College of Nanoscale Science and Engineering

The continued research and development effort on silicon ultrashallow junctions has posed a great challenge to materials characterization, due to the need for depth profiling of dopants and silicon lattice defects with a subnanometer resolution. In this work, we report on a method combining ion beam analysis (Rutherford backscattering, ion channeling, and nuclear reaction analysis) with room-temperature UV-assisted oxidation and chemical wet etching for obtaining high-resolution ($0.5 nm) depth distributions of total boron atoms, electrically activated boron atoms and silicon lattice defects in silicon ultrashallow junctions. The application of this method was demonstrated by profiling silicon junctions as shallow as 8 nm, created by 200-eV boron ion implantation followed annealing by various techniques. The capability to profile boron at such high resolution has resulted in observation of boron segregation. Additionally, the ability for depth profiling Si lattice defects offered by this method has aided in the understanding of thermal and laser annealing effects on defect formation in Si junctions. Our experimental results are compared with those obtained using other techniques such as secondary ion mass spectroscopy and four-point probe, and differences are discussed in detail.

A scanning force microscope tip is used to write ferroelectric domains in He-implanted singlecrystal lithium niobate and subsequently probe them by piezoresponse force microscopy. Investigation of cross-sections of the samples showed that the buried implanted layer, ∼ 1 µm below the surface, is non-ferroelectric and can thus act as a barrier to domain growth. This barrier enabled stable surface domains of < 1 µm size to be written in 500 µm-thick crystal substrates with voltage pulses of only 10 V applied to the tip.

In was implanted into bulk ZnO creating a square profile with a thickness of about 100 nm and an In concentration of about 1 ϫ 10 20 cm −3. The layer was analyzed with Rutherford backscattering, temperature-dependent Hall effect, and low-temperature photoluminescence measurements. The implantation created a nearly degenerate carrier concentration n of about 2 ϫ 10 19 cm −3 , but with a very low mobility , increasing from about 0.06 cm 2 / V s at 20 K to about 2 cm 2 / V s at 300 K. However, after annealing at 600°C for 30 min, n increased to about 5 ϫ 10 19 cm −3 , independent of temperature, and increased to about 38 cm 2 / V s, almost independent of temperature. Also, before the anneal, no excitons bound to neutral In donors, called I 9 in literature, were observed in the photoluminescence spectrum; however, after the anneal, the I 9 line at 3.3568 eV was by far the dominant feature. Analysis of the Hall-effect data with a parametrized, two-layer model showed that the conduction before the anneal was mainly due to very high concentrations of native donors and acceptors, produced by the implantation, whereas the conduction after the anneal was due to In ions that were nearly 100% activated. These results show that strongly degenerate conductive layers with designed profiles can be created in ZnO with implantation and relatively low-temperature anneals.

The continued research and development effort on silicon ultrashallow junctions has posed a great challenge to materials characterization, due to the need for depth profiling of dopants and silicon lattice defects with a subnanometer resolution. In this work, we report on a method combining ion beam analysis (Rutherford backscattering, ion channeling, and nuclear reaction analysis) with room-temperature UV-assisted oxidation and chemical wet etching for obtaining high-resolution ($0.5 nm) depth distributions of total boron atoms, electrically activated boron atoms and silicon lattice defects in silicon ultrashallow junctions. The application of this method was demonstrated by profiling silicon junctions as shallow as 8 nm, created by 200-eV boron ion implantation followed annealing by various techniques. The capability to profile boron at such high resolution has resulted in observation of boron segregation. Additionally, the ability for depth profiling Si lattice defects offered by this method has aided in the understanding of thermal and laser annealing effects on defect formation in Si junctions. Our experimental results are compared with those obtained using other techniques such as secondary ion mass spectroscopy and four-point probe, and differences are discussed in detail. V

In was implanted into bulk ZnO creating a square profile with a thickness of about 100 nm and an In concentration of about 1 ϫ 10 20 cm −3. The layer was analyzed with Rutherford backscattering, temperature-dependent Hall effect, and low-temperature photoluminescence measurements. The implantation created a nearly degenerate carrier concentration n of about 2 ϫ 10 19 cm −3 , but with a very low mobility , increasing from about 0.06 cm 2 / V s at 20 K to about 2 cm 2 / V s at 300 K. However, after annealing at 600°C for 30 min, n increased to about 5 ϫ 10 19 cm −3 , independent of temperature, and increased to about 38 cm 2 / V s, almost independent of temperature. Also, before the anneal, no excitons bound to neutral In donors, called I 9 in literature, were observed in the photoluminescence spectrum; however, after the anneal, the I 9 line at 3.3568 eV was by far the dominant feature. Analysis of the Hall-effect data with a parametrized, two-layer model showed that the conduction before the anneal was mainly due to very high concentrations of native donors and acceptors, produced by the implantation, whereas the conduction after the anneal was due to In ions that were nearly 100% activated. These results show that strongly degenerate conductive layers with designed profiles can be created in ZnO with implantation and relatively low-temperature anneals.

Gracias a sus excepcionales propiedades mecánicas y eléctricas, los nanotubos de carbono son en la actualidad de gran interés para la comunidad industrial mundial. Sus diversas aplicaciones y su capacidad para mejorar o reemplazar materiales o compuestos tradicionales, han generado inmenso interés por estudiar y optimizar su producción y funcionalización. En la siguiente revisión se resumen los procesos actuales de producción industrial de nanotubos de carbono, se explican las capacidades de dichos procesos para la producción selectiva de nanotubos de capa sencilla (SWNTs) o de pared múltiple (MWNTs) y se mencionan algunos avances en la producción de nanotubos de carbono con quiralidades específicas. Posteriormente se hace un recuento de las metodologías de funcionalización química y de su impacto en la solubilidad, dispersión, reactividad y conductividad de los nanotubos de carbono. Finalmente se describen las aplicaciones tradicionales de los nanotubos de carbono no funcionalizados y se relacionan los avances en funcionalización con las nuevas perspectivas para su aplicación en diversos campos industriales.

En un sistema de arco de descarga eléctrica se sintetizan fulleritas en presencia de un catalizador como hierro, itrio, cobalto y níquel. Estas fulleritas se someten a un proceso de purificación de oxidación para eliminar otras clases de fullerenos, como tambien para eliminar las partículas catalizadoras. Las muestras puri¯cadas se caracterizan usando microscopía electrónica de transmisión y de barrido. La caracterización óptica permite sacar conclusiones sobre la influencia del catalizador en la formación de los nanotubos.

While engineered nanomaterials (ENMs) are increasingly incorporated into industrial processes and consumer products, the potential biological effects and health outcomes of exposure remain unknown. Novel advanced direct visualization techniques that require less time, cost, and resource investment than electron microscopy (EM) are needed for identifying and locating ENMs in biological samples. Hyperspectral imaging (HSI) combines spectrophotometry and imaging, using advanced optics and algorithms to capture a spectrum from 400 to 1000 nm at each pixel in an enhanced dark-field microscopic (EDFM) image. HSI-EDFM can be used to confirm the identity of the materials of interest in a sample and generate an image &quot;mapping&quot; their presence and location in a sample. Hyperspectral mapping is particularly important for biological samples, where ENM morphology is visually indistinct from surrounding tissue structures. While use of HSI (without mapping) is increasing, no studies to da...

We investigate the multiphoton ionization and the subsequent fragmentation of SF 6 molecules with intense x-ray-free electron laser pulses at different photon energies. We observe highly charged molecular and atomic ions which were absent in previous experiments with conventional x-ray sources. The observation of fully stripped fluorine ions suggests sequential multiphoton ionization processes with intermediate 1s electron excitation. We measure the average momentum and kinetic energy of each fragment, the comparison of which implies many-body fragmentation pathways of the molecular ions. The observed non-monotonic dependence of the kinetic energy on the charge states of fluorine indicates the multiphoton ionization of 'isolated' atomic neutral fluorine or fluorine ions resultant from bond cleavages and also implies the fragmentation of highly charged molecular ions that are produced at a later time during a single pulse.

Sequential multiple photoionization of the prototypical molecule N 2 is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for singlepulse imaging applications.

Spin-singlet Cooper pairs convert to spin-triplet Cooper pairs on passing through a magnetically noncollinear structure at a superconductor(S)/ ferromagnet(F) interface. In this context, the generation of triplet supercurrents through intrinsic ferromagnetic domain walls, which are naturally occurring noncollinear magnetic features, was proposed theoretically in the past decade. However, an experimental demonstration has been lacking in the literature, particularly because of the difficulty in accessing a single domain wall, which is typically buried between two domains in a ferromagnetic material. By patterning a ferromagnetic nanoconstriction, we have been able to realize a nanoscale S/F/S planar junction, where a single domain wall (pinned at the nanoconstriction) acts as a Josephson barrier. In this geometry, we are able to show the predicted longrange triplet supercurrent across a ferromagnetic barrier exceeding 70 nm. Using this technique, we have demonstrated a ferromagnetic planar nano-SQUID device consisting of two Nb/Ni/Nb spin-triplet Josephson junctions.

Over the last decade, it has been shown that magnetic non-collinearity at an s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bilayer structure can function as a field tunable non-collinear magnetic structure, which may offer magnetic field tunability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers, we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring T c with and without a noncollinear magnetic structure between Co and Py. The sensitivity of T c in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.