In order to clearly highlight the effect of h-BN on graphene’s electronic structure, ARPES meas- urements were carried out. Figure 5 shows the valence band structure around the K point of the first graphene Brillouin zone, for the pristine graphene layer and the h-BN/graphene heterostructure. On the pristine graphene layer (Fig. 5(a)), the linear dispersion of the π band can be observed. As it has already been observed many times in the case of epitaxial graphene with the Si termination, the π bands formed a cone, for which the π branches crossed at the Dirac point (E D ) at − 0.3 eV below the Fermi level (E F ). This n-type doping is typical of epitaxial graphene and is due to the charge transfer from the SiC substrate. After h-BN deposition (Fig. 5(b)), these π branches were still clearly defined, proving that the growth of h-BN did not affect the 2D structure of the graphene. The Dirac point was located at − 0.3 eV below the Fermi level, demonstrating the absence of electronic transfer between h-BN and graphene layers, corroborating the absence of an h-BN doped graphene layer. The full width at half maximum (fwhm) of the Dirac cone branches probably increased because of the interaction between the graphene and h-BN layers. It has been proposed that a bandgap would be induced in graphene aligned in a com- mensurate way to a h-BN substrate 46 . To confirm these features, we performed DFT calculations on the h-BN/graphene vanderWaalsheterostructure (see Methods). We considered here a commensurate h-BN monolayer on top of a graphene/SiC(0001) substrate (Fig. 6(a)). Structural determinations yielded an h-BN-graphene distance of 3.3 Å with a binding energy of 77 meV/atom, and a graphene-buffer layer of 3 Å, with a binding energy of 166 meV/atom, in agreement with the expected values for such systems. Our DFT calculations confirmed that, in this case, a gap of about ~315 meV is opened at the K point (Fig. 6(b)). This gap opening was due to the electrostatic interaction between the h-BN and the graphene planes, breaking the symmetry of the graphene lattice. Moreover, as shown in (Fig. 6(c)), h-BN main- tained its insulating properties (calculated band gap of ~5.5 eV). In the present case, as the experimental data showed no gap openings for graphene, this suggested the two following hypotheses: (i) the h-BN monolayer was lying with a rotated structure with respect to the graphene plane, preventing from sym- metry breaking (ii) several layers of h-BN were present on graphene, the interaction between the BN planes reducing the electrostatic interaction between h-BN and graphene, which forbids the gap opening. Figure 6. (a) DFT and vdW optimised geometrical structure of the h-BN/graphene/ZLG/SiC vanderWaals
According to the above values from XPS and APRES measurements, we are able to figure out the band offsets resulting from the junction formation. In fact, one of the most determining properties of a metal-semiconductor interface is its Schottky-barrier height (SBH), which is a measure of the mismatch of the energy levels for the majority of carriers across the metal- semiconductor interface. In the case of p-doped few TL GaSe /n-doped graphene heterostucture, the lack of dangling bonds on both surfaces implies the absence of surface states. Moreover, no bonds are present at the interface between the two materials, i.e. the heterostructure is held only by vanderWaals interactions. Alike a standard metal-semiconductor interface, the particular properties of these vdW heterostructure ensure the formation of a quasi-ideal (semi) metal – semiconductor interface with a behavior approaching the Schottky limit where the SBH is given by the difference between the metal work function (𝜙 𝑚 ) and the electron affinity (𝜒) of the
This quest to revisit semiconductor electronicsraises the question on the possibility to design elementary devices such as light sources, diodes and photodetectors from 2D materials. In addition the obtained devices need to stay compatible with existing driving technologies and consequently need to be operated at fairly low bias.To design such devices, control of the carrier density of the system is highly desirable since it can further be used as a switch of the material properties. Nevertheless in VanderWaalsheterostructure most of the gate effect remains based on the capacitive coupling through a thick dielectric which requires large operating biases. Here we propose to use electrolyte gating to reduce the operating bias of a graphene-MoS 2 heterojunction. We saw that its behavior can be tuned from a diode to a resistive
of this paper. Figure 3(a,b) show a direct comparison of the calculated band structures (see method) and the cor- responding ARPES spectra of the monolayer and bilayer MoS 2 on epitaxial graphene, along the K-Γ -K direction in the hexagonal Brillouin zone, the respective second derivative are shown in Fig. 3(c,d). Comparisons with our Density Functional Theory (DFT) calculations clearly show that the monolayer, bilayer-dependent band structure evolution shows excellent agreement with theoretical calculations. Monolayer MoS 2 presents only one band at the Γ point (maximum at Binding Energy (BE) ~ − 1.68 eV ± 0.05 eV), and this structure evolves into two branches in the case of bilayer MoS 2 (maximum at BE ~ − 1.25 eV ± 0.05 eV). This evolution is representative of the splitting of the bands due to the weak vanderWaals interaction between the two MoS 2 layers. The relative position of the top of the valence bands at the Γ point in the bilayer film is closer to the Fermi level than the one obtained from the monolayer (Figure S2). This indicates that MoS 2 undergoes a crossover from an indirect to a direct bandgap in monolayer 9,23,26 , as predicted theoretically (Figure S3). Indeed, we can observe from Figure S3 (a) to (c) the evolution of the band structure of MoS 2 from monolayer to bi- and trilayer, calculated in DFT. Even though the trilayer MoS 2 has not been considered in details here experimentally, we show the corresponding DFT result to exhibit the evolution of the band structure when considering multilayer MoS 2 . In the bi- and trilayer systems, the top of the valence band is located at the Γ point, yielding an indirect band gap with the bottom of the conduction band between the K (K′ ) and Γ points. However, when considering the monolayer band structure, the top of the valence band becomes very flat near the Γ point, leading to a direct gap at the K (K′ ) point. The evolution of the valence band at the Γ point provides a straightforward method to identify the thickness of ultrathin MoS 2 films, and also proves the high quality of MoS 2 transferred on epitaxial graphene. The epitaxial graphene underlayer does not affect the MoS 2 band structure, as expected for a vanderWaalsheterostructure. However, we do not exclude the presence of the universal buckled form of 2D crystal in our MoS 2 layer 3 .
Each step of sample preparation was checked with homemade room temperature STM.
In conclusion, by combining two vanderWaals interactions-based approaches, namely in-plane supramolecular self-assembly and out-of-plane layer-to-layer attractive forces, we have provided a methodology to realize a new class of hybrid multilayer vdW heterostructure through a soft process. Host–guest chemistry in two-dimensional supramolecular porous networks promotes the functionalization of the heterostructure. Arbitrary sequences can be elaborated through the library of available 2D crystals and the versatility of the organic building blocks, e.g. the presence or not (as shown here) and nature [24-25] of guest functional molecules at each level.The demonstration of further self-assembly on top of the stack shows the possible extension of this principle to bulk distributed heterojunctions with record density. Altogether, these results might promote a new paradigm in vanderWaals heterostructures of 2D materials.
modi ﬁcation of interlayer orientation [7 e9] . Since the properties of MoS 2 /graphene heterostructure depend strongly on the quality of
the interface between the underlying substrate and the top-layer as well as the interlayer orientation, the development of such vanderWaals (vdW) heterostructure should start with a high quality substrate material such as graphene layer. Among various graphene substrates, epitaxial graphene (EG) on silicon carbide (SiC) provides several potential advantages for designing such heterostructures including high electronic mobility, tunable substrate coupling,
extracted from the ARPES map of Figure 2(d) at the point. The sharpness of the different bands can be attributed to the high quality of the transferred flake. Figure 2(e ) shows the measured band structure corresponding to the graphene underlayer. The single and robust Dirac cone confirms that the graphene monolayer at the heterostructure preserves the Dirac linear dispersion and the massless relativistic character of the graphene carriers close to the Fermi level. The Dirac point (E D ) is located at 0.40 eV below the Fermi level as in the case of pristine graphene on
In summary, we have studied the electronic properties and the band structure of single layer h-BN grown on graph- ite using MBE. We find that this heterostructure gives rise to sharp bands, in particular for the h-BN valence bands at the K points. Our ARPES measurements of the heterostructure showed that graphite and h-BN largely retain their original electronic structure. The obtained epitaxial films of single layer h-BN demonstrate very high structural perfection as confirmed using HR-XPS and NEXAFS. ARPES experi- ments demonstrate clearly the energy dispersion of the valence band of single layer h-BN below the Fermi level. The h-BN layer is electronically decoupled from the graphite surface, as expected in vanderWaals epitaxy of a layered material such as h-BN.
because the bubbles’ mobility increases with temperature. Based on the same prin- ciple, they suggested another procedure to push out bubbles: laminating the vdW stacks onto a substrate at 180°C, with an ironing motion controlled by the micro- actuators. Without the use of extra heat, Kim et al. [ 59 ] proposed a AFM con- tact mode treatment to help remove bubbles with mechnaical force after the vdW heterostructure is completed. Since all of these methods were demonstrated with graphene-based heterostructures, some tweaking is required before applying them to the different vdW materials used in our project. They do, however, provide some plausible directions for fabrication improvements.
ductivity [22,23] . Over the past few decades, researchers have attempted to unravel correlation physics in 2D materials of heterostructure interfaces and/or oxide ultra- thin films, and have reported a variety of interesting observations [24–28] . Layered magnetic vdW materials offer an interesting platform for exploring correlation- induced phenomena. Intrinsic magnetism in TMTC implies the presence of localized electrons, so correlation physics is expected in these vdW materials. The electronic structure of TMTC can provide insights into the electronic correla- tions in layered magnetic vdW materials. However, few experimental studies have focused on the electron corre- lation in TMTC.
The aim of the present work is twofold: firstly it is to present briefly in Section 2 a new numerical tool able to compute the vanderWaals interaction energy within the additivity hypothesis but for arbitrary complex geometries and without any other approxi- mation. The scope of this article is to present this useful numerical method to the colloid community and to illustrate it by an exam- ple. The details of the numerical technique will thus be developed more in depth in a journal specialized in numerical analysis. The second aim of the present work is to assess the range of validity of both well-known and new DLVO approximations adapted the context of colloid filtration by using the numerical code as a vali- dation tool. Hence the interaction energy between a sphere and a slit of finite depth is computed analytically and validated numeri- cally in Section 3. The interaction energy between a sphere and a cylindrical pore is then computed numerically in Section 4 and the validity of the sphere/slit approximation as a model of a the real sphere/pore solution is examined.
Abstract. As a result of recent studies on unidimensional low discrepancy sequences, we can assert that the original vander Corput sequences are the worst distributed with respect to var- ious measures of irregularities of distribution among two large families of (0, 1)–sequences, and even among all (0, 1)–sequences for the star discrepancy D ∗ . We show in the present paper that it is not the case for the extreme discrepancy D by producing two kinds of sequences which are the worst distributed among all (0, 1)–sequences, with a discrepancy D essentially twice greater. In addition, we give an unified presentation for the two general- izations presently known of vander Corput sequences.
Bryan R. Goldsmith, 1,2* Jacob Florian, 2 Jin-Xun Liu, 2 Philipp Gruene, 1 Jonathan T. Lyon, 1,3 David M.
Rayner, 4 André Fielicke, 1* Matthias Scheffler, 1 and Luca M. Ghiringhelli 1*
1 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany 2 Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109‑2136, USA 3 Department of Chemistry and Biochemistry, Kennesaw State University, 370 Paulding Avenue NW, MD#1203,
Band gap modulation of bilayer graphene by single and dual molecular doping: A vanderWaals density-functional study Tao Hu a , b , Iann C. Gerber a , ∗
a Université de Toulouse; INSA, UPS, CNRS; LPCNO 135 avenue de Rangueil, F-31077 Toulouse, France b Department of Physics, Pukyong National University, Busan 608-737, South Korea
O(1) + O(log N) = O(log N),
where the constants implied by the O-symbols do not depend on y. With D(N, [y, z)) = D(N, [0, z))−D(N, [0, y)), we get sup I D(N, I) = O(log N), and the theorem is proved. As Section 3 shows, the β-adic vander Corput sequences defined by Pisot numbers β with irreducible β-polynomial considered in [Nin98a, Nin98b] are special cases of the abstract vander Corput sequences in Theorem 1. By Lemma 2, e S = (e L, A, ≤) is another, usually different, abstract numeration system satisfying the conditions of Theorem 1 whenever e
paramètre ; le prix à payer est que, à chaque application de la transforma- tion A, le gain doit être pris à la puissance un demi.
L’utilisation du paramètre de la transformation A conduit à la méthode de Vander Corput pour les sommes doubles d’exponentielles. Malgré d’énormes complications, celle-ci n’amène que de très légères améliorations
Figura 1: Copertina della rivista della Fédération des Sociétés pour la Protection des Sites et des Monuments de la Belgique
Vander Swaelmen incise in maniera decisiva nello sviluppo dell’idea di conservazione, come anche nella trasformazione di parte di territorio urbano belga devastato dai «continui imbruttimenti» attuati «dagli appetiti utilitaristici del secolo, dai lavori antiestetici degli ingegneri, dalle fantasie degli amministratori locali e dai discutibili maneggi degli architetti, grandi maneggiatori di edifici antichi» 9 . In particolare nel periodo della ricostruzione successiva alla prima guerra mondiale, egli si sforzò di agire sul territorio elaborando una pianificazione moderna e d’insieme del costruito, che costituiva il primo tentativo, in ambito belga, di conferire un’espressione collettiva al paesaggio urbano.
l'entablement et le podium du portique.
*Quant à l'ordre géant du devant de l ’Altes Muséum, i l est également rappelé de manière très vive dans les antes plates des angles, juste en deçà desquelles s'arrêtent le bandeau entre les étages et l'ouvrage rustique des murs. On peut voir un prototype d'un travail semblable de finition dans le Propylée d'Athènes, sans doute familier à Schinkel par les publications; on trouve une filia tio n au vingtième siècle -ou pour le moins un parallèle superbe- dans la manière dbnt Mies vander Rohe a traité la
VanderWaals heterostructures (VDWHs), obtained via the controlled assembly of two- dimensional atomically thin crystals, exhibit unique physico-chemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom-up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS 2 are fabricated. Field-effect transistors (FETs) based on such