Out-of-plane preferential growth of 2D molybdenum diselenide nanosheets on laser-induced periodic surface structures

Applied Surface Science 669 (2024) 160567 Available online 19 June 2024 0169-4332/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Full Length Article Out-of-plane preferential growth of 2D molybdenum diselenide nanosheets on laser-induced periodic surface structures A. Fern´ andez García a , R. Ariza b , d , J. Solis b , F. Agull´ o-Rueda c , M. Manso Silvan a , * , M. GarciaLechuga b , * a Departamento de Física Aplicada, Centro de Microan´ alisis de Materiales and Instituto de Ciencia de Materiales Nicol´ as Cabrera, Universidad Aut´ onoma de Madrid, Madrid, Spain b Laser Processing Group, Instituto de ´ Optica “Daza de Vald´ es” (IO), CSIC, 28006 Madrid, Spain c Instituto de Ciencia de Materiales de Madrid, (ICMM), CSIC, 28049, Madrid, Spain d cMACS, Department of Microbial and Molecular Systems, KU Leuven, Leuven 3001, Belgium ARTICLE INFO Keywords: 2D nanosheets Molybdenum diselenide Ultrashort laser structuring Isothermal closed space vapor deposition SERS ABSTRACT In this study, we explore the morphology and orientation of molybdenum diselenide, a Van der Waals 2D material, through isothermal closed space vapor deposition on both pristine and laser-structured substrates. Laser structuring is conducted on dielectric (sapphire), semiconductor (silicon), and conductive (titanium nitride) substrates using ultrashort laser pulses, resulting in smooth topographic changes such as laser-induced periodic surface structures (LIPSS) or selective ablation. Scanning electron microscopy (SEM) reveals the pivotal role of surface structuring in the growth of out-of-plane MoSe 2 nanosheets. This effect is particularly pronounced on monocrystalline substrates like sapphire and silicon, exhibiting in-plane growth on pristine substrates. Additionally, Raman spectroscopy confirms the vertical orientation of flakes on structured substrates and highlights the presence of active edge sites by demonstrating an increased abundance of deposited material. Overall, our findings emphasize the controllability of directing the growth of MoSe 2 flakes through appropriate pre-treatment of the substrate, with potential applications in various fields, including Surface-Enhanced Raman Scattering (SERS). Furthermore, the scalability, reproducibility, and applicability to any substrate make ultrashort laser structuration a promising general strategy for orienting 2D materials. 1. Introduction Over the past decade, the exploration of two-dimensional (2D) layered materials has sparked tremendous scientific interest owing to their exceptional applications, including electronics, photonics, mechanics, and catalysis [1–6]. Among these materials, transition-metal dichalcogenides (TMDs) have emerged as a prominent class, characterized by their chemical formula MX 2 , where M represents a IV–VI transition metal atom (such as Ti, Zr, V, Nb, Mo, or W) and X denotes a chalcogen element (S, Se, or Te). The synthesis of TMDs can be achieved through two primary methods: top-down and bottom-up techniques. Top-down approaches involve mechanical and chemical exfoliation of bulk material, often assisted by sonication or ion intercalation [7–9]. Bottom-up approaches involve deposition methods, such as chemical vapor deposition (CVD) and Isothermal Closed Space Vapor Deposition (ICSVD), which probed to be effective to synthesize a wide range of TMDs (MoS 2 , MoTe 2 or WS 2 among others) [10–12]. Among the plethora of TMD compounds, in this work we focus on MoSe 2 , showing superior electrical conductivity when compared to extensively studied MoS 2 [4,13]. Furthermore, the band gap of MoSe 2 can be readily tuned through chemical composition alterations or exposure to external stimuli, such as electric fields or pressure [14–17]. Beyond their utility in optoelectronic applications requiring planar layer structures [4,6], TMDs flakes with standing configurations offer significant potential in diverse fields. Flake sharp edges, rich in dangling bonds, are chemically active and crucial for catalytic reactions with functionality in energy systems (hydrogen evolution [18], and methane conversion [19]), chemical synthesis (oxygen reduction [20]) and environmental remediation (hydrodesulfurization [21], photodegradation of organic dyes [22]). Additionally, vertically standing flakes, thanks to a high surface-to-volume ratio, are particularly suitable for energy storage devices as electrodes in Li-ion batteries [23], gas * Corresponding authors. E-mail addresses: [email protected] (M. Manso Silvan), [email protected] (M. Garcia-Lechuga). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc https://doi.org/10.1016/j.apsusc.2024.160567 Received 25 March 2024; Received in revised form 29 May 2024; Accepted 18 June 2024 Applied Surface Science 669 (2024) 160567 2 sensors [24], and optoelectronics due to their strong photon absorption and efficient photon transport [25–27]. Therefore, understanding the mechanisms and developing strategies to obtain out-of-plane flakes is crucial for achieving enhanced performance. Among the successful strategies, vertical MoS 2 nanoflakes were obtained by CVD with high Mo vapor concentration [26]. However, an oversupply of Mo vapor results in significantly thicker flakes and a variety of parameters need to be controlled for the synthesis, such as the types of precursors and substrates, the effects of substrate angles and gas flow rates [24,29,30]. This dependence causes the samples to become inhomogeneous, due to the depletion of vapor concentration as it travels across the substrate [26,31]. Achieving out-of-plane MoSe 2 nanosheets enriched with both Mo and Se edges presents a formidable challenge due to the low reactivity of Se with Mo precursors, necessitating prolonged reaction times. Alternative approaches, such as the reaction of Mo precursors with rapidly vaporized Se, have shown promise in guiding MoSe 2 growth perpendicular to the substrate [13,28], but requiring a high level of control over the experimental conditions to manage the reaction kinetics [13]. Apart from synthesis strategies, TMDs orientation shows a clear dependency on substrate properties [32,33]. Studies suggest that substrate roughness induces the vertical standing growth of MoS 2 flakes, implying a correlation between surface morphology and resulting TMD structures [33]. The surface morphology of SiO 2 with “hill and valley” structures in the range of sub-millimetre scale was found to promote the formation of large single-crystal MoS 2 structures [32]. These findings highlight the potential for tailoring TMD properties through substrate surface engineering techniques. In this work, we seek to explore the effect of 2D flakes’ orientation on substrates preconditioned by laser processing. In particular, the structuration achieved by ultrashort laser, with minimal thermal damage and debris reduction [34], shows ideal conditions for inducing controlled (smooth and reproducible) topographic changes. The so-called LaserInduced Periodic Surface Structures (LIPSS) [35], structures exhibiting regular periodic modulation close to or even smaller than the irradiation wavelength, has shown already promise for material deposition purposes, as for preferential growth of ZnO assemblies over amorphouscrystalline LIPSS on Si [36] or enhancement of metal nanostructure deposition over ablative LIPSS on Si [37]. To our knowledge, even if ultrafast laser processing directly applied on 2D materials has already been explored [38,39], the effect of 2D material growth on laserstructured surfaces has not been studied. Apart from the interest in the possibility of allowing growth in an out-of-plane configuration, there is also interest in exploring possible enhancement in TMD deposition and if this deposition follows the geometry of LIPSS (leading to anisotropic properties). Therefore, in this work, we carried out a systematic study on the influence of MoSe 2 nanosheet orientation on LIPSS. First, we produce LIPSS on three different substrates: sapphire (dielectric), silicon (semiconductor) and TiN thin-film (conductor). Next, MoO 2 is grown via ICSVD on LIPSS, followed by a selenization process that involves reacting Mo precursors with rapidly vaporized selenium. This two-step synthesis ensures the production of homogeneous samples across the substrate [10]. By means of scanning electron microscopy and Raman spectroscopy, the formation of MoSe₂ nanosheets and their orientation in both LIPSS and pristine regions are analysed. The observation of preferential out-of-plane flake formation on LIPSS, regardless of the substrate explored, highlights surface texturing by ultrafast laser processing as a promising approach for out-of-plane 2D material deposition. 2. Experimental methods 2.1. Femtosecond laser structuration of substrates The micro-structuring of dielectric (c-plane sapphire substrate), semiconductor (Silicon (100), and metallic thin-films (200-nm TiN over 40-nm Ti on silicon) used as substrates is conducted using a Satsuma HP2 Yb fiber laser from Amplitude. The laser operates at a wavelength of 1030 nm with a pulse duration of 350 fs. For all structuring conditions in this work, the repetition rate is set to 500 kHz. A schematic of the setup is presented in Fig. 1(a). The irradiation pulse energy (E) is controlled either by an acousto-optic modulator in the laser and/or by a motorized half-wave plate (HWP) in conjunction with a pair of thin-film polarizers (TFP). A second HWP allows adjusting the polarization orientation (horizontal or vertical) on the sample. The sample is irradiated by scanning the beam across the surface using a galvanometric mirror scanner (ScanCube 14) and focusing with an F-theta lens (FTL) with a focal length of 100 mm. The control of the beam spot size at the sample position can be achieved by either introducing a pinhole at the entrance of the scanner (for larger spots) or/and by introducing a telescope to expand the beam at the entrance of the scanner (for smaller spots). The Gaussian spot beam diameter (2w 0, at 1/e 2 ) for processing sapphire and TiN thin-films is 2w 0 =18.4 µm and for processing silicon is 2w 0 =39.0 µm, obtained by following the Liu’s method [40,41].The irradiation peak fluence (F 0 ) is calculated by using F 0 =2⋅E/ ( π ⋅w 0 2 ). The kind of transformation produced in this work are millimeter long vertical lines with micrometric lateral dimension (d line ), generated by partial spatial overlap of individual spots. As defined in [42] the effective number of pulses (N eff ) is calculated by: Neff =dline⋅RR vscan Being v scan the vertical scanning speed set by the scanner and RR the repetition rate of the laser (500 kHz). 2.2. Molybdenum oxide evaporation The graphite crucible used for MoO 2 growth comprises a lower section housing the MoO 3 powder source and an upper section serving as Fig. 1. (a) Sketch of the laser irradiation set-up. Dashed boxes indicate the removable elements under certain irradiation conditions. HWP: half wave plate. TFP: thin-film polarizers. PH: pinhole. FTL: F-Theta lens. (b) Schematic representation of the growth of MoSe 2 via an Isothermal Closed Space Vapor Deposition and selenization process. A. Fern´ andez García et al. Applied Surface Science 669 (2024) 160567 3 the substrate support. The substrate support features a square-shaped relief to stabilize the substrate during the growth process. The distance between the source and substrate was maintained at six millimetres, representing the typical isothermal geometry for vapor transport in close-space conditions. A scheme of the system is represented in Fig. 1 (b) left. Positioned within the temperature plateau region of a quartz reactor, the crucible is heated by a resistive tube furnace, ensuring uniform temperature distribution over the orifice’s graphite crucible, as validated in prior experiments [10,43,44]. Prior to the MoO 3 evaporation, the reactor underwent a 50-minute rinse with high-purity Ar gas to eliminate residual air. The reactor operates on a reduction gas mixture of H 2 :Ar in a 1:5 ratio, supplied at a rate of 25 ml/min and maintained at atmospheric pressure. This environment facilitates the reduction of MoO 3 to MoO 2 . Optimal growth conditions were achieved at a temperature of 600 ◦C for a duration of 90 min [10,45]. Following the growth process, the furnace was allowed to cool within the Ar environment. 2.3. MoSe 2 preparation: Selenization The selenization process was conducted using the same graphite crucible and quartz reactor employed for molybdenum evaporation. Within the system, the lower part of the crucible was filled with 50 mg of Se powder (99.99 % purity from Aldrich). A scheme of the system is represented in Fig. 1(b) left part. Due to the semi-closed nature of the system, Se vapors were confined within the small growth compartment, resulting in a concentration of vapor pressure close to the equilibrium vapor pressure at the annealing temperature. This concentration of Se vapor is expected to enhance the efficiency of selenization compared to an open tube process where Se vapor is more diluted in the carrier gas. Given these conditions, a lower temperature for selenization than the typical CVD process is anticipated, allowing annealing to be employed [46–48]. Selenization was conducted at 600 ◦C for 15 min in an atmosphere consisting of a gas mixture of H 2 :Ar in a 1:3 ratio. Hydrogen is a critical component for the growth of MoSe 2 ; without it, selenization does not occur [48,49]. 2.4. Characterization A morphological analysis was performed using a Field Emission Scanning Electron Microscope FEI VERIOS 4i. Raman spectra were obtained with a Renishaw Ramascope 2000 spectrometer and an Argon ion laser emitting at a wavelength of 514.5 nm. The laser power was maintained low to avoid sample damage. A metallographic optical microscope Olympus BH-2 was used to focus the laser light on the film and to collect the light scattered by the sample in backscattering geometry. The optical axis was perpendicular to the sample surface. The recording spectrum time was 500 s for each spectrum. 3. Results and discussion 3.1. Results on laser-structured sapphire (dielectric) Fig. 2(a–b) shows the surface of laser-structured sapphire under two similar irradiation conditions (F 0 =4.3 J/cm 2 , v scan =0.5 m/s, N eff ≈8), only differing on laser polarization direction. In the case of setting the laser polarization parallel to the laser scan direction, LIPSS in the direction perpendicular to the direction of the laser scan appears, showing a periodicity of Λ=0.96 µm. The value of the periodicity is close to the irradiation wavelength, corresponding therefore to the generation of low-spatial frequency LIPSS (LSFL). The obtained LSFL periodicity and orientation, perpendicular to the polarization, are consistent with previously reported data by Ashkenasi et al. [50], with the only difference of having a larger period due to the use of a 1030-nm laser instead of 800-nm one (Λ=0.76 µm in [50]). However, when crossing the polarization perpendicular to the laser scan direction, the periodic pattern does not propagate homogeneously along the sample, as observed previously (among other materials) on silicon [42]. Nevertheless, these two different morphologies provide access to two distinctly modified substrates of interest for the ulterior MoSe 2 growth. SEM images in Fig. 2(c–f) illustrate the growth of MoSe 2 nanosheets onto the sapphire substrate’s LIPSS and ablated regions. In contrast, Fig. 2(g) presents SEM images of MoSe 2 flakes deposited on a pristine sapphire substrate. A comparison between these images and those in (cf) enables the evaluation of the influence of surface structures on MoSe 2 growth and morphology. In both irradiated areas of the substrate, the flakes exhibit vertical growth, in contrast to the horizontal growth observed on the pristine sapphire. This phenomenon is attributed to the topography and the inherent change of crystallinity orientation induced by laser treatment, which conditions active nucleation sites, promoting the proliferation of out-of-plane flakes and inhibiting the growth of large flakes along the surface. The high density of these nucleation sites favours growth perpendicular to the substrate, akin to the mechanism observed in Chemical Vapor Deposition (CVD) by enhancing the reactivity of selenium to generate numerous nucleation sites [28]. Conversely, on pristine sapphire, the low density of nucleation sites promotes the formation of layers consisting of large-lying MoSe 2 flakes. A limited number of large-lying flakes are formed, reminiscent of typical Fig. 2. (a-b) SEM images of the LIPSS and ablated lines, respectively, induced by femtosecond laser on sapphire. The only change between the two kinds of modification is the laser polarization direction (F 0 =4.3 J/cm 2 , v scan =0.5 m/s, N eff ≈8). (c, e) SEM images of the MoSe 2 flakes growth onto LIPSS, being (e) a detail of the box indicated in (c). (d, f) SEM images of the MoSe 2 flakes grown onto the ablated region, being (f) a detail of the box indicated in (d).(g) SEM images of the same flakes onto a pristine region. A. Fern´ andez García et al. Applied Surface Science 669 (2024) 160567 4 CVD growth processes [47,48]. Additionally, surface-level modification of the crystalline structure of the sapphire substrate critically affects the acquisition of out-of-plane flakes. Several studies on the epitaxial growth of MoSe 2 on (0001) Al 2 O 3 substrates demonstrate that for the crystalline structure of sapphire (cut along this axis), the interfacial energetics and kinetics, and the intra-layers Van der Waals forces of the TMDs, induce the growth of highly oriented MoSe 2 flakes on the sapphire plane [51–54]. Therefore, tilt grains are energetically unfavourable, and the domains tend to align epitaxially [54]. Laser irradiations, besides inducing topography, also generate a superficial crystalline modification on sapphire. This breaks the crystalline symmetry and alters the surface interactions responsible for well-oriented growth. This change in growth conditions also favours the out-ofplane growth of MoSe 2 flakes. In both Fig. 2(c) and (d), it is noted that the largest vertical flakes are situated at the boundary (step) between the irradiated and unmodified zones, suggesting that the specific type of modification is not pivotal; rather, inducing topography or roughness is crucial. This observation is reiterated when comparing both types of modifications, wherein a greater abundance of vertical flakes is observed within the LIPSS line compared to the ablated region. This underscores the notion that a specific substrate topology is requisite for obtaining out-of-plane flakes. Fig. 2(f) presents flakes of nanometrescale dimensions interspersed among larger flake clusters. Within the ablated region, the laser induces significant roughness, which acts as additional nucleation sites, resulting in the emergence of nanoflakes. This further substantiates the proposition that roughness plays a crucial role in facilitating the growth of vertical flakes. The geometric shape of the flakes is directly related to their chemical composition. The presence of triangular and hexagonal flakes signifies the composition of MoSe 2 and MoOSe, respectively [31,47]. This combination is frequently encountered during the growth processes of MoSe 2 , either through Chemical Vapor Deposition (CVD) or MoO 2 selenization [47]. Fig. 3(a) presents Raman spectra analysis of the samples on LIPSS (blue), ablated line (red) and pristine (black) sapphire regions. The distinctive vibration modes of Mo-Se bonds characteristic of MoSe 2 , namely the out-of-plane A 1g and in-plane E 1 2g modes, at 244 and 288 cm −1 , respectively, are visible. A comparison of Raman signals across different regions reveals an augmentation in the peak corresponding to the A 1g out-of-plane mode. In Fig. 3(a) a detail of this peak is shown in the inset. These findings corroborate the vertical orientation of MoSe 2 flakes observed via SEM. Specifically, this enhancement is 1.8-fold for the ablated region and 3.5-fold for the LIPSS. The A 1g peak is stronger for scattering on edge-terminated structures in the flakes, while the E 1 2g peak is stronger for scattering perpendicular to the flakes. The relative intensity ratio of these two modes offers insights into the density of active sites [28,55]. In this case, it is observed a significantly lower intensity of the E 1 2g peak compared to the A 1g peak, with respective values of 13%, 18%, and 20% for flakes grown on LIPSS, the ablated line, and pristine sapphire, respectively. This suggests the presence of a substantial number of active edge sites in MoSe 2 flakes, with substrate structuring further augmenting their abundance, thus enhancing material performance. In addition to these Raman characteristics, an additional peak at approximately 355 cm −1 , corresponding to MoSe 2 interlayer interaction, is observed [28,56]. This peak (assigned as the B 2g vibrational mode) is inactive in a single monolayer and in bulk MoSe 2 but becomes active for a few layers due to translation symmetry breaking [57,58]. Additional secondary peaks observed include the E 1g mode of MoSe 2 at 169 cm −1 and the longitudinal acoustic phonon mode LA(M) centred around 146–150 cm −1 [59,60]. Furthermore, bands corresponding to various molybdenum oxides are evident, notably Mo 4 O 11 at 309 cm −1 and 439 cm −1 and MoO 2 at 583 cm −1 . This observation aligns with the SEM findings indicating the presence of two distinct types of flakes: MoSe 2 exhibiting triangular morphology and MoOSe displaying hexagonal morphology [47]. In Fig. 3(b), it is represented the ratio between the peak intensities on irradiated regions and pristine regions. In both, the LIPSS and ablated region, it is noteworthy that the entirety of the spectrum (baseline) experiences an increase, attributed to a relative increase in the deposited material. Of importance, it is the larger augmentation observed in the A 1g mode that is particularly remarkable, which cannot be only associated with a greater material thickness but with the highest density of vertical flakes, being the most pronounced increase in the LIPSS region. Finally, a last piece of evidence of the dominance of vertical alignment, is that in-plane vibrational mode, E 1 2g , increased factor in both irradiated regions, is less intense than the baseline. In simpler terms, there is a greater quantity of material but a lower percentage of in-plane orientation. 3.2. Results on laser-structured silicon (semiconductor) Fig. 4(a) shows the surface of laser-structured silicon (F 0 =0.35 J/ cm 2 , v scan =1 m/s, N eff ≈8), forming LIPSS with a periodicity of Λ= 0.90 µm (extracted from magnified images). Overlaying the SEM image is an optical microscope image captured under monochromatic illumination (LED of 460 nm). In this image, a dark central region corresponding to an ablative process (resulting in debris ejection observed near the lines) can be observed, along with a brighter region at the Fig. 3. (a) Raman spectra of MoSe 2 grown on LIPSS (blue), ablated (red) and pristine sapphire regions (black). Insight: detail of enhancement of the A 1g mode. (b) Graphical comparison of the Raman signal increase of different peaks between modified (the ablated and LIPSS regions) and pristine regions. A. Fern´ andez García et al. Applied Surface Science 669 (2024) 160567 5 borders of the lines, indicative of amorphous silicon formation [42]. The observation of this amorphization region is an indicator of a processing condition that enables the induction of smooth topographic changes. Fig. 4(b–e) illustrates the material grown on silicon, in LIPSS and on pristine regions. Fig. 4(b) already illustrates the disparity between LIPSS region and the pristine surrounding area, confirmed upon increasing magnification in Fig. 4(c) and (e). It’s worth noting that, in slight contrast to the sapphire substrate findings, there’s a possibility of observing some out-of-plane flakes in the pristine region. We attribute this effect, at least partially, to the presence of ablation debris on the pristine region. Nevertheless, the growth of the flakes in an out-of-plane manner is dominant in the LIPSS region, as exemplified in Fig. 4(d). Moreover, beautiful desert rose-like structures are formed (Fig. 4(e)), being attributed to the anisotropic structure of TMDs. The rapid diffusion of selenium vapours along the van der Waals gaps, compared to diffusion across the layers, facilitates vertical orientation for nanoflowers [13,28]. The vertically aligned discrete MoSe 2 nanosheets aggregate to minimize surface energy, resulting in the formation of nanoflower morphology [28]. In Fig. 4(f), Raman spectra are presented for the region with LIPSS and the pristine silicon region. It can be observed that the change in substrate, from sapphire to silicon, does not significantly affect the results, with similar vibration modes and intensities obtained. Furthermore, the effects associated with the growth of out-of-plane flakes in the LIPSS region are reproduced. An increase in signal is attributed to the relative thickness increase and an extraordinary enhancement of the A 1g vibration mode. However, for LIPSS, the increase is only by a factor of 1.3; half of what was obtained on sapphire. A redshift of the entire spectrum is observed. It has been established that pressure or mechanical stress induces a redshift of vibrational modes towards higher energies [56,59]. Both the interaction of MoSe 2 flake edges with silicon and the microstructures resembling desert roses may generate mechanical stress, thus explaining this redshift. 3.3. Results on laser-structured TiN thin-film (conductor) Fig. 4(a–b) show the surface of laser-structured on TiN thin-film (F 0 =0.11 J/cm 2 , v scan =0.5 m/s, N eff ≈4), forming LIPSS with a periodicity of Λ=0.07 ±0.02 µm. This periodicity, as observed in the magnified image in Fig. 4(b), is much smaller than the values fabricated in sapphire and silicon. This periodicity is consistent with the values reported by Bonse et al. [61] in titanium (Λ =65–95 nm, under 800 nm radiation), corresponding to high-spatial frequency LIPSS (HSFL). However, we find that the orientation of the HSFL is perpendicular to the laser polarization. In a very recent report by Liu et al. on 100-nm thick TiN [62], the origin of this HSFL orientation is associated after FDTD-based numerical simulation to the strong metallic properties of TiN and the oxidative nature of the HSFL (Λ =90 nm, under 1030 nm radiation). As for sapphire and silicon, a clear difference is observed (Fig. 5 (c–e)) between the LIPSS and pristine region. A comparison between both regions reveals a quantity increase and a more homogeneous size distribution of vertical flakes in the LIPSS region. Conversely, in the pristine zone, where vertical flakes are present probably due to the columnar polycrystalline nature of the thin film, flakes vary in size (from 100 ′ s of nm to few 10 ′ s of nm). The small flake size is most likely due to the low mobility of the Mo atoms during growth because of the intrinsic conductivity of the substrate [51,53]. The topography characterized by “hills and valleys” in the irradiated area promotes the growth of larger and more homogenous flakes [32]. Notably, the geometry and order of the flakes are consistent in both regions. Fig. 5(f) illustrates the Raman spectra of various zones corresponding to the TiN substrate. Similar vibration modes with comparable intensities to those observed on the other two substrates have been identified. Additionally, a 1.3-fold increase in the A 1g out-of-plane vibration mode is noted However, a notable difference lies in the absence of an increase in the Raman signal associated with the relative thickness augmentation compared to silicon and sapphire substrates, consistent with observations from SEM. With a rough TiN substrate, vertical flakes are inherently obtained without additional laser structuring. Laser structuring serves to enhance the density of these flakes; thus, while the signal of the out-of-plane vibration mode increases, the overall Raman signal remains unaffected. 4. Conclusions This work presents a study on the growth of MoSe 2 nanosheets on various laser-structured substrates, with a focus on understanding and exploiting the technological possibilities to control their morphology and orientation. A general conclusion drawn is that regardless of the substrate’s nature (conductive or dielectric, monocrystalline or polycristalline) and the type of smooth topographic modification induced (such as LSFL, HSFL, or ablation), laser treatments promote the emergence of preferential nucleation sites. Nonetheless, larger differences in in-plane orientation on pristine material and out-of-plane orientation on laser-modified regions are observed on monocrystalline samples such as silicon and, most notably, sapphire. In this case, a 3.5-fold increase in the Raman intensity signal of the out-of-plane A 1g vibration mode is observed in the LIPSS region. This increase is partially attributed to the Fig. 4. (a) Optical image (top) and SEM images (bottom) of the LIPSS induced by femtosecond laser on silicon (F 0 =0.35 J/cm 2 , v scan =1 m/s, N eff ≈8). (b-e) SEM images of the MoSe 2 flakes grown onto these LIPSS. (f) Raman spectra of MoSe 2 grown on silicon LIPSS (blue) and on pristine regions (black). A. Fern´ andez García et al. Applied Surface Science 669 (2024) 160567 6 deposition of more material (a 2.2-fold increase), but crucially, it is mostly attributed to the preferential out-of-plane orientation. Therefore, we propose a pioneering multiple-step synthesis approach designed to yield dense and finely controllable out-of-plane MoSe 2 structures, and potentially to any other Van der Waals 2D material, which can become crucial for technological topics such as catalysis and electronic devices. CRediT authorship contribution statement A. Fern´ andez García: Writing – original draft, Methodology, Investigation, Formal analysis. R. Ariza: Writing – review & editing, Investigation. J. Solis: Writing – review & editing, Investigation, Funding acquisition. F. Agull´ o-Rueda: Writing – review & editing, Investigation. M. Manso Silvan: Writing – review & editing, Investigation, Funding acquisition, Conceptualization. M. Garcia-Lechuga: Writing – review & editing, Supervision, Investigation, Conceptualization. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Mario Garcia Lechuga reports financial support, administrative support, and article publishing charges were provided by Consejo Superior de Investigaciones Científicas. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper]. Data availability Data will be made available on request. Acknowledgments The current research was funded through grants PID2020112770RB-C21 and PID2020-112770RB-C22 funded by MCIN/AEI/ 10.13039/501100011033. We acknowledge the service from the MiNa Laboratory at IMN, and funding from CM (project S2018/NMT-4291 TEC2SPACE), MINECO (project CSIC13-4E-1794) and EU (FEDER, FSE). The initial part of the research leading to these results has received funding from LASERLAB-EUROPE (grant agreement no. 871124, European Union’s Horizon 2020 research and innovation programme. Grant project CNRS-LP3002707). References [1] T. Kuila, S. Bose, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Chemical functionalization of graphene and its applications, Prog. Mater Sci. 57 (2012) 1061–1105. [2] B. Radisavljevic, A. 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