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At least 253 records · Page 14

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DOE OSTI2020

Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations establish the large work function and narrow bands of α-RuCl 3 enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe 2 , and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl 3 . Here, this leads to the best-reported monolayer graphene mobilities (4900 cm 2 /(V s)) at these high hole densities (3 × 10 13 cm -2 ) and yields larger charge transfer to bilayer graphene (6 × 10 13 cm -2 ).

2D atomic crystals↗

DOE OSTI2021

Imaging Quantum Interference in Stadium-Shaped Monolayer and Bilayer Graphene Quantum Dots

Experimental realizations of graphene-based stadium-shaped quantum dots (QDs) have been few and have been incompatible with scanned probe microscopy. Yet, the direct visualization of electronic states within these QDs is crucial for determining the existence of quantum chaos in these systems. Here we report the fabrication and characterization of electrostatically defined stadium-shaped QDs in heterostructure devices composed of monolayer graphene (MLG) and bilayer graphene (BLG). To realize a stadium-shaped QD, we utilized the tip of a scanning tunneling microscope to charge defects in a supporting hexagonal boron nitride flake. The stadium states visualized are consistent with tight-binding-based simulations but lack clear quantum chaos signatures. The absence of quantum chaos features in MLG-based stadium QDs is attributed to the leaky nature of the confinement potential due to Klein tunneling. In contrast, for BLG-based stadium QDs (which have stronger confinement) quantum chaos is precluded by the smooth confinement potential which reduces interference and mixing between states.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower - to - Higher Bandgap 2D Material

High light absorption (~15%) and strong photoluminescence (PL) emission in monolayer (1L) transition metal dichalcogenides (TMDs) make them ideal candidates for optoelectronic device applications. Competing interlayer charge transfer (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ET can survive up to several tens of nm, unlike the CT process. Our experiment shows that an efficient ET occurs from the 1Ls WSe 2 -to-MoS 2 with an interlayer hexagonal boron nitride (hBN), due to the resonant overlapping of the high-lying excitonic states between the two TMDs, resulting in enhanced HS MoS 2 PL emission. This type of unconventional ET from the lower-to-higher optical bandgap material is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increased electron–phonon scattering, destroying the enhanced MoS 2 emission. Our work provides new insight into the long-distance ET process and its effect on the photocarrier relaxation pathways.

2D material↗

DOE OSTI2023

Polaritonic Probe of an Emergent 2D Dipole Interface

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl 3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl 3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Finally, our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Bifacial Raman Enhancement on Monolayer Two-Dimensional Materials

Understanding the charge interaction between molecules and two-dimensional (2D) materials is essential for the design of functional devices. Here, we report the bifacial Raman enhancement of molecules on monolayer graphene and hexagonal boron nitride (h-BN). Taking advantage of the atomically thick layered structure, we show that both surfaces of 2D materials can interact with molecules and simultaneously enhance their Raman scattering. Different enhancement features were observed for monolayer graphene and h -BN. The intensity decrease of particular Raman modes of copper phthalocyanine (CuPc) on both surfaces of h -BN suggests that z -dipoles exist and are partially canceled out between the two interfaces, while the twice Raman intensities of the characteristic Raman modes of CuPc on both surfaces of graphene compared to that on one surface evidenced the charge transfer process. Furthermore, these results provide an approach to modify 2D materials by bifacial adsorption of molecules, and the findings can inspire the design of functional 2D material-based devices.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Phonon Polaritonics in Two-Dimensional Materials

Extreme confinement of electromagnetic energy by phonon polaritons holds the promise of strong and new forms of control over the dynamics of matter. To bring such control to the atomic-scale limit, it is important to consider phonon polaritons in two-dimensional (2D) systems. Recent studies have pointed out that in 2D, splitting between longitudinal and transverse optical (LO and TO) phonons is absent at the Γ point, even for polar materials. Does this lack of LO–TO splitting imply the absence of a phonon polariton in polar monolayers? To answer this, we connect the microscopic phonon properties with the macroscopic electromagnetic response. Specifically, we derive a first-principles expression for the conductivity of a polar monolayer specified by the wave-vector-dependent LO and TO phonon dispersions. In the long-wavelength (local) limit, we find a universal form for the conductivity in terms of the LO phonon frequency at the Γ point, its lifetime, and the group velocity of the LO phonon. Our analysis reveals that the phonon polariton of 2D is simply the LO phonon of the 2D system. For the specific example of hexagonal boron nitride (hBN), we estimate the confinement and propagation losses of the LO phonons, finding that high confinement and reasonable propagation quality factors coincide in regions that may be difficult to detect with current near-field optical microscopy techniques. Finally, we study the interaction of external emitters with 2D hBN nanostructures, finding an extreme enhancement of spontaneous emission due to coupling with localized 2D phonon polaritons and the possibility of multimode strong and ultrastrong coupling between an external emitter and hBN phonons. This may lead to the design of new hybrid states of electrons and phonons based on strong coupling.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Proximity Spin–Orbit Torque on a Two-Dimensional Magnet within van der Waals Heterostructure: Current-Driven Antiferromagnet-to-Ferromagnet Reversible Nonequilibrium Phase Transition in Bilayer CrI 3

The recently discovered two-dimensional magnetic insulator CrI 3 is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using the first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of the bilayer-CrI 3 /monolayer-TaSe 2 van der Waals (vdW) heterostructure will induce spin–orbit torque and thereby drive the dynamics of magnetization on the first monolayer of CrI 3 in direct contact with TaSe 2 . By combining the calculated complex angular dependence of spin–orbit torque with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI 3 from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI 3 carries current due to evanescent wave functions injected by metallic transition metal dichalcogenide TaSe 2 , while concurrently acquiring strong spin–orbit coupling via such a proximity effect, whereas the second monolayer of CrI 3 remains insulating. Furthermore, the transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by a bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find a tunneling magnetoresistance of ≃240%.

2D magnetic materials↗

DOE OSTI2024

Role of Heterointerface in Lithium-Induced Phase Transition in T d -WTe 2 Nanoflakes

A new polytype of WTe 2 with a bandgap has been recently discovered through the intercalation of lithium into the van der Waals gaps of T d -WTe 2 . Here, we report the effects of reduced thicknesses and heterointerfaces on the intercalation-induced phase transition in WTe 2 . Using in situ Raman spectroscopy during the electrochemical lithiation of WTe 2 flakes as a function of flake thickness, we observe that additional electrochemical energy is required for the phase transition of WTe 2 from the T d phase to the new lithiated T d ' phase, going from 0.8 V of the applied electrochemical voltage for a thick flake to 0.5 V and 0.3 V for 7- and 5-layered samples, respectively. We ascribe this suppression of the phase transition to the interfacial interaction between the nanoflake and SiO 2 /Si substrate, which plays an increasing role as the sample thickness is reduced. The suppressed kinetics of the phase transition can be mitigated by placing the WTe 2 flake on a hexagonal boron nitride (hBN) flake, which facilitates the release of the in-plane strain induced by the phase transition. Importantly, our study underscores the significance of interfacial effects in modulating phase transitions in two-dimensional (2D) materials, suggesting heterogeneous transition pathways, as well as interfacial engineering to control these phase transitions.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Low-Temperature 2D/2D Ohmic Contacts in WSe 2 Field-Effect Transistors as a Platform for the 2D Metal–Insulator Transition

in this work, we report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multiterminal WSe 2 Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe 2 devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe 2 devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature (T) and density (n s ) dependence of the conductivity σ(T, n s ) demonstrate scaling behavior consistent with a metalinsulator quantum phase transition driven by electron-electron interactions, but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering.

2D materials↗

DOE OSTI2021

Localized Orbital Excitation Drives Bond Formation in Plasmonic Catalysis

Localized surface plasmons generated on metallic nanostructures can be used to accelerate molecular transformations; however, the efficiency is limited by the challenge to control the energy/charge transfer at the interfaces. Here, we combine density functional theory (DFT) calculations and experiments to reveal the mechanism of nitrophenol reduction on Au nanoparticles under visible-light irradiation and propose a strategy to further enhance the reaction rates. DFT calculations show a reduced activation barrier under electronic excitation on Au(111), thus explaining the measured higher rates under visible-light irradiation. Furthermore, we propose a heterostructure with Au nanoparticles covered by a thin film of hexagonal boron nitride; the latter is used to decouple the molecular orbitals from the metal to enable charge localization in the molecule. DFT calculations show that, by this electronic decoupling, the activation barrier can be lowered by a factor of five. Furthermore, this work thus provides a valuable strategy for optimizing catalytic efficiency.