Atomic layer deposition (ALD) is a thin-film deposition technique that allows for even growth of a material over three-dimensional surfaces. It is especially useful because the growth occurs atomic layer by atomic layer in a self-limiting chemical reaction. This allows for the growth of ultrathin film with pristine quality and excellent conformity. We are utilizing this technique to fabricate novel supercapacitors by growing thin, high-k dielectric materials over patterened three-dimensional surfaces.
My research interest has been focused on the development of graphene transparent conductors and related electronic as well as optoelectronics devices, such as FET, graphene photodetector. Most recently, we developed transparent conductors of graphene nanohole array (Figure 1) with tunable transmittance and conductrance and plasmonic graphene (Figure 2) in which the surface plasmonic effect has been incorporated.
My current research involves improving carbon nanotube films for infrared detectors by manipulating the way that heat and charge carriers flow through the film. The film is made up of a lattice-like network of nanotubes and we are looking to improve the sensitivity of the detectors by improving the tube-to-tube contact and the film to substrate interface.
Nanopatterned 2D and 3D photonic crystal Fluorine-doped Tin Oxide (FTO) electrodes were created with nanoimprinting lithography. Au particles (~50-200 nm in diameter) formed on the nanopatterned FTO electrodes through evaporation followed by annealing. The morphology and particles size were characterized by SEM. The plasmonic effects were investigated via UV-Vis spectra transmission mode. Localized surface plasmon resonance position depends on the Au particles structure and size irrespective to FTO pattern while transmittance relies on the pattern of FTO, Au particles structure and size. Simulation predicts an enhanced scattering on such a structure.
Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important to optimization of the device performance. Despite being a commonly used TC in photovoltaic devices, fluorine-doped tin oxide (FTO) has limited transmittance in solar spectrum. Improving the light transmittance and light scattering properties of FTO may lead to increased light absorption in the active volume of the photovoltaic device. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on FTO using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO photonic crystal exhibits enhanced light scattering as compared to the unpatterned FTO, which agrees well with simulations based on Finite Difference Time-Domain method. Dye sensitized solar cells fabricated on the patterned FTO exhibited improved performance (fill factor and efficiency), which can be attributed to enhanced incident photon to-current conversion in the range 400 - 650 nm as suggested also by theoretical simulations.
The major focus of my research has been on the fabrication of carbon-based nanomaterials for the application of energy conversion and storage devices. Recently, we have prepared graphene on Cu substrate with various textures using chemical vapor deposition. Doped graphene nanohole arrays were fabricated for flexible transparent conductor, as shown in Figure 1. Also we have developed an oxide-assisted self-assembly method for the growth of triangular graphene grains on cubic texture Cu substrates and the investigation of growth mechanism of graphene, as shown in Figure 2. By engineering substrates, we can prepare graphene film with uniformity, high conductivity and transparency.
Ion beam assisted deposition (IBAD) is widely used on developing textured templates on amorphous substrates in last few years. It should be realized that majority of thin films used for electronical and electrical devices need to be grown epitaxially, which normally requires a single crystal substrate with perfect lattice match and chemical compatibility. This has been a great limitation in practical applications. The IBAD texturing technique provides a novel approach for epitaxy of thin films on nontextured substrates. IBAD-MgO has been successfully used to build texture on long metallic tapes for large scale application of HTS YBCO coated conductors. High quality texture can be built within few minutes on about 10 nm IBAD-MgO deposition. Homo-epitaxial MgO layer and other buffer layers can be used to further improve the epitaxy and surface compatibility with YBCO.
IBAD-MgO on non-metallic flexible Ceraflex. AC loss has been a serious concern of YBCO coated conductor applications since most of HTS electrical devices need to work in a strong AC field. Non-metallic flexible Ceraflex is a possible candidate as substrate to reduce AC loss of YBCO coated conductors. It has very high resistivity so the eddy current loss will be totally removed; it also has excellent compatibility with YBCO since its composition is YSZ. It is flexible but the surface is very rough with Ra~100nm, which is not suitable for IBAD-MgO texturing. We used multi-layer Spin-on-Glass (SOG) coat the original surface and reduce the Ra to about 1nm. Textured IBAD-MgO template has been developed on the SOG smoothened Ceraflex; then highly textured SrTiO3 film has been grown on top. Preliminary YBCO growth experiment on IBAD-MgO textured Ceraflex shows the phase is pure and the Tc is about 88K, optimization is still in progress.
IBAD-MgO on glass. Amorphous buffer layers are normally necessary for IBAD-MgO texturing process. These buffer layers will induce additional loss and decrease the efficiency. We tried to use Ar+ pre-bombarding to activate the substrate surface and obtained highly textured IBAD-MgO template on unbuffered glass. By optimizing the pre-bombarding time, the best texture quality, in-plane ??~6.5o and out-of-plane FWHM~2o, has been obtained.
Correlation of different physical properties at microscopic to nanometer scales is crucial to understanding the basic physics in nano-matters. It should be realized that most existing SPM probes are single-channel and therefore can only image one physical property at a time. This motivated us to develop multi-channel SPM. The first microwave/optical dual-channel microprobe was demonstrated recently in our laboratory for simultaneous mapping of microwave and optical properties. The key element of this probe is the open-ended coaxial resonator terminated by a tapered and metal-coated fiber optic tip. Microwave is emitted/collected at the tip via the metal coating and light is emitted/collected via the core of the fiber. We extended the application of this metal-coated fiber tip to scanning tunneling microscopy (STM). Since the metal-coating (Gold) terminates at the tip apex but not covering the optical aperture, it facilitates the injection of tunneling electron. With this probe, STM and near-field scanning optical microscopy (NSOM) experiments can be carried out. There is no doubt that these multi-channel SPM probes will become versatile tools for material research and nanoscience and we are currently carrying out some exciting applications.
Imaging of different physical properties of a sample can also be realized by combining SPM techniques with macroscopic measurements. For example, we have integrated the microwave microprobe with electrical current-voltage (I-V) measurement. This allows us to excite the sample locally using the focused microwave from the probe and measure the sample voltage response. By recording this voltage response as a function of probe position, we can visualize the current flow pattern in a conductor. We can use this technique to map the current distribution in HTS coated conductors and locate defects that impede the current flow. Moreover, other information about the sample area under investigation can be acquired. A microwave map reflecting the nonuniformity of structure and chemical composition through the HTS layer will reveal location and form of defects. This method may provide a unique room-temperature technique for electrical current mapping in long-length HTS coated conductors.
Nanoscale devices are promising for next generation electronics. Our focus has been on exploring the controllable growth of nanowire arrays and studying their electrical transport properties. We are particularly interested on the boron-based nanowires because of their unique and special characteristics. Single-crystal boron nanowires have been obtained from quenching the quartz tube in which a thermal vapor transfer method is used to deposit nanowires on Au-coated Si substrates. Furthermore, nanoscale Y-junctions and nano chains have been synthesized in our group.
The next step is to find out is whether this light and high-temperature semiconductor can have high electrical conductivity in the nanowire or nanotube form as predicted by theoretical calculations. We are fabricating boron nanotubes and devising microscopic measurement procedures to verify this novel character.
Epitaxy and microstructure are important properties of high temperature superconductors that directly impact their current carrying capabilities. For example, microstructural defects can improve supercurrent carrying capacity by providing normal state regions by which magnetic vortices are �pinned�. Because defects or non-superconducting regions in a type-II superconductor form energetically favorable penetration sites for magnetic vortices, an attractive force between the magnetic field line and the defect results, and the supercurrent impeding motion of vortices is dampened. Therefore, to improve current carrying properties of materials such as YBa2Cu3Ox (YBCO), analysis and understanding of defect properties and mechanisms is crucial.
Since it is desirable that such defects are on the order of a coherence length (?ab ~ 2 nm in YBCO), transmission electron microscopy (TEM) is a valuable technique for characterization of the microstructure. High resolution TEM can characterize crystals to very small dimensions through both imaging and diffraction. TEM characterization of defects that are either intentionally inserted (such as nanoparticles) or those that result from other structural modification (such as substrate-level surface modulations) gives direct evidence of their size and extent through the YBCO film, and gives some understanding of the mechanisms responsible for interesting effects such as the porous films observed by our group.
Another advantage of TEM characterization is that cross-sectional images can be obtained, thus showing the physical properties through the thickness of the film. For example, the image below shows of a cross section of a vicinal YBCO film on a miscut SrTiO3 substrate. Pores can be seen to extend through a significant portion of the film thickness. This illustrates an example of microstructurally modifying YBCO for the enhancement of its critical current.
Coated conductor (see Fig. 4) research has been a central topic in the HTS research during the past few years. The so-called second-generation HTS tapes are expected to have tremendous potentials for various electrical applications. One of the outstanding problems is the dramatic decrease of Jc with increasing HTS coating thickness. This problem must be solved for coated conductors to carry large currents. Our group has taken two approaches�bottom up by growing HTS films of different thickness and top down by thinning them using ion milling�to understand the physics related. We have also developed new scheme of making porous HTS thick films to tailor the current flow. In collaboration with researchers at the Air Force research laboratory and Oak Ridge national laboratory, we have developed a microstructure-engineering scheme using vicinal growth induced strain plus Y2BaCuO5 nanoparticle insertion and achieved uniformly porous-structured YBCO thick films with much improved Jc.
The relatively slowly decreasing Jc with increasing thickness could be attributed to microstructure degradation for films thicker than 0.8 �m. However the major Jc reduction happens in the thin film regime (film thickness less than 0.8~1.0 �m), where the microstructure degradation is not a concern. Our approach to address this issue is to apply the multilayer structure in which a thick YBCO films is evenly chopped into thin films with insulating spacers. The experiment result on a trilayer YBCO/CeO2/YBCO, where a 0.5-�m-thick YBCO layer was divided into two 0.25-�m-thick YBCO layer by a thin insulator layer of CeO2, shows that the Jc increases with the thickness of CeO2. This observation suggests that, with gradually decoupling the fluxons in two adjacent YBCO layers, the thin film pinning efficiency is achieved which is in line with the 2D collective pinning model.
High critical current density (Jc) is the most critical specification for high temperature superconductor coated conductors as required by numerous electric power-related applications. This has motivated an intensive research effort on the effects of microstructure on Jc. By growing YBa2Cu3O7-? (YBCO) films at a small vicinal angle we have recently obtained a highly porous structure in these films accompanied with a significantly enhanced Jc and smaller Jc reduction at larger film thickness. Furthermore, we tailored the porosity by inserting Y2BaCuO5 (211) nanoparticles in vicinal YBCO thick films to alter the strain at the nanometer scale. A nearly doubled pore density was obtained. A further improved Jc as the consequence of the enhanced pore density in these films suggests a direct correlation between microstructure and Jc and projects an even higher Jc in YBCO films with microstructure engineered optimally at a nanometer scale
The discovery of the first high temperature superconuctor (HTS) by Bednoz and Miller in 1986, which won them the Nobel prize, triggered a worldwide effort in HTS research because of the tremendous potential of these materials in applications. Among many other HTSs, Hg-based HTS (HgBa2Can-1CunO2n+2, n=1,2,3,�), or Hg-HTSs, have the highest superconducting transition temperature Tc of 135 K. The highly volatile nature of the Hg-based compounds, however, makes epitaxy of Hg-HTS films the toughest challenge so far in the HTS material research. We have developed several new processes including an alkaline doping assisted process to promote liquid phase formation which accelerates the formation of Hg-HTS phase, and a fast temperature ramping process to bring the processing temperature directly to the window so as to minimize the formation of the impurity phases. Using these new processes, we have demonstrated high-quality Hg-1223 films with Tc>130K and followed with many interesting studies on these films.
Despite the many exciting results on Hg-HTS films that her group achieved early on, two problems hindered further progress: poor run-to-run reproducibility and severe film/substrate reaction. These generic problems associated with epitaxy of volatile compounds in conventional material processing prompted us to invent a non-conventional cation exchange process. This process employs a precursor matrix of similar crystalline structure and chemical composition to the desired material, but without the volatile cations such as Hg. By providing perturbations to the guest cations on the sites near the final sites of the volatile cations, the guest cations can be replaced with volatile cations without collapsing the crystal structure, like an �atomic surgery� over an existing crystal lattice. The microscopic mechanism of the cation exchange has been a focus of our group in recent years and question we would like to answer is how it occurs at microscopic scale, what are the relevant processing parameters, and can it be applied to design a new material using exiting ones.
The increased research interest in the microwave applications of high temperature superconductors (HTS) was brought about by the perceived potential on the marketability of superconducting electronics, especially in the wireless communications industry. Excellent surface morphology and high reproducibility of the HgBa2CaCu2O6+d (Hg-1212) thin films made from cation exchange process has motivated us to develop microwave bandpass filters, which can be operated at above 77 K and therefore are much more cost effective. High quality Hg-1212 three-pole filters have been successfully fabricated. The filters exhibited lower insertion loss even at higher operating temperature, compared with bandpass filters made of YBCO and copper with the same mask. The power handling capability of the filter was characterized by monitoring the third-order intermodulation signals. The power scaling of IM3 products in the Hg-1212 filter with regard to input power was around 2.8:1, well consistent with theoretical predicted scaling: 3:1. The IP3 of the Hg-1212 filter was 51 dBm at 90K, which was comparable to that of YBCO at 77K. This improved performance was attributed mainly to its higher Tc, which makes Hg-1212 a promising alternative material for passive microwave devices.