A van der Waals heterostructure between all-inorganic CsPbBr 3 halide perovskite quantum dots (CPB QDs) and Ti 3 C 2 T x MXene (MXN) QDs is realized, and the photophysics of the composite is examined. A strong quenching of the steady-state photoluminescence of the perovskite QDs is observed, which is further examined by a time-resolved photoluminescence study. It is attributed to the charge transfer from CPB to MXN QDs followed by rapid energy relaxation of hot electrons and hot holes toward the band edge, leading to their recombination with photon energy in the far-IR energy domain. It is further shown that the CPB− MXN QD/QD system acts as an excellent on−off−on photoluminescence probe for cadmium ion detection and also as an active material for photodetector application.
In view of their immensely intriguing properties, 2D materials are being intensely researched in search of novel phenomena and diverse application interests; however, studies on the realization of 2D/2D nanocomposites in the application-worthy thin film platform are rare. Here we have grown MoS2-hBN 2D/2D composite thin films on different substrates by the pulsed laser deposition (PLD) technique and made comparative studies with the pristine MoS2 and hBN films. The Raman and x-ray photoelectron spectroscopy (XPS) techniques as well as high-resolution transmission electron microscopy (HRTEM) confirm the concomitant presence of both the 1T (conducting) and 2H (semiconducting) polymorphs of MoS2 in the composite film. Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is seen in the MoS2-hBN 2D/2D composite film which is absent in the MoS2 film, and it correlates well with the signatures of phonon softening seen in temperature dependent Raman spectroscopy. Furthermore, electrostatic force microscopy (EFM) reveals the presence of three distinct regions (metallic, semiconducting and insulating) in the MoS2-hBN composite film with differing contact potentials and enhanced propensity for charge transfer with respect to pristine MoS2. A triboelectric nanogenerator (TENG)device containing biphasic MoS2-hBN composite film as an electron acceptor exhibits more than two-fold (six-fold) enhancement in peak-to-peak output voltage as compared to the pristine MoS2 2 (hBN) film. These observations bring out the potential of 2D/2D nanocomposite thin films for unfolding emergent phenomena and technological applications.
In this work we address the question of growth of thin films of a soft van der Waals solid (2D MoS 2 ) on a hard, crystalline metal oxide substrates. Thus, MoS 2 thin films are grown on different single-crystal metal oxide substrates (SrLaAlO 4 , c-Al 2 O 3 , SrTiO 3 , LaAlO 3 ) by pulsed laser deposition (PLD) and characterized by different techniques to confirm and quantify their phase constitution (2H, 1T′). We observe that, on the SrLaAlO 4 (001) substrate, a mixed 1T′+2H phase of MoS 2 grows with a dominant (∼75%) 1T′ phase, while on c-Al 2 O 3 (0001) pure 2H phase grows. On the SrTiO 3 and LaAlO 3 substrates also the mixed phase grows, but with a much lesser component of the 1T′. Higher 1T′ contribution in SLAO and STO points to the chemical role of strontium in the early growth. It is also noted that, with increased film thickness on SrLaAlO 4 , the contribution of the pure 2H phase is enhanced, indicating the pivotal role of lattice strain in stabilizing the initial layer(s). The mixed 1T′+2H phase in the ultrathin film shows significantly lower room temperature resistivity (∼2 mΩ cm) with respect to that of the pure 2H phase (∼14 mΩ cm). The substrate selective polymorphism with distinct electronic features could invite multiple application interests.
The mechatronic functionality of lead iodide hybrid perovskite thin films grown on the flexible substrate is investigated via the study of current-perpendicular-to-plane charge transport modulation under flex-mode compressive and tensile strains (CS and TS) for multiple flexing cycles. It is shown that the transport is significantly, reversibly, and asymmetrically modulated. Typically, for a strain of 0.088% (0.23%), a remarkable current modulation of +196% (+393%) is achieved for compressive strain and −49% (−53%) for tensile strain at an applied potential of 1 V. For low levels of bending, the response is robust for a large number of bending cycles. The effects of the change of organic cation from methylammonium to formamidinium and the grain size on the response are also examined. A comparative study of the structural, morphological, and optical properties of the pristine sample and the samples subjected to multiple bending cycles is performed to understand and elucidate the possible mechanisms of the straininduced changes in the transport properties.
The dynamics and control of charge transfer between optoelectronically interesting and size-tunable halide perovskite quantum dots and other juxtaposed functional electronic materials are important issues for the emergent device interest involving such a family of materials in heterostructure configurations. Herein, we have grown bimetallic Au−Ag thin films on glass by pulsed laser deposition at room temperature, which bear nanoparticulate character, and the corresponding optical absorption spectra reveal the expected surface plasmon resonance signature(s). Subsequently, spin-coated CsPbBr 3 nanoparticle films onto the bimetallic Au−Ag films exhibit surface-enhanced Raman scattering as well as strong photoluminescence quenching, the latter reflecting highly efficient transfer of photo-generated carriers across the CsPbBr 3 /Au−Ag interface. Surprisingly, when an ultrathin MgO (insulating) layer of optimum thickness is introduced between the CsPbBr 3 and Au−Ag films, the charge transfer is further facilitated with the average lifetime of carriers becoming even shorter. By changing the thickness of the thin MgO layer, the carrier lifetime can in fact be tuned; with the charge transfer getting fully blocked for thick enough MgO layers, as expected. Our study thus throws light on the charge-carrier dynamics in halide perovskites, which is of importance to emergent optoelectronic applications.
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