Graphene growth of high crystal quality and single-layer thickness can be achieved by low pressure sublimation (LPS) on SiC(0001). On SiC(0001), which is the C-terminated polar surface, there has been much less success growing uniform, single-layer films. In this work, a systematic study of surface preparation by hydrogen etching followed by LPS in an argon ambient was performed. Hydrogen etching is an important first step in the graphene growth process because it removes damage caused by polishing the substrate surface. However, for SiC(0001), etching at too high of a temperature or for too long has been found to result in pit formation due to the preferential etching of screw dislocations that intersect the surface. It was found that temperatures above 1450°C in 200mbar of hydrogen result in pitting of the surface, whereas etch temperatures at and below 1450°C can result in atomically at terraces of ~ 1 µm width. Following the hydrogen etch optimization, argon-mediated graphene growth was carried out at several different temperatures. For the growth experiments, pressure and growth time were both fixed. Regardless of growth temperature, all of the films were found to have non-uniform thickness. Further, x-ray photoelectron spectroscopy and low energy electron diffraction measurements reveal that trace amounts of oxygen, which may be present during growth, significantly affects the graphene growth process on this polar surface.
Soliton-like pulses with a 1984-nm center wavelength are produced from a Tm-doped mode-locked fiber laser. The linear cavity has a graphene saturable absorber mirror at one end and a fiber Bragg grating as the output coupler. The laser operates without dispersion compensation, and the repetition rate was tuned from 20 to 5 MHz by the addition of SMF-28 fiber. The dry transfer process used to place the graphene on a mirror could be extended to any optical substrate. This enables integration of graphene with optics such as an optical window coated with a graphene filter or a graphene-saturable absorber placed directly on a semiconductor laser facet.
Surface-enhanced Raman scattering (SERS) from trinitrotoluene and other nitro-based explosives is important for the
development of a reliable detection scheme exhibiting low false-positive rates. However, the interaction of these
compounds with Ag and Au causes the molecules to orient in ways such that the primary vibrations of the nitro groups,
the main identifying Raman marker of these compounds, are inhibited in addition to causing a reduction in the SERS
response. It has recently been shown that cysteamine, which contains amine functional end groups, will electrostatically
attract the nitro groups of TNT. Therefore, as the thiol functional group of cysteamine chemically bonds this molecule
to the plasmonically-active Au and Ag nanoparticles studied, SERS of TNT can be obtained following the nitro-amine
functional group complex formation. It is observed that the cysteamine adsorbs in one of two configurations on the
metal surface, with the trans configuration consisting of bonding at the S end of the molecule and the cysteamine is
perpendicular to the metal surface, while in the Gauche configuration S bonding occurs, but the molecule bends over
towards the metal film surface, approaching the parallel configuration allowing the amine groups interact with the
surface. We find that the trans configuration is best for the detection of SERS from TNT. Experiments compare well
with DFT calculations of the cysteamine and TNT complex and their adsorption on Ag.
Photoluminescence (PL) and electron paramagnetic resonance (EPR) are high-resolution techniques used to study donors and acceptors in optoelectronic materials. Zinc oxide (ZnO), with a room-temperature band gap of 3.37 eV, has significant potential for applications ranging from light emission to sensors and detectors. The low-temperature near-edge PL of ZnO is rich in detail, with sharp-line emissions from bound excitons related to various donors and acceptors. Strong phonon couplings in this material produce a series of LO and TO phonon sidebands at slightly lower energies. Donor-acceptor pair and electron-acceptor recombinations related to nitrogen (EA = 209 meV) and lithium (EA ~ 0.6 eV) are detected. Copper and iron impurities show characteristic luminescence spectra in the visible. Thermal anneals in air induce significant changes in the PL spectra. Complementary information can be obtained from EPR and photoinduced EPR experiments performed at low temperature. In ZnO, EPR spectra have been observed from neutral nitrogen acceptors, neutral copper acceptors, neutral lithium acceptors, hydrogenic shallow donors, as well as deeper donors such as nickel and iron. In previous work, EPR spectra have been assigned to singly ionized oxygen vacancies and singly ionized zinc vacancies in electron-irradiated crystals.
Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material useful for frequency conversion applications in the midinfrared. A broad absorption band peaking near 1.2 microns and extending past 2 microns is often observed. To identify the defects responsible for these absorption losses, we have performed an optical absorption investigation from 10 to 296 K on bulk crystals of ZnGeP2 grown by the horizontal gradient-freeze method. Three broad absorption bands in the spectral range from 1 to 4 microns are observed that are due to native defects. Comparison of photoinduced changes in absorption with photoinduced changes in EPR spectra allowed specific defects to be associated with each of the three absorption bands. A band peaking near 1.2 microns and another band peaking near 2.2 microns involve transitions associated with singly ionized zinc vacancies. A third absorption band, peaking near 2.3 microns and extending from 1.5 microns to beyond 4 microns, involves neutral phosphorus vacancies. Absorption bands due to anion-site donor impurities Se and S have also been studied.
CdGeAs2 is an important nonlinear optical infrared material. Room-temperature absorption and temperature-dependent photoluminescence (PL) of as-grown p-type bulk crystals and crystals doped with indium and tellurium have been measured. The intensity of an intervalence band absorption near 5.5 microns (0.225 eV) is correlated with the intensity of a PL band near 0.55 eV. Both of these optical features indicate the presence of a native shallow acceptor level at 120 meV above the top valence band. The 0.55-eV PL band is donor-acceptor-pair recombination between shallow donors and the shallow acceptor level. A second PL band peaking near 0.35 eV is donor-acceptor-pair recombination between shallow donors and a deeper acceptor at 300 meV above the top valence band. Doping with indium and tellurium produces n-type material. The intervalence band absorption at 5.5 microns is completely eliminated in the n-type samples. Indium donors are incorporated on the Cd site and Te donors are incorporated on the As site.
Electron paramagnetic resonance (EPR) has been used to identify and characterize point defects in lithium triborate (LiB3O5) crystals grown for nonlinear optical applications. As-grown crystals contain oxygen vacancies and lithium vacancies (as well as trace amounts of transition-metal ions in a few samples). Exposing a crystal to ionizing radiation at 77 K produces “free” electrons and holes. These electrons are trapped at the pre-existing oxygen vacancies and give rise to an EPR signal with a large hyperfine from one 11B nucleus. The corresponding holes become self-trapped on oxygen ions as a result of the significant lattice relaxation of a nearest-neighbor fourfold-bonded boron ion. This gives rise to an EPR signal with a smaller 11B hyperfine pattern due to the oxygen’s threefold bonded boron neighbor. Warming the crystal to approximately 130 K destroys the self-trapped hole centers that were initially formed, and allows a second holelike signal to be observed (which in turn decays between 150 and 200 K). The structure of the second hole center is very similar to the self-trapped hole center and a neighboring lithium vacancy makes this latter center more thermally stable. The EPR spectra from Ni+ and Cu2+ ions are also reported.
Cadmium zinc telluride (CdZnTe) is being developed for room- temperature x-ray and gamma ray detectors. Identification and control of point defects and charge compensators are currently important issues. We have used electron paramagnetic resonance (EPR) and photo-induced EPR to evalute shallow-donor defects in CdZnTe crystal grown by two different techniques. Samples grown by the high-pressure Bridgman technique and a crystal grown by horizontal Bridgman at IMARAD and doped with indium were included in this study. Prior to the EPR investigations, we performed liquid-helium photoluminescence (PL) in order to examine the radiation recombination paths and identify the presence of other defects in these crystals. Spectra were obtained showing sharp excitonic lines, shallow and deep DAP emission bands, and a deeper 1.1 eV emission. The PL data help define the optical excitation range used in photo-EPR measurements. The photo-EPR data obtained from our samples is used to determine the concentration of isolated donor centers, while the EPR signal present under no illumination gives a measure of the net compensation. We also report the excitation wavelength dependence of the isotropic EPR signal from the shallow donors.
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