Thin film properties on the nanoscale: from oligomers to polymer, from 2d materials to ferroelectrics

The scientific investigations of thin films are a very fascinating subject regarding the properties of confined matter. These studies are also very important for a range of nanotechnology applications. Scanning Probe Microscopies (SPM) represent powerful tools for such a quest. In general, alone or better combined with other surface techniques, they allow one to correlate the film morphology with their functionalities, giving the access to the understanding and the control of physical phenomena on the nanoscale.
The SPM setups at disposal in our laboratories allow us to perform a series of analyses from the glass transition temperature (T<sub>g</sub>) [1-2] to wear and tribology of polymers [3-5], from the initial growth stages of conjugated oligomers [6-11] to the piezo-response characterization of Ferroelectric or Multiferroic films [12-14], from the mechanical response of more compliant [15-17] to stiffer materials [18-19].
Specifically, one of our homemade systems allows us to vary the sample temperature while indenting the film versus time. A second setup allows Piezo-response Force Microscopy (PFM) [20] and Ultrasonic Force Microscopy (UFM) [21] to be performed in the same place, with no need to withdraw the tip. In this case, the samples are rigidly mounted on a piezo-disc, which can be biased to operate in UFM or PFM and collect the respective data. In the following we show a couple of examples of our work, carried out in the recent years.
In Figure 1, we report images obtained in a study of the adsorption of Sexithiophene (T6) molecules on a Silicon oxide (SiO<sub>2</sub>) substrate [10]. A thin film with a coverage around 0.4 ML (Mono Layer, with vertical backbone axis) is reported: (a) morphology and (b) UFM signal. The analysis of the formation of ultra-thin organic films is a very important issue. In fact, it is known that the properties of organic light emitting diodes and field effect transistors are strongly affected by the early growth stages. For instance, in the case of T6, the presence of domains made of molecules with the backbone parallel to the substrate surface can be highly detrimental to the functioning of such devices. In the past, this presence has been indirectly evidenced by photoluminescence spectroscopy and confocal microscopy [7]. On the contrary, conventional SPM both in contact and intermittent contact modes have failed to detect such domains.
UFM, that is sensitive to nanomechanical properties, allows one to directly identify the structure of sub-monolayer thick films. T6 flat domains have been imaged for the first time with nanometer scale spatial resolution. In the UFM contrast, the terraces (vertical domains) are darker with respect to the interstitial regions. At the same time, the UFM contrast is not homogeneous in the SiO<sub>2</sub> regions. For control, in the inset we report the UFM contrast obtained on bare SiO<sub>2</sub>. In particular, this proves that UFM is highly suitable for investigations where high sensitivity to material mechanical properties, low specimen damage and high spatial resolution are required.
In Figure 2, we show a study of mixed-phase BiFeO<sub>3</sub> films by means of PFM and UFM to characterize the elastic response at the boundaries between rhombohedral-like monoclinic (M<sub>I</sub>) and tilted tetragonal-like monoclinic (M<sub>II,tilt</sub>) phases [14]. Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour. In this case the elastic modulation in mixed-phase areas has been resolved at the nanoscale level. Low/high stiffness values perfectly coincide with the M<sub>I</sub>/M<sub>II,tilt</sub> phases. The rhombohedral-like M<sub>I</sub> phase is found to be more compliant than the other phases. A large elastic modulation at the phase boundary on the M<sub>I</sub> side suggests a high strain gradient due to compressive stress. The SPM conclusions have been also confirmed by X-ray diffraction strain analysis (RSM). This evidences how UFM and PFM are highly suitable for the investigations of complex samples and that correlating the film grain structure with the macro response of the whole film is highly valuable.

[1] F. Dinelli, C.K. Buenviaje, R.M. Overney, ‘Glass transition of thin polymeric films: Speed and load dependence in lateral force microscopy’, Journal of Chemical Physics, 113 (5), 2043‑8, 2000
[2] F. Dinelli, A. Ricci, T. Sgrilli, P. Baschieri, R. Manjuhath, P. Kingshott, P. Pingue, ‘Nanoscale Viscoelastic Behavior of the Surface of Thick Polystyrene Films as a Function of Temperature’, Macromolecules 44, 987, 2011
[3] F. Dinelli, G.J. Leggett, P.H. Shipway, and M.R. Alexander, ‘Scanning Force Microscopy of plasma polymerised hexane: information on in-plasma and downstream-plasma deposition regimes from nanowear analysis’, Journal of Applied Physics, 91 (6), 3841, 2002
[4] F. Dinelli, P.H. Shipway, G.J. Leggett, ‘Nanowear of polystyrene surfaces: molecular entanglement and bundle formation’, Nanotechnology, 16, 675, 2005
[5] F. Dinelli, C. Menozzi, P. Baschieri, P. Facci, P. Pingue, ‘Scanning probe nanoimprint lithography’, Nanotechnology 21, 75305, 2010
[6] F. Dinelli, M. Murgia, P. Levy, M. Cavallini, D. de Leeuw, F. Biscarini, ‘Spatially correlated charge transport in organic thin films’, Physical Review Letters, 91, 6802, 2004
[7] M. A. Loi, E. Da Como, F. Dinelli, M. Murgia, R. Zamboni, F. Biscarini, and M. Muccini ‘Supra‑molecular organization in ultra-thin films of a-sexithiophene on silicon dioxide’, Nature Materials, 4, 81, 2005
[8] F. Dinelli, R. Capelli, M. A. Loi, M. Murgia, M. Muccini, A. Facchetti, T. Marks, ‘Light Emission in Organic FETs based on DHT4/P13 heterostructures’, Advanced Materials, 18, 1416, 2006
[9] F. Dinelli, J.-F. Moulin, M. A. Loi, E. Da Como, M. Massi, M. Murgia, M. Muccini, F. Biscarini, J. Wie, P. Kingshott, ‘Effects of surface chemical composition on the early growth stages of a-sexithienyl films on silicon oxide substrates’, J. Phys. Chem. B, 110, 258, 2006
[10] F. Dinelli, C. Albonetti, O. V. Kolosov, ‘Ultrasonic force microscopy: Detection and imaging of ultra-thin molecular domains’, Ultramicroscopy 111, 267, 2011
[11] R. Capelli, F. Dinelli, M. Gazzano, R. D’Alpaos, S. Stefani, M. Riva, M. Montecchi, A. Giglia, and L. Pasquali, ‘Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors (OLET)’, Advanced Functional Materials, in press doi: 10.1002/adfm.201400877, 2014
[12] E. B. Araujo, B. O. Nahime, M. Melo, F. Dinelli, F. Tantussi, P. Baschieri, F. Fuso, M. Allegrini, ‘Processing and structural properties of random oriented lead lanthanum zirconate titanate thin films’, Materials Research Bulletin 61, 26–31, 2014
[13] B. Nahime, E. Araujo, M. Melo, E. Lima, F. Dinelli, F. Tantussi, P. Baschieri, F. Fuso, M. Allegrini, ‘Nanoscale physical properties of La-modified Pb(Zr0.65Ti0.35)O3 thin films’ Submitted to Journal of Physics D: Applied Physics
[14] Cheng-En Cheng, Heng-Jui Liu, Yi-Chun Chen, Franco Dinelli, Chen-Shiung Chang, Forest Shih-Sen Chien, and Ying-Hao Chu, ‘Probing Elastic Modulation of Mixed-Phase Boundary in Epitaxial BiFeO<sub>3</sub> Thin Films by Ultrasonic Force Microscopy’, Accepted for publication on Scientific Reports
[15] F. Dinelli, H.E. Assender, N. Takeda, G.A.D.Briggs, and O.V. Kolosov, ‘Elastic mapping of heterogeneous nano-structures with ultrasonic force microscopy (UFM)’, Surface and Interface Analysis, 27 (5-6), 562-7, 1999
[16] F. Dinelli, H.E. Assender, K. Kirov, and O.V. Kolosov, ‘Surface morphology and crystallinity of biaxially stretched PET films on the nanoscale’, Polymer, 41 (11), 4285-9, 2000
[17] C. Dionigi, T. Posati, V. Benfenati, A. Sagnella, A. Pistone, S. Bonetti, G. Ruani, F. Dinelli, G. Padeletti, R. Zamboni, M. Muccini, ‘A nanostructured conductive bio-composite of silk fibroin-single walled carbon nanotubes’, J. Materials Chemistry B, 2(10),1424-1431, 2014
[18] F. Dinelli, M.R. Castell, N.J. Mason, G.A.D. Briggs, and O.V. Kolosov, ‘Mapping surface elastic properties of stiff and compliant materials on the nanoscale using ultrasonic force microscopy (UFM)’, Philosophical Magazine A, 80 (10), 2299‑323, 2000
[19] F. Dinelli, S.K. Biswas, G.A.D. Briggs, and O.V. Kolosov, ‘Measurements of stiff material compliance on the nanoscale with ultrasonic force microscopy’, Physical Review B, 61 (20), 13995-14006, 2000
[20] D. A. Bonnell, S. V. Kalinin, A. Kholkin, andA. Gruverman,’Piezoresponse Force Microscopy: a window into electromechanical behavior at the nanoscale’,MRS Bulletin, September, 2009
[21] O.V. Kolosov, K. Yamanaka, Jap. J. Appl. Phys. Part 2-Letters 32 (8A), L1095-L1098, 1993

Research & Technical staff:
Barbini AlessandroDinelli Franco

Associated Researchers:
Ascoli Cesare