Doping-free complementary WSe2 circuit via van der Waals metal integration.

Two-dimensional (2D) semiconductors have attracted considerable attention for the development of ultra-thin body transistors. However, the polarity control of 2D transistors and the achievement of complementary logic functions remain critical challenges. Here, we report a doping-free strategy to modulate the polarity of WSe2 transistors using same contact metal but different integration methods. By applying low-energy van der Waals integration of Au electrodes, we observed robust and optimized p-type transistor behavior, which is in great contrast to the transistors fabricated on the same WSe2 flake using conventional deposited Au contacts with pronounced n-type characteristics. With the ability to switch majority carrier type and to achieve optimized contact for both electrons and holes, a doping-free logic inverter is demonstrated with higher voltage gain of 340, at the bias voltage of 5.5 V. Furthermore, the simple polarity control strategy is extended for realizing more complex logic functions such as NAND and NOR

CVD Graphene Contacts for Lateral Heterostructure MoS2{_2} Field Effect Transistors

Intensive research is carried out on two-dimensional materials, in particular molybdenum disulfide, towards high-performance transistors for integrated circuits. Fabricating transistors with ohmic contacts is challenging due to the high Schottky barrier that severely limits the transistors' performance. Graphene-based heterostructures can be used in addition or as a substitute for unsuitable metals. We present lateral heterostructure transistors made of scalable chemical vapor-deposited molybdenum disulfide and chemical vapor-deposited graphene with low contact resistances of about 9 k{\Omega}{\mu}m and high on/off current ratios of 10${^8}. We also present a theoretical model calibrated on our experiments showing further potential for scaling transistors and contact areas into the few nanometers range and the possibility of a strong performance enhancement by means of layer optimizations that would make transistors promising for use in future logic circuits.Comment: 23 page

Layer by layer printing of nanomaterials for large-area, flexible electronics

Large-area electronics, including printable and flexible electronics, is an emerging concept which aims to develop electronic components in a cheaper and faster manner, especially on those non-conventional substrates. Being flexible and deformable, this new form of electronics is regarded to hold great promises for various futuristic applications including the internet of things, virtual reality, healthcare monitoring, prosthetics and robotics. However, at present, large-area electronics is still nowhere near the commercialisation stage, which is due to several problems associated with performance, uniformity and reliability, etc. Moreover, although the device’s density is not the major concern in printed electronics, there is still a merit in further increasing the total number of devices in a limited area, in order to achieve more electronic blocks, higher performance and multiple functionalities. In this context, this Ph.D. thesis focuses on the printing of various nanomaterials for the realisation of high-performance, flexible and large-area electronics. Several aspects have been covered in this thesis, including the printing dynamics of quasi-1D NWs, the contact problem in device realisation and the strategy to achieve sequential integration (3D integration) of the as-printed devices, both on rigid and flexible substrates. Promisingly, some of the devices based on the printed nanomaterial show a comparable performance to the state-of-the-art technology. With the demonstrated 3D integration strategy, a highly dense array of electronic devices can be potentially achieved by printing method. This thesis also touches on the problem associated with the circuit and system realisation. Specifically, graphene-based logic gates and NW based UV sensing circuit has been discussed, which shows the promising applications of nanomaterial-based electronics. Future work will be focusing on extending the UV sensing circuit to an active matrix sensor array

Black phosphorus: narrow gap, wide applications

The recent isolation of atomically thin black phosphorus by mechanical exfoliation of bulk layered crystals has triggered an unprecedented interest, even higher than that raised by the first works on graphene and other two-dimensional, in the nanoscience and nanotechnology community. In this Perspective we critically analyze the reasons behind the surge of experimental and theoretical works on this novel two-dimensional material. We believe that the fact that black phosphorus band gap value spans over a wide range of the electromagnetic spectrum that was not covered by any other two-dimensional material isolated to date (with remarkable industrial interest such as thermal imaging, thermoelectrics, fiber optics communication, photovoltaics, etc), its high carrier mobility, its ambipolar field-effect and its rather unusual in-plane anisotropy drew the attention of the scientific community towards this two-dimensional material. Here we also review the current advances, the future directions and the challenges in this young research field.Comment: Updated version of the perspective article about black phosphorus, including all the feedback received from arXiv users + reviewer

Towards barrier free contact to MoS2 using graphene electrodes

The two-dimensional (2D) layered semiconductors such as MoS2 have attracted tremendous interest as a new class of electronic materials. However, there is considerable challenge in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as back electrodes to achieve Ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily modified by gate potential to ensure a nearly perfect band alignment with MoS2. We demonstrate, for the first time, a transparent contact can be made to MoS2 with essentially zero contact barrier and linear output behaviour at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator-transition (MIT) can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barrier and only seen in four-terminal devices in conventional metal-contacted MoS2 system. With further passivation y born nitride encapsulation, we demonstrate a record high extrinsic (two-terminal) field effect mobility over 1300 cm2/Vs in MoS2