Recent Topics

Last update on May 25, 2011


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Charge Transfer and Molecular Orientation of Tetrafluorotetracyanoquinodimethane on a Hydrogen-Terminated Si(111) Surface Prepared by a Wet Chemical Method [Masayuki Furuhashi and Jun Yoshinobu, J. Phys. Chem. Lett. 1(2010) pp.2917-2921]
We investigated the chemical state and molecular orientation of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) adsorbed on a hydrogen-terminated Si(111)(1x1) surface using transmission infrared (IR) spectroscopy. We deposited F4-TCNQ molecules on H-Si(111) by a wet chemical method. Similar to evaporated F4-TCNQ molecules on various substrates in vacuum, we observed anionized F4-TCNQ on the H-Si(111) substrate. The incident angle dependence of the IR spectra reveals that this F4-TCNQanion lies flatly on the surface. On the other hand, minority neutral F4-TCNQ species assume random orientation, judging from the comparison between s- and p-polarized IR spectra. We conclude that the first layer on the H-Si surface is a flat-lying anion species and the upper layers consist of randomly oriented neutral molecules.

Infrared spectroscopy of the organic monolayer sandwiched between a Hg electrode and a Si substrate [Masayuki Furuhashi and Jun Yoshinobu, Rev. Sci. Instrum. 81, 053103 (2010); doi:10.1063/1.3422256 (6 pages) ]

We have successfully observed the vibrational spectra of organic monolayers sandwiched between a liquid Hg electrode and a Si substrate by means of a newly developed reflection absorption (RA) device(Fig. 1). The vibrational spectra of organic monolayers between two electrodes can be observed under a certain bias voltage. The monolayers were fabricated by the reaction of hydrogen-terminated Si(111) with 1-octadecene. A metal/insulator/semiconductor structure was prepared using liquid Hg as a metal electrode and the organic monolayer as an insulator. Infrared (IR) light entered from the Si substrate side with an incident angle of 75°. The reflected IR light from the metallic Hg was detected by a mercury-cadmium-telluride detector. We obtained RA spectra using a bare H&endash;Si(111) substrate as a reference. The absorbance of the RA spectrum (Fig. 2b) was comparable with that of the transmission spectrum (Fig.2a) for the octadecyl-terminated Si(111) without Hg. The C&endash;H stretching modes in the CH2 group show blueshifts, and the C&endash;H antisymmetric stretching modes in the CH3 are broadened in comparison with the transmission spectrum. Under a certain bias voltage, we observed changes in band shape. We concluded that the variation was due to the temperature increase by resistive heating of the substrate.
Fig.1 Fig.2

Low-temperature observation of the softened C-H stretching vibrations of cyclohexane on Rh(111)
[Takanori Koitaya, Atsushi Beniya, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu, Phys. Rev. B 80, 193409 (2009) DOI: 10.1103/PhysRevB.80.193409 ].

The C-H stretching vibrations of cyclohexane on Rh(111) were investigated using infrared reflection absorption spectroscopy between 20 and 89 K. At 20 K, the softened C-H stretching band consists of several sharp peaks, ranging from 2500 to 2700ÝcmÄ|1. The wide-range distribution of the softened C-H stretching peaks results from inhomogeneity of adsorption environments. With increasing the substrate temperature, each softened C-H stretching peak becomes significantly broadened, but the normal C-H stretching peaks are little changed. These results indicate that the local interaction between the softened C-H species and the Rh(111) surface is sensitive to a thermally excited low-energy mode. The temperature-dependent broadening of a soft mode at low temperature is analyzed using a vibrational dephasing model, where the softened C-H stretching mode is anharmonically coupled with a thermally excited frustrated translation mode.


The growth process of first water layer and crystalline ice on the Rh(111) surface [Atsushi Beniya, Yuji Sakaguchi, Tetsuya Narushima, Kozo Mukai, Yoshiyuki Yamashita, Shinya Yoshimoto, and Jun Yoshinobu; J. Chem. Phys. 130, 034706 (2009); doi:10.1063/1.3060952 (10 pages)

The adsorption states and growth process of the first layer and multilayer of water (D2O) on Rh(111) above 135ÝK were investigated using infrared reflection absorption spectroscopy (IRAS), temperature programed desorption, spot-profile-analysis low-energy electron diffraction, and scanning tunneling microscopy (STM). At the initial stage, water molecules form commensurate (3Ý3)R30° islands, whose size is limited for several hexagonal units; the average diameter is Ý 2.5Ýnm. This two-dimensional (2D) island includes D-down species, and free OD species exist at the island edge. With increasing coverage, the D-up species starts to appear in IRAS. At higher coverages, the 2D islands are connected in STM images (Fig.1). By the titration of Xe adsorption we estimated that the D-down domain occupies about 55% on Rh(111) at the saturation coverage. Further adsorption of water molecules forms three-dimensional ice crystallites on the first water layer; thus, the growth mode of crystalline water layers on Rh(111) is a Stranski&endash;Krastanov type. We have found that an ice crystallite starts to grow on D-down domains and the D-down species do not reorient upon the formation of a crystalline ice.
Fig. 1

Microscopic diffusion processes of NO on the Pt(997) surface
[Noriyuki Tsukahara, Kozo Mukai, Yoshiyuku Yamashita, and Jun Yoshinobu, J. Chem. Phys. 128(2008)054701]

The microscopic diffusion processes of NO molecules on Pt(997) at low coverage were investigated using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). When NO molecules adsorb on Pt(997) at low temperature, each molecule transiently migrates on the surface from the first impact point to a possible adsorption site. At 11  K, the molecules are trapped at four adsorption sites on Pt(997): the on-top sites on the (111) terrace (OT), the hollow sites on the (111) terrace (HT), the bridge sites at the step (BS) and the hollow sites at the step downstream (HS). Based on the initial population ratio for these sites, the mean lateral displacement by transient migration is estimated to be 4.1  Å. By heating the surface to 45  K, the HS species migrate up to the BS sites; the migration barrier is roughly estimated to be 120  meV. In the temperature range from 70  to  77  K, TR-IRAS measurements were carried out to observe the site change of OT species to the adjacent HT sites at isothermal conditions; the activation barrier and the preexponential factor are estimated to be 200  meV and 2.0x1011  s-11, respectively. In the temperature range from 100  to  110  K, the HT species migrate across the terrace and finally reach the BS sites. The activation barrier between the HT sites and the preexponential factor are estimated to be 290  meV and 6.5x1011  s-11, respectively, from the TR-IRAS data together with kinetic Monte Carlo simulations. On the whole, the quantitative microscopic picture of NO migration on Pt(997) has been established.


Regioselective Cycloaddition Reaction of Alkene Molecules with the Asymmetric Dimer on Si(100)c(4x2) [K. Oguchi et al., J. Am. Chem. Soc. 129(2007) 1242-1245.]

We investigated the adsorption states of 2-methylpropene and propene on Si(100)c(4x2) using low-temperature scanning tunneling microscopy. We have found that regioselective cycloaddition reactions (di-sigma bond formation) occur between the asymmetric alkene molecules and the asymmetric dimers on Si(100)c(4x2). First-principles calculations have elucidated that the regioselectivity is closely related to the structures of precursor species and these precursor species have carbocation-like features. Thus, we conclude that Markovnikov's rule is applicable for the cycloaddition of asymmetric alkene with the asymmetric dimer on Si(100)c(4x2).


Transient diffusion and cluster formation of water molecules on Rh(111) at 20 K [ Atsushi Beniya, Kozo Mukai, Yoshiyuku Yamashita and Jun Yoshinobu, J. Chem. Phys. 126 (2007)141102]

We investigated the initial stage of water adsorption on Rh(111) at 20 K using infrared reflection absorption spectroscopy (IRAS). In this low coverage region, isolated water molecules and small water clusters are observed. Since thermal diffusion is suppressed at 20 K, the formation of water clusters at low coverage is controlled by both coverage and transient diffusion on the surface. Within a simple random walk model as transient diffusion and clustering process, we estimate the mean lateral displacement from the first impact point to the final adsorption site to be 0.76 nm.


Precursor mediated cycloaddition reaction of ethylene to the Si(100)c(4x2) surface [M.Nagao et al., JACS 126(2004) 9922-9923]

We report the direct observation of a precursor state for the cycloaddition reaction (the di-s bond formation) of ethylene on Si(100)c(4x2) using high-resolution electron energy loss spectroscopy at low temperature, and the meta-stable precursor state is identified as a weakly bonded p -complex type. The activation energy from the p-complex precursor to the di- s bonded species is experimentally estimated to be 0.2 eV. First-principles calculations support the p-complex precursor mediated cycloaddition reaction of ethylene on Si(100)c(4x2).


Low-Energy Electron-Stimulated Chemical Reactions of CO in Water Ice [S. Yamamoto et al., Chem. Phys. Lett. 388(2004) 284-288]

We investigated low-energy electron-stimulated chemical reactions between CO and water molecules in low-temperature ice using infrared reflection absorption spectroscopy. Carbon dioxide, the formyl radical, formaldehyde, and methanol were produced by electron irradiation of the water/CO/water layered ice.The electron energy threshold and temperature dependence for the chemical reactions were investigated to elucidate the reaction mechanisms.

CO in water ice + e- -> CO2, H2CO, CH3 OH


Selective functionalization of the Si(100) surface by converting the adsorption linkage of a bifunctional organic molecule [Md. Zakir Hossain et al., Chem. Phys. Lett. 388(2004) 27-30]
Selective functionalization by converting the adsorption linkage of bi-functional organic molecule, 1-dimethylamino-2-propyne (DMAP), on Si(100) has been studied using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). The HREELS and STM results clearly indicate that DMAP adsorbs at the down dimer atom of Si(100)c(4x2) through an N-Si dative bond leaving the acetylene unit of DMAP free at low temperature (65 ~ 90 K). The adsorption linkage is converted from the N-Si dative bond to the Si-C di- s bonds by annealing the adsorbed surface to 300 K, and thus the tertiary amino group becomes free.

Purely site-specific chemisorption and conformation of trimethylamine on Si(100)c(4x2)
[Md. Zakir Hossain et al., JACS 125(2003) 9252-9253]
We have investigated the adsorption of trimethylamine (TMA) on Si(100)c(4x2) using scanning tunneling microscopy (STM) at 80 K. The adsorbed TMA appears as a triangle shaped bright protrusion in the occupied state STM image. The triangle shaped protrusion is ascribed to three methyl groups in the adsorbed TMA. The center of the protrusion is located on the down atom site, which indicates that the adsorption of TMA occurs only on the down dimer atom. Thus, TMA adsorption on Si(100)c(4x2) is found to be purely site-specific on the down dimer atom and can be categorized in Lewis acid-base reaction.


Lateral displacement by transient mobility in chemisorption of CO on Pt(997) (Yoshinobu et al., Phys. Rev. Lett. 90(2003)248301 )

We investigated adsorbed states of CO on Pt(997) at 11 K using infrared reflection absorption spectroscopy. At 11 K, thermal migration is suppressed and thus the initial chemisorption at terrace sites and step sites is controlled by the transient mobility of the adsorbing molecule. The initial occupation ratio between atop CO on the terrace and atop CO at the step is directly determined to be 3.6:1.With a simple isotropic migration model, we estimated the mean lateral displacement from the first impact point to the initial chemisorption site to be 0.68nm .We also discuss the origin of transient mobility of CO on metal surfaces.


Elucidation of c-type defect on Si(100) (Md.Zakir Hossain et al., Phys. Rev. B. 67(2003)153307. )

The origin and atomic structure of C-defect on Si(100) have been unambiguously identified. Two pairs of enantiomorphic protrusions of C-defect (U,U',D,D') have been observed by low temperature scanning tunnelling microscopy (STM). These are attributed to the dissociative adsorption of single water molecule on two adjacent dimers. Two unreacted dangling bonds on these dimers have different electronic states which are visualized in unoccupied state STM images.


Si 2p photoelectron spectrum of Si(100)c(4x2) at 30K (S. Machida et al., Surf. Sci. 532/535 (2003) 716-720)

This spectrum was measured with hn=129eV using a Phoibos100 analyzer at KEK-PF BL16B. The overall instrumental resolution was ~40meV, so that the width of each component can be estimated as a function of temperature.


Direct evidence for asymmetric dimer of Si(100) surface at low temperatures by means of high resolution Si 2p photoelectron spectroscopy (Y. Yamashita et al., Jpn. J. Appl. Phys 41(2002)L272-L274.)

Surface atoms on Si(100) are reconstructed to form a (2x1) dimer row structure. Many experimental and theoretical studies have supported the buckled (asymmetric) dimer with c(4x2) phase as a ground state, although some quantum chemical studies including electron correlation have supported a symmetric dimer as the most stable state. Recently, several low temperature STM studies have reported symmetric dimer images. With decreasing the temperature from 80 K to 20 K, the area of apparent symmetric dimer increases. The origin of symmetric images is a controversial issue (dynamic or static). In this study, using high resolution Si 2p photoelectron spectroscopy, we have investigated the electronic states of Si(100) at 140 K, 100 K and 55 K (Fig.1). By careful analysis of surface core level shift and area intensity of each component in the spectra, the number of asymmetric dimer atoms does not change in this temperature range. Thus, we conclude that the dimer on Si(100) is asymmetric down to 55 K, and the origin of symmetric images in STM may be due to dynamic effects.


Fig.1 Si 2p PES spectra at low temperature. (hn=129eV, normal emission)


High-resolution core-level photoelectron spectroscopy of organic and inorganic molecules on Si(100)
[ISSP Yoshinobu laboratory: since December 1999]

Chemical bond between adsorbed unsaturated hydrocarbon molecules and the Si(100)(2x1) surface has been investigated by means of HRPES at PF BL-16B. This undulater beam line has a 24-m High-resolution Spherical Grating Monochromator (H-SGM), which covers the photon energy range from 40 eV to 550 eV. We have carried in our UHV chamber including a LEED optics (Varian) and an old electron analyzer (VSW HA100) and a new SPECS Phoibos100.


Microscopic behavior of atoms and molecules on surfaces by using a combined low temperature STM and IRAS system
[ISSP Yoshinobu laboratory: since August 1999]

In order to elucidate microscopic behavior and nano structures of atoms/molecules on solid surfaces, we are constructing a new apparatus which includes low temperature STM and IRAS. At the present moment, the IRAS system shows a satisfactory good S/N ratio (<3x10-5 abs: 1000scans, 4cm-1).


Towards the fabrication of hybrid system with organic molecules on Si(100)
[ISSP Yoshinobu laboratory & IMS-UVSOR]

In the cases of cyclopentene and cyclohexene, the dangling bond state just below the Fermi level in valence PES spectra has decreased in intensity with increasing the coverage and at the saturation the dangling bond peak disappears almost completely. On the other hand, the HOMO derived states which are clearly observed for the multilayer of molecules at 90K have been significantly attenuated at the original position. Thus, we conclude that the p bond of cyclopentene (and cyclohexene) interacts with the Si dangling bonds. According to the Si 2p spectra, it seems that the SiC bonds are formed between the Si surface and the molecule. This is also supported from the information about the C 1s peak in XPS spectra taken at ISSP.
1,4-cyclohexadiene has two C=C double bonds. There are two possible adsorption structures on Si(100)(2x1); (1) two p bonds interact with Si dangling bonds to form four SiC sigma bonds, and (2) one p bond reacts with one Si dimer to make di-sigma bond and another C=C bond is still intact. Preliminary results from PES in UVSOR-BL5A and XPS in ISSP suggest that the case (2) is plausible. In this case, additional molecular development/modification is possible using organic chemical reactions with 1,4-cyclohexadiene/Si(100). However, further experiments are needed to confirm the adsorption structure and realize this idea.


A new photoelectron spectroscopy apparatus for solid surfaces at low temperature
[ISSP Yoshinobu laboratory]

In order to elucidate electronic properties of adsorbed atoms/molecules and surfaces, a new UHV system including a hemispherical electron analyzer (VG CLAM4), X-ray sources (VG XR3), a UV source (VG UVL-Hi) with a polarizer, LEED (VSI), QMS (Balzers Prisma) and the home-made gas dosing system (a continuous doser with a variable temperature tungsten tube and a MCP doser with a pulse valve) is constructed. The sample is cooled down to <30 K using a closed cycle He refrigerator, and can be rotated around two axis.

(From left: Ms. Hamaguchi, Dr. Yamashita and Mr. Machida)


Internal structure of STM images in the Pd(110)c(4x2)benzene [J. Yoshinobu et al., Phys. Rev. Lett., 79 3942-3945 (1997)]

Evidence for a molecule-substrate hybridized state near the Fermi level (EF) is presented for Pd(110)c(4x2)-benzene. Observed images of adsorbed benzene near EF by STM consist of two elongated protrusions separated by a single nodal depression with C2 symmetry. The existence of a benzene derived state near EF is also observed by metastable atom electron spectroscopy, and it is assigned to the antibonding states between the 1e1g molecular orbital of benzene and the Pd 4d orbitals by ab initio molecular orbital calculations.


Determination of adsorption potential energy surface of CO on Ni(100) [T. Moriwaki, R. Kishi, J. Yoshinobu and M. Kawai, submitted to J. Chem. Phys.]

The microscopic surface diffusion of CO molecules on Ni(100) has been studied with infrared reflection absorption spectroscopy (IRAS). We have observed low temperature kinetics approaching equilibrium between isolated adsorbed CO and c(2x2)-CO domains at a low coverage. The upper limit of the diffusion barrier Ed = 60 ± 7 meV and the preexponential factor A = 106.8±1.1 s-1 were estimated. While the adsorption energy (Ead) of CO on Ni(100) is approximately 1.3 eV, the diffusion barrier is less than 5% of Ead. It is shown that the barrier is consistent with the postulated potential energy surface [J. Yoshinobu et al., Phys. Rev.B49,16670-16677 (1994)] derived from the frustrated translation modes and the binding energy difference with a harmonic parabola assumption, and also with the recent microscopic diffusion barrier obtained by quasielastic He atom scattering (QHAS) measurements.

NiCOpotential.JPG


Thermal excitation of oxygen species as a trigger for CO oxidation on Pt(111) [ J. Yoshinobu and M. Kawai, J. Chem. Phys. 103 (1995) 3220-3229]
Thermal excitation of adsorbed oxygen species is found to initiate the CO oxidation on Pt(111). We have prepared three different coadsorption systems to study the reactivity of different oxygen species: (1) CO on the O2 preadsorbed Pt(111) surface, (2) CO on the nearly perfect Pt(111) p(2x2)-O surface and (3) CO on the disordered atomic oxygen-preadsorbed Pt(111) surface. Four CO2 desorption peaks (a-CO2 at 125 K, b3-CO2 at ~225 K, b2-CO2 at ~260 K and b1-CO2 at 320 K) are observed. The desorption temperatures of CO2 strongly depend on the adsorbed states of oxygen species. We have shown that the a-CO2 state, b2,3-CO2states and b1 - CO2 state are correlated with adsorbed O2, disordered oxygen atoms and p(2x2) oxygen atoms, respectively. The difference in CO2 desorption temperature is related to thermal excitation of each oxygen species, which is derived from the structural information of coadsorbed states during thermal evolution by means of LEED and IRAS.

This proposed mechanism can explain most experimental data of CO2 formation dynamics on Pt(111), and is supported by a recent first principle calculation [A.Alavi et al., Phys. Rev. Lett. 80, 3650 (1998)].


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