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PubMed2004

Vibrational spectra of germanium-carbon clusters. II. GeC(7) and GeC(9).

Experimental and theoretical studies of a novel family of germanium-carbon clusters (Ge(n)C(m)) that were initiated with our earlier identification of the GeC(3)Ge cluster have now been extended to the GeC(7) and GeC(9) chains. The new clusters, which were formed by laser ablation and trapped in solid Ar at approximately 10 K, have been identified using Fourier-transform infrared (FTIR) measurements coupled with density-functional theory (DFT) calculations. The nu(1)(sigma) vibrational fundamental of linear GeC(7) has been identified at 2063.6 cm(-1), and an absorption at 1928.3 cm(-1) has been assigned to the nu(4)(sigma) fundamental of linear GeC(9). FTIR measurements of the isotopic shifts for the assignments are in good agreement with the DFT predictions.

Journal Article↗
PubMed2005

Search for global minimum geometries for medium sized germanium clusters: Ge12-Ge20.

We have performed an unbiased search for the global minimum geometries of small-to-medium sized germanium clusters Gen(12< or =n< or =18) as well as a biased search (using seeding method) for Gen(17< or =n< or =20). We employed the basin-hopping algorithm coupled with the plane-wave pseudopotential density functional calculations. For each size, we started the unbiased search with using several structurally very different initial clusters, or we started the biased search with three different seeds. Irrespective of the initial structures of clusters we found that the obtained lowest-energy clusters of the size n=12-16 and 18 are the same. Among them, the predicted global minima of Gen(12< or =n< or =16) are identical to those reported previously [Shvartsburg et al., Phys. Rev. Lett. 83, 167 (1999)]. For n=17-20, we have identified two or three nearly isoenergetic low-lying isomers (for each size) that compete for the global minimum. Nearly all the low-lying clusters in the size range of 12< or =n< or =20 contain the tri-caped trigonal prism motif and are all prolate in geometry, in agreement with the experiment.

Journal Article↗
PubMed2005

Theoretical study on germanium cyanide radical GeCN and its ions.

A detailed theoretical study is performed on the hitherto unknown germanium cyanide radical and its ions. The (2)Pi state GeCN lies 5.0 kcal/mol lower than the (2)Pi state GeNC at the coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations [CCSD(T)]/6-311++G(3df)//quadratic configuration interaction with single and double excitations (QCISD)/6-311G(d)+zero-point vibrational energy (ZPVE) level. For interconversion between them, two electronic state pathways (2)A(') and (2)A(") are located, with the latter being 0.7 kcal/mol more favorable than the former. On the (2)A(") path, the GeCN-->GeNC and GeNC-->GeCN conversion barriers are 14.5 and 9.5 kcal/mol, respectively. The detailed singlet and triplet potential-energy surfaces of both the cationic and anionic GeCN species are also investigated. On the ground-state electronic hypersurface, singlet GeNC(+) is 4.6 kcal/mol more stable than singlet GeCN(+), whereas triplet GeNC(-) is 10.0 kcal/mol less stable than triplet GeCN(-). The relative energy difference between the GeCN(0,+/-) and GeNC(0,+/-) can be well correlated with the number of vacant orbitals on the Ge atom. The stability of the neutral and ionic CGeN and cyclic cGeCN is also discussed. The predicted structures, spectroscopies, ionization, and affinity energies as well as the Renner-Teller properties are expected to provide reliable estimates for future characterization of the potential GeCN and GeNC radicals as well as their ionic counterparts both in the laboratory and in the interstellar space.

Journal Article↗
PubMed2005

Photoluminescence of oxygen-containing surface defects in germanium oxides: a theoretical study.

Photoabsorption and photoluminescence properties of nonbridging oxygen -O-Ge[triple bond](NBO), -OO-Ge[triple bond] (peroxy radical), O=Ge=, and (O2)Ge= defects in germanium oxides have been investigated by high-level ab initio calculations. Geometry optimization for excited electronic states of model clusters simulating these defects was carried out at the complete-active-space self-consistent-field level, and relative energies were calculated by various methods including time-dependent density-functional theory, outer-valence Green's functions, equation-of-motion coupled cluster theory with single and double excitations, symmetry-adapted cluster configuration interaction, multireference second-order perturbation theory, and multireference configuration interaction. The results demonstrate that the considered excited states of the aforementioned defects normally exhibit large Stokes shifts and that, with few exceptions, UV photoabsorption is accompanied by red or IR photoluminescence.

Journal Article↗
PubMed2006

In search of the germanium halomethylidyne (Ge=C-X; X=F,Cl,Br) free radicals: Ab initio studies of their spectroscopic signatures.

A variety of ab initio methods have been used to calculate the X (2)Pi and A (2)Sigma(+) state spectroscopic parameters of the GeCX (X=F,Cl,Br) free radicals. The theoretical methods and basis sets were tested on GeCH, for which extensive experimental data are available, and found to give predictions sufficiently reliable to guide experimental searches for spectra. In all cases, the linear Ge=C-X species was found to be the global minimum on the potential energy surface, with the bent X-Ge=C ((2)A(')) isomer as a local minimum much higher (62-36 kcal/mol) in energy. In both the ground and excited states, the GeC moiety is very similar to that of GeCH, with a double bond in the lower state and a triple bond in the excited state, indicating that halogenation does not radically perturb the energetics or structure of germanium methylidyne. Ground state GeCX radicals have suitable rotational constants for microwave studies, although they suffer from only modest dipole moments. Matrix infrared experiments are most likely to detect the nu(1) fundamentals in the 1450-1100 cm(-1) region or the nu(3) fundamentals at the transition between the mid- and far-infrared regions. We have used the ab initio values for the Renner-Teller parameter, the average bending frequency, and the spin-orbit coupling constant to calculate the ground state energy levels, which will be helpful in the interpretation of A-X single vibronic level emission spectra, if they can be observed. The electronic absorption spectra of the (2)Pi(32) spin component of the 0(0) (0) bands of all three radicals have been calculated assuming typical jet-expansion conditions and should be useful in future laser-induced fluorescence, resonance enhanced multiphoton ionization, or cavity ringdown searches for the electronic band systems.

Journal Article↗
PubMed2006

Vibrational spectra of germanium-carbon clusters in solid Ar: identification of the nu4(sigma(u)) mode of linear GeC5Ge.

The linear GeC(5)Ge cluster has been detected in Fourier transform infrared spectra observed when the products from the dual laser evaporation of carbon and germanium rods were trapped in solid Ar at approximately 10 K. Comparison of (13)C isotopic shift measurements with the predictions of density functional theory calculations at the B3LYP/cc-pVDZ level confirms the identification of the nu(4)(sigma(u)) mode of GeC(5)Ge at 2158.0 cm(-1).

Journal Article↗
PubMed2006

Photoluminescence of oxygen-deficient defects in germanium oxides: a quantum chemical study.

The photoabsorption and photoluminescence (PL) properties of the surface E(') center, -GeX(3), and the combined E(')-center-oxygen vacancy, X(3)Ge-GeX(2), defects in substoichiometric germanium oxides have been investigated by high-level ab initio calculations, including complete active space self-consistent field, multireference configuration interaction, and symmetry-adapted cluster configuration interaction methods. Both defects have been shown to give rise to photoabsorption bands between 4 and 6 eV. Geometry relaxation is significant and the Stokes shifts are large for all calculated excited states. A removal of an electron from the Ge-Ge bond leads to its destruction, whereas the creation of an electron hole at lone pairs of O atoms results in elongations of the Ge-O-Ge bonds in the corresponding bridges. Most often, deexcitations of excited electronic states proceed radiationlessly, through crossing points of their potential energy surfaces with those of the lower states. The -GeX(3) defect is able to generate several PL bands in the UV ( approximately 3 eV) and IR (1.2-1.4 and 0.5-0.6 eV) spectral ranges, whereas the X(3)Ge-GeX(2) defect gives only one red/orange PL band at 2.0-2.1 eV. No intense PL band was found in the blue spectral region of 2.5-2.7 eV, and the two defects are not likely to contribute to the intense blue photoluminescence observed for GeO(2) nanowires.

Journal Article↗
PubMed1990

Temporal instabilities associated with a planar high purity germanium detector.

Experiments have been performed which show that a grooved planar high purity germanium semiconductor detector may give rise to time variant counting rates in energy regions below prominent spectral peaks. It is postulated that this effect is due to prolonged or incomplete charge collection in the vicinity of the bottom of the groove, and is caused by distortions in the applied electric field in this region. Such effects may lead to changes in the minimum detectable count rate in x-ray fluorescence measurements; these can be reduced by collimating incident photons on the central region of the detector but the resulting reduction in efficiency lowers the overall sensitivity.

Radiometry↗
PubMed1999

The effect of lung deposition patterns on the activity estimate obtained from a large area germanium detector lung counter.

Lung counters for in vivo detection of low energy photon emitters are typically calibrated using phantoms containing lung tissue equivalent material with the radioactivity homogeneously distributed throughout the material. If the activity in a measurement subject is heterogeneously distributed, the activity estimate for that subject will be uncertain due to the assumptions of distribution. The magnitude of the uncertainty for a four-detector germanium array, using the Lawrence Livermore National Laboratory torso phantom with a newly designed lung set that allows the activity to be localized in one or more of 16 areas, was estimated. The results show that detector arrays will reduce the uncertainties arising from the geometry of the lung deposition compared to single detectors. The estimated activity of an internal deposition that emits 17.5 keV photons can be overestimated by a factor of three, or underestimated by a factor of infinity (i.e., the activity is missed completely). As the photon energy rises to 59.5 keV the uncertainty in the activity decreases so that the maximum overestimate (underestimate) will be a factor of two (five). As the energy rises to 344.3 keV only the maximum underestimate changes: it becomes a factor of three.

Humans↗
PubMed2000

Study of phonon dispersion in silicon and germanium at long wavelengths using picosecond ultrasonics

We have studied the dispersion of long wavelength longitudinal phonons in silicon and germanium using ultrasonic techniques. For long wavelengths, the acoustic phonon dispersion relation is of the form omega(k) approximately ck-gammak(3), where c is the speed of sound and gamma measures the lowest-order phonon dispersion. By sending an ultrasonic pulse of length a few hundred angstroms into a crystal and measuring the change of the pulse shape with propagation distance, we are able to determine the parameter gamma. The results are compared with lattice dynamics models.

Journal Article↗
PubMed2000

A three-dimensional open-framework germanate containing four-, five- and six-coordinated germanium

A new three-dimensional open-framework germanate, namely ethylenediamine bis(ethylenediammonium) tetrahydroxooctadecaoxononagermanate, (C(2)H(8)N(2))(C(2)H(10)N(2))(2)[Ge(9)O(18)(OH)(4)], has been synthesized hydrothermally and its structure determined by single-crystal X-ray diffraction. The framework is built of [Ge(9)O(22)(OH)(4)] units formed by four-, five- and six-oxygen-coordinated germanium and templated by ethylenediamine. Three types of intersecting channels are formed in the framework, one by eight-membered rings running along the b axis and the other two by ten-membered rings running parallel to the a and c axes, respectively.

Journal Article↗
PubMed2002

Germanium(IV) chloride at 193 K.

Single crystals of germanium(IV) tetrachloride, GeCl(4), were grown at 193 K. In the crystal structure, the isolated molecules have very well realised tetrahedral geometry.

Journal Article↗
PubMed2005

Three-beam resonant X-ray diffraction in germanium - Laue transmission cases.

Perturbation of the two-beam diffracted power owing to the influence of a third lattice node has been examined for various three-beam cases in a small finite germanium crystal in the vicinity of the K-absorption edge. Although the crystal was slightly imperfect, the main parts of the experimental results are very well described within the framework of the fundamental theory of X-ray diffraction in conjunction with Cromer-Liberman calculations for the resonant scattering terms. Beam divergence and dynamical block size are treated as adjustable parameters in the analysis. Observed changes in the three-beam profile asymmetry are mainly attributed to size and not to resonance effects associated with the triplet phase sum of the involved reflections. Close to the absorption edge there is however some evidence indicating that f' values should be reduced in magnitude compared to the tabulated ones.

Journal Article↗
PubMed1999

Determination of mean inner potential of germanium using off-axis electron holography.

Off-axis electron holography has been used to determine the mean inner potential of germanium using cleaved 90 degrees wedge samples, where the wedge thickness profiles were checked by weak-beam dark-field extinction fringes. Dynamical contributions to the phase of the image were minimized by tilting to weakly diffracting conditions, as confirmed by reference to convergent-beam electron diffraction patterns. Small residual corrections were determined using multislice calculations. From a total of 18 separate measurements, it is concluded that the value of the mean inner potential is 14.3(2) V, which agrees with recent theoretical calculations to within experimental error.

Journal Article↗
PubMed1998

Shape transition of germanium nanocrystals on a silicon (001) surface from pyramids to domes

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been shown to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change.

Journal Article↗
PubMed2003

Development of a gallium-doped germanium far-infrared photoconductor direct hybrid two-dimensional array.

To our knowledge, we are the first to successfully report a direct hybrid two-dimensional (2D) detector array in the far-infrared region. Gallium-doped germanium (Ge:Ga) has been used extensively to produce sensitive far-infrared detectors with a cutoff wavelength of approximately equal to 110 microm (2.7 THz). It is widely used in the fields of astronomy and molecular and solid spectroscopy. However, Ge:Ga photoconductors must be cooled below 4.2 K to reduce thermal noise, and this operating condition makes it difficult to develop a large format array because of the need for a warm amplifier. Development of Ge:Ga photoconductor arrays to take 2D terahertz images is now an important target in such research fields as space astronomy. We present the design of a 20 x 3 Ge:Ga far-infrared photoconductor array directly hybridized to a Si p-type metal-oxide-semiconductor readout integrated circuit using indium-bump technology. The main obstacles in creating this 2D array were (1) fabricating a monolithic Ge:Ga 2D array with a longitudinal configuration, (2) developing a cryogenic capacitive transimpedance amplifer, and (3) developing a technology for connecting the detector to the electronics. With this technology, a prototype Ge:Ga photoconductor with a direct hybrid structure has shown a responsivity as high as 14.6 A/W and a minimum detectable power of 5.6 x 10(-17) W for an integration time of 0.14 s when it was cooled to 2.1 K. Its noise is limited by the readout circuit with 20 microV/Hz(1/2) at 1 Hz. Vibration and cooling tests demonstrated that this direct hybrid structure is strong enough for spaceborne instruments. This detector array will be installed on the Japanese infrared satellite ASTRO-F.

Journal Article↗
PubMed2003

Analysis of thermal decay and prediction of operational lifetime for a type I boron-germanium codoped fiber Bragg grating.

The thermal decay of a type I fiber Bragg grating written at 248 nm in boron-germanium codoped silica fiber was examined in terms of its reflectivity and Bragg wavelength change. In addition to the decay in reflectivity, which was observed, a shift in Bragg wavelength over the temperature range considered was seen. A mechanism for the decay in the reflectivity was developed and modeled according to a power law, and the results were compared with those from the aging curve approach. The wavelength shift was simulated by modification of the power law, which was also found to fit well to the experimental data. Temperature-induced reversible and irreversible change in the grating characteristic were observed and considered to be a means to predict the working lifetime of the grating at comparatively low temperatures. Accelerated aging was also reviewed and compared in terms of reflectivity and Bragg wavelength shift. It was shown that the temperature-induced irreversible shift in the Bragg wavelengths could not be predicted by use of the isothermal decay of the refractive-index modulation. The results were discussed within the framework of the current theoretical approaches for predicting the stability of gratings of this type.

Journal Article↗
PubMed2004

Refractive-index measurements of zinc germanium diphosphide at 300 and 77 K by use of a modified Michelson interferometer.

A method to determine the absolute refractive index of materials available in the shape of flat wafers with parallel sides by using interferometric techniques is presented. With this method, nondestructive, sample-specific measurements can be made. The method is tested by using silicon, germanium and zinc selenide, and measurements for both the ordinary and extraordinary axes of ZnGeP2 for temperatures of 300 and 77 K are reported.

Journal Article↗