Paul Finnie

Single-Walled Carbon Nanotube Growth on Silicon Substrates Using Nanoparticle Catalysts

We investigated the growth of carbon nanotubes (CNT) directly on silicon substrates by nanoparticle-catalyst-assisted chemical vapor deposition. Single-walled CNTs were almost selectively obtained when methane was used in combination with Fe2O3 nanoparticles at the growth temperature of around 950°C. In the growth of single-walled CNTs, this temperature is essential in order to avoid silicidation of the catalyst. The growth direction was parallel to the substrate surface, which is useful for device applications. For Fe-metal nanoparticles, grown nanotubes always contained thick multiwalled CNTs. The selective single-walled CNT growth for Fe2O3 nanoparticles is attributed to the fact that the particles remain small even after the chemical vapor deposition (CVD) process.

Quantum dot infrared photodetectors

Abstract We discuss key issues related to quantum dot infrared photodetectors. These are the normal incidence response, the dark current, and the responsivity and detectivity. It is argued that the present devices have not fully demonstrated the potential advantages. The dominant infrared response in devices so far is polarized in the growth direction. The observed dark currents are several orders of magnitude higher than those for quantum well photodetectors; while ideally they should be lower. The areas that need improvements are pointed out.

Excited excitonic states in single-walled carbon nanotubes.

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.

Aligned island formation using step-band networks on Si(111)

We have achieved control of island formation using a patterned Si(111) surface with a periodic array of atomic-step bands and holes. Liquid metals, Au–Si or Ga, migrate on the patterned surface by annealing and form an island at a particular position in each pattern unit. The islands show highly uniform positions and narrow size distributions. To obtain such good uniformity, the diffusion length of surface atoms should be comparable with the pattern period. High mobility on step bands is also a necessary factor. Periodic arrays of Au islands are used as seeds for selective growth using a vapor-liquid-solid reaction.

InAs/GaAs(100) self-assembled quantum dots: arsenic pressure and capping effects

We explore growth effects leading to size and compositional limitations in the production of self-assembled quantum dots (QD) emitting at long wavelengths. Molecular beam epitaxy grown QDs are studied as a function of arsenic pressure at a specific InAs coverage, and as a function of InAs coverage for three arsenic pressures. As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Microscopy of uncapped QDs shows that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low-pressure samples. This points to a modification of the QDs during capping. We discuss prior reports pertaining to arsenic pressure and capping effects, and in this context describe our observations of the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping.

Real-time in situ Raman imaging of carbon nanotube growth

In the quest for control over carbon nanotube synthesis in situ imaging has the potential to become a primary tool. Here, we show that global Raman imaging enables the observation of individual nanotubes and ensembles in real time, during growth. Individual nanotubes are detected even at 875 °C. Imaging and spectroscopy measurements of nanotube growth show distinct nucleation and growth phases. The first optical images of individual nanotubes captured during growth are presented.

Raman microscopy mapping for the purity assessment of chirality enriched carbon nanotube networks in thin-film transistors

With recent improvements in carbon nanotube separation methods, the accurate determination of residual metallic carbon nanotubes in a purified nanotube sample is important, particularly for those interested in using semiconducting single-walled carbon nanotubes (SWCNTs) in electronic device applications such as thin-film transistors (TFTs). This work demonstrates that Raman microscopy mapping is a powerful characterization tool for quantifying residual metallic carbon nanotubes present in highly enriched semiconducting nanotube networks. Raman mapping correlates well with absorption spectroscopy, yet it provides greater differentiation in purity. Electrical data from TFTs with channel lengths of 2.5 and 5 µm demonstrate the utility of the method. By comparing samples with nominal purities of 99.0% and 99.8%, a clear differentiation can be made when evaluating the current on/off ratio as a function of channel length, and thus the Raman mapping method provides a means to guide device fabrication by correlating SWCNT network density and purity with TFT channel scaling.

Epitaxy: the motion picture

Abstract The engineering of many modern electronic devices demands control over a crystal down to the thickness of a single layer of atoms—and future demands will be even more challenging. Such control is achieved by the method of crystal growth known as epitaxy, and that makes this method the subject of intense study. More than that, recent advances are revolutionizing our knowledge of how surfaces grow. In fact, growing surfaces show a beautifully rich variety of phenomena, many of which are only now beginning to be uncovered. In the past few years many surface imaging techniques have been used to give us a close look at how crystals grow— while they are growing . The purpose of this article will be to illustrate some of the ways real surfaces grow and change as revealed by some of the latest in situ microscopic imaging technologies. It is often said that crystal growth is more of an art than a science. Here we will show that it is emphatically both.

Photoinduced band gap shift and deep levels in luminescent carbon nanotubes.

Individual air-suspended single-walled carbon nanotubes are imaged both spatially and spectrally in photoluminescence. At low excitation power, photoluminescence is bright and stable with high quantum efficiency; however, higher power initially causes a gradual red shift and then more severe changes. Blinking, the loss of quantum efficiency, and the appearance of new deep levels are all seen and can be explained by the introduction of defects. We propose that optical excitation induces molecular deposition onto the nanotube by optically induced van der Waals interactions, leading to physisorption and ultimately chemisorption which severely degrades the luminescence.

Charge contrast imaging of suspended nanotubes by scanning electron microscopy

A method of rapidly identifying and imaging suspended nanotubes by scanning electron microscopy is reported. Nanotubes are visible in high contrast and even at low magnification. The contrast can be explained by considering the effect that the charge on the nanotube has on the substrate. The proposed mechanism is general and should apply to any charged nanostructure in proximity to a surface or interface. This represents a new contrast mechanism in scanning electron microscopy.