Nonvolatile Modulation of BiOSe/Pb(Zr,Ti)O Heteroepitaxy.

The pursuit of high-performance electronic devices has driven the research focus toward 2D semiconductors with high electron mobility and suitable band gaps. Previous studies have demonstrated that quasi-2D BiOSe (BOSe) has remarkable physical properties and is a promising candidate for further exploration. Building upon this foundation, the present work introduces a novel concept for achieving nonvolatile and reversible control of BOSe's electronic properties.

Quantum-Engineered Devices Based on 2D Materials for Next-Generation Information Processing and Storage.

As an approximation to the quantum state of solids, the band theory, developed nearly seven decades ago, fostered the advance of modern integrated solid-state electronics, one of the most successful technologies in the history of human civilization. Nonetheless, their rapidly growing energy consumption and accompanied environmental issues call for more energy-efficient electronics and optoelectronics, which necessitate the exploration of more advanced quantum mechanical effects, such as band-to-band tunneling, spin-orbit coupling, spin-valley locking, and quantum entanglement. The emerging 2D layered materials, featured by their exotic electrical, magnetic, optical, and structural properties, provide a revolutionary low-dimensional and manufacture-friendly platform (and many more opportunities) to implement these quantum-engineered devices, compared to the traditional electronic materials system.

Graphene-Transition Metal Dichalcogenide Heterojunctions for Scalable and Low-Power Complementary Integrated Circuits.

The most pressing barrier for the development of advanced electronics based on two-dimensional (2D) layered semiconductors stems from the lack of site-selective synthesis of complementary n- and p-channels with low contact resistance. Here, we report an in-plane epitaxial route for the growth of interlaced 2D semiconductor monolayers using chemical vapor deposition with a gas-confined scheme, in which patterned graphene (Gr) serves as a guiding template for site-selective growth of Gr-WS-Gr and Gr-WSe-Gr heterostructures. The Gr/2D semiconductor interface exhibits a transparent contact with a nearly ideal pinning factor of 0.

End-Bonded Metal Contacts on WSe Field-Effect Transistors.

Contact engineering has been the central issue in the context of high-performance field-effect transistors (FETs) made of atomic thin transition metal dichalcogenides (TMDs). Conventional metal contacts on TMDs have been made on top a lithography process, forming a top-bonded contact scheme with an appreciable contact barrier. To provide a more efficient pathway for charge injection, an end-bonded contact scheme has been proposed, in which covalent bonds are formed between the contact metal and channel edges.

Ultrafast Monolayer In/Gr-WS-Gr Hybrid Photodetectors with High Gain.

One of the primary limitations of previously reported two-dimensional (2D) photodetectors is a low frequency response (≪ 1 Hz) for sensitive devices with gain. Yet, little efforts have been devoted to improve the temporal response of photodetectors while maintaining high gain and responsivity. Here, we demonstrate a gain of 6.

Gigahertz Field-Effect Transistors with CMOS-Compatible Transfer-Free Graphene.

High-quality graphene grown on metal-free substrates represents a vital milestone that provides an atomic clean interface and a complementary metal-oxide-semiconductor-compatible manufacturing process for electronic applications. We report a scalable approach to fabricate radio frequency field-effect transistors with a graphene channel grown directly on the sapphire substrate using the technique of remote-catalyzed chemical vapor deposition (CVD). A mushroom-shaped AlO top gate is used to allow the self-aligned drain/source contacts, yielding remarkable increase of device transconductance and reduction of the associated parasitic resistance.

Stable 1T Tungsten Disulfide Monolayer and Its Junctions: Growth and Atomic Structures.

Transition-metal dichalcogenides in the 1T phase have been a subject of increasing interest, which is partly due to their fascinating physical properties and partly to their potential applications in the next generation of electronic devices, including supercapacitors, electrocatalytic hydrogen evolution, and phase-transition memories. The primary method for obtaining 1T WS or MoS has been using ion intercalation in combination with solution-based exfoliation. The resulting flakes are small in size and tend to aggregate upon deposition, forming an intercalant-TMD complex with small 1T and 1T' patches embedded in the 2H matrix.

Twisted bilayer graphene photoluminescence emission peaks at van Hove singularities.

We report on photoluminescence emission imaging by femtosecond laser excitation on twisted bilayer graphene samples. The emission images are obtained by tuning the excitation laser energies in the near infrared region. We demonstrate an increase of the photoluminescence emission at excitation energies that depends on the bilayer twist angle.

Scalable van der Waals Heterojunctions for High-Performance Photodetectors.

Atomically thin two-dimensional (2D) materials have attracted increasing attention for optoelectronic applications in view of their compact, ultrathin, flexible, and superior photosensing characteristics. Yet, scalable growth of 2D heterostructures and the fabrication of integrable optoelectronic devices remain unaddressed. Here, we show a scalable formation of 2D stacks and the fabrication of phototransistor arrays, with each photosensing element made of a graphene-WS vertical heterojunction and individually addressable by a local top gate.

High-Performance Organic Light-Emitting Diode with Substitutionally Boron-Doped Graphene Anode.

The hole-injection barrier between the anode and the hole-injection layer (HIL) is of critical importance to determine the device performance of organic light-emitting diodes (OLEDs). Here, we report on a record-high external quantum efficiency (EQE) (24.6% in green phosphorescence) of OLEDs fabricated on both rigid and flexible substrates, with the performance enhanced by the use of nearly defect-free and high-mobility boron-doped graphene as an effective anode and hexaazatriphenylene hexacarbonitrile as a new type of HIL.

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains.

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)1-3 are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system.

Robust room temperature valley polarization in monolayer and bilayer WS2.

We report robust room temperature valley polarization in chemical-vapor-deposition (CVD) grown monolayer and bilayer WS2via polarization-resolved photoluminescence measurements using excitation below the bandgap. We show that excitation with energy slightly below the bandgap of the multi-valleyed transition metal chalcogenides can effectively suppress the random redistribution of excited electrons and, thereby, greatly enhance the efficiency of valley polarization at room temperature. Compared to mechanically exfoliated WS2, our CVD grown WS2 films also show enhancement in the coupling of spin, layer and valley degree of freedom and, therefore, provide improved valley polarization.

Postsynthesis of h-BN/Graphene Heterostructures Inside a STEM.

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in-plane graphene growth from the step-edge of hexagonal BN (h-BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source.

High-performance and high-sensitivity applications of graphene transistors with self-assembled monolayers.

Charge impurities and polar molecules on the surface of dielectric substrates has long been a critical obstacle to using graphene for its niche applications that involve graphene's high mobility and high sensitivity nature. Self-assembled monolayers (SAMs) have been found to effectively reduce the impact of long-range scatterings induced by the external charges. Yet, demonstrations of scalable device applications using the SAMs technique remains missing due to the difficulties in the device fabrication arising from the strong surface tension of the modified dielectric environment.

In Situ Tuning of Switching Window in a Gate-Controlled Bilayer Graphene-Electrode Resistive Memory Device.

A resistive random access memory (RRAM) device with a tunable switching window is demonstrated for the first time. The SET voltage can be continuously tuned from 0.27 to 4.

Structural and Chemical Dynamics of Pyridinic-Nitrogen Defects in Graphene.

High density and controllable nitrogen doping in graphene is a critical issue to realize high performance graphene-based devices. In this paper, we demonstrate an efficient method to selectively produce graphitic-N and pyridinic-N defects in graphene by using the mixture plasma of ozone and nitrogen. The atomic structure, electronic structure, and dynamic behavior of these nitrogen defects are systematically studied at the atomic level by using a scanning transmission electron microscopy.

Single-Layer ReS₂: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy.

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample.

Three-fold rotational defects in two-dimensional transition metal dichalcogenides.

As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal-chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8-5-5-8 membered rings.

Growth and Raman spectra of single-crystal trilayer graphene with different stacking orientations.

Understanding the growth mechanism of graphene layers in chemical vapor deposition (CVD) and their corresponding Raman properties is technologically relevant and of importance for the application of graphene in electronic and optoelectronic devices. Here, we report CVD growth of single-crystal trilayer graphene (TLG) grains on Cu and show that lattice defects at the center of each grain persist throughout the growth, indicating that the adlayers share the same nucleation site with the upper layers and these central defects could also act as a carbon pathway for the growth of a new layer. Statistics shows that ABA, 30-30, 30-AB, and AB-30 make up the major stacking orientations in the CVD-grown TLG, with distinctive Raman 2D characteristics.

Layer-dependent optical conductivity in atomic thin WS₂ by reflection contrast spectroscopy.

Optical conductivity, which originates from the interband transition due to electron-phonon interaction, is one of the powerful tools used for studying the electronic states in layered transition metal dichalcogenides (TMDCs). Here, we report for the first time the optical conductivity of WS2, one of the emerging classes of TMDCs, prepared directly on SiO2/Si substrate using reflection contrast spectroscopy. The measured optical conductivity at direct excitonic transition point K of the Brillouin zone for monolayer WS2 shows a value of 0.

Gigahertz flexible graphene transistors for microwave integrated circuits.

Flexible integrated circuits with complex functionalities are the missing link for the active development of wearable electronic devices. Here, we report a scalable approach to fabricate self-aligned graphene microwave transistors for the implementation of flexible low-noise amplifiers and frequency mixers, two fundamental building blocks of a wireless communication receiver. A devised AlOx T-gate structure is used to achieve an appreciable increase of device transconductance and a commensurate reduction of the associated parasitic resistance, thus yielding a remarkable extrinsic cutoff frequency of 32 GHz and a maximum oscillation frequency of 20 GHz; in both cases the operation frequency is an order of magnitude higher than previously reported.

Gating electron-hole asymmetry in twisted bilayer graphene.

Electron-hole symmetry is one of the unique properties of graphene that is generally absent in most semiconductors because of the different conduction and valence band structures. Here we report on the manipulation of electron-hole symmetry in the low-energy band structure of twisted bilayer graphene, where symmetric saddle points form in the conduction and valence bands as a result of interlayer coupling. By applying a gate voltage to a twisted bilayer with a critical rotation angle, enhanced electron resonance between the two saddle points can be turned on or off, depending on the electron-hole symmetry near the saddle points.

In situ observation of step-edge in-plane growth of graphene in a STEM.

It is extremely difficult to control the growth orientation of the graphene layer in comparison to Si or III-V semiconductors. Here we report a direct observation of graphene growth and domain boundary formation in a scanning transmission electron microscope, with residual hydrocarbon in the microscope chamber being used as the carbon source for in-plane graphene growth at the step-edge of bilayer graphene substrate. We show that the orientation of the growth is strongly influenced by the step-edge structure and areas grown from a reconstructed 5-7 edge are rotated by 30° with respect to the mother layer.

Direct growth of self-crystallized graphene and graphite nanoballs with Ni vapor-assisted growth: from controllable growth to material characterization.

A directly self-crystallized graphene layer with transfer-free process on arbitrary insulator by Ni vapor-assisted growth at growth temperatures between 950 to 1100 °C via conventional chemical vapor deposition (CVD) system was developed and demonstrated. Domain sizes of graphene were confirmed by Raman spectra from ~12 nm at growth temperature of 1000 °C to ~32 nm at growth temperature of 1100 °C, respectively. Furthermore, the thickness of the graphene is controllable, depending on deposition time and growth temperature.

Scalable graphite/copper bishell composite for high-performance interconnects.

We present the fabrication and characterizations of novel electrical interconnect test lines made of a Cu/graphite bishell composite with the graphite cap layer grown by electron cyclotron resonance chemical vapor deposition. Through this technique, conformal multilayer graphene can be formed on the predeposited Cu interconnects under CMOS-friendly conditions. The low-temperature (400 °C) deposition also renders the process unlimitedly scalable.