PacSurf2014 Session NM-TuP: Nanomaterials Poster Session

TiO_x-based nanostructures are being used as active materials in lithium ion batteries, gas sensors, photocatalysts, dye synthesized solar cells, etc. Much effort has been directed toward understanding their electronic properties [1] because they exhibit physical and chemical properties different from their bulk counterparts. They are challenging to study by analytical electron microscopy techniques since they are highly sensitive to radiation damage. Their nano-scale prevents the use of conventional X-ray absorption spectroscopy (NEXAFS) to study individual structures. Scanning transmission x-ray microscopy (STXM) allows studies of the electronic structure of individual nanomaterials with high spatial resolution (30 nm) and high energy resolution (0.1 Ev) via NEXAFS spectroscopy and its linear dichroism (spectral variation as a function of the E-vector orientation) [2]. Here we report STXM-NEXAFS studies of individual sodium titanate nanoribbons (NaTiOx-NR) prepared by hydrothermal treatment of anatase TiO₂ micro-particles [3]. By measuring the dichroism with the sample at different orientations relative to the X-ray beam, and varying the direction of the linear E-vector by an elliptically polarizing undulator (EPU), the linear dichroism at the O 1s and Ti 2p edges was measured along the 3 principle axes of the anisotropic nanoribbons. Comparison of the spectra and dichroism with high level calculations helps confirm the crystal structure [4] and gives insight into the electronic bonding in these novel layered materials through the orientation dependence of local electronic structure within the crystallite. The methodology for 3D dichroic STXM measurements newly developed in this study is a powerful way to investigate anisotropic nanomaterials.

Research carried out at the Canadian Light Source, which is supported by NSERC, CIHR, NRC and the University of Saskatchewan. Research funded by NSERC, Canada Research Chair.

[1] P. Krüger, Phys. Rev. B 81 (2010) 125121.

[2] A.P. Hitchcock,Soft X-ray Imaging and Spectromicroscopy Ch. 22 in Handbook on Nanoscopy, eds. G. Van Tendeloo, D. Van Dyck and S.J. Pennycook (Wiley, 2012) 745.

[3] P. Umek, R. C. Korošec, B. Jančar, R. Dominko, D. Arčon, J. Nanosci. Nanotechnol.7 (2007) 3502.

Vo_x-TiO₂ nanocomposite samples made by one-pot synthesis showed the various catalytic activities depending on the calcination temperature. Photocatalytic activity of VO_x-TiO₂ nanoparticles fabricated at various temperatures were characterized by the degradation of methylene blue in aqueous solutions under UV irradiation. Nanocomposites show significant enhancement of decomposition of methylene blue and the reason is considered by the synergistic effect between two different materials. In this study, the prepared nanocomposite catalysts were attached on glass fiber by cross-linking method and photodegradation abilities were tested by the change of VOC gas concentration in a closed chamber. For the photocatalytic analysis of the prepared nanoparticles, the absorption spectra are measured by UV-visible spectrophotometer and the bandgap analysis are performed by Kubelka-Munk theory. VOC removal abilities are quantified by the variation of p-xylene concentration in the closed chamber. Obtained results show that the as-prepared Vo_x-TiO₂ nanocomposites have different degradation abilities depending on the calcination temperature and vanadium atomic percentage.

The nanomaterials have been widely applied in the biomedical area, which play important role in the disease diagnosis and treatment, tissue repair and proteins separation. Nano-TiO₂, as a biologically inert material, shows good biocompatibility and bonding biological activity of the protein. In addition, the surface modification of TiO₂ can be achieved by the surface bonding between its surface hydroxyl groups and organic molecules. Furthermore, the research on the selective adsorption of proteins on the functionalized nanomaterials has important theoretical significance and application value in the field of protein separation.

In this work, TiO₂ nanoparticles were prepared by improved hydrothermal method, and then some organic molecule, such as silane KH560 and oleic acid were used to modify the TiO₂ nanoparticles. Some means, such as TEM, XRD, TG-DSC, and FT-IR were used to character the functionalized TiO₂ nanoparticles. The results showed that the organic molecules were successfully combined on the surface of TiO₂ nanoparticles via chemical bond and physical weak interaction.

We study on electrical and luminescent characteristics of ZnO nanorods array when an electric field is applied in lateral direction to the axis of ZnO nanorods.

ZnO nanorods are grown by chemical vapor deposition after laser ablation of gold on substrate. This growth method was developed by us. Glass substrates (26 mm x 26 mm) are coated with SiO₂ film of 100 nm thickness by electron-beam deposition. The length of ZnO nanorods is about 1 micron meter and the diameter is from 50 nm to 80 nm. The direction of nanorods is distributed around a vertical axis to substrate surface.

The mean separation between nanorods on the area between electrodes is about 1.5 micron meter, and nanorods are isolated with each other. This is confirmed by SEM images and the measurement of electrical conduction between indium electrodes. Indium electrodes are electron-beam deposited on ZnO nanorods array with 600 nm thick. Shape of anode is rectangular (4 mm x 4 mm) and that of cathode is triangular with 30 degree vertical angle and 3 mm base, and the separation between the side of anode rectangle and the vertex of cathode triangle is 1.5 mm.

DC voltage is applied between electrodes in vacuum. An electrical potential of the metal chamber is same as that of anode. It is gradually increased from 0 volt. When the applied voltage is low, electric current does not flow. When it reaches about 1500 V, however, the current of about 10^-6 [A] initiates abruptly to flow and bluish-white light emission is observed at many points along the cathode edge. These beams of light extend along lines with directions nearly vertical to cathode edge as if the anode electrode did not exist or the voltage was not applied on anode. The intensity of emitted light is very high. It is confirmed experimentally that electrons are field-emitted from ZnO nanorods being at the cathode edges and the light beams correspond to trajectories of the field-emitted electrons. We estimate that the light may be generated by collisions of these electrons with ZnO nanorods, and are studying on this mechanisms.

Low-dimensional sp² carbon materials including single-walled carbon nanotubes (swCNTs) and graphene have recently received a great deal of attention for potential uses in transparent and flexible nanoelectronics due to their remarkable mechanical, electrical, thermal, and optical properties. Especially, there are many interest in hybrid nanostructures including swCNTs and grapehene. Here, we combined swCNTs and graphene hybrid films with complementary properties for use in high-performance transparent electrodes and field effect transistors (FETs). The hybrid films were synthesized by thermal chemical vapor deposition on Cu foil spin-coated with swCNTs. The density and alignment of swCNTs were simply controlled by adjusting the spin-coating speed or swCNT concentration. Evidence for the formation of the hybrid film was provided by the combined G- and 2D-bands intensity and lineshape Raman features of swCNTs and graphene. Based on the transfer characteristics for the hybrid film-based FETs, an improved I_on/I_off and on-state current are achieved compared with pristine graphene. Notably, the hybrid film had a sheet resistance of 300 Ω/sq with 96.4% transparency, which was comparable to that of hybrid materials in previous reports.

Recently, serially connected transistors with vertical configuration have received considerable attention in memory applications due to their potential to increase integration density to ultra-high values. In transistors with the vertically integrated configuration, the semiconductor channel material is usually composed of polycrystalline Si (poly-Si). Although the poly-Si channel has revealed feasible functionality as the semiconductor channel, degraded mobility, uniformity and reliability concerns related to the presence of grain-boundaries have not yet been completely resolved. In this regard, the amorphous nature and high carrier mobility of In₂Ga₂ZnO₇ (a-IGZO) thin films attract a great deal of attention as the channel material for such applications.

In this study, two serially connected and vertically integrated a-IGZO thin film transistors (V-TFTs) were fabricated using a gate-first fabrication process. The V-TFTs were fabricated with a vertical channel length (L_g) of ~500 nm for the top TFT (t-TFT) and ~400 nm for the bottom TFT (b-TFT). Heavily doped p-type silicon was used as the substrate and gate of b-TFT (G_b). A 100-nm-thick SiO₂ layer was thermally grown as the isolation layer between the t- and b-TFTs. 500-nm-thick poly-Si was deposited by low-pressure chemical vapor deposition to make the gate of the t-TFT (G_t). Then, the poly-Si/SiO₂/Si structure was dry etched sequentially to form the gates. Then, a 100-nm-thick SiO₂ layer was deposited by a PECVD as the gate dielectric layer, and an a-IGZO layer was sputter-deposited at room temperature on this structure with a target thickness of 100 nm on the top surface of the sample, which results in a channel thickness of ~40 nm for the t-TFT, and ~50 nm for the b-TFT on the side walls of the SiO₂. Finally, the Ti source and drain contacts were fabricated by a lift-off process, as the Ti contact with the a-IGZO is quasi-Ohmic.

The t- and b-TFTs show well-behaved transfer characteristics, with an I_on/I_off ratio (~ 10⁸) and an SS value of 0.6 V/dec., which are much improved device parameters compared with the previously reported single-layer V-TFT, for which the gate-last fabrication process was adopted. This is due to the favorable distribution of the electric field by the V_d and V_g of the TFTs, where the influence of V_d on the channel can be minimized compared with that from V_g. While the two serially connected TFTs behave well and rather independently of each other, there were certain cross-influence between them. Details for such cross-talk will be discussed in the presentation. Further study results up to four layer V-TFT will also be presented.

Transition metal dichalcogenides (MoS₂, WS₂, Wse₂, MoSe₂, NbS₂, NbSe₂, etc.) are layered materials that would exhibit semiconducting, metallic and even superconducting behavior. In the bulk, it is semiconducting and has an indirect band gap. Recently, these layered systems have attracted a great deal of attention mainly due to their complementary electronic properties when compared to other two-dimensional materials, such as graphene (a semimetal) and boron nitride (an insulator). However, these bulk properties could be significantly modified when the system becomes mono-layered; the indirect band gap becomes direct. Such changes in the band structure when reducing the thickness of a MoS₂ and WS₂ films has important implications for the development of novel applications, such as valleytronics. In this work, we report for the controlled synthesis of large-area (~cm²) single-, bi-, and few-layer MoS₂ and WS₂ using a two-step process. MoO_x and WO_x thin films were deposited onto a Si/SiO₂ substrate, and these films were then sulfurized under vacuum in a second step occurring at high temperatures (750 °C). The synthesized MoS₂ and WS₂ atomic thin films were analyzed by optical microscopy, Raman spectroscopy and TEM, and their photoluminescence properties were evaluated.

The objective of this work is to report on a study on the preparation of bolometers and their characterization, based on parallel linear arrays of carbon nanotubes (CNTs).

The CNTs present good thermal and electrical properties. That is the reason why they may constitute competitive bolometric devices. A bolometer is a device that absorbs energy, in our case infrared radiation, thus varying its temperature (increasing) and electrical resistance (decreasing). The figure of merit of the bolometers is the temperature coefficient of resistance (TCR).

With the objective of obtaining aligned arrays of CNTs we made a solution in water with polymer (PVP), surfactant (SDBS) and CNTs. For a second group Functionalization was also performed with amino groups (Thymine) to induce better alignment, taking advantage of the hydrogen bonds that form between them.

For both sets of prepared CNT’s copper was evaporated on them to have proper ohmic metal contacts to characterize the bolometers thus created.

The samples morphology and alignment were characterized by Scanning Electron Microscopy (SEM). Kaiser Test was performed to check on the degree of functionalization. The TCR of the constructed bolometers was electrothermally measured.

The results obtained with SEM were in some case alignments as long as 3 mm in length. The functionalization degree obtained was 1100 µmol/g. We obtain values of TCR in the range of -1.7 to -3.0 % K^-1 for non-functionalized CNTs and from -0.4 to -5.4% K^-1 for functionalized CNT’s.These last values are the largest reported for CNTs so far in the literature.

The authors acknowledge the financial support from Consejo Nacional de Ciencia y Tecnología (CONACyT) México, through grant 09-133428, CB-166014, Project CeMIESol 22 from FRC-UASLP, and the access to Laboratorio Nacional de Análisis Físicos, Químicos y Biológicos-UASLP, during the course of this research. MGGV acknowledges the financial support of CONACYT through a Ph.D. scholarship 422974/259064.

Graphene is one of the latest generation of multifunctional materials considered to be applied to a large number of research areas. However, its structure is often modified to anchor it to different polymeric bases. The effect of anchoring groups such as C = O and C-OH, are necessary for the good dispersion of graphene in solution. However the presence of those chemical groups, greatly affect the electrical and optical properties of the final nanocomposite. The present study shows the relationship between the functionalization of graphene and the optoelectronic effect on the nanocomposites obtained. % functionalization Vs Electrical conductivity is reported. Graphene dispersion in the polymer was observed by SEM and UV-Vis and the functional groups are identified by FTIR.

Mechanical deformation (strain) plays an important role in the electronic, optical and transport properties of semiconductor quantum dots (SQDs). Epitaxial SQDs have a spatial deformation profile caused by the mismatch of the lattice parameters of the materials involved. This deformation creates an elastic energy which is one determinant fact in the SQDs grown on Stranski-Kastranow mode. The deformation also changes the potential profiles experienced by the carriers (electron and hole), requiring consideration of them when one calculates the electronics eingenstates. This shows the importance of the deformation profile on the properties of the nanostructures for future applications in electronic and optical devices.

In this work we show one more tool for the calculation of mechanical deformation on Si/Ge semiconductor quantum dots. We used a numerical simulation package to solve the differential equations for the deformation, using Continuous Elasticity Theory. We studied two systems: capped and uncapped. The cap layer is the Si layer grown above the Ge QDs layer. On the capped system, we observed a larger biaxial deformation on the Ge layer plane, when compared with the uncapped system. We also observed the deformation on the Si layer underneath the Ge layer. Again, on the capped system this biaxial deformation was also larger. With these deformation profile, we can also calculate the stress tensor and the elastic energy profiles.

Introduction

In order to utilize hydrogen as new clean energy resources, we have to realize safe, efficient, and low cost hydrogen transportation and storage. One of the current transport methods is the one with high-pressure hydrogen tanks. However, hydrogen is one of the most important elements in the damage process of materials. Many recent studies have reported that hydrogen atoms absorbed in materials can stabilize vacancies[1], which cause reduction of their ductility. From this point, the atomic- and electronic-scale understanding of hydrogen embrittlement process is necessary to develop new long-life materials for hydrogen transportation and storage.

Calculation Methods

In this study, we investigated the adsorption properties of hydrogen isotopes on iron surfaces with the aid of first principles calculations based on spin-polarized density functional theory, in order to understand hydrogen behaviors in commonly-used iron-based materials, for instance, ferritic and austenitic stainless steels. We also adopted the quantum mechanics calculations of hydrogen nuclei, because the importance of delocalization and zero-point energy of hydrogen nuclei has been reported in various papers. [2, 3] Furthermore, we considered hydrogen(H), deuterium(D), tritium(T), and muonium(μ⁺-e^-) as hydrogen isotopes.

Results and Discussion

At first, we investigated the potential energy surfaces of hydrogen on bcc-Fe(110) and fcc-Fe(111) surfaces. These surface orientations are the most stable ones for each crystal structure, respectively. Furthermore, in order to reveal the surface orientation dependence of hydrogen adsorption, we investigated the potential energy surfaces of hydrogen on fcc-Fe(100) surfaces. We revealed that the most stable adsorption sites on bcc-Fe(110), fcc-Fe(111), and fcc-Fe(100) surfaces are the long-bridge, fcc-hollow, and short-bridge site, respectively. The corresponding adsorption energies with these adsorption configurations are 2.99, 2.78, and 2.82 eV, respectively. Thus, we can point that the adsorption energies on bcc-Fe(110) and fcc-Fe(100) surfaces are slightly lager than the one on fcc-Fe(111) surfaces. We also investigated the hydrogen adsorption states on these surfaces by calculating shrödinger equation for hydrogen nuclei. We found the delocalization of H atoms in the ground state. In addition, we clarified the non-negligible isotope effects in delocalization and zero-point energy of hydrogen isotopes.

References

[1] Y. Tateyama and T. Ohno, Phys. Rev. B 67 (2003) 174105.

[2] N. Ozawa, T. A. Roman, H. Nakanishi, and H. Kasai, Surf. Sci. 600 (2006) 3550.

[3] Y. Kunisada and H. Kasai, J. Phys. Soc. Jpn. 82 (2013) 023601.

Si(110)-16×2 is a favorable candidate for a next-generation semiconductor substrate because it takes a single domain (SD) structure with high hole-mobility, which is larger than that of the other Si crystal faces. Recently, Adatom-Buckling (AB) model has been proposed as the surface structure model for clean Si(110)-16×2 surface [1]. The AB model consists of five Si surface components of SC1: the buckled upper atoms, SC2: the unbuckled atoms and the second layer atoms with dangling bonds (DBs), SC3: the first layer and second layers atoms without DBs, SC4: the adatoms, and SC5: buckled lower atoms. In the AB model, the surface states S₁, S_2, S₃, and S₄ with binding energies of 0.2 eV 0.4 eV, 0.7 eV, and 1.0 eV are reported to be located at specific surface sites of SC4, SC1, SC5, and SC3, respectively [1]. However, the SD structure of Si(110)-16×2 disappears when it is exposed to atmospheric air. In this study, we investigated a water-terminated Si(110)-16×2 SD surface with XPS and Auger-photoelectron coincidence spectroscopy. Si-2p photoelectron spectra indicate that water is dissociatively adsorbed and forms Si-H and Si-OH components after exposure to water of 0.1 Langmuir (L). When Si(110)-16×2 surface is exposure to water form 1.0 L to 5.0 L, there is little change in these Si-2p intensity ratio. This result indicates that clean Si(110)-16×2 surface is saturated with water of less than 1.0 Langmuir (L). In addition, water is preferentially dissociative-adsorbed at surface components of the SC1, the SC2, and the SC3 on step edge of SD structure, but is hardly adsorbed at surface components of the SC4 and the SC5 on the terrace of the SD structure. The SD structure of Si(110)-16×2 surface after the exposure to water of 5.0 Langmuir is kept under ultra-high vacuums condition and atmosphere pressure of N₂ gas. This result indicates that Si(110)-16×2 SD surface is inactivated by water dissociative-adsorption. On the other hand, Si-L₂₃VV-Si-2p coincidence spectra suggest that the surface states S₁, S₂, S₃, and S₄ disappear after exposure to water. The valence band maximum energy of water adsorbed Si(110)-16×2 surface is shifted to deeper binding energy side in comparison with that of clean Si(110)-16×2 surface. The water adsorbed Si(110)-16×2 SD surface may be used as a unique semiconductor substrate.

[1] K. Sakamoto, M. Setvin, K. Mawatari, P. E. J. Eriksson, K. Miki, and R. I. G. Uhrberg, Phys. Rev. B 79, 045304 (2009).

One of the problems is tracking of outdoor polymeric silicone rubber(SIR) insulation used in HVDC or HVAC transmission lines. These problem are more serious with dc than with ac voltage, due to the electrostatic attraction of airborne contaminants to the insulator surface. In order to improve the tracking resistance of SIR, inclined plane(IP) tracking test similar to IEC 60587 was conducted under positive and negative dc voltages for SIR filled with micro sized silica and nano sized alumina trihydrate(ATH) fillers. The results show that tracking resistance of SIR nanocomposites were more increased than SIR unfilled micro or nano filler. It was also seen that the tracking resistance of SIR showed polarity effect under dc voltage.

The filling of an initial core hole by an electron from a higher subshell of the same shell is known as a Coster–Kronig transition, and Auger photoelectron coincidence spectroscopy (APECS), in which Auger electrons are measured in coincidence with photoelectrons with a fixed kinetic energy (KE), is an ideal tool for probing Coster–Kronig transitions because Auger electrons originating from a specific core ionization are detected .

However, studies of Coster–Kronig transitions have mainly been restricted to metal surfaces, even though Si surfaces are crucial for surface science applications and the semiconductor industry. Issues such as the assignments of the Si L₁VV Auger peaks, experimental determination of the branching ratio of the Si L₁L₂₃V and Si L₁VV Auger decays, and competition between the delocalization of the valence hole and the Si L₂₃V-VVV Auger decay are, to the best of our knowledge, largely unexplored. In this paper, we report on the decay processes of Si 2s core holes in a clean Si(111)-7×7 surface studied using coincidence measurements of the Si Auger electron and the Si 2s photoelectron.

We measured a coincidence spectrum measured by scanning the ASMA over a KE range of 20­–150 eV, with the DP-CMA fixed at a KE of 76.5 eV (Si-L₂₃VV-Si-2p APECS spectrum). Distinct Auger peaks are observed in a KE region of 50–92 eV with the maximum peak located at KE = 88 eV. We assigned these peaks to Si L₂₃VV Auger electrons emitted in the decay processes of Si 2p holes. Another coincidence spectrum was measured by scanning the ASMA over a KE range of 20­–150 eV with the DP-CMA fixed at KE = 26.3 eV, which corresponds to Si 2s photoelectrons (Si-Auger-Si-2s APECS spectrum). The maximum peak position and shape of the Si-Auger-Si-2s APECS spectrum in the AeKE = 50–92 eV region are almost identical to those of the Si-L₂₃VV-Si-2p APECS. Therefore, we assigned these Auger peaks to Si L₂₃VV Auger processes. This is direct evidence of Si L₂₃VV Auger processes being induced by Si 2s ionization.

The peaks of Si-Auger-Si-2s APECS in the AeKE = 20–50 regions were assigned to Si L₁L₂₃V Auger decays, while the peaks of Si-Auger-Si-2s APECS in the AeKE = 100–150 eV regions were assigned to Si L₁VV Auger decays. These results indicate that there are two nonradiative decay processes of the Si 2s core hole. The first is the Si L₁L₂₃V Coster–Kronig transition followed by delocalization of the valence hole and Si L₂₃VV Auger decay, while the second is Si L₁VV Auger decay. From the integrated intensity of the normalized Si L₂₃VV and Si L₁VV Auger peaks, we estimated the branching ratio of Si L₁L₂₃VV to Si L₁VV Auger processes to be (96.8 ± 0.4):(3.2 ± 0.4).

[1] N. Goncharuk, V. Malyshko, V. Orehovskiy, N. Karushkin, EuMW2013 Proceedings.

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[3] S. M. Sze, Physics of Semiconductor Devices, Vol. 2, Chapt. 10, Sect. 7, J. Wiley & Sons, 1981.

[1] N. M. Goncharuk, Materials Science & Engineering, A353 , pp. 36−40, 2003.