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147,374 Matching Results
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Stable, free-standing Ge nanocrystals
Free-standing Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5 nm are grown in amorphous SiO{sub 2} by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain free-standing nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Free-standing nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For free-standing as opposed to embedded Ge nanocrystals, an additional amorphous-like contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms.
Date:
January 28, 2005
Creator:
Sharp, I. D.; Xu, Q.; Liao, C. Y.; Yi, D. O.; Beeman, J. W.; Liliental-Weber, Z. et al.
Object Type:
Article
System:
The UNT Digital Library
Dopants and Defects in InN and InGaN Alloys
We have performed systematic studies of the effects of high-energy particle irradiation on the properties of InGaN alloys. In agreement with the amphoteric defect model, irradiation of InN produces donor-like defects. The electron concentration increases with increasing radiation dose and saturates at 4 x 10{sup 20} cm{sup -3} at very high doses. We find that the increase of the electron concentration causes a large blue-shift of the absorption edge, which is well-explained by the Burstein-Moss effect. The maximum electron concentration decreases with increasing Ga fraction in irradiated In{sub 1-x}Ga{sub x}N alloys as the conduction band edge approaches the Fermi level stabilization energy (E{sub FS}). For x > 0.66 the conduction band edge moves above E{sub FS} and the irradiation of n-type films produces acceptor-like defects, resulting in a reduced free electron concentration. An analysis of the concentration dependence of the electron mobility in InN indicates that the dominant defects in irradiated InN are triply-charged donors. Finally, we show that InN films doped with Mg acceptors behave like undoped films above a threshold radiation dose.
Date:
April 1, 2005
Creator:
Walukiewicz, W.; Jones, R. E.; Li, S. X.; Yu, K. M.; Ager, J. W., III; Haller, E. E. et al.
Object Type:
Article
System:
The UNT Digital Library
Effects of pressure on the band structure of highly mismatched Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys
None
Date:
September 3, 2003
Creator:
Shan, W.; Yu, K. M.; Walukiewicz, W.; Beeman, J. W.; Wu, J.; Ager, J. W., III et al.
Object Type:
Article
System:
The UNT Digital Library
Exceptional Electron Transport Properties of In-rich InGaN
Recent years have seen an explosion of interest in the narrow band gap end of the InGaN alloy system, particularly in InN. The existence of surface electron accumulation and a tendency for n-type conductivity have been well-established and are explained by an extremely large electron affinity and the location of the Fermi level stabilization energy (E{sub FS}) high in the conduction band [1]. These characteristics pose significant challenges to the integration of In-rich InGaN into devices and demonstrate the need for a better understanding of the relationship between native defects and electronic transport in the alloy system. It has been previously shown that high-energy particle irradiation can predictably control the electronic properties of In-rich InGaN [1]. With increasing irradiation dose, the electron concentration (n) increases and the electron mobility ({mu}) decreases until the Fermi level reaches E{sub FS}, which is the saturation point. The value of n at saturation decreases with decreasing In fraction, due to the raising of the conduction band edge with respect to E{sub FS}.
Date:
October 22, 2006
Creator:
Jones, R. E.; van Genuchten, H. C. M.; Yu, K. M.; Walukiewicz, W.; Li, S. X.; Liliental-Weber, Z. et al.
Object Type:
Article
System:
The UNT Digital Library
High Electron Mobility InN
None
Date:
February 6, 2007
Creator:
Jones, R. E.; Li, S. X.; Haller, E. E.; van Genuchten, H. C. M.; Yu, K. M.; Ager, J. W., III et al.
Object Type:
Article
System:
The UNT Digital Library
High-purity, isotopically enriched bulk silicon
The synthesis and characterization of dislocation-free, undoped, single crystals of Si enriched in all 3 stable isotopes is reported: {sup 28}Si (99.92%), {sup 29}Si (91.37%), and {sup 30}Si (89.8%). A silane-based process compatible with the relatively small amounts of isotopically enriched precursors that are practically available was used. The silane is decomposed to silicon on a graphite starter rod heated to 700-750 C in a recirculating flow reactor. A typical run produces 35 gm of polycrystalline Si at a growth rates of 5 {micro}m/min and conversion efficiency >95%. Single crystals are grown by the floating zone method and characterized by electrical and optical measurements. Concentrations of shallow dopants (P and B) are as low as mid-10{sup 13} cm{sup -3}. Concentrations of C and O lie below 10{sup 16} and 10{sup 15} cm{sup -3}, respectively.
Date:
November 17, 2004
Creator:
Ager, J. W., III; Beeman, J. W.; Hansen, W. L.; Haller, E. E.; Sharp, I. D.; Liao, C. et al.
Object Type:
Article
System:
The UNT Digital Library
Structural Properties of Ge Nanocrystals Embedded inSapphire
None
Date:
September 6, 2006
Creator:
Sharp, I. D.; Xu, Q.; Yi, D. O.; Yuan, C. W.; Beeman, J. W.; Yu, K. M. et al.
Object Type:
Article
System:
The UNT Digital Library
Superheating and supercooling of Ge nanocrystals embedded inSiO2
Free-standing nanocrystals exhibit a size-dependant thermodynamic melting point reduction relative to the bulk melting point that is governed by the surface free energy. The presence of an encapsulating matrix, however, alters the interface free energy of nanocrystals and their thermodynamic melting point can either increase or decrease relative to bulk. Furthermore, kinetic contributions can significantly alter the melting behaviors of embedded nanoscale materials. To study the effect of an encapsulating matrix on the melting behavior of nanocrystals, we performed in situ electron diffraction measurements on Ge nanocrystals embedded in a silicon dioxide matrix. Ge nanocrystals were formed by multi-energy ion implantation into a 500 nm thick silica thin film on a silicon substrate followed by thermal annealing at 900 C for 1 h. We present results demonstrating that Ge nanocrystals embedded in SiO{sub 2} exhibit a 470 K melting/solidification hysteresis that is approximately symmetric about the bulk melting point. This unique behavior, which is thought to be impossible for bulk materials, is well described using a classical thermodynamic model that predicts both kinetic supercooling and kinetic superheating. The presence of the silica matrix suppresses surface pre-melting of nanocrystals. Therefore, heterogeneous nucleation of both the liquid phase and the solid phase …
Date:
August 21, 2006
Creator:
Xu, Q.; Sharp, I. D.; Yuan, C. W.; Yi, D. O.; Liao, C. Y.; Glaeser, A. M. et al.
Object Type:
Article
System:
The UNT Digital Library
Universal bandgap bowing in group III nitride alloys
The energy gaps of MBE-grown wurtzite-structure In{sub 1-x}Al{sub x}N alloys with x {le} 0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of {approx}0.8 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and AlGaN, these results show a universal relationship between the bandgap variations of group III nitride alloys and their compositions.
Date:
August 6, 2002
Creator:
Wu, J.; Walukiewicz, W.; Yu, K. M.; Ager, J. W., III; Li, S. X.; Haller, E. E. et al.
Object Type:
Article
System:
The UNT Digital Library
Direct Observation of Room-Temperature Polar Ordering in Colloidal GeTe Nanocrystals
Ferroelectrics and other materials that exhibit spontaneous polar ordering have demonstrated immense promise for applications ranging from non-volatile memories to microelectromechanical systems. However, experimental evidence of polar ordering and effective synthetic strategies for accessing these materials are lacking for low-dimensional nanomaterials. Here, we demonstrate the synthesis of size-controlled nanocrystals of the polar material germanium telluride (GeTe) using colloidal chemistry and provide the first direct evidence of room-temperature polar ordering in nanocrystals less than 5 nm in size using aberration-corrected transmission electron microscopy. Synchrotron x-ray diffraction and Raman studies demonstrate a sizeable polar distortion and a reversible size-dependent polar phase transition in these nanocrystals. The stability of polar ordering in solution-processible nanomaterials suggests an economical avenue to Tbit/in2-density non-volatile memory devices and other applications.
Date:
December 7, 2009
Creator:
Polking, Mark J.; Zheng, Haimei; Urban, Jeffrey J.; Milliron, Delia J.; Chan, Emory; Caldwell, Marissa A. et al.
Object Type:
Article
System:
The UNT Digital Library
Valence band hybridization in N-rich GaN1-xAsx alloys
We have used photo-modulated transmission and optical absorption spectroscopies to measure the composition dependence of interband optical transitions in N-rich GaN{sub 1-x}As{sub x} alloys with x up to 0.06. The direct bandgap gradually decreases as x increases. In the dilute x limit, the observed band gap approaches 2.8 eV; this limiting value is attributed to a transition between the As localized level, which has been previously observed in As-doped GaN at 0.6 eV above the valence band maximum in As-doped GaN, and the conduction band minimum. The structure of the valence band of GaN{sub 1-x}As{sub x} is explained by the hybridization of the localized As states with the extended valence band states of GaN matrix. The hybridization is directly confirmed by soft x-ray emission experiments. To describe the electronic structure of the GaN{sub 1-x}As{sub x} alloys in the entire composition range a linear interpolation is used to combine the effects of valence band hybridization in N-rich alloys with conduction band anticrossing in As-rich alloys.
Date:
May 4, 2004
Creator:
Wu, J.; Walukiewicz, W.; Yu, K. M.; Denlinger, J. D.; Shan, W.; Ager, J. W., III et al.
Object Type:
Article
System:
The UNT Digital Library
Band anticrossing in Group II-Ox-VI1-x highly mismatched alloys: Cd1-yMnyOxTe1-x quaternaries synthesized by O ion implantation
None
Date:
November 5, 2001
Creator:
Yu, K. M.; Walukiewicz, W.; Wu, J.; Beeman, J. W.; Ager, J. W., III; Haller, E. E. et al.
Object Type:
Article
System:
The UNT Digital Library
Band-gap bowing effects in BxGa1-xAs alloys
None
Date:
September 19, 2002
Creator:
Shan, W.; Walukiewicz, W.; Wu, J.; Yu, K. M.; Ager, J. W., III; Li, S. X. et al.
Object Type:
Article
System:
The UNT Digital Library
Compositional Modulation in InxGa1-xN
Transmission Electron Microscopy and x-ray diffraction were used to study compositional modulation in In{sub x}Ga{sub 1-x} N layers grown with compositions close to the miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200-nm-thick AlN or GaN buffer layer grown on a sapphire substrate. In the TEM imaging mode this modulation is seen as black/white fringes which can be considered as self-assembled thin quantum wells. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in x-ray diffraction in the {theta}-2{theta} coupled geometry. The modulation period was determined using both methods. Larger modulation periods were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. Compositional modulation was not observed for a sample with x = 0.34 grown on a GaN buffer layer. Modulated films tend to have large 'Stokes shifts' between their absorption edge and photoluminescence peak.
Date:
July 20, 2005
Creator:
Liliental-Weber, Z.; Zakharov, D. N.; Yu, K. M.; Ager, J. W., III; Walukiewicz, W.; Haller, E. E. et al.
Object Type:
Article
System:
The UNT Digital Library
Theory of Nanocluster Size Distributions from Ion Beam Synthesis
Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict the existence of a steady state shape for the cluster size distribution that depends only on a characteristic length determined by the ratio of the effective diffusion coefficient to the ion flux. The average cluster size in the steady state regime is determined by the implanted species/matrix interface energy.
Date:
June 13, 2008
Creator:
Yuan, C. W.; Yi, D. O.; Sharp, I. D.; Shin, S. J.; Liao, C. Y.; Guzman, J. et al.
Object Type:
Article
System:
The UNT Digital Library
Unusual properties of the fundamental band gap of InN
None
Date:
February 12, 2002
Creator:
Wu, J.; Walukiewicz, W.; Yu, K. M.; Ager, J. W., III; Haller, E. E.; Lu, Hai et al.
Object Type:
Article
System:
The UNT Digital Library
Advanced Burner Test Reactor Preconceptual Design Report.
The goals of the Global Nuclear Energy Partnership (GNEP) are to expand the use of nuclear energy to meet increasing global energy demand, to address nuclear waste management concerns and to promote non-proliferation. Implementation of the GNEP requires development and demonstration of three major technologies: (1) Light water reactor (LWR) spent fuel separations technologies that will recover transuranics to be recycled for fuel but not separate plutonium from other transuranics, thereby providing proliferation-resistance; (2) Advanced Burner Reactors (ABRs) based on a fast spectrum that transmute the recycled transuranics to produce energy while also reducing the long term radiotoxicity and decay heat loading in the repository; and (3) Fast reactor fuel recycling technologies to recover and refabricate the transuranics for repeated recycling in the fast reactor system. The primary mission of the ABR Program is to demonstrate the transmutation of transuranics recovered from the LWR spent fuel, and hence the benefits of the fuel cycle closure to nuclear waste management. The transmutation, or burning of the transuranics is accomplished by fissioning and this is most effectively done in a fast spectrum. In the thermal spectrum of commercial LWRs, some transuranics capture neutrons and become even heavier transuranics rather than being fissioned. …
Date:
December 16, 2008
Creator:
Chang, Y. I.; Finck, P. J.; Grandy, C.; Cahalan, J.; Deitrich, L.; Dunn, F. et al.
Object Type:
Report
System:
The UNT Digital Library
Configuration Manual Polarized Proton Collider at RHIC
In this report we present our design to accelerate and store polarized protons in RHIC, with the level of polarization, luminosity, and control of systematic errors required by the approved RHIC spin physics program. We provide an overview of the physics to be studied using RHIC with polarized proton beams, and a brief description of the accelerator systems required for the project.
Date:
January 1, 2006
Creator:
Alekseev, I.; Allgower, C.; Bai, M.; Batygin, Y.; Bozano, L.; Brown, K. et al.
Object Type:
Report
System:
The UNT Digital Library
Ionization Thresholds of Small Carbon Clusters: Tunable VUVExperiments and Theory
Small carbon clusters (Cn, n = 2-15) are produced in amolecular beam by pulsed laser vaporization and studied with vacuumultraviolet (VUV) photoionization mass spectrometry. The required VUVradiation in the 8-12 eV range is provided by the Advanced Light Source(ALS) at the Lawrence Berkeley National Laboratory. Mass spectra atvarious ionization energies reveal the qualitative relative abundances ofthe neutral carbon clusters produced. By far the most abundant species isC3. Using the tunability of the ALS, ionization threshold spectra arerecorded for the clusters up to 15 atoms in size. The ionizationthresholds are compared to those measured previously with charge-transferbracketing methods. To interpret the ionization thresholds for differentcluster sizes, new ab initio calculations are carried out on the clustersfor n = 4-10. Geometric structures are optimized at the CCSD(T) levelwith cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations areapplied to both neutral and cation species to determine adiabatic andvertical ionization potentials. The comparison of computed and measuredionization potentials makes it possible to investigate the isomericstructures of the neutral clusters produced in this experiment. Themeasurements are inconclusive for the n = 4-6 species because ofunquenched excited electronic states. However, the data provide evidencefor the prominence of linear structures for the n = 7, 9, …
Date:
July 31, 2007
Creator:
Belau, Leonid; Wheeler, Steven E.; Ticknor, Brian W.; Ahmed, Musahid; Leone, Stephen R.; Allen, Wesley D. et al.
Object Type:
Article
System:
The UNT Digital Library
Remote Detection of Radioactive Plumes Using Millimeter Wave Technology
The reprocessing of spent nuclear fuel, a common method for manufacturing weapons-grade special nuclear materials, is accompanied by the release of fi ssion products trapped within the fuel. One of these fi ssion products is a radioactive isotope of Krypton (Kr-85); a pure β- emitter with a half-life of 10.72 years. Due to its chemical neutrality and relatively long half life, nearly all of the Kr-85 is released into the surrounding air during reprocessing, resulting in a concentration of Kr-85 near the source that is several orders of magnitude higher than the typical background (atmospheric) concentrations. This high concentration of Kr-85 is accompanied by a proportionately high increase in air ionization due to the release of beta radiation from Kr-85 decay. Millimeter wave (MMW) sensing technology can be used to detect the presence of Kr-85 induced plumes since a high concentration of ions in the air increases the radar cross section due to a combination of atmospheric phenomena. Possible applications for this technology include the remote sensing of reprocessing activities across national borders bolstering global anti-proliferation initiatives. The feasibility of using MMW radar technology to uniquely detect the presence of Kr-85 can be tested using commercial ion generators or sealed …
Date:
January 1, 2009
Creator:
Barnowski, Ross; Chien, Haul-Te & Gopalsami, Nachappa
Object Type:
Article
System:
The UNT Digital Library
Interchange and Infernal Fishbone Modes in Plasmas with Tangentially Injected Beams
New energetic particle mode instabilities of fishbone type are predicted. The considered instabilities are driven by the circulating energetic ions. They can arise in plasmas of tokamaks and spherical tori with weak magnetic shear in the wide core region and strong shear at the periphery, provided that the central safety factor is close to the ratio m/n, where m and n are the poloidal mode number and toroidal mode number, respectively. The instability with m = n = 1 has interchange-like spatial structure, whereas the structure of instabilities with m/n > 1 is similar to that of the infernal MHD mode (except for the region in vicinity of the local Alfvén resonance).
Date:
January 1, 2006
Creator:
Kolesnichenko, Ya I.; Marchenko, V. S. & White, R. B.
Object Type:
Report
System:
The UNT Digital Library
User's manual for RESRAD version 6.
This manual provides information on the design and application of the RESidual RADioactivity (RESRAD) code. It describes the basic models and parameters used in the RESRAD code to calculate doses and risks from residual radioactive materials and the procedures for applying these models to calculate operational guidelines for soil contamination. RESRAD has undergone many improvements to make it more realistic in terms of the models used in the code and the parameters used as defaults. Version 6 contains a total of 145 radionuclides (92 principal and 53 associated radionuclides), and the cutoff half-life for associated radionuclides has been reduced to 1 month. Other major improvements to the RESRAD code include its ability to run uncertainty analyses, additional options for graphical and text output, a better dose conversion factor editor, updated databases, a better groundwater transport model for long decay chains, an external ground radiation pathway model, an inhalation area factor model, time-integration of dose and risk, and a better graphical user interface. In addition, RESRAD has been benchmarked against other codes in the environmental assessment and site cleanup arena, and RESRAD models have been verified and validated.
Date:
July 23, 2001
Creator:
Yu, C.; Zielen, A. J.; Cheng, J. J.; LePoire, D. J.; Gnanapragasam, E.; Kamboj, S. et al.
Object Type:
Report
System:
The UNT Digital Library
Physics development of web-based tools for use in hardware clusters doing lattice physics
Jefferson Lab and MIT are developing a set of web-based tools within the Lattice Hadron Physics Collaboration to allow lattice QCD theorists to treat the computational facilities located at the two sites as a single meta-facility. The prototype Lattice Portal provides researchers the ability to submit jobs to the cluster, browse data caches, and transfer files between cache and off-line storage. The user can view the configuration of the PBS servers and to monitor both the status of all batch queues as well as the jobs in each queue. Work is starting on expanding the present system to include job submissions at the meta-facility level (shared queue), as well as multi-site file transfers and enhanced policy-based data management capabilities.
Date:
August 1, 2001
Creator:
Dreher, P.; Akers, Walt; Chen, Jian-ping; Chen, Y. & Watson, William A., III
Object Type:
Article
System:
The UNT Digital Library
Band anticrossing effects in Mg{sub y}Zn{sub 1-y}Te{sub 1-x}Se{sub x} alloys
None
Date:
August 7, 2001
Creator:
Wu, J.; Walukiewicz, W.; Yu, K. M.; Ager, J. W., III; Shan, W.; Haller, E. E. et al.
Object Type:
Article
System:
The UNT Digital Library