The hexagonal silver-nanoparticles (Ag-NPs) array grown in self-assembled anodic aluminum oxide (AAO) nanochannels have been successfully demonstrated to show reliable enhanced Raman-scattering signal. In this work, we systematically explored the roles of multipolar resonances for Ag-NP/AAO substrates in affecting the signal amplification, the background noise, as well as the signal-to-noise ratio at three typical wavelengths for Raman spectroscopy (532 nm, 633nm, and 785 nm). In addition, a duel-resonant system consisting of Ag-NPs embedded in a sinusoidal-shaped AAO/Al substrate was also investigated. The grating structure enables the excitation of surface plasmon polaritons (SPPs) to further couple with the generated hots spots between adjacent Ag-NPs and meanwhile provides a diffraction dispersion which is beneficial for spectral detection.
Extensive interface between donors and acceptors and respective connected networks of both created in bulk
heterojunction (BHJ) solar cells have been proposed to effectively boost the photovoltaic efficiency via facilitating
exciton dissociation and charge transport. The multi-scale nature of intermolecular interaction involved however renders
the fabrication of such nano-morphology to try-and-error. Our recently proposed freeze-dry method to fabricate the BHJ
polymer solar cells has demonstrated comparable efficiencies, regardless the intermolecular interaction strengths of
polymers. A fibrous polymer scaffold, being first concocted with the simultaneously grouted PCBM in solution, sustains
while the solid-phase solvent sublimates at a low temperature. The formation of such polymer structure can only be
unraveled with in-situ monitoring means. Here, we report in-situ characterization of such structure during the initial
cooling process with Raman spectroscopy. Raman spectroscopy – revealing molecular vibrational signatures –
scrutinizes short-range structural regularity. In comparison with the Raman spectrum of the thermally annealed films,
the sequential Raman spectra, acquired during cooling drop-cast o-dichlorobenzene solution of pristine P3HT and its
blend with PCBM, show promptly emergent Raman signatures below -5°C – significantly narrowed peaks and new
prominent peaks, signifying homogeneously packed P3HT agglomerates. These distinct Raman characteristics
accompanied by real-time photoluminescence and absorption measurements suggest extended conjugation and high
homogeneity of the P3HT network formed under the dynamic cooling process. This in-situ study thus opens a new
utility of Raman spectroscopy to investigate intricate molecular packing that is relevant to the efficient transport of
excitons and charges in polymer solar cells.
This report presents an overview of our recent near-field investigation of both local and surface plasmon resonance
(SPR) with a scattering-type scanning near-field optical microscope (s-SNOM) which has sub-10 nanometer resolution.
With the ability to perform near-field optical experiments at multiple excitation wavelengths simultaneously, this
instrument has recorded near-field intensity and phase images of a wide range of subwavelength plasmonic structures:
single nanohole and nanoslit, circular and elliptical hole arrays, etc. The near-field results obtained with different
excitation wavelengths were confirmed by numerical calculation and were made direct correspondence with far-field
observations by comprehensive models. The multi-wavelength s-SNOM proves to be an essential tool to unravel many
interesting plasmonic phenomena in nanometer scale. This work investigates the nature of subwavelength plasmon
optics which potentially will play an important role in the development of many innovative highly efficient optoelectronic
devices (light-emitting devices and solar cells) and highly sensitive sensors based on SPR and surfaceenhanced Raman scattering.
We report a time-of-flight study of drift mobilities of hole and electron in mixed thin films of N,N'-diphenyl-N,N'-
bis(1-napthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ3). Based on Poole-
Frenkel model, the extracted zero-field hole mobility of pure NPB was 2.6x10-4 cm2/Vs which is much larger than that
of pure AlQ3 (9.16x10-10 cm2/Vs). As the AlQ3 concentration is increased, the hole mobility decreases exponentially.
In this case, AlQ3 molecules act as blocking "hills" to the hole transport, since its HOMO energy level is 0.4 eV lower
than that of NPB. In contrast, the difference in the electron mobilities of pure NPB and AlQ3 is much smaller
(5.28x10-6 cm2/Vs vs. 1.51x10-7 cm2/Vs) and the field-free electron mobility of the mixed films exhibits a minimum as
the AlQ3/NPB fraction ratio reaches about 75%. The LUMO energy level of AlQ3 is 0.6 eV lower than that of NPB,
making AlQ3 become "traps" to the electron transport. When the amount of AlQ3 reaches a certain level such that they
form connected transport network, the electrons are then driven mostly in this network and the NPB molecules become
blocking "hills". In summary, the HOMO and LUMO energy levels, the charge mobilities of pure compounds and the
characteristics of their microscopic networks can greatly influence the resultant transport behaviors. These results may
create challenges for existing transport models of disordered organic semiconductors and will be useful in designing
organic light-emitting devices based on mixed-layer structures.
Different arrays of Ag-nanoparticles grown on anodic alumina nanochannels with precisely tunable gaps (5~25 nm) are
exploited for surface-enhanced Raman spectroscopy (SERS). The enhancement becomes significant for gaps below 10
nm and turns dramatically large when gaps reach an unprecedented value of 5 nm. The results are quantitatively
consistent with theories based on collectively coupled surface plasmon. Such nanofabricated substrates with consistently
uniform and large dynamic range have many chemical/biological sensing applications.
In this report, we have investigated electroluminsecence (EL) characteristics and field-induced recovery of organic light-emitting devices (OLEDs) with a mixed emitting layer (EML). The mixed EML which is composed of a mixture of a hole transport layer (HTL), N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), and an electron transport layer (ETL), bis(10-hydroxybenzo[h]quinolinato) beryllium (Bebq2), was fabricated by co-evaporation. Evident recovery of luminance-voltage characteristic was observed in the mixed device. It is explained by a dipole rearrangement model. The lifetime of this mixed layer OLED can reach 348 min. with initial luminance of 11,000 cd/m2 which is two times better that of the comparable heterojunction device.
For the purpose of functional third harmonic optical microscopy, it is necessary to find a method to locally enhance third harmonic generation at specific cellular site. We have demonstrated that by matching the third harmonic generation frequency of a Cr:forsterite laser and the surface plasmon resonance frequency of <50-nm silver nanoparticles, localized enhancement of third harmonic intensity of more than 100-folds can be achieved both in phantom and in real biological tissues. This strongly enhanced third harmonic signal can then be applied to specific molecule imaging by attaching the nanoparticles to the target molecule with the advantages of noninvasiveness and deep penetration capability.
We report a high-power, kilohertz, ultrafast optical parametric amplifier (OPA) that is seeded by white-light continuum and contains three amplification stages. Two 3-mm KTA crystals cut in type-II phase-matching configuration are used in the OPA system which is capable of producing up to 70 μJ, 140 fs infrared laser pulses at wavelength ranging from 2.9 to 4 μm. A full-scaled numerical simulation on the OPA system was performed. Actual white-light seeded signal pulse and finite phase-matching bandwidth were taken into account in the calculation. Material dispersion and linear absorption of all the optical components involved were properly incorporated. The simulation results match the experimental results almost perfectly. Our simulation provides an essential tool to design and optimize the OPA systems. A step-by-step design procedure based on this simulation algorithm is presented.