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Internal photoemission of electrons was used to determine the energy position of the top valence band of mono- and few-layer WS2 on an SiO2/Si substrate. It was found, contrary to density functional theory calculations, that the valence band top in WS2 shifts up in energy with respect to the conduction band minimum of SiO2 with decreasing number of monolayers. At the same time, the band alignment of WS2 with SiO2 appears to be less sensitive to the WS2 synthesis route than in the previously studied case of the MoS2/SiO2 interface, indicating less extrinsic WS2 variability.

Internal photoemission spectroscopy was used to determine the valence band top energy position in few-monolayer WS2 and WSe2 films directly synthesized on top of the SiO2 insulator. It is found that in WS2 the valence band top edge lies systematically higher (by 0.4–0.7 eV) in energy than that in WSe2. This unexpected trend is seen for several synthesis methods suggesting it to be the intrinsic property of these W-based layered dichalcogenides. At the same time, a change in the WS2 synthesis method from metal sulfurization to chemical vapor deposition leads to a ≈0.3 eV barrier change indicating a non-negligible impact of interface charges or dipoles. The influence of synthesis chemistry on the WSe2/SiO2 interface barrier appears to be marginal at least for the selenization and molecular-beam epitaxy growth methods.

2-D material FETs hold the promise of excellent gate control, but the impact of nonidealities on their performance remains poorly understood. This is because of the need, so far, to use computationally intensive nonequilibrium Green’s function (NEGF) simulations. Here, we therefore use a semiclassical model to investigate the role of nonidealities in the scaling of back-gated (BG) and top-gated (TG) monolayer MoS2 FETs. We verify the electrostatics and transport of the semiclassical model with density functional theory-based NEGF simulations and calibrate nonidealities, such as interface traps ( ${D}_{\textsf {it}}$ ) and Schottky contact barrier height ( $\phi _{\textsf {SB}}$ ) to experimental monolayer and bilayer MoS2 FETs. We find that among the nonidealities, ${D}_{\textsf {it}}$ has the strongest subthreshold swing impact with 70 mV/dec obtainable in BG devices for a ${D}_{\textsf {it}}$ of $5\times {10}^{11}$ cm−2eV $^{-1}$ , an equivalent oxide thickness (EOT) of 1 nm, and a channel length ( ${L}_{\textsf {ch}}$ ) of 5 nm. For scaled EOT, $\phi _{\textsf {SB}}$ only strongly impacts ${I}_{ \mathrm{\scriptscriptstyle ON}}$ for the TG case, as the overlapping gate thins the Schottky barriers in the BG case. We show in TG devices that a spacer of only 5 nm results in a 1000-fold drop in ${I}_{ \mathrm{\scriptscriptstyle ON}}$ because of the nonidealities. We propose positive spacer oxide charge as a solution and show that a charge density of above 1013 cm−2 is required to fully recover the device performance.

The effects of oxidants both in the channel and contact regions of MoS2 transistors are discussed through a systematic experimental study. This letter highlights the issues of partial instability in metal-organic chemical vapor deposition MoS2 and proposes a procedure, which considerably improves the electrical characteristics of back-gated transistors. By avoiding ambient exposure and layer oxidation, contact resistance can be reduced and intrinsic mobility increased by 50%.

As Si-based electronic devices are approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively studied as potential new channel materials and fundamental building blocks of emerging sensors and devices.[1-2] In this context, MoS2, WS2 and WSe2 to name a few, are now available for deposition trough different top down approaches. However, their outstanding properties are often degraded during the fabrication processes required for the device integration. For example, the selective growth of 2D TMDs their patterning and the electronics properties fine tuning still remain elusive. Here we report on electrical atomic force microcopy (AFM) and beam analysis techniques which are used to develop a framework of analysis for 2D materials. The latter is applied to understand the local properties of MoS2 comparing pristine material and structures which are selectively grown and patterned.[3] Different growth techniques are investigated. After modelling the tip-sample contact system, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. We study the local electrical properties of grain boundaries and their transport respectively in pristine and patterned structures for FET devices by conductive atomic force microscopy (C-AFM). [1] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,” Nat. Nanotechnol., vol. 9, no. 10, pp. 768–779, 2014. [2] Desai, S. B., Madhvapathy, S. R., Sachid, A. B., Llinas, J. P., Wang, Q., Ahn, G. H., Javey, A. (2016). MoS 2 transistors with 1-nanometer gate lengths, 354(6308), 2–6. [3] Chiappe, D., Asselberghs, I., Sutar, S., Iacovo, S., Afanas’Ev, V., Stesmans, A., … Thean, A. (2016). Controlled Sulfurization Process for the Synthesis of Large Area MoS2 Films and MoS2/WS2 Heterostructures. Advanced Materials Interfaces, 3(4), 1–10.

In the realm of two-dimensional material frameworks, single-element graphene-like lattices, known as Xenes, pose several issues concerning their environmental stability, with implications for their use in technology transfer to a device layout. In this Discussion, we scrutinize the chemical reactivity of epitaxial silicene, taken as a case in point, in oxygen-rich environments. The oxidation of silicene is detailed by means of a photoemission spectroscopy study upon carefully dosing molecular oxygen under vacuum and subsequent exposure to ambient conditions, showing different chemical reactivity. We therefore propose a sequential Al2O3 encapsulation of silicene as a solution to face degradation, proving its effectiveness by virtue of the interaction between silicene and a silver substrate. Based on this method, we generalize our encapsulation scheme to a large number of metal-supported Xenes by taking into account the case of epitaxial phosphorene-on-gold.

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and comprehension of its mechanisms. Low-temperature atomic layer deposition of aluminum oxide is found to n-dope MoS

Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS

Using internal photoemission of electrons, the energy position of the valence band top edge in 1 monolayer WS2 films on top of SiO2 thermally-grown on Si was monitored to evaluate the stability of the WS2 layer with respect to two critically important technological factors: exposure to air and the transfer of WS2 from the growth substrate (sapphire) onto SiO2. Contrary to previous results obtained for WS2 and MoS2 layers synthesized by metal film thermal sulfurization in H2S, the valence band top of metal-organic chemical vapor deposition grown WS2 is found to remain at 3.7 ± 0.1 eV below the conduction band bottom edge of SiO2 through different growth runs, transfer processing, and storage in air for several months. This exceptional stability indicates WS2 as a viable candidate for the wafer-scale technology implementation.

Internal photoemission of electrons from 4- and 2-monolayer thick MoS2 films prepared by sulphurization of metallic Mo on top of SiO2 or HfO2/SiO2 insulating stacks is detected. This enables determination of the energy position of the MoS2 valence band which is found to be at 4.1-4.2 eV below the SiO2 conduction band. At the interface with HfO2, a barrier height of 3.7 eV is found, corresponding to an increase of the electron affinity of MoS2 by 0.5 eV as compared to the SiO2 case. This suggests the presence of interface charges (or dipoles) in the interfacial HfO2 layer. publisher: Elsevier articletitle: Band alignment at interfaces of few-monolayer MoS2 with SiO2 and HfO2 journaltitle: Microelectronic Engineering articlelink: http://dx.doi.org/10.1016/j.mee.2015.04.106 content_type: article copyright: Copyright © 2015 Elsevier B.V. All rights reserved. ispartof: Microelectronic Engineering vol:147 pages:294-297 ispartof: location:ITALY, Udine status: published

For the first time, WS2-based transistors have been successfully integrated in a 300 mm pilot line using production tools. The 2D material was deposited using either area selective chemical vapor deposition (CVD) or atomic layer deposition (ALD). No material transfer was required. The major integration challenges are the limited adhesion and the fragility of the few-monolayer 2D material. These issues are avoided by using a sacrificial Al 2 O 3 capping layer and by encapsulating the edges of the 2D material during wet processing. The WS2 channel is contacted with Ti/TiN side contacts and an industry-standard back end of line (BEOL) flow. This novel low-temperature flow is promising for integration of back-gated 2D transistors in the BEOL.

A low-temperature electron spin resonance study has been carried out on large-area high-purity polycrystalline two-dimensional few monolayer (ML) 2H MoS2 films synthesized by sulfurization of Mo layers, with intent to atomically assess mobility-degrading detrimental point defects. This reveals the presence of a distinct previously unreported anisotropic defect of axial symmetry about the c-axis characterized by g // = 2.00145 and g ⊥ = 2.0027, with corresponding density (spin S = ½) ~3 × 1012 cm−2 for a 4 ML thick film. Inverse correlation of the defect density with grain size points to a domain boundary associated defect, inherently incorporated during sample growth. Based on the analysis of ESR signal features in combination with literature data, the signal is tentatively ascribed to the a (di)sulfur antisite defect (S or S2 substituting for a Mo atom). Beset by these defects, the grain boundaries thus emerge as an intolerable threat for the carrier mobility and layer functionality.

A gold nanowired grid polarizer is studied in the mid-IR range by means of spectroscopic Mueller matrix ellipsometry. This material is also an example of a so-called 2-dimensional hyperbolic metamaterial/metasurface, and produced by low-cost large area scalable manufacturing techniques. We present the IR spectroscopic Mueller matrix optical response of such sample for a full azimuthal rotation. It is observed strong in-plane anisotropy due to the gold wire grid. The main objective is to show a characterization methodology to reveal the complete polarimetric response over a large spectral range, providing information about the effective optical properties of the sample. Three different optical models are proposed that catch the main features of experimental data. The first one is based on the Rigorous Coupled-Wave Analysis (RCWA) assuming the profile of the sample to be periodic, second, a Generalized Effective Medium Approximation (G-EMA) assuming oriented gold ellipsoids aligned along the lines of the wire grid, and third, a biaxial dispersion model. Experimental data is compared with simulated data obtained from the three optical models and a comparative analysis between them is presented. The different models allow to retrieve information on the characteristic dimensions of the nanowires, and also to obtain the two in-plane components of the effective dielectric tensor of the nanostructured layer. The tensor component parallel to the nanowires shows a metallic response whereas the component perpendicular to them behaves as a transparent dielectric. Moreover, we have also shown that the nanowired grid behaves as a hyperbolic material.

Since its discovery, surface-enhanced Raman spectroscopy (SERS) has pushed researchers' interest to develop different kinds of active substrates for high sensitivity molecular detection. Defocused ion beam sputtering (IBS) represents a viable route for the production of large scale, highly reproducible SERS-active substrates consisting of near-field coupled nanowires featuring localized surface plasmon resonances in the visible and the near-infrared. Here we investigate the field enhancement and spatial confinement in the visible and the near-infrared of arrays of optically resonant gold nanowires, using two complementary techniques: SERS and scanning near-field optical microscopy (SNOM). While SERS allows us to quantify the field enhancement factor, SNOM is used to image the localization of the enhanced electromagnetic fields. We show that in the visible (633 nm) the nanowires are SERS active only for excitation polarized parallel to the wire-to-wire nanocavities, yielding enhancement factors of 7 X 10(3). In the near-infrared (785 nm) we observe a 2-fold larger SERS enhancement (1.3 x 10(4)) for excitation parallel to the nanocavities and detect the onset of SERS amplification for excitation polarization parallel to the nanowires long axis. Polarization-sensitive SNOM in the near-infrared (830 nm) enables the correlation of the scattered intensity with the sample morphology at the local scale. We demonstrate that the field enhancement stems from the wire-to-wire nanocavity regions when the excitation field is polarized parallel to the wire-to-wire nanocavity, while we observe more complex field confinement patterns related to the partially inhomogeneous morphology of the substrate when the polarization is parallel to the nanowires long axis. Our experiments strongly suggest IBS-fabricated nanowires as novel substrates for plasmon-enhanced spectroscopies.

Spin-on-diffussant (SoD) processing as a doping technique in two-dimensional semiconductors shows a general process compatibility with materials such as MoS2 and WSe2. The characteristic Raman modes in chemical vapor deposition-grown MoS2 are retained after a phosphorus-based SoD processing at temperatures up to 1000 °C; the evolution of the characteristic Raman peaks with SoD processing indicates a reduction in the intrinsic electron concentration. Electrical measurements show a corresponding p-type shift in the MoS2 MOSFET transfer characteristics and indicate possible oxidation of MoS2 by the SoD processing. Both these effects correlate with XPS measurements which confirm the diffusion of phosphorus atoms into MoS2 and increased atomic percentage of Mo oxide after SoD processing. The p-type shifts in the electrical characteristics correspond to a maximum of 2.4 × 1012 cm–2 change in the intrinsic carrier concentration. Repeating the experiment on WSe2 flake devices leads to similar trends: an increased p-type and a decreased n-type conduction indicating p-type doping and a significant increase in the OFF-state leakage current.Spin-on-diffussant (SoD) processing as a doping technique in two-dimensional semiconductors shows a general process compatibility with materials such as MoS2 and WSe2. The characteristic Raman modes in chemical vapor deposition-grown MoS2 are retained after a phosphorus-based SoD processing at temperatures up to 1000 °C; the evolution of the characteristic Raman peaks with SoD processing indicates a reduction in the intrinsic electron concentration. Electrical measurements show a corresponding p-type shift in the MoS2 MOSFET transfer characteristics and indicate possible oxidation of MoS2 by the SoD processing. Both these effects correlate with XPS measurements which confirm the diffusion of phosphorus atoms into MoS2 and increased atomic percentage of Mo oxide after SoD processing. The p-type shifts in the electrical characteristics correspond to a maximum of 2.4 × 1012 cm–2 change in the intrinsic carrier concentration. Repeating the experiment on WSe2 flake devices leads to similar trends: an increased...

In this study we present results on the AC admittance response of bilayer MoS2 films grown using chemical vapor deposition. A new MOS capacitor design for ultra-thin body 2D materials is proposed. We show that along with the density of interface traps (D it ), a transverse electric field distribution in the semiconductor and parasitic capacitance also cause frequency dispersion in measured capacitance. D it extracted using the conductance method in 40 devices indicates reliable measurements for channel length, L 10 m, an increase in D it is an artifact of access resistance in the semiconductor. Temperature measurements show an increasing defect distribution from cm−2 eV−1 around mid-gap to cm−2 eV−1 close to the conduction band minimum.

Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS2 as an example material, we are able to probe grain boundaries in top and buried layers using conductive atomic force microscopy. We find that the electrical radius of the grain boundary extends approximately 2 nm from the core into the pristine material. The presence of grain boundaries affects electrical conductivity not just within its own layer, but also in the surrounding layers. Therefore, electrical grain size is always smaller than the physical size, and decreases with increasing thickness of the MoS2. These results signify that the number of layers in synthetically grown 2D materials must ideally be limited for device applications.

The interaction of localized surface plasmon resonances (LSPRs) with the photonic modes in an opal photonic crystal is reported. Gold nanocrescents are evaporated at glancing angles over both microsphere monolayers and artificial opals. Microsphere monolayers allow the three localised plasmonic resonances observed in the nanocrescent optical spectra to be identified. By changing the microsphere diameter, the opal photonic stop band and a selected plasmonic resonance can be spectrally overlapped causing a dramatic change of the properties of both excitations. The creation of new modes is observed, whose spectral position and dispersion properties are a combination of those localised in nanoparticle plasmons and in the photonic crystal.

We investigated the deposition of MoS2 multilayers on large area substrates. The pre-deposition of metal or metal oxide with subsequent sulfurization is a promising technique to achieve layered films. We distinguish a different reaction behavior in metal oxide and metallic films and investigate the effect of the temperature, the H2S/H2 gas mixture composition, and the role of the underlying substrate on the material quality. The results of the experiments suggest a MoS2 growth mechanism consisting of two subsequent process steps. At first, the reaction of the sulfur precursor with the metal or metal oxide occurs, requiring higher temperatures in the case of metallic film compared to metal oxide. At this stage, the basal planes assemble towards the diffusion direction of the reaction educts and products. After the sulfurization reaction, the material recrystallizes and the basal planes rearrange parallel to the substrate to minimize the surface energy. Therefore, substrates with low roughness show basal plane assembly parallel to the substrate. These results indicate that the substrate character has a significant impact on the assembly of low dimensional MoS2 films. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information identifier: M. H. Heyne (ORCID) identifier: M. H. Heyne (ResearcherID) identifier: E. C. Neyts (ORCID) identifier: E. C. Neyts (ResearcherID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0) history: Received 2 December 2015; Accepted 4 January 2016; Accepted Manuscript published 5 January 2016; Advance Article published 18 January 2016; Version of Record published 4 February 2016 ispartof: Journal of Materials Chemistry C vol:4 issue:4 pages:1295-1304 status: published

Hyperbolic metamaterials use the concept of controlling the propagative modes through the engineering of the dispersion relation, and are considered highly promising to reach different meta-properties. Spectroscopic Mueller Matrix Ellipsometry with variable angle of incidence and full azimuthal rotation of the sample is a powerful optical technique to characterize both anisotropic and bi-anisotropic materials. We here discuss the experimentally extracted uniaxial and biaxial optical properties of two self-assembled plasmonic systems that appear to have the appropriate meta-dispersion relations. The metasurface was produced by oblique incidence angle ion beam sputtering of glass followed by shadow deposition of Au [1]. The second bulk metamaterial was a block-copolymer based self-assembled hyperbolic metamaterial of nanocomposites based on metal nanoparticles embedded in a self-assembled anisotropic polymer host, presenting a strong spectrally selective optical anisotropy [2]. The extracted effective dielectric functions and the resulting dispersion relations are presented.

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS2 film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, transition metal dichalcogenides (TMDs) with a unique layered structure have attracted tremendous interest in recent years mainly motivated by the characteristic 2D nature together with distinctive and tunable optoelectronic properties which make them appealing for a wide variety of applications. Their ultra-thin body nature, in particular, is expected to provide superior immunity to short channel effects therefore extending the potential to scale transistors down to the few-nanometer-scale. [1-4] Another key feature of 2D materials is the absence of surface dangling bonds. The latter property has the potential to eliminate lattice mismatch constraints thus paving the way for hybrid integration of TMDs into artificial heterostructures with sharp interfaces and designed band alignment. As researchers explore the physics and applications of layered semiconductors, it is now becoming important to find a wafer-scale path towards technology implementation and integration of these novel materials. In this context, it has been recently demonstrated that metal-organic chemical vapor deposition (MOCVD) can be used to manufacture large area 2D semiconductor materials. [5]. The goal of this work is to unravel some of the fundamental aspects of film formation and provide a pathway towards a layer-controlled, wafer-scale synthesis of TMD films. More in details, we will provide insights on the different growth mechanisms of TMD films on amorphous and crystalline templates. In this framework, we will discuss the key aspects to enable and control a real van der Waals epitaxy of TMD films. The target, of course, is to achieve unidirectional and high quality monocrystalline domains by forcing an epitaxial relationship between the 2D film and the underlying substrate. In-depth structural AFM, XPS, TEM analyses along with Raman, Photoluminescence and lifetime measurements are used in our study. In order to establish a direct link between electrical performance and material quality, FET devices using our layers are also fabricated. A careful link between the structural analyses and the electrical results is made in order to asses material quality and offer a step further on the understanding of 2D TMDs for nanoelectronics. B. Radisavljevic et al. Nature Nanotechnol., 6, 147 (2011). R. Ganatra and Q. Zhang, ACS Nano, 8, 4074 (2014). A. Nourbakhsh et al. Nano Lett., 16 7798 (2016). A. Nourbakhsh et al. Nanoscale, 18, 6122 (2017). K. Kang et al. Nature volume 520, 656 (2015)

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, transition metal dichalcogenides (TMDs) with a unique layered structure have attracted tremendous interest in recent years mainly motivated by the characteristic 2D nature together with distinctive and tunable optoelectronic properties which make them appealing for a wide variety of applications. Their ultra-thin body nature, in particular, is expected to provide superior immunity to short channel effects therefore extending the potential to scale transistors down to the few-nanometer-scale. [1-4] Another key feature of 2D materials is the absence of surface dangling bonds. The latter property has the potential to eliminate lattice mismatch constraints thus paving the way for hybrid integration of TMDs into artificial heterostructures with sharp interfaces and designed band alignment. These ingredients, as recently demonstrated, make TMDs ideal building blocks for the fabrication of tunnel field effect transistors with very steep sub-threshold slope which is mandatory for low voltage device operation. [5] Based on these premises, there are some practical issues that need to tackled in order to enable a TMD-based technology. Given their 2D nature, the electronic properties of TMDs critically depend on the physico-chemical characteristics of the interfaces. Therefore, interface/surface engineering represents a logical route to control the electrical performance of TMD-based devices. Surface quality control is mainly a material growth-related issue. From that standpoint, the development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. However, another important step still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale over the entire wafer surface to obtain TMD films with atomically flat, self-passivated surfaces. This challenge is further complicated by the fact that powder vaporization techniques and CVD processes used to grow TMD films do not exhibit a self-limiting character. Because of that, local thickness fluctuations and layer discontinuities are commonly observed in synthetic TMD films. In this work, we will discuss fundamental aspects of film formation and provide a pathway towards layer-controlled, wafer-scale synthesis of TMD films. The high temperatures involved in the synthesis process as well as the use of growth templates require the development of a reliable wafer-scale transfer technology. Such a heterogeneous integration, on the one hand, is useful to overcome the incompatibility of the growth process with processing requirements for the BEOL interconnect structures. On the other hand, it reduces the level of control over the interface between the 2D layer and the dielectric substrate. It is not surprising that electronic transport in transferred TMD films is strongly affected by hydrocarbon residues, non-homogeneities (e.g. uncontrolled strain and contaminated regions), dipole formation, and charge transfer effects. Monitoring environmental effects in ultra-thin TMD films and minimizing the impact of layer transfer on the electrostatic potential distribution across the interface is of utmost importance. A wafer-scale transfer process and related experimental challenges and opportunities will be also presented. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nature Nanotechnol., 6, 147 (2011). R. Ganatra and Q. Zhang, ACS Nano, 8, 4074 (2014). A. Nourbakhsh, A. Zubair, R. N. Sajjad, A. Tavakkoli K. G., W. Chen, S. Fang, X. Ling, J. Kong, M. S. Dresselhaus, E. Kaxiras, K. K. Berggren, D. Antoniadis, and T. Palacios, Nano Lett., 16 7798 (2016). A. Nourbakhsh, A. Zubair, S. Joglekar, M. Dresselhaus, and T. Palacios, Nanoscale, 18, 6122 (2017). D. Sarkar, X. Xie, W. Liu, W. Cao, J. Kang, Y. Gong, S. Kraemer, P. M. Ajayan and K. Banerjee, Nature, 526, 91 (2015).

Ultrafast transient reflectivity measurements were performed in epitaxial 4x4 silicene grown on Ag(111). Comparison with bulk silicon and silver response highlighted the occurrence of peculiar photo-physical mechanisms, suggesting a metallic-like behavior in silicene. © 2014 OSA.

Free-standing silicene, a silicon analogue of graphene, has a buckled honeycomb latticeand, because of its Dirac bandstructurecombined with its sensitive surface, offers the potential for a widely tunable two-dimensional monolayer, where external fields and interface interactions can be exploited to influence fundamental properties such as bandgapand band characterfor future nanoelectronic devices. The quantum spin Hall effect, chiral superconductivity, giant magnetoresistanceand various exotic field-dependent stateshave been predicted in monolayer silicene. Despite recent progress regarding the epitaxial synthesis of siliceneand investigation of its electronic properties, to date there has been no report of experimental silicene devices because of its air stability issue. Here, we report a silicene field-effect transistor, corroborating theoretical expectations regarding its ambipolar Dirac charge transport, with a measured room-temperature mobility of∼100 cm2 V–1 s–1attributed to acoustic phonon-limited transportand grain boundary scattering. These results are enabled by a growth–transfer–fabrication process that we have devised—silicene encapsulated delamination with native electrodes. This approach addresses a major challenge for material preservation of silicene during transfer and device fabrication and is applicable to other air-sensitive two-dimensional materials such as germaneneand phosphorene. Silicene's allotropic affinity with bulk silicon and its low-temperature synthesis compared with graphene or alternative two-dimensional semiconductors suggest a more direct integration with ubiquitous semiconductor technology.

Few monolayer (ML) thin transition metal dichalcogenide (TMD) semiconductors attract considerable interest because of their tunable properties relevant to a broad range of device applications. In particular, few-layer MoS2 has already been shown to deliver significant advantages in terms of transistor channel downscaling. The performance of the MoS2-based devices critically depends on the band alignment with other materials since the alignment determines the electrostatics of the stack, height of the tunneling barriers, contact resistance, etc. However, barrier characterization at interfaces of few-layer two-dimensional (2D) materials represents an experimental challenge: The electronic transport across these interfaces is strongly affected by hydrocarbon residues of the flake transfer process, non-homogeneities (e.g., domain boundaries), and defects in the material itself. Therefore, it is important to find a technique allowing one to reliably determine the 2D-semiconductor bandgap edge energies with respect to the bands of other materials. In the present work we will show that this goal can be achieved using the spectroscopy of Internal PhotoEmission (IPE) of electrons from MoS2 and WS2 into an insulating layer. By using 2- and 4-ML thick films of these TMDs grown on top of a SiO2 layer as the prototype interfaces, we observe electron IPE from the 2D photo-emitters and determine the corresponding energy barriers. The studied samples were prepared by thermal evaporation of a thin Mo or W film on SiO2/Si or HfO2(2 nm)/SiO2/Si substrates. Next, the samples were transferred to a furnace and sulfurized in pure H2S(10 or 100 mbar) at 800 °C resulting in the formation of a large-area (in the range of cm2) poly-crystalline MoS2 or WS2 films with crystallites of 50 nm size [1]. Cross-sectional TEM images reveal the characteristic layered structure. The Raman spectrum exhibits two distinct peaks corresponding to the in-plane (E2g) and the out-of-plane (A1g) vibrations in TMD layer, respectively. This observation reveals conversion of the metal film into 2H-MoS2 or 2H-WS2 hexagonal polytypes. By varying the thickness of the initially deposited metal, it appears possible to produce samples with 4 or 2 ML thick MoS2 and WS2 films without contamination of the sulfide/oxide interfaces. Electrical contacts were fabricated by evaporation of 100-nm thick Au or Al pads on top of the TMD films [2]. Fabrication of large area laterally uniform MoS2 and WS2 few-ML thin films enabled us to perform IPE experiments which address one of the fundamental issues concerning energy band alignment between different TMD materials. By observing electron photoemission from the valence band (VB) of MoS2 or WS2 into the conduction band (CB) of SiO2 underlayer, one can find the energy barriers. These correspond to the VB top energies of the two photo-emitting TMD materials referenced to the same energy level of the oxide CB bottom edge [3]. The IPE quantum yield spectra reveal the same field-dependent energy onset of the IPE current in the energy range 3.5-4.0 eV for both MoS2 and WS2 as well as in their stacked heterojenctions. Since the same photocurrent spectra were also obtained when using Al as the contact pad material instead of Au, the TMD layer can be identified as the sole source of photoelectrons [2]. This conclusion is independently affirmed by the observed significant decrease of the quantum yield if reducing the thickness of MoS2 electrode from 4 ML to 2 ML. The observed field dependence of the IPE spectra agrees well with the image-force barrier lowering at the TMD/SiO2 interface. The energy barrier of Fe =4.2±0.1 eV between the top of the MoS2 VB and the bottom of the SiO2 CB can be found by extrapolating this dependence to zero field. No significant difference between spectral thresholds from MoS2 and WS2 is found suggesting that the VBs of of these TMDs are nearly aligned. The same conclusion can be reached if analyzing the IPE yield spectra from MoS2/WS2 and WS2/MoS2 heterostructures. This result would suggest the validity of the “common anion rule” for sulfides of metals similarly to the earlier studied case of insulating metal oxides [3]. Recent IPE experiments addressing the effects of TMD layer transfer on top of insulating substrate will also be discussed. D. Chiappe et al., Adv. Mater. Interfaces 1500635 (2015). V. V. Afanasev et al. Microelectron. Eng. 147 294 (2015). V. V. Afanas’ev, Adv. Condens. Matter Phys. 301302 (2014).

Molybdenum disulfide (MoS2) is a promising potential replacement for Si in future microelectronic devices. Integration in electronic devices will likely involve the growth or transfer of large-area MoS2 films onto substrates and subsequent isolation of devices. In this paper, the effect of ion implantation on the electrical properties of MoS2 is reported. Large-area ∼4 layer MoS2 films were implanted by low energy phosphorus plasma at biases of 100, 200, and 300 V and a dose of 1 × 1014 cm−2. Electrical measurements using patterned Ni/Au contacts show that after implantation, independent of bias, there is greater than a 104 increase in resistivity. TEM and Raman spectroscopy suggest that the film is crystalline prior to and after ion implantation and annealing and that there is no measurable sputtering following implantation. This suggests that the increase in resistivity is likely the result of radiation damage in the MoS2. The thermal stability of the increase in electrical resistivity was assessed by a...

MoS2 is a promising two-dimensional material that is being considered as a replacement for Si in sub-5 nm device technologies. Integration could include the growth or transfer of large areas of MoS2 films onto Si wafers. Subsequent isolation of devices fabricated on MoS2 films is an open research area that has received little attention. Although reactive ion etching or chemical etching can be used to create isolated channels, these techniques are problematic due to the weak van der Waals interaction between the two-dimensional films and their substrates. Subsequent wet processing during lithography can cause delamination from the substrate, resulting in mechanical damage to the films or even displacement of the channels. An alternative to physical isolation of MoS2 was the focus of this study. It is known that, in many semiconductors, radiation damage can be used for isolation. In this study, large-area 3-5 layer MoS2 films were grown on sapphire and subsequently transferred onto SiO2/Si wafers. The MoS2 was then exposed to low energy phosphorus plasma implantation at biases of 100, 200, and 300 V and a dose of 1 x 1014 cm-2. Electrical measurements using patterned Ni/Au contacts show that after implantation, independent of bias, there is a 105 increase in resistivity and a similar increase in specific contact resistivity of the MoS2. TEM shows that the film is still crystalline and there is no measurable etching of the films after implantation, suggesting that the increase in resistivity is likely the result of radiation damage in the MoS2. The thermal stability of the increase in electrical resistivity was assessed by a series of 15 minute anneals beginning at 325°C in a sulfur overpressure and progressing up to 525°C under an Al2O3 ALD cap. The resistivity increase remained unchanged after annealing, suggesting that this is a stable alternative to physical isolation in MoS2 devices.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation. ispartof: Journal of Chemical Physics vol:146 issue:5 pages:052810-052810 ispartof: location:United States status: published

The electronic and vibrational properties of three different reconstructions of silicene on Ag(1 1 1) are calculated and compared to experimental results. The 2D epitaxial silicon layers, namely the (4 x 4), (root 13 x root 13) and (2 root 3 x 2 root 3) phases, exhibit different electronic and vibrational properties. Few peaks in the experimental Raman spectrum are identified and attributed to the vibrational modes of the silicene layers. The position and behavior of the Raman peaks with respect to the excitation energy are shown to be a fundamental tool to investigate and discern different phases of silicene on Ag( 1 1 1). (C) 2013 Elsevier B.V. All rights reserved.

The structural and electronic properties of a Si nanosheet (NS) grown onto a MoS2 substrate by means of molecular beam epitaxy are assessed. Epitaxially grown Si is shown to adapt to the trigonal prismatic surface lattice of MoS2 by forming two-dimensional nanodomains. The Si layer structure is distinguished from the underlying MoS2 surface structure. The local electronic properties of the Si nanosheet are dictated by the atomistic arrangement of the layer and unlike the MoS2 hosting substrate they are qualified by a gap-less density of states.

Silicene, the Si counterpart of graphene, grows on Ag(111) forming domains. Investigation, by means of scanning tunneling microscopy, of morphological properties is carried out by considering post-deposition process. Particular attention is here addressed to the post-deposition annealing temperature, which plays an important role in determining the resulting morphology. On the other hand, electronic properties are probed by scanning tunneling spectroscopy and a position-dependent local density of states results, which can be understood in terms of symmetry breaking in the honeycomb lattice. © 2013 Elsevier B.V.

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN. © 2013 AIP Publishing LLC.

Structural features and magnetic behaviour of TbPc2 thin films sublimated on LSMO and on cobalt surfaces have been investigated by synchrotron-based XNLD and XMCD techniques. Different orientation of the molecules is observed for the two substrates. No significant magnetic interaction with the ferromagnetic substrates is detected.

The generalized Snell's law of refraction was introduce by F. Capasso in 2011. One consequence of the law is that resonant metallic nanostructures at the interface between two dielectrics can bent the light in a controlled way. Alternatively, by measuring the bent of a beam of light induced by a nanopatterned metallic surface it is possible to retrieve information on the surface morphology. Here we show that deviation measurements of the light passing through curved metallic nanowires deposited on a glass slide gives information on the morphology only when the resonance is excited. Full Text: PDF References N. Yu, P. Genevet, M.A. Kats, F. Aieta, J.-P. Tetienne, F. Capasso, Z. Gaburro, "Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction", Science 334, 333 (2011). CrossRef X. Ni, N.K. Emani, A.V. Kildishev, A. Boltasseva, V.M. Shalaev, "Broadband Light Bending with Plasmonic Nanoantennas", Science 335, 427 (2012). CrossRef N. Engheta, "Antenna-Guided Light", Science 334, 317 (2011). CrossRef F. Aieta, P. Genevet, M.A. Kats, N. Yu, R. Blanchard, Z. Gaburro, F. Capasso, "Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces", Nano Lett. 12, 4932 (2012). CrossRef A. Belardini, F. Pannone, G. Leahu, M.C. Larciprete, M. Centini, C. Sibilia, C. Martella, M. Giordano, D. Chiappe, F. Buatier de Mongeot, "Evidence of anomalous refraction of self-assembled curved gold nanowires", Appl. Phys. Lett. 100, 251109 (2012). CrossRef A. Belardini, F. Pannone, G. Leahu, M.C. Larciprete, M. Centini, C. Sibilia, C. Martella, M. Giordano, D. Chiappe, F. Buatier de Mongeot, "Asymmetric transmission and anomalous refraction in metal nanowires metasurface", J. Europ. Opt. Soc. Rap. Public. 7, 12051 (2012). CrossRef A. Belardini, C. Sibilia, Optoelectron. Adv. Mater. ? Rapid Commun. 7, 184 (2013). DirectLink A. Sinibaldi, N. Danz, E. Descrovi, P. Munzert, U. Schulz, F. Sonntagd, L. Dominici, F. Michelotti, "Direct comparison of the performance of Bloch surface wave and surface plasmon polariton sensors", Sensors and Actuators B 174, 292?298 (2012). CrossRef A. Belardini, A. Bosco, G. Leahu, M. Centini, E. Fazio, C. Sibilia, M. Bertolotti, S. Zhukovsky, S.V. Gaponenko, "Femtosecond pulses chirping compensation by using one-dimensional compact multiple-defect photonic crystals", Appl. Phys. Lett. 89, 031111 (2006). CrossRef A. Belardini, M.C. Larciprete, M. Centini, E. Fazio, C. Sibilia, D. Chiappe, C. Martella, A. Toma, M. Giordano, F. Buatier de Mongeot, "Circular Dichroism in the Optical Second-Harmonic Emission of Curved Gold Metal Nanowires", Phys. Rev. Lett. 107, 257401 (2011). CrossRef A. Belardini, M.C. Larciprete, M. Centini, E. Fazio, C. Sibilia, M. Bertolotti, A. Toma, D. Chiappe, F. Buatier de Mongeot, "Tailored second harmonic generation from self-organized metal nano-wires arrays", Opt. Expr. 17, 3603 (2009). CrossRef

Gold nanowires in general demonstrate very interesting plasmonic properties. Here, by applying the generalized Snell’s law introduced by F. Capasso in 2011, we study how the resonant behavior of the nanowires and their geometrical feature such as the radius of curvature can produce a bent in the propagation direction of a transmitted light beam. The measurements that were performed at a wavelength larger than the nanopatterned features reveal information on the meatusurface morphology.

Nanostructured glass substrates endowed with high aspect ratio one-dimensional corrugations are prepared by defocused ion beam erosion through a self-organized gold (Au) stencil mask. The shielding action of the stencil mask is amplified by co-deposition of gold atoms during ion bombardment. The resulting glass nanostructures enable broadband anti-reflection functionality and at the same time ensure a high efficiency for diffuse light scattering (Haze). It is demonstrated that the patterned glass substrates exhibit a better photon harvesting than the flat glass substrate in p-i-n type thin film a-Si:H solar cells. © 2013 IOP Publishing Ltd.

By combining experimental techniques with ab initio density functional theory calculations, we describe the Si/Ag(111) 2D systems in terms of a sp 2-sp3 form of silicon characterized by a vertically distorted honeycomb lattice provided by the constraint imposed by the substrate. The Raman spectrum reflects the multihybridized nature of the 2D Si nanosheets (NSs) resulting from a buckling-induced distortion of a purely sp2 hybridized structure. We show that vibrational and electronic properties of 2D Si-NSs are tightly linked to the buckling arrangement. © 2013 American Chemical Society.

The chemical stability of buckled silicene, i.e., the silicon counterpart of graphene, is investigated then resulting in a low reactivity with O 2 when dosing up to 1000 L and in a progressive oxidation under ambient conditions. The latter drawback is addressed by engineering ad hoc Al- and Al2O3-based encapsulations of the silicene layer. This encapsulation design can be generally applied to any silicene configuration, irrespective of the support substrate, and it leads to the fabrication of atomically sharp and chemically intact Al/silicene and Al2O 3/silicene interfaces that can be functionally used for ex situ characterization as well as for gated device fabrication. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold nanoparticles deposited on self-organized nano-ripple quartz substrates have been studied by spectroscopic Mueller matrix ellipsometry. The surface was found to have biaxial anisotropic optical properties. For electric field components normal to the ripples the periodic and disconnected nature of the in plane nanowires gives rise to an optical response dominated by the localized plasmon resonance. In the direction parallel to the ripples the gold nanoparticles are aligned closely leading to localized plasmon resonances in the infrared. As Au was deposited at an angle oblique to the surface normal, the gold nanoparticles were formed on the side of the ripples facing the incoming evaporation flux. This makes the gold particles slightly inclined, correspondingly the principal coordinate system of the biaxial dielectric tensor results tilted. The anisotropic plasmonic optical response results in a strong polarizing effect, making it suitable as a plasmonic nanowired grid polarizer. © 2013 Optical Society of America. Open access.

We report on the electrical characterization of synthetic, large-area MoS2 layers obtained by the sulfurization technique. The effects of dielectric encapsulation and localized defect states on the intrinsic transport properties are explored with the aid of temperature-dependent measurements. We study the effect of dielectric environment by transferring as-grown MoS2 films into different dielectrics such as SiO2, Al2O3, HfO2, and ZrO2 with increasing dielectric permittivity. Electrical data are collected on a statistically-relevant device ensemble and allow to assess device performances on a large scale assembly. Our devices show relative in-sensitiveness of mobility with respect to dielectric encapsulation. We conclude that the device behavior is strongly affected by several scattering mechanisms of different origin that can completely mask any effect related to dielectric mismatch. At low temperatures, conductivity of the devices is thermally activated, a clear footprint of the existence of a mobility e...

By using self-organized patterning of polycrystalline Au electrodes, we demonstrate that the electrical resistivity of the film can be forced to become strongly anisotropic by tailoring a nanoscale ripple undulation at the metal-vacuum interface. Electrons traveling orthogonally to the nanoripples are affected by a strong increase in resistivity, while when electrons travel parallel to the undulations of the rippled film, the resistivity is almost unaffected and even shows a slight reduction. The observations are quantitatively interpreted within the semiclassical size-effect regime in terms of an increase of surface scattering from the large-scale corrugations and of a reduction of diffuse scattering from atomic-scale roughness.

A self-organised approach for the synthesis of transparent metal nanowire arrays is based on defocused ion beam sputtering. The nanowire arrays, supported on low-cost dielectric substrates (glass slides), feature a dual functionality: they exhibit anisotropic conductivity, with sheet resistances which are reduced in comparison to those of transparent conductive oxides, and additionally they support localised plasmon resonances. The latter represents an attractive feature in view of plasmon enhanced photon harvesting applications, in which the nanostructured metal electrodes are employed as an alternative to conventional transparent conductive oxides.

IO N Exploring new physical-chemical properties of atomically thin graphitic materials is retaining a booming interest and it is expected to have a tremendous impact on the development of future nanoelectronic devices. So far, the enormous consideration for graphene [ 1 , 2 ] has overshadowed other two dimensional (2D) counterparts which may overcome the intrinsic limitations of graphene as active material. Indeed, despite the recent advances either in graphene bandgap opening [ 3–5 ] or integration in ad hoc device structures, [ 6 , 7 ] the zero-gap character of free standing graphene poses a severe drawback for an electric fi eld modulation suitable to graphene based logic devices. A strong effort is currently made to provide theoretical and experimental evidence of stable graphene-like nanolattices of the other group-IV semiconductors, the so called silicene and germanene, [ 8–11 ] which might take benefi t from being naturally compatible with the Si technology and from an intrinsically higher spin orbit coupling. Unlike graphene, the spontaneous formation of silicene is not expected to be energetically favorable because the sp 3 hybridization of Si atoms is more stable than the sp 2 one. However, a graphene-like form of silicene has been theoretically predicted [ 8 , 9 , 12 ] and, in just a few years, experimental investigations have moved up this fascinating hypothesis to concrete evidence. Recent experiments [ 13 , 14 ] report on the epitaxial growth of silicene on Ag(111) substrates with a non-trivial surface arrangement and relevant electronic features such as a linear electronic dispersion at the K points and an estimated Fermi velocity v F = 1.3 × 10 6 ms − 1 . [ 13 ]

Resonant metallic nanostructures, located at the interface between two dielectrics, can produce abrupt phase discontinuities on propagating light that will be anomalously refracted by following the generalized Snell’s law. In this work, we show evidence of anomalous refraction arising when such an interface is nano-patterned with self-assembled bent gold nano-wires having sub-wavelength periodicity.

We present the application of a near infra red Mueller matrix imaging ellipsometer to the characterization of plasmonic polarizers. The samples are prepared by evaporation of Au onto SiO2 ripples. The nanostructured ripple surface has been produced by ion beam sputtering at an off normal angle of incidence. Au was thereafter evaporated onto the surface at an grazing angle. As a result, thin lines of nearly connected Au nanoparticles form along the illuminated side of the ripples, resulting in a large in-plane anisotropy of the structure. Mueller matrix imaging is used to determine the lateral uniformity of the optical signal in correlation to the real space topography of the sample, and to determine to what degree the nanoparticles tend to form a connected wire, or whether there are well separated Au particles. The success of this method in order to produce polarizers, lies in controlling the process to allow well connected lines of Au particles along the ripples, with a high degree of homogeneity. Mueller Matrix images of the sample recorded at normal incidence are shown, and the information that can be extracted from such images is discussed.