Pristine Crystal Research Articles

Density Functional Theory Studies of Hole Mobility in Picene and Pentacene Crystals

We have investigated the electronic properties and hole mobilities of picene and its isomer pentacene using the density functional theory and classical Marcus charge transfer theory. In pristine crystal with herringbone structure, pentacene and picene have drift hole mobilities of 2.147 and 0.644 cm2 V–1 s–1, respectively, which are consistent with recent experimental results. We also show that picene crystal can exhibit maximum mobility up to 2.629 cm2 V–1 s–1 along the π–π stacking direction, while the highest mobility of pentacene crystal exhibits along the herringbone stacking direction. Also the anisotropy of the mobility in the ab plane is 2.4 and 5.6 for picene and pentacene crystals, respectively. Since the air stability of picene is better than pentacene due to lower HOMO levels in picene, we suggest that picene and its homologous phenacenes series are promising candidates toward high mobility organic semiconductor devices with good air stability. This work also sheds light on the favorable or un...

Damage recovery of FIB modified Si for directed-assembly of semiconductor nanostructures

Focused ion beam (FIB) techniques have previously been shown to have applications in templating semiconductor nanostructure growth on Si. To assess crystalline quality in this method, or in other FIB-based nano-fabrication methods in Si, we assess in this work FIB-implantation damage in Si and subsequent recovery by annealing. Specifically, we study Si substrates implanted to 1 × 1012–5 × 1015 ions cm−2 fluences of 30 kV Si2+, Ge2+ and Ga+ incident normally at room temperature, covering a structural damage regime from almost no damage to amorphization. Raman spectroscopy with incident photons of wavelength 405 and 514 nm were used for probing structural damage in different depths in as-implanted and thermally annealed substrates. Annealing was performed for varying times at 730–900 °C in ultra-high purity nitrogen ambient. Structural damage quantification was performed through measurements of peak height and position of the crystalline Si peak at 520 cm−1. Our analysis shows that a structural damage parameter, D, (defined by subtracting from unity, the ratio of the height of the Si Raman peak in implanted/annealed samples to that for the Si peak in an unimplanted standard, such that D = 0.00 and D = 1.00 correspond to pristine single crystal and amorphous Si respectively) is 0.05 or less in all the Si and Ge implants after annealing to 900 °C, 1800 s. However, D for the Ga implants is close to zero for Ga fluences up to 3 × 1013 cm−2 but increases steadily to 0.19 for 5 × 1015 Ga cm−2 under these annealing conditions. The underlying damage recovery mechanisms are discussed.

Thermoelectric Properties of p-Type Clathrate Ba8.0Ga15.9ZnySn30.1 Single Crystals with Various Carrier Concentrations

To match the excellent performance of n-type type-VIII clathrate Ba8Ga16Sn30 single crystals in thermoelectric (TE) generator design, synthesizing a large size p-type counterpart with a high TE figure of merit ZT continues to be a challenge for many years. In this work, we obtained 7-mm-diameter single crystals of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 (Zn-BGS) with various carrier concentrations by Ga-flux method. Though the actual substituted compositions of the single crystals Ba8Ga15.9ZnySn30.1 (y = 0–0.019) are much lower than the nominal ones, the Zn atoms work as a “spur” and effectively suppress the nucleation rate of BGS in the Ga-flux, which is vital to achieving a similar large size crystal with the n-type counterpart. The y = 0.006 and 0.007 crystals share nearly the same or a slightly lower value of resistivity with a pristine crystal in the whole temperature. However, the Hall coefficient measurement revealed that Zn substitution triggers a profound transition from a “low-carrier-concentration high-carrier-mobility” regime to a “high-carrier-concentration low-carrier-mobility” one in p-type BGS. As a consequence, both the Seebeck coefficient and thermal conductivity benefit from the relief of the negative impact of minority carrier by shifting the “trigger temperature” of intrinsic excitation to a higher temperature. Figure of merit ZTs of 0.97 and 1.07 are observed for x = 0.006, 0.007 at 500 K, which are more than 25% and 40% higher than the figure of merit for pristine BGS at the same temperature zone, respectively. The improvement demonstrates the potential of making TE modules from n- and p-type single crystal BGS for waste heat recovery at medium temperatures.

Dynamics of ultrashort pulsed laser radiation induced non-thermal ablation of graphite

We report on the dependence of a laser radiation induced ablation process of graphite on the applied pulse duration of ultrashort pulsed laser radiation smaller than 4 ps. The emerging so-called non-thermal ablation process of graphite has been confirmed to be capable to physically separate ultrathin graphitic layers from the surface of pristine graphite bulk crystal. This allows the deposition of ablated graphitic flakes on a substrate in the vicinity of the target. The observed ablation threshold determined at different pulse durations shows a modulation, which we ascribe to lattice motions along the c axis that are theoretically predicted to induce the non-thermal ablation process. In a simple approach, the ablation threshold can be described as a function of the energy penetration depth and the absorption of the applied ultrashort pulsed laser radiation. Based on the analysis of the pulse duration dependence of those two determining factors and the assumption of an invariant ablation process, we are able to reproduce the pulse duration dependence of the ablation threshold. Furthermore, the observed pulse duration dependences confirm the assumption of a fast material specific response of graphite target subsequent to optical excitation within the first 2 ps.

How disorder affects the Berry-phase anomalous Hall conductivity: A reciprocal-space analysis

The anomalous Hall conductivity of "dirty" ferromagnetic metals is dominated by a Berry-phase contribution which is usually interpreted as an intrinsic property of the Bloch electrons in the pristine crystal. In this work we evaluate the geometric Hall current directly from the electronic ground state with disorder, and then recast it as an integral over the crystalline Brillouin zone. The integrand is a generalized k-space Berry curvature, obtained by unfolding the Berry curvature from the small Brillouin zone of a large supercell. Therein, disorder yields a net extrinsic Hall contribution, which we argue is related to the elusive side-jump effect. As an example, we unfold the first-principles Berry curvature of an Fe3Co ordered alloy from the original fcc-lattice Brillouin zone onto a bcc-lattice zone with four times the volume. Comparison with the virtual-crystal Berry curvature clearly reveals the symmetry-breaking effects of the substitutional Co atoms.

Crystal structure search and electronic properties of alkali-doped phenanthrene and picene

Alkali doped aromatic compounds have shown evidence of metallic and superconducting phases whose precise nature is still mysterious. In potassium and rubidium doped phenanthrene, superconducting temperatures around 5 K have been detected, but such basic elements as the stoichiometry, crystal structure, and electronic bands are still speculative. We seek to predict the crystal structure of M3-phenanthrene (M = K, Rb) using ab-initio evolutionary simulation in conjunction with density functional theory (DFT), and find metal but also insulator phases with distinct structures. The original P21 herringbone structure of the pristine molecular crystal is generally abandoned in favor of different packing and chemical motifs. The metallic phases are frankly ionic with three electrons acquired by each molecule. In the nonmagnetic insulating phases the alkalis coalesce reducing the donated charge from three to two per phenanthrene molecule. A similar search for K3-picene yields an old and a new structure, with unlike potassium positions and different electronic bands, but both metallic retaining the face-to-edge herringbone structure and the P21 symmetry of pristine picene. Both the new K3-picene and the best metallic M3-phenanthrene are further found to undergo a spontaneous transition from metal to antiferromagnetic insulator when spin polarization is allowed, a transition which is not necessarily real, but which underlines the necessity to include correlations beyond DFT. Features of the metallic phases that may be relevant to phonon-driven superconductivity are underlined.

Critical current anisotropy in Nd-1111 single crystals and the influence of neutron irradiation

We report on angle-resolved magnetization measurements on NdFeAsO0.65F0.35 (Nd-1111) single crystals. The field dependence of the critical current density, Jc, is non-monotonic in these crystals at all orientations and temperatures due to the fishtail effect, which strongly influences the angular dependence of Jc. The currents decrease as the field is tilted from the crystallographic c-axis at low fields, but increase at high fields. A peak occurs in the angular dependence of Jc at intermediate fields. The critical currents are significantly enhanced after irradiation with fast neutrons and the fishtail disappears. The different current anisotropies at low and high fields, however, persist. We discuss the data in the framework of the anisotropic scaling approach and propose a transition from dominant pinning by large defects of low density at low fields to pinning by small defects of high density at high fields in the pristine crystal. Strong pinning dominates at all fields after the irradiation, and the angular dependence of Jc can be described via anisotropic scaling only after an appropriate extension to this pinning regime.

The Effect of 320 MeV Au Irradiation in K-doped Ba-122

We report the effect of 320 MeV Au irradiation in (Ba0.6K0.4)Fe2As2 single crystals including its dose dependence. We found that the suppression of Tc is weak up to a certain level of irradiation in agreement with the previous report. Critical current density (Jc) is strongly enhanced by the introduction of defects up to 15 MA/cm2 at 2K under self- field, which is 10 times larger than the pristine crystal. The field dependence of Jc changes from H-0.5 to H-1, which indicates that columnar defects are effectively introduced in the crystal by the irradiation.

Phonons in two-dimensional soft colloidal crystals

The vibrational modes of pristine and polycrystalline monolayer colloidal crystals composed of thermosensitive microgel particles are measured using video microscopy and covariance matrix analysis. At low frequencies, the Debye relation for two-dimensional harmonic crystals is observed in both crystal types; at higher frequencies, evidence for van Hove singularities in the phonon density of states is significantly smeared out by experimental noise and measurement statistics. The effects of these errors are analyzed using numerical simulations. We introduce methods to correct for these limitations, which can be applied to disordered systems as well as crystalline ones, and we show that application of the error correction procedure to the experimental data leads to more pronounced van Hove singularities in the pristine crystal. Finally, quasilocalized low-frequency modes in polycrystalline two-dimensional colloidal crystals are identified and demonstrated to correlate with structural defects such as dislocations, suggesting that quasilocalized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids.

Controllable fabrication of amorphous Si layer by energetic cluster ion bombardment

We report on the fabrication of an amorphous Si (a-Si) thin layer by means of bombardment of a Si(100) surface using monoenergetic C60 cluster ions with energies from 50 keV to 400 keV. The C60 cluster implantation produces nanogranules on the surface of a-Si layer detected by atomic force microscopy. The structural disorder and thickness of the modified layer were identified using Raman spectrometry, ion channelling, spectroscopic ellipsometry (SE) and transmission electron microscopy (TEM). According to SE and TEM data the thickness of a-Si layer gradually increases with cluster ion energy reaching to about 30 nm in the 200 keV C60-bombarded Si sample. There is also thin layer of nanocrystalline Si found between the a-Si layer and pristine Si crystal. The obtained results represent an attractive method for creation of the a-Si layer as a functional material for opto- and nano-electronics. The study describes nanostructure created by cluster ion implantation as well as demonstrates the structural consequences of fast cluster energy dissipation in solids such as local heating and shock waves.

Room temperature, high-efficiency, noncritical phase-matching fourth harmonic generation in partially deuterated ADP crystal

High-efficiency frequency conversion into the deep ultraviolet in a partially deuterated ammonium dihydrogen phosphate (ADP) crystal is reported. For a 60% deuterated ADP crystal, the spontaneous Raman scattering intensity decreased by 38% compared with that of a pristine ADP crystal. At room temperature (24°C), noncritical phase-matching fourth harmonic generation of a 1053 nm laser was realized. The angular acceptance was 55.7 mrad, and the external conversion efficiency from 526 to 263 nm reached 85.3%.

Growth and Characterization of High-Quality Single Crystals of Ni- and Zn-Doped Bi2Sr2Ca(Cu2−xMx)O8+δ (M = Ni or Zn) High-Temperature Superconductors

High-quality large single crystals of Ni- and Zn-doped Bi2Sr2Ca(Cu2−xMx)O8+δ (M = Ni or Zn) have been successfully grown by the traveling solvent floating zone technique. The single crystals are characterized by compositional and structural analyses and their physical properties are investigated by magnetic susceptibility and resistivity measurements. A record high critical temperature with a Tc = 97.5 K has been achieved in the annealed pristine Bi2212 single crystal. Substitution of Cu by Ni or Zn reduces the superconducting transition temperature when compared with pristine Bi2Sr2CaCu2O8+δ (Bi2212) grown under similar conditions. The successful growth of such pristine Ni- and Zn-doped Bi2212 single crystals will facilitate studies of the relationship between the magnetism and superconductivity in high-temperature cuprate superconductors.

Photoluminescence and positron annihilation spectroscopic investigation on a H+ irradiated ZnO single crystal

Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6 MeV H+ ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378 eV (10 K). The irradiation creates an intense and narrow emission at 3.368 eV (10 K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368 eV emission indicate its origin as a ‘hydrogen at oxygen vacancy’ type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164 ± 1 ps) and irradiated crystal (175 ± 1 ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ∼4 × 1017 cm−3 (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of ∼175 ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed.

Effect of cadmium sulfide nanorod content on Freedericksz threshold voltage, splay and bend elastic constants in liquid-crystal nanocomposites

This report describes how doping liquid crystals (LC) with rod-like hexagonal semiconductor nanoprisms alters the dielectric and elastic properties of the composites as compared with a pristine nematic liquid crystal (NLC). Cadmium sulfide nanorods were synthesized via the solvothermal process and blended with a NLC. Nanorods were highly miscible with NLC and produced a topological defect-free texture up to a certain limit. A good dark state was achieved during the homeotropic configuration of the cell within that limit. Appreciable changes in splay and bend elastic constants of the LCs were observed after blending with nanorods. Long-range order was established in the hybrid system, and consequently the anisotropy was increased. The threshold voltage decreased dramatically by ∼31%. Dielectric study revealed a high-frequency mode, which might be due to anchoring of the LC with nanorods.

Phase transition behavior in the mixed crystal of pristine and mono‐methyl substituted EDO‐TTF

AbstractWe report the mixed crystals of pristine and monomethyl substituted EDO‐TTF with PF6, [(EDO‐TTF)1–x(MeEDO ‐TTF)x ]2PF6 (x = mole fraction of MeEDO‐TTF, x = 0.095, 0.210, 0.345). While three mechanisms of Peierls, charge order (CO), and anion order (AO) transiton concurrently cause the metal‐insulator (M‐I) transion at about 280 K in the pristine crystal (x = 0), the cooperativity was modulated in the mixed crystals. The mixed crystals of x = 0.095‐0.210 showed the M‐I transition originated only from Peierls mechanism and that of x = 0.345 did not show the unit cell‐doubling down to 100 K. The analysis of anion motion revealed that AO transition does not cooperate with Peierls transition for the mixed crystals with intermediate x. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of H + ion implantation on structural, morphological, optical and dielectric properties of l-arginine monohydrochloride monohydrate single crystals

l-arginine monohydrochloride monohydrate (LAHCl) single crystals have been implanted with 100 keV H + ions at different ion fluence ranging from 10 12 to 10 15 ions/cm 2. Implanted LAHCl single crystals have been investigated for property changes. Crystal surface and crystalline perfection of the pristine and implanted crystals were analyzed by atomic force microscope and high-resolution X-ray diffraction studies, respectively. Optical absorption bands induced by colour centers, refractive index and birefringence, mechanical stability and dielectric constant of implanted crystals were studied at different ion fluence and compared with that of pristine LAHCl single crystal.

Effects of conducting polymer poly(3, 4-ethylenedioxythiophene) nanotubes on the electro-optical and dielectric properties of a nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl host

We report the results of the optical transmission and the capacitance behavior as a function dc electric field of a pristine liquid crystal and conducting polymer nanotube-liquid crystal composite measured in twisted nematic cells. The threshold and driving voltages have been determined from transmission-voltage curve. There is remarkable reduction in the threshold and driving voltage in the polymer nanotube doped liquid crystal cell which is good from application point of view. The residual dc is also reduced significantly in the doped cell and the reduction is even more than that observed in the carbon nanotube doped same liquid crystal system.

Dielectric relaxation dynamics in liquid crystal – dye composites

Dielectric relaxation spectroscopic studies were undertaken to investigate the role of the resistivity of a host liquid crystal in the relaxation dynamics of dyed systems. Two different types of liquid crystal mixture were used, namely, one that was characterized by high ion contamination and low resistivity and the other which was of high resistivity and virtual ion-free; both types were doped using an azo or anthraquinone dye. Dielectric spectra of both the pristine liquid crystals and their composites at different external fields were measured in homogenously-aligned cells. The use of both dyes on an ion-enriched liquid crystal not only enhanced ion concentration but also slowed down the relaxation process. In contrast, the relaxation process for composites with a high-resistivity liquid crystal host was faster than that of the pristine liquid crystal.

Electronic Evolution of Poly(3,4-ethylenedioxythiophene) (PEDOT): From the Isolated Chain to the Pristine and Heavily Doped Crystals

Poly(3,4-ethylenedioxythiophene) (PEDOT) is the prototypical conjugated polymer used in the doped state as the hole injection/transport layer in organic (opto)electronic devices. Numerous experimental studies have been successful only in drawing a partial microscopic picture of PEDOT due to its complex morphology, which has also hampered application of theoretical approaches. Using density functional theory methods, combined with refined structural models built upon crystallographic data of PEDOT and other substituted polythiophenes, our work seeks to establish a comprehensive understanding of the electronic and geometric structures of PEDOT, as an isolated chain and in the pristine and doped bulk phases. We find that ethylenedioxy substitution planarizes the polythiophene backbone but the experimentally observed bandgap reduction is caused mainly by a stronger destabilization of the valence band than the conduction band via donor-type substitution. The calculated crystal of pristine PEDOT has a monoclinic lamellar structure consisting of inclined pi-stacks. The impact of interchain interactions on the charge carrier effective masses is greater than that of the ethylenedioxy substitution and leads to the reversal of the relative masses; the electrons are lighter than the holes in the pristine crystal. The small interchain electron effective mass is comparable to the hole effective masses found in high mobility organic crystals. Tosylic acid-doped PEDOT (PEDOT:Tos), which is receiving renewed interest as an anode material to replace indium tin oxide, is calculated to be a two-dimensional-like metal. The PEDOT:Tos crystal is found to have an embedded mirror plane in the tosylate monolayer that is sandwiched between PEDOT stacks, and thus to have twice the size of the unit cell proposed earlier. Doping is seen to remove the intrastack inclination of the PEDOT chains.

Femtosecond laser and swift-ion damage in lithium niobate: A comparative analysis

Relevant damage features associated with femtosecond pulse laser and swift-ion irradiations on LiNbO3 crystals are comparatively discussed. Experiments described in this paper include irradiations with repetitive femtosecond-laser pulses (800 nm, 130 fs) and irradiation with O, F, Si, and Cl ions at energies in the range of 0.2–1 MeV/amu where electronic stopping power is dominant. Data are semiquantitatively discussed by using a two-step phenomenological scheme. The first step corresponds to massive electronic excitation either by photons (primarily three-photon absorption) or ions (via ion-electron collisions) leading to a dense electron-hole plasma. The second step involves the relaxation of the stored excitation energy causing bond breaking and defect generation. It is described at a phenomenological level within a unified thermal spike scheme previously developed to account for damage by swift ions. A key common feature for the two irradiation sources is a well-defined intrinsic threshold in the deposited energy density Uth required to initiate observable damage in a pristine crystal: Uth≈1.3×104−2×104J/cm3 for amorphization in the case of ions and Uth≈7×104 J/cm3 for ablation in the case of laser pulses. The morphology of the heavily damaged regions (ion-induced tracks and laser-induced craters) generated above threshold and its evolution with the deposited energy are also comparatively discussed. The data show that damage in both types of experiments is cumulative and increases on successive irradiations. As a consequence, a certain incubation energy density has to be delivered either by the ions or laser photons in order to start observable damage under subthreshold conditions. The parallelism between the effects of laser pulses and ion impacts is well appreciated when they are described in terms of the ratio between the deposited energy density and the corresponding threshold value.