• Dr. Elena Matei
• Dr. Ionut Enculescu
• Dr. Monica Enculescu
• Dr. Andreea Costas
• Drd. Melania Loredana Onea
• Dr. Mihaela Beregoi
• Dr. Mihaela Bunea

Stage 1 (2021): Deposition of uni-segment nanowires and testing of oxidation processes

Summary of the stage: In this stage of the project, one of the objectives was to obtain single-segment metallic nanowires, which will later be used in the production of metal- metallic oxide -metal (MOM) nanowires The metallic segment will be either positioned at the extremities of the segmented nanowires  or to obtain the central oxidic component. For this, Ni, Cu, Co, Sn, Au, Pt metallic nanowires were prepared using a polymer template and electrodeposition. This approach allows obtaining of nanostructures with well-controlled characteristics. Very important is the possibility to control the structure/structural orientation of the nanowires only through the electrodeposition potential. For the oxidation tests of the metal nanowires, three methods were employed, namely: thermal treatment, plasma exposure and electrochemical oxidation, the purpose being to determine the parameters of obtaining the oxidic material. The parameters used in the oxidation tests were chosen considering the reduced dimensions of the starting material. The samples subjected to the oxidation processes were characterized from the morphological, structural, compositional and optical properties point of view. Thus, the characterization data showed metallic nanowires covered with an oxide layer with a weakly crystallized structure and a band gap close in value to the results reported in the literature. The morphology of oxidized nanowires is strongly influenced by the method used, an interesting result being obtained after thermal treatment of Co nanowires, resulting in secondary cobalt oxide nanowires that decorate the surface of the initial nanowire. It is interesting to note that these secondary nanowires probably have diameters of only a few nanometers, the equivalent of several tens of atoms, being probably some of the thinnest structures synthesized.

Stage 2 (2022): Deposition of multi-segment metallic nanowires and testing of selective oxidation processes

In this stage, multi-segment nanowires of the Au(Pt)-M (Ni, Cu, Co, Sn)-Au(Pt) type were prepared by electrodeposition using the template method. The nanowires were deposited sequentially from two different baths, one containing Au (Pt) to obtain the edge segments and another one containing metal ions for the central one. Multi-segmented metallic nanowires were prepared replicating the shape of the cylindrical pores, the desired lengths of the segments, having smooth surfaces and a good Au(Pt)-M interface. The next step was the testing of the selective oxidation processes of the metallic multi-segment nanowires considering the results of the previous step. Thus, for oxidation were tested thermal treatment and plasma with discharge in humidified air. Besides the final goal, for the choice of the parameters of the two techniques, it was considered that the integrity of the nanowires to not be affected. All types of multi-segment nanowires were characterized from the point of view of physico-chemical properties. A particularly strong influence of the thermal process on the central segment of the nanowires was observed, obtaining almost complete oxidation. The morphology of the oxidized nanowires is strongly influenced by the method used. The most interesting result obtained was after the thermal treatment of Au(Pt)-Co-Au(Pt) nanowires, of course, when the CoOx segment present the specific morphology with secondary nanowires decorating the surface.

Multi-segment nanowires with an oxide central segment and noble metal segments can be the basis for the development of sensor platforms with superior performances due to their characteristics.

Stage 3 (2023). Preparation of nanowires with MOM structure and investigation of their electrical properties

In the final stage, Au(Pt)-M(Ni, Cu, Co, Sn)Ox-Au(Pt) nanowires were prepared using sequential electrodeposition in nanoporous polymeric membranes and oxidation of the central segment material through heat and plasma treatment. The tests showed that a plasma treatment in a humidified air atmosphere, under certain conditions, can change the composition of the central segment. The thermal oxidation conditions in the air led to spectacular results from morpho-structural point of view. Thus, in the case of the Co segment, the tendency to decorate the surface of the resulting oxide with platelets and nanowires with dimensions of a few nanometers was preserved. The thermal oxidation of the Cu segment led to the formation of a tubular CuO segment and that of the Sn segment to a monocrystalline SnO₂ segment, the other resulting oxides being polycrystalline. The obtained MOM nanowires were contacted using photolithography and electron lithography, thus allowing the investigation of electrical properties.

A device that can be successfully used as a sensor in a field-effect transistor type configuration with a liquid gate, was made. Thus, the fact that the device responds to the potential applied to the gate, demonstrates that the current transported through the channel can be modulated based on the potential of a liquid medium around the device. Devices with an important impact in the health field can be developed later, especially point-of-care biomedical devices, devices that can be integrated into different types of wearables. These devices are all the more important as the population aging trends increase and the costs of medical care become higher and higher.

Publications:

1. Electrochemical Deposition of ZnO Nanowires on CVD-Graphene/Copper Substrates, authors: I. Boukhoubza, E. Matei, A. Jorio, M. Enculescu, I. Enculescu , Nanomaterials 2022, 12, 2858, https://doi.org/10.3390/nano12162858.
2. The interface structural, electronic and optical properties of ZnO nanowires/Graphene nanohybrid (ZnO NWs/G): Experimental and theoretical DFT investigations, authors: I. Boukhoubza, M. Achehboune, I. Derkaoui, M. Enculescu, A. Jorio, I. Zorkani, I. Enculescu, E. Matei,  Jornal of Alloys and Compunds, accepted
3. New insights specific magnetic configurations of polycrystalline Ni nanowires  fabricated by the template method at different electrodepostion potentials, authors: M.L. Onea, Nicusor Iacob, Gabriel Schinteie, Victor Kuncser, M.E. Toimil-Molares, E. Matei, I. Enculescu, manuscript to be submitted.
4. Effect of SiO₂ passivation on electrical characteristics of ZnO nanowire FETs, authors: M.L. Onea, M. Enculescu, G. Boni, M.E. Toimil-Molares, E. Matei, I. Enculescu, manuscript to be submitted.

Presentations at conferences:

1. M.L. Onea, E. Matei, M. Enculescu, V. Kuncser, I. Enculescu,  Ni Nanowires Propertie.s for Functional Magnetic Materials Applications , CREMS 2021, 21-22 October 2021, National Institute of Materials Physics, Magurele, Romania.
2. M.L. Onea, E. Matei, M. Enculescu, M. Beregoi, I. Enculescu, The influence of process parameters on structural and morphological properties of different metallic nanowires, 12^th APMAS 2022 – International Advances in Applied Physics & Materials Science Congress & Exhibition, 13-19 oct. 2022, Oludeniz, Turkey.
3. Melania Loredana Onea, Elena Matei, Ionut Enculescu, A comparison of electric properties nanowire FET fabricated on 2 electrode system vs .electrode system, 13th International Advances in Applied Physics and Materials Science Congress and Exhibition, Mugla, Turkey. 11-17.10.2023
4. Elena Matei, Andreea Costas, Melania Onea, Victor Diculescu, Adrian Enache, Ionut Enculescu, Nanowire based electronic devices for sensor applications, 74th Annual Meeting of the International Society of Electrochemistry”, Lyon, Franta, 3-8 septembrie 2023.