Derivative Line Shape Research Articles

Trion Formation Resolves Observed Peak Shifts in the Optical Spectra of Transition-Metal Dichalcogenides.

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data. Our approach provides an excellent, global description of the potential-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

Electroreflectance study on optical anisotropy in β-Ga2O3

Electroreflectance (ER) spectra of β-Ga2O3 were measured using Schottky barrier diodes. The ER spectra were well fitted to a third derivative line shape function by assuming three excitonic transitions. The transition energies exhibited moderate blueshifts with an external reverse bias by reflecting the contribution of transitions at M0 type critical points. Optical anisotropy was selectively observed in the polarized ER spectra. The fittings for the third derivative line shape function gave relatively large broadening parameters (0.11–0.89 eV) due mainly to the large exciton-longitudinal-optical-phonon interaction in β-Ga2O3. The ER spectra exhibited a third derivative line shape with a Franz-Keldysh oscillation superimposed on it. The results demonstrate that ER measurements are useful for investigating optical anisotropy in β-Ga2O3 and related alloys.

Validation of reconstructed component spectra from non-parametric derivative envelopes: comparison with component lineshapes from parametric derivative estimations with the solved quantification problem

Of late, we have put forward a new branch called high-order derivative signal processing. This investigative strategy is universally relevant for all spectroscopies, the progress of which ultimately depends on resolution improvement and noise suppression. The high-order, non-parametric derivative fast Padé transform (dFPT) simultaneously solves these two problems of utmost importance. The present work goes one critical step further than our previous two studies on this particular topic, by setting up the goal of validating the non-parametric dFPT by its parametric counterpart. This is done by comparing the full lineshapes of derivative envelopes from the non-parametric dFPT with the corresponding derivative component spectra from the parametric dFPT. The non-parametric dFPT, as a shape estimator, never solves the quantification problem (or equivalently, the spectral analysis problem via e.g. an eigenvalue problem, rooting characteristic/secular equations, etc.). The parametric dFPT first solves the quantification problem from which the lineshapes of components and envelopes are plotted. Thus, if the derivative component spectra from the parametric dFPT could be fully reconstructed by the derivative envelopes from the non-parametric dFPT, the goal of achieving quantification would be done by derivative lineshape processing alone. This would amount to providing stand-alone quantification without actually solving the quantification problem (and, of course, without fitting, either). The present study accomplishes this goal, with an important application to data encountered in magnetic resonance spectroscopy for clinical diagnostics of breast cancer.

Explicit extraction of absorption peak positions, widths and heights using higher order derivatives of total shape spectra by nonparametric processing of time signals as complex damped multi-exponentials

Sole lineshape estimation of non-parametrically computed higher-order derivatives of spectral envelopes in different modes (complex, real, imaginary, magnitude) is investigated. The processed time signals are sums of complex attenuated exponentials (harmonics). The peak parameters of the derivative spectra are directly connected to those of the customary (non-derivative) absorption lineshapes. Crucially, this permits the reconstruction of the latter from the former parameters (the latter are sought since they are unknown, whereas the former are extractable from the derivative envelopes). The cross-checking relationships of the lineshapes for the magnitude modes with the real and imaginary parts of the complex-valued envelopes (total shape spectra) are established. The explicit procedure and the necessary analytical expressions are presented for reconstruction of the exact locations, widths and heights of all the retrieved physical resonances (spectral peaks). These facets are illuminated in the derivative fast Padé transform (dFPT) using its non-parametric version, i.e. without solving the quantification problem (no polynomial rooting, no tackling of eigen-value problems, etc.). Two kinds of illustrations for derivative spectra are reported. One deals with the general Breit-Wigner resonance formula and its first three derivatives. The other is concerned with the dFPT in clinical diagnostics of relevance to breast cancer detection by magnetic resonance spectroscopy. A systematic parallel between these two examples is drawn to highlight, in a stepwise manner, the role of paramount importance played by derivative lineshapes, especially for disentangling overlapping resonances that invariably plague all quantitative analyses of spectra.

Probing the Interaction of Quantum Dots with Chiral Capping Molecules Using Circular Dichroism Spectroscopy.

Circular dichroism (CD) induced at exciton transitions by chiral ligands attached to single component and core/shell colloidal quantum dots (QDs) was used to study the interactions between QDs and their capping ligands. Analysis of the CD line shapes of CdSe and CdS QDs capped with l-cysteine reveals that all of the features in the complex spectra can be assigned to the different excitonic transitions. It is shown that each transition is accompanied by a derivative line shape in the CD response, indicating that the chiral ligand can split the exciton level into two new sublevels, with opposite angular momentum, even in the absence of an external magnetic field. The role of electrons and holes in this effect could be separated by experiments on various types of core/shell QDs, and it was concluded that the induced CD is likely related to interactions of the highest occupied molecular orbitals of the ligands with the holes. Hence, CD was useful for the analysis of hole level–ligand interactions in quantum semiconductor heterostructures, with promising outlook toward better general understanding the properties of the surface of such systems.

Electrical detection of ferromagnetic resonance in single layers of permalloy: Evidence of magnonic charge pumping

The generation of a DC voltage in single layers of permalloy (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) when the magnetization is undergoing ferromagnetic resonance is investigated in a series of samples with thickness varying from 4.0 to 150 nm. By sweeping the external field at a fixed microwave frequency, we measure a DC voltage at the ends of the layer as a function of the in-plane angle for each sample. The asymmetric voltage signal generated at the resonance field is a superposition of symmetric Lorentzian and antisymmetric Lorentzian derivative line shapes. The in-plane dependence of both symmetric and antisymmetric signals cannot be explained as due to spin rectification (SRE) only. The results are well explained by a model that takes into account in addition to the SRE the contribution of the recent discovered effect of magnonic charge pumping that converts magnetization dynamics into charge current by means of the spin orbit coupling.

Excited-State Dynamics of Diindenoperylene in Liquid Solution and in Solid Films

The excited-state dynamics of diindenoperylene (DIP) are investigated in dilute solution and in a solid film at room temperature using picosecond photoluminescence and femtosecond transient absorption measurements. In solution, DIP undergoes a rapid (0.89 ns) internal conversion back to its ground state, with no detectable formation of triplet or other long-lived states. In the solid state, multiple emissive species are formed. The time-resolved photoluminescence signal is dominated by an intrinsic exciton that decays on a time scale of 166 ps. Emission from lower energy excimer-like species then persists for >10 ns. Transient absorption experiments indicate that the majority of the excited-state population relaxes to the ground state on the 166 ps time scale, but a smaller fraction (<10%) survives in longer-lived trap or defect states. The rapid internal conversion leads to transient heating that results in a derivative line shape in the transient absorption signal at longer delays. DIP does not appear t...

Optical properties of pseudomorphic Ge1−xSnx (x = 0 to 0.11) alloys on Ge(001)

The characterization of the optical properties of pseudomorphic Ge1−xSnx/Ge/Si (x = 0 to 0.11) alloys from the IR to UV is presented. The Ge1−xSnx alloys were epitaxially grown on relaxed Ge grown on Si. Rutherford backscattering (RBS) and RBS ion channeling methods were used to confirm the Sn composition and substitutional nature of the Sn into the Ge lattice. The pseudomorphic nature of the Ge1−xSnx on Ge is confirmed using high resolution x-ray diffraction (HRXRD) and transmission electron microscopy. Although HRXRD reciprocal space maps indicated that the Ge1−xSnx was pseudomorphic to Ge, the shape of the Bragg peaks indicated that the sample surface was rough. The rough surface morphology is confirmed using atomic force microscopy. The complex dielectric function is reported in the IR, visible, and UV spectrum in the wavelength range of 0.2–5.06 eV. The E1, E1 + Δ1, E2, and E0 critical points are extracted using second and third derivative line shape fitting and are compared with the elastic theory calculations of strained Ge1−xSnx (x = 0 to 0.11) alloys and fully relaxed Ge1−xSnx (x = 0 to 0.11) alloys. The E0 critical point energies are observed to have slightly larger values than those calculated for completely relaxed Ge1−xSnx alloys due to the presence of compressive strain.

Characterization of damage structure in ion implanted SiC using high photon energy synchrotron ellipsometry

The optical properties of ion implantation induced disorder in SiC have been investigated in the photon energy range of 5–9eV using spectroscopic ellipsometry (SE). The most characteristic interband transitions of SiC are located between 5 and 8eV. This photon energy region is extremely important for the sensitive characterization of lattice order in SiC. The dielectric function of the disordered layer has been calculated taking into account the surface overlayer consisting of oxide and roughness using complementary characterization tools. The dielectric function of the damaged region has been analyzed using different techniques like second derivative analysis and effective medium approximation (EMA) based on reference dielectric functions. The disorder determined by SE has been verified by Rutherford backscattering spectrometry combined with channelling (RBS/C). Using derivative lineshape analysis combined with simulations, the track size can be estimated. The results can give insight into the effect of the decreasing characteristic size of the unchanged crystalline regions on the optical properties. We created near-surface damage using heavy ions, since the penetration depth of light at photon energies around the direct interband transitions is very small (in the range of 10nm). We used 100-keV Xe at fluences ranging from a slight damage to full amorphization (between 2.0×1013cm−2 and 1.6×1014cm−2).

Dynamic nuclear polarization of 29Si via spin S=1 centers in isotopically controlled silicon

We report the results of the experiments on dynamic nuclear polarization (DNP) of 29 Si nuclei under saturation the electron paramagnetic resonance (EPR) transitions of the photoexcited spin S =1 states of the oxygen+vacancy complexes in irradiated 29 Si isotopically controlled silicon. The effect of isotope 29 Si abundance on the line width and hyperfine structure of the Si-SL1 EPR spectra was observed. It was shown clearly that the decrease of the 29 Si abundance leads to the transformation the DNP mechanism from the “differential” to “resolved” solid-effect accompanied with the increase of DNP degree. High steady-state 29 Si nuclear polarization of 6% due to “resolved” solid-effect was achieved in silicon crystals with the 29 Si isotope abundance below 4.7%. It was found that the DNP induced by saturation the hyperfine structure EPR lines of triplet centers does not follow the symmetric first derivative EPR line shape, showing the additional contribution to the DNP process.

Electroabsorption in triphenylamine-based hole-transporting materials for organic light-emitting diodes

Electric-field modulated absorption (EA) spectra have been studied in solid films of triphenylamine (TPA)-based compounds, commonly used as hole-transporting materials in organic light-emitting diodes. The electroabsorption spectra of triphenyldiamine derivatives (TPD and TAPC) and a starburst amine dendrimer m-MTDATA are compared with those of TPA which is the building block of the molecules. The EA results indicate that properties of excited states of m-MTDATA and TAPC can be qualitatively rationalized in the terms of exciton interaction between TPA constituents. The lowest energy electronic excitations of m-MTDATA dendrimer are strongly delocalized within the area of the whole molecule. In contrast to m-MTDATA and TAPC, the TPD behavior in the electric field shows individual features that can not be derived from the optical properties of TPA monomers alone. The influence of excited state degeneracy on EA spectra is discussed. The consistent qualitative interpretation of EA spectra for compounds under investigation has been reached assuming that the second derivative lineshapes of EA signal originate from degenerate (in TPA and m-MTDATA) and possible quasi-degenerate states (in TAPC and TPD).

Stationary and modulated absorption spectroscopy of copper phthalocyanine (CuPc) layers grown on transparent substrate

Copper phthalocyanine thin films grown on glass and quartz substrates by the thermal evaporation method were investigated using stationary and modulated photoabsorption as well as photoreflectance spectroscopy. The stationary and modulated studies at room temperature were performed in the wavelength range 200–1200 nm and 400–1200 nm, respectively. The absorption coefficient of investigated layers was determined directly from the transmission spectrum and fitted in terms of the mixed Lorentz–Lorenz model, which assumed five oscillator components. Sensitive modulated spectroscopy, which enabled us to emphasize weakly visible transitions in the Q-band spectral region, was also performed to estimate the electronic structure of metallophthalocyanine layers. The complex nature of excited states of the metallophthalocyanine layers gave us abundant modulated spectra. These spectra were fitted using the third derivative line shape given by the Aspnes formula. Modulated photoabsorption and photoreflectance spectra for CuPc thin layers are presented for the first time in this paper.

Characterization of nanocrystalline porous CdSe thin films by electrolyte electroabsorption spectroscopy

Nanocrystalline porous CdSe thin films with particle diameter of 6.0 and 8.0 nm were prepared by chemical deposition on SnO 2 conducting glass substrates. The broadening of the excitonic transitions in the absorption spectrum due to the size distribution and an increase of bandgap energy compared with the bulk CdSe are observed. Electrolyte electroabsorption (EEA) spectrum measured in polysulfide solution exhibits the characteristic second derivative lineshape of the quantum-confined CdSe nanoparticles. A quadratic modulation voltage dependence of EEA signals indicates that the electrical field distribution of nanocrystalline porous CdSe thin films is different significantly from that of bulk CdSe thin films.

EPR study of substrate binding to the Mn(II) active site of the bacterial antibiotic resistance enzyme FosA: a better way to examine Mn(II).

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.

Electric field effects on the Davydov components of a strong intramolecular transition:α-sexithiophene single crystals

Low-temperature absorption and electroabsorption spectra of a thin single crystal of $\ensuremath{\alpha}$-sexithiophene reveal a distinctly different line shape and sensitivity to electric fields of the Daydov components of the first intramolecular $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ transition. The weak component of very small linewidth responds by a quadratic Stark shift which is attributed to a molecular polarizability of $900{\AA{}}^{3}.$ The strong Davydov component of much larger linewidth responds by an additional effect which dominates its spectrum and results in an electroabsorption spectrum of second derivative line shape. This signal is attributed to coupling of the field to the large transition dipole of the four molecules of the unit cell. The spectra show no response of a charge-transfer state and no Franz-Keldysh effect of a band continuum. The data indicate that Davydov components of the same molecular state are not mixed by the field.

Electron-spin-resonance STM on iron atoms in silicon

Electron-spin-resonance--scanning tunneling microscopy (ESR)-(STM) of iron atoms in silicon was observed: Si(111) surfaces were covered with iron atoms and annealed. This gives surfaces covered with small islands of different silicide phases. We could detect an ESR-STM signal that corresponds to a g value of 2.07. The signal was split by magnetic field modulation, and phase-sensitive detection was applied. This shows that electron-spin-resonance (ESR)-STM signals can be detected on a surface that is imaged with atomic resolution and that it is possible to observe a $g\ensuremath{\ne}2$ spin center. Getting absorption (instead of the expected derivative) line shapes with phase-sensitive detection is explained by the asymmetry in the line shape, which is a result of a rapid scan of the signal. Then, when the integration time of the phase sensitive detector is close to the time it takes to sweep the linewidth of the signal, absorption line shapes are observed.

Lineshape of Field Modulated Spectra of Ordered and Disordered Polymers

Abstract Electric field modulated spectra of polymer films are compared with those of single crystals to study the influence of disorder on size and shape of the effects. The quadratic Stark effect is similar as in crystals but reveals even in isotropic films pronounced anisotropy. Disordered films show an additional response quadratic with the field but of second derivative line shape which is interpreted as linear Stark effect due to dipole moments induced by asymmetric boundaries of chain segments.

Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state

The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 Å in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d≳70 Å), the dipole moment decreases. The dipolar character is lost in clusters less than 25 Å. These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.

Lineshape analysis for optical transitions in III-V semiconductors using a sequential simplex procedure

A computer lineshape analysis for optical transitions in III-V semiconductors is presented. The routine utilizes the Nelder–Mead version of the sequential simplex optimization procedure embodied in a pascal program that can be executed on a personal computer. The sequential simplex routine allowed the optimum fitting of experimental data with a function or group of functions with n-independent parameters. The routine was successfully applied to two types of optical spectra. First, convolved Gaussian and asymmetric Gaussian bands in low temperature (4.2 K) Fourier-transformed photoluminescence spectra of lattice-matched InxGa1−xAs on InP substrates were fit. As a result of applying the sequential simplex routine to the InxGa1−xAs/InP system, three optical transitions (excitonic band, donor-to-acceptor pair band, and defect/impurity or exciton LO replica band) were identified in all spectra and a fourth transition was found in some samples. A second application of the routine was the fitting of the Aspnes third derivative lineshapes (TDLS) to room temperature, low electric field photoreflectance spectra of high purity GaAs. In applying the routine to photoreflectance spectra, it was found that the two-dimensional critical point, n=3 form of the TDLS best fit the photoreflectance spectra of high-purity GaAs at room temperature. In both applications of the sequential simplex routine, the lineshape analysis contributed valuable insight into the nature of the optical phenomena occurring in the analyzed material.

Ferroelectric Transition in Vinylidene Fluoride and Trifluoroethylene Copolymers Studied by NMR Method II. 52 and 65 mol% Vinylidene Fluoride Copolymers

NMR derivative line shapes f′(ΔH) in vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymers with VDF contents of 52 and 65 mol% were measured as a function of the alignment angle γ between the drawn axis and magnetic field over a temperature range between 20 and 130°C. The fine structure of f′(ΔH) and the motional narrowings of the line widths in the each copolymer were investigated by comparing them with those for VDF 72 mol% copolymer. In the ferroelectric phase, f′(ΔH)’s in VDF 65 and 72 mol% copolymers consisted of three components of narrow, intermediate and broad line widths (ΔHs, ΔHm, and ΔHb), ΔHm and ΔHb which were due to the TrFE and VDF groups, respectively; f′(ΔH) in the VDF 52 mol% copolymer had two components of ΔHs, and ΔHb. The value of ΔHb was almost equal to ΔHm in 65 and 72 mol% copolymers. When VDF content decreased, the flip-flop motion of TrFE groups in the VDF 72 mol% copolymer changed into rotational motion around the chain axis. Near the Curie temperature, Tc two kinds of trans VDF group motion around the chain axis and TrFE group motion coordinate with the VDF rotational motion disappeared in the VDF 52 mol% copolymer. At the Tc, ΔHb discontinuity for each copolymer was observed, but the thermal hysteresis of ΔHb vs. temperature decreased as VDF content decreased, and was not significant in the VDF 52 mol% copolymer. These findings suggest that the ferroelectric phase transition is of the first order above VDF 65 mol%, and of the first order close to the second order for VDF 52 mol%.