Conference Agenda

Active implantable medical devices have experienced enormous

development over the past years. However, one limiting factor remains a

challenge, its reliance on external batteries. Furthermore, in the case of

neural interfaces, the use of bulky batteries limits the advance of our

understanding of the nervous system, since the presence, on the

head/body of a moving animal under study, of a bulky head-stage

originates the large variability found in the existing results on this regard. In

an ideal scenario, medical implants should be powered using long-lasting

power suppliers that pose no mechanical or chemical danger to the body.

We work towards the development of an energy autonomous neural

stimulation technology by exploring the coupling of triboelectric

nanogenerators and our neural graphene-based microelectronics.

Abstract

The study of two-dimensional (2D) materials, particularly molybdenum disulfide (MoS2), is crucial for future electronic and optoelectronic devices. Single-layer MoS2 (1L-MoS2) exhibits a 1.8 eV bandgap, making it ideal for such applications. However, achieving large-scale synthesis with consistent properties remains a challenge. The synthesis method and substrate choice significantly affect the material’s electronic and optical properties, influencing strain and doping. To address this, controlled post-synthesis treatments are explored. Previous studies showed that 1L-MoS2 thermal treatments on gold substrates can effectively tune strain-doping properties and fluorescence without altering morphology, though the effects depend on synthesis routes. This work systematically compares 1L-MoS2 samples obtained via Chemical Vapor Deposition (CVD), deposited on an insulating substrate of SiO2, and Gold-Assisted Exfoliation (GAE). Thermal treatments in O2 and Ar atmospheres were applied to refine strain, doping, and fluorescence, showing potential for defect remediation, doping control, and luminescence enhancement.

Introduction

The study of two-dimensional (2D) materials has a huge interest and influence on the future electronic and optoelectronic devices based on nanostructures[1]. One of the most promising and studied 2D structures is molybdenum disulfide (MoS2), due to its interesting electronic and optical properties[2, 3]. Single layer MoS2 (1L-MoS2) presents semiconductive properties with a bandgap of 1.8 eV from which a fluorescence emission is obtained by direct exciton recombination[4]. These features make 1L-MoS2 an ideal candidate as material for optoelectronic devices. However, to reach this step an industrial-scale synthesis with homogeneity in structural and physical properties is required but not yet achieved. Several methods of synthesis exist to produce this kind of material and each of them has a different impact on the electronic and optical MoS2 features[5].

Furthermore, the nature and the superficial features of the substrate on which 1L-MoS2 is placed influence significantly the initial strain and doping of the nanostructure, highlighting that the choice of the substrate is crucial according to the application or the heterostructure needed[6]. For this reason, finding a controlled procedure after synthesis to remediate and tune the MoS2 properties is nowadays a challenge.

As shown in our previous work[7], thermal treatments of 1L-MoS2 on gold substrates are promising for tailoring the strain-doping properties and properly tuning the optical features. The best condition found underlines that there is a critical temperature over which the physical properties have been changed without ruining the morphology of the sample. However, all these advantages cannot be extendible automatically for each kind of sample independently of the route of synthesis.

In this work, we present a systematic study of 1L-MoS2 obtained by CVD on SiO2 comparing the results with the samples made by GAE[6] in our previous work, exploring the physical differences raised from the different procedures and substrates. We applied thermal treatments in a controlled atmosphere (O2 and Ar) to fine-tune strain, doping and fluorescence properties after synthesis. These treatments seem to be promising in remediating defects occurring naturally, controlling the doping type and enhancing the luminescence of 1L-MoS2.

Experimental Details

Several flakes of 1L-MoS2 have been prepared by CVD using liquid molybdenum precursors, at a temperature of 820 °C, under an ambient pressure, nitrogen atmosphere and grown on a SiO2/Si substrate (silicon with native silicon oxide on top)[8]. All the samples were treated in a Linkam cell from the temperature of 100 °C up to 300 °C, for 2 hours with 2 atm of O2 or Ar atmosphere.

We performed non-destructive techniques such as Raman spectroscopy and steady-state micro-photoluminescence using a Horiba LabRaman HR-Evolution with a 532 nm laser source, to study initial strain and doping conditions and the effects of the thermal treatments. A Bruker FastScan Dimension Atomic Force Microscope (AFM) was employed for observing the structure and morphology of the flakes in tapping mode.

Results and discussion

The E12g (in-plane) and A1g (out-of-plane) vibrational modes of MoS2 are detectable through Raman spectroscopy. As known in the literature, the frequency of vibration of these two Raman modes is connected to the strain and doping of the MoS2 flakes[9]. Figure 1 shows a strain-doping map of two different samples, the first one thermally treated in Ar (Figure 1a) and the second one in O2 (Figure 1b), studying the effect as a function of the temperature.

In general, both pristine samples characterized at room temperature (20°C) show a compressive strain condition and an initial n-type doping. Thermal treatments in Ar (figure 1a) at 100°C do not affect the flake. The first significant change appears at 150°C and 200°C, in which is possible to observe an increase of n-type doping and a small reduction of the compressive strain. Over these two temperatures, the 1L-MoS2 maintains the same doping of the pristine sample, but with a smaller compressive strain than untreated flake. A similar behaviour has been observed in the sample treated in O2. Increasing the temperature above 200°C there is a significant and systematic decrease of the n-doping. Strain variations are the same in both experiments, as this is probably only affected by temperature. Otherwise, except at the lowest temperatures, variations in doping seem to be accelerated by the presence of an active atmosphere like O2.

Additional effects of thermal treatments in the optical properties have been recorded through the acquisition of micro-photoluminescence steady-state spectra (data not shown), in which modifications in intensity and band position have been recorded. In detail, a significant variation of the photoluminescence has been observed treating the flake in the presence of O2. By contrast, under an Ar atmosphere, no significant enhancement of the intensity of luminescence is observed. Instead, in the O2 environment an increase from 2 up to 7 times the intensity has been measured, notably improving the luminescence of the 1L-MoS2. AFM and optical images (data not shown) have been used to observe the sample morphology before and after the thermal treatments.

We have found that temperatures higher than 225°C showed some variations in the structure of the flake. These findings allowed us to set the best conditions of treatment at 225°C with 2 atm, finding a controlled protocol to tailor the strain-doping properties (Ar, or O2 treatment) and/or the luminescence properties (only O2 treatment).

Conclusions

In conclusion, we have performed a systematic study on single-layer Molybdenum Disulfide prepared by CVD on an insulating SiO2 substrate, in which a controlled process aimed at modifying the strain, doping and luminescence properties has been optimized. This study can open a path towards the improvement of the features of two-dimensional materials regardless of the synthesis route chosen.

Acknowledgements

This research was funded by the Italian MUR PNRR project SAMOTHRACE (ECS00000022) and Funded by the European Union - Next Generation EU, Mission 4 Component 1 CUP B53D23004460006 (Finanziato dall’Unione europea- Next Generation EU, Missione 4 Componente 1 CUP B53D23004460006).

References

[1] Huang, X. et al., NPJ 2D Mater Appl, vol. 6, no. 1, 2022.

[2] Serron, J. et al., ACS Appl Mater Interfaces, vol. 15, no. 21, pp. 26175–26189, May 2023.

[3] Scheuschner, N. et al., Phys Rev B Condens Matter Mater Phys, vol. 89, no. 12, p. 125406, Mar. 2014.

[4] Splendiani, A. et al., Nano Lett, vol. 10, no. 4, 2010.

[5] Samy, O. et al., Crystals 11, no. 4: 355. 2021.

[6] Panasci S.E. et al., Appl Phys Lett, vol. 119, no. 9, Aug. 2021.

[7] Sangiorgi, E. et al., Nanomaterials, 15, 160, 2025.

[8] Esposito, F. et al., Appl Surf Sci, vol. 639, 2023.

[9] Michail, A. et al., Appl. Phys. Lett., 108, 173102, 2016.

This paper reports on the large area growth of ultrathin monolayer (1L) MoS2 onto homoepitaxial n--GaN on n+-GaN substrates by sulfurization of a pre-deposited MoOx film. A highly uniform and conformal MoS2 coverage and a nearly ideal van der Waals interface between MoS2 and Ga terminated GaN crystal was demonstrated by the combination of several high-resolution analyses. Finally, the vertical current-voltage characteristics showed a highly rectifying behaviour with an average turn-on voltage Von=1.7 V under forward bias, indicating the formation of p+/n heterojunction diode at 1L-MoS2/GaN interface.

In this paper, the growth of uniform few layers MoS2 on single crystalline diamond substrates has been obtained by sulfurization of a pre-deposited ultra-thin Mo film at an optimal temperature of 750°C. The MoS2 thickness uniformity, interface structural, and electrical behaviour of the MoS2/diamond heterostructures has been investigated by the combination of TEM, micro-Raman, AFM and several electrical scanning probe techniques (C-AFM, KPFM). The obtained results indicate the formation of a n-MoS2/p-diamond heterostructure.

In this work, we report on the fabrication of an ultrathin superabsorber in the visible spectral range based on transition metal dichalcogenide (TMDC) nanostructures. We designed and patterned a periodic nanohole arrays on MoSe₂ thin films deposited on metallic foils. Our results show broadband absorption above 90% over the visible range, attributed to multiple optical resonances such as diffraction, guided modes, Fabry–Pérot interference, and plasmonic excitations. The overall approach demonstrates a simple, scalable method to produce efficient light-trapping photonic structures suitable for optoelectronic devices. As a proof of concept, we employed a fast, clean-room-free laser ablation.

The exceptional properties of graphene promised a candidate to overcome the fundamental limitations of silicon technologies in photonics and optoelectronics. Especially when data transferring is at stake, novel graphene devices have been reported. These hybrid structures exploit the integration of graphene on a CMOS compatible process. In this work, the simulation of a mode filter based on SiN photonics is presented together with the developments carried out to achieve yield and control of the fabrication process. Finally, we present the characterization of the power transmitted in the TE0 mode through the graphene-on-silicon nitride (SiN) integrated waveguides. Our devices can potentially boost the development of efficient Mode Division Multiplexing systems for ultra-fast on-chip interconnections.