] Of relevance to this work is the binary metal perovskite CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1]. [30,31] Interestingly, the bandgap bows and becomes lower when Sn 2+ is substituted by Pb 2+ for samples with 80% and 60% Sn content compared to 100% Sn-based perovskite, in line with previous observations. [30,31] While such tin-based perovskites offer tunable bandgaps down to 1.1 eV, the fabrication of efficient optoelectronic devices has been impeded by factors including poor semiconductor quality and low surface coverage. [30] As a consequence, solar cells made using these perovskites often exhibit very low efficiencies, with typical PCEs < 1% obtained for planar heterojunction devices. [30] To overcome this challenge, we have developed a novel elevated temperature processing method (depicted in Figure 1A), [32] for preparing CH 3 NH 3 (Pb x Sn 1-x )I 3 perovskites on a Poly(3,4-ethylenedioxythiophene):poly(styrenesulf onate) (PEDOT:PSS)/nickel oxide (NiO) bilayer, which results in the formation of large micron-sized grains ( Figure 1B) with almost complete substrate coverage. Our semiconductors not only exhibit relatively low energetic and structural disorder but also impart high PCEs when fabricated into a PV device. For PVs prepared using the lowest bandgap perovskites, open circuit voltages (V OC 's) approaching the prediction of the Shockley-Queisser (S-Q) model are demonstrated. Such promising performance metrics are obtained against a backdrop of fast radiative recombination and low photoluminescence quantum efficiencies (PLQEs), pointing toward the crucial role of high intrinsic charge carrier mobility in these low-bandgap semiconductors.To study the optical properties of the CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1] perovskite thin films, linear absorption and photoluminescence (PL) were measured as shown in Figure S1 (Supporting Information). It can be observed in Figure 1C that the bandgap bows as we substitute Pb 2+ in place of Sn 2+ (until 40% Sn 2+ ions are replaced by Pb 2+ ) and results in a nonmonotonic bandgap lowering similar to what was observed previously by Hao etal. [31] Briefly, the bandgap of the 60% and 80% Sn content films exhibit a lower bandgap than the 100% Sn-substituted films. A similar trend can also be traced in the PL spectra (see Figure S1B of the Supporting Information) where the PL spectra of 80% and 60% Sn content thin-film samples are red-shifted compared to the 100% Sn content thinfilm sample, which is consistent with the absorption spectra. Such anomalous bandgap bowing and lack of conformity with Vegard's law [31,33] have been attributed to the competition The performance of organometallic halide (hybrid) perovskite solar cells has improved dramatically in just a few years, with photovoltaic (PV) power conversion efficiencies (PCEs) now exceeding 22% for state-of-the-art devices. [1][2][3][4][5] This remarkable result, coupled with their low cost, tunability, and versatile lowtemperature preparation methods, makes hybrid perovskites one of the most promising semiconduct...

Despite sustained research, application of lead halide perovskites in field-effect transistors (FETs) has substantial concerns in terms of operational instabilities and hysteresis effects which are linked to its ionic nature. Here, we investigate the mechanism behind these instabilities and demonstrate an effective route to suppress them to realize high-performance perovskite FETs with low hysteresis, high threshold voltage stability (ΔVt < 2 V over 10 hours of continuous operation), and high mobility values >1 cm2/V·s at room temperature. We show that multiple cation incorporation using strain-relieving cations like Cs and cations such as Rb, which act as passivation/crystallization modifying agents, is an effective strategy for reducing vacancy concentration and ion migration in perovskite FETs. Furthermore, we demonstrate that treatment of perovskite films with positive azeotrope solvents that act as Lewis bases (acids) enables a further reduction in defect density and substantial improvement in performance and stability of n-type (p-type) perovskite devices.

Optoelectronic terahertz modulators, operating by actively tuning metamaterial, plasmonic resonator structures, have helped to unlock a myriad of terahertz applications, ranging from spectroscopy and imaging to communications. At the same time, due to the inherently versatile dispersion properties of metamaterials, they offer unique platforms for studying intriguing phenomena such as negative refractive index and slow light. Active resonance frequency tuning of a metamaterial working in the terahertz regime is achieved by integrating metal coupled resonator arrays with electrically tunable graphene. This metamaterial device exploits coupled plasmonic resonators to exhibit an electromagnetically induced transparency analog, resulting in the splitting of the resonance into coupled hybrid optical modes. By variably dampening one of the resonators using graphene, the coupling condition is electrically modulated and continuous tuning of the metamaterial resonance frequency is achieved. This device, operating

We report the fast amplitude modulation of a quantum cascade laser emitting in single-mode operation in the terahertz frequency range by employing compact, integrated devices based on the interplay between plasmonic antenna arrays and monolayer graphene. By acting on the carrier concentration of graphene, the optical response of these plasmonic resonances was modified. The modulator’s characteristics have been studied by using both time domain spectroscopic laser systems, yielding the broad frequency response of these resonant arrays, and quantum cascade lasers, providing us with a narrow and stable laser source, a mandatory prerequisite for the determination of the modulation speed of these devices. The measured modulation speed exhibits a cutoff frequency of 5.5 MHz ± 1.1 MHz. These results represent the first step toward the realization of fast integrated circuitry for communications in the terahertz frequency range.

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapour deposited graphene films on sapphire, silicon dioxide/silicon and germanium. We validate the technique against measurements performed with previously established conventional transmission based THz-TDS and are able to resolve conductivity changes in response to induced back-gate voltages. Compared to the transmission geometry, measurement in reflection mode requires careful alignment and complex analysis, but circumvents the need of a terahertz transparent substrate, potentially enabling fast, contactless, in-line characterisation of graphene films on non-insulating substrates such as germanium.

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm 2 /V•s at 4 K, indicative of a surfacedominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×10 12 cm -2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state. Device fabrication and electrical transport:Optical-lithography was used to define a microscale Hall bar of dimensions 1400 µm × 80 µm. This was followed by the deposition of 15 nm Ti and 90 nm Au and a standard lift-off process to obtain ohmic contacts to the mesa. The device was then packaged and measured in a He-3 cryostat with a base temperature of 300 mK. Hall measurements were obtained using an a.c lock-in four-terminal setup with an input current of 1 µA at a frequency of 133 Hz. ASSOCIATED CONTENTSupporting information: XRD and XRR measurements to characterize the quality, phase and thickness of the Bi2Se3 film; field dependence of the longitudinal and Hall resistance measurement at corresponding temperatures; THz time domain response of the (0001)-oriented sapphire substrate at different temperatures.

Active control of the amplitude and frequency of terahertz sources is an essential prerequisite for exploiting a myriad of terahertz applications in imaging, spectroscopy, and communications. Here we present a optoelectronic, external modulation technique applied to a terahertz quantum cascade laser which holds the promise of addressing a number of important challenges in this research area. A hybrid metamaterial/graphene device is implemented into an external cavity set-up allowing for optoelectronic tuning of feedback into a quantum cascade laser. We demonstrate powerful, all-electronic, control over the amplitude and frequency of the laser output. Full laser switching is performed by electrostatic gating of the metamaterial/graphene device, demonstrating a modulation depth of 100%. External control of the emission spectrum is also achieved, highlighting the flexibility of this feedback method. By taking advantage of the frequency dispersive reflectivity of the metamaterial array, different modes of the QCL output are selectively suppressed using lithographic tuning and single mode operation of the multi-mode laser is enforced. Side mode suppression is electrically modulated from ~6 dB to ~21 dB, demonstrating active, optoelectronic modulation of the laser frequency content between multi-mode and single mode operation.

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