A Supramolecular Nanofiber-Based Passive Memory Device for Remembering Past Humidity

Memorizing the magnitude of a physical parameter such as relative humidity in a consignment may be useful for maintaining recommended conditions over a period of time. In relation to cost and energy considerations, it is important that the memorizing device works in the unpowered passive state. In this article, we report the fabrication of a humidity-responsive device that can memorize the humidity condition it had experienced while being unpowered. The device makes use of supramolecular nanofibers obtained from the self-assembly of donor–acceptor (D–A) molecules, coronene tetracarboxylate salt (CS) and dodecyl methyl viologen (DMV), respectively, from aqueous medium. The fibers, while being highly sensitive to humidity, tend to develop electrically induced disorder under constant voltage, leading to increased resistance with time. The conducting state can be regained via self-assembly by exposing the device to humidity in the absence of applied voltage, the extent of recovery depending on the magnitude of the humidity applied under no bias. This nature of the fibers has been exploited in reading the humidity memory state, which interestingly is independent of the lapsed time since the humidity exposure as well as the duration of exposure. Importantly, the device is capable of differentiating the profiles of varying humidity conditions from its memory. The device finds use in applications requiring stringent condition monitoring

Intrinsic Nature of Graphene Revealed in Temperature-Dependent Transport of Twisted Multilayer Graphene

Graphene in its purest form is expected to exhibit a semiconducting to metallic transition in its temperature-dependent conductivity as a result of the interplay between Coulomb disorder and phonon scattering, the transition temperature, <i>T</i>_c, depending sensitively on the disorder induced carrier density (<i>n</i>_c). Even for good quality graphene, the <i>n</i>_c can be quite high (∼10¹² cm^–2) and the transition temperature may be placed well above the ambient, practically rendering it to be only semiconducting over a wide range of temperature. Here we report an experimental study on the transport behavior of twisted multilayer graphene (tMLG) exhibiting <i>T</i>_c well below the ambient temperature. The graphene layers in these tMLG are highly decoupled with one another due to the angular rotation among them; as a result, they exhibit very high Raman I_2D/I_G values (up to 12) with narrow 2D width (16–24 cm^–1). The observed <i>T</i>_c values seem to go hand in hand with the Raman I_2D/I_G values; a multilayer with mean I_2D/I_G value of 4.6 showed a <i>T</i>_c of 180 K, while that with mean I_2D/I_G of 4.9 showed lower a <i>T</i>_c of 160 K. Further, another multilayer with even higher mean I_2D/I_G value of 6.9 was metallic down to 5 K, indicating a very low disorder. The photoresponse behavior also corroborates well with the transition in transport behavior

Homojunction Interface Boosts Hole-Carrier Injection in p‑Type CuI Nanoribbon Field-Effect Transistors

Despite substantial advancements in n-type 1D and 2D nanostructures, achieving p-type field-effect transistors (FETs) using 1D nanostructures remains a formidable challenge due to surface defects and doping limitations. This study presents a scalable approach for fabricating the p-type homojunction (p/p+) CuI nanoribbons (CuI NRs) with enhanced charge injection. Characterization of iodide-exposed (I-rich) CuI thin films reveals improved crystallinity and significantly higher carrier concentration compared with pristine CuI thin films. Leveraging the unique carrier tuning property of CuI, localized iodine exposure facilitated by electron beam lithography at the source/drain electrode interface of CuI NRs leads to the formation of a homojunction CuI NR. The homojunction CuI NR p-type FETs exhibits performance improvements, with three-orders of magnitude lower contact resistance and high mobility (5.6 cm2V–1s–1) with an on/off ratio of 104. Temperature-dependent studies reveal the presence of shallow traps and a reduced Schottky barrier height in the homojunction CuI NR FETs, contributing to efficient charge transfer at the metal–semiconductor interface. These findings establish CuI NR as a promising material for developing reliable p-type semiconductor devices. The fabrication of homojunction CuI NRs represents a significant advancement in the field of 1D nanostructures, holding immense potential for cost-effective and scalable device fabrication

Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal–halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl–. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability