This page contains publications and articles written by member(s) of our group, arranged by year.
A. Wadhawan, R. E Stallcup II, K. F. Stephens II, J. M. Perez, and I. A. Akwani, "Effects of O2, Ar, and H2 gases on the field-emission properties of single-walled and multiwalled carbon nanotubes," Appl. Phys. Lett. 79(12), 1867 (2001).
[Abstract or pdf (69 kb)]R. E Stallcup II and J. M. Perez, "Scanning Tunneling Microscopy Studies of Temperature-Dependent Etching of Diamond (100) by Atomic Hydrogen," Phys. Rev. Lett. 86(15), 3368 (2001).
[Abstract or pdf (341 kb)]A. Wadhawan, R. E Stallcup II and J. M. Perez, "Effects of Cs Deposition on the Field-Emission Properties of Single-Walled Carbon-Nanotube Bundles," Appl. Phys. Lett. 78(1), 108 (2001).
[Abstract or pdf (68 kb)]
J. M. Perez, R. E Stallcup II and I. A. Akwani, "Defect and Raman Spectroscopy of CVD Grown Diamond Films," in Flat-Panel Displays and Sensors - Principles, Materials and Processes, Materials Research Society Symposium Proceedings Volume 558 (MRS, Warrendale, 2000), pp. 589-594.
R. E. Stallcup II, "Scanning Tunneling Microscopy of Homo-epitaxial Chemical Vapor Deposited Diamond (100) Films," PhD Dissertation, University of North Texas, 2000.
[Abstract or pdfs: Abstract, front matter and Chapter 1 (50 kb), Chapter 2 (11.2 Mb), Chapter 3 (366 kb), Chapter 4 (375 kb), Chapter 5 (2.4 Mb), Bibliography (13 kb)]
S. C. Lim, R. E. Stallcup II, I. A. Akwani, and J. M. Perez, "Effects of O2, H2, and N2 Gases on the Field Emission Properties of Diamond-Coated Microtips," Appl. Phys. Lett. 75(8), 1179 (1999).
[Abstract or pdf (70 kb)]
I. Akwani, E. D. Sosa, S. C. Lim, R. E. Stallcup II, J. N. Castillega, J. Bernhard, D. E. Golden and J. M. Perez, "Effect of SP3/(SP2 + SP3) Carbon Fraction on the Photoelectric Threshold and Electron Affinity of Diamond Films," in Materials Issues in Vacuum Microelectronics, Materials Research Society Symposium Proceedings Volume 509 (MRS, Warrendale, 1998), pp. 137-141.
I. Akwani, E. D. Sosa, J. Bernhard, S. C. Lim, R. E. Stallcup II, J. M. Perez and D. E. Golden, "Variation of the Photoelectric Threshold of Boron Doped Diamond Films," in Materials Issues in Vacuum Microelectronics, Materials Research Society Symposium Proceedings Volume 509 (MRS, Warrendale, 1998), pp. 143-148.
R. E. Stallcup II and J. M. Perez, "UHV STM of Cesium on Oxygenated Epitaxial Diamond (100) Films," in Materials Issues in Vacuum Microelectronics, Materials Research Society Symposium Proceedings Volume 509 (MRS, Warrendale, 1998), pp. 161-164.
S. C. Lim, R. E. Stallcup II, I. Akwani and J. M. Perez, "Structural and Electronic Properties of Negative Electron Affinity Epitaxial Diamond (110) Films Studied using Atomic Resolution UHV STM," in Materials Issues in Vacuum Microelectronics, Materials Research Society Symposium Proceedings Volume 509 (MRS, Warrendale, 1998), pp. 165-170.
S. C. Lim, "
Scanning Tunneling Microscopy of Epitaxial Diamond (110) and (111) Films and Field Emission Properties of Diamond Coated Molybdenum Microtips," PhD Dissertation, University of North Texas, 1998. [Abstract]I. Akwani, "
Structural and Photoelectron Emission Properties of Chemical Vapor Deposition Grown Diamond Films," PhD Dissertation, University of North Texas, 1998. [Abstract]
A. F. Aviles, R. E. Stallcup I, W. Rivera and J. M. Perez, "Scanning Tunneling Microscopy of Chemical Vapor Deposition Diamond Film Growth on Highly-Oriented-Pyrolytic Graphite and Si," Atomic Force Microscopy/Scanning Tunneling Microscopy 2 (Plenum Press, New York, 1997), pp. 45-51.
R. E. Stallcup II, A. F. Aviles and J. M. Perez, "Atomic Resolution Ultrahigh Vacuum Scanning Tunneling Microscopy of Diamond (100) Epitaxial Films," Atomic Force Microscopy/Scanning Tunneling Microscopy 2 (Plenum Press, New York, 1997), pp. 59-64.
J. M. Perez and J. L. Large, "Current versus Voltage Characteristics for Deposition and Removal of Gold Nanostructures on Gold Surface using Scanning Tunneling Microscopy," Atomic Force Microscopy/Scanning Tunneling Microscopy 2 (Plenum Press, New York, 1997), pp. 233-239.
R. E. Stallcup, L. M. Villarreal, S. C. Lim, I. Akwani, A. F. Aviles, and J. M. Perez, " Atomic Structure of the Diamond (100) Surface Studied Using Scanning Tunneling Microscopy," J. Vac. Sci. Technol. B 14(2), 929 (1996).
[Abstract or pdf (538 kb)]
W. Rivera, J. M. Perez, R. S. Ruoff, D. C. Lorents, R. Malhotra, S. Lim, Y. G. Rho, E. G. Jacobs and R. F. Pinizzotto, "Scanning Tunneling Microscopy Current-Voltage Characteristics of Carbon Nanotubes," J. Vac. Sci. Technol. B 13(2), 327 (1995).
[Abstract or pdf (695 kb)]R. E. Stallcup, A. F. Aviles, and J. M. Perez, " Atomic Resolution Ultrahigh Vacuum Scanning Tunneling Microscopy of Epitaxial Diamond (100) Films," Appl. Phys. Lett. 66(18), 2331 (1995).
[Abstract or pdf (191 kb)]J. M. Perez, J. Steinshnider, R. E. Stallcup II and A. F. Aviles, "Control of Chaos in a CO2 Laser," in Proceedings of the 2nd Experimental Chaos Conference, edited by W. Ditto, L. Pecora, M. Shlesinger, M. Spano and S. Vohra (World Scientific, Singapore, 1995), pp. 147-152.
J. M. Perez, J. Steinshnider, R. E. Stallcup and A. F. Aviles, "Control of Chaos in a CO2 Laser," Appl. Phys. Lett., 65(10), 1216, (1994).
[Abstract or pdf (335 kb)]R. F. Pinizzotto, H. Yang, J. M. Perez and J. L. Coffer, "The Observation of Silicon Nanocrystals in Siloxene," J. Appl. Phys. 75(9), 4486 (1994).
[Abstract or pdf (475 kb)]Z. Yu, J. Steinshnider, C. L. Littler, J. M. Perez and J. M. Kowalski, "Stabilization of Quasiperiodic Orbits for Line-Coupled Diode Resonator Systems," Phys. Rev. E 49(1), 220 (1994).
[Abstract or pdf (519 kb)]W. Rivera, J. M. Perez, R. S. Ruoff, D. C. Lorents, R. Malhotra, S. Lim, Y. G. Rho, E. G. Jacobs and R. F. Pinizzotto, "Scanning Tunneling Microscopy and Spectroscopy of Carbon Nanotubes," in Atomic Force Microscopy/Scanning Tunneling Microscopy, edited by S. Cohen, M. Bray and M. Lightbody (Plenum Press, New York, 1994), pp. 137-142.
J. M. Perez, R. Rivera, C. Lin, R. C. Hyer, M. Green, S. C. Sharma, D. R. Chopra and A. R. Chourasia, "Scanning Tunneling Microscopy of the Structural and Electronic Properties of Chemical Vapor Deposited Diamond Films," in Atomic Force Microscopy/Scanning Tunneling Microscopy, edited by S. Cohen, M. Bray and M. Lightbody (Plenum Press, New York, 1994), pp. 203-210.
J. M. Perez, C. Lin, W. Rivera. R. C. Hyer, M. Green, S. C. Sharma, D. R. Chopra and A. R. Chourasia, "Scanning Tunneling Microscopy of the Electronic Structure or Chemical Vapor Deposited Diamond Films," Appl. Phys. Lett. 62(16), 1889 (1993).
[Abstract or pdf (402 kb)]
J. M. Perez, J. Villalobos, P. McNeill, J. Prasad, R. Cheek, J. Kelber, J. P. Estrera, P. D. Stevens and R. Glosser, "Direct Evidence for the Amorphous Silicon Phase in Visible Photoluminescent Porous Silicon," Appl. Phys. Lett. 61(5), 563 (1992).
[Abstract or pdf (346 kb)]X. N. Song, Z. Yu, G. Albert, J. M. Kowalski, C. L. Littler and J. M. Perez, "Autonomous and Externally Driven Periodic and Chaotic Hall Voltages Oscillations in N-InSb," in Proceedings of the First Experimental Chaos Conference, edited by S. Vohra, M. Spano, M. Shlesinger, L. Pecora and W. Ditto (World Scientific, Singapore, 1992), pp. 219-224.
J. M. Perez, Z. Yu, J. M. Kowalski, G. Albert, C. L. Littler and X. N. Song, "Synchronization of Chaos in Coupled Tunnel Diode Relaxation Oscillators," in Proceedings of the First Experimental Chaos Conference, edited by S. Vohra, M. Spano, M. Shlesinger, L. Pecora and W. Ditto (World Scientific, Singapore, 1992), pp. 327-332.
We compare the effects of O2, Ar, and H2 gases on the field-emission (FE) properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. The higher operating voltages in an O2 environment cause a permanent decrease of the FE current and an increase in the turn-on field of MWNTs. The ratios of the slopes before and after O2 exposure are approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively.
We present a technique for obtaining atomic resolution ultrahigh vacuum
scanning tunneling microscopy images of diamond (100) films, and use this
technique to study the temperature dependence of the etching of epitaxial
diamond (100) films by atomic hydrogen. We find that etching by atomic hydrogen
is highly temperature dependent, resulting in a rough and pitted surface at
T![[approximate]](images/ap.gif){kind=small}
200 and 500 °C, respectively. At
T1000 °C etching results in a smooth
surface and is highly anisotropic, occurring predominantly in the direction of
dimer rows. This observation supports recent theoretical models that propose
anisotropic etching as the mechanism for the growth of smooth diamond (100)
films.
We report the effects of Cs deposition on the field-emission (FE) properties of single-walled carbon-nanotube bundles. We observe that Cs deposition decreases the turn-on field for FE by a factor of 2.1–2.8 and increases the FE current by six orders of magnitude. After Cs deposition, the FE current versus voltage (I–V) curves show non-Fowler–Nordheim behavior at large currents, consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I–V curves before and after Cs deposition is approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current.
Atomic resolution images of hot-tungsten filament chemical-vapor-deposition (CVD) grown epitaxial diamond (100) films obtained in ultrahigh vacuum (UHV) with a scanning tunneling microscope (STM) are reported. A (2´1) dimer surface reconstruction and amorphous atomic regions were observed on the hydrogen terminated (100) surface. The (2´1) unit cell was measured to be 0.51±0.01 ´ 0.25±0.01 nm2. The amorphous regions were identified as amorphous carbon. After CVD growth, the surface of the epitaxial films was amorphous at the atomic scale. After 2 minutes of exposure to atomic hydrogen at 30 Torr and the sample temperature at 500 ºC, the surface was observed to consist of amorphous regions and (2´1) dimer reconstructed regions. After 5 minutes of exposure to atomic hydrogen, the surface was observed to consist mostly of (2´1) dimer reconstructed regions. These observations support a recent model for CVD diamond growth that is based on an amorphous carbon layer that is etched or converted to diamond by atomic hydrogen. With further exposure to atomic hydrogen at 500 ºC, etch pits were observed in the shape of inverted pyramids with {111} oriented sides. The temperature dependence of atomic hydrogen etching of the diamond (100) surface was also investigated using UHV STM, and it was found that it was highly temperature dependent. Etching with a diamond sample temperature of 200 ºC produced (100) surfaces that are atomically rough with no large pits, indicating that the hydrogen etch was isotropic at 200 ºC. Atomic hydrogen etching of the surface with a sample temperature of 500 ºC produced etch-pits and vacancy islands indicating an anisotropic etch at 500 ºC. With a sample temperature of 1000 ºC during the hydrogen etch, the (100) surface was atomically smooth with no pits and few single atomic vacancies, but with vacancy rows predominantly in the direction of the dimer rows, indicating that the 1000 ºC etch was highly anisotropic. Raman spectroscopy was used as a temperature probe, and for determining film quality.
We report the effects of O2, H2, and N2 residual gases on the field emission properties of uncoated and diamond-coated individual Mo microtips. The microtips are made using electrochemical etching techniques and positioned 5 µm from the anode using a scanning tunneling microscopy system. We observe that the field emission (FE) current and turn-on voltage of diamond-coated microtips are significantly less degraded by O2 exposure than those of uncoated Mo microtips. H2 exposure enhances the FE properties of both uncoated and diamond-coated microtips, while N2 exposure does not have any significant effect.
The growth mechanism of chemical vapor deposition (CVD) grown
homo-epitaxial diamond (110) and (111) films was studied using ultrahigh vacuum
(UHV) scanning tunneling microscopy (STM). In addition, the field emission
properties of diamond coated molybdenum microtips were studied as a function of
exposure to different gases.
We observe that homo-epitaxial diamond (110) films grow very rough due to
striations. The striations are found to be due to the appearance of (111) faces
and contain (100) steps. UHV STM atomic resolution images of the diamond (110)
films show a (1x1) zigzag structure that measures
1.5 Å´
1.5 Å, in agreement with theoretical predictions for the hydrogen terminated diamond
(110) surface.
We observe that homo-epitaxial diamond (111) films grow in a defective manner
characterized by islanding and steps. The defective structures are not due to
microtwinning. The island size and step width decrease as the growth temperature
decreases. This supports a model in which growth is mediated by the diffusion of
carbon atoms on the surface. Atomic resolution imaging of the islands shows a
(1´1)
six-fold atomic structure that measures
2.54 Å´2.54 Å.
In the field emission studies, diamond coated and uncoated Mo microtips were exposed
to O2, H2, and N2. A novel positioning system was
designed and constructed that allows a comparison of the field emission properties
of different microtips. Exposure of uncoated Mo microtips to 10 L of O2
is found to degrade the emission current significantly. After exposure to 100 L of
O2, the emission current from the uncoated Mo microtips disappears.
However, diamond coated Mo microtips show very stable field emission properties
even after exposure to 1000 L of O2. The field emission current of both
coated and uncoated Mo microtips increases after exposure to H2.
N2 slightly increases the emission current from uncoated Mo microtips.
However, diamond coated Mo microtips do not show any changes after exposure to
N2. These results show that diamond coated Mo microptips exhibit
significantly more stable and reliable field emission characteristics than
uncoated Mo microtips in the presence of these gases.
The effects of methane (CH4), diborone (B2 H6)
and nitrogen (N2) concentrations on the structure and photoelectron
emission properties of chemical vapor deposition (CVD) polycrystalline diamond
films were studied. The diamond films were grown on single-crystal Si substrates
using the hot-tungsten filament CVD technique. Raman spectroscopy and x-ray
photoelectron spectroscopy (XPS) were used to characterize the different forms of
carbon in the films, and the fraction of sp3 carbon to sp3
plus sp2 carbon at the surface of the films, respectively. Scanning
electron microscopy (SEM) was used to characterize the surface morphology of the
films. The photoelectron emission properties were determined by measuring the
energy distributions of photoemitted electrons using ultraviolet photoelectron
spectroscopy (UPS), and by measuring the photoelectric current as a function of
incident photon energy.
We found that the structure and photoelectron emission properties of the
polycrystalline diamond films are strongly dependent on the growth conditions.
We observed for the first time, a continuous and significant decrease in the
photoelectric threshold as the fraction of sp3 carbon to sp3
plus sp2 carbon in the film decreases. By varying the CH4
concentration from 0.10% to 0.70%, we observed a decrease in photoelectric
threshold from 4.8 eV to 3.9 eV as the percent of sp3 carbon at the
surface decreases from 91% to 55%.
We observed that the percent of sp3 carbon at the surface is
independent of the boron concentrations used during growth. Measurement of the
photoelectric current and electron energy distributions show that the
photoelectric threshold is insensitive to the boron concentration, although, the
boron doping modified the surface morphology and photoemission intensity. The
photoemission intensity increases with microcrystlline diamond content and the
photoelectric threshold is about 4.4 eV.
By varying the partial pressure of the N2 gas used during growth,
from 15 to 25 millitorr, we observed a decrease in photoelectric threshold from
approximately 4.27 eV to 4.09 eV as the percent of sp3 carbon at the
surface decreased from 76% to 50%.
Analyses of the photocurrent measurements show that the electron emission is due
to valence band emission. Following these observations, we propose that the
decrease in photoelectric threshold is due to a decrease in the band gap of the
sp3 - sp2 carbon networks at the grain boundaries, in
agreement with recent theoretical calculations.
Atomic resolution ultrahigh-vacuum scanning tunneling microscopy studies of chemical-vapor-deposition-grown epitaxial diamond (100) films are reported. After growth, the surface of the epitaxial films is amorphous at the atomic scale. After 2 min of exposure to atomic hydrogen at 30 Torr, the surface is observed to consist of amorphous regions and (2 × 1) dimer reconstructed regions. After 5 min of exposure to atomic hydrogen, the surface is observed to consist mostly of (2 × 1) dimer reconstructed regions. These observations are compared with a recent model for chemical-vapor-deposition diamond growth. Tunneling current versus voltage spectroscopy of undoped and boron-doped epitaxial diamond (100) films is also reported.
Scanning tunneling microscopy (STM) has been used to obtain images and current–voltage (I–V) curves of carbon nanotubes produced by arc discharge of carbon electrodes. The STM I–V curves indicate that carbon nanotubes with diameters from 2.0 to 5.1 nm have a metallic density of states. Using STM, we also observe nanometer-size graphene sheets which are four graphite layers thick. The STM images of carbon nanotubes are in good agreement with transmission electron microscope images.
We report atomic resolution images of chemical vapor deposition grown epitaxial diamond (100) films obtained in ultrahigh vacuum (UHV) with a scanning tunneling microscope. A (2 × 1) dimer surface reconstruction and amorphous atomic regions are observed. The (2 × 1) unit cell is measured to be 0.51 ± 0.01 × 0.25 ± 0.01 nm2. The amorphous regions are identified as amorphous carbon. A radial structure 1.5 nm in diameter is observed on a plane at a 20° slope to the (2 × 1) surface. Tunneling current versus voltage spectra in UHV and Raman spectra are also obtained.
We report the experimental control of chaos in an optically modulated CO2 laser. The CO2 laser was driven into chaos by injecting a feedback beam modulated by an electro-optical modulator. Control of chaos was achieved using a modified proportional feedback technique in which the control pulses were delayed by approximately one relaxation period. Using this technique, it was possible to control unstable periodic orbits up to period 6.
We report the direct observation of silicon nanocrystals in unannealed siloxene using high resolution transmission electron microscopy. The microstructure consists of an amorphous matrix plus silicon crystallites with dimensions of a few nanometers. This is additional evidence that the photoluminescence of silicon-based materials is due to quantum confinement.
Chaotic transitions in line-coupled diode resonator systems were experimentally found to follow the Curry-Yorke model. [The Structure of Attractors in Dynamical Systems (Springer, Berlin, 1977), p. 48]. The standard diode model from the computer program SPICE was used to simulate these systems; the complete Lyapunov spectra from simulated systems are in good agreement with the spectra computed from the experimental time series. By applying the proportional-feedback technique to these systems, we can stabilize periodic orbits of increasing periods as well as unstable quasiperiodic orbits densely covering a torus.
Scanning tunneling microscopy has been used to characterize the electronic structure and surface morphology of diamond films grown using the hot filament and microwave plasma chemical vapor deposition techniques. We observe a significant difference between the current-voltage (I-V) curves for the two types of films. The I-V curves for the hot-filament grown films are characterized by a well-defined zero-current region from which a surface band gap of 4.1 eV is measured. The I-V curves for the microwave plasma grown films exhibit a rectifying behavior which can be modeled by surface band bending. We compare the surface density of states obtained from the I-V curves with those obtained from x-ray photoelectron and appearance potential spectroscopies.
We report on micro-Raman spectroscopy studies of porous silicon which show an amorphous silicon Raman line at 480 R cm–1 from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm–1 is also observed. X-ray photoelectron spectroscopy data is presented which shows a high silicon-dioxide content in porous silicon consistent with an amorphous silicon phase.