The ability to pattern, structure, re-shape and actuate hydrogels is important for biomimetics, soft robotics, cell scaffolding and biomaterials. Here we introduce an 'ionoprinting' technique with the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions via their directed injection and complexation, assisted by electric fields. The ionic binding changes the local mechanical properties of the gel to induce relief patterns and, in some cases, evokes localized stress large enough to cause rapid folding. These ionoprinted patterns are stable for months, yet the ionoprinting process is fully reversible by immersing the gel in a chelator. The mechanically patterned hydrogels exhibit programmable temporal and spatial shape transitions, and serve as a basis for a new class of soft actuators that can gently manipulate objects both in air and in liquid solutions.
This article describes the fabrication of self-healing stretchable wires formed by embedding liquid metal wires in microchannels composed of self-healing polymer. These stretchable wires can be completely severed with scissors and rapidly self-heal both mechanically and electrically at ambient conditions. By cutting the channels strategically, the pieces can be re-assembled in a different order to form complex microfluidic networks in 2D or 3D space.
We perform photoluminescence experiments at 4 K on two different transition metal diselenide monolayers, namely MoSe2 and WSe2 in magnetic fields Bz up to 9 T applied perpendicular to the sample plane. In MoSe2 monolayers the valley polarization of the neutral and the charged exciton (trion) can be tuned by the magnetic field, independent of the excitation laser polarization. In the investigated WSe2 monolayer sample the evolution of the trion valley polarization depends both on the applied magnetic field and the excitation laser helicity, while the neutral exciton valley polarization depends only on the latter. Remarkably we observe a reversal of the sign of the trion polarization between WSe2 and MoSe2. For both systems we observe a clear Zeeman splitting for the neutral exciton and the trion of about ±2 meV at Bz ∓ 9 T. The extracted Landé-factors for both exciton complexes in both materials are g ≈ −4.
Stimuli responsive polyelectrolyte hydrogels may be useful for soft robotics because of their ability to transform chemical energy into mechanical motion without the use of external mechanical input. Composed of soft and biocompatible materials, gel robots can easily bend and fold, interface and manipulate biological components and transport cargo in aqueous solutions. Electrical fields in aqueous solutions offer repeatable and controllable stimuli, which induce actuation by the re-distribution of ions in the system. Electrical fields applied to polyelectrolyte-doped gels submerged in ionic solution distribute the mobile ions asymmetrically to create osmotic pressure differences that swell and deform the gels. The sign of the fixed charges on the polyelectrolyte network determines the direction of bending, which we harness to control the motion of the gel legs in opposing directions as a response to electrical fields. We present and analyze a walking gel actuator comprised of cationic and anionic gel legs made of copolymer networks of acrylamide (AAm)/sodium acrylate (NaAc) and acrylamide/quaternized dimethylaminoethyl methacrylate (DMAEMA Q), respectively. The anionic and cationic legs were attached by electric field-promoted polyion complexation. We characterize the electro-actuated response of the sodium acrylate hydrogel as a function of charge density and external salt concentration. We demonstrate that "osmotically passive" fixed charges play an important role in controlling the bending magnitude of the gel networks. The gel walkers achieve unidirectional motion on flat elastomer substrates and exemplify a simple way to move and manipulate soft matter devices and robots in aqueous solutions.
We investigate valley exciton dynamics in MoSe2 monolayers in polarization-and time-resolved photoluminescence (PL) spectroscopy at 4K. Following circularly polarized laser excitation, we record a low circular polarization degree of the PL of typically ≤ 5%. This is about 10 times lower than the polarization induced under comparable conditions in MoS2 and WSe2 monolayers. The evolution of the exciton polarization as a function of excitation laser energy and power is monitored in PL excitation (PLE) experiments. Fast PL emission times are recorded for both the neutral exciton of ≤ 3 ps and for the charged exciton (trion) of 12 ps.Monolayers (MLs) of the transition metal dichalcogenides (TMDCs) MoS 2 , MoSe 2 , WS 2 and WSe 2 are semiconductors with a direct bandgap in the visible region [1][2][3]. Their optical properties are dominated by excitons, strongly Coulomb-bound electron hole pairs [4][5][6][7][8][9][10][11][12]. TMDC MLs have emerged as very promising materials for optical, electronic and quantum manipulation applications [13,14]. In TMDC MLs crystal inversion symmetry breaking together with the strong spin-orbit (SO) interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (σ + or σ − ) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two non-equivalent K valleys (K + or K − , respectively) [15][16][17][18][19][20][21]. These chiral optical selection rules leading to strong valley selectivity are expected to be a common feature for MoS 2 , MoSe 2 , WS 2 and WSe 2 . High values of the order of 50% for the circular polarization P c of the stationary photoluminescence (PL) emission corresponding to successful valley polarization have been reported in MoS 2 [17][18][19]22], WSe 2 [10, 20] and WS 2 [23], albeit with very different dependences on laser excitation energy i.e. on the excess energy of the initial excitation compared to the exciton emission energy.So ideal conditions for valley polarization generation in PL experiments need to be investigated also for ML MoSe 2 . This material is very promising for valley index manipulation [24,25] with bright, well separated emission lines for neutral (X 0 ) and charged excitons (trions T) [11] shown in Fig. 1, which allows to investigate the valley physics for these complexes individually at low temperature. In time-integrated PL experiments at 4K we record essentially unpolarized emission P c 0 following excitation as close as 100 meV above the A-exciton with a σ + polarized laser. One possible origin for the low PL polarization would be efficient depolarization with a typical time τ s much shorter than the PL emission time τ . Our time resolved measurements show PL emission times τ in the ps range, just as in the case of ML MoS 2 [26] and WSe 2 [27]. This hints at either faster polarization relaxation in MoSe 2 in the sub-picosecond range or inefficientDet. optical polarization generation due to anomalies in the bandstructure. MoSe 2 ML flakes are o...
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