Now showing 1 - 6 of 6
  • Publication
    Characterization, tuning and fabrication by nanoscale stress on ferroelectric thin films
    (University College Dublin. School of Physics, 2020) ;
    0000-0001-6302-3574
    Atomic force microscopy (AFM) based techniques have been used widely to study functional properties of ferroelectrics. In this thesis, we investigated the application of AFM tips loaded with different stress ranges (from ~nN to ~µN) on ferroelectric thin films at the nanoscale. AFM tips applied with tens of nN force were used for careful characterization of ferroelectric switching on an inhomogeneous Pb(Zr,Ti)O3 (PZT) thin film. Different switching loops and domains were obtained after performing band excitation piezoelectric spectroscopy (BEPS) on the film at adjacent nanoscale areas. By combining BEPS, piezoresponse force microscopy (PFM), transmission electron microscope (TEM) and machine learning, abnormal loops were clustered and proved to be induced from various mechanisms including electrostatic, ferroelastic, and charge injection, which was mediated by defect-led microstructural variations. Applied stress in a large force range up to ~1 µN on ferroelectric BiFeO3 (BFO) thin films showed significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching via conductive atomic force microscopy. This injected current can be effectively modulated by applying mechanical force. As the loading force increases from tens of nN to hundreds of nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by several orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of injected current may be attributed to the mechanical force-induced changes in barrier height and width of the interfacial layer. AFM tips with these forces can characterize or modulate ferroelectric related properties. We therefore expected that larger force (tens of µN) can further remove ferroelectric materials for nanomechanical machining as ferroelectrics are important candidate materials for a wide range of applications including data storage and actuators. AFM-based machining of ferroelectric nanostructures offers advantages such as low damage and low-cost modification for already-fabricated thin films. Through a systematic investigation of a broad range of AFM parameters, we demonstrate that AFM-based machining provides a low-cost option to rapidly modify local regions of the BFO thin film, as well as fabricate a range of different nanostructures, including a nanocapacitor array with individually addressable ferroelectric elements.
      245
  • Publication
    Electric Field-Induced Chemical Surface-Enhanced Raman Spectroscopy from Aligned Peptide Nanotube–Graphene Oxide Templates for Universal Trace Detection of Biomolecules
    Semiconductor-graphene oxide-based surface-enhanced Raman spectroscopy substrates represent a new frontier in the field of surface-enhanced Raman spectroscopy (SERS). However, the application of graphene oxide has had limited success because of the poor Raman enhancement factors that are achievable in comparison to noble metals. In this work, we report chemical SERS enhancement enabled by the application of an electric field (10-25 V/mm) to aligned semiconducting peptide nanotube-graphene oxide composite structures during Raman measurements. The technique enables nanomolar detection sensitivity of glucose and nucleobases with up to 10-fold signal enhancement compared to metal-based substrates, which, to our knowledge, is higher than that previously reported for semiconductor-based SERS substrates. The increased Raman scattering is assigned to enhanced charge-transfer resonance enabled by work function lowering of the peptide nanotubes. These results provide insight into how semiconductor organic peptide nanotubes interact with graphene oxide, which may facilitate chemical biosensing, electronic devices, and energy-harvesting applications.
      831Scopus© Citations 36
  • Publication
    Flexing Piezoelectric Diphenylalanine-Plasmonic Metal Nanocomposites to Increase SERS Signal Strength
    Piezoelectric quasi-1D peptide nanotubes and plasmonic metal nanoparticles are combined to create a flexible and self-energized surface-enhanced Raman spectroscopy (SERS) substrate that strengthens SERS signal intensities by over an order of magnitude compared to an unflexed substrate. The platform is used to sense bovine serum albumin, lysozyme, glucose, and adenine. Finite-element electromagnetic modeling indicates that the signal enhancement results from piezoelectric-induced charge, which is mechanically activated via substrate bending. The results presented here open the possibility of using peptide nanotubes on conformal substrates for in situ SERS detection.
      529Scopus© Citations 20
  • Publication
    Fibril size-dependent control of polar ordering in type I collagen membranes
    The most abundant protein in the human body, collagen, is widely used in tissue culture and engineering applications, spanning from substrate functionalization to fibrillar architectures and three-dimensional constructs. Collagen piezoelectricity provides an opportunity to exploit electromechanical coupling in these applications, wherein an applied mechanical stress generates charge, which might influence ion screening, protein absorption, and cell response. In type I collagen, the polarization direction follows the fibril orientation. Thus, control of fibril orientation and size in a collagen film or membrane may provide control of the polarization, enabling the creation of regions of uniform polarization direction. Here, aligned substrate-supported type I collagen membranes having fibril sizes from ∼100-500 nm are deposited using different osmotic concentrations (90, 190, and 290 mOsm/kg, from low to high ionic strength) to investigate the correlation between fibril size and piezoelectric properties. Lateral piezoresponse force microscopy is used to show that regions of uniform polarization orientation, as determined through 2D correlation analysis, decrease with increasing fibril size.
      232Scopus© Citations 2
  • Publication
    Investigation of AFM-based machining of ferroelectric thin films at the nanoscale
    Atomic force microscopy (AFM) has been utilized for nanomechanical machining of various materials including polymers, metals, and semiconductors. Despite being important candidate materials for a wide range of applications including data storage and actuators, ferroelectric materials have rarely been machined via AFM. AFM-based machining of ferroelectric nanostructures offers advantages over established techniques, such as bottom-up approaches and focused ion beam milling, in select cases where low damage and low-cost modification of already-fabricated thin films are required. Through a systematic investigation of a broad range of AFM parameters, we demonstrate that AFM-based machining provides a low-cost option to rapidly modify local regions of the film, as well as fabricate a range of different nanostructures, including a nanocapacitor array with individually addressable ferroelectric elements.
      286Scopus© Citations 15
  • Publication
    Energy harvesting with peptide nanotube-graphene oxide flexible substrates prepared with electric field and wettability assisted self-assembly
    Piezoelectric diphenylalanine peptide nanotubes (PNTs) have recently been demonstrated in energy harvesting applications, typically based on vertically aligned PNTs that generate charge when pressed. In this work, we use a wettability difference and an applied electric field to align PNTs and PNT-based composites on flexible substrates. Open-circuit voltages and short-circuit currents exceeding 6 V and 60 nA, respectively, are achieved by bending the substrate, opening up the use of horizontally aligned PNTs as flexible energy harvesting substrates.
      207Scopus© Citations 7