The large-area monolayer of MoS₂, MoSe₂, WS₂ and WSe₂ can be synthesized directly on arbitrary insulating substrates with CVD method using corresponding metal trioxides and S or Se powders as the reactants. To realize the high efficiency solar cells or other optoelectronic devices based on the TMD monolayers, it is important to develop a strategy to tune the optical band gap of these TMD monolayers. I would like to discuss several synthetic methods of obtaining monolayer alloys such as MoS_xSe_1-x, WS_x,Se_1-x, WS_xTe_1-x and so on. The band gap energy of these alloys is controllable depending on their composition.

Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.

2D materials beyond graphene such as hexagonal boron nitride (h-BN) have recently attracted a lot of research interest.[1] While, chemical vapour deposition (CVD) of h-BN on transition metal catalysts has emerged as a preferred route for synthesis, the mechanism underlying the growth of h-BN are still very much unclear.
Here, using a combination of high-pressure time and depth resolved in-situ X-ray photoelectron spectroscopy (XPS)[2] and in-situ X-ray diffraction (XRD)[2] at realistic CVD conditions of pressure (~0.001 - 1 mbar) and extreme temperatures (700-1000oC) we analyse the behaviour of some of the most popular poly-crystalline metallic catalyst films and foils (eg: Cu, Ni, Co) for h-BN growth during exposure to precursors (both gaseous and liquid precursors).
These measurements allow for a clear understanding of the B and N incorporation in the nanostructure as it happens by identifying the catalyst state at any point of time during CVD. These coupled with ex-situ experiments [3,4] allow for development of a comprehensive growth mechanism to rationally engineer CVD processes to produce high quality material for device applications.
1. Kidambi et al. (manuscript in preparation)
2. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
3. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
4. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

Owing to their unusual electronic properties, one atom-thick layers of hybridized hexagonal boron nitride (h-BN) and graphene have recently attracted a great deal of attention [1]. However, many important issues still remain open. Among them, addressing at the atomic scale the in-plane continuity as well as the interfacial electronic properties between h-BN and graphene domains is important for application in nanolectronics.
Here we demonstrate the growth of h-BN/graphene heterostructures that comprise graphene nanoribbons (GNRs) with width of ~ 1nm via a two-step process. Firstly, we produce high-quality h-BN flakes on a Au(111) substrate. Due to the chemical inertness of bulk Au, it is extremely difficult to synthesize either BN or graphene on such a substrate via the conventional chemical vapor deposition process. We overcome this challenge by using a recently developed alternative growth process - reactive magnetron-sputtering of B in N2/Ar gas mixtures - for the h-BN&’s synthesis [2]. Using this method, we are able to obtain high-quality sub-micron size h-BN flakes. Subsequently, we produce GNRs by employing the bottom-up surface-assisted method introduced in Ref. [3]. We use in-situ scanning tunneling microscopy (STM) to investigate the different steps of this process toward graphene/h-BN nano-heterostructures. Tunneling spectroscopy is applied to probe the local electronic properties, in particular near the interface of GNRs stitched to surrounding BN. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are used to assess the quality of the synthesized nanostructures.
Detaching the h-BN/GNR nanohybrids from the Au substrate after the growth, through the nondestructive bubbling transfer method [4], allows their transfer to other substrates of technological interest.
References
[1] Ci, L. J.; et al. Nat. Mater. 2010, 9, 430
[2] Sutter, P.; Nano Lett. 2013, 13, 276
[3] Cai, J.; et al. Nature 2010, 466, 470
[4] Gao, L.; et al. Nat. Comm. 2012, 3, 699

Topological insulators (TI), Bi2Se3 in particular, exhibit robust 2D surface states in which spin is strongly coupled to the orbital motion creating the prospect for the realization of electronic devices with novel switching mechanism and novel functionalities. An important requirement is that Bi2Se3 can be grown epitaxially in the form of very thin films, so that the surface properties dominate over the unwanted bulk conduction. Unfortunately in most cases, in order to observe the surface states, the films must be at least 6 quintuple (QL) layer thick (~6 nm) [1, 2], which limits their applicability in functional devices. Here in this paper we show by ARPES that well defined 2D surface metallic states are formed in ultrathin (only about 3 QL thick) Bi2Se3 grown epitaxially on AlN(0001)/Si(111) substrates by Molecular Beam Epitaxy (MBE). The use of AlN, not reported before, opens the possibility of employing insulating substrates first to improve electrical conduction measurements, second to integrate Bi2Se3 with gate dielectrics in order to exploit the quantum capacitance of TIs. The Bi2Se3/AlN will be compared with our control Bi2Se3/Si (111) samples more typically obtained in the literature [3, 4] in order to assess the advantages of growth on AlN. Employing RHEED, XRD/XRR and HRTEM we demonstrate a high epitaxial quality of Bi2Se3 on AlN. Using HRTEM, we identify interfacial misfit dislocations (MDs) that accommodate the high lattice mismatch between Bi2Se3 and AlN as well as rotation and lamellar twins. Strain mapping is performed to reveal the distribution of the strain field of the MDs at the atomic scale. Using in-situ XPS we show that, Bi2Se3 does not react with AlN allowing the growth of high quality ultrathin films at an optimum temperature of 300 oC without the need to grow low temperature buffer layers, as in the case of Si substrates. HRTEM observations reveal a crystalline, albeit distorted, interface between the two materials. Using ARPES we perform a systematic investigation of the dependence of the 2D surface states as a function of the Bi2Se3 thickness. On both substrates we observe a shift of the Dirac point closer to the Fermi level as thickness increases, but remarkably, in the case of Bi2Se3/AlN, the shift is much larger, bringing the Dirac point at about 0.15 eV below EF for a 12 QL sample, which is highly desirable for several envisaged applications. Finally, we will show that Bi2Se3 on AlN may be suitable substrate for the growth of a few layer MoSe2 dichalcogenide, yielding good quality epitaxial growth with well-defined valence bandstructure as evidenced from ARPES.
A. Dimoulas acknowledges an ERC Advanced Grant through project SMARTGATE-291260
[1] Y. Zhang, et al., Nature Physics, 6, 584-588 (2010); [2] Y. Sakamoto, et al., Phys Rev. B 81, 165432 (2010); [3] G. H. Zhang et al., Appl. Phys. Lett. 95, 053114 (2009); [4] A. A. Taskin, et al., Adv. Mater. 24, 5581 (2012).

The isolation of graphene constituted a new paradigm in next generation electronic technologies, and even though graphene is considered transformational, it is only the “tip of the iceberg.” Transition metal dichalcogenides (TMDs), could have an even greater impact on next generation technologies. Similar to graphene, TMDs are composed of vertically stacked, weakly interacting layers that are scalable down to sub-nanometer thicknesses. This, in addition to reported carrier mobilities up to 1000 cm2/Vs, and subtreshold slopes nearing the theoretical limit of 60mV/dec, make them an impressive material candidate for analog and digital electronics. Molybdenum disulfide (MoS2) is currently a leading TMD for scientific exploration, but there are a variety of other suitable, less explored, TMDs and TMD heterostructures that exhibit very attractive bandgaps, charge carrier effective masses, and mobilities for electronic applications. Transition-metal dichalcogenides (TMDs) in the form of MeX2 (where Me = a transition metal such as Mo, W, Ti, Nb, etc. and X = S, Se, or Te) also exhibit extreme flexibility, possession of tunable band gaps, modest electron mobilities, and wide variety of band-offsets. The ability to grow high quality, large area TMDs is the first critical step in the realization of electronics requiring atomic layers with tailored electronic band alignments. In this talk I will discuss the applications of TMDs and TMD heterostructures, discuss our research in the synthesis and characterization of WSe2, WTe2, and TMD/graphene van der Waal solids, and show that heterostructures could be key to advanced electronic applications.

Synthesis capability for uniform growth of 2D materials over large areas at lower temperatures without sacrificing their unique properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into novel devices. We have demonstrated, for the first time, vapor phase growth techniques for precisely controlled synthesis of continuous, uniform molecular layers of all MoX₂ and WX₂ transition metal dichalcogenide (TMD) compounds on diverse substrates, including graphene, hexagonal boron nitride, highly oriented pyrolitic graphite (HOPG), SiO₂, and metal substrates over several square centimeters. Preliminary results show MoX₂ and WX₂ transition metal dichalcogenide materials grown in a novel ultra-high vacuum (UHV) physical vapor deposition (PVD) process demonstrate properties identical or even superior (e.g., electron mobilities >500 cm² V^-1 s^-1) to exfoliated layers. This plasma-based physical vapor deposition (PVD) process for growth of high-quality mono-, bi- and few-layer TMDs is characterized by incident constituent atoms with maximum kinetic energies that are > 10 times higher than those produced in a thermal chemical vapor deposition process, yet below the threshold known to produce defects in the growing film materials. The kinetic energy and arrival rate of atoms is selected by modulating the pulse characteristics of power to the plasma cathode, while the flux of ions is tunable with an external magnetic field. The energetic atoms comprising the TMD film possess high surface mobility upon first contact with the substrate, reducing the processing temperatures necessary for continuous film coverage of the substrate. Further reduction of processing temperatures for application on flexible substrates are expected as computational modeling of growth modes guides experiments to reduce growth temperature without compromising properties via high defect densities. Examples such MoS₂/WS₂ heterostructures and bi-layer MoS₂ on few-layer graphene with a 30% lattice mismatch and TMD/TMD heterostructures are shown to demonstrate how natural accommodation of stresses at 2D van der Waals interfaces has the remarkable potential to transform the way materials selection is considered for synthetic heterostructures, as concerns regarding lattice constant matching can be abandoned with preference given to desired properties and performance. Further development of synthetic heterostructures comprised of TMDs and previously unexplored materials known to have layered structures in bulk form for new 2D sensors and devices with tailored functionality is anticipated.

Two-dimensional (2D) nanomaterials with single or few atomic layers show many unique physical and chemical properties compared with their bulk counterparts. As the most widely and intensively studied 2D materials, graphene shows exotic electrical properties and great