Electric Field Modulation Spectroscopy Research Articles

Extremely large electro-optic effect of TPPS J-aggregates in the PVA or PVP polymer matrix and aqueous solution.

The molecules of tetra-phenyl porphyrin tetra-sulfonic acid (TPPS) form a J-aggregate via self-organization in aqueous solution. The J-aggregates formed in an aqueous solution with added hydrochloric acid were dispersed in PVP polymer and subjected to electric field modulation spectroscopy. The observed difference in the static polarizability Δαp between the excited and ground states in the TPPS J-aggregates reached a value 2.66 × 104 times and 1.22 × 105 times larger than in the monomer molecules in PVA and PVP polymer matrices, respectively. This large enhancement factor of the electro-optic effect (EOE) may be induced by the higher-order structure of the J-aggregate. The different enhancement factor for each polymer matrix may be introduced by the structural differences between the polymer matrices. Electric field modulation spectroscopy of the J-aggregate in aqueous solution showed an absorbance change of 25% of the magnitude of absorbance peak.

(Invited) Transition Metal Dichalcogenide Monolayers As Quantum Wells

Recently, transition metal dichalcogenide (TMDC) monolayers, such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), have attracted strong attention as novel two-dimensional (2D) semiconducting materials due to their large bandgap (1−2 eV) and excellent transport properties. Moreover, the thickness of monolayer TMDCs is less than 1 nm, which is one of the thinnest materials, and it leads to strong confinement effects, resulting in large binding energy of exciton (> 100 meV) and formation of charged excitons. Particularly, due to their layered structure, there are no dangling-bond states on the surface of TMDC monolayers and it could be an ideal quantum well. Here, we will report the optical and thermoelectric properties of TMDC monolayers as quantum wells. In this study, we fabricated ion-gel-gated EDLTs using large-area TMDC monolayers, MoS2 and WSe2, grown by chemical vapor deposition [1-8]. The Fermi level of TMDCs can be continuously shifted by applying gate voltage, and we can induce both hole and electron transport in these devices. The hole mobility of WSe2 can be enhanced up to 90 cm2/Vs at high carrier density of 1014 cm-2, whereas the MoS2 showed electron mobility of 60 cm2/Vs. Using EDLT technique, we investigated the electric field modulation spectroscopy of TMDC monolayers and the clear signature of quantum well, such as quantum confined Stark effect, was observed. Very importantly, it is well-clarified that, in quantum wells, the bandgap reduction by electric field is simply proportional to width of quantum well. Therefore, in ultimately thin TMDC monolayers, we need extremely strong electric field to realize obvious spectral modulation, which is very difficult for conventional field-effect transistors with oxide dielectric layers such as SiO2 and Al2O3. To break this limitation, we used EDLT technique and firstly observed the obvious evidence of quantum confined Stark effect. Seebeck coefficient, which is the proportional constant to the voltage generation against induced temperature gradient, is a significant factor to design thermoelectric materials. Importantly, according to the Mott equation, Seebeck coefficient is proportional to the energy derivative of the electronic density of states at around Fermi energy and, therefore, low-dimensional structures/materials are suitable for thermoelectric applications. We also investigated Fermi energy dependence of Seebeck coefficient in TMDC monolayers using EDLT technique. As the results, we observed clear enhancement in thermoelectric properties due to the low-dimensional effect. [1] J. Pu, L.-J. Li, T. Takenobu, et al., Nano Lett. 12, 4013 (2012). [2] J.-K. Huang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 923 (2014). [3] J. Pu, L.-J. Li, T. Takenobu, et al., Appl. Phys. Lett., 103, 23505 (2013). [4] J. Pu, L.-J. Li and T. Takenobu, Phys. Chem. Chem. Phys., 10.1039/C3CP55270E (2014). [5] Y.-H. Chang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 8582 (2014). [6] C.-H. Chen, T. Takenobu, L.-J. Li, et al., 2D Materials, 1, 034001 (2014). [7] L. Chu, T. Takenobu, G. Eda , et al., Scientific Reports, 4, 7293 (2014) [8] K. Funahashi, L.-J. Li, T. Takenobu, et al., Jpn. J. Appl. Phys. 54, 06FF06 (2015).

Nanoscale-resolved near-infrared photoabsorption spectroscopy and imaging of individual gallium antimonide quantum dots

Near-infrared photoabsorption spectra of individual dots of gallium antimonide (GaSb) epitaxially grown on a silicon substrate were investigated by nanoscale-resolved spectroscopic measurements and by imaging through scanning tunneling microscopy (STM). With a short acquisition time of a few minutes, photomodulated current spectroscopy (STM-PMCS), based on STM without a tunneling current, showed that our samples exhibited photoabsorption with a peak centered at about 0.78 eV. Spatial variations in STM-PMCS imaging arose from differences in dot sizes. However, the STM-PMCS images were insufficiently unambiguous to permit interpretation of the signal origin in the tunneling region, because a tip-expansion effect could have arisen through photointensity modulation. Spectral features revealed by STM-based electric field modulation spectroscopy (STM-EFMS), which is free of the tip-expansion effect, showed a significant size-dependent energy shift at around 0.8 eV that was attributed to a quantum-confinement effect on the bound states of the GaSb quantum dots. Our approach, which utilizes the merits of both STM-PMCS and STM-EFMS, is efficient in elucidating the photoabsorption properties of nanoscale objects.

Electroabsorption Spectra of Lead Sulfide (PbS) Quantum Dots in a Polymer Film

The electric field response of lead sulfide (PbS) quantum dots with near-infrared absorption of 1000–1600 nm (corresponding to diameters of 2–5 nm) embedded in a PMMA film has been systematically studied by using electric field modulation spectroscopy. For all PbS quantum dots samples, the electroabsorption spectra across the first and second exciton bands are similar in shape to the second derivative of the absorption spectra, indicating the enhancement of the electric dipole moment following the optical transition to these exciton bands. The analysis of the electroabsorption spectra shows that the dipole moments for both first and second exciton bands increase a little bit with increasing particle size. The quadratic field dependence is observed between the modulated absorption spectrum and the electric field of PbS quantum dots doped in a PMMA film.

Gigantic Electric Dipole Moment of Organic Microcrystals Evaluated in Dispersion Liquid with Polarized Electroabsorption Spectra

Polarized electroabsorption spectra of the 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) microcrystal, which is well-known as an organic nonlinear optical material, were measured in decalin liquid using electric-field modulation spectroscopy. On the basis of the results, the magnitude of the electric dipole moment in the ground state of DAST microcrystals whose size was about 0.35 μm was determined to be as large as ∼4.5 × 104 D, and the magnitude of the difference in the dipole moment between the ground state and excited state was estimated to be ∼2.5 × 103 D, though the local field correction was not made. The transition moment of the DAST crystal was also found to be a little affected by application of electric fields.

Electroabsorption spectra of PbSe nanocrystal quantum dots

The quantum-confined Stark effect of PbSe nanocrystal quantum dots (QDs) in a polymer film has been studied for the near-infrared absorption in the region of 1300–1600 nm (corresponding to diameters of 4.0–7.0 nm) by using electric field modulation spectroscopy. Electroabsorption spectra of QDs show the electric-field-induced energy shift in the exciton state, resulting in the spectral broadening given by the second derivative of the absorption spectra. It is shown that the magnitude of the change in electric dipole moment following excitation into the first exciton state increases with an increase in the size of QDs.

External Electric Field Effects on Optical Property and Excitation Dynamics of Capped CdS Quantum Dots Embedded in a Polymer Film

Nanocomposites of benzyl mercaptan (BM)-capped cadmium sulfide (CdS) quantum dots (QDs) have been prepared and investigated by using electric field modulation spectroscopy. Applied electric fields on absorption and photoluminescence (PL) spectra as well as PL decays of BM-capped CdS QDs embedded in a poly(methyl methacrylate) (PMMA) film have revealed a field-induced alteration. Electro-photoluminescence (E-PL) exhibits field-induced quenching of photoluminescence in the presence of electric fields. The observed E-PL spectra and field-induced change in decay profile suggest that the PL intensity alternation originates from the electric field effect on emitting state population rather than PL quantum yield, probably as a result of the presence of field-assisted dissociation into hole and electron at the photoexcited state having a charge-separated character. Field-induced change in absorption spectra showed the spectral broadening caused by the stark shifts, indicating a large electric dipole moment in the...

External Electric Field Effects on Emission of a Dendrimer-encapsulated Gold Nanocluster in a Polymer Film

Abstract External electric field effects on emission spectrum of a dendrimer-encapsulated gold cluster in a PVA film have been examined with electric field modulation spectroscopy. Besides the Stark shift, a field-induced quenching of the emission is observed, indicating the field-induced enhancement of the rates of nonradiative processes from the emitting state.

Electric field modulation nanospectroscopy for characterization of individual β-FeSi2 nanodots

Electronic structures of individual β-FeSi2 nanodots were investigated using electric field modulation spectroscopy combined with scanning tunneling microscopy (STM-EFMS) by optimizing experimental conditions to enhance the inherently small signal due to a small number of photocarriers generated in a single nanodot. The obtained STM-EFMS spectra in different schemes for electric field modulation exhibited common features around 0.73 eV, which agreed well with the indirect absorption edge of β-FeSi2 bulk crystal. Maps of the STM-EFMS signal prove that the 0.73 eV signal arises from individual nanodots with a spatial resolution of a few nanometers. Those results strongly suggest that the present β-FeSi2 nanodots are indirect semiconductors.

Self-Assembled Epitaxial Growth of High Density β-FeSi2 Nanodots on Si (001) and Their Spatially Resolved Optical Absorption Properties

A self-assembly technique for high density (∼1 × 1011 cm−2) β-FeSi2 nanodots epitaxially grown on Si (001) substrates was developed using a codeposition method of Fe and Si on ultrathin SiO2 films with Si nuclei. Photoabsorption spectra of individual nanodots and their photoabsorption maps at the direct-transition photoabsorption edge were obtained using electric field modulation spectroscopy combined with scanning tunneling microscopy with a nanometer spatial resolution.

Electric Field effects on Excitation Dynamics in Electron Donor and Acceptor Pairs of Porphyrin and Fullerene

Electroabsorption and electrofluorescence spectra have been measured using electric field modulation spectroscopy for a mixture of tetraphenylporphyrin (TPP) and fullerene (C60) in a PMMA film. Fluorescence of TPP is enhanced and de-enhanced by an electric field in zinc and free-base compounds, respectively, indicating that the electron transfer from the excited state of TPP to C60 is decelerated in zinc compound and accelerated in free-base compound by an electric field. TPP was also found to form complex upon mixing with C60 which exhibits a broad emission in the wavelength region longer than that of TPP emission. The electroabsorption spectra show that a charge separation occurs upon photoexcitation to the absorption band located near the Soret band. The complex emission is significantly quenched by an external electric field in both zinc and free base compounds, suggesting that the nonradiative decay of the excited complex is accelerate d by an electric field.

The origin of spectral distortion in electric field modulation spectroscopy based on scanning tunneling microscopy

Electric field modulation spectroscopy using scanning tunneling microscopy (STM-EFMS) measurements were performed for a Si(1 1 1) surface with epitaxially-grown β-FeSi 2 islands. STM-EFMS spectra acquired around the indirect energy gap of Si reproduced the photon energy peak position observed in conventional macroscopic EFMS experiments. However, a considerable discrepancy was found in the energy position of the accompanying spectral dip. We examined two possibilities for the cause of this discrepancy. The first interpretation is that the STM-‘EFMS’ spectra may simply reflect the local density of states based on essentially the same principle as that of tunneling spectroscopy. However, this interpretation is ruled out by the facts that almost identical STM-EFMS spectra are obtained also out of the regime of tunneling. The second interpretation is a spectral distortion due to a large electric field steadily built in the sample surface, which is supported experimentally by a spectral shift of the dip energy that is induced by altering the tip-induced band bending.

Electric-Field-Induced Changes in Absorption and Emission Spectra of CdS Nanoparticles Doped in a Polymer Film

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.

External Electric Field Effects on Absorption and Fluorescence Spectra of a Fullerene Derivative and Its Mixture with Zinc-Tetraphenylporphyrin Doped in a PMMA Film

Electroabsorption and electrofluorescence spectra of a fullerene derivative, C60(C18)2, and its mixture with zinc-tetraphenylporphyrin (ZnTPP) have been measured by using electric field modulation spectroscopy. The change in dipole moment is significant in the electroabsorption spectra both of C60(C18)2 and of a complex composed of C60(C18)2 and ZnTPP, indicating that the excited states both of C60(C18)2 and of a complex between C60(C18)2 and ZnTPP have a large charge-transfer character. The fluorescence quantum yield of C60(C18)2 decreases in the presence of an electric field, which probably arises from the field-induced acceleration of the intramolecular nonradiative process of C60(C18)2 in the fluorescent state. In a mixture between ZnTPP and C60(C18)2, electrofluorescence spectra show the field-induced enhancement for the fluorescence of ZnTPP and the field-induced de-enhancement for the fluorescence both of C60(C18)2 and of the complex between ZnTPP and C60(C18)2. A theoretical analysis clearly shows that the field-induced enhancement of the ZnTPP fluorescence in a mixture results from the field-induced deceleration of the rate of the electron transfer from the excited ZnTPP to C60(C18)2. The standard free energy gap for the photoinduced electron-transfer process is estimated based on the theoretical simulation of the field-dependent fluorescence intensity.

Electric Field Effects on IR Absorption of α-Helical Polypeptide

Abstract External electric field effects on the IR absorption spectrum of α-helical poly(γ-benzyl-l-glutamate) have been examined with electric field modulation spectroscopy. The electric-field-induced change in each vibrational band is reproduced by a superposition of the absorption spectrum, and its first and second derivative spectra, suggesting that the electric dipole moment and the molecular polarizability change following vibrational excitation. The N–H stretching band at 3291 cm−1 shows the largest change in the magnitude of dipole moment following vibrational excitation.

EXCITATION DYNAMICS AND ITS ELECTRIC FIELD DEPENDENCE OF MESO,MESO-LINKED PORPHYRIN ARRAYS IN A POLYMER FILM

External electric field effects both on absorption spectra and on fluorescence spectra of meso,meso-linked porphyrin arrays doped in a polymer film have been examined using electric field modulation spectroscopy with the number of linked porphyrins (N) from 1 to 48. The magnitudes of the change in electric dipole moment (Δμ) and in molecular polarizability (Δα) are evaluated for excitation into each of the exciton splitting components of the Soret band, and excitation dynamics has been discussed. Field-induced quenching of fluorescence occurs, as the number of N increases, suggesting that nonradiative process(es) at the emitting state is significantly enhanced by an electric field, as N increases.

The strain field around a single point defect in semiconductors spatially resolved by electric field modulation scanning tunneling spectro-microscopy

We have developed a novel instrumentation that allows us to image with a nano-scale resolution the strain field localized around single point defects in semiconducting crystals. The instrument that coupled the electric field modulation spectroscopy with the scanning tunneling microscopy was applied to point defects in low-temperature-grown GaAs (LT-GaAs) epi-films. The strain field image constructed by the present method exhibited features consistent with other experimental results.

Electric field modulation spectroscopy by scanning tunneling microscopy with a nanometer-scale resolution

Instrumentation has been devised by coupling electric field modulation spectroscopy (EFMS) with scanning tunneling microscopy (STM) that enables the investigation of electronic band structures in semiconductors with a nanometer-scale spatial resolution. Model experiments using low-temperature-grown GaAs (LT–GaAs) epifilms show that a difference as small as 0.01 eV in electronic energy gaps between the GaAs substrate and the LT–GaAs epilayers can be distinguished in the EFMS spectra, demonstrating the high energy, as well as spatial, resolution of the STM–EFMS.

Energy-level localization in Bragg-confined asymmetric coupled quantum wells studied by electric field modulation spectroscopy

Electronic Bragg mirrors are used to confine carriers at energy levels above the barrier height in asymmetric coupled quantum wells. An electric field inside the quantum structure is created by transferring carriers from a wide quantum well into a narrow one. Two classes of above-the-barrier states are resolved by using modulated resonant Raman spectroscopy. The first level is the resonant one, which is highly localized by the Bragg reflector above the asymmetric quantum well and is redshifted when a photogenerated local electric field is created in the asymmetric quantum well region. The second class of levels, which extend mainly above the reflectors region, is seen by photoluminescence and photoluminescence excitation measurements. It is less shifted than the resonant level, when the photogenerated local field is applied, which is due to the smaller localization of these states in the asymmetric coupled quantum wells. We used modulated photoluminescence and Raman spectroscopy to resolve the Stark shifts of the bound and continuum levels as a function of the infrared photoexcitation intensity. Our results indicate that because of the photoinduced electric field, the shifts of the above the barrier levels are linked to their degree of localization. These shifts are much stronger than those of the bound states inside the well and a model is proposed to explain the shifts of the continuum and bound levels using perturbation theory.

電場変調分光法によるメロシアニンＬＢ膜の評価

電場変調分光法によるメロシアニンＬＢ膜の評価