Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. The most well known material that has piezoelectric effect is the provskite structured Pb(Zr, Ti)O3 (PZT), which has found huge applications in electromechanical sensors, actuators and energy generators. But PZT is an electric insulator and it is less useful for building electronic devices. Wurtzite structures, such as ZnO, GaN, InN and ZnS, also have piezoelectric properties but they are not extensively used as much as PZT in piezoelectric sensors and actuators due to their small piezoelectric coefficients. In fact, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. For materials such as ZnO, GaN, InN in the wurtzite structure family, the effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science and sensors.[1] Z.L. Wang and J.H. Song “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, 312 (2006) 242-246.[2] Z.L. Wang “Nano-piezotronics”, Adv. Mater., 19 (2007) 889-992.[3] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder and Z.L. Wang “Designing the electric transport characteristics of ZnO micro/nanowire devices by coupling piezoelectric and photoexcitation effects”, ACS Nano, 4 (2010) 1234–1240.[4] Z.L. Wang et al. “Lateral nanowire/nanobelt based nanogenerators, piezotronics and piezo-phototronics”, Mater. Sci. and Engi. Reports.

Surface plasmons are electromagnetic waves that couple to the free electrons in a metal and oscillate collectively at the interface of the metal and a dielectric. Due to the capability of surface plasmons in localizing and guiding light in sub-wavelength metal structures, surface plasmon-based photonics, or “plasmonics”, offers an opportunity to merge photonics and electronics at nanoscale dimensions. Thus, through plasmonics, the realization of very large scale electronics and photonics integration (VLSEPI) becomes possible. In the past years, our research has been centered on designing, modeling, and prototyping a new class of active plasmonic materials and devices based on active nanostructures such as molecular motors. In particular, we have: (1) developed high-throughput and cost-effective nanofabrication and nanoengineering tools for producing metal nanostructures of the desired plasmonic properties; (2) measured and modeled the localized surface plasmon resonances (LSPRs) of both individual nanostructures and nanostructure arrays; (3) achieved dynamic control of the LSPRs and their interactions with molecules resonances; and (4) realized prototype plasmonic switches and modulators based on active nanostructures such as artificial molecular machines. Once established, these molecular-level active plasmonic devices could achieve unprecedented performance and become integral components for future, ultrasmall, energy-saving photonic integrated circuits and VLSEPI, benefiting a range of applications, from optical communications to medical diagnosis.

Abstract:Piezoelectrics are an important class of functional materials that provide important properties for energy converting and actuating during motion or deformation. Existing applications, which all involve planar, rigid layouts, include memory and MEMS devices, pyroelectric infrared detectors, and high-frequency ultrasound transducers. However, conventional piezoelectric ceramics such as PZT are brittle and prone to fracture, thereby severely limiting applications to those that involve only very small deformations. As with silicon circuits that have been the subject of our past work, piezoelectric devices do not enable natural integration with the soft, curvilinear surfaces of biological systems. Relaxing this requirement would enable many device opportunities, especially for energy harvesting from bio-motion such as elbow and knee bending. The ultimate goal of the energy harvestor based on stretchable piezoelectrics is the development and integration of a novel piezioelectric (PZT) energy scavenging device to capture human motion energy, such as the motions of elbow and knee during walk, the pulse, heart beat and breath. In this paper, the feasibility study focused on commercially available PZT solutions in order to meet high-quality energy conversion goals. Immediate challenges include (1) developing the innovatively flexible and stretchable device structure for non-planar human body and (2) proving success of novel approach to PZT mechanical energy capture. These two challenges form the main objectives of this paper. Piezoresponse Force Microscope (PFM) is used to characterize the piezoelectricity and ferroelectricy of stretchable PZT ribbons.

If we bring a glass rod electrically charged by rubbing with silk, near a hair, the hair will be attracted to the rod. Here we demonstrate an approach to predict similar electrostatic phenomena occurring in nanoscale. This approach expands the applications of carbon nanotubes and graphene nanoribbons in nanoelectromechanical systems (NEMS), which allow direct conversion from electrical energy to nanoscale mechanical energy. The nanostructures are deflected in response to transverse gate voltages as a consequence of electric polarization. We demonstrate a strong dependence of the electrostatic deformation on both the field strength and the geometry of nano-objects. This field-induced deflection allows the nanostructures to oscillate at a frequency in a gigahertz range. References:[1] Z. Wang and L. Philippe, Phys. Rev. Lett. 102, (2009)215501 .[2] Z.Wang, Phys. Rev. B 79 (2009)155407.[3] Z.Wang and M. Devel, Phys. Rev. B 76 (2007)195434.

There has been much interest recently directed to in-situ measurements during nanomechanical testing. Measurement of the electrical properties during nanoindentation are one such parameter that can be monitored alongside the mechanical response of the system. Such in-situ electrical measurements have been shown to provide great insight into the mechanical behavior of a range of materials and nanostructures including nanoindentation-induced phase transformations in silicon. However, quantitative electrical measurements have not been possible to date as a result of issues concerning the doping uniformity and electrical contact area of boron-doped diamond nanoindentation tips used in such experiments. Thus, to fully realize the potential in-situ capabilities of such an electrical system, detailed a-priori knowledge of the boron-doped diamond tips, is vital. We have studied a range of diamond tips used in a Hysitron Triboindenter with the aim to understand the electrical variability observed between tips and possible changes in tips with use. This provides the basis to extend the instrument’s current capabilities by providing quantitative in-situ measurements. A combination of cathodoluminescence, scanning electron microscopy and micro-secondary ion mass spectrometry have been used to characterize the different tips. An inhomogeneous boron doping profile is observed for most tips with boron-rich and boron-poor regions leading to variability in conductivity over the surface of the tips. The electrically resistive areas contain boron in the form of clusters instead of electrically active substitutional boron. In addition, higher concentrations of boron lead to destabilization of the structural integrity of the tip. Finally, by the use of material standards we have correlated the tip characterization with data measured during indentation tests and have extracted an electrical contact area function similar to the typical mechanical area function.

In response to a stimulus, a soft material deforms, and the deformation provides a function. We call such a material a soft active material (SAM). This talk focuses on one class of soft active materials: dielectric elastomers. Subject to a voltage, a membrane of a dielectric elastomer reduces thickness and expands area, possibly straining over 100%. The phenomenon is being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. This talk reviews the theory of dielectric elastomers, coupling large deformation and electric potential. The theory is developed within the framework of continuum mechanics and thermodynamics. The theory attempts to answer commonly asked questions. How do mechanics and electrostatics work together to generate large deformation? How efficiently can a material convert energy from one form to another? How do molecular processes affect macroscopic behavior? The theory is used to describe nonlinear and nonequilibrium behavior, such as electromechanical instability and viscoelasticity. It is hoped that the theory will aid the creation of soft active materials and soft machines. This talk is based on a recent article: Zhigang Suo, Theory of dielectric elastomers, http://www.seas.harvard.edu/suo/papers/243.pdf.

With precise, rigid synthetic molecules and assemblies, we seek to understand and to mimic nature’s complex molecular machines. We use these synthetic molecular machines capable of performing mechanical motion in response to external stimuli to understand the limits of controlled and cooperative motion at the nanoscale. We use molecular design, tailored syntheses, and intermolecular interactions to create functional and interacting nanostructures on solid substrates. We explore and exploit adsorbate-adsorbate and substrate-adsorbate interactions to fabricate precise nanostructures, patterns, and assemblies on surfaces. Using custom-built scanning tunneling microscopes (STMs), we measure the motion of molecules driven by light, electric field, or electrochemical potential. We vary the molecular design, environmental conditions, and other aspects to understand and to optimize the function and motion at molecular scale. In order to understand the concerted operation of the functional molecules, we assemble the functional molecules into linear one-dimensional arrays or two dimensional clusters. We discuss families of three artificial molecular machines that utilize light, electric field, or electrochemical potential to perform mechanical motion. We drive reversible photo- and STM probe tip-induced switching of single or bundles of azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111}. We employ tethers that decouple the functional moiety from the conductive substrate thereby eliminating quenching of surface states. We assemble precisely defined 1D linear chain structures of azobenzene-functionalized molecules using self- and directed-assembly. We control in situ photo- and electron-induced switching of these linear chains and study the effects of local environments and coupling on switching efficiency. Oligo(phenylene ethynylene) molecules assembled on Au{111} are shown to undergo stochastic and driven conductivity switching in the highly localized electric field of the STM tunneling junction. We establish that the contacts between the substrate and the molecule play a critical role in switching. Switching of the rigid, conjugated molecules is due to changes in the molecule-substrate bonds, which involves motion of the molecules and of substrate atoms. Furthermore, by controlling the concerted nanoscale motion of billions of palindromic bistable rotaxane molecules bound to gold film, we demonstrate that the cumulative nanoscale motion of the artificial muscle molecules can be harnessed to induce motion at a macroscale.

Subject to a voltage, a layer of a dielectric polymer reduces thickness and expands area, so the same voltage will induce an even higher electric field. The positive feedback may cause the polymer to thin down drastically, resulting in an electrical breakdown. This electromechanical instability has long been recognized in the electrical power industry as a failure mode for polymer insulators. However, since the electromechanical instability is generally followed by an electrical breakdown of the polymer, it is very difficult to directly observe its initiation and propagation. Here we report a simple method to observe whole evolution process of the electromechanical instability in polymer films with thickness ranging from millimeters to nanometers in real time. Experimental results indicate that the instability involve transitions of the polymer between homogeneous and multiple inhomogeneous deformation states. The experimental results are consistent with our theoretical predictions. Further, we show that the instability can be harnessed with promising applications in many areas including high-breakdown-field organic capacitors, electrostatic lithography, dynamic pattern formations, and fabrication of semi-permeable membranes.

An ionic polymer metal composites (IPMC) consists of an ionic gel plated between two metal electrodes. The IPMC is soft and can work in aqueous environment with low voltage input, offering new possibilities for soft sensors and actuators. Typical IPMC actuators have a large length-to-height ratio, exhibiting large deformation during bending and relaxation processes. A multiphysics modeling of IPMC actuator was carried out, incorporating the electrokinetics, electrostatics and nonlinear large deformation of the actuators. The vibration of a self-oscillating bending actuator and a micropump with IPMC were simulated successfully by using the mulitiphysics model.

We study both by means of Molecular Dynamics (MD) simulations and theoretically the response of a single polyelectrolyte chain, grafted on a solid interface, to electrostatic field. An external force, f, is applied to the free end of the chain, which counteracts its adsorption on the oppositely charged interface. We analyze the dependence of the size of the bulk and adsorbed parts of the chain on applied force and electric field. The theoretical predictions are found to agree well with simulation results. In addition to constant load, f=const., we performed MD simulations with a few types of load among them linear spring and Hertzian force. It is demonstrated that the electric field generates a mechanical force that is determined by the degree of chain contraction. While it is rather difficult to develop an analytical theory for the case of non-constant load, a simple mechanical approach provides a reasonable semi-quantitative description with f=NqeE, where N is the number of non-adsorbed chain segments (bulk part), q is the valency of monomers, E is the applied electric field and e the elementary charge. There is a variety of possible applications, in particular in nano-technology, where it may be important to generate a force by an applied electric field. One can also consider the polymer contraction regulated by electric field as a prototype of an artificial muscle.