Wenbing Peng

Tao Xu

Pascale Diener

Louis Biadala

Maxime Berthe

Xiaodong Pi

Yves Borensztein

Alberto Curcella

Romain Bernard

Geoffroy Prévot

Bruno Grandidier

Resolving the Controversial Existence of Silicene and Germanene Nanosheets Grown on Graphite

Keywords: silicene, germanene, synthesis, HOPG, scanning tunneling microscopy, electron-density superstructure

The highly oriented pyrolytic graphite (HOPG) surface, consisting of a dangling bond-free lattice, is regarded as a potential substrate for van der Waals heteroepitaxy of twodimensional layered materials. In this work, the growth of silicon and germanium on HOPG is investigated with scanning tunneling microscopy by using typical synthesis conditions for silicene and germanene on metal surfaces. At low coverages, the deposition of Si and Ge gives rise to tiny and sparse clusters that are surrounded by a honeycomb superstructure.

From the detailed analysis of the superstructure, its comparison with the one encountered on the bare and clean HOPG surface and simulations of the electron density, we conclude that the superstructure is caused by charge density modulations in the HOPG surface. At high coverages, we find the formation of clusters, assembled in filamentary patterns, which indicates a Volmer-Weber growth mode instead of a layer-by-layer growth mode. This coverage-dependent study sets the stage for revisiting recent results alleging the synthesis of silicene and germanene on the HOPG surface.

One of the main impetuses for studying graphene stemmed from analogies between the lowenergy physics of the material and relativistic quantum mechanics. [START_REF] Polini | Artificial Honeycomb Lattices for Electrons, Atoms and Photons[END_REF] While the honeycomb lattice of graphene gives rise to the formation of Dirac cones, features that do not exist in conventional two-dimensional (2D) semiconductors, the absence of an electronic gap and a weak spin-orbit coupling hampers its extensive use in electronic and spintronic applications.

Inspired by this peculiar band structure, a quest has emerged to synthesize other 2D crystals, [START_REF] Novoselov | Two-Dimensional Atomic Crystals[END_REF] since they have raised the hope to find new classes of quantum particles and create more robust quantum computation playgrounds. In particular, it has been shown that other group IV atoms can arrange into 2D atomic crystals with a honeycomb lattice. By analogy with graphene, they have been coined 2D-Xenes. [START_REF] Molle | Buckled Two-Dimensional Xene Sheets[END_REF] Nature usually forces 2D crystallites consisting of group IV elements other than carbon to morph into stable three-dimensional forms as their lateral size increases. Only an epitaxial growth on top of a substrate can suppress the deleterious thermal vibrations that are responsible for such transformations. This concept turned out to be successful for the growth of silicene, [START_REF] Vogt | Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon[END_REF][START_REF] Feng | Evidence of Silicene in Honeycomb Structures of Silicon on Ag (111)[END_REF][START_REF] Fleurence | Experimental Evidence for Epitaxial Silicene on Diboride Thin Films[END_REF] germanene, [START_REF] Dávila | Germanene: a Novel Two-Dimansional Germanium Allotrope akin to Graphene and Silicene[END_REF][START_REF] Derivaz | Continuous Germanene Layer on Al(111)[END_REF][START_REF] Gou | Strained Monolayer Germanene with 1×1 Lattice on Sb (111)[END_REF] and stanene. [START_REF] Zhu | Epitaxial Growth of Two-Dimensional Stanene[END_REF][START_REF] Gou | Strain-Induced Band Engineering in Monolayer Stanene on Sb(111)[END_REF] But, most of the syntheses occurred on metallic substrates, that electronically couple with the 2D-Xenes and hinder the unique physical phenomena expected for such 2D layers. An alternative consists in using substrates, for which a van der Waals interaction is expected. As a result, silicon and germanium have been adsorbed on semimetal, semiconductor and insulating surfaces and there have been claims of the successful formation of 2D crystallites. [START_REF] Chiappe | Two-Dimensional Si Nanosheets with Local Hexagonal Structure on a MoS2 Surface[END_REF][START_REF] Zhang | Structural and Electronic Properties of Germanene on MoS2[END_REF][START_REF] D'acapito | Evidence for Germanene Growth on Epitaxial Hexagonal (h)-AlN on Ag(111)[END_REF] But recent works related to the synthesis of silicene on MoS 2 and germanene on gold have proved that the interpretation of the experimental results might be different, casting doubt on the previous claims. [START_REF] Van Bremen | Intercalation of Si between MoS2 Layers[END_REF][START_REF] Wang | Investigation of the Atomic and Electronic Structures of Highly Ordered Two-Dimensional Germanium on Au(111)[END_REF] Similar contradictions exist for the growth of Si and Ge on highly oriented pyrolytic graphite (HOPG). Due to their weak interaction with the HOPG surface maintained at room temperature, [START_REF] Nath | Chemical-State Analysis for Low-Dimensional Si and Ge Films on Graphite[END_REF] Si and Ge adatoms could have enough freedom to bond in a honeycomb lattice. Such hypothesis has been recently proposed in Ref. 18 and 19, where honeycomb superstructures were observed and assigned to silicene and germanene islands. But several studies had previously reported the formation of clusters instead of 2D sheets, [START_REF] Nezhdanov | The Morphology, Electron Structure, and Optical Properties of Self-Assembled Silicon Nanostructures on the Surface of Highly Oriented Pyrolytic Graphite[END_REF][START_REF] Shimonaka | Self-Assembly of Ge Clusters on Highly Oriented Pyrolytic Graphite Surfaces[END_REF] indicating an island growth mode, also called Volmer-Weber. As the growth of silicene and germanene has been shown to occur in a narrow temperature window with a very small flux of Si or Ge atoms, [START_REF] Derivaz | Continuous Germanene Layer on Al(111)[END_REF][START_REF] Prévot | Formation of Silicene on Silver: Strong Interaction between Ag and Si[END_REF][START_REF] Díaz Álvarez | Scanning Tunnelling Spectroscopy and Raman Spectroscopy of Monolayer Silicene on Ag(111)[END_REF] such specific growth conditions may not have been precisely considered in all studies. Moreover, as proposed in Ref. 18 and 19, both monolayers and 3D islands could be present on the substrate surface. Therefore, we have studied the adsorption and growth of silicon and germanium on HOPG with scanning tunnelling microscopy (STM) in ultrahigh vacuum (UHV) in a more comprehensive manner.

First, we show that a (√3x√3)R30° honeycomb superstructure, induced by the presence of defects such as step edges, can be found on the pristine HOPG surface, consistent with the literature. [START_REF] Giunta | Direct Observation of Graphite Layer Edge States by Scanning Tunneling Microscopy[END_REF][START_REF] Niimi | Scanning Tunneling Microscopy and Spectroscopy of the Electronic Local Density of States of Graphite Surfaces near Monoatomic Step Edges[END_REF] Then we reproduce the experimental conditions described in Ref. 18 for the growth of silicon on the HOPG surface. It is observed that areas with a similar honeycomb structure in the vicinity of Si clusters and step edges occur. As a third step, we have investigated the growth of germanium considering different growth conditions and coverages up to one monolayer (ML). At small Ge coverages, Ge clusters are formed and are also surrounded by a honeycomb structure. Interestingly, for either Si or Ge, the analysis of the honeycomb superstructure reveals the same (√3x√3)R30° structure with respect to the triangular lattice of graphite. This is in marked contrast with the lattice structures expected for silicene and germanene on HOPG. [START_REF] De Crescenzi | Formation of Silicene Nanosheets on Graphite[END_REF][START_REF] Persichetti | Der Waals Heteroepitaxy of Germanene Islands on Graphite[END_REF][START_REF] Cai | Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene[END_REF] At high Ge coverages, the growth gives rise to filamentary patterns of Ge clusters. Similar results are also obtained for high Si coverages.

Based on the limited extent of the honeycomb structure at small coverages and the growth of clusters at higher coverages, we conclude that no 2D growth mode takes place, regardless of the growth conditions. This result questions the putative epitaxial growth of germanene and silicene on the HOPG surface. Conversely, we show that the (√3x√3)R30° honeycomb superstructure is well reproduced by taking into account the interferences resulting from electron wave functions scattered by defects on the HOPG surface.

RESULTS AND DISCUSSION

Figure 1 shows a typical STM image of the bare HOPG surface, which consists of atomically flat terraces and step edges. Far away from the step edges, the atomicallyresolved image reveals the triangular lattice of graphite with a lattice constant of 2.46 Å, which matches the second-nearest neighbour distance of carbon atoms (left inset of Figure 1). This triangular lattice is in agreement with the sub-lattice asymmetry expected for Bernal stacked graphite, for which the three atoms belonging to the first sub-lattice of the surface layer are stacked above the same three atoms of the second layer, while the other three atoms of the surface layer are above the hollow sites of the carbon hexagon of the second layer. [START_REF] Tománek | First-Principles Calculation of Highly Asymmetric Structure in Scanning Tunneling Microscopy Images of Graphite[END_REF] By contrast, in the vicinity of step edges, where the edge of the terrace appears fuzzy, the triangular lattice is no longer visible. Instead a (√3x√3)R30° superstructure appears on the upper terrace (right inset of Figure 1). Based on the lattice constant of the triangular lattice, we measure a periodicity of 4.3 ± 0.1 Å, consistent with a lattice constant of √3 x 2.46 Å = 4.26 Å. This superstructure agrees with the same superstructure usually After the deposition of Si atoms on the HOPG surface maintained at room temperature, bright protrusions, similar to the ones shown in Ref. 18, were found on the surface (Figure 2 and Figure 2-SI). We attribute these protrusions to small Si clusters, as demonstrated in Ref.

22. The apparent height of the clusters A and B is about 4 Å. We note that cluster B grew along a substrate step edge. While the major part of the atomically flat surface scanned between the clusters shows the triangular lattice of the HOPG surface, the areas in the vicinity of the clusters exhibit different reconstruction patterns. It is worth noticing that, in Ref. 18, the measured lattice constant of the honeycomb areas is similar (4.1 ± 0.2 Å) to the one measured here. We must stress that in both cases, the measured lattice constant of the (√3x√3)R30° honeycomb superstructure disagrees with the one expected for silicene. For a 30° rotation between silicene and HOPG, ab-initio simulations performed in Ref. 18Erreur ! Signet non défini. predict that a weak interaction leads to a silicene lattice constant equal to 3.79 Å, close to the one of free-standing silicene (3.83 Å). It is 11% smaller than the measured value (experimental uncertainty < 2%), which is not compatible with the observed (√3x√3)R30° superstructure cell. Moreover, in the close case of silicene on graphene, Cai et al. [START_REF] Cai | Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene[END_REF] predicted that multiple phases of single crystalline silicene with different orientations relative to the graphene substrate could coexist at room temperature. A large number of simulated configurations have been found to have nearly the same free energy, the most stable one corresponding to the coincidence between a (√21x√21)R10.9° silicene supercell and a (2√13x2√13)R13.9° graphene supercell. Whereas such a variety of reconstructions are observed for Si/Ag(111), [START_REF] Prévot | Formation of Silicene on Silver: Strong Interaction between Ag and Si[END_REF][START_REF] Díaz Álvarez | Scanning Tunnelling Spectroscopy and Raman Spectroscopy of Monolayer Silicene on Ag(111)[END_REF] all reconstructions observed on HOPG after Si evaporation correspond to (√3x√3)R3 superstructures.

Therefore, this superstructure is unlikely to be a silicene nanosheet weakly interacting with HOPG.

Additionally, it is also clear in Figure 3 that there is a continuous modification from the unreconstructed areas of graphite displaying the typical triangular arrangement (right side) to the honeycomb superstructure area close to the cluster (left side). This change takes along with a slight increase of the corrugation, as seen in the height profiles of Figure 3b, acquired along similar directions than the ones shown in Ref. 18. If the latter area were caused by silicene, we would not expect such a graduate change from the unreconstructed area to the honeycomb superstructure, extending over a distance of 30 Å. Instead a sharp transition would rather be obtained. Interestingly, an atom is located at the centre of the honeycomb.

Its contrast progressively fades out from the unreconstructed region to the honeycomb superstructure one. This central atom is not expected if the superstructure consists of a silicene nanosheet located 3.3 Å above the graphite surface, as proposed from calculation in Ref 18. In order to better evidence these observations, Figure 3b displays the measured profile along the white line indicated in Figure 3a, which shows the variation of the apparent height of this central atom. Focussing on the left part of the profile, that corresponds to the superstructure, it appears that two neighbouring atoms do not have exactly the same height, suggesting a buckling effect, similar to the one described in Figure 3c of Ref. 18. However, the positions of the up and down atoms of the honeycomb superstructure match perfectly the It has recently been demonstrated that germanene can be grown on Al(111) when this crystal is maintained at 85°C during Ge evaporation. [START_REF] Derivaz | Continuous Germanene Layer on Al(111)[END_REF] Hence, we first study the growth of Ge on HOPG heated at 85°C. After evaporation of Ge at a coverage of 0.1 ML, the surface is covered with small bright protrusions. However, the STM images obtained at 77K are usually fuzzy, as shown in the Figure 1a-SI. Although the protrusions are randomly found on the surface, they show a higher probability to reside close to step edges. As they are not seen prior to the Ge evaporation, we attribute them to Ge adsorbates. In the vicinity of areas corresponding to step edges, where the diffusion of the Ge protrusions is much reduced, superstructures with a honeycomb lattice could be observed. But the instability of the tunnelling junction, certainly due to the diffusion of Ge atoms or tiny Ge clusters under the STM tip, does not allow for accurately determining the origin of the pattern. Therefore, the sample is further annealed at 150°C for 2 hours.

After the annealing, sparse islands are found on the atomically flat terraces of the HOPG surface consistent with those shown in Ref. 19. Such islands with limited size and rounded boundaries are in contrast with the extended terraces obtained after the exfoliation of the top HOPG layer. [START_REF] Li | Scanning Tunneling Spectroscopy of Graphene on Graphite[END_REF] Therefore we can reasonably wonder if these islands could be due to stable germanene nanosheets. From the height calibration obtained from the different terraces observed in the inset of Figure 4a, we find an apparent island height of 3.3 ± 0.1 Å, which is close to the predicted separation of 3.28 Å between germanene and HOPG. [START_REF] Persichetti | Der Waals Heteroepitaxy of Germanene Islands on Graphite[END_REF] Nevertheless, this value corresponds to the interlayer spacing of graphite.

In order to disclose the nature of the island, high-resolution STM images are acquired on the largest island seen in Figure 4a. Interestingly, superstructures are observed at the boundaries of the islands (Figure 4b andd). In particular, Figure 4d shows a honeycomb structure with a lattice constant of 4.3 ± 0.1 Å, in very good agreement with the one reported in Ref. 19 and consistent with the ones predicted for a hypothetical germanene/graphene bilayer. [START_REF] Cai | Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene[END_REF] However, the geometry of the observed pattern matches the one measured in Figure 1 and 3b, corresponding to a (√3x√3)R30° honeycomb structure. When the tip scans a few nanometres away from the island edge (Figure 4c), the structure changes to show the typical triangular lattice of graphite with a periodicity of 2.46 ± 0.10 Å. The latter is seen everywhere in the centre of the islands. Again, as shown in Figure 4b, the transition between the superstructures (top left) and the triangular lattice (bottom right) is smooth without any significant height variation. Moreover, the observation of the island in Figure 4a clearly reveals that the top of the island is atomically flat in this region. Therefore, the existence of a triangular lattice with the unit cell of graphite in the major part of the island indicates a graphitic island. Although the adsorption of Ge atoms on the surface may be involved in the observation of these islands, since they were not visible prior to the Ge deposition (see section 3 of the supporting information), the (√3x√3)R30° honeycomb structure cannot be related to the epitaxial growth of a germanene nanosheet. At high coverages, keeping the same temperature range of the typical synthesis of germanene on the Al(111) surface, [START_REF] Derivaz | Continuous Germanene Layer on Al(111)[END_REF][START_REF] Stephan | Tip-Induced Switch of Germanene Atomic Structure[END_REF] the STM observation of the HOPG surface reveals the presence of small flat islands and clusters. As shown in Figure 5a, the clusters decorate the step edges of the HOPG surface, giving rise to filamentary patterns in the middle of atomically flat terraces. Their mean size is about 2 nm (see the height profile in Figure 3b-SI). They are much more stable than the Ge protrusions obtained at a coverage of 0.1 ML.

Since the HOPG surface has a triangular lattice away from the area filled with clusters, we focus on the analysis of the atomic lattice of small cluster-free flat islands (Figure 5b), looking for germanene nanosheets. However, from the atomically resolved STM images (Figure 5c Ge and Si were also evaporated at a higher coverage with the HOPG maintained at room temperature. In the STM image of Figure 6a, obtained after the deposition of 0.6 ML of Ge, a filamentary pattern is observed again. It consists of irregular clusters with a mean size of 2 nm (Figure 3d-SI). Similarly, in the scanning electron microscope image of Figure 6b, acquired after the deposition of 1.0 ML of Si, numerous clusters are seen. Some of them are sprinkled on the atomically flat terraces. But the majority decorates the step edges of the HOPG surface. Due to the very poor reactivity of the HOPG surface, adsorbed Ge and Si adatoms can easily diffuse onto this surface and then aggregate to form clusters. The formation of clusters is thus consistent with the results found by previous works, which studied the growth of Ge on the HOPG surface, despite the fact a higher Ge / Si flux was used in the previous studies. [START_REF] Kushvaha | Self-Assembled Ge, Sb and Al Nanostructures on Graphite: Comparative STM Studies[END_REF][START_REF] Nezhdanov | The Morphology, Electron Structure, and Optical Properties of Self-Assembled Silicon Nanostructures on the Surface of Highly Oriented Pyrolytic Graphite[END_REF] Away from these clusters, the analysis of atomically flat areas does not reveal a lattice spacing that is different from the triangular lattice of graphite in regions free of Ge or Si islands. Such an observation excludes the formation of germanene and silicene nanosheets on top of the HOPG surface. Only in the surroundings of isolated clusters, superstructures appeared, as shown in the inset of Figure 6a. Again, the area showing the superstructure is limited, which cannot be attributed to the growth of a 2D layered materials. With or without the presence of Si and Ge on the HOPG surface, most of the superstructures show the same (√3x√3)R30° honeycomb lattice with respect to the unit cell of pristine graphite. Moreover, such areas are always limited, which can be found in the vicinity of clusters and step edges, that involve a breakdown of the graphite atomic lattice periodicity. The superstructures could thus result from a long-range perturbation of the charge density in HOPG due to interference phenomena between the surface electron waves and scattered waves from defects or clusters. [START_REF] Mizes | Long-Range Electronic Perturbations Caused by Defects Using Scanning Tunneling Microscopy[END_REF][START_REF] Shedd | A Simple Wave Function Interference Model for the Electron-Density Superstructures Observed during Scanning Tunneling Microscopy of Perturbed Graphite Surfaces[END_REF][START_REF] Takeuchi | Electronic Superstructures on the Graphite Surface Studied by First-Principles Calculations[END_REF] Similar patterns were also observed after the adsorption of other chemical elements (e. g., ozone [START_REF] Mizes | Long-Range Electronic Perturbations Caused by Defects Using Scanning Tunneling Microscopy[END_REF] and hydrogen [START_REF] Ruffieux | Hydrogen Atoms Cause Long-Range Electronic Effects on Graphite[END_REF] ), particles (e. g., Pt, [START_REF] Xhie | Periodic Charge-Density Modulations on Graphite near Platinum Particles[END_REF][START_REF] Kondo | Formation of Nonbonding π Electronic States of Graphite due to Pt-C Hybridization[END_REF] Mo, [START_REF] Xu | STM Study of Mo Growth and Induced Surface Structure Changes on HOPG[END_REF] and Ag clusters [START_REF] Francis | Deposition and Growth of Noble Metal Clusters on Graphite[END_REF] ), and/or the creation of defects induced by Ar +bombardment [START_REF] Bai | Characteristics of the (√3x√3)R30° Superstructure of Graphite by Scanning Tunneling Microscopy[END_REF][START_REF] Tapasztó | Complex Superstructure Patterns near Defect Sites of Carbon Nanotubes and Graphite[END_REF][START_REF] López | Surface Superstructures in Highly Oriented Pyrolytic Graphite Surfaces after Ar+ Bombardment[END_REF] and heating-induced pits. [START_REF] Tapasztó | Complex Superstructure Patterns near Defect Sites of Carbon Nanotubes and Graphite[END_REF] In particular, honeycomb superstructure images identical to our observations and those in Ref. 18 have been obtained in the case of Pt clusters [START_REF] Kondo | Formation of Nonbonding π Electronic States of Graphite due to Pt-C Hybridization[END_REF] and Ar + induced defects. [START_REF] López | Surface Superstructures in Highly Oriented Pyrolytic Graphite Surfaces after Ar+ Bombardment[END_REF] In all these cases, the charge density modulations decay from the defects along a distance around 20 to 50 Å, depending on the type of defects. This is similar to what is observed here and in Ref. 18.

In order to simulate the modulation of the electron density, we have used a wave function interference model similar to the one developed in Ref. 32 and 33, keeping in mind that, at low bias, the STM images reflect the wave functions of the electron states near the Fermi surface. Therefore, we have restricted the wave planes to the six plane waves corresponding to the different K and K' points of the Brillouin zone at the Fermi energy. The electron density can then be rewritten as:

2 3 1 2 3 1 2 2 ) sin( ) cos( ) (       +       = + = ∑ ∑ = = i i i i i i a s r K b r K a r n ψ ψ (1)
where s ψ and a ψ are the symmetric and antisymmetric linear combinations of the six plane waves. For an unperturbed surface, one expects all coefficients to be equal, giving rises to the trigonal pattern in Figure 7a, with a (1x1) periodicity. As soon as the coefficients are not all equal, a (√3x√3)R30° pattern forms, as shown in Figure 7b, c andd We have also simulated the combined effect of a linear step edge and a silicon island (see Figure 8). For that purpose, we have introduced two perturbation waves, exponentially attenuated from the position of a linear defect mimicking a step edge, and from the position of a circular defect mimicking a Si island. The same attenuation coefficient of 20 Å has been used for both waves. With no attempt of performing a quantitative fit, we reproduce the variations observed in the vicinity of the island that has grown near a step edge. In particular, the transition between a hexagonal pattern and a zigzag chain is well reproduced by the model. Similarly, the transition between the different reconstructions observed in Ref. 18 corresponds to the interference between perturbation waves originating from neighbour silicon islands and does not requires the formation of a silicene nanoribbon between the clusters to account for the existence of a superstructure.

CONCLUSIONS

In conclusion, the accurate analysis of our STM data demonstrates that the deposition of Si and Ge on HOPG does not produce silicene and germanene islands for growth conditions that are known to lead to the synthesis of silicene and germanene on metal substrates. At low coverage, the well-known honeycomb (√3x√3)R30° superstructure that is observed at step edges and clusters, reflects surface charge density modulations induced by a breaking of periodicity. At high coverage, the formation of small clusters indicates a Volmer-Weber growth mode which is opposite to van der Waals heteroepitaxy. The current results highlight the challenges of synthesizing Si and Ge 2D nanosheets on the HOPG surface.

METHODS

The investigations of silicon and germanium deposition on HOPG were performed in two different ultrahigh vacuum (UHV) systems, on different samples. The preparation of clean and well-ordered HOPG surfaces consisted of two steps: first the top layers of a HOPG substrate were peeled off in air and then the substrate was loaded in UHV to be annealed at 300°C for 1 hour. For the deposition of Si and Ge on the HOPG surface, a monolayer (ML) is referred to the silicene and germanene monolayers that are obtained when the (3x3) silicene unit cell coincides with the (4x4) Ag(111)-(1x1) area and the (2x2) germanene unit cell coincides with the (3x3) Al(111)-(1x1) area. They respectively correspond to 15.7x10 [START_REF] D'acapito | Evidence for Germanene Growth on Epitaxial Hexagonal (h)-AlN on Ag(111)[END_REF] Si atoms.cm -2 and 12.6x10 [START_REF] D'acapito | Evidence for Germanene Growth on Epitaxial Hexagonal (h)-AlN on Ag(111)[END_REF] Ge atoms.cm -2 . Si was evaporated at rates below 0.02 ML/min, using an Omicron Nanotechnology e-beam evaporator and a base pressure of 1.6×10 -10 Torr. The deposition was achieved with the HOPG substrate already fixed on the STM head of a variable-temperature microscope working at room-temperature. The evaporation of Ge onto the HOPG substrate was performed with a direct-current heating of a Ge(001) substrate held at 800°C and a base pressure of 2.0×10 -10 Torr, yielding small deposition rates of ~ 0.008 ML/min. During the evaporation process, the HOPG substrate was either left at room temperature or heated at temperatures in the range 85~120°C to adopt similar growth conditions as the ones encountered for the synthesis of germanene on Al(111). [START_REF] Derivaz | Continuous Germanene Layer on Al(111)[END_REF] In the latter case, an Al(111) crystal was used to ensure that similar growth conditions led to the successful formation of germanene, as shown in Figure S2-SI. After Ge deposition, the sample was transferred into the analysis chamber of the UHV system and imaged with a lowtemperature scanning tunnelling microscope (STM) working at 77 K. In all experiments, several W STM tips, electrochemically etched and then heated in UHV, were used to ensure the reproducibility of the experimental findings described below and avoid possible tip artefacts. [START_REF] Mizes | Multiple-Tip Interpretation of Anomalous Scanning-Tunneling-Microscopy Images of Layered Materials[END_REF]

Figure 1 . 3 x 6 .3 nm 2 .

 1362 Figure 1. Large scale STM image of the clean HOPG surface obtained at 77 K. Tunneling conditions: Vsample = -1 V, Isetpoint = 50 pA. Inset: (left) Atomic-resolved STM image revealing the triangular lattice of pristine graphite. Tunneling conditions: Vsample = -0.5 V, Isetpoint = 100 pA. Image size: 4.8 x 4.8 nm 2 . (right) High-resolution STM image of a step edge on the HOPG surface. Tunneling conditions: Vsample = -0.2 V, Isetpoint = 10 pA. Image size: 6.3 x 6.3 nm 2 . The unit cells for both lattices are superimposed to the STM images.

Figure 2 .

 2 Figure 2. Room temperature STM observation of HOPG after deposition of about 0.2 ML of Si. Two Si clusters discussed in the main text are labelled A, B. The unit cell of HOPG and the one of a (√3x√3)R30° lattice are superimposed to the STM images. Tunneling conditions: Vsample =0.20 V, Isetpoint =50 pA.

Figure 3 .

 3 Figure 3. (a) High magnification STM view of the left down part of Figure 2. (b) Height profile along the white and pink segments shown in (a). The STM has been calibrated with respect to the unreconstructed HOPG. Tunneling conditions: Vsample =0.20 V, Isetpoint =50 pA.

Figure 3

 3 Figure 3 is a high resolution image of the bottom left part of Figure 2b. After accurate

Figure 4 .

 4 Figure 4. (a) STM image of the HOPG surface after the deposition of 0.1 ML of Ge at 85°C and a subsequent annealing at 150°C. Inset: STM image of the same area at a larger scale where arrows highlight steps of the HOPG surface that were used to calibrate the vertical axis of the scanner. Tunneling conditions: Vbias = +1 V, Isetpoint = 5 pA. The squares in (a) indicate the position where the atomically resolved STM images in (b), (c) and d) were obtained. Tunneling conditions: Vbias = 150 mV (b), 100 mV (c), 150 mV (d), Isetpoint = 10 pA. The images were obtained at 77 K.

Figure 5 .

 5 Figure 5. (a) Large scale STM image of the HOPG surface after the deposition of 1.0 ML of Ge at 120°C. (b) Zoom on the small island seen at the bottom of (a). (c) Atomic-resolved STM image obtained in the center of the island showing the triangular lattice of the HOPG surface. Tunneling conditions: (a) and (b) Vbias = +5V, Isetpoint = 5pA, (c) Vbias = +0.15V, Isetpoint = 30pA. The images were obtained at 77 K.

  ), we unambiguously show that these islands have the triangular lattice of graphite. This observation indicates that the synthesis of 2D islands of germanene does not take along with the nucleation of clusters at this particular temperature of 120°C for the HOPG surface and a low flux during the deposition of Ge. Importantly, in the case of Ge deposition on Al(111) surfaces, similar growth conditions lead to a full coverage of the Al terraces by germanene nanosheets (see Figure 5-SI).

Figure 6 .

 6 Figure 6. a) STM image of the HOPG surface after the deposition of 0.6 ML of Ge at room temperature. Tunneling conditions: Vsample = +5.0V, Isetpoint = 10pA. Inset : Area showing the honeycomb structure in the vicinity of isolated Ge clusters. Tunneling conditions: Vsample = 200 mV, Isetpoint = 8pA. The images were obtained at 77 K. b) Scanning electron micrograph of the HOPG surface after the deposition of 1.0 ML of Si at room temperature.

Figure 7 .

 7 Figure 7. Simulated local density of states at the Fermi level, for various set of ai and bi coefficients. a) ai = bi = 1, b) a1 = a2 = 0, a3 = bi = 1, c) a1 = a2 = b2 = b3 = 1, a3 = 2, b1 = 0, d) a1 = b2 = 1, a2 = b1 = 0, a3 = b3 = 2.

19 Figure 8 .

 198 Figure 8. Comparison of (a) the experimental STM image of Figure 3a with (b) a simulated STM image where electronic interferences are considered.

ACKNOWLEDGMENTS

We thank C. Pirri and M. Derivaz for their assistance in the sample preparation. This study is financially supported by the Natural Science Foundation of China (Grant No. 6147409), the French National Research Agency (Germanene project ANR-17-CE09-0021-03), the French state funds managed by the ANR within the Investissements d'Avenir programme LABEX MATISSE (ANR-11-IDEX-0004-02) and EQUIPEX Excelsior (ANR-11-EQPX-0015), the RENATECH network.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: Supplemental STM images and height profiles.

AUTHOR INFORMATION

Corresponding authors. *E-mail: bruno.grandidier@isen.iemn.univ-lille1.fr; geoffroy.prevot@insp.sorbonne_universite.fr; xdpi@zju.edu.cn


Lina Maria Romero Millan
email: lina.romero_millan@mines-albi.fr

Fabio Emiro

Sierra Vargas
email: fesierrav@unal.edu.co

Lina María

Romero Millán

Ange Nzihou
email: ange.nzihou@mines-albi.fr

Unraveled mechanisms in energy production from bioresources using steam gasification

Keywords:

Introduction

Each year, billion tons of agricultural and agroindustrial wastes are generated all around the world, representing one of the most abundant and inexpensive bioresources on earth [1]. Being widely available, agrowastes constitute a valuable and renewable source of energy in developed and developing countries, in the framework of an environmentally sustainable approach, representing more than 7 million terajoules of energy, or the equivalent of more than 7.5 billion barrels of diesel [2-5]. Among the existing agrowaste conversion pathways, steam gasification is particularly interesting, as it allows the production of high heating value fuel gases (10 and 18 MJ/m 3 ), for the generation of heat or power [6-8], as well as the synthesis of advanced chemicals [9-11]. However, the sustainable use of these bioresources for energy production needs to overcome different challenges related to their variability. In particular, the volume of residues generated from agricultural and agroindustrial activities is not constant throughout the year, and is regularly determined by the seasonality of crops. Therefore, the associated gasification facilities should work intermittently, or employ multiple kinds of residues with heterogeneous characteristics.

Along with the reactor configuration and the process operating conditions like temperature and steam flowrate, the characteristics of the feedstock may also impact the performance of gasification, and particularly, the produced gas composition and yield [12-14]. Regarding the steam gasification product distribution and gas composition, several studies related to different kinds of lignocellulosic feedstocks have been developed [15-17], dealing with the analysis of residues individually, without comparing different materials. A few approaches working with several feedstocks described some divergences in the gas production rate and composition during gasification, attributed to the samples ash content and composition, volatile matter, particle porosity, or particle size, without defining the mechanisms associated with the described behavior [12,18,19].

In general, lignocellulosic materials are a complex mixture of natural polymers, mainly hemicellulose, cellulose, and lignin, tightly bonded by physical and chemical interactions [20,21]. They may have different proportions of the constituent polymers depending on the plant species, origin, and age, as well as several inorganic elements present in their structure [22]. In this regard, it has been proven that the biomass gasification reactivity may depend on the sample morphology, structure, and composition [23,24]. Among these parameters, the biomass inherent inorganic composition has been identified as the most influencing one [25][26][27]. In particular, alkali and alkali earth metals (AAEM) like K, Na, Ca and Mg, are related to a catalytic effect on biomass

- • • • - ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± - + -• - Char yield(%) = m char m biomass + m steam 100
Liquid yield(%) = m liquid m biomass + m steam 100

Gas yield(%) = m gas m biomass + m steam 100

E
α(t) = m 0 -m(t) m 0 -m ash α R(α) (app) = 1 1 -α(t) dα dt R(α) (app)average = 1 n ∑ n t=1 1 1 -α(t) dα dt μ ' θ = θ = • • • • • • • • + → + + • • • + → + -

  prod = E solid + E gas + E liquid

	Gas eff (%) =	m gas HHV gas m biomass HHV biomass + m steam h steam(Tr )	100
			E solid = m char	( C pchar T r + HHV char	)
			E gas = m gas	(	h gas(Tr ) + HHV gas	)
			E liquid = m steam	( h steam(Tr )	)	+ m tars	( h tars(Tr ) + HHV tars	)
			-
			E.frac gas (%) =	E gas E prod	100
			E.frac char (%) =	E char E prod	100
			E.frac liquid (%) =	E liquid E prod	100


Pedro Garcia Eijo

Roberto Sosa

Jean-Luc Aider

Juan Martin

Juan Martin Cabaleiro
email: jmcabaleiro@gmail.com

Control of

come

Introduction

Micro-jets are jets whose at least one dimension is lower than 1 mm. Their study and use have become more and more popular because of their great potential for flow control, especially control of separated flows. The pulsed micro-jets are a new and promising type of actuator which have been developed as an extension of the stationary micro-jets used for the control of external flow separation [START_REF] Kumar | Use of high-speed microjets for active separation control in diffusers[END_REF][START_REF] Aubrun | Separation flow control on a generic ground vehicle using steady microjet arrays[END_REF] and for active control of supersonic impingement tones in [START_REF] Choi | Active control of supersonic impingement tones using steady and pulsed microjets[END_REF]. Synthetic Micro-jets have been also successfully developed for aerodynamic flow control in [START_REF] Gerbedoen | Elaboration of compact synthetic micro-jets based on micro magneto-mechanical systems for aerodynamic flow control[END_REF]. The interaction of synthetic jets with an external cross flow over the surface in which they are mounted can displace the local streamlines and induce an apparent or virtual change in the shape of the surface [START_REF] Glezer | Synthetic jets[END_REF]. Indeed, if pulsed jets have proved their efficiency to control separated flows, their miniaturization is mandatory in the perspective of industrial applications. Moreover, it is necessary to use low-cost, low-consumption, low-weight devices to ensure a positive energy balance to the flow control. A realistic solution is to use MEMS (Micro Electro Mechanical Systems) actuators to pulse micro-jets. For instance, it was shown recently that using MEMS pulsed micro-jets to control the separated flow over the rear slant of an Ahmed body ensure a much better energy gain compared to macro-jets [START_REF] Joseph | Flow control using mems pulsed micro-jets on the ahmed body[END_REF]. It was also demonstrated that the same MEMS pulsed micro-jets could also decrease the lift coefficient and suppress the recirculation bubble over the rear window of a full-scale vehicle [START_REF] Aider | Active flow control using pulsed micro-jets on a full-scale production car[END_REF]. Unfortunately, pulsed micro-jets are both difficult to design and build using the MEMS technology. Small moving mechanical parts are also an issue when considering industrial applications which require robustness.

An interesting alternative is to use Dielectric Barrier Discharge (DBD) to generate a well-controlled jet parallel to the wall. The DBD requires electrodes which can be easily integrated over a wall without disturbing the flow and without the need of upstream pressurized air circuit. Because of their high frequency response with very low power consumption, plasma actuators have become popular for active flow control experiments.

They have been successfully used for separation control on flat plates [START_REF] Roth | Boundary layer flow control with a one atmosphere uniform glow discharge surface plasma[END_REF][START_REF] Jukes | Turbulent Boundary-Layer Control for Drag Reduction Using Surface Plasma[END_REF], bluff bodies [START_REF] Thomas | Plasma Actuators for Bluff Body Flow Control[END_REF][START_REF] Mclaughlin | Plasma-Based Actuators for Cylinder Wake Vortex Control[END_REF], airfoils [START_REF] Post | Separation control on high angle of attack airfoil using plasma actuators[END_REF][START_REF] Corke | Overview of Plasma Flow Control: Concepts, Optimization, and Applications[END_REF][START_REF] Sosa | Steady control of laminar separation over airfoils with plasma sheet actuators[END_REF], cylinders [START_REF] Artana | Control of the near wake flow around a circular cylinder with electrohydrodynamic actuators[END_REF], to induce mixing enhancement of jets [START_REF] Narayanaswamy | Characterization of a high-frequency pulsed-plasma jet actuator for supersonic flow control[END_REF], and cooling enhancement [START_REF] Audier | Flow control of an elongated jet in cross-flow: Film cooling effectiveness enhancement using surface dielectric barrier discharge plasma actuator[END_REF], among others. Moreover, in [START_REF] Wang | Three-dimensional simulation of a microplasma pump[END_REF] a three-dimensional simulation of dielectric barrier discharge has been used to predict the complicated flow structure inside a plasma induced micropump, generating an induced flow rate of the order of a few mL•min -1 . Furthermore, Debiasi and Jiun-Ming [START_REF] Debiasi | Experimental study of a dbd-plasma driven channel flow[END_REF] present an experimental study to investigate the flow induced by DBD actuators placed on a channel walls, whose distance (channel height) they varied between 2-30 mm. In [START_REF] Riherd | Measurements and simulations of a channel flow powered by plasma actuators[END_REF] a numerical and experimental study was carried out to characterize a DBD-plasma driven channel flow in a 20 mm high rectangular channel, indicating the possibility of using such channel for pumping small flows as demanded by control applications. In [START_REF] Narayanaswamy | Characterization of a high-frequency pulsed-plasma jet actuator for supersonic flow control[END_REF] a pulsed-plasma jet has been characterized for potential use in supersonic flows (Mach 3) and the fraction of power that goes to gas heating was estimated to be about only 10% of the input power, which is consistent with the values available in the literature. Comprehensive and extended reviews of this subject can be found in [START_REF] Corke | Dielectric barrier discharge plasma actuators for flow control[END_REF][START_REF] Benard | Electrical and mechanical characteristics of surface ac dielectric barrier discharge plasma actuators applied to airflow control[END_REF].

If the DBD actuators have a strong potential for many applications because of their small size and easy integration over a wall, their limited velocity and flow rate make them difficult to use in high-speed aerodynamics. The maximum velocity of a DBD is about 8 m • s -1 in a small region confined into the lower part of the boundary layer. This is the reason why they are also well-suited to confined geometries, such as micro-channels or micro-nozzles. This is why we think DBD can be an efficient way to control micro-jets. The objective of this work is to evaluate the potential of DBD to pulse a 2D micro-jet without any mechanical moving parts.

In the first section, we present the method used to build the micro nozzle and the electrical assembly to generate the DBD discharges. We also describe the experimental setup that allowed us to visualize the micro-jet and to compute the fluid velocity fields by using Particle Image Velocimetry (PIV). Then we characterize the micro-jets without and with DBD actuation. We finally discuss the potential and limitations of DBD actuation to control and pulse efficiently micro-jets.

Experimental Set-up

We have built an experimental set-up that allowed us to:

• generate the micro-jet with controlled pressure and flow rate values (pneumatic circuit).

• produce the plasma discharges needed to pulse the micro-jet (electric circuit and DBD actuator channel).

• obtain high quality images for PIV (particle image velocimetry).

A global overview of the experimental set-up is shown in Fig. 1. It is composed of three main parts:

• First, the jet generation circuit (compressed air, jet nozzle, DBD actuator and glass microchannel).

• Second, the DBD signal generation and power amplification.

• Third, the visualization set-up using a smoke generation device, a laser-sheet and a high-speed camera.

Pulsed-jet generation

The pressure is increased (4 to 7 Bar) using an air compressor while the regulation is performed by a needle valve (0.5 to 4 mBar at the outlet) connected as shown in Fig. 1.

Flow rate and pressure are measured by a flowmeter and a U-tube water manometer.

A plastic chamber with a convergent (Fig. 2.a) has been built using a 3D printer to allow a smooth transition of the flow into the glass micro-channel. The micro-channel following the convergent section has been built using two 1 mm thick microscope glass slides. The channel height h is 0.39 mm while its width w is 18 mm and its length L is 75 mm. The electrodes are made with copper sheets glued to the glass slides, as illustrated in Fig. 2.b. With this set of electrodes, two types of discharges can be generated depending on the wire connection. In one case, a "positive" discharge is induced along the direction of the flow (electrodes 1 and 2 were grounded while 3 and 4 were connected to the power supply, 5 and 6 were floating) as shown on Fig. 2.c). In the second case "negative" discharges are generated in the opposite direction to the main flow (electrodes 1 and 2 were floating, 3 and 4 were connected to the power supply, 5 and 6 were grounded).

DBD electrical signals

The plasma is generated on both sides of the glass plate when a high-tension (several kV ) signal is applied to the electrodes. To avoid the plasma generated by the external electrode, it has been insulated with epoxy.

The signal applied to the electrode is generated by a wave generator (f ac = 3 kHz), an amplifier, and an ignition coil. The ignition coil amplify the voltage a thousand times up to 5 signal is measured and controlled by an oscilloscope.

Instead of applying a constant signal, it is modulated using the burst mode with a period T b . It allows to turn on and off the plasma discharge at a frequency f pulse = 1/T b with a duty cycle DC defined as DC = T on /T b (Fig. 3).

Flow visualization

The main purpose of this part of the setup was to generate high quality images for Particle Image Velocimetry (PIV) post processing. A transparent sealed box was built to retain smoke particles generated by the smoke generator while a 470 nm Laser diode was used to illuminate the smoke particles and allowed the high-speed camera to capture high-resolution images. A cylindrical lens was used to open the collimated light beam and generate a Laser sheet. The jet itself is not seeded, mainly because smoke particles would deposit on the walls on the electrodes zone, probably interfering with the DBD.

Moreover, even if we succeeded in seeding the entire flow, we would need either more light and a faster camera, or to do phase reconstruction instead of time resolved PIV. Indeed, we use a PCO 1200s high-speed camera and a continuous blue 1.5 W laser diode which let us acquire snapshots with a suitable signal to noise ratio up to 1 kHz. To obtain good velocity fields, one has to keep particle displacement between two snapshots below 5 pixels.

It corresponds to a maximum velocity around ∼ 0.2 m.s -1 . The estimated velocities in the core of the jet are ranging between 0.5 to 4 m.s -1 which could not be measured with our experimental setup.

Additionally, it is worth calculating the Stokes number (eq. 1) of the seeding particles to ensure that they will correctly follow the flow streamlines. It is defined as:

Stk = ρ p D 2 p U 0 18µ a h (1)
Where ρ p is the particle density (10 3 kg m 3 ), D p is the nominal particle size (∼ 4 µm), µ a is the air dynamic viscosity (1.81 10 -5 kg m.s ), U 0 the fluid velocity (∼ 1 m.s -1 ), and h the height of the micro-channel (0.39 mm) (see Fig. 2). The resulting stokes numbers is Stk = 0.12 which means that particles will follow fluid streamlines closely with good tracing accuracy [START_REF] Raffel | Particle Image Velocimetry: A Practical Guide[END_REF].

The laser sheet was specifically placed in the mid-section of the glass channel outlet as shown in Fig. 2. It is important to notice that in order to confirm that the micro-jet is 2D, we took snapshots of the flow in the two main nozzle symmetry planes (front and side views). The flow in the middle section remains fairly two dimensional within the vicinity of the outlet section. Axis-switching [START_REF] Cabaleiro | Axis-switching of a micro-jet[END_REF] phenomenon was not observed in our experiments within the visualized zone (See Fig. 2.a).

Results and Discussion

The micro-jet is pulsed at different frequencies f pulse varying from 1 to 80 Hz, with what we called positive and negative discharges, associated to a ionic wind in the direction, or opposite to the main flow, respectively. We also applied different voltages leading to an increase of the volume force applied inside the micro-channel. The Reynolds number is varied from 25 to 200, based on the pressure and flow-rate, and studied the effect of the DBD discharges on the jet at the outlet of the nozzle through visualizations and PIV.

The results and discussion section is be divided in two parts. First, a qualitative analysis with the experimental results is carried out pointing out the influence of some of the main parameters such as Reynolds number and DBD frequency, that characterize the shape of the resultant jet structure. In the second part a deeper analysis of the DBD frequency and "polarity" role is presented in a quantitative manner.

Qualitative analysis

In the following, the jet Reynolds number is defined as:

Re = u jet h ν (2)
where ν = 1.52 × 10 -5 m 2 .s is the kinematic viscosity at ambient temperature. u jet is the mean air velocity inside the microchannel calculated using the flowrate Q: u jet = Q hw , leading to to Re = 15 for Q = 0.24 l.min -1 .

The transient effect of the discharges on the fluid dynamics of the micro-jet can be seen in Fig. 4 for a flowrate of Q = 0.24 l/min. The time-step between the snapshots is 0.31 s for a pulsing frequency f pulse = 1 Hz. The first snapshot corresponds to the continuous free micro-jet (DBD off). The following snapshots show clearly the transport and growth of a vortex induced by the DBD actuator. It confirms that it is possible to induce a wellcontrolled perturbation of the free micro-jet. If the DBD pulsing frequencies is increased, it changes the size and the spacing between the induced vortices, as shown on Fig. 5). A higher pulsing frequency leads to smaller and closer vortices for a given flow rate. For frequencies higher than a certain value (in this case 40 Hz) the effect of the DBD starts to decrease. It is important to note that the effect of the DBD will also diminish if the Reynolds number is increased. In this case, the DBD momentum contribution becomes smaller compared to the main flow momentum.

Quantitative analysis of the pulsing jets

From the resulting experimental snapshots we obtained the velocity fields time-series through PIV computations as illustrated in Fig. 6. To quantify some of the effects of the DBD discharges, we define a new variable U y out (t), the jet velocity averaged along the outlet of the microchannel. It is obtained simply through the finite integration along the red line marked in Fig. 6 of the vertical velocity U y (t) divided by the length L of that line (eq. 3). In other words, U y out (t) is simply the mean vertical velocity over the red line at each time-step.

U y out (t) = 1 L -L/2 L/2 U y (t) • dx (3)
The distance between the horizontal line and the outlet is 3 mm (see Fig. 6) in order to obtain a clear and stable signal.

It is worth noticing that the subscript out in U y out (t) is indicating that the velocity mentioned has been measured just downstream of the microchannel. A typical U y out (t) time-series is shown on Fig. 7 for different actuation frequencies f pulse for positive or negative DBD discharges. It is important to mention that we centered each time-series around zero in order to be able to compare more easily the effect of the discharges.

In all cases we observe that the predominant frequency of the resulting signal is the DBD pulsing frequency. It clearly demonstrates that it is possible to pulse the micro-jet with the frequency of the DBD actuators, as expected. We performed a FFT analysis of each signal showed in 7. The results can be seen in Fig. 8.

In each case, the dominant frequency matches the DBD pulsing frequency as expected.

Moreover, the rest of the harmonics are uneven multiples of the dominant frequency. Another interesting result, is that the FFT of the positive discharge presents more noise than the negative one, pointing out that blowing against the micro-jet flow is more efficient than blowing in the same direction. In Fig. 9 the U y out (t) amplitudes are plotted versus the frequency for different Reynolds numbers and for positive and negative discharges. As mentioned before, the effect of the DBD pulsing frequency plays a major role on the jet velocity fluctuations.

For the three Reynolds numbers, increasing the actuation frequency reduces the amplitude of the fluctuations. Moreover, in each case there is an optimal frequency maximizing the amplitude of the fluctuations, ranging between 10 to 20 Hz.

Also, for lower frequencies, the "negative" polarity leads to higher amplitudes than the "positive" polarity. However this trend needs to be confirmed with more experiments. Finally, it can be noticed that for a given frequency larger effects are observed for lower Reynolds numbers (the blue line, which corresponds to Re = 52, has the lowest amplitudes). According to our measurements, we can increase the U y out (t) up to 20 % for a DBD pulsing frequency of 19 Hz. We have seen that the amplitude of the pulsation depends on the DBD frequency, with a maximum value between 10 Hz and 20 Hz. These results strengthen the hypothesis that the fluid response and effectiveness depends on the DBD pulsing frequency.

Frequency response analysis

To explain why the discharges have a more noticeable effect on the jet for certain frequencies, a frequency analysis has been carried out. The flow is perturbed with a single pulse of the DBD discharge in order to obtain the fundamental frequencies of the resultant jet response. A typical example is shown on Fig. 11 where U y out (t), measured at y 0 = 10mm, is plotted with the response to a single DBD pulse. This times-series are collected for different y 0 and three different Reynolds numbers. Each time-series are analyzed through FFT to find the dominant frequency.

It is worth noticing that each "y 0 " line (see Fig. This is also illustrated in Fig. 13 where the obtained dominant frequencies are plotted in a contour plot, next to a real image of the jet. One can see that most of the domain is coloured in blue, which means that the frequency spectrum ranges between 28 to 10 Hz. From these measurements, it can be concluded that there are indeed some frequencies which will be amplified by the jet. if we excite the domain with periodic perturbations, the resonance frequency of the jet profile will depend on the distance from the outlet nozzle (y 0 ). According to Fig. 12, the range of the amplified frequencies are between 55 and 10 Hz. Looking at the frequency response further away, more than 7 mm from the nozzle, the dominant frequency ranges between 28 and 10 Hz (See Fig. 13). These results are in agreement with the experimental results shown in the previous sections, where we have shown that the jet pulsation is maximum for frequencies ranging between 10 to 50 Hz.

Conclusions

We have presented experimental results showing that a DBD can be used to modulate an air microjet for 25 < Re < 200. Through the quantitative analysis of the experimental results, we found out that the predominant frequency of the pulsed micro-jet is in fact the DBD pulsing frequency. Moreover, the DBD pulsing frequency also plays an important role on the pulsation efficiency. For lower frequencies, it seems that the "negative" polarity shows a higher impact on the jet. In the cases presented here, the frequency that leads to the highest perturbation amplitude is around 20 Hz. This result is in accordance with the Fourier analysis. Finally, we found out that the jet modulation results in an increase of the total flowrate (including entrainment). In our experiments, we increased the flowrate up to 20% for a DBD pulsing frequency of 19 Hz. All these results confirm that it is possible to use a DBD to efficiently pulse a micro-jet without any mechanical or moving parts.

Acknowledgements

This work has been funded through the University of Buenos Aires grant 20020130200291BA.

Figure 1 :

 1 Figure 1: Experimental Set-up Diagram. 1) Oscilloscope, 2) Signal generator, 3) voltage amplifier, 4) Ignition coil, 5) High-voltage probe, 6) Air compressor and tank, 7) Flowmeter , 8) Needle valve, 9) Manometer, 10) Plastic nozzle, 11) Glass microchannel and electrodes(orange), 12) High-speed camera, 13) Laser.

Figure 2 :

 2 Figure 2: a) Plastic Nozzle, glass microchannel and laser iluminated plane, b) glass microchannel together with the set of electrodes, c) configuration of the copper electrodes to generate the plasma jets inside the microchannel walls. Dimensions are in mm.

Figure 3 :

 3 Figure 3: Schematic voltage signal vs time. DC = 50 %

Figure 4 :

 4 Figure 4: Visualization of the micro-jet pulsed at f pulse = 1Hz in the opposite direction, with Q = 0.24 l/min (∆V = 12 kV , Re = 15). The time-step between each snapshot is ∆T = 0.31 s.

(a) f = 5 Figure 5 :

 55 Figure 5: Influence of the pulsing frequency on the creation of the induced vortices (h = 390 µm, ∆V = 12 kV , negative, Q = 0.24 l/min, Re = 15).

Figure 6 :

 6 Figure 6: Size and location of the PIV window used to compute U y out (t) along the marked red line.

Figure 7 :

 7 Figure 7: Typical U y out (t) time series obtained for ∆V = 12 kV , and Re = 15.

Figure 8 :

 8 Figure 8: FFT analysis of each signal obtained in figure 7

Figure 9 :

 9 Figure 9: Amplitude of the U y out (t) signal, as a function of the pulsing frequency f pulse for Re = 15 (black), Re = 26 (red) and Re = 52 (blue). The DBD voltage is ∆V = 12 kV for all experiments.

3. 3 .

 3 Increase of the mean outlet entrained flow.The typical signal response is represented in Fig.10. The first part of the time-series shows the U y out (t) at the section indicated in Fig.6when the discharges are turned off. The second part shows clearly the effect of the plasma discharges, resulting in an increase in the U y out (t). Finally, the last part of the time-series represents the signal when the discharges are turned off again. This strategy to measure the changes in the U y out (t) or, as explained before, in the total flow rate, allowed us to have a clear view of the discharges effect by comparing the first and second stage. During the actuation time, one can see clearly the sharp increase of velocity associated to each DBD actuation. Once the short DBD pulse is turned off, the velocity starts to decrease until the DBD is turned on again. One can see a slow increase of the jet velocity as the DBD pulses are actuated successively. It shows that the actuation time also plays a role. If the actuation frequency was lower, the jet velocity would have time to decrease. If the actuation time (time during which the DBD is on) had been longer, maybe the maximum jet velocity would have been slightly higher at the end of the actuation time.This time-series shows that the choice of the DBD actuation frequency and the burst frequency both play an important role to optimize the jet pulsations.

Figure 10 :

 10 Figure 10: Time series of U y out (t) showing the three detail of a burst period, centered on the actuated part of the signal for ∆V = 12 kV , Re = 26, f pulse = 19 Hz, negative.

  6) has a different velocity profile and, consequently, a different dominant frequency. The Fourier analysis of the different impulse responses lead to the dominant frequencies for each Reynolds number and each vertical position. In Fig. 12 the frequencies are plotted as a function of y 0 for Re = 15, Re = 26 and Re = 52.

Figure 11 :

 11 Figure 11: Example of the response of the jet to a negative DBD pulse U y out (t) for ∆V = 15 kV , Re = 26 and y 0 = 10mm. The voltage as a function of time is added to show the time-delay between the DBD pulse and the jet response.

Figure 12 :

 12 Figure 12: Natural frequencies of the jet in response to a DBD pulse as a function of the vertical position y 0 for Re = 15, Re = 26 and Re = 52.

Figure 13 :

 13 Figure 13: Snapshot of the pulsed jet (left) for f = 10Hz, together with a contour plot of the dominant frequencies (right), for ∆V = 12 kV and Re = 15.