Nature of Self-Trapped Exciton Emission in Zero-Dimensional Cs2ZrCl6 Perovskite Nanocrystals.

Low dimensional perovskite-inspired materials with self-tapped exciton (STE) emission have stimulated a surge of cutting-edge research in optoelectronics. Despite numerous efforts on developing versatile low-dimensional perovskite-inspired materials with efficient STE emissions, there is little emphasis on the intrinsic dynamics of STE-based broad emission in these materials. Here, we investigated the excited state dynamics in zero-dimensional (0D) Cs2ZrCl6 nanocrystals (NCs) with efficient blue STE emission. By using femtosecond transient absorption (fs-TA) spectroscopy, the ultrafast STE formation process within 400 fs is directly observed. Then, the formed STEs relax to an intermediate STE state with a lifetime of ∼180 ps before reaching the emissive STE state with a lifetime of ∼15 µs. Our work offers a comprehensive and precise dynamic picture of STE emission in low-dimensional metal halides and sheds light on extending their potential applications.

Investigating Design Rules for Photoinduced Electron Transfer Quenching in Triangulenium Probes.

Fluorescent probes based on photoinduced electron transfer (PET) quenching of long lifetime triangulenium fluorophores have found multiple applications. For such probes a successful design relies on the right balance between the rate of PET quenching and fluorescence. In a series of ADOTA (A) and DAOTA (D) triangulenium fluorophores appended with aniline-like quencher moieties, we have investigated the rate of quenching and its relation to thermodynamic driving force, distance, and conjugation within the quencher moiety. Three different quenchers, a short (1), a long (2), and a long twisted (3), 4-aminophenyl, 4'-aminobiphenyl, and 2,2'-dimethyl-4'-aminobiphenyl, respectively were investigated. Steady-state spectroscopy and electrochemistry confirms that the quencher moieties are electronically decoupled from the dyes and have similar oxidation potentials and thus driving force for PET quenching, irrespectively of their different length and conjugation. Time-resolved fluorescence measurement was used to measure the fast PET quenching, with rate constant kPET ranging from >4×1011 to 2×109  s-1 . Interestingly, PET quenching is equally efficient/fast from 1 and 2, even with increase in distance between the donor and the acceptor. However, when twisting the biphenyl in 3, a 20-fold decrease in quenching is found. Even with this decrease in kPET, the quenching in 3 A/D is still highly efficient, with nearly 99 % quenching. The study show that long lifetime fluorophores can be efficiently switched even by relatively slow PET processes and that PET quencher moieties can be removed far from the fluorophore if conjugated linkers are applied.

Interfacial THz generation from graphene/Si mixed-dimensional van der Waals heterostructure.

Even though Si is the most cost efficient and extensively used semiconductor in modern optoelectronics, it is not considered to be an effective THz emitter due to its low carrier drift velocity and small saturated built-in electric field from the inversion layer. Herein, we present an effective way to enhance THz generation using a graphene/Si Schottky junction (GSSJ) excited with a femtosecond laser under electrical gating without rapid saturation and with high carrier drift velocity. This mixed-dimensional van der Waals interface demonstrates large saturation pump fluence with an invalid inversion layer by removing the native oxide on the Si surface. The THz emission amplitude from GSSJ effectively increases with the gate voltage. The THz emission from GSSJ under the same excitation conditions is stronger than that from the surface of InAs (100) and GaAs (100). The results not only show an efficient THz emission from GSSJ but also demonstrate the ability of THz generation for probing the mixed-dimensional van der Waals interface.

Active broadband terahertz wave impedance matching based on optically doped graphene-silicon heterojunction.

Broadband terahertz (THz) impedance matching is important for both spectral resolution improvement and THz anti-radar technology. Herein, graphene-silicon hybrid structure has been proposed for active broadband THz wave impedance matching with optical tunability. The main transmission pulse measured in the time domain indicates a modulation depth as high as 92.7% totally from the graphene-silicon interface. The interface reflection from the graphene-silicon junction implies that an impedance matching condition can be actively achieved by optical doping. To reveal the mechanism, we propose a graphene-silicon heterojunction model, which gives a full consideration of both the THz conductivity of graphene and the loss in doped junction layer. The theory fits well with the experimental results. This work proves active THz wave manipulation by junction effect and paves the way for active anti-reflection coating for THz components.

Terahertz surface and interface emission spectroscopy for advanced materials.

Surfaces and interfaces are of particular importance for optoelectronic and photonic materials as they are involved in many physical and chemical processes such as carrier dynamics, charge transfer, chemical bonding, transformation reactions and so on. Terahertz (THz) emission spectroscopy provides a sensitive and nondestructive method for surface or interface analysis of advanced materials ranging from graphene to transition metal dichalcogenides, topological insulators, hybrid perovskites, and mixed-dimensional heterostructures based on 2D materials. In this review paper, we start with the THz radiation mechanisms under ultrafast laser excitation. Then we concentrate on the recent progresses of THz emission spectroscopy on the surface and interface properties of advanced materials, including transient surface photocurrents, surface nonlinear polarization, surface states, interface potential, and gas molecule adsorption/desorption processes. This novel spectroscopic method can not only promote the development of new and compact THz sources, but also provide a nondestructive optical method for surface and interface characterization of photocurrent and nonlinear polarization dynamics of materials.

Angular dependent strong coupling between localized waveguide resonance and surface plasmon resonance in complementary metamaterials.

We give direct evidence of both surface plasmon resonance (SPR) and localized waveguide resonance (LWR) contribution to the extraordinary optical transmission in complementary metamaterials. Strong coupling between SPR and LWR are also observed with clear evidence of Rabi splitting and anti-crossing phenomena. The splitting introduces sharp phase shift, which in turn enhances group velocity delay by the incident angle without geometric parameter change. The results not only clarify SPR and LWR effects in the extraordinary optical transmission, but also provide a novel route to control light-metamaterial interaction by angular modulation for on-chip slow light devices.

Interface Properties Probed by Active THz Surface Emission in Graphene/SiO2/Si Heterostructures.

Graphene/semiconductor heterostructures demonstrate an improvement of traditional electronic and optoelectronic devices because of their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from the interface of graphene on SiO2/Si (Gr/SiO2/Si). The active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface in the heterostructure. By analyzing the interface electric-field-induced optical rectification (EFIOR) in a strong inversion case, the intrinsic build-in potential is identified as -0.15 V at Gr/SiO2/Si interface. The interface depletion layer presents 44% positive THz intrinsic modulation by the reverse gate voltage and 70% negative THz intrinsic modulation by the forward gate voltage. Moreover, a time-dependent THz generation measurement has been used to deduce the charge detrapping decay time constant. The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis but also demonstrate the potential for the efficient THz intrinsic modulation as well as the enhancement of THz emission by the heterostructures.

Band alignments and heterostructures of monolayer transition metal trichalcogenides MX3 (M = Zr, Hf; X = S, Se) and dichalcogenides MX2 (M = Tc, Re; X=S, Se) for solar applications.

Knowledge of band alignments and heterostructure formations is fundamental for a new generation of optoelectronics based on two-dimensional layered materials. Herein, band alignments and heterostructures of IVB-VIA monolayer MX3 (M = Zr, Hf; X = S, Se) and VIIB-VIA monolayer MX2 (M = Tc, Re; X = S, Se) are calculated by density functional theory with hybrid functionals. The results indicate that for monolayer MX3, the valence bands mainly depend on the p state of the chalcogens and the conduction bands mainly depend on the d state of the transition metals. In contrast, for monolayer MX2, both valence and conduction bands depend on the d state of the transition metals. This suggests that their work functions are obviously different. Meanwhile, the characteristics of the band alignments and the planar-averaged local density of states show that ZrS3, HfS3, TcSe2 and ReS2 could be favorable candidates for photocatalytic water splitting. ZrS3, HfS3 and MX2 with the same structures are able to form type II heterostructures at their interfaces, which could be used for solar energy conversion. The power-conversion efficiency of an MX3 thin-film solar cell is approximately 16-18%, which is higher than those of MX2 thin-film solar cells. In addition, for heterostructures composed of MX3, both of the two kinds of material (M and X) play an important role in every band formation. Meanwhile, for MX2 heterostructures, almost every band depends only on a single material. The charge density difference of the heterostructures demonstrates a higher charge accumulation at the interface of MX3 heterostructures than that of MX2 heterostructures. These phenomena show that type II heterostructures formed of MX3 are more stable than those of MX2.

Efficient mixed-solvent exfoliation of few-quintuple layer Bi2S3 and its photoelectric response.

A scalable liquid exfoliation of layered Bi2S3 employing a mixed-solvent strategy was used for the fabrication of Bi2S3 nanosheets. We found that 10% deionized water in 90% isopropyl alcohol is the best mixed solvent for the efficient and effective exfoliation of layered Bi2S3. These results are consistent with the absorbance spectra and enthalpy of mixing theory. The obtained Bi2S3 nanosheets had few-quintuple layers and were investigated by transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. These Bi2S3 nanosheets can be used to fabricate large-scale thin films by filtration method; the films demonstrated sensitive photoelectric response with the rise and decay response of photocurrent on the sub-second scale under visible light excitation. The electronic structures of bulk and one-quintuple layer Bi2S3 are calculated by first-principle calculation for better understanding of the photoelectric response. A green mixed-solvent processing of Bi2S3 opens up the potential application of Bi2S3 optoelectric films to photoelectric detection and solar energy conversion devices.