- T. Isoniemi, A. Johansson, T. K. Hakala, M. Rinkiö, P. Törmä, J. J. Toppari, and H. Kunttu: Surface plasmon effects on carbon nanotube field effect transistors, Applied Physics Letters, 2011 (submitted).
Abstract: Herein we experimentally demonstrate surface plasmon polariton (SPP) induced changes in the conductivity of carbon nanotube field effect transistor (CNT FET). SPP excitation is done via Kretschmann configuration while the measured CNT FET is situated on the opposite side of the metal layer away from the laser, but within reach of the launched SPPs. We observe a shift of ~0.4 V in effective gate voltage. SPP-intermediated desorption of physisorbed oxygen from the device is discussed as a likely explanation of the observed effect. This effect is visible even at low SPP intensities and within the near-infrared range
- T. Kiljunen, E. Popov, H. Kunttu, and J. Eloranta: Rotation of methyl radicals in molecular solids, Journal of Physical Chemistry A, 2010, 114, 4770.
Abstract: Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in solid carbon monoxide, carbon dioxide, and nitrogen matrices. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm ArF excimer laser pulse during the gas condensation on the substrate. The ESR spectra exhibit anisotropic features that persist over the temperature range examined and in most cases this indicates a restriction of rotation about the C2 symmetry axis. A nonrotating CH 3 was also observed in CO2 matrix. The intensity ratiobetween the symmetric (A) and antisymmetric (E) nuclear spin states was recorded as a function of temperature for each molecular matrix. The rotational energy levels are modified from their gas phase structure with increasing crystal field strength. An anomalous situation was observed where the A/E ratio extended below the high temperature limit of 1/2.
- T. K. Hakala, J. J. Toppari, A. Kuzyk, M. Pettersson, H. Tikkanen, H. Kunttu, and P. Törmä: Vacuum Rabi splitting and strong coupling dynamics for surface plasmon polaritons and Rhodamonen 6G molecules, Physical Review Letters, 2009, 103, 053602.
Abstract: We report strong coupling between surface-plasmon polaritons (SPP) and Rhodamine 6G (R6G) molecules, with double vacuum Rabi splitting energies up to 230 and 110 meV. In addition, we demonstrate the emission of all three energy branches of the strongly coupled SPP-exciton hybrid system, revealing features of system dynamics that are not visible in conventional reflectometry. Finally, in analogy with tunable-Q microcavities, we show that the Rabi splitting can be controlled by adjusting the interaction time between waveguided SPPs and R6G deposited on top of the waveguide. The interaction time can be controlled with sub-fs precision by adjusting the length of the R6G area with standard litography methods.
- T. Kiljunen, E. Popov, H. Kunttu, and J. Eloranta: Rotation of methyl radicals in a solid krypton matrix, The Journal of Chemical Physics, 2009, 130, 164504. Abstract: Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in a solid krypton matrix at 17 - 31 K temperature range. The radicals were produced by dissociating methane by plasma bursts generated by focused 193 nm excimer laser radiation during krypton gas condensation on the substrate. The ESR spectrum exhibits only isotropic features at the temperature range examined and the intensity ratio between the symmetric (A) and antisymmetric (E) spin state lines exhibit weaker temperature dependence than in solid argon matrix. However, the general appearance of the methyl radical spectrum depends strongly on temperature due to the pronounced temperature dependency of the E state linewidths. The rotational energy level populations are analyzed based on the static crystal field model, pseudorotating cage model, and quantum chemical calculations for an axially symmetric planar rotor. Crystal field stregth parameter values -140 cm-1 in Ar and -240 cm-1 in Kr match most closely the experimentally observed rotational energy level shifts from the gas phase value. In the alternative model, considering the lattice atom movement in a pseudorotating cage, the effective lowering of the rotational constants B and C by 80 - 90% leads to similar effects.
- E. M. S. Maçôas , P. Myllyperkiö, M. Pettersson, and H. Kunttu: Vibrational relaxation of matrix isolated carboxylic acid dimers and monomers, Journal of Physical Chemistry A, 2009, 113, 7227.
Abstract: Femtosecond mid-IR transient absorption spectroscopy was used to probe the vibrational dynamics of formic and acetic acid isolated in solid argon following excitation of the fundamental transition of the carbonyl stretching mode. Carboxylic acids form extremely stable H-bonded dimers hindering the study of monomeric species. The low-temperature rare-gas matrix isolation sampling technique allows for a unique control over aggregation enebling the study of the monomer vibrational dynamics, as well as the dynamics of two distinct dimer structures (cyclic and open chain), This study provides insights into the role of the methyl rotor and hydrogen bonding in the vibrational dynamics of carboxylic acids. In the monomer of FA, depopulation of the initially excited state is characterized by a time constant of approximately 500 ps, and it is followed by the energy transfer from intermediately populated intramolecular states into the phonon modes of the argon lattice (vibrational cooling) in a much longer time scale (estimated to be longer than 5 ns). The methyl rotor in acetic acid monomer accelerates both processes of population transfer and vibrational cooling, with time constants of 70-90 ps. Hydrogen-bonding in formic acid dimers, decreases the time constant associated with the dominant relaxation process by more than two orders of magnitude. Unlike formic acid, hydrogen-bonding in acetic acid has no apparent effect on the vibrational cooling.
- T. K. Hakala, J. J. Toppari, M. Pettersson, A. Kuzyk, H. Tikkanen, H. Kunttu, and P. Törmä: Frequency conversion of propagating surface plasmon polaritons by organic molecules, Applied Physics Letters, 2008, 93, 123307.
Abstract: We demonstrate frequency conversion of surface plasmon polaritons (SPP) by utilizing the coupling between organic dye-molecules and SPP. Launching of SPPs intoplasmonic waveguide is done in two ways: by optically excited molecules and quantum dots (QD). QDs are demonstrated to overcome the major problem of bleaching occuring with molecules. The SSP propagates tens of micrometers and clear frequency conversion is observed in the SPP spectrum after passing the are of converter molecules. The use of QDs and molecules as elements of all-plasmonic elements has the potential for high integration and use of self-assembly in the fabrication.
- S. L. Fiedler, H. M. Kunttu, and J. Eloranta: Application on mean-field and surface-hopping approaches for interrogation of the Xe 3 + molecular ion photoexcitation dynamics The Journal of Chemical Physics, 2008, 128, 164309 (1-10).
Abstract: The dissociation dynamics of the excited Xe3(+) molecular ion through the Pi(1/2)(u) and Pi(1/2)(g) conical intersection was interrogated by computational simulation in which no adjustable parameters were used. The electronic ground and excited state potential energy surfaces were generated by the diatomics-in-molecules method, and the Ehrenfest mean-field and Tully surface-hopping approaches treated the nonadiabatic interactions. Reproduction of the experimental spectrum of the symmetric photofragmentation as a function of excitation energy was obtained within the region of interest (2.5-3.75 eV), with the exception of a 0.25 eV width on the red side of the spectral apex. Good agreement was obtained with the experimental dissociated photofragment kinetic energy spectra. It was determined that the greatest contribution to the nonadiabatic coupling between the two states originated from the bending vibrational mode of the molecule in the Sigma(1/2)(u), ground electronic state before excitation.
- V. V. Khmelenko , H. Kunttu, and D. M. Lee: Recent progress in studies on nanostructured impurity helium solids, Journal of Low Temperature Physics, 2007, 148, 1-31. Abstract: Impurity-helium (Im-He) solids are porous materials formed inside superfluid He-4 by nanoclusters of impurities injected from the gas phase. The results of studies of these materials have relevance to soft condensed matter physics, matrix isolation of free radicals and low temperature chemistry. Recent studies by a variety of experimental techniques, including CW and pulse ESR, X-ray diffraction, ultrasound and Raman spectroscopy allow a better characterization of the properties of Im-He solids. The structure of Im-He solids, the trapping sites of stabilized atoms and the possible energy content of the samples are analyzed on the basis of experimental data. The kinetics of exchange tunneling reactions of hydrogen isotopes in nanoclusters and the changes of environment of the atoms during the course of these reactions are reviewed. Analysis of the ESR data shows that very large fraction of the stabilized atoms in Im-He solids reside on the surfaces of impurity nanoclusters. The future directions for studying Im-He solids are described. Among the most attractive are the studies of Im-He solids with high concentrations of stabilized atoms at ultralow (10-20 mK) temperature for the observation of new collective quantum phenomena, the studies of practical application of Im-He solids as a medium in neutron moderator for efficient production of ultracold (similar to 1 mK) neutrons, and the possibilities of obtaining high concentration of atomic nitrogen embedded in N-2 clusters for energy storage.
- E. Maçôas , R. Kananavicius, P. Myllyperkiö, M. Pettersson, and H. Kunttu: Ultrafast vibrational energy relaxation of Fe(acetylacetonate) 3 in solution, The Jounal of Physical Chemistry A, 2007, 111, 2054-2061.
Abstract: Transient mid-infrared spectroscopy is used to probe the dynamics initiated by excitation of ligand-to-metal (400 nm) and metal-to-ligand (345 nm) charge transfer states of Fe-III complexed with acetylacetonate (Fe(acac)(3), where acac stands for deprotonated anion of acetylacetone) in solution. Transient spectra in the 1500-1600 cm(-1) range show two broad absorptions red-shifted from the bleach of the nu(CO) (approximate to 1575 cm(-1)) and nu(CC) (approximate to 1525 cm(-1)) ground state absorptions. Bleach recovery kinetics has a time constant of 12-19 ps in chloroform and tetrachloroethylene and it decreases by 30-40% in a 10% mixture of methanol in tetrachloroethylene. The transient absorptions experience band narrowing simultaneously with blue-shifting of the absorption maxima. Both phenomena have time constants of 3-9 ps with no evident dependence on the solvent. The experimental observations are ascribed to fast conversion of the initially excited charge transfer states to the ligand field manifold, and subsequent vibrational cooling on the lowest ligand field excited state prior to electronic conversion to the ground state. The analysis of time dependent bandwidths and positions of the transient absorptions provides some evidence of mode specific vibrational cooling.
- E. M. S. Maçôas , P. Myllyperkiö, M. Pettersson, and H. Kunttu: Excited state dynamics of Cr(acac) 3 probed by ultrafast infrared spectroscopy, Journal of the American Chemical Society, 2007, 129, 8934.
Abstract: Ultrafast infrared spectroscopy is used to probe the dynamics of Cr(acac)(3) upon ligand field (400 nm) and charge transfer state (345 nm) excitation. At both pump wavelengths, the ground state absorption bands are strongly bleached at zero delay, and new broad transient absorption bands appear red shifted from the bleached bands. Recovery of ground state bleach is dominated by a fast time constant (15 ps), while a small percentage recovers within 760-900 ps. Despite the extensive studies on Cr(acac)(3) photophysics, the fast recovery of the ground state as a major channel is reported here for the first time. As a general result, the present communication emphasizes the great value of ultrafast infrared spectroscopy in inorganic photophysics.
- A. Kuzyk, M. Pettersson, J. J. Toppari, T. Hakala, H. Tikkanen, H. Kunttu, and P. Törmä: Molecular coupling of light with plasmonic waveguides, Optics Express, 2007, 15. 9908 -9917.
Abstract: We use molecules to couple light into and out of microscale plasmonic waveguides. Energy transfer, mediated by surface plasmons, from donor molecules to acceptor molecules over ten micrometer distances is demonstrated. Also surface plasmon coupled emission from the donor molecules is observed at similar distances away from the excitation spot. The lithographic fabrication method we use for positioning the dye molecules allows scaling to nanometer dimensions. The use of molecules as couplers between far-field and near-field light offers the advantages that no special excitation geometry is needed, any light source can be used to excite plasmons and the excitation can be localized below the diffraction limit. Moreover, the use of molecules has the potential for integration with molecular electronics and for the use of molecular self-assembly in fabrication. Our results constitute a proof-of-principle demonstration of a plasmonic waveguide where signal in- and outcoupling is done by molecules.
- E. Popov, T. Kiljunen, H. Kunttu, and J. Eloranta: Rotation of methyl radical in argon matrix, The Journal of Chemical Physics, 2007, 126, 134504. Abstract: Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in a solid argon matrix at 14-35 K temperatures. The radicals were produced by dissociating methane by plasma bursts generated either by a focused 193 nm laser radiation or a radio frequency discharge device during the gas condensation on the substrate. The ESR spectrum exhibits axial symmetry at the lowest temperature and is ascribed to ground state molecules with symmetric total nuclear spin function I=3/2. The hyperfine anisotropy (A(parallel to)-A(perpendicular to)) was found to be -0.01 mT, whereas that of the g value was 2.5x10(-5). The anisotropy is observed for the first time in Ar and is manifested by the splitting of the low-field transition. Elevation of temperature leads reversibly to the appearance of excited state contribution having antisymmetric I=1/2. As a function of the sample temperature, the relative intensities of symmetric and antisymmetric spin states corresponding to ground and excited rotor states, respectively, proton hyperfine and electron g-tensor components, and spin-lattice relaxation rates were determined by a numerical fitting procedure. The experimental observations were interpreted in terms of a free rotation about the C-3 axis and a thermal activation of the C-2-type rotations above 15 K. The ground and excited rotational state energy levels were found to be separated by 11.2 cm(-1) and to exhibit significantly different spin-lattice coupling. A crystal field model has been applied to evaluate the energy levels of the hindered rotor in the matrix, and crystal field parameter epsilon(4)=-200 cm(-1), corresponding to a 60 cm(-1) effective potential barrier for rotation of the C-3 axis, was obtained.
- A. Siitonen, H. Kunttu, and Mika Pettersson: Temperature dependence of electronic transitions of single-wall carbon nanotubes: Observation of an abrupt blueshift in the near-infrared absorption, The Jounal of Physical Chemistry C, 2007, 111, 1888-1894.
Abstract: Near-infrared (NIR) absorption spectra of single-wall carbon nanotube (SWNT) films are studied between 10 and 293 K. The most prominent effect is the shift of bands with temperature. Some nanotubes show a redshift of transition upon increasing temperature while some show blueshift and others show no shift. The shift is interpreted to originate mainly from the effect of strain induced in the tubes because of interaction with the environment. In particular, at temperatures T = 175-225 K, for some bands, there is an abrupt large blueshift, which is interpreted to originate from interaction of the nanotubes with water. Two models could be considered to explain the effect: (1) strain induced by a phase transition of water confined inside the nanotubes or (2) adsorption -desorption of water on the surfaces of nanotubes, but the current experimental data does not allow to distinguish unambiguously between the two possibilities. Some evidence of the presence of adsorbed water is obtained from coupled thermogravimetric and Fourier transform infrared (FTIR) measurements which indicate that water is released from purified nanotubes upon heating in synthetic air atmosphehre.
- E. Vehmanen, K. Kelo, J. Rintala, A. Siitonen, M. Pettersson, and H. Kunttu: Raman spectrum of nitrogen-helium solid, Journal of Low Temperature Physics, 2006, 114, 135.
Abstract: Structural and thermal properties of nanocrystalline nitrogen-helium solids are studied by Raman spectroscopy at temperature range 1.8-41 K. The N-2 vibrational line possesses spectral structure very similar to what is observed in bulk solid nitrogen, indicating ordered structure inside the nanocrystallites. The spectral observations show that the structure of the solid is dependent on the N2 content in the gas mixture used for sample preparation. Evidence for disordered nitrogen on the surfaces and interfaces of nanocrystallites can be extracted from the Raman spectra of the most diluted samples. Removing superfluid helium from the sample and annealing at 21 K did not affect the structure of the solid, whereas higher annealing temperatures yielded strong increase of density as evidenced by the up to similar to 10-fold increase of Raman signal. The alpha-solid-beta -solid transition, evidenced by a similar to 0.8 cm(-1) shift in the peak position, was seen in the collapsed nitrogen samples approximately at the same temperature as in a solid nitrogen film.
- J. Ahokas, K. Vaskonen, and H. Kunttu: Structure and Matrix Isolated Infrared Spectrum of Formyl Fluoride Dimer: Blue-Shift of the C-H Stretching Frequency, The Journal of Physical Chemistry, 2006, A 110, 7816. Abstract: Infrared spectroscopy (IR) of formyl fluoride (HCOF) dimer is studied in low-temperature argon and krypton matrixes. New IR absorptions, ca. 17 cm(-1) blue shifted from the monomer C-H stretching fundamental, are assigned to the HCOF dimer. The MP2/6-311++G** calculations were utilized to define structures and harmonic frequencies of various HCOF dimers. Among the four optimized structures, the dimer having two C-H center dot center dot center dot O hydrogen bonds possesses strongest intermolecular bonding. The calculated harmonic frequencies of this dimer structure are shifted from the monomer similarly as observed in the experiment. Thus, we suggest that the experimentally observed blue shifted C-H bands belong to the dimer with two C-H center dot center dot center dot O hydrogen bonds. This observation includes the HCOF dimer to the class of hydrogen bonded complexes showing blue shift in their vibrational energies.
- J. Ahokas, K. Vaskonen, and H. Kunttu: Photodissociation of formyl fluoride in rare gas matrixes, Journal of Physical Chemistry, 2006, 110, 6208-6215. Abstract: Photodissociation of formyl fluoride (HCOF) is studied in Ar, Kr, and Xe matrixes at 248 and 193 nm excitation by following spectral changes in the infrared absorption spectra. In all matrixes, the main photodissociation products are CO/HF species, including CO-HF and OC-HF complexes and thermally unstable CO/HF species (a distorted CO/HF complex or a reaction intermediate), which indicate negligible cage exit of atoms produced via the C-F and C-H bond cleavage channels. However, the observation of traces of H, F, CO, CO2, F2CO, FCO, and HRg(2)(+) (Rg = Kr or Xe) in Kr and Xe matrixes would imply some importance of other reaction channels too. The analysis of the decay curves of the precursor shows that dissociation efficiency of HCOF increases as Ar < Kr < Xe, the difference being the factor of 10 between Ar and Xe. Moreover, HCOF dissociates 20-50 times faster at 193 nm compared to 248 nm. Interestingly, whereas the CO/HF species are stable with respect to photolysis in Ar, they photobleach in Kr and Xe matrixes at 248 and 193 nm, even though the first excited states of CO and HF are not energetically accessible with 193 and 248 nm photons. In krypton matrix, the photodissociation of CO/HF species at 248 nm is observed to be a single photon process. Quantum chemical calculations of electronic excitation energies of CO-HF and OC-HF complexes show that the electronic states of HF and CO mostly retain their diatomic nature in the pair. This clearly demonstrates that photodissociation of CO/HF complexes is promoted by the surrounding rare gas lattice.