Andrew J. Stollenwerk

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Ostwald ripening of manganese silicide islands on Si(001)

The deposition of Mn onto Si(001) in the submonolayer regime has been studied with scanning tunneling microscopy to gain insight into the bonding and energetics of Mn with Si. The as-deposited Mn films at room temperature are unstructured. Upon annealing to 300–700u2009°C three-dimensional islands of Mn or MnxSiy form while between the islands the Si(001)-(2×1) reconstruction becomes visible. With increasing annealing time the density of islands per surface area decreases while the average height of the remaining islands increases. The large islands grow in size at the expense of the small ones, which can be understood in the context of Ostwald [Z. Phys. Chem. 34, 495 (1900)] ripening theory. The average island height shows a time dependence of H∼t1∕4, indicating that surface diffusion is the growth limiting process.

Ballistic electron transport properties of Fe-based films on Si(001)

Thickness dependent ballistic electron emission microscopy (BEEM) studies have been performed on Au∕Fe81C19∕Si(001) and Au∕Si(001) Schottky diodes at 80K. The Schottky height was measured to be 0.70±0.02eV for the Fe81C19∕Si(001) interface. Electron attenuation lengths were extracted from the slope of the semilog BEEM current versus the thickness of the Fe81C19 layers for electron energies ranging from 1.0to1.5eV. In this range the attenuation length was found to decrease with increasing energy from 4.1±0.9to2.5±0.6nm, which indicates that some electron-electron scattering is occurring in the metal overlayer. This decrease is slightly greater than predicted for a free electron gas system, resulting from the complex structure of the Fe81C19 film.

Hot electron transport across manganese silicide layers on the Si(001) surface

Ballistic electron emission microscopy (BEEM) has been performed on MnSi∕Si(001) Schottky diodes at 80K to study the hot electron transport properties. The BEEM spectra best fit the thermally broadening 5∕2 power law model with two threshold heights at 0.71 and 0.86eV, indicating a complex interface band structure. In addition, the normalized BEEM current in the MnSi overlayer was found to be approximately seven times less than is observed in Au∕Si(001) samples of similar thicknesses, indicating a larger amount of hot electron scattering in the MnSi∕Si(001) samples.

Measurement of the clustering energy for manganese silicide islands on Si(001) by ostwald ripening

The rate of growth during annealing of manganese silicide islands in the submonolayer coverage regime on the Si(001) surface has been measured by scanning tunneling microscopy. The fourth power of the growth rate is linearly dependent upon the annealing time, consistent with a diffusion limited Ostwald ripening mechanism for island growth. The growth rate has been determined for four different annealing temperatures to extract the activation energy for clustering, which has been found to be 2.6±0.2eV.

Ballistic Electron Transport Properties Across the Manganese/Silicon Interface

Ballistic electron transmission is used to investigate electron transport across the Au/Mn/Si and Au/Si interfaces. The Au/Mn/Si spectra exhibit multiple threshold voltages above the Schottky barrier. The energetic spacing of these threshold voltages is found to vary with Mn thickness. These features are believed to be the result of resonant transport. Transmission calculations match the experimental data exceedingly well, but only when one accounts for reflections at the Au/Mn interface. Interestingly, scattering at the Mn/Si interface is over an order of magnitude less than at the Au/Si interface, suggesting a better matching of available states at the Mn/Si interface.

Room Temperature Formation of Carbon Onions via Ultrasonic Agitation of MoS2 in Isopropanol

Ultrasonic agitation is a proven method for breaking down layered materials such as MoS2 into single or few layer nanoparticles. In this experiment, MoS2 powder is sonicated in isopropanol for an extended period of time in an attempt to create particles of the smallest possible size. As expected, the process yielded a significant quantity of nanoscale MoS2 in the form of finite layer sheets with lateral dimensions as small as a few tens of nanometers. Although no evidence was found to indicate a larger the longer sonication times resulted in a significant increase in yield of single layer MoS2, the increased sonication did result in the formation of several types of carbon allotropes in addition to the sheets of MoS2. These carbon structures appear to originate from the breakdown of the isopropanol and consist of finite layer graphite platelets as well as a large number of multi-walled fullerenes, also known as carbon onions. Both the finite layer graphite and MoS2 nanoplatelets were both found to be heavily decorated with carbon onions. However, isolated clusters of carbon onions could also be found. Our results show that liquid exfoliation of MoS2 is not only useful for forming finite layer MoS2, but also creating carbon onions at room temperature as well.

Universal Method for Creating Optically Active Nanostructures on Layered Materials

The ability to form patterned surface nanostructures has revolutionized the miniaturization of electronics and led to the discovery of emergent behaviors unseen in macroscopic systems. However, the creation of such nanostructures typically requires multiple processing steps, a high level of technical expertise, and highly sophisticated equipment. In this work, we have discovered a simple method to create nanostructures with control size and positioning in a single processing step using a standard scanning electron microscope. The technique can be applied to a wide range of systems and was successful in every layered material tested. Patterned nanostructures were formed on graphite, topological insulators, novel superconductors, and layered transition metal dichalcogenides. The nanostructures were formed via the incorporation of carbon nanoparticles into the samples in a novel form of intercalation. It appears that the electron beam interacts with residual organic molecules available on the sample surface, making it possible for them to intercalate between the layers in their crystal structure and break down into carbon. These carbon nanoparticles have strong broad-wavelength interactions in the visible light range, making these nanostructures easily detectable in an optical microscope and of interest for a range of nanoscale electro-optical devices.

Electronic structure of multi-walled carbon fullerenes.

Despite an enormous amount of research on carbon based nanostructures, relatively little is known about the electronic structure of multi-walled carbon fullerenes, also known as carbon onions. In part, this is due to the very high computational expense involved in estimating electronic structure of large molecules. At the same time, experimentally, the exact crystal structure of the carbon onion is usually unknown, and therefore one relies on qualitative arguments only. In this work we present the results of a computational study on a series of multi-walled fullerenes and compare their electronic structures to experimental data. Experimentally, the carbon onions were fabricated using ultrasonic agitation of isopropanol alcohol and deposited onto the surface of highly ordered pyrolytic graphite using a drop cast method. Scanning tunneling microscopy images indicate that the carbon onions produced using this technique are ellipsoidal with dimensions on the order of 10u2009nm. The majority of differential tunneling spectra acquired on individual carbon onions are similar to that of graphite with the addition of molecular-like peaks, indicating that these particles span the transition between molecules and bulk crystals. A smaller, yet sizable number exhibited a semiconducting gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. These results are compared with the electronic structure of different carbon onion configurations calculated using first-principles. Similar to the experimental results, the majority of these configurations are metallic with a minority behaving as semiconductors. Analysis of the configurations investigated here reveals that each carbon onion exhibiting an energy band gap consisted only of non-metallic fullerene layers, indicating that the interlayer interaction is not significant enough to affect the total density of states in these structures.