EDLTs using three newly developed ion conductors. My first contribution was demonstrating the removal of polymeric resist residue from the channel using atomic force microscopy (AFM) in contact mode. This technique provides a molecularly clean 2D surface for depositing a nanometer-thin ion conductor and for achieving the strongest EDL gating possible. My second contribution was the development of a monolayer electrolyte field-effect transistor as non-volatile memory (MERAM) based on WSe2. The electrolyte is a single molecule thick and has two stable states which can be modulated by a gate bias. After programming, MERAM has an On-Off ratio exceeding 10^4 at a 0V read voltage, which is repeatable over 1000 program/erase cycles; the retention time for each state exceeds 6 hours (maximum cycles and time measured). The third contribution was the development of a single-ion conductor where anions are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the channel. Experiments and modeling support that the single-ion conductor gating can create an electrostatic imbalance that induces strain on a suspended MoTe2 channel to exploit the semiconductor-to-metal phase transition for low-power 2D transistors. The last contribution was demonstrating EDL “locking” using a doubly polymerizable ionic liquid (DPIL) developed by our collaborators. Ions are drifted into place and immobilized by thermally/photo-induced polymerization. This concept has been used on graphene to lock a lateral p-n junction. A thermally triggerable ion release was also demonstrated for ion-unlocking