NitingZ

Direct growth of transition metal dichalcogenides over large areas within the back-end-of-line (BEOL) thermal budget limit of silicon integrated circuits is a significant challenge for 3D heterogeneous integration. In this work, we report on the growth of MoS 2 films (∼1–10 nm) on SiO 2 , amorphous-Al 2 O 3 , c-plane sapphire, and glass substrates achieved at low temperatures (350 °C–550 °C) by chemical vapor deposition in a manufacturing-compatible 300 mm atomic layer deposition reactor. We investigate the MoS 2 films as a potential material solution for BEOL logic, memory and sensing applications. Hall-effect/4-point measurements indicate that the ∼10 nm MoS 2 films exhibit very low carrier concentrations (10 14 –10 15 cm −3 ), high resistivity, and Hall mobility values of ∼0.5–17 cm 2 V −1 s −1 , confirmed by transistor and resistor test device results. MoS 2

Atomic layers of black phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared (IR). In this work, we investigate the IR photoluminescence (PL) of BP single crystals at very low temperature. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier–Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2 K.

Studying local variations in the Seebeck coefficient of materials is important for understanding and optimizing their thermoelectric properties, yet most thermoelectric measurements are global over a whole device or material, thus overlooking spatial divergences in the signal and the role of local variation and internal structure. Such variations can be caused by local defects, metallic contacts or interfaces that often substantially influence thermoelectric properties, especially in two dimensional materials. Here, we demonstrate scanning thermal gate microscopy, a non-destructive method to obtain high resolution 2-dimensional maps of the thermovoltage, to study graphene samples. We demonstrate the efficiency of this newly developed method by measuring local Seebeck coefficient in a graphene ribbon and in a junction between single-layer and bilayer graphene.

Two-dimensional (2D) ferromagnetic semiconductors with a room-temperature Curie temperature ( T c ) are required for next-generation spintronic devices, but the current candidates suffer from a low T c and poor chemical stability. Here, a new layered compound RhI 3 is discovered to be an above-room-temperature ferromagnetic semiconductor. This compound crystallizes in a monoclinic crystal system of space group C 2/ m , with the unit cell of a = 6.773(8) Å, b = 11.721(2) Å, c = 6.811(8) Å and β = 108.18(4) °. The structure consists of honeycomb rhodium layers separated by iodine–iodine van der Waals gap. Chemically stable RhI 3 possesses an optical bandgap of 1.17 eV. Its robust ferromagnetism with a T c of above 400 K, which is far higher than 61 K for the well-known CrI 3 and the highest among the bulk 2D ferromagnetic semiconductors. The robust intrinsic fer...

WSe 2 has demonstrated potential for applications in thermoelectric energy conversion. Optimization of such devices requires control over interfacial thermal and electrical transport properties. Ti, TiO x , and Ti/TiO x contacts to the MBE-grown WSe 2 are characterized by XPS and transport measurements. The deposition of Ti is found to result in W-Se bond scission yielding metallic W and Ti-Se chemical states. The deposition of Ti on WSe 2 in the presence of a partial pressure of O 2 , which yields a TiO x overlayer, results in the formation of substoichiometric WSe x ( x < 2) as well as WO x . The thermal boundary conductance at Ti/WSe 2 contacts is found to be reduced for greater WSe 2 film thickness or when Au/TiO x interface is present at the contact. Electrical resistance of Au/Ti contacts is found to be higher than that of Au/TiO x contact...

Two-dimensional (2D) materials hold great promise for future nanoelectronics as conventional semiconductor technologies face serious limitations in performance and power dissipation for future technology nodes. The atomic thinness of 2D materials enables highly scaled field-effect transistors (FETs) with reduced short-channel effects while maintaining high carrier mobility, essential for high-performance, low-voltage device operations. The richness of their electronic band structure opens up the possibility of using these materials in novel electronic and optoelectronic devices. These applications are strongly dependent on the electrical properties of 2D materials-based FETs. Thus, accurate characterization of important properties such as conductivity, carrier density, mobility, contact resistance, interface trap density, etc is vital for progress in the field. However, electrical characterization methods for 2D devices, particularly FET-related measurement techniques, must be r...

The intrinsic magnetism has long been pursued in two-dimensional (2D) materials down to one-atomic layer thickness. But only very recently, the intrinsic magnetism of monolayer CrI 3 , Fe 3 GeTe 2 , FePS 3 , VSe 2 and bilayer Cr 2 Ge 2 Te 6 are verified in experiment by optical measurement, Raman spectrum and conventional magnetism measurement. Among them, the intralayer exchange interaction of FePS 3 is antiferromagnetic while all the others are ferromagnetic. Most of the ferromagnetic orders in these materials are induce by super exchange interaction. Monolayer Fe 3 GeTe 2 and VSe 2 exhibit metallic character while all the others are semiconductor or insulator. Stable spontaneous magnetization can exist in these monolayer 2D materials because of their strong anisotropy. The anisotropy is mostly from the strong spin–orbit coupling of heavy atoms (CrI 3 , Cr

The World Health Organization reported that 4.2 million deaths every year were a direct result of exposure to ambient air pollution (NO 2 , SO 2 , NH 3 , CO 2 , CO, CH 4 ). There is a well-demonstrated global need for high sensitivity, low cost and low energy consumption miniaturised gas sensors to be deployed in a dense network and to be used in an attempt to pinpoint and avoid high pollution hot spots. The high sensitivity of graphene to the local environment has shown to be highly advantageous in sensing applications, where ultralow concentrations of adsorbed molecules induce a significant response on the electronic properties of graphene. This is commonly attributed to the π electrons of graphene, being directly exposed to the surrounding environment. The unique electronic structure makes graphene the ‘ultimate’ sensing material for applications in environmental monitoring and air quality. In this review, we present the fro...

Raman spectroscopy is one of the most important optical techniques for the study of two-dimensional systems, providing fundamental information for the development of applications using these materials in optoelectronics and valleytronics. The emerging area of two-dimensional layered materials demands the characterization and understanding of the basic physical properties of the material under study and is indispensable to pave the way for the engineering of devices. In this review we cover the recent development of resonance Raman spectroscopy on transition metal dichalcogenides, discussing the exciton-phonon coupling and intervalley double-resonance Raman scattering process. A brief discussion of the effect of defects and disorder on the Raman spectra of these materials is also presented. The results of Raman spectroscopy in TMDs are compared to those observed in graphene, showing that this technique also provides physical information about TMDs that were previously reported in...

We propose a reversible mechanism for switching Heisenberg-type exchange interactions between deposited transition metal adatoms from ferromagnetic to antiferromagnetic. Using first-principles calculations, we show that this mechanism can be realized for cobalt atoms on the surface of black phosphorus by making use of electrically-controlled orbital repopulation, as recently demonstrated by scanning probe techniques [Nat. Commun. 9 , 3904 (2018)]. We find that field-induced repopulation not only affects the spin state, but also causes considerable modification of exchange interaction between adatoms, including its sign. Our model analysis demonstrates that variable adatom-substrate hybridization is a key factor responsible for this modification. We perform quantum simulations of inelastic tunneling characteristics and discuss possible ways to verify the proposed mechanism experimentally.

Growth of two-dimensional metals has eluded materials scientists since the discovery of the atomically thin graphene and other covalently bound 2D materials. Here, we report a two-atom-thick hexagonal copper-gold alloy, grown through thermal evaporation on freestanding graphene and hexagonal boron nitride. The structures are imaged at atomic resolution with scanning transmission electron microscopy and further characterized with spectroscopic techniques. While the 2D structures are stable over months in vacuum, electron irradiation in the microscope provides sufficient energy to cause a phase transformation—atoms are released from their lattice sites with the gold atoms eventually forming face-centered cubic nanoclusters on top of 2D regions during observation. The presence of copper in the alloy enhances sticking of gold to the substrate, which has clear implications for creating atomically thin electrodes for applications utilizing 2D materials. Its practically infinite surfac...

Strong second harmonic generation (SHG) in monolayer transition metal dichalcogenides demonstrates great promise for nonlinear photonic applications. However, to utilize these materials in photonic applications, it is imperative to investigate the robustness of the nonlinear optical properties against various defects introduced during or post synthesis. Here we study the SHG in hexagonal monolayer WS 2 with synthesis defects, in triangular monolayer WS 2 with focused ion beam induced structural defects, and in WS 2 under ambient and aqueous environmental conditions. We find that WS 2 hexagonal monolayers exhibit significantly more uniform SHG intensity across the sample with a coefficient of variation, ##IMG## [http://ej.iop.org/images/2053-1583/7/4/045020/tdmaba564ieqn1.gif] {${c_v} = 4.9\% $} than the varying photoluminescence (PL) intensity ( ##IMG## [http://ej.iop.org/images...] {${c_v} = 32.83\% $}

sp 2 -hybridized boron nitride is identified as a strategic material for many purposes related to the integration of graphene and two-dimensional materials in devices and the fabrication of van der Waals heterostructures. Thus, it becomes mandatory to have scalable synthesis and characterization procedures for providing suitable and reliable boron nitride material according to these identified needs. We report here on the growth of sp 2 -hybridized boron nitride film on polycrystalline nickel substrate by chemical vapor deposition with borazine as precursor. We propose a complete study of the influence of the underlying nickel grain orientation on the BN structure layers, in terms of thickness, crystallographic orientation, domain size and stacking. We show the heteroepitaxial growth of continuous, single crystalline hexagonal boron nitride multilayer film on nickel (111)-like grains. We highlight its ABC stacking sequence with AB stacking faults and show how i...

There has been a massive growth in the study of transition metal dichalcogenides (TMDs) over the past decade, based upon their interesting and unusual electronic, optical and mechanical properties, such as tuneable and strain-dependent bandgaps. Tungsten disulphide (WS 2 ), as a typical example of TMDs, has considerable potential in applications such as strain engineered devices and the next generation multifunctional polymer nanocomposites. However, controlling the strain, or more practically, monitoring the strain in WS 2 and the associated micromechanics have not been so well studied. Both photoluminescence (PL) spectroscopy and Raman spectroscopy have been proved to be effective but PL cannot be employed to characterise multilayer TMDs while it is difficult for Raman spectroscopy to reveal the band structure. In this present study, PL and Raman spectroscopy have been combined to monitor the strain distribution and stress transfer of monolayer WS 2

That one can stack van der Waals materials with atomically sharp interfaces has provided a new material platform of constructing heterostructures. The technical challenge of mechanical stacking is picking up the exfoliated atomically thin materials after mechanical exfoliation without chemical and mechanical degradation. Chemically inert hexagonal boron nitride (hBN) has been widely used for encapsulating and picking up vdW materials. However, due to the relatively weak adhesion of hBN, assembling vdW heterostructures based on hBN has been limited. We report a new dry transfer technique. We used two vdW semiconductors (ZnPS 3 and CrPS 4 ) to pick up and encapsulate layers for vdW heterostructures, which otherwise are known to be hard to fabricate. By combining with optimized polycaprolactone (PCL) providing strong adhesion, we demonstrated various vertical heterostructure devices, including quasi-2D superconducting NbSe 2 Josephson junctions with ato...