- Bicrystal Boundary
- Controlling Interface Properties for Advanced Energy Applications
- Size-Dependent Interface Properties
- Porphyrin-Metal Interface Properties
- Electronic Transport in Porphyrin Supermolecule-Gold Nanoparticle Assemblies
Oxide bicrystals provide a convenient object for local SPM studies. In many cases, quantitative information on grain boundary properties can be obtained. Imaging can be performed on static grain boundary to image space charge layer at the interface. Imaging of the laterally dc biased interface allows current-voltage characteristic to be reconstructed. Finally, using recently developed Scanning Impedance Microscopy, ac transport properties can be quantified.
Static grain boundary
Transport properties of ceramic materials are strongly influenced by grain boundary structure and topology. Dopant or vacancy segregation as well as intrinsic interface states result in the interface charge compensated by the band bending and formation of adjacent depletion regions. Such structures referred to as Double Schottky Barrier (DSB) give rise to non linear I-V (varistor) behavior and constitute the basis of numerous industrial applications. Properties of grain boundaries vary strongly depending on relative crystallographic orientation of the adjacent grains, presence of second phase wetting layers, etc. Scanning probe microscopy based techniques are able to detect the stray fields above DSB structure and thus be used to identify grain boundary structure and properties. Shown below are crystallographic structure of 2 simple grain boundaries in SrTiO3. Note that surface potential contrast is different for different grain boundaries. We have shown that the presence of mobile surface charges alters experimental contrast; nevertheless, position and type of grain boundary in some cases can be determined.
Biased grain boundary
Shown below is surface topography and surface potential on grounded, forward and reverse biased SrTiO3 bicrystal surface. Surface topography is essentially flat. Grain boundary is associated with a number of pores that renders its detection in transmission optical microscope possible. Surface potential of grounded surface exhibits a small feature associated with screened double Schottky barrier at the grain boundary. Application of lateral bias shows that grain boundary resistivity is significantly less than that of the bulk.
Extremely simple equivalent circuit in this measurements allows to reconstruct the non-linear transport characteristic of grain boundary fro SSPM measurements. Shown below is potential drop at the grain boundary as a function of external bias and equivalent dc circuit for the grain boundary. From the mathematical analysis of the data we estimate the nonlinearity coefficient as ~3 and ohmic part of grain boundary conductivity as ~250 Ohm.
Internal interfaces in materials play an important role in the performance of many devices used in energy applications including solar cells, LEDs, passive electronics, and fuel cells. Efficiencies in energy and power consumption may be realized by optimizing and often miniaturizing these devices. Our studies show that internal boundaries and biomaterial interfaces cause local property variations. These effects will dominate device performance as the systems become smaller. A fundamental understanding of the effect of atomic structure on local properties is a prerequisite to device optimization. Developing this understanding requires new probes that access local properties, controlled interface structure, atomic resolution electron microscopy and first principles calculations of geometric and electronic structure.
New Probes of Local Properties
The two new multiple modulation techniques, developed at Penn, have been shown to successfully overcome barriers to quantifying local electrical behavior.
Scanning Impedance Microscopy SIM
The modulating electric signal is applied laterally across the surface rather than to the tip, allowing R, C, and trap state time constants to be accessed.
Nanoimpedance Microscopy NIM
Monitoring the frequency dependence over 6 orders of magnitude yields the real and imaginary contributions to impedance.
Structure and Properties of Internal Interfaces
The charge at SrTiO3 grain boundaries is due to periodic under coordinated Ti in the boundary so the amount at any boundary depends on the atomic structure, i.e. there is one electron associated with each Ti polyhedron. This interface charge causes local dielectric constant suppression adjacent to the interface. In addition the charge induces ferroelectric dipole formation and alignment at a mid temperature phase transition in SrTiO3.
Extension to Interfaces of Nanocontacts
Characterizing the properties of a metal-semiconductor nano-contact is crucial to the emergence of nano-electronic devices. We have shown that contact potential can be size dependent. Discovering the mode of conduction through the contact and the effects of surface states on the interface barrier are two of the most important aspects of understanding the nature of the contact. By combining SIM with STM and spectroscopy, the fundamental properties governing the interaction of metals and semiconductors can be determined.
The electronic properties of metal-ceramic interfaces are found to vary with the size of the interface in the range of 3-100 nm. Interface properties can be extracted from variations in apparent heights of metal clusters, as measured by scanning tunneling microscopy, of small clusters on atomically smooth oxide surfaces. Relations that describe the size dependence in terms of parameters related to interfacial bonding are developed. The effect of the size dependent interface properties on metal cluster stability are explored by modeling the effect of the parameters that control contact potential and comparing the size dependence with that of the other contributions to the total energy.
Click the images to enlarge:
Deposition of a nominal monolayer of TET – H2 – TET porphyrin on HOPG results in islands with 2 step heights: 0.6nm and 1.55 nm, Fig. a. Layer thicknesses imply that in the islands with ~ 0.7 nm height the plane of the porphyrin ring is perpendicular to the substrate, while in the islands with ~ 1.6 nm height the ring is parallel to the substrate.
Both layers (A and B) assemble into laterally ordered structures. Fig. d and e compare molecular resolution nc-AFM contrast of both layers. Within the layer A the porphyrin ring is oriented perpendicular to the substrate, superposition of the molecular structure oriented to match the lattice dimensions and the topographic contrast is shown in d. The structure on island B is determined in similar fashion. Based on the lattice parameters of the monolayer and the fact that the porphyrin ring is parallel to the substrate from c, the arrangement of porphyrin molecules in e is proposed. The phenyl rings are situated at the highest contrast since the alkane chains extend above them.
In order to relate these structures to interface properties, KFM was performed consecutively to nc-AFM, Fig. b. Variations in surface potential are correlated with the locations of the porphyrin monolayers. The difference between the potential of step A and the graphite substrate was below the energy resolution, i.e. <5 mV. The potential on island A, Vstep A = 0.64 V, and island B, Vstep B = 0.59 V, differs by 50mV.
The difference in work function with orientation reflects a difference in the coupling between the molecule and the substrate. A graphite surface presents a satisfied p-orbital which is not chemically reactive. The fact that the porphyrin oriented perpendicular to the substrate does not alter the work function implies the absence of reaction with the surface. In this case self assembly would be dominated by van der Walls interactions. The decrease in work function that occurs when the molecule is oriented parallel is indicative of a substrate-molecule interaction. The orientation of the p-systems in the graphite and molecule in this configuration might be amenable to overlap, but the distance above the surface (0.8 nm) makes direct interaction unlikely. A time varying induced dipole across this separation would result in a work function difference.
We examine the transport properties of unique supermolecule/nanoparticle assemblies and take a different approach to the transport analysis, though based on familiar fundamental principles. We experimentally investigate the electronic properties of random arrays of two-dimensional gold nanoparticles (AuNPs) consisting of metal junctions linked by optically active dithiol-PZnn supermolecules. The conductance of the assemblies was determined as a function of bias voltage, particle size, particle distribution, and the dithiol-PZnn supermolecule. Using normalized differential conduction analysis, we find that the mechanism is thermally assisted tunneling (TAT), where the response is independent of the particle size and distribution.
Fig. a shows a general device configuration, in which Au NPs disordered bimodal array is deposited on the substrate, interconnected with dithiol-PZnn linkers. The temperature dependence of conductance of different samples were shown in Fig. b and fit well to an Arrhenius model or a variable-range hopping model. Fig. c compares the apparent activation energies obtained from Arrhenius analysis.
Differential analysis of transport of functionalized AuNPs shows temperature dependence of d ln(I)/dV for a variety of AuNP arrays and supermolecule linkers and led us to propose that thermally assisted tunneling is the mechanism controlling transport. This transport process is illustrated in Figure d-g, which shows an idealized band diagram as a function of molecular length. The energy at which the majority of tunneling occurs is above the Fermi energy (at temperatures above 0 K) but below the LUMO and, therefore, likely associated with an energetic metal or molecular state, as expected for a thermally assisted tunneling mechanism.