Kaoru Nakajima

In-line aligned and bottom-up Ag nanorods for surface-enhanced Raman spectroscopy

We have demonstrated surface-enhanced Raman spectroscopy on arrays of Ag nanorods aligned in line by a dynamic oblique deposition technique. For the light polarized along the major axis of the nanorods, the plasma resonance of the Ag nanorods has been tuned to a wavelength suitable for Raman spectroscopy. The average width and the length-to-width ratio of the resulting nanorods are 56nm and 3.5, respectively, and the nanorods align in line with small gaps of a few 10nm. The Raman scattering for the polarized light along the nanorods is enhanced significantly as compared with that perpendicular to the nanorods. This polarization dependent Raman enhancement is attributed to the local field concentration at the ends of the nanorods. Since the preparation process is physical and completely bottom up, it is robust in its selection of the materials and is useful in providing the surface-enhanced Raman scattering sensors at low cost.

Vapor phase growth of Al whiskers induced by glancing angle deposition at high temperature

The authors demonstrate the growth of unusual Al whiskers by glancing angle deposition on a high temperature (HT-GLAD) substrate, while the usual columnar structures completely disappear due to accelerated surface diffusion. HT-GLAD is essential for the nucleation of the whiskers and efficient supply of Al atoms on the side surface of the vertically growing whiskers. HT-GLAD will, for the first time, reveal the mechanisms for the vapor growth of metal whiskers.

Observation of surface structure of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide using high-resolution Rutherford backscattering spectroscopy

The surface structures of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)MIM][TFSI], n=2,4,6) are studied by high-resolution Rutherford backscattering spectroscopy. The average composition of the surface molecular layer is very close to the stoichiometric composition, showing that neither ion is enriched in the surface layer. A detailed analysis indicates that both cations and anions have preferential molecular orientations at the surface. The alkyl chains of the [C(n)MIM] cations protrude to the vacuum and the CF(3) groups of the [TFSI] anions are also pointing toward the vacuum. While the orientation of the [TFSI] anion becomes weaker with increasing alkyl-chain length, the protrusion of the alkyl chain occurs irrespective of the chain length. It was also found that the N(SO(2))(2) moiety is located nearly at the same depth as the imidazolium ring, suggesting that one of oxygen atoms in [TFSI] is bonded to the hydrogen of the C(2) carbon atom of the imidazolium ring.

Si emission from the SiO2/Si interface during the growth of SiO2 in the HfO2/SiO2/Si structure

HfO2∕SiO2∕Si(001) structures were annealed in dry oxygen, and compositional depth profiles were measured by high-resolution Rutherford backscattering spectroscopy. Growth of the interfacial SiO2 layer and simultaneous surface accumulation of Si were observed. The observed result indicates that silicon species are emitted from the SiO2∕Si interface to release the stress induced by oxidation as was predicted by recent theoretical studies.

Hydrogen depth-profiling in chemical-vapor-deposited diamond films by high-resolution elastic recoil detection

We have measured the hydrogen depth profiles in chemical-vapor-deposited diamond films by elastic recoil detection. Depth resolution of ∼0.23 nm is achieved using a high-resolution magnetic spectrometer. The hydrogen depth profile shows a sharp peak at surface, and the hydrogen coverage is estimated to be 1±0.3 ML, indicating formation of the monohydride structure. The surface peak has a small tail toward deeper region, which is ascribed to hydrogen atoms incorporated in a subsurface region. These subsurface hydrogen atoms might be the origin of the surface conductivity.

Observation of molecular ordering at the surface of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide using high-resolution rutherford backscattering spectroscopy.

The surface structure of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]) is studied by high-resolution Rutherford backscattering spectroscopy at room temperature. The results provide direct evidence of the molecular ordering at the surface. The C1 conformer of the [TFSI] anion is dominant among two stable conformers, and the anions are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer. The anions in the second molecular layer also show preferred orientation although it is rather weak.

Observation of surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate using high-resolution Rutherford backscattering spectroscopy.

The surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate is studied by high-resolution Rutherford backscattering spectroscopy (HRBS) at room temperature. Elemental depth profiles are derived from the observed HRBS spectrum through spectrum simulation. While the obtained carbon profile has a sharp peak at the surface, the nitrogen profile shows a broader peak at a depth approximately 0.3 nm. These observations indicate that the butyl chain protrudes from the bulk liquid to the vacuum at the surface. The profiles of phosphorous and fluorine also have a broad peak at almost the same depth as the nitrogen profile, indicating that the anions are located near the imidazolium rings. These results are in good agreement with recent molecular dynamics simulations.

Strain profiling of HfO2/Si(001) interface with high-resolution Rutherford backscattering spectroscopy

Strain depth profiles in HfO2 (3 nm)/Si(001) prepared by atomic-layer chemical vapor deposition have been measured using high-resolution Rutherford backscattering spectroscopy in combination with a channeling technique. It is found that the Si lattice is compressed in the vertical direction around the interface. The observed maximum strain is about 1% at the interface and the strained region extends down to ∼3 nm from the interface.

Surface structures of binary mixtures of imidazolium-based ionic liquids using high-resolution Rutherford backscattering spectroscopy and time of flight secondary ion mass spectroscopy

Surface structures of binary mixtures of imidazolium-based ionic liquids having a common anion (bis(trifluoromethanesulfonyl)imide ([TFSI]), namely [C2MIM]1-x[C10MIM]x[TFSI] (x = 0.5 and 0.1), are studied using high-resolution Rutherford backscattering spectroscopy (HRBS) and time of flight secondary ion mass spectroscopy (TOF-SIMS). Although both measurements show surface segregation of [C10MIM] the degrees of the segregation are different. The surface fraction xsurf of [C10MIM] is estimated to be 0.6 ± 0.05 and 0.18 ± 0.02 by HRBS for x = 0.5 and 0.1, respectively. On the other hand, TOF-SIMS indicates much stronger surface segregation, namely xsurf = 0.83 ± 0.03 and 0.42 ± 0.04 for x = 0.5 and 0.1, respectively. The observed discrepancy can be attributed to the difference in the probing depth between HRBS and TOF-SIMS. The observed surface segregation can be roughly explained in terms of surface tension.

Influence of elastic scattering of photoelectrons on angle-resolved x-ray photoelectron spectroscopy

The validity of the electron effective attenuation length database developed by National Institute of Standards and Technology (NIST) is examined for x-ray photoelectron spectroscopy (XPS) measurement of HfO2 (2.7nm)∕SiON (0.8nm)∕Si. The angular dependences of photoelectron yields are calculated using the NIST database and composition depth profiles measured by high-resolution Rutherford backscattering spectroscopy. The calculated result reproduces the observed XPS result fairly well even at larger emission angles up to 80°, indicating that the accuracy of XPS depth profiling can be improved using the NIST database.