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2020 Highlights

2DCC User Research Highlights 2020

In this project, we use a novel approach for next generation ultra-low-power sensor design by embracing the evolutionary success of animals with extraordinary sensory information processing capabilities that allow them to survive in extreme and resource constrained environments. Stochastic resonance (SR) is one of those astounding phenomena, where noise, which is considered detrimental for electronic circuits and communication systems, plays a constructive role in the detection of weak signals. Here, we show SR in a photodetector based on wafer-scale monolayer MoS2 for detecting ultra-low-intensity subthreshold optical signals from a distant light emitting diode (LED). We demonstrate that weak periodic LED signals, which are otherwise undetectable, can be detected by a MoS2 photodetector in the presence of a finite and optimum amount of white Gaussian noise at a frugal energy expenditure of few tens of nano-Joules. The concept of SR is generic in nature and can be extended beyond photodetector to any other sensors.

Topological insulators are materials that are insulating in the bulk but have conducting surface states with a helical Dirac dispersion. These properties are created by the combined effects of strong spin-orbit coupling and time-reversal symmetry. When a topological insulator is interfaced with a magnetic insulator, the magnetic interactions across the interface can break time-reversal symmetry, opening a gap at the Dirac point of the topological surface states. Experiments demonstrating the impact of magnetism on topology in this manner have been well documented. An unresolved question is whether the inverse situation is possible: can topological surface states influence magnetism? In a user project led by Mingzhong Wu (Colorado State), 2DCC scientists used molecular beam epitaxy (MBE) to synthesize topological insulator (Bi2Se3) thin films on ferrimagnetic insulator Y3Fe5O12 (YIG) thin film substrates grown by sputtering at Colorado State. Wu’s group then used ferromagnetic resonance measurements to show that the magnetic anisotropy, gyromagnetic ratio, and damping of the YIG were modified by interfacing with Bi2Se3. To prove that the topological surface states in Bi2Se3 are crucial for the observed changes in the magnetic properties, the 2DCC team interfaced YIG with (Bi,In)2Se3, a “trivial” insulator, and found no significant modification of the ferromagnetic resonance signal.

Atomic defects are controllably introduced in suspended single layer molybdenum disulfide (1L MoS2) using helium ion beam. Vacancies exhibit one missing atom of molybdenum and a few atoms of sulfur. Quantification was done using a Scanning Transmission Electron Microscope (STEM) with an annular detector. Experimentally accessible inter-defect distance was employed to measure the degree of crystallinity in 1L MoS2. Correlation between the appearance of an acoustic phonon mode in the Raman spectra and the inter-defect distance was established, which introduces a new methodology for quantifying defects in 2D materials.

ZrS2 has received increased attention due to its indirect band gap that matches with the visible spectrum and which is predicted to undergo an indirect-to-direct transition with strain. Alloying with ZrSe2 to produce ZrSxSe2-x (x=0...2) would provide continuous control over key optical and electronic parameters for photonics. Here, we combine Raman spectroscopy, spectroscopic ellipsometry (SE), and density functional theory to conduct the first comprehensive exploration of phonons and excitons in this system. We find that long-range Coulomb interactions activate optically-silent infrared phonons that completely dominate Raman spectra in some alloys. SE measurements of excitons reveal strong light-matter interactions with low optical losses for near-infrared wavelengths. This study suggests ZrSxSe2-x alloys will find applications in atomically-thin, tunable photodetectors and photovoltaics.

This review provided an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and how they can be utilized to assist/guide the design and synthesis of 2D materials. It focuses on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. It also discusses how machine learning can be combined with computation and experiments to guide the discovery of new 2D materials.

Plasmon-exciton coupling in hetero-bilayer of WSe2 and WS2 transferred onto Au nanorod arrays is studied. Dark-field scattering measurements reveal that the in-plane dipole moment of excitons in monolayer WS2 allows only the narrow spectral range of 30 nm for the resonant coupling between the localized particle plasmons from Au nanorods and the bright excitons from WS2. We demonstrate that the q-parameter that represents the asymmetry of Fano resonances from plasmon-exciton coupling can be controlled by the polarization states of incident light. Surface lattice resonances in between individual Au nanorods play a role to diminish the damping factor of plasmon-exciton coupling in the arrays.  

Dirac fermions with highly-dispersive linear bands are usually considered weakly correlated, due to relatively large bandwidths (W) compared to Coulomb interactions (U). With the discovery of nodal-line semimetals, the notion of Dirac point has been extended to lines and loops in the momentum space. The anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. However, experimental evidence for anticipated enhanced correlations in nodal-line semimetals is sparse. Through optical studies on the ZrSiSe single crystals provided by 2DCC, Basov’s group at Univ. of Columbia find prominent correlation effects in nodal-line semimetals.

They observed spectroscopic hallmarks of electronic correlations: strong reduction  of the Drude weight and the Fermi velocity.

2DCC Research Highlights 2020

Doping is the cornerstone of semiconductor technology, enabling the success of modern digital electronics. Successful realization of wafer-scale, electronic grade, intrinsic 2D TMDCs via common deposition methods is rapidly progressing, however, advances in scalable doping still remain in the “proof-of-concept” stage, delaying the large-scale fabrication of logic circuits based on extrinsic 2D semiconductors. This work is presenting a wafer-scale synthesis of rhenium doping of WSe2 films via MOCVD at front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures. By controlling the partial pressures of the precursors, doping concentrations as low as 0.0001% can be achieved. Rhenium atoms substitutionally replace W atoms in the WSe2 lattice and introduce discrete defect levels that lie close to the conduction band minima confirming the n-type nature of the dopants. However, the transport properties of field-effect transistors degrade as a function of doping concentration which is attributed to the large binding energy of electrons due to less effective dielectric screening in monolayer 2D films.

Among the high-mobility two-dimensional transition metal dichalcogenides PtSe2 is of particular interest due to its record high carrier mobility of 1,000 cm2/Vs, sizeable band gap and air stability to address the current need for low-power, high-performance and ultra-thin body electronics. A scalable fabrication process with highly precise thickness control on ultra-smooth insulating gate oxides however has not been achieved to date. This work is addressing this gap by presenting ultra-thin growth of Pt layers on insulating Al2O3 and the subsequent conversion into PtSe2 by direct selenization in molecular beam epitaxy. Due to the chosen growth technique, the approach is scalable and offers precise control over Pt and thus PtSe2 layer thickness. We engineered deposition parameters to fabricate smooth, single crystal Pt films in the mono-/few-layer limit and optimized selenization conditions to convert a maximum amount of Pt into PtSe2. A post selenization anneal in Se proved essential to enhance crystal quality of the PtSe2 layers.

MnBi2Te4 has recently been established as the first intrinsic antiferromagnetic (AFM) topological insulator. Although quantum anomalous Hall effect (QAHE) has been observed in MnBi2Te4, it is only realized with odd numbers of septuple layers due to the AFM interlayer coupling. Therefore, it is crucial to stabilize ferromagnetic (FM) phase in MnBi2Te4. We have discovered a new FM phase with the Curie temperature of 26 K in the MnSb1.8Bi0.2Te4 sample through tuning growth conditions, in contrast to the AFM order seen in the Mn(Bi1-xSbx)2Te4 family. We have investigated magnetotransport properties of the FM thin flakes and observed features similar to topological Hall effect. Our work pushes forward the realization of intrinsic FM topological insulator and establishes a new platform to explore novel topological quantum states arising from the interplay between magnetism and band topology.

  • Atomically-thin metals Ga, In, and Sn are stabilized at the interface of epitaxial graphene and SiC through confinement heteroepitaxy
  • Metal atoms generated via thermal evaporation of metal powders migrate through defects in graphene layers and passivate the surface of SiC
  • 2D Ga layers exhibit BCS-type superconductivity and enhanced Tc of ~4 K (compared to 2 K in bulk, α-Ga)
  • The process of stabilizing 2D superconductors via CHet can be applied to elements beyond p-block metals, opening opportunities to study unconventional properties that enable exploration of new physics and devices.