Photo-FETs: phototransistors enabled by 2D and 0D nanomaterials

The large diversity of applications in our daily lives that rely on photodetection technology requires photodetectors with distinct properties. The choice of an adequate photodetecting system depends on its application, where aspects such as spectral selectivity, speed, and sensitivity play a critical role. High-sensitivity photodetection covering a large spectral range from the UV to IR is dominated by photodiodes. To overcome existing limitations in sensitivity and cost of state-of-the-art systems, new device architectures and material systems are needed with low-cost fabrication and high performance. Low-dimensional nanomaterials (0D, 1D, 2D) are promising candidates with many unique electrical and optical properties and additional functionalities such as flexibility and transparency. In this Perspective, the physical mechanism of photo-FETs (field-effect transistors) is described and recent advances in the field of low-dimensional photo-FETs and hybrids thereof are discussed. Several requirements for the channel material are addressed in view of the photon absorption and carrier transport process, and a fundamental trade-off between them is pointed out for single-material-based devices. We further clarify how hybrid devices, consisting of an ultrathin channel sensitized with strongly absorbing semiconductors, can circumvent these limitations and lead to a new generation of highly sensitive photodetectors. Recent advances in the development of sensitized low-dimensional photo-FETs are discussed, and several promising future directions for their application in high-sensitivity photodetection are proposed.Peer ReviewedPostprint (author's final draft

Graphene against Other Two‐Dimensional Materials: A Comparative Study on the Basis of Electronic Applications

The evolution of the electronics industry since almost 75 years ago has depended on the novel materials and devices that continuously are introduced. In first decades of this century, 2D materials are impelling this development through materials such as graphene, graphane, graphone, graphyne, graphdiyne, silicene, silicane, germanene, germanane, stanene, phosphorene, arsenene, antimonene, borophene, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), and MXenes. In this work, the main strategies to modify electrical properties of 2D materials are studied for obtaining dielectric, semiconducting, or semimetallic properties. The effects of doping, chemical modification, electrical field, or compressive and/or tensile strains are considered. In addition, the light‐matter interaction to develop optoelectronic applications is analyzed. In next three decades, a lot of scientific research will be realized to completely exploit the use of 2D materials either as single monolayers or as stacked multilayers in several fields of knowledge with a special emphasis on the benefit to the electronic industry and ultimately our society

Substrate Effects And Dielectric Integration In 2d Electronics

The ultra-thin body of monolayer (and few-layer) two dimensional (2D) semiconducting materials such as transitional metal dichalconiges (TMDs), black phosphorous (BP) has demonstrated tremendous beneficial physical, transport, and optical properties for a wide range of applications. Because of their ultrathin bodies, the properties of 2D materials are highly sensitive to environmental effects. Particularly, the performance of 2D semiconductor electronic devices is strongly dependent on the substrate/dielectric properties, extrinsic impurities and absorbates. In this work, we systematically studied the transport properties of mechanically exfoliated few layer TMD field-effect transistors (FETs) consistently fabricated on various substrates including SiO2,Parylene –C, Al2O3, SiO2 modified by octadecyltrimethoxysilane (OTMS) self-assembled monolayer (SAMs), and hexagonal boron nitride (h-BN). We performed variable temperature transport measurements to understand the effects of various scattering mechanisms such as remote surface phonon scattering, coulomb scattering, surface roughness scattering on the mobility of these devices. To reveal the intrinsic channel properties, we also investigated TMD devices encapsulated by h-BN. To further optimize the dielectric interface and electrostatic control of the TMD channels, we developed a novel thermal-oxidation method to turn few-layer 2D metals into ultrathin and atomically flat high –κ dielectrics. In order to optimize the performance of TMD electronic devices, it is also critical to fabricate low resistance ohmic contacts required for effectively injecting charge carriers into the TMD channel. Along this direction, we developed a new contact strategy to minimize the contact resistance for a variety of TMDs by van der Waals assembly of doped TMDs as contacts and undoped TMDs as channel materials. The developed unique method for low-resistance ohmic contacts achieved using the 2D/2D contact strategy and novel technique for high-k dielectric integration is expected to open the path to explore the rich quantum physics in TMDs 2DEGs and 2DHGs

Transition metal dichalcogenide MoSe₂ nanostructures

Transition metal dichalcogenides (TMDs) are a family of van der Waals (vdW) layered materials exhibiting unique electronic, optical, magnetic, and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures. Here I present three kinds of nanostructures of semiconducting TMD MoSe₂, created by molecular beam epitaxy (MBE) and characterized by scanning tunneling microscopy and spectroscopy (STM/STS). The three kinds of nanostructures are two-dimensional (2D) nanoislands, quasi one-dimensional (1D) nanoribbons, and heterostructures. The successful growth of 2D nanoislands lays the foundation for the preparation of the other two structures. By properly controlling the substrate temperature and Se over-pressure, the MoSe₂ atomic layers undergo a dramatic three-stage shape transformation: from fractal to compact 2D nanoislands, and eventually to nanoribbons, in stark contrast to the traditional two-stage growth behaviour involving only the transformation from the fractal to compact regime. Experimentally, it is found that the Se:Mo flux ratio during MBE growth plays a central role in controlling the nanoribbon formation. Theoretically, first-principles calculations show that the abundance/deficiency of extra Se atoms at different island edges significantly modifies the relative step energies between zigzag and armchair edges, which in turn impacts the island shape evolution during nonequilibrium growth. The successful preparation of MoSe2/hBN/Ru(0001) heterostructure is a demonstration that MBE technique is suitable for fabricating vdW heterostructures. Surprisingly, we found that the quasi-particle gap of the MoSe₂ on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. The surface of MoSe₂ exhibits Moiré pattern that replicates the Moiré pattern of hBN/Ru. In addition, the electronic structure and the work function of MoSe₂ are modulated electrostatically with an amplitude of ~ 0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitudePhysic

High Mobility N-Type Field Effect Transistors Enabled By Wse2/pdse2 Heterojunctions

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have emerged as a promising candidate for post-silicon electronics. Few-layer tungsten diselenide (WSe2), a well-studied TMD, has sown high hole mobility and ON/OFF ratio in field effect transistor (FET) devices. But the n-type performance of WSe2 is still quite limited by the presence of a substantial Schottky Barrier. Palladium diselenide, (PdSe2) is a newly discovered TMD that is of interest because of its high electron mobility, and moderate ON/OFF ratios. However, despite its relatively small bandgap, the n-type performance of few-layer PdSe2 FETs has also been limited by a Schottky barrier, which is likely due to Fermi-level pinning. In this work, we report high performance n-type FETs enabled by a few-layer WSe2/PdSe2 heterojunction. We show that the current through few-layer WSe2 or PdSe2 alone is quite small, but across the heterojunction WSe2 serves as a “buffer layer” at the drain/source contacts for few-layer PdSe2 FETs. We observe a high ON/OFF ratio of 105, with an electron mobility of ~139 cm2 V-1 s-1. The mobility continues to rise at cryogenic temperatures, indicating a substantial reduction in the Schottky Barrier height. A heterojunction consisting of 3-layer PdSe2 and 3L WSe2 showed an ON/OFF ratio approaching 107, while still maintaining a moderate mobility of ~ 57 cm2 V-1 s-1. We believe the significantly improved device performance enabled by our contact engineering technique will facilitate further study of the intrinsic properties of few-layer 2D materials

Piezoelectricity and flexoelectricity in 2D transition metal dichalcogenides

Two-dimensional materials are only on to a few atoms thick, making them the thinnest possible material known to man. Their combination of electrical, optical, and mechanical properties allows for unique electrical devices with a wide range of future applications, from being a post-silicon material option, creating high-speed communication systems, allowing the advancement of flexible electronics, and even creating transparent electronics. Among their amazing characteristics is the coupling of electrical and mechanical properties. Although not unique to 2D materials, electromechanical coupling could be used in 2D materials to create a class of sensors, actuators, and energy harvesters at a scale not previously possible. Specifically, 2D materials could be utilized in flexible, wearable electronics as an energy harvester to convert the motion of the body into electrical energy. In this dissertation, the electromechanical coupling properties known as piezoelectricity and flexoelectricity are studied in 2D materials both to advance the development of 2D materials in general, and to improve the understanding of the relatively novel effect of flexoelectricity. This work focuses on a class of 2D materials known as transition metal dichalcogenides (TMDs), which are semiconducting and intrinsically piezoelectric. To begin, the adhesion between the TMDs and soft substrates is studied. Soft substrates could be used in flexible and wearable electronic systems, so adhesion of TMDs to soft substrates is important. It was found that the adhesions between the TMD molybdenum disulfide and polydimethylsiloxane is roughly 18 mJ m⁻². Next, the out-of-plane electromechanical coupling of molybdenum disulfide and other TMDs was studied. Piezoelectric theory predicts that there should be zero out-of-plane response, but a signal is measured in all TMDs, suggesting the presence of flexoelectricity. The measured effective out-of-plane piezoelectric response is on the order of 1 pm V⁻¹ and the estimated flexoelectric response is on the order of 0.05 nC m⁻¹. Additionally, it was found that the magnitude of the out-of-plane electromechanical response of different TMDs roughly follows a trend predicted by a simple model of flexoelectricity. The work presented in this dissertation provides the first experimental evidence of a flexoelectric effect present in 2D TMDs.Electrical and Computer Engineerin

Functional inks of graphene, metal dichalcogenides and black phosphorus for photonics and (opto) electronics

We discuss the emerging role of solution processing and functional ink formulation in the fabrication of devices based on two dimensional (2d) materials. By drawing on examples from our research, we show that these inks allow 2d materials to be exploited in a wide variety of applications, including in photonics and (opto)electronics. Keywords: Graphene, Transition Metal Dichalcogenides, TMDs, Black Phosphorous, Phosphorene, Functional Inks, 2d Materials, Inkjet Printing, Ultrafast Lasers, Flexible Electronic

Physics-Based Modeling and Validation of 2D Schottky Barrier Field-Effect Transistors

In this work, we describe the charge transport in two-dimensional (2D) Schottky barrier field-effect transistors (SB-FETs) based on the carrier injection at the Schottky contacts. We first develop a numerical model for thermionic and field-emission processes of carrier injection that occur at a Schottky contact. The numerical model is then simplified to yield an analytic equation for current versus voltage ($I$-$V$) in the SB-FET. The lateral electric field at the junction, controlling the carrier injection, is obtained by accurately modeling the electrostatics and the tunneling barrier width. Unlike previous SB-FET models that are valid for near-equilibrium conditions, this model is applicable for a broad bias range as it incorporates the pertinent physics of thermionic, thermionic field-emission, and field-emission processes from a 3D metal into a 2D semiconductor. The $I$-$V$ model is validated against the measurement data of 2-, 3-, and 4-layer ambipolar MoTe$_2$ SB-FETs fabricated in our lab, as well as the published data of unipolar 2D SB-FETs using MoS$_2$. Finally, the model's physics is tested rigorously by comparing model-generated data against TCAD simulation data

Engineering grain boundaries at the 2D limit for the hydrogen evolution reaction

Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs

Engineering grain boundaries at the 2D limit for the hydrogen evolution reaction

Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs