Session C56: Devices from 2D Materials: Function, Fabrication and Characterization - I

Live

A conventional optical cavity supports one or more modes in the cavity core, which are unable to leak through the cavity cladding. The unconventional alternative, known as bound state in continuum (BIC) cavities, are modes which have an available channel to escape the cavity, but nevertheless remain confined due to destructive interferences. BICs have never been demonstrated in nanophotonics, utilizing high momenta plasmon or phonon polaritons.
Here, we introduce and implement the first deep subwavelength BIC cavity, and realize this by employing a novel multi-mode interference concept of hyperbolic phonon polaritons. We demonstrate mid-IR cavities with volumes more than a billion below the free-space mode volume, while maintaining quality factors above 100.
In general, the alliance of HyM with BICs provides a radically new way to confine light and is expected to have far reaching consequences wherever strong optical confinement is being used, from light-matter interaction experiments, to mid-IR nonlinear optics and a range of sensing applications.   

Live

Graphene nanoribbons (GNRs) have attracted considerable interest due to their largely modifiable electronic properties, including width-dependent bandgaps for armchair GNRs and spin-polarized edge states for GNRs with zigzag edges.^1,2,3,4 Manifestation of these properties requires atomically precise GNRs, which can be achieved through a bottom-up synthesis approach under ultrahigh vacuum conditions. We show that 5-atom wide armchair GNRs as well as pyrene-GNRs can be processed under ambient conditions and incorporated as the active material in a field effect transistor.^5,6 At room temperature, a film like behavior is observed while at cryogenic temperatures coulomb blockade and single electron tunnelling can be seen. Our recent results may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.

¹ Cai, J. et al., Nature 466, 470–473, (2010)
² Chen, Y.-C et al., ACS Nano 7, 7, 6123–6128, (2013)
³ Ruffieux, P. et al., Nature 531, 489–492, (2016)
⁴ Gröning, O. et al., Nature 560, 209–213, (2018)
⁵ El Abbassi, M. et al., ACS Nano, 14, 5, 5754–5762 (2020)
⁶ Sun, Q. et al., Advanced Materials, 32, 1906054 (2020)   

Live

Recently, we discovered that the layered Mott insulator α-RuCl₃ can induce a large population of holes of order few 10¹³ cm^-2 in graphene and other layered materials, demonstrating its capability as a strong 2d crystalline acceptor. Graphene doped by contact with α-RuCl₃ shows the highest reported transport mobilities at such large carrier densities due to the clean and uniform interface between the materials. Unexpectedly, we find that the charge transfer persists to a lesser degree even when α-RuCl3 and graphene are separated by an insulating spacer layer up to a few nm thick, achieving a separation of the charge origin from the active conducting layer that is analogous to modulation doping in epitaxially-grown semiconductors. Thus α-RuCl₃ enables a degree of control to selectively dope specific layers within a van der Waals heterostructure. It can also be readily patterned to engineer a custom potential landscape. In this talk, we will discuss transport characteristics of α-RuCl₃/graphene heterostructures including details of modulation doping through layers of hexagonal boron nitride and aluminum oxide, as well as prototype devices that showcase advantages of using α-RuCl₃ as a 2d crystalline acceptor.   

Live

The collective vibrational motions of biomolecules at the terahertz/far infrared frequencies can be employed as a unique fingerprint for molecular recognition. The conventional far infrared spectroscopy methods show very weak absorption signals of biomolecules. At present, a detection method is needed with enhanced sensitivity that can be utilized to identify biomolecules in dilute concentrations required for immunological diagnostic in clinical applications. We fabricate graphene field-effect transistor devices for biomolecular detection at the terahertz frequencies. The intensity of the collective vibrational modes of biomolecules is efficiently enhanced in a desired frequency range by tuning the gate voltage. Fourier transform infrared spectroscopy is used as a simultaneous authentication of vibrational modes. The device can measure a low concentration of biomolecules with plasmons induced amplification factor of ∼ 40 as compared to FTIR signals.   

Live

Ionic liquids (IL), molten salts at room temperature with mobile cations and anions, have demonstrated the ability to electrostatically dope MoS₂ and MoSe₂ top gated field effect transistors to increase charge density, carrier mobility and induce superconductivity in MoS₂ and MoSe₂ [1]. When a gate voltage is applied to the ionic liquid, an electric double layer (EDL) is formed on the surface of the MoS₂/MoSe₂ sample as a layer of cations or anions accumulate at the sample/IL interface to create a high capacitance, highly doping the surface. Here, the ability to induce changes in the polymorphs of MoS₂ and MoSe₂ from the semiconducting 2H phase to the semi-metallic 1T’ and the metallic 1T phase through IL gating will be discussed. The fabrication and induction of phase-switching of MoS₂/MoSe₂ field effect transistors gated with the ionic liquid DEME-TFSI will be detailed. Finally, the use of in-situ Raman spectroscopy for simultaneous transport measurements and Raman spectroscopy throughout the phase switching process to understand the underlying mechanism will be discussed.

[1] Shi, W., Ye, J., Zhang, Y. et al. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating. Sci Rep 5, 12534 (2015). https://doi.org/10.1038/srep12534   

Live

There has been extensive investigation of low-dimensional materials, such as quantum dots, 2D materials, and nanowires. The reduced dimensionality and van der Waals bonding nature of these materials can lead to unique electronic and transport properties of relevance scientifically and in technological applications. This potential has led to many studies exploring the properties of 2D heterostructures, but mixed dimensional heterostructures, e.g., 1D nanowires sandwiched between 2D layers, are significantly less understood. In this talk, we will present a high throughput computational study of mixed dimensional heterostructures. We classify these systems based on the mixture of dimensions and compute the crystal structures with density functional theory calculations including long-range van der Waals forces. We identify the thermodynamic stability in these systems and the change in crystal structure due to van der Waals forces, then compare these changes within each class of heterostructure. We follow by calculating the electronic band structure of these materials and performing chemical substitution on a subset to identify potential avenues of engineering their properties.   

Live

Ferroelectric (FE) thin films have been investigated for many years due to their broad applications in electronic devices. It was recently demonstrated that FE functionality persists in ultrathin films, possibly even in some monolayers. However, the feasibility of 2D-based FE functional devices remains an open challenge. Here, by using DFT calculations, we propose and document the possible integration of graphene (Gr) with 2D FE materials on metal substrates in the form of functional FE devices. We show that monolayers of proposed M₂O₃ (M = Al, Y) in the quintuple-layer (QL) In₂Se₃ structure (five atomic planes in a monolayer) are stable 2D FE materials. The Gr/QL-M₂O₃/Ru heterostructure can function as a prototype ferroelectric tunnel junction, in which QL-M₂O₃ works as a ferroelectric tunnel barrier and the barrier width can be modulated by its polarization direction. Moreover, alternating the polarization of QL-M₂O₃ modulates the doping type of Gr, enabling the fabrication of Gr p–n junctions. By design, the proposed heterostructures can in principle be fabricated by intercalation, which is known to produce high-quality, large-scale 2D-based heterostructures. [1]

[1] X. Jin et al., Nanoscale Horiz. 5, 1303 (2020)   

Live

h-BN has proven to be an essential 2D material in fabrication of vast array of high quality 2D heterostructures. However, use of h-BN in the field of NEMS in conjunction with other 2D materials like graphene etc. has not been explored extensively. Here, we report the fabrication and characterization of h-BN graphene derived heterostructure resonators. The devices are measured at cryogenic temperatures. A continuum mechanics model is employed to analyze the various modes of h-BN- monolayer graphene heterostructures. The heterostructure resonators behave quite differently vis-à-vis their counterpart graphene resonators. We will discuss the various kinks features observed in the resonance frequency as a function of gate voltage whose origin is yet not clear. We will also discuss other heterostructure resonators like twisted bilayer graphene – h-BN.   

Live

Heterostructures of graphene with transition metal dichalcogenides (TMDCs) offer a plethora of interesting electronic properties. The presence of layer degree of freedom in bilayer graphene (BLG) provides unprecedented control over layer polarization. Here, we establish the band-structure evolution from an interplay between proximity induced strong spin-orbit interaction (SOI) and the layer-polarizability in BLG/WSe2 heterostructure through magnetoconductance measurement. The effective valley-Zeeman SOI in this heterostructure can be switched on/off by applying a transverse displacement field or can be controllably transferred between the valence and the conduction band. This results in an evolution from weak localization to weak anti-localization at a constant electronic density as the net displacement field is tuned from a positive to negative a value with a concomitant SOI-induced splitting of the low-energy bands of the BLG near the K(K’)–valley which is a unique signature of the theoretically predicted spin-orbit valve effect. Our analysis shows that quantum correction to the Drude conductance in Dirac materials with strong induced SOI can only be explained satisfactorily by a theory which accounts for the SOI induced spin-splitting of the BLG low-energy bands   

Live

In the last years 2D materials have attracted a huge attention for spintronics thanks to the amazing properties that arise when thickness approaches the single layer level and thanks to the large number of functionalities that they offer. The recent introduction of 2D materials in magnetic tunnel junctions (2D-MTJs) offers very promising properties such as atomically defined interfaces, spin filtering, perpendicular anisotropy and spin-orbit torques modulation. Nevertheless, the difficult integration of exfoliated 2D materials in spintronic devices has limited so far their exploration and performances, maintaining experimental results still far from theoretical expectations¹. Here, we will show successful fabrication of NiFe/MoS₂/Co MTJs thanks to an in-situ fabrication process leading to the highest results reported so far for MTJs based on TMDCs 2D family.² Moreover, we will further discuss a path to alleviate fundamental technological and physics issues encountered for the integration of 2D-MTJs.<sup>2,3

1</sup> Wang et al. Nano Lett. 15, 5261 (2015); Dankert et al. ACS Nano, 11, 6389 (2017); Wong et al., IEEE Trans. Magn. 53, 1600205 (2017); Dolui et al. PRB, 90, 041401(R) (2014)
² Galbiati et al. PRAppl. 12, 044022 (2019).
³ Galbiati et al. ACS Appl. Mater. Interf. 10, 30017 (2018)   

Live

Recently, group-10 transition metal dichalcogenides (TMDs) such as PtSe₂ and PdSe₂ have emerged as 2D materials with a theoretically predicted electron mobility significantly higher than that of group-6 TMDs. However the performance of field-effect transistors (FETs) based on few-layer PdSe₂ has been limited by the presence of a Schottky barrier at the drain/source contacts. In this work, we utilize a 2D-semiconductor interlayer at the metal/PdSe₂ contacts to significantly lower the Schottky barrier. As a result, our FETs based on PdSe₂/WSe₂ heterostructures exhibit a two-terminal effective mobility exceeding 200 cm² V^-1 s^-1 at room temperature and approaching 700 cm² V^-1 s^-1 at 77 K, consistent with phonon-limited electron transport. By contrast, the two-terminal effective mobility of FETs based on few-layer PdSe₂ (without WSe₂) decreases with decreasing temperature, suggesting that the electron transport is limited by the contacts. Our PdSe₂/WSe₂ heterostructure FET consisting of a trilayer PdSe₂ and a bilayer WSe₂ concurrently exhibits a high ON/OFF ratio of ~ 10⁷ and significantly enhanced two-terminal electron mobility compared to FETs based on a trilayer PdSe₂ or a bilayer WSe₂ alone.   

Live

Both In₂Se₃ and InSe feature several structural modifications with distinct electronic properties of the material, demonstrating an interesting structure-property relationship. α-In₂Se₃ is ferroelectric while γ-InSe is a noncentrosymmetric semiconductor featuring very high electronic mobility (>1000 V/cm² s at room temperature). Both semiconductors possess a van der Waals interlayer bonding. We have explored novel ferroelectric semiconductor field effect transistors (FeSmFETs) employing 2D crystals of α-In₂Se₃ as a channel material and measured the devices from room to the liquid-helium temperatures. We observed the effect of the reorientation of the polarization by gating and also found evidence for an electric field induced metallic state in this material system. We also studied very thin films of γ-InSe grown by molecular beam epitaxy and fabricated FET devices to test whether the MBE grown film have a mobility similar to the 2D crystal obtained by mechanical exfoliation.   

Live

This study illustrates the nature of electronic transport and its transition from direct tunneling to Fowler Nordheim tunneling between a metal electrode and MoS2 channel interface in a vertical tunnel device configuration. We developed a fabricating procedure to prepare vertical tunnel junction based on 2D transition metal dichalcogenides sandwiched between two ultra-flat Au substrates. We prepared ultra-flat gold electrodes using template stripping. The devices were prepared by dry transfer technique. We studied many devices with varying thickness from monolayer to few layers MoS2. We studied the transition between Fowler-Nordheim tunneling and direct tunneling signatures at different temperature ranging from 77K to room temperature allowing us to extract the Schottky barrier height precisely. We found that the Schottky barrier height depends on the temperature and layer numbers.