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The fine-tuning of the growth conditions and post-deposition treatments is of fundamental importance to improve the efficiency of photovoltaic devices based on all-inorganic metal halide perovskites like CsPbBr3. In this work, we used time-resolved terahertz spectroscopy (TRTS) in combination with optical characterization techniques, x-ray diffraction (XRD), and scanning electron microscopy, to probe the different properties induced by a low-temperature (180 °C) annealing treatment on evaporated CsPbBr3 thin-films. We observed a faster build-up and relaxation dynamics in the annealed sample, accompanied by a remarkable decrease of the photoluminescence intensity and minor changes in the photoconductivity and XRD measurements as compared to the as-deposited sample. We estimated for both the samples a mobility of μ = 1.7 ± 0.5 × 10 2 cm2 V−1 s−1. Our results suggest that the lattice reorganization induced by low-temperature annealing of evaporated CsPbBr3 could lead to a different charge carrier-phonon coupling and to an increased contribution of non-radiative recombination channels. We found that TRTS can be effectively used to follow the changes induced by post-deposition thermal annealing of CsPbBr3.

We describe the implementation of nonlinear time-domain spectroscopy of rotovibrational IR-active modes in methane through broadband Four-Wave Mixing driven by resonant high-energy mid infrared laser pulses. At high driving pulse intensities we observe an efficient vibrational ladder climbing triggered in the molecules. This study opens the possibility to impulsively and selectively excite molecules of biological interest to high-lying vibrational states and to characterize their dynamics.

In this work we briefly review the studies of the optical constants of monolayer transition metal dichalcogenides and few layer black phosphorus, with particular emphasis to the complex dielectric function and refractive index. Specifically, we give an estimate of the complex index of refraction of phosphorene and few-layer black phosphorus. We extracted the complex index of refraction of this material from differential reflectance data reported in literature by employing a constrained Kramers-Kronig analysis. Finally, we studied the linear optical response of multilayer systems embedding phosphorene by using the transfer matrix method., Comment: 11 pages, 3 figures

We exploited ultrafast THz spectroscopy to explore the carrier dynamics in 2D perovskite and estimate the transient dielectric properties. The conductivity spectra show the presence of polaronic carriers diluted in an excitonic bath.

The graphane analogues of group 14 are a unique family of 2D materials due to the necessity of a terminal ligand for stability. Here we highlight how changing the surface ligand can lead to nonobvious interactions with other chemical species. We show using XRD, FTIR, and TGA that GeCH3 reversibly absorbs water into the van der Waals space, whereas GeH does not intercalate water. Molecular dynamics and density functional theory simulations predict that water datively interacts with the Ge-C ?* pocket on the Ge framework, resulting in local structural distortions. Surprisingly, these distortions give rise to an intense above band gap luminescence state of 1.87 eV, with an average lifetime of hundreds of picoseconds. This work opens potential applications for exploiting surface functionalization chemistry of 2D materials to create membrane and separation technologies.

We investigate high-order harmonic generation (HHG) in ZnTe crystals driven by a mid-IR optical parametric amplifier as a function of the angle between the polarization direction of the driving laser field and the crystal axis.

We measured the chiral relaxation of photoexcited Fenchone at the Carbon K-edge. Our results demonstrate that ultrafast chiral dynamics can be probed using core level spectroscopy with circularly polarized free-electron laser pulses.

We generated high-order harmonics in ZnTe crystal driven by a mid-IR optical parametric amplifier. The dependence of the harmonic signal as a function of the crystal orientation is investigated and interpreted within the re-collision framework.

We report on the implementation of time-domain spectroscopy through broadband Four-Wave Mixing in the study of roto-vibrational excitation of hydrocarbons by resonant high-energy mid-IR pulses.

We study the nature of photo-excited charge carriers in CsPbBr3 nanocrystal thin films by ultrafast optical pump - THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

We unveil the large polaron fingerprints in the transient THz dielectric response of lead-halide perovskites. We clarify the mechanism underlying the physics of charge transport of full-inorganic lead-halide perovskites by combining ultrafast Thz spectroscopy with DFT calculations.

Si:C and Si:C:P alloys are potential candidates for source-drain stressor applications in n-type Fin Field Effect Transistors (FinFETs). Increasing the C content to achieve high strain results in the arrangement of C atoms as third nearest neighbors (3nn) in the Si: C lattice. During thermal annealing, the presence of C atoms as 3nn may promote clustering at the interstitial sites, causing loss of stress. The concentration of C atoms as 3nn is reduced by the incorporation of a small amount of Ge atoms during the growth, whereas in-situ P doping does not influence this 3nn distribution [J Solid State Sci. Technol vol 6, p 755, 2017]. Small amounts of Ge are provided during low temperature selective epitaxial growth scheme, which are based on cyclic deposition and etching (CDE). In this work, we aim to provide physical insights into the aforementioned phenomena, to understand the behavior of 3nn C atoms and the types of defects that are formed in the annealed Si: C films. Using ab-initio simulations, the Ge-C interaction in the Si matrix is investigated and this insight is used to explain how the Ge incorporation leads to a reduced 3nn distribution of the C atoms. The interaction between C and P in the Si: C: P films is also investigated to explain why the P incorporation has not led to a reduction in the 3nn distribution. We then report on the Raman characterization of Si: C layers subjected to post epi annealing. As the penetration depth of the laser is dependent on the wavelength, Raman measurements at two different wavelengths enable us to probe the depth distribution of 3nn C atoms after applying different annealing conditions. We observed a homogeneous loss in 3nn C throughout the layer. Whereas in the kinematic modeling of high resolution X-ray diffraction spectra, a gradient in the substitutional C loss was observed close to the epitaxial layer/substrate interface. This gradient can be due to the out diffusion of C atoms into the Si substrate or to the formation of interstitial C clusters, which cannot be distinguished in HR-XRD. Deep Level Transient Spectroscopy indicated that the prominent out-diffusing species was interstitial CO complex while the interstitial C defects were also prevalent in the epi layer. (c) 2019 The Electrochemical Society.

The evolution of semiconductor nanoelectronics demands for a progressive miniaturization of the device feature size. Downscaling the silicon channel down to the ultra-thin film regime is a current trend in the semiconductor technology roadmap for digital devices such as ultra-thin body silicon-on-insulator field (UTB-SOI) or Fin field effect transistors. Future technology nodes are to require a turning point in the choice of alternative semiconductors replacing silicon. One approach is to make use of mobility boosters such as germanium or III-V’s in order to overcome the physical limits of silicon in the CMOS technology. However, new emerging materials are yet to come which imply a reduced dimensionality of the silicon channels (see the Figure below for a silicon roadmap).[1] This route inspired the concept of silicene as a silicon at the extreme two-dimensional (2D) limit. Silicene can be thought of as a structural analogue of graphene as well as a graphene follower in the periodic table. Silicene has recently attracted a tremendous attraction for its potential to bring the current silicon technology down to an ultimate thickness limit. Apart from the technology drivers, the existence of a silicene monolayer paves the way to the exploration of new exotic physical effecs such as the quantum spin Hall effect in an elementary crystal and to a new class of 2D materials, the so-called Xenes (X denoting other group IV semiconductors such as Ge and Sn). Despite the natural tendency of silicon to hybridize sp3 instead of sp2as carbon in graphene, the concept of silicene was previously proposed in its theoretical terms as a pseudo planar free-standing lattice. However, unlike graphene, silicene is not free-standing in nature. Silicene can be artificially forced to grow upon epitaxy on substrates with a commensurate surface lattice. The details of the silicene epitaxy are reported from the monolayer to the multilayer regime taking the growth onto Ag(111) substrates as paradigmatic example.[2] According to the ideal case of free-standing silicene, the epitaxial silicene is buckled in nature and can be grown in number of differently buckled phase depending on the growth temperature and the deposition rate. A full exploitation of silicene for device applications is hurdled by stability issues at its top and bottom interfaces. At its top interface, silicene is highly reactive under environmental condition thereby suffering from fast structural degradation and oxidation. Encapsulation with an Al2O3 ultra-thin capping films is shown to prevent silicene from degradation thus allowing for ex situ characterization and further processing steps. At the bottom interface of silicene is the hosting substrate. Previously studied hosting substrates are metallic in character, serve as stabilizers for the silicene lattice and strongly interact with it thereby suppressing the formation of Dirac cones in the silicene band structure and ultimately causing a metallization of the silicene itself. The latter fact is a detrimental obstacle for silicene handling. Two approaches are outlined to bypass this drawback. A) On one hand, a methodology for the silicene encapsulation, delamination, and isolation is presented as enabling technology for device integration. The key-point is this procedure relies on the use of epi-Ag(111) film on mica as cleavable substrates for the silicene epitaxy. A silicene-based transistor can be then fabricated by patterning the native Ag as source/drain contacts of a pure silicene channel.[3] Room temperature operation of field effect transistor incorporating silicene monolayer and multilayer as active channel materials is demonstrated. The observation of an ambipolar behavior in the transistor transfer curves turns out to be the electrical hallmark of silicene. B) On the other hand, template engineering is also explored as alternative approach to create a metal-free platform for the technological implementation of silicene. As a representative case, the epitaxy of silicene on MoS2 templates is reported and the electronic band alignment between the two terms described. [4] Silicene-on-MoS2 is then integrated into a heterosheet junction transistor making evidence of an effective electrical transport at the Si/MoS2interface. Possible advances in the functionalization of silicene are discussed which pave the way to a multifunctional applications based on the same silicene platform. Finally, an outlook of silicene contemporaries, germanene, stanene, etc., is proposed aiming at the exploitation of their non-trivial topological properties in nanoelectronics. [1] “2D Materials for Nanoelectronics”, Eds. M. Houssa, A. Dimoulas, A. Molle, CRC Press, Taylor&Francis Group, 2016 Boca Raton, FL [2] C. Grazianetti, E. Cinquanta, and A. Molle, 2D Materials 3, 012001 (2016) (Topical Review). [3] L. Tao, et al., Nature Nanotech. 10, 227 (2015). [4] A. Molle, et al., Adv. Mater. Interf. (2016), DOI: 10.1002/admi.201500619. Figure 1

Besides theoretical studies, experimental investigations on silicene began with the synthesis of silicene on ceramic or metallic catalyst substrates such as ZrB2, Ir and Ag. Among various reported methods, the epitaxial growth of silicene sheet atop Ag(111) has received increasing attention and a derivative approach of using evaporated Ag(111) film as catalyst on a cleavable substrate will be specifically discussed in this Chapter for the ease of following device studies. Despite these research progresses in silicene synthesis, there is a lack of experimental investigation on silicene devices. One of the most key challenges is the material preservation during device fabrication and measurement process. This chapter will summarize recent understanding and progress in air-stability of silicene and viable device fabrication choices, to enable the debut of the first silicene field-effect transistor. A survey will be conducted on experimental probing of electrical properties of silicene via scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and experimental transport measurement on field-effect transistors. These results not only provide experimental feedback to existing theoretical studies, but also encourages further interest in novel device concepts and prospects of silicene and other emerging 2D materials like germanene, stanene and phosphorene.

Two-dimensional transition metal dichalcogenide (MX 2 ) are receiving lot of attention in recent years due to their excellent electronic and optical properties which make them feasible for advanced photonics applications [1]. In particular Molybdenum-Disulfide (MoS 2 ) has shown large exciton binding energy, strong spin-orbit interaction, and high carrier mobilities.

The structural and electronic properties of nanoscale Si epitaxially grown on Ag(111) can be tuned from a multilayer silicene phase, where the constitutive layers incorporate a mixed sp2/sp3bonding, to other ordinary Si phases, such as amorphous and diamond-like Si. Based on comparative scanning tunneling microscopy and Raman spectroscopy investigations, a key role in determining the nanoscale Si phase is played by the growth temperature of the epitaxial deposition on Ag(111) substrate and the presence or absence of a single-layer silicene as a seed for the successive growth. Furthermore, when integrated into a field-effect transistor device, multilayer silicene exhibits a characteristic ambipolar charge carrier transport behavior that makes it strikingly different from other conventional Si channels and suggestive of a Dirac-like character of the electronic bands of the crystal. These findings spotlight the interest in multilayer silicene as a different nanoscale Si phase for advanced nanotechnology applications such as ultrascaled nanoelectronics and nanomembranes, as well as for fundamental exploration of quantum properties.

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of π and π* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. Here we elucidate the role of the metallic support in determining the physical properties of the Si/Ag interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out contributions arising from different localization. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.

The structural and electronic properties of nanoscale Si epitaxially grown on Ag(111) can be tuned from a multilayer silicene phase, where the constitutive layers incorporate a mixed sp(2)/sp(3) bonding, to other ordinary Si phases, such as amorphous and diamond -like Si. Based on comparative scanning tunneling microscopy and Raman spectroscopy investigations, a key role in determining the nanoscale Si phase is played by the growth temperature of the epitaxial deposition on Ag(111) substrate and the presence or absence of a single-layer silicene as a seed for the successive growth. Furthermore, when integrated into a field-effect transistor device, multilayer silicene exhibits a characteristic ambipolar charge carrier transport behavior that makes it strikingly different from other conventional Si channels and suggestive of a Dirac-like character of the electronic bands of the crystal. These findings spotlight the interest in multilayer silicene as a different nanoscale Si phase for advanced nanotechnology applications such as ultrascaled nanoelectronics and nanomembranes, as well as for fundamental exploration of quantum properties.

Manipulating the anisotropy in 2D nanosheets is a promising way to tune or trigger functional properties at the nanoscale. Here, a novel approach is presented to introduce a one-directional anisotropy in MoS2 nanosheets via chemical vapor deposition (CVD) onto rippled patterns prepared on ion-sputtered SiO2 /Si substrates. The optoelectronic properties of MoS2 are dramatically affected by the rippled MoS2 morphology both at the macro- and the nanoscale. In particular, strongly anisotropic phonon modes are observed depending on the polarization orientation with respect to the ripple axis. Moreover, the rippled morphology induces localization of strain and charge doping at the nanoscale, thus causing substantial redshifts of the phonon mode frequencies and a topography-dependent modulation of the MoS2 workfunction, respectively. This study paves the way to a controllable tuning of the anisotropy via substrate pattern engineering in CVD-grown 2D nanosheets.

The silicon counterpart of graphene, the so called “silicene”, has been so far a fascinating theoretical hypothesis, but only recent efforts have pushed it to concrete evidence thus triggering a tremendous interest in silicene for electronic applications and fundamental investigations. Indeed, in addition to graphene, silicene is expected to provide an intimate affinity with the Sibased technology but, unlike graphene, the silicene appears as a buckled lattice where various superstructures take place depending on the periodic distribution of buckled bonds. Here we elucidate the atomistic details of the 4x4 and √13 × √13 silicene superstructures epitaxially grown on Ag(111) substrates. These silicene layers have been successfully encapsulated with Al2O3 in order to access ex situ Raman spectroscopy. The Raman spectrum of the encapsulated silicene has been self-consistently interpreted upon comparison with ab initio calculated spectra. In addition to the presence of the double degenerate E2g mode, typical for honeycomb lattices, the non-uniform bond length and the nonsymmetric buckling in the silicene layer introduce an intrinsic disorder which gives rise to disorder activated Raman extra-peaks.

Manipulating the anisotropy in 2D nanosheets is a promising way to tune or trigger functional properties at the nanoscale. Here, a novel approach is presented to introduce a one-directional anisotropy in MoS

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission., Impurities and vacancies are commonly found within the crystalline lattice of transition metal dichalcogenides, however they are usually seen as detrimental for their optical properties. Here, the authors demonstrate that sulfur vacancies in MoS2 can give rise to a near-infrared emission peak.

In this Letter, varying the thickness of the ALD precursor films, we systematically investigate the properties of MoS2 by means of Raman and X-ray photoelectron spectroscopies (XPS) in addition to atomic force microscopy (AFM). We show that the use of the ALD technique allows the growth of MoS2 films with thicknesses down to four layers. On the contrary, at very low precursor thicknesses (

In the realm of layered materials beyond graphene, MoS2 gains a primary role due to its semiconducting nature and n-type transport down to the 2D limit that makes it extremely appealing for electronic and optoelectronic applications. The intrinsic presence of defects causes MoS2 to undergo localization effects. In the present work, solid evidence of Cs impurities in bulky MoS2 crystals in a concentration well beyond the sensitivity threshold of independent compositional spectrometry probes is brought. Unlike conventional intercalation of alkali metals in MoS2, on the basis of the measured crystal structure and ab initio calculations, it is proposed that the incorporation of Cs is stabilized by complex where one Cs atom is associated with a double S vacancy therein resulting in an overall n-type doping of the MoS2. The field effect transistor based on this kind of Cs-doped MoS2 multilayer flakes exhibits a variable range hopping transport and a metal-insulator transition.

Silicene is sometimes thought of as the Si alter ego of graphene. However, experimental evidence indicates that silicene is substantially different from graphene in terms of its stability, atomic structure, electronic properties, and device process issues. Some of these aspects hamper the feasibility of silicene for practical application, but at the same time they may offer routes to engineer or functionalize silicene as a complementary material to graphene if a good control of the material can be achieved. As such, the research on silicene runs along the cutting edge between unsurmountable limitation and pioneering opportunities. In the present review, we examine the issues that are representative of this dual edge and try to make a preliminary balance of the state-of-the-art features of this material. Each relevant topic will be explored in a dedicated section. We start with the introduction of 'experimental' silicene in the so-called 'flatland' from the point of view of technology drivers and of its conceptual precursor, freestanding silicene. We then explore the following: specific aspects of the silicene on substrates; the tendency of silicene to have multiple structural forms (what we call the polymorphic nature of silicene) the role of the strong hybridization with the substrate in the electronic band structure of silicene; the Raman spectrum of silicene, and silicene processing and integration into a transistor. Finally we conclude by proposing an investigation into silicene's emerging contemporaries in the realm of elementary two-dimensional materials. Mindful of ongoing discussions and current issues, we try to go to the heart of the problems by treating each topic objectively and scientifically and we then provide our personal views in the discussion.

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.

We report a combined experimental and theoretical study of the optical response of epitaxial silicene on silver. The silicene/Ag(111) ultraviolet-visible absorption spectra, which turn out to be strongly nonadditive, are analyzed in the framework of ab initio calculations. Electronic transitions involving silver states are found to provide huge contributions to the optical absorption of silicene, compatible with a strong Si-Ag hybridization. The results are independent of the specific silicene configuration and are also worked out for thin amorphous silicon. This points to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of transient-reflectance spectroscopy. The latter shows a metalliclike relaxation time, hence demonstrating the effects of the strong hybridization arising in silicene/Ag(111) systems on charge carriers dynamical properties.

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.

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.

By using first-principles simulations, we investigate the interaction of a 2D silicon layer with two classes of chalcogenide-layered compounds, namely MoX2 and GaX (X = S, Se, Te). A rather weak (van der Waals) interaction between the silicene layers and the chalcogenide layers is predicted. We found that the buckling of the silicene layer is correlated to the lattice mismatch between the silicene layer and the MoX2 or GaX template. The electronic properties of silicene on these different templates largely depend on the buckling of the silicene layer: highly buckled silicene on MoS2 is predicted to be metallic, while low buckled silicene on GaS and GaSe is predicted to be semi-metallic, with preserved Dirac cones at the K points. These results indicate new routes for artificially engineering silicene nanosheets, providing tailored electronic properties of this 2D layer on non-metallic substrates. These non-metallic templates also open the way to the possible integration of silicene in future nanoelectronic devices.

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.

Summary form only given. We investigated experimentally and theoretically the photo-physical properties of a class of linear sp-carbon chains, the α,ω-dinaphthylpolyyne Ar-C2n-Ar, where Ar is the naphthyl group and n is the number of triple bonds. The dynamics of the excited states has been investigated by ultrafast transient absorption spectroscopy and DFT//TDDFT calculations.We found that the role of molecular conformers, in which end-capped naphthalene rings are planar or perpendicular to the polyyne plane, is fundamental for understanding the steady state properties, like the UV-Vis absorption spectra and vibronic transitions, and the ultrafast transient absorption features. Owing to DFT//TD-DFT analysis, we were able to quantify the contributions of each conformer in the UV-Vis absorption spectra (not shown in the figure). Accordingly ultrafast transient absorption measurements revealed the excited states spectral features and dynamics. Figure 1(a) shows the 2D map of the pump-probe signal. The excitation pulse was 150-fs long and centered at 390 nm while the probe pulse was supercontinuum white light from 450 to 750 nm. Figure 1(b) shows two cuts of the 2D map for two different wavelengths. Over the whole spectral bandwidth it is possible to observe a broad photo-induced absorption, which appears instantaneously upon impulsive photo-excitation. A photo-induced absorption signal forms at 600 nm with a time constant of 30 ps and a narrow-band spectral shape.In light of DFT//TD-DFT, these features can be assigned to different transitions. The photo-induced absorption with a fast formation time can be assigned to optical transitions from the excited singlet state to higher molecular singlet levels. The narrow feature centered at 600 nm can be related to an intersystem crossing event bringing to the formation of a triplet state followed by transition between triplet states. In this picture the conformers play a prominent role: we observed a selective pathway of triplet formation, determined by the peculiar conformational structure of dinaphthylpolyyne in the ground state. Understanding the role of end-capped groups opens the way for other possible chemical functionalizations of sp-carbon chain systems in order to govern their chemical stability and allow optical control of their photo-excited properties.

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.

MoS2 multi-layer flakes, exfoliated from geological molybdenite, have been exposed to high dose electron irradiation showing clear evidence of crystal lattice and stoichiometry modifications. A massive surface sulfur depletion is induced together with the consequent formation of molybdenum nanoislands. It is found that a nanometric amorphous carbon layer, unwillingly deposited during the transmission electron microscope experiments, prevents the formation of the nanoislands. In the absence of the carbon layer, the formation of molybdenum grains proceeds both on the top and bottom surfaces of the flake. If carbon is present on both the surfaces then the formation of Mo grains is completely prevented.

The electronic band line-up between a uniform 2D silicon layer and a MoS2 substrate is shown to result in a distortion of the MoS2 bands. This effect is reflected in the admittance and electrical transport responses measured from the field-effect transistor incorporating the Si/MoS2 heterosheet interface and fabricated from MoS2 multilayer flakes on SiO2/Si++ substrates. In particular, the gate modulation of the capacitance curve and the observation of a double-peak feature in the transconductance profile make evidence of the built-in of two active channels in the transistor: one at the MoS2/SiO2 interface and the other at the Si/MoS2 heterosheet interface. The emergence of a gate modulated conductive channel at the Si/MoS2 heterosheet interface is rationalized in terms of an effective electron accumulation at the Si/MoS2 interface that is consistent with the electronic band bending deduced from high-resolution synchrotron radiation photoemission spectroscopy.

About 10 years ago the synthesis of random carbon schwarzites by supersonic cluster beam deposition has endowed the rich sp2 carbon family with its three-dimensional member. Its reluctance to grow as a three-periodic minimal surface according to topological and physical predictions still prevents schwarzites from being a hot topic, although spongy carbon is already having countless applications. Understanding the links between topology and quantum structure, possibly with the help of large-scale quantum molecular dynamics simulations should trace the route to the synthesis of periodic schwarzites. In this perspective, after a brief account on the growth and characterization of spongy carbon, we review the elementary topology of schwarzites, their stability and growth conditions as derived from pure topological arguments, the electronic structure and the electron-phonon interaction of the smallest periodic schwarzites and what can be learnt by the topological monitoring of quantum molecular dynamics.

Linear chains made by a single row of sp-hybridized carbon are predicted to display fascinating mechano-electronic properties connected with their termination and stabilization inside realistic carbon structures. The present work describes how the computed vibrational properties of cumulenic and polyynic carbon chains allow one to interpret the carbynic features observed in Raman spectra of cluster-assembled sp-sp2 films. The overall picture is consistent with the measured decay of the sp components induced by air or oxygen exposure., Comment: in print in Physica Status Solidi

We report a combined study on the synthesis, spectroscopic characterization and theoretical modelling of a series of {\alpha},{\omega}-dinaphthylpolyynes. We synthesized this family of naphtyl-terminated sp carbon chains by reacting diiodoacetylene and 1-ethynylnaphthalene under the Cadiot-Chodkiewicz reaction conditions. By means of liquid chromatography (HPLC), we separated the products and recorded their electronic absorption spectra, which enabled us to identify the complete series of dinaphthylpolyynes Ar-C2n-Ar (with Ar = naphthyl group and n = number of acetilenic units) with n ranging from 2 to 6. The longest wavelength transition (LWT) in the electronic spectra of the dinaphthylpolyynes red shifts linearly with n away from the LWT of the bare termination. This result is also supported by DFT-LDA simulations. Finally, we probed the stability of the dinaphthylpolyynes in a solid-state precipitate by Fourier-transform infrared spectroscopy and by differential scanning calorimetry (DSC)., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in [J. Phys. Chem. B], copyright \c{opyright} American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021%2Fjp104863v

Ab initio calculations within density-functional theory combined with experimental Raman spectra on cluster-beam deposited pure-carbon films provide a consistent picture of sp-carbon chains stabilized by sp;{3} or sp;{2} terminations, the latter being sensitive to torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level and to switch on and off the on-chain pi-electron magnetism.

Silicene grown on Ag(1 1 1) is characterized by several critical parameters. Among them, the substrate temperature plays a key role in determining the morphology during growth. However, an unexpected important role is also equally played by the post-deposition annealing temperature which determines the self-organization of silicene domains even in the submonolayer coverage regime and consecutive transitions between silicene with different periodicity. These temperature-driven phase transitions can be exploited to select the desired majority silicene phase, thus allowing for the manipulation of silicene properties.

[Invited] Nanoelectronics Based on Silicene In the past decade, two-dimensional (2D) materials have evolved into a big family with emerging new members beyond graphene, such as hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), and elemental monolayers (e.g. silicene, phosphorene). Silicene, the group IV elemental cousin of graphene, has a buckled honeycomb lattice1 with predicted Dirac band structure2, 3. It has the potential to be a widely tuneable 2D monolayer, where external fields and surface interactions can influence fundamental properties like the band gap4, for future innovative nanoelectronics5. For instance, several interesting phenomena in silicene has been predicted such as quantum spin Hall effect3, piezo-magnetism6 and strain-related thermal conductivity7. Despite the aforementioned exciting theoretical studies and the developing epitaxial synthesis of silicene8, air-stability is still a central issue9 for experimental material and device studies. Here, we report our recent progress addressing the air-stability issue by a unique growth-transfer-fabrication process named silicene encapsulated delamination with native electrodes (SEDNE). SEDNE process preserves silicene during transfer and device fabrication, and enables a short time window for Ag-removed silicene device studies. As a result, we demonstrated the first silicene field-effect transistors (FETs), corroborating theoretical expectations on ambipolar Dirac charge transport. Monolayer silicene FETs exhibit ON/OFF ratio over one order of magnitude with extracted mobility ~100 cm2/V-s. Our preliminary data suggest that bilayer silicene FETs also comply to ambipolar transport but with lower ON/OFF ratio and reduced mobility. The mobilities are currently limited by acoustic phonon scattering, and phase and grain boundary scattering. This work suggests a promising route to minimizing the risk of degradation during transfer and device fabrication for air-sensitive elemental 2D materials such as phosphorene and germanene. Importantly, the allotropic affinity of silicene with crystalline bulk silicon and its relatively low synthesis temperature suggest a more direct path for silicene integration with ubiquitous semiconductor technology. Acknowledgement This work is supported in part by the Army Research Office under contract W911NF-13-1-0364 and the Future and Emerging Technologies (FET) program # 270749 within the Seventh Framework Program for Research of the European Commission. References 1. Takeda, K. and Shiraishi, K., Phys. Rev. B 50, 14916-14922 (1994). 2. Cahangirov, S., et al., Phys. Rev. Lett. 102, 236804 (2009). 3. Liu, C.-C., et al., Phys. Rev. Lett. 107, 076802 (2011). 4. Ni, Z., et al., Nano Lett. 12, 113-118 (2011). 5. Padova, P. D., et al., J. Phys.: Condens. Matter. 24, 223001 (2012). 6. Nelson, Y. D., et al., J. Phys. Cond. Matter 22, 375502 (2010). 7. Pei, Q.-X., et al., J. Appl.Phys. 114, - (2013). 8. Aufray, B., et al., Appl. Phys. Lett. 96, 183102 (2010). 9. Molle, A., et al., Adv. Funct. Mater. 23, 4340-4344 (2013).

We report and solidly interpret the infrared spectrum of both pristine and oxidized carbynes embedded in a pure-carbon matrix. The spectra probe separately the effects of oxidation on sp- and on sp(2)-hybridized carbon, and provide information on the stability of the different structures in an oxidizing atmosphere. The final products are mostly short end-oxidized carbynes anchored with a double bond to sp(2) fragments, plus an oxidized sp(2) amorphous matrix. Our results have important implications for the realization of carbyne-based nano-electronics devices and highlight the active participation of carbynes in astrochemical reactions where they act as carbon source for the promotion of more complex organic species.