Optical-pump Terahertz-probe Spectroscopy Research Articles

Ultrafast Photocarrier Dynamics in Vertically Aligned SnS2 Nanoflakes Probing with Transient Terahertz Spectroscopy.

By employing optical pump Terahertz (THz) probe spectroscopy, ultrafast photocarrier dynamics of a two-dimensional (2D) semiconductor, SnS2 nanoflake film, has been investigated systematically at room temperature. The dynamics of photoexcitation is strongly related to the density of edge sites and defects in the SnS2 nanoflakes, which is controllable by adjusting the height of vertically aligned SnS2 during chemical vapor deposition growth. After photoexcitation at 400 nm, the transient THz photoconductivity response of the films can be well fitted with bi-exponential decay function. The fast and slow processes are shorter in the thinner film than in the thicker sample, and both components are independent on the pump fluence. Hereby, we propose that edge-site trapping as well as defect-assisted electron-hole recombination are responsible for the fast and slow decay progress, respectively. Our experimental results demonstrate that the edge sites and defects in SnS2 nanoflakes play a dominant role in photocarrier relaxation, which is crucial in understanding the photoelectrochemical performance of SnS2 nanoflakes.

Ultraviolet-to-infrared broadband photodetector and imaging application based on a perovskite single crystal.

The organic-inorganic hybrid perovskite CH3NH3PbBr3(MAPbBr3) has been well developed in the X-ray to visible light band due to its superior optoelectronic properties, but this material is rarely studied in the infrared band. In this paper, a UV-NIR broadband optical detector based on MAPbBr3 single crystal is studied, and the response range can reach the near-infrared region. In the visible light band, the optical response of the device is mainly caused by the photoelectric effect; in the near-infrared band, the optical response of the device is mainly caused by the thermal effect. The carrier response of MAPbBr3 material under different wavelengths of light was investigated using a non-contact measurement method (optical pump terahertz (THz) probe spectroscopy). This paper also builds a set of photoelectric sensor array components, and successfully realizes the conversion of optical image signals to electrical image signals in the visible light band and infrared band. The experimental results show that MAPbBr3 crystals provide a new possibility for UV-NIR broadband photodetectors.

Revealing charge carrier dynamics and transport in Te-doped GaAsSb and GaAsSbN nanowires by correlating ultrafast terahertz spectroscopy and optoelectronic characterization

Recent advances in the growth of III-V semiconductor nanowires (NWs) hold great promise for nanoscale optoelectronic device applications. It is established that a small amount of nitrogen (N) incorporation in III-V semiconductor NWs can effectively red-shift their wavelength of operation and tailor their electronic properties for specific applications. However, understanding the impact of N incorporation on non-equilibrium charge carrier dynamics and transport in semiconducting NWs is critical in achieving efficient semiconducting NW devices. In this work, ultrafast optical pump-terahertz probe spectroscopy has been used to study non-equilibrium carrier dynamics and transport in Te-doped GaAsSb and dilute nitride GaAsSbN NWs, with the goal of correlating these results with electrical characterization of their equilibrium photo-response under bias and low-frequency noise characteristics. Nitrogen incorporation in GaAsSb NWs led to a significant increase in the carrier scattering rate, resulting in a severe reduction in carrier mobility. Carrier recombination lifetimes of 33 ± 1 picoseconds (ps) and 147 ± 3 ps in GaAsSbN and GaAsSb NWs, respectively, were measured. The reduction in the carrier lifetime and photoinduced optical conductivities are due to the presence of N-induced defects, leading to deterioration in the electrical and optical characteristics of dilute nitride NWs relative to the non-nitride NWs. Finally, we observed a very fast rise time of ∼2 ps for both NW materials, directly impacting their potential use as high-speed photodetectors.

Probing Charge Transport in Sp 3-Functionalized Single-Walled Carbon Nanotubes with Terahertz Spectroscopy

The covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp 3-hybridized defects gives rise to tunable, narrowband emission in the near-infrared and increases their photoluminescence quantum yields [ACS Nano 2019, 13, 9259]. To enable their application in electrically driven light sources, a detailed understanding of the impact of sp 3 defects on charge transport within individual nanotubes and nanotube networks is required. Here, we investigate the ultrafast carrier dynamics in sp 3-functionalized, polymer-sorted (6,5) SWCNT dispersions and thin films using optical-pump terahertz-probe (OPTP) spectroscopy. OPTP spectroscopy relies on the photo-generation of charge carriers that are subsequently probed by a single-cycle terahertz pulse, rendering it a highly suitable technique for the determination of intrinsic charge transport properties (e.g., carrier mobility) in a quantitative and contact-free fashion. We observe a large transient photoconductivity of pristine and functionalized nanotubes that is dominated by the contribution of free charge carriers. The photoconductivity intensity and lifetime decrease upon functionalization, corroborating the impact of sp 3 defects on intra-nanotube charge transport. Through combination of (temperature-dependent) OPTP measurements of sp 3-functionalized SWCNT dispersions and films with electrical measurements of ambipolar, light-emitting field-effect transistors based on functionalized SWCNT networks [ACS Nano 2021, 15, 10451], we can separate the contributions of sp 3 defects (intra-nanotube) and nanotube junctions (inter-nanotube) to charge transport in such networks and distinguish the short- and long-range transport characteristics. Fig. 1: Ultrafast optical-pump terahertz-probe spectroscopy is a highly suitable technique to study the intrinsic charge transport properties of sp 3-functionalized (6,5) SWCNTs in dispersions and thin films. Figure 1

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy.

The controlled introduction of covalent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp3 defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp3 functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp3 defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

Observation of Ultrafast Interfacial Exciton Formation and Relaxation in Graphene/MoS2 Heterostructure.

Heterostructures constructed from graphene and transition metal dichalcogenides (TMDs) have established a new platform for optoelectronic applications. After a large number of studies, one intriguing debate is the existence of the interfacial exciton in graphene/TMDs. Hereby, by combined optical pump-terahertz probe spectroscopy and transient absorption spectroscopy, we report the observation of the interfacial exciton in graphene/MoS2 heterostructure. With the photon energy well below the band gap of monolayer MoS2, the hot electrons of graphene are transferred to MoS2 within 0.5 ps; subsequently, the relaxation of the holes in graphene and electrons in MoS2 shows an identical time scale of 15-18 ps, which manifests the formation and relaxation of the interfacial exciton in the heterostructure following photoexcitation. Moreover, a model of the carrier heating and photogating effect in graphene is proposed to estimate the amount of transferred charge, which agrees well with the experimental results. Our study provides insights into the dynamics of graphene-based heterostructure interfacial non-equilibrium carriers.

Boosted Charge‐Carrier Transport in Triple‐Cation Perovskites by Ultrasonic Vibration Post Treatment

AbstractUltrasonic vibration imposed on the substrate of a drying perovskite solution film has previously been proposed as a nearly annealing‐free method to improve film quality and thus the photovoltaic performance for perovskite solar cells. However, an in‐depth understanding of the underlying mechanism of the improved film quality via ultrasonic vibration is still lacking. In this work, the effects of substrate vibration post treatment on the carrier lifetime and mobility are studied in triple‐cation perovskite films. With 80 s of annealing‐free vibration, the perovskite film demonstrates much stronger photoluminescence intensity and much longer carrier lifetime up to 2.634 µs, 2 orders of magnitude longer than that of the thermal annealed films. Optical pump terahertz probe spectroscopy reveals that the charge‐carrier mobility increases to 121 ± 44 cm2 V−1 s−1 when subjected to 80 s vibration followed by annealing. Such mobility is about 80 ± 40 cm2 V−1 s−1 higher than that of other polycrystalline organic–inorganic hybrid perovskite thin films of similar composition. The diffusion length is improved to nearly 1.5 times. The new understanding on the vibration‐induced charge‐carrier transport properties paves the way for the application of ultrasonic vibration toward the performance improvement of perovskite‐based electronic devices.

Optical-Pump Terahertz-Probe Spectroscopy of the Topological Crystalline Insulator Pb1–xSnxSe through the Topological Phase Transition

Topological crystalline insulators -- topological insulators whose properties are guaranteed by crystalline symmetry -- can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy of topological crystalline insulators, using them to study the dynamics of Pb1-xSnxSe as a function of temperature. At low temperatures, excitation of Dirac fermions leads to an increase in terahertz transmission; from this negative photoconductivity, the intrasubband relaxation rate of 6 ps is extracted. At high temperatures where only massive fermions exist, the free-carrier losses induced by the pump reduce the terahertz transmission for the duration of the 27 ps interband lifetime. Both effects are present at temperatures near the topological-to-trivial transition. Our experimental observations provide critical details for potential applications of Pb1-xSnxSe and provide a direct measurement of the topological character of Pb1-xSnxSe heterostructures.

Photogenerated carrier density dependence of ultrafast carrier dynamics in intrinsic 6H-SiC measured by optical-pump terahertz-probe spectroscopy

Ultrafast carrier dynamics and terahertz conductivity of intrinsic 6H-SiC are investigated by optical-pump terahertz-probe spectroscopy at room temperature. The amplitude of photoinduced absorption increases gradually and followed by a biexponential recovery with photogenerated carrier densities from 7.43 × 1017 to 2.81 × 1018 cm −3 at 400 nm. With the increase of photogenerated carrier density, the fast decay time decreases may be due to higher order recombination (i.e., Auger recombination), and the slow one increases is related to the band-filling effect. The evolution of complex conductivity with photogenerated carrier density at the delay time of about 50 ps can be better fitted with Drude–Smith​ model than Drude–Lorenz model. The Drude–Smith scattering time τDS and Smith​ parameter c1 increase with photogenerated carrier density implies carriers backscattering is dominant. Our analysis provides important basic data for research and improvement ultrafast semiconductor devices of intrinsic 6H-SiC.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Nature of Local Charge Carrier Motions in Porphyrin-based Bulk Heterojunction Films Revealed by Time-resolved Optical Pump-terahertz Probe Spectroscopy

Local charge carrier dynamics in diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP)-based bulk heterojunction (BHJ) films are investigated by optical pump-terahertz probe spectroscopy. Here we focus on the effect of the alkyl chain lengths of DPP on the carrier dynamics. In contrast to the large difference in the power conversion efficiency, similar dynamics are observed between C10-DPP-BP and C4-DPP-BP BHJ films. We discuss the origin of similarity in terms of nature of the high frequency response of photo-generated charge carriers.

Metal-insulator transition and interfacial thermal transport in atomic layer deposited Ru nanofilms characterized by ultrafast terahertz spectroscopy

Ruthenium nanofilms are a promising material for wide applications in nanoelectronics, such as ultrathin electrodes and metallization. However, little is known about their electrical and thermal properties. Here, we utilize ultrafast optical-pump terahertz-probe (OPTP) spectroscopy to characterize thickness- and substrate-dependent properties of ruthenium nanofilms. Atomic layer deposition produces ruthenium nanofilms with precisely controlled thicknesses and provides different morphologies of ruthenium islands on different substrates. First, we directly observe a thickness-dependent metal–insulator transition, revealed by a sign change in the OPTP signals near a critical film thickness of about 10 nm. This phase transition originates from the formation of electrical percolation networks of ruthenium islands, providing key information for film-thickness optimization in nanodevice designs. Second, OPTP reveals interfacial thermal transport from ruthenium to various substrates (sapphire, fused silica, and MgO). The thermal dissipation exhibits strong substrate dependence, attributed to the different morphologies of ruthenium islands and quality of the ruthenium-substrate interface. The observed dynamics are reproduced by simulation of spatiotemporal temperature distributions, providing the interface thermal conductance, a key design parameter in the thermal management of nanodevices. This work provides novel insight into the physical properties of ruthenium nanofilms and their dependence on the thickness and the morphology of the films.

Charge-Carrier Mobility and Localization in Semiconducting Cu2AgBiI6 for Photovoltaic Applications.

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V–1 s–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications.

Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite.

Cs2AgBiBr6 is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs2AgBiBr6 using a combination of temperature-dependent photoluminescence, absorption and optical pump–terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm2 V–1 s–1 at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron–phonon coupling in this material induces rapid charge-carrier localization.

Transient evolution of quasifree electrons of plasma in liquid water revealed by optical-pump terahertz-probe spectroscopy

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Rapid, facile synthesis of InSb twinning superlattice nanowires with a high-frequency photoconductivity response.

We present a self-seeded (with indium droplets) solution–liquid–solid (SLS) synthesis route for InSb nanowires (NWs) using commercially available precursors at a relatively low temperature of about 175 °C, which takes only 1 min upon the injection of reductant. Structural characterization reveals that the InSb nanowires are high quality and have twinning superlattice structures with periodically spaced twin planes along the growth direction of 〈111〉. Notably, we have measured an ultrafast conductivity lifetime in the NWs of just 9.1 ps utilizing time-resolved optical pump-terahertz probe (OPTP) spectroscopy, which may facilitate the development of high-frequency nanoscale integrated optoelectronic systems related to twinning superlattice structures.

Single Copper Atoms Enhance Photoconductivity in g-C3N4.

Graphitic carbon nitride (g-C3N4) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis; however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C3N4 (Cu-g-C3N4) and demonstrate via extended X-ray absorption fine structure that Cu+ incorporates as an individual ion. Time-resolved optical pump terahertz probe spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu+ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. This study provides valuable insight into the ultrafast photoconductivity dynamics of g-C3N4 materials, which is essential toward developing efficient g-C3N4 photocatalysts.

Photoinduced carrier dynamics in bulk ZnTe using optical-pump terahertz-probe spectroscopy

We have utilized optical-pump terahertz-probe spectroscopy to analyze the relaxation process of photoinduced nonequilibrium carriers in bulk ZnTe with a crystal orientation of ⟨ 110 ⟩ . The experimental results showed that the concentration of nonequilibrium carriers increases with the increase in the energy of pump laser. When the pump energy calculated by a single exponential fitting function reached 63 μJ, the concentration of nonequilibrium carriers attained saturation. In addition, for low pump energy, the relaxation time did not change with increasing pump energy. On the contrary, when the pump energy was high, the relaxation time increased in proportion to the pump laser energy.

Quasi-two-dimensional van der Waals semiconducting magnet CrSiTe<sub>3</sub> studied by using THz spectroscopy

Quasi-two-dimensional van der Waals ferromagnetic semiconductor CrSiTe<sub>3</sub> with wide potential applications in optoelectronics and nanospintronics has aroused the immense interest of researchers due to the coexistence of intrinsic magnetism and semiconductivity. By combining untrafast femtosecond laser and terahertz spectroscopy, including terahertz time-domain spectroscopy, optical pump-terahertz probe spectroscopy and terahertz emission spectroscopy, we carry out systematic investigation into the van der Waals ferromagnetic semiconductor CrSiTe<sub>3</sub> crystal. The experimental results indicate that the conductivity of the sample is robust against the temperature change and isotropic terahertz transmission in the <i>ab</i>-plane. Moreover, it is also observed that the photocarriers induced by 800 nm optical pump exhibit a relaxation in the biexponential form and the complex photoconductivity can be well reproduced by the Drude-Smith model. The main relaxation channel of photocarriers is the recombination of electron-hole pairs. With femtosecond pulse illuminating the surface of sample, a strong terahertz radiation signal with a broad band of 0–2 THz is observed. The present study provides the responses of CrSiTe<sub>3</sub> to optical and terahertz frequency and offers crucial information for the future design of CrSiTe<sub>3</sub>-based electronic and optoelectronic devices.

Ultrafast Auger process in few-layer PtSe2.

Enhanced many-body interactions due to strong Coulomb interactions and quantum confinement are one of the most prominent features of two-dimensional systems. The Auger process is a representative many-body interaction typically observed in two-dimensional semiconductors, determining important physical properties of materials, such as carrier lifetime, photoconductivity, and emission quantum yield. Recently, platinum dichalcogenides, represented by PtSe2 and PtS2, have attracted great attention due to their superior air stability, thickness-dependent semimetal-to-semiconductor transition, and exotic magnetic characteristics. However, the Auger process in platinum dichalcogenides has not been investigated to date. Here, we utilized ultrafast optical-pump terahertz-probe spectroscopy to explore carrier dynamics in few-layer semiconducting PtSe2. Most of the excited carriers are trapped by defects within ∼10 ps after excitation due to high defect density. We overcome this challenge by raising the excitation intensity to saturate trap sites with carriers, and observed a many-body process involving the carriers that survived the rapid trapping. This process is not band-to-band Auger recombination, but rather defect-assisted Auger recombination in which free carriers interact with trapped carriers at defects. Theoretical simulations show that this three-body Auger process can be approximated as bimolecular recombination at the rate of ∼3.3 × 10-3 cm2 s-1. This work provides insights into the interplay between ultrafast many-body processes and defects in two-dimensional semiconductors.