Yoichi Miyahara

Cantilever-based sensing: the origin of surface stress and optimization strategies

Many interactions drive the adsorption of molecules on surfaces, all of which can result in a measurable change in surface stress. This article compares the contributions of various possible interactions to the overall induced surface stress for cantilever-based sensing applications. The surface stress resulting from adsorption-induced changes in the electronic density of the underlying surface is up to 2-4 orders of magnitude larger than that resulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal that the surface stress associated with the formation of high quality alkanethiol self-assembled monolayers on gold surfaces is independent of the molecular chain length, supporting our theoretical findings. This provides a foundation for the development of new strategies for increasing the sensitivity of cantilever-based sensors for various applications.

Strong electromechanical coupling of an atomic force microscope cantilever to a quantum dot.

We present theoretical and experimental results on the mechanical damping of an atomic force microscope cantilever strongly coupled to a self-assembled InAs quantum dot. When the cantilever oscillation amplitude is large, its motion dominates the charge dynamics of the dot which in turn leads to nonlinear, amplitude-dependent damping of the cantilever. We observe highly asymmetric lineshapes of Coulomb blockade peaks in the damping that reflect the degeneracy of energy levels on the dot, in excellent agreement with our strong coupling theory. Furthermore, we predict that excited state spectroscopy is possible by studying the damping versus oscillation amplitude, in analogy to varying the amplitude of an ac gate voltage.

Energy levels of few-electron quantum dots imaged and characterized by atomic force microscopy

Strong confinement of charges in few-electron systems such as in atoms, molecules, and quantum dots leads to a spectrum of discrete energy levels often shared by several degenerate states. Because the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum results from a change in cantilever resonance frequency and dissipation when an electron tunnels on/off a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the quantitative measurement of predicted temperature-dependent shifts of Coulomb blockade peaks. Scanning the surface shows that several quantum dots may reside on what topographically appears to be just one. Relative coupling strengths can be estimated from these images of grouped coupled dots.

Determination of the local contact potential difference of PTCDA on NaCl: a comparison of techniques

There has been increasing focus on the use of Kelvin probe force microscopy (KPFM) for the determination of local electronic structure in recent years, especially in systems where other methods, such as scanning tunnelling microscopy/spectroscopy, may be intractable. We have examined three methods for determining the local apparent contact potential difference (CPD): frequency modulation KPFM (FM-KPFM), amplitude modulation KPFM (AM-KPFM), and frequency shift-bias spectroscopy, on a test system of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on NaCl, an example of an organic semiconductor on a bulk insulating substrate. We will discuss the influence of the bias modulation on the apparent CPD measurement by FM-KPFM compared to the DC-bias spectroscopy method, and provide a comparison of AM-KPFM, AM-slope detection KPFM and FM-KPFM imaging resolution and accuracy. We will also discuss the distance dependence of the CPD as measured by FM-KPFM for both the PTCDA organic deposit and the NaCl substrate.

Implementation of atomically defined field ion microscopy tips in scanning probe microscopy

The field ion microscope (FIM) can be used to characterize the atomic configuration of the apices of sharp tips. These tips are well suited for scanning probe microscope (SPM) use since they predetermine the SPM resolution and the electronic structure for spectroscopy. A protocol is proposed for preserving the atomic structure of the tip apex from etching due to gas impurities during the period of transfer from the FIM to the SPM, and estimations are made regarding the time limitations of such an experiment due to contamination with ultra-high vacuum rest gases. While avoiding any current setpoint overshoot to preserve the tip integrity, we present results from approaches of atomically defined tungsten tips to the tunneling regime with Au(111), HOPG (highly oriented pyrolytic graphite) and Si(111) surfaces at room temperature. We conclude from these experiments that adatom mobility and physisorbed gas on the sample surface limit the choice of surfaces for which the tip integrity is preserved in tunneling experiments at room temperature. The atomic structure of FIM tip apices is unchanged only after tunneling to the highly reactive Si(111) surface.

Room-Temperature Single-Electron Charging Detected by Electrostatic Force Microscopy

We use atomic force microscopy to measure electron addition spectra of individual Au nanoparticles that exhibit Coulomb blockade at room temperature. The cantilever tip charges individual nanoparticles supported on an ultra-thin NaCl film via single-electron tunneling from the metal back electrode. The tunneling is detected by measuring frequency shift and damping of the oscillating cantilever. Finite element electrostatic calculations indicate that the total nanoparticle capacitance is dominated by mutual capacitance to the back electrode.

Conductivity of an atomically defined metallic interface

A mechanically formed electrical nanocontact between gold and tungsten is a prototypical junction between metals with dissimilar electronic structure. Through atomically characterized nanoindentation experiments and first-principles quantum transport calculations, we find that the ballistic conduction across this intermetallic interface is drastically reduced because of the fundamental mismatch between s wave-like modes of electron conduction in the gold and d wave-like modes in the tungsten. The mechanical formation of the junction introduces defects and disorder, which act as an additional source of conduction losses and increase junction resistance by up to an order of magnitude. These findings apply to nanoelectronics and semiconductor device design. The technique that we use is very broadly applicable to molecular electronics, nanoscale contact mechanics, and scanning tunneling microscopy.

Excited-state spectroscopy on an individual quantum dot using atomic force microscopy.

We present a new charge sensing technique for the excited-state spectroscopy of individual quantum dots, which requires no patterned electrodes. An oscillating atomic force microscope cantilever is used as a movable charge sensor as well as gate to measure the single-electron tunneling between an individual self-assembled InAs quantum dot and back electrode. A set of cantilever dissipation versus bias voltage curves measured at different cantilever oscillation amplitudes forms a diagram analogous to the Coulomb diamond usually measured with transport measurements. The excited-state levels as well as the electron addition spectrum can be obtained from the diagram. In addition, a signature which can result from inelastic tunneling by phonon emission or a peak in the density of states of the electrode is also observed, which demonstrates the versatility of the technique.

High-aspect ratio metal tips attached to atomic force microscopy cantilevers with controlled angle, length, and radius for electrostatic force microscopy

We demonstrate a method to fabricate a high-aspect ratio metal tip attached to microfabricated cantilevers with controlled angle, length, and radius, for use in electrostatic force microscopy. A metal wire, after gluing it into a guiding slot that is cut into the cantilever, is shaped into a long, thin tip using a focused ion beam. The high-aspect ratio results in considerable reduction of the capacitive force between tip body and sample when compared to a metal coated pyramidal tip.

Revealing Energy Level Structure of Individual Quantum Dots by Tunneling Rate Measured by Single-Electron Sensitive Electrostatic Force Spectroscopy

We present theoretical and experimental studies of the effect of the density of states of a quantum dot (QD) on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitavely coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. We present experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K. In contrast, our analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.