Stephen S. Nonnenmann

Wafer-Scale Nanopatterning and Translation into High-Performance Piezoelectric Nanowires

The development of a facile method for fabricating one-dimensional, precisely positioned nanostructures over large areas offers exciting opportunities in fundamental research and innovative applications. Large-scale nanofabrication methods have been restricted in accessibility due to their complexity and cost. Likewise, bottom-up synthesis of nanowires has been limited in methods to assemble these structures at precisely defined locations. Nanomaterials such as PbZr(x)Ti(1-x)O(3) (PZT) nanowires (NWs)--which may be useful for nonvolatile memory storage (FeRAM), nanoactuation, and nanoscale power generation--are difficult to synthesize without suffering from polycrystallinity or poor stoichiometric control. Here, we report a novel fabrication method which requires only low-resolution photolithography and electrochemical etching to generate ultrasmooth NWs over wafer scales. These nanostructures are subsequently used as patterning templates to generate PZT nanowires with the highest reported piezoelectric performance (d(eff) ∼ 145 pm/V). The combined large-scale nanopatterning with hierarchical assembly of functional nanomaterials could yield breakthroughs in areas ranging from nanodevice arrays to nanodevice powering.

Finite curvature-mediated ferroelectricity.

We demonstrate that ferroelectric (FE) polarizations oriented along the finite thickness direction in ultrathin films are enhanced by the introduction of extreme curvature, thereby suppressing the finite-size-driven evolution of the FE phase transition temperature T(C). The measured responses within individual nanoshells possess magnitudes nearly three times that for their planar counterparts while exhibiting finite curvature-dependent offsets in FE switching hystereses. In stark contrast to the expected scaling of a depression of T(C) with inverse thickness, results based on modified Landau-Ginzburg model calculations indicate geometric curvature-driven polarization gradients in ultrathin films result in significant increases in T(C).

Excitation of Local Field Enhancement on Silicon Nanowires

The interaction between light and reduced-dimensionality silicon attracts significant interest due to the possibilities of designing nanoscaled optical devices, highly cost-efficient solar cells, and ultracompact optoelectronic systems that are integrated with standard microelectronic technology. We demonstrate that Si nanowires (SiNWs) possessing metal-nanocluster coatings support a multiplicatively enhanced near-field light-matter interaction. Raman scattering from chemisorbed probing molecules provides a quantitative measure of the strength of this enhanced coupling. An enhancement factor of 2 orders of magnitude larger than that for the surface plasmon resonance alone (without the SiNWs) along with the attractive properties of SiNWs, including synthetic controllability of shape, indicates that these nanostructures may be an attractive and versatile material platform for the design of nanoscaled optical and optoelectronic circuits.

Air–Liquid Interfacial Self-Assembly of Conjugated Block Copolymers into Ordered Nanowire Arrays

The ability to control the molecular packing and nanoscale morphology of conjugated polymers is important for many of their applications. Here, we report the fabrication of well-ordered nanoarrays of conjugated polymers, based on the self-assembly of conjugated block copolymers at the air-liquid interface. We demonstrate that the self-assembly of poly(3-hexylthiophene)-block-poly(ethylene glycol) (P3HT-b-PEG) at the air-water interface leads to large-area free-standing films of well-aligned P3HT nanowires. Block copolymers with high P3HT contents (82-91%) formed well-ordered nanoarrays at the interface. The fluidic nature of the interface, block copolymer architecture, and rigid nature of P3HT were necessary for the formation of well-ordered nanostructures. The free-standing films formed at the interface can be readily transferred to arbitrary solid substrates. The P3HT-b-PEG films are integrated in field-effect transistors and show orders of magnitude higher charge carrier mobility than spin-cast films, demonstrating that the air-liquid interfacial self-assembly is an effective thin film fabrication tool for conjugated block copolymers.

Redox-based resistive switching in ferroelectric perovskite nanotubes

Hysteresis in current and ferroelectric piezoelectric phase were collected across the walls of individual, electrically interfaced lead zirconate titanate (PZT) nanotubes. The nanotubes exhibit average on/off current ratios of ∼10 and ∼1000 in static local probe and top-electroded configurations, respectively. Reversibility in conduction state of an individual nanotube following different stages of an O2-rich/O2-deficient/O2-rich anneal cycle provide evidence of an oxygen vacancy concentration-based conduction mechanism.

Piezoresponse through a ferroelectric nanotube wall

We report on the controlled local switching and imaging of local ferroelectric polarizations oriented perpendicular to the long axis of a lead zirconate titanate (PZT) nanotube. Piezoresponse force microscopy and ferroelectric piezoelectric hysteresis data indicate stable polarizations oriented along the radial, finite-thickness direction can be formed in a nanoshell geometry. The results of infrared spectroscopy and of the character of as-found polarizations are consistent with recent findings linking surface chemical environment to ferroelectric stability and to orientation of ferroelectric polarizations.

Direct in situ probe of electrochemical processes in operating fuel cells.

The function of systems and devices in many technologically important applications depends on dynamic processes in complex environments not accessible by structure and property characterization tools. Fuel cells represent an example in which interactions occur under extreme conditions: high pressure, high temperature, in reactive gas environments. Here, scanning surface potential microscopy is used to quantify local potential at electrode/electrolyte interfaces in operating solid oxide fuel cells at 600 °C. Two types of fuel cells are compared to demonstrate two mechanisms of ionic transport at interfaces. Lanthanum strontium ferrite-yttria-stabilized zirconia (LSF-YSZ) and lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) cross-sectional electrode assemblies were measured to compare mixed ionic electronic conducting and electronic conducting mechanisms. Direct observation of the active zones in these devices yields characteristic length scales and estimates of activation barrier changes.

Magneto-elastic tuning of ferroelectricity within a magnetoelectric nanowire

Nanowires each consisting of a magnetostrictive Co core and a PbZr0.52Ti0.48O3 or BiFeO3 ferroelectric oxide shell exhibit a magnetic field-tunable piezoelectric response and ferroelectric coercivity owing to magneto-elastic coupling through the interfacial core-shell boundary. The observed magneto-elastic tuning of the ferroelectric switching is analyzed using a renormalized Landau-Ginzburg stiffness in which a magnetic field-tunable stress concentration is incorporated. These results provide insight into the design of integrated functional devices and magnetoelectric sensors.

Tip loading effects on AFM-based transport measurements of metal–oxide interfaces

Here we demonstrate the effects of tip loading force on the contact quality and local current-voltage character between conductive AFM tips and individual noble metal nanoparticle-strontium titanate (NP-STO) interfaces. These results show that though contact quality may improve with increased loading force, nanoparticle deformation remains negligible for loading forces in the nN-μN range. Maintaining a moderate loading force in the tens to hundreds of nN therefore enables size-dependent transport of individual NP-STO interfaces to be determined.

A transition in mechanisms of size dependent electrical transport at nanoscale metal-oxide interfaces

As device miniaturization approaches nanoscale dimensions, interfaces begin to dominate electrical properties. Here the system archetype Au/SrTiO3 is used to examine the origin of size dependent transport properties along metal-oxide interfaces. We demonstrate that a transition between two classes of size dependent electronic transport mechanisms exists, defined by a critical size e. At sizes larger than e an edge-related tunneling effect proportional to 1/D (the height of the supported Au nanoparticle) is observed; interfaces with sizes smaller than e exhibit random fluctuations in current. The ability to distinguish between these mechanisms is important to future developments in nanoscale device design.