Two-dimensional semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in surface passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. We have developed a passivation technique that allows for observation of near-unity photoluminescence quantum yield in monolayer semiconductors. I will discuss the mechanism by which non-radiative recombination can be fully removed in monolayers. The work presents the first demonstration of an optoelectronically perfect monolayer, and highlights one of their unique properties. Finally, I will discuss an AC carrier injection mechanism to enable bright light emitting devices using monolayers, overcoming the problem of Schottky contacts.

Graphene transistors are considered to be the successor’s basic element for the next generation of advanced integrated circuits. However, graphene material suffers from the absence of bandgap to behave as semiconductor. The present paper deals with the investigation on the bandgap engineering approach aiming an increase of the switching characteristics of the graphene transistors.

The main obstacle for graphene transistor is the material zero bandgap that worsens the switching characteristics of the GFETs. Several techniques have been proposed to open a bandgap in graphene, among these engineering techniques, we can cite the Substrate induced bandgap, Bandgap engineering using h-BN/Ni (111). It is known that in theory a maximum of 0.50 to 0.53 eV can be obtained. Such bandgaps are observed on Graphene Bi-Layer (GBL) sheets grown on silicon carbide (SiC).

Other methods are the substitutional doping (SD), Nitrogen doping (NB). In any case graphene engineering should be considered in chemistry and physics view points. A high selective hydrogenation of graphene grown by lithography under the form of nanoruban showed a very interesting result of 0.7 eV. This process is part of selective chemical graphene functionalization techniques (SCGF).

In this paper we will deal with the graphene nanoruban and the opening of a bandgap capable of inducing an appreciable switching current ratio of at least I_ON/I_OFF>10⁶.

The Graphene Nano Ribbon (GNR) structure used in the form of GNRFET for logic circuits and RF devices combines the high field, high mobility and the possibility of opening a bandgap. The higher carrier mobility of graphene is the basis of all electrical characteristics of graphene transistors.

In this paper we have used a semi-classical device model including the band to band tunneling that is described in Ref⁸ to emphasize on the bandgap engineering. Device performances are studied based on the current-voltage characteristics with respective bandgap width variations. I_OFF current estimated and the performance ratio deduced.

Tungsten di-selenide (WSe₂), a classic representative of two dimensional (2D) layered materials has recently drawn much attention due to its unique optoelectronic properties, offering a potential platform to construct hetero-structure photodetector (PD) for ultrafast optoelectronic devices on low-cost, flexible substrates [1,2]. As WSe₂ exhibits a weak van der Waals interlayer bonding, one of the approaches to obtain 2D WSe₂ is through top-down liquid phase exfoliation (LPE), where the bulk crystal is dispersed in a solvent through appropriate sonication and centrifugation conditions [1]. In this work, we report on the synthesis of WSe₂ via LPE and the first-ever assembly of an all inkjet printed WSe₂-graphene hetero-structure PD on flexible polyimide film, where the WSe₂ acted as a photo-active semiconductor and graphene was the carrier collector. The inkjet printed PD was photo-responsive to broadband incoming radiation in the visible regime, and exhibited a high photoresponsivity R ~ 0.70 A/W, and detectivity D ~ 3 × 10¹⁰ Jones. The strain-dependent measurements were conducted with bending for different curvatures, indicating the feasibility of such devices for large format arrays printed on flexible substrates. The capacitance-frequency (C-f) measurements were performed to investigate the trap states. In conclusion, this unique all inkjet printed 2D hetero-junction photodetector formed on flexible and conformable substrate was successfully shown to be highly photo-responsive to a wide range of light intensities and strain levels, making it a promising prospect for scalable flexible electronic and optoelectronic devices and circuitry.

References:

[1] Kelly, A. G., Hallam, T., Backes, C., Harvey, A., Esmaeily, A. S., Godwin, I., ... & Kinge, S. (2017). All-printed thin-film transistors from networks of liquid-exfoliated nanosheets. Science, 356(6333), 69-73.

[2] Pradhan, N. R., Ludwig, J., Lu, Z., Rhodes, D., Bishop, M. M., Thirunavukkuarasu, K., ... & Balicas, L. (2015). High photoresponsivity and short photoresponse times in few-layered WSe2 transistors. ACS applied materials & interfaces, 7(22), 12080-12088.

This research was performed while S.V.S held a National Research Council fellowship at NRL. This work was supported by core programs at NRL.

References

[1] A.L. Friedman et al., Sci. Reports 7, 3836 (2017)

[2] P.M. Campbell et al., Appl. Phys. Lett. 113, 163106 (2018)

Electrical detection is a very robust technique to transduce the adsorption of charged protein to a biological selective layer (i.e. biosensing). Electrolyte gated field effect transistors (EGFET) integrating graphene monolayers as the transducing element have shown outstanding electrical sensitivity in liquid compared to silicon and diamond based EGFET. In order to build graphene EGFET as effective biosensing unit it is important to attach at its surface a functional layers of biological molecules that will carry the task of enforcing specific detection of compound. Protein are widely used as specific bioreceptor for sensor biological functionalization yet it has been shown that protein lose their function when simply adsorbed on graphene. Covalent binding being out of the way for 2D dimensional crystals such as graphene (for the inherent deterioration of mechanical and electrical properties) we have shown that custom made tripodal compound attaching the graphene basal plane through Pi-stacking of aromatic moieties could be used to attach specific biomolecules to graphene while maintaining their biological function hence their specificity. In this report we present an optimized fabrication process for graphene EGFET that includes patterning and passivation of electrical contact. The devices reproducibly show state of the art electrical performances. We demonstrate that the process can be simply transferred to different host substrates to integrate graphene EGFET ubiquitously on Silicon, glass or printed circuit board with similar performances. Finally, we implemented biological functionalization of the sensors by attaching streptavidin to the sensor thanks to the non-covalent tripodal compound. We report consistent changes in the Dirac peak of graphene due to the adsorption of tripodal compound and streptavidin as well as the binding of biotin, specifically bound to streptavidin. We show the detection to be specific and reproducible.

Na₃Bi in bulk form represents a zero-bandgap topological Dirac semimetal (TDS), but when confined to few-layers is predicted to be a quantum spin Hall insulator with bulk bandgap of 300 meV.¹ Furthermore, application of an electric field to few-layer Na₃Bi has been predicted to induce a topological phase transition from conventional to topological insulator.²

I will discuss our efforts to grow epitaxial few-layer Na₃Bi via molecular beam epitaxy, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy and angle-resolved photoelectron spectroscopy. We demonstrate that monolayer and bilayer Na₃Bi are wide bandgap quantum spin Hall insulators (E_G>300 meV) that can be tuned with an electric field to semi-metallic, and at higher electric fields re-opened as a conventional insulator.³ This is the first experimental demonstration of such an electric field tuned topological phase transition in any material. Finally, I will discuss our most recent efforts to perform transport measurements on few-layer Na₃Bi at doping levels corresponding to bulk conduction and edge conduction, with and without an applied magnetic field.

References

[1] C. Niu et al., Phys. Rev. B (2017) 95, 075404

[2] H. Pan et al., Scientific Reports (2015) 5, 14639

[3] J. Collins et al., Nature 564, 390 (2018)

Combining two-dimensional materials with transparent conductive oxide can result in a fully transparent 2D electronics with outstanding device performance. It has been shown that monolayer MoS₂ in contact with AZO has 85% transmittance in the visible region, a low threshold voltage (0.69V) and a large switching ratio (4 × 10⁸). Another study has shown that using 2D MoS₂ as an active channel, tin doped indium oxide as a backgated electrode and zinc doped indium oxide (IZO) as source-drain electrodes, 85% transmittance in the visible region can be obtained. The contact resistance between the IZO and MoS₂ was reduced after laser annealing which in turn enhances µ_eff and I_on/I_off. Thus, it becomes essential to study the performance of different transparent conductive oxides in contact with various 2D materials in a transistor. Black phosphorus is an interesting 2D materials since it combines unique attributes of both transition metal dichalcogenides and graphene. CVD is a preferred method compared to other techniques such as mechanical exfoliation, liquid exfoliation, and PVD to synthesize 2D materials since the materials produced are of great quality as is a scalable technique. However, CVD of black phosphorus is challenging. Thus, CVD in addition to exfoliation will be used to develop 2D materials. In this work opto-electronics performance of various transparent conductive oxides in contact with different 2D materials is evaluated with an emphasis on black phosphorus.