Thin films and multilayers based on oxides or clusters: microelectronics, environment & energy storage

CSM develop a strong know-how in the engineering of multifunctional thin films materials with the two following goals:

• the control of the growth, structuration and integration of materials in hetero-structures in order to generate new functionalities.
• the multiscale crystal chemistry investigation of thin films in order to improve physical properties.

The control of materials in thin films is a prerequisite for their integration in new devices for microelectronics, sensors or energy conversion. We are working in particular on multifunctional oxides and metal cluster compounds thin films grown by pulsed laser deposition (PLD), sputtering (PVD) and chemical solution deposition (CSD) using spin or dip coating. Recently we developed a new expertise in electrophoretic deposition (EPD) or Langmuir-Blodgett (LB) deposition processes. Our group develops specific skills in structural and chemical characterizations such as X-ray diffraction (XRD).

Lead free ferroelectric thin films

The RoHS (Restriction of Hazardous Substances) directive that took effect in Europe 2006 motivates research on piezoelectric and ferroelectric lead-free thin films. In the KxTa1-xNbxO3 (KTN) system the Curie Temperature (Tc) can be tuned by the composition (Ta/Nb ratio) that is a great advantage to tune the properties at room temperature. We have obtained epitaxial thin films on various substrates such as R-plane sapphire that is suitable for microwave applications. Indeed a smart way to realize high frequency reconfigurable devices is to tune the dielectric permittivity of a ferroelectric thin film by application of a bias voltage. We have controlled KTN thin films by PLD and CSD (spin-coating polymeric precursors route) on which reconfigurable coplanar devices have been printed. In this field, we work in collaboration with IETR (F. Cissé PhD in co-supervision IETR-ISCR). A twofold approach combining chemical doping and design of the device led to a good compromise between dielectric loss reduction and device tunability.

Recently we investigated the KxNa1-xNbO3 (KNN) material that is identified as a possible alternative to lead based piezoelectric materials because of its high piezoelectric coefficient. The phase diagram of the potassium sodium niobates is very rich but few phases were studied in thin films. We obtained the perovskite phase that is the well-known ferroelectric with a high Tc. We have studied the effect of the substrate on the structural characteristics and their influence on the high frequency dielectric properties (ArMin DGA – Region Bretagne project, B. Aspe PhD co-supervised ISCR – IETR, 8th October 2019, University of Rennes 1).

The epitaxial growth on MgO enhances the tunability and reconfigurable miniaturized antennas have been obtained. We have also succeeded in the growth of pure thin films of the TTB (Tetragonal Tungsten Bronze) phase that is one of the low alkaline content phases little studied, or even not at all, in thin films (A. Waroquet and B. Aspe PhD theses). Its epitaxial growth was studied in detail and its dielectric properties were determined. The investigation of lead-free piezoelectric materials thin films is going on in the frame of a collaboration between ISCR and the CEA LETI for transducer application (CEA LETI PhD H. Kuentz, co-supervision 2020-2023).

J Mater Sci 2018, 53:13042–13052
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2018, 65 665 – 671
Journal of Alloys and Compounds 2020, 827, 154341
Crystal Growth & Design, 2020, 20, 2356-2366
Journal of Alloys and Compounds, 2021, 856, 158138

Metal atom cluster thin films

The investigation of the outstanding insertion properties of molybdenum cluster thin films is one of the main research axis of our group. In particular we studied Chevrel phase type Mo₆X₈ (X = S, Se) thin films for miniaturized electrochemical sensor dedicated to the detection of heavy metal ions in water, in collaboration with IJL – Metz. The electrochemical behaviour of M_xMo₆S₈ thin films grown by CSD have shown the intercalation capacity of the Cu₂Mo₆S₈ films with the best results on TiN/Si substrate: fast and reversible intercalation process with a cyclability higher than 100 cycles for an aqueous solution of CdSO₄ 1M. For lower concentration differential pulse voltammetry allowed to detect Cd²⁺ with a minimal sensitivity of 10^-6 M.

In order to address more accurately the selectivity of the insertion in multications solutions, it is necessary to investigate in detail the insertion mechanisms in reference compounds. Electrochemical characterizations have been combined with X-ray diffraction using synchrotron facilities (ESRF) to study molybdenum clusters powders. The crystal structure of intercalated phases have been determined. Unexpectedly, Cd₂Mo₆Se₈ presents a trigonal crystal structure with only cavity 2 occupied, which has not been encountered before for Chevrel phases. In addition a cinetic effect was evidenced (Inorg. Chem. 2019, 58, 2158-2168).

Recently a new research topic emerged in CSM team, to develop new transition metal cluster materials for solar energy conversion. The activities deal with photoelectric properties of semiconducting oxides and sulfides for their integration in photovoltaic cells as new rare-metal free non-toxic absorbers.

Several routes of synthesis and thin films deposition are investigated to control the composition and microstructure of thin films. The optical and electronic properties are determined by complementary characterization methods (UV-Vis-NIR spectroscopy, electrochemistry, XPS). These experiments enable to evaluate the potentiality of such materials for the targeted application and to design the architecture of PV devices.

Electrochimica Acta 2017, 257, 436–443
Inorg. Chem. 2019, 58, 2158-2168

Layered phases and 2D nanosheets

Layered compounds have received considerable attention for several years due to their numerous catalytic properties, such as intercalation and exfoliation, induced by their particular structure. This structuration into negatively charged sheets with positively charged species intercalated between them allows the mobility of the interlayered cations giving rise to interesting exchange and/or intercalation properties, in particular at low temperature, and thus leading to new materials. If the layered niobates are intensively studied in the form of powders, few studies relate to these materials in thin layers opening the way to further research. Among these compounds, the KNb₃O₈ and K₄Nb₆O₁₇ potassium niobate phases show excellent UV photocatalytic activity for degradation of dyes. Synthesis of thin films of these niobates was achieved by pulsed laser deposition although this method is challenging for potassium-based compounds due to the high volatility of K₂O. Such films are epitaxially grown on single-crystalline substrates and present ion-exchange properties that shift their optical band-gap towards visible range.

Exfoliation of layered powder phases leads to colloid solutions of two-dimensional (2D) nanosheets. These nanosheets are transferred by means of Langmuir-Blodgett method on low cost substrates such a glass, silicon and mica and play the role of crystalline templates for the growth of subsequent functional oxides thin films. Synthesis and transfer of Ca₂Nb₃O₁₀^-, K₂Nb₆O₁₇^2-, and Cs₄W₁₁O₃₆^2- nanosheets were achieved allowing the growth of preferentially oriented La_0.67Sr_0.33MnO₃, KNbO₃, BiFeO₃, SrVO₃, CaVO₃ and Pt thin films, by pulsed laser ablation, chemical deposition solution and sputtering. The project is mainly developed in the ANR project PolyNash (ANR-17-CE08-0012) in collaboration with the CRISMAT based at Caen and the GeMAC based at Versailles.

Solid State Sci. 2016, 54, 17–21
RSC Adv. 2017, 7, 15482–15491
ACS Appl. Mater. Interf. 2019, 11, 37302–37312
Mater. Lett. 2019, 253, 392–395
Thin Solid Films. 2020, 693, 137682
Thin Solid Films. 2020, 693, 137687
SN Appl. Sci. 2020, 2, 453
 

Solution-processed nanocolloidal have emerged as a versatile platform for building electronic and optoelectronic devices. The concept has been challenged by the development of materials and processes that allow bottom up fabrication. These demands are spurring the design of nanomaterials that exhibit engineered physical properties and that can enable fabrication method for low cost, large area and/or flexible devices. The device parts are assembled from nanoscale components (nanoclusters, nanocrystals, nanoparticles) by using CSD (spin-coating, dip-coating, LB, EPD…). However, the shift from materials that are bulk single crystals to assemblies of many components introduces challenges, most notably the increase role of the interfaces.

Metal atom clusters nanocomposite thin films by EPD

In a strong collaboration with the international research laboratory (IRL) for Innovative Key Materials and Structures (IRL3629 LINK) and the National Institute for Material Science (NIMS) based at Tsukuba (Japan), we focused on the development of a new strategy using an EPD process to fabricate highly transparent and colored nanocomposite photoelectrodes thin films based on inorganic transition metal atom clusters. EPD offers great combinations of cost effectiveness (EPD is conducted at room temperature under ambient conditions), long-range consistency in film thickness and surface morphology, size-scalability and high deposition rates. Recently, we have demonstrated for the first time that EPD is a very relevant process for the fabrication of new nanocomposite photoelectrodes on transparent conductive oxide glass substrate using Mo6 metal atom clusters as building blocks1-3.

[1] Bull. Chem. Soc. Jpn. 2018, 91, 1763
[2] Electrochimica Acta 2019, 317, 737e745
[3] ACS Appl. Mater. Interfaces 2020, 12, 40495

Oxides thin films based on nanocolloids

In the group, we develop, in collaboration with IRL3629 LINK, a strategy to fabricate thin films based on colloidal solutions by CSD process. We target in priority the compounds ZnO, HfO2 and -Fe2O3. These compounds are attractive building blocks for the solution-based assembly of nanocrystals with emergent physical properties that are derived from the size, shape, and composition-dependent characteristics and the collective interactions between the nanocrystals in the thin film. Thin films of ZnO and HfO2 with thickness between 50 to 100 nm are already prepared.

This project is mainly based and develop in the ANR project DUVNANO (ANR-18-CE08-0022: Fabrication of functional thin films by combining deep UV nanolithography and solution colloidal nanocrystals processes) with IS2M at Mulhouse (France) as partner.

The team has been developing for several years within the frame of multidisciplinary collaborations (ISCR and IPR / Rennes, ITQ Valencia) an activity aimed at functionalizing different types of surfaces by clusters of transitional elements. Depending on their nature (Si, Au, amorphous carbon or graphene) and the method of grafting implemented, the immobilization of clusters makes it possible either to modify their electronic properties, or to promote new properties such as luminescence or photo-catalysis.

Red-NIR luminescence of Mo6 monolayered assembly directly anchored on Au(001)

Gold surfaces were functionalized by monolayers of Mo6 clusters using thiocyanate linkers (NCS). This bidentate ligand is covalently grafted onto the cluster and interacts strongly with the gold surface via sulfur. This is the first example of a luminescent pigment grafted directly onto a surface whose luminescence properties are not extinguished following grafting.
Mater. Horizons 2019, 6, 1828-1833

Supramolecular anchoring of octahedral molybdenum clusters onto graphene and their synergies in photocatalytic water reduction

In collaboration with the ITQ in Valencia (Spain), several grafting strategies have been implemented to immobilize clusters on graphene or graphene oxide. The systems thus obtained exhibit photo-reduction properties of water. Dihydrogen (H2) production from sunlight should become one of the most important energy production means in the future. To reach this goal, low-cost and efficient photocatalysts still need to be discovered. We showed that red near-IR luminescent metal cluster anions, once combined with pyrene-containing cations, are able to photocatalytically produce molecular hydrogen from water. The pyrene moieties act simultaneously as energy transmitters and as supramolecular linkers between the cluster anions and graphene. This association results in a hybrid material combining the emission abilities of pyrene and cluster moieties with the electronic conduction efficiency of graphene. Hydrogen evolution reaction (HER) studies show that this association induces a significant increase of H2 production compared to that produced separately by clusters or graphene. Considering the versatility of the strategy used to design this photocatalytic hybrid material, transition-metal clusters are promising candidates to develop new, environmentally friendly, and low-cost photocatalysts for HER.
Inorg. Chem. 2019, 58, 15443-15454
CHEMSUSCHEM 2016,, 9,1963-1971