R. Reifenberger

Top-gated graphene field-effect-transistors formed by decomposition of SiC

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400cm2∕Vs, respectively, which are much larger than the corresponding values from conventional SiC or silicon.

Writing nanometer‐scale symbols in gold using the scanning tunneling microscope

The conditions required to electroetch nanometer‐sized craters in flat gold substrates with a scanning tunneling microscope operating in air are identified. Reproducible nanometer‐scale modifications of the substrate are possible. Letters and complex symbols with linewidths as small as 2 nm have been written. Experiments show that a good tunneling tip is not destroyed by the writing process.

Enhanced mass sensing using torsional and lateral resonances in microcantilevers

We present a method to detect, with enhanced sensitivity, a target mass particle attached eccentrically to a microcantilever by measuring multiple three-dimensional modes in the microcantilever vibration spectrum. Peaks in the spectrum reveal a complex coupling between the bending, torsional, and lateral motions and detailed finite element models assist in their interpretation. The mass sensitivities of the torsional and lateral mode frequencies are an order of magnitude greater, and their Q factors significantly higher, than that of the conventionally used fundamental bending mode. These modes offer significantly enhanced mass sensing capabilities within the realm of existing microcantilever technology.

Nonlinear dynamic perspectives on dynamic force microscopy.

Dynamic force microscopy (DFM) utilizes the dynamic response of a resonating probe tip as it approaches and retracts from a sample to measure the topography and material properties of a nanostructure. We present recent results based on nonlinear dynamical systems theory, computational continuation techniques and detailed experiments that yield new perspectives and insights into DFM.A dynamic model including van der Waals and Derjaguin-Müller-Toporov contact forces demonstrates that periodic solutions can be represented as a catastrophe surface with respect to the approach distance and excitation frequency. Turning points on the surface lead to hysteretic amplitude jumps as the tip nears/retracts from the sample. New light is cast upon sudden global changes that occur in the interaction potential at certain gap widths that cause the tip to stick to, or tap irregularly the sample. Experiments are performed using a tapping mode tip on a graphite sample to verify the predictions.

Nonlinear tapping dynamics of multi-walled carbon nanotube tipped atomic force microcantilevers

The nonlinear dynamics of an atomic force microcantilever (AFM) with an attached multi-walled carbon nanotube (MWCNT) tip is investigated experimentally and theoretically. We present the experimental nonlinear frequency response of a MWCNT tipped microcantilever in the tapping mode. Several unusual features in the response distinguish it from those traditionally observed for conventional tips. The MWCNT tipped AFM probe is apparently immune to conventional imaging instabilities related to the coexistence of attractive and repulsive tapping regimes. A theoretical interaction model for the system using an Euler elastica MWCNT model is developed and found to predict several unusual features of the measured nonlinear response.

Correlating the location of structural defects with the electrical failure of multiwalled carbon nanotubes

Electrical failure of carbon nanotubes was investigated by obtaining I(V) data with a voltage ramp from a rope of multiwalled carbon nanotubes. Noncontact scanning force microscope images were obtained before and after each I(V) curve until electrical failure of the tube resulted. Following this procedure, it was possible to correlate a defect on the surface of a nanotube with the exact location of the tube failure.

Shape of nanometer-size supported gold clusters studied by scanning tunneling microscopy

Abstract The shape of preformed spherical Au clusters with radii varying from 1 to 6 nm has been studied with the scanning tunneling microscope after deposition onto flat Au substrates. The supported clusters are found to resemble spherical caps with radii of curvature greater than and not strongly correlated with the original free-space radii. Measurements revealed that ∼ 80% of the clusters studied have a radius of curvature lying between 10 and 30 nm. A continuum model to interpret this result indicates that clusters with radii less than a critical value, characteristic of the cluster material, are stressed beyond their elastic limit and can deform so as to decrease their surface free energy even at temperatures well below their melting point.

Strain energy and lateral friction force distributions of carbon nanotubes manipulated into shapes by atomic force microscopy

The interplay between local mechanical strain energy and lateral frictional forces determines the shape of carbon nanotubes on substrates. In turn, because of its nanometer-size diameter, the shape of a carbon nanotube strongly influences its local electronic, chemical, and mechanical properties. Few, if any, methods exist for resolving the strain energy and static frictional forces along the length of a deformed nanotube supported on a substrate. We present a method using nonlinear elastic rod theory in which we compute the flexural strain energy and static frictional forces along the length of single walled carbon nanotubes (SWCNTs) manipulated into various shapes on a clean SiO(2) substrate. Using only high resolution atomic force microscopy images of curved single walled nanotubes, we estimate flexural strain energy distributions on the order of attojoules per nanometer and the static frictional forces between a SWCNT and SiO(2) surface to be a minimum of 230 pN nm(-1).

A field emission technique to measure the melting temperature of individual nanometer-sized clusters

Abstract The utility of the field emission microscope in studying individual nanometer-sized clusters is demonstrated. Preformed Au clusters of controlled size are deposited on a W(110) field emitter. The field emission characteristics of the supported clusters are studied after each of a series of heating cycles in which the temperature of the W tip is raised from room temperature to a predetermined high temperature and then cooled again to room temperature. An abrupt change in the current-voltage characteristics from a supported cluster is found to occur at a definite transition temperature. This phenomenon, which is accompanied by a change in the field emission pattern, is interpreted as a melting transition that results in the cluster spreading on the substrate.

Direct imaging of 13‐Å‐diam Au clusters using scanning tunneling microscopy

Controlled size clusters of Au with a diameter of 13 A, prepared using a multiple expansion cluster source, have been supported on highly oriented pyrolytic graphite and observed with a scanning tunneling microscope. A reliable constant‐current signature for a metallic cluster supported on a graphite substrate is identified. Images of the supported Au clusters are found to exhibit a narrow size distribution and a diameter which is in close agreement with the diameter predicted from conditions in the multiple expansion cluster source.