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LEONARDO MARTINI

Ricercatore t.d. art. 24 c. 3 lett. A
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2023 - Thermoelectric and Structural Properties of Sputtered AZO Thin Films with Varying Al Doping Ratios [Articolo su rivista]
Isram, Muhammad; MAGRIN MAFFEI, Riccardo; Demontis, Valeria; Martini, Leonardo; Forti, Stiven; Coletti, Camilla; Bellani, Vittorio; Mescola, Andrea; Paolicelli, Guido; Rota, Alberto; Benedetti, Stefania; DI BONA, Alessandro; Ribeiro, Joana M.; Tavares, C. J.; Rossella, Francesco
abstract

2022 - Ultra-clean high-mobility graphene on technologically relevant substrates [Articolo su rivista]
Tyagi, Ayush; Mišeikis, Vaidotas; Martini, Leonardo; Forti, Stiven; Mishra, Neeraj; Gebeyehu, Zewdu M; Giambra, Marco A; Zribi, Jihene; Frégnaux, Mathieu; Aureau, Damien; Romagnoli, Marco; Beltram, Fabio; Coletti, Camilla
abstract

Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 cm(2) V-1 s(-1) at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO2/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility mu(h) up to similar to 9000 cm(2) V-1 s(-1) and electron mobility mu(e) up to similar to 8000 cm(2) V-1 s(-1), with average values mu(h) similar to 7500 cm(2) V-1 s(-1) and mu(e) similar to 6300 cm(2) V-1 s(-1). The carrier mobility of ultraclean graphene reaches values nearly double than those measured in graphene processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics.

2021 - Synthesis of Large-Scale Monolayer 1T′-MoTe2and Its Stabilization via Scalable hBN Encapsulation [Articolo su rivista]
Pace, S.; Martini, L.; Convertino, D.; Keum, D. H.; Forti, S.; Pezzini, S.; Fabbri, F.; Miseikis, V.; Coletti, C.
abstract

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T′ possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. Indeed, 1T′-MoTe2 is a semimetal with superconductivity, which has been predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. Large instability of monolayer 1T′-MoTe2 in environmental conditions, however, has made its investigation extremely challenging so far. In this work, we demonstrate homogeneous growth of large single-crystal (up to 500 μm) monolayer 1T′-MoTe2 via chemical vapor deposition (CVD) and its stabilization in air with a scalable encapsulation approach. The encapsulant is obtained by electrochemically delaminating CVD hexagonal boron nitride (hBN) from copper foil, and it is applied on the freshly grown 1T′-MoTe2 via a top-down dry lamination step. The structural and electrical properties of encapsulated 1T′-MoTe2 have been monitored over several months to assess the degree of degradation of the material. We find that when encapsulated with hBN, the lifetime of monolayer 1T′-MoTe2 successfully increases from a few minutes to more than a month. Furthermore, the encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties. The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T′-MoTe2 by both Raman spectroscopy and electrical measurements. The growth and encapsulation methods reported in this work can be employed for further fundamental studies of this enticing material as well as facilitate the technological development of monolayer 1T′-MoTe2.

2020 - Deterministic direct growth of WS2 on CVD graphene arrays [Articolo su rivista]
Piccinini, G.; Forti, S.; Martini, L.; Pezzini, S.; Miseikis, V.; Starke, U.; Fabbri, F.; Coletti, C.
abstract

The combination of the exciting properties of graphene with those of monolayer tungsten disulfide (WS2) makes this heterostack of great interest for electronic, optoelectronic and spintronic applications. The scalable synthesis of graphene/WS2 heterostructures on technologically attractive substrates like SiO2 would greatly facilitate the implementation of novel two-dimensional (2D) devices. In this work, we report the direct growth of monolayer WS2 via chemical vapor deposition (CVD) on single-crystal graphene arrays on SiO2. Remarkably, spectroscopic and microscopic characterization reveals that WS2 grows only on top of the graphene crystals so that the vertical heterostack is selectively obtained in a bottom-up fashion. Spectroscopic characterization indicates that, after WS2 synthesis, graphene undergoes compressive strain and hole doping. Tailored experiments show that such hole doping is caused by the modification of the SiO2 stoichiometry at the graphene/SiO2 interface during the WS2 growth. Electrical transport measurements reveal that the heterostructure behaves like an electron-blocking layer at large positive gate voltage, which makes it a suitable candidate for the development of unipolar optoelectronic components.

2020 - Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides [Articolo su rivista]
Miseikis, V.; Marconi, S.; Giambra, M. A.; Montanaro, A.; Martini, L.; Fabbri, F.; Pezzini, S.; Piccinini, G.; Forti, S.; Terres, B.; Goykhman, I.; Hamidouche, L.; Legagneux, P.; Sorianello, V.; Ferrari, A. C.; Koppens, F. H. L.; Romagnoli, M.; Coletti, C.
abstract

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene/polymer/graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity ∼8 × 1010 cm-2 at the charge neutrality point, and a large Seebeck coefficient ∼140 μV K-1, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices.

2019 - Structure-dependent electrical properties of graphene nanoribbon devices with graphene electrodes [Articolo su rivista]
Martini, L.; Chen, Z.; Mishra, N.; Barin, G. B.; Fantuzzi, P.; Ruffieux, P.; Fasel, R.; Feng, X.; Narita, A.; Coletti, C.; Mullen, K.; Candini, A.
abstract

Graphene nanoribbons (GNRs) are a novel and intriguing class of materials in the field of nanoelectronics, since their properties, solely defined by their width and edge type, are controllable with high precision directly from synthesis. Here we study the correlation between the GNR structure and the corresponding device electrical properties. We investigated a series of field effect devices consisting of a film of armchair GNRs with different structures (namely width and/or length) as the transistor channel, contacted with narrowly spaced graphene sheets as the source-drain electrodes. By analyzing several tens of junctions for each individual GNR type, we observe that the values of the output current display a width-dependent behavior, indicating electronic bandgaps in good agreement with the predicted theoretical values. These results provide insights into the link between the ribbon structure and the device properties, which are fundamental for the development of GNR-based electronics.

2019 - Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene [Articolo su rivista]
Mishra, N.; Forti, S.; Fabbri, F.; Martini, L.; Mcaleese, C.; Conran, B. R.; Whelan, P. R.; Shivayogimath, A.; Jessen, B. S.; Buss, L.; Falta, J.; Aliaj, I.; Roddaro, S.; Flege, J. I.; Boggild, P.; Teo, K. B. K.; Coletti, C.
abstract

The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction (√31 × √31)R±9° of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm2 V−1 s−1. The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.

2017 - Fabrication and characterization of nanometer-sized gaps in suspended few-layer graphene devices [Articolo su rivista]
Lumetti, S.; Martini, L.; Candini, A.
abstract

Graphene nanodevices, such as ultra-narrow constrictions and nanometer-spaced gaps, are emerging as appealing candidates for various applications, ranging from advanced quantum devices to single-molecule junctions and even DNA sequencing. Here, we present the realization and characterization of nanometer-sized gaps in suspended few-layer graphene devices via feedback-controlled electroburning at room temperature. By analyzing the electrical behavior after the electroburning process, we identify two distinct regimes for the resulting devices, deriving a simple yet effective quantitative criterion to determine the complete opening of the nanogaps.

2017 - High Photoresponsivity in Graphene Nanoribbon Field Effect Transistor Devices Contacted With Graphene Electrodes [Articolo su rivista]
Candini, Andrea; Martini, Leonardo; Chen, Zongping; Mishra, Neeraj; Convertino, Domenica; Coletti, Camilla; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus; Affronte, Marco
abstract

Ultra-narrow graphene nanoribbons (GNRs) with atomically precise structures are considered a promising class of materials for the realization of optoelectronic and photonic devices with improved functionalities. Here we report the opto-electronic characterization of a field effect transistor devices made of a layer of bottom-up synthesized GNRs contacted with multilayer graphene electrodes, showing high photoresponsivity of 5 × 105 A/W for small incident power in the visible-UV range. Our results show that combining the properties of intrinsic graphene with that of semiconducting GNRs is a viable route to realize novel devices for optoelectronic and sensing applications.

2016 - Experimental and theoretical analysis of Landauer erasure in nano-magnetic switches of different sizes [Articolo su rivista]
Martini, L.; Pancaldi, M.; Madami, M.; Vavassori, P.; Gubbiotti, G.; Tacchi, S.; Hartmann, F.; Emmerling, M.; Hofling, S.; Worschech, L.; Carlotti, G.
abstract

Bistable nano-magnetic switches are extensively used in storage media and magnetic memories, associating each logic state to a different equilibrium orientation of the magnetization. Here we consider the issue of the minimum energy required to change the information content of nano-magnetic switches, a crucial topic to face fundamental challenges of current technology, such as power dissipation and limits of scaling. The energy dissipated during a reset operation, also known as "Landauer erasure", has been accurately measured at room temperature by vectorial magneto-optical measurements in arrays of elongated Permalloy nanodots. Both elliptical and rectangular dots were analysed, with lateral sizes ranging from several hundreds to a few tens of nanometers and thickness of either 10nm or 5nm. The experimental results show a nearly linear decrease of the dissipated energy with the dot volume, ranging from three to one orders of magnitude above the theoretical Landauer limit of kBT×ln(2). These experimental findings are corroborated by micromagnetic simulations showing that the significant deviations from the ideal macrospin behavior are caused by both inhomogeneous magnetization distribution and edge effects, leading to an average produced heat which is appreciably larger than that expected for ideal nanoswitches.

2016 - Fabrication of three terminal devices by ElectroSpray deposition of graphene nanoribbons [Articolo su rivista]
Fantuzzi, Paolo; Martini, Leonardo; Candini, A.; Corradini, V.; Del Pennino, Umberto; Hu, Y.; Feng, X.; Müllen, K.; Narita, A.; Affronte, Marco
abstract

Electrospray deposition (ESD) in ambient conditions has been used to deposit graphene nanoribbons (GNRs) dispersed in liquid phase on different types of substrates, including ones suitable for electrical transport. The deposition process was controlled and optimized by using Raman spectroscopy, Scanning Probe Microscopies and Scanning Electron Microscopy. When deposited on graphitic electrodes, GNRs were used as semi-conducting channel in three terminal devices showing gate tunability of the electrical current. These results suggest that ESD technique can be used as an effective tool to deposit chemically synthesized GNRs onto substrates of interest for technological applications.

2016 - Synthesis of Graphene Nanoribbons by Ambient-Pressure Chemical Vapor Deposition and Device Integration [Articolo su rivista]
Chen, Zongping; Zhang, Wen; Palma, Carlos Andres; LODI RIZZINI, Alberto; Liu, Bilu; Abbas, Ahmad; Richter, Nils; Martini, Leonardo; Wang, Xiao Ye; Cavani, Nicola; Lu, Hao; Mishra, Neeraj; Coletti, Camilla; Berger, Reinhard; Klappenberger, Florian; Kläui, Mathias; Candini, Andrea; Affronte, Marco; Zhou, Chongwu; DE RENZI, Valentina; DEL PENNINO, Umberto; Barth, Johannes V.; Räder, Hans Joachim; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus
abstract

Graphene nanoribbons (GNRs), quasi-one-dimensional graphene strips, have shown great potential for nanoscale electronics, optoelectronics, and photonics. Atomically precise 47 8 GNRs can be "bottom-up" synthesized by surface-assisted assembly of molecular building blocks under ultra-high-vacuum conditions. However, large-scale and efficient synthesis of such GNRs at low cost remains a significant challenge. Here we report an efficient "bottom-up" chemical vapor deposition (CVD) process for inexpensive and high-throughput growth of structurally defined GNRs with varying structures under ambient-pressure conditions. The high quality of our CVD-grown GNRs is validated by a combination of different spectroscopic and microscopic characterizations. Facile, large-area transfer of GNRs onto insulating substrates and subsequent device fabrication demonstrate their promising potential as semiconducting materials, exhibiting high current on/off ratios up to 6000 in field-effect transistor devices. This value is 3 orders of magnitude higher than values reported so far for other thin-film transistors of structurally defined GNRs. Notably, on-surface mass spectrometry analyses of polymer precursors provide unprecedented evidence for the chemical structures of the resulting GNRs, especially the heteroatom doping and heterojunctions. These results pave the way toward the scalable and controllable growth of GNRs for future applications.