Research | Material science, Superconductivity & Nanotechnologies

Graphene and 2D Materials Research Laboratory

Since the discovery of graphene in 2004 via mechanical exfoliation, 2D materials have attracted much attention in the scientific community. The research activity is focused on constantly pushing forward the knowledge on nanomaterials for nano-electronic devices. The activities are focused on the electrical and optoelectrical properties of graphene, carbon nanotubes (CNTs), black phosphorus, GeAs, and two-dimensional transition metal dichalcogenides (TMDs) such as MoS 2, ReS 2, WSe 2, WS 2, PdSe 2, PtSe 2, ReSe 2, etc. Due to their atomically thin structure, 2D materials enable device architectures and applications beyond conventional 3D materials. Examples are ultra-scaled atomically thin channels for field-effect transistors (FETs) and tunneling FETs, van der Waals heterostructures, vertical stacked hybrid heterostructures, etc. In this regard, the role of materials, device architecture, contacts, and environmental conditions on the fabricated devices are being investigated. Further, research in optoelectronic measurements and sensors deals with several aspects related to methodology, experimental characterization, the proposal of novel designs and cost-effective fabrication of chip devices, the development of innovative optical sensors, photodetectors, etc. Our research group members are experts in the synthesis, fabrication, characterization of nanowires, nanotubes, graphene, and 2D materials and their applications in new electronic devices and sensors. The fundamental electrical properties and the potential applications of devices based on two-dimensional material are systematically investigated. Our fabricated devices are investigated in different conditions such as temperature from 77 K to 450 K, air pressure from atmospheric to 10 -6 mbar, controlled gas environment, as well as under irradiation by electron or ion beam. Other areas of major interest to the laboratory are the investigation of low dimensional materials for applications in field emission, non-volatile memories, solar cells, and sensors.

Responsible: prof. A. Di Bartolomeo, E-Mail: adibartolomeo@unisa.it

Members of the research group:

• Dr. F. Giubileo (CNR-SPIN), E-Mail: filippo.giuboile@unisa.it
• Dr. N. Martucciello (CNR-SPIN), E-Mail: nadia.martucciello@unisa.it
• Dr. A. Kumar, E-Mail: akumar@unisa.it
• Dr. O. Durante, E-Mail: odurante@unisa.it

LAMBDA Laboratory

Among the experimental research activities in Condensed Matter Physics, the study of the magnetic properties of different types of materials, in presence of both DC and AC magnetic fields, is faced at the “LAMBDA” (“Laboratory for Analysis of Materials Behaviour in Dc and Ac fields”) laboratory. The materials are studied in a temperature range between 3 K and 400 K, with DC fields up to 9 Tesla, and AC fields in a frequency range between about 1 Hz and 10 kHz. In particular, the superconducting materials are analyzed, focusing the attention on the dynamics of the magnetic flux quanta that are generated in the materials in presence of an external magnetic field, and on the interaction between the flux quanta and the impurities and the lattice defects in the materials. This analysis is devoted not only to acquiring the knowledge related to the basic physical properties of these systems but also to obtaining information about the fabrication parameters to act upon, in order to improve their electric transport properties and their potential application wherever the use of high electric currents is required (just as examples, in the production and technical management of the electric energy, in the carriage of passengers and/or goods, in the medical diagnosis, in the research,…). Moreover, the magnetic properties of different types of magnetic materials are studied, in form of thin films for their application in optoelectronic devices, and of magnetic nanoparticles with various coatings for the functionalization and the protection of their surface, addressed to be applied in the areas of biomedicine, environment and energy storage.

Responsible: prof. M. Polichetti, E-Mail: mpolichetti@unisa.it

Field Effect Transistors (FETs) based on 2D materials

Graphene and 2D materials are being extensively investigated as a reliable alternative to Si for the channel of field effect transistors (FETs) at the nanometric scale. The ultrathin channel enables devices immune from short channel effects; the high carrier mobility of graphene and other 2D materials can result in a high operational speed. Thus, the need to improve the performances of FETs has made graphene and some specific 2D materials excellent candidates as a channel material in nano-electronic applications. We investigate the behavior of the fabricated devices focusing on the role of their intrinsic and extrinsic defects as well as on the interaction with metal contacts. Further, the transport properties of the device are explored over a wide range of temperatures, pressures, and under electron irradiation.

Photodetectors

The research group also deals with the development and characterization of a new generation of photodetectors with a wide spectral response that are compatible with the current microfabrication processes, based on heterojunctions between 2D materials such as graphene, TMDCs, and silicon. Over the past several years, the efforts were focused on the studies of physics and performance improvement of photodetectors based on 2D materials. For instance, recently, the group reported that 2D-platinum diselenide (PtSe 2 ) field-effect transistors exhibit p-type electrical conductivity and, upon exposure to a light source, reveal the coexistence of positive and negative photoconductivity. The dominance of one type of photoconductivity over the other is controlled by environmental pressure. Indeed, positive photoconductivity observed in high vacuum converts to negative photoconductivity when the pressure is raised. Further, density functional theory calculations confirm that physisorbed oxygen molecules on the PtSe 2 surface act as acceptors. The desorption of oxygen molecules from the surface, caused by light irradiation, leads to decreased carrier concentration in the channel conductivity. The understanding of the charge transfer occurring between the physisorbed oxygen molecules and the PtSe 2 film provides an effective route for modulating the density of carriers and the optical properties of the material.

Memories with 2D materials

It is being predicted that the conventional available computing architecture will not be capable of data storage for future data-intensive computing applications. To resolve this issue, extensive research and calculations are being performed for the non-volatile memory units. Non-volatile memories can store data even when power is removed and thus are mostly employed in electronic gadgets. In this regard, 2D materials, due to their unique and remarkable features can provide solutions to design next-generation memory devices. On the other hand, various challenges, and potential strategies for the selection of 2D materials, devices, stability, circuit, and architecture level, are also being investigated. The research group deals with 2D materials and fabricated devices for memory applications to explore the capability for charge storage, endurance, data retention time, and measurements of the device under various conditions to examine the degradation behavior.

Schottky Barrier Diodes

In the past years, graphene has been one of the most studied materials for several unique and excellent properties. Due to its physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the existing semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building- block of more advanced electronic systems. A thorough understanding of the physics and the potentiality of the graphene/Silicon heterojunction is still missing. The studies of the past few years, to which the group has given a significant contribution, have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc.

Sensors

Graphene, carbon nanotubes (CNTs), black phosphorus, hexagonal boron nitride (h-BN), and 2D transition metal dichalcogenides (TMDCs), have attracted significant attention as supporting materials over a large range of sensing technologies, for temperature, pressure, gas, chemicals, etc. Due to advancements in the synthesis route and structural engineering of 2D materials, new device functionalities are being exploited by creating defect engineering, doping, and heterostructures with various nanomaterials. Due to the planar nature of the 2D materials, they are easy to fabricate and are compatible with device fabrication. Apart from the 2D materials, other nanomaterials such as carbon nanotubes also exhibit some degree of tunability. Due to the large surface-to-volume ratio and a large number of reaction sites, 2D materials are very sensitive to the surface state. While with the functionalization or defects of 2D materials, one can modify the surface chemistry and thus can tailor the selectivity to respond in certain conditions with high sensitivity. Recently, we have employed buckypaper as a low-cost sensing element for the detection of temperature, pressure, water droplets, and alcohol. The electrical conduction of the buckypaper is highly sensitive to environmental conditions. It increases with rising temperature or when pressure is applied; conversely, it is decreased under tensile strain or by exposure to water droplets.

Field Emission Properties

Field emission is a quantum tunneling phenomenon in which electrons pass from an emitting material (cathode) to an anode through a vacuum barrier by the effect of a high electric field. For a given material, cathodes with higher aspect ratios and sharper edges produce higher FE currents. Field-emission cold cathodes are the key components of vacuum nanoelectronics that offer great advantages over other electron sources based
on the thermionic or photoelectric effects. Nanostructures are considered promising field emitters for commercial applications such as flat-panel displays, vacuum electronics, microwave power tubes, electron sources, etc. We generally apply two different approaches: the first is related to the application of scanning probe microscopy techniques for their precision in scanning and manipulation, realizing field emission devices by using the microscope probe as counter-electrode, which allows access to the characterization of electronic properties on the nanometer scale; the second experimental approach is to characterize the field emitter device within a scanning electron microscope in which are installed two nanomanipulators. one of which is used as the anode. Moreover, the use of a source-measurement unit gives access to high-resolution measurements. In the effort to realize new electron sources, the group has been performing a systematic investigation of the field emission properties of low dimensional materials, such as graphene, CNTs, SnO 2 nanofibers, MoS 2, GeAs, etc. These materials exhibit high and stable emission current demonstrating their suitability as field emitters.

The MaSTeR-Lab

The MaSTeR-Lab Laboratories include three facilities created in collaboration with the Physics Department and the CNR-SPIN Institute for the fabrication and characterization of innovative materials. The fabrication line is active in the production of electron-doped high critical temperature superconducting oxide thin films for the production of electronic devices. Films are produced by DC or AC sputtering in a high vacuum chamber equipped with a heated platform capable of reaching temperatures up to 800 °C. The facility is completed by a horizontal furnace for the treatment of samples in a vacuum or in a controlled atmosphere with temperatures up to 1000 °C, and a station for electrical transport properties characterization in cryogenic liquids. In the two characterization facilities are installed cryogen-free cryomagnetic systems for the analysis of materials electrical and thermal transport properties in high field values and at very low temperatures. The CFM9T facility has a cryostat equipped with a variable temperature insert operating in the range of 1.6 - 300 K and with a superconducting magnet generating fields up to 9 T. In this system, measurements are carried out for the study of phenomena related to the transport of electric current in a wide range of materials. In particular, research is carried out in the fundamental physics of superconducting materials. The CFM16T facility, built within the NAFASSY project by CNR-SPIN in collaboration with the Department, hosts a cryogen-free cryomagnetic system designed for the characterization of electrical and thermal transport properties of superconducting materials relevant for high-field and/or high-power applications. In this system, it is possible to perform measurements in an extremely wide temperature range ranging from 50 mK to 650 K and in high magnetic fields (up to 16 T), also varying the relative position of the sample with respect to the direction of the applied magnetic field.

Responsibles:

• prof. A. Nigro, E-Mail: anigro@unisa.it
• Dr. Gaia Grimaldi (CNR-SPIN), E-Mail: gaia.grimaldi@spin.cnr.it

Members of the research group:

• Dr. Antonio Leo, E-Mail: aleo@unisa.it
• Dr. Masood R. Khan, E-Mail: mkhan@unisa.it