Publications by Thorsten Hesjedal

Methane chemical vapor deposition on transition metal/GaAs samples - A possible route to Haeckelite carbon nanotubes?

Surface and Interface Analysis 44 (2012) 456-465

MJ Burek, T Hesjedal

We present a systematic study of atmospheric chemical vapor deposition growth of carbon nanotubes (CNTs) on patterned, transition metal/GaAs samples employing methane as the carbon feedstock. Controlled CNT growth was found to occur from the exposed metal-semiconductor interface, rather than from the metal or semiconductor surfaces themselves. A fast sample loading system allowed for a minimization of the exposure to high temperatures, thereby preventing excessive sample damage. The optimum growth temperature for CrNi/GaAs interfaces is 700 °C (at a methane flow rate of 700 sccm). Possible growth scenarios involving the Ni-As-Ga system and its interaction with C is discussed. Raman spectroscopy of the CNTs revealed the presence of pentagon-heptagon defects. Closer analysis of the spectra points towards a mixture of so-called Haeckelite CNTs. © 2011 John Wiley & Sons, Ltd.

Methane chemical vapor deposition on transition metal/GaAs samples - a possible route to Haeckelite carbon nanotubes?

Surface and Interface Analysis (2011)

MJ Burek, T Hesjedal

Interface Characterization of Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions

Journal of Nanoscience and Nanotechnology 12 (2012) 1006-1023

SG Wang, RCC Ward, T Hesjedal, XG Zhang, C Wang, A Kohn, QL Ma, J Zhang, HF Liu, XF Han

Magnetic susceptibility of n-type GaAs

Semiconductor Science and Technology IOP 27 (2012) 055018

T Hesjedal, U Kretzer, A Ney

An undergraduate nanotechnology engineering laboratory course on atomic force microscopy

IEEE Transactions on Education 54 (2011) 428-441

D Russo, RD Fagan, T Hesjedal

The University of Waterloo, Waterloo, ON, Canada, is home to North America's first undergraduate program in nanotechnology. As part of the Nanotechnology Engineering degree program, a scanning probe microscopy (SPM)-based laboratory has been developed for students in their fourth year. The one-term laboratory course Nanoprobing and Lithography is accompanied by a preceding one-term lecture course, Nanoprobing and Lithography. The lecture course lays the theoretical foundation for the concepts covered in the laboratory course. The students work in groups of two and obtain hands-on experience in biweekly 3-h laboratory sessions. The labs use a dedicated undergraduate SPM teaching facility consisting of five atomic force microscope stations. The laboratory course covers all common standard modes of operation, as well as force spectroscopy, electrostatic force microscopy, magnetic force microscopy, and scanning probe lithography by electrochemical oxidation and scratching/ploughing of resist. In light of the breadth of the nanotechnology engineering educational program in terms of synthesis and characterization of nanomaterials, the authors designed a dedicated SPM lab with a capacity of up to 130 students per term. © 2010 IEEE.

Development of an electronic nose sensing platform for undergraduate education in nanotechnology

European Journal of Physics 32 (2011) 675-686

DV Russo, MJ Burek, RM Iutzi, JA Mracek, T Hesjedal

The teaching of the different aspects of a sensor system, with a focus on the involved nanotechnology, is a challenging, yet important task. We present the development of an electronic nose system that utilizes a nanoscale amperometric sensing mechanism for gas mixtures. The fabrication of the system makes use of a basic microfabrication facility, as well as an undergraduate chemistry laboratory for material synthesis and preparation. The sensing device consists of an array of cross-reactive sensors composed of metal-oxide semiconducting nanoparticles. Each sensor in the array produces a unique response in the presence of a target gas, allowing the sensor to determine the identity and concentration of multiple gases in a mixture. The educational aspects include microheater simulation and fabrication, design and fabrication of interdigitated electrodes, development of interfacing circuitry and software, development and calibration of a sensory array, sol-gel processing of nanoparticle films and their characterization, and details of the fundamental chemical sensing mechanism. © 2011 IOP Publishing Ltd.

Continuous roll-to-roll growth of graphene films by chemical vapor deposition

Applied Physics Letters 98 (2011)

T Hesjedal

Few-layer graphene is obtained in atmospheric chemical vapor deposition on polycrystalline copper in a roll-to-roll process. Raman and x-ray photoelectron spectroscopy were employed to confirm the few-layer nature of the graphene film, to map the inhomogeneities, and to study and optimize the growth process. This continuous growth process can be easily scaled up and enables the low-cost fabrication of graphene films for industrial applications. © 2011 American Institute of Physics.

Micromagnetic analysis of unusual, V-shaped domain transitions in MnAs nanowires

Journal of Magnetism and Magnetic Materials 323 (2011) 1840-1845

R Engel-Herbert, T Hesjedal

V-shaped domain transitions in αMnAs nanowires were investigated by micromagnetic simulations. These rather unusual domain patterns are commonly observed experimentally by surface-sensitive magnetic imaging techniques. It has been speculated that the accompanying inclined domain walls in MnAs are the result of either an exchange biasing effect between ferromagnetic αMnAs wires and antiferromagnetic βMnAs wires or possibly due to competing exchange mechanisms in MnAs. Here we present evidence that these domain features are in fact transitions between three-dimensional flux-closure domains of opposite chirality and can therefore rule out the involvement of an antiferromagnetic biasing effect or anisotropic exchange. The formation of the energetically unfavorable V-shaped domain transitions is discussed in the context of the magneto-structural phase transition of the sample. © 2011 Elsevier B.V.

Are carbon nanotubes a naturally occurring material? Hints from methane CVD using lava as a catalyst

Current Nanoscience 7 (2011) 294-296

J Mracek, RD Fagan, RM Stengelin, T Hesjedal

Single-walled carbon nanotubes (SWNTs) were grown using methane CVD with lava as a catalyst and substrate. Metal-oxide phases embedded in the lava are reduced in the presence of hydrogen, thereby promoting catalytic growth. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show a correlation between the growth of carbonaceous nanomaterials and the presence of iron in the alumina matrix. Raman spectroscopy of the carbon deposits proves the occurrence of SWNTs. Although this growth route lacks ef-ficiency, it provides evidence for the claim that SWNTs are a natural allotrope of carbon and that volcanoes may provide an environment for their synthesis. © 2011 Bentham Science Publishers Ltd.

An Undergraduate Nanotechnology Engineering Laboratory Course on Atomic Force Microscopy

IEEE Transactions on Education (2010)

D Russo, RD Fagan, T Hesjedal

Magnetic logic element based on an S-shaped Permalloy structure

Applied Physics Letters 96 (2010)

T Hesjedal, T Phung

Magnetic devices have shown the potential to be used not only as storage elements but also as nonvolatile and programmable logic devices. We present a magnetic logic device element-the S state element-that consists of a single magnetic layer. Its output can be controlled by orthogonal magnetic inputs. The reconfigurable logic element can be easily integrated with common magnetoresistive device concepts, such as spin valves or magnetic tunnel junctions. Using Permalloy as an example, we demonstrate the feasibility of magnetologic operation through micromagnetic simulations. © 2010 American Institute of Physics.

Surface acoustic wave-assisted scanning probe microscopy - A summary

Reports on Progress in Physics 73 (2010)

T Hesjedal

Elastic properties of nanoscopic materials, structures and thin films are important parameters controlling their growth, as well as their optical and electronic properties. Acoustic microscopy is a well-established method for elastic imaging. In order to overcome its micrometer-scale diffraction-limited lateral resolution, scanning probe microscopy-based acoustic near-field techniques have been developed. Among the acoustic modes used for microscopy, surface acoustic waves (SAWs) are especially suited for probing very small and thin objects due to their localization in the vicinity of the surface. Moreover, the study of SAWs is crucial for the design of frequency filter devices as well as for fundamental physical studies, for instance, the probing of composite fermions in two-dimensional electron systems. This review discusses the capabilities and limitations of SAW-based scanning probe microscopy techniques. Particular emphasis is laid on the review of surface acoustic waves and their interaction with elastic inhomogeneities. Scattering, diffraction and wave localization phenomena will be discussed in detail. Finally, the possibilities for quantitative acoustic microscopy of objects on the nanoscale, as well as practical applications, are presented. © 2010 IOP Publishing Ltd.

Investigation of slanted and V-shaped domain walls in MnAs films

Journal of Applied Physics 105 (2009)

R Engel-Herbert, T Hesjedal

The magnetic domain structure of MnAs stripes exhibits a number of domain transitions that are inclined with respect to the system's easy axis. Among them are laterally confined slanted and V-shaped domain walls, as well as extended zigzag shaped transitions that are running along the stripe axis. The nature of these unusual domain transitions was investigated with the help of micromagnetic simulations and compared with experimental magnetic force microscopy (MFM) images. All types of inclined walls result from the underlying three-dimensional domain structure of the wires. In the domain transition region, the underlying flux closure pattern gets asymmetrically distorted in the cross-sectional wire plane as the system changes its domain types. The results of the simulations are in excellent qualitative agreement with the stray field patterns observed with MFM. © 2009 American Institute of Physics.

Magnetic coupling of ferromagnetic stripe arrays: Analytical model for the α-β -phase coexistence regime of MnAs/GaAs(001)

Physical Review B - Condensed Matter and Materials Physics 78 (2008)

R Engel-Herbert, T Hesjedal

We investigate the temperature-dependent hysteresis of the stripe state of MnAs thin films on GaAs(001) in the phase coexistence regime. The underlying magnetic domain structure is described employing an analytic model for stripe arrays with perpendicular anisotropy. In the framework of this model the magnetic properties of the MnAs stripe array can be unraveled as a combined effect of magnetostatic coupling of neighboring ferromagnetic stripes and the tendency to form antiparallel magnetic domains within the individual ferromagnetic stripes. The detailed analysis reveals the balance of demagnetization energy and domain-wall energy for the domain structure. It is capable to quantitatively predict the temperature dependency of the coercive field of MnAs thin films on GaAs(001) in the phase coexistence regime. Further, the analytic model allows for an understanding of the unusual magnetic reversal properties as a consequence of the temperature-driven geometrical variations in the stripe array. Here, it is the energy difference of the single and the multidomain states associated with the geometrical variations, which is the driving factor, rather than the temperature dependence of the magnetic properties themselves. Although the stripe array of MnAs thin films can be in an interstripe as well as in an intrastripe coupling state, the magnetization reversal is entirely determined by interstripe coupling. © 2008 The American Physical Society.

Comparative study of the influence of the solvent on the catalytic growth of carbon nanotubes

Microelectronic Engineering 85 (2008) 156-160

R Engel-Herbert, H Pforte, T Hesjedal

The catalytic growth by chemical vapor deposition is a well-established route to single-wall carbon nanotubes (SWNTs). In this process, the choice and preparation of the metal catalyst determines the nanotube growth. The system Fe/Mo is known to yield a large percentage of SWNTs. In order to make use of SWNTs in electronic or electromechanical devices, the patterned growth relies on lithography techniques like photolithography or electron beam lithography. Many standard lithographic processes, i.e. the combination of photoresist and lift-off procedure, are not compatible with Fe/Mo catalyst solutions, resulting in low SWNT yield. We present a systematic study of the influence of the catalyst solvent on the patterned SWNT growth. Most remarkably, the use of water as a solvent is the basis for integrating SWNT with the established processing techniques due to its compatibility with any lithographic process and the simultaneous high SWNT yield. © 2007 Elsevier B.V. All rights reserved.

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

Physica Status Solidi (C) Current Topics in Solid State Physics 4 (2007) 1763-1766

T Hesjedal, R Engel-Herbert, DM Schaadt, KH Ploog

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magneto-structural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Growth of carbon nanotubes on GaAs

Materials Letters 61 (2007) 4631-4634

R Engel-Herbert, Y Takagaki, T Hesjedal

Hybrid carbon nanotube-semiconductor systems offer unique properties by combining the advantages of one-dimensional conductors with the broad opportunities of semiconductor technology. Thus, it is desirable to incorporate the nanotube growth in III-V semiconductor systems. We present the directed growth of carbon nanotubes from prepatterned CrNi catalyst structures on GaAs. © 2007 Elsevier B.V. All rights reserved.

Systematic investigation of Permalloy nanostructures for magnetologic applications

Journal of Applied Physics 101 (2007)

R Engel-Herbert, SA Haque, T Hesjedal

Magnetic logic gates based on magnetostatically coupled ferromagnetic structures show a great promise for future information processing. The absolute size and shape of the elementary structures determine their intrinsic and interaction properties and thus have to be chosen appropriately in order to allow for binary logic operations. Here, we present a systematic study of the switching behavior, stray field, and coupling of Permalloy nanostructures as a function of geometrical parameters using micromagnetic simulations. We show that bow-tie shaped elements are promising for magnetic logic gates. © 2007 American Institute of Physics.

Three-dimensional magnetic flux-closure domain patterns in MnAs thin films on GaAs(001)

Journal of Applied Physics 101 (2007)

R Engel-Herbert, T Hesjedal, DM Schaadt

The magnetic microstructure of single-crystalline MnAs films on GaAs(001) has been investigated. Magnetic force microscopy (MFM) reveals a three-dimensional magnetization pattern that is in disagreement with the simple domain picture observed by surface-sensitive magnetic imaging. Here, we present a consistent micromagnetic picture of MnAs thin films in the ferromagnetic stripe phase, which appears in the course of the phase transition. A number of equilibrium magnetization patterns of the stripes are found that are, in fact, based on flux-closure domain patterns in the basal plane of MnAs. The simulation of a stripe array yields excellent agreement with the measured surface magnetization. The experimentally observed stray field contrast was confirmed by MFM contrast simulations based on these equilibrium magnetization patterns. © 2007 American Institute of Physics.

CVD synthesis and purification of single-walled carbon nanotubes using silica-supported metal catalyst

Materials Letters 61 (2007) 2589-2593

R Engel-Herbert, H Pforte, T Hesjedal

We report the growth of single-walled carbon nanotubes (SWNTs) from silica aerogel by methane chemical vapor deposition (CVD). Bimetallic Fe/Mo supported by amorphous silica forms nanostructures that are catalytically active at the growth temperature. Despite the flexibility gained by using silica as a support matrix, the as-grown nanotube samples need to be further purified for applications in nanodevices. Our experiments show that the treatment in hydrofluoric acid selectively removes the silica matrix,while the amorphous carbon deposits can be selectively removed by oxidation. The influence of the oxidation and etching parameters on the SWNT yield is discussed. The optimized purification procedure renders SWNTs only. © 2006 Elsevier B.V. All rights reserved.