R. E. Walkup

Atomic scale desorption through electronic and vibrational excitation mechanisms

The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Carbon nanotubes : nanomechanics, manipulation, and electronic devices

Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, atomic force microscopy (AFM) and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate–nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography.

Breaking individual chemical bonds via STM-induced excitations

We present experimental and theoretical results on the STM-induced SiH bond-breaking on the Si(100)-(2 × 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that SiH bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the σ→σ∗ transition of SiH. The desorption yield is ∼2.4 × 10−6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ∼1015 s−1, competes with desorption. Most of the desorbing atoms originate from the “hot” ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiple-vibration excitation of the SiH bond. Such excitation becomes possible because of the very high current densities in the STM, and the long SiH stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process — the ultimate lithographic resolution.

Laser‐induced fluorescence studies of excimer laser ablation of Al2O3

We have used laser‐induced fluorescence to measure the energy distributions of Al atoms and AlO molecules produced by excimer laser ablation of Al2O3. Excimer laser fluences close to the threshold for ablation were used to minimize the effects of gas phase collisions. The kinetic energies of both species were high, ∼4 eV for Al and ∼1 eV for AlO, but the AlO rotational and vibrational energies were quite low, corresponding to a temperature of ∼600 K. These results rule out thermal vaporization and provide indirect support for an electronic ablation mechanism.

STM-induced H atom desorption from Si(100): isotope effects and site selectivity

Abstract We investigate the scanning tunnelling microscopy-induced H and D atom desorption from Si(100)-(2 × 1):H(D). The desorption of both atoms shows the same energy threshold that corresponds well with the computed σ → σ ∗ excitation energy of the SiH group. The H desorption yield, however, is much higher than the D yield. We ascribe this to the greater influence of quenching processes on the excited state of the SiD species. We use wavepacket dynamics to follow the motion of H and D atoms, and conclude that desorption occurs, for the most part, from the ‘hot’ ground state populated by the quenching process. Site-selective excitation-induced chemistry is found in the desorption of H from Si(100)-(3 × 1):H.

Dissociation of individual molecules with electrons from the tip of a scanning tunneling microscope.

The scanning tunneling microscope (STM) can be used to select a particular adsorbed molecule, probe its electronic structure, dissociate the molecule by using electrons from the STM tip, and then examine the dissociation products. These capabilities are demonstrated for decaborane(14) (B10H14) molecules adsorbed on a silicon(111)-(7 x 7) surface. In addition to basic studies, such selective dissociation processes can be used in a variety of applications to control surface chemistry on the molecular scale.

In situ measurements of SiO(g) production during dry oxidation of crystalline silicon

We report in situ measurements of SiO(g) evolution during the oxidation of silicon by O2 for a range of experimental conditions including the transition from active to passive oxidation. The results show that this transition occurs when the SiO(g) partial pressure reaches the equilibrium vapor pressure for the reaction Si(s)+SiO(s)⇄2SiO(g). During the growth of a SiO2 film, there is no significant transport of SiO molecules into the gas phase.

Studies of excimer laser ablation of solids using a Michelson interferometer

A Michelson interferometer has been used as a direct quantitative probe for gas phase plasma formation in the UV excimer laser ablation of solids. Excimer laser fluence thresholds for plasma formation are determined and correlated with optical emission from electronically excited ablation fragments.

Real space imaging of electron scattering phenomena at metal surfaces

Real space studies of the interaction of the two‐dimensional electron gas provided by metal surface states with localized scatterers are presented. The results involve electron scattering by steps and point defects (adsorbates) at Au(111) and Ag(111) surfaces. These scattering events lead, through interference, to an oscillatory local density of states (LDOS), which is imaged in maps of (dI/dV)/(I/V). Analysis of the LDOS oscillations provides insights into the scattering phenomena involved. We show that the decay of the amplitude of the oscillations as a function of distance from the scatterer can be accounted for by a model that describes the loss of coherence as a result of the wave number (k∥) spread of the states probed by the STM. This model also explains the energy dependence of the amplitude of the oscillations and provides a basis for comparing results from different metal surfaces. Analysis of the properties of the oscillations shows that at low k∥, steps act very much like hard walls isolating ...

Laser-induced fluorescence study of laser sputtering of graphite

Abstract Pulsed laser irradiation of graphite surfaces has been known for some time to lead to the ejection of C, C2, and C3 neutrals as well as related ions. Since most relevant thermodynamic quantities are known, graphite represents an ideal system for further study. We report on the sputtering of pyrolytic, polycrystalline, and vitreous graphite by 20 ns pulses of laser light at 351 nm. The threshold energy density for sputtering is found to be 0.5 to 0.6 J/cm2. At this fluence, the material removal rate is of the order of a monolayer/pulse. This is consistent with pulsed evaporation provided that the surface reaches a peak temperature of ~ 4000 K. The emitted particles are probed using laser-induced fluorescence (LIF). Kinetic (i.e. translational) energies are obtained by time-of-flight and correspond, for the lowest fluences, to ~ 4600 K. Rotational and vibrational distributions are obtained by analysis of the LIF spectra for the D1Σu+X1Σg+ Mulliken bands of the dimer, C2. Detailed analysis indicates a rotational temperature of 4100 ± 300 K and a vibrational temperature of 3650 ± 350 K. Since the temperatures are all similar at the lowest it is concluded that the laser sputtering of graphite involves thermally activated vaporization, i.e. is what is normally termed “thermal sputtering”. At higher fluences, the time-of-flight information appears to be significantly perturbed by Knudsen layer formation.