Drude-Smith Model Research Articles

Ultrafast photoinduced insulator-metal transition in epitaxial samarium nickelate thin films investigated by time-resolved terahertz spectroscopy

We investigate the ultrafast dynamic behavior of photoinduced insulator-metal phase transition in an epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film by using time-resolved terahertz (THz) spectroscopy at different excitation fluences. It is found that the relaxation of the photoexcited carriers is characterized by two components with different time scales: (i) an ultrafast one with time constant varying between 0.3 ps (at 4.4 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and 0.35 ps (at 0.88 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and (ii) a slow one with a time constant varying between 1.7 ps (at 2 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) and 4.7 ps (at 0.88 $\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$). These relaxation times could be primarily attributed to the coexistence of conducting and insulating charge-ordering domains. The observed behavior of the transient THz conductivity of $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film is well fitted by the Drude-Smith model, revealing that insulating domains emerge between the metallic domains, which in turn increase the carrier confinement during the relaxation process. Furthermore, the ultrafast dynamic behavior at different temperatures suggests that the insulating gap shrinks gradually between the Ni $3d$ and O $2p$ states as the temperature applied to the film increases. Our experimental findings pave the way to scientific opportunities and to technological applications of epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_{3}$ thin film such as ultrafast optical switching and modulation devices.

Terahertz Spectroscopy of Amorphous WSe₂ and MoSe₂ Thin Films.

Time domain spectroscopy is used to determine the THz electromagnetic response of amorphous transition metal dichalcogenides WSe2 and MoSe2 in thin-film form. The dielectric function is obtained using a rigorous transmission model to account for the large etalon effect. The Drude–Smith model is applied to retrieve the dielectric function, and from there, the sample conductivity.

Numerical and Experimental Time-Domain Characterization of Terahertz Conducting Polymers

A comprehensive framework for the theoretical and experimental investigation of thin conducting films for terahertz applications is presented. The electromagnetic properties of conducting polymers spin-coated on low-loss dielectric substrates are characterized by means of terahertz time-domain spectroscopy and interpreted through the Drude-Smith model. The analysis is complemented by an advanced finite-difference time-domain algorithm, which rigorously deals with both the dispersive nature of the involved materials and the extremely subwavelength thickness of the conducting films. Significant agreement is observed among experimental measurements, numerical simulations, and theoretical results. The proposed approach provides a complete toolbox for the engineering of terahertz optoelectronic devices.

Investigation of the optical and conductive properties of antimony-doped titanium dioxide using terahertz time-domain spectroscopy

The refractive index, absorption coefficient, dielectric constant and conductivity of titanium dioxide at different doping concentrations of antimony (Sb) have been investigated using the non-contact and damage-free technique of terahertz time-domain spectroscopy. Simple effective medium theory has been applied to composite samples to extract the dielectric and conductive response in the range of 0.2–1.2 THz. The frequency-dependent values of these parameters are observed to increase with doping concentration and also with calcination temperature. Drude–Smith model fitting with the measured conductivity curves reveals that back-scattering is the major conductivity mechanism.

Classical modelling of grain size and boundary effects in polycrystalline perovskite solar cells

A theoretical approach based on the Matthiessen rule for scattering lifetime is presented to formulate the effective characteristics of polycrystalline solar cells. Application of this method to Perovskite solar cells obtains polycrystalline bulk defect density for diverse grain sizes along with the determination of surface recombination velocity at grain boundaries. Unlike previous works on Silicon based solar cells that use an independently weighted average equation for extracting mobility, we introduce a variation of Drude-Smith model for calculating the mobility from carrier lifetime and defect density. In agreement with the experiments, introduced method reveals sub-picosecond background scattering times associated with phonon-lattice vibrations. Obtained carrier diffusion length spans over multi-micron to multi-millimeter scale for grain sizes ranging from 100 nm to 1 mm. Calculated monomolecular recombination lifetimes explains elevated Photoluminescence Yield and peak position in larger grain sizes. Presented method is verified by feeding extracted parameters into Drift-Diffusion equations and fitting with reported experimental photovoltaic conversion efficiency data. Finally, through employing a Gaussian distribution for grain sizes, we also study the reduction of device efficiency caused by non-uniform grain size distribution as a more realistic case.

Optically induced abnormal terahertz absorption in black silicon**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574408, 11504439, 61627814, and 61675238), the National Key Research and Development Program of China (Grant No. 2017YFB0405402), the National Instrumentation Program of China (Grant No. 2012YQ14000508), and the Young-talent Plan of State Affairs Commission, China (Grant No. 2016-3-02).

The absorption responses of blank silicon and black silicon (silicon with micro/nano-conical surface structures) wafers to an 808-nm continuous-wave (CW) laser are investigated at room temperature by terahertz time-domain spectroscopy. The transmission of the blank silicon shows an appreciable change, from ground state to the pump state, with amplitude varying up to 50%, while that of the black silicon (BS) with different cone sizes is observed to be more stable. Furthermore, the terahertz transmission through BS is observed to be strongly dependent on the size of the conical structure geometry. The conductivities of blank silicon and BS are extracted from the experimental data with and without pumping. The non-photo-excited conductivities increase with increasing frequency and agree well with the Lorentz model, whereas the photo-excited conductivities decrease with increasing frequency and fit well with the Drude–Smith model. Indeed, for BS, the conductivity, electron density and mobility are found to correlate closely with the size of the conical structure. This is attributed to the influence of space confinement on the carrier excitation, that is, the carriers excited at the BS conical structure surface have a stronger localization effect with a backscattering behavior in small-sized microstructures and a higher recombination rate due to increased electron interaction and collision with electrons, interfaces and grain boundaries.

Effects of surface nanostructuring and impurity doping on ultrafast carrier dynamics of silicon photovoltaic cells: a pump-probe study

We present femtosecond optical pump-terahertz probe studies on the ultrafast dynamical processes of photo-generated charge carriers in silicon photovoltaic cells with various nanostructured surfaces and doping densities. The pump-probe measurements provide direct insight on the lifetime of photo-generated carriers, frequency-dependent complex dielectric response along with photoconductivity of silicon photovoltaic cells excited by optical pump pulses. A lifetime of photo-generated carriers of tens of nanosecond is identified from the time-dependent pump-induced attenuation of the terahertz transmission. In addition, we find a large value of the imaginary part of the dielectric function and of the real part of the photoconductivity in silicon photovoltaic cells with micron length nanowires. We attribute these findings to the result of defect-enhanced electron–photon interactions. Moreover, doping densities of phosphorous impurities in silicon photovoltaic cells are also quantified using the Drude–Smith model with our measured frequency-dependent complex photoconductivities.

Dielectric Properties of DMSO‐Doped‐PEDOT:PSS at THz Frequencies

Poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) is a promising candidate for terahertz (THz) metamaterial devices. Its dielectric properties can be changed by the dopant dimethylsulfoxide (DMSO). In this study, the dielectric properties of DMSO‐doped‐PEDOT:PSS films are investigated using terahertz time‐domain spectroscopy (THz‐TDS) measurements. The values of the carrier density and mobility are also determined by fitting the dielectric permittivity to the Drude–Smith model. A simple THz frequency selective surface (FSS) is designed based on these DMSO‐doped‐PEDOT:SS films. Strong band rejection can be achieved at resonant frequencies. These results may inspire the development of new materials for manipulating THz waves in future studies.

Microscopic origin of the Drude-Smith model

The Drude-Smith model has been used extensively in fitting the THz conductivities of nanomaterials with carrier confinement on the mesoscopic scale. Here, we show that the conventional 'backscattering' explanation for the suppression of low-frequency conductivities in the Drude-Smith model is not consistent with a confined Drude gas of classical non-interacting electrons and we derive a modified Drude-Smith conductivity formula based on a diffusive restoring current. We perform Monte Carlo simulations of a model system and show that the modified Drude-Smith model reproduces the extracted conductivities without free parameters. This alternate route to the Drude-Smith model provides the popular formula with a more solid physical foundation and well-defined fit parameters.

Probing Charge Carrier Dynamics in Porphyrin-Based Organic Semiconductor Thin Films by Time-Resolved THz Spectroscopy.

To improve the power conversion efficiency of solar cells, it is important to understand the underlying relaxation mechanisms of photogenerated charge carriers in organic semiconductors. In this work, we studied the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin thin films where the diketopyrrolopyrrole unit has two n-butyl groups, abbreviated as C4-DPP-BP. We used time-resolved terahertz (THz) spectroscopy to track charge carrier dynamics with excitations at 800 and 400 nm. Compared with tetrabenzoporphyrin (BP), the extension of π-electron delocalization to the diketopyrrolopyrrole peripherals leads to an increase in absorption in the near-infrared region. Following the excitation at 800 nm, we found that the transient THz signals in C4-DPP-BP thin films decay with time constants of 0.5 and 9.1 ps, with small residual components. With excitation at 400 nm, we found that the transient THz signals decay with time constants of 0.4 and 7.5 ps. On the basis of the similarity of the decay profiles of the transient THz signals obtained with excitations at 400 and 800 nm, we considered that the decaying components are due to charge carrier recombination and/or trapping at defect sites, which do not depend on the excess energy of the photoexcitation. In contrast to BP, even without an electron acceptor, we observed the finite offset of the transient THz signals at 100 ps, demonstrating the existence of long-lived charge carriers. We also measured the photoconductivity spectra of C4-DPP-BP thin films with the excitation at both 800 and 400 nm. It was found that the spectra can be fitted by the Drude-Smith model. From these results, it was determined that the charge carriers are localized right after photoexcitation. At 0.4 ps, the product of the quantum yield of charge generation and mobility of charge carriers at 400 nm is approximately twice that obtained at 800 nm. We discuss the implications of the excess excitation energy in organic semiconductor-based devices.

Terahertz time-domain spectroscopy characterization of carbon nanostructures embedded in polymer

This work presents results of terahertz time-domain spectroscopy (THz-TDS) investigations of two types of polymer-based nanocomposites, consisting of bulk samples of graphene nanoflakes embedded in a polymer matrix and thin films where single-walled carbon nanotubes (SWCNTs) with a chiral index (7,5) were wrapped in single strains of a polymer. Our THz-TDS setup is a room-temperature system with a frequency range of 0.1 to 3.5 THz, based on photoconductive switches as both the emitter and detector, excited by 100-fs-wide optical pulses. We have studied THz spectra of both types of samples and compared them to the ones obtained for the pure polymer reference specimens and fitted the experimental complex conductivity data to the Drude–Smith model. The Drude–Smith model fits our data well, demonstrating that graphene nanoflakes and SWCNTs, exhibit highly localized intra grain/tube electron backscattering with a femtosecond relaxation time.

Impact of tungsten doping on the dynamics of the photo-induced insulator-metal phase transition in VO2 thin film investigated by optical pump-terahertz probe spectroscopy

The influence of tungsten (W) doping on the ultrafast dynamics of the photo-induced insulator-metal phase transition (IMT) is investigated at room temperature in epitaxially grown vanadium dioxide (VO2) thin films by means of optical pump-terahertz (THz) probe spectroscopy. It is observed that the THz transmission variation of the films across the IMT follows a bi-exponential decrease characterized by two time constants, one corresponding to a fast process and the other to a slower process. W-doping (i) reduces the photo-excitation fluence threshold required for triggering the IMT, (ii) accelerates the slow process, and (iii) increases the THz transient transmission variation for corresponding fluences. From the Drude-Smith model, it is deduced that a strong carrier confinement and an enhancement of the transient conductivity occur across the IMT. The IMT is also accompanied by an increase in the carrier concentration in the films, which is enhanced by W-doping. Our results suggest that W-doped VO2 could be advantageously exploited in applications such as ultrafast THz optical switching and modulation devices.

Morphological, Structural, and Charge Transfer Properties of F-Doped ZnO: A Spectroscopic Investigation

Different spectroscopic techniques have been applied to fluorine doped ZnO powders prepared through hydrothermal synthesis, to discern the effective capability of F atoms to improve ZnO conductivity. From XRD analysis, no lattice distortion was observed up to F doping at 5 at. % concentration. Photoluminescence measurements and electron paramagnetic resonance data show that F atoms tend to occupy oxygen vacancies, inducing the onset of luminescence centers. The resulting doping effect consists into the increment of localized charge, as also proved via THz spectroscopy, where the Drude–Smith model has been applied to extract quantitative information on the electrodynamic parameters of ZnO:F samples. Results show that F doping does not produce any substantial change of plasma frequency but only the enhancement of scattering rate due to an increase of grain boundary density. Our measurements are in agreement with theoretical calculations asserting that the energy required to excite donor levels is on the ord...

Identification of Combination Phonon Modes in Pure and Doped GaSe Crystals by THz Spectroscopy

Terahertz transmission of (i) pure gallium selenide, (ii) 500-ppm indium-doped gallium selenide, and (iii) 500-ppm chromium-doped gallium selenide is taken in the range 0.2–3.0 THz (6.67–100 cm−1 ) to identify the effect of doping on phonon modes. Fourier transform of time domain THz transmission data is fitted with a modified Drude–Smith model to derive phonon mode oscillator strength, phonon relaxation rate, plasma frequency, and momentum relaxation time of free carriers. It is observed that the effect of doping on defect states and carrier concentration rightly accounts the measured plasma frequency and momentum relaxation time. The THz absorption lines are appropriately attributed to the combination of phonon modes obtained by Raman spectroscopy.

Twisted nematic liquid crystal cells with rubbed poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) films for active polarization control of terahertz waves

We fabricated a terahertz (THz) polarization converter using a twisted nematic (TN) liquid crystal (LC) cell. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films coated on quartz glass substrates were used as electrode layers in the TN LC cell. The PEDOT/PSS films were rubbed unidirectionally using a rayon cloth to align the nematic LC, thereby also serving as an alignment layer. The azimuthal surface anchoring strength of the PEDOT/PSS films was measured to be 5 × 10−4 J/m2 using the Néel wall method, which is similar to that of typical polymeric alignment layers. The optical constants of the PEDOT/PSS film in the THz range were also characterized using the Drude-Smith model, and the results indicated that the PEDOT/PSS films could be used both as transparent electrodes in the THz range and as alignment layers for the LC. The electro-optical properties of the fabricated TN LC cell were also investigated using a polarized visible laser and THz time-domain spectroscopic system. In particular, the transmission spectra and polarization conversion property of the TN LC cell in the THz range were theoretically analyzed based on a stratified model that considers optical anisotropy, absorption, and multiple interference. This work substantiates the advantages of TN LC cells with rubbed PEDOT/PSS films useful for THz polarization converters with electrical tunability.

Flexibility and non-destructive conductivity measurements of Ag nanowire based transparent conductive films via terahertz time domain spectroscopy.

Highly stable and flexible transparent electrodes are fabricated based on silver nanowires (AgNWs) on both polyethylene-terephthalate (PET) and polyimide (PI) substrates. Terahertz time domain spectroscopy (THz-TDS) was utilized to probe AgNW films while bended with a radius 5 mm to discover conductivity of bended films which was further analyzed through Drude-Smith model. AgNW films experience little degradation in conductivity (<3%) before, after, and during 1000 bending cycles. Highly stable AgNW flexible electrodes have broad applications in flexible optoelectronic and electronic devices. THz-TDS is an effective technique to investigate the electrical properties of the bended and flattened conducting films in a nondestructive manner.

Tuning the terahertz low-energy charge dynamics by simultaneous effect of epitaxial and anisotropic strain in PrNiO3 thin films

The interplay of charge, spin, and lattice correlations strongly influence the insulator-metal (I-M) transition and magnetic ordering in rare earth nickelates. In this context, we explored the low-energy charge dynamics in structurally modulated $\mathrm{PrNi}{\mathrm{O}}_{3}$ (PNO) thin films to unravel the complexity of ground state across I-M transition using terahertz (THz) spectroscopy. The THz optical constants of compressive film on $\mathrm{LaAl}{\mathrm{O}}_{3}$ (100) substrate and the tensile films on $\mathrm{NdGa}{\mathrm{O}}_{3}$ (100), (001), (110), and (111) substrates with varying orthorhombic distortion exhibit remarkably distinct features as a function of frequency and temperature. The THz conductivity of compressive film sans any I-M transition follows the Drude model. In contrast, the tensile strained films exhibit non-Drude THz conductivity, a giant positive dielectric permittivity, and negative imaginary conductivity, all of which can be explained by the Drude-Smith model. This rich variety of low-energy dynamics manifests as a function of temperature, strain, and crystal orientation. Such distinct THz spectral features, as induced by a subtle variation in strain while crossing over from tensile to compressive strain and with varying degree of orthorhombicity coupled with oxygen vacancies, reveal a novel facet of structure-property relationship of PNO.

THz conductivity of semi-insulating and magnetic CoFe2O4 nano-hollow structures through thermally activated polaron

Herein, terahertz (THz) time domain spectroscopy is used to measure the complex conductivity of semi-insulating CoFe2O4 nanoparticles (NPs) and nano-hollow spheres (NHSs) with different diameters ranging from 100 to 350 nm having a nanocrystalline shell thickness of 19 to 90 nm, respectively. Interestingly, the magnitude of conductivity for CoFe2O4 NPs and NHSs of same average diameter (∼100 nm) for a given frequency of 0.3 THz is found to be 0.33 S/m and 9.08 S/m, respectively, indicating that the hollow structure exhibits greater THz conduction in comparison to its solid counterpart. Moreover, THz conductivity can be tailored by varying the nano-shell thickness of NHSs, and a maximum conductivity of 15.61 S/m is observed at 0.3 THz for NHSs of average diameter 250 nm. A detailed study reveals that thermally activated polaronic hopping plays the key role in determining the electrical transport property of CoFe2O4 nanostructures, which is found to solely depend on their magnitude of THz absorptivity. The non-Drude conductivity of all CoFe2O4 nanostructures is well described by the Polaron model instead of the Drude-Smith model, which is relevant for backscattering of free electrons in a nanostructured material. The Polaron model includes intra-particle and interparticle polaronic conductivities for closely spaced magnetic nanostructures and provides a mean free path of 29 nm for CoFe2O4 NPs of diameter 100 nm, which is comparable with its average crystallite size, indicating the applicability of the developed model for nanomaterials where charge transport is determined by polaronic hopping. Finally, we have demonstrated the morphology and size dependent magnetic measurements of ferrimagnetically aligned CoFe2O4 nanostructures through a vibrating sample magnetometer in the temperature range of 80–250 K, revealing that the disordered surface spin layer of nanostructures significantly controls their magnetism.

Terahertz transport dynamics in the metal-insulator transition of V2O3 thin film

The dynamic behavior of thermally-induced metal-insulator transition of V2O3 thin film on Si substrate grown by reactive magnetron sputtering was investigated by the terahertz time-domain spectroscopy. It was found that the THz absorption and optical conductivity of the thin films are temperature-dependent, and the THz amplitude modulation can reach as high as 74.7%. The complex THz optical conductivity in the metallic state of the V2O3 thin films can be well-fitted by the Drude-Smith model, which offer the insight into the electron transport dynamic during the metal-insulator transition of the thin film.

Intragrain charge transport in kesterite thin films—Limits arising from carrier localization

Intragrain charge carrier mobilities measured by time-resolved terahertz spectroscopy in state of the art Cu2ZnSn(S,Se)4 kesterite thin films are found to increase from 32 to 140 cm2 V−1 s−1 with increasing Se content. The mobilities are limited by carrier localization on the nanometer-scale, which takes place within the first 2 ps after carrier excitation. The localization strength obtained from the Drude-Smith model is found to be independent of the excited photocarrier density. This is in accordance with bandgap fluctuations as a cause of the localized transport. Charge carrier localization is a general issue in the probed kesterite thin films, which were deposited by coevaporation, colloidal inks, and sputtering followed by annealing with varying Se/S contents and yield 4.9%–10.0% efficiency in the completed device.